Biogeochemical Processes of Biogenic Elements in China Marginal Seas

ADVANCED TOPICS IN SCIENCE AND TECHNOLOGY IN CHINA ADVANCED TOPICS IN SCIENCE AND TECHNOLOGY IN CHINA Zhejiang Univer...

Author: Jinming Song

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ADVANCED TOPICS IN SCIENCE AND TECHNOLOGY IN CHINA

ADVANCED TOPICS IN SCIENCE AND TECHNOLOGY IN CHINA Zhejiang University is one of the leading universities in China. In Advanced Topics in Science and Technology in China, Zhejiang University Press and Springer jointly publish monographs by Chinese scholars and professors, as well as invited authors and editors from abroad who are outstanding experts and scholars in their ﬁelds. This series will be of interest to researchers, lecturers, and graduate students alike. Advanced Topics in Science and Technology in China aims to present the latest and most cutting-edge theories, techniques, and methodologies in various research areas in China. It covers all disciplines in the ﬁelds of natural science and technology, including but not limited to, computer science, materials science, life sciences, engineering, environmental sciences, mathematics, and physics.

Jinming Song

Biogeochemical Processes of Biogenic Elements in China Marginal Seas With 288 ﬁgures

Author Prof. Jinming Song Institute of Oceanology Chinese Academy of Sciences Qingdao 266071, China E-mail: [email protected]

ISSN 1995-6819 e-ISSN 1995-6827 Advanced Topics in Science and Technology in China ISBN 978-7-308-06592-4 Zhejiang University Press, Hangzhou ISBN 978-3-642-04059-7 e-ISBN 978-3-540-04060-3 Springer Heidelberg Dordrecht London New York Library of Congress Control Number: 2009933161 © Zhejiang University Press, Hangzhou and Springer-Verlag Berlin Heidelberg 2010 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, speciﬁcally the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microﬁlm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations are liable to prosecution under the German Copyright Law. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a speciﬁc statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Cover design: Frido Steinen-Broo, EStudio Calamar, Spain Printed on acid-free paper Springer is a part of Springer Science+Business Media (www.springer.com)

Preface

Marine biogeochemistry is the study of the interactions of the biology, chemistry, and geology of the ocean, i.e., the roles of distribution, transformation, removal, enrichment and dispersal of chemical components in the ocean controlled by marine biological processes. The marine biogeochemical process is one of the most important controlling procedures of global change. Marine biogeochemical process research in China has made progress in the past 20 years, although the biogeochemical study of biogenic elements in China marginal seas started relatively late. The global research programs launched from the 1980s have greatly accelerated the development of marine biogeochemistry and made it one of the main concerns in the studies of oceanography. Since then, scientists from different ﬁelds throughout the world have been devoted to this study, and have made unprecedented progress, which was clearly shown in two aspects, i.e., an unparalleled cross-link between all the speciﬁc research ﬁelds in oceanographic research, and the systematic new results achieved up to date. Though the oceanic process is very complex, it has been understood more clearly than before. Nowadays, oceanic problems cannot be resolved using only the knowledge from one single ﬁeld. We can say that, in the past 10 years, the progress in marine research was largely demonstrated by the development of marine biogeochemistry. Global oceanic evolution research, which consists of research of diﬀerent regions, is concerned mainly with the ocean’s role in the global climate and changing marine environment. So the regional response to global oceanic change is the groundwork of that research. China marginal seas, including the Bohai Sea, the Yellow Sea (YS), the East China Sea (ECS) and the South China Sea (SCS), have their particular environmental characteristics. Extending from the continental shelf to the continental slope, from tropical to temperate seas, with the input from world-famous rivers, and with many developing and developed cities located in the coastal regions, China’s seas almost contain every kind of typical ecosystem, such as an estuarine ecosystem, a continental shelf ecosystem, an upwelling ecosystem, a coral reef ecosystem and a mangrove ecosystem. They are typical regions for studying marine bio-

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geochemical processes. The biogeochemistry process of China marginal seas has been studied continuously with great eﬀort by Chinese scientists and great achievements have been made in the ﬁeld. This monograph, which is the ﬁrst book on the biogeochemistry of biogenic elements in China marginal seas in the world, deals with the variations and change mechanisms of biogenic elements such as carbon, nitrogen, phosphorus, silica, sulfur, oxygen, in China marginal seas. It is the main research achievement of the project on “The process and mechanism of ecosystem variation in China marginal seas” which is a “Fund for Creative Research Groups” of the NSFC (No. 40821004), “The degenerated mechanism of main biological resources under composite-pollutant stress in Bohai Bay” which is a National Key Project for Basic Research in China (No. 2007CB407305), “Chemical processes in the sediment-seawater interface and biogenic elements cycling of China’s seas” which is a National Science Foundation for Outstanding Young Scientists in China project (No. 49925614), and the “100 Talents Project” of the Chinese Academy of Sciences (No. 2003-202). The monograph is a landmark in marine biogeochemistry development in China. It also lays a good foundation for further study in this ﬁeld. It is believed that many scientists and others, who are concerned with the environment, will be interested in the book. This monograph includes 6 chapters. Chapter 1 describes the basic status of China marginal seas, including the Bohai Sea, the Yellow Sea, the East China Sea and the South China Sea, and the research progress in marine biogeochemistry in China. From chapter 2 to chapter 5, the research results of biogeochemical processes on biogenic elements in China marginal seas including the Bohai Sea, the Yellow Sea, the East China Sea and the South China Sea are summarized. Chapter 6 provides the main key biogeochemical processes in China marginal seas, the prospects for biogeochemistry in China marginal seas, and the methods, concepts and focus on marine biogeochemical process research in China. There were many people who contributed to the research eﬀort over the past 30 years and their great work has contributed so much to a growing and dynamic ﬁeld in China. Now, nobody should be in any doubt that the importance of marine biogeochemical processes has been recognized where it counts. Therefore, I want to acknowledge the contributions of the numerous people who made this monograph possible. I owe a special debt of gratitude to my colleagues and students, from whose insights and understanding I have beneﬁted greatly and borrowed freely. These include Dr. Xuegang Li, Dr. Huamao Yuan, Dr. Ning Li, Dr. Peng Zhang, Dr. Liqin Duan, Dr. Yayan Xu and Dr. Sisi Xu. I also greatly appreciate the contributions of Dr. Xuelu Gao, Dr. Jicui Dai, Dr. Guoxia Zheng, and Dr. Xiaoxia L¨ u, for their diligent library research. I thank Miss Hanfeng Lin, Mr. Ian McIntosh, Mrs. Helen (Yuehong) Zhang, and Mr. Jianzhong You at Zhejiang University Press, whose consistent encouragement, hard work, and careful attention to details contributed much to the

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clarity of both the text and the ﬁgures. I also express my thanks to authors for permitting their papers published partly cited in this book. Finally, I am grateful beyond measure to my family and friends, without whose patience, understanding and forbearance this monograph would never have been written. I hope that it will be of interest to all those working in the ﬁeld.

Jinming Song Qingdao, China August 8, 2009

Contents

1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Basic Status of China Marginal Seas . . . . . . . . . . . . . . . . . . . . . . . 1 1.1.1 The Bohai Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1.2 The Yellow Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 1.1.3 The East China Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 1.1.4 The South China Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 1.2 Progress in Marine Biogeochemical Process Research in China 67 1.2.1 Progress in the Studies in Marine Biogeochemical Processes before 2000 in China . . . . . . . . . . . . . . . . . . . . . . 69 1.2.2 Progress in Biogeochemical Processes of Marine Carbon Cycles since 2000 in China . . . . . . . . . . . . . . . . . . 80 1.2.3 Biogeochemical Cycle of Biogenic Elements . . . . . . . . . . . 94 1.3 Functions of China Marginal Sea Sediments in Cycles of Biogenic Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 1.3.1 Biogenic Elements in China Marginal Sea Sediments . . . 112 1.3.2 Chemical Environments of China Marginal Sea Sediments and Early Diagenesis of Biogenic Elements . . 114 1.3.3 Contribution of Settling Particles to Biogenic Element Recycling in China Marginal Seas . . . . . . . . . . . . . . . . . . . 116 1.3.4 Contributions of China Marginal Sea Sediments in the Recycling of Biogenic Elements . . . . . . . . . . . . . . . . . . . . . 119 1.3.5 Inﬂuences of Biological Productions in China Marginal Sea Sediments on the Recycling of Biogenic Elements . . 123 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

2

Biogeochemical Processes of the Bohai Sea . . . . . . . . . . . . . . . . 139 2.1 Change Processes of Carbon in the Bohai Sea . . . . . . . . . . . . . . 140 2.1.1 Partial Pressure of CO2 in Sea Water . . . . . . . . . . . . . . . . 140 2.1.2 Riverine Sources and Estuarine Fates of Particulate Organic Carbon in Seawaters . . . . . . . . . . . . . . . . . . . . . . . 144

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2.1.3 Inorganic Carbon in Liaodong Bay Sediments of the Bohai Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 2.1.4 Biogeochemical Process of Organic Carbon in Sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 2.2 Distributions and Transformations of Nitrogen in the Bohai Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 2.2.1 Nitrogen in Seawaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 2.2.2 Evolution of Nutrients and Primary Production . . . . . . 175 2.2.3 Nitrogen Forms and the Decomposition of Organic Nitrogen in Sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 2.2.4 Sediment-Water Exchange of Inorganic Nitrogen . . . . . . 193 2.3 Biogeochemical Processes of Phosphorus and Silicon in the Bohai Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 2.3.1 Distribution of Phosphorus and Silicate in Seawaters . . . 195 2.3.2 Forms of Phosphorus and Silicon in Surface Sediments . 197 2.3.3 Processes of Nutrients across the Sediment-Water Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206 2.4 Behaviour of Heavy Metals in the Bohai Sea . . . . . . . . . . . . . . . . 214 2.4.1 Distribution of Dissolved Heavy Metals in Seawaters . . . 214 2.4.2 Dissolved Heavy Metal Pollution in Bohai Bay . . . . . . . . 219 2.4.3 Heavy Metals in Bohai Bay Sediments . . . . . . . . . . . . . . . 223 2.5 Persistent Organic Pollutants in the Coastal Areas of the Bohai Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 2.5.1 Distributions of Persistent Organic Pollutants in Sediments and Mollusks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 2.5.2 Composition and Sources of Persistent Organic Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 2.5.3 Potential Risk of Persistent Organic Pollutants . . . . . . . . 250 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252 3

Biogeochemical Processes of the Yellow Sea . . . . . . . . . . . . . . . 263 3.1 Dynamic Processes of the Yellow Sea . . . . . . . . . . . . . . . . . . . . . . . 264 3.1.1 Yellow Sea Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 3.1.2 Water Exchange Between the Yellow Sea and the East China Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 3.2 Carbon Biogeochemical Processes in the Yellow Sea . . . . . . . . . . 273 3.2.1 Carbon Processes across the Air-Sea Interface . . . . . . . . . 273 3.2.2 Biological Carbon Fixation in the South Yellow Sea Seawater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282 3.2.3 Initial Carbon Fixed Production . . . . . . . . . . . . . . . . . . . . 290 3.3 Dimethylsulﬁde and Its Fluxes across the Sea-Air Interface of the Yellow Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295 3.3.1 Characteristics of Dimethylsulﬁde and Dimethylsulfoniopropionate . . . . . . . . . . . . . . . . . . . . . . . . . 296 3.3.2 Sea-to-Air Flux of Dimethylsulﬁde . . . . . . . . . . . . . . . . . . . 299

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3.3.3 Source and Sink of Dimethylsulﬁde in the Microlayer . . 301 3.4 Biogeochemical Characteristics Nitrogen and Phosphorus in the Yellow Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 3.4.1 Variations of Nitrogen and Phosphorus in Seawaters . . . 303 3.4.2 Dry and Wet Fluxes of Nutrients . . . . . . . . . . . . . . . . . . . . 308 3.4.3 Nutrients in the South Yellow Sea Sediments . . . . . . . . . . 311 3.4.4 Nitrogen in the North Yellow Sea Sediments . . . . . . . . . . 323 3.4.5 Biogeochemical Processes of Phosphorus . . . . . . . . . . . . . 332 3.5 Biogeochemical Processes of Jiaozhou Bay, South Yellow Sea . . 337 3.5.1 Behaviour and Variation of Carbon . . . . . . . . . . . . . . . . . . 337 3.5.2 Historical Variation of Nitrogen . . . . . . . . . . . . . . . . . . . . 347 3.5.3 Historical Variation of Phosphorus . . . . . . . . . . . . . . . . . . 366 3.5.4 Biogenic Silica in the Sediments . . . . . . . . . . . . . . . . . . . . 374 3.5.5 Nutrients (N, P, Si) in the Seawaters . . . . . . . . . . . . . . . . 381 3.6 Biogeochemical Characteristics of Heavy Metals in Yellow Sea Sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384 3.6.1 Distributions of Heavy Metals . . . . . . . . . . . . . . . . . . . . . . 384 3.6.2 Annual Variations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387 3.6.3 Controlling and Inﬂuencing Factors . . . . . . . . . . . . . . . . . 388 3.6.4 Pollution Characteristics and Ecological Risk Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 3.7 Biogeochemistry of PAHs and PCBs in the Yellow Sea Sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396 3.7.1 Polycyclic Aromatic Hydrocarbons in the Sediments of the Northern Yellow Sea . . . . . . . . . . . . . . . . . . . . . . . . . 396 3.7.2 Polychlorinated Biphenyls in the Sediments of the South Yellow Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399 3.7.3 Contamination History of Polycyclic Aromatic Hydrocarbons and Polychlorinated Biphenyls in the 20th Century . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418 4

Biogeochemical Processes of the East China Sea . . . . . . . . . . . 425 4.1 Dynamic Processes in the East China Sea and Its Adjacent Ocean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426 4.1.1 Circulation and Sea-Air Interaction in the Southern Yellow Sea and East China Sea . . . . . . . . . . . . . . . . . . . . . 426 4.1.2 The Kuroshio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428 4.1.3 Currents East of the Pyukyu Islands . . . . . . . . . . . . . . . . . 430 4.2 Carbon Cycling in the East China Sea . . . . . . . . . . . . . . . . . . . . . 431 4.2.1 Spatial Distributions of Inorganic Carbon in Seawaters . 431 4.2.2 Organic Carbon (Dissolved Organic Carbon and Particulate Organic Carbon) in seawaters . . . . . . . . . . . . 440 4.2.3 Key Biogeochemical Processes of Carbon in Seawaters . 443 4.2.4 Inorganic Carbon in Sediments . . . . . . . . . . . . . . . . . . . . . . 449

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4.2.5 Biogeochemical Characteristics of Organic Carbon in Sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 460 4.3 Nitrogen Variations and Budgets in the East China Sea . . . . . . 467 4.3.1 Seasonal Variations of Nitrogen in Seawaters . . . . . . . . . 467 4.3.2 Nitrogen Distribution and Its Inﬂuencing Factors in the Sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475 4.3.3 Fluxes of Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478 4.4 Phosphorus Biogeochemistry in the East China Sea . . . . . . . . . . 482 4.4.1 Distribution of Phosphorus in the Seawater . . . . . . . . . . . 483 4.4.2 Distribution of Phosphorus in the Sediments . . . . . . . . . . 488 4.4.3 Phosphorus Burial Fluxes . . . . . . . . . . . . . . . . . . . . . . . . . . 490 4.4.4 Phosphorus Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491 4.4.5 Cycling of Phosphorus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 494 4.5 Silicate and Biogenic Silica in the East China Sea . . . . . . . . . . . 498 4.5.1 Spatial Distribution of the Dissolved Silicate in Seawaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499 4.5.2 Distribution of Biogenic Silica in Sediments . . . . . . . . . . . 504 4.5.3 Silica Balance on the East China Sea Shelf . . . . . . . . . . . 506 4.6 Dissolved Oxygen and O2 Flux across the Sea-Air Interface of the ECS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509 4.6.1 Dissolved Oxygen Distributions in Seawaters . . . . . . . . . . 511 4.6.2 O2 Flux across the Sea-Air Interface . . . . . . . . . . . . . . . . . 517 4.6.3 Factors Inﬂuencing Dissolved Oxygen Concentration . . . 520 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523 5

Biogeochemical Processes of the South China Sea . . . . . . . . . 529 5.1 Water Dynamical Processes in the South China Sea . . . . . . . . . . 529 5.1.1 Circulation and Eddies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 532 5.1.2 Water Exchange via the Straits . . . . . . . . . . . . . . . . . . . . . 537 5.1.3 Dynamics of the Mixed Layer and Thermocline of the South China Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 539 5.2 Nutrient Budgets in the Seawaters of the South China Sea . . . . 541 5.2.1 Nitrogen Budgets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 542 5.2.2 Phosphorus Budgets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 543 5.2.3 Silicate Budgets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544 5.3 Biogeochemical Processes in the Pearl River Estuary . . . . . . . . . 546 5.3.1 Nutrients in Coastal Waters of the Pearl River Estuary 546 5.3.2 Carbon in the Pearl River Estuary . . . . . . . . . . . . . . . . . . . 555 5.4 Biogenic Elements in the Northern South China Sea . . . . . . . . . 570 5.4.1 Carbon in the Northern South China Sea . . . . . . . . . . . . . 570 5.4.2 Distributions of Inorganic Nutrients in the Northern South China Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 575 5.5 Biogeochemical Processes in the Nansha Islands Waters . . . . . . 575 5.5.1 Coral Reefs and Their Aﬀected Factors . . . . . . . . . . . . . . . 577

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5.5.2 Simulated Drift-Net Theory: The New Viewpoint on the High Productivity Supporting the Nansha Coral Reef Ecosystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 579 5.5.3 Nitrogen in Sediments of the Nansha Islands Waters . . . 583 5.5.4 Carbon Cycling in the Nansha Coral Reef Ecosystem . . 590 5.5.5 Vertical Transferring Process of Major and Rare Elements in the Nansha Coral Rreef Lagoons . . . . . . . . . 605 5.5.6 Sulﬁde (−2 Valence) in Lagoon and Oﬀ-Reef Sediment Interstitial Waters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 616 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 620 6

Prospects for Marine Biogeochemical Process Research in China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 627 6.1 Marine Biogeochemical Process Research in China . . . . . . . . . . . 627 6.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 627 6.1.2 Focus on Marine Biogeochemical Process Research in China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 631 6.1.3 Research Methods of China Marginal Seas’ Biogeochemical Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . 639 6.2 Main Key Biogeochemical Processes in China Marginal Seas . . 642 6.2.1 River Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643 6.2.2 Coastal Anthropogenic Activities . . . . . . . . . . . . . . . . . . . . 646 6.2.3 Biological Pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 647 6.2.4 Ecological Disasters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 648 6.3 Prospects for Biogeochemistry in China Marginal Seas . . . . . . . 649 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 651

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 657

1 Introduction

Abstract: In this chapter the basic status of China marginal seas, including the Bohai Sea, the Yellow Sea, the East China Sea, and the South China Sea, and the research progress into marine biogeochemistry in China are described. China marginal seas are typically an ideal area to study marine biogeochemical processes and the progress is signiﬁcant, especially in biogenic elements such as carbon, nitrogen, phosphorous, and silica.

1.1 Basic Status of China Marginal Seas China marginal seas, which lie between 15°∼42° N and 105°∼134° E including the East China Marginal Sea (Fig. 1.1, Naimie et al., 2001) and the South China Marginal Sea (Fig. 1.2, Kuo et al., 2000) in the western Paciﬁc Ocean, form one of the largest marginal seas in the world. The East China Marginal Sea includes the Bohai Sea, the Yellow Sea, the East China Sea, while the South China Marginal Sea is the South China Sea, extending from temperate, subtropical to tropical zones. China marginal seas form one of the most productive parts of the world’s oceans and have a total area of 4.73×106 km2 , with a continental coastline of 1.8×104 km. The coastal ocean adjacent to three of the largest rivers in the world, the Huanghe River (Yellow River), the Changjiang River (Yangtze River), and the Zhujiang River (Pearl River), is quite active. Recent studies provided evidence that estuaries in China act as a source of nutrients and we can trace species to the ecosystem of the adjacent shelf region. For instance, the total nitrogen input from the Changjiang River was approximately 7.8×109 kg in 1997, which is a threefold increase over the level of 1968. In the northern area (the Bohai Sea and the Yellow Sea), the seasonal variations of sea surface temperature (SST) are large and vary from 0 to 28 ◦ C. In the East China Sea, the SST averages about 21 ◦ C and ranges from 7 to 28 ◦ C. In the southern area (north of the South China Sea), the

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1 Introduction

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Fig. 1.2. Map of the South China Marginal Sea. Isobaths are in meters (Kuo et al., 2000) (With permission from Elsevier’s Copyright Clearance Center)

1.1 Basic Status of China Marginal Seas

3

SST is high and its seasonal variation is small, varying from 21 to 29 ◦ C. Figs. 1.3 and 1.4 (Chen, 2008) display the SST and sea surface salinity (SSS) of three regions in winter and summer. In winter, the salinity in the Bohai Sea, Yellow Sea, East China Sea, and South China Sea is 26‰∼31.5‰, 31‰∼31.5‰, 19‰∼34.7‰, and 29‰∼34.5‰, respectively. In summer, the salinity in the Bohai Sea, Yellow Sea, East China Sea, and South China Sea is 28.5‰∼30.5‰, 29‰∼32‰, 25‰∼33.5‰, and 30.5‰∼34‰, respectively. Seawater temperatures are much lower in winter (Fig. 1.4a) whereas salinity values are generally higher (Fig. 1.4b). Cooling and stronger wind-induced mixing, supplemented by coastal upwelling, generally causes the concentrations to markedly exceed those in summer (Figs. 1.4c∼e). In general, a higher temperature (Fig. 1.3a) or salinity (Fig. 1.3b) corresponds to lower nutrient concentrations (Figs. 1.3c∼e). On the other hand, a lower temperature or salinity corresponds to higher nutrient concentrations. The monthly wind pattern is displayed in Fig. 1.5 (Lee and Chao, 2003). In winter (Fig. 1.5a), a northwesterly wind was observed in the Bohai Sea and the Yellow Sea. The wind was then roughly parallel to the coast. It became stronger and changed to a northerly wind in the East China Sea and the Taiwan Strait. It again changed to a northeasterly wind in the northern South China Sea. In spring, the wind became weaker and southerly in the Bohai Sea and the Yellow Sea. The wind then became easterly in the East China Sea toward coastal areas (Fig. 1.5b). In summer, the wind almost reversed in comparison to the winter season: a southeasterly wind in the Bohai Sea and the northern Yellow Sea, and a southwesterly strong wind in the South China Sea (circle in Fig. 1.5c). The wind in late fall gradually became similar to that in winter (Fig. 1.5d), and a strong northeasterly wind was observed on the Taiwan bank and in the northern South China Sea. In general, the wind is stronger during fall-winter (Figs. 1.5a and d) than during spring-summer (Figs. 1.5b and c) in the China seas. And the Taiwan Strait seems to have strong winds during the fall-winter season. 1.1.1 The Bohai Sea The Bohai Sea is one of the main China marginal seas, which has its own particular state and hydrography characteristics. This includes the topography, chemical environment, hydrography, sediment pattern, riverine input, and biological characteristics. 1.1.1.1 Topography The Bohai Sea is a semi-enclosed continental shelf sea of the NW Paciﬁc Ocean in northern China, located between 37°07 N to 41° N and 117°35 E to 121°10 E, with a surface area of 77×109 m2 , an average depth of 18.7 m and a coastline of nearly 3,800 km. The size of the Bohai Sea is about 500 km from north to south and 300 km from east to west. The Bohai Sea is a

4

1 Introduction

Fig. 1.3. Distributions of (a) temperature (◦ C), (b) salinity (‰), (c) nitrate concentration (μmol/L), (d) phosphate concentration (μmol/L), and (e) silicate concentration (μmol/L) at surface water in the East China Marginal Sea in August (Chen, 2008) (With permission from Springer)

1.1 Basic Status of China Marginal Seas

5

Fig. 1.4. Distributions of (a) temperature (◦ C), (b) salinity (‰), (c) nitrate concentration (μmol/L), (d) phosphate concentration (μmol/L), and (e) silicate concentration (μmol/L) at surface water in the East China Marginal Sea in February (Chen, 2008) (With permission from Springer)

6

1 Introduction N 40 (a)

(c)

(b) 1 dyne/cm

2

1 dyne/cm

Korea

35 30

(d)

2

1 dyne/cm

Korea Shikoku Kyushu

2

1 dyne/cm

Shikoku Kyushu

2

Korea

Korea

Shikoku Kyushu

Shikoku Kyushu

Yangtze River

Yangtze River

Yangtze River

Yangtze River

China

China

China

China

25 120

125

130 E

120

125

130

E

120

125

130

E

120

125

130 E

Fig. 1.5. Monthly climatological wind-stress ﬁelds in: (a) October, (b) December, (c) April, and (d) August (Lee and Chao, 2003) (With permission from Elsevier’s Copyright Clearance Center)

shallow coastal area with a depth of less than 30 m in 95% of the area. The water depth of the Bohai Sea is shallow at around 10∼20 m in the coastal area, including Liaodong Bay (Li-B, north of 40° N), Bohai Bay (Bo-B, west of 118.6° E), Laizhou Bay (La-B, south of 37.6° N), central Bohai (Cn-B), and the Bohai Strait (Bo-S). The area with a depth of more than 30 m is mainly located in the northern section of the Bohai Strait. The maximal water depth of 70 m is found in the northern part of the Bohai Strait, the connection with the Yellow Sea. The topography of the Bohai Sea is given in Fig. 1.6 (S¨ undermanna and Feng, 2004). N 41

40

39 Bohai Sea Bohai Strait

38

37 Depth 70

118 60

119 50

121

120 40

30

20

122 10

E 0 (m)

Fig. 1.6. Bathymetry of the Bohai Sea (S¨ undermanna and Feng, 2004) (With permission from Elsevier’s Copyright Clearance Center)

1.1 Basic Status of China Marginal Seas

7

The Bohai Sea is surrounded by land to the north, west and south and is connected only to the Yellow Sea through the narrow Bohai Strait, approximately 90 km wide. Rivers empty into the Bohai Sea including the Huanghe River, Haihe River, Liaohe River, and Luanhe River. The Bohai Strait is situated in the region between the Liaodong Peninsula and the Shandong Peninsula. At its narrowest part, the Bohai Strait is about 105 km wide. It is separated into several channels by the Miaodao Islands. The north channel is the main channel which has a maximum depth of 86 m. The seabed of the north channel consists of gravel and exposed bedrocks. The southern channels are shallow, with maximum water depths between 20 and 30 m. A wide terrace, with a gradient of only 1/2,000, extends from the north coast of the Shandong Peninsula to the 25 m isobath. The central part of the Bohai Strait is deeper than 40 m, and is characterized by a relatively ﬂat bottom. 1.1.1.2 Hydrographical and Chemical Environment The water temperature of the Bohai Sea is vertically homogeneous from November to March. A thermocline is gradually formed from April to May in the deep area of the Bohai Sea. The thermo-stratiﬁcation intensiﬁes and becomes greater than 1 ◦ C/m from June to August. Then it decreases rapidly in September. The average annual precipitation in the Bohai Sea is about 500∼600 mm (Feng et al., 1999). In spring, the temperatures of surface waters and sea bottom waters range from 11.6 to 19.8 ◦ C and 10.5 to 19.7 ◦ C, respectively, with the average values being 16.1 ◦ C and 15.1 ◦ C. In summer, the temperatures of surface waters and sea bottom waters range from 23.4 to 28.6 ◦ C and 16.8 to 27.7 ◦ C, respectively, with the average values being 26.1 ◦ C and 22.9 ◦ C. In autumn, the temperatures of surface waters and sea bottom waters range from 17.2 to 22.6 ◦ C and 18.1 to 22.8 ◦ C, respectively, with the average values being 21.2 ◦ C and 21.3 ◦ C. Overall, the temperature is higher nearshore than oﬀshore, but there are some areas of exception. Conductivity-temperature-depth (CTD) data in the Bohai Sea show three low-temperature centers in summer in the middle and lower layers: the central Bohai Straits, Liaodong Bay Mouth, and Bohai Bay Mouth. In addition, there is an evenly high-temperature center in the middle Bohai Sea. In winter the isotherms extend northwestward in the Bohai Sea with a cold tongue-shaped mass oﬀ Qinhuangdao towards the southeast, presenting a saddle-like isotherm pattern in the central Bohai Sea. The salinity of the Bohai Sea has obvious temporal and spatial variations. In spring, the salinity in surface and sea bottom waters ranges from 27.75‰ to 31.9‰ and 29.83‰ to 32.46‰, respectively, with the average values of 31.49‰ and 31.40‰. In summer, the salinity in surface and sea bottom waters ranges from 15.39‰ to 31.45‰ and 27.96‰ to 31.47‰, with the averages of 29.73‰ and 30.74‰, respectively. In autumn, the salinity in surface and sea

8

1 Introduction

bottom waters ranges from 23.08‰ to 31.68‰ and 23.9‰ to 31.65‰, with the averages of 29.75‰ and 29.82‰, respectively. In addition, the salinity is higher in the Bohai Bay and lower in the Bohai Straits (Fig. 1.7) (Bao et al., 2004). N 41

N 41

(a)

(b)

40

40

39

39

38

38

37

37 118

119

120

121

122

E

118

119

120

121

122

E

Fig. 1.7. Salinity and temperature of sea bottom waters in (a) winter and (b) summer in the Bohai Sea. Solid and dotted lines denote salinity (‰) and temperature (◦ C), respectively (Bao et al., 2004) (With permission from Bao XW)

Long-term variations of temperature and salinity in the Bohai Sea are displayed in Fig. 1.8 (Lin et al., 2001). The annual mean SSS and SST of the Bohai Sea both show ascending trends during 1960∼1997. The linear trends were 0.074‰ per year for SSS and 0.011 ◦ C per year for SST respectively. The long-term variations of these annual means both showed climate-jump years or inﬂection years. The dissolved oxygen (DO) and pH are important chemical parameters of seawaters. The DO also varies with the change in seasons. In spring, the DO and pH range from 7.2 to 11.2 mg/L and 7.97 to 8.33, respectively, with averages of 9.59 mg/L and 8.13. In summer, the DO and pH range from 5.4 to 8.1 mg/L and 7.99 to 8.25, respectively, with averages of 6.6 mg/L and 8.14. In autumn, the DO and pH range from 5.2 to 8.3 mg/L and 8.00 to 8.60, respectively, with averages of 7.1 mg/L and 8.31. The silicate (SiO3 -Si), phosphate (PO4 -P), and inorganic nitrogen (DIN) are the nutrients of planktons. In spring, SiO3 -Si and PO4 -P concentrations in surface waters are 6.08∼38.7 and 0∼1.96 μmol/L, with averages of 14.1 and 0.42 μmol/L, respectively. In summer, SiO3 -Si and PO4 -P concentrations in surface waters are 5.16∼47.63 and 0.11∼0.69 μmol/L, with averages of 20.08

1.1 Basic Status of China Marginal Seas Sea surface temperature ( )

Sea surface salinity ( )

34 (a) 32 30 28 26 24 1960

1970

1980 Year

1990

1997

14

9

(b)

13 12 11 10 1960

1970

1980 Year

1990 1997

Fig. 1.8. Long-term variation of annual mean (a) SSS and (b) SST of the Bohai Sea. The solid lines are from the averages of the respective annual means of the Bohai Sea and the dashed lines are their linear regressions (Lin et al., 2001) (With permission from Elsevier’s Copyright Clearance Center)

and 0.27 μmol/L, respectively. In autumn, SiO3 -Si and PO4 -P concentrations in surface waters are 7.35∼47.61 and 0.07∼1.34 μmol/L, with averages of 19.16 and 0.4 μmol/L, respectively (Cheng et al., 2004). DIN mainly includes nitrite (NO2 -N), nitrate (NO3 -N), and ammonium (NH4 -N). In spring, NO3 -N, NO2 -N, and NH4 -N concentrations in surface waters range from 0.85 to 22.23 μmol/L, 0.03 to 5.4 μmol/L, and 0 to 5.24 μmol/L, with average values of 8.65, 0.68, and 1.16 μmol/L, respectively. In summer, NO3 -N, NO2 -N, and NH4 -N concentrations in surface waters are 0.32∼5.78, 0∼8.21, and 0∼4.84 μmol/L, with averages of 3.86, 0.95, and 1.02 μmol/L, respectively. In autumn, NO3 -N, NO2 -N, and NH4 -N concentrations in surface waters are 2.72∼10.71, 0.08∼3.22, and 0∼4.34 μmol/L, with averages of 4.89, 0.93, and 0.84 μmol/L respectively (Cheng et al., 2004). 1.1.1.3 Tides In the Bohai Sea, the tidal movement is one of the prominent hydrodynamic processes. The tidal dynamics is characterized by a syntonic tide system deduced by the introduction of the tidal wave system of the northern Yellow Sea, which propagates through the Bohai Strait into the Bohai Sea. The largest part of the Bohai Sea exhibits a mixed semidiurnal tide. The tides are predominately semi-diurnal showing a tidal range of about 4 m with surface tidal currents reaching maximum values of 0.7 and 2 m/s during neap and spring tides, respectively. The M2 tide is the principal tidal constituent in the Bohai Sea. There are two amphidromic points, one close to the coast near Qinhuangdao and the other oﬀ the Huanghe River delta. The K1 tide has an amphidromic point at the southern part of the Bohai Sea. The maximum amplitude of the (M2+K1) tide is about 2 m. The maximum velocity of (M2+K1) tidal current is about 1 m/s. Fig. 1.9 (Jiang et al., 2000) shows

10

1 Introduction

the M2 tidal ellipses in the surface layer. There is a (left) rotating tidal ﬂow entering the Bohai Sea from the Yellow Sea and propagating into the three inner bays almost as an alternating current (Feng et al., 1999). N 41

Tidal ellipses of M2 Layer: 1, 0~3 m d t =6 min barotropic of HAMSOM

40

Qinhuangdao Changxingdao Tanggu

39

38 25 cm/s

R Y.

. Longkou

50 cm/s 100 cm/s

37 118

119

120

121

122

E

Fig. 1.9. Tidal ellipses of the M2 in the surface layer (Jiang et al., 2000) (With permission from Elsevier’s Copyright Clearance Center)

In summer and autumn, typhoon surges usually occur in the southeast and south Chinese coastal zones, but there are some typhoon surges in the Bohai Sea. In winter there are strong wind surges in Bohai Bay and Laizhou Bay (Feng et al., 1999). In addition, two counter-clockwise semidiurnal (M2, S2) amphidromic systems are formed (Sun and Xi, 1988). In the Bohai Sea the maximum surface current velocity is 1∼1.5 m/s. In Laizhou Bay the ﬂow is weaker and the maximum current velocity is less than 1 m/s. The currents are stronger in the northern part of the Bohai Strait where the maximum current velocity can reach 2.5 m/s. Wind and abundance structure also contribute to the residual currents. The general circulation in the Bohai Sea consists of an inﬂow through the northern section of the Bohai Strait and an outﬂow through the southern part. The vertical stratiﬁcation occurs only in summer and can easily be eroded by strong wind events. 1.1.1.4 Circulation In the Bohai Sea, ﬁve water masses are recognized. They are the Bohai Sea and the Yellow Sea mixed water, the North Yellow Sea bottom cold water, the Bohai Sea central water, the Bohai Sea coastal water and the continental diluted water. There is a strong seasonal signal in temperature with a smaller

1.1 Basic Status of China Marginal Seas

11

amplitude in the cold water at the bottom. Salinity exhibits a clear seasonality only for the Bohai Sea and the Yellow Sea mixed water. In winter, the distribution of water mass at the sea surface and on the bottom layer is basically identical (Fig. 1.10, S¨ undermanna and Feng, 2004). Gradual modiﬁcation occurs from November to April at the sea surface and from December to March on the bottom layer. Distribution of the water mass at the sea surface and on the bottom layer in spring and autumn is transitional and patchy. The distribution of temperature and salinity and the circulation pattern in the Bohai Sea are displayed in Fig. 1.11 (S¨ undermanna and Feng, 2004). The “Yellow Sea Warm Current Extension”, like a jet, enters the Bohai Sea through the Bohai Strait, and moves westward along the central part until it meets the coast and there splits into two branches. One branch moves towards Liaodong Bay, forming a clockwise gyre, due to the merging of a current oﬀ the Huanghe River delta in a northeast direction and a current from the northeast in western Liaodong Bay, while the other current moves towards Bohai Bay, forming a counterclockwise gyre. 1.1.1.5 Wind Wind waves are a dominant feature in the Bohai Sea. They are important to the suspended particle matter (SPM) dynamics. Because there is a strong monsoon signal in the meteorological data, especially in the eastern part of the Bohai Sea, the wave direction changes accordingly. In winter, southerly waves prevail and in summer northerly waves dominate. The signiﬁcant wave height is 0.3∼0.7 m in the near shore area of the Bohai Sea. In the Bohai Strait and the central part of the Bohai Sea it can reach 1.1 m (Qin et al., 1985). The East Asia Monsoon dominates the meteorology of this region. In winter, under the inﬂuence of Asian High Pressure and Aleutian Low Pressure, strong winds with a mean wind speed of 6∼7 m/s frequently blow over the Bohai Sea from the north. In summer, southerly winds with a mean speed smaller than 4∼6 m/s blow. The air temperature over the Bohai Sea reaches its lowest value of −4 to ∼0 ◦ C in January and highest value of 25 ◦ C in July. 1.1.1.6 Distribution Pattern of Sediments The sediments in the Bohai Sea are generally ﬁne (Fig. 1.12, Jiang et al., 2004). They consist of soft clay mud (sediment with sizes smaller than 0.01 mm amounts to a portion of more than 70%), ﬁne silt mud (sediment with sizes smaller than 0.01 mm amounts to a portion of 50%∼70%), coarse silt, and ﬁne sand. In Liaodong Bay the coarse silt (0.1∼0.05 mm) and ﬁne sand (0.25∼0.1 mm) dominate in the sediment. In Laizhou Bay the sediment consists of silt deposits, whereas in the central basin ﬁne sand spreads widely. At the Laotieshan waterway (entrance to the Bohai Sea), the tidal ﬂow is very strong, the area is often eroded and therefore the sediment particles are coarse there. The constituents of SPM in the Bohai Sea are mainly inorganic

12

1 Introduction

Fig. 1.10. The Bohai Sea water mass at the sea surface and on the bottom layer. (a) Winter, surface; (b) Winter, bottom; (c) Summer, bottom. B: Bohai Sea central water; BS: Bohai Sea coastal water; BY: Bohai Sea and Yellow Sea mixed water; F: continental diluted water; YC: North Yellow Sea bottom cold water (S¨ undermanna and Feng, 2004) (With permission from Elsevier’s Copyright Clearance Center)

1.1 Basic Status of China Marginal Seas

13

N 40

35

30

25

20 105

115

125

135 E

Fig. 1.11. Schematic diagram of the circulation in the Bohai Sea, Yellow Sea, and East China Sea. (a) Winter; (b) Summer (S¨ undermanna and Feng, 2004) (With permission from Elsevier’s Copyright Clearance Center)

Yingkou N 40

Tianjin

Qinhuangdao

Tanggu

38

Hu an gh eR ive r

Dalian

Penglai

1

6

2 3

7 8

4

9

5 118

120

122

E

Fig. 1.12. Sediment type distribution of the Bohai Sea. 1, Gravel; 2, Medium sand; 3, Fine sand; 4, Coarse silt; 5, Fine silty mud; 6, Silty clay mud; 7, Clay mud; 8, Shell; 9, Nodule (Jiang et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

14

1 Introduction

particles from the land. Most of the particles are smaller than 0.01 mm in size (Qin et al., 1985). The sources of SPM are the adjacent rivers and resuspension. Atmospheric dust transported by the wind is a further source of SPM in the Bohai Sea (Qin and Li, 1982). The seasonal and regional variation of SPM concentration is evident, which is closely related to the ﬂow and the sediment concentration of the rivers. 1.1.1.7 Riverine Discharge There are several rivers (e.g., Huanghe River, Haihe River, Liaohe River, and Luanhe River) which ﬂux into the Bohai Sea with a total annual water discharge of 68.5×109 m3 /yr and annual suspended matter input of 1.1×109 t/yr. The Huanghe River is famous for its high concentration of sediments in the water. The average run-oﬀ is 42×109 m3 /yr and the sediment transport per year is 1.0×109 t, accounting for more than 50% of the fresh water and 90% of the sediment input into the Bohai Sea through rivers. But there is strong seasonal variation in both sediment and water discharge from the Huanghe River, whereas those from the Luanhe and Haihe Rivers are about 2.67×107 and 6×106 t/yr, respectively (Qin et al., 1985). The river discharges concentrate in July to October, the ﬂood season in China (Li GX et al., 2004; Qin et al., 1985). After entering the Bohai Sea, the Huanghe River sourced suspended particle matter (SPM) is carried onto the western bank of Liaodong Bay, while most of the sediments settle down in estuary regions. Deposition ﬂux decreases gradually with the distance from land, which has been demonstrated in the mineral distribution in surface sediments of the Bohai Sea (Qin et al., 1985). Sediments from the Luanhe River are transferred into Liaodong Bay along the western coast of the Bohai Sea, while those of the Liaohe River move along the eastern coast into the southern Bohai Sea. Most of the water and sediment discharge occur in the ﬂood season (July to September). Although historical observation of nutrients in this region started in the late 1950s, including several national and international cooperative programs in the 1980s, most of these research activities were focused on distribution and variation of nutrients in the water column. Benthic metabolism and nutrient regeneration processes at the sediment-water interface are still poorly known in the study of the region. Decadal-average annual freshwater ﬂux into the Bohai Sea was shown in Fig. 1.13 (Lin et al., 2001). Most of the rivers have been dammed for irrigation and municipal water supply; for example, over the last two decades for many days in the year the Huanghe River discharged no water into the Bohai Sea, because of the constant withdrawal of water upstream along the river. Moreover, sedimentation aﬀects the estuaries, the coastal zone and the sea enormously. This eﬀect is speciﬁcally important at the Huanghe River delta. Here each year the coastline extends towards the sea 150∼420 m and,

1000 800 600 400 200 0 200 400 600 800 1000

15

34

Sea surface salinity ( )

Fresh water flux ( 104 m3)

1.1 Basic Status of China Marginal Seas Sea surface salinity 32 30 28 26 Fresh water flux 24 1960 1970 1980 Year

1990

1997

Fig. 1.13. Annual variations of the (partial) net fresh water ﬂux into the Bohai Sea and annual mean SSS of the Bohai Sea. The dashed lines are their respective linear regression representations (Lin et al., 2001) (With permission from Elsevier’s Copyright Clearance Center)

on average, 23 km2 of land is created. It should be pointed out that in recent years, owing to the dry weather on the middle and low reaches of the Huanghe River and the retaining and drawing of water from the Huanghe River, the runoﬀ of the river decreased dramatically. For example, in 1997 the lower river was dry for as many as 226 days (Feng et al., 1999). 1.1.1.8 Biological Characteristics Long-term variations of SST and SSS may aﬀect the ecosystems in the Bohai Sea. Limited observations show that the population of phytoplankton and zooplankton, benthos community biomass, ﬁsh community biomass, the index of species diversity, and the recruitment of penaeid shrimp all showed an outof-phase change with the variations in SSS and SST of the Bohai Sea (Table 1.1). (1) Chlorophyll a and primary production The concentration of chlorophyll a is used as the most convenient index of phytoplankton biomass. In 1982/1983, the annual cycle of chlorophyll a concentration in the Bohai Sea was of a double peak type, and two blooms occurred in spring and autumn. The chlorophyll a concentration ranged from 0.60 to 1.95 mg/m3 . An exception was in the Bohai Bay where the chlorophyll a concentration was low in March and increased after April until the maximum in August. The spring bloom spread out from Laizhou Bay and south of the central Bohai Sea in March into Liaodong Bay, and into the central Bohai Sea in April. Finally it spread into Bohai Bay in August. In May, high chlorophyll a concentration existed in Laizhou Bay and south of Bohai Bay, while low chlorophyll a concentrations were found in Liaodong Bay and north of the central Bohai Sea. High chlorophyll a concentrations were found along the coast of Bohai Bay and west of the Laizhou Bay, while the chlorophyll

16

1 Introduction Table 1.1. Biological population change in the Bohai Sea∗

Observation period Phytoplankton biomass (×104 cells/m3 ) Zooplankton biomass (mg/m3 ) Zooplankton density (ind./m3 ) Coscinodiscaceae (×104 cells/m3 ) Chaetoceros (×104 cells/m3 ) Mean catch per haul of benthos (kg/haul) Mean density per haul of benthos (ind./haul) Fish community biomass (t) Index of species diversity (Shannon-Weiner index, H ) Recruitment of penaeid shrimp (106 number of individuals) ∗

1959 188 205 8 129

1982∼1983 222 160 800∗∗ 20 90 22.91 2,683

1992∼1993 99 70 100 9 8 13.99 1,428

82,074 3.6092

72,120 2.5296

361

72

The data originate from Cheng and Guo (1998), Deng et al. (1999), Jin and Tang (1998),

Meng (1998) and Wang and Kang (1998).

∗∗

1985 data

a concentration was extremely low in the central Bohai Sea in September. In the year 1984/1985, high chlorophyll a concentration appeared in summer and autumn. The chlorophyll a concentration ranged from 0.5 to 4.0 mg/m3 . The highest chlorophyll a concentration occurred in spring and summer. In autumn, the chlorophyll a concentration was higher in east of the Laizhou Bay than in the other areas. The chlorophyll a concentration in the Bohai Strait was low during the whole year, except in autumn. The annual average net primary production was high in Laizhou Bay and the central Bohai Sea (>200 mg C/(m2 ·d)), while it was lowest in Bohai Bay (<100 mg C/(m2 ·d)). In the data set of 1992/1993, high chlorophyll a concentration appeared in spring and summer though the highest value existed in autumn in the east corner of Laizhou Bay; the chlorophyll a concentration ranged from 0.5 to 7.5 mg/m3 . Compared with the chlorophyll a distribution in 1982/1983, the pattern did not change much, while the concentrations were higher in February and May but lower in August and October, thus making the annual primary production of the total Bohai Sea decrease by 30%. In the year 1998/1999, chlorophyll a concentration showed a similar pattern as the monthly data of 1982/1983; the chlorophyll a concentration ranged from 0.05 to 8.19 mg/m3 (Fig. 1.14). The amount of primary production of the 1998/1999 cruises varied between 10 and 740 mg C/(m2 ·d), with the average value of 244 mg C/(m2 ·d). In general, it was higher than that in 1982/1983 because the chlorophyll a concentration was low in 1982/1983. In autumn, the high primary production was observed in the Bohai Strait (>420 mg C/(m2 ·d)), while the low primary production was located in the southwest of the observed area (<30 mg C/(m2 ·d)). The distribution pattern of primary production in spring was

1.1 Basic Status of China Marginal Seas N 41.0

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40.5 40.0

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119

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121

122 E

Fig. 1.14. Concentration distribution of chlorophyll a in (a) September, cruise 1998 and (b) May, cruise 1999 (Wei et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

similar to that in autumn. Maximum values of more than 700 mg C/(m2 ·d) were observed near to the Bohai Strait. (2) Phytoplankton Diatoms and dinoﬂagellates are the major components of the phytoplankton community in the Bohai Sea. Cyanobacteria, green algae, and silico-ﬂagellates are also common in some areas. Concerning size fractional features of the phytoplankton community in the Bohai Sea, nanophytoplankton was a major component and the picophytoplankton took up 21.3% and 28.4% of the biomass in autumn 1998 and spring 1999, respectively. Three phytoplankton provinces can be discerned: the Huanghe River Estuary, north of Bohai Bay, and the Bohai Strait. The annual variation of the phytoplankton community in the Bohai Sea exhibits the typical double-peak of the northern hemisphere. The higher cell abundance peak is in April and the lower

18

1 Introduction

peak in September. In spring, phytoplankton biomass is 132.2×104 cells/m3 and consists of 31 species—diatoms 24 species and dinoﬂagellates 7 species. Coscinodiscus asteromphalus Ehrenberg, Noctiluca scintillans (Macartney) Kofoid et Swezy, Chaetoceros sp., Nitzschia spp., Pleurosigma spp., Coscinodiscus radiatus Ehrenberg, and Biddulphia sinensis Greville were the dominant species (by abundance), accounting for 21.1%∼86.6%. In summer, phytoplankton biomass is 12.9×104 cells/m3 and consists of 44 species— diatoms 36 species and dinoﬂagellates 8 species. Coscinodiscus asteromphalus Ehrenberg, Noctiluca scintillans (Macartney) Kofoid et Swezy, Chaetoceros sp., Coscinodiscus radiatus Ehrenberg, Biddulphia sinensis Greville, Ceratium tripos (O.F. Mueller) Witzisch Pleurosigma spp., Thalassiothrix frauenfeldii (Grun.) Grunow, and Coscinodiscus lineatus Ehrenberg were dominant species, accounting for 21.1%∼89.5%. In autumn, phytoplankton biomass is 25.1×104 cells/m3 and consists of 46 species—diatoms 38 species and dinoﬂagellates 8 species. Coscinodiscus asteromphalus Ehrenberg, Biddulphia sinensicus Greville, Pleurosigma spp., Coscinodiscus lineatus Ehrenberg, Thalassiothrix frauenfeldii (Grun.) Grunow, Chaetoceros sp., Noctiluca scintillans (Macartney) Kofoid et Swezy, Coscinodiscus radiatus Ehrenberg, Ditylum brightwellii (West) Grunow, Chaetoceros castracanei Karsten, Nitzschia pungens Grunow, and Coscinodiscus granii Gough were the dominant species, accounting for 21.1%∼100%. The species succession was the major process within the seasonal changes of the phytoplankton community development in the Bohai Sea. Compared with historical data, the replacement of diatoms by dinoﬂagellates is the main feature of the phytoplankton community changes in recent years, which may be determined by the N/P ratio increasing and the Si/N ratio decreasing. (3) Zooplankton According to the investigation in May, June, August, and October, zooplankton consist of 46 species, mainly including Sgitta crassa Tokioka, Calanus sinicus Brodsky, Labidocera euchaeta Giesbrecht, Centropages mcmurrichi Willey, Acrtia paciﬁca Steuers, etc. In spring, summer, and autumn, zooplankton biomass are 618, 293, and 115 mg/m3 , respectively (Cheng et al., 2004). (4) Benthos species According to the three seasons’ investigation, the average benthos biomass is 57 g/m2 , and the average abundance is 450 ind./m2 . The mollusk biomass is highest, with an average value of 33 g/m2 , accounting for 58% of the total. The average abundance is also the largest, 258 ind./m2 , accounting for 57%, followed by polychaeta with an average biomass of 7 g/m2 , accounting for 12%, and an average abundance of 90 ind./m2 , accounting for 20%. Echinodermata average biomass is 7 g/m2 , and the abundance is 30 ind./m2 . The biomass and abundance of carapace are the lowest, 4 g/m2 and 19 ind./m2 , respectively. In spring, the average benthos biomass and abundance are 55 g/m2 and 509 ind./m2 . In summer, the average benthos biomass and abundance are 50 g/m2

1.1 Basic Status of China Marginal Seas

19

and 304 ind./m2 . In autumn, the average benthos biomass and abundance are 70 g/m2 and 552 ind./m2 (Cheng et al., 2004). A total of 460 macrofaunal species have been recorded in the Bohai Sea, 249 species in Laizhou Bay, 271 species in the central Bohai Sea, and 168 species in Bohai Bay. In the Bohai Sea, Polychaeta and Crustacea were the two groups consisting of more than 60% of total faunal species (Fig. 1.15). There was a decreasing trend in the relative number of Polychaeta species, whereas the relative number of Crustacea species tended to increase from the 1980s to the 1990s. Overall, the average macrofaunal abundance in the Bohai Sea was estimated to be 1,700 ind./m2 , of which Polychaeta and Bivalvia were the most dominant groups, together contributing more than 50% of the total abundance (Fig. 1.16). One exception is for Bohai Bay in the 1980s when Echinodermata had a quite high dominance of up to 25%. Noteworthy is the fact that the relative abundances of Echinodermata declined at all locations in the study area from the 1980s to 1990s and a t-test showed that their abundance was also signiﬁcantly reduced in the central Bohai Sea. On the other hand, there were signiﬁcant increases in the abundance of Polychaeta and Bivalvia in both the central Bohai Sea and Bohai Bay, and Gastropoda in the central Bohai Sea and Crustacea in Bohai Bay, respectively. However, dominance of these groups may have increased, decreased or remained unchanged (Fig. 1.16). 1980s 1990s

1980s 1990s

1980s 1990s

L

C Location

B

Relative number of species (%)

100 90 80 70 60 50 40 30 20 10 0

Polychaeta

Bivalvia

Gastropoda

Crustacea

Echinodermata

Others

Fig. 1.15. Species composition of major macrofaunal groups in the Bohai Sea. L, Laizhou Bay; C, central Bohai Sea; B, Bohai Bay (Zhou et al., 2007) (With permission from Elsevier’s Copyright Clearance Center)

20

1 Introduction

Fig. 1.16. Relative abundance of major macrofaunal groups in the Bohai Sea. L, Laizhou Bay; C, central Bohai Sea; B, Bohai Bay (Zhou et al., 2007) (With permission from Elsevier’s Copyright Clearance Center)

Over a decade from the 1980s to 1990s, the macrofaunal community structure in the Bohai Sea changed notably. Most of the species/families increased in abundance consistently over the three geographic locations. These are small-sized species including four polychaetes (Sternaspis scutata, Paralacydonia paradoxa, Ancistrasyllis constricta, Nephtys oligobranchia), three bivalves (Moerella jedoensis, Modiolus elongates, Leptomya minuta) and one gastropod (Yokoyamaia argentata). On the other hand, only one bivalve (Musculista senhousia) and two echinoderms (Amphioplus japonicus, Echinocardium cordatum) showed consistently and notably reduced abundance after 10 years. E. cordatum, ever abundant in Laizhou Bay, nearly disappeared from the study area during the 1990s’ survey although its distribution was still recorded in the Bohai Strait. The rest of the species showed an inconsistent pattern over space in temporal change of abundance. The most evident increases in abundance at the family level were identiﬁed from ﬁve Polychaeta families (Sternaspidae, Lacydoniidae, Nephtyidae, Pilargiidae, and Goniadidae) and two families of bivalve (Tellinidae, Semelidae). Clearly this is a mirror of the temporal pattern of the dominant species in the respective families. There appeared an increasing trend from the 1980s to 1990s for the diversity indices and total faunal abundance. Relative elevation of k -dominance curves clearly suggested that after 10 years the diversity decreased in the central Bohai Sea and Bohai Bay but increased in Laizhou Bay (Fig. 1.17a) and the macrobenthic diversity in the Bohai Sea as a whole was only slightly

1.1 Basic Status of China Marginal Seas

21

Cumulative dominance (%)

100

Cumulative dominance (%)

100

Cumulative dominance (%)

decreased (Fig. 1.17b). A general spatial pattern of diversity could also be identiﬁed if samples were pooled for each region (Fig. 1.17c), i.e., a diversity ranking of the central Bohai Sea, Laizhou Bay, and Bohai Bay.

100

(a) 1980s C

80

1980s B

60

1980s L

40

1990s C

20

1990s B

0

1990s L 1

10 100 Species rank

1000

(b)

80

80s 90s

60 40 20 B

0 1

10 100 Species rank

1000

(c) C B L

80 60 40 20 0 1

10 100 Species rank

1000

Fig. 1.17. k -dominance curves to compare diversity between decades and among geographic locations. (a) Pooled samples for the two decades (1980s, 1990s) within each of the three regions; (b) Pooled samples for the two decades; (c) Pooled samples for the three regions. C: central Bohai Sea; B: Bohai Bay; L: Laizhou Bay (Zhou et al., 2007) (With permission from Elsevier’s Copyright Clearance Center)

(5) Fish In the Bohai Sea there are 10 species of ﬁsh whose biomasses account for 1%. They are Konosirus punctatus, Engraulis japonicus, Setipinnaes taty, Pseu-

22

1 Introduction

dosciaena polyactis, Eupleurogrammus muticus, Pampus argenteus, Scomberomorus niphonius, Thrissa kammalensis, Chaeturichthys stigmatias, etc., which account for 92.8%. In spring, those biomasses accounting for 1% include Konosirus punctatus, Engraulis japonicus, Setipinna taty, Pseudosciaena polyactis, Eupleurogrammus muticus, Pampus argenteus, Scomberomorus niphonius, Cynoglossus joyneri, Callionymus beniteguri, etc. In summer, those biomasses accounting for 1% include Konosirus punctatus, Engraulis japonicus, Setipinna taty, Pseudosciaena polyactis, Argyrosomus argentatus, etc., which account for 96.6%. In autumn, those biomasses accounting for 1% include Konosirus punctatus, Engraulis japonicus, Setipinna taty, Pseudosciaena polyactis, Eupleurogrammus muticus, Pampus argenteus, Scomberomorus niphonius, Thrissa kammalensis, Chaeturichthys stigmatias, etc., which account for 95.3% (Cheng et al., 2004). (6) Trophic levels Mean trophic level (TL) of high TLs calculated by using dominant species occupying 80% of total biomass showed that mean TL at high TLs in the Bohai Sea declined clearly from 4.06 in 1959∼1960 to 3.41 in 1998∼1999, about 0.17 a decade during the past 40 years. The mean TL declined by 0.16 a decade from 1959∼1960 to 1982∼1983, 0.19 a decade from 1982∼1983 to 1992∼1993, and 0.17 a decade from 1992∼1993 to 1998∼1999 (Fig. 1.18). Dominant species accounting for 80% of the total biomass in the Bohai Sea included one kind of planktivorous, omnivorous, and piscivorous species, respectively, no benthivorous species in 1959∼1960, and four kinds of planktivorous species, two kinds of omnivorous species, no benthivorous or piscivorous species in 1998∼1999. In the Bohai Sea, the percentage of planktivorous species increased from 4.75% in 1959∼1960 to 58.02% in 1998∼1999, while the percentage of piscivorous species decreased remarkably from 29.28% in 4.20 4.00

TL

3.80 3.60 3.40 3.20 3.00 1959~1960 1982~1983 1985~1986 1992~1993 1998~1999 2000~2001 Year Mean TL of dominant species accounting for 80% of total biomass Mean TL of dominant species accounting for 85% of total biomass Mean TL of dominant species accounting for 90% of total biomass

Fig. 1.18. Decadal variations of mean TL of the Bohai Sea (Zhang et al., 2007) (With permission from Elsevier’s Copyright Clearance Center)

1.1 Basic Status of China Marginal Seas

23

1959∼1960 to 4.27% in 1982∼1983 and no piscivorous species in 1992∼1993 or 1998∼1999 (Figs. 1.19a∼d). 4.75% 20.6%

21.95%

24.62%

29.47% 7.9%

14.55% 25.19%

Planktivorous 4.27%

29.28%

17.42%

Omnivorous

(b)

(a)

Benthivorous Piscivorous

18.98%

20.71%

Invertebrates Others

10.49% 58.02%

53.14%

15.58%

12.51% 10.57%

(c)

(d)

Fig. 1.19. The feeding habit composition of dominant species accounting for 80% of total biomass: (a) 1959∼1960 in the Bohai Sea; (b) 1982∼1983 in the Bohai Sea; (c) 1992∼1993 in the Bohai Sea; (d) 1998∼1999 in the Bohai Sea (Zhang et al., 2007) (With permission from Elsevier’s Copyright Clearance Center)

1.1.2 The Yellow Sea The Yellow Sea is the broadest marginal sea of the China seas. There are prominent diﬀerences and connections between other China marginal seas at a basic level, including topography, chemical environment, hydrography, sediment patterns, riverine input, and biological characteristics. 1.1.2.1 Topography The Yellow Sea is commonly considered to be among the broadest marginal seas in the world (Fig. 1.20, Yang et al., 2003). It is a semi-enclosed sea of the northwest Paciﬁc Ocean with a surface area of 380×109 m2 and average depth of 44 m, surrounded by the west coast of the Korean Peninsula and the east coast of China and connected to the East China Sea in the south and to the Bohai Sea in the north. The size is about 870 km from north to south and 556 km from east to west. It is quite shallow, with depths ranging from 90 m in the central trough to less than 20 m within 50 km of the coast. The deepest

24

1 Introduction

water is conﬁned to the north-south trough of 140 m which runs from the northern boundary of the Yellow Sea to the continental shelf break southwest of Cheju. Another feature of the topography of the sea bottom is the east-west asymmetry of the bathymetric gradients. Generally, the bathymetric gradients are relatively small on the Chinese side than on the Korean side. Owing to the very shallow depth and the complex topography of the sea bottom, the currents and tides are very complex here and diﬃcult to model and predict (Liu ZL et al., 2008).

N 40

35

120

125

130

E

Fig. 1.20. Bathymetric chart of the Yellow Sea. Isobaths are in meters. The major Chinese and Korean rivers enter the Yellow Sea (Yang et al., 2003) (With permission from Elsevier’s Copyright Clearance Center)

The Yellow Sea is divided into the northern Yellow Sea and the southern Yellow Sea by a line linking the Chengshanjiao of the Shandong Peninsula and Changshanchuan of the Korean Peninsula, whose names come from their positions. 1.1.2.2 Hydrographical and Chemical Environment The temperature and salinity of the Bohai Sea have obvious temporal and spatial variations. The temperature in the Yellow Sea waters is highest in summer, followed by autumn, and the lowest value exists in winter. Whereas the highest salinity is found in spring, followed by winter, and the lowest value exists in summer. In spring, the temperature of surface waters and sea bottom waters ranges from 9.58 to 17.6 ◦ C and 6 to 15.6 ◦ C, respectively, with average values of 15.12 ◦ C and 9.53 ◦ C. The salinity in the surface waters and sea bottom waters ranges from 29.67‰ to 34.06‰ and 31.22‰ to 34.41‰,

1.1 Basic Status of China Marginal Seas

25

respectively, with average values of 32.52‰ and 32.86‰. In summer, the temperature of the surface waters and sea bottom waters ranges from 21.81 to 27.17 ◦ C and 20.91 to 27.17 ◦ C, respectively, with average values of 24.27 and 23.47 ◦ C. The salinity in surface waters and sea bottom waters ranges from 31.64‰ to 32.06‰ and 31.8‰ to 32.16‰, respectively, with average values of 31.9‰ and 31.94‰. In autumn, the temperature of surface waters and sea bottom waters ranges from 18.38 to 24.48 ◦ C and 7.02 to 22.91 ◦ C, respectively, with average values of 21.38 ◦ C and 13.18 ◦ C. The salinity in surface waters and sea bottom waters ranges from 30.89‰ to 33.14‰ and 31.25‰ to 33.08‰, respectively, with average values of 31.63‰ and 32.21‰. In winter, the temperature of surface waters and sea bottom waters ranges from 7.01 to 14.67 ◦ C and 8.45 to 14.97 ◦ C, respectively, with average values of 10.65 ◦ C and 10.81 ◦ C. The salinity in surface waters and sea bottom waters ranges from 30.56‰ to 33.01‰ and 31.57‰ to 33.99‰, respectively, with average values of 32.16‰ and 32.29‰. Plots of surface waters and near-bottom properties for January (“winter”) and July (“summer”) appear in Figs. 1.21 and 1.22. In January, cold conditions prevail throughout, with minimal vertical structure. The salinity shows a sea bottom water intrusion of saline East China sea water. This is the climatological signature of the Yellow Sea Warm Current. In July, the sea bottom water salinity shows the accumulated impact of this wintertime intrusion and the Yellow Sea Cold Water is clearly evident in the deep waters, with thermal stratiﬁcation of the order of 20 ◦ C. Hence, both temperature and salinity contribute to the abundance stratiﬁcation of the order of 3σt for the central portion of the Yellow Sea, penetrating to near the Bohai Strait. Additionally, the reduced surface waters salinity near Shanghai indicates the eastward spreading of the Changjiang River discharge. The dissolved oxygen (DO) and pH are important chemical parameters of seawaters. The DO distribution decreases from north to south, with 8.19 mg/L in the north, 7.86 mg/L in the middle, and 7.56 mg/L in the south. The average DO is 7.78 mg/L. In addition, the DO also varies with seasons. In spring, the DO and pH range from 8.17 mg/L to 10.22 mg/L and 8.2 to 8.58, respectively, with averages of 8.66 mg/L and 8.44. In summer, the DO and pH range from 6.04 mg/L to 8.5 mg/L and 8.06 to 8.4, respectively, with averages of 7.02 mg/L and 8.27. In autumn, the DO and pH range from 6.92 mg/L to 8.19 mg/L and 8.13 to 8.37, respectively, with averages of 7.67 mg/L and 8.27. In winter, the DO and pH range from 7.08 mg/L to 8.81 mg/L and 8.02 to 8.31, respectively, with averages of 8.02 mg/L and 8.19 (Jin et al., 2005). The silicate (SiO3 -Si), phosphate (PO4 -P), and inorganic nitrogen (DIN) are the nutrients of planktons. Their concentrations vary with the areas and seasons. Silicate and DIN concentrations decrease from north to south. While the highest phosphate concentration is found in the south and the lowest value in the north. In spring, SiO3 -Si, PO4 -P, and DIN concentrations in surface waters are 0.59∼13.6, 0.1∼0.28, and 1.91∼11.6 μmol/L, with averages of 3.43, 0.16, and 3.73 μmol/L, respectively. In summer, SiO3 -Si, PO4 -P, and DIN con-

26

1 Introduction

N 40

35

30

Unit: g/L 26.0

Unit: g/L 26.0

25.5

25.5

25.0

25.0

24.5

24.5

24.0

24.0

23.5

(a)

23.0

23.5 (b) 23.0

N 40 Unit:

35

30

24

Unit: 24

21

21

18

18

15

15

12

12

9 (c)

6

9 (d)

6

N 40

35

30

Unit: 35

Unit: 35

34

34

33

33

32

32

31

31

30 (e)

30 (f)

29

29 120

125

E

120

125

E

Fig. 1.21. January climatological hydrography: surface and near-bottom abundance. (a) Bottom density; (b) Surface density; (c) Bottom temperature; (d) Surface temperature; (e) Bottom salinity; (f) Surface salinity (Naimie et al., 2001) (With permission from Elsevier’s Copyright Clearance Center)

1.1 Basic Status of China Marginal Seas

27

N 40 Unit: g/L 26

Unit: g/L 26 35

30

25

25

24

24

23

23

22

22

21 (a) N 40

35

30

21

(b)

20

20

Unit: 27

Unit: 27

24

24

21

21

18

18

15

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9 (c)

6

9 (d)

6

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Unit: 35

Unit: 35

34

34

33

33

32

32

31

31

30

(e)

30

(f)

29 120

125

E

29 120

125

E

Fig. 1.22. July climatological hydrography: surface and near-bottom abundance. (a) Bottom density; (b) Surface density; (c) Bottom temperature; (d) Surface temperature; (e) Bottom salinity; (f) Surface salinity (Naimie et al., 2001) (With permission from Elsevier’s Copyright Clearance Center)

centrations in surface waters are 0.04∼11.6, 0.26∼0.68, and 0.54∼22 μmol/L, with averages of 3.02, 0.35, and 5.67 μmol/L, respectively. In autumn, SiO3 -Si, PO4 -P, and DIN concentrations in surface waters are 0.1∼137.1, 0.26∼1.11, and 1.52∼16.4 μmol/L, with averages of 11.6, 0.6, and 6.38 μmol/L, respec-

28

1 Introduction

tively. In winter, SiO3 -Si, PO4 -P, and DIN concentrations in surface waters are 1.17∼14.1, 0.3∼2.73, and 1.19∼7.62 μmol/L, with averages of 7.75, 0.16, and 3.42 μmol/L, respectively (Jin et al., 2005). 1.1.2.3 Tides The Yellow Sea is dominated by a semidiurnal tidal regime with a tidal range exceeding 4 m in many coastal areas. Strong tidal currents are largely rotary with the long axis of the tidal ellipse oriented NE-SW in the mid-eastern part (north of 35◦ 30 N) and N-S to NW-SE in the southeastern part of the sea, respectively. Tidal current ellipses also display a distinctive radial pattern near the Jiangsu coast and a rectilinear pattern in the Changjiang River mouth. In contrast, a relatively high ellipticity of tidal currents is observed in the east of the Changjiang River mouth. Tidal currents readily exceed 1 m/s in the nearshore and gradually diminish oﬀshore, with an N-S trend in the central Yellow Sea. The inferred paleotidal current model shows that the tidal current at 10 kyr was stronger and oscillated more rectilinearly than it does today. As the sea level rose, the region of intense tidal bottom stress migrated from around Jeju Island toward the southwestern coast of Korea and along the retreat path of the paleo-Changjiang River mouth, suggesting that tidal processes played a signiﬁcant role in the reworking and deposition of sediments. Tidal and sub-tidal currents were examined using current proﬁles from three bottom-moored Sontek Acoustic Doppler Proﬁlers (ADPs) deployed in the southern Yellow Sea in the summers of 2001 and 2003. The measured current time series were dominated by tidal currents. The maximum velocities were 40∼80 cm/s at the mooring stations. The M2 current was the dominant primary tidal constituent, while the MS4 and M4 components produced the most signiﬁcant shallow water tidal currents with much weaker amplitudes. The measured mean sub-tidal velocities were less than 5 cm/s. The mean ﬂows in the lower layer implied that an anti-cyclonic circulation pattern might exist in the deeper central Yellow Sea (Liu ZL et al., 2008). The tidal fronts near the boundary of the shallow coastal area and channel are developed during summer when thermal stratiﬁcation is established in the channel and a vertically homogenous water mass developed by strong tidal currents is sustained in the shallow coastal area. These fronts may aﬀect the transport of terrestrial materials (including N) to oﬀshore locations, and hence the biogeochemical activity. 1.1.2.4 Currents The Yellow Sea is aﬀected by warm and saline oceanic currents and less saline coastal currents in a basin-wide scheme of a cyclonic gyre. In general, the former ﬂow northward whilst the latter ﬂow southward. On the east side of the Yellow Sea, the Kuroshio and Tsushima Warm Currents and the Yellow Sea

1.1 Basic Status of China Marginal Seas

29

Warm Current (YSWC) dominate, whereas various coastal currents prevail in the Chinese shallow waters (Fig. 1.23, Yang et al., 2003). Coastal currents ﬂow southward along the west coast of Korea particularly during winter. In the southern Yellow Sea and East China Seas, pronounced coastal currents switch their current direction seasonally, i.e., southward in winter and northward in summer, although the direction of the oﬀshore Taiwan Warm Current is constantly northward. In particular, during the summertime ﬂooding season, the northward Taiwan Warm Current meets the discharge of the Changjiang River, forming the “Changjiang diluted water (CDW)” oﬀ the river mouth. The CDW extends southeast to eastward and may reach the oﬀshore west of Jeju Island during peak ﬂoods. A seasonal cold water mass (Yellow Sea Cold Water) occurs in the lower water column of the central Yellow Sea during the warm half of the year (April to November). It is characterized by lower temperature and higher salinity compared with those of surrounding waters and varies in distribution and volume with the season. It is considered that Yellow Sea Cold Water is derived from the surface water of the previous winter sinking through a cooling process. The coastal current along the Korean coast, reinforced by strong northerly winds during late fall and winter, ﬂows southward and then eastward through the Jeju Strait. Diluted by summer runoﬀ and chilled by outbreaks of cold air, the Korean coastal current forms a strong and stable thermohaline front embracing the coastal archipelago, including Heuksan Island oﬀ the southwestern coast of Korea.

N 40

35

1 20

1 25

1 30 E

Fig. 1.23. Schematic map of the regional circulation patterns in the Yellow Sea. BCC: Bohai coastal current; LDCC: Liaodong coastal current; KCC: Korea coastal current; YSCC: Yellow Sea coastal current; YSWC: Yellow Sea Warm Current; CDFW: Changjiang diluted freshwater; TC: Tushima current; YSCW: Yellow Sea Cold Water (Yang et al., 2003) (With permission from Elsevier’s Copyright Clearance Center)

30

1 Introduction

A striking physical feature is the Yellow Sea Cold Water Mass (YSCWM), which entrenches in the deep trough below the thermocline from late spring to fall and performs as a bottom nutrient pool, providing rich nutrient to the euphotic layer through entrainment (Wei et al., 2002). The tidal fronts separate well-mixed turbid coastal water and seasonal stratiﬁed deep water on both sides of the basin. These two processes were exempliﬁed by temperature distributions at a section along the 35◦ N parallel, where the cold water mass core locates, from May to July of 2001 in the studies of Wei et al. (2003). The thermocline in May, 10 m thick, locates at 15 m deep with a vertical gradient of 0.5 ◦ C/m. By July it has strengthened to 1 ◦ C/m with a thickness of 5 m and a center at 10 m depth. The sea surface temperature rises from 16 to 23 ◦ C, but the bottom temperature remains at 8 ◦ C, which is a characteristic of YSCWM. Tidal fronts locate at the edge of the YSCWM. The horizontal gradient of the front is 0.034 ◦ C/km at 35 m deep in May and 0.13 ◦ C/km in July. The tidal front becomes very strong and moves to shallow areas as the surface water heats. Their observations at the station of YSCWM (34◦ 30 N, 123◦ 06 E) on October 23∼25, 2000 showed that, during the late period of thermocline, the vertical eddy diﬀusivity was still very low, less than 10−3 cm2 /s near the thermocline and its maximum located at the bottom, more than 4 cm2 /s. The Yellow Sea was subaerially exposed during the last glacial period when the sea level was lowered to about 120 m below the present level. In the central part of the sea, marine transgression initiated at about 11∼12 kyr (Wang et al., 1985) and the sea level reached the present position at about 6 kyr. Depositional processes of quaternary sediments have largely been controlled by changes in sea level in concert with spatio-temporal variations in water-mass circulation and along-shore and seasonal currents, waves and tidal currents. A high amplitude rise in sea-level had an especially strong inﬂuence on sedimentation during the erosional retreat of shore faces and river mouths when pre-Holocene deposits were intensively eroded to form transgressive sheets and sediment ridges. As the rate of rise in sea-level decreased, depositional processes were aﬀected by sediment supply, topography, and prevailing currents. 1.1.2.5 Wind Strong northerly winds generally prevail over the Yellow Sea during winter months, while the summer is characterized by weak southerly winds. During winter, wind generated turbulence enhances deep cooling by thermal convection. During summer, the weaker and less persistent winds combine with surface heating to result in very strong thermal stratiﬁcation, isolating the sea bottom water. Fig. 1.24 displays the model results at the horizontal resolution of the data set (2◦ ). Strong northerly winds prevail during winter, from November to March. On balance during the year, weaker winds are characteristic of

1.1 Basic Status of China Marginal Seas

31

N 40

35

0.2 Pa

0.2 Pa

30 15 cm/s

15 cm/s

(a)

(b)

N 40

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0.2 Pa

0.2 Pa

15 cm/s

15 cm/s

30

(d)

(c) N 40

35

0.2 Pa

0.2 Pa

15 cm/s

15 cm/s

30

(e) 120

(f) 125 E

120

125

E

Fig. 1.24. Annual cycle of the Yellow Sea wind. Wind stress (heavy vectors, plotted at the spatial resolution (2.0◦ ) of the data), model predicted vertically averaged residual velocity (thin vectors, interpolated to a 0.25◦ grid for display purposes in this and subsequent ﬁgures), and the associated stream function (increasing from white to black in 0.05 Sv increments, circulation around local minima is anticyclonic). (a) January; (b) March; (c) May; (d) July; (e) September; (f) November (Naimie et al., 2001) (With permission from Elsevier’s Copyright Clearance Center)

32

1 Introduction

the climatology. July is our representative “summer” season, with southerly winds. May and September are transition seasons. 1.1.2.6 Distribution Pattern of Sediments Ridge-and-swale topography is dominant in the inner-middle shelf area of the eastern Yellow Sea (Fig. 1.25, Yang et al., 2003). A thin sheet of sand drapes erosional ridges in the nearshore area, which appear to have originated from erosion of older deposits by energetic tidal activity during the period of post-glacial transgression. Some ridges, located far oﬀshore, are considered to be depositional in origin as a result of sand migration, as represented by southwestward-dipping internal reﬂectors.

N 40

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130

E

Fig. 1.25. A map showing surface sediment distribution in the Yellow Sea and adjacent areas. CYSM: central Yellow Sea mud; (1) NYSM: North Yellow Sea mud; (2) SEYSM: southeastern Yellow Sea mud; (3) SWYSM: southwestern Yellow Sea mud; (4) SWCIM: southwestern Cheju Island mud. Sediment types oﬀ the coast of the North Korea remain unclear and are not available from the literature (Yang et al., 2003) (With permission from Elsevier’s Copyright Clearance Center)

1.1.2.7 Riverine Discharge Nearly 30 rivers run into the Yellow Sea, mainly including the Aprock River, the Han River, and the Keum River, and the Yellow Sea receives a vast amount of fresh water from both Chinese and Korean rivers, such as the Changjiang River, Huanghe River, Yalujiang River, Daliaohe River, and Haihe River of China and the Han River, Kum River, and Yeongsan River of Korea. The Yellow Sea receives a great quantity of sediments discharged through the adjacent rivers, especially the Huanghe River (1.08×109 t/yr) and Changjiang River (ca. 5×109 t/yr). The Huanghe River is a major agent in depositing

1.1 Basic Status of China Marginal Seas

33

recent muddy sediments in the Yellow Sea. And the rivers (the Huanghe, the Aprock, the Han, the Keum, the Haihe, and the Luanhe) drain freshwater of about >160 km3 and suspended materials of 1.1×109 t annually into the Bohai and Yellow Sea systems. The Yalujiang River is a major river directly running from the east coast of Liaodong Island into the Yellow Sea. It has a catchment area of 61,889 km2 and stretches for 790 km, with a water discharge of 25×109 m3 /yr, suspended materials of 113×104 t/yr, a rainfall of 1,050 mm and a sediment delivery of 5∼10 mg/(L·yr) (Qin et al., 1989). The Huanghe River-derived deltaic muddy sediments (9%∼15% of sediment discharge) are transported southward and accumulate in the central Yellow Sea. Echo-character mapping and sediment coring have revealed that the mud unit is up to 6 m thick and thins out southeastward. A thick clinoform mud deposit occurs at the eastern tip of the Shandong Peninsula, referred to as the Shandong mud wedge. It was mostly deposited early in the Holocene (prior to 9.0 cal. kyr) before the Huanghe River mouth shifted to the Bohai Sea, which is the reason why the Shandong mud wedge is not connected to the present Huanghe River delta. Large birdfoot-like sand bodies occur north of the Changjiang River mouth oﬀ the Jiangsu coast, which were formed prior to the southward shift of the river mouth by tidal-current winnowing. On the other hand, the sand bodies were deposited during the regressive phase of the sea level and were formed by tidal currents and longshore transportation of sands from the abandoned Huanghe River and modern Changjiang River deltas. Further north, along the Jiangsu coast, muddy sand, sandy mud, and mud were deposited by the old Huanghe River delta prior to 1855. These sediments have been partly winnowed by waves and tidal currents and acted as a source of the mud patch southwest of Jeju Island. The Changjiang River-derived sediments are mostly conﬁned to the south and seasonally transported oﬀshore by a plume event. The east of the Changjiang River mouth is covered with sandy sediments, either relict sands of the continental shelf or an active oﬀshore tidal sand sheet. The Korean rivers discharge relatively small amounts of sediments into the eastern Yellow Sea. The ﬁne-grained sediments, largely derived from the Geum River (discharge, 5.6×106 t/yr), are transported southward, forming a distinct mud belt, the Heuksan mud belt, in a water depth of 20∼70 m. Although it is known that ﬁne-grained sediments largely originate from the Geum River, along with other small-scale rivers and coastal erosion, an integrated study of the origin and depositional processes of this mud belt is warranted. Besides, billions of tons of terrestrial materials are discharged annually through rivers (including the Huanghe, Aprock, Han, Keum, and others), tens of million of tons of mineral dusts (otherwise known as “yellow sand”) are deposited annually into surface seawaters from the atmosphere. It has been suggested that the atmospheric dust ﬂux at the Yellow Sea may be comparable to the river input. The sea receives more than 60% of precipitation in the

34

1 Introduction

period from June to September, under the inﬂuence of the northeast Asian monsoon. 1.1.2.8 Biological Characteristics (1) Chlorophyll a and primary production The concentration of chlorophyll a is used as the most convenient index of phytoplankton biomass. The average value is 0.56 mg/m3 . It varies in diﬀerent seasons and areas. The highest value is found in spring, with 0.66 mg/m3 . In spring, the concentration of chlorophyll a in surface waters ranges from 0.09 to 2.16 mg/m3 . The highest value is found in the Changjiang River Estuary. In summer, the concentration of chlorophyll a in surface waters ranges from 0.06 to 3.39 mg/m3 . The highest value is found in the Changjiang River Estuary and the coast of Jiangsu. In autumn, the concentration of chlorophyll a in surface waters ranges from 0.12 to 2.8 mg/m3 . The highest value is found in the southeast of the south Yellow Sea and the northeast of the north Yellow Sea. In winter, the concentration of chlorophyll a in surface waters ranges from 0.2 to 2.25 mg/m3 . The highest value is found in the north of the north Yellow Sea and the east of Jiaozhou Bay (Jin et al., 2005). The amount of primary production (PP) in diﬀerent areas and seasons varies. The highest value is found in spring and summer and the lowest value is found in winter. In spring, PP ranges from 8.14 to 177.87 mg C/(m2 ·h). High primary production was observed in the middle of the north Yellow Sea. In summer, PP ranges from 10.85 to 261.65 mg C/(m2 ·h). High primary production was observed near Diandao Island. In autumn, PP ranges from 5.75 to 139.24 mg C/(m2 ·h). High primary production was observed in the north of the north Yellow Sea. In winter, PP ranges from 4.05 to 247.57 mg C/(m2 ·h). High primary production was observed near Changshan Islands. (2) Phytoplankton The phytoplankton of the Yellow Sea includes 63 species—diatoms 55 species, dinoﬂagellates 8 species, accounting for 87.3% and 12.7% of total, respectively. In spring, the dominant species include Coscinodiscus radiatus, Coscinodiscus asteromphalus, Melosira sulcata, Navicula, Pleurosigma, Ceratium, Ceratium tripos, Peridinium, etc. The phytoplankton biomass is 2.24×104 cells/m3 . Diatom biomass is 1.57×104 cells/m3 , accounting for 70.1% of total phytoplankton. In summer, the dominant species include Chaetoceros compressus, Chaetoceros lorenzianus, Cheatoceros subsecundus, Chaetoceros paradox, Cheatoceros debilis, Chaetoceros didymus, Chaetoceros curvisetus, Chaetocers teres, Chaetoceros aﬃnis. The phytoplankton biomass is 20.17×104 cells/m3 . Diatom biomass is 18.73×104 cells/m3 , accounting for 90% of total phytoplankton. In autumn, the dominant species include Chaetoceros compressus, Chaetoceros castracanei, Chaetoceros lorenzianus, Chaetoceros aﬃnis, Chaetoceros densus, Eucampia zoodiacus, Biddulphia regia, Thalassionema nitzschioides. The phytoplankton biomass is 7.92×104 cells/m3 . Diatom biomass

1.1 Basic Status of China Marginal Seas

35

is 7.3×104 cells/m3 , accounting for 92.2% of total phytoplankton. In winter, the dominant species include Coscinodiscus asteromphalus, Coscinodiscus radiatus, Melosira sulcata, Thalassionema nitzschioides, Chaetoceros curvisetus, Chaetoceros castracanei, Rhizosolenia styliformis, Rhizosolenia styliformis, Chaetoceros densus. The phytoplankton biomass is 18.25×104 cells/m3 . Diatom biomass is 17.97×104 cells/m3 , accounting for 98.4% of total phytoplankton (Jin et al., 2005). (3) Zooplankton According to investigation during the four seasons of 1998 to 2000, zooplankton includes 67 species. In autumn, the zooplankton structure is most complicated, including 28 species. In spring, summer, and winter, Carapace species change little. It is found that Calanus sinicus, Labidocera bipinnata, Oithona similis, Themisto gracilipes, Euphausia paciﬁca, Sagitta crassa, etc. always exist in each season. In addition, the dominant species vary in diﬀerent seasons (Table 1.2). In spring, Copepoda is the dominant species. In other seasons, the dominant species include Calanus sinicus Brodsky and Sagitta crassa Tokioka (Jin et al., 2005). Table 1.2. The dominant species of the Yellow Sea in each season (unit: ind./m3 ) (Jin et al., 2005) (With permission from Jin XS) No. 1 2 3 4 5 6 7 8 9

Species Spring Summer Autumn Winter Percentage (%) Calanus sinicus Brodsky 58.20 48.03 40.84 428 52.01 Sagitta crassa Tokioka 0.51 22.68 18.53 40.29 22.46 Sagitta enﬂata Grassi ND ND 4.48 1.80 1.72 Penilia avirostris Dana ND 21.15 ND ND 5.79 Oithona similis Claus 11.43 ND ND ND 3.13 Labidocera euchaeta ND 1.11 8.36 0.32 2.68 Giesbrecht Euphausia paciﬁca Hansen 1.26 1.67 3.31 1.88 2.22 Euchaeta concinna Dana ND ND 6.50 6.01 3.43 Parathemisto gaudichaudi ND 1.89 1.03 4.95 2.17 Guerin

Dominant species: 1>5>7>2 in spring, 1>2>4>9>7>6 in summer, 1>2>6>8>3>7>9 in autumn, 1>2>8>9>3>6 in winnter

The zooplankton biomasses vary in diﬀerent areas and seasons. The spatial zooplankton biomass decreases from south to north; the average biomasses in the north, middle area and south are 28.8, 31.7, and 74.3 mg/m3 , respectively. The highest value is found in summer, followed by spring, and the lowest value is in winter. In spring, the average biomasses in the north, middle area, and south are 12.9, 14.3, and 134.7 mg/m3 , respectively. In summer, the average biomasses in the north, middle area, and south are 50.1, 45.4, and 27.1 mg/m3 , respectively. In autumn, the average biomasses in the north, middle area, and

36

1 Introduction

south are <40, 20, and 84.9 mg/m3 , respectively. In winter, the biomass ranges from 20 to 96.6 mg/m3 (Jin et al., 2005). (4) Benthos species The benthos of the Yellow Sea has 414 species, including Polychaeta 194 species, Mollusk 86 species, Carapace 90 species, Echinodermata 21 species, and others 23 species. Polychaeta, Mollusk, and Carapace account for 89.4% of the total benthos species. According to the amount and present frequency, the dominant species in the Yellow Sea include Sigambra hanaokai, Nereis longior, Goniada maculata, Aricidea fragilis, Ophelia acuminate, Ninoe palmate, Lumbrineris sp., Terebellides stroemii, Thyasira tokunagaii, Raetellops pulchella, Calliodentalium crocinum, Episiphon kiaochowwanensis, Natica janthostomoides, Philine japonica, Eudorella paciﬁca, Harpiniopsis sp., Ampelisca brevicornis, A. miharaensis, Byblis japonicus, Eriopisella sechellensis, Photis hawaiensis, Caprella equilibra, Callianassa japonica, Ophiura sarsii, etc. (Jin et al., 2005). The benthos species vary with seasons: spring (247 species)>summer (206 species)>autumn (181 species)>winter (178 species). The benthos biomass also varies with seasons: spring (50.75 g/m2 )>autumn (35.35 g/m2 )>summer (32.64 g/m2 )>winter (29.94 g/m2 ). The sequence of abundance is: spring (359 ind./m2 )>winter (290 ind./m2 )>summer (186 ind./m2 )>autumn (165 ind./m2 ) (Table 1.3). Table 1.3. The benthos biomasses of the Yellow Sea in diﬀerent seasons (Jin et al., 2005) (With permission from Jin XS) Season Polychaeta Mollusk Carapace Echinodermata Others Spring 11.07 8.54 3.81 18.99 8.60 Summer 10.49 3.19 6.50 6.41 6.07 Autumn 10.75 1.78 2.56 11.56 8.69 Winter 7.89 3.69 1.56 9.48 7.33 Average 10.05 4.30 3.61 11.61 7.67 Abundance Spring 202 57 70 24 5 (ind./m2 ) Summer 108 16 55 5 2 Autumn 130 7 13 11 3 Winter 131 72 37 11 38 Average 143 38 44 13 12 Biomass (g/m2 )

Total 50.75 32.64 35.35 29.94 37.17 359 186 165 290 250

(5) Fish During 1985∼2002, species diversity showed a decreasing tendency before 1992 and an increasing trend thereafter for the whole ﬁsh assemblage. In 1985, there were seven dominant species, accounting for 72.5% of the total catch, of which the snailﬁsh Liparis tanakae dominated (34.2%). In 1986, the three dominant species accounted for 78.9% of the total catch by weight, of which the proportion of anchovy increased and amounted to 61.5%. In 1987∼1998, there were

1.1 Basic Status of China Marginal Seas

37

usually 1∼3 dominant species, accounting for 81.4%∼94.1% of the total catch, of which anchovy dominated (58.8%∼94.1%) and small yellow croaker Pseudosciaena polyactis usually followed (9.3%∼22.2%). In addition, the cardinal ﬁsh Apogonichthys lineatus accounted for 11.1% of the total catch in 1991. In 1999∼2002 there were 4∼8 dominant species, accounting for 78.7%∼86.6% of the total catch. Compared with 1986∼1998, the proportion of anchovy decreased, amounting to 17.4%∼45.4%. Small yellow croaker also accounted for a relatively high proportion, ranging from 11.4% to 31.7%. The percentages of snailﬁsh and anglerﬁsh Lophius litulon amounted to 12.4%∼31.4% of the total catch. Some planktotrophic species, such as blackgill croaker Collichthys niveatus, silver pomfret Pampus argenteus, half-ﬁn anchovy Setipinna taty, chub mackerel Scomber japonicus, long-tailed anchovy Coilia mystus, and gizzard shad Konosirus punctatus, accounted for relatively small proportions, varying at 2.6%∼9.6% (Table 1.4). According to investigation in 1997 to 2000, it was found that 124 species of ﬁsh exist in the Yellow Sea. The most dominant species is Engraulis japonicus (65.5 kg/h), followed by Trachurus japonicus (8.9 kg/h), then Pseudosciaena polyactis (3.2 kg/h), Trichiurus lepturus (1.5 kg/h), Pampus argenteus (1.4 kg/h). In spring, the number of species is 90, with an average abundance of 27.5 kg/h. The dominant species is Engraulis japonicus (15.7 kg/h), accounting for 57.0%. The second dominant species is Ammodytes personatus (3.7 kg/h), accounting for 13.4%. In summer, the number of species is 70, with an average abundance of 238.3 kg/h. The dominant species is Engraulis japonicus (225.8 kg/h), accounting for 94.8%. In autumn, the number of species is 74, with an average abundance of 93.4 kg/h. The dominant species is Trachurus japonicus (35.7 kg/h), accounting for 38.2%. The second dominant species is Scomber japonicus (21 kg/h), accounting for 22.5%. In winter, the number of species is 85, with an average abundance of 43.4 kg/h. The dominant species is Engraulis japonicus (114 kg/h), accounting for 26.2%. The second dominant species is Pseudosciaena polyactis (6.9 kg/h), accounting for 15.9%. (6) Trophic levels Mean TL of dominant species in the Yellow Sea declined from 3.61 in 1985∼1986 to 3.40 in 2000∼2001, or by about 0.14 per decade (Fig. 1.26). Dominant species in the Yellow Sea included four kinds of planktivorous species, four kinds of omnivorous species, one kind of benthivorous species, and no piscivorous species in 1985∼1986. In 2000∼2001 there was one kind of planktivorous, omnivorous, and benthivorous species, respectively, also no piscivorous species. In the Yellow Sea, the percentage of planktivorous species also increased from 50.70% in 1985∼1986 to 60.08% in 2000∼2001 and the percentage of omnivorous species decreased from 14.50% in 1985∼1986 to 7.41% in 2000∼2001 (Fig. 1.27).

Species Anchovy Small yellow croaker Snailﬁsh Plaice Anglerﬁsh Olive ﬂounder Paciﬁc cod Cardinal ﬁsh Blackgill croaker Silver pomfret Half-ﬁn anchovy Chub mackerel Hairtail Long-tailed anchovy Smallhead hairtail Gizzard shad Total

1985 Engraulis japonicus 4.1 Pseudosciaena polyactis 6.5 Liparis tanakae 34.2 Cleisthenes herz ensteini 10.7 Lophius litulon 8.6 Paralichthys oliv aceus 5.0 Gadus macrocephalus 3.5 Apogonichthys lineatus Collichthys niveatus Pampus argenteus Setipinna taty Scomber japonicus Trichiurus lepturus Coilia mystus Eupleur ogrammus muticus Konosirus punctatus 72.5 4.7 9.0 4.4 7.0

12.1

5.8

7.5

4.7

3.4 9.6

6.0 2.6 78.9 93.0 88.0 93.7 81.4 94.1 86.7 87.6 82.9 78.7 80.7 82.2 86.6

11.1

1986 1987 1988 1990 1991 1992 1993 1994 1998 1999 2000 2001 2002 61.5 80.2 88.0 71.5 60.7 94.1 86.7 78.3 58.8 28.1 45.4 17.4 22.4 22.2 9.6 9.3 19.4 17.8 17.4 31.7 11.4 10.2 12.7 8.4 9.9 14.1 7.3 4.0 11.0 17.3

Table 1.4. Dominant ﬁsh species determined by Hill’s N2 and their percentages in the total catch by weight in the southern Yellow Sea over the period 1985∼2002 (Xu and Jin, 2005) (With permission from Elsevier’s Copyright Clearance Center)

38 1 Introduction

1.1 Basic Status of China Marginal Seas

39

4.20 4.00

TL

3.80 3.60 3.40 3.20 3.00 1959~1960 1982~1983 1985~1986 1992~1993 1998~1999 2000~2001 Year Mean TL of dominant species accounting for 80% of total biomass Mean TL of dominant species accounting for 85% of total biomass Mean TL of dominant species accounting for 90% of total biomass

Fig. 1.26. Decadal variations of mean TL of the Yellow Sea and the Bohai Sea (Zhang et al., 2007) (With permission from Elsevier’s Copyright Clearance Center) 20.2% 9.3% 14.5%

19.63% 50.7%

5.25% 7.41%

60.08%

7.63% 5.3% (a)

Planktivorous Benthivorous Omnivorous Piscivorous Invertebrates Others

(b)

Fig. 1.27. The feeding habit composition of dominant species accounting for 80% of total biomass. (a) 1985∼1986 in the Yellow Sea; (b) 2000∼2001 in the Yellow Sea (Zhang et al., 2007) (With permission from Elsevier’s Copyright Clearance Center)

1.1.3 The East China Sea The East China Sea (ECS) is known for its broad continental shelf, rich natural resources, and tremendous river runoﬀ from China, which is surrounded by several countries and has a complicated status, such as topography, chemical environment, hydrography, sediment pattern, riverine input, and biological characteristics. 1.1.3.1 Topography The East China Sea is one of the largest marginal seas of the continental shelf areas of the world, surrounded by Korea, Japan, Chinese mainland, and Taiwan region. Its size is about 1,300 km from north to south and 740 km from east to west (Fig. 1.28, Valle-Levinson and Matsuno, 2003). It lies over the broad shelf of the Northwest Paciﬁc Ocean, with a surface area of 770×109 m2 and average depth of 370 m. More than 70% of the surface area of the East China Sea is covered by continental shelf. The ECS is bordered by the Okinawa Trough, with a maximum depth exceeding 2,000 m. Together with the

40

1 Introduction

Okinawa Trough, the Ryukyu island arc, and the Ryukyu trench, it makes up another Trench-Arc-Basin system in the West Paciﬁc Ocean, which links northwards with the Japanese island arc and connects southwards with the Taiwan-Philippine island arc. The northwest shelf connects with the Yellow Sea and between them is the Changjiang River delta, with a water depth of about 50∼100 m.

N 35

30

25 120

125

130 E

Fig. 1.28. The East China Sea showing the water depth (Valle-Levinson and Matsuno, 2003) (With permission from Springer)

1.1.3.2 Hydrographical and Chemical Environment The SST of ECS ranges from 27 to 29 ◦ C in summer, but some cold eddies were found oﬀ northeast Taiwan and to the south of the mouth of the Changjiang River. SST anomalies at the center of these eddies were about 2∼5 ◦ C. The strongest front usually occurs in May each year and its temperature gradient is about 5∼6 ◦ C over a cross-shelf distance of 30 nautical miles. The Yellow Sea mixed with cold water also provides a contrast from China Coastal waters shoreward of the 50 m isobath, the cross-shore temperature gradient is about 6∼8 ◦ C over 30 nautical miles.

1.1 Basic Status of China Marginal Seas

41

The seasonal variation of the evaporation minus precipitation (E–P) had a secondary eﬀect on the SSS distribution (Fig. 1.29).

N 40

N 40

35

35

30

30

Salinity (

25

)

0~20 20~26 26~30 30~32 32~34 >34

25 120

125 130 E Jul. 28, 1998

120

N 40

N 40

35

35

30

30

25

125 130 E Aug. 1, 1998

25 120

125

130 E

120

Aug. 8, 1998

125

130 E

Aug. 11, 1998

N 40

N 40

35

35

30

30

25

25 120

125

130 E

Aug. 16, 1998

120

125

130 E

Sept. 7, 1998

Fig. 1.29. Temporal variations in horizontal distributions of salinity from July 28 to September 7, 1998 during the 1998 ﬂood (Chen et al., 2006) (With permission from Elsevier’s Copyright Clearance Center)

42

1 Introduction

The dissolved oxygen (DO) and pH are important chemical parameters of seawaters. The average DO and pH are 6.7 mg/L and 8.22, respectively (Jin et al., 2005). The silicate (SiO3 -Si), phosphate (PO4 -P), and inorganic nitrogen (DIN) are the nutrients of planktons. In winter, the SiO3 -Si, PO4 -P, and DIN concentrations in surface waters are 3∼40, 0.1∼1.6, and 1∼25 μmol/L, respectively. In summer, the SiO3 -Si, PO4 -P, and DIN concentrations in surface waters are 5∼50, 0.2∼1.0, and 1∼50 μmol/L, respectively. 1.1.3.3 Tides The East China Sea continental margin is characterized by high tidal currents, frequent and intense storm events, and extremely high sediment supply relative to other margins (e.g., Atlantic Coast, Gulf of Mexico). Semidiurnal tides are dominant in the East China Sea, with the spring tidal range between ∼1.25 and 1.80 m. The tidal wave propagates from the northwestern Paciﬁc Ocean through the straits between Taiwan and Kyushu as a progressive northeast-southwest-oriented wave front. Tides can also locally aﬀect surface sediment distribution. Tides are particularly dominant in the East China Sea of the mouth of the Changjiang River and oﬀ southwestern Korea. In both instances tidal currents are suﬃciently strong (1∼2 knot(s)) to erode and transport sediments. The intrusion of the Taiwan Warm Water (TWW) between the Changjiang Coastal Water (CJCW) and the Jiangsu Coastal Water (JCW) results in a seaward transition from turbid (CJCW) to clear (TWW) to turbid (JCW) waters east of the Changjiang River Estuary. As a result, Changjiang River sediments tend to be transported to the south by the CJCW, while the sediments to the east of the TWW are predominantly Huanghe River sediments transported south by the JCW. 1.1.3.4 Currents The East China Sea circulation is dominated by the northward ﬂow of two loops of the Kuroshio Current (KC): the Taiwan Warm Water (TWW) in the west and the Yellow Sea Warm Water (YSWW) in the east. Both water masses are characterized by high salinity and warm water temperatures. In contrast, a southward ﬂow in close to sea bottom water occurs from the ﬂow of the Changjiang (CJCW) and Jiangsu Coastal Waters (JCW) along the Chinese coast, the Korean Coastal Waters (KCW) in the east, and the Yellow Sea Cold Waters (YSCW) in the north (Fig. 1.30, Lee and Chao, 2003). The coastal currents in particular appear as seasonally cold and brackish water masses. To the east of ECS, the Kuroshio Current, a strong western boundary current, ﬂowing along the Paciﬁc Margin of northeastern Asia, borders the shelf slope of the East China Sea. When passing through the ECS, it has

1.1 Basic Status of China Marginal Seas N 40

N

(a)

40

(b)

50 cm/s

50 cm/s

Korea

Korea

35

35

Shikoku Kyushu

Shikoku Kyushu Yangtze River

Yangtze River

30

30 China

China

25

25 120 N 40

43

125

130 E

120

N

(c)

40

125

130 E

(d)

50 cm/s

50 cm/s

Korea

Korea

35

35 Shikoku Kyushu

Shikoku Kyushu

Yangtze River

Yangtze River

30

30 China

China

25

25 120

125

130 E

120

125

130 E

Fig. 1.30. Model-derived depth-averaged surface currents in: (a) October, (b) December, (c) April, and (d) August. The depth average extends from sea surface to 830 m or bottom, whichever is shallower (Lee and Chao, 2003) (With permission from Elsevier’s Copyright Clearance Center)

a great eﬀect on the ocean environment of the continental shelf area of the ECS. In addition, the Changjiang River dilution and the Kuroshio upwelling are two principal sources of materials of the ECS. 1.1.3.5 Wind Summer winds tend to be gentle and come from the south, except during major storms (typhoons). Resuspension by wind-generated waves is infrequent.

44

1 Introduction

Moreover, except in very shallow waters, the water column is stratiﬁed and therefore resuspended sediment tends to remain conﬁned to nearbottom waters below the pycnocline. Resuspension of sediment is largely the result of storm activity and the subsequent transport of this sediment by regional currents. Graber et al. (1989) proposed that the present-day distribution of sediments in the East China Sea is largely related to storm-generated surface waves during the winter season. Furthermore, these authors suggest that erosion of sand along the Chinese coast is largely the product of surface wave action. 1.1.3.6 Distribution Pattern of Sediments Surface sediments in the ECS inner shelf are characterized by gray mud and generally contain 40%∼45% clay, 40%∼60% silt, and less than 5% sand (Fig. 1.31, Liu et al., 2006). Surface sediments are composed generally of well sorted, positively skewed silts, clayey-silts, and silty clays with median diameter (MD, φ) from 6.0 to 8.0. Clay mineral assemblages are dominated by illite (generally more than 70%), with small amounts of chlorite (12%), kaolinite (9%), and smectite (3%). The high illite concentrations and low smectite concentrations (<5%) are a good indicator of the Changjiang River derived material. N 34

32

30

28

26

24

118

120

122

124

126

128 E

Fig. 1.31. Distribution of surface sediments in the ECS and Okinawa Trough (Liu et al., 2006) (With permission from Elsevier’s Copyright Clearance Center)

1.1 Basic Status of China Marginal Seas

45

The contents of silt and sand in the surface sediments of the ECS outer shelf are over 80%, mainly including ﬁne sand, medial-ﬁne sand and ﬁnemedial sand. Among them, the medial-ﬁnd sand sediment covers larger areas of the ECS outer shelf, whereas ﬁne-medial and ﬁne sand sediments present only in small areas, mainly concentrating on the Changjiang River Estuary and the southeast of outer shelf. The Okinawa Trough sediment is mainly composed of slit-mud. It distributes along the Trough bottom and parallelly with the Trough proﬁle (Qin et al., 1987). 1.1.3.7 Riverine Discharge There are eight major rivers in this region (from high to low latitude) that run into the East China Sea. They are the Han River, Yalu River, Liaohe River, Haihe River, Huanghe River, Changjiang River, Qiantangjiang River, and Minjiang River. The Changjiang River is the major river discharging directly into the East China Sea. It ranks the third in length (6,300 km), the ﬁfth in fresh water discharge (9.24×1011 m3 /yr), and the fourth in sediment discharge (4.86×108 t/yr) directly into the East China Sea in the world, which accounts for almost 90% of the whole river discharge into this region. This sediment is conﬁned to the coastal zones of the East China Sea and is ultimately transported southward and southwestward by the CJCW. The Huanghe River annually discharges 1.08×109 t of sediment into the Bohai Sea. A portion of this sediment load is then transported southward via the Yellow Sea Coastal Waters (YSCW) and the Jiangsu Coastal Waters (JCW) into the East China Sea. Surface concentrations of suspended sediments in the East China Sea generally range from 1 to 100 mg/L in winter and 0.5 to 5 mg/L in summer, although in winter nearbottom concentrations tend to be considerably higher than surface values due largely to the resuspension of bottom sediments during intense winter storms. 1.1.3.8 Biological Characteristics (1) Chlorophyll a Spatial variations in the total Chl a amount are mainly reﬂected in those diatoms in both spring and summer, and this tendency was most apparent in the samples with high total Chl a over 1 μg/L (Fig. 1.32). In contrast, Chl a of other phytoplankton groups ﬂuctuated much less, and their maximum abundance was limited to a certain level, even when the total Chl a reached 2∼4 μg/L. The volume of the Synechococcus cells ranged from 0.13 to 8.18 mm3 in the East China Sea. The mean carbon biomass varied signiﬁcantly between the cold (<0.2 g C/m2 ) and warm seasons (>0.8 g C/m2 ). The average total

46

1 Introduction

chlorophyll a was 31 mg/m2 in the cold season and 42 mg/m2 in the warm season, respectively (Fig. 1.33). Based on chlorophyll a and the Synechococcus biomass, the Synechococcus population accounts for about 6%∼25% of total phytoplankton in the cold season and 44%∼59% in the warm season (Fig. 1.33). Total chlorophyll a changes with the seasonal variation. The chlorophyll a concentrations increased from 31 mg/m2 in the cold season to 42 mg/m2 in the warm season (Fig. 1.33), which is equivalent to an increase in phytoplankton carbon biomass from 1.77 to 2.38 g C/m2 (Table 1.5). After Synechococcus carbon biomass is subtracted, the remaining carbon biomass of other phytoplankton shows an inconspicuous diﬀerence between the 1.63 g C/m2 in the cold season and 1.50 g C/m2 in the warm season. Table 1.5. The carbon biomass (g C/m2 ) of Synechococcus and larger-sized phytoplankton in the warm and cold seasons in the East China Sea (Chiang et al., 2002) (With permission from Elsevier’s Copyright Clearance Center)

Synechococcus Other phytoplankton Total phytoplankton

Winter 0.19 1.31 1.50

Cold season Spring Mean 0.09 0.14 1.94 1.63 2.03 1.77

Warm season Summer Fall Mean 0.85 0.90 0.88 0.94 2.07 1.50 1.79 2.97 2.38

In addition, the total chlorophyll a concentration also changes in the different water masses. The Chl a concentration is the lowest in the Kuroshio surface water (0.5 mg/m3 ), then that in the Taiwan Warm Current surface water (1.0∼1.5 mg/m3 ). The Chl a concentration is higher in the Changjiang diluted water and Jiangzhe oﬀshore water (>1.5 mg/m3 ); there then exist some high value (>1.5 mg/m3 ) areas such as the one where the weak current speeds and Kureahio splits, others are in upwelling current areas such as the Taiwan northeast oﬀshore, middle and north of Fujian oﬀshore and Zhejiang oﬀshore (Lu et al., 1997). (2) Phytoplankton During spring in the shelf water, diatoms were the most abundant taxa followed by chlorophytes, cryptophytes, chrysophytes, and prymnesiophytes. In summer, diatom became sharply less abundant towards the mid and oﬀshelf (Fig. 1.34). It was found that the total abundance obvious seasonal variations from 1997 to 2000 in the East China Sea (23◦ 30 ∼33◦ N and 118◦ 30 ∼128◦ E). It peaked in autumn with a mean value of 211.91×104 cells/m3 , followed by summer (50.40×104 cells/m3 ) and winter (11.34×104 cells/m3 ). The lowest abundance occurred in spring (2.01×104 cells/m3 ). Over the 4 seasons excluding winter, the mean density of phytoplankton was over 100×104 cells/m3 . For the horizontal distribution, the abundance in summer and spring was

1.1 Basic Status of China Marginal Seas 4

47

(a)

3

2

Spring Summer

Chl a ( m g/L)

1

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(b)

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55 50 45 40 35 30 25 20 15 10 5 0

100 80 60 40 20

Contribution (%)

Biomass (g C/m 2)

2.0

Total Chl a (mg/m 2)

Fig. 1.32. Relationship between total chlorophyll a and algal group speciﬁc chlorophyll a. (a) Diatoms; (b) Eukaryotic ﬂagellates; (c) Procaryotic alage (Furuya et al., 2003) (With permission from Elsevier’s Copyright Clearance Center)

0

Fig. 1.33. Seasonal variation in average integrated carbon biomass of Synechococcus (◦), average integrated total chlorophyll a (), and Synechococcus contribution to total phytoplankton biomass (♦) from Dec. 1997 to Oct. 1998. Vertical bars represent standard error (Chiang et al., 2002) (With permission from Elsevier’s Copyright Clearance Center)

48

1 Introduction

Total Chl a (mg/m2)

100

(a)

(b)

80 60 40 20

Relative abundance (%)

0 100

Prochlorophytes

80

Cyanobacteria

60

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40

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Chrysophytes

Cryptophytes 20 0

Dinoflagellates Diatoms 1 2 3 4 5 6 7 8 9 10 11 Station PN

1 2 3 4 5 6 7 8 9 10 11 12 Station PN

Fig. 1.34. Horizontal variations in integrated total chlorophyll a (chlorophyll a plus divinyl chlorophyll a) amount with contribution of various algal classes in (a) spring and (b) summer. Stations PN-2 and PN-12 were not sampled during the spring cruise (Furuya et al., 2003) (With permission from Elsevier’s Copyright Clearance Center)

higher in nearshore than that in oﬀshore areas of the East China Sea. In autumn and winter, the abundance in the oﬀshore of the north was higher than that in the nearshore, while in the south this was reversed. Over the 4 seasons, phytoplankton abundance showed a signiﬁcant correlation with the water temperature. Eleven dominant species were observed in 4 investigated seasons, in which Chaetoceros lorenzianus and Thalassiosira subtilis dominated in winter, Chaetoceros lorenzianus and Noctiluca scientillans in spring, Chaetoceros pseudocurvisetus and Rhizosolenia alataf gracillima in summer, and only Chaetoceros socialis in autumn (Luo et al., 2007). (3) Zooplankton The seasonal mean value of total biomass of zooplankton was 65.32 mg/m3 from 1997 to 2000. Among them the total biomass in autumn (86.18 mg/m3 ) was greater than those in summer (69.18 mg/m3 ), spring (55.67 mg/m3 ), and winter (50.33 mg/m3 ); average value of diet biomass of zooplankton was 40.9 mg/m3 , which was about 60% of total biomass of zooplankton. Among them, total diet biomass of zooplankton in autumn (56.84 mg/m3 ) was greater than those in summer (46.10 mg/m3 ), winter (30.82 mg/m3 ), and spring (29.82 mg/m3 ). Total biomass distribution trends toward to similar with diet biomass (Xu et al., 2004).

1.1 Basic Status of China Marginal Seas

49

Based on the results of autumn 2000 and spring 2001, the total 210 species with occurrence >2 were identiﬁed. There were 112 common species to occur in both of seasons. The most common species in autumn were Sagitta nagae, Calanus sinicus, Sagitta enﬂata, and Euckaeta concinna, and in spring were Calanus sinicus, Sagitta crassa, Sagitta nagae, Paracalanus parvus, and Euphausia paciﬁfa. In autumn, the average abundance was 6,018 ind./m2 and the most abundant species were Calanus sinicus, Sagitta nagae, and Paracalanus aculeatus. In spring, the average abundance was 9,271 ind./m2 , and the most abundant species was Calanus sinicus, which accounted for 87% of total abundance (Zuo et al., 2005). The main dominant species of zooplankton were Calanus sinicus and Macrura larva in summer, and Calanus sinicus, Sagitta enﬂata, and Doliolum denticulatum in autumn. In summer 2003, the distribution of zooplankton was not uniform, and the higher biomass presented in the outer sea. The main dominant species were Calanus sinicus, Macrura larva, Labidocera euchaeta, and Sagitta larva. In summer 2004, the higher zooplankton biomass occurred in the north of the ECS. The main dominant species were Calanus sinicus, Macrura larva, and Themisto gracilipes. In summer 2005, the higher zooplankton biomass occurred in the north of the ECS. The main dominant species were Calanus sinicus and Invertebrate egg. In autumn 2003, the distribution of zooplankton also was not uniform, with the higher biomass in the north of the ECS. The main dominant species were Doliolum denticulatum, Calanus sinicus, Sagitta enﬂata, and Nannocalanus minor. In autumn 2004, the higher zooplankton biomass occurred in the north of the ECS. The main dominant species were Calanus sinicus, Sagitta enﬂata, Euchaeta marina, and Salpa fusiformis. In autumn 2005, the higher zooplankton biomass appeared in the Yellow Sea-East China Sea interface. The main dominant species were Calanus sinicus and Sagitta enﬂata (Li HY et al., 2007). (4) Benthos species The benthos species in the East China Sea includes 855 species. The dominant species mainly include Polychaeta, Mollusk, and Carapace. The average biomass and abundance are 21.36 g/m2 and 283 ind./m2 , respectively. The biomass and abundance vary with the change of seasons. The highest biomass is found in spring (41.27 g/m2 ) and the lowest value is found in winter (10.23 g/m2 ), while the highest abundance is found in autumn (461 ind./m2 ) and the lowest abundance is found in winter (146 ind./m2 ) (Jin et al., 2005). Based on the data obtained in the autumn of 2000 and spring of 2001, the secondary production of macrobenthos from the East China Sea was calculated with Brey’s empirical formula. The results showed that the mean abundance in autumn (2000) was 87 ind./m2 , lower than that in spring (2001), 138 ind./m2 ; the mean biomass in ash-free dry weight (AFDW) in autumn was 1.40 g/m2 , higher than that (1.25 g/m2 ) in spring; the mean annual secondary production in the study area was 1.62 g/(m2 ·yr) (AFDW), much lower than those in the Bohai Bay and the south Yellow Sea. The production of mac-

50

1 Introduction

robenthos should be aﬀected by water temperature and water depth (Li XZ et al., 2005). (5) Fish On the basis of the data from bottom trawl surveys in 4 seasons of 2000 in the East China Sea and the Yellow Sea, studies of the seasonal characteristics of ﬁsh quantity distribution are conducted by the method of factor analysis. By R type analysis, the results show that there are 4 species, Raja kenojei, Coilia mystus, Muraenesox cinereus, and Collichthys niveatus, which are correlated closely in spring; 5 species, Engraulis japonicas, Pseudosciaena polyatis, Lophius litulon, Chelidonichthys kumu, and Saurida elongate, in summer; 5 species, Trichiurus haumela, Pampus nozawae, Erisphex potti, Pneumatophorus japonicas, and Pseudosciaena polyatis, in autumn; and 3 species, Trichiurus haumela, Champsodon capensis, and Acropoma japonicum, in winter. By the approbation of historic survey information, the main relationship among the 5 species in spring is the relation between prey and predators. By the promoted analysis, it was discovered that the water areas where correlative species of each season assemble together correspond to those areas where these species take physiology cycling migrations in each season. Based on Q type analysis, the synthesis of the predominant species in each season and their mainly distributed water area can be obtained. The results show that Trichiurus haumela and Pseudosciaena polyatis are the absolutely predominant species in the East China Sea and the Yellow Sea. Except for these two syntheses of the predominant species, the others are all small types of ﬁsh which have low value and quick growth characteristics (Liu and Cheng, 2008). 1.1.4 The South China Sea The South China Sea is the second largest marginal sea in the world, which resembles other subtropical oceans in a warm and oligotrophic condition, and has complex coastlines and bottom topography as well as a chemical environment, hydrography, sediment pattern, riverine input, and biological characteristics. At the same time, the South China Sea is the most sensitive to environmental changes. 1.1.4.1 Topography The South China Sea (SCS) is the largest semi-enclosed marginal sea oﬀ East Asia in the western tropical Paciﬁc Ocean, spreading from the equator to 20◦ N and spanning zonally about 15◦ in longitude with a broad shelf and a deep basin and an area of about 3.5×106 km2 (Fig. 1.35, Liu QY et al., 2008). The size is about 3,330 km from north to the south and 1,670 km from east to west. SCS lies between the South China coast and the maritime continent, located between the Asian land mass to the north and west, the Philippine Islands to the east, Borneo to the southeast, and Indonesia to

1.1 Basic Status of China Marginal Seas

51

N 25

20

15

10

5

0

100

105

110

115

120 E

Fig. 1.35. Bathymetry of the South China Sea. The 300-m isobath is indicated by the heavy solid line (Liu QY et al., 2008) (With permission from Springer)

the south. It includes the shallow Gulf of Thailand and connections to the East China Sea (through the Taiwan Strait), the Paciﬁc Ocean (through the Luzon Strait), the Sulu Sea, the Java Sea (through the Gasper and Karimata Straits), and the Indian Ocean (through the Strait of Malacca). All of these straits are shallow except the Luzon Strait, the maximum depth of which is 1,800 m. The South China Sea has complex topography including the broad shallows of the Sunda Shelf in the south/southwest, the continental shelf of the Asian landmass in the north, extending from the Gulf of Tonkin to the Taiwan Strait, a deep, elliptically shaped basin in the center, and numerous reef islands and underwater plateau scattered throughout. The SCS basin is the deepest in the center with a maximum depth exceeding 5,000 m. The shelf, which extends from the Gulf of Tonkin to the Taiwan Strait, is consistently nearly 70 m deep and averages 150 km in width, the central deep basin is 1,900 km along its major axis (northeast-southwest) and approximately 1,100 km along its minor axis, and extends to over 4,000 m depth. The Sunda Shelf is the submerged connection between southeastern Asia, Malaysia, Sumatra, Java, and Borneo, and reaches 100 m depth in the middle. The center of the Gulf of Thailand is about 70 m deep. The Luzon Strait is rather wide, but a series of small islands cross its width. Other important connections are to the Sulu Sea through the Mindoro Strait and Balabac Strait, to the East China Sea through the Taiwan Strait, to the Java Sea through the Karimata Strait and to the Andaman Sea through the Malacca Strait, which is one of the most important marine transportation routes in the world.

52

1 Introduction

In general, the SCS can be divided into the northern continental margin, the central oceanic basin, and the southern continental margin. The Zhujiang River Mouth Basin (ZRMB) is largely distributed on the continental shelf of the northern SCS. 1.1.4.2 Hydrographical and Chemical Environment Fig. 1.36 (Chern and Wang, 2003) depicts the horizontal distribution of temperature in the second layer, 25∼50 m, averaged over a year. The 25 ◦ C isotherm separates the SCS into two regions. The cold water, <25 ◦ C, occurs in the northwestern part of the basin, corresponding to the main cyclonic eddy. The core of the eddy, to the west of Luzon, has a mean temperature lower than 24 ◦ C. The warm water, >25 ◦ C, occurs in the southern SCS, including the Gulf of Thailand, the Luzon Strait, and the region of the anticyclonic eddy to the southwest of Taiwan region.

27

N 22

D

20 18

C 24

16

25

E 25

14

10

27

6

26 28

8

26

12

F

4 100 102 104 106 108 110 112 114 116 118 120 122 E

Fig. 1.36. Horizontal distribution of annual mean temperature (◦ C) of the 25∼50 m layer (Chern and Wang, 2003) (With permission from Springer)

Fig. 1.37 shows the seasonal variation in the vertical temperature distribution at four diﬀerent sites shown in Fig. 1.36. The temperature variation in the SCS mainly occurs in the upper eight layers, 0∼400 m. The thermal structure to the west of Luzon, station C, and in the southern SCS, station F, shows distinct seasonal variations, cold in winter and warm in summer. The seasonal change in the temperature on the continental slope to the southwest of Taiwan, station D, has quite a diﬀerent pattern. The Kuroshio intrusion across the Luzon Strait in October increases the upper layer temperature at station D. The water temperature in this slope area shows a major drop only when the cyclonic eddy to the west of Luzon extends into this area after February

1.1 Basic Status of China Marginal Seas

53

and then maintains a slow warm-up during the spring and summer period. To the east of Vietnam, station E, the warming and cooling of the upper ocean has a much shorter period. The upwelling associated with the southwest wind in summer cools the upper ocean and then the water temperature increases slowly during the northeast wind period. The temperature at station E drops again during March and April, consistent with the westward spreading of cold eddy water along the continental slope south of China (from station D to station E). During May and June, the apparent temperature increase at stations C, E, F and the mild warming at station D indicate that this is a warming period for the upper ocean in the SCS. Sta. C 2 4 6

Levels

8 10

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24

Sta. E

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12 14

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16 18 120 240 360

120 240 360

120 240 360

120 240 360

Days

Fig. 1.37. Seasonal variation of vertical temperature distribution at four sites in Fig. 1.36. Vertical coordinate is the layer number and horizontal coordinate is the Julian day in a year. Shaded area denotes land mask. Contours are in ◦ C. The study stations were C (17◦ 57.53 N, 118◦ 46.60 E), D (21◦ 23.62 N, 118◦ 15.80 E), E (15◦ 11.72 N, 111◦ 14.16 E), and F (6◦ 56.64 N, 111◦ 11.79 E) (Chern and Wang, 2003) (With permission from Springer)

The dissolved oxygen (DO) and pH are important chemical parameters of seawaters. The average DO and pH values are 5.8 mg/L and 8.12 in northern waters, and 5.19 mg/L and 8.1 in middle and southern waters. The average DO is 7.78 mg/L (Jin et al., 2005). The silicate (SiO3 -Si), phosphate (PO4 -P), and inorganic nitrogen (DIN) are the nutrients of planktons. In winter, the SiO3 -Si, PO4 -P, and DIN concentrations in surface waters are <5∼25, <0.1∼0.4, and <2∼20 μmol/L, respectively. In summer, the SiO3 -Si, PO4 -P, and DIN concentrations in surface waters are 5∼20, 0.1∼0.4, and <1∼4 μmol/L, respectively.

54

1 Introduction

1.1.4.3 Tides The amplitude of the semi-diurnal tide, M2, decreases, while the amplitude of the diurnal tide, K1, increases similar to the Helmholtz resonance after the tidal waves propagate from the western Paciﬁc into the SCS through the Luzon Strait (LS). Analyses of the energy studies show that the LS is a place where both M2 and K1 tidal energy dissipates the most, and strong M2 tidal dissipation also occurs in the Taiwan Strait (TS). The work rate of the tidal generating force in the SCS basin is negative for M2 and positive for K1. It is found that the responses of tides in the SCS are largely associated with the propagating directions of the tides in the Paciﬁc, the tidal frequency, the wavelengths, the local geometry, and the bottom topography. The M2 tide propagates mainly from the Paciﬁc into the SCS through the LS, and it is subsequently directed southwestward into the interior of the SCS and northward into the Taiwan shoal. The M2 tidal amplitude (about 0.2 m) is markedly diminished after passing through the LS from the Paciﬁc (about 0.6 m), which is associated with strong tidal energy dissipation by the local topography. Similar to the conditions of the M2 tide, the relatively high amplitude of the K1 tide appears on the continental shelf, particularly the Sunda shelf. However, unlike M2, no extremely high amplitude of K1 exists in the TS, and the largest amplitude of the K1 tide occurs in the Gulf of Tonkin (about 0.7∼0.9 m). Three other areas with relatively high K1 amplitudes are located in the northern part and mouth of the Gulf of Thailand and south of the Karimata Strait (about 0.6 m). The corresponding strong K1 currents are found in the Gulfs of Tonkin and Thailand and over the continental shelf in the northern and southwestern parts of the basin (Fig. 1.38, Fang et al., 1999).

N 24

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(a)

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E

Fig. 1.38. Numerical results of the M2 (a) and K1 (b) tide currents of SCS (Fang et al., 1999) (With permission from Elsevier’s Copyright Clearance Center)

1.1 Basic Status of China Marginal Seas

55

The K1 currents are stronger in the Gulfs of Tonkin and Thailand, the LS, and the Karimata Strait, but weaker in the TS, as compared with currents induced by the M2 tide. Note that, contrary to the condition of the M2 tide, the amplitude of K1 is markedly increased in the SCS basin (about 0.2∼0.4 m) after propagating from the Paciﬁc (about 0.1∼0.2 m) through the LS. 1.1.4.4 Currents The basic circulation pattern of the South China Sea is as follows: there are two cyclonic eddies, one located east of Vietnam and the other oﬀ northwest Luzon, occurring in the upper layer of the SCS (Fig. 1.39, Liu QY et al., 2008). The structure of the upper layer circulation in the SCS depends on the relative importance of these two eddies. In winter both eddies are developed and there is a generally cyclonic gyre over the entire deep basin of the SCS, while during

Fig. 1.39. Seasonal distribution of the original positions of mesoeddies (circles and stars representing the cyclonic and anticyclonic eddies, respectively) generated during January 1993 and December 2000 (unit: m) (Liu QY et al., 2008) (With permission from Springer)

56

1 Introduction

summer the eddy to the west of Luzon decays and there are a cyclonic gyre north of about 12◦ N and an anticyclonic gyre along the continental slope area in the northwest part of the northern SCS. The southwest monsoon makes the appearance of the South China Sea Warm Current dynamically possible from May to August (Fig. 1.40, Liu QY et al., 2008). In winter like months, especially from December to April, the appearance of the warm current against the local northeasterly winds of 0.2 Pa mean strength is counterintuitive. Guan (1986) proposed that the northeastward Winter Counter-wind Currents (WCWC) originate from the oﬀshore area east of the Hainan Island, ﬂow over the shelf-slope region of the northern SCS, pass through the TS and ﬁnally enter the southwestern part of the ECS. These currents, successively from SW to NE, consist of three components, namely the South China Sea Warm Current (SCSWC), the Taiwan Strait Warm Current (TSWC), and the Taiwan Warm Current (TWC). The SCSWC and the TWC were initially proposed as independent currents in the SCS and ECS, respectively, and afterwards, considering their relations with the current in the TS and emphasizing their counter-wind characteristics in winter, these three currents as a whole were named the WCWC oﬀ the southeastern China Coast. On the west side of the western boundary currents in the world ocean, only the oﬀshore area of China has a broad continental shelf, so this counter-wind current system is also a prominent, unique phenomenon in the shelf circulation of the world ocean. N 25

N 25

GC

(a)

(b)

SC SW C

20

K

SC SW C

20

LC E 15

15

10

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5

C

KC 0 100

105

110

115

VCE

5 KC 120

125 E

0 100

105

110

115

120

125 E

Fig. 1.40. Diagrams of the surface current patterns on the climatological map in the SCS for (a) winter and (b) summer. K, Kuroshio; KC, Karimata current; SCSWC, South China Sea Warm Current; GC, Guangdong coastal current; LCE, Luzon cold eddy; VCE, Vietnam cold eddy (Liu QY et al., 2008) (With permission from Springer)

1.1 Basic Status of China Marginal Seas

57

From spring to fall, the intrusion water from the Paciﬁc is narrowly conﬁned to the continental slope south of China, only in winter, when the northeast monsoon becomes fully developed, can it spread in the southern South China Sea. The seasonal cycle over most of the SCS basin is determined predominantly by the regional dynamics within the SCS, and is forced mainly by surface wind stress curl on baroclinic Rossby waves. Furthermore, the Kuroshio also exerts a signiﬁcant inﬂuence on the SCS. However, the water exchange in the Luzon Strait is very complex. The Kuroshio, the western boundary current of the subtropical North Paciﬁc, begins to form to east of the Philippines. It ﬂows northwards along the coast of Luzon and continues northward east of Taiwan after making a slight excursion into the Luzon Strait. It has a signiﬁcant impact on the ocean circulation of northern SCS. It is noted that the pressure ﬁeld across the Luzon Strait and around Taiwan Island is an important dynamic mechanism governing both the circulation in the northern SCS and the intrusion of the Paciﬁc waters into the SCS through the Luzon Strait. Yuan et al. (2005) pointed out that the Kuroshio intrudes into the SCS to ﬂow northwestward through the Luzon Strait at 200 m and 500 m levels, but the ﬂow direction at the 800 m level diﬀers very much from that at the 200 m and 500 m levels. Model representations show that Luzon Strait Transport (LST) displays a maximum in winter (6.1×106 m3 /s, westward) and a minimum in summer (0.9×106 m3 /s, eastward). On the interannual time scale, LST tends to be larger during El Ni˜ no years and smaller during La Ni˜ na years. The results from many numerical models show that the intruded Kuroshio forms an anticyclonic current loop or a Kuroshio branch. In addition, due to the formation of local eddies along most of the central Vietnam shelf, a southward along-shelf current dominates throughout the year. The southward longshore current ﬂows with a maximum velocity exceeding 1 m/s near the sea bottom. Mean wave height ranges between 0.5 m and 2.0 m, with a maximum of 7.5 m during typhoons, which strike the coast on average 2.5 times per year. 1.1.4.5 Wind The general circulation in the SCS is largely seasonal and is driven primarily by the distinct seasonal monsoon winds. The annual cycle of the mean wind stress ﬁeld is illustrated in Fig. 1.41. With the East Asian landmass to the north and west and the Philippine and Indonesian archipelagoes to the east and south, the SCS is at a unique geographical location, the junction of the three major northern Hemisphere summer monsoon components: the Indian (or South Asian) summer monsoon, the western North Paciﬁc summer monsoon, and the East Asia subtropical monsoon. Inﬂuenced by equatorial disturbances, and linked to the Australian winter monsoon through cross-equatorial ﬂows, this unique geographical location of the SCS in a typical monsoon region determines the important role the SCS plays in the monsoon and climate

58

1 Introduction N 20

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100

Fig. 1.41. Monthly averaged wind stress, derived from the ECMWF model winds during 1984∼1999, over the South China Sea. Wind stress curl contours are in 10−7 N/m3 . Shaded area is positive contour values (Chern and Wang, 2003) (With permission from Springer)

1.1 Basic Status of China Marginal Seas

59

of the East Asian and the western Paciﬁc region. The SCS lies between the Indian Ocean and the Paciﬁc and is part of the eastern Indian Ocean-western Paciﬁc warm pool. However, Zhou and Wang (1999) found that SCS SST variability appeared discontinuous in the phase domain over the western Paciﬁc on an interannual time scale but continuous in an annual cycle. Recently, new research revealed that the Indo-Paciﬁc warm pool (IPWP) in boreal winter showed a conspicuous gap over the SCS where the SST was considerably lower than that over the oceans both to the west and east, which suggested that the coupled monsoon-ocean interaction over the SCS may be an important factor in SCS climate variability (Huang et al., 2008). In winter (November to February), the strong northeast monsoon winds dominate the region and produce a cyclonic circulation. In contrast, in summer (June to August), the weaker southwesterly monsoon winds set up a weaker anticyclonic circulation across the basin. In spring and fall, there is a transition period between the two monsoons. The onset and length of the transition period vary each year. Wind magnitudes from the National Aeronautics and Space Administration (NASA) QuikSCAT scatterometer show peak seasonal wind speeds of 11 m/s in winter and 8 m/s in summer. The southwest monsoon ﬁrst appears in the central basin in May and expands over the entire basin in July and August. From April to August, the weaker southwesterly summer monsoon winds result in a wind stress of about 0.1 N/m2 . Winds prior to September are dominated by the southwest monsoon. In September, the northeast monsoon begins to appear in the seas north of 20◦ N. South of that latitude, the southwest monsoon still prevails. The northeast monsoon expands southward against the diminishing southwest monsoon in October, reaching its maximum strength and covering the entire South China Sea in December. From November to March, the stronger northeasterly winter monsoon winds correspond to a maximum wind stress of 0.3 N/m2 . April marks the end of the winter monsoon. The more powerful winter winds are distributed across the basin while summer winds are more pronounced in the southern part of the SCS near Vietnam. These seasonal monsoon winds also drive mesoscale circulation variability in the SCS. The SCS monsoon is one of the important subsystems of the East Asian monsoon. In boreal winter, high mountains on Taiwan Island and the Philippine Islands block the northeasterly monsoon. Wind speed maxima exceeding 10 m/s have been recorded in the Taiwan Strait and the Luzon Strait. The third wind maximum is located oﬀshore, southwest of Manila Bay. In boreal summer, wind speeds reach a maximum around 11 m/s oﬀ the coast of South Vietnam, which is a mountain range that rises above 500 m and runs in a north-south direction on the east coast of the Indochina Peninsula. The southwesterly winds are blocked by this coastal mountain range, giving rise to a wind jet oﬀshore. A northeast-southwest oriented zero-curl contour extends approximately from the Taiwan Strait to the region oﬀshore of central Vietnam, separating the SCS into a large southeastern region and a small northwestern region, with positive and negative wind-stress curls, respectively,

60

1 Introduction

in winter. The large standard deviation of monthly wind stress curls from the annual mean value is also seen to southeast of Vietnam. The action of monsoons, solar radiation, topography, and so on, mean that the structure of the circulation in the SCS is very complex. 1.1.4.6 Distribution Pattern of Sediments The South China Sea (SCS) is a marginal sea bounded to the west by variable continental shelf types. In the northern (Gulf of Tonkin) and southern (Sunda) sectors, the shelf is wide, extending over hundreds of km. It is covered with relict deposits, or thin Holocene sediments, except for the Ba Lat and Mekong River deltas and prodeltas, where ongoing sediment accumulation is high. By contrast, the shelf along the central sector of Vietnam is much narrower (only about 40 km wide), and is covered with almost 10 m of Holocene sediments. Mountainous drainages adjacent to the central Vietnam shelf represent the type responsible for a large fraction of the global sediment delivery to coastal oceans, characterized by steep morphological gradients, high sediment yields, and seasonally high precipitation with frequent ﬂood events. Moreover, in the last century a rapid deforestation has increased soil erosion and, consequently, the sediment ﬂux to the ocean. From previous studies, it is evident that at least a portion of the sediment delivered to the sea is accumulated on the narrow Vietnam shelf. According to 14 C-based, long-term linear sediment accumulation rates were determined for 12 irregularly spaced gravity cores from the central Vietnam shelf. The rates ranged from 0.05 cm to 0.1 cm per year. Modern rates have been reported only for two closely spaced cores dated with 210 Pb chronology from radioactive fallout. These were 0.33 cm and 0.37 cm per year for muds on the mid shelf oﬀshore Nha Trang. Preliminary results from recently deployed deep-sea sediment traps at the station (12.09◦ N, 109.98◦ E) oﬀ the Vietnam shelf, indicate that signiﬁcant amounts of lithogenic material are probably advected from the shelf. Most of the coastline at the western margin of the SCS is rocky, sandy beaches or tidal ﬂats are found only inside several bays, and along the northern part where open sandy beaches prevail. Vietnam shelf surface sediments are composed mainly of sand on the inner shelf, and mud, with variable amounts of sand, more oﬀshore. 1.1.4.7 Riverine Discharge There are several large rivers delivered into the SCS, including the Zhujiang River and Mekong River. The Zhujiang River is the second largest river with a water discharge of ∼330×109 m3 /yr in China, which has a sediment delivery of ∼80×106 t/yr. The river has a catchment area of 4.5×105 km2 and stretches for 2,200 km. The entire drainage basin is located south of 27◦ N and it is considered as a subtropical large river. After the Mekong River, it provides the largest annual freshwater input to the South China Sea, the

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largest marginal region in the North Paciﬁc. The Zhujiang River Estuary is among the most complex estuaries in the world. It is composed of three subestuaries, namely Lingdingyang, Modaomen, and Huangmaohai, and is fed by three major Zhujiang River tributaries, namely the Xijiang, the Beijiang, and the Dongjiang. The partitioning of water discharge among these major tributaries is listed in Table 1.6, in which the Xijiang has the dominant water discharge. It is estimated that 50%∼55% of the Zhujiang River freshwater is discharged into the Lingdingyang through four outlets, namely the Humen (HUM), the Jiaomen (JIM), the Hongqili (HQL), and the Hengmen (HEM) (Table 1.6). The shelf areas in the northern part of the South China Sea are greatly inﬂuenced by freshwater discharge from the Zhujiang River, and a freshwater plume typically forms in spring and summer. Table 1.6. Basic hydrographic data of major tributaries and outlets of the Zhujiang River (Pearl River) Estuary and Lingdingyang Estuary, which is the major subestuary of the Zhujiang River Estuary (Yao et al., 2006) (With permission from Elsevier’s Copyright Clearance Center) Basin area (×103 m2 ) Tributaries Xijiang Beijiang Dongjiang Others Total Outlet Humen Jiaomen Hongqili Hengmen Total

351.5 44.7 25.3

Wet

Discharge (×109 m3 /yr) Dry Mean Percentage of the total (%)

335 52.6 33.8

83.1 16.2 13.8

215 48.9 23 25 312

76.3 72.6 28.1 47.8 224.8

24.8 21.5 6.2 13.5 66.1

56.9 52.6 18.6 32.5 160.6

421.5

35.5 32.7 32.7 11.6 20.2

1.1.4.8 Biological Characteristics (1) Chlorophyll a and primary production (PP) The chlorophyll a concentrations during 1998 to 2002 are displayed in Fig. 1.42. In 1998, the chlorophyll a concentration was low, particularly in winter. In April, before the onset of the southwest monsoon, the coastal zone color scanner (CZCS) image (Fig. 1.43) shows very low chlorophyll a concentration almost everywhere in the SCS except in the coastal zone. In the central basin, the CZCS-derived concentration is mostly below 0.1 mg/m3 , especially in the inner portion of the central basin (<0.05 mg/m3 ). In most coastal areas, the relatively high chlorophyll a concentrations may be attributed to DIN provided by river runoﬀ. Under the summer monsoon in August, the Sea-viewing

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Fig. 1.42. Time series of monthly Ch1 a content of the SCS from 1998 to 2002 (Li XB et al., 2006) (With permission from Shi P)

Fig. 1.43. Sea-surface chlorophyll a distribution for April (Liu et al., 2002) (With permission from Elsevier’s Copyright Clearance Center)

Wide Field of View Sensor (Sea-WiFS) image (Fig. 1.44) shows a band of high chlorophyll a concentration about 100 km wide extending 600 km northeastward from the Mekong River mouth. Elevated chlorophyll a concentrations also appear along the coast of southwestern China and extend into the southwestern reaches of the Taiwan Strait (Fig. 1.44). During the inter-monsoon period in October, the chlorophyll a concentrations again dropped to very low levels (Fig. 1.45), similar to the condition during the spring inter-monsoon period (Fig. 1.43). When the winter monsoon peaks in December, the CZCS image (Fig. 1.44) shows patches of elevated chlorophyll a concentrations oﬀ northwest Luzon. In the southern SCS, the CZCS image of December (Fig. 1.46) shows a very strong signal in the coastal zone oﬀ southern Vietnam (up to 2∼3 mg/m3 ), oﬀ the Malay Peninsula and north of Borneo (up to 5 mg/m3 ).

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63

WiFS Chl (August) N 22 20 Unit: mg/m3 5 1.5 0.5

18 16 14

0.3 0.2 0.1 0.05

12 10 8 6 4

100

104

108

112

116

120 E

Fig. 1.44. Sea-surface chlorophyll a distribution for August. CZCS data have been replaced by the monthly composite Sea WiFS Chl a data for August 2000, because serious cloud coverage masked a major portion of the CZCS data. Rectangles oﬀ Vietnam are upwelling areas (Liu et al., 2002) (With permission from Elsevier’s Copyright Clearance Center) CZCS (October) N 22 20 Unit: mg/m3 5 1.5 0.5 0.3 0.2 0.1 0.05

18 16 14 12 10 8 6 4

100

104

108

112

116

120

E

Fig. 1.45. Sea-surface chlorophyll a distribution for October (Liu et al., 2002) (With permission from Elsevier’s Copyright Clearance Center)

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1 Introduction CZCS (December) N 22 20 Unit: mg/m3 5 1.5 0.5

18 16 14

0.3 0.2 0.1 0.05

12 10 8 6 4

100

104

108

112

116

120 E

Fig. 1.46. Sea-surface chlorophyll a distribution for December. Rectangles oﬀ northwest Luzon and southeast of Vietnam are upwelling areas (northwest of Luzon (L) and north of Sunda Shelf (S)) (Liu et al., 2002) (With permission from Elsevier’s Copyright Clearance Center)

The PP during 1998 and 2002 is displayed in Fig. 1.47. The average PP is 200∼250 g C/(m2 ·yr). The PP value was lower in 1998 (217.1 g C/(m2 ·yr)) than those in the other four years (230 g C/(m2 ·yr)) (Fig. 1.48, Li GX et al., 2006). In four seasons, the highest PP value is found in winter and the lowest is found in summer (Fig. 1.47, Li XB et al., 2006). Fig. 1.49 shows the monthly variation of PP, the mean depth-integrated primary production over the entire output domain follows the seasonal variation of chlorophyll, with high values in winter and summer and low values in between (Fig. 1.49). It varies between 207 and 373 mg C/(m2 ·d) with an

PP (g C/m 2 )

27.5 25.0 22.5 20.0 17.5 15.0 1 7 1998

1 7 1999

1 7 1 7 2000 2001 Time

1 7 Month 2002 Year

Fig. 1.47. Time series of monthly primary production of SCS from 1998 to 2002 (Li XB et al., 2006) (With permission from Shi P)

PP (g C/(m 2 .yr))

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65

250 240 230 220 210 200

1998

1999

2000 Year

2001

2002

Fig. 1.48. Annual variation of primary production of SCS from 1998 to 2002 (Li GX et al., 2006) (With permission from Elsevier’s Copyright Clearance Center)

Fig. 1.49. Monthly variation of basin-wide averages of depth-integrated values of primary production and grazing for the top 135 m of the basin and area-averaged export ﬂux of POC at 125 m in the SCS. The time series are model outputs at 10day intervals (Liu et al., 2002) (With permission from Elsevier’s Copyright Clearance Center)

monthly mean of 280 mg C/(m2 ·d). The highest productivity is in February, and the summer peak is considerably lower. (2) Phytoplankton The average biomass is 83.69 mg/m3 . In winter, surface water of the SCS is laden with nitrate that supported the growth of typical non-diazotrophs such as diatoms and coccolithophores (Table 1.7). In the basin, coccolithophores and diatoms are more abundant in winter than in summer. Emiliania huxleyi is the most dominant coccolithophore in winter, and so are Umbellosphaera irregularis and U. tenuis in summer. In winter Chaetoceros spp. and Nitzschia spp. are the most prevalent diatoms while in summer Nitzschia spp. alone is the most dominant. The tendency of the phytoplankton assemblage in the SCS to have more Chaetoceros and E. huxleyi in winter, but more Umbellosphaera irregularis and U. tenuis, and no Chaetoceros in summer, is in compliance

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Table 1.7. Comparison of winter and summer phytoplankton assemblages in the basin and on the shelf of the South China Sea (Chen, 2005) (With permission from Elsevier’s Copyright Clearance Center) Phytoplankton Diatom (×103 cells/L) Centric Pennate Dinoﬂagellates (×103 cells/L) Coccolithophorids (×103 cells/L) Blue-green algae (×103 trichomes/L) Richelia intracellularis Trichodesmium spp. Microﬂagellates (×103 cells/L)

Shelf Summer Winter 2.78 3.94 0.02 2.90 2.76 1.04 1.44 1.72 1.80 3.24

Basin Summer Winter 0.31 1.34 0.09 0.36 0.22 0.98 0.83 0.54 0.04 19.37

0 0.01 NA

0.01 0.01 4.00

0 0.02 4.58

0 0 1.82

NA: not measured. The summer samples were collected in June 2001 (shelf) and July 2000 (basin), respectively, whereas the winter samples were collected in January 2003

with the phytoplankton assemblage development during ecological succession from mesotrophic to oligotrophic environments. Moreover, the abundance of diatoms on the shelf, regardless of season, is higher than that of the basin in winter (Table 1.7, Chen, 2005). The fact that the diatom genera Thalassiosira and Rhizosolenia are dominant in winter and Nitzcshia is dominant in summer supports the assertion that the SCS shelf is more eutrophic in winter than in summer. (3) Zooplankton According to investigation in 1997 to 2000, the zooplankton biomass was 22.05 mg/m3 in the South China Sea. Li CH et al. (2004) reported the zooplankton in the north of the South China Sea from 1997 to 2001. In total, 709 species of zooplankton were identiﬁed and the results showed that the composition of the dominant species was diﬀerent in various waters, and eight dominant species occurred during the surveys, which were Temora discaudata, Undinula vulgaris, Canthocalanus pauper, Centropages furcatus, Eucalanus subcrassus, Euchaeta concinna, Sagitta enﬂata, and Lucr intermedius. In four seasons the total biomass of zooplankton ranged from 18.08 to 38.27 mg/m2 , and the average was 25.27 mg/m2 . The biomass in winter was the highest, followed by summer and spring, while in autumn it was the lowest. The horizontal distribution of zooplankton was seen to be uneven and speckled and changed with the seasons. In spring and autumn the dense area occurred mainly in the upwelling waters along the Taiwan coast and the northern inshore waters of Beibu Bay, and in summer the dense area was distributed in the coastal waters from the Taiwan Bank to West Guangdong, while in winter the distribution was relatively even. The density ranged from 0.24 to 621.13 ind./m2 with an average of 27.52 ind./m2 , and the densest area occurred in the coastal waters. The horizontal distribution varied with the diﬀerent seasons, and the

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dense area occurred along Taiwan coastal waters, north of Beibu Bay, and West Guangdong coastal waters. The biodiversity index varied from 1.63 to 5.55, while the average was 3.90, and it showed an increasing trend from north to south. In summer and spring the diversity index was higher, while in autumn and winter it was lower. The range of the diversity threshold value was 0.02∼4.63, while the average was 2.93. The diversity level was in the second class, which showed that zooplankton in the north of the South China Sea was rich in diversity. (4) Benthos species The benthos species in the South China Sea includes 690 species. The dominant species mainly include Polychaeta, Mollusk, and Carapace. The average biomass and abundance are 10.83 g/m2 and 122 ind./m2 . The biomass and abundance vary with the change of seasons. The highest biomass is found in spring (13.26 g/m2 ) and the lowest value is found in winter (7.88 g/m2 ), while the highest abundance is found in winter (130 ind./m2 ) and the lowest abundance is found in spring (110 ind./m2 ) (Jin et al., 2005). Li XZ et al. (2007) investigated the Zhubi Reef of the Nanshan Islands in the southern South China Sea. 314 macrobenthic species occurred in the stations, including 130 mollusc species, 110 crustaceans, and 30 polychaetes. The dominant species were Pilodius scabriculus and Alpheus spp., and the indices of relative importance (IRIs) of the dominant species were low. (5) Fish Ma et al. (2006) analyzed ﬁsh biodiversity in the South China Sea. 2,321 species of ﬁsh inhabit the South China Sea belonging to 35 orders. Fishes in the order Perciformes almost dominate the ﬁsh species in this region (979 species). Labridae, representative of warm waters, inhabiting among the corals, form the dominating families (117 species). Yet Chaetodon (34 species) and Epinephelus (31 species) absolutely predominate in this area. In the South China Sea, the dominant species is Epinephelus, accounting for 91.31%, whereas, Kareius bicoloratus accounts for only 0.15% (Fig. 1.50, Ma et al., 2006).

1.2 Progress in Marine Biogeochemical Process Research in China Biogeochemistry is the science which studies the relationship between an organism and its environment, through tracing the transport and transform of chemical elements. Marine biogeochemistry is such a science that introduces the concept and research approach of biogeochemistry into the research ﬁeld of marine sciences. It is an intersecting discipline that emerges from the synthesis of chemical oceanography, marine geology, and marine biology. It mainly focuses on the geochemical process involved in marine organisms and on the eﬀect of the marine geochemical environment on marine organisms,

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1 Introduction 8.54% Warm temperature species

0.15% Cold temperature species

91.31% Warm-water species

Fig. 1.50. The proportions of ﬁsh species in the South China Sea seawaters (Ma et al., 2006) (With permission from Ma CH)

to reveal the correlation of the element composition of marine organisms and their environment. The formation of marine biogeochemistry is the consequence of an increasing synthesis of marine sciences and the extension of the multi-disciplinary intersection. Its importance is also due to the increasing attention paid by human society to research into geosphere-biosphere interaction and global change. Marine biogeochemistry has been one of the most active frontiers in the marine sciences in the past two or three decades. Marine biogeochemistry studies perform important functions in research into important scientiﬁc problems such as the ocean ﬂuxes in the China Sea and its adjacent sea area, the shallow sea ecosystem dynamics, the land-ocean interaction of the coastal zone, and the red tide process dynamics. Chinese marine biogeochemistry studies began at the end of the 1970s. By and large, they can be divided into three stages. The ﬁrst stage was from the end of the 1970s to the end of the 1980s, i.e., the ﬁrst decade, when several international cooperative projects had been carried out such as the Sino-American joint investigation starting from 1980, the Sino-American and Sino-French Huanghe River Estuary investigation starting from 1985 and the Sino-French Changjiang River Estuary biogeochemical investigation into pollutants and nutrient elements. At this stage Chinese scientists kept widely abreast of the newest technologies and methods of marine biogeochemistry research abroad. It not only greatly promoted research into Chinese estuarine chemistry and marine biogeochemistry, but also fostered a large number of young academic heads who have been active in ﬁrst-line work in Chinese marine science research, which created a sound foundation in the development of Chinese marine biogeochemistry research. The second stage was from the end of the 1980s to the end of the 1990s, i.e., the second decade. Chinese marine biogeochemistry research had been greatly developed in this decade, and some important frontier subjects were made independently, for example the NSFC key projects “Studies on Ocean Flux of the East China Sea Shelf Margin”

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(1992∼1995), “Studies on Key Process of the East China Sea Ocean Fluxes” (1996∼1999), and “Studies on Biogeochemical Process of Biogenic Elements in the Taiwan Strait” (1997∼2000). Rich achievements in biogeochemistry have been obtained and a research system with Chinese characteristics has been formed gradually since then. The third stage is from 2000 up to now, and it is still in progress. This section tries to review the major advances in marine biogeochemistry studies in China in recent years. 1.2.1 Progress in the Studies in Marine Biogeochemical Processes before 2000 in China Much important progress into marine biogeochemical processes was made before 2000 in China. These works laid a foundation for the development of marine biogeochemistry. 1.2.1.1 Estuaries, Coasts and Continental Shelf The research made by Chinese scientists into estuarine nutrients in recent years mainly focused on the large estuaries such as the Zhujiang River Estuary, the Changjiang River Estuary, the Huanghe River Estuary, and the Jiulong River Estuary, and they ﬁxed their attention on the nutrient ﬂuxes discharging into the sea, the transport mechanism, and the controlling process. Of all estuaries, more attention has been paid to the Zhujiang River Estuary because of its special geographical position and complexity. Based on the ﬁeld observations and in situ incubation experiments in the summer of 1998 and 1999 along the Zhujiang River Estuary and adjacent coastal waters of south Hong Kong, the nutrient limiting factor shifted across the coastal plume from the P limitation in the estuary to the N limitation in the oceanic waters in the Zhujiang River Estuary. The potential P limitation was observed in the estuary; the P and Si co-limiting occurred at the edge of the coastal plume, and N was the limiting factor in the oceanic side. This kind of limitation characteristic was also found in the Shuangtaizihe River in northern China. In eutrophic estuarine and coastal waters, trace elements of Cu and Fe(II) have an important impact on the photosynthesis, with Fe(II) being a more important limiting element than Cu. According to the calculation, about 774.90×103 , 55.38×103 , and 144.55×103 t of dissolved inorganic nitrogen (DIN) were discharged into their respective estuaries each year by the Changjiang River, Huanghe River, and Zhujiang River in 1980∼1989, mainly in the form of nitrate (>80%). A positive relationship was observed between the annual DIN transport amount of the Changjiang River and the annual application amount of chemical fertilizers in its catchment area, and the annual DIN loads of the Huanghe River and Zhujiang River were inﬂuenced mainly by runoﬀ and also by the amount of chemical fertilizers applied. In recent years, more attention has been paid to the changes in nutrient concentrations and structure under the inﬂuence of human activities. This

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1 Introduction

is the prominent characteristic of nutrient research in bay and coastal seas in China. For example, based on the analysis of two observation reports in the Bohai Sea in 1998∼1999 and 20-year time-series data of nutrients and biological parameters, both the concentration and relative content of nutrients have changed dramatically in the central Bohai Sea in the past 20 years. The increase in nitrogen and decrease in phosphate and silicate led to a dramatic increase in the N/P ratio and a decrease in the Si/N ratio. The nutrient limiting factor in the central Bohai Sea is gradually changing from nitrogen in importance to a relative lack of phosphate and silicate. They indicated that the decrease in silicate might be the major factor in the high frequency of red tide in the Bohai Sea in recent years. The prominent feature is the notable increase in nitrogen and phosphate concentration, and the possibility that nitrogen and/or phosphate as primary production limiting factors in Jiaozhou Bay have been decreased or eliminated and that of silicate limitation has increased. As far as associating the changes in nutrient concentrations and structure (composition) with the structural evolution of the ecosystem on a larger time scale are concerned, these researches are undoubtedly of beneﬁt. Chinese scholars have started research into the transportation of nutrients to the ocean via the atmosphere and its inﬂuence on the marine ecosystem in recent years. There were clear seasonal variations for most of the ions, and the concentrations of major ions from urban area rainwater were apparently higher than those in remote regions. By in situ incubation experiments in the coastal Yellow Sea, the atmospheric deposition with high nitrogen and low phosphorus in the Yellow Sea area was the major nutrient resource for phytoplankton in the mixed layer during the water stratiﬁcation period in summer. The study of carbon biogeochemistry in the South China Sea has been a very active ﬁeld in recent years, and one of the most important advances is the discovery of the inﬂuence of the East Asia monsoon on primary production through the analysis of the sediment trap experiments data. Chen J et al. (1998) estimated the primary productivity in the South China Sea and its output at the surface layer based on the data of sediment trap experiments, and they found that these two kinds of productivity increased obviously in the monsoon period. On the basis of the data of 1993∼1995, the seasonal variations of radiolarian and diatom ﬂuxes in the central South China Sea were overwhelmingly controlled by the monsoon climate, and they increased during the northeast (from November to February) and the southwest (from June to September) monsoons and decreased during the periods between the monsoons. The high radiolarian ﬂux corresponds to the high surface primary productivity. The change in circulation driven by the monsoons improves water exchange in the diﬀerent areas that brings rich nutrients for the surface phytoplankton, thereby enhancing primary productivity and increasing diatom ﬂuxes.

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1.2.1.2 Cycling Processes of Biogenic Elements in the Euphotic Zone Song et al. (1997b) studied the sectional and vertical distributions of dissolved oxygen (DO) and the O2 ﬂuxes across the air-sea interface in the East China Sea (ECS) waters. Their research showed that the waters were in a steady state and that the diﬀerence in DO was great in upper and bottom waters in Apr. 1994, but seawater mixing was strong and the diﬀerence in DO was small in upper and bottom waters in Oct. 1994. The above conclusions were shown to be valid for continental shelf waters below 100 m. The DO maximum in subsurface layer waters appeared only at several stations and in general the DO in the waters decreased with depth. The horizontal distributions of O2 ﬂuxes across the air-sea interface appeared in strings in Leg 9404 when most regions covered were supersaturated with O2 seawater to air ﬂux being large; the ﬂux on section No. 1 was 1.594 L/(m2 ·d). The horizontal distribution of O2 ﬂuxes across the air-sea interface appeared lumpy in Leg 9410, when most regions covered were unsaturated with O2 . O2 was dissolved from air to seawater, and the ﬂuxes were 0.819 L/(m2 ·d) on section No. 1 in Leg 9310 and 0.219 L/(m2 ·d) in Leg 9410. The main reasons for DO change in the surface layer seawaters were the mixture of upper and bottom layer waters and the exchange of O2 across the air-sea interface. The variation in DO by biological activity was only 20% of the total change in DO (Song et al., 1997b). Mean particulate organic carbon (POC) value in the ECS was about one order of magnitude greater than that in the Atlantic and Paciﬁc oceans. Living POC accounted for about 10% of the total POC in spring and 4% in autumn. The maximum values of POC occurred in the shelf center in spring, but moved to the Changjiang River mouth in autumn. In spring, the distribution of POC in surface waters coincided with that of ATP and the ratio of carbon to nitrogen in particulate organic matter (POM) was relatively low (C:N=7.63), suggesting that POC came mainly from the local biological production. In autumn, however, POC in surface waters, which decreased seaward from the Changjiang River mouth, had little in common with the distribution of ATP, and the C:N value in POM was very high (C:N=15.23) in this season. This indicated that most of the POC was not provided by biological production in autumn. However, the strom-caused resuspension of sediments in the inner shelf area and the relatively large Changjiang River runoﬀ during autumn were probably the main sources of PC in the ECS (Liu and Wang, 1997). The CO2 vertical ﬂux in seawater is mainly dependent on the CO2 concentration diﬀerence between seawater and the atmosphere. Its exchange velocity is aﬀected by friction, velocity, solubility, and resistivity. Agreement between this exchange velocity and recent wind tunnel experimental results has been observed with a wind speed in the range of 4∼10 m/s. This ﬂux can increase in temperature. During the three cruise expeditions in October 1993 and April and October 1994, the CO2 vertical ﬂux at the sea-surface was directed downward over the shelf area of the East China Sea. Because

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1 Introduction

the CO2 partial pressure diﬀerence between seawater and atmosphere had a negative value, the ﬂux value was −45.5 μg/(m2 ·h) on average (Zhang and Sun, 1996). In autumn 1994, DOC distribution tended to decrease gradually from the northwest to the southeast. DOC concentrations varied from 40 to 170 μmmol/L. The DOC distribution is mainly aﬀected by continental input and mixing of water masses in the East China Sea. It was found that DOC and salinity in the 32◦ N section correlated well (Zhang et al., 1997). Measurements of dimethyl sulﬁde (DMS) along surface transects and on vertical proﬁles across the East China Sea (ECS) continental shelf show that its concentrations in the surface seawater ranged from 64 to 180 ng/L and that its vertical distribution was divided into 3 types. Model calculations of a stagnant ﬁlm show a DMS ﬂux of 10.6 μmol/(m2 ·d) across the air-sea interface (Yang et al., 1996). Jiaozhou Bay is a typical coastal region. Si/P ratio in surface waters of Jiaozhou Bay decreased signiﬁcantly from the mid-1980s to the early 1990s, and Si/N and N/P ratios also decreased. The relative frequency of probable silicon limitation in the mid-1980s was about 36.6%, but it increased to about 69.6% in the early 1990s. On the other hand, because dissolved inorganic nitrogen and phosphorus concentrations were higher, the relative frequency of nitrogen and phosphorus limitations in Jiaozhou Bay was zero or nearly zero from the mid-1980s to the early 1990s. Consequently, the nutrient structure in Jiaozhou Bay surface waters underwent signiﬁcant changes, and the potential for silicon limitation in Jiaozhou Bay increased (Zhang and Shen, 1997). The monthly average contents of total dissolved free amino acids (TDFAA) in surface seawater at seven stations from Jan. to Nov. range from 1.24 to 2.28 μmol/L. Most stations exhibit in Feb., generally lower than 1.0 μmol/L, with the highest value of 5.0 μmol/L. The lowest values (0.4 μmol/L) always appear in Nov. Among all DFAAs determined, Glu, Gly, Arg, Leu, Orn, and Ser are the most dominant amino acids. The average contents of neutral, basic, and acidic amino acids to total amino acids in surface seawater at station C5 are 55%, 30%, and 15%, respectively. The DFAA contents in the eastern station of Jiaozhou Bay near the city coast give much higher values in surface seawater than in other stations. The contents of dissolved combined amino acids (DCAA) are generally higher than those of TDFAA and both compositions of amino acids are somewhat diﬀerent. The TDFAA contents in the bottom water are usually lower and this shows that in the course of the decomposition experiment of particulate organic matter (POM) the DFAA and DCAA are ﬁrstly and dominantly released and easily inﬂuenced by the biological process (Lu and Ji, 1996). In the coastal sea oﬀ Tianheng Island, the rich nutrient contents are suited for phytoplankton growth and mariculture. The coastal sea waters are of good quality based on the national standard of seawater quality in terms of pH, oxygen, and nutrients. The main controlling factors in the investigated area are hydrographic (salinity, temperature, river runoﬀ, etc.), biological (photo-

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synthesis of plankton), and anthropogenic (fertilizing in the nearby land and sea). In the central and northern parts of the Taiwan Strait during the period from 1983 to 1988, the nutrient concentration in the coastal and bottom waters was relatively high with an upward and oﬀward decrease, and the upwelling in the Fujian coastal area controlled the distribution of nutrients in summer. The upwelling water is characterized by 2.29 μmol/L of nitrate, 2.83 μmol/L of silicate, 0.20 μmol/L of dissolved inorganic phosphorus, respectively. Signiﬁcant correlation between nutrients and dissolved oxygen concentration and its degree of saturation, temperature, and salinity, respectively, are found in the upwelling area. The average of the N/P ratio in the area (15.9) approaches the Redﬁeld ratio. Their vertical ﬂux is estimated to be 23.6 mg/(m2 ·d) for PO4 -P, 223 mg/(m2 ·d) for NO3 -N, and 302 mg/(m2 ·d) for SiO3 -Si, respectively, which is the main source of nutrients in the areas in summer. The means of the ﬂuxes of PO4 -P and NO3 -N are about 86% and 73% necessary for phytoplankton in the euphotic zone, respectively (Chen and Ruan, 1995). The biogeochemical behavior of DIN, phosphate, and silicate in the Minjiang River Estuary is discussed based on data obtained from May 1990 to Feb. 1991 oceanographic surveys in the area. The annual ﬂuxes of nutrients in the Minjiang River Estuary were estimated to be 326.8×103 t for silicon, 771.0 t for phosphate, 45.7×103 t for DIN (42.1×103 t for nitrate, 3.0×103 t for ammonia, 600 t for nitrite), respectively (Chen, 1997). In the Bashi Strait, the ﬂuxes of nutrients were near zero near the surface and increased with depth. For oxygen and carbonates, the distributions of ﬂux had structures similar to those of the current speed ﬁeld. By estimating the gross transport of oxygen, nutrients, and carbonate by the Kuroshio Current, the ﬂux of dissolved oxygen is 6.7×106 mol/s northward and 0.9×106 mol/s southward. The net ﬂux equals 5.8×106 mol/s downstream. The northward ﬂux of phosphate is 22.6×103 mol/s; the southward ﬂux is 1.4×103 mol/s. The net phosphate ﬂux is 21.2×103 mol/s northward. The ﬂux of silicate is 967×103 mol/s northward and 59×103 mol/s southward; the net transport is 908×103 mol/s downstream. The ﬂux of alkalinity is 75.5×106 mol/s northward and 10.8×106 mol/s southward, and the net ﬂux is 64.7×106 mol/s northward. For total CO2 the transport is 73.4×106 mol/s northward and 10.8×106 mol/s southward, or a net transport of 62.6×106 mol/s northward (Chen et al., 1994). The solid-liquid carbon ﬂuxes in the South China Sea have been calculated by using the primary productivity, deposition rate, percentage of carbon in sediments, and the average rate of consumption of dissolved oxygen in abyssal waters. Based on the equations of mass conservation of carbon, the carbon ﬂuxes in Box I, Box II, and Box III have been derived. A model of carbon ﬂuxes in the South China Sea is established. The calculated results show that the carbon input taking up 99% of the total in the South China Sea was mainly from the middle layer and the bottom layer. This was brought into the upper layer by sea water upwelling, then met the carbon input from

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river and rain, accounting for 1%. The total carbon input in the South China Sea is calculated to be 601×104 mol/s. Carbon output due to deposition is calculated to be 3.8×104 mol/s, accounting for 0.6% of the total input carbon. The residence time of carbon in Box I, Box II, and Box III have been derived to be 1.5, 44, and 79 yr, respectively (Han et al., 1996). In the Nanshan Island coral reef ecosystems, the deﬁciencies of 234 Th with respect to 238 U were observed within the euphotic zone and the nitracline zone, which responded to two maximums of photosynthetic pigments. All of these reﬂected the diﬀerential zones where two vertically distinct phytoplankton assemblages lived. Excessive total 234 Th over 238 U was found at depth just below the euphotic zone, which might be due to particle regeneration and/or retardation of particle settling. Estimated by the irreversible scavenging model, the residence time of dissolved and particulate 234 Th was in the ranges of 7∼93 d and 3∼141 d, respectively. The calculated parameters suggested that the study station has a lack of a clear two-layer thorium structure in the euphotic zone (Chen et al., 1997a). The residence time of dissolved and particulate 234 Th was in the ranges of 84∼125 d and 16∼96 d, respectively. The diﬀerence in scavenging ﬂuxes (J ), removal ﬂuxes (F ), and residence time between the mixed layer and the lower euphotic zone suggested the study station had a stratiﬁed two-layer euphotic structure, which was further conﬁrmed by the POC export ﬂuxes out of diﬀerent boxes. The POC export ﬂux out of the euphotic zone (export production) was 19.3 mmol/(m2 ·d). The ways how nutrients are transported into the euphotic zone for supporting this export production were discussed. The covariance of the residence time of particulate 234 Th and POC shows that 234 Th is an excellent tracer for the POC cycle. The estimated export production within this region in autumn and winter ranged from 2.9 to 28.7 mmol/(m2 ·d) and 3.8 to 32.4 mmol/(m2 ·d), respectively. The results derived from two diﬀerent approaches agree to within 30%, which shows that 234 Th ﬂux has also been discussed (Chen et al., 1997b). DOC content ranges from 1.00 to 4.85 mg/L; POC content ranges from 155 to 592 μg/L. Most of the suspended organic carbon is detrital carbon. The ratio of C/N in the organic suspended matter is 9∼20; POC has a close, positive relation to PON, while it is not closely related to other environmental factors (Cai et al., 1997). Net primary productivity in the Zhubi Reef Lagoon was estimated to be 1.81 g/(m2 ·d), of which 3% would be lost with the seawater ﬂowing out of the lagoon; 2.3% also would be lost by being deposited on the sea ﬂoor; about 95% would be changed into secondary productivity to enter the food chain. Therefore, it was reﬂected that the Zhubi Reef Lagoon was an area with high productivity and high nutrient conversion eﬃciency (Lin et al., 1997).

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1.2.1.3 Marine Biogeochemical Process of Settling and Suspended Particulates The vertical ﬂux of sedimentary particles in coral reef lagoons of the Nansha Islands is relatively high. In the Xingyi, Zhubi, and Yongshu Reefs the ﬂuxes of sedimentary particles are 14.62, 3.92, and 22.93 g/(m2 ·d), respectively. The contents of POC are 3.17%, 1.87%, and 0.96%, respectively. In the euphotic layer outside these lagoons, the sedimentary particle ﬂux decreases with depth, but the POC content of sedimentary particles increases with depth. The inorganic matter in sedimentary particles of these lagoons accounts for about 95% of total weight (Wu et al., 1997). The vertical ﬂuxes of sinking particulate carbon, nitrogen, and ﬁve forms of phosphorus were measured with sediment traps in the Zhubi and Yongshu Lagoons of the Nansha Coral Reef, the South China Sea, during three cruises (1993∼1994). The results indicate that, the vertical ﬂuxes of particulate organic carbon, nitrogen, and phosphorus in the Zhubi Reef Lagoon are 77.8, 7.1, and 2.1 mg/(m2 ·d), respectively, those of the total carbon, nitrogen, and phosphorus are 489.0, 11.8, and 3.9 mg/(m2 ·d), respectively, and signiﬁcant variations in particulate are 123:10:1. The higher ratio of C to N implies that N may be the factor limiting plankton growth in the lagoons. The forms of sinking particular phosphorus and the release rate of POC are also discussed (Li CH et al., 1997). The total P and organic P in the biogenous detritus were transported down to the bottom at the rate of 3.60 and 2.07 mg/(m2 ·d) in the Zhubi Reef Lagoon and 11.96 and 4.82 mg/(m2 ·d) in the Yongshu Reef Lagoon, respectively. The release rate of particulate P was estimated at about 61% and 90.2% of organic P was released in lagoon water in vertical transportation and the sediment process by decomposition and biodegradation. The vertical transportation and release of phosphorus play an important role in the supplying and recycling of nutrients and in maintaining the high biomass and productivity of the coral reef. According to the diﬀerent functions of nutrient recycling and the reef lagoon, ﬂat and seaward slopes are divided into three zones where the nutritional particulate matter is produced, decomposed, and released (Li CH et al., 1997). About half of the sinking particulate organic matter in the above two areas is consumed before reaching a depth of 5 m from the sea ﬂoor and the degree of this consumption in the Yongshu Reef Lagoon is larger than that on the continental shelf of the East China Sea. The distributions of hydrocarbons and fatty acids indicate that the minor diﬀerence in biological sources of sinking particulate organic matter exists between the Yongshu Reef Lagoon and the continental shelf of the East China Sea, but mainly comes from marine plankton. Stronger biological and biochemical transformations of sinking particulate organic matter are also observed and the intensity of this transformation in the Yongshu Reef Lagoon is greater than that on the continental shelf of the East China Sea. It is found that the occurrence of C25 highly branched isoprenoid (HBI) diene may be related to the composi-

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tion of diatom species (Duan et al., 1998). The sinking particulate materials contain abundant isoprenoid ketone, aldehyde, and alcohol lipid compounds, which take part in marine chemical cycling. The compositional features of these compounds show that sinking particulate organic matter mainly comes from autochthonous marine organisms and that the Yongshu Reef Lagoon has more submerged macrophytes-derived components, while the continental shelf of the East China Sea contains more dinoﬂagellate-derived constituents. The contribution of higher land plants to sinking particulate organic matter may be very small. In contrast, it is greater in the Yongshu Reef Lagoon than that on the continental shelf of the East China Sea. Sinking particulate organic matter in the two regions undergoes strong biochemical transformation processes before reaching a depth of 5 m from the sea ﬂoor, and its intensity is greater in the Yongshu Reef Lagoon than that in the researched marine chemistry, marine biology, and marine sedimentology in China, and reveals the importance of organic geochemistry in the study of oceanology (Duan et al., 1997). In the sediments of the Nansha Islands waters, compositional characteristics of nalkanes, isoprenoid alkanes, alkanes, steranes, and hopanes indicate that they come from marine planktons. Lower Pr/Ph ratios show that the depositing environment is anoxic. With increasing burial depth, the relationship of diagenetic transformation between hopenes and hopanes and that between hopane isomers are found (Duan et al., 1996). The contents and vertical ﬂuxes of major elements in the Nansha coral reef lagoons were diﬀerent. Vertical ﬂuxes of most major elements in the Yongshu Reef lagoon were higher than those in the Zhubi Reef Lagoon. Ca had the highest ﬂux, about 1.4 g/(m2 ·d). Na, K, Ca, Mg, and Sr were transferred onto the sea ﬂoor mainly in carbonate in the Yongshu Reef Lagoon, Fe, I, and Ba in iron-manganese oxides and Al, Br in silicate, and Cl transformation tended to be in the forms of carbonate and silicate. Most of Br, K, Al, I in sinking particulate can recycle into seawater. Half of the total Mg, Na, and Cl can be released into seawater. Most of Ca, Sr, Ba, and Fe can be really precipitated into sediment. The order of the precipitation ratio was Ca>Mg>I>Al>Br>K. Vertical ﬂuxes of major elements have a close relationship to SST. Na and other 8 elements decreased exponentially with SST, but K and Ba had no relationship. The order of sensitivities to SST was Fe>Br>Sr>Ca>Na>Cl>Mg>I>Al. The vertical ﬂux changes of Fe and Br can react to SST variations (Song, 1997a). The vertical transferring ﬂux of most of the measured rare elements in the Yongshu Reef Lagoon was higher than that in the Zhubi Reef Lagoon. The vertical transferring forms of rare elements were mainly in the carbonate form, but Ta, As, Th mainly in the ion-exchange form, Ag in iron-manganese oxides form, and Sb in the organic matter+sulphide form. None of the 18 rare elements was transferred mainly in the form of detritus silicate to the sea ﬂoor. This proved that the rare elements originating from the earth’s crust were redistributed in sinking particulates after they were brought into

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the ocean. The relation between the ﬂuxes and surface seawater temperature (SST) was also studied; the sensitivity of rare elements to SST was in the order of Rb>V>As>Ti>U>Zn>Sb>Hf>Ag>Cs (Song and Li, 1998). In the East China Sea, the content of PIC in the surface sediments is higher than that of POC. Although there is a considerable high vertical ﬂux of particulate carbon (PC) in the bottom seawater at St. 410, the maintenance of PC in the surface sediments is almost impossible due to the low depositing rate, relatively high energy dynamic condition, and oxidative environment of the sea-ﬂoor in that area. The PC may be transferred from the seawater to the surface sediments during the blooming season (i.e., the summer) of marine organisms, but part of them will be transferred back to the seawater again. Possibly, surface sediments dominate in the dis-carbon process during the winter. Therefore, the budget of PC on the sea-ﬂoor sediment-seawater interface is in dynamic equilibrium, making the contribution of the carbon’s catch very limited in the middle continental shelf sand area. The cold eddy mud area and the Zhejiang coastal mud area are the PC sinks since the two areas have relatively high vertical ﬂuxes of PC and the conditions for PC’s maintenance on the surface sediments are satisﬁed. The storage of carbon in the Zhejiang coastal mud area is much higher than that of the cold eddy mud because the former area has a much higher depositing rate. The PC sinks in the seawater controlled by the marine organism productuin and other carbon origins do not always coincide with the PC sinks of the sea-ﬂoor surface sediments in the East China Sea. 1.2.1.4 Biogeochemical Behavior near the Sediment-Water Interface The nutrients (NO3 -N, NO2 -N, NH4 -N, PO4 -P, SiO3 -Si) in sediment interstitial waters and their diﬀusion ﬂuxed across the sediment-water interface are studied in the ecological system of the lagoon and outer-reef in the sea waters of Nansha Islands, the South China Sea. The main results show that: (1) The nutrient concentrations are high in interstitial waters of the seawaters and ΣN/P of interstitial waters is higher in the outer-reef than in the lagoon. The distribution of nutrients is diﬀerent in the interstitial waters of the outer-reef. (2) The activity of sediment in the sea region of the Nansha Islands is higher than that in the East China Sea. A large number of nutrients diﬀuse from sediment to overlying water. H4 SiO4 is the main diﬀusion ﬂux component of + nutrients in the outer-reef and NO− 3 or NH4 is the main diﬀusion ﬂux component of nutrients in lagoons. The diﬀusion ﬂuxes of H4 SiO4 and NO− 3 are higher in the outer-reef than in the lagoon. The nutrient diﬀusion characteristic is decided by the nutrient feature and sediment environment. (3) The high temperature in the sea waters of the Nansha Islands is the main reason for a large number of nutrients produced in sediment to be released to the overlying water. The apparent active energy decreases under high temperature and the sediment activity increases, so a large number of nutrients are produced

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in sediment and diﬀused from sediment to the overlying water (Song et al., 1997a; Song, 1999). HS− concentrations in the lagoon interstitial waters were much higher than those in the oﬀ-reef sediment interstitial waters. Concentrations in interstitial waters of lagoons without human activities and in the near-reef sediment interstitial waters were much higher than those in the oﬀ-reef sediment interstitial waters. Concentrations tended to increase with the depth of the oﬀ-reef sediment interstitial waters. HS− and S2− diﬀused from sediment to overlying seawater, but it was the opposite for SO2− 4 . The average diﬀusion ﬂux in the lagoon was higher than that oﬀ-reef. The average diﬀusion ﬂux in the lagoons was 61.34 μmol/(m2 ·d) for HS− and –0.41 μmol/(m2 ·d) for SO2− 4 . The average diﬀusion ﬂux oﬀ-reef sediments was 14.96 μmol/(m2 ·d) for HS− and –0.35 2− μmol/(m2 ·d) for SO2− 4 . A –2 valence sulfur was controlled by the S+2e→S redox pair in sediment interstitial waters of the water region of the Nansha Islands. The Eh values calculated from the redox pair corresponded to the measured Eh values. Sulfur as a sub-stable form can exist in sediments and induce pyrite (FeS2 ) precipitation (Song and Li, 1996). A monograph “Chemistry of Sediment-Seawater Interface of China’s Seas”, which concerns the marine biogeochemical process, was published in 1997 (Song, 1997a). The monograph contains 7 chapters and the main contents are as follows: An outline of the Bohai Sea, the Yellow Sea, the East China Sea, the South China Sea and the feature of surface sediments in China’s seas are given in chapter 1. The contents and methods of studying the marine sedimentseawater interface are summarized in chapter 1 and chapter 2. The chemical features of overlying waters of marine sediments are described in chapter 3. The diﬀusion ﬂuxes across the sediment-seawater interface of China’s seas are discussed using Fick’s Law. Two main conclusions should be paid much attention to: (1) In the location of volcanic activities, the diﬀusion ﬂuxes of elements from sea bottom to water are very high, for instance in the Okinawa Trough the diﬀusion ﬂux of Cl− from sediment to water is as high as 10.24 mmol/(m2 ·d). (2) In the coral reef ecosystem of the Nansha Islands, a large number of nutrients such as N, P, and Si are diﬀused to seawater, so it is apparent that the diﬀused nutrients are key factors for maintaining the high production of the coral reef ecosystem. The thermodynamic equilibria of elements near the sediment-water interface of China’s seas is elucidated in chapter 5. By analyzing Fe, Mn, and S systems in the sediment interstitial waters and focusing on the redox features of sediments, a series of new concepts and theories have been proposed by “Degree of Relative Equilibrium (DRE)” to quantitatively study the equilibrium degree of the redox pair in sediments and “Redox Degree (ROD)” to assess redox features of marine sediments. The redox pair of China’s Sea sediments are in quasi-equilibrium, and the redox interface is of 200 mV and ROD is 15. The reduction in lagoon deposits of the Nansha Islands is stronger than that in locations on the out-reefs, and the pure chemical standard “Grain Size La-

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bel (GSL)” is also proposed to classify marine sediment types. All these new concepts, theories, and methods are the bases for studying chemical equilibria near the marine sediment-water interfaces. Early diagenesis models of elements near the sediment-water interface of China’s seas are discussed in chapter 6. The elements include P, Si, S, N, F, Cl, Br, and I. Vertical ﬂuxes of biogenic elements, rare and rare-earth elements, and major elements are described in chapter 7. The elemental biogeochemical processes were systematically studied, which are a key problem in oceanographic research into global climate changes and the frontiers of oceanographic research in the world. 1.2.1.5 Function of Small Organisms in Marine Biogeochemical Cycling If three dissolved organic phosphorus (DOP) compounds available as nutrient sources for the experimental culture of three algae were studied, results indicated that these compounds could be utilized by algae, and that dissolved inorganic phosphorus (DIP) was ﬁrst to be taken up when various forms of phosphorus (DIP and DOP) co-existed. Dicrateria zhanjiangensis’ uptake of sodium glycerophosphate was faster than that of D-ribose-5-phosphate. The increase in sodium glcerophosphate had little eﬀect on the maximum uptake rate (Vmax ) of Chlorella sp., but increased the semi-saturation constant (Ks ) remarkably; the photosynthesis rates (PR) of Dicrateria zhanjiangensis and Chlorella sp. were rarely aﬀected by using various forms of phosphorus in the culture experiments. The possible DOP pathways utilized by algae are discussed (Hong et al., 1995). Microzooplanktons including Foraminifera, pelagic Molluscs, Copepoda, and Copelata are major resident members of the coral reef lagoons in the Nansha Islands. Microzooplanktons are resident members with a mean density of 4,246 ind./m3 in these lagoons. They are an important source of food and nutrients for the coral reef lagoons. The carbon, nitrogen, phosphorus, hydrogen, and calcium contents and atomic ratios of these elements (C:N:P:H) of the major taxonomic groups in the microzooplankton were measured (Zhang and Li, 1997). The contents and composition of the nutrients elements of the carbon, nitrogen, phosphorus, silicium, and calcium of zooxanthellae from the coral reef lagoons in the Nansha Islands were reported ﬁrstly. The average contents of C, N, P, Si, and Ca in the cells are 522.1, 87.4, 3.68, 0.17, and 3.40 mg/g dry wt, respectively. The average atomic ratio of these biologically active elements C:N:P:Si:Ca is 366:53:1:0.051:0.71. It is remarkably diﬀerent from the stoichiometric Redﬁeld ratio. The enrichment factors of C, N, P, and Si in the cells relative to the ambient water are 2.24×104 , 3.45×106 , 4.96×105 , and 7.79×103 , respectively (Li PC et al., 1997). It was reported that the fact probably relates to the biological species community in the coral reef lagoon. With the ball-jar process, the in situ test for corals and algae in coral reef lagoons of the Nansha Islands was conducted in May 1990. It was found that

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the coral will ﬁnally reduce the oxygen, but the algae will increase the oxygen much more than the chlorophyll in the waters. The productivity of four algae (Turbinaria sp., Padina sp., Boodla sp., Caulerpa sp.) was estimated in diﬀerent weather conditions: sunny, overcast, cloudy, to be 3.48, 0.35, and 0.86 mg/(g·h), respectively. In addition, if there are algae of 150 g/m2 of reef, and the average sunshine time is 10 h/d, the productivity will reach 1,300 mg/(m2 ·d) (Wu and Li, 1997). The four parts described above show that marine chemistry research in China at the end of the 20th century takes much interest in biogeochemical processes in such interfaces as the seawater-air, seawater-particulate, seawater-sediment, and seawater-organisms interfaces. The theoretical research is closely related to the application of resources and environments. Chinese marine chemists will pay much attention to new methodologies and new discoveries in marine chemistry research in China’s seas in the 21st century. 1.2.2 Progress in Biogeochemical Processes of Marine Carbon Cycles since 2000 in China The biogeochemical process of the marine carbon cycle is one of the key links controlling global change. With the development of the key international plans of JGOFS, GLOBEC, SOLAS, etc. in the past ten years, the carbon cycle study has made great progress. It may be said that the biogeochemical process of the marine carbon cycle has been understood more systematically than ever before. In particular, the marine biological pump process and the mechanism of CO2 absorbed in oceans have been quantitatively recognized and understood. This part focuses mainly on the progress of biogeochemical processes of marine carbon cycles after the year 2000 in China. It includes 3 parts: the CO2 ﬂuxes and processes between atmosphere and seawater; carbon and its biogeochemical cycles; functions of sediment and soils around estuaries in marine cycles. 1.2.2.1 CO2 Fluxes and Processes Between Atmospheres and Seawaters The aquatic ecosystem, especially the ocean, is in total a huge CO2 reservoir. According to a recent estimation, human behavior contributes 5.5×109 t of CO2 to the atmosphere annually, of which about 2.0×109 t is absorbed by the ocean, accounting for 35% of the total discharge, and about 0.7×109 t is absorbed by the terrestrial ecological system, accounting for 13%. It is shown that the ocean and land hold about half of the CO2 from human activity, and the other half of the CO2 is emitted into the atmosphere. It is clear that the ocean could weaken the greenhouse eﬀect from CO2 , and play an important role in regulating the levels of atmospheric CO2 and hence global climate. The study of biogeochemical processes of marine carbon cycling has become the

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key issue in studying the marine carbon cycle and global climate change, and it also will be an important study in international oceanography in the 21st century. and HCO− Carbon in the oceans mainly exists in the forms of CO2− 3 3. The total dissolved CO2 (TCO2 ) in most seawaters is about 2 mmol/kg and about ten times that of the dissolved organic carbon (DOC), and much higher than the particulate organic carbon (POC). The ocean’s role in regulating the uptake capacity of CO2 and the carbon exchange between atmosphere and ocean depends on the mixed layer carbonate chemistry, the advection transfer of carbon dissolved in seawaters, the CO2 diﬀusion across the waterair interface, the various biological processes and settling of organic carbon from biological production, the dissolving and settling of carbonates around the sediment-seawater interface, etc. Many models have been established and developed in order to evaluate the CO2 sink in the oceans. The net ocean sink is estimated to be in a range from 1.2 to 2.4 Gt C/yr based on the box model and the general circulation model, which is generally accepted as 2.0 Gt C/yr. CO2 in the atmosphere is driven by biological pump into the ocean. In the marine ecosystem CO2 is then changed into OC due to biological carbonates of biological photosynthesis in the mixed layer, and is further transferred from the surface to the deep layer, which is the main process of marine carbon cycles (Fang et al., 2001). In order to gain a deeper insight into the global carbon cycle, the ﬁrst thing is to study the variations of CO2 in the surface water and the diﬀerences of PCO2 (ΔPCO2 ) between the sea and the air. The changes in total dissolved CO2 (TCO2 ) in the surface water in the tropical Paciﬁc (10◦ S, 20◦ N, 120◦ E, 90◦ W) during the El Ni˜ no and the La Ni˜ na events have been numerically simulated using a 3D global ocean carbon cycle model with biological pump. The results showed that the changes in the total dissolved CO2 and the partial pressure diﬀerence between the sea and the air (ΔPCO2 ) in the northwest Paciﬁc (0∼20◦ N, 120∼150◦ E) and in the central and east equatorial Paciﬁc no events, (10◦ S, 10◦ N, 150◦ E, 90◦ W) were noticeable. During the El Ni˜ the changes in TCO2 in the surface water increased in the northwest Paciﬁc and decreased in the central and east equatorial Paciﬁc; there were opposite changes in both regions during the La Ni˜ na events (Xing and Wang, 2001). A 3D global ocean carbon cycle model with the ocean biological pump was developed. In this model, the atmosphere is represented as a well-mixed box of CO2 , where CO2 from the surface water is exchanged. The carbon cycle model has been numerically integrated for 1,200 years and ﬁnally reached a quasiequilibrium state. Under the quasi-equilibrium state condition of the model, the computed TCO2 , alkalinity, the dissolved oxygen concentration in seawaters, the distribution of new production and the diﬀerences in PCO2 between the sea and the air are close to the observed results. CO2 absorbed by the sea is 42% and 7% with and without the ocean biological pump, respectively, which shows that there are signiﬁcant eﬀects due to the ocean biological pump on the capacity of the ocean to absorb CO2 in the air. A 3D ocean carbon

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cycle model and a simple terrestrial biosphere model were used to simulate the anthropogenic CO2 uptake by the ocean and terrestrial biosphere under the Intergovernmental Panel on Climate Change (IPCC) scenarios to predict the atmospheric TCO2 levels in future. It was estimated that the anthropogenic carbon emissions must be reduced in order to stabilize CO2 in the atmosphere at various PCO2 levels ranging from 350×10−6 to 750×10−6 . All the stabilization scenarios require a substantial future reduction in emissions (Jin and Shi, 2001). Song (2004) reported that the relation between partial pressure of CO2 water (PCO ) and temperature (T ) in the surface water was obtained from the simu2 water =6.62T +221.03. lated laboratory experiments, which showed the formula PCO 2 water The relative error between the estimated PCO2 and the measured values is lower than 4.5%. The air-sea ﬂux seasonal distributions and strength of source/sink of CO2 in the East China Sea were obtained for the ﬁrst time based on the data of surface seawater temperatures and partial pressure of the atmosphere. The seawater could take in CO2 from the atmosphere in the Bohai Sea, the Yellow Sea, and the East China Sea and the ﬂux values are higher in winter than those in spring. In summer, the situation is reversed and CO2 is released into the atmosphere. In autumn, the seawaters can take in CO2 in the Bohai Sea and the northern Yellow Sea, but release CO2 into the atmosphere in the East China Sea and the southern Yellow Sea. The minimum and maximum of air-sea ﬂux of adsorbed CO2 appear in autumn in the northern Yellow Sea (5.3 g C/(m2 ·yr)) and in winter in the Bohai Sea (106.0 g C/(m2 ·yr)), respectively, and the minimum and maximum of released CO2 appear in summer in the northern Yellow Sea (–1.9 g C/(m2 ·yr)) and the East China Sea (–18.8 g C/(m2 ·yr)), respectively. The annual mean ﬂuxes from seawater to air are 36.8, 35.2, 21.0, and 3.5 g C/(m2 ·yr) in the Bohai Sea, the northern Yellow Sea, the southern Yellow Sea, and the East China Sea, respectively (the Yellow Sea ﬂux is 23.7 g C/(m2 ·yr)), the East China Sea is the net sink of atmospheric CO2 in spring and winter, which can take in 7.69 and 13.56 million tons of carbon, respectively, and is the source of the release of CO2 into the air with 4.59 million tons of carbon. The Bohai Sea and the northern Yellow Sea are the sink of atmospheric CO2 and can take in 0.27 million tons of carbon. The southern Yellow Sea and the eastern China Sea are the source of CO2 , which releases into the air 3.24 million tons of carbon in autumn. As a result, the net carbon sink strength of the East China Sea is 3.24 million tons of carbon in autumn. The annual mean sink strength of atmospheric CO2 in the seas east of China is 13.69 million tons of carbon. In conclusion, in the past 4 years the studies into exchanges of carbon between air and water in Chinese marginal seas have made some progress, especially on exchanges of CO2 . Various ocean carbon cycle models from different points of view have been proposed and have been applied when studying the carbon cycles between air and water.

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1.2.2.2 Carbon and Its Biogeochemical Cycles in Seawaters The presence of CO2 in the marine environment is a crucial link for the global carbon cycle, and plays a predominant role in the exchange and ﬂow of carbon between the atmosphere, hydrosphere, ecosphere, and lithosphere. The research into the transportation and sink of CO2 in the marine environment, which includes the adsorption and transportation ability of CO2 by the marine environment and the cyclic mechanism of CO2 in the marine environment, has been an important research content in international marine science nowadays. The marginal sea and continental slope, especially estuaries and bays impacted greatly by human activities, have played a signiﬁcant role in the whole carbon cycle. In recent years, the research in these regions has made great progress. Now our national study focuses mainly on CO2 exchange between sea water and the atmosphere, colorful dissolved organic matter (CDOM), inorganic carbon in sediment in typical regions, such as Jiaozhou Bay, Zhujiang River Estuary, and Changjiang River Estuary (Song et al., 2008). The research in Jiaozhou Bay in June and July 2003 showed that the average dissolved inorganic carbon (DIC) of surface water is 2,066 μmol/L in June and 2,075 μmol/L in July. In the outside bay, the average DIC is 1,949 μmol/L for surface water and 2,147 μmol/L for sea bottom water. In June, the DIC values of inner Jiaozhou Bay is higher than that of the outside bay, but the result is reversed in July. The concentration is the highest in the northeast and decreases to a minimum toward the west. The total trend of vertical distributions is for a gradual increase from the surface to the bottom, which has some relation to the particulate N and P. The average CO2 seawateratmosphere ﬂux in June in Jiaozhou Bay is 0.55 and 0.725 mol/(m2 ·yr) in July. In addition, Jiaozhou Bay is the source of atmospheric CO2 in both of the two months. The total CO2 ﬂux from seawater into the atmosphere is 62 and 81 t in June and July, respectively (Li CH et al., 2004). As for the north part of the South Sea, it is the source of atmospheric CO2 and the average seawater-atmosphere ﬂux in summer is 7 mmol CO2 /(m2 ·d) and 1∼3 mmol CO2 /(m2 ·d) in spring and autumn. The PCO2 of surface seawater is inﬂuenced greatly by the seawater temperature. CDOM is the substance with optical activity. It plays an important role in the marine cycle, which has distinctive optical characteristics. However, up to now, the biochemical structure and the biochemical substance of the complicated compound are not clear, and the possible constitutes may be amino acids, sugar, glucidamin, fatty acid, carotene, and phenol. In Jiaozhou Bay, the total fatty accounts for 86.90% of CDOM, total sugar is 5.82%, free amino acids is 7.22%, and glucidamin is 0.06%, but the percentages of the carotene and phenol are small, and several orders lower than those of the total fatty, amino acids, and total sugar. The CDOM in Jiaozhou Bay is mainly land-derived; the biochemical substances are further diﬀused and transported under the inﬂuence of river freshwater and inner bay circulation. From the research on CDOM and DOC in the Zhujiang River Estuary in 1999, it is

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revealed that the CDOM content in freshwater is the highest and in seawater is the lowest, which indicates that river water is the primary source of the CDOM in the Zhujiang River Estuary. However, the CDOM in this region is lower than that of the other estuaries in the world, and the CDOM does not show the conservative mixture behavior. This behavior cannot be explained by the removal eﬀect (such as ﬂocculation and photodegeneration), and it may be the result of the diﬀerent constitute and optical characteristics of various water masses. In another way, DOC concentrations vary little with salinity change, and as a result the CDOM and DOC distributions in the Zhujiang River Estuary are diﬀerent. The diﬀerence between CDOM and DOC shows that the contribution of CDOM to DOC is mobile, so it is infeasible to evaluate the DOC concentration in the Zhujiang River Estuary by remote sensing techniques. The research into DOC in the Zhujiang River Estuary shows that the adsorption coeﬃcient at 355 nm in the Zhujiang River Estuary is lower than that of America and Europe due to the fact that the Zhujiang River Estuary is impacted greatly by human activities. The CDOM produced by land-derived human activities is apparently more than that of natural plant degradation. Additionally, the location of CDOM ﬂuorophore varies with the source, and in this way it can decide the constitute characteristics and trace the sources of CDOM. It is especially important to study carbon cycles in the East China Sea, which is a marginal sea with its coastal regions aﬀected signiﬁcantly by human activities. It includes a typical continental shelf, continental slope, and semi-deep-sea regions. Studies into DOC and POC along a cross-shelf transect in the southern East China Sea showed that the DOC concentrations were higher (>85 μmol/L) in the inner shelf and slope waters but lower (ca. 65 μmol/L) around the shelf break, where the Kuroshio upwelling occurred. Such a distribution pattern showed a little temporal variation. The coastal water contained less colloidal organic carbon (COC) than the oligotrophic slope water, suggesting a lower production rate and/or a higher breakdown rate in the coastal water. The POC distribution showed a decreasing trend from the inner shelf to the slope with a local maximum at the shelf break, where POC was enriched due to the enhanced primary productivity induced by upwelling. The average POC content corresponded to about 1/10 of the average DOC content. There was a maximum POC in the mid-depth over the slope, which indicated the lateral transport of POC going oﬀshore from the shelf. The net POC exports of DOC and POC from the shelf were estimated to be 414 and 106 Gmol C/yr, respectively (Chen, 1999). Heterotrophic bacterial biomass, production, and turnover rates were investigated in transect across the continental shelf of the southern East China Sea during spring and autumn. In the coastal and upwelling areas, bacterial biomass was 350∼200 mg C/m2 , production was 28∼329 mg C/m2 , and the averaged turnover rates were 0.09∼0.22 d−1 , which were at least 2-fold those in the Kuroshio waters. Production and turnover rates were positively correlated with primary production (90∼2,133 mg C/m2 ) and POC (1,415∼4,682 mg C/m2 ). Dissolved

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organic carbon from the non-phytoplankton and allochthonous sources might play a signiﬁcant role in supporting bacterial carbon demand in the shelf area of the East China Sea. The southern East China Sea continental shelf region is characterized by relatively low organic carbon concentration with a fast sedimentation rate. The organic carbon concentration ranged from 0.3 to 0.6 wt% and the sedimentation rate from 0.2 to 0.7 cm/yr. In addition, the normal marine S/C ratios were observed. Up to 96% of pyrite-sulfur was reoxidized before its ﬁnal burial. The sulfate reduction rate and the pyrite-sulfur burial rate increased linearly with the increase in the organic carbon burial rate, which indicated that the organic carbon deposition controlled the pyrite formation in the East China Sea continental shelf sediments. The organic carbon utilized by the sulfate reduction and its burial represented a signiﬁcant but a relatively small fraction of the primary production in the studied East China Sea region. According to carbonate and the related parameters, the low-temperature, low-salinity water mass in the summertime northern East China Sea originates from the Yellow Sea Cold Water, which is formed farther northward. There is no apparent annual variation in the carbonate parameters in the Kuroshio east of the shelf break. The partial pressure of CO2 calculated from the pH, TA or TCO2 data in this study shows that the surface water in the shelf area is undersaturated with CO2 in spring and summer. Taking the above data, combined with the other data collected in diﬀerent seasons into consideration, it is shown that the shelf area of the East China Sea is indeed a net sink for atmospheric CO2 , and that it absorbs as much as 0.013∼0.030 Gt C/yr (Wang et al., 2000b). The vertical ﬂuxes and the molar ratios of carbon of the suspended particulate matter in the Yellow Sea were studied based on the analysis of the suspended particulate matter. It was shown that most of the particulate organic matter in the Yellow Sea water column comes from marine life rather than the continent, and that the vertical ﬂuxes of POC in the Yellow Sea are much higher than those in other seas around the world. There was high primary production in this region (Wang et al., 2002). The average carbon biomass of Sybechococcus in the East China Sea ranged from 0.09 g C/m2 (early spring) to 0.90 g C/m2 (autumn). The upward ﬂux of nitrate into the euphotic zone in the South China Sea was calculated by the coupled Ra-nitrate approach, and further converted into a new production of 4.4 mmol C/(m2 ·d) based on a Redﬁeld ratio of 6.6 for C:N. The 234 Th-238 U disequilibrium and the measured ratio of POC to particulate 234 Th yield a POC export ﬂux of 5.7 mmol C/(m2 ·d), and it is consistent with the new production calculated by nutrient budget. Based on the 234 Th-228 Ra disequilibrium, POC export ﬂux was estimated to be 1.7 mmol C/(m2 ·d), signiﬁcantly lower than the derived new production. The discrepancy can be caused by the uncertainty of the DOC transport or accumulation of the data obtained in diﬀerent ways and over diﬀerent seasons and durations. Donghu Lake is a typical shallow eutrophic lake along the Changjiang River’s middle reaches. The mean concentrations of DOC were (15.11±3.26), (15.19±4.24), (14.27±3.43), (13.31±3.30) mg/L in four stations during 1996∼

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1997, respectively. The DOC concentrations of the study area were very similar to those in other lakes along the Changjiang River’s middle reaches. The mean POC of the whole lake was 5.01 mg/L due to the large amount of organic detritus from both local origin and allochthonous origin. A signiﬁcant linear relation was found between POC and chlorophyll a at all of the four stations, which presumably shows that phytoplankton and its exudates and metabolic products are the main contributors to the POC in the water column. DOC/POC ratio (mean value of 4.40) indicated that the organic detritus would be the most important component of the particulate organic matter. phytoplankton is also a factor dominating the particulate organic matter in Donghu Lake (Liu et al., 2000). Han (1998) studied the component of OC and carbon cycle in Daya Bay and the Zhujiang River Estuary. They classiﬁed OC into the dissolved OC (DOC), the particulate OC (POC), and the sedimentary OC (SOC). It is found that both the East China Sea and the Taiwan Strait are reservoirs of CO2 in winter, and the Taiwan Strait is a weaker source of CO2 in spring. Research into DOC and COC in the Zhujiang River Estuary shows that COC accounts for 3%∼32% of DOC, the maxima are lower than those of low salinity regions (<5%) not only in winter but also in summer (Chen, 1999). Colloid matter is formed on the spot where it is found. The phytoplankton biomass and primary productivity, and their annual variations and the photosynthetic carbon ﬂow in the Taiwan Strait were investigated during cruises in Aug. 1997, Feb., Mar., and Aug. 1998, and Aug. 1999. Their results showed that nanophytoplankton (NANO) and picophytoplankton (PICO) dominated the community, with a contribution of 34%∼48% and 34%∼40%, respectively, while microphytoplankton (MICRO) contributed only 12%∼27%. Seasonal and annual variations occurred for size-structure and size-fractionated phytoplankton biomass. PICO dominated the phytoplankton productivity with 45%∼50%, both NANO and MICRO contributed 19%∼32%. The 25% of photosynthetic carbons (PC) were incorporated into the microbial food web (MFW) via secondary production by heterobacteria, 36% of PC into MFW via gazing by heteroﬂagellate. Thus, approximately 60% of PC went into MFW via the two paths, which indicated that MFW would play an important role in the transformation of organic carbon in the Taiwan Strait (Huang et al., 2002). Hong and Wang (2001) studied the biogeochemical processes of biogenic elements in the Taiwan Strait based on the marine dynamics, the coupling of physical, chemical, and biological processes and the contribution of microplankton to the carbon cycle. The results show that on the large spacial and temporal scales, biogeochemical cycling of carbon and phosphate is regulated by marine dynamics in this area. It is shown that the southern area of the Taiwan Strait is a strong source of CO2 in the air in summer, the dissolved organic carbon is the major organic species and the 60% particulate organic carbon comes from the continent. Also, the nano- and picophytoplanktons dominated phytoplanktons in this sea area. Their contributions to biomass

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and primary productivity were over 60% and 80%, respectively. Most of the primary productivity was consumed by bacteria and by hetero-dinoﬂagellates. The POC concentration in Xiamen Bay ranged from 14.4 to 34.6 mmol/m3 with an average value of 21.6 mmol/m3 . The contributions of living organic carbon (phytoplankton) and organic detritus were estimated at about 8%∼26% and 74%∼92% of the TPOC, respectively. The proﬁles of POC showed a gradual decrease with the depth, and its temporal variation showed that the POC concentrations in daytime are higher than those at night. Both features suggested that POC would be closely related to biological processes in Xiamen Bay. The primary production in the study area varied up to 5 times in 1 d, which was consistent with that of the biomass. Primary production also decreased with depth, which was coincident with the patterns of chlorophyll a and POC. In the meantime, the eﬀect of the incubation time on the determination of primary production was studied. The primary production calculated from short-time incubation (2 h) is higher than that from long-time incubation (24 h), indicating that some of the new ﬁxed carbon is preferentially respired and then excreted. Based on the particulate export ﬂuxes from disequilibria and the POC/PTh (particulate OC/particulate Th) ratios in the particles, the POC export ﬂux from the euphotic zone is estimated at 16.0 mmol/(m2 ·d), in which the export ﬂuxes of living organic carbon and detrital organic matter are 2.7 and 13.3 mmol/(m2 ·d), respectively. The ratio between POC export and primary production, referred as the ratio, is 0.31. Both POC export ﬂuxes and the ratios are consistent with the predictive value from the relative formula presented by Aksnes and Wassmann in 1993, but not with other models. The residence time of POC in the euphotic zone was estimated at 11 d, indicating a rapid regeneration rate of POC in the study region. The DOC concentration was 142∼239 μmol/L in the freshwater taken in March 1997 from the four Zhujiang River tributaries ﬂowing into the Lingdingyang Estuary. A high concentration was observed in the Human tributary located near Guangzhou. The rapidly increased DOC concentration at low salinities (∼5‰) may be attributed to the exchange between macroparticulate and dissolved organic matter during the early stage of estuarine mixing. Overall, the DOC concentration followed the mixing line until salinity 25‰, where the deep Bay is located and where DOC was elevated. The elevated DOC may suggest a local organic source from Shenzhen. COC in the study area ranged from 5 to 85 μmol/L, representing DOC of 3% to 32%. The highest COC percentage was found at low salinities (<5‰) in both summer and winter. This suggests that the terrestrial organic matter, which is similar to the hydrokinetic factors, seasons, and salinity, be an important factor controlling the carbon cycling (Dai et al., 2001). Despite the South China Sea being a reservoir of CO2 , its area is large and there are many upwellings favorable to the transfer of CO2 from the lower layer to the upper layer (Dai et al., 2001). Based on the organic carbon concentrations, the stable isotope analyses, and the analyses of the distributions of benthic foraminifera in gravity and the piston cores from two observed stations

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of the northern and southern continental slopes of the South China Sea, the changes in the surface paleoproductivity and the variations of the East Asian Monsoon over the last 40,000 yr have been evaluated. The distribution patterns and accumulation rates of some deep-sea benthic foraminiferal species may be primarily controlled by the organic carbon ﬂux to the seaﬂoor in the South China Sea. Two major subgroups of these species serve as proxy to distinguish two diﬀerent ranges of organic carbon ﬂuxes (>2.5 mg C/(m2 ·yr) and >3.5 mg C/(m2 ·yr)). When organic carbon ﬂux increased to above 3.5 in the southern South China Sea during the Last Glacial Maximum and in the northern South China Sea during the early Holocene, a group of detritus feeders such as Bulimina aculeate and Uvigerina dominated over the others. However, the suspension feeders such as Cibicidoides wuellerstorﬁ and Chilostomella ovoidea gradually became more important than detritus feeders as soon as the organic carbon ﬂux decreased to 2.5∼3.5 mg C/(m2 ·yr). During the LGM, the high organic carbon ﬂux and the increased abundance and accumulation rates of B. aculeate and U. peregrina in the southern South China Sea were mainly caused by the enhanced NE winter monsoon-driven upwelling and the associated productivity, and partly by the increased input of terrigenous nutrients as a result of the lowered sea level. However, during the ﬁrst part of the Holocene, around 10,000 a B.P., the remarkably increased abundance and accumulation rates of B. aculeate and U. peregrina especially in the northern South China Sea, together with the high organic ﬂux, point to increasing productivity, probably driven by a maximum intensity in the SW summer monsoon (Jian et al., 1999). Carbon biogeochemical processes in the special environment have also been studied. For example, the hydrothermal vent communities are quite diﬀerent from the typical deep-sea communities in many aspects, whose food chain structure is chemosynthetic bacteria, which feed on reduced inorganic chemicals like hydrogen sulﬁde in order to synthetize OC to provide energy for macroanimals by means of endosymbioses. The DOC and POC at 12 stations in the Yantai Sishili Bay in May, August, November of 1997 and March and May of 1998 were investigated. The DOC concentrations varied from 1.14 to 5.35 mg/L. The average values at all stations in each cruise varied from 1.52 to 2.12 mg/L. The POC concentrations varied from 0.049 to 1.411 mg/L. The average values of POC in each cruise varied from 0.159 to 0.631 mg/L. Horizontal distribution of DOC was inﬂuenced by several factors, such as terrestrial input, organism activity, temperature, aquiculture environment. The higher POC concentration occurred along the coast. The vertical distribution of DOC and POC changed obviously in spring and summer, but not in autumn or winter. The DOC concentration was the highest in summer and POC in spring; both DOC and POC were the lowest in winter. The seasonal variation of DOC was consistent with that of primary productivity. The seasonal variation of POC was consistent with that of Chl a. There was a signiﬁcant seasonal variation trend of C/N ratio in the dissolved organic matter, but the C/N ratio of particulate organic matter had no signiﬁcant trend (Zhao et al., 2001).

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Carbon biogeochemistry is a science that studies the correction and role of carbons that aﬀect lives and their environments by exploring carbon’s transfer and recycling. Biogeochemical abundance, currents, coupling, and ﬁeld are its four elementary ideas. The transfer and recycling of CO2 , CH4 , N2 O, etc. have been paid close attention to because of abnormal global climates. Li (2001) adopted a DNDC model to study the biogeochemical factors and processes associated with C and N cycling, and to simulate the interactions among global climate change, human activities, and terrestrial ecology. DNDC represents the denitriﬁcation and decomposition which are two main reactions that cause C and N transfer from soil to atmosphere. This model has been used to forecast fertility of the soil and the emission of greenhouse gases in some countries. There are some reports about biogeochemical cycling models of C and N abroad, but so far there is no such systematic or comprehensive model which can simulate the C cycle and N cycle in the atmosphere-ocean-land system. To develop such a comprehensive model we need to set up a multidisciplinary project in which all scientists from marine chemistry, biology, geology, and physics can make a joint study. A theoretical study for imitating the carbon cycling test in a laboratory has also been conducted. Wang et al. (2000b) found that pH can aﬀect adsorption of DOC on goethite. The adsorption percentage of DOC shows a maximum at pH 5∼6, and above 50% at pH 8.1. It shows that the adsorption can aﬀect distributions of DOC in seawater. A simple dynamical box-model was constructed in order to test the seasonal variation features of phytoplankton, zooplankton, DIN, DIP, DOC, POC, as well as dissolved oxygen (DO) in the northern part of Jiaozhou Bay in 1995. The annual variations of the phytoplankton production show two high value periods (Mar. to Apr. and July to Aug.) and two low value periods (May to July and after Oct.). DOC shows the common features: it is high in summer and low in winter (Wu and Yu, 1999). Their model equations describing the cycles of phytoplankton and OC in the ocean are as follows: (1) Phytoplankton (P: mg C/m3 ) dP/dt = B1 − B2 − B3 − B4 − B6 − B7 , (2) Zooplankton (Z: C mg/m3 ) dZ/dt = B4 − B8 − B9 − B10 , (3) Particulate organic carbon (POC: mg C/m3 ) d[POC]/dt = B6 + B8 + B10 − B12 − B13 − B14 , (4) Dissolved organic carbon (DOC: mg C/m3 ) d[DOC]/dt = B2 + B13 − B15 + QDOC , where B1 stands for photosynthesis; B2 , external secretion; B3 , exhalation; B4 , zooplankton assimilating; B6 , natural death; B7 , settling; B8 , dejection; B9 , excretion; B10 , natural death including assimilation; B12 , POC decomposed by bacterium into inorganic matter; B13 , POC decomposed by bacterium into DOC; B14 , POC settling; B15 , DOC decomposed by bacterium into inorganic

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matter. QDOC is the discharge of terrestrial wastewater and the dissolution in sediment. The methods of getting colloid organic carbon (COC) analyzed and separated have also been studied at this stage. The cross ﬂow-over ﬁlter technology (CFF) is a common means that separates COC from the total dissolved organic carbon (TDOC). COC and TDOC are usually determined by high temperature combustion (HTC) and the UV/persulfate method (Wang et al., 2000a). It may be found from the above review that the main aspect of marine carbon cycling studies in China in recent years is the carbon geochemistry in the China Seas, which includes the forms, transfers, distributions, and changes of carbon, the biological productions, the models of carbon cycling, etc. So far, comprehensive or systematic studies of carbon cycles in China marginal seas, especially the process studies of carbon cycles in the carbon biology-chemistrydynamics system, have not been reported and this should be studied in depth in the coming years. 1.2.2.3 Carbon and Its Biogeochemistry in Marine Sediments Marine sediment is the primary part of the marine environment. Carbon in the sediment exchanges unceasingly with water, organisms, the atmosphere, and rivers that empty into the sea. The gas carbon in the atmosphere will transform into dissolved carbon in the water body by a complicated marine biogeochemical progress, and then particulate carbon ﬁnally becomes sediments by deposition. However, under suitable conditions, the reverse process will happen. Therefore, carbon in marine sediments plays a very important role in the carbon cycle. Carbon in sediments includes mainly organic carbon and inorganic carbon, and the main study is of organic carbon at present. The organic carbon in sediments is mainly land-derived and autogenetic, but the percentage is not the same in diﬀerent sea areas. According to the spatial distribution of OC, TN, δ 13 C, and δ 15 N in sediments, the land-derived particulate organic matter accumulation was faster than that of marine organism debris in the Zhujiang River Estuary and its adjacent South Sea sediments, and the contribution of marine organic matter was 31%∼67%; as far from the estuary the contribution of marine organic matter was higher (Hu et al., 2005). Organic carbon derived from algae was lower than 0.06% in the estuary, but higher than 0.57% in the inner shelf. Carbonate is an important composition of inorganic carbon in marine sediments. Up to now, the research into inorganic carbon concentrates mostly on the source, distribution, dissolution, and precipitation of carbonate in sediments. For example, in the western South China Sea, the contents of carbonate in the north and mid-southern areas are high, but low in the middle and southeast areas. The distribution characteristics are controlled by terrigenous material supply and are in close relationship with the extent of the shelf and the gradient of the slope. The contents of carbonate are highest in the area

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of water depth between 400 m and 600 m. The average content in the area with a water depth of 500∼600 m is as high as 44.37%. The dissolution of carbonate intensiﬁes in a water depth of more than 1,300 m, and the contents decrease distinctly. Carbonate concentration becomes stable near a depth of 3,500 m, which suggests that 3,500 m should be a CCD of the sea area (Li XJ et al., 2004). However, the role of inorganic carbon in sediments for the marine carbon cycle has not been paid enough attention to. In order to evaluate the contribution to the marine carbon cycle, the inorganic carbon in sediments is divided into several forms by Li XG et al. (2004a) who studied them in detail in Jiaozhou Bay and the Changjiang River Estuary, for example. The characteristics of diﬀerent inorganic carbon in sediments are obvious, that is NaCl form
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CO2 . Every year there are about 4.99×1010 g IC that empty into Jiaozhou Bay sediments. Among them there are 1.47×1010 g IC at least which are buried for a long time and 3.51×1010 g IC can return to water and take part in the recycle. For the Changjiang River Estuary, there are 11.17×1011 g IC that are emptied into sediment, and there are 5.93×1010 g IC buried for a long time and 5.23×1010 g IC can be returned to water to recycle. 1.2.2.4 Impact of Soil from River and Marine Sediment on Carbon Cycles In order to gain a deeper insight into the ocean carbon cycle, it is very important to study the inﬂuences of marine sediments and soils from rivers and land on carbon cycles. The potential of soil and sediment providing DOC for natural waters depends on the content and the sorption coeﬃcient of the soluble organic carbon. Soil from river is an important factor in regional and global carbon budgets because it serves as a reservoir of a large amount of OC. The δ 13 C isotope analysis of particulate organic matter from the Changjiang River Drainage Basin has shown that the particulate organic matter input from the southern tributary mainly comes from the higher plants, and the particulate organic matter input from the northern tributary mainly comes from soils with the higher organic matter, and less depends on the higher plants. Soils from the land also aﬀect the global carbon cycles. Duan et al. (2002) showed that the total storage of organic carbon in the 0∼50 cm soil layer of the desertiﬁed lands is 855 Mt. In the last 40 years, the total CO2 amount released by the land-desertiﬁcation processes to the atmosphere was 150 Mt C, while the CO2 amount sequestered from the atmosphere by the anti-desertiﬁcation processes was 59 Mt C. Hence, the net CO2 amount released from the desertiﬁed lands of China was 91 Mt C, which indicated that CO2 sequestered by the anti-desertiﬁcation reversing processes in the desertiﬁed land had greater potential than that in the other soils. Yuan et al. (2003) made a study of OC in the core sediments in the 6 stations located in 3 typical regions of the Bohai Sea, i.e., the region of Bohai Bay, the region oﬀ the Huanghe River Estuary and the region of Liaodong Bay. They further approached the proﬁle distribution of OC and the inﬂuences of redox environments (Eh◦ , Es◦ , and Fe3+ /Fe2+ ratio) on OC in the Bohai Sea sediment. The organic carbon content in the sediments with natural grain size is 0.38%∼0.86% in the Bohai Sea. Its variations are greater in the surface and subsurface layers, and less in the deep layer, which is due to the organic carbon diagenesis mechanism in sediments and the sediment origin. A correlation analysis shows that the sedimentary reduction in the middle layer is greater than that in the surface layer, where the oxidizing and reducing environments coexist, the organic matter is oxidized, the OC concentrations tend to decrease, the Fe3+ /Fe2+ ratios also tend to decrease, and a signiﬁcant negative correlation exists between the OC concentrations and the Fe3+ /Fe2+ ratios. In the bottom layer, the reduction takes precedence, a great deal of

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OC cannot be oxidized and reserved, and the Fe3+ /Fe2+ ratios also tend to decrease, which results in a more signiﬁcant negative correlation between the OC concentrations and the Fe3+ /Fe2+ ratios. In the surface layer, the correlation of OC concentrations and Fe3+ /Fe2+ ratios is complex and nonlinear because of some biochemical and physical factors. The surface and subsurface OC sediments are controlled by material sources and physical disturbances. The middle and bottom OC sediments are controlled by redox environments, which lead to a signiﬁcant mineralization and the OC concentration gradually decreases. It is evident that the OC contents in the diﬀerent layers are regulated by material sources, settling environments, redox processes, and different biological and chemical processes. It is found from the observed proﬁles in Liaodong Bay that the OC contents from the surface and subsurface sediments show vertical laminating distributions, which is due to the gradual settling and depositing. The OC contents below the 25-cm sediment surface show level-gradient distribution, which may be due to the special depositing event such as great ﬂood scouring and depositing. The decomposition constant of OC in the core sediment of the Bohai Sea is 0.00479 yr−1 , and the decomposition rates of biogenic elements C, N, P, Si have the sequence N>P>C>Si. The OC/TN ratio is much lower than the OC/ON rario, which indicates that the sediment preserves plenty of inorganic nitrogen (IN) and/or ﬁxed nitrogen, and the decrease in the OC/ON ratio with depth is due to the ON reservation in sediments (Song et al., 2002). It is clear that the function of sediment in carbon cycles is well correlated with the other biogenic elements. It is necessary to study the inﬂuences of the relevant elements on carbon cycles, in order to gain a deep insight into the functions of sediments in ocean carbon cycle. Radiocarbons of carbonate (PIC) and of organic carbon (POC) in the sediment trap samples from the Okinawa Trough were measured by AMS. Concentrations of 14 C in PIC and POC (δ 14 C-PIC and δ 14 C-POC) ranged approximately from +40‰ to −80‰ and their averages were approximately −32‰ over a full two years. These values were much lower than that of the dissolved inorganic carbon (δ14 C-DIC) in the upper 200 m of the water column (+100‰ on an average). The variations in δ 14 C-PIC and δ 14 C-POC were positively correlated with concentrations of inorganic and organic carbons, respectively, and negatively correlated with concentration Al. This suggests that variabilities in δ 14 C-PIC and δ 14 C-POC may be associated with the input of lithogenic materials, and they had a seasonal variation during two years with lower values in winter and higher values in summer. The contents of OC in the two sediment cores from the Nansha Islands region average 0.7% and 0.53%, respectively, and are higher than those of the other sedimentary environments in this region. The distributions of various lipid compounds indicate that most of the sedimentary organic matter in the two cores is derived from marine plankton and bacteria, with a smaller amount of land-derived organic matter. It is evident that the relative abundance of

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shorter chain lipids decreases with depth, as is the biochemical conversion of stenols to stanols in some samples. Apparently, there are many studies about the distribution, content, transfer, and transformation of carbon in sediment or soil to be performed. As one of three interdependent basic links in sediment (including soil-water-atmosphere systems), marine sediment plays an important role in oceanic or global environments. From now on, more attention should be paid to research into the functions of sediments in carbon biogeochemical cycles (Sun and Song, 2002). It is expected that in the coming years Chinese scientists will lay emphasis on research into the carbon-ﬁxed mechanism of marine organisms, the carbon exchange processes between China marginal seas and its adjacent oceans, the coupled mechanism of dynamical transformation and biochemistry, and the functions of sediments in carbon cycles, etc. The Chinese Academy of Sciences has carried out a new project “Carbon Sinks and Sources Study in China’s Land and Marginal Sea Ecosystems” since 2001, which has made great progress in the study of Chinese marine carbon biogeochemical cycle processes, and plays an important role in studying the functions of oceans in global carbon cycles (Sun and Song, 2003). 1.2.3 Biogeochemical Cycle of Biogenic Elements As a basic constituent of the marine food chain, nutrient has prominent signiﬁcance in the marine biogeochemical cycle. On the one hand, eutrophication of river water and coastal water is more serious with the faster development of industrial society in recent years, and red tide bloom is frequent in the marine environment. On the other hand, the lack of some kinds of nutrient can limit the growth of phytoplankton and may become one limiting factor for phytoplankton growth. Therefore, the study on nutrient cycling is a basis and precondition for sustainable utilization of marine biology resources. 1.2.3.1 Biogeochemical Cycle of Nitrogen The forms of nitrogen in the marine environment include the organic, the inorganic, and its concrete forms (NH4 -N, NO3 -N, and NO2 -N). In general, organic nitrogen is higher than inorganic nitrogen in sediments. However, only inorganic nitrogen can be utilized directly by phytoplankton, so the research into inorganic nitrogen is relatively greater than that into organic nitrogen. In surface sediments of Bohai, the NH4 -N in the Bohai central basin increased from the northeast toward the southwest and there were two abnormally high values in the central region. The NH4 -N in Laizhou Bay was distributed uniformly and there were relatively low values in the south of Bohai Bay. Moreover, the values increased toward the north. The NO3 -N in the Bohai central basin increased from the east and west sides towards the center, but for Bohai Bay and Laizhou Bay, the NO3 -N decreased the further the distance from the shore. Results showed that the adsorbed inorganic nitrogen in

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surface sediments accounts for only 3.28% of total nitrogen, the adsorbed inorganic nitrogen is mainly NO3 -N accounting for 83.7%. The NH4 -N adsorbed in surface sediments is primarily derived from the organic matter anoxic mineralization, and its distribution is controlled by the content of organisms and Es. The pH and the characteristics of clay have an important inﬂuence too. The adsorbed NO3 -N is from the overlying water and is controlled by the NO− 3 concentration and its distribution of overlying water. At the same time, the clay and other factors may exert their inﬂuence on NO3 -N. The content of NO3 -N was higher than that of NH4 -N and NO3 -N was the predominant species of adsorbed nitrogen in this region, which showed that the mineralization of organisms was weak and could not provide much nutrient for primary productivity. The terrestrial input of NO3 -N was the dominating nutrient source (Song et al., 2004). The changes in organic matter in sediments have some relation with the NH4 -N change. However, high organic matter does not mean high NH4 -N, and the organic matter contents, nitrogen contents in sediments, and degradation degree will inﬂuence the NH4 -N contents. Additionally, the slim change of salinity may aﬀect greatly the NH4 -N adsorption. Sediment is the important sink and source of nitrogen for seawater. It exchanges unceasingly with the overlying water by nitriﬁcation and denitriﬁcation. Nevertheless, in diﬀerent regions the sink and source characteristics, mass exchange rate, and controlling factors are not the same. Therefore, besides the distributions and sources of nitrogen in sediments, national scientists studies focused mainly on the sediment-seawater exchange ﬂux and their controlling factors in diﬀerent regions. The organic material decomposition in the surface sediments of the abyssal equatorial northeastern Paciﬁc is mainly through nitriﬁcation. The silicate, phosphate, and nitrate of the seawater-sediment interface nearby have an extremely steep concentration gradient and the benthic ﬂuxes of silicate, phosphate, and nitrate are −886.45∼42.62, −3.04∼5.83, and −189.43∼21.05 μmol/(m2 ·d), respectively. Moreover, nitrate and silicate diﬀuse from sediment to overlying water, which is the main source of nutrients in sea bottom water. The results also revealed temporal and spatial variations for benthic ﬂuxes of silicate, phosphate, and nitrate, which may be related to sedimentation environment changes caused by global climatologic change (Ni et al., 2005). The result investigated in the Changjiang River Estuary indicates that nitrous oxide (N2 O) production is very low in the water, and the intertidal ﬂat sediments are the source of the water N2 O during the submerging period and N2 O comes from several processes such as nitriﬁcation and denitriﬁcation of the nitrogen cycle. The natural production rate of N2 O in sediment is between 0.10 and 8.50 μmol/(m2 ·h), and the denitriﬁcation rate changes between 21.91 and 35.87 μmol/(m2 ·h). During the ebb tide, the middle tidal ﬂat is the source of atmospheric N2 O with the exchange ﬂux between –11.03 and 13.17 μmol/(m2 ·h). The ground temperature above the depth of 5∼10 cm is the signiﬁcant factor controlling the emission ﬂux. The N2 O emission in the low

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tidal ﬂat-atmosphere interface is –5.75∼0.49 μmol/(m2 ·h). Overall, the middle tidal ﬂat is the source of air N2 O, while the low tidal ﬂat obviously adsorbs air N2 O. The N2 O emission and absorption through the intertidal ﬂat-atmosphere interface has a signiﬁcant positive correlation to the emission and adsorption of CO2 (Wang et al., 2006). The research in the Changjiang River Estuary revealed that the sediment-water exchange of inorganic nitrogen appeared to have complicated spatial diﬀerences and seasonal variations. The measured ﬂuxes of NO3 -N and NH4 -N could change over a large range, which varied from −32.82 to 24.13 μmol/(m2 ·d) and from −18.45 to 10.65 mmo1/(m2 ·d), respectively. However, the ﬂuxes of NO2 -N were very low, and varied only from −1.15 to 2.82 mmo1/(m2 ·d). The spatial and seasonal diﬀerences between the upper and lower estuary for NO3 -N ﬂuxes were observed clearly, but the NH4 N exhibited the spatial and seasonal diﬀerences between the south and north bank. It had been recognized that the NH4 -N exchange behavior was mainly controlled by the salinity, whereas the NO3 -N exchange behavior was inﬂuenced by the sediment grain size, nitrate concentration in overlying water, organic matter content, water temperature, and dissolved oxygen concentration. Additionally, benthos-burrowing activities increased NH+ 4 release and NO− 3 eﬄux from sediments to overlying water. The bioturbation and excretion can promote NH+ 4 release to overlying water and stronger nitriﬁcations in sedimentation oxidation layers. The vertical distributions of inorganic nitrogen in sediment were changed by benthos disturbance and bioirrigation near the sediment-water interface, then accelerated organic matter mineralization and NH+ 4 exchange between overlying water and pore water, resulting in a larger release from the NH+ 4 pool in the top-sediment to overlying water and evidently modiﬁed the nitrogen dynamics in sediment-water systems (Chen ZL et al., 2005). There are strong nitriﬁcation and denitriﬁcation in Zhujiang River Estuary sediments and the average nitriﬁcation, denitriﬁcation, and nitrate reduction rates ranged from 0.32 to 2.43 mmol/(m2 ·h), 0.03 to 0.84 mmol/(m2 ·h), and 4.17 to 13.06 mmol/(m2 ·h), respectively. The vertical proﬁles of the sediments showed that the nitriﬁcation and denitriﬁcation processes mainly took place in the depth from 0 to 4 cm and there were diﬀerences at diﬀerent sampling sites. The rates of nitriﬁcation, denitriﬁcation, and nitrate reduction were dominated by Eh, nitrate, and ammonium concentrations in sediment and DO in overlying water (Xu et al., 2005). − + The research into the NO− 3 , NO2 , NH4 , P, and Si in the pore water and their exchange between the sediment-water interface in the Yellow Sea and East Sea sediments revealed that both of the two regions sediments were in a weak oxide-reduction environment. Moreover, the nutrient distribution in pore water was controlled by the sedimentary environment, and NH4 -N was the main organic nitrogen in pore water. According to Fick’s First Law, the exchange ﬂuxes of ammonium, dissolved inorganic nitrogen, phosphorus, silicon between sediment and seawater were 299.3∼2,214.8, 404.4∼2,159.5, 5.5∼18.8, 541.3∼1,781.6 μmol/(m2 ·d), respectively. All these were the main

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sources of dissolved inorganic nitrogen, phosphorus, and silicon. At most sites the ﬂuxes of NO− 3 were transported from seawater to sediments. The bottom sediment in the central Yellow Sea provides a large amount of nitrate to the seawater by means of sediment-water interface exchange, which contributes 56% of nitrogen for the new production (Tian et al., 2003). Additionally, the nutrient ﬂuxes in coastal areas are higher than those of shelf areas and when the distance is further from land, the exchange ﬂux is larger. However, for the whole East Sea and Yellow Sea, their exchange ﬂuxes are negative; that is, sediment will absorb dissolved inorganic nitrogen from water. Except for ammonium, whose ﬂuxes decreased with increasing depth and increased exponentially with the water temperature, not all other nutrients showed any similar rules (Ni et al., 2006; Qi et al., 2006). In Jiaozhou Bay, the benthic exchange rates across the sediment-seawater interface ranged from –0.5 to 1.6 mmol/(m2 ·d) for NH4 -N, from 0.005 to 0.67 mmol/(m2 ·d) for NO2 -N, from –2.0 to 2.8 mmol/(m2 ·d) for NO3 -N, respectively. The NH4 -N diﬀusion between pore water and overlying water was the dominant process of DIN exchange. Furthermore, NH4 -N released from sediments to water at most stations. The NO3 -N mainly came from the nitriﬁcation of NH4 -N, while NO2 -N was the intermediate produced in the transformation process. The exchange ﬂux of DIN between sediment and seawater was estimated as 9.68×108 mmol/d, which is about 50% of river input DIN. The ﬂux of DIN can provide 52% of nitrogen required by phytoplankton growth in Jiaozhou Bay. The diﬀusion ﬂuxes in Sanggou Bay sediments − − were 376.33, 33.02, 6.41, and 10.08 μmol/(m2 ·d) for NH+ 4 , NO3 , NO2 , and 3− PO4 , respectively. It can be estimated that the total inorganic nitrogen from sediments to overlying water was 281.7 t, which can supply 28.73% of the nitrogen requirement for phytoplankton primary production. The total released inorganic phosphorus quantity was 416.2 t, which can satisfy the demand of phytoplankton (Cai et al., 2004). Although inorganic nitrogen in sediment plays an important role in the mass exchange between sediment and seawater, the contribution of organic nitrogen to the marine nitrogen cycle should not be neglected. In the early digenesis of sediments, organic nitrogen is released by microorganism degradation, which is an important source of inorganic nitrogen. In order to evaluate synthetically the contribution of nitrogen to the marine nitrogen cycle, nitrogen in natural grain size Bohai sediments was divided into transferable nitrogen and nontransferable nitrogen by a sequential extraction technique for the ﬁrst time. The transferable nitrogen includes four forms: ion exchangeable form (IEF-N), carbonate form (CF-N), iron-manganese oxides form (IMOFN), and organic matter-sulﬁde form (OSF-N). Their distributions were studied and their contribution to the nitrogen cycle across sediment-water exchange was estimated. The results showed that transferable nitrogen accounted for 30.85% of TN in the Bohai Sea sediments and nontransferable nitrogen was 69.15%. Thereinto, IEF-N, CF-N, IMOF-N, and OSF-N account for 3.67%, 0.31%, 0.43%, and 26.45% of TN, respectively. The release order of diﬀerent

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forms is consistent with their combined strength in the sediment and their relative contribution to the nitrogen cycle varies with the time scale. The absolute contribution is decided by its content in sediments, which has the sequence of OSF-N (84.6%)>IEF-N (13.0%)>IMOF-N (1.4%)>CF-N (1.0%). The nontransferable nitrogen accounted for 69.15%, of which about 49% was caused by sediment particulate (Ma et al., 2003). The nitrogen in diﬀerent grain sizes in the South Yellow Sea sediment is divided into four forms. They are ion exchangeable form of nitrogen (IEF-N), weak acid extractable form of nitrogen (WAEF-N), strong alkaline extractable form of nitrogen (SAEF-N), and strong oxidant extractable form of nitrogen (SOEF-N). Among them, the content of SOEF-N was the highest and IEF-N was the primary form of transferable inorganic nitrogen, which was also the easiest participant in the cycle. When all forms of transferable nitrogen of the same grain sizes can take part in a cycle, their contributions to the nitrogen cycle followed the order: SOEF-N>IEF-N>SAEF-N>WAEF-N. For diﬀerent grain size sediments, the absolute contents for diﬀerent transferable forms of nitrogen in ﬁne sediments are the highest and the lowest in coarse sediments. If the proportions of each grain in the sediments are the same, the contents of transferable nitrogen in ﬁne sediments can occupy 60% of the total transferable nitrogen, which is two times that in medium size sediments and almost seven times that in coarse sediments. Thus, ﬁne sediment has the highest potential contribution to nitrogen cycling. In addition, the relative contents of organic transferable nitrogen increase with a ﬁner grain size, while those of inorganic nitrogen decrease (L¨ u et al., 2004b). Grain size is also important for the nitrogen distribution in the southern Yellow Sea sediments. The contents of various forms of nitrogen were higher when the ﬁne grain size sediment increased. Nutrient burial eﬃciency was the highest in ﬁne grain size sediment. Moreover, the highest burial eﬃciency of TN was 30.21%, indicating that an excess of 70% nitrogen in the southern Yellow Sea surface sediments could be released to take part in biogeochemical recycling. The nitrogen released from sediments could supply 6.54% nitrogen for the new primary productivity of the southern Yellow Sea (L¨ u et al., 2005b). However, the ecological function of diﬀerent nitrogen in diﬀerent grain sizes has great diﬀerences. Generally, the various transferable nitrogens in ﬁne grain sizes sediment had close relations with phytoplankton and benthos, and the transferable nitrogen of medium and coarse sediments was mainly correlated with zooplankton. In the four transferable nitrogens of diﬀerent grain sizes in the southern Yellow Sea surface sediment, SOEF-N and SAEF-N had close correlation with the growth and breeding of phytoplankton and promotion of productivity. For the inorganic nitrogen compounds, chlorophyll a, the total phytoplankton abundance and the primary productivity had a positive relationship with the contents of the two inorganic existence forms of nitrogen (NH4 -N and NO3 -N), which indicated that the transferable NH4 -N and NO3 -N could accelerate the phytoplankton growth and promote the primary productivity. Moreover, they were the primary forms absorbed by phytoplankton. Also, NO3 -N was more

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useful than NH4 -N and the ﬁner the grain size the more obvious the function. On the whole, NH4 -N and NO3 -N cannot obviously improve the growth of zooplankton or benthos. This was because that they could not be assimilated by zooplankton or benthos directly but carried into the food chain. No matter what form of nitrogen, its ecological role was easily realized when these nitrogens are transformed into inorganic form (L¨ u et al., 2004a). Finegrained components are the predominant composition in the research area in the northern Yellow Sea core sediment, and their structure and characteristics directly aﬀect the form, content and distribution of nitrogen in natural sediments, so that the vertical distribution of nitrogen in natural sediments is very similar to that in ﬁne sediments. The grain size of sediments has an important role in the early digenesis of nitrogen. The ﬁner the sediments are, the smaller the decomposition rate of organic nitrogen will be, i.e., the decomposition rate of organic nitrogen is the lowest in ﬁne sediment, so organic nitrogen is easily enriched in ﬁne-grained sediments. The burial ﬂuxes of various forms of nitrogen are diﬀerent because of the varying sedimentation rates in diﬀerent sampling stations. The higher the sedimentation rate, the greater the burial ﬂux of nitrogen (L¨ u et al., 2005a). 1.2.3.2 Biogeochemical Cycle of Phosphorus Phosphorus is not only one of the essential components for the growth and breeding of phytoplankton, but also the fundamental element for marine primary productivity and the food chain. Sediment is one of the important sources of phosphorus in seawater; moreover, it is a buﬀer for the phosphorus in overlying water. Therefore, it is of great signiﬁcance for the dynamic cycle, the transformation at the sediment-seawater interface, and the subsequent digenesis to study phosphorus and its forms. Phosphorus in sediments includes mainly organic phosphorus (OP) and inorganic phosphorus (IP), and IP is the primary form. However, the percentage of IP in diﬀerent regions is diﬀerent, for example the IP percentage of total phosphorus (TP) was more than 60% (Li XG et al., 2005) for Jiaozhou Bay sediments and was over 50% for Bohai Bay (Zhao ZM et al., 2005). It cannot reﬂect fully the biogeochemical behavior of phosphorus in sediments if phosphorus was divided only into inorganic and organic phosphorus. Therefore, phosphorus in sediments is generally divided into 5 forms: adsorption form (Ad-P), iron-bound form (Fe-P), calcium-bound form (Ca-P), detrital form, and organic form (OP). The contents of total phosphorus (TP), organic phosphorus (OP), and the iron-bound phosphorus (Fe-P) are primarily controlled by the source matter. Ad-P and OP belong to bioavailable phosphorus and their cycle in sediments is dependent on iron-oxides. Calciumbound phosphorus (Ca-P) in sediments mainly comes from marine plankton. Song (2000b) divided the transferable phosphorus in natural sediments into ion-exchange form, carbon-bound form, iron-manganese oxide form, organic matter-sulphide form. Only these phosphorus forms can take part in the geo-

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chemical cycle. For the Bohai Sea sediment, the transferable phosphorus accounted for 19.2% of total phosphorus, and the organic matter-sulphide form was the primary form of transferable phosphorus accounting for 10.7% (Song et al., 2003). The spatial distribution of phosphorus in sediments is primarily controlled by source matter and the hydrodynamic condition, which has clear regional characteristics. The P distribution presented clear spatial and seasonal characteristics, which were dominated by the resources of P input, sediment texture, and a number of biogeochemical processes in diﬀerent hydrodynamic and environmental conditions of the Changjiang River Estuarine and coastal zone (Gao et al., 2003). The phosphorus combined by apatite in the Nansha Islands sea area has the same source as dissolved phosphorus in seawaters and can be utilized by organisms when it returns to the biogeochemical phosphorus recycle; therefore, the phosphorus was mainly controlled by biological action. Total phosphorus and organic phosphorus contents were correlated observably to FeO and apatite combined phosphorus was correlated to CaO. The phosphorus source and its means of input were controlled by the precipitation of detrital particles in water. The total phosphorus in the Zhujiang River Estuary sediments had a general tendency to increase from sea bottom to surface and the maximums mostly emerged at the surface or at a layer of about 10 cm depth. The vertical proﬁles of organic phosphorus were similar to those of total phosphorus with the maximums mostly emerging at a depth of 5∼10 cm. The contents of Fe-P and Al-P were higher and decreased downward, but the trend was not obvious. TP, OP, Fe-P, and Al-P have a distinct synchronous eﬀect on their sedimentation process, especially total phosphorus and organic phosphorus (Yue and Huang, 2005). Sediment is one important source and sink of phosphorus, and its adsorption and release are always the key points that scientists are focusing on. For the East China Sea, the ﬂuxed PO3− 4 and total dissolved phosphorus (TDP) were removed from overlying water to sediment. The dissolved phosphorus diﬀuses from seawater to sediment in most stations near land. Moreover, the closer the distance to land, the larger the exchange ﬂux. For the whole of the Yellow Sea and the East China Sea, the exchange ﬂuxes of PO3− 4 are negative; that is, sediment absorbs dissolved phosphorus from water and the sediment 3− is a sink of PO3− 4 (Qi et al., 2006). The PO4 absorbed from water every year accounted for 67% of Changjiang River inputs in the region (Qi et al., 2003). In Jiaozhou Bay, PO4 -P was transferred from sediment to seawater at most stations and the exchange rates usually ranged from 0.1 to 90 μmol/(m2 ·d). By considering the area percentage of diﬀerent patterns out of the total area of Jiaozhou Bay, the exchange ﬂux of PO4 -P from the sediments to seawater in Jiaozhou Bay was estimated as 9.76×106 mmol/d, which occupied 24% of the river input. The exchange ﬂux can provide (9±3)% of phosphate required by phytoplankton in Jiaozhou Bay (Jiang et al., 2003). The PO3− 4 exchange ﬂux of Laizhou Bay of the Bohai Sea was 6.7∼6.8 μmol/(m2 ·d). The biological activity inﬂuenced the phosphate exchange greatly.

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In order to understand the phosphorus release behavior in sediments, considerable simulation in the lab and investigation in the ﬁeld have been done. Studies showed that the adsorption capacities of diﬀerent sediment samples have “stable pH scope”, and that this scope is consistent on the whole, which is between 6.5 and 9.5. The maximum adsorption capacity of diﬀerent sediment samples occurs at salinity 6‰ with a range of 5∼7. The phosphate adsorption capacities on most of the sediments increase with the increase in temperature, showing the characteristics of endothermic reactions, while some samples show the characteristics of exothermic reactions (Li M et al., 2005). Under a static environment, phosphorus released from sediment reaches its highest level after vibration for about 10 min, and then maintains stability after 3 h. Adsorption kinetics could be ﬁtted to both the Elovich equation and the two-constant-rate equation. The most released quantity is closely related to the composition of the sediment. Those sediments consisting mainly of silty and muddy components are higher in phosphorus released than those dominated by sandy composition. In the adsorption test, Fe-P is the most active one, with a releasing ratio higher than other types of phosphorus, followed by Ad-P and OP. The varieties of dissolved inorganic phosphate (DIP) and dissolved organic phosphate (DOP) were the same under static conditions. In the lower vibration frequency (60 times/min), DOP and DIP have a similar variable trend with those in quiescence. However, in the higher vibration frequency (120 times/min, 150 times/min), the variety of DOP’s concentration has an opposite trend to that of DIP’s concentration. The concentration of DIP is increased with the increase in the vibration frequency, but the concentration of DOP is reduced with the increase in the vibration frequency. The adsorption of sediment to phosphorus can reach a percentage of over 85% of balanced quantity after 4 h. The adsorption almost did not increase as the time extended beyond 12 h. The inﬂuence of temperature on the adsorption balance time and balanced adsorption quantities is not obvious. The suspended sediment content is the most important factor, and the inﬂuences of pH, temperature, and salinity on adsorption quantity are similar. Adsorption happens on ﬁne grain size sediment, the higher the ﬁne particle sediment contents are, the higher the adsorption quantity is. The study of the phosphorus adsorption in the Changjiang River Estuary sediments showed that the sediments were buﬀers to phosphate and the balance time of phosphorus adsorption (desorption) was about 6 h. The saturated adsorption quantity of phosphorus was about 600 μg/g and the saturated desorption was about 126.37 μg/g. The apparent adsorption heat of ﬁne sediments (ΔH ) was 47.59 kJ/mol. The adsorption of phosphate was in accordance with Freundlich’s isothermal equilibrium and it was a heat absorption reaction. The contents of sand and mud in sediments had some inﬂuence on adsorption and the adsorption would decrease with the increase in sand or mud content. In general, the increase in pH favors adsorption. Moreover, the increase in salinity does not favor adsorption but favors desorption. The research shows that the release of phosphorus in the sediments of the Changjiang River Estuary

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is small. The re-release of phosphorus is slow after great adsorption by the sediments. The balancing point of adsorption and release of phosphorus in the sediments is 0.038∼0.085 μg/ml by measurement, which is higher than that in sea bottom water. So the Changjiang River Estuary sediments are inclined to release phosphorus to overlying water (Li and Yang, 2004). In addition, the research on sediment phosphorus can supply information about the paleoenvironment. The information about the environment and biogeochemistry of phosphorus extracted from the South Sea shelf slope sediments showed that the supply of terrigenous phosphorus for the sea was steady overall. The change in phosphorus contents in diﬀerent depths was the result of climate and environmental changes. The vertical distribution of phosphorus had an opposite trend to calcium carbonate and Cd in sediment. The ebb and ﬂow between CO2 and PO3− 4 in seawater calculated by chemical balance indicated that the accumulation of phosphorus in marine sediments was related to the atmospheric CO2 change, and the decrease in sediment phosphorus accumulation and the increase in calcium carbonate contents may be one of the key factors leading to a decrease in CO2 in the atmosphere in the ice age. 1.2.3.3 Source and Characteristics of Biogenic Silicate Silicate is a very important nutrient in the ocean. Unlike other major nutrients such as phosphate and nitrate or ammonium, which are needed by almost all marine plankton, silicate is an essential chemical only for certain biota such as diatoms, radiolarian, silicoﬂagellates, and siliceous sponges. However, this biology is one of the most important producers in marine. The estimation shows that diatoms contribute more than 40% of the entire primary production. Therefore, silicate cycling has received signiﬁcant scientiﬁc attention in recent years and many scientists have studied silicate behavior in marine environments. Biogenic silicate is the amorphous content extracted by chemical methods, which is named as biogenic opal or opal in brief. The concentration of dissolved silicate in the world ocean is about 70.6 μmol/L and the net input of dissolved silicate from land to ocean is (6.1±2.0)×1012 mol (calculated by Si) every year, and the primary contribution (about 80%) comes from river. Silicic phytoplankton is the essential constitute of marine primary productivity and their reliquiae is the main source of biological silicate in bottom sediment. The accumulation of biogenic silica (BSi) in sediments can reﬂect the inﬂuence of the nutrient changes on the growth of diatom and other phytoplankton; moreover, it also records the occurrence and development of the eutrophication process. At the same time, the biogenic silica accumulation in sediment can reﬂect the long time and spatial changes in overlying water primary productivity. In the East China Sea and the Yellow Sea surface sediments, the BSi contents ranged from 0.21% to 0.70%, which was in accordance with the primary productivity. For the Changjiang River Estuary, the accumulation of BSi had a close relation with Chl a and the primary productivity

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of waters and the change in the BSi content in the sediments recorded the annual changes in the nutrient ﬂuxes of N, P, Si transported by the Changjiang River and its runoﬀ in the last 20 years (Ye et al., 2006). Silica in sediments, which is mostly paid attention to, is the part that can be dissolved and takes part in the biogeochemical cycle under natural conditions. Song et al. (2003) divided silica in natural grain size sediment into ion-exchange form, carbonate-bound form, iron-manganese oxide form, and organic matter-sulﬁde form, and thought that only these forms can take part in the biogeochemical cycle. In the Bohai Sea sediment, the content of the transferable form of silicon accounted for 0.12% of total silica, and the carbonate-bound silica was the primary form accounting for 0.05% of total silica. In the East China Sea and the Yellow Sea, SiO2− 3 will transform from released from sediment sediment to seawater, and the contribution of SiO2− 3 to the primary productivity was 13% and 10%∼18%, respectively. Compared with river input and atmospheric deposition, the SiO2− 3 contribution of sediment was 90% and 86%, which indicated that sediment was the main source of SiO2− 3 (Qi et al., 2006). The regeneration of silica in sediments is the main supply source for the ocean, and the silica supplied to the world’s ocean from biogenic silica dissolution in sediment is four times that from rivers. However, for the marginal sea, sediment silica behavior is diﬀerent from that in the ocean due to the shallow water depth, high primary productivity, and considerable terrigenous inputs. Long-term research on Jiaozhou Bay nutrients showed that the nutrients are characteristic of high nitrogen and low phosphorus and silica. In the past four decades the nitrogen has increased unceasingly but the silica contents decreased continually and, as a result, silica may have become the primary possible limiting factor for phytoplankton growth in Jiaozhou Bay. Studies of the biogenic silica in Jiaozhou Bay sediments showed that the biogenic silica in three surface sediments samples was 1.58%, 1.44%, and 1.48%, respectively, higher than those of the East China Sea and the Yellow Sea (biogenic silica contents were lower than 1% (Zhao YF et al., 2005)). The BSi/TN ratios in Jiaoxhou Bay sediments were much greater than 1 and BSi/TP ratios were >16 too, which were the opposite of the ratios of BSi/TN<1 and BSi/TP<16 in water. At one time, the OC/BSi ratios in sediments were lower than the Redﬁeld ratio (106:16). These indicate that the decomposition rate of OC is much higher than that for BSi and the biogenic silicate decomposition rate is very slow. In addition, during the course of deposition, only 15.5% of biogenic silicate formed by diatom is decomposed into water, thus approximately 84.5% of biogenic silicate is buried and enters into sediments. The rate of silica in Jiaozhou Bay sediments returning to water across sediment-seawater is lower than the biogenic silica sedimentation rate. All this indicates that silica in Jiaozhou Bay seawater transports to sediments and, as a result, the silica in water keeps a low concentration. This is the key reason for silicate limiting phytoplankton growth in Jiaozhou Bay (Li XG et al., 2005).

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1.2.3.4 Environment Chemical Process of the Persistent Organic Pollutants Persistent organic pollutants (POPs) could hardly be degraded by microorganism under natural circumstances, for it takes a long time to be biodegraded. The popular POPs include: halogenated hydrocarbons, halogenated ester, single-ring aromatic compounds, phenol and alkylphenol, phthalate, polycylic aromatic hydrocarbon, polychlorinated biphenyls, organochlorinated pesticides (OCPs), organophosphorous pesticides, carbamate insecticide, herbicide, and so on. They are liable to accumulate in some natural mediums, such as water, soil, and sediment environments and then reach humans and animals via the food chain and bioaccumulate gradually through the trophic pyramid. POPs can induce irreversible transformation in cells, endangering health. Due to their carcinogenic, teratogenic, and mutagenic properties and their long-term negative eﬀect on the quality of the environment, POPs have attracted global attention and their behavior has been studied deeply. Environmental scientists from our country have done much research into POPs in the coastal areas and primary estuaries in China, which showed that the source, distribution, evolvement, and controlling factors of the material in different locations, mediums, and types present obviously local characteristics. Most of the POPs are productions of anthropogenic origin and enter the environment by two main ways. One is that POPs transfer from the atmospheric environment to the aquatic environment by way of dry or wet sedimentation; the other way is that POPs enter the aquatic environment via surface eﬄuent release. In the oﬀshore areas, the primary route by which POPs enter the marine environment is by surface riverine input. In the diﬀerent coastal areas, the ﬂux and species of POPs dragged along by rivers are quite diﬀerent. For example, in the Human tidal channel of the Zhujiang River, the total PAH ﬂux (dissolved and particulate phase) is 4.384×105 kg/yr, the particulate phase ﬂux is 5.25×104 kg/yr, and the ﬂux of 16 prior controlling PAHs is 2.479×105 kg/yr. Among them, some PAH compounds with less rings, such as naphthalene and acenaphtylene, are transported mainly in the dissolved phase, while the corresponding compounds with more rings, such as benzo(a)pyrene, are transported mainly via movement of particles, whose proportion exceeds 92%. The total ﬂux of OCPs is 2.6×103 kg/yr with equal proportions of the dissolved and particulate phase. The ﬂuxes of hexachlorocyclohexane and DDT are 1.1×103 and 0.3×103 kg/yr, respectively, and the ﬂux of the rest of OCPs is 1.2×103 kg/yr (Yang et al., 2004a). However, in the polar coastal areas, POPs accumulated in the polar aquatic environment came from atmospheric deposition mainly via the volatilization of the global distillation eﬀect in the Torrid Zone and Semitropical Zone, then the deposition of the condensation eﬀect in the high latitude Frigid Zone (Lu et al., 2005). In an aquatic environment, POPs are mainly adsorbed in the suspended particles, and then enter the sediment environment by deposition, which causes the POP concentration in the sediment to be one or two orders of

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magnitude higher than that in seawater. Whereas the spatial distribution patterns in sea water agree well with those in sediments, namely the POP concentrations decrease continuously as the distance from the coast to open sea increases. For example, in the surface water and sediment of the Zhujiang River Estuary and its adjacent northern South Sea, a continuous reduction in the alkylphenol concentrations is shown from river to estuary and then to the open sea in the direction of the ﬂow. Moreover, the concentration sequences of PAHs and OCPs are in the river, in the Zhujiang River Estuary, in Lingdingyang, in the South China Sea (Luo et al., 2005). Likewise, in the Changjiang River Estuary, the concentration sequence of the PCBs is in hightidal ﬂats, in middle-tidal ﬂats, in low-tidal ﬂats. Even so, the components and transformation of POPs in the above overlying seawater do not agree well with those in the sediments. Firstly, it is possible that the components of POPs in the sediment are diﬀerent to those in seawater. For instance, in Daya Bay, the PAH components in seawater constituted the individual PAH with 3 rings mainly, while in the sediments the majority of PAHs had 4 rings. Secondly, under the eﬀects of the season, the composition and species of POPs present signiﬁcant seasonal changes in water. Yang et al. (2004b) studied the distribution and seasonal changes of polycyclic aromatic hydrocarbons (PAHs) in surface water from the Humen tidal channel. He found that the PAH concentration in the dry season was one order of magnitude higher than that in the monsoon season and the proportion of the PAHs with 2 rings exceeded 95% existing in the dissolved phase, while, in the monsoon season, the PAH pollutant level rose obviously, transported in the particulate phase mainly (Yang et al., 2004b). Compared to sediments, the POP concentration in suspended particles is likely higher than that in sediment. The research into the POP distribution in water near the Changjiang River Estuary and coast showed that the PCB concentration in the suspended particles was signiﬁcantly higher than that in the corresponding sediment, but no obvious regional distribution tendency related to the discharging. Furthermore, no signiﬁcant relationship between PCBs and TOC or the grain size of suspended particles was found, but the source of organic matter and the composition of the inorganic mineral matter may aﬀect their distribution to some extent. At most of the sampling stations in the study area, low chlorinated level PCBs (3∼5 Cl) account for more than 70%, whereas the predominant PCB congeners in the suspended particle are two chlorinated substituted compounds (Cheng et al., 2006). The result of research into OCPs is quite similar. Sediment is the primary source or sink of the POPs. Some previous studies focusing on the pore water reported that the POP concentration in the pore water is much higher than that in the overlying seawater. However, the POP concentration in sediments is higher than that in pore water and overlying water. POPs transfer from the bottom sediment to the overlying water via a resuspension eﬀect. Consequently, it is vital to conﬁrm the POP sources,

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composition, and controlling factors for evaluating the bio-serviceability and environmental risk and thus constitute remediation technology. Hao et al. (2006) investigated the concentrations and distributions of free and bound PAHs in sediment core from Nam Van artiﬁcial Lake of Macao under natural circumstances, whose research indicated that the free PAHs were predominated by 4-ring, 6,7-ring, and 5-ring PAHs, with mass percentages of 28.7%∼40.6%, 17.6%∼29.6%, and 13.2%∼28.2%, respectively, and for bound PAHs, the main components are 4-ring, 3-ring, and 2-ring ones, with mass percentages 42.3%∼55.8%, 20.2%∼35.8%, and 7.8%∼l8.8%, respectively, which suggested that PAHs with low molecular weight were more prone to entering the micro-pores of sediment organic matter. Vertical proﬁle of free PAH concentrations in the sediment core, to some degree, was correlated with the degree of regional economic development and the results of environmental management. The bound PAH concentrations in the sediment core were controlled by the total PAH mass input to the sediments and the structure (especially the polymerization) of sedimentation organic matter. Sediment burying favors the transformation of free form to bound form for PAH compounds. The sorption behavior of phenol in the sediment was extremely complex, and Freundlich, Langmuir, and linear models were all suitable for describing the sorption behavior of phenol in marine sediments. The sorption behavior of phenol in the sediments was aﬀected by some factors such as organic carbon content of sediments, aqueous media, temperature, and acidity. The saturated sorption amount of phenol increased with increasing organic carbon content, salinity, and acidity, but decreased with increasing temperature. Wu and Jia (2005) used a wet sieving method to separate the sediments into ﬁve size fractions and further separated them into low and high density fractions with cesium chloride solutions. They found that OCP enrichment in diﬀerent types of components was inﬂuenced by important factors, especially the structure of diﬀerent fractions and the sources and structures of the pollutant play an important role. The organic low density fraction processes a higher enrichment capacity in OCPs than a high density fraction (inorganic mineral matter and amorphous organic matter), and the sorption capacity of organic matter on hydrophobic organic pollutants (HOPs) was far larger than that of inorganic mineral matter and amorphous organic matter. However, they were limited to explaining the horizontal distribution of OCPs in the sediment in terms of the total organic carbon (TOC) and black carbon (BC) contents, because the distribution of OCPs in the diﬀerent grain size sediment related well to the species and structures of the organic matter in sediments. A report on the OCP (HCH and DDT) distribution in the sediments showed that the signiﬁcant positive relationship between DDT concentrations and TOC contents occurred in the sediment of the East China Sea, while no obvious relationship between HCH concentration and TOC contents was found. According to the OCP distribution pattern, it showed that sediments from the vicinity of the estuary or near shore had higher TOC contents and higher OCPs residue con-

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centrations, which indicated OCPs from the Changjiang River might greatly aﬀect the OCP content. Taking typical POPs of HCHs, DDTs, and PCBs as the indexes of environmental pollution, we can trace the usage over time of organic matter from the start, summarize the characteristics of all the phases over time and follow the eﬀects of OCPs on the marine environment. The analysis of the PAH vertical distribution in the core sediment sample from Lingdingyang west tidal land reconstructed the sediment history of the PAHs in the past hundred years. An initial increase in ΣPAH concentration was found around the 1860s, followed with a ﬁrst peak in the 1950s. There was a slight decrease in total PAH concentration and ﬂux in the 1960s and 1970s. A sharp increase in PAH levels was observed from the 1980s and a peak was found in the 1990s. PAH diagnostic ratios indicate that PAH in the sediment core is mainly of pyrolytic origin. PAH concentrations are found to correlate positively with gross domestic production, the number of vehicles, and power generation in the surrounding regions, indicating that the PAHs in the sediment core are mainly anthropogenic. Atmospheric deposition and land runoﬀ may serve as the important pathway of PAH input to the sediment. In a sediment core from Nam Van artiﬁcial lake of Macao, the concentrations of nonylphenols (NPs) ranged from 2.17 to 5.91 μg/g with a mean value of 3.66 μg/g in the estuarine sedimentary environment from the 1970s to 1980s and from 0.69 to 3.04 μg/g, with a mean value of 2.08 μg/g in the lagoon sedimentary environment in the 1990s. The concentrations of octylphenol (OP) during the initial stages of the lagoon environment in the early 1990s were similar to those in the estuarine environment, and ranged from 14.33 to 39.1 μg/g. Subsequently, the concentration of OP rapidly decreased with a range of 6.52∼2.58 μg/g. Sources of alkylphenols (Alas) in the estuarine environment included urban runoﬀs from the upstream waters of the Zhujiang River delta and Macao City, but just from Macao in the lagoon environment. Consequently, the concentration of APs in the estuary environment was higher than that in the lagoon environment. The vertical distribution of the concentration of APs in the sediment core to some degree was correlated with the degree of development of the regional economy and the process of wastewater treatment. The results also showed that the alkylphenols in the overlapping silty layer preferred to transfer downward to ﬁll a sandy layer (Hao et al., 2004). 1.2.3.5 Major and Minor Elements in Marine Environments In marine chemistry, major and minor elements in marine system are the key points studied. According to the combination of major and minor elements in sediments, we can trace their source and evolve paleoenvironmental changes inversely. However, heavy metals still draw comprehensive attention because of their accumulation, which could aﬀect the growth and reproduction of aquatic animals and plants. Moreover, they can enter the human body through the food chain to threaten the health and development of humans. In recent years,

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Chinese researchers continued to study the contents, distribution and sources of heavy metals in typical Chinese seawaters and deeply studied their forms and bioavailability. Normally, the heavy metals could be divided into an exchangeable form, carbonate bond form, Fe-Mn oxidate form, organic matter form, and residue form. Results showed that Cu, Pb, Zn, Fe, and Cr in sediments near the Bailong River sewage discharging mouth in the Changjiang River Estuary were mainly in residue form. Mn was mainly in carbonate form, which was 50% of the total amount. The contents of heavy metals in rechangeable and organic matter form had no obvious seasonal changes; however, carbonate form might transform to Fe-Mn oxidate form in autumn. Cu, Pb, Fe, and Mn in carbonate form and Fe-Mn oxidate form had similar transformation characteristics. Nevertheless, the cycle of Fe and Mn was the main control factor in the behavior of heavy metals. According to the sequential extraction procedure, four extraction fractions of sediments were obtained, named acid soluble phase (include exchangeable ion form and carbonate bond form), reducible phase (ironmanganese oxide form), oxidable phase (sulﬁde-organics form), and residual phase (metals bound in lithogenic minerals). In the East China Sea sediment, more than 90% of the total concentrations of V, Cr, Mo, and Sn existed in the residual fraction. Fe, Co, Ni, Cu, and Zn mainly (more than 60%) occurred in the residual fraction, while Mn, Pb, and Cd were dominantly present in the non-residual fractions in the top sediments. For most heavy metal except Fe, the acid soluble phase and reducible phase in surface sediment were higher than those in deep sediment. The total contents of four familiar heavy metals (Cu, Zn, Pb, and Cd) in rhizosphere sediment around Suaeda heteroptera were also in a similar condition (Zhu et al., 2005). The chemical species of the heavy metals in rhizosphere sediment were investigated in 5 forms (the exchangeable, carbonate, Fe-Mn oxide, organic complex, and residual). Considering the bioavailability, the above mentioned 5 forms can be classiﬁed into two groups, the active group (involving the exchangeable, carbonate, Fe-Mn oxide, and organic complex) and the residuals. The transformation of diﬀerent chemical forms in rhizosphere sediment was related to the character of the elements. Thus, the trend of Cu and Pb to transform from the active form to the residuals was found, which led to bioavailability reduction. Nevertheless, Zn was in the reverse direction, which means its bioavailability was increased. In every form of heavy metals, the acid soluble phase is the most active. It controls the distribution of heavy metals in sediments-pore waters and their availability. The diﬀerence between acid-volatile sulﬁde and simultaneous extracted metals (AVS-SEM) was investigated to explain the biological toxicity of zinc in the sediments to benthic organisms (Han et al., 2003). When the molar diﬀerence between SEM and AVS (i.e., SEMZn -AVS) was <0 μmol/g, the concentration of zinc in the sediment interstitial water was low and few toxic eﬀects were observed. Conversely, when SEMzn -AVS exceeded 0 μmol/g, a dose-dependent increase in the relative concentration of zinc in the pore water was detected and apparent toxic eﬀects in the organisms were observed.

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Due to the aeration time in high tideland longer than that in low tideland, the depth of oxidative layer changed to be shallow from high tideland to low tideland. The maximal value of acid extracted metals (AEM) that were extracted using 1 mol/L HCl in tideland sediment became higher from high tideland to low tideland and accumulated in the oxide-reduced layer. AEM mainly came from the phase combined with S2− , but part of AEM in low tideland came from the Fe-Mn form or organic form. The heavy metals were easy to release to overlaying water in the medium tideland. Diﬀerent forms of heavy metals in sediments have diﬀerent activities. The factors inﬂuencing the distribution of heavy metals are mainly oxidativereductive conditions and pH besides the sediments themselves. In the oxidative environment, heavy metals are mainly adsorbed by Fe-Mn oxidate. In the reductive environment, they are combined with organic matter and S2− . Therefore, the study of the activity of heavy metals is focused on the changes in oxidative-reductive conditions and pH. In the surface sediments, the oxygen content determined the oxidative-reductive ability. The changes in Fe-Mn oxidate below the subsurface sediments mostly inﬂuence the oxidativereductive conditions. The vertical distribution of Fe-Mn is controlled by the reduction, diﬀusion, and re-precipitation of Fe-Mn oxidate. In the upper sediments, the content of Fe-Mn is the highest, which is related to the diﬀusion depth of oxygen and the diﬀusion ﬂuxes of Mn2+ and Fe2+ . Mn bacteria play important roles in the cycle of Fe-Mn in the solid phase. They can oxidate dissolved Mn2+ and Fe2+ to Mn4+ and Fe3+ in an oxidative condition and can reduce Mn4+ and Fe3+ to Mn2+ and Fe2+ in an oxygen deﬁcient condition. There are many methods for determining the oxidative-reductive conditions in sediments. Active iron and Fe3+ /Fe2+ were usually used and could obtain satisfactory results (Li XG et al., 2003; 2004b). To determine the source of heavy metals in sediments, mathematical statistics can be used to obtain the results except when it comes to the combination characteristics of elements. Factor analysis is employed to analyse the data of heavy metals in Bohai Bay sediment. Results showed that Cd, Zn, and As are seriously contaminated by human pollution. Pb, Cr, and Hg are partly contaminated by humans, while Cu, Fe, and Ni are from natural sources. Principal component analysis (PCA) was used to estimate source of heavy metal contamination in Jiaozhou Bay sediments. Results showed that heavy metal contamination sources in this bay could be divided into three groups, such as industrial wastewater, degradation of organic matter, and erosion of rocks, respectively. The Q-cluster analysis indicated that the degree of pollution near the estuary was heavy, but was light far away from the estuary (Li Y et al., 2006). Except for normal elements, the typical elements were also studied such as rare earth elements, radionuclide, Te, I, and Al in seawater. Rare earth elements (REEs) have similar chemical characteristics and low dissolubility and are not easy to migrate in weathering, erosion, transportation, deposition, and early diagenetic processes and have little fractional dis-

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tillation in the transportation process. Therefore, REEs are usually used to trace source matter in marine sediments and geochemical characteristics characteristics in the source matter area. The abundance of REEs in the Chukchi Sea is very similar to that in ﬁne-grained sediment in the East China Sea shelf, which indicates that REEs in sediments mainly originate from the terrene. The distribution patterns of REEs in water, suspended particles, and pore water from the intertidalite of Tianjin are similar to that in inland freshwaters, which are quite diﬀerent from that of the ocean. The average contents of REEs in sediment and suspended particles are obviously lower than that in freshwater rivers but higher than that in the sediment in the East China Sea shelf. These indicated that sediment and suspended particles of intertidalite mainly originate from terrestrial soil that is contiguous to the sea and their evolutionary processes are similar to those in rivers while they are diﬀerent from the deep-sea sediment from the East China Sea (Liang et al., 2003). According to the distribution of rare earth elements and transition elements, the Nansha Islands sea area can be divided into three parts, i.e., near-shelf residual sediment, deep-sea sediment, and carbonate sediment. Generally, REE content shows a big diﬀerence in diﬀerent sediments, but the distribution pattern is the same, which is light REE enrichment and Eu deﬁciency. REEs in sediments are highest related with Ti and Zr but are not obviously related with other metals, which reveals that the sediment in the Nansha Islands sea area is mainly from the landmass. Based on the change of ΣREE, ΣLREE, ΣHREE, ΣLREE/ΣHREE, δEu and δCe in sediments at the E2 core in the south Yellow Sea and the rare earth element of marine sediments in the south Yellow Sea are controlled by the same source and the distribution of REE presents the character of the upper terrestrial crust. The REE model of the E2 hole in the south Yellow Sea is similar to the eastern China upper continent, but quite diﬀerent from deep sea sediment, indicating that Yellow Sea sediment has a good relationship with the Chinese mainland. The REE source mainly comes from weathering materials of upper continent rock, not from seawater. The radionuclide tracing method is widely applied in the study of marine science. Through determining the distribution of radionuclide in seawater and sediments and their unbalanced relationship with natural radionuclide, we can study the transport process of particulate matter and the erosion process of the seacoast, and obtain a sedimentation rate and an erosion rate. Some researcher studied radioisotopes in a part of the Chinese sea area. Jia et al. (2003) determined the radioisotopes 40 K, 137 Cs, 210 Pb, 226 Ra, 228 Ra, 228 Th, and 238 U in Jiaozhou Bay surface sediments. The radionuclide contents in a portion of size <0.063 mm are higher than those in size >0.063 mm except 210 Pb, 226 Ra, 228 Ra, 228 Th, 238 U in the east bay. There are imbalances for U and Th radioactive series. In the total samples, the portion of sediment which is of grain size <0.063 mm, 210 Pb is in excess of 226 Ra, 226 Ra is deﬁcient in 238 U, and 228 Th is in excess of 228 Ra. Radionuclides contents in surface sediments of Jiaozhou Bay are consistent with those in the soil of the drainage basin, which indicates that sediments in Jiaozhou Bay are mainly

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terrigenous. Natural radioactive series in sediments of Xiamen intertidal mudﬂats are in disequilibrium. 234 Th related to 238 U, 210 Pb related to 226 Ra are in excess, respectively, while 222 Rn related to 226 Ra, 228 Ac related to 228 Ra, 224 Ra related to 228 Th are deﬁcient, respectively. High values of 210 Pb activity were found in the central area of the Bohai Sea, the ﬁne-grained sediment areas of the northern Yellow Sea center, and the mud sedimentation area of the southern Yellow Sea center. However, 210 Pb activities were low in Bohai Bay and Laizhou Bay and the west coast of the northern Yellow Sea. The enrichment and spatial distribution of 210 Pb are mainly controlled by hydrodynamic conditions and the grain size of sediments in the Yellow Sea and Bohai Sea. The vertical distributions of 210 Pb activity appear as two-segment models regularly in the ﬁne-grained sediment areas of the Yellow Sea and Bohai sea characterized by high 210 Pb activity and sedimentation rates are low in this region, showing that the sedimentary environments in the ﬁne-grained sediment areas are stable (Qi et al., 2005). Tellurium is a sort of scattered rare element on the Earth. Its concentration is very low in the Earth’s crust, only 1.0 ng/g. However, it is in extremely high abundance in Co-rich crusts, marine polymetallic nodules, deep-sea sediments, and aerolites. The extreme enrichment of tellurium in deep-sea sediments, like helium isotope anomalies, probably results from the input of interplanetary dust particles (IDPs). Similarly, the extreme enrichment of tellurium in marine polymetallic nodules and Co-rich crusts is possibly related to IDPs. Iodine is an important trace element, which is highly related to humans. Iodine deﬁciency can lead to abnormal growth in humans. In nature, iodine becomes enriched in marine organisms and sediments that provide the largest reservoir. The distribution of iodine in marine sediments has a trend of increasing from low latitude to high latitude. The distribution of Al in seawater is primarily controlled by external sources and can be rapidly scavenged from the water column. Therefore, it is often applied as a sensitive tracer of water mass in coastal and oﬀshore seawater. Fluvial input has been suggested as an important source of dissolved Al in the coastal regions, whereas the partial dissolution of eolian dust has been proposed as the dominant source of dissolved Al in the surface water of oceans. The distribution of aluminum in the Yellow Sea showed that it had clear seasonal variations and its main source is the Changjiang River and adjacent rivers. The contribution of Changjiang River fresh water was the highest (56.6%) near the Changjiang River Estuary, then decreased seaward along the PN section. At a distance of 250 km from the Changjiang River mouth, the freshwater input was hardly seen and the incursion of Kuroshio waters became dominant. Combining the diﬀerent inputs from the Changjiang River, atmospheric deposition, Kuroshio waters, and Taiwan Current Warm Waters with the total amount of Al, the average residence time is about (339±118) d for dissolved Al in the East China Sea shelf. In addition, an experiment using biogenic barium to study the chemical components of the modern and ancient marine environments and the transi-

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tion of marine productivity was conducted. Chen SX et al. (2005) calculated the paleoproductivity in the Nansha Islands sea area by using barium as a reconstructing proxy. However, Ni et al. (2006) thought there is little obvious correlation between surface oceanic productivity and biogenic Ba in sediment in the equatorial northeastern Paciﬁc.

1.3 Functions of China Marginal Sea Sediments in Cycles of Biogenic Elements One important item of modern chemical oceanography is the study of the marine biogeochemical processes relating to global changes, and the key is to study the recycling of biogenic elements in marine ecological systems. The marginal sea sediments play an important role in the recycling of biogenic elements. Studies on the transfer, release, and recycling of biogenic elements in China’s sea sediments have made great progress in the past ten years or more, which involve contents and changes in the biogenic elements C, N, P, Si, and S in sediments and in sinking particulates, the recycling of biogenic elements in sinking particulates, chemical mass transfer and recycling across the sediment-seawater interface, the relations between the recycling of biogenic elements and their biological production, etc. The following discussion focuses on the main progress in studying the transfer, release, and recycling of biogenic elements in China marginal sea sediments (Song, 2004). 1.3.1 Biogenic Elements in China Marginal Sea Sediments Biogenic elements in marine environments are mainly concentrated in sediments, and their contents are related to grain sizes, river inputs, etc. Distributions of the biogenic elements are regulated by the origin of matter, water dynamics and sedimentary grain size, etc. In general, the ﬁner the grain sizes, the higher the contents of OC, N, P, and organic matter, and the lower the contents of S and Si. In China marginal seas, the highest OC contents were found in muddy sediments around the inner continental shelf of the southern Yellow Sea and of the East China Sea. Lower OC contents were found in the middle regions of the Bohai Sea, in southwestern Cheju Island and in the Zhujiang River Estuary, and the lowest were in some sandy sediments, such as on Liaodong beach, the outer-bank of the Changjiang River Estuary, the Taiwan bank and middle North Bay. The eastern Okinawa Trough had a lower content of OC, and the others had higher contents of OC. The distributions of N and P were the same as those of OC and Si, and the content in shallow sea sediment was higher than that in abyssal sediments. Lower Si contents were found on the inner continental shelf of the East China Sea, the middle southern Yellow Sea, the Huanghe River Estuary, southwestern Cheju Island and eastern North Bay, etc. Lower Si contents were found in regions such

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as the Taiwan bank, middle North Bay, Liaodong bank, the eastern South Yellow Sea, middle Jiaozhou Bay, outer continental shelf of the South China Sea, outer big bank of the Changjiang River Estuary, etc. The contents of SiO2 were: continental shelf (68.2%), continental slope (34.93%), abyssal plain (51.30%). P2 O5 contents were: continental shelf (0.10%), continental slope (0.17%), abyssal plain (0.17%). S was enriched in the South China Sea and the Yellow Sea. High S contents were found on the continental shelf of the Zhujiang River Estuary, northeastern North Bay, and the middle southern Yellow Sea. Low S contents were found in other sandy regions. The average abundances of OC, ON, and OP in the East China Sea continental surface sediments (not including the Taiwan Strait) were 0.516%, 0.054%, and 0.048%, respectively. OC and TN were lower in the northern East China Sea continental shelf surface sediments, higher in the southern and near continental shelf. While medium contents of OC, ON and very low content of P were found in the northeastern East China Sea continental shelf surface sediments. In addition, high content of P exists in the Zhejiang coastal regions (Wang, 1995). Lower OC, ON, and OP contents and higher Si content in the southern Taiwan Strait than those in its near regions were determined. Higher Si content was caused by higher silica content in sedimentary particulates and less river input. Positive relations between OC, ON and OP contents were found, negative relations between OC, ON, OP contents and Si content were distinctly found, which showed the dilution of silica. A common characteristic of proﬁle distribution was that the contents of OC, ON, OP, and Si decreased with depth, and swiftly decreased in the 20 cm surface layer. This suggested that the decomposition of organic matter mainly occurred in the surface layer and subsurface layer (Hong, 1994). The contents of some biogenic elements in China marginal sea sediments were listed in Table 1.8. Table 1.8. Contents of biogenic elements in China marginal sea sediments C (%) S (×10−6 ) N (×10−6 ) P (×10−6 ) OC IC Sand Silt Clay Bohai Sea 0.66 0.66 250 340 520 556 527 Yellow Sea 0.71 0.69 350 420 820 624 522 East China Sea 0.61 1.21 350 530 730 615 516 South China Sea 0.59 1.32 450 580 1,000 620 425 Okinawa Trough 0.97 1.66 730 1, 130 630 Regions

Sand 33.42 32.44 30.66 31.04

Si (%) Silt Clay 30.13 26.15 30.00 26.48 29.07 26.17 29.00 26.36 19.21

The total characteristic is that the changes in the regularities of OC, TN, TP in China marginal sea sediments are similar. The contents of OC, TN, TP are higher in regions such as the Okinawa Trough and the Minjiang River Estuary, lower in the southern Taiwan Strait, and show no change in other regions. Sometimes the change in P is not related to C and N, such as in the western Paciﬁc Ocean and near-shore regions of the East China Sea. This re-

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sults can be obtained from their diﬀerent terrigenous inputs in estuary regions (Bao, 1992). S and Si accord with the role of grain size regulation. 1.3.2 Chemical Environments of China Marginal Sea Sediments and Early Diagenesis of Biogenic Elements The chemical environments of sediments mainly involve the physical and chemical factors which aﬀect the biogeochemical processes of biogenic elements, such as contents and activities of organic matter, contents of metallic ions correlating with sedimentation and dissolution, redox degree (ROD), pH, temperature, pressure, etc. Organic carbon participates in the consumption of oxygen, reduction of Fe, Mn, and S ions in the processes where organic matter is decomposed in the sedimentary environments. The content of organic matter directly reﬂects the degree of the reactions, pH, Eh, Es, and concentrations of SO2− 4 in interstitial waters are the characteristic functions of sediments, and are also important factors in appraising redox environments. The inﬂuencing degree of organic matter on its regions can be deduced from diﬀerent characteristic factors. The driving reaction of early diagenesis is the oxidation of organic matter. It inﬂuences the recycling of CO2 , which is correlative to the global climates and the biogeochemical recycling of biogenic elements by a series of biogeochemical processes, such as denitriﬁcation and dissolution of minerals. Unstable organic matter is changed and decomposed into NH3 , CO2 , and H3 PO4 through deammoniation, decarboxylation, depolymerization, isomerization, and redox. At the pH and Eh of interstitial waters, the decomposed − 3− products mainly include NH+ 4 , HCO3 , and ΣPO4 . Accordingly, whether or not the decomposition of organic matter occurs and to what degree can all be estimated by changes in the ions in seabed sediment interstitial waters. Only a few elements such as C, N, O, S, Fe, and Mn can participate in redox reactions in the marine environment. The reactions and their degrees directly control and regulate the properties of redox reactions in sediments. The oxidizing agents can be arranged in oxidizing property order according to their standard free energies of decomposing organic matter: O2 >NO− 3 >MnO2 > Fe(OH)3 >SO2− >CH O. China marginal sea sedimentary environments had 2 4 been systematically researched and it was found that the sedimentary redox environments not only related to Eh, organic carbon, the radio of Fe3+ /Fe2+ , but also related to the grain sizes of sediments and the sulfur system in interstitial waters. Hereby, a new function—redox degree (ROD) was introduced to appraise the redox environments of sediments. The magnitude of ROD quantiﬁcationally reﬂected the degree of redox reactions. The analysis of ROD shows that the sedimentary reduction properties in the northern Huanghe River Estuary are stronger than those in the South Yellow Sea and the Okinawa Trough, etc. The reducing property of sediment is stronger than that of seawater; the reducing property in regions far oﬀshore is stronger than that close inshore.

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The S system is one of the main systems for regulating sedimentary redox processes. It participates in a series of diagenesis such as complexation, exchange adsorption, precipitation. The existing forms of S are regulated by environmental pE and pH. S in the East China Sea sediment interstitial waters − mainly exists in the form of SO2− 4 , it amounts to 99% of total S, HS amounts 2− to only 1% of total S, the others are a little H2 S, S , etc. Sulfate is reduced to − 0 sulﬁde through microbial processes in anoxic circumstances. SO2− 4 / HS (Sx ) is one of the main redox electrode pairs which regulate marine environments. The greatest reduction rate of sulfate occurs in surface layer sediment interstitial waters, and it is regulated by the content of organic matter in sediment and inﬂuenced by the season. S with –2 state mainly results from the diﬀusion from sedimentary interstitial waters to overlying waters, the diﬀusing ﬂux of HS− in the Huanghe River Estuary regions reached 8.95 μmol/(m2 ·d) (Qiu et al., 1999). Behavior of OC in sediments is mainly controlled by depth and redox environment. OC from sulfur mineralization accounts for 60%∼80% of average net primary production. Phosphorus is controlled by pH and dissolved oxygen of water columns. The more the phosphorus released, the higher the pH and DO. Silicon is mainly controlled by some inorganic dissolution. For example, it can precipitate in the formation of meerschaum mixing with Mg(OH)2 , and decrease enormously in dissolved silica concentration in interstitial waters. Only a little N in sediments is buried, and the others are regenerated through − + mineralization and are released in the forms of NO− 3 , NO2 , NH4 , N2 or N2 O via early diagenesis (Song, 1997b). 2− 3− Song (1997b) studied the early diagenesis models of NH+ 4 , SO4 , PO4 , and H4 SiO4 in East Sea China sediment interstitial waters. The decomposition, precipitation, and adsorption of ammonium, dissolved silicate, phosphate, and sulfate in interstitial waters were ubiquitous, and they could be treated with the ﬁrst dimension kinetic reaction model. The dissolution of silicate and reduction of sulfate occurred in the upper surface layer (0∼10 cm). Proﬁles of silicate in interstitial waters showed a decreasing trend with depth. Proﬁles of sulfate showed that the further the distance was, the higher were the concentrations of sulfate. Alkalinity and ΣPO3− 4 in sedimentary interstitial waters increased with depth, suggesting that the origin of NH+ 4 came from the decomposition of bottom sediments and the release of the decomposed productions, not from the diﬀusion or reduction of NO− 3 in overlying waters. The decomposition of organic matter depended upon the activity of organic matter in sediment and the eﬃcient actions of electron acceptors in special sedimentary circumstances, which included O2 , NO− 3 , MnO2 (or Mn2 O3 , MnOOH), Fe2 O3 (or FeOOH), SO2− 4 , and C. Daya Bay is rich in biological resources. Most of its sediments are soft mud, but contents of nutrients are not high. Daya Bay has high primary production and is prone to bring about red tide. The determined contents of NH+ 4, HPO2− , and H SiO were arranged in the order: interstitial waters>overlying 4 4 4 waters>bottom waters. No signiﬁcant change had been found for NO− 2 and

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NO− 3 , and no signiﬁcant change had been found in the concentration proﬁles − 2− of NO− 3 , NO2 , HPO4 , and H4 SiO4 in sedimentary interstitial waters, but the + proﬁles of NH4 concentrations decreased with depth in the exponential function. The content of NH+ 4 in sedimentary interstitial waters was 2∼3 numeric − orders greater than those of NO− 2 and NO3 . It suggested that the surface sediment of Daya Bay was in a reducing state. The high concentration of NH+ 4 suggested the recycling utilization ratio of N was very high, and NH+ 4 might have been assimilated before it was converted. The concentration of NH+ 4 in Daya Bay interstitial waters was higher than that in the East China Sea (Qiu et al., 1999). The transferable phosphorus in the natural grain size sediments was in the range of 58.5∼69.8 mg/g in Huanghe River Estuary regions, accounting for 9.1%∼11.0% of the total phosphorus. In comparison, the phosphorus in totally-ground sediments was 454.8∼529.2 mg/g, accounting for 9.1%∼11.0% of total phosphorus. It suggests that the transformed P in totally-ground sediments mostly could not participate in the biogeochemical recycling. It was found from the investigated results in situ from benthos that the biomass of big or small benthos correlated well to the non-detrital state P in natural grain sizes of sediments. Bad correlations were found between the biological mass and the phosphoric content in totally-ground sediments. It showed that the transformed P was the P of the non-detrital state in natural grain size sediments, and not the P in the totally-ground sediments, which had been taken for granted before (Song, 2000c). Accordingly, the recycling of biogenic elements in China marginal sea sediments is correlated with pH, ROD, temperature, water dynamics, and biological activity in sedimentary circumstances. In addition, it is very important to know whether the means of study adopted and its results can really reﬂect the biogeochemical processes and their contributions to marine changes or not. 1.3.3 Contribution of Settling Particles to Biogenic Element Recycling in China Marginal Seas The sedimentation of marine settling particles is an important source of sediment on the seabed. It plays an important role in sediment-seawater interface processes. It is signiﬁcant that we research the regulation of settling particles on material transfer, transforming, and depositing. Fluxes of some biogenic elements in China marginal sea settling particles are listed in Table 1.9. The settling ﬂuxes of particle carbons in the euphotic zone of the southern Taiwan Strait regions were 73.76 (5 m) and 98.50 (60 m) mg/(m2 ·d), respectively, the annual ﬂuxes in these regions were 2.45×109 kg C/yr, the total removal rates in the euphotic zone were about 2.5×109 kg C/yr, and total carbons in the euphotic zone of the region were 6.48×1010 kg C/yr. Accordingly, the retention period of total carbon in the euphotic zone was estimated to be about 26 years, which showed that the recycling rate of OC in the region was rather quick. It was not rich in inputs of exterior nutrients in Luoyuan

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Table 1.9. The ﬂuxes of biogenic elements of settling particulates (mg/(m2 ·d)) Regions Nansha coral reef lagoons Xinyi Lagoon Zhubi Lagoon Zhubi Lagoon Yongshu Lagoon Nansha Islands Taiwan Strait Luoyuan Bay Xiamen Bay East China Sea

Depth (m)

5 24 16 5 80 25 50 60 5 5 5

POC

463.40 53.38 90.16 220.10 193.50 99.25 73.80 98.50 5,054 856 213.80

TPC

402.3 575.7 2,735

PON TPN POP TPP

8.5 5.6 32.1

9.8 13.8 45.9

C:N:P

50.4: 6.3: 1 2.98 4.1 199.1: 9.6: 1 1.15 3.07 119.4: 14.9: 1 4.82 12

5.64 47.70

89.4:11.1:1

20.10

Bay regions, whereas about 91% of N, 97% of P, and 68% of Si for biological activities came from inner nutrient recycling in the seawater. The contents of POC decreased successively from suspended matter to settling particulates and to surface sediments, 15%∼40% of POC in suspended matter had participated in recycling in the settling process from water to sediment. The annual average content of PP in Luoyuan Bay waters was (0.342±0.249) μmol/L, and this amounted to about 40% of TP. The distribution of PP was mainly controlled by resuspension and continental inputting processes in winter, and was mainly controlled by biological processes in the waters in summer. Upwelling played an important role in the recycling of biogenic elements of settling particles in the East China Sea continental shelf, sometimes it was the main contributor, and the biological activity was less. About 80% of surface sediments were constituted of suspended matter, so upwelling also inﬂuenced the interfacial behavior of biogenic elements. Upwelling increased the provision of nutrients, and also increased biological production. 25%∼40% of primary production came from seabed sediments in the processes, and ultimately led to 76%∼83% of PP to regenerate. Strong upwelling occurred at the near-shore in summer and existed at the south edge of the Taiwan bank throughout the year. Suspended matter from 0∼10 m upwelling water layer on the south edge of the Taiwan bank mainly consisted of biological particles in summer. The PP accounted for 50% of TP in Xiamen Harbor and Jiulong River Estuary regions in spring, and the correlativities between PP and TP were very good. Transformations among diﬀerent forms of P mainly occurred in solution, and were also inﬂuenced by biological activities (Hong, 1994). Resuspension of sediment had a signiﬁcant inﬂuence on the compositions of setting particles in the East China Sea continental shelf regions. Resuspensions of sediments from storm, tides, bottom currents, and benthos irrigation were greater in the East China Sea continental shelf regions than those in the Changjiang River Estuary. There were signiﬁcant diﬀerences in the material vertical ﬂuxes in the East China Sea continental shelf regions from other regions and depths. The vertical transferring of C in the regions mainly depended on POC, which accounted for 98% of TC in surface layer

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waters and 68% of TC in the bottom waters. POC/PON ratios in planktons and suspended particles both were close to 10.6, much lower than that in sediments. The particle inorganic carbons (PIC) in planktons and suspended particles accounted for 0.20%∼1.9% of PC. PIC/TP value in bottom layer suspended matter was much greater than that in surface layer suspended matter. Therefore, resuspension of bottom layer matter had a signiﬁcant inﬂuence on the composition, contents, and ﬂuxes of settling particles in the East China Sea. The C in seawater of sediment came from the CO2 exchanged across the seawater-air interface, and then the DIC was translated into DOC and POC through photosynthesis of phytoplankton and production of zooplankton, which constituted the main vertical transferring matter and C deposits on the East China Sea continental shelf. The settling particles in the Nansha Islands coral reef ecosystems mainly came from biological origins, their setting rates were greater than those in near-shore regions, and it reﬂected the higher biological production in the regions. C:N:P was 123:10:1 in the particles on average, having a greater diﬀerence from the Redﬁeld ratio (106:16:1), and was higher than that in Luoyuan Bay (89.4:11.1:1). About 86%∼89% of OC in lagoon setting particles could be decomposed and released, about 11%∼14% of which might be buried. The ﬂux of OC was in direct proportion to TC, and was in inverse proportion to water depth. In contrast with this, the ﬂuxes of ON were in inverse proportion to TN, and were in direct proportion to the water depth. The content of PP in the coral reefs was about 3.13×1018 g, which accounted for 96.2% of TP, and played an important role in the recycling of P in the ocean. About 86%∼89% of OC in the setting particle had been released before sinking to the seabed, which played an important role in the recycling of OC and in maintaining higher coral reef biological production. The ﬂux of OP was between OC and ON. The recycling rates of setting PP in lagoons were rather quick, 35 d in the Zhubi Reef and 7 d in the Yongshu Reef. POP in setting particles of Nansha coral reef lagoons accounted for 37.9%∼72.6% of TP. The releasing ratios of setting PP could be estimated by the contents of TP and POP of setting particles and through surface sediments of lagoons. The releasing ratio of OP was 97.5% in Zhubi Reef regions, which accounted for 90.7% of TP. Therefore, 97.5% of OP was released into waters for recycling. The releasing ratio of POP was 81.8% in the Yongshu Reef region. Accordingly, 2.5%∼18% of OP was buried and because of its burial aggradation IP could also be released to a certain extent. It mainly came from the regeneration and dissolution of solid calcium phosphate in bones by bacterial activity. The Nansha Islands coral reef ecosystems were the exporter of organic particles and nutrients. In conclusion, 15%∼40% of OC and 82%∼97.5% of OP had been released into sea waters for recycling before biogenic elements in the setting particles reached the seabed in China marginal seas. The researches of biogenic elemental recycling processes in China’s seas vertical setting particles are not systemic. It is necessary to systematically study the recycling and controlling mechanisms of biogenic elements. Considering the past research in China’s

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seas, biological processes and water dynamical processes must be two important factors. 1.3.4 Contributions of China Marginal Sea Sediments in the Recycling of Biogenic Elements Terrestrial detrital organic matter and suspended particles are the main sources of marine sedimentary biogenic elements, and they are brought into the ocean by river runoﬀs, biological activities, and living activities, and ultimately reach the seabed. The biogenic elements carry through mass transfer mainly by molecular diﬀusion across the sediment-seawater interface. Molecular diﬀusion results from a series of chemical reactions and processes, such as mineralization of organic matter, dissolution and precipitation of minerals, adsorption, and exchange of matter, which alter the composition of interstitial waters, and result in a concentration gradient in interstitial waters. Recycling of biogenic elements between sediments and interstitial waters is carried through endlessly along the gradient. The physical chemical environments of sediments play an important regulating role in the recycling. The mass exchanges between interstitial waters and overlying waters are important material conditions to maintain the ecosystem in surface sediments and in bottom waters. The diﬀusing ﬂuxes of biogenic elements across sediment-seawater interfaces mainly depend on the diﬀusion of the concentration diﬀerence caused by the concentration gradient near the interfaces. The net diﬀusing ﬂuxes of biogenic elements across the sediment-water interface in some of China’s sea regions can be estimated by the First Fick Law, and the results are listed in Table 1.10. Table 1.10. Diﬀusion ﬂuxes across the sediment-seawater interfaces in China’s seas Regions East China Sea

HCO− SO2− S2− NH+ NO− 3 4 4 2 −0.634 ∼ −2.055 ∼ −0.022 ∼ −34.081 ∼ 0.387 1.214 0.141 0.193 −0.66 2.24 1.410

NO− 3

HPO2− H4 SiO4 4 −1.199 ∼ −324.9 ∼ 1.938 148.800

Liaodong Bay Nansha Islands Lagoon (1993) −0.39 0.70 630.26 30.22 103.86 9.13 255.10 Lagoon (1994) −0.43 14.70 197.61 4.42 212.86 6.71 151.63 Oﬀ-reef (1993) −0.36 0.36 619.77 19.82 345.15 8.35 2, 637.70 Oﬀ-reef (1994) −0.33 0.22 485.3 14.81 585.05 15.78 1, 627.32 Xiamen Bay 720 7.74 2800 Okinawa Trough −4.488 1.131 2.209 Outer-Wenzhou −6.640 0.133 111.90 South Yellow Sea −1.321 −0.102 −55.91 Luoyuan Bay 0.37 314.80 Daya Bay 302 ∼ 0.06 ∼ 1.82 2.53 47.96 Unit: HCO− and SO2− are in mmol/(m2 ·d); S2− is in pmol/(m2 ·d); the others are in 3 4 μmol/(m2 ·d). “−” stands for the ﬂux direction from overlying waters to sediment

The diﬀusing behavior of nutrients depends upon their inner characteristics and the environment of the regions. The whole oceanic annual input

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ﬂuxes of PO4 -P, SiO4 -Si, NH4 -N, and Mn2+ to overlying waters are 3.8×1011 , 1.1×1013 , 1.0×1013 , and 1.4×1011 g/yr, amounting to 54%, 2.2%, 4.8%, and 40% of total river inputting ﬂuxes, respectively. Nutrients released from organic matter decomposition in bay sediments of China’s seas contribute 80% of N and 200% of P that plankton needs in the bays. Due to shallower water depth, stronger seasonal changes in temperatures and biogeochemical actions, stronger changes in early diagenesis processes near interfaces, and quicker production or adsorption of nutrients from sediments in the Bohai Sea, the Yellow Sea, and the East China Sea, the directions of nutrient diﬀusions are diﬀerent in China marginal sea sediments. Most of the East China Sea continental shelf surface sediments were covered with sandy sediments. Content of H4 SiO4 in surface sediments was 24 times greater than that of surface layer waters, and 23 times greater than that of bottom layer waters. The value of PO4 -P was 19 times and 3 times greater, respectively. The annual releasing ﬂuxes of P and Si into seawaters from sediments in the Bohai Sea were 1.02×107 kg P/yr and 1.91×108 kg Si/yr, respectively, accounting for 86.4% of recycling TP and 31.7% of recycling TSi, respectively. Accordingly, the processes near sediment-seawater interfaces in the Bohai Sea played an important role in the recycling of nutrients, especially for P (Song, 2000b). Liaodong Bay lies in the northern part of the Bohai Sea. Ammonium and sulﬁde diﬀused from sediments to overlying waters, SO2− 4 diﬀused from overlying waters to sediments, and this reﬂected their diﬀerent chemical diagenesis processes (Song, 1997b). All the diﬀusion directions of nutrients near sediment-seawater interfaces in Nansha Islands regions were from sediments to overlying waters, their ﬂuxes were greater than those in the Bohai Sea, the Yellow Sea, and the East China Sea regions. Due to the high water temperature in Nansha Islands regions year after year, the apparently active energy of nutrients diﬀusing and releasing from sediments decreased, and the sediment activities increased. This resulted in the production of a great deal of nutrients in sediments and the diﬀusion from sediments to the overlying waters. HS− concentrations in Nansha lagoon interstitial waters were much higher than those in the outer-reef sediment interstitial waters. The concentrations in interstitial waters of the lagoons from human activities were higher than those in interstitial waters of lagoons without human activities, and were higher in the near-reef sediment interstitial waters than in the oﬀ-reef sediment interstitial waters. HS− and S2− diﬀused from sediments to overlying seawaters, but it was the opposite for SO2− 4 . The average diﬀusing ﬂuxes in the lagoons were higher than those in the outer-reef. H4 SiO4 was the greatest diﬀusing + composition in the outer-reef nutrient compositions, and NO− 3 and NH4 were the greatest diﬀusing compositions. The diﬀusing ﬂux of H4 SiO4 across the outer-reef sediment-water interfaces was 10.5 times greater than that in the lagoons, and that of NO− 3 was 3 times greater. All the diﬀusion directions of S2− , HS− , H4 SiO4 were from sediment to + overlying waters in the East China Sea, and the opposites were SO2− 4 , NH4 ,

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PO3− 4 , namely from overlying water to sediments. The diﬀerent diﬀusing directions reﬂected their diﬀerent early diagenesis processes in the sediments (Song, 1997b). The diﬀusions of HCO− 3 from seawater to sediment in the outer-Changjiang River Estuary and the Okinawa Trough regions showed that HCO− 3 must be transferred in the surface layer sediment. The diﬀusions of HCO− 3 from sediments to seawaters in the northern Taiwan Strait were due to their short distances from the bank, high contents of organic matter in sediments, low pH value, the dissolution of CaCO3 and the oxidation of organic matter. The diﬀusions of NH+ 4 were from sediments to seawaters in the outer-Changjiang River Estuary, the Okinawa Trough, and the northern Taiwan Strait, which reﬂected the fact that the transfer of NH+ 4 was due to its adsorbtion in clay minerals in the East China Sea sediments. There was rather a lot of H4 SiO4 in sediments diﬀused to seawater in the Okinawa Trough and the Taiwan Strait because of their low pH in sediments. HPO2− 4 could be transferred through adsorption or through forming apatite in the Okinawa Trough and the southern Taiwan Strait. HPO2− 4 could diﬀuse from sediment to seawater in the Okinawa Trough sediments, which was due to the characteristics of volcanic activity and oceanic sedimentation in the regions. There was much more organic matter from biological activities in the sediments of the Changjiang River Estuary − near-shore, which could decompose and release more NH+ 4 and HCO3 to enter interstitial waters, and then be diﬀused into overlying waters, which provided more chemical substances for overlying waters than in the oﬀ-shore regions. The transfer of Si existed in the sediments of the outer-Changjiang River Estuary. The diﬀusions of SO2− 4 across the sediment-water interface were from overlying waters to sediments in the regions of Nansha Islands, the Okinawa Trough, the Huanghe River Estuary, and the South Yellow Sea, but also in outer-Wenzhou. It was due to the stronger reduction of SO2− in surface 4 sediments that led to the consumption and diﬀusion of SO2− 4 . The diﬀusion directions of S2− and HS− were from sediments to overlying waters, which reﬂected the stronger reduction of the two elements in the sediments than that in the overlying waters. S2− from the reduction of SO2− 4 diﬀused into seawaters. Most of S2− was produced by chemical diagenesis in initial stages when S2− was buried in sediments. Diﬀusion of S2− into overlying waters showed that S2− in the surface sediments could be dissolved and freed from reduction. The magnitude of transferring ﬂuxes of HS− reﬂected the reductions of sediments to a certain degree. The sedimentary reduction could be listed below according to the diﬀusion ﬂuxes of HS− : Huanghe River outer-shore region>Huanghe River near-shore region>southern Yellow Sea region>outerWenzhou region>Okinawa Trough region, which mainly depended upon the input of terrigenous matter, especially of terrigenous organic matter (Song, 1997b). Luoyuan Bay belongs to a sub-tropical region, and is a typical semi-closed bay. Annually P and Si diﬀusing into overlying waters from the bay sediments are 6.61×102 g P/yr and 4.95×105 g Si/yr, respectively. The average ﬂuxes

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of P and Si into the ocean from the river were about 7.13×106 g P/yr and 1.97×109 g Si/yr, so the diﬀusions accounted for about 10% of P and 25% of Si in the input ﬂuxes, respectively. The diﬀusing ﬂuxes of Si in Luoyuan Bay sediments were less than those in Xiamen Bay sediments, and greater than those on the East China Sea continental shelf and in the outer-Changjiang River. The diﬀusion ﬂuxes of P in Luoyuan Bay sediments were less, and this was due to the content of P in its interstitial waters being only 1.0 μmol. The content was comparable with the contents in Xiamen Bay sediments, and was tens to millions of times less than those in the world’s oceans sediments. The diﬀusing ﬂuxes of P into overlying waters in the western Xiamen Bay were 0.9 μmol/(m2 ·d). The diﬀusing ﬂuxes of P across sediment-seawater interface were 0.026 mg/(m2 ·d) in Luoyuan Bay and 0.24 mg/(m2 ·d) in Xiamen Bay, respectively. The ﬂuxes of PP determined from the sampling implements under the euphotic zones were 5.64∼47.7 mg/(m2 ·d). Accordingly, most of the P released participated in recycling in the sinking processes. PP that reached the seabed was strongly adsorbed as solid, only about 0.5% of which diﬀused into waters in the form of solute through early diagenesis processes, even though some P had been released through mineralization of organic matter. From the Huanghe River Estuary to the Bohai Sea, the ﬂuxes of dissolved P were 1.6×106 kg/yr, the ﬂuxes of dissolvable P were 2.81×108 kg/yr, and the ﬂuxes of buried P were 5.88×108 kg/yr. The direct contributions of dissolved P to the Bohai Sea were less than the release of dissolvable P from suspended particles and from sediments for higher suspended matter transportation and lower eﬄux. About 2/3 of TP transferring from the Huanghe River Estuary sank and was buried in marine sediments. OP released from the decomposition of river organic matter was the main source of dissolved P and dissolvable P in the region (Li and Yu, 1999). The nutrients supplied from sediments to seawaters in Daya Bay were more important than those in the other regions. The cultivating regions of Daya Bay were covered with soft mud, the vertical concentration distributions of − 2− NO− 3 , NO2 , H4 SiO4 , HPO4 had no changes in the interstitial waters of the regions, and the distribution of NH+ 4 decreased distinctly with depth. The vertical distribution trend of NH+ 4 was similar with that in the East China Sea, but the content of NH+ 4 in Daya Bay sedimentary interstitial waters was greater than that in the East China Sea. The diﬀusing ﬂuxes into seawater of 2− NH+ 4 , HPO4 , and H4 SiO4 in Daya Bay sediments were much less than those in Xiamen Bay and the Nansha Islands, but greater than those in many near− shore regions. The diﬀusing directions of NO− 3 and NO2 were from overlying waters to sediments, which showed the enrichment of organic matter near the surface sediment interfaces in the cultivating regions. The contents of H4 SiO4 in the interstitial waters of Daya Bay were higher than those in other regions, but changed much less than those in other regions. In conclusion, the diﬀerences in biogenic element diﬀusing ﬂuxes near sediment-seawater interfaces in diﬀerent regions are great, and their directions are not all the same. It reﬂects the diﬀerences in their early diagene-

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sis processes, interfacial geological circumstances, and biological and physical chemical environments. 1.3.5 Inﬂuences of Biological Productions in China Marginal Sea Sediments on the Recycling of Biogenic Elements Of all marine organic matter, life particle organic matter accounts for 2%, non-life particle organic matter accounts for 9%, and dissolved organic matter accounts for 89%. Life activities are the most active factors in the ocean, and almost involve all the geochemical processes. It is the 2% of the life organic particles in the ocean that regulate the existence of 89% of dissolved organic matter. 1.3.5.1 Inﬂuences of Planktons on the Recycling of Biogenic Elements Planktons make an important contribution to marine primary production. Marine biological productions are initiated from the photosynthesis of phytoplanktons, which utilize sunlight and CO2 to synthesize 40%∼50% of global primary productions. 2∼20 μm micro-phytoplanktons and those less than 2 μm account for a great proportion of marine primary producers, and their contributions sometimes exceed the diatoms—the traditional primary producers, especially in the tropical ocean. The diatoms with tens of μm were the main producers of marine primary production in the past, especially in the spring when algal bloom occurs. The primary production estimated according to chlorophyll a in the Nansha regions was 406 mg/(m2 ·yr), and the proportion of the vertical ﬂuxes of organic carbon in primary production was also estimated. The proportions were 17.7% in the Nansha Zhubi Reef and 54.2% in the Yongshu Reef. It showed that 17.7%∼54.2% of primary productions in the lagoons sank to the seabed through biological particles, and 45.8%∼82.3% of OC participated in recycling in waters. The values were lower than those in Luoyuan Bay, and higher than those in general sea regions (Song, 1997b). The nutrient concentrations of surface layer waters were 0.2 μmol/L, lower than those of bottom waters, and 0.4 μmol/L higher than those of oﬀshore regions and, moreover, increased with depth. The regenerating rates of phosphates between 200 m and 600 m in depth were 6.7∼20 μmol/(L·yr), and the average sinking rates were 15∼80 d (Zhu et al., 1999). The inﬂuence of planktons on the recycling of biogenic elements was also embodied in seasonal changes in the contents and the recycling of biogenic elements. The ﬂuxes of amino acids and organic matter in the northern South China Sea setting particles showed intense seasonal changes, and the increase and decrease of planktons such as diatom, coccolith, and foraminifer were similar to the changes in total ﬂuxes. The siliceous and calcareous planktons were the main contributors of organic matter. The organic matter was assembled and rapidly settled through mutual actions between organic particles

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and inorganic particles, decomposed and dissolved by microbes, and led to the particles returning back to the waters for recycling. Nutrients had an important eﬀect on the growth rates of pelagic diatoms, whereas the biological production processes of pelagic diatoms also had an important eﬀect on the recycling of biogenic elements. The concentrations of nutrients in marine ecosystems not only inﬂuenced the total biomass, but also led to changes in the composition, structure, and characteristics of particles. Some studies showed that waters would have become eutrophic when IN>0.2 mg/L, IP>0.045 mg/L, and COD>0.2∼0.3 mg/L. The optimal nutrient ratios (N:P:Fe:Si) for the growth of Cocconeis scutellum var. parva, Amphora coﬀeaeformis, and N. millis were 50:0.25:0.5:2.0, 2.5:1.0:1.0:2.0, and 5.0:0.15:0.5:2.0, respectively. The primary production in estuary regions reached 500∼1,000 g/(m2 ·yr), in contrast to 100 g/(m2 ·yr) in continential shelf regions. The primary production had an important inﬂuence on the transfer of OC and on the other biogenic elements. The diﬀerent microbial and plankton activities, diﬀerent biogeochemical processes across particle-water interfaces, and diﬀerent circumstances leading to the changes in forms and the transforms of phases of the biogenic elements C, N, P in the Changjiang River Estuary, further altered the surface compositions, properties of particles, and the components of the medium, and consequently inﬂuenced the stability of the colloidal particle. The biogeochemical process was an important factor and also one of the main mechanisms in regulating the ﬂocculations of particles in the Changjiang River Estuary regions (Lin et al., 1995). About 50% of particle organic matter had been used up before they reached the 5 cm waters of the upper seabed in the Yongshu Reef and Zhubi Reef. The consuming rate was greater in the Yongshu Reef than that in China marginal sea continental shelf. The main source of organic matter in the Yongshu Reef was marine planktons (Duan et al., 1998). Upwelling in the Taiwan Strait regions led to net nutrient ﬂuxes, higher nutrient consumption rates, and net production rates of particle organic matter and dissolved oxygen. POC in planktons provided 25% of total POC. POC from the aggradation in euphotic zones was about 2×10 g C/(m2 ·s), accounting for 35% of the primary production (Hong, 1994). In conclusion, plankton is very important in the recycling of setting particle, its production processes inﬂuenced the releases, burials, and transfers of biogenic elements in China’s seas. 1.3.5.2 Benthos Inﬂuences on the Recycling of Biogenic Elements At present, studies on the recycling of biogenic elements in protozoans and minitype animals are not enough, but studies on the predations of several macrobenthos have been very distinct. Most of the macrobenthos prey on suspensions or sediments, they incept the adsorded sediments (for instance,

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echinus species), and another species prey on deep-water particles (for instance, star worms). The benthos near marine sediments include microbes (such as bacteria and epiphytes), protozoans, minitype animals, macrobenthos, etc. Their inﬂuences on the recycling of biogenic elements in marine sediments are mainly in: (1) inﬂuencing the evolvement of waters and the transfers of nutrients; (2) inﬂuencing the stability and transfer of biogenic elements in sediments; (3) inﬂuencing the recycling of global biogenic elements. The benthos near the sediment-seawater interfaces sometimes irrigate their own caves by taking oxygen and food near sediment-seawater interfaces, thereby facilitating the mixing and exchanging of biogenic elements between surface interstitial waters and overlying waters (Song, 2000a). Preying processes were the basis for the study of benthos ecological functions and biogeochemical processes. Funguses could decompose ligncellulose and chitin, and bacteria could decompose particle OC, and most of the particles were diatoms and the animal residues. Hydrolyzing bacteria started this process and created dissolved organic carbons (DOC). DOC could be oxi4+ dized to CO2 by oxidants (O2 , NO− , Fe3+ , SO2− 3 , Mn 4 , etc.) in sedimentary circumstances, and could be decomposed by zymosis too. The most typical bacteria were the sulfate deoxidizing bacteria. Mn2+ , Fe2+ , and SO2− 4 could be oxidized by some bacteria during the oxidizing processes of OC. Because the processes provided less energy, the production was also limited. However, it was the process that ﬁnished the recycling of C, N, S, Fe, and Mn in sediments, and further formed their oxides. Benthos inﬂuenced and regulated the recycling of N, for example the irritations of benthos strongly worked on the contents and its vertical distribution + of NH+ 4 . In general, the concentrations of NH4 in interstitial waters increased with depth up to a depth of 5∼10 cm or 10∼15 cm while, reversely, the + production rates of NH+ 4 decreased with depth. NH4 from surface sediments diﬀused into overlying waters prior to that from deep layer waters, and the benthos irritations accelerated the process. The water depth where the benthos located was in direct proportion to the acting time when the sediments were buried. Thereby, in spite of the lower production rate of NH+ 4 in deeper waters, there was much time for the releasing and accumulating of NH+ 4 from the decomposition of organic matter. Biological processes reduced the gradients between sediment interstitial waters and their overlying waters, further − reducing the ionic diﬀusing ﬂuxes of NH+ 4 and HCO3 calculated by gradient means. On the other hand, due to overestimating the quantity of NH+ 4 that entered overlying waters, the ﬂux calculated by gradient means increased. It − further uncovered the relations between distributions of NH+ 4 and HCO3 and benthos animal mass (Song, 1997a; 2000a). The biogeochemical regeneration and transfer of P were two signiﬁcant controlling factors in primary production, and the biological activities played an important role in the transfer of phosphoric forms. The releasing of P was especially important in estuaries and near-shore circumstances. The regeneration of nutrients in near-shore sediments evidently provided what primary

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production needed in the overlying waters. Planktons per se have certain depositing functions to N and P in waters. Due to the decompositions of biological detritus and excretions in the waters under water columns, nutrients such as P and others gained regeneration. Of the nutrients, the regeneration rates of active P commonly were 3.88 μg/(L·d), and the net biological utilization rates were 3.10 μg/(L·d), so the biological inputting and outgoing processes of P basically were in a state of balance (Hong, 1994). Through surveying in situ and laboratory cultivation, the greatest rates of biological adsorptions and excretions of P in Xiamen Bay and the Jiulong River Estuary in spring were 0.191 h−1 and 0.063 h−1 , respectively. The distributions of alkaline phosphatase (ALPase) activities in western Xiamen Harbour sediments and Hongkong Victoria sediments had a certain relation to the degree of pollution indicated by the microbial characteristic factors, and had a notable relation to the anaerobic circumstances of sediments (Hong, 1994). The ﬂuxes of nutrients in the Minjiang River Estuary were about 3.3×1011 g for silicate, 7.7×1011 g for phosphate, 4.6×1010 g for DIN, respectively. The average primary production was about 192.4 mg/(m2 ·d) (8.59∼685 mg/(m2 ·d)). The photosynthesis of planktons was related to the nutrient mass, and C:N:P: Si=106:16:1:15. The consumption of nutrients accounted for 14.5% N, 53.6% P, and 1.9% Si of the total river inputs (Chen, 1997). The recycling of S in sediments was related to biological activities, and the existing forms of S were related to its environment. The reduction in sulfate and the oxidation of sulﬁde were the two main processes in which S was metabolized in sediments, and the bacteria played a key role in the processes. It was estimated that benthos played a main role in the accumulation of S in sediments by information recorded on the fossils. Such a function might be carried out by regulating dissolved oxygen and the content of unstable carbons. In the light of the whole world, S was not a limiting element, but it played an important role in the recycling and in the bacterial evolution. The distributions of S2− were mainly controlled by bacteria, the ratios of S2− /S2− 2 increased from estuary to continental shelf. The oxidation of S from S2− to S0 was also carried out by S bacteria, a kind of oxidizing bacteria, so the ratio in the proﬁles of sediments could be used as a measurement for of S2− /S2− 2 estimating the settling rates in modern oceanography (Qiu et al., 1999). It is obvious that biological production inﬂuences the releasing, burying, and transferring of biogenic elements. It is also an important controlling factor for the recycling of biogenic elements in sediments. Planktons and benthos participate in the biogeochemical processes of biogenic elements in marine sediments, and also inﬂuence the oceanic environments and global marine changes. In conclusion, the functions of China marginal sea sediments in the recycling of biogenic elements are very diﬀerent in diﬀerent regions, and are controlled by contents of organic matter, biological irritations, physical chemical environments, geology, water dynamics, etc. The redox environments of biogenic elements and the ecological characteristics near the sediment-seawater

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interfaces are two main factors regulating the recycling. In the future, studies should be concentrated in particular on the distributions, spatial and temporal variabilities, transferring mechanisms of biogenic elements in sediments, and their inﬂuences on ecological environments.

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Song JM, Li PC (1996) –2 valence sulfur of lagoon and oﬀ-reef sediment environment in the South China Sea. Oceanol Limnol Sin 27(6):597-603 (in Chinese with English abstract) Song JM, Li PC (1998) Vertical transferring process of rare elements in coral reef lagoons of Nansha Islands, South China Sea. Sci Chin (Ser D) 41(1):42-48 Song JM, Li PC, Wu LX (1997a) Diﬀusion ﬂuxes of N, P, Si across sediment-water interface in the coral reef ecosystem of the Nansha Islands. In: Studies on the Chemistry and Biology in Coral Reef Lagoons of the Nansha Islands. China Ocean Press, Beijing, pp.84-92 (in Chinese) Song JM, Zhan BQ, Li PC (1997b) Dissolved oxygen distribution and O2 ﬂuxes across the sea-air interface in the East China Sea waters. Chin J Oceanol Limnol 14(4):228-232 Song JM, Ma HB, L¨ u XX (2002) Nitrogen forms and decomposition of organic carbon in the southern Bohai Sea core sediments. Acta Oceanol Sin 21(1):125133 Song JM, Luo YX, L¨ u XX, Li PC (2003) Forms of phosphorus and silicon in the natural grain size surface sediments of the southern Bohai Sea. Chin J Oceanol Limnol 21(3):286-292 Song JM, Ma HB, Li XG, Yuan HM, Li N (2004) Geochemical characteristics of adsorbed inorganic nitrogen in the south Bohai Sea sediments. Oceanol Limnol Sin 35(4):315-322 (in Chinese with English abstract) Song JM, Xu YF, Hu WP, Ni LY (2008) Biogeochemistry of Carbon in China’s Seas and Lakes. Science Press, Beijing, pp.1-553 (in Chinese) Sun WX, Xi PG (1988) The results of the 3D nonlinear tidal boundary problem and its analysis. J Shandong College Oceanogr 18(2):51-52 (in Chinese with English abstract) Sun YM, Song JM (2002) Functions of China’s marginal sea sediments in the cycles of biogenic elements. Mar Environ Sci 21(1):26-33 (in Chinese with English abstract) Sun YM, Song JM (2003) Progress on biogeochemical processes of marine carbon cycles in China (1998-2002). In: 1999-2002 China National Report on Geodesy and Geophysics for the 23rd General Assembly of IUGG. China Meteorological Press, Beijing, pp.116-128 (in Chinese) S¨ undermanna J, Feng SZ (2004) Analysis and modelling of the Bohai Sea ecosystem: A joint German-Chinese study. J Mar Syst 44(3-4):127-140 Tian T, Wei H, Su J, Zheng CS, Sun WX (2003) Study on cycle and budgets of nutrients in the Yellow Sea. Adv Mar Sci 21(1):1-11 (in Chinese with English abstract) Valle-Levinson A, Matsuno T (2003) Tidal and subtidal ﬂow along a cross-shelf transect on the East China Sea. J Oceanogr 59(55):573-584 Wang BD, Zhan R, Xu MD (2002) Molar radios of C, N, P of particulate matter and their vertical ﬂuxes in the Yellow Sea. Chin J Oceanol Limnol 20(1):90-96 Wang CH (1995) Geochemistry of Sediments in East Sea Seabed. China Ocean Press, Beijing, pp.59-72 (in Chinese with English abstract) Wang DQ, Chen ZL, Wang J, Xu SY, Yang HX, Chen H, Yang LY, Hu LZ (2006) Denitriﬁcation, nitrous oxide emission and adsorption in intertidal ﬂat, Yangtze Estuary, in summer. Geochimica 35(3):271-279 (in Chinese with English abstract)

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Wang J, Kang Y (1998) Study on population dynamics of phytoplankton in the Bohai Sea. Mar Fish Res 19(1):43-52 (in Chinese with English abstract) Wang JT, Zhao WH, Tan LJ, Zhang ZB (2000a) A comparison of HTC method and UV/persulphate method to determine colloidal organic carbon in sea water. Acta Oceanol Sin 22(2):67-70 (in Chinese with English abstract) Wang JT, Zhao WH, Zhang ZB (2000b) Adsorption of natural dissolved organic carbon in seawater on needle ferric mineral. Oceanol Limnol Sin 31(3):309-313 (in Chinese with English abstract) Wang P, Min Q, Gao J (1985) A preliminary study of foraminiferal and ostracod assemblages in the Huanghai Sea. In: Marine Micropaleontology of China. China Ocean Press, Beijing, pp.115-132 (in Chinese) Wei H, Tian T, Zhou F, Zhao L (2002) Numerical study on the water exchange of the Bohai Sea: Simulation of the half-life time by dispersion model. J Ocean Univ Chin 32(4):519-525 (in Chinese with English abstract) Wei H, Sun J, Moll A, Zhao L (2004) Phytoplankton dynamic in the Bohai Sea– Observations and modelling. J Mar Syst 44(3-4):233-251 Wei ZX, Li CY, Fang HG, Wang YX (2003) Numerical diagnostic study of the summertime circulation in the Bohai Sea and the water transport in the Bohai Strait. Adv Mar Sci 21(4):454-464 (in Chinese with English abstract) Wu LX, Li HY (1997) Contribution of alga to the coral reef lagoon’s primay productivity of the Nansha Islands by determining oxygen with Bell-Jar process. In: Studies on the Chemistry and Biology in Coral Reef Lagoons of the Nansa Islands. China Ocean Press, Beijing, pp.78-83 (in Chinese) Wu LX, Wang HK, Li PC, Song JM (1997) The vertical ﬂuxes of sedimentary particles in coral reef lagoons of the Nansha Islands. In: Studies on the Chemistry and Biology in Coral Reef Lagoon of the Nansha Islands. China Ocean Press, Beijing, pp.43-48 (in Chinese) Wu QH, Jia HL (2005) The grain-size distribution of organochlorine pesticides in sediments. J Guangzhou Univ (Nat Sci Ed) 4(1):40-45 (in Chinese with English abstract) Wu ZM, Yu GY (1999) A pelagic ecosystem model and simulation of the northern part of Jiao Zhou Bay. I. A simulation study on the pelagic ecosystem seasonal variations. J Ocean Univ Chin 29(3):429-435 (in Chinese with English abstract) Xing RN, Wang ZG (2001) Numerical modeling of the response of tropical Paciﬁc surface water to event. Acta Meteorol Sin 59(3):308-316 (in Chinese with English abstract) Xu BD, Jin XS (2005) Variations in ﬁsh community structure during winter in the southern Yellow Sea over the period 1985∼2002. Fish Res 71(1):79-91 Xu JR, Wang YS, Yin JP, Wang QJ, Zhang FQ, He L, Sun CC (2005) Transformation of dissolved inorganic nitrogen species and nitriﬁcation and denitriﬁcation processes in the near sea section of Zhujiang River. Acta Sci Circum 25(5):686692 (in Chinese with English abstract) Xu ZL, Chao M, Chen YQ (2004) Distribution characteristics of zooplankton biomass in the East China Sea. Acta Oceanol Sin 26(3):93-101 (in Chinese with English abstract) Yan J, Li J (1997) The climate change in the last 100 years over the East China Sea and its adjacent region. Acta Oceanol Sin 19(6):121-126 (in Chinese with English abstract)

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Yang GP, Zhang ZB, Liu LS, Liu XT (1996) Study on the analysis and distribution of dimethyl sulﬁde in the East China Sea. Chin J Oceanol Limnol 14(2):141-147 Yang QS, Mai BX, Fu JM, Sheng GY, Luo XJ, Lin Z (2004a) Fluxes of persistent organic pollutants from Humen tidal channel of pearl river to Lingdingyang Estuary. Sci Geogr Sin 24(6):704-709 (in Chinese with English abstract) Yang QS, Ou SY, Xie P, Mai BX, Fu JM, Sheng GY (2004b) Distribution and seasonal changes of polycyclic aromatic hydrocarbons (PAHs) in surface water from Humen tidal channel. Acta Oceanol Sin 26(6):37-48 (in Chinese with English abstract) Yang SY, Jung HS, Lim DI, Li CX (2003) A review on the provenance discrimination of sediments in the Yellow Sea. Earth Sci Rev 63(1-2):93-120 Yao QZ, Zhang J, Qin XG, Dong LX (2006) The behavior of selenium and arsenic in the Zhujiang (Pearl River) Estuary, South China Sea. Estuar Coast Shelf Sci 67(1-2):170-180 Ye XW, Liu SM, Zhao YF, Zhang J (2006) The distribution of biogenic silica in the sediments of the East China Sea and the Yelow Sea and its environmental signiﬁcation. Chin Environ Sci 24(3):265-269 (in Chinese with English abstract) Yuan HM, L¨ u XX, Li XG, Li N, Sun YM, Zhan TR, Song JM (2003) Geochemical factors of organic carbon in natural grain size sediments of Bohai Sea. Environ Chem 22(2):115-120 (in Chinese with English abstract) Yuan YC, Liu YG, Liao GG, Lou RY, Su JL, Wang KS (2005) Calculation of circulation in the South China Sea during summer of 2000 by the modiﬁed inverse method. Acta Oceanol Sin 24(1):14-30 (in Chinese with English abstract) Yue WZ, Huang XP (2005) Distribution characteristics of phosphorus in core sediments from Zhujiang River Estuary and its environmental signiﬁcance. J Trop Oceanogr 24(1):21-27 (in Chinese with English abstract) Zhang B, Tang Q, Jin X (2007) Decadal-scale variations of trophic levels at high trophic levels in the Yellow Sea and the Bohai Sea ecosystem. J Mar Syst 67(34):304-311 Zhang H, Sun F (1996) Dynamics analysis for ﬂux of carbon dioxide across seasurface. Acta Oceanol Sin 14(4):447-455 Zhang JS, Shen ZL (1997) A study of change in nutrient structure of Jiaozhou Bay. Oceanol Limnol Sin 28(5):535-541 (in Chinese with English abstract) Zhang SZ, Li CH (1997) The nutrient role of microzooplankton in the lagoons ecosystem from coral reefs of the Nansha Islands. In: Studies on the Chemistry and Biology in Coral Reef Lagoons of the Nansha Islands. China Ocean Press, Beijing, pp.64-70 (in Chinese) Zhang ZB, Wang JT, Liu LS (1997) Dissolved organic carbon in the East China Sea in autumn. Chin J Oceanlo Limnol 15(2):137-144 Zhao WH, Wang JT, Jiao NZ, Zhao ZX (2001) Dissolved and particulate organic carbon in Yantai Sishili Bay aquiculture waters. Chin J Oceanol Limnol 19(2):175-185 Zhao YF, Liu SM, Ye YW, Zhang J (2005) The analysis of biogenic silica in the sediments of the East China Sea and the Yellow Sea. Period Ocean Univ Chin 35(3):423-428 (in Chinese with English abstract) Zhao ZM, Zhang L, Zheng BH, Guo KQ, Qin YW, Wang Y (2005) Spatial distribution of phosphorus and nitrogen in core sediments of Bohai Bay. J Northwest Sci Tech Univ Agric For (Nat Sci Ed) 33(4):107-111 (in Chinese with English abstract)

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Zhou FX, Wang ZT (1999) Phase discontinuity of the SST interannual oscillation between the South China Sea and its adjoining west Paciﬁc region. Oceanol Limnol Sin 30(6):679-686 (in Chinese with English abstract) Zhou H, Zhang ZN, Liu XS, Tu LH, Yu ZS (2007) Changes in the shelf macrobenthic community over large temporal and spatial scales in the Bohai Sea, China. J Mar Syst 67(3-4):312-321 Zhu CJ, Shi XY, Li T (1999) The distribution and regeneration of phosphate in the East China Sea. In: Hu D, Tsunogai S (eds.) Margin Flux in the East China Sea. China Ocean Press, Beijing, pp.219-227 (in Chinese) Zhu MH, Ding YS, Zheng DC, Tao P, Qu YJ, Cui Y, Gong WM, Ding DW (2005) Bioavailability and distribution of the familiar heavy metals around rhizosphere sediment of Suaeda heteroptera. Mar Environ Sci 24(3):21-24 (in Chinese with English abstract) Zuo T, Wang R, Chen YZ, Gao SW, Wang K (2005) Net macro-zooplankton community classiﬁcation on the shelf area of the East China Sea and the Yellow Sea in spring and autumn. Acta Ecol Sin 25(7):1531-1540 (in Chinese with English abstract)

2 Biogeochemical Processes of the Bohai Sea

Abstract: In this chapter the biogeochemical processes in the Bohai Sea are described. The main contents deal with the distributions, transformation, and their controlling factors of carbon, nitrogen, phosphorus, silicon, and pollutants such as heavy metals and persistent organic pollutants (POPs) in the seawaters and sediments. In the Bohai Sea, human activity and riverine input are the most important controlling processes in the variation of biogenic elements and pollutants. The Bohai Sea is a semi-closed marine area of the NW Paciﬁc Ocean, with a surface area of 77×109 m2 and an average depth of 18.7 m. The area with a depth of less than 30 m constitutes 95% of the total area of the sea (Fig. 2.1, Mao et al., 2008). The water depth of the Bohai Sea is shallow at 10∼20 m

Fig. 2.1. Bathymetry of the Bohai Sea. Boundaries between diﬀerent parts of the Bohai Sea are indicated by solid lines. Isobaths are in meters (Mao et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

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in coastal areas, including Liaodong Bay, Bohai Bay, Laizhou Bay, central Bohai, and the Bohai Strait. The maximal water depth of 70 m is found in the northern part of the Bohai Strait. The rivers discharge into the Bohai Sea with various amounts of water and sediment loads, notably the Liaohe River, Shuangtaizihe River, Luanhe River, and Huanghe River (Yellow River).

2.1 Change Processes of Carbon in the Bohai Sea The continental shelf zones are usually active in biological production as summarized in IGBP and may have a signiﬁcant role in the net absorption of atmospheric CO2 , although it comprises only 7.6% of the surface area of the world oceans. In general, the coastal oceans tend to absorb CO2 in winter, when the water cools, and in spring, as a consequence of biological processes. In summer and fall, the processes of warming, respiration of marine organisms, and decomposition of organic matter release CO2 back into the atmosphere. Finally, direct and indirect human perturbations to the continental margins (e.g., pollution, eutrophication) are large and have dire consequences for marine ecosystems. Unfortunately, owing to the diversity and therefore complexity of the shelf systems, their precise roles in the carbon cycle have yet to be quantiﬁed with any degree of certainty. There is still, in fact, no consensus on the simple question posed by Land-Ocean Interaction in the Coastal Zone project (LOICZ) in its ﬁrst report: “Are continental shelves carbon sources or sinks?” 2.1.1 Partial Pressure of CO2 in Sea Water The surface water CO2 partial pressure (PCO2 ) at grid stations in the Bohai Sea in August, 2006 and in Laizhou Bay in July, 2005, was used to discuss the distribution of the PCO2 (Fig. 2.2, Zhang and Zhang, 2008). 2.1.1.1 Distribution of PCO2 in Surface Waters Fig. 2.3 (Zhang and Zhang, 2008) shows the distribution of PCO2 in the Bohai Sea in summer. The magnitude of PCO2 ranged from 313 to 1,118 μatm, and the mean was 537 μatm. PCO2 values in the central area were a minimum and a maximum of 313 and 621 μatm, respectively, with an average value of 435 μatm. Bohai Bay, Liaodong Bay, and Laizhou Bay all showed a higher level of PCO2 and acted as a source of CO2 . The region wholly acted as a source of CO2 except for the area along the western and central shore of the sea (38.5◦ ∼40◦ N, 119◦ ∼120.5◦ E) and the east of Liaodong Bay (39.9◦ ∼40.1◦ N, 120.7◦ ∼121.2◦ E) which acted as a sink of CO2 and the sum of the sink area accounted for one ﬁfth of the whole area. Estuaries are known for signiﬁcant supersaturation of CO2 with respect to the atmosphere. The value of PCO2 in the Huanghe River Estuary and

2.1 Change Processes of Carbon in the Bohai Sea

141

N 40.5

39.5

38.5

37.5 117.5

118.5

119.5

120.5

121.5

122.5

E

Fig. 2.2. Sampling stations in the Bohai Sea (Zhang and Zhang, 2008) (With permission from Zhang LJ) N 40.5

39.5

38.5

37.5 117.5

118.5

119.5

120.5

121.5

122.5

E

Fig. 2.3. Horizontal distribution of PCO2 (in μatm) in the Bohai Sea in summer (Zhang and Zhang, 2008) (With permission from Zhang LJ)

Laizhou Bay (622∼950 μatm) was much higher than that in other areas and gradually decreased from the estuary to the east, which showed a distinct gradient distribution. The highest value of PCO2 existed near the estuary with the salinity lower than 24‰. The mechanism by which estuarine systems can sustain such high levels of PCO2 remains unclear. Abril et al. (2000) indicated that heterotrophic activity and acidiﬁcation due to nitriﬁcation within the estuarine zone were major factors in the total estuarine emission to the atmosphere, while the excess CO2 transport by rivers followed by ventilation in the estuary was a minor one. Cai and Wang (1998) believed that the combined eﬀects of pelagic and benthic respiration, photodegradation, and the mixing of seawater and acidic river water were insuﬃcient to sustain the high PCO2 values and then high water-to-air ﬂuxes in the estuaries they studied, and they suggested that the CO2 input from organic carbon respiration in tidally ﬂooded salt marshes controlled the CO2 concentration. This explanation is consistent with a subsequent mass balance study of biogenic gases (Cai et al., 1999). Therefore, PCO2 distribution pattern in the estuary results from

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2 Biogeochemical Processes of the Bohai Sea

the combination of various processes: the production/degradation/export of organic carbon, the production/dissolution/export of carbonates, the input of dissolved inorganic carbon by vertical mixing processes and/or freshwater runoﬀ and the thermodynamic eﬀects related to both water temperature variations and water mass mixing (Song, 2004; Song et al., 2008). In general, PCO2 in the Bohai Sea showed an uneven distribution which fully reﬂected the impact of riverine input on Bohai Sea waters although the whole Bohai area is not very large. 2.1.1.2 Relationship between PCO2 and Temperature, Salinity and Chl a Temperature and salinity are two important controlling factors of PCO2 in seawaters. Fig. 2.4 showed that, in the central area, the surface water salinity was between 31.0‰ and 31.6‰, while it was below 30‰ in Laizhou Bay and Liaodong Bay due to the eﬀect of the freshwater discharge. The whole Bohai Sea can be divided into two parts according to the salinity distribution. One part is that with salinity higher than 31‰ (S >31‰) and another part is that with salinity lower than 31‰ (S <31‰). In the S >31‰ area, PCO2 showed a signiﬁcant positive correlation with salinity (R 2 =0.705, n=28; Fig. 2.5a). This may be due to low PCO2 in the midwest area where intense phytoplankton bloom can consume signiﬁcant amounts of dissolved CO2 and high PCO2 in the old Huanghe River subaqueous delta. In the S <31‰ area, PCO2 showed a negative correlation with salinity (R 2 =0.669, n=24; Fig. 2.5b) except the two stations with low water exchange and high silt content at the north of the Huanghe River Estuary. Temperature aﬀects the equilibrium constants of dissolved inorganic carbon and, in particular, the solubility coeﬃcient of CO2 , so that PCO2 rises by about 4% with an increase of every 1 ◦ C in temperature (Borges and Frankignoulle, 2002). In the S >31‰ area, PCO2 all showed positive correlation with temperature both in the sink area (R 2 =0.594, N 40.5

39.5

38.5

37.5 117.5

118.5

119.5

120.5

121.5

122.5

E

Fig. 2.4. Horizontal distribution of salinity (‰) in the Bohai Sea in summer (Zhang and Zhang, 2008) (With permission from Zhang LJ)

2.1 Change Processes of Carbon in the Bohai Sea

143

n=13; Fig. 2.6a) and in the source area (R 2 =0.520, n=17; Fig. 2.6b). The correlation coeﬃcient in the source area was higher than that in the sink area, which indicated that PCO2 in the source area were more inclined to be inﬂuenced by temperature than in the sink area. In the S <31‰ area, similar to the relationship between PCO2 and salinity, PCO2 showed a negative correlation with temperature (R 2 =0.645, n=23; Fig. 2.6c). It is noticeable that PCO2 had negative correlation with both the salinity and the temperature in the S <31‰ area, which was totally diﬀerent from the central area of the Bohai Sea. Such a situation is believed to result from the discharge of the Huanghe River. The Huanghe River is characterized by high silt content, high DIC content and a high PCO2 level. The average silt content is 24.8 kg/m3 , the average DIC content is 36∼44.28 mg/L, and the average PCO2 y =501.89 x 15302 R 2=0.7051

650

y = 72.076 x +2686.2 R 2=0.6694 1100

P CO ( m atm)

450

2

2

P CO ( m atm)

550

350

900 700 500

(a) 250 31.0

(b) 300

31.2

31.4 S( )

31.6

20

24

28 S (

32

36

)

Fig. 2.5. Relationship between PCO2 and salinity in the Bohai Sea. (a) Salinity>31‰; (b) Salinity<31‰ (Zhang and Zhang, 2008) (With permission from Zhang LJ)

390

y =47.12 x 670.19 R 2 =0.5202

y =10.271 x +112.72 R 2=0.5942

y = 53.508 x +2096.7 R 2=0.6446

2

500

2

350

1100

330

P CO ( m atm)

P CO ( m atm)

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P CO ( m atm)

600 370

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Z12

900 700 500

Z21 310 22

23 25 24 Temperature ( ) (a)

26

300 20 22 24 26 28 30 Temperature ( ) (b)

300 18

26 22 Temperature ( (c)

30 )

Fig. 2.6. Relationship between PCO2 and temperature in the Bohai Sea. (a) Salinity>31‰, sink area; (b) Salinity>31‰, source area; (c) Salinity<31‰ (Zhang and Zhang, 2008) (With permission from Zhang LJ)

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level is 1,342.0 μatm in the Huanghe River. The freshwater discharged into the Bohai Sea ﬂows into Laizhou Bay and is mixed with the seawaters in an easterly direction under the action of the circulation. With the mixing of freshwater and seawater, salinity increased and PCO2 decreased. In the S >31‰ area, PCO2 showed a signiﬁcant negative correlation with salinity except for a maximum of Chl a in station D2 (R 2 =0.738, n=31; Fig. 2.7). In the sink area, the content of Chl a was high, with a maximum of 2.4 μg/L, and the transparency was higher than the average level for the Bohai Sea (4 m). In this situation, the photosynthesis of phytoplankton is strong and can adsorb more CO2 in surface water. y = 182.95 x +800.88 R 2=0.738

700

500

2

P CO ( m atm)

600

400 300 200 1

2

3

Chl a (mg/m 3)

Fig. 2.7. Relationship between PCO2 and Chl a in the Bohai Sea in summer (Zhang and Zhang, 2008) (With permission from Zhang LJ)

2.1.2 Riverine Sources and Estuarine Fates of Particulate Organic Carbon in Seawaters The scientiﬁc database of organic carbon regimes for large world rivers has been greatly enhanced in the last 10∼20 years. Indeed, the organic materials transported by large rivers can exert a signiﬁcant impact on the carbon cycle and ecosystems in coastal and shelf regions. The organic material carried by small- and middle-size rivers, however, may aﬀect the marine ecosystems and environment on a more regional scale. The Luanhe River and Shuangtaizihe River are two major freshwater sources from North China draining into the Bohai Sea. The long-term averaged water discharge, sediment load, and the general features of these rivers and estuaries are summarized in Table 2.1 (Zhang et al., 1998a). Zhang et al. (1998a) reported the following POC results in suspended particulates from the Luanhe River Estuary and Shuangtaizihe River Estuary.

2.1 Change Processes of Carbon in the Bohai Sea

145

Table 2.1. Drainage area, water discharge, sediment load and description of estuarine features for the Luanhe River and Shuangtaizihe River (Zhang et al., 1998a) (With permission from Elsevier’s Copyright Clearance Center) Water Sediment Drainage Estuarine features discharge load area (km2 ) (×109 m3 /yr) (×106 t/yr) Luanhe 54,412 6 20 The estuary has several shoals and sand-bars. The tidal range is 1∼2 m with currents of 0.5∼1 m/s. The tide is limited to 5∼10 km inland from the river mouth Shuang- 57,104 5.1 9.1 The estuary consists of two taizihe branches that join at the upper estuary. The tidal range is 3∼4 m on average, with currents of 1.0∼2.0 m/s. The area inﬂuenced by the tides is limited to 30∼40 km inland from the river mouth River

2.1.2.1 Particulate Organic Carbon In the Luanhe River Estuary, the organic carbon in suspended sediments shows an overall decrease with increasing chlorinity, indicating a rather simple dilution of riverine POC (1.3%∼2.4%) by local organic poor particle populations, although the data show considerable scatter (Fig. 2.8, Zhang et al., 1998a). A similar distribution has been found for the absolute concentration of POC (mg/L), which reduces from ∼2 mg/L in the river to 0.5 mg/L in the lower estuary (Fig. 2.8). It is diﬃcult, however, to determine the real marine POC end-members since the sampling was ended at a chlorinity of 14‰∼15‰. Clearly, the POC levels in weight percentage in the Shuangtaizihe River Estuary remain low and relatively stable, varying between 1.1% and 1.6% over the whole salinity range sampled, except for one sample collected upstream of the Raoyanghe River which has a POC value of 6.7% indicating a diﬀerent freshwater organic pool with low total suspended matter of 50 mg/L before joining the main stream (Fig. 2.9, Zhang et al., 1998a). A plot of absolute POC concentration in solution (i.e., mg/L) against chlorinity indicates a rapid removal of particulate organic materials at early stages of mixing between fresh and marine waters (Fig. 2.9). However, a value of 5∼10 mg/L for POC is typical in the lower estuary with chlorinity 10‰∼15‰, corresponding to a concentration of suspended sediments well above 100 mg/L. The absolute POC concentration (i.e., mg/L) in the Shuangtaizihe River Estuary can be up to a factor of 100 higher than in the Luanhe River Estuary (comparing Fig. 2.9 with Fig. 2.8).

146

2 Biogeochemical Processes of the Bohai Sea 2.5

3 (a)

POC (mg/L)

2 1.5 1 0.5

0

3 2.5

POC (%)

(b)

5 10 Chlorinity ( )

1.5 1 0.5

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60 80 100 120 140 160 Suspended sediment (mg/L)

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2.5

15

(d)

2 1.5 1 0.5

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60 80 100 120 140 Suspended sediment (mg/L)

160

Fig. 2.8. Distribution of POC in the Luanhe River Estuary. (a) POC (%) vs chlorinity; (b) POC (mg/L) vs chlorinity; (c) POC (%) vs turbidity; (d) POC (mg/L) vs turbidity (Zhang et al., 1998a) (With permission from Elsevier’s Copyright Clearance Center)

Fig. 2.9. Distribution of POC in the Shuangtaizihe River Estuary. (a) POC (%) vs chlorinity; (b) POC (mg/L) vs chlorinity; (c) POC (%) vs turbidity; (d) POC (mg/L) vs turbidity (Zhang et al., 1998a) (With permission from Elsevier’s Copyright Clearance Center)

2.1 Change Processes of Carbon in the Bohai Sea

147

2.1.2.2 Relationship between POC and Suspended Matter The plots of POC in weight-percent of suspended matter vs total suspended matter concentrations in the estuaries demonstrate a general reverse relationship between these two parameters, at least at the low end of the turbidity range (Figs. 2.8, 2.9). However, the absolute concentrations of POC (mg/L) in these two estuaries show a strongly positive relationship with suspended matter loads (Figs. 2.8, 2.9). The average POC to suspended sediment ratio is 0.013 for the Shuangtaizihe River Estuary and 0.015 for the Luanhe River Estuary. The correlation coeﬃcient (R2 ) for the relationship between POC and total suspended matter (TSM) is 0.99 for the Shuangtaizihe River and 0.60 for the Luanhe River, which again indicates that POC distribution is regulated by the concentration of suspended sediments in the Bohai Sea estuaries. 2.1.2.3 Source of POC in the Estuary The POC can be reduced to 1.5%∼2.5% or lower while the suspended sediment concentrations exceed 50∼100 mg/L (Figs. 2.8 and 2.9). A value of 1.5%∼2.0% is typical of organic carbon concentrations in soils (1.3%∼1.8%) from drainage areas of these rivers. We hypothesize, however, that in low turbidity waters, photosynthesis can be an important contributor to the observed POC. The datasets obtained show that at a concentration greater than 50∼100 mg/L for suspended sediments, photosynthesis is probably strongly reduced, and the POC in the river is hence regulated by the organic materials supplied from soil erosion, with in situ primary production being in a state of radiation limitation (Figs. 2.8 and 2.9). While photosynthetic carbon may play a role in regulating POC in the estuary, there are still no existing datasets or other evidence to examine this hypothesis. The positive POC-TSM relationship in these estuaries suggests presumably a common terrigenous source for most of the POC. Further oﬀshore, the POC value of ∼2 mg/L indicates a rather important input from the marine pool. The distribution of POC clearly illustrates distinctive processes and events taking place in both estuaries, with a simple dilution of the terrigenous organic pool by organic poor particles in the Luanhe River and a rather stable POC level irrespective of chlorinity in the Shuangtaizihe River. In both estuaries, the POC remains low and somewhat stable when the amount of suspended sediments is superior to 100 mg/L, presumably indicating a radiation limitation for in situ photosynthesis at these turbidity levels. 2.1.3 Inorganic Carbon in Liaodong Bay Sediments of the Bohai Sea The ocean is evidently a major sink of carbon dioxide and plays an important role in the global carbon cycle. However, the carbon ﬂux between seawater and sediment in the coastal seas is still poorly understood (Song, 2003; Li

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2 Biogeochemical Processes of the Bohai Sea

et al., 2004a). Knowledge of the carbonate mineral dissolution in sediment during the processes of diagenesis, lithiﬁcation, and evolution is of central importance to develop an insight into the carbon ﬂux (Morse and Arvidson, 2002). The pattern of calcium carbonate accumulation rates can be used to decipher the Pliocene-Pleistocene history of biogenic production and its relationship with global and local changes in oceanic circulation and climate (Beek et al., 2004). Calcium carbonate dissolution or precipitation is controlled mainly by the bottom water (or pore water) saturation state, sediment pH, and metabolic release of carbon dioxide. Moreover, salinity also aﬀects the chemical reactions that occur in sediment, like precipitation/dissolution of CaCO3 . Inorganic carbon (IC) concentrations in sediment vary little, or increase with depth due to the dissolution near the water-sediment interface. The direct tracer of CaCO3 dissolution is the increase in the calcium concentration of the pore water below the sediment-water interface. Most studies focused on the total inorganic carbon in sediment, few on IC forms. In fact, there are many kinds of carbonate mineral in sediment, such as aragonite, siderite, calamine, cerusite, phosgenite, magensite, and dialogite (Preda and Cox, 2004) with diﬀerent dissolvabilities under diﬀerent pH solutions; for example, calamine can be dissolved in NH3 ·H2 O cerusite, phosgenite can be dissolved in NaOH, and calcite and aragonite can be dissolved in acid. To understand the burial and diagenesis of inorganic carbon in marine sediments, it is necessary to identify, separate, and quantify the various solid-phase reservoirs of deposited carbon, but it is very diﬃcult because of the ﬁne-grained nature of most marine sediments. So it is necessary to ﬁnd an indirect means to determine the identity and size of sedimentary IC reservoirs. One approach is physical separation of diﬀerent sedimentary fractions by grain size, and measurement of total inorganic carbon (TIC) in the diﬀerent fraction after each has been separately dissolved (Yang et al., 2002). However, this method can easily lead to ambiguous or incorrect identiﬁcation of the C-bearing phase. Complete physical separation of diﬀerent phases from ﬁne-grained sediment rarely can be achieved and surface coating of various sorts, potentially important in IC removal to sediments, can remain undetected and unidentiﬁed in such treatment. The most promising methods for separating and quantifying the various IC reservoirs in marine sediment are by sequential extraction. So according to IC characters, IC in sediment is divided into ﬁve forms: NaCl form, NH3 ·H2 O form, NaOH form, NH3 OH·HCl form, and HCl form (Fig. 2.10, Li et al., 2004b). 2.1.3.1 Inorganic Carbon Forms in the Liaodong Bay Sediments Liaodong Bay (Fig. 2.11) is located in the northeastern part of the Bohai Sea (longitude 119◦ 50 00 ∼121◦ 08 01 E, latitude 38◦ 20 00 ∼39◦ 59 05 N). More than 10 rivers enter the bay, and a mass of sand inputs the bay every year from these rivers. The sediments in the eastern bay mainly consist of sand and muddy sand. But in the central and southern bay, sediments are sandy mud

2.1 Change Processes of Carbon in the Bohai Sea Step I-A I-B

Residue

II-A

Residue

Residue

III-B IV Residue

H2O wash 10 min, 25

NaCl IC

NH3 H2O (0.1 mol/L) 2 h, 25 H2O wash Residue 10 min, 25

II-B III-A

Extracted

Extractant NaCl (1 mol/L) 1.0 g sediment 2 h, 25

NH3 H2O IC

NaOH (0.1 mol/L) 2 h, 25 H2O wash Residue 10 min, 25

NaOH IC

NH2OH HCl (0.2 mol/L)

NH2OH HCl IC

1 h, 25 HCl (1+1) 1 h, 25

V Residue

149

HCl IC

Fig. 2.10. Scheme of the sequential extraction method for diﬀerent forms of IC in marine sediment (Li et al., 2004b) N 41

GS1 GS1 GS3 GS5 Liaodong Bay

40.5 GS3

40 39.5

GS5

39 Bohai Sea

38.5 38 37.5

Yellow River

118

119

120

121

122

E

Fig. 2.11. Location of the sampling stations

and sandy clay, part of which comes from the Yellow River. Three sediment cores in Liaodong Bay were collected with a gravity corer in December, 2001 (Fig. 2.11). The cores were successively cut into 3.0 cm thick slices from surface to bottom for DIC measurement. The contents of diﬀerent IC forms and their vertical distributions in core sediments of Liaodong Bay were shown in Fig. 2.12. The contents of the NaCl form were similar in 3 core samples with a range of 0.16∼0.23 mg/g, and its vertical distribution maintained stability from surface to sea bottom. The contents of the NH3 ·H2 O form ranged from 0.16 to 1.96 mg/g, and the

2 Biogeochemical Processes of the Bohai Sea Content (mg/g) 3 1 2

40 80

120 160

4

0

0

Content (mg/g) 3 1 2

40 80 120 160

GS1

4

Depth (cm)

Depth (cm)

0

0

Depth (cm)

150

200

0

Content (mg/g) 3 1 2 4

5

0 40 80 120 160

GS3

200

GS5

Fig. 2.12. Vertical proﬁles of diﬀerent inorganic carbon forms in Liaodong Bay sediments. NaCl form, ; NH3 ·H2 O form, ; NaOH form, ; NH2 OH·HCl form, ×; HCl form, *; TIC, ◦

vertical distribution was basically consistent in 3 stations with prominent ﬂuctuation in upper layers (above 66 cm at station GS1, 99 cm at station GS3, 36 cm at station GS5), but stable in the under layers. The content of the NaOH form was slightly higher than that of the NaCl form with a range of 0.19∼0.43 mg/g. Its vertical distribution was simple with a slight decrease from surface to sea bottom. The contents of the NH2 OH·HCl form, and HCl form were similar with a range of 0.36∼1.70 mg/g. Their vertical distributions were similar too, but the ﬂuctuation of the HCl form was bigger than that of the NH2 OH·HCl form. Compared to regions of the Yangtze River Estuary and Jiaozhou Bay (Table 2.2), the content of TIC in Liaodong Bay was lower because its carbonate was diluted by a large amount of matter from the Liaohe River and other surrounding rivers. So the contents of every IC form were lower accordingly. However, the percentage of every IC form was similar to that in the Yangtze River Estuary and Jiaozhou Bay except for the NH3 ·H2 O form. The percentage of the NaCl form and NaOH form was similar, which accounted for a minority of the total inorganic carbon. The percentage of the Table 2.2. The characteristics of diﬀerent IC forms in Liaodong Bay, Jiaozhou Bay and Yangtze River Estuary sediments Liaodong Bay Jiaozhou Bay∗ Yangtze River Estuary∗ Content Ratio Content Ratio Content Ratio (mg/g) (%)†† (mg/g) (%)†† (mg/g) (%)†† NaCl form 0.16∼0.23 4.49∼10.58 0.29∼0.54 1.6∼14.1 0.064∼0.518 0.86∼4.86 0.16∼1.96 10.47∼46.68 0.17∼0.56 1.25∼15.6 0.17∼0.79 2.33∼7.38 NH3 ·H2 O form NaOH form 0.19∼0.43 5.68∼19.41 0.29∼0.68 1.8∼17.8 0.20∼1.39 2.41∼14.19 NH2 OH·HCl form 0.36∼1.35 17.21∼39.74 1.03∼11.1 9.9∼82.4 1.30∼5.86 20.65∼49.59 HCl form 0.46∼1.70 18.67∼42.39 0.59∼21.1 6.0∼86.6 2.93∼7.58 35.80∼69.86 TIC 1.62∼4.24 3.03∼24.7 4.99∼13.72 ∗ The data of Jiaozhou Bay and Yangtze River Estuary are from Li et al. (2004a); †† The ratio of one form of IC to TIC in every sample IC form

2.1 Change Processes of Carbon in the Bohai Sea

151

NH3 ·H2 O form, NH2 OH·HCl form, and HCl form was similar; they accounted for most of the total inorganic carbon. 2.1.3.2 Relationship between Organic Carbon and Inorganic Carbon Organic matter can be oxidized to CO2 by O2 and NO− 3 in upper layers of sediment, and results in a decrease in pH, and this then promotes CaCO3 dissolution. However, in an environment where O2 is lacking, the organic matter will be oxidized by Fe, Mn oxide, and sulfate, and then will increase in alkalinity and result in CaCO3 precipitation. Therefore, the eﬀect of organic matter diagenesis on inorganic carbon is more complicated and presents diﬀerent characteristics in diﬀerent regions. The study of Li et al. (2000) showed that lower CaCO3 concentrations corresponded exactly to the higher contents of total organic carbon (TOC), which suggested that the higher TOC contents may have played an important role in CaCO3 dissolution. The higher proportions of organic matter in sediments increased the accumulation of higher concentrations of CO2 in interstitial waters through bacterial degeneration, thus enhancing the dissolution of carbonate (Yuan et al., 2004). The molar ratio of organic and calcium carbonate in particulate material that reaches the seaﬂoor can aﬀect carbonate preservation. A higher content of carbonate in sediment will promote carbonate preservation, while higher content of organic matter and the release of CO2 due to organic carbon oxidation will lead to enhanced carbonate dissolution. When the ratio of the molar Corg /CaCO3 in sediment exceeded a maximum value of approximately 1.25, the organic matter oxidation may promote 100% CaCO3 dissolution in theory (Pfeifer et al., 2002). The content of organic carbon (OC) in Liaodong Bay sediments is shown in Table 2.3, which indicated that the discrepancy is small in diﬀerent stations. The ratios of OC and diﬀerent IC forms in Liaodong Bay sediments are very small (Table 2.4), which is predicted to promote 100% CaCO3 dissolution in theory. The ratios of the same IC form were similar at diﬀerent stations, especially at station GS1 and station GS3, which showed that the inﬂuence of OC on carbonate dissolution is similar over the whole region. Table 2.3. The content of organic carbon in Liaodong Bay sediments (%) Station GS1 GS3 GS5

Min 0.57 0.56 0.39

Max 0.86 0.73 0.77

Average 0.69 0.64 0.59

Although much of the literature reported that organic carbon oxidation will promote carbonate dissolution, we do not know which part of carbonate will be dissolved because we cannot separate carbonate individually from their

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2 Biogeochemical Processes of the Bohai Sea

Table 2.4. The ratios of organic carbon (OC) and every IC form in Liaodong Bay sediments Sample

Depth (cm)

NaCl/OC

NH3 ·H2 O/OC

NaOH/OC

NH2 OH·HCl/OC

HCl/OC

GS5

0 6 12 24 36 66 126 186 0 6 12 24 42 69 129 189 0 6 12 24 36 66 126

0.023 0.033 0.027 0.028 0.031 0.044 0.031 0.038 0.032 0.035 0.030 0.028 0.034 0.030 0.031 0.032 0.029 0.035 0.027 0.027 0.033 0.020 0.024

0.10 0.14 0.11 0.27 0.20 0.29 0.18 0.22 0.13 0.15 0.14 0.12 0.18 0.14 0.05 0.05 0.12 0.13 0.22 0.13 0.09 0.02 0.02

0.02 0.07 0.06 0.05 0.07 0.08 0.04 0.04 0.05 0.07 0.06 0.05 0.05 0.04 0.05 0.06 0.06 0.06 0.05 0.06 0.06 0.03 0.02

0.10 0.18 0.16 0.11 0.20 0.35 0.10 0.12 0.14 0.13 0.15 0.13 0.12 0.10 0.14 0.08 0.09 0.11 0.09 0.06 0.09 0.04 0.14

0.13 0.31 0.16 0.14 0.17 0.18 0.12 0.14 0.21 0.10 0.12 0.13 0.11 0.11 0.18 0.09 0.18 0.16 0.09 0.06 0.11 0.08 0.14

GS3

GS1

minerals on acount of their ﬁne grain. However, in Liaodong Bay sediments, only the correlation between organic carbon and the NH2 OH·HCl form of IC is signiﬁcant at the 0.05 level and the correlation is negative (Table 2.5), which showed that higher organic carbon will promote the NH2 OH·HCl form of IC dissolution ﬁrst. Table 2.5. Correlation coeﬃcients between iron, OC, and every IC form in Liaodong Bay sediments (n=23) Fe2+ Fe3+ OC

NaCl form 0.190 0.288 0.207

NH3 ·H2 O form 0.113 0.251 −0.095

NaOH form 0.430∗ 0.203 0.053

* Correlation is signiﬁcant at the 0.05 level (2-tailed)

NH2 OH·HCl form 0.041 −0.256 −0.383∗

HCl form 0.484∗ −0.324 −0.052

2.1 Change Processes of Carbon in the Bohai Sea

153

2.1.3.3 Relationship Between Fe and Inorganic Carbon Iron in sediments has been documented in much literature and has become recognized as an important constituent of the global carbon cycle (Cooper et al., 2005). The role that iron plays in biogeochemical cycles depends greatly on its redox species of iron (II) and iron (III). Their redox transformations at the oxic-anoxic boundary aﬀect the distribution and cycle of carbon, sulfur, phosphorus, and other elements in sediments. Under well oxygenated condition Fe(III) is the stable oxidation state, and at neutral pH it forms highly insoluble oxides and hydroxides. Ferrous Fe is stable in an anoxic condition, and in the presence of high carbonate, sulﬁde, and orthophosphate concentrations it forms insoluble salts. In Liaodong Bay sediments, because the main redox environment characteristic is a relatively weak oxidation (Li et al., 2004b), the ratios of Fe3+ :Fe2+ were bigger than 1. Recent studies have shown that Fe(III) reduction can also occur in oxygenated, high pH, although this usually results in much lower steady state Fe(II) concentrations (Shaked et al., 2002). These redox reactions should have an impact on organic carbon reduction and carbonate dissolution or precipitation. In particular, the labile IC form may be aﬀected strongly. Fe inﬂuence on inorganic carbon is embodied two ways. On the one hand, Fe3+ can oxidate organic matter into CO2 , and then cause carbonate dissolution or precipitation. Model application revealed that 3% of organic matter mineralization was due to Fe(OH)3 (Pfeifer et al., 2002). On the other hand, Fe3+ and Fe2+ can combine CO2− 3 ions and form carbonate. According to correlation between iron and IC (Table 2.5), diﬀerent coeﬃcients with diﬀerent IC forms indicated that iron has diﬀerent inﬂuences on diﬀerent forms of IC in Liaodong Bay sediments. If iron has an eﬀect on IC, the eﬀect will drive the same IC form change as a steady mode. So the ratio of IC to iron will change regularly. The ratios of diﬀerent IC forms to iron in Liaodong Bay sediments were shown in Fig. 2.13 and Fig. 2.14 (Niu et al., 2006). Their vertical distributions had obvious regularities. In general, the ratios of NaCl form, NH3 ·H2 O form, and NaOH form to Fe3+ had a clear trend of increasing with depth, especially in upper sediment layers. But the trends of the NH2 OH·HCl form and HCl form were not clear. So we thought the eﬀects of Fe3+ on the NaCl form, NH3 ·H2 O form, and NaOH form were stronger than those on the NH2 OH·HCl form and HCl form, and the eﬀects in upper sediment layers were greater than in lower sediment layers. The vertical distributions of ratios of diﬀerent IC forms to Fe2+ were more complicated than those of Fe3+ . The distribution of the ratios of the NaCl form to Fe2+ was similar to that of the NaOH form, with more prominent characteristics than other IC forms. They decreased with depth in upper sediment layers (above 40 cm at stations GS3 and GS5, 12 cm at station GS1), then increased slightly below the depth of sediment at stations GS3 and GS5 or increased between the depths of 12∼36 cm and decreased below the depth of 36 cm at station GS1. The vertical distribution of the ratios of the NH3 ·H2 O form at station GS1 was similar to that at station GS3 but was the reverse of that at station

154

2 Biogeochemical Processes of the Bohai Sea

GS5. The vertical distributions of the ratios of the NH2 OH·HCl form and HCl form were diﬀerent at the three stations. So we reached a similar conclusion to Fe3+ that the eﬀect of Fe2+ on the NaCl form, NH3 ·H2 O form, and NaOH form was stronger than that on the NH2 OH·HCl form and HCl form. Based on Li et al. (2005)’s studies, the NaCl form, NH3 ·H2 O form, NaOH form, and NH2 OH·HCl form were relatively labile IC, and their activities decreased from the NaCl form to NH2 OH·HCl form. The HCl form of IC was the steadiest. So we can conclude that the eﬀect of iron on the labile IC form was stronger than on the stable IC form.

1.000

40 80 120 160

0

0.200

Ratios 0.600

1.000

0

40

Depth (cm)

Ratios 0.600

Depth (cm)

Depth (cm)

0

0.200

80 120 160

GS1

Ratios 0.500 1.000

1.500

40 80 120 160

GS3

200

GS5

200

Fig. 2.13. The ratios of every IC form and Fe in Liaodong Bay sediments. NaCl form, ; NH3 ·H2 O form, ; NaOH form, ; NH2 OH·HCl form, ×; HCl form, * (Niu et al., 2006) 3+

80 120 160

40 80 120

200

Ratios 2.000

3.000

80

120

160 GS1

1.000

40

Depth (cm)

40

Depth (cm)

Depth (cm)

Ratios Ratios 0.000 0.200 0.400 0.600 0.800 0.000 0.500 1.000 1.500 2.000 0.000 0 0 0

160 GS3

200

GS5

Fig. 2.14. The ratios of every IC form and Fe2+ in Liaodong Bay sediments. NaCl form, ; NH3 ·H2 O form, ; NaOH form, ; NH2 OH·HCl form, ×; HCl form, * (Niu et al., 2006)

2.1.3.4 Assessment of Inﬂuencing Factors The concentration of inorganic carbon in marine sediments is mainly controlled by the following factors: (1) The supply speed of autogenetic CaCO3 by

2.1 Change Processes of Carbon in the Bohai Sea

155

the ocean itself. (2) The supply speed of terrestrial matter. Terrestrial matter controls the content of CaCO3 in sediments. Under the condition of a steady supply of carbonate, the inputs of terrestrial matter will dilute the content of carbonate. The more the inputs of terrestrial matter, the lower the concention of carbonate. (3) Carbonate dissolution or precipitation in the early diagenetic process (Wu et al., 2001). In Liaodong Bay, the primary production was high and a mass of biogenic CaCO3 was produced (Wang and Gao, 2002), but it accepted a large amount of terrestrial matter from the Liaohe River and other surrounding rivers. So the carbonate concentration in Liaodong Bay sediments was lower than in other regions in the Bohai Sea (Yang et al., 1989). Except for the inﬂuence of the above-mentioned factors, all forms of IC in sediments were inﬂuenced by the sedimentary environment, such as pH, Eh, Es, water content, organic carbon, Fe3+ /Fe2+ , and so on. However, their inﬂuence was inconsistent and represented diﬀerent characteristics in diﬀerent environments (Li et al., 2005). Besides the eﬀect of organic carbon and iron discussed above, pH, Eh, Es, and water content may inﬂuence the change of IC in sediments, but it is diﬃcult to estimate the relative strength of their eﬀect on IC. Cluster analysis encompasses a number of diﬀerent algorithms and methods for grouping objects of similar kinds into respective categories. In other words, cluster analysis is an exploratory data analysis tool aimed at sorting diﬀerent objects into groups in a way that the degree of association between two objects is maximal if they belong to the same group, and minimal otherwise. So cluster analysis can be used to discover structures in data without providing an explanation/interpretation, and may reveal deeper associations in data which, though not previously evident, nevertheless are sensible and useful once found. In this part, the aim of using cluster analysis is to sort all inﬂuencing factors into groups, and reveal the association between inﬂuencing factors and diﬀerent IC forms. Fig. 2.15 showed the results of cluster analysis for all forms of IC and their inﬂuencing factors, which could indicate which had close relationships with which IC form. On the whole, all the IC forms and their inﬂuencing factors could be divided into two groups: pH group and iron group. The iron group could be subdivided into Fe3+ group and Fe2+ group. The pH group included the NH2 OH·HCl form, HCl form, and pH, which indicated that the NH2 OH·HCl form and HCl form of IC were inﬂuenced directly by pH and inﬂuenced indirectly by other factors. The Fe3+ group included Fe3+ , Es, and OC, which indicated that the deoxidization of Fe3+ has close relationships with the oxidation of S2− and organic carbon. They had similar eﬀects on all IC forms. The Fe2+ group included the NaCl form, NaOH form, and NH3 ·H2 O form of IC, inﬂuencing factors of Fe2+ , Eh, and water content, which indicated that Fe2+ , Eh, and water content had close relationships with the NaCl form, NaOH form, and NH3 ·H2 O form of IC. Their eﬀect on the NH3 ·H2 O form was stronger than on the NaCl form and NaOH form, and the Fe2+ relationship with the NH3 ·H2 O form of IC is weaker than that with Eh and water content. Comparing the inﬂuence of Fe3+ and Fe2+ on diﬀerent IC forms, Fe3+ and Fe2+ had a stronger eﬀect on the NaCl form, NaOH form, and

156

2 Biogeochemical Processes of the Bohai Sea 0

Rescaled distance cluster combination 5 10 15 20 25

NH3 H2O form Eh Water content Fe2+ NaCl form NaOH form Fe3+ Es NH2OH HCl form OC HCl form pH

Fig. 2.15. The result of cluster analysis for IC inﬂuencing factors in Liaodong Bay sediments (Dendrogram using average linkage between groups, Pearson correlation)

NH3 ·H2 O form than on the NH2 OH·HCl form and HCl form, and Fe2+ had a stronger eﬀect on the NaCl form, NaOH form, and NH3 ·H2 O form than Fe3+ . The inorganic carbon in Liaodong Bay sediments was divided into 5 forms: NaCl form, NH3 ·H2 O form, NaOH form, NH2 OH·HCl form, and HCl form. The contents of the NaCl form and NaOH form were similar and occupied the minority of TIC. However, the NH3 ·H2 O form, NH2 OH·HCl form, and HCl form were the principal forms of TIC and accounted for more than 80% of TIC. In particular, the percentage of the NH3 ·H2 O form was much higher than that in the Changjiang River Estuary and Jiaozhou Bay sediments. The concentration of Fe3+ was higher than Fe2+ and Fe3+ /Fe2+ ratios showed that the main redox environmental characteristic in this region was relatively weak oxidation. Iron had little eﬀect on the NH2 OH·HCl form and HCl form of IC which was inﬂuenced mainly by pH. However, iron had a stronger inﬂuence on the NaCl form, NaOH form, and NH3 ·H2 O form of IC; the inﬂuence of Fe2+ was higher than Fe3+ and its eﬀect on the NH3 ·H2 O form was stronger than on the NaCl form and NaOH form. In a word, the redox action of iron aﬀected mainly the labile IC form. 2.1.4 Biogeochemical Process of Organic Carbon in Sediments Organic matter enters the world’s oceans from two primary sources: marine primary productivity and terrestrial river runoﬀ. Organic material is produced in marine surface waters by the photosynthetic processes of phytoplankton. This primary production is grazed upon by herbivorous zooplankton which releases biochemical compounds and excreta to the water column. As the detritus sinks through the water it is subject to breakdown, or remineralization, by bacterial activity releasing dissolved nutrients into the water. The original organic material produced in the surface waters is repeatedly recycled and sinks in particulate form to the sea bottom. The amount of organic material

2.1 Change Processes of Carbon in the Bohai Sea

157

that reaches the sediment is proportional to the depth of the water column and the amount of primary production in surface waters. The percentage of organic matter that makes it to the sediments is generally <10% depending on the depositional conditions (Beazley, 2003). The organic material that is transported to the oceans by rivers is a complex mixture of humic substances derived from plant and animal detritus. Much of this material consists of nonlabile lignin structures which can be deposited on river deltas and fans. Due to high sedimentation rates, deposited organic material may have insuﬃcient time to be completely oxidized under aerobic conditions before being buried by continuing sedimentation (Beazley, 2003). Once organic matter reaches the sediment, whether from marine or terrestrial sources, it will be exposed to degradation processes and can be completely respired back to CO2 , transformed to by-products, or preserved in the sediment. Several factors aﬀect the preservation of organic material in marine sediments including, but not limited to, bottom-water oxygen levels (Canﬁeld, 1994), organic matter origin (Hedges et al., 1988), water column depth (Suess, 1980), geopolymerization (Berner, 1980), microbial dynamics (Lee, 1992), and adsorption to mineral surfaces (Beazley, 2003). Six sediment cores in Bohai Bay (A), Huanghe River Estuary (B), and Liaodong Bay (C) were collected with a gravity corer in December, 2001 (Fig. 2.16). The cores were successively cut into 3.0 cm thick slices from surface to bottom for OC, Fe3+ , and Fe2+ content measurement.

Fig. 2.16. Location of the sampling stations

2.1.4.1 Vertical Distribution of OC in Bohai Sea Sediments The vertical distribution of OC in Bohai sea sediments is shown in Fig. 2.17. The core sediments can be divided into three parts: surfacial/subsurfacial

158

2 Biogeochemical Processes of the Bohai Sea

(0∼15 cm), middle (15∼72 cm), and bottom (under 72 cm) sediments. OC in sites B1, B2, and C2 showed a slight increase with core depth in the surfacial/subsurfacial sediments, but in sites C1, C3, and A it showed a slight decrease. The high values of OC almost exist in the surfacial/subsurfacial sediments, which prove that the OC is mainly produced by the biochemical process (including detritus and metabolite of organisms, transfers to the bottom by biological pump and physical and chemical accumulation) and by the new delivery of terrestrial input which has not decomposed in time in the sedimentation process. Bacteria are mainly responsible for OC mineralization in surfacial/subsurfacial sediments; meio- and macrofauna aﬀect OC degradation both directly, through feeding on it, and indirectly through bioturbation. Bioturbation can foster sediment irrigation, resulting in the increase in aerobic metabolism and organic matter decomposition rates. Moreover, it has been argued that grazing of bacteria by meiofauna has enhanced the rate of OC decomposition (Accornero et al., 2003). In the middle and bottom sediments, OC has a decreasing trend with sediment depth except at sites A and C3. In this layer OC is degraded by microorganisms so that the sedimentation environment changes from a weak oxidative state to a relatively reductive state. That is proved by the decrease in Eh and Es. OC (%) 0.50 0.70

OC (%) 0.30 0.50 0.70 0.90 0

0.90

150 200 250 300

B2

150

OC (%) 0.50 0.70 0.90

Depth (cm)

100

0.70

50 100

C2 200

0.50

C1

200

0.90

OC (%) 0.30 0.50 0.70 0.90 0 50 100 150

150

150

100

150 OC (%)

0.30 0

50

B1

250

50

Depth (cm)

100

200

350 0.30 0

Depth (cm)

50

100

Depth (cm)

Depth (cm)

50

OC (%) 0.30 0.50 0.70 0.90 0

Depth (cm)

0.30 0

C3

200

A

Fig. 2.17. Vertical proﬁles of OC in the Bohai Sea sediments. A: Bohai Bay; B: Huanghe River Estuary; C: Liaodong Bay

2.1 Change Processes of Carbon in the Bohai Sea

159

OC (%)

Fig. 2.18 showed the diﬀerence of OC in A, B, and C areas. In the surfacial/subsurfacial layer, OC in B area is lower than in A and C areas. The terrestrial input in B area was mainly from the Huanghe River which is characterized by high silt content and a high sedimentary rate. The organic matter was deposited with lots of silts which led to the low OC contents in the B area. In addition, OC contents were similar in 6∼9 cm, 24∼27 cm, 66∼72 cm, 126∼132 cm, and 189∼192 cm in B and C areas, which indicated a similar sedimentary environment in B and C areas. 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00

B Area C Area A Area

3

9

15

27 45 72 132 192 Depth (cm)

Fig. 2.18. Diﬀerence in average OC contents in the three areas with the same depth. A: Bohai Bay; B: Huanghe River Estuary; C: Liaodong Bay

2.1.4.2 Section Distribution in C Area Because the three stations in C area are located at one section in an NE-SW direction, studying the section distribution (Fig. 2.19) could help to understand the source and mineralization of OC. OC in 0∼20 cm varied between

0.83 0.79 0.75 0.71 0.67 0.63 0.59 0.55 0.51 0.47 0.43 0.39 0.35

Depth (cm)

40 60 80 100 120 140 160

OC contents (%)

0 20

180 C3

C2 Station

C1

Fig. 2.19. Section distribution of OC in sediments of C area. C: Liaodong Bay

160

2 Biogeochemical Processes of the Bohai Sea

0.55% and 0.75% and was present in laminated distribution, which indicated that OC originated from the sedimentation in seawater. Below 25 cm, OC was present in regular gradient horizontal distribution and decreased clearly from C1 to C3. It is possible that a ﬂood happened in a corresponding deposition time. Freshwater with a great quantity of OC ﬂowed into this area and led to the accumulation of OC in this direction. 2.1.4.3 Correlative Factors of OC and OC Decomposition Dynamics (1) pH From the measured results (Table 2.6), it can be seen that pH shows an increasing trend with deposition depth. The process of mineralization is shown: the degradation of OC makes the sedimentation environment more reductive so that acidity decreases and pH increases. However, the distribution of OC in surfacial/subsurfacial sediments is complicated, and pH shows an obviously increasing trend. Therefore, OC is related to not only pH but also the biomass and distribution of the microorganisms in surfacial/subsurfacial sediments. In addition, due to the shallow water column and active dynamics of the seawater, organic-rich sediment can be easily re-suspended into the water column and be utilized by organisms. The middle sediments are the mineralization area and OC decreases with an increase in pH. Table 2.6. pH in the Bohai Sea sediments Depth (cm) 1 5 10 15 20 25 30 40 70 130 190 250 310

A 7.54 7.58 8.08 8.06 8.08 8.01 8.21 8.14 8.25 8.53 8.65 – –

B1 7.34 7.60 7.67 7.83 7.94 7.95 7.97 7.91 8.06 8.20 8.08 7.90 7.92

B2 7.45 7.57 7.57 7.86 7.75 7.82 7.54 7.65 7.60 7.68 7.86 – –

C1 – – 7.54 – 7.69 – 7.72 7.73 7.71 8.02 – – –

C2 – – 7.84 – 8.22 – 8.43 8.25 8.33 8.06 – – –

C3 – – 7.80 – 7.97 – 8.06 8.04 8.05 8.21 7.96 – –

A: Bohai Bay; B: Huanghe River Estuary; C: Liaodong Bay

(2) Fe3+ /Fe2+ , Eh, and Es Commonly, values of Fe3+ /Fe2+ , Eh, and Es in surfacial sediments are high and the OC contents are high too. With the increase in deposition depth,

2.1 Change Processes of Carbon in the Bohai Sea

161

Fe3+ /Fe2+ , Eh, and Es tend to decrease and the OC content also decreases. From the results of Eh and Es (Fig. 2.20), this trend is obvious and can also reﬂect the enhancement of mineralization of OC and the reductive environment. However, the vertical distribution of the ratio of Fe3+ /Fe2+ is also complex with the increase in deposition depth, and the whole trend is consistent with the rule. This trend is consistent with the change in OC. 3+

2+

Eh (mV)

Fe /Fe 1.00

2.00

3.00

0

100 C1 C2 200

C3

0

50

100

150

200

100 C2 C1

A B1

300

4.00

Depth (cm)

Depth (cm)

0.00 0

200

A C3

B2

B2 B1

400

300

Fig. 2.20. Vertical distribution of Fe3+ /Fe2+ ratio and Eh in the Bohai Sea sediments

2.1.4.4 Decomposition of OC Phytoplankton can transfer CO2 in the atmosphere to organic carbon by photosynthesis. The ﬁxed carbon phytoplankton is almost cycled in the euphotic layer. Only a small part of OC is transferred by the food chain and particulate organic carbon (POC) is produced in the complicated process. Coarse POC precipitates from the euphotic layer to the seaﬂoor and then becomes the main source of OC. Before the mineralization of OC in the surfacial sediment, various components in the soft sediment are changed in serials of the oxidative process which is called the early diagenetic process. The main driver reaction in the process is the decomposition of the organic mass occurring in the sediment-water interface. The decomposed OC are diﬀused into water to recycle in the form of dissolved inorganic carbon by incessant mass exchange. The survey was carried out studying the OC decomposition in September 2002 in a 17×11 km2 area oﬀ the Huanghe River Estuary (Fig. 2.21). Stations A, B, C, D, E, F, and S are the central sampling stations and at each central

162

2 Biogeochemical Processes of the Bohai Sea

station’s four corners there are also four stations named a, b, c, and d. In addition, in the middle of two adjacent central stations, one sampling station is set. Altogether, 14 cores from A-b, B-d, D-c, F-d, S, D-E, and E-F stations have been collected. N 38.28 A A-b A-a A-c A-d A-B a e B-a B B-b ai S B-c B-d Boh B-S CB-C C-a C-b r e v i R C-c C-d ghe Huan D-S D-a D D-b D-E F F-b E-F E-a E E-b D-c D-d F-a F-c E-c E-d F-d

38.23

S-a S S-b S-c S-d

38.18 119

119.05

119.1

119.15

119.2

E

Fig. 2.21. Location of the sampling stations for OC decomposition study

The OC decomposition rate may be estimated based on the degree of OC decrease, assuming the decomposition is a one-grade reaction. ΔZ (cm) is the diﬀerence in deposition depth, C Z is the content of OC at Z cm depth, K (yr−1 ) is the decomposition rate constant, and S (cm/yr) is the sedimentation rate (S =0.33 cm/yr, ΔZ=6 cm). Then K=

ln(C0 /CZ ) ΔZ/S

(2.1)

Based on the formula, we estimate the decomposition rate constant K1 in surfacial sediments (3∼9 cm) at sites D-c, S, D-E, and E-F and the decomposition rate constant K2 in subsurfacial sediments (9∼15 cm) at sites A-b, B-d, F-d (Table 2.7). Due to the coarse sediment that is a disadvantage in the preservation of OC, the bioturbation of benthos and the bacteria that are mainly responsible for OC mineralization in surfacial/subsurfacial sediments, the estimated values of K1 and K2 are high in contrast with other sites (Ma et al., 2003). However, K1 and K2 are very diﬀerent: K1 in site D-E is 1.89 times higher than K1 in site S and K2 in site A-b is 2.45 times higher than K2 in site F-d, which exhibits the feature of regional diﬀerence in the decomposition rate.

2.2 Distributions and Transformations of Nitrogen in the Bohai Sea

163

Table 2.7. OC decomposition rate constants (yr−1 ) in surface/subsurface sediments OC contents in Site sediments (%) C3 D-c 1.54 S 0.87 D-E 0.99 E-F 0.74

surface C9 1.06 0.73 0.67 0.44

K1 0.021 0.0097 0.021 0.028

OC contents in Site sediments (%) C9 A-b 1.72 B-d 1.25 F-d 1.75

subsurface C15 0.43 0.56 1.17

K2 0.076 0.044 0.022

2.2 Distributions and Transformations of Nitrogen in the Bohai Sea It is generally considered that nitrogen availability is one of the major factors regulating primary production in temperate coastal marine environments. Coastal regions often receive large anthropogenic inputs of nitrogen that cause eutrophication, resulting in harmful blooms and other damage to the marine ecosystems. Rivers discharge into the Bohai Sea with various amounts of water and sediment loads, notably the Daliaohe River, Shuangtaizihe River, Luanhe River, and Huanghe River (Yellow River). Riverine concentrations in this region are high for nutrients, with 100 μmol/L for DIN − + 3− (DIN=NO− 3 +NO2 +NH4 ) and 0.5∼1.0 μmol/L for PO4 in general, and the N/P ratio can be 100∼500 (Zhang et al., 2004). 2.2.1 Nitrogen in Seawaters 2.2.1.1 Distributions of Nitrogen Table 2.8 summarises the diﬀerent phase distributions of nitrogen in the water column during autumn and spring cruises of Dongfanghong (DFH) 2 in 1998 and 1999 (Fig. 2.22, Raabe et al., 2004). The mean values of repeated grid samplings within a fortnight of dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), particulate nitrogen (PN), and total nitrogen (TN, i.e., the sum of total dissolved and particulate nitrogen) as well as the contribution of each species to TN are compiled for both seasons (Raabe et al., 2004). TN values ranged from 16 to 21 μmol N/L, with DON constantly averaging 10∼11 μmol N/L and generally being the main fraction (55%∼69% contribution to TN). For each season, there were no signiﬁcant diﬀerences either between mean surface and bottom concentrations of TN or between the averages of the repeated grid samplings. Comparing both seasons, TN was about 3.5 μmol N/L higher in spring 1999 than in autumn 1998. This was mainly caused by higher spring values for DIN and PN, each averaging 5 μmol N/L compared to 3 μmol N/L in autumn. In addition to Table 2.8, Fig. 2.23 (Raabe et al., 2004) shows the horizontal distribution of TN as well as the contributions of DIN, DON, and PN to TN at the surface and on the bottom

164

2 Biogeochemical Processes of the Bohai Sea N 41

40

39

38

37 118

119

120

121

122

E

Fig. 2.22. Grid stations during ﬁeld investigations in the Bohai Sea (Raabe et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

of the Bohai Sea during autumn 1998 and spring 1999. Again, TN concentrations near the bottom were very similar to the surface concentrations at most stations, all ranging between 16 and 21 μmol N/L. Only at stations B1 (Laizhou Bay in 1998 and 1999) and A1 (southern Bohai Strait in 1998), was TN signiﬁcantly higher at the bottom than at the surface, resulting in a vertical diﬀerence of >15 μmol N/L. As already shown by Table 2.8, DON fractions generally accounted for more than 50% of TN. Exceptions were observed in September 1998 at stations B1 and C1 (west and north of the mouth of the Huanghe River, Fig. 2.23), where only 28%∼43% was reached and where DIN became the main fraction, reaching about 56% and 43%, respectively. Here also, the TN concentrations were between 25 and 30 μmol/L, and thus almost twice the concentration was found at the other stations. Further autumn maxima of DIN of >7 μmol/L were found in the bottom near layers at the eastern stations A2 through A4 (Laotieshan Channel), reaching >40% of TN. As for dissolved inorganic nitrogen, higher concentrations are found in the Huanghe River Estuary, Bohai Bay and the northern part of the central Bohai Sea (Fig. 2.24, Zhao et al., 2002). The highest concentration of DIN appears in the Huanghe River Estuary. It indicates that the Huanghe River was the most important source of DIN in the Bohai Sea in October 1998. During May 1999, dominating high DIN contributions to TN of >50% were found in the northwest, at the outer Bohai Bay (stations E1 and E2). Here, concentrations

2.2 Distributions and Transformations of Nitrogen in the Bohai Sea

165

Table 2.8. Mean concentrations (μmol N/L) of DIN, DON, PN, and TN in the Bohai Sea (Raabe et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

Surface DIN DIN/TN (%) DON DON/TN (%) PN PN/TN (%) TN Bottom DIN DIN/TN (%) DON DON/TN (%) PN PN/TN (%) TN

September 1998, grid 1

September 1998, grid 2

May 1999, grid 1

May 1999, grid 2

2.81±3.29 15.24±11.26 10.18±2.19 62.49±12.03 3.51±0.85 22.27±7.69 16.50±4.33

2.54±3.16 13.72±12.01 10.97±1.52 68.90±10.28 2.79±0.76 17.38±4.52 16.30±3.42

5.3±4.73 24.00±15.95 10.87±2.94 57.30±16.06 3.86±3.77 18.70±8.45 20.08±8.18

4.73±4.10 21.20±14.12 11.84±2.28 62.80±14.47 3.26±2.02 16.00±5.69 19.84±6.26

3.75±3.55 19.78±13.95 10.99±3.89 61.12±13.67 3.28±0.98 19.10±4.99 17.91±4.76

3.39±3.70 18.48±15.90 11.03±2.82 65.45±13.28 2.66±0.59 16.07±3.84 17.08±4.77

5.42±4.37 22.96±13.59 11.28±4.53 55.00±17.19 5.05±4.94 22.04±9.81 21.75±10.27

4.87±4.03 21.15±13.25 11.22±2.12 59.04±14.55 4.20±2.72 19.81±6.17 20.27±7.07

N 41

40

39

38

37 118

119

120

121

122

E

Fig. 2.23. Composition of total N (μmol N/L) in surface and bottom water layers of the Bohai Sea (Raabe et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

166

2 Biogeochemical Processes of the Bohai Sea

of TN were at least twice as high as in the central Bohai Sea. DIN contributions of >30% were found at stations G3 through G5 and at F4/F5 west of L¨ ushun. In the eastern part of the investigated area, at stations A1 through A4 and C4/D5 near the Bohai Strait and the Laotieshan Channel, the contribution of DIN to TN was in most cases <6%. N 40 39.5

(a)

39 38.5 38 37.5 37 N 40

118

119

120

121

122

E

119

120

121

122

E

(b) 39.5 39 38.5 38 37.5 37 118

Fig. 2.24. Horizontal distribution of nitrogen (μmol/L) at surface layer (5 m) in the Bohai Sea. (a) October 1998; (b) May 1999 (Zhao et al., 2002) (With permission from Wei H)

In the autumn cruise, a relatively high concentration (8.0∼10.0 μmol/L) of nitrate was found in Laizhou Bay, which suggests an inﬂow of nitrate from the Yellow River. The concentration of nitrate falls almost linearly with increasing distance oﬀ Laizhou Bay and reaches 0.5∼1.0 μmol/L in the central Bohai Sea, while Bohai Bay and Liaodong Bay have low levels of nitrate (Fig. 2.25, Zhang et al., 2004). Nitrite averages 2.5∼3.0 μmol/L oﬀ Laizhou Bay and falls to ca. 0.50 μmol/L in the central Bohai Sea, suggesting a dispersion of nitrite from the coast. Ammonia ranges from 0.40 to ca. 0.50 μmol/L in the central Bohai Sea to 1.0∼1.5 μmol/L in Laizhou Bay, with levels in Liaodong Bay and Bohai Bay having a concentration in the range of 0.40∼1.50 μmol/L. In the spring cruise, nitrate showed a high value in Bohai Bay with a mean concentration of 10.0 μmol/L, followed by Laizhou Bay (6.0∼8.0 μmol/L) and Liaodong

2.2 Distributions and Transformations of Nitrogen in the Bohai Sea

167

Bay (6.0∼7.0 μmol/L), with a low level of 2.0∼4.0 μmol/L for the central Bohai Sea and Bohai Strait (Fig. 2.25). Ammonia and nitrite are similar to nitrate, both decreasing from the coast to the central Bohai Sea, although the concentrations are quite diﬀerent between these two species. At station D1 in the southeastern area of Bohai Bay, PN was the dominating fraction during spring, amounting to 46% of TN. In this area, TN exceeded 50 μmol/L and thus reached threefold concentrations at the western stations. Furthermore, PN concentrations in the shallow areas along stations C1 through G1 including D2 and E2, i.e., in the eastern part of Bohai Bay, were >2 μmol/L higher on the bottom than at the surface and thus accounted for most of the local TN increase between the surface and the bottom (Fig. 2.23). Around stations F1, E1, and E2, this diﬀerence between the bottom and the surface partly exceeded 15% of the total PN contribution to TN. Also, at the inner part of the Laotieshan Channel, at stations E5, F5, and G5, the contributions of PN to TN were >10% higher in the bottom waters than at the surface.

Fig. 2.25. Dissolved inorganic nitrogen (μmol/L) of the Bohai Sea in spring (BH99) and autumn (BH9) cruises (Zhang et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

2.2.1.2 Vertical Variation of Dissolved Inorganic Nitrogen In a section across the central Bohai Sea (section L) of the BH98 cruise, nutrient species showed a somewhat vertically well-mixed proﬁle (Fig. 2.26,

168

2 Biogeochemical Processes of the Bohai Sea

1.0 0

Depth (cm)

BH99 Nitrate

0.0

0.0

0.60 0.80 1.00

0.40

A4

1.00

0.60

0.35 0.35 0 .40 0.45 0.50

0.40

40 50

0.40

0.80

30

0.35

0.35

1.40

0.60

Depth (cm)

0 0.6

0.40

A3

0 1.2

0.40

30

20

A1 A2 0 1.0 10 0 00 20 1. 1.20 0 1.4 30 40 50 60 BH98 Ammonium 0 10 20 1.0 1.0 30 2.0 3.0 40 4.0 50 BH98 Nitrate 60 0.80

0.40

20

G4

F4

0.60

10

10

Depth (cm) Depth (cm)

0.45

A4

0.60

40 BH99 Ammonium 50 0

Depth (cm)

0.30

7.00

0.30 0.4 5

4.00 5.0 0 6.0 0

(a) A3

A2

E4 0.40

A1

0.60

0.4 5

C1 D3 0 10 20 30 BH98 Ammonium 0 10 20 BH98 Nitrate 30 0.80

0 0.60.75

G4

F4

1.50 3.00 4.50 6.00 7.50 8.00

0

Depth (cm)

E4

0 7.0

C1 D3 0 10 20 BH99 Ammonium 30 0 3.00 3.00 10 20 BH99 Nitrate 30

0.30

Depth (cm) Depth (cm)

Zhang et al., 2004). Concentrations of nitrate, nitrite, and ammonia fell from stations C1 to G4. In the section across the Bohai Strait (section A) from south to north, nitrate was almost vertically homogenous in the upper waters of ca. 20 m, corresponding to the depth of pycnocline. Concentrations of nitrate increased then with depth and reached 6.0∼7.0 μmol/L, in near-bottom waters, higher in the south than in the northern part of the Bohai Strait. Ammonia and nitrite showed somehow a vertically mixed picture with a higher concentration at station A1 than at station A4.

1.0 2.0 5.0

3.0 4.0

6.0

(b)

Fig. 2.26. Vertical variation of nitrogen (μmol/L) at sections L (a) and A (b) of the Bohai Sea in spring (BH99) and autumn (BH98). L: central Bohai Sea; A: Bohai Strait (Zhang et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

At section L of the BH99 cruise, the nitrate averaged 7.5 μmol/L at station G4, then reduced southwestwards to C1 (Fig. 2.26). Ammonia and nitrite showed well-mixed vertical proﬁles in coastal waters when taking into account the nature of stratiﬁcation in the central Bohai Sea, with 1.0 μmol/L for ammonia in near-bottom waters and 0.12 μmol/L for nitrite at the surface. At sections across the Bohai Strait (i.e., A1 to A4), stratiﬁcation is identiﬁed for nitrogen. Concentrations of ammonia and nitrite increase considerably from

2.2 Distributions and Transformations of Nitrogen in the Bohai Sea

169

surface to near-bottom waters. Nitrate shows a value of 0.40∼0.60 μmol/L in this region, similar between surface and near-bottom waters. 2.2.1.3 Seasonal Diﬀerences of Nitrogen In spring 1999, the DIN concentrations from Bohai Bay through the central Bohai Sea to the stations west of L¨ ushun were higher than in autumn and— through enhanced phytoplankton activity—caused generally higher values of total N itself. Fig. 2.27b (Raabe et al., 2004) shows distinct maxima of primary production in the Bohai Sea with the highest values of >700 mg C/(m2 ·d) at the Laotieshan Channel and with further maxima at stations G2 (southern Liaodong Bay) and C1 (north of the Huanghe River mouth). Simultaneously, there was an accumulation of organic matter with high concentrations of particulate N (Fig. 2.23) in the western Bohai Sea, especially near Bohai Bay. The production maxima in Fig. 2.27b seemed to have been moved by the main inﬂow current, triggered by the Yellow Sea Warm Current YSWC. From grid 1 to grid 2, the maximum located in the Laotieshan Channel around station D5 (Fig. 2.27b) moved in a western direction to station G4 west of L¨ ushun (Fig. 2.27d), while the maxima at station G2 (southwestern Liaodong Bay) and at station C1 (mouth of the Huanghe River) followed the anticlockwise south current in southern and eastern directions, respectively. It was considered that this is a typical spring-time situation with the developing phytoplankton originating from the Yellow Sea, fuelled by nutrients from deeper water layers and driven northwest by the YSWC (Fig. 2.27d). Following the anticlockwise circulation, the production maxima reached then the very shallow Bohai Bay and built up a high biomass, fed by the nutrient input of the Huanghe River. Apart from the Huanghe River supply and the stations west of L¨ ushun city fuelled by the YSWC (Fig. 2.23), the generally low DIN concentrations in 1998 in the central part of the Bohai Sea reﬂected the autumn season, when most of the phytoplankton blooms had ended and nutrient concentrations were exhausted. In particular, nitrate limited the primary production, for example at station B2 (Laizhou Bay): Although there was no actual maximum in primary production (Fig. 2.27a), the nitrate minimum of <0.1 μmol/L must have been caused by a previous bloom, proved by high chlorophyll concentrations of >4 mg/L (Wei et al., 2003) as well as by higher concentrations of particulate N (Fig. 2.23). Only in region inﬂuenced by the Huanghe River (from the eastern Bohai Bay to the northern Laizhou Bay) and in the area west of L¨ ushun city, did higher DIN concentrations persist—again fuelled by the YSWC and by the Huanghe River—and caused moderate increases in primary production in these areas between grids 1 and 2. At the same time, the production maximum observed in the Bohai Strait (stations C4/D5 in Fig. 2.27a) moved in an eastern direction (Fig. 2.27c). From this it can be assumed that with the seasonal decline in phytoplankton blooms and due to the overall nutrient exhaustion in the Bohai Sea, the phytoplankton and part of the particulate organic material left the Bohai Sea along the south coast through the Bohai

170

2 Biogeochemical Processes of the Bohai Sea

41

Fig. 2.27. Primary production (mg C/(m2 ·d)) at the surface in the Bohai Sea in autumn 1998 (a) and in spring 1999 (b), and diﬀerences between two grids in autumn 1998 (c) and in spring 1999 (d) (Raabe et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

2.2 Distributions and Transformations of Nitrogen in the Bohai Sea

171

Strait. The origin of PN was mostly coupled with phytoplankton standing stocks, because PN and chlorophyll correlated with the highest signiﬁcance (P >99.99%) during both seasons (Figs. 2.28a and 2.29a, open squares). The correlation was strongest in spring 1999. There was also a correlation of PN with SPM of highest signiﬁcance (P >99.99%, Fig. 2.29b, open squares) during spring, while the signiﬁcance in autumn was much lower (P >95%, Fig. 2.28). This means that most of the SPM in autumn did not consist of fresh organic material, but originated from resuspended sediments and detritus.

Particulate N (mmol/L)

10

R2=0.2933, n=391

(a)

8 6 4 2 0

0

1

8

5

6

R2=0.1087, n=326

(b)

Particulate N (mmol/L)

2 3 4 Chlorophyll a (mg/L)

6 4 2 0

0

2

4 6 8 10 12 14 Total suspended matter (mg/L)

16

18

Fig. 2.28. Correlation of particulate N with chlorophyll a (a) and with total SPM (b) of the Bohai Sea in autumn 1998 (Raabe et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

In autumn 1998, no regional diﬀerences for the correlations were found (Figs. 2.28a and b), indicating that the biogeochemical processes were more or less in the same range throughout the investigated area. Support for this assumption came also from the PC and DOC fractions that did not show strong lateral gradients in the Bohai Sea (Fig. 2.23). In spring 1999, two main groups of data points could be identiﬁed (Fig. 2.29a). The samples from the central Bohai Sea inclusive of the southern and eastern regions (open squares) showed highly signiﬁcant correlations of PN with chlorophyll. In Bohai Bay (ﬁlled squares), a highly signiﬁcant correlation was also found, but with a much steeper slope; i.e., the samples had a much higher content of PN

172

2 Biogeochemical Processes of the Bohai Sea Particulate N (mmol/L)

30 25

Central, southern and eastern Bohai Sea Bohai Bay

20

R =0.4754, n=107 2

15 10 5 0

0

20

Particulate N (mmol/L)

(a)

R =0.7303, n=349 2

1

2

5 3 4 6 Chlorophyll a (mg/L)

7

8

Central, southern and eastern Bohai Sea Bohai Bay

9

(b)

15 10 R2=0.7747, n=434

5 0

0

20

40 60 80 100 Total suspended matter (mg/L)

120

140

Fig. 2.29. Correlation of particulate N with chlorophyll a (a) and with total SPM (b) of the Bohai Sea in spring 1999 (Raabe et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

compared to chlorophyll. These Bohai Bay samples were also characterised by high SPM values having a very close relationship with PN (Fig. 2.29b, ﬁlled squares). This was an indication of the production of fresh biomass with high N content in Bohai Bay during spring 1999, partly originating from in situ plankton production, partly from resuspension of the sediments or from imports via the residual currents from the coastal regions of Liaodong Bay into Bohai Bay. The biogeochemical processes were also strongly enhanced by interactions between sediments and the water column in this very shallow area and resulted in the described strong accumulation of particulate organic material during spring (Figs. 2.23 and 2.30). 2.2.1.4 Distinguishing between Transport and Conversion Processes of Nitrogen In order to identify net import and export processes into/from the Bohai Sea, it was necessary to examine whether changes in biogeochemical parameters were caused by transport processes or by phase-transfer and conversion processes. For this, analysis of changes in SPM data was a supportive tool, in particular when SPM concentrations showed a close relationship to particulate nitrogen (Figs. 2.28b and 2.29b). During both seasons, suspended matter was generally higher in the bottom layers than at the surface and, consequently, equilibrium between sedimentation and erosion was assumed (Jiang et al., 2004). In the Laotieshan Channel, currents and higher water density

2.2 Distributions and Transformations of Nitrogen in the Bohai Sea

173

N 41

40

39

38

37 118

119

120

121

122

E

Fig. 2.30. Composition of total C in surface and bottom water layers of the Bohai Sea on grid 1 in autumn 1998 and in spring 1999 (Raabe et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

of the YSWC caused sediment erosion and an increase in SPM in the bottom water. Then, via residual currents, the SPM was transported to the northeast of the Bohai Sea, west and north of L¨ ushun (Liaodong Bay), causing SPM maxima of 10∼20 mg/L throughout the water column (Jiang et al., 2004). In general, major parts of the suspension would settle here (Huang et al., 1999). Further maxima of total suspended matter found in the coastal zones from Bohai Bay to Laizhou Bay were caused by discharges of silt by the Huanghe River and Haihe River, partly indicated by lower salinity values as well as by stronger vertical salinity gradients, as reported by Zhang et al. (1998b) and Jiang et al. (2004). Apart from these local SPM maxima originated by riverine input and by sediment erosion by the YSWC, wind-induced local resuspension had played a major role in increasing SPM content in the shallow areas of the Bohai Sea: Jiang et al. (2004) ascribed the diﬀerent SPM concentrations in spring and autumn to diﬀerent meteorological forcing, namely the wind speed. Consequently, apart from the import of nitrogen by the YSWC and by the Huanghe River, resuspension processes in Bohai Bay released nitrogen into the water column. In spring and autumn, decreases in PN were observed in the coastal area from Bohai Bay to Laizhou Bay near the mouth of the Huanghe River during the two successive grids. These decreases were higher during spring, and they were probably caused by continuing decomposition

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2 Biogeochemical Processes of the Bohai Sea

as well as by sedimentation, indicating highly dynamic processes, especially in the shallow areas of the Bohai Sea. Signiﬁcant increases in PN (>0.15 μmol N/(L·d)) observed during spring at stations G3/G4 and F4 were well correlated with increases in primary production (Fig. 2.27d) as well as with decreases in DIN (>0.28 μmol N/(L·d)) and thus were caused by an enhanced biogeochemical conversion of nitrogen. In contrast to this, increases of >0.33 μmol PN/(L·d) throughout the water column at station B1 (northern Laizhou Bay) were accompanied by strong decreases in primary production (>100 mg C/(m2 ·d), Fig. 2.27d). Here, PN concentrations were apparently aﬀected by the mentioned strong resuspension of surface sediments (Jiang et al., 2004). Other strong evidence for the resuspension of organic material near Bohai Bay in spring came from the very high particulate carbon content, especially at station D1 (Fig. 2.30). Furthermore, DOC concentrations were much higher in this region, indicating strong remineralisation processes followed by the beginning of decomposition and oxidation. As a consequence, oxygen values were lower (<9 mg/L) in Bohai Bay than in the central and eastern Bohai Sea (>10 mg/L). This was conﬁrmed by low oxygen values generally observed in Bohai Bay during early summer (Cui et al., 1994). Since the DON was generally >60% of TN in autumn and spring in the investigated area (Fig. 2.23), it can be assumed that the pelagic ecosystem was in a steady state of ongoing production and decomposition, including sorption processes. That means the situation during both seasons was comparable with the postspring situation in temperate waters described by Butler et al. (1979). High loads of DON observed in areas of high primary production (stations D5 and C4) can be related to DON release during exponential phytoplankton growth (Bronk et al., 1994). Short-time changes between the successive grid samplings indicated that besides changes induced by advection and resuspension a variable turnover of nitrogen species occurred. However, as already indicated by SPM data, major parts of the observed changes must be referred to as advection of gradients near the bottom in the deeper part of the Bohai Sea. This assumption was strongly supported by the high primary production of partly >700 mg C/(m2 ·d) at the Laotieshan Channel (Figs. 2.27a and b), requiring a net turnover of nitrogen of >9 μmol/(L·d). Due to the low DIN concentrations of <0.7 μmol N/L near the surface of stations A3/A4 (Fig. 2.23), nitrogen supply could have partly come from the fast regenerating N pool (DON) being in the range of 9∼10 μmol/L, but a major source must have been the entering YSWC, increasing at the bottom near DIN concentrations at stations A3/A4 to >6 μmol N/L in autumn and to 2 μmol N/L in spring and thus feeding primary production by nitrogen import. During spring 1999, the high primary production of 5 μmol C/(L·d) near the Huanghe River did not ﬁt in with the observed DIN decreases of 0.1 μmol N/(L·d) from grid 1 to grid 2 at stations C1/D1. Here also, a net import of nitrogen by the Huanghe River had to be postulated (Raabe et al., 2004).

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175

2.2.2 Evolution of Nutrients and Primary Production The Bohai Sea is the main spawning and feeding site for many ﬁsh and shrimps. Fishery resources were once very abundant, but they declined signiﬁcantly over the past 30 years. There are many factors inﬂuencing the variation of ﬁshery resources. Phytoplankton, as the foundation of the whole ecosystem, are very important for ﬁshery resources. The primary productivity (PP) in the Bohai Sea was high in the 1980s, but it became low from the 1990s onwards. On the other hand, the phytoplankton composition changed gradually from diatoms with absolute predominance to diatoms coexisting with dinoﬂagellates since the 1980s (Sun et al., 2002). Diatoms support carnivorous ﬁshes which have a long food chain, while dinoﬂagellates are the main food for zooplankton, such as jellyﬁsh that support a short food chain. Therefore, such a change in PP and phytoplankton community structure in the Bohai Sea can cause a change in the ﬁsh yield and a variation in the ﬁshery composition, and can also aﬀect the stability and function of the ecosystem. Nutrients, such as nitrogen and phosphorus, are fundamental for phytoplankton growth. They changed notably in the past 30 years in the Bohai Sea. Many studies were conducted to study the nutrients uptake of phytoplankton and the limitation of nutrients on phytoplankton by means of modeling in the laboratory or by in situ enclosure experiment. However, few researches were conducted to analyze the relationship between PP and nutrients over a timescale of several decades, and systematic analysis of the eﬀect of nitrogen and phosphorus on PP in the past 30 years in the Bohai Sea has not been reported yet. Many comprehensive investigations of the Bohai Sea in the past 30 years provided lots of basic data about concentrations of nitrogen and phosphorus, and data of PP and phytoplankton composition. Mainly based on these basic data, this section tries to investigate the role of nitrogen and phosphorus in the variation of PP and phytoplankton community structure, in order to provide a scientiﬁc foundation for marine environmental protection. The Bohai Sea is the main receiving water body for contaminants from the Bohai Sea Economic Circle region which is composed of the surrounding provinces and cities, such as Shandong, Liaoning and Hebei provinces, and the city of Tianjin. It has about 21% of the national population and contributes 22% of the national GDP in 1999 (Zhang Z et al., 2006). The region is the most developed in northern China. With the rapid development of industrial society, the Bohai Sea Economic Circle region discharged many contaminants into the Bohai Sea. These pollutants have broken the balance of the Bohai Sea ecosystem. 2.2.2.1 Evolution of DIN and DIP in Bohai Sea Waters Evolution of DIN and DIP in Bohai Sea waters exhibited three stages in the past 30 years: in the early and middle 1980s; from the late 1980s to the middle 1990s; from the middle 1990s onwards. In the ﬁrst stage, DIN was low and stable, while DIP decreased gradually; in the second stage, both DIN and

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DIP increased; in the third stage, both DIN and DIP decreased in variational tendency; however, DIN was still above its level of the early 1980s, while DIP was already below its level of the early 1980s and even reached the lowest level in the past 30 years (as shown in Figs. 2.31 and 2.32).

DIN (mmol/L)

20 15 10 5 0 1978

1988

1998

2008

Year

Fig. 2.31. Evolution of DIN in Bohai Sea waters in the past 30 years

DIP (mmol/L)

1.5 1.0 0.5 0 1978

1988

1998

2008

Year

Fig. 2.32. Evolution of DIP in Bohai Sea waters in the past 30 years

The quantity of chemical fertilizer used can aﬀect the DIN in the Bohai Sea greatly. Based on the series of China Statistical Yearbooks, the quantity of chemical fertilizer used in farmland increased by an average ratio of 6.7% every year from 1980 to 2000. There was a close relationship between DIN and the quantity of chemical fertilizer used in farmland from the early 1980s to the middle 1990s (r =0.907, n=14, sig.<0.001). Therefore, the quantity of chemical fertilizer was an important source of DIN in Bohai Sea waters. The quantity of chemical fertilizer increased constantly from the middle 1990s. However, concentrations of DIN in Bohai Sea waters decreased during the same period. The sewage disposal plants can reduce the discharge of DIN into the Bohai Sea by denitrogenation technology. There was a close negative relationship between sewage disposal ability in the Bohai Sea Economic Circle region and concentrations of DIN in the Bohai Sea from 1995 to 2004 (r =−0.602, n=10, sig.<0.05). With the increasing sewage disposal ability in the Bohai Sea Economic Circle region (as shown in Fig. 2.33), concentrations of DIN in

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177

the Bohai Sea decreased signiﬁcantly from the middle 1990s. Consequently, sewage disposal ability was an important factor inﬂuencing concentrations of DIN in Bohai Sea waters from the middle 1990s.

Fig. 2.33. Total annual sewage disposal ability of the Bohai Sea Economic Circle region

In addition, the ﬂux of DIN of the Yellow River into the Bohai Sea has the same variation trend as the evolution of the concentration of DIN in Bohai Sea waters (as shown in Fig. 2.34), especially from the middle 1990s. There was a close relationship between the ﬂux of DIN of the Yellow River and concentrations of DIN in Bohai Sea waters from the middle 1990s (r =0.820, n=9, sig.<0.01). Consequently, the Yellow River input of DIN was an important source of nitrogen in Bohai Sea waters. The runoﬀ of the Yellow River was also an important factor inﬂuencing the concentration of DIP in the Bohai Sea waters. The runoﬀ of the Yellow River has exhibited a decreasing trend over the past 30 years (as shown in Fig. 2.35). There was a close correlation between the runoﬀ of the Yellow River

Fig. 2.34. Flux of DIN to the Bohai Sea from the Yellow River and concentration of DIN in the Bohai Sea waters in the past 30 years. Solid circle means ﬂux of DIN; hollow circle means concentration of DIN

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500

1.0

400

0.8

300 0.6 200 0.4

100

0.2

0 1980

1985

1990 1995 Year

2000

2005

Runoff of the Yellow River ( 109 m3/yr)

Concentration of DIP (mmol/L)

and concentrations of DIP in the Bohai Sea waters from the early 1980s to the middle 1990s (r =0.734, n=8, sig.<0.05), indicating that DIP discharged by the Yellow River was an important source of DIP in Bohai Sea waters during this period. The runoﬀ of the Yellow River still exhibited a decreasing trend from the middle 1990s, while concentrations of DIP in Bohai Sea waters ﬁrstly increased slightly then decreased during the same period, indicating that the importance of the eﬀect of the concentration of DIP in the Bohai Sea waters from the runoﬀ of the Yellow River was disturbed by other factors from the 1990s.

Fig. 2.35. Runoﬀ of the Yellow River and concentration of DIP in Bohai Sea waters in the past 30 years. Solid circle means concentration of DIP; hollow circle means runoﬀ of the Yellow River

The concentrations of DIP in Bohai Sea waters increased slightly in the middle 1990s. It was related to the use of many phosphorus detergents. The yield of synthetic detergents has increased by an average ratio of 9.6% every year in China over the past 30 years. The industry standard of phosphatefree detergents was promulgated in 1995 in China. However, the prohibition of sales and use of phosphorus detergents in the Bohai Sea Economic Circle region began in 2000 in part of the region, and the prohibition was only realized in the whole Bohai Sea Economic Circle region in 2002. Therefore, although the runoﬀ of the Yellow River decreased signiﬁcantly from the middle 1990s, the use of many phosphorus detergents meant that the discharge of DIP did not decrease with the reduction in the runoﬀ of the Yellow River but exhibited a slightly increasing trend in the middle 1990s. The sewage disposal ability of the Bohai Sea Economic Circle region increased greatly from the middle 1990s. By phosphorus removal technology, the sewage disposal plants can reduce the discharge of DIP into the Bohai Sea. There was a signiﬁcant negative correlation between concentrations of DIP in Bohai Sea waters and the increasing sewage disposal ability of the Bohai Sea Economic Circle region (r =−0.795, n=10, sig.<0.01). Consequently, although many phosphorus de-

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179

tergents were used in the Bohai Sea Economic Circle region from the middle 1990s, the increasing sewage disposal ability in the region played an important role in reducing the discharge of DIP into the Bohai Sea and it was an important factor inﬂuencing the decrease in DIP in Bohai Sea waters from the middle 1990s. With many sewage disposal plants built from the middle 1990s and the increasing eﬀectiveness of sewage disposal from then on (as shown in Fig. 2.33), the discharge ﬂux of phosphorus into the Bohai Sea was reduced greatly. There was a close negative relationship between sewage disposal ability in the Bohai Sea Economic Circle region and the concentrations of DIP in the Bohai Sea from 1995 to 2004 (r =−0.795, n=10, sig.<0.01). Therefore, the increasing ability to handle sewage disposal in the Bohai Sea Economic Circle region played an important part in reducing the discharge of phosphorus into the Bohai Sea. 2.2.2.2 Evolution of PP and Phytoplankton Community Structure Many comprehensive investigations of the Bohai Sea have been conducted since the 1980s. Based on these data, it was found that the PP of the Bohai Sea changed signiﬁcantly over the past 30 years (as shown in Fig. 2.36). It was high in the 1980s and became low in the 1990s. It has exhibited an increasing trend since the start of the 21st century. However, the eﬀective part of PP that supports ﬁshery resources might not have increased yet because of the frequent bursts of the red tides during the same period. The phytoplankton community structure in the Bohai Sea has changed much in the past 30 years. Although diatoms were still the main component, the phytoplankton community structure changed gradually from diatoms that had absolute predominance to diatoms coexisting with dinoﬂagellates during the past 30 years (Sun et al., 2002). Speciﬁcally speaking, the relative cell abundance of Chaetoceros spp. and Skeletonema costatum declined from the 1980s, while that of Coscinodiscus spp., Eucampia zoodiacus, Ceratium spp.,

PP (g C/(m2 yr))

150

120

90

60 1980 1984 1988 1992 1996 2000 2004 2008 Year

Fig. 2.36. Variation of PP in the Bohai Sea in the past 30 years. No data in some years

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2 Biogeochemical Processes of the Bohai Sea

and Pseudo-nitzschia pungens increased during the same period. The most dominant species were Skeletonema costatum, Paralia sulcata, and Asterionellopsis glacialis in spring of the early 1980s, the early 1990s and the late 1990s, respectively. The main dominant species in autumn and winter in the past 30 years are shown in Table 2.9. Table 2.9. Primary dominant species in autumn and winter in the past 30 years Season Autumn

Year 1982 1992 1998 2000

Winter

1982 1992 2001

Primary dominant species Chaetoceros spp., Coscinodiscus spp., and Asterionellopsis glacialis Ceratium spp., Coscinodiscus spp., and Eucampia zoodiacus Coscinodiscus spp., Ceratium spp., Chaetoceros spp., and Eucampia zoodiacus Eucampia zoodiacus, Coscinodiscus spp., Ceratium spp., and Chaetoceros spp. Skeletonema costatum, Chaetoceros spp., and Asterionella japonica Eucampia zoodiacus, Thalassiosira nordenskioldi, and Skeletonema costatum Pseudo-nitzschia pungens, Eucampia zoodiacus, Cossinodiscus excentricus, Paralia sulcata, and Lauderia annulata

2.2.2.3 Inﬂuence on PP of Nitrogen and Phosphorus Nitrogen and phosphorus are fundamental for phytoplankton. The spatial distribution of nitrogen and phosphorus in the Bohai Sea is similar to that of the phytoplankton biomass (Song, 2000). An increase in their concentrations is good for maintaining a larger PP, while a low concentration will limit the increase in PP. There were diﬀerent methods to determine which kind of nutrient limited the growth of phytoplankton. In (Song, 2000), two methods were taken into consideration together, to determine the limiting nutrient. One was based on the concentration of the nutrients, the other was based on the concentration ratio of the nutrients. As to the ﬁrst method, the lower optimal concentration limits were 5.71 and 0.58 μmol/L for inorganic nitrogen and inorganic phosphorus, respectively (Cui et al., 1996). The N/P ratio (atom ratio) was often used to determine the limiting nutrient of phytoplankton. The N/P ratio was about 16 in the ocean in general, the same ratio at which phytoplankton uptakes nitrogen and phosphorus from the seawater. Both a high and a low N/P ratio will cause phytoplankton to be limited by a nutrient at a relatively low concentration. So it was suggested that phosphorus was the limiting nutrient when the N/P ratio was above 16; otherwise, the limiting nutrient was nitrogen. It was convenient to determine the limiting nutrient from the concentration ratio of nutrients. But the fact that concentrations of

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181

nutrients had a great inﬂuence on the growth of phytoplankton was neglected. For instance, whatever the N/P ratio was, it could not support a high biomass or PP if concentrations of both nitrogen and phosphorus were very low. Thus, when both concentrations of nitrogen and phosphorus are below their lower optimal concentration limits, nitrogen and phosphorus are the limiting nutrients, whatever are the concentration ratios of nitrogen to phosphorus. The limiting nutrients of phytoplankton were nitrogen, nitrogen-phosphorus, and phosphorus in the early 1980s, in the late 1980s and in the period from the 1990s, respectively. In the early 1980s, concentrations of DIN in the Bohai Sea were below 5.71 μmol/L, while concentrations of DIP were above 0.58 μmol/L during the same period (as shown in Figs. 2.31 and 2.32). On the other hand, in the early 1980s the average N/P ratio was 4.2, which was far below 16. Therefore, nitrogen was the limiting nutrient of phytoplankton in the early 1980s. In the late 1980s, nitrogen and phosphorus were below 5.71 and 0.58 μmol/L, respectively, so both of them became the limiting nutrients. From the 1990s onwards, DIN was higher than 5.71 μmol/L, while DIP was below 0.58 μmol/L in general and ﬂuctuated around 0.48 μmol/L at the same time (as shown in Figs. 2.31 and 2.32). On the other hand, the average N/P ratio was 22, which was far above 16. Therefore, phosphorus was the limiting nutrient of phytoplankton from the 1990s. Phosphorus was the main nutrient that inﬂuenced PP over the past 30 years. Correlations of DIN and DIP with PP in the past 30 years showed that there was a signiﬁcant negative correlation between PP and DIN (r =−0.760, sig.<0.05, n=7), and there was a significant positive correlation between PP and DIP (r =0.914, sig.<0.05, n=6). The coeﬃcient of PP with phosphorus was larger than that with nitrogen. Thus PP was closely related to DIP and phosphorus was the primary nutrient limiting growth of phytoplankton. The negative correlation between PP and DIN indicated that the carbon ﬁxation ability decreased with the increase in DIN. By indoor simulation experiments, Zhang NX et al. (2006) concluded that the intensity of the carbon sink weakened and the intensity of the carbon source was enhanced when nitrate was added to the seawater. On the other hand, with the increase in DIN, phytoplankton that was prone to be limited by phosphorus decreased over the past 30 years, while phytoplankton that was prone to be limited by nitrogen increased during the same period. Diﬀerent species of phytoplankton have a diﬀerent carbon ﬁxation ability, and thus a variation in phytoplankton community structure, along with the change in DIN, may be the reason for the actual carbon ﬁxation ability of phytoplankton over the past 30 years. Low phosphorus was responsible for the low PP over the past 20 years in the Bohai Sea. Yang D et al. (2004) and Yin et al. (2007) studied the eﬀect of phosphorus on Chaetoceros spp. and Skeletonema costatum, respectively. Based on their research, the growth multiple of the cell density of both Chaetoceros spp. and Skeletonema costatum increased with the increasing concentration of phosphorus in the range of 0∼10 μmol/L, and a regression curve was obtained (as shown in Fig. 2.37). The equation for the curve was

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2 Biogeochemical Processes of the Bohai Sea

y = 8.77608 − 6.71774 ∗ exp(−x/2.79047), R2 = 0.99765, n = 5

(2.2)

In the equation, y was the growth multiple of cell density, x was the concentration of phosphorus. It could be seen in Fig.2.37 that the relationship between phosphorus and the growth multiple of the cell density of Skeletonema costatum could be ﬁtted very well by equation (2.2). On the basis of equation (2.2) and concentrations of phosphorus over the past 30 years in the Bohai Sea, we calculated accordingly the growth multiple of phytoplankton cell density over that period. It was found that the average growth multiple of the cell density of phytoplankton in the 1980s was 1.5 times as large as that in the period after 1990.

Growth multiple

10 8 6 4 2 0

2

4 6 Phosphate ( m mol/L)

8

10

Fig. 2.37. Fitting of the relationship between concentration of phosphorus and growth multiple of cell density of Skeletonema costatum

Furthermore, the phytoplankton biomass has a signiﬁcant linear correlation with PP (r =0.7415, sig.<0.01, n=12) (Liu et al., 2003). Therefore, it could be supposed that equation (2.2) could describe the relationship between the phytoplankton biomass and the PP of the Bohai Sea. Y =a∗X +b

(2.3)

In equation (2.3), Y stands for PP; X stands for phytoplankton biomass; a and b are constants. The average PP of the Bohai Sea in the 1980s and from then on was denoted as Y1 and Y2 , respectively, and PP was denoted as Y0 when X=0. The average phytoplankton biomass of the Bohai Sea in the 1980s and from then on was denoted as X1 and X2 , respectively. Phytoplankton were denoted as X 0 when X=0. Based on equation (2.3), Y 1 −Y 0 =a(X 1 −X 0 ), Y 2 −Y 0 =a(X 2 −X 0 ). So equation (2.4) could be obtained: (Y2 − Y0 )/(Y1 − Y0 ) = (X2 − X0 )/(X1 − X0 )

(2.4)

2.2 Distributions and Transformations of Nitrogen in the Bohai Sea

183

(X 2 −X 0 )/X 0 , (X 1 −X 0 )/X 0 equalled the average growth multiple of the cell density of phytoplankton in the 1980s and from then on, respectively, and it had been calculated in the last paragraph that (X 1 −X 0 )/X 0 was 1.5 times as large as (X 2 −X 0 )/X 0 . Based on equation (2.4), it could be obtained that the value of (Y 1 −Y 0 )/(Y 2 −Y 0 ) was 1.5. When the phytoplankton biomass was zero, the PP was very low. So Y 0 could be neglected in the expression (Y1 −Y0 )/(Y 2 −Y 0 ), and the value of Y 1 /Y 2 was 1.5 approximately. Therefore, it could be estimated that the average PP in the 1980s was about 1.5 times as high as that in the period after 1990, while the actual average PP in the 1980s was about 1.3 times as high as that in the period after 1990. The estimated value was very near to the actual one. So the low PP in the past 20 years in the Bohai Sea was greatly inﬂuenced by the low phosphorus in the seawater. 2.2.2.4 Inﬂuence on Phytoplankton Community Structure of Nitrogen and Phosphorus Phytoplankton community structure changed greatly during the past 30 years. Chaetoceros spp., Skeletonema costatum, Coscinodiscus spp., Eucampia zoodiacus, Ceratium spp., and Pseudo-nitzschia pungens were the primary species that changed. Diﬀerent kinds of phytoplankton have diﬀerent half saturation constants for nutrients. The higher the half saturation for nitrate of the plankton, the easier it will be limited by the nitrate. Nitrate is the most important form among DIN in the Bohai Sea waters. Therefore, phytoplankton which has the lower half saturation constant of nitrate will be more easily limited by nitrogen. Skeletonema costatum and Chaetoceros spp. have relatively low half saturation constants for nitrate than Coscinodiscus spp. and Eucampia zoodiacus (as shown in Table 2.10). Consequently, they are less easily limited by the low concentration of nitrogen than Coscinodiscus spp. and Eucampia zoodiacus. The concentration of nitrogen was signiﬁcantly lower in 1982 than in 1992, 1998, and 2000. The concentration of nitrogen was higher in 2000 than in 1992 and 1998. With the increase in the concentration of nitrogen, the relative cell abundance of Coscinodiscus spp. and Eucampia zoodiacus increased and the relative importance of Skeletonema costatum and Chaetoceros spp. decreased. On the other hand, Pseudo-nitzschia pungens preferred nitrogen (Wang and Li, 2006). Therefore, with the increase in nitrogen from 1982 to 1992, 1998, and 2000, the relative cell abundance of Pseudo-nitzschia pungens increased over the past 30 years. Ceratium fusus and Ceratium furca also have a low half saturation value of nitrate, similar to Skeletonema costatum and Chaetoceros spp. However, they did not have the same variation trend as Skeletonema costatum and Chaetoceros spp., and they became gradually an important part of the phytoplankton community in the Bohai Sea in recent 30 years. Ceratium fusus and Ceratium furca had very low half saturation constants for phosphorus (as shown in Table 2.11). Therefore, they had more advantage in phosphorus-limited sea

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Table 2.10. Half saturation constants (Ks ) for nitrate uptake of phytoplankton Phytoplankton Skeletonema spp. Chaetoceros spp. Ceratium fusus Ceratium furca Coscinodiscus wailesii Eucampia zoodiacus ∗

At 9 ◦ C;

∗∗

Ks (μmol/L) 0.4∼0.5 0.1∼0.3 0.32 0.49 2.1∼5.1 2.59∗ , 2.92∗∗

Reference Eppley (1977) Eppley (1977) Baek et al. (2008) Baek et al. (2008) Eppley (1977) Nishikawa et al. (2009)

At 20 ◦ C

Table 2.11. Half saturation constants (Ks ) for phosphorus uptake of phytoplankton Phytoplankton Skeletonema costatum Chaetoceros curvisetus Ceratium fusus Ceratium furca

Ks (μmol/L) 0.722 0.944 0.03 0.05

Reference Jørgensen et al. (1991) Jørgensen et al. (1991) Baek et al. (2008) Baek et al. (2008)

waters. The concentration of phosphorus decreased from 1982 to 1998. The concentration of phosphorus was a little higher in 2000 than in 1998, but it was much lower in 2000 than in 1982. With the reduction in the concentration of phosphorus in the past 30 years, Ceratium fusus and Ceratium furca had stronger competitive ability for phosphate uptake than Skeletonema costatum, Chaetoceros spp., Eucampia zoodiacus, and so on. Therefore, the evolution of DIN and phosphorus was a very important factor inﬂuencing the variation in the structure of the phytoplankton community. With the decrease in DIP and increase of DIN, the relative cell abundance of Chaetoceros spp. and Skeletonema costatum decreased, while that of Coscinodiscus spp., Eucampia zoodiacus, Ceratium spp., and Pseudo-nitzschia pungens increased over the past 30 years. On the other hand, Paralia sulcata had stronger competitive ability for light and often was found in the bottom waters (McQuoid and Nordberg, 2003). The relative cell abundance of Paralia sulcata was high in the early 1990s, which might reﬂect the fact that the vertical mixing process was stronger in this period. Paralia sulcata obtained from the Bohai Sea was net-phytoplankton, so it has large cells, while big Paralia sulcata frequently appeared in seawater of low salinity or inadequate nutrients (McQuoid and Nordberg, 2003). The Bohai Sea was in a poor nutrient condition in the early 1990s (Jiang et al., 2005), which provided suitable environmental conditions for Paralia sulcata to become the dominant species. 2.2.2.5 Indicative Function to Eco-environmental Evolution by Phytoplankton The cell abundance ratio between Chaetoceros spp. and Coscinodiscus spp. (denoted by F ) could reﬂect nitrogen and phosphorus conditions, and it could

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185

be used to estimate the PP trend in the Bohai Sea. Chaetoceros spp. was prone to be limited by phosphorus (with a relatively large half saturation constant for phosphorus uptake and a relatively small half saturation constant for nitrogen uptake, as shown in Table 2.10 and Table 2.11), while Coscinodiscus spp. was prone to be limited by nitrogen (with a very high half saturation constant for nitrogen uptake, as shown in Table 2.10). Correlation analysis of the ratio F and the N/P ratio showed that they had signiﬁcant negative correlation. Thus, when the relative cell abundance of Coscinodiscus spp. increases or that of Chaetoceros spp. decreases, it could be concluded that the N/P ratio in the water may have an increasing trend. On the other hand, when the N/P ratio becomes high, relative cell abundance of Chaetoceros spp. may decrease while that of Coscinodiscus spp. may increase. There was a signiﬁcant positive relationship between PP and the ratio F , so it can be used to estimate the trend in PP. Therefore, the ratio F could be used to judge the nutrient status and estimate the trend of PP preliminarily, and it could be used to indicate the eco-environmental evolution. The concentrations of nitrogen and phosphorus changed much over the past 30 years. The quantity of chemical fertilizer and the Yellow River discharge were the important sources of DIN in the Bohai Sea waters from the early 1980s to the middle 1990s and from the middle 1990s onwards, respectively. The increasing sewage disposal ability was an important factor inﬂuencing the decreasing trend in concentrations of DIN in the Bohai Sea waters from the middle 1990s. Discharge from the Yellow River was an important source of DIP in the Bohai Sea waters from the early 1980s to the middle 1990s. The use of many phosphorus detergents contributed to the slightly increasing trend in the concentration of DIP in the Bohai Sea in the middle 1990s. Although many phosphorus detergents were used in the Bohai Sea Economic Circle region from the middle 1990s, the increasing sewage disposal ability in the region played an important role in reducing the discharge of DIP into the Bohai Sea and it was an important factor inﬂuencing the decreasing trend in DIP in the Bohai Sea waters from the middle 1990s. The PP in the Bohai Sea was signiﬁcantly lower from the 1990s onwards than that in the 1980s. The PP exhibited an increasing trend even in the early 21st century, but the eﬀective part played by PP in supporting ﬁshery resources might not have increased because of the frequent occurrence of red tides during the same period. There was a close positive correlation between PP and DIP and a close negative correlation between PP and DIN in the Bohai Sea in the past 30 years. The limiting nutrients of phytoplankton in the Bohai Sea changed gradually from nitrogen to nitrogen-phosphorus, and then to phosphorus over the past 30 years. Phosphorus was the main factor inﬂuencing PP in the past 30 years. Low DIP was the main reason for low PP from the 1990s onwards. So we should increase the phosphorus and control the nitrogen discharge in order to enhance PP in the Bohai Sea. Over the past 30 years, phytoplankton that was prone to be limited by phosphorus declined, such as Chaetoceros spp., Skeletonema costatum, while phytoplankton that was prone to be limited by

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nitrogen increased in the Bohai Sea, such as Coscinodiscus spp., Eucampia zoodiacus, Ceratium spp., and Pseudo-nitzschia pungens. The change in DIN and phosphorus in the Bohai Sea was an important reason for such variation. Furthermore, the cell abundance ratio between Chaetoceros spp. and Coscinodiscus spp. could be used to indicate the eco-environmental evolution of the sea preliminarily. 2.2.3 Nitrogen Forms and the Decomposition of Organic Nitrogen in Sediments Marine sediments are the main pool of nitrogen in the marine environment, and have great signiﬁcance on nitrogen cycling. Most previous research into sediment nitrogen was concentrated on total nitrogen (TN), total organic nitrogen (ON), and total inorganic nitrogen (IN), in which sediments were grinded according to a certain size and then TN, ON, and IN were determined. This method is necessary for studying the background value of nitrogen in sediments, but cannot give valuable information about nitrogen cycling. Research shows that not all nitrogen in sediments can be involved in cycling, on the one hand because of the diﬀerent combination (including physical and chemical) intensities between nitrogen and sediment, and on the other hand because of the sediment size eﬀect such that bigger grains are not liable to be crushed even during intense environmental change, and nitrogen contained cannot be released. That means only a small part of nitrogen in natural grain size sediments is involved in cycling. Therefore, it is very important to research nitrogen form characteristics of natural grain size in a nitrogen sediment cycling study. Yet there has been no quantitative research in this ﬁeld. Accordingly, this section is devoted to studying nitrogen form characteristics of southern Bohai Sea sediments of natural grain size with a sequential extraction process. The decomposition constants of ON, OC/TN (C/N), and OC/TN (OC: organic carbon) change in ratio during early diagenesis are discussed at the same time (Song et al., 2002b). 2.2.3.1 Characteristics of Nitrogen Forms The promoted Ruttenburg (1992)’s Sequential Extraction Process was used to determine contents of four diﬀerent nitrogen forms: ion-exchangebale form (IEF), carbonate form (CF), iron-manganese oxides form (IMOF), and organic matter-sulﬁde form (OSF), all of which were deﬁned as transferable nitrogen. The alkaline potassium persulphate oxidation method was used to determine the total nitrogen (TN), and the ﬁxed nitrogen was gained by TN minus the transferable nitrogen. Fig. 2.38 displays the vertical distributions of nitrogen forms in ﬁve sediment cores, and their geochemical characteristics in the early diagenesis process are as follows (Song et al., 2002b): The average contents (μg/g) of IEF-N in 5 core sediments are M9-12 (60.36)<M3-7 (71.24)<M6-5 (79.48)<M9-5 (126.76)<M10-1 (131.18). IEF-N is correlative to OC and grain

2.2 Distributions and Transformations of Nitrogen in the Bohai Sea Content (mg/g)

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Fig. 2.38. Vertical distribution of nitrogen form. M3-7 (38◦ 45.06 N, 120◦ 23.46 E), M6-5 (38◦ 32.87 N, 119◦ 37.00 E), M9-5 (38◦ 27.43 N, 118◦ 39.95 E), M9-12 (37◦ 41.13 N, 119◦ 55.38 E), M10-1 (38◦ 46.13 N, 117◦ 48.13 E) (Song et al., 2002b)

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size. The higher the OC content and the smaller the grain size the sediments have, the higher the IEF-N content they contain. Every core sediment shows a distinct vertical change trend. NO3 -N is the predominant state in IEF-N, and is mainly from overlying seawater, since the mixing intensity between sediment and seawater weakens with depth. NO3 -N content decreases with depth, which causes IEF-N to decrease with depth. M6-5, M9-5, and M10-1 display such characteristics and their IEF-N reduces by 68.2%, 76.8%, and 45.5% respectively within 0∼100 cm. In a depth of more than 100 cm, IEF-N shows little change. The concentration of CF-N is the lowest, ranging from 4.41 to 10.38 μg/g in the transferable nitrogen. Its distribution is mainly correlated to carbonate; i.e., a higher carbonate content corresponds to a higher CF-N content. M6-5, M9-12, and B63 are in the area with high carbonate content and display a high CF-N value. Located by the eastern shore of Bohai Bay, with abundant organic matter and little carbonate, M10-1 shows the least CF-N value. Except for M10-1, in which the CF-N decreases with depth, other sites display complex trend changes. The IMOF-N content is mainly aﬀected by the redox environment. Among the 5 sites, M9-12 shows the least IMOF-N content, 2.8 μg/g, and the other four sites were between 11.39 and 16.18 μg/g. Generally, vertical distributions of M6-5, M9-12, and M10-1 show a decreasing trend with depth, which is because the redox environment tends to be more reductive with sediment depth, causing IMOF-N release. Moreover, an abrupt transition of IMOF-N with depth in some core sediments indicates the mutation of the redox environment relative to the depth. OSF-N is the predominant state of transferable nitrogen. The average concentrations (μg/g) of OSF-N in studied core sediments are as follows: M10-1 (611.00)>M9-5 (531.14)>M3-7 (417.93)>M9-12 (297.58)>M6-5 (271.06). The diﬀerence in OSF-N content in core sediments reﬂects diﬀerent substances in origin and diﬀerent sediment environments. Aﬀected by terrestrial input, the OSF-N value of M10-1, located by the western shore of Bohai Bay, is twice that of M6-5, which is in the middle of Bohai Bay. Generally, the OSF-N of M37, M10-1, and M9-12 increases with depth, indicating that decomposition of organic matter weakens with depth. OSF-N distribution of M6-5 and M9-5 does not show obvious regularity. The TN distribution of M9-12, M9-5, and M6-5 displays the same trend with depth, ﬁrstly increasing remarkably and then decreasing, but with diﬀerent changes in depth. M9-5 and M6-5 reach the max at 100 cm, increasing by 88.3% and 216% respectively, indicating that early diagenesis of nitrogen is very active and reduces with depth. It also can be concluded that the sedimentation speed in this area is very high. The OSF-N in M9-12 reaches its max at 25 cm, indicating that its diagenesis depth is much lower than those in M9-5 and M10-1. The decrease of TN in depth under 100 cm indicates that the early terrestrial input contained less nitrogen than today’s. IN in M3-1 and M10-1 displays a complex change and generally increases with depth.

2.2 Distributions and Transformations of Nitrogen in the Bohai Sea

189

2.2.3.2 OC Decomposition and Early Diagenesis in Core Sediments The distribution of diﬀerent nitrogen forms changing with depth can indicate the transformation process during early diagenesis. The ON change with depth displays the rate and course of mineralization, and the variation of the C/N and OC/ON ratio can indicate the composition of organic matter, mineralization course and organic matter replenishment rate. Research on them can provide valuable information about early diagenesis. (1) Decomposition rate constant of OC and ON It can be concluded from the vertical distribution of nitrogen forms that the contents of OC, ON, BP, and BSi decrease remarkably within a depth of 0∼10 cm (Song et al., 2000b), indicating that organic matter decomposition mainly occurs at the surface and sub-surface. The decomposition rate constant of OC can be evaluated by its decrease. Taking M9-5 as an example, the decomposition rate constant of OC, ON, BP, and BSi can be estimated. It is supposed that, OC decomposition is a ﬁrst degree reaction, Z is the sediment depth (cm), C 0 and Z 0 are OC contents at a depth of 0 and Z cm respectively, K is the decomposition rate constant (yr−1 ), and S is the sedimentation rate (cm/yr). The formula is as follows:

K=

ln(C0 /CZ ) Z/S

(2.5)

According to the formula above, the decomposition rate constants of OC, ON, BP, and BSi are calculated (Table 2.12). It is concluded that the decomposition rate constant of ON is 15.51×10−3 yr−1 , and the constants of biogenic elements have the following sequence: N>P>C>Si, which is accordant with Hong et al. (1996) in studying the biogenic element of southern Taiwan Strait sediment, and but the absolute value of the former is much higher than the latter, which indicates that a diﬀerent sedimentation environment has a diﬀerent eﬀect on diagenesis. The M9-5 is located in Bohai Bay, belonging to silt-clay sediment, and is abounding in organic matter due to terrestrial input, and benthos activities are very active. All these factors are favorable to organic matter mineralization. However, sediment grain in the Table 2.12. Decomposition rate constants of C, N, P, Si OC (%) ON (μg/g) BP (μg/g) BSi (mg/g)

C0 0.493 776 6.7 18.7

Sedimentation rate S is 0.19 cm/yr, Z is 10 cm

C10 0.383 343 5.1 17.9

K (×103 ) 4.789 15.512 5.185 0.831

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2 Biogeochemical Processes of the Bohai Sea

southern Taiwan Strait is much bigger due to its strong hydrodynamic activity, and concentrations of OC, ON, P, and Si are much lower, which causes its mineralization to be weakened. Furthermore, the decomposition rate can be used as an index of organic matter reservation time in sediment. A higher decomposition rate constant means a higher decomposition rate, which indicates that the sediments are relatively new and their reservation time is correspondingly short. The reservation time of every biogenic element can be obtained by taking a reciprocal of the decomposition rate constant. According to this, the reservation time of OC, ON, P, and Si in the southern Bohai Sea sediment is 208, 64.5, 192, and 1,204 yr, respectively. It is known that the OC reservation time in marine sediment is in a range from several weeks to 106 yr, while regeneration of P is relatively quick and is regenerated partly, even in the course of the sedimentation process. Nitrogen is between carbon and phosphate, and Si has a long reservation time. Obviously, the result in this study is not completely consistent with this conclusion in that phosphate displays a long reservation time, which is mainly because of its complex diagenesis. In spite of its quick degradation, not all phosphate is degradated on release; instead, it may be retained in sediment with various combination styles through diagenesis transition and redistribution. As a result, phosphate displays a low decomposition rate and a long reservation time on a large time scale. (2) C/N and OC/ON ratio change in diagenesis The C/N ratio in marine sediment depends on the composition of the organic matter, mineralization course and replenishment rate of the organic matter. In general, the C/N ratio of sediment coming mainly from marine biogenic matter is lower than that of sediment from terrestrial input. Over a certain range of depth, the C/N ratio increases with depth due to organic matter mineralization, and OC/ON also shows an increase because of microorganism preferential utilization to ON relative to OC. Therefore, the ratio change of C/N and OC/ON with depth can reﬂect early diagenesis and nitrogen reservation. Fig. 2.39 shows the C/N and OC/ON ratio change of four core sediments in the southern Bohai Sea. It can be seen from Fig. 2.39 that M3-7 and M10-1 have a similar change in trend in the C/N and OC/ON ratios: the C/N ratio decreases slightly at the beginning and then remains constant, and the OC/ON ratio decreases to a minimum and then increases slightly. The average C/N and OC/ON ratios of M3-7 are 1.04 and 3.73 respectively, and those of M10-1 are 1.67 and 8.55 respectively. M9-5 and M9-12 have a similar trend: ratios of the former are 1.3 and 11.24, and the latter are 0.7 and 8.3. The C/N ratio of the four core sediments is much lower than OC/ON, indicating that there is plenty of inorganic nitrogen of ﬁxed ON contained in the sediments, and the change in the OC/ON ratio indicates the amount of reserved nitrogen. It can also be shown that OC/ON ratios of the four core sediments decrease with depth, which is inconsistent with the conclusion concerning ON’s preferential degradation

2.2 Distributions and Transformations of Nitrogen in the Bohai Sea 0

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Fig. 2.39. Variation of C/N and OC/ON ratios with depth in four core sediments in the southern Bohai Sea. Station M9-7 is located at 38◦ 19.55 N, 119◦ 6.11 E and the others are the same as in Fig. 2.38

relative to OC, indicating that sediment retains ON in some way. There are two main reasons for such enrichment. One is the diﬀerent composition of ON in sediments. It is known that organic matter in sediment can be divided into two parts: an active part and an inactive part. The active part includes amino acid, carbohydrate, fatty-acid, etc. The activity of these components can be shown as their contents change with depth. It has been shown that inactive components increase distinctly with depth. Therefore, a large amount of inactive components in ON is one reason for an OC/ON ratio reduction with depth. Another cause is organic nitrogen compounds adsorption to clay

192

2 Biogeochemical Processes of the Bohai Sea

minerals, which prohibits degradation, and the extent of OC/ON reduction reﬂects the amount of adsorbed organic nitrogen. 2.2.3.3 Transferable Nitrogen in Core Sediments Generally, transferable nitrogen, including IEF-N, CF-N, IMOF-N, and OSFN, can be transformed and involved in recycling under appropriate conditions. Usually, the proportion of transferable nitrogen to TN in surface sediment is higher than in deep layer sediment, since nitrogen in deep sediments transforms into more stable states during early diagenesis. Table 2.13 gives the proportion of transferable nitrogen to TN in diﬀerent depths of ﬁve studied core sediments (Song et al., 2002b). Table 2.13. Proportion of transferable nitrogen in TN (%) (Song et al., 2002b) Depth (cm) 0∼2 49∼50 95∼100 190∼200 Average

M3-7 53.75 19.13 14.79 – 19.50

M6-5 52.20 9.31 9.1 2.71 12.83

M9-5 25.36 20.12 12.54 19.02 18.37

M9-12 13.55 15.68 17.9 − 12.47

M10-1 26.41 28.8 33.11 24.68 25.79

Only a part of the results are listed. The average value is gained by the proportion of all samples in the sampling depth of every core sediment

It can be seen that the transferable nitrogen in surface sediments of the southern Bohai Sea accounts for 13.55% to 53.75% of TN. In sediments of M37, M6-5, and M9-5, the proportion increases remarkably with depth, which means the proportion in the surface layer is much higher than in the deep layer. But M9-12 and M10-1 display diﬀerent characteristics, in which the proportion in the surface layer and the deep layer is almost the same. This diﬀerence reﬂects the geochemical environmental diﬀerence. From the location points of the ﬁve core sediments, the main sources of sediments are the Huanghe, Luanhe, and Haihe Rivers, especially the Huanghe River (Song et al., 2000b). The eﬀective strength of the Huanghe River on the ﬁve cores has the sequence: M9-5>M9-12≈M6-5>M3-7≈M10-1. Aﬀected by the Huanghe River input enriched with organic matter, as for M9-12, the stable nitrogen formed in early diagenesis can be reactivated in a deeper layer sediment, and became a transferable form, which is the main reason why the transferable nitrogen in the deep layer is almost the same as in the surface layer. M10-1, mainly aﬀected by the Haihe River inputs, shows the same characteristic. Song (1997) has found that “active ferrous” concentration in deeper layer sediment is higher than in surface layer sediment in this area, proving that such activation exists. Nitrogen in sediment may have a similar activation to ferrous concentration. In the core sediments with natural grain size of the southern Bohai Sea, OSF-N, and IEF-N are preponderant forms in transferable nitrogen. The core sediments show diﬀerent vertical distributions: IEF-N decreases

2.2 Distributions and Transformations of Nitrogen in the Bohai Sea

193

with depth and IMOF-N has an abrupt transition with depth, due to redox enviroment change, and OSF-N is mainly aﬀected by sediment resources and the mineralization process. Decomposition rate constants of ON and OC in the sediment are 15.51×10−3 and 4.79×10−3 yr−1 respectively and constants of C, N, P, Si have the sequence N>P>C>Si. Because there is plenty of nitrogen combined in the sediments in early diagenesis, the C/N ratio is much lower than ON/ON. A large amount of inactive components contained in organic nitrogen and ON adsorption to clay minerals result in ON enrichment in sediment, which is the main reason for OC/ON reduction with depth. In the surface sediment, there is about 34.25% of nitrogen that can be involved in recycling. As to core sediments, the proportion of transferable nitrogen to TN in the surface layer is higher than in the deeper layer, but some special sedimentation environments with organic matter can induce “stable” nitrogen to be active and, as a result, the two proportions become almost the same (Song et al., 2002b). 2.2.4 Sediment-Water Exchange of Inorganic Nitrogen Inorganic nitrogen regeneration in sediment plays an important role in the budget and in the dynamics of nutrients in a water column. The Bohai Sea is aﬀected greatly by continental detritus input by river, which is a very important pathway of nitrogen to be the marine recipient. Nitrogen regeneration from sediments may also contribute signiﬁcantly to marine primary production in the Bohai Sea. It shows the processes aﬀecting the exchange ﬂuxes at the sediment-water interface. The nitrogen release from and adsorption onto surface sediments were also examined via simulation. Based on the incubation experiments, the exchange ﬂux of nutrients at the sediment-water interface is established in Table 2.14. In the BH98 cruise, data of incubation with airﬂow show an average of 0.34 mmol/(m2 ·d), indicating a release of NO− 3 from sediment to overlying seawater at stations E1, E3, and A2, with the exception of station G2 (Table 2.14, Zhang et al., 2004). In the case of incubation with N2 -ﬂow, the NO− 3 exchange ﬂuxes have a geometric average of −0.88 mmol/(m2 ·d), underlying a removal from seawater at stations G2, E1, and A2 except for E3 (Table 2.14). 2 The exchange ﬂux of NO− 2 shows an average of −0.041 mmol/(m ·d), which indicates a removal from seawater in the incubations with air sup2 ply. The NO− 2 ﬂuxes average 0.10 mmol/(m ·d) in N2 case, indicating a net − + increase of NO2 into seawater. The NH4 ﬂuxes vary from −0.37 to 0.065 mmol/(m2 ·d) in case of air supply, suggesting a removal from seawaters. In 2 the N2 case, the NH+ 4 shows an exchange ﬂux of 0.24∼0.52 mmol/(m ·d), + suggesting that NH4 turns out to be an important nitrogen species at the sediment-water interface in anaerobic conditions (Table 2.14). In the BH99 cruise, the observed sediment-water exchange ﬂuxes at station A2 are 0.007, − + 0.009, and 0.034 mmol/(m2 ·d) for NO− 3 , NO2 , and NH4 , respectively. At stations E3 and A4, the exchange ﬂuxes of N species in aerobic incubations

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Table 2.14. The exchange ﬂux (mmol/(m2 ·d)) of nitrogen at sediment-water interface in the Bohai Sea (Zhang et al., 2004) (With permission from Elsevier’s Copyright Clearance Center) Station BH98 cruise A2 E1 E3 G2 BH99 cruise A2 A4 E3

Series

NH+ 4

NO− 2

NO− 3

DIN

Air N2 Air N2 Air N2 Air N2

−0.370 0.520 0.065 0.260 −0.160 0.240 −0.070 0.520

0.290 −3.48 0.145 −0.655 1.05 1.08 −2.33 −3.55

−0.120 0.410 0.009 0.019 −0.033 −0.009 −0.020 0.050

−0.190 −2.55 0.235 −0.380 0.860 1.31 −2.41 −2.98

Air Air N2 Air N2

0.034 −0.016 −0.065 −0.082 0.012

0.007 −0.004 −0.034 −0.048 −0.255

0.009 −0.007 −0.005 0.003 0.034

0.050 −0.026 −0.104 −0.129 −0.210

Negative values indicate the removal of nutrients from near-bottom waters, while positive values show the release of nutrients from sediment to near-bottom waters

indicate a net removal from seawaters to sediments, with a net exchange ﬂux of − + −0.04, 0.003, and −0.06 mmol/(m2 ·d) for NO− 3 , NO2 , and NH4 , respectively (Table 2.14). With respect to the anaerobic incubation (i.e., N2 case), the aver− + 2 age ﬂuxes of NO− 3 , NO2 , and NH4 are −0.2, 0.024, and −0.01 mmol/(m ·d), − respectively. The NO3 dominates in DIN and shows a removal from seawa+ ters. Data of NO− 2 and NH4 indicate an apparent release from sediment to seawater at station E3 but a net removal from seawater at station A4 (Table 2.14). In the Bohai Sea, the benthic ﬂuxes of NO− 3 in N2 incubation can be 5∼10 fold those in aerobic conditions, suggesting an increase in nitrogen removal from seawater in anaerobic conditions. Since the NH+ 4 ﬂux in anaerobic incubation is twice as high as that in the aerobic condition, the coupling of + NO− 3 and NH4 exchange ﬂuxes indicates that denitriﬁcation happens at the sediment-water interface in N2 incubations (Rysgaard et al., 1993). The concentrations of dissolved oxygen (DO) in anaerobic incubation can be as low as ca. 0.91 mg/L, corresponding to a saturation of ca. 15%, and the DO concentration in aerobic incubations is 7.51 mg/L, with a saturation of 120%. It seems, however, that a simple relationship cannot be established between + NO− 3 and NH4 with the data in Table 2.14. In the BH98 cruise, the DO concentration of near-bottom seawaters is 5.60∼7.00 mg/L with a saturation of 80%∼99% at our incubation stations. In the BH99 cruise, the near-bottom water DO concentration increases to 9.63∼10.5 mg/L, having a saturation of 105%∼112%. Some of the ammonia will be transformed to nitrate via nitriﬁcation reaction.

2.3 Biogeochemical Processes of Phosphorus and Silicon in the Bohai Sea

195

2.3 Biogeochemical Processes of Phosphorus and Silicon in the Bohai Sea Despite the role of phosphorus as one vital nutrient, the distributions and dynamics of marine phosphorus pools are less well characterized in comparison with those of the bioelements carbon and nitrogen (Suzumura and Ingall, 2004). Phosphorus is the key element for phytoplankton growth and one of the factors in coastal estuarine and seawater eutrophication. Recently, phosphorus has been observed to limit phytoplankton production in both coastal waters and the open sea. 2.3.1 Distribution of Phosphorus and Silicate in Seawaters In coastal ecosystems, the anthropogenic inputs of phosphorus in coastal seas induce serious problems, such as reducing web diversity, altering phytoplankton compositions, and increasing the intensity and frequency of red tides (Nixon, 1993). Survey data in the past 40 years have shown that the content of inorganic nitrogen has greatly increased and the contents of inorganic phosphorus and silicon have decreased to a low level. The average phosphorus content was 1.06 μmol/L in 1982∼1983. It fell to 0.33 μmol/L in 1992∼1993 and was 0.50 μmol/L in 2000∼2001, half of that in the early 1980s, although it has slightly increased compared with its value in the early 1990s. Silicate concentration also shows a substantial decrease. The present silicate concentration (8.12 μmol/L) is only 35% that of the early 1980s (Li et al., 2003). Some researchers pointed out that these changes have caused an unbalanced proportion of nutrients, such as the ratio of N/P changing from 1.7 in the beginning of the 1980s to 23.7 in the late 1990s, Si/N from 13.7 to 2.4, and Si/P from 23.7 to 57 (Yu et al., 2000). The nutrient regime in the Bohai Sea ecosystem has changed from N-limitation to P-limitation (Tang et al., 2003). 2.3.1.1 Horizontal Distribution of Phosphate and Silicate The horizontal distributions of phosphate and silicate are shown in Fig. 2.40. In the autumn cruise, relatively high levels of reactive phosphate (i.e., PO3− 4 ) were found in Liaodong Bay with a mean value of ca. 0.35 μmol/L both at the surface and in near-bottom waters, followed by Bohai Bay (0.20 μmol/L) and Laizhou Bay (0.10 μmol/L). Concentrations of phosphate were relatively low in the central part of the Bohai Sea, where PO3− 4 averages 0.1∼0.2 μmol/L. The concentration of silicate was higher in the coastal waters than in the central part of the Bohai Sea, and the inﬂuence from riverine discharges could be identiﬁed from the horizontal proﬁles across the shelf. For example, concentrations of silicate decreased from 8.0∼10.0 μmol/L oﬀ the Yellow River Estuary to ca. 4.0 μmol/L in the central Bohai Sea. High levels of silica were also found in the area oﬀ Liaodong Bay (Fig. 2.40, Zhang et al., 2004).

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2 Biogeochemical Processes of the Bohai Sea

Fig. 2.40. Horizontal distribution (μmol/L) of phosphate and silicate of the Bohai Sea in spring (BH99) and autumn (BH98) (Zhang et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

In the spring cruise, phosphate in Laizhou Bay was 0.10∼0.20 μmol/L for both surface and near-bottom waters. The phosphate increased toward the north part of the Bohai Sea and reached 0.50∼0.60 μmol/L in Liaodong Bay. Compared to the autumn cruise, the concentration of silica was lower in the spring 1999 cruise. The inﬂuence of Bohai Bay could be seen from a relatively high level of 8.0∼9.0 μmol/L for silica, followed by Liaodong Bay. Concentration of silica was low at Laizhou Bay, with a mean level of ca. 2.0 μmol/L; this silica deﬁcit water mass apparently dominated the southern part of the Bohai Sea (Fig. 2.40). 2.3.1.2 Vertical Distribution of Phosphate and Silicon On the autumn cruise, in the section across the Bohai Strait (section A) from south to north (i.e., A1 to A4), phosphate was almost vertically homogenous in the upper waters of ca. 20 m, corresponding to the depth of pycnocline. The concentrations of phosphate increased then with depth and reached 0.65 μmol/L, in near-bottom waters (Fig. 2.41), higher in the south than in the northern part of the Bohai Strait. With respect to silica, the concentration reduced from south to north across the strait; a high level of up to 12.0 μmol/L could be distinguished from near-bottom waters at station A2. At section L in the spring cruise, phosphate increased regularly from 0.10 μmol/L in the southwest (C1) to 0.60 μmol/L in the northeast (G4). Silica averaged 10.0 μmol/L at station G4, and then reduced southwestwards to C1. At sections

2.3 Biogeochemical Processes of Phosphorus and Silicon in the Bohai Sea

197

across the Bohai Strait (i.e., A1 to A4), stratiﬁcation was identiﬁed for nutrients. Concentrations of phosphate increased considerably from surface to near-bottom waters. For silica, a vertical proﬁle across the strait provided evidence of a silica-poor water mass moving up to the surface in the central part of the Bohai Strait, with high levels (2.5∼3.0 μmol/L) of this species found in both the northern and southern parts (Fig. 2.41). C1

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A3 A1 A2 0 10 20 0.2 0.2 30 0.3 0.3 0.4 40 0.5 50 BH98 Phosphate 60 0 8.0 9.0 6.0 10 10.0 .0 20 11 9.0 8 40 .0 11. 30 0 50 BH98 Silicate 60

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Fig. 2.41. Vertical distribution (μmol/L) of phosphate and silicate of the Bohai Sea in spring (BH99) and autumn (BH98) (Zhang et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

2.3.2 Forms of Phosphorus and Silicon in Surface Sediments The geochemical phases of phosphorus are the important parameters that reﬂect the bioavailability and ecological eﬀects of phosphorus. As is well known, aqueous phosphate shares with nitrate the characteristic of being a major growth-limiting nutrient in the global biosphere, and apatite and iron oxide weathering are the major aquatic supplies.

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2.3.2.1 Forms of Phosphorus and Silicon in the Surface Sediments Phosphorus and silicon are both major biogenic elements in the marine biogeochemical process and hence occupy an important place in oceanographic research (Song et al., 2000b; Song, 2001). At present, research into the cycling of phosphorus and silicon near the sediment-seawater interface tends to focus on the conversion and burial of phosphorus and silicon near the interface in combination with the eﬀects of biological activities (chieﬂy of benthos and microorganisms) near the interface, such as the eﬀects of the phytoplankton bloom period on the interface ﬂuxes of phosphorus, silicon, and other biogenic elements (Conley and Johnstone, 1995), the relationship of bacterial production and biomass at the sea bottom to interface exchange rates of phosphorus and other nutrients (van Duyl et al., 1993). Tarapchak and Moll (1990) considered bacteria as the major user of dissolved organic phosphorus (DOP), but research conducted by Conter and Waizel (1992) demonstrated that the capability of bacteria to absorb DOP is poorer than that of phytoplankton. Functions of bacteria in the biogeochemical cycling of phosphorus in the marine environment remain to be further studied. Disturbance by benthos is another important factor aﬀecting the interface cycles of phosphorus and silicon. Aller and Yingst (1985) investigated the inﬂuence of disturbances by such marine benthonic animals as Heteromastus ﬁliformis, Macoma baltica and Tellina texana on the interface exchange and exchange rate of phosphorus. On the whole, biological activities near the sediment-water interface will aﬀect to a great extent the exchange between the sediment and its overlying water (Gehlen et al., 1995). In fact, the forms of phosphorus and silicon must be studied closely to determine the functions of phosphorus and silicon in sediments (Song et al., 2003). The distributions of various forms of phosphorus and silicon in the surface sediments of the southern Bohai Sea are shown in Table 2.15. Their major geochemical features were as follows. (1) Ion-exchangeable form (IEF) The content of IEF-P in the surface sediments ranged from 0.033 to 0.137 μmol/g (mean of 0.077 μmol/g), and that of IEF-Si ranged from 0.54 to 2.30 μmol/g (mean of 1.17 μmol/g), accounting for 0.9% and 0.01% of their respective total amounts, being the lowest percentages of all the forms. Compared with other forms of inorganic phosphorus and silicon, IEF was more closely related to the aqueous environment. Therefore, under the same physicochemical conditions, IEF would be utilized by organisms more easily than other forms of inorganic phosphorus and silicon. This was also demonstrated by the fact that the distribution areas with higher contents of IEF-P and IEF-Si appeared in such high production areas as Bohai Bay, Laizhou Bay and its neighbouring waters.

IEF

IMOF

Transferable P and Si OSF TIF BF T Si P Si P Si P Si BP BSi P Si1) 2.35/0.02 0.12/2.1 7.84/0.06 0.50/8.9 2.15/0.02 0.44/7.8 11.43/0.09 1.18/20.9 5.64 5.91 3.43/0.03 0.23/3.6 6.91/0.06 0.82/12.9 1.81/0.01 0.64/10.1 11.07/0.1 3.21/50.6 15.18/0 .11 6.34 5.72 2.57/0.02 0.16/2.5 0.56/0.004 0.37/5.8 0.82/0.006 0.59/9.3 4.08/0.03 2.99/47.2 16.60/0 .12 6.34 5.86 6.55/0.06 0.43/4.7 4.77/0.05 0.90/9.8 3.80/0.04 0.82/9.0 11.86/0.12 2.05/22.4 4.20/0 .04 9.16 4.82 7.87/0.08 0.44/4.5 2.29/0.02 0.84/8.5 2.53/0.03 1.01/10.2 11.33/0.11 1.25/23.8 20.63/0 .20 9.86 4.42 7.04/0.07 0.40/4.3 0.11/0.001 0.81/8.8 2.76/0.03 1.01/1.1 8.77/0.09 4.35/47.5 19.58/0 .21 9.16 4.93 5.64/0.06 0.25/3.6 0.35/0.003 0.87/12.4 3.26/0.03 0.71/10.1 7.29/0.10 11.33/0 .12 7.04 5.32 3.55/0.03 0.16/2.3 2.35/0.02 0.91/12.8 2.31/0.02 0.54/7.6 6.95/0.06 3.90/55.4 6.65/0 .06 7.04 5.54 8.13/0.08 0.45/4.0 7.28/0.08 0.76/6.7 2.20/0.02 0.86/7.6 16.65/0.17 6.12/54.3 40.84/0 .41 11.27 4.41 3.38/0.03 0.26/4.1 2.60/0.02 0.70/11.0 2.10/0.02 0.67/10.8 6.83/0.06 3.71/58.5 22.02/0 .21 6.34 5.56 7.86/0.08 0.52/4.9 4.95/0.05 0.90/8.5 2.48/0.03 1.10/10.4 13.52/0.14 5.71/54.2 31.31/0 .33 10.57 4.26 7.24/0.07 0.45/4.3 2.29/0.02 0.99/9.4 3.14/0.03 1.01/9.6 10.89/0.11 4.56/43.1 41.77/0 .43 10.57 4.48 3.67/0.03 0.22/2.4 4.55/0.04 1.18/12.9 4.29/0.04 0.59/6.4 9.51/0.08 4.16/45.4 31.51/0 .34 9.16 5.34 2.37/0.02 0.22/2.1 0.03/0.00 1.50/14.2 4.01/0.03 0.55/5.2 7.74/0.06 3.37/31.9 42.87/0 .42 10.57 5.48 8.94/0.09 0.64/5.7 1.09/0.01 0.93/8.3 2.99/0.03 1.22/10.8 11.59/0.12 2.15/19.1 33.21/0 .41 11.27 4.29 7.60/0.08 0.48/4.3 1.36/0.01 1.03/9.1 3.54/0.04 1.04/9.2 10.56/0.11 4.82/42.6 42.72/0 .51 11.27 4.37 6.51/0.07 0.56/4.7 0.56/0.006 1.13/9.4 3.70/0.04 1.16/9.6 8.29/0.09 5.89/49.2 43.78/0 .51 11.98 4.10 7.41/0.08 0.52/4.3 1.78/0.02 0.90/7.5 2.52/0.03 1.03/9.6 9.89/0.11 6.74/56.3 17.92/0 .21 11.98 4.05 – 0.51/4.5 – 1.03/9.1 – 1.05/9.3 – 2.59/23.0 – 11.27 – – 0.37/3.7 – 1.16/11.8 – 0.84/8.5 – 4.17/42.3 – 9.86 – 4.37/0.04 0.34/3.2 2.61/0.03 1.30/11.8 2.75/0.03 0.81/7.7 9.09/0.09 3.58/33.9 22.50/0 .21 10.57 4.90 1.55/0.01 0.18/2.0 1.11/0.01 1.08/11.8 3.58/0.03 0.31/3.4 2.44/0.03 6.10/66.6 12.20/0 .11 9.16 5.53 8.89/0.09 0.57/4.8 4.03/0.04 1.10/9.2 2.64/0.03 1.05/8.7 14.08/0.14 7.42/61.9 26.40/0 .32 11.98 4.07 7.26/0.07 0.50/4.2 1.78/0.02 1.14/9.5 2.99/0.03 1.05/8.7 10.2/0.11 5.07/42.3 55.76/0 .62 11.98 4.16 1.84/0.02 0.12/1.2 1.32/0.01 1.57/15.9 3.92/0.03 0.37/3.7 4.15/0.04 1.96/19.9 43.09/0 .42 9.86 5.36 5.36/0.05 0.40/3.8 2.27/0.02 1.26/11.9 2.48/0.03 0.89/8.4 8.58/0.08 3.96/37.5 30.87/0 .32 10.57 4.41 2.47/0.02 0.08/1.6 2.44/0.02 1.53/16.7 4.63/0.04 0.41/4.5 5.65/0.05 3.86/42.1 3.45/0 .03 9.16 5.25 8.62/0.09 0.59/6.0 3.28/0.03 0.93/9.4 2.65/0.03 1.07/10.9 12.82/0.13 5.94/60.2 40.27/0 .42 9.86 4.20 3.12/0.03 0.26/3.1 1.71/0.02 1.25/14.8 2.87/0.03 0.61/7.2 5.94/0.06 4.06/48.0 9.25/0 .09 8.45 4.90 5.56/0.05 0.36/3.7 2.81/0.03 1.01/10.7 2.91/0.03 4.14/42.8 28.33/0 .33 P or Si, and B is the ratio percentage of the content of each form of P or Si to total P or Si in sediment. 1) unit:

CF

P Si P M3-5 0.069/1.2 1.24/0.01 0.25/4.4 M3-7 0.093/1.5 0.73/0.006 0.32/5.1 M3-9 0.092/1.5 0.95/0.008 0.34/5.4 M5-1 0.044/0.5 0.54/0.005 0.35/3.8 M5-3 0.087/0.9 1.17/0.01 0.48/4.9 M5-5 0.140/1.5 1.62/0.02 0.47/5.1 M5-7 0.074/1.1 1.30/0.01 0.39/5.5 M5-9 0.078/1.1 1.05/0.01 0.30/4.3 M6-5 0.051/0.5 1.24/0.01 0.36/3.2 M7-2 0.086/1.4 0.85/0.008 0.34/5.4 M7-4 0.078/0.7 0.71/0.007 0.50/4.7 M7-6 0.083/0.8 1.36/0.01 0.48/4.5 M7-8 0.096/1.0 1.29/0.01 0.27/2.9 M7-10 0.070/0.7 1.31/0.01 0.26/2.5 M8-2 0.077/0.7 1.56/0.02 0.50/4.4 M9-1 0.067/0.6 1.60/0.02 0.49/4.3 M9-3 0.085/0.7 1.22/0.008 0.51/4.3 M9-5 0.052/0.4 0.70/0.02 0.46/3.8 M9-7 0.096/0.9 – 0.44/3.9 M9-9 0.079/0.8 – 0.39/4.0 M9-11 0.100/0.9 2.11/0.008 0.37/3.5 M9-12 0.033/0.4 0.79/0.01 0.10/1.1 M10-1 0.066/0.6 1.17/0.01 0.41/3.4 M10-3 0.088/0.7 1.17/0.01 0.46/3.8 M10-7 0.066/0.7 0.99/0.01 0.18/1.8 M11-2 0.082/0.8 0.95/0.01 0.41/3.9 M11-4 0.053/0.6 0.74/0.007 0.21/2.3 M11-6 0.081/0.8 0.92/0.01 0.40/4.1 M11-8 0.060/0.7 1.11/0.01 0.29/3.4 Avg2) 0.077/0.9 1.17/0.01 0.37/3.9 ∗ A/B, A is the content of each form of mmol/L. 2) Avg is short for Average

Site

Table 2.15. Contents of diﬀerent forms of phosphorus and silicon in the surface sediments (μmol/g)∗

2.3 Biogeochemical Processes of Phosphorus and Silicon in the Bohai Sea 199

200

2 Biogeochemical Processes of the Bohai Sea

(2) Carbonate bound form (CF) CF-Si was the dominant form of transferable silicon. This was probably due to the transport of large quantities of silt with a high concentration of carbonates from the Huanghe River each year. The content of CF-Si measured at station M11-6 was the highest. CF-P was also the major existence form of inorganic phosphorus. In earlier researches on the bioavailability of various forms of phosphorus in sediments, CF-P was considered as an inapplicable form. However, the fact is that when environmental changes are large (for example, the pH value drops down), considerable amounts of carbonates are soluble and can enter into waters to participate in the biogeochemical cycling. In this research, we found that the high-value area of CF-P appeared in Bohai Bay and the waters oﬀ the Luanhe River mouth, thus providing some supporting evidence for the above conclusion. (3) Iron-manganese oxide form (IMOF) The content of IMOF-P was relatively high in Bohai Bay, and the waters oﬀ the Huanghe River Estuary and the Luanhe River mouth. This was related to two factors: the sediments’ redox environment and the constant transport of a high content of iron in large quantities of silt in the coastal runoﬀs. It is generally accepted that IMOF-P cannot be easily utilized by organisms in a non-reductive environment, whereas in a reductive environment Fe(III) is easily reduced to Fe(II) and causes the release of P to the overlying water. This phenomenon was reported to occur in the estuary of the Hooghly River (Vaithiyanathan et al., 1993). Due to the inﬂuence of human activities in the coastal region (mainly the discharge of pollutants and nearshore aquaculture) more quantities of organic matter settle down and bacterial activity is more active in Bohai Bay and Laizhou Bay, thus making the environment reductive in part of the bottom sediments in these areas, hence the lower distribution of IMOF-P here. However, in most parts of Bohai Bay and the western part of Laizhou Bay, the very prominent reductive environment led to a high value of IMOF-P there (Song, 1997). Obviously, the transfer of IMOF-P was inﬂuenced not only by the redox environment but also by other factors, which in some cases are not negligible. (4) Organic matter+sulphide form (OSF) The OSF-P was the main form of transferable phosphorus, and was inﬂuenced to a considerable degree by the redox properties of the sedimentary environment. (5) Biogenic form (BF) The contents of BSi and BP were much higher than those of silicon and phosphorus in transferable form, indicating that through a series of early diagenesis after the precipitation of organisms or their metabolic products, most of the phosphorus and silicon contained in them had transformed into relatively stable forms and no longer participated in the biogeochemical cycling.

2.3 Biogeochemical Processes of Phosphorus and Silicon in the Bohai Sea

201

Of the forms of transferable phosphorus in surface sediments, OSF-P was the predominant form, accounting for 10.7% of the total phosphorus; CF-Si was the predominant form of transferable silicon, accounting for 0.05% of the total silicon. There was a certain similarity in the diagenesis of various forms of phosphorus. The content of the transferable form of silicon accounted for less than 0.12%, indicating that previous research on the biogeochemical process of silicon by means of the determination of the total silicon content cannot give us any valuable information (Song et al., 2003). 2.3.2.2 Biogeochemical Processes of Phosphorus and Silicon of Surface Sediments in the southern Bohai Sea Earlier research indicated that many factors aﬀect the contents and distributions of P and Si in sediments as well as P and Si transfer across the sediment seawater interface. Marine sediments are an important source and sink of P and Si in sediments and have more important functions than scientists considered before. Hence, it is necessary to ascertain which factor has the largest impact on the cycling (release and burial) of P and Si in the southern Bohai Sea. In other words, the major factor controlling the interface cycle of P and Si must be determined. (1) Correlation analysis of diﬀerent forms of phosphorus in the surface sediment The forms of phosphorus and silicon in the southern Bohai Sea surface sediments are described clearly above. The questions concerning the factors that control their forms and what are the functions of these P and Si forms are answered in the following sections. In general, the coeﬃcients of the correlation r between various forms of P or Si contained in sediments and the sedimentary parameters may reveal some possible correlations among other constituents (Tables 2.16 and 2.17) and help provide some answers to the above questions. Of all forms of phosphorus, IMOF-P exhibits a signiﬁcant positive correlation with CF-P, BP, TIP, and TP, indicating that IMOF-P plays an important role in the biogeochemical process of phosphorus, that the transfer of phosphorus near the interface is chieﬂy related to the oxidative-reductive process, and that the diagenesis processes of various forms of phosphorus are similar, whereas TSi exhibits a negative correlation with silicon in other phases (CFSi, BSi) or exhibits no signiﬁcant correlation, indicating that very little of silicon in sediments can participate in the cycling and that no valuable information can be derived from previous studies on the biogeochemical process of silicon based on determination of the total silicon. BSi, as a main source of dissolved silicon, shows a good positive correlation with CF-Si and IEF-Si and a signiﬁcant negative correlation with TSi. The possible dissolution eﬀect on the transfer of silicon and the diﬀerences between the transfer mechanisms of phosphorus and silicon make them produce totally diﬀerent results in the geochemical process. The relations of various forms of phosphorus and silicon

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2 Biogeochemical Processes of the Bohai Sea

Table 2.16. Correlation coeﬃcients for diﬀerent forms of phosphorus in surface sediment with respect to sedimentary parameters (n=29, P <0.05) (Song et al., 2002a) IEF CF IMOF OSF BF TP Fe Corg CaCO3 Eh S pH PO4 -P

IEF 1 0.43 0.11 −0.18 −0.19 −0.10 0.15 −0.12 −0.37 0.08 0.23 −0.05 0.18

CF IMOF OSF 1 0.83 −0.27 0.23 0.49 0.62 0.45 0.11 −0.25 0.01 −0.01 0.24

1 −0.07 0.44 0.75 0.77 0.54 0.35 −0.34 −0.17 −0.18 0.08

1 0.08 0.47 −0.04 −0.03 0.57 −0.13 −0.67 −0.36 −0.75

BF

1 0.50 0.30 0.13 0.47 −0.16 −0.21 −0.18 −0.28

TP

Fe

Corg CaCO3 Eh

S

pH

1 0.45 1 0.32 0.55 1 0.74 0.17 0.07 1 −0.21 −0.30 −0.15 −0.32 1 −0.45 −0.26 −0.18 −0.81 0.36 1 −0.28 −0.25 −0.17 −0.50 0.21 0.42 1 −0.37 0.03 0.25 −0.51 0.01 0.59 0.50

Table 2.17. Correlation coeﬃcients for diﬀerent forms of silicon in surface sediments with respect to sedimentary parameters (n=29, P <0.05) (Song et al., 2002a) IEF CF IMOF OSF BF TSi Fe Corg CaCO3 IEF 1 0.14 0.04 −0.14 CF 0.24 1 0.73 0.90 0.21 IMOF −0.05 0.1 1 −0.09 0.02 −0.25 OSF 0 −0.13 −0.28 1 0.02 −0.07 0.14 BF 0.39 0.38 0.02 0.1 1 0.33 −0.08 0.40 TSi −0.21 −0.87 0.03 −0.07 −0.5 1 −0.71 −0.08 −0.62 0.48 0.35 −0.07 −0.21 −0.33 0.33 0.58 −0.19 SiO3 -Si −0.17

Eh −0.24 −0.28 −0.15 0.08 −0.03 0.38 0.1

S pH −0.05 −0.07 −0.1 −0.15 0.27 0.36 −0.4 −0.08 −0.27 0.03 0.39 0.26 0.36 0.28

in sediments to CaCO3 and organic carbon (Corg ) may reveal a certain degree of regularity. (2) Function of inorganic carbon (CaCO3 ) in surface sediments Generally speaking, CaCO3 has an important eﬀect on the variation in the phosphorus concentration. Atkinson (1987) held that if phosphorus exists in the form of Ca3 (PO4 )2 , the change in phosphorus is mainly attributed to the calcifying degradation of organic matter: (CH2 O)106 (NH3 )16 (H3 PO4 ) + 138O2 + 124CaCO3 106HCO− 3

+

16NO− 3

+

HPO2− 4

+ 124Ca

2+

→ + 16H2 O

In case phosphorus is absorbed onto the surface of calcite (CaCO3 ), P change will be controlled mainly by PO4 -P. Shukla et al. (1971) compared the diﬀerence in phosphorus absorption capacities of the calcium (lime)-rich and calcium-poor sediments in freshwater lakes, and found that the absorption capacity of non-lime sediment was higher than that of lime sediment.

2.3 Biogeochemical Processes of Phosphorus and Silicon in the Bohai Sea

203

The absorption capacity of calcareous sediment decreases with the increase in CaCO3 content. In this research, we found that there was no correlation between CF-P and CaCO3 (r =0.11) and positive correlation between OSFP and CaCO3 (n=29, r =0.57, P <0.05). Therefore, in the southern Bohai Sea, due to the input of a higher concentration of CaCO3 from the Huanghe River (Song, 1997), it is quite possible that the former mechanism controls the change in phosphorus; namely, OSF-P caused by CaCO3 in sediments will inﬂuence the interface behaviour of P during its mineralization. In some cases, CaCO3 often serves as the crystal nuclei and place for the growth of CF-P too, whereas OSF-P is generally regarded as being relevant to such organic matter as humus and lipid. Therefore, this kind of “calciﬁcation” might be a process of the overlaying of organic matter on CaCO3 particles, particularly in a reductive environment. BP and BSi exhibit a positive correlation with CaCO3 (Fig. 2.42). This indicates that the behaviour of BP and BSi is similar to that of CaCO3 . CaCO3 mainly assumes a detrital form in the waters oﬀ the Huanghe River mouth, Bohai Bay, and Laizhou Bay, so most of both BP and BSi after being preserved will convert into relatively stable forms. If 100% of CaCO3 has been converted (i.e., CCaCO3 =0 μmol/g), the contents of BP and BSi irrelevant to CaCO3 will be about 0.86 and 6.99 μmol/g, respectively, and these parts of BP and BSi are in an exchangeable form, accounting for 21% and 25% of the total BP and BSi, respectively. In other words, after being preserved in sediments, about 79% of BP and 75% of BSi will be converted into relatively stable forms and will no longer participate in the biogeochemical cycling in a shorter period of settlement in sediments. 8

n=16, R=0.82, P<0.05

50

BSi (mmol/g)

BP (mmol/g)

6 5 4 3 2

40 30 20 10

1 0 0

n=16, R=0.64, P<0.05

60

7

200

400

CaCO3 (mmol/g)

600

0

0

200

400

600

CaCO3 (mmol/g)

Fig. 2.42. Correlation of CaCO3 with BP and BSi

(3) Eﬀect of Fe on P and Si forms in surface sediments Among various forms of inorganic phosphorus, both CF and IMOF exhibit signiﬁcant positive correlations with Fe (n=29, r =0.62 for CF, r =0.89 for IMOF, P <0.05), and CF-Si also exhibits a signiﬁcant positive correlation

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2 Biogeochemical Processes of the Bohai Sea

with Fe (n=29, r =0.73, P <0.05), indicating that the contents of inorganic phosphorus and silicon in sediments are mainly inﬂuenced by the content of ferric oxides and that at least parts of CF-P and CF-Si are from ferric phosphate and silicate. Fe3 (PO4 )2 ·8H2 O was found in the surface sediment from Chesapeake Bay (Bray et al., 1973). Whether CF-P in sediments in the southern Bohai Sea was from the immediate locality or from outside sources, the detrital particles of apatite or biological debris are probably their major sources. Gelatinous ferric hydroxide is closely related to phosphorus and silicon, because phosphorus or silicon probably serves as a substitute for oxygen in metal hydroxides. Therefore, all factors that make dissolved iron or suspended gelatinous ferric hydroxide settle down within the water column and that make iron in sediments dissolve will cause changes in the contents of phosphorus and silicon in sediments. The isoelectric point for non-crystalline ferric hydroxide [Fe(OH)3 or Fe2 O3 ·H2 O] is pH=8.5. When the pH value falls below this value of IEP, about 5% of the total phosphorus will settle down from the water body (Stamm and Kohlsch¨ utter, 1965), and the contents of Fe2 P and Fe2 Si will increase in sediments; the molecular formula of the products can be written as Fen (O, P)m and FeAl(O, Si)m , deﬁned as “complex compound of ferric oxide-orthophosphate” and “complex compound of ferric oxide-orthosilicate”, respectively. Sediment in a reductive environment releases phosphorus and silicon. If the overlying water is in a hypoxic state, the released phosphorus and silicon will diﬀuse into the water body; otherwise, the redox environment of the overlying water will cause much of the released phosphorus and silicon to return to the reductive surface sediment. The contents of IMOF-P, CF-P, and CF-Si in the surface sediment are still very high due to the oxidative environment of the waters (mean of Eh is 297 mV). Hence, it can be concluded that the interface behaviour of elements is inﬂuenced not only by the sedimentary environment but also by the environment near the interface (for instance, the overlying water), and that the previous research method of taking the sediment and seawater as two separate systems cannot reveal any valuable information. Only after all the particles have been buried with continuous settlement (reductive environment), can they be reduced and decomposed ﬁnally, and participate in the cycling through interstitial water. In the reductive area, phosphorus can be preserved in the form of vivianite [Fe3 (PO4 )2 ·8H2 O], and silicon can probably be preserved in the form of SiAl-Fe. Vivianite is easily soluble in a citrate-hydrosulﬁte-bicarbonate sodium (CDB) reagent, and hence it is often a constituent part of IMOF-P. Of course, vivianite is not a unique form resulting from combining phosphorus with iron in the reductive environment. (4) Relationship of organic carbon and P in surface sediments Much research showed that the ratio Corg /Porg will change in the process of diagenesis (Ingall and Cappellen, 1990; Ingall et al., 1993; Berner et al., 1993; Berner and Rao, 1994), whereas Match et al. (1987) and Ramirez and Rose (1992) showed that this ratio is constant and that the deviations resulted

2.3 Biogeochemical Processes of Phosphorus and Silicon in the Bohai Sea

205

Porg (mmol/g)

from analytical errors. The results of our research on the surface sediment in the southern Bohai Sea showed that, i) there was no signiﬁcant correlation between Corg and Porg (n=29, r =−0.03, P <0.05) (Fig. 2.43), ii) diagenesis had a deﬁnite inﬂuence on the ratio of Corg /Porg in this sea area even after consideration of possible analytical errors, and iii) the transfer of Porg and that of Corg (re-key) were two independent processes. Statistical data showed that the ratio of Corg /Porg was 340 (mol ratio), lower than the results (490, 522, and 1,200) obtained by Match et al. (1987), Ingall and Cappellen (1990), and de Lange (1992). This diﬀerence was mainly due to the shorter period of settlement of organic matter in the Bohai Sea compared with that in the deep ocean. When organic matter settles down ﬁnally on the surface of sediments, the degradation of detrital matter in the surface sediment has not yet been completed. The content of Corg is generally higher in the surface sediment but, in the southern Bohai Sea, the content of Corg in the surface sediment showed no signiﬁcant diﬀerence compared with that in lower layers (Fig. 2.43), suggesting that Corg had been preserved. Additionally, the ratio Corg /Porg was three times that in phytoplankton (Redﬁeld’s ratio). This also indicated that during subsidence and the early process of diagenesis, the phosphorus rich parts in the organic matter had been degraded preferentially, and hence Corg had already been preserved. 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 0

Surface sediments Deep-sea sediments 100

200

300 400 Corg (mmol/g)

500

600

Fig. 2.43. Correlation between Corg and Porg

The signiﬁcant positive correlation between Corg and CF-Si (n=29, r =0.90, P <0.05) indicated that CF-Si came chieﬂy from biological detritus, including metabolic matter, such as excrements, corpses, and cells of phytoplankton, and that they both had similar cyclic controlling mechanisms. TSi and Corg showed a signiﬁcant negative correlation (n=29, r =−0.80, P <0.05), suggesting that the main form of TSi was inorganic. IMOF-P, OSF-P, IMOF-Si, and CF-Si in sediments play important roles in the biogeochemical cycling of phosphorus and silicon, and CaCO3 , Fe(III), and Corg have considerable inﬂuence on the interface cycling of P and Si. The calciﬁcation of organic matter leads to the mineralization of OSF-P and hence the release of P; the reduction of Fe(III) causes the release of P and Si from IMOF-P, CF-P, and CF-Si. The

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2 Biogeochemical Processes of the Bohai Sea

diagenesis of Corg has an inﬂuence on CF-Si. About 79% of BP and 75% of BSi will convert into relatively stable forms after sedimentation, and will not participate in the cycle, at least within a short timespan. In studying the cycling of marine biogenic matter, it is necessary to investigate the forms of biogenic elements in their natural grain size sediments. The method of thoroughly grinding the samples cannot be used (Song et al., 2002a). 2.3.3 Processes of Nutrients across the Sediment-Water Interface The change rate of phosphorus concentrations in the water column indicated the net exchanges between sediment and overlying water during the incubation period. The exchange ﬂuxes were obtained from the change in the amount of phosphorus and silicon divided by the surface area and time of incubation, as described by Lerat et al. (1990) and Liu et al. (1999). According to that calculation, a negative ﬂux would indicate solute intake by sediment, whereas a positive ﬂux would indicate solute release from sediment. 2.3.3.1 Fluxes across the Sediment-Water Interface of P and Si in the Bohai Sea The ﬂuxes of phosphorus and silicon exchange between the sediment and seawater in the Bohai Sea are shown in Table 2.18 (Liu et al., 2004). Liu et al. (2004) reported the following results. In the autumn cruise, the dissolved oxygen concentration of near-bottom waters was 5.60∼6.99 mg/L, saturation of 80%∼99%, at our incubation stations. In the spring cruise, the dissolved oxygen concentration of near-bottom waters increased to 9.63∼10.5 mg/L, saturation of 105%∼112%. This implied that the exchange ﬂuxes of nutrients in the Bohai Sea were better approached by incubation under air conditions, while the exchange ﬂuxes of nutrients under N2 conditions should be considered as an approach of anaerobic conditions. (1) Autumn cruise For incubation with air, the phosphate, DOP, and TDP ﬂuxes varied from –0.043 to –0.001, –0.068 to 0.001, and –0.071 to –0.042 mmol/(m2 ·d), respectively, which showed that phosphorus moved from water to sediment. In the incubation with N2 , the ﬂuxes of phosphate ranged from –0.003 to 0.063 mmol/(m2 ·d), indicating a transport from sediment to water. The ﬂuxes of DOP and TDP varied from –0.036 to 0.087 and from –0.071 to 0.151 mmol/(m2 ·d), respectively, which indicated that they moved from sediment to water at stations A2 and E3, and in the opposite direction at stations E1 and G2 (Table 2.18). The sediments in the Bohai Sea are mainly terrestrial detritus, on average, composed of 71% clay, 23% silt, and 6% sand. Taking into account the sediment type, the geometric mean ﬂuxes of phosphorus were calculated and shown in Table 2.18. The average ﬂuxes of PO3− 4 , DOP, and TDP

2.3 Biogeochemical Processes of Phosphorus and Silicon in the Bohai Sea

207

Table 2.18. The benthic ﬂuxes of phosphorus and silicate at the sediment-water interface in the Bohai Sea (mmol/(m2 ·d)) (Liu et al., 2004) (With permission from Liu SM) Station BH98 cruise (September, 1998) A2 E1 E3 G2 Mean BH99 cruise (May, 1999) A2 A4 E3 Mean

Series

PO3− 4

DOP

TDP

Air N2 Air N2 Air N2 Air N2 Air N2

−0.043 0.063 −0.002 0.018 −0.001 0.014 −0.030 −0.003 −0.014 0.037

0.001 0.087 −0.068 −0.036 −0.047 0.012 −0.037 −0.013 −0.083 0.002

−0.042 0.151 −0.071 −0.019 −0.047 0.026 −0.067 −0.016 −0.097 0.039

Air Air N2 Air N2 Air N2

0.024 0.001 0.020 −0.065 −0.030 −0.043 −0.018

−0.072 −0.002 0.050 0.045 0.020 0.033

−0.071 0.019 −0.015 0.015 −0.029 0.016

SiO2− 3

3.01 −0.107 0.982 −0.366 −0.731 0.845 0.126

A negative ﬂux indicates the solute intake by the sediment and a positive one for the solute release from the sediment

showed exchange from sediment to overlying water under anoxic conditions and in the opposite direction under oxic conditions. (2) Spring cruise At station A2 only data on the nutrient incubation with air is available. The benthic ﬂuxes of phosphorus at station A2 were all from sediment to the overlying seawater. In the incubation bubbled with air at stations E3 and A4, all the ﬂuxes of P and Si species (TDP and SiO2− 3 ) transferred from overlying water to sediment, while DOP transferred from overlying seawater to sediment at station A4, and from sediment to overlying seawater at station at station E3 were 3 times that at station E3. The benthic ﬂuxes of SiO2− 3 A4. This explained partly the fact that the concentrations of SiO2− 3 at near bottom waters at station E3 were 5 times those at station A4. The exchange ﬂuxes of nutrients at station A2 were all from sediment to overlying water, which may in part be related to the fact that the nutrient concentrations at near bottom waters at station A2 were lower than those at stations A4 and E3. In the incubation bubbled with N2 , the ﬂuxes of DOP and TDP were released from sediment to water at station E3 but from water to sediment at station A4 2− (Table 2.18). The geometric mean ﬂuxes of PO3− 4 , SiO3 ﬂowed from water to sediment, while the ﬂuxes of DOP and TDP were released from sediment

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2 Biogeochemical Processes of the Bohai Sea

to water. If the sediments transferred from oxic into anoxic conditions with eutrophication and anoxia, at station A4, P and Si compounds were freed from sediments. At station E3, DOP and TDP were freed from sediment to 2− water and the benthic ﬂuxes of PO3− 4 and SiO3 were from water to sediment. 3− However, only the ﬂux of PO4 decreased further. Under both oxic and anoxic conditions, the ﬂuxes of DOP were released from sediment to water, but under anoxic conditions the ﬂuxes were greater than those under oxic conditions. The benthic ﬂuxes of PO3− from water to sediment under oxic conditions 4 were greater than those under anoxic conditions. 2.3.3.2 Seasonal Variations of Phosphorus and Silicon Fluxes Incubation experiments were carried out in both spring and autumn cruises at stations A2 and E3. In the second cruise the incubation at station A2 was done only under oxic conditions, and dissolved inorganic phosphorus was analyzed (Liu et al., 2004). When incubated with air supply at station A2, samples showed that the ﬂuxes of P compounds in the BH98 cruise moved from water to sediment, while dissolved inorganic phosphorus indicated a ﬂux from sediment to water in the BH99 cruise. At station E3, DOP in the BH99 cruise was from sediment to water, while ﬂuxes for PO3− 4 , DOP, TDP in the 2− BH98 cruise, and PO3− , TDP, and SiO in the BH99 cruise were from water 4 3 to sediments. When incubated with N2 supply, the benthic ﬂuxes of PO3− 4 , DOP, and TDP in the BH98 cruise and DOP and TDP in the BH99 cruise were from sediment to overlying seawater at station E3. It seemed that at station E3, if the redox condition changed from oxic to anoxic conditions, the benthic from sediment to seawater. To activities would induce the release of PO3− 4 some extent, this is in agreement with the result that the ammonium release from sediments increased with increasing temperature and decreasing DO concentrations in Chesapeake Bay. In redox conditions in our case, the benthic ﬂuxes of SiO2− 3 were from sediment to water, while the benthic ﬂuxes of PO3− , DOP, and TDP were from water to sediment. The implication was 4 that the exchanges of nutrients between sediment and water induced a change in limiting phosphorus characters in the Bohai Sea, and retarded the depletion of silicon following the decrease in riverine discharge (Yu et al., 1999). An anoxic situation arises from quick growth in economic development and population around the Bohai Sea, increasing the release of PO3− 4 , DOP, and TDP deteriorated water quality, and reduces the release of silicate from sediment, which results in a decrease in the diatom biomass, but helps the formation of dinoﬂagellates bloom. 2.3.3.3 Budgets of Phosphate and Silicate The turnover times for phosphate and silicate in the water column were calculated to assess the importance of benthic recycling based on the work of

2.3 Biogeochemical Processes of Phosphorus and Silicon in the Bohai Sea

209

Friedl et al. (1998) and Warnken et al. (2000). The phosphate and silicate inputs from sediments to the water column calculated from diagenesis equations were −0.0056×109 ∼0.084×109 mol/yr for phosphate, 0.787×109 ∼0.804×109 mol/yr for silicate with averages of 0.029×109 and 4.18×109 mol/yr for phosphate and silicate. Based on Redﬁeld’s ratios, phosphate turnovers of 0.139∼0.324 mmol/(m2 ·d) phosphorus can be estimated. Comparison with the benthic ﬂuxes of phosphate and silicate reveals that the benthic ﬂuxes correspond to a few percent of the phosphate and silicate turnover, which is indicative of obvious denitriﬁcation in sediments, intense phosphate and silicate recycling in the water column, or important riverine and atmospheric inputs. Many rivers (e.g., the Huanghe River, Haihe River, Liaohe River, and Luanhe River) discharge into the Bohai Sea with various amounts of water and sediment loads, notably the Daliaohe River, Shuangtaizihe River, Luanhe River, and Huanghe River. Diﬀerences in phosphate and silicate concentrations exist among these rivers. High levels of silicate were observed in the Huanghe River, whereas the Daliaohe River surpasses the others in DIP, and other rivers show intermediate values. Riverine phosphate and silicate inputs into the Bohai Sea are calculated based on Zhang (1996) and Zhang et al. (1997) (Table 2.19, Liu et al., 2003). Table 2.19. Phosphate and silicate inputs into the Bohai Sea (×109 mol/yr) (Liu et al., 2003) (With permission from Springer) Riverine input Atmospheric deposition Benthic ﬂux

PO3− 4 0.14±0.07 (20) 0.49 (72) 0.00∼0.14 (8)

SiO2− 3 9.78±4.89 (38) 0.59 (2) 8.80∼24.4 (60)

Values in brackets are a relative contribution (%) to the water column as a whole

Reports on the phosphate and silicate concentrations in wet and dry depositions, however, are scarce in the literature. Rainwater samples were collected by the laboratory on the Changdao, an island located in the Bohai Strait in 1995, which covered a period of 5 months with 20 consecutive rain samples in the wet season. The wet deposition ﬂuxes based on rain volume weighted average concentrations are 0.016×109 mol/yr for PO34– and 0.18×109 mol/yr for SiO23– . Dry deposition ﬂuxes of 0.006 μmol/m3 for PO34– and 0.00467 μmol/m3 for H4 SiO4 were used as an approximation to the measurements of aerosols over the Kuroshio area and northeastern China (Chen et al., 1992; Dong and Yang, 1998). Suppose that the dry deposition ﬂux of aerosols is 36.8 g/(m2 ·yr). A rough estimate of the dry deposition of nutrient species can be made (Zhang et al., 1993), and this gives an estimate of dry deposition ﬂuxes of 0.47×109 mol/yr for PO34– and 0.41×109 mol/yr for SiO23– . The total deposition ﬂuxes were estimated accordingly (Table 2.19). A preliminary estimate for the relative contribution of the nutrient inputs was made, even though it is

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2 Biogeochemical Processes of the Bohai Sea

based on limited data (Table 2.19). Ammonium and phosphate derived mainly from atmospheric deposition, nitrate was mainly transported by riverine input into the sea, and silicate from sediment regeneration accounts for up to 60% of the total. This demonstrates that nutrient regeneration in sediments contributes more silicate than riverine input and atmospheric deposition together, but benthic ﬂux contributes very much less phosphate and nitrate relative to riverine input and atmospheric deposition. Since the 1960s in the Bohai Sea, concentrations of nitrate have obviously increased from 0.30 to 3.55 μmol/L, but concentrations of phosphate and silicate have decreased from 0.76 to 0.31 and 26.6 to 6.6 μmol/L, respectively. The ratio of Si/N decreased and the N/P ratio increased (Lin et al., 1991; Yu et al., 2000). It should be born in mind that rivers in northern China are apt to suﬀer drought periods over the drainage basin and hence there was a reduction in water discharge in the 1990s (Yang et al., 1998; Wang, 2000). Moreover, the water discharge of some of the rivers is greatly modiﬁed by anthropogenic activities. For example, the Haihe River was dammed in the 1980s at the river mouth, so that almost no freshwater has been discharged into the Bohai Sea since then. The number of days without freshwater inﬂow in a year on the lower reaches of the Huanghe River was 10∼20 d in the 1980s, reaching 3∼5 months in the late 1990s. The reduction in fresh water discharge led to a decrease in nutrients input into the Bohai Sea, especially silicate which mainly comes from weathering. However, the wastewater discharge input into the Bohai Sea in 1998 was 15.24×108 t/yr with 47.4×1010 mol/yr for phosphate. The N/P ratio of wastewater was 38/1, which is much higher than that in the water column of the Bohai Sea (∼15 to 17) (Yu et al., 2000). This may contribute to a decrease in nutrient concentrations by a factor of 2 for phosphate and 4 for silicate since the 1960s. As discussed above, the benthic ﬂuxes of nutrients may lead to an increase in phosphate and silicate in the water column. The release of silicate from sediments may compensate the decrease in silicate due to the reduction in the riverine discharge. As the Bohai Sea is surrounded by areas with a rapid population increase and economic development, nutrient regeneration in sediment may have an important inﬂuence on the eutrophic character of the coastal waters in this region (Liu et al., 2003). 2.3.3.4 Release and Adsorption of Nutrients (1) Release of nutrients Nutrient-enriched sediment can release nutrients into seawater under various thermodynamic conditions. In this study, the amount of nutrients released was calculated from the change in concentration of dissolved nutrients in the experiment. Fig. 2.44 shows the nutrients release against sediment dilution; i.e., the amount of nutrients released by a unit mass of sediment is plotted against the reciprocal of the sediment concentration in water. The amount of nutrients release was estimated via regression of the experimental data.

2.3 Biogeochemical Processes of Phosphorus and Silicon in the Bohai Sea

211

The release of nutrients increased rapidly and reached a maximum with sediment dilutions up to 0.07 L/g except for NO− 2 . The maximum releases of + 2− 3− NO− , NH , SiO , and PO were 0.246, 0.026, 0.487, and 0.012 μmol/g, 3 4 3 4 respectively (Fig. 2.44, Liu et al., 2004).

Fig. 2.44. Released nutrients (μmol/g) versus solid dilution (L/g) (Liu et al., 2004) (With permission from Liu SM)

The concentrations of suspended particle matter (SPM) near bottom waters at stations B1 and E3 were 9.6∼14.2 and 19.3∼24.0 mg/L with averages of 12.2 and 22.2 mg/L, respectively. The reciprocal of the SPM near bottom waters was 0.08 L/g at station B1 and 0.045 L/g at station E3. The annual variation of SPM between the BH98 and BH99 cruises was 25% and 11% at stations B1 and E3, respectively. This implied that the nutrient releases from sediments could be very close to the maximum release. In the experiment, the atom ratios of released nutrients were Si:DIN:P=40:25:1, which indicated that the phosphorus amount was relatively low compared to the Redﬁeld ratio (Si:N:P≈16:16:1). Fig. 2.45 (Liu et al., 2004) is the plot of time dependent desorption/release of phosphate and silicate from sediments. When surface sediment and seawater were mixed, they were released from sediments (Fig. 2− 3− 2.45). At station E3, the releases of NO− 3 , SiO3 , and PO4 reached a maximum in 3 h, being 0.006, 0.083, and 0.001 mmol/g, respectively. At station 3− B1, the release of NO− 3 and PO4 reached a maximum in 2 h. The concentra+ tion of NH4 showed a rapid initial increase, followed by a slow fall indicating readsorption. Again, NO− 2 at this station was similar to that at station E3. 3− + − The overall release of NO− 3 , PO4 , NH4 , and NO2 was 0.016, 0.001, 0.008, 2− and 0.001 mmol/g, respectively. The release of SiO3 increased gradually and reached a maximum of 0.055 mmol/g within 5∼8 h. + 2− 3− at station B1 The maximum release of NO− 2 , NH4 , SiO3 , and PO4 (Fig. 2.44) was 15, 3.5, 7, and 12 times that released from the sediment-water

0.02 0 0

5 10 Time (h)

15

0.0015 0.001 0.0005 0 0

5 10 Time (h)

15

0.015 0.01

0.002 0.0015 0.001 0.0005 0

0.1

0.05

0.005 0 0

Silicate (mmol/g)

0.06 0.04

Ammonia (mmol/g)

2 Biogeochemical Processes of the Bohai Sea

Phosphate (mmol/g)

Nitrite (mmol/g)

Nitrate (mmol/g)

212

5

10 Time (h)

15

0

0

5 10 Time (h)

15

Station E3 Station B1 0

5 10 Time (h)

15

Fig. 2.45. Time dependence of phosphate and silicate releases from sediments in seawater at stations E3 and B1 in the Bohai Sea (Liu et al., 2004) (With permission from Liu SM)

mixture at a deﬁnite solid/water ratio (Fig. 2.45). The atom ratios of Si:DIN:P released from sediment at stations B1 and E3 were (50∼67):(20∼38):1. At both stations phosphorus, compared to Si and N, appeared to be less mobile from sediments. These ﬁndings indicated that nutrients were released from sediments at stations E3 and B1 when sediments were resuspended by wave and current (e.g., storm). Owing to the shallow water depth of the Bohai Sea, catastrophic events, such as wind storms, may play an important role in aﬀecting bottom sediment resuspension and sediment mass-balance in this region. Rapid response of the current circulation and suspended sediment distribution to storms was observed. Since these catastrophic events were temporal and episodic, it was diﬃcult to estimate the sediment transport due to individual wind storms. The calculation of SPM variation was, therefore, based on all grids and anchor stations’ investigations at stations E3 and B1 in the BH98 and BH99 cruises. The concentrations of resuspended SPM were estimated to be 1.97∼4.48 mg/L at station E3 and 2.53∼7.82 mg/L at station B1 by calculation of the variation of SPM concentrations integrated over the water depth. The phosphate and silicate loads at stations E3 and B1 were calculated by integrating phosphate and silicate concentrations over the water depth, which showed that released nutrients after resuspension could account for 1% for phosphate, 3%∼6% for silicate, 0.2%∼0.4% for nitrate and 4%∼9% for ammonium at station E3, and 4%∼52% for phosphate, 4%∼83% for silicate, 2%∼6% for nitrite, 0.6%∼28% for nitrate and 2%∼16% for ammonium at station B1. The diﬀerence between stations E3 and B1 was due to the fact that the resuspension released nutrients at station E3 were from experiments at a constant solid-solution ratio, while those at station B1 were from the maximum release experiments at station B1. It was clear that nutrient loads in the water column were aﬀected by resuspension of sediment, especially in shallow water areas. Resuspension

2.3 Biogeochemical Processes of Phosphorus and Silicon in the Bohai Sea

213

can lead to nutrient release to the water column from sediment particles and porewater. However, resuspension changes the diﬀusive sediment water ﬂuxes of nutrients. It was reported that in the southwestern Kattegat, Scandinavia, ﬂuxes of nutrients from sediment to water after resuspension were reduced or changed in an exchange direction. It was therefore very diﬃcult to determine if the resuspended particles were similar to the surface sediments in chemical composition, grain-size, etc., and had characteristics depending on consolidation and resuspension history, and cohesion. The change in net nutrients exchange ﬂuxes between sediment and water after resuspension in the Bohai Sea needs to be studied further. (2) Adsorption of phosphate The results of this preliminary approach to determine the phosphate adsorption are shown in Fig. 2.46 (Liu et al., 2004). As seen from these data, the adsorption of phosphate by sediments at stations B1 and E3 can be simulated by a simple linear adsorption isotherm, which is a special case of Langmuir and Freundlich isotherms (Krom and Berner, 1980). Applying linear adsorption isotherm here would yield: Cs = Cso + Cse = K ∗ C

(2.6)

0.6

Phosphate adsorption (99HB-B1)

Adsorption (mmol/g)

Adsorption (mmol/g)

Here C s is the mass adsorbed per unit mass of total solids, C (μmol/L) is the concentration in solution at equilibrium, K (L/g) is the linear adsorption coeﬃcient, C se is the mass of adsorbed phosphate (per gram of dry sediment) added during the experiment, and C so (μmol/g) is the original quantity of phosphate on the surface of the sediment, which is exchangeable with the phosphate added in the experiment. A graph of C s vs C should be a straight line with slope of K and intercept of C so .

0.4 0.2 0 0

1

2

Concentration (mmol/L)

3

Phosphate adsorption (99HB-E3) 0.2

0.1

0

0

0.5 1.0 Concentration (mmol/L)

1.5

Fig. 2.46. Plot of mass of phosphate adsorbed during experiment (in μmol/g dry weight) against equilibrium concentration of dissolved phosphate (in μmol/L) (Liu et al., 2004) (With permission from Liu SM)

The linear regression coeﬃcients were R 2 =0.99 for stations E3 and B1. The linear adsorption coeﬃcients were 189.3 and 248.9 ml/g at stations E3

214

2 Biogeochemical Processes of the Bohai Sea

and B1, respectively, ranked at the lower end in the range for estuarine and oceanic oxic sediments (50∼5,000 ml/g). The concentrations of exchangeable phosphate on the sediments were 0.06 and 0.03 mmol/g at stations B1 and E3, respectively, which were 40 and 30 times the released mass of PO3− 4 at stations B1 and E3, respectively, and 5 times the maximum released mass of PO3− 4 at station B1. This indicated that the values of exchangeable phosphate were much higher than those obtained from release experiments on PO3− 4 , which was probably partly due to the fact that not all phosphate combined onto the sediments can be released via sediment resuspension (Liu et al., 2004).

2.4 Behaviour of Heavy Metals in the Bohai Sea Pollutants entering the marine environment through rivers aﬀect estuarine and deltaic systems in various ways. The impact of anthropogenic activities on marine environments, especially on enclosed systems such as the Bohai Sea, can be determined by measuring various chemical markers including nutrients, organic compounds and toxic heavy metals in the water column, biota, and sediments (Tuncel et al., 2007). Of the many pollutants found in coastal and estuarine sediments, heavy metals are amongst the most persistent because they cannot be destroyed or broken down. Therefore, they are useful as markers of environmental change (Dai et al., 2007). In coastal zones, metals can enter surface waters principally through atmospheric deposition, industrial efﬂuent discharge and streams. When metals enter marine environments, they are adsorbed onto the sediments of adjacent shelf regions. Marine sediments act as scavengers of trace metals and often provide an excellent record of human impact (Guevara et al., 2005). Although the Bohai Sea makes up only 1.6% of the total of China marginal seas, it receives about 36% of the wastewater and 47% of the solid pollutants in China marginal seas. Large amounts of heavy metals are transported into this sea through direct discharges or loads from rivers. For example, the Liaohe River discharged 390 t of heavy metals into the sea in 2002 while, in 2003, the Yellow River totally discharged 200 t of heavy metals and the Luanhe River 120 t (Wang and Wang, 2007). Metal pollution is an important environmental problem because of its potential toxicity and accumulation in aquatic food webs (Cravo and Bebianno, 2005). It has been shown that heavy metals profoundly inﬂuenced the marine ecosystem structure and function, and the health of human beings (Chen et al., 2004). 2.4.1 Distribution of Dissolved Heavy Metals in Seawaters The investigation was conducted from August 12 to August 25, 2003, along with the sea monitoring operations made by the North Sea Branch of the National Ocean Bureau of China. The heavy metal water samples at surface and bottom layers were collected from 48 stations in the whole area of the Bohai Sea (Fig. 2.47, Wang and Wang, 2007).

2.4 Behaviour of Heavy Metals in the Bohai Sea

215

N 41

40

39

38

37

118

119

120

121

122

E

Fig. 2.47. Study stations for heavy metals investigation in the Bohai Sea waters (Wang and Wang, 2007) (With permission from Elsevier’s Copyright Clearance Center)

The average concentration of the dissolved Pb was (1.1±0.4) μg/L (mean ±SD) in August 2003, which was slightly higher than the grade-one Sea Water Quality Standard of China (SWQSC-1). The estimated ratio of over-standard waters was utilized and their integral sea area surveyed (EROSA) was up to 66% at the surface and 51% at the bottom layer, but there was no station where the concentration was higher than the grade-two Sea Water Quality Standard of China (SWQSC-2). The increased concentrations of dissolved Pb mainly appeared in three bays and the Bohai Straits, and the isopleths generally descended from the bays and straits to the central areas (Fig. 2.48, Wang and Wang, 2007). The average concentrations in Liaodong Bay and the Bohai Straits were the highest, those in Bohai Bay and Laizhou Bay were very similar, while Pb concentration in the central area was the lowest. In addition, there was no diﬀerence between Pb in the surface and bottom layers (Paired Samples t-test, P =0.35). The geometric mean concentration of dissolved Hg was 0.03 μg/L in August 2003, which was less than SWQSC-1. The EROSA was less than 1% at the surface while up to 12% at the bottom layer. Again, there was no station with Hg concentrations higher than SWQSC-2. The spatial distribution of dissolved Hg showed a pattern diﬀerent from that obtained for Pb; over-standard stations did not only occur in Liaodong Bay, Bohai Bay, Laizhou Bay, and the Bohai Straits, but also in the central area; viz., they appeared not only inshore but also in oﬀshore areas (Fig. 2.48). The vertical distribution of Hg concentration was not uniform (Paired Samples ttest, P =0.05): both the average concentration and the EROSA of dissolved Hg at the bottom layer were higher than those at the surface. The average concentrations of dissolved Cd, Cu, and As were (0.31±0.12), (1.9±0.8), and (1.5±0.3) μg/L (mean±SD) in August 2003, which were less than SWQSC-1, respectively, and there was no station with concentrations

216

2 Biogeochemical Processes of the Bohai Sea

Fig. 2.48. Horizontal distribution of concentration of dissolved heavy metals in the Bohai Sea in August, 2003 (isopleths are in μg/L) (Wang and Wang, 2007) (With permission from Elsevier’s Copyright Clearance Center)

2.4 Behaviour of Heavy Metals in the Bohai Sea

217

higher than SWQSC-1. Increased concentrations mainly appeared in three bays and the Bohai Straits. Similar to that of Pb, their isopleths largely decreased from the bays to the central waters (Fig. 2.48). Vertical distributions did not show diﬀerences between bottom and surface layers for Cd (P =0.98), Cu (P =0.26), and As (P =0.3) in the Bohai Sea. The spatial distribution of dissolved Pb was similar to that of Cd, Cu, and As. Isopleths largely descended from the bays to the central areas, indicating strong continental inputs. Only Hg had a diﬀerent distribution pattern, being discovered with relatively high concentrations located not only inshore but also in oﬀshore areas, which suggested that processes other than continental inputs were probably inﬂuencing the dissolved concentrations of Hg. Some areas with relatively high concentrations were staggered and resulted in a complicated pattern of the horizontal distribution of dissolved heavy metals. We infer that such complicated patterns are related not only to the input of pollutants from land but also to the dynamics of sea water masses and the biochemical processes inﬂuencing dissolved heavy metals in the water column as well. The pollutants of the Bohai Sea mainly derive from land sources. Taking Liaodong Bay for example, the Liaohe River is the biggest source of pollutants, discharging 390 t of heavy metals in 2002, and another important source of pollutants is the Luanhe River, which discharged 120 t of heavy metals into the Bohai Sea in 2003. The annual average import of Pb from sewage along the coast of the Bohai Sea was about 613 t (Zhang, 2001), of which 55.7% were discharged into Liaodong Bay. As a result, the average concentration of Pb found in this study for Liaodong Bay was very high. The Yellow River, discharging 216 t of heavy metals annually, was assumed to be the most important source for Cd (Liu and Zhang, 1996). Consequently, the concentration of Cd found in this study was higher at the estuary of the Yellow River and its adjacent waters. Last but not least, the direct discharge of sewage from ambient industrial zones was shown to be an important source for heavy metals in the Bohai Sea, as there were 66 main point sources identiﬁed around the Bohai Sea, accounting for 30.4% of the main inputs in China (Gao and Liu, 1999). On the other hand, as can be seen in Fig. 2.49 (Wang and Wang, 2007), the salinity was high in the west but low in the east, high in the bays but low in the central area and the Bohai Straits, and there were no regions with relatively low salinity near the estuaries. The above phenomenon indicated that the freshwater inputs of rivers were relatively small in summer 2003, leading to an increase in the salinity. This eﬀect was ampliﬁed by relatively high evaporation due to relatively shallow waters. Moreover, high concentrations of heavy metals did not occur only in the estuaries, as shown in Fig. 2.48, indicating that direct discharges are also an important inﬂuencing factor of the spatial distribution of heavy metals. The dynamics of sea water masses can also be regarded as an important factor inﬂuencing the spatial distribution of metals. The Bohai Sea exchanges sea water with the Yellow Sea through the Bohai Strait (mainly “in from the north, out from the south”). Liaodong Bay is basically controlled by a cyclonic

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2 Biogeochemical Processes of the Bohai Sea

gyre, while the waters of the estuary of the Yellow River and Bohai Bay are basically controlled by an anticyclonic gyre. The special sea currents of the Bohai Sea resulted in the fact that sea water exchange is signiﬁcantly diﬀerent in various areas. For example, Laizhou Bay has the highest exchange capacity with a half-life of 6 months, while Liaodong Bay, especially the northwestern part of it, has the lowest one with a half-life of 3 yr. Consequently, the average concentration of Hg found in this study for the whole Bohai Sea was 0.05 μg/L in August 2002, while Laizhou Bay showed the highest concentrations up to 0.07 μg/L, but in August 2003, it decreased to 0.03 μg/L for both the Bohai Sea and Laizhou Bay. It can be assumed that biochemical processes are also important for the spatial distribution of dissolved heavy metals in sea water. Phytoplankton, microbes, and other ocean life-forms can adsorb, accumulate, transport, and release dissolved heavy metals, furthermore complicating their distributions. Hish dissolved metal concentrations within oﬀshore regions could be the results of such biogeochemical processes. The vertical proﬁles of dissolved heavy metals may be also determined by release from bottom sediments owing to oxidation-reduction cycles and degradation of organic materials. As already reported, the Bohai Sea had been badly contaminated in the last decade, with high values of Hg in sea water and sediments (Zhang, 2001). When the concentration of dissolved Hg dramatically decreased, the release of Hg from bottom sediments might have resulted in a higher concentration of dissolved Hg in the bottom rather than in the surface layer, which was the likely explanation for the vertical distribution of Hg. Although there were few all-around investigations of dissolved heavy metals in the whole Bohai Sea, some papers had reported data of concentrations of the main dissolved heavy metals in inshore areas over several decades (Zhao and Kong, 2000; Zhang, 2001). So we can make some conclusions by comparing the data of diﬀerent N 41

40

39

38

37

118

119

120

121

122

E

Fig. 2.49. Horizontal distribution of the salinity (‰) in the Bohai Sea in August 2003. Solid lines: surface salinity; dotted lines: bottom salinity (Wang and Wang, 2007) (With permission from Elsevier’s Copyright Clearance Center)

2.4 Behaviour of Heavy Metals in the Bohai Sea

219

years. As can be seen from Fig. 2.50 (Wang and Wang, 2007), average concentrations of dissolved heavy metals tended to increase from the beginning of the 1980’s to the middle of the 1990’s, but then declined and became stable in recent years, which is consistent with the results of (Li et al., 1996) who had investigated the history of heavy metal pollution in the Bohai Sea by the sediment 210 Pb age dating method.

Concentration ( m g/L)

10

Pb

Hg

Cd

1

0.1

0.01 1975

1980

1985

1990 Year

1995

2000

2005

Fig. 2.50. Changing trend in concentration of dissolved heavy metals in inshore areas of the Bohai Sea in the past 30 years (the data before 2000 are referred to (Zhang, 2001)) (Wang and Wang, 2007) (With permission from Elsevier’s Copyright Clearance Center)

2.4.2 Dissolved Heavy Metal Pollution in Bohai Bay Bohai Bay is located in the west of the Bohai Sea and near the city of Tianjin and the provinces of Hebei and Shandong. Accounting for about 20% of the extent of the Bohai Sea, and with a mean depth of 12.5 m, Bohai Bay is a typical semi-enclosed coastal sea and has limited water exchange with the ocean. Large quantities of wastewater are discharged into Bohai Bay each year from rivers of the Beijing-Tianjin, which are polluted with eﬄuent and storm runoﬀ. Pollution and environmental management of enclosed coastal seas is an important area of research, which has received a good deal of attention worldwide. Since the 1970s, the environmental quality of Bohai Bay has declined as economic development, pollution, and city expansion have increased in the Bohai Sea coastal area. Surﬁcial seawater samples were collected from 20 stations in coastal areas of Bohai Bay (Fig. 2.51, Meng et al., 2008). Sediment samples were collected from 11 stations in Bohai Bay, including 3 samples collected from the tidal belt closer to terrestrial discharge outlets and 8 samples from coastal areas of Bohai Bay.

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2 Biogeochemical Processes of the Bohai Sea

Fig. 2.51. Study areas and sampling locations in Bohai Bay near Tianjin, northern China (Meng et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

2.4.2.1 Distribution Characteristics of the Main Polluting Metals in Surface Waters The metal concentration ranges and averages are summarized in Table 2.20 (Meng et al., 2008). The National Seawater Quality Standard of China (GB 3097-1997) was used to assess the levels of heavy metals in coastal waters from Tianjin Bohai Bay. The quality of seawater is plotted by four levels corresponding to the diﬀerent function zones according to the standard GB 3097-1997. Table 2.20. Concentration ranges and average values of heavy metals in Bohai Bay waters (Meng et al., 2008) (With permission from Elsevier’s Copyright Clearance Center) Element Cu Pb Zn Cd Cr As Hg

Concentration range (μg/L) 1.60∼4.10 3.63∼12.65 3.0∼55.0 0.08∼0.19 0.11∼1.15 0.79∼2.06 0.004∼0.09

Mean (μg/L) 2.54±0.77 7.18±2.57 26.9±14.2 0.12±0.03 0.40±0.26 1.26±0.41 0.04±0.02

2.4 Behaviour of Heavy Metals in the Bohai Sea

221

Most metals except Zn and Pb in surface waters of Bohai Bay attained the ﬁrst level, indicating concentrations that are not out of the range of clean seawater. Consequently, Zn and Pb were the main contaminative elements in coastal waters from Tianjin Bohai Bay. The total concentrations of Zn ranged from 3.0 to 55.0 μg/L, and the average Zn concentration in coastal waters was 26.9 μg/L. The highest Zn concentration (55.0 μg/L) was found at site 8 (Fig. 2.52, Meng et al., 2008). According to the National Seawater Quality Standard (GB 3097-1997), the concentration of Zn in 70% of the sample sites attained the second level (50 μg/L), and 10% of sites attained the third level (100 μg/L) (Fig. 2.52a). The total Pb concentrations ranged from 3.63 to 12.65 μg/L, and the average in coastal waters was 7.2 μg/L. According to the standard GB 3097-1997, the concentrations of Pb in 55% of sampled sites attained the third level (10 μg/L), 25% of sites attained the fourth level (50 μg/L) and the remaining sites reached the second level (5 μg/L) (Fig. 2.52b). 60

Second level

Zn total (mg/L)

50 40 30 20

First level

10 0

1

Pb total ( m g/L)

14.0 12.0 10.0 8.0 6.0 4.0 2.0 0

5

3 2

7 6

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9

12 15 13 A25 A18 10 11 16 A7 A12 A19 Station

9

12 15 13 A25 A18 10 11 16 A7 A12 A19 Station

Third level

Second level

1

5

3 2

4

7 6

8

Fig. 2.52. Distribution of Zn and Pb in Bohai Bay waters (Meng et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

The distribution characteristics of heavy metal pollutants can be used to determine likely pollution sources and the physicochemical properties of elements. The distribution of Zn in surface waters exhibited a strong declining trend in concentration with the distance from the estuary mouth to the outer parts of Tianjin Bohai Bay. The stations with higher Zn concentrations were located near the mouths of the Beitang and Dagu estuaries, suggesting that the Zn contamination in Tianjin Bohai Bay was the result of terrestrial sewage discharge (Fig. 2.52). The distribution of Pb in coastal waters exhib-

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2 Biogeochemical Processes of the Bohai Sea

ited a diﬀerent trend to that of Zn (Fig. 2.52). First, sites with higher Pb concentrations were distributed in the inner embayment beside inshore waters. This distribution characteristic can be attributed to pollution sources and the migratory paths of Pb. Pb pollution in aqueous environments has various origins including the deposition of leaded atmospheric dust from the burning of fuels and the emission of mobile pollution sources in the sea and river discharge. The deposition of leaded atmospheric dust would result in Pb pollution in the center of the bay. 2.4.2.2 Historical Changes of Pb and Zn in Surface Waters

Zn (mg/L)

9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0

90.0 80.0 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0

Annual concentration of Pb Annual runoff input

1994

1996

1998

2000

2002

2004

60 50 40 30 20 10 0

Runoff ( 108 m3)

Pb (mg/L)

Historical data of contaminative elements (Pb and Zn) in Bohai Bay from 1987 to 2004 were collected and the trends were examined. The annual average concentrations of Pb and Zn in surface waters and the corresponding annual runoﬀ of Tianjin Bohai Bay are shown in Fig. 2.53 (Meng et al., 2008). For coastal waters in Bohai Bay before 2001, the annual average concentration of Pb was well correlated with annual river runoﬀ (R 2 =0.77), indicating that Pb pollution in these waters originated primarily from river discharge during this period. However, the levels of Pb did not decrease after 2001, when annual runoﬀ levels declined, indicating that Pb pollution by atmospheric deposition had increased because of the use of leaded petrol in motorcars. According to the standard GB 3097-1997, the concentrations of Pb in coastal waters near Tianjin have exceeded the ﬁrst level since 1994. Moreover, the Pb levels in coastal waters have attained the third level since 2001, despite the continued decrease in annual runoﬀ. As a whole, Pb concentrations

Annual concentration of Zn

1987 1989 1990 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 Year

Fig. 2.53. Annual average concentrations of Pb and Zn in surface waters and the corresponding annual runoﬀ into Bohai Bay (Meng et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

2.4 Behaviour of Heavy Metals in the Bohai Sea

223

increased from 2.6 μg/L in 1994 to 6.2 μg/L in 2004, with a sharp rise occurring after 2001, probably due to the use of leaded petrol. Annual runoﬀ declined continually before 2001 owing to the construction of reservoirs upstream on input rivers and the continually decreasing rainfall in the drainage basin of northern China. The Zn concentrations in Tianjin Bohai Bay coastal waters exhibited three climax phases from 1987 to 2004. The ﬁrst climax was from 1989 to 1990 and the average concentrations of Zn were 68.6 μg/L. The second climax appeared in 1999, with concentrations of 63.7 μg/L, and the third appeared in 2004 with concentrations of 74.9 μg/L, within the range of the third level (100 μg/L). However, the annual concentrations of Zn showed little correlation with annual runoﬀ input, suggesting that Zn contamination had various origins: river discharge, atmospheric deposition, salt-water advection from the ocean and industrial discharge. 2.4.3 Heavy Metals in Bohai Bay Sediments The heavy metals in Bohai Bay sediments mainly come from the rivers input, such as the input from the Dagu River, the Duliujian River, the Qihe River, and the Haihe River. Their distributions are aﬀected by many factors (Song, 2004). 2.4.3.1 Geochemical Characteristics of Core Sediments (1) Grain size The grain sizes of three core sediments have been determined by the UdderWentworth grade scale. According to the Udder-Wentworth grade scale, the grain size range of silt sand, very ﬁne sand, ﬁne sand, and clay are 0.125∼0.25 mm, 0.063∼0.125 mm, 0.004∼0.063 mm, and below 0.004 mm, respectively. At station C1 (Fig. 2.54, Qin et al., 2006), the average percentage contents of clay, silt sand, very ﬁne sand, and ﬁne sand are 6.5%, 30.2%, 39.5%, and 22.4%, respectively. At station C3 (Fig. 2.54), the proportion of very ﬁne sand is increasing, arriving at 53.4%, while the other compositions like silt sand and ﬁne sand have similar proportions, 21.5% and 21%, respectively, and the proportion of clay is decreasing to 3.2%. However, the grain size of the Qihe River Estuary is ﬁner, where the proportion of clay has reached nearly 8%, the silt sand is 48%, and the very ﬁne sand is 31%. On average, the distributions of grain size for C1 and C3 are similar, where the dominant form is very ﬁne sand, followed by silt sand and then ﬁne sand. By contrast, at station C4 (Fig. 2.54), the powder sand has turned into the dominant form, followed by the very ﬁne sand and then the ﬁne sand and the clay. The analysis of the grain size has indicated that sediments of Bohai Bay become ﬁner from the north to the south. Besides, the Qihe River Estuary lies at the top of Bohai Bay, where the hydrodynamics and the dilution of seawater are faint, so contaminants can easily accumulate there. The vertical distributions

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2 Biogeochemical Processes of the Bohai Sea

N 39 C1 20

C3 C4 02

5

10

38 118.0

118.5 E

Fig. 2.54. Schematic map of core sediments sampling stations in tidal zones of Bohai Bay. C1: Dagu Estuary; C3: Duliujian Estuary; C4: Qihe River Estuary (Qin et al., 2006) (With permission from Qin YW) 0

Grain size (mm) 50 100 150 200

Depth (cm)

0

40 0

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Grain size (mm) 60 80 100 120

0

Grain size (mm) 100 200 300

400

0 40

80

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C1

120

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160 C3

200

C4

Fig. 2.55. Vertical distribution of the average grain size for core sediments in tidal zones of Bohai Bay. C1: Dagu Estuary; C3: Duliujian Estuary; C4: Qihe River Estuary (Qin et al., 2006) (With permission from Qin YW)

2.4 Behaviour of Heavy Metals in the Bohai Sea

225

of the average grain size are shown in Fig. 2.55 (Qin et al., 2006), indicating that the grain size becomes ﬁner with depth. (2) Hydrous iron, manganese, and aluminum oxides Hydrous iron, manganese, and aluminum oxides are important inorganic colloids in sediments, which will aﬀect the content of heavy metals in sediments due to some physical chemistry interactions, like adsorption and co-deposition (Qin et al., 2004). The correlative results of Fe, Mn, Al, and grain size are shown in Table 2.21 (Qin et al., 2006). The good positive correlations among Fe, Mn, and Al indicate that these three elements are the constituents of sediment, and they originate mostly from rock weathering, not anthropogenic input. The average contents of Al, Fe, and Mn are 63, 23.9, and 0.52 mg/g, respectively. These results are in accordance with the literature values in the 1980s (Guo et al., 1983). The distribution of Al, Fe, and Mn has a highly negative correlation with the grain size, indicating that the ﬁner sediments will have more hydrous Fe, Mn, and Al oxides. Moreover, the distribution of Al, Fe, and Mn has a good positive correlation with the average proportion of clay and silt sand, whereas there is a good negative correlation with the average proportion of ﬁne and very ﬁne sand, suggesting that most hydrous metal oxides are important components for the clay and silt sand (Table 2.21). Table 2.21. Correlation (Pearson) coeﬃcient matrices between the hydrous metal contents and grain proportion of the core sediments in the Dagu Estuary (Qin et al., 2006) (With permission from Qin YW) Al Fe Mn Clay Silt sand Very ﬁne sand Al 1.000 Fe 0.931 1.000 Mn 0.866 0.946 1.000 Clay 0.726b 0.728b 0.746b 1.000 Silt sand 0.759b 0.799b 0.880a 0.822 1.000 Very ﬁne sand −0.645b −0.445 −0.422 −0.642 −0.626 1.000 Fine sand −0.672 −0.777b −0.874a −0.793 −0.967 0.443 α=0.01; n=21; a. highly positive correlation; b. remarkably positive correlation

Fine sand

1.000

(3) Distribution of polluted heavy metals in the tidal core sediment Five areas in the coastal region of Bohai Bay were delineated, based on hydrodynamic and sewage discharge conditions, to analyze the pollution levels of metal elements in surﬁcial sediments (Table 2.22, Fig. 2.56, Meng et al., 2008). The ﬁve study regions representing the diﬀerent sewage discharge conditions for Bohai Bay were the Dagu Estuary region, the Lvjuhe River region, the Duliujian Estuary region, the Qihe River Estuary region, and the inner embayment region. Four main estuaries are distributed in Tianjin Bohai Bay from north to south, namely Beitang, Dagu, Duliujian, and Qihe River estuaries, and most sewage rivers, such as the South Sewage River and the Beijing Sewage River, discharge into these estuaries. The South Sewage River and the Beijing Sewage River carry a great amount of industrial and municipal

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2 Biogeochemical Processes of the Bohai Sea

sewage from the cities of Tianjin and Beijing, respectively. Consequently, all sediment samplings were attributed to the corresponding study region (Table 2.22). Each study region also comprised two kinds of sediment samples: tidal sediments and oceanic sediments. The metal element (Pb, Zn, Cu, As, Cd, and Cr) contents of sediments from the ﬁve Bohai Bay study areas are presented in Table 2.23 (Meng et al., 2008). The corresponding upper limits of environmental background values were used to determine the pollution levels of individual elements (Qin et al., 2006). The average Zn content was 131.8 mg/kg, and the highest level appeared in tidal sediments from the tidal zones of the Dagu Estuary, which had levels about ﬁve times that of the environmental background value (Table 2.23). The average Pb content of surﬁcial sediments from Tianjin Bohai Bay was 22.4 mg/kg and the maximum content was found in tidal sediments from the Qihe River Estuary, which had an average Pb content of 34.9 mg/kg and doubled the corresponding upper limit for environmental background values (Table 2.23). The Cd content was higher in tidal sediments closer to the main Table 2.22. Compartmentalization of sampling stations (Meng et al., 2008) (With permission from Elsevier’s Copyright Clearance Center) Study region Dagu Estuary (R1) Tidal zones of Dagu Estuary (R1-2) Lvjuhe River (R2) Duliujian Estuary (R3) Tidal zones of Duliujian Estuary (R3-2) Qihe River Estuary (R4) Tidal zones of Qihe River Estuary (R4-2) Inner region of Bohai Bay (R5)

Sampling station S5 S1 S9, S10 S14, S15 SC3 S20, S21 SC4 S11

Table 2.23. Average concentrations of metal elements in surﬁcial sediments from Bohai Bay and the corresponding environmental background values (Meng et al., 2008) (With permission from Elsevier’s Copyright Clearance Center) Study region

Zn Dagu Estuary 90.9 Tidal zones of Dagu Estuary 392.8 Lvjuhe River 96.6 Duliujian Estuary 89.9 Tidal zones of Duliujian Estuary 68.7 Qihe River Estuary 91.4 Tidal zones of Qihe River Estuary 112.9 Inner region of Bohai Bay 110.9 Background values∗ 75.0 ∗

Referenced from Li et al. (1990; 1994)

Pb 17.9 28.4 17.5 21.5 22.5 17.9 34.9 17.9 16.6

Total metal (mg/kg) Cd Cu Cr As 0.32 21.9 117 6.70 1.01 11.4 42 7.50 0.25 27.3 191 7.00 0.20 20.9 107 6.64 1.07 12.3 18 8.50 0.20 22.5 110 6.49 1.82 15.6 59 16.5 0.14 23.0 110 6.40 0.14 25.9 60.0 13.0

Hg 0.30 0.03 0.65 0.51 0.02 0.85 0.04 0.25 0.05

2.4 Behaviour of Heavy Metals in the Bohai Sea

227

estuary than in coastal sediments from inshore sites. The highest Cd level appeared in tidal sediments from the Qihe River Estuary, which had a Cd level (1.82 mg/kg) 12 times greater than the environmental background value. In contrast, the Cd contents in sediments from the inner embayment were closer to the environmental background value and gave no sign of anthropogenic enrichment (Table 2.23). The contamination degree of heavy metals was assessed on the basis of the contamination factor Cfi . The contamination factor is measured by the following equation (Hakanson, 1980): i Cfi = C0∼1 /Cni i where C0∼1 is the mean content of metals from sampling sites, and Cni is the pre-industrial concentration of individual metal. In this study, the background value of individual metal in sediments from Bohai Bay is applied as the preindustrial concentration of Cni . The background value of individual metal is shown in Table 2.23 (Meng et al., 2008). Values of the contamination factor are characterized as follows: Cfi <1 indicating a low contamination of the sediment with the examined substance, 1Cfi <3 a moderate contamination factor, 3Cfi <6 a considerable contamination factor, and 6Cfi a very high contamination factor. Cfi of metal elements is shown in Table 2.24 (Meng et al., 2008). First, the contamination extent was set oﬀ according to the contamination factor of metal elements, Cd>Zn>Pb>Cr>Cu. Consequently, Pb, Zn, and Cd were considered the main polluting elements in surﬁcial sediments from Bohai Bay, and their distributions in sediments are shown in Fig. 2.56 (Meng et al., 2008).

Table 2.24. The contamination factor of metal elements in surﬁcial sediments from Bohai Bay (Meng et al., 2008) (With permission from Elsevier’s Copyright Clearance Center) Study region Dagu Estuary Tidal zones of Dagu Estuary Lvjuhe River Duliujian Estuary Tidal zones of Duliujian Estuary Qihe River Estuary Tidal zones of Qihe River Estuary Inner region of Bohai Bay

Zn 1.21 5.24 1.29 1.20 0.92 1.22 1.50 1.48

Contamination factor Cfi Pb Cd Cu 1.08 2.35 0.85 1.71 7.41 0.44 1.05 1.86 1.06 1.29 1.47 0.81 1.35 7.90 0.47 1.08 1.43 0.87 2.10 13.35 0.60 1.08 1.06 0.89

Cr 1.95 0.70 3.18 1.79 0.29 1.83 0.99 1.83

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2 Biogeochemical Processes of the Bohai Sea Zn total (mg/kg)

450

Zn

300 150 0

R1 R1-2 R2

R3 R3-2

R4 R4-2 R5

Pb total (mg/kg)

40

Pb

30 20 10 0

Cd total (mg/kg)

B

2.0 1.6 1.2 0.8 0.4 0

R1 R1-2 R2

R3 R3-2

R4 R4-2 R5

B Cd

R1 R1-2 R2

R3 R3-2 R4 R4-2 R5 Study region

B

Fig. 2.56. Levels of metal contaminants in surﬁcial sediments of Bohai Bay. The description of R1, R1-2, · · · , R5 is referred to Table 2.22. B: background (Meng et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

2.4.3.2 Vertical Distribution of Heavy Metals (1) Vertical variations Although the inventories of heavy metals for three sediment cores were analyzed, the vertical distribution of heavy metals at CS1 station is presented and discussed in detail in the current work. The correlation among heavy metal concentrations, the geochemical characteristics (expressed by the contents of Fe, Al, and Mn) and the grain size can be used to distinguish natural and anthropogenic enrichments. According to the correlation results (data not shown), the vertical distribution trend can be divided into three types. The ﬁrst type is highly correlative element (R 2 0.8), represented by Cu and Ni. The depth distribution of the ﬁrst type is dominated strongly by the grain size and the contents of hydrous iron, manganese, and aluminum oxides, which means that the ﬁrst type of elements originates mainly from natural sedimentation. Furthermore, there is the remarkably positive correlation between the contents of the ﬁrst type element (Cu and Ni) and the proportion of clay and silt sand, indicating that Cu and Ni can easily associate with clay and silt sand. The second type has partial correlation with the geochemical features of sediment, represented by Cr, As, and Hg. The three elements come from the natural and anthropogenic enrichments simultaneously. The last type is the

2.4 Behaviour of Heavy Metals in the Bohai Sea

229

anthropogenic contaminative element, represented by Zn, Pb, and Cd. These kinds of elements have little correlation with the grain size and the content of Fe, Al, and Mn in the sediment core. For Zn, the depth of 50 cm is the turning point (Fig. 2.57, Qin et al., 2006). Below 50 cm, total Zn concentrations range slightly from 82.6 to 115.5 mg/kg, whereas from 50 cm to the sediment-water interface, Zn concentrations increase abruptly with an average concentration of 408.4 mg/kg. This indicates that the exogenous Zn input is still present in the vicinity coastal zones of the Dagu Estuary and that the 0∼50 cm layer is contaminated by zinc pollution. Cu (mg/kg) 10.0 20.0 30.0

20

20

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60 80 120

80.0

Hg (mg/kg)

5.00 6.00 7.00 8.00 9.00 0

0.01 0 20

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Depth (cm)

20

Depth (cm)

20

100

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120

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0.04

Cd (mg/kg) 0.00 1.00 2.00 3.00 0 1963 20

Depth (cm)

Pb (mg/kg) 0.0 20.0 40.0 60.0 80.0 0 100 200 300 400 500 0 0 1980 20 20 1955 40 40 1952

0.03

80

120

Zn (mg/kg)

0.02

60

100

60

30.0

As (mg/kg)

Depth (cm)

Depth (cm) Depth (cm)

40.0

Ni (mg/kg) 10.0 20.0

100

Cr (mg/kg) 0.0 0

0.0 0

Depth (cm)

Depth (cm)

0.0 0

40 1955 60 80

100

100

100

120

120

120

Fig. 2.57. Vertical proﬁles of heavy metals in the cored sediment of the Dagu Estuary (CS1) (Qin et al., 2006) (With permission from Qin YW)

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2 Biogeochemical Processes of the Bohai Sea

(2) Historical pollution of heavy metals As a good repository for pollutants, the core sediments can serve as a record of the temporal changes in contamination, and thus can be used to determine local contaminant histories. Because of the high sedimentary rate and the strong extent of environmental variance, the estuary has been considered an ideal research region for the historical reconstruction of marine pollution (Chen et al., 1998). The sediment core in the adjacent region of the Dagu Estuary has been dated by the application of radioactive isotopes 210 Pb and 137 Cs (Meng et al., 2005). The dating results show that the sedimentation rate from 1955 to 1963 is 3.43 cm/yr, and 0.65 cm/yr since 1963. The results above suggest that the sandy area of the intertidal zone had experienced a rapid sedimentation from the 1950s to 1960s, and the sedimentation slowed down after the 1960s. The rapid sedimentation in the 1950s to 1960s in the sandy area was due to the fact that northern China had plentiful precipitation during this period and the intertidal zone was supplied with abundant material sources for sedimentation. After the 1960s, the weakened sedimentation may have been the result of the reduction in sand transported to the intertidal zone, which was a consequence of diminished rainfall in northern China and intensive human activities in the Haihe River Basin (Meng et al., 2005). Zn, Pb, and Cd represent the anthropogenic contaminative elements at the CS1 station, and the contaminant history of these elements near the Dagu Estuary has been determined by the dating results. The contamination by Zn has continued to rise since the middle 1950s and the ascending trend is still happening today. Similarly, the contamination by Cd also started in the middle 1950s, reaching a maximum in the sediments deposited between 1955 and 1963, and then decreased in the late 1960s. The pollution of Zn and Cd in coastal environments mainly originates from sewage discharge, including metal melting, electroplating, battery production, and mining sewage. There is a great deal of lead and zinc mining distributed across the region surrounding the Bohai Sea, such as the Laiyuan and the Xinglong lead and zinc mines. Mining deposits and melting sewage with high concentrations of zinc and cadmium had been discharged into the river without any treatment prior to the mid 1970s. In particular, large numbers of reservoirs had been built on the river since the middle 1950s, so the runoﬀ from these rivers (like the Haihe River) continued to decrease. Without so many reservoirs, the sewage could easily enter Bohai Bay from the rivers before the mid 1950s. Consequently, the mid phase of the 1950s becomes the jump-oﬀ point for the contamination of Zn and Cd in Bohai Bay. The sources of Pb contamination are numerous (fossil-fuel burning, automobile exhausts, industrial activity, sewage, etc.) and their mode of introduction into the environment is also variable (air, water). Depending on all these variables, the resulting pollution by Pb is either locally or globally distributed. The dating results show that contamination by Pb appeared in the early 1940s, and a sharp increase occurred before the early 1950s. The accumulation of Pb in the sediment deposited between 1952 and 1963 began to decrease slightly. However, a new climax of contamination

2.5 Persistent Organic Pollutants in the Coastal Areas of the Bohai Sea

231

appeared in the sediment deposited around 1980, due mostly to atmospheric deposition. After 1980, the contamination by Pb in the sediment has been reduced following the implementation of laws regulating the use of leaded gasoline in automobiles. (3) Anthropogenic enrichment of polluting metals in sediments The upper limit of environmental background values for sediments was used to estimate the main anthropogenic enrichment metals in sediments of Tianjin Bohai Bay. The main anthropogenic enrichment metals in sediments were Zn, Pb, and Cd. The distribution of Zn in surﬁcial sediments was similar to that of Zn in water, and higher Zn contents centered on the tidal sediments near the Dagu Estuary. However, the distribution of Pb in sediments showed a declining trend from the coastal zones to the inner parts of the embayment, and the highest levels of Pb appeared in tidal sediments near the Qihe River Estuary. The ﬁner grain size and faint hydrodynamics and dilution conditions in this estuary likely resulted in the enrichment of Pb in these sediments. Relative to Zn and Pb, Cd is prone to accumulation in sediments. Although the Cd concentration in coastal waters was very low, an obvious anthropogenic enrichment of Cd in coastal sediments was observed, indicating that the sediments were major repositories for Cd. The largest quantity of wastewater and pollutants entering the Bohai Sea was discharged from estuaries, followed by municipal sewerage outlets. Most pollutants (including heavy metals) are prone to adhere to ﬁner grains and accumulate in regions far from the Dagu sewage mouth. Thus, the contents of most elements analyzed (Cu, Cd, Pb, Ni, and Cr) were higher in the Qihe River Estuary because of the ﬁner grain size, faint hydrodynamics, and less dilution by seawater. Grain size analysis has shown that sediments from Tianjin Bohai Bay become ﬁner from north to south (Qin et al., 2006), and the Qihe River Estuary is the furthest point from open seawaters in Tianjin Bohai Bay, where the hydrodynamics and dilution by seawater are faint, allowing contaminants to easily accumulate there. However, the spatial distribution of Zn showed an opposing trend, with the highest concentrations in tidal sediments near the Dagu Estuary. The reason for this distribution is not clear, but it is suggested that a new source of zinc contamination may occur near the Dagu Estuary.

2.5 Persistent Organic Pollutants in the Coastal Areas of the Bohai Sea Persistent organic pollutants (POPs) have been of increasing concern for several decades all over the world. POPs have high toxicity, last for a long time in environment, and may travel a long distance far from their source of usage, release, and emission. Furthermore, they can accumulate in fatty tissues of living organisms, leading to undesirable eﬀects linked to the occurrence of immunologic and teratogenic dysfunctions, reproductive

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impairments, and endocrine disruption (Colborn and Smolen, 1996). Eight kinds of pesticides, including dieldrin, aldrin, endrin, chlordane, heptachlor, dichlorodiphenyltrichloroethane (DDT), toxaphene, and mirex, two kinds of industrial chemicals including polychlorinated biphenyls (PCBs) and hexachlorobenzene (HCB), and two kinds of byproducts including dibenzo-pdioxins and dibenzofurans (PCDDs and PCDFs) were listed as a dirty dozen at the Stockholm Convention (2001). Chemicals having some of the POPs characteristics, such as polycyclic aromatic hydrocarbons (PAHs), are proposed as potential POPs and POPs-like chemicals by many agencies. DDTs, perhaps the best known of the POPs, were widely used in World War II to protect soldiers and civilians from malaria, typhus, and other diseases spreaded by insects. It continues to be applied against mosquitoes in several countries to control malaria (UNEP, 1999). DDTs and hexachlorocyclohexanes (HCHs) have been the most popular pesticides used in agriculture since 1945. From 1981 to 1984, about 184,000 t of HCH and 311,000 t of DDT were consumed in 103 countries annually (Jones and Voogt, 1999). PCBs were ﬁrstly used as heat exchange ﬂuids in electric transformers and capacitors, and as additives in paint, carbonless copy paper, sealants, and plastics in America from circa 1929 after they had been synthesized in 1881 by the Germans (UNEP, 1999). PCDD/Fs are produced unintentionally in some Cl-containing chemicals with incomplete combustion, as well as during the manufacture of certain pesticides and other chemicals. PAHs are formed during carbonaceous material burning and the petrogenic process. 2.5.1 Distributions of Persistent Organic Pollutants in Sediments and Mollusks The behavior, bioaccumulation, and ecological toxicities of DDTs, HCHs, PAHs, PCBs, and PCDD/Fs have been the subject of much debate over the past 20 years, and this continues, especially their spatial and temporal distribution, which is reported in depth and in detail. POPs have been detected worldwide from the tropics to polar areas and in a wide range of environmental media, such as sediment, soil, water, air, aquatic biota, even in human beings (Iwata et al., 1993; Kannan et al., 1997; Tanabe, 2000; Tanabe et al., 2000). Studies of the levels of POPs in the global environment show that emission sources of a number of POPs (such as DDTs and HCHs) in the past 20 years have shifted from the industrialized countries of the northern hemisphere to less developed countries in tropical and sub-tropical regions, owing to delays in banning their production and use, where they are still being used both legally and illegally in agriculture and for the control of diseases such as malaria, typhus, and cholera (Tanabe, 1991; Iwata et al., 1993; Loganathan and Kannan, 1994). The POPs concentrations in human tissue and milk resulting from the food intake of vegetables and marine products have also been reported in many countries (Moon and Ok, 2006; Mato et al., 2007). In order to quantify the ecological risk of POPs, when animals and humans are exposed to POPs,

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the “toxicity equivalency” concept was developed in the mid 1980s with toxic equivalent factors (TEFs) and toxic equivalent quantity (TEQ) proposed. It was even reported that the average daily intake of dioxins in industrialized countries is estimated at about 0.3∼5 pg TEQ/kg body weight per day. The dose associated with a cancer risk of 1 in 1 million has been estimated at between 6.4 and 1,400 fg/(kg·d) (Liem, 1999). Because of the lack of experiment data, it was widely accepted that there are two approaches for quantifying bioaccumulation phenomena. In the ﬁrst, the bioaccumulation ratio can be deduced from concentrations in the organism and a measured experimental or ﬁeld concentration. The second and more demanding approach is to compile a mechanistic mass balance model in which the various uptake and loss processes are quantiﬁed (Mackay and Fraser, 2000). The oral bioaccessibility can be simulated through using extraction methodology normally found in analytical laboratories for the recovery of total POPs from soils (Dean and Ma, 2007). 2.5.1.1 PAHs Several previous papers have reported the horizontal distribution of PAHs in the surface sediment of the Bohai Sea, the most detailed one of which is Lin et al. (2005). Fifty-six surﬁcial sediment samples along a 5 m isobath and 10 outer sea sediment samples were collected in the Bohai Sea to investigate PAHs occurrence. Ten parent PAHs components, including naphthalene, ﬂuorene, phenanthrene, anthracene, ﬂuoranthene, pyrene, benzo(a)anthracene, chrysene, benzo(a)pyrene, and benzo(e)pyrene were studied, whose total concentrations were summed up (ΣPAHs) based on dry weight. To simplify the overview results from Lin et al. (2005), the PAHs concentrations of 10 typical areas were brieﬂy summarized in Fig. 2.58. A great variance in ΣPAHs concentrations occurred between sampling sites, ranging from 24.7 to 2,079.4 ng/g with a mean value of 130.6 ng/g. Two peaks of ΣPAHs concentrations (2,079.4 and 964.3 ng/g, dw) were detected in the stations adjacent to Qinhuangdao City, which is a large multifunction harbour trading in diesel, aero-kerosene, and petroleum. The highest average PAHs level (1,082 ng/g) was predominantly due to wastewater containing oil discharged from ﬁshing boats, passenger ships, and freighters, as well as sewage released from aquaculture, industry, and municipal engineering institutions nearby, such as bump stations and disposal plants. The PAHs level of Liaodong Bay immediately followed that of Qinhuangdao City, with a mean concentration of 143.4 ng/g. In particular, in Jinzhou Bay, a higher PAHs level (250 ng/g) was presented because of the Liaohe River, Dalinghe River, and Xiaolinghe River input. The third highest level was located in areas close to Laizhou Bay and Penglai City (mean value of 120 ng/g). The investigations into PAHs residual levels in the organisms of the Bohai Sea are much fewer, and only two papers have been published. Ten parent PAHs, including naphthalene, ﬂuorene, phenanthrene, anthracene, ﬂuoranthene, pyrene, benzo(a)anthracene,

234

2 Biogeochemical Processes of the Bohai Sea 250.4 Liaohe River Dalinghe River 1082.0

N 41

Jinzhou Xiaolinghe River

(a)

Huludao Yingkou

40.5 Luanhe River

40

Qinhuangdao Jiyunhe River

39.5

120.2 Tianjin

39

Haihe River

Bohai Sea

Dalian

38.5

20 ng/g, dw

38

0

37.5

Penglai Laizhou

118 N 41

119

120

Yantai

121

Weihai

122 E

Xiaolinghe River Liaohe River Jinzhou Dalinghe River Yingkou Huludao

(b)

40.5 Qinhuangdao Luanhe River

40 39.5

Jiyunhe River Tianjin

39 Haihe River

38.5

Dalian

Bohai Sea

10 ng/g 0

38 37.5

Penglai Laizhou

118

119

120

Yantai

121

Weihai

122 E

Fig. 2.58. Horizontal distribution of PAHs in the surface sediments (a) and mollusks (b) of the Bohai Sea (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center)

chrysene, benzo(a)pyrene, and benzo(e)pyrene, were investigated in mollusks along the Bohai Sea coastline due to the requirements of the State Oceanic Administration of China (SOA) (Liu WX et al., 2007). Two higher PAHs levels in mollusks occurred in Jinzhou Bay and Laizhou Bay. PAH concentrations in the Bohai Sea were over one order of magnitude less than in Weihai and Yantai (Fung et al., 2004). ΣPAHs remaining in Scapharca subcrenata Lischke reached a maximum value of 21.1 ng/g in Liaodong Bay, while the highest average level appeared in Ruditapes philippinarum (Adams & Reeve) and Meretrix meretrix Linnaeus in Laizhou Bay, 20.3 and 16.5 ng/g, respectively. The reasons for a high PAHs level in some mollusk species were associated with their diﬀerent ingestive behaviors, internal metabolic processes, and so

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235

on (Liu WX et al., 2007). Comparing Figs. 2.58a and b, a big diﬀerence in the distributions in sediments and mollusks was that PAHs levels in the whole Bohai Bay remained at an almost equal level without extremely high values. 2.5.1.2 Organic Chlorine Pesticides (OCPs) (1) DDTs There has been great interest in DDTs in recent years in China. However, their concentrations in the sediments of the Bohai Sea are just occasionally reported. In Ma et al. (2001), the total concentrations of p,p -DDT, o,p-DDT, p,p -DDE, and p,p -DDD based on dry weight were summed up and expressed with ΣDDTs, as shown in Fig. 2.59. The ΣDDTs concentrations ranged from 0.38 to 1,417.08 ng/g with a mean value of 177 ng/g, followed by Yingkou City in Liaodong Bay (129.87 ng/g), and ΣDDTs concentrations in the other coastal areas were in the same order of magnitude without signiﬁcant diﬀerence (Ma et al., 2001). However, the results reported by Liu et al. (2005) were about three orders of magnitude less. Because the sampling stations in Liu’s investigation covered those in Ma’s, the inﬂuence of sampling areas could be ruled out. Nevertheless, the large diﬀerence was hard to explain in terms of the result of bio-degradation over 4 years. The organism samples were collected from 10 coastal cities in order to investigate HCHs and DDTs residue in mollusks and evaluate their pollution levels (Yang RQ et al., 2004). Since the dominant species varied substantially with the sampling locations, DDTs concentrations in all the species were summed up (ΣDDTs) to elucidate their distribution. ΣDDTs concentrations in the mollusks of the Bohai Sea ranging from 133.7 to 300.77 ng/g (wet weight, ww) occurred in Yangkou and Tianjin respectively with a mean value of 199.52 ng/g, while the DDTs level in Tianjin was lower than in Yantai and Weihai along the coastline of the Yellow Sea. No well-deﬁned trend was found in the bioaccumulations of all species. In Yingkou, Huludao, and Penglai, Ruditapes philippinarum accumulated the highest level of DDTs, while the highest level existed in Neverita didyma in Jinzhou and Laizhou, and the residual levels in Neptunea cumingi, Sinonovacula constricta, and Scapharca broughtonii were higher than the other species in Dalian, Tianjin, and Yangkou respectively. Wang YW et al. (2007) also reported their investigation into DDTs residual levels from 2002 to 2004. The Independent Sample t-test method was employed to compare these two reports. F =1.451, P =0.244>0.05 supported the equal variance assumption, and t=−1.529, P =0.144>0.05 indicated that no signiﬁcant diﬀerence existed in the two groups, as Table 2.25 shows. Furthermore, Wang YW et al. (2007) compared the DDTs and HCHs concentrations in mollusks from 2002 to 2004 using the one-way ANOVA method, in which every signiﬁcance higher than 0.05 suggested that no signiﬁcant variations in the residual levels of DDTs and HCHs appeared.

236

2 Biogeochemical Processes of the Bohai Sea 129.87

N 41

Xiaolinghe River Liaohe River

(a)

Dalinghe River 1417.08

40.5

Jinzhou Yingkou

Huludao

Qinhuangdao

40

Luanhe River Jiyunhe River

39.5 39

Tianjin Haihe River

Bohai Sea

38.5

Dalian 20 ng/g

38

0

37.5

Penglai Laizhou

119

118 N 41

120

(b)

Yantai

Weihai

121

Dalinghe River

122

E

Jinzhou Huludao Yingkou

40.5 Qinhuangdao

40

Luanhe River

39.5 39

Jiyunhe River Tianjin Haihe River

Bohai Sea

Dalian 100 ng/g, wt

38.5

0

38 37.5

Yangkou

Penglai Laizhou

118

119

120

Yantai

121

Weihai

122

E

Fig. 2.59. Horizontal distribution of DDTs in the surface sediments (a) and mollusks (b) of the Bohai Sea (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center) Table 2.25. The results of the Independent Sample Test for the variation in DDTs concentrations reported by Yang RQ et al. (2004) and Wang YW et al. (2007) (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center) Levene’s test for equality of variance F Sig.

t-test for equality of means

t df Sig.∗ Mean Std. 95% CI DDT equal variance 0.413 0.529 −0.046 18 0.964 −2.5190 54.3362 −116.6752∼111.6372 assumed Equal variance −0.046 16.632 0.964 −2.5190 54.3362 −117.3520∼112.3140 not assumed ∗ 2-tailed. CI: conﬁdence interval

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237

(2) HCHs Until now, there has been lack of information concerning HCHs distribution in the sediments of the Bohai Sea. Only Yang RQ et al. (2004) reported the HCHs residual levels in mollusks in the same paper as discussing DDTs. It is likely that HCHs concentrations in all species were summed up as ΣHCHs based on wet weight to evaluate their contamination status. ΣHCHs were in the range of 4.68∼18.75 ng/g with a mean of 10.91 ng/g (Fig. 2.60). The highest mean ΣHCH was found in Laizhou Bay (18.75 ng/g) of Shandong Province, followed immediately by Dalian. In the whole Bohai Sea, the HCH levels in Laizhou Bay, Weihai, Yantai, and Penglai were slightly higher than in other sea areas where the HCH concentrations were almost even. With respect to the species, HCHs residue in Sinonovacula constricta was higher than in other species, which elicited its higher bioaccumulation capacity for HCHs compared to other selected mollusk species. ANOVA analysis was also employed to judge the annual variation from 2002 to 2004. As with DDTs, the residual levels and the polluted character of HCHs in mollusks did not signiﬁcantly change from 2002 to 2004 (Wang YW et al., 2007). Xiaolinghe River Liaohe River

N 41

Dalinghe River Jinzhou Yingkou

Huludao

40.5 Luanhe River

40 39.5

Qinhuangdao Jiyunhe River Tianjin

39

Haihe River

Dalian

Bohai Sea

38.5

5 ng/g 0

38 37.5

Penglai

Yangkou Laizhou

118

119

120

Yantai Weihai

121

122

E

Fig. 2.60. HCH concentrations in the mollusks of the Bohai Sea, 2004 (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center)

2.5.1.3 Dioxin-like Chemicals (PCBs, PCDDs and PCDFs) (1) PCBs The PCB congeners with International Union of Pure and Applied Chemistry (IUCAP) numbers 28, 52, 101, 112, 118, 138, 153, 155, 180, and 198 were detected in the sediments of the Bohai Sea, whose total concentration ranged from 0.30 to 14.95 ng/g based on dry weight (dw) with a mean of 3.4 ng/g

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2 Biogeochemical Processes of the Bohai Sea

(Ma et al., 2001). The maximum value was detected in the station located at the northeastern corner of Bohai Bay, followed by 8.68 ng/g at the station close to Dalian Bay. As shown in Fig. 2.61, the PCBs levels of Dalian Bay and Liaodong Bay were higher than in other coastal areas in the Bohai Sea. Liu et al. (2001) proposed that the high PCBs level in the sediment of Dalian Bay was attributed to two evident sources, these being the Dalian petroleum chemical factory and the Dalian coal wharf. N (a) 41

Xiaolinghe River Liaohe River Jinzhou Dalinghe River

40.5 40 39.5

Huludao Yingkou

Qinhuangdao

14.95 Luanhe River

8.68

Jiyunhe River Tianjin

39 Haihe River

Dalian

Bohai Sea

38.5

1 ng/g

38

0

37.5

Penglai Yantai Laizhou

118 N 41

119

120

(b)

Xiaolinghe River Liaohe River Huludao

38.5

Jinzhou Yingkou

Qinhuangdao

40

39

122 E

Dalinghe River

40.5

39.5

Weihai

121

Luanhe River Jiyunhe River Tianjin Haihe River

Bohai Sea

Dalian 5 ng/g

38

0

37.5 Penglai Yantai Laizhou

118

119

120

121

Weihai

122 E

Fig. 2.61. Horizontal distribution of PCBs in the surface sediments (a) and mollusks (b) of the Bohai Sea (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center)

Similarly, the same 10 PCB congeners mentioned above were measured in the mollusks of the Bohai Sea. The PCB concentrations in Laizhou Bay were much higher than those in the other areas. The maximum value was detected in Hutouya adjacent to Laizhou, exceeding 15 ng/g (wet weight),

2.5 Persistent Organic Pollutants in the Coastal Areas of the Bohai Sea

239

followed immediately by the site positioned in Yangjiagou adjacent to the Yellow River Estuary. In view of the residual level in mollusk tissue, Rapana venosa (Valenciennes) and S. subcrenata Lishke had higher bioaccumulation capacities. Zhao et al. (2005) reported PCB levels in the mollusks of Yangkou, Tianjin, Yantai, Weihai, Qinhuangdao, Dalian, Huludao, and Yingkou based on wet weight, dry weight, and lipid weight, in the ranges of 0.47∼3.4, 1.9∼21, and 66∼345 ng/g, respectively. The highest level was obtained in the mollusks in Yantai. Nevertheless, it was about 4 times less than the maximum value in Laizhou Bay. Comparing their residual levels in sediments and mollusks (Fig. 2.61), PCBs distribution trend in sediments was the reverse of that in mollusks generally, which may be due to the fact that their sampling stations were not consistent. Because the information on PCB congeners in sediments we possessed was limited and fragmented, we failed to uncover the interaction between PCBs distribution in sediments and mollusks. (2) PCDD/Fs The investigation into PCDD/Fs in China was just arranged recently. Therefore, the information on the sample areas, matrices, and behavior of PCDD/Fs we possessed was quite limited. Until now, their distributions in the Haihe River (Liu HX et al., 2007) and Nanpaiwu River (Hu et al., 2005) were available only in the literature. In order to illuminate the horizontal distribution of PCDD/Fs in the surface sediment of Bohai Sea coastal areas of Bohai Bay, their concentrations in the Yongdingxin River mouth, Haihe River mouth, and Dagu Drainage River mouth were presented (Fig. 2.62). As Fig. 2.62 shows, the PCDD/Fs concentrations in the surface sediments varied significantly with sampling stations. The PCDD/Fs level in the Dagu Drainage River mouth was the highest (533,134 pg/g, dw, Liu HX et al., 2007), 2 orders of magnitude higher than that of Bohai Bay (177 pg/g, dw, Hu et al., 2005) and the Yongdingxin River mouth (430 pg/g, dw, Liu HX et al., 2007).

N 39.3

39.1

38.9 117.2

117.4

117.6

117.8

E

Fig. 2.62. The concentrations of PCDD/Fs in the surface sediment of the Yongdingxin River Estuary, Haihe River Estuary, Dagu Drainage River Estuary, and Bohai Bay (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center)

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2 Biogeochemical Processes of the Bohai Sea

Liu and his colleagues reported that both the Haihe River and Dagu Drainage River ﬂowed through the chemical industrial zone where PCP and PCP-Na had been manufactured for nearly a half century. Moreover, the pollution of PCDD/Fs sharply increased along the ﬂow direction of the Dagu Drainage River and, consequently, PCDD/Fs might come from the chemical industry (Liu HX et al., 2007). Zhao et al. (2005) collected some bivalves and gastropods species from eight cities along the Bohai Sea, i.e., Yangkou, Tianjin, Yantai, Weihai, Qinhuangdao, Dalian, Huludao, and Yingkou. The collected bivalves and gastropods, including Rapana venosa, Neptunea arthritica cumingii, Neverita didyma, Solen grandis, Scapharca subcrenata, Meretrix meretrix, Sinonovacula constricta, Chlamys farreri, Amusium spp., and Clinocardium californiense, are routine seafood for most Chinese people. The PCDD/Fs concentrations (lipid wet) were summarized in Table 2.26. Both the highest values in gastropods and bivalves were detected in Yingkou. The residue trends of PCDD/Fs in gastropods were diﬀerent from those in bivalves. In general, bivalves accumulated more PCDDs than gastropods, while the residue level of PCDFs in gastropods was higher than in bivalves. Table 2.26. The concentrations of PCDDs and PCDFs in the gastropods and bivalves in the study areas (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center) Sampling site Yangkou Tianjin Yantai Weihai Qinhuangdao Dalian Huludao Yingkou

Gastropods (pg/g, lipid weight) PCDDs PCDFs 58.1 N.D. 24 N.D. 16.7 41.2 39.8 N.D. 34.8 3.5 95 N.D. 61 75.5 282.5 15, 317

Bivalves (pg/g, lipid weight) PCDDs PCDFs 630 N.D. 41 10.7 148.8 N.D. 159 N.D. 158 N.D. 23.2 3.5 85 N.D. 2, 719 72

N.D.=not detected

2.5.1.4 Vertical Distribution of PCDD/Fs and PCBs Until now, only Hu et al. (2005) reported the PCDD/Fs concentrations in the core sediments collected from the Nanpaiwu River and Bohai Sea. Therein, C3 (38◦ 57 N, 117◦ 44 E) core sediment located at Bohai Bay along the Nanpaiwu River course was collected to reveal the PCDD/Fs and PCBs vertical distribution trend. The Nanpaiwu River was the major source of industrial and domestic wastewater from the cities of Beijing and Tianjin to Bohai Bay

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(Hu et al., 2005). The core samples were sliced at 5 cm intervals, and the vertical distribution of ΣHCHs, ΣDDTs, PCDD/Fs, and co-PCBs were reported in Fig. 2.63. Fig. 2.63a exhibits the vertical proﬁles of PCBs and PCDD/Fs concentrations. From the surface to the bottom (145 cm), the PCDFs concentrations were almost constant in the whole core, and the down-cored co-PCBs concentrations also remained stable except for the abrupt peak occurring at 85 cm. However, PCDDs varied tremendously with depth. First of all, the PCDDs level increased linearly with a slope rate of 29.355 (R 2 =0.9968) from the surface to 85 cm, where the maximum value was detected. Then, a sharp decrease followed immediately and, ﬁnally, it decreased linearly with a slope rate of 24.25 (R 2 =0.9997) after a transient platform from 85 to 105 cm. Fig. 2.63b presents the vertical distributions of ΣDDTs and ΣHCHs in the C3 core. As with PCDFs, the ΣDDTs concentrations remained at a low and stable level, while the HCHs’ distribution trend was relatively complex. Four obvious concentration peaks were detected at 25, 45, 65, and 150 cm, and their concentrations were at a very low level elsewhere in the core. It was common that the maximum values of PCDDs and HCHs were both detected in the layer at 65∼70 cm. Concentra tion (ng/g) PCDDs

Depth (cm)

0

co-PCBs

PCDFs

1000 2000 3000

0

SHCHs S DDTs 40

80

120 160

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Fig. 2.63. The vertical proﬁles of PCDDs, PCDFs, co-PCBs, ΣDDTs, and ΣHCHs (ng/g, dry weight) in the core sediment of Bohai Bay (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center)

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2 Biogeochemical Processes of the Bohai Sea

2.5.1.5 Summary for the Spatial and Temporal Distributions of POPs The horizontal distributions of the POPs we discussed above are not the same. Comparing Figs. 2.58, 2.59, and 2.61, it could be concluded that the highest level PAHs and DDTs in the surface sediments accumulated in the areas close to the northeast corner of Bohai Bay and Qinhuangdao, followed by Liaodong Bay, while there was little diﬀerence in PCBs distribution trends, and that the PCBs level in Dalian was somewhere between that in the northeast corner of Bohai Bay and Liaodong Bay. The residue levels of DDTs, HCHs, PCBs, PCDDs, and PCDFs in mollusks collected from the cities along the Bohai Sea coastline were summarized on account of the deep insight they provided about their distributions, including Dalian, Yingkou, Jinzhou, Huludao, Qinhuangdao, Tianjin, Yangkou, Yantai, Weihai, and Penglai. Correlation analysis was employed, with the results presented in Table 2.27. Therein, the concentrations of DDTs and HCHs were expressed as wet weight and those of PCBs, PCDDs, and PCDFs were calculated based on lipid weight. As Table 2.27 shows, DDTs and HCHs did not correlate with each other signiﬁcantly, while strong linear relationships were found among PCBs, PCDDs, and PCDFs at the 0.05 level. This phenomenon showed that they might have the same sources or similar bioaccumulated characteristics. As we discussed above, some POP concentrations in similar studies reported by diﬀerent researchers were not coincidental. For example, the distribution trend in mollusks of PCBs shows a little variation in Liu HX et al. (2007) and Zhao et al. (2005). The lack of agreement among the residue levels of POPs in mollusks reported by diﬀerent work teams’ investigations indicated that large diﬀerences existed in the sampling sites, sampling species, individual compounds and analytical methods, which had a direct inﬂuence on the results. Although close attention to POPs has been paid worldwide for several decades, no unique criterion is applied for environmental quality evaluation. For example, Environmental Quality Standards (EQSs) are only applied to water and there are no equivalent standards used for sediments in the UK. Meanwhile, in the USA Table 2.27. The correlations among the concentrations of DDTs, HCHs, PCBs, PCDDs, and PCDFs in the mollusks of the Bohai Sea (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center) DDTsa HCHsa PCBsb PCDDsb PCDFsb ∗

DDTs 1.000

HCHs 0.491 1.000

Signiﬁcant at the 0.05 level (2-tailed).

a

PCBs −0.157 −0.604 1.000

wet weight;

b

lipid weight

PCDDs −0.449 −0.672 0.783∗ 1.000

PCDFs −0.647 −0.834 0.819∗ 0.999∗ 1.000

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and Canada, the approach for evaluating sediment quality is the deviation of Threshold Eﬀect Levels (TELs) and Probable Eﬀect Levels (PELs) from an extensive data base containing direct measurements of the toxicity of contaminated sediments to a range of aquatic organisms exposed in laboratory tests and under ﬁeld conditions, which belongs to the Weight of Evidence Approach (WEA). This approach is restricted on account of four shortcomings: ﬁrstly, a large database is required; secondly, it is not possible to establish cause and eﬀect relationships; thirdly, the amalgamation of data from multiple sources could result in unknown biases in the database; lastly, bioavailability is not considered. The yardsticks of PCBs, HCHs, and DDTs for sediment quality have been established in China in GB 18668-2002, as Table 2.28 shows, in which sediment quality was classiﬁed in four grades, Grades I, II, III, and IV, according to the pollutants concentration, i.e., clean sediment, weakly contaminated sediment, contaminated sediment and heavily contaminated sediment, respectively. Comparing the DDTs yardsticks and their concentrations in sediments, 22% of samples were of clean character, located in the vicinities of Jinzhou and Dalian, 44% of samples were weakly contaminated, located in the vicinities of the Haihe River mouth, Jiyunhe River mouth, and Laizhou Bay, and 11% of samples, including the only sample collected near to Weihai, were deﬁnitely contaminated sediment. Moreover, the two peak values in the Liaohe River mouth and the northeast corner of Bohai Bay were heavily contaminated. Comparing PCBs concentrations in sediments with GB 18668-2002, all the samples were clean, while no data focusing on the residual levels of HCHs in sediments is available at present. We therefore cannot assess sediment quality with respect to HCHs contamination. Table 2.28. Sediment quality (×10−6 , dry weight) recommended by GB 18668-2002 (China) PCBs 0.02 0.02∼0.20 0.20∼0.60 >0.60

HCHs 0.50 0.50∼1.00 1.00∼1.50 >1.50

DDTs 0.02 0. 02∼0.05 0. 05∼0.10 >0.10

Grade I II III IV

2.5.2 Composition and Sources of Persistent Organic Pollutants It was widely accepted that the low molecular weight (LMW) PAHs (with 2 and 3 rings) were due primarily to petroleum contamination, while high molecular weight (HMW) PAHs (4-ring) were the product of the combustion process. In particular, the 4-ring PAHs derived from incomplete combustion, and the PAHs with 5 or more rings were usually synthesized in high temperature combustion or by pyrogenic process.

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2 Biogeochemical Processes of the Bohai Sea

2.5.2.1 PAHs A box-plot diagram supplied detailed information of the PAHs composition in the Liaodong Peninsula, Liaodong Bay, Qinhuangdao, Bohai Bay, and Laizhou Bay, and this data was also obtained from Liu et al. (2005). As Fig. 2.64 shows, the data was distributed irregularly in the 5 areas of the Bohai Sea, suggesting that the PAH compositions were relatively complex and one who used the average composition to demonstrate the whole area was somewhat fragmentary. In the Liaodong Peninsula and Bohai Bay, 5-ring PAHs were predominant constituents, indicating that PAHs originated from the pyrogenic process; 4-ring PAHs accounted for the majority of total PAHs in Qinhuangdao, indicating that Qinhuangdao obtained PAHs from incomplete combustion. However, the data of Liaodong Bay and Laizhou Bay exhibited haphazard characteristics so that it was safe to believe that PAHs from a combination of sources varied signiﬁcantly according to location, which was in line with the real industrial and economic situation. Liaodong Bay and Laizhou Bay, well known as the two key areas of economic development, possess famous oilﬁelds, harbors, and manufacturing plants, where PAHs might originate from petroleum and combustion. Molecular indices based on isomeric ratios were often used to apportion PAHs sources. Liu et al. (2005) supplied the information on PAHs sources according to isomeric ratios (ﬂuoranthracene/pyrene and pyrene/benzo(a)pyrene) as follows: the PAHs input on the coast adjacent to Jinzhou in Liaodong Bay was predominantly composed of petroleum that derived from the sewage water discharged from the Jinzhou Oilﬁeld and Jinxi oil plants. PAHs in the coastal areas adjacent to Qinhuangdao and Laizhou Bay were predominantly due to gasoline combustion and the majority of PAHs in the other areas of the Bohai Sea originated from coal combustion. 302 soil samples were collected in the areas around the west coast of the Bohai Sea, including Beijing, Tianjin, and the provinces of Hebei and Shandong, to investigate PAHs distribution and sources (Zuo et al., 2007). In general, the PAH levels in Bohai Bay were the highest, especially in Tianjin (8,428 ng/g), Bejing, Tanggu, and their vicinities in Hebei Province, while lower level PAHs were detected in the alluvial plain caused by the Yellow River in northwest Shandong Province and northern Hebei Province, which are located in the south corner and north corner of Bohai Bay, respectively. Therefore, it was found that a similarity existed in PAH distributions in soils and sediments, indicating that PAHs in sediments of Bohai Bay derived from terrestrial materials input through river or air deposition. To reveal PAHs distribution in the outer sea area, 10 surface sediment samples were collected close to several estuaries, whose PAHs concentrations were homogenous in the range of 25.6∼47.0 ng/g, indicating that they were free from the eﬀect of river sources (Liu et al., 2005).

2.5 Persistent Organic Pollutants in the Coastal Areas of the Bohai Sea

245

80

Percentage (%)

60

40 20 2,3-ring

0

4-ring -20 N= 5 5 5 16 16 16 4 4 4 4 4 4 Liaodong Peninsula Qinhuangdao Liaodong Bay Bohai Bay Area

5-ring 5 5 5 Laizhou Bay

Fig. 2.64. The box-plot diagram of PAHs composition in the surface sediment of the Bohai Sea (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center)

2.5.2.2 HCHs Two types of HCH products have been manufactured throughout the world: technical HCH (containing about 60%∼70% α-HCH, 5%∼14% β-HCH, 10%∼15% γ-HCH and minor proportions of minor isomers) and lindane (γHCH99%) (UNEP, 1995). In China, the production and application of technical HCH was restricted in 1983, while lindane is currently used for pest control (Cai et al., 2008). The proportions of HCH isomers in diﬀerent mollusk species according to Yang RQ et al. (2004) are presented in Fig. 2.65. No well-deﬁned interspeciﬁc variability of the bioaccumulations of HCH isomers were found. However, β-HCH was the major contaminant residue in the mollusks of the 10 cities, and isomers of α-, γ-, and δ-HCH were observed to contribute on average about 27.1%, 13.3%, and 4.3%, respectively. The contribution of β-HCH was elevated about 3∼11 times compared with the original composition, which might be attributed to diﬀerent properties of the isomers. Additionally, migration and transformation might contribute to βHCH being the major contaminant among the observed isomers. Fig. 2.65a suggested that Yingkou, Jinzhou, and Tianjin might have new HCHs in use because all the HCH isomers detected in Mytilus edulis (ME), Meretrix meretrix (MM), Ruditapes philippinarum (RP), Rapana venosa (RV), and Mactra veneriformis (MV) were α-HCH but these species did not accumulate α-HCH only. The HCH levels in the seawater of the Bohai Sea were also reported in previous papers. For example, 21 water samples were collected, 12 samples from along the Haihe River major course and 9 samples from an area extending to Bohai Bay along the Haihe River major course (Wang T et al., 2007).

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2 Biogeochemical Processes of the Bohai Sea

Fig. 2.65. HCHs composition in the mollusks of the costal cities along the Bohai Sea. (a), (b), (c), and (d) represent the percentage of α-, β-, γ-, and δ-HCH, respectively (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center)

The HCH concentrations in the surface water of the Haihe River ranged from 0.295 to 1.07 μg/L (mean value 0.660 μg/L), and 0.047 to 0.190 μg/L (mean value 0.160 μg/L) in Bohai Bay. The discovery that the α-HCH proportion increased along the Haihe River major course suggested a new input of HCHs into the Haihe River at present. Partly because sewage from Tianjin was discharged into the Haihe River, the HCHs might be derived from Tianjin, which supports the deduction concerning the proportion of HCH isomers discussed above. Tan et al. (2006) reported that the average total HCHs concentrations in the water of Laizhou Bay was 1.6 μg/L, about 2 times higher than in the Haihe River and 10 times higher than in Bohai Bay. Comparing HCHs distribution in water and mollusks, it was found that they were coincident. HCHs residue in mollusks in Bohai Bay was lower than in Laizhou Bay partly because of HCHs usage. Additionally, the species and bioaccumulation capacity contributed to the higher residue level in Laizhou Bay to a large extent.

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2.5.2.3 DDTs As manufactured, DDTs contain the byproducts DDD and DDE. Technically, DDT generally contains 75% p,p -DDT, 15% o,p -DDT, 5% p,p -DDE, and below 5% others (Kim et al., 2002). Since DDT could undergo biological degradation to DDD under anaerobic conditions and to DDE under aerobic conditions, the ratio of p,p -DDT to the sum of two metabolites (p,p -DDD and p,p -DDE) could be used as an indicator to identify a recent input of technical DDT or retarded degradation due to environmental conditions. In addition, the concentration ratio between p,p -DDD and p,p -DDE could reveal the inﬂuence of benthic surrounding the pathway of DDT degradation (Liu et al., 2005). As shown in Fig. 2.66, signiﬁcant diﬀerences existed in the percentages of p,p -DDT, o,p -DDT, p,p -DDD, and p,p -DDE. At the stations near to the Liaodong Peninsula, i.e., the estuaries of the Liaohe River and Shuangtaizihe River, Jinzhou Bay and Liaodong Bay, the principal contributor of ΣDDTs concentration in sediments was p,p -DDD, accounting for 33.5%∼39.3%. Whereas, the o,p -DDT levels were higher than other isomers in the sediments of Bohai Bay, Laizhou Bay, and the outer sea, ranging from 29.8% to 36.6%. Especially in Qinhuangdao, p,p -DDT was the dominant component of DDT isomers in sediments with a percentage of 30.2%. We therefore drew the conclusion that the Bohai Sea was divided into two regions based on DDT compositions; i.e., in the areas north of Qinhuangdao, the predominant component was p,p -DDD, while o,p -DDT took the place of p,p -DDD as the major component in the areas south of Qinhuangdao. Generally, the DDTs residue in the sediments of the Bohai Sea had suﬀered degradation to a large extent, which could be deduced from the ratios of p,p -DDT/(p,p -DDE+p,p DDD) being in the ranges of 0.36∼0.71 and 9∼14 times less than the original technical DDTs composition. Outer sea Laizhou Bay Bohai Bay p,p'-DDT o,p'-DDT p,p'-DDD p,p'-DDE

Qinhuangdao Liaodong Bay Jinzhou Bay Liaohe River and Shuangtaizihe River Liaodong Peninsula 0%

20% 40% 60% 80% 100%

Fig. 2.66. DDT composition in the sediment of the costal cities along the Bohai Sea (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center)

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2 Biogeochemical Processes of the Bohai Sea

It was reported that there were two chemical plants located in the Haihe River Basin. A chemical production plant that produced dicofol pesticides using DDTs as raw material was located in the upper reaches of the Yongdingxin River, and another plant producing DDTs was on the upper reach of the Jiyun River (Wan et al., 2005). Thus, these two speciﬁc sources contributed to the DDTs residue in Bohai Bay. Hites and Day (1992) had pointed out that a relatively high percentage of p,p -DDT may indicate new inputs into the environmental media. Therefore, it was safe to conclude that new DDTs were entering the sea areas close to Qinhuangdao and Liaodong Bay because p,p DDT percentiles were high there than in Bohai Bay. The percentages of p,p -DDE, p,p -DDD, o,p -DDT, and p,p -DDT in mollusks were shown in Fig. 2.67, whose original data was obtained from Yang RQ et al. (2004). The residual levels in the same species in diﬀerent cities were diﬀerent, but in the same city the residual levels of p,p -DDE, p,p -DDD did not vary signiﬁcantly, indicating that the bioaccumulation capacities were not

80

Dalian Yingkou Jinzhou Huludao Tianjin Yangkou Laizhou Penglai

60 40

34

Percentage (%)

30 26 22 18 14

(b)

50 40

Dalian Yingkou Jinzhou Huludao Tianjin Yangkou Laizhou Penglai

30 20

20 0

60

Percentage (%)

(a)

10 0 ME RP SC PA NC CV SSU MA SP MM OT CF ND RV MV SB SST Species

Dalian Yingkou Jinzhou Huludao Tianjin Yangkou Laizhou Penglai

ME RP SC PA NC CV SSU SB MM OT CF ND RV MV MA SST Species

90 (d) 80

(c)

Percentage (%)

Percentage (%)

100

70 60

Dalian Yingkou Jinzhou Huludao Tianjin Yangkou Laizhou Penglai

50 40

10

30

6

20 10

2 ME RP SC PA RV MA SST MM OT CF ND SSU SB Species

ME RP SC PA NC MV SB SST MM OT CF ND RV SSU MA Species

Fig. 2.67. DDT composition in the mollusks of 8 cities along the Bohai Sea coastline. (a), (b), (c), and (d) represent the percentages of p,p -DDE, p,p -DDD, o,p DDT, and p,p -DDT, respectively (Yang RQ et al., 2004; Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center)

2.5 Persistent Organic Pollutants in the Coastal Areas of the Bohai Sea

249

hugely diﬀerent in the diﬀerent mollusk species. According to Fig. 2.67, p,p DDE was the predominant component in 6 of 10 coastal cities around the Bohai Sea, especially in Yingkou where the p,p -DDE percentage exceeded 80%. However, the p,p -DDT percentage was the highest in Jinzhou, Tianjin, Yantai, and Weihai, ranging from 41% to 55%. Since the general half-life biodegradation in marine organisms is about 10∼20 yr (Kurt and Ozkoc, 2004), the p,p -DDT residue in mollusks should be very low after their banning in 1983 in China. However, its percentages in Tianjin, Yantai and Weihai were quite high, indicating that some DDTs were still in use. 2.5.2.4 PCBs The PCB composition in the mollusks of 8 cities along the Bohai Sea coastline was summarized in Fig. 2.68. In 7 cities except Huludao, 4-Cl PCB congeners were the predominant contributors to the total PCB concentrations in sediments, while the majority of PCBs detected in the sediment of Huludao were 5-Cl PCB congeners. The principal component analysis (PCA) method was used to study the contaminant characteristics of POPs in mollusks collected from 8 cities by Zhao et al. (2005), whose results showed that the samples near Qinhuangdao, Huludao, and Yingkou cities had a similar PCB pattern to 3-Cl PCB. These sampling sites were located near a former capacitor production factory that used 3-Cl PCB as the impregnant of the power capacitor from the 1960s to the 1970s. The samples from the sampling sites near Tianjin and Dalian cities were characterized by higher chlorinated PCBs and had a similar pattern independent of mollusk species. The patterns of PCBs in the two principal components were diﬀerent from those of 3- and 5-Cl PCB. Sources analysis pointed to the former Tianjin Paint General Factory and the Dalian

Yingkou Huludao Dalian 3-Cl 4-Cl 5-Cl 6-Cl 7-Cl

Qinhuangdao Weihai Yantai Tianjin Yangkou 0%

20%

40%

60%

80%

100%

Fig. 2.68. PCBs composition in the mollusks of 8 cities along the Bohai Sea coastline (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center)

250

2 Biogeochemical Processes of the Bohai Sea

Paint Factory, which used 5-Cl PCB as a paint additive in the 1960s (Zhao et al., 2005). 2.5.3 Potential Risk of Persistent Organic Pollutants On the basis of a large number of previous ﬁeld studies, Long et al. (1995) proposed Eﬀects Range Low (ERL) and Eﬀects Range Median (ERM) to evaluate the potential eco-risk of organic pollutants in sea and estuary sediments. The ERL and ERM values are intended to deﬁne chemical concentration ranges that are rarely, occasionally or frequently associated with adverse biological eﬀects, in which ERL stands for the potential eco-risk probability of 10%. 50% for ERM is another transition point. Now, both of them are considered as eco-risk guidelines. 2.5.3.1 PAHs It is noted that the correlation between impacts and chemical concentrations is fairly good for individual PAHs and total PAH, but poor for p,p -DDE, total DDT, and total PCB. Nevertheless, ERM and ERL values are useful in addressing sediment quality issues and provide qualitative guidelines on water that needs to be done to eﬀectively protect the aquatic environment (Mai et al., 2002). Lin et al. (2005) compared the measured concentrations of PAHs to the ERL and ERM values and draw the conclusion that the concentrations of ﬂuorene in Liaodong Bay, of ﬂuorene, anthracene, chrysene, and benzo(a)anthracene in Qinhuangdao were between ERL and ERM values, indicating that a negative eco-risk eﬀect was occasionally posed in the areas near Liaodong Bay and Qinhuangdao, but other individual PAH concentrations were below ERL elsewhere in the Bohai Sea. Therefore, the other areas of the Bohai Sea were unlikely to suﬀer biological impairment as far as PAHs are concerned (Lin et al., 2005). 2.5.3.2 DDTs and HCHs OCPs can cause a range of adverse human health eﬀects, such as cancer, reproductive eﬀects, and acute and chronic injury to the nervous system. Many previous investigations show that the increasing consumption of contaminated seafood has resulted in an elevated pollutants residual in human tissues compared with that in older generations (Asplund et al., 1994). The World Health Organization (WHO) and Food and Agriculture Organization (FAO) have proposed an acceptable daily intake (ADI) for DDTs at 5,000 ng/(kg·d) (Zhulidov et al., 2002). For a 70-kg human, if he/she consumes l kg aquatic product a day from the Bohai Sea with a mean DDTs level of 199 or 52 ng/g, his/her daily intake dose is 2,850 ng/kg. This value is well below the proposed ADI. However, the ADIs of DDT are 400 and 600 ng/(kg·d) in

2.5 Persistent Organic Pollutants in the Coastal Areas of the Bohai Sea

251

Canada and the USA respectively. Therefore, the DDTs residual level is about 6 times higher than that in Canada and 4 times higher than that in the USA. Similar guidelines do not exist for ΣHCH, but it is reported by the US DHHS (2005) that it has assigned a γ-HCH oral reference dose of 300 ng/(kg·d). If it is presumed that a 70-kg person consumes 1 kg of mollusks from the Bohai Sea in view of the maximum γ-HCH of 2.12 ng/g in Yingkou, his/her daily intake of HCHs is 31 ng/(kg·d), about 1/10 of the oral reference dose. 2.5.3.3 PCDD/Fs and PCBs The complex nature of PCDD/F and PCB mixtures complicates the risk evaluation for human, ﬁsh, and wildlife. For this purpose, the concept of toxic equivalent factors (TEFs) has been developed and introduced to facilitate risk assessment and regulatory control of exposure to these mixtures. TEF values, in combination with chemical residue data, can be used to calculate toxic equivalent quality (TEQ) concentrations in various environmental samples, including animal tissues, soil, sediment, and water. It is now common practice to use the TEQ and associated TEFs directly to characterize and compare contamination by dioxin-like chemicals of abiotic environment samples, such as sediment, soil, industrial waste, soot, ﬂy ash from municipal incinerators, and waste water eﬄuence. However, the application of these ‘intake or ingestion’ TEFs for calculating TEQs in these matrices has limited toxicological relevance and use for risk assessment, unless the aspect of reduced bioavailability and environmental fate and the transport of the various dioxin-like compounds are taken into account. If human risk assessment is done for abiotic matrices, it is recommended that congener-speciﬁc equations be used throughout the whole model, instead of using a total TEQ basis, because fate and transport properties diﬀer widely between congeners (van den Berg et al., 2006). TEQ concentrations in samples are calculated using the following equations (van den Berg et al., 1998): T EQPCDD =

n1 [P CDDi × T EFi ] i=1

T EQPCDF =

n2

[P CDFi × T EFi ]

i=1

T EQPCB =

n3

[P CBi × T EFi ]

i=1

Consequently, TEQs of PCDD/Fs and PCBs in Bohai Bay are summarized in Table 2.29.

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2 Biogeochemical Processes of the Bohai Sea

Table 2.29. TEQs for PCDD/Fs and PCBs using WHO-TEFs (pg TEQ/g dw) (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center) Sediment sample T EQPCDD T EQPCDF T EQPCB Bohai Baya 1.301 0.826 1.0 Yongdingxin River Estuary 1.901 0.776 0.1 Haihe River Estuary 11.52 4.55 0.2 Dagu Drainage River Estuary 645.3 178.3 21 6.760 − 2.42 Yangkoub 6.018 1.07 14.15 Tianjinb Yantaib 4.327 3.195 22.7 0.100 − 3.3 Weihaib Qinhuangdaob 21.90 0.001 3.07 47.32 0.035 12.08 Dalianb Huludaob 19.59 6.098 1.69 32.26 6.845 8.49 Yingkoub

Reference Hu et al., 2005 Liu et al., 2007b Liu et al., 2007b Liu et al., 2007b Zhao et al., 2005 Zhao et al., 2005 Zhao et al., 2005 Zhao et al., 2005 Zhao et al., 2005 Zhao et al., 2005 Zhao et al., 2005 Zhao et al., 2005

a The station located in Bohai Bay in the extension direction where the Nanpaiwu River enters the Bohai Sea. b The TEQ was the product of the concentrations based on lipid weight and TEFs

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3 Biogeochemical Processes of the Yellow Sea

Abstract: In this chapter the biogeochemical processes in the Yellow Sea (YS) are described. The focus is on C, N, P, and Si in the seawaters and sediments, heavy metals and POPs in the sediments, and the main research areas are the South Yellow Sea and Jiaozhou Bay. In the Yellow Sea, the biological pump and riverine input are very important controlling processes in the variation of biogenic elements. The Yellow Sea (YS) is a shallow epicontinental sea surrounded by the Chinese mainland and Korean Peninsula (Fig. 3.1) (Yuan et al., 2008). It is connected with the East China Sea (ECS) to the south, and with the Bohai Sea (BS) to the northwest. Water depth is generally less than 80 m with an average of 44 m. The major sediment sources are the Huanghe River and Changjiang (Yangtze) River, providing an annual sediment load of about 1.1×109 and 4.9×108 t, respectively. A maximum of 1.6×108 t of sediments, which is about 15% of the Huanghe River discharge, is accumulating in the YS annually. Approximately 60% of these sediments are deposited in the Shandong subaqueous delta; the remaining sediments are transported further southward and deposited in the central YS. Most sediment from the Changjiang River is transported southward to the ESC by the Jiangsu coastal current. A number of small rivers draining the Korean Peninsula contribute less than 5.0×106 t of suspended sediments to the YS annually. The YS is divided into two parts by a line from Chengshan Cape in Shandong, China to Changshan in Korea. According to previous investigations, the sedimentation rates in the central part of the southern YS ranged from 0.094 to 0.17 cm/yr, but the sedimentation rates were high and varied from 0.12 to 1.65 cm/yr in the eastern and western regions of the southern YS.

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N 40 38 36 34 32 30 28 26

118

120

122

124

126

128

E

Fig. 3.1. The currents and isobath (m) lines of the Yellow Sea (Yuan et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

3.1 Dynamic Processes of the Yellow Sea The water movement of the YS is aﬀected strongly by semidiurnal tides. The tidal range varies from 1.5 to 8 m, with maximum amplitudes recorded in Kyonggi Bay, the western central coast of Korea. The tidal current velocity ranges from 40 to 110 cm/s, smaller in the center. Because of the shallow water depth, wave action in the YS is strong and important in redistributing coastal sediments derived from rivers. Northward ﬂowing currents dominate the circulation, among which is the YS Warm Current (YSWC) in the central YS. Southward ﬂowing currents are the Jiangsu coastal current along the Chinese coast and the Korean coastal current along the Korean coast. 3.1.1 Yellow Sea Currents The YS is aﬀected by warm and saline oceanic currents and less saline coastal currents in a basin-wide scheme of a cyclonic gyre. In general, the former ﬂow northward whilst the latter ﬂow southward. On the east side of the YS, the Kuroshio and Tsushima Warm Currents and the YSWC dominate, whereas various coastal currents prevail in the Chinese shallow waters.

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3.1.1.1 Yellow Sea Warm Current The YS Warm Current is a main component of the YS circulation, and brings the external warm saline water into the YS to aﬀect the YS environment. The YS Warm Current used to be thought of as a branch separated from the Tsushima Warm Current in the sea area southeast of Cheju Island and to have features that were stronger in winter and weaker in summer. (1) Indicative thermohaline characteristics of the YSWC The warm saline water tongue intruding into the YS has long been considered as a main indicative feature of the YSWC, but it is shown from many satellite remote sensing images and observed temperature distributions that the warm water tongue in winter and spring stretches northwestward in the area southwest of Cheju Island, and then turns to the north, pointing to the eastern end of the Shandong Peninsula, which indicates that the YS Warm Current ﬂows northward along the isobaths of 50 to 60 m rather than along the YS trough. It is shown from the comparison between the YS temperature distribution in October of 1996 and that of February of 1997 (Fig. 3.2, Guo et al., 2000) that the temperature in the warm water tongue area in FebruN 37 6.0

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ary of 1997 was about 1.5 ◦ C higher than that in October of 1996, and the temperature on two sides of the warm water tongue in winter (February of 1997) was obviously lower than that in autumn. In particular on the west side it was lower by over 5 ◦ C than that in autumn, which indicates that there was obvious thermal advection in the warm water tongue in winter, so the YS warm water tongue can certainly be taken as an indicator of the YS Warm Current. It is shown from the analysis of saline water tongues intruding into the YS in diﬀerent months that the saline water tongue exists in the bottom layer almost all the year round, and its northward stretching extent has an annual variation cycle, namely it stretches northward to only 34◦ N in January; it stretches northward quickly to 35◦ 40 N in February (Fig. 3.3, Guo et al., 2000) with a pattern similar to that of the warm water tongue, which stretches northward slowly in spring and summer. The 33.0‰ (saline water) isohaline goes as far north as 37◦ N, but its tongue looks less sharp; the saline water tongue shrinks southward in autumn, the tongue in the bottom layer shrinks southward quickly to the YS trough northwest of Cheju Island in December because the convectional mixing in December has reached the bottom so as to result in a vertically uniform temperature proﬁle, and the saline water tongue keeps on shrinking southward until January. It is suggested that the reason why the warm water tongue in January was not synchronized with the saline water tongue in January is that the temperature ﬁeld in the YS in autumn has a distribution pattern diﬀerent from that of the salinity ﬁeld, i.e., the isotherms have an east-west trend, and the isohalines have a north-south trend, so after the YS Warm Current brings the warm saline water into the YS, the warm water tongue appears at once, but the saline water tongue does N 38 (c)

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not appear until the salinity value in the warm current water has exceeded that on both sides of the warm current. In summer and autumn, the YS Cold Water Mass occupies the YS trough bottom layer so as to make the warm water tongue disappear, but the saline water tongue still exists because the weak water exchange in the YS trough area enables the saline water tongue in the bottom layer to be maintained. Therefore, it is suggested that the warm water tongue in the YS is more indicative of the YSWC than the saline water tongue. (2) Seasonal characteristics of the YSWC In winter the Subei coastal current intrudes into the northern East China Sea along the Changjiang River shoal due to the driving of a strong northerly wind, and the YS Warm Current intrudes into the southern YS as a compensation current. In spring the Subei coastal current intruding into the East China Sea weakens, and the YSWC ﬂowing northward weakens as a consequence; in summer the YS Cold Water Mass occupies the whole YS basin, and the Subei coastal current has no longer intruded into the East China Sea and joins the YS circulation, so the YSWC has disappeared. It is shown from the observed data in October of 1996 that there is an oceanic front in the area near the line connecting the Changjiang River mouth and Cheju Island in autumn, with the temperature front being dominant in the eastern part of the front and the salinity front being dominant in the western part (Fig. 3.4, Guo et al., 2000), which indicates that there is no signiﬁcant water exchange between the YS and the East China Sea in autumn, so there is no YSWC entering the YS. The above-mentioned facts indicate that the YSWC is a seasonal compensation current, which occurs mainly in winter, quickly weakens in spring, and disappears in summer and autumn. These are contrary to the traditional thoughts on the YSWC. (3) Cheju Warm Current and the origins of the YSWC The current passing through the Cheju Strait ﬂows eastward all the year round, and is named the Cheju Strait Current. Lie et al. (1999) named the current ﬂowing round Cheju Island as the Cheju Warm Current. Zang et al. (2001) made a further analysis of the water mass characteristics west of Cheju Island and pointed out that the sea area west of Cheju Island was demarcated by the 34.0‰ isohaline in winter with saline water (salinity is 34.0‰ to 34.5‰) east of the isohaline being the Cheju Warm Current and mixed water (salinity is 33.0‰ to 34.0‰) west of the isohaline being the origin of the YSWC (Fig. 3.5, Guo et al., 2000). (4) Observed current evidence of the YS Warm Current The current observations for three consecutive days were made at the ﬁxed point of 34◦ N, 123◦ 30 E which the YS warm water tongue reached in April of 1996, and the observed data indicated that the surface and bottom residual current directions at the point were roughly north by west, the residual current

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Fig. 3.5. Trajectories of satellite-tracked drifters with daily mean currents at 3 days interval and distributions of 34.0‰ isohaline (Guo et al., 2000) (With permission from Guo BH)

direction at the 20 m depth layer was northeast, and its residual current speed was about 2 to 3 cm/s. In addition, Argos drifters were deployed in the YS warm water tongue in April of 1996 and February of 1997. Among them, the Argos drifter deployed at 34◦ N, 122◦ E in February of 1997 moved northwestward at a speed of 2.4 cm/s for a long period of time and then turned to the south west. Therefore, it is shown from the direct current observations that the YS Warm Current is quite weak with a speed of less than 5 cm/s. And it is an intermittent northwestward current during the winter monsoon when the strong northerly winds prevail.

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3.1.1.2 Coastal Currents Coastal currents ﬂow southward along the west coast of Korea particularly during winter. In the southern Yellow and East China seas, pronounced coastal currents switch their current direction seasonally, i.e., southward in winter and northward in summer, although the direction of the oﬀshore Taiwan Warm Current is constantly northward. In particular, during the summertime ﬂooding season, the northward Taiwan Warm Current meets the discharge of the Changjiang River forming the “Changjiang diluted water (CDW)” oﬀ the river mouth. The CDW extends southeast to eastward and may reach the oﬀshore west of Jeju Island during peak ﬂoods. The coastal current along the Korean coast, reinforced by strong northerly winds during late fall and winter, ﬂows southward and then eastward through the Jeju Strait. Diluted by summer runoﬀ and chilled by outbreaks of cold air, the Korean coastal current forms a strong and stable thermohaline front embracing the coastal archipelago including Heuksan Island oﬀ the southwestern coast of Korea. 3.1.1.3 Cold Water A striking physical feature of YS is the YS Cold Water Mass (YSCWM), which is entrenched in the deep trough below the thermocline from late spring to fall and performs as a bottom nutrient pool, providing rich nutrient to the euphotic layer through entrainment. The seasonal cold water mass (YS Cold Water) occurs in the lower water column of the central YS during the warm half of the year (April to November). It is characterized by a lower temperature and higher salinity compared with that of surrounding waters and varies in distribution and volume with the season. It is considered that the YS Cold Water is derived from the surface water of the previous winter, sinking through a cooling process. The intermediate cold water in the sea area southeast of the Shandong Peninsula was ﬁrst found in April of 1996. The water mass distributions in the upper layer of the YS in spring are shown in Fig. 3.6, where the intermediate cold water area is the region enclosed by a dashed line. Fig. 3.6 illustrates the proﬁles observed at 6 CTD stations, which show that the temperature in the intermediate layer is lower by 0.5 to 2.5 ◦ C than that in both the upper and lower layers. Zou et al. (2000) pointed out that intermediate cold water occurred in the shelf-front area (Fig. 3.7), whose formation mechanism could be stated as follows. In early spring, the YSWC gets weaker ﬁrst in the upper layer, while the temperature variations get slower in the bottom layer; then the upper layer shelf-front moves eastward along with a weakening of the northward ﬂowing warm current, and the mixed water in the western YS extends southeastward. After that mixed water in early spring is formed in a shallow water area of a depth of about 30 m with a signiﬁcant rise in water temperature in the upper layer and at the same time the lower layer (at depths

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3.1 Dynamic Processes of the Yellow Sea

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of more than 15 m) keeps its wintertime temperature, which is lower by about 2 ◦ C than that of the neighboring YS Warm Current water, and its salinity and density are all lower than those of the warm current water. Thus, when the mixed water extends southeastward and overlies the YS Warm Current water, the lower layer mixed water becomes the intermediate cold water with a water temperature lower than that in the upper and lower layers. The strong bottom boundary mixing makes the thermocline domed. Furthermore, the strength of the bottom boundary mixing inﬂuences the temperature structures but has less of an eﬀect on the velocity structure. The circulation of YSCWM has a two-layer structure, the upper layer is cyclonic, while the lower layer is anticyclonic. The lower layer is thinner (about 10∼20 m thick) and weaker than the upper layer, and the depth-integrated circulation is cyclonic. 3.1.2 Water Exchange Between the Yellow Sea and the East China Sea In winter, the Subei coastal water extends southeastward to intrude into the northern ECS, and the YSWC intrudes into the southern YS as a compensation current to form a pattern of S-shaped isothermal lines, which indicate the features of water exchange (Fig. 3.8a). In spring, the Subei coastal water begins to retreat, the Taiwan Warm Current water extends northward to affect the Changjiang River Estuary, which results in a water pattern without a tongue of low temperature and low salinity in the surface layer (0∼10 m). However, the low temperature and low salinity water tongue at depths of more than 30 m still remain. Moreover, the northern ECS Cold Water (cold eddy) and the associated cyclonic circulation occur in the area southwest of Cheju Island. The YSWC water intruding into the southern YS has been cut oﬀ to form an isolated warm water mass. Therefore, a pattern of distribution composed of a cold water mass in the northern ECS and a warm water mass in the southern YS has been formed (Fig. 3.8b). In summer, the northern ECS cold eddy moves eastward due to the strengthened Taiwan Warm Current, the cold eddy edge water is mixed with the neighboring warm water to result in shrinkage in the cold eddy range (Fig. 3.8c), and a distribution pattern composed of a high salinity water mass in the north and a low salinity water mass in the south occurs in the bottom layer (Fig. 3.9). In autumn, the northern ECS cold eddy disappears (Fig. 3.8d) and an oceanic front occurs along the line connecting the Changjiang River mouth with Cheju Island, where the temperature front is dominant in the eastern part of the front and the salinity front is dominant in the western part. Therefore, there is almost no water exchange between the YS and the ECS in autumn, and a new cycle of water exchange between the YS and the ECS begins in late autumn and early winter.

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3.2 Carbon Biogeochemical Processes in the Yellow Sea Important processes in the carbon cycling system always happen in the airsea interface (Fig. 3.10). As a result, it is critical in studying the multilayer distribution of CO2 system across the air-sea interface. In recent years, scientists have made a series of advances in studying the carbon biogeochemical process across the air-sea interface and its physical-chemical properties determinations. CO 2 (g )

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Fig. 3.10. The sketch map of carbon biogeochemical process across air-sea interface. The interface was divided into three multi-layers, whose names and description were as follows: SML: surface micro-layer, thickness50 nm; SSL: subsurface layer, 50 cm; SL: surface layer, 2 m (Gong et al., 2007) (With permission from Gong HD)

3.2.1 Carbon Processes across the Air-Sea Interface The YS is an important part of the marginal sea in the northwestern part of the Paciﬁc. It was estimated that the ECS and YS could absorb atmospheric CO2 of approximately (13∼30)×106 t C based on PCO2 data calculated from the pH, Alk (alkalinity) and DIC (dissolved inorganic carbon) data observed in the southeast of the ECS. 3.2.1.1 Horizontal Distributions of Dissolved Inorganic Carbon and Alkalinity in the Surface Water The isograms of DIC and Alk of the SML, SSL, and SL are shown in Fig. 3.11 (Gong et al., 2007). The distribution of DIC and Alk in the surface seawater in March is similar to that of May. DIC and Alk decrease in turn from SML, SSL to SL, showing obvious enrichment of DIC and Alk in the SML. The results are consistent with the conclusion that inorganic matter is enriched in the SML near the Nansha Islands of the South China Sea. In addition, there is clearly a positive relationship between DIC and Alk (Fig. 3.12). It could be explained by basic deﬁnition formulas of DIC and Alk, in which their main

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Fig. 3.12. Relationship between DIC and Alk in SML, SSL, and SL. (a) March; (b) May. SML: surface micro-layer, thickness50 nm; SSL: subsurface layer, 50 cm; SL: surface layer, 2 m (Gong et al., 2007) (With permission from Gong HD) 2− components are HCO− taking up 95% of the total. Comparing 3 and CO3 the concentrations of DIC and Alk in March and May, it was found that DIC and Alk in March are obviously higher than those in May. This could be explained by seawater warming up from March to May, with more and stronger biological activities. As a result, biological photosynthesis consumed more inorganic carbon, resulting in decreases in DIC and Alk. Scientists also found that biological activity decreases the concentration of DIC in CO2 in the Taiwan Strait.

3.2.1.2 Horizontal Distributions of pH and PCO2 in the Surface Water The isograms of pH and PCO2 of the SML, SSL, and SL are shown in Fig. 3.13 (Gong et al., 2007). Apparently, the distribution patterns in March are similar to those in May. PCO2 decreased but pH increased in turn from SML, SSL to SL. Clearly, a negative relationship exists between pH and PCO2 (Fig. 3.14, Gong et al., 2007). As PCO2 in the seawater is lower than that in the atmosphere, the YS in spring behaves as a sink for atmospheric CO2 . The PCO2 in seawater will go up and a balance is reformed between the CO2 dissolution and hydrolyzation of H2 CO3 when atmospheric CO2 comes into the surface seawater through the air-sea interface. As a result, the H+ concentration increases and pH decreases in seawater. The pH and PCO2 of seawater has a negative relationship.

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3.2 Carbon Biogeochemical Processes in the Yellow Sea 300

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The distribution of PCO2 also shows that PCO2 decreased from the outer sea to the coast. The stations close to the coast would be aﬀected mostly by terrestrial materials with more nutritional input. Consequently, the richer the nutrition, the stronger the biological activity, and the greater the consumption of CO2 , which is also the case in the southern YS. Comparison of pH and PCO2 between March and May showed that pH in March was lower than that in May but the opposite to PCO2 . In 2004, I studied the distribution of PCO2 in the ECS and found that the degree of biological activity in the marginal sea is one of the important eﬀective factors (Song, 2004). Meanwhile, the photosynthesis of phytoplankton consumes the CO2 in seawater and releases O2 , which leads to a decrease in PCO2 and an increase in pH. Therefore, the reason for the changes in pH and PCO2 in March and May should be possibly the enhancement of biological photosynthesis with a rise in the temperature of the seawater. 3.2.1.3 Relationships Between PCO2 and Temperature, Salinity, Longitude and Latitude For isolating the factors that control the temporal variation in PCO2 with certain parameters, statistical analyses were performed. The relationships between PCO2 and temperature, salinity, longitude, and latitude are shown in Fig. 3.15 (Gong et al., 2007). For each month PCO2 increased with rising temperature and salinity. An earlier study on the ECS showed a good positive relationship between PCO2 and temperature/salinity, but it was local and transient because of the eﬀects of current and climate. Although the temperature in May was obviously higher than that in March, so obviously was the

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amount of PCO2 due to biological activities. This phenomenon shows that over diﬀerent time spans, the distribution of PCO2 in the marginal sea is complex and shows special patterns due to the combined eﬀects of temperature and biological activity. A possible explanation is that the eﬀect of biological activities was stronger than that of the temperature increase because of lower PCO2 in May. A similar result was also found in the study of the annual variation (especially in March and May) of PCO2 in the Mid-Atlantic: much of the variation could be explained by the annual solar cycle. Because of the shallow depth and relatively long residence time, seasonal heating and cooling dramatically aﬀect the seasonal surface water temperature, which impacts biological activity directly in a regular temporal pattern. SML 350

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3.2.1.4 Carbon Flux across Seawater Interface of the Yellow Sea Two facts determined that the YS is a sink of atmospheric CO2 in spring. Firstly, in March and May 2005, the values of PCO2 in the YS were much below 370 μatm. Secondly, the pH value decreased from SL to SML. The CO2 ﬂux in SML (FCO2 (SML) ) through the sea-atmosphere interface in the YS in spring was calculated on the basis of the SML data. Four models were used in this calculation, including Liss and Merlivat (1986), Peng and Takahashi (1989), Tans et al. (1990) and Wanninkhof (1992). The calculated FCO2 were compared for the cases of SSL, SL, and SML. The formulae are: FCO2 = K × PCO2 (unit of FCO2 : mol/(m2 ·yr))

(3.1)

PCO2 = PCO2 (SML) − PCO2 (atmsp)

(3.2)

K is the constant of CO2 transfer at the interface of the sea-atmosphere, and PCO2 (atmsp) regarded as 370 μatm. FCO2 , calculated in the model of Liss and Merlivat, is the smallest, whereas that of Peng and Takahashi is the largest (Table 3.1, Gong et al., 2007). The average values obtained from four models between the atmosphere and SML were about –6.105 mol/(m2 ·yr). And FCO2 in May was greater than that in March. Recently, in a study on the carbon cycle, the PCO2 data in SL are always used to calculate FCO2 . It was reported that the material ﬂux calculated on the basis of the SML data is more reliable than that on the basis of the SL, because the SML is the closest seawater interface to the atmosphere. Table 3.1 (Gong et al., 2007) shows obvious diﬀerences in FCO2 among SML, SSL, and SL, in an order of FCO2 (SML)
Liss and Merlivat −2.152 −2.449 −2.713 −4.680 −4.916 −5.111 −3.416 −3.683 −3.912

Model Peng and Tans Takahashi −4.551 −4.495 −5.176 −5.119 −5.733 −5.667 −9.663 −9.589 −10.149 −10.074 −10.549 −10.469 −7.107 −7.042 −7.663 −7.595 −8.141 −8.068

Average Wanninkhof −4.267 −4.845 −5.383 −9.443 −9.913 −10.321 −6.855 −7.379 −7.852

−3.866 −4.396 −4.874 −8.344 −8.763 −9.113 −6.105 −6.580 −6.994

Models from Liss and Merlivat (1986), Peng and Takahashi (1989), Tans et al. (1990), and Wanninkhof (1992)

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3 Biogeochemical Processes of the Yellow Sea

between FCO2 (SL) and FCO2 (SML) was 0.873, which would cause a big bias in FCO2 calculation. Setting the area of the YS at 3.8×1011 m2 and the time of spring being 1/4 year, the FCO2 (in tons of carbon, or t C) of the entire sea in the season would be about –6.96×106 t C, yielded from: Cﬂux = FCO2 (SML) × Sarea × 0.25 × 12 × 10−6 = −6.105 × 3.8 × 1011 × 0.25 × 12 × 10−6 = −6.96 × 106 t C 3.2.2 Biological Carbon Fixation in the South Yellow Sea Seawater Consecutive cruise data along three transects (added to four transects since 2003) in the southern Yellow Sea (SYS, Fig. 3.16), maintained by the YS Environmental Cooperative Research between China and Korea, were used. The research concerned marine physics, chemistry, geology, biology, and environment. The cruise data included the ocean temperature (T ), salinity (S), plankton biomass (chlorophyll a concentration and zooplankton biomass, etc.) and biogenic elements. N 38 37

China

A6

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36 35

C9

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C2 C1

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34 33 120

121

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Fig. 3.16. A map of the southern YS. A1∼D9 were the sampling stations (Zheng et al., 2006)

3.2.2.1 Water Masses Based on the average temperature and salinity calculated from 1999∼2005 cruise data, 33 stations can be distinguished in four distinct water masses and were plotted with diﬀerent symbols in Fig. 3.17; the YS and ECS mixed water with relatively high salinity (32.5‰∼33.7‰) and surface temperature

3.2 Carbon Biogeochemical Processes in the Yellow Sea

283

(18.5∼21.5 ◦ C) in the transition area of YS and ECS; YS surface water with low salinity (31.0‰∼32.5‰) in the shallow water (depth<50 m) of YS; YS Cold Water with an extremely low bottom temperature (<11 ◦ C, intermediate salinity of 32‰∼33‰ and depth>50 m in the central part of the YS Coastal Water with the lowest salinity (<31‰). Although diﬀerent water types had quite diﬀerent T -S characteristics, natural boundaries which distinguished one from another were not always present. The boundaries were then taken from previous studies according to certain chemical characteristics observed, in order to diﬀerentiate hydrographic conditions under which different biogeochemical processes prevailed. According to the prevailing water massed in the SYS, hydrographic data are presented in three groups, representing the inshore (YS Surface Water and Coastal Water), central (YS Cold Water) and mixing (YS and ECS Mixed Water) regions. The corresponding localities are represented by stations A1, A2, A6, B8, B9, C7∼C9, D7∼D9; A3∼A5, B1∼B7, C1∼C6 and D1∼D5. YS Costal Water YS Cold Water

21

Surface temperature ( )

D9

20 19 18

D8

YS surface water YS and ECS mixed water D1D3 D2D4 D5

D7 D6 C9C8C7

C6C4C3 C1 C2 C5 B7 B6 B2 B3 B5 B1 B4

B9 B8 A6

17

A5A4 A3

16 A2

15 A1

14 20 Bott 18 16 om 14 tem 12 pera ture 10 ( ) 8

34 33 32 ) 31 ( 30 ty 29 salini 28 om 27 Bott

Fig. 3.17. Plot of surface temperature, bottom temperature and salinity of stations. Stations of four distinct water masses were represented with diﬀerent symbols (Zheng et al., 2006)

3.2.2.2 Carbon Fixation Production Many researchers have noticed that the SYS is deﬁcient in nutrients, both the chlorophyll a (Chl a) and primary production levels being very low. For better understanding the response of the carbon ﬁxed production (CFP) of phytoplankton to environmental changes in the SYS, daily carbon ﬁxed production of phytoplankton, namely primary productivity, was estimated as follows (Zheng et al., 2006):

284

3 Biogeochemical Processes of the Yellow Sea

CF P =

Ps ED 2

(3.3)

where CFP is daily carbon ﬁxed production of phytoplankton (mg C/(m2 ·d)); Ps is the potential productivity of phytoplankton in surface waters (mg C/(m3 ·h)); E is the euphotic depth (m) and D is the illumination period (h). Euphotic depth (E) was calculated as three times as transparent. The potential productivity of phytoplankton in surface waters (within 1 m) (Ps ) was computed as Ps = Ca Q

(3.4)

where Ca is the Chl a concentration in surface waters (mg/m3 ) and Q is the assimilation rate (mg C/(mg Chl a·h)) which means the maximum rate of carbon ﬁxation within a water column. Because the photosynthesis of chlorophyll concentration is dominated by enzymes and the activity of enzymes is mainly dominated by temperature, Q can be commonly considered as the function of the sea surface temperature (SST). Using the experienced relation of Q and the SST (◦ C) provided, we successfully estimated the values of Q and the ocean’s primary productivity for ECS. In a similar way, the values of Q for the SYS were calculated in the present study: Q = 1.13 (SST < 1.0) Q = 4.00 (SST > 28.5) Q = 1.2956 + 2.749 × 10−1 SST + 6.17 × 10−2 SST 2 − 2.05 × 10−2 SST 3 +2.462 × 10−3 SST 4 − 1.348 × 10−4 SST 5 + 3.4132 × 10−6 SST 6 −3.27 × 10−8 SST 7 (1.0 SST 28.5) (3.5) In 1999 and 2000, the high values of carbon ﬁxed production (CFP) of phytoplankton did not follow the general pattern with a coastal maximum, but were found in the central region of the SYS which was generally occupied by YS Cold Water (Figs. 3.18a, b). This may be attributed to the nutrients regime in this period in the SYS (e.g., Fig. 3.18a). The YS Cold Water is a seasonal water mass with low temperature which has great eﬀects on the distribution and transportation of nutrients. It is one of the main nutrient sources in the SYS which caused nutrients enrichment at a depth below the pynocline and thermocline rather than in the euphotic zone during the stratiﬁed period (from April to November). In November, a vertical eddy intensiﬁes its intrusion and the YS Cold Water disappears gradually, thus the nutrients stored in the water mass could be diﬀused from the deep to the euphotic zone and become the main nutrient source to support phytoplankton growth rather than riverine inputs. It is interesting to note that high values of CFP in SYS have transferred clearly from the central region to the inshore area since 2001 (Figs. 3.18c∼g).

3.2 Carbon Biogeochemical Processes in the Yellow Sea N 38

N 38

37

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36

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285

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34 (c)

33 120

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N 38 37 36 35 34 (g)

33 120

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Fig. 3.18. Isolines of carbon ﬁxed production (CFP) of phytoplankton for the SYS in (a) 1999, (b) 2000, (c) 2001, (d) 2002, (e) 2003, (f) 2004, (g) 2005 (unit: mg C/(m2 ·d)) (Zheng et al., 2006)

286

3 Biogeochemical Processes of the Yellow Sea

In coastal waters, nutrients accumulation occurred easily owing to freshwater infusion, together with the limitation of diﬀusion of coastal currents. High nutrients promoted the growth of phytoplankton and therefore formed high CFP regions. It indicated that since 2001 terrigenous nutrients became the main nutrient source to support phytoplankton growth rather than the nutrients stored in the YS Cold Water (e.g., Fig. 3.19b). As shown in Figs. 3.18 c∼g, CFP in the study waters dynamically changed spatially with two signiﬁcantly high value regions located in the inshore areas of the Shandong Peninsula and the Korean Peninsula, respectively. The former occurred in the area occupied by YS Surface Water and Coastal Waters (Lubei and Subei Costal waters), and the latter was the center current composed of the YSWC (from the northbound branch of the Kuroshio in the ECS) and the southbound Korea coastal current. At the same time, low values of CFP were widespread in the inner region of the nutrient-deﬁcient YSWC and outer sea area adjacent to the Changjiang River plume. N 38

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36

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34 Cheji Do

(a) 33 120

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33

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(b) 128

E

Fig. 3.19. Isolines of means of vertically upper layer phosphate in (a) 1999 and (b) 2005 in the SYS (unit: μmol/L). Because for a long time past and until recently the environment of the YS has been experiencing a limitation of P, the phosphate distributions were described as a nutrient condition in the SYS (Zheng et al., 2006)

It was noticeable that the distribution pattern of CFP in SYS in recent years was characterized by having more correlation with hydrological conditions: the high values were observed in the coastal waters and frontal area; low values occurred in the Kuroshio area and turbid area adjacent to estuary. Considering that nutrient supply and other hydrographic conditions were quite diﬀerent in diﬀerent water types for the SYS, so were the prevailing biogeochemical processes. In order to diﬀerentiate controlling factors for CFP in diﬀerent water types, the correlation between nutrient supply, light penetration and CFP in diﬀerent water masses was discussed.

3.2 Carbon Biogeochemical Processes in the Yellow Sea

287

(1) The inshore region Transparency increased accompanied by an increase in salinity for the inshore region (e.g., Fig. 3.20a, Table 3.2). Apparently, the inshore region was experiencing a violent hydrodynamic condition (riverine discharge, tide, and coastal water), which results in turbidity and light limitation. It means that there might be more light available a little further away from the coast. The carbon

50 (a)

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30 )

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2003 2004 2005

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1000 800 600

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1999 2000 2001 2002 2003 2004 2005

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CFP (mg C/(m2 d))

P (mmol/L)

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0 26

(b)

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20 30 Tr (m) (f)

40

50

1999 2000 2001 2002

400

2003 2004

200

2005

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 P (mmol/L)

Fig. 3.20. The transparency (Tr)-salinity (S ) (a), dissolved inorganic nitrogen (DIN)-S (b), phosphate (P)-S (c), carbon ﬁxed production (CFP)-Tr (d), CFPDIN (e), and CFP-P (f) diagrams in the inshore region (YS Surface Water and YS Costal Water) for the SYS in various years (Zheng et al., 2006)

288

3 Biogeochemical Processes of the Yellow Sea

Table 3.2. Regression results of nutrient supply, light penetration and CFP in diﬀerent water mass for the SYS in various years (Zheng et al., 2006) Cruises 1999.11

Region Inshore region

Water types YS surface water and coastal water

Central YS Cold Water region 2000.11

Inshore region

YS surface water and coastal water

Central YS Cold Water region 2001.11

Inshore region

YS surface water and coastal water

Central YS Cold Water region 2002.11

Inshore region

YS surface water and coastal water

Central YS Cold Water region 2003.11

Inshore region

YS surface water and coastal water

Central YS Cold Water region

2004.11

Mixed region

YS and ECS mixed water

Inshore region

YS surface water and coastal water

Central YS Cold Water region 2005.11

Inshore region

YS surface water and coastal water

Central YS Cold Water region Mixed region

YS and ECS Mixed Water

Parameters Tr (m)-S (‰) DIN (μmol/L)-S (‰) P (μmol/L)-S (‰) CFP (mg C/(m2 ·d))-Tr (m) CFP (mg C/(m2 ·d))-DIN (μmol/L) CFP (mg C/(m2 ·d))-P (μmol/L) CFP (mg C/(m2 ·d))-Tr (m) CFP (mg C/(m2 ·d))-DIN (μmol/L) CFP (mg C/(m2 ·d))-P (μmol/L) CFP (mgC/(m2 ·d))-Tr (m) CFP (mg C/(m2 ·d))-DIN (μmol/L) CFP (mg C/(m2 ·d))-P (μmol/L) CFP (mg C/(m2 ·d))-Tr (m) CFP (mg C/(m2 ·d))-DIN (μmol/L) CFP (mg C/(m2 ·d))-P (μmol/L)

Regression results Y =11.7X−344.57 Y =−2.17X+73.68 * Y =23.214X−77.54 * * * Y =158.6X+127.19 Y =242.64X+488.11

R 0.68 −0.67 ∗ 0.78 ∗ ∗ ∗ 0.64 0.33

n 8 8 8 8 8 8 16 16 16

Y =14.539X−54.64 Y =−138.36X+751.27 Y =−929.82X+566.54 Y =13.237X−89.256 * *

0.68 −0.66 −0.36 0.69 ∗ ∗

8 8 8 16 16 16

C/(m2 ·d))-Tr (m) C/(m2 ·d))-DIN (μmol/L) C/(m2 ·d))-P (μmol/L) C/(m2 ·d))-Tr (m) C/(m2 ·d))-DIN (μmol/L) C/(m2 ·d))-P (μmol/L) C/(m2 ·d))-Tr (m) C/(m2 ·d))-DIN (μmol/L) C/(m2 ·d))-P (μmol/L) C/(m2 ·d))-Tr (m) C/(m2 ·d))-DIN (μmol/L) C/(m2 ·d))-P (μmol/L)

Y =12.806X−133.92 Y =−777.62X+771.33 Y =−878.27X+708.9 * Y =74.203X+20.161 Y =858.55X+150.87

0.55 −0.69 −0.69 ∗ 0.46 0.65

12 12 12 16 16 16

Y =9.4432X−63.559 Y =−44.39X+414.59 * * * Y =1019.4X+155.47

0.71 −0.60 ∗ ∗ ∗ 0.60

12 12 12 16 16 16

CFP (mg C/(m2 ·d))-Tr (m) CFP (mg C/(m2 ·d))-DIN (μmol/L) CFP (mg C/(m2 ·d))-P (μmol/L) CFP (mg C/(m2 ·d))-Tr (m) CFP (mg C/(m2 ·d))-DIN (μmol/L) CFP (mg C/(m2 ·d))-P (μmol/L) CFP (mg C/m2 ·d)-Tr (m) CFP(mg C/(m2 ·d))-DIN(μmol/L) CFP (mg C/(m2 ·d))-P (μmol/L) CFP (mg C/(m2 ·d))-Tr (m) CFP (mg C/(m2 ·d))-DIN (μmol/L) CFP (mg C/(m2 ·d))-P (μmol/L) CFP (mg C/(m2 ·d))- Tr (m) CFP (mg C/(m2 ·d))-DIN(μmol/L) CFP (mg C/(m2 ·d))-P (μmol/L)

Y =8.6257X−23.466 Y =−7.4256X+198.26 Y =−948.13X+294.99 * * Y =211.23X+43.87 Y =25.148X+318.88 Y =40.952X+18.91 *

0.59 −0.37 −0.52 ∗ ∗ 0.69 0.85 0.98 ∗

12 12 12 16 16 16 5 5 5

Y =8.6257X−23.466 Y =−15.632X+405.03 * * Y =123.09X+147.7 Y =1596.4X+82.355 Y =5.32X−148.85 Y =−8.97X + 281.92 Y =−0.048X−1.67 Y =17.467X−48.992 Y =−6.747X+331.89 * * * Y =1967.3X+153.37 Y =30.726X−23.245 Y =65.068X+16.314 Y =186.87X+34.693

0.73 −0.50 ∗ ∗ 0.56 0.55 0.66 −0.91 −0.54 0.87 −0.41 ∗ ∗ ∗ 0.78 0.88 0.75 0.82

12 12 12 16 16 16 12 12 12 12 12 12 16 16 16 5 5 5

CFP CFP CFP CFP CFP CFP

(mg (mg (mg (mg (mg (mg

CFP CFP CFP CFP CFP CFP

(mg (mg (mg (mg (mg (mg

Tr (m)-S (‰) DIN (μmol/L)-S (‰) P (μmol/L)-S (‰) CFP (mg C/(m2 ·d))-Tr (m) CFP (mg C/(m2 ·d))-DIN (μmol/L) CFP (mg C/(m2 ·d))-P (μmol/L) CFP (mg C/(m2 ·d))-Tr (m) CFP (mg C/(m2 ·d))-DIN (μmol/L) CFP (mg C/(m2 ·d))-P (μmol/L) CFP (mg C/(m2 ·d))-Tr (m) CFP (mg C/(m2 ·d))-DIN (μmol/L) CFP (mg C/(m2 ·d))-P (μmol/L)

ﬁxed production of phytoplankton in the coastal waters has been positively related to transparency with a reasonably good correlation (Fig. 3.20d, Table 3.2). The phytoplankton photosynthesis must have been limited by light penetration rather than by other factors in the inshore region. The lack of correlation between nutrients (N and P) and CFP (Figs. 3.20e, f, Table 3.2) for the coastal waters indicated that nutrients were not the limiting factor to phytoplankton growth in the inshore region which was always enriched with nutrients. Furthermore, transparency increased with increasing salinity and a

3.2 Carbon Biogeochemical Processes in the Yellow Sea

289

decrease in nutrients concentration (Figs. 3.20a, b) which could explain the negative correlation between nutrients and CFP for the inshore region.

1200

(a)

1999 2000 2001 2002 2003 2004 2005

1000 800 600

CFP (mg C/(m2 d))

CFP (mg C/(m2 d))

(2) The central region There was almost no correlation between transparencies (Tr) and CFP for the central region (Fig. 3.21a, Table 3.2) suggesting that light availability was not the predominant controlling factor for phytoplankton photosynthesis in the outer shelf water. Studies showed that light availability was one of the dominant limiting factors to phytoplankton growth in coastal waters and adjacent areas to the estuary. However, in the highly transparent middle area of the SYS, transparency was no longer the limitation, but it was nutrient supplement that became the dominant limiting factor for phytoplankton growth. It was interesting to note that CFP had no correlation with nutrients concentration for the central region in 1999 and 2000 (Figs. 3.21b, c, Table 3.2). It might be attributed to suﬃcient nutrients stored in YS Cold Water during the stratiﬁed period. In November, YS Cold Water disappeared gradually and nutrients could be supplemented from the benthic ﬂux to the euphotic zone and support phytoplankton growth. However, for most studied years, the CFP in the coastal waters has been positively related to phosphate concentration with a reasonably good correlation but had a lack of correlation with the DIN

400 200 0

1200

(b)

1000

1999 2000 2001 2002 2003 2004 2005

800 600 400 200 0

5

15

25

35 Tr (m)

45

55

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 DIN (mmol/L) (c)

CFP (mg C/(m2 d))

1200

1999 2000 2001 2002 2003 2004 2005

1000 800 600 400 200 0

0

0.2

0.4 0.6 P (mmol/L)

0.8

1.0

Fig. 3.21. The CFP-Tr (a), CFP-DIN (b), and CFP-P (c) diagrams in the central region (YS Cold Water) for the SYS in various years (Zheng et al., 2006)

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3 Biogeochemical Processes of the Yellow Sea

concentration, which indicated that the SYS ecosystem has probably been experiencing a limitation of P rather than N.

400 350 300 250 200 150 100 50 0

CFP (mg C/(m2 d))

CFP (mg C/(m2 d))

(3) The mixed region The relationships of Tr-CFP, P-CFP, and DIN-CFP in the YS and ECS mixed water are the direct proportion, i.e., the CFP increases while the Tr (transparency), P, and DIN concentrations increases (Fig. 3.22). The mixed region is characterized by a more violent hydrodynamic condition, which causes turbidity and low transparency of seawater. Furthermore, the nutrients in the mixed region water far away from land are poor from terrestrial input. Besides, the mixed region water was diluted by the saline and oligotrophic waters from the ECS. Therefore, the factors controlled CFP in the YS and ECS mixed water were low transparency and nutrients (such as P and DIN).

2003 2005

(a)

400 350 300 250 200 150 100 50 0

CFP (mg C/(m2 d))

8 10 12 14 16 18 20 22 24 26 28 Tr (m)

2003 2005

(b) 1

2

3

4 5 6 7 DIN ( m mol/L)

8

9

400 350 2003 300 2005 250 200 150 100 50 (c) 0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 P ( m mol/L)

Fig. 3.22. The CFP-Tr (a), CFP-DIN (b), and CFP-P (c) diagrams in the mixed region for the SYS in various years (Zheng et al., 2006)

3.2.3 Initial Carbon Fixed Production The ocean, which is by far the largest active reservoir of carbon, covering 71 percent of the globe, is the main sink in the global carbon cycle and hence the ultimate repository for fossil fuel CO2 . Phytoplankton is the major organism that ﬁxes CO2 , either in particulate or dissolved forms via the photosynthetic process. This constitutes the basis of the marine food chains and

3.2 Carbon Biogeochemical Processes in the Yellow Sea

291

thus may aﬀect the dynamic of higher trophic levels due to the variability in their biomass and productivity. The dead phytoplankton, a byproduct of zooplankton metabolic activity through fecal pellet production and the vertical migration of zooplankton through feces production represent ﬂuxes of carbon at depth. The deep sea is the ultimate sediment trap. Once deposited, the majority of the particulate matter reaching the sea ﬂoor is remineralized on a time scale of weeks up to even decades. It has been clear that the transport of biogenic particles from the productive ocean layers to the interior sink is the key driver of the oceanic biological pump. Globally, the magnitude and eﬃciency of the biological pump will in part modulate levels of atmospheric CO2 . The net production of the euphotic zone and carbon ﬂux data for each step of the biological pump in global oceans have been obtained by JGOEFS. Where the annual carbon production is ﬁxed by phytoplankton photosynthesis, the initial stage of the biological pump exceeds 300×108 t, which is about 5 times more than annual anthropogenic emissions, and thus photosynthesis plays an important role in the global carbon cycle. Furthermore, carbon ﬁxed production of zooplankton through the prey of the primary producer, which is the second step of the biological pump, is also the basis of the oceanic carbon cycle. In the past, based on the data obtained from the end of the 1990s, the values of initial carbon ﬁxed production (ﬁxed by phytoplankton and zooplankton) for the SYS were estimated and their trends were discussed in detail. 3.2.3.1 Estimation of Carbon Fixed Production of Phytoplankton According to the equations in section 3.2.2.2, the assimilation rate (Q) of phytoplankton changes consequentially with temporal-spatial variation. Besides phytoplankton adaptability, there are so many environmental factors having an eﬀect on the assimilation rate such as nutrients, light availability, temperature, and so on. In order to compare the spatial and temporal variation of the photosynthetic capacity of phytoplankton, the measured values of the assimilation rate in diﬀerent periods and sea areas are exhibited in Table 3.3. In the following study, we use the assimilation rates of phytoplankton in autumn of the SYS obtained by Zheng et al. (2006) after comparing the data from Ning et al. (1995) and Sun et al. (2003). 3.2.3.2 Estimation of Carbon Fixed Production of Zooplankton Carbon ﬁxed production of zooplankton through zooplankton biomass could be undertaken from the following procedures. Firstly, the dry weight for a zooplankton unit is calculated: DW =

ZB ZA

(3.6)

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3 Biogeochemical Processes of the Yellow Sea

Table 3.3. Assimilation rates in diﬀerent periods and sea areas (Zheng et al., 2006) Assimilation rate References (mg C/(mg Chl a·h)) Kuroshio Current (low nutrients) 0.2∼1.0 Lalli and Parson, 1997 Oyashio Current 3.7 Saijo and Tehimum, 1960 <5 Letelier et al., 1996 JGOFS-WOCE station in the subtropical eddy zone of the North Paciﬁc Tropical inshore water (high 9∼17 Lalli and Parson, 1997 temperature and nutrients) Woods Hole coastal water area 5.7 Liu et al., 2002 Prydz Bay (summer) 1.53 Central Bohai Sea (autumn) 0.26∼4.65 Sun et al., 2003 Southern YS and northern ECS 8.0, 3.7, 2.4, 1.6 Ning et al., 1995 (May, Aug., Nov., and Dec.) SYS (autumn) 3.4∼4.5 Zheng et al., 2006 Costal waters of the ECS (winter 2.6∼5.8, 3.1∼11.2 Fei and Mao, 1987 and autumn) Fukuyama Bay 0.44∼17.0 Fei and Mao, 1987 Sea area

where DW is the dry weight for a zooplankton unit (mg/cell), ZB is the zooplankton biomass (in dry weight) for each station (mg/m3 ), and ZA is the zooplankton abundance (cell/m3 ). Secondly, we computed the respiration rate of zooplankton (RO2 ) and converted it into the respiration rate of zooplankton (in carbon) (RC ) according to the following formulas: lnRO2 = 0.7886lnDW + 0.0490t − 0.2512 RC =

RQ × 12 × 24 × RO2 22.4 × 1000

(3.7) (3.8)

where RO2 is the respiration rate of zooplankton (μl O2 /(cell·h)); DW is the dry weight for a zooplankton unit (mg/cell); t is the water temperature (◦ C); RC is the respiration rate of zooplankton (in carbon) (mg C/(cell·d)) and RQ is the respiration quotient and assumed 0.8. Then, the daily production of a zooplankton unit for each station could be calculated by the following equation: Pz =

Gr × RC Q z − Gr

(3.9)

where Pz is the daily production of a zooplankton unit for each station (mg C/(cell·d)); Gr is the gross growth eﬃciency of zooplankton (%) and Qz is the assimilation eﬃciency of zooplankton (%) (Gr and Qz in this study were assumed to be 30% and 70%, respectively). The conversion from the daily

3.2 Carbon Biogeochemical Processes in the Yellow Sea

293

production for a zooplankton unit to daily production of zooplankton could be described as follows: Pz = Pz × ZA × h

(3.10)

3.2.3.3 Changes in Carbon Fixed Production and Ecosystem The negative trends of carbon ﬁxed production of phytoplankton (both measured and estimated values) have been observed in the SYS during 1983∼2004, namely the role the SYS played in the process of carbon ﬁxation has been declining since the 1980s (Table 3.4). In this study, we suggest that changes in the key nutrients may have an important impact on the carbon ﬁxed production and ecosystem of the SYS. Since the 1970s, concentrations of DO, P, and Si have clearly decreased and concentrations of N have obviously increased [the change rates (μmol/(L·yr)) of the parameters: DO (–2.451), P (–0.003), Si (–0.219), N (0.136)] (Lin et al., 2005). Until the end of the 1990s, these trends were not alleviated but accelerated [the change rates (μmol/(L·yr)) of the parameters: DO (–9.141), P (–0.023), N (0.497)]. There has been an increasing occurrence in P limitation of phytoplankton growth in coastal and shelf waters, particularly at the end of phytoplankton blooms. These phenomena have even happened in the dilution zone of the Changjiang River Estuary and in the SYS. When P was deﬁcient and N was suﬃcient, the dominant species of phytoplankton communities readily changed from diatoms to dinoﬂagellates. The increase in N/P ratio may lead to a reduction of phytoplankton species diversity. The deﬁciency in phosphorus may also have resulted substantially from bioactivity. In natural seawater, diatoms could uptake P as Table 3.4. Changes of carbon ﬁxed production (CFP) of phytoplankton (mg C/(m2 ·d)), Chl a (mg/m3 ) and phytoplankton abundance (cell/m3 ) in autumn of the SYS during 1983∼2004a (Zheng et al., 2006) Parameter CFP of phytoplankton (measured values) CFP of phytoplankton (estimated from Chl a) CFP of zooplankton Chl a Phytoplankton abundance Bacillariophyta Coscinodiscus Chaetoceros Pyrrophyta Bacillariophyta (%)c Pyrrophyta (%)c Zooplankton abundance a

1983∼ 1996∼ 1986 1998 426.0 66.3 394.7

71.04

0.73 11.0 5.6 0.3 2.3 0.7 50.9 6.4

0.25 4.1 1.6 0.8 b

0.7 39.0 17.1

1999

2000

2001

2002

2003

2004

2005

536.52 354.22 422.96 293.40 113.76 371.19 289.54 34.98 21.93 37.14 25.03 23.49 20.47 0.61 0.50 0.55 0.45 0.46 0.16 1.9 1.7 5.4 1.6 0.5 0.6 0.8 0.4 0.1 0.048 0.019 0.013 0.2 0.3 35.2 14.8 11.7 5.5 624 355 460

b

b

0.4 25.0 25.0 603

0.2 20.0 40.0 370

0.40 0.65 0.15

b

0.3 23.1 47.7 304

The data in the two periods of 1983∼1986 and 1996∼1998 were taken from Zhu et al. (1993) and Wang et al. (2000; 2003) and Wang and Jiao (1999). The else data were from Zheng et al. (2006); b Very few; c Percentage in phytoplankton abundance (%)

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3 Biogeochemical Processes of the Yellow Sea

much as 30 times more than that is really needed and store it in its cells for use when P is deﬁcient. Since the 1980s, the SYS ecosystem has probably been experiencing a limitation of P and Si. Investigation in this area indicated that there were some ecological signs, such as Chl a and phytoplankton abundance dramatically decreasing from 1983 to 2004, which have probably led to a trend of decreases in the carbon ﬁxed production of phytoplankton and zooplankton abundance (Table 3.4). Contemporarily, due to the decrease in the photosynthetic rate, DO in the SYS also showed a decreasing trend during 1983∼2004. The percentage of Bacillariophyta in the total phytoplankton abundance dropped from 50.9% in 1986 to 20.0% in 2003, while that of Pyrrophyta increased from 6.4% to 40.0% in the same period (Table 3.4). Furthermore, the percentage of Chaetoceros in the Bacillariophyta decreased enormously, while that of Coscinodiscus increased by 1.7 times and became a dominant genus in the phytoplankton communities. These ecological responses clearly resulted from the limitation of P and/or Si, which induced the shift in phytoplankton species composition dominated by diatoms to non-diatoms, such as dinoﬂagellates and cyanophytes, thus increasing the proportion of small-size structure phytoplankton communities. Moreover, through analyzing the data collected from bottom trawl surveys conducted in autumn 1985 and 2000, some evidence was found of inter-annual changes in the ﬁsh community structure, species diversity, and succession of dominant species in the YS. First, the dominant species in the ﬁsh community of the YS shifted from Spanish mackerel (Scomberomorus niphonius) and eel-pout (Enchelyopus elongates) to small yellow croaker (Pseudosciaena polyactis) and Pomfret (Stromateus argenteus). Second, the proportion of each species changed greatly (Table 3.5); the percentage of horse-scad mackerel (Trachurus japonicus) and chub mackerel (Pneumatophorus japonicus) in the total ﬁsh biomass increased, whereas that of seasnail (Liparis tanakai) and Japanese anchovy (Engraulis japonicus) declined (Table 3.5). Third, obvious temporal variations in biodiversity were also observed. All the ecological indexes such as the species number, the species richness, the diversity, and the evenness in 2000 pronouncedly declined in comparison with those in 1985. This is consistent with the unexpected signs of ecosystem response mentioned above, especially the carbon ﬁxed production of phytoplankton. Since the decreases in the biomass and production of primary producer in the SYS, the grazer abundance and ﬁsheries resources have inevitably declined, which probably resulted from environmental changes induced not only by climate changes, but also by anthropogenic impacts. It suggests that the SYS has been evolving and we have an ecosystem with P limitations, due to an increase in N and a decrease in P.

3.3 Dimethylsulﬁde and Its Fluxes across the Sea-Air Interface of the Yellow Sea

295

Table 3.5. Percentage (%) of dominant ﬁsh species in total ﬁsh biomass and some ecological indexes in the YS during 1985∼2000a (Zheng et al., 2006) Dominant species and ecological index Horse-scad mackerel (Trachurus japonicus) Chub mackerel (Pneumatophorus japonicus) Japanese anchovy (Engraulis japonicus) Seasnail (Liparis tanakai) Spanish mackerel (Scomberomorus niphonius) Plaice (Cleisthenes herzensteini) Eelpout (Enchelyopus elongates) Small yellow croaker (Pseudosciaena polyactis) Pomfret (Stromateus argenteus) Number of species Richness Diversity Evenness

1985 2000 Centerb YS Centerb YS 15.4 38.2 7.2 22.5 19.7 12.1 36.6 9.6 23.9 20.1 33.7 12.9 8.1 6.9 NDSc NDSc 8.5 3.0 8.9 NDSc NDSc NDSc 6.9 3.7 4.9 NDSc 93 73 7.81 5.62 2.80 1.95 0.62 0.45

a

The data are after Lin et al. (2005) and Wang (2003); b Center means the central region of the YS, which is generally in accordance with the present study area; c Nondominant species

3.3 Dimethylsulﬁde and Its Fluxes across the Sea-Air Interface of the Yellow Sea Dimethylsulﬁde (CH3 SCH3 , DMS) is the dominant volatile biogenic sulfur compound emanating from the ocean, which accumulates on the sea surface, particularly in the microlayer and drives a signiﬁcant sea-to-air ﬂux of DMS, making a major contribution to the global sulfur cycle. The latest estimation of DMS ﬂux from the ocean to the atmosphere is (20.7±5.2) Tg/yr, accounting for 85% of global DMS sources (Hu et al., 2003). In the atmosphere, DMS is oxidized to sulfate (and methanemethanesulfonate), producing aerosol that generates cloud-condensation nuclei leading to reﬂection of solar radiation and contributes to the acidity of atmospheric particles and rainfall. The production of DMS occurs via enzymatic cleavage of its precursor, dimethylsulfoniopropionate (DMSP). As a cryoprotectant and an osmolyte in algae, DMSP is an abundant intra-cellular component in some marine phytoplankton taxa. Production of DMSP from algae shows large seasonal and geographic variations, linked to seasonal variations in phytoplankton abundance and speciation. Diatoms generally contain less DMSP than Dinophyceae and Prymnesiophyceae. However, in some diatom-dominated waters with high biomass, DMS concentrations were as high as in those dominated by major DMSP-producing groups. DMSP and DMS are given oﬀ to some extent by live phytoplankton cells, but this release is accelerated during senescence, grazing, or viral attack. DMSP is then cleaved into DMS and other products by intra- and extra-cellular, algal or bacterial DMSP-lyase enzymes. Recent investigation

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3 Biogeochemical Processes of the Yellow Sea

has indicated that DMSP also plays an important role in the transfer and cycling of sulfur and carbon among trophic levels in the microbial food chains. Oceanic emissions of DMS are controlled by its concentration in surface water, which is determined by complex interactions of biological, physical, and chemical parameters. Biological processes are largely responsible for the in situ production and sink of DMS, but physical factors such as temperature, vertical mixing, light, and ﬂux to the atmosphere are also critical for the ultimate distribution and concentration of dissolved DMS in the water column. Earlier research suggested the photochemical oxidation of DMS, to either dimethylsulfoxide or other products, accounts for 7%∼40% of the total removal of DMS from the surface mixed layer. Zhang et al. (2008) reported the following results of dimethylsulﬁde and its ﬂuxes across the sea-air interface of the Yellow Sea. 3.3.1 Characteristics of Dimethylsulﬁde and Dimethylsulfoniopropionate In this part the characteristics of DMS and DMSP were studied from horizontal distributions, biological production, and consumption of DMS, and roles of phytoplankton biomass in controlling the distribution of biogenic sulfur. 3.3.1.1 Horizontal Distributions The horizontal distributions of DMS and DMSP in the microlayer and subsurface waters are shown in Fig. 3.23 (Zhang et al., 2008). Dissolved and particulate DMSP are expressed as DMSPd and DMSPp. In the center of the YS, the spring bloom was accompanied by increased DMSPp, DMSPd, and ultimately DMS, and these peaks were linked to the concurrently increased phytoplankton biomass. High levels of DMSPd (410 nmol/L) and DMSPp (425 nmol/L) were observed during the bloom, which were obviously higher than those observed at the coastal areas. The increases in DMSPp are likely a manifestation of the relative contribution of DMSP containing phytoplankton in the bloom populations. Maximum concentrations of DMSPd occurred concurrently with or just after the peaks in production of DMSPp. Similar to the case of DMSP, DMS concentrations were also elevated in the central regions of the YS. The relationships between the DMS and DMSP pools can be applied for making inferences about the processes that control the distribution patterns. As a consequence, no signiﬁcant correlation appeared between DMS and DMSP concentrations. This may be due to some diﬀerences in the nature of the dynamics of DMS and DMSP production. DMSP production is species-speciﬁc and DMS production is controlled by many ecological and physiological processes, such as zooplankton grazing, microbial activity and the release of algal lysis upon senescence or cell breakage. These are possible reasons why the concentrations of DMSP and DMS were not tightly coupled in time. In comparison, a signiﬁcant correlation was found between DMSPd

3.3 Dimethylsulﬁde and Its Fluxes across the Sea-Air Interface of the Yellow Sea

297

and DMSPp concentrations from all stations, reﬂecting the dependence of DMSPd concentrations on the DMSPp levels in the ocean (for the subsurface water: R2 =0.7582, n=43, P <0.0001; for the microlayer: R2 =0.6112, n=43, P <0.0001). N 40

N 40

N 40

39

39

39

38

38

38

37

37

37

36

36

36

35

35

35

34

34

34

33

33

33

32 32 32 120 120 122 124 E 122 124 E 120 122 124 E DMSPd in the subsurface water DMS in the subsurface water DMSPp in the subsurface water N N N 40 40 40 39

39

39

38

38

38

37

37

37

36

36

36

35

35

35

34

34

34

33

33

33

32 120 122 124 E DMSPd in the microlayer

32 120

122 124 E DMS in the microlayer

32

120 122 124 E DMSPp in the microlayer

Fig. 3.23. Horizontal distributions of DMSPd, DMS, and DMSPp (nmol/L) in the microlayer and subsurface water (Zhang et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

3.3.1.2 Biological Production and Consumption of Dimethylsulﬁde The sea-surface microlayer not only enriches more chemicals in this interface but also displays more distinct biological activity relative to the underlying water. As a biological sulfur compound, the source and sink of DMS in the

298

3 Biogeochemical Processes of the Yellow Sea

ocean are closely linked with biological activity. For these reasons, the rates of biological production and consumption of DMS in the surface microlayer and subsurface water were measured. The production and consumption of DMS were determined by means of a selective consumption inhibitor, chloroform (CHCl3 ). A volume of 4 ml CHCl3 liquid was added to 100 ml seawater in a gastight glass syringe with a ﬁnal concentration of 500 mmol/L. The sample was incubated at in situ temperatures in the dark to avoid any photochemical loss of DMS. The DMS metabolism by bacteria could be selectively inhibited by CHCl3 of this level. Aliquots of 5∼10 ml were taken for the determination of DMS at t=0, 3, and 6 h during incubation. Calculations of DMS production were based on the rates of increase in DMS concentration in the CHCl3 amended samples. The DMS consumption rates were estimated from the diﬀerences in the DMS accumulation rates between the samples with and without CHCl3 . The production rates of DMS in the microlayer and subsurface water ranged from 2.41 to 10.35 nmol/(L·d) and from 2.96 to 13.53 nmol/(L·d), with average values of 7.31 and 5.39 nmol/(L·d), respectively. By contrast, the consumption rates of DMS in the microlayer and subsurface water varied within one order of magnitude with averages of 5.56 and 4.09 nmol/(L·d), respectively. Overall, the production and consumption rates of DMS in the microlayer were mostly higher than those in the subsurface water. These observations imply that the sea-surface microlayer is biologically active relative to the underlying water. It has been found that biological parameters (such as total bacteria, culturable bacteria, and pigments) are signiﬁcantly enriched in the microlayer samples collected by a screen sampler. The existence of a high density of bacteria in the microlayer may strongly inﬂuence the production and consumption of DMS. On the one hand, DMS production from the enzymatic cleavage of DMSP is expedited due to the accumulated bacteria. On the other hand, the higher DMS consumption rates observed in the microlayer could be due to more populations of bacteria that can metabolize DMS quickly. Linear regression analysis showed that the production rates of DMS were closely correlated to DMSPd concentrations in the microlayer (R2 =0.5563, n=8, P =0.03) as well as in the subsurface water (R2 =0.6220, n=8, P =0.02). The DMS production through enzymatic DMSP cleavage generally exceeded its microbial consumption rates, leading to net DMS production. The imbalance in these two processes might be caused by other sink pathways for DMS such as photochemical oxidation and sea-to-air emission. 3.3.1.3 Roles of Phytoplankton Biomass in Controlling the Distribution of Biogenic Sulfur Since DMSP and DMS in turn are intimately linked to phytoplankton activity, many eﬀorts have been made to ﬁnd a possible relationship between DMS(P)

3.3 Dimethylsulﬁde and Its Fluxes across the Sea-Air Interface of the Yellow Sea

299

and chlorophyll a, a general indicator of total phytoplankton biomass. The difﬁculties in obtaining a general correlation have usually been attributed to the highly dynamic and variable physical environment and diﬀerent phytoplankton structure. Not only do diﬀerent phytoplankton species produce unequal amounts of chlorophyll a, they also diﬀer in their ability to form DMSP. The DMSPp/chlorophyll a ratio has been used as an estimate of the proportion of DMSP producers in the total phytoplankton assemblage. In the YS, the ratios of DMSPp to chlorophyll a in the microlayer and subsurface water ﬂuctuated from 8.02 to 57.98 mmol/g and from 6.84 to 41.39 mmol/g, with average values of 21.11 and 18.54 mmol/g, respectively. Such low ratios indicated that the dominant species in the study area were not high DMSP producers. Data obtained on the same cruise indicated that diatoms were utterly dominant in the phytoplankton biomass in the surface water, accounting for 84.48% in the identiﬁed phytoplankton species. Despite the highly dynamic and variable physical environment of the YS and resultant large ranges in algal biomass, two consistent relationships with chlorophyll a integrated values from the compiled data set were observed; one linked to DMS(P) concentrations, the second related to DMS production rates. Firstly, DMSPp concentrations correlated well with chlorophyll a concentrations in the microlayer as well as in the subsurface water when all stations were plotted (Fig. 3.24). There was also a close relationship between DMS and chlorophyll a concentrations in the microlayer and subsurface water (Fig. 3.24, Zhang et al., 2008). Sim´ o et al. (2002) proposed an empirical equation relating the global ocean monthly distribution of surface DMS concentration to the ratio of surface chlorophyll a and mixed layer depth (MLD). Based on the measured chlorophyll a and MLD data extracted from the CTD (15∼20 m), predicted DMS concentrations were calculated using the formula and compared with measured DMS. Consequently, a signiﬁcant correlation was found between predicted concentrations and measured ones (R2 =0.6283, n=43, P <0.0001). These observations suggest that phytoplankton biomass might play an important role in determining the distribution of biogenic sulfur compounds, even if diatoms dominating the biomass are not high DMSP producers. Secondly, the results show that high production rates of DMS generally appeared in the samples containing high levels of chlorophyll a. A signiﬁcant relationship was found between DMS production rates and chlorophyll a concentrations (R2 =0.8828, n=8, P <0.0001). This ﬁnding agrees well with a growing body of evidence to support the role of phytoplankton biomass in controlling the concentration of DMS in the YS. 3.3.2 Sea-to-Air Flux of Dimethylsulﬁde The sea-to-air ﬂux of DMS (FDMS) can be calculated according to the equation (Liss and Merlivat, 1986) F DM S = KDMS C = KDMS [DMS]

(3.11)

3 Biogeochemical Processes of the Yellow Sea 45 R2=0.5285, n=43, P<0.0001

40

Microlayer DMSPp (mmol/L)

Subsurface water DMSPp (nmol/L)

300

35 30 25 20 15 10 5

2

30 20 10

5 3 1 2 4 Subsurface water Chl a (mg/L)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Microlayer Chl a (mg/L)

14

12 R2=0.4550, n=43, P<0.0001

12

Microlayer DMSP (nmol/L)

Subsurface water DMS (nmol/L)

R =0.5216, n=43, P<0.0001 40

0 0

10 8 6 4 2 0

50

0

5 3 1 2 4 Subsurface water Chl a (mg/L)

6

10

R2=0.4068, n=43, P<0.0001

8 6 4 2 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Microlayer Chl a (mg/L)

Fig. 3.24. Relationships between DMSPp, DMS, and chlorophyll a (Chl a) concentrations in the microlayer and subsurface water (Zhang et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

where KDMS is a kinetic factor known as the gas transfer velocity, ΔC is the concentration diﬀerence of DMS in the surface seawater and the atmosphere and [DMS] is the concentration of dissolved DMS in seawater. The above approach assumes that the sea surface DMS concentration may be used as an estimate of the concentration diﬀerence across the air-sea interface, since atmospheric DMS concentration is generally negligible compared to the dissolved DMS level in seawater. The KDMS value was calculated as a function of wind speed and DMS Schmidt number (Sc), which was derived from the local water temperature from the equation put forward by Saltzman et al. (1993). The wind data used in this study were measured on board at about 10 m elevation from the sea surface. Accurate estimates of the sea-to-air ﬂux of DMS are essential to our understanding of the global cycle of biogenic sulfur and its eﬀect on the Earth’s radiation budget. For an accurate quantiﬁcation of the sea-to-air ﬂux, near surface concentrations, preferably surface microlayer values should be taken

3.3 Dimethylsulﬁde and Its Fluxes across the Sea-Air Interface of the Yellow Sea

301

into account. The FDMS values estimated according to the above approaches range from 0.04 to 20.76 μmol/(m2 ·d), with KDMS values in the range of 0.04∼11.87 cm/h. The calculated ﬂuxes were uncertain by at least a factor of 2, due to uncertainties arising from the microlayer concentration of DMS and KDMS value that depends excessively on in situ wind speed. We believe that the ﬂux values may be an underestimation since microlayer concentrations were underestimated. Nevertheless, the ﬂuxes are useful for at least a rough estimate of the spring emission ﬂux of DMS from the YS to the atmosphere. In the present study, the sea-to-air ﬂuxes of DMS varied widely from 0.04 to 20.76 mmol/(m2 ·d), with an average of 6.41 mmol/(m2 ·d). The mean ﬂux is approximately higher by a factor of 2 than that (3.14 mmol/(m2 ·d)) estimated during the spring 2005 cruise, mainly due to the higher DMS concentrations in the spring bloom during this cruise. Our result was comparable to the values reported for its adjacent shelf regions, e.g., 6.05 mmol/(m2 ·d) from the ECS in July (Uzuka et al., 1996). The tendency towards higher emissions from coastal/shelf regions suggests that human perturbation of the ecosystem, enhancing the primary production, increases DMS emissions and therefore may have a signiﬁcant inﬂuence on the global climate. 3.3.3 Source and Sink of Dimethylsulﬁde in the Microlayer The most likely sources of DMS in the microlayer are in situ production from phytoplankton within the microlayer and vertical transport by turbulent diffusion from the underlying water. As shown above, biological production and consumption led to net DMS production in the microlayer. However, the net production was too small to maintain the balance of DMS concentrations in the microlayer. Consequently, the input from the underlying water is thought to be a major source of DMS in the microlayer. The relationship between the microlayer and subsurface water concentrations can be useful to support this viewpoint. A statistically signiﬁcant relationship was found between the DMS microlayer concentration and its subsurface water concentration (Fig. 3.24). Similar to the case of DMS, the DMSP and chlorophyll a concentrations in the microlayer were also related to their corresponding subsurface water concentrations (Fig. 3.25). These results provide favorable evidence demonstrating that the materials in the microlayer could be directly related to and transported from the underlying water. It has been generally believed that DMS microbial degradation, photochemical oxidation, and escape to the atmosphere are three principal mechanisms controlling the fate of DMS in the surface ocean. In this study, the DMS biological turnover time is enumerated by dividing the DMS concentrations by their corresponding biological consumption rates. It should be noted here that the calculated biological turnover time was relatively lower than the “real” one, since the consumption rates were overestimated due to the use of the inhibitor (CHCl3 ). The DMS biological turnover time in the subsurface water varied from 1.06 to 3.44 d with an average of 1.93 d. In contrast, the DMS biological turnover times in the microlayer changed

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3 Biogeochemical Processes of the Yellow Sea

from 0.65 to 2.39 d with a mean of 1.12 d. The more rapid DMS biological turnover time in the microlayer could be explained by the higher biological consumption rates observed here. Moreover, we also calculated the turnover time for sea-air transfer of DMS, which was obtained by dividing the burden of DMS in the microlayer (unit: mmol/m2 ), determined from the product of the DMS concentration and the average microlayer thickness of 200 mm, by the estimated sea-to-air ﬂux of DMS. Comparing microlayer with sea-air interface showed that the former was over three orders of magnitude greater than the latter. Thus the above observations lead to a clear conclusion that the biological consumption of DMS is negligible relative to its sea-to-air emission in controlling the fate of DMS in the microlayer (Zhang et al., 2008).

Fig. 3.25. Relationships between microlayer chlorophyll a (Chl a), DMS, DMSPd, and DMSPp concentrations and their subsurface water concentrations (Zhang et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

3.4 Biogeochemical Characteristics Nitrogen and Phosphorus in the Yellow Sea

303

3.4 Biogeochemical Characteristics Nitrogen and Phosphorus in the Yellow Sea Distributions of nutrient elements, especially nitrogen and phosphorus, observed in the Yellow Sea provided relevant information that can be linked to the dynamics and biological/chemical reactions taking place in the region. 3.4.1 Variations of Nitrogen and Phosphorus in Seawaters N and P exist in various chemical forms in the sea waters. Deferent forms of N and P biogeochemical behaviors in the Yellow sea waters were discussed in this work. 3.4.1.1 Evolution of N and P from 1999 to 2005 The concentrations of NO3 -N, NO2 -N, NH4 -N, and dissolved inorganic nitrogen (DIN) in the SYS were obviously increasing during the observation period (Fig. 3.26; Table 3.6). The annual increasing rates of average nitrogen concentration in the water column, sea surface, and sea bottom were 0.449, 0.417, and 0.650 μmol/(L·yr), respectively. The water column average value of DIN from 2000 to 2005 increased to 3.31 μmol/L. The climate trend coeﬃcients were 0.64∼0.91 with a signiﬁcance level of 99%. Table 3.6. The annual change rates of the related environmental parameters in the SYS from 1997 to 2004 Parameter T S DO NO2 -N NO3 -N NH4 -N DIN PO4 -P N/P

Sea surface layer 0.321 0.058 −22.881 0.017 0.347 0.053 0.417 −0.022 3.981

Sea bottom layer 0.518 0.144 −23.090 0.013 0.582 0.054 0.650 −0.012 3.023

Average for water column 0.230 0.078 −23.427 0.016 0.379 0.055 0.449 −0.020 3.256

The units are ◦ C/yr for temperature (T ), ‰/yr for salinity (S) and μmol/yr for the rest, except for N/P

Since the 1980s, P concentrations exhibited decreasing trends in the SYS, especially 1983∼1989 and 1995∼1996 near the ecological threshold essential for diatom growth 0.2 μmol/L (Lin et al., 2005). During our observation period (1997∼2005), the decreasing trends of P have been continuous in the SYS (Fig. 3.27). Their annual rates were –0.020 μmol/(L·yr) for the water column,

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3 Biogeochemical Processes of the Yellow Sea

DIN (mmol/L)

7 6

r = 0.81,

5 4 3 7 6

P < 0.01

(a)

r = 0.82,

P < 0.01

5 4 (b)

3 7 6

r = 0.86,

P < 0.01

5 4

(c)

3 1997 1998 1999 2000 2001 2002 2003 2004 2005 Year

PO4-P (mmol/L)

Fig. 3.26. Variation trends of DIN concentration (μmol/L) in the SYS. (a) Average DIN concentration of the water column (DINav ); (b) Average DIN concentration of the sea surface (SSDIN); (c) Average DIN concentration of the sea bottom layer (BDIN) (The dashed lines show their linear regression) 0.35 0.30 0.25 0.20 0.15 0.35 0.30 0.25 0.20 0.15 0.10 0.38 0.36 0.34 0.32 0.30 0.28 0.26 0.24

(a) r = - 0.81, P < 0.01

(b) r = - 0.69, P < 0.05

(c) r = - 0.85, P < 0.01 1997 1998 1999 2000 2001 2002 2003 2004 2005 Year

Fig. 3.27. Variation trends of P concentration (μmol/L) in the SYS. (a) Average P concentration of the water column (Pav ); (b) Average P concentration of the sea surface (SSP); (c) Average P concentration of the sea bottom layer (BP) (The dashed lines show their linear regression)

3.4 Biogeochemical Characteristics Nitrogen and Phosphorus in the Yellow Sea

305

–0.021 μmol/(L·yr) for the sea surface, and –0.012 μmol/(L·yr) for the sea bottom. The water column average value of P decreased to 0.18 μmol/L in 2005, lower than the ecological limitation of diatom growth and decreased by 0.18 μmol/L since 1997. As illustrated in Fig. 3.28, the N/P ratios increased at annual rates of 3.02∼3.98 yr−1 . The climate coeﬃcients were 0.91∼0.94 with a signiﬁcance level of P <0.01. We noted that the N/P ratios increased from approximately 6 in 1997 to over 16 in 2001. There was an indication that, despite the N/P ratio having rapidly increased, it was still at the Redﬁeld ratio that is suitable for phytoplankton growth (Lin et al., 2005). Until the most recent years the N/P ratios increased abruptly and the environment of the SYS experienced limitations of P, as noted in (Wang and Jiao, 1999; L¨ u et al., 2004; 2005).

N /P

28 24 20 16 12 8 28 24 20 16 12 8 24 20 16 12 8

r = 0.83, P < 0.01

(a)

r = 0.87, P < 0.01

(b)

r = 0.87, P < 0.01

(c) 1997 1998 1999 2000 2001 2002 2003 2004 2005 Year

Fig. 3.28. Variation trends of N/P ratio the SYS. (a) Average N/P ratio of the water column; (b) Average N/P ratio of the sea surface; (c) Average N/P ratio of the sea bottom layer (The dashed lines show their linear regression)

3.4.1.2 Reasons for the Environmental Variation The warming of seawater in the SYS during 1997∼2004 (Fig. 3.29) is consistent with the increase in the mean air temperature observed throughout northern China (Chen et al., 1998) and the increase in SST found in both the Bohai Sea and the ECS. In the SYS, however, the average temperature of the water column (Tav ), the average temperature of the sea surface (SST) and the average temperature of the sea bottom layer (BT) all increased more sharply than the SST of both the Bohai Sea and the ECS. In these seas, the

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3 Biogeochemical Processes of the Yellow Sea

linear trends of SST were 0.011 and 0.010 ◦ C/yr, respectively (Song, 1997), which coincided with the linear trend (0.01 ◦ C/yr) of the annual mean AT (atmospheric temperature) in northern China (Chen et al., 1998).

Fig. 3.29. Variation trends of seawater temperature (◦ C) in the YS. (a) Average temperature of the water column (Tav ); (b) Average temperature of the sea surface (SST); (c) Average temperature of the sea bottom layer (BT) (The dashed lines show their linear regression)

The positive trends of DIN in the SYS during the observation period were consistent with the rise of DIN observed throughout the global marginal seas. Along with the quick development of the economy of China, DIN concentration in the seawater of the coastal zone and shelf of the China Sea has dramatically increased, due to the increasing urbanization near the coastal areas, chemical fertilizer release from farmlands, animal excretion from marine-culture sites, etc. Signiﬁcant inputs of DIN into the SYS and ECS have occurred through atmospheric deposition (mainly precipitation) and also through river discharges, such as the Changjiang River runoﬀ, which contributed about 10% of the DIN input to the YS. The major source of dissolved inorganic nutrients is deposition from atmospheric precipitation in the central region of the YS. Precipitation has exhibited increasing N concentration resulting from the atmospheric transport of chemical fertilizer. It was observed that the concentration of DIN in rainfall increased by 2.29 times on the east coast (Anshan, Korea) of the SYS from 1992 to 1998, and 2.84 times on the west coast during the same period. Also, DIN input from the Changjiang River was one order

3.4 Biogeochemical Characteristics Nitrogen and Phosphorus in the Yellow Sea

307

of magnitude higher in 1990 than in 1960; the concentration of NO3 -N increased by 1.9 times in the Changjiang River Estuary from 1985 to 1998, and its ﬂux increased by 1.3 times during the same period (Table 3.7). All these anthropogenic environmental impacts have been leading to eutrophication in the coastal zone and oﬀshore, particularly in the Changjiang River Estuary. Furthermore, the high DIN concentration in the Changjiang River Estuary is brought to the SYS and even to the entire YS by the Changjiang River plume. Its tongue extends northeastward, in the direction of the tip of the Korean Peninsula, from late spring to early autumn, which signiﬁcantly enriches the DIN concentration of seawater in the YS and increases the N/P ratio. The N/P ratio increase also resulted from the decreased P concentration in the seawater of the area. The negative trend in P concentration over 1997∼2004 was probably induced by decreasing river discharges in recent years, resulting from the retention of freshwater for use in the upstream regions of rivers like the Changjiang River and Huanghe River. In the case of the latter, the decrease in the freshwater discharge was most serious. As an example, in 1997 the Huanghe River dried up for more than 200 days; its salinity in the estuary, located in the Bohai Sea, reached up to >32‰ (usually <30‰, Su and Tang, 2002). Table 3.7. Changes in the concentration of inorganic nitrogen in precipitation (CINP) and the DIN export from the Changjiang River Estuary∗ (Chen et al., 1998) (With permission from Chen L) Parameter and geographic position CINP in the eastern coast of the SYS (Anshan, μmol/L) CINP in the western coast of the SYS (Qianliyan, μmol/L) CINP in the middle of the SYS (μmol/L) CINP in the middle Changjiang River catchment (μmol/L) CINP in the Donghu Lake of middle Changjiang River (μmol/L) Concentration of NO3 -N in the Changjiang River Estuary (mg/L) Flux of NO3 -N in the Changjiang River Estuary (×104 t/yr) Flux of DIN in the Changjiang River Estuary (1) (×104 t/yr) Flux of DIN in the Changjiang River Estuary (2) (×104 t/yr) ∗

The ﬁgures in brackets are observation years

9.4 (1992)

Parameter value 21.5 (1997)

12.5 (1992)

35.5 (1998)

2.62 (1990∼1991) 62.3 (1985∼1986)

3.77 (1999∼2000) 91.3 (1998)

31.9 (1962∼1963)

64.8 (1998)

2 (1985∼1986)

5.8 (1998)

62.5 (1985∼1986)

143.8 (1998)

87.3 (1985∼1986)

174.6 (1998)

16.1 (1972), 77 (1982)

149.9 (1998)

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3 Biogeochemical Processes of the Yellow Sea

3.4.2 Dry and Wet Fluxes of Nutrients The chemical material ﬂux from atmosphere to ocean plays an important role in biogeochemical cycling, and the acceptance of atmospheric deposition as an important source of nutrients, particularly to oligotrophic oceans, is received widely now. Previous works have suggested that in certain oceanic regions biological productivity may be inﬂuenced by direct input of inorganic nutrients from atmospheric deposition. Eutrophication related to land drainage and atmospheric deposition of nutrients in the marginal sea is believed to be the major cause of frequent toxic plankton blooms (red tide). Moreover, the impact of the atmospheric deposition of nutrients on new productivity in the central waters of the YS was also studied. There is still lacking complete knowledge of the atmospheric deposition in the YS and the ECS from the data available up until now. Therefore, it is necessary to deeply analyze the ﬂux of nutrient elements via the atmosphere to the ocean. Wan et al. (2003) reported the following results of dry and wet ﬂuxes of nutrients in the SYS. 3.4.2.1 Dry Deposition Flux of Nutrients The dry deposition velocity of particles depends on many factors, such as the size of particles, wind speed, relative humidity and the stability of the surface layer atmosphere. So there are many diﬃculties in estimating the dry deposition velocity accurately. It is assumed that the dry deposition velocity of nutrient elements is 2.0 cm/s according to the research results. The concentrations of aerosols in each season are shown in Table 3.8 (Wan et al., 2003) and estimated by weighted average of data from representative stations. Silicate aerosol is not taken into account here, since the silicate concentration observed in aerosol is too low in the YS. Cheju Island (33◦ 17 N, 126◦ 10 E) is taken as the representative station in the southern YS but, for lack of information on phosphate aerosol, its concentration is estimated using observed data from its vicinity. The concentrations of phosphate in aerosol is about 0.06 μg/m3 over the Xiamen area and about 0.307 μg/m3 in northeastern China. Table 3.8. Average concentrations (μg/m3 ) and dry deposition ﬂuxes of nutrient elements in the SYS (Wan et al., 2003) (With permission from Wan XF) Seasons Spring Summer Autumn Winter Average

NO− 3 1.54/79.8 0.82/42.5 1.01/52.4 1.19/61.7 1.14/59.1

NH+ 4 1.37/71.0 1.17/60.7 1.34/69.5 1.06/55.0 1.24/64.3

PO3− 4 0.0681/3.53 0.0519/2.69 0.0938/4.85 0.1133/5.87 0.0818/4.24

3.4 Biogeochemical Characteristics Nitrogen and Phosphorus in the Yellow Sea

309

As shown in Table 3.8 (Wan et al., 2003), all substances in this region show a similar seasonal cycle. Basically, the concentration is the highest in spring and the lowest in summer in the SYS. Distinguished seasonal variability of nutrients in this region is due to its characteristic geological position which, as is known, is under the control of the eastern Asian monsoon. In late winter the prevailing wind is from the north, and the SYS is inﬂuenced by the air coming from the Korean Peninsula. In spring the air ﬂows are mainly from northern China under the inﬂuence of anthropogenic activities and Asian dust storms. In summer the ﬂows are from southern China and Japan, Paciﬁc maritime conditions. In addition, summer is the wettest season and rain scavenging also contributes to the summer minimums. It tends to be the minimum in the summer time. Nitrate also shows a similar cycle. According to the concentrations of nutrients and sulphate described above, the dry deposition ﬂuxes can be obtained in each season. The seasonal variability of dry deposition ﬂuxes is similar to that of its concentration. The dry deposition ﬂux of nitrate in the southern YS is at a maximum in spring and reaches a minimum in summer. Other nutrient elements in the southern YS have similar seasonal cycles to that of nitrate. 3.4.2.2 Wet Deposition Fluxes of Nutrients The basic concentration data for precipitation in the SYS are obtained from Qianliyan Island and Maidao Island in the west part of the YS and Ansan in the eastern part of the YS. In contrast to that in the western YS, the average concentrations and annual rainfall in the eastern YS are much higher. The concentration of nitrate is more or less the same in both parts of the YS, phosphate and ammonium are also the same, but the silicate concentration shows a big diﬀerence in both areas. Based on the above studies, the concentrations in the SYS by weighted average are obtained, as listed in Table 3.9 (Wan et al., 2003). Similar to the spatial distribution of aerosol concentrations, the element concentrations for precipitation in the SYS are a bit larger than those in the ECS. In detail, the phosphate concentration is extremely low in precipitation, because there are no signiﬁcant vapor phase forms of phosphorus in the atmosphere. The concentration of silicate is also low resembling phosphate. It still displays a seasonal cycle for the concentration in precipitation. In fact, their values in winter are higher than in other seasons in the SYS. From the concentrations in precipitation and monthly rainfall data in Table 3.10, we can easily conclude the monthly average wet deposition ﬂuxes in the southern YS (Table 3.10, Wan et al., 2003). The seasonal variability of wet deposition ﬂuxes in the southern YS is contrary to that of dry deposition ﬂuxes. It tends to reach a maximum in summer and a minimum in winter. Though the concentrations are relatively low in summer, the wet deposition ﬂuxes are still the greatest in the whole year due to enormous rainfall. By comparing dry and wet deposition ﬂuxes, it can also be found that the wet

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Table 3.9. The monthly average precipitation (mm) and the concentrations of nutrient (μmol/L) in the YS (Wan et al., 2003) (With permission from Wan XF) Seasons Spring Summer Autumn Winter Average

Precipitation 48.52 122.74 60.80 36.25 67.08

NO− 3 9.8 17.4 21.6 34.4 20.9

NH+ 4 41.5 35.7 41.8 52.4 44.8

PO3− 4 0.50 0.53 0.76 0.81 0.65

SiO2− 3 4.90 3.51 5.17 5.35 4.14

Table 3.10. The monthly average wet deposition ﬂuxes of nutrient (mg/(m3 ·month)) towards the YS (Wan et al., 2003) (With permission from Wan XF) Seasons Spring Summer Autumn Winter Average

NO− 3 29.48 132.4 81.5 77.3 86.9

NH+ 4 44.3 96.4 55.9 41.8 59.6

PO3− 4 2.27 6.11 4.34 2.76 4.10

SiO2− 3 18.0 32.7 23.9 14.7 27.3

deposition ﬂuxes are greater than dry deposition ﬂuxes in the summertime. Depositions of nitrate are dominated by wet deposition, while deposition of phosphate is mainly dry deposition because of its low concentration in precipitation. 3.4.2.3 Comparison Between the Atmospheric Fluxes and Riverine Inputs The total atmospheric ﬂuxes to the southern YS are respectively calculated based on the results as we discussed above. In order to account for the importance of air to sea ﬂuxes in the area, the values are compared with the riverine inputs (Table 3.11, Wan et al., 2003). The Changjiang River ﬂows into the southern YS in a diﬀerent ratio for each season. Based on the drainage areas and concentrations of nutrients, the riverine inputs of nutrient elements to the YS can be obtained. As the SYS has a smaller area, the annual atmospheric input to the SYS is smaller. Rivers are obviously the major source of nitrate and silicate, especially for the latter, as about 90% of them are supplied by rivers. Comparing atmospheric inputs with riverine inputs, it is found that although the atmospheric inputs of phosphorous are smaller, they are still greater than riverine inputs in both sea areas, and the nutrient elements input via the atmosphere may be important for new productivity.

3.4 Biogeochemical Characteristics Nitrogen and Phosphorus in the Yellow Sea

311

Table 3.11. The comparison between annual atmospheric and riverine inputs of nutrient elements to the SYS and the ECS (×1010 g/yr) (Wan et al., 2003) (With permission from Wan XF) Sea areas The southern YS The ECS

Source Atmospheric input Riverine input Atmospheric input Riverine input

NO− 3 49.1 85.2 75.0 127.9

NH+ 4 41.6 13.4 99.7 17.0

PO3− 4 2.8 2.3 3.8 3.2

SiO2− 3 9.2 301.6 36.4 485.6

3.4.3 Nutrients in the South Yellow Sea Sediments Four forms of nitrogen could be obtained by an improved sequential extraction method. The four forms of nitrogen are nitrogen in ion extractable form (IEFN), nitrogen in weak acid extractable form (WAEF-N), nitrogen in strong alkali extractable form (SAEF-N), and nitrogen in strong oxidant extractable form (SOEF-N), respectively. At the same time, the total nitrogen was determined. 3.4.3.1 Distribution Characteristics of Nitrogen The distributions of diﬀerent forms of nitrogen in the SYS surface sediments are shown in Fig. 3.30, from which the distribution characteristics of diﬀerent forms of nitrogen may be obtained. The four forms of extractable nitrogen are also named as transferable nitrogen because they can be released to the water column to take part in cycling by bioturbations, hydrodynamics or when the temperature, salinity, or pH of overlying water changes (Ma et al., 2003). Nitrogen in ion extractable form (IEF-N): the content of IEF-N in the surface sediments of the SYS ranges from 1.44 to 3.61 μmol/g, with an average of 2.30 μmol/g, and accounts for 4.00% of TN on average. Fig. 3.30 shows that the average content of IEF-N in the west sea area (2.83 μmol/g) (except for Haizhou Bay) is higher than that in the east (2.03 μmol/g). At the station close to Jiaozhou Bay, the content of IEF-N is the highest (3.61 μmol/g), and it is much higher around the site of the continental current. Another higher content is around the old Yellow River Estuary for residual sediments, which can be up to 3.00 μmol/g. In Haizhou Bay, IEF-N content is lower, and averages 1.66 μmol/g, which only amounts to 72% of the average content of the whole sea. In the middle part, the distribution of IEF-N is uniform, and the average content (2.74 μmol/g) is similar to that of the whole sea. On the whole, the content of IEF-N in the surface sediments of the SYS decreases eastward and southward, for the Bohai eutrophic matter accumulates in the northern part and the excessive terrestrial matter is imported into the west sea area. Nitrogen in weak acid extractable form (WAEF-N): the content of WAEFN is the lowest among all the nitrogen forms, and ranges from 0.44 to 1.25

312 N 39

3 Biogeochemical Processes of the Yellow Sea IEF-N

WAEF-N

N 39

38

38

37

37

36

36

35

35

34

34

33

33

32

32

31

31

119 120 121 122 123 124 125 126 127 N SAEF-N 39

E N 39

38

38

37

37

36

36

35

35

34

34

33

33

32

32

31

119 120 121 122 123 124 125 126 127 SOEF-N

E

119 120 121 122 123 124 125 126 127

E

31 119 120 121 122 123 124 125 126 127 N 39

E TN

38 37 36 35 34 33 32 31

119 120 121 122 123 124 125 126 127

E

Fig. 3.30. Distributions of various forms of nitrogen (μmol/g) in natural sediments in the SYS (L¨ u et al., 2004)

3.4 Biogeochemical Characteristics Nitrogen and Phosphorus in the Yellow Sea

313

μmol/g (average is 0.67 μmol/g), which accounts for 0.70%∼2.70%, or 1.17% on average of the TN. The highest content is around Cheju Island of Korea and averages 0.80 mmol/g; the lowest is in Haizhou Bay (0.55 μmol/g), and the content in the middle of the SYS is relatively uniform. Interestingly, at the boundary between the YS and the ECS, the distribution is in the pattern of the adjacent eddy (see Fig. 3.30), with the WAEF-N content of the eastern eddy being lower than that of the western one, which might be caused by freshwater from the Changjiang (Yangtze) River and the Taiwan Warm Current. On the whole, the content of WAEF-N increases gradually eastward. Nitrogen in strong alkali extractable form (SAEF-N): SAEF is mainly the nitrogen absorbed by ferric, ferrous or manganic oxides. The content ranges from 0.55 to 3.81 μmol/g, with an average of 1.24 μmol/g, which accounts for 1.13%∼7.08% or on average accounts for 2.15% of TN. The content of SAEF-N is higher in the northern part of the SYS. The distribution appears in an outward radiating pattern centered on 37◦ N, 123.4◦ E, and the average content is 1.91 μmol/g in this area. And in the southern part, the content is low and relatively uniform, and the average is 1.09 μmol/g. Nitrogen in strong-oxidant extractable form (SOEF-N): SOEF-N is mainly the organic form of nitrogen. It is the predominant form that can be leached out to take part in recycling. The content ranges from 6.43 to 50.02 μmol/g with an average of 17.99 μmol/g which accounts for 31.25% on average of TN respectively. The SOEF-N content is higher in the middle part (23.98 μmol/g) than along the sea coast (16.49 μmol/g). And the content along the west coast is higher (18.99 μmol/g) than that along the east coast (11.98 μmol/g). Importantly, the highest content (50.02 μmol/g) is in the eastern part of Yancheng City (Jiangsu Province in China), where the distribution of SOEF-N appears in a centered pattern. Total nitrogen: The content of TN ranges from 21.88 to 96.87 μmol/g, with an average of 57.60 μmol/g. The content of the exchangeable form is 11.48∼54.80 μmol/g, which accounts for 38.81% of TN. The content of TN is higher along the west coast (63.59 μmol/g) and the middle part of the sea (68.82 μmol/g), lower along the east coast (47.72 μmol/g). The highest content (96.87 μmol/g) appears at the station for the old Yellow River matter. The lowest content is in Haizhou Bay, which is only 32.92 μmol/g, 57% of the average content of the whole sea. Non-transferable nitrogen: Non-transferable nitrogen refers to the nitrogen which cannot be extracted by the extractant. Generally, the non-transferable nitrogen includes two parts: one enters the mineral crystal lattice, and the other is packaged in the big particles. The content of non-exchangeable nitrogen is from 10.40 to 52.23 μmol/g, with an average of 35.24 μmol/g, which may account for 61.19% of TN. By non-transferable nitrogen we do not mean that the nitrogen cannot absolutely take part in cycling, but that it cannot be easily released. Under a certain condition, it can also be released or be ingested by halobios in particles to take part in recycling.

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3 Biogeochemical Processes of the Yellow Sea

In addition, outside the Changiiang River Estuary is a northward tongueshaped distribution of each nitrogen form (except SAEF-N), which results from the Taiwan Warm Current. The Taiwan Warm Current disintegrates after its northward journey from the Taiwan Strait through the ECS; the residual current enters the SYS together with the freshwater from the Changiiang River, and the bottom suspended matter and surface sediments are transported or dispersed to some extent by hydrodynamics. During the long sinking and resuspension processes, some sediments extend northward to a place where the Taiwan Warm Current can spread, so special distributions of different forms of nitrogen in this area are formed. Above all, in the four forms of nitrogen, the content of SOEF-N is the highest and that of WAEF-N is the lowest, and that of IEF-N is higher than that of SAEF-N. If all the transferable nitrogen could be released to take part in cycling under suitable conditions, their relative contributions to cycling would decrease gradually in the order SOEF-N (81%), IEF-N (10%), SAEF-N (6%), and WAEF-N (3%). 3.4.3.2 Relationships of Diﬀerent Forms of Nitrogen In order to clarify the reciprocity between diﬀerent forms of nitrogen, the correlative coeﬃcient between them is calculated. Table 3.12 shows that the diﬀerent forms of nitrogen may interact with each other and the reciprocity is diﬀerent between diﬀerent forms. The content of IEF-N is positively relative to that of all other forms of nitrogen. The correlative coeﬃcient between IEF-N and TN content is the highest, showing that the content of IEF-N is obviously relative to that of TN. IEF-N is the adsorptive nitrogen. If the content of TN in sediment is higher, that of organisms is also higher and then the adsorptive spots and capacity will be higher. In addition, more organic nitrogen will be mineralized to an inorganic form, so the content of IEF-N is higher. The contents of WAEF-N and SAEF-N are not relative to those of SOEF-N and TN, but is relative to that of IEF-N. The positive relativity between the contents of SOEF-N and TN shows that the nitrogen in sediment is mainly organic nitrogen. Table 3.12. The correlation coeﬃcients (r) between the contents of diﬀerent forms of nitrogen in surface sediments of the southern Huanghai Sea (n=48, P <0.05) IEF-N WAEF-N SAEF-N SOEF-N TN

IEF-N 1.00 0.32 0.31 0.26 0.59

WAEF-N

SAEF-N

SOEF-N

TN

1.00 0.30 0.06 0.15

1.00 0.06 0.16

1.00 0.76

1.00

3.4 Biogeochemical Characteristics Nitrogen and Phosphorus in the Yellow Sea

315

3.4.3.3 Predominant Factors Aﬀecting Nitrogen Distributions The distributions and forms of nitrogen are aﬀected by many factors. Both circumstances and sediment characteristics can become the determinant under a certain condition. Table 3.12 shows the correlation coeﬃcients (r) among the contents of diﬀerent forms of nitrogen and environmental parameters and ﬁne sediment (<31 μm) proportions of the SYS surface sediments. The correlation between the contents of diﬀerent forms of nitrogen and the aﬀecting factors are discussed as follows. IEF-N: IEF-N is the adsorptive nitrogen. Its bonding strength is the weakest and it can take part in cycling easily. Its distribution is aﬀected by bioturbation in sediments, organisms of the overlying water, some environmental factors such as temperature, salinity, and pH, and the contents of DO (Table 3.13). In addition, the content of ﬁne sediments (<31 μm) may also inﬂuence the distribution of IEF-N. The adsorptive capacity and spots increase with ﬁne sediment because of the large surface area and higher content of organic matter, so the content of N is higher where that of ﬁne sediment is higher. This further shows that the distribution of IEF-N is signiﬁcantly aﬀected by sediment grain size. Table 3.13. The correlation coeﬃcients (r) between the contents of diﬀerent forms of nitrogen in sediments and environmental parameters and ﬁne sediment (<31 μm) proportions (n=48, P <0.05) IEF-N WAEF-N SAEF-N SOEF-N TN Temperature −0.34 −0.10 −0.41 0.27 0.11 Salinity −0.10 0.24 0.09 0.03 0.16 pH 0.18 0.10 0.09 0.08 0.18 Dissolved oxygen 0.19 0.01 0.07 −0.26 −0.24 Contents of the ﬁne sediments (<31 μm) 0.68 0.35 0.11 0.58 0.71

WAEF-N: The formation of WAEF-N is mainly controlled by pH change during the organisms’ mineralization processes, but not related to the pH of overlying water. The pH change would cause CaCO3 to dissolve or deposit, and ammonia or nitrate could be bonded with carbonate to form WAEF-N ultimately. In general, if the content of carbonate is higher in sediments, that of OC would be lower, OC mineralization would be weak, and consequently pH variation would be smaller; the dissolution and deposition of CaCO3 would be weak, so the content of WAEF-N would be lower. Table 3.13 shows that the content of ﬁne sediments may also aﬀect the formation of N besides the inﬂuence of the salinity of overlying water. The OC content is higher with ﬁne sediment, and the higher OC content in ﬁne sediments results in the stronger mineralization of OC which would cause a wide variation in pH and a higher WAEF-N content, so the content of WAEF-N is positively relative with that of ﬁne sediments. Unfortunately, the distribution of WAEF-N is not consistent with the above conclusion in the middle part of the SYS, where WAEF-N

316

3 Biogeochemical Processes of the Yellow Sea

content is relatively low, which might be due to the sampling time and the YS Cold Water Mass. In the water under the euphotic layer of the YS Cold Water Mass, the distribution of DO is mainly aﬀected by the decomposition of organisms and a closed or half closed area would be easily formed at the bottom. The intense activities of plankton and benthos in spring lead to heavy consumption of O2 in the water column, the sediments are still in anaerobic condition although O2 content is relatively high for the vertical water perturbation in winter, so the mineralization of organic matter is weak and the pH variation is small. This means the carbonate content would be higher in sediments, OC content would be lower, OC mineralization would be weak and, consequently, pH variation would be smaller; the dissolution and deposition of CaCO3 would be weak, so the content of WAEF-N would be lower. Table 3.13 shows that the content of ﬁne sediments may also aﬀect the formation of N besides the inﬂuence of the salinity of overlying water. The OC content is higher with ﬁne sediment, and the higher OC content in ﬁne sediments results in the stronger mineralization of OC which would cause a wide variation in pH and higher WAEF-N content, so the content of WAEF-N is positive relative to that of ﬁne sediments. Unfortunately, the distribution of WAEF-N is not consistent with the above conclusion in the middle part, where WAEF-N content is relatively low, which might be due to the sampling time and the YS Cold Water Mass. In the water under the euphotic layer of the YS Cold Water Mass, the distribution of DO is mainly aﬀected by the decomposition of organisms and a closed or half closed area would be easily formed at the bottom. The intense activities of plankton and benthos in spring lead to heavy consumption of O2 in the water column, the sediments are still in anaerobic condition although O2 content is relatively high for the water vertical perturbation in winter, so the mineralization of organic matter is weak and the pH variation is small, which results in a lower content of WAEF-N in the middle part. SAEF-N: The distribution of SAEF-N is mainly controlled by the oxidativereductive condition of sediment. The sediment oxidative-reductive condition is inﬂuenced by organisms content, the ratios of Fe3+ to Fe2+ , Mn4+ to Mn2+ , to H2 S, sediment size, sulfate content of pore-water (especially and SO2− 4 S2− ), and so on. When the oxidative-reductive condition is mainly aﬀected by the sediment grain size, the content of SAEF-N will be negatively relative to that of ﬁne sediments. That is to say, the content of SAEF-N is lower when that of ﬁne sediments is higher, while the lower content of ﬁne sediments leads to higher content of SAEF-N. The SAEF-N content is higher in the middle part east of Chengshan Cape (in the Shandong Peninsula) than anywhere else. In the middle part east of Chengshan Cape, the sediment is in an oxidative condition because the O2 content and Eh are higher due to the coarse scraping sediments and the higher porosity. In addition, the content of Mn may be up to 1,420 μg/g, and the autogenic Mn amounts to 86% (1,220 μg/g). SAEF-N is mainly the nitrogen which could be bonded with the ferric of manganic oxides. If the content of ferric or manganic oxide is higher, the

3.4 Biogeochemical Characteristics Nitrogen and Phosphorus in the Yellow Sea

317

amount of nitrogen which could be bonded with autogenied material would be higher, consequently the content of S would be higher and the adsorptive domino eﬀect would be weakened, so the content of SAEF-N is not relative to that of ﬁne sediments. In addition, the temperature is negatively relative to the content of SAEF-N, which is another reason why the content of SAEF-N is higher in the northern part. SOEF-N: The distribution of SOEF-N is relative to the sediment of origin and grain size, the content and import rate of organic matter, sedimentary rate, and the oxidative-reductive condition. Table 3.13 shows that the content of SOEF-N is positively relative to temperature and negatively relative to the content of DO, and is obviously relative to the content of ﬁne sediment. If the temperature of overlying water is higher, the marine halobios growth would be active, and the egesta and dead detritus would be greater, so the content of SOEF-N is higher if the decomposition rate of organisms is lower than the sedimentary rate. If the DO content of overlying water is higher, the sediment is in a relatively oxidative condition; there would be more organisms decomposed to take part in cycling through the sediment-water interface, so the content of SOEF-N would decrease. Usually, ﬁne sediments accumulate more compactly, and ventless anaerobic conditions are easily formed. Organisms would be maintained in an anaerobic condition, which makes their mineralization and deposition diﬃcult to take place, so the content of SOEF-N would be higher. This is why in the dense area of ﬁne sediments in the SYS (the middle part of the sea and the sea area east of Yancheng City of Jiangsu Province) the content of SOEF-N is high. In the sea area east of Yancheng City, the sediments are a mixture of ancient Yellow River matter and modern sediments. Alongshore currents thoroughly washed out, crashed, and ﬁltered sediments; a large quantity of ﬁne sediments remain in the plain where the old Yellow River and the North Yellow Sea (NYS) matter rich in organisms accumulates. In this area, ﬁne sediments accumulate rapidly, which makes the mineralization of organisms diﬃcult, so the content of SOEF-N is the highest. TN: The content of TN is that of all the forms of nitrogen (including transferable and non-transferable nitrogen). The content of TN is usually used to measure the potential productivity in an area. The distribution of TN is mainly aﬀected by the sediment grain size and origin. The content of TN is negatively relative to that of DO, and has an obviously positive relativity with that of ﬁne sediments. In the SYS, terrestrial input has little eﬀect on the content of TN except along the west coast of the sea. Organic carbon (OC), as the predominant component of marine organisms, could be transported and cycled in the shallow continental shelf with ﬁne sediment movement; i.e., the OC transportation and accumulation mainly occur in ﬁne sediments, where the contents of organisms and nitrogen are high. So the sediment grain size would be an important factor inﬂuencing the distribution of TN. Above all, we may conclude that the contents of diﬀerent forms of nitrogen have a positive relativity to the DO content and temperature of the overlying water, but no relativity to the salinity and pH, and have a markedly positive relativity to the

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3 Biogeochemical Processes of the Yellow Sea

content of ﬁne sediments. The contents of IEF-N, SOEF-N, and TN present deﬁnite linearity with the content of ﬁne sediments. Their imitation equations could be obtained based on the regression calculation: Y = ax + b, where Y is the content of diﬀerent forms of nitrogen; x is the content of ﬁne sediments. That is to say, ﬁne sediments can carry and adsorb IEF-N, SOEF-N, and TN strongly, and are the carrier of their transportation and cycling. According to the formula, the contents of the three forms of nitrogen in diﬀerent grain size sediments could be calculated quickly (Table 3.14). The content of WAEFN still could be calculated cursorily though the relativity is not obvious. The content of SAEF-N is not relative to that of ﬁne sediments, is aﬀected by other environmental factors and is not dependent on the content of ﬁne sediments. Table 3.14. Correlative relationships of parameters between the content of diﬀerent forms of nitrogen in sediments and ﬁne sediment proportion (<31 μm) of the SYS IEF-N WAEF-N SAEF-N SOEF-N TN

Correlative coeﬃcient 0.68 0.35 0.11 0.58 0.71

n 48 48 48 48 48

a 2.1833 0.2071 – 30.089 61.419

b 1.4226 0.5816 – 5.8487 32.774

3.4.3.4 Release of Nitrogen The southern YS was divided into three regions: regions I, II, and III according to the proportions of ﬁne grain size sediments (<31 μm) >65%, 35%∼65%, and <35% (Fig. 3.31). The proportion of ﬁne sediments was more than 70%, with an average of 80.55% in the middle part of the southern YS and the eastern part of the ancient Yellow River Estuary. The proportions of ﬁne particles were less than 30%, and the average was only 19.66% in the parts near the Korean Peninsula and between Qingdao and Shijiusuo (Shandong Peninsula, China). Surface sediment distribution was inﬂuenced by modern sedimentary circumstances, such as temperature, salinity, overlying water pH, hydrodynamics, the size, species and distributions of suspended matter, and the productive processes of organisms. The contents of diﬀerent forms of nitrogen could also be inﬂuenced by the sedimentary environment. The southern YS surface sediments, a mixture of modern sediments and residual sediments, were continuously eroded, washed out and transported by hydrodynamics. As a result, the sediment grain size became ﬁner from the coast to the central part, and the average proportion of ﬁne sediments increased from 20.40% to 44.32% to 81.41% with the distance from the shore, except in the sea area east of Yancheng City. In the west coastal area east of Yancheng City, through long-time corroding, washing out, and transportation by the China coastal

3.4 Biogeochemical Characteristics Nitrogen and Phosphorus in the Yellow Sea

319

current, the sediments accumulated with ancient Yellow River and Yangtze River materials and were split into ﬁne particles. The average proportion of ﬁne sediments accounted for 75.82%. N 39

N 38 37 36

III

I 35

III

34

I II

33 32 31

>65% 35% to 65% <35%

III

70 km

119 120 121 122 123 124 125 126 127

E

Fig. 3.31. Sediment regions of the SYS according to the proportion of ﬁne grain size sediments (<31 μm). I, >65%; II, 35%∼65%; III, <35% (L¨ u et al., 2004)

The areas of the southern YS three regions were 72,308 km2 (region I), 95,077 km2 (region II), and 141,615 km2 (region III) respectively, by computational simulation using a grid. The amount of various forms of nitrogen in diﬀerent region sediments would be accounted for from the formula (Ma et al., 2003) Gij = C¯ij × Ai × hi × ρd = C¯ij × Ai × hi × (1 − C¯wi )/((1 − C¯wi )/ρs + C¯wi /ρw ) (3.12) with Gij (mol): the quantum of j form of nitrogen in region i; C¯ij (μmol/g): the average content of j form of nitrogen in region i; Ai (km2 ): the area of region i; hi (cm): the sediment depth for releasing nitrogen in region i (Sediment depth was assumed to be 3 cm in the southern YS because the mineralization of organic matter was mainly taking place in surface sediments (Song, 2002; L¨ u and Song, 2002)); ρd (g/cm3 ): the sediment dry bulk density; 3 ρw (g/cm ): the seawater density (1.027 g/cm3 ); C¯wi (%): the average water content of sediments in region i; ρs (g/cm3 ): the sediment grain density (The ρs in regions I, II, and III was 2.3, 2.5, and 2.6 g/cm3 respectively (Li et al., 2002) because the sediment grain size was diﬀerent in diﬀerent regions and the porosity was also diﬀerent (Table 3.15)).

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3 Biogeochemical Processes of the Yellow Sea

Table 3.15. The quanta of diﬀerent forms of nitrogen in the sediments of the three regions of the SYS (×1010 mol) Forms IEF-N WAEF-N SAEF-N SOEF-N TN

Region I 1.55 0.37 0.72 13.61 38.76

Region II 1.76 0.48 0.99 13.40 44.10

Region III 2.08 0.67 1.14 14.53 50.40

Whole area 5.39 1.52 2.85 41.54 133.26

The DIN exchange ﬂux at sediment-water interface was 1.31 mmol/(m2 ·d) (from sediment to overlying water) by simulating examination in Chung et al. (1999). Assuming the DIN ﬂux of ﬁne sediment (<31 μm), medium sediment (31∼63 μm) and coarse sediment (>63 μm) full ﬂux, 1/2 of full ﬂux and 1/8 of full ﬂux respectively, that of natural sediments in diﬀerent regions could be obtained from the formula (Song et al., 2003) Fi × x i (3.13) F¯i = with F¯i (mmol/(m2 ·d)): the average DIN exchange ﬂux of region i; Fi (mmol/(m2 ·d)): the DIN exchange ﬂux of diﬀerent grain size sediment; xi (%): the proportion of each grain size sediment. The DIN exchange ﬂux at the sediment-water interface in the three regions was 1.146, 0.779, and 0.463 mmol/(m2 ·d) respectively. If all the extractable nitrogen could be released to take part in the interface exchange, the releasing time of various forms of extractable nitrogen in diﬀerent regions surface sediments of the southern YS could be accounted for from the formula (Song, 2004) (Table 3.16) Tij =

Gij Fi × Ai

(3.14)

with Tij (yr): the releasing time of j nitrogen form in region i; Gij (mmol): the amounts of j nitrogen form in region i; Fi (mmol/(m2 ·d)): the DIN ﬂux in region i; Ai (km2 ): the area of region i. Table 3.16. The releasing time of diﬀerent forms of extractable nitrogen in the three regions surface sediments of the SYS (yr) Forms IEF-N WAEF-N SAEF-N SOEF-N

Region I 0.51 0.12 0.24 4.50

Region II 0.65 0.18 0.37 4.96

Region III 0.87 0.28 0.48 6.07

3.4 Biogeochemical Characteristics Nitrogen and Phosphorus in the Yellow Sea

321

The releasing time of diﬀerent forms of nitrogen was the shortest in region I, and the longest in region III. This resulted from the highest DIN exchange ﬂux at the sediment-water interface in region I. So all the extractable nitrogen in region I could be released to take part in cycling in a short time, while that in regions II and III needed a long time, indicating that ﬁne sediment made a greater contribution to nitrogen cycling than coarse sediment. All the extractable nitrogen in the southern YS sediments could be released completely to take part in cycling in 6.07 years. The contributions of various forms of extractable nitrogen to cycling could be obtained assuming they all could take part in cycling (Table 3.16). The contributions of various forms of nitrogen decreased from SOEF-N to IEF-N to SAEF-N to WAEF-N, and were 80% (SOEF-N), 11% (IEF-N), 6% (SAEF-N), and 3% (WAEF-N) respectively, indicating that the organic nitrogen (SOEF-N) was the predominant form of extractable nitrogen and had the highest potential contribution to nitrogen recycling. 3.4.3.5 Burial Flux and Eﬃciency of Nitrogen Particles in the water column sank to the bottom ultimately through a long period of transportation and sedimentation. Through complex mineralization of organic matter, one part of nitrogen in sediment is released into the water column for supporting the plankton reproduction, and the other part remains in the sediments. The remaining part could also be released in diﬀerent forms to overlying water in suitable conditions, and is the potential “nutrient source” of the water column. Nitrogen burial ﬂux could be calculated by the formula BF = Ci × S × ρd =

C i × S × (1 − Cw ) (1 − Cw )/ρs + Cw /ρw

(3.15)

with BF (μmol/(cm2 ·yr)): the burial ﬂux of nitrogen in sediments; Ci (μmol/g): the content of i form of nitrogen in sediments; S (cm/yr): the sedimentary rate; ρd (g/cm3 ): the sediment dry bulk density; ρw (g/cm3 ): the seawater density (1.027 g/cm3 ); Cw (%): the water content in sediments; ρs (g/cm3 ): the sediment grain density (The ρs in regions I, II, and III was 2.3, 2.5, and 2.6 g/cm, respectively (Li et al., 2002)). The burial ﬂuxes of diﬀerent forms of nitrogen in surface sediments of the three regions of the southern YS are shown in Table 3.17. The burial ﬂuxes of nitrogen in the three regions were similar, indicating that sediment granularity was not the predominant factor to aﬀect nitrogen burial ﬂux, and the nitrogen burial ﬂux might be aﬀected by other factors. The correlation between nitrogen burial ﬂuxes and diﬀerent circumstancial factors showed that burial ﬂuxes of diﬀerent forms of nitrogen had a positive correlation to the sedimentary rate, had a negative correlation to NO3 -N content and the salinity of overlaying water, and had little negative correlation

322

3 Biogeochemical Processes of the Yellow Sea

Table 3.17. The burial ﬂuxes of diﬀerent forms of nitrogen in the three regions surface sediments of the SYS (μmol/(cm2 ·yr)) Region I Range Average IEF-N 0.16∼0.80 0.34 WAEF-N 0.04∼0.19 0.09 SAEF-N 0.05∼0.35 0.17 SOEF-N 0.83∼4.85 2.61 TN 2.77∼14.54 8.34 Forms

Region II Range Average 0.16∼0.78 0.33 0.04∼0.29 0.10 0.09∼0.38 0.17 1.31∼5.80 2.51 4.19∼16.83 8.47

Region III Range Average 0.11∼0.66 0.33 0.04∼0.21 0.11 0.08∼0.32 0.17 0.59∼4.80 2.34 2.81∼15.51 8.34

to pH and the temperature of the overlaying water (Table 3.18). The sedimentary rate was the main factor in determining the nitrogen burial ﬂux, so the burial ﬂuxes of various forms of nitrogen were higher where the sedimentary rate was higher. This was because sediment had accumulated before the organic matter decomposed, or the mineralized matter had been buried with quickly subsiding sediments before they were released to, or exchanged with, the water column. In addition, the hydrodynamics had a strong eﬀect on the distribution and burial ﬂux of nitrogen, which we will not discuss here. In general, the diﬀusive ﬂuxes (DF) of diﬀerent forms of nitrogen in the sediments of the same region are the same. The nitrogen burial eﬃciency in diﬀerent regions would be accounted for by the formula (Table 3.19, L¨ u et al., 2002) BE =

BF × 100% (BF + DF )

(3.16)

Table 3.18. The correlation between nitrogen burial ﬂuxes and various circumstantial factors Forms IEF-N WAEF-N SAEF-N SOEF-N TN

S Cw C<31 μm Temp. Sal. 0.92 0.07 −0.05 0.06 −0.44 0.87 −0.11 −0.14 0.27 −0.36 0.91 0.13 0.09 0.07 −0.36 0.72 −0.06 0.01 0.30 −0.19 0.91 0.02 0.01 0.24 −0.26

S: sedimentary rate; Cw : water content; C <31 oxygen in overlying water

μm :

pH 0.28 0.20 0.29 0.31 0.36

DO 0.17 0.03 0.03 −0.12 −0.04

NO− 3 −0.39 −0.29 −0.46 −0.13 -0.40

NH+ 4 0.00 0.11 0.26 0.04 0.03

the content of ﬁne sediment; DO: dissolved

Table 3.19. The nitrogen burial eﬃciency in the three regions surface sediments of the SYS Regions Region I Region II Region III

Diﬀusive ﬂux (mol/(cm2 ·yr)) 42.12 29.64 19.27

IEF-N 0.8 1.1 1.68

Burial eﬃciency (%) WAEF-N SAEF-N SOEF-N 0.21 0.4 5.84 0.34 0.57 7.81 0.57 0.87 10.83

TN 16.52 22.22 30.21

3.4 Biogeochemical Characteristics Nitrogen and Phosphorus in the Yellow Sea

323

with BE (%): the burial eﬃciency; BF (μmol/(cm2 ·yr)): the burial ﬂux; DF (μmol/(cm2 ·yr)): the diﬀusive ﬂux. Nitrogen burial eﬃciency was the lowest in region I where the ﬁne sediment proportion was the highest, and the burial eﬃciency of total nitrogen was 16.52%, indicating that the great mass of nitrogen in ﬁne sediments could take part in recycling through the sediment-water interface, and the releasing eﬃciency could exceed 83.48%. Fine sediments were strongly aﬀected by water dynamics, bioturbation, and so on. In addition, the comparative surface area of ﬁne sediment was the largest and the osculating area with overlying water was the biggest, and much of nitrogen was released to take part in recycling through the sediment-water interface, so a little nitrogen was buried in region I. On the contrary, coarse sediment was less aﬀected by water dynamics and bioturbation, and was not the main carrier of nitrogen. Furthermore, a part of the nitrogen was packed in big particles or entered into the mineral crystal lattice, and was not easily released by water dynamics and bioturbation, so nitrogen burial eﬃciency in coarse sediment was higher. The burial eﬃciency of total nitrogen could be up to 30.21% in region III, indicating that about 69.79% of nitrogen could be released to take part in recycling. It could be that more than 70% of nitrogen in the surface sediments of the southern YS could ultimately be released to the water column to take part in recycling (L¨ u et al., 2004). 3.4.4 Nitrogen in the North Yellow Sea Sediments Five core sediment samples were collected with the box sampler, and then cut at 2 cm intervals. The sediments location was as shown in Fig. 3.32. Four exchangeable forms of nitrogen were analyzed in every sub-sample, including IEF-N, WAEF-N, SAEF-N, and SOEF-N. 3.4.4.1 Exchangeable Nitrogen Fig. 3.33 illustrates the vertical proﬁles of 4 exchangeable nitrogen forms in the studied core sediments, which are discussed in detail as follows: IEF-N: The concentrations of IEF-N in all core sediments declined with depth generally in a top to bottom of the core direction, except C3; meanwhile, all core sediments except C3 presented the highest surﬁcial concentrations and IEF-N concentrations did not vary signiﬁcantly with depth. However, the maximum concentrations of IEF-N in C3 were detected 23 cm below the surface. The vertical distribution of IEF-N suggested that the mineralization of organic matter in the sediments always happened in the surﬁcial oxygencontaining layers. The concentration sequence of all sediments was: C3 (5.08 mmol/g)>C4 (3.38 mmol/g)>C2 (2.97 mmol/g)>C7 (2.61 mmol/g)>C1 (2.35 mmol/g). IEF-N consisted of NH4 -N and NO3 -N. The ratios of NO3 -N to NH4 -N were: C3 (1.69)>C1 (1.59)>C4 (1.39)>C2 (1.33)>C7 (1.02).

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3 Biogeochemical Processes of the Yellow Sea

Fig. 3.32. The map of sampling stations (C1, C2, C3, C4, and C7) in the NYS (L¨ u et al., 2004)

WAEF-N concentrations: The average concentrations of WAEF-N in 5 cores were 1.11 (C1), 1.15 (C2), 1.07 (C3), 1.07 (C4), and 1.18 μmol/g (C7), respectively, which were the lowest among the four forms. In the whole vertical proﬁles, WAEF-N concentrations did not vary signiﬁcantly with depth, ranging from 0.64 to 1.47 μmol/g. The highest concentrations were detected in 0∼6 cm of C1 and C7, while the maximum concentrations were detected in the middle or bottom of the other cores. These distributions might be controlled by the sedimentation process and texture. Similar to IEF-N, NO3 -N was the predominant component in all the samples, with the ratios of NO3 -N to NH4 -N as follows: 1.64 (C1), 2.11 (C2), 1.18 (C3), 1.10 (C4), and 1.59 (C7). SAEF-N: The primary form of SAEF-N was Fe-Mn oxide combined form, whose formation and distribution were aﬀected by the oxidation-reduction characteristics of the sediment environment. The average concentrations in 5 cores were: 9.18 μmol/g in C1, 11.95 μmol/g in C2, 5.66 μmol/g in C3, 6.27 μmol/g in C4, and 4.71 μmol/g in C7, reﬂecting the diﬀerence in the oxidationreduction characteristics in the 5 cores. SAEF-N concentrations presented complex variations with depth, as well as the ratios of NO3 -N to NH4 -N (Table 3.20). SOEF-N: SOEF-N was the primary organic nitrogen form, whose average concentration was the highest in exchangeable nitrogen. The sequence of SOEF-N average concentrations in the core was as follows: C2 (116.06 μmol/g)>C1 (98.93 μmol/g)>C4 (97.78 μmol/g)>C7 (92.82 μmol/g)>C3 (89.16 μmol/g). In the 5 vertical proﬁles, the distribution tendencies of SOEFN were diﬀerent, while all of them presented the highest surﬁcial concentrations (0∼3 cm) and then the concentrations declined with depth slightly, in-

3.4 Biogeochemical Characteristics Nitrogen and Phosphorus in the Yellow Sea

325

326

3 Biogeochemical Processes of the Yellow Sea

Fig. 3.33. The vertical distribution of various forms of nitrogen in natural sediments (L¨ u et al., 2004) Table 3.20. The average contents of NH4 -N and NO3 -N of the three inorganic forms of nitrogen in the sediments of the NYS (μmol/g) (L¨ u et al., 2004)

C1 C2 C3 C4 C7

NH4 -N 0.91 1.25 1.89 1.42 1.29

IEF-N NO3 -N 1.45 1.72 3.19 1.97 1.32

NH4 -N 0.42 0.37 0.49 0.51 0.46

WAEF-N NO3 -N 0.69 0.78 0.58 0.56 0.73

NH4 -N 2.33 2.12 2.98 3.28 3.14

SAEF-N NO3 -N 6.83 9.82 2.68 2.99 1.58

3.4 Biogeochemical Characteristics Nitrogen and Phosphorus in the Yellow Sea

327

dicating that the mineralization of the organic matter always occurred in the surﬁcial oxygen-containing sediments and the oxygen-containing sediments in the SYS were more shallow than those in other seas. The complex distribution tendencies from the sub-surﬁcial layer to the bottom of the core suggested that the mineralization procedure and sedimentation rate represented diﬀerent apparent strengths in diﬀerent periods. TN: TN concentrations in 5 core sediments followed the order: C2 (743.75 μmol/g)>C3 (674.99 μmol/g)>C4 (591.95 μmol/g)>C1 (521.81 μmol/g)>C7 (453.96 μmol/g), indicating the sediment sources would pose an inﬂuence in TN distributions. As shown in the vertical proﬁles of C1, C2, and C3, TN concentrations varied widely and increased with depth generally, while TN concentrations decreased with depth in C4 and C7 cores. TN vertical distribution was similar to SOEF-N in C4 and C7, the opposite of that in C3, which might be related to the distance of the stations and the material sources. In the coastal area, the terrestrial-origin material was abundant, and thus the organic matter varied with TN concentrations. When the bio-origin material was the primary source of the sediment, the stronger the mineralization process of organic matter in sediment, the lower the organic nitrogen concentration, and TN concentrations would decline with the mineralization. This viewpoint might explain why TN and IN shared a common distribution tendency in contrast to that of SOEF-N. In conclusion, SOEF-N was the predominant form of exchangeable nitrogen in the sediment, followed immediately by ASEF-N, and the WAEF-N level was the lowest. The average concentrations and percentages accounting for the total nitrogen of all the 4 exchangeable nitrogen forms are listed in Table 3.21. Table 3.21. The average content (μmol/g) of various forms of extractable nitrogen and its percentages relative to total nitrogen (%) in northern YS sediment (L¨ u et al., 2004)

C1 C2 C3 C4 C7

IEF-N Aver Perc 2.35 0.45 2.97 0.40 5.08 0.75 3.38 0.57 2.61 0.57

WAEF-N Aver Perc 1.11 0.21 1.15 0.15 1.07 0.16 1.07 0.18 1.18 0.26

SAEF-N Aver Perc 9.18 1.76 11.95 1.61 5.66 0.84 6.27 1.06 4.71 1.04

SOEF-N Aver Perc 98.93 18.96 116.06 15.60 89.16 13.21 94.78 16.01 92.82 20.45

Exchangeable N Aver Perc 111.58 21.38 132.13 17.76 100.96 14.96 105.50 17.82 101.32 22.32

3.4.4.2 Inﬂuence of Grain Size of the Sediment Particles The grain size and components of sediments played important roles in the distribution of elements in the sediments, and aﬀected the local biogeochemical

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3 Biogeochemical Processes of the Yellow Sea

characteristics of the sediment and eco-environment. The texture and grain size of the 5 studied cores were listed in Table 3.22. Coarse particles (>63 μm) were only detected in C1; the middle grain size sediments (31∼63 μm) were higher in C1 and C7; ﬁne particles were the predominant components in all samples in the range of 76.14%∼94.25%. Table 3.22. The range and average content of diﬀerent grain size sediments in the NYS (%) (L¨ u et al., 2004)

C1 C2 C3 C4 C7

>63 Range 1.62∼12.88 – – – –

μm Average 7.14 – – – –

31∼63 Range 10.85∼21.74 4.43∼9.81 6.64∼10.33 4.47∼6.38 20.86∼28.40

μm Average 16.47 7.07 8.24 5.78 23.83

<31 μm Range Average 68.40∼87.14 76.39 90.05∼95.57 92.93 89.67∼93.36 91.76 93.62∼95.53 94.25 71.60∼79.14 76.14

The concentrations of all forms of nitrogen in sediments varied with the sediments texture. Generally, ﬁne-grained components directly aﬀect the form, content, and distribution of nitrogen in natural sediments. The distributions of 4 exchangeable nitrogen forms in diﬀerent components of sediments were listed in Table 3.23. It was noticeable that the concentrations of 4 exchangeable Table 3.23. The average content (μmol/g) of various forms of nitrogen in diﬀerent grain size sediments (L¨ u et al., 2004) Station C1

C2

C3

C4

C7

Grain size >63 μm 31∼63 μm <31 μm Natural 31∼63 μm <31 μm Natural 31∼63 μm <31 μm Natural 31∼63 μm <31 μm Natural 31∼63 μm <31 μm Natural

IEF-N 2.61 1.78 2.56 2.35 3.11 2.96 2.97 3.58 5.21 5.08 3.03 3.41 3.38 1.79 2.86 2.61

WAEF-N 0.95 0.79 1.22 1.11 0.70 1.18 1.15 0.63 1.11 1.07 0.60 1.09 1.07 0.94 1.26 1.18

SAEF-N 2.13 1.29 11.56 9.18 2.66 12.66 11.95 2.37 5.94 5.66 3.17 6.46 6.27 1.15 5.81 4.71

SOEF-N 90.25 64.21 109.60 98.93 83.45 118.24 11.606 71.32 90.80 89.16 80.03 95.69 94.78 73.89 98.52 92.82

TN 425.43 477.06 539.53 521.81 521.68 759.28 743.75 548.32 686.17 674.99 451.21 600.48 591.95 384.06 475.42 453.96

3.4 Biogeochemical Characteristics Nitrogen and Phosphorus in the Yellow Sea

329

nitrogen forms in the middle size sediment (31∼63 μm) were less than those in the ﬁne sediment (<31 μm) of all studied cores, suggesting that the adsorption and carriage of exchangeable nitrogen primarily occurred in the ﬁne sediment particles. Furthermore, the concentrations of 4 exchangeable nitrogen forms in natural sediments were close to those in ﬁne sediments, which resulted from the high percentages of ﬁne sediments particles, as Fig. 3.34 shows.

Depth (cm)

C1

0

Depth (cm) Depth (cm)

C3

SOEF-N (mmol/g) 15

0

50

100

TN (mmol/g)

150

0

0

10

10

10

20

20

20

30

30

40

40

50

50

50

60

60

60

30

IEF-N WAEF-N SAEF-N

40

C2

IN (mmol/g) 5 10

0 5 10 15 20 25 30 35 40 45 50

0 5 10 15 20 25 30 35 40 45 50

0

0

SOEF-N (mmol/g) 100 200

IN (mmol/g) 5 10 15 20

IN (mmol/g) 6 2 4

00 5 10 15 20 25 30 35 40 45 50

8

0

TN (mmol/g) 0 5 10 15 20 25 30 35 40 45 50

SOEF-N (mmol/g) 86 88 90 92 94 96

0 5 10 15 20 25 30 35 40 45 50

0 200 400 600 800

0

0 0 5 10 15 20 25 30 35 40 45 50

500

1000

TN (mmol/g) 500 1000

330

3 Biogeochemical Processes of the Yellow Sea SOEF-N (mmol/g)

IN (mmol/g)

Depth (cm)

C4

0

0

5

10

15

0

0

50

100

150

TN (mmol/g) 0

10

10

10

20

20

20

30

30

40

40

40

50

50

50

60

60

60

IEF-N WAEF-N SAEF-N

30

C7

Depth (cm)

0

0

IN (mmol/g) 6 2 4

8

SOEF-N (mmol/g) 90 95 100 105 110 0

0

400 0

2

2

2

4

4

4

6

6

6

8

8

8

10

10

10

12

12

12

14

14

14

16

16

16

18

18

18

500

1000

TN (mmol/g) 450 500 550

Fig. 3.34. The vertical distribution of various forms of nitrogen in ﬁne sediments (<31 μm) of diﬀerent stations (L¨ u et al., 2004)

3.4.4.3 Early Diagenesis of Nitrogens The vertical distribution of nitrogen reﬂected the reaction variation in the early diagenesis process and recorded the information on the variation of nitrogen forms, concentrations, and the environmental conditions. Based on the SOEF-N decomposition degree, the decomposition rates could be calculated. It was assumed that the decomposition of organic nitrogen was a ﬁrst order reaction. Z was the depth, C 0 and CZ were the concentrations at the surface and Z cm respectively, K was the decomposition rate constant (yr−1 ), and S was the sedimentation rate (cm/yr). K value could be calculated with the following equation and tabulated in Table 3.24: K=

ln(C0 /CZ ) Z/S

(3.17)

As shown in Table 3.24, K values declined with the decrease in grain size, which might be explained by the fact that the inﬁltration capacity of O2 in

3.4 Biogeochemical Characteristics Nitrogen and Phosphorus in the Yellow Sea

331

coarse particle sediments was higher than that in ﬁne particle sediments. The ﬁner the sediments are, the smaller the decomposition rate of organic nitrogen will be; i.e., the organism was easily enriched in ﬁne-grained sediments. The burial ﬂuxes of various forms of nitrogen are diﬀerent because of the varying sedimentation rates in diﬀerent sampling stations. The higher the sedimentation rate, the greater the burial ﬂux of nitrogen will be. The burial ﬂuxes of various forms of nitrogen are the highest at station C4 where the sedimentation rate is the highest. Consequently, the sediment environment presented high oxidizability, which could support the oxidation-reduction reaction of organic matter in the sediments. The organic matter consumption due to oxidationreduction reaction was one of the reasons that SOEF-N concentrations in coarse particle sediments were lower than those in ﬁne sediments. Table 3.24. Decomposable velocity constant (K) (×103 ) of organic nitrogen in diﬀerent grain-size sediments (L¨ u et al., 2004) Grain size >63 μm 31∼63 μm <31 μm Natural

C1 16.24 9.93 2.67 4.13

C2 – – 5.36 4.84

C3 – 8.85 2.32 2.48

C4 – 42.33 27.60 25.56

C7 – – 6.86 5.16

3.4.4.4 Buried Fluxes The calculation method and expression were discussed previously (equations (3.15) and (3.16)); therefore the buried ﬂuxes were calculated and tabulated in Table 3.25. The buried ﬂux in C4 was the highest, which related to the sedimentation rates to a large extent. It means that the higher the sedimentation rate, the faster the deposition of sediment particles and the greater the deposition and buried ﬂuxes. Although the decomposition rate of organic matter was high in the high-deposition area, there would be abundant organic matter joining the recycling process. Therefore, a large amount of aliquot nitrogen was buried as “inert nitrogen” (L¨ u et al., 2005). Table 3.25. The burial ﬂuxes (μmol/(cm·yr)) of various forms of nitrogen in the northern Yellow Sea surface sediment (L¨ u et al., 2004) Station S (cm/yr) C1 0.07 C2 0.18 C3 0.25 C4 0.45 C7 0.21

IEF-N 0.25 0.45 0.94 1.54 0.42

WAEF-N 0.07 0.15 0.18 0.43 0.15

SAEF-N 0.53 1.39 1.20 2.56 0.65

SOEF-N 6.51 16.74 15.79 36.72 13.63

TN 26.91 73.28 113.32 284.23 67.60

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3 Biogeochemical Processes of the Yellow Sea

3.4.5 Biogeochemical Processes of Phosphorus A six-step extraction method was used to operationally discriminate the chemical species of P (e.g., authigenic apatite and refractory organic P). The sixstep extraction procedure separates the major reservoirs of sedimentary P into six pools, including loosely adsorbed P (LSor-P), iron-bound inorganic P (Fe-P), leachable organic P (Lea-OP), authigenic apatite (CFAP), detrital apatite (FAP), and refractory organic P (Ref-OP). The location of studied cores was as shown in Fig. 3.35.

N 40

38

118

120

122

124

126 E

Fig. 3.35. The locations of the core sediments in the NYS (Liu et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

3.4.5.1 Phosphorus Concentrations Analytical results for the concentrations of IP, OP, and TP in surface sediments are listed in Table 3.26 (Liu et al., 2004). The results indicate that IP is the most important P speciation representing 73%∼98% of the TP. The Table 3.26. The concentrations (μmol/g) of IP, OP and TP in surface sediments from the YS (Liu et al., 2004) (With permission from Elsevier’s Copyright Clearance Center) Stations

IP

OP

TP

BH-A2 BH-A4 BH-D5 YS-1 YS-2 YS-3 YS-T4 YS-5

13.5 10.4 10.7 8.62 10.5 12.8 12.0 5.61

4.26 3.37 0.68 0.33 0.26 2.38 4.57 1.90

17.8 13.7 11.4 8.95 10.7 15.2 16.5 7.51

3.4 Biogeochemical Characteristics Nitrogen and Phosphorus in the Yellow Sea

333

diﬀerences in IP, OP, and TP among stations in the NYS were signiﬁcant at the 95% conﬁdence level. In the YS, the concentrations of OP were high in the southern part of the Shandong Peninsula and the central basin. The distribution of OP is diﬀerent from IP, illustrating that the sources of OP diﬀer from those of IP, and OP can be aﬀected diﬀerently to IP by biological and physical factors. Liu et al. (2004) reported the following results. The variations of IP, OP, and TP at depth were signiﬁcant at the 95% conﬁdence level. The concentrations of OP, IP, and TP were slightly higher at the surface than at depth at station A2, and/or slightly higher at the subsurface than at depth at station A4 (Fig. 3.36, Liu et al., 2004). The depth distribution of P is related to the physical condition and exterior input of P. In the YS, the concentrations of IP increased slightly with depth, whereas OP and TP fell gradually from the surface to depth at station T4. This might indicate that slow decomposition of P occurs at station T4 and a transfer within P fractions exists at station T4. BH-A2

BH-A4

Contents (mmol/g)

Depth (cm)

0

5

YS-T4

Contents (mmol/g)

10 15 20

0

Contents (mmol/g)

5 10 15 20 25

0

0

0

0

5

5

50

10

10

15

15

100 150 200

5 10 15 20 25

IP TP OP

250 300

Fig. 3.36. The depth distribution of IP, OP, and TP (μmol/g) in core sediments at stations BH-A2, BH-A4, and YS-T4 (Liu et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

3.4.5.2 Phosphorus Speciation The diﬀerences in phosphorus speciation among stations were signiﬁcant at the 95% conﬁdence level. Sediment phosphorus distribution ranges were 0.8%∼2% for LSor-P, 3%∼10% for Fe-P, 4%∼19% for Lea-OP, 3%∼15% for CFAP, 35%∼66% for FAP, and 14%∼36% for Ref-OP (Fig. 3.37). This can partially explain the higher proportions of FAP relative to the other species of P in the sediments of the YS. Given the tremendous amount of loess-type sediment transported by rivers into the study area, FAP decreased from ∼90% in loess to ∼50% of TP in marine sediments. The other species of P obviously increased, especially Ref-OP,

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3 Biogeochemical Processes of the Yellow Sea

Fig. 3.37. The distribution plot of phosphorus speciation in the surface sediments (μmol/g) (Liu et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

which increased from <10% in loess to ∼30% of TP in sea areas. This indicates that more than 50% of TP in sediments of the Bohai Sea and YS might be derived from P naturally found in the source materials (e.g., loess), and less than 50% can be due to anthropogenic eﬀects (e.g., runoﬀ and sewage). Phosphorus load associated with eroded soil (i.e., detrital apatite) could be larger than the anthropogenic load. The CFAP and Ref-OP decreased by a factor of 2∼6 from the Bohai Sea to the YS. This is in agreement with the distribution of chlorophyll a illustrating that CFAP and Ref-OP may come from phytoplankton ﬁxation (Zhu et al., 1993; Sun et al., 2003). The relative contributions of the P speciation to the TP in the present sediments diﬀer greatly from those of deep-sea sediments (Song, 1997). Authigenic apatite was the dominant Pbearing component (61%∼86%), followed by Fe-bound P (7%∼17%), organic P (3%∼12%), adsorbed P (2%∼9%), and detrital apatite (0%∼1%). The differences in phosphorus speciation in depths of core sediments are signiﬁcant at the 95% conﬁdence level. The FAP remained stable with depth, while the other forms of phosphorus decreased by a factor of 1.3∼4.3 with depth at stations G2 and A2. There was a tendency to decrease with depth for all phosphorus forms at station A4, while most forms of phosphorus remained stable with depth at stations E1 and E3 (Fig. 3.37, Liu et al., 2004). FAP was the major form of phosphorus, accounting for ∼50% of the TP, followed by the Ref-OP which represented 20%∼30% of the TP. The sum of the ﬁrst three steps of phosphorus represented <10% of the TP, except at station G2 (ca. 17%). The phosphorus forms remained stable with depth in the central Bohai Sea, where sediments were mainly composed of clay and ﬁne silt, and the deposition environment was relatively stable (Song, 2002). At the Bohai Strait and the region near the Luanhe River Estuary, the various phosphorus forms decreased with depth, where the sediments were mainly sand/sand silt and/or

3.4 Biogeochemical Characteristics Nitrogen and Phosphorus in the Yellow Sea

335

ﬁne silt with strong dynamic conditions (e.g., tidal currents and storms). The stable distribution of various phosphorus species indicated that diagenesis of P is not signiﬁcant in core sediments and accordingly, the benthic phosphate ﬂux is very low, with diﬀusion from water to sediment. It has been reported that the concentrations of phosphate in the Bohai Sea have decreased by a factor of 2 since the 1960s, owing to a decrease in the Huanghe River discharge. This, however, was not supported by the measurements of the core sediment samples, and the direct contribution of dissolved P carried by the Huanghe River discharge to the Bohai Sea is shown in Fig. 3.36. The depth distribution of phosphorus speciation in core sediments at stations A4 (a), A2 (b), E1 (c), E3 (d), and G2 (e) (μmol/g) is rather minor (<0.1%) when compared with the P transported by suspended river matter. In ﬁeld simulation experiments, it is reported that most of P released from loess tillage during rainfall runoﬀ erosion processes is absorbed into suspended matter, and more than 99% of P was transported by suspended matter (Chen and Zhang, 1991). Experimental studies also indicated that the adsorption of phosphate by sediments of the Huanghe River was very rapid. This demonstrates that sediment loads transported by rivers play an important role in regulating the levels of dissolved P in the Bohai Sea and YS. In addition, the high proportion of FAP in TP that is not bioavailable (discussed subsequently) and the high sediment accumulation rate can partially explain why the concentration of phosphate in the Bohai Sea and YS has decreased or remained stable over the past 20∼40 years with the increase in population and more wastewater discharge. 3.4.5.3 Bioavailable Phosphorus Although the estimate of phosphorus availability is largely dependent on the extraction methods, knowledge of phosphorus speciation is required to understand the potential bioavailability of this element in coastal oceans. By means of bioassays it has been shown that LSor-P is available for algae growth, and that CFAP and FAP are almost insoluble under the physiochemical conditions of marine waters. Apatite formation kinetics is very slow and hence is not considered to be a major component to interstitial or overlying water enrichment in phosphate. In relatively well-oxygenated waters like the Bohai Sea and YS, Fe-P is considered to be insoluble and non-reactive. Ref-OP could become bioavailable by remineralization. Therefore, initially only LSor-P and organic P should be taken as potentially bioavailable in aquatic environments. In this case, 16%∼35% of TP in the Bohai Sea and NYS will be potentially bioavailable, which represents only a minor proportion of the sedimentary phosphorus pool. The diﬀerences in phosphorus speciation in depths of core sediments as revealed by incubation data are signiﬁcant at the 95% conﬁdence level. The diﬀerences in phosphorus speciation observed between incubations with either air or nitrogen are not signiﬁcant in the time scale of our experiments, except the change in concentration of Fe-P and Lea-OP after incubation with air

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and/or nitrogen. The Fe-P and Lea-OP increased by a factor of 1.7 and 1.5 after incubation, respectively, indicating a source from the overlying seawater; the benthic phosphate ﬂux in this region shows the dominance from water to sediment. The variations in phosphorus speciation were not obvious during a period of <50 h incubation. 3.4.5.4 Sedimentation and Burial of Phosphorus Removal of phosphorus from the ocean is governed solely by its accumulation in sediments. The P accumulation in sediments was estimated by sediment deposition rates, density, and the percentage of phosphorus in sediments. The sediment accumulation rates in the Bohai Sea and YS have been described extensively. The P accumulation rates of IP, OP, and TP were 0.79∼62.0, 0.19∼13.6, and 1.14∼75.6 μmol/(cm2 ·yr) in the Bohai Sea, and 0.52∼4.62, 0.02∼1.89, and 0.54∼6.19 μmol/(cm2 ·yr) in the YS, respectively. The P accumulation rates were the highest near the Huanghe River Estuary, followed by stations adjacent to the Changjiang River Estuary, and then the central basin. The P accumulation rate shows a gradient from coastal to central basin, and it is higher in the Bohai Sea than in the YS. The TP accumulation rates ranged between 0.09 and 8 μmol/(cm2 ·yr) estimated from some maginal seas (Song, 2004). The TP accumulation rates in this study are within this range except that samples in the Huanghe River Estuary illustrate very high accumulation rates. To evaluate regeneration and burial of P in sediments of the Bohai Sea and YS, the P burial eﬃciencies (PBE), that is the ratio of P accumulation rate to the sum of P accumulation rate plus the benthic phosphate ﬂux, were calculated. The TP accumulation rates were (260±179) μmol/(cm2 ·yr) in the YS. The benthic phosphate ﬂuxes determined by incubating the core top sediment with overlying water on board were (–1.1±0.4) μmol/(cm2 ·yr) from overlying water to sediment in investigation periods (1998∼1999) in the Bohai Sea. The benthic phosphate ﬂuxes were (–0.086±0.031) μmol/(cm2 ·yr) from water to sediment in spring and autumn cruises from 1998 to 2001 in the YS. The PBE for TP in the Bohai Sea and YS is almost 100%. The high values of PBE in study areas could be related to the low benthic phosphate ﬂux. This is due to a high percentage of FAP that is not bioavailable (ca. 50%) and a high sediment accumulation rate. Since the estimate of the vertical ﬂux of materials by sediment trap technique is not feasible in shallow coastal waters, e.g., the Bohai Sea, only regeneration of P in the water column of the YS was evaluated. Sediment trap studies showed that the vertical ﬂux of P measured 15 m above the bottom during 1996∼2000 was 21.2∼73.1 μmol/(cm2 ·yr) with an average of 39.0 μmol/(cm2 ·yr) in the YS. The P regeneration eﬃciency, that is the ratio of P vertical ﬂux minus P accumulation rate relative to P vertical ﬂux, is calculated to be >95%. It appears that most of the phosphorus regenerated in the water column, and almost all of the remaining P will be buried in sediments after accumulation. This occurs because the ben-

3.5 Biogeochemical Processes of Jiaozhou Bay, South Yellow Sea

337

thic phosphate released from sediments was relatively low compared to the accumulation of detrital P that is not readily regenerated (Liu et al., 2004).

3.5 Biogeochemical Processes of Jiaozhou Bay, South Yellow Sea Jiaozhou Bay is a semi-enclosed bay situated in the western part of the Shandong Peninsula, northern China. The bay is surrounded by Qingdao City with an area of about 340 km2 and an average water depth of about 7 m. The bay mouth is narrow, only about 2.5 km wide connecting the bay with the south YS. There are more than 10 small rivers along the Jiaozhou Bay coast, and the largest river is the Dagu River with an annual average runoﬀ of 6.61×108 m3 . Most of these rivers have become discharge trenches for industrial and human waste from Qingdao City. The water quality, biological species and abundance, as well as nutrient concentrations in Jiaozhou Bay have changed recently along with the rapid development of Qingdao City. 3.5.1 Behaviour and Variation of Carbon The three core samples were collected by box-sampler, whose locations were as shown in Fig. 3.38.

Fig. 3.38. Jiaozhou Bay map (Li et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

3.5.1.1 Carbon Processes across the Air-Sea Interface The role of shelf seas has not been fully understood yet. Some results indicated that the shelf sea is a source for atmospheric carbon dioxide. However, other

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3 Biogeochemical Processes of the Yellow Sea

studies showed that it can absorb 0.2∼1 pg C every year (Song, 2004). That is to say, whether the shelf sea acts as a net sink or a source of atmospheric CO2 is uncertain now, because the shelf sea is strongly aﬀected by both nature and human activities. On the one hand, the gradient of partial pressure between air and seawaters will be reversed with the continual increase in atmospheric CO2 , so the character of source or sink may change. For example, the Weddell Sea was a relatively strong source of atmospheric CO2 in pre-industrial times, but it turned into a CO2 sink in recent times because of the steadily rising atmospheric CO2 . On the other hand, the shelf seas and coastal areas carbon budget has been altered dramatically by human activities. The change in air-sea ﬂuxes for the coastal ocean since pre-industrial times was modeled in detail and it suggested that the shallow-water ocean environment has served as a net source of CO2 into the atmosphere throughout most of the past 300 years, but its role as a source has substantially decreased and the net ﬂux is expected to reverse at some point in time. The main reason is down to human activities (Anderson and Mackenzie, 2004; Song et al., 2008). Sabine and Mackenzie (1991) estimated that the ﬂux of nutrient from river to coastal sea has increased 2.5-fold due to human activities. In addition, particle organic carbon, dissolved organic carbon and dissolved inorganic carbon showed an apparent increase, too. Additional mass input has resulted in conﬂicting conclusions as to whether the shelf sea acts as a net sink or as a source of CO2 into the atmosphere. For example, the SW of the Caribbean Sea was a source of CO2 into the atmosphere throughout 2002, but the NE was a sink during winter and spring and a source during summer and autumn. Although some surveys have estimated the air-sea CO2 ﬂux of the coastal sea, we need more data to understand fully the coastal contribution to marine carbon cycling. (1) Spatial and temporal distributions of DIC The contents of DIC and the diﬀerent components of the CO2 system in Jiaozhou Bay surface water for four seasons during 2003∼2004 are shown in Table 3.27. The seasonal distributions of DIC are shown in Fig. 3.39. The seasonal variation of DIC contents was not clear, and its average content varied from 2,025 to 2,044 μmol/kg. Although the horizontal distribution showed diﬀerent patterns in diﬀerent seasons, they showed a general trend that DIC in the inner bay was higher than that in the outer bay except in July. The characteristics of DIC distribution were mainly determined by the growth of plankton and the hydrographic conditions in Jiaozhou Bay. In particular, the Haibo River, the Licun River, and the Loushan River were main discharge rivers for industrial waste and sewage from Qingdao City on the northeastern shore of Jiaozhou Bay. The Dagu River discharged industrial and human waste and sewage from urban districts on the northwestern shore. Meanwhile, the western and northern shores were mollusk cultivation regions, and the southeastern shore and southwestern shore were port regions. Therefore, the seawater in the inner bay accepted a large mount of industrial and domestic sewage, which was decomposed ultimately into CO2 . On the other hand, the

3.5 Biogeochemical Processes of Jiaozhou Bay, South Yellow Sea

339

water exchange to the YS is slow and the average water residence time is about 80 days in the inner bay. These two reasons resulted in DIC in the inner bay being higher than that in the outer bay. However, the reason that DIC in the outer bay is higher than that in the inner bay might be related to phytoplankton blooming in July. In July, the phytoplankton grew faster in Jiaozhou Bay and it decreased from the inner to outer bay (Wu et al., 2004), so the consumed DIC in the inner bay is relatively high and resulted in DIC in the inner bay being lower than that in the outer bay. Table 3.27. Carbon dioxide system concentrations and CO2 ﬂuxes in Jiaozhou Bay (Li et al., 2008) (With permission from Elsevier’s Copyright Clearance Center) Month 2

Range Average 6 Range Average 7 Range Average 11 Range Average

DIC

Salinity

(μmol/L)

(‰)

1765∼2465 2044 1913∼2171 2027 1856∼2414 2042 1642∼2318 2025

32.37∼32.97 32.71 32.03∼32.61 32.33 29.19∼31.11 30.60 29.07∼30.64 30.01

Water temperature (◦ C) 3.87∼4.63 4.16 15.22∼19.21 17.5 20.22∼22.22 21.20 10.52∼14.79 12.43

Wind speed (m/s) 5.1∼6.8 5.9 4.3∼5.8 5.1 4.2∼5.3 4.6 5.8∼7.1 5.9

PCO2

Chl a 3

(mg/m ) (μatm) 2.0∼20.0 92∼595 317 1.5∼6.5 452∼864 640 1.5∼6.0 500∼903 695 1.0∼4.5 279∼593 419

CO2 ﬂux (mmol/(m2 ·d)) −25.6∼21.6 −2.91 5.35∼30.6 16.9 7.23∼28.8 17.7 −8.57∼22.6 5.40

N 36 10'

36 00'

N 36 10'

36 00' 120 10'

120 20'

120 30' E

120 10'

120 20'

120 30' E

Fig. 3.39. Horizontal distributions of sea surface DIC (μmol/L) in the diﬀerent seasons in Jiaozhou Bay (Li et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

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3 Biogeochemical Processes of the Yellow Sea

(2) Seasonal variability of PCO2 The distribution and the contents of PCO2 in Jiaozhou Bay seawater are shown in Fig. 3.40 and Table 3.27, which indicated strong seasonal variation. PCO2 was the lowest with a value of 311 μatm in February and was the highest with a value of 695 μatm in July. The CO2 contents in most waters were oversaturated with respect to atmospheric CO2 . However, the under-saturated waters were found in the southern inner bay during February and the bay mouth during November. To discuss such a seasonal variation in PCO2 , three major processes can be envisaged: (i) the river input from ambient rivers; (ii) the variation in temperature; (iii) the biological activity in the water column. Because the yearly change of river input and DIC contents in Jiaozhou Bay is little, we thought the river input was not the main reason for the seasonal variation of PCO2 . However, the average PCO2 decreased from July, June and November to February. The higher the surface seawater temperature, the higher the PCO2 . Fig. 3.41 showed that water temperature shows a marked change as the seasons alternate. Temperature aﬀects the equilibrium constants of dissolved inorganic carbon and, in particular, the solubility coeﬃcient of CO2 . The seasonal change in PCO2 showed that the increase in physical solubility of CO2 , which was caused by the decrease in seawater temperature, was one important reason for the variability of the CO2 source/sink for the coastal sea. Therefore, temperature plays a key role in PCO2 variation. Similar to DIC, phytoplankton may inﬂuence PCO2 in its bloom season. Although the salinity in the inner bay is a little lower than that in the outer bay because of

N 36 10'

36 00'

N 36 10'

36 00' 120 10'

120 20'

120 30' E

120 10'

120 20'

120 30' E

Fig. 3.40. Horizontal distributions of sea surface PCO2 (μatm) in the diﬀerent seasons in Jiaozhou Bay (Li et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

3.5 Biogeochemical Processes of Jiaozhou Bay, South Yellow Sea

341

river-water input in all seasons, the distribution was clearly diﬀerent to DIC and PCO2 (Fig. 3.42), which showed that salinity has an apparent inﬂuence on DIC and PCO2 .

N 36 10'

36 00'

N 36 10'

36 00' 120 10'

120 20'

120 30' E

120 10'

120 20'

120 30' E

◦

Fig. 3.41. Horizontal distributions of sea surface temperature ( C) in the diﬀerent seasons in Jiaozhou Bay (Li et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

N 36 10'

36 00'

N 36 10'

36 00' 120 10'

120 20'

120 30' E

120 10'

120 20'

120 30' E

Fig. 3.42. Horizontal distributions of sea surface salinity (‰) in the diﬀerent seasons in Jiaozhou Bay (Li et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

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3 Biogeochemical Processes of the Yellow Sea

(3) Seasonal variety of CO2 ﬂuxes In order to assess the CO2 seasonal cycle, the monthly CO2 ﬂuxes in June, July and November 2003 and February 2004 were calculated. The uncertainty of the ﬂux was less than 15% due to the variety of PCO2 . In February 2004, average of CO2 ﬂuxes in the Jiaozhou Bay was (–2.91±0.44) mmol C/(m2 ·d), which indicated that Jiaozhou Bay absorbed atmospheric CO2 . However, it was a weak sink compared with the adjacent YS CO2 ﬂuxes of –14.79 mmol C/(m2 ·d) in winter. In June 2003, average CO2 ﬂuxes were (16.9±2.5) mmol C/(m2 ·d), which indicated that Jiaozhou Bay released CO2 into the atmosphere. It was a strong source of atmospheric CO2 and similar to the YS in spring. In July 2003, average CO2 ﬂuxes were (17.7±2.6) mmol C/(m2 ·d), which indicated that Jiaozhou Bay released CO2 into the atmosphere, but the source of atmospheric CO2 was stronger than that in summer in the YS. In November 2003, CO2 ﬂuxes were (5.4±0.81) mmol C/(m2 ·d), which indicated that Jiaozhou Bay released CO2 into the atmosphere. This was consistent with that (about 3 mmol C/(m2 ·d)) in the YS in autumn (Song, 2004), but the source was weaker than that in summer. Although Jiaozhou Bay released CO2 into the atmosphere in summer and autumn or absorbed atmospheric CO2 in winter which was consistent with the YS, the discrepancy in strength was very great. In addition, Jiaozhou Bay released CO2 into the atmosphere in spring, which is reversed in the YS. The main reason was that the seawater temperature in spring increases faster than that in the YS because of the shallow water in Jiaozhou Bay. Although Jiaozhou Bay sometimes acted as a source or sink of atmospheric CO2, the variability in diﬀerent regions was obvious lately, especially in February and November. CO2 ﬂuxes in diﬀerent regions are shown in Fig. 3.43. The characteristics of the carbon source/sink were absolutely the opposite in diﬀerent regions of Jiaozhou Bay. In February, seawater in the inner bay absorbed atmospheric CO2 , but released CO2 into the atmosphere in the bay mouth and outer bay. The distribution in June was similar to that in July. The whole region released CO2 into the atmosphere and the strength decreased from the inner bay to bay mouth and to the outer bay. In November, seawater released

Fig. 3.43. Average CO2 ﬂuxes in diﬀerent regions of Jiaozhou Bay (Li et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

3.5 Biogeochemical Processes of Jiaozhou Bay, South Yellow Sea

343

CO2 into the atmosphere in the inner bay and outer bay, but absorbed atmospheric CO2 in the bay mouth. (4) The strength of the carbon source/sink and its inﬂuencing factors Using the CO2 ﬂuxes as mentioned above and taking the water area of Jiaozhou Bay as 340 km2 , the strength of the carbon source/sink for 4 measured months was calculated as follows: 1.53×1011 mmol C in June, 1.61×1011 mmol C in July, 4.90×1011 mmol C in November but absorbed 2.65×1011 mmol C in February. Many researchers reported that the surface seawater temperature (SST) is a primary factor that controls the variation of sea surface PCO2 . Between 40◦ and 60◦ latitude in both the northern and southern hemispheres, heat ﬂux resulting in low temperature has long been recognized to be a major mechanism that causes the area to be a net sink of atmospheric CO2 . So SST should play a key role in CO2 ﬂuxes across the seawater-air interface. According to long-range observation data of the hydrography, wind, and salinity, which are the important inﬂuencing factors for CO2 ﬂuxes, these changed little in one season, and only the temperature changed very much. Because the ﬂux of CO2 is a function of various factors such as wind speed, salinity, temperature, and so on, Fig. 3.44 shows the statistical results between FCO2 and the measured temperature in June, July and November 2003 and February 2004, which indicated that they had a good linear relationship. This good correlation suggested that temperature was the most important force for CO2 ﬂux variations in Jiaozhou Bay. Based on the statistical results, Jiaozhou Bay was a source of atmospheric CO2 when the surface seawater temperature is higher than 6.6 ◦ C, and a sink when the surface seawater temperature is lower than that. SST also aﬀects the equilibrium constants of dissolved inorganic carbon and, in particular, the solubility coeﬃcient of CO2 . Solubility, the ﬁrst and second apparent ionization constants of H2 CO3 decreased with the increase in SST, so PCO2 rises with them. It is reported that PCO2 may rise by ∼4% with temperature increases of 1 ◦ C. In Jiaozhou Bay, 6.6 ◦ C was the critical temperature, PCO2 in seawater is higher than atmospheric CO2 partial presCO2 fluxes ( 107 mmol C)

25.0 20.0 15.0 10.0 5.0 0.0 -5.0 -10.0

y=1.1783x-7.7904 2 R =0.9723

0

5

10 15 20 Temperature ( )

25

30

Fig. 3.44. Relationship between CO2 ﬂuxes and SST in Jiaozhou Bay (Li et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

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3 Biogeochemical Processes of the Yellow Sea

sure when the temperature is higher than 6.6 ◦ C; otherwise, PCO2 is lower than atmospheric CO2 partial pressure. In order to assess the annual variability of carbon source/sink, CO2 ﬂuxes were calculated from January to December respectively using the regressive equation mentioned above and the average surface seawater temperature between 2002 and 2004 (Table 3.28 and Fig. 3.45).

Winter 2 3 6.0 5.5 4.3 6.0 31.9 −2.17 −2.66 −0.84

Spring 4 5 6 5.8 5.3 5 9.6 13.9 18.2 31.55 3.86 9.39 15.0

CO2 fluxes ( 107 mmol C)

Table 3.28. Hydrographic and meteorological parameters and CO2 ﬂuxes in Jiaozhou Bay during one year (Li et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

4

1 6.1 4.9

Summer 7 8 9 4.7 4.7 5.1 22.0 24.7 24.5 30.80 19.9 23.5 23.2

Autumn 10 11 12 5.5 6.4 6.3 20.0 14.8 10.0 31.63 17.3 10.7 4.38

Wind speed* SST (◦ C)** Salinity (‰)*** CO2 ﬂuxes (mmol/(m2 ·d)) 18.7 22.1 21.1 16.3 9.67 4.11 Total CO2 ﬂuxes −2.03 −2.50 −0.79 3.51 8.83 13.7 (×107 mol C) * From China bay records (The edit committee of China bay records, 1992), average wind speed between 1960 and 1979; ** Average SST was observed at a ﬁxed station every day by the Jiaozhou Bay Marine Research Station between 2002 and 2004; *** From Yang and Wu (1999)

25.0 20.0 15.0 10.0 5.0 0.0 -5.0

1

2

3

5

6 7 Month

8

9 10 11 12

Fig. 3.45. Monthly CO2 ﬂuxes in Jiaozhou Bay (Li et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

Jiaozhou Bay was a sink of atmospheric CO2 in winter due to the low temperature, which absorbed about 5.32×1010 mmol C. In spring, it was a source of atmospheric CO2 and released 2.60×1011 mmol C. In summer, it was a strong source for higher temperature and released 6.18×1011 mmol C. In autumn, as in the spring, it was a source and released 3.01×1011 mmol C. Over the year, Jiaozhou Bay acted a net source of atmospheric CO2 , and released 1.13×1012 mmol C. On a global scale, some researchers reported that marginal seas acted as an atmospheric CO2 sink, while some others did not, they considered marginal seas to be the source of CO2 . For example, the Caribbean Sea was a source

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345

of CO2 into the atmosphere throughout 2002, but the northern and eastern parts were net annual sinks of CO2 of around 0.5∼1 mol C/(m2 ·yr), and the southwestern parts were net sources of CO2 of about 0.5∼1 mol C/(m2 ·yr). The South Atlantic Bight (SAB) is a strong source of CO2 into the atmosphere with an average rate of 2.5 mol C/(m2 ·yr) (Wang, 2003). The northern South China Sea acts as a source with an average sea-to-air CO2 ﬂux of 7 mmol CO2 /(m2 ·d) in the summer and 1∼3 mmol CO2 /(m2 ·d) in the spring and fall (Zhai et al., 2005). Compared to these regions, Jiaozhou Bay is a strong source of atmospheric CO2 on the whole. Fig. 3.45 shows the monthly change in the carbon source/sink for Jiaozhou Bay, and indicated that it was a sink of atmospheric CO2 from January to March, and acted strongest in February. Then, it was a source from April to December and especially from July to September. In one year, Jiaozhou Bay changed from source to sink between December and January, and it changed from sink to source between March and April. The growth of phytoplankton will continually consume CO2 in seawater, and then it is propitious to transport CO2 from the atmosphere to seawater. Sequentially, the strength of the carbon source will be weakened, or the strength of the carbon sink will be enhanced. The growth in phytoplankton had obvious seasonal variability in Jiaozhou Bay. Long-term observation of the quantitative variation for chlorophyll a showed that the peak of chlorophyll a value frequently occurred in winter and summer, with a mean of (4.72±3.15) and (4.33±2.57) mg/m3 respectively. In spring, the mean of chlorophyll a dropped to (2.78±2.43) mg/m3 and the lowest concentration was obtained in autumn, only (1.95±0.80) mg/m3 (Wu et al., 2004). However, the highest phytoplankton concentration was in February, and then it decreased gradually. From May, the concentration of chlorophyll a increased again with the increase in water temperature, and reached its second peak in August, but the values were only half of those in February. The seasonal changes in chlorophyll a are higher in the northern shallow area than those in the southern bay and outer bay. The highest value occurred in February (monthly average concentration was 15.52 mg/m3 ), which was nearly 30 times higher than that in December (0.55 mg/m3 ) in the north of the bay (Li FY et al., 2006). The horizontal distribution patterns showed that the concentration of chlorophyll a decreased from the northern to southern inner bay, which is higher than that in the outer bay. Comparing the distribution of chlorophyll a to PCO2 , they were completely consistent in February. However, the strength of the carbon source/sink did not go all the way with the total phytoplankton biomass. For example, Jiaozhou Bay was a stronger source in September but the phytoplankton bloom did not turn it from source to sink. Therefore, it was obvious that the inﬂuence of phytoplankton was limited in months when the carbon source was stronger, and it could not decide the strength of the carbon source in high temperature seasons. Nevertheless, phytoplankton might still play an important role in the transformation of the carbon source or sink in regions where the source or sink is weaker. For example, phytoplankton abundance

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in the inner bay is higher than that in the bay mouth. Moreover, the inner bay was a sink of atmospheric CO2 and the bay mouth was a weaker source in February. Here, phytoplankton played a key role in the transformation of the carbon source or sink. The monthly variability of the carbon source/sink was consistent with the change in seawater temperature. The annual change in seawater temperature is very prominent in Jiaozhou Bay. The seawater temperature in August is the highest in the year and it is the lowest in February. According to the consistency of the distribution of the surface seawater temperature with the strength of the carbon source/sink, Jiaozhou Bay is a strong carbon source in August with the highest seawater temperature and it is a sink in February with the lowest temperature. The strength of the carbon source continually decreases or turns to a carbon sink ultimately as the temperature changes from the highest to the lowest. It is clear that the inﬂuence of temperature on the carbon source/sink is very important in Jiaozhou Bay. Under the conditions of global warming, the seawater temperature will increase along with global warming, the increase in the strength of the carbon source will be inevitable and Jiaozhou Bay may turn from sink to source in lower temperature months. The contents of dissolved inorganic carbon (DIC) had no obvious seasonal variations with an average of 2,035 μmol/kg, but the horizontal distributions had diﬀerent patterns in diﬀerent seasons. The partial pressure of CO2 in surface seawater changed with the seasons. PCO2 was the lowest in February with a value of 317 μatm and was the highest in July with a value of 695 μatm. The higher the temperature of the surface seawater, the higher was PCO2 . Jiaozhou Bay absorbed atmospheric CO2 with CO2 ﬂuxes of –2.91 mmol C/(m2 ·d) in February. It released CO2 into the atmosphere with CO2 ﬂuxes of 16.9 mmol C/(m2 ·d) in June, 17.7 mmol C/(m2 ·d) in July, and 5.4 mmol C/(m2 ·d) in November. The inner bay absorbed atmospheric CO2 , but the bay mouth and outer bay released CO2 into the atmosphere in February. The distribution in June was consistant with that in July but the strength of the source was diﬀerent. All regions of Jiaozhou Bay released CO2 into the atmosphere, and the released CO2 decreased from the inner bay to bay mouth and to the outer bay. The inner bay and outer bay released CO2 into the atmosphere and the former ﬂux was higher than the latter, but the bay mouth absorbed atmospheric CO2 in November. The water temperature plays a key role in carbon source/sink changes in Jiaozhou Bay. Jiaozhou Bay released 2.60×1011 mmol C into the atmosphere in spring, 6.18×1011 mmol C in summer, and 3.01×1011 mmol C in autumn, while 5.32×1010 mmol C was absorbed from the atmosphere in winter. 1.13×1012 mmol C was released into the atmosphere over one year. Although phytoplankton had a smaller inﬂuence on the carbon source/sink than the seawater temperature did, it could change the character of the carbon source or sink in the local region in the lower temperature season (Li et al., 2008).

3.5 Biogeochemical Processes of Jiaozhou Bay, South Yellow Sea

347

3.5.1.2 Carbon Burial Process in the Sediments from Jiaozhou Bay Coastal areas play a vital role in the global carbon cycle either as sources of organic matter (OM) to the open ocean or as carbon sinks due to accumulation of OM in sediments. As an accumulation of “geochemical fossils”, the organic matter content of coastal sea sediments provides information that is important to interpretation of coastal sea paleoenvironments, histories of climatic change, and the eﬀects of human activities on local and regional ecosystems. The primary source of organic matter in coastal sea sediments is from the particulate detritus of plants; only a few percent came from animals. Plants can be divided into two geochemically distinctive groups on the basis of their biochemical compositions: (1) nonvascular plants that contain little or no carbon-rich cellulose and lignin, such as phytoplankton, and (2) vascular plants that contain large proportions of these ﬁbrous tissues, such as grasses, shrubs, and trees. Because the reactivities of higher plants and microbial OM are quite diﬀerent, assessing autochthonous marine and allochthonous terrestrial inputs in coastal areas is of great importance. The relative contributions from these two plant groups to sedimentary records are inﬂuenced strongly by morphology, watershed topography, climate, abundance of coastal sea plants and watershed plants. Studies of organic matter in the sediments from diﬀerent parts of the world have been used to help reconstruct records of regional and continental paleoclimates. An important component of paleolimnologic investigations is to identify the sources of organic matter in sediments deposited at diﬀerent time in the past. Variations of C:N ratios within sediments have been used to determine historical changes in sources of organic matter. Algae have a C:N ratio between 4 and 10, whereas terrestrial organic matter has a C:N ratio greater than 20. Increase in the C:N ratio within sediment proﬁles has been interpreted to identify periods in sedimentary history when sediments received a high proportion of terrestrial organic matter. Conversely, a decrease in C:N ratios has been used to identify periods when sediments have received a high proportion of algal organic matter. The C:N ratio was used to trace the variations in the organic matter source in Jiaozhou Bay sediment, and to examine the changes in the organic matter burial rate during the past one hundred years. 3.5.2 Historical Variation of Nitrogen To gain information of environmental changes and to gain insight into development trends in Jiaozhou Bay, studies were made based ﬁrstly on the determination of chrono-geochemical record, and then useful environmental information from sedimentary sequences of a known age was extracted. The age-depth relationships in sediment can be estimated using short-lived isotope 210 Pb dating. On the basis of sedimentation dating, the abundance and temporal changes of nitrogen could be obtained.

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3 Biogeochemical Processes of the Yellow Sea

3.5.2.1 Organic Carbon Source and Burial (1) 210 Pb chronology and sedimentation rate The core sediment was dated using 210 Pb chronology from radioactive fallout. The 210 Pb and excess 210 Pb activities were plotted on a log scale versus the depth (Fig. 3.46, Dai et al., 2007a). As for B3, the 210 Pb activity reached equilibrium with excess 210 Pb of 0.77 dpm/g at 59 cm, and 210 Pb exhibited obvious exponential fallout above 59 cm. Below 59 cm, it was the background value of the core sediment. For D4, the 210 Pb activity assumed exponential fallout above 17 cm. Moreover, there is another fallout segment indicating that there is environmental change. For D7, located outside of the bay, it showed that there is perfect fallout and there are no mixed layers on top sediments. The fallout of 210 Pb exhibited clear distribution with two periods: fallout and background. In the study, a Least Square was employed to calculate the average sedimentation rate and the equilibrium can be expressed as follows (Li et al., 2003; 2006): DR =

Hλ ln IIh0

(3.18)

The excess 210 Pb activities were calculated by subtracting the 210 Pb background activity (226 Ra-supported) from the total activities. The background activity at the core was estimated as 210 Pb activities which have decayed to a low constant level at a certain depth. According to equation (3.18), equation (3.19) could be deduced as follows: lg AH = lg A0 −

λ×H 2.303 × S

(3.19)

Equation (3.19) could be considered as a One-Place Linear Equation, i.e., y = b + kx. In the equation, lgAH and H were the dependent variable and independent variable respectively, and then the slope was represented by k and expressed in equation (3.20). A Least Square Algorithm was employed to obtain the linear regression equation, and then the average sedimentation rate could be calculated according to equation (3.21). λ (3.20) 2.303 × S λ S=− (3.21) 2.303 × k The rapid increase in sedimentation was observed in the sediment record of Jiaozhou Bay, which is also a common phenomenon in coastal areas, especially in recent years. Intensiﬁcation of land use, rapid population growth, deforestation, and urbanization have resulted in a signiﬁcant increase in the delivery of sediments to the bay since the 1980s. Before the 1980s, the sedimentation rates of the inner bay and outer bay maintained a relatively steady k=−

3.5 Biogeochemical Processes of Jiaozhou Bay, South Yellow Sea

0.1 0

210

Pb activity (dpm/g) 10 1

0.1 0

Pb activity (dpm/g) 10 1

0.1 0

40 60

349

Pb activity (dpm/g) 10 1

10

20

Depth (cm)

Depth (cm)

20

210

Depth (cm)

210

40

20 30

60 40

80 B3 100

80

D7

D4 50

Fig. 3.46. 210 Pb activity in three cores in Jiaozhou Bay sediments (Dai et al., 2007a) (With permission from Elsevier’s Copyright Clearance Center)

increase indicating a comparatively stable sedimentary environment. However, the sedimentation rate in the bay mouth sharply decreased from the 1930s to 1940s because the Xin-an River emptied into the bay almost dried up in the period. Climatic changes such as large ﬂood events were not observed in the past one hundred years in Jiaozhou Bay, so the general accelerating trends in the sedimentation rate from the 1980s would rather reﬂect the increasing eﬀect of human impact. With the rapid urbanization and economic growth from the 1980s, increased eﬄuent from industry and agriculture will induce an increase in sediment discharge, which may be related to long-term increasing trends in the sedimentation rate between 1980 and 2003. Because Qingdao City was selected to be the co-host of the 2008 Beijing Olympic Games, Licun, Haipohe, and Tuandao wastewater treatment plants, which are distributed on the east bank of the bay, had been built to improve the water quality, and the garbage from industry and agriculture was not allowed to be dumped into the bay directly. These measures might have accounted for the decrease in the sedimentation rate since the beginning of this century. In addition, the deforestation and dam construction in the rivers entering the bay can also aﬀect the input of matter from the rivers. For example, dams have been built since the 1950s, which have reduced the sand inputs to the bay to some degree. But the inﬂuence is rather weak compared with the sewage and wastewater discharge into the bay. Thus, the sedimentation rates in Jiaozhou Bay increased on the whole. (2) Total organic carbon (TOC) TOC in Jiaozhou Bay sediment was less than 0.5% which decreased from the inner bay to bay mouth and then to the outer bay in the surface sediment (Fig. 3.47). In general, organic carbon content in core sediments decreased in the upper 30 cm, then maintained a relatively constant level below this.

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3 Biogeochemical Processes of the Yellow Sea

However, the vertical distribution of organic carbon content is complicated in Jiaozhou Bay core sediment. At station B3, the organic carbon content decreased from 2 to 0 cm (2000 to 2003), stabilized from 8 to 2 cm (1993 to 2000), and then increased from 26 to 8 cm (1951 to 1993). Thus, the highest value (0.5%) was attained between 1993 and 2000. At station D4, the organic carbon content increased from 12 to 0 cm (1977 to 2003), decreased from 22 to 12 cm (1948 to 1977), but was relatively constant prior to 1948. At station D7, the organic carbon content increased from 4 to 0 cm (1997 to 2003) and decreased below the depth (prior to 1997). It is obvious that the organic carbon content variation was not caused by organic matter remineralization but had a close relationship with the input of solid waste and sewage from Qingdao City because the variation is highly consistent with the solid waste and sewage input.

0 0

Organic carbon content (%) 0.2 0.4

0.6

20

Depth (cm)

40

60

80

100

B3 (inner bay) D4 (bay mouth) D7 (outer bay)

Fig. 3.47. Organic carbon contents in Jiaozhou Bay (Dai et al., 2007a) (With permission from Elsevier’s Copyright Clearance Center)

(3) Burial ﬂux of organic carbon The burial ﬂux of organic carbon in marine sediments can be determined by the supply and preservation of organic matter into the sediments. However, the mechanisms that govern OC burial in margins are controversial and have historically focused on the importance of marine processes. Productivity, the bottom water oxygen level, the sediment accumulation rate, sediment porosity, microbial activity, bioturbation rates, and organic matter composition can all inﬂuence OC preservation in sediments. In general, three pieces of information are required to calculate the burial ﬂux in marine sediments: the organic carbon content of sediments accumulating below a deﬁned horizon, the recent

3.5 Biogeochemical Processes of Jiaozhou Bay, South Yellow Sea

351

sedimentation rate and the dry bulk density. The burial ﬂux (BF) can be calculated based on equations (3.15) and (3.16). The burial ﬂuxes of organic matter were calculated and described in Fig. 3.48. The burial ﬂuxes of organic carbon began to increase in the 1980s with a peak at the end of the last century. At the beginning of this century, with measurements adopted to manage pollution and eutrophication, the environmental quality has been improved greatly with a drop in the organic carbon burial ﬂux. The variation in the burial ﬂux of organic carbon in the past one hundred years can be divided into the following three stages: (1) a relatively steady stage before the 1980s, (2) a rapidly increasing stage from 1980 to a peak in the 1990s, and (3) a decreasing stage from the 1990s to the present. The change is consistent with the amount of solid waste and sewage discharged into the bay. In the middle of the 1980s, the amount of industrial solid waste emptied into the bay was approximately 290.43×104 t/yr and municipal garbage was approximately 33×104 t/yr. However, the total solid waste decreased to 60×104 t/yr in recent years. The amount of sewage discharged into the bay was approximately 83.5×106 t in 1980, increased to 145.6×106 t in 1987, and 193×106 t in 1995. Although the sewage increased steadily from 1980, the amount of sewage cleared by wastewater treatment work increased from 2000. At present the percentage of treated sewage has reached 75.63%. To reduce the amount of waste and sewage, many techniques such as increased regulation of wastewater discharge, reduction of nitrogen, phosphorus fertilizers, increased industrial waste recycling, and cleaner industrial processes have been applied from the 1990s. Thus, the burial ﬂuxes of organic matter decreased from the 1990s to the present.

Burial flux (mg/(cm2 yr))

6

B3 (inner bay) D4 (bay mouth) D7 (outer bay)

4

2

0 1800

1850

1900 Year

1950

2000

Fig. 3.48. Burial ﬂuxes of organic matter during diﬀerent periods in Jiaozhou Bay sediment (Dai et al., 2007a) (With permission from Elsevier’s Copyright Clearance Center)

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3 Biogeochemical Processes of the Yellow Sea

(4) Variations in organic carbon sources in the past one hundred years The ratio of the nutrient is regarded as an indicator of whether the source of organic carbon is autogenetic or allochthonous. If all the organic matter in sediment comes from marine phytoplankton, the ratio of C:N:P would be close to the Redﬁeld value (106:16:1), and the ratio of OC:TN could be about 6.6. If the organic matter comes from terrestrial sources, the ratio of OC:TN would be more than 20, in general. The higher the percentage of terrestrial organic matter, the greater the ratio of OC:TN. The validity of using C:N ratios to discern changes in organic carbon sources has been questioned because the C:N of terrestrial organic matter decreases during diagenesis, whereas that of algae increases. In addition, the C:N ratios record of a particular coring site may not provide an accurate representation of changes in the entire sea. However, many studies show that C:N ratios in sediments can be used reliably to identify historical sources of sedimentary organic matter, and indicate human disturbance of watersheds. Milliman et al. (1984) used the ratio of OC:TN to evaluate the source of organic matter in the Yangtze River Estuary. It was suggested that if the ratio of OC:TN is above 12, the organic matter was landderived, and if the ratio is below 8, the organic matter would be indicated as being from marine autogeny. In addition, it was also observed that the organic matter in particles came mainly from the Yangtze River Estuary in winter. Subsequently, Cai et al. (1992) evaluated the source of organic matter in the particles in the Yangtze River Estuary using the method of stable isotope (δ 13 C) and the ratios of OC:TN. The results of the two methods lead to the same conclusion, which improved the applicability of OC:TN ratios for indicating the source of organic matter. Large diagenetic changes in C:N ratios, which would have led to an overlap of terrestrial and algal C:N ratios, were not evident in either the surface or core sediments. Therefore, the signiﬁcant increase in C:N for surface sediments was most likely caused by an increase in the proportion of terrestrial organic matter in sediments. The proportion of terrestrial organic matter could have risen because of increased particulate matter loads and discharges of streams directly following deforestation. Organic matter in Jiaozhou Bay sediment was land-derived or marine autogenic. The former came from industrial and agricultural activities along the Jiaozhou Bay coast from surrounding rivers; the latter was mainly from primary production in the bay. Qian et al. (1997) brought forward a formula that was used to evaluate the approximate percentage of the terrestrial source or autogenic source, and then the nitrogen or phosphorus source. The formula uses the ratio of TOC:TN to determine quantitatively if the organic carbon and nitrogen were hydrophytic or terrestrial. With this method, it is proposed that the ratio of C:N in hydrophytic organic matter and in terrestrial organic matter is 5 and 20, respectively (Jia et al., 2002). The formulae are given as T OC = Cl + Ca T N = Na + Nl

(3.22) (3.23)

3.5 Biogeochemical Processes of Jiaozhou Bay, South Yellow Sea

Ca /Na = 5 Cl /Nl = 20

353

(3.24) (3.25)

where TOC and TN are the measured values, Cl (l means from land) is the content of organic carbon from land, Ca (a means from autogenous) is the content of organic carbon from marine autogeny, Nl is the content of nitrogen from land, and Na is the content of nitrogen from marine autogeny. Then, the following equations can be induced from the above formulae: Ca = (20T N − T OC)/3 Cl = 4(T OC − 5T N )/3

(3.26) (3.27)

The ratio of OC:TN in Jiaozhou Bay sediment ranged between 6.48 and 17.92 (Fig. 3.49). It was much greater at station B3 in the inner bay and station D4 in the bay mouth than that in the outer bay because the former stations had more intake of terrestrial matter in the sediments from rivers. On the contrary, the outer bay received more marine autogenic organic matter in sediment that resulted in a low OC/TN ratio. The similar variation between station B3 in the inner bay and station D4 in the bay mouth indicated that they have similar sources of organic carbon over the past one hundred years. However, the ratios of OC:TN in station D7 are slightly higher than those in station B3. In the bay mouth the area is small and wastewater produced by industry and agriculture in Huangdao and its surrounding area was drained into this region by the Xin-an River. In addition, a number of ships shuﬄing

0

0

5.00

OC/TN ratio 15.00 10.00

20.00

Depth (cm)

20

40

60

80

100

B3 D4 D7

Fig. 3.49. Vertical distributions of OC:TN ratio in Jiaozhou Bay sediment (Dai et al., 2007a) (With permission from Elsevier’s Copyright Clearance Center)

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3 Biogeochemical Processes of the Yellow Sea

2~4

2003 2000

4~6

1994

0~2 2~4

2003 1999

4~6

1997

8~10

1993

10~12

1989

12~14

1983

8~10

1992

10~12

1984

12~14

1977

14~16

1962

14~16

1976

18~20

1955

18~20

1960

1948 22~24 0 50 100 0 Content of organic carbon from land (%) 22~24

50

Year

D7 (outer bay)

D4 (bay month) 0~2

Year Depth (cm)

B3 (inner bay) 2003 0~2 2000 2~4 1999 4~6 1993 8~10 1990 10~12 1987 12~14 1985 14~16 1977 18~20 1964 22~24 1951 26~28 1938 30~32 1912 38~40 0 50 100

Year Depth (cm)

Depth (cm)

through the port near the bay mouth might bring a mass of organic matter, and result in a higher percentage of external organic matter to the bay mouth. The organic carbon from the inner bay (station B3) and the bay mouth (station D4) has a predominantly terrestrial source, and more than 70% terrestrial organic carbon input (Fig. 3.50). However, the terrestrial matter is about half of the source in the outer bay (station D7). In the inner bay (station B3), the terrestrial source was steady with a range of 69%∼77% before 1990. The percentage of organic carbon from land increased to 93% in 2000, which is consistent with the increase in solid waste and sewage discharged into the bay in this period. From 2000, the percentage of organic carbon from the land decreased due to the decrease in terrestrial input. In the bay mouth (station D4), the percentage of organic carbon from land reached the highest value with 94% in 1994. The source matter remained steady in the outer bay. The lower percentage of organic carbon from the land in 1946 and 1999 may be caused by phytoplankton bloom in this period.

1946 100

Fig. 3.50. Variation of organic matter source in Jiaozhou Bay over the past 100 years (Dai et al., 2007a) (With permission from Elsevier’s Copyright Clearance Center)

(5) Ratio of organic carbon to inorganic carbon The ratio of organic carbon to inorganic carbon (Corg /Cinorg ) is important for quantifying the eﬃciency of the biological pump in drawing down the PCO2 in the surface water because photosynthesis (assimilation) decreases the PCO2 of surface water, whereas more organic matter is produced. By drawing down the PCO2 of the surface water, the biological pump enhances the potential uptake of atmospheric CO2 and the production of organic matter by the ocean. Tsunogai and Noriki (1991) summarized sediment-trap data for the global ocean and found that Corg /Cinorg ratios of sediment-trap samples collected in the mesopelagic and pelagic layers (deeper than 500 m) are greater than 1 on average. On the basis of stoichiometry, Kano (1990) suggested that 0.6 is the boundary Corg /Cinorg ratio, showing that the PCO2 of seawater increases

3.5 Biogeochemical Processes of Jiaozhou Bay, South Yellow Sea

355

when the particulate carbon is biologically produced and its Corg /Cinorg ratio is less than approximately 0.6. In Jiaozhou Bay, the ratio of organic carbon to inorganic carbon (Corg /Cinorg ) in sediment can be used to evaluate marine autogenic organic matter production because the water depth is shallow and the organic matter produced in water can sink to the bottom before it decomposes. The ratio of Corg /Cinorg in the inner bay is similar to that in the bay mouth with an average of 0.81 and 0.88, respectively, but higher than that in the outer bay with an average of 0.17 (Fig. 3.51). The Corg /Cinorg indicates that the inner bay and bay mouth can absorb more atmospheric CO2 and produce more organic matter than the outer bay. This is consistent with the conclusion that the inner bay and the bay mouth can absorb atmospheric CO2 in winter (phytoplankton bloom season), but the outer bay cannot (Li et al., 2007). According to the vertical variation of the ratios in the past hundred years, the marine autogenic organic carbon was stable in the outer bay, but increased in the inner bay and bay mouth, especially after the 1980s.

0

0

Corg/Cinorg ratio 0.5 1.0

1.5

20

Depth (cm)

40

60

80

100

D4 B3 D7

Fig. 3.51. Vertical distributions of Corg /Cinorg ratio in Jiaozhou Bay (Dai et al., 2007a) (With permission from Elsevier’s Copyright Clearance Center)

The sources and burial of organic carbon during the past one hundred years were studied with three core sediments in Jiaozhou Bay. The variation of the burial ﬂux of organic carbon over the past one hundred years can be divided into the following three stages: (1) a relatively steady stage before the 1980s, (2) a rapidly increasing stage from 1980 to a peak in the 1990s, and (3) a decreasing stage from the 1990s to the present. The change is consistent with the amount of solid waste and sewage emptied into the bay. According to the ratio of OC:TN, the organic carbon mainly came from terrestrial sources in the

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3 Biogeochemical Processes of the Yellow Sea

inner bay (station B3) and the bay mouth (station D4), and only about half of the organic carbon was from a terrestrial source in the outer bay (station D7). In the inner bay (station B3), the terrestrial source was steady with a range of 69%∼77% before 1990. Then, the percentage of organic carbon from the land increased to 93% in 2000. From 2000, the percentage of organic carbon from the land decreased due to the decrease in terrestrial input. In the bay mouth (station D4), the percentage of organic carbon from the land reached the highest value with 94% in 1994. In the outer bay, the source matter has remained steady over the past one hundred years. 3.5.2.2 Vertical Distributions of Nitrogen The pretreatment and analysis processes have been reported in the previous chapter (Song et al., 2002), thus we will not describe them in this part for the sake of brevity. The vertical proﬁles of diﬀerent forms of nitrogen in the core sediments of B3, D4, and D7 in Jiaozhou Bay are summarized in Fig. 3.52. As an attempt to understand the relationships with other factors, we calculated liner correlation coeﬃcients for the various nitrogen forms and other environmental factors and tabulated them in Table 3.29. Table 3.29. The correlation coeﬃcients between nitrogen and environmental factors (Dai et al., 2007a) (With permission from Elsevier’s Copyright Clearance Center) IEF-N WAEF-N SAEF-N SOEF-N TN NH4 -N NO3 -N

φ −0.176 −0.322 −0.363 −0.272 −0.368 0.013 −0.276

pH −0.525 0.218 −0.435 −0.673 −0.474 −0.488 −0.692

Eh 0.385 −0.214 0.642 0.514 0.457 −0.492 0.552

Es 0.185 −0.018 0.439 0.479 0.394 0.115 0.500

OC 0.393 −0.287 0.253 0.323 0.411 0.404 0.208

(1) TN TN is the total quanta of nitrogen in sediments, which can be considered as the maximum amount of nitrogen taking part in nitrogen recycling. Accordingly, the TN can be a superior measurement for primary production. For Jiaozhou Bay sediments, TN in diﬀerent core sediments (D4, D7, and B3) had similar distribution patterns but the distributions were diverse and complicated. The average concentrations of TN for three core sediments were 0.323 (B3)>0.217 (D4)>0.213 mg/g (D7). Moreover, both D4 and D7 had similar distribution patterns and close concentrations. Generally speaking, the distribution of TN is controlled by the source and particle size of sediments. There was no signiﬁcant correlation observed between TN and OC, considering all the stations selected.

3.5 Biogeochemical Processes of Jiaozhou Bay, South Yellow Sea 0.01

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Fig. 3.52. Vertical proﬁles of nitrogen in Jiaozhou Bay sediments (Dai et al., 2007a) (With permission from Elsevier’s Copyright Clearance Center)

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(2) IEF-N The ion exchangeable form of nitrogen (IEF-N) is the most active part of nitrogen in sediments, which plays an essential role in the nitrogen cycle. And the NO3 -N was the dominant state in IEF-N in Jiaozhou Bay sediments. Factors inﬂuencing the distribution and concentrations of IEF-N in sediments include mainly temperature, salinity, pH, OC (organic carbon) and the characteristics of clay minerals, etc. The correlative coeﬃcients of IEF-N and other sedimentary environmental parameters were calculated and tabulated in Table 3.29. NH4 -N is the ﬁrst product of organic matter mineralization in sediments, which can be nitriﬁed or assimilated by microorganism, or be released to overlying water, or absorbed by clay minerals. When there is a relatively high exchange capacity, the NH4 -N concentration produced by mineralization is very low when entering the overlying water. Moreover, especially in coastal sediments, where it is in a comparatively anoxic condition, the NH+ 4 absorbed onto the sediments surface can reach up to 2/3 of the total NH4 -N and only a little can be released into the water body. Accordingly, it will exert great inﬂuence on the transformation ﬂux in the sediment-water interface, the supply of nutrients, and the primary production. For Jiaozhou Bay sediments, the average IEF-N concentrations of the selected three core sediments were 0.019 (D7)>0.016 (B3)>0.010 mg/g (D4), which accounted for 9.22% (D7), 5.38% (B3), and 4.42% (D4) of the TN average, respectively. The highest concentrations of IEF-N were 0.027 (D7), 0.018 (B3), and 0.015 mg/g (D4) and were 22, 78, and 18 cm below the surface sediments, respectively. And the lowest ones were 0.012 (D7), 0.013 (B3), and 0.006 mg/g (D4), which were 30, 22, and 68 cm below the surface sediments, respectively. The vertical distributions of IEF-N of the three core sediments were diﬀerent but almost had the same distribution. However, the patterns, which were the IEF-N concentrations, decreased with depth. However, the pattern was not striking for B3 core sediments and varied greatly for D7. Moreover, below 40 cm, the IEF-N concentrations varied vertically by less than 0.001 mg/g, which showed there is little variation in the supply of IEF-N and, on the other hand, it also indicated that the mineralization of organic matter in sediments took place generally at the oxygenous surface and NO3 -N was the dominant form of IEF-N. As Table 3.29 shows, IEF-N had good positive correlation with φ (r=0.652, n=19, P <0.05) and OC (r=0.393, n=19, P <0.05) but a negative correlation with pH (r=–0.382, n=19, P <0.05), Eh (r=–0.471, n=19, P <0.05), and Es (r=–0.346, n=19, P <0.05). In detail, both IEF-NH4 and IEF-NO3 had positive correlations with φ and OC but negative correlations with pH, Eh, and Es. The inﬂuence of organic matter on NH4 -N concentrations can be expressed in two ways. On the one hand, the fresh inputs of organic matter will be mineralized on surface sediments, which can add to the NH4 -N concentrations; on the other hand, the degradation of organic matter in sediments can supply active adsorption spots for NH4 -N. Under similar conditions, the NH4 -N concentrations are higher in sediments which are abundant in organic matter

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than in those that are aﬄuent in carbonate. In Jiaozhou Bay, the inner bay, where B3 core sediments are located, accepts aﬄuent input from dozens of rivers. The pH had a relatively negative correlation with IEF-NH4 ; that is, the lower the pH, the more propitious its adsorption of NH+ 4 . The horizontal distribution of pH is the mirror of diﬀerent inputs from diﬀerent rivers. The high inputs of organic matter from land-derived sediment can strengthen the reduction ability of the sedimentary environment due to the oxidation of organic matter, which will make the pH much lower and, as a result, sediments will absorb more NH+ 4 , which is in accordance with the inﬂuence that OC exerts . on NH+ 4 Diﬀering from IEF-NH4 , IEF-NO3 had no clear correlation with OC, Es, Eh but had a positive correlation with φ (r=0.308, n=19, P <0.05) and NH+ 4, which accounts for the fact that IEF-NO3 in Jiaozhou Bay sediments is mainly derived from overlying water but not from the nitriﬁcation of NH+ 4 . Because sediments in this paper were sampled at the beginning of autumn, after the depletion of dissolved oxygen by bloom microorganism activities, making the surface sediments relatively anoxic, nitriﬁcation is diﬃcult as a result to carry through and it will be absorbed onto the surface sediments. From another point of view, in summer, because of the enhanced runoﬀ of rivers, it will bring more NO3 -N into the bay and it will be absorbed onto the sediments as a high concentration of NO− 3 when there has been a concentration gradient between the water body and sediments. (3) WAEF-N The WAEF-N accounted for the least part of the nitrogen in Jiaozhou Bay sediments. It is mainly composed of carbonate. The distribution of WAEF-N relies on the changes in pH in the process of the mineralization of organic matter. So it has a negative correlation with pH (r=–0.296, n=19, P <0.05) and OC (r=–0.395, n=19, P <0.05). In the process of the mineralization of organic matter, with the pH increase or decrease, carbonate will dissolve or + precipitate, which will lead to a combination of carbonate with NO− 3 or NH4 . In Jiaozhou Bay sediments, there is a high concentration of CaCO3 and a low concentration of OC. Moreover, the mineralization is very weak and the pH is not vulnerable to change, so it is diﬃcult for CaCO3 to dissolve or precipitate. As a result, WAEF-N is generally in very low concentration. For Jiaozhou Bay sediments, the concentrations of WAEF-N in the three selected core sediments were very close to each other. Except for D7, the vertical distributions of each core sediment did not change greatly and the pattern was relatively mild. NO− 3 was the dominant state. The vertical patterns of D4 and B3 sediments were very similar and did not change much with depth. For Jiaozhou Bay sediments, the average WAEF-N concentrations of the selected three core sediments were 0.013 (B3)>0.007 (D7)>0.001 mg/g (D4), which accounted for 4.37% (B3), 3.11% (D7), and 0.67% (D4) of the TN on average, respectively. The highest concentrations of IEF-N were 0.014 (B3),

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0.020 (D7), and 0.002 mg/g (D4) and they were 30, 18, and 38 cm below the surface sediments, respectively. And the lowest ones were 0.011 (B3), 0.002 (D7), and 0.001 mg/g (D4), which were 92, 38, and 26 cm below the surface sediments, respectively. (4) SAEF-N SAEF-N was the predominant form of inorganic nitrogen and mostly controlled by the reduction/oxidation environment of sediments. Moreover, the reduction/oxidation environment of sediments is aﬀected by many factors such − as Eh, OC, Fe3+ /Fe2+ , Mn4+ /Mn2+ , SO2− 4 /HS and particle size (Ma et al., 2003). The method for extracting SAEF-N was adopted from the SEDEX technique by Ruttenberg (1992) which deﬁned the state as Fe-abound phosphorus because iron/manganese has a strong aﬃnity to phosphorus and silica. Similar to phosphorus, SAEF-N is easily absorbed by ferromanganese oxide. So SAEF-N is inevitably inﬂuenced by the concentration of ferromanganese oxide in sediments. For the Jiaozhou Bay sediments, the vertical distribution of three selected core sediments was very complicated and the changes were very great, and it was acknowledged therefore that in the process of the formation of SAEFN, there had been diﬀerent and complicated inﬂuences. Similar to WAEF-N, NO3 -N was the dominant state for SAEF-N. The average SAEF-N concentrations in the selected three core sediments were 0.036 (B3)>0.028 (D7)>0.024 mg/g (D4), which accounted for 12.34% (B3), 11.32% (D7), and 13.39% (D4) of the TN on average, respectively. The highest concentrations of IEF-N were 0.047 (B3), 0.038 (D7), and 0.035 mg/g (D4) and were all 2 cm below the surface sediments. And the lowest ones were 0.028 (B3), 0.013 (D7), and 0.023 mg/g (D4), which were 92, 14, and 106 cm below the surface sediments, respectively. Generally speaking, SAEF-N can easily be utilized in a non-reduction/oxidation condition in biology. In a reduction condition, Fe3+ is transformed into − Fe2+ and it will arouse the diﬀusion of NH+ 4 , NO3 in overlying water. Impacted by human activity, the sedimentation of organic matter will be enhanced, bacterial activities will ﬂourish and ultimately, as a result, this will make sediments reductive. So there is a low concentration of SAEF-N. According to this, SAEF-N has a negative correlation with pH (r=–0.435, n=19, P <0.05) and Es (r=–0.439, n=19, P <0.05) and a positive correlation with Eh (r=0.642, n=19, P <0.05) (as shown in Table 3.29). (5) SOEF-N SOEF-N, the predominant form of nitrogen, was mainly in organic form and easily adsorbed by organic matter. For Jiaozhou Bay sediments, the three selected core sediments were very strikingly similar to each other and decreased with depth. Moreover, at the surface sediments, the concentrations decreased rapidly but below 20 cm, the concentration remained unchanged and large, which was accounted for the fact that the mineralization of organic matter

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occurs mainly in the surface oxygenous zone. The average SOEF-N concentrations of the three core sediments were 0.052 (D4)>0.051 (D7)>0.049 (B3) mg/g and accounted for 23.53% (D4), 24.10% (D7), and 16.20% (B3) of the TN on average, respectively. The highest concentrations of the three core sediments were 0.098 (D4, 18 cm), 0.085 (D7, 18 cm), and 0.080 mg/g (B3, 2 cm). The lowest concentrations were 0.020 (D4, 38 cm), 0.031 (D7, 38 cm), and 0.031 (B3, 48 cm) mg/g, respectively. The distributions of SOEF-N are inﬂuenced by the source of sediment, sedimentation rate, organic matter, particle size, diﬀusion rate, reduction/oxidation environment, and so on. For example, when there is a high concentration of dissolved oxygen (DO), which makes the sediments rich in oxygen, more organic matter will be decomposed and involved in the transformation with the interface and participate in the whole biogenic cycle. As a result, the concentration of SOEF-N will decrease. So, in this paper the SAEF-N had a negative correlation with Eh (r=–0.512, n=19, P <0.05) and a positive correlation with OC (r=0.454, n=19, P <0.05), which further validated the fact that SOEF-N and OC share a similar mineralization process and transform mechanism. (6) NH4 -N and NO3 -N NH4 -N and NO3 -N are the dominant forms of nitrogen, which can participate in the process of early diagenesis. Because of diﬀerent sources and characteristics, they are under diﬀerent inﬂuences due to the sedimentary environment. NH4 -N is the ﬁrst product of organic matter and a reductive environment is optimal for NH4 -N. Under aerobic conditions, NH4 -N is nitriﬁed into NO3 -N by nitrifying bacteria. The vertical proﬁles of NH4 -N and NO3 -N can reveal the degree of nitrogen mineralization and the nitrogen adsorption by sediments as well as the transformation between NH4 -N and NO3 -N. As for Jiaozhou Bay sediments, the NH4 -N abundance at B3 and D7 is very complicated, with a rapid decrease above 20 cm, but sharp increases at 22 cm for D7 and 38 cm for B3. Moreover, the average NH4 -N concentration follows the order: B3>D4>D7, that is it decreases from the inner bay to the mouth to the outer bay. NO3 -N is the predominant nitrogen form in Jiaozhou Bay sediments. The NO3 -N concentrations are 0.070∼0.184 mg/g (B3)>0.054∼0.161 mg/g (D4)>0.052∼0.159 mg/g (D7) with complicated vertical proﬁles exhibiting a ﬂuctuant decrease downward and the maximum values are always in the surface or subsurface layers. The transferable nitrogen in marine sediments is the relatively weak bonded part and its concentration controls the transformation of nitrogen between sediment and overlying water. If all the transferable nitrogen in surface sediments may take part in nitrogen recycling, the percentage of transferable nitrogen in total nitrogen in Jiaozhou Bay surface sediments can be calculated (Table 3.30). As Table 3.30 shows, SOEF-N is the dominant part of total nitrogen accounting for 24.02%∼40.31% of TN. TIN is the primary inorganic transferable nitrogen accounting for 14.65%∼19.33% of TN. For all transferable ni-

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Table 3.30. The percentage (%) of transferable nitrogen in total nitrogen in Jiaozhou Bay surface sediments (Dai et al., 2007a) (With permission from Elsevier’s Copyright Clearance Center) Station B3 D4 D7

IEF -N 3.54 3.54 6.47

WAEF-N 2.81 0.35 3.55

SAEF-N 9.69 10.76 9.32

SOEF-N 26.83 40.31 24.02

TIN 16.04 14.65 19.33

trogen, WAEF-N is the least, IEF-N follows, accounting for 0.35%∼3.55%, 3.54%∼6.47% of TN, respectively. 3.5.2.3 Human Impact on the Sedimentary Environment Organic matter constitutes a minor fraction of lacustrine sedimentary records, but it is an important component of the paleoenvironmental evidence. Organic matter is derived from both marine productivity and terrestrial inputs. That is, sediments receive organic matter from both autochthonous (phytoplankton, bacteria, aquatic macrophytes) and allochthonous sources (terrestrial plant debris, pollen). C/N ratios can be used as indicators of sediment provenance or diagenetic changes. Increases in C:N ratios in sediment proﬁles can be interpreted to identify periods when sediments received a high proportion of terrestrial organic matter. Conversely, decreases in C:N ratios may be used to identify periods when sediments have received a high proportion of algal organic matter. The OC/TN ratios in Jiaozhou Bay sediments are shown in Fig. 3.53. As it shows, the OC/TN ratios in Jiaozhou Bay sediments were all higher than Redﬁeld ratios indicating that the primary source of Jiaozhou Bay sediments is land-derived. The OC/TN ratios were 9∼53 (D7), 6∼55 (B3), and 19∼72 (D4), respectively. And they follow the order: D4>B3>D7. Qian et al. (1997) presented the expressions for calculating the autochthonous or allochthonous organic matter for sediments on the assumption that the OC/TN is based on equations (3.22)∼(3.27), with the results tabulated in Table 3.31. As Table 3.31 showed for D4 and B3, the allochthonous inputs are the dominant source of sediments and the percentage can even reach as much as 86.8% and 78.8%, respectively. However, for D7, which located in the outside Bay, autochthony is the primary source of sediments and the allochthonous matter accounted for 48.0% of organic matter. On the whole, the percentage of allochthonous inputs accounting for total organic matter follows the same order as OC/TN ratios, that is, D4>B3>D7. Overall, the sediments of D4 and B3 are land-derived input and D7 is mainly autochthonous. Generally speaking, the Si/DIN and Si/P ratios can be used to identify the nutrient limiting factors for the growth of phytoplankton. With Si/P<16 or Si/DIN<1 of overlying water in Jiaozhou Bay, silica is the limiting factor for phytoplankton growth in the bay. The high ratio of DIN:PO4 -P and low ratios

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363

Fig. 3.53. Variation of elemental ratios with depth (cm) in the core sediments of Jiaozhou Bay (Dai et al., 2007a) (With permission from Elsevier’s Copyright Clearance Center) Table 3.31. The Ca and C1 values in Jiaozhou Bay sediments (Dai et al., 2007a) (With permission from Elsevier’s Copyright Clearance Center)

B3

Depth (cm) 0∼2 2∼4 8∼10 10∼12 12∼14 14∼16 18∼20 22∼24 26∼28 38∼40 48∼50 58∼60 68∼70 88∼90 Average

C1 (%) 71.2 86.7 69.3 62.2 60.2 69.7 92.8 75.2 86.0 87.5 88.4 77.8 80.0 95.8 78.8

Ca (%) 28.8 13.3 30.7 37.8 39.8 30.3 7.2 24.8 14.0 12.5 11.6 22.2 20.0 4.2 21.2

D4

Depth (cm) 0∼2 2∼4 4∼6 8∼10 10∼12 12∼14 14∼16 18∼20 22∼24 26∼28 38∼40 68∼70 78∼80 98∼100 Average

C1 (%) 87.0 91.4 87.5 95.8 72.9 51.3 81.5 94.3 96.6 94.1 96.3 97.0 81.1 88.9 86.8

Ca (%) 13.0 8.6 12.5 4.2 27.1 48.7 18.5 5.7 3.4 5.9 3.7 3.0 18.9 11.1 13.2

D7

Depth (cm) 0∼2 8∼10 10∼12 12∼14 14∼16 18∼20 22∼24 26∼28 30∼32 38∼40 48∼50 80∼82 Average

C1 (%) 38.1 44.4 42.4 51.3 41.0 46.2 19.6 26.7 57.8 75.0 54.9 79.2 48.0

Ca (%) 61.9 55.6 57.6 48.7 59.0 53.8 80.4 73.3 42.2 25.0 45.1 20.8 52.0

of SiO3 -Si:PO4 -P (7.6±8.9) and SiO3 -Si:DIN (0.19±0.15) in the water body showed the nutrient structure of Jiaozhou Bay has changed from a balanced to an unbalanced structure during the past 40 years. The reason may lie in the fact that phytoplankton consume the silica in overlying water and then are transported downward by the biological pump, and ﬁnally deposited in the sediments. As Fig. 3.53 showed, take B3 core for example, the Si/TN and Si/16P ratios in the sediments are all more than 1, Si/N ratios are in the range of 12∼83 and Si/P ratios are 37∼124 indicating that the silica is inclined to be deposited in the sediments. The vertical proﬁles of Si/N and Si/P ratios have similar trends with a slight increase downward. However, the Si/N and Si/P ratios in surface sediments are larger than those of subsurface sediments

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indicating that in the process of early diagenesis the decomposition rates of nitrogen and phosphorus are larger than that of silica and, as a result, the silica accumulates gradually in the sediments. Therefore silica is relatively scarce in overlying water. D4 and D7 have a similar pattern. Also, this reveals that the accumulation rate of silica in sediments has been enhanced in the past two decades. The decrease tendency of Si/TN and Si/P in sediments after 2000 means the increase of Si and decrease of TN or P in seawater, which may be caused by improvement of the environmental quality. Silica concentration limitation for phytoplankton in seawater was no longer as strong as before (Li XG et al., 2006). 3.5.2.4 Burial Flux of Nitrogen To be buried with the sediments is an end but also the start of marine recycling of biogenic elements. That is to say, sediments act as both source and sink of biogenic elements. Therefore, the biogenic elements will exert their inﬂuence on the primary production and carbon cycle, and moreover, biogenic elements buried by the sediments can be released to overlying water under appropriate conditions, so it is essential to know how much biogenic element is buried and what factors controlling burial ﬂux are. The burial ﬂux can be an indicator of environmental change over a speciﬁc period. Based on equations (3.15) and (3.16), the burial ﬂuxes of nitrogen are descriptive in Fig. 3.54. As is shown, the burial ﬂuxes of all nitrogen forms began to increase in the 1980s with a peak at the end of the last century. For example, the burial ﬂux of SOEF-N of B3 was 0.82 μmol/(cm2 ·yr) at the beginning of the 1980s, but with a rapid increase of 1.22 μmol/(cm2 ·yr) from 1998 to 2000. Fortunately, at the beginning of the century, with measurements adopted to manage pollution and eutrophication, the environmental quality has been improved greatly with the SOEF-N burial ﬂux dropping to 0.89 μmol/(cm2 ·yr), even lower than that in the 1980s when the rapid development of Qingdao occurred. Similar to SOEF-N, the burial ﬂux of TN and other forms of nitrogen increased from the 1980s until around 2000. This suggests that the burial ﬂux of nutrient is a more eﬀective proxy indicating the eutrophication history of Jiaozhou Bay compared to the abundance of nutrients. It can be divided into three periods of eutrophication in Jiaozhou Bay in the past 100 years, according to the burial ﬂux changes in nitrogen. In detail, the quantity and burial ﬂuxes of nitrogen have similar trends: (1) increasing steadily from 1900 to 1980, (2) increasing rapidly after 1980, (3) accelerating from 1900 to a peak in 2000, and (4) decreasing from 2000 to the present. Fertilizers applied in the catchments areas are a dominant source of eutrophication in coastal areas. The amounts of nitrogen fertilizers applied annually in the catchment area of Jiaozhou Bay increased by three times from 1980 to 1998 indicating that the strong increase in fertilizer consumption that started in the early 1980s was followed by an increase in nutrients concentrations. The burial ﬂuxes of nitrogen decreased from 2000

365

Burial fluxes ( m mol/(cm 2 yr))

3.5 Biogeochemical Processes of Jiaozhou Bay, South Yellow Sea

.

Fig. 3.54. The burial ﬂuxes of nitrogen at diﬀerent periods in Jiaozhou Bay sediments (Dai et al., 2007a) (With permission from Elsevier’s Copyright Clearance Center)

to the present. This trend is attributed to source reduction associated with increased regulation of wastewater discharge, industrial production of nitrogen, phosphorus fertilizers, increased industrial waste recycling, and cleaner industrial processes. Much regulatory attention has also been given to sewage eﬄuent and its detrimental environmental eﬀects. Sewage treatment practices have been upgraded and monitored to reduce the concentrations of nutrients being introduced to the sediment record. The improvement in the Jiaozhou Bay sedimentary environment can be attributed to the measurements presented by municipal agencies of Qingdao, which also appropriated nearly 1.7 million RMB in 2005. Anthropogenic activities have accelerated cycling and increased the nitrogen delivery to Jiaozhou Bay. It is therefore of great importance to understand how human activities change the nitrogen inputs. To interpret recent humaninduced impacts on nitrogen burial in Jiaozhou Bay, we consider the average value of burial ﬂuxes before the 1980s as the natural background. The average value of the burial ﬂuxes from 1980 to 2000 subtracting that before the 1980s

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can be considered as the human-induced inputs. We deﬁne the ratios as the anthropogenic factor (AF), which can be expressed as follows: Fa − Fb (3.28) Fb where Fa is the average value of burial ﬂuxes of nitrogen from 1980 to 2000, Fb is the average value of burial ﬂuxes before the 1980s as the background values. The AFs of nitrogen in Jiaozhou Bay sediments are tabulated in Table 3.32. AF =

Table 3.32. The anthropogenic factors (AF) of nitrogen inputs in Jiaozhou Bay (Dai et al., 2007a) (With permission from Elsevier’s Copyright Clearance Center) AF IEF-N WAEF-N SAEF-N SOEF-N TIN Transferable-N TN B3 0.98 1.15 1.28 1.23 1.18 1.21 1.06 D4 4.24 2.72 2.67 1.78 3.04 1.35 3.31 D7 0.82 4.70 0.86 0.97 1.16 1.58 1.21 Average 2.01 2.86 1.60 1.99 1.79 1.38 1.86

As shown, the relative anthropogenic inputs of nitrogen to the bay have been 1.38∼2.56 times greater than the values of natural inputs before the 1980s, which may indicate that there are considerable changes in nitrogen burial ﬂuxes between the 1980s and 2000 and perhaps it is the result of nitrogen fertilizers and agricultural discharge and other sources derived from Qingdao City. The fact that IEF-N and WAEF-N are especially sensitive to human impact on their inputs to the bay maybe due to their weak bonding strength to sediment particles. On the other hand, the AFs of inorganic nitrogen are larger than those of organic nitrogen indicating that inorganic nitrogen is more sensitive than organic nitrogen and inorganic nitrogen is one of the main sources of nitrogen derived from allochthonous inputs such as the nitrogen fertilizers which are mainly composed of inorganic nitrogen (Dai et al., 2007b). 3.5.3 Historical Variation of Phosphorus The study demonstrates the geochemical characteristics of phosphorus, combined with the lead-210 dating for interpreting the environmental changes of Jiaozhou Bay. 3.5.3.1 Total Phosphorus and Organic Phosphorus The concentrations of TP of the three cores are 0.197∼0.312 mg/g (D4), 0.247∼0.408 mg/g (D7), and 0.167∼0.309 mg/g (B3), respectively. The TP concentrations increase little with depth but are relatively constant both over

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the two cores and over time. The vertical proﬁles of TP in D4 core sediment are similar to those of B3 but somewhat diﬀerent from those of D7. The concentrations of TP do not change remarkably in the D7 sediment column. The variability of the concentration of phosphorus forms along the vertical sediment proﬁle shows a typical distribution with maximal values on the surface. For D4, it is comparatively high with 0.271 mg/g, and the highest concentration of total phosphorus appears 68 cm below the surface and the lowest of TP was at 2 cm below surface. For D7, the highest concentration of total phosphorus is at 8 cm under water and the lowest is 10 cm under the surface. Similar to D4, the concentrations of total phosphorus in the surface sediments of D7 are not the highest but relatively high. Generally speaking, it can be postulated that high phosphorus levels may be indicative of high anthropogenic input from urban, industrial and agriculture sources. For the D7 station, below 10 cm, the TP concentration is relatively constant, only about 0.292 mg/g on average. The TP vertical proﬁle of B3 is similar to D4 (Fig. 3.55). Organic phosphorus is quantitatively one of the most common phosphate phases buried in the sediments and thus directly aﬀects the availability levels of dissolved phosphorus for primary production (Edlund and Carman, 2001) and organic phosphorus is formed primarily by biological processes and may be produced in situ or enter via sewage eﬄuent containing body wastes and food residues. Sediment OP is a rough measure of organic production in the basin and decomposes slowly. It might be a better indicator of eutrophication than TP. Only 4.06%∼55.17% (D4), 4.80%∼44.91% (D7), and 5.59%∼55.25% (B3) of TP were in the form of OP, which comprised a minor constituent in the sediments compared to inorganic phosphorus. Therefore, it can be supposed that OP is a relatively minor reactive P sink in Jiaozhou Bay. The OP concentrations in Jiaozhou Bay sediments are 0.0098∼0.155 mg/g (D4), 0.009∼0.171 mg/g (D7), 0.012∼0.180 mg/g (B3), respectively. For D7, vertical changes in OP concentration are very distinct and always decrease with depth. The persistent increase of OP from the bottom to the surface may be due to the anthropogenic inputs and river runoﬀ. However, at the surface layer, the OP concentrations decrease rapidly because of the strong mineralization of OP potentially in the surface sediments. For D4, the concentrations of organic phosphorus are complicated with many ﬂuctuations, especially at 38 cm under water. Similar to D4, the organic phosphorus of B3 is also complicated and has no distinct rules to follow. 3.5.3.2 Inorganic Phosphorus Fractions Inorganic phosphorus is the principal solid-phase in the sediments of Jiaozhou Bay accounting for 44.83%∼95.54% (D4), 55.09%∼95.20% (D7), and 44.57%∼ 94.41% (B3) of TP. As Fig. 3.55 shows, the concentrations of IP decrease gradually with depth. In Jiaozhou Bay sediments, the relatively high concentration is still in the surface sediments. The vertical distribution patterns of the three

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Fig. 3.55. Vertical distributions of diﬀerent forms of phosphorus in the sediments of Jiaozhou Bay (Dai et al., 2007a) (With permission from Elsevier’s Copyright Clearance Center)

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selected sites were very similar. The concentrations of inorganic phosphorus follow the same order, that is Det-P>Ca-P>Fe-P>Oc-P>Ex-P>Al-P. And the authigenic apatite-bound phosphorus and detrital phosphorus were the dominant components of inorganic phosphorus. (1) Exchangeable phosphorus (Ex-P) Loosely sorbed P or exchangeable phosphorus (Ex-P) is often omitted because the concentration of Ex-P is very low in normal sediments. The Ex-P is mainly the phosphorus that is absorbed into the oxide and oxyhydrate and onto the surface of clay mineral particles in sediments, which is very labile in oxic conditions. The physical-chemical parameters such as temperature, pH, hydrodynamics and biological turbations have great inﬂuence on the release and adsorption of phosphorus. The concentrations of Ex-P are 0.006∼0.016 mg/g (D4), 0.024∼0.049 mg/g (D7), and 0.007∼0.014 mg/g (B3) with a percentage of 2.20%∼6.62% (D4), 6.85%∼18.50% (D7), and 2.94%∼6.53% (B3) of TP, respectively. The Ex-P concentrations vary vertically and are comparatively complicated and drastic (as Fig. 3.55 shows), which indicates that there may have been larger variations in the supply of Ex-P. Take B3 as an example, the highest concentration of Ex-P is in the surface sediments but the lowest is at 78 cm below surface sediments. However, for D4 and D7, the trends are not very clear, the highest concentrations both appeared in the middle of sediments and the lowest appeared in the deep sediments. Generally, the Ex-P concentrations decrease from the inner bay and mouth of the bay to the outside bay. (2) Aluminum-bound P (Al-P) Aluminum-bound P (Al-P) is the lowest among the six inorganic phosphorus types. The concentrations of Al-P are 0.0018∼0.0036 mg/g (D4), 0.0020∼0.0030 mg/g (D7), and 0.006∼0.009 mg/g (B3), accounting for 0.61%∼ 1.32% (D4), 0.51%∼1.10% (D7), and 2.01%∼4.07% (B3) of TP, respectively. The highest Al-P concentrations mostly appear in the surface and subsurface sediments. Except for D7, the lowest Al-P concentrations of D4 and B3 were at the bottom of sediments. On the whole, the Al-P concentrations decrease from the inner bay to the outside bay; that is, they follow the order B3>D4>D7. Except for B3, the Al-P concentrations of D4 and D7 vary vertically by less than 0.001 mg/g, which showed there are little variations in the supply of Al-P. (3) Iron-bound phosphorus (Fe-P) Phosphorus also occurs in minerals containing iron in diﬀerent forms, such as vivianite [Fe3 (PO4 )2 ·8H2 O] and strengite [FePO4 ·2H2 O]. The Fe oxides have a high aﬃnity for phosphorus and Fe oxides present in the oxic layer of the sediments may act as a trap for upward diﬀusing phosphate. This fraction is usually considered as a potentially mobile pool of P and is algal available. The Fe-P concentrations in Jiaozhou Bay sediments are 0.018∼0.030 mg/g (D4), 0.012∼0.067 mg/g (D7), and 0.015∼0.042 mg/g (B3) with a percentage

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of 6.92%∼12.25% (D4), 4.70%∼18.66% (D7), and 7.44%∼20.93% (B3) of the total inorganic phosphorus, respectively. The Fe-P concentrations in the surface sediments are not the highest but are still relatively high and decrease with depth. The vertical proﬁles of Fe-P in Jiaozhou Bay sediments are similar to those of Al-P with the highest concentrations in surface or subsurface sediments and the lowest ones are at the bottom except for D7. The average Fe-P concentrations follow the order B3 (0.032 mg/g)>D7 (0.031 mg/g)>D4 (0.024 mg/g). The Fe-P concentrations are greatly aﬀected by the sources, characteristics of sediments, mineral compositions, particle size of sediments, human activities, and other environmental factors. The redox cycle Fe greatly aﬀects P geochemistry after burial (Cha et al., 2005) and the oxidation-reduction cycle is important in controlling the fate of iron in most aquatic systems (El-Azim and El-Moselhy, 2004). Generally speaking, redox-sensitive phosphorus forms include phosphorus absorbed on Fe-oxides or in humic Fe(III)complexes, Fe(III)-phosphates and phosphorus occluded in Fe oxides. Under reducing conditions, the oxidized form of iron, deﬁned as Fe3+ , is transformed to the reduced form, what is called Fe2+ . The former is a hardly soluble compound with phosphate ions and the latter a soluble one. Therefore, the Fe-P fraction is ready to be deposited in sediments under oxidized conditions and released to overlying water under reduced conditions. The pH is also an important factor inﬂuencing the vertical distributions of Fe-P concentrations. When the pH increases in sediments, it creates intense competition between hydroxyl and phosphorus ions at the same time, thereby weakening the combination of phosphorus and Fe3+ , and leading to the release of phosphates to the overlying water. (4) Occluded phosphorus (Oc-P) Occluded phosphorus (Oc-P) is the phosphorus that is tightly sealed by the oxyhydroxides of Fe and Al and it is diﬃcult to release and utilize. The Oc-P concentrations in Jiaozhou Bay sediments are 0.015∼0.025 mg/g (D4), 0.011∼0.019 mg/g (D7), and 0.014∼0.031 mg/g (B3) accounting for 5.29%∼11.17% (D4), 4.19%∼7.26% (D7), and 6.53%∼13.86% (B3) of TP, respectively. The high concentrations of Oc-P in Jiaozhou Bay account for the high concentration of oxides in the zone. The vertical distributions of Oc-P concentrations are more variable with no marked features but, by and large, they decrease with depth and decrease from the inner bay to the outside bay. Moreover, the surface sediments have the highest concentrations in the whole column. 3.5.3.3 Authigenic Apatite-Bound Phosphorus and Detrital Phosphorus Apatite group minerals are by far the most prevalent phosphate in sediments. General marine apatites can be hydroxylapatite [Ca10 (PO4 )6 (OH)2 ], ﬂuoroa-

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patite [Ca10 (PO4 )6 (FOH)2 ], and chlorapatite [Ca10 (PO4 )6 Cl]. However, ﬂuoroapatite (CFA) is considered to be the primary diagenetic sink in tropical marine carbonate sediments. The calcium-bound phosphorus in the sediments is generally made up of two parts, detrital carbonate-bound P and authigenic apatite-bound P. Calcium-bound P is a relatively stable fraction of phosphorus and contributes to the permanent burial of phosphorus in sediments. In all phases, the calciumbound phosphorus is the most dominant form in the sediments of the D4 and D7 columns. In general, neutral or alkaline sediments contain calcium phosphate as the dominant phosphorus phase as water soluble phosphates react to form calcium phosphate precipitates. The authigenic apatite-bound phosphorus (ACa-P), consisting of the active phosphorus fractions, is mainly the parts of biogenic phosphorus, CaCO3 -bound phosphorus. And the ﬂuor apatite presents itself only below 2 cm, so the marine plankton is the main source of the surface sediments. The ACa-P concentrations ranged from 0.025 to 0.058 mg/g (D4), from 0.039 to 0.079 mg/g (D7), and from 023 to 0.102 mg/g (B3) with a percentage distribution range of 9.09%∼24.00%, 11.00%∼27.03%, and 9.07%∼41.05% of the TP, respectively. The detrital phosphorus (Det-P) concentrations of D4, D7, and B3 stations vary from 0.042 to 0.109 mg/g, from 0.051 to 0.116 mg/g, and from 0.042 to 0.063 mg/g accounting for 20.39%∼42.14%, 14.52%∼40.28%, and 14.87%∼30.96% of TP, respectively. The vertical distributions of the two forms of Ca-P are very obvious, which increase with depth and it shows that the Ca-P tends to be buried. The CaP concentrations in the sediments of Jiaozhou Bay are very high, which can illuminate the same sources and they are calcium orthophosphates from sea facies. The Ca-P is the inert component in the sediments. For a high solubility product (Ksp : 1.66×1044 ), it has a minor inﬂuence on the overlying water. The Ca-P concentrations, including detrital carbonate-bound P and authigenic apatite-bound P, generally increase with depth, though there are some ﬂuctuations, which may be due to the bioturbation. Generally speaking, the Ca-P is ready to be released to the overlying water when the pH of sediments decreases, but the process is diﬃcult to realize. The ACa-P and the Det-P concentrations in Jiaozhou Bay sediments also have obvious turning points at 38 cm below the surface, shown by a sudden increase in the two Ca-P concentrations and this can account for the bloom of the plankton and further emphasizes the changes in the sedimentary environment so that the pH may increase abruptly. On the other hand, the sediment of D7 changed from clayey silt to ﬁne sand below 38 cm, so the grain size is important for the phosphorus concentrations and distributions. From the discussions given above, we may infer that the forms and distributions of phosphorus vary with diﬀerent sedimentary environments. Generally speaking, calcium and iron have a diﬀerent reactivity in the pore water of sediments. The reactivity of iron becomes weaker during transformation from the freshwater sediments to the sea facies sediments, whereas calcium shows the reverse pattern, whose reactivity becomes stronger along with the

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change from freshwater sediments to sea facies sediments. Therefore, in the freshwater sediments, Fe-P is the main component and Ca-P is the primary part in the sea facies sediments. In the sediments of two cores in Jiaozhou Bay, the Ca-P accounts for the dominant part of TP and the Fe-P is the relatively subdominant portion, but still an important part of TP. Thus, it can be concluded that Jiaozhou Bay is inﬂuenced by human activity and the open sea, the YS, and it is a typical transition zone between land and ocean but, on the whole, the greater inﬂuence is what imposed on it by the whole marine system. 3.5.3.4 Burial Fluxes of Phosphorus To be buried with the sediments is not only the end but also the start of marine recycling of phosphorus. That is to say, sediments also act as both source and sink of phosphorus. We should know the phosphorus release from sediments and phosphorus buried by the sediments because the phosphorus is very important for the primary production in seawater. In some sense, the burial ﬂux can be an indicator of environmental change at a deﬁnite time. Based on equations (3.15) and (3.16), the burial ﬂuxes of phosphorus were descriptive in Fig. 3.56. As it showed, the burial ﬂuxes of all nitrogen forms began to increase in the 1980s with a peak at the end of the last century. For example, the burial ﬂux of inorganic phosphorus (IP) of B3 was 1.37 μmol/(cm2 ·yr) at the beginning of the 1980s, but with a rapid augmentation of 3.48 μmol/(cm2 ·yr) in the middle of the 1990s. And the burial ﬂux of organic phosphorus (OP) at the beginning of the 1980s was 0.46 μmol/(cm2 ·yr) with a rapid increase of 1.57 μmol/(cm2 ·yr) at the end of the last century. Correspondingly, for B3, the burial ﬂux of total phosphorus (TP) increased from 1.83 μmol/(cm2 ·yr) to 5.04 μmol/(cm2 ·yr) during the same period. Fortunately, at the beginning of this new century, with measurements adopted to manage pollution and eutrophication, the environmental quality has been improved greatly with the IP, OP, and TP burial ﬂuxes dropping to 1.67, 0.15, and 1.82 μmol/(cm2 ·yr), respectively, even lower than those in the 1980s when the rapid development of Qingdao occurred. Therefore, the decrease in the extent of the OP burial ﬂux is the greatest and the level may therefore even be the lowest in the past hundred years. It suggests that the burial ﬂux of nutrient is a more eﬀective proxy indicating the eutrophication history of Jiaozhou Bay compared with the abundance of nutrients. We can divide the eutrophication in Jiaozhou Bay in the past 100 years into three periods, according to the burial ﬂux changes in nitrogen. In detail, the quantity and burial ﬂuxes of phosphorus have similar trends: (1) increasing steadily from 1900 to 1980, (2) increasing rapidly after 1980, (3) accelerating from 1900 to a peak in 2000, and (4) decreasing from 2000 to the present. Fertilizers applied in the catchments areas are a dominant source of eutrophication in coastal areas. The amount of fertilizers applied annually in the catchment area of Jiaozhou Bay increased by three times from

373

2 Burial fluxes ( m mol/(cm yr))

3.5 Biogeochemical Processes of Jiaozhou Bay, South Yellow Sea

.

Fig. 3.56. The burial ﬂuxes of phosphorus at diﬀerent periods in Jiaozhou Bay sediments (Li et al., 2006) (With permission from Springer)

1980 to 1998 indicating that a strong increase in fertilizer consumption that started in the early 1980s was followed by an increase in nutrient concentrations. The burial ﬂuxes of phosphorus decreased from 2000 to the present. This trend is attributed to source reduction associated with increased regulation of wastewater discharge, industrial production of phosphorus fertilizers, increased industrial waste recycling and cleaner industrial processes. Much regulatory attention has also been given to sewage eﬄuent and its detrimental environmental eﬀects. Sewage treatment practices have been upgraded and monitored to reduce the concentrations of nutrients being introduced to the sediment record. The improvement in the Jiaozhou Bay sedimentary envi-

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ronment can be attributed to the measurements presented by the municipal agencies of Qingdao, which have also appropriated nearly 1.7 millions RMB in 2005. Anthropogenic activities have accelerated cycling and increased the nitrogen delivery to Jiaozhou Bay. It is therefore of great importance to understand how human activities change the nitrogen inputs. To interpret recent humaninduced impacts on phosphorus burial in Jiaozhou Bay, we consider the mean burial ﬂux before the 1980s as the natural background and the mean between the 1980s and 2000 after subtracting the mean before the 1980s as the humaninduced input. We deﬁne the ratios as the anthropogenic factor (AF) based on equation (3.28). The AFs of phosphorus in Jiaozhou Bay sediments are tabulated in Table 3.33. Table 3.33. The anthropogenic factors (AF) of phosphorus inputs in Jiaozhou Bay (Dai et al., 2007a) (With permission from Elsevier’s Copyright Clearance Center) AF B3 D4 D7 Average

Ex-P 1.17 1.62 0.83 1.21

Al-P 0.97 1.94 1.78 1.56

Fe-P 1.21 2.52 2.76 2.16

Oc-P 0.73 3.38 0.96 1.69

ACa-P 1.18 2.14 0.85 1.39

Det-P 1.22 2.84 1.41 1.82

IP 1.11 2.59 1.27 1.66

OP 0.21 0.57 2.94 1.24

TP 0.74 1.84 1.70 1.43

As it shows, the relative anthropogenic inputs of phosphorus to the bay have been 1.21∼2.16 times greater than the values of natural inputs before the 1980s, which may indicate there are considerable changes in phosphorus burial ﬂuxes between the 1980s and 2000 and perhaps it is the result of phosphorus fertilizers and agricultural discharge and other sources derived from Qingdao City. In particular, Det-P and Fe-P are sensitive to human impact in their inputs to the bay and the AF values were 1.82 and 2.16, respectively. As it shows, the AFs of inorganic phosphorus are larger than those of organic phosphorus. Moreover, the AFs of TP in diﬀerent stations follow the order: D4>D7>B3 (Dai et al., 2007b). 3.5.4 Biogenic Silica in the Sediments Silicate, or silicic acid (H4 SiO4 ), is a very important nutrient in the ocean. Unlike other major nutrients such as phosphate and nitrate or ammonium, which are needed by almost all marine plankton, silicate is an essential chemical requirement only for certain biota such as diatoms, radiolarian, silicoﬂagellates, and siliceous sponges. But siliceous phytoplankton contributes signiﬁcantly to the primary production in the world’s oceans. More than 40% of the entire primary production is contributed by diatoms, which reveals a close coupling of silica and carbon in the ocean. Therefore, silicate cycling has received sig-

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niﬁcant scientiﬁc attention in recent years and many scientists have studied silicate behavior in the marine environment. Biogenic silica is one kind of amorphous silica or is termed biogenic opal, or opal in brief. Biogenic silica in sediments mainly consists of fecal pellets and reliquiae of siliceous plankton such as diatoms, radiolarian, silicoﬂagellates, and siliceous sponges. It is estimated that about 240×1012 mol silica was ﬁxed by marine silica biota every year. Nevertheless, most of the biogenic silica produced in the euphotic zone dissolves during settling. Nelson et al. (1995) estimated that on a global scale at least 50% of the silica produced by diatoms in the euphotic zone dissolves in the upper 100 m of a water column. Diatom frustules that escape dissolution during settling reach the sea ﬂoor, where a major fraction of the silica dissolves at the sediment-water interface or in surface sediments, and only 3% of the silica produced in the euphotic zone is ﬁnally buried in the sediment. Calculation shows that the silica supplied to the world’s oceans from biogenic silica dissolution in sediment is 4 times that from rivers, and the regeneration of biogenic silica in sediment is the main source of silica in seawater. There are a number of factors that inﬂuence the dissolution of biogenic silica in sediment. Besides the source of silica, the dissolution is related to the water depth, water temperature, the degree of the saturation of silicate in pore water and overlying water too. Silica accumulation in sediment will strongly inﬂuence the silica concentration in seawater. Indeed, silica may limit the growth of diatoms under some particular conditions. (1) High content of biogenic silica in Jiaozhou Bay sediment The biogenic silica contents in three cores sediment from Jiaozhou Bay are shown in Table 3.34 and Fig. 3.57, by which an overall low discrimination of biogenic silica content is indicated. From the contrast between the biogenic silica content in surface sediment (0∼10 cm) and subsurface sediment (10∼20 cm), supposing the sedimentation rate is constant, the content of biogenic silica in recent years is obviously higher than that in earlier years in the inner bay, indicating a faster accumulation of biogenic silica in recent years. On the whole, the average of biogenic silica in locations B3, D4, and D7 is 1.54%, 1.48%, and 1.39%, respectively, showing that the biogenic silica content decreases from the inner bay to the bay mouth and outer bay, which is the same trend as that of phytoplankton distribution (Wu et al., 2004). Therefore, the biogenic silica content in Jiaozhou Bay sediment is positively correlated to the biomass of phytoplankton. The vertical changes are small at the bay mouth and outer bay. But the change is bigger at the inner bay, and several high value intervals at 4∼6 cm, 22∼24 cm, 68∼70 cm, and 92∼94 cm are denoted, which is due to slow water exchange with the YS in the inner bay, and massive N, P nutrient inputs from surrounding rivers, which would bring about eutrophication and result in algae bloom if conditions are favorable. Consequently, biogenic silica that accumulated in sediment could ﬂuctuate with the bloom, because the

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Table 3.34. The content (%) of biogenic silica in Jiaozhou Bay sediments (Li et al., 2006) (With permission from Springer) Location 0∼10 cm 10∼20 cm Total

B3 (inner bay) 1.58 1.29 1.54

D4 (bay mouth) 1.44 1.49 1.48

D7 (outer bay) 1.48 1.40 1.39

Biogenic silica content ( % ( 0.5 0

1.5

2.5

3.5

Depth (cm)

20 40 60 80 100

D4 (bay mouth) B3 (inner bay) D7 (outer bay)

120

Fig. 3.57. The vertical distribution of biogenic silica in Jiaozhou Bay sediment (Li et al., 2006) (With permission from Springer)

frequency and scale of algae bloom in the inner bay are often higher than those in the bay mouth and outer bay. Therefore, the biogenic silica content in the inner bay is more variable. In addition, water exchange continuously increases from the inner bay to outer bay and the concentration of nutrient can decrease. As a result, the frequency and scale of the bloom will decrease from the inner bay to outer bay. Therefore, the content of biogenic silica in sediment remains more stable in the outer bay than in the inner bay. According to the sedimentation rate we calculated for Jiaozhou Bay, the high content of biogenic silica at 4∼6 cm in location B3 may probably be related to the event of a large red tide in 1998.

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(2) The ratios of Si:N and Si:P The ratios of Si:N and Si:P can be used to estimate limitation nutrient factors for phytoplankton growth. In general, when Si:P<10 and Si:DIN<1 in seawater, silica will become an important limiting factor to phytoplankton growth. Since Si:DIN and Si:16P are less than 1 in Jiaozhou Bay seawater, it was thought that silica is a limiting factor in Jiaozhou Bay to phytoplankton growth. However, although this silica limitation is complicated, the most important reason is that silica in seawater is absorbed and delivered to the bottom in biological pumping, and buried ﬁnally by sediment. The accumulation of silica in sediment may reﬂect the ratios of Si to C, N or P. The ratio of Si:N:P in marine alga is 16:16:1 (Wang, 2003), and if all Si, N, and P in sediment come from phytoplankton, the ratio of Si:N:P in sediment should be close to 16:16:1. The ratios of BSi:TN and BSi:TP in Jiaozhou Bay core sediment were listed in Table 3.35, showing that the ratios of BSi:TN at every location are much greater than 1, and the ratios of BSi:TP are well above 16, which indicates that biogenic silica is accumulated in sediment. In general, BSi:TN and BSi:TP increase with depth, illustrating that the rate of N and P decomposition is faster than that of biogenic silica in early diagenesis, and silica continuously accumulates in sediment resulting in silica depletion in seawater. In addition, the fact that BSi:TN and BSi:TP are greater in the surface layer than in the subsurface layer in location B3 conﬁrmed that the silicate accumulation rate in the inner bay has been increasing in recent years. (3) The relationship between biogenic silica and organic carbon Silica plays an important role in controlling the carbon cycle. It controls the absorption and release of carbon by regulating phytoplankton growth; whether from its source or by preservation, biogenic silica in sediment has a close relationship with organic matter. Remero and Hebbeln (2003) observed a good correlation between biogenic silica in sediment and primary production in overlying water in the region of the Peru-Chile upwelling; viz., higher biogenic silica content means higher primary production. Preservation of silicate in sediment has a close relationship with its dissolution rate. And the dissolution rate depends mainly on factors including under saturation of Si(OH)4 in bottom and pore water, depth pressure, temperature, pH, the content of organic carbon, and the protective organic coating. In fact, the generation of Si(OH)4 from biogenic silicate increases with sediment permeability. In general, organically poor sands have relatively high mineralization rates. Silica depletion in overlying water can boost the dissolution of biogenic silica. Therefore, the relationship between biogenic silicate and organic carbon in sediment is a very important issue in this regard for scientists, as shown in the case of Jiaozhou Bay (Fig. 3.58). In Fig. 3.58, the correlation coeﬃcient between biogenic silica and organic carbon at B3 (inner bay) is obviously greater than that at D7 (outer bay), and the ratios of OC:BSi in the inner bay and bay mouth are close to but higher than that in the outer bay, which might result from diﬀerent hydrodynamics,

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Table 3.35. The ratios of Si/TN and Si/TP in Jiaozhou Bay sediments (Li et al., 2006) (With permission from Springer) B3 Layer (cm) 0∼2 2∼4 4∼6 8∼10 10∼12 12∼14 14∼16 18∼20 22∼24 26∼28 30∼32 38∼40 48∼50 58∼60 68∼70 78∼80 88∼90 92∼94

(inner bay) BSi BSi OC TN TP BSi 18 56 0.30 20 60 0.35 43 85 0.25 15 55 0.34 13 46 0.37 12 42 0.37 18 73 0.29 24 64 0.35 30 124 0.19 23 47 0.31 43 62 0.27 20 47 0.36 21 50 0.35 29 37 0.21 83 98 0.08 31 73 0.06 30 67 0.29 54 92 0.26

Average 29

65

0.28

D4 (bay mouth) Layer BSi BSi OC (cm) T N TP BSi 0∼2 37 84 0.20 2∼4 28 57 0.28 4∼6 28 54 0.26 8∼10 41 74 0.22 10∼12 23 65 0.24 12∼14 24 65 0.17 14∼16 28 69 0.23 18∼20 32 71 0.27 22∼24 30 70 0.30 26∼28 34 62 0.25 30∼32 40 77 0.28 38∼40 37 67 0.25 48∼50 43 67 0.29 58∼60 52 55 0.30 68∼70 34 67 0.27 78∼80 25 59 0.26 88∼90 49 60 0.24 98∼100 42 59 0.18 106∼108 45 57 0.25 Average 35 65 0.25

D7 Layer (cm) 0∼2 2∼4 4∼6 8∼10 10∼12 12∼14 14∼16 18∼20 22∼24 26∼28 30∼32 38∼40 48∼50 58∼60 68∼70 80∼82

(outer bay) BSi BSi OC TN TP BSi 33 67 0.11 25 66 0.07 42 84 0.07 32 75 0.12 53 93 0.07 41 76 0.08 41 135 0.09 36 97 0.11 18 70 0.16 41 80 0.08 37 91 0.12 31 80 0.18 32 93 0.13 33 117 0.06 60 154 0.19 57 110 0.11

Average 38

93

0.11

structures, and compositions of sediment between the inner bay and outer bay. Sediment in the outer bay is coarser than that in the inner bay and has good permeability from which OC and BSi could leak out and low contents from them be observed. In addition, the exchange between pore water and overlying water results in not only low OC content and silica unsaturation but also an acceleration in biogenic silica dissolution. This is why very low biogenic silica content is observed in sediment and a smaller correlation coeﬃcient in the outer bay than in the inner bay. In the OC:BSi ratio, the maximum value is 0.37 in Jiaozhou Bay sediment, which is much smaller than the Redﬁeld ratio, indicating that OC decomposed much faster than biogenic silica did in the same environment. Most OC would be decomposed and released to seawater and then participate in the carbon recycle in seawater. The biogenic silica is preserved in sediments and the silicate concentration in seawater is low, which may explain why Si becomes a limiting factor for phytoplankton in Jiaozhou Bay. According to Ma et al. (2002), the decomposition rate of the biogenic element is N>P>OC>Si, which is consistent with the phenomenon that the

3.5 Biogeochemical Processes of Jiaozhou Bay, South Yellow Sea 3

379

n=15, r=0.65

2.5 2 1.5 1

Biogenic silica (%)

0.5 0 0 2

B3 0.2

0.4

0.6

0.8

0.4

0.6

0.8

n=15, r=0.79

1.5 1 0.5 D4 0 0

0.2

2

n=13, r=0.54

1.5 1 0.5 D7 0 0

0.1

0.2 0.3 0.4 Organic carbon (%)

0.5

Fig. 3.58. Relationship between biogenic silica and organic carbon in Jiaozhou Bay sediments (Li et al., 2006) (With permission from Springer)

decomposition rates of C, N, and P are faster than that of biogenic silica indicated by the ratios of OC:BSi, BSi:TN, and BSi:TP in sediment of Jiaozhou Bay, which improved the BSi that accumulated in the Jiaozhou Bay sediment. (4) Flux of biogenic silica sedimentation in Jiaozhou Bay sediment The sedimentation ﬂux can reﬂect quantitatively how much sediment may accumulate. Being compared with primary production, the amount of matter that decomposed and came back to the water during settling can be estimated. By comparing it with the ﬂux across the interface of sediment-water, we can calculate how much matter is buried. Therefore, how much biogenic silica can be buried and how much biogenic silica can be taken back to the seawater in Jiaozhou Bay can be determined based on equations (3.14) and (3.15), with the results tabulated in Table 3.36.

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Table 3.36. The ﬂuxes of biogenic silica sedimentation in Jiaozhou Bay (Li et al., 2006) (With permission from Springer) Region Inner bay Bay mouth Outer bay

Water content (%) 65.6 65.4 80.3

Sedimentation rate (cm/yr) 0.85 1.63 0.45

Sedimentation ﬂux (mmol/(m2 ·d)) 5.34 9.87 2.08

According to the primary production of (41.9±15.3) mmol/(m2 ·d) (Wang, 2003), the biogenic silica production from diatom, calculated in the Redﬁeld ratio is (6.32±2.35) mmol/(m2 ·d). Using the sedimentation ﬂux at B3 (inner bay) as the average ﬂux of biogenic silica sedimentation in the bay, we found that only about 15.5% of diatom-produced biogenic silica can be decomposed during sinking down to the bottom, about 84.5% of biogenic silica can reach the bottom and be deposited. Because the water depth of Jiaozhou Bay is small (7 m on average), most primary production may be deposited before decomposition takes place. The ﬂux of biogenic silica from the sedimentwater interface to water, which can represent the silicate release rate, is 3.3 mmol/(m2 ·d) (Jiang et al., 2002), less than the inner bay sedimentation ﬂux. Consequently, silica continuously transported from sea water to sediment results in low silica concentration in seawater for phytoplankton growth. Above all, the accumulation of biogenic silica in sediment is a major mechanism for Si limitation in phytoplankton growth in Jiaozhou Bay. Based on information about biogenic silica revealed from three sediment cores from Jiaozhou Bay, the reasons for silica limitation in local phytoplankton growth were discussed. The main conclusions can be summarized as follows: •

The biogenic silicate content in Jiaozhou Bay sediment is obviously much higher than that in the YS and the Bohai Sea. The sediment of the bay is characterized by a high content of biogenic silicate, being 1.58%, 1.44%, and 1.48% in locations B3, D4, and D7, respectively. In the inner bay, the content at the sediment surface is higher than it is underneath, indicating that the accumulation rate in recent years is greater than that in earlier years. • In Jiaozhou Bay sediment, OC, N, and P decompose much faster than BSi does in similar conditions, which means that most biogenic silicate will be buried and separated from silicate recycling. • Only 15.5% of biogenic silicate is hydrolyzed during the journey from the surface to the bottom in seawater, thus ca. 84.5% can be deposited on the bottom. The silicate release rate at the sediment-seawater interface is quite a bit lower than its accumulation rate from water to sediment. They are the main reasons for a constantly low silicate concentration in sea water and Si-limitation in phytoplankton primary production. In one

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word, silicate accumulation in sediment is the key factor to explain silicate limitation in phytoplankton growth in Jiaozhou Bay (Li XG et al., 2006). 3.5.5 Nutrients (N, P, Si) in the Seawaters We all know that marine sediment acts as source or sink for nitrogen and phosphorus in seawater. The sediment as the source will release nitrogen and phosphorus into the water for the phytoplankton use. However, when the nitrogen and phosphorus in seawater are oversupplied by river input mainly from human activities, the phytoplankton will grow fast to bloom, and produce more faecal pellets and reliquiae into sediments, the sediment becomes the sink of the nitrogen and phosphorus in seawater (Li et al., 2004). Therefore, nitrogen and phosphorus in sediment play a key role in a marine ecosystem, on which subject many studies have been published. Nitrogen and phosphorus in sediment can be divided into an organic form and an inorganic form. Both can be utilized by phytoplankton as well as zooplankton and microorganisms. The distributions of nitrogen and phosphorus in the two forms in sediment are obviously diﬀerent. In general, the contents of IN and IP increase with depth in surface sediments, and then gradually stabilize in the subsurface. However, the vertical distributions of ON and OP are the reverse, and the corresponding distributions are clearer with an increase in depth. The reason is that most microorganisms live in surface sediment, causing rich organic nitrogen and phosphorus, which decrease with depth in surface sediment, or remain roughly stable in deeper sediment. The content and distribution of the nutrients bear rich environmental and biogeochemical information about the nitrogen and phosphorus cycle, which is presented in this paper taking Jiaozhou Bay as an example (Fig. 3.59).

Fig. 3.59. Investigation sites in Jiaozhou Bay (Yang et al., 2003) (With permission from Yang DF)

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3 Biogeochemical Processes of the Yellow Sea

N and P inputted into Jiaozhou Bay by river and by sewage eﬄuents of cities have made the bay become more and more eutrophic day by day. It was thought that phytoplankton growth was limited by the change from nitrogen to phosphorous, and that the silicate concentration in Jiaozhou Bay is important for limiting diatom growth. Inorganic phosphorous and total inorganic nitrogen were thought to be abundant enough to meet the requirements of phytoplankton growth and reproduction; i.e., N and P of Jiaozhou Bay were not limiting nutrients for phytoplankton growth. Zhang and Shen (1997) indicated that the probability of dissolved inorganic element nitrogen and phosphorous as the limiting factors in phytoplankton growth was very small or close to zero at the surface layer of Jiaozhou Bay. (1) Seasonal variation and horizontal distribution of nutrients N, P, and Si DIN (the sum of nitrate, nitrite, and ammonium) and phosphate concentrations did not have an obvious annual, less than annual, or even seasonal cyclical variation. Throughout Jiaozhou Bay in one year, the DIN values were >2.36 μmol/L and the phosphate values were >0.16 μmol/L. Silicate seasonal variation showed a repeated annual cyclic pattern. Every year there was only one highest value of silicate concentration in summer. Throughout Jiaozhou Bay, silicate values were <2 μmol/L in spring, autumn, and winter, but in summer were >2 μmol/L. For example, at stations 1 and 4 in Jiaozhou Bay, the seasonal variations of DIN concentration, phosphate concentration and silicate concentration were as shown in Figs. 3.60, 3.61, and 3.62.

DIN1

DIN4

Fig. 3.60. Seasonal variations of DIN concentration at stations 1 and 4 (Yang et al., 2003) (With permission from Yang DF)

(2) Seasonal variation and horizontal distribution of the nutrient Si:DIN ratio From December to May, Si:DIN ratios were <0.5; Si:DIN ratios became >0.5 generally from June to November (Fig. 3.63), with such a distribution pattern being repeated each year. The variation of nutrients N and P in Jiaozhou Bay is mostly due to human activity. In the last 30 years the population around Jiaozhou Bay has increased

0.6

P1

383

P4

0.5 0.4 0.3 0.2

1994-2

1993-11

1993-8

1993-5

1993-2

1992-11

1992-8

1992-5

1992-2

1991-11

0

1991-8

0.1

1991-5

Phosphate concentration (mmol/L)

3.5 Biogeochemical Processes of Jiaozhou Bay, South Yellow Sea

Date (Year-Month)

8

Si1

Si4

6 4

1994-2

1993-11

1993-8

1993-5

1993-2

1992-11

1992-8

1992-5

1992-2

1991-11

0

1991-8

2

1991-5

Silicate concentration (mmol/L)

Fig. 3.61. Seasonal variations of phosphate concentration at stations 1 and 4 (Yang et al., 2003) (With permission from Yang DF)

Date (Year-Month)

Si:DIN and Si:16P ratios

Si:DIN and Si:16P ratios

Fig. 3.62. Seasonal variations of silicate concentration at stations 1 and 4 (Yang et al., 2003) (With permission from Yang DF) Si:DIN2

Si:16P2

Si:DIN5

Si:16P5

Fig. 3.63. Seasonal variations of Si:DIN and Si:16P ratios at stations 2 (a) and 5 (b) (Yang et al., 2003) (With permission from Yang DF)

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3 Biogeochemical Processes of the Yellow Sea

rapidly, and the amount of industrial waste and daily living sewage inputted into Jiaozhou Bay has increased rapidly. Among the rivers ﬂowing into the bay in 1980, the Haibo River carried 140×103 t/yr of sewage into Jiaozhou Bay. Also, in recent years, the long time application of nitrogenous fertilizers to the bay’s beach lands and coastal waters favorable for agriculture resulted in an input of much N and P into bay waters. The direct and indirect intervention of human activity has broken down the ecological cycle of nutrients N and P so that they now show an increasing trend (Yang et al., 2003).

3.6 Biogeochemical Characteristics of Heavy Metals in Yellow Sea Sediments Since the 1960s, China and Korea have experienced rapid industrial developments, resulting in the degradation of the marine environmental quality mainly due to the increase in population, production of pollutants and deterioration of natural habitats. The rapid marine environmental change with respect to its oceanographic characteristics has drawn increasing international attention over the last three decades (Song, 2004). A certain level of heavy metals in the ocean may result in changes in the ecological equilibrium. With the call for sustainable development and marine environmental conservation, it was critical to understand the characteristics of pollution and the ecological risk of heavy metals. 3.6.1 Distributions of Heavy Metals The distributions of Cu and Zn were similar (Fig. 3.64), which showed a bimodality trend that went from low to high, was then low, and ﬁnally a high trend from west to east. There was a descending trend from north to south. The YS trough was the dividing line separating the south YS from the western and eastern parts. In the west, distributions resembled grain size distribution, and in the east they were parallel to the Korean coastline. A high content area existed in 35◦ ∼36◦ N, 122◦ ∼123.5◦ E district. B7 and C5 stations were high centers of Cu while A6, B6, and B7 were Zn’s high centers, with a highest ﬁgure of 186.2 mg/kg at the B7 station. In those high content stations, clay (d<4 μm) and silt clay (4 μm
3.6 Biogeochemical Characteristics of Heavy Metals in Yellow Sea Sediments

385

they were between 35◦ ∼36◦ N, 32.5◦ ∼33.5◦ N, and 122◦ ∼125◦ E, and the low content areas were closed and ran parallel to the shoreline evenly, which were diﬀerent from Pb’s lowest area in Jiangsu Province’s adjacent sea. The average content of Cd was 0.159 mg/kg, which was still much lower than ﬁrst class marine sediment quality, but most of the stations exceeded the background value (0.103 mg/kg). The average content of Pb was 13.85 mg/kg, slightly lower than the background value (14.5 mg/kg), and far smaller than the ﬁrst class Marine Sediment Quality value of 60.0 mg/kg. The gap between the high and low values of Pb diﬀered not very much, only ﬂuctuating by 30% of average value, so Pb’s distribution was the most even in the 6 heavy metals. The distribution of As showed a northern part higher than the south and trends in the east (near Korea) a little higher than that in the west (near China). In detail, A section, which is near the northern YS, was the high content area, most of the above background value stations appeared there, and A1 station was the peak. The inshore areas near Jiangsu Province of China, as well as D section which faced the Yangtze River Estuary, were low content areas, with the average only half that of A section. This unique distribution pattern exposed the diﬀerent characteristics between metalloid and metals, such as sorption, enrichment, transformation, and chemical reaction. The distribution of Hg was in a class by itself. It can be divided by 34◦ N into north and south parts. In the north, Hg was similar to Cu in the same area, with two sub-high content areas centered around B3-B4 and B7-B8. In the north, there was a large-scale high area in D section whose content decreased from estuary to oﬀshore area. Seeking the cause, this may be the result of a fresh and salt water mixture, which accelerated the absorbed Hg in the solids so as to be deposited in sediments by coacervation. The average of Hg was 0.025 mg/kg, and most of the stations went beyond the background value of 0.016 mg/kg, but still much less than the ﬁrst class limit of Marine Sediment Quality (0.20 mg/kg). On the whole, although the distributions of studied heavy metals were not homogeneous, high and low content areas followed the same pattern as ﬁne grain size sediments, only diﬀered in range. They were high in central areas and low inshore, except Zn. According to their details, we can summarize three patterns for heavy metals distribution. A high content area was to be found in the central YS mud area, and a low area in the modern sand sedimentation area. Cu and Zn belonged to this pattern. Positions of high and low content areas were similar to the former, but the range was broadened. Cd and Pb were in this pattern. A high content area was close to the estuary, and some sub high areas were in the central YS mud area. This corresponded to the model of Hg. According to the similarity or dissimilarity of the heavy metal distribution types, we can divide Jiaozhou Bay into several geochemical areas. Every area was consistent with a speciﬁc sedimentary environment, water dynamics, and sediment type. We compared those areas and abstracted four distinct areas.

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3 Biogeochemical Processes of the Yellow Sea

Unit: mg/kg

Unit: mg/kg

Unit: mg/kg

Unit: mg/kg

Unit: mg/kg

Unit: mg/kg

Fig. 3.64. Distributions of heavy metals (mg/kg) in surface sediments in 2003 (He et al., 2006) (With permission from Springer)

(1) High Cd-Cu-Pb-Zn area This area accorded with central YS mud, had a modern sedimentary environment and relatively stable water dynamics (in the cold eddy of SYS currents). Fine grain size sediment (clay) dominated the sediments, and most of the metals existed in detritus form.

3.6 Biogeochemical Characteristics of Heavy Metals in Yellow Sea Sediments

387

(2) High Hg, low As-Cu-Zn area This area was located in D section, near the boundary of the SYS and ECS. Input from the Yangtze River and some Korean rivers might be the origin of this silt-clay area. At the same time, the early sediments’ corrosion, resuspending and redeposition can be the origin as well. We concluded that it has a multiple origin modern sedimentary environment. (3) High As, low Cd-Hg-Zn area This area was located in 36◦ ∼37◦ N, near the northern YS. It accorded with a relict sand area, weak in modern sedimentation, but has strong water dynamics. It was dominated by coarse sediment, as the ﬁne grain size materials were diﬃcult to deposit. Being in an oxidizing environment, there were authigenic sediments, such as manganese oxides, ferric oxide, and phosphate of iron in the coarse sediment. This area was much inﬂuenced by anthropogenic heavy metals, because its geographic location was adjacent to land in the west and east, and also it was easy to exchange water and suspended solids with the northern YS. (4) Low As-Cd-Cu-Hg-Pb-Zn area This area was parallel to the coastline, including the sea areas near the Shandong Peninsula, Jiangsu Province of China and western Korea. Although this area did not cover all the low belts of the six metals, it was the most concentrated low content area. The complicated distributions were the results of the coupling of anthropogenic production and environmental factors. 3.6.2 Annual Variations The annual trends of heavy metals can be depicted as relatively stable coupled with a little ﬂuctuation (Fig. 3.65). The ﬂuctuations were 10%∼30% of 8-year-averages, which were (7.17±1.70), (0.108±0.024), (17.61±1.65), (0.024±0.008), (18.44±4.26), and (70.53±5.73) mg/kg for As, Cd, Cu, Hg, Pb, and Zn, respectively. There was little regularity, except that Hg showed a good linear variation trend [y=0.0033x–6.50, r=0.75, y is the average value of Hg every year, x is the survey year (1998∼2005)], which meant that the

C (mg/kg)

100

As

Cu

Pb

Zn

80 60

0.20

Cd

Hg

0.15 0.10

40 20 0 1998 1999 2000 2001 2002 2003 2004 2005

0.05 0.00 1998 1999 2000 2001 2002 2003 2004 2005 Year

Fig. 3.65. The annual variation of six heavy metals in the SYS (He et al., 2006) (With permission from Springer)

388

3 Biogeochemical Processes of the Yellow Sea

content of Hg was rising slowly during those years. Cd ﬂuctuated a bit unusually, but it was still acceptable because the concentration level of Cd was about 1‰ of Cu or Zn, so the measuring error was easily imported. Putting those characteristics together, the steady ongoing trends were the result of a small sedimentation rate, which was 2∼5 mm/yr in most of the survey area. The surface sediment samples were collected by grab bucket sampler in the 1∼3 cm sediment layer, and they needed about 3 years to be deposited, so the time span actually reduced the diﬀerences in the data which, in the end, caused a similarity in two years contents. As there was not so big an increase or decrease, we concluded that the heavy metal environment of the SYS was still relatively stable rather than deteriorating or improving. In other words, the anthropogenic heavy metals of China and Korea were still within the environmental capacity of the SYS. Besides this preliminary study, further work should be placed on 210 Pb dating to analyze the core sediment, in order to illustrate the heavy metals evolvement mode over a time span of 100 years. 3.6.3 Controlling and Inﬂuencing Factors Studies showed that the sediment grain size and TOC inﬂuenced the heavy metal distributions. According to the profound analysis of the heavy metal concentrations in the sediments of SYS, it was safe to reach a conclusion that the grain size of sediment was the most important factor controlling the heavy metal distributions in SYS. 3.6.3.1 Sedimentation Type and Sedimentation Rate The grain size of sediments was classiﬁed according to the percentage of the ﬁne grain size sediments (d<63 μm). In order to discuss the sediment type distinctly, 3 areas, i.e., areas I, II, and III, were split according to the percentage of the ﬁne grain size (Fig. 3.31). In area I, clay dominated the surface sediments, and in the central part clay took up more than 85.4%, which descended outward with the silt component increasing. This type of sediment was similar to pelagian sediments in the benthic basin. Area I had stable material sources and no disturbance caused by an abrupt strong current existed. In those 3 areas, the adsorption quantity of heavy metals was diﬀerent relative, ﬁrst of all, to diﬀerent grain size, then to diﬀerent constituents, and ﬁnally to diﬀering adsorbility of the sediment grains to heavy metals. The sediments grain size is controlled by the distance from the material source, the transportation medium, transportation mode and the sediments environmental characteristics. B2 and the A line stations were near to the Han River Estuary, so that the sand and silt schlepped by the Han River contributed enormously to high concentrations. Additionally, due to the eﬀects of the ancient Yellow River and Yangtze River in the W¨ urm glaciations, a thick layer of sand sediments developed in the west coast area north of the Yangtze

3.6 Biogeochemical Characteristics of Heavy Metals in Yellow Sea Sediments

389

River Estuary, represented by B9, C7, C8, and C9 in the study area. Therefore, the sand concentrations of the stations mentioned above were higher than those of other stations. The ﬁner sediments area in the SYS middle area reﬂected the eﬀects of the transportation mode and sediments characteristics on the sediments types. The clay sediments of the middle area in the SYS continental shelf tended to transport to the mid-north sediments area center, consistent with the cold vortex center by and large, located at about 123.5◦ E, 35.5◦ N. The weak hydrodynamic energy controlled by the cyclonic circumﬂuence at the surface and cold vortex at the bottom resulted in the centrality of transportation (Shi et al., 2002). Therefore, ﬁne sediment particles were transported to the SYS middle area with the ocean current and settled down very tardily. Li (1995) pointed out that the majority of the sediments in the SYS west area and the area adjacent to the ancient Yellow River came from the ancient Yellow River. However, in the east area, the sediments originated from the modern Yellow River, and the material corralled by the ancient Yellow River had not been conveyed to the east sediments area. The Yangtze River contributed to the sediments distribution to some extent, but the material of sediments in the east of the SYS could not embody the characteristics of Yangtze River material explicitly. Therefore, the sediments in the eastern area of the SYS had several sources, partly from the Yangtze River, partly from Korean Rivers, or from the sediments formed before corrosion, resuspending, and redeposition under the complex hydrodynamic conditions (Li, 1995; Song, 1997). There were signiﬁcant relationships between heavy metal concentrations and the sediment grain size and sediment type; i.e., heavy metal concentrations increased as the clay concentrations increased and the average grain size decreased; the adsorption capacity of clay to heavy metals was the strongest, followed by silt and sand in turn. The ﬁner the sediments were, the larger the potent surface area would be, which would ﬁnally absorb more heavy metals. This was embodied in the similarity between distributions of ﬁne grain size sediments (Fig. 3.66) and the distribution of Cd, Cu, Pb, and Zn. The concentrations of heavy metals had a good correlativity with the sediment average grain size, for instance, [Cu]=0.0136x +0.2243, r=0.53; [Zn]=10.922x –14.575, r=0.70; [Hg]=0.0014x +0.0157, r=0.51. In the formulae, x was the average grain size of sediments (in μm). Whereas As, Cd, and Pb showed no good relativity statistically, as expected, their contents were consistent with the average grain size in the local area. We can draw a conclusion from the similarity between Figs. 3.31 and 3.66 that the grain size of sediments was the uppermost controlling factor in the distribution of heavy metals in the SYS. Even though ﬁne grain size sediments were the main carriers of heavy metals, it was still possible to form certain high areas of coarse grain size sediments. Heavy metal deposition was through the sediment-seawater surface: in ﬁne sediments, heavy metals were easily released to the overlying water and the burial of heavy metals would decrease; in coarse sediments the adsorption

390

3 Biogeochemical Processes of the Yellow Sea N 39 38

Unit: % A6 A5 A4 A3 A2A1

37 36

B9 B8 B7 B6 B5 B4 B3 B2 B1

1.4 35

C9 C8 C7 C6 C5 C4 C3 C2C1

34

D9 D8 D7 D6 D5 D4 D3 D2 D1

0.8

33 32 31 119

0.2

120 121

122 123

124 125

126 127

E

Fig. 3.66. The distribution (%) of TOC in the SYS sediments in 2003. A1∼D9 were the sampling stations) (He et al., 2006) (With permission from Springer)

was weak, and the heavy metals were wrapped in part in the mineral crystal lattices or the inner part of the sediment, so the impact of water dynamics and bioturbation on resuspension was restricted. Finally, it was diﬃcult to release heavy metals into seawater following the changes in the environment. It turned out that although coarse sediments have low carrier ability, their burial eﬃciency can be high. The high content area of As was formed in area III by this mechanism. The sedimentation rate was relative to grain size too. Using the calculation of sediment of natural grain size, the sedimentation rate of coarse size (>63 μm) was 16 times that of the ﬁne grain size (<31 μm), and the middle grain size (31∼63 μm) was 4 times that of the ﬁne. Based on this theory, the ﬁne sediment area generally had a low sedimentation rate. The clay area in the central and east of the SYS, and the sea area near the ancient Yellow River Estuary, were low sedimentation areas, whereas sea areas near the Shandong Peninsula in the western part of the SYS were low sedimentation rate areas (Li et al., 2002). Given the determined sedimentation rate, it is still very complicated to understand its impact on the content of heavy metals. For one reason, it played a less important role, and for another, its inﬂuence could be becoming positive or negative. In the low sedimentation rate area, in certain formations of heavy metals which need chemical reaction or ion exchange, the probability of collisions between particles is small, then the deposition process is slow, chemical equilibrium is hard to break and, ﬁnally, the heavy metals in sediments are diﬃcult to release. The high concentrations of heavy metals appear in the region, such as the central part of area I. Fig. 3.31 probably showed a reﬂection of this mechanism.

3.6 Biogeochemical Characteristics of Heavy Metals in Yellow Sea Sediments

391

In the high sedimentation rate area, although the decomposition rate of organic material is high, most organic materials had been buried before they were decomposed and mineralized because of the high sedimentation rate. Heavy metals were contained in those buried organic materials and became inert. The sedimentation rate in the eastern part of the clay area was of 1.65 cm/yr maximum, much larger than that in the central YS mud area. The high sedimentation rate inﬂuences were exhibited in distributions of As, Cd, Cu, and Hg in east clay area. Also, Hg in the Yangtze River Estuary and B7 station near the Shandong Peninsula represented high content areas, both of which were related to the fast deposition process as a result of the high sedimentation rate. 3.6.3.2 Interaction Between Metals and Their Chemical Properties According to the chemical bonded intensity of heavy metals, we may divide the heavy metals into diﬀerent bonded forms of the geochemical phase in sediments. Diﬀerent forms of heavy metals have diﬀerent characteristics of biological utilization and toxicity. Exchangeable form heavy metals are most active in a neutral environment, so they are easy to be released and transformed to other forms by chemical reaction, and have a strong biological toxicity. Changes of ionic composition in water likely aﬀect adsorption-desorption processes of heavy metals. The heavy metals bonded by iron and manganese oxides will be released into water in a reduction environment (i.e., low Eh). The heavy metals bonded by organic carbon are released slowly, but they are also released while organic matter can be degraded in the oxidizing conditions of natural waters. The heavy metals boned ﬁrmly residual form in sediments are diﬃcult to be released, and they are not expected to be released under normally nature conditions. The anthropogenic activities or soil erosion may made the heavy metals into runoﬀs mainly by suspended solid dissolution and adsorption. The suspended solid particulate are good adsorbents and carriers of inorganic ions, and the particulate adsorption capacity increases with the surface area increase. Cu, Pb, and Zn in the Yangtze River water existed mainly in solid particulate form (>86%), and they were input into the estuary. The ﬂuvial heavy metals were released in a region far away from the estuary rather than in mixed water region, so the high content heavy metals appear in the region away from the coast. Obviously, the high content of Hg in 122◦ ∼124◦ N was the embodiment of this elucidation. The linear relationship analysis results of heavy metals and the coeﬃcient matrix are shown in Table 3.37, Cu-Cd and As-Cd showed a strong correlation. Cu and Cd take a diagonal position in the periodic table, and thus their chemical properties are similar and have comparable controlling factors. They displayed similar high content areas in the south of the Shandong Peninsula and in central SYS clay area, and their low content area in China’s inshore area; As and Cd showed a strong negative linear correlation, because As was a

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3 Biogeochemical Processes of the Yellow Sea

metalloid, which behaves diﬀerently in both chemical and physical properties and, as a consequence, the distribution controlling factors and their existing forms in the sediments diﬀered a lot. Pb-Hg, Pb-As, and As-Cu showed moderate correlations, while the former two pairs showed a positive corelation and the third pair showed a negative correlation. Other pairs showed no notable correlation. Interestingly, Zn showed no obvious correlation to the other ﬁve heavy metals and this implied that the enrichment process in the sediments is very complicated. Table 3.37. Correlation coeﬃcients (r) between heavy metals in surface sediments (P <0.05) (He et al., 2006) (With permission from Springer) As Cd Cu Hg Pb Zn

As 1.000 −0.700 −0.551 0.029 0.553 0.191

Cd

Cu

Hg

Pb

Zn

1.000 0.915 −0.037 −0.357 −0.318

1.000 −0.216 −0.324 0.042

1.000 0.539 −0.296

1.000 0.123

1.000

r>0.60, strong correlation; 0.50
3.6.3.3 Inﬂuences of Total Organic Carbon As a whole, total organic carbon (TOC) was high in the center and low in the inshore area of the northern SYS, and TOC in the inshore area near northern Jiangsu was also high, while the highest TOC was in D section (Fig. 3.66). Generally speaking, the ﬁner the sediments, the tighter the sediments will be after deposition. The environment will be in an anaerobic condition, Eh is low, so TOC is easy to be conserved in this kind of reducing environment. As a result, the TOC is high in the ﬁne grain size sediment (area I), and there were many analogical areas between Fig.3.31 and Fig. 3.66. Heavy metals, especially Hg, have a connection with TOC. Hg had a linear correlation with TOC, [Hg]=0.006[TOC]+0.0197 (r =0.41, P <0.05), others do not have a good correlation, but in most of the survey areas they had a similar distribution pattern with TOC: they generally shared the same high content areas, for instance, the central SYS and D section; their low content areas were of the same pattern, like the inshore area of the Shandong Peninsula and Korea. The humic substances in TOC are macromolecular compounds, and they have a strong adsorption and complex eﬀect on the heavy metals. In Thomas and Meybeckm’s study (Chapman, 1992), in unpolluted conditions most heavy metals exist in mineral crystal lattices or ion and manganese oxides; in polluted conditions, the anthropogenic heavy metals are mainly

3.6 Biogeochemical Characteristics of Heavy Metals in Yellow Sea Sediments

393

absorbed on the solids surface or combined with TOC of solids. Take Hg as an example, the background value of Hg in the lower reaches of the Yangtze River was 0.004 μg/L, and its content was 0.03 μg/L, much higher than that of SYS water 0.0036 μg/L, and the content of TOC in D section was the highest in the SYS, so Hg was coupled with TOC in the suspended solids and deposited on the seaﬂoor, ﬁnally showing a similar distribution to TOC, so the large area of high content Hg was determined by TOC. The TOC in sediment was absorbed in the colloid of clay, and its content increased as the grain size became ﬁner. The TOC’s inﬂuence on heavy metals was in synergy with grain size; i.e., the rise in TOC content, as well as the rise in ﬁne grain size content, will lead to an increase in heavy metal contents. This feature showed in certain sea areas for all heavy metals. 3.6.4 Pollution Characteristics and Ecological Risk Evaluation The average contents of As, Cd, Cu, Hg, Pb, and Zn in 2003 were 3.64, 0.159, 22.20, 0.025, 13.85, and 61.35 mg/kg, respectively. Compared to the 8-yearaverage, the contents of 2003 were on the same level except that As was only half of that (Table 3.38). The average contents in most of the sea areas in the SYS were under the background values, with only those of a small number of stations near to or exceeding the background values. Compared to the Marine Sediment Quality of China’s ﬁrst class, the pollution indices of heavy metals were under 1.00, which means that the quality of SYS sediments was relatively good. The averages of the potential ecological risk factor (Eri ) were less than 40 and SYS belonged to a low risk area. As various calculation methods lead to diﬀerent results, and the foregoing methods were too simple for understanding the pollution status, we used therefore the potential ecological risk index method to evaluate the pollution and ecological risk of SYS in an integrated way, which can meet demands for accuracy, simplicity, and rapidity. Table 3.38. Heavy metal concentrations (mg/kg) and their simple pollutionevaluations in the sediments of 2003 (He et al., 2006) (With permission from Springer) Year 2003 Average of 8 years (Cs ) Background value of SYS First class of Marine Quality Standard (Cn ) Pollution index Average of potential ecological risk index (Eri ) Cs /Cn =pollution index

As 3.64 6.89 5.78 20.0

Cd 0.159 0.135 0.103 0.50

0.344 0.27 1.82 19.11

Cu 22.20 17.72 15.92 35.0

Hg 0.025 0.022 0.0159 0.20

0.506 0.11 3.17 5.07

Pb Zn 13.85 61.35 16.27 69.95 14.54 60.00 60.0 150.0 0.271 1.15

0.4663 0.41

TOC (%) 0.931 0.93 0.43 2.0

1.82

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3 Biogeochemical Processes of the Yellow Sea

The distribution of Cd was presented in Fig. 3.67, which was a veriﬁcation of grain size as the main controlling factor for heavy metals. An area of two longitudes from inshore to oﬀshore had a low total pollution level (Cd <8), but southeast of the Shandong Peninsula the sea area was an exception. The Cd of the oﬀshore area was from 8 to 16, and the area with the highest Cd was near B7. Those areas had a moderate pollution level. In the B7 station, the contents of Cu and Zn were 67.53 and 186.2 mg/kg, respectively, which were three times their own averages. Integral calculus of the SYS revealed that 38.7% was in a state of moderate pollution, and no considerable or high pollution areas appeared (Table 3.39). N 39 38 37 14.5

36 35

11.5

34 8.5 33 5.5

32 31 119

120 121

122 123

124 125

126 127

E

Fig. 3.67. Cd of the six heavy metals in the surface sediments of SYS (He et al., 2006) (With permission from Springer)

Table 3.39. The criterion of the contamination degree and potential ecological risk (He et al., 2006) (With permission from Springer) Cf <1 1∼3 3∼6 6

Description Cd Low <8 Moderate 8∼16 Considerable 16∼32 High 32

Description Eri Low <40 Moderate 40∼80 Considerable 80∼160 High 160∼320

Description Low Moderate Considerable High

ERI <150 150∼300 300∼600 600

Description Low Moderate Considerable High

The distribution pattern of ERI was consistent with Cd , only diﬀerentiated in the high area around 33.2◦ N, 123.5◦ E, which meant ERI distribution had two high areas. On the whole, the moderate potential ecological risk area enlarged a lot compared to the moderate Cd area, for example the inshore

3.6 Biogeochemical Characteristics of Heavy Metals in Yellow Sea Sediments

395

area of the Shandong Peninsula and D section belonged to the moderate ERI area (Fig. 3.68). The moderate ecological risk area was 77.8% of the SYS, and no considerable or high risk area existed either. Such diﬀerences were due to the inﬂuence of Tri , which was high in Cd, Hg, and As. This result reﬂected the SYS environment more realistically. N 39 38 37 36

200

35 34

150

33 125

32 31 119

120 121

122 123

124 125

126 127

E

Fig. 3.68. ERI distribution of heavy metals in the SYS (He et al., 2006) (With permission from Springer)

To illustrate the response of biomass to the pollution and ecological risk, we analyzed the relationship between the dry weight of phytoplankton in seawater and the degree of the total contamination level, y=0.0033Eri +8.5808 (r=0.039, P =0.825), and dry zooplankton with potential ecological risk, y=0.0343ERI +175.15 (r=0.026, P =0.898). In the same way, we calculated the ERI of seawater, and there were no correlation to biomass. Such formulae showed there was no correlation between sediment heavy metals and the biomass as yet. The content of heavy metals was still within the tolerance scope for organisms, nevertheless, in some stations, such as A2 and B7, ERI were 128 and 255, while the dry weight of phytoplankton was 12.73 mg/m3 and 41.58 mg/m3 respectively. This implied that the relationship between heavy metals and the biomass was complicated and, as in the SYS, the content levels of heavy metals did not inﬂuence the biomass as yet. Putting the above depictions together, at least in the view of heavy metals, the ecological risk in the SYS was low and indicated that the sediment quality was relatively good in general. Even so, the potential ecological risk was moderate in most areas (ERI >50), which should be paid continuous attention to and studied thoroughly, in case the heavy metals “time bomb” comes into being (He et al., 2008).

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3.7 Biogeochemistry of PAHs and PCBs in the Yellow Sea Sediments PCBs and PAHs, as well-known hydrophobic, potentially toxic, and persistent pollutants, are ubiquitous in terrene and marine environments. Once produced, PAHs and PCBs will be widely dispersed into the environment through air transportation or stream pathway, and then associated with ﬁne particles in the environment, such as the soil, suspended solids, and suspended particulate matter, drained into lakes and the sea through rivers and accumulated eventually in the sediment. Therefore, the proﬁle of these substances in the sediment column can provide insights into local and global time trends of past and present inﬂow changes and the use of organochlorine pollutants, because undisturbed sediments in relatively closed environments (i.e., lakes) can be used as temporal integrators. Consequently, the usage history of PAHs and PCBs could be reconstructed via the analysis of their distributions in the core sediments, with valid ageing methods, such as 210 Pb and 137 Cs methods. Based on these methods, much research focusing on the date sediment trap has been carried out. 3.7.1 Polycyclic Aromatic Hydrocarbons in the Sediments of the Northern Yellow Sea In this work, we mainly talked about the distributions of the PAHs in the northern Yellow Sea, and the source of PAHs was discussed. 3.7.1.1 Horizontal Distribution of Polycyclic Aromatic Hydrocarbons Seventeen kinds of PAHs compounds in the range of 3∼5 rings in the surface sediments of the NYS were inspected quantitatively with the results illustrated in Fig. 3.69 (Li et al., 2002). The concentrations of PAHs inspected and measured were 222.1∼776.3 ng/g. The maximum was detected at the station close to the northeast corner of the Shandong Peninsula, while the minimum was detected at the station on the cross line between the NYS and the SYS. In general, PAHs concentrations in the sediments from the north and south of the YS were higher than those from the middle, and PAHs concentrations in the sediments from the west coast were higher than those from the east coast, which might have resulted from riverine inputs. For example, Yalujiang River pours into the NYS with huge amounts of sediment particles where PAHs are adsorbed. After the ﬂow rate declined in the estuary, the majority of sediment particles settled down, resulting in PAHs deposition with sediment particles. Additionally, dense PAHs concentration isolines suggested the occurrence of a high concentration gradient. This phenomenon also resulted from the sediment particle deposition. Southwards the Yalujiang River

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397

encountered the northward YSWC in the south of the NYS, thus the terrestrial sediment particles were deposited rather than transferred southwards. Consequently, the residual level of PAHs was relatively low; furthermore, a high PAHs concentrations gradient occurred.

Fig. 3.69. Distribution of PAHs (ng/g) in surface sediment in the northern YS (Li et al., 2002) (With permission from Zhang J)

3.7.1.2 Polycyclic Aromatic Hydrocarbons Sources Another approach to apportion petroleum/pyrogenic PAHs origin was the use of molecular indices based on isomeric ratios. Because some PAHs compounds had stronger thermo-dynamical stability than their isomeric compounds, their molecular ratios could supply the clue for judging whether they were the production of high temperature combustion, such as ANT/178, FLR/202, B(a)A/228, and INP/276. For mass 178, the ratio of anthracene to anthracene plus phenanthrene (ANT/(ANT+PHEN) or ANT/178) <0.10 was usually taken as an indication of petroleum, while a ratio >0.10 indicated a dominance of combustion. For mass 202, ﬂuoranthene to ﬂuoranthene plus pyrene (FLR/(FLR+PYR) or FLR/202) ratio of 0.50 was usually deﬁned as the petroleum/combustion transition point. The ratios between 0.40 and 0.50 were more characteristic of liquid fossil fuel (vehicle and crude oil) combustion whereas ratios >0.50 were characteristic of grass, wood or coal combustion. B(a)A/228 ratio less than 0.35 or INP/276 ratio less than 0.20 likely indicated petroleum. INP/276 ratios for wood soot were likely to be more representative of material that was accumulating in sediments. Accordingly, INP/276 ratios <0.20 likely imply petroleum, ratios between 0.20 and 0.50 imply liquid fossil fuel (vehicle and crude oil) combustion, and ratios >0.50 imply grass, wood, and coal combustion (Yunker et al., 2002).

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3 Biogeochemical Processes of the Yellow Sea

From the indicators, the dominant source of PAHs in the NYS was the incomplete combustion of fossil fuel. Perylene was identiﬁed from most surface sediments, which indicated that the substances from a terrestrial source were accumulating in the northern YS. The main source was the terrestrial substance transported from the Yalujiang River Estuary into the sea. 3.7.1.3 Contribution of Terrestrial Input It was widely accepted that perylene was a preface index to reﬂect the material input of terrestrial origin. In the surface sediments from the NYS, perylene concentrations ranged from 21.6 to 126 ng/g, accounting for 6.9%∼24% of the total PAHs concentrations. Fig. 3.70 (Li et al., 2002) illustrated the ratios of perylene to PAHs.

Fig. 3.70. The ratios of perylene/PAHs from the NYS (Li et al., 2002) (With permission from Zhang J)

As Fig. 3.70 showed, the ratios of perylene/PAHs exceeded 10% in most of the NYS, wherein the highest ratio (22.5%) was detected at the station close to the Yalujiang River mouth, while the ratios were lower than those from the southeast of the NYS. Consequently, perylene in the NYS mainly derived from the terrestrial material carried by the Yalujiang River. The majority was deposited after entering the NYS, and the rest was transferred southwards. The sediment provenance and distribution also improved the sediments of the NYS, these being mainly from the Yalujiang River. The material carried by the branch of the YSWC and the coastal current of west Korea diluted the perylene from the Yalujiang River.

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3.7.2 Polychlorinated Biphenyls in the Sediments of the South Yellow Sea Twenty-seven PCBs congeners with 4∼9 Cl atoms were determined. The PCBs congeners’ total concentration was chosen to reﬂect the PCBs pollution level in the SYS, whose distribution was shown in Fig. 3.71. 3.7.2.1 Horizontal Distribution of Polychlorinated Biphenyls Total PCBs concentrations were not homogenous, and they extended over 2 orders of magnitude from 518 to 5,848 pg/g. The sequence of PCBs concentrations in the four lines was: B line (2,638 pg/g)>D line (1,618 pg/g)>C line (1,334 pg/g)>A line (1,120 pg/g), with the maximum value of 5,848 pg/g at the B2 station and the minimum value of 518 pg/g at the C4 station. The PCBs concentrations of B4, B5, C2, and C7 were grouped into a higher value team, whose value exceeded 3,000 pg/g, consistent with the reports of recent years, whereas exceptionally low concentrations appeared at the C4 and C5 stations, 518 pg/g and 589 pg/g respectively, which was not consistent with the earlier reports, so this phenomenon was worth while studying deeply. The average value of PCBs concentration in the study area was 1,715 pg/g, decreasing by 1,775 pg/g compared with last year. PCBs, as a kind of very complex synthesized macromolecule compound, were industrially produced from 1965 and banned until the end of the 1980s in our country. Under biodegradation and chemo-degradation circumstances, PCBs were decomposed and transformed so that PCBs pollution level kept on declining. Nonetheless, it would still take a very long time for PCBs to disappear in the sediments because their special steric structure resulted in their poor self-cleaning ability. In this study, all the PCBs determined had 4∼9 Cl atoms, the majority of which were recommendatory marine pollutants of the International Council for Explorations (ICEs). The average PCBs concentration in the SYS surface sediments was lower than the pollutant control standards of the National Environment Protection Agency (50 mg/kg, GB 13015 Σ91), in the lower PCBs pollution level of the global coastal surface sediments (200∼400,000 pg/g) (Fowler, 1990). The average value of the A6, B9, C9, and D9 stations adjacent to the Chinese coast was designated to evaluate the Chinese coastal pollution level; similarly, A1, B2, C1, and D6 values were taken for Korea. The result showed that the PCBs pollution status on Korean coast (2,244 pg/g) was more serious than that on the Chinese coast (1,124 pg/g). It was reported that the PCBs concentration on the suspended particles was 4∼10 times more than that in the sediments (Haque et al., 1993). The SYS covers 309,000 m2 , with an average depth of 46 m, thus the total PCBs weight ranged from 125 to 276 t. Based on previous ﬁeld researches, Long put forward the Eﬀects Range Low (ERL, bio-nocuousness probability <10%) and Eﬀects Range Median (ERM, bio-nocuousness probability >50%) values to estimate the latent eco-risk of PCBs in the marine and estuary sediments, which were

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3 Biogeochemical Processes of the Yellow Sea

50 ng/g and 400 ng/g (Long et al., 1995) respectively. The average PCBs concentration was one order of magnitude less than ERL; consequently, PCBs rarely imposed negative eﬀects on the SYS eco-environment. From Fig. 3.71, the horizontal distribution of PCBs in the SYS surface sediments revealed the trend as follows: the PCBs concentrations in the middle area were highest, followed by the Korean coast, and that on the Chinese coast was the lowest among them; the PCBs concentrations in the northern zone were higher than that in the southern zone; on both coasts the PCBs concentration isolines were nearly parallel to the coastlines. These above phenomena suggested that the eﬀects of the factors controlling PCBs distribution were stronger than those of the sources. N 38 0 120

37

2400

3600

36

121

1200

1600

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3200 2800 240 0 00 20

1200

34

1600

800

1600

20 00

35

33 120

80 0 1200 1500 2000 2800

125 126

127

128 E

Fig. 3.71. PCBs horizontal distribution (pg/g) in the SYS surface sediments (Zhang et al., 2007) (With permission from Elsevier’s Copyright Clearance Center)

3.7.2.2 Relationships Between Polychlorinated Biphenyls Distribution and Environmental Parameters (1) Sediment type and grain size Based on the grain size, the sediments were classiﬁed as gravel, sand, silt, and clay. No gravel was found at every station, and the sands and clays were the dominant components in the diﬀerent areas. The high values of sand concentrations in the surface sediments appear in both coasts, A line and D line were quite high, which at A1, A2, A3, A4, B2, C1, C7, and C8 ranged from 56.6% to 99.2%, while a large area of clay sediments developed in the SYS middle area, whose concentrations exceeded 66%. The sediment grain size was controlled by the distance of the material source, transportation medium, transportation mode and the sediment environmental characteristics. B2 and the A line stations were near to the Han River Estuary, so that the sand and silt schlepped by the Han River contributed enormously. Additionally, due to the eﬀects of the ancient Yellow

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401

River and Changjiang River in the W¨ urm glaciations, a thick layer of sand sediments developed in the west coast area north to the Changjiang River Estuary, represented by B9, C7, C8, and C9 in the study area. Therefore, the sand concentrations of the stations mentioned above were relatively high. The ﬁner sediment area in the SYS middle area reﬂected the eﬀects of the transportation mode and sediments characteristics on the sediment types. The clay sediments of the middle area in the SYS continental shelf tended to be transported to the mid-north sediments area center, consistent with the cold vortex center by and large, locating at about 123.4◦ E, 35.1◦ N. The weak hydrodynamic energy controlled by the cyclonic circumﬂuence in the surface and cold vortex in the bottom resulted in the centrality in PCBs transportation (Shi et al., 2002). Therefore, ﬁne sediment particles were transported to the SYS middle area with the ocean current and settled down very tardily. In order to discuss the relationship between PCBs and sediment type, the SYS was divided into three parts, namely I, II, and III regions (Fig. 3.31). In the three regions, due to the diﬀerences in sediment types and grain size, the adsorbility of the sediment particles to PCBs is diﬀerent, and thus the PCBs concentrations in the three regions are diﬀerent. PCBs concentration sequence in the sediments of the three regions was: I region (2,545 pg/g)>II region (1,387 pg/g)>III region (1,193 pg/g). There were signiﬁcant relationships between PCBs concentrations and the sediment grain size and types; i.e., PCBs concentrations increased with the increase in clay concentrations and the decrease in average grain size; the adsorption capacity of clay to PCBs was the strongest, followed by silt and sand in turn. In general, the interaction between the sediment particles and HOPs molecules was a relatively weak composition of force caused by van der Waals force and the entropy gradient forcing the HOPs molecules to leave the water phase. Therefore, the PCBs congeners’ structures and the sediment environmental conditions aﬀected the sorption process a lot. There was a negative relationship between sediment grain size and PCBs concentrations. Concretely, the less the sediments grain size, the bigger the surface area, the more active the sorption spots, and the more the PCBs were enriched. However, the sorption of PCBs in the sediment particles was considered as two eﬀects working together. One was the inorganic mineral material sorption, and the other was the organic material sorption. Because of the polarization eﬀect of the inorganic mineral material and the existence of a polar water molecule, the active sorption spots were always occupied by water molecules, so it was widely accepted that the organic material sorption process was dominant (Luo et al., 2004; Song et al., 2004). Meanwhile, in general, the greater surface area of the smaller particles also provided a larger area for the adsorption of organic matter. As discussed above, explaining the relationship between PCBs and sediments in terms of grain size and sediment types was limited and the contents, composition, and types of organic matter in sediments maybe had more potential contribution to this system. So the discussion about TOC contents was really necessary.

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3 Biogeochemical Processes of the Yellow Sea

(2) TOC in sediments The TOC contents in the SYS surface sediments followed this order: B line (0.91%)>C line (0.531%)>A line (0.247%), with the maximum value of 2.01% at B5 and minimum value of 0.04% at A1. The average value of the whole study area was 0.554%, and the TOC contents in the middle area were obviously higher than those on both coasts, similar to the PCBs horizontal distribution, and the stations with higher TOC contents were almost all located in I region, which was the clay sediments dominant region. The atomic ratio of C/N could be used to deﬁne the general characteristics of organic matter input. This approach had been supported by stable isotope measurements on sites elsewhere. In this respect, C/N>12 was taken as being from a terrigenous input. Planktonic organisms had lower C/N ratios with a value between 4 and 7, with zooplankton and phytoplankton having an average ratio of 5∼6 and bacteria and benthic organisms, which were generally rich in protein, had a C/N ratio of 4.1∼4.2 (Muller and Mathesius, 1999). The C/N ratio in the SYS was 16, which conﬁrmed that the majority of organic matter in the SYS sediments originated from terrestrial material and the zooplankton and phytoplankton biomass acted as an assistant source. TOC contents in the three regions described as Fig. 3.31 were as follows: I region (1.12%)>II region (0.47%)>III region (0.23%), so an obvious relationship between TOC contents and the sediments type in the study area occurred, and a negative relationship between TOC contents and the sediment grain size was discovered, too. It was also reported that TOC contents not only aﬀected the PCBs distribution, but also aﬀected the congeners’ composition. In the clay sediments area, the interstices among small sediment particles were small, from which correspondingly little dissolved oxygen resulted and, furthermore, TOC contents there were of the higher group and were inclined to consume a lot of dissolved oxygen to be degraded. Hence the redox potential (Eh) in the clay settling region was the lowest among the three regions, which encouraged a low degree of chlorinated PCBs to biodegrade and transform, and meanwhile kept the more highly chlorinated PCBs (Cl>4) in the region. In the SYS surface sediments, the PCBs Cl atom number ranged from 4 to 9 with the highest concentrations in I region, so our study proved the theory again as a piece of new evidence. Since the TOC horizontal distribution was quite similar to that of PCBs’, the assumption about whether a linear relationship between TOC contents and PCBs concentrations existed was presented. After analyzing the linear relationship, it was found that a signiﬁcant linear relationship between TOC contents and PCB concentrations occurred, with r =0.61. Much research pointed out that the concentrations of organic pollutants adsorbed by the small particle sediments linearly related to the TOC contents there (Song, 2004), while Colombo gave a more complex conclusion in his research about the R´ıo de la Plata Estuary (Colombo et al., 2005). PCBs concentrations were signiﬁcantly linearly related to TOC contents within 1 km of the coast (r =0.96), reﬂecting the contribution of anthropogenic sources to PCBs dis-

3.7 Biogeochemistry of PAHs and PCBs in the Yellow Sea Sediments

403

tribution, while the linear relationship diminished at 2.5 km (r =0.92), and disappeared at a distance of 4 km from the coast. Historically, the question of the sorption of organic pollutants from gas or liquid to a solid surface has been debated among the two diﬀerent mechanisms from physical adsorption to chemical sorption and the interaction eﬀect. Chiou published his paper in which he proposed that the transfer of hydrophobic organic carbons (HOCs) from water to soil might be described in terms of a hypothesis of solute partition in the soil organic material where the isotherm was linear (Chiou et al., 1979). At the present time, lots of ﬁeld and laboratory experiments have conﬁrmed and expanded his theory, one of which was the explanation of the non-linear phenomenon of the isotherm; i.e., the organic carbon contents and the microstructure of particles would aﬀect the sorption behavior of HOCs. If the organic carbon content in the soils or sediments was high, partition mechanisms would mainly function; otherwise, the microstructure of particles would impose a primary eﬀect. Likewise, it was reported that the organic materials in the sediments were a kind of extraordinary heterogeneous sorbent, to them. If TOC contents were more than 0.1%, the TOC contents would well relate to the HOCs phase partition coeﬃcient Kp , namely, the ratio of the concentration of a chemical (e.g., PCBs congers) in sediments to the concentration of the same chemical in water. It had been demonstrated that the sorption of a non-polar compound in laboratory experiments correlates to organic carbon (OC) contents in the sorbent. As a consequence, the equilibrium partition concept for hydrophobic organic contaminants was normalized to the percentage of organic C, giving the equation: Kp = Koc · foc

(3.29)

where Koc was the normalized C partition coeﬃcient and foc was the weight fraction of organic matter (% organic/100). This was based on the assumption that OC from diﬀerent sources had similar aﬃnities for PCBs congeners. So it was easy to deduce that low TOC contents corresponded to a low Kp value, though the sorbent had a larger surface area. Additionally, many scientists also proposed that the sediment adsorption capacity to PCBs was aﬀected by the following factors: the sediment positions, sources, petrogenic eﬀect, and the degree of PCBs congeners chlorination. In conclusion, adsorbing PCBs on the sediment particles was a very complex process. Thus further work, such as TOC components and relative abundance, PCBs congeners characteristics, and so on, should be carried out to address a more detailed evaluation of their inﬂuence. (3) Heavy metals in sediments Six heavy metals of As, Cd, Cu, Hg, Pb, and Zn were detected in this investigation. Their concentration ranges were as follows: 3.80∼33.0 mg/kg, 0.042∼0.416 mg/kg, 6.80∼33.0 mg/kg, 0.011∼0.066 mg/kg, 4.91∼35.6 mg/kg, and 32.7∼141.0 mg/kg respectively, with average concentrations of 7.09 mg/kg, 0.130 mg/kg, 17.2 mg/kg, 0.040 mg/kg, 16.7 mg/kg, and 72.4 mg/kg

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3 Biogeochemical Processes of the Yellow Sea

respectively, and without signiﬁcant changes compared to last year’s reports. The maximum detection values were obtained at A1, A1, B7, C2, B3, and B5, adjacent to both coasts, except for B5, where the highest Zn concentration was detected. It suggested that the six heavy metals in the SYS surface sediments came mainly from terrestrial input and the source eﬀect functioned mainly in their transference and enrichment, except for Zn. The heavy metals of Cd, Cu, Hg, Pb, and Zn were enriched in the SYS middle area, especially in the B line, while As was distributed dispersedly without a strong tendency. Taking into account the three regions, the relationships between the sediment types and the six heavy metals were investigated. The P and r values of the linear analyses on the relationships between sediment representative characteristics (TOC contents and relative proportions) and the six heavy metals concentrations are summarized in Table 3.40, which are quite complex. Besides As and Cd, the concentrations of the remaining four heavy metals were in the following order from the most abundant to the least: I region>II region>III region; i.e., the concentrations of the four heavy metals of Cu, Hg, Pb, and Zn increased as the clay proportions increased and the sediment grain size decreased, and As and Cd vary irregularly without any consistent pattern with the sediment characteristics. Moreover, the relationships between heavy metal concentrations in the TOC contents, as well as sand, silt, and clay proportions, did not display consistent trends in the three regions either. For example, in I region, all the heavy metal concentrations positively related to the clay concentration except for Pb, with the r value ranging from 0.39 to 0.90, while in III region, a negative relationship occurred, with the r value ranging –0.67∼–0.34. The status of the sand proportion was even reversed. Inside, Cd and Cu showed well their positive linear relationships with clay in I region, and it was also true of TOC contents because they were subject to chelate with the organic matter. Cd and Cu can be transformed with the complex compounds, which lead to an increase in Cd and Cu concentrations in sediments. Analyzing from the general to the speciﬁc, not all the heavy metals related well with the sediments characteristics. The factors controlling metal distributions in the sediments were mainly classiﬁed according to natural conditions and anthropogenic sources, wherein the anthropogenic sources include various contamination inputs caused by human dispersal into the environment, including dry and wet deposition, industrial and municipal sewage, marine transportation pollutant and the exploitation of underground mining. The natural conditions include the proportion of sediment types caused by hydrodynamic conditions, the salinity, pH and Eh of seawater and sediments, and so on. According to the above discussion, it is clear that the source and the sediments type did play the dominant role. Therefore, further studies about the other factors will be quite necessary, and emphasis should be placed on the salinity, pH and Eh of sediments and the hydrodynamics conditions of the SYS. Analyzing the linear relationship between a pair of heavy metals, the coefﬁcients matrix of the linear relationship was shown in Table 3.40. As and Cd,

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405

Table 3.40. Correlation coeﬃcients between heavy metal concentrations and the sediment types and TOC concentration of the SYS surface sediments (Zhang et al., 2007) (With permission from Elsevier’s Copyright Clearance Center) Heavy

Range

metal

(mg/kg)

TOC vs. heavy (mg/kg) metal P r Average

Sand vs. heavy metal P r

I region (6 stations) As 5.20∼8.28 6.85 0.3 0.5 0.03 Cd 0.081∼0.239 0.188 0.6 0.25 0.4 Cu 19.1∼33.0 25.6 0.5 0.34 0.02 Hg 0.046∼0.066 0.057 0.6 0.29 1 Pb 13.74∼26.35 20.35 0.9 0.07 0.06 Zn 74.3∼141.0 105.2 0.03 0.84 0.5 II region (12 stations) As 3.80∼8.35 5.88 0.8 0.1 0.4 Cd 0.042∼0.227 0.0969 0.3 0.33 0.1 Cu 8.2∼25.6 15.88 < 0.01 0.81 0.3 Hg 0.011∼0.058 0.037 0.04 0.59 0.6 Pb 4.91∼35.59 16.29 0.01 0.86 0.5 Zn 32.7∼108.0 64.35 0.1 0.47 0.3 III region (9 stations) As 4.34∼33.0 8.85 0.2 −0.55 0.3 Cd 0.061∼0.416 0.135 0.1 −0.58 0.1 Cu 6.8∼20.0 13.4 0.06 −0.69 0.09 Hg 0.015∼0.047 0.03 0.9 −0.07 0.1 Pb 7.9∼29.11 14.78 0.5 −0.26 0.2 Zn 44.0∼76.6 61.2 0.9 −0.06 0.1

Silt vs. heavy metal P r

Clay vs. heavy metal P r

−0.86 −0.43 −0.87 −0.01 0.79 −0.34

0.1 0.69 0.4 0.47 0.5 −0.37 0.02 0.90 0.4 0.44 0.1 0.71 0.5 −0.31 0.6 0.30 0.1 −0.7 0.5 −0.38 0.9 0.06 0.4 0.39

−0.29 0.5 0.31 0.19 0.22 0.33

0.2 0.08 0.1 0.3 0.2 0.3

0.37 −0.52 −0.47 −0.33 −0.44 −0.35

0.7 −0.14 0.7 −0.12 0.6 0.18 0.5 0.19 0.3 0.31 0.8 −0.067

0.38 0.54 0.60 0.54 0.45 0.57

0.5 0.3 0.09 0.07 0.3 0.07

−0.24 −0.42 −0.59 −0.64 −0.37 −0.64

0.1 0.05 0.2 0.4 0.1 0.3

−0.54 −0.67 −0.52 −0.34 −0.53 −0.41

Cu and Cd, Cu and Hg, Cu and Zn, and Zn and Hg presented signiﬁcant linear relationships. Their correlation indices range from r =0.60 to r =0.76, which indicated that such elements had the same sources and a similar occurrence in the same samples. Correlation analysis results presented by Fig. 3.72 showed that the six heavy metals, except for As, positively related to PCBs concentrations, and there was no signiﬁcant relationship between As concentration and PCBs concentrations. The positive relationships suggested that they had the same source, and the factors controlling PCBs distribution also worked on the heavy metals of As, Cd, Cu, Hg, Pb, and Zn. In addition, their function medium, mode, and outcomes displayed similarities. (4) Phytoplankton and zooplankton biomass Organic matter in sea surface sediments was mainly terrigenous and biogenous. So the phytoplankton biomass impacting on organic matter in the water column and on the PCBs distribution in the surface sediments of the study area was discussed in detail as follows. Chlorophyll a concentrations and phytoplankton cell abundance in seawater were commonly used as indicators of

3 Biogeochemical Processes of the Yellow Sea PCBs concentration (pg/g, dw)

35

5000 4000 3000 2000 1000 0

0

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PCBs concentration (pg/g, dw)

r=0.36 P=0.07 6000

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10 15 20 25 30 35 40 Pb concentration (mg/g)

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35

PCBs concentration (pg/g, dw)

r= 0.13 P=0.51

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PCBs concentration (pg/g, dw)

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406

r=0.18 P=0.38

6000 5000 4000 3000 2000 1000 0

0

0.10 0.20 0.30 0.40 Cd concentration (mg/kg) r=0.14

P=0.48

6000 5000 4000 3000 2000 1000 0

0.01 0.02 0.03 0.04 0.05 0.06 0.07 Hg concentration (mg/g) r=0.32 P=0.10

6000 5000 4000 3000 2000 1000 0 20

40

60 80 100 120 140 Zn concentration (mg/g)

Fig. 3.72. Relationships between PCBs concentration and the concentrations of As, Cd, Cu, Hg, Pb, and Zn in the surface sediments of the SYS (Zhang et al., 2007) (With permission from Elsevier’s Copyright Clearance Center)

phytoplankton biomass and regarded as important parameters to describe ocean ecosystems and environmental characteristics. The horizontal distributions of chlorophyll a concentration, phytoplankton cell abundances, and zooplankton wet weights in the SYS were shown in Fig. 3.73. As indicated in Fig. 3.73, there were general downtrends from northwest to southeast with the higher values of chlorophyll a concentrations and phytoplankton cell abundance observed in inshore surface seawater, especially around stations A1 and D9. The horizontal distribution of zooplankton wet weight was similar to the distributions of chlorophyll a concentrations and phytoplankton cell abundance. It showed a good correlation with PO4 -P and

3.7 Biogeochemistry of PAHs and PCBs in the Yellow Sea Sediments N 38

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Chl a-0m Cells density 33 33 120 121 122 123 124 125 126 127 128 E 120 121 122 123 124 125 126 127 128 N 38

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37 36 35 34 Wet weight 33 120 121 122 123 124 125 126 127 128

E

Fig. 3.73. Horizontal distribution of the chlorophyll a (μg/L), plankton cell density (×103 cells/L), and zooplankton biomass based on wet weight (mg/m3 ) in the surface seawater of the SYS (Zhang et al., 2007) (With permission from Elsevier’s Copyright Clearance Center)

a negative correlation with TOC. There were so many environmental factors imposing their eﬀects on the phytoplankton biomass and primary productivity, such as nutrients, light availability, temperature, and so on (Falkowski and Woodhead, 1992). It was reported that in respect of controlling the photosynthetic capacity of the phytoplankton community, nutrients occupied the driving seat rather than temperature. Since the 1980s, the YS ecosystem has been suﬀering the P limit (N:P=25>16). Apparently, A1 and D9 came under the strong inﬂuence of the Changjiang River’s eﬄuent waters and the surface runoﬀ in the SYS western coast, where the P concentrations were very high. High nutrients promoted the growth of phytoplankton, and thus formed high chlorophyll a concentration regions. However, in the middle area away from the seaboards, nutrients could not be supplied from terrestrial input. Therefore, nutrient limitation restrained the growth of phytoplankton, thus forming a plume and a closed center with a signiﬁcantly low chlorophyll a concentration. Studies showed that the nutrients level in seawater had a great impact on the phytoplankton biomass, sedimentation rate, suspended particle concen-

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3 Biogeochemical Processes of the Yellow Sea

tration, particle organic carbon, and dissolved organic carbon. Phytoplankton were the major organisms that ﬁx CO2 , in either particulate or dissolved forms via the photosynthetic process. They constituted the base of the marine food chain and thus might aﬀect the dynamics of higher trophic levels due to the variability in their biomass and productivity. The dead phytoplankton, the byproduct of zooplankton metabolic activity through fecal pellet production and the vertical migration of zooplankton through feces production, represented ﬂuxes of carbon at depth. The deep sea was the ultimate sediment trap. In the productive sea area, the vertical ﬂux of organic matter was relatively high. Besides, O2 was almost exhausted by plankton and the remineralization of organisms in sediments was weak. So, the relatively high concentration of the particulate matter reaching the sea ﬂoor was maintained and TOC in surface sediments was high. In the middle of the SYS where there was a nutrient deﬁcit, TOC in sediments was relatively low. So the PCBs concentrations were low, correspondingly. However, we saw a contrary trend in PCBs contribution. The discrepancy between biomass and TOC distribution indicated that the phytoplankton biomass was not a key factor to aﬀect PCBs distribution, and the factors controlling suspended particles distribution played a more important role in the TOC concentration. (5) Impact of water hydrodynamics on PCBs in sediments Many previous reports proposed that the PCBs abundance fell precipitously as the distance increased from the contaminant source in many estuaries and coastal areas, while the PCBs horizontal distribution tendency in the SYS was very special (Mora et al., 2005; Chen et al., 2003; Colombo et al., 2005). From Fig. 3.31, it was easy to ﬁnd that the middle sediments area presented a special characteristic, where the ﬁne particle sediments developed and, through the above discussion, it could be indicated that the factors working mainly on PCBs distribution had direct and indirect contact with the sediment characteristics to some extent, such as sediment components and proportions, TOC contents, plankton biomass, and so on. According to earlier publications, hydrodynamics was the dominant driving power to aﬀect the sediment types and distribution (Li et al., 2003; Dai et al., 2007). In the YS, the function of the warm current, coastal current, and circumﬂuence presented a piece of valid evidence that hydrodynamic conditions controlled the sediment deposition process. In general, the intensity of the current corresponded to the sediment grain size (Tang et al., 2000). It was thought widely that the sediments of the SYS middle and east areas were aﬀected by Kuroshio and the Cheju Channel (Lan et al., 2005). In summer, a stable southerly coastal current in the west SYS existed; at the meeting of the YS and the East Sea, the ﬂash water mainly originated from the Changjiang River and obviously expanded to Cheju Island, while in the east of the SYS, a northerly coastal current developed. Consequently, as a whole, the surface water approximately formed a marine basin scale anticlockwise circumﬂuence. In the bottom, a cold water mass developed and occupied the whole SYS. By way of CTD ob-

3.7 Biogeochemistry of PAHs and PCBs in the Yellow Sea Sediments

409

servation and the excursion of the man-made acaleph, the bottom current of the SYS approximately took on an anticlockwise cyclonic movement, too. In winter, a warm current with high salinity ﬂowed from south to north in the SYS middle area, the so-called traditional YS Warm Current, on both sides of which there were two southerly coastal currents, forming a little circumﬂuence on the west coast. Furthermore, in the east, the YS Warm Current ﬂowed on the outside continental shelf from south to north. A nearly circular weak current area came into being centered on the point of 36◦ N, 123◦ E, with the velocity of ﬂow less than 0.4 m/s (Tang et al., 2000). Because of the eﬀect of the cold mass and marine circumﬂuence, a large clay sediment region developed. In conclusion, the PCBs distribution was aﬀected by the characteristics of sediments, such as grain size, clay proportion and TOC content, but the hydrodynamic condition was the most important factor aﬀecting the sediment mode and sediment transportation tendency. In the middle sediment region, whose dominant component was clay, the surface area and sorption capacity increased with a decrease in the grain size. Meanwhile, due to the high TOC contents in ﬁne particle sediments, the sorption capacity of the organic pollutant increased further. Thus the conclusion could be drawn safely that the characteristics of the sediments were the dominant factor controlling the PCBs distribution, and the driving power of the hydrodynamic conditions was the reason that led to this result (Zhang et al., 2007). 3.7.3 Contamination History of Polycyclic Aromatic Hydrocarbons and Polychlorinated Biphenyls in the 20th Century The natural sediment cores used to measure PAHs and PCBs were reported in many previous studies. In China, the studies on dating sediment trap experiments designed to reconstruct the historical usage of PAHs and PCBs were not carried out frequently for various reasons, and the sampling areas where they were studied mainly focused on the Pearl River Estuary, Changjiang River Estuary and Bohai Bay. Although many investigations have been presented on the distribution and sources of PAHs and PCBs in the surface sediment of the YS, the sedimentation rate and ﬂux of core sediment samples, no available investigation was documented on the vertical distribution of PAHs and PCBs in the core sediment and this aspect of historical usage had not been included in the targets. D6 station (33.855◦ N, 123.033◦ E) was sampled in II region controlled by weak hydrodynamic conditions and covered with middle grain size sediment particles (Fig. 3.31). 3.7.3.1 Sediment Aging Quantitative reconstruction of contaminant spatial and temporal trends requires precise chronology. A common approach of 210 Pb has been in use. The

410

3 Biogeochemical Processes of the Yellow Sea

vertical proﬁles of total and excess 210 Pb activities have shown that the logarithm of excess 210 Pb activities is signiﬁcantly and linearly related to depth (r =0.99, P <0.01). The linear regression equation of lgAH and H was obtained using the Least Square method as follows: lgAH =0.20–0.04H. The average sedimentation rate was 0.34 cm/yr and every layer in D6 was deposited for 6 years. The sample was collected in 2004, suggesting that the surface sediment was deposited in 2004, thus the sediment chronology was obtained. 3.7.3.2 PAHs in the 20th Century (1) PAHs concentrations The vertical proﬁles of PAHs and PCBs that reﬂected their variance with depth and time were shown in Fig. 3.74. Sixteen individual PAH compounds recommended as the priority pollutants by US EPA were detected, with the results tabulated in Table 3.41. The concentrations of all individual PAH compounds (ΣPAHs) in each layer were summed up to evaluate PAHs levels. From surface to bottom, ΣPAHs ranged from 26.31 (24∼26 cm) to 76.92 ng/g (14∼16 cm) based on dry weight. In general, benzo(b)ﬂuoranthene (B(b)F) was the prevalent component in most samples, accounting for 7%∼16%. Table 3.41. Sixteen individual PAHs and total PAH (tPAH) concentrations (ng/g, dw) in every layer (cm) core sediment (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center) Layer (cm) 0∼2 NAP ACY ACE FL PHEN ANT FLR PYR B(a)A CHY B(b)F B(k)F B(a)P INP D(ah)A B(ghi)P tPAHs

2∼4

4∼6

6∼8 8∼10 10∼12 12∼14 14∼16 16∼18 18∼20 20∼22 22∼24 24∼26 26∼28

5.38 2.53 2.88 2.23 2.04 0.44 0.15 0.20 0.09 0.08 0.65 N.D. 0.06 N.D. 0.06 4.90 2.85 3.10 2.50 2.62 11.03 7.47 8.62 8.16 7.79 N.D. 0.50 0.55 N.D. 0.20 7.49 5.65 6.07 5.76 4.88 5.09 3.65 3.96 3.85 3.45 0.91 0.51 0.87 1.03 0.85 2.57 1.49 1.76 2.07 1.44 9.96 10.12 9.36 7.89 7.84 3.16 3.11 2.94 2.56 2.69 1.90 1.95 1.77 1.44 1.45 4.87 5.26 5.26 4.11 3.12 1.68 1.45 1.50 1.56 1.55 3.59 3.71 3.82 3.21 2.41 63.63 50.39 52.71 46.47 42.47

2.02 0.07 N.D. 2.68 9.50 0.22 5.62 3.76 1.02 1.55 7.90 2.06 1.40 3.67 1.52 2.70 45.70

3.04 0.14 0.10 4.37 11.02 0.40 6.51 4.13 1.22 1.69 8.99 2.48 1.61 3.60 1.84 2.56 53.70

7.49 0.24 2.23 14.49 25.00 1.59 5.32 3.83 0.91 1.50 5.14 1.51 1.26 2.53 1.81 2.07 76.92

1.82 N.D. 0.08 2.91 7.39 N.D. 2.61 2.18 0.68 1.05 3.97 1.07 0.83 1.83 1.13 1.55 29.11

3.60 0.07 0.58 6.77 18.29 0.37 3.83 3.00 0.79 1.14 4.22 1.58 1.00 1.76 1.49 1.68 50.17

9.93 0.27 0.84 6.81 14.47 N.D. 5.39 4.16 1.36 1.82 6.67 5.27 1.76 2.36 2.40 2.63 66.15

2.59 N.D. N.D. 2.15 8.06 N.D. 2.24 2.10 0.73 0.97 3.05 1.18 0.85 1.56 N.D. 1.22 26.69

2.29 N.D. N.D. 1.97 6.85 0.11 1.99 2.10 0.58 0.80 2.92 2.45 0.74 1.15 1.19 1.15 26.31

2.87 N.D. N.D. 2.94 10.19 N.D. 2.38 2.40 0.63 0.81 2.44 1.94 0.69 0.88 N.D. 1.21 29.40

Naphthalene (NAP), Acenaphthylene (ACY), Acenaphthene (ACE), Fluorene (FL), Phenanthrene (PHEN), Anthracene (ANT), Fluoranthene (FLR), Pyrene (PYR), Benzo(a)anthracene (B(a)A), Chrysene (CHY), Benzo(b)ﬂuoranthene (B(b)F), Benzo(k)ﬂuoranthene (B(k)F), Benzo(a)pyrene (B(a)P), Indeno(1,2,3.cd)pyrene (INP), Dibenz(a,h)anthracene (D(ah)A), Benzo(g,h,i)perylene (B(ghi)P). N.D.: not detectable

(2) Temporal trends of PAHs We evaluated the ages of every layer by virtue of the depth and sediment rate. From bottom to surface, the core sample was deposited from 1914 to 2004 with about 6 years interval at every layer. Fig. 3.74 showed the vertical proﬁle of ΣPAHs varying with depth and time. In general, ΣPAHs decreased with depth

0

5

Depth (cm)

10

15

20

25

30

tPAHs (ng/g, dw) and tPCBs (pg/g, dw) 40 80 120 160 200 2004 1998 1992 1986 1980 1974 1968 1962 1956 1950 1944 1938 1932 1926 1920 1914

411

Year

3.7 Biogeochemistry of PAHs and PCBs in the Yellow Sea Sediments

PAHs concentrations (ng/g, dw) PCBs concentrations (pg/g, dw)

Fig. 3.74. The vertical variations of total PAHs (tPAHs) and total PCBs (tPCBs) concentrations with the depth and year (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center)

gently and two peaks divided the proﬁle into three segments. Firstly, ΣPAHs declined slightly from the surface to 10 cm, and then increased signiﬁcantly up to 16 cm, where the maximum value was detected. Secondly, ΣPAHs decreased sharply up to 18 cm, with the decrement high to 47.81 ng/g. Subsequently, it increased abruptly to the second well-deﬁned peak value (66.15 ng/g). Lastly, a steep decrease occurred at 24 cm and then ΣPAHs remained at a relatively low level (less than 30 ng/g) from 22 to 28 cm. With respect to the temporal variation, ΣPAHs remained at a low level from 1920 to 1932; the synthesis and discharge of PAHs peaked in the periods from 1938 to 1944 and from 1956 to 1962; PAHs level increased slightly in the last 4 decades. In the beginning of the 1900s, China was a colony of many countries and was in underdeveloped economy, which was predominantly agricultural. During the period from 1938 to 1944, the YS was one of the main war zones in the Paciﬁc Ocean in the Second World War. Consequently, the consumption and spillage of fossil fuel from warships raised the possibility that PAHs accumulation in the southern YS was much higher. After the war, China underwent its ﬁrst economic development from the 1950s to the 1960s. The development process was interrupted by the ten-year Cultural Revolution from 1966 to 1976. The second economic development started at the end of the 1970s because of an adjustment in national policies. As described above, the temporal variation in PAHs concentrations was consistent with the history of Chinese economic development.

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3 Biogeochemical Processes of the Yellow Sea

(3) Sources of PAHs The anthropogenic release of the PAHs could be attributed to petroleum and combustion origins. To identify the source of PAHs, the principal component analysis (PCA) method was employed, which provided a means of reducing the complexity of the total PAH data set. Principal component (PC) loading values provided information about the relationships among the variables. In our study, PCA was used to study the correlation among PAH compounds. 16 individual PAH compounds were extracted from three principal components of PC1, PC2, and PC3 in light of the principle that the eigenvalue was higher than 1, accounting for 48.907%, 27.703%, and 16.217% respectively, which could provide 92.827% of the information of total variances. The result of the rotation sum of square loading are presented in Fig. 3.75. PC1 was mainly contributed by 4-, 5-, and 6-ringed PAHs (FLU, PYR, CHR, B(b)F, B(a)P, INP, and B(ghi)P) and ACY (3-ringed), which were produced in the process of combustion including low temperature and pyrogenic combustion, while 3-ringed PAHs (ACE, FL, PHEN, ANT) had high loading in PC2. Consequently, PC1 was treated as a combustion source and PC2 was essentially petroleum contamination. Additionally, NAP (2-ringed), B(a)A (4-ringed), B(k)F (5-ringed), and D(ah)A (5-ringed) were related to PC3, suggesting multiple sources of petroleum and combustion. In conclusion, PAHs in the whole core were from the combustion product and petroleum leakage. Furthermore, combustion production was considered the dominant source.

1.0

D(ah)A B(a)A

0.8

NAY

B(k)F

PCA3

0.6 0.4

PHEN FL ACE INP

0.2

ACY PYR

0.0

FLU ANT

0.2 0.4 0.4 0.2 0.0 0.2 PCA 0.4 1 0.6

B(b)F

B(ghi)P

INP

0.8

1.0

0.6

1.0 0.8 0.40.6 0.2 0.0 0.2 CA2 0.4 P

Fig. 3.75. Plot of the factor loadings of the variables on three PCs (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center)

The isomeric ratio of PAH compounds was also employed to apportion PAHs sources. Because anthracene was not detected in 6 layers in our study for unknown reasons, we chose B(a)A/228 and INP/276 as the isomeric ratio

3.7 Biogeochemistry of PAHs and PCBs in the Yellow Sea Sediments

413

indices. According to Fig. 3.76, two transition points were found, 8 cm and 20 cm. From the surface to 8 cm, PAHs were characteristic of mixed sources of petroleum and combustion, and the aliquot of combustion-originating PAHs was the production of grass/wood/coal combustion process. 20 cm was the transition point of the liquid fossil fuel and grass/wood/coal combustion. From 8 cm to 20 cm, all ratios of INP/276 were above 0.5, which implied that grass/wood/coal combustion was the prevalent source of PAHs, below 20 cm the ratios of INP/276 ranged 0.4 to 0.5, suggesting they derived from a liquid fossil fuel combustion process. Petroleum Mixed sources 0.6

b

INP / 276

0.5

a

c d

Combustion f e

h i

j

g l

Grass/wood/ coal combustion

m k n

0.4

Liquid fossil fuel combustion

0.3 0.2 Petroleum

0.1 0.1

0.2

0.3 0.4 B(a)A / 228

0.5

Fig. 3.76. PAHs cross plots for the ratio of B(a)A/228 vs. INP/276. In which, the letters a∼n represented the layers of core sediments: 0∼2 cm, 2∼4 cm, 4∼6 cm, 6∼8 cm, 8∼10 cm, 10∼12 cm, 12∼14 cm, 14∼16 cm, 16∼18 cm, 18∼20 cm, 20∼22 cm, 22∼24 cm, 24∼26 cm, 26∼28 cm in order corresponding to the periods of 2004∼1998, 1998∼1992, 1992∼1986, 1980∼1974, 1974∼1968, 1968∼1960, 1960∼1954, 1954∼1948, 1948∼1942, 1942∼1938, 1938∼1932, 1932∼1926, 1926∼1920, and 1920∼1914 (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center)

As a result of the above discussion, it was found that a variation in energy types could be examined during three periods: ﬁrstly, the period that liquid fossil fuel was consumed began from 1914 until 1938, which might result from the naval battles continuously fought in the First World War. Secondly, the period that grass/wood/coal was the predominant fuel began from 1938 until 1974, attributed to the transfer of the war zone from sea to land and economic development after the war. Lastly, after 1974, the PAHs sources reﬂected the eﬀorts being taken to ﬁnd new energy sources to substitute fossil fuels.

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3 Biogeochemical Processes of the Yellow Sea

3.7.3.3 PCBs in the 20th Century (1) PCBs concentrations In this study we were to detect 28 PCBs congeners, but not all of them were obtained in every layer. Only PCB congeners with IUCAP numbers 8, 18, 28, 44, 66, 101, 153, and 137 were detected in most of the samples, and they were detected from the surface to 26 cm. 28 PCBs congeners’ concentrations are presented in Table 3.42. Similarly, total PCBs (tPCBs) concentrations of all the subsamples were calculated to evaluate the PCBs level. tPCBs concentrations were not homogeneous, and they ranged over 2 orders of magnitude from N.D. at the depth of 26 cm to 179.13 pg/g (dw) at the 14∼16 cm layer (Table 3.42). The PCBs level in the southern YS was far lower than the pollutant control standards of the National Environment Protection Agency (50 mg/kg, GB 13015-91). A smaller portion of PCBs congeners, PCBs 77, 81, 105, 114, 118, 123, 126, 156, 157, 167, 169, and 189 tended to be “dioxin-like” PCBs, which were very stable and resistant to biodegradation and metabolism. Therefore, they were the priority pollutants in PCBs monitoring. In this study, all “dioxin-like” PCBs except for PCB81 were under detection level limitations, and PCB81 was only found in the layers of the surface, 4∼6 cm, and 6∼8 cm, with concentrations of 17.91, 19.42, and 19.92 pg/g (dw). Table 3.42. Twenty-eight PCBs congeners concentrations (pg/g, dw) in every layer (cm) core sediment (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center) Layer (cm) 0∼2 2∼4 PCB8 9.14 7.96 PCB18 10.61 8.24 PCB28 14.29 11.42 PCB52 N.D. N.D. PCB44 9.93 N.D. PCB66 26.27 20.70 PCB101 7.40 N.D. PCB77 N.D. N.D. PCB81 17.91 N.D. PCB114 N.D. N.D. PCB118 N.D. N.D. PCB123 N.D. N.D. PCB153 2.90 4.47 PCB105 N.D. N.D. PCB138 N.D. 18.43 PCB126 N.D. N.D. PCB187 N.D. N.D. PCB128 N.D. N.D. PCB167 N.D. N.D. PCB156 N.D. N.D. PCB157 N.D. N.D. PCB180 N.D. N.D. PCB169 N.D. N.D. PCB170 N.D. N.D. PCB189 N.D. N.D. PCB195 N.D. N.D. PCB206 N.D. N.D. PCB209 N.D. N.D. tPCBs 98.45 71.22 N.D.: not detectable

4∼6 7.47 11.49 10.29 N.D. N.D. 23.80 6.73 N.D. 19.42 N.D. N.D. N.D. 4.16 N.D. 20.10 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 103.47

6∼8 8.39 10.89 17.94 N.D. N.D. 27.93 8.90 N.D. N.D. N.D. N.D. N.D. 3.92 N.D. 19.94 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 97.92

8∼10 11.17 13.21 N.D. N.D. N.D. 24.64 7.35 N.D. 19.72 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 76.09

10∼12 9.58 N.D. 17.12 N.D. N.D. 28.41 8.88 N.D. N.D. N.D. N.D. N.D. N.D. N.D. 21.79 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 85.78

12∼14 15.13 15.44 N.D. N.D. N.D. 28.71 7.91 N D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 67.20

14∼16 47.94 N.D. 74.13 N.D. 18.18 38.88 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 179.13

16∼18 8.15 8.91 11.11 N.D. N.D. 20.32 5.58 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 54.07

18∼20 20.66 N.D. 28.23 N.D. 13.70 27.51 11.05 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 101.15

20∼22 14.13 N.D. N.D. N.D. N.D. 42.92 11.13 N.D. N.D. N.D. N.D. N.D. N.D. N.D. 38.27 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 106.45

22∼24 8.11 N.D. 16.40 N.D. N.D. 24.73 4.59 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 53.83

24∼26 5.82 N.D. N.D. 14.47 11.03 20.57 5.11 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 57.01

26∼28 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D.

(2) Temporal trends of PCBs As Fig. 3.74 shows, ΣPCBs varied complicatedly from surface to bottom, with three peaks occurring in the vertical proﬁle. Similar to PAHs, ΣPCBs declined

3.7 Biogeochemistry of PAHs and PCBs in the Yellow Sea Sediments

415

slightly with depth, in general. Common to PAHs was the fact that two peaks were also detected in the layers at 14∼16 cm and 18∼22 cm. The proﬁle presented a higher surface concentration, and then ΣPCBs declined abruptly at 2∼4 cm. Below 4 cm, it rose to a well-deﬁned peak in the 4∼8 cm layer. After temporal decrease, PCBs concentration reached the highest measured level in the 14∼16 cm layer, where the maximum ΣPAHs was also detected. The last peak was presented in the 18∼22 cm layer, which was higher than that in the 4∼8 cm layer. The chorology of the three peaks corresponded to the periods of 1980∼2004, 1956∼1962, and 1932∼1944, respectively. Although PCBs production was legally banned in 1974 in China, some PCB-containing equipments are still in service. The increase in PCBs level in the period of 1981∼2004 was likely due to PCB-containing ﬂuid volatilization and spillage in the disposal processes of outdated PCB-containing equipments rather than in production from related industrial processes. An inconsistency was worthy of noticing in that PCBs, as a class of anthropogenic compounds, were ﬁrstly introduced to the environment in 1929 approximately, while some PCBs congeners showed their trace at a depth of 24∼26 cm, corresponding to the period from 1926 to 1932 in China, which was prior to widespread use of PCBs in the world. It was not the ﬁrst time that measurable PCBs were reported to be detected in the pre-production period (Cantwell et al., 2007). Two explanations were proposed to account for the phenomenon. On the one hand, the downward percolation might result from rainfall or bio-turbulence. On the other hand, it might be due to the contamination of samples during collection and pretreatment periods because the PCBs in air usually deposited in the samples when they were exposed to air for several hours. The latter explanation was preferred. Because the transposition abilities of PCBs in soil and sediments were poor, it was not persuasive to believe that PCBs can transport downward about 10 cm over several decades. (3) Composition of PCBs Twenty-eight PCBs congeners had attempted to detect ranging from 1 to 10 chlorinated PCBs, but only the congeners with 2 to 6 Cl atoms were obtained. In order to acquire deeper information about PCBs, the PCBs homologue percentages were calculated and summarized in Table 3.43. One chlorinated PCBs congener might evaporate in the pretreatment process, and the congeners with 7 to 10 Cl atoms were below their detection limits. The compositions of PCBs detected in the core did not present signiﬁcant trends with depth, but it was clear that the 3 and 4 chlorinated congeners were the majority constituents, exceeding to 40%, similar to the PCBs composition in the air. Consequently, PCBs in the study core were probably obtained from air deposition, or high chlorinated PCBs were degraded because the D6 core was in a deoxidized environment, which requires support for future work on sediment environment characteristics.

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3 Biogeochemical Processes of the Yellow Sea

Table 3.43. The compositions and average Cl percentages of PCBs in every layer in the core sample (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center) Layer (cm) 0∼2 2∼4 4∼6 6∼8 8∼10 10∼12 12∼14 14∼16 16∼18 18∼20 20∼22 22∼24 24∼26 26∼28

2-CB (%) 9.3 11.2 7.2 8.6 14.7 11.2 22.5 26.8 15.1 20.4 13.3 15.1 10.2 0

3-CB (%) 25.3 27.6 21.1 29.4 17.4 20.0 23.0 41.4 37.0 27.9 0 30.5 0 0

4-CB (%) 55.0 29.1 41.8 28.5 58.3 33.1 42.7 31.9 37.6 40.7 40.3 45.9 80.8 0

5-CB (%) 7.5 0 6.5 9.1 9.7 10.3 11.8 0 10.3 10.9 10.5 8.5 9.0 0

6-CB (%) 2.9 32.2 23.4 24.4 0 25.4 0 0 0 0 36.0 0 0 0

Cl (%) 46.1 48.1 48.7 48.1 45.6 48.6 43.9 41.1 44.0 43.8 50.9 44.4 47.7 0

(4) Relationship between PAHs, PCBs, and TOC It was found in Fig. 3.74 that the vertical distribution of PAHs in the downcore direction was quite similar to that of PCBs, thus their relationship was studied in order that it would give some insight into their historical deposition model and the factors eﬀecting their distribution. As a result of linear correlation analysis between PAHs and PCBs in the core, it was safe to draw the conclusion that the increase in PAHs concentrations was signiﬁcantly in positive proportions to that of PCBs (R2 =0.714, P <0.01), suggesting that the distributions of PAHs and PCBs were controlled by similar factors. Many factors inﬂuenced the sorptive nature of sediment, including clay mineral content, particles size and surface area, pH, temperature, and cation exchange capacity, but the presence of organic matter had been shown to be the dominant force controlling the sorption of hydrophobia organic pollutants (Dunnivant et al., 2005). Many previous papers had reported the existence of the strong linear correlations between TOC contents and hydrophobia organic pollutant concentrations (Song et al., 2004), but some studies showed a diﬀerent tendency. Consequently, a lot of theories and models were proposed based on the data of ﬁeld and laboratory experiments to explain the adsorption of hydrophobia organic pollutants in soils and aquatic sediments. According to the theories and models, the adsorption was a quite complicated process and was inﬂuenced by many factors, including the types and quantity of sorbents, the bio-disturbance and the character of sediments, the ambient environment, and physical-chemical characteristics of the pollutants themselves. Table 3.44 summarized the results of linear correlation analysis. tPAHs, 4- and above ringed individual PAHs compounds linearly correlated strongly with TOC contents in the sediments, except B(a)A, B(k)F, and D(ah)A, with

3.7 Biogeochemistry of PAHs and PCBs in the Yellow Sea Sediments

417

pertinency coeﬃcients ranging from 0.587 to 0.881, while no linear correlations were found between 2- and 3-ringed PAHs and TOC contents. Table 3.44. The results of linear correlation analysis between TOC contents and the concentrations of individual PAHs compounds (Zhang et al., 2009) (With permission from Elsevier’s Copyright Clearance Center) Individual PAHs NAP ACY ACE FL PHEN ANT FLR PYR B(a)A CHY B(b)F B(k)F B(a)P INP D(ah)A B(ghi)P tPAHs tPAHs vs. tPCBs

n 14 10 8 14 14 14 14 14 14 14 14 14 14 14 12 14 14 14

P 0.392 0.065 0.610 0.605 0.796 0.815 <0.01 <0.01 0.052 <0.01 <0.01 0.108 <0.01 <0.01 0.153 <0.01 0.04 <0.01

r 0.248 0.603 −0.214 0.151 0.076 0.069 0.881 0.866 0.529 0.836 0.837 0.448 0.809 0.708 0.439 0.719 0.587 0.845

The adsorption of PAHs was a combination of two factors, van de Waals force and a thermodynamic gradient determined by the hydrophobicity driving them out of the solution. Adsorption was dominated by van der Waals attractive forces between instantaneous and induced dipole moments of molecules (Gauthier et al., 1987). Dipole moments depended upon the polarizability and could be estimated by the sum of bond moments in the molecule. An increase in the numbers of C=C double bonds should contribute to molecular polarizability and thereby increase the van der Waals attraction strength. As a result, heavy molecular weight PAHs with more C=C double bonds should be adsorbed more eﬃciently to sediment particles. Linear correlation analysis was done to the TOC content and PCB 8, 18, 66, 101 and tPCBs concentrations, but no signiﬁcant relationships were found. The rest of PCBs congeners were unﬁt for linear correlation analysis because of the paucity of data. Although PAHs concentrations were signiﬁcantly linearly related to PCBs concentrations and TOC content, no signiﬁcant linear correlation occurred between PCBs and TOC contents. No all-encompassing explanation could account for the relationship between PCBs and TOC. The concentrations of PAHs ranged from 26.31 to 76.92 ng/g based on dry weight in the core sample. From the surface to the bottom of the core, PAHs

418

3 Biogeochemical Processes of the Yellow Sea

concentrations declined in the rough, with two peaks appearing at the depth of 14∼16 cm and 20∼22 cm. The two concentration peaks illuminate two periods when a large amount of PAHs was produced and used in the periods of 1956∼1962 and 1938∼1944 respectively. In the whole core scale, PAHs had multiple sources of combustion products and petroleum contaminant, and based on the isomeric ratio indices, PAHs mainly deriving from combustion with two transition sections of 8 cm and 20 cm were found. From the surface to 8 cm, PAHs originated from petroleum contaminant and combustion products. Downward to 20 cm, they were the product of grass/wood/coal combustion, and below 20 cm, PAHs derived from petroleum combustion. Strong linear correlations were found between high molecular weight PAHs (4-, 5-, and 6ringed) except B(a)A, B(k)F, and D(ah)A. Thus, the adsorption of PAHs in the sediment was dominated by TOC content. The concentrations of PCBs ranged from N.D. to 179.13 pg/g based on dry weight in the core sample, with 2 to 6 chlorinated PCBs congeners detected. The “dioxin-like” PCBs, except PCB81, were undetectable in the core sample. The variance in PCBs concentrations with depth were diﬀerent to PAHs, and only a sharp peak was obtained in the layer at 14∼16 cm, the same layer as PAHs peak. The production and usage of PCBs came to a head in the period 1946∼1952. The PCBs composition was similar to that of the mixture of Aroclor 1254 and Aroclor 1242, suggesting their sources were dielectrical and from heat-transfer ﬂuid. PAHs and PCBs concentrations in the core linearly correlated strongly with each other, and high molecular weight PAHs were also linearly related to TOC content, but no signiﬁcant relationship occurred between TOC content and PCBs concentrations (Zhang et al., 2009).

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4 Biogeochemical Processes of the East China Sea

Abstract: In this chapter the biogeochemical processes in the East China Sea (ECS) are described. The focus is on C, N, P, Si, and O in the seawaters and sediments, and the main research area is the Changjiang River Estuary. In the ECS, the riverine input and biological pump are very important controlling processes in the variation of biogenic elements. The role of the coastal environment in global carbon and nutrient cycles has been an important focus in the argument about global environmental change. In spite of their low coverage of the ocean surface (8%), coastal systems contribute a signiﬁcant portion (14%) of ocean production and may largely determine the fate of terrestrial materials in oceans (Hung and Hung, 2003). In the coastal regions of shallow shelf seas, environmental problems due to increased nutrient inputs have been a focus of research since the 1970s. Problems due to these inputs have generally been studied in terms of the effects of nutrient load and stoichiometry on the development of harmful algal blooms. The ECS, located at 26◦ ∼31◦ N and 121◦ ∼126◦ E, is surrounded by China to the west, the Kuroshio Current to the east, Taiwan and the Taiwan Strait to the south, and the Yellow Sea to the north. The ECS continental shelf is one of the largest shelves in the world. Because the Changjiang River, the third in length (6,300 km), the ﬁfth in fresh water discharge (9.24×1011 m3 /yr), and the fourth in solid discharge (4.86×108 t/yr) in the world, discharges into it, the inner and middle shelves of the ECS form one of the highest primary production areas in the world (Tian et al., 1992). Many industrial and urban centers are located in the Changjiang River watershed, especially along its lower reaches and estuary. High population densities, the extensive use of chemical fertilizers, and domestic waste have led to the Changjiang River Estuary receiving a high loading of anthropogenic nutrients in recent decades. As a result, eutrophication has become increasingly serious and noxious algal blooms have been of more frequent occurrence in the estuary. One quarter of recorded algal blooms in China occur in the Changjiang River Estuary and adjacent area (Zhou et al., 2003). Over the last

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4 Biogeochemical Processes of the East China Sea

two decades, many investigations of nutrients and the phytoplankton biomass in the ECS have been conducted. The purpose of this chapter is to introduce the results of nutrient biogeochemical processes obtained in recent years.

4.1 Dynamic Processes in the East China Sea and Its Adjacent Ocean The ECS circulation is dominated by the northward ﬂow of two loops of the Kuroshio Current: the Taiwan Warm Water in the west and the Yellow Sea Warm Water in the east. Both water masses are characterized by high salinity and warm water temperatures. In contrast, the southward ﬂow in near bottom water occurs from the ﬂow of the Changjiang River and Jiangsu Coastal Waters along the Chinese coast, the Korean Coastal Waters in the east, and the Yellow Sea Cold Water in the north (Fig. 4.1, Lee and Chao, 2003). The coastal currents in particular appear as seasonally cold and brackish water masses. 4.1.1 Circulation and Sea-Air Interaction in the Southern Yellow Sea and East China Sea The stratiﬁed water occurs in this continental area during summer. The Changjiang diluted water spreads towards Cheju Island, whose depth is about 10 m. If we consider the 31 isohaline as the boundary of Changjiang diluted water, it reaches the region east of 124◦ E while the Yellow Sea Coastal Current ﬂows to the southeast. Then it makes a cyclonic turn along the boundary of the Yellow Sea Cold Water, and ﬂows northeastward. And a cyclonic eddy occurs southwest of Cheju Island, at 125◦ ∼126◦ 30 E, 30◦ 40 ∼31◦ 50 N. The hydrographic distributions show it has a high density and low temperature. Its center moves to the east in the lower layer. The inshore branch of the Taiwan Warm Current through the ECS is about 0.4×106 m3 /s. The Taiwan Warm Current much aﬀects the currents on the continental shelf. For example, the Yellow Sea Coastal Current ﬂows southeastward and enters into the northwestern part of the study region, and after that it ﬂows cyclonically, then it ﬂows northeastward, which is due to the inﬂuence of the Taiwan Warm Current and topography. There is a cyclonic cold eddy southwest of Cheju Island, and it is located in the north of the area, in which the Yellow Sea Coastal Current ﬂows cyclonically. It has a high density and cold water. The cold and high density water appeared in the layer from about 30 m in depth to the bottom. It is the southwestern part of the Yellow Sea Cold Water Mass. The high density water with a cold core is located in the region south of Cheju Island between 125◦ 30 E and 127◦ E. Large and positive latent and sensible heat ﬂuxes are found in the Kuroshio and the warm eddy east of the Kuroshio core, which is a high temperature region. This shows that the distributions of latent and sensible heat ﬂuxes

4.1 Dynamic Processes in the East China Sea and Its Adjacent Ocean N 40

N 40

35

35

30

30

25

25 120

125

130

E

N 40

N 40

35

35

427

120

125

130

E

120

125

130

E

(c)

30

30

25

25 120

125

130

E

Fig. 4.1. Model-derived depth-averaged surface currents in (a) October, (b) December, (c) April, (d) August. The depth average extends from the sea surface to 830 m or the bottom, whichever is shallower (Lee and Chao, 2003) (With permission from Elsevier’s Copyright Clearance Center)

are closely related to the hydrographic characteristics and circulation in the Yellow Sea and ECS. The above relationship of air-sea interactions is found for the ﬁrst time in the Yellow Sea and ECS. It may be remarked from the numerical simulation about the developing process of a cyclone moving eastward that the heat ﬂuxes in front of the cyclone may be one of the important causes leading to the cyclone moving eastward, since there are very large and positive heat ﬂuxes in the Kuroshio and the region southwest of Japan, which support enough heat ﬂuxes transferred from the ocean to the atmosphere.

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4 Biogeochemical Processes of the East China Sea

The monthly mean net ﬂux has a distinct seasonal variation and the heat ﬂux is transferred from ocean to atmosphere in January and from atmosphere to ocean in July in the Yellow Sea and ECS. The early development process of cyclones is very important in the ECS and a deﬁnite fact is that the heat ﬂuxes are transferred from ocean to atmosphere, in which the latent heat ﬂux is not only more important than the sensible heat ﬂux but is about 20 times that of the sensible heat ﬂux. The heat ﬂuxes transferred from ocean to atmosphere accelerate the instability of the atmosphere in the lower layer, which is one of the important causes leading to a cyclone developing in the ECS. Regarding the seasonal variability of the sea water exchange between the Yellow Sea and the ECS, the Subei Coastal water runs southeastward into the northern ECS, and the corresponding Yellow Warm Current intrudes into the southern Yellow Sea as a compensation current of water. The process of water exchange becomes gradually weak from the beginning of spring. 4.1.2 The Kuroshio On the east of the ECS, The Kuroshio Current, a strong western boundary current, ﬂowing along the Paciﬁc Margin of northeast Asia, borders the shelf slope of the ECS. When passing through the ECS, it creates great eﬀects on the ocean environment of the continental shelf area of the ECS. In addition, the Changjiang River dilution and the Kuroshio upwelling are two principal sources of materials of the ECS. The Kuroshio region can be divided into two parts, namely the Kuroshio east of Taiwan and the Kuroshio in the ECS. 4.1.2.1 The Kuroshio East of Taiwan The net northward volume transports (VTs) of the Kuroshio through section K2 (from 21.4◦ N, 121.1◦ E to 20.2◦ N, 121.2◦ E) southeast of Taiwan were about 57.8×106 and 44.6×106 m3 /s in October of 1995 and the early summer of 1996, respectively. However, they were about 37.5×106 and 27.6×106 m3 /s in July and December of 1997, respectively. This means that the VT of the Kuroshio and its maximum velocity southeast of Taiwan obviously decreased during the stronger 1997 El Ni˜ no year. The decrease in the Kuroshio VT through section K2 in July 1977 may be due to the following two reasons. First, the North Equatorial Current (NEC) weakened during the El Ni˜ no event. As 1977 was a strong El Ni˜ no year, the Kuroshio VT at section K2 may have decreased with the weakening of the NEC. Second, comparing the strength of the anticyclonic recirculating eddy east of the Ryukyu Islands in July 1997 with that in another year, such as May-June 1996, the decrease in the Kuroshio VT through section K2 may be associated with the weakening of the anticyclonic recirculating eddy east of the Ryukyu Islands in July 1997. During October of 1995 and early summer of 1996, there should have been two or three branches of the Kuroshio east of Taiwan. The main branch of the

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Kuroshio rode on a submarine ridge east of Taipei and Suao, and was deﬂected anticyclonically. The easternmost branch of the Kuroshio ﬂowed northeastward to a region east of the Ryukyu Islands during October of 1995 and the early summer of 1996. However, there was no branch of the Kuroshio east of Taiwan to ﬂow northeastward to the region east of the Ryukyu Islands during July and December of 1997. This showed that the patterns of circulation during October of 1995 and the early summer of 1996 were diﬀerent from those during July and December of 1997. At 290 and 594 m depths at the west of Yonakuni Island, the Kuroshio was quite steady during the period of May 18 to June 1. The rotary spectral estimates of the current data, by the maximum entropy method, showed that there were peaks with the periods from 3 to 7 d. There is a signiﬁcant coherence between the time series of currents at 290 and 594 m depths in the range from 3 to 5 d. There were some cyclonic and anticyclonic eddies east of Taiwan. The TS curve of Kuroshio water was in an S-shape in the region east of Taiwan. There are four kinds of water masses from the surface to the lower layers, i.e., the high-temperature and subhigh-salinity surface water, the high-salinity subsurface water, the low-salinity mid-layer water and the low-temperature water. However, the water temperature and salinity properties of the 4 water masses vary with the seasons. In the region south of Taiwan and near the Bashi Channel, the Kuroshio water and the South China Sea water intruded alternately, resulting in a complicated distribution of the water masses there, which vary with the seasons. A countercurrent was found below the 500 m level in the region near the southeastern part of Taiwan. Thus, the position of the main branch of the Kuroshio was further to the east here than that in the upper 500 m during the early summer of 1996. The Kuroshio east of Taiwan separates into two branches; i.e., the main branch ﬂows through the ridge northeast of Taiwan and then ﬂows along the continental slope of the ECS, and the other eastern branch ﬂows to the east of the Ryukyu Islands. These two branches are joined together in the region south of Japan. 4.1.2.2 The Kuroshio in the East China Sea There are some anticyclonic and cyclonic eddies east and west of the Kuroshio, respectively, all the time. The net northward VT through the section PN (from 30◦ 30 N, 122◦ 45 E to 27◦ 55 N, 127◦ 30 E) in the ECS is largest in summer and smallest in autumn with an average of 28.0×106 m3 /s in the four cruises of 1992, and the VT at section TK (from 28.5◦ N, 129.6◦ E to 30.2◦ N, 130.8◦ E) (at the Tokara Strait) was also largest in summer of 1992. However, the VT of the Kuroshio in the ECS was smallest in spring and largest in summer during 1993 and 1994. The average net northward VT through sections PN and TK were 27.1×106 and 25.0×106 m3 /s during 1993 and 1994, respectively.

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As for variability of the Kuroshio in the ECS in 1995, the VT through section PN was the largest in spring but the smallest in the summer of 1995. However, from the statistical average results, its VT is the largest in summer but the smallest in autumn. This means that 1995 is an anomalous year, as also shown from T-S distributions. According to 1997 and 1998 investigations of the ECS, when the El Ni˜ no event occurred in May of 1997, both the VT of the Kuroshio in the summer of 1997 and the average VT of the Kuroshio in 1997 decreased. There appeared two diﬀerent patterns in the circulation in the ECS in January and JuneJuly of 1997 corresponding to the periods before and after the El Ni˜ no event, respectively. The Kuroshio through section PN had multi-current cores during the period from April to November of 1997, in particular it had three current cores in October and November of 1997. The position of the main current core of the Kuroshio moves eastward in the autumn. Both 1995 and 1998 are anomalous years for the Kuroshio in the ECS, which may be due to the following two reasons: (a) the increase of the Kuroshio VT may be associated with the strengthening of the anticyclonic recirculating eddy south of Okinawa Island; (b) the transformation from the El Ni˜ no event to the La Ni˜ na event occurred in the summer of 1995 and 1998. The Kuroshio plays a very important role in the early development stage of cyclones in the ECS, when the heat ﬂux is transferred from ocean to atmosphere. The VT of the Kuroshio ranges in the ECS from 27.5×106 m3 /s in winter to 32.9×106 m3 /s in summer, which is in contrast with that derived from the Sverdrup relation. In April 1994 the VT and HT through section PN in the ECS was 30.6×106 m3 /s and 2.42×1015 W, respectively, and the horizontal ﬂuxes of TCO2 , DOC and POC through the section PN were 65×106 , 2.2×106 and 0.17×106 mol/s, respectively. 4.1.3 Currents East of the Pyukyu Islands There are two origins of the western boundary current east of the Ryukyu Islands. One came from the easternmost branch of the Kuroshio east of Taiwan during October of 1995 and early summer of 1996. However, in summer and winter of 1997 there appeared to be no branch of the Kuroshio east of Taiwan to ﬂow northeastward to the region east of the Ryukyu Islands. Another origin comes from the anticyclonic recirculating gyre south of the Ryukyu Islands. The anticyclonic recirculating gyre always existed in the region southeast of Okinawa Island during all cruises. The seasonal variation in the currents east of the Ryukyu Islands from 1992 to 1998 has been studied. It was found that from 1992 to 1996 the Ryukyu current had two current cores. On the seasonal variation of VT through sections east of the Ryukyu Islands, it was also found that the northeastward VT was larger in autumn but smaller in spring in every cruise year except in 1996, and the southeastward VT was larger in autumn except in 1995. There were southwestward countercurrents east of the Ryukyu Current in the deep

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layer under the Ryukyu Current. 1995 was an anomalous year for the currents east of the Ryukyu Islands, which could be indicated by the velocity and VT variations as well as by their T-S diagram. The Ryukyu Current was stronger in summer and autumn, but weaker in the spring of 1997, and its northeastward VT was about 5×106 , 16×106 and 17×106 m3 /s in spring, summer and autumn of 1997, respectively. In addition, there were anticyclonic warm and cyclonic cold eddies, respectively. These two eddies were composed of a dipole, which was discovered ﬁrst in the region southeast of Okinawa Island during the summer of 1996. The maximum velocity of the Ryukyu Current at section OK was stronger in July, but weaker in February and April of 1998. The northeastward VT of the Ryukyu Current in February, April and July of 1998 was about 10×106 and 8×106 m3 /s, respectively. The VT was smaller in April, in part due to the inﬂuence of a cyclonic eddy east of the Okinawa Islands.

4.2 Carbon Cycling in the East China Sea The ECS is one of the largest continental shelves in the world and has high primary production all year round, as observed on the Coastal Zone Color Scanner images. As a transition zone between the Asian continent and the open ocean, this shelf sea is a very active site of carbon cycling. Some measurements of the partial pressure of CO2 (PCO2 ) and other carbon parameters have been conducted by other studies since the 1990s primarily in the central and southern part of the ECS. Among the marginal seas in the northwest Paciﬁc area, the ECS is the only one composed mainly of a continental shelf zone. Four diﬀerent water masses that aﬀect the ecosystem of the ECS have been identiﬁed, including a large amount of nutrient-laden riverine runoﬀ with strong seasonal variation of temperature and salinity from the west, intrusion and upwelling of the Kuroshio surface and subsurface waters from the east, Yellow Sea waters from the northern boundary, and warm oligotrophic Taiwan Strait waters from the south. Signiﬁcant amounts of allochthonous organic carbon and inorganic nutrients have been supplied to the ECS from riverine runoﬀ, upwelled Kuroshio waters, or both. Recent studies have shown that the physical, hydrographical, and biological features in the ECS include strong gradients that change dramatically with the seasons. Besides summarizing the results of other researchers in the ECS, the focus of the chart is to introduce our results acquired in the Changjiang River Estuary and its adjacent regions (Fig. 4.2). 4.2.1 Spatial Distributions of Inorganic Carbon in Seawaters CO2 can enter seawater in two ways: from the atmosphere and from the breakdown of organic materials in seawater and sediment by organisms. In an aque2− ous solution, CO2 exists in many forms: HCO− 3 , CO3 , H2 CO3 , and the sum

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of gaseous CO2 . The sum of all forms of CO2 in water is called DIC in marine chemistry, which is the major carbon reservoir in the ocean. 4.2.1.1 PCO2 and Its Eﬀecting Factors The CO2 system in seawater is an important and complicated balance system in oceans; it is composed of some sub-balance systems and is inﬂuenced by atmospheric, biological, geologic and other processes. The PCO2 in seawater is an important parameter of the sea’s CO2 system and is very sensitive to physicochemical and biological processes in oceans. Its distribution and change are closely related to factors such as water mass and biological activity. The distribution of PCO2 in the ECS surface water takes on apparent seasonal and spatial variation. PCO2 is lower in the coastal sea than in the open sea in spring and summer, whereas the situation is the opposite in autumn and winter (Fig. 4.3, Zhang et al., 1997). PCO2 in the surface water of the Kuroshio region does not show signiﬁcant seasonal variation compared with the other areas. The region near the Changjiang River Estuary, where salinity is below 20‰, has a high PCO2 in summer; the highest value measured was 81.1 Pa. The abundant inﬂux of Changjiang River freshwater is thought to be the reason this phenomenon occurs. The region around Changjiang River Estuary is a perennial source of atmospheric CO2 , the region in the middle ECS in a southwest-northeast direction is a perennial sink, and the region near the Kuroshio is a source most times of the year (spring, summer, and winter). In every season, the PCO2 in the ECS surface water is more diverse than that in the oceans, demonstrating the complexity of marginal seas (Zhang et al., 1997; Hu and Yang, 2001). For a representative estuary, the Changjiang River Estuary can be divided into two sections, namely the inner and outer estuaries. The inner estuary is a region between the limit of the tidal inﬂuence and the river mouth; the outer estuary is the plume of freshened water which ﬂoats on denser coastal seawater and can be traced miles away from the geographical mouth of the

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estuary and is also called an “estuarine plume”. The surface area of a plume is commonly deﬁned on the basis of the salinity in surface waters. A salinity value of 1‰ lower than the adjacent oceanic basin is arbitrarily used as the oﬀshore boundary. The characteristic value of surface salinity of the ECS, with which the Changjiang River Estuary and Hangzhou Bay are connected, is higher than 33‰ (Wang and Xu, 2004). On August 5 through 10, during the 2004 cruise, the distribution of isohaline indicated that the investigated area covered both the Changjiang River and Qiantang River estuaries and their plumes (Fig. 4.4, Gao et al., 2008). N 32.0

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Fig. 4.5 (Gao et al., 2008) shows the distribution of PCO2 across the sampled region. A distinct spatial diﬀerence in overall distribution of PCO2 values

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was shown with a minimum and maximum of 168 and 2,264 μatm respectively. The PCO2 decreased oﬀshore while salinity rises. However, the maximum PCO2 did not coincide with the minimum salinity (at the mouth of the Changjiang River Estuary) as expected, if the input of over-saturated water from the Changjiang River was the only process controlling the distribution of PCO2 . The inner Changjiang River Estuary and Hangzhou Bay were entirely over-saturated in CO2 . The variation in PCO2 values was >1,000 μatm. The estuarine plume region was also over-saturated in CO2 , except for a small area in the east. N 32.0

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Estuaries are known with signiﬁcant supersaturation of CO2 with respect to the atmosphere. PCO2 as high as 15,500 μatm was once reported in the Scheldt Estuary (Belgium and Netherlands), which is >40 times the PCO2 of the present atmospheric equilibrium (Hellings et al., 2001). Such high PCO2 values are believed to result from intricate biological and physicochemical processes that characterize estuarine dynamics (Hellings et al., 2001). Investigations in the 1990s indicated that, in Europe, the estuaries emit between 30 and 60 million tons of carbon per year into the atmosphere, representing 5%∼10% of anthropogenic CO2 emissions for western Europe at that time (Frankignoulle et al., 1998). The surface PCO2 in the inner Changjiang River Estuary was similar to those of some European estuaries in spring such as the Elbe (Germany) and the Gironde (France) (Frankignoulle et al., 1998), and was much lower than some well-studied estuaries such as the Pearl River Estuary (China) and the Scheldt Estuary (Frankignoulle et al., 1998; Hellings et al., 2001; Zhai et al., 2005).

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The mechanism by which estuarine systems can sustain such high levels of PCO2 remains unclear. Abril et al. (2000) indicated that heterotrophic activity and acidiﬁcation due to nitriﬁcation within the estuarine zone is a major factor in the total estuarine emission to the atmosphere, while the excess CO2 transport by rivers followed by ventilation in the estuary was a minor one. Cai and Wang (1998) believed that the combined eﬀects of pelagic and benthic respiration, photodegradation, and the mixing of seawater and acidic river water were insuﬃcient to sustain the high PCO2 values and the high water-toair ﬂuxes in the estuaries they studied. They suggested that the CO2 input from organic carbon respiration in tidally ﬂooded salt marshes controls the CO2 concentration. This explanation is consistent with a subsequent mass balance study of biogenic gases (Cai et al., 1999). To sum up, the PCO2 distribution pattern in estuaries results from a combination of various processes: the production/degradation/export of organic carbon, the production/dissolution/export of carbonates, the input of dissolved inorganic carbon by vertical mixing processes and/or freshwater runoﬀ, and the thermodynamic eﬀects related to both water temperature variations and water mass mixing. Compared with the inner estuary, the outer estuary has substantially different properties in terms of CO2 , where intense phytoplankton bloom can consume signiﬁcant amounts of dissolved CO2 . PCO2 in the estuarine plume depends not only on its primary production/respiration balance but also on the quantity of excess dissolved CO2 advected from the inner estuary. Thus, CO2 atmospheric exchanges in plumes are aﬀected by many parameters, among which river discharge, the degree of heterotrophy in the inner estuary, the availability of nutrients and light, and the stratiﬁcation of the water column are important (Abril and Borges, 2005). In the sampling area of this study, light penetrates deeper and deeper into water from the west to the east with the depositing of suspended particles, and the region near the eastern boundary of the studied area has similar characteristics to the sea with which it is connected. The light and nutrients from the surroundings of the Changjiang River Estuary and Hangzhou Bay are favorable for phytoplankton blooms. So this was a possible factor causing the PCO2 distribution to show such a pattern in this study. Temperature aﬀects the equilibrium constants of dissolved inorganic carbon and, in particular, the solubility coeﬃcient of CO2 , so that PCO2 rises by ∼4% with an increase of 1 ◦ C in temperature. In the sampling region, the surface water temperature decreased from the west to the east, similar to that of PCO2 (Fig. 4.5). Thus this was another possible factor causing the PCO2 distribution to show such a pattern in this study. Table 4.1 compares PCO2 ranges in various estuaries of the world (Gao et al., 2008). PCO2 concentration instead of CO2 ﬂux was compared for removing the uncertainties in using diﬀerent k values. The Changjiang River Estuary and Hangzhou Bay fell in the low range of PCO2 reported for estuaries. Part of the discrepancy may be due to the lack of seasonal PCO2 data in

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Table 4.1. PCO2 ranges reported in estuaries of the world (Gao et al., 2008) (With permission from Springer) PCO2 range (μatm) Reference ∼ 100∼15, 500∗∗ Hellings et al. (2001); Borges Scheldt (Belgium/Netherlands) and Frankignoulle (2002) Saja-Besaya (Spain) 264∼9, 728# Ortega et al. (2005) Cai and Wang (1998); Cai et # Satilla (US-Georgia) 420.8∼200 al. (1999) Cai and Wang (1998) Altamaha (US-Georgia) 380∼7, 800# Sado (Portugal) 450∼5, 700∗∗ Frankignoulle et al. (1998) Frankignoulle et al. (1998) Thames (UK) 465∼5, 200# Pearl River (China-Guangdong) ∼ 360∼4, 785∗∗ Zhai et al. (2005) Frankignoulle et al. (1998) Ems (Germany/Netherlands) 525∼3, 755∗∗ Loire (France) ∼ 600∼2, 900∗∗ Abril et al. (2003) Frankignoulle et al. (1998) Gironde (France) 440∼2, 860∗∗ Mandovi-Zuavi (India)∗ 400∼2, 500# Sarma et al. (2001) Hudson (US-New York) 503∼2, 270# Raymond et al. (1997) Changjiang River 168∼2, 264∗∗ This study (China-Shanghai/Jiangsu) Frankignoulle et al. (1998) Douro (Portugal) 385∼2, 200∗∗ Rhine (Netherlands) 375∼1, 990∗∗ Frankignoulle et al. (1998) 352∼1, 896# Raymond et al. (2000) York (US-Virginia)∗ Tamar (UK) 380∼2, 200# Frankignoulle et al. (1998) 474∼1, 613# Raymond et al. (2000) Rappanhannock (US-Virginia)∗ Urdaibai (Spain) 256∼1, 569# Ortega et al. (2005) James (US-Virginia)∗ 284∼1, 361# Raymond et al. (2000) Frankignoulle et al. (1998) Elbe (Germany) 340∼1, 100∗∗ Columbia (US-Oregon) 560∼950# Park et al. (1969) 646∼878# Raymond et al. (2000) Potomac (US-Maryland)∗ As´ on (Spain) 246∼436# Ortega et al. (2005) Estuary (location)

The PCO2 range was obtained by taking the lowest and highest values for each estuary reported in the reference(s) except for those marked with an asterisk, which was obtained by averaging the lowest and highest values for each transect. The estuaries are ranked by the high limit. ∗∗ denotes that the data were gained from both inner and outer estuaries; # denotes that the data were gained only from the inner estuary

many estuaries, including the ones we researched. Besides, large diﬀerences in organic matter concentrations among estuaries are another important reason for the variation in estuarine CO2 supersaturation. For example, the dissolved organic carbon (DOC) concentrations in the Satilla and Altamaha River estuaries were 25∼50 and 10 mg/L, respectively (Cai and Wang, 1998), and 5 mg/L on average in the low salinity region of the York River Estuary (Raymond et al., 2000), whereas the maximum was only 3.5 mg/L obtained in May 2003 in the entire salinity gradient of the Changjiang River Estuary (Gao and Song, unpublished data).

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2− 4.2.1.2 HCO− 3 , CO3 , CO2 , and Dissolved Inorganic Carbon in the Changjiang River Estuary Seawaters

(1) Distributions 2− The spatial distribution of HCO− 3 , CO3 , CO2 , and dissolved inorganic carbon (DIC) concentration along transect A was shown in Fig. 4.6. Like salinity, there existed sharp horizontal gradients for all of the four parameters, but their positions were not the same. The gradients appeared generally at the same and CO2 . For HCO− position as salinity for CO2− 3 3 and DIC, the gradients appeared at about a similar position from stations 7 to 11, approximately three stations earlier than those of CO2− 3 and CO2 . On the whole, all of the 2− HCO− , CO , and DIC increased in a downriver direction while it was the 3 3 reverse for CO2 . In the relatively deeper water column between stations 17 and 20, HCO− 3 , CO2 and DIC increased generally from the surface to the bottom except for CO2− 3 , which showed a reverse pattern. Compared with within the freshwater mass was homogeneous and other parameters, CO2− 3 only about 5 μmol/L.

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The transect B was situated at the estuarine mixed water mass and the water depth of stations located here was shallow, only three of them were a bit deeper than 10 m. So for most stations only water samples of the 0 m layer and 2 m above the bottom layer were taken. The concentration variability of 2− HCO− 3 , CO3 , CO2 , and DIC along transect B is shown in Fig. 4.7. It was clear that no matter whether at 0 m or 2 m above the bottom layer, most of the HCO− 3 and DIC concentration values were >2,000 μmol/L. Like that of transect A, the variation in HCO− 3 and DIC concentration along transect B took on a very similar pattern, and this was probably owing to the fact that HCO− 3 was the dominant component of DIC. All of the four parameters exhibited revealed that at most stations their concentrations at the 0 m layer were lower than those at 2 m above the bottom layer, especially for CO2 . The diﬀerence between 0 m and 2 m above the bottom layer apparently indicated that the water was not mixed well, although the depth was shallow.

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2− Fig. 4.7. Concentration variability of HCO− 3 , CO3 , CO2 , and DIC along transect B

The data indicated that in the Changjiang River Estuary waters, HCO− 3 accounted for 84.93% to 97.43% of the DIC with an average of (94.41±2.92)%; CO2− 3 accounted for 0.28% to 14.77% of the DIC with an average of (3.95± 3.68)%; CO2 accounted for 0.30% to 3.56% of the DIC with an average of (1.63±0.89)%. The ratios of HCO− 3 and CO2 to DIC decreased in a downriver direction while the trend was the reverse for CO2− 3 . In the freshwater mass, namely the water column before station 10 in this study, the ratio of CO2 to DIC was higher than that of CO2− 3 . This indicated that CO2 was the secondary dominant species of DIC after HCO− 3 . But in the estuarine mixed water mass and seawater mass, CO2− 3 became the secondary dominant species and its ratio to DIC reached a maximum of 14.77% at station 19.

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(2) Diurnal variations For anchor stations 13 and 20, each was investigated for 48 h. Parameters were measured at about 4-h intervals, so a total of 13 sets of data were gained for each parameter. The water depth at station 13 was only 8 m. Two layers, namely the 0 m layer and 2 m above the bottom layer, were studied. 2− HCO− 3 , CO3 , CO2 , and DIC against time variation all showed that no matter whether at the 0 m layer or 2 m above the bottom layer, they changed within a very large range. In most cases 2 m above the bottom layer had higher values, and sometimes diﬀerences between them were big in spite of the shallow depth. For 0 m and 2 m above the bottom layer, HCO− 3 ﬂuctuated within a range from 1,630 to 2,540 μmol/L and from 1,523 to 2,893 μmol/L with a mean value of (2,004±283) (mean±standard deviation) and ﬂuctuated within a range from 49 (2,441±365) μmol/L, respectively; CO2− 3 to 125 μmol/L and from 49 to 141 μmol/L with a mean value of (82±22) and (102±26) μmol/L, respectively; CO2 ﬂuctuated within a range from 20.0 to 30.1 μmol/L and from 24.2 to 48.5 μmol/L with a mean value of (26.0±3.3) and (33.4±7.6) μmol/L, respectively; DIC ﬂuctuated within a range from 1,714 to 2,693 μmol/L and from 1,596 to 3,062 μmol/L with a mean value of (2,112±304) and (2,576±388) μmol/L, respectively. Furthermore, diﬀerences between the two samples gained in succession at 4-h intervals sometimes were very large, especially for the 2 m above the bottom layer. Take DIC for example. Its value at 2 m above the bottom layer increased from 1,596 to 2,835 μmol/L during the period 1:30 to 6:00, 23 May 2003; its value at the 0 m layer decreased from 2,693 to 2,095 μmol/L during the period 18:00 to 22:00, 23 May 2003. The DIC of one layer increasing from one sample to the next did not mean that the other would vary in the same way, and vice versa. Anchor station 20 was located near the estuarine front. Its depth was 43 m, according to which a total of ﬁve layers, namely 0, 5, 10, 20 and 2 m above the bottom, were investigated. On the whole, like that of station 13, values at 2 m above the bottom were higher than the upper layers in most cases for 2− HCO− 3 , CO2 , and DIC. But for CO3 the trend was diﬀerent, and generally higher values appeared at the upper two layers. Comparing the ﬁve layers, although at each layer values ﬂuctuated in a wide range, ﬂuctuation at 2 m above the bottom was the weakest for every parameter excluding CO2 (Table 4.2). Concentration values varied in the widest range at the 0 m layer for CO2− 3 and CO2 but at the 5 m layer for HCO− 3 and DIC. Similar to station 13, for each layer the diﬀerence between the two samples obtained in succession was sometimes very big. Data from anchor stations 13 and 20 indicated that HCO− 3 was the dominant form of dissolved inorganic carbon species during the investigated 48 h, followed by CO2− 3 and CO2 in sequence. For station 13, diﬀerences in the ratio of HCO− 3 to DIC between the two layers were not distinct. It was the same for CO2− 3 and CO2 . Their diﬀerences between the ﬁve layers of station 20 were much bigger than that at station 13, perhaps because the former was much deeper than that the latter, leading to the inhomogeneous distribution

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4 Biogeochemical Processes of the East China Sea

of dissolved inorganic carbon species. The maximum contribution of HCO− 3 and CO2 to DIC appeared at 2 m above the bottom, resulting in the appearance of a corresponding minimum contribution of CO2− 3 . Values of the 2− , CO , and CO to DIC for station 13 all varied in contribution of HCO− 2 3 3 narrower ranges than those at station 20. Considering the whole data for station 13, HCO− 3 accounted for 93.98% to 95.51% of the DIC with an average of (94.85±0.43)% during the investigated 48 h; CO2− 3 accounted for 2.57% to 5.11% of the DIC with an average of (3.87±0.63)%; CO2 accounted for 0.92% to 1.92% of the DIC with an average of (1.28±0.22)%. Considering the whole data for station 20, HCO− 3 accounted for 85.26% to 95.51% of the DIC with an average of (91.67±2.60)% during the investigated 48 h; CO2− 3 accounted for 2.57% to 14.42% of the DIC with an average of (7.52±2.93)%; CO2 accounted for 0.32% to 1.92% of the DIC with an average of (0.81±0.35)%. 2− Table 4.2. Ranges, means, and standard deviations (SD) of HCO− 3 , CO3 , CO2 , and DIC in water samples collected at the ﬁve layers of anchor station 20

HCO− 3 (μmol/L) Layers Range Mean SD 0m 1716∼2797 2008 290 5m 1494∼2695 1990 315 10 m 1807∼2934 2084 276 20 m 1331∼2438 2000 270 2 m* 1882∼2594 2267 193 * Above the bottom

CO2− (μmol/L) CO2 (μmol/L) 3 Range Mean SD Range Mean SD 112∼306 227 55.7 6.5∼25.0 12.2 5.1 167∼316 230 38.0 6.2∼15.6 11.1 3.3 130∼238 187 30.4 10.4∼25.9 15.0 3.9 133∼316 192 52.2 8.6∼21.2 14.1 3.8 129∼216 181 24.4 14.5∼26.0 19.7 3.5

DIC (μmol/L) Range Mean SD 1922∼3119 2248 307 1708∼3025 2232 334 1971∼3168 2286 290 1473∼2688 2207 301 2063∼2822 2467 207

4.2.2 Organic Carbon (Dissolved Organic Carbon and Particulate Organic Carbon) in seawaters 4.2.2.1 Dissolved Organic Carbon Non-living organic matter in seawater is one of the four largest active organic carbon reservoirs on the Earth’s surface and holds approximately 0.70×1018 g C (Hedges, 1992). It includes both dissolved organic carbon (DOC) (<0.5 μm in diameter) and particulate organic carbon (POC) (>0.5 μm in diameter). The dominant form is DOC. The organic carbon content of living marine organisms (∼0.002×1018 g C) is trivial compared with both DOC and non-living POC. Dissolved and particulate organic matter profoundly inﬂuence the marine biogeochemical cycling of carbon. The cycling of organic matter in the open ocean has received a lot of attention over the last two decades. Research indicates that ﬂuctuations in the concentration of DOC in seawater potentially have a great eﬀect on carbon cycling in the marine system. A change of less than 10% in the size of the DOC pool would be comparable to the annual primary productivity in the whole ocean. Despite greater productivity at the continental margin than in the open ocean, the role of marginal seas in regulating carbon generation, loss, and export remains poorly understood.

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Even though it is one of the largest marginal seas in the world, the role of the ECS in regulation of organic matter is relatively unknown because reliable DOC and POC data are scarce. In the surface mixed layer (<5 m) of the ECS, DOC concentrations vary approximately from 780 to 900 μg/L during autumn (Ogawa et al., 2003). The DOC concentrations are consistently high in the surface mixed layer and decrease rapidly with increasing depth. Below 250 m, DOC concentrations are low and relatively constant around 504∼600 μg/L. This range and proﬁle are similar to those obtained from other oceanic regions, such as the equatorial Paciﬁc and the Gulf of Mexico. The weak relationship between DOC and chlorophyll a (Chl a) distribution in the surface water column during autumn demonstrates that the temporal abundance of phytoplankton does not lead directly to a net accumulation of DOC. In general, the distribution of DOC seems to be controlled by hydrography rather than biology (Chl a). However, small variations (usually <36 μg/L) within the surface mixed layer are detected, implying photochemical degradation of DOC in the top surface and a temporal accumulation of DOC of intermediate reactivity. During spring, DOC concentrations vary from 696 to 900 μg/L throughout the entire water column. The distribution is relatively homogenous compared with the autumn. Depth-integrated DOC increases by 13 g C/m2 from autumn to spring in the 100∼200 m layer around the shelf edge, which is comparable to the annual particle ﬂux from the euphotic zone, suggesting substantial downward export of DOC in this area. Terrestrial DOC input is estimated to be 4.8×1012 g C/yr in the shelf area, relatively close to the Changjiang River Estuary (Gao and Song, 2006). In the entire ECS, surface distributions of DOC varied spatially, ranging from 1,020 to 1,440 μg/L in coastal waters, from 900 to 1,020 μg/L in Kuroshio waters, and from 720 to 840 μg/L in upwelled waters. The relatively high DOC concentration in coastal waters (∼1,020 μg/L oﬀ the southern China coast [26◦ ∼30◦ N] during autumn and winter and 1,320∼1,440 μg/L close to the Changjiang River Estuary [30.5◦ ∼32◦ N, 123◦ ∼124◦ E] during summer) might derive mostly from terrestrial inputs and less from in situ production because China’s coastal waters are sometimes limited in light and therefore low in primary productivity (Gong et al., 2000). DOC concentrations in most shelf mixed-water regions are between 864 and 1,020 μg/L. Temporal variations are insigniﬁcant, with the exception of coastal waters, in which the concentration is greater in summer and autumn than that in spring and winter. The horizontal transports of DOC and POC across the shelf in the ECS can be derived from the mean DOC and POC concentrations and exchanged water volumes. The cross-shelf export of DOC (414 Gmol C/yr) is roughly four times that of POC (106 Gmol C/yr) and more than double the riverine ﬂux of DOC (155 Gmol C/yr) to the shelf. DOC concentrations are relatively high (>1,020 μg/L) in the inner shelf and slope waters, but low (about 780 μg/L) around the shelf break, where Kuroshio upwelling occurs. Such a distribution pattern shows little temporal variation. DOC distribution in the shelf waters appears

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to follow a trend of decreasing DOC concentration with increasing salinity. Such a trend evidently indicates mixing between the DOC-rich coastal waters in the inner shelf and the DOC-poor upwelled waters (which are upwelled mainly from Kuroshio subsurface water and mixed with ECS shelf water) from the shelf break. The lowest concentration of DOC is always found in the upwelled waters. The POC distribution shows a decreasing trend from the inner shelf to the slope, with a local maximum at the shelf break, where POC is enriched because of enhanced primary productivity induced by upwelling. The distribution patterns of POC reveal much greater temporal variability than those of DOC. A mid-depth maximum is repeatedly observed above the slope, indicating lateral transport of POC oﬀshore from the shelf. The average POC inventory is about one-tenth that of the DOC. The southern ECS continental shelf is characterized by a low organic carbon concentration with a relatively fast sedimentation rate (Lin et al., 2000). 4.2.2.2 Particulate Organic Carbon Distribution characteristics of POC in seawater of the whole ECS were studied (Huang et al., 1997; Liu et al., 1997). In both spring and autumn, the horizontal distribution characteristics of the 0- and 10-m layers indicate that the highest POC content is in the coastal waters and tends to decrease oﬀshore (Fig. 4.8 and Fig. 4.9, Huang et al., 1997). The POC value is low in the Kuroshio Current area. POC distribution in the deep water area farther oﬀ the coast is comparatively even, owing to the weakening inﬂuence of the Changjiang River runoﬀ. In autumn, the POC contour tends to be parallel with the coast and decreases oﬀshore, and it is thought to be controlled by continental runoﬀ and biological activities. In spring, POC distribution shows evidence that it is moved by the water masses; the maximum values of POC occur in the shelf center, and its high value is consistent with the high value area of Chl a in surface waters. The vertical distribution characteristics indicate comparatively consistent POC distribution in both surface and bottom water in the shallow waters along the coast because of the even mixture of surface and bottom water. POC values in the oﬀshore deep-sea area are high at the surface and decrease with depth, but the lowest values do not appear at the bottom because of the inﬂuence of resuspended sediment. Whether it is in surface or bottom waters, the mean POC value in deep-sea areas is approximately 135 μg/L, in both spring and autumn. The mean POC value is less than 50 μg/L when water depths exceed 20∼35 m. The data collected from the continuous diurnal monitoring of two typical stations in the ECS indicate that, during spring and autumn, POC content in the euphotic zone usually reaches its highest value at midnight (2,400 μg/L), whereas Chl a at the surface reaches its highest value at 1,800 μg/L. That is perhaps a result of biological activities. POC content diurnal variation in the euphotic zone is 86∼153 μg/ L in spring, but only 11 μg/L in autumn (Huang et al., 1997).

4.2 Carbon Cycling in the East China Sea

( )

443

( )

Fig. 4.8. Distributions of POC (μg/L) in layers 0 m (a) and 10 m (b) of the ECS in spring (Huang et al., 1997) (With permission from Huang ZQ)

( )

Fig. 4.9. Distributions of POC (μg/L) in layers 0 m (a) and 10 m (b) of the ECS in autumn (Huang et al., 1997) (With permission from Huang ZQ)

The POC content in the ECS is about one order of magnitude greater than that in the Atlantic and Paciﬁc Oceans (54 and 30 μg/L, respectively). The mean POC value in the ECS is 417∼280 μg/L in spring and 541 μg/L in autumn. Living POC accounts for about 10% of total POC in spring and 4% in autumn. POC in surface waters is mainly from biological production in the local water columns in spring. However, storm-caused resuspension of sediments in the inner shelf area and a relatively large Changjiang River runoﬀ are probably the main sources of POC in the ECS in autumn (Liu et al., 1997). 4.2.3 Key Biogeochemical Processes of Carbon in Seawaters Carbon has a lifetime in the sedimentary reservoir measured in hundreds of millions of years and is released to the atmosphere by volcanic eruptions and

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by weathering, following the uplift of sedimentary rocks. Carbon is then exchanged rapidly, on time scales of tens to hundreds of years, between the atmosphere, biosphere, soils and the upper ocean, with occasional more prolonged periods of residence in the deep sea pending ultimate return to the sediments. The oceans are by far the largest active reservoir of carbon. One way carbon enters the ocean from the atmosphere in the form of CO2 , mainly at middle and high latitudes. It is transformed in the sea from CO2 to HCO− 3 (with a mole fraction of 90%), with some additional production of (with a mole fraction of <10%), which are dominant forms of DIC. It is CO2− 3 transferred to the deep sea as either organic detritus, in the structural CaCO3 of organisms living in the mixed layer, or inorganic carbon, as a constituent of cold dense waters, sinking to depth at high latitudes. Another way is through the “biological pump”. The fall of organisms from surface waters, primarily through the sinking of particulates, and the subsequent release of constituent carbon at depth has a powerful inﬂuence in apportioning carbon between the deep oceans on one hand and the surface oceans and atmosphere on the other hand. If the pump acts with high efﬁciency, the level of atmospheric CO2 will be relatively low; conversely, an ineﬃcient pump will serve to raise the level of CO2 . The eﬃciency of the biological pump depends on the supply of nutrients to surface waters, food web dynamics, and sinking losses of particulates to the deep sea (Gao and Song, 2006). The overall cycle of carbon among the atmosphere, biota, soils, and ocean has an associated residence time of about 100,000 years, with most of this time spent in deep seas. The cycle involving the atmosphere, biota, soils, and upper ocean is mediated, for the most part, by biological processes, limited in many cases by the supply of constituents other than C, N, P, and water. The biogeochemical cycles of these elements are thus inextricably connected. 4.2.3.1 Sea-Air CO2 Fluxes Enhanced by abundant substrate, high biological productivity with seasonal and spatial variation has been observed in the ECS. High primary production tends to enhance the biological pump and draws down the concentration of dissolved CO2 . On the basis of the sea-air PCO2 diﬀerence, the sea-air ﬂux was calculated by three methods. The results were greatly diﬀerent, but the trend was identical (Table 4.3, Gao and Song, 2006). The ECS is a sink of atmospheric CO2 in spring and summer and a source in autumn and winter. The distribution of the sea-air net ﬂux of CO2 is inhomogeneous in continental marginal seas. Because of the inﬂuence of the Changjiang River input, the character of the carbon sink/source for the Changjiang River Estuary was clearly diﬀerent from those of the other regions of the ECS. We calculated the CO2 sea-air ﬂux of the Changjiang River Estuary (Fig. 4.10, Gao et al., 2008) The results indicated that the ﬂux values varied between −10.0 and

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Table 4.3. Four-season sea-air interface net carbon ﬂux of the ECS (Gao and Song, 2006) (With permission from the Coastal Education & Research Foundation Inc. Terms & Conditions) Calculation

Grid† Statistic‡ Truabgle§ Average Laboratory simulation

Carbon ﬂux (×106 t)∗ Spring Summer Autumn Winter

2.5 1.2 6.7 3.5 2.91

−2.2 −0.7 −2.7 −1.9 −2.86

1.7 2.2 4.5 2.8 −3.53

30 13∼30 2.0 2.5 8.3 4.3 1.88

−0.00 −0.2 −0.2 −0.1

Reference

Year

Tsunogai et al. (1999) Wang et al. (2000) Hu and Yang (2001)

Song (2004)

* A positive number means the sea is an atmospheric CO2 sink; a negative number, a CO2 source. † The studied region was divided into 0.5◦ (longitude) by 0.5◦ (latitude) grids. All the sea-air CO2 net ﬂuxes were calculated individually and the results were then averaged. ‡ Because the stations were well distributed on all transects, all data were averaged. § Because the studied regions were not the same in all seasons, only the data of the triangle region studied in every season were used

N 32.0 N 31.5

30

30.0

12.5 15

22.5 25

2.5

27.5

30

27 2 .5 5

30

60 40

50 70

30.5

25

0 5

30

31.0

20

Shanghai

17.5

40

10 2.5 5

40

20

Ningbo 29.5 121.0

121.5

122.0

122.5

123.0

E

Fig. 4.10. Distribution of ﬂux isolines (mmol/(m ·d)) in surface waters (Gao et al., 2008) (With permission from Springer) 2

88.1 mmol/(m2 ·d) with an average of (24.4±16.5) (mean±standard deviation) mmol/(m2 ·d). Many factors can inﬂuence surface seawater PCO2 . Among them, physical and biological factors are primary. Early studies have indicated that physical factors such as temperature and salinity are the most important factors. Bio-

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4 Biogeochemical Processes of the East China Sea

logical inﬂuences are relatively unimportant and partly overlap with physical inﬂuences, so temperature and salinity are the essential factors controlling surface seawater PCO2 . After obtaining the relationship between surface seawater PCO2 and temperature from laboratory simulation experiments, the seasonal sea-air ﬂuxes and source/sink strength of carbon in the eastern seas of China (the Bohai Sea, the Yellow Sea, and the ECS) were calculated with the surface seawater PCO2 and temperature data (Song, 2004). Results for the ECS research are shown in Table 4.3. Because the salinity gradient of the region around the Changjiang River Estuary plume and coast is strong, the inﬂuence of salinity in these areas cannot be ignored, making results obtained by this method a poor reﬂection of the actual situation. More than 98% of the carbon in the sea-air system is stored in oceans as DIC. CO2 in seawater exists mainly in the form of dissolved CO2 , HCO− 3, CO2− 3 , and H2 CO3 . In the northern part of the ECS during summer, research indicated that the concentrations of total CO2 and HCO− 3 in the deep water column are higher than those in surface waters, but the distribution of CO2− is the opposite. Reasons for this phenomenon can be ex3 plained as follows. In surface water, the concentration of dissolved CO2 decreases because of higher photosynthesis. According to the balance equation − 2− CO2 (l)+CO2− 3 +H2 O=2HCO3 , it is evident that the concentration of CO3 − increases with a decrease in total CO2 and HCO3 concentrations. As the depth increases, CO2 is released from the sinking organic detritus decomposed by bacterioplankton. Also, because CaCO3 on the sea bottom redissolves to generate CO2− 3 , total CO2 concentration increases in the deep water column. Furthermore, it is probable that more CO2 is released than CO2− is gener3 2− ated, leading to the generation of more HCO− concentration 3 , so the CO3 decreases while the HCO− 3 concentration increases. To sum up, the content of various CO2 components and their distribution are connected with the matter transported from land, the activity of marine life, the topographic and geomorphologic conditions, and the interaction among various masses. The present study did not take the diel ﬂuctuation into account in calculating the CO2 ﬂux as all measurements were made in daytime. Higher ﬂux values could be gained if diel variation was considered, particularly in summer, because night time CO2 levels are generally higher owing to the absence of primary production. However, limited studies in diel cases in the Hudson and York Rivers indicated that the range of PCO2 over a summer diel cycle was considerably smaller than the observed range in seasonal and spatial measurements (Raymond et al., 1997; 2000). As the sea-air exchange of CO2 is a dynamic process in estuarine regions, to estimate the ability of a region to absorb atmospheric CO2 accurately, suﬃcient spatial and temporal data about the diﬀerence of CO2 partial pressure between water and air as well as synchronizingly obtained hydrological, meteorologic, chemical, and biological data are necessary. In view of the great geographical heterogeneity of PCO2 distribution in continental shelf sea areas, to estimate the CO2 ﬂux between water and air

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interface of a whole area based on limited observations, deviation will come into being inevitably. The deviation would be even more if the role of global marginal shelf seas on sources and sinks of atmospheric CO2 was evaluated based on these data. Though signiﬁcant CO2 sinks were indeed observed in the ECS shelf using the sea-air PCO2 diﬀerence and it is generally considered that the ECS is an annual net carbon sink, controversies still exist about whether the ECS is a source or sink of carbon in a given season and about the ability of the ECS to absorb atmospheric CO2 (Tsunogai et al., 1999; Song, 2000b). Except for the fact that they used diﬀerent calculation methods, which can cause a diﬀerence to some extent, the diﬀerence in the studied time and area is another reason leading to the controversies, owing to the fact that spatio-temporal diversity is an important characteristic for PCO2 distribution in surface seawaters. The data from this study show that the sea-air CO2 ﬂux was quite high in the Changjiang River Estuary, Hangzhou Bay and their adjacent areas. So, although the studied area was estimated to be only 2×104 km2 , it could emit (5.9±4.0)×103 t of carbon to the air per day. Therefore, these areas must be taken into account to gain a proper understanding of the ability of ECS to absorb atmospheric CO2 . However, none of the sea-air CO2 ﬂux researches performed in the ECS have covered the whole Changjiang River Estuary and nearby oligohaline and mesohaline areas. 4.2.3.2 Biological Pump Photosynthetic marine organisms continuously remove some CO2 from the mixed layer and convert it to organic carbon with energy provided by the sun. Although much of it is recycled by respiration within the mixed layer, some falls by gravity through the thermocline as POC: dead phytoplankton and zooplankton and detritus. In deep water, a small fraction of this POC (<1%) is trapped in sediments and removed from the ocean-air system. On a geological timescale, this is a major biological control on atmospheric CO2 . However, most of the organic carbon is remineralized by bacteria in the deep water, thus raising TIC in deep water relative to the average in surface water. The remineralized CO2 is brought back to the surface in upwelling regions. The spring vertical ﬂuxes in ECS continental shelf waters vary by area and water depth (Song, 1997). Vertical carbon transport is mainly in the form of POC. In surface waters, more than 98% of total carbon is transported in the form of POC; the number exceeds 68% in water near the bottom. Fluxes apparently can be inﬂuenced by resuspension of the substrate. In ECS continental shelf waters, 98% of carbon is stored as organic carbon, more than 83% as DOC (mean 87.5%) and the rest as POC (mean 11%). Therefore, ECS carbon originates from the exchange of CO2 at the sea-air interface, and DIC becomes DOC and POC through phytoplankton photosynthesis and zooplankton secondary production. In plankton and depositing particles, particulate

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inorganic carbon (PIC) content ranges from 0.2% to 1.9% of particulate carbon. The ratio of PIC to total carbon in the suspended mass near the sea bottom is much higher than at the surface layer. It has been suggested that microbial and protozoan communities, especially bacteria, play important roles in organic carbon consumption in aquatic ecosystems. On the basis of bacterial production, Shiah et al. (2000) estimated that bacteria might consume carbon equivalent to all the in situ particulate primary productivity of the ECS shelf. To evaluate the respective roles played by microbes (heterotrophic bacteria and ciliates) in organic carbon consumption on the continental shelf of the ECS, autumn planktonic community respiration rates were measured with the oxygen method (Chen et al., 2003). The ECS shelf was divided into mesotrophic ([NO− 3 ]>0.3 μmol/L) and ]<0.3 μmol/L) systems for comparative purposes. Bacteoligotrophic ([NO− 3 rial biomass and production, as well as POC concentrations, are signiﬁcantly higher in the mesotrophic system, whereas protozoa are more abundant in the oligotrophic system. Planktonic community respiration rates range from 127.6 to 4,728.6 mg C/(m2 ·d), and the rates are either linearly related to protozoan biomass or multiply regressed with both bacterial and protozoan biomass. Further analysis shows that planktonic community respiration is dominated by distinct microbial components in diﬀerent trophic systems, with bacteria and protozoa contributing 72% and 85% of planktonic community respiration in meso- and oligotrophic systems, respectively. The low primary production to planktonic community respiration ratio (0.33±0.30) suggests that the ECS is net heterotrophic during the study period, implying that in situ organic carbon production could not sustain consumption by respiration. Allochthonous supplies of organic carbon, in addition to in situ production, are required to support these high respiration rates. Riverine inputs, resuspension from surface sediments, or both, are potential sources of this allochthonous organic carbon. In a conﬁned region on the inner shelf oﬀ the Changjiang River in the ECS [29◦ 00 ∼32◦ 30 N, 122◦ 00 ∼123◦ 20 E], harmful algal blooms (HABs, or “red tides”) frequently occur from May through August. The formation of phytoplankton blooms is controlled by a complex interplay of physical, geological, biological, and chemical processes associated with the Changjiang River discharge, sediment deposition, and Taiwan Warm Current intrusion (Chen et al., 2003). The concentration of sand and mud in the river discharge to the ocean will be signiﬁcantly reduced after the Three Gorges Dam, the largest hydroelectric project built so far on the Changjiang River, begins operation. It will provide a deeper euphotic zone for the growth of phytoplankton over the shallow shelf connected to the Changjiang River. Moreover, power generation will regulate the river discharge, with a reduction in October but an increase in January through April (TGDPWG, 1987). Because nutrients are not factors limiting the growth of phytoplankton, under improved light conditions, HABs would become more frequent and intense around the mouth of the Changjiang River and on the inner shelf of the western ECS, which would

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seriously inﬂuence the carbon cycling processes of the ECS. Moreover, many observations have demonstrated that Changjiang diluted water can be carried into Kuroshio surface waters and even reach the Korea/Tsushima Strait and enter the Japan/East Sea (Beardsley et al., 1985; Chen et al., 1994), which means that the Three Gorges Dam can inﬂuence not only ECS carbon cycling processes but also those of areas at a distance. 4.2.4 Inorganic Carbon in Sediments The ocean is evidently a major sink of carbon dioxide and plays an important role in the global carbon cycle. However, the carbon ﬂux between seawater and sediment in the coastal seas is still poorly understood. Knowledge of the carbonate mineral dissolution in sediment during the processes of diagenesis, lithiﬁcation, and evolution is of central importance to develop an insight into the carbon ﬂux. The pattern of calcium carbonate accumulation rates can be used to decipher the Pliocene-Pleistocene history of biogenic production and its relationship with global and local changes in oceanic circulation and climate. Calcium carbonate dissolution or precipitation is controlled mainly by bottom water (or pore water) saturation state, sediment pH, and metabolic release of carbon dioxide. Moreover, salinity also aﬀects the chemical reactions that occur in sediment, like precipitation/dissolution of CaCO3 . Inorganic carbon (IC) in sediment varies little, or increases with depth due to dissolution near the sediment-water interface. The direct tracer of CaCO3 dissolution is the increase in the calcium concentration of the pore water below the sediment-water interface. Most studies focused on the TIC in sediment, a few on IC forms. In fact, there are many kinds of carbonate minerals in sediment such as aragonite, siderite, calamine, cerusite, phosgenite, magensite, and dialogite with diﬀerent dissolvabilities in diﬀerent pH solutions, for example: calamine can be dissolved in NH3 ·H2 O; cerusite and phosgenite can be dissolved in NaOH; calcite and aragonite can be dissolved in acid. To understand the burial and diagenesis of inorganic carbon in marine sediments, it is necessary to identify, separate and quantify the various solid-phase reservoirs of deposited carbon, but it is very diﬃcult because of the ﬁne-grained nature of most marine sediments. So it is necessary to ﬁnd an indirect means to determine the identity and size of sedimentary IC reservoirs. One approach is physical separation of diﬀerent sedimentary fractions by grain size, and measurement of TIC in the diﬀerent fractions after each has been separately dissolved. However, this method can easily lead to ambiguous or incorrect identiﬁcation of the C-bearing phase. Complete physical separation of different phases from ﬁne-grained sediment rarely can be achieved and surface coating of various sorts, potentially important in IC removal to sediments, can remain undetected and unidentiﬁed in such treatment. The most promising methods for separating and quantifying the various IC reservoirs in marine sediment are sequential extraction. So according to IC characters, IC in sedi-

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4 Biogeochemical Processes of the East China Sea

ment is divided into ﬁve forms (Fig. 2.10): NaCl form, NH3 ·H2 O form, NaOH form, NH2 OH·HCl form and HCl form (Li et al., 2006b). The purpose of this section was to study the biogeochemical characteristics of IC in the Changjiang River Estuary sediments and its contribution to the marine carbon cycle. 4.2.4.1 Spatial Distribution of Carbonate in the East China Sea Sediment In the ECS continental shelf surface sediments, carbonate is the most abundant in the outer continental shelf sediments (Lin et al., 2002b). The highest content, up to 90%, was found in the area where the Kuroshio Current intruded on the ECS continental shelf. Common carbonate content on the shelf is in the range of 10% to 30%. Values greater than 25% were observed mostly in areas directly under the path of the Kuroshio Current. Most sediments are biogenic carbonate along the southeastern part of the outer shelf (Gao et al., 2006). This resulted in that the carbonate is rich in coarse sediment and poor in ﬁne sediment (Fig. 4.11, Yang et al., 2002).

111

Fig. 4.11. Carbonate contents in the diﬀerent grain-size sediments of the ECS station 111 (32◦ 0.08 N, 125◦ 59.41 E) and station 410 (29◦ 19.38 N, 124◦ 59.57 E) (Yang et al., 2002) (With permission from Yang ZS)

In the northern ECS mud area and its surrounding area, carbonate content in the surface sediments is less than 5%, much lower than that in other areas of the ECS. Evidence indicated that the sediments in this area are a mixture of the Yellow River, the Changjiang River, and resuspended matter adjacent to the mud area (Yang et al., 2002). Distribution of carbonate in the surface sediments of these areas can be divided into four zones (Fig. 4.12): a relatively high carbonate content zone (>1.2%) in the coastal area, a low carbonate content zone in the middle continental shelf sand area (<1.2%; in most of this area, >1.0%), a high carbonate content zone in the middle continental

4.2 Carbon Cycling in the East China Sea

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shelf mud area (range 1.2%∼2.0%; in most of this area, >1.5%), and a highest carbonate content zone in the outer continental shelf sand area (>2.0%). The distribution pattern of carbonate is inﬂuenced by the source of carbon and corresponds well with the distribution of the circulation system (Guo et al., 1999).

ECS

N 34

1.5

32

2.0

1.2 1.0

A

1.2

<1.0

>2.0 1.5

B

C

D

126

128

3 2 . 5. 0

3.5

30

28

120

122

124

130

E

Fig. 4.12. Distribution of IC (%) in the surﬁcial sediments (Guo et al., 1999) (With permission from Yang ZS)

4.2.4.2 Inorganic Carbon Forms in the Changjiang River Estuary Sediments In the Changjiang River Estuary, a sequential extraction method which is based on the diﬀerence in IC chemical combined strength was used to divide IC in sediment into ﬁve forms: NaCl form, NH3 ·H2 O form, NaOH form, NH2 OH·HCl form and HCl form. The content of diﬀerent IC forms in surface sediment of the Changjiang River Estuary is shown in Table 4.4 (Li et al., 2006b). The frequency of the content in diﬀerent forms is shown in Fig. 4.13 (Li et al., 2006b). The content of diﬀerent IC forms increased obviously from the NaCl form to the HCl form. In particular, the content of the NaCl form ranged mostly from 0.20 to 0.40 mg/g with an average of 0.29 mg/g, and accounted for 2.84% in total IC. The content of the NH3 ·H2 O form ranged mostly from 0.49 to 0.78 mg/g, with an average of 0.54 mg/g, and accounted for 5.75% in total IC. The content of the NaOH form ranged mostly from 0.60 to 1.00 mg/g with an average of 0.80 mg/g, and accounted for 7.88% in total IC. The content of the NH2 OH·HCl form ranged mostly from 2.00 to 4.00 mg/g with an average of 3.11 mg/g, and accounted for 30.33% in total IC. The content of the HCl form ranged mostly from 4.00 to 6.50 mg/g with a mean of 5.38 mg/g, and accounted for 53.61% in total IC. The NH2 OH·HCl and HCl forms accounted for most of the total IC in the Changjiang River Estuary sediments.

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4 Biogeochemical Processes of the East China Sea

Fig. 4.13. Frequency of the content for diﬀerent IC forms (Li et al., 2006b) (With permission from Springer)

4.2.4.3 Factors Inﬂuencing the Distribution of Inorganic Carbon The change in diﬀerent IC forms in sediment was inﬂuenced by many factors, such as pH, Eh, Es, organic carbon (OC), total phosphorus (TP), total nitrogen (TN), organic phosphorus (OP), organic nitrogen (ON), inorganic phosphorus (IP), inorganic nitrogen (IN), and water content. The inﬂuence of various factors was not isolated, but mutually restricting. So cluster analysis and factor analysis were used to estimate synthetically the inﬂuence of all factors in this chapter.

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Table 4.4. The content of diﬀerent inorganic carbon forms in surface sediment of the Changjiang River Estuary (mg/g) (Li et al., 2006b) (With permission from Springer) Station 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 22 23 25 26 28 29 30 31

NaCl form 0.064 0.129 0.229 0.280 0.319 0.224 0.109 0.216 0.381 0.289 0.284 0.251 0.248 0.253 0.286 0.287 0.313 0.353 0.338 0.311 0.381 0.301 0.353 0.349 0.357 0.518 0.357 0.311

NH3 ·H2 O form 0.173 0.271 0.319 0.259 0.495 0.463 0.267 0.487 0.577 0.573 0.658 0.512 0.491 0.543 0.566 0.538 0.686 0.679 0.732 0.646 0.563 0.616 0.791 0.692 0.749 0.677 0.590 0.542

NaOH form 0.198 0.363 0.930 0.645 0.597 0.736 0.234 0.624 0.970 0.995 0.769 0.913 0.605 0.886 0.875 1.113 1.306 1.386 1.273 0.723 0.702 1.184 0.939 0.798 0.815 0.730 0.434 0.611

NH2 OH·HCl form 1.798 1.301 2.933 2.241 2.713 1.962 2.313 2.735 3.620 2.030 3.481 2.754 2.922 2.527 5.863 4.187 3.841 3.405 3.994 4.516 4.818 2.229 2.718 2.903 3.876 2.837 2.661 3.965

HCl form 5.177 2.929 6.392 7.427 5.356 4.639 6.768 5.460 6.012 4.800 5.246 5.766 5.921 5.242 4.233 4.257 7.575 4.188 4.772 4.486 5.326 4.015 5.948 6.048 5.875 5.903 5.986 4.825

Total IC 7.410 4.994 10.802 10.852 9.481 8.023 9.691 9.522 11.561 8.686 10.439 10.197 10.187 9.451 11.824 10.382 13.721 10.011 11.108 10.682 11.789 8.345 10.748 10.789 11.671 10.665 10.028 10.255

(1) Cluster analysis Cluster analysis encompasses a number of diﬀerent algorithms and methods for grouping objects of a similar kind into respective categories. In other words, cluster analysis is an exploratory data analysis tool aimed at sorting diﬀerent objects into groups in a way that the degree of association between two objects is maximal if they belong to the same group and minimal otherwise. So, cluster analysis can be used to discover structures in data without providing an explanation/interpretation and may reveal deeper associations in data which, though not previously evident, nevertheless are sensible and useful once found. In this chapter, the aim of using cluster analysis is to sort all inﬂuencing factors into groups, and to reveal the association among inﬂuencing factors and diﬀerent IC forms.

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4 Biogeochemical Processes of the East China Sea

Fig. 4.14 (Li et al., 2006b) shows the results of cluster analysis for the factors which inﬂuence IC in sediment. The cluster analysis classiﬁed the factors into two homogenous clusters: physical factors and chemical factors, which can be subdivided into N-cluster, P-cluster, and OC. Physical factors can be subdivided into Eh-, Es-, and pH-cluster. The inﬂuence of all factors belonging to one cluster (or sub-cluster) on IC is similar. TN ON IN TP OP IP OC pH Water content Eh Es

Fig. 4.14. Results of cluster analysis for IC inﬂuencing factors in the Changjiang River Estuary sediment (Li et al., 2006b) (With permission from Springer)

Fig. 4.15 (Li et al., 2006b) shows the results of cluster analysis for all forms of IC and their inﬂuencing factors, which can indicate which factor has a close relationship with which IC form. On the whole, all the IC forms are inﬂuenced by physical factors indirectly through acting with chemical factors. But chemical factors inﬂuence IC directly and, moreover, OC has a close relationship with the NaOH form and a weak relationship with the HCl form. Its relationship with the NH2 OH·HCl form and TIC is weaker than its relationship with the NaCl form, but closer than its relationship with the HCl form. OP and TP have a close relationship with the NH2 OH·HCl form and TIC, but the weakest relationship with the HCl form and a weaker relationship with other forms of IC. TN, IN, and ON have only a weak relationship with all IC forms. All these revealed that the association between the HCl form TN ON IN NaCl IC NH 3 H 2O IC NaOH IC OC NH 2OH HCl IC TIC TP OP IP HCl IC pH Water content Eh Es

Fig. 4.15. Results of cluster anaylsis for all forms of IC and their inﬂuencing factors in the Changjiang River Estuary sediments (Li et al., 2006b) (With permission from Springer)

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455

and the most inﬂuencing factors was weak, and that the HCl form may play a special role in carbon cycling. (2) Factor analysis The main applications of factor analysis are to reduce the number of variables and detect structure in the relationships between variables. Therefore, we used factor analysis to reveal which factor plays the main role in carbon cycling. Table 4.5 (Li et al., 2006b) shows the result of factor analysis for inﬂuencing factors of IC in the Changjiang River Estuary sediment. Four factors can be used to represent all inﬂuencing parameters. They contribute 80% of all information, and every factor’s contribution gradually decreases from factor 1 to factor 4. Table 4.6 (Li et al., 2006b) is a factor loading matrix of factor analysis. Factor 1 mainly includes TN, IN, and ON, factor 2 mainly includes pH, Es, water content and OC, factor 3 mainly includes TP and OP, and factor 4 mainly includes Eh and IP. These results indicated that TN, IN and Table 4.5. Results of factor analysis for inﬂuencing factors of IC in Changjiang River Estuary sediment (Li et al., 2006b) (With permission from Springer) Factor

Eigenvalue

1 2 3 4 5 6 7 8 9 10 11

3.629081 2.439065 1.559507 1.189572 0.704013 0.651339 0.437943 0.264626 0.124853 3.56E–16 –2.8E–17

Percentage of accumulated variance 32.99164 55.16496 69.34230 80.15659 86.55671 92.47797 96.45928 98.86497 100 100 100

Percentage of variance 32.99164 22.17332 14.17734 10.81429 6.400117 5.921263 3.981304 2.405695 1.135028 3.24E–15 –2.5E–16

Table 4.6. Factor loading matrix of factor analysis (after rotation) (Li et al., 2006b) (With permission from Springer) Factor pH Eh Es Water content OC TN IN ON TP IP

1 −0.15 −0.25 0.29 −0.17 0.12 0.98 0.75 0.89 0.02 −0.27

2 0.83 0.49 0.63 0.67 −0.78 −0.03 −0.22 0.08 −0.05 0.10

3 −0.04 0.11 0.11 −0.48 0.41 0.12 0.16 0.07 0.80 0.12

4 0.02 −0.68 −0.22 0.32 0.10 −0.07 0.04 −0.11 0.58 0.88

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4 Biogeochemical Processes of the East China Sea

ON provided the biggest contribution to all IC forms in the Changjiang River Estuary, followed by pH, Es, water content and OC, then TP and OP, while Eh and IP’s contribution was the least. So we can conclude that nitrogen, pH, Es, water content and OC may play important roles in the change in IC. 4.2.4.4 Transformation of Diﬀerent IC Forms All matter in sediment might be changed at a diﬀerent rate during early diagenesis when organic matter was degraded by benthic macro- and microorganisms and accompanied by the consumption of oxygen and other electron acceptors, and by the production of CO2 and other metabolites. For example, with OC’s continuing oxidation, nitrogen (N) may transform to some other N form by nitriﬁcation or de-nitriﬁcation. And phosphorus (P) may transform to PO3− or form apatite (Song, 1997). Due to the continued dissolution or 4 participation of carbonate, any IC form in sediment may also be changeable (Song et al., 2008). The vertical distribution of the ratio of every IC form to total IC should be stable when the source matter is stable. A change in the ratio of one IC form should indicate that the content of this form of IC was increased or decreased. The increase in content indicates that other forms of IC transformed to it; otherwise, this form will transform to another form. Fig. 4.16 showed the ratio of every IC form to total IC in the Changjiang River Estuary sediment. The ratios of the NaCl form, NH3 ·H2 O form, NaOH form, and NH2 OH·HCl form have a similar vertical distribution in the Changjiang River Estuary core samples, but the HCl form’s vertical distribution is the reverse of them, which indicates that all IC forms have a trend of transforming from the NaCl form, NH3 ·H2 O form, NaOH form, and NH2 OH·HCl form to the HCl form. That is to say, the NaCl form, NH3 ·H2 O form, NaOH form and NH2 OH·HCl form are unstable, and easy to change in matter exchange in sediment, and will be changed into the HCl form ultimately. 4.2.4.5 Contribution of Every IC Form to Marine Carbon Cycling Part of the IC in sediment will return to water to participate in the carbon recycle (Song, 2004). Precipitation or dissolution of IC in sediment is aﬀected by physical and chemical factors in overlying water except in sediment, such as DIC, temperature, salinity, dissolved oxygen (DO), NO3 -N, PO4 -P, active silicon, Chl a, phytoplankton, zooplankton. Although these factors in overlying water play an important role, the function of every factor is diﬀerent. Some factors such as nitrogen and phosphorus may improve the dissolution of IC, whose consumption will promote the absorption of carbon by phytoplankton and then the dissolution of IC in sediment, while other factors such as zooplankton may improve IC precipitation, whose growth will increase the deposition of IC. The correlation between one factor and IC can reﬂect this relationship. The correlation between diﬀerent IC forms and every aﬀecting

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457

0 0 2 4 6 8 10 12 14 0 0 2 4 6 8 10 12 14

Station 7 Ratio (%) 20 40

60

80

Station 10 20

Ratio (%) 40 60

Station 13

80

Depth (cm)

0

Station 2 0

Ratio (%) 20 40

0

Station 5 Ratio (%) 20 40 60

0 2 4 6 8 10

60

0 1 2 3 4 5 0 1 2 3 4 5 6 7

80

Station 8 0

Ratio (%) 20 40

0 0 2 4 6 8 10 12 14

60

Station 11 Ratio (%) 20 40

Station 14

0 0.5 1.0 1.5 2.0 2.5

Depth (cm)

0 1 2 3 4 5 6 7

80

Ratio (%) 40 60

Depth (cm)

0 1 2 3 4 5 6 7

20

Depth (cm)

0

Station 4 Ratio (%) 20 40 60

80

Depth (cm)

Depth (cm)

0 1 2 3 4 5

Ratio (%) 20 40 60

0

60

Depth (cm)

Depth (cm)

0 1 2 3 4 5

Depth (cm)

Depth (cm)

0 0.5 1.0 1.5 2.0 2.5

Depth (cm)

80

Station 1 0

Depth (cm)

Ratio (%) 40 60

20

Depth (cm)

0 1 2 3 4 5

0

Depth (cm)

Depth (cm)

factor of IC in overlying water indicates every IC form has a diﬀerent ability to return to water from sediment. In other words, the contribution of diﬀerent IC forms to carbon cycling is diﬀerent. So the correlation can be used to deduce one IC form’s contribution to carbon cycling. The correlation between diﬀerent IC forms in the Changjiang River Estuary sediment and physical, chemical, and biological factors in overlying water are listed in Table 4.7 (Li et al., 2006b). In order to assess the contribution, a new parameter, contribution score, was designed. The correlation between every IC form and DIC, temperature, salinity, DO, NO3 -N, PO4 -P, active silicon, Chl a, phytoplankton, and zooplankton was used to determine the contribution synthetically. The calculation of the contribution score is explained below.

0

0

0 0 2 4 6 8 10 0 0 2 4 6 8 10 12 14

20

Ratio (%) 40 60

80

Station 3 Ratio (%) 40 60

80

Station 6 Ratio (%) 20 40

60

20

Station 9 Ratio (%) 20 40

60

Station 12 Ratio (%) 20 40

Station 15

60

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4 Biogeochemical Processes of the East China Sea

Fig. 4.16. Vertical change for ratio of every IC form to total IC in the Changjiang River Estuary sediments Table 4.7. Correlation between diﬀerent IC forms in the Changjiang River Estuary sediment and physical, chemical, and biological parameters in overlying waters (Li et al., 2006b) (With permission from Springer) IC form NaCl IC NH3 ·H2 O IC NaOH IC NH2 OH·HCl IC HCl IC

Temperature Salt −0.57 −0.69 −0.60 −0.53 0.23

0.47 0.65 0.60 0.61 −0.17

DO

− NO3

−0.27 −0.46 −0.64 −0.32 0.08

−0.45 −0.61 −0.51 −0.58 0.12

Active P −0.27 −0.39 −0.41 −0.57 0.23

Active Si −0.40 −0.46 −0.60 −0.61 0.13

Chl a −0.09 −0.13 −0.14 0.01 0.15

Zooplankton Cell counts of DIC dry weight phytoplankton 0.13 0.03 0.64 0.30 0.18 0.75 0.49 0.29 0.61 0.63 0.23 0.47 −0.09 −0.03 −0.19

The coeﬃcients of correlation between the same factor and ﬁve diﬀerent IC forms were calculated and sorted by their absolute values, and given a score of 5, 4, 3, 2, 1 respectively, in the order of maximum (score 5) to minimum (score 1). Negative correlation indicates that this factor can improve IC dissolution, and more IC may take part in carbon cycling, and its score will be positive. Positive correlation indicates that this factor improves IC precipitation, so less IC participates in carbon cycling and its score will be negative. The sum of all

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459

scores of one IC form to every inﬂuencing factor is the contribution score of this IC form. The results are shown in Table 4.8 (Li et al., 2006b). According to the value of the contribution score, the contribution of the NaOH form to carbon cycling is the biggest, the NH2 OH·HCl form is the second, and the contribution of the NH3 ·H2 O form is smaller than that of the NH2 OH·HCl form. The contribution of the NaCl form is smaller than that of the NH3 ·H2 O form. The contribution of the HCl form is the smallest. The contribution of the NaOH form to carbon cycling being the biggest may be due to its weaker combined strength, and its higher content compared with those of the NaCl form and NH3 ·H2 O form. The contribution of the NH2 OH·HCl form being bigger than that of the NaCl form may be due to its much higher content compared with that of the NaCl form, and its advantage of higher content compensates for the inadequacy of its bigger combined strength. The combined strength of the HCl form is the biggest, so it cannot easily participate in carbon recycling. Table 4.8. Contribution scores of diﬀerent IC forms in Changjiang River Estuary sediments (Li et al., 2006b) (With permission from Springer) Temper- Salt DO ature NaCl IC 3 –2 2 NH3 ·H2 O IC 5 –5 4 NaOH IC 4 –3 5 NH2 OH·HCl IC 2 –4 3 HCl IC –1 1 –1

− NO3 2 5 3 4 –1

ActiveActive Chl P Si a 2 2 2 3 3 3 4 4 4 5 5 1 –1 –1 –5

Zooplankton dry weight 2 3 4 5 –1

Cell counts of phytoplankton 2 3 5 4 –1

DIC Contribution score –4 8 –5 14 –3 23 –2 19 1 –9

From the positive and negative contribution scores, the NaCl form, NH3 ·H2 O form, NaOH form, and NH2 OH·HCl form may take part in carbon cycling during early diagenesis, and may be a potential carbon source. The HCl form may not participate in carbon recycling within the short term, and will be buried in the long term. It is well worth noting that HCl IC may be one of the ﬁnal resting places of atmospheric CO2 . Regarding the source of HCl IC, if it came from organisms, the equivalent CO2 would surely be depleted in its genetic process; if it came from terrestrial sources, carbonate would deplete CO2 in the weathering process because CO2− might be transformed into HCO− 3 3 by absorbing CO2 (Wong, 1995). So, wherever it came from, HCl IC will deplete atmospheric CO2 . If the amount of all HCl IC in global marine sediment is known, the amount of absorbed atmospheric CO2 can be calculated from the sediment rate. 4.2.4.6 Sedimentation Flux of Diﬀerent IC Forms in the Changjiang River Estuary According to the above results, oﬀshore sediment is the sink of atmospheric CO2 . But its capacity for sinking is controlled mainly by the input of IC

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4 Biogeochemical Processes of the East China Sea

from rivers, a high level of primary productivity, the sedimentation rate and hydrodynamic conditions, and other factors. In the Changjiang River Estuary, the enormous primary production increased from west to east. Abundant matter from the Changjiang River was deposited here, where the sedimentation is faster, at about 2.0 cm/yr. In addition, the not too strong wave, tide, and coastal current are favorable for sedimentation. So there is a strong ability to ﬁx carbon in the Changjiang River Estuary. Sedimentation ﬂux can be used to assess the strength of sinking. Sedimentation ﬂux of every IC form can be calculated as follows: BF = Ci × DR × S × ρd ρd =

1 − Wc (1 − Wc )/ρs + Wc /ρw

where BF is the sedimentation ﬂux of carbon in sediment, Ci is the content of every IC form, DR is the sedimentation rate, S is the area of sub-aqueous delta, ρd is the dry density of sediment, Wc is the water content, ρs is the density of sediment, and ρw is the density of seawater. The results (Table 4.9, Li et al., 2006b) showed that the sedimentation ﬂux of TIC is 11.17×1011 g C/yr, which is more than organic carbon ﬂuxes. About half (HCl IC) will be buried for a long time; the other half may be the potential source of carbon cycling. So the Changjiang River Estuary sediment may absorb at least about 40.96×1011 g atmospheric CO2 every year, which indicates that oﬀshore sediment plays an important role in absorbing atmospheric CO2 . Table 4.9. Sedimentation ﬂux of diﬀerent IC forms in the Changjiang River Estuary (×1011 g C/yr) (Li et al., 2006b) (With permission from Springer) OC 7.11

NaCl IC 0.32

NH3 ·H2 O IC 0.60

NaOH IC 0.88

NH2 OH·HCl IC 3.43

HCl IC 5.93

TIC 11.17

4.2.5 Biogeochemical Characteristics of Organic Carbon in Sediment Although present day shelf seas make up less than 8% of the total ocean surface area, about one ﬁfth to one third, or 8.3×109 t, of organic carbon of the global annual marine primary production takes place in these areas (Wollast, 1991). Shelves, therefore, are considered potentially important sinks for large amounts of organic carbon. de Haas et al. (2002), however, concluded that the role of shelf seas as sinks for organic carbon is overestimated at times. Under present day conditions, large areas of the continental shelves do not

4.2 Carbon Cycling in the East China Sea

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show any accumulation of organic matter. Most of the organic particles that settle on the sea ﬂoor are decomposed through early diagenesis in the surface sediment layer and released as dissolved matter back to the bottom water through interstitial water. 4.2.5.1 Spatial Distributions of Organic Carbon in Sediments For the majority of the ECS continental shelf sediments, the organic carbon content is between 0.0% and 0.4%, except in the Changjiang River delta and inner shelf region (>0.9%), which is lower than the world shelf average with 0.75% (Berner, 1982). Organic carbon content in ECS continental shelf sediments takes on large spatial variation with a band-type distribution (Lin et al., 2002a). The band-type distribution generally follows the coastline, with little variation on the north-south axis. Higher organic carbon content appears in inner shelf sediments and lower content in outer shelf sediments. Organic carbon content in sediments is controlled by grain size and increases linearly with an increasing proportion of ﬁne grained sediment (Song et al., 1996; Guo et al., 1999). Terrigenous sediments from the Changjiang River are a dominating factor controlling the spatial variation of organic carbon content. Fine-grained sediments from the Changjiang River deposit mostly on the inner shelf and decrease rapidly away from land. As a result, organic carbon concentrations also decrease in a seaward direction. Another possible factor for lower organic carbon content in the middle and outer continental shelf sediments is rapid organic carbon oxidation in sediments through sulfate reduction (Lin et al., 2000). The stable carbon isotope δ 13 C in sediments near the Changjiang River Estuary indicates a predominately terrigenous source, whereas sediments away from the Changjiang River Estuary are characterized by organic carbon with a marine signature. To sum up, the distribution of organic carbon in the sediment is an integrated result of the source, sedimentary dynamic environment associated with the circulation system, and physicochemical conditions of the seaﬂoor (Song, 2000a; Lin et al., 2002a). 4.2.5.2 Contribution of the Changjiang River to Organic Matter in Sediments Several sources of organic carbon may contribute to the particulate organic matter on the ECS shelf: (1) terrestrial organic carbon from the Changjiang River Estuary, (2) alive or dead organic matter of planktonic origin related to primary production, (3) the contribution from the Kuroshio water, and (4) resuspension of organic matter from bottom sediments. Compared with particulate organic matter of the ECS shelf, organic matter in sediments of the ECS mainly originates from terrestrial organic carbon from the Changjiang River Estuary (terrestrial source), plankton and benthos (marine source). The deposited particulate organic matter from the Changjiang River, could be suﬀering from diﬀerent sediment dynamics and various biogeochemical eﬀects.

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4 Biogeochemical Processes of the East China Sea

Some of the marine source organic matter originates from native biological production, and others may derive from biological productivity transported from other sea areas. The contribution of terrestrial and marine organic matter to the ECS sediment is shown in Fig. 4.17 (Gao et al., 2007). The highest contribution (up to 50%) of the terrestrial organic carbon sources appears near the Changjiang River Estuary, and their isolines project towards the northeast which indicate the inﬂuence of the Changjiang diluted water, west of 31◦ N. Whereas, near 123.5◦ E, the contribution of terrestrial organic carbon quickly decreases to 30%. To the south of 31◦ N, the terrestrial organic carbon proportion lessens rapidly to only 30% near 123◦ E, and about 10% near 124◦ E. Wu et al. (2003) have calculated the contribution of organic matter from the Changjiang River to particulate organic matter on the ECS shelf. Their results indicate that, within a distance of <100 km oﬀ the river mouth, the terrigenous particulate organic matter dominates, being 70%∼80% of the total particulate organic matter; the percentage of terrestrial particulate organic matter decreases quickly to 50% at a distance around 150 km oﬀ the mouth; further oﬀ shore (>250 km) the contribution of terrigenous particulate organic matter is less than 20%. Compared with their results, the contribution of terrestrial organic carbon to particulate organic matter is obviously greater than that of organic matter in sediments in the same place. Terrestrial particulate organic matter

Fig. 4.17. Proportion of terrestrial organic matter in sediments (%) of the Changjiang River Estuary and its adjacent sea area (Gao et al., 2007) (With permission from Gao JH)

4.2 Carbon Cycling in the East China Sea

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suﬀers from eﬀects of diagenesis, benthos and continuous inputting of dead organic matter of plankton, after depositing on the seabed (Gao et al., 2007). Therefore, the proportion of terrestrial organic matter in sediments decreases constantly, because of the above factors. 4.2.5.3 Organic Carbon Mineralization In the ECS, benthic ﬂuxes from muddy shelf sediments are larger than those from sandy shelf sediments (Kato and Terumuma, 1995). Benthic ﬂuxes are inﬂuenced by the oxygen concentration of surrounding water and sediment physical properties, such as water content, grain size. Benthic ﬂuxes of total carbon on the shelf increase rapidly with an increase in surface sediment water content. Guo and Yang (1997) studied the sediments in three typical areas in the ECS, including the cold eddy mud area in the northern ECS [32◦ 00 N, 126◦ 00 E], the middle continental shelf sand area [29◦ 19 N, 125◦ 00 E], and the Zhejiang coastal mud area [30◦ 45 N, 122◦ 45 E]. In the middle continental shelf sand area, though, there is a considerably high vertical ﬂux of particulate carbon in the bottom seawater. Maintenance of carbon in the surface sediment is almost impossible because of the low depositional rate and an oxidative environment on the seaﬂoor. Particulate carbon transfers from seawater to surface sediment during the bloom period for marine organisms, but carbon loss from the sediment process is dominant in other seasons. Therefore, the annual budget of carbon through the sediment-seawater interface is in dynamic equilibrium, limiting the carbon ﬁxation contribution in this area. The cold eddy mud area and Zhejiang coastal mud area are carbon sinks because their physicochemical conditions are advantageous for carbon maintenance. The amount of carbon ﬁxation in the Zhejiang coastal mud area is much more than that in the cold eddy mud area because the former has a much higher depositional rate. Quantitative determination of the organic carbon deposition and oxidation rate is important in understanding the fate of organic carbon in the marine environment. Anoxic sulfate reduction and permanent burial are two primary pathways that determine the fate of sedimentary organic carbon in shelf sediments. The southern ECS continental shelf is characterized by a low organic carbon concentration with a relatively fast sedimentation rate and is an eﬃcient pyrite sulfur burial environment. The primary factor controlling pyrite formation is the supply of organic carbon, because both sulfate reduction rates and pyrite burial rates increase linearly with an increase in the organic carbon burial rate, and because reactive iron is abundant in southern ECS continental shelf sediments. The fast sedimentation rate makes more organic matter available as the sulfate reducer; it will undergo a shorter period of oxic and suboxic degradation, allowing more pyrite to be formed and buried in sediments. Before its ﬁnal burial, up to 96% of the pyrite-sulfur produced from sulfate reduction is reoxidized. The organic carbon utilized in sulfate

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4 Biogeochemical Processes of the East China Sea

reduction and organic carbon burial is 37.7 g C/(m2 ·yr), representing only (20.6±10.4)% of the primary production rate. A signiﬁcant fraction of the annual productivity appears to be missing, and not deposited into the sediments. The supply of organic carbon limits not only the rate of sulfate reduction but also the ﬁnal burial of pyrite in the ECS continental shelf sediments (Lin et al., 2000; 2002b). 4.2.5.4 Dynamic Processes Stoichiometric patterns of dissolved organic matter (DOM) in the shelf-wide vernal ECS were investigated by Hung et al. (2003). The ratios of DOC, dissolved organic nitrogen (DON), and dissolved organic phosphorus (DOP) in the ECS deviate signiﬁcantly from the Redﬁeld ratio (C:N:P=106:16:1) derived from biota. The elemental ratio ranges from 8.9 to 15.3 for DOC:DON, from 19.0 to 83.6 for DON:DOP, and from 200 to 853 for DOC:DOP in ECS shelf and Kuroshio surface waters (<150 m). Compared with the Redﬁeld value, DOM is considerably rich in carbon relative to nitrogen and phosphorus and in nitrogen relative to phosphorus. Elemental ratios exhibit a large range for each water type; no clear spatial pattern can be identiﬁed, although slightly greater values appear in the upwelled water and Kuroshio waters. C:N, N:P, and C:P all increase gradually up to a depth of 100 m but increase markedly below 100 m in Kuroshio waters, suggesting a preferential decay of DOP over DOC and DON, and DON over DOC in the ECS. Such patterns of ranges and depth-increased ratios are consistent with previous results obtained from marginal seas and open oceans. It is obvious that the elemental ratios of DOM do not necessarily conform to the Redﬁeld ratio. Mineralization rates of DOC and DOP are much greater in ECS shelf waters than that in Kuroshio waters. The residence time estimated for shelf DOC, DON, and DOP (about 1.10, 0.98, 0.92 yr, respectively) is close to that of shelf water, implying little DOM escaped from recycling in the shelf. The ECS shelf is likely to be a small source of degradable DOM for oceanic waters. It has been estimated that the ECS can adsorb considerably more CO2 carbon in the inner and middle shelves adjacent to the Changjiang River mouth than the estimated ﬂux of organic carbon burial in shelf sediments (Gao et al., 2007). Apparently, a major fraction of absorbed carbon must be exported out of the ECS shelf to balance the carbon budget. The Okinawa Trough, which is located at the seaward edge of the shelf, has been noted as an important depocenter for POM from the ECS. Plenty of evidence indicates that the southern Okinawa Trough (SOT) is an important site for POC export from the shelf. In the SOT region, sedimentation rates and POC ﬂuxes are signiﬁcantly higher than those observed in the central and northern trough. In addition to sediments from the ECS shelf, sediments from Taiwan (Hsu et al., 1998) and primary production in the water column (Liu et al., 1995) are potential sources of sedimentary organic matter in the SOT.

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465

To determine which of the potential sources is the most important, Kao et al. (2003) analyzed the TOC and total nitrogen content and their isotopic compositions of surﬁcial sediment samples obtained from a wide region on the ECS shelf and the SOT in depths from 50 to 1,500 m. Distributions of TOC content, 13 Corg , and nitrogen content all show a similar spatial pattern, resembling the distribution of ﬁne-grained sediments. The coastal belt of elevated organic carbon content extends southward from the mouth of the Changjiang River and veers oﬀshore toward the SOT just north of Taiwan, suggesting a pathway for ﬁne grained sediments from the inner shelf to the depocenter. This distribution pattern is consistent with shelf circulation. The isotope compositions (13 Corg , 15 N) of sediments from the SOT fall between those of riverine particulate organic matter and the mid-outer shelf sediments, but overlap with those of the inner shelf sediments (Fig. 4.18, Kao et al., 2003). The previously reported 13 Corg values of sinking particles collected by sediment traps in the SOT (Sheu et al., 1999) are also close to those of the inner shelf sediments, especially during high-ﬂux conditions. Therefore, the isotopic evidence strongly supports the notion that a major fraction of the sedimentary organic matter in the SOT originates from the inner shelf. Jeng et al. (2003) examined n-alkanes, n-fatty alcohols, and sterols in the sediments from SOT and concluded that most of the extracted lipids are of terrestrial origin. 7

Mid-outer shelf sediments Inner-shelf sediments Near shore sediments

d 15N ( )

6 5

(1)

4

(3)

(2) (4) 3 2 26

25

24

23 22 d 13 C org ( )

21

20

19

Fig. 4.18. 13 Corg -15 N plot of potential sources and sedimentary organic matter in the SOT. Rectangles (1), (2), (3), and (4) represent the isotopic composition ﬁeld of marine end-member, riverine POM, SOT sediments, and trapped material collected at SST1 (in the northern trough) and SST2 (in the central trough), respectively (Kao et al., 2003) (With permission from Elsevier’s Copyright Clearance Center)

It was found that both sediment and organic carbon accumulation rates are the highest in the inner shelf area (1.8 g C/(cm2 ·yr) and 3.1 mg C/(cm2 ·yr) for sediment and organic carbon accumulation rates, respectively). Sediment accumulation rates decreased oﬀshore, whereas organic carbon accumulation

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4 Biogeochemical Processes of the East China Sea

rates were relatively high on the shelf edge and the slope (Fig. 4.19, Oguri et al., 2003). Most of the depositing particles accumulated in the area from the shelf edge to the Okinawa Trough, except part of the lower slope. On the other hand, organic carbon is less preserved, suggesting that most is re-transported, re-mineralized, or both. In the trough area, both sediment and organic carbon accumulation rates are higher than the ﬂuxes gained by the sediment trap, so lateral near bottom transport could be a key process in material transport into the trough sediments from the shelf; that is, the seasonality of particle ﬂuxes in the deep waters of the trough is linked to seasonal events (local climate and oceanographic conditions) in shelf waters rather than the surface biology in the Okinawa Trough. The information of when and how suspended matter is transported from the inner shelf of the ECS to the shelf edge was gained by numerical model simulations. The models indicated that suspended matter is transported from the shelf edge to the inner shelf in summer and from the inner shelf to the shelf edge in other seasons as a result of vertical circulation, mainly from monsoons. Maximum transport of suspended matter from the inner shelf to the shelf edge occurs in autumn, whereas in spring and summer transport is very small. It has been reported that at other continental margins (e.g., Mid-Atlantic Bight, North Sea, Northwest Mediterranean), storms also play

Fig. 4.19. Budgets of depositing particles (a) and organic carbon (b) on the seaﬂoor from the shelf to the trough areas (Oguri et al., 2003) (With permission from Elsevier’s Copyright Clearance Center)

4.3 Nitrogen Variations and Budgets in the East China Sea

467

an important role in the delivery of shelf sediments to the continental slope, particularly in winter because of a higher frequency of storms and enhanced mixing from cooling at that time compared with other seasons.

4.3 Nitrogen Variations and Budgets in the East China Sea The ECS shelf receives a rich supply of nutrients from the Changjiang River and other rivers as well as from the Kuroshio. The total runoﬀ from the major rivers to the ECS shelf is about 1.1×1012 m3 /yr, 85% of which is from the Changjiang River. The riverine discharges are rich in nitrogenous nutrients and silicate but low in dissolved phosphate, for the biogeochemical process is strongly inﬂuenced by the Changjiang River. 4.3.1 Seasonal Variations of Nitrogen in Seawaters The tremendous dissolved inorganic nitrogen (DIN) from the Changjiang River to the ECS has caused elevated primary production, frequent harmful algal blooms, and a large area of hypoxia in the Changjiang River Estuary and the adjacent ECS. Moreover, the eutrophication in this region exhibits strong seasonal variability, with the worst situation occurring in summer. This seasonal variability might be coupled with the timing of riverine nutrient inputs. 4.3.1.1 Seasonal Distributions of Dissolved Inorganic Nitrogen in Seawaters The distributions of DIN were similar in diﬀerent seasons (Fig. 4.20, Wang et al., 2003), which were strongly inﬂuenced by the Changjiang diluted water (CDW). The waters with elevated concentrations of DIN covered the whole coastal area from 35◦ N in the north to 26◦ N in the south, and to the southwest of Cheju Island at about 126◦ E in the east. In contrast, the concentrations of nutrients in the outer continental shelf area of the ECS were very low. In spring, under the combined actions of the higher runoﬀ and the prevailing southerly winds, a part of the CDW ﬂows to the northeast into the Subei coastal area, the other part extends southward along the coast, and some of the southward extending CDW spreads over the outer continental shelf of the ECS in a narrow band (Fig. 4.20a). During the summer of 1998 a severe ﬂood event occurred in the Changjiang River Valley, the largest since a similar catastrophic ﬂood in 1954. The annual discharge of the Changjiang River was as high as 1,240 km3 /yr in 1998, which was 30% greater than the average discharge; the discharge during 65 day’s ﬂooding period (from 25th June to the end of August) was 422 km3 , which was about one-third of the annual discharge; the net discharge of nitrate into the ECS from the Changjiang River

468

4 Biogeochemical Processes of the East China Sea

Fig. 4.20. Horizontal distributions of DIN (μmol/L) in the upper water of the ECS. (a) May, 1998; (b) Aug., 1998; (c) Nov., 1998; (d) Jan., 1999 (Wang et al., 2003) (With permission from Elsevier’s Copyright Clearance Center)

4.3 Nitrogen Variations and Budgets in the East China Sea

469

reached a value of 103×109 mol/yr in 1998, which was twice the annual mean discharge from 1980 to 1990 (46×109 mol/yr). It is shown from Fig. 4.20b that CDW ﬂows to the northeast toward Cheju Island in a tongue-like shape with salinity increasing from 6.0‰ to 26.0‰, and then it diﬀuses southeastward in the area east of 125.5◦ E, and then eastward continually. The surface 27.0‰ isohaline can reach the area east of 127◦ E, and the surface water in the Kuroshio turning area was also inﬂuenced by CDW that reduced the salinity to less than 31.0‰. In the southern ECS, the extension of CDW can reach 28◦ N. The 1 μmol/L isopleth of DIN coincided approximately with the isohaline of 30.0‰ in shape, but its range was still larger than that of the latter. In autumn, the combination of the low discharge and the prevailing northeasterly wind conﬁned the inﬂuence of the Changjiang River water to a narrow band southward along the coast. Therefore, waters with high concentrations of DIN (>5 μmol/L) were in the coastal area of ECS, but conﬁned to a narrow band (Fig. 4.20c). In winter, the surface distribution of temperature was more representative of the water circulation pattern than that of salinity. The saline warm surface waters intruding into the ECS came primarily from the Kuroshio that forms the eastern boundary of the ECS and secondly from the Taiwan Warm Current in the south, and the latter could reach as far north as 31◦ N. The cold water of the Subei coastal current extended southeastward to enter the northern ECS to form an obtuse tongue. The combination of the low discharge and the prevailing northeasterly wind in winter conﬁned the inﬂuence of the Changjiang River water to a narrow band along the coast. The waters with high concentrations of DIN (>5 μmol/L) were found in the coastal area of ECS (Fig. 4.20d). The distribution pattern of nutrients in winter is quite similar to that in autumn. It was still nutrient deﬁcient in the oﬀshore area of the ECS, but the area in winter was larger than that in the other three seasons. The approximately linear relationship between DIN and salinity in the surface water in the continental shelf of the ECS in winter (south of 33.5◦ N) indicates a two-end-member mixing between the low salinity but high DIN coastal water and the high salinity but low DIN Kuroshio water. The approximately linear relationship between DIN and salinity in the surface water with a salinity of less than 31.0‰ in summer indicates that DIN was conservative during the mixing of the Changjiang River water with the seawater. However, points on the diagrams of DIN vs. salinity show considerable scatter in spring and autumn, which might imply that DIN was removed and/or regenerated as the Changjiang River plume travels from the river mouth to the Yellow Sea (YS) and the continental shelf area of the ECS. 4.3.1.2 Concentrations of NO3 -N, NO2 -N, and NH4 -N in the Changjiang River Estuary An observation was conducted in the Changjiang River Estuary from May 19 to 26, 2003 (Fig. 4.2). In spatial and diurnal variations, DIN was examined. Fig. 4.21 showed the distribution of NO3 -N, NO2 -N along transect A. The dis-

470

4 Biogeochemical Processes of the East China Sea

(a)

(b)

(c)

Fig. 4.21. Concentration isolines of NO3 -N (a), NO2 -N (b), and NH4 -N (c) along transect A (μmol/L)

tribution of NO2 -N was similar to that of NH4 -N, and high values were located in stations 6∼12; their maximal values are in the surface water on station 9 near the Changjiang River mouth with 3.14 and 31.43 μmol/L respectively which account for 2.0% and 20.2% of DIN respectively. Generally, in the downriver direction, the concentrations of NO3 -N, NO2 -N, NH4 -N increased ﬁrst, reaching a maximum at 130.0, 3.14 and 31.43 μmol/L respectively in the area where the freshwater and saline water met, and then decreased. NO3 -N concentrations were above 100 μmol/L in the freshwater sector and decreased quickly as salinity increased; the minimum, about 2 μmol/L, was in the eastern part of the study area (Fig. 4.21). In a south-north direction, distribution of nutrient showed overall the same trend, that diﬀerences between the surface and bottom layers were more apparent in the northern part than in the southern part (Fig. 4.22). The data obtained from anchor stations indicated that for NO3 -N, NO2 -N, the concentration could vary greatly within 4 h (Fig.

4.3 Nitrogen Variations and Budgets in the East China Sea

471

4.23 and Fig. 4.24). In the mixed water sector, the shallow depth made water mixing easy; therefore, the mean nutrient concentrations showed no signiﬁcant diﬀerences among the studied layers. In the seawater sector, the average concentration of NO3 -N and NO2 -N decreased downward (Table 4.10). 2 m above bottom 70

0.6

60

0.5

NO2-N (mmol/L

NO3-N (mmol/L

0m

50 40 30 20

0.4 0.3 0.2 0.1

10 21 22

0.0 20 21 22 23 24 25 26 27 28 29 30 Station

23 24 25 26 27 28 29 Station

2 m above bottom 0.5

60

0.4

50 40

0.3 0.2

30

0.1

20

0.0

Sampling time

0m

May 22 9:50 13:50 17:50 21:50 May 23 1:30 6:00 10:00 14:00 18:00 22:00 May 24 2:00 6:00 10:00

NO2-N (mmol/L

70

May 22 9:50 13:50 17:50 21:50 May 23 1:30 6:00 10:00 14:00 18:00 22:00 May 24 2:00 6:00 10:00

NO3-N (mmol/L

Fig. 4.22. Spatial distributions of NO3 -N and NO2 -N concentration along transect B

Sampling time

Fig. 4.23. The variation trends of NO3 -N and NO2 -N concentrations vs. time for each layer at station 13 (Time: month, day, hour, minute)

The ratios of NO3 -N, NO2 -N, and NH4 -N (Fig. 4.25, Chai, 2006) show that nitrate, which originated from fertilizers extensively used in the Changjiang River basin, was the dominant form of DIN in the Changjiang River Estuary and the adjacent ECS throughout the year. With the exception of winter,

NO 3-N ( m mol/L)

4 Biogeochemical Processes of the East China Sea

NO 2-N ( m mol/L)

472

Fig. 4.24. The variation trends of NO3 -N and NO2 -N concentrations vs. time for each layer at station 20. From top to bottom: 0, 5, 10, and 20 m from the surface; 2 m above the bottom Table 4.10. The variation range, mean value, and standard deviation (SD) of NO3 -N and NO2 -N concentrations for each layer at stations 13 and 20 Station 13 20

Layer 0 2 m above the bottom 0m 5m 10 m 20 m 2 m above the bottom

NO3 -N (μmol/L) Range Average 24.93∼62.14 47.94 21.93∼58.43 42.63 4.50∼19.79 13.36 8.00∼20.14 14.41 2.00∼23.29 15.38 1.36∼20.29 12.40 6.21∼14.14 10.46

SD 11.12 13.31 4.38 4.23 5.44 6.59 1.88

NO2 -N (μmol/L) Range Average 0.21∼0.43 0.30 0.07∼0.43 0.21 0.14∼0.57 0.43 0.21∼0.57 0.41 0.07∼0.57 0.39 0.14∼0.36 0.29 0.14∼0.29 0.19

SD 0.08 0.13 0.12 0.09 0.12 0.08 0.05

4.3 Nitrogen Variations and Budgets in the East China Sea

473

nitrate accounted for more than 90% of DIN inshore and more than 80% oﬀshore. Inﬂuenced by domestic activities, the mean ammonia concentration was the highest in zone 1 in winter (32 μmol/L) and accounted for 25% of DIN. One reason may be that the delivery of nitrate from farmland was lower in the dry season. Nitrite concentrations were low (0.2∼1.5 μmol/L) throughout the year and thus represented a very small contribution to the DIN.

Date

Fig. 4.25. The ratios of NO3 -N, NO2 -N, and NH4 -N in the Changjiang River Estuary and the adjacent ECS. Zone 1: upper estuary; Zone 2: lower estuary; Zone 3: adjacent marine area (Chai, 2006) (With permission from Yu ZM)

4.3.1.3 Long Term Change in Nitrogen in the East China Sea Fig. 4.26 shows the long-term trend for nitrate and soluble reactive phosphorus (SRP) (Chai, 2006). “Inshore” in Fig. 4.26 mainly refers to the upper estuary (the turbid zone with salinity <3‰ and its nearby area, and “oﬀshore” represents the area east of the estuary mouth. Concentrations of nitrogen increased both inshore and oﬀshore from the 1960s to 2004. Nitrate concentrations inshore increased from 11 μmol/L to about 97 μmol/L, nearly 9-fold since the early 1960s. Nitrate concentrations inshore demonstrated a fast ascending trend from the 1960s to 1980s, which plateaued by the 1990s and, since then, the nitrate concentration has increased slowly. Nitrate concentrations oﬀshore also exhibited an increasing trend; in the summer of 1963 the concentration was just above the detection limit, while it was more than 30 μmol/L in 2003. The increasing ﬂux of nitrate to the estuary can be attributed to the increase in human population density and fertilizers. The quantity of nitrogen fertilizers applied increased more rapidly from the 1960s to 1980s, when nitrate exhibited a fast ascending trend, indicating that fertilizers are a key cause of nutrient enrichment in the Changjiang River Estuary. Previous studies show that nitrate and silicate were conservative in the Changjiang River Estuary. The approximately linear relationship between

4 Biogeochemical Processes of the East China Sea 140

NO3-N (mmol/L)

120 100

Inshore Offshore Range Regression of inshore Regression of offshore

2.0

(a)

80 60 40

SRP (mmol/L)

474

1.5

2.61 mmol/L Inshore Offshore Range Regression of inshore Regression of offshore

(b)

1.0 0.5

20 0

1960 19651970 19751980 19851990 1995 20002005

Year

0

1955 196019651970197519801985199019952000 2005

Year

Fig. 4.26. Mean concentrations and range of (a) nitrate, (b) SRP inshore and oﬀshore and the associated regression lines from the 1960s to 2004 (Chai, 2006) (With permission from Yu ZM)

DIN and salinity in the surface water in the continental shelf of the ECS in winter (south of 33.5◦ N, Fig. 4.27, Wang et al., 2003) indicates a twoend-member mixing between the low salinity, but high DIN coastal water, and the high salinity but low DIN Kuroshio water. The approximately linear relationship between DIN and salinity in the surface water with a salinity of less than 31.0‰ in summer indicates that DIN was conservative during the mixing of the Changjiang River water with the seawater. However, points on the diagrams of DIN vs. salinity show considerable scatter in spring and autumn, which might imply that DIN was removed and/or regenerated as

Fig. 4.27. Relationship between DIN and salinity in the surface water over the continental shelf of ECS (south of 33.5◦ N) in January 1999 (a) and in the surface water where the salinity was less than 31.0‰ in August 1998 (b) (Wang et al., 2003) (With permission from Elsevier’s Copyright Clearance Center)

4.3 Nitrogen Variations and Budgets in the East China Sea

475

the Changjiang River plume travels from the river mouth to the YS and the continental shelf area of ECS. 4.3.2 Nitrogen Distribution and Its Inﬂuencing Factors in the Sediment Sediment, as a seawater nutrient buﬀer, can continuously receive and release biogenic elements to balance the water chemically or physically. The nitrogen in sediments includes two kinds: one is released into water to take part in the cycling, and the other is buried in sediments. In addition, the burial part can also release when environment changed sharply. So the burial part is a “potential source” of nitrogen for water. 4.3.2.1 Nitrogen Concentration in the Sediment The distribution of TN in surﬁcial sediments on the ECS shelf from the Changjiang River Estuary to the SOT (Fig. 4.28) shows that most of the shelf area has less than 0.06% TN. A strong seaward gradient appears on the inner shelf with higher TN values (>0.10%) near the coast, and the maximum value (=0.17%) occurs near the Changjiang River Estuary. Outside the inner shelf the TN percentage decreases seaward in general and drops to less than 0.03% near the shelf break. Two areas with distinctively high TN stand out against the generally low TOC background away from the coastal zone. One is located at the northeastern corner (around 32◦ N, 126◦ E) of the contour map and the other is in the SOT. The areas with TN >0.1% match well with the mud patches. The coastal belt with elevated TN content extends southward from the Changjiang River N 33 32 31 30 29 28 27 26 25 24 120 121 122 123 124 125 126 127 128 129 130 131 E

Fig. 4.28. Distribution of the nitrogen content (TN%) in sediments in the ECS shelf

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4 Biogeochemical Processes of the East China Sea

Estuary. It is noteworthy that this belt veers oﬀshore and extends toward the SOT as it approaches the northern end of the Taiwan Strait. Separate investigation of IN, ON, and TN in the Changjiang River Estuary sediments showed that ON occupied the majority of TN (about 64%) and that IN occupied 36% in the Changjiang River Estuary. The contents of IN, ON, and TN were about 0.02∼0.33, 0.04∼0.57, and 0.06∼0.67 mg/g, with mean values of 0.15, 0.26, and 0.41 mg/g, respectively (Li et al., 2006b). The spatial distribution of IN, ON, and TN was similar in the Changjiang River Estuary sediments. But the change in IN was smaller than those of ON and TN. Fig. 4.29 (Li et al., 2006b) shows the distribution of IN, ON, and TN in sections A and B (Fig. 4.2). Their distribution can be divided into three regions (stations 1∼4, 4∼11 and 11∼20) in section A according to their regular variation, while in section B the change in IN, ON, and TN was more stable.

Fig. 4.29. Distribution of nitrogen in the Changjiang River Estuary sediments (Li et al., 2006b) (With permission from Springer)

4.3.2.2 Eﬀect of Grain Size and TOC on TN in Sediments The mean grain size of seabed sediments, ranging from 4φ to 7.5φ (Fig. 4.30, Gao et al., 2007), increases from land to sea in the ECS, and the isolines project towards the northeast and east outside of the Changjiang River Estuary and Hangzhou Bay, respectively. To the east of 123◦ E, mean grain size isolines appear in circular ringed distribution, and the highest value occurs at 123.5◦ E and 30.5◦ N. Sediment components are mainly composed of sand and silt (Fig. 4.30). To the east of 123◦ E, the sandy component increases from land to sea, and the silty component increases from sea to land. Subsequently, isolines of silt and sand project towards the Changjiang River Estuary, running parallel in a northwest to southeast direction. To the east of 123◦ E, sand and silt both show circular distributions, with their centers situated at 123.5◦ E and 30.5◦ N, respectively.

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477

Fig. 4.30. Distributions of (a) mean grain size (ø), (b) sand (%), (c) silt (%), and (d) seabed sediment types in the Changjiang River Estuary and its adjacent sea area (Gao et al., 2007) (With permission from Gao JH)

The TN shows a signiﬁcant linear correlation with TOC (Fig. 4.31, Gao et al., 2007), and sediments with high TOC content have C/N ratios higher than the Redﬁeld ratio of 6.7, while those with lower TOC contents have C/N ratios approaching the Redﬁeld ratio. Similarly, from 123◦ E to 124◦ E, most of the C/N ratios are situated around the Redﬁeld ratio (Fig. 4.32). Whereas the ﬁtting straight line between TOC and TN to the north of 31◦ N nearly parallels that to the south of 31◦ N, and C/N ratios in the north are greater than those in the south.

478

4 Biogeochemical Processes of the East China Sea y=7.0769x+0.0712 2 R =0.90

TOC (%)

0.8

y=5.8739x+0.0644 R2=0.90

0.6 y=7.1102x+0.0141 2 0.4 R =0.76

West of 122.5 E 122.5 E 123 E 123 E 124 E

0.2 0.0 0.00

0.02

0.04

0.06 0.08 TN (%)

Redfield ratio

0.10

0.12

TOC (%)

0.8 y=6.9256x+0.0317

0.6 R2=0.91

Redfield ratio West of 122.5 E 122.5 E 123 E 123 E 124 E

0.4 0.2 0.0 0.00

0.02

0.04

0.06 0.08 TN (%)

0.10

0.12

TOC (%)

Fig. 4.31. The relationship between contents of TOC (%) and TN (%) in sediments (Gao et al., 2007) (With permission from Gao JH)

0.8

y=7.3973x 0.0231

0.6

y=7.4892x 0.0119

0.4 Redfield ratio (6.7) 0.2

0.0 0.00

South of 31 N North of 31 N 0.02

0.04

0.06 TN (%)

0.08

0.10

Fig. 4.32. The relationship between contents of TOC (%) and TN (%) in sediments south of 31◦ N and those north of 31◦ N (Gao et al., 2007) (With permission from Gao JH)

4.3.3 Fluxes of Nitrogen 4.3.3.1 Nutrient Fluxes from the Changjiang into the Sea The inﬂow from the Changjiang River is the major source of nutrients to the Changjiang River Estuary and coastal waters. The nutrient concentrations as well as nutrient loads of the Changjiang River, notably DIN, increased exponentially and by a factor of seven from the 1960s to the end of the 1990s, mainly due to the increasing amount of fertilizer application and eﬄuents

4.3 Nitrogen Variations and Budgets in the East China Sea

479

from cities in the river basin. Liu et al. (1997) found that the concentration of DIN increased by a factor of seven from 15 μmol/L in 1968 to 118 μmol/L in 1997. Compared to the result of Liu et a1. (1997), although there were some diﬀerences in the nutrient ﬂuxes of the Changjiang River estimated by diﬀerent investigators (Table 4.11, Wang and Tu, 2005) due to the diﬀerences in sampling time, place, methods of analysis as well as the annual and seasonal variations of the nutrients ﬂuxes, it is widely believed that the nutrient ﬂuxes from the Changjiang River into the sea have been increasing signiﬁcantly since the 1960s. Table 4.11. Nutrient ﬂuxes from the Changjiang River (t/yr) (Wang and Tu, 2005) (With permission from Wang BD) Reference DIN (×105 ) NO3 -N (×105 ) PO4 -P (×105 ) SiO3 -Si (×106 ) Liu et al. (2002) 3.15 5.40 5.69 3.17 Shen (1991) 7.84 6.91 15.10 2.22 Edmond et al. (1985) – 8.40 – – Zhang (1996) 6.25 4.27 16.40 2.47

4.3.3.2 Atmosphere Input The eﬀect of atmospheric inputs of nutrients on biological cycles is particularly important for oligotrophic oceanic provinces, and episodic deposition may even induce algal blooms. The ECS adjacent to the East Asia mainland is inﬂuenced by a monsoon climate and strong emissions of natural and anthropogenic compounds into the atmosphere. Soil-derived dust and anthropogenic compounds are transported over the ECS via atmospheric circulation, while aerosols from the ocean also play an important role in the chemical characteristics of precipitation. − (1) Dry deposition of particulate NH+ 4 and NO3 Nakamura et al. (2005) estimated the dry deposition ﬂux of particulate NH+ 4 + − and NO− 3 in order to assess the eﬀect of atmospheric NH4 and NO3 supply on the biological productivity in the surface ocean. A summary of the ambient concentrations, deposition velocities, and mean dry deposition ﬂuxes of aerosols is given in Table 4.12. The average contributions of coarse particle aerosol dry depositions were 52% and 99% of the total depositions of NH+ 4 + and NO− 3 , respectively. Although coarse particle NH4 represented a small fraction (less than 11%) of the total in aerosols, its percentage deposition ﬂux was comparable to that of the ﬁne particle. The average total atmospheric − concentration of NH+ 4 was higher than that of NO3 , but the dry deposition − + ﬂux of NO3 was higher than that of NH4 . The deposition ﬂux over the ECS + was higher by 2.8 times for NO− 3 and 8.0 times for NH4 than those over the western Paciﬁc Ocean.

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Table 4.12. Mean concentrations and depositional velocities of aerosols considered and dry deposition rates of combined nitrogen over the ECS (Nakamura et al., 2005) (With permission from Elsevier’s Copyright Clearance Center)

Ambient concentration (μg N/m3 ) Dry deposition velocity (cm/s) Dry deposition ﬂux (μg N/(m2 ·d))

Paciﬁc Ocean (n=7) Percentage NO− NH+ 3 4 of NO− 3 (%) 0.17 0.30 36

East China Sea (n=20) Percentage NO− NH+ 3 4 of NO− 3 (%) 0.48 2.3 17

1.7

0.19

–

1.7

0.22

–

250

66

83

720

450

64

As aerosols in the atmosphere originate from multiple sources, the composition of nutrients changes considerably over the four seasons. Nitrate, NO− 2, and NH+ 4 are deﬁned as secondary aerosol-associated species and are not associated with primary aerosols. The combustion of fossil fuels is a signiﬁcant source of NOx , whereas NH+ 4 may originate from anthropogenic emissions such as animal waste and the application of chemical fertilizers. Fig. 4.33 shows seasonal variations in nutrient concentrations within the aerosols (mol/kg, dry weight) (Zhang, 2004). At both sites, aerosols recorded − high values of NO− 3 +NO2 in winter due to the combustion of fossil fuels and a predominant northwest wind from the mainland, whereas low values were recorded in summer because of washing-out associated with frequent rain events. In spring, soil dust is the major source of aerosols because of frequent dust storms at this time of year over the East Asian mainland and the move-

1.2

Qianliyan Shengsi

0.8 0.4 0

3.0

Nitrate+Nitrite

Spring Summer Autumn Winter

Concentration (mol/kg)

Concentration (mol/kg)

1.6

Ammonium Qianliyan Shengsi

2.0

1.0

0

Spring Summer Autumn Winter

Fig. 4.33. Seasonal variations in nutrients within aerosols (mol/kg) based on seasonal divisions of spring (March∼May), summer (June∼August), autumn (September∼November), and winter (December∼February) at Qianliyan Island and Shengsi Island. The error bars represent analytical errors (Zhang, 2004) (With permission from Zhang GS)

4.3 Nitrogen Variations and Budgets in the East China Sea

481

ment of air masses toward the mid-latitude Paciﬁc Ocean; consequently, high concentrations of aerosols are recorded in spring. Similar trends have been observed for atmospheric samples collected from the southeast Mediterranean, where the concentration of nutrients in aerosols dropped to 1/160∼1/64 of normal levels during dust storm events. In summer, high temperatures and the related decomposition of biomass leads to large emissions of NH3 ; consequently, NH+ 4 in aerosols also shows high values for this period. − (2) Wet deposition of NH+ 4 and NO3 Zhang (2004) collected rainwater samples at Qianliyan Island, South Yellow Sea, from May 2000 to August 2002 and at Shengsi Archipelago in the ECS from May 2000 to May 2002, and studied the wet deposition of NH+ 4 and NO− (Fig. 4.34). In general, the monthly average concentrations of NO− 3 3 at Qianliyan Island during winter are at least 3∼5 times higher than those measured during summer. This trend is related to the frequency of rainfall, the changing sources of air masses, and the variable concentration of aerosols,

160

Concentration (mmol/l)

Rainfall (mm)

120

200

Rainfall Qianliyan Shengsi

80

40

400

200

0

50

5.0

Concentration (mmol/l)

Concentration (mmol/l)

600

Qianliyan Shengsi

100

0

0 Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. 800 Qianliyan Shengsi

150

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. Qianliyan Shengsi

4.0 3.0 2.0 1.0

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

0

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

Fig. 4.34. Monthly volume-weighted average concentrations of nutrient species in rainwater samples from Qianliyan Island and Shengsi Island (Zhang, 2004) (With permission form Zhang GS)

482

4 Biogeochemical Processes of the East China Sea

and results from the combined scavenging eﬀect in clouds and washing-out + eﬀect below the cloud base. The variable sources of NO− 2 and NH4 mean that concentrations for these nutrients do not show clear seasonal signatures. At Qianliyan Island, 50%∼70% of annual rainfall and 40%∼55% of rain events occur during summer. The dilution and washing-out eﬀects mean that low concentrations of nutrients are exclusively observed during summer. In contrast, only 2%∼8% of annual rainfall and about 10% of rain events occurred during winter during the sampling years; consequently, high concentrations of nutrients are recorded during winter. At Shengsi Island, the relatively even seasonal distribution of rainfall means that nearly 40% of the annual deposition occurs in summer. As discussed above, high concentrations of nutrients in rainwater samples are recorded during winter and spring at Shengsi Island and relatively low values recorded during summer and autumn. 4.3.3.3 Benthic Flux of Dissolved Nutrients at the Sediment Water Interface Nutrients in sediment are an important source or sink for overlying seawater. The ﬂux across the sediment-water interface is one of the important parameters for nutrient balance. The benthic ﬂuxes of dissolved nutrients at the sediment water interface in the ECS were listed in Table 4.13 (Shi et al., 2004). The benthic ﬂuxes of nutrients at the sediment-water interface were acquired in laboratory incubation experiments. The average exchange rates of SiO3 -Si, PO4 -P, NH4 -N, NO2 -N, and NO3 -N at the sediment-water interface were 4.12, −0.01, 0.48, −0.02 and −0.07 mmol/(m2 ·d), respectively. By considering the area percentage of the diﬀerent pattern out of the total area of the ECS, the ﬂuxes of SiO3 -Si, PO4 -P and DIN from the sediments to seawater in the ECS were estimated at 3.18×1012 , −7.37×109 , and 2.95×1011 mmol/d, respectively. The exchange ﬂuxes can provide 55% of silicate and 5.1% of DIN required by phytoplankton in the ECS. Table 4.13. Diﬀerent ﬂuxes of nutrients (mmol/d) at the sediment-water interface between two cruises in the ECS (Shi et al., 2004) (With permission from Shi F) Cruises 2002-04-05 2002-08-09

SiO3 -Si (×1012 ) 3.15 3.20

PO4 -P (×109 ) 5.47 –2.02

NH4 -N (×1011 ) 2.71 4.71

NO2 -N (×1010 ) –1.58 –2.09

NO3 -N (×1011 ) –1.15 0

DIN (×1011 ) 1.40 4.50

4.4 Phosphorus Biogeochemistry in the East China Sea Nutrient cycling and food web dynamics in oceans are recognized as central problems in chemical and biological oceanography. Compared to studies

4.4 Phosphorus Biogeochemistry in the East China Sea

483

of carbon and nitrogen, the distribution and dynamics of phosphorus in the world’s ocean are less characterized. Recently, P has been observed to limit phytoplankton production in both coastal waters and the open ocean. Even when phosphorus is not a limiting factor, increasing anthropogenic phosphorus inputs in coastal ecosystems may substantially change the dynamics within these fragile communities. In the heavily populated areas of the world’s coastlines, the anthropogenic inputs of P into the coastal ocean are 10∼100 times greater than those in pre-industrial times. The consequences of such an increase are numerous and include reducing food web diversity, altering phytoplankton compositions, and an increase in the intensity and frequency of red tides. These changes could also greatly aﬀect phytoplankton growth and zooplankton grazing, thereby potentially altering the extent of particulate matter production in the coastal ocean. It is clear that elucidating the cycling of all nutrients in marine systems is extremely important if one is to understand current controls on primary production and particulate carbon export in the world’s oceans. In coastal areas, the supply of phosphorus is mainly dominated by riverine and atmospheric inputs. The ECS receives a rich supply of phosphorus from the discharge of the Changjiang River, 1.4×104 t P/yr (Shen, 1993) and from the upwelling of the Kuroshio intermediate water (KIW), 4.43×105 mol P/yr (Chen et al., 1995). However, Wang et al. (2003) investigated the nutrient distribution in the ECS and found high N/P ratios (>30) in the Changjiang River Estuary and its adjacent areas of the ECS. These authors suggested that phosphorus is the limiting factor for phytoplankton production. In contrast, some study indicated that primary production in the ECS is limited by N deﬁciency in summer and by sunlight in winter. The incompatible results indicated that our knowledge about phosphorus in the ECS is insuﬃcient. The aim of this section is to synthesize the results of phosphorus in recent years and enhance our understanding of it. 4.4.1 Distribution of Phosphorus in the Seawater The pattern of PO3− 4 distribution in diﬀerent seasons (Fig. 4.35, Wang et al., 2003) is similar in the ECS, where high values are found in the Changjiang River Estuary. However, P concentration decreases seaward in general. Phosphate concentrations in the ECS have increased about 3- to 4-fold since the 1960s. The mean phosphate concentration inshore in the middle of the 1980s was around 0.8 μmol/L, about a 2-fold increase since the 1960s. The phosphate concentration inshore increased slightly after the 1990s and reached up to 0.95 μmol/L in 2004. The phosphate concentration oﬀshore also increased from 0.16 μmol/L in 1963 to 0.8 μmol/L in 2004 (Chai, 2006). The concentrations of dissolved inorganic phosphorus (DIP) and dissolved organic phosphorus (DOP) in the ECS ranged from 0.05 to 3.01 and from 0.01 to 0.54 μmol/L, respectively. The concentrations of DIP and DOP in the inner shelf were signiﬁcantly higher than those in the middle and the outer shelves. The concentrations of DIP were signiﬁcantly higher than those

484

4 Biogeochemical Processes of the East China Sea

N 38

N 38

36

36

34

34

32

32

30

30

28

28 (a)

26 N 120 38

26 122

124

126

128

E

N 38

36

36

34

34

32

32

30

30

28

28

26

(c) 120

(b) 120

124

126

128

E

124

126

128

E

122

124

126

128

E

(d)

26 122

122

120

Fig. 4.35. Distribution of surface water phosphorus concentration in the ECS. (a) May, 1998; (b) Aug., 1998; (c) Nov., 1997; (d) Jan., 1999 (Wang et al., 2003) (With permission from Elsevier’s Copyright Clearance Center)

4.4 Phosphorus Biogeochemistry in the East China Sea

485

of DOP and accounted for more than 70% of the total dissolved phosphorus (TDP) at stations in the inner and middle shelf waters. In contrast, DOP concentrations generally exceeded DIP concentrations in the surface layer (<100 m) at stations in the outer shelf water. Moreover, the concentration of DIP in the outer shelf water gradually increased with depth and became the dominant species, representing more than 90% of the TDP concentration in deeper water (>1,000 m) (Fang, 2004). The spatial and diurnal variations of phosphorus in the Changjiang River Estuary were investigated from May 19 to 26, 2003. The results showed that PO4 -P distribution is similar to that of NO3 -N and NO2 -N along transect A (Fig. 4.36). PO4 -P concentration is stable with a value of 1.3 μmol/L before station 6, then increased clearly until station 8, with a maximal value of 2.05 μmol/L, and then decreased in a downriver direction. On the whole, PO4 -P concentration in the region between stations 7∼11 is higher than that in the rest of the region. The reasons causing the distribution might be the input from rivers and drain outlets along coast, desorption of suspended particles and decomposition of phosphorus organic compounds in seawater and sediment. 0 5 10

Depth (m)

15 20 25 30 35 40 45 50 1 2 3 4 5 6 7 8 9 10 11 1213 14 15 16 17 18 19 20 Station

Fig. 4.36. The concentration isolines of PO4 -P along transect A (μmol/L)

The diﬀerence in PO4 -P concentration between 0 m and 2 m above the bottom layers in the northern stations is greater than that in the southern stations along transect B (Fig. 4.37). Except for stations 24 and 25, PO4 -P concentration in the 0 m layer was higher than that in the 2 m above the bottom layer. The data obtained from anchor station 13 indicated that the diﬀerence in PO4 -P concentration between 0 m and 2 m above the bottom layers in

486

4 Biogeochemical Processes of the East China Sea 1.35

0m 2 m above the bottom

PO4 P (mmol/L)

1.20 1.05 0.90 0.75 0.60 0.45 21

22

23

24

25 26 Station

27

28

29

Fig. 4.37. The concentration isolines of PO4 -P along transect B (μmol/L)

the ﬁrst 24 h is greater than that in the second 24 h (Fig. 4.38). During the investigation period, PO4 -P concentration in the 0 m layer was higher than that in the layer of 2 m above the bottom. The data obtained from anchor station 20 indicated that the PO4 -P concentration increased from the surface to the bottom and the concentration in the 0 m layer was 3.59 times that in the layer of 2 m above the bottom (Fig. 4.39). 1.5 1.4

0m 2 m above the bottom

PO4-P (mmol/L

1.3 1.2 1.1 1.0

0.8

May 22 9:50 13:50 17:50 21:50 May 23 1:30 6:00 10:00 14:00 18:00 22:00 May 24 2:00 6:00 10:00

0.9

Sampling time

Fig. 4.38. The variation trends of PO4 -P concentrations vs. time for each layer at station 13

The N/P ratio has been widely used to determine whether the growth of phytoplankton in seawater is N- or P-limited. Fig. 4.40 (Wang et al., 2003) shows that the distribution pattern of N/P ratios in the ECS are quite similar to that of nitrate; that is, a high value of the N/P ratio was found where the

0.4 0.3 0.2 0.1 0.0 0.4 0.3 0.2 0.1 0.0 0.8 0.6 0.4 0.2 0.0 0.8 0.6 0.4 0.2 0.0 0.9 0.8 0.7 0.6 0.5 0.4

487

May 19 16:00 20:00 May 20 0:00 4:00 8:00 12:00 16:00 20:00 May 21 0:00 4:00 8:00 12:00 16:00

PO4-P (mmol/L)

4.4 Phosphorus Biogeochemistry in the East China Sea

Sampling time

Fig. 4.39. The variation trends of PO4 -P concentrations vs. time for each layer at station 20 (from top to bottom: 0, 5, 10, 20 m from the surface, and 2 m above the bottom)

nitrate concentration was high. Hu et al. (1989) put forward a criterion of nutrient limitation for the phytoplankton in the Changjiang River Estuary based on an in-situ incubation experiment of phytoplankton; that is, the growth of phytoplankton would be P-limited when the N/P ratio is larger than 30 and N-limited when the N/P ratio is less than 8. If this criterion is applicable to the inﬂuenced areas of the CDW in the YS and ECS, it would be P-limited in the area to the west of 126◦ E from 33◦ N in the north to 30◦ N in the south and the coastal area of the ECS in summer, and the two areas are the largest P-limited in one year. In spring, the P-limited area covered the coastal area west of 123◦ E from 34◦ N in the north to 29◦ N in the south, and in autumn, the P-limited area was conﬁned to a narrow band in the coastal area of the ECS, but there was no P-limited area in winter.

488

4 Biogeochemical Processes of the East China Sea

N 38

N 38

36

36

34

34

32

32

30

30

28

28 May, 1998

26 N 38 36

36

34

34

32

32

30

30

28

28 Nov., 1997

26 120

122

124

126

Aug., 1998

26 N 38

Jan., 1999

26 128

E

120

122

124

126

128

E

Fig. 4.40. Horizontal distributions of N/P ratios in the upper water of the ECS (Wang et al., 2003) (With permission from Elsevier’s Copyright Clearance Center)

4.4.2 Distribution of Phosphorus in the Sediments Phosphorus in the marine environment is mainly present in the solid phase, accounting for 90%∼95% of the total P pool. Phosphorus in the sediment consisted of organic and inorganic forms. However, numerous studies have indicated that signiﬁcant diagenetic reorganization of P occurs during burial, especially in the continental margin sediments. For better understanding the diagenesis of P in marine sediments, it is usual to separate phosphorus in marine sediment into reactive-P (potentially bio-available) and refractory-P in sediments by a selective sequential extraction method (SEDEX).

4.4 Phosphorus Biogeochemistry in the East China Sea

489

Phosphorus in the surface sediment in the middle shelf of ECS was sequentially extracted into diﬀerent forms: loosely sorbed P and iron-bound P (PCDB ), inorganic P associated with francolite (carbonate ﬂuorapatite, CFA), biogenic hydroxyapatite, smecite, and CaCO3 (PCFA ), detrital P (PDetrital ), organic P (POrganic ) and total P (Fang et al., 2007). The concentration of different forms of sedimentary P in the surface sediments are shown in Fig. 4.41 (Fang et al., 2007). The total concentration of P in the surface sediments in the middle shelf of the ECS ranged from 13.5 to 22.3 μmol/g. Among the four forms of sedimentary phosphorus, the P concentration ranges were 0.89∼1.87 μmol/g (average 1.45 μmol/g) for PCDB , 0.53∼1.77 μmol/g (average 1.0 μmol/g) for PCFA , 9.34∼17.21 μmol/g (average 12.28 μmol/g) for PDetrital , and 1.09∼5.88 μmol/g (average 2.75 μmol/g) for POrganic . The average percentage of each fraction of P followed the sequence: PDetrital (70%)>POrganic (15.5%)>PCDB (8.4%)>PCFA (5.8%). Basically, the inorganic P was the major form, accounting for 72%∼93% of the total P pool, and the contribution of POrganic was relatively small at the study stations. The concentration of PCDB in the surface sediments seemed to increase eastward. The minimum contour of PCDB appeared near 123.5◦ E and 30◦ N, which is outside of Hangzhou Bay. In contrast, the maximum contour of PCFA was located outside of Hangzhou Bay, and the concentration of PCFA decreased eastward. The distribution of PDetrital diﬀered from those of PCDB and PCFA and there were two higher N 33 32 31 30 29 28

120

121 122

123 N 33

124

125

E

120

121 122

123 N 33

32

32

31

31

30

30

29 28 121 122

123

124

125

E

120

121 122

123

124

125

E

29

3

120

124

125

E

28 120

121 122

123

124

125

E

Fig. 4.41. The distributions of diﬀerent forms of phosphorus in the surface sediments in the middle shelf of the ECS (μmol/g). (a) PCDB ; (b) PDFA ; (c) PPetrital ; (d) POrganic ; (e) Total P (Fang et al., 2007) (With permission from Elsevier’s Copyright Clearance Center)

490

4 Biogeochemical Processes of the East China Sea

values of PDetrital in the ECS. One was close to 123.5◦ E and 32◦ N, approximately 200 km east of the mouth of the Changjiang River Estuary, and the other was located 125◦ E and 29◦ N, in the south-western part of Hangzhou Bay. The distribution pattern of total P was similar to that of PDetrital . The maximum contour of POrganic also appeared about 200 km east of the mouth of the Changjiang River Estuary. Inorganic phosphorus (IP), organic phosphorus (OP), and total phosphorus (TP) in the Changjiang River Estuary sediment were also studied separately. Similar to that in the middle shelf of ECS, IP occupied the majority of TP (about 77%) while OP occupied 23% in the Changjiang River Estuary. The content of IP, OP, and TP was about 0.25∼0.5, 0.02∼0.25, and 0.29∼0.61 mg/g, respectively, with a mean value of 0.36, 0.11, and 0.49 mg/g, respectively. Fig. 4.42 (Li et al., 2006b) shows the distribution of P in sections A and B in the Changjiang River Estuary sediment, and that the change in P was smaller in all regions except the area of stations 1 to 5 in section A.

Fig. 4.42. P distributions in the Changjiang River Estuary sediments (Li et al., 2006b) (With permission from Springer)

4.4.3 Phosphorus Burial Fluxes Mass accumulation rate (MAR) in the ECS varied from >2 to 0.05 g/(cm2 ·yr). The maximum MAR appeared in the mouth of the Changjiang River, and the value generally decreased southward along the inner shelf and eastward oﬀshore. Based on this valuable published data, Fang et al. (2007) calculated the phosphorus burial ﬂux in the ECS. To facilitate the calculation, the calculated area is divided into ﬁve boxes: estuary (box I), inner shelf (box II), middle shelf (boxes III and IV), and outer shelf (box V) (Fig. 4.43), according to the value of MAR in each box observed by Huh and Su (1999) and to the phosphorus content in surface sediments found by Fang et al. (2007). The total P concentration in the sediments in the ECS area remained within a narrow range, 14∼23 μmol/g. Thus, the error in calculation caused by the variation in the total P concentration in sediments in diﬀerent boxes was probably less than 20%. Table 4.14 (Fang et al., 2007) shows the values of the parameters used in the calculations and the calculated results. The

4.4 Phosphorus Biogeochemistry in the East China Sea

491

Fig. 4.43. The area within the ECS was divided into ﬁve boxes for burial ﬂux calculations. The ﬁve boxes representing diﬀerent MAR area and phosphorus content are: (I) estuary; (II) inner shelf; (III) middle shelf; (IV) middle shelf; (V) outer shelf (Fang et al., 2007) (With permission from Elsevier’s Copyright Clearance Center) Table 4.14. The area, mass accumulation rate, P total concentration, P accumulation rate, and P burial ﬂux for each box of the ECS (Fang et al., 2007) (With permission from Elsevier’s Copyright Clearance Center) Box I II III IV V Total

Mass accum. Area (km2 ) rate (g/(cm2 ·yr)) 42,400 0.6∼1.0 (0.8) 38,200 0.2∼0.4 (0.3) 57,400 0.2∼0.5 (0.35) 69,700 0.1∼0.4 (0.25) 197,000 0.05∼0.1 (0.075) 404,700

P total conc. (μmol/g) 15.8∼17.2 (16.1) 14.5∼19.8 (16.5) 13.5∼22.2 (17.5) 17.5±3.6 17.5±3.6

P accum. rate (μmol/(cm2 ·yr)) 9.48∼17.2 (12.88) 2.90∼7.92 (4.95) 2.72∼11.13 (6.13) 1.42∼8.41 (4.38) 0.71∼2.12 (1.31)

P burial ﬂux (×109 mol/yr) 4.02∼7.29 (5.46) 1.11∼3.03 (1.89) 1.55∼6.37 (3.52) 0.98∼5.85 (3.05) 1.37∼4.13 (2.59) 9.03∼26.68 (16.50)

P burial ﬂux was found to be in the range of 9.03×109 ∼26.68×109 mol/yr (average 16.5×109 mol/yr) for the calculated area. 4.4.4 Phosphorus Balance The water ﬂuxes of the ECS are chieﬂy inﬂuenced by the Kuroshio surface water (KSW), the Kuroshio tropical water (KTW), and the Taiwan Strait water (TSW) entering into the ECS, and by the shelf surface water (SSW) leaving the ECS. The annual variation in concentration of DIP in these waters is small. If the geochemical cycle of phosphorus in the ECS is assumed to have reached a steady state, then the P budget of the ECS can be calculated using water and phosphorus mass-balance, using data obtained in previous studies.

492

4 Biogeochemical Processes of the East China Sea

The major water masses near the ECS continental shelf break were divided into four groups, namely, the KSW (0∼75 m), the KTW (76∼200 m), the KIW (201∼600 m), and the SSW. The annual ﬂux of each water mass was as follows: the KSW, 22,248×109 m3 /yr; the KTW, 11,827×109 m3 /yr; the KIW, 3,942×109 m3 /yr; the SSW, 50,573×109 m3 /yr (Chen and Wang, 1999). Although the major Kuroshio currents are parallel to the isobaths, the SSW has net transport oﬀshore because of fresh water discharged from rivers, mainly the Changjiang River, and net precipitation, while the KSW, KTW, and KIW have net onshore transport. In addition, there is input from the TSW. The annual water ﬂux ﬂowing through the Taiwan Strait is still uncertain and ranges from 0.2 to 2.31 Sv (1 Sv=1×106 m3 /s). The annual water ﬂux of the Taiwan Strait, 0.2 Sv, calculated by Chen and Wang (1999) is relatively small. Thus, a reasonable value, approximately 1.0 Sv, was adopted in the present study. In addition, Liu et al. (2000) estimated that based on two ﬁeld surveys conducted in August 1994 and in March 1997, the Kuroshio upwelling water intrusion into the southern ECS oﬀ northern Taiwan was about 0.6∼0.8 Sv. The value of 0.7 Sv was substituted here for the water ﬂux of the KIW by Chen and Wang (1999). The water ﬂuxes of SSW, KSW, KTW, and KIW are estimated to have an uncertainty of ±20%, mainly due to uncertainties involved in estimating the relative proportions of the Kuroshio water masses (Chen and Wang, 1999). For matching the water mass balance calculation, the uncertainty of the Changjiang River (R) and TSW ﬂuxes is also considered as ±20% (Fang et al., 2007). Based on the above information, a simple box model (Fig. 4.44, Fang, 2004) was used to calculate the P budget of the ECS according to the water and P mass balance, as shown in the following equations: QR + QTSW + QKSW + QKTW + QKIW = QSSW QR (CR−DIP + CR−DOP + CR−PIP )

(4.1)

+QTSW (CTSW−DIP + CTSW−DOP + CTSW−PIP ) +QKSW (CKSW−DIP + CKSW−DOP + CKSW−PIP ) +QKTW (CKTW−DIP + CKTW−DOP + CKTW−PIP ) +QKIW (CKIW−DIP + CKIW−DOP + CKIW−PIP ) = QSSW (CSSW−DIP + CSSW−DOP + CSSW−PIP ) + SPP

(4.2)

where Q and C are the water ﬂux and diﬀerent fractions of the P concentration. Subscripts R, TSW, KSW, KTW, KIW, and SSW denote the river input from the Changjiang River, the Taiwan Strait water, the Kuroshio surface water, the Kuroshio tropical water, the Kuroshio intermediate water, and the shelf surface water, respectively. SPP represents the P ﬂux deposited in the sediment. The concentrations of DIP, DOP, PIP, and SPM in each water mass were taken as the average concentrations in the samples taken from

4.4 Phosphorus Biogeochemistry in the East China Sea

493

each water mass. The error constraint of each parameter was calculated as one standard deviation of the concentration in all samples of each water mass. QR

QKSW

QSSW

QKTW

SPP

QTSW

QKIW

Sediment

Fig. 4.44. Schematic diagram of the box model used to calculate the phosphorus budget in the ECS (Fang, 2004) (With permission from Elsevier’s Copyright Clearance Center)

The import ﬂux of DOP from the Changjiang River and the inputs of atmospheric P ﬂuxes into the ECS were excluded from the calculation because measurements were not available. Table 4.15 (Fang, 2004) summarizes the parameters used to calculate the P budget of each water mass in the ECS and the calculated results. The high annual ﬂux of these parameters was calculated from the high value of water ﬂux and high concentration in each column, and low annual ﬂux was calculated from the low value of the water ﬂux and low concentration. The average value was calculated from the value without error constraint. The results show that the P input ﬂux from the various water masses into the ESC was (78.8±48.8)×109 mol P/yr. The inputs of the TSW and the KIW were the major sources, and both contributed more than 65% of Table 4.15. Values of parameters used and phosphorus ﬂuxes calculated using the box model for the ECS (Fang, 2004) (With permission from Fang TH) Annual ﬂux (×109 mol/yr)

Concentration Water mass

River water (QR )

DOP PIP SPM Water ﬂuxes DIP (μmol/L) (μmol/L) (μmol/g) (mg/l) (×109 3 m /yr)

0.53± 0.1

PIP

TP

11.2± 2.3

11.8± 2.3

0.57

No data

23.98

504

Kuroshio surface 22,248± water (QKSW ) 4,450

0.138± 0.072

0.212± 0.112

0.586± 0.179

1.23± 8.6± 0.50 5.7

0.02± 0.015

8.6± 5.7

Kuroshio 11,827± tropical 2,365 water (QKTW )

0.343± 0.31

0.145± 0.066

0.340± 0.113

1.77± 6.7± 0.69 5.6

0.009± 0.007

6.7± 5.6

Kuroshio 22,075± intermediate 4,415 water (QKIW )

0.854± 0.682

0.301± 0.094

0.519± 0.125

1.09± 28.9± 0.45 22.2

0.015± 0.011

28.9± 22.2

Taiwan Strait 313,536± water (QTSW ) 6,307

0.450± 0.133

0.181± 0.112

5.56± 3.78

4.41± 21.4± 2.87 11.7

1.3± 1.2

22.8± 13.0

88,614± Shelf surface water (QSSW ) 17,723

0.356± 0.145

0.099± 0.069

1.348± 1.006

2.01± 44.1± 0.63 27

0.35± 0.31

44.5± 27.3

Spp

928.2± 185

TDP

34.3± 21.5

494

4 Biogeochemical Processes of the East China Sea

the total ﬂuxes. The inﬂux from the KSW and KTW, and from the Changjiang River, accounted for 19% and 15%, respectively, of the total ﬂuxes. The ECS is a phosphorus sink, and approximately (34.3±21.5)×109 mol P, 43% of the total P ﬂux, is deposited annually in the sediments. The SSW exports about (44.5±27.3)×109 mol P/yr out of the ECS. The import and export of P in each water mass, except for the Changjiang River, are dominated by the dissolved phase. The ﬂuxes of the particulate phase are very small and can be neglected. In contrast, the annual import of particulate P from the Changjiang River is about (11.2±2.3)×109 mol P, which accounts for over one third of the total P ﬂuxes deposited in the sediment in the ECS, because the particulate P input from the Changjiang River was omitted from the calculation of (Chen and Wang, 1999). 4.4.5 Cycling of Phosphorus It is clear that elucidating the cycling of all nutrients in marine systems is extremely important if one is to understand current controls on primary production and particulate carbon export in the world’s oceans. The intensity of coastal P cycling, in particular, can have a direct impact on a wide range of processes, ranging from nutrient limitation to particulate matter exported from the euphotic zone to underlying sediments. Hence, long-term changes in P cycling will aﬀect the residence time of P over geological time scales. 4.4.5.1 Phosphorus Residence Time The time scales for P turnover are very important in understanding the mechanisms that control plankton distributions, particulate export, and the biological cycling of nutrients in marine systems. Using an initial 33 P/32 P input ratio of 0.99 and the steady-state model, the residence time of TDP was calculated to range between 3.7 and 13.9 d (Fig. 4.45a, Zhang et al., 2004). The TDP residence time was similar to the PO4 -P residence time (1.5∼24 d) determined in the sea of Okhotsk using artiﬁcial radiolabeled 32 PO4 -P, but shorter than that (20∼52 d) determined previously using naturally produced 32 P and 33 P in the North Paciﬁc Subtropical Gyre. Furthermore, the residence time is also much shorter than expected in the ECS based on the phosphate advection-diﬀusion model, which is in the order of 15∼80 d. In fact, the TDP in seawater is divided into two main pools: SRP and soluble non-reactive phosphorus (SNP). The SNP is referred to as the fraction that is not available for biological uptake, and generally ranges from 0% to 50% of the TDP pool in coastal marine environments to as great as 75% in the open ocean. If SRP instead of TDP was used for the application of P residence time estimation, then it would be predicted that SRP residence time was much shorter than that of TDP. The dissolved P turnover rates varied signiﬁcantly in diﬀerent marine environments, and the residence time of TDP increased from 3∼4 d in the coastal

495

P assimilation rates (d)

4.4 Phosphorus Biogeochemistry in the East China Sea

Fig. 4.45. The phosphorus residence time, the uptake rates of phytoplankton and zooplankton and POC export ﬂuxes at three sampling stations in the ECS. (a) Residence time for TDP, suspended matter, and net-plankton; (b) The phosphorus assimilation rates and carbon assimilation ﬂuxes of phytoplankton; (c) The phosphorus grazing rates and carbon grazing ﬂuxes of zooplankton; (d) POC export ﬂuxes from the upper 35 m (Zhang et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

regime and mid-shelf area, to about 14 d in the outer-shelf area. Diﬀerences in residence time between environments are most likely related to rates of biological assimilation and/or nutrient concentrations, since residence time is a function of the concentration and the ﬂux of a substance in seawater. During the course of this study, surface TDP concentrations dropped from 0.99 μmol/L at the inshore station to 0.40 μmol/L at the oﬀshore station, whereas phytoplankton biomass in the water column decreased from 4×106 to 4×104 cells/m3 . The relative fraction of phytoplankton biomass in the outer-shelf decreased by about a factor of two compared to the coastal water, whereas surface TDP concentrations decreased only by 60%. The decrease in the outershelf phytoplankton biomass, however, could not be accounted for by TDP concentrations alone. The present ﬁnding provides the in situ evidence that TDP has rapid turnover rates in the coastal water, which suggests that low TDP concentrations can support relatively high levels of biological production than previously thought. If correct, the present results also imply that the role of P in supporting primary production has been underestimated. This information is key for current ecological models of the food web in the upper ocean, especially since many of these models assume P limitation to be based only

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on inorganic P concentration relative to N. The rapid turnover and regional variability of P within the dissolved pools reﬂect the dynamic nature of P in marine ecosystems. The short residence time of TDP suggests that studies of dissolved nutrient concentrations alone are insuﬃcient for determining either nutrient limitation or maximal primary production. It has been shown that the abundance and assemblage structure of the marine ecosystem can greatly aﬀect the residence time of nutrients in the upper ocean. Turnover models provide a unique tool with which to investigate the in situ variability of P turnover in marine ecosystems. Assuming that the dominant P source of phytoplanktonic pools was from TDP, it is calculated that the 0.45∼77 μm size class had P residence time of 15∼30 d. In the coastal waters and mid-shelf area, this 0.45∼77 μm size class consisted predominantly of the phytoplankton: Chaetoceros aﬃnis, Chaetoceros debilis, Chaetoceros compressus, Chaetoceros laciniosus, and Nizschia pungens, and had a short residence time of 15∼25 d. On the outer shelf, however, the 0.45∼77 μm size class contained mixed assemblages of organisms dominated by Pyrocystis fusiformis, Ceratium contortum, Ceratium gravidum, etc., as well as decomposing fecal matter. These mixtures of phytoplankton and decomposing fecal matter had a longer P residence time of about 30 d. Assuming all the P uptake of the >77 μm size class was derived from the phytoplanktonic pools (0.45∼77 μm), residence time in this size class was relatively short, ranging from 9.7 to 15.4 d. In the coastal waters, plankton (>77 μm) were dominated by mature copepods, such as Calannus ﬁnmarchicus, and some developmental stages of Euphausids and P residence time of about 10 d. In the mid- and outer-shelf area, residence time in this size class increased to about 15 d, and was dominated by zooplankton: Eucalanus crassus, Eucalanus subcrassus, Eucalanus subtenuis, Rhincalanus cornutus, Pareuchaeta russelli, etc. The present estimate of in situ zooplanktonic P residence time is comparable to 19 d found by laboratory studies in which copepods were fed with 32 P-labeled phytoplankton. Furthermore, the estimates were within the 12∼21 d range determined in the Gulf Stream using mass balance estimates. 4.4.5.2 Phosphorus Uptake Rates The rates in which the nutrient is ﬁxed by phytoplankton and transferred upward in the food web to zooplankton can largely determine the productivity of ﬁsh in the marine system. Traditionally, studies of nutrient ﬂux rates have been conducted using incubation experiments with artiﬁcial C or N isotope tracer and the transfer of activity over time into the various size classes of phytoplankton and zooplankton examined. The diﬃculty with such types of research is that they involve signiﬁcant perturbations to the system of interest. For example, samples are ﬁrst separated from the ecosystem before incubation. Bottle incubations will, at best, miss sporadic bloom events and provide rate estimates that are only valid for discrete depths and time. Recently, naturally produced 32 P and 33 P have been used to investigate directly the uptake rates

4.4 Phosphorus Biogeochemistry in the East China Sea

497

of phytoplankton and zooplankton. The advantage of using naturally produced 32 P and 33 P is that they not only are a nutrient directly utilized by organisms, but also enable an examination of the uptake rate of P without disturbing the regime of interest. Furthermore, results will integrate over all of the processes that have aﬀected the distribution of 32 P and 33 P over the prior 20 to 35 d. The estimated radioactive P assimilation rate of phytoplankton was low and ranged from 0.014 d−1 oﬀshore waters to 0.035∼0.043 d−1 in nearshore waters (Fig. 4.45b, Zhang et al., 2004). The high variability in radioactive P assimilation rates suggests that dynamic uptakes of TDP decreased with increasing distance from the coast. Furthermore, assuming the C/P assimilation ratio of phytoplankton follows the Redﬁeld ratio (C/P=106/1), phytoplankton assimilation ﬂuxes for carbon from the calculated assimilation rates and measured inventory of TDP can be estimated. The ﬂuxes of carbon assimilated by phytoplankton were thus estimated to be in the range of 189∼814 mg C/(m2 ·d), with a mean of 593 mg C/(m2 ·d) (Fig. 4.45). These estimates of 32 P (or 33 P) derived carbon assimilation rates of phytoplankton were about 2 times lower than average 14 C derived primary production of 1,100 mg C/(m2 ·d) from the same general area and in the same seasons. Although the 32 P (or 33 P) method might have large uncertainties associated with a number of factors, such as compartmentalizing phytoplankton and zooplankton into discrete size classes inappropriately, it is believed that the reason for the discrepancy was most likely due to the diﬀerent mechanisms of C and P uptake by phytoplankton. The 14 C method measures mostly photosynthesis; the C ﬁxation rates are usually close to net primary production. By contrast with the 14 C method, 32 P (or 33 P) derived P assimilation rates seem to be independent of photosynthetically available radiation; several studies have shown little eﬀect of light on the rate of P assimilation by phytoplankton. The measurements made in this study have allowed comparisons of carbon ﬁxation and phosphate uptake rates by natural plankton assemblages, where each method gives a diﬀerent measure of plankton dynamics. In a similar manner, if it is assumed that the 32 P and 33 P activities in net-plankton and suspended particulate matter were similar to those in zooand phytoplankton, zooplankton grazing rates for radioactive P can at this point be derived from the steady-state model. The estimated values ranged from 0.025 per day in mid-shelf water to as high as 0.103∼0.077 per day in coastal and outer-shelf waters (Fig. 4.45c). The diﬀerences in grazing rates may be a result of diﬀerent assemblage structure and life periods in each area. Although the zooplankton grazing rate for radioactive P was the highest in the outer shelf, the net grazing ﬂux for P was the smallest due to extremely low phytoplankton biomass in the outer-shelf. It has been found that P was ingested by zooplankton as eﬃciently as C; zooplankton grazing ﬂuxes for carbon can thus be calculated from the measured inventory of particulate P (PP) and the Redﬁeld ratio. The estimated value was the highest in coastal waters and ranged between 10 and 161 mg C/(m2 ·d), with a mean of 64 mg C/(m2 ·d) (Fig. 4.45c). Given the estimated phytoplankton assimilation rates

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4 Biogeochemical Processes of the East China Sea

for carbon above, it is possible to estimate the ecological eﬃciency of this carbon transfer from the ratio of zooplankton carbon uptake to phytoplankton carbon uptake. The ecological transfer eﬃciency thus ranged from 3% to 20%, which means that grazing ﬂuxes of carbon by zooplankton corresponded to 3%∼20% of primary production (Zhang et al., 2004).

4.5 Silicate and Biogenic Silica in the East China Sea In recent years, the transformation, retention, and transport of nutrients through the vast continuum of rivers, lakes, wetlands, and estuaries to coastal waters was the subject of many large research projects including the LOICZ-Program (Land-Ocean-Interactions-in-the-Coastal-Zone-Program), a core project of the International Geosphere-Biosphere Programme (IGBP). Compared to our knowledge concerning N and P processing, transport and cycling of dissolved silica in the aquatic continuum is signiﬁcantly less known. In contrast to N and P, with large human inputs, anthropogenic input of Si to estuarine systems is negligible. The amount of dissolved silica that eventually reaches coastal waters through estuaries is, however, essential in inﬂuencing the occurrence of eutrophication problems in the coastal zone. High anthropogenic inputs of N and P can eventually induce dissolved silica limitation of diatoms and subsequent succession of a phytoplankton community dominated by diatoms to a nondiatom phytoplankton community. Biogenic silica in sediments is known to be an important parameter to understand the biogeochemical processes and paleoenviromental records in estuarine and coastal ecosystems. The content of biogenic silica in sediments is found to have a close link with biosiliceous productivity in overlying waters. Alterations to phytoplankton species composition are likely to have a large eﬀect on levels of CO2 in the atmosphere, and have a strong potential as a powerful proxy for paleoproductivity reconstructions. Therefore, it is of great signiﬁcance to investigate the accumulation and distribution of biogenic silica in estuarine and coastal sediments. However, data on biogenic silica contents in estuarine and coastal sediments are relatively scarce, and factors controlling the preservation of biogenic silica in such complex systems still remain unclear. The Changjiang River is regarded as an important source of materials in the North Paciﬁc coastal seas, and the materials carried by the river have a signiﬁcant inﬂuence on coastal environmental health, sustaining the productive ﬁsheries in the adjacent seas. With the development of the economy and the increased demands for preventing ﬂoods and saving water resources in China, thousands of water conservancy projects have been constructed in the Changjiang River drainage basin during recent decades. Anthropogenic perturbations have caused considerable changes in riverine nutrient concentrations and ﬂuxes to the sea. However, few comprehensive data are available on the variations in nutrient concentrations and ﬂuxes and the eﬀects on the coastal ecosystem, in particular in the riverine silicon variation. Despite the

4.5 Silicate and Biogenic Silica in the East China Sea

499

considerable eﬀorts that have been devoted to the study of the Changjiang River and the ECS, our understanding of the dissolved silicate and biogenic silica in this region is still limited. The main objectives of the section are to introduce the spatio-temporal variations of the dissolved silicate and biogenic silica in the ECS and to explore the factors and mechanisms that aﬀect the distributions of the dissolved silicate and biogenic silica. 4.5.1 Spatial Distribution of the Dissolved Silicate in Seawaters The surface distributions of dissolved silicate (DSi) in the ECS are shown in Fig. 4.46 (Wang et al., 2003). Concentrations of DSi decreased gradually from the estuary to the adjacent sea, indicating that they were inﬂuenced by Changjiang diluted water. DSi concentrations underwent strong seasonal cycles, especially in the Changjiang River Estuary. DSi was signiﬁcantly higher in summer than that in autumn, winter, and spring. Previous research showed that DSi showed conservative behavior in winter and a slight depletion due to biological uptake in summer. Shen et al. (2001) pointed out that the DSi concentration was generally high during the transition period from the wet to dry season (November) while it was low during the transition period from the dry to wet season (April∼May). Since the river runoﬀ and ﬂood deliver terrestrial materials, the seasonal pattern of the DSi can be an indicator of terrestrial origin. In spring, the distributions of silicate were quite similar to those of DIN and phosphorus (Fig. 4.46a). The waters with elevated concentrations of silicate covered the whole coastal area from 35◦ N in the north to 26◦ N in the south, and to the southwest of Cheju Island at about 126◦ E in the east. In contrast, the concentrations of silicate in the outer continental shelf area of the ECS were very low. During the summer, the surface distribution of silicate was quite similar to that of DIN, but was diﬀerent from that of phosphorus. The waters with elevated concentrations of silicate covered the whole coastal area from 35◦ N in the north to 27◦ N in the south (Fig. 4.46b). In autumn, the combination of the low discharge and the prevailing northeasterly wind conﬁned the inﬂuence of the Changjiang River water to a narrow band southward along the coast. Waters with high concentrations of silicate (>10 μmol/L) were found in the coastal area of the ECS, but conﬁned to a narrow band (Fig. 4.46c). In winter, the surface distribution of temperature was more representative of the water circulation pattern than that of salinity. The waters with high concentrations of silicate (>10 μmol/L) were found in the coastal area of the ECS (Fig. 4.46d). The distribution pattern of nutrients in winter is quite similar to that in autumn. It was still nutrient deﬁcient in the oﬀshore area of the ECS, but the area in winter was larger than that in the other three seasons. The spatial and diurnal variations of dissolved silicate in the Changjiang River Estuary were investigated from May 19 to 26, 2003. The results showed

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4 Biogeochemical Processes of the East China Sea

N 38

N 38

36

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26 N 38

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E120 N 38

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128

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26 128

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Fig. 4.46. Distribution of surface water silicate concentration in the ECS. (a) May, 1998; (b) Aug., 1998; (c) Nov., 1997; (d) Jan., 1999 (Wang et al., 2003) (With permission from Elsevier’s Copyright Clearance Center)

4.5 Silicate and Biogenic Silica in the East China Sea

501

that SiO3 -Si distribution is diﬀerent clearly from that of NO3 -N, NO2 -N, NH4 N, and PO4 -P along transect A, which decreased continually seawards (Fig. 4.47). SiO3 -Si concentration in the inner Changjiang River Estuary is about 101.97 μmol/L. However, the variation in SiO3 -Si concentration at station 15 and its eastern stations was small and SiO3 -Si concentration was less than 30 μmol/L commonly. The distribution of SiO3 -Si along transect B indicated that the diﬀerence in SiO3 -Si concentration between 0 m and 2 m above the bottom layers in southern stations is smaller than that in northern stations (Fig. 4.48), which was similar to that of NO3 -N, NO2 -N, and PO4 -P. The variation in SiO3 -Si concentrations vs. time for each layer at anchor station 13 indicated that SiO3 -Si concentrations on the 0 m layer was higher than that in the layer of 2 m above the bottom most of the time (Fig. 4.49). 0 5 10

Depth (m)

15 20 25 30 35 40 45 50

1 2 3 4 5 6 7 8 9 1011121314151617181920 Station

Fig. 4.47. The concentration isolines of SiO3 -Si along transect A (μmol/L) 120

SiO3-Si (mmol/L/)

105 90 75 60 45 0m 2 m above the bottom

30 15 21 22 23 24

25 26 Station

27

28 29

Fig. 4.48. The concentration isolines of SiO3 -Si along transect B (μmol/L)

502

4 Biogeochemical Processes of the East China Sea 110 0m 2 m above the bottom

SiO3-Si (mmol/L)

100 90 80 70 60

6:00

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May 24 2:00

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May 23 1:30

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May 22 9:50

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Fig. 4.49. The variation trends of SiO3 -Si concentrations vs. time for each layer at station 13

The variation trends of SiO3 -Si concentrations vs. time in the two layers were similar; viz., if SiO3 -Si concentrations in one layer decreased or increased, they might decrease or increase simultaneously in another layer. The variation in SiO3 -Si concentration might be signiﬁcant during 4 h, especially in the layer of 2 m above the bottom where its variation might be more than 60 μmol/L. On May 22, an SiO3 -Si concentration of 107.50 μmol/L decreased to 47.14 μmol/L from 13:50 to 17:50. The variation in SiO3 -Si concentrations vs. time for each layer at anchor station 20 indicated that its variation is similar in the layers 0 m and 2 m above the bottom, especially in the ﬁrst 24 h when the variation went all the way (Fig. 4.50). SiO3 -Si concentration was the highest in the layer at 5 m, and the lowest at 20 m. During the investigation time, the variation was the smallest in the layer at 10 m, but the biggest in the layer at 5 m. Diﬀerent to the change in N and P nutrients, silicate ﬂux in the Changjiang River has decreased remarkably over the last 50 years and dissolved silicate concentration at Datong Hydrographic station decreased by 53.3 μmol/L from 1959 to 1984. Especially after the Danjiangkou Dam was completed in 1968, silicate concentration decreased sharply. Up to now, there were 48,000 reservoirs in the Changjiang River basin. Li and Cheng (2001) attributed the decrease in silicate concentration to the dam constructions and the eutrophication caused by industrial and domestic sewage, fertilizers, etc. What is more important, the Three Gorges Project is being built in order to make use of the abundant resources of the Changjiang River. The Three Gorges Project is the largest hydroelectric project ever built in China, and in the world. China’s Three Gorges Dam began to ﬁll as its sluice gates started closing in 2003, with

60 50 40 30 20 10 125 120 40 30 20 10 50 40 30 20 10 60 50 40 30 20 10 90 75 60 45 30 15 0

503

May 19 16:00 20:00 May 20 0:00 4:00 8:00 12:00 16:00 20:00 May 21 0:00 4:00 8:00 12:00 16:00

SiO3-Si (mmol/L)

4.5 Silicate and Biogenic Silica in the East China Sea

Sampling time

Fig. 4.50. The variation trends of SiO3 -Si concentrations vs. time for each layer at station 20 (from top to bottom: 0, 5, 10, 20 m from the surface, and 2 m above the bottom)

the water level reaching 139 m. The export of Si to the sediments in reservoirs has resulted in declines in dissolved silicate in many river waters after dam closure and the decline in Si transport to the coastal seas can change the nutrient ratios available for the phytoplankton community. In order to represent the nutrient conditions in the euphotic zone, the average concentrations of nutrients in the upper 20 m in the coastal area of the ECS, and the upper 50 m in the outer continental shelf of the ECS were used for calculation, for the depth of the euphotic zone is about 20∼30 m in the coastal area of the ECS, and 50 m or so in the outer continental shelf area of the ECS. The Si/N ratios were calculated by using the average concentrations of silicate and total DIN (DIN=nitrite+nitrate+ammonia) in the upper layer at each station. The maximal Si/N ratios were found in the

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Changjiang River mouth, and the maximal average Si/N ratios were found in spring and summer, which implies that the Si/N ratios in the ECS were mainly inﬂuenced by the runoﬀ of the Changjiang River (the runoﬀ of the Changjiang River during the rainy season (from May to October) is 70% of the total runoﬀ in a year). The Si/N ratios were larger than 1 at most stations. The stations with Si/N ratios less than 1 were located mostly in the Changjiang River mouth, but the actual concentrations of silicate were higher than 5 μmol/L. Therefore, it seems that silicate was not the limiting nutrient for the growth of phytoplankton in the ECS. 4.5.2 Distribution of Biogenic Silica in Sediments The biogenic silica (BSi) record in marine sediments acts as a potential proxy for oceanic changes and has a close link to the carbon cycle. BSi is preferentially transported over organic carbon throughout the water column and preserved in sediments; it has been used to reconstruct past production of surface waters. However, lateral advection of water masses, sediment redistribution and spatial variations in the preservation of silica frustules aﬀect the signature produced in the water column and its burial in marine sediments. Because of spatial variations in BSi preservation, and coupling/uncoupling between Si and C in marine biogeochemical cycles, calibration of the BSi proxy in the modern ocean is required, which requires a full understanding of the mechanisms that control the Si cycle. In the coastal environment, the BSi record in core sediments provides evidence of eutrophication. It has been shown that the ratio of plant nutrients and availability of silicate control both the silica productivity and the phytoplankton species composition. Silicon limitation in marine systems has been examined in a variety of marine environments. Knowledge of the origin and fate of BSi in the ECS is relevant to the understanding of the population dynamics and community function. It was reported that the riverine ﬂux of dissolved silica has decreased, but those of DIN and phosphate have increased in the Changjiang River, leading to decreasing Si:N and Si:P ratios in the adjacent coastal waters, with potential silica limitation of diatom growth. The Si:N ratios in the Changjiang River may decrease in the near future, which will inﬂuence the ecosystem in the adjacent coastal environment after the construction of the Three Gorges Dam is completed. Benthic regeneration of dissolved silica is known to be an important source in supporting the production of diatoms. Silicate released from sediment can provide 55% of silicate required by phytoplankton in the ECS (Shi et al., 2004). So it is important to enhance our understanding of biogenic silicate in the ECS. 4.5.2.1 Biogenic Silica in the SPM BSi concentrations in 2002 surface waters varied between 0.21 and 2.76 μmol/L, with an average±SD of (1.3±0.98) μmol/L. The highest concentrations of BSi were observed in coastal areas (Fig. 4.51), which were aﬀected by

4.5 Silicate and Biogenic Silica in the East China Sea

505

abundant terrestrial sediment discharge from the Changjiang River (Liu et al., 2005). Biogenic opal produced by vascular plants, diatoms, and siliceous sponges has been found in soils and terrestrial sediments of continents, with opal ranging from 2% to more than 5%, except in Antarctica. This can partly explain the high content of BSi near the coast, where SPM was high. Elevated concentrations of BSi in the Changjiang River Estuary are also related to the strong vertical mixing that conveys diatom cells upward from the near-bottom and/or sediments back to euphotic surface waters. Silica production extending to depths greater than that where there is 1% of light radiation has been reported in most coastal environments. Abundant nutrient supply, shallow water depth, and strong vertical mixing are prerequisites for the success of large diatoms in estuarine environments. It has been reported that freshwater diatoms contain 10 times more silica per cell volume than marine diatoms. In addition, diatoms are abundant in the surface sediments of coastal areas, including freshwater species.

N 32

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Fig. 4.51. Horizontal distributions of BSi in surface and near-bottom waters in 2002 (μmol/L) (Liu et al., 2005) (With permission from Marine Ecology Progress Series)

BSi increased with depth, varying from 0.26 to 5.10 μmol/L, with an average of (1.9±1.63) μmol/L, in the intermediate layer, and 1.47 to 9.19 μmol/L, with an average of (3.2±2.08) μmol/L, in the bottom waters (Fig. 4.51) (Liu et al., 2005). High concentrations of BSi in near-bottom waters are presumably due to high sedimentation rates and resuspension of bottom sediments. Planktonic diatoms are abundant in ECS, with Pseudo-nitzschia delicatissma, P. pungens, and Melosira sulcata being the dominant species (Gao et al., 2003). Phytoplankton abundance was low (<5×103 cells/L) in the coastal areas, where the SPM was high (>100 mg/L), and the dominant species are

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4 Biogeochemical Processes of the East China Sea

Paralia sulcata and Coscinodiscus jonesianus. The distribution of chlorophyll (Chl) a during the 2002 cruise was diﬀerent from that of BSi, in that the content of Chl a was high near 123◦ E, but low in the surface layer of the coastal areas. This is probably caused by the fact that only a small amount of BSi was bound to identiﬁable diatom cells in the river plume, while a large fraction of the remaining BSi consisted of disintegrated diatom cells. The concentrations of silicate during the 2002 cruise showed higher values in the coastal areas than in oﬀshore areas, especially in surface waters, owing to high freshwater discharge from the Changjiang River, and the silicate levels were higher at the near-bottom than in surface waters due to resuspension of sediments. In summary, the distribution of BSi is closely related to that of silicate, and the silica kinetics associated with SPM is a factor aﬀecting the distribution of BSi in coastal areas. Compared with the BSi content of settling particulate matter at an open-shelf station further oﬀshore in the ECS, the BSi content showed a signiﬁcant gradient from the coast to the shelf edge and the Okinawa Trough. 4.5.2.2 Biogenic Silica in Sediments BSi varied from 0.2% to 0.82% in ECS core sediments. The variation of BSi at diﬀerent depths for a given sediment core and the diﬀerences in BSi between stations were both signiﬁcant at the 95% conﬁdence level, based on the application of ANOVA. High concentrations of BSi were observed at stations DC10, D34, E4, and E5, while low BSi concentrations were found at stations E6 and DB6 (Fig. 4.52) (Liu et al., 2005). Field observations at stations E6 and DB6 showed the bottom sediments consisted of silt and sand, with considerable shell fragments (e.g., DB6). Accordingly, the pigment content (e.g., Chl a plus pheo-pigments) was higher at stations E4, E5, and DC10 than at stations E6 and DB6. The variations of BSi within a given core are probably related to changes in primary production and resuspension of bottom sediments. No obvious trend with depth was observed in core sediments, except at stations DC10, E5 and, to a lesser extent, D34. The BSi content in ECS sediments was less than 1%, which is similar to those from the Bohai and Yellow Seas, but lower than that in Jiaozhou Bay sediment (Li et al., 2006a). The low concentrations of BSi can be attributed to the high content of SPM and the shallow euphotic zone. It has been reported that iron may stimulate diatom growth and enhance nitrate uptake in coastal areas, but limited increase in silica uptake leads to rapid dissolution of BSi. 4.5.3 Silica Balance on the East China Sea Shelf The silica balances in the ocean are controlled by many processes, such as terrestrial input, silica dissolution in sediment, and biogenic silica production. In the ECS, the silica balance is related to the river inputs, atmospheric deposition, BSi deposition and dissolution in sediments, export toward the deep sea and the input from Kuroshio water, etc.

4.5 Silicate and Biogenic Silica in the East China Sea

507

Fig. 4.52. BSi in core sediments at stations E4 [122.61◦ E, 30.95◦ N], E5 [122.48◦ E, 28.94◦ N], E6 [125.00◦ E, 29.58◦ N], DC10 [122.47◦ E, 30.99◦ N], D34 [122.77◦ E, 30.53◦ N], DB6 [122.47◦ E, 31.48◦ N] in October 2000, May 2001, and April∼May 2002 (Liu et al., 2005) (With permission from Marine Ecology Progress Series)

4.5.3.1 Inputs of Silicate The main land-source input to the ECS is the Changjiang River. Part of the discharge from the Changjiang River is carried into the Yellow Sea, especially in summer, when the Changjiang River plume reaches Cheju Island. The annual freshwater discharge into the ECS is 86% (i.e., 794.8×109 m3 ) of the Changjiang River runoﬀ, with a silicate discharge of 81×109 mol/yr to the ECS (FR ) based on a previous investigation (Fig. 4.53) (Liu et al., 2005). Total deposition ﬂuxes of silicate for dry (i.e., aerosol) and wet (i.e., rain) depositions were (1.01±0.61)×109 mol/yr (Fig. 4.53, Liu et al., 2005). In the exchange between the ECS and the Kuroshio, the Kuroshio surface water (KSW), Kuroshio subsurface water (KSSW), and Kuroshio intermediate water (KIW) have net onshore transport, while shelf mixed water (SMW) has net oﬀshore transport, because net precipitation and freshwater discharge from rivers exceed evaporation. In addition, there is silicate input through the Taiwan Strait water (TSW). The silicate input was estimated to be 22.3×109 mol/yr from the KSW, 47.3×109 mol/yr from the KSSW, and 237×109 mol/yr from the KIW, with a total input ﬂux of 306.6×109 mol/yr from Kuroshio water. The silicate input ﬂux was 56×109 mol/yr from the TSW. In contrast, the silicate oﬀshore transport was 100.4×109 mol/yr (Fig. 4.53) (Chen and Wang, 1999; Liu et al., 2005).

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4 Biogeochemical Processes of the East China Sea

F E(export)

Fig. 4.53. Biogeochemical cycle of Si on the ECS shelf (×109 mol/yr). River inputs, FR ; atmospheric deposition, FA ; net deposition of BSi in sediments, FB ; BSi gross production, FP(gross) ; silicate ﬂux recycled in the surface layer, FD(surface) ; BSi ﬂux exported toward the deep layer, FE(export) ; silicate ﬂux recycled in the deep layer, FD(deep) ; silicate ﬂux transferred from the deep layer to the surface layer, Fup ; silicate ﬂux at the sediment-water interface, FD(benthic) ; BSi ﬂux that reaches the sedimentwater interface, FS(rain) ; silicate input through the Taiwan Strait water, FTSW , and Kuroshio water, FKW ; oﬀshore transport of silicate, FSMW (Liu et al., 2005) (With permission from Marine Ecology Progress Series)

4.5.3.2 Production of Biogenic Silica Silicate is required by planktonic organisms to grow (e.g., diatoms), and a signiﬁcant amount of BSi is remobilized by dissolution of silica cells. Production of BSi was estimated by a combination of factors: 14 C primary production, the relative contribution of diatoms, and Si/C ratios. However, use of measured Si/C ratios for natural particle assemblages to transform 14 C productivity data into BSi productions is subject to uncertainties. Average primary production in the ECS was (9.0±3.9), (9.3±4.8), (15.7±9.6), and (11.3±4.7) mol C/(m2 ·yr) in winter, spring, summer, and autumn, respectively, with an annual mean primary productivity of (12.1±5.7) mol C/(m2 ·yr) (Gong et al., 2003). Estimation of a representative BSi/POC ratio for diatoms in natural environments is diﬃcult; diatoms growing in nutrientreplete conditions show a reasonably constant BSi/ POC atomic ratio of (0.13±0.05), but in nutrient-poor conditions large variations of the BSi/POC ratio can occur, in both oceanic and coastal areas. The BSi/POC atomic ratio in settling particles in this study was 0.22 to 0.45, and the BSi/POC atomic ratio in settling parti-

4.6 Dissolved Oxygen and O2 Flux across the Sea-Air Interface of the ECS

509

cles was 0.12 to 1.4 on the ECS shelf. The average BSi/POC atomic ratio was hence estimated to be 0.48. Therefore, the annual BSi production on the ECS shelf was (3.1±0.6)×1012 mol/yr, given a surface area of ca. 53×104 km2 over the shelf region of water depth <200 m (Fig. 4.53, Liu et al., 2005). 4.5.3.3 Silica Recycling in Sediments and Silica Budget The BSi deposition ﬂux below the euphotic zone was 17.9 to 85.5 mmol/(m2 ·d), with an average of (42±37.5) mmol/(m2 ·d). The average BSi deposition ﬂux in the upper water column over the whole ECS shelf was estimated to be (9.4±10.1) mmol/(m2 ·d). The annual ﬂux of BSi exported toward the deep reservoir (F E ) on the ECS shelf was determined to be (1.8±1.95)×1012 mol/yr. Therefore, the ﬂux of recycled silicate that occurred in the surface layer [F D(surface) ] was 1.27×1012 mol/yr, accounting for 41% of BSi production (Fig. 4.53, Liu et al., 2005). The distribution of surface-sediment types over the ECS shelf is patchy, consisting of muddy silt at the inner and mid-shelf to muddy sand at the shelf edge. By using data for in situ incubations and pore-water proﬁles, the sediment-to-water exchange ﬂux over the entire shelf [FD(benthic) ] was estimated to be 0.416×1012 mol/yr. The BSi accumulation rate [FB(netdeposit) ] on the ECS shelf was estimated to be 0.348×1012 mol/yr. The BSi ﬂux that reaches the sediment-water interface [F S(rain) ] was 0.764×1012 mol/yr, and the silicate ﬂux recycled in the deep reservoir [FD(deep) ] was 1.05×1012 mol/yr. The silicate ﬂux transported from the deep layer to the surface layer (Fup ) was calculated to be 1.752×1012 mol/yr, by assuming that the silicate ﬂux from TSW and the Kuroshio was well distributed vertically (Fig. 4.53). The input of benthic silicate ﬂux accounts for 54% of the BSi that reaches the sediment-water interface, and the silicate ﬂux recycled in the deep reservoir [FD(deep) ] accounts for 58% of the BSi ﬂux exported toward the deep reservoir (FE ). The removal of silica is mainly by BSi accumulation in sediment, which accounts for 78% of the total output. Considering all input budgets of silicate into the ECS, benthic silicate ﬂux [FD(benthic) ] alone accounts for 48%, followed by the Kuroshio onshore transport, accounting for ca. 36%, then the riverine input (FR ), representing 9%; the input through the TSW accounts for the remaining 7% (Liu et al., 2005).

4.6 Dissolved Oxygen and O2 Flux across the Sea-Air Interface of the ECS Dissolved oxygen (DO) level in natural aquatic systems is a highly informative variable which can elucidate atmosphere-ocean interactions, water mass movements, net primary productivity and carbon remineralization processes (Gao et al., 2005). It is also strongly representative of an ecosystem’s functionality and behavior. The DO level indicates how well the water is aerated

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4 Biogeochemical Processes of the East China Sea

and is a commonly measured parameter because it is an immediate sign that indicator-inadequate oxygen levels will quickly aﬀect aquatic life. The atmosphere and aquatic plant photosynthesis are two main sources from which oxygen enters the water. Oxygen is essential not only for marine life but also for the decomposition of organic matter, a process which consumes oxygen. When the DO level is less than 3 mg/L in seawater, the environment is deﬁned as hypoxia. Hypoxia and anoxia have been widely observed in many estuarine and coastal regions over the past few decades. The occurrence of hypoxia and anoxia in shallow, coastal, and estuarine areas is most likely accelerated by human activities, through over-enrichment of anthropogenic nutrients. Excess nutrient loading often leads to eutrophication, which can result in oxygen depletion through decomposition of elevated organic matter from enhanced primary production. Major ecological impacts of hypoxic and anoxic environments include reduced biodiversity and alteration of community structure and ecology. Hypoxia and anoxia have rarely been documented in the ECS which is one of the largest continental shelves in the world. DO in the ECS or Changjiang River Estuary has been attracting the attention of many marine scientists. DO in the ECS was investigated in Oct. 1993 (Leg 9310), Apr. 1994 (Leg 9404), from Oct. to Dec. 1994 (Leg 9410) (Fig. 4.54, Song et al., 1996), and in the Changjiang River Estuary in 2003 (Fig. 4.2).

N 32

111 Profile 1 Profile 2

30 410

Pro

file

28

Pro

26

file

4

5

Taiwan 24 121

123

125

127

129

E

Fig. 4.54. Sampling station in ECS in Oct. 1993, Apr. 1994, and from Oct. to Dec. 1994 (Song et al., 1996)

4.6 Dissolved Oxygen and O2 Flux across the Sea-Air Interface of the ECS

511

4.6.1 Dissolved Oxygen Distributions in Seawaters 4.6.1.1 General Distribution on Dissolved Oxygen in ECS In the <100 m depth coastal region, the mixture of upper and lower layers was strong so there was little diﬀerence between DO in the upper and lower layer seawater. But the seawater mixture in Leg 9410 was better than that in Leg 9404, so the DO was very low in the bottom waters in Leg 9404. In <100 m depth stations, the DO decreased with depth. The DO in seawater in Leg 9310 was similar to that in Leg 9410, but was obviously diﬀerent from that in Leg 9404. The DO concentrations in Leg 9404 were higher than those in Leg 9310 and Leg 9410. It was obvious that the DO in seawater was mainly controlled by seasonal temperature. The subsurface maxima of DO appeared at only several stations. In Proﬁle 1, the DO maxima appeared near the surface at 122◦ E and ◦ 125 30 E in Leg 9404, near the subsurface at 122◦ E in Leg 9410, and near the surface at 128◦ E, and could extend to a 50 m depth. The DO vertical mixture was fairly uniform under a 100 m depth, but in a still deeper region DO decreased rapidly with depth. Proﬁle 2 was in deep water along 129◦ E. The DO maximum appeared near the surface at 31◦ 30 N in Leg 9404, and the DO vertical change was strong at 0.6 ml/100 m. In Leg 9410, the DO maxima appeared near the surface at 29◦ 45 N and the DO vertical change was only 0.3 ml/100 m. During Leg 9410, the ECS was beset by strong waves (wind force always >6 m/s), so the higher DO water could extend to 300 m in depth. The DO in Leg 9410 was higher than that in Leg 9404, especially at the bottom. Proﬁle 4 showed a great change in depth. DO maxima appeared near 124◦ E in Leg 9410, near 113◦ 30 E and the subsurface at 126◦ E in Leg 9410. The DO vertical mixture in Leg 9410 was apparently better than that in Leg 9404, especially in the deep region of Proﬁle 4. Proﬁle 5 was in deep water with a large change in depth. DO maxima appeared near the surface at 122◦ E in Leg 9404, and near the surface at 123◦ 30 E in Leg 9410. The DO concentrations in Leg 9404 were higher than those in Leg 9410 on the same layer. DO sectional distribution diﬀerences in various Legs could be due to: (1) Temperature eﬀect and climate conditions. Leg 9404 was measured in spring when seawater temperature was low, and still a little like that in winter. The climate was calm and vertical mixture was not strong, so DO concentrations were high in Leg 9404. Leg 9410 was measured in late fall and the seawater temperature was high. The marine environment was characterized by cold currents and occasional tropical storms. The vertical mixture of seawater was very strong. The higher the seawater temperature, the lower the DO concentration. (2) Marine hydrodynamics. In the ECS, there were very big variations in velocity and direction of sea current in April and October in Proﬁle 4 and Proﬁle 5 located in the Kurashio mainstream and Proﬁle 1 in its branch. There

512

4 Biogeochemical Processes of the East China Sea

were strong coastal currents that also aﬀected the sectional distributions of DO in seawater. (3) Biological activity in the euphotic zone. The euphotic zone biological activity (for example, breeding) was obviously diﬀerent in April and October. Photosynthesis of phytoplankton varied seasonally. The biological species in the euphotic zone have an important eﬀect on DO concentration in seawater. In summary, seasonal temperature and climatic conditions were the main factors controlling ECS DO variation in Leg 9404 and Leg 9410 in the ECS. 4.6.1.2 Dissolved Oxygen Concentration in the Changjiang River Estuary (1) Spatial distribution of DO concentration Dynamic patterns of DO emerge in estuaries from complex interactions among physical, chemical, and biological processes. The DO concentration of a water mass is largely determined by a balance among the exchange of atmospheric oxygen with the upper mixed layer, net increases due to photosynthetic processes, and net decreases due to respiratory demands and heterotrophic processes. Variations in temperature, freshwater discharge, saltwater intrusion, bathymetry, circulation, meteorology, and biological production and respiration combine to produce strong estuarine DO gradients. In the Changjiang River Estuary, the tidal wave reaches as far as 640 km inland, and the freshwater inﬂuence extends ca. 200 km into the sea. The saline interface shows considerable variability in depth and width owing to the balance between river discharge and marine driving forces. The mixing of fresh and saline water occurs from the head of the mouth bar to the mouth of the estuary. Generally, waters around the Changjiang River Estuary can be classiﬁed as three principal water masses: (1) freshwater from the Changjiang River; (2) seawater (continental shelf waters) entering the ECS, either with the Yellow Sea along shore current from the north or with the Taiwan Warm Current and its branches from the south; (3) the transition zone between freshwater and seawater, resulting in estuarine mixed water. Based on the salinity data gained during the ﬁeld investigation, salinity between stations 10 and 16 changed quickly, indicating that the freshwater and seawater interacted with each other in this area (Fig. 4.55, Gao and Song, 2008). Basically, station 10 could be regarded as the salt-wedge, and waters ahead of and behind it could be classiﬁed as freshwater mass and estuarine mixed water mass, respectively; station 16 could be regarded as the estuarine front, and waters ahead of and behind it could be classiﬁed as estuarine mixed water mass and seawater mass, respectively. Variation in water temperature exhibited a trend that showed a decrease as salinity increased in a downriver direction, which meant that salinity and temperature in the Changjiang River Estuary during this investigation showed summer characteristics; i.e., seawater is cooler than river water (Fig. 4.55).

4.6 Dissolved Oxygen and O2 Flux across the Sea-Air Interface of the ECS

513

Fig. 4.55. The spatial distributions of salinity (‰) (a), temperature (◦ C) (b), and DO (mg/L) (c) isolines along transect A (Gao and Song, 2008) (With permission from Elsevier’s Copyright Clearance Center)

On the whole, DO concentrations increased downriver with the rise in salinity and reached high values in the area between stations 17 and 20 (Fig. 4.55). In the freshwater and mixed water before station 14, DO concentrations were relatively stable and ﬂuctuated basically within the narrow range of 7.33∼8.10 mg/L, and after that they increased rapidly. In the relatively deep water column between stations 17 and 20, DO concentrations decreased with water depth from a highest of 14.88 mg/L at the 0 m layer of station 19 to a lowest of ∼4 mg/L at 2 m above the bottom layer of the same station. Transect B was situated among the estuarine mixed water mass. Water depths of stations located along it were shallow; only three of them were a bit deeper than 10 m. So, for most stations, only water samples at the 0 m layer and 2 m above the bottom layer were taken. Results indicated that at the four northern stations, namely stations 21 to 24, DO concentrations at the 0 m layer were markedly higher than those at the 2 m above the bottom

514

4 Biogeochemical Processes of the East China Sea

Temperature ( )

Salinity ( )

layer. Even for station 22, where the diﬀerence between the two layers was the smallest among the four northern stations, the DO concentration at the 0 m layer was still 1.44 mg/L higher than that at the 2 m above the bottom layer, while the diﬀerences between the two layers were small and all were <0.5 mg/L at each of the other ﬁve stations (Fig. 4.56). Average DO concentrations were (8.07±0.65) and (7.18±0.60) mg/L for 0 m and 2 m above the bottom layers, respectively. The variation pattern of DO along transect B was similar to that of temperature, while it was to some extent the inverse of that of salinity (Fig. 4.56, Gao and Song, 2008). All these three parameters indicated that water in the southern part of transect B mixed better than that in the northern part. DO is of concern in aquatic ecosystems because depressed levels can be stressful for ﬁsh and other limnetic or marine organisms. A minimum level of 5.0 mg/L is considered crucial for aquatic life. When DO levels drop to 3 to 5 mg/L, most organisms become stressed, and mobile species must move to areas with higher oxygen concentrations to survive, while immobile species often perish. Below 3 mg/L, a condition called hypoxia sets in. Hypoxic

0m 2 m above the bottom 35.0 30.0 25.0 20.0 15.0 10.0 5.0 0.0 22.5 21.0 19.5 18.0 16.5 15.0 10.0

DO (mg/L)

(a)

(b)

9.0 8.0 7.0 (c) 6.0 21 22 23 24 25 26 27 28 29 Station

Fig. 4.56. The distributions of salinity (a), temperature (b), and DO (c) along transect B (Gao and Song, 2008) (With permission from Elsevier’s Copyright Clearance Center)

4.6 Dissolved Oxygen and O2 Flux across the Sea-Air Interface of the ECS

515

levels are regularly seen in the area around the Changjiang River Estuary, particularly in the summertime. The National Standard GB 3097-1997, which is established by the Standardization Administration of China (SAC), has deﬁned four grades of seawater. Grade I is suitable for nature reserves with a DO concentration of 6 mg/L; Grade II is suitable for mariculture and leisure activities such as swimming with a DO concentration of 5 mg/L; Grade III is suitable for tourism and industrial purposes except food processing with a DO concentration of 4 mg/L; Grade IV is harbour quality with a DO concentration of 3 mg/L. Based on this standard, DO concentrations measured up to Chinese seawater quality grade I in the brackish water region oﬀ the Changjiang River mouth. (2) Temporal distribution of DO concentration To investigate the trend of DO variation over time, two representative anchor stations viz. stations 13 and 20 were monitored over a period of 48 h in mixed water and seawater regions, respectively. Parameters were measured at about 4-h intervals. The water depth of station 13 was only 8 m. Two layers, namely 0 m and 2 m above the bottom, were studied. The plot of DO against the time variation shows that there was only one time when the two layers were recorded with the same DO concentration, and apart from that the 0 m layer had a higher DO concentration than the 2 m above the bottom layer; the biggest diﬀerence between the two layers, which was recorded at 17:50, May 22, was 1.23 mg/L (Fig. 4.57, Gao and Song, 2008). This indicates that the waters were not well mixed most of the time despite the shallow depth. The same conclusion could also be drawn from salinity data. DO concentrations ﬂuctuated more widely at 2 m above the bottom layer than at the 0 m layer,

Fig. 4.57. Temporal variations in DO within 48 h for station 13 (Gao and Song, 2008) (With permission from Elsevier’s Copyright Clearance Center)

516

4 Biogeochemical Processes of the East China Sea

but at both of the two layers the magnitude of the ﬂuctuations was usually less than 0.5 mg/L, namely 6%, from one sampling time to the next. The average DO concentrations were (8.20±0.19) and (7.72±0.32) mg/L for 0 m and 2 m above the bottom layers, respectively. Anchor station 20 was located near the estuarine front. Its depth was 43 m, according to which a total of ﬁve layers, namely 0 m, 5 m, 10 m, 20 m, and 2 m above the bottom, were investigated. For the ﬁve samples collected at the same time, DO concentration decreased gradually with the increase in water depth except for only a few cases; the maxima nearly always appeared at the top two layers, especially at the 0 m layer, while the minima appeared at the 2 m above the bottom layer with no exception (Fig. 4.58, Gao and Song, 2008). The biggest diﬀerence among the ﬁve samples collected at the same time was 8.38 mg/L (12:00, May 21), more than twice the smallest 4.04 mg/L (04:00, May 20), which means the average vertical oxygen gradient reached 0.20 mg/(L·m) at that time. Compared with other layers, DO concentration

Fig. 4.58. Temporal variations in DO within 48 h for station 20 (from top to bottom: 0 m, 5 m, 10 m, 20 m, and 2 m above the bottom) (Gao and Song, 2008) (With permission from Elsevier’s Copyright Clearance Center)

4.6 Dissolved Oxygen and O2 Flux across the Sea-Air Interface of the ECS

517

was more variable from one sampling time to the next at the 0 m layer and a variation from 8.77 to 12.66 mg/L was recorded during the period of 08:00 to 12:00, May 21. That means the DO concentrations varied 44.4% in 4 h. The variation trends of DO concentration at the top two layers were very similar and showed a signiﬁcant correlation (R2 =0.8734, P <0.001). Among the ﬁve layers studied, DO concentrations at 2 m above the bottom layer were the most stable, and their variation ratios were 6.1% for any two sequential sampling times. From 0 m to 2 m above the bottom layer, the average DO concentrations were (9.65±1.31), (9.45±1.18), (7.80±0.67), (6.86±0.69), and (4.37±0.14) mg/L, respectively. DO diﬀerences between the top two layers for the samples collected at the same time were usually <0.3 mg/L, namely <3.1% of the averaged DO concentration at these two layers, which means that water in the upper 5 m was well mixed in terms of DO concentration. This could also be proved by the salinity data gained at the same time. DO variation with time indicates that, at station 13 and the upper 20 m of station 20, its concentrations were signiﬁcantly >6.0 mg/L except for a very few cases, and measured up to Chinese seawater quality grade I; at the 2 m above the bottom layer of station 20, its concentrations only measured up to Chinese seawater quality grade III. 4.6.2 O2 Flux across the Sea-Air Interface DO concentration in the upper layer of the ocean is aﬀected by the transport of oxygen into and out of this layer. Oxygen will be transported horizontally by advective forces and vertically by air-sea gas exchange as well as by mixing in the water column. In addition, biological production and remineralization will also alter the oxygen concentration in the water column. To understand the status of air-water oxygen exchange in ECS and the Changjiang River Estuary area, the oxygen ﬂuxes across the air-water interface were calculated. The water-air O2 ﬂux (F, g/(m2 ·d)) in the ECS was calculated as follows: F (O2 ) = E ·

ΔDO O2 (sat)

where F (O2 ) is the O2 ﬂux in L/(m2 ·d) across the sea-air interface. E is the exchange coeﬃcient in 382 mol/(m2 ·d), which is estimated from in situ temperature and pressure, and is 23.44 L/(m2 ·d) under standard conditions. ΔDO is the diﬀerence between O2 solubility and the DO determined, i.e., ΔDO=O2 (sat)−DO. O2 (sat) is the solubility of O2 in situ (Song et al., 1996). The water-air O2 ﬂux (F, g/(m2 ·d)) in the Changjiang River Estuary was calculated as follows: F = k([O2 ]m − [O2 ]s )

518

4 Biogeochemical Processes of the East China Sea

where k is the gas transfer velocity for oxygen, [O2 ]m is measured oxygen content in the surface water and [O2 ]s is the corresponding saturation concentration calculated according to the equation of Weiss (Weiss, 1970; Gao and Song, 2008). 4.6.2.1 O2 Flux in the ECS Fig. 4.59 (Song et al., 1996) shows the horizontal distributions of O2 ﬂux across the sea-air interface in the ECS. The positive values of F (O2 ) indicate O2 in air dissolving into seawater. The negative values indicate DO in seawater released to air. In Leg 9310 an O2 supersaturated area appeared in the outer region of the Changjiang River Estuary and the unsaturated area near the 32◦ N, 124◦ E region where O2 tended to dissolve into seawater. In Leg 9410, a supersaturated region also appeared in the outer part of the Changjiang River Estuary, and near the coast. A large supersaturated region appeared in the southeast sea region of Japan. There was an unsaturated region of O2 in the Kurashio branch, where a large amount of O2 was dissolved from air into seawater. The exchange of O2 was small in the other regions. A striped supersaturated region of O2 appeared in the north of the Taiwan Strait where oxygen was released into air. The side areas of the striped region were unsaturated with O2 , but the amount of O2 dissolved into seawater was small. The exchange of oxygen was controlled by the sea current in April, but the lumpy horizontal distribution of the O2 ﬂux was controlled by meteorological factors (such as wind, temperature) in October. Table 4.16 (Song et al., 1996) shows the average O2 ﬂux in four sections. It can be seen that the O2 diﬀusion direction for Proﬁle 1 and Proﬁle 4 in Leg 9310 and Leg 9410 were the same, but the O2 ﬂuxes were diﬀerent. Oxygen released to air mainly occurred in Leg 9404. The biggest ﬂux appeared in Proﬁle 1, but the exchange amount was small in Proﬁle 4.

0 0 .5 0 .5

2.0 3.0 2.0 0

2.0

0

1.0 1.0

5 0. 0.5 0

0.5 5 0.

00 1 .5 2.0.5 2.0 1.5

0

0

0.5

2.0 0.5

0

0

3.5

1.0

0.5 0

0. 0. 5 5

0.5.0 1 1.50.5 0.5

0

1.0

2.5

2.0

1.0.0 1

0 0.5

2.0 1.0 3.5 1.0 2.5 2.0

0.5

1.01.5 .0 1

2.5 2.0

Taiwan

Leg 9310

Taiwan

Leg 9404

Taiwan

Leg 9410

Fig. 4.59. The horizontal distribution of O2 ﬂux across the sea-air interface in the ECS (L/(m2 ·d)) (Song et al., 1996)

4.6 Dissolved Oxygen and O2 Flux across the Sea-Air Interface of the ECS

519

Table 4.16. Fluxes across the sea-air interface in the ECS (L/(m3 ·d)) (Song et al., 1996) Proﬁle Proﬁle Proﬁle Proﬁle

1 2 4 5

Leg 9310 0.819 – 0.109 –

Leg 9404 −1.594 −0.448 −0.020 −0.0719

Leg 9410 0.219 −0.919 0.189 0.311

Fig. 4.60 (Song et al., 1996) shows the daily variation of O2 ﬂux across the sea-air interface at the continuous stations 410 and 111. It can be seen that oxygen released to air was the main process in one day and night, and that the amount released in daytime was more than that at night. This was because photosynthesis produced more oxygen in daytime, and the high temperature of seawater also increased the release of oxygen. This feature was very obvious in Leg 9410. At station 410, the oxygen in air dissolved into seawater from 18:00 to 06:00, but it was O2 from seawater to air from 06:00 to 18:00. The oxygen released into air was −8.63 L/(m2 ·d) in Leg 9404. This reﬂected the fact that the seawater was supersaturated and the photosynthesis was very

2 0

Time 0:00 6:00 12:00 18:00 24:00

2

Flux (L/(m2 d))

4 2

Leg 9404-410 Time 0:00 6:00 12:00 18:00 24:00

0 Leg 9410-410 2 2 0

Time 0:00 6:00 12:00 18:00 24:00

2 4

Leg 9404-111

Fig. 4.60. Daily variations of oxygen ﬂux across the sea-air interface (Song et al., 1996)

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4 Biogeochemical Processes of the East China Sea

strong during the observation. Because the meteorological factors (such as wind speed) have an important role in the daily variation of the O2 ﬂux, the daily variation of the O2 ﬂux was diﬀerent during diﬀerent observation periods. 4.6.2.2 O2 Flux in the Changjiang River Estuary Spatial data indicate that an oxygen sink gradually shifted to a source in a downriver direction, and the strength of oxygen ﬂuxes ranged from −14.95 to 5.98 g/(m2 ·d) (Fig. 4.61, Gao and Song, 2008). Stations 1 to 15 of transect A and all the stations of transect B except station 21 were sinks for atmospheric oxygen, and their average strength was (−5.77±5.00) g/(m2 ·d) during this cruise; the rest were sources for atmospheric oxygen, and their mean strength was (3.64±1.61) g/(m2 ·d).

(a)

1 2 3 4 5 6 7 8 9 10 1112 14 15 17 18 19 Station

Flux of oxygen (g/(m2 d))

Flux of oxygen (g/(m2 d))

3 6 4 3 0 2 4 6 8 10 12 14 16

2

(b)

1 0 1 2 3 4 5 2122 23 24 25 26 27 28 29 Station

Fig. 4.61. Variation of O2 ﬂux strength along transects A (a) and B (b) (Gao and Song, 2008) (With permission from Elsevier’s Copyright Clearance Center)

Temporal data indicate that, for both stations 13 and 20, the strength of the oxygen ﬂux varied within a wide range, and even the direction changed from one sampling time to another (Fig. 4.62, Gao and Song, 2008). Considering the whole studied period, station 13 was a net sink for atmospheric oxygen and the average F value was (−0.32±0.19) g/(m2 ·d), while station 20 was a net source and the average F value was (2.10±2.34) g/(m2 ·d). 4.6.3 Factors Inﬂuencing Dissolved Oxygen Concentration The ability of water to hold oxygen is dependent on salinity, temperature, time of day, season, and so on. Oxygen is not very soluble in water, and is even less soluble in saltwater. Temperature also plays a big role. Water at higher temperatures cannot hold as much oxygen as colder water. DO levels vary greatly during a diurnal cycle. Generally, the water at noon holds high

0.3 0.4 0.5

Ma y2

20

9:5 13 0 :5 17 0 :50 Ma y 2 21:5 30 0 1:3 06 0 :0 10 0 :00 14 :0 18 0 :00 Ma y 2 22:0 40 0 2:0 06 0 :0 10 0 :00

0.6

Sampling time

6

(b)

4 2 1.2 0.9 0.6 0.3 0.0 0.3 91 6:0 0 Ma y 2 20:0 00 0 0:0 04 0 :0 08 0 :00 12 :0 16 0 :00 Ma y 2 20:0 10 0 0:0 04 0 :0 08 0 :00 12 :0 16 0 :00

Flux of oxygen (g/(m2 d))

0.08 0.06 (a) 0.04 0.02 0.00 0.2

521

Ma y1

Flux of oxygen (g/(m2 d))

4.6 Dissolved Oxygen and O2 Flux across the Sea-Air Interface of the ECS

Sampling time

Fig. 4.62. Variations of oxygen ﬂux strength within 48 h for stations 13 (a) and 20 (b) (Gao and Song, 2008) (With permission from Elsevier’s Copyright Clearance Center)

levels of DO due to oxygen generated from photosynthesis; once night falls, photosynthesis stops and plants consume oxygen as they respire, decreasing DO levels. DO levels are also aﬀected by depth. Temperature diﬀerences at diﬀerent depths aﬀect how much oxygen the water can hold. Waters can become stratiﬁed in late spring and summer, which means a layer of warmer, fresher water forms over a colder, saltier layer. Thus, oxygen is unable to reach the lower depths, resulting in lower DO levels at deeper depths and higher levels near the surface. Nutrient availability can impact on DO levels in several ways. Excess nutrients lead to an increase in phytoplankton and other types of algae. When the algae die and decompose, they use up oxygen in the estuary, resulting in low DO conditions. In some cases, living phytoplankton or algae can also cause low DO conditions. Historically Chl a has been used as a measure of phytoplankton standing stock. Also, when related to photosynthetic carbon production, the concentration of Chl a and its distribution in the water column allow one to estimate an index of the eﬃciency of phytoplankton in harvesting light, consuming CO2 and releasing oxygen. Obviously, based on the whole spatial data obtained in this study, the highly signiﬁcant correlation between DO and Chl a indicates that phytoplankton photosynthesis was a major factor inﬂuencing DO spatial distribution (Table 4.17, Gao and Song, 2008). In the Changjiang River Estuary, nutrient concentrations generally decreased seaward, which is mainly because they were diluted by seawater and consumed by autotrophy, especially phytoplankton during their export to the sea. When only the data obtained at the water surface are considered, signiﬁcant negative correlations exist between

522

4 Biogeochemical Processes of the East China Sea

3− DO and the parameters of NO− 3 and PO4 concentrations, which seem to further prove the conclusion that phytoplankton photosynthesis has a signiﬁcant inﬂuence on DO spatial distribution from another aspect (Table 4.17). The inﬂuence of phytoplankton photosynthesis on DO spatial distribution must be rather strong at the water surface resulting in a positive correlation between DO and salinity, for it should be negatively correlated under normal conditions (Table 4.17).

Table 4.17. Correlation coeﬃcients between DO and some biogeochemical parameters (Gao and Song, 2008) (With permission from Elsevier’s Copyright Clearance Center)

DO vs S DO vs T DO vs Chl a − DO vs NO3 3− DO vs PO4

Data of anchor station 13 Data of Data of 2 Whole 0 m m above data layer the (n=26) (n=13) bottom (n=13) ns –0.912∗∗∗ –0.866∗∗∗ ns 0.899∗∗∗ –0.764∗∗∗

Data of anchor station 20 Data of 0 Data of Whole m layer other data (n=13) layers (n=65) (n=52)

Data of other stations Data of 0 Data of Whole m layer other data (n=26) layers (n=78) (n=52)

–0.909∗∗ 0.478∗∗∗ ns

0.527∗∗ ns 0.896∗∗∗ –0.501∗∗

–

ns

– 0.619∗

–0.741∗∗ ns 0.787∗∗

0.501∗∗

ns

ns

ns

ns

ns

ns

–0.907∗∗∗ 0.517∗∗∗

ns ns ns ns ∗∗∗ 0.688 0.726∗∗∗

ns ns ns –0.863∗∗∗ –0.852∗∗∗ –0.688∗∗∗ ns –0.244∗ ∗ ∗∗ S: salinity; T : water temperature; ns: not significant; significant at P <0.05; significant at P <0.01; ∗∗∗ significant at P <0.001

Data obtained from station 13 show that DO variation with time can be mainly attributed to the inﬂuence of physical factors in the mixed water region (Table 4.17). The inﬂuence of salinity on DO variation with time was greater than that of temperature based on the correlation coeﬃcients between them and DO. The signiﬁcant positive correlation between DO and water temperature is inconsistent with thermodynamic knowledge, which suggests that the inﬂuence of other factors on DO variations with time is more signiﬁcant than the thermodynamic factor. When only the data obtained at the water surface are considered, none of the ﬁve parameters, viz., salinity, water temperature, 3− Chl a, NO− 3 and PO4 concentrations, has signiﬁcant correlation with DO. Data obtained from station 20 show that DO variation with time can be attributed to the conjunct inﬂuence of physical and biological factors in seawater regions (Table 4.17). Advective transport, namely the movement of the water body with higher DO concentration from other regions to station 20, must have an important inﬂuence on DO variations with time in the upper two layers, because two of the three marked peak values appeared at nighttime when the inﬂuence of photosynthesis stopped (Fig. 4.58). It seems that PO3− 4 concentration is more likely to become a limiting factor of phytoplankton photosynthesis than NO− 3 concentration considering the whole water column, since it had a signiﬁcant negative correlation with DO while NO− 3 concentration did not. This coincides with other reports (Harrison et al., 1990; Pu et al., 2000; 2001).

References

523

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Huang ZQ, Fu TB, Zhang YH (1997) Distributive characteristics of POC in sea water in the East China Sea. J Oceanogr Taiwan Strait 16(2):145-152 (in Chinese with English abstract) Huh CA, Su CC (1999) Sedimentation dynamics in the East China Sea elucidated from Pb-210, Cs-137 and Pu-239, Pu-240. Mar Geol 160(1-2):183-196 Hung JJ, Hung PY (2003) Carbon and nutrient dynamics in a hypertrophic lagoon in southwestern Taiwan. J Mar Syst 42(3-4):97-114 Hung JJ, Chen CH, Gong GC, Sheu DD, Shiah FK (2003) Distributions, stoichiometric patterns and cross-shelf exports of dissolved organic matter in the East China Sea. Deep Sea Res Part II 50(6-7):1127-1145 Jeng WL, Lin S, Kao SJ (2003) Distribution of terrigenous lipids in marine sediments oﬀ northeastern Taiwan. Deep Sea Res Part II 50(6-7):1179-1201 Kao SJ, Lin FJ, Liu KK (2003) Organic carbon and nitrogen contents and their isotopic compositions in surﬁcial sediments from the East China Sea shelf and the southern Okinawa Trough. Deep Sea Res Part II 50(6-7):1203-1217 Kato Y, Terunuma Y, 1995. Recycling of biogenic materials in sediments from the ECS: Results of MASFLEX pore-water study. In: Tsunogai S, Iseki K, Koike I, Oba T (eds.) Global Fluxes of Carbon and Its Related Substances in the Coastal Sea-Ocean-Atmosphere System. Proceedings of the 1994 Sapporo IGBP Symposium, Hokkaido University, Japan. Yokohama, M & J Int, Japan, pp.238245 Lee HJ, Chao SY (2003) A climatological description of circulation in and around the East China Sea. Deep Sea Res II 50:1065-1084 Li MT, Cheng HQ (2001) Changes of dissolved silicate ﬂux from the Changjiang River into sea and its inﬂuence since late 50 years. China Environ Sci 21(3):193197 (in Chinese with English abstract) Li XG, Song JM, Dai JC, Yuan HM, LiN, Li FY, Sun S (2006a) Biogenic silicate accumulation in Jiaozhou Bay sediments. Chin J Oceanol 24(3):270-277 Li XG, Song JM, Yuan HM (2006b) Inorganic carbon of sediments in the Yangtze River Estuary and Jiaozhou Bay. Biogeochemistry 77(2):177-197 Lin S, Huang KM, Chen SK (2000) Organic carbon deposition and its control on iron sulﬁde formation of the southern East China Sea continental shelf sediments. Cont Shelf Res 20(4-5):619-635 Lin S, Hsieh IJ, Huang KM, Wang CH (2002a) Inﬂuence of the Yangtze River and grain size on the spatial variations of heavy metals and organic carbon in the East China Sea continental shelf sediments. Chem Geol 182(2-4):331-336 Lin S, Huang KM, Chen SK (2002b) Sulfate reduction and iron sulﬁde mineral formation in the southern East China Sea continental slope sediment. Deep Sea Res Part I 49(10):92-96 Liu KK, Lai ZL, Gong GC, Shiah FK (1995) Distribution of particulate organic matter in the southern East China Sea: Implications in productivity and transport. Terr Atmos Ocean Sci 6(1):27-45 Liu KK, Tang TY, Gong GC, Chen LY, Shiah FK (2000) Cross-shelf and alongshelf nutrient ﬂuxes derived from ﬂow ﬁelds and chemical hydrography observed in the southern East China Sea oﬀ northern Taiwan. Cont Shelf Res 20(4-5):493523 Liu SM, Zhang J, Li RX (2005) Ecological signiﬁcance of biogenic silica in the East China Sea. Mar Ecol Pro Ser 290(Apr.):15-26

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Liu WC, Wang R, Ji P (1997) Study on particulate organic carbon in the East China Sea. Oceanol Limnol Sin 28:139-143 (in Chinese with English abstract) Liu XC, Shen HT, Huang QH (2002) Concentration variation and ﬂux estimation of dissolved inorganic nutrient from the Changjiang River into its estuary. Oceanol Limnol Sin 33(3):332-340 (in Chinese with English abstract) Nakamura T, Matsumoto K, Uematsu M (2005) Chemical characteristics of aerosols transported from Asia to the East China Sea: An evaluation of anthropogenic combined nitrogen deposition in autumn. Atmos Environ 39(9):17491758 Ogawa H, Usui T, Koike I (2003) Distribution of dissolved organic carbon in the East China Sea. Deep Sea Res Part II 50(2):353-366 Oguri K, Matsmoto E, Yamada M, Saito Y, Iseki K (2003) Sediment accumulation rates and budgets of depositing particles of the East China Sea. Deep Sea Res II 50(2):513-528 Ortega T, Ponce R, Forja J, Gomez-Parra A (2005) Fluxes of dissolved inorganic carbon in three estuarine systems of the Cantabrian Sea (north of Spain). J Mar Syst 53(1-4):125-142 Park PK, Gordon LI, Hager SW, Cissell MC (1969) Carbon dioxide partial pressure in Columbia River. Science 166(3907):867-868 Pu X, Wu Y, Zhang Y (2000) Nutrient limitation of phytoplankton in the Changjiang Estuary I. Condition of nutrient limitation in autumn. Acta Oceanol Sin 22(4):60-66 Pu X, Wu Y, Zhang Y (2001) Nutrient limitation of phytoplankton in the Changjiang Estuary II. Condition of nutrient limitation in spring. Acta Oceanol Sin 223(3):57-65 Raymond PA, Caraco NF, Cole JJ (1997) Carbon dioxide concentration and atmospheric ﬂux in the Hudson River. Estuaries 20(2):381-390 Raymond PA, Bauer JE, Cole JJ (2000) Atmospheric CO2 evasion, dissolved inorganic carbon production, and net heterotrophy in the York River Estuary. Limnol Oceanogr 45(8):1707-1717 Sarma V, Kumar MD, Manerikar M (2001) Emission of carbon dioxide from a tropical estuarine system, Goa, India. Geophys Res Lett 28(7):1239-1242 Shen ZL (1991) A study on the eﬀects of the Three Gorge Project on the distributions and changes of the nutrients in the Changjiang River Estuary. Oceanol Limnol Sin 22(6):540-546 (in Chinese with English abstract) Shen ZL (1993) A study on the relationships of the nutrients near the Changjiang River Estuary with the ﬂow of the Changjiang River water. Chin J Oceanol Linmol 11(3):260-267 Shen ZL, Liu Q, Zhang SM, Miao H, Zhang P (2001) The dominant controlling factors of high content inorganic nitrogen in the Changjiang River and its mouth. Oceanol Limnol Sin 32(5):465-473 (in Chenese with English abstract) Sheu DD, Jou WC, Chung YC, Tang TY, Hung JJ (1999) Geochemical and carbon isotopic characterization of particles collected in sediment traps from the East China Sea continental slope and the Okinawa Trough northeast of Taiwan. Cont Shelf Res 19(2):183-203 Shi F, Wang XL, Shi XY, Zhang CS, Jiang FH, Zhong CJ, Li KQ (2004) Benthic ﬂux of dissolved nutrients at the sediment water interface in the East China Sea. Mar Environ Sci 23(1):5-8 (in Chinese with English abstract)

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5 Biogeochemical Processes of the South China Sea

Abstract: In this chapter, the biogeochemical processes in the South China Sea are described. The Zhujiang River (Pearl River) plays an important part in the biogeochemical cycling of chemical components in the northern South China Sea. The biogenic element characteristics in the Nansha coral ecosystem are intensively concerned.

5.1 Water Dynamical Processes in the South China Sea The South China Sea (SCS) has an area of about 3.5 million km2 and depths ranging from the shallowest coastal fringe to 5,567 m, with an average depth of 1,212 m (Fig. 5.1, Morton and Blackmore, 2001). It is also studded with numerous islets, atolls, and reefs, many of which are just awash at low tide. The water dynamics plays an important part in the biogeochemical processes of the SCS. The SCS is surrounded by land and countries that have a major inﬂuence on, and claims to, the sea including China, Malaysia, the Philippines, and Vietnam, although Thailand and Indonesia have some claims too. The SCS occupies the northern tropics, almost exactly between the equator and the Tropic of Cancer, at 22◦ N. It thus experiences a monsoon climate created by the inﬂuences of the southwest monsoon in summer and the northeast monsoon in winter. The latter is a stronger and more constant dry wind, increasing average wave heights in Hong Kong, for example, by about 1 m. The former is rain-bearing, deriving moisture from evaporation over the SCS. Furthermore, in summer there are often intense low pressure cells built up in the western Paciﬁc, which develop into severe tropical storms or, in extreme cases, into typhoons (or hurricanes) which characteristically pass over the Philippines and either veer northeast towards Taiwan region and Japan, or cross into the SCS and head towards Vietnam in the west or the coast of southern China in the north.

530

5 Biogeochemical Processes of the South China Sea

Fig. 5.1. A map of the South China Sea (Morton and Blackmore, 2001) (With permission from Elsevier’s Copyright Clearance Center)

There is the relationship between the interannual variability in the index of the SCS Warm Water and the monsoon break over the SCS. The SCS Warm Water, and the warm pools in both the western equatorial Paciﬁc and the Indian Oceans are in the same coupled system on a large scale, sharing a long period of oscillation of about 4.8 years. During the years when colder water occurs in the SCS, an atmospheric anticyclone maintains itself to the east of the Philippines in summer. A low frequency activity associated with the anticyclone results in low frequency oscillation in the precipitation ﬁeld over this region, whereas the weak subtropical high over the western Paciﬁc travels eastward in summer, which is responsible for the anomalous distribution of meridional vapor transport. The coastal fringes of the SCS are home to about 270 million people, which have had some of the fastest developing and most vibrant economies on the globe. Consequently, anthropogenic impacts, such as over-exploitation of resources and pollution, are anticipated to be huge although, in reality, relatively little is known about them. The Indo-West Paciﬁc biogeographic province, at the center of which the SCS lies, is probably the world’s most diverse shallow-water marine area. Of three major near shore habitat types, i.e., coral reefs, mangroves, and sea grasses, 45 mangrove species out of a global total of 51, most of the currently recognized 70 coral genera, and 20 of 50 known sea grass species have been recorded in the SCS. The northern and northwestern regions are wide shelves leading to the East China Sea (ECS) through the 50-m deep Taiwan Strait. Furthermore, the SCS

5.1 Water Dynamical Processes in the South China Sea

531

is a major ﬁshing ground, with the ﬁshing industry providing the livelihood for millions of ﬁshermen in the surrounding territories. Like the ECS, the SCS receives large amounts of nutrients in the form of river discharge to support productivity. The ECS receives only a small portion of its nutrient input from rivers. Instead, upwelling is the major source of nutrients. And the seasonally reversing monsoon winds play an important role in determining the upper ocean circulation, but the combination of such variable atmospheric forcing and the complex geometry contributes to the complicated dynamics of the ﬂow in the SCS. The most important oﬀshore systems in the SCS are coral reefs (Fig. 5.2). The most abundant reefs lie in the waters to the south of Hainan, that is, the more than 20 atolls and islets of the Macclesﬁeld Bank (Zhongsha Qundao) and the approximately 36 islands of the Paracel Islands (Xisha Qundao) (Morton and Blackmore, 2001).

1. North Reef 2. Pattle Island 3. Robert Island 4. Passu Keah 5. Triton Island 6. Tree Island 7. Rocky Island 8. Woody Island 9. Lincoln Island 10. Valaddore Reef 11. Bombay Reef 12. Bankok Schoal 13. Cathay Shoal 14. Pygmy Shoal 15. Egeria Bank 16. Plover Shoal 17. Balfour Shoal 18. Scarborough Reef 19. North Danger Reef 20. Chu-pi Tao 21. Loarta Island 22. Trident Shoal 23. Reed Tablemout 24. Chung-yeh Tao 25. Nanshan Island 26. Heng Chiao 27. Amy Douglas Bank 28. Seahorse Shoal 29. Western Reef 30. Tizard Bank 31. Sabina Shoal 32. Discovery Great Reef 33. Fiery Cross Reef 34. Sin Cowe Island

35. Second Thomas Shoal 36. Alicia Annie Reef 37. London Reefs 38. Cuarterog Reef 39. Parson Reef 40. Cornwallis South Reef 41. Chien-Chang An-sha 42. Ladd Reef 43. Spratly Island 44. Ivestigator Shoal 45. Half Moon Shoal 46. Prince Consort Bank 47. Prince of Wales Bank 48. Bombay Castle 49. Owen Shoal 50. Lizzie Webber 51. Tu-hu An-Sha 52. Hsi-wei Tao 53. Vanguard Bank 54. Granger Bank 55. Rifleman Bank 56. Kingston Shoal 57. Amboyna Cay 58. Royal Charlotte Reef 59. Louisa Reef 60. North Luconia Shoals 61. Seahorse Breakers 62. South Luconia Shoals 63. Herald Reef 64. Tseng Mu An-Sha 65. North Vereker Bank 66. South Vereker Bank 67. Pratas

Fig. 5.2. A map of the South China Sea showing the continental mainland of Southeast Asia, the Philippines, East Malaysia and the islands of the sea (Morton and Blackmore, 2001) (With permission from Elsevier’s Copyright Clearance Center)

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5 Biogeochemical Processes of the South China Sea

5.1.1 Circulation and Eddies Circulation and eddies in SCS are complicated and have its own special characteristics. The general circulation over the entire basin shows a dipole structure. 5.1.1.1 Regional Dynamical Processes of the South China Sea When large-scale circulation is concerned, the northeast monsoon will drive a basin-scale cyclonic circulation with each cyclonic gyre in the northern and southern SCS (NSCS, SSCS), respectively, and the circulation displays a double-gyre structure. In summer, the circulation in the NSCS will still maintain a weak cyclonic gyre. However, due to the southwest monsoon, the circulation in the SSCS displays an anticyclonic gyre. (1) Dynamical interpretation to the SCS circulation pattern By assuming the SCS to be an enclosed basin, the basin-scale circulation pattern can be obtained from the Sverdrup stream function. And from the Sverdrup stream function in the interior region, the mass transport of the western boundary current (WBC) is 5∼6 Sv and 3∼4 Sv in winter and summer, respectively. They suggested that the upper layer basin-scale circulation can be mainly regarded as a wind-driven circulation forced by the wind stress vorticity, which indicates that the SCS circulation has strong regional characteristics. The pattern of surface ﬂow in the SCS in winter is shown in Fig. 5.3 (Morton and Blackmore, 2001). In winter (Fig. 5.3a), the northeast monsoon creates an anticlockwise pattern of circulation. The wind pushes cooler, coastal waters down through the Taiwan Straits to circulate west and southwards along the coast of China and Vietnam and to either depart the SCS via the Karimata Straits or be turned northeasterly and run along the coast of Borneo and Palawan and thus return to the northern rim of the sea. This creates an anticlockwise gyre in the central area. The Taiwan Current is cold coastal water but the Kuroshio Current entering the SCS via the Straits of Luzon, dampens its cooling eﬀects, especially above the northern continental shelf by inputting into it north equatorial water which is much warmer (26∼29 ◦ C). This joins the incurring Taiwan Current at the surface. In summer (Fig. 5.3b), from May to September, under the inﬂuence of the southwest monsoon, current ﬂow is reversed in the SCS and water enters from the Java Sea via the Karimata Straits to sweep up into the central area and exits through the Taiwan Straits. In so doing, a clockwise gyre is established oﬀ the coast of Borneo, above the Spratlys, and a smaller anticlockwise one oﬀ the coast of Vietnam. The southern coasts of the SCS, i.e., Borneo and much of the western Philippines, are aﬀected by a prevailing northeasterly ﬂow while the coasts of the northern rim, i.e., the southern coast of China and most of Vietnam, are

5.1 Water Dynamical Processes in the South China Sea

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inﬂuenced by a southeasterly-oriented ﬂow. The latter is particularly inﬂuenced by the Coriolis force so that, for example, the prevailing ﬂow of water from the Pearl River, the major drainage of southern China, is to the west.

N

N

20

20

10

10

0

0 100

110

120 E

100

110

120 E

Fig. 5.3. The surface currents of the South China Sea in (a) winter and (b) summer (Morton and Blackmore, 2001) (With permission from Elsevier’s Copyright Clearance Center)

(2) Circulation in the SSCS The circulation in the SSCS is much diﬀerent from that in the NSCS due to the existence of the Nansha Islands. The circulation in the SSCS mainly consists of some cyclonic and anticyclonic eddies, which show a multi-eddy structure. In general, a large scale gyre consists of two or more small scale eddies. The circulation in the SSCS was calculated by using the temperature and salinity data from 1959∼1988, and it was found that an anticyclonic circulation in winter dominates the region southeast of the Nansha Islands, and a cyclonic circulation dominates the rest of the area. The circulation in the upper layer in the region around the Nansha Islands has an independent enclosed structure. The deep circulation in the SSCS together with the central SCS forms an enclosed circulation system. The direction of circulation in the upper layer is always contrary to that in the lower layer. The reversal of the upper and lower circulations indicates that the SSCS is dominated by a typical baroclinic structure; as a consequence, stronger vertical motions will occur. 5.1.1.2 Eddies in the South China Sea There is a cyclonic eddy located oﬀ northwestern Luzon, and this eddy exists all the year round, while the cold eddy oﬀ the southeast coast of Vietnam is thus named the Vietnam cold eddy. Cross correlation analysis shows that the local air-sea feedback mechanism is the major cause of the formation of the

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5 Biogeochemical Processes of the South China Sea

Vietnam cold eddy. There exist cold eddies in the region east of Vietnam in the central SCS and in the region oﬀ the northwest Luzon Strait, and there perennially exists a cold eddy in the region around the Dongsha Islands. There also exist some stable eddies in the regions around the Nansha Islands and southeast of the Mekong Estuary. There exist some clear seasonal sea surface height (SSH) signals of the Kuroshio’s intrusion in the northeastern SCS. Regarding the relationship between the SCS eddy and the seasonal adjustment of the SCS circulation, the appearance, decline, and migration of the sub-basin scale and mesoscale eddies lead to the seasonal adjustment of the large scale circulation. The dynamics of the generation of the multieddy structure connects tightly with the process of the energy transformation among the large-scale circulation, sub-basin scale circulation, and the mesoscale eddy. The mesoscale and small scale eddy, which are mainly generated by monsoon forcing, the lateral boundary and topographical restrictions, have an important role in the energy transformation of the larger scale gyre system. This includes the Kuroshio forcing at the Luzon Straits and the local and remote eﬀects caused by the generation of Rossby waves. However, until now, knowledge of the vertical structure of the eddy and the possible role of thermocline dynamics in the generation, maintenance, and development of the SCS multi-eddy structure, especially an understanding of the energy transformation among those eddies, is deﬁcient (Wang et al., 2001). 5.1.1.3 Dynamics of the South China Sea Circulation Adjustment The ocean circulation will respond to any change in the wind ﬁelds, which usually occurs in terms of large-scale planetary wave adjustment. This kind of process can be simpliﬁed as follows. The wind stress acts on the ocean and generates a barotropic Rossby wave at ﬁrst, which migrates through the SCS basin in several days, and then generates barotropic velocity ﬁelds, which have uniform property in the whole water column. It will take about 1 month (in the SSCS) and 3 months (in the NSCS) for the ﬁrst mode of the baroclinic Rossby wave to propagate from east to west, which enhances the baroclinic structure in the Sverdrup stream ﬁeld. After the coastal trapped Kelvin waves (CTKW) propagate along the lateral boundary completely, and the Rossby wave initiated by the CTKW at the tip of the eastern coast reaches the western coast, the adjustment of the SCS basin-scale circulation will be accomplished as a preliminary stage. As a consequence, the Sverdrup transport mainly concentrates in the upper layers, and the full Sverdrup relationship degenerates to an upper layer Sverdrup balance. Theoretically, the observed SSH seasonal cycle represents the variation in the mean thermocline in the SCS, where the response to Ekman pumping is concerned. The circulation in the NSCS is relatively stable. However, the SSCS circulation has a strong annual variation. One reasonable interpretation is that the seasonal signal in the SSCS is much stronger than that in the NSCS. In winter, there exists a cyclonic vorticity center over both the SSCS and NSCS.

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In summer the vorticity center over the NSCS weakens, whereas wind stresses over the SSCS reverse to an anticyclone. Another reasonable interpretation is that the propagation of the Rossby wave in the SSCS is much faster than that the NSCS. As a result, the SSCS can respond to the external forcing more quickly than the NSCS (Yang, 2000). In winter, the cyclonic wind stress in the northwestern SCS forces a cyclonic gyre to the west of the Luzon Strait, whose typical spatial scale is similar to the Rossby deformation scale. Yang (2000) and Liu et al. (2001) suggested that the westwards propagation of the wind force leads to a similar shift of sea surface height anomaly (SSHA), which is often regarded as a Rossby wave signal. In the matter of the wave propagation in the interior SCS, the systematic eastward migration of wind stress curl will drive a westward-propagating forced Rossby wave. Thus it presents an eastward migration of SSHA, whereas the westward-propagating signal carried by free Rossby waves is always obscured by the eastward-propagating forced Rossby wave. It is very diﬀerent to that in the open oceans. As a consequence, the seasonal variation in the SCS basin-scale circulation is mainly attributed to the internal dynamical process. SSHA migrates from the Luzon Strait to the western boundary of the NSCS with a speed of 11∼12 cm/s, which resembles the Rossby wave propagating from the Paciﬁc Ocean to the SCS. The velocity of the baroclinic Rossby wave with interannual time scale is about 30 cm/s (Gan and Cai, 2001). 5.1.1.4 Interannual Variability of the South China Sea Circulation The air-sea system over the SCS connects with the global climate system to a certain degree. The upper layer circulation in the SCS has signiﬁcant seasonal adjustment responding to seasonal change in wind stresses. Interannual variation of the large-scale wind ﬁeld mainly causes interannual variability of the upper layer circulation. Interannual variability of the SCS relates closely with the southern Oscillation Index, which stands for interannual variability of the tropical ocean and atmosphere. This reveals that the SCS interannual variability may have a planetary scale background associated with the global low frequency oscillation. Some researchers found that sea surface temperature (SST) in the SCS can be taken as a kind of forecasting index of the Asian monsoon onset, as much as for ENSO (El Ni˜ no-southern Oscillation) events. (1) Upper layer oceanic thermodynamics during ENSO In the view of climatology, the sea waters gain heat in the SSCS and lose heat in the NSCS. As an integral, the SCS is a neutral system with the heat exchange balanced between air and sea, till ENSO events destroy this equilibrium. The pronounced anomalies appear in the SCS during ENSO. This response appears not only in the thermal structure of the upper layer, but also in the SCS circulation and in the wind ﬁeld. The upper thermal structure in the SCS has signiﬁcant variations in periods of around 2 years, 3∼5 years and interdecadal oscillation. He et al. (1997) found that the upper layer

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5 Biogeochemical Processes of the South China Sea

heat content in the SCS increases signiﬁcantly in the El Ni˜ no years, with a delay of 4∼12 months compared with the eastern equatorial Paciﬁc. Wang et al. (2000) supported this point and found that whenever a warm/cold event occurred in the SCS, an El Ni˜ no/La Ni˜ na event with 5 months delay happened in the eastern equatorial Paciﬁc. (2) Anomalous wind and modiﬁed circulation during ENSO Accompanied with the occurrence of ENSO, there are northeasterly anomalies before ENSO events and southwesterly anomalies during ENSO. Wu et al. (1998) decomposed the simulated SCS circulation ﬁeld in 1992∼1995 by the means of EOF, and found that there was a southern center in the signature structure in the ﬁrst mode, corresponding with a seasonal reversal of circulation in the SSCS, in April and October, respectively. Two centers are found in the second mode in the SSCS and NSCS respectively, corresponding to the seasonal and interannual variations in circulation in the NSCS. The NSCS circulation changed greatly in the El Ni˜ no year, for instance, the winter time circulation weakened and shrunk southward, whereas the summer time circulation displayed no dipole structure. The EOF of wind stress curls shows that the ﬁrst two modes in the SCS circulation can be explained by the two leading modes in the wind stress curl. In the El Ni˜ no year the second mode of wind stress curls decreased greatly. In another research, changes in the circulation ﬁeld during El Ni˜ no were also noted. By EOF decomposing TOPEX SSH data of 1992∼1995, Shaw et al. (1999) found that the ﬁrst mode, corresponding to the basin-scale oscillation in the SCS, has a weaker interannual signal. The weakened wind stress curl in the winters of 1992∼1993, 1994∼1995 and summer of 1995 resulted in weak winter and summer circulations respectively, while the reduced low center oﬀ Vietnam showed a suppressed eastward Vietnam jet current. These studies show interannual variability in the basinscale circulation in the SCS connected with the East Asian monsoon. Wang et al. (2001) found that the anomalous stream function in summer during the El Ni˜ no events mainly strengthens the seasonal mean pattern, e.g., strengthens the anticyclonic gyre in the SSCS and the cyclonic gyre in the NSCS, but weakens the entire cyclonic gyre in winter. In the La Ni˜ na case, the anomalous stream function in summer mainly has an eﬀect in the NSCS, i.e., weakens cyclonic gyre. But in winter it strengthens the entire cyclone. Distinct changes occurred in the vertical mode in the SCS circulation during ENSO. Chao et al. (1996a) revealed the modiﬁcation of the SCS circulation in El Ni˜ no 1982/1983 by using a numerical model. A weaker surface ﬂow leads to a strengthened upwelling in the central basin and a weakened downwelling in the surrounding area during August to November of 1982, which resulted in a weakened vertical advection of heat and warmer SST. Kuo et al. (2000) carried out a study on the SST anomalies in the upwelling region in the western SCS in 1997∼2000. He found that upwelling in 1997 was stronger than that in other years, so did SSH in the SCS in El Ni˜ no 1997/1998. By means of qualitative analysis and quantitative diagnosis of SST, wind stress, SSH,

5.1 Water Dynamical Processes in the South China Sea

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and thermocline in the SCS during El Ni˜ no 1997/1998 and during La Ni˜ na 1998, Wang et al. (2002) found that the positive correlation between SSTA and the meridional wind anomaly occurred only in the stages of the onset and development of the 1997/1998 warm event, whereas the maintenance of the warm event is controlled by the downwelling mode. In other words, anomalous downwelling pushes thermocline deeper, so the positive SSTA maintained in the SCS can last longer. 5.1.2 Water Exchange via the Straits Several straits connect the SCS with the surrounding waters. However, water exchange at the Taiwan, Kalimantan, and Palawan Straits is mainly concentrated in the surface layer, whose eﬀect on the intermediate circulation can be negligible. In the Luzon Strait, which connects the SCS with the western Paciﬁc at the northern tip of Luzon Island, waters in the NSCS are inﬂuenced by the Kuroshio directly. The Strait’s water depth and geographical location, the outside western boundary current (WBC), and the western boundary countercurrent (WBCC) mean that the transport at the Luzon Strait has a signiﬁcant variation in the vertical direction. 5.1.2.1 Water Exchange at the Luzon Strait The water exchange between the SCS and Paciﬁc Ocean at the surface layer relates the monsoon to the exchange between the Kuroshio water and the SCS water. The winter monsoon forces the Kuroshio water to enter the Luzon Strait and even to reach the western coast of the NSCS. However, the summer monsoon drives the SCS water into the Paciﬁc. The Philippines seawater enters the SCS perennially above the submarine sill in the Luzon Strait. In a word, the Kuroshio surface water does enter the SCS. The Kuroshio enters the SCS in the form of a loop current. After shedding oﬀ the Kuroshio, the water mass intrudes into the NSCS in an eddy style all the year round, which is analogous to the Mexico Gulf Stream. The horizontal scale of a cyclonic ring away from the Kuroshio is about 150 km, and the vertical scale is about 1,000 m. The maximum surface velocity is about 1 m/s (Li et al., 1997). The backward and forward movements of Kuroshio waters (also named the Philippine Sea waters), which pass through the Luzon Strait at the intermediate layer, can be well conﬁned by the 34.6‰ isohaline. The maximum (minimum) intrusion of the salinity tongue arises in June (October). The warm, saltier Kuroshio water enters the SCS through the Luzon Strait from October to March, whose maximum (minimum) intrusion arises in February (September) with a volume transport of about 13.7 Sv (1.4 Sv), and the annual mean volume transport is about 6.5 Sv (Chu and Li, 2000). The saltiest North Paciﬁc water (NPW) enters the SCS all the year round with a strong intrusion present in winter and summer, and a weak intrusion present

538

5 Biogeochemical Processes of the South China Sea

in spring and autumn. On the other hand, the freshest NPW enters the SCS only in spring, when the saltiest NPW enters the SCS very little. However, the annual mean mass transport through the Luzon Strait is 3 Sv, and the maximum (minimum) arises during January and February (June and July) with a volume transport of about 5.3 Sv (0.2 Sv) (Qu et al., 2000). The volume transport through the Luzon Strait is about 4.4 Sv as an annual mean with an amplitude of 2.5 Sv from a coarse resolution modeling result. 5.1.2.2 Dynamic Interpretation of the Kuroshio Intrusion The loop current grows when a four-day average of the local wind stress component directed to the south exceeds 0.08 N/m2 . When this average wind stress component drops below the critical value, the Kuroshio returns to its northward path. In fact, the Kuroshio, SCS gyre, monsoon, and local topography all inﬂuence circulation in the Luzon Strait area. The loop current’s strength in the strait reduces as the strongly stratiﬁed SCS water is driven northward by the southwest winds. Numerical results also indicate that the Kuroshio is separated by a nearly meridional ridge east of Luzon Strait. Moreover, the water ﬂowing from the SCS contributes primarily to the near shore part of the Kuroshio. The volume transport through the Luzon Strait is mainly attributed to the pressure gradient fed by mass convergence due to monsoon forcing. Ekman pumping driven by wind stress curl does not aﬀect the transport through the Luzon Strait. Topography also plays an important role in the transport. There is no evidence of any relationship between the interannual variability of the Kuroshio intrusion and the meridional or zonal components of wind or wind stress strength or wind stress curl in this area. However, it can be found that the seasonal cycle of the transport through the Luzon Strait has a close relationship with the seasonal reversal of the northeasterly and southwesterly monsoon. Some other independent researches also suggested that the pressure gradient is the major factor inﬂuencing the Kuroshio intrusion, with the enhanced inﬂuence concerning the β eﬀect. 5.1.2.3 Vertical Structure of Water Exchange Through the Luzon Strait The water property below the 3,500 m depth in the SCS is similar to that of the Philippine seawater in the layer from 1,900 m to 2,000 m, which shows that the bottom water in the SCS originates from the western Paciﬁc. The same result could also be obtained from chemical tracer analysis. Using a box model, Han (1998) suggested the renewal time of the deep-water below 2,000 m is about 76 years. Fang and Wei (2002) pointed out that the circulation between Paciﬁc Ocean and Indian Ocean through the SCS is a branch of the throughﬂow. From their numerical results, the annual mean transport is about (4±1.5) Sv, and the renewal time of the SCS water is (40±15) years. All the

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evidence mentioned above shows that the renewal rate of the SCS water is much faster than that of the open ocean. Regular observations indicate that the Kuroshio water (SCS water) ﬂowing into (out of) the SCS basin changes seasonally in the upper layer (from surface to 350 m). In the intermediate layer, the SCS water ﬂows to the northwestern Paciﬁc all the year round. In the deeper layers, what has been conﬁrmed by distribution of isothermals and relationship between temperature and salinity, is that the northern Paciﬁc bottom water enters the NSCS in the layer from 1,500 m to the bottom of the Luzon Strait. Therefore, that SCS water originates from the Paciﬁc can be inferred. The intruding bottom water acts as an anticyclonic circulation. The transport of the bottom water (1,500 to 2,500 m) intrudes into the SCS at about 0.7 Sv. However, direct observations show that the transport of the deep water through the Luzon Strait into the SCS is about 1.2 Sv. Some research has been done about water exchange between the Kuroshio and the SCS in the upper layer. However, the issue about how and why the deep and intermediate waters go through the Luzon Strait is an open question. Based on the analysis of the temperature and salinity data, the salinity of the saltiest SCS subsurface water is higher than that in the Philippine Sea, while salinity of the freshest intermediate SCS water is lower than that in the Philippine Sea. This kind of salinity distribution hints that there exist strong mixing processes in the interior SCS. Yuan (2002) argued that the SCS works as a “mixing mill” that stirs the surface and deep waters to return them to the Luzon Strait at the intermediate depth. Based on the assumption above mentioned, he inferred that the volume transport through the Luzon Strait may be dominated by diapycnal mixing. 5.1.3 Dynamics of the Mixed Layer and Thermocline of the South China Sea Driven by the monsoon, a signiﬁcant seasonal reversal occurs not only in the SCS circulation, but also in heat storage. There are signiﬁcant exchanges of mass and energy between the mixed layer and thermocline via the process of entrainment and detrainment due to turbulent mixing in the mixed layer (or subduction/ventilation), which aﬀects the circulation in the thermocline and below. It was one of the key contents of ocean circulation theory, with signiﬁcant implications in marginal seas. The most prominent interannual variability in the SCS lies at the level of 100 m instead of 300 m, in contrast to the Kuroshio. Controlled by internal dynamical processes, the interannual change at the surface is weaker than that at the subsurface in the SCS. The intraseasonal variation is usually restrained at the surface and subsurface, and also modiﬁed by the dynamic process (Gao et al., 2002).

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5.1.3.1 Climatic Features of the Mixed Layer and Thermocline in the South China Sea The mixed layer depth had a negative correlation with SST, and the surface heat ﬂux played an important role in the seasonal circle of SST, whereas the inﬂuence of the oceanic dynamical process could not be ignored. The mixed layer is in good agreement with the large-scale circulation. For example, the monsoon imposes itself on the temporal and spatial variation of the mixed layer via the associated advection in the ﬂow ﬁeld. They also found that among such dominant factors controlling the mixed layer as wind stress, surface heat ﬂux, and fresh water ﬂux, the wind stress is the most important one. The thermocline became deeper and thinner under the action of surface cooling in winter (Liu et al., 2001). It can also be found that the intraseasonal variation in the thermocline is mainly determined by geostrophic currents. Moreover, in the intraseasonal time scale, thermocline and SSH are out of phase. The main leading factors in seasonal dynamics of the mixed layer and thermocline of the SCS were direction, strength of wind, and wind stress curl, while both eﬀects had the same order of magnitude. The thermal structure reveals the existence of a barrier layer in the SCS. The barrier layer usually weakens the cooling eﬀect entrained at the bottom of the mixed layer. There are barrier layers in both the NSCS and SSCS, but they are thinner than that in the western equatorial Paciﬁc. A barrier layer in the SSCS has a seasonal variation, and its depth has a positive correlation with temperature in the mixed layer. In addition, the barrier layer often exists in summer and autumn. The structure of the barrier layer in the SSCS is signiﬁcantly modulated by the wind ﬁeld, as well as by development of the mixed layer. In summer, relatively fresh water in the upper layer in the SSCS piles up in the southeast SCS because of the combined action of southeastward Ekman transport and downwelling in the eastern SCS. The high temperature water at the bottom of the mixed layer remains in a thermally uniform layer after separating from the mixed layer. The deepest barrier layer lies in the southeastern SCS, at about 30 m depth. The location of the thickest barrier layer almost overlaps the SCS Warm Water, which suggests that the heat barrier eﬀect may stimulate the development of the SCS Warm Water. 5.1.3.2 Warm Waters of the South China Sea The monsoon has a close relationship to the upper water with high temperature formed in spring in the SCS. Both enhanced radiation in spring and convergence/downwelling induced by the anticyclone circulation in the upper layer can stimulate the appearance of warm water, over 29.5 ◦ C oﬀ the west Philippines. It will disappear after the onset of the summer monsoon. The fading of the warm pool during the monsoon in the SCS is a main feature associated with monsoon reversal. However, the warm water mentioned above is formed in a short time and is located in a small sea area.

5.2 Nutrient Budgets in the Seawaters of the South China Sea

541

Warm water of the SCS can be deﬁned as SST over 28 ◦ C. With the threedimensional structure concerned, the seasonal cycle in the SCS Warm Water can be classiﬁed in four stages, namely development, maintenance, weakness, and disappearance stages. Local surface heating usually causes development. In the maintenance stage turbulent mixing and the Ekman eﬀect maintain the SCS Warm Water, whereas SST decreases rapidly in autumn and winter. Then the weakness and disappearance are caused by entrainment of cold water at the bottom of the mixed layer. The upper layer circulation over spring to winter also favors the development and maintenance of the SCS Warm Water. Variability in the SCS Warm Water feeds back to the air-sea interaction on a broad scale. A coupled teleconnection exists among the SCS Warm Water, ENSO, and the subtropical high in the western equatorial Paciﬁc. On the other hand, its variations coincide with SST in the warm pool of the western equatorial Paciﬁc and the central and eastern equatorial Paciﬁc. 5.1.3.3 Ventilated Thermocline in the Northern South China Sea in Winter The thermocline was ventilated in the NSCS, which was accompanied by the occurrence of potential vorticity with a speciﬁc circulation pattern. The thermocline ventilation is a seasonal phenomenon in the SCS actually. The potential vorticity in winter has a high value center and its distribution looks like a thin and ﬂat ellipsoid. Around the edge of this center, a horizontal circulation movement can be tracked. Water subducted into a thermocline from the mixed layer moves southward in the path of the cyclone along the edge of a seasonal potential vorticity pool. Research shows that signals in terms of the monthly temperature increment seem to come from the intrusion of the Kuroshio into the SCS. These show that seasonal variation in the SCS can deeply eﬀect the thermocline. Thus, a study of the SCS upper circulation should include thermocline dynamics. Moreover, research into the dynamics of the thermocline and the mixed layer can help interpret the seasonal variation in the SCS circulation.

5.2 Nutrient Budgets in the Seawaters of the South China Sea The surface Kuroshio water that ﬂows into the SCS is salty, but relatively nutrient poor, and what raised questions is that the surface water that ﬂows out of the SCS is relatively fresh, but slightly nutrient rich. A net seawater ﬂux is not at all informative in itself as it may actually be in the opposite direction to the salt and nutrient ﬂuxes. It is clear that both the inward and outward ﬂuxes through the Bashi Channel must be investigated. Further, the SCS is a major ﬁshing ground, with the ﬁshing industry providing the livelihood for millions of ﬁshermen in the surrounding territories. Like the East China

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Sea (ECS), the SCS receives large amounts of nutrients in the form of river discharge to support productivity. Chen et al. (2001) reported the nutrient budgets results shown as follows. 5.2.1 Nitrogen Budgets The nitrogen ﬂuxes are more complicated since acid deposition (QP ) adds nitrogen to the SCS. Further, nitrogen ﬁxation by cyanobacteria utilizes N2 , thereby contributing additional nitrogen to the SCS. On the other hand, denitriﬁcation converts nitrate to NH3 , N2 O, and N2 , which degas at the air-sea interface. No data regarding either nitrogen ﬁxation or denitriﬁcation in the SCS is available. The unknown net air-sea exchange is taken to be QN2 , which is calculated by balancing the nitrogen budget:

QRi × YRi + QSSW × YSSW + QKSW × SKSW + QDW × YDW + QP + QRe = QTSW × YTSW + QSCSW × YSCSW + QIW × YIW + QB + QN2 (5.1) where Q is the water ﬂux in weight unit, and the subscripts Ri, P, SSW, MSW, KSW, DW, E, TSW, SCSW, and IW denote river input, precipitation, Sunda Shelf water, Mindoro Strait water, Kuroshio surface water, deep water, evaporation, Taiwan Strait water, SCS surface water, and intermediate water, respectively. Y denotes the concentration of N in each medium, QP is the aerosol input, and QRe and QB are the amounts of N released from and deposited to the sediments. All ﬂuxes are average values for the wet and the dry seasons, each 6 months long. The relevant ﬂuxes are given in Table 5.1 and Fig. 5.4. Here it was assumed that the imbalances are due to the diﬀerences in the rates of nitrogen ﬁxation and denitriﬁcation. Accordingly, the N budget suggests net denitriﬁcation in both wet and dry seasons, totaling 114 (±129)×109 mol/yr or 0.03 (±0.04) mol N/(m2 ·yr). This (Fig. 5.4), although with a large uncertainty, is

QRi (24) QSSW (44) QMSW(32)

QB

QTSW (6) QP (8) QN2 (33) QSCSW (45) QKSW (106) Q Re

QIW (743)

QB _ QRe =14

QDW (716)

NO3- , 109 mol (a)

QRi (73) QSSW (58) QMSW (7)

QP (23) QN2 (80) QB

QRe

QTSW (16) QSCSW (358) QKSW (288) QIW (668)

QB_ QRe =42

QDW (716)

NO3- , 109 mol (b)

Fig. 5.4. N budgets in the (a) dry and (b) wet seasons (Chen et al., 2001) (With permission from Elsevier’s Copyright Clearance Center)

5.2 Nutrient Budgets in the Seawaters of the South China Sea

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smaller than that recorded in the ECS (0.1 mol N/(m2 ·yr)) or the North Sea (0.05∼1.46 mol N/(m2 ·yr)) (Chen and Wang, 1999). It is assumed that denitriﬁcation in the SCS occurs only on the shelf, then the net rate is (0.11±0.12) mol N/(m2 ·yr). As with phosphate, a large amount of nitrate both in the SCSW and in the intermediate water is exported through the Bashi Channel. Table 5.1. Concentrations (μmol/kg) and ﬂuxes (×109 mol for 6 months) of N for the South China Sea (Chen et al., 2001) (With permission from Elsevier’s Copyright Clearance Center)

Ri QP SSW MSW TSW SCSW KSW IW DW B-Re Net denitriﬁcation

Wet season Concentration Flux 60 73 (±15) − 23 2 58 2 7 2 −16 1.6 −358 (±47) 1.4 288 (±26) 23 −668 (±134) 37 716 (±143) − −42 (±15) − −80 (±91)

Dry season Concentration Flux 60 24 (±15) − 8 0.9 −44 2 32 2 −6 1.6 −45 (±4) 1.4 106 (±10) 23 −743 (±148) 37 716 (±143) − −14 (±4) − −33 (±92)

Postive and negative numbers represent inﬂow and outﬂow, respectively

5.2.2 Phosphorus Budgets The P balance is represented as: QRi × YRi + QSSW × YSSW + QKSW × SKSW + QDW × YDW + QP + QRe = QTSW × YTSW + QSCSW × YSCSW + QIW × YIW + QB

(5.2)

The relevant ﬂuxes are given in Table 5.2 and Fig. 5.5. There is an imbalance of 1.1×109 mol in the wet season, i.e., an accumulation of P. The imbalance of P in the dry season is −1.1×109 mol, i.e., an export of P. These ﬁgures and the zero annual imbalances all fall within the uncertainties of the calculation, lending support to the internal consistency of the water ﬂuxes. It is noteworthy that the SCS exports P through the Bashi Channel because the import of P by the relatively P-poor KSW is not suﬃcient to compensate for the export by the relatively P-rich SCSW. In addition, the outﬂowing intermediate water, with essentially all of its P supplied by the deep water, turns northward after leaving the Bashi Channel. Subsequently, this water upwelling onto the ECS shelf provides P to the shelf water which is short of P relative to

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5 Biogeochemical Processes of the South China Sea

N (Chen and Huang, 1995; Chen, 1996). Rivers play a relatively insigniﬁcant role in sustaining high productivity in either the ECS or the SCS (Chen and Wang, 1999). The net P deposition in the sediment alone is greater than the riverine input. Supplies from the Sulu Sea and from upwelling provide the P needed for new production. Table 5.2. Concentrations (μmol/kg) and ﬂuxes (×109 mol for 6 months) of P for the South China Sea (Chen et al., 2001) (With permission from Elsevier’s Copyright Clearance Center)

Ri SSW MSW TSW SCSW KSW IW DW B-Re

Wet season Concentration Flux 0.7 0.9 (±0.2) 0.2 5.8 0.2 0.7 0.2 −1.6 0.12 −26.8 (±3.5) 0.1 20.6 (±1.9) 1.67 −48.5 (±9.6) 2.71 52.5 (±10.5) − −2.6 (±0.8)

Dry season Concentration Flux 0.7 0.3 (±0.06) 0.12 −5.8 0.2 3.2 0.2 −0.6 0.12 −3.4 (±0.3) 0.1 −7.6 (±0.7) 1.67 −54.0 (±10.8) 2.71 52.5 (±10.5) − −0.9 (±0.3)

Positive and negative numbers represent inﬂow and outﬂow, respectively

QRi (0.9) QSSW(5.8) QMSW(0.7)

QTSW (1.6)

QP (0.3) QB

QSCSW (26.8) QRi (0.3) QKSW(20.6) QSSW (5.8) QMSW (3.2) QIW (48.5)

QRe

QB _ QRe=26

QDW (52.5)

PO43- , 109 mol (a)

QTSW (0.6)

QP (0.1) QB

QSCSW (3.4) QKSW (7.6)

QRe

QIW (54.0) QB _ QRe=0.9

QDW (52.5)

PO43- , 109 mol (b)

Fig. 5.5. P budgets in the (a) wet and (b) dry seasons (Chen et al., 2001) (With permission from Elsevier’s Copyright Clearance Center)

5.2.3 Silicate Budgets The Si balance can also be represented by equation (5.2). The relevant ﬂuxes are given in Table 5.3 and Fig. 5.6. Unlike P and N, far less Si is transported out of the SCS by the intermediate water than is brought in through deepwater inﬂux. This is because siliceous particles do not dissolve as readily as

5.2 Nutrient Budgets in the Seawaters of the South China Sea

545

organic particles decompose. This means that a larger portion of Si reaches the sediment. The net burial of biogenic silica is taken as the residual in the box model and is 155 and 132×109 mol for 6 months for the wet and dry seasons, respectively. These ﬂuxes are large compared with P and N, but are small compared with the total sediment deposit. This is because only the biogenic siliceous particles produced from dissolved Si in the SCS were taken into account. Aeolian dust particles and suspended particles, such as sand, transported by rivers, were not included. Note the SCSW transports about 2×1012 mol of Si a year out of the SCS vs. 0.56×1012 mol input from KSW. The net export is an order of magnitude more than the riverine input. Much of this excess comes from the upwelling of deep waters (Chen et al., 2001). Table 5.3. Concentrations (μmol/kg) and ﬂuxes (×109 mol for 6 months) of Si for the South China Sea (Chen et al., 2001) (With permission from Elsevier’s Copyright Clearance Center)

Ri QP SSW MSW TSW SCSW KSW IW DW B-Re

Wet season Concentration Flux 121 147 (±29) − 0.5 4 116 4 13 5 −40 8 −1,790 (±232) 2 412 (±37) 60 −1,742 (±157) 140 2,710 (±542) − −155 (±47)

Dry season Concentration Flux 121 49 (±10) − 0.2 5 −242 4 65 5 −16 8 −227 (±20) 2 151 (±14) 60 −1,940 (±175) 140 2,710 (±542) − −132 (±40)

Positive and negative numbers represent inﬂow and outﬂow, respectively

(16) 6

Fig. 5.6. Si budgets in the (a) dry and (b) wet seasons (Chen et al., 2001) (With permission from Elsevier’s Copyright Clearance Center)

546

5 Biogeochemical Processes of the South China Sea

5.3 Biogeochemical Processes in the Pearl River Estuary Coastal eutrophication is a recently recognized phenomenon, and scientiﬁc investigation of this human disturbance has progressed for a few decades. Recent research results indicated that the eutrophication had an inﬂuence on transportation and transformation of contaminants in an aquatic environment, which includes potential key factors, such as biomass dissolving functions, staying period in waters, sediment embedding, and structures of the food net. 5.3.1 Nutrients in Coastal Waters of the Pearl River Estuary The Pearl River Estuary (PRE) is located in southern China (Fig. 5.7, Huang et al., 2003), and it is the area of the Pearl River ﬂowing into the South China Sea. The PRE supported a large population of marine organisms and contributed signiﬁcantly to the ﬁsheries in the SCS, and it is also the main receiving water of land-based pollutants of the SCS. In the last decades, massive economic growth and urban development in the region have led to an excessive release of waste into the estuary. The environmental quality of the PRE is vital for future sustainable development in the region. The PRE is a complicated system with respect to geography and hydrodynamics. The coastal waters in the estuary are profoundly inﬂuenced by three water regimes: the Pearl River discharge, oceanic water from the SCS and coastal water from the South China coastal current. There are many diﬀerent sources of nutrients in the estuary regions, mainly including runoﬀ of rivers, estuarine land-based N 23 00'

Guangzhou

0

20

40 km

Humen Jiaomen

1

Hongqimen 2 Hengmen

Zhuhai Macau

Pearl River Eatuary (Lingdingyang)

22 30'

Shenzhen

3 4 zhen en Sh 6

5 7

y Ba

Hong Kong

d Islan tau 8 Lan 9 10

22 00' 113 30'

South China Sea 114 00' E

Fig. 5.7. Location of sampling sites in the Pearl River Estuary (Huang et al., 2003) (With permission from Elsevier’s Copyright Clearance Center)

5.3 Biogeochemical Processes in the Pearl River Estuary

547

discharge, coastal land-based pollutants coming with the South China coastal current and atmospheric deposition. Therefore, the dynamical variation and biogeochemical processes of nutrients in the estuary are very complicated. Huang et al. (2003) reported the nutrient results in coastal waters of the PRE shown as follows. 5.3.1.1 Variations of Dissolved Inorganic Nitrogen Fig. 5.8a shows the concentration variation of DIN from north to south (Huang et al., 2003). It can be seen that in spite of ﬂood tide or ebb tide, the conce-

DIN (mg/L)

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Fig. 5.8. (a) Spatial-temporal variation of DIN; (b) Vertical variation of DIN in ebb tide; (c) Vertical variation of DIN in ﬂood tide; (d) Form variation of DIN in ebb tide; (e) Form variation of DIN in ﬂood tide (Huang et al., 2003) (With permission from Elsevier’s Copyright Clearance Center)

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5 Biogeochemical Processes of the South China Sea

ntration of DIN in the northern sea area was generally higher than that in the southern part; the situation was more evident in the course of the ebb tide. The results indicated that DIN mainly came from the runoﬀ of the Pearl River. The concentration of DIN was generally over 0.30 mg/L. In most areas, the concentration was over 0.50 mg/L. The highest concentration was more than 1.6 mg/L. It can also be seen that the concentrations of DIN in the ebb tide and ﬂood tide were very diﬀerent; the variation range of concentration can reach up to two times in some areas. So in those estuarine areas aﬀected seriously by tide, the analysis results from one time sampling were always short of being representative. Figs. 5.8b and c show the variation in DIN concentrations in a vertical direction in the ebb and ﬂood tides respectively. The results indicated that both in the northern area and southern area of the estuary, the variation was not apparent in a vertical direction, while the concentration in the surface layer was higher than that in the bottom layer in the middle area. The reason is that the middle area is inﬂuenced by the invasion of the briny wedge, the freshwaters from runoﬀ mainly concentrate on the surface, and the seawater is distributed at the bottom, while the water system in the southern and northern areas is well mixed in a vertical direction. This also indicated that the runoﬀ of the Pearl River was the chief source of the DIN from another point of view. Figs. 5.8d and e indicate the form variation of DIN in the ebb and ﬂood tides respectively. It can be seen that NO3 -N was the main form in most sea areas except station 6 both in the ebb and ﬂood tides, and the concentration of NO3 -N was generally over 60% of DIN, it was even over 90% in some stations. The second one was NH4 -N, and the third one was NO2 -N (below 10%). However, in the area near station 6, NH4 -N was the main form of DIN, and NO3 -N was the next one. This indicated that this area was mainly inﬂuenced by domestic wastewater. It may be related to the inﬂuence of domestic wastewater in Shenzhen (including the submarine wastewater discharge project in the west of Shenzhen) and Hong Kong near this area. 5.3.1.2 Variation Characteristics of Phosphate The spatial-temporal variation patterns of phosphate in the ﬂood and ebb tides are shown in Fig. 5.9a (Huang et al., 2003). The results indicated that the phosphate concentration in the seawater near the area of Shenzhen Bay was the highest, which was over 0.030 mg/L. This implied that land-based pollutants near Shenzhen Bay contribute greatly to phosphate. Phosphate concentration in the other area was 0.010∼0.015 mg/L in the ebb tide, 0.007∼0.017 mg/L in the ﬂood tide. What should be emphasized was that the concentration of phosphate in the southern seaward area was obviously higher than that in the northern riverward area in the ﬂood tide, while the diﬀerence in concentrations in the northern part and southern part was not obvious in the ebb tide (except the middle area near Shenzhen Bay). This result implied

5.3 Biogeochemical Processes in the Pearl River Estuary

549

Phosphate (mg/L)

that the contributions of land-based phosphate along the PRE was great, and also the phosphate discharged from the area outside the Lingdingyang. It can also be seen that the variation in phosphate concentration in the ebb tide and ﬂood tide was obvious, but the range of variation was smaller than that of DIN. This maybe relates to decentralization of the sources of phosphate. Figs. 5.9b and c show the variation in phosphate concentrations in a vertical direction in the ebb tide and ﬂood tide respectively. This proved further that the contribution of phosphate from river runoﬀ was not large in the sea area of the PRE. 0.04

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Fig. 5.9. (a) Spatial-temporal variation of phosphate; (b) Vertical variation of phosphate in ebb tide; (c) Vertical variation of phosphate ﬂood tide; (d) Variation of N:P ratios (Huang et al., 2003) (With permission from Elsevier’s Copyright Clearance Center)

550

5 Biogeochemical Processes of the South China Sea

(1) Variation of N/P ratio In the sea area of eutrophication, N and P were often supplied plentifully. The N:P ratio was often regarded as the dominant factor, and when it was about 16 in the outer sea, and 5∼15 in coastal area, phytoplankton could attain their growth climax. The N:P ratio was always higher than 100 in the northern sea area, the highest was over 300, about 40∼100 in the middle sea area, and 30∼40 in the southern sea area. It can be concluded that DIN in the PRE was in surplus comparatively, and phosphate may be the limiting factor. Such a similar conclusion can be seen from other research results in the PRE. (2) Input amounts of DIN and phosphate from the runoﬀ of four river channels The runoﬀ of the main PRE mainly comes from four river channels: the Humen, Jiaomen, Hongqimen, and Hengmen. According to the results of monitoring and analysis in 1996 (Li, 2000), Table 5.4 shows the speciﬁc input amounts of DIN and phosphate from the four river channels. Thus it can be seen that DIN and phosphate transported by the runoﬀ of the Pearl River mainly came from the Humen. The amount of DIN from the runoﬀ was very large, while the amount of phosphate was small comparatively. Table 5.4. Input amounts of DIN and phosphate from four river channels (t/yr) (Huang et al., 2003) (With permission from Elsevier’s Copyright Clearance Center) Item DIN Phosphate

Humen 77, 208 1, 808

Jiaomen 44, 629 1, 525

Hongqimen 18, 553 651

Hengmen 38, 903 1, 004

(3) Source for nutrients The nutrients in the PRE mainly came from domestic sewage, industrial wastewater, agriculture fertilizer, and marine culture. The Pearl River delta is densely populated with a population density of more than 1.0×104 persons per km, there are a lot of small towns located around the big or middle cities, and all kinds of industries as well. In the delta (not including Hong Kong and Macau), domestic sewage amounted to more than 6.0×108 m3 /yr, and industrial wastewater was about 9.0×108 m3 /yr. More than 70% of domestic wastewater in the Pearl River delta was not treated, and discharged into river or coastal waters directly. The industrial wastewater discharged from Macau amounted to 0.45×108 m3 /yr and domestic sewage was about 0.46×108 m3 /yr. All the wastewater was being discharged directly into the PRE from more than 10 discharge openings without any disposal. In Hong Kong, there was about 7.8×108 m3 of wastewater per year (Wen et al., 1999). Of the total volume, 10% was treated by a biochemical treatment method, 40% was disposed incompletely, and the remaining 50% was discharged without any treatment, of which 80% was domestic sewage, and was discharged into the coastal water directly.

5.3 Biogeochemical Processes in the Pearl River Estuary

551

In the middle and low reaches of the Pearl River, fertilizer was widely used in agriculture, and this was also a main source of nutrients for the estuary. This kind of non-point pollution of surface water was also one of the main sources of nutrients in other countries. There was a lot of marine culture in the PRE. The surplus feedstuﬀ and the excretion of ﬁsh also contributed much to nutrients. (4) Yearly variation trend of nutrients Based on data from the government gazette, unpublished survey results and published literature, Figs. 5.10a and b show the yearly variation trend of DIN and phosphate in the estuary over the decade from 1992 to 2001. It can be seen that both DIN and phosphate had a decreasing trend, although there was some ﬂuctuating, and a decreasing trend in phosphate was more obvious. However, the concentrations of nutrients still stayed at a high level. 0.8

DIN (mg/L)

(a) 0.6 0.4 0.2 0.0

1992 19931994 1995 1996 1997 1998 1999 2000 2001

Phosphate (mg/L)

0.04 0.03

(b)

0.02 0.01 0.00 1992 19931994 19951996 1997 1998 1999 2000 2001 Year

Fig. 5.10. (a) Yearly variation of DIN and (b) yearly variation of phosphate (Huang et al., 2003) (With permission from Elsevier’s Copyright Clearance Center)

In the past two decades, the PRE has received a high loading of nutrients. The results indicated that DIN was mainly from the runoﬀ of four river channels, at the same time the land-based pollutants near Shenzhen Bay also contributed to it markedly. The concentration of DIN exhibited a tendency to reduce from north to south, and the variation was great during one tidal period. The main form of DIN in most of the sea area was NO3 -N, but was NH4 -N in the area near Shenzhen Bay. The concentration of DIN in the PRE was generally over 0.30 mg/L, while it was over 0.50 mg/L in most areas. The contribution of phosphate from runoﬀ was not evident, but land-based sources from the area near Shenzhen Bay or along the estuary were obvious, and other land-based sources outside the estuary brought by coastal current

552

5 Biogeochemical Processes of the South China Sea

and ﬂood tide current were also the main contributions. The concentration of phosphate varied less evidently in one tidal period. The concentration of phosphate was about 0.015 mg/L in most sea areas except the area near Shenzhen Bay, where it exceeded 0.030 mg/L. The N:P ratio in this estuary was comparatively high, and it was higher in the northern area than in the southern area. Its highest value was over 300, and the lowest value was over 30. The concentration of chlorophyll a was about 0.8∼7.8 mg/m3 . Turbidity and phosphate may be the main two limiting factors for algal bloom in the estuary (Huang et al., 2003). 5.3.1.3 Silicon Speciation and Release in Pearl River Estuary Surface Sediments (1) Distribution of silicon speciation in the initial sediments The silicon released from the surface sediments of the PRE was exposed in vitro under diﬀerent environmental conditions of agitation time, pH value, and salinity. Then the consequent speciation distribution of silicon in the leached sediments was also analyzed through comparison with the silicon speciation in the initial (unleached) sediments, and the sampling stations are shown in Fig. 5.11 (Qin and Weng, 2006).

Guangzhou

Pearl

N 23

ng Dongjia

River

River

22 50'

Pearl

22 40'

Shenzhen

22 30' River

22 20'

Hong Kong N

D3 D2

Zhuhai

ry Estua

Macao

22 10'

D1

China

21 50'

0

113 20'

16

So uth Ch ina Se a

22 32 km

113 40'

114

114 20' E

Fig. 5.11. Map of the Pearl River Estuary showing the sampling locations (D1∼D3) (Qin and Weng, 2006) (With permission from Elsevier’s Copyright Clearance Center)

As shown in Table 5.5, it can be concluded that the main pool of noncrystalline silicon is bound to Fe-Mn oxides, accounting for 63.8%∼66.2%; the second is bound to organic matter, involving 27.1%∼30.1%; silicon bound

5.3 Biogeochemical Processes in the Pearl River Estuary

553

Table 5.5. Speciation distribution of silicon in the initial sediments of the Pearl River Estuary (μg Si/g) (Qin and Weng, 2006) (With permission from Elsevier’s Copyright Clearance Center) Samples D1 D2 D3 Mean

Exchangeable Content Wt.% 17.91±0.54 0.865 18.18±0.55 0.922 20.57±0.62 1.058 18.89±0.57 0.947

Bound to carbonate Content Wt.% 117.1±3.5 5.66 112.4±3.4 5.70 97.58±2.9 5.02 109.0±3.3 5.46

Bound to Fe-Mn oxides Content Wt.% 1,371±41.1 66.23 1,282±38.5 65.01 1,241±37.2 63.82 1,298±38.9 65.05

Bound to organics Content Wt.% 563.9±11.3 27.14 559.3±11.2 28.36 585.4±11.7 30.10 569.5±11.4 28.54

to carbonates is ∼5%; exchangeable silicon is the least fraction, ranging from 17.91 to 20.57 μg/g, which accounts for ∼1% of the non-crystalline silicon (Qin and Weng, 2006). Because the latter two fractions are sensitive to the environment and are readily leached and utilized by siliceous phytoplankton, the sum of them may be operationally deﬁned as bioavailable silicon based on the sequential extraction procedure. With respect to the three sediments, the speciation distribution of silicon is quite similar in samples D1 and D2 due to close composition of clay minerals, whereas sample D3, retrieved close to Lantau Island, Hong Kong, is ﬁne sand, exhibiting relatively low bioavailability. In addition, aﬀected likely by anthropogenic activities, silicon bound to organics in sample D3 is relatively high. Silicon speciation distribution in sediments is hence related to the deposit environments and sediment composition. (2) Variation of silicon speciation in the leached sediments Exchangeable: Fig. 5.12a1 exhibited the distribution of the exchangeable fraction in the sediments liberated during diﬀerent agitation time. It tended to decrease slowly and ﬁnally approached a dynamic balance. Because the leaching degree varied with diﬀerent agitation time, about 13.6 mg/g of the exchangeable fraction remained after leaching for 24 h (for instance of sample D1), which accounts for ∼76% of the exchangeable fraction in the initial sediments. Fig. 5.12a2 showed the exchangeable distribution in the sediments leached with pH-adjusted de-ionized water. The exchangeable fraction varied slightly and averaged 15.37 μg/g, which accounted for ∼85% of that in the initial sediments. The sediments leached at lower pH values had relatively high exchangeable silicon because silicon bounded to carbonates was activated at the attack of H+ and was partially transformed into a loosely adsorbed or exchangeable form. Fig. 5.12a3 demonstrated the exchangeable distribution in the sediments leached with seawater of diﬀerent salinities. The exchangeable fraction varied slightly with an average of 12.2 μg/g, which approximated to ∼68.1% of the initial sediments. In contrast with the other leaching conditions, enhanced salinity not only inhibited silicon release, but also led to the unfavorable extraction of the exchangeable pool from the leached sediments. The exchangeable fraction left in sample D3 varied more obviously and regularly than that of samples D1 and D2, which suggested that the eﬀect of salinity exhibited an obvious selectivity on sediment composition, or rather,

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5 Biogeochemical Processes of the South China Sea

the eﬀect of salinity was more apparent in dominating silicon release from sandy sediments. Bound to carbonates: As shown in Fig. 5.12b1, silicon bound to remaining carbonates varied slightly from 104.3 to 110.9 μg/g (sample D1) in the sediments leached with de-ionized water for diﬀerent agitation time. Also, this fraction in samples D1 and D2 was quite a bit higher than that of Sample D3 due to the diﬀerent lithology. Its distribution in the sediments leached with diﬀerent pH values was exhibited in Fig. 5.12b2. A portion of this pool was dissolved by 5∼13 μg/g in the case of pH <5, accounting for 5%∼10%, whereas from pH 5 to pH 9 its content remained relatively constant. Bound to Fe-Mn oxides: Silicon bound to Fe-Mn oxides was predominant in the sediments either unleached or leached under the three conditions. As shown in Fig. 5.12c, it varied relatively little in the leached sediments. Bound to organics: It could be found from Fig. 5.12d that the distribution of silicon bound to organics in the liberated sediments was not aﬀected in comparison with that in the initial sediments.

Fig. 5.12. Speciation distribution of silicon in the sediment samples D1, D2, and D3 from the Pearl River Estuary after leaching under diﬀerent conditions of agitation time (a1∼d1), pH (a2∼d2), and salinity (a3∼d3). (a) Exchangeable; (b) Bound to carbonates; (c) Bound to Fe-Mn oxides; (d) Bound to organics (Qin and Weng, 2006) (With permission from Elsevier’s Copyright Clearance Center)

5.3 Biogeochemical Processes in the Pearl River Estuary

555

According to the above description and analyses, the exchangeable silicon was not remarkably reduced after the sediments were leached under the conditions of the three parameters. This pool might remain from 66% to 87% of the initial sediments. These results suggested that a loosely bound fraction extracted with de-ionized water was more important to silicon bioavailability and its liberation did not exert a prevailing inﬂuence on the subsequent exchangeable extraction. Sequential extraction methods were operationally deﬁned, and there was a lack of standardization between laboratories because of the variety of extraction protocols used by the geochemical community. Besides the selectivity of chemical reagents, silicon concentration liberated was theoretically dependent on the partition coeﬃcient between the leaching solution and the solid phase, and a higher ratio of leaching solution versus sediment sample may readily improve the extracted quantity. De-ionized water was more favorable for leaching exchangeable silicon than 1 mol/L MgCl2 solution (pH 7.0), as mimiced somewhat in the reversible process of silicic acid ﬂocculation predominated by the seawater electrolyte eﬀect and conﬁrmed, in part, by the inhibition of salinity to silicon liberation. Commonly, if silicon mobility was only operationally evaluated by the exchangeable and bound to carbonates based on the sequential extraction method generally utilized, the quantiﬁcation might be obviously underestimated in natural aquatic environments, since loosely bound silicon represents a higher activity and should not be neglected. Phase transformations were prevalent especially among amorphous or poor crystalline phases in deltaic sediments in response to ambient alterations, and equilibration with these phases was one possible control on pore-water Si(OH)4 concentration, though these processes might take place on diﬀerent time scales. In the presence of the concentration gradient, exchange was evident and continuous between interstitial water and the overlying water column. Although these gradual phase transformations were slow, they might also be essential for Si recycling in world oceans in the long run (Qin and Weng, 2006). 5.3.2 Carbon in the Pearl River Estuary Carbonaceous particles, usually classiﬁed into two categories—organic carbon (OC) and elemental carbon (EC)—were the most important constituents of the ﬁne fraction of particulate material (PM). OC represented a large variety of organic compounds that could be classiﬁed into general compound classes such as aliphatic, aromatic compounds, acids, etc. EC was actually a mixture of graphite-like particles and light-absorbing organic matter. Moreover, the surface of EC particles contained numerous adsorption sites that were capable of enhancing catalytic processes. As the result of its catalytic properties, EC might intervene in some important chemical reactions involving atmospheric sulfur dioxide (SO2 ), nitrogen oxides (NOx ), ozone (O3 ), and other gaseous compounds. Carbonaceous species in particles also played an important role in global climate change by aﬀecting radiative forcing. The Pearl River Delta

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5 Biogeochemical Processes of the South China Sea

Region (PRDR), including Hong Kong and Macao, was one of the fastest growing regions in China. The PRDR was one of the ﬁrst regions of China to experience massive industrialization and urbanization in recent years. 5.3.2.1 Carbon in Atmosphere and Seawaters

EC concentration (mg/m3)

OC concentration (mg/m3)

The statistics for OC and EC concentrations at eight sampling sites in summer are shown in Table 5.6, and spatial and seasonal distributions of OC/EC are illustrated in Fig. 5.13 (Cao et al., 2004). In brief, eight sites were selected in the four cities including three sites in Hong Kong (The Hong Kong Polytechnic University (PU), Baptist University (BU), Hok Tusi (HT)), three sites in Guangzhou (Zhongshan University (ZU), Huangpu (HP), Longgui (LG)), one site in Shenzhen (Luohu (LH)), and one site in Zhuhai (Xiangzhou (XZ)). EC, which has a chemical structure similar to impure graphite, originates primarily from direct emissions of particles, predominantly during combustion. OC, from primary anthropogenic sources and from formation by chemical reactions in the atmosphere, rendered the higher concentrations of OC than EC at eight sampling sites. The average OC and EC concentrations in PM2.5 (d 2.5 μm) for the PRDR were (9.2±6.5) and (4.1±2.7) μg/m3 for the summer period, respectively. The majority of carbonaceous aerosols was in the PM2.5 fraction. Among these eight sites, average OC and EC at HP (Guangzhou) had the highest concentrations, which is attributable to the mixed contribution of high traﬃc ﬂows and industrial emissions. The lowest OC and EC concentrations were found at HT (the background site in Hong Kong) because it is located upwind of the anthropogenic emission sources. In contrast, PM2.5 PM2.5

PM10 Winter Summer

10

1

10

1 HK

GZ

SZ

ZH

HK

GZ

SZ

ZH

Fig. 5.13. Box plots of OC and EC concentrations (μg/m ) in PM2.5 and PM10 samples during winter and summer in four cities in PRDR (Cao et al., 2004) (With permission from Elsevier’s Copyright Clearance Center) 3

5.3 Biogeochemical Processes in the Pearl River Estuary

557

OC and EC concentrations at the LG background site in Guangzhou were 5∼8 times those at HT. The OC and EC concentrations at an urban site (ZU) in Guangzhou were around 1∼3 times higher than those at other urban sites in the three cities (BU, LH, and XZ), implying carbonaceous pollution in Guangzhou. Table 5.6. Average of the concentrations of OC and EC at eight sites during the summer in PRDR, China (Cao et al., 2004) (With permission from Elsevier’s Copyright Clearance Center) City

Hong Kong

Guangzhou Shenzhen Zhuhai PRDR

Site PU BU HT AVER HP ZU LG AVER LH XZ AVER

OC (μg/m3 ) PM2.5 PM10 6.3±2.3 7.4±2.9 5.6±0.8 6.7±1.1 3.4±0.3 4.1±0.6 5.3±2.1 6.3±2.5 20.0±2.8 28.5±5.2 13.1±3.9 17.8±6.9 17.0±10.6 24.7±18.9 15.8±6.4 22.2±11.2 7.6±4.9 10.4±6.5 5.4±3.4 6.9±4.3 9.2±6.5 12.3±10.1

EC (μg/m3 ) PM2.5 PM10 3.9±1.6 4.7±2.1 3.2±0.4 3.9±0.5 0.7±0.1 1.1±0.1 3.2±2.6 3.9±2.9 7.9±1.1 10.5±1.9 4.6±1.2 5.9±1.8 6.5±2.5 8.8±4.0 5.9±2.1 7.8±3.1 4.2±3.1 5.0±3.5 1.9±0.9 2.5±1.0 4.1±2.7 5.2±3.4

OC/EC PM2.5 PM10 1.7 1.6 1.8 1.7 4.7 3.8 1.9 1.8 2.5 2.7 2.8 3.0 2.6 2.8 2.7 2.9 1.8 2.1 2.9 2.7 2.5 2.5

Values represent average ± standard deviation

OC concentrations in PM2.5 and PM10 ranked in the order Hong Kong< Zhuhai<Shenzhen
558

5 Biogeochemical Processes of the South China Sea

summer concentrations might be due to enhanced thermal convection during summer, which could be attributed to the inﬂuence of the Asian monsoon. The southwesterly summer monsoon brought cleaner oceanic aerosols from the oceans (South China Sea and tropical Paciﬁc), while the northeasterly winter monsoon brought polluted air masses from mainland China. In addition, both the increased emissions and the occurrence of stable atmospheric conditions during winter might also lead to the high carbon concentrations. 5.3.2.2 High P CO2 in Surface Water of the Pearl River Estuary and Its Maintaining Mechanism Recent observations have shown that river-estuary systems may release a signiﬁcant amount of CO2 into the atmosphere in addition to the commonly recognized ﬂuvial export of inorganic/organic matter. Very high partial pressure of CO2 (P CO2 , 2,000∼8,000 μatm in the mainstream, >10,000 μatm in some tributaries) and signiﬁcant outgassing of CO2 have been reported for the Amazon River. The combined eﬀects of pelagic and benthic respiration, photodegradation, and the mixing of seawater and acidic river water were insuﬃcient to sustain the high P CO2 values. Zhai et al. (2005) reported the results of the high P CO2 in surface water of PRE and its maintaining mechanism shown as follows. (1) Spatial distribution of P CO2 in surface waters Typical distributions of temperature, salinity, pH, DIC, P CO2 , and DO across the estuary are presented in Fig. 5.14 for the June 1, 2001 survey (Zhai et al., 2005). Upstream from the Humen outlet, P CO2 ranged 3,380∼4,785 μatm, while DO was 76∼143 μmol O2 /kg. DIC was ∼1,100 μmol/kg and pH was 7.0∼7.2. Temperature was relatively high (28.1∼28.5 ◦ C). Between the Humen outlet and shortly before the Hongqimen outlet, i.e., in the inner Lingdingyang Bay, temperature and P CO2 rapidly declined while pH and DO substantially increased within a narrow salinity range (0.25‰∼1.3‰). In this salinity range, DIC apparently increased. However, when we normalized DIC with salinity to S =5‰ (nDIC in Fig. 5.14b) according to Friis et al. (2003), nDIC declined and followed the similar trend of P CO2 . Note that the dramatic drop in nDIC in the low salinity range as compared to P CO2 may be a reﬂection of multiple inputs of DIC from the branches of the Pearl River. Around the outlets of Hongqimen and Hengmen, both P CO2 and DO ﬂuctuated moderately, while DIC leveled oﬀ to <1,200 μmol/kg, due to the additional freshwater input from these two outlets. Downstream from these two outlets, where the inﬂuence of fresh water starts to diminish, a salinity front was observable of salinity 3‰∼20‰. Within the front, P CO2 dropped rapidly, from >1,000 to <500 μatm, and pH rose from ∼7.8 to >8.0. DIC also rose to ∼1,600 μmol/kg, while DO did not change signiﬁcantly. In the outer Lingdingyang Bay, P CO2 remained at <500 μatm, while DO was ∼210 μmol O2 /kg, DIC was 1,600∼1,700 μmol/kg, pH was 8.2∼8.4, and the

5.3 Biogeochemical Processes in the Pearl River Estuary

559

temperature reached a relatively low value of 27.0∼27.5 ◦ C. The strong inverse relationship between P CO2 and DO observed in the low salinity region (S <4‰) became less signiﬁcant in the outer Lingdingyang Bay (Fig. 5.14c).

Fig. 5.14. Surface water P CO2 and relevant parameters measured during underway pumping experiments in the Pearl River Estuary on 1st June 2001 (Zhai et al., 2005). Vertical lines represent the major outlets. In (b), nDIC values are DIC being normalized to S =5‰ according to Friis et al. (2003). In (c), P CO2 data of >3,700 μatm have a higher uncertainty than the rest of our reported data due to the LI-6,252 reading being out of range and we had to change its sensitivity. See text for details. (a) Temperature and salinity; (b) DIC, salinity normalized DIC (nDIC) and pH; (c) P CO2 and DO (With permission from Elsevier’s Copyright Clearance Center)

Further summarized in Fig. 5.15 are P CO2 and DO data from both the 2000 and 2001 cruises, plotted here against salinity. As a general feature described above, the P CO2 level rapidly declined along with the increase in salinity, reaching <1,000 μatm at salinity 2‰∼3‰, while the DO level rose along with the salinity increase, reaching near-saturation in saline waters. A signiﬁcant diﬀerence in P CO2 and DO distribution can be observed, however, for the survey on May 23, where lower DO and higher P CO2 signals were observed further downstream as compared with what we observed during other surveys (Fig. 5.15). This is most likely related to heavy precipitations immediately before the survey, which transported much more oxygen-consuming organic pollutants from the upper stream of the Humen into inner Lingdingyang Bay and resulted in the high P CO2 and low DO signals further downstream as

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5 Biogeochemical Processes of the South China Sea

compared to typical hydrological conditions. Indeed, the total precipitation in the upstream watershed was ∼120 mm during 15∼18 May 2001 and ∼21 mm on 21∼22 May 2001 according to the weather records from the Guangzhou Weather Observatory.

5000 13-14 Jul 2000 15 Jul 2000 17 Jul 2000 01 Jun 2001 23 May 2001

( m atm)

4000 3000

(a)

2000 1000

air

DO (mmol O2/kg)

0 300 DO saturation

250 200

(b)

DO

150 100 50 0

0

2

4

6

8 10 Salinity (%o)

20

30

Fig. 5.15. Distributions of P CO2 and DO concentration in the estuary (Zhai et al., 2005). In (a), P CO2 data of >3,700 μatm of 2001 surveys have a higher uncertainty than the rest of our reported data due to the LI-6,252 reading being out of range and we had to change its sensitivity. See text for details. (a) Surface water P CO2 vs. salinity; (b) DO concentration vs. salinity. The upper DO saturation line in panel b is of spring while the lower one is of summer (With permission from Elsevier’s Copyright Clearance Center)

(2) The upper limit of P CO2 in the PRE and its implication From plots in Fig. 5.16, an anoxic condition (i.e., DO=0) and an estimation of an upper limit of P CO2 to be ∼7,000 μatm were calculated. The agreement of this prediction with ﬁeld measurements provides more conﬁdence that the P CO2 in this system (upstream region, near zero salinity) is indeed controlled by DO consumption through organic carbon oxidation. P CO2 values of >10,000 μatm were reported in the Amazon River (Richey et al., 2002). The source processes, however, are diﬀerent from the Pearl River case. In the Amazon River, much more CO2 * was produced than predicted by a theoretical ΔCO2 :(−ΔO2 ) of 0.83. The “extra” CO2 was presumably produced anaerobically, mostly from methanogenesis (Richey et al., 1988). In upper es-

5.3 Biogeochemical Processes in the Pearl River Estuary

561

Excess CO2 (mmol CO2 /kg)

210 180 150 120 90

5500~7000 matm

tuarine waters of some southeastern U.S. rivers, very high P CO2 values of >6,000 μatm have been shown to be the result of humic-rich, acidic river water, in situ respiration, and DIC supplied from the anaerobic degradation of organic matter from adjacent salt marshes. In the water column of the PRE, DO never reached zero, thus anaerobic processes are not expected to be major (Zhai et al., 2005).

01 Jun 2001 23 May 2001 22 May 2001 17 Jul 2000 15 Jul 2000 14 Jul 2000

60 30 0 -30 -50

DO=0

0

50 100 150 AOU (mmol O2/kg)

200

250

Fig. 5.16. Excess CO2 vs. AOU in the Pearl River Estuary (Zhai et al., 2005). Excess CO2 data of >120 μmol/kg of 2001 surveys have a higher uncertainty than the rest of our reported data due to the LI-6,252 reading being out of range and we had to change its sensitivity. See text for details. The regression lines, ﬁtted by minimizing the sum of the squares of the y-oﬀsets, are: 14 July 2000: y=0.752x −14.09, R 2 =0.982; 15 July 2000: y=0.706x +14.74, R 2 =0.967; 17 July 2000: y=0.624x +8.05, R2 =0.985; 22 May 2001: y=0.527x +14.28, R 2 =0.966 (at the Humen station and upstream); 01 June 2001: y=0.778x +8.27, R 2 =0.984. The two dashed lines show the upper limit (slope=0.90) and the lower limit (slope=0.62) for stoichiometric ratio of aerobic biological respiration in the environment with abundance of HCO− 3 . The grey region on right side represents anoxic condition, i.e., DO=0 (With permission from Elsevier’s Copyright Clearance Center)

5.3.2.3 Non-Aromatic Hydrocarbons in Surface Sediments near the Pearl River Estuary The Pearl River, which is the largest river in southern China, ﬂows across the PRDR and supplies a great deal of natural and anthropogenic substances to the South China Sea annually. The northern South China Sea (NSCS) records various substantial anthropogenic inﬂuences from the PRD economic region. The characteristics of the sediments collected from the NSCS are summarized in Table 5.7 (Gao et al., 2007). The TOC and TN contents range from 0.26% to 1.69% and from 0.030% to 0.231% respectively, and are distributed in the same trends. The atomic ratio of TOC to TN (hereafter abbreviated as C/N ratio) is one of the parameters frequently used to identify changes in the proportions of

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5 Biogeochemical Processes of the South China Sea

Table 5.7. Some bulk geochemical parameters of the sediments sampled (Gao et al., 2007) (With permission from Elsevier’s Copyright Clearance Center) Range Average

TOC (%) 0.26∼1.68 0.88

TN (%) 0.030∼0.231 0.103

EOM1 (μg/g) 18.70∼38.58 27.61

EOM2 (%) 0.20∼0.72 0.425

NAH (μg/g) 3.43∼8.46 6.04

C/N 7.6∼20.4 11.65

sedimentary organic matter originating from marine autogenic and terrigenous plants. Compared to cellulose- and lignin-rich vascular land plants, marine algae are rich in proteins. Therefore, fresh marine autogenic organic matter shows C/N ratios typically between 5 and 8, whereas organic matter from terrestrial sources is characterized by C/N ratios of 20 and greater. The C/N ratios indicate that organic matter in the NSCS surface sediments is derived from both marine autogenic and terrestrial sources in diﬀerent proportions (Table 5.7). The C/N ratio is much higher at site E3 than at the other three sites, indicating qualitatively that there is more organic matter originating from terrestrial higher plants in the sediment at site E3, and more sedimentary marine autogenic algal organic matter in the surface sediments at the other three sites. The C/N ratios are within the range of those in the Pearl River Estuary and its adjacent shelf except for E3, which shows a much higher C/N ratio (Hu et al., 2006). 5.3.2.4 Suspended Particulate Matter from the Pearl River Delta to the Coastal Ocean Terrestrial organic carbon is transported to oceanic environments mainly via rivers and the atmosphere. Transport of organic carbon via rivers is a signiﬁcant process in global carbon cycling. Deltaic and coastal regions play a key role in global carbon cycling because they are the main repositories of terrestrial organic matter delivered by rivers. In general, DOC can increase the solubility and thus the mobility of organic contaminants, while POC can act as a carrier to convey organic chemicals along rivers. Therefore, quantifying riverine input of TOC is a crucial step toward a better understanding of global carbon cycling processes and assessments of organic pollution. (1) Temporal and spatial distributions The concentrations of SPM, POC, and DOC in the water from March 2005 to February 2006 in riverine runoﬀ collected from the eight outlets, which included Humen (HM), Jiaomen (JM), Hongqilimen (HQ), Hengmen (HE), Modaomen (MD), Jitimen (JT), Hutiaomen (HT), and Yamen (YM), varied within a big range (Ni et al., 2008). The temporal variations in POC and SPM concentrations were similar, reﬂecting the close association between POC and terrestrial particulate matter. Higher POC and SPM concentrations occurred during the wet season (from April to September), rather than in the dry season (from October to March), with POC ranging from 1.0 to 3.8 mg/L and SPM ranging from 70.1 to 247 mg/L. This indicates that the total riverine discharge

5.3 Biogeochemical Processes in the Pearl River Estuary

563

amount is the major factor regulating SPM levels. During the wet season, large amounts of terrestrial particulate matter are transported to rivers by rainfall, resulting in higher POC concentrations in the wet season than in the dry season. In contrast to the results for the POC, higher DOC concentrations with a range of 1.9∼3.5 mg/L were found in the dry season rather than in the wet season, especially in December 2005. The high level of DOC in the dry season should not be ascribed solely to minor dilution eﬀects, which nevertheless was one of the factors contributing to increased TOC contents in riverine runoﬀ. River water was much clearer in the dry season than in the wet season because the lack of rainfall in the dry season minimizes soil erosion and sediment resuspension. As a result, contributions of photosynthesized POC to TOC in fresh water should not be neglected, as this may result in increased levels of DOC via partitioning of organic carbon between the water and SPM. In addition, high nutrient concentrations also elevated the levels of TOC (both of DOC and POC) in riverine runoﬀ. Resuspension of river sediment that might contain high organic carbon content derived from algae and microbes might also contribute to DOC. An exception was at Yamen (YM) where higher DOC (2.7 mg/L) appeared in August. The reason for the diﬀerence may be because the runoﬀ source at YM is not exactly the same as those found at the other outlets. The Tanjiang River is another important discharge source for YM in addition to the Xijiang River. The discharge sources for the other seven outlets are mainly the three main tributaries of the PRD, i.e., the Beijiang River, Xijiang River, and Dongjiang River. Higher POC/TOC ratios (50%∼80%) were observed during the wet season rather than in the dry season (17%∼49%), suggesting that the contribution of POC to TOC was greater than that of DOC during the wet season. During the wet season, the PRD region experienced a prolonged ﬂood period (from June to August, 2005). Consequently, large amounts of land particulate organic matter (POM) associated with terrestrial particulate materials were carried into the rivers by rainwater, resulting in a common mechanism that delivered SPM and POC to the aquatic system. Fluvial organic carbon was also the dominant contributor to POM, especially during ﬂood events. Spatially, the concentrations of POC and DOC varied slightly at diﬀerent outlets for the same sampling month. From the four western outlets to the four eastern outlets, the POC/TOC ratios decreased slightly for all sampling time frames. The lowest average POC/TOC (28%) occurred at HM in March 2005. These results imply that POC and DOC were the major contributors to TOC at the western and eastern outlets, respectively. Two reasons may explain the diﬀerence. First, the main sources of water ﬂow to the eastern outlets are the Beijiang River and Dongjiang River that run through several densely populated regions such as Guangzhou, Dongguan, and Zhongshan. Consequently large amounts of industrial eﬄuent, agricultural runoﬀ, and domestic sewage are discharged into the surface water, resulting in increased levels of DOC in riverine runoﬀ. As well, other natural sources may contribute DOC to river-

564

5 Biogeochemical Processes of the South China Sea

ine runoﬀ. High levels of organic nutrients and bacteria were thought to be associated with municipal wastewater discharge leading to more rapidly increasing DOC concentrations compared to POC. Second, the Xijiang River, the main source of water ﬂow to the four western outlets, transfers 86% of total suspended sediment within the PRD, as compared to 3.3% and 9.2% of total sediment transported by the Dongjiang River and Beijiang River, respectively. Therefore, POC that is closely associated with terrestrial particulate matter becomes the main source of TOC to the western outlets. (2) Sources and variation of organic carbon The C/N ratio can be a sensitive indicator of organic matter sources, with signiﬁcantly higher C/N ratios (>12) in terrestrial organic matter than in phytoplankton. Average C/N ratios of SPM from some diverse rivers such as the lower Amazon, Danube, rivers of the US and St. Lawrence are 11.1, 13.1, 11.2, and 12.1, respectively (Onstad et al., 2000), demonstrating the importance of the contribution from soil organic matter. The annual mean C/N values ranged between 7.2 and 9.3, which were considerably lower than those for vascular plant tissues and debris (20∼400) and close to or slightly higher than those for plankton or bacteria biomass (5∼8). Individually, the C/N ratios ranged from 3.8 to 15.4, with the highest values found in June 2005 at all outlets (10∼16) and the lowest values occurring typically during the dry season, i.e., January for HE (5.9), March for HT (3.8) and YM (5.6), October for JT (5.5), November for HM (5.2) and JM (4.6), and December for HQ (4.5) and MD (4.2). Apparently, POM within the PRD originates from soil as heavy rainfall during the wet season carries large amounts of POM to rivers and streams, resulting in higher C/N ratios. Conversely, lower rainfall during the dry season transports less soil organic matter into the aquatic systems. Also, contributions of POM from plankton or bacteria biomass may be more important in the dry season than in the wet season. The POC concentrations dropped to 1.5%∼2.5% or lower as the SPM concentrations rose to more than 50 mg/L. Similarly, the percentage weight of soil organic carbon ranged between 0.1% and 4.0% with an average of 1.9% throughout the drainage area of the PRD (Gan et al., 2003). This provides further evidence that soil organic material may be the dominant source of POC in riverine runoﬀ of the PRD. (3) Riverine ﬂuxes of SPM and TOC The monthly SPM ﬂuxes from each runoﬀ outlet were dependent upon the monthly variation in rainfall events (Fig. 5.17, Ni et al., 2008). The maximum monthly SPM ﬂuxes from all outlets occurred in June 2005 with a range of 4.3×105 ∼6.4×106 t. This resulted from extremely high levels of precipitation in June 2005 that caused the worst ﬂood event in the past 100 years in most areas of southern China. The ﬂood crests from upstream of the PRD, speciﬁcally the Xijiang River and Beijiang River, carried large amounts of terrestrial particles into the coastal ocean via the eight runoﬀ outlets. On the other hand, the lowest monthly SPM ﬂuxes from most outlets were obtained

5.3 Biogeochemical Processes in the Pearl River Estuary

565

TOC flux ( 103 t)

SPM flux ( 104 t)

mainly during the dry season (from October to March), especially in January 2006. The exceptions were the ﬂuxes from HM to JT where the lowest SPM ﬂuxes occurred in November 2005 and February 2006, respectively. Fig. 5.17a also illustrates that the monthly SPM ﬂuxes were largely proportional to the monthly water discharge amounts. The total annual ﬂux of SPM from the PRD to the coastal ocean was approximately 2.5×107 t during 2005∼2006.

(a) 700

SPM=-22+1.64 runoff R2=0.78

400 100 (b) 90

TOC=-1.78+0.39 runoff 2 R =0.92

40

0

100 200 River runoff ( 108 m3)

Fig. 5.17. Correlations of runoﬀ amounts with (a) monthly SPM ﬂuxes and (b) monthly TOC ﬂuxes (Ni et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

The monthly TOC ﬂuxes also showed a positive correlation with discharge amounts (Fig. 5.17b). The highest monthly TOC ﬂuxes from all runoﬀ outlets occurred in June and the lowest ﬂuxes were found in January, except for HM and MD, where the lowest monthly TOC ﬂuxes were found in February. The highly linear relationship between the monthly TOC ﬂuxes and river discharge amounts can be described by (5.3) FTOC = −1.64 + 0.39Fwater (R2 = 0.92) Equation (5.3) may be used to estimate monthly TOC ﬂux for a speciﬁc runoﬀ outlet if the monthly water ﬂux is available. In addition, the annual TOC ﬂux from the PRD to the coastal ocean was approximately 9.2×105 t during 2005∼2006. (4) Hydrological control on SPM, POC, and DOC concentrations The temporal distribution of SPM concentrations correlated nicely with the variability of discharge amounts for the entire sampling period (Fig. 5.18a). For example, the highest SPM concentration occurred in June 2005 when the water discharge peaked. These results suggest that soil discharge in the PRD may be gauged by riverine ﬂows. In addition, both POC and DOC concentrations also varied synchronously with water discharge amounts (Figs.

566

5 Biogeochemical Processes of the South China Sea

(a) 800

SPM POC DOC Runoff

100 50

800

(b)

3 2

400 1 0 (c)

3 2

400

1

0

0

M a Apr. 20 r 0 M . 20 5 ay 05 Ju 20 n. 05 Ju 200 Au l. 20 5 g 0 Se . 20 5 pt 05 Oc . 20 t 0 No . 20 5 v 0 De . 20 5 c. 05 Ja 20 n. 05 Fe 200 b. 6 20 06

800

POC (mg/L)

0

DOC (mg/L)

8

3

Runoff ( 10 m )

400

150

SPM (mg/L)

5.18b and c), but with nearly opposite proﬁles. POC concentrations increased with increasing discharge amount and peaked in June 2005 (2.6 mg/L) when the highest level of water discharge occurred. The exception is that the second highest POC concentration occurred in March 2005 when the discharge amount was not the second highest. On the other hand, DOC concentrations increased with decreasing discharge amounts and peaked in December 2005 (2.4 mg/L) when the river runoﬀ was the lowest. As river runoﬀ increases, increasing amounts of particulates are expected to be transported from drainage areas to rivers, leading to enhanced mobility of terrestrial POC stock (Figs. 5.18a and b). As a result, higher water discharge should lead to higher SPM and POC concentrations. On the other hand, DOC concentrations are lower with higher discharge amounts because of dilution eﬀects and the tendency of organic matter to adsorb onto particulates.

Fig. 5.18. Correlations of runoﬀ amounts with (a) SPM concentrations (mg/L), (b) POC concentrations (mg/L), and (c) DOC concentrations (mg/L) (Ni et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

The temporal trends of SPM, POC, and DOC concentrations diﬀered greatly from those of the Oubangui River, which is a major tributary of the Congo River (Coynel et al., 2005); this likely results from the diﬀerent geographical settings of the two regions. The center of the Congo Basin is covered by evergreen forests and surrounded by savannah. Swamp forests are typical in the central depression, and most forests are ﬂooded during the wet season, but dry out during the dry season (Coynel et al., 2005). On the contrary, there is almost no forest within the watershed of the PRD. Mountainous regions and hills account for approximately 94.5% of the total area.

5.3 Biogeochemical Processes in the Pearl River Estuary

567

(5) Mass inventory of TOC in the PRE and northern SCS To estimate the inventories of TOC in the PRE and northern SCS, a procedure reported previously (Chen et al., 2006) was adopted, i.e, the inventory (I, in tons) is calculated by (5.4) I= (kCi Ai dρ) where C i is the TOC concentration in sediment at site i (ng/g) (Chen et al., 2006), Ai is the area of the compartment represented by site i (km2 ), d is the thickness of the sediment sampled (cm), ρ is the average density of the dry sediment particles (g/cm3 ), and k is a conversion factor. With an assumed sediment density of 1.5 g/cm3 and a sediment thickness of 5 cm, the inventories were 4,312 and 9,577 t for TOC within the PRE and the northern SCS, respectively. Sedimentary rates in marine environments varied from 0.1 to 0.3 cm/yr (Tolosa et al., 1996; J¨ onsson et al., 2003), and a median value of 0.2 cm/yr was used. Consequently, the inventories estimated for the northern SCS represent approximately 25 years of deposition. A sedimentary rate of 1 cm/yr is a reasonable estimate for the PRE from the results of a previous study (Chen et al., 2006). Therefore, the mass inventories in the PRE represent deposits from the last 5 years. If the same time interval (e.g., within one year) was used to estimate the inventories in the PRE and northern SCS sediments and the vertical concentration ﬂuctuation in sediments was neglected, the average annual inventories of TOC in the PRE and the northern SCS were 862 and 383 t, respectively. Compared with the riverine ﬂux of TOC (920,000 t; Table 5.8) from the PRD, the amounts of organic carbon deposited in the PRE and northern SCS are negligible (Ni et al., 2008). Table 5.8. Comparison of ﬂuxes (F ) and yields for several major river systems in the world (Ni et al., 2008) (With permission from Elsevier’s Copyright Clearance Center) Areaa Amazon 6.4 Congo 3.7 Mississippi 3.0 Yangtze 1.8 Orinoco 1.1 Yellow 0.8 Pearl River Deltaf 0.5

Flowb 6, 600 1, 325 580 995 1, 135 48 280

a

b

d

Drainage area, in 106 km2 ; Yield, in g C/(m2 ·yr)

c FSPM 600∼1,150 31.7 500 480 107 900 25

c c c FPOC FDOC FTOC 6.1 37.6 43.8 2.0 12.4 14.4 1.1 1.9 3.0 6.0 2.1 8.1 1.7 5.0 6.7 6.3 0.1 6.4 0.5 0.4 0.9

Annual discharge amount, in 109 m3 /yr;

c

d d d YPOC YDOC YTOC 1.0 5.9 6.9 0.5 3.3 3.8 0.4 0.6 1.0 3.3 1.2 4.5 1.5 4.5 6.0 8.4 0.1 8.5 1.2 0.8 2.0

Flux, in million t/yr;

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5 Biogeochemical Processes of the South China Sea

5.3.2.5 Composition and Sources of Organic Matter in Surface Sediments of Daya Bay Daya Bay is a subtropical drowned valley bay of the northern SCS located on the eastern coast of Guangdong Province, southern China. It is one of a series of large embayments along the southern coast of China and covers an area of ∼600 km2 with a width of about 20 km and a north-south length of about 30 km. The water depth in Daya Bay averages 11 m, and generally ranges from 6 to 16 m, with the western part being deeper than the eastern part. Although there are no large rivers discharging into the bay, there are over 10 seasonal streams that lead to the bay within a short distance of each other along the coast. The average water temperature in the coastal region of Daya Bay is 29.3 ◦ C in summer (July to September) and 17.3 ◦ C in winter (December to February); the weighted mean wind velocity is 4.8 m/s in summer and 4.6 m/s in winter; the daily average salinity is about 28.0‰; the tidal current in Daya Bay is dominated by an irregular semidiurnal tide with an average tidal day of about 24.7 h; the mean tidal range is 1.01 m and the maximum is 2.57 m; the current velocities are lower than 0.5 m/s and currents form a clockwise gyre system in spring and autumn. The surface water has an average resident time of about 3.2 days. The annual mean precipitation in Daya Bay is 1,827 mm; dry and rainy seasons can be distinguished. In recent decades, the mean sedimentary rate of the bay is 0.9 cm/yr. (1) Grain size The sand content in ﬁve of the nine sampling sites was <5% (Table 5.9, Gao et al., 2008), indicating that clay and silt are the main components of the surface sediments in Daya Bay. There was clearly higher clay content in the northern part of the bay than in the southern part. The relatively high and stable amount of clay in the northern part of the bay probably indicates that the hydraulic conditions in this area are relatively weak. Table 5.9. The bulk geochemical parameters of the sediments sampled (Gao et al., 2008) (With permission from Elsevier’s Copyright Clearance Center) Site 1 2 3 4 5 6 7 8 9

Clay (%) 4.3 5.7 5.8 15.6 28.9 34.6 31.7 36.8 34.5

Silt (%) 93.8 86.9 89.1 73.5 55.6 65.3 66.3 61.8 63.4

Sites 1∼9 are located in Daya Bay

Sand (%) 1.9 7.4 5.1 10.9 15.5 0.1 2.0 1.4 2.1

TOC (%) 0.86 1.08 1.40 0.87 1.06 1.60 1.42 1.53 1.46

TN (%) 0.105 0.118 0.166 0.096 0.109 0.180 0.138 0.206 0.196

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5.3 Biogeochemical Processes in the Pearl River Estuary

569

(2) TOC and TN TOC content varied in the range of 0.86%∼1.60% of the dry sediment weight with an average of 1.25% (Table 5.9, Gao et al., 2008). The highest and lowest TOC values were recorded at the sites located in the most extensive aquacultural region in the northwest part of Daya Bay–Aotou Cove and the western nearshore part of the bay mouth, respectively. The total nitrogen content varied in the range of 0.096%∼0.206% of the dry sediment weight with an average of 0.146% (Table 5.9). In the surface sediments of Daya Bay, the average TOC content is higher than that of the Yangtze River and PRE and their adjacent shelves; the average TN content is higher than that of the Yangtze River surrounding area and is comparable to that of the PRE and their adjacent shelves. The percentages of TOC and TN are signiﬁcantly correlated (r =0.911). Generally, the hydraulic conditions of an estuary with signiﬁcant fresh water discharge are stronger than those of a semi-closed bay like Daya Bay; this makes it more diﬃcult for ﬁne suspended particles to be deposited in the sediments of an estuary. Perhaps for this reason, the grain sizes of Daya Bay sediments are ﬁner than those of the Yangtze River and PRE obtained by analyzing the correlations between TOC and the percentages of diﬀerent grain size fractions. As shown in Table 5.9, a positive correlation exists between TOC and the ﬁnest fraction of the sediment, and negative correlations exist between TOC and the other two coarser fractions of the sediment. Similar results can be achieved by analyzing the correlations between TN and the percentages of diﬀerent grain size fractions. Besides the inﬂuence of sediment grain size composition, the diﬀerence in organic supply as well as variation in sedimentary inorganic materials (dilution eﬀect) from one area to another can also lead to the ﬂuctuation of TOC and TN concentrations. Although inﬂuenced by extensive early diagenesis, the atomic ratio of C/N is one of the parameters frequently used to identify changes in the proportions of sedimentary organic matter originating from marine autogenic and terrigenous inputs. Compared to cellulose- and lignin-rich vascular land plants, marine algae are rich in proteins. Therefore, fresh marine autogenic organic matter shows C/N ratios typically between 5 and 8, whereas organic matter from terrestrial sources is characterized by C/N ratios of 20 or greater. The C/N ratios in this study varied in a narrow range of 8.7∼12.0 with an average of 10.2, indicating a weak spatial diﬀerence (Table 5.9). Most of the C/N ratios were slightly higher than the typical high end of marine autogenic organic matter (5∼8), suggesting a predominance of marine-derived organic matter in Daya Bay. Hydrodynamic sorting of sediment by grain size can aﬀect the C/N ratios. In general, the C/N ratios of organic matter in ﬁne-sized sediments are lower than those in coarse sediments, partially because ﬁne sediment fractions contain larger proportions of clay minerals, which have both large surface areas and negative surface charges and therefore adsorb ammonia well. No correlation is found between C/N ratio and the diﬀerent components of sediment, indicating the weak inﬂuence of sediment grain size on C/N ratios. Combined

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with the narrow variation range of C/N ratios, it seems that organic matter in the surface sediments of Daya Bay is from similar non-point sources. The evidence of C/N ratios indicates that the organic matter at all sampling sites is derived from both marine autogenic and terrestrial origins in diﬀerent proportions. The marine autogenic input seems to be the major origin of organic matter. And organic matter in sediments from diﬀerent areas of Daya Bay has experienced diﬀerent degrees of degradation (Gao et al., 2008).

5.4 Biogenic Elements in the Northern South China Sea Dissolved organic carbon (DOC) is the second largest pool (685 Gt C) of carbon in the ocean, the same size as the inorganic carbon in the atmosphere. The pool of particulate organic carbon (POC), although small, is active in exchanging with other phases of oceanic carbon. 5.4.1 Carbon in the Northern South China Sea Previous studies have emphasized that both DOC and POC play important roles in biogeochemical processes and cycles of carbon and other bio-reactive elements in open oceans and marginal seas. 5.4.1.1 Dissolved Organic Carbon and Particulate Organic Carbon Distributions of DOC in the northern South China Sea (NSCS) were shown in Fig. 5.19 (Huang et al., 2007). DOC concentrations in the surface layer were generally higher in shelf and upper slope waters than in deep-basin waters (water depth 41,000 m) for all seasons. The concentrations ranged from 70 to 85 μmol/L in the mixed layer and decreased generally with depth in deep-basin waters. Also, DOC distributions were rather uniform ((43±3) μmol/L) at depths greater than 1,000 m. DOC concentrations were higher in summer than in fall and winter, particularly for coastal and shelf waters. Concentrations could be elevated up to 132 μmol/L in the Zhujiang River plume during summer with the highest river discharge. The concentration of DOC in the Zhujiang River Estuary was up to 180 μmol/L in late spring and fall seasons (Callahan et al., 2004), and the concentration may be higher in summer. DOC input from biological production may not be neglected because China coastal waters had a moderate concentration of Chl a (0.45∼1.98 mg/m3 ). The overall distributions of DOC in the NSCS were not much diﬀerent from those, respectively, in the coastal and deeper zones of temperate, subtropical and tropical marginal seas, such as the Mediterranean Sea (Doval et al., 1999), the East China Sea (Hung et al., 2003), and the Arabian Sea (Hansell and Peltzer, 1998). The horizontal and vertical distributions of POC resembled the pattern of DOC distributions. The concentrations of POC ranged from 1.6 to 4.5

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Fig. 5.19. Contours of DOC distributions in three transects of fall (left panel), summer (middle panel), and winter (right panel) seasons (Huang et al., 2007). H: 21.8◦ N, 113.5◦ E; G: 22.1◦ N, 115.9◦ E; A: 22.2◦ N, 118.0◦ E; 1: 22.5◦ N, 119.7◦ E; I: 20.2◦ N, 114.2◦ E; C: 18.0◦ N, 115.7◦ E; J: 15.7◦ N, 116.8◦ E; D: 18.1◦ N, 117.6◦ E; F: 20.3◦ N, 118.5◦ E; M1: 21.5◦ N, 119.5◦ E (With permission from Elsevier’s Copyright Clearance Center)

μmol/L in the surface and decreased monotonously with depth down to about (1.1±0.2) μmol/L below a depth of 2,000 m for most studied areas without considering the shelf zone (Fig. 5.20). The surface POC concentrations were also higher than those in Kuroshio (2.5 μmol/L). POC concentrations were always higher in the shelf zone than in the deep basin and higher in summer than in fall and winter seasons. Particularly high concentrations were found in the Zhujiang River plume (up to 13 μmol/L) and other shelf areas (up to 15 μmol/L) during the summer season, which were likely derived from terrestrial inputs. Similar POC concentration ranges were also found in other deep marginal seas (Gismondi et al., 2002). 5.4.1.2 CO2 TCO2 remains fairly constant from ∼1,887 to ∼1,898 μmol/kg throughout the period of this study except for the highest value of ∼1,925 μmol/kg in winter (Fig. 5.21b). The TCO2 maximum thus coincides with the deepening of the mixed-layer (Fig. 5.21a) and the winter cooling. Such a close association suggests that the strong northeast monsoon not only causes vigorous downward mixing to deepen the mixed-layer but also brings in more cold, TCO2 -rich subsurface waters from the deep. The drawdown of NTCO2 in spring-summer (Fig. 5.21c) manifests the biological uptake in the mixed-layer.

572

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Fig. 5.20. Vertical proﬁles of POC and PN (particulate nitrogen) in each station during the summer season (CR688). The sampling sites are the same as in Fig. 5.19 (Huang et al., 2007) (With permission from Elsevier’s Copyright Clearance Center)

Fig. 5.21d shows the seasonal variability of titration alkalinity (TA) measured in the mixed-layer at SEATS site (South-East Asian Time-series Study, 18◦ 15 N, 115◦ 35 E). TA values vary from ∼2,190 to ∼2,220 μmol/kg with higher values in January, July, and September and lower values in March, April, and November. The observed seasonal oscillation of TA is thus controlled primarily by the same factors aﬀecting salinity. A similar relationship between TA and salinity is also documented at BATS (Bermuda Atlantic Time-series Study, 31◦ 50 N, 64◦ 10 W) and HOT (Hawaii Ocean Time-series, 22◦ 45 N, 158◦ 00 W) sites (Bates et al., 1998). However, as revealed in Fig. 5.21e, NTA remains variable throughout the year, suggesting the potential

5.4 Biogenic Elements in the Northern South China Sea

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contribution of the formation and/or dissolution of CaCO3 and the consumption and/or remineralization of nitrate to the observed NTA variability. A strong seasonality with an amplitude of ∼35 μatm during the annual cycle is found in the mixed-layer P CO2 at the SEATS site (Fig. 5.21f). The P CO2 increases progressively in spring to summer with the maximum in July (∼382 μatm) followed by a decrease in fall to winter with the minimum (∼347 μatm) in January. The seasonal changes of P CO2 are closely in phase with temperature (Fig. 5.21f) but are inversely correlated with NTCO2 (Fig. 5.21c). These relationships suggest that the variation in P CO2 at SEATS is controlled ultimately by seawater temperature change, and will be examined in detail in the following sections.

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Fig. 5.21. Seasonal ﬂuctuations of (a) the mixed-layer depth, (b) TCO2 , (c) NTCO2 (salinity-normalized total CO2 , solid line) and potential temperature (dashed line), (d) TA (solid line) and salinity (dashed line), (e) NTA (salinity-normalized total TA), and (f) P CO2 (solid line) and potential temperature (dashed line) in the mixedlayer at the SEATS site from March 2002 to April 2003. The data points represent the averaged values of all samples measured in the mixed-layer from each cruise. The vertical bars indicate the analytical errors (±1σ) (Chou et al., 2005) (With permission from Terrestrial, Atmospheric and Oceanic Sciences (TAO))

Fig. 5.22 shows the seasonal variability of ΔP CO2 at the SEATS site, in which the maximum positive and negative ΔP CO2 occur in summer (∼+20

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μatm; July 2002) and in winter (∼–15 μatm; January 2003), respectively. In other words, there has been an eﬄux of CO2 from the SCS in summer and fall but an inﬂux of CO2 into the surface SCS in winter. The seasonal variability also reveals that the ΔP CO2 increases gradually from spring to summer, and then decreases progressively from fall to winter. Values of the calculated sea-to-air CO2 ﬂux at the SEATS site using diﬀerent formulations are listed in Table 5.10. As seen, they vary remarkably in diﬀerent seasons and depend on which formula is used in calculations, ranging from (0.00±0.01) to (−0.02±0.05) (spring), (+0.03±0.01) to (+0.23±0.06) (summer), (+0.18±0.10) to (+0.45±0.25) (fall), and (−0.62±0.20) to (−1.42±0.46) (winter) mol C/(m2 ·yr). It further shows that the annual sea-to-air CO2 ﬂux is dominated by the inﬂux of CO2 from the atmosphere onto the surface waters in winter. It is worth pointing out that despite the bigger positive ΔP CO2 in summer, the modest wind speed results in a relatively small CO2 eﬄux. In contrast, the high wind speed in winter gives rise to higher CO2 inﬂux regardless of the smaller ΔP CO2 . The annual sea-to-air CO2 ﬂux at the SEATS site is estimated to be around (−0.11±0.08) to (−0.23±0.18) mol C/(m2 ·yr) during the observed period (Table 5.10).

Fig. 5.22. Seasonal variations of ΔP CO2 (surface seawater P CO2 -atmospheric P CO2 ) in the mixed layer at SEATS site from March 2002 to April 2003. The vertical bars represent the range of potential errors (Chou et al., 2005) (With permission from Terrestrial, Atmospheric and Oceanic Sciences (TAO))

Similar seasonal patterns have also been documented at BATS and HOT (Bates et al., 1998). The ﬂuxes are less than −0.7 mol C/(m2 ·yr) found at HOT or −0.3 to −0.6 mol C/(m2 ·yr) found at BATS (Bates et al., 1998). The lower values at SEATS can be attributed to the higher sea surface temperature at SEATS, which increases P CO2 in the surface water and depresses the capacity for CO2 uptake, and the general upwelling circulation in the SCS (Chao et al., 1996b; Chen et al., 2001). Finally, if the estimated annual sea-to-air CO2 ﬂux at SEATS is extrapolated to the entire SCS (3.5×106 km2 ), it could take up (4.5±3.3)∼(9.6±7.7) TgC/yr, i.e., only about (0.20±0.15)% to (0.44±0.35)% of the total CO2 uptake by the global oceans (∼2.2 PgC/yr). These values

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575

Table 5.10. Estimates of seasonal CO2 ﬂux at the SEATS site using formulations of Liss and Merlivat (1986), Tans et al. (1990), and Wanninkhof (1992). The seasonal wind speed data are calculated from monthly averaged values for 1985∼1999 from the ECMWF database (Chou et al., 2005) (With permission from Terrestrial, Atmospheric, and Oceanic Sciences (TAO)) CO2 ﬂux (F, mol C/(m2 ·yr)) and exchange coeﬃcient Wind (K, mol C/(m2 ·yr·μatm)) speed Liss and Merlivat (1986) Tans et al. (1990) Wanninkhof (1992) (m/s) K F K F K F Spring −1.9±5 3.1 0.001 0.00±0.01 0.002 0.00±0.01 0.011 −0.02±0.05 Summer 20.1±5 3.2 0.001 0.03±0.01 0.003 0.06±0.02 0.012 0.23±0.06 Fall 9.0±5 6.1 0.018 0.18±0.10 0.050 0.45±0.25 0.042 0.38±0.21 Winter −15.3±5 8.8 0.041 −0.62±0.20 0.093 −1.42±0.46 0.087 −1.33±0.43 Average −0.11±0.08 −0.23±0.18 −0.19±0.19 ΔPCO2 (μatm)

are small, considering the SCS’s share that occupies 0.97% of the total ocean area. 5.4.2 Distributions of Inorganic Nutrients in the Northern South China Sea − The horizontal distributions of the nutrients, including NO− 2 , TIN, NO3 , 2− + 3− SiO3 , NH4 , and PO4 in surface water from the northern South China − Sea in 2004 are shown in Fig. 5.23. The concentrations of NO− 2 , NO3 , and + NH4 were in the ranges of 0.02∼0.44, 0.04∼12.29, 0.17∼7.66 mol/L (Long et al., 2006). The concentrations of TIN were expressed through summing up the − + concentrations of NO− 2 , NO3 , and NH4 , ranging from 0.87 to 14.50 mol/L. Among the 3 forms of nitrogen, the sequence of the proportion from high + − 2− and to low was as follows: NO− 3 >NH4 >NO2 . The concentrations of SiO3 3− PO4 ranged from 1.49 to 6.49 and from 0.15 to 0.42 mol/L respectively. In the NSCS, the horizontal distributions of the inorganic nutrients were blocklike. However, the concentrations of inorganic nutrients in the seawater from the east coast were higher than those from the west coast generally, except that the distributions of TIN and NO− 2 presented the opposite tendencies. It was notable that the maximum nutrient concentrations were detected at stations some distance from the coastal sea area rather than on both coasts. It was possible that the coastal upwellings of Yuedong and Yuexi brought abundant nutrients.

5.5 Biogeochemical Processes in the Nansha Islands Waters The Nansha Islands waters, located in the SSCS, are under the inﬂuence of the East-Asian monsoon. It is an example of a coral reef environment that might

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3

Fig. 5.23. Distributions of nutrients of the NSCS surface seawater in Sept. to Oct., 2004 (Long et al., 2006) (With permission from Long AM)

be directly aﬀected by typhoon activity. In fact, typhoon- or strong storminduced redistribution of massive reef blocks (most >2 m3 ) is widespread on the reef ﬂats (Fig. 5.24). The Nansha Islands waters are densely scattered with hundreds of islands, reef banks, and shoals. They are bounded by the oceanic sub-basins of the SCS (the central sub-basin and the southwest sub-basin) in the north, the Nansha Trough in the oﬀshore northwest, Palawan to oﬀshore northwest Borneo in the southeast, and neighbored with the Sunda Shelf (including west Borneo and southern peninsula of Indochina) in the southwest. The bathymetry changes

5.5 Biogeochemical Processes in the Nansha Islands Waters

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from tens of meters to 3,000 m, mostly at ca. 1,000 m. The Nansha waters, underlain largely by thinned continental crust, are also named the Nansha Microcontinent Block. The basements are Paleozoic and Mesozoic. Intermediate and acid volcanic rocks of the Upper Jurassic and Cretaceous periods are widely distributed over the coastal area of south Vietnam; early to middle Permian sediment and meta-sediment rocks outcrop at the northern end of Palawan Island; late Triassic-early Jurassic fern-rich sand, siltstone, and early to late Cretaceous schist were dredged from the southwest to the Liyue Bank. The overall tectonic trend of the Nansha Microcontinent Block is in a northeast direction. According to Taylor and Hayes (1980), the Nansha Block drifted apart from South China when seaﬂoor spreading occurred to form the South China Sea during the late Oligocene to mid-Miocene periods. They proposed that an oceanic paleo-SCS existed to the southeast of the Nansha Block before the southward separation of the Nansha Microcontinent Block, but was subducted and consumed completely later. Since the mid-Miocene period, the wide region kept subsiding. During seaﬂoor spreading, the counter clockwise rotating Borneo collided with the Nansha Block, and led to northwestward thrust structures along the NE-trending Nansha Trough. However, in contrast to the subduction model, Hinz and Schluter (1985) proposed that the Nansha Trough represents a downwarping of drifted continental crust in response to the upthrust sedimentary load. In Nansha Islands waters, there developed a series of large and deep Cenozoic sedimentary basins, such as Liyue (Reed) Bank Basin, Nansha Trough Basin, Zengmu Basin, and Wan’an Basin fringing the waters and several small basins in the central part. Based on drilling data from various basins circling the Nansha region, several stratigraphic schemes have been established by various researchers. In oﬀshore Palawan, Mohammad, the deepwater area oﬀshore of Sarawak (Sarawak Basin), and in the west Wan’an Basin (Nam Con Son Basin) oﬀshore of southeast Vietnam. Within these basins, thick Cenozoic deposits developed pervasively. 5.5.1 Coral Reefs and Their Aﬀected Factors Coral reefs are outstanding examples of marine ecosystems. Reefs embody a mixture of the minute and the grand, the ephemeral and the permanent, and the simple and the complex that we associate with the natural systems of this planet. More than 80% of the Nansha coral reefs (also called the Spratly Archipelago) are composed of Lutjanus. In the total coral reef area almost 90% of the shallow coral reefs in the SCS constitute the main habitat of Lutjanus. The Nansha coral reefs comprise about 200 small coral reefs mostly located in the water area from 7◦ N to 12◦ N latitude and from 112◦ E to 118◦ E longitude, where larvae of Lutjanus are dominantly emerged in the southeast water area covering the two coral reefs named Zhubi and Yongshu, respectively, during their propagation period. Therefore, the water area between the two coral reefs is inferred to as the spawn area of Lutjanus. Fish of Lutjanus are commercially

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valuable, and some species are cultured in Southeast Asia especially in South China. Nevertheless, artiﬁcial breeding techniques for most Lutjanus species have not yet been available; therefore, the wild larvae, captured from Nansha coral reefs, remain the major source for mariculture in South China. The Yongshu Reef (9◦ 32 ∼9◦ 40 N, 112◦ 20 ∼113◦ 40 E), located in the northern part of Nansha Islands, is about 25 km long in NEE-SWW direction and 6 km wide in NW-SE direction, and covers 110.4 km2 . A closed lagoon (about 380 m long, 150 m wide, and with maximum water depth of 12 m) is situated in the center of the southwest reef ﬂat, which lies about 0∼2 m below the low tidal sea level. The area around the lagoon is referred to as the ‘small atoll’). The Yongshu Reef is composed of two large reef ﬂats distributed on both sides of the reef with a lagoon between them. The reef is separated by a deep sea basin into surrounding islands and mainland. The Yongshu Reef is characterized by a tropical marine season climate with annual average temperature of 29 ◦ C and annual precipitation rain fall of 1,998.4 mm. Surface water temperature around the Yongshu Reef is 26.6 ◦ C during January and 30.2 ◦ C in May. The surface water mass is distributed at a depth of 0∼90 m. The Yongshu Reef was developed since the Tertiary period over a basement consisting of Proterozoic-Palaeozoic metamorphic and Mesozoic magmatic rocks, similar in rock assemblages to the continental slope in the NSCS. The basement was revealed at a depth of 1,500∼1,900 m during deep drilling by petroleum exploration companies (Zhu et al., 1997). The coral reef growth was initiated during the Holocene period about 7,350 to 8,000 years ago, and unconformably overlies weathered Pleistocene coral reef limestone at a depth of 17∼18 m. Radiocarbon data of these cores indicates that the reef was continuously developing throughout the Holocene period, probably as a result of continued subsidence, combined with a rise in sea level initially. The total subsidence over the last 1,000 years is about 2∼3 m (Yu et al., 2006). The marine environmental conditions for living coral are mostly controlled by the monsoon climate. The isotopic composition of living and fossil coral can provide high-resolution records of monsoon climate history. Previous work has revealed the role of temperature on the oxygen isotopic composition of massive corals from the southern SCS (Yu et al., 2001) and the Leizhou Peninsula (Yu et al., 2005). Work on the coral record from Xisha Islands in the SCS suggested the role of wind strength, hence the intensity of the Asian winter monsoon, on the oxygen isotopic composition of Porites coral (Peng et al., 2003). Recent work on Holocene fossil coral from oﬀshore and to the east of Hainan Island has demonstrated the association between coral δ 18 O and monsoonal climate (Sun et al., 2005). In the 1970s and early 1980s, the misconception was widespread that the high productivity of coral reefs was dependent upon rich nutrient supplies. However, it was also recognized latterly that coral reefs were dependent not only upon warm ocean waters, but also upon low-nutrient environments. Smith and Kinsey (1976) observed that the very marginal reefs in the eastern tropical Paciﬁc (ETP) were exposed to no greater temperature extremes

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579

than the well developed reefs of Australia’s Great Barrier Reef, attributing the diﬀerence in reef development to nutrients from upwelling in the ETP. Muscatine and Porter (1977) concluded, in reference to reef corals with zooxanthellae, that the cardinal feature of algae invertebrate symbioses is their ability to survive in nutrient-poor environments. Many mathematical models suggested that such symbioses provide the holobiont with, quite literally, orders of magnitude of energetic advantage over non-symbiotic plants and animals, but only in environments where nutrients are scarce. These models and other biological principles were applied, along with simple illustrations of how water transparency inﬂuences the depth distributions of coral communities, to argue that changes in nutrient ﬂux not only inﬂuence shallow-water carbonate sedimentation, but can also be used to interpret drowned reefs in the geological record (Hallock, 2005). 5.5.2 Simulated Drift-Net Theory: The New Viewpoint on the High Productivity Supporting the Nansha Coral Reef Ecosystem Coral reef ﬂats have a high area of gross productivity which rivals the most productive ecosystem. They represent the marvelous architecture of life which can thrive in the very low nutrient level water of the trophic seas, where a coral reef is reputed to be the marine equivalent of an ‘oasis in the desert’ (Song, 2004). Therefore, coral reef ecosystems have long been regarded as paradoxical because their high biomass and gross primary productivity far exceeded that expected for ecosystems in tropical oligotrophic waters. Previous authors have explained the paradox by emphasizing eﬃcient recycling, and conservation and storage of nutrients within the reef ecosystem (Song, 1999). However, the fact that reefs are net explorers of nutrients and organic matter means that for sustained productivity new nutrients must be imported. Three reasons are proposed to explain why high productivity is maintained in a coral reef ecosystem: the nutrients circulate at a high rate; the seawater motion accelerates the assimilation of nutrients; the geothermic-originated upwelling currents supply extrinsic nutrient. (1) High cycle rate of nutrients Coral reef lagoons are a relatively enclosed environment, and the high-rated cycle of nutrients in a coral reef has been observed. 14 C-labled NaH14 CO3 is studied to reveal the utilization by organism in a coral reef. 50% of NaH14 CO3 is consumed overnight, 67% of NaH14 CO3 was consumed in the ﬁrst month. Therefore, it was considered that the cycle of carbon in a coral reef was 12 times per year, which was much higher than that in the general waters. The organisms of a coral reef assimilate nutrient quickly, and the nutrients were released into seawater through metabolization, excretion, and decomposition of bodies.

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(2) Accelerations of nutrients uptake by seawater motion In general, organisms could uptake the nutrients freely when the nutrients are adequate, while the growth and reproduction are restricted when the nutrients + are absent. PO3− 4 and NH4 were used to represent the nutrients. It was found that the increment in seawater ﬂow velocity positively related to the nutrient uptake by organisms. The result suggested that organisms of a coral reef could obtain the adequate nutrients by virtue of the water exchange from in- and oﬀ-reef waters. (3) Extrinsic nutrient supplied by geothermal endo-upwelling currents Comparisons of the chemical properties of interstitial waters from shallow boreholes in atolls, barrier and lagoon pinnacle reefs in French Polynesia indicate that their nutrient concentrations are similar or superior to those in Antarctic Water (A.I.W.) at 500∼1,000 m depth. By the geothermal endoupwelling process, A.I.W. enters the porous reef framework, is driven by the local geothermal gradient and emerges at the reef crest to provide nutrients to the ﬂourishing algal-coral ecosystem. The components there were quite similar to those of the middle layer (500∼1,000 m) water from the South Pole. The water with a high level of nutrients leached into the coral reef through the volcano, which maintains the relatively high level of nutrients in coral reef ecosystems. Continuing research on the reef nutrient controversy suggests that there are several paths presently converging on a solution: among them the endoupwelling model seems an adequate explanation for barrier reefs located in clear oligotrophic waters such as the Polynesian Ocean. Therefore, we must answer why high productivity was maintained in the Nansha coral reef ecosystem. (1) Cycling rates of the biogenic elements The cycling rates of the biogenic elements were calculated according to the equation as follows: h · Cwater (5.5) F where h was the depth, i.e., the depth of the catcher was deployed; C water was the content of the biogenic elements; F was the vertical ﬂuxes of the biogenic elements. The retention time of biogenic elements was calculated and tabulated in Table 5.11. According to Table 5.11, it was found that the retention time of carbon, nitrogen, and phosphor in the Zhubi Reef was longer than that in the Yongshu Reef. The retention time of total carbon, nitrogen, and phosphor in Yongshu Reef was 0.44, 0.60, and 0.37 yr, respectively, and in the Yongshu Reef was 2.61, 2.52, and 1.29 yr, respectively. The diﬀerences in the retention time of biogenic elements resulted from their topographic features because the Yongshu Reef was open and the Zhubi Reef was enclosed. It showed that the average retention time of carbon in Nansha coral reef ecosystem was 61.0 yr. Comτ=

5.5 Biogeochemical Processes in the Nansha Islands Waters

581

Table 5.11. Retention time of biogenic elements in the Nansha coral reef lagoons (Song, 1999) Biogenic elements Total

Carbon

Inorganic

Organic

Total

Nitrogen

Inorganic

Organic

Total

Phosphor

Inorganic

Organic

C F τ C F τ C F τ C F τ C F τ C F τ C F τ C F τ C F τ

Yongshu Reef Dec. 1993∼Jan. 1994 28, 947 2, 735, 100 158.8 25, 824 2, 514, 970 154.0 2, 123 220, 130 212.8 675 45, 900 220.7 283 13, 800 307.6 392 32, 100 183.3 108 11, 960 135.2 49 7, 140 102.8 59 4, 820 183.1

Yongshu Reef May 1993 28, 947 402, 300 1, 151.2 25, 824 348, 920 1, 184.2 3, 123 53, 380 936.0 675 9, 800 1, 102.5 283 1, 300 3, 483.1 392 8, 500 738.4 108 4, 120 418.6 49 1, 140 686.9 59 2, 980 316.0

Zhubi Reef Apr. 1994 28, 947 575, 700 754.2 25, 824 485, 540 797.8 2, 123 90, 160 519.6 675 13, 800 734.0 283 8, 200 517.7 392 5, 600 1, 050.8 108 3, 080 525.0 49 1, 920 382.3 59 1, 150 767.6

The units of C, F, and τ were μg/L, μg/(m2 ·d), and day, respectively

pared with the results of this study, the cycle rates of carbon in the Yongshu Reef and the Zhubi Reef were 137.6 and 22.4 times higher than those in the outer sea, which indicated that the cycles of biogenic elements in coral reef ecosystems were rather fast. (2) Budget of biogenic elements in the Nansha coral reef ecosystem The uptake contents of biogenic elements were quantitatively obtained on the basis of the follow equations (Bilger et al., 1995): For

PO3− 4 : Ca = 1.96U + 0.71

(5.6)

For

NH+ 4 : Ca = 7.14U + 3.21

(5.7)

582

5 Biogeochemical Processes of the South China Sea

where C a was the uptake of biogenic elements, with the unit of mmol/(m2 ·d), and U was the ﬂow velocity of seawater (m/s). The Yongshu Reef Lagoon area is 105.62 km2 and that of the Zhubi Reef Lagoon is 9.68 km2 . The ﬂow velocities were 18 cm/s and 34 cm/s, respectively. Therefore, the uptake ﬂuxes (F 1 ) of the biogenic elements by organisms originating from water movement were calculated and shown in Table 5.12. Song and Li (1996c) had reported the diﬀusion ﬂuxes (F 2 ) of nutrients from the sediment-water interface. The ﬂuxes that were discharged by bioactivities into seawater could be obtained through F 2 subtracted from F 1 , which was expressed as F 3 . According to Table 5.12, F 3 was the predominate fraction of F 1 , ranging from 99.02% to 99.05% for PO3− 4 and from 88.62% to 93.54% for NH+ . The high percentages of F indicated that the overwhelming majority 3 4 of nutrients bioconcentrated from water were discharged into water through vital behavior, such as metabolization, excretion, and decomposition of the bodies, while the aliquot of nutrients derived from water exchange was rather little. Table 5.12. Fluxes (F 1 , F 2 , and F 3 , mol/d) of PO3− and NH+ 4 4 in the Yongshu Reef and the Zhubi Reef (Song, 1999) PO3− 4 F1 F2 F3

Yongshu Reef Flux % 11, 960 100 1, 095 0.98 110, 865 99.02

Zhubi Reef Flux % 13, 360 100 127 0.95 13, 233 99.05

NH+ 4 Yongshu Reef Flux % 475, 290 100 30, 715 6.46 444, 575 93.54

Zhubi Reef Flux % 54, 600 100 6, 214 11.38 48, 386 88.62

F 1 : the uptakes of biogenetic elements by organisms caused by seawater movement; F 2 : the diffusion ﬂuxes of nutrients from the sediment-water interface; F 3 : the ﬂuxes those were discharged by bioactivities into seawater

Taking the whole coral reef ecosystem as an open box, the budgets of biogenic elements were presented as shown in Fig. 5.24. The budget was as follows: F 1 =F 2 +F 3 . F 1 were composed of two parts: the increment of the uptake ﬂuxes originated from seawater movement (F ma ) and the uptake ﬂuxes by organisms (F ba ), i.e., F 1 =F ma +F ba . The sum of F 2 and F 3 was also composed of two parts: the ﬂuxes involved in cycling of the ecosystem inside (F cy ) and the export ﬂuxes from the ecosystem (F ex ). The value of F ma was far higher than F ba and F cy was far higher than F ex . Consequently, with respect to the organisms in the Nansha coral reef, the inputs of nutrients were mainly derived from the seawater movement, while the metabolization, excretion, and decomposition of bodies played the predominant role in export ﬂuxes. Based on the discussion above, the simulated drift-net theory is proposed to explain why a high level of productivity was retained. The ﬁxed tropical organisms there can bioconcentrate a great amount of nutrients in the oligotrophic water coming out from the reef. Since the coral reef organisms

5.5 Biogeochemical Processes in the Nansha Islands Waters F1

Coral reef ecosystem

583

F2 F3

Fig. 5.24. The diagram form of the budget of the nutrients diﬀusion ﬂuxes (Song, 1999)

are relatively ﬁxed and cannot drift with the moving water, they can uptake many nutrients and produce nutrient enrichment of nutrient-poor waters. As a metaphorical meaning, the ﬁxed reef-building coral organisms function like a drift-net. The simulated drift-net theory is summarized in one sentence: the reason for high gross productivity supporting the coral reef ecosystem is that the special biochemical reactor made up of relatively ﬁxed tropical organisms enriches with a great amount of nutrient the oligotrophic water coming out of the reef. 5.5.3 Nitrogen in Sediments of the Nansha Islands Waters Marine sediments are diﬀerent in grain size and in content as a result of washing, erosion, transportation, and sorting in seawater with their own particular physical-chemical characteristics. 5.5.3.1 Site Description The site of the study represents one of the most typical semi-deep sea areas of the world, as a transition area from shoal water to abyssal zone. It is located in the southwestern part of the Nansha Trough bordered by the northwestern continental slope of Kalimantan in the east, Nansha submarine plateau in the west, and Nankang submerged bank in the south, and named the southwestern Nansha Trough in our study. The trough is mainly aﬀected by rivers such as the Mekong River, the paleo-Sunda River, and also short rivers in Palau Kalimantan, and thus serves as a reservoir for sediments basically made up of terrestrial, biological, volcanic, and island materials with diﬀerent origins formed in diﬀerent periods. The sediments in the trough mainly consist of ﬁne silt and clay. The mineral assemblage is dominated by montmorillonite, illite, and bioclastic calcite. The stations visited during the cruise in April and May 1999 were almost located at a depth ranging from 1,000 to 3,000 m, the overall depth of the Nansha Trough (Fig. 5.25, Zheng et al., 2008). 5.5.3.2 Sequential Extraction and Determination of Nitrogen Four forms of extractable nitrogen are obtained using the method described below: nitrogen in ion exchangeable form (IEF-N), nitrogen in weak acid extractable form (WAEF-N), nitrogen in strong alkali extractable form (SAEFN), nitrogen in strong oxidation extractable form (SOEF-N), and total nitrogen (TN).

584

5 Biogeochemical Processes of the South China Sea N 6.5

3 1

2

14

4

22 20

Tr ou gh

6 Beikang submcrgcd bank 7

5.5 Hainan

8

South China Sea

5.0

20009

1580 1000

Kalimantan

4.5

12

112.5

113

113.5

114

114.5

115

115.5 E

Nansha Islands

8

Sulu Sea gh rou aT h ns Na Kalimantan

6 4 2 106

50

500

10

Nankang submerged bank

14

10

11

12

18 16

16

15

13

Na ns ha

6.0

17

2500

5

N 24

19 20

18

110

114

118

122 E

Fig. 5.25. Map of the South China Sea and the location of the study sites (1∼20) (Zheng et al., 2008) (With permission from Springer)

5.5.3.3 Sediment Type Distribution Sediment characteristics varied with bathymetry (Fig. 5.26, Zheng et al., 2008), and also with topography. The surface sediments of stations 8 and 9 located on the upper continental slope showed a relatively large bulk fraction and low proportion of clays (<40%) which might be considered as a mixture between outer shelf and bathyal sediments. It appears to have mainly resulted from particulate transportation from the Kalimantan continental shelf receiving input from short rivers in Palau Kalimantan. Sediments of low porosity were observed in the stations near the slope of Beikang submerged bank (i.e., stations 2, 5, and 6) where sediments belong to coral clastic deposit. Sediments from all other stations were dominated by silt and clay (>90%) and can be characterized as a bathyal-abyssal sedimentary environment, where sediments in the trough area were a mixture of diﬀerent origins and periods and were mainly composed of volcanic clastics and deposits transported from paleoSunda River or the modern Kalimantan shelf. The clay fraction was greater than that in the northwestern slope of the Nansha Trough, in which place the sediments are dominated by relict deposits of particulate transportation by large rivers such as the Mekong River and the paleo-Sunda River.

5.5 Biogeochemical Processes in the Nansha Islands Waters N 6.5

18 C 14

3 1

A

4

2 5

6.0

13

17

1920 15

B

585

16 C

6

11 12 7

5.5

10 9

8

B

A

5.0 4.5

>60% 40% to 60% <40%

112.5

113

Kalimantan

113.5

114

114.5

115

115.5 E

Fig. 5.26. Distribution of clay sediments (<4 μm) (at stations 1∼20) and the regions (A, B, and C) of the southwestern Nansha Trough according to the proportions of clay sediments (%) (Zheng et al., 2008) (With permission from Springer)

5.5.3.4 Regional Distribution of Nitrogen The southwestern Nansha Trough was divided into three Regions (Regions A, B, and C) according to clay (<4 μm) content being <40%, 40%∼60%, and >60%, respectively (Fig. 5.26). Sediment characteristics varied in the regions as the result of hydrodynamics conditions, diagenesis phases, and depositional environments. Therefore, forms and distributions of nitrogen were also diﬀerent. Table 5.13 shows the distributions of nitrogen in diﬀerent forms of nitrogen in the three regions. In general, all nitrogen forms and TN were the highest in region C and the lowest in region A, indicating that the ﬁner the sediments grain, the richer the nitrogen. However, SAEF-N showed a reversed trend from the above pattern, probably due to the nature of SAEF-N, which was absorbed at the surface of the oxide of Fe or Mn that can be extracted by strong alkali. On the other hand, the Eh of ﬁne-grained sediment was relatively low, and the sediment was usually reductive. Therefore, region C with high clay content was low in SAEF-N content. Furthermore, it was worth mentioning that the increase in nitrogen in diﬀerent forms was diﬀerTable 5.13. Distributions of diﬀerent nitrogen forms in the three regions’ surface sediments of the southwestern Nansha Trough (μmol/g for dry weight) (Zheng et al., 2008) Region A Range Average IEF-N 0.72∼1.20 0.94 WAEF-N 0.47∼0.69 0.55 SAEF-N 1.04∼1.67 1.35 SOEF-N 7.12∼12.88 10.02 TN 24.86∼43.31 35.31 Forms

Region B Range Average 1.19∼1.59 1.34 0.56∼0.81 0.69 0.84∼1.35 1.07 7.92∼14.04 12.69 41.61∼49.24 46.03

Region C Range Average 1.41∼2.45 1.82 0.57∼1.03 0.83 0.75∼1.06 0.94 14.92∼19.72 17.29 52.70∼60.69 58.24

586

5 Biogeochemical Processes of the South China Sea

ent between regions A and C. Usually the increase in the amounts of IEF-N, SOEF-N, and TN was higher, while that of WAEF-N was lower. This might be related to the nature of IEF-N, WAEF-N, and SOEF-N. Moreover, clay in sediments was rich in elements. Clearly the amount of OC decreased from the trough close to the Kalimantan continental slope towards the west and north (coral clastic and relict deposit areas). Hence, the role of ﬂuvial inputs (organic compounds), their dispersion and the enrichment in sediments were evident. SOEF-N, the main form of nitrogen, was often in organic form and easily absorbed by organic matter. This is why SOEF-N and TN content increased from regions A to C. IEF-N existed in absorbed form, and it was controlled by the speciﬁc surface area and adsorption capacity of carrier minerals. A smaller sediment grain size would lead to a larger speciﬁc surface area, larger adsorption capacity, higher content of organic matter, and more adsorption spots. So, IEF-N was highest in region C where ﬁne-grained sediments concentrated. However, the increase in WAEF-N was low because its content was dependent on not only the adsorption capacity but also other factors such as hydrodynamics, the pH of overlying water, and the oxidative-reductive condition of sediment. The correlation in contents of diﬀerent nitrogen forms and clay proportions among the three regions is shown in Fig. 5.27 (Zheng et al., 2008). The contents of IEF-N, SOEF-N, and TN were signiﬁcantly related to clay fractions (P <0.001) with correlation coeﬃcients of 0.7134, 0.7128, and 0.7684, respectively, and the signiﬁcance of correlation of contents of other nitrogen forms (WAEF-N and SAEF-N) to clay contents was relatively low. It indicated that IEF-N, SOEF-N, and TN existed mainly in ﬁne-grained sediments and showed the diﬀerent increases in the amounts of various nitrogen forms among the three regions. The sediment source and grain-size appeared to be the two main factors that aﬀected nitrogen contents in sediments (Song, 2000). In general, ﬁnegrained sediments were richer in nitrogen (IEF-N, SOEF-N, and TN) than coarse-grained ones (Table 5.14, Zheng et al., 2008). In sea areas of violent hydrodynamic conditions in which eﬄuents, coastal currents, tide, and strong currents are often found, such as estuaries, straits, and upper shelves, the sediments were largely dominated by coarse-grained sediments that bear less nitrogen. However, in the central Bohai basin, in northern China, the outer shelf and central area of the South Yellow Sea, and the southwestern Nansha Trough, the sedimentary environments are relatively stable, in which sediments are dominated by ﬁne-grained deposits, and nitrogen can be easily enriched. Furthermore, nitrogen content is signiﬁcantly higher in coastal sediments than in abyssal sediments, showing that terrigenous inputs are important to nitrogen content in surface sediment. However, the nitrogen content in sediments from the study area was lower than that in costal sediments, higher than that in abyssal sediments, and similar to that in the outer shelf of the South Yellow Sea and the Baltic Sea. Both seas can be considered as a transition zone for nitrogen content between shoals and the deep-water

5.5 Biogeochemical Processes in the Nansha Islands Waters

587

Fig. 5.27. Correlations between clay sediment contents (%) and contents of various nitrogen forms (μmol/g) (Zheng et al., 2008) (With permission from Springer)

area. Therefore, the Nansha Trough was an important channel transporting terrigenous materials from the continental shelf to the deep sea. 5.5.3.5 Biogeochemical Characteristics of Nitrogen (1) Release and burial of nitrogen The study area is 25,390 km2 calculated by girding simulated computation. The amount of diﬀerent forms of nitrogen in surface sediments in the study area was calculated by the formula below (Table 5.15, Zheng et al., 2008): Qi = ci Ahγ = ci Ah(1 − Cw )/((1 − Cw )/G + Cw /ρ)

(5.8)

588

5 Biogeochemical Processes of the South China Sea

Table 5.14. Comparison of contents of various nitrogen forms in the southwestern Nansha Trough surface sediments with previous published data in the literature (Zheng et al., 2008) (With permission from Springer) Location Bohai Bay Bohai central basin Yellow River Estuary and adjacent shelves South Yellow Sea outer shelf Central South Yellow Sea Changjiang River Estuary and adjacent shelves East China Sea shelf Southwestern Nansha Trough Southern Taiwan Strait Central Paciﬁc Ocean Baltic Sea West Atlantic Data were from Song (2004)

Sediment type Silt ooze Silt-clay ooze Silt-sand

IEF-N SAEF-N WAEF-N SOEF-N TN 3.00 0.17 0.33 37.51 191.19 7.58 0.21 0.47 36.94 173.33 5.95 0.21 0.57 22.44 159.29

Silt-clay silt Silt clay-clay Silt-silt clay

1.88 2.78 −

0.61 0.715 −

1.03 1.42 −

13.15 23.04 −

45.63 69.77 52.86

Silt Clay silt Sand-grit Abyssal clay Silt Calcareous clay

− 1.37 − − − −

− 0.69 − − − −

− 1.12 − − − −

− 13.33 − − − −

32.85 46.53 14.28 27.85 40.71 31.43

where Qi (mol) is the amount of i form of nitrogen; ci (μmol/g), the average content of i form of nitrogen; A (km2 ), the area of the study region; h (cm), the sediment depth of nitrogen release (the depth was assumed to be 3 cm in this area because the mineralization of organic matter took place mainly within the depth (Song et al., 2002)); γ (g/cm3 ), the sediment dry bulk density; Cw (%), the water content of sediments; G (g/cm3 ), the sediment grain density; ρ (g/cm3 ), the density of sea water (1.027 g/cm3 (Song, 2004)). Table 5.15. The quanta and cycling of extractable nitrogen in surface sediments of the southwest Nasha Trough (Zheng et al., 2008) (With permission from Springer) Forms Quanta (×109 mol) Burial ﬂuxes (μmol/(cm2 ·yr)) Burial eﬃciency (%)

IEF-N 0.49 0.10 1.17

WAEF-N 0.25 0.05 0.60

SAEF-N 0.40 0.08 0.96

SOEF-N 4.77 0.95 10.35

TN 16.66 3.32 28.79

Particles in water would deposit on the bottom at last after a long time of transportation. Experiencing a series of complex mineralizations of organic matter, parts of nitrogen in the sediment would release into the water column to replenish nutrients by hydrolic or biological disturbance. Therefore, nitrogen burial ﬂux and eﬃciency can be calculated by the following formulae (Ingall and Jahnke, 1994): BF = ci Sγ = ci S(1 − Cw )/((1 − Cw )/G + Cw /ρ)

(5.9)

BE = BF/(BF + DF ) × 100%

(5.10)

where BF (μmol/(cm ·yr)) is the burial ﬂux of nitrogen in sediments; ci (μmol/g) is the average content of i form of nitrogen; S (10−5 m/yr) is the 2

5.5 Biogeochemical Processes in the Nansha Islands Waters

589

sedimentary rate (5.6×10−5 m/yr (Song, 2004)); γ (g/cm3 ) is the sediment dry bulk density; Cw (%) is water content of sediments; G (g/cm3 ) is the sediment grain density; ρ (g/cm3 ) is the density of sea water (1.027 g/cm3 (Song, 2004)); with BE (%) as the burial eﬃciency and DF (μmol/(cm2 ·yr)) the diﬀusive ﬂux. The burial eﬃciency of total nitrogen was 28.79% in the study area, which indicated that the great mass of nitrogen in sediments could take part in recycling through the sediment-water interface, and the releasing eﬃciency could exceed 70%. As described above, the site of the study is a typical semi-deep sea area, a transition from shoal water to abyssal zone, in which sediments are mainly ﬁne silt and clay. Fine sediments were strongly aﬀected by water dynamics, bioturbation, and so on. In addition, the speciﬁc surface area of ﬁne sediment was largest, and the contact area with overlying water was the biggest, which resulted in matter being more easily released through the sediment-water interface (Song, 1997b). So most of the nitrogen was released into water in the study area and only a little (<30%) nitrogen was buried. (2) Contribution of released nitrogen to primary production Nitrate that diﬀuses from the deep is a new nitrogen source supporting phytoplankton growth in the upper euphotic oceans, and results in new productivity. In the study, the spring primary productivity was about 500 mg C/(m2 ·d) (Huang and Chen, 1997) with which primary productivity can be labeled by 15 N (mg N/(m2 ·d)), according to the Redﬁeld ratio (C:N:P=106:16:1). Cai and Huang (2002) determined the ratio of new productivity to primary productivity as being about 0.16 in the southwestern Nansha Trough using 228 Ra-NO3 method. Therefore, the new productivity in the study area should be 14 mg N/(m2 ·d) and the contribution of released nitrogen to new productivity in the study area was calculated by the formula (Song, 2004) n = (DF/P ) × 100%

(5.11)

with n (%), the contribution of nitrogen released; DF (μmol/(cm ·yr)), the diﬀusive ﬂux, and P (mg N/(m2 ·d)), the new productivity. Generally speaking, nutrient sources supporting phytoplankton growth in a euphoric layer are mainly from: diﬀusion from the deep, land-based import, dry-wet sedimentation, and N2 ﬁxation. In the study area, land-based sources were limited due to long distance from coast and short rivers in Kalimantan. New production from N2 -ﬁxation in the SCS was also very low (Chen, 2005). Moreover, an earlier measurement in the study area found that high primary productivity occurred in coastal areas or an area where sea masses meet, such as the southwestern part of the Nansha Trough, where the currents from the Sulu Sea and the Nansha Trough meet. As a result of the conﬂuence, nutrients were supplemented from aged water that invaded by upwelling. Therefore, nitrogen diﬀused from deep aged water was an important source to support primary productivity in the study area; the contribution was calculated for 22% to new productivity (Zheng et al., 2008). 2

590

5 Biogeochemical Processes of the South China Sea

5.5.4 Carbon Cycling in the Nansha Coral Reef Ecosystem Carbon cycling research is one of the key issues in research into the coral reef ecosystem, which relates to the sustainable development of the reef’s ecosystem (Song et al., 2008). 5.5.4.1 Carbon in Settling Particulars of the Coral Reef Ecosystem The main scientiﬁc problems include the carbon form and type in the cycle, the carbon budget in a coral reef ecosystem, and the function of organisms. Most research undertaken has concerned abiogenic particulate carbon in the coral reefs of the Nansha Islands over the past 10 years. A series of scientiﬁc investigations on Nansha Islands were carried on from April to May, 1999. The sampling areas included 5 typical atolls, the Yongshu Reef, the Zhubi Reef, the Huayang Reef, the Chigua Reef, and the Nanxun Reef. The sediment traps were deployed for collecting settling particles, with their positions. A drag net with a size of 90 meshes was employed for collecting samples from the Yongshu Reef, the Huayang Reef, the Chigua Reef, and the Nanxun Reef. The coral samples included Isis sp., Fungia fungites, Goniastrea retiformis, Cyphastrea serailia, Platygyra crossland, Favia palauensis, and Acropora formosa. Total carbon (TC) and organic carbon (Corg ) were analyzed and shown in Table 5.16. In general, TC contents in the samples collected using the drag net ranged from 211.3 to 306.6 mg/g, about 50% higher than those of other solid samples, and no signiﬁcant diﬀerences existed in suspended particles, algae, and coral. The sequence of Corg contents was as follows: samples collected with the drag net>suspended particles>algae>coral>sand of lagoons, so were the ratios of Corg to TC. Table 5.16. Carbon content (mg/g) and the percentage (%) of organic carbon in the solid samples Samples Suspended particle Alga Drag net Coral Sand

TC 139.9∼149.4 149.5∼201.2 211.3∼306.6 100.6∼169.4 110.4∼117.8

Corg 32.71∼61.36 29.99∼31.94 96.81∼206.68 8.81∼48.33 1.32∼2.11

IC 78.54∼116.69 117.6∼171.2 99.92∼114.5 88.28∼121.1 108.4∼116.1

Corg /TC 21.9∼43.9 14.9∼21.4 45.8∼67.4 6.9∼28.5 1.1∼1.9

TC: total carbon; Corg : organic carbon; IC: inorganic carbon

The inorganic carbons adsorbed in the lagoon sands mainly derived from the dissolved CO2 and inorganic carbonate sediment produced by hermatypic organisms. CaCO3 , as the majority component of inorganic carbon, as well as dissolved inorganic carbon (DIC), is the major form in carbon cycling and existence. Inorganic carbon contents were determined by calciﬁcation and the

5.5 Biogeochemical Processes in the Nansha Islands Waters

591

dissolution equilibrium. Therefore, no signiﬁcant diﬀerence in inorganic carbon contents occurred in the diﬀerent samples. Organic carbon could qualitatively present the level of productivity in the coral reef ecosystem. Although organic carbon played a minor role in carbon cycling, its content was the key link in the chain of carbon and nutrient cycling in the coral reef ecosystem. The average organic carbon content in lagoon sands was very low, while it elevated 25 factors in suspended particles, indicating that a tiny aliquot of organic carbon matter could sink to the bed sediment and the majority was decomposed or utilized before the deposition (Table 5.17). Table 5.17. Release of particle carbon in the Zhubi Reef Suspended particles (mg/g) PTC POC 149.40∼146.60 32.71∼48.95

Release (mg/g) RPTC RPOC 31.60∼36.20 30.60∼46.91

Percentages (%) RPTC /PTC RPOC /POC 21.2∼24.7 93.6∼95.8

PTC: total carbon in the suspended particles; POC: organic carbon in the suspended particles; RPTC : release of total carbon in the suspended particles; RPOC : release of organic carbon in the suspended particles

According to Table 5.18, particle organic carbons were consumed quickly in the predation and decomposition situations. Before suspended particles were deposited in the bed sediments, 93.6%∼95.8% organic carbon had been discharged (or consumed) in the Zhubi Reef. Table 5.18. Vertical ﬂuxes of particulate organic carbon and its percentage in gross sinking particulate carbon both in the Zhubi Reef Lagoon and outside the atoll Cruises 99-4 93-5 94-4

Stations In-reef Oﬀ-reef In-reef In-reef

SF (mg/(m2 ·d)) 410∼1,850 350∼1,030 3,280 4,600

FPOC (mg/(m2 ·d)) 20.11∼60.49 21.62∼51.21 53.38 90.16

FPOC/FTC 0.219∼0.334 0.348∼0.439 0.133 0.157

SF: ﬂux of suspended particles; FPOC: ﬂux of the organic carbon in the suspended particles; FTC: ﬂux of the total carbon in the suspended particles

It was generally considered that particulate organic carbon (POC) mainly originated from terrigenous input, biodetritus from the marine food chains and the adsorption of DOC in seawater. The OC contents in plankton were the highest, in the range of 96.8∼206.7 mg/g, and the OC contents in the lagoon sands ranged from 1.32 to 2.11 mg/g, about 1% of those in plankton. The huge diﬀerences in contents suggested that about 99% of biodetritusoriginated POC was transformed to inorganic carbon during the predation and decomposition processes and rejoined the cycle. Therefore, a conclusion could be draw that almost all the sinking POC might be discharged and reutilized by organisms in the seawater, which could also be helpful to understand

592

5 Biogeochemical Processes of the South China Sea

the phenomenon whereby the gross productivity was high in the coral reef ecosystem but the net productivity was low. A great deal of carbon was deposited in the form of inorganic carbonates, and would not be involved in the carbon cycle gradually. About 21%∼25% of carbon discharged from suspended particles and sediments was recycled. Therefore, it was vital for coral development to retain stable carbon transportation to the coral reef ecosystem. In a situation of adequate DIC, the quantity and density of zooxanthellae have a direct inﬂuence on the stability of the coral reef ecosystem, which is in symbiosis with the coral. Zooxanthellae can produce organic matter and discharge O2 during the photosynthesis process. The majority of organic matter was excreted from coral and O2 was supplied to coral for respiration. Some organic matter was captured by coral as one food source. Some CO2 discharged from coral respiration combined with Ca2+ in seawater and constructed the coral skeleton. In the tropical sea areas, much research showed that the coral reef served as a source of CO2 and the ﬂuxes from the surface of water-air were dominated by calciﬁcation. Therefore, the carbon of the coral reef had multi-sources, i.e., water-air exchange, as well as seawater and organisms outside. The behavior of outside organisms supplying large amounts of carbon for coral reef ecosystems could be considered as suspension ﬁshing strategy in the ﬁshing ﬁeld. Suspension ﬁshing strategy can restore the excessively consumed primary productivity. The organism and its metabolic products play an import role in the carbon continuous input process. For coral reef production more attention should be paid to water circumstances and biotic multiformity. 5.5.4.2 Organic Matter Fluxes and Distributional Features of Hydrocarbon Compounds and Fatty Acids Sinking particulate material in the sea contains abundant organic matter deriving mainly from marine organisms and partly from higher land plants transported by aeolian dusts and rivers. Such organic matter is not only a food source of marine organisms and an organic matter source of marine sediments, but also a major factor inﬂuencing the marine chemistry. Organic geochemical studies of sinking particulate material in the sea can be conducive to understanding the vertical ﬂux, the source and process of chemical and biochemical degradation for sinking particulate organic matter (Song, 2004). These processes result in changes in the quantity and composition of sinking particulate organic matter so that they aﬀect the food source of marine organisms and the composition of marine sedimentary organic matter. Therefore, many such studies have been carried out abroad. We have systematically analyzed the sinking particulate material in two diﬀerent marine environments in the Yongshu Reef Lagoon and the continental shelf of the East China Sea by the organic geochemical method. The ﬂuxes of sinking particulate organic matter and distributional features of hydrocarbons and fatty acids are present,

5.5 Biogeochemical Processes in the Nansha Islands Waters

593

followed by the distributions of aldehyde, ketone and alcohol lipid compounds. These data provide important evidence for studies in marine biology, marine chemistry, and marine sedimentology (Duan et al., 1998). (1) Organic matter ﬂux Sinking particulate material in the sea was collected in the center of the Yongshu Reef Lagoon (9◦ 35 N, 113◦ 00 E) during the period from December 19, 1993 to March 25, 1994 and the continental shelf of the East China Sea (29◦ 02 N, 125◦ 00 E) from April 19, 1994 to April, 20, 1994. The ﬂux of sinking particulate organic matter in the sea is represented by organic carbon content. As shown in Table 5.19, organic carbon ﬂuxes of the Yongshu Reef Lagoon and the continental shelf of the East China Sea are similar. They are 220.1 mg/(m2 ·d) and 213.8 mg/(m2 ·d), respectively. However, the ﬂuxes of sinking particulate material diﬀer signiﬁcantly. This shows that organic matter content in sinking particulate material in the Yongshu Reef Lagoon is much higher than that in the continental shelf of the East China Sea. Organic matter content in sinking particulate material of the sea is controlled by many factors, such as primary productivity of plankton, the amount of the supply of inorganic particles, and the extent of chemical and biochemical degradation to organic matter. Previous research shows that the primary productivity of plankton is 490 mg C/(m2 ·d) in the continental shelf of the East China Sea and 406 mg C/(m2 ·d) in the Nansha Islands sea area (Huang, 1991). As primary productivity of plankton in the lagoon is higher than that in the Nansha Islands sea area, primary productivity of plankton in the two study areas does not diﬀer signiﬁcantly. Therefore, it is suggested that a greater supply of reef-building organism detritus from the Yongshu Reef ﬂat not only makes the atoll lagoon have a higher ﬂux of sinking particulate material, but also dilutes its organic content of sinking particulate material. Table 5.19. Fluxes of sinking particulate material and organic matter (mg/(m2 ·d)) (Duan et al., 1998) (With permission from Duan Y) Area Particle material TOC TON Extract Fatty acid Alkenes C 25 HBI diene Nansha Lagoon 22,930 220.1 32.1 60.7 2.9 0.69 0.15 East China Sea 6,439 213.8 20.1 125.0 4.1 0.72 −

Sinking particulate organic matter of the sea is an especially dynamic component of nutrimental, geochemical, and biochemical cycles. Most of it, before sinking to the sea ﬂoor, is consumed by various processes. Generally speaking, organic matter in sinking particulate material of the sea mainly comes from marine plankton. Therefore, the consumption of particulate organic matter during sinking can be estimated according to the primary productivity of plankton in the surface water and the ﬂux of sinking particulate organic carbon. Such estimates show that the percentage of particulate organic matter before reaching a depth of 5 m from the sea ﬂoor is only 43.6% for the continental shelf of the East China Sea and less than 54.2% for the Yongshu Reef

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5 Biogeochemical Processes of the South China Sea

Lagoon, indicating that about half of particulate organic matter in the two regions is consumed during sinking. From the extent of decreases in the ﬂux of particulate organic matter per 1 m of sinking, the Yongshu Reef Lagoon is more than 12.4 mg/(m2 ·d), whereas the continental shelf of the East China Sea is only 3.7 mg/(m2 ·d), showing that the degree of consumption of sinking particulate organic matter in the Yongshu Reef Lagoon is larger than that in the continental shelf of the East China Sea. Generally, soluble organic matter and its various fractions are biochemicals in marine organisms. Therefore, it is important to understand their ﬂuxes. The ﬂux of soluble extracted organic matter in the two study areas is very high but the ﬂuxes of fatty acid and hydrocarbon fractions are low. However, these ﬂuxes in the continental shelf of the East China Sea are higher than those in the Yongshu Reef Lagoon. This diﬀerence may be related to their distinct marine environments. (2) n-Alkanes and isoprenoid alkanes The carbon numbers of n-alkanes in sinking particulate material in the Yongshu Reef Lagoon and the continental shelf of the East China Sea are from C14 to C35 and n-alkanes exhibit a bimodal distribution with C17 and C25 as the maxim (Table 5.20). However, nC− / nC+ 21 21 ratio is 1.49 for the Yongshu Reef Lagoon and 0.25 for the continental shelf of the East China Sea, showing that the components of n-alkanes in samples from the Yongshu Reef Lagoon are short chain n-alkanes while n-alkanes in samples from the continental shelf of the East China Sea are dominated by a long chain component. In general, low carbon number n-alkanes are derived from plankton while high carbon number n-alkanes with strong odd-over-even predominance come from higher land plants. However, odd-over-even predominance of higher n-alkanes in the two study areas, with CPI values of 1.29 and 1.17, respectively, is not obvious, implying that only a small amount of long chain n-alkanes come from higher land plants. Although higher n-alkanes from ancient sediment by weathering have no predominance of odd carbon compounds, such n-alkane input is unlikely, because the Yongshu Reef is separated from land by a deep sea. Therefore, most of the higher n-alkanes in the two study areas are derived most probably from diatom and bacteria, for these organisms contain higher n-alkanes with little or no predominance of odd carbon (Volkman et al., 1980). Table 5.20. n-Alkane and isoprenoid alkane parameters (Duan et al., 1998) (With permission from Duan Y) P

Area NS ECS

P

nC− 21 nC+ 21

1.49 0.25

CPI25∼34 1.29 1.17

Crange

Cmax

C14 ∼C34 C17 , C25 C14 ∼C35 C17 , C25

CP 25 HBI diene nC14∼34

0.22 −

Pr/Ph Pr/nC17 Ph/nC15 0.97 0.83

NS: Nansha Lagoon; ECS: East China Sea; Pr: pristane; Ph: phytane

0.48 0.86

0.97 0.91

5.5 Biogeochemical Processes in the Nansha Islands Waters

595

Isoprenoid alkanes detected in the samples are mainly pristane (Pr) and phytane (Ph). Pr/Ph ratio is 0.97 for the Yongshu Reef Lagoon and 0.83 for the continental shelf of the East China Sea, showing that the relative abundance of pristane is lower than that of phytane. For sedimentary organic matter, the formations of pristane and phytane are mainly controlled by the sedimentary environment. For sinking particulate organic matter, their formations also depend on biological and biochemical processes. Previous research shows that phytol can be transformed to pristane by copepod grazing chlorophyll a so that their excretive material provides pristane for sinking particulate material, whereas phytane is formed by anaerobic microbia reprocessing to phytol (Risatti et al., 1984). Therefore, the presences of pristane and phytane in the samples reﬂect the biological and biochemical transformations of sinking particulate organic matter before falling to the sea ﬂoor. (3) Isoprenoid alkene Abundant C25 HBI diene is detected in the sinking particulate material from the Yongshu Reef Lagoon. Previous studies hold that C25 HBI alkene compounds are of marine origin, but more recent data show that they probably derive mainly from diatom (Simoneit, 1977). C25 HBI diene compound in sinking particulate material in the Yongshu Reef Lagoon is very abundant for it is the major peak in the gas chromatogram of saturated fraction and the ratio of this compound to n-alkanes reaches 0.22. Phytoplankton in the Nansha and East China Sea area are mainly diatom but their dominant species are diﬀerent. The former are Chaetoceros and Rhizosolenia while the latter are Coscinodiscus and Melosira. If this compound derives from diatom, our data will show that its biological source is related to the composition of the diatom species, and Chaetoceros and Rhizosolenia may be a main biological source of this compound. (4) Fatty acids The carbon number range of saturated n-fatty acids with a maximum at C16:0 is from C9:0 to C28:0 for the Yongshu Reef C9:0 to C20:0 for the Lagoon and + continental shelf of the East China Sea. C− / C 20:0 20:0 ratio in the Yongshu Reef Lagoon samples is 8.24, but no fatty acids beyond C20 are detected in East China Sea samples (Table 5.21). This distribution of saturated n-fatty acids shows that fatty acids in sinking particulate organic matter from the Yongshu Reef Lagoon mainly come from marine plankton and bacteria, and Table 5.21. Fatty acid parameters (Duan et al., 1998) (With permission from Duan Y) P

Area

P

nC− 20:0 nC+ 20:0

NS 8.24 ECS not>C20:0

CPI16∼30 CU (%) CP (%) a+iC (%) 7.27 27.52

35.63 51.20

7.20 9.27

13.57 3.57

C18:1 Δ11 C18:1 Δ9

C16:0 C18:0

C16:1 C18:1

1.87 0.29

2.40 7.24

0.46 1.57

NS: Nansha Lagoon; ECS: East China Sea; CU , total unsaturated fatty acids; CP , total polyunsaturated fatty acids; a+iC, anteiso- and iso-fatty acids

596

5 Biogeochemical Processes of the South China Sea

a small amount of higher carbon number fatty acids (>C20:0 ) deriving from higher land plants could be transported by aeolian dusts (Simoneit, 1977). However, the absence of fatty acids (>C20:0 ) deriving from higher land plants implies an exclusive marine source of fatty acids in sinking particulate organic matter from the continental shelf of the East China Sea. Unsaturated fatty acids from C14 to C22 detected in the two studied regions exist in abundance. They comprise 35.63% of the total fatty acids for the Yongshu Reef Lagoon and 51.20% for the continental shelf of the East China Sea (Table 5.22). Polyunsaturated fatty acids are mainly C18:2 , C20:4 , C20:2 , and C22:4 ones which originate mainly from marine plankton, particularly from diatom. The relative content of these fatty acids in the Yongshu Reef Lagoon (7.20 % of the total fatty acids) is lower than that in the continental shelf of the East China Sea (9.27% of the total fatty acids), showing that the latter has more diatom-derived fatty acids. Monounsaturated fatty acids are mainly C16:1 Δ9 , C18:1 Δ9 , and C18:1 Δ11 ones. In particular, C18:1 Δ11 acids are considered to derive mainly from bacteria and the C18:1 Δ11 /C18:1 Δ9 ratio is used for estimating the input of bacterial organic matter. C18:1 Δ11 /C18:1 Δ9 ratio is 1.87 for the Yongshu Reef Lagoon and 0.29 for the continental shelf of the East China Sea, indicating that more fatty acids in the Yongshu Reef Lagoon sample derive from bacteria. The further evidence is that the relative content of iso- and anteiso-fatty acids in the Yongshu Reef Lagoon (13.57% of the total fatty acids) is higher than that in the continental shelf of the East China Sea (3.57% of the total fatty acids). The fatty acids are contributed to by bacteria. These results clearly show that microbial transformations and biogeochemical reactions of particulate organic matter in the study areas strongly occur during sinking. The degree of these processes in the Yongshu Reef Lagoon is greater than that in the continental shelf of the East China Sea (Duan et al., 1998). (5) Carbon isotopic studies of individual lipids in organisms from the Nansha Islands waters Sediments contain abundant lipid compounds in general, which are used as biomarker compounds to study organic matter sources and reconstruct the palaeo-environments. However, lipid compounds in sediments are generally a mixture of various genetic components so it is diﬃcult to correctly decouple their biological sources only by the results of biochemical research. Carbon isotopic studies of individual sedimentary lipid compounds can discover their genetic information, which provides a new way to understand their origins. The reasonable interpretation of carbon isotopes of sedimentary lipid compounds relies heavily on the carbon isotopic studies of individual lipids in the modern organisms. Biological lipids are precursors of sedimentary lipids and their carbon isotopic compositions are controlled by the environmental conditions, lipid biosynthetic pathways, and so on (Thompson and Calvert, 1994; Popp et al., 1998; Laws et al., 1997). However, the carbon isotopic studies of individual lipids in extant organisms are limited (Monson and Hayes, 1982;

5.5 Biogeochemical Processes in the Nansha Islands Waters

597

Table 5.22. Percentage content of fatty acid (Duan et al., 1998) (With permission from Duan Y) Peak number

Fatty acid

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

C9:0 C10:0 C11:0 C12:0 iC13:0 aiC13:0 C13:0 iC14:0 C14:1 Δ7 C14:0 iC15:0 aiC15:0 C15:0 iC16:0 C16:1 Δ9 C16:0 iC17:0 aiC17:0 C17:1 C17:0 C18:2 Δ9,12 C18:1 Δ9 C18:1 Δ11 C18:0 iC19:0 C19:0 C20:4 Δ5,8,11,14 C20:4 C20:2 Δ11,13 C20:1 C20:0 C21:0 C22:4 Δ7,10,13,16 C22:1 C22:0 C23:0 C24:0 C26:0 C28:0

ND, not delected; NS, Nansha; ECS, East China Sea

Contents (%) NS 1.40 0.16 0.16 1.90 0.16 0.16 0.33 1.31 0.65 5.89 5.23 3.44 3.11 0.98 8.02 18.49 1.31 0.82 0.81 2.62 2.13 6.05 11.29 7.69 0.16 0.49 2.13 1.64 0.98 0.49 2.29 0.32 0.32 0.65 3.11 0.15 1.47 0.30 0.32

ECS 0.21 Trace Trace 0.84 ND ND Trace 0.21 0.42 15.37 0.63 2.52 1.47 0.21 24.84 22.95 ND ND 0.84 0.42 2.74 12.21 3.57 3.16 ND 0.21 0.42 6.11 ND ND 0.63 ND ND ND ND ND ND ND ND

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5 Biogeochemical Processes of the South China Sea

Summons et al., 1994; Collister et al., 1994; Schouten et al., 1998; van Dongen et al., 2002). The Nansha sea area of China is a special marine ecological environment. This is because it is located in the low-latitude tropical zone and semi-enclosed drainage area of the SCS which is far away from the continent and has abundant tropical organisms, especially coral organisms. A carbon isotopic study of individual lipids in marine organisms from this environment can provide new data for research on the carbon isotope of lipid compounds in marine organisms. On the other hand, we have determined carbon isotopic compositions of individual sedimentary lipids in the Nansha sea area (Duan and Wang, 2002). However, their genetic interpretations are made only by extrapolation from more conventional studies on bulk carbon isotopic compositions of organisms grown under diﬀerent conditions. If we can know the carbon isotopic compositions of lipid compounds in various organisms from this marine region and compare them with those of these sedimentary lipid compounds, we will establish the genetic relationship between them. With this object in mind, plankton, coral, and macro-algae in the Nansha sea area were collected, and their soluble organic matter was extracted and fractionated. δ 13 C values of individual lipid compounds were determined using a new technique of gas chromatography combined with isotopic ratio mass spectrometry (GC-IRMS) and the features of their carbon isotopic compositions and isotopic genetic relationship between biological and sedimentary lipid compounds were discussed (Duan et al., 1998). 5.5.4.3 Carbon Isotopic Compositions of Fatty Acids The carbon isotope analytical results of lipids in plankton, coral, and macroalgae in the low-latitude tropical region of the Nansha sea area are listed in Table 5.23. Carbon isotopic compositions of C10 ∼C22 saturated fatty acids in plankton range from −25.6‰ to −28.6‰ with a mean of −27.4‰. Compared with other fatty acids, C16:0 fatty acid is enriched in 13 C, while C10:0 and C20:0 fatty acids are enriched in 12 C. δ 13 C values of C10:0 ∼C22:0 fatty acids in Gorgonia are in the range of −26.0‰∼−29.7‰ averaging −28.2‰. Fatty acids in Gorgonia have similar carbon isotopic compositions with the exception of C14:0 fatty acid enriched in 13 C. Carbon isotopic compositions of C14:0 ∼C22:0 fatty acids in macro-algae Laurencia vary from −24.4‰ to −28.1‰ with an average of −26.4‰. It is also observed that C14:0 fatty acid is enriched in 13 C and C20:0 fatty acid is enriched in 12 C relative to C16:0 and C18:0 fatty acids which are quite similar isotopically. If δ 13 C value of C16:0 fatty acids in the studied organisms is used as a carbon isotopic reference point and compared with other fatty acids in each studied organism, their isotopic diﬀerences are mostly within ±2.0‰(Table 5.23), reﬂecting that these fatty acids are biosynthesized by carbon chain elongation in the same ecological environment (Schouten et al., 1998). The mean carbon isotopic composition is light for Gorgonia and heavy for Laurencia, and the diﬀerence in mean δ13 C

5.5 Biogeochemical Processes in the Nansha Islands Waters

599

values of saturated fatty acids between the studied organisms is 1.8‰ showing that carbon isotopic compositions of saturated fatty acids do not diﬀer signiﬁcantly in diﬀerent organisms from the Nansha sea area (Schouten et al., 1998). Table 5.23. Carbon isotopic compositions of fatty acids and n-alkanes in organisms from the Nansha sea area (‰ PDB) (Duan et al., 2004) (With permission from Duan Y) Comp.

Organisms C10 C12 C14 C15 C16 C18 C20 Plankton −28.6 −27.5 −26.7 −27.7 −25.6 −26.6 −28.3 Fatty acid Gorgonia −27.3 −29.7 −26.0 −27.5 −29.0 −28.2 −29.1 Laurencia −24.4 −26.4 −26.7 −28.1 Means −28.0 −28.6 −25.7 −27.6 −27.0 −27.2 −28.5 Plankton −29.7 −28.2 −28.7 Gorgonia −29.7 −29.7 −27.5 n-Alkanes Laurencia −29.2 −29.0 −28.0 Means −29.5 −29.0 −28.1 δ 13 C values are the mean of 1∼3 measurements for each sample

C22 Means C= C= 16 18 −27.3 −27.2 −20.1 −24.7 −28.1 −28.2 −20.5 −24.4 −26.4 −20.1 −23.9 −27.7 −27.3 −20.2 −24.3 −28.6 −28.9 −28.9 −28.8

The carbon isotope analytical data of individual fatty acids in organisms are limited. van Dongen et al. (2002) have determined carbon isotopic compositions of individual fatty acids in Sphagnum cuspidatum from the Bargerveen peat bog, the Netherlands, which are from −33.1‰ to −35.0‰. We have measured δ 13 C value of individual fatty acids in herbaceous plants and tree leaves from the Gansu Marsh and obtained similar isotopic data, ranging from −33.0‰ to −35.4‰. If carbon isotope compositions of individual saturated fatty acids in marine organisms from the Nansha sea area are compared with those studied results above, we will ﬁnd that the former have heavier carbon isotopic compositions than the latter, which is consistent with the studied results that marine organisms are enriched in 13 C (Deines, 1980). One ﬁnding is that C16 and C18 unsaturated fatty acids in the three studied organisms are evidently enriched in 13 C and have similar carbon isotopic compositions in various organisms, respectively. δ 13 C values range from −20.0‰ to −20.5‰ for C16 unsaturated fatty acid (C= 16 ) and from −23.9‰ to ). The data show that C16 unsat−27.4‰ for C18 unsaturated fatty acid (C= 18 urated fatty acid is isotopically heavier than C18 homologue. Compared with C16:0 fatty acid, C16 and C18 unsaturated fatty acids are enriched in 13 C by a range from 0.9‰ to 8.5‰ (Table 5.24). It shows that unsaturated fatty acids have lighter carbon isotope compositions as compared with saturated fatty acids, and the cause is interpreted from the biosynthetic pathway. However, the results here indicate that desaturation of fatty acids also causes enrichment in 13 C of fatty acids. The reasons for the enrichment are unclear, and thus further investigation is required.

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5 Biogeochemical Processes of the South China Sea

Table 5.24. Carbon isotopic compositions of fatty acids and n-alkanes relative to the C16 fatty acid and C16 n-alkane, respectively (‰ PDB) (Duan et al., 2004) (With permission from Duan Y) Compounds Organisms C10 C12 C14 C15 C18 C20 C22 C= C= 16 18 Plankton −3.0 −1.9 −1.1 −2.1 −1.0 −2.7 −1.7 +5.5 +0.9 Fatty acid Gorgonia +1.7 −0.7 +3.0 +1.5 +0.8 −0.1 −0.9 +8.5 +4.6 Laurencia +2.0 −0.3 −1.7 +6.3 +2.5 Plankton +1.5 +1.9 +1.0 n-Alkanes Gorgonia 0.0 +1.0 +2.2 Laurencia +0.2 −0.4 +1.2

(1) Carbon isotopic compositions of n-alkanes The carbon number distributions of n-alkanes range from C14 to C22 . δ 13 C values of abundant C16 ∼C22 even-carbon-number n-alkanes are from −27.8‰ to −29.7‰ for plankton, from −27.5‰ to −29.7‰ for gorgonia, and from −28.0‰ to −29.2‰ for Laurencia. Their average δ 13 C values, ranging from −28.6‰ to −28.9‰, are quite similar. The isotopic compositions of diﬀerent carbon-number n-alkanes do not diﬀer signiﬁcantly, whose diﬀerences are within a range of ±2.2‰. Similar to fatty acid, if δ13 C value of C16 n-alkanes is compared with that of other homologues, the variance range of δ 13 C values in the three studied organisms is from −0.4‰ to 2.2‰. These data show that carbon isotope compositions between n-alkanes do not diﬀer signiﬁcantly in the three studied organisms from the ecological environment of the Nansha sea area, reﬂecting that carbon isotopic compositions of n-alkanes in organisms from the same ecological environment are similar. The mean δ13 C values of n-alkanes in C3 plants analyzed by Collister et al. (1994) are from −31.4‰ to −38.6‰ and from −30.1‰ to −38.4‰, respectively. Our measured n-alkanes in herbaceous plants and tree leaves from the Gansu Marsh have carbon isotopic compositions from −31.3‰ to −34.9‰. Compared with those mentioned above, n-alkanes analyzed here are enriched in 13 C, indicating the carbon isotopic characteristics of the marine aquatic organisms. On the other hand, the mean δ 13 C diﬀerence between n-alkanes and saturated fatty acids is only 1.5‰, reﬂecting that they are biosynthetically similar. (2) Carbon isotope genetic relationship of individual lipids in organisms and sediments Sedimentary lipids can have multiple origins, so that their carbon isotopic compositions are various. The previous recognition of carbon isotope genesis of sedimentary lipids relies mostly on the extrapolation from the sedimentary lipid-formed environments and the studied results of bulk carbon isotopic compositions of organisms grown under diﬀerent conditions. The comparative study of the carbon isotope genetic relationship of individual lipids in organisms and sediments from the same region and same ecological environment can provide a scientiﬁc basis for correctly understanding the carbon isotopic

5.5 Biogeochemical Processes in the Nansha Islands Waters

601

origin of sedimentary lipids. We have determined carbon isotopic compositions of individual fatty acids, alcohols, and n-alkanes in the sediments from the Nansha sea area (Duan and Wang, 2002). The results indicate that mean δ 13 C values of C14 and C15 saturated fatty acids in sediments are −31.9‰ and −31.4‰, respectively, which is 6.2‰ and 3.8‰ lighter than those in algae and coral (−25.7‰ and −27.6‰), respectively. This demonstrates that C14 and C15 saturated fatty acids are derived mainly from bacteria, not from marine algae or animals. Carbon isotopic compositions of C16:0 and C18:0 fatty acids in sediments are −28.2‰ and −27.9‰ on average, respectively, which are similar to those in algae and coral (averaging −27.0‰ and −27.2‰, respectively), especially coral. No presence of long chain (>C22 ) fatty acids in algae and coral indicates that those compounds in sediment come mainly from higher-land plants. δ 13 C values of short chain alkanols (C16 ∼C18 ) in sediments range from −25.0‰ to −28.8‰ averaging at −27.7‰, which are close to those of fatty acids in plankton and coral (Schouten et al., 1998). From a biosynthetic stand point, alkanols and fatty acids in the same organisms should have similar carbon isotopic compositions. Therefore, the characteristics of carbon isotopic compositions of short chain alkanols in sediments reﬂect that they mainly originate from plankton and coral. δ 13 C values of short chain n-alkanes (C16 ∼C20 ) in the three sediments from the Nansha sea area range from −29.8‰ to −32.9‰, with an average of −32.0‰. This mean value is 3.2‰ lighter than that of n-alkanes in marine organisms in the Nansha sea area, reﬂecting that these n-alkanes are derived mainly from bacteria, since bacteria and algae contain abundant low-carbon-number n-alkanes. A predominance of C16 ∼C20 n-alkanes and lack of >C22 homologues in the studied organisms further demonstrates that the abundant long-chain n-alkanes in sediments from the Nansha sea area with lighter carbon isotopic compositions come from higher plants and bacteria (Duan and Wang, 2002). The Nansha sea area, which is located in a low-latitude tropical zone and is far away from the continent and has abundant tropical organisms, especially coral organisms, is a special marine ecological environment. It was determined with carbon isotopic compositions of individual fatty acids and n-alkanes in plankton, Gorgonia and Laurencia from the Nansha sea area. δ 13 C values of individual saturated fatty acids are from −25.6‰ to −28.6‰ for plankton, from −26.0‰ to −29.7‰ for Gorgonia, and from −24.4‰ to −28.1‰ for Laurencia, respectively. Their averages are −27.2‰, −28.2‰, and −26.4‰, respectively. These data show that Laurencia is relatively enriched in 13 C and Gorgonia is relatively enriched in 12 C. However, their mean diﬀerence is small, whose range is only 1.8‰. C16 and C18 unsaturated fatty acids have heavy carbon isotopic compositions compared with the same carbon-number fatty acids and their diﬀerences are 6.8‰ and 2.9‰, respectively. The reasons for the enrichment are unclear. δ 13 C values of n-alkanes in the three studied organisms range from −27.8‰ to −29.7‰, from −27.5‰ to −29.7‰, and from −28.0‰ to −29.2‰, respectively. Their mean values are quite similar, ranging from −28.6‰ to −28.9‰. Small diﬀerences in δ 13 C values between

602

5 Biogeochemical Processes of the South China Sea

saturated fatty acids and n-alkanes in the same organism reﬂect that they are biosynthesized by carbon chain elongation. Heavy carbon isotopic compositions of lipids in marine organisms from the Nansha sea area, as compared with land C3 plants, reﬂect the carbon isotopic characteristics of marine aquatic organisms. The comparative studies of carbon isotopic compositions of individual lipids in organisms and sediments from the Nansha sea area have established the carbon isotopic genetic relationships between the biological and sedimentary lipids, which provides a scientiﬁc basis for carbon isotopic applied research of individual lipids (Duan et al., 2004). 5.5.4.4 Geochemical Signiﬁcance of Compositional Features of Ketone, Aldehyde, and Alcohol Compounds (1) Acyclic isoprenoid ketones, aldehydes, and alcohols Abundant C18 isoprenoid ketone, Z/E pristenal, and Z/E phytenal as well as phytol biomarker compounds were detected in the sinking particulate materials from the Yongshu Reef Lagoon and the continental shelf of the East China Sea (Figs. 5.28 and 5.29). This appears to be the ﬁrst report of these compounds in China marine environment. Detection of these compounds in the studied regions is of some importance to understand the evolutional processes of acyclic isoprenoid compounds in the seawater column and their formative pathways in marine sediments. Their identiﬁcations were based on chromatographic retention time, mass spectrum features (Figs. 5.28 and 5.29), and comparison with those reported previously. The mass spectrum of phytol trimethylsilyl ether exhibits a base peak at m/z 143 and a molecular ion at m/z 368. C18 isoprenoid ketone has a base peak at m/z 58, a molecular CH3 O Si CH3 143 CH3 Mass cleavage

Relative abundance (%)

100

Phytol

Sterols

n-Alkanols

50 nC16

nC22

nC12

0 20

30

40

50

60

t (min)

Fig. 5.28. Total ion chromatogram of the alcohol fraction in sinking particulate material from the Yongshu Reef Lagoon (Duan et al., 1997) (With permission from Duan Y)

5.5 Biogeochemical Processes in the Nansha Islands Waters

603

Relative abundance (%)

100

A

B

50 C

0

10

20

30

D E

40 t (min)

50

60

Fig. 5.29. Total ion chromatogram of ketone+aldehyde fractions in sinking particulate material from the Yongshu Reef Lagoon. A, C18 isoprenoid ketone; B, Z pritenal; C, E pristenal; D, Z phytenal; E, E phytenal (Duan et al., 1997) (With permission from Duan Y)

ion at m/z 268, and other characteristic fragment ions at m/z 165, 210, and 250. The base peaks of Z/E pristenals and Z/E phytenals are m/z 84 and their molecular ion peaks are m/z 280 and 294, respectively. Z and E isomers were determined according to the diﬀerent intensity of m/z 97 in the mass spectrum. It is generally considered that in the water column phytol is formed mainly from chlorophyll a by biological and biochemical hydrolyses, while C18 isoprenoid ketone, Z/E pristenals, and Z/E phytenals are intermediates in the conversion of phytol to other isoprenoid compounds and two pathways of biochemical and photochemical degradations have been proposed. Therefore, the presence of these abundant isoprenoid compounds shows that sinking particulate organic matter is strongly transformed before reaching a depth of 5 m from the sea ﬂoor. The phytol/Σn-alkanol ratio is 0.48 for the Yongshu Reef Lagoon and 0.03 for the continental shelf of the East China Sea, indicating that the relative content of phytol in the Yongshu Reef Lagoon is higher than that in the continental shelf of the East China Sea. However, the content of chlorophyll a does not diﬀer signiﬁcantly. This diﬀerence in their phytol contents may be related to the extent of biological and biochemical processes. These imply that phytol and its degraded products are formed by biological and biochemical pathways. (2) n-Alkanols Sinking particulate materials in the Yongshu Reef Lagoon and the continental shelf of the East China Sea contain abundant n-alkanols. Their carbon numbers range from C12 to C28 with a strong even-over-odd predominance (with a CPI16∼28 value of 11.88 and 4.69, respectively), indicating the feature of modern organism genesis. n-Alkanols, with a maxmum at C16 and

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5 Biogeochemical Processes of the South China Sea

− a ΣnC+ 22 /ΣnC22 ratio of 6.10 and 6.70 respectively, are dominated by lower carbon number n-alkanols (Tables 5.25 and 5.26). This distribution shows that they mainly come from marine plankton. However, a higher abundance of C22 n-alkanol is also present in the Yongshu Reef Lagoon, indicating the contribution of submerged macrophytes to n-alkanol. Higher carbon number n-alkanols (>C22 ) are generally thought to be derived from the surface wax of higher plants, but carbon isotope data show that they can also come from bacteria. The relative content of higher carbon n-alkanols is 14.08% for the Yongshu Reef Lagoon and 12.65% for the continental shelf of the East China Sea, showing that the former has higher plant or bacterium-derived n-alkanols. This is identical with the studied results of fatty acids.

Table 5.25. Relative contents of n-alkanols and sterols (Duan et al., 1997) (With permission from Duan Y) Number

n-Alkanols

1 2 3 4 5 6 7 8 9 10 11 12 13

nC12 nC13 nC14 nC15 nC16 nC17 nC18 nC19 nC20 nC21 nC22 nC23 ∼nC28

Relative content (%) NS ECS 7.04 1.15 1.41 0.86 4.23 6.31 2.82 3.16 29.58 28.16 1.41 7.18 5.63 20.98 1.41 2.87 8.45 7.76 1.41 2.30 22.53 6.61 14.08 12.65

Sterols C27 Δ5.22 C27 Δ22 C27 Δ5 5α-C27 Δ0 C28 Δ5.22 C28 Δ22 C28 Δ5 5α-C28 Δ0 C29 Δ5.22 5α-C29 Δ22 C29 Δ5 5α-C29 Δ0 5α-C30 Δ22

Relative content (%) NS ECS 6.44 1.39 2.03 1.85 28.81 37.96 21.02 12.50 14.24 9.25 3.73 4.17 5.08 6.94 1.36 2.32 6.78 2.32 1.02 4.17 7.12 3.70 1.02 0.46 1.36 12.96

Δ5.22 , stenols; Δ0 , stanols; NS, Nansha Sea; ECS, East China Sea

Table 5.26. The parameters of n-alkanol and sterols (Duan et al., 1997) (With permission from Duan Y) Region NS ECS

Σ nC+ 22 /ΣnC− 22 * 6.10 6.70

CPI16∼28 11.88 4.69

Phytol/ Σn-alkanol 0.48 0.03

sterols (C27 Δ0 /C27 Δ5 ) 0.56 0.33

Relative content of sterols (%) C27 C28 C29 C30 58.30 24.41 15.94 1.36 53.70 22.68 10.65 12.96

− *ΣnC+ 22 /ΣnC22 , Σ(C12 −C22 )/Σ(C23 −C28 ) ratio of n-alkanols; NS, Nansha Sea; ECS, East China Sea

(3) Sterols Thirteen sterols have been detected in sinking particulate materials from the Yongshu Reef Lagoon and the continental shelf of the East China Sea. Their

5.5 Biogeochemical Processes in the Nansha Islands Waters

605

carbon numbers range from C27 to C30 . The most abundant sterols of the two regions are cholest-5-en-3β-ol (3) and 5-cholestan-3-ol (4), which account for about 50% of the total sterols (Table 5.25). They are generally thought to originate primarily from zooplankton. This suggests a similarity in the contribution of zooplankton to sinking particulate organic matter in the Yongshu Reef Lagoon and the continental shelf of the East China Sea. C28 sterols in the two regions represent 24.41% and 22.68% of the total sterols respectively (Table 5.26), and the most abundant C28 sterol is 24-methyl cholest-5,22-dien3β-ol (5) which is generally considered to be derived mainly from diatom and used as a biomarker for diatom lipids. C29 sterols are present in low abundances, and 24-ethylcholest-5,22-dien-3β-ol (9) and 24-ethylcholest-5-en-3β-ol (11) are major components of the sterols in higher plants. However, for the marine environment, the two sterols can also come from some phytoplankton. Combining the results with those of hydrocarbon compound and fatty acid distributions, it indicates that the two sterols in sinking particulate materials from the Yongshu Reef Lagoon and the continental shelf of the East China Sea partially originate from higher land plants and their relative contents in the Yongshu Reef Lagoon are higher than those in the continental shelf of the East China Sea (Table 5.25). The input of the higher land plant component in sinking particulate material of the Yongshu Reef Lagoon is most probably in the form of aeolian dusts or polluted materials formed by human activity on the Yongshu Reef island, as the Yongshu Reef is separated from land by deep sea. C30 4α, 23,24-triethyl-5α-cholest-22-en-3β-ol (13) derived from dinoﬂagellates in Pyrrophyta comprise 1.36% and 12.96% of the total sterols in the Yongshu Reef Lagoon and the continental shelf of the East China Sea respectively, showing that sinking particulate organic matter in the continental shelf of the East China Sea has a more dinoﬂagellate-derived component. Sterols can come from both organisms and biochemical conversion of stenols to stanols. A C27 Δ0 /C27 Δ5 ratio of 0.1∼0.2 is thought to represent the constitution of stenols and stanols in plankton. The C27 Δ0 /C27 Δ5 ratio is 0.56 for the Yongshu Reef Lagoon and 0.33 for the continental shelf of the East China Sea, showing that the biochemical transformation process of sterols is greater in the Yongshu Reef Lagoon than in the continental shelf of the East China Sea. This is consistent with the result for isoprenoid compounds (Duan et al., 1997). 5.5.5 Vertical Transferring Process of Major and Rare Elements in the Nansha Coral Rreef Lagoons In geology, rare elements (REs) are deﬁned as the elements whose contents in the earth are low, or have diﬃculty in forming signal mineral in nature although their total amount in the earth is very huge (Zhao and Yan, 1994). The major elements in the various environment matrixes are one of the most important subjects in the environment sciences.

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5 Biogeochemical Processes of the South China Sea

5.5.5.1 Rare Elements Most REs were discovered in the 19th century. More than one hundred years has passed since their discovery. However, some geochemical issues are still paradoxical (Song and Li, 1997). The contents of REs in the earth are so small that they seem more sensitive to environmental changes than common elements do. For example, common elements usually change in content from an order of magnitude of 10−3 to 10−2 when the circumstances suddenly change; however, the contents of REs will change from tens to hundreds of times (Song and Li, 1998). Studying the changes in REs in environments would infer accurately what had happened to the environmental changes in the past. So, studying the geochemistry of REs becomes more and more signiﬁcant in the context of global climate change. Previous research into REs in the oceans mostly concentrated on sediments and have made great progress in understanding the origin of sediments and the contributing factors of authigenic mineral forming. However, RE research in seawater is far from satisfactory; what is more, in some sea areas the research is still rare. Over a long period, RE research in seawater was limited due to their extraordinarily low content (10−10 μg/L) and diﬃculty in determining this. But, in any case, the study certainly plays an important part in the chemical oceanography of REs. By weathering, dripping and other processes, part of the REs in the earth’s crust ﬁnally enter oceans from rivers, and then take part in the ocean’s biogeochemical cycle. REs in seawater are transferred from ions into particulates. The transformation of REs in seawater leads to their redistribution in diﬀerent phases. It is obvious that REs take part in almost all biogeochemical processes in the ocean. The vertical transformation of REs in sinking particulates is one of the most important processes. The characteristics of the vertical transformation of REs would reﬂect the information about environmental changes in certain contents. (1) Characteristics and components of sinking particulates In the sinking particulates collected from coral reef lagoons in the Nansha Islands, SCS, during three cruises great diﬀerences were found to exist in the characteristics and the chemical components compared to those in open oceans and compared to sediments. More organic particulates, such as secretion, excreta, and dead bodies, were trapped. Chemical analysis showed that organic carbon content in sinking particulates reached 0.96%∼96%, organic nitrogen, 0.11%∼0.26%, and organic phosphorus, 209.17×10−10 ∼912.46×10−10 μg/L. Biogenic CaCO3 was the main component in mineral detritus and its content reached 90%. The vertical ﬂuxes of sinking matter during three cruises (93-5, 94-4, 93-94) were 3.26, 4.60, and 22.93 g/(m2 ·d) respectively. (2) Vertical ﬂuxes and contents of REs in sinking particulates The vertical ﬂuxes and contents of REs in sinking particulates collected during three cruises are listed in Table 5.27. The contents of 18 elements in sinking particulates are greatly diﬀerent; for instance, Ti is higher than 150 mg/g, As,

5.5 Biogeochemical Processes in the Nansha Islands Waters

607

Zn, and V are higher than 1 mg/g, respectively, and the rest are below 1 mg/g. The data indicate that the ﬂuxes of most elements in the Yongshu Reef Lagoon are higher than those in the Zhubi Reef Lagoon except the ﬂuxes of Zr, Co, Zn, and Sb. Results obtained from the same lagoon (the Zhubi Reef) during two cruises are similar. The nature of REs and environmental conditions in the coral reef determined the distribution of REs in diﬀerent phases of each element. Geography, density of water exchange species, biomass of biocoene, etc., have a strong inﬂuence on the distributions of REs. The Yongshu coral reef is a nearly open lagoon, parts of the sinking particulates would come from the external sea by water exchange, so the mass ﬂux of materials is higher than that in the Zhubi Reef, a typical closed lagoon. The water dynamic factor greatly inﬂuenced the ﬂux. The sea current and tide observed at the same time indicated that current sea velocity and tide in the Yongshu Reef were obviously higher than those in the Zhubi Reef. Strong water dynamics would increase the collision of particles, so lots of sinking particulates would form. The average velocity of the current in the Zhubi Reef was 34 cm/s during the 94-4 cruise and 16 cm/s during the 93-5 cruise (5 m layer) (Yu and Xu, 1994). The vertical ﬂuxes of the elements during the 94-4 cruise were evidently higher than those during the 93-5 cruise except for Au, Cr, Co, Zn, Sb, and Se, as shown in Table 5.27. The details will be discussed below. Table 5.27. Contents (μg/g) and vertical ﬂuxes (μg/(m2 ·d)) of REs in sinking particulates (Song and Li, 1998) Element Zr Hf Ta V Ti Rb Cs Ag Au Cr Co Zn As Sb Sc Se U Th

93-5 Content Flux 1.00 3.28 0.002 0.007 0.0038 0.0125 2.47 8.10 200 656 0.006 0.020 0.001 0.003 0.004 0.013 0.0027 0.0089 1.16 3.81 0.019 0.062 17.3 56.7 1.0 3.3 0.33 1.09 0.0053 0.0174 0.024 0.079 2.66 8.72 0.0032 0.0105

94-4 Content Flux 0.76 3.5 0.006 0.025 0.0006 0.0028 5.84 26.86 200 920 0.060 0.276 0.003 0.014 0.004 0.018 0.0160 0.0736 0.33 1.51 0.017 0.078 5.0 22.9 2.4 10.9 0.15 0.69 0.0057 0.0262 0.009 0.041 2.66 12.24 0.0047 0.0216

93-94 Content 0.03 0.002 0.0034 5.09 180 0.054 0.001 0.004 0.0024 0.09 0.002 0.7 1.8 0.03 0.0032 0.081 2.35 0.0008

Flux 0.66 0.041 0.0790 116.81 4, 127 1.238 0.021 0.083 0.0557 2.13 0.041 16.3 41.3 0.70 0.0722 0.186 53.86 0.0186

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5 Biogeochemical Processes of the South China Sea

(3) Vertically transferring forms of REs Table 5.28 lists the ﬂuxes of REs in diﬀerent forms and their relative ratios in sinking particulates in the Yongshu Reef Lagoon. The data indicate that Ta, As, Th were transferred to the sea ﬂoor mainly in the ion-exchange form. The elements transferred in this form are likely to be combined with particulates in a weak chemical bond, such as adsorption. A living organism as “a swimmer” which enriched REs was trapped. This is possibly another method of transfer (Song and Li, 1996a). The elements transferred in the carbonate form are Zr, Hf, V, Ti, Rb, Cs, Au, Cr, Co, Zn, Sc, Se, and U. Their transformation could include two ways: one is in detritus wrapped REs, the other is in a compound of carbonate. Zr, Hf, Rb, Cs, and Sc transferred in the former way, while V, Ti, Table 5.28. Contents and ﬂuxes of diﬀerence RE forms in sinking particulates of the Yongshu Reef Lagoon (Song and Li, 1998)

Zr Hf Ta V Ti Rb Cs Ag Au Cr Co Zn As Sb Sc Se U Th

C ND ND 0.0029 ND 1.4 ND ND ND ND ND ND 0.09 0.80 ND ND 0.0001 0.179 0.0061

IEF F ND ND 0.0663 ND 32.1 ND ND ND ND ND ND 1.97 18.34 ND ND 1.0014 4.140 0.0399

R ND ND 64.2 ND 0.6 ND ND ND ND ND ND 12.1 44.4 ND ND 0.7 8.6 75.3

C 0.614 0.0021 0.0004 4.86 166.0 0.0502 0.0035 0.0025 0.0024 0.046 0.0018 0.35 0.33 0.004 0.0013 0.0054 0.043 0.0003

CF F 14.074 0.0482 0.0087 113.86 3, 806.4 1.1511 0.0800 0.0573 0.0555 1.043 0.0413 7.93 7.61 0.083 0.0287 0.1247 46.846 0.00069

OSF C F R Zr 0.035 0.812 5.3 Hf ND ND 0 Ta ND ND 0 V 0.11 2.51 1.6 Ti 27.2 622.8 13.4 Rb 0.0006 0.0138 1.1 Cs 0.0001 0.0023 2.6 Ag 0.0080 0.0183 38.60 Au ND ND 0 Cr 0.020 0.459 19.1 Co 0.0009 0.0197 20.5 Zn 0.02 0.54 3.3 As 0.06 1.45 3.5 Sb 0.018 0.413 49.2 Sc 0.0012 0.0284 34.5 Se 0.026 0.0605 30.9 U 1.166 1.516 2.8 Th 0.0010 0.0220 12.3 ND, not detected; C, content; F, ﬂux; R, relative ratio (%); iron-manganese oxides form; OSF, organic matter+sulphide

R 92.5 77.5 8.4 69.9 81.5 93.0 88.8 12.4 92.0 43.5 42.9 48.7 18.4 9.8 34.8 63.6 87.0 3.7

C ND 0.0004 ND 1.94 ND 0.0005 0.0001 0.0100 0.0001 0.032 0.0011 0.25 0.56 0.001 ND ND 0.026 0.0001

IMOF F ND 0.0089 ND 44.72 ND 0.0115 0.0023 0.2298 0.0022 0.743 0.0252 5.66 12.82 0.023 ND ND 0.596 0.0028

SIF C F 0.015 0.335 0.0002 0.0050 0.0012 0.0282 0.05 1.05 9.1 209.1 0.0027 0.0619 0.0002 0.0055 0.0002 0.0041 0.0001 0.0028 0.007 0.151 0.0004 0.0101 0.01 0.19 0.05 1.05 0.014 0.321 0.0011 0.0252 0.0004 0.0094 0.035 0.805 1.0006 0.0147 IEF, ion exchangeable form; form; SIF, silicate form

R 0 14.4 0 27.8 0 0.9 2.5 18.4 3.4 31.0 26.2 34.8 31.1 2.7 0 0 1.1 1.3 R 1.2 9.1 27.4 0.7 4.5 5.0 6.1 0.6 4.6 6.4 10.4 1.1 2.6 38.3 30.7 4.8 1.5 7.4 IMOF,

5.5 Biogeochemical Processes in the Nansha Islands Waters

609

Cr, Co, Zn, and U did so in the latter way. The trend in the transformation of Ag, V, Cr, Zn, As, and Hf was to the form of iron-manganese oxides, such as the co-precipitation of AgO or AgOH combining with iron-manganese oxides. The ﬁne and close con-structure of the co-precipitation makes it easy to sink. Sb was transferred in a combination of organic matter and sulphide, where it was likely that the Sb-sulphide form was dominant. Another transferring forms of Sc, Ag, Se, and Th would be their organic compounds in sinking particulates. The main transferring forms of 18 elements in the Yongshu coral reef are listed in Table 5.29. Table 5.29. Main vertical transferring forms of REs in sinking particulates of the Yongshu Reef Lagoon (Song and Li, 1998) Element Zr Hf Ta V Ti Rb Cs Ag Au

Main forms CF CF, IMOF IEF, SIF CF, IMOF CF, OSF CF CF IMOF, OSF, CF CF

Elements Cr Co Zn As Sb Sc Se U Th

Main forms CF, IMOF, OSF CF, IMOF, OSF, SIF CF, IMOF, IEF IEF, IMOF, CR OSF, SIF CF, OSF, SIF CF, OSF CF IEF, OSF

(4) Comparison of RE contents in sinking matter and in sediment of the Nansha Islands Table 5.30 shows the ratios of RE contents in sinking matter (SM) to those in sediment (S). REs may be classiﬁed in ﬁve groups according to the ratios. Table 5.30. The classiﬁcation of the REs based on the ratios of RESM /RES (%) (Song and Li, 1998) Clssiﬁed number 1 2 3 4 5

RESM /RES (%) >100 10∼100 1∼10 0.1∼1 <0.1

Elements U Zn, As, Sb, Se V, Ti Ta, Au, Cr, Co Zr, Hf, Rb, Cs, Ag, Sc, Th

RESM : RE contents in sinking matter; RES : RE contents in sedimemt

In general, most REs were carried into the ocean from the earth’s crust in ﬁne particles, so most of them may precipitate at an early stage. REs in sinking particulates were redistributed, so the higher the surviving ratios of

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REs in sinking matter, the higher the active parts of REs, i.e., the lower the silicate form. Comparing Tables 5.28 and 5.29 with Table 5.30, most of REs corresponded to the ratios, except for Sb. The high ratio of Sb, 34.31%, showed that Sb was enriched in the combining form of organic matter and sulﬁde in sinking particles. Table 5.31 also illustrates the point. Because of the strong reduction in the Nansha lagoons, S2− may co-precipitate Sb (Song and Li, 1996a; 1996b). Living organisms enriched with Sb may further change it into a silicate crystal lattice after they come into sediment traps. Consequently, the main vertical transformation form of Sb was the organic matter sulﬁde form and the silicate form. Table 5.31. Ratios (%) of RE concentrations in sinking matter to those in sediment of the South China Sea (Song and Li, 1998) Element Zr Hf Ta V Ti Rb Cs Ag Au Cr Co Zn As Sb Sc Se U Th

93-5 0.0043 0.03 0.35 4.0 5.9 0.007 0.016 0.007 0.34 2.19 0.21 28.36 13.89 66.80 0.07 21.82 133.0 0.03

94-4 0.0032 0.08 0.05 9.6 5.9 0.067 0.047 0.007 2.00 0.62 0.19 8.15 32.92 30.0 0.07 8.18 133.0 0.04

93-94 0.00013 0.03 0.31 8.4 5.3 0.060 0.014 0.006 0.30 0.17 0.02 1.17 25.00 6.12 0.05 73.64 117.5 0.01

Average 0.0025 0.05 0.24 7.3 5.5 0.045 0.026 0.007 0.88 0.99 0.14 12.52 23.94 34.31 0.06 34.55 127.8 0.03

RE (μg/g) 235 6.5 1.1 61 3, 400 90 6.4 60 0.8 53 9 61 7.2 0.5 8 0.11 2.0 11.9

(5) Relationship between vertical ﬂuxes of REs and SST Vertical transferring processes of REs are controlled by many factors. SST, one of the factors, can accelerate or reduce the velocity and ﬂux of REs vertical transformation. Global climate change can cause SST variation, and SST variation can be deduced from the change in REs vertical ﬂux, therefore global climate change in the past might be deduced. There are four kinds of changes in RE vertical ﬂux with SST (unit: T (◦ C), F (μg/m2 ·d)): (i) Exponential increase Zn : F = 6.10 × 10−15 e1.20T + 16.00

(5.12)

5.5 Biogeochemical Processes in the Nansha Islands Waters

611

Sb : F = 8.83 × 10−32 e2.31T + 0.68

(5.13)

Hf : F = −0.0133T + 0.412

(5.14)

Cs : F = −0.00656T + 0.197

(5.15)

(ii) Linear decrease

(iii) Exponential decrease Ti : F = 2.98 × 1032 e−2.39T + 650

(5.16)

As : F = 5.17 × 1036 e−2.85T + 3.20

(5.17)

Rb : F = 5.46 × 1046 e−3.69T + 0.019

(5.18)

Ag : F = 5.54 × 105 e−0.62T + 0.01

(5.19)

U : F = 6.15 × 1022 e−1.77T + 8.5

(5.20)

V : F = 1.67 × 1042 e−3.26T + 8.0

(5.21)

(iv) No relation Au, Se, Th, Cr, Zr, Sc, Ta, Co. Table 5.32 shows the relationship between RE vertical ﬂux and SST. Vertical ﬂuxes of 10 elements (Zn, Sb, etc.) with SST have a close correlation, but the sensitivity of each rare element SST is diﬀerent. The order of sensitivity is Rb>V>As>Ti>U>Zn>Sb>Hf>Ag>Cs. Vertical ﬂux changes of Rb, V, As, Ti, U, Zn can be used for forecasting SST variation. Table 5.32. The relationship between RE vertical ﬂux (μg/(m2 ·d)) in lagoon and SST (◦ C) (Song and Li, 1998) Month 1 SST 27.6 Zn 17.48 Sb 0.68 V 1410 Ti 7355 As 359.6 Rb 321.3 Ag 0.03 U 45.89 Cs 0.02 Hf 0.04

2 3 4 5 6 7 8 9 10 27.5 28.2 29.6 30.6 30.2 29.7 29.5 29.5 29.4 17.31 19.03 32.27 70.03 49.43 34.35 30.43 40.43 28.80 0.68 0.69 0.72 1.12 0.86 0.74 0.72 0.72 0.71 1945 206.7 10.07 8.08 8.30 9.52 10.86 10.86 11.96 9166 2248 706.3 655.2 663.4 694.3 721.5 721.5 740.8 477.1 67.65 4.39 3.27 3.42 4.10 4.79 4.79 5.31 464.7 35.12 0.21 0.002 0.002 0.15 0.31 0.31 0.44 0.03 0.02 0.02 0.01 0.01 0.02 0.02 0.02 0.02 53.13 21.43 9.58 8.68 8.88 9.41 9.80 9.80 9.94 0.02 0.01 0 0 0 0 0 0 0 0.05 0.04 0.02 0.01 0.01 0.02 0.02 0.02 0.02

11 12 29.0 28.2 23.92 19.03 0.69 0.69 22.61 206.7 886.2 2248 9.79 67.65 1.85 35.12 0.02 0.02 11.64 21.43 0 0.01 0.03 0.04

612

5 Biogeochemical Processes of the South China Sea

5.5.5.2 Biogeochemical Process of Major Elements in Sinking Particulates of the Nansha Coral Reef Lagoons, the South China Sea The majority of major elements will be deposited in estuaries and coastal sediments after they have been dredged through precipitation and runoﬀ. Several major elements are transferred to the open sea or to the seabed sediments as dissolved forms. By virtue of these detailed and deep researches, the distribution and variation rules of major elements in seawater and sediments have been revealed. However, no detailed information on the processes that take place when major elements are transferred from seawater to sediments is available now. After being transferred to the seabed sediments through the deposition of suspended particulates, the concentrations of major elements in particulate matter would generally diﬀer from those in sediments. On the one hand, the transformation between the particulate form and dissolved form will change the major elements concentrations. The uptake and excretion processes of organisms also have an inﬂuence on the geochemistry characteristics of the major elements. When the major elements were transferred to the seabed sediments, they were redistributed, therefore their concentrations varied subsequently. Table 5.33 summarizes and tabulates their concentrations and ﬂuxes. Table 5.33. The concentrations and ﬂuxes of major elements in the sinking particulates in Nansha coral reef lagoons (Song, 1997a) Element Na K Cl Br I Ca Mg Ba Fe Sr Al

93-05 Concentrations Fluxes 4, 250 13, 940 8, 790 28, 831 1, 250 4, 063 192 630 73.8 242.1 450, 000 1, 476, 000 32, 900 107, 192 4.09 13.42 7.63 25.03 72.2 236.8 679 2, 227

94-04 Concentrations Fluxes 4, 200 19, 320 2, 000 9, 200 2, 146 9, 872 352 1, 619 74.1 340.9 442, 500 2, 035, 500 31, 780 146, 188 2.14 9.84 69.90 321.54 82.0 377.2 840 3, 864

93-94 Concentrations Fluxes 3, 170 72, 693 1, 786 40, 935 1, 264 28, 974 197 4, 526 52.0 1, 192.8 398, 700 9, 140, 219 26, 879 616, 342 0.58 13.21 59.13 1, 355.82 82.0 1, 880.0 236 5, 407

The concentrations and ﬂuxes varied by 4 orders of magnitude among the 11 major elements. The sequence of the concentrations from high to low is: Ca>Mg>Na, K>Cl>Al>Br>I>Sr>Ba, so are the ﬂuxes. With respect to sinking ﬂuxes, Ca exceeds 1.4 g/(m2 ·d), and Mg exceeds 0.1 g/(m2 ·d). In particular, the concentrations and ﬂuxes of Ca during the three cruises was about 13 factors higher than those of Mg. In the Nansha Sea area, a large number of coral reefs are developed, where reef-building corals live and produce structurally complex calcareous skeletons using inorganic Ca and Mg. Due to the majority of suspended particulate samples is coral crumb in Nansha coral reef, this raises the concentrations of Ca and Mg in suspended particulate matter. In addition, Br and I, as typical bio-modiﬁed elements,

5.5 Biogeochemical Processes in the Nansha Islands Waters

613

have high sinking ﬂuxes in the range of 0.63∼4.5 and 0.24∼1.19 g/(m2 ·d) respectively. Five forms are classiﬁed to reveal the major elements geochemistry characteristics as follows: ion exchangeable form (IEF), carbonate bound form (CF), iron and manganese oxides bound form (IMOF), organic matter and sulﬁde bound form (OSF), and silicate bound form (SIF). As shown in Table 5.34, the CF ﬂuxes of Na, K, Ca, Mg, and Sr account for 97.8%, 93.0%, 99.8%, 99.6%, and 99.8% of all the sinking ﬂuxes respectively, indicating that CF is the predominant form during their sinking process. Ca, Mg, and Sr are deposited on the bed sediments as CaCO3 , MgCO3 , and SrCO3 , while Na and K may be entrained in deposition. Al and Br are transferred from seawater to sediments with the major form of SIF, while Fe, I, and Ba are deposited mainly in IMOF. It is notable that the dominant forms of Cl in the sinking process are CF and SIF; OSF is the second important form for Br in its deposition; CF and SIF play equal roles followed by IMOF for I in deposition. The percentages of Cl, Br, and I forms suggest that Br is modiﬁed mainly in an organic form, while I is modiﬁed mainly in an inorganic form. Table 5.34. The concentrations, ﬂuxes and percentages of four forms of the major elements in the sinking particulates (Song, 1997a) CF IMOF OSF C F R C F R C F R C Na 3,102 71,132 97.8 9.3 213 0.29 7.2 166 0.23 51.5 K 1,660 38,064 93.0 17.2 394 0.95 17.6 404 0.99 91.2 Cl 1,063 24,375 56.3 ND ND 0 ND ND 0 825 Br 44.1 1,012 22.4 7.7 175 3.87 60.6 1,390 30.7 85.0 I 18.1 414 22.9 43.4 994 54.9 ND ND 0 17.6 Ca 432,273 9,912,029 99.8 619 14,204 0.14 121 2,774 0.03 91.2 Mg 26,753 613,453 99.6 17.2 394 0.06 17.6 394 0.07 91.2 Ba 0.26 5.98 37.1 0.39 8.99 55.7 ND ND 0 0.05 Sr 84.31 1,933 99.8 0.13 3.03 0.16 0.01 0.18 0.01 0.03 Fe 22.11 507 31.0 45.3 1,038 63.4 3.21 73.7 4.50 0.78 Al 118.0 2,705 45.5 8.8 201 3.38 21.0 491 8.11 112 C, content; F, flux; R, relative ratio (%). The total IEF of major elements was not detected (ND) Element

SIF F 1,182 2,091 18,926 1,948 403 2,091 2,091 1.22 0.65 18.0 2,562

R 1.63 5.11 43.7 43.0 22.2 0.02 0.34 7.15 0.03 1.10 43.0

After being transferred downward, part of the major elements will be involved in recycling, and the other part will be eventually deposited. Except for the molecular ratio of Br/I, all the molecular ratios of other cognation elements in SPM were lower that those in the lagoon sediments. Taking the major element concentration in the sediments as the ﬁnal concentration and the concentration of SPM as the ﬁnal concentration, the diﬀerence between the ﬁnal concentration and the original concentration was the part of major elements involved in the recycling process. Likewise, taking the concentrations in the block coral as the ﬁnal concentrations and those in SPM as the original concentrations, the diﬀerence between them was the concentrations in the sandy detritus. Subsequently, the proportions of the recycling and buried concentrations were calculated and represented in Tables 5.35 and 5.36. Table 5.35 showed that the majority of Br, I, Al, and K were involved in the recycle; about half of Mg would be recycled. In contrast, Ca was ﬁnally

614

5 Biogeochemical Processes of the South China Sea

buried mainly in the sediments. The sequence of the major elements buried in the sediments from high to low was: Ca, Mg, I, Al, Br, and K. The ratio of the percentage of major elements in sandy detritus to those in block coral showed that Ba, Sr, and Fe predominately existed in block coral, while Na and Cl were distributed evenly in block coral and sandy detritus. Considering Tables 5.35 and 5.36 together, a series conclusions could be drawn, namely that: (1) the majority of Ca and Sr playing an important role in coral reef building were buried in the sediment eventually; (2) Br and I were biomodiﬁed and the majority were recycled; (3) the majority of Fe and Ba were buried in the sediments; (4) Na, K, Mg, and K were deposited mainly by coral detritus, and more than half of them would be involved in the recycling process. Table 5.35. The proportions of the major elements involved in recycling and burial in the sinking particles of the Yongshu Reef (Song, 1997a) Element Mg K Al Ca Br I

Recycling proportions (%)/buried proportions (%) 93-05 94-04 93-94 Average 62.3/37.7 61.0/39.0 53.9/46.1 59.1/40.9 98.1/1.9 91.6/8.4 90.6/9.4 93.4/6.6 93.2/6.8 94.5/5.5 80.4/19.6 89.4/10.6 17.9/52.1 16.5/83.5 7.4/92.6 13.9/86.1 90.1/10.9 94.0/6.0 90.4/10.6 96.8/9.2 77.0/23.0 77.1/22.9 67.3/32.7 73.8/26.2

Table 5.36. The proportions of the major elements in the sandy detritus and in the block coral of the Yongshu Reef (Song, 1997a) Element Cl Fe Na Sr Ba

93-05 33.8/66.2 4.6/95.4 58.2/41.8 1.4/98.6 45.7/54.3

Sandy detritus/block coral 94-04 93-94 58.0/42.0 34.2/65.8 42.1/57.8 35.6/64.4 57.5/42.5 43.4/56.6 1.6/98.4 1.6/98.4 23.9/76.1 6.4/93.6

Average 42.0/58.0 27.4/72.6 53.0/47.0 1.5/98.5 25.3/74.7

The deposition process of the major elements was strongly aﬀected by the SST. Fig. 5.30 illustrates that the deposition processes of 11 major elements except K and Ba decreased exponentially with an increase in SST, while the deposition processes of K and Ba were SST free. The correlation between SST and the deposition ﬂuxes was obtained based on the curvilinear regression as follows:

5.5 Biogeochemical Processes in the Nansha Islands Waters

615

Na : F = 5.24 × 1033 e−2.38T + 1.35 × 104

(5.22)

Cl : F = 1.87 × 1025 e−1.72T + 4000

(5.23)

Ca : F = 4.58 × 1032 e−2.12T + 1.40 × 106

(5.24)

Mg : F = 1.86 × 1025 e−1.557 + 1.00 × 106

(5.25)

Sr : F = 1.17 × 1024 e−1.72T + 220

(5.26)

Br : F = 1.35 × 1035 e−2.60T + 700

(5.27)

I : F = 1.02 × 1023 e−1.67T + 240

(5.28)

Fe : F = 1.37 × 1032 e−2.39T + 10

(5.29)

Al : F = 3.09 × 1019 e−1.32T + 2400

(5.30)

The monthly monitor of the vertical ﬂuxes and SST is represented in Table 5.37, in which it could be concluded that the sensitivities of major elements to SST are in the following sequence: Fe>Br>Sr>Ca>Na>Cl>Mg>I>Al (Song, 1997a). Table 5.37. The average vertical ﬂuxes (μg/(m2 ·d)) of the major elements and SST in the Nansha Coral Reef Lagoon over 12 months (Song, 1997a) Month 1 2 3 4 5 6 7 8 9 10 11 12

SST Na Cl Ca Mg (◦ C) (×104 ) (×104 ) (×104 ) (×104 ) 27.6 16.45 5.31 17.75 5.90 27.5 20.51 6.23 21.61 6.73 28.2 4.97 2.15 5.97 2.94 29.6 1.48 0.56 1.63 1.22 30.6 1.36 0.43 1.43 1.05 30.2 1.38 0.46 1.47 1.09 29.7 1.45 0.53 1.59 1.19 29.5 1.51 0.59 1.69 1.26 29.5 1.51 0.59 1.69 1.26 29.4 1.56 0.62 1.76 1.30 29.0 1.89 0.84 2.24 1.56 28.2 4.97 2.15 5.97 2.94

Sr

Br

3,299 10,738 3,876 35,365 1,319 2,813 319 756 238 704 255 712 304 743 338 772 338 772 360 794 498 965 1,319 2,813

I 1,335 1,534 643 279 247 254 273 286 286 295 346 643

Fe 3,081 3,910 742 36 12 16 30 42 42 52 118 742

Al 7,188 7,863 4,570 2,742 2,492 2,555 2,700 2,791 2,791 2,846 3,155 4,570

5 Biogeochemical Processes of the South China Sea

2 F ( m g/(m d))

616

.

Fig. 5.30. The relationships among the vertical ﬂuxes (μg/(m2 ·d)) of major elements and SST (◦ C) (Song, 1997a)

5.5.6 Sulﬁde (−2 Valence) in Lagoon and Oﬀ-Reef Sediment Interstitial Waters The concentrations of H2 S, HS− , and S2− were used to calculate the total −2 valence sulfur (sulﬁde) concentrations ( S(–II)) according to the following equation: (5.31) S(−II) = H2 S + HS− + S2− The vertical proﬁles of −2 valence sulﬁde ( S(−II) and HS− ) in the pore water from the in- and oﬀ-reefs are presented in HS− was the predominant content of total S(−II), exceeding 90%. The concentrations of S(−II) and HS− increased with depth except the 94-8, as Fig. 5.31 shows. Results of the two year monitor showed that the average S(−II) of the pore water from in-reef lagoon sediments was much higher than that from oﬀreef sediments. S(−II) of in-reef lagoon pore water was higher by about 2

5.5 Biogeochemical Processes in the Nansha Islands Waters

10 0

11

12

SS(-II) (mmol/L) 23 0

24

25

Depth (m)

(a) 10

20

20 30

40

0

(c) 10

26 (b)

10

20 0

617

8.0

8.5

9.0

9.5 (d)

10

SS(-II) 20

20

HS-

Fig. 5.31. Vertical proﬁles of sulﬁde (–2 valence) in oﬀ-reef sediment interstitial waters in the water region of the Nansha Islands, South China Sea (Song and Li, 1996b). (a), (b), (c), and (d) illuminated the vertical distribution of –2 valence sulfur in the water sample from 94-8, 94-12, 94-16, and 94-23 stations respectively. The solid and dashed lines represented the ΣS(−II) and HS− respectively

factors than that in oﬀ-reef lagoons in 1993 and by over 4 factors in 1994. The reason might be that in-reef lagoons were characterized by a high nutrients level and high productivity, and the vital activities of organisms living there, such as growth, propagation, and excretion, could raise the reducibility of the sediment environment. In particular, Eh values of the sediments from the Banyue Reef and the Xinyi Reef were 5 mV and −26 mV, respectively, indicating the high reducibility of the sediment environments. Considering the whole study area, S(−II) of the pore water from the in-reef lagoon declined southward with latitude in general. In the investigation performed in March∼April 1994, S(−II) at 97-Y located in the Yongshu Reef, the most northerly station, was as high as 353.12 μmol/L and declined sharply to 3.90 μmol/L at 94-10H, the most southerly station in the study area. In an east-west direction, S(−II) declined with the increase in longitude. For the pore water from the oﬀ-reef sediments, S(−II) was distributed in a radial pattern, i.e., the higher S(−II) was, the closer the station was to the coral reef. For example, the S(−II) detected in 93-6 (the closest station to the Yongshu Reef, 135.14 μmol/L) and 93-6A (the closest station to the Sanjiao Reef, 36.55 μmol/L) were quite a bit higher than those in other stations. In the investigation performed in March∼April 1994, the maximum value occurred at 94-16 (29.92 μmol/L), close to the Banyue Reef.

618

5 Biogeochemical Processes of the South China Sea

Anthropogenic activities would increase the reducibility of the sediment environment to some extent. For example, the Zhubi Reef and the Yongshu Reef were explored in recent years; therefore, the S(–II) values of the two reefs were 1∼2 orders of magnitude higher than other reefs, as high as 235.06 and 353.13 μmol/L, respectively (Table 5.38). A large amount of sewage was discharged into the lagoons, where the organic matter decomposed, thus the reducibility of the sediment environment increased and −2 valence sulfur was released. Table 5.38. Sulfur of interstitial water in lagoon and oﬀ-reef sediments in the Nansha Islands waters, South China Sea (Song and Li, 1996b) P S(−II) HS− (μmol/L) (μmol/L) Lagoon 21.03∼235.06 20.78∼232.0 (93.02) (91.88) 1993 Oﬀ-reef 19 25.93∼135.14 25.93∼133.46 (43.03) (43.03) Lagoon 15 3.90∼353.12 3.84∼349.96 (82.75) (81.86) 1994 Oﬀ-reef 4 8.61∼29.92 8.48∼29.46 (18.76) (18.41) All data expressed as range (average) Year

Stations

Sample number 6

H2 S (μmol/L) 0.12∼2.93 (1.14) 0.30∼1.68 (0.64) 0.04∼3.16 (0.89) 0.13∼0.50 (0.35)

PS0

SO2− (g/L) 4

10.41∼11.35 (10.91) 10.86∼11.31 (11.17) 10.00∼12.28 (11.04) 11.41∼11.83 (11.70)

2.60∼2.80 (2.68) 2.59∼2.76 (2.70) 2.57∼2.91 (2.75) 2.66∼2.84 (2.74)

5.5.6.1 Diﬀusion Fluxes of Sulfur across the Sediment-Seawater Interface The diﬀusion ﬂuxes of sulfur were calculated based on Fick’s ﬁrst role and shown in Table 5.39. Comparing the results of 1993 and 1994, it was found that the average diﬀusion ﬂuxes of HS− in the pore water from in-reef sediments did not vary signiﬁcantly in the two years, which were 60.56 and 60.12 μmol/(m2 ·d), neither did SO2− 4 . However, no well-deﬁned diﬀerence occurred between the average diﬀusion ﬂuxes of S2− and SO2− 4 of the pore water from oﬀ-reef sediments in 1993 and 1994. Nevertheless, the diﬀusion ﬂuxes of HS− decrease from 22.64 μmol/(m2 ·d) in 1993 to 7.27 μmol/(m2 ·d) in 1994. 5.5.6.2 Thermal Dynamic Balance of (−2 Valence) Sulfur The −2 valence sulfur in the seawater and sediment derived from the reductive product of SO2− 4 . The reaction formula was as follows: −→ H2 S + 2HCO− 2CH2 O + SO2− 4 3 bacteria

(5.32)

where the CH2 O presents organic matter. The correlation between total or ganic carbon contents with S(−II) was studied, as Fig. 5.32 shows. S(−II) concentrations were in linearly inverse proportion to TOC contents, which

5.5 Biogeochemical Processes in the Nansha Islands Waters

619

Table 5.39. Diﬀusion ﬂuxes of sulfur across sediment-seawater interface in the Nansha coral reef lagoon waters, South China Sea (Song and Li, 1996b) Year

Station

1993

Lagoon

1994

Lagoon

1993

Oﬀ-reef

1994

Oﬀ-reef

H2 S (μmol/(m2 ·d)) −0.90∼181.60 (60.56) −5.32∼297.32 (62.12) 3.55∼96.46 (22.64) −1.31∼16.81 (7.27)

S2− (pmol/(m2 ·d)) −1.90∼0.28 (0.70) −52.70∼80.96 (14.70) −1.88∼6.67 (0.36) −1.22∼1.21 (0.22)

SO2− 4 (mmol/(m2 ·d)) −1.08∼0.63 (−0.39) −1.71∼1.35 (−0.43) −1.17∼0.36 (−0.36) −1.08∼0.35 (−0.33)

All data expressed as range (average)

0.8

TOC (%)

0.7 0.6 0.5 0.4 0.3 15

20

S

25

30

35

S( - II) ( m mol/L)

P Fig. 5.32. Relation of organic carbon (Corg ) and S(−II) of interstitial waters of the oﬀ-reef sediments in the Nansha Islands waters, South China Sea (Song and Li, 1996b)

suggested a new piece of evidence to show that S(−II) derived from SO2− 4 reduction (Song and Zhao, 2001). The pH value detected in Nansha Islands ranged from 7.28 to 8.67, and the Eh value was in the range of −242∼333 mV. SO2− 4 was the predominant content, accounting for 90%∼99.8% of all the sulfur. The reduction reaction (+6 valence) to S2− (−2 valence) under bacterial activities was a of SO2− 4 2− complex process, with the intermediate products of S2 O2− 5 (+4), SO3 (+4), 2− 2− S4 O5 (+2), S2 O3 (+2), and S (0) produced incidentally. In diﬀerent situations, this reduction reaction would occur step by step, or skip some intermediate reaction steps to a lower valence product. It was assumed that sulfur in the pore water from Nansha Islands was reduced to S2− from elementary substance form sulfur, whose reduction reaction could be expressed as follows:

620

5 Biogeochemical Processes of the South China Sea

S + 2e → S2−

(5.33)

The calculated Eh based on the equation and the detected Eh values are shown in Table 5.40. The calculated Eh values were close to the corresponding detected Eh values. Therefore, S2− in the pore water from the Nansha Islands was dominated by the balance of the S and S2− transformation. Moreover, elementary substance form sulfur existed in the sediment from the Nansha Islands. However, elementary substance form sulfur was a kind of unstable substance and inclined to co-precipitate FeS to FeS2 , which had be improved in the sediments from Nansha Island in the previous literature (Song and Li, 1996b). Table 5.40. Comparison of Eh values calculated from S/S2− redox pair (Ehc ) and Eh values measured (Ehd ) (Song and Li, 1996b)

Ehc Ehd

Zhubi Banyue Ren’ai 93-6 93-7 93-8 93-6A 94-12 Reef Reef Reef −0.175 0.050 0.055 0.050 0.102 0.260 0.340 0.225 −0.119 0.010 0.025 0.060 0.061 0.203 0.327 0.163

94-16

94-23

0.210 0.196

0.235 0.292

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Risatti JB, Rowland SJ, Yon D (1984) Stereochemica studies of acyclic isoprenoids. VII. Lipids of methanogenic bacteria and possible contributions to sediments. In: Schenck, PA (ed.) Advance in Organic Geochemistry. Pergamon Press, Oxford, pp.93-103 Schouten S, Breteler WCMK, Blokker P, Schogt N, Rijpstra WIC, Grice K, Baas M, Damste JSS (1998) Biosynthetic eﬀects on the stable carbon isotopic composition of algal lipids: Implications for deciphering the carbon isotopic biomarker record. Geochim Cosmochim Acta 62(8):1397-1406 Shaw PT, Chao SY, Fu LL (1999) Sea surface height variations in the South China Sea from satellite altimetry. Acta Oceanol Sin 22(1):1-17 Simoneit BRT (1977) Diterpenoid compounds and other lipids in deep sea sediments and their geochemical signiﬁcance. Geochim Cosmochim Acta 41(4):463476 Smith SV, Kinsey DW (1976) Calcium carbonate production, coral reef growth, and sea level change. Science 194(4268):937-939 Song JM (1997a) Biogeochemical process of major elements in sinking particulate of Nansha coral reef lagoons, South China Sea. Acta Oceanol Sin 16(4):557-562 Song JM (1997b) Chemistry of Sediment-Seawater Interface of the China Seas. China Ocean Press, Beijing, pp.1-222 (in Chinese) Song JM (1999) The new viewpoint of the high productivity supporting Nansha coral reef ecosystem-sediment drift-net theory. Stud Mar Sin 41:79-85 (in Chinese with English abstract) Song JM (2000) Marine Chemistry in China. China Ocean Press, Beijing, pp.1-210 (in Chinese) Song JM (2004) Biogeochemistry of China Seas. Shandong Science and Technology Press, Jinan, pp.1-591 (in Chinese) Song JM, Li PC (1996a) Iron and manganese in interstitial waters and sediment environments of Nansha Islands, South China Sea. Acta Sci Circum 16(3):294301 (in Chinese with English abstract) Song JM, Li PC (1996b) –2 valance sulfur in sediment interstitial waters of Nansha Islands waters. Oceanol Limnol Sin 27(6):642 (in Chinese with English abstract) Song JM, Li PC (1996c) Studies on characteristics of nutrient diﬀusion ﬂuxes across sediment-water interface in the district of Nansha islands, South China Sea. Mar Sci 20(5):43-50 (in Chinese with English abstract) Song JM, Li PC (1998a) Vertical transferring process of rare earth elements in coral reef ecosystem of Nansha Islands waters, South China Sea. Stud Mar Sin 40:125-130 (in Chinese with English abstract) Song JM, Li PC (1998b) Vertical transferring process of rare elements in coral reef lagoons of Nansha Islands, South China Sea. Sci Chin (Ser D) 41(1):42-48 (in Chinese with English abstract) Song JM, Zhao WD (2001) A new biogenic sulﬁde chemical sensor for marine environmental monitoring and survey. Chin J Oceanol Limnol 19(3):282-286 Song JM, Ma HB, L¨ u XX (2002) Nitrogen forms and decomposition of organic carbon in the southern Bohai Sea core sediments. Acta Oceanol Sin 21(1):125133 (in Chinese with English abstract) Song JM, Xu YF, Hu WP, Ni LY (2008) Biogeochemistry of Carbon in China Seas and Lakes. Science Press, Beijing, pp.1-553 (in Chinese)

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Yu KF, Chen TG, Huang DC, Zhao HT, Zhong JL, Liu DS (2001) The highresolution climate recorded in the delta O-18 of Porites lutea from the Nansha Islands of China. Chin Sci Bull 46(24):2097-2102 Yu KF, Zhao JX, Xei GL, Cheng XR, Chen TG, Felise T, Wang PX, Liu TS (2005) δ 18 O, Sr/Ca and Mg/Ca records of Porites lutea corals from Leizhou Peninsula, northern South China Sea, and their applicability as paleoclimatic indicators. Palaeogeogr Palaeoclimatol Palaeoecol 218(1-2):57-73 Yu KF, Zhao JX, Wang PX, Shi Q, Meng QS, Collerson DK, Liu TS (2006) Highprecision TIMS U-series and AMS 14 C dating of a coral reef lagoon sediment core from southern South China Sea. Quatern Sci Rev 25(17-18):2420-2430 Yuan DL (2002) Dynamics of the Luzon Strait transports. Acta Oceanol Sin 21(2):175-185 Zhai WD, Dai MH, Cai WJ, Wang YC, Wang ZH (2005) High partial pressure of CO2 and its maintaining mechanism in a subtropical estuary: The Pearl River Estuary, China. Mar Chem 93(1):21-32 Zhao YY, Yan MC (1994) Geochemistry of sediments of the China shelf sea. Science Press, Beijing, pp.50-155 (in Chinese) Zheng GX, Song JM, Sun YM, Dai JC, Zhang P (2008) Characteristics of nitrogen forms in the surface sediments of southwestern Nansha Trough, South China Sea. Chin J Oceanol Limnol 26(3):280-288 Zhu YZ, Sha QA, Guo LF, Yu KF, Zhao HT (1997) Cenozoic Coral Reef Geology of Yongshu Reef. Nansha Islands. Chinese Science Press, Beijing, pp.1-134 (in Chinese)

6 Prospects for Marine Biogeochemical Process Research in China

Abstract: In this chapter the main key biogeochemical processes in China marginal seas, the methods, concepts, and focus on marine biogeochemical process research in China are presented. Then we put forward the prospects for biogeochemistry in China marginal seas. Marine biogeochemical process research in China has made progress in the past 20 years. The research methods and concepts of marine biogeochemical processes have been established. Some key processes, including river input, biological action, and others, have been illustrated to some degree (Song, 2004). These processes play an important part in biogenic elements and other chemical components cycling.

6.1 Marine Biogeochemical Process Research in China The methods and concepts of marine biogeochemical processes are very important although our knowledge of marine biogeochemical processes in the marginal seas is still sketchy and we do not yet have a clear outline sketch of the marginal seas’ biogeochemical processes. Therefore, it is necessary to summarize the methods and concepts of marine biogeochemical processes. 6.1.1 Introduction Biogeochemistry is the study of the interactions of the biology, chemistry, and geology of the Earth. In the case of a large body of water such as the ocean, biogeochemistry can be thought of as a huge experiment or a set of reactions. Instead of happening in a clean glass beaker, the reactions have the ocean ﬂoor as the container. Oceans cover 71% of the Earth’s surface area and contain more than 97% of all water on the planet. Their capacity to store and redistribute heat and water around the globe is of profound importance in maintaining the Earth’s environment.

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Marine biogeochemistry is challenging because chemical concentrations and transformation rates are determined by complex combinations of transportation and reaction. To a ﬁrst approximation, the chemical structure of the oceans is determined by the ubiquitous sinking of biogenic particles and local upwelling of solutes regenerated from the chemical and biological dissolution of such particles, but the details are far more intriguing than what this broad-brush summary can reveal. The distribution and circulation of biogenic elements such as carbon (C), nitrogen (N), phosphorus (P), silicon (Si), and sulfur (S) in the ocean are regulated by both physical transportation processes and biochemical transformation by various organisms. These elements may occur in volatile, dissolved, or particulate forms, and thus their biogeochemical cycles in the ocean are closely linked with those in the atmosphere and the lithosphere (Song, 1997; 2000a). Because of its large capacity, the ocean plays a crucial role in maintaining the global cycles and balance of these elements. The upper ocean is the interface between the deep ocean, the atmosphere and terrestrial components of the planet. It plays a crucial role in the regulation of the global environment. The surface water is an essential component in the “conveyor belt” of waters ﬂowing through the ocean basins and is also a major reservoir of life on the planet. Associated exchanges of gases, organic matter biominerals, and trace elements between the atmosphere and deep ocean have profound eﬀects on the global cycles of many important elements. Marine snow is a critical component of marine biogeochemical cycles because it transports organic and inorganic material from the water column to the ocean ﬂoor. The oceanographers use bongo nets to collect the falling debris, which is faintly visible and somewhat resembles falling snow on land (Song, 1997). The surface of the water is open to the air, and every day more dust and dirt from land blows over the ocean and falls in. Moreover, the surface of the water contains many small plant forms that are continually growing and being consumed by animals that are themselves consumed by other animals. As this life and death drama continues, the scraps and leftovers drift downward towards the ocean ﬂoor like a snowfall, hence the name “marine snow”. Around the edges of the ocean, rivers empty water and sediment to the ocean, and mud-dwelling creatures wait for the arrival of their next meal from the falling biological debris (marine snow). These events are linked to each other, to the history of life on the Earth, and to variations in the Earth’s climate. Scientists who study biogeochemistry usually consider the cycling of materials through the diﬀerent parts of the system. To do this, they deal with reservoirs of materials and the ﬂuxes of a substance from one reservoir to another. For example, they examine reservoirs such as the surface ocean water versus the deep ocean water, or the transfer of masses of materials per unit time (ﬂuxes). An example of this kind of approach to biogeochemical cycles in the ocean can be seen in the Joint Global Ocean Flux Study (JGOFS) results, where the reservoirs represented are the atmosphere, lithosphere, terrestrial (land-based) biosphere, surface ocean, phytoplankton, and deep ocean. The

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JGOFS results show the global carbon cycle, a network of interrelated processes that transport carbon between diﬀerent reservoirs on the Earth. Most scientiﬁc studies have focused on the carbon cycle. Carbon, after all, is the basis of life on the Earth, and its gaseous form, carbon dioxide, is linked to the greenhouse eﬀect and changes in the Earth’s climate over time. For these reasons, understanding the carbon cycle has been the focus of several large research programs such as the JGOFS, GLOBal ocean ECosystems dynamics (GLOBEC), Surface Ocean-Lower Atmosphere Study (SOLAS), Integrated Marine Biogeochemistry and Ecosystem Research (IMBER), and Global Carbon Program (GCP). The biogeochemical processes in the oceans, such as CO2 uptake and release, exchange of biogenic trace gases, ﬂuxes of particulate and dissolved organic carbon to diﬀerent depths, variable bacterial activities, and phytoplankton blooms, are critical to the global biogeochemical cycling. The extreme heterogeneity, a characteristic of the coastal zone, which harbors more than 50% of the human population, inﬂuences the carbon cycling and carbon storage on a global scale. Human-driven changes in nutrient availability are known to increase the frequency of toxic algal blooms, development of hypoxia and anoxia, and changes in biomass and productivity. Bays are of particular interest for understanding the linkages between land and sea, as they are heavily impacted by human activity. Since the dynamics of the marine ecosystems are closely related to climate variability, changes in climate are bound to have a signiﬁcant eﬀect on marine ecosystems. For example, an increasing load of CO2 in the atmosphere will lead to increased CO2 concentration in the upper layer and consequently will change the carbonate chemistry, aﬀecting adversely the reef organisms. Also, changes in temperature and circulation patterns will aﬀect the geographical distribution of ﬁshes, their prey, and their predators. Other substances also have well-studied cycles. Water, of course, is constantly moving into, through, and out of the ocean. Some of the atmospheric gases such as oxygen and carbon dioxide are vitally important to life. Nutrient elements such as nitrogen, phosphorus, and silicon are necessary to the phytoplankton, and form the basis of the oceanic food web. The presence of life forms on the Earth is tremendously important in the cycling of elements through the major reservoirs. Take the ocean as an example. If one focuses on the impact of a single diatom on the ocean, the following story emerges. Diatoms are a group of algae living by the millions in each cubic centimeter of surface ocean water. In this water each alga has access to the sunlight needed for photosynthesis, the CO2 (carbon dioxide), N (nitrogen), and P (phosphorus) needed to make its soft tissue, the Si (silicon) needed for its shell-like covering, and a number of rare or trace substances in sea water, including Cu (copper) and Fe (iron). To reproduce, it undergoes cell division. Its life processes produce O2 (oxygen) that can be used by other organisms, organic tissue that becomes food for the next higher creatures in the food web, and often an exudate or slime.

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Once the diatom has been consumed by an animal (a copepod, for example), its life is over, but its eﬀect on the ocean is not. The copepod digests and derives energy from the diatom’s soft tissue, and then packages the remains into a fecal pellet that is discharged as waste to become part of the falling debris (marine snow) headed for the ocean ﬂoor. The pellet lands on the ocean ﬂoor, forming a site for bacteria to live as well as food for them to consume. The inorganic part of the diatom that remains (the silicon shell) will begin to dissolve on the way to the ocean bottom, and Si taken out of the surface water is returned to deeper water as the shell dissolves. Decomposition of sinking organic matter by bacteria returns N, C, and P to the water and removes dissolved O2 . Ocean water itself is changed by life processes. During the growth of diatoms and the consumption of diatoms by zooplankton, carbon is removed from ocean water and in turn from the atmosphere as the diatoms use it to grow. The transfer of this carbon towards the ocean ﬂoor and its partial burial in the sediments are often referred to as the carbon pump; it is one of the processes that slow the accumulation of CO2 in the atmosphere (Song, 2000b). The silicon (Si) used in the diatom shell enters the ocean from rivers, from the hot springs along mid-ocean ridges and by diﬀusion from deep-sea sediments. Diatoms remove Si so eﬃciently from the ocean surface water that it is a very scarce element there, and mixing and upwelling processes are necessary to redistribute enough Si back to the surface to provide vertical proﬁles that illustrate the changing concentration with depth. As common constituents in the ocean, silicon dioxide (SiO2 ), nitrogen as nitrate (NO− 3 ), phosphorus as phosphate (PO3− 4 ), and oxygen (O2 ) are important for diatom growth and are largely consumed. For that reason, Si, N, P, and other biologically important elements are in low concentrations in the surface water of the ocean, and increase with depth. Another consequence of ocean biogeochemistry can be seen in the distribution of O2 with depth. The oxygen content at the surface is relatively high (about 6 ml/L) and is replenished from the air. Deeper in the water, the O2 content begins to decrease with depth, until at about 1,000 m, the value reaches a minimum. The reason for the decrease is the consumption by bacteria of the rain of organic debris (marine snow) falling through the water. The process requires O2 , and below the surface there is no immediate source to return the O2 being used up. The exact amount of O2 at the O2 minimum varies with location in the ocean; below the minimum, O2 content begins to increase again with depth (Song, 2004). The increase is related to water circulation in the ocean. The deep water in the ocean starts out at the surface in polar regions, where it becomes very dense because of the extreme cold, and sinks to great depths in the ocean, carrying with it dissolved oxygen from the surface waters. This cold, dense, deep water ﬂows along the ocean ﬂoor close to the bottom, well beneath the depths of the O2 minimum. These factors combine to give the observed shapes of O2 proﬁles in the ocean.

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There are other processes that play a role in determining the nature of the ocean. For example, hydrothermal activity at mid-ocean ridges results in signiﬁcant changes in the chemistry of ocean water. The water that comes out of these hot springs comes from normal deep-ocean water that runs down into deep cracks on the ocean ﬂoor alongside the ridges. As the water penetrates into the oceanic crust, it becomes heated to very high temperatures, and reacts with the rocks. The water that comes out of the vents is very hot. It contains sulﬁde (S2− ) instead of sulfate (SO2− 4 ), contains no Mg (magnesium) or O2 (oxygen), and contains large amounts of Si (silicon). Because the entire volume of the ocean circulates through the mid-ocean ridge system every 10 million years, these changes are of great signiﬁcance to the oceans and the organisms that live in them. 6.1.2 Focus on Marine Biogeochemical Process Research in China The biogeochemical study of biogenic elements in China marginal seas started relatively late in the world. However, there has been much progress in recent decades (Song, 2004). Research into ecosystem dynamics in the Bohai Sea, particulate ﬂuxes of the East China Sea, biogeochemical processes of biogenic elements in the Taiwan Strait, carbon ﬂuxes in the South China Sea, and the coral reef ecosystem in the Nansha Islands, has pushed forward marine biogeochemical studies in China (Song, 2000a). 6.1.2.1 Biogeochemical Research on Biogenic Elements in China Marginal Seas Biogenic elements include carbon, nitrogen, phosphorus, silicon, oxygen and sulfur, and so on. They have a very close relationship with organisms in the sea, and have always been the focus of study in the biogeochemistry of China marginal seas. With the implementation of global and national biogeochemical programs, the biogeochemistry of biogenic elements in China marginal seas has made much progress over the last 20 years. (1) Carbon biogeochemistry At present, research on the intensity of the carbon source/sink of China marginal seas mainly focuses on the Yellow Sea and the East China Sea. Because of the diﬀerent area and methods used in the study, diﬀerent researchers have diﬀerent results. However, all of the Bohai Sea, the Yellow Sea, the East China Sea, and the South China Sea are the sink of CO2 , and they can absorb 284×104 , 1,665×104 , 896×104 , and 188×104 t/yr carbon from the atmosphere, respectively. The intensity of the carbon source/sink of China marginal seas has the characteristic of great seasonal variation. In spring and winter, all of the Bohai Sea, the Yellow Sea, and the East China Sea are the sink of CO2 ; these three seas altogether can absorb 769×104 and 1,356×104 t carbon in spring and winter, respectively. In summer, all of China marginal

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seas are the source of CO2 to the atmosphere, and they can release 459×104 t carbon. In autumn, the Bohai Sea and the North Yellow Sea are the sink of CO2 , and they can absorb 27×104 t carbon from the atmosphere. However, the South Yellow Sea and the East China Sea are the source of CO2 in autumn, and they can release 324×104 t carbon to the atmosphere (Song et al., 2008). Thus, all of eastern China marginal seas release CO2 to the atmosphere in autumn. In China, the cycling of organic carbon in the open ocean has received a lot of attention over the last two decades. Organic carbon profoundly inﬂuences the marine biogeochemical cycle of carbon. Organic carbon is mainly composed of dissolved organic carbon (DOC) and particulate organic carbon (POC). DOC and POC represent 89% and 11% of the total organic carbon in the ocean, respectively. The average DOC inventory was about 10 times that of the POC in the southern East China Sea, and DOC represented the major portion of total organic carbon (80%∼95%) in the northern South China Sea. In the seawater DOC mainly comes from decomposition products of organisms, excrement during metabolism and terrestrial input. It can be transformed into POC through biological activities, and plays an important role in the vertical transportation of carbon. The concentrations of DOC in China marginal seas are higher in coastal waters than in the open seas. The mean DOC concentration in the surface water of the South Yellow Sea was the highest in China marginal seas with a value of 2.02 mg/L. DOC concentrations were relatively high (>85 μmol/L) in inner shelf and slope waters but low (ca. 65 μmol/L) around the shelf break in the East China Sea (Hung et al., 2000; Song et al., 2008). The DOC concentration in the South Yellow Sea was characterized by low concentration in the central oﬀshore area, which was controlled mainly by the dilution eﬀect of a low DOC current from the East China Sea. POC comprises living and non-living POC. Living POC is composed of micro-phytoplankton, bacteria, fungus, zooplankton, small ﬁsh and shrimp, and so on; non-living POC, i.e., organic detritus, mainly comes from biological activities. The proportions of living and non-living POC in the total organic carbon have been investigated in China marginal seas. Living and non-living POC represent about 2% and 9% of the total organic carbon in the ocean, respectively. In the East China Sea, living POC occupies 10% of the total organic carbon in spring, while in autumn it occupies only 4% of the total organic carbon (Liu et al., 1997). Vertical transportation of POC is a very important process. Diﬀerent parts of China marginal seas have diﬀerent concentrations of POC in the water, different sedimentation rates, and diﬀerent hydrodynamic environments. Thus, characteristics of transportation of POC vary among diﬀerent parts of China marginal seas. In the Yellow Sea, the bottom settling particulate matter comes mainly from the resuspension of sediments with the resuspension rate up to 90%∼96% (Song, 2004; Zhang et al., 2004). In the northern South China Sea, most POC is dissolved before being buried in sediments; organic carbon in sediments mainly comes from lateral transition and the contribution of verti-

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cal transition is limited. In the East China Sea, the POC export ratios were about 48%∼77% and 15%∼21% in the upwelling zone and the middle continental shelf, respectively; the resuspension ratios in the bottom layer averaged 66.50% and 88.52% at the stations in the upwelling zone and the middle continental shelf, respectively (Zhang YS et al., 2006). The downward ﬂuxes of total organic carbon were nearly balanced by new productivity derived from upward ﬂuxes of nutrients in the northern South China Sea. In addition, the net export of POC from the East China Sea shelf was estimated to be 106 Gmol C/yr (Hung et al., 2000), and the shelf export of POC was equivalent to <6.4% of carbon ﬁxed biologically on the shelf (Hung et al., 2003). In the Pearl River Estuary and the adjacent shelf of the South China Sea, fatty acids derived from algae were eﬀectively recycled during the whole settling and depositing processes. Knowledge of the sources of organic matter and their controlling factors is important to the understanding of global biogeochemical cycles. Together with the analysis of total organic carbon and total nitrogen, the application of biogeochemical techniques involving stable isotopes of carbon and nitrogen (e.g., δ 13 C and δ 15 N) has been widely used to determine sources of sediments and particulate organic matter in China marginal seas. There are two primary sources of organic matter in particulate organic matter and sediments in China marginal seas, which are terrigenous and marine autochthonous materials. In estuaries and coastal zones, the contribution of terrestrial organic matter is obviously greater than that of the marine contribution. However, in the outer shelf of the seas, the marine origin is often the primary contributor. The particulate organic matter in the Yangtze River Estuary originates from riverine and deltaic sources, and organic matter from marine sources is rather limited. In the Pearl River Estuary and the adjacent shelf of the South China Sea, the accumulation of terrestrial particulate organic matter is greater than that of marine phytodetritus; algal-derived organic carbon content was estimated to be low (0.06%) at the river mouth and higher (up to 0.57%) on the adjacent inner shelf (Hu et al., 2006). Marine origin is the predominant source of organic carbon in the top 200 m of the northern South China Sea where δ 13 C values range from ∼25.2% to ∼21.3%, and the weighted mean value C/N ratio is 6.74, which is very close to the Redﬁeld ratio (Liu et al., 2007). There are many factors that can aﬀect the distributions of total organic carbon, total nitrogen and organic carbon and nitrogen stable isotopes, based on which the contribution of terrestrial and marine sources are estimated. Grain size is an important factor. There is linear correlation of grain size with total organic carbon, total nitrogen, and organic carbon and nitrogen stable isotopes in sediments, respectively. In addition, the terrestrial particulate organic matter suﬀers from the eﬀects of diagenesis, benthos, and incessant input of the dead organic matter of plankton after deposition on the seabed. Therefore, the contribution of terrestrial organic carbon to organic matter in sediments is obviously lower than that of particulate organic matter in the same place.

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(2) Nutrients biogeochemistry Nutrient cycling is recognized as one of the central problems in chemical and biological oceanography. As the basis of the food chain in marine ecosystems, nutrients play an important part in the whole marine biogeochemical cycle. On the one hand, with the development of the economy and society, eutrophication has become a serious problem for coastal environment. On the other hand, the deﬁciency in some nutrients will limit the growth of phytoplankton and become the limiting factor for phytoplankton. Because of the importance of nutrients, there has been much research into the biogeochemistry of nutrients in China marginal seas, and much progress has been made in recent decades. Sources and the distributions of nutrients in China marginal seas are key problems in nutrients biogeochemistry. Riverine input, atmospheric deposition, sedimentary release and exchange with other waters are the primary sources of nutrients in the seawater of China marginal seas. Because of the terrestrial inﬂuence, the concentrations of nutrients are often high in the coastal and inner-shelf region on account of the complex circulation regime. In the Yangtze River and Pearl River estuaries, nutrients are aﬀected greatly by riverine input. There are high concentrations of nutrients in the east and northern-east part of the Yangtze River Estuary. Concentrations of total phosphorus in the Bohai Sea and Yellow Sea are comparable to those in the other worldwide coastal areas. The phosphorus levels in sediments of the Bohai Sea are higher than those in the Yellow Sea (Song et al., 2000c). In the East China Sea, nutrient species in surface waters decrease from eutrophic coastal to oligotrophic open shelf waters, although biological uptake and regeneration in the upper water column can produce a patchy character of nutrient distribution. The outﬂow of subsurface waters from the South China Sea, however, is the major source of new nutrients to the East China Sea continental shelves. These inputs are larger than the aggregate of all the rivers that empty into the East China Sea, contributing 49% of the externally sourced nitrogen, 71% of the phosphorous, and 54% of the silicon for the East China Sea (Chen, 2008). In addition, it was estimated that the annual atmospheric aerosol deposition − ﬂuxes of NH+ 4 and NO3 to the East China Sea were 270 and 160 Gg N/yr, respectively, which were comparable to the riverine inputs of the Yangtze River (Nakamura et al., 2005). Nutrients entering China marginal seas are further modiﬁed within the water body. Regeneration in the water body and deposition as a form of particulate particles are the two important processes. Nutrient regeneration in sediments of the Bohai Sea contributes more silicate than riverine input and atmospheric deposition together, but the benthic ﬂux contributes very much less phosphate and nitrate relative to riverine input and atmospheric deposition. Phosphorus has a rapid regeneration rate relative to nitrogen in the East China Sea. Biogenic silicon regeneration eﬃciency, that is, the ratio of the biogenic silicon vertical ﬂux minus the biogenic silicon accumulation rate relative to biogenic silicon vertical ﬂux, is ca. 80% for the East China Sea

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(Liu SM et al., 2005). More than 70% of nitrogen in the South Yellow Sea surface sediments could be released to the water body. The nitrogen released from sediments could supply 6.54% nitrogen of the new primary productivity of the South Yellow Sea (L¨ u et al., 2005). With respect to the water body, the residence time of phosphorus in the dissolved pools of coastal water and mid-shelf area is very short, and thus low phosphorus could support relatively high levels of primary production in the East China Sea. In addition, it was estimated that regeneration in the water body and excretion of plankton occupied about 17.6% of the total nitrogen input in the Bohai Sea (Zhao et al., 2002). In the Bohai Sea and the Yellow Sea, most of the phosphorus was regenerated in the water column, and almost all of the remaining phosphorus was buried in the sediments after its accumulation. Sediment plays a very important part in the biogeochemistry of nutrients. It can be a source or sink of nutrients in China marginal seas. The sediments of the East China Sea and the Yellow Sea are sinks of nitrogen and phosphorus and sources of silicon. The sediments of the South China Sea are sinks of phosphorus, nitrogen, and silicon in both wet and dry seasons. In the Yellow Sea, the water body deposits 19.6×109 , 87.5×109 and 1.53×109 mol/yr of nitrate, ammonium, and phosphorus to the sediments, respectively; the sediments release 23.8×109 mol/yr of silicon to the water body. In the East China Sea, the water body deposits 65.05×109 and −0.66×109 mol/yr of nitrate and phosphorus to the sediments, respectively; the sediments contribute 53.08×109 and 460×109 mol/yr of ammonium and silicon to the water body, respectively (Qi et al., 2006). The net deposition rates of phosphorus, nitrogen, and silicon are 2.6×109 , 42×109 , and 155×109 mol for six months in the wet season, respectively; those are 0.9×109 , 14×109 , and 132×109 mol for six months in the dry season (Chen et al., 2001). The Bohai Sea sediments are the source of silicon, phosphorus, and nitrogen in summer, with ﬂuxes of 2.59×1013 , 2.95×1011 , and 8.62×1012 mmol, respectively. The nutrients contribution from the sediments accounts for 65%, 12%, and 22% of the silicon, phosphorus, and nitrogen, respectively, which are demanded to maintain the primary productivity (Wang et al., 2007). (3) Biogeochemistry of oxygen and sulfur The dissolved oxygen (DO) level in natural aquatic systems is a highly informative variable which can elucidate atmosphere-ocean interactions, water mass movements, net primary productivity, and carbon remineralization processes. It is also strongly representative of an ecosystem’s function and behavior. The DO level indicates how well the water is aerated and is a commonly measured parameter because it is an immediate indicator that inadequate oxygen levels will quickly aﬀect aquatic life. The atmosphere and aquatic plant photosynthesis are two main sources from which oxygen enters the water. Oxygen is essential not only for marine life but also for decomposition of organic matter, a process which consumes oxygen. Research into the biogeochemistry

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of oxygen mainly focuses on the ﬂux of oxygen at the sea-air interface and the distribution of oxygen in China marginal seas. At present, studies on ﬂuxes of oxygen mainly focus on the South China Sea. Based on the data observed from 1984 to 1994, the total input of oxygen to the South China Sea is 280.4×104 mol/s, and the input of oxygen through the air-sea interface occupies about 17% of the total input (Lin and Han, 1998). In the summer of 1998 and winter of 1999, the air-sea exchange rates of oxygen were −0.346∼0.226 mol/(m2 ·d) and −0.234∼3.123 mol/(m2 ·d), respectively (Liu XY et al., 2005). The average rate of oxygen transferred from the atmosphere to the sea water was about 5.2×10−7 ml/(cm2 ·s) during September to February of the next year in the South Yellow Sea (Sun and Yu, 1980). Based on data observed from February 1984 to February 1985, oxygen released from the sea water to the atmosphere was much greater than that dissolved from the atmosphere to the sea water in the western part of the Taiwan Strait. There have been lots of studies on the distribution of oxygen and its primary inﬂuencing factors in China marginal seas in recent decades. In the Yellow Sea and the East China Sea, the dissolved oxygen is high in the west and the north, low in the east and the south. The East China Sea has the largest low-oxygen area in the world, with the hypoxia area greater than 12,000 km2 . Hypoxia in the East China Sea might be mainly due to decomposition of elevated inputs of organic matter, which resulted in a reduction of oxygen levels in the bottom water, in conditions of strong pycnocline which restricted vertical re-aeration. Distribution of dissolved oxygen in the coastal Yellow Sea in spring is mainly controlled by the temperature, while that of the outer Yellow Sea is mainly aﬀected by temperature and salinity. The distribution of dissolved oxygen in the south of the South Yellow Sea is mainly controlled by the Yellow Sea Warm Current and the frontier of the Taiwan Warm Current. However, the dissolved oxygen in the upper water of the south of the South Yellow Sea is primarily controlled by photosynthesis in summer. In the central and northern South Yellow Sea, dissolved oxygen in the euphotic zone is mainly aﬀected by photosynthesis in spring and summer, and it is primarily controlled by the temperature in autumn and winter. In the cold water body below the euphotic zone of the Yellow Sea, the concentration and distribution of the dissolved oxygen are mainly aﬀected by the consumption of organic materials. As to the South China Sea, the horizontal distribution of dissolved oxygen is very uniform in summer. However, the dissolved oxygen is low in the south and high in the north of the South China Sea, which is mainly aﬀected by the climate and temperature. One important phenomenon of the distribution of oxygen is that there are maximum values in vertical distributions of dissolved oxygen in spring or summer in the East China Sea and the Yellow Sea, and a maximum value in the vertical distribution of dissolved oxygen all the year in the Nansha Islands sea area, South China Sea. With respect to the maximum value in the Yellow Sea, a widely accepted explanation in the 1980s and the early 1990s was that it was maintained from the winter. This explanation was also used to explain

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the vertical maximum values in the East China Sea and the South China Sea. However, Wang et al. (2001) argued that the vertical maximum values of dissolved oxygen were the joint eﬀect of thermocline and photosynthesis in the Yellow Sea. Jiang et al. (1991) and Liu XY et al. (2005) argued that the maximum values were the joint eﬀect of thermocline and biological eﬀects in the East China Sea and the South China Sea, respectively. According to Lin et al. (2003), the maximum value in spring in the Nansha Islands was mainly due to biological activities. However, it was mainly due to the coastal currents in winter and summer. Studies on sulfur mainly focus on the concentration and distribution of dimethylsulﬁde (DMS) in China marginal seas. The average concentration and ﬂux of DMS in the Bohai Sea on the cruise from Dalian to Tianjin were about 1.31 nmol/L and 0.85 μmol/(m2 ·d) in March 1993, respectively; those in the Yellow Sea on the cruise from Shanghai to Qingdao were about 2.89 nmol/L and 7.94 μmol/(m2 ·d) in September 1994, respectively (Hu et al., 2003). In March 2005, the average concentrations of DMS in the surface microlayer and subsurface water at 21 grid stations (which covered most parts of the Yellow Sea) were 2.31 and 2.26 nmol/L in the subsurface water and the microlayer water, respectively. The average ﬂux from the seawater to the atmosphere was 3.14 μmol/(m2 ·d) in the Yellow Sea (Yang et al., 2006). From September to October 1994, the mean concentration of DMS in surface seawater was 82 ng S/L, and the mean ﬂux of DMS from the South China Sea to the atmosphere was estimated to be 5.5 μmol/(m2 ·d) (Yang, 2000). The average concentration of DMS in the subsurface water of the South China Sea was 1.74 nmol/L in May 2005, and the mean ﬂux of DMS from the investigated area to the atmosphere was estimated to be 2.06 μmol/(m2 ·d). This low DMS emission ﬂux, together with the low DMS surface concentrations, was attributed to the low productivity in this sea. Sulﬁde has also been paid attention to in biogeochemical studies in China marginal seas in recent decades. The southern East China Sea continental slope environment is an eﬃcient pyrite sulfur burial environment. The sulfate reduction rate and sulﬁde burial rate increase linearly with the increasing organic carbon burial rate in sediments of the southern East China Sea continental slope, indicating that the deposition of organic carbon on the slope is the primary controlling factor for pyrite formation. The concentrations of sulﬁde in the surface sediment of Rushan Bay were high in the north and low in the south; the sulﬁde contents increased generally with the increase in sediment depth. In addition, the shellﬁsh culture had a signiﬁcant eﬀect on sulﬁde contents in sediments, resulting in the fact that the sulﬁde contents in the culture area were higher than those in uncultured areas.

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6.1.2.2 Biogeochemical Research on Biologically Toxic Compounds in China Marginal Seas Biogeochemical research on organisms mainly focuses on the interactions of nutrients and heavy metals with the organisms. Nutrients, such as nitrogen, phosphorus, and silicon, are fundamental for phytoplankton. An increase in their concentrations is good for maintaining larger primary production, while the low concentration will limit growth of phytoplankton. Phytoplankton community structure and concentrations of nitrogen and phosphorus have changed a lot since the 1980s. In the past 30 years, phytoplankton that were prone to be limited by phosphorus declined, while phytoplankton that were prone to be limited by nitrogen increased in the Bohai Sea. Nitrogen, nitrogenphosphorus, and phosphorus were the limiting nutrients of phytoplankton in the Bohai Sea in the early 1980s, the late 1980s, and the period from the 1990s, respectively, and the low active phosphorus was the main reason for low primary productivity ever since the 1990s. In the Yangtze River Estuary, a sharply decreasing dissolved silicon ﬂux and quickly increasing dissolved inorganic nitrogen and dissolved inorganic phosphorus ﬂuxes into the sea have enhanced eutrophication and caused frequent harmful algal blooms in coastal waters. On average, the red-tide frequency was from 0.04 times/yr during 1933∼1979 to 7.0 times/yr during 2000∼2002. The cell abundance percentage of Skeletonema costatum, the red-tide-predominant species that is in positive proportion to dissolved silicon ﬂux, decreased from 33% during the 1980s to 24% during 2000∼2002. The present data are evidence of an increase in the cell abundance percentage of Prorocentrum dentatum from 12.5% in the 1980s to 36% in 2000∼2002, which tends to be the dominant species of the red tides oﬀ the Yangtze River Estuary (Li et al., 2007). Heavy metals can be divided into essential and non-essential heavy metals, based on the relationship between them and organisms. Essential heavy metals, such as Zn and Cu, are very important in maintaining the normal biochemical activities of organisms; non-essential heavy metals, such as Cd, Hg, Cr, and Pb, are of no biological signiﬁcance. If the amount of heavy metals, including the essential and non-essential, reaches a certain concentration, the organisms will suﬀer the toxicity. Thus, research into the eﬀect of heavy metals on organisms is of great importance. There have been many studies on the eﬀect of heavy metals, which are mainly Cu, Pb, Zn, Cd, Hg, Cr, and As, in China marginal seas in recent decades. In the Yangtze River Estuary and its adjacent coastal waters, about 49% of the total studied area had a Cu level exceeding no detected toxic concentration, leading to an average of 5% decrease in the biomass of Prorocentrum donghaiense Lu and up to a 24% decrease in the area of the highest Cu concentration; Pb, Zn, and Cd had no signiﬁcant inﬂuence on the Prorocentrum donghaiense Lu in the Yangtze River Estuary (Wang et al., 2008). Based on a toxicity test, inhibition rates of Hg, Pb, and Cd on Skeletonema costatum were not more than 1% under present concentrations and the annual

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average concentrations in recent decades in the Bohai Sea. Even in the highest concentrations of heavy metals in the mid 1990s in the local waters of the Bohai Sea, the inhibition rates of Hg, Pb, and Cd on Skeletonema costatum were only about 1%, 4%, and 3%, respectively. However, when the concentration of Pb in seawater was about 200 μg/L, its inhibition rate on Skeletonema costatum could reach approximately 9%. But on the whole, the inhibition of Hg, Pb, and Cd on dominant phytoplankton species was not signiﬁcant in the Bohai Sea (Wang and Li, 2006). 6.1.3 Research Methods of China Marginal Seas’ Biogeochemical Process There were mainly three kinds of methods of study of biogeochemical processes in China marginal seas: ﬁeld investigation, laboratory simulated experiments, and model construction. Field investigation and laboratory simulated experiments are the basic methods in biogeochemical studies, and much progress has been made based on the two methods. In recent years, biogeochemical models of China marginal seas have been quickly developed, which push forward greatly the advancement of biogeochemistry in China marginal seas. 6.1.3.1 Field Investigation Based on the ﬁeld investigation, we can obtain many data, such as the concentrations of biogenic elements, particulate materials, chlorophyll a, and so on. Therefore, we can study the distributions of substances and estimate the ﬂuxes of some substances among diﬀerent pools, which is very essential in biogeochemical studies. However, the nature of the marine water is very complicated. There is much diﬃculty in direct research on biogeochemical processes in the sea. Adopting an enclosure ecosystem can simulate the real marine conditions and simplify the study of marine biogeochemistry. Thus, the study of enclosure ecosystems has made great progress over the past 20 years. In China, studies of the marine enclosure ecosystem mainly include the following aspects: the mutual interactions between nutrients and phytoplankton, the transportation of heavy metals, eﬀects of heavy metals on the plankton, the metabolism of organic carbon, the grazing pressure on bacteria of zooplankton, and so on. With respect to the mutual interactions between nutrients and phytoplankton, both the continuous and intermittent input of nutrients can ﬁrstly cause diatom bloom and then dinoﬂagellate bloom. However, the intermittent input of nutrients can aﬀect the occurrence time and dominant species of dinoﬂagellate bloom. Lin et al. (1994) found that after phosphorus addition into enclosure ecosystems, the macro-phytoplankton were the major contributor to the total biomass of phytoplankton in October, and micro-phytoplankton in May.

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Heavy metals can cause inhibition of plankton, even in concentrations at ppb level. Among plankton, zooplankton, especially larvae zooplankton, are very sensitive to heavy metal pollution. In an enclosure ecosystem, concentrations of dissolved heavy metals decreased exponentially with deposition of particulates, with the removal eﬃciency Pb>Hg>Zn>Cu>Cd; then the dissolved heavy metals were transformed to particulate forms by biological activities. After 27 d, Cd and Cu existed mostly in the dissolved form, most of Pb and Hg were transported to the bottom, and about half of Zn was transported to the bottom. The release of heavy metals from the sediments was determined by both the nature of the metals themselves and the environmental conditions. An anoxic environment is beneﬁcial to the release of heavy metals. In an oxidizing environment, release ﬂuxes of Cu and Zn were 1∼2 order(s) of magnitude higher than those of Cd and Pb. There has been much research on organic carbon metabolism, suspended particulate materials, and the grazing pressure of zooplankton on bacteria in a shrimp cultural enclosure ecosystem. In the shrimp cultural enclosure ecosystem, about 35% of the total organic carbon income was mineralized through the process of plankton community metabolism during the cultural period (Liu et al., 2002). Particulate organic matter occupies 62% of the total suspended particulate materials. The percentage of humic substances in particulate organic matter can reach up to 98%, and the remaining 2% are plankton among which the biomass of phytoplankton is 4.5 times larger than that of zooplankton (Liu et al., 1999). Bacteria can be preyed upon by zooplankton through the microbial loop, which is a very important part of the biological pump. The grazing pressure of zooplankton can reach up to 73%∼175% of the total biomass of bacteria. The feeding amount of zooplankton (<3 μm) can occupy 17% of the total feeding amount of zooplankton, and that of the other zooplankton (3 μm) account for 83% (Liu et al., 2000). 6.1.3.2 Laboratory Simulated Experiments Field work in China marginal seas is very complicated. Every process is affected by many factors. It is very diﬃcult to conduct directly a biogeochemical study in the sea, especially relating to the ﬂuxes and biogeochemical studies near the sediment-seawater interface. By indoor laboratory simulation, environmental conditions in the ﬁeld in China marginal seas can be simulated, and many variables can be controlled. Therefore, much progress in understanding the biogeochemistry of China marginal seas by indoor laboratory simulation has been made in recent decades. The sediment-seawater interface process is important in the biogeochemical cycling of phosphorus and silicon in China marginal seas. Based on simulated experiments, Song et al. (2000b) estimated the ﬂuxes of phosphorus and silicon across the sediment-seawater interface in the Bohai Sea. The estimated ﬂuxes of phosphorus and silicon from sediment to seawater were 10.2×106 and 190.6×106 kg/yr, accounting for 86.4% and 31.7% of the total cycling

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phosphorus and silicon in the Bohai Sea, respectively. On the basis of laboratory incubation experiments, Shi et al. (2004) estimated the ﬂuxes of silicon and nitrogen in the sediment-seawater interface in the East China Sea. It was estimated that the exchange ﬂuxes of SiO3 -Si and dissolved inorganic nitrogen could provide 55% and 5.1% of the silicate and dissolved nitrogen required by phytoplankton in the East China Sea, respectively. Bacteria in marine sediment play an important role in the removal of nitrogen. Quan et al. (2005) isolated the denitrifying bacteria from the sediment in the Yangtze River Estuary and simulated the removal eﬃciency of nitrate from seawater. It was found that there was a correlation between the concentration of nitrate and the number of denitrifying bacteria in the water. The removal eﬃciency reached about 70% within one day when the initial concentration of nitrate was 1 mg/L. About 90% of the nitrate was removed by denitrifying bacteria from seawater in a week when the initial concentration of nitrate was 100 mg/L. In addition, based on laboratory simulated experiments, Zhang NX et al. (2006) and Zheng et al. (2006) studied the inﬂuence of nutrients and heavy metals on marine inorganic carbon systems. Zhang NX et al. (2006) found that the external source nitrate could cause a variation in plankton quantity. However, the variation in the content for the inorganic carbon system such as pH, dissolved inorganic carbon and HCO− 3 was little. The relative variation in dissolved inorganic carbon was lower than 1%, but the increase in PCO2 in seawater was up to 10%. The nitrate from external source may weaken the intensity of carbon sink; in other words, the intensity of carbon source was enhancing. Zheng et al. (2006) found that seawaters with low infusions of heavy metal represented sinks for the atmosphere of CO2 . These sinks would probably convert into CO2 sources after a period of time. Seawaters with a comparatively high amount of heavy metal were always CO2 sources, and their release ﬂuxes of CO2 augmented along with increasing infusions of heavy metal. 6.1.3.3 Biogeochemical Models Biogeochemical cycles of carbon, nitrogen, phosphorus, and silicon are complicated. They are composed of the assimilation of carbon and the other biogenic elements, the mutual transformation of particulate, dissolved, and colloidal organic carbon, the feeding activities of zooplankton, deposition of particulate materials, exchange among the interfaces, and so on. Study of these processes is fundamental to understanding the complicated system. Model construction, based on these processes, is a very powerful means. It can integrate the dispersed processes into a whole system to quantify the contributions of different processes to the system, and to get an insight into the mechanism of the biogeochemical cycle. And thus we can predict the variation in the system. In China, studies of the biogeochemical model have been developed since the 1990s. These studies were mainly related to budgets of biogenic elements

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in diﬀerent parts of China marginal seas. Based on the models, it was found that production and respiration were the most important sinks and sources of nutrients in the Bohai Sea and the Yelllow Sea, and that the remineralization of the detritus pool could compensate 30% of the consumption of nutrient by the production process in the Bohai Sea; the bottom sediment in the central Yellow Sea provides a large amount of nitrate to the sea water by means of sediment-water interface exchange, which contributes 56% of nitrogen for the new production; the nutrients compensated by the atmospheric sedimentation account for 6% of nitrogen and 1.5% of phosphorus consumed by annual primary production (Zhao et al., 2002; Tian et al., 2003). In addition, zooplankton in the eastern South Yellow Sea can uptake 71% of the bacteria in winter; phytoplankton occupy 67.5% of the total feeding amount of zooplankton in spring and summer (Xia and Gao, 2006). Biogeochemical models have developed greatly since the 1990s in China. However, there are still many problems with the models: diﬀerent models for the same problem sometimes have contradictory results; models in the local region can hardly be extended to the global scale; the global models can hardly realize quantitative simulation; anthropogenic inﬂuence and response to global changes are diﬃcult to predict. Therefore, the models should be perfected and the following ﬁve aspects should be considered in the future: accurate expression of biological-chemical processes, perfection of the physical models, enough emphasis on the spatial and temporal variation in the mesoscales, accurate deﬁnition of the exchange ﬂuxes among the interfaces, and strengthening of data acquisition and processing technology.

6.2 Main Key Biogeochemical Processes in China Marginal Seas The marginal seas form the linkage between the continent and the ocean. They mark the areas of interactions of rivers, land, oceans, the atmosphere, and sediments. Despite their relatively modest surface areas, they are important domains that inﬂuence global biogeochemical cycles. Marginal seas play a considerable role in the biogeochemical cycles of carbon, nitrogen, and phosphorus because they receive massive inputs of these elements through upwelling and terrigenous inputs (Song, 2004). The marginal seas are also among the most biologically and geochemically active areas of the biosphere, and exchange large amounts of matter with the open oceans. The marginal seas have high sedimentation rates and act as ﬁlters and traps for both natural and anthropogenic materials transported from the continents to the open ocean. At least 80% of the terrigenous material delivered to the ocean today is trapped on the proximal shelves of the continents. On the other hand, the marginal seas are regions of higher net primary production relative to that of average global oceanic surface waters and play a signiﬁcant role in the mitigation of the eﬀects of anthropogenic perturbations through organic carbon

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storage and recycling within the global framework. The ocean margins can contribute a greater amount, in order of magnitude, of dissolved and particulate organic matter to the interior of the Paciﬁc than that derived from the surface of the open ocean. It was estimated that about half of the organic carbon input to the seabed of the North Paciﬁc occurs within 500 km of the margin (Jahnke et al., 1990). The marginal seas are also especially susceptible to anthropogenic inﬂuence at a time when humans are strongly interfering in the global biogeochemical cycles of carbon, nitrogen, and phosphorus. This interference has led to substantially increased loadings of the land and atmosphere with chemicals, such as nutrients from these activities, especially in the North Paciﬁc. China marginal seas, including the Bohai Sea, the Yellow Sea, the East China Sea, and the South China Sea, have a total area of 4.73×106 km2 . They form the link between the largest continent and the largest ocean in the world. Two of the largest rivers in the world, the Yangtze River and the Yellow River, together with the Pearl River, whose ﬂow is the second largest among rivers in China, empty into China marginal seas with large and ever increasing nutrient and carbon inputs. Biogeochemical cycles can be dominated by diﬀerent factors owing to the geomorphological and current systems in marginal seas. The amount of freshwater ﬂow is an important factor due to its impact on buoyancy ﬂux; upwelling is an important factor which is characteristically episodic in nature and has been identiﬁed as having an important inﬂuence on retention and exchange along those usually narrow shelves. The seasonal ice coverage also plays an important role in the air-sea exchange of gases and the land-to-sea ﬂux of matter, especially in the polar margins. With respect to China marginal seas, the dominant factors are the freshwater input, the Kuroshio intermediate water which originates in the nutrient-rich South China Sea intermediate water and upwells onto the East China Sea continental shelf, the river input of nutrients and dissolved and particulate materials, the biological pump and coastal anthropogenic activities such as mariculture and marine engineering, and ecological disasters such as red tides. 6.2.1 River Input The continental ﬂuxes of nutrients and organic matter have an important impact on marine ecosystems. In combination with freshwater discharge and the resulting stratiﬁcation they can be crucial determinants of the productivity in coastal areas, especially in the estuaries of large rivers. Furthermore, continent-ocean ﬂuxes are a principal source of nutrients to the world’s oceans, whose productivity is controlled by the availability of a few elements. A major fraction of terrigenous nutrients is bound in organic molecules and available to most primary producers only after bacterial mineralization. Rivers serve as a major source of buoyancy and dissolved and particulate materials. They contribute signiﬁcantly to the freshwater ﬂow and terrestrial

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materials. The riverine ﬂuxes of nutrients and organic matter have an important impact on marine ecosystems. The input of high concentrations of biologically important elements, along with the complex physical structure of buoyant freshwater plumes, leads to strong gradients in concentrations of and transformations among biogeochemical constituents in plume environment and can be crucial determinants of the productivity in marginal seas, especially in the estuaries of large rivers. The impacts of large rivers are important both on a regional/continental scale and on a global scale but especially signiﬁcant to the continental shelf regions that receive the river input. It is primarily due to large rivers that approximately 40% of the global marine burial of organic matter occurs in deltaic environments. The world’s 10 largest rivers transport approximately 40% of the freshwater and particulate materials entering the ocean. The Mississippi River drains about 40% of the conterminous US, carries approximately 65% of all the suspended solids and dissolved solutes that enter the ocean from the US, and eﬀectively injects these materials onto the continental shelf as a point source in the northern Gulf of Mexico (Dagg et al., 2004). The annual discharge of rivers in China contributes 7% of the total global riverine discharge. They play a signiﬁcant role in biogeochemical processes in China marginal seas (Song, 1997; 2004). Freshwater runoﬀ from rivers into the seas is an important element of the dynamics over many continental shelves. Rivers serve as a major source of buoyancy which is a key mediating factor in the transformation process in the coastal margins. On the one hand, buoyancy prevents the cold water from sinking there to large depths from the surface at high latitudes. On the other hand, buoyancy produces plumes and coastal currents locally. Since estuary outﬂows tend to be less saline and hence lighter than the ambient shelf water, a plume typically forms as the buoyant water spreads away from the mouth of the estuary and coastal currents develop downstream. Chemical and biological activities are greatly enhanced by the changed physical and optical environment within buoyant plumes. With respect to China marginal seas, freshwater inputs of the Yellow River, the Yangtze River, and the Pearl River have a signiﬁcant inﬂuence on the intensity of the carbon source/sink in estuaries and the adjacent seas. With the development of the economy and increasing demands for preventing ﬂoods and saving water resources in China, thousands of water conservancy projects have been constructed in the large river basins in recent decades, which reduced the freshwater ﬂow to marginal seas. For instance, cutting back the Yangtze River outﬂow by a mere 10% will reduce the cross-shelf exchange by about 9% because of a reduced buoyancy eﬀect and, at the same time, it will cut the onshore nutrient supply by nearly the same amount (Chen, 2002). It can therefore be expected that primary production and the ﬁsh catch in the East China Sea will decrease proportionately. From fresh water to salty water, the Yangtze River mouth has gradually changed from a strong carbon source to a weak carbon sink.

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Rivers contribute substantial amounts of dissolved and particulate materials to continental margins and subsequently to the ocean. These materials can largely inﬂuence the carbon source/sink in marginal seas. The riverine input of nutrients fuels a high biological production in coastal environments, making them an important layer in the global carbon cycle. The organic matter in suspended particulate material can be largely trapped in the estuary turbidity maxima, which intensiﬁes microbial and micro-crustacean grazing activities. And thus the biological pump is strengthened. The riverine dissolved organic matter can be distributed throughout the ocean water column and its oceanic residence time is much shorter than that for marine dissolved organic matter. The regeneration of nutrients during rapid cycling of riverine dissolved organic matter could contribute to a high rate of primary production in the coastal ocean. Signiﬁcant inputs of nutrients, which are the foundation of high production in the continental margins, are discharged into the coastal zone via rivers. N, P, and Si are important limiting nutrients for production in marginal seas. Increasingly, these inputs are being greatly perturbed by anthropogenic activities. Rivers act as natural integrators of surﬁcial processes, including human activities, within their drainage basins. Anthropogenic perturbations have caused considerable changes in riverine nutrient concentrations and ﬂuxes to the sea. From the 1950s to 1970s, dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) concentrations and ﬂuxes have shown a slow increase, but a great increase after the 1980s due to the intensive application of chemical fertilizers and a large number of chemical plants which came into operation after the 1980s. The chemical fertilizer applied to the Yangtze River drainage basin in 1991 was 48 times more than in 1962. The annual dissolved silicon ﬂux has decreased sharply since the 1950s, due to the decreased dissolved silicon concentration. The impoundment of reservoirs on rivers was responsible for the decreased dissolved silicon concentrations. On the one hand, part of the dissolved silicon is ﬁxed by phytoplankton in the reservoirs. On the other hand, the reservoirs have reduced the sediment load to the sea, which has resulted in a decrease in dissolved silicon concentrations. In addition, the N:P:Si ratios of the inputs have been changed. It is well known that silicon plays an important role in the primary productivity of the aquatic ecosystem. Diatoms, accounting for 60% of all species of phytoplankton, take up silicon to form cell walls. Silicon is linked strongly to the carbon cycle through the activities of diatoms, the “workhorses” of primary productivity. The sharply decreasing dissolved silicon ﬂux and quickly increasing DIN and DIP ﬂuxes into the sea have enhanced eutrophication and caused frequent harmful algal blooms in coastal waters. The highest record of red tides in the Yangtze River mouth is 19 times a year and the longest cumulative time is about 60 d, which caused great damage to mariculture and ﬁshery production. The inﬂuence on biogeochemical processes of red tides will be discussed separately in the following part.

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6.2.2 Coastal Anthropogenic Activities Anthropogenic activities along China marginal seas consist mainly of mariculture, sea reclamation, and oil drilling projects. China is a developed country in mariculture, with the largest mariculture area and the largest yield in the world. The mariculture yield was 1.2×107 t in 2002, approximately two thirds of the total yield in the world. With respect to sea reclamation, there were three series of large sea reclamation projects in the history of China, which were carried out for solar salt, to enlarge agricultural land, and for mariculture, respectively. From 1949 to the end of the 20th century, the annual sea reclamation area was about 230∼240 km2 /yr. Now the fourth large series of sea reclamation projects is important. It is planned that sea reclamation projects will add 700 km2 of land annually, which is nearly equivalent to the area of Singapore. Mariculture in China is primarily composed of shellﬁsh and seaweeds, which constitute about 90% of the total yield. Seaweeds and shellﬁsh can greatly aﬀect the biogeochemical processes. Algae aﬀect the biogeochemical processes mainly in two ways: they transform the dissolved inorganic carbon to organic carbon via photosynthesis and the PCO2 decreases; the absorption of nitrate and phosphate by phytoplankton will enlarge the alkalinity. Both of these ways stimulate the diﬀusion of CO2 into the seawater. In Sanggou Bay in the coastal area of the Yellow Sea, the culture seaweeds could contribute to 37% of the total productivity in the bay, and it was estimated that about 3.3×105 t carbon was removed from the seawater by the mariculture of seaweeds in China’s coastal seas in 2002 (Zhang et al., 2005). Shellﬁsh can aﬀect the biogeochemical processes in the following ways: they can use HCO− 3 to form the shell; they can utilize organic material by ﬁlter feeding so that the quantity and composition of suspended particulate materials will be regulated; they can transport a large amount of particulate materials by the forming and deposition of fecal pellets, and thus the sedimentation rate will be enlarged; they excrete lots of nitrogen and phosphorus into the seawater. Part of the shell is transported into the deep sea, and most of the shell is harvested by humans, both of which remove carbon from the water. It was estimated that about 8.6×105 t carbon was removed from the seawater by the mariculture of shellﬁsh in China’s coastal seas in 2002 (Zhang et al., 2005). In addition, in the mariculture area, the excretion of nitrogen and phosphorus by shellﬁsh can satisfy 50% of the nitrogen and phosphorus needed by phytoplankton, among which excretion of NH+ 4 and dissolved inorganic phosphorus can provide 40% and 39% of the nitrogen and phosphorus needed by phytoplankton, respectively (Zhou Y et al., 2003). It was estimated that about 850 t nitrogen and 78 t phosphorus would diminish because of the mariculture of shellﬁsh and seaweeds (Zhou et al., 2002). Furthermore, with respect to artiﬁcial bait, which is primarily applied in shrimp mariculture and cage culture nowadays, bait feeding is an important factor aﬀecting the marine environment. Approximately 20% of the baits are

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not utilized, and will be left in the water together with the excretions of shrimps and ﬁsh. In addition, equipment in the mariculture area, such as cages and raft frames, can have an eﬀect on sea currents. The suspended mariculture of shellﬁsh in Sanggou Bay reduced the tide speed by 35%∼40%, which would inevitably exert an inﬂuence on carbon cycle processes. Sea reclamation projects have reduced the capacity of the estuary to retain nutrients, as a result of the loss to reclamation of accommodation space for intertidal sedimentation. The huge loss of the intertidal-wetland environment has had a severe impact on sediment and material ﬂuxes. Reclamation and commercial/urban development in the estuary in the last 300 years has reduced the organic carbon storage capacity of the Humber Estuary from about 3.2×105 t in the palaeo-estuary to no more than 2.5×103 t today, more than a 99% reduction in potential Corg storage capacity (Andrews et al., 2000). The total modern yearly sulphur deposition is approximately 2% of its value 2000 years ago. The decreased organic carbon storage capacity and sulphur deposition rate in the Humber Estuary are a warning that similar variations could possibly happen to the coastal area where sea reclamation projects are huge. 6.2.3 Biological Pump A better understanding of the ability of the ocean to control atmospheric CO2 levels will only be gained from a quantitative investigation of the mechanism known as the “biological pump” (Song et al., 2008). The marine organisms act as a “biological pump”, thus removing CO2 and nutrients from the surface ocean and transferring these elements into the deeper ocean and ocean bottom (Song et al., 2008). The eﬃciency of the biological pump can be aﬀected by various factors. By altering the number, size, and density of particles in the ocean, the activities of diﬀerent phytoplankton, zooplankton, and microbial species control the formation, degradation, fragmentation, and repackaging of rapidly sinking aggregates of POC and are responsible for much of the variation in the eﬃciency of the biological carbon pump. Photosynthetic production by phytoplankton in the euphotic layer supplies organic material and energy to the aquatic food chain. Furthermore, a possible decrease in PCO2 in the surface sea water by phytoplankton photosynthesis can accelerate the transfer of CO2 from the atmosphere to sea water. Thus, primary production in the euphoric zone is one of the most important processes, not only in material cycling in aquatic environments but also in carbon cycling in relation to global change. The carbon ﬁxed by phytoplankton is about 2.22×108 t/yr in the Bohai Sea, the Yellow Sea, and the East China Sea, while it is 4.16×108 t/yr in the South China Sea, about twice that in the Bohai Sea, the Yellow Sea, and the East China Sea (Song, 1997; 2004). Inorganic carbon ﬁxed by phytoplankton can be transformed into particulate materials after a series of biogeochemical processes (Song et al., 2008).

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Biological activities play an important role in the vertical transition of settling particulate matter. The Bohai Sea is a region of high productivity and turnover rate (Song et al., 2000c). Zooplankton have high grazing pressure on phytoplankton. The growth rate of phytoplankton is about 0.43∼0.73 d−1 , and the feeding rate of micro-zooplankton to phytoplankton is approximately 0.42∼0.69 d−1 . At the bottom, grazing pressure of micro-zooplankton is 3 times that of primary productivity (Zhang and Wang, 2000). According to Zhao et al. (2002), respiration can contribute 62% of the nutrients needed by photosynthesis, and the excretion of zooplankton and regeneration in the water column can contribute 30% of the nutrients needed by photosynthesis. The high turnover rate of materials in the Bohai Sea results in little POC deposition on the bottom, which can be conﬁrmed by the low concentration of organic carbon in the sediments of the Bohai Sea. In the Yellow Sea, particulate organic matter decreases from the surface to the bottom, and resuspension of the sediment is the primary source of particulate matter in the bottom water. Particulate matter in the northern South China Sea is mostly from organic materials of plankton. Vertical transportation of carbon in the East China Sea primarily relies on the particulate matter, and POC is mostly aﬀected by biological activities in spring and by terrestrial input in autumn. Biogeochemical studies suggest very eﬃcient recycling of organic carbon by bacterial and protozoan consumption in the shelf water of the East China Sea, but a ﬁnite amount of particulate organic carbon with a signiﬁcant terrigenous fraction is exported from the shelf. The net ﬂux of POC is larger in the East China Sea than in the Yellow Sea and south of the Taiwan Strait. 6.2.4 Ecological Disasters Marine ecological disasters, such as red tides, pathogenic microorganisms of marine ﬁsheries, and an invasion of alien species, can aﬀect the environment of the seawater, the normal growth of marine organisms, and the marine ecological health conditions. Therefore, the transportation of materials in the marine environment and along the food chain will be changed, which can aﬀect biogeochemistry in China marginal seas greatly. Red tides are the primary marine ecological disaster in China. The frequency of red tides in China marginal seas has increased signiﬁcantly in recent years. There were 955 red tides recorded in China’s coastal area during 1960∼2007, i.e., 4 times in the 1960s, 20 times in the 1970s, 75 times in the 1980s, 200 times in the 1990s, and 656 times during 2000∼2007. The total area of red tides during 2001 to 2007 reached 124,850 km2 , which showed a signiﬁcant expansion when compared with historical records. Red tide is a kind of phenomenon that some marine micro-algae, protozoa, or bacteria accumulate and reproduce rapidly in the water column. Algae bloom is a kind of phenomenon that algae reproduce rapidly in the water column. Commonly, algae blooms whose density of algae is up to a certain level are called red tides. Red tides assimilate lots of nutrients, increase

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dissolved oxygen in the beginning, and largely reduce the dissolved oxygen in the later period. During the red tides, dissolved oxygen, the transformation of nutrients, the primary productivity and sedimentation of chlorophyll a will be aﬀected. There are often some species of phytoplankton which are poisonous and harmful to zooplankton. Through the food chain, the transformation of materials will be aﬀected, such as a reduction in the biomass of organisms at high levels, and thus the carbon transformation will be disturbed. In the regions where red tides often occur, the primary productivity and transformation of nutrients are largely aﬀected. Taking one such region in the East China Sea for example, the average primary productivity of the red tide tracking stations was 399.984 mg/(m3 ·h), while that of the surface water of the East China Sea was 10.091 mg/(m3 ·h) (Zhou W et al., 2003); the sediment in the region is the sink of dissolved inorganic nitrogen and phosphate, while in other areas of the East China Sea dissolved inorganic nitrogen and phosphate mostly diﬀuse from the sediment; in addition, the sediment in the region is the source of silicon, while in other regions of the East China Sea the sediment is mostly the sink of silicate; the sediment in the region can absorb 5.9% and 67% of the dissolved inorganic nitrogen and phosphate delivered by the Yangtze River, respectively, while silicate released from the sediment in the region can contribute to 7.8% of the total silicate in the water column (Song, 1997; Qi et al., 2003; Li et al., 2004). Red tides can also aﬀect the feeding activities of the zooplankton. Some species of phytoplankton in the red tides are harmful. Through selective predation, zooplankton can refuse eating harmful algae or reduce the feeding amount of such algae. With respect to the harmful algae that are taken in by zooplankton, the assimilation eﬃciency, the feeding rate of the zooplankton to the nontoxic algae and the total productivity will also be reduced. And thus the material transportation eﬃciency along the food chain will also be reduced, which will decrease the carbon absorption.

6.3 Prospects for Biogeochemistry in China Marginal Seas With the development of society and the economy, anthropogenic activities (such as the discharge of pollutants, mariculture and coastal engineering, and over ﬁshing) have greatly aﬀected biogeochemical processes in China marginal seas in recent decades, which has negatively inﬂuenced the social development (Song, 2000a). In order to promote sustainable development of people and China’s seas, it is very important to understand the key marine biogeochemical cycles and related ecosystem processes that will be impacted by anthropogenic activities, the roles of ocean biogeochemistry and ecosystems in regulating anthropogenic activities, and the responses of key marine biogeochemical cycles, ecosystems, and their interactions to anthropogenic activities

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(Song, 2000b). In order to solve the problems, the study of the biogeochemistry of China marginal seas in the future should focus on the following issues: (1) understanding how biogeochemical cycles interact with food web dynamics, (2) eﬀects of increasing anthropogenic CO2 and acidiﬁcation on marine biogeochemical cycles, ecosystems, and their interactions, (3) the varying capacity of the ocean to store anthropogenic CO2 , (4) eﬀects of changing supplies of macro- and micronutrients on the biogeochemistry of marginal seas, (5) quantifying material transport within and across the continental shelf, transformation of materials within the water column and sediments, storage of materials in the coastal zone and air-sea exchange, (6) deﬁning the terrestrial boundary condition for nutrient ﬂuxes by a better integration of river basin information, including sediment dynamics and organic inputs, (7) developing budgets and ﬂux estimates for China marginal sea waters in order to understand and predict the impacts of anthropogenic activities and the variation of biogeochemical cycles, and (8) biogeochemistry in aquaculture ecosystems. In the Bohai Sea, biogeochemical processes are mainly aﬀected by anthropogenic activities. In recent decades, freshwater discharge by the Yellow River into the Bohai Sea has decreased greatly, lots of materials have been transported into the Bohai Sea, nutrients regimes and the structure of the ecosystem have greatly changed, and ﬁshery resources have declined signiﬁcantly. Bohai Bay has been the most aﬀected part of the Bohai Sea in recent decades. It was the key region for biogeochemical study in the Bohai Sea. Study of the biogeochemistry of the Bohai Sea in the future should focus on the following aspects: (1) deﬁning the eﬀect on material ﬂuxes of the Yellow River by anthropogenic activities in the Yellow River drainage basin, (2) determining ﬂuxes of terrestrial input of materials, (3) transfers of matter across the Bohai Sea interfaces (including air-sea interface, sediment-seawater interface, and the interface between the Bohai Sea and Yellow Sea), (4) transformation of organic matter in marine food webs, (5) impacts of marine harvesting on end-to-end food webs and biogeochemical cycles, and (6) eﬀects of changing supplies of macro- and micronutrients on the biogeochemistry of the Bohai Sea. In the Yellow Sea, biogeochemical processes are mainly aﬀected by anthropogenic activities along the coastal zones in China and Korea. Mariculture is a kind of important anthropogenic activities in the Yellow Sea (Song et al., 2000a). There are many mariculture zones along the coast of the Yellow Sea. These coastal areas are the key regions of study on biogeochemistry in the Yellow Sea. There are several key biogeochemical processes of the Yellow Sea that should be focused on in the future: (1) transformation mechanisms of nutrients in the water body of the aquaculture ecosystem, (2) budgets and controlling processes of primary biogenic elements in the aquaculture ecosystem, (3) the coupling mechanism of carbon-nitrogen-phosphorus in the aquaculture ecosystem, (4) the generating mechanism and ways of eliminating toxic substances in the aquaculture ecosystem, (5) the biogeochemistry of oxygen (mainly in-

References

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cluding ﬂuxes and concentrations of oxygen), and (6) the biogeochemistry of sulfur (mainly related to the sulﬁde in sediments) in the Yellow Sea. With respect to the East China Sea, biogeochemical processes are greatly aﬀected by the river input, sea currents, and the interactions with the open ocean. Because of the complicated environments in the Yangtze River Estuary, which is greatly aﬀected by the riverine input, the upwelling and coastal currents, and anthropogenic activities (Li et al., 2004), it is the key region of study in both estuarine biogeochemistry of China marginal seas and biogeochemistry of the East China Sea. The study of biogeochemistry in the East China Sea in the future should focus on the following issues: (1) human inﬂuences on the River Basin-Coastal Zone interactions, (2) eﬀects on the nitrogen cycle (especially transformations involving N2 O) by changes in low-oxygen zones, (3) impacts on biogeochemistry in the East China Sea by variations in physical forcing induced by climate change, (4) the varying capacity of the East China Sea to store anthropogenic CO2 , and (5) quantifying material transport within and across the continental shelf. As to the South China Sea, the Pearl River Estuary, and the northern South China Sea, biogeochemical processes can be greatly aﬀected by anthropogenic activities; the coral ecosystem in the Nansha Islands is of high primary productivity and of distinct biogeochemical characteristics and can be aﬀected greatly by variation in pH and anthropogenic activities (Song, 2004). These three regions are the important zones for the study of biogeochemistry in the South China Sea. The following aspects are very important for biogeochemical research in the South China Sea in the future: (1) eﬀects of increasing anthropogenic CO2 and acidiﬁcation on the coral ecosystem in the Nansha Islands and their interactions, (2) transformation of materials within the sediments, storage of materials in the coastal zone, and air-sea exchange, (3) regeneration of nutrients in the water column, and (4) biogeochemistry of oxygen and sulfur. Biogeochemical processes are very complicated and are aﬀected by many factors (Song, 2004). It needs a good synthesis of marine chemistry, marine ecology, marine geology, and physical oceanography to get a better understanding of these processes. Therefore, scientists from a wide range of disciplines are needed to work together in order to solve the above problems and to get a deeper insight into the biogeochemistry of China marginal seas in the future.

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Chen CT (2002) The impact of dams on ﬁsheries: Case of the Three Gorges Dam. In: Steﬀen W, Jager J, Carson DJ, Bradshaw C (eds.) Challenges of a Changing Earth. Springer, Berlin, pp.97-99 Chen CT (2008) Distributions of nutrients in the East China Sea and the South China Sea connection. J Oceanogr 64(5):737-751 Chen CT, Wang SL, Wang BJ, Pai SC (2001) Nutrient budgets for the South China Sea basin. Mar Chem 75(4):281-300 Dagg M, Benner R, Lohrenz S, Lawrence D (2004) Transformation of dissolved and particulate materials on continental shelves inﬂuenced by large rivers: Plume processes. Cont Shelf Res 24(7-8):833-858 Hu J, Peng PA, Jia G, Mai B, Zhang G (2006) Distribution and sources of organic carbon, nitrogen and their isotopes in sediments of the subtropical Pearl River Estuary and adjacent shelf, southern China. Mar Chem 98(2-4):274-285 Hu M, Tang X, Li J, Ma Q (2003) Distributions of dimethylsulﬁde in the Bohai Sea and Yellow Sea of China. J Environ Sci 15(6):762-767 (in Chinese with English abstract) Hung JJ, Lin PL, Liu KK (2000) Dissolved and particulate organic carbon in the southern East China Sea. Cont Shelf Res 20:545-569 Hung JJ, Lin CS, Chung YS, Hung GW, Liu WW (2003) Lateral ﬂuxes of biogenic particles through the Mien–Hua canyon in the southern East China Sea slope. Cont Shelf Res 23(10):935-955 Jahnke R, Reimers C, Craven D (1990) Intensiﬁcation of recycling of organic matter at the sea ﬂoor near ocean margins. Nature 348(6296):50-54 Jiang G, Wang Y, Tang R (1991) Vertical distribution and seasonal variation of dissolved oxygen in the East China Sea. Acta Oceanol Sin 13(3):348-355 (in Chinese with English abstract) L¨ u XX, Song JM, Yuan HM, Li XG, Zhan TR, Li N, Gao X (2005) Distribution characteristics of nitrogen in the southern Yellow Sea surface sediments and nitrogen functions in biogeochemical cycling. Geol Rev 51(2):212-218 (in Chinese with English abstract) Li M, Xu K, Watanabe M, Chen Z (2007) Long-term variations in dissolved silicate, nitrogen, and phosphorus ﬂux from the Yangtze River into the East China Sea and impacts on estuarine ecosystem. Estuar Coast Shelf Sci 71(1-2):3-12 Li XG, Song JM, Yuan HM (2004) Inorganic carbon of sediments in the Yangtze River Estuary and Jiaozhou Bay. Biogeochemistry 77(2):177-197 Lin HY, Han WY (1998) A study on the dissolved oxygen ﬂux in the South China Sea. Oceanol Limnol Sin 29(1):61-66 (in Chinese with English abstract) Lin HY, Cheng SW, Han WY, Wang HK (2003) Intensity of vertical distributed maximum DO in Nansha Islands sea area. J Trop Oceanogr 22(3):9-15 (in Chinese with English abstract) Lin Y, Tang SM, Chen XL, Zhuang DF (1994) Eﬀect of inorganic nitrogen on phytoplankton succession in marine ecosystems. Acta Ecol Sin 14(3):323-326 (in Chinese with English abstract) Liu G, Li D, Lu J, Chen Z, Dong S (1999) Suspending particular matters of shrimp culture in enclosure ecosystems. Chin J Appl Ecol 10(3):350-352 (in Chinese with English abstract) Liu G, Li D, Dong S (2000) Zooplankton feeding on bacteria of shrimp cultural enclosure ecosystems. J Oceanogr Huanghai & Bohai Seas 18(1):45-49 (in Chinese with English abstract)

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Index

Biocommunity coral, 67, 74–76, 78–80, 117, 118, 529–531, 575, 577–584, 586, 590–592, 598, 601, 605–607, 609, 612–615, 617, 619, 631, 651 zooxanthellae, 79, 579, 592 phytoplankton, 15–18, 34, 35, 45–48, 65, 66, 70, 72–74, 85–87, 89, 94, 97–100, 102, 103, 118, 142, 144, 156, 161, 169, 171, 174, 175, 179–185, 195, 198, 205, 218, 279, 283–286, 288–291, 293–296, 298, 299, 301, 305, 334, 339, 340, 345–347, 352, 354, 355, 362–364, 374, 375, 377, 378, 380–382, 395, 402, 405–408, 426, 435, 441, 447, 448, 456, 457, 482, 483, 486, 487, 495–498, 503–505, 512, 521, 522, 550, 553, 564, 589, 595, 605, 628, 629, 632, 634, 638–642, 645–649 species, 1, 15, 16, 18–23, 34–39, 48–50, 66–68, 76, 79, 86, 88, 95, 104–106, 108, 125, 153, 163, 167, 174, 180, 181, 183, 184, 193, 197, 207, 209, 234, 235, 237, 240, 242, 245, 246, 248, 249, 293–296, 299, 318, 332, 333, 335, 337, 438–440, 480, 481, 485, 498, 504, 505, 512, 514, 530, 555, 578, 595, 607, 634, 638, 639, 645, 647–649 zooplankton, 15, 16, 18, 35, 48, 49, 66, 67, 89, 98, 99, 118, 156, 175, 282, 291–294, 296, 381, 395,

402, 405–408, 447, 456, 457, 483, 495–498, 605, 630, 632, 639–642, 647–649 Biogenic element carbon carbon dioxide (CO2 ), 71–73, 80–83, 85–87, 89, 92, 96, 102, 114, 118, 123, 125, 140–144, 147, 148, 151, 153, 157, 161, 273, 276, 278–282, 290, 291, 338–346, 354, 355, 408, 431, 432, 434–440, 444–447, 449, 456, 459, 460, 464, 498, 521, 558–561, 571, 573, 574, 590, 592, 629–632, 641, 646, 647, 650, 651 carbon source/sink, 342, 344–346, 631, 644, 645 colloid organic carbon (COC), 84, 86, 87, 90 dissolved inorganic carbon (DIC), 83, 93, 118, 142, 143, 149, 161, 273, 275, 276, 338–341, 346, 432, 435, 437–440, 444, 446, 447, 456, 457, 459, 558, 559, 561, 590, 592, 641, 646 dissolved organic carbon (DOC), 72, 74, 81, 83–89, 92, 118, 125, 171, 174, 430, 436, 440–442, 447, 464, 562–566, 570, 571, 591, 629, 632 methane, 295 particulate inorganic carbon (PIC), 77, 93, 118, 448

658

Index

particulate organic carbon (POC), 65, 71, 74, 75, 77, 81, 84–89, 93, 117, 118, 124, 144–147, 161, 430, 440–443, 447, 448, 464, 495, 508, 509, 562–566, 570–572, 591, 593, 632, 633, 647, 648 nitrogen dissolved inorganic nitrogen (DIN), 8, 9, 25, 27, 28, 42, 53, 61, 69, 72, 73, 89, 96, 97, 126, 163–167, 169, 174–177, 181, 183–186, 194, 211, 212, 287–290, 303, 306, 307, 320, 321, 362, 363, 377, 382, 383, 467–471, 473, 474, 478, 479, 482, 499, 503, 504, 547–551, 638, 641, 645, 649 inorganic nitrogen (IN), 91, 93–98, 124, 180, 186, 188, 190, 193, 195, 327, 381, 452, 454, 455, 476 organic nitrogen (ON), 91, 93, 94, 96, 97, 99, 113, 118, 186, 189–193, 314, 321, 324, 327, 381, 452, 454–456, 476, 606 nutrient, 3, 14, 30, 68–70, 72–75, 77, 79, 85, 94, 95, 102, 117, 120, 123, 124, 126, 167, 169, 180, 181, 184, 185, 195, 197, 207, 209–213, 269, 284, 286, 289, 308–311, 321, 337, 338, 352, 362–364, 372–377, 382, 407, 408, 425, 426, 431, 469–471, 473, 478–483, 487, 494–499, 503–505, 508, 510, 521, 531, 541, 542, 547, 563, 575, 578–580, 583, 589, 591, 629, 634, 642–645, 650 oxygen dissolved oxygen, 8, 25, 42, 53, 71, 73, 81, 89, 96, 115, 124, 126, 194, 206, 315, 322, 359, 361, 402, 456, 509, 510, 514, 517, 630, 635–637, 649 phosphorus dissolved inorganic phosphorus (DIP), 79, 89, 101, 175–179, 181, 184, 185, 208, 209, 483, 485, 491, 492, 638, 645, 646 inorganic phosphorus (IP), 73, 91, 97, 99, 118, 124, 180, 195, 198, 200, 203, 204, 332, 333, 336, 367, 372, 374, 381, 452, 455, 456, 490

organic phosphorus (OP), 91, 99–101, 107, 113, 118, 122, 332–336, 366, 367, 372, 374, 381, 452, 454–456, 490, 606 silicon biogenic silicon, 634 dissolved silicate, 102, 115, 499, 502, 503 sulfur –2 valence sulﬁde, 616 hydrogen sulﬁde, 88 sulfate, 85, 115, 125, 126, 151, 295, 316, 461, 463, 464, 631, 637 sulﬁde, 97, 103, 108, 115, 120, 126, 153, 610, 613, 616, 631, 637, 651 Biomarker n-alkane, 594, 599, 600 n-alkane, 594, 595, 600–602 sterol, 604, 605 alcohol lipid compound, 76, 593 aldehyde, 76, 593, 602, 603 fatty acid, 75, 83, 592–602, 604, 605, 633 hydrocarbon compound, 592, 605 isoprenoid alkane, 76, 594, 595 ketone, 76, 593, 602, 603 C18 isoprenoid ketone, 602 Z/E phytenal, 602, 603 Z/E pristenal, 602, 603 n-alkane, 465 sterol, 465 Change annual variability, 344 climate change, 79, 81, 82, 89, 294, 555, 606, 610, 651 daily variation, 519, 520 global change, 68, 80, 642, 647 variation characteristics, 548 China margenal sea the South China Sea the northern South China Sea, 575 China marginal sea the Bohai Sea, 1, 3, 6–11, 13–24, 33, 39, 45, 70, 78, 82, 92, 93, 97, 100, 103, 111, 112, 120, 122, 139–144, 147, 148, 155, 160, 161, 163–166, 169–186, 193–197, 205–210, 212–215, 217–219, 230,

Index 231, 233–238, 240, 242, 244–252, 263, 305, 307, 334–336, 380, 446, 631, 632, 634, 635, 637–643, 647, 648, 650 Bohai Bay, 6–8, 10, 11, 15–17, 19–21, 49, 92, 94, 99, 109, 111, 140, 157, 164, 166, 167, 169, 171–174, 188, 195, 196, 215, 218–228, 230, 231, 235, 238–248, 251, 252, 409, 588, 650 Bohai Strait, 6, 7, 9–11, 16, 17, 20, 25, 140, 164, 166–169, 171, 196, 197, 217, 334 Huanghe River Estuary, 68, 69, 112, 114–116, 121, 122, 140, 142, 157, 161, 164, 200 Laizhou Bay, 6, 10, 11, 15, 16, 19–21, 94, 100, 140–142, 144, 164, 166, 169, 173, 174, 195, 196, 198, 200, 203, 215, 218, 233, 234, 237–239, 243, 244, 246, 247 Liaodong Bay, 6, 7, 11, 14, 15, 92, 93, 119, 120, 140, 142, 147–157, 166, 167, 169, 172, 173, 195, 196, 215, 217, 218, 233–235, 238, 242, 244, 247, 248, 250 the southern Bohai Sea, 14, 190, 192, 198, 201, 203–205 the Bohai Sea sediments, 158 the East China Sea, 1, 3, 23, 39, 42, 45, 46, 48–51, 68, 69, 71, 72, 75–78, 82, 84–86, 100, 102, 103, 106, 108, 110–113, 117, 118, 120, 122, 263, 267, 271, 425, 426, 429, 431, 450, 467, 473, 498, 506, 509, 518, 530, 542, 570, 592–596, 602–605, 631–637, 641, 643, 644, 647–649, 651 Changjiang River Estuary, 45, 68, 69, 108, 156, 271, 307, 409, 425, 450, 451, 453, 455–457, 459, 460, 469, 473, 475, 476, 485, 487, 490, 499, 588 Hangzhou Bay, 433–435, 447, 476, 489, 490 Okinawa Trough, 39, 40, 44, 45, 78, 93, 112–114, 119, 121, 464, 466, 506 the South China Sea

659

Daya Bay, 86, 105, 115, 116, 119, 122, 568–570 Nansha Islands waters, 76, 575, 576, 583, 596, 618, 619 the northern South China Sea, 3, 88, 123, 345, 529, 561, 568, 570, 575, 578, 632, 633, 648, 651 Zhujiang River Estuary, 69, 83, 84, 86, 100 the Yellow Sea, 1, 7, 10, 23–25, 28, 30–37, 39, 40, 42, 49, 50, 70, 78, 82, 85, 96, 100, 111, 169, 217, 235, 263, 264, 271, 273, 295, 296, 303, 425–428, 507, 512, 631, 632, 634–637, 643, 646–648, 650, 651 Jiaozhou Bay, 34, 70, 72, 83, 89, 91, 92, 97, 99, 100, 103, 109, 110, 113, 150, 156, 263, 311, 337–382, 384, 506 the northern Yellow Sea, 9, 24, 82, 99, 111, 396, 632 the southern Yellow Sea, 24, 28, 82, 98, 111, 112, 282, 426, 428, 632, 635, 636 Coral reef lagoon, 74–79, 107, 117–120, 578, 580, 582, 590–596, 602–605, 607–609, 611, 613, 615–619 oﬀ-reef, 78, 119, 120, 580, 591, 616–619 Yongshu reef, 75, 76, 118, 123, 124, 578, 580–582, 590, 592–596, 602–605, 607–609, 614, 617, 618 Zhubi reef, 67, 74, 76, 118, 123, 124, 580–582, 590, 591, 607, 618, 620 Determination chemical analysis, 606 sequential extraction, 97, 108, 148, 149, 186, 311, 449, 451, 488, 553, 555, 583 Distribution horizontal distribution, 46, 52, 66, 71, 88, 106, 141, 142, 160, 163, 166, 195, 216–218, 233, 234, 236, 238, 239, 297, 338, 345, 359, 382, 396, 399, 400, 402, 406–408, 433, 442, 518, 636 section distribution, 159

660

Index

vertical distribution, 71, 72, 88, 99, 102, 107, 109, 122, 125, 149, 150, 153, 154, 157, 161, 187, 189, 196, 215, 218, 224, 228, 240, 241, 323, 326, 327, 330, 350, 360, 367, 376, 381, 409, 416, 442, 456, 617 vertical proﬁle, 72, 96, 100, 106, 150, 158, 168, 197, 218, 229, 241, 323, 324, 327, 356, 357, 361, 363, 367, 370, 410, 414, 572, 616, 617, 630 Dynamics Changjiang diluted water (CDW), 29, 46, 269, 426, 449, 462, 467, 469, 487, 499 Changjiang River and Jiangsu Coastal Waters, 426 current Cheju Warm Current, 267 coastal current, 28, 29, 42, 263, 264, 267, 269, 286, 319, 398, 408, 409, 426, 460, 469, 512, 546, 547, 551, 586, 637, 644, 651 ENSO, 535, 536, 541 Taiwan Warm Current, 29, 46, 56, 269, 271, 313, 314, 426, 448, 469, 512, 636 the currents east of the Pyukyu Islands, 430 the Kuroshio Current, 42, 73, 425, 426, 428, 442, 450, 532 the Kuroshio in the ECS, 286, 428–430 Tsushima Warm Current, 264 Tsushima Warm Currents, 28 Yellow Sea Coastal Current, 426 Yellow Sea Warm Current (YSWC), 29, 169, 173, 174, 264, 286, 397, 398 eddy the warm eddy east of the Kuroshio core, 426 nutrient dynamics regeneration, 14, 74, 87, 103, 118, 125, 126, 190, 193, 210, 336, 375, 504, 634, 635, 645, 648, 651 release, 75, 82, 96, 97, 100–102, 104, 109, 112, 115, 121, 122, 140, 148, 151, 174, 188, 190, 193, 194, 200, 201, 205, 206, 208, 210–214, 218,

231, 295, 296, 306, 318, 369, 370, 372, 377, 380, 381, 390, 412, 444, 449, 519, 546, 552–554, 558, 587, 588, 591, 629, 632, 634, 635, 640, 641 the mixed layer, 70, 74, 81, 444, 447, 539–541, 570, 574 the northern ECS Cold Water (cold eddy), 271 the Vietnam cold eddy, 56, 533, 534 the Yellow Sea Cold Water, 42, 85, 426 the Yellow Sea Cold Waters, 25 thermocline, 7, 30, 269, 271, 284, 447, 534, 537, 539–541, 637 thermodynamics, 535 tropical oligotrophic water, 579 water dynamic process, 174, 264, 426, 446, 464, 539 water exchange, 57, 70, 95–97, 142, 193, 218, 219, 267, 271, 339, 375, 376, 428, 509, 537–539, 580, 582, 607 Yellow Sea Cold Water Mass, 30, 426 Ecosystem ecological risk, 232, 384, 393–395 Flux diﬀusion ﬂux, 77, 78, 109 vertical ﬂux, 71, 73, 75–77, 336, 408, 463, 592, 610, 611, 634 Form biogenic form, 200 carbonate form (CF), 76, 97, 98, 108, 188, 192, 200–206, 608, 613 ion-exchangeable form (IEF), 97, 98, 186, 188, 192, 198, 201, 202, 311, 314, 315, 318, 320–324, 326–328, 331, 356, 358–360, 362, 366, 583, 585, 586, 588, 608, 613 ion-exchangeable form, IEF, 98, 321, 358, 359 iron and manganese oxides form (IMOF), 97, 98, 188, 192, 193, 200–205, 608, 613 organic matter and sulﬁde form (OSF), 97, 98, 188, 192, 193, 200–203, 205, 608, 613

Index Heavy metals arsenic (As), 76, 77, 109, 215, 217, 226, 228, 387, 389, 391–393, 395, 403–406, 606–611 cadmium (Cd), 102, 108, 109, 217, 220, 226, 227, 229–231, 384–389, 391–393, 395, 403–406, 638–640 dissolved Cd, 215 chromium (Cr), 108, 109, 220, 226–228, 231, 607–611, 638 copper (Cu), 69, 108, 109, 215, 217, 220, 226–228, 231, 384–389, 391–394, 403–406, 629, 638, 640 heavy metal pollution, 219, 640 lead (Pb), 60, 108–111, 215, 217, 219–222, 226, 227, 229–231, 347–349, 384–389, 391–393, 396, 403–406, 409, 410, 638–640 dissolved Pb, 215, 217 mercury (Hg), 109, 215, 217, 218, 220, 228, 385, 387–389, 391–393, 395, 403–406, 638–640 dissolved Hg, 215, 218 zinc (Zn), 77, 108, 109, 220–223, 226, 227, 229–231, 384–389, 391–394, 403–406, 607–611, 638, 640 Interface microlayer, 295–302, 637 sea-air interface, 295, 296, 445, 447, 517–519, 636 atmospheric input, 209, 310, 311, 479, 483 dry deposition, 209, 308–310, 479, 480 wet deposition, 209, 309, 310, 404, 481 sediment-seawater interface, 77, 78, 81, 97, 99, 112, 116, 119, 120, 122, 125, 127, 198, 380, 463, 618, 619, 640, 641, 650 Marine biogeochemistry biogeochemical cycle, 80, 83, 94, 99, 103, 153, 444, 504, 508, 606, 628, 632–634, 641–643, 649, 650 biogeochemical process, 1, 67, 69, 75, 78–80, 86, 88, 100, 112, 114, 116, 124–126, 139, 156, 171, 172, 195,

661

198, 201, 218, 263, 273, 283, 286, 332, 337, 425, 426, 443, 467, 498, 529, 546, 547, 570, 575, 606, 612, 627, 629, 631, 639, 642, 644–647, 649–651 biological pump, 80, 81, 158, 263, 291, 354, 363, 377, 425, 444, 447, 640, 643, 645, 647 early diagenesis process, 110, 120–123, 186 geochemical characteristics, 110, 186, 223, 228 Marine sediment clay, 11, 13, 44, 95, 113, 121, 149, 189, 191, 193, 206, 223, 225, 228, 334, 358, 369, 384, 386–391, 393, 400–402, 404, 405, 409, 416, 553, 568, 569, 583–589 core sediment, 92, 93, 99, 107, 149, 157, 186, 188–190, 192, 193, 223–225, 230, 240, 241, 323, 327, 332–335, 348–350, 352, 355, 356, 358–361, 363, 367, 377, 388, 396, 409, 410, 413, 414, 504, 506, 507 grain size, 78, 92, 96–99, 101, 103, 105, 106, 110–112, 114, 116, 148, 186, 188, 192, 206, 223, 225, 228, 229, 231, 315, 317–320, 327, 328, 330, 371, 384–390, 392–394, 400–402, 404, 408, 409, 449, 461, 463, 476, 477, 568, 569, 586, 633 gravel, 7, 13, 400 silt, 11, 13, 44, 45, 113, 142, 143, 159, 173, 189, 200, 206, 223, 225, 228, 334, 335, 371, 384, 387–389, 400, 401, 404, 405, 476, 477, 506, 509, 568, 583, 584, 588, 589 surface sediment, 14, 42, 44, 45, 60, 77, 78, 91, 93–95, 98, 102, 103, 108–110, 113, 116–122, 125, 163, 174, 192, 193, 197, 198, 201–205, 211, 213, 233, 234, 236, 238, 239, 242, 244, 245, 311, 314, 315, 318–323, 331, 332, 334, 349, 352, 358–363, 367, 369–371, 375, 381, 386, 388, 392, 394, 396–400, 402, 404–406, 408–410, 448, 450, 451, 453, 461, 463, 489, 490, 505, 552,

662

Index 561, 562, 568–570, 584–588, 635, 637

Particle marine snow, 628, 630 settling particle, 116–118, 508, 509, 590 suspended particle, 14, 104, 105, 110, 118, 119, 122, 211, 399, 407, 408, 435, 485, 545, 569, 590–592 Persistent organic pollutants (POPs), 104, 105, 107, 139, 231–233, 242, 243, 249, 250, 263 polychlorinated biphenyls (PCB), 105 polychlorinated biphenyls (PCBs), 104, 105, 107, 232, 237–243, 249, 251, 252, 396, 399–403, 405, 406, 408–410, 414–418 polycyclic aromatic hydrocarbons (PAHs), 104–107, 232–235, 242–245, 250, 396–398, 409–418, 557 Productivity biomass, 15–19, 21, 22, 34–36, 39, 45–49, 65–67, 75, 84–87, 116, 124, 160, 169, 172, 180–183, 198, 208, 282, 291, 292, 294–296, 298, 299, 345, 375, 395, 402, 405–408, 426, 448, 481, 495, 497, 546, 564, 579, 607, 629, 638–640, 649 new productivity, 308, 310, 589, 633 primary productivity, 70, 73, 74, 84, 86–88, 95, 98, 99, 102, 103, 156, 175, 283, 284, 407, 440–442, 448, 460, 508, 509, 579, 589, 592, 593, 635, 638, 645, 648, 649, 651 assimilation rate, 284, 291, 292, 495, 497 chlorophyll a, 15–17, 34, 45–48, 61, 86, 98, 171, 172, 282, 283, 299–302, 334, 345, 407, 441, 552, 639, 649 photosynthesis, 69, 73, 79, 81, 89, 118, 123, 126, 144, 147, 161, 276, 279, 284, 288, 289, 291, 354, 446, 447, 497, 510, 512, 519, 521, 522, 592, 629, 635–637, 646–648

photosynthetic process, 156, 290, 408, 512 River Changjiang River (Yangtze River), 25, 28, 29, 32, 33, 42, 44, 45, 69, 71, 103, 107, 111, 122, 263, 267, 269, 271, 306, 307, 310, 401, 408, 425, 448, 450, 460–462, 464, 465, 467, 469, 471, 474, 478, 479, 483, 490, 492–494, 498, 499, 502, 504, 507, 512 Dagu River, 223, 337, 338 Duliujian River, 223 fresh water, 14, 32, 45, 111, 210, 425, 492, 540, 558, 563, 569, 643, 644 Haihe River, 7, 14, 32, 173, 192, 209, 223, 239, 240, 243, 245, 246 Huanghe River (Yellow River), 7, 9, 11, 14, 15, 32, 33, 45, 69, 121, 142–144, 163, 164, 169, 173, 174, 192, 200, 203, 209, 263, 307 Qihe River, 223 riverine input, 3, 23, 39, 50, 104, 139, 142, 173, 209, 210, 263, 284, 310, 311, 396, 425, 448, 509, 544, 545, 562, 634, 645, 651 Yalujiang River, 14, 33, 396, 398 Zhujiang River (Pearl River), 52, 60, 69, 87, 104, 107 Seawater interstitial water, 77, 78, 108, 114–116, 119–122, 125, 151, 204, 461, 555, 580, 616–619 overlying water, 77, 78, 95–97, 99, 100, 102, 105, 115, 119–122, 125, 126, 198, 200, 204, 206, 207, 311, 315–318, 320–323, 335, 336, 358–364, 370, 371, 375, 377, 378, 389, 456–458, 498, 555, 586, 589 surface seawater, 72, 77, 82, 83, 273, 276, 300, 340, 343, 344, 346, 406, 407, 445, 446, 574, 637 Simulated drift-net theory eﬃcient recycling, 579, 648 high cycle rate, 579 oasis in the desert, 579