Biogeochemical Processes at the Land-Sea Boundary (Elsevier Oceanography Series)

BIOGEOCHEMICAL PROCESSES AT THE LAND-SEA BOUNDARY FURTHER TITLES IN THIS SERIES 1 J.L. MERO THE M I N E R A L RESOURC...

Author: Pierre Lasserre

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BIOGEOCHEMICAL PROCESSES AT THE LAND-SEA BOUNDARY

FURTHER TITLES IN THIS SERIES 1 J.L. MERO THE M I N E R A L RESOURCES OF THE SEA 2 L.M.FOMlN THE DYNAMIC METHOD I N OCEANOGRAPHY 3 E.J.F.WOOD MICROBIOLOGY OF OCEANS A N D ESTUARIES 4 G.NEUMANN OCEAN CURRENTS 5 N.G.JERLOV OPTICAL OCEANOGRAPHY 6 V.VACQUIER GEOMAGNETISM I N MARINE GEOLOGY 7 W.J. WALLACE THE DEVELOPMENTS OF THE C H L O R l N l T Y l S A L l N l T Y CONCEPT I N OCEANOGRAPHY 8 E. L l S l T Z l N SEA-LEVEL CHANGES 9 R.H.PARKER THE STUDY OF BENTHIC COMMUNITIES 10 J.C.J. N I H O U L (Editor) MODELLING OF MARINE SYSTEMS 1 1 0.1. MAMAYEV TEMPERATURE-SALINITY ANALYSIS OF WORLD OCEAN WATERS 12 E.J. FERGUSON WOOD and R.E. JOHANNES TROPICAL MARINE POLLUTION 13 E. STEEMANN NIELSEN MAR I N E PHOTOSY NTH ESlS 14 N.G. JERLOV MARINE OPTICS 15 G.P.GLASBY MARINE MANGANESE DEPOSITS 16 V.M. KAMENKOVICH FUNDAMENTALS OF OCEAN DYNAMICS 17 R.A.GEYER SUBMERSIBLES A N D THEIR USE I N OCEANOGRAPHY AND OCEAN ENGINEERING 18 J.W. CARUTHERS FUNDAMENTALS OF MARINE ACOUSTICS 19 J.C.J. NIHOUL (Editor) BOTTOM TURBULENCE 20 P.H. LEBLOND and L.A. MYSAK WAVES I N THE OCEAN 21 C.C. VON DER BORCH (Editor) SYNTHESIS OF DEEP-SEA D R I L L I N G RESULTS I N THE I N D I A N OCEAN 22 P. DEHLINGER MARINE G R A V I T Y 23 J.C.J. N I H O U L (Editor) HYDRODYNAMICS OF ESTUARIES A N 0 FJORDS 24 F.T. BANNER, M.B. COLLINS and K.S. MASSIE (Editors) THE NORTH-WEST EUROPEAN SHELF SEAS: THE SEA B E 0 A N D THE SEA I N MOTION 25 J.C.J. N I H O U L (Editor) MARINE FORECASTING 26 H.G. RAMMING and 2. KOWALIK NUMERICAL MODELLING MARINE HYDRODYNAMICS 27 R.A. GEYER (Editor) MARINE ENVIRONMENTAL POLLUTION 28 J.C.J. NIHOUL (Editor) MARINE TURBULENCE 29 M. WALD1CHUK.G.B. KULLENBERG and M.J. ORREN (Editors) MARINE POLLUTANT TRANSFER PROCESSES 30 A. VOlPlO (Editor) THE BALTIC SEA 31 E.K. OUURSMA and R . OAWSON (Editors) MARINE ORGANIC CHEMISTRY 32 J.C.J. NIHOUL. (Editor) ECOHY ORODY NAMlCS 33 R. H E K l N l A N PETROLOGY OF THE OCEAN FLOOR 34 J.C.J. N I H O U L (Editor) HYDRODYNAMICS OF SEMI-ENCLOSEO SEAS 3 5 8. JOHNS (Editor) PHYSICAL OCEANOGRAPHY OF COASTAL A N 0 SHELF SEAS 36 J.C.J. N I H O U L (Editor) HYDRODYNAMICS OF THE EQUATORIAL OCEAN 37 W. LANGERAAR SURVEYING A N D CHARTING OF THE SEAS 38 J.C.J. N I H O U L (Editor) REMOTE SENSING OF SHELF SEA HYDRODYNAMICS 3 9 T. ICHIYE (Editor) OCEAN HYDRODYNAMICS OF THE JAPAN A N D EAST CHINA SEAS 4 0 J.C.J. NIHOUL (Editor) COUPLE0 OCEAN-ATMOSPHERE MODELS 41 H. KUNZENOORF (Editor) MARINE MINERAL EXPLORATION 42 J.C.J. NIHOUL (Edltorl MARINE INTERFACES ECOHYDRODYNAMICS

Elsevier Oceanography Series, 43

BIOG EOCHEMICAL PROCESSES AT THE LAND-SEA BOUNDAR’I Edited by

PIERRE LASSERRE Station Marine de Roscoff CNRS & Universitt?de Paris VI Roscoff, Nord-Finistere, France and

JEAN-MARIE MARTIN lnstitut de Biogkochimie Marine CNRS & Ecole Normale Supkrieure rue d’Ulm, Paris, France

ELSEVIER Amsterdam - Oxford

- New York

- Tokyo 1986

ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 21 1,1000 AE Amsterdam, The Netherlands Distributors for the United States and Canada:

ELSEVIER SCIENCE PUBLISHING COMPANY INC. 52, Vanderbilt Avenue New York, N Y 10017, U.S.A.

Library o f Congress Catalogingin-Publicalion Data

Biogeockemical processes at the land-sea boundary. (Elsevier oceanography series ; 43) Bibliography: p . 1. Biogecchemical c y c l e s . 2. Seashore ecology. 3. Coastal ecology. I. Lasserre, Pierre. 11. Martin, Jean-Marie. 111. Tit3e: The land-sea boundary. I V . Series. QH344.B57 1986 574.5 '2638 66-16702 ISBN 0-444-42675-2 (U.S.)

ISBN 0-444-42675-2 (Vol. 43) ISBN 0 4 4 4 4 1 6 2 3 4 (Series)

0 Elsevier Science Publishers B.V., 1986

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted i n any form or b y any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V./Science 81Technology Division, P.O. Box 330, 1000 A H Amsterdam, The Netherlands. Special regulations for readers in the USA - This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of part o f this publication may be made in the USA. A l l other copyright questions, including photocopying outside of the USA, should be referred t o the publishers. Printed in The Netherlands

V

CONTRIBUTING AUTHORS

M. Bianchi

Laboratoire d e Microbiologie Marine, CNRS & Universit; Provence, 1331, Marseille, France.

S. Chamroux

Station Biologique d e Roscoff, Universit; d e P a r i s VI & CNRS, 29211 Roscoff, France.

E.T. Degens

Geologisch-PalaontologischesInstitut, Bundesstrasse 55, D-2000 Hamburg 13, F e d e r a l Republic of Germany.

J.G. Field

Marine Biology Research Institute, Zoology Department, University of C a p e Town, 770 Rondebosch, South Africa.

B.R. Folsom

Gray F r e s h w a t e r Biological Institute, University of Minnesota, Navarre, MN 55392, United S t a t e s of America.

C.D. Hunt

G r a d u a t e School of Oceanography, University of Rhode Island, Kingston, RI 02881, United S t a t e s of America

V. I t t e k k o t

Ceologisch-PalaontologischesInstitut, Bundesstrasse 55, D-2000 Hamburg 13, F e d e r a l Republic of Germany.

J. Jednacak-Biscan

Laboratory of Electrochemistry, Rudjer Boskovic Institute, P.O. Box 1013, Zagreb, Yugoslavia.

C.J.M. K r a m e r

MT-TNO, D e p a r t m e n t f o r Marine Research, P.O. Box 57, 1780 AB Den Helder, T h e Netherlands.

P. Lasserre

S t a t i o n Biologique d e Roscoff, U n i v e r s i t i d e P a r i s VI & CNRS, 29211 Roscoff, France.

K.H. Mann

Marine Ecology Laboratory, Bedford I n s t i t u t e of Oceanography, Dartmouth, N.C. B2Y 4A2, Canada.

J.-M.

Institut d e Biogkochimie Marine, UA 386 CNRS, Ecole Normale Supkrieure, 46, r u e d'Ulm, 75230 Paris, France.

Martin

de

VI

L.-A. Meyer-Reil

I n s t i t u t fur Meereskunde a n d e r Universitat Kiel, Marine Mikrobiologie, 2300 Kiel I , F e d e r a l Republic of Germany.

W. Michaelis

Geologisch-Palaontologisches Institut, Bundesstrasse 55, D-2000 H a m b u r g 13, F e d e r a l Republic of Germany.

C.L. Moloney

Marine Biology R e s e a r c h Institute, Zoology D e p a r t m e n t , University of C a p e Town, 770 Rondebosch, S o u t h Africa.

F.M.M. Morel

D e p a r t m e n t of Civil Engineering, R.M. P a r s o n s L a b o r a t o r y , M a s s a c h u s e t t s I n s t i t u t e of Technology, Cambridge, Mass., United S t a t e s of America.

J.M. Mouchel

I n s t i t u t d e Biogkochirnie Marine, UA 386 CNRS, E c o l e N o r m a l e SupCrieure, 46, r u e d'Ulrn, 75230 Paris, F r a n c e .

S.W. Nixon

G r a d u a t e School of Oceanography, University of Rhode Island, Kingston, RI 02881, United S t a t e s of America.

B.L. Nowicki

G r a d u a t e School of Oceanography, University of R h o d e Island, Kingston, RI 0 2 8 8 1 , U n i t e d S t a t e s of America.

T. Tour&

S t a t i o n Biologique d e Roscoff, Universitk d e P a r i s VI & CNRS, 29211 Roscoff, F r a n c e .

D.R. T u r n e r

Marine Biological Association of t h e U.K., P l y m o u t h PLI 2PB, U n i t e d Kingdom.

P.A. Wickens

Marine Biology R e s e a r c h Institute, Zoology D e p a r t m e n t , University of C a p e Town, 770 Rondebosch, S o u t h Africa.

J.M. Wood

G r a y F r e s h w a t e r Biological I n s t i t u t e , University of Minnesota, N a v a r r e , MN 55392, United S t a t e s of America.

C i t a d e l Hill,

VII

CONTENTS

iX

Preface 1. Behaviour of c h e m i c a l species a n d modelling.

Approaches in chemical speciation studies in marine waters. C.J.M. K r a m e r

3

River inputs into oceans. W. Michaelis, V. I t t e k k o t a n d E.T. Degens

37

Surface properties of particles at the land-sea boundary. J.-M. Martin, J.-M. Mouchel a n d J. Jednacak-Biscan

53

11. Nutrient cycling and p a t h w a y s of o r g a n i c transformations.

Modelling studies of material flows in a shallow ecosystem compared to the open ocean. J.G. Field, P.A. Wickens a n d D.L. Moloney

75

The retention of nutrients (C,N, P.), heavy metals (Mn, Cd, Pb, Cu), and petroleum hydrocarbons in Narragansett Bay. S.W. Nixon, C.D. Hunt a n d B.L. Nowicki

99

The role of detritus at the land-sea boundary. K.H. Mann

123

Spatial and temporal distribution of bacterial populations in marine shallow water surface sediments. L.-A. Meyer-Reil

141

Heat production of microorganisms in eutrophied estuarine systems An experimental study. P. Lasserre, T. Tourni;, M. Bianchi a n d S. C h a m r o u x

161

-

111. U p t a k e of t r a c e e l e m e n t s by living organisms.

Trace metals-phytoplankton interactions :a n overview. F.M.M. Morel

177

Biological availability of trace elements. D.R. Turner

191

Predictions for t h e mobility of elements in the estuarine environment. B.R. Folsom and J.M. Wood

20 1

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IX

PREFACE

The connection between terrestrial and oceanic systems is a functional one and the consequences of this linkage on the very large variety of coastal systems are profound. I t is widely recognized that many o f the major processes which influence the biological properties and chemical forms of elements, and their biogeochernical cycles in the ocean occur a t the landsea boundary, especially in estuaries, coastal lagoons, the coastline and the shelf. Over 80% of living systems and their fisheries take place in near shore waters am' the consequential production of organic matter produced triggers off the high level o f activity. Hence, the last fifteen years, there has been considerable stimulus to provide framework to evaluate the intemctionsand effects of human activities There is an intuitive belief among scientists that each estuary or lagoon is unique and different Similarities d o exist, however, and this book attempts t o highlight some common properties and perhaps some innovative views on biogeochemical processes, and biological fluxes which are of central concern in the understanding o f the landsea boundary. The contents are based upon lectures given a t a Seminar organised as the scientific component of the 1 7 t h Ceneml Meeting o f SCOR, held a t the Station Biologique de Roscoff, France, on 22-24 October 1984. The original lectures have been substantially extended and revised in order to give a fuller treatment o f the suject The contributions identify important processes influencing (1) Behaviour of chemical species, (II) Nutrient cycling and mechanisms of organic transformations and (III) Uptake o f tmce elements by living systems. Special attention was paid to modelling and to the problem of anthropogenic influence on the natuml behaviour, with an overview o f the methodology and experimental techniques used in labomtory and field research.

X

The editors wish to acknowledge the sustained support of the Scientific Committee on Oceanic Research (SCOR), the International Association for Biological Oceanography (IABO) and the UNESCO Division of Marine Sciences. Financial support was also provided by the Centre National de la Recherche Scientifique (CNRS 'Pirocean' and 'Piren'), the Universite de Paris VI, the lnstitut franGais de Recherche pour I'Exploitation de la Mer [IFREMER), and the Ministire des Relations Exterieures We specifically thank Mrs Bernadette Lasserre for providing excellent copy editing and Mrs Nicole Guyad for secretaria 1 assistance. The selection of topics is the responsibility of the editors and has undoubtedly been influenced by the many friends and colleagues met through mutual interests in marine biogeochemistry. The editors are most grateful t o the authors for their collaboration and to all participants to the SCOR Seminar, in Roscoff, for their involvement in the lively discussions, the results of which are reflected in the final form of contributed chapters. We hope this book will provide insights which will point the way for future research.

Roscoff, April 1986

Pierre Lasserre Jean-Marie Mart in

1

I

BEHAVIOUR OF CHEMICAL SPECIES AND MODELLING

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3

APPROACHES IN CHEMICAL SPECIATION STUDIES IN MARINE WATERS

C.J.M. KRAMER* Netherlands Institute for Sea Research, P.0.Box 59, den Burg, Texel (The Netherlands)

ABSTRACT Kramer, C.J.M., 1986. Approaches in chemical speciation studies in marine waters. In: P. Lasserre and J.M. Martin (eds), Biogeochemical Processes at t h e Land-Sea Boundary. Elsevier, Amsterdam. After a brief introduction on terminology, this overview summarizes t h e experimental and theoretical modelling methods applied in t r a c e metal speciation studies, emphasizing the dissolved fraction as defined by 0.45 p n filtration. The experimental approach comprises interactions with organic - and inorganic ligands, speciation schemes, biological experiments and interactions with particles and colloids. To illustrate one type of speciation research, i.e. t h e determination of t h e apparent complexation capacity for copper ( C C c u ) and the conditional stability constant (K'), examples a r e given for three marine areas, viz. t h e Scheldt estuary, t h e Southern Bight of t h e North Sea and t h e open north Atlantic Ocean. A hypothetical model is presented giving t h e complexation capacity across t h e land-sea boundery : from river t o ocean.

INTRODUCTION Interest

in t r a c e element speciation studies in natural waters has increased

considerably during t h e last decade. I t has become apparent t h a t d a t a on total concentrations of any element rather than on individual well defined chemical entities, a r e often inadequate to identify transport mechanisms, ultimate fate and toxicity of particular elements to organisms. A study of the different t r a c e metal species and their relative distribution will assist in understanding t h e chemical processes t h a t t a k e place in t h e highly reactive estuarine zone and in t h e open sea. These processes include t h e r a t e at which chemical processes t a k e place,

t h e participation

in geochemical processes

(precipitationldissolution,adsorption/desorption). Research in t h e field of t r a c e metal speciation has been developed in application and improvement of two main directions : a) experimental techniques to distinguish specific or operationally defined forms of t r a c e elements, either in laboratory studies under controlled conditions or in situ ; b) mathematical models.

*Present adress : MT-TNO, Dept. for Marine Research, P.O. Box 57, 1780 AB Den Helder, The Netherlands.

4

R e c e n t reviews on c h e m i c a l speciation a r e published by e.g. S t u m m a n d Brauner (1975), F l o r e n c e and Batley (1980) a n d L e p p a r d (1983) s o m e t i m e s , with s p e c i a l r e f e r e n c e to metal-organic i n t e r a c t i o n s (Mantoura, 1982) o r complexation in n a t u r a l w a t e r s ( K r a m e r

and Duinker,l984b). Bruland (1983) s u m m a r i z e d t h e distribution and behaviour of t r a c e e l e m e n t s in o c e a n waters. T h e o c c u r r e n c e of c e r t a i n s p e c i e s is largely d e p e n d e n t on t h e e n v i r o n m e n t a l conditions. T h e r e e x i s t s a s t r o n g c o m p e t i t i o n of t r a c e m e t a l s with H+ o r major c a t i o n s like C a 2 + and Mg2+ in s e a w a t e r , b u t a l s o with o t h e r t r a c e m e t a l s which might f o r m m o r e s t a b l e c o m p l e x e s with t h e ligand in question ; on t h e o t h e r side, many potential ligands o r c h e l a t o r s c o m p e t e f o r o n e t r a c e e l e m e n t . A t t h e land-sea boundary, especially n e a r o u t l e t s a n d in e s t u a r i e s , a rapid c h a n g e in environmental conditions o c c u r s during mixing of f r e s h

-

and s e a w a t e r . R e a c t i v i t y in

response to g r a d i e n t s in i m p o r t a n t p a r a m e t e r s like ionic s t r e n g t h , r a t i o s of major components, pH, Eh, turbidity, etc. a n d t h e availability of t r a c e e l e m e n t s and complexing a g e n t s will affect t h e distribution of t h e d i f f e r e n t s p e c i e s of a t r a c e metal. E.g. in f r e s h w a t e r , c a d m i u m will b e p r e s e n t in ionic f o r m , as c a r b o n a t o - a n d hydroxide c o m p l e x e s and bound to (organic) h u m a t e s , in s e a w a t e r chloro-complexes a r e d o m i n a n t (Raspor, 1980b ; Mantoura et al., 1978 b). In t h e following s e c t i o n s a n overview will b e p r e s e n t e d of t h e various a p p r o a c h e s t h a t h a v e been followed. T h e y a r e briefly s u m m a r i z e d in T a b l e 1. Additionally s o m e e x p e r i m e n t a l r e s u l t s o n t h e complexing behaviour of c o p p e r in t h e marine e n v i r o n m e n t will b e p r e s e n t e d c o v e r i n g t h e land-sea boundary : e s t u a r y , c o a s t a l sea and open ocean, to i l l u s t r a t e t h e possibilities of t h i s t y p e of research.

TABLE 1 D i f f e r e n t a p p r o a c h e s in t r a c e m e t a l s p e c i a t i o n s t u d i e s (with s o m e examples). 1- Valency s r a r e 2 - Metal - inorganic i n t e r a c t i o n s 3 - M e t a l - organic i n t e r a c t i o n s a) d i r e c t d e t e r m i n a t i o n of s p e c i e s b) f u c t i o n a l groups c ) well defined o r g a n i c compounds d) e x p e r i m e n t a l l y defined groups of o r g a n i c m a t t e r e) model compounds f) n a t u r a l s a m p l e s 4 - Speciation s c h e m e s 5 - E f f e c t s of o r g a n i s m s o n c h e m i c a l s p e c i a t i o n a ) bio-assays (uptake, e f f e c t s ) b) e x c r e t i o n p r o d u c t s c) i n t r a c e h l a r o r g a n i c m a t t e r 6 - I n t e r a c t i o n with p a r t i c l e s a n d colloids a) complexation b) ion e x c h a n g e c ) adsorption desorption d) p r e c i p i t a t i o n - dissolution e ) coagulation, flocculation 7 - kinetics 8 c h e m i c a l modelling

-

-

( F e (Il)/(III), C r (III)/(VI) (CI-, ~ 0 3 2 - )

(Pb( Me)) (RCOO-, RzNH, RS4) (amino acids, c a r b o h y d r a t e s ) (humics) (EDTA, NTA)

5

GENERAL TERMINOLOGY In t h e ( a q u a t i c ) e n v i r o n m e n t e l e m e n t s o c c u r in particulate-, colloidal- and dissolved forms. T h e s e f o r m s a r e usually distinguished by f i l t r a t i o n o r c e n t r i f u g a t i o n . Traditionally,

a 0.45 um (membrane). f i l t e r s e p a r a t e s t h e p a r t i c u l a t e f r o m t h e dissolved forms. This may result in t h e passage of colloidal f r a c t i o n s through t h e f i l t e r , classifying colloidal m a t t e r incorrectly within t h e dissolved fraction. Although t h e i n t e r a c t i o n b e t w e e n dissolved and p a r t i c u l a t e ( s u r f a c e ) f r a c t i o n s c a n n o t b e neglected, it i s c o m m o n in speciation studies t o consider t h e "dissolved" fraction. T h e dissolved f o r m s of t r a c e e l e m e n t s a r e mainly present as :

- f r e e h y d r a t e d ions; - ion p a i r s ( w h e r e t h e c o o r d i n a t e d w a t e r is retained); - inorganic- a n d o r g a n i c c o m p l e x e s (with a c o v a l e n t bond). An anion o r molecule containing f r e e e l e c t r o n pairs forming a coordination compound (complex) with a c e n t r a l a t o m (cation, M) is c a l l e d a ligand (L), containing a donor a t o m (Stumm and Morgan, 1981). Complex f o r m a t i o n w i t h anions o r molecules with m o r e than o n e donor a t o m i s called chelation. C h e l a t o r s c a n b e i m p o r t a n t w h e r e l a r g e organic molecules (e.g. e n z y m e s ) h a v e a c o m b i n a t i o n of a f a v o r a b l e t e r t i a r y s t r u c t u r e and several donor a t o m s . T h e c o m p l e x e s t h u s f o r m e d a r e s o m e t i m e s s p e c i f i c for a t r a c e e l e m e n t , due

to i t s s p e c i f i c s i z e and charge. Binding of m o r e t h a n o n e m e t a l to o n e ligand o r c h e l a t o r results in multi- or polynuclear complexes. S t e p w i s e a n d c o n s e c u t i v e c o m p l e x f o r m a t i o n r e a c t i o n s b e t w e e n a m e t a l M a n d a ligand

L, c a n b e r e p r e s e n t e d in a g e n e r a l n o t a t i o n by e q u a t i o n s (1) and (2) :

L

M L n- 1

+ L-

MLn

kd

T h e corresponding ( t h e r m o d y n a m i c ) s t a b i l i t y c o n s t a n t K and t h e (thermodynamic) cumulative stability constant

as :

a r e , a c c o r d i n g t o Sillen a n d Martell (1964, 19711, defined

6

They a r e i n t e r r e l a t e d by : 8 = K 1 . K 2 . K3 . . .

(5)

Kn

T h e f o r m a t i o n r a t e is also i m p o r t a n t for understanding c o m p l e x a t i o n m e c h a n i s m s in t h e a q u a t i c environment. In t h e e q u a t i o n ( s e e also (eq.1)) :

t h e r e a c t i o n r a t e c o n s t a n t kf f o r t h e second o r d e r r e a c t i o n (eq.1) is r e l a t e d to t h e r e a c t a n t half l i f e t i by : 1

w h e r e (MLn-l)i

r e p r e s e n t s t h e initial c o n c e n t r a t i o n of

a p p r o x i m a t e l y 1:l

r e a c t a n t a n d assuming a n

c o n c e n t r a t i o n of t h e r e a c t a n t s . T h e n o t a t i o n for t h e so-called

conditional s t a b i l i t y c o n s t a n t and conditional r a t e c o n s t a n t is K' a n d kf' respectively. Analyses in e n v i r o n m e n t a l samples, involve r e a c t i o n s w i t h s e v e r a l ligands. T h e stability- a n d r a t e c o n s t a n t s t h u s o b t a i n e d d o not r e f l e c t t h e i n t e r a c t i o n with a single ligand b u t r a t h e r w i t h t h e c o m p l e x multi-ligand system. DIFFERENT A P P R O A C H E S D e t e r m i n a t i o n o f d i f f e r e n t oxidation state A n u m b e r of e l e m e n t s c a n b e p r e s e n t in t h e m a r i n e e n v i r o n m e n t in m o r e t h a n o n e oxidation state, e.g. t h e m e t a l s Fe(lI)/(llI), Mn(II)/(IV), Co~Il)/(llI), Cr(lIl)/(Vl), TI(I)/(III), Sn(II)/(IV) a n d t h e m e t a l l o i d s As(III)/(V),

Se(IV)/(VI) and Sb(llI)/(V).

Under a e r o b i c

conditions at pH 8 m o s t m e t a l s b e will thermodynamically s t a b l e in t h e oxidized form. Under a n o x i c c o n d i t i o n s ( s o m e b o t t o m w a t e r s and sediments) t h e lower oxidation state may b e c o m e i m p o r t a n t . K i n e t i c e f f e c t s o r t h e a c t i v i t y of o r g a n i s m s m a y r e s u l t in a deviation f r o m t h e t h e r m o d y n a m i c a l l y e x p e c t e d situation. In t h e e u p h o t i c z o n e t h e f o r m a t i o n of As(II1) is r e l a t e d to p r i m a r y production, m a y b e as a r e s u l t of detoxifying processes (Andreae, 1979). T h e o x i d a t i o n of Mn(I1) a p p e a r s to b e kinetically hindered ; while Se(1V) and Se(V1) a r e , as only e l e m e n t , p r e s e n t in s i m i l a r c o n c e n t r a t i o n s in o c e a n i c w a t e r s (Bruland, 1983). In t h e case of c h r o m i u m i t i s u n c e r t a i n , w h e t h e r t h e oxidation state (111) o r (VI) is predominant i n s e a w a t e r . T h i s is mainly d u e to t h e observed v e r y slow oxidation r a t e of Cr(II1) (Elderfield, 1970 ; N a k a y a m a et a1.1981 ; Gould, 1982 ; van d e r Weyden a n d R e i t h , 1982). I n t e r e s t i n g i s t h e possibility of t h e e x i s t e n c e of Cu(1) in m a r i n e s u r f a c e waters. R e d u c t i o n of Cu(I1) would b e initiated by photochemically produced H 2 0 2 . T h e back oxidation is so slow t h a t Cu(1) s p e c i e s h a v e a s u f f i c i e n t l y long l i f e t i m e to f o r m complexes,

7

predominantly chlorides; therefore, in fresh- and low salinity waters the Cu (I) content will be low (Zika, 1982 ; Moffet and Zika, 1983). Considering the micro-environment close to the surface of plankton cells the existence of Fe(I1) complexes has been observed, even in an aerobic environment (Morel, this volume). The macro-environment represents evidently not necessarily t h e same conditions as the micro-environment close to t h e organisms.

Metal - inorganic interactions The concentrations of the major inorganic ions in seawater a r e well known ; in estuarine and coastal areas as well as in interstitial waters anomalies in their constant ratios may occur. The major cations a r e Na+, Mg2+, Ca2+, K+ and Sr2+, t h e major anions C1-, SO42-, HCO3-, B(OH)4-, F- and Br-. Ion pairs involving these elements and H+, OH-, Under anoxic conditions the S2- - ion and bi- and poly-

C032-, P043- and SiO44-.

sulphides become important. A summary of t h e major ion speciation in seawater is given by Kester et al. (1975). For species distribution calculations stability constants of metal-ligand complexes a r e required (e.g. Stiff, 1971 ; Bilinski et al., 1976 ; Wedborg, 1979 ; S h o e s Goncalves et al., 1981). The accuracy of many d a t a is insufficient and moreover not all d a t a of the potentially important equilibria a r e known, resulting in large discrepancies in species distribution suggested by various authors (Tables 2 and 3).

TABLE 2 Calculated copper species distribution in seawater by different authors (in %).

Reference

Species cu2+ CuOH+ Cu(OH)2

cuco3

Cu(C03)22CuHCO cuc1+3 cuc12 CuOHCl cusoq CUL

(I)

(2)

(3)

(4)

(5)

(6)

(7)

1.0 1.0 90.0 7.7 0.1 0.2 0.1

0.7 3.7

17 22

21.6

49

0.6 0.3 0.3 3.5

3 5 2 87

3 6 32 50 6

10

3.5 95.5

0.7 0.8 85.0 5.6 0.1 0.2 0.1

0.2 -

-

5.8 1.6 65.2

-

0.08

-

-

2 -

-

0.1

1

I 1

0.1

-

l

2.3 7.7 40.7 46.1

-

1 0.3

-

(8)

I

-

2.5

-

< I 0.4 o -

(9) (10) (11) (12)

I

-

-

9.2 5.3 I1 71.4 - 10.1

-

-

9 0.6 3 75 -

-

-

--

5 4

-

79 8

-

-

1.2 3.1 0.4 56

-

1.1 0.3 0.2 40

1, Zirino and Yamarnoto (1972) ; 2, Dyrssen and Wedborg (1974) ; 3, Ahrland (1975) ; 4, Sibley and Morgan (1975) ; 5, Paulsen (1978) ; 6, Mantoura et al. (1978) ; 7, Dyrssen and Wedborg (1979) ; 8, Sylva and Florence (1980) ; 9, Whitfield and Turner (1980) ; 10, van den Berg (1983 b) ; 11, Morel and Morel-Laurens (1983) ; 12, Zuehlke and Kester (1983).

8 There seems t o b e a yenera1 a g r e e m e n t on t h e major cadmium, zinc, lead and c ~ - ; , e r species in seawater. Species with an abundancy > l o % arc ronsidered CdCI+, ZnC1+

21,’

Zn2+, P b C 0 3 and CuCO3 (Table 3).

TABLE 3 Predominant species (> 10 %) of cadmium, zinc, lead and copper in seawater.

Species Cadmium

CdC1+

CdC12

+ + +

+ + +

+

+

+

+

ZnC1+

ZnC12

+

+ Zinc

Zn

2+

+ +

+ +

+

-

+ PbC1+

-

Copper

+

+

-

ZnOHC 1

-

+

&

PbC03C1

-

PbC03

+ + + +

CuC03

-

+

+

-

10 16

+ CuOHCL

-

8 2 3 9

-

CuCl+

+

Zn (OH)

-

+ -

ZnCO,,

-

-

+

-

-

+

I’bC1,

+

cu2+

-

+

-

-

CdC13-

+ +

+

-

+ +

Lead

-

-

-

-

+

+

-

-

-

+ + +

+ + + +

CuL

-

+

-3 3

4 11

12 13

-

14 9 15

+

17

5

6

I, Baric & Branica (1967) ; 2, Zirino & Yamamoto (1972) ; 3, Dyrssen & Wedborg (1974) ; 4, Ahrland (1975) ; 5, Dyrssen & Wedborg (1980) ; 6 , Morel & Morel-Laurens (1983) ; 8, Stumm & Bilinski (1972) ; 9, Whitfield & Turner (1980) ; 10, Sipos et al. (1980) ; 11, Sibley & Morgan (1975) ; 12, Paulsen (1978) ; 13, Mantoura et al. (1978a) ; 14, Sylva & Florence (1980) ; 15, van den Berg (1983b) ; 16, Niirnberg (1983) ; 17, Zuehlke & Kester (1983). Few t r a c e e l e m e n t species c a n b e analysed in natural samples directly. Ion selective electrodes (ISE) allow measurements of m e t a l ion activity (Cu2+, Cd2+) ; however, their use in t h e marine environment is limited due to low sensitivity and i n t e r f e r e n c e by C1-.

9

Metal-organic i n t e r a c t i o n s T h e m i x t u r e of o r g a n i c c o n s t i t u e n t s in t h e m a r i n e e n v i r o n m e n t is e x t r e m e l y complex. Their origin i s p a r t l y terrigenuous ; many compounds a r e produced in t h e m a r i n e environment itself. T h e d i f f e r e n t s o u r c e s and t h e i r r e l a t i v e i m p o r t a n c e for t h e complexation of t r a c e m e t a l s in e s t u a r i e s , c o a s t a l seas a n d o p e n o c e a n a r e : riverine input, runoff f r o m t h e c o a s t a l zone, resuspension, i m p o r t of w a t e r mases, a t m o s p h e r i c input a n d in situ biological production. Although many groups of compounds c a n b e analysed in s e a w a t e r q u a l i t a t i v e l y and quantitatively, e.g. f a t t y acids, sugars, a m i n o acids, sterols, v i t a m i n s (Degens and I t t e k o t , 1983 ; Laane, 1982), t h e i r sum c o v e r s only roughly 10-20% of t h e t o t a l DOC. Most compounds a r e t h u s unidentified.

T h e y include e x c r e t i o n - and d e c a y products of

organisms, a n d decomposition p r o d u c t s of o r g a n i c m a t t e r in various stages of decay, subject t o t r a n s f o r m a t i o n s a n d recombinations. T h e composition a n d distribution of t h e dissolved o r g a n i c m a t t e r v a r i e s t h e r e f o r e widely. Many of t h e s e o r g a n i c compounds c o n t a i n o n e o r m o r e functional groups ; t h e s e c a n f o r m c o m p l e x e s w i t h t r a c e metals. This m a y a f f e c t t h e speciation of t h e s e t r a c e e l e m e n t s significantly. P r e d i c t i o n s based o n s t a b i l i t y c o n s t a n t s f o r m e t a l ions with s o m e t y p e s of functional organic groups s u g g e s t t h a t , in s e a w a t e r , only f e w t r a c e m e t a l s f o r m c o m p l e x e s with t h e s e o r g a n i c compounds to a considerable e x t e n t . T h e s e e l e m e n t s a r e copper, iron and, to a lesser e x t e n t , zinc, l e a d a n d probably nickel. Copper, being t h e most likely to f o r m o r g a n i c c o m p l e x e s h a s b e e n i n v e s t i g a t e d extensively. T h e r e is no a g r e e m e n t on which f r a c t i o n of t o t a l c o p p e r is organically-bound, i t r a n g e s b e t w e e n 0 and nearly 100% (Mantoura, 1982 ; v a n den Berg, 1984) ; Mills and Quinn (1984) found a d e c r e a s e (70-14%) with increasing salinity f o r t h e N a r a g a n s e t t Bay estuary. I t is i n t e r e s t i n g t o see t h a t f e w model s t u d i e s (Table 2) h a v e considered copper-organic i n t e r a c t i o n s (Dyrssen a n d Wedborg, 1974 ; Mantoura et al., 1978 a; Zuehlke and Kester, 1983),

although

their

appreciable concentrations

in s e a w a t e r s e e m s to b e well

documented. Various a t t e m p t s h a v e been m a d e t o i m p r o v e t h e knowledge on t r a c e m e t a l

-

o r g a n i c interactions. D i r e c t d e t e r m i n a t i o n of species. With a combination of t e c h n i q u e s (e.g. G C and AAS) s e v e r a l d i s t i n c t o r g a n i c s p e c i e s c i n b e isolated and d e t e r m i n e d , e.g.

organo-lead,

-

m e r c u r y a n d -tin compounds (Chau a n d Wong, 1983). Also t h e distinction of s e v e r a l organic a n d inorganic s p e c i e s of A s (Braman et al., 1977 ; Andreae, 1977) a n d S b (Andreae, 1983) h a v e been reported. F u n c t i o n a l groups. T h e s t u d y of well defined ligand s i t e s (RCOO-,RzNH, RO-, RS-, -NH2) is based o n t h e p r e f e r e n c e of t h e r e s p e c t i v e t r a c e m e t a l s f o r t h e d i f f e r e n t donor

a t o m s , as s u m m a r i z e d by S t u m m and Brauner (1975). S t u d i e s of r e l a t i v e simple organics g i v e insight i n t o t h e m u c h m o r e c o m p l e x organics l i k e h u m i c s u b s t a n c e s and proteins (Gamble a n d S c h n i t z e r , 1973).

10

Well-defined organic compounds. Some of t h e relatively f e w organic niolecules in seawater with a known s t r u c t u r e have been examined for t h e i r binding ability with t r a c e metals, e.g. amino acids (Simoes Goncalves et al., 1982, 1983 ; Valenta et al., 1984) and phtalic-, citric- and salycilic acid (Stumm and Brauner, 1975).

Experimentally defined groups of organic matter. These types of organic m a t t e r c a n b e divided into bio- and geo-polymers. Bio-polymers (produced by living organisms) a r e e.g. polysaccharides, polypeptides and proteins, enzymes, pigments and vitamins (Reuter and Perdue, 1977). Geo-polymers originate from decay and decomposition processes. They c a n b e divided into operationally defined subgroups :

- humic acid, only soluble in alkaline solutions; - fulvic acid, soluble in acidic and alkaline solutions; - humin, insoluble in acidic and alkaline solutions. These groups of organics with molecular weights ranging 500 to

>

100,000, o f t e n contain

many complexing sites, predominantly carboxylic-, phenolic- and a m i n e t y p e functional groups. Another experimentally defined group of organic compounds able to complex t r a c e metals, is yellow substance (Gelbstoffe), organic m a t t e r showing fluorescent behaviour when e x c i t a t e d with blue light (Shapiro, 1964 ; Ghassemi and Christman, 1968). A problem f o r both humic- and yellow substances is t h a t for these groups of experimentally defined components of different sources, e a c h analysis will b e ambiguous in t e r m s of relative composition and molecular weight distribution. Additionally i t appears t h a t almost every scientist working in this field has developed his own extraction procedure (Weber and Wilson, 1975 ; Mantoura and Riley, 1975 a; Schnitzer, 1976 ; Stuermer and Harvey, 1977). Different extraction t i m e s and -procedures result in different compositions of t h e organic constituents (Laane and Kramer, 1984). Soil humicand fulvic acids, o f t e n used for studies on t h e interaction with t r a c e elements, and those derived from w a t e r have certainly not t h e s a m e composition and contain not t h e s a m e distribution of functional groups. Therefore, results should b e compared with c a r e (Buffle, 1980 ; Buffle et al., 1984). Almost all t r a c e m e t a l s of i n t e r e s t have been investigated with respect t o their complexation with humic- and fulvic acids and to their stability c o n s t a n t s (e.g. Ernst et al., 1975. Mantoura et al., 1978 b; Bresnahan et al., 1978 ; Takamatsu and Yoshida, 1978 ; Raspor et al., 1984). Results w e r e summarized and compared by Mantoura (1982) and Buffle et al. (1984).

Model compounds. Extensive studies have been carried o u t concerning t h e binding of m e t a l s (Zn, Cd, P b and Cu) with model ligands like EDTA, NTA or DTPA (e.g. Raspor and Branica, 1975 ; Raspor et al., 1977, 1978 ; Stolzberg, 1977 ; Plavsic et al., 1980), and with polyelectrolytes (PAA, PMA) as model compounds f o r humic substances (van Leeuwen e t al., 1981 ; Cleven, 1984).

Natural samples.

T h e species distribution of a t r a c e e l e m e n t in t h e natural

environment is a f f e c t e d by t h e t o t a l assemblage of all inorganic and organic ligands. An

11

approach h a s been developed w h e r e t h e n e t binding effect of a l l t h e s e ligands is studied by t h e d e t e r m i n a t i o n of t h e a p p a r e n t c o m p l e x a t i o n c a p a c i t y e.g. f o r c o p p e r ( C C c u ) . This is a measure f o r t h e c o n c e n t r a t i o n of o r g a n i c ligands (L) a b l e to bind ionic m e t a l i n t o nonlabile complexes. T h e a p p a r e n t c o m p l e x a t i o n c a p a c i t y c a n b e d e t e r m i n e d by

titration

with a n ionic t r a c e e l e m e n t . Conditional s t a b i l i t y c o n s t a n t s (K') c a n b e c a l c u l a t e d f r o m t h e t i t r a t i o n curve. However i t should b e r e a l i z e d t h a t m o r e t h a n o n e ligand is involved. Several a n a l y t i c a l p r o c e d u r e s h a v e been developed f o r t h e d e t e r m i n a t i o n of t h e CCcu a n d

K' (see later). Speciation schemes S e v e r a l a u t h o r s h a v e designed a s p e c i a t i o n s c h e m e , i.e.

a s e q u e n c e of a n a l y t i c a l

techniques to distinguish t h e various c h e m i c a l f o r m s in solution (0.45 o r 0.2 p m filtration). T h e s a m p l e is divided i n t o s e v e r a l fractions, a c c o r d i n g to size/density (ultra-filtration, (ultra)-centrifugation,

dialysis),

c h a r g e (electrophoresis), e l e c t r o c h e m i c a l

behaviour

(distinction b e t w e e n labile and non labile f o r m s (ASV)) o r c h r o m a t o g r a p h i c c h a r a c t e r i s t i c s (Chelex resin, XAD-2, Sephadex). T h e distinction m a y include UV-oxidation. T h e s c h e m e s result in 2-9 fractions. E x a m p l e s of d i f f e r e n t s p e c i a t i o n s c h e m e s a r e given in T a b l e 4. T h e significance of t h e individual f r a c t i o n s is not e a s y to understand as t h e y a r e usually e x p e r i m e n t a l l y defined. E a c h s c h e m e r e s u l t s in i t s own s e p a r a t i o n products. T h i s should b e t a k e n i n t o a c c o u n t when c o m p a i r i n g r e s u l t s f o r t h e n a t u r a l e n v i r o n m e n t obtained with d i f f e r e n t s p e c i a t i o n schemes.

TABLE 4 Use of d i f f e r e n t s e p a r a t i o n t e c h n i q u e s applied to t r a c e m e t a l speciation s c h e m e s (n = n u m b e r of fractions).

REFERENCES

I Benes & S t e i n n e s (1974) Guy & C h a k r a b a r t i (1976) Smith (1976) B a t l e y & F l o r e n c e (1976) H a r t & D a v i e s (1977) O d a & O k a b e (1977) Sugai & H e a l y (1978) Sugimura et a1.(1978) Harrison & L a x e n (1980) Hasle & Abdullah (1981) Niirnberg (1984)

* *

*

*

* * * *

2

*

*

*

* *

3

*

* *

TECHNIQUES 4 5 6

*

n 7

*

*

* * * *

9

1

0

*

* *

8

* *

* * *

5 8 5 7

*

*

3 5 4 4 9 8 3

1, 0.45 o r 0.2 tm f i l t r a t i o n ; 2, ultra-filtration ; 3, dialysis ; 4, (ultra)-centrifugation ; 5 , e l e c t r o c h e m i c a l distinction labile - non labile (ASV) ; 6, e l e c t r o p h o r e s i s ;7, UV-oxidation ; 8, c h e l e x resin ; 9, XAD-2 ; 10, sephadex.

12

Biological interactions Many s t u d i e s h a v e been c o n c e r n e d with u p t a k e by a n d e f f e c t s of t r a c e e l e m e n t s o n organisms, s o m e t i m e s in r e l a t i o n to d i f f e r e n t c h e m i c a l s p e c i e s of t h e e l e m e n t c o n c e r n e d (e.g. Langston and Bryan, 1984). As a n e x a m p l e , t h e e x t e n t to which c o p p e r is c o m p l e x e s h a s b e e n found to c o n t r o l i t s t o x i c i t y to organisms (Sunda a n d Cuillard, 1976 ; Anderson and Morel, 1978 ; J a c k s o n and Morgan, 1978). Of c o n c e r n f o r t h i s p a p e r is t h e e f f e c t of organisms on t r a c e m e t a l speciation. Most s t u d i e s h a v e b e e n c a r r i e d o u t in b a t c h cultures, flow-through s y s t e m s o r l a r g e p l a s t i c bags, usually containing o n e o r a f e w diatom-, algal- o r phytoplankton species.

In situ

d a t a a r e scarce. T h e i m p a c t of living m a r i n e organisms o n t h e c o m p l e x a t i o n of t r a c e m e t a l s in solution, c a n b e explained in d i f f e r e n t t e r m s : adsorption o r c o m p l e x a t i o n to t h e c e l l wall a n d c o m p l e x a t i o n with e x c r e t i o n p r o d u c t s and/or i n t r a c e l l u l a r fluids. D e c a y products h a v e been discussed before. T h e adsorption of c o p p e r o n t o phytoplankton a n d m a c r o p h y t e s w a s r e p o r t e d by Hunt and F i t z g e r a l d (1983). E x c r e t i o n p r o d u c t s of organisms m a y b e rich in complexing a g e n t s , and

the

production of

e x u d a t e s might p a r t l y b e induced by

high

trace

metal

concentrations. J o h n s t o n (1964) and Khailov (1964) a l r e a d y mentioned t h e i r possible c h e l a t i n g e f f e c t s . Which w e r e e x p e r i m e n t a l l y c o n f i r m e d in c u l t u r e s for phytoplankton a n d a l g a e (Cnassia-Barelli et al., 1978 ; Swallow et al., 1978 ; R a g a n et al., 1979 ; M c Knight and Morel, 1979 ; Sueur et al., 1982 ; Collier and Edmond, 1984 ; S e r i t t i et al., 1985). D i a t o m s w e r e a l s o found to produce c h e l a t i n g organics (Fischer a n d Fabris, 1982 ; I m b e r and Robinson, 1983 ; Imber et al., 1984). S u r f a c e a c t i v e compounds c a n b e c h e l a t o r s as well (Wallace, 1982) and h a v e a s t r o n g r e l a t i o n with t h e a c t i v i t y of plankton blooms, b o t h in b a t c h c u l t u r e and in t h e field ( Z u t i c et al., 1981).

Interactions with particles and colloids R e a c t i o n s of dissolved s p e c i e s with p a r t i c u l a t e and colloidal suspended m a t t e r include adsorption/desorption, complexation, ion-exchange, precipitation/dissolution, coprecipit a t i o n during coagulation and flocculation (Morgan, 1966 ; S t u m m and Morgan, 1981 ; Parks, 1975). T h e s e processes a r e p a r t i c u l a r l y i m p o r t a n t at t h e land-sea boundary in e s t u a r i e s (Duinker, 1980 ; M a r t i n et al., t h i s volume). T h e i n t e r a c t i o n with p a r t i c l e s

>

0.45

y m is n o t discussed here.

Colloids Colloids include p a r t i c l e s w i t h hydrophobic, hydrophilic and i n t e r m e d i a t e f o r m s with a s i z e r a n g e 1 - 400 nm. Both o r g a n i c (including macromolecules) a n d inorganic (hydrolyzed silica and m e t a l oxides) colloids o c c u r in t h e m a r i n e e n v i r o n m e n t (Sigleo a n d Helz, 1981). T h e i r s u r f a c e s o f t e n c o n t a i n s u i t a b l e s i t e s f o r i n t e r a c t i o n s with t r a c e m e t a l s (adsorption, complexation). In t h e m a r i n e e n v i r o n m e n t a l l p a r t i c l e s h a v e a n e g a t i v e s u r f a c e c h a r g e (Neihof and Loeb, 1972 ; H u n t e r a n d Liss, 1982). I n c r e a s e of t h e e l e c t r o l y t e c o n c e n t r a t i o n d e c r e a s e s t h e s t a b i l i t y of t h e colloidal particles. A s a r e s u l t t h e

13

instable colloids will agglomerate t o larger particles and eventually to large flocs (Stumm and Morgan, 1981 ; Mayer, 1982). As t h e s e processes already t a k e place at slightly increased e l e c t r o l y t e concentration, t h e lower salinity range of an estuary is t h e most important a r e a for coagulation processes including organics (Sholkovitz, 1976, 1978). The coagulates and flocs formed may act as scavengers for t r a c e elements. Aggregation of fulvic acid s e e m s t o b e enhanced by copper ions, as was found by Underdown et al. (1981) and Gamble et al. (1984) using laser light Rayleigh scattering. Relatively l i t t l e is known about natural colloids. Their particle size distribution and the changes therein under variable natural conditions (estuaries) is poorly documented. One of t h e problems is their very small size and t h e i r susceptibility to changes during sample handling : filtration, storage, etc. (Sigg et al., 1984). Concentration techniques (centrifugation and/or ultrafiltration) a r e applied for collection of colloids to study their chemical composition, s i z e distribution and their chemical reactions with metals (Lammers, 1967 ; Boonlayangoor et al., 1979 ; Kramer and Duinker, 1984a). The fulvic- and humic acids in rivers and estuaries a r e a n important p a r t of t h e colloidal fraction, as was demonstrated with small angle X-ray s c a t t e r i n g (Thurman et al., 19821, i t is uncertain whether high molecular weight organics (e.g. humics) a r e in t r u e solution or in colloidal forms (Stumm and Morgan, 1981). Iron has also been found t o be predominantly present in colloidal particles in estuaries, e.g. in t h e Beaulieu estuary (Moore et al., 1979), t h e P a t u x e n t e s t u a r y (Sigleo et al., 19821, t h e Y a r r a estuary (Hart and Davies, 1981) and several rivers in S W England (Mill, 1980). Important complexation and/or sorption reactions between colloids and t r a c e elements may occur. In studies on t h e interactions of copper with EDTA and Y A l 2 0 3 as modelligand and -particles, t h e apparent complexation c a p a c i t y was increased by t h e addition of t h e particulates (Plavsic et al., 1980). This was also observed for t h e influence of MnO2colloids on t h e complexation c a p a c i t y f o r lead (Sigg et al., 1984). In natural s y s t e m s t h e r e f o r e p a r t of t h e complexation capacity might b e caused by colloidal material. This w a s demonstrated in experiments on t h e complexation capacity of samples from t h e Scheldt estuary at different salinities, determined as function of several concentration steps, using a hollow fiber ultrafiltration set up with a theoretical c u t off of M W 5000 (Kramer and Duinker, 1984a). T h e CCcu appeared t o b e linearly dependent on t h e concentration f a c t o r only in t h e upper p a r t of t h e estuary. N o increase in t h e C C c u could b e observed a f t e r concentration

at higher salinities. T h e riverine colloidal m a t e r i a l apparently coagulates t o particles and flocs at increasing salinity in t h e upper p a r t of t h e estuary. These flocs a r e retained by t h e 0.45 pm filter, thus no longer contributing to t h e complexation c a p a c i t y of t h e "dissolved" fraction. This explains t h e non conservative behaviour of t h e CCcu in this p a r t of t h e estuary (see later). Samples taken from t h e north Atlantic Ocean did not show an increase in C C c u , even a f t e r a concentration f a c t o r 200 times.

14

Reaction kinetics Recently, Pankov and Morgan (1981a,b) emphasized t h e importance of various mechanisms for regulating kinetics in t h e a q u a t i c environment. Examples showed t h e wide range of first- and second order r a t e constants (kf) and half lifes ( t i ) f o r different reactions t h a t might t a k e place in natural waters. T h e r a t e constants f o r several first order t r a c e m e t a l hydrolysis reactions, second order redox- and complexation reactions of i n t e r e s t for a q u a t i c studies a r e summarized by Hoffmann (1981). His comparison of kinetic d a t a on t h e oxidation of HS- under only slightly different conditions shows considerable variations ; e.g., t + ranges from 7 -600 min for s e a w a t e r media. The r a t e of a given reaction in t h e marine environment depends on t e m p e r a t u r e and pH,

concentration

of

the

r e a c t a n t s and

competing compounds

and

effects

of

(auto)catalysis. This may have severe e f f e c t s in regions where large gradients in these p a r a m e t e r s occur (estuaries, c o a s t a l regions), e.g. t h e change of t + f o r t h e oxidation reaction of Fe2+ with pH; found was 10-1 s at pH 6 and lo3 s at pH 8 (Stumm and Lee, 1961 ; Roekens and van Grieken, 1983). Catalysis will influence (usually improve) t h e speed of t h e overal reaction. Also surfaces a r e known as catalysts. Evidence has been found t h a t autocatalysis of Fe(I1) c a n t a k e place under c e r t a i n circomstances by Fe(II1) precipitates (Sung and Morgan, 1980). T h e kinetics of t h e oxidation of Cr(II1) and Cu(1) have been discussed before. Cr(V1) is reduced by dissolved organic m a t t e r , t h e slow re-oxidation resulting in a large enough t + for an existence of Cr(II1). Also t h e existence of Cu(1) in seawater is a steady s t a t e between t h e reduction- and back-oxidation reactions. T h e lifetime is dependent on pH, P 0 2 , complexing ligands and redox i n t e r m e d i a t e s such as H 2 0 2 (Moffet and Zika, 1983). Taking advantage of their high sensitivity and selectivity f o r easily reducible t r a c e m e t a l species, several electrochemical techniques a r e used to obtain kinetic d a t a of complexation reactions. Application of dc-ASV at a slowly dropping mercury e l e c t r o d e (SDME) t h e e f f e c t of different model solutions on t h e t i m e dependence of t h e chelation of Cd(I1) with EDTA was studied (Rasport et al., 1977, 1980a). T h e reaction r a t e c o n s t a n t kf ranged f r o m unmeasurably f a s t in 0.59 M NaCl to 4.2 x 102 M-1. s-1 in 0.01 M CaC12. The value in natural s e a w a t e r was (3.6

t

0.5) x 102 M-1. s-1, t h e t + for t h e second order

reaction was found to depend on t h e initial concentration of t h e reactants, in a c c o r d a n c e with theory (see eq. 7). Depending on t h e p a r a m e t e r s used in ASV analysis t h e values of kf which c a n b e determined a r e 102

-

106 M-1. s-1, if one asumes an initial concentration

range of 10-6 -10-8 M (Valenta, 1983). T h e application of a rotating ring-disc e l e c t r o d e in ASV resulted in a kd f o r t h e CdEDTA complex of 12.4 s-1 ; t h e kf determination was less reliable however (Shuman and Michael, 1975). With a rotating disc e l e c t r o d e applied at different rotation speeds, t h e s a m e authors (Shuman and Michael, 1978) determined a first order dissociation r a t e constant f o r copper c h e l a t e s in coastal Atlantic w a t e r s (in t h e order of 2 s-1). Based on a kinetic criterion t h e chelation of copper in t h e s e samples was estimated.

15

T i t r a t i o n c u r v e s of n a t u r a l s a m p l e s for c o m p l e x a t i o n c a p a c i t y d e t e r m i n a t i o n s c a n b e influenced by t h e k i n e t i c s of t h e c o m p l e x a t i o n reaction(s). If t h e t i m e allowed f o r equilibration a f t e r spiking of t h e s a m p l e i s s h o r t with r e s p e c t to t h e t i m e required f o r establishment of c h e m i c a l equilibrium, a deviation f r o m t h e t w o i n t e r s e c t i n g lines will occur (Duinker and K r a m e r , 1977). In a R u z i c plot (Ruzic, 1982) MF vs. MF/(MT-MF), w h e r e MT a n d M F a r e t o t a l a n d f r e e m e t a l respectively, a t y p i c a l c h a n g e of slope bulb appears under t h e s e c i r c o m s t a n c e s , giving information on t h e r a t e c o n s t a n t (Ruzic and Nicolic,1982). T h i s m e t h o d w a s applied to N o r t h A t l a n t i c s u r f a c e w a t e r samples, f o r t h e complexation of ionic c o p p e r by n a t u r a l ligands. T h e s e r e s u l t s i n d i c a t e slightly d i f f e r e n t conditional r a t e c o n s t a n t s f o r d i f f e r e n t a r e a s , kf' ranging f r o m 3.6 x 104 in t h e north to 1.4 x 104 M-1 s-1 in m o r e s o u t h e r n a r e a s , T a b l e 7 ( K r a m e r , 1986).

.

In m a n y e x p e r i m e n t s c h e m i c a l equilibrium

is assumed, however d y n a m i c non-

equilibrium processes in s e a w a t e r m a y r e s u l t in products d i f f e r e n t f r o m t h o s e e x p e c t e d under equilibrium conditions. Equilibria c a n b e c o m p l e x and m a y involve s e v e r a l t y p e s of r e a c t i o n s simultaneously. Kinetics c a n t h e r e f o r e a f f e c t speciation of t r a c e e l e m e n t s and should b e t a k e n i n t o account.

Chemical modelling In c h e m i c a l modelling, physicochemical principles a r e applied f o r t h e i n t e r p r e t a t i o n of n a t u r a l hydrogeochemical s y s t e m s (Jenne, 1979). In m o s t cases t h e p r e s e n c e of c h e m i c a l equilibrium of t h e s y s t e m i s assumed. T h i s is, of course, n o t a l w a y s t h e case in n a t u r a l s y s t e m s b u t t h i s a p p r o a c h i s m u c h less c o m p l i c a t e d t h a n a n approach f o r a d y n a m i c system. C e r t a i n d y n a m i c s i t u a t i o n s c a n b e t r e a t e d however by c h e m i c a l equilibrium theory, especially in cases w h e r e v e r y slow o r v e r y fast r e a c t i o n s o c c u r (Pankov and Morgan, 198 1b). T h e equilibrium models a t t e m p t , usually by i t e r a t i v e calculations, to find t h e m o s t s t a b l e s i t u a t i o n f o r a given set of pressure, t e m p e r a t u r e and composition of a s y s t e m or, in t h e r m o d y n a m i c t e r m s , to minimize t h e v a l u e f o r t h e Gibb's f r e e energy. C a l c u l a t i o n s c a n b e c a r r i e d o u t by a ) minimizing t h e f r e e e n e r g y f u n c t i o n ( f r e e energy minimizing method) o r b) solving a set of non-linear e q u a t i o n s consisting of equilibrium c o n s t a n t s a n d m a s s b a l a n c e c o n s t r a i n t s (equilibration c o n s t a n t method). T h e l a t t e r m e t h o d is p r e f e r r e d , as t h e values of t h e equilibration c o n s t a n t d a t a a r e m o r e reliable t h a n t h e f r e e e n e r g y d a t a (Nordstrom a n d Ball, 1984). T h e models t r y to p r e d i c t a s p e c i e s distribution of a s y s t e m given a set of c o m p o n e n t s t o g e t h e r w i t h t e m p e r a t u r e and pressure. T h e r e s u l t will b e a f i r s t approximation of t h e r e a l s y s t e m , which might yield useful i n f o r m a t i o n f o r t h e n a t u r a l situation. T h e validity of t h e results, o r how c l o s e i s t h e r e a l (natural) s y s t e m a p p r o x i m a t e d by t h e m o d e l predictions is, a p a r t f r o m t h e quality of t h e t h e o r e t i c a l model ( i n t e r a c t i o n s involved, r e f i n e m e n t of calculation), largely dependent o n t h e a c c u r a c y a n d reliability of t h e d a t a used, a n d t h e c h o i c e of physicial and c h e m i c a l c o m p o n e n t s involved. T h e c a u s e s of d i s c r e p a n c i e s b e t w e e n model and r e a l s y s t e m s a r e s u m m a r i z e d by S t u m m a n d Morgan (1981).

16

S o m e r e s u l t s of application of t h e o r e t i c a l models to t r a c e m e t a l speciation w e r e p r e s e n t e d in t h e s e c t i o n o n metal-inorganic i n t e r a c t i o n s ( t a b l e s 2 a n d 3) ; a c o l l e c t i o n of papers dealing with c h e m i c a l modelling in aqueous systems, including speciation, sorption, solubility a n d k i n e t i c s w a s e d i t e d by J e n n e (1979). R e c e n t l y N o r d s t r o m and Ball (1984) s u m m a r i z e d 58 aqueous c h e m i c a l equilibrium c o m p u t e r p r o g r a m s of which 19 w e r e dealing with t r a c e metals.

STUDIES ON NATURAL SAMPLES Even b e f o r e a n e x p e r i m e n t o r analysis is c a r r i e d out, m a n y f a c t o r s m a y a f f e c t t h e speciation of t r a c e e l e m e n t s in t h e s a m p l e under investigation.

Sampling, storage and experimental conditions Sampling. A t t h e t r a c e levels of most c o n s t i t u e n t s in s e a w a t e r i t is d i f f i c u l t n o t t o c o n t a m i n a t e t h e s a m p l e during t h e sampling procedure, n o t only f o r t h e t r a c e element(s) in question, b u t a l s o f o r possible ligands. W a t e r s a m p l e r s and hydrographic w i r e s have been i n t e r c o m p a r e d (Batley a n d C a r d n e r , 1977 ; Bruland et al., 1979 ; B e w e r s a n d Windom, 1982; Spencer et al., 1982 ; B e w e r s and Windom, 1983), s u r f a c e sampling h a s to b e c a r r i e d o u t a w a y f r o m t h e ship, u p s t r e a m of a slowly moving s m a l l r a f t (Mart, 1979b).

Filtration. T o s e p a r a t e t h e dissolved f r o m t h e p a r t i c u l a t e f r a c t i o n f i l t r a t i o n o r c e n t r i f u g a t i o n will b e necessary. T h i s c a u s e s a s e v e r e risk of c o n t a m i n a t i o n ; it i s t h e r e f o r e o f t e n not c a r r i e d o u t with open o c e a n samples, w h e r e t h e c o n c e n t r a t i o n s of suspended m a t e r i a l a r e low. T h e p r e s e n c e of phytoplankton o r a v a r i a b l e c o n c e n t r a t i o n of suspended m a t t e r a f f e c t s t h e t o t a l c o n c e n t r a t i o n and a comparison of s a m p l e s c a n t h u s b e c o m e difficult. In s p e c i a t i o n Studies t h e p r e s e n c e of p a r t i c l e s m a y influence t h e r e s u l t s e v e n m o r e (complexation, adsorption), t h e r e f o r e f i l t r a t i o n o v e r a c i d washed m e m b r a n e o r s c r e e n f i l t e r s in a n a p p r o p r i a t e f i l t r a t i o n a p p a r a t u s is r e c o m m e n d e d f o r a l l n a t u r a l s a m p l e s (Bewers et al., 1985). High pressure during f i l t r a t i o n should b e avoided : r u p t u r e d (plankton-) c e l l s will c o n t r i b u t e o r g a n i c m a t t e r , n u t r i e n t s and t r a c e m e t a l s to t h e solution. A pressure

< 25 k P a is r e c o m m e n d e d ( F l o r e n c e a n d Batley,

1980).

Clogging of f i l t e r s when high loads of suspended m a t t e r a r e p r e s e n t (rivers, e s t u a r i e s , c o a s t a l waters), will t r a p m u c h smaller p a r t i c l e s (including colloids) t h a n corresponds with t h e initial f i l t e r p o r e size, causing a s e v e r e influence on t h e speciation of t r a c e e l e m e n t s (Kennedy et al., 1974 ; Danielson, 1982). F i l t r a t i o n of a l i m i t e d (known) a m o u n t of w a t e r o r t o t e r m i n a t e f i l t r a t i o n o n c e a required over-pressure h a s built up a b o v e t h e f i l t e r (Laxen and Chandler, 1982) might i m p r o v e t h e f i l t r a t i o n procedure. U s e of cross-flow o r tangential-flow f i l t r a t i o n i n s t e a d of t h e normally used dead-end f i l t r a t i o n might r e s u l t in a b e t t e r defined dissolved f r a c t i o n , w i t h t h e additional a d v a n t a g e of i n c r e a s e d f i l t r a t i o n speed. However as f a r as w e known, no ( t r a c e m e t a l ) c o n t a m i n a t i o n f r e e s y s t e m f o r limited volumes (+ I l i t e r ) is available yet.

17 Centrifugation. P a r t i c l e s e p a r a t i o n by c e n t r i f u g a t i o n i s based o n p a r t i c l e density a n d consequently, c e n t r i f u g a t e d s a m p l e s d i f f e r f r o m t h e e x p e r i m e n t a l definition of t h e dissolved f r a c t i o n , as based on filtration. C e n t r i f u g a t i o n m a y r e s u l t in a r e l a t i v e e n r i c h m e n t of l e s s d e n s e p a r t i c l e s in t h e dissolved f r a c t i o n , especially when a flowthrough c e n t r i f u g e is used. T h e r e i s no g a r a n t e e t h a t a l l p a r t i c l e s a r e r e m o v e d f r o m t h e dissolved fraction. S m a l l b u t significant d i f f e r e n c e s w e r e found b e t w e e n f i l t r a t i o n and centrifugation t e c h n i q u e s (Duinker et al.,

1979). C e n t r i f u g a t i o n h a s been applied to

s e p a r a t e s a m p l e s i n t o d i f f e r e n t f r a c t i o n s (Benes and Steinnes, 1974 ; Salbu, 1981). In t h e field of biochemistry (ultra-)centrifugation has b e c o m e a powerful tool f o r separations, which might b e applied in t r a c e m e t a l speciation studies.

Storage.

Samples for

total

t r a c e metal content a r e frequently stored after

acidification t o pH 1. T h i s m e t h o d is of c o u r s e n o t possible in s p e c i a t i o n analyses, w h e r e direct analysis i s t h e o p t i m a l choice. Unfortunatly, t h i s i s n o t a l w a y s possible. Small d i f f e r e n c e s d u e to prolonged s t o r a g e h a v e b e e n r e p o r t e d ( B a t l e y a n d Gardner, 1977). F o r storage, b e s t r e s u l t s c a n b e e x p e c t e d using polythene o r Teflon bottles, a c i d c l e a n e d a n d equilibrated with ( s e a ) w a t e r prior to use, to e l i m i n a t e adsorption to t h e wall (Mart, 1979a). S a m p l e s should b e s t o r e d in t h e d a r k a t a b o u t 4OC ; d e e p f r e e z i n g will result in changing of equilibria (Batley a n d C a r d n e r , 1977 ; Krom and Sholkovitz, 1978). E x p e r i m e n t a l conditions. Conditions during analysis should r e f l e c t n a t u r a l s y s t e m conditions as c l o s e as possible, o r t h e y should b e c o n t r o l l e d in such a w a y t h a t t h e r e s u l t s c a n b e i n t e r p r e t e d properly. D u e to t h e r e l a t i v e l y l a r g e H+ c o n c e n t r a t i o n to t r a c e e l e m e n t s

-

- with

respect

a c h a n g e in pH c a n h a v e serious influence o n t r a c e e l e m e n t s p e c i e s

distribution (Zirino and Y a m a m o t o , 1972). T w o major m e t h o d s a r e used f o r pH control. T h e f i r s t involves t h e application of a pH-stat, using C 0 2 as convenient r e a g e n t ( K r a m e r and Manshanden, 1982). In t h e second method, a b u f f e r is added to t h e sample. T o avoid serious c h a n g e s in t h e distribution of t h e s p e c i e s ( m o s t b u f f e r s c o n t a i n complexing constituents) t h e u s e of s p e c i a l b u f f e r s is recommended. J a r d i m and Allen (1984) used PIPES (Piperazine-N-N'-bis(2-ethanesulfonic acid)) at pH 6.6, which h a s a low stability c o n s t a n t with copper. B e c a u s e (almost) no c h e m i c a l s a r e a d d e d and t h e m e a s u r e m e n t i s c a r r i e d o u t at n a t u r a l pH, t h e f i r s t m e t h o d s e e m s to b e preferable. C o m p l i c a t e d biological s y s t e m s (bioassays) at t r a c e e l e m e n t c o n c e n t r a t i o n levels typical f o r o f f s h o r e w a t e r s , a r e s u b j e c t to serious d a n g e r of contamination. Without e x t r e m e p r e c a u t i o n s e.g. C a r p e n t e r a n d Lively (1980) and F i t z w a t e r et al. (1982) found t h e t o x i c e f f e c t (inhibition of p r i m a r y production) of c o n t a m i n a t i o n by t h e incubation bottles. E f f e c t s of adsorption to walls and p a r t i c u l a t e m a t t e r (sediment) should n o t b e underestimated. U s e of c l e a n l a b t e c h n i q u e s and regular c h e c k of t h e t r a c e e l e m e n t c o n c e n t r a t i o n s throughout t h e (biological) e x p e r i m e n t s is n e c e s s a r y t o g e t a n indication of t h e a c t u a l c o n c e n t r a t i o n and possible distribution of t h e d i f f e r e n t e l e m e n t s . Depending on t h e t y p e of e x p e r i m e n t s i t could b e possible t h a t o t h e r p a r a m e t e s should b e known o r e v e n controlled : pE, P02, ionic s t r e n g t h , t e m p e r a t u r e , D O C etc.

18

Complexation c a p a c i t y and conditional stability c o n s t a n t s in m a r i n e waters. Techniques. Methods which a r e useful for identification of individual species w e r e classified by Stumm and Brauner (1975). Several techniques have been developed f o r studies of t h e apparent complexation c a p a c i t y (CCcu). T h e technique most often applied is differential pulse anodic stripping voltammetry (DPASV) at various e l e c t r o d e types (Duinker and Kramer, 1977 ; Srna et al., 1980 ; Plavsic et al., 1982 ; S e r i t t i et al., 1983 ; Kramer and Duinker, 1984a ; Kramer, 1985, 1986). Ligand competition with c a t e c h o l followed by cathodic stripping voltammetry (CSV) detection of Cu-catechol complex ions makes use of another electrochemical technique (van den Berg, 1984). Other techniques include t h e solubilisation of copper sulphide (Kerr and Quinn, 19801, Mn02-adsorption (van den Berg and Kramer, 1979), ligand exchange with EDTA (Hirose et al.,

19821, ion

exchange (Stolzberg and Rosin, 1977 ; Crosser and Allen, 1977 ; Buffle et al., 1977 ; M c Crady and Chapman, 1979 ; Buffle et al., 1980 ; Wood et al.,

1983) and t h e use of

biological response (bioassay) studies (Cillespie and Vaccaro, 1978 ; Brand et al., 1983 ; Sunda and Ferguson, 1983). Ion selective electrodes (ISE) have not been used in seawater, due to their lack in sensitivity. Other techniques like dialysis (Truitt and Weber, 1981 ; del Castilho et al., 19841, gel filtration (Mantoura and Riley, 1975 b ; Evans et al., 1979) and fluorescence quenching (Ryan and Weber, 1982 ; Berger and Ewald, 1984) have been applied t o fresh w a t e r s only. The techniques for t h e determination of t h e CCcu a r e described and compared by H a r t (1981) and Neubecker and Allen (1983). A summary of t h e CCcu and K' in marine w a t e r s is given by Krarner (1986).

O t h e r techniques applied to speciation studies a r e potentiometric stripping analysis (PSA) (Jagner and Aren,

1982), pseudopolarography (Brown and Kowalski, 19791,

electrophoresis (Roh1,1982), several chromatographic techniques, XAD-I and -2 (Mackey, 1982) and SEP-PAK-Clg (Mills et al., 1982 ; Plavsic and Branica, 19851,HPLC (Mackey, 1983), size exclusion chromatography (Gardner et al.,

1982), equilibration diafiltration

(Lee, 1983) and light s c a t t e r i n g (Underdown et al., 1981).

CCcu and K' determination by DPASV analysis. To illustrate t h e possibilities of speciation studies, an example of t h e application to natural w a t e r s will b e presented in this section, with special r e f e r e n c e to t h e determination of t h e cornplexation c a p a c i t y for copper of marine waters. Voltarnmetric measurements were m a d e with a PAR 174 polarographic analyser with a PAR 315 electroanalysis controller (EC & C ) and a jet-stream mercure film e l e c t r o d e (JMFE, Magjer and Branica, 1977). T h e mercury film was applied t o t h e Classy Carbon s u r f a c e (tdep = 10 min, 8 x 10-5 M Hg, 50 Hz) and formed prior to t h e analyses. An Ag/AgCl (sat. KCI) r e f e r e n c e e l e c t r o d e (Metrohm) and a platinum wire counter e l e c t r o d e w e r e f i t t e d in a 100 ml F E P Teflon cell. Typical s e t t i n g for CCcu titrations was tcond = 90

S,

t d e p = 180 s, Econd = 0 mv, Edep = -900 mV, scan speed 10 mVs-l and a modulation

amplitude of 50 mV. All C C c u and K' results a r e based on t h e d i r e c t titration method

19

with copper (Duinker and Kramer, 1977) and calculated according t o t h e method proposed by Ruzic (1982). For t h e analytical procedure and further details see Duinker and Kramer (1977) and Kramer (1986).

Scheldt estuary. S u r f a c e w a t e r samples in t h e Scheldt estuary w e r e collected at high - 30.3 x 10-3.

water slack at 9 stations (Fig. I). Salinities covered w e r e in t h e range 0.7

The C C c u data, based on triplicate measurements a r e illustrated in Fig. 2. The plot does not reflect conservative behaviour, unlike e.g.,

dissolved fluorescing organic matter. A

rapid d e c r e a s e is observed between salinities 0.7 and 10 x 10-3. I t could b e concluded from ultra-filtration eperiments (see before), t h a t colloids and/or large organic molecules play an important role in t h e complexation of ionic copper. These a r e removed in t h e early stages of estuarine mixing.

b

THE

NETHERLANDS

Fig. 1. Sampling s t a t i o n s in t h e river Scheldt estuary.

TABLE 5 Complexation Capacity (CC), conditional stability c o n s t a n t (K'), temperature, pH, oxygen and suspended m a t t e r c o n t e n t in t h e river Scheldt e s t u a r y at different salinities.

Station

S(xlO3)

CC(nMCu2+) log K'

T(OC)

02(ppm)

pH

SM(mg1-1) mF1

............................................................................... I 2 3 4 6 7 8 9 10

0.7 3.0 5.9 9.2 14.9 18.7 21.7 25.2 30.3

299 245 22 1 I73 159 146 I52 I30 108

7.4 7.7 7.5 7.3 7.4 7.4 7.3 7.2 7.4

7.2 7.3 6.8 6.4 5.1 5.4 5.4 5.5 5.6

3.2 8.1 9.7 11.8 11.3 11.0 10.4 10.6 10.6

7.40 7.62 7.76 7.85 7.95 8.03 7.99 8.00 8.00

83 65 117 67 45 54 47 46 47

93 102 92 78 67 58

50 35 25

20

Fig. 2. Means of triplicate analysis of apparent Scheldt estuary copper cornplexation capacity in the river Scheldt estuary at various salinities. salinities.

The conditional stability constants calculated f o r t h e different salinity ranges a r e given in Table 5. I t looks as if in t h e more saline samples complexes with lower K' a r e formed. Southern North Sea. S u r f a c e watersamples w e r e collected in t h e southern p a r t of t h e

North Sea, in coastal w a t e r s and offshore in October 1982 (Fig. 3). In Table 6 t h e C C c u and log K' a r e presented. I t appears t h a t c o a s t a l w a t e r s (stations 1 and 2) show a higher complexation capacity (82 nM Cu2+) then t h e samples with less contribution of river water (stations 3 and 4) with a CCcu of 44 nM Cu2+, with an i n t e r m e d i a t e value for t h e station in t h e German Bight.

Fig. 3. Sampling stations in t h e Southern North Sea.

21

Fig. 4. S c h e m a t i c r e p r e s e n t a t i o n of s u r f a c e c u r r e n t s in t h e n o r t h A t l a n t i c O c e a n and s t a t i o n s w h e r e s u r f a c e w a t e r s w e r e collected. A r e a s I to V i n d i c a t e regions with c o m m o n t e m p e r a t u r e , salinity a n d n u t r i e n t c h a r a c t e r i s t i c s . T h e conditional s t a b i l i t y c o n s t a n t s (K') show s m a l l d i f f e r e n c e s f o r t h e s e samples. C o a s t a l samples, at lower salinities, t e n d to h a v e lower K' values (log K' = 7.6) t h a n offshore w a t e r s (log K' = 7.81, with again a n i n t e r m e d i a t e v a l u e f o r s t a t i o n 5.

TABLE 6 Complexation c a p a c i t y f o r c o p p e r (CC), conditional s t a b i l i t y c o n s t a n t (K'), salinity and pH of S o u t h e r n N o r t h S e a samples.

North Atlantic Ocean. S u r f a c e s a m p l e s in n o r t h A t l a n t i c w a t e r s w e r e c o l l e c t e d at 20 s t a t i o n s in July-August 1983. T h e a r e a i s influenced mainly by t h e G u l f s t r e a m , t h e N o r t h A t l a n t i c D r i f t and t h e m u c h c o l d e r E a s t Greenland C u r r e n t . T h e s u r f a c e s a m p l e s c a n b e grouped i n t o f i v e s a r e a s with c o m m o n c h a r a c t e r i s t i c s of t e m p e r a t u r e , salinity, phosphate and s i l i c a t e c o n c e n t r a t i o n s , Fig. 4 (Kramer, 1986).

22

The C C c u of t h e samples ranged 20-66 nM Cu2+, t h e log K' 7.73

- 8.41. A

resemblance

can b e seen between t h e different a r e a s (Figs. 5a and 5b). In Table 7 t h e mean values of t h e CCcu, log K' and kf for the different a r e a s a r e given. The CCcu's of a r e a IV a r e lower than found for t h e a r e a s I and 11, while an increase can b e s e e n in t h e Southern Bight of t h e North Sea, a r e a V. These values compare well with results given before. Each a r e a is c h a r a c t e r i z e d by a practically c o n s t a n t K', but appreciable differences exist between t h e regions. More s t a b l e complexes of copper with natural ligands a r e formed in t h e western (111) and northern (I) sections, t h e stability constants of a r e a IV a r e t h e lowest.

Fig. 5. A) Apparent copper complexation c a p a c i t y ( C C c u ) in nM Cu2+ ; B) Conditional stability constants (K')presented as log K'.

TABLE 7 Mean values of complexation capacity (CC), conditional stability c o n s t a n t s (K') and r a t e constants (kf') for different a r e a in north A t l a n t i c waters.

T h e differences between a r e a s c a n possibly b e explained, with several assumptions, in t e r m s of plankton activity patterns. Based on t h e hypothetical model of Fig. 7, t h e most important source of organic ligands in t h e s e open ocean w a t e r s is t h e in situ production, originating from excretion products and/or intracellular fluids. T h e amount of ligands

23

produced d e p e n d s t h e n largely on t h e biological s p e c i e s present, population d e n s i t y and t h e i r activity.

.....

.... . ...... . . ..... .

30

0

Fig. 6. Simplified r e p r e s e n t a t i o n giving t h e a b b u n d a n c e and distribution of t h e 6 major diatom s p e c i e s (A) and 3 major phytoplankton s p e c i e s (B). T h e Continuous Plankton R e c o r d e r P r o j e c t (Edinburgh, t h e O c e a n o g r a p h i c L a b o r a t o r y , 1973) resulted in a P l a n k t o n Atlas, c o v e r i n g p a r t of t h e N o r t h A t l a n t i c and t h e N o r t h Sea. Since no d a t a a r e a v a i l a b l e for t h e y e a r of our sampling w e h a v e to a s s u m e t h a t no d r a s t i c changes o c c u r f r o m y e a r to y e a r in t h e distribution of t h e organisms, and u s e t h e m e a n d a t a o v e r 10 y e a r s of t h e Plankton Atlas. S o m e observations with regard to t h e C C c u and K' c a n b e made. In Fig. 6a and b t h e c a l c u l a t e d a b u n d a n c e of 6 resp. 3 m o s t a b u n d a n t d i a t o m a n d phytoplankton s p e c i e s a r e p r e s e n t e d s c h e m a t i c a l l y . B o t h d i a t o m s and phytoplankton s p e c i e s a r e m o s t a b u n d a n t in t h e a r e a b e t w e e n t h e U K a n d Iceland ; an a r e a with very low c o n c e n t r a t i o n s of organisms e x i s t in t h e w e s t e r n and s o u t h e r n p a r t of our sampling a r e a s (111 a n d IV). I t s e e m s f r o m t h e s e f i g u r e s t h a t a positive r e l a t i o n e x i s t s b e t w e e n plankton abundance and t h e c o m p l e x a t i o n c a p a c i t y . Both e x u d a t e s and i n t r a c e l l u l a r fluids c o n t a i n p o t e n t i a l ligands, t h e r e f o r e no i n f o r m a t i o n on t h e i r r e l a t i v e c o n t r i b u t i o n c a n b e obtained f r o m t h e C C c u . D i f f e r e n c e s in K' however might b e explained in t e r m s of seasonal v a r i a t i o n s of plankton a c t i v i t y ( K r a m e r , 1985).

CCc-

across the land-sea boundary. In a t r a n s e c t covering t h e riverine e n v i r o n m e n t

and t h e o p e n o c e a n including a r e a s with v e r y d i f f e r e n t e n v i r o n m e n t a l conditions, c h a n g e s in physical and c h e m i c a l p a r a m e t e r s and c h a n g e s in t h e r e l a t i v e s o u r c e s of complexing a g e n t s o c c u r , as p r e s e n t e d s c h e m a t i c a l l y in Fig. 7. In t h e r i v e r i n e and e s t u a r i n e e n v i r o n m e n t t h e t e r r i g e n u o u s influence is dominant (both for t r a c e e l e m e n t s and organic m a t t e r ) ; t h e resuspension of s e d i m e n t s h a s a n additional i n f l u e n c e in shallow c o a s t a l

24

waters. In situ biological production and a l s o a t m o s p h e r i c input b e c o m e progressively, relatively m o r e i m p o r t a n t towards t h e open ocean.

in situ biologtcal production atmospheric inpul import 01 water masses resuspension runoff from coastal zone

riverine inputs

complexing capacity lor copper nM

Fig. 7. S c h e m a t i c m o d e l of t h e distribution of t h e a p p a r e n t c o m p l e x a t i o n c a p a c i t y in river- and estuarine- (a), c o a s t a l - (b) and open o c e a n w a t e r s (c). T h e right p a r t shows t h e situation L > M (based on Wood et al., 1983), t h e l e f t p a r t t h e hypothetical s i t u a t i o n M > L for d e e p o c e a n i c - (d) and e s t u a r i n e w a t e r s (e). Bar graph shows t h e rnajor s o u r c e s f o r each system. This is r e f l e c t e d in t h e complexation capacity. Usually a high o r g a n i c m a t t e r c o n t e n t of river and e s t u a r i n e w a t e r s will, t o g e t h e r with t h e colloids in t h e "dissolved" f r a c t i o n , result in a high C C c u (100 - 500 nM Cu2+), Fig. 7a. C o a s t a l w a t e r s (Fig. 7b), as a result of mixing with s e a w a t e r , h a v e a lower C C c u (60 - 150 nM Cu2+). Open o c e a n s u r f a c e w a t e r s of t h e North A t l a n t i c h a v e a C C c u of 20

-

70 nM Cu2+, which in case of low in situ

biological a c t i v i t y might b e well below t h i s v a l u e (Fig. 7c). T h e h y p o t h e t i c a l model of Fig. 7 includes t w o s c e n a r i o s : t h e s i t u a t i o n found so f a r in the aquatic environment where L situation c a n o c c u r w h e r e M

> L.

>

M, resulting in a positive C C c u . T h e o r e t i c a l l y a

O n e c a n a s s u m e a s a t u r a t i o n of t h e p o t e n t i a l ligand s i t e s

and/or, a f t e r many y e a r s of transformations, polycondensations and polymerisations of t h e organic m a t t e r , as o n e c a n e x p e c t in old d e e p o c e a n i c w a t e r s , a r e l a t i v e d e c r e a s e in a c t i v e sites, with (almost) n o input of new organic compounds. T h i s c a n t h u s r e s u l t in a "negative" CCcu, o r a s i t u a t i o n w h e r e t h e t r a c e m e t a l is p a r t l y p r e s e n t in labile uncornplexed f o r m , with n o m o r e donor a t o m s being a v a i l a b l e f o r f u r t h e r c o m p l e x a t i o n (Fig. 7d). T h e l i m i t of this "negative"

C C c u is obviously t h e t o t a l t r a c e m e t a l

25

concentration ; in Fig. 7d, t h e m a x i m u m of t h e c u r v e being 5 nM Cu a c t u a l l y found in deep o c e a n i c w a t e r s (Bruland, 1983). Alrnost n o C C c u d a t a a r e available on d e e p waters. One d e e p cast in t h e N E A t l a n t i c shows a d e c r e a s e in t h e C C c u below t h e e u p h o t i c zone towards 22 nM Cu2+ at 4820 m (Kramer, 1986). Theoretically in r i v e r s and e s t u a r i e s under s e v e r e t r a c e m e t a l pollution s t r e s s t h e s a m e situation (M > L) might occur. In t h i s case t h e l i m i t to t h e possible "negative" C C c u is d e t e r m i n e d by t h e a c t u a l d e g r e e of c o n t a m i n a t i o n (Fig. 7 el. T h e r e s u l t s f o r t h e C C c u , r e p o r t e d in t h i s paper, c o m p a r e well with t h e l i t e r a t u r e d a t a s u m m a r i z e d by K r a m e r (1986) ; most values found f o r o f f s h o r e w a t e r s d i f f e r not more t h a n o n e o r d e r of magnitude, regardless t h e t e c h n i q u e used. T h e conditional s t a b i l i t y c o n s t a n t s found f o r t h e d i f f e r e n t m a r i n e a r e a s a r e not very different. A t r e n d c a n b e observed of K' increasing t o w a r d s t h e o p e n ocean. T h u s relative stronger c o m p l e x e s a r e f o r m e d in t h e open o c e a n t h a n in e s t u a r i n e and coastal waters. Comparable v a l u e s h a v e b e e n observed by o t h e r authors, using t h e s a m e technique. Application of d i f f e r e n t t e c h n i q u e s however, c a n r e s u l t in s e v e r a l o r d e r s of magnitude higher K' ( K r a m e r , 1986). T h i s c a n n o t only b e a t t r i b u t e d to geographical differences. Another f a c t o r is, t h a t d i f f e r e n t t e c h n i q u e s h a v e d i f f e r e n t sensitivities f o r t h e various species of t h e e l e m e n t concerned.

CONCLUSION During t h e l a s t d e c a d e many e x p e r i m e n t a l t e c h n i q u e s and t h e o r e t i c a l models for studies of s p e c i f i c c h e m i c a l e n t i t i e s of t r a c e e l e m e n t s h a v e been developed o r improved. Their

application

to

t h e natural

e n v i r o n m e n t h a s a l r e a d y resulted

in improved

understanding of biogeochemical processes in t h e m a r i n e environment, and i t is likely t h a t further d e v e l o p m e n t in t h e n e a r f u t u r e will b e significant.

ACKNOWLEDGMENTS T h e a u t h o r is g r a t e f u l to Dr. J.C. Duinker f o r t h e c r i t i c a l review of t h e manuscript.

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and Brick, J.C., 1977. inorganic a r s e n i c and

Brand, L.E., Sunda, W.G. a n d Guillard, R.R.L., 1983. L i m i t a t i o n of m a r i n e phytoplankton r e p r o d u c t i v e r a t e s by zinc, m a n g a n e s e a n d iron. Limnol. Oceanogr., 28: 1182-1 198. Bresnahan, W.T., G r a n t , C.L. a n d Weber, J.H., 1978. S t a b i l i t y c o n s t a n t s f o r t h e c o m p l e x a t i o n of c o p p e r (11) ions w i t h w a t e r a n d soil f u l v i c a c i d s m e a s u r e d by a n ion s e l e c t i v e e l e c t r o d e . Anal. Chern., 50: 1675-1679. Brown, S.D. a n d Kowalski, B.R., 1979. Pseudopolarographic d e t e r m i n a t i o n of m e t a l c o m p l e x s t a b i l i t y c o n s t a n t s in d i l u t e solution by rapid s c a n ASV. Anal. Chem., 51: 21 33-2 139. Bruland, K.W., 1983. T r a c e e l e m e n t s in s e a w a t e r . In: J.P. Riley and R. C h e s t e r (eds), C h e m i c a l O c e a n o g r a p h y , A c a d e m i c Press, London, Vo1.8, pp. 157-220. Bruland, K.W.,

Franks, R.P.,

Knauer, G.A. a n d Martin, J.H.,

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RIVER INPUTS INTO OCEANS

W. MICHAELIS, V. ITTEKKOT a n d E.T. DEGENS

Geologisch-Palaontologisches Institut, Bundesstrasse 5 5 , D-2000 Hamburg 13 (Federal Republic of G e r m a n y )

ABSTRACT Michaelis, W., I t t e k k o t , V. and Degens, E.T., 1986. R i v e r inputs into oceans. In: P. L a s s e r r e a n d J.M. Martin (eds), Biogeochemical P r o c e s s e s at t h e Land-Sea Boundary. Elsevier, A m s t e r d a m . D a t a c o l l e c t e d during a n ongoing S C O P E / U N E P study on c a r b o n transport in major world r i v e r s a r e presented. T h e a n n u a l global t r a n s p o r t r a t e s a r e 0.42 x lo9 t C / a for POC and 0.11 x l o 9 t C/a f o r DOC. S t r o n g seasonal f l u c t u a t i o n s w e r e observed in POC and D O C t r a n s p o r t , indicating t h e s i g n i f i c a n c e of continuous monthly sampling programs. INTRODUCTION T h e r i v e r s play a major r o l e in t h e t r a n s f e r t of c a r b o n a n d mineral n u t r i e n t s from land to t h e sea a n d influence significantly t h e biogeochemical processes o p e r a t i n g in coastal

waters. Q u a n t i f i c a t i o n of t h e m a t e r i a l t r a n s p o r t , b o t h in t h e dissolved and particulate forms, h a s b e e n a t t e m p t e d by s e v e r a l a u t h o r s in t h e p a s t (Clarke, 1924 ; Holeman, 1968 ; G a r r e l s & McKenzie, 1971 ; M a r t i n et al., 1980 ; Meybeck, 1982 ; Milliman & Meade, 1983). Depending o n t h e t y p e of sampling techniques and m e t h o d s of calculations employed t h e r e a r e d i f f e r e n c e s in t h e r e p o r t e d fluxes. A major problem in such c a l c u l a t i o n s is t h e p a u c i t y of r e l i a b l e d a t a f r o m s o m e of t h e major r i v e r s of t h e world especially of Asia ( s e e e.g.

Milliman & Meade, 1983). Additionally t h e difficulty of

obtaining r e p r e s e n t a t i v e s a m p l e s f r o m t h e r i v e r s will adversely affect flux calculations.

Most of t h e i n f e r e n c e s d r a w n o n t h e n a t u r e and t r a n s p o r t of riverine m a t e r i a l s r e s t on - at d i f f e r e n t points in t i m e and space. Seasonal variations in t h e

d a t a c o l l e c t e d randomly

t r a n s p o r t of m a t e r i a l s a r e v e r y c o m m o n in s o m e of t h e major world rivers, and in some

cases m o r e t h a n 60 % of t h e m a t e r i a l t r a n s p o r t o c c u r s within a v e r y s h o r t period of time. F u r t h e r m o r e , a v a i l a b l e d a t a a r e n o t a l w a y s c o m p a r a b l e s i n c e t h e a n a l y t i c a l techniques used d i f f e r f r o m r i v e r to river.

To o v e r c o m e s o m e of t h e s e d i f f i c u l t i e s w e s t a r t e d a p r o g r a m to monitor t h e inorganic a n d o r g a n i c c o n s t i t u e n t s in m a j o r world rivers. T h e s t u d y "Carbon T r a n s p o r t in Major World Rivers" is sponsored by t h e S c i e n t i f i c C o m m i t t e e f o r P r o b l e m s of t h e Environment (SCOPE) a n d t h e United N a t i o n s E n v i r o n m e n t P r o g r a m (UNEP). Within i t s f r a m e w o r k over

50 % of t h e t o t a l w a t e r discharged to t h e sea i s being monitored at l e a s t o n c e a month.

38

The major rivers investigated a r e given in Fig. 1. In this article we report on t h e nature and content of organic carbon carried by the major world rivers based on d a t a collected during t h e first two years of the study. Some d a t a on studies on the european estuaries a r e also presented. For sampling locations and basic d a t a see t h e chapters on individual rivers in Degens (19821, Degens et al. (1983, 1985).

Fig. 1. Rivers being studied during the SCOPE/UNEP Program.

POOLS OF ORGANIC CARBON The major pools of organic carbon on land and in t h e sea a r e depicted in Fig. 2a and b. Soil organic carbon is by far t h e most substantial pool followed by dissolved organic

Land Biomass

t

Ovanic Biomass 0.003 a 10'9-C

Fig. 2a. The major pools of organic carbon on land and in the sea.

39

Fig. 2b. Distribution of o r g a n i c pools in t h e ocean.

Fig. 3. Distribution of volume of water above different depth i n t e r v a l s in t h e sea a n d t h e inputs of o r g a n i c c a r b o n to t h e s u r f a c e w a t e r s a b o v e t h e s a m e depths.

0.21

2

3

1

Depth Interval

5

6 > 6 Ckml

40

carbon in t h e sea a n d by land biomass. In c o n t r a s t , t h e o c e a n i c biomass is minimal. T u n i n g to t h e o c e a n i c c a r b o n pool, t h e major role is played by t h e dissolved organic carbon of t h e d e e p sea. Although this is a major sink quantitatively, i t is r a t h e r inertbiologically and a p p e a r s t o have originated mainly f r o m inputs f r o m rivers (Degens and Ittekkot, 1984). P a r t i c u l a t e organic carbon, plankton and b a c t e r i a c o n t r i b u t e t o t h e r e s t of t h e o c e a n i c organic c a r b o n pool. Riverine contribution of p a r t i c u l a t e organic carbon t o t h e open sea is minimal, b e c a u s e i t is mainly t r a p p e d in e s t u a r i e s and d e l t a s (Deuser, 1979). F i g u r e 3 p r e s e n t s d a t a on t h e distribution of volume of w a t e r a b o v e d i f f e r e n t d e p t h intervals, a n d t h e inputs of o r g a n i c c a r b o n to s u r f a c e w a t e r s a b o v e t h e s a m e depths. High inputs of organic m a t t e r to a s m a l l volume of w a t e r over t h e shelves a r e registered. This is due t o inputs f r o m rivers, not only of p a r t i c u l a t e carbon, but also of mineral n u t r i e n t s which i n c r e a s e t h e primary productivity and o r g a n i c input in t h e s e regions. Previous e s t i m a t e s of t h e input of t o t a l organic c a r b o n (TOC) t o t h e o c e a n a r e in t h e range 100- 1000 x 1012 g C / a (Duce a n d Duursma, 1977 ; Richey et al.,

1980). T h e

e s t i m a t e s of Richey et al. (1980) a r e based on m e a s u r e m e n t s m a d e in t h e Amazon river. A m o r e r e c e n t e s t i m a t e given by Schlesinger and Melack (1981) gives t h e global T O C transport to b e around 400 x 1012 g C/a. None of t h e rivers used in t h e s e calculations w a s studied during a l l s t a g e s of t h e river flow comprising a hydrological year. A prerequisite f o r flux calculations is t h e availability of reliable d a t a on w a t e r and s e d i m e n t discharge.

\-

Fig. 4. Annual s e d i m e n t discharge f r o m various drainage basins of t h e world ( a f t e r Milliman & Meade, 1983).

41

Although a d e q u a t e information is available f r o m r i v e r s of t h e t e m p e r a t e regions, t h e y a r e

very s c a r c e for t h o s e of t r o p i c a l rivers. D a t a reviewed by Milliman a n d Meade (1983) show that about 70 % of t h e t o t a l s e d i m e n t d i s c h a r g e to t h e o c e a n s o c c u r via t h e S o u t h e a s t Asian rivers (Fig. 4). Inclusion of s e d i m e n t d a t a a n d t h e o r g a n i c m a t t e r a s s o c i a t e d with these sediments might c h a n g e d r a s t i c a l l y t h e n e t P O C t r a n s p o r t f r o m land to sea.

ORGANIC CARBON TRANSPORT D a t a collected o v e r t h e f i r s t t w o y e a r s of our study on DOC and P O C w e r e used to calculate annual fluxes of c a r b o n f r o m land to sea. T h e d a t a a r e p r e s e n t e d not f o r t h e individual rivers, b u t f o r r i v e r s of a p a r t i c u l a r c o n t i n e n t (Fig. 5). T h e m e a s u r e m e n t m a d e in t h e major rivers of a c o n t i n e n t w e r e e x t r a p o l a t e d to include t h e t o t a l w a t e r and sediment discharge f r o m t h a t p a r t i c u l a r continent. In t e r m s of q u a n t i t y maximum P O C is being transported by t h e r i v e r s of Asia, followed by t h o s e draining N o r t h America, Oceania, Africa and Europe, in t h a t order. South A m e r i c a n r i v e r s c a r r y maximum a m o u n t s of DOC, followed by t h e rivers of Asia, N o r t h America, t h e A r c t i c , Africa, Europe and Oceania.

I 3o01

200

100

Fig. 5. Annual inputs of o r g a n i c c a r b o n f r o m t h e c o n t i n e n t s via t h e r i v e r s draining them. NA-North America, SA-South A m e r i c a , AS-Asia, AF-Africa, AR-Arctic USSR, OCOceania, EU-Europe.

42

In t e r m s of P O C yield, Asia a n d O c e a n i a t o p t h e list (Fig. 6 ) . Although r i v e r s draining t h e s e t w o regions c a r r y P O C in c o n c e n t r a t i o n s similar to o r less t h a n t h o s e m e a s u r e d in t h e r i v e r s of A r c t i c and Europe, b e c a u s e of t h e i r high s e d i m e n t yield t h e POC yield is also t h e highest. F o r DOC, maximum yield is f o r S o u t h A m e r i c a a n d t h e A r c t i c , followed by Asia, N o r t h A m e r i c a , Africa, Europe a n d Oceania. High c o n c e n t r a t i o n s of D O C in r i v e r s draining t h e A r c t i c (above 10 mg/l) c o n t r i b u t e to t h i s observed high yield. T h e inclusion of t h e r i v e r s of Asia a n d O c e a n i a in t h e flux c a l c u l a t i o n s r e s u l t s in an i n c r e a s e in t h e q u a n t i t y of P O C c a r r i e d by t h e r i v e r s i n t o t h e o c e a n c o m p a r e d to DOC.The e s t i m a t e s global t r a n s p o r t r a t e s a r e 0.42 x lO9t/a f o r POC a n d 0.1 I x 1 0 9 t / a f o r D O C ( s e e also D e g e n s et al., 1984).

poc

CARBON YIELD

t.kni2a

Fig. 6. Annual c a r b o n yield (in t o n s p e r s q u a r e k i l o m e t e r ) f o r a r e a s d e p i c t e d i n Fig. 3.

SEASONALITY OF ORGANIC CARBON TRANSPORT T h e S C O P E / U N E P study h a s a l s o shed considerable light on seasonal f l u c t u a t i o n s in t h e t r a n s p o r t of o r g a n i c m a t t e r . Almost a l l t h e r i v e r s studied show such f l u c t u a t i o n s (see e.g. Chowdhury et al., 1982 ; Paolini et al., 1982 ; Arain h Khuhawar, 1982). T o i l l u s t r a t e t h e n a t u r e of s u c h f l u c t u a t i o n s w e p r e s e n t d a t a on t h e O r i n o c c o a n d t h e Ganges. In Orinocco t h e D O C c o n c e n t r a t i o n s vary b e t w e e n 5 m g C/1 in F e b r u a r y and 10 m g C/1 in June-July (Fig. 7). T h e lowest c o n c e n t r a t i o n s a r e observed during S e p t e m b e r . An i n c r e a s e in suspended load with increasing d i s c h a r g e is also observed. T h e P O C c o n c e n t r a t i o n s f l u c t u a t e b e t w e e n 0.5 m g C/I a n d m o r e t h a n 8 m g C/l.Similar

f l u c t u a t i o n s a r e also

observed in t h e G a n g e s (Fig. 8). T h e c o n c e n t r a t i o n s of suspended m a t t e r varied b e t w e e n a

43 Discharge

mquc a 103

50

C mp/l

10-

LO

suspended (cad

mo"

160

8-

Fig. 7. Seasonal fluctuations in organic rnatter transport in the Orinocco River.

30 120

6-

M 80

L-

(0

LO

2-

F M A M J J A S O N D J

-DOC

-SL

-Discharge

-

-Pot

Fig. 8. Seasonal fluctuations DOM transport in the Ganges River. *OI

J

. J

. F

, Y

, A

M

J

J

'

A

'

5

.

0

'

N

'

D

'

44 few milligrams during January-March a n d 1 mg/l in July. T h e P O C c o n c e n t r a t i o n s increased with increasing suspended load a n d showed variations in t h e r a n g e of a f r a c t i o n of a milligram to m o r e t h a n 10 mg/l. T h e maximum D O C c o n c e n t r a t i o n s (up t o 9 m g C/1) w e r e observed in July-August. With a view to a s c e r t a i n i n g t h e n a t u r e of t h e t r a n s p o r t e d o r g a n i c m a t t e r , d e t a i l e d organic geochemical analyses w e r e c a r r i e d out. T h e d a t a on sugars and amino acids associated with DOC in t h e a b o v e r i v e r s a r e given in Figs. 9 a n d 10. Variations similar to D O C a r e also observed here. In t h e Orinocco c o n c e n t r a t i o n s of sugars and a m i n o acids varied b e t w e e n 10 a n d 300 @I

a n d 100 a n d 970 &I

respectively (Fig. 9). Maximum

c o n c e n t r a t i o n s w e r e observed in July. T h e sugar f r a c t i o n c o m p r i s e s mostly of glucose. Similar t r e n d s could a l s o b e discerned f o r t h e Ganges, with maximum sugar and a m i n o a c i d c o n c e n t r a t i o n s at t h e rising w a t e r s t a g e (Fig. 10).

lrn1

I ' J

ORINOCCO

' A ' S ' O ' N ' D ' J ' 1981

Fig. 9. Seasonal f l u c t u a t i o n s in dissolved sugars and a m i n o a c i d s in t h e O r i n o c c o River.

45

--....

600-

o 2l .

I

In

0

a 0 LOO E

-

E

a

P

200-

.L?

0

'

'Jan'

Jul'

I

'

I

Dec

'

GANGES

\ v

'Jon'

'

'

'

'

'I

Fig. 10. Seasonal fluctuations in dissolved sugars and amino acids in the Ganges River.

ESTUARY

RIVER

. .... *... .. . . . ,. . Fig. 1 1 . Estuarine processes affecting organic matter input to the oceans.

46

T h e a b o v e d a t a i n d i c a t e t h a t t h e r e is a s t r o n g seasonality in t h e t r a n s p o r t of m a t e r i a l s by t h e world's r i v e r s (see a l s o Degens et al.,

1985). F u r t h e r m o r e ,

it underscores t h e

i m p o r t a n c e of c o l l e c t i n g s a m p l e s at l e a s t through a hydrological year, especially f o r flux calculations. During t h e ongoing S C O P E / U N E P P r o g r a m d e t e r m i n a t i o n s of c h e m i c a l p a r a m e t e r s w e r e mainly c a r r i e d o u t on s a m p l e s c o l l e c t e d f r o m t h e r i v e r s in a r e a s a b o v e t h e influence of s e a w a t e r . However, in c a l c u l a t i n g t h e t o t a l f l u x e s o r o r g a n i c m a t t e r f r o m river to sea i t is i m p e r a t i v e to t a k e i n t o a c c o u n t t h e e s t u a r i n e processes. T h e s e processes, such as flocculation, adsorption, desorption and s e d i m e n t a t i o n , a l t e r t h e n a t u r e and q u a n t i t y of o r g a n i c m a t t e r u l t i m a t e l y e n t e r i n g t h e sea (Fig. 11). Detailed investigations w e r e c a r r i e d o u t in t h e e s t u a r i e s of s o m e of t h e European r i v e r s such as t h e Elbe, Weser and t h e E m s (Fig. 12) in o r d e r to understand t h e n a t u r e of t h e s e processes. T h e major e m p h a s i s during t h e s e s t u d i e s w a s t h e behaviour of o r g a n i c m a t t e r during e s t u a r i n e mixing.

rI

.

P

1

-xi-

Fig. 12. Map d e p i c t i n g sampling l o c a t i o n s in t h e e s t u a r i e s of t h e r i v e r s Elbe, Weser and t h e E m s Rivers. T h e s a m p l e s w e r e c o l l e c t e d f r o m t h e F/S Valdivia c r u i s e during 1981 (see Degens, 1982). In F i g u r e 13 t h e distribution of DOC with increasing salinity in t h e Elbe, Weser and

t h e E m s a r e depicted. T h e plots a r e similar to t h o s e r e p o r t e d previously (e.g. M a n t o u r a a n d Woodward, 1983) and a r e i n d i c a t i v e of c o n s e r v a t i v e behaviour.

47

The salinity-dependent v a r i a t i o n s of P O C m e a s u r e d in t h e r i v e r s E l b e and E m s a l s o show d e c r e a s e with increasing salinity (Fig. 14) ( s e e also Lohse a n d Michaelis, 1983). However, it is d i f f i c u l t to d i f f e r e n t i a t e b e t w e e n t h e effect of s i m p l e dilution and sedimentation of particles. Individual c o n s t i t u e n t s of dissolved o r g a n i c m a t t e r such as s u g a r s in t h e a b o v e r i v e r s also show a p a t t e r n similar to t h a t observed f o r dissolved o r g a n i c m a t t e r (Fig. 15). Interestingly for both D O C and dissolved s u g a r s t h e r e is a l a r g e s c a t t e r in c o n c e n t r a t i o n s in t h e low salinity regions (less t h a n 10

"/"a).

This is possibly to t h e complex

biogeochemical processes o c c u r r i n g at t h e c o n t a c t of river and seawater. Such processes appear to d e t e r m i n e t h e distribution of organic m a t t e r in o t h e r e s t u a r i e s too (Eisma et al., 1982, 1983).

o f

0

0 10

1

15

I

I

I

lo

20

30

-

1

20 salinity 1%

1

25

1

30

...

Fig. 13. P l o t s of salinity vs. D O C in t h e e s t u a r i e s of t h e r i v e r s Elbe, Weser and t h e Ems.

48

E s t u a r i e s with high inputs of o r g a n i c w a s t e p r o d u c t s m a y e x h i b i t p r o c e s s e s which a r e d i f f e r e n t f r o m t h o s e described above. Decomposition of o r g a n i c m a t t e r is found to o c c u r in such estuaries. This is s e e n f r o m s t u d i e s done o n s a m p l e s c o l l e c t e d f r o m t h e EmsDollart e s t u a r y (Fig. 12). T h e s a m p l e s w e r e c o l l e c t e d at i n t e r v a l s of a p p r o x i m a t e l y

Io/oo

salinity. Dissolved organic c a r b o n a n d c a r b o h y d r a t e s d e t e r m i n e d on t h e s a m p l e s i n d i c a t e (Fig. 17) t h a t b e t w e e n salinities 0 a n d Q O / o o t h e r e is a n e t consumption of t h e s e t w o species. T h i s is c a l c u l a t e d f r o m t h e a r e a b e t w e e n t h e t h e o r e t i c a l mixing l i n e and t h e observed concentrations. T h e c o n c e n t r a t i o n s of individual s u g a r s (Fig. 17) show t h a t t h e r e is a s e l e c t i v e degradation of dissolved sugars, w i t h glucose and g a l a c t o s e being mineralized rapidly in t h e salinity region of 0 to 4"/00. Beyond t h i s region t h e i r distribution is uniform suggesting t h e i r s t a b l e n a t u r e ( L a a n e & I t t e k k o t , 1983). I t a p p e a r s t h a t t h e decomposed monomers a r e in a f o r m which c a n b e easily utilized by h e t e r o t r o p h i c organisms.

:

20]

a

1

Elbe

o lo{m

a

ma ma a

a

a]

a

Ems

15

a a a

0 0

n

8

a

a

5L 8 a

a

0

0

10

ralinlty [KO] 20

30

Fig. 14. P l o t s of salinity vs. POC in t h e r i v e r s E l b e and t h e Ems.

0,

0 w - *

*

I

t

3

L

9

.

i w E CR

*I

Fig. 15. Plots of salinity vs. dissolved sugars in the rivers Elbe, Weser and Ems.

Fig. 16. Plots of salinity vs. DOC, dissolved sugars and oxygen in the Ems-Dollart estuary. 49

50

The d a t a presented show t h a t processes controlling t h e behavior of organic m a t t e r in an. estuary is dependent, among Figure 16. Plot of salinity vs. DOC, dissolved sugars and oxygenothers, on t h e nature and quantity of organic input. This will also d e t e r m i n e t h e quantity and n a t u r e of organic m a t t e r reaching t h e sea. Rough e s t i m a t e s based on sugars and amino acis in world rivers (Ittekkot et al., 1982) suggest up to 30 % of riverine organic carbon may b e labile. Taking these d a t a t o b e average from world rivers, t h e amount of DOC entering t h e oceans is of t h e order of ca. 0.08 x lo9 t C/a. Up t o 70 % of t h e POC has a potential for degradation in t h e coastal marine environment (Degens and Itekkot, 1985). iool

,

,

I

I

,

, ,

.

,

,

I

,

,

80

20

--

i

04

0

Glu I

1

2

I

I

L

1

1

1

6 soliruty

1

8

I

I

10

I

I

,

1

1 2 1 L

[%I

Fig. 17. Plot of salinity vs. individual sugars in t h e Ems-Dollart. CONCLUSION D a t a collected from an ongoing SCOPE/UNEP program on major world rivers a r e presented. T h e total quantity of organic m a t t e r carried by t h e major world rivers a r e 0.42

x lO9t C / a f o r POC and 0.11 x lO9t C/a for DOC. Significant seasonal fluctuations in both t h e POC and DOC transport w e r e observed, indicating t h e importance of sampling through

at least one hydrological year. Studies carried out in t h e estuaries of some North European rivers show DOC to behave conservatively. However, decomposition of organic m a t t e r t a k e s place in estuaries with high input of organic matter. On a global s c a l e approximately 30 96 of t h e DOC is labile and may b e decomposed.

REFERENCES Chowdhury, M.I., Safiullah, S., Iqbal Ali, S.M., Mofizuddin, M. and Enamul, S., 1982. Carbon transport in t h e Ganges and t h e Brahmaputra : preliminary results. In: E.T. Degens (ed.), Transport of Carbon and Minerals in Major World Rivers. Pt. 1, Mitt. Geo1.-Palaont. Inst. Univ. Hamburg, SCOPE/UNEP Sonderbd., pp. 457-468. Clarke, F.W., 1924. T h e d a t a of geochemistry. U.S. Geological Survey Bulletin, 770 pp.

51

Degens, E.T. (ed.), 1982. Transport of Carbon and Minerals in Major World Rivers. Pt. 1. Mitt. Geo1.-Palaont. Inst. Univ. Hamburg, SCOPE/UNEP Sonderbd., 52: 766 pp. Degens, E.T. and Ittekkot, V., 1983. Dissolved organic carbon and overview. In: E.T. Degens et al. (eds), Transport of Carbon and Minerals in Major World Rivers. Pt. 2, Mitt. Geol-Palaont. Inst. Univ. Hamburg, SCOPE/UNEP Sonderbd., 55: 295-314.

-

Degens, E.T. and Ittekkot, V., 1985. P a r t i c u l a t e organic carbon and overview. In: Degens et al. (eds.), Transport of Carbon and Minerals in Major World Rivers. Pt. 3, Mitt. Geo1.-Palaont. Inst. Univ. Hamburg, SCOPE/UNEP Sonderbd. (in press). Degens, E.T., Kempe, S. and Herrera, R. (eds.), 1985. Transport of Carbon and Minerals in Major World Rivers, Pt. 3. Mitt. Ceo1.-Palaont. Inst. Univ. Hamburg, SCOPE/UNEP Sonderbd. (in press). Degens, E.T., Kempe, S. and Ittekkot, V., Environment, 26: 29-33.

1985. Monitoring carbon in world rivers.

Dengens, E.T., Kempe, S. and Soliman, H. (eds), 1983. Transport of Carbon and Minerals in Major World Rivers, Pt. 2. Mitt. Ceo1.-Palaont. Inst. Univ. Hamburg, SCOPE/UNEP Sonderbd., 55: 535 pp. Deuser, W.G., 1979. Marine biota, nearshore sediments, and t h e global carbon balance. Org. Geochem., 1: 243-247. Duce, R.A. and Duursma, E.K., 1977. Inputs of organic m a t t e r t o t h e ocean. Mar. Chem.,

5: 319-339. Eisma, D., Cadee, G.C. and Laane, R., 1982. Supply of suspended m a t t e r and particulate and dissolved organic carbon from t h e Rhine to t h e coastal North Sea. In: E.T. Degens (ed.), Transport of Carbon and Minerals in Major World Rivers. Pt. 1, Mitt. Geo1.Palaont. Inst. Univ. Hamburg, SCOPE/UNEP Sonderbd., 52: 483-505. Eisma, D., Boon, J., Gronewegen, R., Ittekkot, V., Kalf, J. and Mook, W.G., 1983. In: E.T. Degens et al. (eds), Transport of Carbon and Minerals in Major World Rivers. Mitt. Geo1.-Palaont. Inst. Univ. Hamburg, SCOPE/UNEP, Sonderbd., 55: 295-314. Carrels, R.M. and McKenzie, G., 1971. Evolution of sedimentary rocks. Chichester, New York, Brisbane, Wiley: 397 pp. Holeman, J.N.,

1968. Sediment yield of major rivers of t h e world. Water Resources Res.,

4: 737-747. Ittekkot, V., Spitzy, A. and Lammerz, U., 1982. Valdivia Cruise October 1981. Dissolved organic m a t t e r in t h e Elbe, Weser and Ems Rivers and t h e German Bight. In: E.T. Degens (ed.), Transport of Carbon and Minerals in Major World Rivers. Pt. I, Mitt. Geo1.-Palaont. Inst. Univ. Hamburg, SCOPE/UNEP Sonderbd., 52: 749-764. Laane, R.W.P.M. and Ittekkot, V., 1983. Behaviour of dissolved organic w a s t e in a part of t h e Ems-Dollart Estuary : t h e Dollart. In: E.T. Degens et a1 (eds), Transport of Carbon and Minerals in Major World Rivers. Pt. 2, Mitt. Geo1.-Palaont. Inst. Univ. Hamburg, SCOPE/UNEP Sonderbd., 55: 343-363. Lohse, J. and Michaelis, W., 1983. Carbohydrates in p a r t i c u l a t e m a t t e r of t h e Elbe estuary. In: E.T. Degens and S . Kempe (eds), Transport of Carbon and Minerals in Major World Rivers. Pt. 2. Mitt. Geo1.-Palaont. Inst. Univ. Hamburg, SCOPE/UNEP Sonderbd., 55: 371-383. Mantoura, R.F.C. and Woodward, E.M.S., 1983. Conservative behaviour of riverine dissolved organic carbon in t h e Severn estuary : chemical and geochemical

52

implications. Geochim. Cosmochim. Acta, 47: 1293-1309. Martin, J.M., Burton, D. and Eisma, D., 1980. River inputs t o ocean systems. Switzerland: UNEP and UNESCO. Meybeck, M., 1982. Carbon, nitrogen and phosphorus transport by major world rivers. Amer. J. Sci., 282: 401-501. Millirnan, J.D. and Meade, R.H., 1983. World-wide delivery of river sediment t o t h e ocean. J. Geol., 91: 1-21. Paolini, J., Herrera, R. and Nemeth, A., 1983. Hydrochemistry of t h e Orinocco and Caroni Rivers. In: E.T. Degens and S. Kempe (eds), Transport of Carbon and Minerals in Major World Rivers. Pt. 2, Mitt. Geo1.-Palaont. Inst. Univ. Hamburg, SCOPE/UNEP Sonderbd., 55: 223-236. Richey, J., Brock, J.T., Naiman, R.J. et al., 1980. Organic carbon: oxidation and transport in t h e Amazon River. Science, 207: 1348-1415. Schlessinger, W.H. and Melack, J.M., rivers. Tellus, 31: 172-187.

1981. Transport of organic carbon in t h e world

53

S U R F A C E PROPERTIES OF PARTICLES A T THE LAND-SEA BOUNDARY

J.M. MARTINI, J.M. MOUCHELl a n d J. JEDNACAK-BISCAN2 I n s t i t u t d e BiogCochimie Marine - UA-386 2 L a b o r a t o r y of

Electrochemistry,

- E.N.S.,

46, r u e d'Ulm, 75230 P a r i s ( F r a n c e )

Rudjer Boskovic Institute,

POB

1013, Z a b r e b

(Yugoslavia)

ABSTRACT Martin, J.M., Mouchel, J.M. and Jednacak-Biscan, J., 1986. S u r f a c e p r o p e r t i e s of p a r t i c l e s at t h e land-sea boundary. In: P. L a s s e r r e a n d J.M. Martin (eds), Biogeochemical P r o c e s s e s at t h e Land-Sea Boundary. Elsevier, A m s t e r d a m . T h e d e t e r m i n a t i o n of s u r f a c e p r o p e r t i e s of p a r t i c l e s is a n i m p o r t a n t key to understanding i n t e r a c t i o n s of t r a c e e l e m e n t s a n d o r g a n i c compounds b e t w e e n p a r t i c u l a t e and dissolved phases in e s t u a r i n e and c o a s t a l systems. S p e c i f i c s u r f a c e a r e a (SSA), c a t i o n i c e x c h a n g e c a p a c i t y (CEC) and h e a t of immersion ( A H ) h a v e been m e a s u r e d on n a t i v e a n d t r e a t e d suspended s e d i m e n t and a f t e r oxidation with 15% H 2 0 2 . SSA and A. H have also been m e a s u r e d on s a m p l e s l e a c h e d with NaOH and Na-dithionite in o r d e r to r e m o v e amorphous aluminosilicates. R e s u l t s indicated t h a t SSA is controlled by amorphous oxides and is r e l a t e d to t h e A1 c o n t e n t of s a m p l e s a n d t h e i r granulornetry, suggesting a n a l m o s t uniform c o a t i n g of t h e d e t r i t a l c o r e by amorphous oxides. O n t h e o t h e r hand, C E C and AH,which a p p e a r to b e highly c o r r e l a t e d , a r e controlled by both organic m a t t e r and amorphous oxides. Studies on e l e c t r o p h o r e t i c mobility h a v e provided additional d a t a on t h e e x c e s s of c h a r g e at t h e i n t e r f a c e b e t w e e n suspended m a t t e r and e l e c t r o l y t i c medium. P a r t i c l e s in suspension in fresh, sea and e s t u a r i e w a t e r s a p p e a r ubiquitously to exhibit a s m a l l r a n g e of n e g a t i v e s u r f a c e charge. This uniformity is a t t r i b u t e d to t h e p r e s e n c e of organic s u r f a c e c o a t i n g s on t h e particles. INTRODUCTION An e s t u a r y provides a p a r t i c u l a r l y varied c h e m i c a l e n v i r o n m e n t which i s c h a r a c t e r i z e d by s t r o n g ionic s t r e n g t h and pH g r a d i e n t s ranging f r o m river to sea e n d m e m b e r s values. In addition o n e c a n usually o b s e r v e a slight pH and dissolved oxygen minimum in t h e e s t u a r y itself which i s e n h a n c e d in polluted estuaries. With r e g a r d s to solid phase, most of t h e e s t u a r i e s a r e c h a r a c t e r i z e d by a "turbidity maximum". T h e f r e s h w a t e r flow g e n e r a t e s a n e n t r a i n m e n t f r o m t h e b o t t o m w a t e r , t h e

sea w a t e r which is e n t r a i n e d f r o m t h e lower l a y e r by t h i s flow being c o m p e n s a t e d by a residual landward flow along t h e bottom. T h e river-borne p a r t i c l e s which s e t t l e in t h i s b o t t o m l a y e r a r e t a k e n up by t h e residual landward c u r r e n t and t r a p p e d in t h e middle p a r t

of t h e estuary. T h i s p r o c e s s g r e a t l y e n h a n c e s t h e r e s i d e n c e t i m e of particles, especially in tidal estuaries.

54

Moreover, b o t h river induced up-welling and river d i s c h a r g e of n u t r i e n t s c r e a t e a f e r t i l e e n v i r o n m e n t which e n h a n c e s t h e p r i m a r y production of o r g a n i c m a t t e r in off-shore d i r e c t i o n of estuaries. I t c a n b e p r e d i c t e d f r o m t h e s e o b s e r v a t i o n s t h a t h e t e r o g e n e o u s r e a c t i o n s b e t w e e n dissolved a n d b o t h mineral p h a s e and b i o t a will b e p r e d o m i n a n t in e s t u a r i e s and c o a s t a l zones. T h e s e r e a c t i o n s will primarily affect t h o s e e l e m e n t s a n d compounds which a r e l o c a t e d at t h e p a r t i c u l a t e s u r f a c e . T h e d e t e r m i n a t i o n of s u r f a c e p r o p e r t i e s of p a r t i c l e s a p p e a r to b e a n i m p o r t a n t k e y to u n d e r s t a n d t h e i n t e r a c t i o n s of t r a c e e l e m e n t s a n d o r g a n i c c o m p o u n d s b e t w e e n p a r t i c u l a t e and dissolved phases in e s t u a r i n e and c o a s t a l systems. S p e c i f i c s u r f a c e a r e a (SSA), c a t i o n i c e x c h a n g e c a p a c i t y (CEC) a n d H e a t of immersion

(AH)h a v e b e e n m e a s u r e d o n n a t i v e u n t r e a t e d suspended s e d i m e n t and a f t e r oxidation with 15% aqueous solution of H202. In addition, SSA and H e a t of immersion h a v e been

measured o n s a m p l e s which h a v e b e e n l e a c h e d with NaOH and Na-dithionite in o r d e r to r e m o v e amorphous aluminosilicates.

MATERIALS AND METHODS T h e e s t u a r i e s chosen for t h e s t u d i e s w e r e t h e Gironde a n d L o i r e e s t u a r i e s ( F r a n c e ) both c a r a c t e r i z e d by r e l a t i v e l y long flushing a n d r e s i d e n c e t i m e s (Martin, Mouchel a n d Thomas, 1984) as well as by a well developped t u r b i d i t y maximum. T h e i r g e n e r a l c h a r a c t e r i s t i c s h a v e been described previously (Allen, 1972; Barbaroux, 1980; ElbazPoulichet et al., 1982, 1984). S a m p l e s w e r e t a k e n during 3 s u r v e y s in t h e Gironde, s e l e c t e d during d i f f e r e n t hydrological conditions and 1 survey in t h e Loire. L a r g e volume s a m p l e s h a v e been e i t h e r continuously c e n t r i f u g a t e d (11 000 o r 32 000 rpm) o r t a n g e n t i a l l y f i l t r a t e d in o r d e r to r e c o v e r t h e suspended r n a t t e r for radionuclide a n d s u r f a c e p r o p e r t i e s analysis. T h e s e p a r t i c u l a t e s a m p l e s h a v e been lyophilized and c r u s h e d in an a g a t h e mortar. A s e l e c t i v e leaching procedure, a d a p t e d f r o m H a s h i m o t o a n d J a c k s o n (1960) and

J a c k s o n and Mehra (1960) w a s used to e v a l u a t e Fe, Mn, A1 and Si a s s o c i a t e d with t h e "amorphous" aluminosilicates. I t is based o n t w o time-controlled h o t digestions by 0.5 N NaOH, followed by a reduction s t e p using t h e d i t h i o n i t e - c i t r a t e s y s t e m b u f f e r e d in N a H C 0 3 . T h i s p r o c e d u r e w a s supposed to e x t r a c t poorly c r i s t a l l i z e d o x i d e s and hydrous oxides without a n y significant d e s t r u c t i o n of t h e well cristallized p u r e m i n e r a l s (Elbaz et al., 1982). T h e o r g a n i c c o a t i n g w a s rernoved a f t e r t r e a t m e n t with 15 % H202 a t 150°C f o r

12 hours. P.O.C.

w a s d e t e r m i n e d using a L E C O c a r b o n analyser.

S p e c i f i c s u r f a c e a r e a s w e r e d e t e r m i n e d by t h e voluirietric m e t h o d of adsorption of argon at liquid N2 t e m p e r a t u r e , using t h e BET e q u a t i o n ( P a r f i t t and Sing, 1976). H e a t s of immersion w e r e m e a s u r e d using a b a t c h c a l o r i m e t e r (Pravdic, 1976). I t w a s a n isoperibolic i n s t r u m e n t w i t h a t h e r m i s t o r d e t e c t o r of a n a v e r a g e sensitivity of f 1 mJ (which corresponds to a n e s t i m a t e d _+3tJJ/cm2). A s e d i m e n t s a m p l e s e a l e d in a s m a l l glass bulb w a s brought i n t o t h e m i c r o c a l o r i m e t e r

c e l l and c a r e f u l l y e q u i l i b r a t e d to a

55

t e m p e r a t u r e of 25 5 0.2"C (kept, however s t a b l e to + 0.005DC). T h e bulb w a s t h e n crushed and t h e h e a t of immersion a u t o m a t i c a l l y recorded. In e a c h e x p e r i m e n t , t h e h e a t evolved was c a l i b r a t e d by m e a n s of an input of a m e a s u r e d a m o u n t of c u r r e n t to a small in situ heater ( J u r a c i c a n d P r a v d i c , 1981). T h e e l e c t r o p h o r e t i c mobility of n a t u r a l suspended s e d i m e n t s h a s been m e a s u r e d on t h e field a f e w h o u r s a f t e r sampling using a P e n Kern's Model 501 L a s e r Z e e M e t e r which uses

a r o t a t i n g prism design enabling s i m u l t a n e o u s m e a s u r e m e n t s of m a n y particles. Model p a r t i c l e mobility h a s been d e t e r m i n a t e d with t h e Tiselius m e t h o d (Tiselius, 1937, 1938). T h i s m e t h o d a l s o allows t h e i n t e g r a t i o n of t h e mobility of a l a r g e number of p a r t i c l e s e v e n if t h e r e f r a c t i v e index is v e r y c l o s e t o t h a t of t h e e l e c t r o l y t e medium, allowing

to

minimize

the

experimental

errors

inherent

to

the

classical

m i c r o e l e c t r o p h o r e t i c techniques. T h e e l e c t r o p h o r e t i c mobilities will n o t b e t r a n s f o r m e d into s u r f a c e c h a r g e s b e c a u s e t h e t h e o r e t i c a l relationship b e t w e e n t h e s e p a r a m e t e r s is highly d e p e n d a n t o n t h e p a r t i c l e radius of c u r v a t u r e a n d t h e e l e c t r o l y t e c o n c e n t r a t i o n in t h e vicinity of t h e p a r t i c l e ( H u n t e r and Wright, 1971). F o r both methods, t h e a n a l y t i c a l e r r o r f a l l s below 5 %, however, i t i n c r e a s e s up t o 10 ?/c f o r n a t u r a l c o m p o s i t e s a m p l e s and/or low mobilities. C.E.C.

w a s d e t e r m i n e d by using NH4+, s a t u r a t i o n m e t h o d (Busenberg a n d Clemency,

1973) a n d s p e c i f i c a m m o n i a e l e c t r o d e (Orion R e s e a r c h Inc., Cambridge). T h e s e d i m e n t s a m p l e s (ca. 150 mg) w e r e f i r s t s a t u r a t e d in I N a m m o n i u m acetate solution overnight, t h e n f i l t e r e d through 0.45 urn f i l t e r s a n d l e a c h e d with I N NH4CI. E x c e s s a m m o n i u m saIts w e r e r e m o v e d by washing with isopropyl alcohol. A final 8 0 ° C drying m a d e t h e s a m p l e s ready f o r C.E.C.

d e t e r m i n a t i o n . Available a m m o n i u m w a s displaced in 0.1 M NaOH, t h e

corresponding p o t e n t i a l w a s measured. C a l i b r a t i o n m e a s u r e m e n t s w e r e p e r f o r m e d with NHuCI a n d NaOH. T h e reproductibility of t h e e l e c t r o d e itself is 2 2 %, a n d t h e precision obtained o n r e p l i c a t e s a m p l e s w a s up to ? 5 %.

RESULTS AND DISCUSSION E l e c t r o p h o r e t i c mobility A s pointed o u t by H u n t e r (1981) m a n y of t h e i m p o r t a n t p r o p e r t i e s of colloidal s y s t e m s a r e d e t e r m i n e d d i r e c t l y or i n d i r e c t l y by t h e e l e c t r i c a l c h a r g e (or p o t e n t i a l ) o n t h e particles. Adsorption of ions a n d dipolar molecules is d e t e r m i n e d by, and also d e t e r m i n e s , this c h a r g e a n d p o t e n t i a l distribution. T h e p o t e n t i a l distribution itself d e t e r m i n e s t h e e n e r g y of i n t e r a c t i o n s b e t w e e n t h e p a r t i c l e s , a n d is in m a n y cases responsible f o r t h e s t a b i l i t y of p a r t i c l e s t o w a r d s c o a g u l a t i o n a n d f o r m a n y a s p e c t s of t h e flow s t a b i l i t y of t h e colloidal suspension. C o n t r o v e r s i a l p o t e n t i a l d a t a h a v e b e e n obtained during t h e l a s t d e c a d e (Pravdic, 1970 ; Martin, J e d n a c a k and Pravdic, 1971 ; H u n t e r and Liss, 1982) b u t i t is now established t h a t river suspended m a t t e r i s n e g a t i v e l y c h a r g e d with a slight d e c r e a s e of t h e e l e c t r o p h o r e t i c mobility during e s t u a r i n e mixing. W e h a v e r e p e a t e d t h e s e e x p e r i m e n t s in t h e t w o studied e s t u a r i e s (Fig. I). T h e e l e c t r o p h o r e t i c mobilities a p p e a r

56

almost similar in t h e t w o e s t u a r i e s d e s p i t e d i f f e r e n t mineralogical compositions. Although t h e e l e c t r o p h o r e t i c mobility of m a n y m i n e r a l s is n e g a t i v e a t n a t u r a l pH, this suggests, as pointed o u t by H u n t e r and Liss (1982), t h a t s o m e c o a t i n g s c o n t r o l t h e s u r f a c e c h a r g e of t h e particles. T h e e l e c t r o p h o r e t i c mobilities in t h e Gironde a n d t h e L o i r e e s t u a r i e s indicate a n uniformly n e g a t i v e c h a r g e f r o m t h e river to t h e ocean. T h e mobility r e m a i n s c o n s t a n t over t h e whole t i d a l e s t u a r y l o c a t e d a b o v e t h e s a l t intrusion. T h i s s u g g e s t s t h a t those p a r t i c l e s which h a v e been t r a n s p o r t e d landwards f r o m t h e middle e s t u a r y to t h i s a r e a a r e quickly reequilibrated with t h e surrounding w a t e r . T h e e l e c t r o p h o r e t i c mobility d e c r e a s e s sharply by a f a c t o r to t w o during t h e f i r s t s t e p of mixing. Then, it gradually d e c r e a s e s f o r higher chlorinities, b u t r e m a i n s n e g a t i v e as previously mentioned. T h e s e d a t a fall in t h e r a n g e of t h o s e r e p o r t e d f o r o t h e r e s t u a r i e s (Hunter and Liss, 1982 ; L o d e r and Liss, 1982).

b

r>-+,

Loire e s t u a r y (ICOLO 35)

Gironde estuary ( B A G 83)

1 Fig. 1. Evolution of t h e e l e c t r o p h o r e t i c mobilities of suspended s e d i m e n t f r o m t h e Gironde and L o i r e e s t u a r i e s (France). May 1983. W e have t r i e d to s p e c i f y t h e influence of d i f f e r e n t physico-chemical p a r a m e t e r s upon

t h e e l e c t r o p h o r e t i c mobility, using mode1 p a r t i c l e s such as silica (Aerosil 380

-

Degussa)

which is c h a r a c t e r i z e d by a c h a r g e very c l o s e to t h a t of n a t u r a l suspended s e d i m e n t f r o m Loire a n d Cironde. a) Influence of pH. F i g u r e 2 shows t h e evolution of t h e e l e c t r o p h o r e t i c mobility (UE)

with t h e pH variation in a 10-2 mole/l N a C l solution. In such a medium, t h e i s o e l e c t r i c point (IEP) h a s been o b s e r v e d at a pH = 3.4 Z 0.5. T h i s v a l u e is in good a g r e e m e n t with o t h e r r e s u l t s (Bolt, 1957 ; Ahmed, 1966). An IEP, d u e to t h e a c i d i c p r o p e r t i e s of t h e surfaces, is commonly o b s e r v e d f o r o t h e r oxides, however v e r y f e w d a t a a r e a v a i l a b l e f o r natural suspensions. H u n t e r (1979) d e t e r m i n e d t h e influence of pH o n t h e e l e c t r o p h o r e t i c mobility of s y n t h e t i c p a r t i c l e s c o a t e d with sea w a t e r organics, h e suggested t h e major influence of carboxylic a n d phenolic groups. I t is c l e a r f r o m t h e s e d a t a t h a t t h e s l i g h t pH

57

d e c r e a s e usually observed along t h e Gironde o r Loire e s t u a r i e s c a n n o t explain t h e charge d e c r e a s e observed in t h e low chlorinity z o n e b u t c a n c o n t r i b u t e to t h i s variation.

m o b i l i t y (u)

electrophoretic (cm2.V-'.

+

10

-

20

V l n

10-5

i

I

- 30

I

lsoelectric point at-pH3.4

- 40

I

I

- 50 0

I

I

t

1

2

3

I 4

I 5

I

I 7

6

I

8

I 9

1

I 0

dissolution occurs

I

l

b I

~

H

Fig. 2. E l e c t r o p h r e t i c mobility w i t h t h e pH variation in a NaCl solution (10-2 mole/l)

T

electrophoretic

( cm2 V-'. S - 1 )

mobility ( u ) 105

t -30

t 1

-40 '

1.10-2

2.10-2

I 5.10-2

1 1.10-1

I 2.10-1

5.10-1

[NaClj

1

I 1.10'

2.10'

YM,

Fig. 3. E l e c t r o p h o r e t i c mobility of Aerosil-380 silica p a r t i c l e s as a function of N a C l concentration (pH = 6 ) .

58

b) I n f l u e n c e

of

NaCl

concentration.

Figure

3

shows t h e

evolution

of

the

e l e c t r o p h o r e t i c mobility of t h e s i l i c a a s a function of t h e N a C l c o n c e n t r a t i o n in solution at pH = 6. F o r s u c h pH, t h e silica s a m p l e is strongly dissociated, a n d t h e effect of t h e n e g a t i v e c h a r g e of t h e p a r t i c l e s c a n only b e n e u t r a l i z e d at high NaCI c o n c e n t r a t i o n s , b e t w e e n 0.5 and 1 M/I. D a t a a r e very sitnilar to t h e s e obtained for suspended m a t t e r in estuaries.

c) I n f l u e n c e of o r g a n i c substances. T h e i m p o r t a n c e of adsorbed o r g a n i c m a t t e r in a f f e c t i n g t h e e l e c t r i c a l c h a r g e of p a r t i c l e s in sea w a t e r h a s been d e m o n s t r a t e d by Martin, J e d n a c a k and P r a v d i c (19711, Neihof and L o e b (1972, 1974). R e c e n t work by H u n t e r and Liss (1979, 1982) h a s c o n f i r m e d this i m p o r t a n c e . Finally Loder and Liss (1982, 1985) and H u n t e r (1980) c l e a r l y d e m o n s t r a t e t h a t , whenever iron oxides o r a l u m i n a a r e considered, t h e i r s p e c i f i c positive c h a r g e s a r e changed radically to usual n e g a t i v e values by t h e f o r m a t i o n of o r g a n i c c o a t i n g s in s e a or e s t u a r i n e w a t e r s (Fig. 4).

T h e y also pointed o u t

t h e s p e c i f i c adsorption of major c a t i o n s o n t h e s e positively c h a r g e d particles, s i n c e t h e i r e l e c t r o p h o r e t i c mobility i n c r e a s e s with t h e salinity when t h e sea w a t e r is f i r s t UVoxidized, whilst a Couy-Chapman model would h a v e p r e d i c t e d a decrease.

-20,

Fig. 4. E l e c t r o p h o r e t i c mobilities (UE)Of natural (untreated) - curve A - and treated p a r t i c l e s as a f u n c t i o n of salinity ( S O / o o ) for t w o sets of s a m p l e s f r o m Keithing Burn (KB 1 : o p e n symbols - 31 March 1982 ; K B 2 : closed symbols - 30 J u n e 1982). S h a d e d a r e a B i n d i c a t e s t h e spread of r e s u l t s f r o m o t h e r e s t u a r i e s ( r e d r a w n f r o m Fig. 3 of H u n t e r and Liss 1979). C u r v e D - suspended p a r t i c l e s c e n t r i f u g e d a n d resuspended in UV- oxidized s a m p l e s u p e r n a t a n t a n d t h e n UV-oxidized. C u r v e C - n a t u r a l s a m p l e s ( p a r t i c l e s plus s u p e r n a t a n t ) UV-oxidized. s a m p l e s u p e r n a t a n t UV-oxidized Curve E to f o r m new p a r t i c l e s (UV-PPT). S e v e r a l UV-PPT s a m p l e s f r o m KB2 w e r e c e n t r i f u g e d and t h e p a r t i c l e s resuspended in t h e i r original u n t r e a t e d s a m p l e supernatant. T h e resulting c h a n g e s in UE a r e indicated by t h e dashed lines (asterisks - f i n a l values). Keithing Burn suspended m a t t e r is m o s t l y composed of iron o x i d e s ( a f t e r L o d e r a n d Liss, 1985).

-

,

59

Tipping (1981) showed t h a t t h e adsorption of t e r r e s t r i a l h u m i c s u b s t a n c e s could r e v e r s e t h e positive e l e c t r o p h o r e t i c mobility of iron oxides. Thus, b o t h t e r r e s t r i a l and marine o r g a n i c s a r e s t r o n g adsorbates, a b l e to r e v e r s e zeta potential. d) Application to e s t u a r i n e data. D a t a for t h e C i r o n d e and L o i r e e s t u a r i e s a r e reported

in Fig. 5, with d a t a f r o m H u n t e r and Liss (1982) for t h e Conwy estuary. Clearly t h e main f a c t o r explaining t h e evolution of zeta p o t e n t i a l is salinity, while t h e globally negative c h a r g e is d u e to both t h e n e g a t i v e c h a r g e of m o s t s t r u c t u r a l c o n s t i t u e n t s of e s t u a r i n e p a r t i c u l a t e m a t t e r a n d to o r g a n i c coatings. T h e s e d a t a a r e difficult to r e l a t e to an evolution of e l e c t r o s t a t i c t e r m of ion adsorption. I t i s c o m m o n l y a d m i t t e d t h a t , for colloidal systems, t h e zeta potential obtained f r o m t h e e l e c t r o p h o r e t i c mobility and t h e Smoluchkowski equation, possibly c o r r e c t e d by a s i z e o r s h a p e f a c t o r , i s a good approximation f o r t h e O H P p o t e n t i a l at r e l a t i v e l y high ionic s t r e n g t h (> 10-3 M) (Foissy et al., 1982). R e c e n t l y , a model based on t h i s c o n c e p t h a s b e e n successfully used to i n t e r p r e t adsorption d a t e as a function of pH for anions o n t o iron oxide (Haussmann a n d Anderson, 1985). In t h e case of pH, at c o n s t a n t ionic s t r e n g t h , t h e g r e a t variability of t h e e l e c t r o k i n e t i c p o t e n t i a l h a s to b e r e l a t e d to a variation of t h e OHP potential. However, in t h e case of salinity e f f e c t , and f o r natural suspended m a t t e r t h e slight evolution which h a s been observed in s e v e r a l e s t u a r i e s is not

I

Gironde and

Loire

01 0

0.1

0.2

0.3

0.4

0.5

0.6

b

VT

Fig. 5. Plot of t h e logarithm of t h e e l e c t r o p h o r e t i c mobility of n a t u r a l p a r t i c l e s in t h e Gironde, L o i r e a n d Conwy e s t u a r i e s ( D a t e f o r Conwy f r o m H u n t e r and Liss, 1982).

60 necessarily d u e to a variation of t h e OHP p o t e n t i a l b e c a u s e of t h e n u m e r o u s i n c e r t i t u d e s r e l a t e d to t h e Smoluchkowski e q u a t i o n and b e c a u s e a very s m a l l d i s t a n c e b e t w e e n t h e 0

0

OHP and t h e p l a n e of s h e a r (2 A to 7 A) in a sirnple Couy-Chapman model could

the

explain t h e evolution. A c a r e f u l study by S m i t h (1976) d e m o n s t r a t e d t h a t t h e r e w a s n o e v i d e n c e of t h e s h e a r plane being a t a f i n i t e d i s t a n c e f r o m t h e OHP t o a sensitivity of 0

o r d e r 5 A. Thus, it is n o t c l e a r w h e t h e r t h e evolution of t h e e l e c t r o p h o r e t i c mobility is r e l a t e d t o a n evolution of t h e e l e c t r o s t a t i c t e r m of t h e i n t e r a c t i o n s b e t w e e n dissolved species and particulate matter. In s i t u e s t u a r i n e m e a s u r e m e n t s of e l e c t r o p h o r e t i c mobility would b e i n t e r e s t i n g l y c o m p l e t e d by e s t i m a t i o n s of t h e effect of ionic s t r e n g t h and of s o m e specified dissolved c o m p o n e n t s of sea w a t e r on t h e s a m p l e d particles.

2. Specific surface area T h e r e s u l t s of t w o surveys a r e r e p o r t e d on Fig. 6, S.S.A.

a p p e a r s to b e lower in t h e

river p a r t , especially for t h e Gironde estuary. I t usually i n c r e a s e s as soon as t h e t i d a l influence is observed with a c l e a r maximum l o c a t e d around t h e s a l t intrusion. Many processes c a n affect p a r t i c u l a t e m a t t e r in t h i s p a r t of estuaries. T h e i n c r e a s e could b e d u e to t h e d e s t r u c t i o n of s o m e flocs f o r m e d u p s t r e a m as observed by Eisina (personal c o m m u n i c a t i o n ) o r to a c h e m i c a l evolution of t h e superficial composition of p a r t i c l e s as discussed below.

.S.A m2/g

\ \L

I

Km l o o

50

5

10

15

20

CI'BO

b

Fig. 6 . Longitudinal evolution of S.S.A., in t h e Gironde a n d L o i r e estuaries.

61

POC

t 6

50

5

40

4

30

3

20

2

I 10

1

IlPP

I

0 100

Km

5

0

SO

CI-

10

J %.

7a: Gironde estuary.

POC

50

40

30

native

20

coated

10

U

100

Km

50

0

oxides lree

5

10

15

20

CI- %o 7b : L o i r e estuary.

Fig. 7. S p e c i f i c s u r f a c e a r e a (BET Argon adsorption) of native, o r g a n i c s f r e e and amorphous oxides f r e e samples.

62

Fig. 8 shows t h e evolution of t h e m a x i m u m f l o c s i z e in t h e Gironde e s t u a r y , a n i m p o r t a n t d e c r e a s e a p p e a r s around t h e s a l t intrusion (R. Gibbs personal communication). T h e maximum used to b e a s s o c i a t e d with a n i n c r e a s e of turbidity so t h a t t h e S.S.A. p e r l i t e r is e v e n m o r e pronounced. T h i s m a x i m u m could g r e a t l y e n h a n c e t h e availability of adsorption s i t e s for dissolved t r a c e m e t a l s and radionuclides. T h e maximum observed in t h e upper e s t u a r t is followed by a d e c r e a s e in t h e l o w e r p a r t of both L o i r e and Gironde. Possible s e g r e g a t i o n s of c h e m i c a l and mineralogical composition of suspended s e d i m e n t s could p a r t l y explain t h i s evolution. S y s t e m a t i c analysis of c l a y mineralogy as well as q u a r t z a n d c a r b o n a t e c o n t e n t did n o t r e v e a l a n y significant t r e n d along t h e estuary. T h e aluminium c o n t e n t is c o m m o n l y used as a n indicator of t h e g r a n u l o m e t r y of t h e sample, s i n c e most of i t is involved in t h e alumino-silicates which a r e well known to b e t h e most i m p o r t a n t p a r t of t h e s m a l l e r fraction. A r a t h e r good c o r r e l a t i o n (Fig. 9) c a n b e found b e t w e e n S.S.A. and A1 c o n t e n t . This c o n f i r m s t h e utility of normalizing to A1 to c o m p a r e t r a c e m e t a l s o r radionuclides c o n c e n t r a t i o n s e v e n if o n e a s s u m e s t h a t t h e y a r e l o c a t e d at t h e p a r t i c l e surface. N o t i c e also t h a t t h e normalization c a n i n t r o d u c e s o m e bias in c o m p a r i n g e s t u a r i n e and riverine d a t a (in t h e case of t h e Gironde), s i n c e a11 t h e r i v e r i n e d o t s a r e g a t h e r e d (circle) under t h e main c o r r e l a t i o n line.

S.S.A

m*/g

- 30

- 20

-10

Fig. 8. C o m p a r a t i v e evolution of t h e m a x i m u m f l o c s i z e a n d t h e s p e c i f i c s u r f a c e a r e a in t h e C i r o n d e estuary. In o r d e r to i n v e s t i g a t e if s u r f a c e c o a t i n g s such as hydrous oxides and/or o r g a n i c m a t t e r w e r e involved in t h e a c t u a l S.S.A.

distribution, t h e analysis h a v e been r e p e a t e d

with o r g a n i c s f r e e a n d a m o r p h o u s f r e e samples.

63

The S.S.A. raises to higher values if t h e organic m a t t e r is stripped of t h e samples. The d a t a issued from t h e Gironde estuary (Fig. 7a) d e m o n s t r a t e t h a t t h e P.O.C. related to an increase of S.S.A..

is directly

This means t h a t t h e organic m a t t e r somehow blocks t h e

available s i t e s for Argon molecules. In t h e Loire (Fig. 7b), t h e phenomenon is less observable ; t h e increase of P.O.C. greaterincrease of t h e S.S.A.

at sea end member is not correlated with a

a f t e r t r e a t m e n t . But i t has t o b e noticed t h a t organic

m a t t e r composition of this sample, is q u i t e different from river organic m a t t e r as shown by t h e 13C d a t a (R. Letolle, personal communication) this difference c a n explain i t s lower activity in blocking t h e available s i t e s of t h e inorganic matrix.

6

7

8

9

lo

Al %

Fig. 9. Correlation between specific s u r f a c e a r e a and A1 c o n t e n t of t h e samples. Upstream Gironde d a t a a r e circled. Marine sediments as well as deposited estuarine sediments never show such differences. Usually, t h e organic m a t t e r c o n t e n t is lower in estuarine than in river sediment, and S.S.A. is always t h e s a m e or slighthly lower from t r e a t e d as compared t o untreated deposited and marine sediments. This supports t h e idea t h a t t h e organic coating of t h e river p a r t i c u l a t e m a t t e r drastically changes i t s c h a r a c t e r depending upon the environment,

modifying i t s adsorptive

surface properties

radionuclides. T h e generally negative influence of P.O.C.

for

trace

metals

on t h e S.S.A.

demonstrated ; it should b e interesting now to r e l a t e t h e organics f r e e S.S.A.

and

has been t o some

o t h e r p a r a m e t e r characterizing t h e inorganic core. T h e amorphous f r e e samples, which

64

a r e probably a l s o p a r t l y organic f r e e , show a considerably lower S.S.A.

(Fig. 71,

d e m o n s t r a t i n g t h a t , in as much t h e l e a c h i n g is specific, t h e amorphous oxides g r e a t l y c o n t r i b u t e t o t h e S.S.A..

T h e arnorphous f r e e s a m p l e s show an a l m o s t c o n s t a n t S.S.A. o v e r

t h e e s t u a r y (= 10rn2/g). It is higher t h a n t h e c r i s t a l l i z e d S i 0 2 S.S.A. (3.3 m2/g, Benjamin, 1980) b u t lower t h a n t h e illite S.S.A. (70 m2/g, O'Corinor, 1975). Globally, t h e "amorphous

specific" t r e a t m e n t r e m o v e s alrnost 1.2

oh

F e 2 0 3 , 0.8 % A1203, 4-8 "0 S i 0 2 (expressed a s

p e r c e n t a g e s of t h e t o t a l mass of t h e sample). T h e s e p e r c e n t a g e s a r e not c o r r e l a t e d t o t h e global a l u m i n i u m o r silicum c o n t e n t of t h e s a m p l e showing a t least s o m e s p e c i f i c i t y of t h e leaching. "Organics f r e e " S.S.A. should b e d u e to w h a t is removed by t h e leaching. Indeed, t h e r e is a global positive c o r r e l a t i o n b e t w e e n t h e leached q u a n t i t i e s of "amorphous" oxides and especially in t h e case of iron oxide (Fig. 10). Existing d a t a of t h e

t h e o r g a n i c s f r e e S.S.A.,

l i t t e r a t u r e on a m o r p h o u s oxides S.S.A. (Davis, 1978) c a n n o t a c c o u n t f o r t h e g a p b e t w e e n organics f r e e and a m o r p h o u s f r e e samples, b u t t h e S.S.A. of a p u r e p h a s e c a n hardly b e c o m p a r e d to t h e S.S.A. of t h e s a m e phase c o a t e d around a n i n e r t core. I t c a n b e concluded t h a t t h e S.S.A.

of e s t u a r i n e suspended m a t t e r is mainly d u e to

amorphous o x i d e s coatings, arnong t h e m , t h e iron oxides could b e predominant, and t h e organic c o a t i n g s b e h a v e as a mask for Argon rnolecilles during BET adsorption. A m o d e l p a r t i c u l e c a n b e considered as a n inorganic d e t r i t i c c o r e surrounded by a n a c t i v e amorphous l a y e r r e l a t i v e l y t o g a s adsorption, p a r t l y r e c o v e r e d by a less a c t i v e o r g a n i c film. "amorphous"

0

200 0

150

ICOLO 3 5 + B A G 8 3

0

0

100

20

UPSTREAM r e f e r e n c e 3

I

I

I

30

40

50

S.S.A

org.free

Fig. 10. C o r r e l a t i o n b e t w e e n t h e o r g a n i c s f r e e s p e c i f i c s u r f a c e a r e a a n d t h e o p e r a t i o n a l l y d e f i n e d "amorphous" iron c o n t e n t of t h e samples.

65

3. C a t i o n E x c h a n g e C a p a c i t y C a t i o n e x c h a n g e c a p a c i t y provides an additional p a r a m e t e r t o u n d e r s t a n d t h e behaviour of s o m e t r a c e m e t a l s and radionuclides in estuaries. Keeping in mind t h e method of d e t e r m i n a t i o n (NH4 s a t u r a t i o n and r e l e a s e by a s t r o n g base), i t provides an e s t i m a t i o n of t h e n u m b e r of s i t e s available for ion e x c h a n g e o r for s o m e o t h e r process involved with t h e s a m e s u r f a c e a c t i v e sites. I t is now c l e a r t h a t such a c t i v e s i t e s e x i s t in both t h e mineral p h a s e of t h e s e d i m e n t and in t h e organic phase. This l a t e r c o n t r i b u t e s to t h e t o t a l CEC f r o m 25 to 80 % (Schnitzer, 1965 ; Rashid, 1969 ; Hunt, 1981). This means t h a t t h e o r g a n i c c o a t i n g s m a y h a v e a s p e c i f i c CEC. O u r r e s u l t s support s u c h findings. T h e values o b t a i n e d in Gironde and Loire a r e shown Fig. 11. T h e L o i r e a p p e a r s slightly higher t h a n Gironde b u t both s e t s of d a t a correspond t o slightly higher v a l u e s thar? t h e s e s measured by H u n t (1981) for Long Island Sound at t h e s e d i m e n t s u r f a c e . In general, t h e r e a r e no s y s t e m a t i c c h a n g e s f r o m t h e river to t h e sea, e x c e p t a possible maximurn in t h e e s t u a r y as c o m p a r e d to t h e t w o end-members. Whenever t h e values a r e nortnalized t o the s u r f a c e a r e a s , t h e r e is no m o r e d i f f e r e n c e s b e t w e e n L o i r e and Gironde and a l l t h e c u r v e s indicate a minirnum within t h e e s t u a r y .

1

06

CEC r n e q l g

0 4

0.2

100

Km

50

0

10

*O

CI -

* %o

Fig. 11. C a t i o n e x c h a n g e c a p a c i t y (NH4 s a t u r a t i o n method) in t h e Gironde and L o i r e estuaries. As in t h e case of S.S.A.,

w e m e a s u r e d t h e d i f f e r e n c e b e t w e e n t h e "native" and

"organics f r e e " samples. I t h a s to b e reminded t h a t t h e m e a s u r e m e n t s w e r e p e r f o r m e d

66

with dry s a m p l e s (SOOC), so t h a t a c e r t a i n f r a c t i o n of organic s i t e s and a m o r p h o u s s i t e s may disappear. However, t h e global effect of t h e o r g a n i c p a r t on C E C i s c l e a r l y expressed. Fig. 10 shows t h a t t h e C E C v a l u e s f o r o r g a n i c s f r e e s a m p l e s a r e s y s t e m a t i c a l l y lower than t h o s e of n a t i v e samples. T h e residual values for C E C a f t e r t h e r e m o v a l of t h e organics (15-20 meq/100 g) should correspond to t h e CEC of t h e mineral phase. However, i t has to b e noticed t h a t t h e w a r m oxidation a t t a c k s t h e s u b s u r f a c e l a y e r mainly consisting of a m o r p h o u s oxides. T h e r e f o r e , i t c a n n o t b e a r e l e v a n t indication of m i n e r a l phase contribution to t h e t o t a l CEC.

4. H e a t of i m m e r s i o n T h e h e a t t h a t is evolved on i m m e r s i n g a solid in a liquid yields a p i c t u r e of i t s s u r f a c e including a l l i t s heterogeneities. In m a n y g e o c h e m i c a l problems involving solid-liquid interactions, t h e h e a t of immersion t e c h n i q u e provides a very promising method. T h e h e a t of immersion is t h e e n t h a l p y c h a n g e A H observed when a solid s u r f a c e is suddenly immersed i n t o s o m e liquid and c a n b e expressed as an e n e r g y i n t e r a c t i o n by s u r f a c e a r e a , describing t h e e n e r g e t i c s of adsorption of a s o l u t e w i t h t h e p a r t i c u l a t e s u r f a c e (Zettelmoyer, 1965 ; Pravdic, Jednacak-Biscan a n d J u r a c i c , 1981). Whenever t h e liquid used is w a t e r , A H will give s o m e idea of t h e hydrophilicity of t h e

t

- 'native'

AH J i g

- .-.

'organics free'

50

0

0 0

0

lot

I

1

-100

I

Km

-50

I

I

I

I

0

5

10

15

CI-Xo

b

Fig. 12. Evolution of t h e h e a t of immersion in t h e Gironde (closed circles) a n d L o i r e (open squares) estuaries. S t a r s : "amorphorus f r e e

samples, Gironde estuary.

67

solid s u r f a c e and i t s resulting a f f i n i t y for a l a r g e v a r i e t y of o r g a n i c p o l l u t a n t s a n d n a t u r a l ligands. T h e higher t h e s u r f a c e hydrophilicity t h e higher is t h e e n e r g y of w e t t i n g per unit surface. T h e g e n e r a l p i c t u r e of d a t a for t h e t w o e s t u a r i e s is shown in f i g u r e 12. I t c a n b e s e e n t h a t t h e h e a t of immersion d a t a follow t h e CEC d a t a . A highly s i g n i f i c a n t c o r r e l a t i o n e x i s t s b e t w e e n t h e s e t w o s e t s of d a t a , including n a t i v e and o r g a n i c s f r e e s a m p l e s (Fig. 13). T h e slope of t h e s t r a i g h t line c a n g i v e u s s o m e information a b o u t t h e kind of processes which a m m o n i u m adsorption. In t h i s case t h e e n e r g y involved is 105 k J / m o l e

H20. T h e e n e r g i e s of s u r f a c e hydration of oxides a r e 80-105 k J / m o l e for TiO2, 27 k J / m o l e f o r s i O 2 (Morimoto, 1968) o r 84 k J / m o l e for A1203 (Fubini et al., 1978). T h e h e a t s of w e t t i n g as found in o u r r e s u l t s a r e higher, suggesting m o r e hydrophilic interactions. N u m e r o u s p r o c e s s e s c a n t a k e p l a c e when w e t t i n g c l a y minerals with w a t e r , such as swelling, ion e x c h a n g e , chemisorption o r c h e m i c a l i n t e r a c t i o n s ( Z e t t l e m o y e r , 1965), but t h e y c a n hardly h e resolved in t h e case of n a t u r a l inhomogenious sediments. Globally, t h e value of 105 k J / m o l e c o r r e s p o n d s t o s t r o n g physical sorption o r e v e n cheinisorption.

500

400

300

'treated"

200

'native'

081

0

081

G8lB A

A

G81B

BAG 83

0

L-

D

BAG83 ICOLO 3 5

C.E.C peq/m2 10

20

30

40

50

60

Fig. 13. C o r r e l a t i o n b e t w e e n h e a t of i m m e r s i o n and c a t i o n e x c h a n g e c a p a c i t y in t h e G i r o n d e and L o i r e estuaries. D a t a f r o m y e a r s 1981 and 1983. All d a t a f r o m t h e C i r o n d e e s t u a r y b u t ICOLO 3 5 (Loire estuary).

68

As for t h e o t h e r parameters, w e performed measurements of h e a t of immersion on organics f r e e and amorphous f r e e samples. As in t h e case of CEC, t h e organic m a t t e r greatly contributes t o t h e p a r a m e t e r and t h e marine O.M. is less active. This confirms t h e different reactivities of different types of organic m a t t e r , which we already observed for S.S.A..

In general, t h e absolute values of h e a t of wetting for both native and amorphous f r e e samples a r e higher than for pure oxides or mineral (20-100 pJ/cm2, Zettlemoyer, 1965). However, for more basic oxides, having a higher pHzpc, t h e h e a t of wetting becomes (Fubini, 1978). The typical range for organics f r e e samples is

higher reaching 200

50-100 pJ/cm2, higher values being found for sea end members samples. Native and amorphous f r e e samples show approximately t h e s a m e h e a t s of immersion, this similitude clearly indicates t h a t A H is basically governed by organic coatings.

CONCLUSION The role of organic m a t t e r and amorphous oxides has long been considered as a key to understand t h e geochemical interactions between dissolved and particulate phases. Indeed, this influence must b e carefully assessed and greatly depends upon t h e considered property. As f a r as electrophoretic mobility is concerned, we previously underlined t h e obvious lack of interaction between humic acids and negatively charged silica particles, however t h e s a m e kind of experiments with model particles of higher isoelectric point as for example A1203 and iron oxides, indicate a strong e f f e c t of organics. The numerous studies performed by Liss and co workers showed t h a t t h e electrophoretic mobility of both natural or synthetic particles was greatly influenced by fresh marine organic matter. There a r e strong evidences t h a t a negatively charged organic film c o a t s most of t h e natural particles. Organic m a t t e r deposited onto t h e suspended sediment has a large influence on t h e specific surface area. On one hand, i t appears t h a t organic m a t t e r blocks s o m e s i t e s available for physical adsorption of i n e r t g a s (BET adsorption) and, on t h e o t h e r hand, i t probably partly causes flocculation and agglomeration of particles in t h e upper estuary, such flocs and a g r e g a t e s being destroyed downstream in t h e salinity intrusion zone. Two o t h e r properties, C E C and h e a t of

immersion a r e strongly related. T h e

comparison of t h e influence of coatings upon t h e values of t h e s e different p a r a m e t e r s allows t h e distinction of t w o categories : a) on t h e one hand, S.S.A. s e e m s to b e controlled by t h e amorphous oxides and is related to t h e A1 c o n t e n t of t h e samples (which c a n b e considered as a n index of t h e fine grained

fractions

mostly composed of

alumino-silicates),

and accordingly to i t s

granulometry, suggesting a n a l m o s t uniform coating of t h e d e t r i t i c c o r e by amorphous oxides ; conversly, P.O.M.

has a negative influence on this parameter.

b) on t h e o t h e r hand, C.E.C.

and H e a t of immersion which appear to b e strongly

69

c o r r e l a t e d a r e controlled by both organic m a t t e r and amorphous oxides. C E C has long been used as a basic p a r a m e t e r to model t h e e x c h a n g e of m e t a l s at t h e p a r t i c u l a t e surfaces. F o r m e r s t u d i e s by Hodgson et al. (1963) showed t h a t t h e removal of organic m a t t e r , which p a r t l y c o n t r o l s C E C , f r o m soils m i g h t e i t h e r i n c r e a s e o r d e c r e a s e adsorption of Cobalt. I t is likely t h a t s e v e r a l p a r a m e t e r s which a r e commonly associated with t h e a d s o r p t i v e p r o p e r t i e s of p a r t i c u l a t e m a t t e r d o n o t depend t h e s a m e way on t h e c o n s t i t u t i v e p r o p e r t i e s of t h e samples. Nevertheless, i t is still difficult : i) to d e t e r m i n e t h e main f a c t o r controlling adsorption (amorphous o r organic coatings) keeping in mind t h a t i t is n o t necessarily t h e s a m e for a l l m e t a l s , ii) to d e t e r m i n e t h e b e s t p a r a m e t e r to e s t i m a t e t h e a d s o r p t i v e p r o p e r t i e s of particulate matter.

ACKNOWLEDGMENTS T h e a u t h o r s a r e indebted to t h e g r o u p f o r Electrophoresis of t h e C e n t e r for Marine R e s e a r c h Zagreb, "Rudjer Boskovic" I n s t i t u t e , Zagreb, C r o a t i a , Yugoslavia, for C E C measurements. T h i s study h a s been c a r r i e d o u t with t h e financial s u p p o r t of C.N.R.S. ( G R E C O i n t e r a c t i o n s Continent-Ocean).

REFERENCES Ahmed, S.M., 1966. S t u d i e s of t h e dissociation of oxide s u r f a c e s a t t h e liquid-solid i n t e r f a c e . Canc. J. Chem., 44 : 1163-1170. Allen, G.P., 1972. E t u d e d e s processus s d d i m e n t a i r e s d a n s I'estuaire d e la Gironde. T h k e d e D o c t o r a t d'Etat. Universitd d e Bordeaux I, F r a n c e , 314 pp. Barbaroux, L.H., 1980. Evolution d e s propri6tCs physiques et chimiques d e s s 6 d i m e n t s dans l e p a s s a g e continent-oc6an. L ' e f f e t estuarien. (Estuaire d e la L o i r e et d e ses parages). Th6se d e D o c t o r a t d'Etat, Universitd d e Nantes, France. 433 pp. Benjamin, M.M. and Leckie, J.O., 1980. Adsorption of m e t a l s at oxide i n t e r f a c e s : e f f e c t s of t h e c o n c e n t r a t i o n s of a d s o r b a t e a n d c o m p e t i n g metals. In: R.A. Baker (ed.), C o n t a m i n a n t s a n d Sediments. Ann Arbor Pub, Vol. 2. Bolt, G.M., 1957. D e t e r m i n a t i o n of t h e c h a r g e density of Silica sols. J. Phys. Chem., 61: 1166-1169. Busenberg, E. and Clemency, C.V., 1973. D e t e r m i n a t i o n of t h e C a t i o n Exchange Capacity of c l a y s a n d soils using a n a m m o n i a e l e c t r o d e . Clays, Clay Miner, 21, 213. Davis, J.A. a n d Leckie, J.O., 1978. S u r f a c e ionization and complexation a t t h e o x i d e / w a t e r i n t e r f a c e . P a r t 11. J. of Colloid a n d I n t e r f a c e Science, 67: 90-107. Elbaz-Poulichet, F., Huang, W.W., Jednacak-Biscan, J., Martin, J.M. and Thomas, A.J., 1982. T r a c e m e t a l behaviour in t h e Gironde e s t u a r y : t h e problem revisited. Thalassia Jugoslavica, 18: 61-96.

70

Elbaz-Poulichet, F., Holliger, P., Huang, W.W. and Martin, J.M., 1984. Lead cycling in estuaries, illustrated by t h e Gironde estuary, France. Nature, 308: 409-414. Foissy, A., M'Pandou, A., Lamarche, J.M. and Jaffrezeic-Renault, N., 1982. Surface and diffuse-layer c h a r g e of t h e TiO2-electrolyte interface. Colloids and Surf., 5: 363-368. Fubini, B., Della G a t a , G. and Venturello, G., 1978. Energetics of adsorption in aluminawater systems. Microcalorimetric study on t h e influence of adsorption t e m p e r a t u r e on surface processes. J. of Colloid and I n t e r f a c e Sci., 64: 470. Hashimoto, 1. and Jackson, M.L., 1960. Rapid dissolution of allophane and KaoliniteHalloysite a f t e r dehydration. Clay and Clay Minerals, 7 t h Conf., pp. 102-113. Haussmann, D.D. and Anderson, M.A., 1985. Using electrophoresis in modelling sulfate, salinity and phosphate adsorption on t o goethite. Environmental Science and Technology, 19: 544-551. Hunt, C.D., 1981. Regulation of sedimentary cation exchange capacity by organic matter. Chemical Geology, 34: 131. Hunter, K.A., 1980. Microelectrophoretic properties of natural surface-active organic m a t t e r in c o a s t a l sea water. Limnol. Oceanogr., 25: 807-822. Hunter, R.J., 1981. Z e t a potential in colloid science. Academic Press, 386 pp. Hunter, K.A. and Liss, P.S., 1982. Organic m a t t e r and t h e surface c h a r g e of suspended particles in e s t u a r i n e water. Limnol. Oceanogr., 27: 322-335. Hunter, K.A. and Liss, P.S., 1979. The s u r f a c e c h a r g e of suspended particles in estuarine and c o a s t a l water. Nature, 282: 823-825. Hunter, R.J. and Wright, H.J.L., 1971. T h e dependance of e l e c t r o k i n e t i c potential on concentration of electrolyte. J. of Colloid and I n t e r f a c e Sci., 37, no 3: 564-580. Jackson, O.P. and Mehra, M.L., 1960. Iron oxide removal from soils and clays by a dithionite-citrate s y s t e m buffered with sodium bicarbonate. Clay and Clay minerals, 7 t h Conf., 317. Juracic, M. and Pravdic, V., 1981. Geochemical and physico-chemical studies on sediments of t h e Rijeka bay. T h e properties of sediments as depositories of pollutants. Thalassia Jugoslavica, 17: 339-349. Loder, T.C. and Liss, P.S., 1982. The role of organic m a t t e r in determining t h e surface charge of suspended particles in estuarine and ocean waters. Thallasia Jugoslavica, 18: 433-448. Loder, T.C. and Liss, P.S., 1985. - Control by organic coatings of t h e s u r f a c e c h a r g e of estuarine suspended particles. Limnology and Oceanography, 30: 418-421. Martin, J.M., Jednacak, J. and Pravdic, V., 1971. The physico-chemical a s p e c t s of t r a c e e l e m e n t behaviour in m a r i n e environments. Thalassia Jugoslavica, 7: 619-637. Martin, J.M., Mouchel, J.M. and Thomas, A.J., 1984. T i m e concepts in hydrodynamic systems : a n application to t h e Cironde estuary. Invited c o n f e r e n c e paper : VIII Int. Symp.: "Chemistry of t h e Mediterranean". Residence t i m e of microconstituents in coastal waters, Primosten, Yugoslavia, May 16-24, Marine Chemistry (in press). Morimoto, T., Nagao, M. and Omori, T., 1969. H e a t of immersion of titanium dioxyde in water. I. E f f e c t of t h e hydratation t r e a t m e n t of Titanium dioxide. Bull. Chem. SOC. Japan, 42: 943.

71 Neihof, R.A. a n d Loeb, G.I., 1972. S u r f a c e c h a r g e c h a r a c t e r i z a t i o n of p a r t i c u l a t e m a t t e r in s e a w a t e r . Limnol. Oceanogr., 17: 7-16. Neihof, K.A. a n d Loeb, G.I., 1974. Dissolved o r g a n i c m a t t e r in s e a w a t e r a n d t h e e l e c t r i c c h a r g e of i m m e r s e d surfaces. J. Mar. Res., 32: 5-12. O'Connor, T.P. a n d Kester, D.R., 1975. Adsorption of C o b a l t f r o m f r e s h and marine systems. Geoch. Cosmoch. A c t a , 39: 1531-1543. P a r f i t t , G.D. a n d Sing, K.S.W. (eds), 1976. C h a r a c t e r i z a t i o n of powder surfaces. A c a d e m i c Press, London, 476 pp. Pravdic, V., 1970. S u r f a c e c h a r g e c h a r a c t e r i z a t i o n of sea sediments. Limnol. Oceanogr., 15: 230-235. Pravdic, V., Jednacak-Biscan, J. and J u r a c i c , M., 198 1. Physico-chemical p a r a m e t e r s describing t h e role of p a r t i c u l a t e m a t e r i a l s in e s t u a r i n e w a t e r s in R i v e r Inputs to O c e a n Systems. J.M. Martin, D. E i s m a a n d J.D. Burton (eds). UNEP a n d UNESCO, pp. 188-196. Smith, A.L., 1976. E l e c t r o k i n e t i c s at t h e oxide-solution interface. J. of Colloid and I n t e r f a c e Science, 55: 525-530. Tiller, K.C., Hodgson, J.F. a n d P e e c h , M., 1963. Specific sorption of C o b a l t by soil clays. Soil Science, 95: 392-399. Tipping, E., 1981. T h e adsorption of h u m i c substances by ion oxides. Geoch. Cosmoch. A c t a , 45: 191-199. Tiselius, A., 1937. A new a p p a r a t u s f o r e l e c t r o p h o r e t i c analysis of colloidal mixtures. Trans. F a r a d a y SOC., 33: 524-531. Tiselius, A., 1938. Elektrophoretische Messungen a m Eiweiss. Kolloid Z., 55: 129-137. Z e t t l e m o y e r , A.C., 1965. Immersional w e t t i n g of solid surfaces. Industrial a n d Engng. Chemistry, 57: 27-36.

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73

NUTRIENT CYCLING AND PATHWAYS OF ORGANIC TRANSFORMATIONS

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75

MODELLING STUDIES OF MATERIAL FLOWS IN A SHALLOW ECOSYSTEM COMPARED T O THE OPEN OCEAN

J.G. FIELD, P.A. WICKENS and C.L. MOLONEY

Marine Biology Research Institute, Zoology Department, University of C a p e Town, 7700 Rondebosch (South Africa)

ABSTRACT Field, J.G., Wickens, P.A. and Moloney, D.L., 1986. Modelling studies of m a t e r i a l flows in a shallow ecosystem compared t o t h e open ocean. In: P. Lasserre and J.M. Martin (eds), Biogeochemical Processes at t h e Land-Sea Boundary. Elsevier, Amsterdam. Two nitrogen flow simulation models a r e presented to c o n t r a s t t h e e f f e c t s of horizontal water transport which is believed to be important in shallow-water marine systems,with vertical transport in a n open ocean plankton community, including t h e microplankton. A kelp-bed community dominated by filter f e e d e r s is modelled as a closed system with no transport, and under both upwelling and downwelling conditions. In t h e closed system, feedback of organic nitrogen through d e t r i t u s formation from animal f a e c e s is important in t h e maintenance of a food supply, a situation analogous t o closed bays and estuaries. Under upwelling conditions, s u r f a c e w a t e r transports m a t e r i a l o u t of t h e system and cool, c l e a r upwelling w a t e r dilutes t h e suspension of detritus. Under downwelling conditions when phytoplankton-rich water from offshore is imported, phytoplankton supplements t h e d e t r i t a l d i e t of shallow-water filter-feeders and their growth is f a s t e r ; this situation parallels t h a t found on exposed rocky intertidal shores. A nitrogen flow model of a microplankton community is used to investigate t h e e f f e c t of sedimentation o u t of t h e mixed layer. The simulation shows a series of peaks of phytoplankton, bacteria, zooflagellates, larger protozoa and micro-and meso-zooplankton over a 13-day period a f t e r an upwelling or mixing event. Two phytoplankton peaks develop, corresponding to "new" and "regenerated" production respectively. Increasing r a t e s of sedimentation of phytoplankton and zooplankton f a e c e s o u t of t h e mixed layer result in slightly slower development of t h e bloom, f l a t t e n e d and smaller peaks of all components, increasing relative importance of "regenerated" production, and maintenance of a larger phytoplankton P/B ratio. T h e series of peaks in t h e model suggests t h a t different components of t h e community may dominate t h e biomass at different successional s t a g e s in t h e development of a bloom. This may help to resolve t h e controversy over t h e significance of micro-organisms in t h e plankton community. INTRODUCTION During t h e 1970's a good d e a l of research was done on energy flows through coastal ecosystems, tracing t h e flow of energy or carbon f r o m primary producers to consumers. These studies included measurements of primary production, o f t e n expressed as mean annual values (Table 1). I t c a n b e seen from Table 1 t h a t t h e highest primary production tends to occur in systems with a large proportion of macrophytes, regardless of latitude.

76

Presumably t h e high productivity is d u e to a t t a c h e d m a c r o p h y t e s maintaining position and exposure t o sunlight in t h e face of tidal, wind- o r wave-induced w a t e r movement. This allows recycling of m i n e r a l s f r o m t h e sea floor to t h e plants, in c o n t r a s t to micro-algae o n t h e s e d i m e n t s and phytoplankton which a r e likely to b e t r a n s p o r t e d a n d / o r shielded f r o m light by turbid w a t e r s when t h e r e is w a t e r m o v e m e n t at shallow depths.

TABLE 1 S o m e primary production e s t i m a t e s in c o a s t a l ecosystems. T h e v a l u e s of p r i m a r y production h a v e been compiled f r o m t h e s o u r c e s i n d i c a t e d a n d p e r c e n t a g e s of p r i m a r y production a l l o c a t e d to t h r e e c a t e g o r i e s of primary producer.

SYSTEM

%

%

%

Macrophytes

Attached Micro-algae

Phytoplankton

1976

89 %

-

11 %

1445

84 %

10 %

6%

1270

60 %

-

40 %

890

100 %

gC.rn-2yr-1

(1) Kelp-bed

(Nova S c o t i a ) (2) Salt-marsh (Sapelo Island) (3) Kelp-bed (Benguela) (4) Mangal (Florida) ( 5 ) Enclosed Bay (Narragansett) (6) North S e a

-

400

100 %

320

-

(7) Baltic S e a

266

28 %

-

72 %

(8) Mud f l a t (Cornwall, U.K.) (9) Tropical E s t u a r y (Sierra Leone)

225

-

64 %

36 %

186

100 %

100 %

Authors : ( I ) Miller et al. (1971) ; (2) P o m e r o y & Weigert (1981) ; (3) Newell et al. (1982) ; (4) Lug0 et al. (1978) ; ( 5 ) Nixon et al. (this volume) ; ( 6 ) J o i r i s et al. (1982) ; (7) Jansson et al. (1982) ; ( 8 ) Warwick et al. (1979) ; (9) Longhurst (1983). Many of t h e e c o s y s t e m s t u d i e s r e f e r r e d to in T a b l e 1 h a v e been s u m m a r i s e d by d e s c r i p t i v e models of m e a n annual e n e r g y flow, which h a v e been useful f o r quantifying t h e a v e r a g e flows in food webs involved in such ecosystems. T h e y h a v e a l s o led to s t u d i e s on t h e r e l a t i v e i m p o r t a n c e of d e t r i t u s in c o a s t a l e c o s y s t e m s ( s e e Man, t h i s volume). If w a t e r m o v e m e n t is as i m p o r t a n t for c o a s t a l s y s t e m s as suggested above, t h e n i t s e f f e c t should b e studied. This h a s been done in s o m e i n s t a n c e s ; in t h e B a l t i c Sea, f o r e x a m p l e , Jansson et al. (1982) h a v e d e m o n s t r a t e d t h a t w a t e r m o v e m e n t s and fish migration provide t h e links b e t w e e n pelagic, b o t t o m a n d l i t t o r a l subsystems t r a n s p o r t i n g e n e r g y a n d m a t e r i a l s to m a i n t a i n n u t r i e n t cycling in t h e whole system. S m e t a c e k (1984) h a s r e l a t e d b e n t h i c processes to t h o s e in t h e w a t e r column a b o v e and, similarly, J o i r i s et al. (1982)

and Nixon ( t h i s volume) h a v e shown t h a t t h e t r a n s p o r t of m a t e r i a l b e t w e e n t h e w a t e r column and s e d i m e n t s and back m a i n t a i n s t h e productivity of both subsystems. In a review of Benguela kelp-beds on t h e w e s t coast of South Africa, Newell et al. (1982) p r e s e n t e d a n annual b u d g e t of p r i m a r y production and e n e r g y r e q u i r e m e n t s by consumers, summarising d e t a i l e d analyses of biomass, distribution and eco-physiological s t u d i e s of k e y f i l t e r - f e e d e r s such as t h e ribbed mussel Aulacomya

e, a n d p r e d a t o r s such

a s t h e c o m m e r c i a l l y exploited rock l o b s t e r Jaws lalandii. T h e b a l a n c e b e t w e e n p r i m a r y production and c o n s u m m e r r e q u i r e m e n t s in t h a t budget might a p p e a r t o suggest t h a t t h e kelp beds f o r m a closed s y s t e m , o r a l t e r n a t i v e l y , t h a t i m p o r t s b a l a n c e exports. M e a s u r e m e n t s of w a t e r m o v e m e n t s through t h e kelp beds (Field et al., 198Ob) i n d i c a t e t h a t wind-driven upwelling m a y result in t h e w a t e r column turning

over up t o s e v e n t i m e s per day, w i t h detritus-laden w a t e r being e x p o r t e d at t h e s u r f a c e and c l e a r , phytoplankton-free w a t e r f r o m below t h e e u p h o t i c z o n e replacing i t f r o m beneath. In c o n t r a s t ,

under downwelling conditions phytoplankton blooms which develop

o f f s h o r e in w a r m e d plumes of previously upwelled w a t e r , m o v e back t o w a r d s t h e coast and e n t e r t h e fringing kelp-beds at t h e s u r f a c e , displacing denser w a t e r beneath. T h u s under downwelling conditions, f i l t e r - f e e d e r s a r e provided with a rich supply of i m p o r t e d phytoplankton. T h e systern is t h e r e f o r e essentially a n open one, with i m p o r t s and e x p o r t s governed by w a t e r transport. I t i s e x t r e m e l y difficult to m e a s u r e t h e e f f e c t of a d y n a m i c physical process such as w a t e r t r a n s p o r t on t h e r a t e a t which a n ecological c o m m u n i t y functions, b u t hypotheses c a n b e t e s t e d f o r consistency using simulation models as has been done for a Benguela kelp-bed by Wulff a n d Field (1983). In t h e d e e p e r

ocean,

phytoplankton p r i m a r y

production v a l u e s a r e

generally

considerably lower t h a n t h o s e given f o r c o a s t a l w a t e r s in T a b l e 1 (Ryther, 1969 ; Pace et al., 19841, with t h e obvious e x c e p t i o n of upwelling s y s t e m s such as t h e Peruvian (Walsh et al.,

1981) and Benguela (Brown, 1984 ; Shannon a n d Field, 1985) systems. T h e high

productivity of upwelling s y s t e m s is a s c r i b e d to v e r t i c a l t r a n s p o r t of nutrient-rich w a t e r i n t o t h e p h o t i c zone, and t h i s is o f t e n a c c o m p a n i e d by downward flux of o r g a n i c m a t t e r a f t e r phytoplankton blooms in upwelling a r e a s (Walsh et al.,

1981). In t h e C e l t i c S e a

s e d i m e n t a t i o n of phytoplankton h a s been observed as i t falls to t h e b o t t o m (Billett et al., 1983). Thus, in t h e open o c e a n v e r t i c a l flows a r e i d e n t i f i a b l e and a p p e a r to b e i m p o r t a n t , w h e r e a s t h e y t e n d to b e combined with w a v e mixing o r horizontal t r a n s p o r t in shallow m a r i n e systems. I t is t h e a i m of t h i s c o n t r i b u t i o n to c o n t r a s t t h e e f f e c t of v e r t i c a l fluxes of m a t e r i a l in

t h e open o c e a n with h o r i z o n t a l t r a n s p o r t which is likely to b e i m p o r t a n t in shallow-water systems. T h e models described below a r e based on a number of simplifying assumptions, t h e r e f o r e t h e r e s u l t s m u s t b e t r e a t e d with caution. In p a r t i c u l a r t h e magnitude of predictions m a y n o t a l w a y s b e realistic, although t h e r e l a t i v e proportions and s e q u e n c e of e v e n t s modelled a r e believed to r e f l e c t t h e r e a l situations.

NITROGEN FLOW IN A BENCUELA KELP BED Velimorov et al. (1977) have described Benguela kelp bed communities which a r e subjected to pulsing w a t e r movement due t o alternating upwelling and downwelling conditions in t h e Benguela c u r r e n t (Field et al., 1980 b) and thus pulsing availability of phytoplankton t o filter-feeders. T h e primary producers in t h e kelp bed a r e phytoplankton and macrophytes. The dominant rnacrophytes a r e Laminaria pallida and Ecklonia maxima, with occasional Macrocystis angustifolia and various understorey algae and kelp epiphytes described by Field et al. (1980 a). T h e phytoplankton and macrophytes yield 86.5 g N m-2 yr -1 and 58.8 g N m-2 yr-1 respectively (Newell and Field, 1983 a). The production by macrophytes is released as p a r t i c u l a t e organic m a t t e r (POM) and dissolved organic m a t t e r (DOM) in carbon : nitrogen proportions of 9:l.

T h e nitrogen component of primary

production by phytoplankton constitutes 68 % of t h e total, while in t e r m s of carbon, phytoplankton only c o n s t i t u t e s 35 % of all primary production. Macrophytes have a C:N r a t i o approximately t h r e e t i m e s t h a t of phytoplankton (Newell and Field, 1983 a ) which accounts f o r phytoplankton being t h e more important primary producer in t e r m s of nitrogen. A c h a r a c t e r i s t i c f e a t u r e of t h e kelp bed community is t h e scarcity of g r a z e r s directly utilizing t h e living kelp, as they only c o n s t i t u t e some 4% of t h e t o t a l kelp consumer community (Velimirov et al., 1977 ; Field et al., 1977 and Newell et al., 1982). Therefore, most of t h e macrophyte production passes along t h e d e t r i t a l food chain t o t h e filter-feeders which form 72 % of t h e t o t a l faunal standing stock in t h e community. Of this dominant community, 46 % is t h e mussel, Aulacomya

e, 13 % sponges (Tehya spp.),

6 % ascidians (Pyura spp.) and t h e remaining 36 % a variety of o t h e r small filter-feeders.

Carnivores include t h e rock lobster, Jaws lallandii, anemones, t h e fish, Pachymetopan blochii and t h e isopod, Cirolana imposita (Shafir and Field, 1980). Bacteria and f a e c e s a r e components of t h e microbial loop in which d e t r i t a l m a t e r i a l is degraded by bacterial action into a f o r m t h a t c a n b e utilized by t h e filter-feeders. Newell (1965) showed t h a t b a c t e r i a can colonise f a e c a l m a t e r i a l causing an increase in f a e c a l nitrogen content, enhancing i t s nutritional value. R i c e (1982), however, found t h a t bacteria accounted f o r very l i t t l e of t h e nitrogen and t h e percentage increase in nitrogen was largely due to aging of t h e d e t r i t u s itself. There a r e t w o f a c t o r s which make b a c t e r i a and f a e c e s important components. Firstly, particles c a n b e refiltered numerous times, creating cyclic flows through t h e system. Secondly, bacteria a r e significant in converting dissolved organic m a t t e r into p a r t i c u l a t e form for consumption by filter-feeders which cannot utilize dissolved m a t t e r directly.

Kelp bed model. D a t a used for t h e model a r e based on a hypothetical kelp bed in t h e southern Benguela region stretching f o r 10 km of coast with a mean width of 500 m extending t o 20 rn depth, with a mean depth of 10 m as in Newell et al. (1982). All units a r e expressed per square metre.

19 The kelp-bed system may b e depicted in t e r m s of flows of energy, carbon or nitrogen, although in this model t h e currency is nitrogen, which Newell and Field (1983 b) found t o b e a potentially limiting nutrient in this system. The flow pathways show t h e feedback loops provided by animal faeces contributing to detritus formed of kelp material and bacteria shown separately, as indicated in Figure 1. a)

DOWNWELLING

MACROPHVTES

EXPORT

b)

--

UPWELLING

EXPORT 0

MACROPHVTES

-

+

1

FILTER-FEEDERS

CARNIVORES

FAECES

Fig. 1. Model depicting nitrogen flows in a kelp bed community. Primary production by macrophytes is partitioned into p a r t i c u l a t e (POM) and dissolved (DOM) components. Filter-feeders feed on d e t r i t u s consisting of POM, b a c t e r i a and animal faeces. Recycling of nitrogen via t h e feedback loop provided by f a e c e s is indicated by heavy lines. Fig. l a ) shows t h e model under downwelling conditions, when phytoplankton is imported with surface water f r o m offshore. Fig. l b r s h o w s t h e model under upwelling conditions when i t is assumed t h a t phytoplankton in t h e upwelling w a t e r is negligible and excess detritus is exported in s u r f a c e water. The system includes macrophytes (kelp, understorey and epiphytic algae), bacteria, filterfeeders, carnivores and animal faeces. G r a z e r s a r e negligible in biomass and o m i t t e d from t h e model. Phytoplankton is regarded as being outside t h e system and both macrophyte and phytoplankton primary production a r e assumed for simplicity to b e constant. Nitrogen flow through t h e system originates as macrophyte primary production in t h e form of particulate and dissolved organic m a t t e r (POM and DOM, respectively).

80

Phytoplankton p r i m a r y production is a v a i l a b l e in t h e kelp-bed under downwelling conditions (Fig. l a ) when i t is r e g a r d e d as being i m p o r t e d e v e n if t h e r e is no flow ("downwelling with z e r o transport"). Under upwelling conditions no phytoplankton is included in t h e m o d e l (Fig. lb), s i n c e cool upwelling w a t e r is norinally c l e a r and c o n t a i n s negligible phytoplankton (Field et al., 1980b). Nitrogen is lost f r o m t h e s y s t e m through w a t e r e x c h a n g e and c a r n i v o r e mortality. For

ease of description, t h e primary producers (phytoplankton, POM, DOM), b a c t e r i a and faeces will b e r e f e r r e d to as food c o m p o n e n t s in c o n t r a s t to t h e f i l t e r - f e e d e r s and c a r n i v o r e s which will b e r e f e r r e d to as c o n s u m e r components. T h e s e q u e n c e of c o m p u t a t i o n s for t h e food c o m p o n e n t s at e a c h t h e s t e p is : (1) w a t e r e x c h a n g e (loss/gain); (2) f i l t e r - f e e d e r consumption (loss) ; (3) b a c t e r i a l d e g r a d a t i o n (loss) ; a n d (4) primary production (gain). D e t a i l s of equations a r e given in Wickens and Field (1985). T h e c h a n g e s of t h e standing s t o c k s of food c o m p o n e n t s a t t h e end of e a c h day a r e calculated as : F i n a l s t a n d i n g s t a c k = initial standing s t o c k

_+

losses/gains d u e to w a t e r t r a n s p o r t

-

losses to f i l t e r - f e e d e r s - losses to b a c t e r i a + production

(1)

Water e x c h a n g e affects t h e standing s t o c k of e a c h food c o m p o n e n t and depends on t h e d i f f e r e n c e b e t w e e n t h e e x t e r n a l and i n t e r n a l standing stocks. T h e loss/gain through n e t t r a n s p o r t of t h e c o m p o n e n t is a hyperbolic f u n c t i o n of t h e e x c h a n g e r a t e , t a k i n g i n t o a c c o u n t t h e t i m e step. N e t t r a n s p o r t = (1 - + e x c h a n g e r a t e / t i m e s t e p ) x (internal

- e x t e r n a l standing stocks)

(2)

T h e f i l t e r - f e e d e r s a r e assumed to f i l t e r c o n s t a n t proportions of phytoplankton, b a c t e r i a , faeces and POM f r o m t h e w a t e r column, so t h a t losses to f i l t e r - f e e d e r s a r e c a l c u l a t e d for e a c h c o m p o n e n t as : Food f i l t e r e d = Standing s t o c k x e l e c t i v i t y index

(3)

B a c t e r i a l break-down of t h e remaining phytoplankton, faeces and POM o c c u r s a f t e r t h e f i l t e r - f e e d e r s h a v e f i l t e r e d o u t t h e i r ration. In addition t h e dissolved o r g a n i c m a t t e r (DOM), which c a n n o t b e utilised d i r e c t l y by f i l t e r - f e e d e r s , is c o n v e r t e d to b a c t e r i a l production. T h u s b a c t e r i a l consumption of e a c h c o m p o n e n t is r e p r e s e n t e d by : B a c t e r i a l consumption = Food a v a i l a b l e x d e g r a d a t i o n r a t e B a c t e r i a l production f r o m e a c h food c o m p o n e n t

(4) is calculated

by

multiplying

consumption by b a c t e r i a l nitrogen conversion efficiencies, a n d t h e s e a r e t h e n s u m m e d to c a l c u l a t e t h e t o t a l daily b a c t e r i a l production. T h e faeces c o m p o n e n t of t h e m o d e l

81

includes b o t h faeces and urine of f i l t e r - f e e d e r s and carnivores. T o t a l f a e c a l production of b o t h is t h e r e f o r e c a l c u l a t e d as t h e t o t a l a m o u n t of food c o n s u m e d by f i l t e r - f e e d e r s and c a r n i v o r e s minus t h a t assimilated, to which must b e added t h e a m o u n t s of nitrogen e x c r e t e d . M a c r o p h y t e and phytoplankton p r i m a r y production a r e a s s u m e d f o r simplicity to b e c o n s t a n t , b u t phytoplankton p r i m a r y production is only available in t h e kelp-bed under downwelling conditions. T h e c o n s u m e r standing s t o c k s (filter-feeders and carnivores) c h a n g e according to t h e balance b e t w e e n n e t g r o w t h a n d motality. T h i s is c a l c u l a t e d as follows :

Final standing stodc = i n i t a l standing s t o c k + food assimilated - nitrogen e x c r e t e d - m o r t a l i t y

(5)

T h e initial a n d final standing s t o c k s a r e t h o s e c a l c u l a t e d f o r t h e beginning and end of o n e day, respectively. An assimilation c o n s t a n t f o r e a c h of t h e f i l t e r e d food c o m p o n e n t s i s used to c a l c u l a t e t h e a m o u n t of food t h a t is a s s i m i l a t e d i n t o filter-feeder standing stock. T h e biomass of f i l t e r - f e e d e r s t h a t a r e consumed by c a r n i v o r e s is c a l c u l a t e d as :

Filter-feeder mortality

I

f i l t e r - f e e d e r biomass x c a r n i v o r e consumption x c a r n i v o r e biomass

(6)

C a r n i v o r e m o r t a l i t y i s c a l c u l a t e d as :

Carnivore mortality = ( c a r n i v o r e biomass)2 x c o n s t a n t

(7)

t h u s allowing f o r curvilinear density dependence. E x c r e t o r y losses f o r both f i l t e r - f e e d e r s and c a r n i v o r e s a r e a s s u m e d to b e c o n s t a n t proportions of food consumed.

Standing stocks. T h e nitrogen e q u i v a l e n t s of biomass of phytoplankton, b a c t e r i a , f i l t e r - f e e d e r s and c a r n i v o r e s w e r e c o n v e r t e d f r o m c a l o r i f i c values, as d e t a i l e d by Wickens and Field (1985). F i l t e r - f e e d e r s produce a n e s t i m a t e d 113 g N m-2 y-1 of faeces and urine combined (Newell a n d Field, 1983 b), w i t h a corresponding f i g u r e of 15,3 g N m-2 y-1 f o r c a r n i v o r e s (Newell a n d Field, 1983a). S u m m i n g t h e s e a n d c o n v e r t i n g t h e m to daily production, a v a l u e of 0.35 gN m-2 d-1 f o r faeces was used in units of gN m-2 to s t a r t e a c h simulation. T h e c a l c u l a t e d v a l u e s of standing s t o c k s of faeces and POM a r e c o n f i r m e d by t h o s e m e a s u r e d in s o u t h e r n Benguela kelp beds by S t u a r t a n d Klumpp (1984). Standing s t o c k s of t h e food c o m p o n e n t s in w a t e r o u t s i d e t h e kelp-bed s y s t e m w e r e set to z e r o under upwelling conditions when cool, c l e a r w a t e r e n t e r s t h e k e l p bed f r o m below t h e p h o t i c zone. Under downwelling conditions phytoplankton b e c o m e s a n i m p o r t a n t c o m p o n e n t of t h e suspended m a t t e r , and t h e v a l u e of 0.237 gN m-2 w a s used, equivalent to t h e m e a n daily phytoplankton production, c a l c u l a t e d f r o m Newell a n d F i e l d (1983 a).

82

Primary production. T h e yearly production values for t h e primary producers were obtained from Newell and Field (1983 a), and converted t o daily production values. During upwelling conditions when i t is assumed t h a t no phytoplankton is present in t h e kelp bed and none is imported, production is set to zero, whereas under downwelling conditions, daily production was calculated to b e 0.237 gN m-2 d-1.

Biological variables. Electivity indices (or filtration efficiencies) for t h e four suspended food components ( I ; 0.8 ; 0.7 ; 0.7 for phytoplankton, bacteria, faeces and POM respectively) w e r e t h e values found to b e most realistic by Wulff and Field (1983) based on eco-physiological work on kelp bed mussels (Griffiths and King, 1979a and b, and Stuart, 1982). Carbon assimilation efficiencies of mussels have been found t o b e 40-50 % f o r kelp POM, 50 % for d e t r i t u s and 67-70 % for bacteria (Stuart et al.,

1982 a, b). No

measurements have been made of t h e corresponding nitrogen assimilation efficiencies, but calculations indicate t h a t they a r e likely to b e higher than for carbon (Newell and Field, 1983b), and values of 0.8 ; 0.8 ; 0.6 ; 0.5 w e r e assumed for phytoplankton, bacteria, POM and f a e c e s respectively. T h e sensitivity t h e model to these assumptions was checked by means of sensitivity analyses. Bacterial degradation r a t e s of components were given t h e values as calculated for carbon by Wulff and Field (19831, namely 0.033 % d-1 f o r phytoplankton, POM and faeces and 0.1 % d-1 of DOM a r e degrated by microbial action. Bacterial carbon conversion efficiencies w e r e shown to b e between 10 % (Newell and Lucas, 1981) during downwelling, and a possible 40 % (Linley and Newell, 1985) during upwelling conditions. Considerably higher nitrogen conversion efficiencies (NC) have been measured for bacteria such as 94.2% for conversion of kelp POM (Koop et al., 1982) and 8 3 % for faeces (Stuart et al., 1982a). In order to obtain a single value f o r nitrogen, t h e proportion of faeces concentration to kelp concentration was calculated and e a c h of t h e efficiency values weighted accordingly. T h e r a t i o of suspended f a e c e s t o kelp POM is 24:1, giving a n e t conversion efficiency of 85 %. Filter feeder- and carnivore- related constants were calculated from t h e l i t e r a t u r e but they were a l t e r e d whilst tuning t h e model (see Wickens and Field, 1985 for details). T h e final tuning was done on t h e carnivore mortality rate. In addition, t h e standing stocks of t h e filter-feeders and carnivores w e r e initiated t o values i) below and ii) above their calculated values, in a closed system with no water transport, and tuned to reach their original values a f t e r several years. Thus t h e model is tuned t o maintain stability of t h e primary and secondary consumers in a closed system. T h e model was programmed in GBASIC for a n Apple I1 Computer.

KELP-BED MODEL OUTPUT T h e nitrogen flow model was t e s t e d for sensitivity to varying t h e "biological parameters" of filter-feeder electivity and assimilation efficiency. Results detailed in Wickens & Field (1985) show t h a t t h e model is not sensitive to varying biological p a r a m e t e r s when t h e system is closed (i.e.

no w a t e r transport through upwelling or

83

downwelling), and t h a t t h e m o d e l is m o s t s e n s i t i v e to varying feeding e f f i c i e n c i e s under upwelling conditions when suspended m a t t e r i s t r a n s p o r t e d by w a t e r movement. A c o m p a r a b l e simulation model of e n e r g y flow through a Benguela kelp-bed (Wulff & Field, 1983) showed t h a t w a t e r e x c h a n g e o n t h e s c a l e m e a s u r e d in t h e f i e l d by F i e l d et al. (1980b) h a s a f a r g r e a t e r e f f e c t on t h e p o t e n t i a l g r o w t h r a t e of f i l t e r f e e d e r s t h a n r e a l i s t i c r a n g e s of biological variables such as f e e d i n g efficiencies. Fig. 2 d e p i c t s t h e r e l a t i v e proportions of d i f f e r e n t suspended food c o m p o n e n t s available to f i l t e r - f e e d e r s with d i f f e r e n t r a t e s of upwelling (transporting s u r f a c e w a t e r o u t of t h e s y s t e m ) and downwelling ( t r a n s p o r t i n g s u r f a c e w a t e r horizontally i n t o t h e nearshore system). When modelled as a "closed system" (i.e. w a t e r t r a n s p o r t ) b a c t e r i a (including o t h e r micro-organisms) c o n t r i b u t e to t h e nitrogen standing s t o c k of suspended material. Under t h e s e c i r c u m s t a n c e s recycling of faeces i s a n i m p o r t a n t process.

a ) UPWELLING

b) DOWNWELLING BACTERIA

BACTERIA

100

50

c

FAECES PHYTOPLANKTON

I

o i

1

5 6 ;

WATER COLUMN TURNOVERS ( d a j ' )

Fig. 2. Model o u t p u t showing t h e p e r c e n t a g e of d i f f e r e n t food c o m p o n e n t s available to f i l t e r f e e d e r s w i t h varying w a t e r t r a n s p o r t (0-7 w a t e r column t u r n o v e r s p e r day). T h e nitrogen c o n t e n t of a l l food a v a i l a b l e t o f i l t e r f e e d e r s is shown below, a n d is much higher when t h e s y s t e m i s "closed" (0 turnovers). Fig. 2a) shows food proportions and q u a n t i t i e s under upwelling conditions when a l l food i s derived f r o m macrophytes. Fig. 2b) d e p i c t s downwelling conditions when phytoplankton i s a n additional c o m p o n e n t ( A f t e r Wickens and Field, 1985).

84 T h e r e a r e t w o c a t e g o r i e s of "closed system" : under upwelling conditions b u t w i t h no w a t e r t r a n s p o r t , i t i s a s s u m e d t h a t upwelling h a s r e c e n t l y c e a s e d leaving c l e a r w a t e r in t h e s y s t e m with negligible phytoplankton c o n t e n t a n d t h e only s o u r c e of p r i m a r y production is derived f r o m macrophytes. O n t h e o t h e r hand, under downwelling conditions with no w a t e r transport, phytoplankton in t h e kelp-bed w a t e r column m a k e a n additional contribution to t h e p r i m a r y production a n d t h e total supply of food to f i l t e r - f e e d e r s is g r e a t e r . T h u s o n e m a y g e n e r a l i s e t h a t in closed n e a r s h o r e s y s t e m s , d e t r i t u s and f a e c e s cycling a r e likely to b e m o r e i m p o r t a n t t h a n in o p e n systems, a n d t h a t micro-organisms a t t a c h e d to f a e c e s and o t h e r d e t r i t u s e n h a n c e i t s food value to filter-feeders. T h e closed k e l p bed is probably a n a l a g o u s to closed b a y s and closed estuaries. Under conditions of continuous downwelling, phytoplankton i m p o r t e d f r o m o u t s i d e t h e s y s t e m b e c o m e s increasingly i m p o r t a n t w i t h f a s t e r r a t e s of w a t e r t r a n s p o r t (Fig. 2b) and at r a t e s f a s t e r t h a n o n e water-column t u r n o v e r per day, phytoplankton c o n t r i b u t e m o r e nitrogen to f i l t e r - f e e d e r f o o d t h a n rnacrophyte p a r t i c u l a t e m a t t e r or r e c y c l e d faeces.

SEASONAL INDEX

UPWELLING OOWNWELLING

- _- - - _-/_- _

-9

-----____

24

,

0

- - _ _. . . . . . . . . . . . . . . . . . . .

1

1

8

I

2

3

4

5

TIME (Years)

Fig. 3. Input a n d o u t p u t of k e l p bed model when r e a l i s t i c pulses of upwelling and downwelling w a t e r t r a n s p o r t a r e f e d i n t o t h e model. Upwelling prevails during s u m m e r a n d downwelling during winter. Model o u t p u t (below) shows t h a t t h e standing s t o c k of f i l t e r f e e d e r s declines f r o m t h e closed s y s t e m level and t h e n oscillates, increasing during winter and d e c r e a s i n g during t h e s u m m e r season. C a r n i v o r e s t a n d i n g s t o c k d e c l i n e s when t h e s y s t e m i s pulsed ( A f t e r Wickens a n d Field, 1985).

85

T h e s e conditions a r e analagous to exposed rocky s h o r e s in which t h e n e a r s h o r e s y s t e m is d o m i n a t e d by c o n s u m e r s which f e e d o n phytoplankton i m p o r t e d f r o m a v a s t reservoir of plankton o v e r t h e c o n t i n e n t a l shelf. H e r e t h e r a t e of t r a n s p o r t of food i n t o t h e nearshore systern is as i m p o r t a n t as t h e c o n c e n t r a t i o n of plankton in d e t e r m i n i n g t h e r a t e of food supply. When continuous upwelling i s modelled, t h e proportion of p a r t i c u l a t e m a t t e r d e r i v e d f r o m m a c r o p h y t e s i n c r e a s e s w i t h f a s t e r w a t e r transport, and recycling of faeces a n d m i c r o b e s b o t h c o n t r i b u t e l e s s to suspended p a r t i c u l a t e nitrogen. T h e horizontal t r a n s p o r t of w a t e r o u t of t h e n e a r s h o r e s y s t e m d e p l e t e s d e t r i t u s a n d faeces in t h e w a t e r column, and t h e c l e a r replacing w a t e r is d e f i c i e n t in phytoplankton. T h u s t h e food supply under upwelling conditions is paradoxically reduced. Such conditions of reduction in t h e n e a r s h o r e food supply by o f f s h o r e t r a n s p o r t m a y b e unique to upwelling regions, b u t t h e r e a r e analogies with e s t u a r i n e flushing while r i v e r s a r e in s p a t e , a n d l a t e r e n r i c h m e n t when c a l m conditions p r o m o t e p r i m a r y production. Under n a t u r a l conditions, n e a r s h o r e w a t e r t r a n s p o r t is seldom continuous b u t f l u c t u a t e s under t h e influence of winds and tides. In t h e s o u t h e r n Benguela region, t h e kelp beds a r e mainly influenced by t h e wind p a t t e r n which v a r i e s seasonally. Typically, t h e annual p a t t e r n c a n b e simplified to e i g h t m o n t h s of spring-summer conditions a n d four months of w i n t e r conditions (Wulff & Field, 1983). T h e s u m m e r p a t t e r n is typified by r e p e a t e d c y c l e s consisting of four d a y s of s t r o n g upwelling, t w o days of m o d e r a t e upwelling, followed by four d a y s of c a l m o r onshore conditions when m o d e r a t e downwelling occurs. In winter downwelling prevails, with a p a t t e r n of 18 d a y s of m o d e r a t e downwelling followed by t w o d a y s of m o d e r a t e upwelling. Sensitivity a n a l y s e s of d i f f e r e n t c o m b i n a t i o n s of pulse l e g t h a n d r a t e s of w a t e r t r a n s p o r t show t h a t t h e model is most sensitive to fast r a t e s of upwelling a n d less sensitive to pulse duration, with longer pulses d e c r e a s i n g t h e food supply m o r e t h a n s h o r t pulses (Wickens & Field, 1985). Fig. 3 shows t h e seasonal input of upwelling/downwelling described above, with t h e o u t p u t of f l u c t u a t i o n s in f i l t e r - f e e d e r and c a r n i v o r e nitrogen standing s t o c k s predicted by t h e

model. T h e filter-feeder standing s t o c k d e c l i n e s f r o m t h e closed s y s t e m s t e a d y - s t a t e value, to f l u c t u a t e annually a b o u t a slightly lower level, increasing during t h e winter (onshore t r a n s p o r t ) season, a n d d e c r e a s i n g during t h e s u m m e r (offshore t r a n s p o r t ) season. In t h e a b s e n c e of c a r n i v o r e s t h e f l u c t u a t i o n is much g r e a t e r (not shown).

MODEL OF A PLANKTON BLOOM Model of plankton blooms d a t e back to t h e classical work of Riley et al. (19491, l a t e r a d v a n c e d by S t e e l e (1974). T h e role of microbial organisms w a s i n c o r p o r a t e d in models of t h e pelagic plankton by P o m e r o y (19791, Pace et al. (1984) and F a s h a m (1985). T h e above works h a v e a l l t r e a t e d plankton c o m m u n i t i e s on a seasonal scale. In t h e model described below, w e d e a l w i t h a s i m p l e model of phytoplankton and micro-organisms c o n c e r n e d with t h e "microbial loop" ( A z a m et al., 1983) on a daily t i m e s c a l e a p p r o p r i a t e to t h e turnover t i m e of t h e s e organisms. T h e model is based on biomass d a t a c o l l e c t e d by Holligan et al.

86

(1984a, b) a n d Newell a n d Linley (1984) in t h e English C h a n n e l in July/August 1981. Methods of c o l l e c t i o n a r e given in d e t a i l by Holligan et al. (1984a). W e model t h e flows of nitrogen in a uniform mixed layer of t h e w a t e r column, w i t h t h e b o t t o m boundary of t h e model s y s t e m at t h e thermocline. T h e plankton c o m m u n i t y is divided i n t o f i v e c o m p o n e n t s on t h e basis of f u n c t i o n and s i z e (Fig. 4).

P r i m a r y producers a r e a l l grouped t o g e t h e r in o n e phytoplankton

c o m p a r t m e n t . T h e c o n s u m e r c o m p a r t e m e n t s c o m p r i s e b a c t e r i a , z o o f l a g e l l a t e s which p r e y o n b a c t e r i a ( A z a m et al., 19831, l a r g e r non-photosynthetic p r o t o z o a which a r e a s s u m e d t o f e e d on z o o f l a g e l l a t e s and phytoplankton, and micro- and meso-zooplankton ( c o l l e c t e d on 80 and 200 um mesh, respectively) which w e a s s u m e to f e e d o n b o t h phytoplankton a n d l a r g e protozoa. Macro-zooplankton and fish a r e excluded f r o m t h e model f o r lack of s u i t a b l e data. F o r simplicity, t h e a b i o t i c nitrogen pool is r e p r e s e n t e d by a single c o m p a r t m e n t , with no distinction being m a d e b e t w e e n urea, a m m o n i a a n d nitrate.

[ZOOPL

c

i

Fig. 4. C o m p a r t m e n t a l model describing t h e cycling of nitrogen in a planktonic c o m m u n i t y in t h e mixed l a y e r of a w a t e r column. Flow p a t h w a y s a r e r e p r e s e n t e d by a r r o w s a n d n u m b e r s which correspond to m a t h e m a t i c a l expressions d e s c r i b e d in T a b l e 2. T h e nitrogen pool r e p r e s e n t s a l l a b i o t i c nitrogen ( n i t r a t e , a m m o n i a and urea), a n d o t h e r c o m p a r t m e n t s r e p r e s e n t b a c t e r i a , zooflagellates, l a r g e r protozoa, a n d micromesozooplankton, giving off w a s t e p r o d u c t s (F+U). A r r o w s (13) and (14) d e p i c t s e d i m e n t a t i o n of zooplankton f a e c e s and phytoplankton cells, r e s p e c t i v e l y ( A f t e r Moloney et al., 1985).

T h e simulation m o d e l d e p i c t s t h e flows of nitrogen b e t w e e n t h e c o m p a r t m e n t s , a n d in p a r t i c u l a r is used t o i n v e s t i g a t e t h e effect of s e d i m e n t a t i o n of phytoplankton and zooplankton faeces o u t of t h e e u p h o t i c z o n e which is assumed t o b e 6 0 m d e e p ; however t h e d e p t h d o e s n o t a f f e c t t h e conclusions d r a w n f r o m t h e results.

Nitrogen fluxes in the plankton model T h e nitrogen flow p a t h w a y s a r e shown in Fig. 4 and numbered f r o m 1-14. All t h e flows a r e r e p r e s e n t e d by o n e of t h r e e expressions :

Specific uptake rate = k l Y (1

- e-k2

X)

(8)

w h e r e t h e r a t e of u p t a k e f r o m t h e donor C o m p a r t m e n t (X) to t h e r e c i p i e n t ( Y ) i n c r e a s e s a s y m p t o t i c a l l y to k l as X increases. T h e expression is similar to t h e Michaelis-Menten expression ( s e e Moloney et al., 1985 f o r details). T h e a s y m p t o t i c function w a s used to r e p r e s e n t nitrogen fluxes f r o m t h e a b i o t i c pool to phytoplankton (Dugdale and Goering, 1967 ; Eppley et al., 1969 ; M a c l s a a c a n d Dugdale, 1972). I t was also used to model t h e c o m p e t i t i v e u p t a k e of a b i o t i c nitrogen by b a c t e r i a (Monod, 19491, p r e d a t i o n o n b a c t e r i a by z o o f l a g e l l a t e s as d e s c r i b e d by F e n c h e l (1982), and t h e a s s u m e d m a n n e r of b a c t e r i a l u p t a k e of nitrogen f r o m moribund phytoplankton cells.

TABLE 2 Expressions used in t h e plankton m o d e l described in F i g u r e 4. N u m b e r s in p a r e n t h e s e s correspond to p a t h w a y s n u m b e r e d in Fig. 4. Equations below d e s c r i b e t h e r a t e of c h a n g e of e a c h c o m p a r t m e n t based on t h e expressions n u m b e r e d above. C o n s t a n t s K1 to K22 a r e given in T a b l e 3. F r o m Moloney et al. (1985).

dN/dt dP/dt dB/dt dF/dt dL/dt dZ/dt

= (4) + (6) + (9) + (12) - (13) = (1) - (3) - ( 8 ) - (11) - (14) = (2) + (3) - (4) - (5) = (5) - ( 6 ) - (7) = (7) + ( 8 ) - (9) - (10) = (10) + (1 I ) - (12)

- (1) - (2)

88

The second form of expression represents Lotka-Volterra interactions : Interaction rate = k3 X Y

(9)

in which t h e donor c o m p a r t m e n t (X) is consumed by t h e recipient (Y)at a r a t e determined by

k3.

The

Sour

pathways

micro/mesozooplankton,

from phytoplankton to large protozoa and to from large protozoa to micro/mesozooplankton and from

zooflagellates t o large protozoa have been modelled in this way. The third t y p e of expression represents release r a t e s of nitrogen : Release rate = k4 Z

(10)

which is used t o describe t h e release r a t e of f a e c e s and e x c r e t a from bacteria, zooflagellates, large protozoa and zooplankton compartments, where 2 is t h e r a t e of nitrogen ingestion by t h e respective compartments. TABLE 3 Values assigned to e a c h of t h e 22 constants in t h e 14 expressions used in t h e simulation model, and t h e relevant sources. T h e flow pathways a r e represented as donor

->

recipient

compartments. N = nitrogen pool ; P = phytoplankton ; B = b a c t e r i a ; F = zooflagellates ; L

= large protozoa ; 2 = micro-mesozooplankton ; (F+U) = f a e c e s and urine (nitrogen pool).

All units expressed in t e r m s of mg, m2 and/or d.

Const. KI K2 K3 K4 K5 Kg K7 K8 K9 K10

K11 K12 K13 K14 K15 16

K17 K18 K19

K20 K2 I K77

Compartments

-> P -> P -> B N -> B P -> B P -> B B -> (F+U) B -> (F+U) B -> F B -> F F -> (F+U) F -> (F+U) F -> L P -> L L -> (F+U) L -> (F+U) L -> 2 P -> 2 2 -> (F+U) 2 -> (F+U) Z(F+U) -> SED P -> SED N N N

Value 0.9 0.0025 0.9 0.0462 4.5 0.0008 0.07 0.63 17 0.0023 0.05 0.65 0.0043 0.026 0.05 0.65 0.002 0.002 0.05 0.40 0 - 0.3 0 - 0.3

Source Harrison et al. (1977) Harrison et al. (1977) By comparison with K1 By comparison with K I Indirectly from Newell and Linley (1984) Indirectly f r o m Newell and Linely (1984) By comparison with carbon respiratory losses Newell and Linley (1984) Fenchel(1982) Fenchel(1982) By comparison with carbon respiratory losses Newell and Linley (1984) Using d a t a f r o m Newell and Linley (1984) Using d a t a f r o m Newell and Linley (1984) By comparison with carbon respiratory losses Newell and Linley (1984) Assumed equal to K 1 8 Using d a t a f r o m Newell and Linley (1984) By comparison with carbon respiratory losses Newell and Linley (1984)

89

The nitrogen released s e r v e s as input to t h e a b i o t i c nitrogen pool as shown in Fig. 4. In addition, a s m a l l s t a r v a t i o n r a t e of 5 % of t h e standing s t o c k per day was a s s u m e d to b e e x c r e t e d in t h e a b s e n c e of food. T h e e q u a t i o n s used in t h e model a r e listed in T a b l e 2 a n d described in m o r e d e t a i l in Moloney et al. (1985). T a b l e 3 gives v a l u e s for t h e c o n s t a n t s employed in t h e equations, t o g e t h e r w i t h t h e flow p a t h w a y s and t h e s o u r c e s of information used to obtain t h e values. T h e main purpose of t h e m o d e l is to i n v e s t i g a t e t h e effect of v e r t i c a l t r a n s p o r t o u t of t h e mixed l a y e r by sedimentation. T h e t w o main c o m p a r t m e n t s t h a t m a y sustain such losses a r e phytoplankton a n d zooplankton f a e c e s ( J o i n t and Morris, 1982 ; Fasham, 1985). In general, phytoplankton sinking r a t e s (Smayda, 1970 ; Bienfang and Harrison, 1984) a r e slower t h a n t h o s e of zooplankton faeces ( J o i n t and Morris, 1982), and Honjo (1978) has suggested t h a t most of t h e v e r t i c a l flux in t h e w a t e r column is d u e to f a e c a l pellets (however, see also B i l l e t t et al.,

1983). T h e r a t e of

s e d i m e n t a t i o n is modelled

simplistically as a c o n s t a n t proportion of t h e t o t a l phytoplankton standing s t o c k and t h e zooplankton f a e c e s , t h e proportion being varied in d i f f e r e n t simulations. T h e set of d i f f e r e n t i a l e q u a t i o n s comprising t h e model a r e solved numerically by a second-order Runge-Kutta approximation, using a t i m e s t e p of 0.05 day. Sensitivity a n a l y s e s showed t h a t t h e model displayed t h e s a m e g e n e r a l behaviour and t r e n d s when t h e c o n s t a n t s used in t h e e q u a t i o n s w e r e varied up and down five-fold (Moloney et al., 1985). However, t h e m o d e l is sensitive to s t a r t i n g values of nitrogen standing s t o c k s in e a c h c o m p a r t m e n t . F o r simplicity, a l l s t a r t i n g v a l u e s of living c o m p a r t m e n t s w e r e set to low v a l u e s w i t h r e a l i s t i c r e l a t i v e proportions, w h e r e a s t h e simulations w e r e i n i t i a t e d with a n a b i o t i c nitrogen pool equivalent to 1.8 Pg.at.N.1-1

to

s i m u l a t e a n upwelling or mixing e v e n t with w a t e r f r o m below t h e thermocline. PLANKTON MODEL OUTPUT Fig. 5 shows t h e levels of nitrogen standing s t o c k in e a c h c o m p a r t m e n t during a series of 13-day simulations. T h e s i m u l a t i o n s a l l c o m m e n c e with a l a r g e upwelling o r mixing

e v e n t in which nitrogen-rich w a t e r w i t h v e r y small standing s t o c k s of phytoplankton, m i c r o b e s and zooplankton is mixed i n t o t h e p h o t i c z o n e f r o m below t h e pycnocline. In Fig. 5A t h e s y s t e m is modelled without a n y s e d i m e n t a t i o n of f a e c e s or phytoplankton o u t of t h e mixed layer. T h e a b i o t i c n i t r o g e n pool, which initially would c o n s i s t largely of n i t r a t e N, d e c l i n e s rapidly as a phytoplankton bloom develops and p e a k s a f t e r a b o u t 4.5 days, corresponding w i t h "new production" (Dugdale a n d Goering, 1967). T h e subsequent decline in phytoplankton biomass is d u e to t h e c o m b i n e d e f f e c t s of g r a z i n g by herbivores and depletion of t h e a b i o t i c nitrogen pool by b o t h phytoplankton and bacteria. T h e b a c t e r i a r e a c h a peak a f t e r s o m e 5.3 days, followed by z o o f l a g e l l a t e s (5.7 days), and larger h e t e r o t r o p h i c p r o t o z o a (7 days). T h e phytoplankton standing s t o c k r e c o v e r s a f t e r 6.5 days when t h e zooplankton h a v e begun to increase. Zooplankton r e a c h m a x i m u m biomass a f t e r a b o u t 9 d a y s a n d t h e r e a f t e r r e m a i n fairly dominant ; t h e zooplankton d e v e l o p m e n t

90

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1200.

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1200

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TIME ( D A Y S ) Fig. 5 . Simulation r e s u l t s showing t h e d e v e l o p m e n t and d e c a y of a phytoplankton bloom and t h e c h a n g e s in t h e standing s t o c k s of t h e a s s o c i a t e d h e t e r o t r o p h i c c o m p a r t m e n t s with increasing daily r a t e s of s e d i m e n t a t i o n o u t of t h e mixed layer. (A) no s e d i m e n t a t i o n , ( 8 ) 10% s e d i m e n t a t i o n , ( C ) 20% s e d i m e n t a t i o n , (D) 30% sedimentation. A x e s give nitrogen standing s t o c k s (mg.N.m-2 i n t e g r a t e d o v e r 60 m d e p t h ) and t h e nitrogen pool c o n c e n t r a t i o n (pg.at.1-1) ( A f t e r Moloney et al., 1985).

91

corresponds to t h e d e c l i n e of t h e phytoplankton bloom, as h a s been observed in n a t u r e (Parsons et al., 1977 ; Sorokin, 1977). T h e phytoplankton bloom persists for a b o u t 11 days f r o m s t a r t to finish, b e f o r e being g r a z e d down by zooplankton and l a r g e protozoa. Blooms of c o m p a r a b l e d u r a t i o n h a v e been observed in t h e N o r t h S e a (Weichart, 1980 ; G i e s k e s and Kraay,

1980) and in t h e C e l t i c S e a ( F a s h a m et al.,

1983). T h e r e c o v e r y of t h e

phytoplankton bloom at day 6 when t h e microbial standing s t o c k s a r e high a n d t h e nitrogen pool increasing s u g g e s t s t h a t microbes and indirectly responsible. A t t h i s stage, phytoplankton g r o w t h is a t t r i b u t e d to "regenerated productionof (Dugdale a n d Goering, 1967; Y e n t s c h et al., 1977). T h e a b i o t i c nitrogen pool r e m a i n e d artificially high during t h e simulation, which m a y b e a t t r i b u t e d to t h e closed model, w i t h no losses of nitrogen f r o m t h e mixed l a y e r by migration, mixing, diffusion o r sedimentation.

To i n v e s t i g a t e t h e v e r t i c a l losses of nitrogen f r o m t h e mixed l a y e r by s e d i m e n t a t i o n , w e s i m u l a t e d daily losses of 10 %, 20 % a n d 30 % of t h e phytoplankton standing s t o c k and zooplankton faeces. Figs 5 8 - D show t h e r e s u l t s of increasing r a t e s of sedimentation. S e v e r a l t r e n d s emerge. T h e nitrogen pool declines with increasing r a t e s of sedimentation and a l l t h e standing s t o c k p e a k s a r e lower. I t i s a l s o n o t a b l e t h a t with increasing r a t e s of s e d i m e n t a t i o n t h e "new production" phytoplankton peak is s m a l l e r and t h e second " r e g e n e r a t e d production" peak a s s u m e s g r e a t e r r e l a t i v e i m p o r t a n c e in t h e whole bloom although i t is n o higher t h a n when t h e r e is no sedimentation. Finally, Fig. 5 shows t h a t a l l t h e standing s t o c k peaks t e n d to b e f l a t t e r and s h i f t e d to t h e r i g h t with increasing sedimentation, b e c a u s e t h e phytoplankton biomass is eroded continuously by s e d i m e n t a t i o n resulting in a d e l a y in d e v e l o p m e n t of t h e bloom. T h e smaller biomass of phytoplankton in t h e upper mixed l a y e r results in less d e m a n d o n t h e nitrogen pool, allowing a s m a l l e r biomass to grow at a f a s t e r m a s s s p e c i f i c r a t e . Thus a f t e r 1 3 d a y s at a s e d i m e n t a t i o n r a t e of 30 % per day, c u m u l a t i v e phytoplankton production w a s 7 7 % of t h a t when no s e d i m e n t a t i o n w a s modelled. V e r t i c a l t r a n s p o r t of phytoplankton and zooplankton faeces o u t of t h e mixed l a y e r t h u s a p p e a r s to maintain t h e phytoplankton in a state of e x p o n e n t i a l growth, with r e d u c e d biomass b u t f a s t e r turnover r a t e (P/B).

CONCLUSION Both of t h e models p r e s e n t e d h e r e a r e based on t h e flow of nitrogen through e c o s y s t e m s : in o n e case a nearshore kelp-bed s y s t e m a n d in t h e o t h e r a g e n e r a l offshore plankton community. T h e k l e p bed model w a s developed to e x p l o r e t h e hypothesis that nitrogen flow i s a f f e c t e d by horizontal w a t e r t r a n s p o r t in shallow w a t e r m a r i n e systeins ; h e r e w a v e a c t i o n o r mixing associated with h o r i z o n t a l t r a n s p o r t a r e likely t o r e t a i n nitrogen in t h e p h o t i c z o n e a n d t h e b e n t h i c c o m m u n i t y is of fixed location so t h a t boundaries of t h e s y s t e m c a n b e e a s i l y defined. In pelagic systems, on t h e o t h e r hand, t h e c o m m u n i t y t e n d s to move horizontally with w a t e r in t h e mixed layer, and v e r t i c a l t r a n s p o r t i n t o and o u t of t h e mixed l a y e r is a n i m p o r t a n t f e a t u r e of t h e s y s t e m dynamics.

T h e m o d e l of n i t r o g e n flow through a k e l p bed shows t h a t w a t e r t r a n s p o r t h a s a s t r o n g influence o n n i t r o g e n flow through b e n t h i c c o m m u n i t i e s which a r e d o m i n a t e d by filterfeeders. T h e m a g n i t u d e of w a t e r t r a n s p o r t m a y b e such t h a t budgets o r models t h a t a r e based o n t h e a s s u m p t i o n t h a t t h e s y s t e m i s closed m a y b e misleading. W a t e r t r a n s p o r t h a s a n e f f e c t o n t h e sensitivity of t h e model to biological variables, such as feeding e f f i c i e n c i e s a n d assimilation efficiencies. When modelled as a closed s y s t e m , t h e k e l p bed c o m m u n i t y is shown to depend largely upon d e t r i t u s and t h e microbial c o m m u n i t y a s s o c i a t e d w i t h t h e breakdown of m a c r o p h y t e d e b r i s and faeces. This is probably analagous to t h e s i t u a t i o n in closed b a y s and e s t u a r i e s with l i t t l e n e t outflow. Under downwelling conditions in t h e kelp-bed, phytoplankton is t r a n s p o r t e d i n t o t h e s y s t e m in a manner a n a l a g o u s to t h a t on exposed i n t e r t i d a l rocky s h o r e s (Field, 1983). Under t h e s e conditions, t h e b e n t h o s f e e d s mainly o n phytoplankton and t h e r a t e of supply of phytoplankton f r o m t h e l a r g e o f f s h o r e reservoir is as i m p o r t a n t as t h e r a t e of phytoplankton p r i m a r y production. If o n e models t h e flow of nitrogen under upwelling conditions when cool, c l e a r w a t e r i s brought i n t o t h e shallow w a t e r f r o m below t h e p h o t i c zone, t h e r e is n e t loss of o r g a n i c nitrogen f r o m t h e system. A diminished food supply of d e t r i t u s is t h u s a v a i l a b l e to t h e b e n t h i c c o m m u n i t y , so t h a t a s m a l l e r s t a n d i n g s t o c k would b e m a i n t a i n e d if s u c h upwelling w a s m a i n t a i n e d continuously. T h i s m a y b e unique to n e a r s h o r e upwelling systems, a l t h o u g h t h e r e a r e s i m i l a r i t i e s w i t h e s t u a r i e s s u b j e c t e d to flushing. In d e e p e r w a t e r s phytoplankton blooms h a v e been modelled on a s e a s o n a l s c a l e (e.g. Riley et al., 1949 ; S t e e l e , 1974 ; P a c e et al., 1984 ; Fasham, 19851, w h e r e a s w e h a v e p r e s e n t e d a m o d e l w i t h a s h o r t t i m e - s c a l e to e x p l o r e t h e build-up a n d d e c l i n e of phytoplankton a n d microbial populations a f t e r a n upwelling o r mixing e v e n t . T h e simulation of c h a n g e s in a plankton c o m m u n i t y on a daily t i m e s c a l e shows t h a t i t is likely t h a t v e r y d i f f e r e n t biomasses of phytoplankton, b a c t e r i a , flagellates, l a r g e r protozoa, and micro/mesozooplankton d o e x i s t at d i f f e r e n t s t a g e s in t h e d e v e l o p m e n t of a bloom a f t e r n i t r a t e e n r i c h m e n t . T h e model d e p i c t s a c o n t i n u u m of successive s t a g e s in which phytoplankton, microbes, a n d zooplankton e a c h f o r m m a j o r c o m p o n e n t s of t h e biomass a t d i f f e r e n t t i m e s , o f f e r i n g a n explanation f o r t h e c o n f l i c t i n g r e s u l t s o b t a i n e d f r o m s a m p l e s t a k e n at sea. Most o b s e r v a t i o n s h a v e been v e r t i c a l profiles a t o n e o r m o r e s t a t i o n s , giving a n i n s t a n t "snapshot" of t h e r e l a t i v e q u a n t i t i e s of d i f f e r e n t c o m p o n e n t s of t h e system. T h e l a r g e p e a k s of b a c t e r i a l s t a n d i n g s t o c k s s i m u l a t e d f r o m a b o u t d a y 6 in s o m e s i m u l a t i o n s o n t h e m o d e l a g r e e w i t h v a l u e s recorded by Sorokin (1975, 1981) a n d S i e b u r t h

et al. (1978). T h e s i m u l a t e d b a c t e r i a l populations e v e n t u a l l y d e c l i n e d u e to g r a z i n g by z o o f l a g e l l a t e s ( s e e Fenchel, 1982 ; A z a m et al., 1983). When s e d i m e n t a t i o n of faeces a n d phytoplankton i s s i m u l a t e d in t h e model, micro- a n d meso-zooplankton a s s u m e g r e a t e r i m p o r t a n c e f r o m a b o u t d a y 9 onwards. This may correspond to t h e quasi-steady state conditions which o c c u r in t e m p e r a t e s t r a t i f i e d w a t e r s during t h e late summer. T h u s d i f f e r e n t c o m p o n e n t s d o m i n a t e t h e plankton c o m m u n i t y at d i f f e r e n t successional stages

93

of t h e model plankton bloom in t h e upper l a y e r s of t h e open sea w a t e r column, a n d t h e

dominance of t h e c o m p o n e n t s in t h e succession is influenced by t h e a m o u n t of m a t e r i a l which s e d i m e n t s through t h e pycnocline and o u t of t h e s u r f a c e layers. I t is hoped t h a t this c o n t r i b u t i o n will a d d emphasis to t h e i m p o r t a n c e of w a t e r

transport a s a f a c t o r influencing t h e s t r u c t u r e and f u n c t i o n i n g of n e a r s h o r e m a r i n e ecosystems, and h e l p resolve t h e c o n t r o v e r s y over t h e s i g n i f i c a n c e of micro-organisms in t h e plankton c o m m u n i t y , which v a r i e s according t o t h e t i m e elapsed a f t e r a n e n r i c h m e n t e v e n t a n d t h e a m o u n t of m a t e r i a l sediinenting o u t of t h e s u r f a c e layer. Both of t h e s e a r e problems which h a v e been difficult t o resolve by d i r e c t observation and m e a s u r e m e n t in t h e sea. S i m p l e simulation models, based o n hypotheses a b o u t t h e processes involved, produce o u t p u t which a p p e a r to b e c o n s i s t e n t with observations, t h u s helping to f u r t h e r understanding.

ACKNOWLEDGMENTS W e thank M.I. Lucas, M.O. Bergh, R.C. Newell, F.V. VYulff and S.J. P a i n t i n g for help

and discussion in developing t h e models, a n d S. Tolosana a n d D. G i a n a k o u r a s for help in preparing t h e manuscript. T h e work w a s supported by t h e South A f r i c a National C o m m i t t e e for O c e a n o g r a p h i c R e s e a r c h as p a r t of t h e S y s t e m s Analysis P r o j e c t of t h e Benguela Ecology P r o g r a m m e .

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THE RETENTION OF NUTRIENTS (C, N, PI, HEAVY METALS (Mn, Cd, Pb, Cu), AND PETROLEUM HYDROCARBONS IN NARRAGANSETT BAY

S.W. NIXON, C.D. HUNT a n d B.L. NOWICKI G r a d u a t e School of Oceanography, University of Rhode Island, Kingston, RI 0288 I (U.S.A.)

ABSTRACT

Nixon, S.W., Hunt, C.D. a n d Nowicki, R.L., 1986. T h e r e t e n t i o n of n u t r i e n t s (C, N , P), h e a v y rnetals (Mn, Cd, Pb, Cu), and p e t r o l e u m hydrocarbons in N a r r a g a n s e t t Bay. In: P. L a s s e r r e and J.M. Martin (eds), Riogeochemical P r o c e s s e s a t t h e Land-Sea Boundary. Elsevier, A m s t e r d a m . Various workers h a v e assembled r e l a t i v e l y c o m p l e t e a s s e s s m e n t s of t h e annual input of t h e m a j o r biological n u t r i e n t s (C, N, P), c e r t a i n heavy m e t a l s !Mn, Cd, Pb, Cu), and p e t r o l e u m hydrocarbons to N a r r a g a n s e t t Bay. O t h e r s t u d i e s h a v e developed i n v e n t o r i e s of t h e a m o u n t s of t h e s e m a t e r i a l s in t h e s e d i m e n t s of t h e Bay. W e h a v e brought t h e s e d a t a t o g e t h e r with i n f o r m a t i o n on s e d i m e n t a c c u m u l a t i o n r a t e s in t h e Bay to d e t e r m i n e t h e d e g r e e to which t h i s o n e e s t u a r y s e r v e s as a sink f o r d i f f e r e n t t y p e s of m a t e r i a l s in t h e i r passage b e t w e e n land and t h e c o a s t a l ocean. I t a p p e a r s t h a t N a r r a g a n s e t t Bay r e t a i n s less t h a n 5 % of t h e n u t r i e n t s , less t h a n 10 % of t h e Mn, a n d p e r h a p s 15-30 % of t h e C d t h a t is input to t h e s y s t e m e a c h year. T h e r e m o v a l of C u (70-95 %) a n d P b (80-100 %) i s m u c h m o r e e f f e c t i v e . S o m e w h e r e between 25-65 % of t h e p e t r o l e u m hydrocarbons e n t e r i n g t h e Bay r e m a i n in t h e sediments. T h e s e e s t i m a t e s a r e in a g r e e m e n t with t h e behavior of t h e d i f f e r e n t m a t e r i a l s in sedimentwater f l u x m e a s u r e m e n t s a n d in e x p e r i m e n t s using t h e l a r g e MERL mesocosms. INTRODUCTION

Because of t h e i r position b e t w e e n t h e upland d r a i n a g e and t h e sea, i t is r e a s o n a b l e to suppose t h a t e s t u a r i e s h a v e a l w a y s b e e n s u b j e c t e d to a p p r e c i a b l e loadings of n u t r i e n t s and other w e a t h e r i n g p r o d u c t s f r o m t h e i r watersheds. However, various o t h e r f e a t u r e s of e s t u a r i e s (fisheries, c o m m e r c e , etc.) h a v e a l s o long m a d e t h e m a t t r a c t i v e f o r human s e t t l e m e n t , w i t h t h e r e s u l t t h a t a f a r l a r g e r a n t h r o p o g e n i c input of nutrients, heavy metals and o t h e r p o l l u t a n t s h a s o f t e n b e e n superimposed o n a n a l r e a d y rich background. The anthropogenic

input

of

nutrients

probably

accelerated greatly

with

the

development of urban w a t e r systems, indoor plumbing, and t h e " w a t e r closet". In his history of pollution in t h e t i d a l T h a m e s , Wood (1982) n o t e d t h a t a f t e r 1815 i t was "mandatory to c o n n e c t cesspools to t h e sewers", a n d t h a t indoor flush t o i l e t s w e r e "widely used in London a f t e r 1830". In t h e United S t a t e s , a b o u t one-third of urban households had running w a t e r a n d flush t o i l e t s by 1830 (Melosi, 1981) and b e t w e e n 1906-1915, Baltimore, Maryland b e c a m e , " t h e last of t h e g r e a t A m e r i c a n c i t i e s to build a modern s y s t e m for

100

sewage" (Capper, P o w e r and Shivers, 1983). While urban a r e a s always drained t h e i r open sewers and road runoff d i r e c t l y to a d j a c e n t w a t e r s , t h e new c i t y s e w e r s y s t e m s b e c a m e a very large point s o u r c e of n u t r i e n t s and o t h e r pollutants t h a t had f o r m e r l y been p u t on or under t h e ground. I t is also likely t h a t n u t r i e n t loadings to e s t u a r i e s h a v e been g r e a t l y increased by t h e

use of c o m m e r c i a l f e r t i l i z e r s in agriculture. In t h e U n i t e d S t a t e s , t h e use of inorganic fertilizer h a s i n c r e a s e d approximately exponentially s i n c e 1860, with t h e result t h a t o v e r

500 t i m e s m o r e nitrogen w a s applied to t h e fields in 1980 t h a n in 1880 (Fig. 1).

I -

NITROGEN ( t o t a l N consumption 1850-1983~216x106Ions1

-

c lo3 1 1850

I900

1950

YEAR

Fig. 1. H i s t o r i c a l u s e of nitrogen and phosphorus f e r t i l i z e r in t h e United S t a t e s . D a t a f r o m t h e F e r t i l i z e r Institute, Washington, DC. T h e i n c r e a s e d input of n u t r i e n t s and o t h e r p o l l u t a n t s to r i v e r s and e s t u a r i e s m a y h a v e been e x a c e r b a t e d by t h e d e s t r u c t i o n of l a r g e a r e a s of f r e s h a n d s a l t w a t e r wetlands, o f t e n for a g r i c u l t u r e (Figs 2 and 3). F o r example, in t h e United S t a t e s , in 1950 t h e r e w e r e o v e r

100,000,000 a c r e s (over 40 x 106 h e c t a r e s ) of w e t l a n d s being drained (Shaw a n d Fredine, 1956). T h e i m p a c t of w e t l a n d s o n w a t e r quality is d i f f i c u l t to q u a n t i f y (Nixon, 1980), b u t

t h e f e w annual m a s s b a l a n c e s t u d i e s t h a t a r e available i n d i c a t e t h a t t h e s e e n v i r o n m e n t s s e r v e as sinks (though of widely varying s t r e n g t h ) f o r n u t r i e n t s (Fig. 4) a n d a number of

101 heavy metals (see reviews by Giblin, 1982, Nixon and Lee, i n press). As the fresh water swamps and marshes lining streams and rivers have been converted t o urban and agricultural land, potential sites for removing sediments and nutrients before they could reach the estuary have been converted i n t o strong sources of additional input.

COASTAL WETLAND LOSS IN U.S.

%

\

I

v)

z 0

'a 0

1922

'

1954

Fig. 2. Rates of coastal wetland loss in the United States. From Gosselink and Baurnann (1980).

1974

YEARS

Fig. 3. Wetlands in "drainage enterprises" in the United States in 1950. Each dot represents 10,000 acres (over 4000 hectares). From Shaw and Fredine (1956).

102

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I

0

SWGMP5

A HOLS 0 S G L T - ERGCKISH MGRSH

A *

FRESH MGRSH

0

a

10

I000

I00

N INPUT, g m 2 y l

a7

2 -4:2 I-

experimentol enrichment

Q 81 *O

0

a

5-A

a

i4:L a

w

27

, a

31

1

1

a a 1

1

1

I

I

I

I

.:I,,,

10

10

I

I

t I , , t , ,

100

P INPUT, g m 2 y 1

Fig. 4. T h e r e s u l t s of annual m a s s b a l a n c e s t u d i e s for a number of f r e s h and s a l t w a t e r wetland a r e a s receiving d i f f e r e n t l e v e l s of n u t r i e n t input. F r o m Nixon a n d L e e (in press). A s a r e s u l t of

t h e i r geographical c i r c u m s t a n c e s a n d t h e s e various historical

developments, m a n y e s t u a r i e s a p p e a r to b e a m o n g t h e m o s t heavily loaded

ecosystems

in t h e world in t e r m s of t h e a m o u n t of nitrogen and phosphorus r e c e i v e d per u n i t a r e a (Fig. 5). E v e n t h e f a r m fields of t h e A m e r i c a n c o r n b e l t d o n o t r e c e i v e t h e f e r t i l i z a t i o n c o m m o n to m a n y estuaries, a n d t h e n u t r i e n t inputs to n a t u r a l t e r r e s t r i a l s y s t e m s and fresh w a t e r s a p p e a r to b e o r d e r s of m a g n i t u d e lower. T h e r e a r e f e w e r d a t a a v a i l a b l e o n t h e inputs of m e t a l s and o t h e r p o l l u t a n t s to e s t u a r i e s , b u t t h e e v i d e n c e at hand suggests t h a t t h e d e l i v e r y of heavy m e t a l s to e s t u a r i e s m a y e q u a l o r e x c e e d t h a t d e p o s i t e d f r o m t h e a t m o s p h e r e o n heavily industrialized urban a r e a s and e x c e e d t h a t deposited o n r u r a l t e r r e s t r i a l e c o s y s t e m s by o r d e r s of m a g n i t u d e (Fig. 6). Given s u c h l a r g e inputs (even though t h e i r e x a c t values must b e s u b j e c t to considerable v a r i a t i o n a n d uncertainly), i t i s i m p o r t a n t to d e t e r m i n e how much of t h e n u t r i e n t s a n d o t h e r m a t e r i a l s which e n t e r a n e s t u a r y pass through to t h e c o a s t a l ocean. Since i t i s e x t r e m e l y d i f f i c u l t to o b t a i n d i r e c t m e a s u r e m e n t s of t h e n e t t r a n s p o r t of dissolved o r p a r t i c u l a t e s u b s t a n c e s at t h e m o u t h of t h e e s t u a r y , t h e m o s t promising

103

lo41 -

%

In

al

1037

-

TYNE ESTUARY

8 AGRICULTURE, U S. 1979-83

0

THAMES

A TERRESTRIAL SYSTEMS + LAKES V WETLANDS ESTUARINE and COASTAL MARINE SYSTEMS

0 ESTUAF

NORT;

/

DELAWARE

FRANCISCO BAY MERL EXPERIMENTAL MOBILE GRADIENT I, 2 , 4 , 8 , 1 6 , 3 2 X PATUXEN T BAY ESTUARY WADDEN PAMLICO POTOMAC SEA

4

-

d

-

TUNDRA, AK

BOG. MA

SALT MARSH, MA

Fig. 5 . Approximate present day annual r a t e s of input of nitrogen and phosphorus per unit a r e a of terrestrial, lake, wetland, agricultural and estuarine or coastal marine ecosystems. Systems for which t h e input of only one e l e m e n t was reported a r e shown. along t h e appropriate axis. T h e estuarine a r e a s with arrows a t t a c h e d only include dissolved inorganic nutrients. Their r a t e s would b e increased considerably if t o t a l N and P w e r e used. Atmospheric and wetland inputs from numerous sources summarized by Nixon and L e e (in press) ; Canadian Lakes from Schindler (1974) ; Lakes Superior and E r i e from Jaworski (1981) ; Lake Washington from Edmondson (1969) ; Agriculture from Andrilenas and Serletis (1984) ; Wadden S e a from P o s t m a and Dijkema (1982) ; o t h e r estuaries from Jaworski (1981) for t o t a l inputs and Nixon (1983) for inorganic inputs. The MERL eutrophication experiment is described in Nixon et al. (1984). The e s t i m a t e for t h e Peruvian upwelling is very approximate based on information given by Dugdale (1976) and Walsh and Dugdale (1971) assuming 6 m o y-1 of a c t i v e transport. approach in addressing this problem is t o e s t i m a t e t h e r a t e s at which any material of i n t e r e s t is accumulating in t h e estuary and/or being transformed into gases which can exchange with t h e atmosphere. T h e difference between t h e sum of accumulation (burial in

104

lo

I-

HUDSON , p A R l T A N BL

NARRAGANSETT A BAY

h. S A N

ON

CHESAPEAKE mcT BAY .MA

0 sc

OAL

SOUTH FRANCISCO BAY

IL

0 PA

DELAWARE BAY

z a 0

I I

I I11111 10

I

I I I I Ill 10'

I

I111111

I

I11111~

I 0'

Fig. 6. Approximate present-day annual r a t e s of input of lead and copper per unit a r e a of various t e r r e s t r i a l and estuarine ecosystems in t h e United States. D a t a f o r upland a r e a s from numerous sources summarized by Nixon and L e e (in press). Estuarine inputs from T.M. Church, University of Delaware (person.communication) f o r Delaware Bay ; Bradford and Luoma (1980) f o r San Francisco Bay ; Bieri et al. (1982) for Chesapeake Bay ; Mueller, Gerrish and Casey (1982) for t h e Hudson-Raritan e s t u a r y ; various sources noted in this paper for Narragansett Bay. sediments) plus losses to t h e atmosphere and t h e input c a n then b e assumed to equal t h e n e t transport to offshore waters. Removal of materials in fishery harvest must usually b e

a very small fraction of t h e mass balance, but could b e relatively easily e s t i m a t e d in any case.

I o4

105

In this p a p e r w e h a v e brought t o g e t h e r much of t h e information t h a t is presently available on c e r t a i n pollutants in N a r r a g a n s e t t Bay (Rhode Island, U.S.A.)

in a n a t t e m p t to

develop annual m a s s b a l a n c e s f o r carbon, nitrogen and phosphorus as well as s o m e heavy m e t a l s (Mn, Cd, Pb, C u ) and p e t r o l e u m hydrocarbons under conditions of r e c e n t input. W e c a n n o t p r e t e n d c o m p l e t e knowledge of t h e q u a n t i t i e s o r behavior of a n y of t h e s e substances in t h e Bay, b u t t h e y h a v e a l l been s u b j e c t e d to considerable study and i t s e e m e d t h a t it would b e i n t e r e s t i n g and i n s t r u c t i v e to c o m p a r e t h e e f f e c t i v e n e s s of this o n e s y s t e m as a t r a p f o r a v a r i e t y of d i f f e r e n t materials. Narragansett Bay

N a r r a g a n s e t t Bay l i e s in t h e s o u t h e r n New England region of t h e n o r t h e a s t e r n United S t a t e s . T h e Bay proper c o v e r s a b o u t 265 k m 2 (with a n additional 6 3 k m 2 in t h e a d j a c e n t and c o n n e c t e d Mt. H o p e Bay a n d S a k o n n e t River), and h a s a n a v e r a g e d e p t h at mean low w a t e r of a b o u t 8.4 m. A hypsographic analysis shows t h a t 7 0 % of t h e Bay is shallower than 10 m. T h e t i d e is semidiurnal and r a n g e s f r o m a b o u t 1.1 m at t h e mouth to 1.4 m at t h e h e a d of t h e Bay. Annual t e m p e r a t u r e s r a n g e f r o m -0.5"C to 24OC. F r e s h w a t e r input for t h e e n t i r e 328 k m 2 a r e a c o m e s f r o m a d r a i n a g e basin of s o m e 4700 k m 2 and a v e r a g e s a b o u t 100 m3s-1. R a i n f a l l d i r e c t l y o n t h e Bay adds, on t h e a v e r a g e , a n o t h e r 5 m3s-I. Salinities a r e high and relatively c o n s t a n t over t h e annual cycle, at l e a s t in t h e lower Bay. S i n c e t h e f r e s h w a t e r e n t e r s o v e r dams, t h e r e is no well-developed mixing zone, and salinities r a n g e f r o m a b o u t 20-24

O/oo

in t h e Providence River to 31-33

O/oo

a t t h e mouth

of t h e Bay. In general, t h e w a t e r s a r e a l s o well-mixed, and t h e r e is only weak v e r t i c a l

s t r a t i f i c a t i o n e x c e p t in t h e upper Bay a n d Providence River. T h e r e s i d e n c e t i m e of t h e w a t e r a p p e a r s t o r a n g e f r o m a b o u t 10-40 days, with a long-term m e a n of 25-30 days. T h e P r o v i d e n c e m e t r o p o l i t a n a r e a i s l o c a t e d at t h e head of N a r r a g a n s e t t Bay and (along with s o m e o t h e r minor inputs) d i r e c t l y discharges i n t o t h e Bay s o m e 0.46 x lo6 m3 d-1 of p r i m a r y and secondary t r e a t e d s e w a g e containing l a r g e a m o u n t s of industrial as well as d o m e s t i c wastes. In addition, s o m e 0.11 x 106 m 3 d-1 of s e w a g e a r e discharged f r o m M a s s a c h u s e t t s to M t . Hope Bay, b u t i t is not known how much of t h i s m a t e r i a l a c t u a l l y e n t e r s N a r r a g a n s e t t Bay. T h e r i v e r s e n t e r i n g t h e Bay h a v e also passed through a number of urban industrial a r e a s and c a r r y a significant burden of pollutants i n t o t h e system. T h e r e is relatively l i t t l e suspended s e d i m e n t in m o s t of t h e lower Bay (4 m g liter-I), though values of 100 m g liter-1 m a y b e found a t t i m e s in t h e P r o v i d e n c e River. Extinction c o e f f i c i e n t s r a n g e f r o m -0.2 m-1 in t h e lower Bay to a b o u t -1m-1 in t h e P r o v i d e n c e River. As a r e s u l t of t h e g e n e r a l l y c l e a r w a t e r and a b u n d a n t n u t r i e n t s (NH4+ = 0-10

pM, NO2,3- =

0-10 pM, PO4- = 0.2-3 PM, Si(0H)k- = 0-25 PM), phytoplankton production i s high (perhaps 300-400 g C m-2 y-1). T h e b o t t o m of t h e Bay is c o v e r e d by silt-clay and sand sediments, and t h e r e a r e only occasional d e n s e a r e a s of m a c r o a l g a e o r s e a g r a s s e s around t h e periphery of t h e area. Dissolved oxygen is a l w a y s n e a r s a t u r a t i o n , e v e n in t h e b o t t o m w a t e r , e x c e p t during s u m m e r in t h e P r o v i d e n c e River.

106

More c o m p l e t e descriptions of these, a n d o t h e r f e a t u r e s of t h e s y s t e m c a n b e found in Nixon a n d K r e m e r (1977), K r e m e r and Nixon (19781, Pilson (1985) a n d t h e numerous r e f e r e n c e s t h e s e reviews contain. MASS BALANCES

Inputs Developing a n inventory of t h e annual inputs of nutrients, heavy metals, and hydrocarbons to N a r r a g a n s e t t Bay is r e l a t i v e l y s t r a i g h t f o r w a r d in c o n c e p t , b u t d i f f i c u l t and tedious in practice. N o overall, c o o r d i n a t e d sampling p r o g r a m h a s led to t h e p r e s e n t level of i n f o r m a t i o n s u m m a r i z e d in T a b l e 1. R a t h e r , various s t u d i e s h a v e been m a d e a t d i f f e r e n t t i m e s of e a c h t y p e of material. As a result, c e r t a i n inputs a r e b e t t e r described f o r o n e m a t e r i a l t h a n a n o t h e r and t h e q u a l i t y of t h e e s t i m a t e s v a r i e s considerably. However, i t would r e q u i r e a lengthy manuscript to d e a l with t h i s problem alone, and our i n t e n t h e r e is to a c c e p t t h e v a l u e s given in e a r l i e r p a p e r s as r e p r e s e n t i n g t h e b e s t e s t i m a t e s available. O u r impression is t h a t t h e i n f o r m a t i o n regarding inputs f o r t h i s e s t u a r y is at l e a s t as good as t h a t f o r most o t h e r systems. T h e position of N a r r a g a n s e t t Bay in F i g u r e s 5 and 6 shows t h a t i t is c o m p a r a t i v e l y heavily loaded in t e r m s of b o t h n u t r i e n t s a n d metals. T h e r e a p p e a r to b e f e w , if any, d e t a i l e d a s s e s s m e n t s of p e t r o l e u m hydrocarbon inputs to o t h e r e s t u a r i e s (Olsen e t al., 19821, b u t d a t a given by Mueller, C e r r i s h a n d C a s e y (1982) show t h a t t h e Hudson-Raritan e s t u a r y off New York c i t y m a y r e c e i v e s o m e 325 g m-2 y-1 of "oil a n d grease". T h e s u m m a r y given by Olsen et al. (1982) also suggests t h a t in t h e past, l o w e r D e l a w a r e Bay may h a v e received hydrocarbon inputs similar in m a g n i t u d e to t h o s e of N a r r a g a n s e t t Bay, b u t t h a t r e c e n t inputs to t h e D e l a w a r e a r e m o r e like 2-3 g m-2 y-1.

Accumulation in the Bay I t is m o r e d i f f i c u l t to d e t e r m i n e t h e fate of m a t e r i a l o n c e i t h a s e n t e r e d t h e Bay t h a n i t is to m e a s u r e i t s r a t e of delivery to t h e system. A c o m m o n g e o c h e m i c a l tool, t h e mixing d i a g r a m (Boyle et al., 19741, i s d i f f i c u l t to use in N a r r a g a n s e t t Bay b e c a u s e t h e r e is only a v e r y w e a k salinity g r a d i e n t along t h e a x i s of t h e B a y and t h e m a j o r s e w a g e inputs o c c u r in r e l a t i v e l y high salinity water. As a result, t h e s y s t e m d o e s not lend itself readily

to a modeling analysis such as t h a t used by Kaul a n d F r o e l i c h (1984). T h e r e a r e n o long-term d a t a on t h e c o n c e n t r a t i o n s of nutrients, m e t a l s , o r hydrocarbons in t h e w a t e r s of N a r r a g a n s e t t Bay, b u t t h e l i t t l e i n f o r m a t i o n t h a t i s available o n n u t r i e n t s in t h e w a t e r during t h e p a s t 25 y e a r s s u g g e s t s t h a t t h e r e i s no c l e a r s e c u l a r c h a n g e which c a n b e s e e n a g a i n s t t h e background of year-to-year v a r i a t i o n (Nixon and Lee, 1979 ; Pilson, personal communication). T h e s e d i m e n t s of t h e Bay s e r v e as a repository f o r a n y m a t e r i a l s t h a t a r e r e m o v e d during t h e p a s s a g e of t h e w a t e r f r o m land to sea. F o r t u n a t e l y , t h e s e d i m e n t s of N a r r a g a n s e t t Bay h a v e b e e n a n a l y z e d f o r a v a r i e t y of m a t e r i a l s , though at only a m o d e s t n u m b e r of l o c a t i o n s around t h e Bay. T h e s e d i m e n t analyses h a v e b e e n c a r r i e d o u t to various d e p t h s at d i f f e r e n t s i t e s and n o t all of t h e

107

materials of i n t e r e s t in t h i s p a p e r h a v e been m e a s u r e d in a l l of t h e c o r e s t h a t h a v e been collected. Nevertheless, t h e d a t a set in hand is l a r g e enough to m a k e a beginning in assessing t h e r a t e at which m a t e r i a l s a r e a c c u m u l a t i n g in t h e b o t t o m sediments. Major sources of d a t a include Goldberg et al. (1977) ; H u n t a n d S m i t h (1983) ; a n d S a n t s c h i et al. (1984) for metals, H u r t t (1978) and H u r t t a n d Quinn (1979) f o r p e t r o l e u m hydrocarbons, and Nixon a n d Nowicki (unpublished) f o r C, N and P. T h e d a t a f o r Cu, P b a n d hydrocarbons from six s i t e s in t h e Bay w e r e used w i t h m e a s u r e m e n t s of radionuclides (234Th, 21oPb and 239,24013~) with known input f u n c t i o n s to c a l c u l a t e t h e n e t a c c u m u l a t i o n of sediments, Cu, P b and hydrocarbons in t h e Bay (Santschi et al., 1984). A similar modeling w a s l a t e r carried o u t by S a n t s c h i (personal c o m m u n i c a t i o n ) f o r C d in t h e sediments. S i n c e C, N, P and Mn d o n o t a p p e a r to show a c l e a r a n t h r o p o g e n i c e n r i c h m e n t in t h e s e d i m e n t s (Fig. 7 ; Nixon and Pilson, 19841, w e c a l c u l a t e d t h e i r a c c u m u l a t i o n as t h e product of t h e a v e r a g e concentration (below 10 c m f o r nutrients) a n d t h e s e d i m e n t deposition rate. T h e uniformity of Mn with d e p t h in t h e s e d i m e n t s of N a r r a g a n s e t t Bay c o n t r a s t s with t h e marked Mn e n r i c h m e n t of s u r f a c e s e d i m e n t s r e p o r t e d f r o m s o m e o t h e r estuaries, including Long Island Sound (Thomson, T u r e k i a n a n d McCaffrey, 1975) a n d t h e St. Lawrence (Sundby, Silverberg and C h e s s e l e t , 1981). T h e r e a r e l a r g e f l u x e s of dissolved Mn from t h e s e d i m e n t s to t h e overlying w a t e r in N a r r a g a n s e t t Bay ( G r a h a m et al.,

1976 ;

Elderfield et al., 1981 a ; Hunt, 1983) a n d i t m a y b e t h a t t h e metabolism of t h e b o t t o m in this e s t u a r y i s s u f f i c i e n t l y g r e a t t h a t reduced conditions a r e m a i n t a i n e d too c l o s e to t h e sediment-water i n t e r f a c e to allow a r e p r e c i p i t a t i o n and a c c u m u l a t i o n of Mn. P e r h a p s t h e w a t e r c i r c u l a t i o n n e a r t h e b o t t o m i s sufficiently vigorous t h a t v e r y f i n e p a r t i c l e s such as those found in t h e St. L a w r e n c e containing r e p r e c i p i t a t e d Mn (Sundby, Silverberg and Chesselet, 1981) a r e dispersed throughout t h e w a t e r column and t r a n s p o r t e d o u t of t h e system. A striking f e a t u r e of t h e solid phase c o n c e n t r a t i o n profiles shown in F i g u r e 7 is t h e early e n r i c h m e n t of t h e s e d i m e n t s with P b a n d C u ( a f t e r a b o u t 1830-1840 at t h i s Upper Bay site), C d ( a f t e r a b o u t 18801, a n d p e t r o l e u m ( a f t e r a b o u t 1920) c o m p a r e d with t h e relatively c o n s t a n t levels of C, N a n d P below a b o u t 10 c m (about 1960). Metal inputs to this e s t u a r y began e a r l y during t h e A m e r i c a n p h a s e of t h e Industrial Revolution, and by

1860 R h o d e Island w a s t h e m o s t highly industrialized state in t h e Union (Mc Loughlin, 1978). A f t e r a b o u t 1850, G.H. Cor4ss a n d o t h e r s m a d e P r o v i d e n c e a major c e n t e r of s t e a m e n g i n e d e v e l o p m e n t a n d m a n u f a c t u r e , with t h e r e s u l t t h a t t h e b a s e m e t a l industry rose to a p r o m i n e n t position i n t h e state's e c o n o m y and t h e o u t p u t of t h e jewerly industry increased m a r k e d l y (Coleman, 1969). S i n c e t h e industrial w a s t e s f r o m t h e s e a c t i v i t i e s were presumably discharged to N a r r a g a n s e t t Bay, i t i s n o t surprising to find m e t a l s relatively d e e p in t h e sediments. However, t h e major s o u r c e of n u t r i e n t s discharged to t h e Bay, t h e Field's P o i n t S e w a g e T r e a t m e n t P l a n t , began c o n s t r u c t i o n in 1890 and o p e r a t i o n in t h e e a r l y 1900's. Y e t t h e r e i s virtually n o r e f l e c t i o n of t h i s e n r i c h m e n t in t h e sediments. T h e higher c o n c e n t r a t i o n s of C, N a n d P in t h e near-surface z o n e r e f l e c t t h e

108

r e c e n t deposition of o r g a n i c m a t e r i a l s which h a v e b e e n mixed to a b o u t 10 c m and which a r e being rapidly decomposed. O t h e r s t u d i e s h a v e shown t h a t t h e s e d i m e n t s of t h e Bay a r e mixed by resuspension a n d b i o t u r b a t i o n to d e p t h s of 10-20 c m ( M c C a f f r e y et al., 1980 ; Elderfield et al., 1981 b ; S a n t s c h i et al.,

19831, a n d t h a t p e r h a p s 5-10 % of f r e s h l y

deposited o r g a n i c m a t t e r i s c a r r i e d to sub-surface l a y e r s of t h e s e d i m e n t s ( G a r b e r , 1984). T h e a n t h r o p o g e n i c input of Mn i s also s i g n i f i c a n t (Table I), b u t c a n n o t b e s e e n in t h e s e d i m e n t profile (Fig. 7). D i f f e r e n c e s in t h e s e d i m e n t profiles of t h e s e various m a t e r i a l s a r e d u e to d i f f e r e n c e s in t h e b a l a n c e b e t w e e n t h e e f f i c i e n c y a n d s p e e d w i t h which t h e y a r e c a r r i e d to t h e b o t t o m a n d t h e r a t e a t which t h e y a r e remobilized a n d r e t u r n e d to t h e overlying w a t e r . P h y t o p l a n k t o n rapidly d e p l e t e t h e dissolved inorganic n u t r i e n t s f r o m t h e w a t e r c o l u m n during t h e winter-spring bloom a n d t h r o u g h o u t t h e s u m m e r (Smayda, 1973 ; Furnas, H i t c h c o c k a n d S m a y d a , 1976 ; Furnas, 19821, a n d s t u d i e s in t h e MERL m e s o c o s m t a n k s ( 5 m e t r e s d e e p ) by S a n t s c h i et al. (1983) a n d Kelly (1983) h a v e shown a rapid t r a n s p o r t of p a r t i c l e s f r o m t h e w a t e r c o l u m n to t h e s e d i m e n t s (Table 2). Similar s t u d i e s using a d d i t i o n s of r a d i o a c t i v e a n d s t a b l e m e t a l s a n d p e t r o l e u m hydrocarbons to t h e MERL t a n k s h a v e a l s o shown a rapid f l u x to t h e s e d i m e n t s of Mn, P b and hydrocarbons, with a s o m e w h a t slower r e m o v a l of C u a n d a considerably less e f f e c t i v e r e m o v a l of C d (Table 2). Thus, s o m e of t h e m a t e r i a l s which a r e m o s t rapidly a n d e f f e c t i v e l y c a r r i e d to t h e b o t t o m (C, N, P, Mn) d o n o t r e m a i n t h e r e , while o t h e r s d o (Pb, p e t r o l e u m hydrocarbons). C o p p e r , which a p p e a r s less particle-reactive

in t h e w a t e r c o l u m n t h a n Pb, i s e f f e c t i v e l y r e t a i n e d in t h e

s e d i m e n t s o n c e i t a r r i v e s on t h e b o t t o m . C a d m i u m i s m a i n t a i n e d p r e d o m i n a n t l y in dissolved f o r m (Adler, A m d u r e r a n d Santschi, 19801, b u t a s i g n i f i c a n t portion of t h e s m a l l f r a c t i o n r e a c h i n g t h e b o t t o m a p p e a r s to r e m a i n as a p a r t t h e s e d i m e n t a r y r e c o r d (Fig. 7). T h e e v i d e n c e f r o m s e d i m e n t profiles a n d w a t e r c o l u m n r e m o v a l e x p e r i m e n t s r e g a r d i n g t h e behavior of t h e s e various m a t e r i a l s is s u p p o r t e d by d i r e c t m e a s u r e m e n t s of t h e i r r e t u r n to t h e overlying w a t e r a f t e r r e m i n e r a l i z a t i o n a n d r e m o b i l i z a t i o n on a n d in t h e b o t t o m s e d i m e n t ( T a b l e 3). In t h e case of p e t r o l e u m hydrocarbons, C a n d N, t h i s r e g e n e r a t i o n involves t h e production of C 0 2 a n d N 2 + N 2 0 which m a y b e lost to t h e a t m o s p h e r e f r o m t h e Bay o r c a r r i e d o f f s h o r e as dissolved gases. T h e i m p o r t a n c e of b e n t h i c n u t r i e n t r e g e n e r a t i o n in e s t u a r i e s h a s b e e n c l e a r f o r s e v e r a l y e a r s (reviewed by Nixon, 19811, b u t i t is r e m a r k a b l e to see t h a t in N a r r a g a n s e t t Bay t h e r e m o b i l i z a t i o n and r e t u r n of Mn f r o m t h e s e d i m e n t s to t h e overlying w a t e r i s 3-4 t i m e s g r e a t e r t h a n t h e input, a t u r n o v e r m u c h f a s t e r t h a n t h a t of t h e major biological nutrients. T h e r e l a t i v e l y w e a k C d signal in t h e s e d i m e n t s i s n o t surprising considering t h a t i t s half-removal t i m e is slow. Deriving a n u m b e r f o r t h e b e n t h i c r e m i n e r a l i z a t i o n of p e t r o l e u m hydrocarbons i s p a r t i c u l a r l y d i f f i c u l t , s i n c e t h e various m e t a b o l i c c h a m b e r i n c u b a t i o n s used to m e a s u r e t h e n e t f l u x of 0 2 , C 0 2 , NH4, NO2,3, N 2 0 , N2, P O 4 a n d m e t a l s a c r o s s t h e s e d i m e n t w a t e r i n t e r f a c e d o n o t t e l l u s w h a t s u b s t r a t e i s being respired.

ORGANIC

ORGANIC

ORGANIC

-'

N. mg 9 - l

P. mg g

C. mg Q

0

1970 10

1950 20

1930

s

30

1910 I--

z W

40

E0 W u)

z r

1890

50 1870

60

1850

I-

n w 0

70

1830 80

1810 90

100

-'

INORGANIC P.

mQ g -'

-hJl4Q -'Cd.W ~ - ' a 1 0 0

-Cu.pg

g-'-

TOTAL HYDROCARBONS,

-'

/.Lg 8 . '

Pb,W Q

I

-i

Fig. 7. Concentrations of nutrients, some heavy metals, and petroleum hydrocarbons at various depths in t h e sediments at Ohio Ledge in Upper Narragansett Bay. Nutrients and metals w e r e analyzed from t h e s a m e c o r e by t h e authors, while hydrocarbons w e r e measured on a s e p a r a t e c o r e from a near-by site by H u r t t (1978). Geochronology of t h e c o r e used f o r nutrients and m e t a l s is described in detail by Santschi et al. (1984).

CL

0 (D

TABLE 1 Estimates of recent inputs to Narragansett Bay (RI, USA), accumulation r a t e s within t h e sediments of t h e Bay, and losses from t h e Bay to t h e atmosphere and offshore w a t e r s f o r major nutrients, various heavy metals, and t o t a l petroleum hydrocarbons.

g m-2 y-1

IN PUTS^ Rivers Sewage Atmosphere Fixation Total ACCUMULATION IN SEDIMENTS~ OUTPUTS t o Atmosphere3 t o Off shore4

C

N

P

Mn

mg m-2 y-1 Cd Pb

47 -56 -0 310

12.50 14.70 0.4 I 0.0 I

2.03 2.64 0.01 0

500 216 35 0

7.4 2.2 ? 0

-

-

_ I

-

75 I

9.6

0.12

53

1.5-2.8

0 4.56b

0 698b

NO 6.8-8.1b

413

27.62

4.68

6

0.46

<339a > 74a

7.2b 19.96b

31.8 135 -75 0

Cu

g m-2 y-1 Petroleum Hydrocarbons

83.5 243 -70 0

2.8 2.9 0.003 0.04 (spills)

-241.8

396.5

193-340 273-379 -0 0-49b

0 18-124b

5.743 1.5-3.8 0.9-2.0C 0-3.3b

1 : C from Furnas et al. (1976) for phytoplankton production and from t h e organic P reported by Nixon (1981) f o r rivers and sewage using a C:P r a t i o of 106:l by atoms. T o t a l N and P inputs from Nixon (1981). Sewage includes urban runoff. Metals from rivers and sewage measured by Hunt (unpublished report). Cu in rivers also includes d a t a f o r small rivers and smaller sewage t r e a t m e n t plants reported by Hoffman and Quinn (1984). Metals from atmosphere estimated from d a t a for nearby a r e a s summarized by Nixon and L e e (in press). Hydrocarbons from Hoffman and Quinn (19841, and E. Hoffman (pers. comm. as reported in Santschi et al. in press) include M t Hope Bay inputs and area.

2 : C, N and P from Nixon and Pilson (1984), P b and Cu from Santschi et al. (in press), Mn and Cd from Santschi (pers. comm.). 3 : (a) Calculated by difference ; (b) denitrification from Seitzinger et al. (1984) ; (c) sum of volatilization and decomposition extrapolated as 47 % of input from a long-term oil addition experiment using MERL microcosms (Gearing et al., 1980). 4 : (a) From Nixon and Pilson (19841, does not include DOC or allocthonous P O C ; if allocthonous DOC is carried conservative1 through t h e system as found by Cifuentes (1982) in t h e Delaware estuary, then export to offshore would b e < 100 g C m-2 y-? and respiration > 303 g C m-2 y-1 ; (b) calculated by difference.

111

TABLE 2. Half-removal t i m e s (days) f o r t h e flux of particles, various m e t a l s and p e t r o l e u m hydrocarbons f r o m t h e w a t e r column (5 m deep) to t h e s e d i m e n t s in t h e MERL microcosms.

Natural Particles1 M n2 Cd3 cu2 Pb3 P e t r o l e u m Hydrocarbons4 F1 fractiona F2 fraction b

Winter

Summer

2

0.1 1 10-50 4-10 2-3

60-100 40 10-20

1.9 1.9

I s a n t s c h i et al. (1983) using t h e t i m e s e r i e s p a r t i c l e c o n c e n t r a t i o n and s e d i m e n t t r a p d a t a ; 2C. Hunt, unpublished d a t a f r o m a s t a b l e m e t a l spike addition ; %antschi, Adler and A m d u r e r (1983) f r o m r a d i o a c t i v e t r a c e m e t a l spikes. Helf-removal t i m e s for 21oPb in N a r r a g a n s e t t Bay ranged f r o m 2.6-5 days in s u m m e r and 22 days in w i n t e r (Santschi et al., 1983) ; 4Gearing and G e a r i n g (1983) f r o m additions of No. 2 f u e l oil : (a) s a t u r a t e d hydrocarbons a n d olefins w i t h o n e o r t w o double bonds--range f o r 9 compounds = 1.1-2 d a y s ; (b) a r o m a t i c hydrocarbons--range f o r 6 compounds = 0.7-2.5 days. T h e e s t i m a t e given in T a b l e 3 i s based on t h e loss of hydrocarbons f r o m c o n c e n t r a t i o n m e a s u r e m e n t s in t h e s e d i m e n t s of t h e MERL mesocosms a f t e r a long-term e x p e r i m e n t involving t h e addition of NO. 2 f u e l oil (Gearing et al.,

1980). T h e n a t u r e of t h e

hydrocarbon m a t e r i a l s reaching N a r r a g a n s e t t Bay s e d i m e n t s is c e r t a i n l y m o r e complex, and t h e MERL number i s probably a n o v e r e s t i m a t e of t h e biological breakdown of t h e s e compounds. Nevertheless, t h e widespread o c c u r r e n c e of p e t r o l e u m as a n e s t u a r i n e pollutant, t h e e x t e n s i v e d a t a o n p e t r o l e u m hydrocarbons in N a r r a g a n s e t t Bay, and t h e unique MERL oil e x p e r i m e n t s m a k e i t compelling to a s s e m b l e t h e b u d g e t as best w e can. Additional e v i d e n c e f o r t h e a l m o s t c o m p l e t e r e g e n e r a t i o n of t h e o r g a n i c m a t t e r reaching t h e b o t t o m of N a r r a g a n s e t t Bay c o m e s f r o m s e v e r a l o t h e r experiments. In o n e case, d i f f e r e n t a m o u n t s of s e s t o n f i l t e r e d f r o m Bay w a t e r w e r e added to i n t a c t c o r e s of s e d i m e n t m a i n t a i n e d in t h e l a b o r a t o r y (Kelly and Nixon, 1984). Even though t h e c o r e s w e r e m a i n t a i n e d at only 15OC ( s u m m e r b o t t o m t e m p e r a t u r e s r e a c h 20°), 24-30 % of t h e organic N a n d 11-20 % of t h e 0 r g a n i c . C added to t h e s e d i m e n t s had been remineralized within o n e month. T h e r e l e a s e of inorganic P w a s e v e n m o r e rapid. In a r e l a t e d e x p e r i m e n t using 15N labelled phytoplankton d e t r i t u s , G a r b e r (1984) concluded t h a t , "Net r a t e s of N-remineralization based on t h e conversion of labelled PON to NH4+ suggest t h a t e s s e n t i a l l y a l l b u t a f e w p e r c e n t of detrital-N reaching t h e s e d i m e n t s u r f a c e could b e r e c y c l e d in less t h a n o n e year". S t u d i e s of t h e decomposition of freshly deposited organic m a t t e r f r o m a s u m m e r phytoplankton bloom in a MERL t a n k also showed rapid initial r e m i n e r a l i z a t i o n r a t e s of 10.5 %, 7.2 % and 3.1 % day-1 f o r C, N and P respectively

112

TABLE 3 I m p o r t a n c e of b e n t h i c r e g e n e r a t i o n , release, o r decomposition, respectively, of various nutrients, heavy m e t a l s a n d petroleum hydrocarbons in N a r r a g a n s e t t Bay. B e n t h i c R e g e n e r a t i o n , R e l e a s e , o r Decomposition Sediment-Water Flux % of Input

CI N1 N + denitrificationz

P3

P e t r o l e u m Hydrocarbons7

g m-L y-1 139.0 12.4 19.6 3.7

28 45 71 79

m g m-2 y-1 1098 2212 2.5-4.5 0-5 1 0-4.5

416 295 26-47 0-13 0-2 55

lNixon et al. (1976) ; 2Seitzinger et al. (1984) ; 3Nixon et al. (1980) ; Elderfield et al. (1981 a ) ; 5 H u n t (1983) ; 6 H u n t and S m i t h (1983) ; 7 C e a r i n g et al. (1980) based on t h e loss r a t e in s e d i m e n t s during t h e f i r s t 100 days a f t e r inputs w e r e t e r m i n a t e d and applying t h i s r a t e during t h e t i m e of oil addition as well. (Kelly, 1983). Even when t h e a m o u n t of o r g a n i c m a t t e r in N a r r a g a n s e t t Bay s e d i m e n t s is e x p e r i m e n t a l l y i n c r e a s e d markedly, i t a p p e a r s t h a t r e s p i r a t o r y r a t e s r i s e sufficiently t h a t only a srnall portion r e m a i n s to a c c u m u l a t e o v e r a n annual cycle. T h i s result is e v i d e n t in t h e organic e n r i c h m e n t of s e d i m e n t s described by Kelly a n d Nixon (1984), in t h e m e t a b o l i c response of t h e s e d i m e n t s in t h e Bay to t h e winter-spring phytoplankton bloom (Nixon et al., 1980), a n in t h e response of t h e MERL mesocosms to a long-term (2.5 yr) n u t r i e n t addition e x p e r i m e n t (described in Nixon et al.,

1984). Among t h e a d v a n t a g e s of t h e

coupled benthic-pelagic s y s t e m s in t h e MERL t a n k s is t h e r e l a t i v e ease w i t h which o n e c a n m e a s u r e t h e t o t a l s y s t e m a p p a r e n t production during t h e day and consumption during t h e night based on c h a n g e s in dissolved 02 o r C 0 2 . T h e r e s u l t s of s u c h m e a s u r e m e n t s obtained by C.A. O v i a t t f r o m r e p l i c a t e m e s o c o s m s receiving d i f f e r e n t l e v e l s of inorganic N, P a n d Si a r e shown in F i g u r e 8. Only when production r o s e to v e r y high l e v e l s w a s t h e r e

a modest e x c e s s of o r g a n i c m a t t e r f o r m a t i o n o v e r consumption, and much of this m u s t h a v e been flushed o u t of t h e t a n k s (Bay w a t e r is added to t h e t a n k s at a r a t e s u f f i c i e n t to e x c h a n g e t h e volume in 27 days). Only a s m a l l portion of t h e e n h a n c e d p r i m a r y production r e m a i n e d in t h e sediments. T h e organic-rich s e d i m e n t s c o m m o n l y a s s o c i a t e d with polluted environments m a y r e q u i r e y e a r s of a c c u m u l a t i o n , a n o x i c o r low oxygen conditions, and/or rapid r a t e s of s e d i m e n t deposition-all conditions which w e r e n o t found in t h e MERL e x p e r i m e n t s o r in most of N a r r a g a n s e t t Bay.

113 2000 o 6/81 -6182 0

6 / 0 2 -6/83

z

0

c

1200

a

m W LL

800 /\-I00

g C rn-'y-' N E T ORGANIC STORAGE + E X P O R T

400

I

0 0

400

APPARENT

1

800

1

I

I

1200

I

1600

I

1

2000

D A Y T I M E PRODUCTION, g 0, rn-,y-'

Fig. 8. T h e relationship b e t w e e n annual t o t a l s y s t e m n e t production during t h e day and consumption during t h e d a r k in t h e MERL mesocosms during t w o y e a r s in which t h e y w e r e receiving d i f f e r e n t l e v e l s of inorganic n u t r i e n t (N, P, Si) input. E s t i m a t e s a r e based on dawn-dusk c h a n g e s in dissolved oxygen and h a v e been c o r r e c t e d f o r air-sea g a s exchange. The solid line r e p r e s e n t s a P/R r a t i o of 1 w i t h n o n e t organic a c c u m u l a t i o n in, o r e x p o r t from, t h e tanks. T h e broken line r e p r e s e n t s a n e t a c c u m u l a t i o n and/or e x p o r t of 100 g C m-2 y-1 assuming a PQ of 1. D a t a f r o m C. Oviatt. A description of t h e mesocosms and t h e enrichment e x p e r i m e n t i s given in Nixon et al. (1984). Export to Offshore

A consideration of t h e annual mass b a l a n c e s s u m m a r i z e d in T a b l e I and t h e differing behavior of t h e various t y p e s of m a t e r i a l discussed in t h e preceding s e c t i o n suggests t h a t there a r e t h r e e g e n e r a l t y p e s of s u b s t a n c e s t h a t d i f f e r markedly in t h e e x t e n t to which they will b e r e t a i n e d within o r t r a n s m i t t e d through a n estuary. a. t h o s e t h a t a r e n o t v e r y e f f e c t i v e l y t r a n s p o r t e d to t h e b o t t o m s e d i m e n t s b e c a u s e t h e y a r e n o t p a r t i c l e r e a c t i v e (for example, C d ) o r b e c a u s e t h e y a r e very rapidly respired o r degraded in t h e w a t e r (for example, low molecular weight soluble o r volatile o r g a n i c compounds). T h e s e m a t e r i a l s a r e largely l o s t f r o m t h e Bay.

114

TABLE 4 R e l a t i v e e f f i c i e n c y (expressed as a % of t h e t o t a l t e r r e s t r i a l , anthropogenic, a n d a t m o s p h e r i c input) w i t h which N a r r a g a n s e t t Bay (RI, USA) r e m o v e s and t r a n s m i t s various nutrients, heavy metals, a n d petroleum hydrocarbons. S e e T a b l e 1 f o r d a t a sources.

C

N

P

Mn

Cd

Pb

Cu

Petroleum Hydrocarbons

% Retained % Transmitted

1.5

1.7

2.6

7

16-29

80-100

69-95

26-67

To Atmosphere* To Offshore

82 18

26 72

0 97

0 93

0 71-84

0 0-20

0 5-31

47 0-27

* S o m e of t h e C02, N 2 a n d N 2 0 gases produced within t h e Bay a n d included in this t e r m a r e c a r r i e d offshore dissolved in t h e w a t e r r a t h e r than a c t u a l l y exchanged a c r o s s t h e airw a t e r i n t e r f a c e in t h e Bay. Hydrocarbon loss includes volatilization and respiration. If allocthonous D O C i s c a r r i e d conservatively through t h e Bay, t h e offshore loss would rise to a b o u t 27 % of input and t h e flux to t h e a t m o s p h e r e would fall to 73 %. b. t h o s e t h a t a r e c a r r i e d to t h e b o t t o m b u t which a r e a c t i v e l y remineralized and recycled (C, N, P ) o r resolubilized (Mn) in t h e s e d i m e n t and released to t h e overlying water. T h e s e m a t e r i a l s a r e also largely lost f r o m t h e Bay. T h e e x p o r t of N f r o m t h e Bay is a l s o reduced considerably because of denitrification in t h e s e d i m e n t s and t h e loss of N2 a n d N 2 0 to t h e a t m o s p h e r e (Seitzinger, Nixon and Pilson, 1984). c. t h o s e which a r e c a r r i e d to t h e b o t t o m b u t which remain firmly bound to

p a r t i c l e s o r o t h e r w i s e t r a p p e d in t h e s e d i m e n t (for example, Pb, Cu). T h e s e a r e r e t a i n e d and a c c u m u l a t e in t h e system. A s u m m a r y of t h e r e l a t i v e efficiency with which N a r r a g a n s e t t Bay t r a n s m i t s and

r e t a i n s t h e s e various m a t e r i a l s is given in T a b l e 4 based on t h e a b s o l u t e numbers reported in T a b l e 1. We e m p h a s i z e again t h a t t h e r e m u s t b e a considerable (though usually undefined) uncertainly in all of t h e s e budgets. O u r s e n s e i s t h a t t h e input data a r e perhaps t h e most reliable p a r t of t h e balance and t h a t f u t u r e e f f o r t s should b e d i r e c t e d toward a b e t t e r description of sedimentation r a t e s and s e d i m e n t c h a r a c t e r i s t i c s in t h e Bay.

COMPARISON WITH OTHER ESTUARIES In s p i t e of t h e long-standing e f f o r t by limnologists, marine ecologists, and geochemists to quantify t h e flux of n u t r i e n t s and o t h e r m a t e r i a l s in rivers (Clarke, 1916 ; Livingstone, 1963 ; Meybeck, 1979 ; symposium volumes e d i t e d by Martin, Burton a n d Eisrna, 1981 ;

Likens et al., 1981 ; Degens, 1982), t h e r e h a v e been remarkably f e w r e p o r t s of t h e m a s s balances in e s t u a r i e s t h a t a r e needed to link riverine o u t p u t s and ocean inputs. As Bewers a n d Y e a t s (1981) concluded in their review of t h e behavior of t r a c e m e t a l s during e s t u a r i n e mixing, "Several global budgets involving e s t i m a t e s of t h e riverine fluxes of m e t a l s to t h e o c e a n h a v e been constructed...

However, t h e s e budgets a n d flux calculations

115

TABLE 5 Sediment input a n d deposition in various estuaries.

g m-2 y-1

Estuary

Input

.................................... Long Island Sound, NY-CTI N a r r a g a n s e t t Bay, R12 C h e s a p e a k e Bay, MD-VA3 Wadden S e a 4 D e l a w a r e Bay5 San F r a n c i s c o Bay C A 6 Knight Inlet, F5.C.j A t c h a f a l a y a Bay, LA8

Deposition

157 250 262 788 1,400

375 3,500

19,250 9,680,00019,360,000

IBenninger (1978) ; %antschi et al. (in press) ; 3Bieri et al. (1982) ; 4 P o s t m a and Dijkema (1982) ; 5Biggs, S h a r p and Howell (1984) ; 6 C o n o m o s and P e t e r s o n (1977) ; Fturbid Fjord draining a glacial a r e a , F a r r o w , Syvitski a n d Tunnicliffe (1983) ; 8shallow bay with a c t i v e d e l t a building, Van H e e r d e n , Wells and R o b e r t s (1983). do n o t t a k e i n t o a c c o u n t t h e r e m o v a l o r additions of m e t a l s in estuaries...

More emphasis

upon t h e c o n s t r u c t i o n of b u d g e t s f o r e s t u a r i e s would b e justified...". B e c a u s e of this lack, i t is d i f f i c u l t to know how c o m m o n t h e r e s u l t s f r o m N a r r a g a n s e t t Bay m a y be. B e f o r e discussing s o m e of t h e comparisons t h a t c a n b e made, however, i t i s worth noting a f e a t u r e of t h e Bay t h a t m a y play a n i m p o r t a n t r o l e in d e t e r m i n i n g t h e r e l a t i v e s t r e n g t h of t h e e s t u a r y as a sink f o r materials. T h e b e s t c u r r a n t e s t i m a t e of new s e d i m e n t deposition is 250 gdw m-2 y-1 a v e r a g e d o v e r t h e Bay b o t t o m (Santschi et al., 1984). W e h a v e n o idea how t y p i c a l such a s e d i m e n t input m a y be, b u t s o m e idea of t h e range found in o t h e r e s t u a r i e s is given in T a b l e 5.

Nutrients In t h e i r synthesis of many s t u d i e s on t h e Wadden Sea, P o s t m a and Dijkema (1982) concluded t h a t ,

'I...

s o m e of t h e phytoplankton produced in situ may b e c a r r i e d away to

t h e N o r t h Sea. A s m a l l f r a c t i o n will b e buried in new s e d i m e n t deposits. However, most of t h e o r g a n i c m a t t e r is mineralized in t h e Wadden Sea...". Salomons (1981) s u g g e s t t h a t ,

'I...

Similarly, van Bennekom and

only a s m a l l p a r t of t h e i n c r e a s e of t h e phosphorus load

of t h e R h i n e is t r a p p e d in c o a s t a l s e d i m e n t s , t h e r e s t being t r a n s p o r t e d to t h e open North Sea". Both r e p o r t s p u t t h e Wadden S e a in line with t h e results f r o m N a r r a g a n s e t t Bay. In t h e i r modeling analysis of O c h l o c k o n e e Bay, Florida (USA), Kaul and F r o e l i c h (1984) again concluded t h a t , "all biological P-uptake is r e g e n e r a t e d within t h e e s t u a r y so t h a t virtually 100 % of t h e fluvial reactive-P e n t e r s t h e ocean". O n t h e o t h e r hand, Loder and Glibert

(1980) applied c o n s e r v a t i v e mixing models to c o n c e n t r a t i o n d a t a f r o m G r e a t Bay, New H a m p s h i r e (USA), a n d concluded t h a t only 1 3 % of t h e dissolved phosphate input w a s e x p o r t e d to t h e c o a s t a l ocean, w i t h t h e remaining f r a c t i o n buried in t h e sediments.

116 T h e m o s t d r a m a t i c d i s a g r e e m e n t with o u r r e s u l t s c o m e s f r o m t h e r e c e n t v e r y l a r g e study of C h e s a p e a k e Bay c a r r i e d o u t by t h e U.S. Environmental P r o t e c t i o n Agency. N u t r i e n t e n r i c h m e n t w a s a major c o n c e r n of t h a t study, a n d annual mass b a l a n c e s f o r N and P w e r e a n i m p o r t a n t p a r t of t h e final result. T h e overall conclusion was t h a t , "Nearly a l l of t h e m a t e r i a l s t h a t e n t e r t h e Bay r e m a i n t h e r e ; n u t r i e n t s t r i c k l e o u t of t h e Bay mouth at a v e r y slow rate" (Smullen, T a f t a n d MacKnis, 1982). T h e a u t h o r s a r r i v e d at this conclusion by c o m p a r i n g w h a t a p p e a r to b e very c a r e f u l e s t i m a t e s of n u t r i e n t inputs with a very u n c e r t a i n c a l c u l a t i o n of n e t t r a n s p o r t a c r o s s t h e mouth of t h e Bay based on a simple box model. Using a n o t h e r approach, however, i t is possible to show t h a t t h e i r result i s alrnost c e r t a i n l y incorrect. If w e a c c e p t t h e inputs of N (123 x 106 Kg y-l), P (10 x 3.106 Kg y-I), and s e d i m e n t (3014 x 106 Kg y-l)given by Smullen, T a f t a n d MacKnis (19821, and t h e i r conclusion t h a t essentially a l l of t h e s e inputs remain in C h e s a p e a k e Bay, t h e n t h e buried s e d i m e n t s m u s t h a v e a n a v e r a g e composition of 40.9 m g N and 3.42 m g P/g d r y sediment. No d a t a a r e given in t h e r e p o r t on t h e a c t u a l N and P c o n c e n t r a t i o n s in t h e s e d i m e n t s , b u t t h e s t e a d y - s t a t e c o n c e n t r a t i o n s in t h e s e d i m e n t s at Ohio L e d g e in upper N a r r a g a n s e t t Bay a r e a b o u t 3 m g N/g and 0.7 m g P/g dry s e d i m e n t (Fig.

7).

This i s a n a r e a with silt-clay

sediments where nutrient

c o n c e n t r a t i o n s in t h e w a t e r a r e higher t h a n in m o s t of C h e s a p e a k e Bay. D i r e c t e v i d e n c e t h a t C h e s a p e a k e Bay s e d i m e n t s a r e lower in N and P t h a n t h e Ohio L e d g e s e d i m e n t s i s in t h e r e p o r t by Bieri et al. (1982) t h a t organic c a r b o n in C h e s a p e a k e Bay s e d i m e n t s a v e r a g e s 2.2 % in Maryland a n d 1 % in Virginia. I t only r e a c h e s 3 % in t h e f i n e s e d i m e n t s of t h e d e e p c e n t r a l Bay channel. O r g a n i c c a r b o n in t h e Ohio L e d g e s e d i m e n t s i s 3-4 % (Fig. 7). If w e t a k e 2 % o r g a n i c C as a working e s t i m a t e for a v e r a g e C h e s a p e a k e Bay s e d i m e n t a n d a s s u m e t h a t N a n d P a r e t h u s 66.6 % of t h e v a l u e at Ohio L e d g e (or 1.99 m g N and 0.466 m g P / g dry sediment), w e c a n c a l c u l a t e t h e a m o u n t of N and P buried with t h e Smullen, T a f t a n d MacKnis (1982) e s t i m a t e of 3014 x

106 Kg of s e d i m e n t y-l. T h e

result is 6 x 106 Kg N and 1.4 x 106 Kg P o r 4.9 % of t h e N input and 13.6 % of t h e P input. If t h i s analysis is at l e a s t nearly c o r r e c t , i t s e e m s t h a t C h e s a p e a k e Bay i s behaving much like N a r r a g a n s e t t Bay, and n o t at all as described by t h e E P A study.

Heavy Metals In his P r e s i d e n t i a l Address b e f o r e t h e A m e r i c a n G e o c h e m i c a l Society, Turekian (1977) noted t h a t , "The overwhelming s e n s e is t h a t in a n e s t u a r i n e s y s t e m t h e r e is continuous m o v e m e n t of s o m e m e t a l s in a n d o u t of solution b u t l i t t l e i s lost f r o m t h e system". H e i l l u s t r a t e d his point with t h e s t u d y by Evans et al. (1977) of Mn in t h e N e w p o r t R i v e r e s t u a r y in N o r t h C a r o l i n a (USA). While t h e P b and C u b u d g e t s f r o m N a r r a g a n s e t t Bay support t h e view of a s t r o n g e s t u a r i n e sink f o r metals, t h e Mn a n d C d d a t a do not. Similarly, Sundby, Silverberg and C h e s s e l e t (1981) h a v e shown a l a r g e loss of Mn f r o m t h e St. L a w r e n c e e s t u a r y in Canada. Likewise, t h e c o n s e r v a t i v e behvior f o r t o t a l Mn in N a r r a g a n s e t t Bay ( G r a h a m et al., 1976) suggests e x p o r t of m o s t Mn e n t e r i n g t h e system.

117 In t h e i r s t u d i e s of m e t a l s in C h e s a p e a k e Bay, however, Bieri et al. (1982) c l a i m t h a t more t h a n 60 % of both t h e P b a n d Mn input is r e t a i n e d in t h e bed sediments. In t h e i r recent s t u d i e s of heavy m e t a l s in D e l a w a r e Bay (USA), Church, T r a m o n t a n o and Murray (1984 and l a t e r personal c o m m u n i c a t i o n ) c a l c u l a t e d r e t e n t i o n of 9 2 % of t h e Mn, 37% of t h e Cu and 32 % of t h e C d input to t h a t estuary. However, losses f r o m t h e e s t u a r y in t h a t analysis w e r e based o n c a l c u l a t i o n s of t h e probable flux o u t of t h e m o u t h of t h e Bay using

a layered flow model. When s e d i m e n t c o n c e n t r a t i o n s and a c c u m u l a t i o n r a t e s w e r e used, only s m a l l a m o u n t s of Mn a n d C d a p p e a r e d to b e r e t a i n e d in t h e s y s t e m (Church, personal communication). A t this point w e a r e n o t a w a r e of a n y convincing e v i d e n c e t h a t c l e a r l y c o n t r a d i c t s t h e findings regarding t h e behavior of Pb, Cu,Mn o r C d in N a r r a g a n s e t t Bay. Unfortunately, t h e number of mass b a l a n c e s for t h e s e e l e m e n t s is so s m a l l t h a t this is not

a particularly reassuring claim. T h e situation with r e g a r d to p e t r o l e u m hydrocarbons is such t h a t t h e r e d o not a p p e a r t o h a v e b e e n a n y o t h e r published a t t e m p t s to develop a n annual m a s s b a l a n c e for an estuary. T h e a c c u m u l a t i o n of s e d i m e n t s in e s t u a r i e s a p p e a r s to b e so e f f e c t i v e t h a t Meade (1981) h a s e s t i m a t e d t h a t , "Probably less t h a n 5 p e r c e n t of a l l river s e d i m e n t discharged into t h e tidal w a t e r s of t h e A t l a n t i c s e a b o a r d is deposited on t h e floor of t h e c o n t i n e n t a l shelf o r t h e d e e p sea". T h e s e s e d i m e n t s c o n t a i n a n o f t e n d r a m a t i c imprint of human a c t i v i t y and i m p a c t o n t h e e s t u a r y and i t s watershed. But i t i s a n i m p r i n t which results not only f r o m t h e input of m a t e r i a l s , b u t also f r o m t h e i n t e r a c t i o n of a g r e a t v a r i e t y of physical, c h e m i c a l and biological processes in t h e estuary. W e a r e only beginning to l e a r n t o r e a d t h a t record, b u t t h e e a r l y r e s u l t s suggest t h a t w e m a y l e a r n m o r e a b o u t w h a t flows b e t w e e n r i v e r s and t h e sea f r o m t h e humble muds on t h e e s t u a r y floor t h a n w e c a n from t h e w a t e r above.

ACKNOWLEDGMENTS W e a r e g r a t e f u l to a number of colleagues f o r s t i m u l a t i n g discussions of t h e questions addressed in t h i s p a p e r and, in s o m e cases, f o r permission to u s e t h e i r unpublished data. W e particularly t h a n k M.E. Q. Pilson,P. Santschi, C.A. O v i a t t , E. Hoffman, J.G. Quinn and

J.N. Gearing. S o m e of t h e c h e m i c a l analyses of N a r r a g a n s e t t Bay s e d i m e n t s w e r e run by S. Northby, D. S w i f t and J. Hopkins. P r e p a r a t i o n of t h i s m a n u s c r i p t w a s made possible

largely by g r a n t s f r o m t h e U.S. Env.ironmenta1 P r o t e c t i o n Agency and t h e S e a G r a n t Program, NOAA.

REFERENCES Adler, D., Amdurer, M. a n d Santschi, P.H., 1980. M e t a l t r a c e r s in t w o m a r i n e microcosms: sensitivity to s c a l e and configuration. In: J.P. Giesy, Jr. (ed.), Microcosms in Ecological Research. T e c h n i c a l Information C e n t e r , U.S. Dept. of Energy, CONF781101, Springfield, VA, pp. 348-368.

118

Andrilenas, P. and Serletis, B., 1984. Inputs outlook and situation, February 1984. USDA, Econ. Res. Ser. U.S. Government Printing Office, Washington, DC. Bennekom, A.J., van, and Salomons, W., 1981. Pathways of nutrients and organic m a t t e r from land to ocean through rivers. In: J.M. Martin, J.D. Burton and D. Eisma (eds), River Inputs to Ocean Systems. UNEP and UNESCO, Switzerland, pp. 33-51. Benninger, L.K., 1978. 21oPb balance in Long Island Sound. In: M.J. Holloway (eds), Geochim. et Cosmochim. Acta, 42 (6)pp. 1165-1 174.

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THE ROLE OF DETRITUS AT THE LAND-SEA BOUNDARY

K.H. MANN D e p a r t m e n t of F i s h e r i e s a n d Oceans, Marine Ecology Laboratory, Bedford I n s t i t u t e of Oceanography, P.O.Box 1006, D a r t m o u t h , Nova S c o t i a , B2Y 4A2 (Canada)

ABSTRACT Mann, K.H., 1986. T h e r o l e of d e t r i t u s at t h e land-sea boundary. In: P. L a s s e r r e and J.M. M a r t i n (eds), Biogeochernical P r o c e s s e s at t h e Land-Sea Boundary. Elsevier, Amsterdam. D e t r i t u s derived f r o m vascular p l a n t s (seagrasses, marsh g r a s s e s a n d mangroves) c o n t a i n s m u c h s t r u c t u r a l m a t e r i a l t h a t is indigestible to most animals. S u c h d e t r i t u s must undergo a lengthy period of f r a g m e n t a t i o n a n d colonization by b a c t e r i a a n d fungi b e f o r e i t c o n s t i t u t e s nutritious food f o r animals. During t h e process, a l a r g e proportion of i t s c a r b o n is oxidized to C 0 2 . On t h e o t h e r hand t h e a l g a e (seaweeds, b e n t h i c m i c r o a l g a e and phytoplankton) f o r m d e t r i t u s t h a t i s d i r e c t l y usable by animals, though i t s nutritional v a l u e m a y b e f u r t h e r e n h a n c e d by microbial colonization. As a result of t h i s d i f f e r e n c e , r a t h e r a s m a l l proportion of t h e c a r b o n production of s a l t marshes, s e a g r a s s beds and mangroves e n t e r s c o a s t a l food webs. Isotope s t u d i e s i n d i c a t e t h a t most of t h e c a r b o n in coastal a n i m a l s i s derived f r o m algal sources, w i t h minor contributions f r o m vascular m a r i n e plants. T h e dissolved o r g a n i c m a t t e r produced by both phytoplankton and m a c r o p h y t e s i s readily t a k e n up by b a c t e r i a a n d t h e s e provide a n i m p o r t a n t food s o u r c e f o r planktonic a n d b e n t h i c animals. B a c t e r i a a s s o c i a t e d with d e t r i t u s food w e b s a r e a n i m p o r t a n t mechanism f o r conserving nitrogen in c o a s t a l ecosystems.

INTRODUCTION D e t r i t u s i s o r g a n i c m a t t e r in t h e process of decomposition. I t w a s o n c e alive, b u t now i s d e a d a n d t h e tissues h a v e begun to d i s i n t e g r a t e both by autolysis a n d under t h e influence of t h e e n z y m e s s e c r e t e d by microbes. Historically, t h e t e n d e n c y w a s to regard decomposition as synonymous with rernineralization, a n d l i t t l e i n t e r e s t w a s shown in d e t r i t u s as a p o t e n t i a l s o u r c e of food f o r t h e a n i m a l s in c o a s t a l food webs. I t is now known t h a t t h e ecological r o l e of d e t r i t u s is twofold. If d e a d organic m a t t e r i s l e f t in c o n t a c t with

m i c r o b e s b u t isolated f r o m higher organisms i t will eventually d e c o m p o s e

c o m p l e t e l y , releasing n u t r i e n t m a t e r i a l s t h a t a r e available f o r new c y c l e s of plant production. But if, at s o m e i n t e r m e d i a t e s t a g e of decomposition t h e m i x t u r e of dead o r g a n i c m a t t e r a n d microbes i s i n g e s t e d by a n animal, i t m a y provide t h e basis f o r a p r o d u c t i v e food web. A t t h e p r e s e n t stage of m a r i n e ecology t h e key questions a r e :What proportion of d e t r i t u s g e n e r a t e d at t h e land-sea boundary i s destined to b e mineralized by b a c t e r i a a n d fungi and w h a t proportion e n t e r s t h e food w e b as a result of ingestion by

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animals ? H o w i m p o r t a n t is d e t r i t u s production c o m p a r e d with live p h o t o s y n t h e t i c production which e n t e r s t h e food web by herbivory ? ,My a t t e n t i o n w a s f i r s t drawn to t h e i m p o r t a n c e of d e t r i t u s when I w a s working in t h e R i v e r Tharnes a b o u t 25 y e a r s ago.

A

limnological colleague,

reviewing p r i m a r y

productivity of plants in lakes, r i v e r s and w e t l a n d s pointed o u t t h a t t h e r e w a s an e n o r m o u s production of plant m a t e r i a l by rooted plants, b u t nothing s e e m e d to b e e a t i n g t h e m a n d t h e y w e r e t h e r e f o r e a "dead-end" as f a r as t h e f r e s h w e t e r food c h a i n s leading to fish w e r e concerned. This proved to b e q u i t e u n t r u e in our studies of t h e R i v e r Thames.

W e followed t h e energy b u d g e t s of s e v e r a l s p e c i e s of fish, month by m o n t h throughout t h e year. In t h e winter m o n t h s t h e only way t h o s e fish balanced t h e i r e n e r g y budgets w a s by ingesting l a r g e q u a n t i t i e s of d e t r i t u s f r o m t h e bed of t h e river. Living in t h a t d e t r i t u s w e r e a f e w i n v e r t e b r a t e s , mainly w o r m s and i n s e c t larvae, b u t our e n e r g y b u d g e t s showed t h a t t h e fish w e r e a l s o taking considerable a m o u n t s of energy and n u t r i e n t s f r o m t h e detritus. Similar o b s e r v a t i o n s c a n b e m a d e in t h e coastal zone. A s w e m o v e t o w a r d s t h e coast f r o m o f f s h o r e w e n o t i c e t h a t with t h e shallowing of t h e w a t e r i t b e c o m e s possible f o r a t t a c h e d plants, s e a w e e d s and s e a g r a s s e s to r e c e i v e enough light to grow on t h e bottorn. Hard b o t t o m s a r e colonized by a l g a e , which h a v e a holdfast b u t no rooting system. S o f t , depositing s u b s t r a t e s a r e colonized by r o o t e d flowering plants, notably t h e seagrasses. In t h e i n t e r t i d a l z o n e w h e r e i t is rocky t h e a l g a e c o n t i n u e to t h e upper limit of t h e t i d a l excursion, e x c e p t in hot, a r i d c l i m a t e s w h e r e t h e y m a y b e excluded by desiccation, o r in northern l a t i t u d e s w h e r e t h e grinding of i c e on t h e s h o r e p r e v e n t s t h e i r colonization. In s e d i m e n t e d i n t e r t i d a l a r e a s s a l t m a r s h e s t e n d to d o m i n a t e in t e m p e r a t e c l i m a t e s while mangroves b e c o m e m o r e i m p o r t a n t in t h e tropics. All t h e s e p l a n t c o m m u n i t i e s produce l a r g e a m o u n t s of new tissue, b u t with a f e w e x c e p t i o n s nothing m u c h eats them. T h e y t e n d to grow f o r a season t h e n d i e off a n d b e c o m e detritus. When w e s t u d y t h e process closely w e find t h a t much of w h a t i s r e l e a s e d is p a r t i c u l a t e a n d readily visible e.g. f r a g m e n t s of l e a v e s o r of algal fronds, b u t t h e r e is also a considerable a m o u n t of dissolved o r g a n i c m a t t e r . T h e d e a d p a r t i c l e s soon b e c o m e colonized by b a c t e r i a a n d fungi, and b a c t e r i a also t a k e up much of t h e released dissolved organic m a t t e r . F o r t h e purpose of ecological b a l a n c e s h e e t s w e h a v e t o consider t h e production of p a r t i c u l a t e o r g a n i c m a t t e r (POM) w i t h a t t a c h e d microorganisms and t h e production of dissolved o r g a n i c m a t t e r (DOM) plus t h e b a c t e r i a a s s o c i a t e d with it, if t h e s e c a n b e identified. F o r c o m p l e t e n e s s w e a l s o h a v e to consider a n y d e a d o r g a n i c m a t t e r added to t h e pool by animals, f o r e x a m p l e f a e c a l pellets, d e a d bodies, and shed exoskeletons. In discussing t h e role of d e t r i t u s in t h i s symposium w e a r e t h e r e f o r e considering t h e r o l e of a l l d e a d o r g a n i c m a t t e r and i t s a s s o c i a t e d microorganisms. W e should n o t overlook t h e f a c t t h a t in c o a s t a l w a t e r s , as in t h e o p e n o c e a n , phytoplankton

also produces d e a d o r g a n i c m a t t e r .

125

WORK PRIOR TO 1970 P e t e r s e n and his c o l l e a g u e s in D e n m a r k m a d e t h e c l a i m as e a r l y as 1918 t h a t i t was t h e e x t e n s i v e s e a g r a s s beds around t h e coast of Denrnark t h a t provided t h e p r i m a r y production t o support t h e c o a s t a l ficheries through a d e t r i t u s food w e b ( P e t e r s e n , 1918). In t h e 1930's t h e r e w a s a g r e a t reduction in e e l g r a s s biomass f r o m what w a s c a l l e d " t h e wasting d i s e a s e of eelgrass" (although t h e c a u s e of t h e m o r t a l i t y is still in dispute, Rasmussen, 1977). When t h e r e w a s no c o n s e q u e n t c o l l a p s e in t h e fisheries, t h e i d e a of a connection b e t w e e n e e l g r a s s productivity and fish productivity w a s dropped. In t h e 1960's t h e r e w a s a revival of i n t e r e s t in t h e ecological role of c o a s t a l macrophytes, following a

detailed study of the fate of the productivity of a salt rnarsh a t Sapelo Island, Georgia, USA. T e a l (1962) m a d e t h e f i r s t e n e r g y budget for such a s y s t e m and showed t h a t while

only a b o u t 7 % of t h e fixed e n e r g y passed t o herbivores, a b o u t 45% w a s e x p o r t e d f r o m t h e marsh as POM. Odum and d e la C r u z (1967) showed t h a t in a s e r i e s f r o m living S p a r t i n a leaves, through dead brown l e a v e s t o fine p a r t i c u l a t e d e t r i t u s , t h e r e was a 2-3 fold increase in nitrogen c o n t e n t . T h e y also found t h a t t h e weight s p e c i f i c r a t e of oxygen consumption of s m a l l p a r t i c l e s (< 6 5 pm) w a s a l m o s t an o r d e r of magnitude g r e a t e r than t h a t of l a r g e p a r t i c l e s (> 240 ym). T h e y a t t r i b u t e d this to an i n c r e a s e in t h e r e l a t i v e amounts of microorganisms on t h e d e t r i t u s , c o n s e q u e n t on a n i n c r e a s e in t h e r a t i o of surface a r e a to v o l u m e as t h e p a r t i c l e s b e c a m e m o r e f r a g m e n t e d . T h e i n c r e a s e in microorganisms w a s thought to explain t h e i n c r e a s e in nitrogen c o n t e n t , a n d i t was pointed o u t t h a t f i n e d e t r i t u s derived f r o m S p a r t i n a w a s a n u t r i t i o u s s o u r c e of food for many f i l t e r feeding anirnals. In a n e l e g a n t e x p e r i m e n t , Newell (1965) showed t h a t when t h e f a e c a l p e l l e t s of t h e snail Hydrobia w e r e held f o r 3 d a y s t h e i r nitrogen c o n t e n t increased by a n order of magnitude. T h e snails could be induced to re-ingest t h e m , and when t h e m a t e r i a l c a m e o u t again t h e nitrogen c o n t e n t w a s a g a i n low, and could a g a i n b e observed to i n c r e a s e when held in s e a w a t e r f o r 3 d a y s (Fig. 1). T h e c h a n g e s in nitrogen c o n t e n t w e r e , of course, a t t r i b u t e d to colonization by m i c r o b e s a n d i t w a s s u g g e s t e d t h a t in passing through t h e g u t t h e Hydrobia d i g e s t e d t h e microbes b u t w a s u n a b l e to d i g e s t t h e main c o n t e n t s of t h e f a e c a l pellet. Marine macrophytes, especially t h e flowering p l a n t s (seagrasses, m a r s h g r a s s e s and mangroves) h a v e a high c o n t e n t of s t r u c t u r a l m a t e r i a l such as cellulose, which most a n i m a l s c a n n o t digest. However, by t h e e n d of t h e 1960's mechanism had been found for t r a n s f e r r i n g t h e production of m a c r o p h y t e s to i n v e r t e b r a t e s and h e n c e to higher l e v e l s in t h e food chain. I t w a s p o s t u l a t e d (for review, see Mann, 1972 a ) t h a t r e f r a c t o r y m a c r o p h y t e t i s s u e w a s colonized by fungi a n d b a c t e r i a , and t h a t t h e i n v e r t e b r a t e s consuming t h e d e t r i t u s d i g e s t e d t h e c o a t i n g of b a c t e r i a a n d fungi. S t a t e d simply, t h e t h e o r y w a s t h a t i n v e r t e b r a t e s don't eat m a c r o p h y t e detritus, t h e y eat m i c r o b e s which e a t t h e detritus. T h i s process is analogous to t h e situation in r u m i n a n t a n i m a l s : t h e y can't d i g e s t g r a s s a n d leaves, t h e m i c r o b e s in t h e r u m e n d o i t f o r t h e m , and t h e r u m i n a n t s d i g e s t t h e m i c r o b e s a n d t h e soluble p r o d u c t s of t h e i r e n z y m e a c t i v i t y . In t h e m a r i n e

126

Fig. 1. Changes in t h e carbon and nitrogen c o n t e n t of faeces of t h e snail Hydrobia vulvae, (i) during 3 days in sea water, (ii) durin passage through t h e gut and (iiif during a second period of exposure t o sea water. From Newell (1965).

DAYS

situation t h e microorganisms a r e usually in t h e environment rather than in t h e gut, but perform t h e s a m e role. With such a mechanism f o r trophic transfer available, people began to link t h e productivity of s a l t marshes, seagrass beds and kelp beds with t h e productivity of t h e coastal fisheries (e.g. Korringa, 1973, McHugh, 1976). Day et al. (1973)e s t i m a t e d t h a t for Barataria Bay, Lousiana, 30% of t h e macrophyte production was exported to t h e Gulf of Mexico ; Valiela et al. (1978) found t h a t 40% of t h e n e t annual production of t h e G r e a t Sipiwisset marsh was exported as particulate m a t t e r t o coastal w a t e r s ; Kikuchi (1971) documented t h e role of seagrass beds in J a p a n as feeding grounds f o r commercially important fish stocks ; Field et al. (1977)showed how kelp production off South Africa is used by mussels and o t h e r invertebrates which in turn support a commercially important stock of lobsters.

RATES OF DECOMPOSITION OF MATERIAL FROM DIFFERENT SOURCES The processing of macrophyte detritus from t h e living conditions t o t h e finest particles proceeds at different r a t e s for material of different origins. Early a t t e m p t s t o investigate this took t h e form of exposing samples of m a t e r i a l to t h e natural environment in replicate coarse-meshed n e t s or bags, and plotting t h e tLme course of disappearance. Absolute r a t e s of disappearance depend on t e m p e r a t u r e , w a t e r movement and t h e ability of invertebrates to e n t e r t h e bags, but under comparable conditions algae decompose most rapidly, followed by marsh grasses, while seagrasses appear most resistant to decay (Odum and d e la Cruz, 1967 ; Wood et al., 1967 ; Zieman, 1968). Harrison and Chan (1980) showed t h a t Zostera leaves t h a t had been dead up to 2 weeks contained a water-soluble substance which inhibited b a c t e r i a and algae. They speculated t h a t this substance might delay t h e

127 decomposition process. Valiela et al. (1979) showed t h a t newly d e a d S p a r t i n a contains considerable a m o u n t s of f e r u l i c a n d c o u m a r i c acids, which a r e d i s t a s t e f u l to amphipods and snails, a n d m i g h t t h e r e f o r e b e e x p e c t e d to slow t h e process of shredding a n d grinding, which o t h e r w i s e s e r v e s to a c c e l e r a t e t h e colonization by microbes. When w e a l s o consider t h a t , c o m p a r e d w i t h t h e algae, t h e v a s c u l a r m a r i n e plants t e n d to h a v e m o r e s t r u c t u r a l m a t e r i a l s u c h as cellulose which is known to b e r e s i s t a n t to t h e digestive e n z y m e s of all b u t a f e w specilized organisms, w e c a n begin to understand why decomposition i s slower in vascular p l a n t s t h a n in t h e algae. T h e colonization of m a r i n e m a c r o p h y t e s by b a c t e r i a a n d fungi begins while t h e y a r e s t i l l growing (Gesner et al., 1972 ; Meyers, 1974 ; Laycock, 1974) b u t t h e breaking up of t h e m a t e r i a l i n t o f i n e r a n d f i n e r p a r t i c l e s by autolysis, w a t e r m o v e m e n t and particularly by t h e shredding a n d grinding of i n v e r t e b r a t e , g r e a t l y i n c r e a s e s t h e s u r f a c e a r e a and h e n c e t h e populations of m i c r o b e s (Fenchel, 1970). T h e microbes t e n d to find in t h e d e t r i t u s a n a b u n d a n t s o u r c e of c a r b o n b u t limiting a m o u n t s of nitrogen and phosphorus. T h e y a r e c a p a b l e of taking t h e s e n u t r i e n t s f r o m t h e surrounding w a t e r , so t h a t r a t e s of decomposition a r e a function of t h e supply of N and P (Fenchel, 1977). E x p e r i m e n t s have also shown t h a t decomposition is f a s t e r in t h e p r e s e n c e of b a c t e r i a l p r e d a t o r s such as ciliates, r o t i f e r s , n e m a t o d e s a n s s m a l l c r u s t a c e a n s (Fenchel and Harrison, 1976). Their m o v e m e n t s probably b r e a k down diffusion g r a d i e n t s of oxygen and n u t r i e n t s around b a c t e r i a , h e n c e s t i m u l a t i n g b a c t e r i a l m e t a b o l i s m and decomposition of t h e p l a n t material. L a r g e r g r a z e r s such as snails and amphipods probably h a v e t h e s a m e e f f e c t .

STUDIES ON THE NUTRITIONAL QUALITY OF DETRITUS

In s p i t e of a l l t h e c a r e f u l work on t h e colonization of d e t r i t u s by microbes, a n d of t h e f a c t o r s t h a t influence t h e a c t i v i t y of t h o s e microbes, t h e r e h a s been a need f o r an i n t e g r a t e d s t u d y of t h e whole process of d e t r i t u s f o r m a t i o n , ingestion by a n i m a l s and production of new a n i m a l tissue. K.R. T e n o r e h a s explored in d e p t h t h e conversion of dead plant t i s s u e to a n i m a l tissue f o r o n e s p e c i e s of d e t r i t i v o r e , t h e small p o l y c h a e t e worm C a p i t e l l a c a p i t a t a . Using s t a n d a r d c u l t u r e s of t h e worm held under controlled conditions, h e f e d t h e m with d e t r i t u s of known origin a n d m e a s u r e d t h e i r growth. H e found t h a t t w o i m p o r t a n t p r o p e r t i e s of t h e d e t r i t u s a p p e a r e d to c o n t r o l t h e e f f i c i e n c y of i t s utilization : nitrogen c o n t e n t a n d "available c a l o r i c content", defined as t h a t portion of t h e c a l o r i c c o n t e n t hydrolyzed by normal hydrochloric acid under s t a n d a r d conditions. Vascular plant d e t r i t u s normally h a s a low nitrogen c o n t e n t (< 3%) and a low c o n t e n t of available calories (< 23% of t o t a l calories) b e c a u s e of i t s high c o n t e n t of s t r u c t u r a l m a t e r i a l s unavailable to

t h e animals. T h e a l g a e commonly h a v e b o t h a higher nitrogen c o n t e n t and a higher c o n t e n t of a v a i l a b l e calories. A t nitrogen c o n t e n t s below 3.5%

t h e e f f i c i e n c y of

incorporation of d e t r i t u s i n t o worm t i s s u e w a s proportional t o t h e nitrogen c o n t e n t , so t h a t nitrogen a p p e a r e d to b e t h e limiting f a c t o r . A t nitrogen levels a b o u t 3.5%

the

nitrogen w a s no longer limiting, a n d t h e e f f i c i e n c y of conversion to w o r m tissue was

128

c o r r e l a t e d w i t h t h e a v a i l a b l e c a l o r i c c o n t e n t (Tenore, 1977, 1981). F o r e x a m p l e , naturally grown S p a r t i n a is normally rich in s t r u c t u r a l m a t e r i a l s and proportionately low in a v a i l a b l e c a l o r i c c o n t e n t (13%). By growing S p a r t i n a under a r t i f i c i a l conditions T e n o r e

(1983) w a s a b l e t o boost t h e c o n t e n t of a v a i l a b l e c a l o r i e s to 44%. T h e e f f i c i e n c y of incorporation i n t o C a p i t e l l a t i s s u e i n c r e a s e d dramatically. T h e proposed explanation of t h e d i f f e r e n c e b e t w e e n vascular plant d e t r i t u s and s e a w e e d d e t r i t u s in e f f i c i e n c y of incorporation i n t o C a p i t e l l a w a s as follows. Vascular plant d e t r i t u s , having a low c o n t e n t of nitrogen and a v a i l a b l e calories, m u s t undergo a lengthy period of f r a g m e n t a t i o n and colonization by microbes b e f o r e it is usable by Capitella. During t h a t process, m u c h of t h e t i s s u e is mineralized by t h e microbes. S e a w e e d d e t r i t u s n e e d s a minimum of such conditioning. Much of it c a n b e used d i r e c t l y by t h e animals, a n d t h e r e m a i n d e r is fairly rapidly c o a t e d with microorganisms a n d r e n d e r e d p a l a t a b l e to t h e animals. T w o e x p e r i m e n t s t e n d e d to c o n f i r m t h i s hypothesis. T e n o r e and Hanson (1980) prepared d e t r i t u s f r o m marsh g r a s s (Spartina), s e a w e e d (Gracilaria) and periphyton. S o m e was f r o z e n soon a f t e r preparation, b u t o t h e r s a m p l e s w e r e a g e d in t h e p r e s e n c e of microbes f o r varying periods b e f o r e being frozen. C a p i t e l l a w a s t h e n grown in e a c h t y p e of d e t r i t u s . M e a s u r e m e n t s w e r e m a d e of t h e r a t e of oxidation of t h e d e t r i t u s , t h e microbial biomass, and n e t incorporation i n t o C a p i t e l l a (Fig. 2).

. .z 20

1.0 -

A

PERIPHYTON GRAClLARlA

-a

0.4 -

1

1

1

I

Iky<./

4

-1

B

PERIPHYTON

GRAClLARlA

SPARTINA

I

0

0

20

50

100

150

200

DAYS OF AGING

Fig. 2. A. R a t i o of worm production of t o t a l microcosm c a r b o n oxidation f o r t h r e e t y p e s of d e t r i t u s a g e d f o r various periods. B. R a t i o of microbial biomass to t o t a l microcosm c a r b o n oxidation f o r t h r e e t y p e s of d e t r i t u s a g e d f o r various periods. Both f r o m T e n o r e and Hanson, 1980.

129

T h e S p a r t i n a oxidized m o r e slowly t h a n t h e algae, and in e a c h c a s e t h e e f f i c i e n c y of conversion of d e t r i t u s t o microbial biomass, and of conversion of d e t r i t u s t o C a p i t e l l a biomass, (both on a daily basis) increased w i t h t h e a g e of t h e detritus. Findlay a n d T e n o r e (19x2) s o c c e e d e d in labelling plant i n a t e r i a l and m i c r o b e s on d e t r i t u s independently. T h e y prepared t h r e e kinds of d e t r i t u s : (i) tvith p l a n t i n a t e r i a l labelled with 15N, a n d microbes unlabelled ; (ii) with plant m a t e r i a l unlabelled, but microbes labelled with 15N ; a n d (iii) with both labelled. They did this for highly vascular m a t e r i a l ( m a r s h grass) a n d f o r seaweed. T h e w o r m s feeding on vascular p l a n t d e t r i t u s w e r e found to h a v e o b t a i n e d most of their I5N f r o m t h e microbes colonizing t h e s u r f a c e , while w o r m s feeding on t h e s e a w e e d w e r e found t o h a v e o b t a i n e d rnost of t h e i r 15N f r o m t h e d e a d p l a n t material. P u t t i n g all t h e s e results t o g e t h e r , it is c l e a r t h a t vascular plant d e t r i t u s when newly f o r m e d is a poor s o u r c e of n u t r i t i o n f o r t h e worms. Only a f t e r a lengthy period of conditioning in t h e e n v i r o n m e n t d o e s i t b e c o m e a good s o u r c e of nitrogen, c a r b o n and calories. But during t h a t conditioning period t h e microbes a r e oxidizing t h e plant tissues, a n d by t h e t i m e it h a s b e c o m e nutritious a good proportion has a l r e a d y been mineralized. Algal d e t r i t u s , on t h e o t h e r hand, is r e l a t i v e l y nutritious when newly f o r m e d , and is rapidly colonized by m i c r o b e s so t h a t it r e a c h e s peak condition in a f e w days. T h e n e t result is t h a t a l g a l m a t e r i a l i s used m u c h m o r e e f f i c i e n t l y in d e t r i t a l food webs t h a n is m a r s h grass, s e a g r a s s o r m a n g r o v e material. We m u s t b e c a r e f u l n o t to g e n e r a l i z e too broadly f r o m work with o n e s p e c i e s of worm, b u t l i m i t e d work with o t h e r s p e c i e s (e.g. Nephtys, T e n o r e e t al., 1977 ; n e m a t o d e s , Findlay, 1982) s u g g e s t s t h a t o t h e r p a r t s of t h e food w e b h a v e similar responses. WORK WITH CARBON ISOTOPE RATIOS a) Salt marshes

I t h a s b e e n found t h a t vascular p l a n t s divide i n t o t w o groups w i t h r e s p e c t to t h e i r s t a b l e c a r b o n isotope ratios. Those with a C 3 p a t h w a y of photosynthesis h a v e a 6 13C value of -24 to -34'/00 while C 4 p l a n t s h a v e less n e g a t i v e values, -6 t o -19'/00, and a l g a e h a v e i n t e r m e d i a t e v a l u e s of -12 to 23"/00 (Smith and Epstein, 1971). A n i m a l s feeding predominantly on o n e food s o u r c e h a v e b e e n found to have a similar 6 13C v a l u e to t h a t of t h e i r food. H a i n e s (1976) a n d H a i n e s a n d Montague (1979) looked at t h e c a r b o n isotope values of a v a r i e t y of p l a n t s and a n i m a l s living in a S p a r t i n a m a r s h a t S a p e l o Island, G e o r g i a (Fig. 3). T h e y found t h a t t h e S p a r t i n a and o t h e r grasses had

613C v a l u e s of -12.3

to -13.6"/00. O t h e r vascular p l a n t s such as J u n c u s w e r e -22.8 to -26.O0/0o while benthic d i a t o m s w e r e -16.2 to -17.9"/00.

T h e POM in t h e c r e e k s had v a l u e s in t h e r a n g e -19.8 to

-

2 2 . 8 " / 0 0 , suggesting t h a t it w a s derived q u i t e largely f r o m phytoplankton (-20 t o -22"/00)

r a t h e r t h a n Spartina. Marsh snails a n d i n s e c t s had 6 13C v a l u e s c l o s e to t h o s e of Spartina, b u t deposit-feeding c r a b s r e f l e c t e d t h e r a t i o s of t h e b e n t h i c diatoms. Mud snails and filter-feeding bivalves showed r a t i o s similar to t h o s e of b e n t h i c and pelagic algae. In t h e

130 subtidal regions of t h e c r e e k s (Hughes and Sherr, 1983) mud crabs and t w o kinds of fish (mummichogs and mullet) had values indicating a sizeable component of Spartina detritus,

SALT MARSH, GEORGIA, U:S.A, JUNCUS SOURCE MATERIAL

DJOMS

PHYTOPLANKTON

SPARTINA

POM I N CREEKS

--

II SAE MARSHTFZECES -~

INTERTIDAL INVERTEBRATES

SUBTIDAL INVERTEBRATES (DUPLIN RIVER)

--

SALT MARSH SNAILS DEPOSIT-FEEDING CRABS MUD SNAILS RIBBED MUSSELS OYSTERS MUD CRABS SQUID SHRIMPS (PALAEMONETES PENAEUS) IBLUE CRABS BROWN SHRIMPS OYSTERS MUYMICHOG (FUNDULUS) MULLET (MUGIL) MENHADEN (BREVOORTIA)

FISH (DUPLIN RIVER)

1

I

Fig. 3. Diagram of t h e stable carbon isotope ratios of organisms and P O M in a S a r t i n a marsh at Sapelo Island, Georgia. D a t a f r o m Haines (1976) and Haines and M o n t a g u e h % CARBON SPECIES

DERIVED SPARTINA

% CARBON

Fig. 4. Estimated percentages of carbon derived from Spartina f o r 30 i n v e r t e b r a t e and fish species in a Georgia salt marsh estuary. D a t a from Hughes and Sherr (1983).

131

while brown shrimp, menhaden and o y s t e r s w e r e a p p a r e n t l y m o r e d e p e n d e n t on t h e phytoplankton food web. T h e s e s t u d i e s a n d o t h e r s on s a l t marsh s y s t e m s ( r e v i e w e d in F r y and Sherr, in press) c l e a r l y i n d i c a t e t h a t e v e n t h e f a u n a living in t h e c r e e k s within t h e s a l t m a r s h e s d e r i v e only a minor p a r t of t h e i r n o u r i s h m e n t f r o m s a l t m a r s h d e t r i t u s (Fig. 4). b) Seagrass beds

Analogous s t u d i e s h a v e been m a d e in t h e vicinity of s e a g r a s s rrieadows. T h a y e r e t al. (1978) found t h a t e e l g r a s s had r a t i o s of -10.6 t o -12.2"/00, while t h e e p i p h y t e s w e r e a b o u t -16"/00. POM in suspension w a s much m o r e n e g a t i v e t h a n t h e e e l g r a s s or i t s epiphytes, in

t h e r a n g e -19 to 22O/oo and e v e n t h e s e d i m e n t in t h e e e l g r a s s bed w a s a b o u t - 1 9 ° / o o , indicating t h a t phytoplankton (usually around - 2 2 ' / 0 0 ) w a s making a major c o n t r i b u t i o n to suspended a n d s e d i m e n t e d POM. T h e i n v e r t e b r a t e s a n d fish spanned t h e r a n g e -13.1 t o 18.9O/oo,

-

which could b e explained by varying c o n t r i b u t i o n s f r o m Zostera, e p i p h y t e s and

plankton (Fig. 5 ) . T h e y concluded t h a t "seagrass c a r b o n plays a significant b u t not d o m i n a n t r o l e in t h e food w e b of t h i s Z o s t e r a bed".

SEAGRASS M E A D O W , N . C A R O L I N A , U.S.A. ZOSTERA SOURCE MATERIAL

= DETRITUS

t INVERTEBRATES

FISH

r I 1

-25

-

EPIPHYTES

SUSPENDED. OPEN OCEAN SUSPENDED, ESTUARY mSEDlMENTED ESTUARY EPIFAUNA O N GRASS BLADES DEPOSIT FEEDERS INVERT FILTER FEEDING BIVALVES m

=

-

SYNGNATHUS. BAIRDIELLA. URYPHIS

I

- 20

I

I

- I5

-1 0

1

I

-5

2

8l3C RATIO Fig. 5 . D i a g r a m of s t a b l e c a r b o n i s o t o p e r a t i o s in a s e a g r a s s m e a d o w in N o r t h Carolina. D a t a f r o m T h a y e r et al. (1978). Similar r e s u l t s w e r e o b t a i n e d f o r I z e m b e k Lagoon, in Alaska by McConnaughey and McRoy (1979). T h e y concluded t h a t b e n t h i c i n v e r t e b r a t e s such as t h e sea s t a r E v a s t e r i a s and t h e c l a m M a c o m a d e r i v e a b o u t 25% of t h e i r c a r b o n f r o m eelgrass, t h e s h r i m p Crangon dalli a b o u t 17%, t h e worm N e p h t y s caeca a b o u t 7%, while fish, depending w h e t h e r they a r e resident o r m i g r a t o r y derived f r o m o n e t h i r d down to a l m o s t none of t h e i r carbon f r o m t h e eelgrass. In arriving at t h e s e f i g u r e s t h e a u t h o r s took i n t o a c c o u n t t h e possibility t h a t c h a n g e s in 1 3 C c o n t e n t o c c u r in t h e food web, b e c a u s e 13C i s r e s p i r e d at a slower r a t e t h a n 12C. O n t h e o t h e r hand lipid s t o r a g e a p p a r e n t l y e n r i c h e s a n a n i m a l i n 1*C. T h e c o r r e c t i o n s required to allow f o r t h e s e s h i f t s a r e still t h e s u b j e c t of d e b a t e (reviewed in F r y a n d S h e r r , in press) so t h a t t h e p e r c e n t a g e s given a b o v e a r e q u i t e approximate. Nevertheless, t h e s e and o t h e r s t u d i e s m a k e i t c l e a r t h a t food w e b s in t h e vicinity of s a l t

132

marshes a n d s e a g r a s s beds a r e by no m e a n s primarily d e p e n d e n t on d e t r i t a l carbon. This d o e s n o t mean t h a t w e should now dismiss as u n i m p o r t a n t t h e role of m a c r o p h y t e d e t r i t u s in e s t u a r i n e a n d c o a s t a l food webs. F r y a n d P a r k e r (1979) c o m p a r e d t h e s t a b l e carbon isotope r a t i o s of a n i m a l s f r o m a T e x a s s e a g r a s s meadow with t h e r a t i o s in c o m p a r a b l e a n i m a l s t a k e n offshore. T h e p o l y c h a e t e worm D i o p a t r a c u p r e a o c c u r r e d in both situations and t h o s e t a k e n f r o m t h e s e a g r a s s m e a d o w s w e r e up to 8.3"/00 e n r i c h e d in 13C r e l a t i v e to t h o s e f r o m offshore. Fish and s h r i m p w e r e enriched o n a v e r a g e by 3.3"/00 and 5.1 O / o o

respectively.

C l e a r l y t h e s e a g r a s s d e t r i t u s w a s making a significant

contribution to t h e food web. Similarly, in t h e Caribbean, F r y et al. (1982) found a 4-6"/00 e n r i c h m e n t in 13C in fish t a k e n f r o m s e a g r a s s m e a d o w s and c o r a l reefs, r e l a t i v e to t h o s e taken offshore, and in t h e T o r r e s S t r a i t (Fig. 6) t h e r e w e r e e n r i c h m e n t s up to So/..

in

c o m p a r a b l e groups of a n i m a l s f r o m t h e s e a g r a s s beds c o m p a r e d with offshore, a l l indicating a s u b s t a n t i a l u p t a k e of d e t r i t a l c a r b o n f r o m t h e s e a g r a s s e s (Fry et al., 1983).

SOURCE

1

SEAGRASS MEADOW, -~ TORRES STRAIT ~. ~

~~

~

SEAGRASS EPIPHYTES

Poc

SEAGRASSES

I L

ANIMALS IN SEAGRASS MEADOW (BAMPFIELD)

1

ANIMALS FROM OFFSHORE

1

~

~

I

1

BENTHIC GASTROPODS I HOLOTHURIANS CRABS SHRIMPS POLYCHAETES = AMPHIPODS BIVALVES NUDIBRANCHS FISH ZOOPLANKTON HOLOTHURIANS CRABS =SHRIMPS =BIVALVES = ZOOPLANKTON FISH I

- 20

I

I

- I5

- I0

I

-5

0

Fig. 6. Distribution of s t a b l e c a r b o n i s o t o p e r a t i o s in a s e a g r a s s m e a d o w in t h e T o r r e s Strait, c o m p a r e d w i t h a n i m a l s f r o m offshore. D a t a f r o m F r y et al., 1983. When F r y (1984) used t h e c a r b o n i s o t o p e t e c h n i q u e to s t u d y t h e utilization of m a c r o p h y t e carbon in a Florida b e d of Syringodium h e found t h a t m o s t of t h e f a u n a had r a t i o s c h a r a c t e r i s t i c of t h e algae epiphytic o n t h e s e a g r a s s blades. In t h i s s i t u a t i o n t h e productivity of t h e e p i p h y t i c algae w a s as high, o r higher t h a n t h a t of t h e seagrass, a n d t h e s e a g r a s s d e t r i t u s a p p e a r e d to b e c a r r i e d a w a y f r o m t h e s e a g r a s s beds by s t r o n g t i d a l action. c) Mangroves

Rodelli et al. (1984) r e c e n t l y r e p o r t e d t h e f i r s t s t a b l e c a r b o n isotope s t u d y of a mangrove community. T h e y o b t a i n e d good s e p a r a t i o n of t h e p r i m a r y producers, with

133

benthic algae at -18.7 2 2.2O/oo, phytoplankton at 21.0 2 0.3"/00, and mangroves at 27.1 ? 1.2O/00. Twenty-nine of t h e 5 1 species of animals examined had ratios m o r e negative than t h e algae, indicating uptake t o mangrove carbon. These included s o m e commercially important species such as bivalves and shrimps. Animals collected from offshore showed no evidence of uptake of mangrove carbon. d) Algal seaweeds

On t h e whole, t h e s t a b l e carbon isotope r a t i o method has not proved t o be a useful approach to tracing pathways of utilization of seaweed detritus. In p a r t this is due t o t h e close similarity between

t h e isotope ratios of

t h e seaweeds and those of the

phytoplankton, but t h e r e is a n additional problem t h a t a particular blade of a single macrophyte may show a considerable variation f r o m one part to another. Thus Stephenson et al. (1984) found t h a t on a blade of t h e kelp Laminaria longicruris isotope ratios varied

spatially f r o m -12 t o -2O0/0o, and t h a t on t o p of this was an irregular and apparently unpredictable seasonal variation. Clearly, this is no basis f o r careful discrimination between kelp d e t r i t u s and phytoplankton (about -2l0/0o with a range of -18 to -24"/00, Fry and Sherr, in press). T h e question of t h e e x t e n t of utilization of kelp d e t r i t u s in food webs has been approached using o t h e r techniques.

STUDIES OF FOOD WEBS BASED ON KELP DETRITUS T h e seaweeds known as kelps and giant kelps (Laminariales) a r e built in such a way t h a t they produce detritus more o r less continuously. All a r e divisible into holdfast, stipe and blade (Fig. 7) and t h e blades normally have a growth zone at t h e base and a zone of erosion near t h e tip. They have been likened (Mann, 1972b) to "moving belts of tissue". Growth at t h e base compensates for erosion at t h e tip, and in t h e process large amounts of detritus, both particulate and dissolved, a r e released into t h e water. This detritus is

readily colonized by b a c t e r i a (Laycock, 1974) and appears t o b e less resistant t o digestion by animals than t h e d e t r i t u s derived from vascular plants. A group at C a p e Town under t h e leadership of Dr. John Field has made a n intensive study of t h e production and f a t e of tissue in a kelp bed dominated by Ecklonia maxima and Laminaria pallida. This system produces about 600 g C m-2 yr-1 as p a r t i c u l a t e organic m a t t e r and about 300 g C m-2 yr-1 as dissolved organic matter. A t t h e s a m e time, about

500 g C m-2 yr-1 is fixed by t h e phytoplankton (Newell and Field, 1983). T h e main consumers a r e filter-feeders : mussels, sponges, and ascidians. Between them, t h e y a r e capable of filtering a volume equal to all t h e water in t h e kelp bed once a day. Seiderer et al. (1982) showed t h a t t h e mussels had enzyme systems capable of digesting t h e carbohydrates contained in t h e kelps, and S t u a r t (1982) showed t h a t t h e mussels could grow well on a d i e t of kelp detritus. By an ingenious double-labelling technique S t u a r t et al. (1982) showed t h a t t h e mussels in t h e kelp bed w e r e actually incorporating carbon from t h e kelp debris i n t o their tissues, and w e r e not just removing t h e b a c t e r i a from i t s surface. Calculations showed t h a t when t h e kelp bed system is

134

"closed",

w i t h a minimal i m p o r t and e x p o r t , t h e c o m b i n e d production of k e l p and

phytoplankton f e l l a l i t t l e s h o r t of t h e c a r b o n r e q u i r e m e n t s of t h e consumers.

BLADE

Fig. 7. D i a g r a m of t h r e e main t y p e s of b l a d e : i) with s i m p l e blade, e.g. L a m i n a r i a longicruris ; ii) with s e c o n d a r y blades, e:g. ~Ecklonia r a d i a t a ; iii) with e l o n g a t e d s t i p e , secondary blades and a float, e.g. Ecklonia maxima.

In p r a c t i c e , t h e r e w e r e a l t e r n a t i n g periods of upwelling, when t h e c o n s u m e r s r e c e i v e d mainly k e l p d e t r i t u s , and downwelling, when t h e y r e c e i v e d mainly phytoplankton i m p o r t e d f r o m offshore. In a c o m p u t e r simulation Wulff a n d F i e l d (1983) showed t h a t k e l p POM provides u p to 70% of t h e food of f i l t e r f e e d e r s under upwelling c o n d i t i o n s and 10-30% of t h e food under downwelling conditions. T h e r e l a t i v e i m p o r t a n c e of phytoplankton and d e t r i t u s to t h e a n i m a l s depends on t h e r e l a t i v e f r e q u e n c y of upwelling and downwelling conditions, and o n t h e r a t e s of w a t e r e x c h a n g e in t h e k e l p beds (Fig. 8). In t h e model, using a "realistic pulsing r e g i m e s i m u l a t i n g conditions in t h e s o u t h e r n Benguela region" c o n s u m e r biomass i n c r e a s e d during w i n t e r and d e c r e a s e d during t h e s u m m e r upwelling season. F a e c a l production f e d b a c k i n t o t h e s y s t e m and w a s re-used by d e t r i t u s f e e d e r s , t h u s providing a continuous supply of d e t r i t u s e v e n under downwelling conditions. T h e main p r e d a t o r in t h e s y s t e m i s t h e r o c k l o b s t e r J a s u s lalandii, a n d t h i s had t h e effect of dampening f l u c t u a t i o n s in f i l t e r f e e d e r biomass. W e m a y n o t e in passing t h a t a high proportion of t h e world's l o b s t e r s t o c k s a r e found in a n d around k e l p beds w h e r e t h e y r e l y on detritus-based food w e b s f o r a good proportion of t h e i r nutrition. T h e C a p e Town s t u d y also included c o n s i d e r a t i o n of t h e fate of kelp cast up on t h e beach by storms. T h i s is n o t a trivial a m o u n t , i t w a s e s t i m a t e d at 1200-1800 t o n n e s per km of s h o r e l i n e per y e a r (Koop and Field, 1980). C r i f f i t h s and Stenton-Dozey (1981) showed t h a t t h e main i n v e r t e b r a t e colonists a r e amphipods, isopods a n d insects, t h e i r f a e c e s being a major c o m p o n e n t of t h e pile f e w d a y s a f t e r i t is deposited on t h e shore. Using a n ingenious s e r i e s of f i e l d e x p e r i m e n t s , Koop et al. (1982a a n d b) found t h a t t h e i n v e r t e b r a t e s consumed 74% of t h e k e l p within a 8-day period while b a c t e r i a c o n s u m e d 26%.

135

Fig. 8. E s t i m a t e s of p e r c e n t a g e of f o o d consumed by k e l p bed f i l t e r f e e d e r s t h a t is derived f r o m phytoplankton, kelp, faeces and bacteria. F r o m t h e simulation model of Wulff and Field (1983).

I n v e r t e b r a t e f a e c e s a m o u n t e d to 67% of t h e initial kelp carbon, and t h e s e w e r e in turn metabolized by b a c t e r i a in t h e sand. Overall, 100 g C of algal biomass yielded 23

- 28

gC

of b a c t e r i a l biomass.

(B)NITROGEN

( A ) CARBON KELP DEBRIS

KELP DEBRIS

INVERTEBRATES

INVERTEBRATES

69.8 LOSS FROM MlCROCOSM

28.0 BACTERIA

L E ACHATES TO SEA

p5J LEACHATES T O SEA

Fig. 9. Mass balances of (a) Carbon and (b) Nitrogen in a pile of decomposing kelp, a r e g carbon o r nitrogen derived f r o m 100 g c a r b o n o r nitrogen kelp. N o t e t h a t 69.8% of t h e c a r b o n is lost f r o m t h e microcosm b u t only 2.3% of t h e nitrogen.

-Ecklonia maxima. F i g u r e s

136

When the nitrogen budget of this system was drawn up, i t was found that the bacterial biomass incorporated 94% of t h e nitrogen present in t h e deposited kelp, even though it contained only 28% of the carbon (Fig. 9). This led to the idea that bacteria play an important role in conserving nitrogen in a detritus system. W e now see that three lines of work point in t h e same direction. Laboratory feeding experiments have shown that detritus derived from marsh grass, seagrass or mangrove has a high content of structural material t h a t is unavailable t o detritivorous animals until i t has been processed for a long time by bacteria and fungi. During t h a t period much of t h e material is mineralized by mirobial action. Macro- and micro-algae on the other hand, have a much lower content of indigestible material and a r e rather rapidly conditioned by t h e microbiota. These facts help explain why, even for animals living in a salt marsh or seagrass bed, the greater part of the carbon in their bodies appears to b e derived from planktonic or benthic algae. In the one kelp bed system that has been studied in sufficient detail, i t appears t h a t the invertebrate filter feeders receive considerable amounts of phytoplankton carbon.

DETRITUS DERIVED FROM PHYTOPLANKTON AND PERIPHYTON While marine macrophytes a r e quite characteristic of t h e land-sea interface, t h e s a m e cannot be said for phytoplankton. Yet the findings from work with stable carbon isotopes a r e that most organisms living even in seagrass beds of saltmarsh creeks derive more than half their carbon from algae, presumably phytoplankton and benthic microalgae. The whole question of the relationship between phytoplankton, their grazers, dissolved organic matter, bacteria, and their grazers, and the apparently changeable equilibrium between POM and DOM is poorly understood. In nearshore waters where t h e events a r e further confounded by additions of POM and DOM from macrophytes, t h e situation is even more confused. After very detailed studies in t h e Straits of Georgia, British Columbia, Parsons (1975) proposed the kind of relationship shown in Fig. 10.

MATERIAL

MATERIAL

Fig. 10. Diagram showing peaks of DOC, POC and bacteria all occurring a f t e r a phytoplankton maximum. After Parsons (1975). // 4

0

/

TIME OR DISTANCE FROM PRODUCTIVE ZONE-

137

Phytoplankton c e l l s g i v e off dissolved o r g a n i c m a t t e r in q u a n t i t i e s t h a t i n c r e a s e when n u t r i e n t s a r e limiting o r c e l l s a r e senescent. B a c t e r i a readily t a k e up this material, b u t t h e r e a r e also s e v e r a l physical processes c a p a b l e of c o n v e r t i n g DOM to POM. T h e b a c t e r i a t h e m s e l v e s m a y give off considerable q u a n t i t i e s of POM in t h e f o r m of "rope-like aggregates" (Paerl, 1978). D e t r i t u s consisting of s e n e s c e n t phytoplankton and d e a d organic m a t t e r derived f r o m a v a r i e t y of sources, a c c u m u l a t e s at t h e s e d i m e n t - w a t e r i n t e r f a c e in a r e a s of low turbulence. To t h i s is added f a e c a l p e l l e t s f r o m planktonic and b e n t h i c animals.

Direct measurements

with s e d i m e n t t r a p s a n d i n d i r e c t e s t i m a t e s f r o m

m e a s u r e m e n t s of c o m m u n i t y metabolism c o n v e r g e on t h e view t h a t t h e input of d e t r i t u s to t h e b e n t h o s r a n g e s f r o m under 100 g C m-2 yr-1 in less productive c o a s t a l a r e a s to o v e r

300 g C m-3 yr-1 in p l a c e s w h e r e t h e r e is a major deposition of m a t e r i a l f r o m t e r r e s t r i a l plants o r m a r i n e macrophytes. T h e d e t a i l s of t h e mechanisms by which d e t r i t u s is processed in t h e b e n t h o s will b e c o v e r e d by o t h e r c o n t r i b u t i o n s to this symposium. CONCLUSION A k e y question i d e n t i f i e d in t h e introduction w a s : What proportion of d e t r i t u s

g e n e r a t e d at t h e land-sea boundary is d e s t i n e d to b e mineralized by b a c t e r i a and fungi, and w h a t proportion e n t e r s t h e food w e b as a r e s u l t of ingestion by a n i m a l s ? T h e answer will, of course, b e s o m e w h a t d i f f e r e n t in e a c h locality, b u t s o m e t r e n d s a r e beginning to b e seen. B e c a u s e d e t r i t u s derived f r o m vascular m a r i n e plants is m o r e indigestible than d e t r i t u s derived f r o m s e a w e e d s o r microalgae, i t is inherently probable t h a t a larger proportion of i t will b e mineralized by microbes without being utilized in a n i m a l food webs. B u t a l m o s t a l l s t u d i e s to d a t e have i n d i c a t e d t h a t e v e n in a r e a s of abundant macrophytes, a l g a l c a r b o n is a n i m p o r t a n t c o m p o n e n t of a l l a n i m a l tissue. I t looks as if d e t r i t u s is a s e c o n d a r y s o u r c e of nutrition f o r m o s t c o a s t a l communities. I t m a y well b e t h a t f o r a n i m a l s t h a t h a v e access to living phytoplankton o r b e n t h i c algae, t h e s e provide t h e main l i f e s u p p o r t during t h e "bloom" seasons. D e t r i t u s is less seasonal in o c c u r r e n c e vascular plant d e t r i t u s r e s i s t s d e c a y f o r many months, and s e a w e e d d e t r i t u s is produced m o r e o r less continuously. I t s e e m s likely t h a t d e t r i t u s provides a r e s e r v e of food f o r t i m e s when f r e s h food i s less plentiful. This w a s t h e t e n t a t i v e conclusion of Heinle and F l e m e r (1975) f o r zooplankton in t h e P a t u x e n t R i v e r E s t u a r y and S t u a r t , H e a d and Mann (in press) f o r amphipods in t h e Bay of Fundy.

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Fenchel, T., 1977. Aspects of decomposition of seagrasses. In: C.P. M c Roy and C. Hellferich (eds) Seagrass Ecosystems: A Scientific Perspective. Marcel Dekker, New York, pp. 123-145. Fenchel, T. and Harrison, P., 1976. T h e significance of bacterial grazing and mineral cycling for t h e decomposition of p a r t i c u l a t e detritus. In: J.M. Anderson and A. Macfadyen (eds), The Role of Terrestrial and Aquatic Organisms in Decomposition Processes. Blackwell Scientific Publications, Oxford, pp. 285-299. Field, J.G., Jarman, N.G., Dieckman, G.S., Griffiths, C.L., Velimirov, B. and Zoutendyck, P., 1977. Sun, waves, seaweed and lobsters: T h e dynamics of a west-coast kelp bed. South African J. Sci., 73: 7-10. Findlay, S.E.G., 1982. E f f e c t of d e t r i t a l nutritional quality on population dynamics of a marine nematode (Diploaimella chitwoodi). Mar. Biol., 68: 226-227. Findlay, S.E.G. and Tenore, K.R., 1982. Nitrogen source for a detritivore: Detritus substrate versus associated microbes. Science, 218: 37 1-373. Fry, G., 1984. 13C/12C ratios and t h e trophic importance of algae in Florida Syringodium filiforme seagrass meadows. Mar. Biol., 79: 11-19. Fry, B., Lutes, R., Northam, M. and Parker, P.L., 1982. A 13C/12C comparison of food webs in Caribbean seagrass meadows and c o r a l reefs. Aquat. Bot., 14: 389-398. Fry, 8. and Parker, P.L., 1979. Animal d i e t in Texas seagrass meadows : 13C/12C evidence for t h e importance of benthic plants. Estuar. Coastal Mar. Sci., 8: 499-509. Fry, B., Scanlan, R.S. and Parker, P.L., 1983. 13C/12C ratios in marine food webs in t h e Torres Strait, Queensland. Aust. J. Mar. Freshwat. Res., 34: 707-715. Fry, B. and Sherr, E.B., in press. 13C/12C measurements as indicators of carbon flow in marine and freshwater ecosystems. Contrib. Mar. Sci.

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Gessner, R.V., GOOS, R.D. and Sieburth J. Mc N., 1972. The fungal microcosm of t h e internodes of Spartina alterniflora. Mar. Biol., 16: 269-273. Griffiths, C.L. and Stenton-Dozey, J., 1981. The fauna and r a t e of degradation of stranded kelp. Estuar. Coastal and Shelf Sci., 12: 645-653. Haines, E.B., 1976. Stable carbon isotope ratios in t h e biota, soils and tidal w a t e r s of a Georgia s a l t marsh. Estuar. Coast. Mar. Sci., 4: 609-616. Haines, E.B. and Montague, C.L., 1979. Food sources of estuarine invertebrates analyzed using 13C/12C ratios. Ecology, 60: 48-56. Harrison, P.G. and Chan, A.T., 1980. Inhibition of t h e growth of microalgae and b a c t e r i a by e x t r a c t s of eelgrass, (Zostera marina) leaves. Mar. Biol., 61: 21-26. Heinle, D.R. and Flemer, D.A., 1975. Carbon requirements of a population of t h e estuarine copepod Eurytemora affinis. Mar. Biol., 31: 235-247. Hughes, E.H. and Sherr, E.B., 1983. Subtidal food webs in a Georgia e s t u a r y : 13C/12C analysis. J. Exp. Mar. Biol. Ecol., 67: 227-242. Kikuchi, T., 1974. J a p a n e s e contributions on consumer ecology in eelgrass (Zostera marina L.) beds, with special r e f e r e n c e to trophic relations and resources in inshore fisheries. Aquaculture, 4: 145-160. Koop, K. and Field, J.G., 1980. The influence of food availability on t h e population dynamics of a supralittoral isopod Ligia d i l a t a t a Brandt. J. Exp. Mar. Biol. Ecol., 48: 61-72.

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Koop, K., Newell, R.C. and Lucas, M.I., 1982a. Biodegradation and carbon flow based on kelp (Ecklonia maxima) debris in a sandy beach microcosm. Mar. Ecol. Prog. Ser., 7: 315-326. Koop, K., Newell, R.C. and Lucas, M.I., 1982b. Microbial regeneration of nutrients from t h e decomposition of macrophyte debris on t h e shore. Mar. Ecol. Prog. Ser., 9: 91-96. Korringa, P., 1973. T h e e d g e of t h e North Sea as a nursery ground and shellfish area. In: E.D. Coldberg (ed.), North S e a Science. M.I.T. Press, Cambridge, Mass, pp. 361-381. Laycock, R.A., 1974. The d e t r i t a l food chain based on seaweeds. I. Bacteria associated with t h e s u r f a c e of Laminaria fronds. Mar. Biol., 24: 223-231. Mann, K.H., 1972a. Macrophyte production and d e t r i t u s food chains in coastal waters. Mem. 1st. Ital. Idrobiol., 29 suppl.: 353-383. Mann, K.H., 1972b. Ecological energetics of t h e seaweed zone in a marine bay on t h e A t l a n t i c c o a s t of Canada. 11. Productivity of t h e seaweeds. Mar. Biol., 14: 199-209. McConnaughey, T. and McRoy, C.P., 1979. 13C label identifies eelgrass (Zostera marina) carbon in an Alaskan estuarine food web. Mar. Biol., 53: 263-269. McHugh, J.L., 1976. Estuarine fisheries : a r e they doomed ? In: M. Wiley (ed.) Estuarine Processes. Academic Press, New York, Vol. 1, pp. 15-27. Meyers, S.P., 1974. Contribution of fungi to biodegradation of Spartina and o t h e r brackish marshland vegetation. Veroff. Int. Meeresforsch Bremerhaven, Suppl. 5: 357-375. Newell, R.C., 1965. T h e role of d e t r i t u s in t h e nutrition of t w o marine deposit feeders, and t h e bivalve Macoma balthica. Proc. 2001. SOC. t h e prosobranch Hydrobia Lond., 144: 25-45. Newell, R.C. and Field, J.G., 1983. The contribution of b a c t e r i a and d e t r i t u s to carbon and nitrogen flow in a benthic community. Mar. Biol. L e t t e r s , 4: 23-36. Odum, E.P. and d e la Cruz, A.A., 1967. P a r t i c u l a t e organic d e t r i t u s in a Georgia saltmarsh-estuarine ecosystem. In: C.H. Lauff (ed.), Estuaries. Amer. Assoc. for t h e Adv. of Sci. Washington, pp. 383-388. Paerl, H.W., 1978. Microbial organic carbon recovery in aquatic ecosystems. Limnol. Oceanogr., 23: 927-935. Parsons, T.R., 1975. P a r t i c u l a t e organic carbon in t h e sea. In: J.P. Riley and C. Skirrow (eds) Chemical Oceanography. Academic Press, New York, Vol. 2, 2nd edn. pp. 365383. Petersen, C.J.C., 1918. T h e sea bottom and i t s production of fish food. A survey done in connection with t h e valuation of t h e Danish w a t e r s from 1883-1917. Rep. Danish. Biol. Sta., 25: 1-82. Rasmussen, E., 1977. T h e wasting disease of eelgrass (Zostera marina) and i t s e f f e c t s on environmental f a c t o r s and fauna. In: C.P. McRoy and C. Hellfferich (eds), Seagrass Ecosystems : A Scientific Perspective. Marcel Dekker, New York, pp. 1-51. Rodelli, M.R., Gearing, J.N., Gearing, P.J., Marshall, N. and Sasekumar, A., 1984. Stable isotope r a t i o as a tracer of mangrove carbon in Malaysian ecosystems. Oecologia, 61: 326-333. Seiderer, L.J., Newell, R.C. and Cook, P.A., 1982. Quantitative significance of style enzymes from t w o marine mussels (Choromytilus meridionalis Krauss and P e r n a perna L.) in relation to diet. Mar. Biol. Letters, 3: 257-271.

140 Smith, R.N. and Epstein, S., 1971. Two categories of 13C/12C ratios for higher plants. Plant. Physiol., 47: 380-384. Stephenson, R.L., Tan, F.C. and Mann, K.H., 1984. Stable carbon isotope variability in marine macrophytes and its implications for food web studies. ;Mar. Biol., 81: 223-230. Stuart, V.. 1982. Absorbed ration, respiratorv costs and resultant scope for growth in the mussel Aulacomya ater (Molina) fed on a -diet of kelp detritus of differeit ages. Mar. Biol. Letters, 3: 289-306. Stuart, V., Field, J.G. and Newell, R.C., 1982. Evidence for absorption of kelp detritus by the ribbed mussel Aulacomya a f t e r using a new 15C-labelled microsphere technique. Mar. Ecol. Prog. Ser., 9: 263-271. Stuart, V., Head, E.J.H. and Mann, K.H., in press. Relative contributions of benthic diatoms and Spartina alterniflora (Loisel) detritus t o t h e nutrition of the amphipod Corophium volutator (Pallas) on a Bay of Fundy mud flat. J. Exp. Mar. Biol. Ecol. Teal, J.M., 624.

1962. Energy flow in the salt marsh ecosystem of Georgia. Ecology, 43: 614-

Tenore, K.R., 1977. Growth of Capitella capitata cultured on various levels of detritus derived from different sources. Limnol. Oceanogr., 22: 936-94 1. Tenore, K.R., 1981. Organic nitrogen and caloric content of detritus. 1. Utilization by the deposit-feeding polychaete Capitella capitata. Estuar. Coastal and Shelf Sci., 17: 733742. Tenore, K.R., 1983. What controls the availability of detritus derived from vascular plants: organic nitrogen enrichment or caloric availability ? Mar. Ecol. Prog. Ser. 10: 307-309. Tenore, K.R. and Hanson, R.B., 1980. Availability of detritus of different types and ages t o a polychaete macroconsumer Capitella capitata. Limnol. Oceanogr., 25: 553-558. Tenore, K.R., Tietjen, J.H. and Lee, J.J., 1977. E f f e c t of rneiofauna on incorporation of aged eelgrass, Zostera marina detritus by the polychaete Nephtys incisa. J. Fish. Res. Board Can., 35: 563-567. Thayer, G.W., Parker, P.L., La Croix, M.W. and Fry, B., 1978. The stable carbon isotope ratio of some components of an eelgrass (Zostera marina) bed. Oecologia, 35: 1-12. Valiela, I., Koumjian, L. and Swain, T., 1979. Cinnamic acid inhibition of detritus feeding. Nature, 250: 55-57. Valiela, I., Koumjian, L., Swain, T., Teal, J.M. and Hobbie, J.E., inhibition of detritus feeding. Nature, 180: 55-57.

1979. Cinnamic acid

Wood, E.J.F., Odum, W.E. and Zieman, J.C., 1967. Influence of seagrasses on the productivity of coastal lagoons. In: Lagunas Costeras, un Symposia. Mem. Simp. Intern. Lagunas Costeras. UNAM-UNESCO, Mexico, D.F., pp. 495-502. Wulff, F.V. and Field, J.C., 1983. Importance of different trophic pathways in a nearshore benthic community under upwelling and downwelling conditions. Mar.-Ecol. Prog. Ser., 12: 217-228. Zieman, J.C., 1968. A study of the growth and decornposition of seagrass Thalassia testudinum. Ms. Thesis, University of Miami.

141

SPATIAL AND TEMPORAL DISTRIBUTION OF BACTERIAL POPULATIONS IN MARINE SHALLOW WATER SURFACE SEDIMENTS

L.-A. MEYER-REIL Institut f u r Meereskunde an der Universitat Kiel, Marine Mikrobiologie, Dusternbrooker Weg 20, 2300 Kiel 1 (Federal Republic of Germany)

ABSTRACT. Meyer-Reil, L.A., 1986. Spatial and temporal distribution of bacterial populations in marine shallow w a t e r s u r f a c e sediments. In: P. Lasserre and J.M. Martin (eds), Biogeochemical Processes at t h e Land-Sea Boundary. Elsevier, Amsterdam. In t h e paper presented, spatial and temporal a s p e c t s of t h e distribution of bacterial populations in marine shallow water surface sediments a r e discussed. Bacterial number and biomass as determined by epifluorescence microscopy turned o u t to b e closely related to sediment properties such as grain s i z e and organic m a t t e r content. For an "average" sediment, o n e may e x p e c t lo9 cells per g of dry weight sediment with a corresponding biomass of 10 ug bacterial C per cell. Benthic bacterial populations underwent strong seasonal variations which could b e t r a c e d back to t h e accumulation of organic material in t h e sediment following sedimentation events. In shallow water s u r f a c e sediments bacterial communities w e r e governed by diurnal rhythms obviously closely related to microphytobenthos primary production. E s t i m a t e s of bacterial production using quite different approaches demonstrated t h a t between 10 and 800 mg C m-2 d-1 w e r e fixed by bacterial secondary production.

INTRODUCTION Coastal sediments play an important role in nutrient regeneration for marine ecosystems.

These processes a r e governed by t h e activity of

benthic bacteria.

Microbiological work in sediments has been concentrated on t h e investigation of nutrient cycles as well as on t h e spatial and temporal distribution of bacteria. In t h e studies of nutrient cycles, t h e activity of benthic bacteria was indirectly concluded f r o m changes in t h e concentrations or turnover r a t e s of inorganic and organic chemical parameters. Generally, corresponding information on t h e development of t h e bacterial populations themselves could not b e included. This paper summarizes information on t h e spatial and temporal distribution of benthic bacterial

populations.

Many

examples

discussed

originate

from

microbiological

investigations of sediments from t h e brackish water Kiel Bight (Baltic Sea, Federal Republic of Germany). Existing d a t a and concepts illustrating t h e spatial and temporal distribution of b a c t e r i a in shallow water s u r f a c e sediments a r e organized under t h e following topics :

142

I) Sediments as h a b i t a t s for b a c t e r i a 2) Bacterial number and biomass

3) Seasonal development of bacterial communities 4) Diurnal fluctuations of bacterial populations 5) E s t i m a t e s of bacterial production. SEDIMENTS AS HABITATS FOR BACTERIA

Sediments represent a complex environment consisting of particles which a r e more or less densely packed and surrounded by interstitial water. In beach sediments which a r e extremely exposed to wave action and tidal activity more than 95 % of t h e bacteria a r e a t t a c h e d to particle surfaces (Meyer-Reil et al., 1978). Deeper sediments c a r r y a much higher percentage of bacteria "free-floating" in t h e interstitial water (up t o 50 % of t h e t o t a l number ; Weise and Rheinheimer, 1978 ; Weise and Rheinheimer, 1979). Bacteria colonize only a small proportion of t h e available particle s u r f a c e a r e a (0.01 to 5 % ; range of t h e d a t a obviously dependent upon t h e method used f o r t h e calculation

of surface a r e a ; cf. Hargrave, 1972 ; Rublee and Dornseif, 1978 ; Weise and Rheinheimer, 1978 ; DeFlaun and Mayer, 1983). Deep pores of wheathered feldspar and clay grains do

not appear to b e inhabited by bacteria (DeFlaun and Mayer, 1983). Bacteria preferentially

Fig. 1. Scanning e l e c t r o n micrograph of a 0.5 mm q u a r t z grain. N o t e t h a t organic m a t e r i a l is concentrated in protected areas. The numbers on t h e q u a r t z grain mark s i t e s of specific investigation (for details cf. Weise and Rheinheimer, 1978).

143

colonize a r e a s of low relief such as depressions and crevices of particles where t h e cells a r e protected against grazing and mechanical demages (Nickels et al., 1981 ; cf. Fig. 1). The habitation in t h e s e a r e a s is an expression of preferential survival rather than preferential colonization as i t could b e demonstrated by DeFlaun and Mayer (1983). The benthic bacterial flora is of a high diversity : rods, cocci and curved cells of different sizes. Single cells or small colonies a r e prevailing. Most of t h e bacteria a r e found in extracellular slime layers consisting of bacterial fibrous webs and mucus produced by diatoms (Weise and Rheinheimer, 1978 ; Moriarty and Hayward, 1982). Through t h e formation of this organic m a t e r i a l of a considerable structural complexity, bacteria may influence t h e t e x t u r e of sediments like i t was demonstrated f o r fungi mycelium in sand dune soils (Clough and Sutton, 1978).

BACTERIAL NUMBER AND BIOMASS As basic members of t h e food chain, benthic b a c t e r i a represent an important nutrient source f o r meio- and macrofauna. T h e distribution of bacterial number and biomass was shown t o be closely related t o sediment properties, from which organic material and grain size a r e probably most important. Number and biomass as determined by epifluorescence microscopy Beside scanning electron microscopy, epifluorescence microscopy permits a reliable e s t i m a t e of number and biomass of b a c t e r i a in sediments. However, prior to counting b a c t e r i a have to b e liberated from t h e particle surfaces. Different extraction techniques have been applied : t r e a t m e n t of sediments with s u r f a c e a c t i v e agents, homogenization (Meyer-Reil et al.,

19781, and sonication (Weise and Rheinheimer, 1978 ; Ellery and

Schleyer, 1984). From t h e s e techniques, sonication gave t h e most reliable results with t h e highest percentage of b a c t e r i a (approximately 95 % of t h e t o t a l number) liberated from t h e particle surfaces (Meyer-Reil, 1983). However, most recently this technique has been discussed with special regard to a possible destruction of bacterial cells (Ellery and Schleyer, 1984). A f t e r sonication sediment samples a r e diluted, filtered onto prestained Nucleopore polycarbonate filters (pore size 0.2 pm) and stained with fluorescence dyes (e.g. acridine orange). Bodies with clear outline, 6 a c t e r i a l shape and distinct fluorescence (orange or green) a r e collnted as bacterial cells. For biomass determinations slides of characteristic microscopic fields can b e prepared. The slides may b e analysed by means of a n image analyser which reduces t h e uncertainty t o group t h e b a c t e r i a into arbitrary size classes by e y e (Krambeck et a1.,1981 ; Meyer-Reil, 1983). Using conversion factors, d a t a can be extrapolated to biomass in t e r m s of carbon (cf. Ferguson and Rublee, 1976 ; Cammen, 1982 ; Bakken and Olson, 1983). Uncertainties arising from inadequacies of t h e sonication technique and t h e conversion f a c t o r s a r e obviously minor as compared t o t h e subjectivity involved in t h e counting

144 procedure. However, up to now, beside scanning electron microscopy, epifluorescence rnicroscopy is the only direct approach t o gain information on bacterial number and biomass (cf. Fig. 2).

Fig. 2. Epifluorescence photograph of bacteria from a sandy sediment of the Kiel Bight (Baltic Sea ; FRG). The bacteria were liberated from the sediments by sonication. The sample was diluted and stained with acridine orange. Bar represents 3 um (from MeyerReil, 1984a).

Local variations of number and biomass Data on local variations in bacterial number and biomass summarized from the literature a r e presented in Fig.'3 and 4. The d a t a set comprises sediments from arctic, antarctic, boreal and tropical regions. Generally, bacterial numbers a r e in the range of 108 to 1011 cells per gram of dry weight sediment with a concentration of t h e d a t a around

lo9 cells (cf. Fig. 3). Numbers turned out to be much more related t o t h e type of sediment than to t h e region where t h e samples were taken. The lowest cell numbers were found in sandy sediments, the highest numbers in muddy sediments (cf. below). With increasing sediment depths, a slight decrease in bacterial numbers becomes obvious. However, even in the I1 m horizon of a sediment profile from the Antarctica, lo9 cells per gram of sediment could be detected, only insignificantly less than in t h e surface horizon (MeyerReil, 1984 a). Because of the time consuming procedure of size fractionation, much less d a t a a r e available on bacterial biomass (Fig. 4). Generally, bacterial carbon is in t h e range of 1 to lo3 Pg per gram of dry weight sediment thus contributing significantly to the total benthic biomass as determined by ATP measurements (for data cf. Graf et al., 1982 ; Graf

et al., 1983). Again, the lowest bacterial biomass values were found in sandy sediments, the highest values in muddy sediments.

145

Bacterial number per g of sediment 109

108

10’0

10” d

9 9

9 9 1

9 9 1 2

3

9

Fig. 3. S u m m a r y of d a t a concerning b a c t e r i a l n u m b e r s in s e d i m e n t s of d i f f e r e n t areas. E a c h of t h e d o t s r e p r e s e n t o n e d a t a point t a k e n f r o m t h e l i t e r a t u r e listed o n t h e right panel : 1-Dale, 1974 ; 2-Kepkay et al., 1979 ; 3G r i f f i t h s et al., 1978 ; 4Meyer-Reil et al., 1981 ; 5Meyer-Reil et al., 1978 ; 6Meyer-Reil et al., 1980 ; 7Meyer-Reil, 1981 ; 8-Weise and Rheinheimer, 1978, 1979 ; 9-Meyer-Reil unpublished d a t a 10-Rheinheimer, unpublished d a t a ; 11-Jones, 1980 ; 12Moriarty, 1980.

Bacterial biomass NgC per g of sediment 100

10’

102

lo3 d

... ... . . .. m

1

C

E

2 41

ul-

I: 0

21 N

.

w

. 12

Fig. 4. S u m m a r y of d a t a concerning b a c t e r i a l biomass in s e d i m e n t s of d i f f e r e n t areas. F o r explanations see

Fig. 3. 12

12

146

111

vn

. . I

XI

_m

In v

+ling

.n. .N. .VI . . .x. m.. ~

.. *

r

.....

n

. . . . A

N

IX XI

month

Fig. 5 . Bacterial numbers per c m 3 of sediment on a hypothetical horizontal profile from sandy sediments ( w a t e r depth 0, 10 m) t o sandy-mud sediments (water depth 18 m) and to muddy sediments (water depth 20 m) of t h e Kiel Bight (Baltic Sea, FRG).

Relationship between number and grain size A horizontal profil from beaches (water depth 0 m) into deeper w a t e r s (18, 28 m) of t h e Kiel Bight revealed an inverse relationship between benthic bacterial number and grain size of t h e sediment (Fig. 5). Cell numbers increased significantly from sandy to sandy-mud and to mud sediments. This general inverse relationship certainly r e f l e c t s both t h e g r e a t e r s u r f a c e a r e a and t h e higher organic m a t t e r c o n t e n t in fine sediments as compared to c o a r s e ones (Dale, 1974 ; Tanoue and Handa, 1979). However, t h e question still remains open whether t h e bacteria simply respond to higher organic m a t t e r level originally present in fine sediments or whether higher bacterial numbers in f i n e sediments results in higher organic m a t t e r accumulations (DeFlaun and Mayer, 1983).

Relationship between biomass and organic material Relationships between bacterial biomass and organic m a t t e r c o n t e n t of t h e sediments seem to be very complex. In sediments of t h e Kiel Bight, bacterial carbon increased significantly from sandy beach sediments to sandy-mud sediments. However, in muddy sediments bacterial biomass did not further increase despite of t h e more than twofold increase in organic m a t t e r as compared to sandy-mud sediments (Fig. 6). On an average, bacterial carbon in sediments of t h e Kiel Bight accounted f o r 0.7 ? 0.2% (standard deviation of t h e mean ; 76 observations) of t h e sediment organic carbon. In

t h e literature, higher values have been reported (1.2 %, Dale, 1974 ; less than 2%,

147

Cammen, 1982) which may be partly due t o methodological differences in calculating bacterial biomass. Bacterial carbon as percentage of the total sediment carbon varied dependent upon the types of sediments in the Kiel Bight. The lowest percentage was found in sandy beaches (0.5 -I 0.1% ; 36 observations), a considerable higher percentage in sandymud sediments (0.9 2 0.2% ; 21 observations), and again a lower percentage in muddy sediments (0.7 2 0.2% ; 19 observations). Parallel, t h e average bacterial cell weight varied in the different types of sediment investigated in t h e Kiel Bight. For sandy sediments an average cell weight of 15.8~10-9 5 2.5~10-9 pg of bacterial carbon was calculated as compared t o 20.4~10-9 t 3.9~10-9 vg (sandy-mud sediments) and 11.3~10-9 -I 1.7~10-9 pg of bacterial carbon (muddy sediments).

+

+ .X

X

x

x

X

x

x

Fig. 6. Bacterial biomass versus total organic matter content on a hypothetical horizontal profile from sandy beaches (x) to sandy-mud sediments (0)and t o muddy sediments (t) of the Kiel Bight (Baltic Sea, FRG). As an explanation for the observations described above, both the concentration and the

suitability of t h e sediment organic material have to be considered. In t h e Kiel Bight, t h e sandy-mud sediments obviously offer optimal conditions for the development of the bacterial populations, sustaining the highest total bacterial biomass, the highest average cell weight and the highest percentage of bacterial carbon as compared to the total sediment carbon. In sandy beach sediments on t h e one hand and in muddy sediments on the other hand, conditions for the development of bacterial populations have t o be regarded as suboptimal. In sandy sediments the concentration of organic material may be t h e limiting factor for bacterial growth. In muddy sediments, however, much of the organic material may consist development.

of

highly

refractory

compounds

thus

restricting

bacterial

biomass

148

Size spectrum of biomass Grouping the bacteria into different size classes revealed a characteristic spectrum of bacterial biomass in sediments of t h e Kiel Bight. Generally, small-size bacteria (volume

<

0.3 pm3) accounted for the major part of the total bacterial biomass, followed by

medium-size bacteria (volume 0.3-0.6

pm3) and large-size bacteria (volume

>

0.6 pm3).

This pattern fits into the average "Sheldon" spectrum, a characteristic distribution of benthic biomass with two main peaks in t h e largest (> 2 mm ; corresponding to macrofauna) and in t h e smallest size classes (< 2 pm ; corresponding to bacteria, respectively ; cf. Shwinghamer, 198 1). Deviations from this typical pattern in the distribution of benthic biomass have been interpreted by t h e author as an expression of t h e effect of exogenous disturbance factors. This interpretation can be related t o deviations from the "normal" bacterial biomass spectrum as well. The input of the phytoplankton blooms into t h e sediments in autumn and spring, respectively, as well as mass abundances of predators l e t t o drastic shifts in the size spectrum of bacterial biomass in sediments of t h e Kiel Bight (cf. below). SEASONAL DEVELOPMENT OF BACTERIAL COMMUNITIES In t h e literature information on t h e seasonal development of benthic bacteria is limited. From the investigations of Montagna (1982) and Cammen (1982) no general trend for seasonal variations of benthic bacteria could be detected. The studies of Rublee (1982) and DeFlaun and Mayer (1983) revealed a positive correlation between benthic bacteria and temperature. Detailed investigations in a sandy-mud sediment of the Kiel Bight using a high time resolution in sampling demonstrated strong seasonal variations in the development of bacterial populations closely related to specific ecological situations and sedimentation events (Meyer-Reil, 1983, 1984b). Input of organic material into t h e sediment In recent years it could be shown that in boreal marine systems the main sedimentation events and therefore t h e main food supply for the benthos occur in autumn and spring, respectively. Studies in the Kiel Bight have demonstrated that large amounts of the primary produced material in autumn and spring do not enter t h e pelagic food web

but rather s e t t l e onto the sediment. This material already represents 2/3 of the total yearly input from t h e pelagic into t h e benthic system (Smetacek, 1980). Investigations of Peinert et al. (1982) and Gral et al. (1983) followed the biomass development of the autumn and spring phytoplankton bloom in t h e water column and the sedimentation of t h e primary produced organic material by sediment traps. In sediments of t h e Kiel Bight, three periods of accumulation of organic material could be distinguished : in autumn, winter and spring (Fig. 7 ; Meyer-Reil, 1983). The enrichment of organic material during November could b e traced back to t h e breakdown and sedimentation of the autumn phytoplankton bloom composed of dinoflagellates and

149

diatoms. T o t a l organic m a t t e r , protein and carbohydrate accumulated in distinct, s e p a r a t e peaks. During winter a slow continuous increase of organic material was observed in t h e sediment surface. The organic m a t t e r consisted of resuspended sediment, material from t e r r e s t r i a l origin as well as of macrophyte debries eroded by winter storms. The breakdown of t h e spring phytoplankton bloom (mainly diatoms) l e t t o an enrichment of organic material in t h e sediment surface during l a t e March to mid April. Again s e p a r a t e peaks

were

recorded for t o t a l organic m a t t e r ,

protein and carbohydrae.

These

accumulation periods t e r m e d "autumn", "winter"- and "spring-input" turned o u t to b e of high relevance for t h e development of t h e benthic communities in sediments of t h e Kiel Bight (Gral et al., 1983 ; Meyer-Reil, 1983, 1984b). SUMLER STACNPIDN -1

UP, AUTUMN INPUIi

N

f

P

l

WINTER INPUT

D

l

J

'

I

F

'

SPRINO INPUT

M

'

A

,FAUNA DEVELOPMNI

I

M

'

J

'

Lj

Fig. 7. Seasonal variations in concentrations and exoenzymatic decomposition r a t e s of organic m a t e r i a l in t h e 0 to I c m horizon of a sandy-mud sediment (water depth 18 m) of t h e Kiel Bight (Baltic Sea, FRC). Illustrated a r e : concentrations of protein and carbohydrate, and activity ofN-amylase (mg of amylopectin a z u r e decomposed per g of dry weight sediment per hour). T h e headline on top c h a r a c t e r i z e s specific ecological situations and e v e n t s a f f e c t i n g t h e sediments.

Heat production as measurement of community metabolism. Direct calorimetry (heat production) represents a reliable method for t h e estimation

of t o t a l benthic community metabolism, because i t is a d i r e c t measurement of t h e energy flow through t h e system. Although t h e h e a t release from t h e activity of extracellular enzymes and f r o m chemical oxidations a r e also included, t h e l a t t e r t w o components a r e thought to b e of minor importance for t o t a l h e a t loss from sediments (Pamatmat, 1982).

As an i m m e d i a t e response to t h e "autumn"- and "spring-input" in sediments of t h e Kiel Bight, h e a t production culminated. A less obvious response was found during "winterinput". However, considering t h e t e m p e r a t u r e dependence of benthic metabolism, t h e r e was still a considerable and long lasting h e a t production during this period (Graf et al., 1983). Whereas h e a t production comprises all types of benthic metabolism, electron-

150

transport-activity (ETS) r e l a t e s to t h e a c t i v i t y of r e s p i r a t o r y chains. As pointed o u t by P a m a t r n a t et al. (198 I), t h e q u o t i e n t b e t w e e n h e a t production a n d ETS-activity should s e r v e as a q u a l i t a t i v e indicator for c h a n g e s in t h e t y p e of metabolism. Following t h e "autumn-"

a n d !'spring-input",

a strong

increase

in

this quotient

w a s observed,

d e m o n s t r a t i n g a s h i f t in t h e t y p e of b e n t h i c metabolism t o w a r d s f e r m e n t a t i o n . This coincided w i t h suboxic conditions in t h e sedirnent s u r f a c e mainly c a u s e d by biological oxygen consumption (Graf et al., 1983).

Decomposition of particulate organic material. T h e main portion of t h e input of o r g a n i c m a t e r i a l i n t o t h e s e d i m e n t is p a r t i c u l a t e organic c a r b o n which h a s to b e e n z y m a t i c a l l y decomposed, at l e a s t partly, prior t o incorporation i n t o cells. In this process, e x t r a c e l l u l a r e n z y m e s a r e involved which a r e s e c r e t e d by living c e l l s (Corpe a n d Winters, 1972) o r l i b e r a t e d during t h e lysis of d e a d and decaying cells. As shown by Burns (1980) s o m e of t h e s e e n z y m e s m a y r e t a i n t h e i r a c t i v i t y o u t s i d e t h e c e l l s by t h e f o r m a t i o n of hurnic-enzyme c o m p l e x e s bound to c l a y particles. T h e a c c u m u l a t i o n of o r g a n i c m a t e r i a l in s e d i m e n t s of t h e Kiel Bight during "autumn-", "winter-"

a n d "spring-input"

l e t to corresponding s t i m u l a t i o n s in

t h e enzymatic

decomposition r a t e s of c a r b o h y d r a t e and protein ( a c t i v i t y of O( -amylase, p r o t e o l y t i c enzymes, Fig. 7). E x o e n z y m a t i c responses w e r e highest in a u t u m n as c o m p a r e d t o winter and spring. T h i s is obviously a r e f l e c t i o n of b o t h t h e higher t e m p e r a t u r e a n d t h e higher b e n t h i c biomass in autumn. During "autumn-"

a n d "spring-input"

a s t i m u l a t i o n of

e n z y m a t i c decomposition r a t e s a l r e a d y o c c u r r e d when c o n c e n t r a t i o n s of p a r t i c u l a t e o r g a n i c m a t e r i a l s t a r t e d to a c c u m u l a t e in t h e s e d i m e n t s u r f a c e indicating a n induction of e n z y m a t i c a c t i v i t i e s by increasing c o n c e n t r a t i o n s of s u i t a b l e s u b s t r a t e s (Meyer-Reil, 1983). T h e r e is s t r o n g e v i d e n c e f r o m l a b o r a t o r y a n d field d a t a t h a t under anoxic conditions t h e e n z y m a t i c decomposition of p r o t e i n is retarded. During s u m m e r stagnation, a n anoxic period in which hydrogen sulfide i s prevailing in s e d i m e n t s of t h e Kiel Bight, protein a c c u m u l a t e d . T h e c o n c e n t r a t i o n of t h e s t o r e d p r o t e i n w a s e v e n c o m p a r a b l e to t h a t m e a s u r e d during "autumn"- a n d "spring-input". Following t h e introduction of oxygen i n t o t h e s e d i m e n t (break up of s u m m e r stagnation), p r o t e i n c o n c e n t r a t i o n s significantly decreased. P a r a l l e l p e a k s in h e a t production a n d A T P imply t h a t t h e s t o r e d protein was rapidly consumed and i n c o r p o r a t e d i n t o b e n t h i c biomass (cf. Graf et al., 1983 ; MeyerReil, 1983).

Bacterial development in autumn and spring. As shown f o r s e d i m e n t s of t h e Kiel Bight, b a c t e r i a r e a c t e d to t h e "autumn-" and

"spring-input",

respectively, with t w o s e p a r a t e peaks. T h e f i r s t peak a l r e a d y o c c u r r e d

when c o n c e n t r a t i o n s of o r g a n i c m a t e r i a l s t a r t e d to a c c u m u l a t e in t h e s e d i m e n t surface. This d e m o n s t r a t e s t h a t b a c t e r i a a l m o s t i m m e d i a t e l y responded to t h e availability of decomposable o r g a n i c material. T h e second peak in b a c t e r i a l p a r a m e t e r s coincided with t h e main input of o r g a n i c m a t e r i a l following t h e f i n a l breakdown and s e d i m e n t a t i o n of t h e phytoplankton blooms (Fig. 8).

151 SUMMER STAGN4TK)N (WEAK Up,AUTWN INPUT I

WlNlER INPUT

I

SPRING INPUT

A

-

,FAUNA O E M L O P M N l

I

M

'

J

'

ii; a

E

120

LO

c4

VI

a

E

m

Fig. 8. Seasonal variations in microbiological p a r a m e t e r s in t h e 0 t o 1 c m horizon of a sandy-mud sediment ( w a t e r depth 18m) of t h e Kiel Bight (Baltic Sea, FRG). Illustrated are: t o t a l number of bacteria, t o t a l bacteria biomass and biomass spectrum. T h e biomass spectrum comprises : small-size bacteria (volume < 0.3 m3 ; closed circles), medium-size b a c t e r i a (volume 0.3-0.6 pm3 ; open circles), and large-size b a c t e r i a (volume > 0.6 vm3 ; crosses). F o r t h e headline on t o p cf. Fig. 7. T h e bacterial population faced with t h e "autumn-input" was derived from an anoxic population ( f e r m e n t a t i v e bacteria, s u l f a t e reducers) which prevailed during summer stagnation. Within this population t h e input of freshly produced organic m a t e r i a l caused a d r a s t i c shift. B a c t e r i a primarily r e a c t e d with a strong increase in c e l l volume (biomass production). Deviating from i t s "normal" distribution (cf. above), t h e s i z e s p e c t r u m was dominated by

medium

and large-size

cells.

Following t h e final breakdown and

sedimentation of t h e autumn phytoplankton bloom, t h e b a c t e r i a subsequently responded with cell division (increase in cell number). Compared to autumn, t h e history of t h e b a c t e r i a l population faced with t h e "springinput" was q u i t e different. During winter oxic conditions prevailed in t h e sediment. Due to erosion of t h e sediment caused by winter storms, t h e b a c t e r i a l population was declining. T h e "spring-input" hit a n impoverished bacterial community which immediately reacted with a strong increase in cell volume, but only a small i n c r e a s e in cell number. Again, deviations f r o m t h e normal s i z e spectrum w e r e observed : small, medium and large-size cells almost equally contributed to t h e t o t a l biomass. Following t h e final breakdown and sedimentation of t h e spring p h y t q d a n k t o n bloom, bacterial number and biomass responded with a second stimulation.

152

Bacterial development in winter. During winter resuspended sediment, t e r r e s t r i a l m a t e r i a l and eroded macrophytes represent an additional food supply f o r benthic bacterial populations of t h e Kiel Bight. Bacterial number;, biomass and cell-division activity showed a slow continuous increase up to values t h a t w e r e even higher as compared to those obtained following t h e "autumn-

input" (Fig. 8). Taking into account t h e more r e f r a c t o r y nature of t h e organic m a t e r i a l and t h e low t e m p e r a t u r e with reduced matabolic activity rates, t h e accumulation of bacterial biomass during winter is surprising (cf. temperature-dependent development of benthic b a c t e r i a reported by DeFlaun and Mayer, 1983). However, t h e relatively long t i m e the bacterial population had available for i t s "undisturbed development'' and t h e limited number of g r a z e r s have to b e considered. With regard to t h e n a t u r e of t h e organic material and t h e slow continuous development, t h e response of t h e bacterial community t o t h e "winter-input" differed basically f r o m t h e spontaneous bacterial development following t h e input of t h e phytoplankton blooms in autumn and spring, respectively. Bacteria and f a u n a development. The development of t h e benthic fauna in spring greatly influenced t h e composition of t h e bacterial community in sediments of t h e Kiel Bight. Through t h e activity of polychaetes t h e sediment s u r f a c e was firmely glued together. This ecological situation was reflected by a bacterial population which consisted of a high number of almost exclusively small-size cells with a corresponding low biomass (Fig. 8). Since t h e b a c t e r i a actively grew as demonstrated by a high number of dividing cells, nutrient deficiency could not be t h e reason f o r t h e impoverishment of t h e bacterial population. More likely, preferential grazing on medium and large-size b a c t e r i a l e t to t h e restriction in t h e size spectrum of t h e bacteria. Although l i t e r a t u r e regarding interrelationships between bacteria and fauna components is sparse, t h e r e is s o m e evidence for t h e stimulation of bacterial activity by grazing (Cerlach, 1978 ; Morrison and White, 1980).

DIURNAL FLUCTUATIONS OF BACTERIAL POPULATIONS The strong diurnal rhythms of benthic primary production (Jorgensen et al., 1979 ; Karg, 1979 ; Revsbech et al.,

1981) imply a coupling between autotrophic and

heterotrophic processes in shallow water sediments. Evidence for t h e existence of diurnal rhythms in benthic bacterial a c t i v i t i e s obviously closely r e l a t e d to primary production was obtained from investigations of

sediments below sea-grass beds in Moreton Bay,

Queensland, Australia (Moriarty and Pollard, 1982) and f r o m studies in sandy sediments of t h e Kiel Bight, Baltic Sea, FRC (Meyer-Reil and Craf, unpublished data). Since t h e l a t t e r study comprises various p a r a m e t e r s related to benthic biomass and activity, t h e discussion of diurnal fluctuations of bacterial population will b e based upon t h e s e results.

153

B e n t h i c biomass and activities. In sandy s e d i m e n t s of t h e Kiel Bight s a m p l e d during a 36 hour c y c l e in s u m m e r , strong diurnal f l u c t u a t i o n s o c c u r r e d in t o t a l o r g a n i c m a t t e r , b e n t h i c biomass a n d a c t i v i t i e s (Fig. 9, 10). T o t a l o r g a n i c m a t t e r and living b e n t h i c biomass (derived f r o m ATP m e a s u r e m e n t s ) a c c u m u l a t e d a r o u n d midnight a n d d e c r e a s e d during t h e day. B a c t e r i a l biomass, however, r e v e a l e d a n opposite fluctuation p a t t e r n : m a x i m a w e r e observed in l a t e a f t e r n o o n and m i n i m a b e t w e e n midnight and e a r l y morning (cf. Fig. 9). A s i n d i c a t e d by t h e a l m o s t c o n s t a n t r a t i o b e t w e e n t o t a l o r g a n i c m a t t e r a n d protein, t h e composition of o r g a n i c m a t e r i a l did not c h a n g e during t h e day/night cycle. Assuming a conversion f a c t o r of 200 for c a l c u l a t i n g o r g a n i c c a r b o n f r o m ATP, and assuming t h a t carbon r e p r e s e n t s 50% of t h e t o t a l o r g a n i c m a t e r i a l , living biomass on a n a v e r a g e a c c o u n t e d f o r 5% of t h e t o t a l organic carbon. B a c t e r i a l biomass m a d e up 5% of t h e t o t a l living biomass. B e n t h i c a c t i v i t i e s w e r e well c o r r e l a t e d a m o n g e a c h o t h e r d e s p i t e of t h e fact t h a t q u i t e d i f f e r e n t p a r a m e t e r s of h e t e r o t r o p h i c a c t i v i t y w e r e m e a s u r e d such as t o t a l m e t a b o l i c a c t i v i t y ( h e a t production), e x o e n z y m a t i c decomposition of p a r t i c u l a t e organic m a t e r i a l ( a c t i v i t y of

o( -amylase), a n d b a c t e r i a l u p t a k e of dissolved o r g a n i c s u b s t r a t e s

(14C-labelled glucose). B e n t h i c a c t i v i t i e s i n c r e a s e d during t h e morning, c u l m i n a t e d around noon a n d d e c r e a s e d during a f t e r n o o n a n d night (cf. Fig. 10). A corresponding diurnal p a t t e r n in b a c t e r i a l a c t i v i t y w a s r e p o r t e d by Moriarty a n d Pollard (1982) f o r s e d i m e n t s below s e a g r a s s b e d s based upon t h e incorporation of t h y m i d i n e i n t o DNA. T r o p h i c interrelationships. A t l e a s t in periods with s u f f i c i e n t light supply, shallow w a t e r s e d i m e n t s m a y r e p r e s e n t self-supporting s y s t e m s governed by b e n t h i c p r i m a r y production. I t m a y b e s p e c u l a t e d t h a t h e t e r o t r o p h i c b e n t h i c a c t i v i t i e s a r e s t i m u l a t e d by t h e e x c r e t i o n of s u b s t a n c e s from t h e b e n t h i c p r i m a r y production (microphytobenthos) which is i n i t i a t e d by light in t h e early morning. B a c t e r i a t a k e up t h e e x c r e t i o n products and respond with a subsequent i n c r e a s e in biomass ( n o t e t h e t i m e l a g b e t w e e n b a c t e r i a l u p t a k e of s u b s t a n c e s and i n c r e a s e in biomass, Fig. 9, 10). B a c t e r i a as basic m e m b e r s of t h e food c h a i n a r e g r a z e d by meio- and macrofauna. A s a consequence, total b e n t h i c a c t i v i t y ( h e a t production) and e n z y m a t i c decomposition of p a r t i c u l a t e o r g a n i c m a t e r i a l in t h e s e d i m e n t as well as in s e l e c t e d meiofauna o r g a n i s m s (cf. F a u b e l a n d Meyer-Reil, 1983) increase. A f t e r maximum values a r e r e a c h e d around noon, b e n t h i c a c t i v i t i e s d e c l i n e t o w a r d s t h e a f t e r n o o n and night obviously d u e to a d e c r e a s i n g supply with p r i m a r y produced material.

154

rl

a c

+ d I

I

+ I

I

+

+

+

+

I

I

I

I

I

I

I

t

+

*

*

+

*

I

1

I

I

1

I

1

L

0

20

2L

L

I

2L 20 16 -June 12-1

16 12 June 13

*

t

* I-June 1L

Fig. 9. Diurnal fluctuations of t o t a l organic m a t t e r , ATP, and bacterial biomass in t h e 0 t o 1 c m horizon of a sandy sediment (water depth 10 m) of t h e Kiel Bight (Baltic Sea, FRG) sampled every 4 hours during a 36 hour cycle (June 12 t o J u n e 14, 1980).

-June

12-1

June 13

I-June 1L

Fig. 10. Diurnal fluctuations of e x o e n z y m a t i c decomposition r a t e s of carbohydrate (activity of &-amylase), bacterial u p t a k e of 14C-glucose, and h e a t production in t h e 0 to 1 c m horizon of a sandy sediment (water d e p t h 10 m) of t h e Kiel Bight (Baltic Sea, FRC) sampled e v e r y 4 hours during a 36 hour c y c l e ( J u n e 12 to J u n e 14, 1980).

155

ESTIMATES OF BACTERIAL PRODUCTION Since b a c t e r i a play an important role as basic members of t h e food chain in sediments (Yingst and Rhoads, 1980 ; Gerlach, 1978), information on bactrial production is urgently needed. In t h e literature, however, reliable d a t a on bacterial production a r e sparse. Certainly this is a reflection of t h e methodological problems involved. Different approaches have been applied to gain information on bacterial production in sediments (cf. Table I). Glucose uptake. In sandy beaches of t h e Kiel Bight, a c t u a l uptake (flux) of glucose by bacteria amounted to 0.1 pg of carbon (glucose) per gram of dry weight sediment per h (summer conditions, a v e r a g e of 12 observations ; Meyer-Reil, 1978 ; Meyer-Reil et al. 1980). In t h e s e sediments approximately 2% of t h e t o t a l dissolved organic carbon (DOC) existed in t h e form of labile carbon (1% amino acids, 1% monosaccharides) from which glucose roughly represents one q u a r t e r (Meyer-Reil et al., 1978). Under t h e assumption t h a t bacteria t a k e up t h e remaining t h r e e q u a r t e r s of t h e DOC with t h e s a m e velocity, total bacterial carbon uptake would amount to 0.4 pg per g per h. Taking into account respiration (40% as an average of t h e respiration of different organic substrates) and excretion ( e s t i m a t e of lo%, no d a t a available) b a c t e r i a would produced 0.2 pg of carbon per g per h, which is equivalent to 28 mg of carbon per m2 of sediment per day (Table 1). TABLE 1. Summary of d a t a concerning bacterial production in sediments of different areas.

Sediment

Method

Production (mg C m-2 d-1)

(I) Sandy beaches,

Glucose uptake

28

(2) Sediment associated with seagrass beds Moreton Bay, Queensland, Australia

T hymidine incorporation

12

(3) Sand, Nearshore western Atlantic Ocean Sapelo Island, Georgia, USA

Thymidine incorporation

100-800

Kiel Bight, Baltic Sea, FRG

(4) Sandy-mud/mud Kiel Bight, Baltic Sea, FRG Autumn Winter Spring ( 5 ) Sand,

Seasonal changes in biomass 140/370 20/10

300/120 Diurnal changes in biomass

80

Kiel Bight, Baltic Sea, FRG Authors : (I), Meyer-Reil et al. (1980) ; (21, Moriarty & Pollard (1982) ; (3), Fallon e t al. (1983) ; (41, Meyer-Reil (1983, and unpublished) ; (51, Meyer-Reil (unpublished).

156

This m e a n s t h a t b e t w e e n 10 and 30% of t h e rnicrophytobenthos p r i m a r y production in this a r e a (Karg, 1979) is fixed by b a c t e r i a l secondary production, a r a n g e which sounds reasonable adding a t l e a s t s o m e c o n f i d e n c e to t h e b a c t e r i a l production d a t a calculated. However, as pointed out above, various assurnptions h a v e to b e considered. T h y m i d i n e incorporation. T h e incorporation of thymidine i n t o DNA w a s used by Moriarty and Pollard (1982) and Fallon et al. (1983) to c a l c u l a t e b e n t h i c b a c t e r i a l production. F o r s e d i m e n t s below s e a g r a s s b e d s in Moreton Bay, Queensland, Australia, t h e f o r m e r a u t h o r s r e p o r t e d a b a c t e r i a l production of 12 m g of c a r b o n p e r m2 of s e d i m e n t per day. N e a r s h o r e s e d i m e n t s f r o m t h e w e s t e r n A t l a n t i c O c e a n , S a p e l o Island, Georgia, USA, r e v e a l e d a much higher b a c t e r i a l production (100-800 m g of c a r b o n per m2 per day ; cf. l a t t e r authors). T h e lower limit of t h e s e d a t a well a g r e e with t h e b a c t e r i a l production e x t r a p o l a t e d f r o m seasonal and diurnal c h a n g e s in b a c t e r i a l biomass in s e d i m e n t s of t h e Kiel Bight (cf. below, cf. T a b l e 1). However, c a l c u l a t i n g b a c t e r i a l production f r o m t h e incorporation of thymidine requires a n u m b e r of assumptions which a r e d i f f i c u l t to verify. Among these, t h e question of isotope dilution and t h e validity of conversion f a c t o r s to c a l c u l a t e production f r o m b a c t e r i a l u p t a k e of t h y m i d i n e s e e m to b e m o s t i m p o r t a n t (for a d e t a i l e d discussion cf. papers mentioned a b o v e a n d l i t e r a t u r e c i t e d therein).

Diurnal and seasonal changes i n biomass. C a l c u l a t i n g b a c t e r i a l production f r o m c h a n g e s in b a c t e r i a l biomass in s a m p l e s c o l l e c t e d f r e q u e n t l y during s h o r t periods of t i m e ( r a n g e of hours, cf. Fig. 9 ) a t a fixed location primises to o f f e r t h e m o s t reliable e s t i m a t e s . By t h i s m e t h o d t h e addition of exogenous s u b s t r a t e s a n d a r t i f i c i a l conditions during t h e incubation in t h e l a b o r a t o r y c a n b e avoided. E x c e p t for t h e conversion f r o m b a c t e r i a l volume to weight in t e r m s of c a r b o n (cf. above), no conversion f a c t o r s o r a s s u m p t i o n s h a v e t o b e applied. However, o t h e r problems arise. A s u f f i c i e n t number of subsamples h a v e to b e c o l l e c t e d by d i v e r s at e a c h t i m e i n t e r v a l f r o m a d e f i n e d s e d i m e n t a r e a to avoid a r t e f a c t s d u e t o patchiness. S i n c e b a c t e r i a l e x c r e t i o n of s u b s t r a t e s and g r a z i n g o n b a c t e r i a b e t w e e n t h e s a m p l e i n t e r v a l s c a n n o t b e a c c o u n t e d for, t h e v a l u e s gained c e r t a i n l y p r e s e n t a n u n d e r e s t i m a t i o n of b a c t e r i a l production which i s d i f f i c u l t to evaluate. B a s e upon s t u d i e s of diurnal f l u c t u a t i o n s at a shallow w a t e r s e d i m e n t in t h e Kiel Bight

(cf. above), a b a c t e r i a l n e t production of 80 m g of c a r b o n per m2 p e r day w a s calculated. F u r t h e r e s t i m a t e s of b a c t e r i a l production w e r e d e r i v e d f r o m investigations of seasonal v a r a i t i o n s in b a c t e r i a l biomass at t w o s e d i m e n t s t a t i o n s in d e e p e r w a t e r s of t h e Kiel Bight. A s response to t h e input of t h e phytoplankton bloom in a u t u m n , b a c t e r i a l production a m o u n t e d to 140 and 370 m g of c a r b o n per m 2 p e r d a y (sandy-mud a n d muddy sediment, respectively). T h e corresponding v a l u e s in spring w e r e 300 a n d 120 m g of c a r b o n per m 2 p e r day, r e s p e c t i v e l y (cf. T a b l e 1). A s i t w a s pointed o u t above, t h e s e v a l u e s well

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a g r e e w i t h t h e lower r a n g e of b a c t e r i a l production d a t a derived frorn t h e incorporation of thymidine i n t o DNA (Fallon et al., 1983).

CONCLUSION F r o m t h e i n f o r m a t i o n a v a i l a b l e it c a n b e a c c e p t e d t h a t b a c t e r i a c o l o n i z e s e d i m e n t s in high n u m b e r and biomass. Although t h e y r e p r e s e n t less than 1% of t h e t o t a l s e d i m e n t o r g a n i c carbon, b a c t e r i a c o n t r i b u t e significantly t o t h e benthic biomass t h u s stressing t h e i r i m p o r t a n t r o l e as n u t r i e n t s o u r c e for t h e b e n t h i c fauna. B e t w e e n t h e distribution of b a c t e r i a and s e d i m e n t p r o p e r t i e s such as grain s i z e and o r g a n i c m a t t e r , c l o s e c o r r e l a t i o n s exist. However, as shown f o r t h e relationship b e t w e e n o r g a n i c m a t e r i a l and b a c t e r i a l biomass, i n t e r p r e t a t i o n s a r e d i f f i c u l t t o derive. P r o c e s s e s within t h e b e n t h i c b a c t e r i a l c o m m u n i t y m a y o c c u r in very s h o r t t i m e scales. T h e s t r o n g diurnal r h y t h m s observed i n d i c a t e a c l o s e coupling b e t w e e n a u t o t r o p h i c and h e t e r o t r o p h i c p r o c e s s e s in shallow w a t e r s u r f a c e sediments. In b o r e a l m a r i n e e c o s y s t e m s t h e s e a s o n a l d e v e l o p m e n t of t h e b e n t h i c b a c t e r i a l populations t u r n e d o u t to b e strongly influenced by c e r t a i n ecological s i t u a t i o n s and e v e n t s in t h e sediment, f r o m which t h e input of t h e phytoplankton blooms in a u t u m n a n d spring, respectively, t h e a c c u m u l a t i o n of o r g a n i c m a t e r i a l during winter, and t h e d e v e l o p m e n t of t h e b e n t h i c f a u n a in spring w e r e most i m p o r t a n t . T h e e n r i c h m e n t of o r g a n i c m a t e r i a l in t h e s e d i m e n t s u r f a c e l e t to corresponding s t i m u l a t i o n s in t h e e n z y m a t i c decomposition r a t e s of p a r t i c u l a t e organic material. B a c t e r i a i m m e d i a t e l y r e a c t e d o n t h e availability of o r g a n i c m a t e r i a l with a d r a s t i c s h i f t in t h e s i z e distribution of biomass. R e l i a b l e d a t a on b e n t h i c b a c t e r i a l production a r e urgently needed. F r o m t h e very f e w d a t a based upon q u i t e d i f f e r e n t approaches, production r a n g e s b e t w e e n 10 a n d s o m e 100 m g of b a c t e r i a l c a r b o n per m 2 of s e d i m e n t per day. T h e s e e s t i m a t e s a l r e a d y i l l u s t r a t e t h e i m p o r t a n c e of b a c t e r i a in t h e turnover of o r g a n i c m a t e r i a l in sediments. Although b a s i c i n f o r m a t i o n on t h e s p a t i a l and t e m p o r a l distribution of b e n t h i c b a c t e r i a l biomass a n d a c t i v i t y is available, a number of q u e s t i o n s s t i l l r e m a i n open. Among these, t h e m e a s u r e m e n t of b a c t e r i a l production, s e a s o n a l v a r i a t i o n s in t h e m e t a b o l i c a c t i v i t y of b a c t e r i a l populations, a n d i n t e r a c t i o n s b e t w e e n b a c t e r i a and t h e b e n t h i c f a u n a need f u r t h e r a t t e n t i o n .

ACKNOWLEDGMENTS T h i s work w a s s u p p o r t e d by t h e G e r m a n R e s e a r c h Council (Sonderforschungsbereich95 d e r D e u t s c h e n Forschungsgemeinschaft). I would like t o t h a n k all of m y c o l l e a g u e s espacially Dr. G. G r a f f o r supplying m e w i t h t h e i r d a t a and t h e i r helpful c o m m e n t s .

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HEAT PRODUCTION OF MICROORGANISMS IN EUTROPHIED ESTUARINE SYSTEMS - AN EXPERIMENTAL STUDY-

P. LASSERREI, T. TOURNIEI, M. BIANCHI2 and S. CHAMROUXI. I s t a t i o n Biologique d e R o s c o f f , lJniversit6 d e P a r i s VI & CNRS, 29211 Roscoff Z L a b o r a t o i r e d e Microbiologie Marine, CNRS, Univ. d e P r o v e n c e , 13331 Marseille ( F r a n c e )

ABSTRACT L a s s e r r e , P., TourniC, T., Bianchi, M. and Chamroux, S., 1986. H e a t production of microorganisms in e u t r o p h i e d e s t u a r i n e s y s t e m s - An e x p e r i m e n t a l study. In: P. L a s s e r r e a n d J.M. Martin (eds), Biogeochemical P r o c e r s e s a t t h e Land-Sea Boundary. Elsevier, Amsterdam. T h e microbially-mediated d e g r a d a t i o n of o r g a n i c m a t e r i a l s in e s t u a r i e s a n d lagoons e x e r t s i m p o r t a n t influences o n t h e c h e m i c a l composition of sedirnents a n d waters. In t h e s e shallow w a t e r systems, t h e biogeochemical p r o c e s s e s o c c u r i n g a t t h e s e d i m e n t w a t e r i n t e r f a c e a r e r e l a t i v e l y well-known, b u t r a t e s a n d m e c h a n i s m s a r e ill defined. T h e t e m p o r a l behaviour of microorganisms colonising t h e w a t e r - s e d i m e n t i n t e r f a c e f r o m d i f f e r e n t e s t u a r i n e a n d lagoonal e n v i r o n m e n t s w a s studied, in microcosms, to c o m p a r e t h e i r r e l a t i v e a b i l i t y to r e c o v e r f r o m e x p e r i m e n t a l eutrophication. T h e m i c r o c a l o r i m e t r i c d a t a a n d s i m u l t a n e o u s s t u d i e s of t h e microbial physiological p o t e n t i a l i t i e s i n d i c a t e t h a t t h e power-time c u r v e s d e s c r i b e dissipative s t r u c t u r e , with reproducible p a t t e r n s in t h e f o r m of t e m p o r a l successions of microorganisms. T h e evolution of t h e s p e c i f i c r a t e of h e a t dissipation (4) showed d i f f e r e n c e s in r e l a t i o n to s e a s o n a l t h e r m a l regimes. A c h a r a c t e r i s t i c f e a t u r e is t h e i n c r e a s e in t h e s p e c i f i c h e a t production r a t e , s h o r t l y a f t e r nitrogen e n r i c h m e n t of t h e microcoms, followed by a d e c r e a s e indicating subsequent a d a p t a t i o n a l c h a n g e s in populations. T h e s e r e s u l t s o f f e r a n e x a m p l e of t h e v a l u e of d i r e c t m i c r o c a l o r i m e t r y in studying h o m e o s t a t i c c a p a b i l i t i e s of ecosystems. H e r e , h e a t dissipation i s a n a d e q u a t e p a r a m e t e r of t h e a b i l i t y of t h e system to return, a f t e r t r a n s i t o r y oscillations, to a new s t e a d y state ( a p a r a m e t e r of "resilience").

INTRODUCTION Biogeochemical p r o c e s s e s in organic-rich e s t u a r i e s a n d lagoons a r e d o m i n a t e d by t h e inf h e n c e of microbially m e d i a t e d d e g r a d a t i o n of r e c e n t l y d e p o s i t e d o r g a n i c m a t t e r . T h e s e p r o c e s s e s a r e r e l a t i v e l y well d e f i n e d a n d t h e y t a k e p l a c e n o t a b l y at t h e sedimentw a t e r i n t e r f a c e (Martens, 1982 ; Nixon, 1984). Nevertheless, t h e d i f f i c u l t y in providing a c h a r a c t e r i z a t i o n of t h e d y n a m i c s of microbiological p r o c e s s e s induced by e u t r o p h i c a t i o n s t i l l remains. T h e k i n e t i c s of biogeochemical transformations, brought a b o u t during t h e d e g r a d a t i o n of c o m p l e x m i x t u r e s of m a t e r i a l of n a t u r a l a n d a n t h r o p o g e n i c origin, by ill d e f i n e d m i x e d populations of m i c r o b e s such as o c c u r s in t h e m a r i n e c o a s t a l systems, a r e n o t e a s i l y studied by t h e reductionist techniques applicable to homogeneous systems.

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Several a t t e m p t s h a v e been m a d e to d e s c r i b e t h e c h a r a c t e r i s t i c s of t h e a e r o b i c and anaerobic p a r t s of t h e metabolism of sediments, using a m p o u l e c a l o r i m e t e r s ( P a m a t m a t ,

1984). F u r t h e r m o r e , flow m i c r o c a l o r i m e t e r s proved to b e a s u i t a b l e m e t h o d for quantifying rapid (hours) c h a n g e s occuring in micoorganisms metabolism o c c u r i n g in shallow w a t e r eutrophied s y s t e m s l o c a t e d in t h r e e c o n t r a s t i n g l o c a l i t i e s : t h e Arcachon basin, t h e Gironde e s t u a r y (Southwestern A t l a n t i c c o a s t ) a n d t h e "Aber" (ria) of Roscoff in north B r i t t a n y (Lasserre, 1980, 1984 ; L a s s e r r e a n d TourniC 1984 ; TourniC and Lasserre,

1984). R e s u l t s a r e r e p o r t e d h e r e which provide a n indication of t h e t e m p o r a l behaviour of complex mixed populations of microorganisms at t h e w a t e r - s e d i m e n t i n t e r f a c e . An understanding of such d y n a m i c p r o c e s s e s m a k e a significant contribution to improving our predictions of responses of s y s t e m s at t h e land-sea boundary to e u t r o p h i c a t i o n resulting from human a c t i v i t i e s as well as n a t u r a l phenomena.

METABOLISM AND HEAT PRODUCTION T r a n s f o r m a t i o n of m a t t e r during m e t a b o l i s m by living organisms is a c c o m p a n i e d by energy change. S o m e c h e m i c a l e n e r g y i s conserved, f o r s h o r t o r long periods, b u t t h e r e is always a n e t loss of e n e r g y m a n i f e s t e d as h e a t loss w h e t h e r o r g a n i c m a t t e r is c o m p l e t e l y mineralized o r not. T h e h e a t flow f r o m living organisms, r e g a r d l e s s of metabolism t y p e is a d i r e c t r e s u l t of metabolism. C a l o r i m e t r y i s a s e n s i t i v e indicator of t h e e n e r g y c h a n g e s in biological systems, giving i n f o r m a t i o n a b o u t t h e r a t e s a n d e x t e n t of reactions, no m a t t e r how c o m p l e x t h e processes occurring. S i n c e a n y c h e m i c a l o r physical process will b e accompanied by a n e n t h a l p y change, t h e m e t h o d i s c o m p l e t e l y general. I t m i g h t s e e m t h a t t h e r e would b e considerable difficulty in i n t e r p r e t i n g results, s i n c e resolution of t h e processes t a k i n g p l a c e would b e c o m e v e r y c o m p l e x in living organisms. However, t h e practical situation i s n o t usually so c o m p l e x as i t a p p e a r s at f i r s t sight. In biological systems, t h e r e a c t i o n c a n b e considered to t a k e p l a c e in solution b o t h at c o n s t a n t p r e s s u r e and c o n s t a n t volume, so t h a t t h e r m o d y n a m i c d i f f e r e n c e s of definition between e n e r g y a n d e n t h a l p y c a n b e d i s r e g a r d e d a n d t h e c h a n g e in e n e r g y c o n t e n t accompanying a r e a c t i o n considered to b e t h e c h a n g e in e n t h a l p y or h e a t of reaction, AH. This q u a n t i t y , t h e n corresponds to t h e e x p e r i m e n t a l l y m e a s u r e d h e a t evolution, Q, during a reaction in a n isolated s y s t e m in which no e x t e r n a l work i s done, with t h e convention t h a t a n e x o t h e r m i c r e a c t i o n corresponds to a d e c r e a s e in energy, so t h a t AH is negative. The a m o u n t of h e a t evolved i s proportional to n t h e number of moles of r e a c t i o n which have t a k e n p l a c e :

Q = - n AH

(1)

or w h e r e s e v e r a l s i m u l t a n e o u s r e a c t i o n s o c c u r t h e total h e a t produced i s t h e s u m of t h e

individual processes : k

Q=-ZnjAHj 0

(2)

163

(subscripts 0 to k c h a r a c t e r i z e t h e k i n t e r n a l processes occurring, n j is t h e n u m b e r of m o l e s of r e a c t i o n j t h a t h a v e o c c u r r e d s i n c e t i m e z e r o and bHj is i t s h e a t of reaction/mole). O v e r t h e last t e n years, t h e t e c h n i c a l d e v e l o p m e n t of c a l o r i m e t r y h a s r e a c h e d such a

stage t h a t e v e n v e r y slow p r o c e s s e s c o n n e c t e d with a low r a t e of h e a t o u t p u t , such as o c c u r in biological systerns, m a y b e d e t e c t e d . A wide v a r i e t y of p a r t i c u l a r designs h a v e been

used

in

modern

biological

c a l o r i m e t r y and applied

to

molecular,

cellular,

multicellular, and c o m p l e x s y s t e m s s t u d i e s ( g e n e r a l reviews by Spink & Wadsii, 1976 ; L a r n p r e c h t & Zotin, 1978 ; B e e z e r , 1980 ; Gnaiger, 1983). M i c r o c a l o r i m e t r i c c h a r a c t e r i z a t i o n of m e t a b o l i c t r e n d s in a q u a t i c s y s t e m s Microcaloriinetry h a s been used to d e m o n s t r a t e t h e microbial a c t i v i t y in n o r m a l and acidified soils (Mortensen e t al., 1973 ; Ljungholrn et al., 1979 a,b). O t h e r w o r k e r s h a v e r e p o r t e d r e s u l t s of e x p e r i m e n t s w h e r e t o t a l h e a t production in t h e m a r i n e sediinent, m a i n t a i n e d in closed c a l o r i m e t r i c vessels, h a s been assessed ( P a m a t m a t , 1980,1984). R e c e n t l y , t h e h e a t production of m a r i n e b a c t e r i a h a s been studied by Wagensberg et al. (19781, Gordon a n d Miller0 (1980). W e h a v e r e c e n t l y developed a m i c r o c a l o r i m e t r i c

e n e r g e t i c c h a n g e s of

t e c h n i q u e f o r quantifying t h e

microorganisms colonizing a sea-water-sediment

i n t e r f a c e in

e x p e r i m e n t a l microcosms. T h e i n h e r e n t low s p e c i f i c i t y of d i r e c t m i c r o c a l o r i m e t r y and, moreover, t h e high sensitivity a n d reliability of m o d e r n i n i c r o c a l o r i m e t e r s proved advantageous, s i n c e unknown and s u b t l e e v e n t s , n o t shown by m o r e s p e c i f i c methods, m a y b e detected.

METER

TI

TRlC

REC( RDER

1

MICROCOSM

DCAMPLI

Fig. 1. D i a g r a m m a t i c view of t h e m i c r o c a l o r i m e t r i c a n d o x i m e t r i c c i r c u l a t i o n s y s t e m for t h e s t u d y of m e t a b o l i c a c t i v i t y in e x p e r i m e n t a l microcosms, at t h e water-sediment i n t e r f a c e :CI, c i r c u l a t i n g i n t e r f a c e (for d e t a i l s cf. L a s s e r r e & TourniC, 1984).

164

I t appears, therefore, t h a t t h e measurement of h e a t flow using d i r e c t calorimetry, is a suitable technique for biogeochemical studies in a q u a t i c systems since under carefully controlled conditions a reproducible "fingerprint" (or power-time curve) is obtained. Our published results (Lasserre, 1980 ; Lasserre & TourniC, 1984 ; Lasserre, 1984 ; TourniC & Lasserre, 1984), report basic methodological problems involved in t h e continuous and simultaneous registration of h e a t production and oxygen pressure changes occuring in a circulating marine i n t e r f a c e pumped f r o m just above t h e sediment. This "circulating interface"

is taken

through t h e microcalorimeter (acting as a

wattmeter), then to an oxygen electrode and is then returned to t h e microcosm (Fig. 1). The technique provides a reliable method by which changes in t h e metabolic activity at t h e seawater-sediment i n t e r f a c e can b e d e t e c t e d and quantified on an empirical level. The underlying metabolic processes have been identified by adding 14C-glucose to t h e circulating i n t e r f a c e and

measuring t h e

ATP

pool

level.

T h e microcosms a r e

experimentally eutrophicated using nitrogen enrichments (bactopeptone or amino acid cocktail). These enrichments (0.5 to 4.6 mmol N L-1) correspond to situations encoutered in estuaries and lagoons subject to excessive organic inputs originating from urban wastes and agricultural runoff. The power-time curves (PTCs) obtained with microcosms experimentally enriched and composed of undisturbed c o r e s of sediment and w a t e r are highly reproducible (Fig. 2).

r summer

responses

winter r e s p o n s e s

~

6 hours Fig. 2. Power-time c u r v e s obtained in replicate experiments a f t e r peptone t r e a t m e n t (4rng/ml) :summer response in August, winter response in February. After nitrogen enrichment, t h e exponential transitory phase of t h e power-time curve is clearly c o r r e l a t e d with aerobic glucose metabolism, ATP pool level and oxygen tension ( ~ 0 2 ) .The microevents noted on PTC during t h e exponential phase, a r e also observed on t h e oxygen-time c u r v e (OTC) and a s t r a i g h t linear correlation is observed between t h e

165 h e a t production and 0 2 utilisation

. Conversely, a f t e r t h e

exponential transitory phase,

t h e system shows g r e a t differences between batch cultures. The decrease in heat production (more than 100 pW/ml in 2-3 h) occuring a f t e r t h e exponential transitory phase and preceeding t h e secondary steady state in s u m m e r experiments is not correlated with a decrease in ATP, in IYC-particulate m a t t e r level and in oxygen uptake rate, as occurs in batch c u l t u r e s (Belaich, 1980 ; Forrest, 1972). T h e amplitude of seasonal variations in p02, measured at t h e end of t h e exponential phase, clearly demonstrates t h a t the reduction of t h e metabolism, in t e r m of h e a t production, is not produced by a r t e f a c t (e.g. oxygen depletion in t h e tubing) b u t r a t h e r corresponds to a seasonal modification in the integrated metabolic c h a r a c t e r i s t i c s of t h e microcosms (Fig. 3).

C

.0 -.

c, 0

-----

SPRING-AUTUMN type

.-I

u -

-

WINTER t y p e

100 -

hours

0

6

12

18

24

30

36

Fig. 3 : Typical seasonal microcalorimetric and oximetric p a t t e r n s produced in experimental microcosms, at t h e sea-water - sediment interface, a f t e r a n experimental eutrophication : A, unimodal summer PTC ; 8, bimodal spring-autumn P T C ; C, sigmoid winter PTC, with t h e i r respective oxygen-time curves (OTCs). C a l o r i m e t r i c and oximetric responses show a l t e r n a t e periods of homogeneity (summer and winter months) and

of heterogeneity (spring and autumn months): periods of

homogeneity w e r e c h a r a c t e r i z e d by highly reproducible PTCs and OTCs ; periods of heterogeneity displayed transition s t a g e s between summer and winter types. The spring heterogeneous PTCs obtained from March to J u n e c a n b e interpreted as a gradual evolution from sigmoid (winter type) to unimodal (summer type) PTCs

.

166

T h e a u t u m n a l t r a n s i t i o n period, e x t e n d i n g f r o m O c t o b e r to I n i d J a n u a r y presented, both f o r PTCs a n d OTCs, a n a l t e r n a t i o n of t h e t h r e e basic s e a s o n a l p a t t e r n s : s u m m e r type, w i n t e r t y p e and transition spring type, with increasing f r e q u e n c y of w i n t e r t y p e responses as w i n t e r approached. O u r r e s u l t s s u g g e s t t h a t h e a t productions and t h e i r o x i d a t i v e c o u n t e r p a r t s a r e c o r r e l a t e d with seasonal trends.

I t is n o t e w o r t h y t h a t

c a l o r i m e t r i c a n d o x i m e t r i c m e a s u r e m e n t s h a v e b e e n p e r f o r m e d at a c o n s t a n t t e r n p e r a t u r e

(18OC 2 I), a l l y e a r round. T h e r e f o r e , t h e d i f f e r e n c e n o t e d b e t w e e n w i n t e r a n d s u m m e r PTCs a n d OTCs a r e v e r y probably r e l a t e d to long t e r m c y c l i c seasonal fluctuations, occuring in t h e e s t u a r i n e and lagoonal e n v i r o n m e n t s studied. Moreover, our r e s u l t s i n d i c a t e t h a t seasonally-induced e n v i r o n m e n t a l s t r e s s ( t e m p e r a t u r e , e u t r o p h i c a t i o n ) could d e t e r m i n e c o n s e r v a t i v e m e t a b o l i c responses of t h e e c o s y s t e m , of a d a p t a t i v e nature. T h e e x p e r i m e n t a l supply of o r g a n i c m a t t e r , s i m u l a t i n g a n a c u t e e u t r o p h i c a t i o n in t h e n a t u r a l environment, will i n d u c e t h e a c t i v a t i o n of a d a p t a t i v e p r o c e s s e s (corresponding to t h e t r a n s i t o r y p h a s e in t h e PTCs). T h e l a t t e r could b e i n t e r p r e t a t e d in t e r m s of physiological p r o c e s s e s (e.g. switch in a e r o b i c / a n a e r o b i c metabolisms) o r d e m o g r a p h i c population increase, succession), leading to a new s t e a d y state, and

processes (e.g.

c h a r a c t e r i z e d by a c o n s t a n t h e a t dissipation (1) v e r y high in w i n t e r (145 pW), indicating a poorly a d a p t e d s y s t e m , i.e. requiring l a r g e e n e r g y a m o u n t s f o r m a i n t e n a n c e a c t i v i t y , (2) t h r e e t i m e s l e s s in s u m m e r (45-50 PW), indicating a low r e q u i r e m e n t in e n e r g y for m a i n t e n a n c e (which m a y i n d i c a t e a good a d a p t a t i o n to e u t r o p h i c a n d a n o x i c conditions). In summer, when no oxygen i s l e f t in t h e microcosm a f t e r a n e n r i c h m e n t e x p e r i m e n t , t h e r e is always a s t a b l e h e a t production level of 40

- 50 pW,

c o m p l e t e l y inhibited by formalin,

during t h e e x p e r i m e n t a l t i m e (until 1 w e e k ) . T h e r e f o r e , i t is probable t h a t t h i s secondary s t e a d y state i s d u e to a n a e r o b i c metabolism.

MICROCALORIMETRIC STUDIES AND MICROBIAL DYNAMICS Although m a n y p r o b l e m s r e m a i n to b e solved, t h e r e s u l t s i n d i c a t e t h a t d i r e c t m i c r o c a l o r i m e t r y provides n e w information o n t h e o v e r a l l t e m p o r a l behaviour of c o m p l e x m i x t u r e s of

m i c r o o r g a n i s m s colonising

environments.

the

water-sediment

interface

of

coastal

T h e o b s e r v e d m i c r o c a l o r i m e t r i c t r e n d s w e r e r e l a t e d , as a working

hypothesis, to season-dependent a d a p t a t i o n of microorganism populations colonising t h e sediment-water i n t e r f a c e (TourniC & L a s s e r r e , 1984). I t w a s e s s e n t i a l to r e a c h a b e t t e r understanding microorganisms.

of

the

population

R e c e n t works,

successions

and

physiological

developed o n t h e basis of

properties

t h i s hypothesis,

of with

microbiologists in A r c a c h o n (M. Bianchi a n d coll.) a n d in Roscoff (5. C h a m r o u x and coll.) showed s p e c i f i c s e a s o n a l a d a p t a t i o n of n a t u r a l m a r i n e b a c t e r i a l populations in response to e x p e r i m e n t a l eutrophication.

Bacteriological studies R a t e s of h e a t production dQ/dt w e r e m e a s u r e d in p a r a l l e l w i t h oxygen tension as described above. W a t e r s a m p l e s w e r e t a k e n simultaneously in t h e m i c r o c o s m s during

167

t h e course of t h e (24-hour duration) f o r bacterial dynamics analysis. For e a c h experiment, w a t e r samples w e r e t a k e n over a 24 hour period from control and experimental microcosms. P l a t e counts (Oppehheimer & ZoBell, 1952) w e r e made in order to determine t h e viable heterotrophic microflora, and d i r e c t c o u n t s using acridine orange method (Hobbie et al., 1977) w e r e used to c a l c u l a t e t h e volume of t h e cells, according to t h e formula of Krambeck et al. (1981). A t Arcachon, changes in t h e qualitative composition of t h e heterotrophic community were assessed by isolating at random 240 pure bacterial s t r a i n s from p l a t e counts f o r each seasonal experiment. Each strain was c h a r a c t e r i z e d by one hundred morphological, physiological, biochemical and nutritional tests including cellular morphology, Gram staining, colonial morphology and colour, production of exoenzymes, ability to oxidize or f e r m e n t glucose, ability to grow at 4, 37 and 44OC, and ability to reduce n i t r a t e and nitrite. Nutritional tests, ability to grow without NaCl and without growth f a c t o r s such as vitamines or amino acids w e r e performed on 38 organic compounds as carbon and energy sources. T h e s e results a r e coded in binary form (0 or I). T h e catabolic potentialities of t h e bacterial communities isolated at each sampling t i m e w e r e described by synthetical a v e r a g e indexes, calculated by averaging t h e c a r a c t e r i s t i c s of t h e pure s t r a i n s cornposing t h e communities. T h e s e indexes a r e t h e average

index

of

exoenzyme

presence

(EAI,

indicating

the

ability t o

utilize

macromolecules) and t h e average index of carbonaceous compounds utilization (UAI, indicating t h e ability t o degrade low weight compounds corresponding to t h e 38 substrates t e s t e d as being t h e sole source of carbon and energy). For e a c h family of substrate, an average index of utilization was calculated : amino acids, carbohydrates, alcohols, f a t t y acids and organic acids of intermediary metabolism. These indexes w e r e calculated as described by Van Wambeke et al. (1984). A t Roscoff, a similar technique using a reduced number of tests (20) was utilized (api 20B method, see Troussellier and Legendre, 1981). T h e d a t a obtained by t h e two methods a r e not directly comparable, nevertheless they a r e both suitable t o describe t h e qualitative evolution of t h e physiological potentialities of t h e communities studied. In both localities, t h e strains were sorted according to their similarities and clustered using t h e KHI2 coefficient and variance analysis. On t h e graphical representation of this sorting (dendrograms), any group or isolate at a level of 70 % (90% f o r t h e api 20 B tests) of similarity was considered as a different bacterial ecotype. On t h e basis of t h e binary codification of t h e responses to t h e previously described tests, a hierarchical classification was done for e a c h w a t e r sample. A partial dendrogram was established for e a c h sample, and t h e totality of t h e strains isolated during t h e experiment w e r e compared in a general dendrogram. T h e analysis of t h e general dendrogram gives a mean to compare different communities and to c h a r a c t e r i z e their d e g r e e of individualization. T h e comparison of t h e strains isolated at successive sampling t i m e s gives an indication of t h e dynamics of t h e

168

bacterial communities (gradual when t h e strains a r e gathered in t h e s a m e cluster, or rapid, when t h e communities isolated at successive sampling t i m e s a r e distinct). Seasonal patterns in Arcachon and in Roscoff

The experiments were performed on samples collected in Arcachon (SeptemberSummer 1982 and February-Winter 1983) and on samples collected in t h e Aber of Roscoff (December-Winter 1984). Microcalorimetric responses obtained f o r Arcachon microcosms displayed t h e typical summer and winter p a t t e r n s as described by Tourni6 & Lasserre (1984). Conversely, at Roscoff, during t w o consecutive years of experiments (1983-19841, only t h e typical "summer" and "spring-autumn" p a t t e r n s w e r e found (i.e. no "winter-type" response was obtained). This stability of t h e calorimetric p a t t e r n s might well b e related to t h e m o r e s t a b l e t e m p e r a t u r e regime of t h e shallow w a t e r s in Roscoff (temperature range of 7 to 15'C) than in t h e Arcachon Bassin or t h e Gironde estuary (temperature range of 3 t o 24°C). The qualitative and quantitative evolution of bacterial populations both in Arcachon and Roscoff w e r e directly related t o t h e microcalorimetric evolution. Equivalent

power-time

curves obtained in Roscoff

(December) and in Arcachon

(Septembre) showed t h e s a m e bacterial dynamics. Conversely, t h e "winter" calorimetric response obtained in February in Arcachon w e r e related to very different bacterial dynamics.

b

[)rW/mll

150

-

100

-

PO2 [%I

= dO/Jt

A

_---. .___

q

IE

6

0

= dQ/dt

PO2 [%I

A

100 150-.,

21

Log[cells/mll

(*)

-

b

pW/mll

0

6

I2

-

100

21

= LogINI

B

I50 Log "1

___--50

6

P4

*

* 0

6

I2

18

24

Roscof f (December)

M hrcochon (September)

Fig. 4 : Typical "summer" p a t t e r n s obtained in Roscoff (December 1983) and in Arcachon (September 1982). A : H e a t production ( Q ) and oxygen tension ( ~ 0 2 evolution. ) B : Specific r a t e of h e a t production (q) and evolution of number of b a c t e r i a (N). In figure 4A, a typical power t i m e curve and t h e simultaneous oxygen tension variation a r e shown with t h e microheterotrophic cell numeration (N :cells/rnl). Consequently to t h e initial organic enrichment (in t h e microcosm), t h e h e a t production increased exponentially

169 and

t h e oxygen concentration decreased proportionally,

metabolism. In figure 4B, t h e specific r a t e of h e a t dissipation

revealing a c t i v e aerobic

4

= 1/N dQ/dt was obtained

from t h e power t i m e curve and t h e cell numeration data. In both Arcachon and Roscoff, very similar values of

4

w e r e found, t h e evolution of

4,

within t h e duration of t h e

experiment (30 hours), indicated important variations in e n e r g e t i c capabilities. O n e remark c a n b e m a d e refering t o t h e significance of t h e specific h e a t dissipation r a t e (dQ/dt per cell). The nutrient enrichment induced t h e appearence of a n energetically expensive metabolic phase followed by a secondary d e c r e a s e which indicated a metabolic recovery. This phenomenon was independent of oxygen availability, i.e. in "summer" microcosms where t h e oxygen tension was severely limited ( a f t e r enrichment), a similar e n e r g e t i c recovery was observed as in "winter" microcosms (figure 5). T h e study of population dynamics showed significant modifications in t h e physiological potentialities. Before enrichment ( t i m e zero) t h e initial bacterial populations both in Arcachon and Roscoff w e r e c h a r a c t e r i z e d by m o d e r a t e catabolic potentialities (see Table

I), a high diversity and an a v e r a g e cell density of 0.1 and 75 lo5 cell/ml respectively. The cell volume was about 2.6 urn3 in Roscoff. TABLE 1.

ARCACHON

ARCACHON

ROSCOFF

Decem ber September February -__--_-_-______-____-_-_-_________________________________________--_---------Macromolecules utilization

33

53

42

37

49

33

59

43

82

Organic compounds utlization 26

22

32

36

35

39

45

13

70

Glucose fermentation

25

50

75

65

20

35

0

9

97

N03 reduction

75

85

95

70

60

65

39

40

82

IP

P

IP

P

IP

P

FSS

FSS

FSS

Average percentage of positive responses t o 4 t y p e s of catabolic and physiological tests. Exoenzymes presence w a s te'sted on 7 macromolecules (Arcachon) and on 2 macromolecules (Roscoff). T h e organic compounds utilization index was calculated on t h e basis of 34 substrates in Arcachon (amino acids, carbohydrates, alcohols, f a t t y and organic acids) and on 13 substrates in Roscoff (principally carbohydrates and alcohols). IP : initial bacterial population. P : microcalorimetric peak. FSS : final steady-state. During t h e increase of t h e h e a t production rate, t h e succession of populations was very rapid, t h e strains isolated at different incubation t i m e s never clustered at a similarly high level. T h e values of index of utilization of small molecules (amino acids, carbohydrates, alcohols) showed a n e t tendency to d e c r e a s e and reach a minimum indicating physiological specialization, paralleled by t h e maximum value of t h e specific h e a t dissipation r a t e

{

.

170 During this phase of specialization, t h e cell volume decreased from 2.6 pm3 t o 1.2 pm3. In t h e sample corresponding to t h e peak, t h e average volume of t h e cells is somewhat higher

(1.5 pm3) due to t h e emergence of a new population of larger cells (LZ pm3). T h e secondary calorimetric s t e a d y state was reached about 3 hours a f t e r t h e peak of h e a t dissipation. The

drastic d e c r e a s e of t h e level of

was related to t h e rapid

emergence of very different dominant strains, formed by larger cells (2.1 pm3) and characterized by very broad metabolic potentialities. This homogeneous population (the diversity was almost nil) displayed a n adaptation t o anaerobic conditions (75 to 100% of t h e population was able to reduce n i t r a t e s or f e r m e n t glucose). The cell population increased rapidly and reached values of 9 l o 7 cells/ml in Arcachon to 108 cells/ml Roscoff. The situation is different in Arcachon, during t h e winter months (January and February, see figure 5).

Log[cells/mll 150

---__

100

-

60.

*

--- - - - .--....___......_.. ~.

P3

A b -*_-----I

loo

. 50

P5 I

'

'0-

20

A -

0

6

12

18

24

:a

*

* _ _ _ _ _ _._--- --*-

_ - -_ -.___.--pz

*

0JO 1

Log"]

L o g "1

6

12

.6

PS

P4

.o

Hours

=

4

Hours I8

24

Fig. 5 : Typical "winter" p a t t e r n s obtained in Arcachon (February 1983). A : He$t ) B : Specific r a t e of h e a t production (q) production (Q)and oxygen tension ( ~ 0 2 evolution. and evolution of number of b a c t e r i a (N). The initial increase of t h e h e a t production 4 was followed by a secondary steady-state without marked d e c r e a s e of t h e h e a t production (as already described). T h e final bacterial community dissipated a

high level of h e a t per biomass-unit and t h e cell population

increased slowly from lo5 cells/ml to 2.5

lo6 cells/ml, 24 hours a f t e r nitrogen

enrichment. Consequently, t h e specific r a t e of h e a t dissipation

4

reached higher levels

than in summer (maximum 60 vW/lO5 cells, against 20 ~ I Win summer). During t h e 24-hour experiment, t h e bacterial diversity was maintained at a high level. This f a c t indicates that, in spite of a n organic input identical to summer enrichment, t h e qualitative response of t h e microoheterotroph community was fundamentally different in winter and in

summer. Moreover, t h e evolution of t h e physiological potentialities of t h e winter populations was not well marked. I t is shown in Table 1 t h a t t h e average ability t o degrade small and large organic molecules remained s t a b l e during t h e experiment. The percentage of bacterial strains able t o induce glucose fermentation as well as n i t r a t e reduction showed a tendency to decrease, as a probable result of unrestricted oxygen availability. No c h a r a c t e r i s t i c bacterial population appeared at t h e end of t h e exponential phase of

h e a t production.

171

CONCLUSION Our ability t o predict changes in t h e biogeochemistry of estuaries and lagoons, as well

as o t h e r shallow w a t e r organic-rich environments, is tied t o our ability to quantitatively model t h e dynamics of microbially-mediated e n e r g e t i c changes closely associated with degradation processes and their chemical end products. Flow microcalorimetry appears well suited f o r quantifying t h e s e e n e r g e t i c changes, in order t o reveal microbial phenomena not shown by a m o r e reductionist approach. This is particularly t r u e for complex situations in highly eutrophied c o a s t a l systems where bacterial populations play a very important role in t h e biogeochemical exchanges. T h e microcalorimetric d a t a and simultaneous studies of t h e physiological potentialities on marine bacterial populations, colonising t h e water-sediment interface, indicated t h a t , at t h e level of t h e population dynamics, t h e power-time curves describes dissipative

s t r u c t u r e s in t h e form of temporal successions of microorganisms. T h e mechanisms underlying power-time curve evolution a r e due to t h e succession of bacterial populations with different dynamics and physiological capabilities. "Summer" t y p e microcalorimetric curves a r e associated with bacterial populations responding

to

eutrophication by rapid and well marked successions ending in t h e selection of a community dominated by anaerobes. These anaerobes a r e characterised by very high catabolic potentialities. T h e l a t t e r a r e responsible f o r t h e observed metabolic recovery shown by a d e c r e a s e in t h e h e a t production, followed by a s t a b l e microcalorimetric plateau. During t h e first p a r t of t h e microcalorimetric curve (from t i m e zero, PO, to t h e PTC peak, P3), aerobes gradually increase their specialization (figure 6).

Fig. 6 : Evolution of microbial catabolic potentialities during a "summer" t y p e microcalorimetric response t o eutrophication. The five bar c h a r t s c h a r a c t e r i z e t h e specialization of t h e bacterial population, expressed as t h e ability to metabolize a variety of organic substrates (carbohydrates, alcohols, amino acids etc.).

172 When "winter" t y p e microcalorimetric curves a r e observed, t h e bacterial successions a r e less marked. From t h e begining of t h e experiment t o t h e secondary steady state, t h e populations remains very similar and their c a t a b o l i c potentialities a r e identical. A s a consequence, t h e specific r a t e of h e a t is significantly higher than in summer. More generally, our studies o f f e r an example of t h e value of d i r e c t microcalorimetry in studying homeostatic properties of ecosystems. Here, h e a t dissipation with respect t o seasons is an a d e q u a t e p a r a m e t e r of t h e ability of t h e system to return, a f t e r transitory oscillations, to a new steady state (a p a r a m e t e r of ecological "resilience"). Mixed bacterial populations displayed different e n e r g e t i c regulation of adaptative nature. In e f f e c t , t h e evolution of t h e specific r a t e of h e a t dissipation (1/N dQ/dt) showed

a difference in relation to seasonal thermal regimes. A c h a r a c t e r i s t i c f e a t u r e is the increase in t h e specific h e a t production rate, shortly a f t e r t h e perturbation (nutrient enrichment) followed by a d e c r e a s e probably indicating subsequent functional adaptional changes in populations, and marked by a final stationary state. This stationary phase indicates a maximum efficiency with probable adaptation of e n z y m a t i c systems to new environmental conditions. According to Schrodinger (1967), t h e specific r a t e of heat dissipation is a good index of metabolic efficiency, in living organisms. Now, one intriging problem is to understand how ecological successions r e l a t e to the minimum entropy production based on linear irreversible thermodynamics (PrigogineWiame, 1946 ; Prigogine 1982). R e c e n t experimental developments have shown t h a t t h e thermodynamics of

non-equilibrium

processes c a n b e used to deduce quantitative

relationships and equations of importance to developmental biology (Lamprecht and Zotin, 1978). For 20th (1985), "the possibility is not t o b e excluded t h a t thermodynamics might

b e used t o obtain phenomenological equations appropriate to t h e theory of ecosystems".

ACKNOWLEDGMENTS This work was supported by t h e C e n t r e National d e la Recherche Scientifique as part of t h e 'Greco' Interaction Continent-Oc&an, with additional contributions from t h e Institut

Francais d e Recherche pour I'Exploitation d e la Mer and t h e Ministire d e I'Environnement.

REFERENCES Beezer, A.E. (ed.), 1980. Biological Microcalorimetry. Academic Press, London and New York, 483 pp. Belaich, J.P., 1980. Growth and metabolism in bacteria. In: A.E. Beezer (ed.), Biological Microcalorimetry. Academic Press, London, pp. 1-42. Fenchel, T., 1969. The ecology of marine microbenthos. IV. Ophelia, 6: 1-182. Fenchel, T., 1970. Studies on t h e decomposition of organic d e t r i t u s from t u r t l e grass Thalassia testudinum. Limnol. Oceanogr., 15: 14-20.

173 Forrest, W.W., 1972. Microcalorimetry. In: J.R. Norris and D.W. Ribbons (eds), Methods in Microbiology, 6B: 285-318. Gnaiger, E. and Forstner, H. (eds), 1983. Polarographic Oxygen Sensors Physiological Applications. Springer-Verlag, Berlin, 370 pp.

-

Aquatic and

Gordon, A. and Millero, F.J., 1980. Use of microcalorimetry to study t h e growth and metabolism of marine bacteria. Thalassia Jugosl., 16: 405-424. Hobbie, J.E., Daley, R.J. and Jasper, S., 1977. Use of Nuclepore filters for counting b a c t e r i a by fluorescence microscopy. Appl. environ. Microbiol., 37: 805-812. Krambeck, C., Krambeck, K.H.J. and Overbeck, J., 198 1. Microcomputer assisted biomass determination of plankton bacteria on scanning electron micrographs. Appl. environ. iMicrobiol., 42: 142-149. Lamprecht, 1. and Zotin, A.I. (eds), 1978. Thermodynamics of Biological Processes. Walter d e Gruyter, Berlin, 428 pp. Lasserre, P., 1980. Energetic role of meiofauna and epifaunal deposit-feeders in increasing level of microbial activity in estuarine ecosystems, at t h e water-sediment interface. Actes Colloq. int. CNRS, Paris., 293: 309-318. Lasserre, P., 1984. The measurement of t h e enthalpy of metabolism in marine organisms. In: M.J.R. Fasham (ed), Flows of Energy and Materials in Marine Ecosystems, Theory and Practice. Plenum Press, New York, pp. 247-269. Lasserre P. and TourniC T., 1984. Use of microcalorimetry for t h e characterization of marine metabolic activity, at t h e water-sediment interface. J. Exp. Mar. Biol. Ecol., 74: 123-139. Ljungholrn, K., Norin, B., Skold, R. and Wadso, I, 1979 a. Use of microcalorimetry for the characterization of microbial activity in soil. Oikos, 33: 15-23. Ljungholm, K., Nor&, B. and Wadso, I, 1979 b. Microcalorimetric observation of microbial activity in normal and acidified soils. Oikos, 33: 24-30. Mann, K. H., 1972. Macrophyte production and d e t r i t u s food chains in coastal waters. Mem. 1st. Ital. Idrobiol., 29 Suppl.: 353-383. Martens, C.S., 1982. Biogeochemistry of organic-rich c o a s t a l lagoon sediments. In: P. Lasserre and H. P o s t m a (eds), Coastal Lagoons. Oceanologica Acta, pp. 161-176. Mortensen, U., Nor&, 8. and Wadso, I., 1973. Microcalorimetry in t h e study of the activity of microorganisms. Bull. Ecol. Res. Comm. (Stockholm), 17: 189-197. Nixon, S.W., Pilson, M.E.Q., Oviatt, C.A., Donaghay, P., Sullivan, B., Seitzinger, S., Rudnick, D. and Frithsen, J., 1984. Eutrophication of a coastal marine ecosystem - An experimental study using t h e MERL microcosms. In: M.J.R. Fasham (ed.), Flows of Energy and Materials in Marine Ecosystems, Theory and Practice. Plenum Press, New York, pp. 105-135. Oppenheimer, C.H. and Zobell, C.E., 1952. The growth and viability of sixty-three species of marine bacteria as influenced by hydrostatic pressure. J. Mar. Res., 11: 10-18. P a m a t m a t , M.M., 1980. T h e annual mineralization of organic m a t t e r in sediments. Present knowledge, persistent technical problems and uncertainities in our measurements. A c t e s Colloq. int. CNRS, Paris., 293: 309-318. P a m a t m a t , M.M.,

1984. Measuring t h e metabolism of t h e benthic ecosystem. In: M.J.R.

174

Fasham (ed.), Flows of Energy and Materials in Marine Ecosystems, Theory and Practice. Plenum Press, New York, pp. 223-246. Prigogine, I., 1982. Physique, Temps et Devenir. Masson, Paris, pp. 275. Prigogine, I. and Wiame, J.M., 1946. Biologie et thermodynamique des phCnomgnes irrchersibles. Experientia, 2: 451-453. Schrodinger, E., 1967. What is Life ? The Physical Aspect of t h e Living Cell. Cambridge University Press, New York. Spink, C. and Wadso I., 1976. Calorimetry as an analytical tool in biochemistry and biology. In: D. Click (ed.), Methods of Biochemical Analyses, 23: 1-159. TourniC, T. and Lasserre, P., 1984. Microcalorimetric characterization of seasonal metabolic trends in marine microcosms. J. Exp. Mar. Biol. Ecol., 74: 111-121. Troussellier, M. and Legendre P., 1981. A functional evenness index f o r microbial ecology. Microb. Ecol., 7:283-296. Van Wambeke, F., Bianchi, M.A. and Cahet, G., 1984. Short-term bacterial reactivity of nitrogen-enriched sea w a t e r of a eutrophic lagoon. Estuarine Coastal Shelf Sci., 19: 291-30 1. Wagensberg, J., Castell, C., Torra, V., Rodellar, J. and Vallespinos, F., 1978. Microcalorimetric study of t h e metabolism of marine bacteria. Detection of rythmical processes. Invest. Pesq., 42: 179-188. Zotin, A.I., 1985. Thermodynamics and growth of organisms in ecosystems. Can. Bull. Fish. Aquat. Sci., 213: 27-37.

175

UPTAKE OF TRACE ELEMENTS

BY LIVING ORGANISMS

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177

T R A C E METALS

F.M.M.

- PHYTOPLANKTON INTERACTIONS :A N OVERVIEW*

MOREL

D e p a r t m e n t of Civil Engineering, R.M.

P a r s o n s Laboratory, M a s s a c h u s e t t s I n s t i t u t e of

Technology, Cambridge, M a s s a c h u s e t t s (U.S.A.)

ABSTRACT Morel, F.M.M., 1986. T r a c e metals-phytoplankton i n t e r a c t i o n s : a n overview. In: P. L a s s e r r e a n d J.M. M a r t i n (eds), Biogeochemical P r o c e s s e s at t h e Land-Sea Boundary. Elsevier, A m s t e r d a m . As shown e a r l y in t h e c e n t u r y , t h e r o l e of t r a c e m e t a l s as n u t r i e n t s o r t o x i c a n t s to phytoplankton i s m e d i a t e d by t h e aqueous s p e c i a t i o n of t h e e l e m e n t s . More r e c e n t l y f r e e m e t a l ion a c t i v i t i e s h a v e been d e m o n s t r a t e d to b e t h e p a r a m e t e r s d e t e r m i n i n g such physiological e f f e c t s . I t i s a r g u e d t h a t t h i s result implies no p a r t i c u l a r role f o r t h e f r e e hydrated m e t a l ions b u t r e p r e s e n t s simply t h e t h e r m o d y n a m i c t e n d a n c y of t h e e l e m e n t s to r e a c t chemically, at pseudo-equilibrium, with c e l l u l a r f u n c t i o n a l groups and h e n c e to b e bioactive. A t p r e s e n t t h e f o c u s is on disequilibrium processes, particularly t h e reduction and photoreduction of t h e m e t a l s Fe a n d Mn at c e l l surfaces. Such a process m a y m a k e t h e s e e s s e n t i a l e l e m e n t s m o r e a v a i l a b l e to algae. I t m a y a l s o e n h a n c e t h e s p e c i f i c i t y of t h e u p t a k e s y s t e m , o t h e r m e t a l s which act as c o o r d i n a t i v e analogs of t h e e s s e n t i a l ones being t o x i c by i n t e r f e r i n g w i t h t h e i r t r a n s p o r t o r utilization. T h e s e physiological processes correspond t o a g e n e r a l ecological view in which a q u a t i c micro-organisms a r e simultaneously l i m i t e d in s t a b l e s y s t e m s by s e v e r a l e s s e n t i a l e l e m e n t s and t h e i r toxic c h e m i c a l analogs. HISTORICAL DEVELOPMENTS T h e field of t r a c e m e t a l - a q u a t i c microorganism i n t e r a c t i o n s h a s a long history d a t i n g b a c k to t h e 20's with e f f o r t s t o c u l t u r e a q u a t i c organisms, principally phytoplankton, in t h e laboratory. Based mostly o n e x h a u s t i v e e m p i r i c a l studies, ca. 1960, r e s e a r c h e r s established t h e i m p o r t a n c e of t r a c e e l e m e n t c h e m i s t r y f o r t h e g r o w t h of m i c r o a l g a e a n d pioneered t h e u s e of a r t i f i c i a l c h e l a t i n g a g e n t s in g r o w t h medium r e c i p e s (Provasoli et al., 1957 ; Droop, 1961 ; Cuillard a n d R y t h e r , 1962 ; Johnston, 1964). T h e emphasis w a s t h e n o n t h e n u t r i t i o n a l r o l e of t h e t r a c e e l e m e n t s and n a t u r a l and a r t i f i c i a l c h e l a t o r s w e r e t h o u g h t to m a k e iron a v a i l a b l e to phytoplankton.

*This work w a s supported in p a r t by NOAA g r a n t NA79 AA-D-00077,

NSF g r a n t O C E

831 7532, a n d O N R c o n t r a c t N00014-80-C-0273. A slightly d i f f e r e n t version of t h i s a r t i c l e w a s used as p a r t of a final r e p o r t to NOAA.

178

D e s p i t e s o m e a t t e m p t s at physiological l a b o r a t o r y s t u d i e s and a f e w field e x p e r i m e n t s , until t h e mid 70's l i t t l e progress w a s m a d e in o u r m e c h a n i s t i c understanding of t h e e f f e c t s of t r a c e e l e m e n t s o n microalgae. A t t h a t t i m e , f u n d a m e n t a l c o n c e p t s of coordination c h e m i s t r y w e r e brought to b e a r q u a n t i t a t i v e l y o n t h e t o p i c ; t h e f o c u s w a s p u t c l e a r l y on t h e issue of c h e m i c a l speciation in t h e g r o w t h medium a n d a s e r i e s of publications d e m o n s t r a t e d t h e c e n t r a l irnportance of f r e e m e t a l ion a c t i v i t i e s in d e t e r m i n i n g physiological effects as measured by g r o w t h r a t e s (Manahan a n d Smith, 1973 ; Sunda and Guillard, 1976 ; Anderson and Morel, 1978). Much of t h e f o c u s w a s t h e n s h i f t e d f r o m t h e issue of t r a c e e l e m e n t nutrition to t h a t of toxicity ; t h e r o l e of c h e l a t i n g a g e n t s w a s viewed to b e chiefly t r a c e metal--namely

copper--detoxification

r a t h e r t h a n iron

nutrition. T h i s is roughly w h a t t h e s i t u a t i o n w a s until t h e l a t e 70's : o u r understanding of t h e r e l e v a n t c h e m i s t r y r e s t e d squarely o n coordination equilibrium c o n c e p t s a n d physiological studies depended on t h e u s e of c h e l a t i n g a g e n t s to b u f f e r t h e c h e m i s t r y of t r a c e metals. T h e physiological a s p e c t s of t h e r e s e a r c h e f f o r t s r e m a i n e d s t r i c t l y phenomenological ; only i n t e g r a t i v e biological p a r a m e t e r s s u c h as g r o w t h r a t e s w e r e being measured and l i t t l e a t t e n t i o n w a s being paid to t h e underlying mechanisms. Based o n l a b o r a t o r y r e s u l t s with m e d i a of defined c h e m i s t r y and s o m e e a r l y f i e l d work, m u c h speculation w a s being p u t f o r t h regarding t h e p o t e n t i a l e n v i r o n m e n t a l i m p o r t a n c e of t r a c e m e t a l s and c h e l a t i n g a g e n t s in a q u a t i c systems. T h e principal t o p i c of t h e s e speculations w a s t h e low productivity, n u t r i e n t rich, upwelling regions of t h e o c e a n s which s e e m e d to exhibit t h e s a m e need f o r o r g a n i c m a t t e r (i.e. complexing a g e n t s ) conditioning as w a s o b s e r v e d in l a b o r a t o r y c u l t u r e s (Barber, 1973).

THE ISSUE OF FREE ION ACTIVITIES T h e e a r l y question t h a t d o m i n a t e s t h e c h e m i s t r y s i d e of t h e t r a c e e l e m e n t - a q u a t i c microbiota i n t e r a c t i o n problem is t h a t of t h e n a t u r e of t h e r e l e v a n t c h e m i c a l species. T h e d e m o n s t r a t e d i m p o r t a n c e of t h e f r e e m e t a l ion a c t i v i t i e s in d e t e r m i n i n g physiological e f f e c t s h a s o f t e n been i n t e r p r e t e d to m e a n t h a t t h e h y d r a t e d ionic m e t a l s p e c i e s is t h e a c t i v e one, t o x i c o r available. This i s a profound a n d widespread misconception p a r t l y based on a f a l s e analogy with aqueous s p e c i e s s u c h as HgCI2 which i s known to c o n t r o l t h e toxicity of m e r c u r y to microorganisms by promoting r e l a t i v e l y rapid t r a n s m e m b r a n e transport. Indeed, t h e p l a s m a m e m b r a n e is t h e principal b a r r i e r b e t w e e n t h e e x t r a c e l l u l a r milieu and t h e various c e l l u l a r s i t e s w h e r e m e t a l s c a n r e a c t a n d a f f e c t cellular physiology. T r a n s p o r t of highly c h a r g e d c a t i o n s through t h e lipid m e m b r a n e is exceedingly slow, so t h a t t h e i m p o r t a n c e of t h e f r e e m e t a l ion a c t i v i t i e s c a n n o t in fact b e analogous to t h a t of t h e HgC12 concentration.

T h e misconception r e s t s on a confusion b e t w e e n t h e t h e r m o d y n a m i c s c a l e of a c t i v i t i e s (which m e a s u r e s t h e t e n d e n c y of a n e l e m e n t to r e a c t ) a n d t h e c o n c e n t r a t i o n s c a l e f o r f r e e , hydrated, ions. Consider f o r e x a m p l e t h e n o t uncommon s i t u a t i o n w h e r e t h e f r e e

f e r r i c ion a c t i v i t y m a y b e of t h e o r d e r of, say, 10-27 M d u e to hydroxide o r c h e l a t e formation. Such low a c t i v i t y m a k e s p e r f e c t s e n s e as a m e a s u r e of t h e r e a c t i v i t y of Fe(lI1); i t hardly r e p r e s e n t s t h e a c t u a l a b u n d a n c e of t h e Fe(H20)63+ ion in a o n e l i t e r solution: o n e thousandth of a single molecule. F r e e m e t a l ion a c t i v i t i e s t h u s c o n t r o l biological e f f e c t s b e c a u s e t h e y d e t e r m i n e t h e r e a c t i v i t y of t h e m e t a l s in s y s t e m s at equilibrium. Such equilibrium b e t w e e n t h e e x t e r n a l medium a n d t h e c e l l c a n only b e e x p e c t e d to o c c u r with s u r f a c e s i t e s

. T h e biological

effect m u s t t h e n b e c o n t r o l l e d by t h e e x t e n t of s o m e s u r f a c e reaction-- e i t h e r b e c a u s e a

s u r f a c e s i t e is itself t h e locus of m e t a l a c t i o n (e.g., inhibition of u p t a k e of s o m e e s s e n t i a l e l e m e n t ) , o r b e c a u s e s u r f a c e binding c o n t r o l s t h e u p t a k e r a t e of a m e t a l whose cellular c o n c e n t r a t i o n (and h e n c e physiological e f f e c t ) is t h u s d e t e r m i n e d by a b a l a n c e b e t w e e n a c c u m u l a t i o n r a t e a n d growth. In a n y case, t h e c r i t i c a l c e l l u l a r reaction--say t h e binding to a t r a n s p o r t protein--must

b e r e l a t i v e l y f a s t c o m p a r e d to t h e m e a s u r e d biological

response so t h a t a state of (pseudo)equilibrium i s observed b e t w e e n t h e organism and t h e surrounding medium and t h e physiological response--e.g.

t h e t r a n s p o r t rate--is d e p e n d e n t

upon t h e f r e e ion activity. Such a r e s u l t t h e n implies no p a r t i c u l a r r e a c t i o n mechanism ; i t d o e s n o t assign a s p e c i f i c physiological r o l e to t h e h y d r a t e d ionic species; i t m e r e l y r e f l e c t s t h e (pseudo)equilibrium condition in which t h e e x t e n t of a l l t h e coordination r e a c t i o n s of t h e c o m p o n e n t of i n t e r e s t is d e t e r m i n e d by i t s f r e e ionic activity. F r o m t h i s point of view, t h e obvious issues t h a t need addressing a r e t h o s e of t h e a c h i e v e m e n t of t h e equilibrium o r pseudoequilibrium condition (i.e. t h e c r i t i c a l c h e m i c a l k i n e t i c s in t h e medium), of t h e n a t u r e a n d r a t e of t h e slow s t e p in t h e chain of processes t h a t link t h e principal m e t a l s p e c i e s to t h e physiological response (i.e.

the actual

physiological mechanism of m e t a l a c t i o n ) and, of course, t h e e n v i r o n m e n t a l meaning of i t all. Accordingly, c o n c e p t u a l a d v a n c e s m a d e o v e r t h e p a s t f e w y e a r s c a n b e classified under t h r e e principal headings :

i) Medium c h e m i s t r y

--

o u r understanding of

t h e a q u a t i c c h e m i c a l processes

i n t e r a c t i n g with biological responses h a s e x p a n d e d to include, in particular, redox and photoredox r e a c t i o n s and t h e f o c u s h a s gradually s h i f t e d f r o m equilibrium to kinetics.

ii) Physiological responses

--

t h e e a r l y phenomenological a p p r o a c h h a s been replaced

by a n increasingly m e c h a n i s t i c o n e with e m p h a s i s on t h e s u b t l e sublethal processes which a r e m o s t d i r e c t l y r e l e v a n t f o r t h e study of o r g a n i s m s in t h e i r n a t u r a l milieu and on t h e f e e d b a c k c o n t r o l p r o c e s s e s by which t h e organisms adjust to t h e t r a c e e l e m e n t c h e m i s t r y of t h e environment.

iii) Ecological implications - s o m e of t h e e a r l y e n v i r o n m e n t a l hypotheses h a v e been t e s t e d in field e x p e r i m e n t s a n d o u r understanding of t h e role of local and global i n t e r a c t i o n s of a q u a t i c microorganisms with t r a c e e l e m e n t s in t h e e c o n o m y of m a r i n e s y s t e m s h a s b e c o m e b o t h m o r e g e n e r a l and m o r e specific.

180

MEDIUM CHEMISTRY S e v e r a l t r a c e e l e m e n t s a r e e s s e n t i a l f o r plant g r o w t h and many a r e toxic. M e t a l s such as iron a n d manganese, which a r e needed in r e l a t i v e l y l a r g e q u a n t i t i e s and a r e r a t h e r insoluble in a q u a t i c s y s t e m s owing to t h e f o r m a t i o n of hydrous oxides, a r e not known to b e toxic to a q u a t i c plants. Most r e a c t i v e m e t a l s such as c a d m i u m , l e a d o r m e r c u r y f o r which t h e r e a r e no known a l g a l r e q u i r e m e n t s b e c o m e t o x i c at s u f f i c i e n t l y high (but still very

low) f r e e ion concentrations. S o m e m e t a l s s u c h as z i n c and c o p p e r c a n b e limiting at very low f r e e ion c o n c e n t r a t i o n s and t o x i c at higher values. F r o m l a b o r a t o r y and f i e l d studies, i t would a p p e a r t h a t iron, m a n g a n e s e and z i n c m a y b e nearly limiting in s o m e m a r i n e systems, while copper, c a d m i u m and a f e w o t h e r s m a y b e approaching t o x i c i t y (Huntsman and Sunda, 1980 ; Morel a n d Morel-Laurens, 1983 ; Morel a n d Hudson, 1985). In a c c o r d a n c e with t h e previous discussion, a m e c h a n i s t i c look at t h e relationship b e t w e e n biological e f f e c t s a n d t r a c e m e t a l c h e m i s t r y in a q u a t i c s y s t e m s m u s t focus, n o t on t h e r e l a t i v e r o l e of various m e t a l l i c s p e c i e s in solution, b u t on t h e equilibrium condition itself. In s o m e instances, w e e x p e c t s o m e r e a c t i o n s in t h e medium or at t h e s u r f a c e of t h e organism to b e slow, particularly t h o s e t h a t involve iron o r m a n g a n e s e oxide. (Iron will s e r v e as o u r chief e x a m p l e throughout t h i s discussion). To e x a m i n e t h e s e possible slow processes and t h e resulting disequilibrium condition, w e m u s t distinguish b e t w e e n l a b o r a t o r y b u f f e r e d s y s t e m s in which low f r e e m e t a l ion a c t i v i t i e s a r e maintained by s t r o n g c h e l a t i n g agents, a n d t h e condition m o r e t y p i c a l of n a t u r a l s y s t e m s in which low f r e e m e t a l ion a c t i v i t i e s r e f l e c t mostly low t o t a l m e t a l concentrations. In t h e f i r s t situation, l a r g e c o n c e n t r a t i o n s of c h e l a t e d m e t a l s a r e p r e s e n t and p r e c i p i t a t e s a r e normally avoided ; in t h e second, hydrous oxide solids of iron and manganese a r e usually found and buffering r e s u l t s simply f r o m t h e r e l a t i v e l y low c o n c e n t r a t i o n s of organisms in t h e system. T h a t equilibrium a p p e a r s to b e r e a c h e d in buffered l a b o r a t o r y s y s t e m s is r a t h e r surprising. T h e k i n e t i c s of c h e l a t e dissociation a r e relatively slow (Table 1) a n d o n e would e x p e c t i t to c o n t r o l t h e r a t e of cellular binding over a t i m e s c a l e of hours to days--which i s typical of t h e m e a s u r e d physiological e f f e c t s . This question m e r i t s additional s t u d i e s ; at t h i s point, s i n c e t h e r e a c t i o n s d o a p p e a r to b e f a s t , w e a r e left speculating on t h e possible r o l e of t e r n a r y c o m p l e x f o r m a t i o n (chelatesmetal-surface ligand), of low s u r f a c e pH a n d redox potential, and of photochernistry in a c c e l e r a t i n g t h e m e t a l e x c h a n g e process. In unbuffered systems, k i n e t i c l i m i t a t i o n m a y c o m e e i t h e r f r o m slow t r a n s p o r t to t h e c e l l s u r f a c e o r f r o m slow dissolution of solid s p e c i e s resulting in depletion of t h e bulk medium. S i m p l e c a l c u l a t i o n s show (Table 1) t h a t , in t h e o p e n ocean, s t e a d y state diffusion of such e l e m e n t s as z i n c of iron to t h e c e l l s u r f a c e m a y m a t c h t h e u p t a k e r a t e of fast growing algae. T h i s i s in a c c o r d a n c e with t h e p o s t u l a t e t h a t , in a s t a b l e environment, a l l potentially limiting e l e m e n t s should e f f e c t i v e l y b e co-limiting p r i m a r y production and t h a t t h e a v e r a g e u p t a k e r a t e s should m a t c h t h e diffusion r a t e s (Morel a n d Hudson, 1985).

181

TABLE I Slow processes in solution (based on Fe uptake by

weissflogii).

1. Rate of cellular F e uptake

Minimum for optimum growth :p min = 2x10-17 mol cell-1 hr-1 In typical c u l t u r e (4 106 cell 1-1 2 0.4 m g 1-11 P

= 4x10-10 mol-1 hr-1

2. Steady State Diffusion in Open Surface Ocean (Fe)T %lo-10 M ; cell d i a m e t e r = 10 pm

J = 2x10-17 mol cell-1 hr-1 3. Time for soluble Fe depletion in unbuffered culture (Fe)T

< 10-9 M

t = (Fe)T/ pc

5 2.5

hr

4. Rate of uncatalyzed F a x dissolution kd = 10-14 mol cm-2 sec-1 (?) ;(FeOX)7 = 2x10-8 M ; a r e a = 3 cm21-l rd = 10-10 mol 1-1 hr-1

5. Rate of exchange from chelator exchange with transferrine

ke = 10-8 sec-1 ; ( F e y ) = 10-7 M re = 2 10-12 mol 1-1 hr-1

In fact ke must b e at l e a s t 100 x g r e a t e r since re m a t c h e s P

In unbuffered (i.e. unchelated) c u l t u r e media, we can c a l c u l a t e t h a t t h e soluble iron can easily b e exhausted in a f e w hours (Table 1) ; t h e s a m e may b e t r u e of field situations when t h e cell concentrations a r e high. Though t h e precise r a t e s of hydrous iron oxide dissolution a r e not precisely known, t h e uncatalyzed process is probably insufficient to k e e p up with biological uptake. In f a c t , t h e limiting r a t e s observed in unbuffered media (Anderson and Morel, 1982) correspond roughly to t h e value calculated in Table 1 (4x10-10 mol 1-1 hr-1 f o r (FeOx)T = 2x10-8 M). This result points at t h e role of chelating agents, especially those t h a t form light absorbing f e r r i c complexes, in making iron available t o algae. Certainly organic chelators do enhance t h e r a t e of iron oxide dissolution, both through t h e r m a l and photochemical reactions (Waite and Morel, 1984a & b). Since t h e r a t e

of delivery of iron from t h e chelator to t h e cell is unexpectedly f a s t (Table I), particularly

when t h e complex is photoactive (Anderson and Morel, 1982), w e must

conclude t h a t t h e early investigators w e r e right : chelating agents, including natural humic substances, m a k e iron more available to algae. This "kinetic role" of t h e chelators must b e c o n t r a s t e d to their equally important "equilibrium role" in repressing t r a c e m e t a l toxicity.

182

As we have discussed, non-equilibrium conditions in t h e medium may b e achieved because of s o m e slow transport or reaction rate. Disequilibrium may also result from fast, energy consuming processes t h a t maintain a steady state supply of unstable chemical species such as those formed by photochemical redox reactions (Fig.1). For example, w e now know t h a t photo-reduction of metal-chelates enhances t h e r a t e of iron uptake by algae (Anderson and Morel, 19821, t h a t organic s u r f a c e complexes on iron oxide induce photoreductive dissolution of t h e m e t a l (Waite and Morel, 1984a & b), and t h a t algae themselves promote t h e photoreduction of iron (Anderson and Morel, 1980). Direct evidence f o r t h e role of iron photoreduction in promoting algal iron uptake in natural systems is lacking however and will be difficult to establish owing to t h e fast kinetics of Fe(I1) oxidation. Vice versa, we know l i t t l e regarding t h e mechanisms of manganese photoreduction which may well occur as t h e result of complicated indirect processes since we do not even know t h e likely chromophores. Nonetheless t h e n e t manganese reduction promoted by light and i t s role in t h e manganese nutrition of phytoplankton has been demonstrated straightforwardly, thanks to t h e slow kinetics of Mn(I1) reoxidation (Sunda et al., 1982).

A.

M L A T I N G AGENT

+

NO

LIGHT

:

Equilibrium

Fe ( I I I >-Y

xi B. M U T I N G AGENT

LIGHT : Rapid disequilibrium Fe (O&+

+

---

Fe ( I I I ) -Y

Fe-X-I

e--

Fe ( I I >-Y C. NO

M U T I N G AGENT FeOx-

a

Slow di6aquilIbri~11

Fe
xi

Fig. 1. Equilibrium and disequilibrium between a cell and i t s medium.

183

T h e r e is truly l i t t l e doubt a t this point t h a t photochemical processes play an important, perhaps a c r i t i c a l role in t h e uptake of iron and manganese by algae. The situation is a good deal more complex than sketched so f a r however. The tight association of m e t a l oxide particles and cell surfaces, t h e existence of l a r g e concentrations of

s u r f a c e binding s i t e s not directly involved in m e t a l transport, and t h e release of r e a c t i v e redox compounds such as hydrogen peroxide by t h e algae (Zepp, personal communication), all

greatly

complicate the

interplay

among

diffusion,

coordination, redox,

and

photochemical processes. This complex situation is obviously a major focus for f u t u r e work and i t may prove fruitful to develop s o m e working hypotheses to unravel t h e problem. F o r example t h e observed strong m e t a l binding to t h e algal s u r f a c e is apparently not directly involved in m e t a l transport and i t s physiological role is presently unclear (Hudson, R. personal communication). Perhaps such binding may b e useful in an environment with regular light/dark periodicity : as illustrated in Fig. 2, photochemically induced reduction and solubilization of

iron and manganese (either directly d u e to reactivity of ferric

chromophores or indirectly d u e t o reactivity of MnOx with H202) may augment considerably t h e specificity of t h e u p t a k e process which would then favor Fe and Mn over t h e o t h e r t r a c e m e t a l s t h a t do not possess similar redox properties. In o t h e r words one may hypothesize t h a t t h e (photo)redox properties of t h e e l e m e n t s a r e utilized to obtain a specificity of u p t a k e t h a t may b e difficult (or costly) to a c h i e v e on t h e basis of coordination alone. Alternatively, t h e light/dark c y c l e and t h e consequent cycle of photosynthesis and respiration may g e n e r a t e a significant die1 variation in pH, resulting in alternating periods of iron and manganese deposition and solubilization which may favor uptake.

I

)@

Dark

xcu

Light

Fig. 2. Hypothesis: The utilization of (photo)redox properties of Fe and Mn at t h e cell s u r f a c e augments t h e specificity of uptake.

184

PHYSIOLOGICAL MECHANISMS T h e question of t h e physiological m e c h a n i s m s of m e t a l effects on microorganisms poses s o m e f u n d a m e n t a l c o n c e p t u a l and e x p e r i m e n t a l difficulties. T h e f i r s t difficulty is partly a m a t t e r of s e m a n t i c s a n d c o n c e r n s t h e notion of "cause", "mechanism" o r "explanation". F o r example, focussing on t h e issue of m e t a l toxicity, is i t a s a t i s f a c t o r y explanation to s a y t h a t c a d m i u m a f f e c t s g r o w t h r a t e in

weissflogii by reducing

photosynthetic r a t e ? T h e r e is a long chain of i n t e r m e d i a r y processes linking t h e primary c a u s e (high Cd2+) to t h e u l t i m a t e phenomenological observation (reduced g r o w t h r a t e ) a n d inhibition of chlorophyll synthesis o r of iron n u t r i t i o n a r e equally valid e x p l a n a t o r y mechanisms. T h e s i t u a t i o n is m a d e m o r e c o m p l i c a t e d by t h e t i g h t couplings a m o n g t h e d i f f e r e n t p a r t s of t h e c e l l u l a r m a c h i n e r y and i t s c o n t r o l by physiological f e e d b a c k processes. As a result, t h e i n t e g r a t e d g r o w t h r a t e response to t h e p r e s e n c e of a t o x i c e l e m e n t typically depends o n a l m o s t a l l i m p o r t a n t e n v i r o n m e n t a l v a r i a b l e s : t e m p e r a t u r e , pH, light, major and t r a c e nutrients, etc. Finally, highly r e a c t i v e t r a c e m e t a l s p e c i e s h a v e multiple e f f e c t s on living cells. Depending on c o n c e n t r a t i o n s (and on physiological and environmental conditions), p r a c t i c a l l y a l l m e t a l r e a c t i v e groups, f r o m highly s p e c i f i c m e t a l c e n t e r s in enzymes, to c a l c i u m channels, o r unspecific ligands in proteins c a n b e a f f e c t e d by t h e p r e s e n c e of a foreign m e t a l ion. Corresponding t o t h e s e d i f f e r e n t r e a c t i o n sites, d i f f e r e n t m e c h a n i s m s of t o x i c i t y will b e exhibited by t h e organism at d i f f e r e n t m e t a l concentrations. To o b v i a t e t h e s e difficulties, w e wish to f o c u s here, as rnuch as possible, on t h e initial s i t e of a c t i o n of a t o x i c m e t a l , at t h e l o w e s t c o n c e n t r a t i o n (activity) t h a t yields a m e a s u r a b l e physiological e f f e c t , under e n v i r o n m e n t a l conditions t h a t a r e t y p i c a l of t h e organism's n a t u r a l milieu. F o r e x a m p l e , i t is t h e p r i m a r y effect of cadmium in inhibiting iron u p t a k e o r assimilation at low c a d m i u m ion a c t i v i t i e s (Harrison and Morel, 1983) which i s of i n t e r e s t to us, n o t t h e resulting chlorosis o r t h e potassium loss t h a t c a n b e induced at high (Cd2+). Of course, n o t a l l ambiguity i s r e m o v e d in t h i s way. F o r e x a m p l e , depending on t h e r e l a t i v e c o n c e n t r a t i o n s of t h e o t h e r e s s e n t i a l m e t a l s , within t h e normal e n v i r o n m e n t a l range, (and possibly o n o t h e r f a c t o r s s u c h as light, etc.) cadmium t o x i c i t y m a y also b e e f f e c t e d though inhibition of m a n g a n e s e o r z i n c nutrition-or t w o o r t h r e e of t h e s e m e c h a n i s m s concurrently. T h e obvious a n d f u n d a m e n t a l c o n c e p t t h a t r e s u l t s f r o m such elucubrations, o n e t h a t i s now supported, d o c u m e n t e d a n d quantified in s e v e r a l e x p e r i m e n t a l d a t a sets, is t h a t t h e toxic e f f e c t of a t r a c e m e t a l is c a u s e d by i t s i n t e r f e r e n c e with t h e uptake, assimilation o r utilization of another, e s s e n t i a l t r a c e metal. T r a c e m e t a l s a r e to various d e g r e e s coordinative analogs of e a c h o t h e r , t o x i c i t y o c c u r s when a m e t a l binds c o m p e t i t i v e l y to a ligand s i t e involved in t h e t r a n s p o r t , assimilation o r utilization of a n e s s e n t i a l m e t a l whose f u n c t i o n i t c a n n o t e m u l a t e . (In s o m e cases m e t a l s c a n r e p l a c e o t h e r s w i t h reasonably good efficiency). As a result, t h e s t u d y of t r a c e m e t a l t o x i c i t y in m i c r o a l g a e must c o m m e n c e with a s t u d y of t r a c e m e t a l nutrition, a scientifically sound if paradoxical situation.

185

T h e a l g a l r e q u i r e m e n t s f o r t h r e e e s s e n t i a l t r a c e metals--Fe, Mn a n d Zn--appear t o d e t e r m i n e t r a c e m e t a l t p x i c i t y in m a r i n e phytoplankton. O t h e r essential m e t a l s such a s c o p p e r o r c o b a l t a r e sufficiently abundant t h a t t h e y m o r e commonly function as toxicants t h a n as limiting e l e m e n t s . Zinc p r e s e n t s t h e p e c u l a r i t y t h a t i t c a n both b e limiting or t o x i c ; in m o s t phytoplankters, t h e thresholds of Zn2+ l i m i t a t i o n and toxicity a r e fairly c l o s e to e a c h o t h e r , leaving a r e l a t i v e l y n a r r o w r a n g e of conditions t h a t result in o p t i m a l growth.

A number of case s t u d i e s h a v e been p e r f o r m e d with e a c h of t h e principal e s s e n t i a l m e t a l s , iron, rnanganese a n d z i n c and a v a r i e t y of t o x i c m e t a l s (e.g. cadmium, copper, nickel) chosen as e x a m p l e s for p r a g m a t i c reasons such as high toxicity, s i m p l e c h e m i s t r y , e x i s t e n c e of radiotracers, etc. ( R u e t e r and Morel, 1981 ; Sunda and Huntsman, 1983). As a n e x a m p l e l e t u s consider t h e studies of iron and c a d m i u m antagonism in t h e d i a t o m Thalassiosira weissflogii ( F o s t e r a n d Morel, 1982 ; Harrison a n d Morel, 1983). In t h e s e s t u d i e s d i f f e r e n t t o x i c e f f e c t s w e r e observed depending o n t h e t i m e scale and o n t h e f r e e iron c o n c e n t r a t i o n . O v e r a t i r n e scale of hours, t h e principal e f f e c t of t h e t o x i c rnetal (Cd) i s to d e c r e a s e t h e u p t a k e of iron whose supply simply b e c o m e s insufficient over t h e long-term if [Fe3+]is low. T h e s i t u a t i o n c a n b e c h a r a c t e r i z e d as a n equilibrium situation b e t w e e n t h e medium and t h e c r i t i c a l t r a n s p o r t s i t e i n a c t i v a t e d by binding of t h e toxic metal. If rFe3.3 (or a n y a f f e c t e d e s s e n t i a l e l e m e n t ) is high enough, t h e u p t a k e r a t e r e m a i n s s u f f i c i e n t to s u p p o r t (theoretically) a high g r o w t h r a t e . In t h i s case however, t h e i n t r a c e l l u l a r buildup of t h e t o x i c m e t a l (Cd a p p a r e n t l y l e a k s through t h e Fe t r a n s p o r t s y s t e m ) e v e n t u a l l y b e c o m e s high enough to inhibit Fe assimilation. T h e c e l l s t h e n becorne e f f e c t i v e l y iron d e f f i c i e n t though t h e i r cellular iron c o n t e n t is abnormally high. In this

case t h e i n t r a c e l l u l a r c o n c e n t r a t i o n of t h e t o x i c e l e m e n t is t h e c r i t i c a l p a r a m e t e r ; i t is d e t e r m i n e d by a b a l a n c e b e t w e e n g r o w t h r a t e and u p t a k e r a t e , t h e l a t t e r being fixed by a pseudo-equilibrium with a m e m b r a n e p o r t e r site. B e c a u s e t h e n e t r e s u l t of t r a c e m e t a l inhibition c a n paradoxically b e e i t h e r decreased o r i n c r e a s e d c e l l u l a r c o n t e n t of a n e s s e n t i a l e l e m e n t , s u c h s t u d i e s r e q u i r e r a t h e r detailed physiological and b i o c h e m i c a l s t u d i e s to establish mechanisms. Vice-versa, if o n e wishes to s t u d y t r a c e m e t a l t o x i c i t y in t h e field, g r o s s c h e m i c a l and biological m e a s u r e m e n t s a r e

c l e a r l y insufficient ; o n e m u s t depend o n s p e c i f i c physiological and biochemical markers. T h e phenomenon of t o x i c m e t a l i n t e r f e r e n c e w i t h e s s e n t i a l m e t a l n u t r i t i o n is c o m p l i c a t e d by t h e physiological c o n t r o l t h a t t h e c e l l s e x e r c i s e o v e r t h e u p t a k e and assimilation of a l l c r i t i c a l nutrients. Studies of t h e regulation of iron (Harrison a n d Morel, in preparation) a n d m a n g a n e s e (Sunda and Huntsman, 1985) t r a n s p o r t in phytoplankton h a v e yielded s i m i l a r r e s u l t s a n d t h e g e n e r a l p a t t e r n i s c o n s i s t e n t with w h a t i s known f o r all e s s e n t i a l algal nutrients. Microalgae r e g u l a t e t h e i r t r a c e e l e m e n t u p t a k e and, as

e x p e c t e d , t h e y d o so by changing t h e i r m a x i m u m u p t a k e r a t e depending o n t h e i r d e g r e e of s t a r v a t i o n o r r e p l e t e n e s s f o r a particular e l e m e n t . In round numbers, t h e maximurn u p t a k e

186

r a t e of starved c e l l s f o r Fe or Mn exceeds t h a t of r e p l e t e cells by a f a c t o r of 10 t o 100. Together with t h e variations in cellular iron and manganese quotas, t h e s e variations in maximum u p t a k e r a t e s allow high growth r a t e s to b e maintained at very low f r e e m e t a l ion concentrations. T h e resulting half saturation c o n s t a n t for growth (limited e i t h e r by Fe or Mn) i s a f a c t o r of 100 t o 1,000 lower than t h e half saturation c o n s t a n t for m e t a l uptake. T h e feedback control on t h e uptake mechanism t a k e s effect in a m a t t e r of several hours (typically a day or so) and is prone to overshoot a f t e r starvation of iron or manganese. The similarities observed among t h e regulation and toxicity mechanisms in t h e various experimental s y s t e m s provides s o m e confidence t h a t one is studying fundamental physiological processes (almost) universally shared among algae. The sound chemical logic t h a t buttresses t h e physiological observations bolsters t h a t confidence. In a few years, we should have elucidated t h e fundamental molecular biochemical processses t h a t underlie these observations. Then w e will have t h e highly specific tools needed to a t t a c k efficiently t h e environmental issues.

ENVIRONMENTAL SIGNIFICANCE While i t is difficult at present to address specific environmental questions (such as whether or not a particular organism or ecosystem is under a particular stress) our improved understanding of microalgae-trace m e t a l interactions provides us with new insights on t h e role of t r a c e m e t a l s in phytoplankton ecology and on t h e role of phytoplankton in t r a c e m e t a l geochemistry. Though much of t h e conclusions must be speculative at this point ( t h e r e have been only few field experiments t h a t could and have been performed) they provide a general framework to view t h e issue of t r a c e m e t a l pollution in c o a s t a l waters. One c e n t r a l ecological hypothesis is t h a t organisms in their natural milieu should necessarily b e at t h e limit of deficiency of several essential e l e m e n t s and of toxicity of their chemical analogs, simultaneously (Morel and Hudson, 1985). This is a generalization of a c o n c e p t proposed by Redfield f i f t y years a g o f o r nitrogen and phosphorus (Redfield,

19341, and is precisely t h e opposite of Liebig's law of t h e minimum in which only one element is normally e x p e c t e d to b e limiting. O n e could consider t h e available evidence t h a t manganese-copper interactions may b e limiting productivity in freshly upwelled water (Sunda et al., 1981 ; Sunda and Huntsman, 1983) to provide partial support to t h e general notion of multiple e l e m e n t limitation and toxicity in t h e sea. T h e consequence of such a situation is t h a t any significant environmental modification must result in significant ecological change. F o r t h e specific case of t r a c e metals, t h e situation is of course complicated by t h e chemistry of t h e e l e m e n t s (e.g.,

complexation, adsorption,

redox, etc.) and by t h e normal c o m p e t i t i v e ("antagonistic") mode of a c t i o n of toxic m e t a l s (e.g., increased C d may have no e f f e c t on Fe nutrition if Fe is increased simultaneously).

The complementary geochemical hypothesis is t h a t binding of m e t a l s by phytoplankton

187

(followed by g r a z i n g a n d f e c a l p e l l e t settling) is t h e major mechanism for r e m o v a l of t r a c e m e t a l s f r o m t h e w a t e r column. In addition to t h e i r geochemical c o n t r o l of t h e c y c l e of t h e t r a c e e l e m e n t s , phytoplankton also e x e r t a c h e m i c a l c o n t r o l by releasing

complexing a n d (photo)reducing a g e n t s and p r o m o t i n g intracellularly a host of c h e m i c a l transformations, methylaAon in particular. According to this hypothesis, t h e t r a c e m e t a l chernistry in c o a s t a l w a t e r s is under r a t h e r t i g h t c o n t r o l by t h e organisms t h a t i t affects. Though d r a m a t i c ecological effects m a y b e observed during episodes of e x t r e m e conditions of t r a c e m e t a l toxicity, in most situations w e should e x p e c t t h e e c o s y s t e m to e x h i b i t s t r o n g h o m e o s t a t i c behavior.

T h e i n t e r d e p e n d e n c e of

t h e ecological and

g e o c h e m i c a l processes a f f e c t i n g phytoplankton ecology and t r a c e m e t a l geochernistry is s t y l i z e d in Fig. 3. tUl

=

P..

-

'

KH

P

KM

n

{ :M 1

+

max

N

= {M? conditioning

succession

g

~ch oem

ica1 cycling

Fig. 3. I n t e r d e p e d e n c e of a q u a t i c c h e m i s t r y and biology : T h e g e o c h e m i c a l cycling a n d t h e biological e f f e c t of a t r a c e e l e m e n t M a r e b o t h modulated by t h e u p t a k e r a t e of M. Through a d a p t a t i o n , evolution a n d s p e c i e s succession, t h e physiology of t h e organisms is m a t c h e d to i t s c h e m i c a l environment. Conversely, t h e a q u a t i c c h e m i s t r y of t h e c r i t i c a l e l e m e n t s is c o n t r o l l e d by g e o c h e m i c a l and c h e m i c a l processes t h a t depend on t h e physiology of t h e organisms. T h e m u t u a l c o n t r o l of t h e a q u a t i c biology a n d c h e m i s t r y c a n b e m a d e m o r e q u a n t i t a t i v e by considering t h a t t h e half s a t u r a t i o n a c t i v i t y of all biologically a c t i v e e l e m e n t s should b e roughly e q u a l to t h e e l e m e n t ' s a c t i v i t y in t h e medium ; p e r h a p s t e n t i m e s higher for e s s e n t i a l e l e m e n t s ; p e r h a p s t e n t i m e s lower for t h e t o x i c e l e m e n t s t h a t act as t h e i r c h e m i c a l analogs.

REFERENCES Anderson, D.M. a n d Morel, F.M.M., Limnol. Oceanogr., 23: 283-295.

1978.

C o p p e r sensitivity of Gonyaulax tamarensis.

Anderson, M.A. a n d Morel, F.M.M., 1980. U p t a k e of iron I1 by a d i a t o m in o x i c c u l t u r e medium. Mar. Biol. Lett., 1: 263-268. Anderson, M.A. a n d Morel, F.M.M., 1982. T h e i n f l u e n c e of a q u e o u s iron c h e m i s t r y on t h e u p t a k e of iron by t h e c o a s t a l d i a t o m Thalassiosira weissflogii. Limnol. Oceanogr., 27: 789-8 13

188

Barber, R.T., 1973. Organic ligands and phytoplankton growth in nutrient-rich seawater. In: P.C. Singer (ed.), T r a c e Metals and Metal - Organic Interactions in Natural Waters. Ann Arbor Science, Michigan, pp 321-338. Droop, M.R., 1961. S o m e chemical considerations in t h e design of synthetic culture media f o r m a r i n e algae. Botanica iMarina, 2: 231-246. Foster, P.L. and Morel, F.M.M., 1982. Reversal of cadmium toxicity in t h e diatom Thalassiosira weissflogii. Limnol. Oceanogr., 27: 745-752. Guillard, R.R.L. and Ryther, J.H., 1962. Studies of marine plankton diatoms.1. Cyclotella nana H u s t e d t and Detonula confervacea (Cleve) Gran. Can. J. Microbiol., 8: 229-238.

-

Harrison, G.I. and Morel, F.M.M., Phycol., 19: 495-507.

1983. Copper sensitivity of Gonyaulax tamarensis. J.

Huntsman, S.A. and Sunda, W.G., 1980. The role of t r a c e m e t a l s in regulating phytoplankton growth with emphasis on iron manganese and copper. In: I. Morris (ed.), The Physiological Ecology of Phytoplankton. Blackwell Scientific, Oxford, pp. 285-328. Johnston, R., 1964. Seawater, t h e natural medium of phytoplankton. 11. T r a c e m e t a l s and chelation, a general discussion. J. Mar. Biol. Ass. U.K., 44: 87-109. Manahan, S.E. and Smith, M.J., Sci. Technol., 7: 829-833.

1973. Copper micronutrient requirements for algae. Env.

Morel, F.M.M. and Hudson, R.J.M., 1985. The geobiological cycle of t r a c e e l e m e n t in a q u a t i c s y s t e m s :Redfield revisited. In: W. Stumm (ed.), Chemical Processes in Lakes. Wiley Interscience,New York, pp. 251-281. Morel, F.M.M. and Morel-Laurens, N.M.L., 1983. T r a c e m e t a l s and plankton in t h e ocean : f a c t and speculations. In: C.W. Wong, E. Boyle, K.W. Bruland, J.D. Burton and E.D. Goldberg (eds), T r a c e Metals in Sea Water. NATO Conference Series, Plenum Press, New York, pp. 841-869. Provasoli, L., McLaughlin, J.J.A. and Droop, M.R., 1957. T h e development of artificial media f o r marine algae. Arch. Mikrobiol., 25: 392-425. Redfield, A.C., 1934. On t h e proportion of organic derivatives in t h e sea water and their relation to t h e composition of plankton. In: J a m e s Johnston Memorial Volume. Liverpool University Press, Liverpool, pp. 176-192. Rueter, J.G. Jr., Chisholm S.W. and Morel, F.M.M., 1981. E f f e c t s of copper toxicity on silicic acid uptake and growth in Thalassiosira pseudonana. J. Phycol., 17: 270-278. Sunda, W.G., Barber, R.T. and Huntsman, S.A., 1981. Phytoplankton growth in nutrient rich s e a w a t e r : Importance of copper-manganese cellular interactions. J. Mar. Res., 39: 567-586. Sunda, W.G. and Guillard, R.R.L., 1976. T h e relationship between iron activity and t h e toxicity of copper to phytoplankton. J. Mar. Res., 34: 511-529. Sunda, W.C. and Huntsman, S.A., 1983. E f f e c t of c o m p e t i t i v e interactions between manganese and copper on cellular manganese and growth in e s t u a r i n e and oceanic species of t h e diatom Thalassiosira. Limnol. Oceanogr., 28: 924-934. Sunda, W.G. and Huntsman, S.A., 1985. Regulation of cellular manganese transport r a t e s in t h e unicellular a l g a Chlamydomonas. Limnol. Oceanogr., 30: 71-80.

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Sunda, W.G., Huntsman, S.A. and Harvey, G., 1983. Photoreduction of Mn oxide in seawater : geochemical and biological implications. Nature, 301: 234-236. Waite, T.D. and Morel, F.M.M., 1984a. Photoreductive dissolution of colloidal iron oxide : effect of citrate. J. Colloid Interface Sci., 102: 121-137. Waite, T.D. and Morel, F.M.M., 1984b. Photoreductive dissolution of colloidal iron oxides in natural waters. Environ. Sci. Technol., 18: 860-868.

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191

BIOLOGICAL AVAILABILITY OF TRACE ELEMENTS

D.R. TURNER Marine Biological Association of t h e U.K.,

Citadel Hill, Plymouth P L l 2PB (United

Kingdom)

ABSTRACT Turner, D.R., 1986. Biological availability of t r a c e elements. In: P. Lasserre and J.M. Martin (eds), Biogeochemical Processes at t h e Land-Sea Boundary. Elsevier, Amsterdam. T h e chemical speciation of t r a c e e l e m e n t s in seawater covers a wide range of chemical t y p e s f rom weakly complexed cations though strongly complexed cations t o strong-acid anions. Although relatively f e w e l e m e n t s have been studied, t h e biologically available chemical species of different e l e m e n t s which have been identified cover t h e s a m e wide range. The development of chemical techniques capable of measuring these biologically available chemical species is reviewed. While reliable methods have been developed for inorganic and alkylated f o r m s of several metalloid elements, t h e measurement of ionic species at t r a c e levels requires f u r t h e r development. INTRODUCTION Aquatic organisms a r e continuously undergoing chemical exchanges with their environment, involving uptake and excretion of both nutrient and toxic elements. These processes have important consequences not only f o r t h e organisms, but also for the e l e m e n t s concerned, since t h e vertical profiles of many t r a c e e l e m e n t s in t h e oceans appear t o b e biologically controlled (Bruland, 1983). Biological u p t a k e from t h e dissolved

state is of particular i n t e r e s t to chemists since i t is strongly influenced by t h e chemistry of t h e surrounding a q u a t i c environment. The recognition t h a t t h e u p t a k e of t r a c e metals by aquatic organisms is a function of t h e chemical speciation of t h e e l e m e n t s concerned led to t h e use of t h e t e r m biological availability, representing t h e fraction of t h e total t r a c e m e t a l available f o r uptake. Biological availability is thus essentially a chemical concept which largely ignores t h e complexity of t h e biological processes involved. Nevertheless i t o f f e r s a means of relating t h e chemistry and geochemistry of a t r a c e element, as manifested in i t s chemical speciation, to i t s interactions with t h e biota. Before considering t h e details of biological availability i t is important to put this p a r a m e t e r in perspective. Long-term uptake i s usually described by an enrichment f a c t o r equal to t h e r a t i o of concentrations of a n e l e m e n t in an organism and in seawater. Enrichment f a c t o r s f o r a wide range of e l e m e n t s in marine organisms cover 4-5 orders of magnitude, f r o m near unity f o r Na, Mg, C1 to lo4 or m o r e for phosphorus and t h e heavy

192

m e t a l s (Trudinger et al., 1979 ; S p a a r g a r e n and C e c c a l d i , 1984). D e s p i t e t h i s r a n g e of e n r i c h m e n t f a c t o r s , t h e biologically e s s e n t i a l e l e m e n t s a p p e a r t o h a v e been s e l e c t e d simply on t h e basis of t o t a l c o n c e n t r a t i o n in s e a w a t e r . T h e v e r t i c a l divisions in T a b l e 1 group t h e s t a b l e e l e m e n t s i n t o t h r e e c o n c e n t r a t i o n r a n g e s in s e a w a t e r , a n d i t c a n b e s e e n t h a t t h e biologically e s s e n t i a l e l e m e n t s correspond to t h o s e with c o n c e n t r a t i o n s g r e a t e r t h a n InM in s e a w a t e r , with a sinall number of exceptions. If w e a s s u m e t h a t t h e biologically e s s e n t i a l e l e m e n t s correspond to t h e most available e l e m e n t s with t h e required r a n g e of c h e m i c a l properties (Williams, 1981) t h e n i t is c l e a r t h a t , o v e r l a r g e s c a l e s of t i m e and s p a c e , t o t a l c o n c e n t r a t i o n in s e a w a t e r gives a n a d e q u a t e e s t i m a t e of availability to t h e biota.

TABLE 1 Inorganic speciation, s e a w a t e r c o n c e n t r a t i o n and biological function of t h e e l e m e n t s a .

[Mn(II)],[Fe(II)], Co,Ni,Cu(II),Zn

free cations complexes

Be,Sc,Ga,Zr,Ag,In,Hf Hg,T l(III),Bi,Th, [Au(I)I, Au(II1)

complexes, negligible free cation neutral s p e c i e s only neutral s p e c i e s and anions f r e e anions

Y,Cd,La-Lu,[Tl(IJl,Pb

8, c,2,p

T J[Sb(IIIfl

_S _n

V,Ge,[As(III)l

Nb,Ta

W

a : e s s e n t i a l e l e m e n t s a r e underlined ; e l e m e n t s of doubtful s t a t u s a r e underlined with dashed lines ; non- equilibrium oxidation states a r e enclosed in s q u a r e brackets. Biological function d a t a f r o m E g a m i (1974) a n d Trudinger et al. (1979) ; s e a w a t e r c o n c e n t r a t i o n s f r o m Brewer (1975). b : t y p e s of c h e m i c a l s p e c i e s p r e s e n t at > 1% in s e a w a t e r (Turner et al., 1981). T h e biologically a v a i l a b l e c o n c e n t r a t i o n [XbJ of a n e l e m e n t X c a n b e w r i t t e n as : [xb] = rxT] f b

(1)

w h e r e [XT] is t h e t o t a l c o n c e n t r a t i o n and f b t h e biological availability o r biologically available fraction. I t is c l e a r f r o m t h e preceding discussion t h a t t h e v a l u e of f b i s n o t

193

significant when o n e c o n s i d e r s t h e whole periodic table, w i t h a r a n g e of [XT] of a t l e a s t 12 o r d e r s of m a g n i t u d e , o v e r t h e evolutionary t i m e s c a l e ; o n t h i s s c a l e [XI-) provides a n

a d e q u a t e e s t i m a t e of availability. On s m a l l t i m e and s p a c e scales, however, t h e s i t u a t i o n is r a t h e r different. H e r e t h e response of individual organisms o r s p e c i e s to c h a n g e s in t h e c h e m i s t r y of a n a t u r a l w a t e r c a n b e important. Marine organisms h a v e only l i m i t e d d e f e n c e s a g a i n s t i n c r e a s e d c o n c e n t r a t i o n s of m a n y t r a c e e l e m e n t s , and b o t h non-essential and m i c r o n u t r i e n t t r a c e e l e m e n t s c a n b e c o m e t o x i c at e l e v a t e d c o n c e n t r a t i o n levels. In such cases i t is i m p o r t a n t to b e a b l e to d e f i n e [ X d m o r e closely (eqn I), which in turn m e a n s obtaining a n understanding of t h e biological availability f b in addition to t h e simpler parameter

h~].

TRACE ELEMENTS AND CHEMICAL SPECIATION T h e c o r r e l a t i o n b e t w e e n s e a w a t e r c o n c e n t r a t i o n and biological function (Table I ) and t h e relatively l i m i t e d r a n g e of e n r i c h m e n t f a c t o r s r e f e r r e d to a b o v e h a s i m p o r t a n t implications f o r t h e c h e m i c a l n a t u r e of biological availability. T h e maximum r a n g e of e n r i c h m e n t f a c t o r s is a b o u t 5 o r d e r s of magnitude, of which a c o n s i d e r a b l e p a r t c a n be a t t r i b u t e d to c o m p l e x a t i o n o r o t h e r s e q u e s t r a t i o n of t h e heavier e l e m e n t s within t h e organisms. T h e v a r i a t i o n a t t r i b u t a b l e to c h e m i c a l speciation o u t s i d e t h e organisms c a n t h e r e f o r e b e e x p e c t e d to b e considerably less, perhaps 2-3 o r d e r s of magnitude. T h u s for e a c h e l e m e n t a t l e a s t o n e biologically available f o r m must b e p r e s e n t to a significant e x t e n t in s e a w a t e r (> I % of t h e t o t a l concentration). T h i s in t u r n implies a wide r a n g e of biologically a v a i l a b l e c h e m i c a l s p e c i e s as c a n b e s e e n f r o m t h e horizontal classification in T a b l e 1. T h e s t a b l e e l e m e n t s in s e a w a t e r h a v e been divided i n t o six speciation t y p e s based o n t h e s t r e n g t h of c o m p l e x a t i o n of a n e l e m e n t a l c a t i o n s i n c e a l m o s t a l l e l e m e n t s in s e a w a t e r a r e p r e s e n t in positive oxidation states. A t o n e e x t r e m e l i e t h e alkali and alkaline e a r t h c a t i o n s which f o r m only v e r y w e a k c o m p l e x e s a n d a r e l a r g e l y p r e s e n t as t h e f r e e c a t i o n , while at t h e o t h e r e x t r e m e h y p o t h e t i c a l c a t i o n s s u c h as N5+ and Cr6+ i n t e r a c t so strongly w i t h w a t e r t h a t t h e y a r e found as s t r o n g a c i d a n i o n s in seawater. The halide ions h a v e b e e n included in t h i s l a t t e r group f o r completeness. Although 1% h a s been chosen as a n a r b i t r a r y cut-off point, only minor c h a n g e s would r e s u l t f r o m cut-offs of 0.1% o r 0.01% corresponding to 3 a n d 4 o r d e r s of magnitude r a n g e in f b respectively.

IDENTIFICATION OF BIOLOGICALLY AVAILABLE CHEMICAL SPECIES T h e s i m p l e definition of biological availability as t h e f r a c t i o n of t h e total t r a c e m e t a l available f o r u p t a k e by t h e b i o t a implicitly a s s u m e s t h a t a l l r e l e v a n t organisms will have similar u p t a k e c h a r a c t e r i s t i c s . In addition t h e u s e of biological availability as a c h e m i c a l p a r a m e t e r involves t h e assumption t h a t i t c a n b e identified w i t h particular c h e m i c a l s p e c i e s o r groups of species. P e r h a p s surprisingly m a n y of t h e s t u d i e s c a r r i e d o u t to d a t e h a v e l e n t s u p p o r t to t h e s e assumptions, though t h e d e t a i l e d m e c h a n i s m s of t h e u p t a k e p r o c e s s e s r e m a i n unclear.

S o m e of

t h e inorganic c h e m i c a l s p e c i e s identified as

194

biologically available a r e listed in T a b l e 2. I t c a n b e s e e n t h a t m u c h of t h e work has c o n c e n t r a t e d o n t h e divalent c a t i o n s of t h e f i r s t transition series, t o g e t h e r with t h e chemically similar lead and cadmium, w h e r e t h e f r e e c a t i o n is g e n e r a l l y found to b e t h e biologically available form. Work in this a r e a h a s been s t i m u l a t e d by t h e combination of an i m p o r t a n t group of m i c r o n u t r i e n t and t o x i c e l e m e n t s t o g e t h e r with readily available techniques f o r t h e study of speciation, though t h e s e techniques a r e still f a r f r o m ideal (see discussion below). S t u d i e s in o t h e r speciation t y p e s h a v e been much m o r e limited, though s o m e results a r e now available for anion-forming e l e m e n t s , and studies of "acid rain" h a v e s t i m u l a t e d work on t h e biological availability of aluminium. In p r a c t i c a l t e r m s , however, t h e s e e l e m e n t s a r e considerably m o r e d i f f i c u l t to work with t h a n t h e divalent cations, owing t o t h e lack of c h e m i c a l techniques for t h e d i r e c t study of speciation in c u l t u r e media and in n a t u r a l w a t e r samples.

TABLE 2 Bioavailable Inorganic C h e m i c a l S p e c i e s of t h e T r a c e Elements.

complexes only

A1 U(V1) Hg

neutral s p e c i e s

-

neutral s p e c i e s + anions

-

anions

AI(OH)2+ OH, C 0 3 c o m p l e x e s CI c o m p l e x e s

(7)

H2VO4HCr04-

(10) (11)

(8) (9)

Key to r e f e r e n c e s : (1) Sunda & Guillard, 1976 ; Anderson & Morel, 1978 ; Sunda & Gillespie, 1979 ; Hodson et al., 1979 ; P e t e r s e n , 1982. (2) Andrew et al., 1977 ; Magnusson et al., 1979 ; Dodge & Theis, 1979. (3) Schulz-Baldes & Lewin, 1976 ; Merlin & Pozzi, 1977. (4) Spencer & Nichols, 1983. ( 5 ) P e t e r s e n , 1982 ; Anderson et al., 1978 ; Allen et al., 1980. (6) Sunda et al., 1978. (7) Helliwell et al., 1983 (8) Pribil & Marvan, 1976 ( 9 ) P e n t r e a t h , 1976 ; Davies, 1976 (10) Stendahl & Sprague, 1982 (11) van d e r P u t t e et a]., 1981.

195 A

f u r t h e r group of

biologically

available c h e m i c a l s p e c i e s c o m p r i s e s organic

compounds of t h e t r a c e e l e m e n t s , which m a y b e divided i n t o t w o g r o u p s ; organic c o m p l e x e s of cations, a n d m e t a l and non-metal alkyls ( T a b l e 3). T h e s e s p e c i e s a r e mainly n e u t r a l molecules, in c o n t r a s t to t h e ionic s p e c i e s i d e n t i f i e d in T a b l e 2, and a r e t h o u g h t t o b e t a k e n up as a c o n s e q u e n c e of t h e i r lipid-solubility (Florence et al., 1983). I t i s c l e a r f r o m t h i s brief survey t h a t biologically available c h e m i c a l s p e c i e s c a n

include f r e e cations, anionic and c a t i o n i c complexes, and n e u t r a l molecules, depending on t h e e l e m e n t concerned. S u c h a d i s p a r a t e collection implies a r a n g e of d i f f e r e n t u p t a k e m e c h a n i s m s ; probably a minimum of t h r e e for positive, n e u t r a l , a n d n e g a t i v e c h a r g e types. F u r t h e r m o r e , t h e e s t i m a t i o n of biological availability i n t e r m s of c h e m i s t r y b e c o m e s m o r e difficult, s i n c e not only will d i f f e r e n t t e c h n i q u e s b e r e q u i r e d f o r d i f f e r e n t e l e m e n t s , b u t s o m e e l e m e n t s a r e a l r e a d y included in m o r e t h a n o n e s p e c i e s t y p e in Tables 2 and 3 (e.g. Cd, Hg, Cu).

TABLE 3 Bioavailable O r g a n i c C h e m i c a l S p e c i e s of t h e T r a c e Elements.

T y p e of S p e c i e s

Element

Bioavailable C h e m i c a l S p e c i e s

Reference

.................................................................................................. metal-organic complexes

CU(I1) Cd Fe(II1)

m e t a l alkyls

Pb Hg Sn

c i t r a t e , oxine, e t h y l e n e d i a m i n e , diethyldithiocarbamate complexes diethyldithiocarbamate complex siderophore ( h y d r o x a m a t e ) c o m p l e x

(1) (2)

tetramethyllead, tetraethyllead m e t h y l m e r c u r y (?hydroxide c o m p l e x ) various

(4) (5) (6)

(3)

K e y to R e f e r e n c e s : (1) Guy & Ross Kean, 1980 ; F l o r e n c e et al., 1983. ( 2 ) Poldoski, 1979.

(3) Murphy et al., 1976 (4) M a r c h e t t i , 1978 ; Wong et al., 1981 (5) R o d e r e r , 1983 (6) Z u c k e r m a n et al., 1978.

CHEMICAL ESTIMATION OF BIOLOGICAL AVAILABILITY Knowledge of t h e biologically available c h e m i c a l s p e c i e s of a p a r t i c u l a r e l e m e n t will b e valuable only if t h o s e s p e c i e s c a n b e reliably m e a s u r e d at n a t u r a l c o n c e n t r a t i o n levels. S i n c e i t is only v e r y r e c e n t l y t h a t t o t a l levels of m a n y e l e m e n t s h a v e been reliably m e a s u r e d in s e a w a t e r , i t is c l e a r t h a t m e a s u r e m e n t of a c h e m i c a l l y defined f r a c t i o n of t h a t total p r e s e n t s a m a j o r a n a l y t i c a l challenge. In addition to a c h i e v i n g t h e necessary d e t e c t i o n l i m i t a n d guarding a g a i n s t c o n t a m i n a t i o n i t i s n e c e s s a r y to m a t c h t h e c h a r a c t e r i s t i c s of t h e speciation-sensitive t e c h n i q u e with t h e s p e c i e s to b e measured. A wide r a n g e of speciation-sensitive t e c h n i q u e s is now a v a i l a b l e including electroanalysis,

196 chromatography, dialysis a n d ion-exchange.

F u r t h e r d e v e l o p m e n t will, however, b e

required s i n c e many t e c h n i q u e s a r e s u b j e c t to i n t e r f e r e n c e s a n d a r t i f a c t s w h e n used at low c o n c e n t r a t i o n levels in n a t u r a l w a t e r s , a n d t e c h n i q u e s a r e n o t y e t a v a i l a b l e f o r a l l of t h e biologically available s p e c i e s so f a r identified. A d e t a i l e d discussion of t h i s problem c a n b e found e l s e w h e r e (Turner, 19841, so only a brief s u m m a r y will b e given here. A k e y problem c o m m o n t o a l m o s t a l l speciation-sensitive m e a s u r e m e n t s is t h a t of

kinetics. Speciation-sensitive m e a s u r e m e n t s c a n b e divided i n t o equilibrium, k i n e t i c and d e s t r u c t i v e techniques. Equilibrium t e c h n i q u e s c a u s e no d i s t u r b a n c e to c h e m i c a l equilibria in t h e s a m p l e a n d r e s u l t in a t r u e equilibrium m e a s u r e m e n t ; ion-selective e l e c t r o d e p o t e n t i o m e t r y is t h e m o s t f a m i l i a r e x a m p l e . K i n e t i c t e c h n i q u e s in c o n t r a s t involve depletion of t h e s p e c i e s being m e a s u r e d in t h e region of t h e sensing s u r f a c e , t h u s disturbing c h e m i c a l equilibria in t h i s region, a n d m a y t h u s p r o d u c e a l a r g e r response t h a n t h e corresponding equilibrium t e c h n i q u e ; no significant c h a n g e s in t h e bulk solution composition o c c u r during t h e s e m e a s u r e m e n t s . All polarographic a n d v o l t a m m e t r i c t e c h n i q u e s f a l l i n t o t h i s class. D e s t r u c t i v e t e c h n i q u e s involve t h e r e m o v a l of a s i g n i f i c a n t q u a n t i t y of m e t a l f r o m t h e bulk of t h e s a m p l e ; ion-exchange,

chromatographic and

c h e m i c a l r e a c t i o n t e c h n i q u e s a r e included here. I n t h e l a s t t w o cases t h e response obtained d e p e n d s not only o n t h e s e l e c t i v i t y of t h e s e n s o r o r p r o c e d u r e used b u t also on t h e k i n e t i c s of t h e equilibria linking t h e a c t i v e s p e c i e s sensed with o t h e r c h e m i c a l species. In t h e l i m i t of v e r y slow k i n e t i c s a l l t e c h n i q u e s will g i v e a n equilibrium response s i n c e no r e a c t i o n s will o c c u r during t h e m e a s u r e m e n t , while a t t h e o t h e r e x t r e m e of v e r y

fast k i n e t i c s b o t h k i n e t i c a n d d e s t r u c t i v e t e c h n i q u e s will g i v e a response c o r r e s p o n d i n g to t h e t o t a l m e t a l in t h e sample. B e t w e e n t h e s e t w o e x t r e m e s t h e response will d e p e n d on

TABLE 4 Measurement T e c h n i q u e s f o r Bioavailable C h e m i c a l Species.

Species

Technique

Nature

Remarks

polarograph y voltammetry dialysis ion-exchange

kinetic kinetic destructive destructive

) reactions a r e

inorganic complexes

speciation modelling

equilibrium

r e q u i r e s good d a t a f o r solution composition a n d stability constants

organic complexes

solvent extraction chromatography

destructive destructive

-) r e a c t i o n s s l o w e r here, ) so f e w e r k i n e t i c p r o b l e m s

m e t a l alkyls

hydride g e n e r a t i o n solvent extraction chromatography

destructive destructive destructive

1 r e a c t i o n s v e r y slow,

.................................................................................................. free cation potentiometry equilibrium l i m i t e d to t o t a l c o n c < 10-6M rapid, so t h e s e d o ) not truly ) m e a s u r e f r e e ions

) so no k i n e t i c ) problems

197

t h e r e l a t i v e t i m e s c a l e s of t h e r e a c t i o n kinetics and t h e m e a s u r e m e n t process, a n d may o f t e n b e d i f f i c u l t to i n t e r p r e t d i r e c t l y in t e r m s of speciation. T h e e v i d e n c e o b t a i n e d o n biological u p t a k e so f a r shows t h a t it is a n equilibrium process, i.e. t h e r a t e of u p t a k e is too slow to d i s t u r b a n y t r a c e m e t a l equilibria e v e n w h e r e r e a c t i o n r a t e s a r e rapid. T h e biologically available c h e m i c a l s p e c i e s identified to d a t e (Tables 2 and 3) c a n b e s u m m a r i z e d in f o u r groups (Table 41, with d i f f e r e n t possibilities f o r m e a s u r e m e n t in e a c h group. P e r h a p s t h e most successful t e c h n i q u e s a r e t h o s e based o n hydride g e n e r a t i o n (Andreae, 1983) which a r e applicable to a r a n g e of hydride-forming

m e t a l s and metalloids. H e r e t h e interconversion r e a c t i o n s b e t w e e n

inorganic and a l k y l a t e d f o r m s of t h e e l e m e n t s a r e so slow t h a t t h e y c a n b e ignored ; and a d e s t r u c t i v e c h e m i c a l t e c h n i q u e (hydride g e n e r a t i o n ) provides a n e f f e c t i v e method of s p e c i a t i o n analysis. As w e move up T a b l e 4, r e a c t i o n r a t e s b e c o m e f a s t e r , and k i n e t i c responses b e c o m e potentially m o r e significant. This is particularly t r u e in t h e case of polarographic and v o l t a m m e t r i c techniques w h e r e t h e k i n e t i c response d u e t o labile c o m p l e x e s c a n c o n s t i t u e t h e major p a r t of t h e m e a s u r e d response. CONCLUSION O u r knowledge of biological availability has been developing rapidly o v e r t h e past few years, a n d in t h e process t h e s u b j e c t h a s b e c o m e considerably m o r e complex. T h e simple conclusion "biological availability = f r e e m e t a l cation" which a r o s e f r o m much of t h e e a r l i e r work c a n b e s e e n to b e t h e result of a f o r t u i t o u s s e l e c t i o n of e l e m e n t s f o r study, and t h e equally f o r t u i t o u s a v o i d a n c e of lipid-soluble o r g a n i c complexes. I t is now c l e a r t h a t w e m u s t consider a m u c h wider r a n g e of biologically available c h e m i c a l s p e c i e s in o r d e r to include t h e full r a n g e of t r a c e e l e m e n t t y p e s shown in T a b l e 1. I t is to b e hoped t h a t , in c o m m o n with o t h e r a r e a s of s c i e n t i f i c work, a naively simple childhood and an a d o l e s c e n c e of ever-increasing c o m p l e x i t y will b e followed by a m a t u r i t y whose elegant simplicity d e r i v e s f r o m a c l e a r understanding of t h e p r o c e s s e s involved. S o m e indications of t h e n a t u r e of t h a t f u t u r e simplicity c a n p e r h a p s b e s e e n already. Inspection of T a b l e 2 shows t h a t t h e c h a r g e d c h e m i c a l s p e c i e s which h a v e been identified as biologically available a r e e i t h e r f r e e m e t a l c a t i o n s o r singly c h a r g e d complex ions. In t h e case of n e u t r a l species, lipid solubility h a s a l r e a d y b e e n mentioned in connection with t h e u p t a k e of o r g a n i c complexes, b u t t h e r e a r e indications t h a t n e u t r a l inorganic c o m p l e x e s m a y be t a k e n up in t h e s a m e way. G u t k n e c h t a n d Walter (1981) r e p o r t t h a t t h e n e u t r a l complex HgCI2 diffuses rapidly through bilayer lipid membranes, and Simkiss (1983) h a s shown t h a t t r a c e m e t a l s f o r m i n g n e u t r a l inorganic c o m p l e x e s (Cu, Zn, Hg, Cd) c a n b e e x t r a c t e d f r o m s e a w a t e r i n t o a lipid phase, while t h e m o r e weakly c o m p l e x e d Na, Sr, and Mn cannot. The significance of t h e s e r e s u l t s f o r t h e biological u p t a k e of t r a c e m e t a l s is not y e t c l e a r , s i n c e f r e e c a t i o n u p t a k e of Cu, Zn and C d i s c l e a r l y d o m i n a n t in most of t h e cases studied (Table 2). In c o n t r a s t m e r c u r y , which i s p r e s e n t in s e a w a t e r only as complexes, must b e t a k e n up as e i t h e r a n e u t r a l o r c h a r g e d complex.

198 Taking t h e e l e m e n t s as a whole, we may e x p e c t t h e c h a r g e and size of ions and molecules t o e m e r g e as t h e major f a c t o r s controlling biological availability. Chemical methods for t h e estimation of biological availability will have to b e selected or developed for e a c h e l e m e n t from a knowledge of i t s chemical and biological interactions.

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201

PREDICTIONS F O R T H E MOBILITY OF ELEMENTS IN T H E ESTUARINE ENVIRONMENT

B.R. FOLSOM a n d J.M. WOOD G r a y F r e s h w a t e r Biological Institute, University of Minnesota, P.O. BOX 100, Navarre, MN 55392 (U.S.A.)

ABSTRACT Folsom, B.R. and Wood, J.M., 1986. P r e d i c t i o n s f o r t h e mobility of e l e m e n t s in t h e e s t u a r i n e environment. In: P. L a s s e r r e and J.M. Martin (eds), Biogeochemical P r o c e s s e s at t h e Land-Sea Boundary. Elsevier, Amsterdam. All c h e m i c a l a n d physical processes, including biologically m e d i a t e d ones, a r e governed by t h e r m o d y n a m i c a n d k i n e t i c principles. T h e composition and distribution of e l e m e n t s at t h e s u r f a c e of t h e e a r t h a r e f a r f r o m t h o s e defined by equilibrium conditions. S t o r e d p o t e n t i a l and c h e m i c a l e n e r g y is released as t h e various r e a c t i o n s p r o c e e d t o w a r d equilibrium states. Biological c a t a l y s i s allows organisms to e x t r a c t s o m e of t h e e n e r g y f r o m t h e s e spontaneous b u t slow processes. Living organisms t h e m s e l v e s a r e thermodynamically u n s t a b l e and r e q u i r e significant a m o u n t s of e n e r g y to allow f o r t h e production, c o m p a r t m e n t a l i z a t i o n , u p t a k e and s e c r e t i o n on t h e many c h e m i c a l s needed, along with e n e r g y d e m a n d s f o r mobility, g r o w t h and reproduction. D u e to t h e c o m p l e x i t y of most n a t u r a l systems, t h e r m o d y n a m i c and k i n e t i c models a r e d i f f i c u l t to u s e in describing t h e c a u s e and effect of s t r e s s e s o n a s p e c i f i c s y s t e m b e c a u s e of insufficient d a t a concerning a l l r e l e v a n t c h e m i c a l s p e c i e s and r e a c t i o n s under t h e n a t u r a l conditions of pH, pE, t e m p e r a t u r e a n d ionic s t r e n g t h , In p a r t , s o m e simplifications and g e n e r a l o b s e r v a t i o n s c a n b e m a d e which allow for less c o m p l e t e , b u t m a n a g e a b l e modeling of n a t u r a l s y s t e m s under stress. T r e n d s h a v e been observed and d o c u m e n t e d f o r a number of c h a n g e s in t h e physical environment. First, c h a n g e s in pH c a n d r a m a t i c a l l y a f f e c t t h e solubility of many m e t a l complexes. Acidification l e a d s to t h e dissolution of Al, Mn , Fe and Zn f r o m s e d i m e n t s and p a r t i c u l a t e s in significant amounts. C h a n g e s in reduction p o t e n t i a l g e n r e a l l y c o r r e l a t e s to t h e dissolved oxygen c o n t e n t of t h e w a t e r which i s m e d i a t e d biologically through photosynthesis and respiration. Fe and Mn oxides/hydroxides b e c o m e soluble under anoxic conditions w i t h m o s t m e t a l ions being insoluble under o x i c conditions. Another physical t r a n s i t i o n o c c u r s in t h e e s t u a r i n e environment. W a t e r salinity i n c r e a s e s as f r e s h w a t e r m i x e s with s a l t water. Transition m e t a l s bound to r e f r a c t o r y o r g a n i c m a t e r i a l s , s e d i m e n t s and c l a y s c a n b e displaced by t h e fast e x c h a n g e alkali and alkaline e a r t h m e t a l s o r c a n b e recomplexed by inorganic ligands. T h e r e f o r e , when predicting t h e e f f e c t s resulting f r o m c h a n g e s in t h e physical e n v i r o n m e n t on e l e m e n t a l mobility, g e n e r a l t r e n d s which follow established p r i n c i p J e s c a n b e used as a guide. Additional effects c a n b e p r e d i c t e d d e p e n d e n t upon t h e a c t i o n s of t h e biological organisms which r e s i d e in t h e p a r t i c u l a r system. T h e ability of various organisms to c o m p e t e is d e p e n d e n t upon t h e e n v i r o n m e n t including t h e availability, c h e m i c a l speciation and distribution of e s s e n t i a l e l e m e n t s along with t h e physical c h a r a c t e r i s t i c s of light, t e m p e r a t u r e , pH, pE a n d ionic strength. In turn, living o r g a n i s m s will modify t h e i r e n v i r o n m e n t as a result of cellular metabolism. P h o t o s y n t h e t i s l e a d s t o t h e consumption of C02 a n d nitrogen compounds w i t h t h e production of oxygen, reduced organic compounds and a n i n c r e a s e in pH. Respiration r e s u l t s in t h e consumption of oxygen,

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nitrate, s u l f a t e and/or oxygen, t h e d e g r a d a t i o n of o r g a n i c molecules to C 0 2 with r e l a t e d changes in pE and pH. B e c a u s e of t h e r e q u i r e m e n t s f o r 0, C, N , S a n d P, biological activity, m e t a l mobility and t h e bulk e l e m e n t a l c y c l e s a r e i n t i m a t e l y linked.

INTRODUCTION I t is understood t h a t c h e m i c a l s p e c i e s in t h e biosphere m a y m o v e f r o m i t s s o u r c e through t h e physical e n v i r o n m e n t a n d i n t o t h e b i o t a w h e r e i t m a y c a u s e e i t h e r a beneficial o r a t o x i c effect at t h e cellular level. T h e p a t h w a y s t a k e n by e a c h e l e m e n t involve many c h e m i c a l s p e c i e s with t h e i r distribution being d e t e r m i n e d by t h e r m o d y n a m i c and k i n e t i c considerations. Of p r i m a r y i m p o r t a n c e to a n understanding of e l e m e n t a l cycling is a knowledge of ion c o m p l e x a t i o n in w a t e r , s i n c e i t is t h e ubiquitous solvent f o r l i f e and t h e major solvent for t h e m o v e m e n t and r e a c t i o n of c h e m i c a l s at t h e s u r f a c e of t h e earth. T r a c e m e t a l s in aqueous solution a r e seldom f r e e ions, b u t e x i s t as ionic c o m p l e x e s utilizing a v a r i e t y of organic, inorganic and hydrated ligands which a f f e c t mobility, r e a c t i v i t y and solubility. Compounds which a r e s t a b l e thermodynamically, c a n o f t e n b e isolated and quantified. B e c a u s e of t h e inadequacies in c u r r e n t a n a l y t i c a l m e t h o d s to s e p a r a t e and q u a n t i f y individual c h e m i c a l species, e l e m e n t a l cycling h a s been l i m i t e d to monitoring e i t h e r t o t a l e l e m e n t a l abundance or t h e a b u n d a n c e of s t a b l e c h e m i c a l structures. F r o m t h e standpoint of metabolism, e l e m e n t s c a n b e considered to b e : 1) non-critical, in t h a t t h e y r a r e l y e l i c i t h a r m f u l e f f e c t s e i t h e r b e c a u s e t h e y a r e e s s e n t i a l to biological s y s t e m s o r t h e y a r e r a r e l y observed in h a r m f u l c o n c e n t r a t i o n s ; 2) t o x i c and relatively accessible o r ; 3) t o x i c b u t very insoluble o r v e r y r a r e (Table 1). E i t h e r non-critical or essential e l e m e n t s c a n e l i c i t h a r m f u l effects if t h e availability is too g r e a t (Fi. 1).

I

Element concentration]

Fig. 1. Metabolic a c t i v i t y vs. c o n c e n t r a t i o n f o r e s s e n t i a l and non-essential e l e m e n t s reprinted f r o m Wood and Wang, 1983. O v e r t h e p a s t 200 y e a r s s i n c e t h e industrial revolution, w e h a v e witnessed t h e utilization of v a s t q u a n t i t i t e s of e n e r g y to mobilize e l e m e n t s a t t h e s u r f a c e of t h e e a r t h . D u e to

203

t h e s e a c t i v i t i e s , t h e abundance of many e l e m e n t s in our w a t e r w a y s h a s increased, leading to t h e expression of t o x i c effects a n d / o r t h e s t i m u l a t i o n of biological a c t i v i t y , severely disturbing e n v i r o n m e n t a l balances. Intrinsic to an understanding of t h e r a m i f i c a t i o n s of e l e m e n t mobilization o n biological s y s t e m s is a need to understand t h e cycling of e l e m e n t s within aqueous systems.

TABLE 1. Classification of e l e m e n t s a c c o r d i n g to t h e i r toxicity.

Designatioda)

Elements

Non c r i t i c a l elements

Na, K, Mg, C a , H, 0, N, C, P, Fe, S, C1, Br(b), F(b), Li Rb, Sr, Ba, Al, Si

Very t o x i c and relatively accessible

Be, Co, Ni, Cu, Zn, Sn, As, Se, T e , Pd, Ag, Cd, Pt, Au, Hg, TI, Pb, Sb, Bi

Toxic b u t very insoluble o r very r a r e

Ti, Hf, Zr, W, Nb, T a , Re, Ga, La, Os, Rh, Ir, Ru

...................................................................................................

(a) Many e l e m e n t s a r e o m i t t e d b e c a u s e t h e y c a n n o t b e designated in this way. (b) S o m e m a y a r u e w i t h t h i s designation, b u t w e do add F- to drinking water. R e p r i n t e d from%ood, 1974. Of t h e e l e m e n t s in t h e periodic t a b l e , 30 h a v e been shown to b e e s s e n t i a l f o r microbial

growth, although n o t a l l of t h e s e 30 e l e m e n t s a r e necessary f o r t h e growth and division of e v e r y microbial s p e c i e s (Fig. 2). T h e r e i s an a p p a r e n t c o r r e l a t i o n b e t w e e n t h e abundance of e l e m e n t s in t h e e a r t h ' s c r u s t a n d t h e nutritional requirernents of microbial c e l l s (Wood, 1984). T h e r a t e s at which e l e m e n t s a r e cycled in a n aqueous e n v i r o n m e n t depends upon t h e availability of t h e bulk e l e m e n t s C, H, N, 0, P and S (Salomons and Baccini, 1984).

Fig. 2. P e r i o d i c t a b l e indicating r e l a t i v e abundance and biological i m p o r t a n c e of e l e m e n t s . Highlighted e l e m e n t s a r e t h e 30 most a b u n d a n t c r u s t a l e l e m e n t s with c o n c e n t r a t i o n s ranging f r o m 46 % f o r oxygen to 0.1 pg/g f o r selenium. Bold f a c e d e l e m e n t s h a v e known biological f u n c t i o n s w i t h * indicating l i m i t e d d a t a and controversy. D a t a t a k e n f r o m Wood and Wang, 1983.

204

Not only d o microorganisms h a v e a profound influence on t h e c o n c e n t r a t i o n of e l e m e n t s in t h e atmospheric,

a q u a t i c a n d t e r r e s t r i a l e n v i r o n m e n t s (Lovelock,

1979), b u t t h e

c o n c e n t r a t i o n and c h e m i c a l speciation of t h e s e e l e m e n t s p r e d i c a t e s which organisms c a n c o m p e t e a n d s u r v i v e within a given environment. Therefore, t h e cycling a n d mobility of t r a c e e l e m e n t s will b e linked to t h e cycling of bulk e l e m e n t s and c o r r e l a t i o n s to t h e various bulk e l e m e n t c y c l e s c a n b e postulated.

THERMODYNAMIC AND KINETIC AFFECTS E l e m e n t s c a n o f t e n b e found in a n y number of c h e m i c a l and physical states. O n e c o m m o n compound f o r m e d f r o m 2 hydrogens and 1 oxygen i s w a t e r . As c o m m o n l y known, w a t e r c a n e x i s t as a solid, liquid o r gas, depending upon t h e t e m p e r a t u r e and pressure.

Also well known is t h e f a c t t h a t w a t e r ionizes t o OH- a n d H3O+ in solution. O t h e r combinations of hydrogen a n d oxygen result in H2, 0 2 , H 2 0 2 , H, O3,Oz- a n d so on. S o m e of t h e s e c h e m i c a l s p e c i e s a r e s t a b l e w h e r e a s o t h e r s a r e highly reactive. Such a s i m p l e and c o m m o n combination of t w o e l e m e n t s l e a d s to a complex and h e t e r o g e n e o u s m i x t u r e of c h e m i c a l compounds. When considering t h a t a q u a t i c s y s t e m s a r e c o m p o s e d of literally hundreds of s t a b l e and r e a c t i v e chemicals, i t is n o wonder t h a t r e s e a r c h e r s h a v e difficulty in q u a n t i f i c a t i o n a n d identification. T h e abundance and r e a c t i v i t y of a n y compound is governed by t h e r m o d y n a m i c and k i n e t i c principles. T h e r e f o r e , a n a l y t i c a l d a t a i s used in conjunction with t h e r m o d y n a m i c and k i n e t i c p a r a m e t e r s to model c o m p l e x systems. O f t e n , t h e m o s t t o x i c e l e m e n t a l f o r m s a r e t h e m o s t r e a c t i v e and t h e r e f o r e r e p r e s e n t t h e lowest c o n c e n t r a t e d c h e m i c a l s s p e c i e s in water. This leads to additional problems in isolation, i d e n t i f i c a t i o n and q u a n t i f i c a t i o n when assessing p o t e n t i a l pollution problems. Thermodynamic principles govern r e a c t i o n s at equilibrium a n d i n d i c a t e t h e d i r e c t i o n s of spontaneous reaction. For example, at pH 7.0, a m m o n i a i s spontaneously oxidized to n i t r a t e under a e r o b i c conditions by t w o organisms Nitrosomonas a n d N i t r o b a c t e r . NH4+ + 3 / 2 0 2 No2-

+

1/202

<--> <-->

+ 2H+ + H 2 0 N03-

A G'(pH 7) =

- 272 k J / m o l

(1)

A G'(pH 7) =

- 76 kJ/mol(2)

(2)

By c o n t r a s t , under a n a e r o b i c conditions, t h e r e v e r s e r e a c t i o n b e c o m e s spontaneous and organisms such a s Micrococcus d e n i t r i f i c a n s and Thiobacillus d e n i t r i f i c a n s c a n r e d u c e n i t r a t e as a n e n e r g y source. 2 NO3- + 5H2 + 2H+

6 NO3- + 5 s

<-->

+ 2 H 2 0 <-->

N2 + 6 H 2 0 5SO$-

+ 3N2 + 4H+

A G1(pH 7) =

- 561 k J / m o l

(nitrate)

(3)

A G'(pH 7) =

- 455 k J / m o l

(nitrate)

(4)

205

T h e s e r e a c t i o n s a r e s p o n t a n e o u s under s t a n d a r d r e a c t i o n conditions of 1 M c o n c e n t r a t i o n s for a l l r e a c t a n t s and p r o d u c t s ( e x c e p t (H+) which is at 10-7 MI. C o n c e n t r a t i o n s of 1 M a r e f a r f r o m r e a l i s t i c in m o s t n a t u r a l w a t e r s , t h e r e f o r e , to d e t e r m i n e if a r e a c t i o n will b e spontaneous under r e a l conditions, t h e f r e e e n e r g y c a n b e c a l c u l a t e d : A G = A G'

+ R T 1n [prod]/[reac]

(5)

Under n a t u r a l conditions, t h e s e organisms d e r i v e l i t t l e e n e r g y f r o m n i t r a t e reduction so t h e t o t a l biomass i n c r e a s e is s m a l l and slow in comparison to t h e t u r n o v e r of r e a c t a n t s . Equilibrium models f r o m t h e r m o d y n a m i c c a l c u l a t i o n s c a n o f t e n provide a useful f i r s t approximation f o r a r e a l system. T h e s e models a r e l i m i t e d by t h e a c c u r a c y and availability of

t h e r m o d y n a m i c d a t a including

temperature,

pressure and

activity

dependencies, knowledge of a l l p e r t i n e n t c h e m i c a l s p e c i e s a n d r e l a t e d equilibria. O n e major assumption, which is o f t e n false, is t h a t t h e spontaneous r e a c t i o n s r e a c h equilibrium quickly. Many r e a c t i o n s in a q u a t i c s y s t e m s a r e irreversible a n d slow with r e a c t i o n h a l f t i m e s of m i n u t e s o r longer. Included in t h i s group of r e a c t i o n s a r e ; metal-ion oxidation, oxidation of sulfides by organics, various m e t a l ion polymerizations, aging of hydroxide a n d oxide p r e c i p i t a t e s , p r e c i p i t a t i o n of metal-ion s i l i c a t e s a n d c a r b o n a t e s a n d non-biological

hydrolysis

of

p o l y m e r s (Stumm

and

Morgan,

1981).

Not

only d o

t h e r m o d y n a m i c c o n s t a n t s s h i f t with c h a n g e s in t h e physical p r o p e r t i e s of t h e solution, b u t k i n e t i c c o n s t a n t s v a r y also. F o r example, t h e u n c a t a l y z e d oxidation of iron(I1) below pH 4 h a s a r e a c t i o n h a l f t i m e of o v e r 3 years, w h e r e a s at a b o u t pH 7.5 t h e r e a c t i o n h a l f t i m e i s in m i n u t e s (Stumm and Morgan, 1981). F o r t h e s e reasons, a c c u r a t e modeling of n a t u r a l s y s t e m s b e c o m e s m o r e difficult. O n e approximation c a n b e m a d e f o r s o m e systems. Usually, o n e r e a c t i o n s t e p i s slow o r r a t e limiting and t h e r e f o r e a l l o t h e r r e l a t e d r e a c t i o n s c a n b e a s s u m e d to b e at equilibrium. This l e a d s to a mixed model w h e r e t h e slow r e a c t i o n is described kinetically w i t h a l l o t h e r r e a c t i o n s being defined by t h e r m o d y n a m i c p a r a m e t e r s a n d m a s s b a l a n c e constraints.

COMPLEX SOLUBILITY AND MOBILITY Metal/ligand i d e n t i t y d e t e r m i n e s w h e t h e r various c h e m i c a l s p e c i e s a r e soluble o r insoluble in water. Iron c o m m o n l y e x i s t s in t w o oxidation states, Fe (11) a n d Fe(II1). In aqueous solution, Fe(I1) c a n b e l i g a t e d by sulfide ion to f o r m a n insoluble iron sulfide complex. Fe(II1) c a n c o m b i n e with t h r e e hydroxide ions to f o r m t h e insoluble complex, Fe(OH)3, which in t u r n c a n c o o r d i n a t e to a n additional hydroxyl a n d f o r m a soluble Fe(OH)4- complex. Positively c h a r g e d m e t a l ions f o r m i n t e r a c t i o n s with f r e e e l e c t r o n p a i r s f r o m ligands. M u l t i d e n d a t e ligands s u c h as t h o s e found in r e f r a c t o r y o r g a n i c m a t e r i a l s and p a r t i c u l a t e s g e n e r a l l y f o r m m o r e s t a b l e m e t a l ligand complexes. M e t a l i s n s g e n e r a l l y h a v e a n e v e n n u m b e r of coordination sites, f r e q u e n t l y 4 o r 6 . T h e h a r d s o f t m e t a l ligand t h e o r y states t h a t c o m b i n a t i o n s of m e t a l s a n d ligands a r e m o r e s t a b l e if t h e y a r e alike, i.e. hard-hard, soft-soft a n d i n t e r m e d i a t e - i n t e r m e d i a t e (Table 2).

206

TABLE 2 Classification of hard and solft acids and base&) Hard a c c e p t o r H+ Na+, K+, Be2+, M 2+ Ca?+, Mn2+, A13+, C r + C03+, Fez+, As3+

9

Hard donor

Intermediate

Soft acceptor

Fez+, Co2+, Ni2+ Cu2+, Zn2+, Pb2+

Cu+, Ag+, Au+, TI+, Hg+, CH3Hg+

Inter mediate

Soft donor

Br-, N02-, 5032-

SH-, S2-, RS-, CN-, SCN-, CO, R2SRSH

................................................................................................. H20, OH-, F-, C1PO^-, ~ 0 4 2 - , co32-, 02-

(a) Source :Pearson, 1968 ;Forstner and Wittman, 1979. Metal ligand complexes c a n b e divided into t h r e e groups based upon t h e stability of t h e complexes and t h e identity of t h e preferred ligands (Buffel, 1984). T h e group 1 and 2 elements (Li, Nay K, Rb, Cs, Mg, Cay Sr and Ba) tend to complex only with oxygen in ligands forming relatively unstable complexes and undergo rapid ligand exchange in water. These elements a r e generally very w a t e r soluble and only cations found in high concentration (Mg, Ca and Na) contribute significantly to t h e solid or sedimentary material. A variety of transition metals, including Mn 11, Fe 11, Co 11, Ni 11, Cu 11, Zn I1 and P b I1 d e m o n s t r a t e strong affinities f o r ligands containing 0, N a n d S (Buffel, 1984). Mn, Fe and Cu c a n easily be converted t o o t h e r redox states with Fe 111 and Mn IV generally forming stable 0x0 or hydroxo complexes. Another group, containing Cu I, TI I, Cd 11, Ag I, Au I and Hg 11, possess strong affinities for ligands containing N and S (Buffel, 1984). In natural systems, t h e concentration of these last t w o groups of transition metals is generally less than t h e t o t a l ligand concentration and t h e r e f o r e form s t a b l e insoluble complexes in water. T h e solubility of t h e various transition m e t a l s is related to t h e stability of soluble and insoluble ionic complexes formed with 0, N and S containing ligands. Fresh water and sea water a r e dominated by different types of ligands. In fresh water, t h e ligand concentration varies immensely and o f t e n has higher concentrations of organic ligand than in marine waters. There a r e t h r e e major groups of organic ligands. First, freshly e x c r e t e d products f r o m primary and secondary producers including carbohydrates, proteins and lipids which a r e decomposed within days or weeks. Secondly, refractory organic m a t t e r which c o n s t i t u t e s 70-80 % of t h e t o t a l dissolved organic matter. Refractory materials a r e primarily composed of a r o m a t i c hydrocarbons, chlorinated hydrocarbons and others which a r e s t a b l e to decomposition. Lastly, humic and fulvic acids which a r e polymorphous p a r t i c u l a t e complexes. Organic pollution can actually increase t h e ability of a fresh w a t e r system to adsorb excess metals and thereby lower t h e expression of e f f e c t s by toxic metals. T h e concentration of ligands in sea water is

207

relatively s t a b l e a n d d o m i n a t e d by inorganic ligands, primarily chloride, c a r b o n a t e , b o r a t e , silicate, s u l f a t e and a m m o n i a (Jjdrgensen a n d Jensen, 1984). T h e c o n c e n t r a t i o n of soluble m e t a l c o m p l e x e s is s o m e w h a t higher in f r e s h w a t e r t h a n in s a l t w a t e r d u e to t h e g r e a t e r a b u n d a n c e of o r g a n i c ligands. Ligand e x c h a n g e and redox r e a c t i o n s c a n a f f e c t t h e solubility of c h e m i c a l s p e c i e s in w a t e r , t h e r e b y influencing e l e m e n t a l mobility. As n o t e d above, soluble s p e c i e s c a n b e t r a n s p o r t e d w i t h bulk w a t e r movements. R e c o m p l e x a t i o n c a n r e s u l t in e i t h e r t h e solubilization of s p e c i e s f r o m s e d i m e n t s o r t h e p r e c i p i t a t i o n of soluble species. C h a n g e s in physical p a r a m e t e r s such as ph, pE, ionic s t r e n g t h o r t e m p e r a t u r e c a n c h a n g e t h e s t a b i l i t y c o n s t a n t s f o r t h e various complexes. In addition, v a r i a t i o n s in ligand c o n c e n t r a t i o n s c a n lead to c o m p e t i t i o n f o r t h e m e t a l ions, leading to recomplexation. In f r e s h w a t e r , t h e pH is generally a b o u t 7.0 a n d m a n y m e t a l s f o r m insoluble c o m p l e x e s with h u m i c a n d fulvic acids, hydroxides a n d s o m e c a r b o n a t e s . If t h e pH i s lowered significantly, t h e s e insoluble c o m p l e x e s c a n r e e q u i l i b r a t e to soluble o r o t h e r insoluble forms. Al, Fe, Mn a n d Zn are a l l mobilized significantly f r o m s e d i m e n t s under acidification induced by a c i d precipitation f r o m fossil f u e l combustion o r d r a i n a g e f r o m mining operations. By c o n t r a s t , as f r e s h w a t e r e n t e r s s a l t w a t e r , t h e inorganic ligand c o n c e n t r a t i o n i n c r e a s e s dramatically. With t h e i n c r e a s e s in chloride, s u l f a t e a n d c a r b o n a t e , m a n y m e t a l c o m p l e x e s e x c h a n g e ligands and a r e e i t h e r p r e c i p i t a t e d o r solubilized in c o a s t a l regions. N a t u r a l w a t e r s a r e in d y n a m i c flux d u e to normal w a t e r m o v e m e n t s a n d inputs of compounds f r o m t e r r e s t r i a l environments. T h e s e m o v e m e n t s of w a t e r a n d fluxes of c h e m i c a l s l e a d s to r e a c t i o n s involving t h e p r e c i p i t a t i o n a n d dissolution of o t h e r compounds a l r e a d y residing in t h e a q u a t i c system. T h e m o v e m e n t of e l e m e n t s depends upon t h e hydrodynamics of t h e w a t e r w a y and t h e solubility of t h e various chemicals. S i n c e t h e soluble m e t a l s d i f f u s e through t h e w a t e r freely, t h e s e s p e c i e s c a n b e t r a n s p o r t e d with bulk m o v e m e n t s of water. O n t h e o t h e r hand, a l a r g e number of t r a c e m e t a l s discussed above, f o r m insoluble compounds. Most transition m e t a l s discharged i n t o f r e s h w a t e r a r e bound to humic o r fulvic a c i d sediments, w h e r e a s m e t a l ions in m a r i n e s y s t e m s generally p r e c i p i t a t e as hydroxides, c a r b o n a t e s and sulfides o r a r e bound to soils and s e d i m e n t s (J#rgensen and Jensen, 1984). Adsorption of m e t a l s o n t o s e d i m e n t s i s g e n e r a l l y fast, a n d c a n b e modeled e i t h e r by equilibrium t e r m s o r by f i r s t o r d e r kinetics. T h e adsorption of m e t a l s i n t o c l a y i s d o m i n a t e d by pH, w i t h t h e c l a y a c t i n g as a n ion e x c h a n g e resin. Insoluble c h e m i c a l s p e c i e s a r e relatively immobile b u t c a n b e t r a n s p o r t e d as particulates. T h e p a r t i c u l a t e and s e d i m e n t c o n t e n t of r i v e r s and s t r e a m s h a s i n c r e a s e d a b o u t t h r e e fold d u e to i n c r e a s e s in mining, a g r i c u l t u r e and industrial a c t i v i t y , with t h e distribution of m e t a l s in solution being a p p r o x i m a t e l y 17% dissolved and 72% as suspended p a r t i c u l a t e s (Bacinni, 1984). T h e d i s t a n c e t h a t s e d i m e n t s t r a v e l depends upon t h e s i z e of t h e p a r t i c l e a n d t h e r a t e of flow in t h e waterway. Smaller p a r t i c l e s t r a v e l g r e a t e r d i s t a n c e s b e f o r e t h e y settle t o t h e bottom. T h e s e s m a l l e r p a r t i c l e s also h a v e a g r e a t e r c a p a c i t y f o r bound o r ligated e l e m e n t s s i n c e t h e y have a

208

larger

surface-to-weight

ratio.

Sediments a r e transported

f r o m t h e fast moving

w a t e r c o u r s e s to l a k e s and e s t u a r i e s w h e r e t h e y reside a n d a r e buried by m o r e siltation. T h e s e d i m e n t s at t h e b o t t o m of l a k e s and e s t u a r i n e s b e c o m e s t o r e h o u s e s for t h e s e complexes. P a r t i c u l a t e s and s e d i m e n t s c a n a l s o undergo m o v e m e n t resulting f r o m various physical disturbances. T h e influx of p a r t i c l e s a n d m o v e m e n t of s e d i m e n t s in rivers o f t e n rises as a result of erosion, flooding o r inputs of g r e a t e r t h a n n o r m a l a m o u n t s of w a t e r during s t o r m s o r snow melts. T h e s e e v e n t s c a n lead to physical c h a n g e s including resuspension, s e t t l i n g and burial of particulates. In c o a s t a l regions, t i d a l m o v e m e n t s also induce m o v e m e n t a n d redistribution of s e d i m e n t a r y material. S t o r m a c t i o n s a m p l i f y w a v e a c t i v i t y which c a n d r a m a t i c a l l y i n c r e a s e erosion and m o v e m e n t of particulates. O n c e p a r t i c l e s s e t t l e o u t of solution, t h e r e a r e random and c y c l i c e v e n t s which r e s u l t in m a s s m o v e m e n t and redistribution of materials. T h e w a t e r in e s t u a r i e s undergoes additional movement. T h e s u r f a c e w a t e r , which is rich in organic n u t r i e n t s a n d a l g a l life, flows o u t of t h e basin t o w a r d s t h e s e a . T h e a l g a e then

s e t t l e to

the bottom

and

t h e o r g a n i c m a t e r i a l is

mineralized

by

other

microorganisms. T h i s w a t e r t h e n flows back i n t o t h e e s t u a r i a l basin by a c o u n t e r - c u r r e n t of w a t e r originating f r o m t h e o c e a n depths. O f t e n in Norwegian fjords, t h i s d e e p c u r r e n t

of w a t e r b e c o m e s anoxic d u e to r e s p i r a t o r y utilization of oxygen (Stumm a n d Morgan, 1981). T h e s e e s t u a r i n e c u r r e n t s and biological a c t i v i t i e s c a n a f f e c t t h e m o v e m e n t of n o t

only dissolved e l e m e n t s b u t a l s o t h o s e e l e m e n t s found in t h e sediments.

BIOLOGICAL INFLUENCE ON ELEMENTAL TRANSPORT O r g a n i c a n d inorganic ligands h a v e similarities and differences. M e t a l ions generally c o o r d i n a t e to unshared e l e c t r o n pairs on 0, S , N and C1. Physical p a r a m e t e r s in t h e aqueous e n v i r o n m e n t will affect t h e s t a b i l i t y of t h e m e t a l ligand i n t e r a c t i o n with organic and inorganic ligands d e m o n s t r a t i n g similar trends.

Due t o t h e c o m p l e x i t y and

h e t e r o g e n e i t y of m a n y o r g a n i c ligands, s t a b i l i t y c o n s t a n t s a r e h a r d e r t o measure. In addition, c o m p l e x a t i o n within living organisms c a n b e controlled through t h e ability of c e l l s to

c o m p a r t m e n t a l i z e compounds

and

to

control

factors

including

ligand

concentration, pH, pE a n d ionic s t r e n g t h within t h e cell. C e l l s c a n also c o n t r u c t s p e c i f i c molecules designed f o r t r a n s p o r t and s t o r a g e of s p e c i f i c metals. C e r t a i n organisms c a n s e c r e t e c h e l a t o r molecules which bind insoluble m e t a l s in t h e s e d i m e n t (Wood and Wang, in press). T h e s e c o m p l e x e s c a n t h e n specifically i n t e r a c t with t h e c e l l m e m b r a n e and t a n s f e r t t h e m e t a l to t h e c e l l w h e r e t h e m e t a l c a n t h e n u l t i m a t e l y b e i n t e r n a l i z e d and s t o r e d f o r use. By t h i s o r similar mechanisms, organisms c a n a c c u m u l a t e insoluble, essential e l e m e n t s t h e r e b y p r o m o t i n g e l e m e n t a l t r a n s p o r t a n d mobility. Metal ligand c o m p l e x e s undergo c o n s t a n t change. M e t a l s bound to p a r t i c u l a t e s o r found in s e d i m e n t s undergo aging as c r y s t a l l a t t i c e s t r u c t u r e s r e o r g a n i z e i n t o m o r e s t a b l e forms. Both insoluble a n d soluble c o m p l e x e s undergo ligand e x c h a n g e resulting in

209

desorption/adsorption a n d dissolution/precipitation reactions. F o r e x a m p l e , p r o t e i n a c e o u s ligands c a n b e degraded, resulting in t h e r e l e a s e of t h e m e t a l ion or a ligand c a n b e p r o t o n a t e d o r d e p r o t o n a t e d w i t h c o n c o m m i t a n t s h i f t s in s t a b i l i t y a n d solubility of t h e m e t a l ligand complex. O t h e r c h a n g e s in c o m p l e x e s r e s u l t f r o m ligand c o m p e t i t i o n and c h a n g e s in o t h e r physical p a r a m e t e r s . C h a n g e s such as t h e s e c a n o c c u r during t h e s u m m e r m o n t h s as v a s t a m o u n t s of C 0 2 a r e utilized by p h o t o s y n t h e t i c organisms. Typically, t h i s a c t i v i t y l e a d s to a n i n c r e a s e pH f r o m 7.2 to 9.5 which r e f l e c t s a d e c r e a s e in t h e Cogc o n c e n t r a t i o n . C d a n d Zn c o m p l e x e s a r e v e r y s e n s i t i v e to c h a n g e s in pH in t h i s region. C a r b o n a t e - m e t a l c o m p l e x e s will a l s o b e a f f e c t e d by t h e utilization of C02. Alkalinity c h a n g e s during photosynthesis b e c a u s e NO3-, NH4+ a n d HPO42- a r e a s s i m i l a t e d with c o n u r r e n t u p t a k e of H+ o r OH- by t h e cells. Biological processes which a f f e c t alkalinity include photosynthesis, n i t r a t e m e t a b o l i s m a n d s u l f a t e metabolism (Table 3).

TABLE 3 P r o c e s s e s A f f e c t i n g Alkalinity

P h o t o s y n t h e t i s and R e s p i r a t i o n : NC (1 b) 106C02+1 6 N o g - + H P 0 ~ ~ 122H20+18H+ -+ $$y~{C106H2630110N16P1}+ "algae" ( l c ) 1 0 6 C 0 2 + 1 6 N H q + + H P O ~ ~1-0+8 H 2 0 Nitrification : (2) NH4+ + 2 0 2

~ ~ ~ i y ~ { C l 0 6 H 2 6 3 0 116P1)+ 1 0 N 10702 + 14H+

__________ N 0 3 - + H20 + 2H+

Denitrification : (3) 5CH20 + 4 N 0 3 - + 4H+

---------- 5 C O 2 + 2N2 + 7 H 2 0

Sulfide Oxidation : (4a) HS- + 2 0 2 ---------- SO42- + H+ (4b) FeSz(s) + 1 5 / 4 0 2 pyrite

+ 3 1/2H20 ---------- Fe(OH)3(s) + 4H+ + 2S02-

Sulfate Reduction : (5) SO42- + 2CH20 + H+

CaCO3 Dissolution : ( 6 ) CaC03 + C 0 2 + H20

---------- 2 C 0 2 + HS- + H20 Ca2+ + 2 ~ ~ 0 3 -

R e p r i n t e d f r o m S t u m m a n d Morgan, 1981. N C = n o c h a n g e ; D = d e c r e a s e ; I = increase.

1380~

I

D D I

D D

I I

210

A s mentioned e a r l i e r , pH c h a n g e s o c c u r f r o m pollution s t r e s s e s such as a c i d rain o r

mine runoff of f r o m g e o c h e m i c a l r e a c t i o n s s u c h as dissolution of c a l c i u m c a r b o n a t e . In acidified regions, s p e c i e s diversity a n d population density d r o p d r a m a t i c a l l y w i t h s o m e lakes a n d s t r e a m s becoming devoid of life. C h a n g e s in m e t a l c o m p l e x e s o c c u r both naturally and biologically, with d i r e c t and i n d i r e c t e f f e c t s on ligand c o n c e n t r a t i o n and complex stability. Besides m e t a l ligand i n t e r a c t i o n s and hydrodynamic m o v e m e n t s of w a t e r , c h e m i c a l equilibria need t o b e considered in understanding e l e m e n t a l m o v e m e n t through a q u a t i c systems. Only a f e w e l e m e n t s , C, N, 0, S, Fe and Mn, a r e p r e d o m i n a n t p a r t i c i p a n t s in a q u a t i c redox processes (Stumm a n d Morgan, 1981). In a n a q u a t i c s y s t e m at equilibrium with a t m o s p h e r i c oxygen, t h e reduction p o t e n t i a l c a n b e d e f i n e d ( P ( 0 2 ) = 0.21 atm., E(H) = 800 mV at pH 7 a n d 25°C) (Stumm a n d Morgan, 1981). Using s t a n d a r d redox p o t e n t i a l s

(Table 4 ) f o r p e r t i n e n t a q u a t i c reactions, virtually a l l biochemically i m p o r t a n t e l e m e n t s should b e found in t h e i r highest naturally o c c u r r i n g oxidation states : C as C 0 2 , HCO3- o r C032-, N as N032-, S as SOQ~-,Fe as FeOOH o r Fez03 and Mn as MnO2 (Stumm and Morgan, 1981). E v e n a t m o s p h e r i c nitrogen should b e largely c o n v e r t e d t o n i t r a t e under n a t u r a l conditions a c c o r d i n g to t h e r m o d y n a m i c constraints. Clearly, n a t u r a l w a t e r s a r e f a r f r o m t h e r m o d y n a m i c equilibrium. Biological s y s t e m s c a n e x t r a c t e n e r g y f o r growth and division by c a t a l y z i n g t h e s e thermodynamically f a v o r e d and kinetically slow reactions. Biological s y s t e m s c a n b e divided i n t o t w o major classifications, p h o t o s y n t h e t i c and respiratory. Photosynthesis c a n b e c o n c e i v e d as a process by which light e n e r g y is c o n v e r t e d to c h e m i c a l e n e r g y (Stumm and Morgan, 1981). T h e compounds f o r m e d a r e thermodynamically u n s t a b l e a n d typically involve t h e conversion of compounds in a high oxidation state, C2, to m o r e reduced forms, CHO. T h i s process g e n e r a t e s a highly n e g a t i v e redox p o t e n t i a l and a reservoir of 0 2 , a good oxidant. Respiration, on t h e o t h e r hand, t e n d s to r e s t o r e equilibrium by c a t a l y t i c conversion of u n s t a b l e compounds to thermodynamically f a v o r e d f o r m s in higher oxidation states. Energy is derived f r o m t h i s conversion which is used to d r i v e r e a c t i o n s necessary for life. O r g a n i s m s m e d i a t e t h e s e spontaneous r e a c t i o n s b u t c a n n o t c a t a l y s e non-spontaneous reactions, though r e a c t i o n conditions within t h e c e l l c a n b e controlled so t h e t h e r m o d y n a m i c s b e c o m e f a v o r a b l e at t h e e x p e n s e of energy.

A s e q u e n c e of redox r e a c t i o n s in a n aqueous s y s t e m c a n b e g e n e r a t e d to correspond to a thermodynamically f e a s i b l e order. In a closed a q u a t i c s y s t e m containing o r g a n i c material, oxidation o c c u r s f i r s t by t h e reduction of oxygen t h e n t h e reduction of n i t r a t e , n i t r i t e a n d manganese dioxide (if present), followed by F e O O H reduction to Fe(I1) and at sufficiently n e g a t i v e redox potential, f e r m e n t a t i o n and reduction of c a r b o n dioxide and s u l f a t e m a y o c c u r (Stumm a n d Morgan, 1981). T h e s e r e a c t i o n s follow a d e c r e a s e in redox p o t e n t i a l level, with t h e organism deriving m o r e e n e r g y f r o m a e r o b i c respiration t h a n f r o m r e a c t i o n s using o t h e r e l e c t r o n acceptors. An ecological s y s t e m follows t h i s

211

TABLE 4 Equilibrium c o n s t a n t s and redox processes p e r t i n e n t in a q u a t i c conditions (25OC).

Reaction

P&Ok log K)

p&O(W) a

(1) 1/4 0 2 ( g ) + H+ + e = 1/2 H 2 0

+20.75

+13.75

(2) 1/5 NO3 + 6/5Ht

+21.05

t12.65 + 3.90 b

t

e = 1/1ON2(g) + 3/5H20

(3) 1/2 MN02(s)+1/2HC03-(10-3)t3/2Ht+e

-

= 1/2MnCO3(s)

+3/8H20 (4) 1/2 NO3-

H+

e = 1/2N02- + 1/2H20

t14.15

t

(5) 1/8 N03- + 5/4H+

t

e = 1/8NH4+ t 3/8 H20

+14.90

+ 6.15

4/3H+

t

e = 1/6 NH4+ + 1/3 H20

+15.14

+ 5.82

t

9.88

+ 2.88

t

6.94

- 0.06 - 0.80 - 3.01 - 3.30 - 3.50 - 3.75 - 4.11 - 4.13

(6) 1/6 NO2-

t

t

t

(7) 1/2 C H 3 0 H + H+ + e = 1/2CHq(g)

t

1/2H20

( 8 ) 1/4 CH2O t H+ + e = 1/4CHq(g) + 1/4H20

(9) FeOOH(s)

HC03-(10-3) + 2H+

t

(10) 1/2 CH20 + H+

+E

t

= 1/2 C H 3 0 H

(11) 1/6 SO4- t 4/3H+ + e = 1/6S(s) + 2/3H20

(12) 1/8 SO42- + 5/4H+ (13) 1/8 5 0 4 2 -

t

t

e = I/SHzS(g) + 1/2H20

9/8H+ + 3 = 1/8HS- + 1/2H20

(14) 1/2 S(s) + H+ + e = I/2H2S(g) (15) 1/8 CO2(g) + H+

t

-

e = F e C 0 3 + 2H20

e = I/SCHo(g) + 1/4H20

+ 3.99 + 6.03 + 5.25 t

4.25

t

2.89

+ 2.87

7.15

b

(16) 1/6 N2(g) + 4/3H+ + e = 1/3NHqt (17) 1/2 (NADP+) + 1/2H+ (18) H+

t

t

e = 1/2 (NADPH)

e = 1/2 H2(g)

0.0

(19) Oxidized ferredoxin (20) 1/4 C 0 2 ( g ) + H+ (21) 1/2 HCOO-

t

t

- 2.0

t

e = reduced ferredoxin

e = 1/24 (glucose) t 1/4H20

3/2H+

t

e = 1/2CH20 = 1/2H20

(22) 1/4 C 0 2 ( g ) + Ht + e = 1/4CH20

t

1/4H20

(23) 1/2 C02(g) + 1/2H+ + e = 1 / 2 H C 0 0 -

- 7.1 - 0.20 + 2.82

- 1.20 - 4.83

- 5.50 - 7.00 - 7.10 - 7.20 - 7.68 - 8.20 - 8.33

C

d

R e p r i n t e d f r o m S t u m m a n d Morgan, 1981 a

Values f o r p&O(W) apply to t h e e l e c t r o n a c t i v i t y f o r unit a c t i v i t i e s of oxidant and r e d u c t a n t in n e u t r a l w a t e r , t h a t is, at pH = 7.0 f o r 25OC. T h e s e d a t a correspond to (HCO3-) = 10-3 M r a t h e r than unity and so a r e not e x a c t l y p p ( W ) ; t h e y r e p r e s e n t typical a q u a t i c conditions m o r e nearly t h a n pEO(W) values do. M. Calvin and J.A. Bassham, T h e photosynthesis of carbon compounds, W.A. Benjamin, Menlo Park, Calif., 1962. NADP = Nicotinanide adenine dinucleotide phosphate.

D.I. Arnon, S c i e n c e 149, 1460 (1965) Lehninger, Bioenergetics, W.A. Benjamin, Menlo Park, Calif., 1965.

212

thermodynamic system fairly closely starting with aerobic heterotrophs and followed by denitrifiers, fermenters, sulfate reducers and finally methane bacteria. This sequence of biological mediated redox processes has been illustrated by Stumm (Fig. 3).

-0.5 -10

+0.5

0 0

-5

+1.0 EH Volt

+5

+I0

I

1

02- Reduction

+I5

+20 pC

1

A

Reductions

Combination AGO pH.7 Kcal/equiv.

Fermentation F+L Sulfate Reduction G+L Methane Fermentation H+L

-6.4

Sulfide Oxidation Nitrif icat ion Ferrous Oxidation M n ( l l ) Oxidation

.

I -10

-5

o

i

-:

5 h

-5.9 -5.6

A+M

-23.8

A+O

-10.3

A+N A+P

-21.0 -7.2

+I0

10

+I5

+20 p t

20 I

Kcal /equivalent

Fig. 3. Biologically mediated redox processes. Reprinted from Stumm and Morgan, 1981. Microorganisms have adapted to efficiently harvest energy from kinetically slow but thermodynamically favorable reactions through the continuum of redox potentials found in nature. Organisms will utilize compounds which provide the most energy under any set of conditions. This prnciple establishes t h e sequence of electron acceptors in redox processes

213

to b e 0 2 , NOg- and SO42-. T h e ecological s y s t e m a b o v e is g e n e r a l l y found s t r a t i f i e d in

sediments. Metabolism requiring oxygen is found in t h e t o p 0.5 to 1.0 c m (Blackburn, 1983, Sorenson, 1984) of s e d i m e n t as a c o n s e q u e n c e of p e r m e a b i l i t y r a t e s a n d t h e r a t e of utilization by microorganisms. T h e n e x t z o n e is occupied predominantly by t h e n i t r a t e r e d u c e r s which is t h e n e x t b e s t e l e c t r o n a c c e p t o r . O n c e t h e n i t r a t e is exhausted, and if t h e o r g a n i c s u b s t r a t e s a r e n o t t h e m s e l v e s depleted, s u l f a t e r e d u c t i o n predominates. T h e a m o u n t and c o m p l e x i t y of o r g a n i c s u b s t r a t e s d e c r e a s e with t h e d e p t h of p e n e t r a t i o n i n t o t h e s e d i m e n t s as a r e s u l t of biological utilization. Anaerobic m e t a b o l i s m and d e p l e t i o n of t h e various s u b s t r a t e s l e a d s to a d e c r e a s e in oxidation p o t e n t i a l and t h e subsequent mobilization of m e t a l s f r o m insoluble complexes. Regions of r i v e r s o r l a k e s high in n i t r a t e , phosphate and o r g a n i c m a t e r i a l , c a n o f t e n b e c o m e d e p l e t e d of dissolved oxygen d u e to r e s p i r a t o r y metabolism, resulting in a n a e r o b i c respiration a n d t h e subsequent mobilization of m e t a l ions f o r m t h e sediments. Mobilized m e t a l s will r e p r e c i p i t a t e as insoluble c o m p l e x e s when t h e oxygen c o n c e n t r a t i o n increases. Typically, reoxygenation o c c u r s within t h e s e d i m e n t s so t h e m e t a l s a r e r e p r e c i p i t a t e d without much m o v e m e n t , b u t when t h e w a t e r b e c o m e s anoxic, t h e s e m e t a l s c a n t r a v e l g r e a t d i s t a n c e s b e f o r e reoxygenation occurs. This mobilization-reprecipitation c y c l e c a n a l s o lead to ligand e x c h a n g e reactions. As a consequence, soluble c o m p l e x e s c a n also f o r m is t h e a p p r o p r i a t e ligands a r e available, s u c h as chloride. In t h i s way, m e r c u r y and c a d m i u m c a n b e solubilized f r o m s e d i m e n t s under a n a e r o b i c conditions and recomplex to chlorine in m a r i n e estuaries. Biological a c t i v i t y influences t h e mobilization and ligand e x c h a n g e of m e t a l c o m p l e x e s

as a result of a e r o b i c and a n a e r o b i c metabolism.Tida1 a c t i o n not only f a c i l i t a t e s p a r t i c u l a t e m o v e m e n t b u t also r e s u l t s in r a t h e r d r a m a t i c c h a n g e s in e n v i r o n m e n t a l conditions f o r t h e organisms living in t h o s e areas. Desulfovibrio is a s u l f a t e reducing microroganism and is t h e d o m i n a n t a n a e r o b e in e s t u a r i a l s a l t marshes, a c c o u n t i n g f o r up to 80 % of t h e a n a e r o b i c population (Bartha, in press). When t h e s a l t m a r s h is flooded, t h i s

organism r e d u c e s s u l f a t e to sulfide which results in t h e p r e c i p i t a t i o n of m e r c u r y as m e r c u r i c sulfide. O n t h e o t h e r hand, when t h e t i d e is o u t and t h e s u l f a t e c o n c e n t r a t i o n is low, Desulfovibrio c a t a l y z e s t h e f o r m a t i o n of m e t h y l m e r c u r y under a n a e r o b i c conditions. Methyl m e r c u r y i s highly p e r m e a b l e to c e l l s a n d i s mobilized i n t o t h e biota. In t h i s case, biological a c t i v i t y a n d e l e m e n t a l m6bility a r e a l s o influenced by c y c l i c changes, such as by t i d a l actiori. Similar c h a n g e s in biological a c t i v i t y a r e s e e n f o r diurnal cycles, seasonal c y c l e s and random effects resulting f r o m weather. Many of t h e r e a c t i o n s a n d i n t e r a c t i o n s mentioned a b o v e o c c u r in e s t u a r i e s as f r e s h w a t e r mixes with s a l t water. T h e influx of f r e s h w a t e r brings sediments, p a r t i c u l a t e s and dissolved n u t r i e n t materials. T h e s a l t w a t e r body provides i n c r e a s e d c o n c e n t r a t i o n s on inorganic ligands which c a n c o m p e t e with o r g a n i c ligands bound to m e t a l s in t h e f r e s h water. E s t u a r i a l c u r r e n t s p r o m o t e t h e breakdown of t h e various c h e m i c a l , p a r t i c u l a t e and t h e r m a l g r a d i e n t s t h e r e b y a f f e c t i n g biological activity. Tidal m o v e m e n t s result in

214

periodic changes in t h e many chemical and physical gradients. Periodic flooding and storm activity dramatically increase m a t e r i a l movement. Additional movements of sediments occur in many river basins as a result of dredging to keep w a t e r transportation routes navigable. Biotic and abiotic reactions c a n easily b e dominated by t h e fresh water input of nutrients with estuaries often being regions of high biological activity. Since many large river estuaries a r e heavily populated by man, t h e s e systems a r e usually heavily used and polluted from sewage effluent, industrial activity and agricultural runoff.

REFERENCES Baccini, P., 1984. Regulation of t r a c e m e t a l concentrations in freshwater systems. In: H. Sigel (ed.), Metal Ions in Biological Systems, Marcel Dekker, New York, Vol. 18, Ch. 8. Bartha, 1985. Science, in press. Blackburn, T.H., 1983. In: W.E. Krumben (ed.), Scientific Publications, Oxford, Ch. 3.

Microbial Geochemistry. Blackwell

Buffel, J., 1984. Natural organic m a t t e r and metal-organic interactions in a q u a t i c systems. In: H. Sigel (ed.), Metal Ions in Biological Systems, Marcel Dekker, New York, Vol. 18, Ch. 6. Fortsner, U., Wittman, G.T.W., 1979. Metal pollution in t h e a q u a t i c environment. Springer-Verlag, New York, Berlin. Jdrgenson, S.E. and Jensen, A., 1984. Process of m e t a l ions in t h e environment. In: H. Sigel (ed.), Metal Ions in Biological Systems, Marcel Dekker, New York, Vol. 18, Ch. 3. Lovelock, J.E., 1979. Gaea, A new look at life on earth. Oxford Univ. Press, Oxford, U.K. Pearson, R., 1968. J. Chem. Educ., 45: 643. Salomons, W., Baccini, P., 1984. T h e importance of chemical speciation in environmental processes. Dahlem Konferenzen, sept. 1-7. Berlin Springer-Verlag, in press. Sorenson, J., 1984. Seasonal variation and control of oxygen, n i t r a t e and s u l f a t e respiration in c o a s t a l marine sediments. In: M.J. Klug and C.A. Reddy (eds), C u r r e n t Perspectives in Microbial Ecology. American Society for Microbiology, Washington, D.C., pp. 447-453. Stumm, W., Morgan, J.J., 1981. Aquatic Chemistry, 2nd Edition. John Wiley and Sons, New York. Wood, J.M., 1984. Metabolic cycles for toxic e l e m e n t s in t h e environment. Science, 1983: 1049-1 052. Wood, J.M., 1984. Evolutionary a s p e c t s of m e t a l ion transport. In: H. Sigel (ed.), Metal Ions in Biological Systems, Marcel Dekker, New York, Vol. 18, Ch. 7. Wood, J.M. and Wang, H.K., 1983. Microbial resistance to heavy metals. Environ. Sci. Technol., 17(2): 582A-590A. Wood, J.M. and Wang, H.K., 1985. S t r a t e g i e s for microbial resistance to heavy metals. In: W. S t u m m (ed.), Chemical Processes in Lakes. Wiley Interscience, New York.

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