MARINE INTERFACES ECOH YDRODYNAMICS
FURTHER TITLES IN THISSERIES 1 J.L. MERO THE MIN E R A L RESOURCES OF THE SEA 2 L.M. FOMIN THE DYNAMIC METHOD I N OCEANOGRAPHY 3 E.J.F. WOOD MICROBIOLOGY OF OCEANS A ND ESTUARIES 4 G.N E U MA N N OCEAN CURRENTS 5 N.G.JERLOV OPTICAL OCEANOGRAPHY 6 V.VACOUIER GEOMAGNETISM I N MARINE GEOLOGY 7 W.J. WALLACE THE DEVELOPMENTS OF THE CHLORINITY/SALINITY CONCEPT I N OCEANOGRAPHY 8 E. Ll S l TZl N SEA-LEVEL CHANGES 9 R.H.PARKER THE STUDY OF BENTHIC COMMUNITIES 10 J.C.J. N IH OU L (Editor) MODELLING OF MARINE SYSTEMS 1 1 0.1. MA MA Y E V 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 A N D OCEAN ENGINEERING 18 J.W. CARUTHERS FUNDAMENTALS OF MARINE ACOUSTICS 19 J.C.J. N IH OU L (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 DRI L L I NG RESULTS I N THE I NDI AN OCEAN 22 P. DEHLINGER MARINE GR A V ITY 23 J.C.J. N IH OU L (Editor) HYDRODYNAMICS OF ESTUARIES A ND FJORDS 24 F.T. BANNER.M.B.COLLINS and K.S. MASSIE (Editors) THE NORTH-WEST EUROPEAN SHELF SEAS: THE SEA BED AN D THE SEA I N MOTION 25 J.C.J. N IH OU 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. N IH OU L (Editor) MARINE TURBULENCE 29 M. WALDICHUK, G.B. KULLENBERG and M.J. ORREN (Editors) MARINE POLLUTANT TRANSFER PROCESSES 30 A. VOlPlO (Editor) THE BALTIC SEA 31 E.K. DUURSMA and R . DAWSON (Editors) MARINE ORGANIC CHEMISTRY 32 J.C.J. N IH OU L (Editor) ECOHYDRODYNAMICS 33 R. H E K l N l A N PETROLOGY OF THE OCEAN FLOOR 34 J.C.J. N IH OU L (Editor) HYDRODYNAMICS OF SEMI-ENCLOSED SEAS 35 B. JOHNS (Editor) PHYSICAL OCEANOGRAPHY OF COASTAL A N D SHELF SEAS 36 J.C.J. N IH OU L (Editor) HYDRODYNAMICS OF THE EOUATORIALOCEAN 37 W. LANGERAAR SURVEYING A N D CHARTING OF THE SEAS 38 J.C.J. N IH OUL (Editor) REMOTE SENSING OF SHELF SEA HYDRODYNAMICS 39 T. ICHIYE (Editor) OCEAN HYDRODYNAMICS OF THE JAPAN A ND EAST CHINA SEAS 40 J.C.J. NIHOUL (Editor) COUPLED OCEAN-ATMOSPHERE MODELS 41 H. KUNZENDORF (Editor) MARINE MIN E R A L EXPLORATION
Elsevier Oceanography Series, 42
MARINE INTERFACES ECOHYDRODYNAMICS Edited by
J.C.J. NIHOUL University of Liige, B5 Sart Tilman, 8-4000 Ligge, Belgium
ELSEVlE R Amsterdam - Oxford - New York - Tokyo 1986
ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 2 5 P.O. Box 21 1 , 1000 A E 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.
ISBN 0 4 4 4 4 2 6 2 6 4 (Vol. 42) ISBN 0 4 4 4 4 1 6 2 3 4 (Series)
0 Elsevier Science Publishers B.V., 1986
A l l rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any f o r m 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 w i t h the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained f r o m the CCC about conditions under which photocopies of part of 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
The I n t e r n a t i o n a l LiPge Colloquia on Ocean Hydrdynamics are organized annually. Their t o p i c s d i f f e r f r m one year t o another and t r y t o address, as much as possible, r e c e n t problens and incentive new s u b j e c t s i n physical oceanography. Assenbling a group of a c t i v e and aninent scientists f r m d i f f e r e n t coun-
tries a d o f t e n d i f f e r e n t d i s c i p l i n e s , they provide a forum f o r discussion and f o s t e r a mutually b e n e f i c i a l exchange of information o p n i n g on t o a survey of
major r e c e n t discoveries, e s s e n t i a l mechanisms, impelling question-marks and valuable r e c m e n d a t i o n s f o r f u t u r e research. The S c i e n t i f i c Organizing C a r u n i t t e e and a l l the p a r t i c i p a n t s wish t o express t h e i r g r a t i t u d e to the Belgian Minister of Education, the National Science F a u d a t i o n of Belgium, t h e University of Liege, t h e Intergoverrmental Oceanographic C m i s s i o n and the Division of Marine Sciences (UNESCO) and the Office of Naval Research f o r their m o s t valuable support. The editor is indebted to D r . Jamart f o r h i s help i n e d i t i n g the proceedings.
Jacques C. J. NIHCUL
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VII
TABLE OF CONTENTS
.................................................................. ......................................................
Foreword L i s t o f participants
BIOLOGICAL PRODUCTION AT MARINE ERGOCLINES by L. Legendre, S. Demers and 0. L e f a i v r e
V XI
.................................
BIOLOGICAL PRODUCTION AT THE ICE-WATER ERGOCLINE by S. Demers, L. Legendre, J.C. T h e r r i a u l t and R.G.
Ingram
1
................
31
STUDYING FRONTS AS CONTACT ECOSYSTEMS by S. F r o n t i e r
............................................................
THE FRONTAL ZONE I N THE SOUTHERN BENGUELA CURRENT by L. Hutchings, D.A. Amstrong and B . A . M i t c h e l l - I n n e s THE DYNAMIC CONTROL OF BIOLOGICAL ACTIVITY REGION by G.B. B r u n d r i t
55
....................
67
IN THE SOUTHERN BENGUELA UPWELLING
..........................................................
FRONTAL ZONES, CHLOROPHYLL AND PRIMARY PRODUCTION PATTERNS I N THE SURFACE WATERS OF THE SOUTHERN OCEAN SOUTH OF CAPE TOWN by J.R.E. Lutjeharms, B.R. Allanson and L. Parker
.........................
95
105
FRONTAL SYSTEMS I N THE GERMAN BIGHT AND THEIR PHYSICAL AND BIOLOGICAL EFFECTS 119 by G. Krause, G. Budeus, 0. Gerdes, K. Schaumann and K. Hesse
.............
ROLE OF THERMAL FRONTS ON GEORGES BANK PRIMARY PRODUCTION by P. K l e i n
141
THE ROLE OF STREAMERS ASSOCIATED WITH MESOSCALE EDDIES I N THE TRANSPORT OF BIOLOGICAL SUBSTANCES BETWEEN SLOPE AND OCEAN WATERS by C.S. Yentsch and D.A. Phinney
153
ON THE DYNAMICS OF A TIDAL M I X I N G FRONT by G.J.F. Van H e i j s t
165
...............................................................
..........................................
......................................................
ZOOPLANKTON I N THE UPWELLING FRONTS OFF PT. CONCEPTION, CALIFORNIA by S.L. Smith, B.H. Jones, L.P. Atkinson and K.H. B r i n k
.
...................
195
OBSERVATIONS OF FINESTRUCTURE FORMED I N A CONTINENTAL SHELF FRONT (SOUTHEASTERN BERING SEA) by L.K. Coachman
..........................................................
215
SOME ASPECTS OF THE LIGURO-PROVENCAL FRONTAL ECOHYDRODYNAMICS ' by J.H. Hecq, J.M. Bouquegneau, S. D j e n i d i , M. F r a n k i g n o u l l e , A. G o f f a r t and M. L i c o t
257
PLANKTON DISTRIBUTIONS AND PROCESSES I N THE BALTIC BOUNDARY ZONES by M. Kahru, S. Nbmrnann, M. Simm and K. V i l b a s t e
273
THE ROLE OF THE LOOP CURRENT I N THE GULF OF MEXICO FRONTS by D.A. Salas de Leon and M.A. Monreal Gomez
295
..............................................................
..........................
..............................
VIIl
PRELIMINARY STUDY OF A FRONT I N THE BAY OF CAMPECHE, MEXICO by S.P.R. C z i t r o m , F. R u i z , M.A. A l a t t o r e a n d A.R. P a d i l l a
................
301
THE INTERACTION OF PHYSICAL AND BIOLOGICAL PROCESSES I N A MODEL OF THE VERTICAL DISTRIBUTION OF PHYTOPLANKTON UNDER S T R A T I F I C A T I O N by A . H . T a y l o r , J.R.W. H a r r i s a n d J. A i k e n 313
.................................
ESTIMATES OF THE NITROGEN FLUX REQUIRED FOR THE MAINTENANCE OF SUBSURFACE CHLOROPHYLL MAXIMA ON THE AGULHAS BANK by R.A. C a r t e r , P.D. B a r t l e t t and V.P. S w a r t
..............................
331
MODELLING THE T I M E DEPENDENT PHOTOADAPTATION OF PHYTOPLANKTON TO FLUCTUATING LIGHT by K.L. Denman and J. M a r r a 341
...............................................
THE EFFECTS OF THE BROAD SPECTRUM OF PHYSICAL A C T I V I T Y ON THE BIOLOGICAL PROCESSES I N THE CHESAPEAKE BAY by A. B r a n d t , C.C. S a r a b u n , H.H. S e l i g e r , M.A. T y l e r
......................
361
ASPECTS OF THE NORTHERN BERING SEA HYDRODYNAMICS by J a c q u e s C.J. N i h o u l
....................................................
385
SIMULATION ANALYSIS OF PLANKTON DYNAMICS I N THE NORTHERN BERING SEA by J.J. Walsh and D.A. D i e t e r l e
401
...........................................
THE TERRESTRIAL-MARINE INTERFACE : MODELLING NITROGEN TRANSFORMATIONS DURING I T S TRANSFER THROUGH THE SCHELDT RIVER SYSTEM AND I T S ESTUARINE ZONE by G. B i l l e n , C. Lancelot, E. D e b e c k e r and P. S e r v a i s 429
.....................
M O B I L I Z A T I O N OF MAJOR AND TRACE ELEMENTS AT THE WATER-SEDIMENT INTERFACE I N THE BELGIAN COASTAL AREA AND THE SCHELDT ESTUARY by W. B a e y e n s , G. G i l l a i n , M. H o e n i g and F. D e h a i r s 453
.......................
SEASONAL NUTRIENT SUPPLY TO COASTAL WATERS by L. R y d b e r g and 3. S u n d b e r g
467
LARVAL SETTLEMENT OF SOFT-SEDIMENT INVERTEBRATES : SOME PREDICTIONS BASED ON AN ANALYSIS OF NEAR-BOTTOM VELOCITY PROFILES by C h e r y l A n n B u t m a n
487
TURBIDITY AND COHESIVE SEDIMENT DYNAMICS by E. P a r t h e n i a d e s
515
ADRIA 84. A J O I N T REMOTE SENSING EXPERIMENT by P. S c h l i t t e n h a r d t
551
.............................................
......................................................
........................................................ ......................................................
I D E N T I F I C A T I O N OF HYDROGRAPHIC FRONTS BY AIRBORNE L I D A R MEASUREMENTS OF GELBSTOFF DISTRIBUTIONS by D i e b e l - L a n g o h r , T. H e n g s t e r m a n n and R. R e u t e r
..........................
569
WATER DEPTH RESOLVED DETERMINATION OF HYDROGRAPHIC PARAMETERS FROM AIRBORNE L I D A R MEASUREMENTS by D. D i e b e l - L a n g o h r , T. H e n g s t e r m a n n and R. R e u t e r 591
.......................
A QUANTITATIVE DESCRIPTION OF THE CHOROPHYLL GLOBAL IRRADIATION I N THE SURFACE LAYER by K.P. G i j n t h e r
A
FLUORESCENCE REDUCTION DUE TO
...........................................................
603
IX NIMBUS-7 COASTAL ZONE COLOR SCANNER PICTURES O F PHYTOPLANKTON GROWTH ON AN UPWELLING FRONT I N SENEGAL by C. D u p o u y , J . P . R e b e r t and D. T o u r e
....................................
619
MANGROVE ECOSYSTEM STUDY OF CHAKORIA SUNDERBANS A T CHITTAGONG W I T H S P E C I A L EMPHASIS ON SHRIMP PONDS BY REMOTE SENSING TECHNIQUES by 0. Q u a d e r , M.A.H. P r a m a n i k , F.A. K h a n and F . C . P o l c y n
.................. 6 4 5
BIOLOGICAL PROCESSES ASSOCIATED WTIH THE SOUTHEASTERN B E R I N G SEA by T.E. Whitledge and 3.5. W a l s h
THE PYCNOCLINE
AND SURFACE FRONTS
..........................................
Iri 655
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XI
LIST OF PAFQ1CIPA"S ALLANSON, B . , Prof. D r . , BAEYENS, W.F.,
BAH, A . ,
ANM-Wetenschappen, V r i j e U n i v e r s i t e i t te B r u s s e l , Belqium
Dr.,
Dgpartement d e Biologie, Universite d e Laval, Canada
Dr.,
BILLEN, G.,
Department of Z o o l q , Rhcdes University, South Africa
Laboratoire de Chimie I n d u s t r i e l l e , Universit6 Libre de
Dr.,
Brwelles, Belgium
BOUKARY, S., D r . , University of Niamey, Niger Dr.,
E?CUQUEGNEAU,J . M . , BRANDT, A.,
Laboratoire d'O&anologie,
Universit6 de Lieqe, Belqium
Applied Physics L a h r a t o r y , The Johns Hopkins University, USA
Dr.,
BRUNDRIT, G.B.,
Prof. D r . ,
Department of Oceanoqraphy, University of Cape Town,
South Africa BVTMAN, C.A.,
Ocean Engineering Department, woods Hole Oceanoqraphic
Dr.,
I n s t i t u t i o n , USA CAREY, D.A.,
Department of E a r t h and Environmental Sciences, Wesleyan
Dr.,
University, USA CAHTER, R.A.,
National Research I n s t i t u t e f o r Oceanology, South Africa
Dr.,
CHABERT D'HIERES, G.,
CLEMENT, F., M r . ,
Ing., I n s t i t u t d e Mgcanique d e Cxenoble (I.M.G.),France
GeoHydrcdynanics a d Environment Research (GHER), University
of Liege, Belgium COACH",
Prof. D r . ,
L.K.,
School of Oceanography, University of Washinqton,
USA I n s t i t u t o d e Ciencias d e l Mar y Limnolcqia, Ciudad
CZITROM, S . , D r . ,
Universitaria , Mexico
, Ing.,
DELEERSNIJDER, E.
CRoHydrodynamics and Envirorment Research (GHER),
University of Liege, Belgium DEMERS, S., D r . ,
DE",
K.L.,
Centre C h p l a i n d e s Sciences de la M a , Canada
Dr.,
I n s t i t u t e of Ocean Sciences, Canada
Mrs., Fachbereich 8-Physik, U n i v e r s i e t Oldenburg, Germany
DIEBEL-LANGOHR, D.,
DIETERLE, D . ,
Mr.,
Department of Marine Sciences, University of South Florida,
USA DISTECHE, A.,
Prof. D r . ,
Laboratoire d ' O c & n o l q i e , Universite d e LiGge, Belgium
DJENIDI, S., Ing., GeoIiyckdynamics and Enviroment Research (GHER), University of LiSqe, Belgium DUPOUY, C . , M i s s , Antenne ORSTOM, Centre de M6tCorolqie S p a t i a l e , France
ESTRADA, M.,
Dr.,
I n s t i t u t o de Investigaciones Pesqueras de Barcelona, Paseo
Nacional, Spain EXERBECQ, E . , Ing., GeoHydrcdynamics a d Envirorment Research (GHER), University of Liege, Belgium
FLEDUS, C . , M r ., CRoHydrodynamics and Environment Research (GIIER), University
of Liege, Belgium
Miss, Universit6 d e Montreal, Canada
FORTIN, M.J.,
FRANKIGNOLLIE, M.,
Laboratoire d'Oc6anologie, Universit6 d e Liege, Belgium
Mr.,
FIlOf\PTIER, S., Prof. D r . , GALLARDO, Y.,
Antenne ORSrOM/IFF3MER, France
Dr.,
D e l f t Hydraulics Laboratory, The Netherlads
Mr.,
GLAS, P.,
Universit6 d e s Sciences e t Techniques d e L i l l e , France
GODEAUX, J . , Prof. D r . , I n s t i t u t d e Zmlogie, Universit6 d e Liege, Belgium -HER,
Fachbereich 8 - Physik, Universitkit Oldenburg, Germany
Dr.,
K.,
HAPPEL, J.J., Ing., GeoHydrcdynamics and Enviroment Research (CSIER), University of Liege, Belgium CSIRO F i s h e r i e s Research, Marine Laboratories, Tasnania
HARRIS, G.P., D r . ,
GeoHydrcdynamics and Environment Research (GHER)
HECQ, J.H., D r . ,
, University
of Liege, Belgium HEIP, C.,
Marine Biology Section, Zmlogy I n s t i t u t e , University of Gent,
Dr.,
Belgium HERMAN, P.,
Marine B i o l o g y Section, Zoology I n s t i t u t e , U n i v e r s i t y of Gent,
Dr.,
Belgium
Consejo Nacional d e Ciencia y Tecnologia ( C O N A W ) , Ciudad
Dr.,
HUERTA, M.A.,
U n i v e r s i t a r i a , Mexico HUTCHINGS, L.,
Dr.,
Sea F i s h e r i e s Research I n s t i t a t e , South A f r i c a Department of Oceanography, M c G i l l University, Canada
INGRAM, R.G.,
Prof. D r . ,
JNQRT, B.M.,
D r . , Unit6 d c Gestion du Kcdele M a t h k t i q u e M e r du Nord e t
E s t u a i r e d e 1 ' E s c a u t ( U r n ) , I n s t i t u t d e Math&tique,
Belgium
D r . , Laboratorium v m r E k o l q i e en Systenatiek, V r i j e U n i v e r s i t e i t
JOIRIS, C.,
t e Brussels, Belgium KLEIN, P., D r . ,
Laboratoire d'Oc6anographie Physique, Universitg d e Bretagne
Occidentale, France KRAUSE, G., LEBON, G.,
Prof. D r . , Prof. D r . ,
LEGENDRE, L.,
I n s t i t u t fiir Meeresforschung, Germany I r r e v e r s i b l e Themcdynamics, University of Liege, Belgium
Prof. D r . ,
GIROQ, D 6 p r t e n e n t d e B i o l o g i e , U n i v e r s i t 6 d e Laval,
Canada LEWIS, M.R.,
MASO, M.A.,
Dr.,
Department of Oceanography, Dalhousie University, Canada
Miss, I n s t i t u t o d e Investigaciones Pesqueras d e Barcelona, Pas-
Nacional, Spain MONREK,,
A.,
Mrs, Consejo Nacional de Ciencia y Tecnologia (CONACYT), Ciudad
U n i v e r s i t a r i a , Mexico MCUCHET, A.,
Miss, GeoHydrdynamics a d Enviroment Research,
of Li&ge, Belgium
(GHER), University
XI11 Prof. D r . ,
NIHOUL, J . C . J . ,
GeoHydrcdynamics and Environnent Research (QIER),
University of Ligge, Belgium PAKTHENIADES, E.,
Prof. D r . , Department of Engineering Sciences, University of
Florida, USA
Mr.,
PHINNEY, D.A., PICIIOT,G.,
B i g e l m Laboratory f o r Ocean Sciences, USA
D r . , Unit6 d e Gestion du Modi.le M a t h k t i q u e M e r du Nord e t E s t u a i r e
d e 1'Escaut (UGYM) , Belgium
WADER, M d . O., M r . , Bangladesh Space Research and Renote Sensing Organization (SPARRSO), Bangladesh REES, J . M . ,
F i s h e r i e s Laboratory, Ministry of Agriculture, F i s h e r i e s ard
Mr.,
Food, UK RDl'ER,
Fachbereich 8 - Physik, U n i v e r s i G t Oldenbwg, Germany
Dr.,
R.,
ROBE, F.L.E., RONDAY, F.C.,
Intergovernmental Oceanographic Cannission, UNESCO, France
Dr., Dr.,
GeoHydrodynamics and Enviroment Research (GHER), University
of LiSge, Belgium RYDBEFG, L., Mr., Institute of Oceanography, University of Gothenburg, %den
M r . , Consejo Nacional de Ciencia y Tecmlcgia (CDNACYT), Ciudad
SALAS, D.A.,
U n i v e r s i t a r i a , Mexico SMITZ, J . , Ing., GeoHydrcdynamics ard Environnent Research (GHER), University of Liege, Belgium
Alfred-Wegener-Institute f o r Polar Research, Germany
SPIES, A.,
Dr.,
SPITZ, Y.,
Miss, Unit6 de Gestion du Modele M a t h h a t i y e Mer du Nord e t Estuaire
de l ' E s c a u t , I n s t i t u t de Mathhaticpe, Belgium P., Cannission of t h e European C m n i t i e s , J o i n t Research
SCHLITTE",
Center, I t a l y
Dr.,
STIGEBRANDT, A.,
Department of Oceanography, University of Gothenburg,
Sweden
SUNDBERG, J . , D r . , TANKE, M.,
Dr.,
I n s t i t u t e of Oceanography, University of Gothenburg, Sweden
Elsevier S c i e n t i f i c P u b l i s h i q Ccrnpany, The Netherlands
TAUPIER-LETAGE, I . , Miss, C e n t r e d'Oc6anologie de Marseille, Centre U n i v e r s i t a i r e de Luminy, France TAYLOR, A.H.,
Mr.,
THERRIAULT, J . C 1 . , TOPLISS, B . J . ,
Dr.,
I n s t i b t e for Marine Environnental Research, UK
Dr.,
Centre Champlain des Sciences de l a Mer, Canada
Department of F i s h e r i e s and Oceans, Bedford I n s t i t u t e of
Oceanoq-raphy , Canada TIJSSEN, S.B.,
VALKE, A.,
Mr.,
M r . , Netherlands Institute f o r Sea Research, The Netherlands GeoHydralynanics and Environment Research (GHER) , University
of Liege , Belgium VAN HEIJST, G.J.F.,
Netherlards
Dr.,
I n s t i t u t e f o r Meteorology a d Oceanography, The
XIV VETH, C .,
Dr.,
N e d e r l a n d s e I n s t i t u u t vcor Onderzoek der Z e e ( N I O Z ) , The
Netherlands WALEFFE, F.,
Ing., G e o H y d r d y n a m i c s and Enviroment R e s e a r c h (GHER), U n i v e r s i t y
of Ligge, B e l g i u m WHITLEGE, T.E.,
Dr.,
Oceanographic Sciences D i v i s i o n , Brcokhaven N a t i o n a l
Laboratory, USA WOODS, J . D . ,
Prof. D r . ,
I n s t i t u t fiir M e e r e s k u n d e , U n i v e r s i a t K i d , C ~ r m a n y
1
BIOLOGICAL PRODUCTION AT MARINE ERGOCLINES*
L. LEGENDREI, S. DEMERSZ and D. LEFAIVRE2
1 D6partement de b i o l o g i e , U n i v e r s i t 6 Laval, Quebec, Qu6bec G1K 7P4 (Canada)
2 Centre Champlain des sciences de l a mer, Qugbec, Quebec G1K 7Y7 (Canada)
C.P.
15500,
901 Cap Diamant,
ABSTRACT
Ergocl i n e s a r e a q u a t i c i n t e r f a c e s which have the common c h a r a c t e r i s t i c o f i n v o l v i n g s p a t i a l and/or temporal g r a d i e n t s where physical processes can produce s t r u c t u r e s associated w i t h enhanced b i o l o g i c a l production. Biologic a l p r o d u c t i o n i s taken here as t h e storage o f primary ( s o l a r ) energy by autotrophs and i t s t r a n s f e r t o o r among heterotrophs. I t i s hypothesized t h a t enhanced b i o l o g i c a l p r o d u c t i o n occurs a t ergocl i n e s as the consequence o f the matching o r resonance o f physical scales w i t h b i o l o g i c a l scales. Physical scales a c t on b i o l o g i c a l production through the proximal agency o f resources. High b i o l o g i c a l production most o f t e n exhausts the resources a v a i l a b l e a t t h e lower t r o p h i c l e v e l ; a u x i l i a r y energy on proper scales makes p o s s i b l e the replenishment o f these l i m i t i n g resources, and thus b i o l o g i c a l I t i s considered t h a t t r a n s i t i o n s on production a t a q u a t i c ergoclines. various time scales (annual, meteorological, t i d a l and so on) and s p a t i a l s t r u c t u r e s such as the ice-water and bottom-sediment i n t e r f a c e s , t h e pycnocline, t i d a l f r o n t s , and others, a l l belong t o the general category o f marine ergoclines. Hypotheses as t o the mechanisms t h a t govern b i o l o g i c a l production a t e r g o c l i n e s are developed, and the c o n d i t i o n s t o t e s t these hypotheses a t sea are discussed.
THE ERGOCLINE HYPOTHESIS Legendre and Demers (1985) have proposed t h a t the i n p u t o f mechanical energy i n the a q u a t i c environment n o t o n l y improves b i o l o g i c a l production b u t i s an e s s e n t i a l
requirement f o r it.
getic considerations.
*
T h i s hypothesis has evolved from ener-
The energy t h a t i s s t o r e d by p h o t o s y n t h e t i c organisms
C o n t r i b u t i o n t o the program o f GIROQ r e c h e r c h e s oceanographiques du Quebec)
(Groupe
interuniversitaire
de
2
and subsequently flows through aquatic ecosystems i s c a l l e d by e c o l o g i s t s "primary
energy".
The primary
photosynthetically active energy,
source
o f energy
for
r a d i a t i o n o f the sun.
ecosystems
I n addition
is
the
t o primary
the p r o d u c t i v i t y o f marine ecosystems i s i n f l u e n c e d by the i n p u t o f
mechanical energy caused by winds, exchanges,
and so on,
tides,
c a l l e d " a u x i l i a r y energy".
of
primary
(autotrophs),
(solar)
energy
and the t r a n s f e r o f
A u x i l i a r y energy i s n o t
b u t i t i s e f f i c i e n t i n increasing the
d i r e c t l y used by l i v i n g organisms, storage
freshwater r u n o f f , air-ocean heat
by
this
the
photosynthetic
stored energy t o heterotrophs o r
between the h e t e r o t r o p h i c components o f the food web.
B i o l o g i c a l production
i s taken here as the f l o w o f primary energy through ecosystems. spatio-temporal
variations
i n marine
organisms
biological
The observed
production are much more
r e l a t e d t o the space and time d i s t r i b u t i o n s o f a u x i l i a r y energy than t o those o f primary energy.
For example, Margalef and Estrada (1980) have shown t h a t
the o v e r a l l d i s t r i b u t i o n o f phytoplankton i n the oceans corresponds t o t h a t o f air-sea heat exchanges.
A s i m i l a r conclusion has been reached by Odum (1980)
f o r the a u x i l i a r y energy o f t i d e s i n estuaries. Legendre (1981) has proposed t h a t phytoplankton blooms g e n e r a l l y occur a t the spatio-temporal t r a n s i t i o n from unstable t o s t a b l e conditions.
This model
i s v e r i f i e d i n the marine environment, where several s p a t i a l and/or temporal i n t e r f a c e s are h i g h l y productive. water-sediment
Tidal fronts,
n u t r i c l i n e s , ice-water and
i n t e r f a c e s , temporal t r a n s i t i o n s i n v e r t i c a l s t a b i l i t y o f the
water column on various scales (annual, meteorological,
tidal)
,
and so on,
have the common c h a r a c t e r i s t i c o f i n v o l v i n g s p a t i a l and/or temporal gradients where
physical
processes can produce s t r u c t u r e s associated w i t h enhanced
b i o l o g i c a l production.
According t o Legendre and Demers (1985) , a l l these
s t r u c t u r e s are s p a t i a l and/or temporal gradients i n a u x i l i a r y energy, they termed "ergoclines"
(EPYOU:
which
work).
The above d e f i n i t i o n o f ergoclines leads t o a general hypothesis: Enhanced b i o l o g i c a l production occurs a t ergoclines as the consequence o f the matching o r resonance o f physical scales w i t h b i o l o g i c a l scales. Before e x p l o r i n g some o f i t s implications, the very concept o f ergoclines and t h e i r associated high b i o l o g i c a l production w i l l be confronted t o data froin a number o f marine systems. reviewed.
Among the temporal
Various types o f ergoclines w i l l be b r i e f l y ergoclines,
semidiurnal scales w i l l be considered. as the n u t r i c l i n e and the water-sediment
the annual,
meteorological
and
I n space, such h o r i z o n t a l ergoclines i n t e r f a c e w i l l be examined.
f r o n t s , as examples o f v e r t i c a l ergoclines, w i l l a l s o be investigated.
Tidal
3
ERGOCLINES I N THE MARINE ENVIRONMENT The s p r i n g phytoplankton bloom (Fig.
la)
is
probably
the b e s t known
b i o l o g i c a l response t o the physical scales o f an aquatic ergocline.
According
t o the " c r i t i c a l depth" model (Sverdrup, 19531, the spring bloom cannot occur before the mixed l a y e r shallows t o such a depth t h a t carbon f i x a t i o n by the phytoplankton exceeds r e s p i r a t i o n per u n i t area.
The bloom i s therefore
determined by the balance between decreased v e r t i c a l mixing and increased s o l a r r a d i a t i o n (Riley,
1942).
However,
changes i n s o l a r r a d i a t i o n do not
play a major r o l e except i n very high l a t i t u d e s ,
since increased v e r t i c a l
s t a b i l i t y o f the water column a t m i d - l a t i t u d e s r e s u l t s i n a phytoplankton bloom even i n wintertime (e.g. 1979).
Thus,
the Canadian Scotian Shelf:
the main f a c t o r o f
Fournier e t al.,
the spring phytoplankton bloom i s the
v e r t i c a l s t a b i l i t y o f the water column. Increased v e r t i c a l s t a b i l i t y o f the water column, a t the end o f the w i n t e r i n temperate regions, can be caused by e i t h e r higher surface temperatures o r lower s a l i n i t i e s . exhanges.
I n the oceans, higher temperatures r e s u l t from air-sea heat
I n coastal waters, reduced surface s a l i n i t i e s have been associated
w i t h the spring phytoplankton bloom.
For instance, Legendre e t a l .
(1981)
have proposed t h a t phytoplankton blooms under the sea i c e probably r e s u l t from the deepening o f the p h o t i c l a y e r (seasonal increase o f under-ice i r r a d i a n c e ) combined w i t h the increased s t r a t i f i c a t i o n caused by the l o w - s a l i n i t y m e l t i n g water.
Similarly,
Ross Sea
the phytoplankton bloom near a receding i c e edge i n the
( A n t a r c t i c a ) was a t t r i b u t e d by Smith and Nelson (1985) t o the
enhanced s t a b i l i t y brought about by the m e l t i n g o f the i c e . freshwater
i n coastal
areas
i s river
runoff,
which
Another source o f
can cause increased
v e r t i c a l s t a b i l i t y o f the water column and has thus been invoked by S i n c l a i r e t al.
(1981) t o e x p l a i n why several estuaries bloom e a r l i e r than adjacent
water bodies.
I n other estuaries, which are blooming l a t e r than the adjacent
waters, the shortened residence time o f the surface l a y e r w i t h i n the estuary during the f r e s h e t would prevent the i n i t i a t i o n o f an e a r l y bloom. S i m i l a r l y , in
such
environments
as
Indian Arm
(a
fjord
of
Northwestern
Canada:
Gilmartin, 19641, the increased r i v e r r u n o f f d e s t a b i l i z e s the water column, so t h a t a phytoplankton bloom only occurs upon reduction o f the freshwater flow. Whatever the actual mechanism, the spring phytoplankton bloom o n l y occurs when there i s matching o r resonance o f physical scales w i t h b i o l o g i c a l scales. A t a s h o r t e r time scale,
t h a t i s the scale o f a few days t h a t corresponds
t o the passage o f f r o n t a l disturbances (Heath,
1973; Walsh e t al.,
19771,
physical t r a n s i e n t phenomena (wind storms, i n t e r m i t t e n t upwell ing, etc. 1 are causing a p e r i o d i c i n p u t s o f a u x i l i a r y energy, and thus o f n u t r i e n t s , i n the water column. I n t e r m i t t e n t phytoplankton blooms (Fig. l b ) , t h a t f o l l o w
4
-
n
k m
E
-1 -1
>. I
(1
0
n
0
-1
I 0
DAYS
NITRATE (STA.A) .. .. . . .. . . . N I T R A T E
JUNE
JULY
( STA. I
1
M J GU ST
S E PT.
OCT.
F i g . 1 Changes i n biomass ( c h l o r o p h y l l ) and i n l i m i t i n g n u t r i e n t s , a s s o c i a t e d w i t h p h y t o p l a n k t o n blooms on two t i m e s c a l e s : ( a ) The s p r i n g bloom ( R i l e y , 1963: model of S t e e l e , 1958, u s i n g a C:Chl r a t i o o f 100). ( b ) Summer blooms 1977; t h e arrows i n d i c a t e t h e blooms); these blooms f o l l o w (Takahashi e t a1 sudden i n c r e a s e s o f n i t r a t e , t h a t a r e caused by e i t h e r winds o r f o r t n i g h t l y tides.
.,
5
stabilization of a water column previously destabilized by strong winds, have been reported in several studies (Iverson e t a l . , 1974; Takahashi e t a l . , 1977; Walsh e t a l . , 1978; Legendre e t a l . , 1982). A t an even shorter time scale, Fortier and Legendre (1979) and FrGchette and Legendre (1982) have reported, i n the lower St. Lawrence Estuary, bursts of phytoplankton photosynthetic a c t i v i t y and biomass that occurred on the semidiurnal cycle of tidal des t r a t i f ica ti on. The mechanisms for phytoplankton blooms on these various time scales are the same. As explained by Legendre and Demers (19851, increased vertical mixing may have several effects on phytoplankton: (1) increased loss rate of c e l l s from the photic layer, ( 2 ) lowered photosynthetic a c t i v i t y and production due to the deepening of the mixed layer ( l i g h t limitation of the vertically mixed c e l l s ) , and (3) nutrient replenishment of the mixed layer. According t o the production model of Legendre (19811, a phytoplankton bloom occurs upon stabilization of the water column, on any time scale. This bloom will l a s t as long as nutrients do not become limiting, which depends on the duration of the stable phase. When the stable phase i s long enough to r e s u l t in nutrient limitation (Fig. Z a ) , nutrients are u t i l i z e d a t the beginning of the stable phase, a f t e r which photosynthetic a c t i v i t y i s limited by the rate of in s i t u nutrient regeneration. During the next unstable phase, photosynthesis becomes limited by light. When the duration of stable periods i s short, nutrient limitation of the phytoplankton seldom occurs (Fig. 2 b ) . On the contrary, in environments where the input of auxiliary energy i s h i g h and persistent (ergocl ines absent or very small 1, hydrodynamics a c t on phytoplankton through the sole agency of l i g h t , hence circadian cycles of phytosynthesis and possible l i g h t limitation (Legendre and Demers, 1985). This general mechanism of phytoplankton blooms, that applies to a l l the time scales (Fig. 11, leads t o the following conclusions: (1) Physical scales do not a c t directly on phytoplankton, b u t rather through the proximal agency of l i g h t and nutrients (Table 1; the concept of "proximal agents" will be further developed below). ( 2 ) A phytoplankton bloom generally occurs a t temporal ergoclines, whatever t h e i r periodicities (annual, meteorological, semidiurnal, or other). This makes phytoplankton blooms a very general biological response to physical processes. As explained by Legendre and Demers (19851, animals can also respond t o temporal changes in physical scales. O n an interannual scale, Sutcliffe (1972, 1973) has related the landings of several commercial species i n the Uulf of S t . Lawrence t o variations in St. Lawrence River discharge years before. This may be explained by the influence of river discharge, i n the springtime, on the production of plankton. Similarly, the early-survival
Fig. 2 Phytoplankton biomass, production and photosynthesis a t schematic temporal ergocl ines, i n response t o v a r i a t i o n s o f n u t r i e n t s and average i r r a d i a n c e (proximal agents) i n the mixed layer, caused by the i n p u t o f a u x i l i a r y energy ( a ) Stable phase ( a u x i l i a r y energy low o r n u l l ) i s long enough t o r e s u l t i n n u t r i e n t l i m i t a t i o n . ( b ) Stable (ghade-)). P ase 1s s h o r t enough t o prevent n u t r i e n t l i m i t a t i o n . L: l i g h t l i m i t a t i o n ; N: n u t r i e n t l i m i t a t i o n . Adapted from R i l e y (1963, Fig. 5) and Legendre and Demers (1985, Fig. 2).
approach i n f i s h e r i e s biology hypothesizes t h a t the recruitment o f commercial species depends on the success o r f a i l u r e o f the annual c o l o n i z a t i o n o f the environment by the larvae. t i o n s i n recruitment,
Most o f the hypotheses e x p l a i n the observed varia-
and thus i n e a r l y s u r v i v a l ,
by the a v a i l a b i l i t y o f
As s u i t a b l e food (e.g. Pearcy, 1962; Cushing, 1972; Lasker, 1975). recognized by F o r t i e r (1982) and Legendre and Demers (19841, food i s the
On a
proximal agent through. which physical scales a c t on l a r v a e (Table 1).
much shorter time scale, Legendre and Demers (1985) have suggested t h a t the metabolic a c t i v i t y o f such pelagic organisms as zooplankton perhaps responds t o semidiurnal t i d a l v a r i a t i o n s i n a u x i l i a r y energy. The dynamics o f vegetal and animal components o f aquatic ecosystems can t h e r e f o r e be r e l a t e d t o temporal changes i n the physical
scales.
In all
cases, one o r several "proximal agents" are involved, and increased b i o l o g i c a l production occurs a t ergoclines. Ergoclines do n o t only e x i s t along the time axis, and they are encountered as well i n space.
S p a t i a l ergoclines can be e i t h e r h o r i z o n t a l o r v e r t i c a l .
Among the h o r i z o n t a l ergocl ines, the water-sediment i n t e r f a c e and the n u t r i c l i n e w i l l now be b r i e f l y reviewed.
A t h i r d type o f h o r i z o n t a l ergoclines,
ice-water i n t e r f a c e , i s discussed by Demers e t a l . ( t h i s book
( t h i s book) and by Lewis
.
The water-sediment
the
i n t e r f a c e , among other i n t e r f a c e s , has been recognized
by m i c r o b i o l o g i s t s as a p r e f e r r e d b i o t i c h a b i t a t (Marshall, 1976). As reported b.y Legendre e t a l . (1985b), there i s , a t the water-sediment i n t e r face, "a sharp gradient o f k i n e t i c energy which enables organisms t o u t i l i z e the
power
of
f l u i d motion f o r
t h e i r mechanical
dissolved o r p a r t i c u l a t e substances, Nixon e t al. heat o r chemical
concentrations
work 1971).
(e.g.
for
pumping
Sharp gradients i n
across these i n t e r f a c e s c r e a t e p o t e n t i a l s
which f a c i l i t a t e the t r a n s p o r t o f food and wastes v i a convective and d i f f u s i v e mechanisms, thus s t i m u l a t i n g metabolic processes." A good example o f the dynamics a t the water-sediment e r g o c l i n e i s provided by a s e r i e s o f studies on the grazing o f phytoplankton by mussels i n the St.
Lawrence Estuary.
Frechette and Bourget (1985)
found t h a t v e r t i c a l
d e p l e t i o n o f p a r t i c u l a t e organic matter occurs f r e q u e n t l y imnediately above the mussel bed, and t h a t the r e s u l t i n g v e r t i c a l gradients can be destroyed by waves and currents, waves.
and even i n v e r t e d by the i n p u t s o f mechanical energy o f
This supports the idea o f Wildish and Kristmanson (1979) t h a t food i s
o f t e n depleted hydrodynamic
imnediately above
processes
are
suspension-feeder
critical
in
populations,
determining
food
and t h a t
availability.
Frechette (ms.a) demonstrated t h a t v e r t i c a l t u r b u l e n t d i f f u s i o n indeed r e s u l t s i n downwards t r a n s p o r t o f phytoplankton.
Both c u r r e n t v e l o c i t y and bottom
8
roughness
influence
vertical
diffusion.
Low c u r r e n t
velocity
obviously
r e s u l t s i n poor downwards t r a n s p o r t o f phytoplankton, which leads t o d e p l e t i o n o f phytoplankton above the mussel bed and u l t i m a t e l y t o reduced mussel growth. On tne o t h e r hand,
the very development o f the mussel bed increases bottom
roughness and thus v e r t i c a l
turbulent diffusion.
The scales o f physical
processes a t the water-sediment ergocl i n e t h e r e f o r e c o n t r o l the p r o d u c t i o n o f suspension feeders. Subsurface c h l o r o p h y l l maxima are o f t e n observed i n the sea, s i g n i f i c a n c e has been reviewed by C u l l e n (1982).
and t h e i r
He f i r s t cautions the reader
a g a i n s t the usual i n t e r p r e t a t i o n o f c h l o r o p h y l l v e r t i c a l p r o f i l e s as i n d i c e s o f phytoplankton biomass:
some h e t e r o g e n e i t i e s i n the v e r t i c a l d i s t r i b u t i o n s
o f c h l o r o p h y l l may we1 1 r e f l e c t v a r i a b l e c h l o r o p h y l l c o n t e n t o f phytoplankton, and n o t changes i n biomass. temperate
regions,
I n shallow seas and on the c o n t i n e n t a l s h e l f o f
subsurface c h l o r o p h y l l maxima occur a t the n u t r i c l i n e ,
o f t e n w i t h i n s h o r t d i s t a n c e o f the pycnocline.
A possible explanation i s t h a t
s i n k i n g phytoplankton become "trapped" i n the pycnocline.
Another e x p l a n a t i o n
r e l i e s on the f a c t t h a t the upper p a r t o f the water column i s a two-layer system, w i t h a n u t r i e n t - d e p l e t e d surface l a y e r and deeper water i n which l i g h t becomes r a p i d l y l i m i t i n g (Dugdale, 1967).
Maximum phytoplankton biomass i s
o f t e n observed a t the boundary between the two layers.
This suggests t h a t
subsurface c h l o r o p h y l l maxima might be a b i o l o g i c a l response t o an e r g o c l i n e , between the upper well-mixed l a y e r and the more s t a b l e u n d e r l y i n g waters. Comparing the n u t r i c l i n e t o one o f the e r g o c l i n e s discussed above ( f o r instance,
a temporal e r g o c l i n e ) leads t o an i n t e r e s t i n g conclusion (Fig. 3).
I n the n u t r i c l i n e c o n f i g u r a t i o n , the two proximal agents ( l i g h t and n u t r i e n t s ) a r e i n v e r t e d r e l a t i v e t o the temporal e r g o c l i n e .
Despite t h i s f a c t , maximum
biomass (and probably p r o d u c t i o n ) seems t o occur a t the e r g o c l i n e , systems.
i n both
This supports the idea t h a t b i o l o g i c a l p r o d u c t i o n responds p r i m a r i l y
t o physical
scales.
The subsurface c h l o r o p h y l l maxima discussed here thus
occur a t the depth a t which both proximal agents ( l i g h t and n u t r i e n t s ) a r e n o t limiting.
T h i s i s s i m i l a r t o what has been found above f o r o t h e r ergoclines.
Whether t h i s same depth a l s o corresponds t o a zone o f h i g h s t a b i l i t y remains t o be demonstrated i n most cases.
Pjngree e t a l .
development o f h i g h concentrations
of dinoflagellates,
(1975) have explained the w i t h i n sharp pycnoc-
l i n e s near t i d a l f r o n t s , by a longer c h a r a c t e r i s t i c mixing time o f the water. Holligan e t al.
(1984) r e p o r t t h a t d i n o f l a g e l l a t e maxima i n t h e G u l f o f Maine
occur below the s t a b i l i t y maximum ( p y n o c l i n e ) b u t are centered i n regions o f zero t o s l i g h t l y p o s i t i v e Brunt-VaTsala frequencies.
Lewis e t a l . (1983) show
t h a t the very presence o f a subsurface c h l o r o p h y l l maximum causes a d i f f e r e n t i a l h e a t i n g of the water column,
w i t h the r e s u l t t h a t the depth s t r a t u m
ERGOCLINE
-1
a
a 0 a
5
I-
TI M E
c
W E L L - MIXED
DEPTH
FRONTAL
WELL
- STRATIFIED
c
Fig. 3 L i g h t (temporal ergocline: average i r r a d i a n c e i n the mixed layer; n u t r i c l i n e : i r r a d i a n c e ) ; concentration of l i m i t i n g n u t r i e n t ( s ) and phytoplankton biomass ( c h l orophyl 1 1 a t temporal and h o r i z o n t a l ergocl ines. Temporal ergocline: see Figs. l a and 2; n u t r i c l i n e : schematized from H o l l i g a n e t al. (1984, Fig. 5 ) .
Fig. 4 Surface and subsurface c h l o r o p h y l l maxima i n the e r g o c l i n e o f a t i d a l f r o n t . N u t r i e n t s are abundant i n the well-mixed waters, i n the f r o n t a l zone and under the pycnocline o f the w e l l - s t r a t i f i e d water column. Schematized from Pingree e t a l . (1975) and Demers e t a l . (1985).
10
c o n t a i n i n g the deeper p o r t i o n o f the c h l o r o p h y l l maximum becomes i n c r e a s i n g l y more s t a b l e w i t h time. and development o f
This increased s t a b i l i t y may permit the maintenance
the subsurface c h l o r o p h y l l maximum.
It i s therefore
p o s s i b l e t h a t a t l e a s t some subsurface c h l o r o p h y l l maxima be other examples o f increased b i o l o g i c a l
production i n resonance w i t h
physical
scales a t an
h o r i z o n t a l ergocline. Other s p a t i a l ergoclines are v e r t i c a l r a t h e r than h o r i z o n t a l . v e r t i c a l ergoclines, l a s t decade.
Among the
t i d a l f r o n t s have received considerable a t t e n t i o n i n the
Such f r o n t s develop during the sumner, between well-mixed and
we1 1 - s t r a t i f i e d waters.
T h e i r e f f e c t s on phytoplankton have been r e c e n t l y
reviewed by Demers e t a l . biomass associated w i t h
(1985).
I n shallow seas,
the f r o n t s
(Fig.
4)
the high phytoplankton
have been explained by the
f o r t n i g h t l y t i d a l excursions o f these f r o n t s , which c o n t r i b u t e t o p e r i o d i c a l l y r e p l e n i s h n u t r i e n t s i n the waters on the s t a b l e side o f the f r o n t (Pingree e t al.,
1975, 1976, 1977, 1978, 1982, 1983; Pingree, 1978a;
1978;
Parsons e t a1
., 1983).
a r e c y c l o n i c eddies distributions
were
to which h o r i z o n t a l phytoplankton
(Pingree 1978a,b), found
Simpson and Pingree,
Additional mechanisms f o r c r o s s - f r o n t a l mixing
t o be r e l a t e d (Pingree e t al.,
f r i c t i o n a l l y induced mean f l o w ( G a r r e t t and Loder, 1981).
19791,
and the
These mechanisms o f
f r o n t a l n u t r i e n t enrichment do n o t apply, however, t o shallow areas where both sides o f the t i d a l f r o n t are n u t r i e n t l i m i t e d and where the high phytoplankton biomass
associated with
the
front
c r o s s - f r o n t a l mixing (Perry e t al.,
cannot t h e r e f o r e
be e x p l a i n e d by
1983).
High phytoplankton biomasses do n o t necessarily r e f l e c t h i g h phytoplankton production, since mechanical aggregation o f the c e l l s does a l s o r e s u l t i n high biomasses.
Both i n d i r e c t evidences
(Savidge,
1976;
Holligan,
1981;
1981) and d i r e c t measurements o f primary production (Parsons e t al.,
Tett, 1983)
i n d i c a t e t h a t n u t r i e n t s e n t e r i n g the f r o n t a l zone from the well-mixed waters a r e the d r i v i n g force f o r new phytoplankton production. by Demers e t a1
. (1985)
nuities i n stability:
I t has been stressed
t h a t the f r o n t a l ergocl i n e coincide w i t h two d i s c o n t i ( 1 ) the f r o n t a l t r a n s i t i o n ,
which i s l o c a t e d between
well-mixed and w e l l - s t r a t i f i e d waters, and ( 2 ) the temporal t r a n s i t i o n ,
that
occurs from s p r i n g t o neap tides. V e r t i c a l f r o n t s have a l s o been associated w i t h high animal concentrations. For example,
I l e s and S i n c l a i r (1982) and S i n c l a i r and I l e s (1985) have
proposed t h a t yearclass v a r i a b i l i t y i n A t l a n t i c h e r r i n g i s l a r g e l y determined by the confinement o f larvae i n " l a r v a l r e t e n t i o n areas". These areas l i e to a l a r g e e x t e n t w i t h i n well-mixed zones bounded by temperature fronts. T i d a l f r o n t s are o f t e n i d e n t i f i e d by contouring the h o r i z o n t a l d i s t r i b u t i o n o f s, the Simpson and Hunter's (1974) s t r a t i f i c a t i o n parameter (e.g.
Garrett
11
e t al.,
1978;
Pingree and G r i f f i t h s , 1978, 1980; Bowman e t al.,
and Esaias, 1981;
G r i f f i t h s e t al.,
1981).
abundances i n terms o f both s t r a t i f i c a t i o n
1980; Bowman
I n order t o e x p l a i n phytoplankton (5)
and water column i l l u m i n a t i o n ,
Pingree e t a l . (1978) have suggested t o p l o t these abundances i n the s-kh diagram, where k i s the l i g h t e x t i n c t i o n c o e f f i c i e n t and h i s the depth o f the station.
This has been t r i e d by Bowman e t a l . (19811, f o r Long I s l a n d Sound,
and by Bah and Legendre (19851,
f o r the Middle S t .
Lawrence Estuary.
In
agreement w i t h the e r g o c l i n e hypothesis, the l a t t e r found t h a t high biomasses were concentrated i n the m a r g i n a l l y s t a b l e p a r t o f the t r a n s i t i o n zone o f the s-kh diagram, between well-mixed and w e l l - s t r a t i f i e d waters. The hypothesis t h a t high b i o l o g i c a l production occurs c h i e f l y a t aquatic ergoclines a l s o a p p l i e s t o the coastal zone (Fig. 51, where high autotrophic production can occur as long as n u t r i e n t s do n o t become l i m i t i n g (Demers e t al.,
ms). I n the coastal zone, the depth o f the well-mixed water column i s shallow enough to prevent l i g h t l i m i t a t i o n o f photosynthesis, so t h a t i t i s an
environment p r o v i d i n g s u i t a b l e spatio-temporal
scales f o r primary production.
Comparison o f the coastal zone and the n u t r i c l i n e shows that, as f a r as the e r g o c l i n e i s concerned, the two systems are homologous. The examples discussed above, o f temporal and s p a t i a l ergoclines, do stress two major concepts:
( 1 ) Where energetics i s concerned,
b i o l o g i c a l production
a t a given t r o p h i c l e v e l i s the same as the f l o w o f primary energy i n t o t h i s trophic level.
( 2 ) This f l o w o f energy can be l i m i t e d by the a v a i l a b i l i t y o f
resources a t the previous t r o p h i c l e v e l .
These resources are the proximal
agents, which have been mentioned above and through which physical scales a c t on b i o l o g i c a l production. High b i o l o g i c a l production most o f t e n leads to the l o c a l (temporal and/or s p a t i a l 1 exhaustion o f the previous t r o p h i c l e v e l . The i n p u t o f a u x i l i a r y energy on proper scales makes possible the replenishment o f the exhausted resources, Outside the ergocline,
and thus b i o l o g i c a l production a t the ergocline.
the scales o f the physical s t r u c t u r e s are e i t h e r too
l a r g e o r too small t o have any d i r e c t i n f l u e n c e on b i o l o g i c a l production, o r the physical s t r u c t u r e s are on such scales t h a t they impede the production. I n the case o f autotrophs (phytoplankton,
etc.),
the l i m i t i n g resources
(proximal agents) can be e i t h e r l i g h t (primary energy) o r n u t r i e n t s ( l i m i t i n g materials).
For heterotrophs, primary energy and m a t e r i a l s are combined i n t o
food resources, (e.g.
so t h a t there i s generally a s i n g l e class o f proximal agents
food) (Table 1).
Making the general e r g o c l i n e hypothesis ( f i r s t section, above) operational a t sea r e s t s on the a b i l i t y f o r b i o l o g i c a l oceanographers t o s p e c i f i y the b i o l o g i c a l scales which are conducive t o high production,
and f o r physical
oceanographers t o i d e n t i f y corresponding scales i n the physical environment.
12 TABLE 1
Proximal agents through which a u x i l i a r y energy a c t s on the various t r o p h i c 1eve1 s
.
Trophic l e v e l (examples)
Proximal agents
Primary (phytoplankton, etc.) Secondary (herbivorous zooplankton, f i s h larvae, molluscs, etc.) Carnivory ( f i s h e s , etc.)
L i g h t and n u t r i e n t s Organic p a r t i c l e s (phytoplankton, d e t r i t u s , etc,) Preys
COASTAL
I
ZONE
I
h
ORGANIC M A T T E R
EUPHOTIC LAYER
DISTANCE
t
MIXED LAYER
D
N UTRlCLlNE I
M I X E D LAYER-; EUPHOTIC Lf\YER I
I
( NUTRIENTS
DEPTH
D
Fig. 5 Schematic representations o f a coastal zone where n u t r i e n t s are n o n l i m i t i n g , and o f a n u t r i c l i n e w i t h a subsurface c h l o r o p h y l l maximum. Fluxes o f n u t r i e n t s and o f organic matter; i n the coastal zone, the f l u x e s can a l s o be along the shore. I n both systems, n u t r i e n t s flow from the stable r e s e r v o i r t o the ergocl ine, where autotrophic production i s maximum, and the p a r t i c u l a t e organic matter u l t i m a t e l y sinks i n t o the s t a b l e r e s e r v o i r where i t i s mineralized.
13
Specifying the
spatio-temporal
scales o f the most s i g n i f i c a n t b i o l o g i c a l
processes i n oceans has been attempted by various authors, among which Haury e t a l . (1978) and H a r r i s (1980).
As f a r as the t r a n s f e r o f primary energy i n
ecosystems (i.e. b i o l o g i c a l production) i s concerned, the physical s t r u c t u r e s o f i n t e r e s t must (1) i n the time domain, be s u f f i c i e n t l y a c t i v e t o resupply the resources depleted by b i o l o g i c a l production and p e r s i s t long enough t o a l l o w some accumulation o f biomass, and ( 2 ) i n the space domain, have such an extension as
to
sustain biological
activity
of
the
trophic
level
under
As explained j u s t above, physical s t r u c t u r e s t h a t are outside
consideration.
the s p e c i f i e d range o f scales e i t h e r do n o t have any d i r e c t i n f l u e n c e on b i o l o g i c a l production or, on the contrary, do impede it. As examples,
possible c h a r a c t e r i s t i c time and space scales are given i n
Table 2 f o r phytoplankton, zooplankton, The range o f
physical
a c t i v e l y growing f i s h e s and mussels.
scales matching these
biological
scales obviously
extends above and below the c h a r a c t e r i s t i c values o f Table 2. domain,
f o r instance,
H a r r i s (1980)
In the time
suggests t h a t the c h a r a c t e r i s t i c
time
p e r i o d f o r each phytoplankton c e l l i s the generation time, t h a t i s the period over which the environment must be i n t e g r a t e d w h i l e one c e l l grows and d i v i d e s
i n two.
Phytoplankton growth r e q u i r e s replenishment o f n u t r i e n t s by v e r t i c a l
processes, which occur i n the oceans on time scales between a few hours and a few days; such replenishment must a l s o p e r s i s t one o r two weeks, f o r s i g n i f i c a n t phytoplankton biomass t o develop.
The range o f physical scales matching
phytoplankton
encompasses
doubling time.
time
scales
therefore
S i m i l a r l y f o r mussels,
vertical
the
mean
phytoplankton
gradients i n p a r t i c u l a t e
organic matter above the bed must be destroyed by waves and c u r r e n t s several times every hour, f o r the mussels t o grow; a t the other end o f the time range, i t takes several months o f renewed food supply before a mussel bed becomes
established.
The same applies mutatis mutandis t o zooplankton and a c t i v e l y
growing fishes, w i t h the added complexity t h a t the p e r i 0 d i c i t . y o f resources uptake can be p a r t i a l l y c o n t r o l l e d by the animals themselves through v e r t i c a l and/or h o r i z o n t a l migrations. on the other hand,
o n l y the production o f those
organisms t h a t cannot v e r t i c a l l y migrate (e.g.
I n the space domain,
some phytoplankters, and a l s o
b e n t h i c organisms) physical structures.
i s c r i t i c a l l y dependent on the v e r t i c a l
extent o f the
V e r t i c a l mixing must maintain phytoplankton w i t h i n the
p h o t i c l a y e r f o r a bloom t o occur, and mussels cannot grow i f food p a r t i c l e s a r e n o t continuously resupplied i n the benthic boundary layer.
This i s n o t t o
say t h a t v e r t i c a l l y m i g r a t i n g organisms are n o t i n f l u e n c e d by the v e r t i c a l scales o f such physical s t r u c t u r e s as the pycnocline,
i n t e r n a l waves, and so
14
TABLE 2 C h a r a c t e r i s t i c time and space scales o f various groups o f marine organisms.
Temporal : Mean doubl ing time (days) o f the biomass Horizontal (km): C h a r a c t e r i s t i c scales o f patches, swarms, schools, etc. V e r t i c a l : E x t e n t o f the physical c o n t r o l (m)
Phytoplankton
Zooplankton
Fishes
Mussels
la
10-40 a
100-900b
120-500c
0.1-ld
0.1-1e
1-100e
5-50f
---
a Parsons ( 1980). Banse and Mosher (1980). Dare (1976). H a r r i s (1980), Legendre and Demers (1984). Haury e t a l . (1978). Depth o f the p h o t i c l a y e r . Thickness o f the p a r t i c l e depleted benthic l a y e r :
---
?
0.59
Frechette (ms.b).
Fig. 6 Relationships between phytoplankton production and biomass, and a l i m i t i n g n u t r i e n t , a t an e r g o c l i n e (see Fig. l a ) .
15
on,
b u t i t does n o t seem t h a t t h e i r p r o d u c t i o n i s c r i t i c a l l y dependent on
these v e r t i c a l scales. On the h o r i z o n t a l , a l l the organisms show c h a r a c t e r i s t i c s p a t i a l scales o f o r g a n i z a t i o n (Table 2 ) .
These r e s u l t from the i n t e r p l a y between b i o l o g i c a l
production and the environment.
Phytoplankton patches are o f t e n explained by
the "KISS" model ( K i e r s t e a d and Slobodkin, 1953; Skellam, 19511, as the r e s u l t o f both h o r i z o n t a l d i f f u s i o n and phytoplankton growth.
This model has been
m o d i f i e d by P l a t t and Denman (19751 and Wroblewski e t a l . the e f f e c t o f zooplankton grazing.
(1975) to i n c l u d e
A l t e r n a t i v e l y , R i l e y (1976) proposed t h a t
phytoplankton patchiness i s caused by d i f f e r e n t i a l grazing, r e s u l t i n g from the i n t e r a c t i o n o f zooplankton v e r t i c a l m i g r a t i o n s w i t h t i d e s and r e s i d u a l d r i f t . I t i s noteworthy t h a t the c h a r a c t e r i s t i c space scale o f phytoplankton patches
( 1 km:
Table 2) i s e q u i v a l e n t t o a time scale o f about 1 day ( H a r r i s ,
19801,
which corresponds t o the mean doubling time o f phytoplankton (Table 2).
The
explanatory mechanisms o f zooplankton heterogeneous d i s t r i b u t i o n g e n e r a l l y r e f e r t o some physical s t r u c t u r e s o r b i o l o g i c a l p r o p e r t i e s o r a combination o f both physical and b i o l o g i c a l
f a c t o r s (Legendre and Demers,
1984).
This i s
a l s o t r u e f o r h e r r i n g l a r v a e and j u v e n i l e s ( I l e s and S i n c l a i r , 1982; S i n c l a i r and
Iles,
1985).
The h o r i z o n t a l
scales o f
the
physical
s t r u c t u r e s are
t h e r e f o r e s i g n i f i c a n t as t o t h e production o f several t r o p h i c l e v e l s i n the oceans. The
general
ergocl i n e
hypothesis,
which
explains
enhanced
biological
production as the consequence o f the matching o r resonance o f physical scales with biological temporal and/or
scales can t h e r e f o r e be operational1.y a p p l i e d t o various s p a t i a l ergoclines.
T h i s i n d i c a t e s t h a t hypotheses can be
developed w i t h r e s p e c t t o the common c h a r a c t e r i s t i c s o f physical s t r u c t u r e s and b i o l o g i c a l
p r o d u c t i o n a t marine
ergocl ines.
These hypotheses c o u l d
p r o v i d e oceanographers w i t h an u n i f i e d b i o l o g i c a l - p h y s i c a l
approach t o the
hydrodynamic mechanisms t h a t c o n t r o l b i o l o g i c a l p r o d u c t i o n i n the sea. THE RESOURCES-ERGOCLINE DIAGRAM I n the f i e l d , biological
t h e r e i s no coincidence between maximum resources,
p r o d u c t i o n and maximum biomass.
This
p r o d u c t i o n uses the resources t o b u i l d up biomass. and 6,
i t i s assumed t h a t ,
as l o n g as the
is
maximum
because b i o l o g i c a l
I n the model o f Figs. l a
resources remain n o n l i m i t i n g ,
production depends o n l y on the accumulated biomass [ p r o d u c t i o n = biomass x s p e c i f i c p r o d u c t i o n r a t e ] ; when the resources become l i m i t i n g ,
production i s
assumed t o be p r o p o r t i o n a l to the c o n c e n t r a t i o n o f resources [ p r o d u c t i o n = biomass x f ( r e s o u r c e s ) ] .
As a r e s u l t , maximum p r o d u c t i o n and biomass occur
somewhere between the maximum and minimum values observed f o r the resources. I n a d d i t i o n , maximum biomass g e n e r a l l y f o l l o w s maximum production (Fig. 6).
16
Studying t h e s p a t i a l o r temporal d i s t r i b u t i o n s o f e i t h e r p r o d u c t i o n o r biomass cannot t h e r e f o r e be used t o l o c a t e e r g o c l i n e s a t sea. E r g o c l i n e s correspond t o g r a d i e n t s o f physical scales i n space and/or time. Sustained b i o l o g i c a l production, a t e r g o c l i n e s , r e q u i r e s h i g h - r a t e r e p l e n i s h ment o f the resources.
This occurs where and when the time scales of the
p h y s i c a l processes a r e s h o r t .
As the time scale lengthen, the i n c r e a s i n g l y
lower r a t e o f supply o f t h e resources p r o g r e s s i v e l y l i m i t s the b i o l o g i c a l An o p e r a t i o n a l approach t o e r g o c l i n e s , a t sea, would be t h e r e f o r e
production.
t o look a t resources. in
the
P l o t t i n g resources along the o b s e r v a t i o n a x i s r e s u l t s
Resources-Ergocline
(R-E)
diagram,
i d e n t i f i e d ( F i g . 7, bottom panels).
where
ergoclines
I n the R-E diagram,
are
easily
the observed values
o f each resource a r e standardized by the l e v e l o f the resource t h a t l i m i t s biological productive resource
production.
(R/Rlim)
=
1 is
then
1) and nonproductive ( < 1 ) waters,
(>
since
(R/Rlim)
is
dimensionless.
the
threshold
which
When
between
i s t r u e f o r any
there
are
several
resources, o n l y the s m a l l e s t o f the standardized values i s p l o t t e d i n the R-E diagram,
since a s i n g l e l i m i t i n g resource i s enough t o l i m i t the b i o l o g i c a l
production.
S p e c i f i c cases o f s p a t i a l e r g o c l i n e s w i l l now be examined using
t h e R-E diagram. I n the coastal beds,
zone (Figs.
benthic microalgal
light, benthic
n o t by
nutrients.
algae)
and
the
5 and 71,
mats,
or
i s generally
l i m i t e d by
Both bottom i r r a d i a n c e (which i s c r i t i c a l average
v e r t i c a l l y mixed phytoplankton) c o a s t a l e r g o c l i n e i s defined, irradiance,
the production o f autotrophs ( k e l p
phytoplankton)
irradiance
in
t h e mixed l a y e r
depend on water depth. f o r autotrophs,
t h a t i s water depth.
for
(for
the
As a r e s u l t ,
the
by the o f f s h o r e g r a d i e n t i n
N u t r i e n t s do n o t become l i m i t i n g i n t h e
c o a s t a l zone when the time scale o f water movements (advection, e t c . ) i s s h o r t enough t o meet the n u t r i e n t requirements o f autotrophs ( F i g . 7, top panel 1. On the o t h e r hand, t h e r e i s no l i g h t l i m i t a t i o n o f primar,y production as long as the dimensionless space scale (depth o f the p h o t i c l a y e r / depth o f the mixed l a y e r ,
t o which
phytoplankton i s known t o respond:
remains above a c r i t i c a l value.
Harris,
1978)
When the depth o f the mixed l a y e r exceeds
t h a t o f the p h o t i c l a y e r , l i g h t l i m i t a t i o n p r o g r e s s i v e l y develops, which sets t h e o f f s h o r e l i m i t o f the coastal e r g o c l i n e ( F i g . 7, bottom panel). A t t i d a l fronts,
the s i t u a t i o n i s somewhat more complex,
since phytoplank-
t o n p r o d u c t i o n can be l i m i t e d by l i g h t and a l s o by n u t r i e n t s (Figs.
4 and 7).
The f r o n t a l e r g o c l i n e corresponds t o changes i n both the space and time scales ( F i g . 7, top panel).
On the inshore side o f the t i d a l f r o n t , the depth o f the
mixed l a y e r g e n e r a l l y exceeds t h a t o f the p h o t i c l a y e r (dimensionless space s c a l e < 11, so t h a t phytoplankton c e l l s can be l i g h t l i m i t e d ; simultaneously,
COASTAL
TIDAL
FRONT
NUTRlCLlNE
MUSSEL
BED
TIUE S U L E ( 1 1 .MINUTE -PAnAL
SPATIAL SCALE
4 0 '
-----TEM-
-HOUR ,104
....... .. . . . .. . . . .CRITICAL .... .. . .., , LEVEL
\
NUTRIENT
C O T
4
NUTRIENT CONC
4
C O W OF PART O R . MAT
I '
I
/LIU WATER AVAILABLE \ TOTME MUSSELS
',
HEIGHT ABOVE BOTTOM
OBSERVATION A X I S
OBSERVATIGN A X I S
OBSERVATION AXIS
Fig. 7 Top panels: schematic changes i n the space and time scales o f physical energy i n p u t s a t f o u r d i f f e r e n t s p a t i a l ergoclines. I n the coastal zone and a t t i d a l fronts, the dimensionless space scale i s [depth o f the photic l a y e r / depth of the mixed l a y e r ] , w h i l e f o r the n u t r i c l i n e i t i s [depth o f the photic l a y e r / depth i n the water column]. Intermediate panels: schematic changes i n l i m i t i n g resources. Bottom panel s: Resources-Ergocl ine (R-E) diagrams corresponding t o the a e l s ab ve. I? these diagrams, standardized resources ( o b s e r v e d / l i m i t i n g l are p l o t t e d along the observation axis; bafues > d e l i m i t a t e the ergocline (shaded). When there are several resources, the smallest o f the standardized values i s plotted.
?
~
4
18
the time scale o f v e r t i c a l water movements i s s h o r t enough t o maintain high n u t r i e n t concentrations i n the mixed layer. front,
On the o f f s h o r e side o f the t i d a l
the dimensionless space scale exceeds 1, so t h a t phytoplankton c e l l s
a r e kept i n the p h o t i c l a y e r and thus do n o t experience l i g h t l i m i t a t i o n ; hovever, as the time scale o f v e r t i c a l water movements increases up to a few hours,
phytoplankton become progressively
nutrient limited.
The f r o n t a l
e r g o c l i n e corresponds to the region where both the physical time and space scales match the p h y s i o l o g i c a l scales o f the phytoplankton. f r o n t ergocline,
where both resources exceed t h e i r
A t the t i d a l
(R-E
l i m i t i n g values
diagram), phytoplankton production and biomass are o f t e n high.
Whether t h i s
primary production i s a c t u a l l y transferred, a t the ergocline, to other t r o p h i c l e v e l s i s s t i l l a s u b j e c t o f a c t i v e research. The n u t r i c l i n e (Figs.
3 and 7) i s l o c a t e d between an upper l a y e r where
phytoplankton i s o f t e n n u t r i e n t l i m i t e d , and n u t r i e n t - r i c h deep waters where primary production i s l i g h t l i m i t e d . l a y e r by the a c t i o n o f winds,
N u t r i e n t s are replenished i n the upper
currents,
i n t e r n a l t i d e s , and so f o r t h .
These
physical processes s e t the time scale o f n u t r i e n t replenishment a t various depths i n the water column.
The upper waters, where the time scale i s below
t h e c r i t i c a l value f o r primary production, experience n u t r i e n t l i m i t a t i o n . t h e other hand, the dimensionless space scale ( d e f i n e d here as:
On
depth o f the
p h o t i c l a y e r / depth i n the water column) sets the v e r t i c a l extension o f the productive zone.
Here again, the e r g o c l i n e i s l o c a t e d i n the l a y e r where the
time and space scales o f the physics match the b i o l o g i c a l scales.
Biological
production occurs i n the n u t r i c l i n e when l i g h t i n t e n s i t y a t t h a t depth i s h i g h The zone o f the R-E diagram where standardized
enough f o r photosynthesis.
resources exceed one (both resources are n o n l i m i t i n g ) defines the depth o f the ergocline. I t has been suggested t h a t the production maximum i s l o c a t e d a few metres above the biomass maximum. phytoplankton c e l l s once produced.
This could be explained by the s i n k i n g o f However, Cullen (1982) r i g h t l y stresses
t h a t most i n v e s t i g a t i o n s do n o t have a f i n e enough v e r t i c a l r e s o l u t i o n t o d i s c r i m i n a t e between two such peaks. I t i s noteworthy that,
production,
i n a l l the cases above which d e a l t w i t h primary
l i g h t l i m i t a t i o n was r e l a t e d t o the physical space scales, w h i l e
n u t r i e n t l i m i t a t i o n depended on the physical time scales.
This i s simply
because l i g h t cannot be p h y s i c a l l y mixed downwards i n the water column, that
light availability
for
the
autotrophic
p o s i t i o n r e l a t i v e t o the l i g h t gradient,
organisms
depends
so
on t h e i r
t h a t i s on the s p a t i a l scales o f
v e r t i c a l water movements. By contrast, enhanced primary production increases the r a t e o f n u t r i e n t uptake, which r e s u l t s i n n u t r i e n t l i m i t a t i o n i f the r a t e o f n u t r i e n t supply i s too low, hence the r o l e o f hydrodynamic time scales.
19
For organisms t h a t do n o t depend on l i g h t ,
i t i s t h e r e f o r e expected t h a t
physical time scales w i l l be more c r i t i c a l than the space scales, as w i l l be v e r i f i e d f o r mussel beds. Frechette (ms. b ) presents data on the concentrations o f phytoplankton above a mussel bed which show (1) t h a t f i l t r a t i o n by the mussels can reduce the concentration o f the resource, concentration,
near the bottom,
t o about h a l f the ambient
and ( 2 ) t h a t the e f f e c t s o f f i l t r a t i o n can be observed up t o
a t l e a s t 0.5 m above t h e bed.
Despite the f a c t t h a t mussels are attached t o
the bottom, t h e i r f i l t r a t i o n a c t i v i t y thus influences the water column up to a t l e a s t 0.5 m.
I t has been explained above t h a t replenishment o f phytoplank-
ton i n the benthic boundary l a y e r i s e f f e c t e d by v e r t i c a l t u r b u l e n t d i f f u s i o n (Fig.
7).
Due t o the f a c t t h a t mussels are attached t o the bottom,
the
ergocline cannot be defined by simply l o o k i n g a t the v e r t i c a l d i s t r i b u t i o n of p a r t i c u l a t e organic matter above the bed.
The decreasing a v a i l a b i l i t y o f the
resource w i t h the distance from the bed must a l s o be taken i n t o account.
In
Fig. 7, the concentration o f p a r t i c u l a t e organic matter i s n o n l i m i t i n g above the mussel bed,
since i f i t was l i m i t i n g the mussels could n o t grow there.
The c r i t i c a l parameter here i s the time required f o r an h o r i z o n t a l l a y e r of p a r t i c l e - r i c h water t o come i n contact w i t h the bed, since phytoplankton must be a c t i v e l y replenished i n the benthic l a y e r by downwards water movements. With increasing distance above the bottom, come i n contact w i t h the mussel bed.
i t takes longer f o r the water t o
As the time scale o f the v e r t i c a l
replenishment o f the benthic l a y e r increases w i t h the h e i g h t above the bottom, i t reaches
a critical
value where
requirements o f the mussel bed.
the
particle
supply
cannot meet the
The actual h e i g h t o f t h i s c r i t i c a l value
above the bottom i s determined by the angle w i t h the a x i s o f ordinates i n the top panel o f Fig.
7.
This angle depends on the dynamics o f the processes o f
v e r t i c a l d i f f u s i o n and advection.
The v e r t i c a l e x t e n t of the e r g o c l i n e i s
thus a f u n c t i o n o f the volume o f water a v a i l a b l e t o mussel f i l t r a t i o n . r e a l l y q u a n t i f y t h i s idea,
To
some measurements o f the v e r t i c a l d i s t r i b u t i o n o f
the p a r t i c u l a t e organic matter a t t a i n a b l e by the mussel bed would be needed. The e r g o c l i n e f o r mussels i s l i m i t e d t o a t h i n layer, since they cannot swim up i n the water column.
The growth o f mussels i n t o hummocks might be a way t o
r a i s e some o f them s l i g h t l y above the bottom. however been found by mussel
growers,
The u l t i m a t e s o l u t i o n has
who suspend the mussels
from the
surface, thus extending the e r g o c l i n e to the whole water column. A l l the above examples show t h a t ergoclines can be i d e n t i f i e d a t sea by
monitoring t h e l i m i t i n g resources, which i s much easier than measuring R-E diagrams can be used t o i d e n t i f y p o t e n t i a l b i o l o g i c a l production. ergoclines, when l i m i t i n g values f o r the resources are known.
The same would
20
be true for temporal ergoclines (Figs. I and 2 ) . The e x p l i c i t assumption of the R-E diagram is that biological production can occur when resources are nonlimi t i n g ; the implicit assumption i s that such conditions are encountered where and when physical scales match biological scales. TESTING HYPOTHESES AT AQUATIC ERGOCLINES The general ergocline hypothesis, t h a t ergoclines in the f i r s t section above, was:
followed
the
definition
of
Enhanced biological production occurs a t ergocl ines as the consequence of the matching or resonance of physical scales w i t h biological scales. One must be aware that t h i s hypothesis does not concern the relationship between ergoclines and biomass, b u t rather the relationship between ergoclines and production. This i s due t o the f a c t that high biomass may e i t h e r r e s u l t from h i g h production or from mechanical aggregation of the organisms. Conversely, low biomass may r e f l e c t low production, b u t i t can also r e s u l t from mechanical dispersion of the biomass, grazing or predation. Primary production a t ergocl ines i s termed "new production", t h a t i s production resulting from allochtonous nutrient inputs, versus "regenerated production", which r e s u l t s from nutrient regeneration in the surface waters (Eppley and Peterson, 1979). The above general hypothesis i s not easily testable, as originally formulated. Testable hypotheses must be so formulated that sampling a t sea be aimed a t falsifying or rejecting them, since there i s no way of accepting a s c i e n t i f i c hypothesis as being univocally true. The f i r s t step in testing the general ergocline hypothesis would be to f a l s i f y the f ol 1ow i ng hy po t h e s i s : The temporal, horizontal or vertical distribution of biological production i s independent from gradients in the scales of physical structures. To t e s t this hypothesis, one must measure a t sea (1) buoyancy and velocity f i e l d s , from which horizontal and vertical fluxes can be computed and spectra of physical scales be specified, as well as ( 2 ) biological production and biomass, in order to characterize the distribution of specific biological production ( i .e. production per u n i t biomass). Biological production has been defined above as the flow of primary energy into a given trophic level. High specific production indicates that the high biological production measured a t sea r e s u l t s from enhanced biological transfer of primary energy and not only from high accumulated biomass. If the hypothesis of independence i s rejected,
21
the spatio-temporal zone o f high b i o l o g i c a l production most l i k e l y corresponds t o an ergocline, as a l l ergoclines have the common c h a r a c t e r i s t i c o f i n v o l v i n g spatial
and/or
temporal
gradients
where
physical
processes
can
produce
structures associated w i t h enhanced b i o l o g i c a l production (see above).
The
general ergocline hypothesis can be f u r t h e r i n v e s t i g a t e d by t e s t i n g t h i s next hypothesis: A t ergoclines, the temporal, h o r i z o n t a l o r v e r t i c a l d i s t r i b u t i o n o f physical scales does n o t necessarily correspond t o the c h a r a c t e r i s t i c scales o f b i o l o g i c a l production. I n order t o t r y f a l s i f y i n g t h i s second hypothesis, one must ( 1 ) determine the c h a r a c t e r i s t i c ergocline (e.g. scales
time and space scales o f b i o l o g i c a l
Table 2) and ( 2 ) compare the d i s t r i b u t i o n o f the physical
associated w i t h If
scales.
production a t the
dominant
the
ergocline
physical
scales
to at
these c h a r a c t e r i s t i c b i o l o g i c a l the
ergocline
(which
is
the
spatio-temporal zone o f enhanced b i o l o g i c a l production) do coincide w i t h the c h a r a c t e r i s t i c b i o l o g i c a l scales,
the second hypothesis can be r e j e c t e d and
the i m p l i c a t i o n s o f the e r g o c l i n e hypothesis can be f u r t h e r explored. The f i r s t i n t e r e s t i n g question r a i s e d by the ergoclines hypothesis concerns the mechanisms through which b i o l o g i c a l production increases a t ergoclines. According t o the well-known papers o f R i l e y (1942) and Sverdrup (19531, the usual hypothesis e x p l a i n i n g phytoplankton blooms i s t h a t they r e s u l t from changes i n the physical environment, and n o t from p h y s i o l o g i c a l changes o f the organisms.
However,
i t has been observed i n the lower S t .
Lawrence Estuary
t h a t photosynthetic c h a r a c t e r i s t i c s o f the phytoplankton do change according t o the semidiurnal c y c l e o f t i d a l mixing, thus leading t o semidiurnal blooms ( F o r t i e r and Legendre, 1979; Frechette and Legendre, 1982). Some physiological p r o p e r t i e s o f phytoplankton can even show endogenous v a r i a t i o n s , phased on the semidiurnal cycles o f a u x i l i a r y energy ( A u c l a i r e t al., a1
., 1985a).
1982;
Legendre e t
Zooplankton physiology perhaps a1 so responds t o the semidiurnal
t i d a l mixing (data from Maranda and Lacroix, 1983, f o r the Middle S t . Lawrence Estuary, solely
r e i n t e r p r e t e d by Legendre and Demers, physical
effect of
ergoclines
on
1985).
The hypothesis o f a
production cannot
therefore
be
accepted before r e j e c t i n g f i r s t the hypothesis that: The high b i o l o g i c a l production t h a t occurs a t ergoclines i s ( i n p a r t ) caused by changes i n the physiological s t a t e o f the organisms, as the consequence o f the matching o r resonance o f physical scales w i t h b i o l o g i c a l scales.
22
-
I00
.. ,.
ap
Z
0 0
c
c
75c1
m /
I
50
Diffusion /Consumption
0 -
number
v-----Reynolds
tl
1
0
10
CURRENT
I
I
I
20
30
40
vELociiY
(cm.s-1)
Fig. 8 R a t i o o f mean v e r t i c a l d i f f u s i o n t o mean consumption o f phytoplankton by a mussel bed, and Reynolds number, as a f u n c t i o n o f c u r r e n t v e l o c i t y near t h e bottom. Curves freehand adjusted. Data selected from F r k h e t t e (ms.a).
To f a l s i f y t h i s hypothesis, one must r e f i n e the measurements o f b i o l o g i c a l production a t sea, t h a t were already needed t o t e s t the previous hypotheses. To do so, one must f i r s t i d e n t i f y the proximal agent(s1 through which a u x i l i a r y energy enhances the b i o l o g i c a l
production.
Once the proximal agent i s
known, i t becomes possible t o measure i t s f l u x across the ergocline, together with
the
physio-
ecological
responses o f the l i v i n g organisms.
A t the
water-sediment ergocl ine, f o r example, Frechette and Bourget (1985) have shown t h a t mussels can deplete the phytoplankton (proximal agent) imnediately above the bed.
Advection and v e r t i c a l d i f f u s i o n insured seston replenishment i n the
benthic boundary l a y e r on physical scales t h a t matched the b i o l o g i c a l requirements ( F r k h e t t e , ms.a).
Phytoplankton d e p l e t i o n above the mussel bed was
n e g a t i v e l y c o r r e l a t e d w i t h c u r r e n t v e l o c i t y ( F r k h e t t e and Bourget, 1985). current velocity
(and thus
turbulence,
i.e.
As
Reynolds number) increased,
d i f f u s i o n d i d account f o r a l a r g e r p r o p o r t i o n o f the replenishment (Fig. 8). F i l t r a t i o n r a t e s o f the mussels d i d tend t o decrease when d e p l e t i o n became more severe
(FrBchette
and Bourget,
ms).
This example
shows t h a t
the
23
measurements needed t o physio-ecological
.
test
the above hypothesis are both physical
Physical
agent(s) a t the ergocline,
and
measurements concern f l u x e s o f the proximal
w h i l e the physio-ecological
such b i o l o g i c a l c h a r a c t e r i s t i c s as uptake rates,
measurements concern
e x c r e t i o n rates,
and so
forth. The enhanced b i o l o g i c a l a c t i v i t y a t ergoclines can modify the physical
As mentioned above, Lewis e t al. (1983) have shown that, due t o the depth stratum c o n t a i n i n g the deeper p o r t i o n o f the subsurface c h l o r o p h y l l maximum becomes i n c r e a s i n g l y more environment.
d i f f e r e n t i a l heating i n the water column, stable i n time.
This may permit the maintenance and development of
chlorophyll maximum.
As another example,
coral
the
r e e f s increase the t i d a l
currents ( a u x i l i a r y energy a v a i l a b l e f o r mechanical work) i n o v e r l y i n g waters, by decreasing the water depth above t h e i r growing calcareous mounds (Odum and Odum, 1955).
S i m i l a r l y , the growth o f mussels, and o f groups o f mussels t h a t
form humnocks i n high density s i t u a t i o n s , increases bottom roughness and thus v e r t i c a l t u r b u l e n t d i f f u s i o n , which might favour phytoplankton replenishment of
the
boundary
layer
and
u l t i m a t e l y mussel
growth
(Frkhette,
ms.a).
Hummocks probably a c t as densit,y-dependent s t r u c t u r e s t h a t e x e r t a p o s i t i v e feedback on the c a r r y i n g capacity o f a bed by enhancing phytoplankton supply (Carefoot, 1977). Sea-grasses and other rnacrophytes can improve t h e i r l i g h t environments by enhancing sediment deposition and reducing resuspension (Ginsburg and Lowenstam, 1958; Ward e t al., 1984). S i m i l a r p r o p e r t i e s are being discussed by Lewis ( t h i s book) as to the growth o f i c e microalgae a t the ice-water i n t e r f a c e .
Such examples o f p o s i t i v e feedbacks l e d Legendre e t al.
(1985b) t o the conclusion that, a t some ergoclines, b i o l o g i c a l a c t i v i t y can modify the physical environment so as to enhance the growth and/or s u r v i v a l o f ecosystem components. Such a general hypothesis cannot be t e s t e d w i t h o u t demonstrating f i r s t t h a t some environmental
characteristics
a c t i v i t y a t the ergocline.
are indeed modified by the b i o l o g i c a l
To do so,
the f o l l o w i n g hypothesis must be
rejected : Environmental c h a r a c t e r i s t i c s a t an e r g o c l i n e a r e n o t influenced by b i o l o g i c a l production. F a l s i f y i n g t h i s hypothesis requires t o simultaneously measure environmental properties and b i o l o g i c a l a c t i v i t y , l e v e l s o f b i o l o g i c a l production.
a t ergoclines characterized by d i f f e r e n t
The environmental p r o p e r t i e s measured would
be those f o r which a response to b i o l o g i c a l production i s p r e d i c t e d by models (e.g. the thermal s t r u c t u r e a t the pycnocline, o r v e r t i c a l t u r b u l e n t d i f f u s i o n
24
a t the water-sediment i n t e r f a c e ) . The hypothesis would be r e j e c t e d i f s i g n i f i c a n t d i f f e r e n c e s i n the environmental p r o p e r t i e s are found as a f u n c t i o n o f b i o l o g i c a l production.
I t i s a l s o possible t o approach the question experi-
mentally, by i n t r o d u c i n g a r t i f i c i a l mimics o f the b i o l o g i c a l systems i n e i t h e r the natural environment o r l a b o r a t o r y conditions, and measuring the r e s u l t i n g changes i n environmental variables. The above hypothesis concerns, however, only the f i r s t p a r t o f the p o s i t i v e feedback
mechanism.
Some researchers would choose t o t e s t independently
whether the environmental changes brought about by b i o l o g i c a l production do enhance production i t s e l f . the p o s s i b i l i t y o f unknown, variables.
Such an approach i s however questionable, due t o and t h e r e f o r e u n c o n t r o l l e d i n t e r a c t i o n s among
The o n l y convincing t e s t o f the p o s i t i v e feedback hypothesis would
be t o measure changes i n b i o l o g i c a l production when a s i n g l e c r i t i c a l environmental c h a r a c t e r i s t i c i s manipulated. For example, one should measure the a s s i m i l a t i o n o f p a r t i c u l a t e organic matter by mussels when manipulating the s o l e bottom roughness o r v e r t i c a l t u r b u l e n t d i f f u s i o n , a1 1 the other charact e r i s t i c s o f the system remaining unchanged.
S i m i l a r l y a t the pycnocline, the
s o l e temperature gradient should be manipulated; a t the ice-water i n t e r f a c e , i t should be the r a t e o f i c e m e l t only;
and so on.
Whether such c r i t i c a l
manipulations o f environmental conditions can be t e c h n i c a l l y achieved remains deba tab1 e. A t ergoclines, by d e f i n i t i o n , b i o l o g i c a l production i s not l i m i t e d by the
resources (proximal agents).
Under such conditions, the biomass can t h e o r e t i -
c a l l y develop t o very high concentrations.
As an example, the average concen-
t r a t i o n o f c h l o r o p h y l l i n the surface waters o f the oceans does n o t exceed
1 mg.w3. A t some t i d a l f r o n t s i n the approaches to the English Channel, c h l o r o p h y l l concentrations can reach 100 m g . r 3 (Pingree e t al., 1975); a t the ice-water
interface,
Apollonio,
1965; Alexander e t al.,
that
very
the
these
concentrations 1974).
h i g h concentrations
can
exceed
In such cases,
locally
impede
500
mg.w3
(e.g.
i t i s n o t impossible
biological
production
(self-shading o f autotrophs, competition f o r space among animals, and so on). The r a t e o f production would then become a f u n c t i o n o f the r a t e o f dispersal o f the biomass a t the ergocline. processes,
The dispersal may be e f f e c t e d by physical
such as sedimentation o r turbulence, o r i t may be the r e s u l t o f
b i o l o g i c a l processes such as grazing,
predation, and so f o r t h .
In order t o
t e s t t h i s idea, i t would be necessary t o f a l s i f y the f o l l o w i n g hypothesis: A t ergoclines, the r a t e o f b i o l o g i c a l production i s n o t l i m i t e d by the accumulated biomass, so t h a t i t remains independent o f the r a t e o f dispersal o f t h i s biomass.
25
I t i s easy enough t o t e s t the hypothesis, i f the r a t e o f biomass dispersal i s not the same everywhere along the ergocline. This may occur n a t u r a l l y , i n
d i f f e r e n t parts o f the e r g o c l i n e o r a t d i f f e r e n t times. also be induced experimentally,
Such v a r i a t i o n s can
by s e l e c t i v e removal o f the biomass.
Testing
the hypothesis then r e q u i r e s t o measure simultaneously both production and biomass dispersal rates, along the ergocline. The above hypotheses are examples o f those t h a t can be derived from the ergocline perspective.
The power o f the e r g o c l i n e approach l i e s i n the f a c t
t h a t several b i o l o g i c a l l y productive s t r u c t u r e s can be studied using common hypotheses, ergoclines.
since a l l
these s t r u c t u r e s belong t o the general
The b i o l o g i c a l -
category o f
physical approach t o be developed i n studying
marine ergocl ines correspond t o the terms o f reference described by Legendre and Demers (1984) f o r "dynamic b i o l o g i c a l oceanography", and i t i s expected t h a t the ergocline perspective w i l l shed a new l i g h t on the mechanisms t h a t govern b i o l o g i c a l production i n the oceans.
ACKNOWLEDGMENTS The d e f i n i t i o n o f ergoclines and the general e r g o c l i n e hypothesis were discussed during a workshop, organized by SCOR Working Group 73 i n Liege, from
17 t o 19 May 1985, f o l l o w i n g the Seventeenth I n t e r n a t i o n a l Liege Colloquium on Ocean Hydrodynamics. The authors wish to thank Prof. E. Bourget ( U n i v e r s i t e Laval, QuBbec), Drs.
M.
Frechette,
L.
Fortier,
J.C.
T h e r r i a u l t and Mr.
M.
Levasseur (P6ches e t Oceans Canada, Quebec) and Prof. S. F r o n t i e r ( U n i v e r s i t 6 des sciences e t techniques de L i l l e ) f o r t h e i r most useful suggestions.
A
grant from the Natural Sciences and Engineering Research Council o f Canada t o the f i r s t author was instrumental i n the completion o f t h i s work.
REFERENCES Horner, R. and Clasby, R.C., 1974. Metabolism o f A r c t i c sea Alexander, V., i c e organisms. Rep. I n s t . mar. Sci. Univ. Alaska, R74-4, 120 p. Apollonio, S., 1965. Chlorophyll i n A r c t i c sea-ice. A r c t i c , 18: 118-122. Auclair, J.C., Demers, S., Frechette, M., Legendre, L. and Trump, C.L., 1982. High frequency endogeous p e r i o d i c i t i e s o f c h l o r o p h y l l synthesis i n estuarine phytoplankton. Limnol Oceanogr. , 27: 348-352. Bah, A. and Legendre, L., 1985. Biomasse phytoplanctonique e t m6lange de maree dans l ' e s t u a i r e moyen du Saint-Laurent. N a t u r a l i s t e can., 112:
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26
Bowman, M.J., Kibblewhite, A.C. and Ash, D.E., 1980. M, t i d a l e f f e c t s i n Greater Cook S t r a i t , New Zeland. J. geophys. Res., 85: 2728-2742. Carefoot, T., 1977. Pac.ific seashores. Douglas, Vancouver, Canada, 208 p. Cullen, J.J., 1982. The deep c h l o r o p h y l l maximum: comparing v e r t i c a l p r o f i l e s o f c h l o r o p h y l l a. Can. J. Fish. aquat. Sci., 39: 791-803. Cushing, D.H., 1972. The-production c y c l e and the number o f marine f i s h . Symp. ZOO^. SOC. London, 29: 213-232. 1976. Settlement, growth, and production o f the mussel, M y t i l u s Dare, P.J., e d u l i s L., i n Morecambe Bay, England. Fishery I n v e s t i g a t i o n s , Minis. A g r i c u l t u r e , F i s h e r i e s and Food, London, Ser. 11, 28: 25 pp. Demers, S., Legendre, L. and T h e r r i a u l t , J.C., 1985. Phytoplankton responses t o v e r t i c a l t i d a l mixing. I n : M.J. Bowman, W.T. Petersen and C.M. Yentsch ( E d i t o r s ) , T i d a l mixing an plankton dynamics. Springer-Verlag, New York, i n press. Demers, S . , T h e r r i a u l t , J.C. and B o u r g e t , E., ms. Phytoplanktonic p r o d u c t i v i t y o f the l i t t o r a l zone: T u r b i d o s t a t analogy. I n preparation. Dugdale, R.C. 1967. N u t r i e n t l i m i t a t i o n i n the sea: dynamics, i d e n t i f i c a t i o n , and s i g n i f i c a n c e . Limnol. Oceanogr. 12: 685-695. Eppley, R.N. and Peterson, B.J., 1979. P a r t i c u l a t e organic matter f l u x and p l a n k t o n i c new production i n the deep ocean. Nature (London), 282: 677-680. F o r t i e r , L., 1982. Environmental and behavioral c o n t r o l o f large-scale d i s t r i b u t i o n and l o c a l abundance o f ichthyoplankton i n the S t . Lawrence Estuary. Ph. D. Thesis, M c G i l l Univ., Montreal, Quebec, 162 p. F o r t i e r , L. and Legendre, L., 1979. Le c o n t r t i l e de l a v a r i a b i l i t e a c o u r t terme du phytoplancton estuarien: s t a b i l it e v e r t i c a l e e t profondeur c r i t i q u e . J. Fish. Res. Board Can., 36: 1325-1335. Fournier, R.O., van Det, M., Wilson, J.S. and Hargreaves, N.B., 1979. I n f l u e n c e o f the shelf-break f r o n t o f f Nova Scotia on phytoplankton standing stock i n winter. J. Fish. Res. Board Can., 36: 1228-1237. The importance o f d i f f u s i o n i n supplying phytoplankton Frechette, M., ms.a. t o benthic suspension feeders. I n preparation. Food a v a i l a b i l i t y f o r an i n t e r t i d a l M y t i l u s e d u l i s L. Frechette, M., ms.b. population : short-term re1a t i onshi ps. I n preparation. Energy f l o w between the pelagic and Frechette, M. and Bourget, E., 1985. benthic zones: f a c t o r s c o n t r o l l i n g p a r t i c u l a t e organic matter a v a i l a b l e t o an i n t e r t i d a l mussel bed. Can. J. Fish. aquat. Sci., 42: 1158-1165. Frechette, M. and Bourget, E., ms. The s i g n i f i c a n c e o f small-scale spatio-temporal heterogeni t y i n phytoplankton abundance f o r benthic energy flow. I n preparation. Frechette, M. and Legendre, L. , 1982. Phytoplankton photosynthetic response t o l i g h t i n an i n t e r n a l t i d e dominated environment. Estuaries, 5: 287-293. Keeley, J.R. and Greenberg, D.A., 1978. T i d a l mixing versus Garrett, C.J.R., thermal s t r a t i f i c a t i o n i n the Bay o f Fundy and G u l f o f Maine. Atm. Ocean 16: 403-423. 1981. Dynamical aspects o f shallow sea G a r r e t t , C.J.R. and Loder, J.W., f r o n t s . P h i l . Trans. R. SOC. Lond., A302: 563-581. G i l m a r t i n , M., 1964. The primary production o f a B r i t i s h Columbia f j o r d . J. Fish. Res. Board Can., 21: 505-538. Ginsburg, R.N. and Lowenstam, H.A., 1958. The i n f l u e n c e o f marine bottom comnunities on the depositional environment o f sediments. J. Geol., 66: 310-318. Pingree, R.D. and S i n c l a i r , M., 1981. Sumner t i d a l f r o n t s i n G r i f f i t h s , D.K., t h e near-Arctic regions o f Foxe Basin and Hudson Bay. Deep-sea Res., 28: 865-873. 1978. Photosynthesis, p r o d u c t i v i t y and growth. The physiologiHarris, G.P., c a l ecology o f phytoplankton. Arch. Hydrobiol. Beih. Ergeb. Limnol., 10: 1-171.
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31
BIOLOGICAL PRODUCTION AT THE ICE-WATER ERGOCLINE
*
LEGENDREZ, J.C. THERRIAULTl and R.G. INGRAM3 1 Centre Champlain des sciences de l a mer, M i n i s t g r e des P k h e s e t des Ocians, C.P. 15500, 901 Cap Diamant. Quebec, Quebec G1K 7Y7, Canada. 2 Gpartement de b i o l o g i e , U n i v e r s i t e Laval, Quibec. Quebec G1K 7P4, Canada.
S. DEMERS1,L.
I n s t i t u t e o f Oceanography, M c G i l l U n i v e r s i t y , 3620 U n i v e r s i t y , Montreal , Quebec H3A 282, Canada.
ABSTRACT The ice-water i n t e r f a c e i s t h e s i t e o f h i g h m i c r o a l g a l p r o d u c t i v i t y . These microalgae c o n s t i t u t e an i m p o r t a n t p a r t o f t h e p r o d u c t i v i t y o f p o l a r seas. The growth o f i c e microalgae d u r i n g t h e s p r i n g and perhaps d u r i n g t h e autumn extends t h e short growing season i n t h e water column. Herbivores have been observed t o a c t i v e l y feed on t h e i c e microalgae. Sea-ice microalgae respond t o v a r i a t i o n s i n s a l i n i t y (which c o n t r o l s biomass and taxonomic composition i n c o a s t a l areas i n f l u e n c e d by freshwater r u n o f f 1 , temperature ( t h e s u r v i v a l o f microalgae depends on t h e i r a b i l i t y t o develop a p r o t e c t i o n mechanism a g a i n s t f r e e z i n g ) . l i g h t ( t h e p h o t o s y n t h e t i c a c t i v i t y o f i c e microalgae i s a f u n c t i o n o f b o t h l i g h t i n t e n s i t y and q u a l i t y ) and n u t r i e n t s ( n u t r i e n t l i m i t a t i o n has been demonstrated even when ambient n u t r i e n t concentrations were h i g h ) . The b i o l o g i c a l p r o d u c t i o n a t t h i s e n e r g e t i c i n t e r f a c e i s examined i n t h e s p e c i f i c c o n t e x t o f t h e ice-water ergocline.
According t o t h e hypothesis o f Legendre and Demers (1985) and Legendre e t a1
.
( t h i s book), e n e r g e t i c i n t e r f a c e s ( e r g o c l i n e s ) a r e p r e f e r e n t i a l s i t e s f o r b i o l o g i c a l production i n t h e oceans. The ice-water i n t e r f a c e i s one example o f such e r g o c l i n e s . I n t h i s paper we w i l l review t h e major c h a r a c t e r i s t i c s o f b i o l o g i c a l p r o d u c t i o n a t the ice-water i n t e r f a c e .
SEA-ICE MICROALGAE The f i r s t mention o f c o l o n i z a t i o n o f t h e A r c t i c sea-ice by microalgae date from
*Contribution t o t h e program o f GIROQ (Groupe i n t e r u n i v e r s i t a i r e d6 recherches oceanographiques du Quebec).
32 the mid 1800's.
when Ehrenberg (1841,
1853) l i s t e d the diatoms c o l l e c t e d during the
research t r i p s o f S i r John F r a n k l i n i n the Canadian A r c t i c Archipelago. observations i n the A n t a r c t i c are a t t r i b u t e d t o Hoocker (1847).
The f i r s t
Since t h a t time, an
impressive body o f l i t e r a t u r e has accumulated on sea-ice microalgae. The papers published before 1960 were however mainly descriptive, and considered the sea-ice microalgae merely as a c u r i o s i t y .
A f t e r 1960, the i n t e r e s t i n the physiological
aspects o f microalgal growth i n the sea-ice rose considerably and researchers began t o i n v e s t i g a t e t h e physiological
adaptations
o f those algae t o such an extreme
environment (Apollonio, 1961). Three types o f i c e microalgal comnunities are u s u a l l y recognized, depending on the l e v e l a t which the maximum biomass i s observed i n the i c e column (Ackley e t al.. 1979).
These c o r n u n i t i e s are defined as ( 1 ) the snow comnunity, ( 2 ) the i c e i n t e r i o r
cornunity and ( 3 ) the "epontic" o r i c e bottom cornunity. The snow comnunity was f i r s t described by Meguro (1962) and i s observed when the weight o f snow depresses the i c e surface below the water l e v e l , causing a f l o o d o f water containing microalgae through the ice. Few algae are seen i n t h i s layer. The i c e i n t e r i o r cornunity (Fig. 1 ) r e s u l t s from the trapping o f i c e bottom c e l l s i n the i c e m a t r i x as the thickness o f the i c e increases over time (Hoshiai, 1969; Ackley e t
.
These microalgae c o n s t i t u t e a1 , 1979). probably n o t growing a f t e r being trapped The r a t e o f freezing o f the i c e determines 1977; Demers e t al., 1984). The highest
v e s t i g i a l populations since the c e l l s are i n the i c e matrix (Ackley e t al., 1979). the c e l l density i n s i d e the i c e (Grainger. chlorophyll biomasses are observed a t the
bottom o f the i c e (Apollonio, 1965; Bunt, 1963, 1968; Bunt and Wood, 1963; Meguro e t al., 1967; Poulin e t al., 1983; Demers e t al., 1984) (Table 1). The i c e bottom cornunity i s the most m e t a b o l i c a l l y a c t i v e (Palmisano e t al.,
1985).
Microalgal
growth occurs a t the ice-water i n t e r f a c e , i n the unconsolidated i c e l a y e r (Meguro e t al.,
1967; Alexander e t al.,
(Bunt, 1963).
1974) and/or i n the i n t e r s t i t i a l water o f the i c e m a t r i x
I n the A r c t i c , the microalgae form a coloured l a y e r 1 t o 4 cm t h i c k a t
the bottom o f the ice,
w h i l e i n the A n t a r c t i c ,
under-ice slush formed by a l a y e r (0.2 (Bunt. 1963; Bunt and Wood,
microalgal growth occurs i n the
t o 1.0 m) o f unconsolidated i c e c r y s t a l s
1963; Andriashev,
1968;
Hoshiai, 1972; Gruzov, 1977).
A t the beginning o f the spring growth period,
the epontic comnunity i s u s u a l l y composed o f a mixed population o f pelagic and benthic species (Horner and Schrader. 1982;
Gosselin e t al.,
1985;
Rochet e t al.,
1985) but, w i t h time,
the benthic
diatoms become l a r g e l y dominant and may represent as much as 99% o f the epontic cornunity (Bunt and Wood, 1963; Meguro, 1962; Bunt, 1963, 1964. 1968; Burkholder and Mandelli 1965; Poulin and Cardinal, 1982a. b, 1983; Hsiao, 1980). This suggests t h a t i c e algae are present i n the water column i n low abundance, perhaps as r e s t i n g spores. Once trapped i n the i c e w i t h other pelagic species, natural s e l e c t i o n favours species adapted t o the i c e h a b i t a t (Horner and Schrader, 1982). Other microorganisms such as b a c t e r i a ( S u l l i v a n and Palmisano,
1981;
Kaneko e t al.,
1978;
33
CHLOROPHYLL p (rng.m-3)
CHLOROPHYLL g (rng.rn- 3, 0 5 10
0.0 0.5 I.o 0 MANITOUNUK SOUND 2 0 - ARCTIC
WEDDELL S E A
h
w n
IOO-
I20-
Fig. 1 V e r t i c a l d i s t r i b u t i o n o f microalgae ( c h l o r o p h y l l a ) i n t h e sea-ice o f two p o l a r environments. Data from P o u l i n e t a l . (1983: A r c t i c ) and Ackley e t a l . (1979: Antarctica).
TABLE 1 Chlorophyll concentrations i n t h e upper and bottom l a y e r s o f t h e i c e , i n v a r i o u s 1o c a t i ons
.
Chlorophyll concentration (mg m- 3 )
.
Location
References
Upper l a y e r (30-120 cm)
Bottom l a y e r (20 cm)
0.33
31.69
Dunbar and Acreman (1980)
0.24
60.31
Dunbar and Acreman (1980)
0.08
60.31
Dunbar and Acreman (1980)
0.75
109.08
Dunbar and Acreman (1980)
1.87
8.08
Dunbar and Acreman (1980)
0.87
4.55
Demers e t a l . (1984)
0.99
52.1 t o 300.5
0.50
656.00
~~
Robeson Channel, Canada Barrow S t r a i t , Canada Austin Channel, Canada Hudson Bay, Canada Gul f o f St. Lawrence, Canada St.Lawrence Estuary, Canada Frobisher Bay, Canada McMurdo Sound, A n t a r t i ca
~
Hsiao (1980) Palmisano and S u l l i v a n (1983)
34
G r i f f i t h s e t al., 19781, protozoa (Lipps and Krebs, 1984) o r even copepods are a l s o present a t the bottom o f the ice, b u t they never c o n s t i t u t e an important p a r t o f the t o t a l carbon biomass. The biomass a t the bottom o f the i c e thus r e s u l t s from the growth and dispersion o f c e l l s and from the mechanical accumulation o f microalgae (Horner and Schrader , 1982). ECOLOGICAL SIGNIFICANCE OF SEA-ICE MICROALGAE The growth o f microalgae i n the sea i c e c o n s t i t u t e s an important p a r t o f the primary p r o d u c t i v i t y o f p o l a r seas (Meguro e t al., 1966; Horner and Alexander, 1972; McConville and Wetherbee, 1983; Alexander, 1974; McRoy and Goering, 1974; Clasby e t al.,
1976).
Microalgal growth s t a r t s during the i n i t i a l period o f i c e formation i n
autum, and o f t e n reaches abundances comparable t o those observed i n the spring ( i n the A r c t i c : Horner and Schrader, 1982; i n the A n t a r c t i c : Hoshiai, 1977). Then, microalgal growth a t the i c e bottom slows down gradually and stops completely when very low w i n t e r l i g h t i n t e n s i t i e s are reached. Heterotrophy has been invoked as a mechanism by which the algae would survive the prolonged darkness o f the A r c t i c w i n t e r (Rodhe, 1955; Wilce,
1967; A l l e n 1971). b u t experiments using a v a r i e t y o f
l a b e l l e d organic substrates f a i l e d t o demonstrate any heterotrophic growth f o r the A r c t i c (Horner and Alexander, 1972) o r the A n t a r c t i c (Bunt and Lee, 1972) i c e algae. I t has been suggested t h a t these autumn algae could be a t the o r i g i n o f the i c e i n t e r i o r v e s t i g i a l conmnunity, a f t e r being trapped i n the i c e m a t r i x f o l l o w i n g i c e formation (Poulin e t al., 19831. However, i t i s d i f f i c u l t t o generalize t h i s conclusion, since autumn data are scarce. A t the end o f the w i n t e r p e r i o d and i n growth generally occurs a t the ice-water e a r l y spring, an intense peak o f microalgal i n t e r f a c e when the under-ice l i g h t i n t e n s i t y reaches a minimum c r i t i c a l l i g h t l e v e l ( I c r ) (Table 2). The a c t i v e growth o f i c e algae during s p r i n g and perhaps during autumn extends the short growing season i n the water column (Andriashev, 1968; Bunt, 1968; Allen, 1971; McRoy and Goering, 1974; Horner, 1976, 1977; Bradford,
TABLE 2 C r i t i c a l l i g h t l e v e l s below which the photosynthetic a c t i v i t y o f i c e microalgae was undetectable, a t various locations. Location Manitounuk Sound, (Hudson Bay) Barrow S t r a i t , (Alaska) Narwhal Island, (A1aska) McMurdo Sound, (Antarctica)
Critical l i g h t level 1Sinst.m-2. s- 1 7.6 2.3 t o 9.3 7.7 0.3 t o 0.9
References Gosselin e t a l . (1985)
.
Alexander e t a1 (1974) Clasby e t a l . (1973) Horner and Schrader (1982) Bunt (1964) Pal m i sano and Sul 1ivan (1983)
35
1978).
As a matter o f fact,
the h i g h concentration o f i c e microalgae a t the ice-
water i n t e r f a c e ( 4 t o 656 mg Chl a.m-3:
Table 1) represents a s i g n i f i c a n t f r a c t i o n
o f the primary biomass o f p o l a r ecosystems, where chlorophyll concentrations i n the water column are generally low ( < 1 mg.m-3:
.,
Horner and Schrader, 1982; Bunt and Lee,
1977). The biomass o f i c e algae i s o f t e n comparable t o 1970; Holm-Hansen e t a1 water column values i n t e g r a t e d over the photic l a y e r i n many productive planktonic systems (Table 3 ) .
The annual carbon production per u n i t area o f i c e microalgae i s
comparable t o the pelagic production o f the A r c t i c Ocean (Table 4). t i v i t y o f the i c e algae i s o n l y
However, produc-
- 3% o f the estimated t o t a l annual primary production
i n the water column, since young i c e o n l y covers a small area o f the A r c t i c Ocean (Table 5 ) .
Some i s o l a t e d studies i n the Southern Bering Sea have, however, reported
very high production values a t the i c e edge (600 t o 725 mg C.m-2.h-1: al.,
Niebauer e t
1981, as reported by Subba Rao and P l a t t . 19841, compared t o values (0.3 t o 57
mg C.m-2.h-1)
reported previously i n the l i t e r a t u r e (Clasby e t al.,
1973; Matheke and
Horner , 1974). Although no comprehensive studies have been c a r r i e d o u t i n s i t u t o evaluate the u t i l i z a t i o n o f the i c e microalgal biomass by d i f f e r e n t grazers, some observations suggest t h a t herbivores can feed on i c e algae during springtime when water column p r o d u c t i v i t y i s low.
Andriashev (1968) r e p o r t s t h a t a dozen animal species use the
lower l a y e r o f the A n t a r c t i c f a s t ice, both as refuge and feeding ground.
Richardson
and Whitaker (1979) a l s o r e p o r t grazing o f i c e microalgae by the amphipod Pontaginia antarctica a t the ice-water interface. Cross (1982) and Grainger e t a l . (1985) found high densities o f nematodes and h a r p a c t i c o i d and cyclopoid copepods associated w i t h the under-ice microalgae. These under-ice organisms are believed t o represent a l i n k i n the t r a n s f e r o f energy from i c e a l g a l production t o amphipods t o sea-birds and other mamals l i v i n g i n t h e A r c t i c Gull iksen , 1984).
(Horner,
1976;
Bradstreet and Cross,
1982;
RESPONSES OF SEA-ICE MICROALGAE TO ENVIRONMENTAL VARIATIONS S a l i n i t y , temperature, l i g h t and n u t r i e n t s have been i d e n t i f i e d as the main factors r e g u l a t i n g the growth o f sea-ice mfcroalgae. i c e algae u s u a l l y
grow under r e l a t i v e l y
However, u n l i k e phytoplankton,
s t a b l e conditions
of
temperature
s a l i n i t y , and they are n o t subjected t o h i g h frequency l i g h t f l u c t u a t i o n s .
and
Nutrients
are generally believed t o be the f a c t o r s which are the most influenced by hydrodynamical variations.
N u t r i e n t concentrations a t the ice-water i n t e r f a c e are however
generally high and most researchers have n o t considered them as l i m i t i n g . Sal in i ty A number o f studies have suggested t h a t s a l i n i t y was n o t important i n r e g u l a t i n g the growth o f the epontic conmunity.
For example, Grant and Horner (1976) reported
t h a t i c e diatoms were able t o grow i n a wide range o f s a l i n i t i e s ( 5 t o 60).
Meguro
36 TABLE 3 Chloro h y l l concentrations i n some productive planktonic systems (values i n t e g r a t e d over tRe p h o t i c zone) and a t the ice-water interface.
Location
Chlorophy!l concentration mg.m-2
~~~~~~~~~~
References
~
Planktonic systems St.Lawrence Estuary Puget Sound Hudson River Estuary
2 t o 230 10 t o 160 150
Therri aul t and Levasseur (1985) Winter e t a1 (1975) Malone (1977)
.
Ice-water i n t e r f a c e Manitounuk Sound, Canada Manitounuk Sound, Canada Robeson Channel, Canada Barrow S t r a i t Alaska Jones Sound, Canada Narwhal Island, Alaska Frobisher Bay, Canada Frobisher Bay, Canada Barrow S t r a i t , Alaska Hudson Bay. Canada McMurdo Sound, A n t a r c t i c a McMurdo Sound, A n t a r c t i c a Factory Cove, Antarctica
0.85 0.76 12.1 12.5 23.0 26.5 30.1 19.6 30.5 39.6 16 t o 34 131 309
.
Gosselin e t a1 (1985) Poulin e t a l . (19831 Dunbar and Acreman 1980) Dunbar and Acreman (1980) Apollonio (1965) Horner and Schrader (1982) Hsiao (1980) Grainger and Hsiao (1982) Clasb e t a1 (1973 Gossefin e t a l . (msl Bunt and Lee (1970) S u l l i v a n and Palmisano (1983) Whi taker (1977)
.
TABLE 4 Annual microal a1 production i n the water column and a t the ice-water i n t e r f a c e o f the A r c t i c 8cean. Location
Annual producti ona gC.m-2.a-1
References
Pelagic production She1f ( < 200 m) Offshore ( > 200 m)
27 9
Moiseev (1971) as reported by Subba Rao and P l a t t (1984) Moiseev (1971) as reported by Subba Rao and P l a t t (1984)
Ice-a1 gal production Barrow A1 aska Chukchr Sea Beaufort Sea Narwhal Is1and Barrow, Alaska Antarctica Syowa Station, Antarctica McMurdo Sound, Antarctica
0.9 t o 22.0 1.4 t o 164.0 5.0 t o 10.0 0.7 5.0 10.0 1.5 t o 3 3 4.16
a Values from Subba Rao and P l a t t (1984)
b
Alexander (1974) Horner and Schrader 1982 Horner and Schrader 119821 Fog (1977) Hoszi a i (1981 Palmisano and S u l l i v a n (1983)
I n Subba Rao and P l a t t (1984), t h i s va!ue i s i n c o r r e c t l y reported as gC.m-2.d-1 and q u a l i f i e d as being s u b s t a n t i a l l y high.
37 TABLE 5 Total annual production o f t h e p e l a g i c system and t h e ice-water i n t e r f a c e o f t h e A r c t i c Ocean, and t h e i r r e s p e c t i v e areas. Areaa
Total production
Location 106 tC.a-1
106 kmz A r c t i c Ocean
205.8
13.1
A r c t i c sea-ice
6.0b
0.6
a Values from Subba Rao and P l a t t (1984) b This value i s c a l c u l a t e d from an average value f o r t h e annual i c e p r o d u c t i o n o f 10 gC.m-2.a-1
e t al.
(1967) found i c e algae t h a t were exposed t o s a l i n i t i e s up t o 45, i n b r i n e
c e l l s near t h e bottom o f t h e i c e i n Barrow S t r a i t . a t very low s a l i n i t y ( - 0.5
t o 7.0)
Grainger (1977) found i c e algae
a t t h e time o f i c e break-up i n F r o b i s h e r Bay.
These studies, however, d i d n o t present any a c t u a l data on t h e growth response o f i c e microalgae t o s a l i n i t y v a r i a t i o n s .
The f a c t t h a t i c e algae do n o t develop i n
freshwater i c e suggests t h a t s a l i n i t y probably has a s t r o n g e f f e c t on t h e growth o f i c e algae i n p o l a r seas.
Suggestions t o t h a t e f f e c t were made by Bunt (1964) and
S u l l i v a n and Palmisano (1981). t h e l a t t e r proposing a s y n e r g e t i c e f f e c t o f temperature and s a l i n i t y on t h e l i g h t response o f t h e algae. P o u l i n e t a1 (1983)
.
observed a decreasing g r a d i e n t o f c e l l abundance w i t h decreasing s a l i n i t y ,
along
transects o f s t a t i o n s i n Manitounuk Sound and i n Hudson Bay o f f t h e mouth o f t h e They explained t h e observed e f f e c t o f s a l i n i t y on Great Whale R i v e r (Fig. 2). microalgae by suggesting ( 1 ) t h a t t h e number o f b r i n e c e l l s , and consequently t h e surface a v a i l a b l e f o r c o l o n i z a t i o n by t h e i c e algae, decreases w it h decreasing s a l i n i t y and ( 2 ) t h a t a taxonomic g r a d i e n t p a r a l l e l s t h e s a l i n i t y g r a d i e n t a t t h e i n t e r f a c e , as fewer taxa were t o l e r a n t t o lower s a l i n i t i e s .
T h i s was supported by
the observation o f a lower number o f taxa i n t h e l e s s s a l i n e waters o f Manitounuk Sound (151 t a x a ) than i n those o f t h e Canadian A r c t i c ( > 200 species). Therefore, i t might be suggested t h a t seasonal v a r i a t i o n s i n s a l i n i t y would n o t s e r i o u s l y a f f e c t the growth o f i c e algae o f f s h o r e , b u t t h a t s a l i n i t y c e r t a i n l y p l a y s an i m p o r t a n t r o l e i n t h e d i s t r i b u t i o n o f i c e algae i n c o a s t a l areas i n f l u e n c e d by freshwater r u n o f f . During t h e p e r i o d o f i c e m e l t i n g , t h e d r a s t i c decrease i n surface s a l i n i t y may have i m p o r t a n t consequences f o r phytoplankton and t h e e n t i r e e p o n t i c community. the time o f i c e m e l t i n g and breakup,
At
t h e a l g a l biomass produced a t t h e ice-water
i n t e r f a c e i s released i n t h e water column.
There i s c o n f l i c t i n g evidence as t o t h e
r e l a t i v e importance o f these i c e microalgae f o r t h e i n i t i a t i - o n and f o r m a t i o n o f t h e s p r i n g bloom i n t h e water column (Horner, 1976; Grainger, 1977).
Some papers suggest
38
Y
I
0.0
0.4
I .2
0.8 ICE
I .6
SALINITY
Fig. 2 Chlorophyll a concentration (sea-ice microalgae) i n Manitounuk Sound, Hudson Bay, as a function o f i c e s a l i n i t y . Data from Poulin e t a l . (1983).
t h a t the phytoplankton bloom i n the water column i s d i r e c t l y i n i t i a t e d by the release o f i c e microalgae from the melting ice, whereas others c l a i m t h a t the release o f c e l l s i n t o the underlying water i s not a major source f o r the spring phytoplankton bloom.
However, according t o A l l e n (19711, low s a l i n i t y k i l l s or seriously a f f e c t s
the diatoms growing under the ice, and the population does n o t f u l l y recover u n t i l v e r t i c a l mixing restores the s a l i n i t y o f the surface waters. (1972). Legendre e t a l .
Using data from Horner
(1981) have shown t h a t low s a l i n i t i e s do not preclude high
photosynthetic a c t i v i t y , which i s contrary t o the contention o f A l l e n (1971). Saito and Taniguchi (1978) consider the m e l t water as a p o s i t i v e t r i g g e r f o r the under-ice bloom because, a t the same time as i c e algae are released from the ice, a bloom occurs i n the shallow surface l a y e r which i s s t a b i l i z e d by the presence o f low A s i m i l a r p o s i t i v e e f f e c t o f reduced s a l i n i t y on phytoplankton s a l i n i t y waters. blooms was also observed a t i c e edges (Alexander and Niebauer, 1981; El-Sayed and
Taguchi, 1981; Smith and Nelson, 1985).
I n Manitounuk Sound, Legendre e t a l . (1981)
suggested t h a t the phytoplankton bloom under the i c e probably r e s u l t e d from the simultaneous deepening o f both the photic l a y e r (seasonal l i g h t increase) and the s t r a t i f i e d l a y e r ( l o w - s a l i n i t y m e l t i n g water).
The co-occurrence o f i c e microalgae
and phytoplankton i n the blooming water column would considerably increase the a l g a l biomass, under conditions o f i c e m e l t .
S i m i l a r r e s u l t s were found i n the Chukchi Sea
(Hameedi. 19781, i n Stefansson Sound and a t Narwhal I s l a n d (Horner and Schrader, 1982). Other studies have suggested t h a t the increased algal biomass (chlorophyll a) during the i c e m e l t does n o t necessarily r e s u l t i n an increase o f the p r o d u c t i v i t y of
39 the water column (Clasby e t al..
1973; Grainger. 1977).
Microscopic examinations o f
c e l l s collected during i c e m e l t i n g i n the A r c t i c have i n d i c a t e d t h a t the i c e microalgae i n the water column were not healthy (Horner, 1977; Horner and Schrader. 1982). I n contrast, !Smith and Nelson (1985) suggested t h a t the epontic algae could play an important r o l e as a bloom inoculum. They found t h a t N i s t c h i a curta, a menber o f the i c e comunity, was present i n s i g n i f i c a n t numbers and photosynthetically active (microautoradiographic analysis) i n the bloom. We believe t h a t the discrepancies between t h o s e s t u d i e s may have r e s u l t e d f r o m t h e d i f f e r e n t methodologies used f o r assessing the a c t i v i t y o f the c e l l s . Also, many o f the contradictions reported above may have been caused by the t i m e o f sampling, r e l a t i v e
-
-
t o the sequence o f m e l t i n g (release o f i c e algae) stratification phytoplankton bloom (other species than the i c e microalgae), o r because no r e a l under-ice bloom o f phytoplankton was present (Legendre e t a1 , 1981). Physical processes t h a t modify the under-ice s a l i n i t y include b r i n e drainage during the period o f i c e growth, m e l t water i n p u t during i c e m e l t and freshwater runoff. The actual s a l i n i t y values depend on ambient water c h a r a c t e r i s t i c s and the degree o f mixing. Since the water temperature i s u s u a l l y close t o freezing, the s a l i n i t y f i e l d determines the temperature a t the interface. Across the i c e cover, downward heat f l u x values vary from negative t o p o s i t i v e over the i c e season.
.
Tenperatur e The a b i l i t y t o grow under conditions o f low i n s i t u i r r a d i a n c e and temperature i s determinant f o r the survival o f sea i c e microalgae and i s an important f a c t o r i n the control o f species succession and c o m u n i t y structure. A nunber o f studies have reported physiological adaptations o f a1 gae growing under extreme l i g h t and temperature conditions (Neori and Holm-Hansen. 1982; P l a t t e t al., 1982; L i e t al., 1984). For example, Bunt (1968) showed i n laboratory t h a t sea i c e algae from the Antarctic were extremely shade-adapted and o b l i g a t e psychrophilic i n t h e i r temperature response. Seaburg e t a l . (1981) found d i f f e r e n t growth rates i n the temperature range from 2 t o 34'C. f o r d i f f e r e n t species among 35 taxa i s o l a t e d from the Antarctic sea ice. It i s not known whether o r t o what extent net photosynthesis i s l i m i t e d by low temperature a t the low l i g h t i n t e n s i t i e s p r e v a i l i n g under the sea ice, b u t n e t carbon f i x a t i o n by sea i c e algae i s d e f i n i t i v e l y temperature dependent, p a r t i c u l a r l y i n the range -1.5' C t o 5' C (Bunt, 1964; S u l l i v a n and Palmisano, 1981). A d i s t i n c t i v e psychrophilic acclimation caused by the seasonally increasing l i g h t i n t e n s i t y was observed by Rochet e t al. (1985) f o r sea i c e algae i n Hudson Bay (Fig.
3). They suggested t h a t t h i s acclimation was occurring because o f the increasing s e n s i t i v i t y o f photosynthesis t o low temperature as l i g h t i n t e n s i t y increases. Rochet e t a l . (1985) also found a seasonal decrease i n species d i v e r s i t y , which might suggest t h a t the survival o f sea i c e algae i n the i c e t r a b i t a t i s s t r o n g l y dependent on t h e i r a b i l i t y t o develop a p r o t e c t i o n mechanism against freezing (Horner and
40
1 Y I
X 0
mE a
X
mE a
0.04
0
I
I
10
20
I
30
LIGHT INTENSITY AT THE ICE WATER INTERFACE ( p E i n s t. rn -2. s I )
-
Fig. 3 Ratio o f maximum photosynthetic r a t e per u n i t c h l o r o p h y l l a o f sea-ice microalgae (PRax 1, measured a t C'3 and -0.5'C (ambient temperature), p l o t t e d as a f u n c t i o n o f irradiance (I,) a t the ice-water i n t e r f a c e i n Hudson Bay. Data from Rochet e t al. (1985).
Schrader, 1982).
Therefore,
natural s e l e c t i o n probably favours species capable o f
growing a t low temperature i n t h e i c e h a b i t a t , seasonally increases. extremely h o t waters
as the ambient l i g h t i n t e n s i t y
A p a r a l l e l can be made w i t h the microalgae t h a t l i v e i n (hot
springs),
environmental temperature (Brock,
where
1967a.b).
optimal
growth a l s o occurs
a t the
As there i s no form o f a d a p t a b i l i t y
e n t i r e l y without b i o l o g i c a l c o s t (Conrad. 19831, continued photosynthesis a t f r e e z i n g temperature,
when the l i g h t i n t e n s i t y increases,
tends t o narrow the range of
temperatures w i t h i n which optimum photosynthesis can occur. t h i s physiological
adaptation
i s n o t y e t known,
Even i f the mechanism of
some studies have suggested a
r e l a t i o n s h i p w i t h i n t r i n s i c c e l l c h a r a c t e r i s t i c s (Ben-Amotz and Gilboa, 1980) o r w i t h the synthesis o f s p e c i f i c enzymes (e.g. 1982).
RuBPC) a t low temperatures ( L i and Morris,
The acclimation o f i c e microalgae t o low temperature conditions opens a new
avenue f o r research, which could be determinant i n understanding the ecology of the ice-water interface.
41
141
I210-
-
h
I
r 1
I
i
8-
F
6-
0
p Y
4-
X
0
mE
a
2-
0 0
I
I
I
I
1
I
I
1 0
20
30
40
50
60
70
Iz (pEinst. m-?s-' 1 Fig. 4 Maximum p h o t o s y n t h e t i c r a t e per u n i t c h l o r o p h y l l a (P#ax) o f sea-ice microalgae i n Manitounuk Sound, Hudson Bay, as a f u n c t i o n o f t h e i r r a d i a n c e (I,) a t the ice-water i n t e r f a c e . Freehand adjusted curve. Data from Gosselin e t a l . (1985 1.
2.4CI
II)
2.0-
-F
1.6-
01
-I > 1.2I a
0
E 0.8
sr
0 0.4-
0
I
I
IZ
I
I
I
I
1
(pEinst. rn-2.s-1 1
Fig. 5 C h l o r o p h y l l a c o n c e n t r a t i o n (sea-ice microalgae) i n Manitounuk Sound. Hudson Freehand Bay, as a f u n c t i o n o f t h e i r r a d i a n c e (I,) a t t h e ice-water i n t e r f a c e . adjusted curve. Data from Gosselin e t a l . (1985).
42
The i n t e n s i t y o f l i g h t a t t h e i c e - w a t e r i n t e r f a c e i s low, t y p i c a l l y 0.021, o f t h a t i n c i d e n t t o the surface o f the ice. For t h i s reason, l i g h t i s considered as a c r i t i c a l f a c t o r f o r r e g u l a t i n g the growth o f i c e algae
-
(Meguro e t al., Hsiao,
1980;
1967; Bunt and Lee,
Horner and Schrader,
1970; Clasby e t al.,
1976; Grainger.
1982; .Gosselin e t al.,
1985).
1979;
The under-ice
i l l u m i n a t i o n i s mainly c o n t r o l l e d by a i r temperature and snow depth
.,
(Gosselin
1985). since the o p t i c a l properties o f sea i c e and snow depend on a i r e t a1 temperature (Grenfell and Maykut. 1977; Grenfell , 1983). The i c e albedo decreases w i t h increasing a i r temperature, while snow albedo decreases w i t h an increase i n the proportion o f i t s l i q u i d phase.
The seasonal changes i n l i g h t
i n t e n s i t y are r e f l e c t e d i n the photosynthetic response o f the i c e algae (Fig. 41,
i n the chlorophyll
5 ) and i n species d i v e r s i t y (Gosselin e t
biomass (Fig.
al., 1985) which could r e s u l t from a synergetic e f f e c t o f l i g h t and temperature ( S u l l i v a n and Palmisano, 1981; Rochet e t al., 1985). Algal growth i n the i c e does n o t s t a r t before some c r i t i c a l
l i g h t level
i s reached (Table 2).
(Icr)
The
difference between A r c t i c and Antarctic c r i t i c a l values may be r e l a t e d t o differences i n species composition o r i n the physical c h a r a c t e r i s t i c s o f the sea i c e (McConville and Wetherbee, 1983; Clarke and Ackley,
I c e algae were found t o be markedly
1984).
shade adapted (Bunt 1964; Bunt, 1968; Burkholder and Mandelli , 1965). o f microalgae a t the ice-water i n t e r f a c e was generally observed a t p h o t o i n h i b i t i o n occured a t
- 10000 l u x (Bunt,
I n t e r e s t i n g l y , Gosselin e t a l .
1964;
Maximal growth
- 1000 l u x and
Burkholder and Mandelli. 1965).
(1985) observed t h a t the photosynthetic parameter I k
was approximately equal t o the under-ice l i g h t (1,) (Fig. 61, suggesting a strong adaptation o f epontic c e l l s t o the ambient l i g h t conditions. As the snow cover presents great v a r i a b i l i t y expected variable.
that
the
horizontal
light
i n time and i n space,
d i s t r i b u t i o n under
the
it i s
be q u i t e
This i s the cause o f an important small-scale s p a t i a l heterogeneity i n the
horizontal d i s t r i b u t i o n o f i c e microalgae (Alexander e t al., 1976; Horner and Schrader, Gosselin e t a l .
1982; Bunt and Lee,
during
1974;
Clasby e t al.,
1970; Sasaki and Watanabe,
1984).
(ms) were able t o demonstrate a r e l a t i o n s h i p between the s p a t i a l
d i s t r i b u t i o n o f i c e algae and snow depth i n Hudson Bay. changed
ice w i l l
the
season.
At
the
beginning
of
This r e l a t i o n s h i p however the
growing season
(when
I , > I c r ) maximum a l g a l biomass was observed under areas covered by the smallest snow depths. Towards the end o f the season, when the s o l a r i n p u t was much higher, maximum algal biomass was observed under areas covered by the deepest snow. This suggests t h a t i c e algae have two c r i t i c a l l i g h t levels: a minimum l e v e l and a
i s the c r i t i c a l l i g h t i n t e n s i t y below which 7.6 &in.m-2.s-1 f o r Hudson (Icr Bay), and the maximum l e v e l a t a given time corresponds t o the l i g h t i n t e n s i t y above
maximum one.
The minimum l e v e l
photosynthetic a c t i v i t y
is
n o t detectable
-
43
which photosynthetic a c t i v i t y i s i n h i b i t e d .
The snow cover thus extends the length
o f the growing season f o r the epontic comunity,
by o f f e r i n g areas where l i g h t
i n t e n s i t y i n springtime i s compatible w i t h the physiological l i m i t s o f the c e l l s . The observed v a r i a b i l i t y
i n the d i s t r i b u t i o n o f i c e algae might correspond t o
d i f f e r e n t stages o f development o f the epontic comunity, as was suggested f o r phytoplankton by Steele (1978). The snow cover, by i t s o p t i c a l and i n s u l a t i n g properties, thus protects the i c e m i c r o f l o r a against photoinhibition, and delays the dispersion o f the c e l l s by desaltation. I n addition t o changing the l i g h t i n t e n s i t y , the snow cover also changes the spectral q u a l i t y o f the under-ice irradiance (Thomas, 1963). Maykut and Grenfell (1975) showed t h a t the spectral d i s t r i b u t i o n o f l i g h t ranged between 430 and 550 M a t the ice-water i n t e r f a c e , so t h a t the red p a r t o f the spectrum was h i g h l y attenuated by the i c e cover. This could have an important impact on the pigment composition and the photosynthetic a c t i v i t y o f the i c e algae, since i t has long been recognized t h a t l i g h t q u a l i t y plays an important r o l e f o r phytoplankton ( A t l a s and Bannister, 1980; Vesk and Jeffrey, 1977; Haxo. 1960). For example, Wallen and k e n (1971a,b) found t h a t the photosynthetic a c t i v i t y o f the diatom C y c l o t e l l a nana and
80
60
-
-
;40 I
E
?;
i
r a0.78
n
1. I IX
.-c w
=t
v
Y
20
I4
0
I
I
I
40
1
20
60
80
Iz ( p Einst. m-2.s-' 1 Fig. 6 Photosynthetic parameter I k o f sea-ice microalgae i n Manitounuk Sound, Hudson Bay, as a f u n c t i o n of the irradiance (Iz) a t t h e ice-water i n t e r f a c e . Data from Gosselin e t al. (1985).
44
t h e green a l g a O u n a l i e l l a t e r t i o l e c t a was h i g h e r under b l u e l i g h t than under green o r white
light.
Faust e t
al.
(1982)
also
found h i g h e r growth
mariae-lebouriae under b l u e and r e d l i g h t than under w h i t e l i g h t . composition (Vesk and J e f f r e y , F u j i t a and H a t t o r i , l i g h t quality.
of
Prorocentrum
Changes i n pigment
1977; Jones and Myers, 1965; Brody and Emerson, 1959;
1959). growth r a t e and r e s p i r a t i o n were a l s o shown t o depend on
The e f f e c t o f l i g h t q u a l i t y on t h e growth o f i c e algae has never been
s t u d i e d before,
b u t Rochet e t a l .
(ms)
r e c e n t l y showed changes i n t h e pigment
composition o f microalgae i n Hudson Bay, i n response t o l i g h t q u a l i t y . Nutrients Several researchers have concluded t h a t n u t r i e n t s probably d i d n o t l i m i t t h e growth o f t h e e p o n t i c c o r n u n i t y (Meguro e t al., 1967; Bunt and Lee, 1970; Alexander e t al., 1974; S u l l i v a n and Palmisano, 1981; Horner and Schrader, 1982; P o u l i n e t al., 1983; Clarke and Ackley, 1984; Holm-Hansen e t al., 1977). The c o n c e n t r a t i o n o f d i s s o l v e d n u t r i e n t s i n t h e i c e was found t o be about one order o f magnitude h i g h e r than i n t h e u n d e r l y i n g water (Horner, 1976; Demers e t a1 1984). Three processes
.,
are
usually
invoked
t o explain
nutrient
abundance
i n the
i c e habitat:
(1)
replenishement o f n u t r i e n t s by exchange between t h e lower p a r t o f t h e i c e and t h e u n d e r l y i n g water, ( 2 ) d e s a l t a t i o n and ( 3 ) i n s i t u b a c t e r i a l r e g e n e r a t i o n (Meguro e t al., 1967; Alexander e t al., 1974; S u l l i v a n and Palmisano, 1981). Only Grainger (1977. 19791 has ever mentioned t h e p o s s i b i l i t y o f n u t r i e n t l i m i t a t i o n o f i c e algae f o r the Arctic.
Recently, however, bioassays c a r r i e d o u t by M a e s t r i n i e t a l . (ms),
i n southeastern Hudson Bay, microalgae.
gave d i r e c t evidence o f n u t r i e n t l i m i t a t i o n o f t h e i c e
N i t r o g e n was i d e n t i f i e d as the l i m i t i n g n u t r i e n t , b u t s u r p r i s i n g l y ,
l i m i t i n g t h r e s h o l d was q u i t e h i g h ( - 15 p o l N.l-1).
the
Such a h i g h l i m i t i n g t h r e s h o l d
f o r n i t r o g e n had a l s o been observed by M a e s t r i n i e t a l . (1982) f o r b e n t h i c diatoms i n an o y s t e r pond.
I n Hudson Bay, t h e observed low N:P r a t i o s ( - 5.2)
suggest a l a r g e
excess o f phosphate over n i t r o g e n .
However, when n i t r o g e n values reach o r exceed t h e
threshold
silicate
level,
( M a e s t r i n i e t al.,
phosphate ms).
and
might
become t h e
limiting nutrients
I n Manitounuk Sound, Gosselin e t a l . (1985) found evidence
f o r a r e l a t i o n s h i p between t h e p h o t o s y n t h e t i c e f f i c i e n c y
( $ 1 o f t h e i c e algae and
phosphate replenishment o f t h e ice-water i n t e r f a c e by f o r t n i g h t l y t i d a l mixing, thus suggesting a p o s s i b l e phosphate l i m i t a t i o n o f m i c r o a l g a l growth. Cota e t a l . (ms), by i n d i r e c t c a l c u l a t i o n s , showed t h a t a s u b s t a n t i a l f l u x o f n u t r i e n t s from t h e u n d e r l y i n g water i s necessary t o s u s t a i n t h e observed r a t e o f increase i n p l a n t biomass. concentrations over 30 mg
Gosselin e t a l . (ms) observed, i n Hudson Bay, c h l o r o p h y l l m-2.
Assuming no losses, we can conclude from t h e N:Chl
r a t i o s t h a t a t l e a s t 150-300 mg N were f i x e d by microalgae per cquare metre. r e g e n e r a t i o n and d e s a l t a t i o n had been involved,
I f only
t h e water forming t h e i c e ( - 2m
t h i c k ) should have contained a t l e a s t 75-150 mg N m-3,
values which a r e much h i g h e r
45
.
than those r e p o r t e d f o r t h i s area (Legendre e t a1 , 19811. Thus. a c t i v e upward n u t r i e n t t r a n s p o r t from t h e water column t o t h e ice-water i n t e r f a c e i s r e q u i r e d f o r the growth o f t h e i c e algae. (19851,
mentioned above,
T h i s i s supported by t h e f i n d i n g s o f Gosselin e t a l .
which
indicated
n u t r i e n t enrichment
of
the
ice-water
i n t e r f a c e by f o r t n i g h t l y t i d a l mixing. I n Barrow S t r a i t , l o w frequency f l u c t u a t i o n s o f the growth o f i c e algae were a l s o r e l a t e d t o n u t r i e n t pulses, d r i v e n by t h e t i d e s and atmospheric events (Cota e t al.,
ms).
The b i o l o g i c a l dynamics o f t h e ice-water
i n t e r f a c e i s t h e r e f o r e coupled t o t h e hydrodynamics o f t h e u n d e r l y i n g waters. The r a t e o f n u t r i e n t t r a n s p o r t a t t h e ice-water boundary depends b o t h on t h e downward f l u x by s a l t r e j e c t i o n and t h e b r i n e c e l l drainage from t h e i c e and t h e upward f l u x from t h e u n d e r l y i n g waters.
During t h e i c e growth p e r i o d ,
brine
r e j e c t i o n and/or b r i n e drainage can cause l o c a l d e s t a b i l i s a t i o n o f t h e water l a y e r a d j o i n i n g the i c e (Lake and Lewis, 1970; Lewis, 1972).
Whereas, d u r i n g t h e n o r t h e r n spring, i c e m e l t i n g both a t t h e lower and upper i c e boundaries can occur. Increased energy absorption on t h e lower boundary can occur i n response t o a l g a l presence (Lewis, t h i s book). The a d d i t i o n o f m e l t water a t t h e ice-water boundary leads t o a s t a b i l i z a t i o n o f t h e adjacent t h i n l a y e r . The degree t o which m e l t i n g occurs depends on the d i s t r i b u t i o n o f snow cover, s o l a r r a d i a t i o n and temperature a t t h e upper and lower
interfaces.
Nutrient
o c c u r r i n g frequently.
input t o
the
interface
r e l i e s on d e s t a b i l i z a t i o n
However, i t i s more l i k e l y t h a t i n t r a n q u i l areas, entrainment
processes associated w i t h i n t e r n a l wave i n t e r f a c e leads t o n u t r i e n t replenishment.
generation on t h e melt-ambient water Undulations i n a t h i n l a y e r i n m e d i a t e l y
under t h e i c e can be seen v i s u a l l y d u r i n g t h e m e l t p e r i o d i n Hudson Bay. The physical processes o c c u r i n g a t t h e ice-water i n t e r f a c e which determine t h e r a t e o f upward n u t r i e n t t r a n s p o r t a r e found t o vary both i n space and time.
The
boundary l a y e r c h a r a c t e r i s t i c s depend on t h e smoothness o f t h e l o c a l and surrounding ice,
the
regularity o f
the current
gravitational s t a b i l i t y i n the layer.
regime and o t h e r
factors
related t o
the
The degree o f smoothness and i t s e f f e c t on t h e
v e l o c i t y s t r u c t u r e i n t h e boundary l a y e r has
been i n v e s t i g a t e d by Langleben (1982)
f o r f i r s t y e a r sea i c e and by C h r i s s and Caldwell (1984) f o r a smooth ocean f l o o r . Obviously, i n areas adjacent t o pressure r i d g i n g , t h e turbulence c h a r a c t e r i s t i c s a r e g r e a t l y modified. The r e g u l a r i t y o f t h e ambient motion depends both on l o w frequency and t i d a l f o r c i n g . fields,
diffusion Kz=Ko
From d e t a i l e d observations o f t h e v e l o c i t y , d e n s i t y and n u t r i e n t
i t i s p o s s i b l e t o estimate t h e v e r t i c a l
coefficient
(l+mRi)-n
where
can KO
be is
estimated
the
nutrient flux.
from
coefficient
for
an
A v e r t i c a l eddy
equation neutral
of
the
stability,
form Ri
is
Richardson the Richardson number and m and n a r e p o s i t i v e numbers (Jones, 1973). number i s defined as Ri= g.p-1 .dp/dz. (dU/dz)-z where p i s density, g i s g r a v i t y , dz i s t h e v e r t i c a l l e n g t h s c a l e and dU i s h o r i z o n t a l v e l o c i t y d i f f e r e n c e over dz (Turner, 1973). Using a F i c k ' s l a w f o r m u l a t i o n , t h e d i f f u s i v e n u t r i e n t f l u x (0) can be estimated from Q= K,
dN/dz where dN i s t h e d i f f e r e n c e i n n u t r i e n t c o n c e n t r a t i o n
46
over dz.
The extended t i m e series required t o c a l c u l a t e actual f l u x e s are i n
p r a c t i c e d i f f i c u l t t o obtain i n the f i e l d . S t a b i l i t y o f the boundary l a y e r varies from convectively unstable during b r i n e r e j e c t i o n t o very stable near fresh water sources. I n areas suff i c i e n t l y f a r from coastal fresh water sources such t h a t only weak s t r a t i f i c a t i o n . i s present o r during periods o f meltwater input, conditions s u i t a b l e f o r the r e t e n t i o n o f algae i n the m e l t l a y e r and i n p u t o f n u t r i e n t s from deeper waters can coexist, the r a t e o f n u t r i e n t i n p u t varying w i t h the l o c a l s t a b i l i t y . I n a Richardson number sense, values o f order one i n d i c a t e onset o f turbulence. Values l a r g e r than one i n d i c a t e a reduction o f v e r t i c a l d i f f u s i o n , as described i n the preceding paragraph. For example, a pycnocline o f 10 an i n thickness separating m e l t and ambient waters requires a v e l o c i t y d i f f e r e n c e o f greater than 15 cm.s-1 t o become unstable. Richardson numbers calculated by Ingram (1981) i n the i n t e r i o r o f the under-ice p l u m of the Great Whale River (Hudson Bay) were s u f f i c i e n t l y l a r g e (30+) t h a t l i t t l e o r no mixing occurred between the 5 m t h i c k fresh water l a y e r and the underlying sea water. S a l i n i t y , n u t r i e n t and a l g a l concentration l e v e l s were n e g l i g i b l e , the l a t t e r i n response t o the low s a l i n i t y . I n contrast, Gosselin e t a l . (1985) showed t h a t n u t r i e n t transport t o the i n t e r f a c e and a l g a l growth occurred i n areas where the Richardson number varied between 1 and 10, the v a r i a b i l i t y r e s u l t i n g from neap-spring t i d a l f o r c i n g and low frequency c u r r e n t changes. The actual s t r u c t u r e o f the boundary l a y e r subject t o t i d a l f o r c i n g shows a departure from a logarithmic p r o f i l e , (Soulsby and Dyer, 1981). Thus, the dynamics o f the i n t e r f a c e not only determine the n u t r i e n t supply, b u t also the s t a b i l i t y o f the a d j o i n i n g f l u i d layer. The a l t e r n a t i o n o f stable and unstable conditions i n the m e l t water l a y e r may a f f e c t a l g a l concentrations i f p o s i t i v e buoyancy i s essential t o t h e i r r e t e n t i o n a t the interface. Figure 7 shows i n sketch form the various f a c t o r s i n f l u e n c i n g the l i g h t , temperature, s a l i n i t y as well as n u t r i e n t and a l g a l l e v e l s a t the interface. The s i m i l a r i t y o f a l g a l growth a t the periphery o f r i v e r - d e r i v e d fresh water sources and i n m e l t water regions may be through the presence o f a l i g h t e r l a y e r o f intermediate s t a b i l i t y r e l a t i v e t o the ambient f l u i d . D e s t a b i l i s a t i o n occurs i n response t o t i d a l o r other c u r r e n t variability. These conditions can be parameterized by the Richardson number values i n the under-ice layer. THE ICE-WATER INTERFACE AS AN ERGOCLINE The model i m p l i c i t i n most o f the l i t e r a t u r e on the ice-water i n t e r f a c e i s t h a t o f a temporal ergocline. located between the autumn v e r t i c a l mixing o f the water column and the spring/sutmner m e l t o f the i c e followed by sutmner s t r a t i f i c a t i o n . I n a sense, the growth of microalgae a t the ice-water i n t e r f a c e was o f t e n considered as an extended spring bloom, frozen i n the i c e sheet. I n t h i s model, n u t r i e n t s used by microalgal growth were assumed t o be replenished by the autumn v e r t i c a l mixing,
47
Fig. 7 Schematic representation o f various factors t h a t influence temperature, s a l i n i t y , irradiance, n u t r i e n t s and microalgal concentration a t the ice-water interface.
48
frozen i n the i c e ,
and made a v a i l a b l e t o the c e l l s by d e s a l t a t i o n and i n s i t u
It has been explained above t h a t such an hypothesis cannot account f o r
regeneration.
t h e observed high concentrations o f epontic microalgae, which leads t o the conclusion t h a t the ice-water i n t e r f a c e must be dynamically l i n k e d t o the underlying water 1984. Gosselin e t a1 1985; Cota e t al., ms). column (e.g. Demers e t a1
.,
.,
I n order t o explain the h i g h p r o d u c t i v i t y o f the ice-water i n t e r f a c e , Demers e t a l . (1984) have compared i t w i t h f r o n t a l areas where microalgal p r o d u c t i v i t y i s o f t e n very high. I n more general terms, i t i s proposed t h a t the ice-water i n t e r f a c e belongs t o the general category o f ergoclines (Legendre and Demers, 1985; Legendre e t al., t h i s book). The ice-water ergocline i s located between the stable i c e and the h i g h l y energetic n u t r i e n t - r i c h water column. The ice-water i n t e r f a c e i s o f t e n described as a m i r r o r image o f the water-sediment ergocline.
Both ergoclines o f f e r a s t a b l e physical substrate on which o r i n which
organisms
can grow;
as
a consequence,
ergoclines are mainly pennate diatoms,
the microalgae which
colonize
the
t h a t can attach t o the substrate.
two One
d i f f e r e n c e between the two environments concerns l i g h t and n u t r i e n t gradients, which are p a r a l l e l a t the water-sediment ergocline and i n v e r t e d a t the ice-water i n t e r f a c e A more s i g n i f i c a n t d i f f e r e n c e l i e s i n the f l u x o f p a r t i c u l a t e organic
(Fig. 8). matter.
P a r t i c u l a t e organic matter flows towards the benthic i n t e r f a c e , where a
complex animal and b a c t e r i a l community has evolved t o manage t h i s abundant resource. On the contrary, p a r t i c u l a t e organic matter f a l l s o f f the ice-water i n t e r f a c e , where a complex animal and b a c t e r i a l cornunity i s therefore unnecessary t o take charge o f t h e primary production. This might explain the s t r i k i n g d i f f e r e n c e i n the r e l a t i v e complexities o f the benthic and the i c e fauna. As explained by Gosselin e t a l . (19851, the production o f microalgae a t the ice-water i n t e r f a c e i s c o n t r o l l e d by both c l i m a t i c and hydrodynamic phenomena. L i g h t a v a i l a b l e f o r photosynthesis depends on the seasonal increase i n s o l a r i r r a d i a n c e and a1 so on the meteorological i n f l u e n c e on the snow-ice cover, n u t r i e n t replenishment r e l i e s on hydrodynamic events. t h e ergocline,
I n a d d i t i o n t o those c o n t r o l s from above and below
the existence o f the i n t e r f a c e i t s e l f depends on the seasonal heat
f l u x i n t o the i c e and a l s o perhaps on the b i o l o g i c a l production a t the i n t e r f a c e (Lewis, t h i s book). The h i g h microalgal
production a t the ice-water
interface,
which can become
n u t r i e n t l i m i t e d , i n d i c a t e s t h a t t h i s environment i s an ergocline (Legendre e t al., t h i s book).
A t t h i s ergocline,
the a u x i l i a r y energy o f the underlying water column
acts on microalgal production through the proximal agency o f n u t r i e n t s .
Inputs of
a u x i l a r y energy i n the water column do n o t a f f e c t the l i g h t regime a t the interface. The f a c t t h a t the ergocline does n o t generally extend downwards i n the water column i s probably r e l a t e d t o the l a c k o f v e r t i c a l s t a b i l i t y (inadequate physical scale)
(1) if r a t h e r than t o l i g h t l i m i t a t i o n . This i s supported by two observations: a r t i f i c i a l substrates are placed a few metres below the ice-water i n t e r f a c e , a t times
49
ICE- WATER
ERGOCLINE
WATER- SEDIMENT ERGO CLINE
I
A U X l L1 A R Y
;Y
I I -
ENERGY
1
I
Fig. 8 Schematic r e p r e s e n t a t i o n o f the f l u x e s o f l i g h t , n u t r i e n t s and p a r t i c u l a t e organic matter a t t h e ice-water and water-sediment ergoclines.
when no phytoplankton a r e found i n t h e water column, these s u b s t r a t e s a r e r a p i d l y colonized by microalgae (M. Gosselin, unpublished data);
(2) when t h e water column
s t a b i l i z e s , as a consequence o f i c e melt, a phytoplankton bloom can occur under t h e i c e (Legendre e t a1
., 1981).
By comparison w i t h o t h e r e r g o c l i n e s t h a t support h i g h
microalgal p r o d u c t i o n (temporal t r a n s i t i o n s , the ice-water
nutriclines,
t i d a l fronts,
i n t e r f a c e thus o f f e r s t h e advantage o f l e s s complexity,
an so on), since t h e
hydrodynamics do n o t i n f l u e n c e t h e l i g h t regime a t t h e same t i m e as i t a c t s on nutrients. The ice-water e r g o c l i n e can t h e r e f o r e be used t o t e s t t h e hypotheses
.
Concerning e r g o c l i n e s proposed by Legendre e t a1 ( t h i s book). There are two major advantages i n u s i n g t h e e r g o c l i n e approach t o study b i o l o g i c a l production a t t h e ice-water i n t e r f a c e . F i r s t , t h e comparison w i t h o t h e r e r g o c l i n e s can evidence general mechanisms t h a t govern b i o l o g i c a l p r o d u c t i o n i n t h e oceans. Second, the e r g o c l i n e p e r s p e c t i v e can suggest new avenues f o r research as w e l l as new sampling and/or experimental designs concerning s p e c i f i c mechanisms t h a t govern b i o l o g i c a l production a t t h e ice-water i n t e r f a c e .
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53
Lewis, M.L., (1986). Radiation, absorption by ice-algae: i n f l u e n c e on the heat budget o f springtime sea-ice. ( t h i s book). and Morris, I., 1982. Temperature adaptation i n Phaeodactylum tricornutum Li, W.K.W. bohlin: photosynthetic r a t e compensation and capacity. J. Exp. Mar. B i o l . ECOl., 58: 135-150. Smith, J.C. and P l a t t , T., 1984. Temperature response o f photosynthetic Li, W.K.W., capacity and carboxilase a c t i v i t y i n A r c t i c marine phytoplankton. Mar. Ecol. Prog. Ser., 17: 237-243. Lipps, J.H. and Krebs, W.N. , 1974. Planktonic foraminefera associated w i t h A n t a r c t i c sea-ice. J. Foraminifera1 Res., 4: 80-85. Rochet, M., Legendre, L., and Demers, S., (ms). N u t r i e n t l i m i t a t i o n Maestrini, S.Y., o f the ice-microal gal biomass (southeastern Hudson Bay, Canadian A r c t i c ) . Malone, T.C., 1977. Environmental r e g u l a t i o n o f phytoplankton p r o d u c t i v i t y i n . the lower Hudson Estuary. Estuar. Coastal mar. Sci., 5: 157-171. Matheke, G.E.M. and Horner, R.A., 1974. Primary p r o d u c t i v i t y o f the benthic microalgae i n the Chukcki Sea near Barrow, Alaska. J. Fish Res. Board Can., 31: 1779-1786. 1975. The spectral d i s t r i b u t i o n o f l i g h t beneath Maykut, G.A. and Grenfell, T.C., f i r s t - y e a r sea i c e i n the A r c t i c Ocean. Limnol. Oceanogr. 20: 554-563. McConville. M.J. and Wetherbee, R., 1983. The bottom-ice microalgal community from natural i c e i n the inshore waters a t east Antarctica. J. Phycol., 19: 431-439 McRoy. C.P. and Goering, J.J., 1974. The i n f l u e n c e o f i c e the primary p r o d u c t i v i t y o f the Bering Sea. In: D. Hood and E. K e l l y (Editors), The Oceanography o f the Bering Sea. U n i v e r s i t y o f Alaska I n s t i t u t e o f Marine Sciences, Fairbanks., pp. 403-421. 1962. Plankton i c e i n the A n t a r c t i c Ocean, Antarct. Record., 14: Meguro, H., 1192-1199. Meguro, H., I t o , K., and Fukushima, H., 1966. Diatoms and the ecological conditions o f t h e i r growth sea-ice i n the A r c t i c ocean. Science (Washington, O.C.). 152: 1089-1090. I c e f l o r a (bottom type): a Meguro. H., Kuniyuki, I. and Fukushima, H., 1967. mechanism o f primary production i n t h e p o l a r seas and growth o f diatoms i n sea ice. A r c t i c , 20: 114-133. Moiseev, P.A., 1971. The l i v i n g resources o f the world Oceans ( t r a n s l a t e d from Russian). Published f o r National Marine F i s h Service, National Oceanic and Atmospheric Administration, U.S. Dept. o f Commerce and the National Science foundation, Washington D.C., by I s r a e l Program f o r S c i e n t i f i c Translation, Jerusalem. Neori. A., and Holm-Hansen, 0.. 1982. E f f e c t o f temperature on r a t e o f photosynthesis i n A n t a r c t i c phytoplankton. Polar Biol., 1: 33-38. Alexander, V. and Cooney, R.T., 1981. Primary production a t the Niebauer, H.J., Eastern Bering Sea-ice edge. The physical and b i o l o g i c a l regimes. In: D.W. Hood and T.A. Colder ( E d i t o r s ) , the Eastern Bering Sea Shelf: Oceanography and Resources. U. S. Deept comnerce , pp. 763-772. Palmisano, A.C., Kottmeier, S.T., Moe, R.L. and Sullivan, C.W., 1985. Sea-ice microbial communities. I V . The e f f e c t o f l i g h t perturbation on microalgae a t the ice-seawater i n t e r f a c e i n McMurdo Sound, Antartica. Mar. Ecol. Prog. Ser.. 21: 37-45. 1983. Sea-ice microbial comnunities (SIMLO). 1. Palmisano, A.C. and Sullivan, C.W., D i s t r i b u t i o n , abundance and primary production o f i c e microalgae i n McMurdo Sound, Antarctic i n 1980. Polar Biol., 2: 171-177. I r w i n , B., Horne, E.P. and Gallegos, C.L., 1982. P l a t t , T., Harrison, W.G., Photosynthesis and photoadaptation o f marine phytoplankton i n the A r c t i c . Deep-sea Res., 29: 1159-1170. 1982a. Sea-ice diatoms from Manitounuk Sound, Poulin, M. and Cardinal, A., southeastern, Hudson Bay (Quebec, Canada). I.Family Naviculaceae. Can. J. Bat., 60: 1263-1278. Poulin, M. and Cardinal, A., 1982b. Sea-ice diatoms from Manitounuk Sound, Southeastern Hudson Bay (Quebec. Canada). 11. Naviculaceae, genus Navicula. Can. J. Bot., 60: 2825-2845.
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54 Poulin, M. and Cardinal, A., 1983. Sea-ice diatoms from Manitounuk Sound, Southeastern Hudson Bay (Quebec, Canada). 111. Cymbellaceae, Entomoneidaceae, Gcmphonemataceae, and Nitzschiaceae. Can. J. Bot., 61: 107-118 Poulin, M., Cardinal, A. e t Legendre, L., 1983. Reponse d'une comnunaute de diatomees de glace un gradient de s a l i n i t e (Baie d'Hudson). Mar. Biol., 76: 191-202. and Whitaker, T.M., 1979. An A n t a r c t i c f a s t - i c e food chain: Richardson, M.G. observations on the i n t e r a c t i o n o f the amphipod Ponto eneia a n t a r c t i c a chevreux w i t h ice-associated microalgae. Br. Antarct. S u r v h : 1 0 7 - r n Rochet, M., Legendre, L. and Oemers, S., 1985. Acclimation o f sea-ice microalgae t o f r e e z i n g temperature. Mar. Ecol. progr. Ser. 24: 187-191. Rochet, M., Legendre, L. and Demers, S., ms. Riponses des microalgues des glaces aux v a r i a t i o n s q u a n t i t a t i v e s e t q u a l i t a t i v e s de l a lumiere 2 1 ' i n t e r f a c e glace-eau: Photosynthhe e t p i gments. Rodhe, W., 1955. Can plankton production proceed during w i n t e r darkness i n subarctic lakes? Verh. i n t . ver. theo. Ang. Limnol., 12: 117-122. Phytoplankton comnunities i n the Bering Sea and S a i t o K and Taniguchi, A., 1978. adjacent seas. 11. Spring and sumner communities i n seasonally i c e covered areas. Astarte 11: 27-35. Sasaki, H., and Watanabe, K., 1984. Underwater observations o f i c e algae Lutzow-Holm Bay, Antarctica. Antarct. Res., 81: 1-8. Seaburg, K.G., Parker, B.C., Nharton, R.A., J r and Simnons, G.M., Jr., 1981. Temperature growth responses o f a l g a l i s o l a t e d from A n t a r c t i c oases. J. Phycol., 17: 353-360. Smith, W.O. and Nelson, D.M.. 1985. Phytoplankton bloom produced by a receding i c e Science, 227: edge i n the Ross Sea: Spatial coherence w i t h the density f i e l d . 163-166. Soulsby, R.L. and Dyer, K.R., 1981. The form o f the near-bed v e l o c i t y p r o f i l e i n a t i d a l l y accelerating flow. J. Geophys. Res., 86: 8067-8074. Primary production o f A r c t i c waters. Polar Subba Rao, D.V. and P l a t t , T., 1984. BiOl., 3: 191-201. Sullivan, C.W. and Palmisano, A.C., 1981. Sea-ice microbial comnunities i n McMurdo Sound. Antarct. J. U.S.. 16: 126-127. Steele, J.H.. 1978. Some comnents on plankton patches. I n : J.H. Steele ( E d i t o r ) , Spatial p a t t e r n i n plankton comnunities. Plenum Press, New-York and London, pp. 1-17. T h e r r i a u l t , J.C. and Levasseur, M., 1985. Control o f phytoplankton production i n the Lower St.Lawrence Estuary: L i g h t and Freshwater runoff. N a t u r a l i s t e Can. , 112: 77-96. On the t r a n s f e r o f v i s i b l e r a d i a t i o n through sea-ice and snow. Thomas, C.W.,1963. J. Glaciol., 4: 481-484. 1973. Buoyancy e f f e c t s i n f l u i d s . Cambridge Univ. Press, New-York, Turner, J.S., 367 pp. 1977. The e f f e c t o f blue-green l i g h t on photosynthetic Vesk, M. and Jeffrey, S.W., Diaments and chloroDlast s t r u c t u r e i n u n i c e l l u l a r marine alaae from s i x classes. j.-Phycol , 13: 280-288. and Geen. G.M.. 1971a. L i a h t a u a l i t v i n r e l a t i o n t o growth. Wallen. D.G.. photosynthetic r a t e s and carbon metabolis; i n 'two Species o f marine piankton algae. Mar. Biol., 10: 34-43. 1971b. L i g h t q u a l i t y and concentration o f proteins, Wallen, D.G. and Geen, G.H., RNA, DNA and photosynthetic pigments i n two species o f marine plankton algae. Mar. Biol., 10: 44-51. Whitaker, T.M., 1977. Sea-ice h a b i t a t s o f Signy I s l a n d (South Orkneys) and t h e i r productivity. In: G.A. Llano ( E d i t o r ) , Adaptations w i t h i n A n t a r c t i c ecosystems. Proceedings o f the t h i r d SCAR Symposium on A n t a r c t i c Biology, Smithsonian I n s t i t u t i o n , Washington, 75-82. 1967. Heterotrophy i n a r c t i c s u b l i t t o r a l seaweeds Bot. Mar., 10: Wilce, R.T., 185-197. 1975. The dynamics o f phytoplankton Winter, D.F., Banse, K. and Anderson, G.C., blooms i n Puget Sound, a F j o r d i n the northwestern United States. Mar. Biol., 29: 139-176.
.
55
STUDYING FRONTS AS CONTACT ECOSYSTEMS Serge FRONTIER S t a t i o n Marine, BP 41, F62930 Wimereux ( F r a n c e )
ABSTRACT
A t t h e l e v e l o f an oceanographic f r o n t a n enhanced p r i m a r y p r o d u c t i v i t y and/or a c c u m u l a t i o n o f biomass can be observed, g e n e r a t i n g a p a r t i c u l a r ecosystem. Two main t y p e s o f f r o n t a l ecosystems a r e t o be d i s t i n g u i s h e d : ( 1 ) When t h e new o r accumulated biomass i s e x p l o i t e d by a t r o p h i c c h a i n i n v o l v i n g v a g i l e ( i . e . , m i g r a t i n g ) organisms, t h e biomass i s e x p o r t e d t o remote o l i g o t r o p h i c areas. Overaccumulation ( e u t r o p h i c a t i o n ) i s a v o i d e d and the e x p l o i t e d community remains a j u v e n i l e and o p p o r t u n i s t i c one.
( 2 ) When no a c t i v e food c h a i n a r i s e s , t h e new o r accumulated biomass i s degraded t h r o u g h b a c t e r i a l a c t i v i t y . A mature, complex and v e r y d i v e r s i f i e d m i c r o h e t e r o t r o p h i c community a r i s e s , w i t h Protozoans, and a l o c a l r e g e n e r a t e d primary p r o d u c t i o n can o c c u r . I n t e r m e d i a t e cases e x i s t , f o r example f i l t e r i n g macroorganisms f e e d i n g on m i c r o h e t e r o t r o p h s , and p o s s i b l y e a t e n b y f i s h . Enhanced v e g e t a l p r o d u c t i o n o r biomass n e c e s s i t a t e s a t u n i n g o f t h e p h y s i c a l regime ( n u t r i e n t a d v e c t i o n o r d i f f u s i o n ; l i g h t s u p p l y ) and t h e b i o l o g i c a l phenomena. An " a u x i l i a r y energy", coming f r o m t h e d e g r a d a t i o n o f t h e k i n e t i c energy o f w a t e r masses u n t i l r e a c h i n g a time-and-space s c a l e c o m p a t i b l e w i t h b i o l o g i c a l phenomena, i s t h e n used. Tuning between v a r i o u s t r o p h i c f l u x e s and m i g r a t i o n s a l o n g t h e t r o p h i c c h a i n system a l s o use an a u x i l i a r y energy a b l e t o b r i n g i n t o c o n t a c t , a t t h e r i g h t rythm, t h e v a r i o u s elements o f t h e t r o p h i c n e t work. That "secondary a u x i l i a r y e n e r g y " a r i s e s f r o m t h e energy a s s i m i l a t e d by consumer organisms which u t i l i z e a p a r t o f t h i s e n e r g y f o r m o v e m e n t s a n d m i g r a t i o n r The i n t e r a c t i o n o f two p a r t s o f t h e system, a p r o d u c t i v e one and a consumer and v a g i l e one, may be c o n s i d e r e d as an e x p l o i t a t i o n ( = biomass e x p o r t a t i o n ) o f an ecosystem by a n o t h e r one. The i n t e r a c t i o n needs an i n t e r p e n e t r a t i o n between them. A u x i l i a r y energy a t v a r i o u s l e v e l s i s used t o e s t a b l i s h t h e f r a c t a l spatio-temporal s t r u c t u r e o f t h e e n t i r e system.
INTRODUCTION T h i s paper g i v e s t h e c o n c l u s i o n o f t h e Working Group "Theme 5"'
having
p a r t i c i p a t e d i n t h e e l a b o r a t i o n o f t h e FRONTAL Program, a French l a r g e - s c a l e oceanographic e x p e r i m e n t (PIROCEAN-INSU, 1985).
x M. B i a n c h i , F. B l a n c , S. F r o n t i e r , F. Ibanez, G. Jacques, P. K l e i n , J. Le Fevre, M. Leveau, P. Mayzaud, L. P r i e u r , F, Rassoulzadegan, A. Sournia,
D. V i a l e
56
Fronts are c o n t a c t zones between two d i f f e r e n t water masses. A t t h i s l e v e l , p a r t i c u l a r b i o l o g i c a l and e c o l o g i c a l phenomena occur, which perhaps determine more the e c o l o g i c a l p r o p e r t i e s o f the area, than the phenomena o c c u r r i n g i n s i d e the two water masses do. I g i v e here an overview o f the f r o n t a l phenomena from the p o i n t o f view o f the General Ecosystem Theory i n i t s present (and p r o v i s i o n a l ! ) s t a t e , and p a r t i c u l a r l y o f the Theory o f c o n t a c t o r i n t e r f a c e between ecosystems, r e c e n t l y emphasized as ErgocZine Theory
I a l s o r e f e r t o t h e Theory of expzoitation o f an ecosystem by another one. Remember t h a t an ecosystem i s n o t "a biomass" b u t i s b e t t e r d e f i n e d as the i n t e r a c t i o n between biomass and the physical medium. T h i s i n t e r a c t i o n has t o be observed a t various observation l e v e l s .
MAIN FEATURES I N FRONTAL ECOSYSTEMS The main c h a r a c t e r i s t i c s o f oceanographic f r o n t s are :
-
o f t e n mesoscale phenomenons, i n f a c t phenomena o f very v a r y i n g range o f
-
r a t h e r w e l l l o c a l i z e d i n some permanent areas, and
-
f r e q u e n t l y impulsional o r q u a s i - p e r i o d i c , t h a t i s , n o t permanent b u t showing
scales;
important tempora 1 v a r i a t i o n s
.
The problem o f f r o n t a l ecosystems can be formulated as f o l l o w s : a t the l e v e l o f any f r o n t a p a r t i c u l a r ecosystem seems t o appear which i s d i s t i n c t from those o f the two adjacent water masses. An enhanced primary p r o d u c t i o n i s observed o r p o s t u l a t e d here b u t i n some cases o n l y a passive accumulation o f biomass occurs. Sometimes, t h a t l o c a l l y increased biomass i s e x p l o i t e d by various t r o p h i c chains which, through the m i g r a t i o n o f the organisms i n v o l v e d , r e d i s t r i b u t e the a s s i m i l a t e d energy i n t o m r e o l i g o t r o p h i c areas. Sometimes, on the contrary, t h e t r o p h i c chains do n o t develop w e l l , and the accumulated biomass t u r n s o u t t o be degraded by m i c r o h e t e r o t r o p h i c organisms. I n a l l cases, a m o d i f i c a t i o n o f t h e community composition i s observed i n the v i c i n i t y o f f r o n t s , b u t i n various ways according t o the type o f f r o n t . I t denotes a p a r t i c u l a r o r g a n i z a t i o n o f the t r o p h i c network f o l l o w i n g t h e enhanced product i o n o f biomass, and adapted t o it. According t o the k i n d o f b i o l o g i c a l organ i z a t i o n , n u t r i e n t s are regenerated e i t h e r immediately w i t h i n the area ( a l l o wing a regenerated p r o d u c t i o n ) o r f a r away a f t e r a m i g r a t i o n over a l o n g p e r i o d o f time. The e c o l o g i c a l phenomena associated w i t h f r o n t s are indeed very d i v e r s e and, moreover, are t o a c e r t a i n e x t e n t unpredictable. T h i s u n p r e d i c t a b i l i t y i s brought about by p h y s i c a l thenomena i n v o l v e d and i n s t a b i l i t i e s , r e s u l t i n g i n the unstable l o c a l i z a t i o n o f the f r o n t and i t s o f t e n sporadic o r quasip e r i o d i c existence. I t f o l l o w s t h a t a v a r i e t y o f d e s c r i p t i o n s o f f r o n t s have been proposed i n t h e 1it e r a t u r e .
57
TUNING OF THE PHYSICAL REGIME AND THE BIOLOGICAL PHENOMENA An a n a l y s i s o f t h e f r o n t a l phenomenon has a l r e a d y been o u t l i n e d f o l l o w a number o f recent works i n v a r i o u s f r o n t a l zones (see f o r ex. G r a l l & a1 1980; Le Fevre & a l . , 1970, 1981, 1983 a and b; Le Fevre, 1985; H o l l i g a n , 1979, 1981; H o l l i g a n & a l . ,
1983, 1984, 1985; Pingree, 1978; Pingree & a1
1974, 1975, 1978 a and b, 1979, 1981; Legendre, 1981; Legendre & Demers, 1984; e t c ...) are
.A
thorough l i s t o f references and a complete review o f t h e s u b j e c t
given i n Le Fevre, 1985. These analyses gave r i s e t o a number o f general ideas a l s o encountered i n
the a n a l y s i s o f non-pelagic ecosystems, p a r t i c u l a r l y o f c o n t i n e n t a l ecosystems. The main common f e a t u r e seems t o be t h e f a c t t h a t , a t t h e spatio-temporal scales i n v o l v i n g t h e p h y s i c a l phenomena a s s o c i a t e d w i t h v a r i o u s i n t e r f a c e s , a great deal o f n o n - t r o p h i c energy i s used by t h e ecosystem as " a u x i l i a r y energy" (Margalef, 1974, 1985), a l l o w i n g t h e "covariance" (Margalef, 1978), t h a t i s , a spatio-temporal
t u n i n g between t h e proximal agents o f t h e produc-
t i v i t y namely l i g h t , n u t r i e n t s and p h o t o s y n t h e t i c pigments. The k i n e t i c
energy o f water masses, when degraded u n t i l b e i n g a b l e t o a c t
d t
a specific
scale compatible w i t h t b e b i o l o g i c a l phenomena, has t h e r o l e o f r e a l i z i n g t h a t spatio-temporal
"covariance". More p r e c i s e l y , t h e u p w e l l i n g o f n u t r i e n t
enriched water i n t o t h e t r o p h i c zone, t h e t u r b u l e n t m i x i n g , eddies and shearings, b r i n g i n t o c o n t a c t t h e complementary elements, hence a l l o w t h e primary p r o d u c t i o n . A u x i l i a r y energy i s i s s u e d from a compartment o f t h e s o l a r r a d i a t i o n o t h e r than the p h o t o s y n t h e t i c energy, namely t h e i n f r a r e d , which i s p r i n c i p a l l y used i n moving f l u i d s . T h i s f e a t u r e has an e x a c t e q u i v a l e n t i n t h e t e r r e s t r i a l ecosystems
F o r example, i t can be shown t h a t i n o r d e r t o produce any vegetal
biomass, a g r a s s l a n d needs 30 t o 100 times more energy f o r i t s evapotranspir a t i o n than f o r i t s photosynthesis i n t h e same time. Obviously, photosynthesis only occurs when a s u f f i c i e n t f l o w o f sap goes upwards through t h e p l a n t s , c a r r y i n g t h e s a l i n e s o l u t i o n o f s o i l up t o t h e l i v i n g and l i g h t e n e d leaves. The evaporation energy d r i v i n g t h i s ascendent f 1ow can be c a l c u l a t e d ( F r o n t i e r , unpublished). We a l s o may c o n s i d e r t h a t t h e energy the primary p r o d u c t i o n
-
-
d e r i v e d from
i n v o l v e d i n t h e e d i f i c a t i o n o f anatomical s t r u c t u r e
a l l o w i n g the f l o w o f sap, such as vessels and wood, i s a l s o an a u x i l i a r y anergy (Fnargalef, pers. com.)
, which
may be c a l l e d "secondary a u x i l i a r y
energy". I n a q u a t i c ecosystems, t h e "primary a u x i l i a r y energy" i s much g r e a t e r than i n t e r r e s t r i a l vegetations,
because i t involves- t h e whole energy which
moves t h e concerned water masses, mixes them and provokes a1 ternances o f s t a b i l i z e d and d e s t a b i l i z e d s i t u a t i o n s (Legendre, 1981; Legendre & Demers,1984).
58
The more o r l e s s favourable p e r i o d i c i t y o f t h e l a t t e r i s more important, f o r t h e ecosystem, t h a t the amount of energy i n v o l v e d . TurbuZence i s a p r o g r e s s i v e degradation o f t h e k i n e t i c energy o f water masses i n t o s m a l l e r and s m a l l e r eddies down t o t h e viscous range, and a t a given scaZe o f space and time, the energy b r i n g s about t h e coincidence a t any p o i n t o f l i g h t , n u t r i e n t s and l i v i n g c e l l s . The synchronism, o r t u n i n g , o f p h y s i c a l and b i o l o g i c a l rythms e x p l a i n s , i n t h a t hypothesis, t h e p a r t i c u l a r dynamism observed i n t h e v i c i n i t y o f f r o n t s .
A sustained p r o d u c t i v i t y demands a covariance accomplished a t the r i g h t rytkm, as suggested by Le Fevre & a l . (1983). The g e n e r a l i z a t i o n o f these n o t i o n s t o o t h e r sharp g r a d i e n t s i n t h e environment gave r i s e t o t h e d e f i n i t i o n o f
ergoclines. Summarizing, t h e proximal f e a t u r e s o f f r o n t a l ecosystems depend on t h e i n t e r a c t i o n between spatio-temporal c h a r a c t e r i s t i c s o f t h e p h y s i c a l regime o f the f r o n t and t h e response time o f b i o l o g i c a l phenomena r e s u l t i n g i n primary p r o d u c t i o n . But n o t o n l y t h e primary p r o d u c t i o n i s concerned i n t h e t u n i n g o f complementary phenomena. Secondary p r o d u c t i v i t y depends on a t u n i n g o r resonance between t h e response time o f successive f l u x e s along t h e t r o p h i c chain. I t f o l l o w s t h a t a g r e a t d i v e r s i t y o f problems o f o r g a n i z a t i o n i n s i d e t h e f r o n t a l biomass i n f l u e n c e the e c o l o g i c a l p r o p e r t i e s o f t h e l a t t e r . D i f f e r e n t cases can be d i s t i n g u i s h e d : a) A divergence o r a mixing leads t o an enhanctd "new" p r o d u c t i o n l i n k e d w i t h t h e advection o f n u t r i e n t s . A " j u v e n i l e " community then a r i s e s , w i t h h i g h P/B,
low d i v e r s i t y , predominance o f r s t r a t e g i e s , predominance o f vegetal
biomass ( p r i n c i p a l l y Chlorophyceas and Diatomeas) and o f small size, f a s t l y
., t h a t i s resource - t h e
m u l t i p l y i n g p l a n k t o n i c herbivores, e t c . .
t y p i c a l l y an o p p o r t u n i s t i c
community which i s c o l o n i z i n g a new
"resource" being here
g e n e r a l l y d e f i n e d as a covariance between the v a r i o u s c o n d i t i o n s f o r p r o d u c t i o n . I f t h a t p r o d u c t i o n i s e x p l o i t e d , t h e regenerated p r o d u c t i o n i s low and the f l u x o f m a t t e r through t h e biomass i s open. The m i c r o h e t e r o t r o p h i c organisms are scarce because they a r e unfavoured i n t h e c o m p e t i t i o n a g a i n s t v a g i l e macroorganisms. Indeed, t h e l a t t e r e x p o r t biomass o u t s i d e t h e area o f enhanced primary p r o d u c t i v i t y , hence t h e regeneration o f n u t r i e n t s occurs elsewhere, sometimes f a r away, and i s dispersed i n a l a r g e o l i g o t r o p h i c area.
A t u n i n g o f the biomass consumption w i t h respect t o t h e speed o f primary production i s necessary i n o r d e r f o r phytoplankton t o be e f f i c i e n t l y grazed by herbivorous zooplankton. Counter-examples e x i s t : along t h e Ushant t i d a l f r o n t , o f f French B r i t t a n y , Ce Fevre & a l . (1983 a) and Le Fevre (1985) suggested t h a t the e x i s t i n g zooplankton i s n o t adapted t o t h e p e r i o d i c v a r i a t i o n s o f the f r o n t a l primary production. The e x p o r t i n g t r o p h i c network hence does n o t
59
e x i s t , and phytoplankton i s r a p i d l y degraded by b a c t e r i a s , as seen i n many other circumstances (Fenchel, 1964; H o l l i g a n & a l . ,
1984; Jordan & J o i n t , 1984;
Newel1 & L i n l e y , 1984; Le Fevre, 1985). The b a c t e r i a l community i s very i m p o r t a n t , and s t i l l l a r g e l y disregarded i n the f r o n t a l zones. T h i s community seems t o adapt v e r y r a p i d l y t o environmental conditions and t o t h e n a t u r e o f t h e o r g a n i c m a t e r i a l , due t o t h e i r r a p i d response (a few hours)
-
an o p p o r t u n i s t i c community
-, and
t h e i r high potential
biochemical d i v e r s i t y . I t r e s u l t s i n a v e r y p r e c i s e matching between anabolism and catabolism (M. Bianchi, Personal communication). b) A convergence., on t h e c o n t r a r y , induces a passive accumulation l e a d i n g t o a degradation o f t h e o r g a n i c m a t t e r i n t h e same area. The r e s p o n s i b l e , o f t e n complex and very d i v e r s i f i e d community i s m a i n l y c o n s t i t u t e d o f b a c t e r i a s , m i c r o z o o f l a g e l l a t e s and c i l i a t e s . I t can be observed i n a q u a t i c ecosystems t h a t a l a r g e c o n c e n t r a t i o n o f o r g a n i c m a t t e r never occurs w i t h o u t b e i n g immediately, o r almost immediately, degraded'.
The l i v i n g m a t t e r has t o be
permanently renewed, and t h e more i n t e n s i v e t h e accumulation process, t h e f a s t e r t h e d e s t r u c t i v e process i s . Consequently i f a " j u v e n i l e " biomass issued from an enhanced p r i m a r y p r o d u c t i o n g e t s exported ( e i t h e r by t r o p h i c chains associated w i t h h o r i z o n t a l and v e r t i c a l m i g r a t i o n s o f t h e successive consumers, o r by p h y s i c a l phenomena such as h o r i z o n t a l d i s p e r s i o n b y c u r r e n t s o r v e r t i c a l s i n k i n g ) , then a s t a t i o n n a r i t y o f t h e whole process can be obtained. When, on t h e c o n t r a r y , t h e produced o r p a s s i v e l y accumulated biomass i s n e i t h e r b i o l o g i c a l l y n o r p h y s i c a l l y e x p l o i t e d , i t r a p i d l y breaks down due t o t h e a c t i v i t y o f b a c t e r i a l and protozoan communities. c) Such a d e s c r i p t i o n i s o f course o v e r s i w p l i f i e d . phenomena can occur. I n some cases, t h e
I n t e r m e d i a t e o r composite
m i c r o h e t e r o t r o p h i c community can,
a f t e r a time, be e x p l o i t e d by f i l t e r i n g macroorganisms, which may i n t u r n be consumed and migrate, o r do n o t . For example, S a l p swarms can appear i n s i d e the phytoplankton blooms and e x p l o i t them a v o i d i n g t h e accumulation o f vegetal biomass. I t can be shown t h a t these swarms, i n t h e f r o n t a l zones, a r e i n no way t r o p h i c deadlocks, f o r t h e y c o n t a i n a g r e a t q u a n t i t y o f s e m i - p a r a s i t i c H y p e r i i d Amphipods (Laval, 1980) which are, i n t u r n , e x p l o i t e d by tunas o r s l i p j a c k s ( F r o n t i e r , unpublished). E x p l o i t a t i o n o f phytoplankton accumulations by D o l i o l i d s , Appendicularians (which a r e eaten by f i s h l a r v a e , s p e c i a l l y f l a t f i s h e s ) and Pteropod Lirnacina has a l s o been described (Southward & B a r r e t t , 1983; Le Fevre, 1985; Deibel, 1985).
x I n c o n t i n e n t a l waters, l i m n o l o g i s t s c a l l " e u t r o p h i c a t i o n " such a huge accumulation o f biomass, which a f t e r a s h o r t t i m e g e t s degraded; i t never occurs w i t h t h e same importance i n marine waters, except sometimes i n t h e red t i d e s (Wyatt & Horwood, 1973; Wyatt, 1975).
60
Inothercases,thereis
l i t t l e g r a z i n g by microorganisms, and t h e l o c a l degra-
d a t i o n o f l i v i n g m a t t e r produces n u t r i e n t s i n t h e same area, i n d u c i n g a regener a t e d p r o d u c t i o n . I n t h i s way, t h e area can be shared i n t o a mosaic o f heterot r o p h i c and a u t o t r o p h i c communities. I n t h e Ushant f r o n t , a permanent convergence i n d u c i n g biomas,
accumulation and regenerated p r o d u c t i o n i s associated
w i t h t h e p e r i o d i c new p r o d u c t i o n l i n k e d t o t i d a l rythms, b u t i s i n s u f f i c i e n t l y e x p l o i t e d by zooplankton. I n such a composite systems, some f e a t u r e s o f a mature community are t o be found ( h i c h d i v e r s i t y , predominance o f d i n o f l a g e l l a t e s , low P/B,
h i g h r a t e o f n u t r i e n t r e c y c l i n g , e t c ...) near o t h e r f e a t u r e s
c h a r a c t e r i s t i c o f o p p o r t u n i s t i c ecosystem (Diatomeas)(Legendre & a l . , t h i s volume). Summarizing, f r o n t ecosystems a r e o f two main types : ( i ) those i n which a macrotrophic network develops, up t o l a r g e f i s h e s and cetaceans; these ecosystems psssess a system o f displacements o f t h e consumer organisms, w i t h a p r e c i s e matching o f these displacements along t h e t r o p h i c chains, so t h a t an o v e r a l l m i g r a t i o n o f biomass occurs; ( i i ) those i n which such a t r o p h i c network does n o t e x i s t , and t h e biomass decays i n t h e f i e l d through t h e a c t i o n o f b a c t e r i a s . This c o n t r a s t i s n o t h i n g b u t a r e a l i z a t i o n o f t h e a n c e s t r a l c o m p e t i t i o n between m i c r o and macroheterotrophic organisms, as i t occurs i n a l l k i n d s o f ecosystems, i n c l u d i n g t e r r e s t r i a l ones. I n t e r m e d i a t e types o f f r o n t s occur when p a r t i c u l a r macroorganisms a r e developing and m u l t i p l y i n g by e x p l o i t i n g t h e m i c r o h e t e r o t r o p h i c organisms. The f i r s t type o f system demands a w e l l a d j u s t e d matching o f t h e v a r i o u s p o p u l a t i o n s o f consumers and t h e i r behaviour. B u t t h e c o n d i t i o n s i n which t h i s matching does appear and m a i n t a i n s i t s e l f , as a w e l l r e g u l a t e d s t r u c t u r e , remains t o be searched. I t i s a r e a l problem o f ecological evolution. BIOLOGICAL EXPLOITATION OF THE FRONTAL PRODUCTIVITY Cominp back t o t h e o r g a n i z a t i o n o f t h e consumer networks which s t a r t s from any f r o n t a l area, we may p r e f e r t o c o n s i d e r two a d j a c e n t ecosystems, t h e one producing a biomass, and t h e o t h e r consuming i t . We r e j o i n here t h e Theory of
e x p z o i t a t i o n of m ecosystem by another one. " E x p l o i t a t i o n " i s here considered i n a general view, as an exportation of biomass whatever t h e cause ( F r o n t i e r , 1978). We have a p i c t u r e o f a h i g h l y p r o d u c t i v e , j u v e n i l e ecosystem e x p l o i t e d by a mature one whose d i v e r s i t y i s h i g h e r , and which p a r t i c u l a r l y c o n t a i n s some h i g h t r o p h i c l e v e l s , a b l e t o come and graze t h e j u v e n i l e biomass a t t h e interface
-
unless i t e n t e r s i t f o l l o w i n g a geometry o f i n t e r p e n e t r a t i o n which
has something t o do w i t h t h e F r a c t a l t h e o r y (Mandelbrot, 1977
.a,
1982, 1985).
61
That e x p l o i t a t i o n o f a j u v e n i l e ecosystem b y a mature one has two consequences :
1.- The j u v e n i l e ecosystem i s m a i n t a i n e d i n t h i s s t a t e , o r even drops i n o r g a n i z a t i o n , w i t h lowered d i v e r s i t y , b e i n g f o r c e d i n t o a r a p i d p r o d u c t i o n role.
2.- The mature ecosystem develops more (and f a s t e r ) t h a n i t would i f l i m i t e d by i t s own r e s o u r c e s i n energy and m a t t e r . By t h e way, t h e r e i s a t r a n s f e r o f energy and m a t t e r (and a l s o o f i n f o r m a t i o n ) f r o m t h e f o r m e r t o t h e l a t t e r , and t h e dissymmetry i s m a i n t a i n e d ( o r even enhanced) between t h e two p a r t s o f t h e c o u p l i n g system, i n s t e a d o f d i s a p p e a r i n g by m i x i n g . T h a t dissymmetry, o r o r g a n i z a t i o n , i s s t r i c t l y depending on t h e permanence o f t h e f l u x e s o f m a t t e r , energy and i n f o r m a t i o n f r o m one system t o t h e o t h e r . I t can be c o n s i d e r e d as
another ergocline, between t h e two d i f f e r e n t ecosystems, and problems o f t u n i n g are t o be i n v e s t i g a t e d h e r e t o o . Once a g a i n a comparison can be made w i t h t e r r e s t r i a l ecosystems. V a r i o u s i n t e r f a c e s have been d e s c r i b e d and a n a l y z e d i n c o n t i n e n t a l systems. A c o n t a c t between a f o r e s t and a savanna i s a f r o n t . A savanna i s an ecosystem w i t h l o w d i v e r s i t y , a r a p i d t u r n o v e r , and i s a c t i v e l y e x p l o i t e d by h e r b i v o r o u s animals comina by n i g h t f r o m t h e f o r e s t , and f e e d i n g a t t h e edge where p r i m a r y product i v i t y i s high; d u r i n g t h e day, t h e s e h e r b i v o r e s a r e e n r i c h i n a t h e f o r e s t w i t h t h e i r d e j e c t i o n s and carcasses. As a consequence, t h e r e i s a f l u x o f energy and m a t t e r f r o m t h e savanna
i n t o t h e f o r e s t . I n o t h e r words, an open f l u x o f
matter i s continuously t r a v e l l i n g through the j u v e n i l e , o r l e s s organized, ecosystem and t h r o u g h t h e i n t e r f d c e , a l l o w i n g a c a r n i v o r o u s biomass t o be more i m p o r t a n t and more d i v e r s i f i e d t h a n i t can be a t some d i s t a n c e f r o m t h e c o n t a c t . Hunters i n d e e d have known f o r a l o n g t i m e t h a t game i s more abundant and more d i v e r s i f i e d i n t h e c o n t a c t zone and i n t h e mosaic f o r e s t - s a v a n n a , as w e l l as tuna and whale f i s h e r s know t h a t t h e i r p r e y s a r e o f t e n l o c a t e d n e a r t h e oceanographic f r o n t s .
An a c t i v e e x p o r t a t i o n o f biomass by a c t i v e consumers, r e s u l t i n g i n a d i s p e r s i o n o f t h e a s s i m i l a t e d energy t h r o u g h o u t a l a r g e , g e n e r a l l y o l i g o t r o p h i c area, i s consuming energy. T h a t energy i s o b v i o u s l y coming f r o m t h e metabolism o f t h e consumer organisms themselves and, a f t e r complete e x a m i n a t i o n , comes f r o m the p r i m a r y p h o t o s y n t h e t i c energy. I t f o l l o w s t h a t a t eachtrophic l e v e l o f an ecosystem i n v o l v i n g v a g i l e organisms, a part o f t h e assimilated energy i s
withdrawn from the growth o r maintenance o f biomass, and devoted t o the e d i f i c a t i o n and functionning o f the exporting s t r u c t u r e s , w h i c h a r e a b l e t o make r e s i l i e n t t h e whole system ( F r o n t i e r , 1978). I c a l l e d "secondary a m i Z i a r y
energy" t h a t p a r t o f a s s i m i l a t e d energy w h i c h a l l o w s t h e m a t c h i n g ( a t a f a v o u r a b l e speed and p e r i o d i c i t y ) between t h e v a r i o u s elements o f t h e ecosystem, and which i s consequently, used as a " c o v a r i a n c e energy" i n t h e same sense as
62
j photosynthesis
Y
I I
energy
e
.E I
m i crohe terotroph organisms
animal
cascade of trophic l e v e l s and migrations II
\competition between micro and macroheterotrophic organisms
63 the "primary" a u x i 1 i a r y energy i s . Figure 1 represents a conceptual model summarizing t h e r e s p e c t i v e r o l e s o f the open energy f l o w s throughout t h e ecosystem, t h e m a t t e r c y c l i n g , and t h e necessary spatio-temporal matching between t h e v a r i o u s f l u x e s . Primary a u x i l i a r y energy guarantees t h e matching o f t h e vegetal biomass and t h e p h y s i c a l environment as secondary a u x i l i a r y energy does between t h e v a r i o u s p a r t n e r s o f the i n t e r a c t i o n system i n t h e community. The compartment c a l l e d "Animal Biomass" symbolizes, i n f a c t , t h e whole cascade o f t r o p h i c l e v e l s and associated m i g r a t i n g cycles, r e s u l t i n g i n a d i s p e r s i o n o f t h e t r o p h i c energy i n t o some large, remote o r deep, o l i g o t r o p h i c areas. AUXILIARY ENERGY, FRONTS AND FRACTALS We can compare f u r t h e r t h e two k i n d s o f a u x i l i a r y energy, t h e one d i r e c t l y issued from t h e k i n e t i c energy o f f l u i d s , t h e o t h e r from t h e p h o t o s y n t h e t i c energy t r a n s m i t t e d through t h e f o o d chains. Both a c t much more t h r o u g h t h e i r f i t n e s s , and p a r t i c u l a r l y through t h e t u n i n g o f t h e temporal and s p a t i a l scales involved, than by t h e t o t a l amount o f energy. Primary a u x i l i a r y energy i s useful by a d v e c t i n g n u t r i e n t s , by d i s p e r s i n g biomass when t h e m a t u r a t i o n o f eocsystem occurs along a d r i f t , and above a l l by g i v i n g r i s e t o turbuZence which mixes water masses. Turbulence has a f r a c t a l geometry (Mandelbrot, 1974, 1975, 1976, 1977 a and b, 1982), t h a t i s , t h e s m a l l e r and s m a l l e r eddies i n d e f i n i t e l y i n c r e a s e t h e c o n t a c t surface, and m u l t i p l y t h e c o n t a c t p o i n t s between t h e p a r t s o f t h e w a t e r masses, i n c r e a s i n g hugely t h e "covariance".
In
the t e r r e s t r i a l ecosystems, such an enhanced c o n t a c t between p l a n t s and t h e medium ( s o i l and atmosphere) i s accomplished by a r a m i f i e d geometry, which i s a l s o a f r a c t a l (Mandelbrot, 1977 a, 1978, 1982). We saw above t h a t these ramif i e d s t r u c t u r e s a r e b u i l t up u s i n g an
amount o f energy coming from t h e photo-
s y n t h e t i c a s s i m i l a t i o n , and t h a t I c a l l e d "secondary a u x i l i a r y energy" too. With r e g a r d t o m o t i l e organisms, as w e l l i n t e r r e s t r i a l as i n a q u a t i c environments, secondary a u x i l i a r y energy i s used i n o r d e r t o p e r m i t d i s p l a c e ments which r e a l i z e an i n t e r p e n e t r a t i o n between t h e two subsystems
-
the
productive one and t h e consumer one. This r e c i p r o c a l p e n e t r a t i o n through more o r l e s s "random" movements, i s a l s o a f r a c t a l . F i n a l l y , t h e v e r y sense o f t h e (primary and secondary) a u x i l i a r y energy seems t o be t o a l l o w t h e e d i f i c a t i o n of a f r a c t a l spatio-temporal geometry o f biomass and surrounding medium, which i s necessary f o r a l l l i v i n g processes because i t c o n d i t i o n s t h e covariance and tuning between a l l t h e p a r t n e r s o f t h e ecosystem ( F r o n t i e r & Legendre, 1985). I n conclusion, I have n o t presented here new r e s u l t s , b u t r a t h e r a new general v i s i o n based on p r e v i o u s observations,
and- demanding new ones. The
a n a l y s i s o f t h e f r o n t a l systems f o l l o w i n g t h e framework o f t h e Ecosystem Theory
64
p r o v i d e s some s p e c i f i c w o r k i n g hypotheses. We have now t o c o n f i r m ( o r negate, o r complete),
t h e model, and t o f i t i t by measuring f l u x e s , b y i n v e s t i g a t i n g
t h e r e l a t i o n s between f l u x e s and s t r u c t u r e s ( p a r t i c u l a r l y
i n the f i e l d o f
e x p o r t a t i o n o f biomass f r o m t h e f r o n t a l i n t o t h e s u r r o u n d i n g a r e a s ) , and f i n a l l y by a t t e m p t i n g t o e s t i m a t e t h e i m p o r t a n c e o f t h e f l u x e s and t h e s t r u c t u r e s i n t h e p r o d u c t i o n balance, f i r s t i n t h e f r o n t a l zone, t h e n i n t h e e n t i r e ocean. ACKNOWLEDGEMENTS I t h a n k D r . Jacques Le F e v r e f o r u s e f u l s u g g e s t i o n s a b o u t t h i s p a p e r .
REFERENCES D e i b e l , D., 1985. Blooms o f t h e p e l a g i c T u n i c a t e D o l i o l e t t a gegenbauri : a r e t h e y a s s o c i a t e d w i t h G u l f Stream f r o n t a l e d d i e s ? J. Mar. Res., 43: 211-236. Fenchel, T., 1968. On " r e d w a t e r s " i n t h e I s e f j o r d ( i n n e r Danish w a t e r s ) caused b y t h e C i l i a t e Mesodinium rubrum. O p h e l i a , 5: 245-253. F r o n t i e r , S., 1978. I n t e r f a c e s e n t r e deux ecGsystemes : exemples dans l e domaine p e l a g i q u e . Ann. I n s t . Oceanogr., P a r i s , 54: 95-106. F r o n t i e r , S., 1977. R e f l e x i o n s p o u r une t h e o r i e des ecosystemes. B u l l . E c o l . ,
8: 445-464.
F r o n t i e r , S . and Legendre, P., 1985. T h e o r i e des f r a c t a l s : a p p l i c a t i o n s a l ' e c o l o g i e , i m p l i c a t i o n s dans l ' e c h a n t i l l o n n a g e . Rapport CNRS, ATP 9.82.65. E v a l u a t i o n e t o p t i m i s a t i o n des p l a n s d ' e c h a n t i l l o n n a g e en e c o l o g i e l i t t o r a l e 32.PP. F r o n t i e r , S. and Legendre, P . . S u b m i t t e d , F r a c t a l s i n E c o l o g y . G r a l l , J.R., Le Corre, P., Le Fevre, J., M a r t y , Y., and T o u r n i e r , B., 1980. C a r a c t e r i s t i q u e s de l a couche d ' e a u s u p e r f i c i e l l e dans l a zone des f r o n t s thermiques Ouest-Bretagne. Dceanis, 6: 235-249. H o l l i g a n , P.M., 1979. D i n o f l a g e l l a t e blooms a s s o c i a t e d w i t h t i d a l f r o n t s around t h e B r i t i s h I l e s . I n D.L. T a y l o r and H.H. S e l i g e r , eds., T o x i c D i n o f l a g e l l a t e Blooms, E l s e v i e r / n o r t h H o l l a n d , 249-256. H o l l i g a n , P.M., 1981. B i o l o g i c a l i m p l i c a t i o n s o f f r o n t s on t h e n o r t h w e s t e r n European c o n t i n e n t a l s h e l f . P h i l . Trans. Roy. SOC. London, s e r . A, 302:
547-562. H o l l i g a n , P.M., V i o l l i e r , M., Dupouy, C . and Aiken, J., 1983. S a t e l l i t e s t u d i e s on t h e d i s t r i b u t i o n o f c h l o r o p h y l l and d i n o f l a g e l l a t e blooms i n t h e Western E n g l i s h Channel. C o n t i n e n t a l S h e l f Res., 2: 81-96. H o l l i g a n , P.M., W i l l i a m s , P.J., P u r d i e , D. and H a r r i s , R . P., 1984. Photos y n t h e s i s , r e s p i r a t i o n and n i t r o g e n s u p p l y i n s t r a t i f i e d f r o n t a l and t i d a l l y mixed s h e l f w a t e r s . Mar. E c o l . P r o g r . Ser., 17: 201-213. H o l l i g a n , P.M., Pingree, R.D. and M a r d e l l , G.T., 1985. Oceanic s o l i t o n s , n u t r i e n t p u l s e s and p h y t o p l a n k t o n growth. Nature, London, 314: 348-350. Jordan, M.B. and J o i n t , I . R . , 1984. S t u d i e s on p h y t o p l a n k t o n d i s t r i b u t i o n and p r i m a r y p r o d u c t i o n i n t h e w e s t e r n E n g l i s h Channel i n 1980 and 1981. C o n t i n e n t a l S h e l f Res., 3: 25-34. L a v a l , Ph., 1 9 8 0 . H y p e r i i d Amphipods as Crustaceans p a r a s o t o i d s a s s o c i a t e d w i t h g e l a t i n o u s z o o p l a n k t o n . Oceanogr. Mar. B i o l . Ann. Rev., 18: 11-56. Le Fevre, J. and G r a l l , J.R., 1970. On t h e r e l a t i o n s h i p o f N o c t i l u c a swarming o f f t h e w e s t e r n c o a s t o f B r i t t a n y w i t h h y d r o l o g i c a l f e a t u r e s and p l a n k t o n c h a r a c t e r i s t i c s o f t h e environment. J. Exp. Mar. B i o l . Ecol., 4: 287-306. Le Fevre, J., Cochard, J.C. and G r a l l , J.R., 1981. P h y s i c a l c h a r a c t e r i s t i c s o f an i n s h o r e a r e a on t h e A t l a n t i c c o a s t o f B r i t t a n y and t h e i r i n f l u e n c e on t h e p e l a g i c ecosystem : t h e case o f t h e " R i v i e r e d ' E t e l " . E s t u a r i n e , c o a s t a l and S h e l f Science, 13: 131-144.
65 Le Fevre, J., Le Corre, P., Morin, P. and B i r r i e n , J.L., 1983 a. The p e l a g i c ecosystem i n f r o n t a l zones and o t h e r environments o f f t h e western coast o f B r i t t a n y Oceanol o g i ca Acta, Proceed. , 17 t h European Mari ne B i o l o g y Symposium, 125-129. Le Fevre, J., V i o l l i e r , M., Le Corre, P . , Dupouy, C . and G r a l l , J.R., 1983 b. Remote sensing observations o f b i o l o g i c a l m a t e r i a l by Landsat along a t i d a l thermal f r o n t and t h e i r r e l e v a n c y t o t h e a v a i l a b l e f i e l d data. E s t u a r i n e , Coastal and S h e l f Science, 16: 37-50. Le Fevre, J . , 1985. Aspects o f t h e b i o l o g y o f f r o n t a l systems. Adv. Mar. B i o l . , i n press. Legendre, L., 1981. Hydrodynamic c o n t r o l o f marine phytoplankton Droduction : the paradox o f s t a b i l i t y . I n J. Nihoul ED., Ecohydrodynamics, E l s e v i e r S c i e n t i f i c Publish., Amsterdam, 191-207. Legendre, L. and Demers, S., 1984. Towards dynamical b i o l o g i c a l oceanography and l i m n o l o g y . Can. J. F i s h . Aquat. S c i . , 41; 2-19. Legendre, L., Demers, S. and L e f a i v r e , D., 1985. B i o l o g i c a l p r o d u c t i o n a t m a r i n e e r g o c l i n e s . Proceed. 1 7 t h I n t e r n a t . L i e g e Colloquium on Ocean Hydrodynamics, t h i s volume, J. Nihoul ed. Mandelbrot, B., 1974. I n t e r m i t t e n t t u r b u l e n c e i n s e l f - s i m i l a r cascades : d i v e r gence o f h i g h moments and dimension o f t h e c a r r i e r . J. F l u i d Mech., 62: 331-358. Mandelbrot, B., 1975. On t h e geometry o f homogeneous turbulence, w i t h s t r e s s on t h e f r a c t a l dimension o f t h e i s o - s u r f a c e o f s c a l a r s . J . F l u i d Mech., 72: 401-416. Mandelbrot, B., 1976. I n t e r m i t t e n t t u r b u l e n c e and f r a c t a l dimension : k u r t o s i s and t h e s p e c t r a l exponent 513 + B. Turbulence and Navier-Stokes equations. Lecture notes i n Mathematics, 565; 121-145. Mandelbrot, B., 1977 a. F r a c t a l s , Form, Chance and Dimension. W.H. Freeman and Co., San Francisco, 365 pp. Mandelbrot, B., 1977 b. F r a c t a l s and t u r b u l e n c e : a t t r a c t o r s and d i s p e r s i o n . Lecture Notes i n Mathematics, New York, 615: 83-93. Mandelbrot, B., 1978. The f r a c t a l geometry o f t r e e s and o t h e r n a t u r a l phenomena. Lecture Notes i n Biomathemactics, New York, 23: 235-249. Mandelbrot, B., 1982. The f r a c t a l geometry o f Nature. W.H. Freeman & Co., San Francisco, 468 pp. Mandelbrot, B., 1985. Les o b j e t s f r a c t a l s . Flammarion, P a r i s , 204 pp. Margalef, R., 1974. E c o l o g i a . Ediciones Omega, Barcelona, 951 pp. Margalef, R., 1978. L i f e forms o f phytoplankton as s u r v i v a l d e t e r m i n a t i v e s i n an u n s t a b l e environment. Oceanol. Acta, 1: 493-510. Margalef, R., 1985. From hydrodynamic processes t o s t r u c t u r e ( i n f o r m a t i o n ) and from i n f o r m a t i o n t o process. I n R.E. Ulanowicz and T. P l a t t eds. Proceed. Sympos. on E c o l o g i c a l Theory i n R e l a t i o n t o B i o l o g i c a l Oceanography (Quebec, March 1984), Can. J . Aquat. F i s h . Sci., 213: 200-220. Newell, R . C . and L i n l e y , E.A.S., 1984. S i g n i f i c a n c e o f m i c r o h e t e r o t r o p h s i n t h e decomposition o f phytoplankton : e s t i m a t e s o f carbon and n i t r o g e n f l o w based on t h e biomass o f p l a n k t o n communities. Mar. E c o l . Progr. Ser., 16: 105-119. Pingree, R.D., F o r s t e r , G.R. and Morrison, G.K., 1974. T u r b u l e n t convergent t i d a l f r o n t s . J. Mar. B i o l . Assoc. U.K., 54: 469-479. Pingree, R.D. and Pennycuik, L., 1975. T r a n s f e r o f heat, f r e s h water and n u t r i e n t s throuqh t h e seasonal thermocline. J . Mar. B i o l . Assoc. U.K., 55: 261-274. Pingree, R.D., 1978. C y c l o n i c eddies and c r o s s - f r o n t a l mixing. J. Mar. B i o l ASSOC. U.K.. 58: 955-963. Pingree, R.D. and G r i f f i t h s , D.K., 1978a. T i d a l f r o n t s on t h e s h e l f seas around the B r i t i s h I s l e s . J. Geophys. Res., 83: 4615-4622. Pingree, R.D., H o l l i g a n , P.M. and M a r d e l l , G.T., 1978 b. The e f f e c t o f v e r t i c a l s t a b i l i t y on phytoplankton growth and c y c l o n i c eddies. Nature, London, 278: 245-247.
.
66
Pingree, R.D. and M a r d e l l , G.T., 1981. Slope turbulence, i n t e r n a l waves and phytoplankton growth a t t h e C e l t i c Sea s h e l f break. P h i l . Trans. Roy. SOC. London, s e r . A, 302: 663-682. PIROCEAN-I.N.S.U., 1985. P r o j e t de f a i s a b i l i t e du Programme "FRONTAL". Doc. C.N.R.S. (PIROCEAN), m u l t i g r . 90 pp. Southward, A.J. and B a r r e t t , R.L., 1983. Observations on t h e v e r t i c a l d i s t r i b u t i o n o f zooplankton, i n c l u d i n g p o s t l a r v a l Teleosts, o f f Plymouth i n t h e p r e s e n c e o f a Chlorophyll-dense l a y e r . J. Plankton Res., 5: 599-618. Wyatt, T. and Horwood, J., 1973. Model which generates r e d t i d e s . Nature, London, 244: 238-240. Wyatt, T., 1975. F u r t h e r remarks on r e d t i d e models. Environmental L e t t e r s , 9: 217-224.
67
THE FRONTAL ZONE IN THE SOUTHERN BENGUELA CURRENT
.
1 2 L HUTCHINGS' , D .A. ARMSTRONG and B .A. MITCHELL-INNES
'Sea Fisheries Research Institute, Private Bag X2, Rogge Bay 8012, Cape Town, South Africa. 2Marine Biology Research Institute, University of Cape Town, Rondebosch 7700, Cape Town, South Africa.
INTRODUCTION The southern region of the Benguela Current is an area of complex mixing, where Agulhas Current and South Atlantic Surface Water meet and mingle with upwelled South Atlantic Central Water.
The relationships between
the water masses (see Fig. 1) are presented in the conceptual image of
30'
s
40
30"
E
Fig. 1. A conceptual image of the Agulhas Current System. Heavy dots indicate warm water, lighter dots colder water and hatching cold, subtropical surface water and upwelled water. The line demarcating the subtropical water at about 4 O o S latitude is the Subtropical Convergence. 1. The embryonic stages of a Natal pulse. 2. Small waves or disturbances on the Agulhas Current border. 3 . Shear edge effects, warm-water plumes or shear edge eddies. 4. Cold upwelled water. 5. The dispersion of a shear edge feature attached to an Agulhas Current meander. 6. The Agulhas Current retroflection. 7. An Agulhas Current Ring being advected northward. 8. The Benguela upwelling regime with frontal eddies in evidence. 9 . A planetary wave on the Agulhas Return Current-Subtropical Convergence. 10. An independent cold-wave eddy spawned by an unstable planetary wave. (From Lutjeharms, 1981a).
68 Lutjeharms (1981a), showing advection of filaments of the Agulhas Current onto the Agulhas Bank, Agulhas Rings breaking off and moving northwards into the Atlantic, and entrainment of Agulhas Bank Water along the west coast offshore of the Benguela upwelling zone by seasonal southeasterly winds.
These factors combine and peak
during summer months to create
an intense front from Cape Agulhas (3YS)in the extreme south of the continent up to Cape Columbine (33OS). Beyond this point gradients generally weaken (Shannon, 1985).
Scientific
attention has been focussed mainly on the inshore area, and numerous transects across the shelf have been made (see Fig. line was the Upwelling Monitoring
2).
The most frequently sampled
(UM) line, sampled 36 times between
March 1971 and March 1983 (Andrews and Hutchings, 1980).
The Bathythermograph
(BT) line at 255' from Cape Town (Bang and Andrews, 19741, and the December 1984 Frontal Zone (FZ) transect at 32' 28 '
due west of Cape Columbine,
were perpendicular to the usual orientation of the front.
s'
F 2 line ...................
20)8V,I4I21)864 2
33O.
-
!O
9 .
8
UM Line 34O'
17O
18O
E
Fig. 2. The transect lines off the Cape Peninsula. Hydrological and plankton data were collected along the Upwelling Monitoring (UM) line and Frontal Zone (FZ) transect, and temperature only along the Bathythermograph (BT) line. SURFACE FEATURES An appreciation of the variability of the front has grown in direct proportion to the frequency with which images were obtainable from shipboard studies, from aircraft and lately, from satellites.
Colour images from
the NIMBUS-7 satellite (Shannon et al., 1984a) show that the thermal front' roughly follows the shelf-break between Cape Columbine and Cape Point, but further south it cuts eastwards across the bottom contours.
Temperature
69 gradients are often l0C per nautical mile but can be several degrees over a few hundred meters (Bang, 1973).
Maximum gradients occur in midsummer
to autumn when southeasterly winds predominate.
Imbedded within the coastal
waters are active upwelling sites (see Fig. 3) off the Cape Peninsula, Cape Columbine and Hondeklip Bay
(Nelson and Hutchings, 1983).
Surface
features are extremely sensitive to changes in wind stress (Bang and Andrews, 1974; Andrews and Hutchings, 1980; Lutjeharms, 1981b; Taunton-Clark, 1985), varying extensively within a few hours of a change in wind direction. During winter the area south of 32OS is subject to westerly winds as cyclones
310
-
32'
23 November 1980
33O
(C)
34'
-
11 January1980
16'
17'
la'
19'
E
Fig. 3. A montage of three upwelling sites in the southern Benguela Current (a, b and c) defined by aerial radiation thermometry. (From Nelson and Hutchings, 1983).
70
associated with depressions move eastward past the southern tip of Africa. Upwelling activity declines and cross-shelf surface temperature gradients decrease from 11°C (10-21OC) down to 1-2°C (13-15OC) (Andrews and Hutchings, 1980).
VERTICAL STRUCTURE Bang (1971, 1973, 1974) did pioneering work on the frontal structure using detailed bathythermography between 3loS and 34OS.
He distinguished
an upwelling front associated with the growing upwelling centre at 10-15 n.miles
offshore, and
a
shelf-break front at
50-60 n.miles
offshore.
After prolonged upwelling these two fronts might coalesce; this in fact occurs further south where the shelf is very steep, resulting in the steepest temperature gradients observed during active upwelling.
The subsurface
structure of the shelf-break front appears to persist perennially (Hutchings et al., 1984), weakening in midwinter but intensifying in August to September with the intrusion of cold water (8OC) onto the shelf, before the local southeasterly wind stress begins, indicating that at least part of the formation of the front is due to larger scale processes.
The narrow shelf
and the convolutions of the shelf-break with two major submerged canyons, the Cape Point valley and the Cape Canyon associated with the Cape Peninsula and
Cape
Columbine
respectively,
facilitate the movement of cold, low
salinity water onto the shelf. Associated with
the steeply sloping isotherms of the frontal region
is a strong equatorward jet current (Bang and Andrews, 1974) with some evidence of compensating flows on
the bottom inshore and also offshore
(see Fig. 4). In a recent description of the currents off the Cape Peninsula, Nelson (1985) indicates that the jet current is not in geostrophic balance; he postulates that vorticity changes, which occur when the South Atlantic gyre is deflected by
the shelf, combine with other large scale forces
to play an important role in maintaining the shelf-edge jet.
Near-surface
frontal features alter rapidly with changes in wind stress and southward flowing currents can occur after prolonged northwesterly winds (Shelton and Hutchings, 1982). North of Cape Columbine the shelf widens and the shelf profile alters, with the shelf-break varying between 200-500m (Shannon, 1985). activity occurs both
Upwelling
in a narrow coastal strip and on the shelf-break
(Hart and Currie, 1960; Shannon, 1966; Bang, 1971; Nelson and Hutchings, 1983), and frontal features become less definite.
Therefore we have focussed
our attention on the front between Cape Point and Cape Columbine (33'50' -34'30's).
71
I-
Fig. 4. Subsurface frontal features along the BT line and the shelf-edge jet west of the Cape Peninsula during January 1973. (From Bang and Andrews, 1974)
.
BIOLOGICAL CHANGES ACROSS THE FRONT Marked changes in species composition and abundance of phytoplankton, zooplankton and ichthyoplankton occur across the front (De Decker, 1973, 1984; Thiriot, 1978; Hutchings, 1979; Shelton, in prep.),
in keeping with
similar findings in other frontal systems (Holligan, 1981; Boucher, 1984; Vinogradov and Shushkina, 1978; Shushkina et al., 1978).
A low diversity,
cool water fauna extends along the coast inshore, from 31OS to 19OS (Kollmer, 1963; Unteruberbacher, 1964; Thiriot, 1978; De Decker, 1984).
The offshore
extensions of the distribution of Centropages brachiatus (sse Fig. 5a), a typical surface member of this inshore community, may indicate occasional advection into the South Atlantic.
Immediately hyond the front is warm
water, often of Agulhas Bank or Current origin, with a high diversity
72
of species (De Decker, 1984; Hutchings, 1979).
Centropages furcatus,
a characteristic Agulhas Current copepod, disappears as this water moves northwards and mixes
(see Fig. 5b), its penetration into the Atlantic being proportional to its tolerance to changing conditions. Further offshore, species from the South Atlantic gyre and occasionally from Subantarctic waters
(e.g. Calanus tonsus and Calanoides macrocarinatus) are apparent
(De Decker, 1984). S 200
300
400
w
I lo"
0"
lo"
20"
30'
40"
50"
E
Fig. 5. The distribution of (a) Centropages brachiatus, a cold water copepod species, and (b) Centropages furcatus, a characteristic Agulhas Current copepod, around the coast of South Africa. (From De Decker, 1984).
73 Nutrients and phytoplankton Vigorous growth of phytoplankton in newly upwelled waters occurs close inshore.
Nutrient concentrations are often low in aged upwelled waters
at the surface immediately inshore of the shelf-break
front, resulting
in the absence of a pronounced nutricline across the front in the upper layers (see Fig. 6). Nutrients increase rapidly below the shallow pycnoclines. Strong gradients in chlorophyll 'a' across the front, however, do exist (see Fig. 6 ) ,
with very
low concentrations in the upper layers on the
warm water side, while higher concentrations inshore sink below the front and extend offshore at subsurface depths for some distance.
Raised chloro-
phyll 'a' levels can extend to a depth of '75-100m at the front, indicating strong sinking motions, as illustrated by the November 19'72 transect of the UM line (see Fig. 7).
-
-(
Y N I
'
E
c p
FZ
Secondary peak in nutrients
4
4
85 v
?
20 u
I
Z Y 7
-k -c- c
3 9 % 9
-
-
150 2.0 20
200 -
15
10
150 -
100 1,5 15
-
350 1.0 10
-
390 0.5 5
100 5
50-
80
20
40
DISTANCE ALONG PLUME (km) Fig. 6. Distributions of nitrate, silicate, phospha-te, oxygen and chlorophyll in the photic layer in a section along the UM line in January 1972. Note the position of the frontal zone (FZ) and the secondary nutrient peak inshore of the front. (From Andrews and Hutchings, 1980).
During quiescent periods or onshore winds, complicated motions occur in the frontal region as a surface layer 0-f warm water floods shorewards. High subsurface chlorophyll 'a' maxima occur, as illustrated by the January 19'73 transect of the UM line (see Fig. 8).
The coarse spacing of stations
74
STATION NO.
Fig. 7. Temperature (a) and chlorophyll 'a' (b) sections along the UM line in November 1972, showing a pronounced front with chlorophyll sinking at the front. in time (monthly) and space (5-10 n.miles apart) preclude detailed analyses of these changes. Results from the December 1984 Frontal Zone cruise (stations were 3 n.miles apart) showed a weakly developed front (see Fig. 9b) after prolonged northwesterly winds prior to sampling.
Currents measured with a Niel-Brown
acoustic current meter showed strong northward-flowing currents on either
. side of the front (see Fig. 9a), up to 1m.sec-1 immediately above the Cape Canyon.
Surface temperatures ranged from 15.2OC inshore, to 18.9OC offshore
and nitrates were low ((1 slightly close inshore.
~ n m o l . m - ~ ) on both sides of the front, rising
However, there were pronounced differences
15
STATION NO.
DISTANCE OFFSHORE ( k m )
Fig. 8. Temperature (a) and chlorophyll 'a' (b) sections along the UM line in January 1973, showing conditions during quiescent periods, with a relaxed front and a pronounced subsurface chlorophyll 'a' maximum. in the phytoplankton standing stock (see Fig. 10) along the transect. Low concentrations occurred offshore of the frontal zone, with approximately equal concentrations of nano- and net-phytoplankton, whereas, net-phytoplankton predominated at the biomass peak close inshore, as well as at the secondary peak in the frontal zone (stations 4 and 5). Microzooolankton During an extensive survey of the distribution of pelagic ichthyoplankton off the south western Cape in 1977-1978 (Cape Egg and Larval Programme, CELP), microplankton were collected at five depths near interfaces (surface, thermoclines etc.) in the upper 75m by sieving 2 litres of water through 37pm nylon mesh.
The main concentration of microzooplankton was located
inshore of the front and comprised mostly egg and naupliar copepod stages in the upwelled waters.
Scattered dinoflagellate blooms occurred on the
76
10
09
STATION NO. 06+ 05 04
08 07
03
02
01
I
,
Fig. 9. Current (a) and density (b) sections along the FZ line in December 1984, showing strong equatorward jets flanking the frontal zone (marked by an arrow). Agulhas Bank, with
low concentrations beyond the front.
No noticeable
accumulation occurred within the frontal zone, which effectively formed a barrier to the offshore dispersion of the abundant young copepod stages. Zooplankton Shelton and Hutchings
(in prep.)
show that high zooplankton stocks,
COASTAL St. 2
OCEANIC
0.2 Z = 3.1
COASTAL 1.0
-
0.8 0.6
St. 3
o. OCEANIC
OCEANIC St. 8
0.2 -
-
OCEANIC
0.2
st. 10
4
8
16 32
64 128
PARTICLE DIAMETER (m) Fig. 10. Particle spectra of samples from the chlorophyll maximum layer along the FZ line in December 1984. Stations are characterized as coastal, frontal or oceanic based on the water characteristics.
78 expressed as displaced volume, are limited to the nearshore region by strong frontal features (see Fig. 11).
If zooplankton standing stock
were plotted in dry weight or carbon units, the inshore, offshore differences would probably be enhanced, as the crustacean-dominated, cool-water plankton inshore is replaced by more gelatinous species offshore. has
shown significantly higher biomasses
of
Pillar (in prep.
euphausiids and
copepods
expressed as dry weight, in coastal waters than in oceanic waters.
-3
Fig. 11. The distribution of zooplankton (dispaced volume, ml. lOOOm ) around the south western Cape coast measured during the CELP surveys, 1977-1978 . (From Shelton and Hutchings, in prep.) Hutchings (1981) examined 13 of 36 monthly transects of the southern Benguela shelf region off the Cape Peninsula, in terms of mesozooplankton dry weight (WP-2 net) in the whole water column, chlorophyll 'a' in the upper lOOm and surface temperature.
Highest zooplankton concentrations
were observed close inshore, and there was no particular association of higher zooplankton stocks with the frontal zone.
Andrews and' Hutchings
(1980) showed a strong shoreward movement of high zooplankton concentrations
79
as oceanic water flooded shorewards during winter and onshore wind periods in summer.
A more detailed analysis of the same data by Armstrong (in
prep.) showed that when a strong front was present above the shelf-break after prolonged upwelling, there was a pronounced peak of mesozooplankton in the upper mixed 12a, b).
layer, immediately inshore of the front (see Figs.
In February 1973, a double front was present, with high zooplankton
biomass at the inshore upwelling front and only a slight increase at the shelf-edge front
(see Fig. 12d).
When the front was weakly developed,
with an intrusion of warm water onshore (e.g. January 1973), relatively low zooplankton concentrations were observed along the entire line (see
Fig. 12c). Hutchings (1979) collected an intensive data set of zooplankton samples over 10 days in the Cape Peninsula upwelling system.
Multivariate analysis
of these data showed that different zooplankton communities occurred across the frontal zone (see Fig. 13), along the front.
Vertical distribution
of species and biomass at a frontal station showed a clear separation in the vertical plane with distinct warm water, thermocline and cool water communities in
separate
layers.
How these communities are maintained
in the presence of the strong water motions in the region (Bang and Andrews, 1974; Shannon et al., 1981; Nelson, 1985), still remains uncertain. Armstrong (in prep.) also found sharp changes in communities across a well defined frontal feature (see Fig. 14) with a low diversity, high biomass community inshore and a high diversity, low biomass commmunity in the warm water offshore.
Pelagic fish Egg and larval species assemblages in the southern Benguela are clearly influenced by the strong front.
Very few fish species spawn inshore of
the front in cool upwelled waters; a larger number (including the commercially important anchovy and pilchard) spawn in warm shelf waters on the Agulhas Bank, and many
more
species (largely mesopelagic
water offshore of the front (Shelton, in prep).
forms) spawn in deep
In December 1984, pelagic
fish eggs and larvae were observed along most of the transect, with peak larval abundance beyond the front in the warm surface waters and eggs closer to the front. The cumulative annual distribution of anchovy eggs and larvae during 1977-1978 (see Fig. 15) clearly show the centre of spawning on the Agulhas Bank, with an advected tail of eggs and larvae entrained in the frontal jet current extending up the west coast.
The subsequent movement of older
larvae and juveniles is the subject of much conjecture (Badenhorst and
80
10
9
STATION NO. 8 7 6 5 4 3 2 1
1
,
1
1
1
1
1
1
Front
(a)
r:d; 2
100
80
60
40
20
DISTANCE OFFSHORE ( km)
Fig. 12. The distribution of mesozooplankton biomass (g.dry wt .rn-2) above the thermocline along the UM line under various frontal conditions. In January 1972 (a) and February 1972 (b) there was a pronounced front; in January 1973 (c) the front was weakly developed and in February 1973 ( d ) a double front was present. Boyd, 1980; Boyd and Hewitson, 1983).
Other species such as round herring
Etrumeus whiteheadi) and mackerel (Scomber japonicus) are associated with warmer waters outside the front.
A pole-and-line fishery for longfin
81
s 50'
34O 0
0
0 0
10'
20'
50'
34O 10'
20'
18"
10'
20'
30'
18O
10'
20' 30'E
Fig. 13. Geographic distribution of sample-groups based on zooplankton species abundance derived using the Bray-Curtis measure of similarity. Group A samples represent the zooplankton community of recently upwelled water; Group B samples are inshore neritic species, Group C samples are frontal zone groups, and Groups D and E samples are warm water groups. (From Hutchings, 1979). and yellowfin tuna exists along the frontal zone in the region of the shelf-break
from 28OS
to 35's.
Fishing boats characteristically cross
the front, steam for a few more miles and then begin trolling parallel with the bottom isobaths until shoals are encountered.
Recently satellite
imagery has been used to concentrate fishirlg activities in frontal regions.
82
FRONTAL ZONE
Fig. 14. Changes in species composition and concentration of mesozooplankton, collected above the thermocline (upper 30-60m), across sharp frontal gradients along the UM line during January 1972 and February 1972. DYNAMIC BIOLOGICAL PROCESSES Phytoplankton development in newly upwelled waters Drogues, placed at 10m depth in large patches of newly upwelled water off the Cape Peninsula, were tracked for 5-9 day periods during six Plankton Dynamics Cruises.
Barlow (1982a, b),
Olivieri et al. (1985), and Brown
and Hutchings (1985) found that nitrate is rapidly removed from the stabilizing upper mixed layer by a developing phytoplankton bloom, usually but not always dominated by diatoms (Olivieri et al., 1985; Hutchings et al., 1984; Mitchell-Innes, in prep.).
Preliminary estimates of grazing in
83
>10 OoO per lorn2 Lorvoe
17'
18'
19'
20'
21'
E
17'
18'
'91
20'
21'
E
Fig. 15. Cumulative distribution of anchovy (Engraulis capensis) eggs and larvae off the south western Cape during 1977-1978; (a) eggs (August 1977-August 1978), (b) larvae (August 1977-February 1978, April and June 1978). (From Shannon et al., 1984b). newly upwelled waters (Olivieri and Hutchings, in prep.) show that only a small proportion of phytoplankton production is grazed by mesozooplankton, with major losses probably due to dispersion over a wide plume area (Boyd, 1982) or to sinking. During these studies none of the initial blooms approached the frontal region, yet Andrews and Hutchings (1980), Shannon et al. (1984a) and Hutchings et al. (1984) showed high chlorophyll 'a1 levels at the front.
This may
be due to rapid offshore transport of developing blooms from the extreme south of the upwelling centre, or the slower development of patches due to poor seeding, leading to increased offshore transport prior to a bloom developing.
An alternative explanation of the peak at the front is the
presence of a mixing cell where the aged upwelled water sinks, with some uplift of water immediately inshore of the front.
This is supported by
the presence of a secondary nutrient peak occasionally detected just inshore of the frontal peak in chlorophyll 'a' (see Fig. 6).
Bang (1973) suggests
mixing processes in the frontal region may be caused by internal waves propagating up the steep isopycnals and "shaking themselves out", creating a series of eddies where phytoplankton may be retained. Phytoplankton production, nutrient uptake and regeneration During the December 1984 Frontal Zone survey daily integrated
84
productivity values at the frontal zone stations were 2096 and 2916 mgC.mb2. day-’.
The production rates in the frontal area, although higher than in
adjoining waters, were nevertheless comparatively low when compared to the production measured in upwelled waters of the Benguela upwelling system. Brown (1984) reports integrated daily production rates in summer ranging from barely detectable levels in newly upwelled water to 11056 mgC.m-2.day-1. Since P:B ratios did not show any enhanced productivity at the frontal stations, the higher production may be due to the accumulation of phytoplankton in an area of lower current speed flanked by stronger currents to either side (see Fig. 9a).
Size fractionation of samples showed that
at oceanic stations the nanoplankton fraction (3-10pm) formed 33-70% of the total carbon production.
At stations in frontal or inshore waters
the nanoplankton fraction only contributed 6-31% of the total carbon fixed. Recent work by Probyn (1985) on nitrogen uptake by different size-fractions of phytoplankton
at
inshore, mid-shelf
(100-200m) and offshore
(1000m)
locations in the Southern Benguela show that, in common with other upwelling zones, net plankton ( > l o p ) dominated inshore in terms of both biomass and nitrogen uptake (mainly nitrate), while offshore, nano- and pico-plankton (1-lOpm
and
< lpm
respectively) were relatively more important, with ammonia
and urea the major sources of nitrogen.
A lower total rate of nitrogen
uptake occurred offshore, beyond the front. The presence of aged, lowbnitrate water and a large quantity of phytoplankton well below the euphotic zone at the front suggests that microheterotrophic processes may be enhanced in the frontal region. Water column stability and phytoplankton growth Although sunwarming and mixing create shallow discs of moderately stable aged upwelled
water
overlying cooler water inshore, the pycnocline is
weaker and shallower than that present offshore of the front, a situation similar to that off Oregon
(Mooers et al., 1978).
Advected filaments
of Agulhas water (Bang, 1971, 1973; Lutjeharms, 1981b) increase the vertical gradients considerably beyond the front.
Strong winds combined with wave
action can periodically cause some mixing of nutrient rich water
into
the euphotic zone across the weak pycnocline inshore of the front, maintaining some growth potential compared with offshore waters. During southeasterly wind relaxations or reversals in wind stress as occurred in January 1973 (see Fig. 8). a layer of warm water can mix across the front (Bang and Andrews, 1974) rapidly stabilizing the water to a degree unusual in most coastal waters.
Legendre (1981) has pointed out
the role of alternation of stable and mixed water columns in stimulating
85
phytoplankton growth. summer months
With
a 3 to 6 day wind cycle characteristic of
(Andrews and Hutchings, 1980; Nelson and Hutchings, 1983;
Nelson, 1985), waters
in close proximity
to the front could be highly
productive for short but frequent periods between wind events during summer.
Sinking processes Sinking processes in frontal regions have been proposed by both biologists (Packard et al., 1978; Andrews and Hutchings, 1980) and physical oceanographers
(Mooers, Collins
and
Smith, 1976; Bang, 1973, 1976; Simpson,
1981; Brink, 1983; Nelson, 1985). containing one,
Many models of upwelling circulation
two and three cells have been proposed, some with sinking
at the front at the end of a wind cycle (Stevenson et al., 1974), and during
active
upwelling
(Andrews and
Hutchings, 1980; Nelson,
1985).
Andrews and Hutchings (1980) show strong evidence for large-scale frontal sinking during active upwelling as well as sinking at the coast during onshore winds.
The November 1972 transect off the Cape Peninsula (see
Fig. 7) illustrates clearly the deepening chlorophyll-rich layer in the vicinity of the front, to depths well below the euphotic zone.
Much of
the phytoplankton would be entrained in the jet current (Bang and Andrews, 1974; Shannon et al., 1981) and transported rapidly northwards. The December 1984 Frontal Zone transect, sampled during quiescent conditions, showed
chlorophyll
'a' and particle
distributions indicative of
coastal downwelling, but even the close (3 n. miles) spacing of stations failed to reveal any frontal sinking motions during this phase. (19811, discussing
tidal
fronts, shows maximum
the stratified side close to
Holligan
phytoplankton stocks on
the thermocline region, where lateral and
vertical diffusion of nutrients may be sufficient to maintain production, combined with some migration of motile species.
Clearly there are very
big differences between upwelling and tidal fronts, although the physical features may l o o k similar.
Transport of pelagic fish eggs and larvae The drogue study of Shelton and Hutchings (1982) and the distribution of anchovy eggs and early larvae (see Fig. 15) show the entrainment and rapid alongshore transport of spawning products to the recruitment grounds
on the west
coast.
Vertical distribution studies (Shelton, 1984) show
the eggs to be closely associated with the frontal zone (see Fig. 16). Successful feeding of early stage larvae in this turbulent environment, the processes
of cross-frontal mixing necessarl to move young fish to
the inshore nursery areas, and the possibility of offshore advection and
86
consequent starvation of a large proportion of the early life history stages of anchovy are of current interest to local scientists, stimulating the focus on frontal processes.
Fig. 16. Vertical temperature structure and distribution of anchovy eggs along a line which transects the front close to Cape Town in November, 1979. Note the maximum egg distribution coinciding with a sharp front. (Modified from Shelton, 1984).
CONCEPTUAL IMAGES OF THE FRONTAL SYSTEMS IN THE SOUTHERN BENGUELA All the models discussed are, of necessity, descriptive Ones as both Brink (1983) and Nelson (1981, 1985) point out the difficulties of modelling frontal systems in regions of rapid change with strong topographic features. Brink (1983) emphasizes the complex nature of cross-shelf and alongshore flow in upwelling regions. ling may
only
Secondary cross-shelf flows and frontal downwel-
occur during
particular phases of the upwelling cycle.
The situation becomes even more complex as topographic effects need to be considered, as well as the shelf-edge jet which appears to be responding to larger oceanic phenomena rather than upwelling processes alone.
For
simplicity we will list the ideas chronologically. Hart and Currie (1960) proposed a two-cell structure over a wide continental
shelf with
inshore
and shelf-break upwelling, and sinking inshoce
of the shelf-break front (see Fig. 17).
STATIONS WS1056 WS1055 WS1054 WS1053 WS1052 WS1051 WS1050
Fig. 17. Distribution of specific volume anomaly along the Orange River line, September 1950, showing cellular structure. (From Hart and Currie, 1960). Bang
(1973, 1976) proposed a complex two front, three cell type of
circulation pattern (see Fig. 18) with: (i) the coastal divergence slightly displaced from the coast by
an inner closed circulation pattern; (ii)
the "upwelling" front at 20-30 n.miles offshore with sinking on the inshore side; (iii) the
shelf-break
front, separated from the upwelling front
by a lens of Agulhas water some 30-50m thick.
There is a subsurface uplift
of cool water at the shelf-break with both shoreward and offshore sinking
88 to either side, in a similar manner to that proposed by Hart and Currie (1960).
Fig. 18. An impression of the three-dimensional structure created when southerly winds force warm, predominantly tropical water away from the coast, thus causing cold water of predominantly Antarctic origin to well up in replacement. Key elements include 1) the Jet, with sporadic eddies and partial upwelling along its inner-side; 2) shallow patches of Agulhas Current water; 3) the Benguela Front, possibly associated with a shallow northward jet; 4) the De Decker undercurrent. Note: scales are not consisNote tent but the block s approximately 70Qm thick and 100 km wide. (From Bang, also that N/S speeds are very much greater than E/W speeds. 1976)
.
Lutjeharms (1981b)
on the basis of infrared satellite imagery, showed
a series of frontal eddies (see Fig. 1) which he considered to be a result of instabilities in the jet current. Shannon et al. (1983), using satellite imagery, showed a similar set of eddies based on water colour, although Nelson (1985) cautions on distinguishing between actual physical processes in the region and biological consequences which may persist long after all physical manifestations have disappeared. Nelson (19851, in a preliminary analysis of the hydrography and currents off the Cape Peninsula, based on drogues, current profiles and moored meters, proposes no less than six different types of fronts (see Fig.
89 19).
Some of these are more
pronounced than others, particularly the
surface front over the shelf-edge between the 230-250m contours.
A strong
jet flowing northwards is associated with this front.
Fig. 19. Conceptual image of isotherm formation typically found during the Cape Upwelling Experiment (CUEX), 1978-1980 (From Nelson, 1985).
.
Andrews
and
Hutchings
(1980) proposed a one-celled
sinking at the front during upwelling conditions.
circulation with
They suggest that much
of the high phytoplankton biomass is entrained into the powerful jet current associated with the front.
However, re-examination of some of the chlorophyll
'a' and nutrient transects along the UM line made in 1971-1973 revealed a peak of chlorophyll 'a' at the front, and a minor peak of nutrients just inshore of the chlorophyll 'a' peak on some transects (see Fig. 6). This is at variance with the downward trend in nutrients along the transect and suggests that sinking at the front may be accompanied by a slight uplift in an anti-cyclonic cell (see Fig. 20a).
There is little evidence,
however, to indicate that any uplift of water is taking place at the outer edge of the front.
In agreement with Andrews and Hutchings (1980), sinking
is shown to occur at the coast during relaxations of the upwelling favourable wind stress (see Fig. 20b). CONCLUSIONS In comparison with newly upwelled water at the inshore sites of active
90
Coastal
downwelling
'I
DOWNWELLING
Fig. 20. Schematic presentation of the main direction of water movement proposed to occur during (a) upwelling and (b) downwelling. upwelling, the frontal region is, despite strong gradients, not a good example of a productive ergocline in terns of the description by Legendre et al. (1985).
Rapid changes in the position of surface features of the
front and the strong alongshore transport within the front, make it a difficult region to study on suitable time and distance scales. is
nevertheless of
considerable ecological
importance
in
the
The front southern
Benguela current, as summarised below:1)
The frontal zone in the southern Benguela current displays very
pronounced gradients compared with many other frontal regions, suggesting strong convergent flow. 2)
Very complicated water motions which fluctuate rapidly with tidal,
91 diurnal, wind and seasonal cycles occur, to the.despairof the local modelling fraternity, 3) The front may affect biological processes in a number of ways:
a)
as a convergent boundary region between cool eutrophic and
warm oligotrophic water masses, restricting energy and material fluxes with little input to the offshore environment. b)
where the strong sinking motions and subsequent re-entrainment
of organisms could be important in providing a mechanism for population maintenance within the system but, paradoxically, could also limit their growth potential in upwelled waters. c)
as a region where a large stock of aged phytoplankton exists
well below the euphotic zone, which may stimulate increased microheterotrophic activity and nutrient regeneration leading to enhanced primary productivity within the frontal zone. d)
as an area of enhanced productivity induced by intermittent
surface stabilization and/or upwelling of nutrients at the shelf-break or from an anticyclonic uplift of nutrients inshore of the front. Vertical migration by zooplankton and mesopelagic fish from the shelf-break region may also increase grazing pressures in the region of the front. e) as the location of a jet current which transports pelagic fish spawning products to the recruitment area. with the attendant problems of:
alongshore transport, offshore dispersion and onshore displacement,
and feeding of and predation on the larvae, which are closely associated with the physical dynamic processes in the frontal zone.
Of particular
interest in this regard are the eddies, common features in satellite images of the frontal zone, which may provide important mechanisms for cross frontal mixing or transport of organisms out of the system. ACKNOWLEDGEMENTS The authors would like to express their appreciation to their colleagues at Sea Fisheries and the University of Cape Town for their useful comments on the original manuscript, and for access to unpublished data.
Thanks
are also due to Mr T. van Dalsen and staff for the artwork and to Ms M. van Niekerk for typing the final document. REFERENCES Andrews, U.R.H. and Hutchings, L., 1980. Upwelling in the southern Benguela current. Prog. Oceanog., 9: 1-81. Badenhorst, A. and Boyd, A.J., 1980. Distributional ecology of the larvae and juveniles of the anchovy Engraulis capensis Gilchrist in relation to the hydrological environment off South West Africa, 1978/79. Fish. Bull. S. Afr., 13: 83-106.
92
Barlow, R.G., 1982a. Phytoplankton ecology in the southern Benguela current. I. Biochemical composition. J. Exp. Mar. Biol. Ecol., 63: 209-227. Barlow, R.G., 1982b. Phytoplankton ecology in the southern Benguela current. 111. Dynamics of a bloom. J. Exp. Mar. Biol. Ecol., 63: 239-248. Bang, N.D., 1971. The southern Benguela current region in February, 1966: Part 11. Bathythermography and air-sea interactions. Deep-sea Res., 18: 209-224. Bang, N.D., 1973. Characteristics of an intense ocean frontal system in the upwell region west of Cape Town. Tellus, 25: 256-265. Bang, N.D., 1974. The southern Benguela system: finer oceanic structure and atmospheric determinants. Ph.D. thesis, University of Cape Tawn, South Africa, 181 pp. Bang, N., 1976. On estimating the oceanic mass flux budget of lateral and cross circulations of the southern Benguela upwelling system. National Research Institute for Oceanology, Internal General Report, SEA IR 7616, 14 pp. Bang, N.D. and Andrews, W.R.H., 1974. Direct current measurements of a shelf-edge frontal jet in the southern Benguela system. J. mar. Res., 32: 405-417. Boucher, J., 1984. Localization of zooplankton populations in the Ligurian marine front: role of ontogenic migration. Deep-sea Res., 31: 469-484. Boyd, A.J., 1982. Small-scale measurements of vertical shear and rates of horizontal diffusion in the southern Benguela current. Fish. Bull. S. Afr., 16: 1-9. Boyd, A.J. and Hewitson, J.D., 1983. Distribution of anchovy larvae off the west coast of southern Africa between 32O30' and 26O30'5, 1979-1982. S. Afr. J. mar. Sci., 1: 71-75. Brink, K.H., 1983. The near-surface dynamics of coastal upwelling. Prog. Oceanog., 12: 223-257. Brown, P.C., 1984. Primary production at two contrasting nearshore sites in the southern Benguela upwelling region, 1977-1979. S. Afr. J. mar. Sci., 2: 205-215. Brown, P. and Hutchings, L., 1985. Phytoplankton distribution and dynamics in the southern Benguela current. In: C. Bas, R. Margalef and P. Rubies (Editors), International Symposium on the Most Important Upwelling Areas off Western Africa (Cape Blanco and Benguela), Barcelona, November 1983. Instituto de Investigaciones Pesqueras, Barcelona, 1: 319-344. De Decker, A.H.B., 1973. Agulhas Bank plankton. In: B. Zeitschel, (Editor), Ecological studies. Analysis and synthesis, Vol. 3. Springer-Verlag, Berlin, pp. 189-219. De Decker, A.H.B., 1984. Near-surface copepod distribution in the southwestern Indian and south-eastern Atlantic ocean. Ann. s. Afr. Mus., 93: 303-370. Hart, T.J. and Currie, R.I., 1960. The Benguela Current. Discovery Reports, 31: 123-298. Hutchings, L., 1979. Zooplankton of the Cape Peninsula Upwelling Region. Ph.D. thesis, University of Cape Town, South Africa, 223 pp. Hutchings, L., 1981. The formation of plankton patches in the southern Benguela Current. In: F.A. Richards (Editor), Coastal Upwelling. Coastal and Estuarine Sciences, American Geophysical Union, Washington D.C., 1: 496-506. Hutchings, L., Holden, C. and Mitchell-Innes, B., 1984. Hydrological and biological shipboard monitoring of upwelling off the Cape Peninsula. S. Afr. J. Sci., 80: 83-89. Holligan, P.M., 1981. Biological implications of fronts on the northwest European continental shelf. Phil. Trans. R. SOC. Lond., A302: 547-562. Kollmer, W.E., 1963. Notes on zooplankton and phxtoplankton $ollections made off Walvis Bay. Invest1 EeF. mar. Iles. Lab. S.VJ. Afr., 8: 78pp. Legendre, L., 1981. Hydrodynamic control of marine phytoplankton production: the paradox of stability. In: J.C.J. Nihoul (Editor), Ecohydrodynamics. Elsevier, Amsterdam, pp. 191-207.
93 Legendre, L., Demers, S. and Lefaivre, D., 1985. Biological production at marine ergoclines. In: J.C.J. Nihoul (Editor), Proceedings of the 17th International Liege Colloquium on Ocean Hydrodynamics. Elsevier, Amsterdam. This volume. Lutjeharms, J.R.E., 1981a. Features of the southern Agulhas current circulation from satellite remote sensing. S. Afr. J. Sci., 77: 231-236. Lutjeharms, J.R.E., 1981b. Satellite studies of the South Atlantic upwelling system. In: J.F.R. Gower (Editor), Oceanography from Space. Plenum Press, New York. Mar. Sci., 13: 195-199. Mooers, C.N.K., Collins, C.A. and Smith, R.L., 1976. The dynamic structure of the frontal zone in the coastal upwelling region off Oregon. J. Phys. Oceanogr., 6: 3-21. Nelson, G . and Hutchings, L., 1983. The Benguela upwelling area. Prog. Oceanog., 12: 333-356. Nelson, G . , 1985. Notes on the physical oceanography of the Cape Peninsula upwelling system. In: L.V. Shannon (Editor), South African Ocean Colour and Upwelling Experiment. Sea Fisheries Research Institute, Cape Town, pp. 63-95. Olivieri, E.T., Hutchings, L., Brown, P.C. and Barlow, R.G., 1985. The development of phytoplankton communities in terms of their particle size frequency distribution, in newly upwelled waters of the southern Benguela current. In: C. Bas, R. Margalef and P. Rubies (Editors), International Symposium on the Most Important Upwelling Areas off Western Africa (Cape Blanco and Benguela), Barcelona, November 1983. Instituto de Investigaciones Pesqueras, Barcelona, 1: 345-371. Packard, T.T., Blasco, D. and Barber, R.T., 1978. Mesodinium rubrum in the Baja California upwelling system. In: R. Boje and M. Tomczak (Editors), Upwelling Ecosystems. Springer-Verlag, Berlin, Heidelberg, New York, pp. 73-89. Probyn, T.A., 1985. Nitrogen uptake by size-fractionated phytoplankton populations in the southern Benguela upwelling system. Mar. Ecol. Prog. Ser., 22: 249-258. Shannon, L.V., 1966. Hydrology of the south and west coasts of South Africa. Invest1 Rep. Div. Sea Fish. S. Afr., 58: 62 ppShannon, L.V., 1985. The Benguela Ecosystem. Part 1. Evolution of the Benguela, physical features and processes. Oceanogr. Mar. Biol. Ann. Rev., 23: 105-182. Shannon, L.V., Nelson, G . and Jury, M.R., 1981. Hydrological and meteorological aspects of upwelling in the southern Benguela current. In: F.A. Richards (Editor), Coastal Upwelling. Coastal and Estuarine Sciences, American Geophysical Union, Washington D.C., 1: 146-159. Shannon, L.V., Mostert, S.A., Walters, N.M. and Anderson, F.P., 1983. Chlorophyll concentrations in the southern Benguela current region as determined by satellite (Nimbus-7 coastal zone colour scanner). J. Plankton Res., 5: 565-583. Shannon, L.V., Schlittenhardt, P. and Mostert, S.A., 1984a. The Nimbus-7 CZCS experiment in the Benguela current region off Southern Africa, February 1980. 2. Interpretation of imagery and oceanographic implications. J. Geophys. Res., 89: 4968-4976. Shannon, L.V., Hutchings, L., Bailey, G.W. and Shelton, P.A., 1984b. Spatial and temporal distribution of chlorophyll in southern African waters as deduced from ship and satellite measurements and their implications for pelagic fisheries. S. Afr. J. mar. Sci., 2: 109-130. Shelton, P., 1984. Notes on the spawning of anchovy during the summer of 1982-3. S. Afr. J. Sci., 80: 69-71. Shelton, P.A. and Hutchings, L., 1982. Transport of anchovy, Engraulis capensis Gilchrist, eggs and early larvae by a frontal jet current. J. Cons. int. Explor. Mer., 40: 185-1%. Shushkina, E.A., Vinogradov, M.Ye., Sorokin, Yu.I., Lebedeva, L.P. and Mikheyev, V.N., 1978. Functional characteristics of planktonic communities in the Peruvian upwelling region. Oceanology, 18: 579-589.
94 Simpson, J.H., 1981. The shelf-sea fronts: implications of their existence and behaviour. Phil. Trans. R. SOC. Lond., A302: 531-543. Stevenson, M.R., Garvine, R.W. and Wyatt, B., 1974. Lagrangian measurements in a coastal upwelling zone off Oregon. J . Phys. Oceanogr., 4: 321-336. Taunton-Clark, J . , 1985. The formation, growth and decay of upwelling tongues in response to the mesoscale wind field during summer. In: L.V. Shannon (Editor), South African Ocean Colour and Upwelling Experiment. Sea Fisheries Research Institute, Cape Town, pp. 47-61. Thiriot, A., 1978. Zooplankton communities in the West African upwelling area. In: R. Boje and M. Tomczak (Editors), Upwelling Ecosystems. Springer-Verlag, Berlin, Heidelberg, New York, pp. 32-61. Unteriiberbacher, H.K., 1964. Zooplankton studies in the waters off Walvis Bay with special reference to the Copepoda. Invest1 Rep. mar. Res. Lab. S.W. Afr., 11: 1-42. Vinogradov, M.E. and Shushkina, E.A., 1978. Some development patterns of plankton communities in the upwelling areas of the Pacific Ocean. Mar. Biol., 48: 357-366.
95
THE DYNAMIC CONTROL OF BIOLOGICAL ACTIVITY I N THE SOUTHERN BENGUELA UPWELLING REGION G.B.
BRUNDRIT
Department o f Oceanography, U n i v e r s i t y o f Cape Town, P r i v a t e Baa, 7700 Rondebosch (South A f r i c a )
ABSTRACT Dynamic processes have been r e c o g n i s e d t o be r e s p o n s i b l e f o r t h e c o n t r o l o f s p a t i a l and temporal v a r i a b i l i t y i n b i o l o g i c a l a c t i v i t y . T h i s i s i n d i c a t e d i n t h e r e s u l t s f r o m a f i e l d s t u d y i n t h e Southern Benguela u p w e l l i n g r e g i o n i n which w a t e r t y p e s t r u c t u r e s can be c o n s i s t e n t l y i d e n t i f i e d on p h y s i c a l , chemical and b i o l o g i c a l c r i t e r i a . V a r i o u s c a n d i d a t e s f o r t h e dynamic processes which c o n t r o l i n t e r a c t i o n s b o t h w i t h i n and between t h e s e s t r u c t u r e s a r e c o n s i d e r e d and assessed. INTRODUCTION Dynamical processes i n t h e ocean have been r e c o g n i s e d t o be fundamental t o an u n d e r s t a n d i n g o f t h e s p a t i a l and temporal v a r i a b i l i t y i n b i o l o g i c a l production.
Indeed, t h e i n t e r - r e l a t i o n s between t h e two have l e d t o t h e emergence
of a new d i s c i p l i n e , w h i c h has been termed "dynamic b i o l o g i c a l oceanography" (Legendre and Demers, 1984).
I n common w i t h o t h e r c o a s t a l u p w e l l i n g r e g i o n s
( B r i n k , 1983), t h e Southern Benguela (Shannon, 1985) p r o v i d e s a d r a m a t i c cont r a s t t o t h e open ocean s u r f a c e norm w i t h .the h i g h l e v e l s o f b i o l o g i c a l a c t i v i t y which a r e t o be f o u n d i n t e r m i t t e n t l y w i t h i n i t s b o u n d a r i e s . I t i s t h u s an i d e a l f i e l d l a b o r a t o r y f o r t h e s t u d y o f dynamic b i o l o g i c a l oceanography. D u r i n g a c t i v e w i n d f o r c i n g o f c o a s t a l u p w e l l i n a , a pronounced s u r f a c e f r o n t
i s e s t a b l i s h e d some 50 km o f f s h o r e , which forms t h e ocean boundary o f t h e r e g i o n i n which t h e b i o l o g i c a l p r o d u c t i o n i s i n i t i a t e d .
Within a matter o f
days, t h e w i n d f o r c i n g a b a t e s and t h e u p w e l l i n g r e l a x e s , w h i l s t t h e b i o l o g i c a l p r o d u c t i o n develops and matures.
I n t h i s p a s s i v e phase, new dynamical proces-
ses l e a d i n g t o t h e e v e n t u a l breakdown o f t h e s u r f a c e f r o n t become i m p o r t a n t , and d i s t i n c t i v e s p a t i a l s t r u c t u r e s become a p p a r e n t i n t h e f r o n t a l r e o i o n . Recent f i e l d s t u d i e s have p r o v i d e d d e t a i l e d i n f o r m a t i o n on t h e p h y s i c a l , chemical and b i o l o g i c a l v a r i a b i l i t y o f t h e w a t e r masses w i t h i n t h e Southern Benguela r e g i o n ( H u t c h i n g s e t a l . ,
1985).
A "snapshot" f r o m one such f i e l d
study i s used t o examine t h e p o t e n t i a l r o l e o f dynamic processes i n c h a r a c t e r i s i n g t h e scope o f t h e b i o l o g i c a l p r o d u c t i v i t y . ' I t i s shown t h a t s u r f a c e a d v e c t i o n o f u p w e l l e d w a t e r , p r o g r e s s i v e l y warmed
96
through i n s o l a t i o n , i s t h e dominant process i n f u e l i n g t h e b i o l o g i c a l product i o n d u r i n g t h e a c t i v e phase o f t h e u p w e l l i n g c y c l e .
Thus t h e p r o d u c t i o n i s
c o n f i n e d t o t h e s u r f a c e mixed l a y e r b e i n g d r i v e n longshore and o f f s h o r e from t h e t o p o g r a p h i c a l l y determined c e n t r e o f u p w e l l i n g .
Thereafter, during the
passive phase, isopycnal i n t e r l e a v i n g o f water masses leads t o a smearing of t h e d e f i n i t i o n o f t h e f r o n t and t o " s i n k i n g " o f p r o d u c t i v e s u r f a c e waters below t h e p h o t i c zone.
These conclusions a r e a i d e d by t h e d e t a i l e d p r o f i l e s
o f s a l i n i t y which i s d i s t i n g u i s h e d as a c o n s e r v a t i v e v a r i a b l e throughout t h e u p w e l l i n g processes. FIELD DATA The f i e l d d a t a was c o l l e c t e d d u r i n g a 40 km t r a n s e c t , o f f s h o r e from Cape Columbine (see F i g . l ) , as p a r t o f t h e F r o n t a l Zone C r u i s e o f t h e South A f r i c a n Sea F i s h e r i e s Research I n s t i t u t e d u r i n g December 1984 (Hutchings e t al.,
1985).
T h i s t r a n s e c t had been chosen so as t o cross t h e Cape Columbine
u p w e l l i n g plume, which i s shown i n i t s a c t i v e phase i n t h e sea s u r f a c e temperatures a l s o marked i n F i g . 1 ( a f t e r Nelson and Hutchings, 1983).
The
t r a n s e c t c o n s i s t e d o f n i n e s t a t i o n s a t a spacing o f 5 km, which crossed an i l l - d e f i n e d temperature f r o n t between s t a t i o n s FZ06 and FZ07.
S t a t i o n FZ05
was reDeated as s t a t i o n FZ05A.
F i g . 1. L o c a t i o n map o f t h e study area i n t h e Southern Benguela u p w e l l i n g r e g i o n , showing t h e l i n e o f t h e Cape Columbine t r a n s e c t and t h e t y p i c a l sea s u r f a c e temperatures found n o r t h o f Cape Columbine and n o r t h of Cape Town i n t h e a c t i v e phase o f t h e u p w e l l i n g c y c l e ( a f t e r Nelson and Hutchings, 1983).
91
A t each s t a t i o n , CTD p r o f i l e s were taken, c u r r e n t s measured, a r o s e t t e sampler c o l l e c t e d b o t t l e samples o f w a t e r f r o m v a r i o u s depths, and n e t samples were taken.
From among t h e r e s u l t s o f t h e v a r i o u s p h y s i c a l , chemical and b i o -
l o g i c a l analyses u n d e r t a k e n l a t e r , t h e p a r t i c u l a r q u a n t i t i e s used i n t h i s s t u d y were temperature, s a l i n i t y , n i t r a t e and c h l o r o p h y l l .
Other studies a r e being
undertaken and w i l l appear i n t h e l i t e r a t u r e ( H u t c h i n g s e t a l . ,
1985).
CTD p r o f i l e s o f t e m p e r a t u r e and s a l i n i t y w i t h d e p t h a t t h e i n n e r m o s t s t a t i o n FZO1, and a t s t a t i o n s FZ05A and FZO9 on e i t h e r s i d e o f t h e f r o n t , a r e t a k e n as t y p i c a l and a r e shown i n F i g . 2 .
There i s a s e p a r a t i o n between s u r f a c e and
subsurface w a t e r s a t a b o u t 30 m d e p t h on each p r o f i l e .
A t s t a t i o n FZO9, t h e
s u r f a c e w a t e r appears t o be a w i n d mixed, sun warmed e x t e n s i o n o f t h e subs u r f a c e water.
T h i s i s n o t t h e case a t s t a t i o n FZOSA, where t h e s u r f a c e w a t e r Takina i n s o l a t i o n i n t o
bears no d i r e c t r e l a t i o n t o t h e w a t e r beneath i t .
account, t h e s u r f a c e w a t e r i s t o o c o o l and n o t s u f f i c i e n t l y s a l i n e t o have been formed by w i n d m i x i n g o f s u b s u r f a c e w a t e r a t t h e s t a t i o n .
There i s t h u s a con-
t r a s t i n b o t h t e m p e r a t u r e and s a l i n i t y between t h e s u r f a c e w a t e r s a t t h e s e two stations.
T h i s c o n t r a s t i s n o t r e f l e c t e d i n s u b s u r f a c e w a t e r s , though t h e s e
A t t h e i n n e r m o s t s t a t i o n FZO1, t h e s u b s u r f a c e
appear t o r i s e towards t h e c o a s t .
water c o n s i s t s o f a b o t t o m mixed l a y e r , w h i l e t h e s u r f a c e w a t e r shows a c t i v e sun warming i n i t s t e m p e r a t u r e p r o f i l e .
Teprature 1p
OC
2
10
, 5 I.
I
.
,
Stn FZ05A
4
.
.
,
,
315
Salinity
,
ppt.
, ,
,
,
,
, , 31
F i g . 2. CTD p r o f i l e s o f t e m p e r a t u r e and s a l i n i t y w i t h p r e s s u r e ( d e p t h ) a t s t a t i o n s FZO1, FZ05A and FZO9.
98
A complete s e t of temperature and s a l i n i t y values from rosette sampler
depths a t each s t a t i o n on the transect are displayed on a T-S diagram (see Fig. 3 ) . These r e s u l t s extend the comments already made concerning the surface and subsurface waters. In particular, i t should be noted t h a t i t i s the increase o r decrease in s a l i n i t y between the surface and subsurface water a t each station which i d e n t i f i e s the surface water on e i t h e r side of the front.
Fig. 3 . T-S diagram, w i t h density as sigma-t, f o r a l l stations of the Cape Columbine transect. The t h i n lines join values a t each s t a t i o n , between 0 and 30 m depth.
99 More i m p o r t a n t l y , i t can be seen t h a t t h e s u b s u r f a c e w a t e r , below 30m depth, e x h i b i t s a c o n s i s t e n t c o r r e l a t i o n between t e m p e r a t u r e and s a l i n i t y .
This axis
o f t e m p e r a t u r e and s a l i n i t y i s South A t l a n t i c C e n t r a l Water, w h i c h i s t h e source o f a l l t h e w a t e r w h i c h e v e n t u a l l y forms t h e s u r f a c e w a t e r s .
A t each
s t a t i o n , t h e T-S v a l u e s i n t h e s u r f a c e l a y e r a r e j o i n e d t o g e t h e r ( s e e F i g . 3 ) . I t can be seen t h a t t h e r e a r e c r o s s - s h o r e g r a d i e n t s i n t h e v a l u e s o f temperat u r e , s a l i n i t y and s i g m a - t a t t h e t o p o f t h e s u b s u r f a c e w a t e r .
This i s useful
i n f o l l o w i n g t h e s u b s u r f a c e w a t e r i n t o t h e s u r f a c e l a y e r above 30 m. D e s p i t e t h e m o d i f i c a t i o n o f t h e t e m p e r a t u r e o f s u r f a c e w a t e r by i n s o l a t i o n , t h e o r i g i n o f t h e s u r f a c e w a t e r can s t i l l be t r a c e d t h r o u g h i t s s a l i n i t y . O u t s i d e t h e f r o n t , t h e s u r f a c e w a t e r i s a sun-warmed e x t e n s i o n o f i t s subsurface counterpart.
I n s i d e t h e f r o n t , i t i s c l e a r t h a t upwelling, i n t h e
sense o f s u b s u r f a c e w a t e r e n t e r i n g t h e s u r f a c e l a y e r , i s o n l y t a k i n g p l a c e a t the innermost s t a t i o n s .
The s u r f a c e w a t e r a t o t h e r s t a t i o n s i n s i d e t h e f r o n t
i s advected f r o m i n s h o r e l o c a t i o n s upstream where u p w e l l i n g has p r e v i o u s l y occurred.
C u r r e n t m e t e r i n g c o n f i r m s t h i s view, and i t may be c o n j e c t e d t h a t
some o f t h e s u r f a c e w a t e r had i t s o r i g i n i n t h e c e n t r e o f u p w e l l i n g a t Cape Town (see F i g . 1).
I n o r d e r t o examine t h e v a r i a b i l i t y i n n u t r i e n t s and b i o l o g i c a l p r o d u c t i v i t y , t h e T-S diagrams a r e r e p e a t e d i n F i g s . 4 and 5, b u t show s p o t v a l u e s o f n i t r a t e and c h l o r o p h y l l r a t h e r t h a n s t a t i o n number.
I n F i g . 4, t h e n i t r a t e v a l u e s
a l o n g t h e C e n t r a l Water a x i s c o n f i r m p r e v i o u s o b s e r v a t i o n s (Andrews and Hutchi n g s , 1980) t h a t n u t r i e n t s a r e c o r r e l a t e d w i t h t e m p e r a t u r e and s a l i n i t y w i t h i n C e n t r a l Water.
The e x c e p t i o n t o t h i s i s t o be f o u n d i n t h e b o t t o m mixed l a y e r
a t t h e i n n e r m o s t s t a t i o n s FZOl and FZ02, where t e m p e r a t u r e and s a l i n i t y a r e c o r r e l a t e d i n t h e manner o f C e n t r a l Water, b u t n u t r i e n t v a l u e s a r e enhanced. I n t h e s u r f a c e w a t e r s , t h e n i t r a t e l e v e l i s reduced below t h e C e n t r a l Water values.
I t remains r e l a t i v e l y h i g h a t t h e i n n e r m o s t s t a t i o i , b u t decreases t o
low l e v e l s o f f s h o r e , i n d i c a t i n g how t h e s u r f a c e w a t e r s have been s t r i p p e d o f n u t r i e n t s by b i o l o g i c a l p r o d u c t i o n .
I t s h o u l d be n o t e d t h a t t h e r e a r e no
gradients i n surface n u t r i e n t values across t h e f r o n t . The T-S diagram w i t h s p o t v a l u e s o f c h l o r o p h y l l ( s e e F i g . 5 ) c o n f i r m s t h a t C e n t r a l Water can be c o n s i d e r e d t o be c o n s e r v a t i v e i n n u t r i e n t s as t h e r e i s no b i o l o g i c a l production present.
I n t h e surface water, the c h l o r o p h y l l l e v e l s
p r o v i d e c o n t r a s t s between t h e i n n e r m o s t s t a t i o n , t h e s t a t i o n s i n s h o r e o f t h e f r o n t , and t h e s t a t i o n s o u t s i d e t h e f r o n t .
This confirms t h e d i s t i n c t i o n s
made on t h e b a s i s o f s a l i n i t y d e s p i t e t h e l a c k o f c o n t r a s t i n n u t r i e n t l e v e l s across t h e f r o n t .
T h i s agreement between s a l i n i t y and c h l o r o p h y l l can be
h i g h l i g h t e d i n an o f f s h o r e t r a n s e c t f o r t h e upper 50 m depth, w h i c h shows s a l i n i t y c o n t o u r s and s p o t v a l u e s o f c h l o r o p h y l l ( s e e F i g . 6 ) .
Except a t t h e
innermost s t a t i o n FZO1, t h e p r o d u c t i o n i s c o n f i n e d t o w a t e r which i s above t h e s a l i n i t y maximir.
100
Fig. 4. T-S diagram with spot values of n i t r a t e f o r a l l s t a t i o n s .
f
3 D
m 7
% m
"
0 00
I
3 0
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3512 salinity
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Fig. 5. T-S diagram with spot values o f chlorophyll f o r a l l s t a t i o n s .
101
Station No
F i g . 6. Contours o f s a l i n i t y and s p o t v a l u e s o f c h l o r o p h y l l f o r a l l s t a t i o n s o f t h e Cape Columbine t r a n s e c t , above 50 m depth.
WATER TYPE STRUCTURES AND D Y N A M I C CONTROL The v a r i o u s w a t e r t y p e s t r u c t u r e s w i t h i n t h e s u r f a c e and s u b s u r f a c e w a t e r s o f t h e Southern Benguela r e g i o n can be i d e n t i f i e d on p h y s i c a l , chemical and b i o l o g i c a l grounds, and t h e r e i s a c o n s i s t e n c y between t h e r e s u l t s o f t h e i d e n t i f i c a t i o n s w h i c h c o n f i r m s t h e b a s i c h y p o t h e s i s o f dynamic b i o l o g i c a l oceanography. South A t l a n t i c C e n t r a l Water can be i d e n t i f i e d on t h e b a s i s o f t h e demonstri+
A l l the other
t e d c o r r e l a t i o n s between i t s p h y s i c a l and chemical p r o p e r t i e s .
water t y p e s t r u c t u r e s can t h e n be c o n s i d e r e d t o be m o d i f i c a t i o n s o f C e n t r a l Water i n w h i c h o u t s i d e f l u x e s r e n d e r t h e v a r i o u s p r o p e r t i e s n o n - c o n s e r v a t i v e . S a l i n i t y i s d i s t i n c t i v e i n r e t a i n i n g i t s conservative character. The s u b s u r f a c e w a t e r i n t h e b o t t o m mixed l a y e r a t t h e i n n e r m o s t s t a t i o n s i s enhanced i n n u t r i e n t s .
The s u r f a c e w a t e r t h e r e i s sun-warmed,
i t s n u t r i e n t s due t o r a p i d l y d e v e l o p i n g p r o d u c t i o n .
and i s l o s i n g
The s u r f a c e w a t e r s i n s i d e
t h e f r o n t a r e i n s h o r e s u r f a c e w a t e r s f u r t h e r m o d i f i e d i n t e m p e r a t u r e and p r o g r e s s i v e l y s t r i p p e d o f n u t r i e n t s as t h e y a r e advected o f f s h o r e . O u t s i d e t h e f r o n t , t h e s u r f a c e w a t e r s a r e a sun warmed m o d i f i c a t i o n o f t h e s u b s u r f a c e waters beneath, w i t h n u t r i e n t s l o s t i n p r o d u c t i o n , l o n g s i n c e completed. Having e s t a b l i s h e d t h i s s e t o f w a t e r t y p e s t r u c t u r e s which a r e c o n s i s t e n t l y i d e n t i f i e d on t h e b a s i s of t h e i r p h y s i c a l , chemical and b i o l o g i c a l p r o p e r t i e s ,
i t i s p o s s i b l e t o examine c a n d i d a t e s f o r t h e dynamic processes which may be
102
responsible f o r the control of these structures. South Atlantic Central Water i s i t s e l f a mixture of S o u t h Atlantic Surface Water and Antarctic Intermediate Water (Shannon, 1985). Over a long period of time, various mixing processes will have been responsible f o r the uniform trend of properties which characterise the water. Residual evidence of these properties can be seen i n the familiar step features present i n the CTD profile of station FZO9 (see Fig. 2 ) . For the remaining water type structures the dynamic controls may a c t within the structure or between adjacent structures. Clearly the wind will mix the surface waters, b u t the evidence i s t h a t only outside the front does the wind mixed layer extend completely t h r o u g h t o the subsurface waters. Indeed, i t i s t h i s contact which has p a r t i a l l y determined the character of the South Atlantic Central Water. Inside the f r o n t , the wind i s not actively entraining subsurface water b u t the onset of high winds may well change t h i s s i t u a t i o n . Rather, i t i s the indirect action of the w i n d in driving the upwelling a t the coast and the offshore advection of the surface water which provides the important dynamic control of the surface water inshore of the front. Insolation must also play an important role in t h i s process, and investigations are needed which take both insolation and wind driven surface transport into account. Only thenwill the detail of the dynamic control of the surface water inshore of the f r o n t become apparent. In the bottom mixed layer a t the innermost s t a t i o n s , i t should be noted that the time scale needed for the nutrient enhancement i s long compared t o the upwelling cycle time scale. Thus t h i s layer must be considered to be relatively permanent and the upwelling must proceed over i t . Candidates f o r the control of t h i s structure should be sought within these constraints. I t i s now possible t o turn to the interaction between the various water type structures. Mention has already been made of the possibility of a deepening wind mixed surface layer entraining subsurface water. The advection of the surface water across the subsurface water beneath can also give r i s e t o a shear flow entrainment of subsurface water. These two mechanisms driving the entrainment and subsequent upward vertical fluxes are interconnected and the r e l a t i v e importance of each a t different phases of the upwelling cycle should be assessed. In the horizontal, advection i s important in brinoina subsurface waters i n shore and upward towards the coast and moving surface waters away from the coast inshore of the front. Across the f r o n t , the evidence s-uggests t h a t in the passive stages of upwelling, the warming of the surface waters inshore of the front will lead t o weakened density (sigma-t) gradients. The possibility of l a t e r a l interleaving o f water type structures across the front then occurs.
103
Station No
F i g . 7 . A conceptual diagram o f t h e s t a t i o n s o f t h e Cape Columbine t r a n s e c t , above 100 m depth, showing c o n t o u r s o f sigma-t f o r S o u t h , A t l a n t i c C e n t r a l Water (SACW), t h e m i x e d l a y e r s ( ) a t t h e s u r f a c e (GML) and on t h e b o t t o m a t t h e i n n e r m o s t s t a t i o n s (IBML), and t h e i n t e r l e a v i n g i n t h e r e g i o n o f t h e f r o n t . The d e p t h o f t h e s a l i n i t y maximum i s denoted b y an a s t e r i s k a t each s t a t i o n .
The s t r e n g t h o f t h i s process may be enhanced b y t h e v e r t i c a l s a l i n i t y g r a d i e n t s which w i l l t e n d t o d e s t a b i l i s e t h e w a t e r column t h r o u g h d o u b l e d i f f u s i o n (Rudd i c k and T u r n e r , 1979).
The i n t e r l e a v i n g w i l l l e a d t o t h e v e r y i m p o r t a n t c r o s s
f r o n t a l f l u x e s which accompany t h e breakdown o f t h e f r o n t .
Evidence o f i n t e r -
l e a v i n g can be seen i n t h e CTD p r o f i l e a t s t a t i o n FZ05A ( s e e F i g . 2 ) and i n t h e s a l i n i t y s e c t i o n (see Fig. 6). The c o n c l u s i o n s reached c o n c e r n i n g t h e i d e n t i f i c a t i o n o f t h e w a t e r t y p e s t r u c t u r e s i n t h e Southern Benguela r e g i o n and t h e dynamic c o n t r o l s o f t h e s e s t r u c t u r e s a r e summarised i n a conceptual diagram (see F i g . 7 ) .
This study
forms p a r t o f t h e Benguela Ecology Programme and acknowledgement s h o u l d be made t o t h e work o f members o f t h e Sea F i s h e r i e s Research I n s t i t u t e d u r i n g and f o l l o w i n g t h e F r o n t a l Zone C r u i s e .
104 REFERENCES Andrews, W.R.H. and Hutchinos, L., 1980. U p w e l l i n g i n t h e Southern Benguela c u r r e n t . Prog. Oceanogr., 9: 1-81. 1983. The near s u r f a c e dynamics of c o a s t a l u p w e l l i n g . Prog. B r i n k , K.H., Oceanogr., 12: 223-257. Hutchings, L., Armstrong, D.A. and M i t c h e l l - I n n e s , B . A . , 1985. The f r o n t a l zone i n t h e Southern Benguela c u r r e n t . I n : J.C.J. Nihoul ( E d i t o r ) , Proceedings o f t h e 1 7 t h I n t e r n a t i o n a l Liege Colloquium on Ocean Hydrodynamics. E l s e v i e r , Amsterdam. Legendre, L. and Demers, S . , 1984. Towards dynamical b i o l o g i c a l oceanography and limnology. Can. J . F i s h . Aquat. Sci., 41: 2-19. Nelson, G. and Hutchings, L., 1983. The Benguela u p w e l l i n g area. Prog. Oceanogr., 12: 333-356. Ruddick, B.R. and Turner, J.S. 1979. The v e r t i c a l l e n q t h s c a l e o f doubled i f f u s i v e i n t r u s i o n s . Deep-sea Res., 26A: 903-013. Shannon, L.V., 1985. The Benguela ecosystem, I E v o l u t i o n o f t h e Benguela, p h y s i c a l f e a t u r e s and processes. Oceanogr. Mar. B i o l . Ann. Rev., 23: 105182.
105
FRONTAL ZONES , CHLOROPHYLL AND PRIMARY PRODUCTION PATTERNS IN THE SURFACE WATERS OF THE SOUTHERN OCEAN SOUTH OF CAPE TOWN J.R.E.
LUTJEHARMS', B.R. ALLANSON'
and L. PARKER'
'National Research Institute for Oceanology, C.S.I.R., P.O.Box 320, Stellenbosch 7 6 0 0 , South Africa 'Department of Zoology and Entomology, Rhodes University, P.O.Box 9 4 , Grahamstown 6 1 4 0 , South Africa
ABSTRACT The enhancing effects of certain frontal regions in the world ocean to biological activity is becoming recognised to an increasing degree. A number of underlying mechanisms for such enhancing effects are being hypothesised. In the Southern Ocean a series of coherent fronts are formed due to a variety of physical factors, making it an ideal area to study the proposed hypotheses. Fronts in the Southern Ocean sector south of Africa are particularly good candidates for such investigations since they exhibit some of the most extreme horizontal gradients in physico-chemical variables. It has, furthermore, been noted that the primary production of the Southern Ocean as a whole is considerably lower than would be suggested by the usual controlling factors. This process is not well understood. Results of recent research cruises in the area between Africa and Antarctica show that many fronts in this area exhibit relatively high chlorophyll concentrations at the sea surface, increases in potential primary production as well as increases in photosynthetic efficiency. These fronts, because of their unique and special characteristics, may thus offer superior areas for studying not only the interaction between physicochemical factors and open ocean biological activity in general, but also Southern Ocean primary productivity in particular.
INTRODUCTION Increased biological activity at ocean fronts has been observed with increasing frequency in the last decade. Pingree g. reported summer blooms at tidal fronts in the English channel in 1975 and showed that this was related to greater stratification in those areas. In most continental shelf seas, primary production
106
b l o o m s are r e s t r i c t e d t o a s h o r t p e r i o d o f s p r i n g when a n i n c r e a s e in
stratification
nutrient
rich
of
t h e water
e u p h o t i c zone.
column
traps
phytoplankton
in
a
Primary production d e c r e a s e s once
t h e n u t r i e n t s are d e p l e t e d . N u t r i e n t r e c y c l i n g is t h e n r e s t r i c t e d Bre a kdow n o f t h i s b y t h e i n c r e a s e d s t a b i l i t y o f t h e water c o l u m n . s t r a t i f i c a t i o n a t shelf-edge
Marra
s &.,
1 9 8 2 ) or d u e
f r o n t s ( P i n g r e e , 1982; S i m p s o n ,
to t i d a l l y
1979;
induced t u r b u l e n t mixing more r e a d i l y a v a i l a b l e a n d may enhance l e v e l s of primary production. I n c r e a s e d p r i m a r y p r o d u c t i o n h a s also been o b s e r v e d a t t h e e d g e s o f d e e p - s e a e d d i e s or c u r r e n t r i n g s . Yentsch and Phinney ( 1 9 8 5 ) show t h a t p h y t o p l a n k t o n p o p u l a t i o n s i n t h e s e p e r i p h e r a l r e g i o n s e x p e r i e n c e a n e a r s t e a d y s t a t e g r o w t h r e l a t e d t o t h e rotational velocity of the rings. T h i s s u p p o r t s t h e f i n d i n g s of L u t j e h a r m s and Walters ( 1 9 8 5 ) who show i n c r e a s e d c o n c e n t r a t i o n s o f c h l o r o p h y l l a t t h e b o r d e r s o f t h e A g u l h a s C u r r e n t a nd o f f i l a m e n t s T h e a f f i n i t y f o r f r o n t s by o r g a n i s m s is o f A g u l h a s C u r r e n t water. n o t r e s t r i c t e d t o t h e lower t r o p h i c l e v e l s . O l s o n and Backus (1985) have described t h e c o n c e n t r a t i o n of f i s h a t t h e border o f a warm-core G u l f S t r e a m r i n g , w h i l e Maul e t a l . ( 1 9 8 4 ) h a v e shown t h a t t h e J a p a n e s e b l u e f i n t u n a f i s h e r i e s i n t h e G u l f o f Me xic o i s most e f f i c i e n t when c o n c e n t r a t e d a t t h e b o u n d a r y o f t h e G u l f Loop Current. S i m i l a r , a n e c d o t a l , r e s u l t s on t u n a f i s h i n g have been r e p o r t e d by L u t j e h a r m s e t a l . ( 1 9 8 5 ) . The c o n c e n t r a t i o n of b i r d s a t some f r o n t s h a s b e e n d e s c r i b e d by Abrams ( 1 9 8 5 ) a nd by A i n l e y and J a c o b s ( 1 9 8 1 ) . V a r i o u s mechanisms have been proposed f o r t h e i n c r e a s e d b i o l o Most r e l y on i n c r e a s e d s t r a t i f i c a t i o n gical a c t i v i t y at fronts. &., 1 9 7 5 ; S i m p s o n , 1 9 7 9 ; S m i t h a nd N e l s o n , 1 9 8 5 ) (Pingree w h i l e some a s s u m e a m e a s u r e of h o r i z o n t a l c o n v e r g e n c e w h i c h is b e l i e v e d to c o n c e n t r a t e buoyant, non-motile organisms a t f r o n t s ( O l s o n and Back u s , 1985; A i n l e y and J a c o b s , 1 9 8 1 ) . S u c h mechan i s m s w o u l d l e a d t o a d e c a y i n g p o p u l a t i o n a t f r o n t s , w h i c h is c e r t a i n l y n o t true o f a l l i n s t a n c e s where i n c r e a s e d p h y t o p l a n k t o n S i m p s o n 3 &. c o n c e n t r a t i o n s were o b s e r v e d a t t h e sea s u r f a c e . ( 1 9 7 9 ) i n f a c t report a h e a l t h i e r p h y t o p l a n k t o n s t a n d i n g crop a t a s h e l f sea f r o n t t h a n i n t h e mi x ed water i n s h o r e . Additional m e c h a n i s m s b a s e d o n c u r r e n t s h e a r h a v e b e e n p r o p o s e d by L u t j e h a r m s et al. (1985). (Dooley,
1981)
s
mak es
nutrients
107
SOUTHERN OCEAN FRONTS The Southern Ocean is divided into zonal, circum-global bands by a series of well-defined, distinctive fronts (Deacon, 1 9 3 7 ) . More closely spaced measurement in the last decade have resolved detail of these fronts and of some surface fronts not recognised before (Sievers and Emery, 1978; Lutjeharms and Emery, 1 9 8 3 ) . This meridional zonation is also reflected in the distribution of organisms as has, for instance, been shown by Hedgpeth ( 1 9 6 9 ) in regards to a selected group of marine invertebrates. Although Southern Ocean fronts may act as biogeographical barriers (Tranter, 1 9 8 2 ) they are by no means totally impervious to the movement of species (Voronina, 1 9 6 2 ) . Certain fronts in the Southern Ocean have also been shown to be areas in which a greater concentration of phytoplankton standing stock (Plancke, 19771, primary productivity (Allanson et al., 1 9 8 1 ) and birds (Ainley and Jacobs, 1981; Abrams, 1 9 8 5 ) are to be found. Fronts also seem to be preferred areas of spawning for krill, the main Antarctic macro-zooplankton (Tranter, 1 9 8 2 ) . Deacon ( 1 9 8 2 ) has discussed each individual frontal area and its biogeographical impact in detail. He shows that the Antarctic Divergence, the line between average easterly and westerly winds, may be an area of local, short-lived upwelling due to Ekman divergence. Tranter ( 1 9 8 2 ) believes that enhanced primary productivity at this front is less likely to be due to nutrient enrichment from deeper layers than to light-associated factors related to the wind regime, that is, vertical movement will inhibit loss of organisms from the euphotic zone. The Antarctic Polar Front is a potent biogeographical barrier. Tranter ( 1 9 8 2 ) considers that this may be partially due to the vertical migration, in the case of krill larvae, between surface waters which have a northerly drift component and deeper waters which have a southerly component in the Antarctic zone. Apart from being a biogeographical boundary, the Antarctic Polar Front also has some species specific enhancing and degrading properties (Deacon, 1 9 8 2 ) . Some species are least abundant at the front, with higher catches north or south of it, while some have their highest concentrations at the front. Deacon ( 1 9 8 2 ) states that the Sub-Antarctic Front, about which little is as yet known, has also been observed to act as a biogeographical front. The Sub-Tropical Convergence, which is the most intense front at the sea surface in a number of Southern Ocean locations,
108
marks the limit of many warm-water species and forms the conventional northern boundary of the Southern Ocean. From the above it seems clear that although all major fronts in the Southern Ocean have been shown to probably possess significant biogeographical effects, data about the exact causes for these effect and the underlying mechanisms are sparse. Enhanced primary productivity, as observed at some fronts, holds special import since the low primary productivity of the Southern Ocean as a whole is not well understood. PRIMARY PRODUCTION IN THE SOUTHERN OCEAN In most parts of the world ocean, primary productivity is nutrient limited. In the Southern Ocean nutrient concentrations, with the exception of silica, are high south of the Sub-Tropical Convergence. South of the Antarctic Polar Front all nutrients, including silica, are present in high concentrations. One would therefore expect high primary production, but this has not been observed (Holm-Hansen % 1977). It was thought that low concentrations of silica might be an inhibiting factor in the Sub-Antarctic zone, between the Sub-Tropical Convergence and the Antarctic Polar Front, while low temperatures of antarctic waters might limit algal growth rates elsewhere. Witek % (1982), however, considered nutrients as being totally non-limiting in the western Antarctic and in fact observed net-phytoplankton only in water exhibiting a well-developed thermal stratification. Fogg and Hayes ( 1 9 8 2 ) also consider hydrographic stratification to be of paramount importance. Tranter ( 1 9 8 2 ) was convinced that primary production in the Southern Ocean is limited primarily by light and recent results seem to support his conclusion (Tilzer et al., 1985). During summer, when sufficient light is available, wind stress is frequently so great that phytoplankton is mixed well below the compensation depth where gains by photosynthesis are lost in respiration. Such a mechanism is not unique to the Southern Ocean. In the area south of 28OS in the southwestern tropical Pacific a combination of available light and mixed-layer depth is the limiting factor to sea surface chlorophyll (Dandonneau and Gohin, 1 9 8 4 ) . Marra and Boardman ( 1 9 8 4 ) , working on the ice edge zone in the Weddell Sea, have also come to the conclusion that phytoplankton distributions are regulated by the aiailability of light. Jennings et al. ( 1 9 8 4 ) have recently used a new productivity estimate for the Weddell Sea, based on the seasonal deple-
c.,
c.
109
tion of nitrate, phosphate and silicic acid in the surface layer. Their estimate is far in excess of most reported measurements of productivity in the open ocean areas of the Southern Ocean casting doubt on previous assessments. In a review on nutrient cycles in marine antarctic ecosystems, Holm-Hansen ( 1985), considering many of the conflicting conjectures on primary productivity in the Southern Ocean mentioned above, comes to the conclusion that our understanding of the interaction between biological-physical-geochemical processes in the Antarctic is meagre. The frontal systems south of Africa, because of their special characteristics, may be particularly amenable to studies that could resolve some of these problems. FRONTS AND BIOLOGICAL ACTIVITY SOUTH OF AFRICA The frontal systems between Africa and Antarctica consist of the Sub-Tropical Convergence, sometimes enhanced by the Agulhas Front, the Sub-Antarctic Front, the Antarctic Polar Front, an ephemeral Antarctic Divergence and the Continental Water Boundary at the edge of the Antarctic continental shelf. The detail of these fronts has been studied extensively in the past few years (Lutjeharms and Emery, 1983; Lutjeharms and Foldvik, 1985; Lutjeharms and Rickett, 1985; Lutjeharms, 1985b). Their average locations, widths and characteristics have been established (Lutjeharms, 1985a) both from measurements by expendable bathythermograph probes and by conductivity-temperature-depth units. The results are in good agreement with those of a statistical analysis of all available sea surface temperature measurements (Lutjeharms and Valentine, 1984). The results show that the Sub-Tropical Convergence south of Africa exhibits some of the strongest sea surface temperature gradients observed. It is an area of extreme mesoscale variability (Lutjeharms and Baker, 1980) which stretches from the Agulhas Retroflection region to the longitude of the Crozet Islands (Lutjeharms and van Ballegooyen, 1984). This variability is due to meanders in the front and to active eddy shedding to both sides of the front (Lutjeharms and Valentine, 1985). The Sub-Antarctic Front was observed on almost all cruises, in the area, lies at 6 ' s (Lutjeharms and Valentine, 1984) and exhibits an about 4 increasing step-like structure in the vertical with the onset of summer (Lutjeharms and Foldvik, 1985). This step-like morphology is also found to develop in the Antarctic Polar Front which
110
furthermore shows little meridional wandering. The characteristic vertical thinning of the subsurface temperature minimum in the Antarctic sector, which is assumed to correspond to the Antarctic Divergence, is seldom found. Few direct measurements of the Continental Water Boundary are available, but when observed it is a very explicit feature. Some measurements of phytoplankton in the Sub-Tropical Convergence area in 1 9 7 3 and 1 9 7 4 showed increases in biomass at the northern edge of the convergence (Plancke, 1 9 7 7 ) . Ichimura and Fukushima ( 1 9 6 3 ) had found very similar features, but have also reported peaks in chlorophyll 2 content at the Antarctic Polar Front. The influence of these fronts was also observed in the distribution of pelagic birds (Abrams, 1 9 8 5 ) .
I 3
tn
c
18-
n W 16
-
14-
z
W I-
Feb
12-
V W
2
310
10
~
Mar 1984
-
3 n
*
a W
86-
-
4-
-
AgF
AgF STC
APF
Fig. 1 . The meridional distribution of sea surface temperature and sea surface concentration of chlorophyll 5 along a transect between Antarctica and Cape Town during the months February to March 1 9 8 4 . These data were collected from the icebreaker Shirase and reported by Hamada et al. ( 1 9 8 5 ) . The significant peaks in surface chlorophyll a t t h 7 Antarctic Polar Front -(APF), the Sub-Tropical Convergence (STC) and the Agulhas Front (AgF) are evident.
111
F u r t h e r s i m u l t a n e o u s m e a s u r e m e n t s o f c h l o r o p h y l l and h y d r o g r a p h i c v a r i a b l e s have shown some v e r y d i s t i n c t i v e p a t t e r n s ( A l l a n s o n et a l . , 1981; L u t j e h a r m s e t a l . , 1 9 8 5 ) . An example is g i v e n i n
a
Fig. 1. A l i n e of continuous c h l o r o p h y l l m e a s u r e m e n t s from a d e p t h o f 8 m was u n d e r t a k e n between Syowa S t a t i o n on t h e A n t a r c t i c c o n t i n e n t and Cape Town.
T h e s e r e a d i n g s formed p a r t of t h e J a p a n -
ese c o n t r i b u t i o n t o t h e i n t e r n a t i o n a l S I B E X programme, were undert a k e n from t h e i c e b r e a k e r S h i r a s e and s u b s e q u e n t l y r e p o r t e d by Hamada e t a l . ( 1 9 8 5 ) . On p r o c e e d i n g n o r t h w a r d t h e s u r f a c e temper a t u r e s rose s l o w l y from a b o u t 1'C t o 3,l'C a t 52' where t h e r e was an n o t a b l e i n c r e a s e o f more t h a n 2OC o v e r a s h o r t m e r i d i o n a l distance. ranges
T h i s c o r r e s p o n d s e x a c t l y t o t h e l a t i t u d i n a l and t h e r m a l
given
Lutjeharms
for
and
the
Antarctic
Valentine
Polar
(1984).
Front
The
sea
south of surface
Africa
by
chlorophyll
r e a d i n g s e x h i b i t a v e r y s i g n i f i c a n t peak a t t h i s s u r f a c e express i o n o f t h e A n t a r c t i c P o l a r F r o n t w i t h v a l u e s t e n times t h a t found i n normal
Antarctic
Two v e r y s t r o n g h o r i z o n t a l
S u r f a c e Water.
t h e r m a l g r a d i e n t s were a l s o found a t 44' and 42'5. The f o r m e r ' s t h e r m a l s i g n a l i s t h a t o f t h e S u b - T r o p i c a l C o n v e r g e n c e , b u t i t is located f u r t h e r south than usual. The l a t t e r i s t h e A g u l h a s Front. Both f r o n t a l f e a t u r e s h a v e c o r r e s p o n d i n g p e a k s i n t h e sea 1). A small peak a t 38's l i e s a t t h e l o c a t i o n o f t h e presumed n o r t h e r n b o r d e r of t h e Agulh a s C u r r e n t . A l t h o u g h i t m i g h t n o t be a s t a t i s t i c a l l y s i g n i f i c a n t f e a t u r e i n t h i s i n s t a n c e , i t is i n t e r e s t i n g t o n o t e t h a t L u t j e harms and Walters ( 1985) o b s e r v e d enhanced c h l o r o p h y l l c o n c e n t r a t i o n s a t t h e same l a n d w a r d e d g e o f t h e A g u l h a s C u r r e n t on a previous occasion. The S u b - A n t a r c t i c F r o n t may have been l o c a t e d a t 49's a c c o r d i n g t o s u r f a c e t e m p e r a t u r e g r a d i e n t s . T h i s corresponds t o an i n c r e a s e d c h l o r o p h y l l g v a l u e , b u t w i t h o u t s u b s u r f a c e readings to determine a c c u r a t e l y the l o c a t i o n of the Sub-Antarctic F r o n t t h i s a p p a r e n t c o r r e l a t i o n c a n n o t be a s c e r t a i n e d unambiguously. T h e s e h i g h v a l u e s found a t t h e a b o v e m e n t i o n e d f r o n t s a r e e x t r e m e by S o u t h e r n Ocean s t a n d a r d s . Their strong geographical c o r r e l a t i o n s w i t h t h e f r o n t s i n d i c a t e t h a t t h e s e a r e a s may be
surface chlorophyll distribution (Fig.
i m p o r t a n t f e a t u r e s of p r i m a r y p r o d u c t i o n i n t h e S o u t h e r n Ocean a s
a whole.
However, s i n c e o n l y s u r f a c e c h l o r o p h y l l
a
was d e t e r m i n e d
i n t h i s p a r t i c u l a r case, i t is q u i t e p o s s i b l e t h a t t h e s e concent r a t i o n s were d u e t o a d v e c t i v e p r o c e s s e s o n l y which c o u l d have accumulated s u r f a c e organisms, productivity.
and
not
due
to enhanced primary
112
e.
Allanson et ( 1 9 8 1 ) have also undertaken such readings between Africa and Antarctica and have presented very similar results. Some of these are portrayed in the two upper panels of Fig. 2. Concurrent subsurface temperature readings were taken to a depth of 5 0 0 m making it possible to locate accurately all fronts, but in particular the Sub-Antarctic Front and the Antarctic Divergence. A full suite of readings was also taken right up to the ice edge of Antarctica, thus resolving the Continental Water Boundary (Fig. 2). Once again significant peaks in the chlorophyll g concentrations were latitudinally correlated with the Sub-Tropical Convergence region, with the Antarctic Polar Front and particularly with the Continental Water Boundary. Smaller increases were observed at the Sub-Antarctic Front and in the stratified surface layer of Antarctic Surface Water. Potential primary production values were also determined, but not with the latitudinal resolution as the surface chlorophyll measurements (Fig. 2 A ) . A statistically significant correlation between chlorophyll g concentrations at the sea surface and potential primary production was established (Allanson et 1981). Although it is a tentative conclusion, based on only one set of data, these results agree with those of Plancke ( 1 9 7 7 ) and do point to underlying mechanisms enhancing the productivity at these fronts and not to mechanical concentration processes. Accumulating all the potential primary production estimates of four cruise tracks between Cape Town and SANAE, the South African Antarctic base, a distribution is obtained which is shown in Fig. 2C. These values have been related to the statistical average locations for these fronts according to Lutjeharms and Valentine (1984). A noticeable overlap between peaks in the combined potential primary productivity from 1 9 8 0 to 1 9 8 2 and the meridional range for the various fronts is found. These data are for different periods and for overlapping, but not identical, longitudinal bands. A perfect correspondence can thus not be expected. It may also be noted that the attenuation coefficient measured at the fronts, and given in Fig. 2C, is an order of magnitude greater than the average value for most of the Antarctic Surface Water, which is about 0,05. The maximum potential primary production at each station was determined by taking & situ samples of the phytoplankton, at various depths and incubating them on board at the light intensity measured at those depths. The depth at which maximum photosynthe-
e.,
113 1 . 30
7
r
A
r
2 0 -
0
-25
t
J I
STC
SAF
AD
APF
CWB
0 5J I00 I
200 I
300
I IL
#
400
500
-
Bouvet Island
Assimilation number
L
p
mox
(034)
(044)
(077)
(064)
- --B - - -10_ _ _I II - _I40 ________
U E
e
Attenuation
0 z
coefficient
I
30° S
0 153
0 147
I 12 0.187
k-'
50's LATITUDE
60°S
7OoS
Fig. 2. Panel A: The latitudinal variation in sea surface concentration of chlorophyll d along a cruise track between Antarctica and Cape Town during January 1 9 8 1 . Data were collected from the research and supply vessel S.A.Agulhas and results reported by Peaks in the surface chlorophyll Allanson et al. ( 1 9 8 1 ) . c o r r e s p o n d t o t h e latitudinal locations of the Antarctic Polar Front (APF), the Sub-Tropical Convergence (STC), to a lesser extent the Sub-Antarctic Front (SAF) but strongly so at the Continental Water Boundary (CWB), all shown in panel B. Panel B: The thermal structure of the upper 500 m of the water column between Antarctica and Cape Town during January 1 9 8 1 . Panel C: The accumulated potential primary production estimates for tracks of the vessel S.A.Agu1ha.s between Cape Town and the South African AntarcAssimilation tic base SANAE from December 1 9 8 0 to January 1 9 8 2 . numbers, in brackets for interfront areas, and high attenuation coefficients reinforce the biological significance of the fronts.
i
114
tic assimilation took place was thus established. This value was then divided by the biomass, as expressed in terms of chlorophyll at that depth, to derive the assimilation numbers for each station. These express the maximum efficiency of the chlorophyll present at the station, or by implication, the health of the phytoplankton population. The values established for frontal and inter-frontal regions are given in Fig. 2C. In all but one instance, the assimilation numbers for frontal areas are double that of the adjacent water masses. This shows that the phytoplankton populations at these fronts are most probably thriving there for specific environmental reasons and have not been accumulated there as part of the average advection patterns at the sea surface. CONCLUSIONS Detailed work on the frontal systems of the Southern Ocean and their interaction with the biota has only just started, but some of the preliminary results, sketched above, indicate that these frontal systems may play an important role in the overall primary productivity of this ocean. These fronts consist of areas of convergence or divergence, of high degrees of stratification and of borders between areas with highly disparate nutrient concentrations. A more intensive and detailed study of these fronts may thus lead to a better understanding of a number of different frontal processes and their impact on surface layer ecology all of which are at present poorly understood. These include problems concerning the production limiting role of silicate in the SubAntarctic zone, the role of stratification in enhancing productivity, the dynamics of the various frontal zones themselves as well as the role of fronts in the overall primary production of the Southern Ocean as a whole. REFERENCES Abrams, R.W., 1985. Energy and food requirements of plegic aerial seabirds in different regions of the African sector of the Southern Ocean. In: W.R. Siegfried, P.R. Condy and R.M. Laws (Editors), Antarctic Nutrient Cycles and Food Webs, Springer-Verlag, Heidelberg, pp. 466-472. Sea-bird affinities for Ainley, D.G. and Jacobs, S.S., 1981. ocean and ice boundaries in the Antarctic. Deep-sea Res., 28A: 1173-1185. Allanson, B.R., Hart, R.C. and Lutjeharms, J.R.E., 1983. Observations on the nutrients, chlorophyll and primary production of the Southern Ocean south of Africa. S. Afr. J. Antarc. Res., lO/ll: 3-14.
115
Dandonneau, Y. and Gohin, F., 1984. Meridional and seasonal variation of the sea surface chlorophyll concentration in the southwestern tropical Pacific (14 to 32"S, 160 to 175OE). Deep-sea Res., 3 1 : 1377- 1393. Deacon, G.E.R., 1937. The hydrology of the Southern Ocean. Discovery Reports, 15: 1-124 Deacon, G.E.R., 1982. Physical and biological zonation in the Southern Ocean. Deep-sea Res., 29A: 1-15. Dooley, H.D., 1981. The role of axially varying vertical mixing along the path of a current in generating phytoplankton production. Phil. Trans. R. SOC. Lond., A302: 649-660. Fogg, G.E. and Hays, P.K., 1982. The relative importance of nutrients and hydrographic features for the growth of Antarctic plankton. Joint Oceanographic Assembly, Halifax, Abstracts, A6.4: 60-61. Hamada, E., Taniguchi, A, Okazaki, M. and Naito, Y., 1985. Report on the phytoplankton pigments measured during the JARE-25 cruise to Syowa Station, Antarctica, November 1983 to National Institute of Polar Research, Japanese April 1984. Antarctic Research Expedition, JARE Data Reports No. 103 (Marine Biology 7) , 89 pp. Introduction to Antarctic Zoogeography. Hedgpeth, J.M., 1969. In: V.D. Bushnell (Editor), Distribution of selected groups of marine invertebrates in waters south of 35's latitude. American Geographical Society, Antarctic Map Folio Series, Folio 11, 44 pp. + 29 plates. Holm-Hansen, O., 1985. Nutrient cycles in Antarctic marine ecosystems. In: W.R. Siegfried, P.R. Condy and R.M. Laws (Editors), Antarctic Nutrient Cycles and Food Webs, SpringerVerlag, Heidelberg, pp. 6-10. 0. , El-Sayed, S.Z., Franceschini, G.A. and Holm-Hansen, Cuhel, R.L., 1977. Primary production and the factors controlling phytoplankton growth in the Southern Ocean. In : G.A. Llano (Editor), Adaptions within Antarctic Ecosystems, Smithsonian Institution, Washington, D.C., pp. 11-50. Ichimura, S. and Fukushima, H., 1963. On the chlorophyll content in the surface water of the Indian and the Antarctic Oceans. Bot. Mag., Tokyo, 76: 395-399. Jennings, J.C., Gordon, L.I. and D.M. Nelson, 1984. Nutrient depletion indicates high primary productivity in the Weddell Sea. Nature, 308: 51-54. Lutjeharms, J.R.E., 1985a. Location of oceanic frontal systems between Africa and Antarctica. Deep-sea Res., in press. Lutjeharms, J.R.E., 1985b. Detail of the upper thermal structure of the Southern Ocean between South Africa and Prydz Bay during March-May 1984. S. Afr. J. Antarc. Res., in press. Lutjeharms, J.R.E. and Baker, D.J., 1980. A statistical analysis of the meso-scale dynamics of the Southern Ocean. Deep-sea Res., 27A: 145-159. Lutjeharms, J.R.E. and Emery, W.J., 1983. The detailed thermal structure of the upper ocean layers between Cape Town and Antarctica during the period Jan-Feb 1978. S. Afr. J. Antarc. Res. , 13: 4-14. Lutjeharms, J.R.E. and Foldvik, A., 1985. The thermal structure of the upper ocean layers between Africa and Antarctica during the period December 1978 to March 1979. S. Afr. J. Antarc. Res. , in press. Lutjeharms, J.R.E. and Rickett, L., 1985. Changes in the structure of thermal ocean fronts south of Africa over a three month period. S. Afr. J. Sci., in preparation.
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Lutjeharms, J.R.E. thermal fronts
and Valentine, H.R., south of Africa.
Southern Ocean Deep-sea Res., 31A:
1984.
1461- 1 4 7 6 .
Lutjeharms, J.R.E. and Valentine, H.R., 1 9 8 5 . The formation of eddies at the Sub-Tropical Convergence south of Africa. J. phys. Oceanogr., in preparation. Topographic Lutjeharms, J.R.E. and van Ballegooyen, R.C., 1 9 8 4 . control in the Agulhas Current system. Deep-sea Res., 31A: 1321- 1 3 3 7 .
Lutjeharms, J.R.E. and Walters, N.M., 1 9 8 5 . Ocean colour and thermal fronts south of Africa. In: L.V. Shannon (Editor), The South African Ocean Colour and Upwelling Experiment, Sea Fisheries Research Institute, Cape Town, in press. 1985. Lutjeharms, J.R.E., Walters, N.M. and Allanson, B.R., Oceanic frontal systems and biological enhancement. In : W.R. Siegfried, P.R. Condy and R.M. Laws (Editors), Antarctic Nutrient Cycles and Food Webs, Springer-Verlag , Heidelberg, pp. 1 1 - 2 1 . Late winter chlorophyll a Marra, J. and Boardman, D.C., 1 9 8 4 . distributions in the Weddell Sea. Mar. Ecol. Prog. Ser., 19: 19 7- 2 0 5 .
Marra, J., Houghton, R.W., Boardman, D.C. and Neale, P.J., 1 9 8 2 . Variability in surface chlorophyll 2 at a shelf-break front. J. mar. Res., 4 0 : 5 7 5 - 5 9 1 . Maul, G.A., Williams, F., Roffer, M. and Sousa, F.M., 1984. Remotely sensed oceanographic patterns and variability of bluefin tuna catch in the Gulf of Mexico. Oceanol. Acta, 7 : 469-479.
Olson, D.B. and Backus, R.H., 1985. The concentrating of organisms at fronts: a cold-water fish and a warm-core Gulf Stream ring. J. mar. Res., 4 3 : 1 1 3 - 1 3 7 . Pingree, R.D. , Mardell, G.T. , Holligan, P.M. , Griffiths, D.K. and Smithers, J., 1 9 8 2 . Celtic Sea and Armorican current structure and the vertical distributions of temperature and chlorophyll. Cont. shelf Res., 1 : 9 9 - 1 1 6 . Pingree, R.D., Pugh, P.R., Halligan, P.M. and Forster, G.R., 1975. Summer phytoplankton blooms and red tides along tidal fronts in the approaches to the English channel. Nature, 2 5 8 : 672-677.
Plancke, J., 1 9 7 7 . Phytoplankton biomass and productivity in the Subtropical Convergence area and the shelves of the western Indian subantarctic islands. In: G.A. Llano (Editor), Adaptions within Antarctic Ecosystems, Smithsonian Institution, Washington, D.C., pp. 5 1 - 7 3 . Sievers, H.A. and Emergy, W.J., 1 9 7 8 . Variability of the Antarctic Polar Frontal Zone in the Drake Passage - Summer 1976-1977. J. geophys. Res. , 8 3 : 3 0 1 0 - 3 0 2 2 . Simpson, J.H., Edelsten, D.J., Edwards, A., Morris, N.C.G. and The Islay Front: physical structure and Tett, P.B., 1 9 7 9 . phytoplankton distribution. Est. coast. mar. Sci., 9: 7 1 3- 7 2 6 .
Smith, W.O. and Nelson, D.M., 1 9 8 5 . Phytoplankton biomass near a receding ice-edge in the Ross Sea. In: W.R. Siegfried, P.R. Condy and R.M. Laws (Editors), Antarctic Nutrient Cycles and Food Webs, Springer-Verlag, Heidelberg, pp. 7 0 - 7 7 . Tilzer, M.M., von Bodungen, 8. and Smetacek, V., 1 9 8 5 . Lightdependance of phytoplankton photosynthesis in the Antarctic Ocean: implications for regulating productivity. In : W.R. Siegfried, P.R. Condy and R.M. Laws (Editors), Antarctic Nutrient Cycles and Food Webs, Springer-Verlag , Heidelberg , pp. 6 0 - 6 9 .
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Tranter, D.J., 1 9 8 2 . Interlinking of physical and biological processes in the Antarctic Ocean. Oceanogr. Mar. Biol. Ann. Rev., 2 0 : 1 1 - 3 5 . Voronina, N.M., 1 9 6 2 . On the dependance of the character of the boundary between Antarctic and Sub-Antarctic pelagic zones on the meteorological conditions. American Geophysical Union Monographs, 7 : 1 6 0 - 1 6 2 . Witek, Z., Pastuszak, M. and Grelowski, A., 1 9 8 2 . Net-phytoplankton abundance in western Antarctic and its relation to environmental conditions. Meeresforshung, 2 9 : 1 6 6 - 1 8 1 . Yentsch, C.S. and Phinney, D.A., 1 9 8 5 . Rotary motions and convection as a means of regulating primary production in ware core rings. J. geophys. Res., 90: 3 2 3 7 - 3 2 4 8 .
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FRONTAL SYSTEMS IN THE GERMAN BIGHT AND THEIR PHYSICAL AND BIOLOGICAL EFFECTS G. KRAUSE, G. BUDEUS, D. GERDES, K. SCHAUMANN and K. HESSE Institut fur Meeresforschung, Am Handelshafen 12, 2850 Bremerhaven (Federal Republic of Germany)
ABSTRACT Results of an interdisciplinary study on fronts in the German Bight are presented. During normal summer conditions a frontal system is a permanent feature north of the East Frisian Islands. It separates well mixed coastal water from stratified North Sea water. In the transition zone one observes a cold salty belt with a front towards the well mixed and a separate front towards the stratified regime. Fronts of the river plume type occur in the eastern part of the bight. Upwelling water in the region of the old Elbe Valley near Helgoland is separated from surrounding water by well developed fronts. Physical observations are discussed together with biological implications as accumulation of organisms and the role of fronts for the distribution of zoo- and phytoplankton communities. It is shown that strong fronts are not necessarily associated with high biomass accumulation. In a case study an intensive phytoplankton bloom is traced back to small horizontal inhomogeneities in the surface layer.
INTRODUCTION Existence and relevance of a frontal zone in the German Bight are well known from early hydrographic observations. In the literature this transition area between coastal water influenced by river discharge, and the open North Sea is described as the “converyence of the German Bight” (Goedecke, 1941; Dietrich, 1950). More recent measurements by the use of modern profiling equipment, towed instruments and remote sensing techniques have revealed a rather complicated structure within this convergence area (Becker and Prahm-Rodewald, 1980). Typical space scales of mesoscale fronts, meanders and eddies range between 5 and 20 km. Some of the fronts have life times in the order o f half a tidal cycle, others may persist throughout summer time. The dynamics of these structures is superimposed on large tidal variability. The compli-
120
cated hydrographic situation is reflected by a large heterogeneity of phytoplankton, zooplankton and benthic communities. In view of the great importance of shelf sea fronts for spreading and mixing processes and as interfaces between different associations of organisms, an interdisciplinary research group of physical and biological oceanographers has been formed to assess the role of fronts for the transport and spreading of substances and organisms in the German Bight. This paper presents results obtained from case studies at different types of fronts found in the area.
STATE OF KNOWLEDGE AND OBJECTIVES OF THE STUDY The German Bight is a shallow shelf sea with water depths between 20 and 40 m. Its remarkable topographic features are the Old River Elbe Valley cutting through the flat bottom towards the Northwest and the existence of large tidal flats.
4 basic processes are responsible for the occurrence of fronts in this area. 1. The competition between tidal stirring and heat input at the surface during summer results in a stratification of the water
column in the deeper part and a well-mixed region near the coast. At places where the stratification parameter h/u3 (h water depth, u amplitude of tidal current) assumes a critical value, thermal fronts may be observed. This process has been studied extensively in the Irish Sea (Simpson and Hunter, 1974) and in the waters around the British Isles. The process has been described by potential energy models and verified by satellite IR imagery (Simpson and Bowers, 1979). 2. The fresh water entering the German Bight by the rivers Elbe and Weser gives rise to river plume fronts which have been studied in many other parts of coastal waters (Bowman and Esaias, 1978). 3 . Upwelling phenomena during easterly winds are frequently ob-
served in the region of the Old Elbe Valley. The upwelling bottom water with rich nutrient content is separated from surface water by a well developed front. This is a particular regional phenomenon of the German Bight. 4. Due to the shallow depth of the area the stirring ac'tion by
strong winds is an important factor in formation and decay of fronts. Fronts formed under the combined action of tidal stirring, wind mixing and fresh water input were studied in Liverpool Bay (Czitrom, 1 9 8 2 ) . In the German Bight the relative importance of the different and combined types of fronts, their influence on the mixing process and on the marine ecosystem were rather unknown at the beginning of this study. The working group agreed to concentrate on the following questions: Physical oceanography The most urgent task was to improve the regional knowledge on fronts. More specifically, where does what type of front occur in the German Bight and what is its persistence? Besides the evaluation of satellite images this work relies on in-situ measurements to obtain the vertical structure and tidal displacement of fronts. Such regional knowledge is not only required to develop or to apply adequate mathematical models but also to assess the biological relevance of fronts. For both purposes, special experiments are required to investigate circulation and cross-frontal mixing. The use of rhodamin as a tracer is very well suited for such investigations, and respective experiments were planned and carried out in the meantime (Franz et al., 1 9 8 2 ) . The larger scale description of spreading and mixing of river water into the German Bight requires sequences of synoptic water mass distribution. As yellow substances are indicators of river water, a third sub-project investigates the use of these substances as a tracer for remote sensing by an air-borne LIDAR s y s tem. This system also provides chlorophyll and transparency distributions. It is described by Diebel-Langohr et al. (1985 b, c), and results are presented in this volume (Diebel-Langohr et al., 1985 a). Chemical and Biological Oceanography Inorganic nutrients constitute one of the major growth factors for the development of phytoplankton. Studies on distribution and dynamics of nutrients is a necessary prerequisite for the understanding of phytoplankton activity, development and distribution in relation to fronts.
122
The oxygen regime in the German Bight is of particular interest because of the repeated observation of low oxygen concentrations in the bottom water (Rachor and Albrecht, 1983). As this phenomenon is closely associated with the upwelling type of front a special project is devoted to the generating mechanisms of such water masses. The scientific goals of the biological projects concentrate on the process of accumulation and crossfrontal exchange of phyto-, myco- and zooplankton in close cooperation with the exchange of physical quantities. The bottom fauna group is particularly interested in the question whether the possible downward transport of larvae in a front is important for the recruitment of benthic communities. The vertical transport of particles and organisms would be important for quality and composition of the bottom substrate and the benthic associations. E.g. we suppose that the occurrence of large mud deposits in the areas where fronts are observed owe their existence to this mechanism.
RESULTS Many ship surveys throughout the year have not only confirmed the existence of the 3 basic types of fronts to be expected, but they have also shown some new phenomena. The eastern part of the Bight (fig. 1) is characterized by a predominant occurrence of fronts of the river plume type, upwelling fronts were found near Helgoland, and fronts in the transition zone between well-mixed and stratified water occur north of the East-Frisian Islands. This subdivision of the Bight into at least 3 different hydrographic regimes is reflected by the bottom sediments (fig. 2) and the associations of the macrozoobenthos (fig. 3 ) . In the water column 4 different zooplankton communities can be identified which are characteristic for - water of the inner German Bight (stations P2 - P4, fig. 3 ) which is directly influenced by the rivers Elbe and Weser
-
North Sea water in the western part of the Bight (stations P12 P16)
-
near-shore water in the south-westerly part (stations PO, P1, P10, Pll), a mixing product of coastal water and water of the
open North Sea
-
123
-
near-shore water in the north-easterly part (stations P17
-
P19), where mixing of coastal water, water of the inner German Bight and North Sea water takes place. Details on the zooplankton communities can be found in Gerdes ( 1 9 8 5 ) and later in the text where their interrelations and accu-
mulation at fronts are discussed. In the following we present results from case studies for the different types of fronts which occur in the area.
Fig. 1. Areas of the German Bight with predominant occurrence of: A : thermal fronts B: fronts caused by upwelling C: river plume type of fronts
Fig. 2 . Distribution of bottom sediments in the German Bight and adjacent areas (after Salzwedel et al., 1985)
124 Fig. 3. Associations of the macrobenthos in the deeper sublittoral of the German Bight and adjacent areas in October 1975 (after Salzwedel et al. 1985)
Fig. 4 4 water masses as identified by 4 different zooplankton communities. (after Gerdes, 1985)
125
s:q
1
I
I
-10.0 10
-
20
-
30 rn
7
T/ "C
Fig. 5. Temperature and salinity distribution on a section perpendicular to the coast along 7"40'E in June 1984. The colder and saltier water in the vicinity of station 505 suggests upwelling. The bottom picture is a record of the ship's thermosalinograph. This record shows the double front system much better than the hydrographic sections.
5 nm
-
The "cold belt", region A A summer section through the transition zone perpendicular to
the coast in North-South direction is presented in fig. 5. Temperature, salinity and density show distributions with the following characteristics:
-
The transition between well-mixed and stratified water is not
-
only true for temperature, as expected, but also for salinity. Salinity has even a greater effect on density than temperature. Most of the isotherms strike the bottom rather than the surface. An area of cold water at the surface separates the stratified
-
and the well-mixed region. The area of low surface temperature is also characterized by higher salinties.
126
Fig. 6 . Temperature and salinity distribution on the same section as in fig. 5 but at a different time (August 1983). The very clear situation of fig. 5 is masked by advection of less salty water in the top layer.
10
20
30
m
5 nm
c
o
’
-
:.
T 2.0 0
.
The isotherms and isohalines tend to approach the surface before bending to the bottom. There is much evidence of upwelling in
-
the transition zone. The surface temperature and salinity was additionally measured by a thermosalinograph from the ship under way. The recordings give more information on the real gradients, and it is seen that there are two well pronounced fronts, one towards the stratified and one towards the well mixed water.
-
From time to time the clear situation in fig. 5 is disturbed by advection of less haline water in the top layer, but the general structure is maintained. The distance between the fronts is 5-10 km. Therefore the cold water in between can easily be detected by satellite IR images.
127
I
A
C
B
D
Fig. 7. Areas of colder surface water in the south western German Bight for 4 cloud-free situations redrawn from IR satellite images of the AVHRR. A: 13.5.1980, 14:20 h ; B: 18.6.1983, 13:28 h ; C: 1.9.1983, 03:24 h; D: 13.5.1984, 14.52 h
Fig. 7 displays the horizontal characteristics of the cold, salty transition zone. It extends parallel to the coastline, and due to its usual shape we call the structure the "cold belt". Towards the stratified North Sea the cold belt exhibits very complex meso-scale structures like meanders and eddies with space scales in the order of 10
-
20 km.
270"
180"
8/ 83
8/83
4/84
6/84
8/84
Fig. 8. Wind directions (dots) during periods of in-situ measurements of the cold belt. The actual time of observation is marked by an asterisk. The hatched area indicates winds in off-shore direction.
128
By evaluation of weather maps together with in-situ measurements and IR-images we have found that the cold belt is not caused by the meteorological forcing by offshore winds (fig. 8) but that it is a permanent feature during normal summer conditions.
As fig. 9 shows, our observations coincide with the transition zone where thermal fronts are predicted to occur between stratified and tidally mixed water (Pingree and Griffiths, 1978). Our observed fronts are found closer inshore than those most commonly observed in the Irish Sea. This is to be expected because the stratification parameter log (h/u3) is derived from a potential energy model which equates heat input and tidal stirring only, whereas the additional presence of river influenced stratification is not taken into account.
Fig. 9. The stratification 55” parameter log (h/c u3) in cgs-units and tfle transition area between stratified and mixed water as predicted by Pingree and Griffith (1978) together with observed fronts in several months indicated by numbers. h : water depth c , : bottom friction coefficient u : vertically averaged horizontal velocity 54”
7‘
8’
The existence of a cold region is easily understood qualitatively. In the deep part the water is stratified. Heat input and wind mixing determine the temperature of the surface mixed layer. In the shallow part the water is well mixed predominantly by tidal stirring. The incoming heat is used for warming of a shallow water column, comparable to the thickness of the well-mixed offshore surface layer.
129 In the transition zone cold bottom water is mixed with warm surface water by tidal stirring resulting in intermediate temperatures, i.e. colder surface water. This argument is supported by the experiments of Hachey (1934) and the numerical model studies of James (1984). In an experimental tank Hachey observed water flowing towards the mixing area in the top and bottom layer, and the mixed water left the area in the intermediate layer.
Fig. 10. Residual circulation and density distribution in the transition area between well-mixed and stratified water. Siniplified picture on the basis of fig. 4 e in James (1984)
A much more detailed information on the residual circulation re-
sults from the numerical model. A simplified circulation pattern is presented in fig. 10. It shows upwelling as well as a convergence zone at the surface. It is interesting that James (1984) has calculated this picture as typical for the Celtic Sea. We can add that the region A (fig. 1) of the German Bight shows very much the same characteristics. On the basis of these results from the physical environment we can expect the cold area to be richer in nutrients, and a considerable influence on the distribution of organisms seems obvious. We have not yet confirmed the nutrient situation, but the following case study on the zooplankton distribution demonstrates the important role of this front for the near-shore ecosystem. During the biological sampling the stratification did unfortunately not resemble the simple picture as in fig. 5 but the situation was that of fig. 6 . The additional seaward front (station 2 ) was caused by the presence of a water mass of higher temperature and lower salinity.
130
Fig. 11. The hydrographic situation on a section perpendicular to the coast along 7"40'E. The biological samples were taken at 4 stations in August 1983.
m150p
250pm
501 I I n
LO
n
TE 30 TI
-
20
10
0
Fig. 12. Abundance and composition of the zooplankton populations in different depths at 4 stations across the cold belt region. Stations 1 to 4 as in fig. 11. s = surface, p= pycnocline, b = close to bottom sample
131
As the abundance distribution of zooplankton organisms (fig. 11) shows, the highest abundance in the surface water is at the front (station 2). Even higher values only occur in the pycnocline at station 1. By means of the composition of the zooplankton populations we can distinguish 3 water masses North Sea water near the bottom of stations 1 and 2, dominated by copepods (90 % ) and characterized by indicator organisms (Sagitta elegans). Near-shore water (station 4) with high values for copelata and lower values for copepods. Water of the inner German Bight (surface water at station 3) with medium dominance of copepods and typical organisms like the cladocera Podon intermedius and Evadne nordrnanni (compare Gerdes, 1985). Also the chlorophyll-a content was more than two times higher as in any other samples. At the front station the composition of the zooplankton population is remarkably different with a very high concentration of echinoderm larvae (33,000 Ind. mW3), which have no locomotive power. Thus, this front appears as a barrier and a region where certain species are accumulated. Fronts induced by upwelling, region B In the region of the Old Elbe valley upwelling situations occur mainly during easterly winds. Cold water of higher salinity, occasionally with lower oxygen content (Rachor and Albrecht, 19831, is separated from the surrounding water by a clear front. One such occasion was investigated, and the hydrographical situation is depicted in fig. 13. station i %a
0 321
327
331 3 3 1 -1?L
Fig. 13. A front at station 2 caused by upwelling of bottom water in region B. The section is 3 nautical miles NW of Helgoland at the slope of the old Elbe valley (August 1982).
132
As indicated in fig. 14 the highest concentration of organisms occurs in the front (station 2). In contrast to the cold belt fronts, some cross-frontal exchange of organisms takes place as it is obvious from the distribution of echinoderm larvae and the crustaceans (fig. 15).
20000
15000
0
Copepoda ubrige Crustacea Polychaeta Echinodermata Copelata Mollusca 0 t o t a l Ind
a ezm
1i i
3
2
1
Station
Fig. 14. Abundances of zooplankton organisms in the surface water of the upwelling front (see fig. 13) and adjacent water masses.
133
Crustacea Echinoderrnata m Mollusca EXI Copelata E m Polychaeta
"1. 100
C(
00 70
60 50 40
30 20
10 3
2
1 S t at i on
Fig. 15. Corresponding composition of the zooplankton populations on the transect crossing the front.
River plume type of fronts, region C For a river plume type of front we present a biological phenomenon in the distribution of phytoplankton. In October 1982 this front was investigated NE of Helgoland. After following the front over a tidal cycle the ship was anchored, and measurements of water temperature and salinity were performed at 15 minute intervals. The physical data were processed immediately so that controlled biological sampling could be achieved on the basis of the physical situation. The isopleth diagram of salinity (fig. 16) shows that the front approached the ship, passed it during flood time and left it after current reversal.
134
Fig. 16. Isopleths of salinity and biological samples during two passages of a front measured from the anchored ship 10 nm NE of Helgoland island in October 1982. HW = time of tidal high water; circles indicate times and positions of phyto- and mycoplankton samples.
A qualitative and quantitative plankton analysis revealed two distinct phyto- and mycoplankton associations inhabiting the two water masses on either side of the front. This is in agreement with earlier findings. However, it was recognized that there was a third association occuring just within the narrow band of frontal convergence between the two water masses. This specific association was dominated neither by one nor by a mixture of the two adjacent associations of plankton organisms, but it exhibited its very own characteristics (see fig. 17). On the other hand the total biomass of all phytoplankton individuals was not significantly different in the frontal samples as compared with the samples from the two adjacent water masses.
135
Fig. 17. Sketch of morphology and distribution of marine phytoplankton species (Bacillariophyceae) in a river plume front 10 nm NE of Helgoland island in October 1982. Illustrated are only dominating and specific organisms of the three different associations resp. water masses. Of course there have been more species, however, these were indifferent in distribution or too rare for proper assignment. Please take into account that the species illustrations have been taken from different text books and therefore the scales are different. The true natural dimensions of the species are given in brackets as follows: 1 2 3 4 5 6 7 8 9 10
= = = = = = = =
Biddulphia sinensis (210 x 140 pm) Rhizosolenia stolterfothii (119 x 27 pm) Rhizosolenia shrubsolei (362 x 10 pm) Lithodesmium undulatum (74 x 61 pm) Biddulphia regia (163 x 59 pm) Coscinodiscus radiatus ( @ 50 pm) Paralia sulcata (7 x 20 pm) Podosira stelliger ( @ 50 pm) = Thalassiosira eccentrica ( @ 63 pm) = Thalassiosira cf. anguste-lineata ( @ 21 pm)
136
These findings offer new and interesting aspects as to the origin and transport of water masses in such a front. One possible explanation would be the presence of a frontal jet carrying water with different plankton organisms just along the frontal interface, but unfortunately at that time this was not investigated by physical measurements, e.g. by current measurements or by dye distribution studies. Last but not least, the existence of very weak local inhomogeneities with frontal characteristics and their significance to the distribution of plankton organisms will be presented in an interesting case study. During one of our cruises an extensive red tide patch was observed, about 25 nm west of Helgoland. A detailed ad hoc study of the physical, chemical and biological characteristics of this patch and the surrounding waters showed, that the orange-red discoloration of the water was caused by an intensive accumulation of Noctiluca miliaris cells, a dinoflagellate, being responsible for the visible phosphorescence of the sea during night time. The Noctiluca bloom was restricted to the upper 3 to 4 meters (see table 1). On the windward side the patch was homogeneous, and it was bordered by a very sharp line. Towards the lee-side the patch was broken up into long parallel rows, most probably due to Langmuir circulations. Table 1. Vertical and horizontal biomass distribution of phytoand protozooplankton in a section across a red tide patch in the German Bight, August 2nd 1984. Station number/ depth 672/
2m 5m
bottom 673/
2m
5m
bottom 674/
2m 5m
bottom
Phytoplankton total
Biomass (mg C/1) Protozooplankton total Noctiluca (excl.)
1.006 1.300 0.056
0.201 0.239 0.026
0.180 0.220 0.020
1.234
-
3.064
3.000
1.060 2.270 0.043
0.119
0.100
0.032 0.023
0.003 0.020
-
-
-
137
There had been a stable meteorological situation with weak winds before the investigation for at least a week. It is difficult to present the results in a properly scaled cross section. The decisive changes occur on a horizontal distance of some tenth of meters, whereas the total patch phenomenon should be drawn on a km-scale (fig. 18).
‘I
.
31.8
.
10-
./ ’ / /
: solen
:
s
Fig. 18. Isopleths of salinity in a section across a red tide patch (stations no. 672-674) in the German Bight 25 nm west of Helgoland island at August 2nd 1984, illustrating a small inhomogeneity in the surface layer with wfiich an intensive accumulation of ~ o ~ t i l ~ EiLigrLs ca cells was associated.
138
In surface water we recognize a very slight local inhomogeneity ("mini-front") with lower salinities to the east. The temperature of this lower salinity water is only 1/10 of a degree higher than in the western waters. Nevertheless, the biological effects are intense as could be traced from photographs and from table 1. This table also shows that east of the Noctiluca bloom there is a secondary biomass maximum (Station 674/5m) being due to the mass development of Ceratium fusus in subsurface waters. A more detailed presentation of the biological and chemical results of this investigation will be given in a separate paper by
Schaumann et al. (1985). In connection with the study of different fronts it is important to note that even very weak fronts may cause surprisingly intense biological phenomena. DISCUSSION, CONCLUSIONS AND OPEN QUESTIONS We have presented a variety of case studies on the physical and biological phenomena which are associated with fronts in the German Bight. The first phase of this interdisciplinary study has especially increased the regional knowledge on fronts in this area. The double-front system which includes a cold belt of water north of the East-Frisian Islands is perhaps the most important new finding. We have not yet modeled the real situation, but it seems that the physics underlying the model of James (1984) for the Celtic Sea (spatial variation in tidal mixing) would also ade quately describe the cold belt fronts. However, spatial variation of tidal mixing on the slope towards the open sea is perhaps not the only important agent for the formation of this front. It might also be possible that deformations of the tidal flow field could cause the front on the sloping bottom. Observations during winter could contribute to this question because under favourable conditions a "warm belt" should exist in the latter case. Inspite of the considerable influx of fresh water the seaward position of the thermal front follows closely the prediction by a theory which only takes heat input and tidal stirring into account. The reason for this rather puzzling result is still under investigation.
Whereas the stratification parameter log (h/u3) provides an operational guideline for position finding of the thermal fronts, our regional knowledge on fronts of the river plume type is still very poor. We can only name the general area of occurence, but we do not understand the reasons why and when these fronts form at a particular place. With regard to the biological implications we have shown that all types of fronts are locations with qualitatively differentiated increased phytoplankton, mycoplankton and zooplankton densities. Of special importance is the existence of front specific plankton communities. Not in all cases it is understood whether selection or advection is the decisive factor for their formation. It is also interesting to note that certain organism groups can be regarded as indicators for fronts like the echinoderm larvae and certain diatoms. The study has clearly shown that strong fronts in physical quantities can but do not necessarily produce likewise high accumulations of biomass. It has been demonstrated that high accumulation of biomass can occur in very weak fronts or local inhomogeneities of the surface layer. This fact will make it very difficult to assess quantitatively the relative importance of the different fronts for the overall productivity of the German Bight. We expect the cold belt area to be more productive than the adjacent regions of the German Bight due to upwelling of bottom water. Besides further studies on the causes of the cold belt and river plume fronts this is the main question currently investigated. ACKNOWLEDGEMENTS Many cruises were necessary to gather the material presented in this article. The authors would like to thank to the crew of RV "Victor Hensen" for their permanent assistance and cooperation. We are also grateful to the Deutsche Forschungsgemeinschaft who is funding this work.
140
REFERENCES Becker, G.A. and Prahm-Rodewald, G., 1980. Fronten im Meer - Salzgehaltsfronten der Deutschen Bucht. Der Seewart 41: Nr. 1. Bowman, M.J. and Esaias, W.E. (Editors), 1978. Oceanic Fronts in Coastal Processes. Springer Verlag, 114 pp. Czitrom, S.P.R., 1982. Density stratification and an associated front in Liverpool Bay. Ph. D. Thesis, University of College of North Wales, Bangor, U.K. Diebel-Langohr, D., Gunther, K.P. and Reuter, R., 1983. Lidar applications in remote sensing of ocean properties. Int. Coll. on Spectral Signatures of Objects in Remote Sensing, Conf. Proc., Bordeaux. Diebel-Langohr, D., Hengstermann, T. and Reuter, R., 1985a. Identification of hydrographic fronts by air-borne- Lidar-measurements of Gelbstoff distributions. Proc. 17th International Lisge Colloquium on Ocean Hydrodynamics. Diebel-Langohr, D., Gunther, K.P., Hengstermann, T., Loquay, K., Reuter, R. and Zimmermann, R., 1985b. An air-borne Lidar system for Oceanographic measurements in: Optoelektronik in der Technik, Tagungsberichte LASER 85 - Optoelektronic, Munchen 1. - 5. Juli 1985, Springer Verlag (in press). Diebel-Langohr, D., Hengstermann, T. and Reuter, R., 1985c. Depth profiles of hydrographic parameters-measurement and interpretation of Lidar signals in: Optoelektronik in der Technik, Tagungsberichte LASER 85 - Optoelektronic, Munchen 1. - 5. Juli 1985, Springer Verlag (in press). Franz, H., Gehlhaar, U., Gunther, K.P., Klein, A., Luther, J., Reuter, R. and Weidmann, H., 1982. Airborne fluorescence lidar monitoring of tracer dye patches a comparison with shipboard measurements. Deep-sea Res. 29: 893. Gerdes, D., 1985. Zusamrnensetzung und Verteilung von Zooplankton sowie Chlorophyll- und Sestongehalte in verschiedenen Wassermassen der Deutschen Bucht in der Jahren 1982/83. Veroff. Inst. Meeresforsch. Brernerh. 20: 119-139. Hachey, H.B., 1934. Movements resulting from mixing of stratified water. J. Biol. Bd. Can l(2): 133-143. James, I.D., 1984. A three-dimensional numerical model with variable eddy viscosity and diffusivity. Cont. Shelf Res. 3: 69-98. Pingree, R.D. and Griffiths, D.K., 1978. Tidal fronts on the Shelf Seas around the British Isles. J. Geophys. Res. 83: 4615-4622. Rachor, E. and Albrecht, I I . , 1983. Sauerstoffmangel im Bodenwasser der Deutschen Bucht. Veroff. Inst. Meeresforsch. Bremerh. 19: 209-227. Salzwedel, H., Rachor, E. and Gerdes, D., 1985. Benthic macrofauna communities in the German Bight. Veroff. Inst. Meeresforsch. Bremerh. 20: 199-267. Schaumann, K., Gerdes, D. and Hesse, K.J., 1985. Biological and chemical characteristics of a Noctiluca miliaris red tide patch in the western German Bight 1984. Botanica Marina (submitted). Simpson, J.H. and Hunter, J.R., 1974. Fronts in the Irish Sea. Nature 250: 404-406. Simpson, J.H. and Bowers, D., 1979. Shelf sea fronts' adjustments revealed by satellite IR-imagery. Nature 280: 648-651.
-
141
ROLE
OF THERMAL FRONTS ON GEORGES BANK P R I M A R Y PRODUCTIOFI
P. KLEIN
L a b o r a t o i r e d'Oci'anographie Physique, F a c u l t i ' des Sciences
29287 b e s t Ci'dex ( F r a n c e )
ASSTRACT A v e r y s i m p l e p h y s i c a l - b i o l o g i c a l model has been used t o s i m u l a t e t h e seasonal c y c l e o f Georges Bank p l a n k t o n ecosystem. Numerical r e s u l t s , which agree on t h e whole w i t h a v a i l a b l e f i e l d d a t a , r e v e a l t h a t Georges Sank works as a chemostat i n w i n t e r b u t t h a t r e c y c l i n g processes a r e dominant i n summer. The t r a n s i t i o n between t h e w i n t e r and t h e summer regimes i s t r i g g e r e d by t h e appearance ( i n s p r i n g ) and disappearance ( i n f a l l ) o f t h e thermal f r o n t s around t h e bank.
I NTRO[1U C T I0 N Georges Dank i s a s h a l l o w bank, 300 km l o n g and 150 km wide, l o c a t e d a l o n g t h e seaward edge o f t h e G u l f o f !laine ( F i g . 1 ) .
L 2 O
LOo N N
mean flow
F i g. 1. Schematic map o f Georges Bank topography and d i r e c t i o n o f mean f l o w f ield.
142 F i g . 2 d i s p l a y s t h e seasonal c y c l e o f i n o r g a n i c n i t r o g e n ( w h i c h i n c l u d e s
-
-
-
b o t h n i t r a t e and ammonia), PI, p h y t o p l a n k t o n , P, and zooplankton, Z, on Georges Bank w i t h i n t h e 60 m i s o b a t h . T h i s c y c l e , w h i c h r e c u r s a n n u a l l y , d i s p l a y s two main d i s t i n c t p e r i o d s : t h e w i n t e r and t h e summer p e r i o d s s e p a r a t e d b y t r a n s i t i o n p e r i o d s . More p r e c i s e l y , i n o r g a n i c n i t r o g e n c o n c e n t r a t i o n i s h i g h i n w i n t e r b u t t h e r e g i o n appears a l m o s t n u t r i e n t d e p l e t e d i n summer. P h y t o p l a n k t o n biomass i s r e l a t i v e l y h i g h y e a r around ; d u r i n g t h e summer p h y t o p l a n k t o n b i o mass i s about h a l f o f t h e w i n t e r v a l u e . Note t h a t p r i m a r y p r o d u c t i o n v a l u e s range f r o m 0.8 gC/m2/d t o 2.5 gC/m2/d ( O ' R e i l l y e t a l . ,
1985) ; h i g h e r v a l u e s
appear between August and December. Zooplankton biomass a t t a i n s a maximum i n J u l y . B u t z o o p l a n k t o n n i t r o g e n compared w i t h p h y t o p l a n k t o n and i n o r g a n i c n i t r o gen i s v e r y l o w t h r o u g h o u t
t h e y e a r and may n o t a f f e c t p h y t o p l a n k t o n - n u t r i e n t
dynamics. Because o f t h e h i g h p h y t o p l a n k t o n biomass, z o o p l a n k t o n may n o t be f o o d - l i m i t e d . Moreover t o t a l n i t r o g e n
7 defined
as
s = 7 + 7+
displays the
same p a t t e r n as i n o r g a n i c n i t r o g e n , t h a t i s , a s t r o n g d e c l i n e i n s p r i n g and summer. These c h a r a c t e r i s t i c s r a i s e some i m p o r t a n t q u e s t i o n s . Why a r e h i g h phyt o p l a n k t o n biomass and p r o d u c t i o n r a t e s n o t r e f l e c t e d a t h i g h e r t r o p h i c l e v e l s ? IJhat a r e t h e s o u r c e o f n u t r i e n t s ? Which process ( p h y s i c a l o r b i o l o g i c a l ) i s r e s p o n s i b l e o f t h e s t r o n g seasonal d e c l i n e o f t o t a l n i t r o g e n ? As a p r e l i m i n a r y t o an e x a m i n a t i o n o f t h e s e q u e s t i o n s , i t i s u s e f u l t o b r i e f l y examine t h e seasonal e v o l u t i o n o f t h e p h y s i c a l processes i n v o l v e d i n t h i s area.
( mga t N.
F i g . 2. Seasonal c y c l e o f n i t r o g e n budget e s t i m a t e d from a v a i l a b l e d a t a : P a s t u r z a c k e t a l . (1982) and O ' R e i l l y e t a l . (1980) f o r t h e i n o r g a n i c n i t r o g e n
(n), O ' R e i l l y plankton
and E v a n s - Z e t l i n (1982) and O ' R e i l l y e t a l . (1980) f o r t h e p h y t o -
(P), and
r a t i o s : mgC/mgat
Davis (1982) f o r t h e z o o p l a n k t o n
(7). Conversions
N
1982).
= 100, mgC/mgChl = 50 ( S t e e l e ,
made use o f
143 D u r i n g t h e w i n t e r , Georges Bank a r e a w i t h i n t h e 60 m i s o b a t h as w e l l as o u t s i d e areas a r e v e r t i c a l l y w e l l - m i x e d ( a t l e a s t t o a d e p t h o f 200 m). There i s a mean ( o r l o w f r e q u e n c y ) c l o c k w i s e c i r c u l a t i o n around t h e bank ( F i g . 1). Corresponding c u r r e n t i s about 5-10 cm/s on t h e s o u t h f l a n k , b u t t h e r e i s a narrow j e t of 20 cm/s on t h e n o r t h f l a n k (Butman e t a l . ,
1982). Eecause a l l i s w e l l -
mixed, v e r t i c a l d i f f u s i o n i s h i g h ; c o n s e q u e n t l y , l o w f r e q u e n c y h o r i z o n t a l d i s p e r s i o n , which r e s u l t s f r o m t h e c o m b i n a t i o n o f t h e v e r t i c a l shear o f t i d a l c u r r e n t s w i t h t h e v e r t i c a l t u r b u l e n t d i f f u s i o n ( K u l l e n b e r g , 1978). i s l a r g e . So, a t t h i s t i m e , exchanges w i t h areas o u t s i d e t h e Georges Bank a r e l a r g e . P u r i n g t h e summer, v e r t i c a l s t r a t i f i c a t i o n o u t s i d e t h e 60 m i s o b a t h i s e s t a b l i s h e d , b u t w i t h i n t h i s i s o b a t h , t h e w a t e r s r e m a i n w e l l - m i x e d because o f t h e s t r o n g t i d a l c u r r e n t s . So t h e Georges Bank w e l l - m i x e d area i s i s o l a t e d from G u l f o f Maine and Slope w a t e r s by s t r o n g thermal f r o n t s around t h e bank. The r e s u l t i n g cross-bank d e n s i t y f i e l d a c c e l e r a t e s t h e mean c l o c k w i s e c i r c u l a t i o n around t h e bank. However, v e r t i c a l t u r b u l e n t d i f f u s i o n near t h e 60 m i s o b a t h i s s m a l l because o f t h e presence o f t h e thermal f r o n t s and t h e a s s o c i a t e d v e r t i c a l s t r a t i f i c a t i o n . Then t h e l o w f r e q u e n c y h o r i z o n t a l d i s p e r s i o n i s l o w , So, d u r i n g t h i s period, exchanges w i t h t h e o u t s i d e a r e s m a l l . T h i s s h o r t d e s c r i p t i o n o f t h e p l a n k t o n ecosystem and p h y s i c a l processes observed i n t h e Georges Eank area r e v e a l s and p u t s i n t o evidence a s t r o n g c o r r e l a t i o n between b i o l o g i c a l and p h y s i c a l processes a t t h e seasonal t i m e s c a l e . I n p a r t i c u l a r t h e t r a n s i t i o n between t h e w i n t e r and t h e summer regimes seems t o be t r i g g e r e d by t h e appearance and disappearance o f t h e thermal f r o n t s around t h e bank. So some o t h e r q u e s t i o n s a r e : how does t h e seasonal e v o l u t i o n o f p h y s i c a l processes a f f e c t t h e p h y t o p l a n k t o n - n u t r i e n t c y c l e ; what i s t h e i m p o r t a n c e o f p h y s i c a l - b i o l o g i c a l i n t e r a c t i o n s ? These q u e s t i o n s have been adressed t h r o u g h a v e r y s i m p l e numerical model. S i m u l a t i o n s have been performed t o q u a n t i f y t h e r e s p e c t i v e e f f e c t s o f p h y s i c a l and b i o l o g i c a l processes f o r t h e d i f f e r e n t per i o d s o f t h e seasonal c y c l e o f Georges Bank p l a n k t o n ecosystem. The model used and i t s numerical i m p l e m e n t a t i o n have been d e s c r i b e d i n d e t a i l i n K l e i n (1985 a, b). A s h o r t d e s c r i p t i o n i s g i v e n here. Then t h e numerical r e s u l t s a r e d i s c u s s e d ; d i s c u s s i o n i s focused on t h e d i f f e r e n t b i o l o g i c a l and p h y s i c a l budget components i n v o l v e d d u r i n g t h e w i n t e r and summer p e r i o d s .
THE MODEL Georges Bank w i t h i n t h e 60 m i s o b a t h can be r e p r e s e n t e d by an e l l i p s e whose major and m i n o r axes a r e 170 and 130 km l o n g ( F i g . 1). For sake o f s i m p l i c i t y , a p p r o p r i a t e t r a n s f o r m a t i o n s ( s e e K l e i n , 1985a) have been used so t h a t t h e t r a n s f o r m domain i s c i r c u l a r 130 km across, 60 m Peep a t t h e p e r i p h e r y and 30 m deep a t t h e c e n t e r . The model developed i s two-dimensional
( x ,y)
, homogeneous-
144 l y mixed v e r t i c a l l y b u t w i t h v a r y i n g d e p t h ( h ) . Podel e q u a t i o n s a r e w r i t t e n i n
c y l i n d a r c o o r d i n a t e s (r,O). v e l o c i t y f i e l d (Ur,Uo)
P h y s i c a l processes a r e d e f i n e d by t h e l o w frequency
and t h e l o w f r e q u e n c y h o r i z o n t a l d i s p e r s i o n (kr,kO).
B i o l o g i c a l v a r i a b l e s a r e i n o r g a n i c n i t r o g e n , N, p h y t o p l a n k t o n , P, and zooplankt o n , Z. An a d d i t i o n a l v a r i a b l e ,
the "phytodetritus",
M, i s i n c l u d e d t o comple-
t e t h e budget. U n i t s a r e i n mgatN/m3. V a r i a b l e s a r e f u n c t i o n s o f t i m e ( t ) and space c o o r d i n a t e s ( r , O ) .
-growth
de(Z) =
Equations a r e :
predation
b2 P Z
d Z2
=
-- grazing
growth f(P)
(1)
-
a N / ( N + Kn) P uptake
f(N) =
"mortality"
b P Z
P
a
regeneration
-a N/(N + Kn) P + b l
P Z + d Z2
t 6
p h y t o d e t r i t u s source
decomposition
a P
8 P
--
S(M)=
P
(3)
(4)
i s an o p e r a t o r f o r t h e t i m e and space e v o l u t i o n d e f i n e d as : advection
$.=
diffusion
I
a.
a
at
ar
r
a0
rh
ar
rhKr
a. ar
1
l a a. (5) -%y r2
ao
Boundary c o n d i t i o n s concern t h e v a l u e s o f t h e b i o l o g i c a l v a r i a b l e s on t h e p e r i p h e r y o f t h e area ( K l e i n , 1985a). The r i g h t hand s i d e o f eqs. 1 t o 4 d i s p l a y t h e b i o l o g i c a l parameters i n v o l ved. Seasonal e v o l u t i o n o f t h e p h y t o p l a n k t o n g r o w t h r a t e , a, g i v e n on F i g . 3-a c o n t a i n s e f f e c t s o f seasonal v a r y i n g l i g h t i n t e g r a t e d o v e r t h e w a t e r column. Kn i s t h e h a l f s a t u r a t i o n c o n s t a n t ( = 0.3 mgatN/m3). E s t i m a t i o n o f t h e seasonal
c y c l e o f z o o p l a n k t o n g r o w t h r a t e , b P , shown on F i g . 3-b,
i s based on t h e works
o f Davis (1982) and S t e e l e and Henderson (1981). Zooplankton g r o w t h i s assumed t o be 20 % e f f i c i e n t ( S t e e l e , 1982). So : b = 5b2. The d i f f e r e n c e bl
= b
-
b2,
w h i c h r e p r e s e n t s e x c r e t i o n f e c a l p e l l e t s and m o r t a l i t y , i s c o n s i d e r e d t o be r e c y c l e d i n t h e system a t t h e seasonal t i m e s c a l e . P r e d a t i o n r a t e on z o o p l a n k t o n i s expressed as dZ, w h i c h means t h a t t h e p r e d a t o r p o p u l a t i o n changes i n p r o p o r t i o n t o h e r b i v o r e s . Zooplankton consumed as p r e y i s supposed t o be r e c y c l e d i n t h e system. The p h y t o p l a n k t o n " m o r t a l i t y r a t e " , a, and t h e " p h y t o d e t r i t u s " r e c y c l i n g r a t e , 6, have been p a r a m e t e r i s e d and e s t i m a t e d ( F i g . 3 - c ) i n r e l a t i o n t o the net loss o f
7+
have focused on a
(%
7 since 7 i s
v e r y l o w ) . M has been i n t r o d u c e d and we
and 6 f o r t h e f o l l o w i n g reasons. The s t r o n g d e f i c i t
7+a
145 d u r i n g t h e summer cannot o c c u r t h r o u g h t h e p h y s i c a l exchanges : t h i s would r e q u i r e a s t r o n g seasonal change o f t h e boundary c o n d i t i o n s , w h i c h i s n o t t h e case ( K l e i n , 1985a). The o n l y p o s s i b l e e x p l a n a t i o n i s t h a t t h e d e f i c i t must go i n t o a n o t h e r b i o l o g i c a l compartment w h i c h i s n o t 2 ; hence t h e i n t r o d u c t i o n o f
M. Then we must assume t h a t t h e r e i s a s i n k f o r P w h i c h p r e v e n t s t o t a l r e c y c l i n g i n t o n u t r i e n t s . Parameters a and 6 a r e t h e o n l y ones w h i c h have been f i t t e d . The chosen v a l u e s a r e : a = 0.08 P
(P
> 1.75),
a = 0.14
(P < 1.75) and
B = 0.08.
a02
0.3
2
P
F i g . 3. Seasonal e v o l u t i o n o f p h y t o p l a n k t o n growth r a t e ( a ) , z o o p l a n k t o n growth r a t e ( b ) and v a r i a t i o n s o f aP and BP ( c ) . The l o w f r e q u e n c y f l o w f i e l d c o n s i d e r e d has been e s t i m a t e d d i r e c t l y from t h e a v a i l a b l e d a t a (Butman e t a l . ,
1982 ; Butman and Beardsley, 1984) and s a t i s f i e s
t h e c o n t i n u i t y e q u a t i o n . I n t h e domain considered, t h e f l o w extends near t h e edges o v e r a band w i t h a w i d t h between 1 0 km and 25 km ( K l e i n , 1985a). V e l o c i t y ranges between 1 . 4 cm/s t o 16 cm/s. H o r i z o n t a l d i s p e r s i o n c o e f f i c i e n t s Kr,
Kg
have been e s t i m a t e d by means o f c l a s s i c a l f o r m u l a s (Csanady, 1974 ; K u l l e n b e r g , 1978) and u s i n g t i d a l c u r r e n t d a t a (Moody and Butman, 1980) and e s t i m a t i o n s o f v e r t i c a l t u r b u l e n t d i f f u s i o n (James, 1 9 7 7 ) . Value f o r t h e h o r i z o n t a l d i s p e r s i o n i n t h e w e l l - m i x e d a r e a i s about 350 m 2 / s y e a r around ; t h i s v a l u e i s c l o s e t o those found b y L o d e r e t a l . (1983). On t h e edges ( 6 0 m i s o b a t h ) v a l u e s o f t h e h o r i z o n t a l d i s p e r s i o n across t h e s t r e a m l i n e s ( i . e . Kr)
range f r o m 350 m2/s du-
r i n g t h e w i n t e r t o 30 m 2 / s d u r i n g t h e summer. RESULTS : PHYTOPLANKTON
A N D NUTRIENT BUDGETS
The model has been used t o s i m u l a t e t h e annual c y c l e o f t h e Georges Bank p l a n k t o n ecosystem. Values a t t h e boundary, y e a r around, a r e Zo = 0.05, Po = 0.05, No = 8,
M0
= 0 ( i n mgatN/m3). F i g . 4 d i s p l a y s t h e s p a t i a l d i s t r i b u -
t i o n o f p h y t o p l a n k t o n i n w i n t e r and summer. P h y t o p l a n k t o n biomass i s h i g h . I n w i n t e r t h e r e i s a " h o t s p o t " o f p h y t o p l a n k t o n i n t h e c e n t e r o f t h e bank ; t h e v a l u e i s 2.65 mgatN/m3. I n summer, t h e r e i s a h i g h s p a t i a l h e t e r o g e n e i t y i n t h e n o r t h e a s t p a r t , due t o a d v e c t i o n o f n u t r i e n t . These s i m u l a t e d p a t t e r n s a r e s i m i l a r t o observed s p a t i a l d i s t r i b u t i o n s ( O ' R e i l l y e t a l . , comparison w i t h f i e l d data, o v e r t h e seasonal cycle,has
1 9 8 0 ) . A more p r e c i s e been done by a v e r a g i n g
146 P (February)
Krn
P (August)
F i g . 4 . P h y t o p l a n k t o n d i s t r i b u t i o n ( i n mgatN/m3) i n w i n t e r and summer.
(mgatN. m-3)
81
I
0-
I
Jan.
1
I
,
April
I
1
1
1
August
1
I
I
Dec
F i g . 5 . Seasonal c y c l e o f n i t r o g e n budget from n u m e r i c a l r e s u l t s .
147 t h e numerical r e s u l t s o v e r t h e Georges Bank area ( F i g . 5 ) . The modeled t i m e e v o l u t i o n o f each mean b i o l o g i c a l component agrees w e l l w i t h f i e l d d a t a ( F i g . 2 ) except f o r t h e c a l c u l a t e d z o o p l a n k t o n biomass, w h i c h i s h i g h e r t h a n t h e observed one, b u t c l o s e r t o more r e c e n t v a l u e s (Sherman e t a l . ,
1985). More d e t a i l e d
r e s u l t s , p r e s e n t e d i n K l e i n (1985a), c o n f i r m t h e r e l a t i v e good agreement, on t h e whole, o f t h e r e s u l t s f r o m t h i s v e r y s i m p l e n u m e r i c a l model w i t h a v a i l a b l e data a t t h e seasonal t i m e s c a l e . I n t h e p r e s e n t s t u d y , t h e model has been used i n o r d e r t o q u a n t i f y t h e r e s p e c t i v e e f f e c t s o f b i o l o g i c a l and p h y s i c a l processes on t h e Georges Bank p l a n k t o n ecosystem. More p r e c i s e l y , a t t e n t i o n has been f o cused on t h e d i f f e r e n t p h y t o p l a n k t o n and n u t r i e n t budget components. Equations w h i c h d r i v e t h e t i m e e v o l u t i o n o f t h e mean v a r i a b l e s can be d e r i ved from eqs. 1 t o 4 b y u s i n g a mean o p e r a t o r d e f i n e d as :
where $ d e s i g n a t e s any b i o l o g i c a l v a r i a b l e ( Z , P, N o r M ) . V and
2 are
respect
v e l y t h e volume o f w a t e r and t h e area o f Georges Bank w i t h i n t h e 60 m i s o b a t h . The r e s u l t i n g e q u a t i o n s a r e : d f / d t = b2
P Z -
d
dF/dt = a N / ( N + Kn) P dF/dt =
-
-
k,
(7- To)
-
b
- aP -
a N/(N+Kn) P + b
-
dR/dt =
km
(a -
(7 kp
P Z + d
+
(7- To) 6
7-
kn
(8)
(F - Fb)
-
(9) (10)
Mo)
where c o e f f i c i e n t s kZ, kp, kn, km a r e d e f i n e d by :
advection k
0
diffusion
i s i n f a c t an exchange r a t e between Georges Bank w a t e r and w a t e r s o u t s i d e
the bank. I n t h i s study, e x p l i c i t c o n t r i b u t i o n s o f t h e a d v e c t i v e and d i s p e r s i v e processes have been t a k e n i n t o account. They a r e d e f i n e d r e s p e c t i v e l y as :
So we have :
-
k$
(7 - To) = 7"
+
TK.
Budget terms f o r p h y t o p l a n k t o n and n u t r i e n t h i v e been e s t i m a t e d f o r t h e w i n t e r and summer p e r i o d s . U s i n g a r a t i o o f 100 f o r mgC/mgatN ( S t e e l e , 1982) and
148 assuming a mean d e p t h o f 40 m, c a l c u l a t e d v a l u e s f o r mean p r i m a r y p r o d u c t i o n a r e 2 gC/m2/d i n w i n t e r and 0.8 gC/m2/d i n summer. These o r d e r s o f magnitude appear r e a l i s t i c compared w i t h f i e l d d a t a (Cohen e t a l . ,
1982 ; O ' R e i l l y e t a l . ,
1985). P h y t o p l a n k t o n and n u t r i e n t budget terms, e s t i m a t e d as percentages o f p r i mary p r o d u c t i o n , a r e d i s p l a y e d on F i g . 6. D u r i n g t h e w i n t e r , z o o p l a n k t o n biomass i s n e g l i g i b l e and has no e f f e c t on t h e p h y t o p l a n k t o n - n u t r i e n t
dynamics. The
p h y t o p l a n k t o n f l u s h i n g - o u t r e p r e s e n t s 35 % o f t h e p r i m a r y p r o d u c t i o n . Phytop l a n k t o n " m o r t a l i t y " i s h i g h (Q 65 9 ) and more t h a n h a l f o f t h e " p h y t o d e t r i t u s " a r e f l u s h e d o u t ; t h e o t h e r p a r t , 28 %,
i s r e c y c l e d i n t h e system. The "new"
p r o d u c t i o n i s h i g h : c o n t r i b u t i o n o f p h y s i c a l processes t o n u t r i e n t s u p p l y i s 72 %.T h e r e f o r e Georges Bank i n w i n t e r seems t o work as a chemostat w i t h nut r i e n t i n p u t and an o u t f l u s h i n g o f p h y t o p l a n k t o n and " p h y t o d e t r i t u s " . Moreover 90 9: o f t h e p h y s i c a l exchanges a r e due t o t h e cross-bank h o r i z o n t a l d i s p e r s i o n ; a d v e c t i v e processes r e p r e s e n t o n l y 10 % o f t h e exchanges. D u r i n g t h e summer, z o o p l a n k t o n biomass i s l o w b u t n o t n e g l i g i b l e ; g r a z i n g s t r e s s r e p r e s e n t s 20 % o f t h e p r i m a r y p r o d u c t i o n ; t h i s i s i n agreement w i t h t h e o r d e r o f magnitude g i v e n b y Walsh (1983). Again, p h y t o p l a n k t o n " m o r t a l i t y " i s h i g h : 75 % o f t h e p r i m a r y p r o d u c t i o n , b u t p h y t o p l a n k t o n f l u s h i n g - o u t i s o n l y 5 %. "New"production r e p r e s e n t s o n l y 40 % o f t h e p r i m a r y p r o d u c t i o n ; r e c y c l i n g processes a r e dominant
(s
60 % ) and concern m a i n l y t h e " p h y t o d e t r i t u s " .
Reduction o f t h e physical
exchanges i n summer concerns m a i n l y t h e cross-bank h o r i z o n t a l d i s p e r s i o n : t h e y r e p r e s e n t o n l y 25 % o f t h e t o t a l p h y s i c a l exchanges.
F i g , 6. Flow c h a r t o f n i t r o g e n budgets i n w i n t e r and summer.
149 From t h e s e r e s u l t s i t appears t h a t t h i s i s t h e seasonal e v o l u t i o n o f t h e d i s p e r s i v e processes a t t h e edges o f t h e bank, produced by t h e appearance and disappearance o f t h e thermal f r o n t s , which d r i v e s t h e seasonal c y c l e o f Georges Bank p l a n k t o n ecosystem. P h y s i c a l exchanges due t o a d v e c t i o n a r e v e r y l o w y e a r around. In summer, t h e o n l y e x p l i c i t e f f e c t o f a d v e c t i o n ( w h i c h i s dominant a t t h i s p e r i o d ) c o u l d be a l o w e r p r i m a r y p r o d u c t i v i t y r e s u l t i n g from t h e s p a t i a l h e t e r o g e n e i t y ( K l e i n and S t e e l e , 1 9 8 5 ) . Appearance o f t h e t h e r m a l f r o n t s l e a d s t o t h e d o m i n a t i o n o f r e c y c l i n g processes whereas t h e i r disappearance l e a d s t o t h e c h e n o s t a t regime. The s t r o n g seasonal change i n p h y s i c a l exchanges a t t h e boundary e x p l a i n s t h e s t r o n g d e f i c i t o f
P
+
( F i g . 2 ) d u r i n g t h e summer. Nume-
r i c a l r e s u l t s ( K l e i n , 1985a) show t h a t t h i s d e f i c i t i s compensated by a s t r o n g increase o f t h e " p h y t o d e t r i t u s " concentration.
CO NCL US I0 I! A v e r y s i m p l e n u m e r i c a l model has been used t o i l l u s t r a t e and q u a n t i f y t h e r e s p e c t i v e e f f e c t s o f b i o l o g i c a l and p h y s i c a l processes on t h e seasonal c y c l e of t h e Georges Bank p l a n k t o n ecosystem w i t h i n t h e 60 m i s o b a t h . Numerical r e s u l t s agree, on t h e whole, w i t h a v a i l a b l e d a t a . D u r i n g t h e w i n t e r p h y s i c a l exchanges, which a r e s t r o n g l y dominated by t h e cross-bank d i s p e r s i v e processes, are v e r y i m p o r t a n t and a r e r e s p o n s i b l e o f t h e chemostat regime. However t h e r e s u l t i n g p h y t o p l a n k t o n f l u s h i n g - o u t cannot b a l a n c e t h e p r i m a r y p r o d u c t i o n . Phytoplankton " m o r t a l i t y " i s h i g h . D u r i n g t h e summer, cross-bank d i s p e r s i v e exchanges a r e s t r o n g l y reduced because o f t h e presence o f t h e thermal f r o n t s around t h e bank. Then, p h y s i c a l exchanges ( m a i n l y due t o a d v e c t i o n a t t h i s per i o d ) a r e l o w and as a consequence, r e c y c l i n g processes a r e dominant. D u r i n g t h i s period phytoplankton " m o r t a l i t y " i s very high. The h i g h p h y t o p l a n k t o n " m o r t a l i t y " and f l u s h i n g - o u t o f " p h y t o d e t r i t u s " i n w i n t e r and summer a r e s u r p r i s i n g . However t h i s phenomenon i s n o t f a r f r o m a r e c e n t c o n c l u s i o n o f Idalsh (1983) w h i c h shows t h e s i g n i f i c a n t e x p o r t o f photos y n t h e t i c carbon o f f Georges Sank. Anyway t h e " m o r t a l i t y " q u e s t i o n seems a f u n damental one t o i n v e s t i g a t e b e f o r e g o i n g f u r t h e r i n m o d e l l i n g work. T h i s s t u d y has shown t h e importance o f t h e l o w f r e q u e n c y d i s p e r s i v e processes a t t h e edges of t h e bank and t h e n o f t h e presence and absence o f t h e thermal f r o n t s around t h e bank. The e s t i m a t e d v a l u e s used f o r t h e s e d i s p e r s i v e
processes seem r e a -
sonable, a t l e a s t a t t h e seasonal t i m e s c a l e . However a t a s h o r t e r t i m e s c a l e , thermal f r o n t s a r e n o n - s t a t i o n n a r y . T h e i r e v o l u t i o n i n t h e s p r i n g and e a r l y i n t h e summer depends on t h e atmospheric f o r c i n g s and on t h e s p r i n g - n e a p c y c l e (Simpson, 1981). A t t h e end o f t h e summer and d u r i n g t h e f a l l , t h e y a r e s i g n i f i c a n t l y a f f e c t e d b y t h e developpment o f b a r o t r o - p i c and b a r o c l i n i c i n s t a b i l i t i e s (Pingree, 1978 ; G a r r e t t and Loder, 1981). These s h o r t t i m e - s c a l e e v e n t s
150 s h o u l d i n v o l v e a s i g n i f i c a n t v a r i a b i l i t y o f t h e cross-bank exchanges and t h e n a s h o r t t i m e v a r i a b i l i t y o f t h e Georges Bank p l a n k t o n ecosystem. I n v e s t i g a t i o n o f t h i s v a r i a b i l i t y could represent another o f f s h o o t o f t h i s study.
AC KNOWLE@GEMENTS
I am v e r y g r a t e f u l t o D r . John S t e e l e f o r s u g g e s t i n g t h i s problem and f o r h i s guidance and encouragement t h r o u g h o u t . T h i s work was done w h i l e t h e a u t h o r was v i s i t i n g t h e Woods H o l e Oceanographic I n s t i t u t i o n . I t was s u p p o r t e d by a FrenchUS exchange award f r o m t h e N a t i o n a l Science Foundation and b y t h e Center f o r
A n a l y s i s o f Y a r i n e Systems o f t h e Woods H o l e Oceanographic I n s t i t u t i o n . I thank C. Maze and P. Doare f o r p r e p a r a t i o n o f t h e m a n u s c r i p t .
REFERENCES Butman, B., Beardsley, R., M a g n e l l , B., Frye, D., Vermersch, J . , S c h l i t z , R., Limeburner, R., ! { r i g h t , W . and Noble, M., 1982. Recent o b s e r v a t i o n s o f t h e mean c i r c u l a t i o n on Georges Fank. J. Phys. Oceanogr., 12 : 569-591. Butman, B. and Beardsley. R., 1984. Long t e n o b s e r v a t i o n s on t h e Southern f l a n k o f Georges Bank : Seasonal c y c l e o f c u r r e n t s . S u b m i t t e d t o t h e J. Phys Oceanogr. Cohen, F., G r o s s l e i n , M., Sissenwine, If., S t e i m l e , F. and I-Jright, M . , 1982. An energy budget o f Georges Bank. I n : M. Mercer ( E d i t o r ) , M u l t i s p e c i e s Approaches t o F i s h e r i e s Kanagement Advice, Canadian S p e c i a l P u b l i c a t i o n i n F i s h e r i e s A q u a t i c Sciences, 59 : 95-107. Csanady, G., 1974. T u r b u l e n t d i f f u s i o n i n t h e environment. D. R e i d e l P u s b l i s h i n g Co., Boston, Mass. Davis, C., 1982. Processes c o n t r o l l i n g z o o p l a n k t o n abundance on Georges Bank. Phn t h e s i s , Boston U n i v e r s i t y M a r i n e Program, MBL, Woods Hole. G a r r e t t , C. and Loder, J., 1981. Dynamical aspects o f s h a l l o w sea f r o n t s . P h i l o s o p h i c a l T r a n s a c t i o n s o f t h e Royal S o c i e t y o f London, A302, pp. 563-581. James, I , , 1977. A model o f t h e annual c y c l e o f t e m p e r a t u r e i n a f r o n t a l r e g i o n o f t h e C e l t i c Sea. E s t u a r i n e and Coastal Y a r i n e Science, 5 : 339-363. K l e i n , P., 1985a. A s i m u l a t i o n o f some p h y s i c a l and b i o l o g i c a l i n t e r a c t i o n s . I n Georges Bank, Y I T Press, Cambridge, MA ( i n p r e s s ) . K l e i n , P., 1985b. A s i m p l e n u m e r i c a l method f o r ecosystem models. J . Computat i o n a l Phys. ( s u b m i t t e d ) . K l e i n , P. and S t e e l e , J., 1985. Some p h y s i c a l f a c t o r s a f f e c t i n g ecosystems. J . Mar. Res., 43 : 337-350. K u l l e n b e r g , G., 1978. V e r t i c a l processes and t h e v e r t i c a l h o r i z o n t a l c o u p l i n g . I n : J.H. S t e e l e ( E d i t o r ) , S p a t i a l P a t t e r n s i n P l a n k t o n Communities, Plenum New York, pp. 43-72. Loder, J . , W r i g h t , 0.. G a r r e t t , C. and Juszko, B., 1982. H o r i z o n t a l exchanges on Georges Bank. Canadian J o u r n a l o f F i s h e r i e s and A q u a t i c Sciences, 39 : 1130- 1137. Moody, J. and Butman, B., 1980. S e m i - d i u r n a l b o t t o m p r e s s u r e and t i d a l c u r r e n t s on Georges Bank and i n t h e M i d - A t l a n t i c B i g h t . US G e o l o g i c a l Survey Report, pp. 80-1137. O ' R e i l l y , J., Evans, C., Zdanowicz, V., O r a x l e r A., Elaldhauer, R. and M a t t e , A., 1980. B a s e l i n e s s t u d i e s on t h e d i s t r i b u t i o n o f p h y t o p l a n k t o n biomass, o r g a n i c p r o d u c t i o n , seawater n u t r i e n t s and t r a c e m e t a l i n c o a s t a l w a t e r s between Cap H a t t e r a s and Nova S c o t i a . F i r s t Annual R e p o r t N o r t h e a s t m o n i t o r i n g p r o gram. NMFS, NE F i s h e r i e s Center.
151 O ' R e i l l y , J . and E v a n s - Z e t l i n , C . , 1982. A comparison o f t h e abundance ( C h l a ) and s i z e c o m p o s i t i o n o f t h e p h y t o p l a n k t o n communities i n 20 subareas o f Georges Bank and s u r r o u n d i n g w a t e r s . I C E S CM.L : 49, pp. 9. O ' R e i l l y , J . , E v a n s - Z e t l i n , C . and Bush, C . , 1985. P r i m a r y p r o d u c t i o n : Georges Bank, G u l f o f Maine and t h e M i d - A t l a n t i c S h e l f . I n Georges Bank, MIT Press, Cambridge, MA ( i n p r e s s ) . Pastuszak, F . , W r i g h t , W . and Patango, D., 1982. One y e a r o f n u t r i e n t d i s t r i b u t i o n i n t h e Georges Bank r e g i o n i n r e l a t i o n t o hydrography. 1975-1976. J . Mar. Res., 40 : 525-542. Pingree, R., 1978. C y c l o n i c e d d i e s and c r o s s - f r o n t a l m i x i n g . J o u r n a l o f t h e Marine B i o l o g i c a l A s s o c i a t i o n o f t h e UK, 58 : 955-963. Sherman e t a l . , 1985. Zooplankton p r o d u c t i o n and t h e f i s h e r y o f t h e n o r t h e a s t s h e l f . I n Georges Bank, MIT Press, Cambridge, MA ( i n p r e s s ) . Simpson, J., 1981. The s h e l f - s e a f r o n t s : i m p l i c a t i o n s o f t h e i r e x i s t e n c e and b e h a v i o r . P h i l o s o p h i c a l T r a n s a c t i o n s o f t h e Royal S o c i e t y o f London, A302 : 531-546. Steele, J. and Henderson, E., 1981. A s i m p l e p l a n k t o n model. The American Natur a l i s t , 117 : 676-691. Steele, J., 1982. The p r o d u c t i o n o f Georges Bank. Unpublished Planuscript. Walsh, J., 1983. Death i n t h e sea : E n i g m a t i c p h y t o p l a n k t o n l o s s e s . Progress i n Oceanography, 1 2 : 1-86.
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THE ROLE OF STREAMERS ASSOCIATED W I T H MESOSCALE EDDIES I N THE TRANSPORT OF B I O L O G I C A L SUBSTANCES BETWEEN SLOPE AND O C E A N WATERS C.S.
YENTSCH a n d D . A .
PHINNEY Boothbay H a r b o r , Maine
B i g e l o w L a b o r a t o r y f o r O c e a n S c i e n c e s , W. 04575 (U.S.A.)
INTRODUCTION
With t h e d i s c o v e r y of m e s o s c a l e e d d i e s ( P a r k e r , 1 9 7 1 ; F u g l i s ter,
1972)
evident
associated with western
a m a j o r mechanism
that
ocean waters had been found.
boundary
for
the
Prior t o
currents,
i n t e r c h a n g e of recognizing
became
it
s l o p e and
the
frequency
and m a g n i t u d e o f t h e s e e d d i e s , s l o p e and o c e a n i c w a t e r i n t e r c h a n g e
was l a r g e l y b e l i e v e d t o b e d u e t o t h e a g e o s t r o p h i c r e l a t i o n s h i p s a s s o c i a t e d w i t h t h e f l o w of a boundary c u r r e n t (Rossby, 1936; F i g . 1).
Although t h e
exchanges
due
to
are slow,
these motions
they
a r e e x t e n s i v e , e s s e n t i a l l y o c c u r r i n g a l o n g t h e e n t i r e p a t h of t h e high
velocity
interchange warm
and
current,
via
cold
this
core type,
w i l l c a l l entrainment meanders
of
hence
the
one
mechanism. offer
(Fig.
Gulf
account
another
exchange
pinch
off
to
t a n c e o f t h e t w o p r o c e s s e s i s n o t known, the
large
scale
considerable both
system
form
of
the
that
we
an
eddy
that
The r e l a t i v e impor-
e n t r a i n s e i t h e r s l o p e o r S a r g a s s o Sea water. made b e t w e e n
for
eddies,
T h i s m e c h a n i s m i s i n i t i a t e d when
2).
Stream
can
Mesoscale
but a d i s t i n c t i o n can b e
i n s h o r e boundary
a flow such as
of
t h e Gulf S t r e a m and i n t e r m i t t e n t m e s o s c a l e e d d i e s t h a t c a u s e more r a p i d exchange. With
t h e advent
temperature,
a
of
third
satellites
mechanism
t h a t measure
for
slopefocean
ocean
color
and
interchange
has
a p p e a r e d which w e w i l l c a l l t h e s t r e a m e r mechanism.
It i s d i s t i n c t
f r o m t h e o t h e r two t r a n s p o r t p r o c e s s e s i n t h a t i t i s v e r y s m a l l i n scale,
o c c u r r i n g a t t i m e s c a l e s of
The a c t i o n o f
mesoscale
eddies
days t o weeks,
g i v e rise
to
r i b b o n s o f w a t e r when t h e r o t a r y m o t i o n s o f
waters
around
edge
its
e v i d e n c e w e h a v e of lite
observations
of
producing
t h e e x i s t e n c e of ocean
color
energetic.
a r i n g wrap
streamer.
a
yet
these high velocity The
outlying
first
real
streamers comes f r o m s a t e l -
and
temperature.
This
d e s c r i b e s a p e r s i s t e n t s t r e a m e r o b s e r v e d i n t h e r e g i o n of
report Georges
154
F i g . 1. T h e R o s s b y ( 1 9 3 6 ) m o d e l o f s l o p e / o c e a n i n t e r c h a n g e i n t h e Cf = C o r i o l i s f o r c e ; Pg = p r e s s u r e g r a d i e n t ; Gulf S t r e a m system. l i n e s of e q u a l d e n s i t y . ut =
Bank.
offer
We
describe the
reasons
as
to
e f f e c t s t h e y have
why on
the
streamers
t h e exchange of
appear
and
phytoplankton
between s l o p e and open-ocean waters. O b s e r v a t i o n s o f a Streamer U s i n g S a t e l l i t e I m a g e r y a n d Oceanog r a p h i c Measurements In
1984 we
G e o r g e s Bank.
observed a warm
c o r e r i n g a n d streamer s o u t h of
F i g u r e 2 i s a n AVHRR ( A d v a n c e d V e r y H i g h R e s o l u t i o n
R a d i o m e t e r ) t h e r m a l i m a g e t a k e n 13 J u l y 1 9 8 4 .
C o l d , t i d a l l y mixed
G e o r g e s Bank w a t e r a p p e a r s l i g h t c o l o r e d a n d warm w a t e r masses a r e darker. large
The ring
streamer,
warm core ring
with
strongly
a
starting
located
in
t h e n o r t h e a s t e r n q u a n d r a n t of
one-half
of
of
the
the ring,
the ring,
G e o r g e s Bank central
200 m i s o b a t h ,
t h e maximum w i d t h
Bank
water.
We
do n o t have
A
occupies
almost t o t h e n o r t h e r n edge over
the
tapering t o
t o t a l length.
streamer a p p e a r s t o b e c o n n e c t e d t o a s o u t h e r l y movement Georges
core.
is a
extending n e a r l y one-half
The streamer i s w i d e r n e a r s h o r e ,
of t h e Gulf Stream. about
warm
delineated
the region
t h e e n t i r e circumference of
s o u t h of
of
The cold
ocean c o l o r (phytoplankton
155
F i g . 2 . AVHRR t h e r m a l i m a g e o f 13 J u l y 1 9 8 5 . C o o l w a t e r a p p e a r s l i g h t c o l o r e d ; warm w a t e r i s d a r k e r . R i n g / s t r e a m e r f e a t u r e s c a n b e s e e n s o u t h o f G e o r g e s Bank. chlorophyll)
imagery
C o a s t a l Zone C o l o r
streamer
for
this
day,
S c a n n e r (CZCS)
however,
on
another
date
imagery c l e a r l y shows t h a t t h e
i s a s s o c i a t e d w i t h marked
changes
in
color
as
well
as
Presumably t h e c o l o r change i s d e r i v e d i n p a r t from
temperature.
more p r o d u c t i v e w a t e r s a d j a c e n t t o G e o r g e s Bank. During August,
a
1984),
cruise the
aboard
the
RfV C a p e H a t t e r a s
streamer
was
sampled
to
(20 July
establish
its
to 2 size,
156 depth and water column characteristics. Figure 3 shows t h e surface temperature and chlorophyll signature of the feature in an east to west
crossing,
The
continuous
temperature
signal
showed
the
streamer t o b e about 25 km wide and 4-5OC colder than surrounding waters,
Surface chlorophyll fluorescence increased by a factor of
two; actual surface concentrations measured were ca.
0.3 pg/R
in
the streamer. Three CTD/pump stations w e r e occupied, (Station S ) , west
in
one immediately east
one in the streamer (Station 6) and one immediately
t h e high
velocity
region
of
the
ring
(Station
7),
to
create a section of physical and biological parameters. I n Figure (Station
temperature feature.
4,
the
61, water and
Density
of
interior
salinity
are
(as sigma-t
horizontal structure. 20 m
of the SCM w a s
27 r
on
either
side
of
the
surfaces in Figure 5) shows little H i g h productivity water carried by (SCM).
At
over
the main
Stations 5 and
6, the
40-45 m, yet in t h e high velocity region of
EAST/ WEST TRANSECT TO STATION 6
STA 6
>
52 35w-
present
20 m
Sharp gradients of
O/oo.
f o r m s an additional maximum
subsurface chlorophyll maximum depth
the streamer is seen at 33.4
Subsurface chlorophyll concentrations (Fig.
5) form a distinctive pattern. the streamer at
of
< 1 7 O C and
334%0
No conlinuous salinily record
'7
5-. 2 5 0
I
I700
1800
I
1900
-
1
2000
10 Km
Fig. 3. Surface temperature and chlorophyll signatures,-eastrwest transect of streamer to Station 6.
157 20 Km L1
7
5
6
7
5
6
I
20 -
6080-
I I20
10035 6
1201
TEMPERATURE "C
SALl N ITY %o
F i g . 4 . T e m p e r a t u r e a n d s a l i n i t y s e c t i o n s f r o m CTDIpump s t a t i o n s . Ring e d g e i s t o t h e l e f t ( S t a t i o n 7 ) . the ring (Station 7), at
35 m.
Yentsch
t h e SCM w a s c o n s i d e r a b l y s h a l l o w e r o c c u r r i n g
and
Phinney
(1985)
found
differences
in
pro-
d u c t i v i t y a t t h e c e n t e r of a r i n g and t h e p e r i p h e r a l h i g h v e l o c i t y region;
t h e upward
bowing
d e n s i t y s u r f a c e s a t t h e edge of
a
ring
to
Georges
Bank
be
described
c a u s e t h e SCM t o o c c u r a t s h a l l o w e r d e p t h s . The p r e s e n c e yields
a
of
a
warm
cross-section
markedly b a r o c l i n i c .
core
of
ring
density
The i s o t h e r m s a t
can
the periphery
sweep upward
toward
cross-section
e x t e n d i n g f r o m G e o r g e s Bank t h r o u g h
of
a
typical
G e o r g e s Bank,
adjacent
which
tidal
front
center
coupled
f e a t u r e s a s s o c i a t e d w i t h a warm c o r e eddy. the
case
of
such
a
ring
adjacent
of
the
thus the characteristic the ring
with
the
We w i l l
ring the
i s one
baroclinic
now
t o G e o r g e s Bank a n d
of
as
consider
t h e condi-
tions f o r streamer formation. C o n c e p t u a l Model f o r S t r e a m e r F o r m a t i o n The g e n e r a l c i r c u l a t i o n a r o u n d G e o r g e s Bank
is believed Gulf
to
be
associated
with
of Maine and t i d a l p r o c e s s e s
Smith,
1981;
Butman e t a l . ,
1982).
basin
i s c l o c k w i s e and
geostrophic
on G e o r g e s B a n k
flow i n the
( B e a r d s l e y and
A conspicuous f e a t u r e of t h i s
c i r c u l a t i o n i s t h e h i g h v e l o c i t y j e t c u r r e n t on t h e n o r t h e r n f l a n k that
f o l l o w s t h e o v e r a l l c o n t o u r s o f G e o r g e s Bank, w i t h a l e s s e n -
158 20 Itm
u 5 7
6
7
5
6
I
r
I
/
Chl pg/4 @ =SCM
I20 I"i
F i g . 5. C h l o r o p h y l l a n d d e n s i t y s e c t i o n s f r o m CTD/pump s t a t i o n s . Ring edge i s t o t h e l e f t ( S t a t i o n 7 ) . Depth of t h e s u b s u r f a c e c h l o r o p h y l l maximum i s d e n o t e d b y c r o s s e d c i r c l e s . i n g of v e l o c i t y near t h e n o r t h e a s t
peak and a c o n t i n u e d
decrease
t o t h e s o u t h and e v e n t u a l l y t o t h e w e s t . D u r i n g t h e summer m o n t h s ,
s a t e l l i t e images of ocean c o l o r and
sea s u r f a c e t e m p e r a t u r e c l e a r l y o u t l i n e t h e Bank, g e n e r a l l y i n t h e
r e g i o n of outlines
the
60 m
isobath
(Yentsch
and G a r f i e l d ,
These
1981).
c o l o r and t e m p e r a t u r e a r e b e l i e v e d t o a r i s e from t h e
of
composite a c t i v i t y of b o t h t h e c l o c k w i s e f l o w and t i d a l s t i r r i n g . We
believe
that
the
general
clockwise
flow around
Georges
Bank
a s s o c i a t e d w i t h t h e f r o n t a l r e g i o n s i s i n t e r r u p t e d by t h e p r e s e n c e of
warm
a
core
ring
6).
(Fig.
Specifically,
the h i g h v e l o c i t y
r e g i o n o f a r i n g moves i n a n o r t h e a s t e r l y d i r e c t i o n a n d i n t e r r u p t s the
southwesterly
flow
on
the
southern
flank
of
Georges
Bank.
P r i o r t o t h e d i s c o v e r y of
streamers by
satellite observation,
it
was
high
productivity
by
our
belief
phytoplankton tidal
that
the
concentrations
erosion
of
thermal
primary
on
Georges
structure
Bank
We
further
transported
believed
that
laterally
off
differences
in
waters.
o t h e r words,
mass
In
with
ut
density greater
this the
augmented
Bank
between mixing
by
the over
was
the
in
turn
which
n u t r i e n t s from deep waters t o t h e overlying
Bank the
result
p h o t i c zone
growth a
suggested
flow and Bank
on
(Fig.
7).
t h e Bank w a s
arising the
of
released
from
the
surrounding
produced.
than surrounding s u r f a c e waters
a
water
and t h i s
159
....
TRANSPORT OF PRODUCTION 2 WITH (obovc) ond WITHOUT( below) A WARM CORE RING (WCR)
F i g . 6 . T r a n s p o r t o f p r o d u c t i o n o f f G e o r g e s Bank w i t h and w i t h o u t t h e p r e s e n c e o f a warm c o r e r i n g . Normal s o u t h w e s t e r l y f l o w o f f t h e Bank i s d i v e r t e d o f f s h o r e d u e t o c o u n t e r f l o w o f t h e r i n g h i g h velocity region.
N
S
F i g . 7 . C o n c e p t u a l m o d e l o f d e n s i t y d r i v e n n u t r i e n t e n r i c h m e n t on G e o r g e s Bank ( f r o m Y e n t s c h , 1 9 8 4 ) .
160
water
mass
sinks
to
a depth
excess p r o d u c t i o n
Any
estimated
that
this
is
that
is
of
identical density not
consumed
is
considerable)
s o u t h a n d w e s t b y t h e mean
locally
transported
circulation.
w a r m c o r e r i n g d i v e r t s some o f
off
Thus
the (and
Although a t
t h e presence
the of
a
t h i s f l o w s e a w a r d a s i n d i c a t e d by
t i m e we have
this
is
it
toward
s a t e l l i t e c o l o r i m e t r y and o u r s e a s u r f a c e measurements of phyll.
Bank.
no
chloro-
d i r e c t measurements of
volume t r a n s p o r t of t h e streamer, w e d i d estimate from s h i p p o s i tion two
that
t h e high velocity
knots.
velocities
This
region
of
t h e r i n g w a s i n e x c e s s of
is sufficient
velocity
to
divert
t h e slower
a s s o c i a t e d w i t h t h e f r o n t a l r e g i o n s on t h e s o u t h f l a n k
o f G e o r g e s Bank. B e n e f i t s t o P r i m a r y P r o d u c t i o n by Ring C o u p l i n g t o T i d a l Regime We b e l i e v e t h a t a r i n g l y i n g a d j a c e n t t o t h e t i d a l f r o n t s on
t h e southern
f l a n k o f G e o r g e s Bank p r o v i d e s a m e a n s f o r t r a n s p o r -
t i n g r e l a t i v e l y d e e p n u t r i e n t r i c h waters t o t h e p h o t i c zone. is
some
region
coalescence of
the
The
t h e ring t o t h e t i d a l f r o n t argues t h a t there
c l o s e p r o x i m i t y of
of
warm
the
core
frontal ring
flow
such
with
that
between t h e r i n g and t h e t i d a l f r o n t (Fig.
the
high
velocity
partnership
a
8).
exists
We h a v e d e s i g n a t e d
t h e s e t w o r e g i o n s a s t i d a l r e g i m e a n d g e o s t r o p h i c r e g i m e b a s e d on t h e p r i n c i p a l f o r c e s of each system.
Due t o t h e r o t a r y m o t i o n o f
the
water
warm
core
ring,
nutrient
rich
is
transported
along
i s o p y c n a l s u r f a c e s i n t o t h e h i g h v e l o c i t y r e g i o n of t h e eddy.
water m a s s
adjacent
to
the
t i d a l and f r o n t a l flow.
high
velocity
region
N u t r i e n t r i c h water
is
The
stirred
is transported
by
into
t h e t i d a l l y s t i r r e d e u p h o t i c zone and enhanced growth of phytoAS l o n g a s t h e w a r m c o r e r i n g s t a y s i n t h i s plankton ensues. p o s i t i o n , w e assume t h a t t h e t i d a l r e g i m e i s s u p p l i e d by n u t r i e n t s A s t r e a m e r forms as a r e s u l t of
in
t h i s manner.
of
the high productivity
around
the opposite direction. to
water
t h e Bank w i t h w a t e r
offset
any
of
the
of
carried
the
mean
circulation
t h e h i g h v e l o c i t y r e g i o n moving
The v e l o c i t i e s density
by
the interaction
appear t o be high
differences
previously
in
enough
discussed.
One c o u l d a r g u e t h a t i f i n d e e d t h i s i s a p a r t n e r s h i p o f p r o c e s s e s , f u r t h e r a u g m e n t a t i o n may o c c u r when p r o d u c t i v i t y o f t h e h i g h v e l o c i t y r e g i o n and t i d a l l y d r i v e n r e g i o n s are coupled. a t sea i n c r o s s i n g t h e s e
Our e x p e r i e n c e
streamers i s t h a t t h e y are n o t e x t r e m e l y
r i c h i n p h y t o p l a n k t o n a n d y e t f i s h a n d sea m a m m a l s a r e a b u n d a n t i n t h e i r presence.
161
C o n c e p t u a l model of r i n g c o u p l i n g t o t i d a l r e g i m e on t h e F i g . 8. s o u t h f l a n k of G e o r g e s Bank. S t i p p l e d a r e a s a r e w a t e r o f e q u a l d e n s i t y ; a r r o w s i n d i c a t e d i r e c t i o n o f f l o w ; Z e = e u p h o t i c z o n e ; N+ a n d N- = n u t r i e n t r i c h a n d p o o r w a t e r r e s p e c t i v e l y . Deep n u t r i e n t r i c h w a t e r from t h e g e o s t r o p h i c regime f l o w s a l o n g i s o p y c n a l surfaces into the tidal regime of t h e G e o r g e s Bank front. Opposing d i r e c t i o n s of f l o w along t h e f r o n t / r i n g h i g h v e l o c i t y region i n t e r f a c e d i v e r t streamer flow. Need f o r S t u d y The change
statement
cannot
imagery
--
be
--
estimates
made
of
effectively
slope
to
without
water
ocean
the
use
inter-
satellite
of
The r e a l v a l u e
is n o t new, h o w e v e r i t n e e d s r e p e a t i n g .
of s a t e l l i t e i m a g e r y comes f o r t h i n t h e e x a m i n a t i o n of a s t r e a m e r ; i t d o e s n o t seem p o s s i b l e t h a t
without
a
great
estimates of
ship crossings space
for
deal
of
slope/ocean
such
alone
to
these could be c l e a r l y elucidated
shiptime.
For
interchange, gather
estimates.
We
data have
it
in
those of
u s who c o n s i d e r
seems f u t i l e the
used
to
rely
on
a p p r o p r i a t e t i m e and satellite
imagery
in
c o n c e r t w i t h s h i p b o a r d t e c h n i q u e s t o make m e a s u r e m e n t s o f a s m a l l , h i g h l y dynamic oceanographic information streamers. gical
and
concerning
feature.
frequency,
S a t e l l i t e imagery p r o v i d e s duration
and
location
of
We s o r e l y n e e d d a t a c o n c e r n i n g t h e i r p h y s i c a l , b i o l o chemical
characteristics,
volume
transport
of m a t e r i a l s t o make t h e p r o p e r e s t i m a t e s o f f l u x .
and amounts
162 I t i s c l e a r t h a t t h e o b s e r v a t i o n o f s t r e a m e r s is c h a n g i n g o u r
ideas
of
are
how
slope
and
ocean
waters
interchange.
Perhaps
our
i d e a s a b o u t e x p o r t f r o m r i c h a r e a s s u c h a s G e o r g e s Bank
original
simplistic.
adjacent
to
The
western
same
may
be
boundary
perturbed
as
eddies
nutrients
and
phytoplankton
move
true
currents
through
all
for
their
area.
w a t e r s may
rich
other
are
which
regions
occasionally Transport
b e more
of
rapid
and
l o c a l i z e d than we a n t i c i p a t e d h e r e t o f o r e . The p r e s e n c e o f a w a r m c o r e r i n g i n t h e v i c i n i t y of t h e s o u t h f l a n k of
G e o r g e s Bank
explains
our
o b s e r v a t i o n s of
s t r o n g baro-
c l i n i c i t y associated with the t i d a l f r o n t regions along t h e south I t r e m a i n s t o b e s e e n how p e r s i s t e n t
flank. this
We a r e now t e m p t e d
region.
to
ring features are in
say t h a t t h e sustained high
p r o d u c t i o n o n G e o r g e s Bank d u r i n g t h e summer m o n t h s i s e n h a n c e d a s a r e s u l t of t h e i m p l a n t a t i o n of w a r m c o r e r i n g s i n t o t h i s r e g i o n . ACKNOWLEDGEMENTS The
authors
wish
Consortium f o r vessel Cape
Hatteras
possible, preparing the
without
and
J.
to
support, whom
Rollins
t h e manuscript.
National
acknowledge
Science
Space Administration,
t h e Duke U n i v e r s i t y M a r i n e
t h e o f f i c e r s and
this and
research
Boisvert
P.
crew o f
would for
not
t h e R/V
have
been
assistance
in
T h i s work w a s s u p p o r t e d by f u n d s from
Foundation,
the
t h e O f f i c e of
National
Aeronautics
and
Naval R e s e a r c h a n d t h e S t a t e
of Maine. This no.
is
Bigelow
Laboratory
for
Ocean
Sciences contribution
85019.
REFERENCES B e a r d s l e y , R.C. a n d S m i t h , P.C. 1981. The mean, s e a s o n a l and s u b t i d a l c i r c u l a t i o n s i n t h e G e o r g e s Bank a n d G u l f o f M a i n e region. In: T h i r d I n f o r m a l Workshop o n t h e O c e a n o g r a p h y o f U n i v e r s i t y o f New t h e Gulf of Maine and A d j a c e n t Seas. H a m p s h i r e , Durham, N H , p p . 9-16. Butman, B . , Beardsley, R.C., Magnell, B., Frye, D., Vermersch, J.A., S c h l i t z , R . , L i m e b u r n e r , R . , W r i g h t , W.R. and Noble, N.A. 1982. R e c e n t o b s e r v a t i o n s o f t h e mean c i r c u l a t i o n o f G e o r g e s Bank. J. P h y s . O c e a n o g r . , 1 2 : 5 6 9 - 5 9 1 . F u g l i s t e r , F. 1972. C y c l o n i c r i n g s formed by t h e Gulf S t r e a m 1965-66. In: A.L. Gordon ( E d i t o r ) , S t u d i e s i n P h y s i c a l O c e a n o g r a p h y -- A T r i b u t e t o G e o r g e Wust o n h i s 8 0 t h B i r t h d a y , V o l . I. Gordon a n d B r e a c h , N e w Y o r k , pp. 137-168. 1971. Gulf Stream r i n g s i n t h e Sargasso Sea. P a r k e r , C.E. Deep-sea Res., 1 8 : 981-993.
163 Rossby, C.G. 1936. Dynamics of s t e a d y o c e a n c u r r e n t s i n l i g h t of experimental f l u i d mechanics. P a p e r s i n Phys. Oceanogr. and M e t e o r o l . , 5 : 3. Y e n t s c h , C.S. a n d G a r f i e l d , N. 1981. P r i n c i p a l areas of v e r t i c a l mixing i n t h e waters of t h e Gulf of Maine, w i t h r e f e r e n c e t o In: J.F.R. Cower t h e t o t a l p r o d u c t i v i t y of t h e area. ( E d i t o r ) , Oceanography from Space. Plenum P u b l . Corp. , pp. 303-312. D.A.. 1985. R o t a r y m o t i o n s and Yentsch, C.S. and Phinney, c o n v e c t i o n as a means of r e g u l a t i n g p r i m a r y p r o d u c t i o n i n w a r m c o r e r i n g s . J. G e o p h y s . R e s . , 9 0 : 3 2 3 7 - 3 2 4 8 .
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165
ON THE DYNAMICS OF A TIDAL MIXING FRONT G.J.F
VAN HEIJST
University of Utrecht, Institute of Meteorology and Oceanography, Princetonplein 5, Utrecht, The Netherlands.
ABSTRACT This paper addresses some aspects of the dynamics of tidal mixing fronts, as commonly occurring in the continental shelf seas. Attention will be focussed on the along-frontal flow structure, the cross-frontal circulation, and the instability behaviour. The results to be discussed were obtained by analytical and numerical models, by oceanographic observations, and by laboratory experiments.
1. INTRODUCTION
The occurrence of
seasonal stratification is a well-known feature of
shallow shelf seas: in areas where the tidal stirring is too weak to keep the water column mixed, thermal stratification can be observed during the spring and summer seasons when the enhanced solar heat flux warms the upper layer. On the other hand, in tidally energetic areas the water column will be kept wellmixed throughout the year. The narrow transition zone between well-mixed and stratified areas is commonly called 'tidal mixing front' and forms the subject of this study. The positions of these fronts are well-defined and can be calculated from an energy criterion derived by Simpson 6 Hunter (1974). The general structure of a tidal mixing front is depicted schematically in Fig. 1. Note the characteristic 'forking' of the isolines (being isotherms or isopycnals) in the transition zone between the two-layer stratified region and the well-mixed water column. The sea-surface temperature is usually highest in the stratified area, whereas the temperature in the bottom layer is lower than the temperature of the mixed water column. It has been observed in many shelfsea situations that the surface front is less pronounced than the bottom front
-
an effect caused mainly by atmospheric forcing. In general terms, a tidal mixing front acts as a physical boundary between
water masses of different thermal, biological and chemical composition. As the exchange of properties between these water masses is locally intense, frontal
166
Stratified
Fig. 1
frontal zone
mixed
A schematic picture of a tidal mixing front (the solid curves
represent isotherms or isopynals). zones usually create particularly favourable conditions for various biological processes. Fronts are therefore of significant biological importance, and enhanced primary productivity is often associated with their occurrence (see e.g. Bowman h Esaias, 1978). Oceanographic observations have revealed that the basic flow in alongfrontal direction is in first approximation geostrophically controlled, the Coriolis force being balanced by the density gradient force. In addition to the along-frontal flow, there is a weak secondary circulation in cross-frontal direction caused by internal frictional processes. It is generally believed that this cross-frontal circulation plays an important role with respect to the enhanced biological activity. Infrared satellite imagery has provided a wealth of information on fronts, both on their position and their dynamical behaviour. A general feature is that most fronts seem to be unstable, as can be observed from the abundant occurrence of
meanders and
eddies
in the
frontal zones.
These eddies
contribute significantly to the exchange and mixing of properties across the front, and are therefore of crucial importance to the biological processes occurring near the frontal boundary. This paper concentrates on a few aspects of the frontal dynamics. To start with, the basic geostrophic along-frontal flow will be considered in the next section. The secondary circulation in the cross-frontal plane is the subject of section 3 , while section 4 addresses the frontal instability. Special attention is given to laboratory experiments that have been performed in order to study the behaviour of unstable bottom and surface fronts.
167 2. THE ALONG-FRONTAL FLOW
In view of their large along-frontal scales, it seems reasonable to assume that most fronts are, at
least in first approximation, geostrophically
balanced, with the vertical shear of the along-frontal flow related to the cross-frontal density gradient. In order to study the basic dynamics of the along-frontal flow structure, a simple three-layer model has been designed, which will be described first. Detailed information about the frontal flow structure was obtained fairly recently by
observations in the North Sea. These revealed a number of
interesting features, and some of the preliminary results will be discussed at the end of this section.
2.1 A geostrophic adjustment model The geostrophically controlled basic state of a tidal mixing front has recently been studied by use of a geostrophic adjustment model (Van Heijst, 1985),
as
schematically
shown
in
Fig.
2.
This
model
considers
the
geostrophically balanced equilibrium state that results after instantaneous withdrawal of the barrier separating a stably stratified two-layer fluid from a
homogeneous
fluid
of
intermediate
density.
When
the
barrier
is
instantaneously lifted, a density-driven flow in cross-frontal direction will arise immediately. As the entire fluid system rotates steadily at angular velocity f/2, with f the Coriolis parameter, this cross-frontal flow is subjected to the Coriolis force, and will be deflected accordingly. After some time the flow will be geostrophically adjusted and be directed parallel to the front, such that the pressure gradient force balances the Coriolis force. Referring
to
a
Cartesian
coordinate
system
x,y,z
(with
x
directed
perpendicular to the barrier into the stratified fluid, y parallel to the barrier, and z vertically upwards), the equilibrium along-frontal flow in the noses of the layers 1 and 3 (the indices refer to the upper and lower-layer fluids, as shown in Fig. 2a) will be in positive y-direction, while the flow in the nose of the intruding layer 2 will be oppositely directed, i.e.
in
negative y-direction. The resulting interfacial shapes in the adjusted state are depicted in Fig. 2b. It is useful to nondimensionalize the problem by scaling the vertical lengths by
the
total water
depth, H, and
the horizontal lengths by the
168 tfl2
/ barrier
if ' 2
I
x=-L,
Fig. 2
,
I
I
-L2
0
R
--t X
The geometry. of the geostrophic adjustment model: (a) The initial state with the barrier separating the two-layer stratified fluid (layers 1 and 3) from the well-mixed fluid (layer 2); the densities satisfy p < p < p (b) The adjusted state, with the along-frontal flows geostropiicalq; 1 balanced.
internal Rossby radius
with g the gravitational acceleration;
p1
and p3 are the upper and lower-
layer fluid densities. The horizontal velocities are then scaled by fR.
The final, geostrophically adjusted state can be calculated by assuming that mixing and frictional effects are negligible during the adjustment process, so that potential vorticity is conserved. When the layer depths are denoted by hj(x), (j=1,2,3), and along-frontal velocities by vj(x) , conservation of potential vorticity can be written in nondimensional terms for each individual layer as
dv
=+1.
1
=
hl
6
dv
2 as;-+
1 = h2
dv 3
h3
rn’
= + 1 =
with 6 = H1/H
. The
velocity
jump
(V -V )
i
j
across an interface is directly
related to the interfacial slope between the layers i and j according to the Margules equations v2
-
v1
v3
-
v2 =
v 3 - v1
=
-
=
- L2,
A dx dhl
for
-
L1 < x <
dh3 A -
for
-
L2 < x < R,
dx
-L2<x‘R,
(2a)
- -dx
with
and L1, L2, R the positions of the surface front, the bottom front, and the intrusion nose (see Fig. 2b).
In addition, it is useful to assume that the
upper surface is covered by a rigid lid,
so
that
h +h2=1
f o r - L 1 < x < - L2
(3a)
h1 + h 2 + h 3 = 1
f o r - L2 < x < R
(3b)
h1 + h 3 = 1
for
1
x > R
.
(3c)
-
( 3 ) general solutions for h. and vj in J The integration constants involved can be
It is now possible to derive from (1)
the regions indicated in (3). determined by matching the solutions across the domain boundaries
-
x = L2 and x = R, and by applying some additional conditions associated with conservation of mass and conservation of along-frontal momentum. Details of the analysis are given in Van Heijst (1985).
110
Fig.
3
Model results for the frontal shape (a) and the along-frontal velocity distributions (b) in the geostrophically adjusted equilibrium state. The parameter values are: 6 = 0.5, p1 = o.95p3, p 2 = 0.99p
A
typical
3'
result
is
shown
in
Fig.
3
for
the
parameter
values
b0.5, p = 0 . 9 5 ~ , ~ p2 = 0 . 9 9 ~ ~ In . this case the nondimensional front 1 Positions are L1 = 0.5737, L2 = 0.2071, and the position of the intrusion is
given by R = 0.3771.
The shape of the fluid interfaces and the corresponding
along-frontal velocities v.(x) J
Fig. 3 , respectively.
are shown in the upper and lower graphs of
In the analysis it is assumed that the flow during the adjustment process is merely a horizontal motion of vertical vortex tubes which are gradually
being stretched or shrinked. The resulting velocity distributions v.(x) 1 then be understood from conservation of along-frontal momentum
can and
conservation of potential vorticity, which imply that:
. horizontal displacement of a fluid column in positive (negative) results in a negative (positive) along-frontal velocity v
.
j
x-direction
, and
stretching (shrinking) of a fluid column results in a positive (negative)
velocity gradient dv./dx 1
.
171
According to these principles it is easily verified why the lower-layer velocity v (x) takes both positive and negative values on its solution domain. 3 Although this model is highly idealized, it provides some insight into the structure of geostrophically adjusted flows. In reality, however, the situation is more complicated (due to the occurrence of turbulent mixing, internal friction, continuous stratification, frontal instabilities), and one should be cautious when drawing detailed comparisons. 2.2. Some observations It is well-known that seasonal stratification occurs in the North Sea (see e.g. Tomczak
&
Goedecke, 1964). During the spring, summer and autumn seasons
of 1981 and 1982 observations were carried out near the southern edge of the seasonally stratified area south-east of the Dogger Bank. A description of this observational work is presented by Van Aken et al. (1986). Apart from CTD measurements at a number of evenly spaced stations along systematic tracks in the research area, a couple of thermistor chains and current meters were moored for some months near the central position (54'30'N, 4'30'E), close to the boundary of the stratified area. In this region the sea depth measures typically 50 m, and the sea bottom, which is approximately flat in the stratified area, slopes gently upwards into the mixed area, the slope being roughly
-
During the 1982 campaign a strong gale developed soon after deployment of the measuring equipment, causing the front to advect slowly into the stratified area, such that the thermistors and the current meters originally located on the stratified side of the front were able to measure the temperature distribution and the flow structure in a frontal cross-section. Large-scale and small-scale hydrographic surveys revealed that the advection velocity was directed roughly perpendicular to the front at a rate of approximately 20 km in 10 days. The magnitude of the advection speed was very small, and could not be measured directly. The time-dependent thermistor and current meter signals were averaged by taking running means, and typical graphs of the data obtained in two different stations are shown in Figs. 4 and 5. Note that in both figures the parameters along the abscissa and the ordinate are time (in Julian days) and depth (in meters).
In Fig. 5 isotherms are shown in (a),
whereas the corresponding
isotachs of the along-frontal velocity component are depicted in (b). As the h e water column is limited (11 number of thermistors and current meters in ! thermistors at equidistant depths ranging from 11 m to 31 m, and 6 current
172
depth
time ( d a y s )
Fig.
4 Graph of
time dependent thermistor signals, showing the thermal structure of a slowly advecting front (taken from Van Aken et al., 1986).
meters as well as thermistors at depths of 12, 18, 24, 30, 37 and 44 m),
some
interpolation had to be made. The isotherms are plotted for differential temperatures of l'C,
ranging from 9'C
near the bottom to 18'C
in the upper
parts of the water column. It appears that there is a well-defined thermocline on the stratified side of the front at roughly 20-25 m depth, with a large temperature jump from 16'C
to 1O'C
over a few meters depth. Although the
'forking' of isotherms, characteristic for tidal mixing fronts, is clearly visible, it appears that the bottom front is much more pronounced than the surface front. This phenomenon was also met at the other stations, and it is believed to be caused by the intense atmospheric forcing. The isotachs shown in Fig. 5b reveal the existence of a jet-like flow feature at the front, with a maximum velocity of almost 15 cm roughly at the steep slope of the 15'C
and 16'C
s-'
occurring
isotherms at day 233. This
flow structure is consistent with the thermal-wind balance, assuming that the density distribution has roughly the same structure as the isotherm pattern: the along-frontal velocity is largest in regions with the steepest crossfrontal (hydrostatic) pressure gradients. The observed currents also show agreement with the velocity distribution found semi-analytically by Garrett & Loder (1981, their figure 5 ) . Because the stratification of the water column is continuous rather than discrete, it is hard to draw a comparison with the results obtained by the geostrophic adjustment model described
in section 2.1.
Nevertheless, the
173 velocity distribution v (x) on the unstratified side of the front as plotted 2
in Fig. 3b shows some agreement with the along-frontal jet shown in Fig. 5. As a matter of fact, along-frontal flows were also calculated by James (1984) in a numerical model of a tidal mixing front: figure 4f of his paper shows a jet structure similar, both in magnitude and position, to the observed one. Besides, James' calculations indicate the presence of a limited region of small, oppositely directed, along-frontal velocities on the stratified side of the bottom front
-
a feature which also shows up in the present observations
(although not visible in the graphical presentation of Fig. 5b). For a more extensive account of the North Sea observations the reader is referred to Van Aken et al. (1986).
NO OBSERVATIONS
I
I-\ I I I I
-
NO OBSERVATIONS
Fig. 5
The temperature distribution and the flow structure of a front as observed in 1982 at the edge of a seasonally stratified area in the North Sea at 54'30'N, 4'30'E ( s e e Van &en et al., 1986). The curves in (a) represent isotherms, whereas the corresponding isotachs of the along-frontal velocity component are shown in (b). The hatched area in (b) represents velocity magnitudes exceeding 12.5 cm s-l.
I
174 3 * THE CROSS-FRONTAL CIRCULATION
Another important aspect of the mixing-front dynamic
is the oc irrence of
a weak flow in the cross-frontal plane. This secondary circulation, arising from the slow frictional spreading of the front under gravity, has been studied numerically by James (1978, 1984) and semi-analytically by Garrett
&
Loder (1981). In these studies it was found that the secondary flow pattern of a tidal mixing front has a two-cell structure as schematically drawn in Fig.6. The circulation in the upper and lower cell are in opposite sense, with the flow along the sea bottom directed towards the well-mixed side of the front. The circulation in the upper cell is usually weaker, as the largest horizontal density differences occur in the deeper parts of the water column. This causes an asymmetry in the circulation pattern, resulting in a convergence at the surface where the cross-frontal velocity is zero, and an upwelling flow on the mixed side of the front. Surface convergence near fronts is a well-known feature, and can be easily observed from the presence of oil slicks and the accumulation of foam, seaweed, jelly fish and other floating material along the frontal zone. The vertical velocities associated with the upwelling flow induced by the lower circulation cell are too small to be measured directly (the magnitudes are less than 1 mm/s).
Fig. 6
However, uplifting of the near-surface isotherms as
A schematic representation of the circulation in the cross-frontal plane. Solid curves represent streamlines, whereas isopycnals are denoted by broken lines.
175
well as a narrow band of minimum sea-surface temperature on the mixed side of the front is frequently observed (see e.g. Simpson et al., 1978; Krause et al.,
1986),
and this is consistent with the tendency of the circulation
pattern to bring dense (cold) bottom water to the sea surface. An interesting feature visible in most of the isotherm plots obtained from the North Sea observations mentioned before, is a characteristic downward 'dip' in the thermocline at some distance from the bottom front (see Fig. 4). It is believed that this 'dip' is another manifestation of the secondary circulation: it is likely to be caused by the downwelling flow in the lower circulation cell on the stratified side of the front. Such a characteristic feature seems not to have been reported thusfar
-
probably because it is
easily lost when the temperature measurements are performed in .too widelyspaced stations. More details of this feature can be found in Van Aken et al. (1986).
4. FRONTAL INSTABILITY Infrared satellite images of sea-surface temperature have revealed that virtually all surface fronts are unstable, as can be concluded from the abundancy of eddy-like structures in frontal zones. A nice collection of unstable fronts can be seen on Fig. 7, showing the North Sea as viewed by the NOAA-7 satellite on 2 November 1984 at 14h24 G.M.T. Higher sea-surface temperatures show up by darker tones. Apparently, a large band of relatively warm water lies across the North Sea, extending from the English channel to the Skagerrak, bounded by cooler areas in the central North Sea, along the English and Dutch coasts, and in the German Bight. The satellite picture clearly shows that the frontal boundaries are unstable, and that most of them are distorted in a more or less wavelike fashion. An interesting feature is the cold-water tongue extending from the Norfolk Banks in ENE direction, which shows a remarkably regular wave pattern at its southern edge. Frontal instabilities can be barotropic (when the horizontal shear across the frontal interface is the
-
kinetic
-
energy source to feed small
perturbations) or baroclinic (when perturbations are fed by the potential energy associated with the horizontal density gradients).
In most practical
situations, however, both energy sources are present, and the instabilities will be of mixed barotropic-baroclinic type. The dynamics of unstable surface and bottom fronts has been simulated in laboratory experiments by Griffiths & Linden-(1981, 1982) and by Linden & Van Heijst (1984),
respectively. This experimental work has provided important
Fig. 7
Infrared satellite picture of the North Sea on 2 November 1984 at 14h24 GMT. (By courtesy of K N M I , De Bilt, The Netherlands).
177 information about the evolution of the instabilities and the formation of eddies. A numerical simulation of unstable fronts has been performed by James (1984); h i s calculations show good agreement with available sea-surface observations as well as with the experimental results obtained by Griffiths & Linden (1981, 1982).
4.1. Laboratory experiments Fronts can be created in the laboratory by allowing a fluid column to collapse in a
rotating, environmental fluid
Saunders, 1973; Griffiths &
Linden, 1982).
of The
different density basic
set-up
(see
for this
experiment is shown schematically in Fig. 8: a central, removable cylinder is filled with a fluid (either homogeneous, or two-layer stratified) and is placed at the centre of a rotating tank containing fluid of a contrasting density. Ideally, the fluids are assumed to be immiscible. When the inner cylinder is withdrawn, a density-driven radial flow will arise in the fluids, until a state of geostrophic balance is reached (similar to the adjustment problem discussed in section 2.1).
The resultant flows in this ultimate,
geostrophically controlled state are
purely azimuthal
(ignoring viscous
effects, which are confined to the Ekman layers at the bottom and at the interfaces).
Provided that the appropriate Rossby radius of deformation is
sufficiently smaller than the tank radius, the interfaces between the fluids will in the adjusted state intersect one of the horizontal flow boundaries (or thus producing a circular bottom and/or surface front. A surface front
both),
is obtained by taking p1 8a),
<
6 Linden, 1982; see Fig. (see Fig. 8b). When the p2 = p 3 1 is chosen in the range p1 < p2 < p3, both a bottom
p2 = p3 (as did Griffiths
while a bottom front arises when p
ambient fluid density p 2 front an a surface front will
arise
>
(see
Fig.
8c).
This particular
configuration resembles the tidal mixing front, and is therefore useful to study its dynamics. In order to visualize the frontal shape, dye may be added to the fluid(s) in the inner cylinder. In addition, small pieces of paper floating on the free surface allow streak photography, which provides essential information about the upper-layer flow. Motions at greater depth can be visualized by dyeproducing particles of potassium permanganate to be dropped into the tank during
the
course
of
a
particular
experiment.
Observations
are
most
conveniently done by using a co-rotating photocamera mounted at some distance above the fluids.
178
Fig.
8
A schematic representation of the laboratory system used for simulation of unstable fronts. When the inner cylinder is withdrawn the fluids will 'collapse' until a geostrophically adjusted state is reached. Depending on the choice of fluid densities; three different types of fronts can thus be generated: (a) a surface front, for p 1 < p2 = p3; (b) a bottom front, for p 1 = p2 < p3; and (c) a 'tidal mixing front', for p i < p2 < p3The tank diameter and tank height measure 92.5 cm and 3 0 . 0 cm, respectively. The radius of the inner cylinder is Ro = 14-4 cm, and the initial layer depths are denoted by Hi. Another parameter is gfid, being the reduced gravity based on the density difference bet een layers i and j.
4.2 Surface fronts Surface fronts produced by the 'collapse technique' as well as by injection of lighter fluid at the surface have been studied in considerable detail by Griffiths & Linden (1981, 1982) and also by the present author (unpublished).
It was observed that these fronts are unstable, and that the circular frontal shape is always distorted by a more or less regular, wavelike pattern. By counting the number of waves (n) the wavelength A of the frontal instabilities is
easily calculated from A = 2rrR In, with Ro
cylinder.
In the early
stages of
the radius of
an experiment the
the inner
(small-pmplitude)
instabilities have a short wavelength, but this length is usually observed to
179 increase until some ultimate, constant value is reached. The experiments of Griffiths
&
Linden (1982) have demonstrated that the 'final' wavelength A of
instability mode is A/2xR = 1.1 f 0.3 , with R^ the geometric mean of the Rossby radii of the upper and lower layer (to be defined the most
unstable
later). This result agrees well with the value 1.15 derived theoretically by Killworth et al. (1984). Careful observations have provided a lot of information about the evolution of the frontal instability, which is illustrated in Fig. 9. Initially, the disturbance has a sinusoidal shape (Fig. 9a); the waves move along the front at a speed intermediate to the along-frontal velocities V 1 and V2 of the fluids on either side of the front. When their amplitudes increase, the waves become slightly asymmetric and show a tendency to break backwards (Fig. 9b).
Fig. 9
A schematic representation of the evolution of an unstable surface
front. The hatched areas represent the ambient, denser fluid. The drawings, representing subsequent stages of evolution, show (a) the initial sinusoidal perturbation, (b) the asymmetry of the wave, (c) the backwards breaking wave, and (d) the formation of a vortex dipole. Regions where vorticity is being concentrated are indicated by arrows, with the plus and minus signs denoting cyclonic and anticyclonic vorticity, respectively.
180
Fig. 10 A photograph showing 'hammer-head'-shaped instabilities at a surface front produced in the laboratory. The picture was taken roughly 6 rotation periods after withdrawal of the inner cylinder (which lower ed e is visible). The parameter values are H 1 = H = 10.0 cm, f = 2.30 s - ~and 8'12 = 0.5 cm s - ~ . Note that the flow in the wave crest (fluid 1) has anticyclonic vorticity, whereas cyclonic vorticity is being concentrated in the 'wake' (fluid 2). As the amplitude growth continues, the waves are observed to break backwards (Fig. 9c),
thereby causing entrainment of denser ambient fluid into the
buoyant upper-layer fluid. At
this stage the formation of a cyclonic-
anticyclonic vortex pair becomes visible.
-
V 2 is not too large the dipole When the velocity difference AV = V 1 structure becomes more pronounced and the waves take on their characteristic 'hammer-head' appearance (Fig. 9d). off',
and
propel
distribution
-
itself
-
The vortex pair may then even be 'pinched
under
the
influence of
its
own vorticity
away from the front, into the ambient fluid. In this way some
upper-layer fluid is carried into the environment
-a
process which is thought
to be of considerable importance to the cross-frontal transfer of properties. Some 'hammer-head'-shaped protrusions can be seen clearly on the satellite picture of the North Sea (Fig. 7 ) , in particular in the frontal zone north of the Netherlands, just at the right-hand side of the white reference mark. Similarly shaped instabilities are visible on Fig. 10, which shows an unstable surface front produced by the collapse of a central column of (dark-coloured) fluid in a rotating fluid of larger density. The observed instability pattern
181
is rather irregular, showing both backwards breaking waves and hammer heads.
When the velocity jump AV
across the front is too large to allow large
protrusions, the backwards-breaking effect is much more pronounced and the waves are curled up cyclonically into a spiral shape. This curling effect is demonstrated in Fig. 11, showing a pair of photographs of an unstable surface front similar to the one shown in Fig. 10, but now under different experimental conditions. (The difference lies in the value of the Froude number F = ( R o / R ) 2 , with R the upper-layer Rossby radius of deformation: for the experiments shown in Figs. 10 and 11 it measures 220 and 91, respectively). The first picture (Fig. lla), taken approximately 9 rotation periods after the collapse of the central fluid column (the inner cylinder is still visible
-
it hangs just above the water surface), nicely shows the
entrainment of ambient fluid at the back of the waves into the dark-coloured buoyant fluid. Because of the intense shearing motion the mixing in the curling waves is significant, and the spirals soon get a 'blurred' appearance. This can be observed in the second picture (Fig. llb), taken 2 rotation only a periods later: details of the curling motion are no longer visible
-
light spot in the centre of the cyclonic vortices indicates the presence of ambient fluid. The presence of denser fluid in the eddy centre is not exclusively caused by entrainment: it is also a result of upwelling driven by the cyclonic upper-layer eddy itself. Similar cyclonic eddies have been frequently observed in the frontal zones as occurring in the shallow seas around the British Isles. Nice examples of 'curling' structures at the Ushant and Flamborough Head fronts as well as some thermal fronts in the Irish Sea and the Celtic Sea have been collected by Pingree (1978, 1979). Detailed hydrographic surveys in the Ushant frontal region revealed that the sea surface density has a maximum value in the central region of such an eddy (see Pingree et al., 1979).
This corresponds to the entrainment/upwelling feature
observed in the laboratory experiment as shown in Fig. 11.
4 . 3 Bottom fronts
As described at the beginning of this section, bottom fronts can also be
produced in the laboratory by applying the 'collapse technique', now with the centre fluid denser than the environmental fluid. Such experiments have been carried out recently and have revealed a number of striking differences as compared to surface fronts. For example, it appeared that bottom frdnts are not necessarily unstable: when the lower-layer Rossby radius R is sufficiently larger than the radius Ro
Fig.
11 A sequence of two photographs showing the 'curling' of backwards breaking waves on a surface front. The pictures are taken (a) 9 and (b) 11 rotation periods after the inner cylinder was withdrawn. Parameter values: HI = H = 10.0 cm, g n 1 2 = .5 cm s-' and f = 1.48 6-1
183
of the inner cylinder, the central fluid column was observed to collapse into a
smoothly
curved
'dome
shape'
which
remained
stable
throughout the
experiment. In fact, one thus produces a single, stable baroclinic vortex (cf. Saunders, 1973). By streak photography and by dropping dye releasing crystals into the fluid information was obtained about the velocity structure in the vortex. It turned out that the upper-layer azimuthal velocity as measured at the free surface was
cyclonic
throughout the experiment.
This can be
understood from conservation of angular momentum, which requires acceleration in the swirl velocity of a contracting ring of upper-layer fluid during the collapse process. According to the same principle one would expect to measure an a n t i c y c l o n i c velocity in the lower layer, and this is indeed what has been observed in the initial stages of the experiment. After a few rotation periods, however, the lower-layer swirl velocity is observed to change sign, and the flow becomes cyclonic as in the upper layer. It is believed that this 'spin-up' is caused mainly by the bottom Ekman layer and
-
-
to a lesser extent
by the interfacial Ekman layer. Observations revealed that at this stage the
vortex had reached a state of equilibrium, with cyclonic velocities in both the upper layer and the lower layer. The flow is then in geostrophic (more precisely: cyclostrophic) balance, the velocities in each layer being depthindependent. A smooth transition between the upper-layer flow and the (weaker) lower-layer flow is accomplished by the thin Ekman layer at the interface. As a result of frictional effects (confined to the Ekman layers) the baroclinic vortex thus produced appeared to decay gradually. This decay process proceeds at an extremely slow rate: in most experiments the vortex was still visible even after a few hundred rotation periods! In experiments with R
<
Ro the bottom front is unstable and wavelike
distortions of the circular shape became visible soon after withdrawal of the inner cylinder. Although initially these distortions have the same appearance as the surface instabilities, after a short while they take a different appearance. When the inner cylinder is lifted, one observes in the lower layer an anticyclonic motion which is confined t o a narrow band, a few Rossby radii wide, at the front. At this stage waves may develop (see Fig. 12a), with a tendency to break backwards (thus producing cyclonic vortices, as at surface fronts).
Within a few rotation periods, however, the anticyclonic motion in
the bottom layer is reduced to zero (due to the spin-up mechanism caused by the Ekman layers mentioned before), and large radial excursions of lower-layer fluid are observed: the frontal distortions take on the appearance of 'bulges' (see Fig. 12b).
As a result of the continuing spin-up action of the Ekman
layers, these 'bulges' were after a few rotation periods seen to move in
184
185
Fig. 12
A sequence of photographs showing the evolution of an unstable bottom front. The pictures are taken (a) 1.5, (b) 4 and (c) 6 rotation periods after the start of the experiment. Parameter values: H j = H = 8.0 cm, f = 1.99 s-' and g'23 = 1 cm s - ~ .
186 cyclonic direction. When their amplitudes are large enough, the waves again show a (weak) tendency to break backwards, thus producing (weak) anticyclonic vorticity in the tip of the protrusions. This can be seen in Fig. 12c. Note also the sharp leading edges, in contrast to the vagueness of the trailing edges - an effect caused by the bottom friction. Application of the earliermentioned visualization techniques revealed
that the motion inside the
'bulges', as well as in the upper layer just above them, is cyclonic. As the amplitude growth continues, the waves can under certain conditions be pinched off ('vortex splitting', see Saunders, 1973). Each pinched off wave crest soon gets organized into a symmetric, cyclonic vortex and is accompanied by a 'Gaussian'-shaped
dome of denser lower-layer fluid at the bottom. Fig. 13
nicely shows the evolution of an unstable bottom front into a symmetric pattern of four vortices. After they were pinched off, the vortices became perfectly circular. In marked contrast to the observed behaviour of an unstable surface front (with its characteristic vortex-pair formation, leading to 'hammer-head'shaped protrusions),
an unstable bottom front seems to produce cyclonic
monopoles rather than cyclonic-anticyclonic dipoles. The main reason for this different behaviour lies in the presence of the bottom Ekman layer, which tends to spin up the fluid in anticyclonic flow regions. Before the frontal protrusions are pinched off, the instabilities usually have a very regular appearance, which allows accurate measurement of the 'final' wavelength of the fastest-growing instability mode. In order to present the observational results graphically, it is useful to define the geometric mean of the upper and lower-layer Rossby radii as
with =
x the Rossby radius of
the upper layer based on its mean depth
h R2/E2, fi being the radius of the circular front before it becomes 0 0
unstable. Following Griffiths & Linden (1982), scaled by
the observed wavelengths A,
have been plotted in Fig. 14 as a function of the Froude number
F, defined as
with R the Rossby radius of the upper layer, based on its initial depth. The broken lines indicate the band containing the data for unstable surface fronts obtained by Griffiths & Linden (1982, their figure 10). It appears that the observed wavelengths in the bottom front experiments lie well within this
Fig- 13 Two photographs showing a central column of dense fluid breaking up into four cyclonic vortices. The pictures are taken approximately (a) 2 and (b) 4 rotation periods after withdrawal of the inner cylinder. Parameter values: Hg = H = 10.0 cm, f = 1.55 s-l and g'23 = 5 cm S-'.
188
I:
2
t
t
,
e-
/ o /
,
0.01
0.1
1
10
100
1000
F
Fig. 14 The observed wavelength A at unstable ,bottom fronts nondimensionalized by the mean Rossby radius' R plotted against the Froude number F. The broken lines indicate the band containing the experimental data for surface fronts as obtained by Griffiths & Linden (1982). band.
At
larger Froude numbers (F
>
1)
the observed wavelengths take
approximately constant values. Averaging over 15 experiments with F
>
1 one
finds h/2n.6= 1.1 f 0.25, which is exactly the value found by Griffiths & Linden for unstable surface fronts. This suggests that bottom friction has only a very limited effect (if any) on the wavelength of the most unstable mode.
4.4 Tidal mixing fronts By choosing the ambient fluid density p2 in the range p
1
<
Fig. 8c) one obtains a flow situation that can be regarded as an too
simplifying
-
p
2
<
p
3
(see
- admittedly
model of a tidal mixing front, provided that the appropriate
Rossby radii are sufficiently small that the upper and lower interfaces intersect the free surface and the bottom, respectively. A number of such experiments has been carried out, mainly in order to study the instability behaviour of this type of fronts. In all cases the parameters were chosen such that the upper and lower-layer Rossby radii had approximately equal values. Figure 15 shows a sequence of photographs illustrating the evolution of the unstable front. In this experiment the upper and lower-layer fluids in the stratified central region were dyed with different colours which unfortunately cannot be distinguished on the black-white photographs. Observation by eye and photography from the side revealed the existence of an extremely- strong vertical coupling between the upper and lower layer, and also revealed that the distortions of the surface front and the bottom front are identical.
189
Looking down into the tank, differences between the upper and lower frontal shapes are hardly visible, even when the flow eventually becomes irregular due to vortex 'splitting' and the subsequent interaction between the vortices. Careful observations made clear that in this particular type of experiment (with identical upper and lower-layer Rossby radii) the upper-layer flow is completely governed by the lower layer, as can be seen clearly in Fig. 15: the surface (and bottom) front visible on the photographs shows exactly the same instability behaviour as the bottom front shown in Fig. 12, which is significantly different from the surface fronts shown in Figs. 10 and 11. The first photograph (Fig. 15a) shows the fluids immediately after withdrawal of the inner cylinder, the surface (and bottom) front still being circular then. When the next picture (Fig. 15b) was taken, approximately 2 rotation periods later, the upper and lower fluids in a narrow band near the fronts were observed to move in anticyclonic direction; the waves on the unstable fronts appear to break backwards, thus producing cyclonic vorticity. The third photograph (Fig. 15c) was taken 13 rotation periods after the start of the experiment, which roughly coincided with the stage at which the relative motion in the upper and lower-layer fluids was reduced to zero by the spin-up effect: the 'bulging' of the waves can be seen clearly (compare with Fig. 12b). The last picture (Fig. 15d) is taken some 7 rotation periods later. It clearly shows that the frontal waves move in cyclonic direction, as can be seen from the diffuse trailing edges of the bottom waves. Note the resemblance to the corresponding picture of the bottom front (Fig. 12c), on which similar features are visible. An interesting aspect of the vertical coupling between the fluid layers is visible on photographs taken from the side (see e.g. Fig. 16), which all show 'threads' of coloured fluid between the 'blobs' in the upper and the lower layer. These 'blobs' correspond to cyclonic eddies in the frontal region; the vortices seem to extend over the full water column. The 'threads' are in fact cylindrical sheets of coloured fluid, and indicate vertical exchange between the upper and the lower layer. This vertical motion is likely to be driven by horizontal shear, similar to the vertical flow arising in the Stewartson shear layer encompassing a Taylor column in a rotating fluid. It might be expected that the instability behaviour is different in cases where the upper and lower-layer Rossby radii have dissimilar values. However, this problem has not been addressed here.
In shallow shelf seas the flow situation is much more complicated, as the stratification is continuous rather than discrete. Also, the occurrence of vigorous turbulent mixing in the individual layers is a complicating factor. These effects may be of significant importance to the vertical structure of
190
Fig. 15
A sequence of photographs showing the evolution of an unstable 'tidal mixing front' as produced by the collapse technique. The first picture (a) is taken immediately after withdrawal of the inner cylinder; the next photographs are taken (b) 2, (c) 13 and (d) 20 rotation periods after the start of the experiment. Parameter values: H = 10.8 cm, H3 = 6.2 cm, f = 2.16 s-', g'13 = 2 cm s-2 and g'23 = 1 cm s-2.
Fig.
16 Two p h o t o g r a p h s t a k e n f r o m t h e s i d e , s h o w i n g ' t h r e a d s ' f l u i d between t h e f r o n t a l e d d i e s i n t h e upper The s e c o n d p i c t u r e ( b ) i s t a k e n d u r i n g a i n s t a b i l i t y p r o c e s s , as c a n b e s e e n f r o m t h e Note t h e r e f l e c t i o n s at t h e f r e e s u r f a c e and a t
of c o l o u r e d and t h e lower l a y e r . l a t e r s t a g e of t h e a b u n d a n c y of e d d i e s . t h e t a n k bottom.
193 tidal mixing fronts, and may change the picture obtained by laboratory experiments considerably.
Nevertheless, it
is
thought
that
the
simple
laboratory model discussed here is helpful in gaining some insight into the intricate dynamics of unstable fronts. As yet, too few laboratory experiments on tidal mixing fronts have been performed to draw reliable conclusions about the observed wavelengths. This work will be extended in the near future, however, and the results will be reported in due time.
5. CONCLUS I O N Although the theoretical and laboratory models described in this paper are rather simplifying in that they are dealing with idealized flow situations, they give some useful insight into the frontal dynamics, particularly with respect to the along-frontal flow structure and the instability behaviour. Unfortunately, detailed observations of the three-dimensional structure of tidal mixing fronts in continental shelf seas are scarce. An observational project with a number of thermistor chains or undulating bat fish to be towed simultaneously along different frontal cross-sections, in combination with a fine array of current meters moored in the frontal zone, backed up by airborne and satellite observation of the sea surface is extremely costly and seems a utopian enterprise. Nevertheless, this paper concludes with a plea for such observations to be made, as these will provide valuable material for a better understanding of the various cross-frontal transport processes. ACKNOWLEDGEMENTS Some of the experiments described in section 4 were carried out in the Hydrodynamics Laboratories of
the Twente University
of
Technology, The
Netherlands, by kind permission of Professor van Wijngaarden. I wish to thank Lieuwe Seinstra for his skilful assistance with the laboratory experiments, and Leo Maas and Hendrik van Aken for providing the observational data plotted in Figs. 4 and 5. Financial support from the Working Group on Meteorology and Physical
Oceanography
(MFO)
of
Advancement of Pure Research (Z.W.O.)
the
Netherlands
Organization
for
is gratefully acknowledged.
REFERENCES Van Aken, H.M.,
G.J.F.
van Heijst and L.R.M.
Maas, 1986. Observations of
fronts in the North Sea. (In preparation).
the
194
Bowman, M.J. and W.E. Esaias (Editors), 1978. Oceanic Fronts in Coastal Processes. Springer, Berlin , 114 pp. Garrett, C.J.R., and J.W. Loder, 1981. Dynamical aspects of shallow sea fronts. Phil. Trans. Roy. SOC. London, A302, 563-581. Griffiths, R.W., and P.F. Linden, 1981. The stability of buoyancy-driven coastal currents. Dyn. Atmos. Oceans, 5, 281-306. Griffiths, R.W., and P.F. Linden, 1982. Laboratory experiments on fronts. Part 1: Density-driven boundary currents. Geophys. Astrophys. Fluid Dyn., 19, 159-187. Van Heijst, G.J.F., 1985. A geostrophic adjustment model of a tidal mixing front. J. Phys. Oceanogr. 15, 1182-1190. James, I.D., 1978. A note on the circulation induced by a shallow-sea front. Estuarine Coastal Mar. Sci., 7, 197-202. James, I.D., 1984. A three-dimensional numerical shelf-sea front model with variable eddy viscosity and diffusivity. Cont. Shelf Res., 3, 69-98. Killworth, P.D., N. Paldor and M.E. Stern, 1984. Wave propagation and growth on a surface front in a two-layer geostrophic current. J. Mar. Res., 42, 761-785. Krause, G., G. Budeus, D. Gerdes, K. Schaumann and K. Hesse, 1986. Frontal systems in the German Bight and their physical and biological effects. (This issue). Linden, P.F., and G.J.F. van Heijst, 1984. Two-layer spin-up and frontogenesis. J. Fluid Mech., 143, 69-94. Pingree, R.D., 1978. Cyclonic eddies and cross-frontal mixing. J. Mar. Biol. ASS. U.K., 58, 955-963. Pingree, R.D., 1979. Baroclinic eddies bordering the Celtic Sea in late summer. J. Mar. Biol. Ass. U.K., 59, 689-698. Pingree, R.D, P.M. Holligan and G.T. Mardell, 1979. Phytoplankton growth and cyclonic eddies. Nature, 278, 245-247. Saunders, P.M., 1973. The instability of a baroclinic vortex. J. Phys. Oceanogr., 3, 61-65. Simpson, J.H., C.M. Allen and N.C.G. Morris, 1978. Fronts on the Continental Shelf. J. Geophys. Res., 83, 4607-4614. Simpson, J.H., and J.R. Hunter, 1974. Fronts in the Irish Sea. Nature, 250, 404-406. Tomczak, G., and E. Goedecke, 1964. Die thermische Schichtung der Nordsee. Deutsche Hydrographische Zeitschrift, ErgXnzungsheft Reihe S ( 4 ) , Nr. 8, 182 pp.
195
ZOOPLANKTON I N THE UPWELLING FRONTS OFF PT. CONCEPTION, CALIFORNIA S.L.
SMITH,l
B.H.
JONESY2 L.P.
ATKINSONY3 and K.H.
BRINK4
1Oceanographic Sciences D i v i s i o n , Department o f Applied Science, Brookhaven National Laboratory, Upton, NY
11973
2Department o f B i o l o g i c a l Sci ences , Uni v e r s i t y o f Southern Cal if o r n i a, Los Angeles, CA
90089
3Department o f Oceanography, Old Dominion U n i v e r s i t y , N o r f o l k , VA 4Woods Hole Oceanographic I n s t i t u t i o n , Woods Hole, MA
23508
02543
ABSTRACT
Surface maps o f s e l e c t e d t a x a o f zooplankton were made o f f P t . Conception, C a l i f o r n i a , d u r i n g t h r e e c o n t r a s t i n g upwell ing s i t u a t i o n s : moderate upwell ing, s t r o n g u p w e l l i n g , and downwelling. Number o f t a x a and number o f i n d i v i d u a l s decreased w i t h i n c r e a s i n g u p w e l l i n g i n t e n s i t y , i n d i c a t i n g replacement o f r i c h e r s u r f a c e waters by r e l a t i v e l y impoverished subsurface waters. The e x c e p t i o n t o t h i s p a t t e r n was i n copepodid stage V o f Calanus p a c i f i c u s which increased i n numbers nearshore as u p w e l l i n g s t r e n g t h increased. Since copepodid V i s t h e deep-living, diapausing stage o f t h i s copepod, i t s i n c r e a s e i n numbers i s c o n s i s t e n t w i t h t h e upward movement o f deep water and t h e l i f e c y c l e o f C. acificus. F r o n t a l zones always had more i n d i v i d u a l s than n o n - f r o n t a l zones, and t ese were p r i m a r i l y copepod n a u p l i i . Estimated i n g e s t i o n by copepod n a u p l i i and Calanus p a c i f i c u s i n f r o n t a l zones d u r i n g u p w e l l i n g was t w i c e t h e i n g e s t i o n by-taxa o u t s i d e t h e f r o n t a l zones, suggesting t h a t t h e f r o n t a l zones associated w i t h u p w e l l i n g o f f P t . Conception a r e s i t e s o f enhanced secondary production.
+
INTRODUCTION The l a r g e and l e n g t h y s e r i e s o f observations o f p l a n k t o n and hydrography i n t h e C a l i f o r n i a Current System and North P a c i f i c C e n t r a l Gyre has been analyzed p r i m a r i l y f o r l a r g e - s c a l e c o r r e l a t i o n s between p h y s i c a l f o r c i n g and p l a n k t o n i c response ( W i c k e t t , 1967; Bernal and McGowan, 1981; Bernal , 1981; Chelton, 1982).
The g e n e r a l i t i e s t h a t have a r i s e n a r e p r i n c i p a l l y t h a t maxima i n zoo-
p l a n k t o n i c biomass i n t h e C a l i f o r n i a c u r r e n t r e g i o n a r e associated w i t h t r a n s p o r t from t h e n o r t h ( W i c k e t t , 1967; Bernal and McGowan, 1981) and n o t w i t h coastal u p w e l l i n g (Bernal and McGowan, 1981; Chelton,
1982).
Although Chelton
(1982) proposes another o f f s h o r e u p w e l l i n g mechanism t o e x p l a i n t h e biomass peak observed approximately 100-200 km from t h e coast, he concedes t h e very i m p o r t a n t p o i n t t h a t t h e CalCOFI surveys upon which t h e s e g e n e r a l i t i e s a r e based d i d not i n c l u d e samples from t h e nearshore, c o a s t a l region.
T h i s narrow c o a s t a l zone,
196 where c l a s s i c a l wind-driven c o a s t a l u p w e l l i n g c o u l d be a c t i n g t o i n c r e a s e zoop l a n k t o n i c stocks,
N; 120'-121'
W),
and l o c a t e d between P o i n t s Conception and A r g u e l l o (-34'-35'
i s t h e area i n which t h e present study was conducted.
The nearshore r e g i o n o f f southern C a l i f o r n i a i s n o t o r i o u s f o r t h e s p a t i a l v a r i a b i l i t y i n i t s p l a n k t o n p o p u l a t i o n s i n s p r i n g and summer (Haury, 1976; Cox, Haury and Simpson, 1982; S t a r and M u l l i n ,
1981), seen c l e a r l y i n s a t e l l i t e
c o a s t a l zone c o l o r scanner images such as t h o s e used t o analyze t h e E l Nifio phenomenon ( F i e d l er, 1984). 80
pm
O f f San Diego, f o r example, zoopl ankton 1 a r g e r t h a n
were most abundant and most patchy i n a s e t o f nearshore samples compared
w i t h s i m i l a r sample s e t s c o l l e c t e d i n t h e western p a r t o f t h e C a l i f o r n i a Current and t h e North P a c i f i c C e n t r a l Gyre ( S t a r and M u l l i n , 1981).
Patch s i z e i n t h e
C a l i f o r n i a Current was l i k e w i s e s m a l l e r t h a n patch s i z e i n t h e c e n t r a l gyre (Haury, 1976).
Thus, t h e r e seems t o be an onshore-offshore g r a d i e n t i n patch
size, w i t h s m a l l e s t patches nearshore.
Surface waters i n t h e P o i n t Conception
area have been shown t o c o n t a i n small s c a l e zooplankton patches, associated w i t h patches o f c h l o r o p h y l l
2,
and i n t e r p r e t e d as t o p o g r a p h i c a l l y induced o f f s h o r e
extensions o f b a n d - l i k e o r f r o n t a l c o a s t a l u p w e l l i n g e f f e c t s (Cox e t al.,
1982).
A prominent member o f t h e nearshore and C a l i f o r n i a Current p l a n k t o n commun i t y i n spr ng and summer i s Calanus p a c i f i c u s ( S t a r and M u l l i n , 1981; F1 emi nger,
964) whose d i e 1 ( E n r i g h t and Honegger,
(Longhurst
,
1967; A l l d r e d g e e t al.,
1977) and o n t o g e n e t i c
1984) v e r t i c a l m i g r a t i o n s may c o n t r i b u t e
s u b s t a n t i a1 y t o observed patchiness.
Die1 d i f f e r e n c e s a t 35 m d u r i n g s i n g l e ,
s h o r t sampl ng i n t e r v a l s a r e not s t r i k i n g ( S t a r and M u l l i n , 1981), b u t m i g r a t i o n by l a t e copepodids ( s t a g e CV) and a d u l t s i n t o t h e upper 20-25 m c o n s i s t e n t l y t o o k p l a c e a t n i g h t d u r i n g s p r i n g and e a r l y summer, although t h e exact t i m e o f ascent v a r i e d ( E n r i g h t and Honegger, migrating
C.
1977).
Thus d u r i n g s p r i n g and summer,
p a c i f i c u s c o n t r i b u t e t o patchiness observed i n t h e s u r f a c e zoo-
p l a n k t o n community, w h i l e a t o t h e r seasons when t h e r e s t i n g stage (copepodid V ) i s i n l a r g e numbers a t depth (Longhurst, 1967; A l l d r e d g e e t al.,
1984), i t can-
n o t c o n t r i b u t e t o observed s h a l l o w patchiness. Two s e t s o f o b s e r v a t i o n s have prompted t h e a n a l y s i s o f s p a t i a l p a t t e r n s herein.
F i r s t , near P t . Conception c h l o r o p h y l l
i n t o a f r o n t a l f e a t u r e (Cox e t al.,
2
a t t h e s u r f a c e i s organized
1982) which may a l s o c o n t a i n n a u p l i i o f
Calanus p a c i f i c u s ( S t a r and M u l l i n , 1981), assuming s p a t i a l c o r r e l a t i o n s nearshore o f f San Diego a r e v a l i d a t Pt. Conception.
Second, i n a study of t h e
u p w e l l i n g area o f f Peru, n a u p l i i tended t o be concentrated i n t h e downstream edges o f p l u m e - l i k e f e a t u r e s o f c h l o r o p h y l l (Boyd and Smith, 1983).
2
a r i s i n g from u p w e l l i n g processes
These " f r o n t a l " areas, which were p o t e n t i a l l y s i t e s o f
197 increased secondary p r o d u c t i o n d u r i n g u p w e l l i n g , were much reduced d u r i n g r e l a x a t i o n (Boyd and Smith, 1983).
We t h e r e f o r e h y p o t h e s i z e d t h a t t h e u p w e l l i n g a r e a
near P t . Conception would have f r o n t a l zones where p h y t o p l a n k t o n and copepod n a u p l i i co-occurred.
Furthermore, t h e s e s p a t i a l f e a t u r e s a r i s i n g f r o m i n t e r m i t -
t e n t u p w e l l i n g episodes m i g h t be c r i t i c a l t o g e n e r a l p r o d u c t i o n i n t h e a r e a because t h e y were s i t e s o f i n c r e a s e d h e r b i v o r e a c t i v i t y ( p r i m a r i l y due t o Calanus p a c i f i c u s ) and secondary p r o d u c t i o n .
S p a t i a l and temporal p a t t e r n s i n
zooplankton a t t h e s u r f a c e i n o t h e r u p w e l l i n g areas have been shown t o be dominated by copepods t h a t have a s t r o n g l y t h r e e - d i m e n s i o n a l d i s t r i b u t i o n d u r i n g a l i f e c y c l e c o n t a i n i n g a diapausing l a t e j u v e n i l e stage ( s i m i l a r t o
C.
pacificus;
B i n e t and S u i s s e de S t . C l a i r e , 1975; Smith, 1982). METHODS Underway samples were c o l l e c t e d f r o m t h e R.V.
New H o r i z o n d u r i n g r a p i d , u n i n -
t e r r u p t e d steaming i n t h e c o a s t a l area between P o i n t s A r g u e l l o and Conception (Fig. 1A). laboratory,
A PVC t h r o u g h - h u l l
f i t t i n g ( 2 m d e p t h ) d e l i v e r e d seawater t o t h e
No sea c h e s t o r o t h e r impediment o b s t r u c t e d t h e s y n o p t i c sampling
o f s u r f a c e waters.
Zooplankton were c o l l e c t e d f r o m t h e f l o w (average f l o w r a t e
was 15 l i t e r s p e r m i n u t e ) f o r 3 m i n u t e s each 15 m i n u t e s o n t o 70-pm mesh screen, washed i n t o v i a l s , and p r e s e r v e d i n 5% b u f f e r e d f o r m a l i n . f o r e , r e p r e s e n t s an i n t e g r a t i o n o f a p p r o x i m a t e l y 0.7 d i s t a n c e covered d u r i n g 3 minutes.
Each sample, t h e r e -
km (0.4
nm) o f h o r i z o n t a l
Eleven such maps o f z o o p l a n k t o n i c abundance
a t t h e s u r f a c e were made d u r i n g t h e OPUS s t u d y i n A p r i l and May 1983.
w i l l be d i s c u s s e d i n t h i s paper.
I n t h e l a b o r a t o r y a l l z o o p l a n k t o n i n each v i a l
were counted, w i t h t o t a l t a x a e q u a l i n g f i f t y .
Calanus p a c i f i c u s and Paracalanus
parvus were staged f o r s i z e s c o l l e c t e d q u a n t i t a t i v e l y ( t o to C I I / I I I for
1. p a r v u s ) ,
Three
CI f o r C. p a c i f i c u s ;
b u t most copepods were grouped i n t o a d u l t s o r cope-
podids o f t h e p a r t i c u l a r species.
No a t t e m p t was made t o i d e n t i f y non-copepod
t a x a t o s p e c i e s and t h u s g r o u p i n g s such as chaetognatha, o s t r a c o d a and so f o r t h are common i n t h e data.
D u r i n g sampling and d a t a a n a l y s i s , we noted a v e r y
sharp r i s e i n t h e abundance o f Calanus p a c i f i c u s a t sunset.
O t h e r d i e 1 problems
were n o t e v i d e n t , b u t most analyses were done on day and n i g h t p o r t i o n s o f maps s e p a r a t e l y , and t h e "sunset s p i k e " sample i n each map was n o t used. Temperature, c h l o r o p h y l l
2,
and n u t r i e n t c o n c e n t r a t i o n s were measured f r o m
t h e s u r f a c e stream by a t h e r m i s t o r i n t h e stream, and by i n - v i v o f l u o r e s c e n c e and c o n t i n u o u s a u t o a n a l y z e r sampling.
P o s i t i o n was recorded d i r e c t l y f r o m a
198 LORAN-C u n i t onto a computer every m i n u t e (see Jones e t al.,
1986, f o r d e t a i l s
o f t h e maps o f nonzooplanktonic v a r i a b l e s ) .
The t h e r m i s t o r , f l u o r o m e t e r , and
autoanalyzer were c a l i b r a t e d by XBT, bucket,
and CTD casts,
r e p o r t e d i n Paluszkiewicz e t al.
(1984).
and d a t a are
Temperatures used i n t h i s a n a l y s i s a r e
t h o s e recorded by XBT's launched d u r i n g t h e 3-minute sampling f o r zooplankton. RESULTS We have s e l e c t e d t h r e e surface maps f o r a n a l y s i s because they represent t h r e e d i s t i n c t s i t u a t i o n s observed d u r i n g t h e u p w e l l i n g season o f f P t . Conception and P t . Arguello.
The f i r s t one was sampled on A p r i l 14-15,
1983, d u r i n g moderate
u p w e l l i n g when t h e winds were approximately 8 ms-l from t h e north-northwest. Upwelled water was from t h e Santa Barbara Channel and south o f P t . Conception ( A t k i n s o n e t al.,
1986).
The second example was from a p e r i o d o f downwelling on A p r i l 19-20,
1983.
Winds were a l s o
approximately 8 ms-l b u t from t h e OPUS-83MAPI. 4-15 APRIL 1983
south-southeast.
'EMPERATURE (daq Cl
d u r i n g s t r o n g u p w e l l i n g on May 6-7,
The t h i r d map was made
1
1983, when u p w e l l i n g extended n o r t h o f P t . A r g u e l l o a l s o and winds were
sustained a t 8 ms-I from t h e n o r t h northwest f o r a 3-day p e r i o d b e f o r e t h e sampling e x e r c i s e and a t 13 ms-l f o r f o u r days subsequent t o i t ( A t k i n s o n e t
al.,
1986).
The behavior o f near s u r -
f a c e ( 5 m and 10 m) c u r r e n t s nearshore ( a t t h e 70 m i s o b a t h ) between P t . Conc e p t i o n and Pt. A r g u e l l o were c o n s i s t e n t w i t h l o c a l l y wind-driven u p w e l l i n g dynamics ( B r i n k and Muench, 1985); t h a t i s , f l o w i n t h e upper 15 m tended t o go 121.00
120.73 120.45 LONGITUOE (degl
120.18
o f f s h o r e (onshore) i n response t o equatorward (poleward) winds,
The
apparent upwell ing c e n t e r was l o c a t e d Fig. 1A.
Surface d i s t r i b u t i o n o f
temperature f o r map 4 (moderate) u p w e l l i n g ) sampled on 14-15 A p r i l 1983.
Maps o f temperature
between t h e P o i n t s a t about 34"30.5'N, 120°36.6'W
(Atkinson e t al.
, 1986).
When maps o f s i x zooplanktonic t a x a were compared w i t h maps o f s u r f a c e tem-
2,
were c o n s t r u c t e d from 5-minute
p e r a t u r e and c h l o r o p h y l l
averages o f t h e data.
suggested t h a t t h e r e were i m p o r t a n t
inspection
199 c o r r e l a t i o n s between s u r f a c e 'temperature and organism d i s t r i b u t i o n s .
F o r exam-
ple, d u r i n g moderate u p w e l l i n g ( A p r i l 14-15) most o f t h e copepod n a u p l i i i n t h e survey area were found on t h e s o u t h e r l y edge o f what m i g h t be c a l l e d a s u r f a c e "streamer" o f c o o l w a t e r e x t e n d i n g o f f s h o r e f r o m between P o i n t s A r g u e l l o and Conception ( F i g s . 1 A and 16). s o u t h e r l y edge (Fig.
1C).
Chlorophyll
2
was s i m i l a r l y h i g h e s t a l o n g t h i s
D u r i n g s t r o n g u p w e l l ing, abundances of copepod
n a u p l i i were g r e a t l y reduced, b u t h i g h e s t c o n c e n t r a t i o n s were a g a i n i n an area where i s o t h e r m s were r e l a t i v e l y c l o s e l y spaced (Figs.
2A and 26).
Chlorophyll
2
was u n i f o r m l y l o w ( < 1 ~ n g * m - ~i )n c o n c e n t r a t i o n t h r o u g h o u t t h e s t u d y area (Fig. 2C). (Fig.
I n t h e d o w n w e l l i n g p e r i o d , s u r f a c e t e m p e r a t u r e was u n i n f o r m a t i v e
3A). even though a d i s t i n c t area o f h i g h abundance o f copepod n a u p l i i
c o i n c i d e d w i t h an area o f e l e v a t e d s u r f a c e c h l o r o p h y l l and 3C).
2
c o n c e n t r a t i o n (Figs.
H i g h e s t abundances of copepod n a u p l i i were never a d j a c e n t t o t h e
coast, and were f a r t h e s t removed f r o m t h e c o a s t d u r i n g d o w n w e l l i n g when t h e y a l s o achieved t h e i r maxima (Figs.
Maps of o t h e r t a x a such as
16, 26, and 36).
OPUS-z
MAP^ APRIL 1 4 - 1 5 . 1 9 e 3
ESTIMATED CHLOROPHYLL I p G l L l
T------
34-45
34'30'
34'15'
14-15 APRIL 1983
34"
I
120'30'
I'W
"
120.73
121.00
izo.ie
120.45
LONGITUOE ldspl
Fig.
16.
Surface d i s t r i b u t i o n o f
F i g . 1C.
Surface d i s t r i b u t i o n o f
2 2
f o r map 4.
Maps o f
Open
chlorophyll
c i r c l e s i n d i c a t e d a y t i m e samples;
chlorophyll
closed c i r c l e s i n d i c a t e night.
5-minute averages o f t h e data.
copepod n a u p l i i f o r map 4.
were c o n s t r u c t e d f r o m
36
200
euphausi i d n a u p l i i
, appendicul a r i a n s ,
t o t a l Paracalanus p a r v u s and O i t h o n a spp.
d i d n o t show such d r a m a t i c c o n t r a s t s i n abundance between u p w e l l i n g and downwe1 1 ing p e r i o d s . Because i n b o t h moderate and s t r o n g u p w e l l ing p e r i o d s t h e copepod naupl ii seemed t o be aggregated i n areas t h a t m i g h t be c h a r a c t e r i z e d as f r o n t a l zones a s s o c i a t e d w i t h t h e u p w e l l i n g f e a t u r e , we must d e f i n e t h e " f r o n t " f o r each map b e f o r e a n a l y z i n g i t s impact on t h e d i s t r i b u t i o n s o f g r a z i n g organisms and t h e f o o d c h a i n dynamics o f t h e P t . C o n c e p t i o n area. chlorophyll
2
To do t h i s , we p l o t t e d s u r f a c e
and n i t r a t e a g a i n s t t e m p e r a t u r e and chose t h e 1°C i n t e r v a l i n
which c h l o r o p h y l l
d
approached zero.
We c a l l t h i s 1°C i n t e r v a l t h e f r o n t a l zone, and i t c o r r e s -
c o n c e n t r a t i o n s achieved t h e i r maximum v a l u e and n i t r a t e
ponded t o t h e area between t h e 12'
and 13'
i s o t h e r m s i n t h e moderate u p w e l l i n g
and d o w n w e l l i n g maps and t h e area between t h e 13" and 14" i s o t h e r m s i n t h e map
6 - 7 MAY 1983 EMPERATURE Ideg C I
)
140
\
34-30' -
34.15'
-
copepod nauplll I nunbar i 3 1 6 - 7 MAY 1983
/-
2ow
o
c)
121.00
120.73
120.45
120.18
LONGITUDE l d e g l
F i g . 2A.
Surface d i s t r i b u t i o n o f
t e m p e r a t u r e f o r map 10 ( s t r o n g u p w e l l i n g ) , sampled on 6-7 May 1983.
F i g . 2B.
Surface d i s t r i b u t i o n of
copepod n a u p l i i f o r map 10.
201 made d u r i n g s t r o n g u p w e l l i n g ( F i g s . 4A-C).
Such s i m p l i f i c a t i o n s a r e necessary
i n o r d e r t o a n a l y z e events i n t h i s i n t e n s e l y heterogeneous a r e a and a t t e m p t t o d e l i n e a t e e f f e c t s a s s o c i a t e d w i t h l o c a l u p w e l l i n g f r o m e f f e c t s c r e a t e d by l a r g e r s c a l e a d v e c t i o n ( C a l i f o r n i a C u r r e n t ) and oceanic i n f l u e n c e s . The s i m p l e s t q u e s t i o n t o ask i s whether t h e t o t a l number o f t a x a and t o t a l number o f i n d i v i d u a l s v a r i e s i n any i d e n t i f i a b l e way w i t h i n c r e a s i n g s t r e n g t h o f u p w e l l i n g , and what t h e r e l a t i o n s h i p i s between f r o n t a l f e a t u r e s and t h e s u r r o u n d i n g waters.
The fewest t a x a and fewest i n d i v i d u a l s were found d u r i n g
s t r o n g e s t u p w e l l i n g (May 6-7) w h i l e t h e l a r g e s t number o f t a x a and h i g h e s t animal abundances were observed d u r i n g downwell ing ( A p r i l 19-20; Tab1 e 1).
In
a l l t h r e e maps t h e number o f t a x a found i n t h e f r o n t a l r e g i o n s was s i m i l a r t o t h e number found o u t s i d e , b u t t h e number o f i n d i v i d u a l s was always g r e a t e s t w i t h i n t h e f r o n t a l zones ( T a b l e 1).
The d e c r e a s i n g number o f i n d i v i d u a l s w i t h
19-20 APRIL 1983 OPUS-2 MAP 10 MAY 6-7. 1983
TEMPERATURE (dsql
7------
ESTIMATED CHLOROPHYLL ( p G I L I
4 c.)
t
q.25
.'.. ;.' .
\ .
.
5,-\:,,
.
.
.
.
.
.
... i.
flo
.(._...
'f
d
00
120.73
120.45
120.18
121.00
Fig. 2C.
Surface d i s t r i b u t i o n o f
estimated c h l o r o p h y l l
5
120.73
120.45
li 1.18
LONGITUDE ldegl
LONGITUDE id091
f o r map 10.
Fig. 3A.
S u r f a c e d i s t r i b u t i o n o f tem-
p e r a t u r e - f o r map 5 ( d o w n w e l l i n g ) , sampled on 19-20 A p r i l 1983.
202
TABLE 1 Mean c h a r a c t e r i s t i c s o f t h r e e maps made d u r i n g v a r y i n g u p w e l l i n g c o n d i t i o n s . ~~~
Downwell ing Characteristic
In front
Out o f front
Number o f t a x a 22(35)+ 22(20) Number of i n d i ~ i d u a l s - m - ~ 20,034 17,514 Percentage copepod n a u p l i i 87.0 81.4 Percentage C. p a c i f i c u s 1.3 1.9 Percentage P. 1.7 3.2 Percentage T i t ona spp. 1.5 2.6 I n g e s t i o n by copepod n a u p l i i * 6.2 5.3 I n g e s t i o n by C. p a c i f i c u s * 1.9 0.7
F
~~~
Moder a t e u p w e l l ing
Strong u p w e l l ing
In front
Out o f front
In front
Out o f front
21(21) 12,135 77.6 1.9 5.4 5.3 3.8 3.8
21(22) 9,654 63.2 2.3 11.0 8.7 2.1 1.6
13(14) 5,421 70.4 3.9 13.6 5.6 1.5 0.9
13(18) 3,474 45.7 4.4 26.2 15.4 0.5 0.7
+( N) *mg C - ~ n - ~ . d a y - l
TABLE 2 C o e f f i c i e n t s o f v a r i a t i o n o f samples c o l l e c t e d a t n i g h t . ~~
Taxon
Downwell ing I n > Out 17.63 > 15.85
Copepod naupl ii Euphausi i d naupl ii
Out > I n
162.16 > 125.66 I n > Out
Appendicularia
65.73 > 57.74
228.85 > 81.21 Out > I n 170.78 > 159.85
172.78 > 146.08
Out > I n
Cal anus p a c i f i c u s C2
c3
Out > I n
235.74 > 97.24
c4 c5
Out > I n Out > I n Out > I n
374.17 > 16.66 Out > I n 176.03 > 164.95
Out > I n
Out > I n
264.58 > 169.39
343.00 > 147.37 Out > I n 282.24 > 196.60
Out > I n
264.58 > 200.25 *None captured.
Out > I n
56.59 > 25.17
I n > Out
Calanus p a c i f i c u s
Out > I n
47.26 > 40.42 Out > I n 151.52 > 97.03
73.26 > 49.53
Out > I n
Paracal anus p a r v u s
~~~
139.59 > 56.95 Out > I n 55.82 > 1.75
57.93 > 45.96 O i t h o n a spp.
~
Moderate u p w e l l ing
Strong u p w e l l ing I n > Out 97.41 > 51.45
*
* Out > I n
151.28 > 144.27 Out > I n
77.07 > 46.16 I n > Out
83.23 > 68.15 I n > Out
170.74 > 134.52
* * * * * *
Out > I n
17p.01 > 173.19
203 i n c r e a s i n g s t r e n g t h of u p w e l l i n g suggests t h a t when s u r f a c e w a t e r s a r e r e p l a c e d by deeper w a t e r d u r i n g u p w e l l i n g , t h e deeper w a t e r i s r e l a t i v e l y i m p o v e r i s h e d i n zooplankton.
S u r f a c e fauna a r e presumably moved o f f s h o r e and d i l u t e d by m i x i n g
w i t h t h e deep w a t e r r i s i n g d u r i n g s t r o n g u p w e l l i n g . F u r t h e r e v i d e n c e o f t h e s u r f a c i n g o f deep w a t e r nearshore, p r i m a r i l y d u r i n g s t r o n g u p w e l l i n g events, can b e seen i n maps o f copepodid s t a g e V o f Calanus p a c i f i c u s (Figs.
5A-C).
I t was found a t t h e s u r f a c e n e a r s h o r e o n l y d u r i n g
moderate and s t r o n g u p w e l l i n g .
D u r i n g d o w n w e l l i n g i t was found i n g r e a t e s t
abundance o f f s h o r e i n c l u s t e r s s u g g e s t i n g o r i g i n a t i o n n o r t h o f P t . A r g u e l l o and Copepodid s t a g e V i s t h e s t a g e i n
i n t h e Santa Barbara Channel ( F i g s . 5A-C).
which d i a p a u s e a t d e p t h i s e x p e r i e n c e d as p a r t o f t h e l i f e c y c l e of t h i s copepod (Longhurst,
1967; A l l dredge e t al.,
Presumably t h e copepodid s t a g e V
1984).
OPUS-2 MAP 5 APRIL 19-20, I983 ESTIMATED CHLOROPHYLL I p G I L I 34.4:
\ c
c 34'30
34-15,
(number m - 3 )
19-20 APRIL 1983 34'N I2I"W
Fig. 38.
lZO"30'
Surface d i s t r i b u t i o n o f
copepod n a u p l i i f o r map 5.
"7
121.00
F i g . 3C.
120.73 120.45 LONGITUDE Idegl
120.18
Surface d i s t r i b u t i o n o f e s t i -
mated c h l o r o p h y l l
2
f o r map 5.
204
s u r f a c e s , m o l t s , and reproduces d u r i n g t h e u p w e l l i n g season i n t h i s a r e a s i n c e t h e CalCOFI a t l a s ( F l e m i n g e r , 1964) shows no o t h e r t a x o n t o be as abundant as
C.
p a c i f i c u s i n t h e Pt. Conception area, and t h e maxima i n abundances o f
p a c i f i c u s were noted i n A p r i l and J u l y .
C.
S t r o n g u p w e l l i n g moves s u r f a c e p l a n k t o n
o u t o f t h e Pt. Conception area and i n t r o d u c e s s u b s u r f a c e organisms such as t h e s t a g e CV o f Calanus p a c i f i c u s .
The p e r c e n t a g e o f t o t a l numbers comprised by
Calanus p a c i f i c u s i n c r e a s e s as s t r e n g t h o f u p w e l l i n g i n c r e a s e s ( T a b l e l ) , a f u r t h e r i n d i c a t i o n o f i t s a s s o c i a t i o n w i t h s t r o n g a d v e c t i o n and s u r f a c i n g o f deep water. D u r i n g downwell ing t h e i m p o r t a n c e o f b i o l o g i c a l e f f e c t s becomes obvious.
0
opus-83
MAP 4
, rather
than physical ,
Numbers o f i n d i v i d u a l s r i s e d r a m a t i c a l l y and t h e
4/14-15/#3
o
OPUS-83
MAP 5
APRIL
19-20.
1901
z r
E /
L
+ 2 12.0
10.0
' II
Lb.0
10.0
12.0
TEMPERATIRE
Fig. 4A.
10.0
12.0
L4.0
l6.b
L8.0
12.0 IDES Cl
14.0
lb.b
lE.0
=I
I
"4'
lb.b
(OC)
14.0
14.0
10.0
1b.O
LO.0
IDEG CI
TEMPERATIRE
C o r r e l a t i o n o f temperature
and e s t i m a t e d c h l o r o p h y l l
3 and
n i t r a t e a t t h e s u r f a c e f o r map 4
Fig. 48. (OC)
C o r r e l a t i o n o f temperature
and e s t i m a t e d c h l o r o p h y l l
2
and
n i t r a t e a t t h e s u r f a c e f o r map 5
(moderate u p w e l l in g ) sampled on
(downwel
14-15 A p r i l 1983.
1983.
ng) sampled on 19-20 A p r i 1
Note t h e v e r y l o w c o n c e n t r a t i o n s
o f n i t r a t e and r e l a t i v e l y h i g h concent r a t i o n s o f estimated c h l o r o p h y l l
2.
205
percentage o f copepod n a u p l i i reaches 87% o f t o t a l numbers i n t h e f r o n t a l zones (Table 1).
I n a l l t h r e e maps, copepod n a u p l i i were more abundant i n t h e f r o n t a l
zones t h a n i n s u r r o u n d i n g w a t e r ( T a b l e 1) i n d i c a t i n g p h y s i c a l e f f e c t s a c t i n g t o aggregate t h e s e buoyant organisms.
The a g g r e g a t i o n o f n a u p l i i i n t h e f r o n t a l
zones suggests t h a t t h e s e areas may be i m p o r t a n t i n o v e r a l l r e c r u i t m e n t o f copepods i n t h i s u p w e l l i n g area.
T o t a l i n g e s t i o n by copepod n a u p l i i , c a l c u l a t e d by
assuming each n a u p l i u s i n g e s t e d 0.4 pg C - d a y - l as has been shown f o r
C.
p a c i f i c u s ( P a f f e n h b f e r , 1971; M u l l i n and Brooks, 1970), was always h i g h e r i n t h e f r o n t a l zones t h a n o u t s i d e ( r e c a l l t h e f r o n t a l zone i s p a r t i a l l y d e f i n e d as t h e area where c h l o r o p h y l l
2 c o n c e n t r a t i o n s a c h i e v e t h e i r maximum v a l u e s ) and was
h i g h e s t d u r i n g d o w n w e l l i n g and l o w e s t d u r i n g s t r o n g u p w e l l i n g , f o l l o w i n g t h e abundance t r e n d s a1 ready n o t e d ( T a b l e 1).
Similarly,
i n g e s t i o n by Calanus
p a c i f i c u s , assuming i n g e s t i o n by each copepodid s t a g e was e q u i v a l e n t t o t h a t r e p o r t e d by M u l l i n and Brooks (1970), was always h i g h e r i n t h e f r o n t a l zones o OPUS-83
t h a n o u t s i d e ( T a b l e 1).
UAP 10 L U V 6-1. 1983
The g e n e r a l c h a r a c t e r i s t i c s show f o u r i n t e r e s t i n g trends. 1.
As s t r e n g t h o f u p w e l l i n g
i n c r e a s e d , number o f t a x a decreased, and t h e r e was no d i f f e r e n c e between f r o n t a l and n o n f r o n t a l zones. 10.0
12.0
11.0
L8.0
16.0
2.
Number o f i n d i v i d u a l s decreased
w i t h i n c r e a s i n g u p w e l l i n g s t r e n g t h , and f r o n t a l zones always had h i g h e r abundances t h a n n o n f r o n t a l zones.
3.
A l t h o u g h t o t a l numbers decreased
as u p w e l l i n g s t r e n g t h increased, t h e p e r c e n t a g e o f t o t a l numbers comprised by copepodids and a d u l t s o f t h r e e copepod 10.0
12.0
TWPERATLRE
14.0
18.0
16.0
[DEG CI
t a x a (Calanus p a c i f i c u s , Paracalanus p a r v u s , and Oithona spp.)
Fig. 4C.
C o r r e l a t i o n o f temperature
(OC) and e s t i m a t e d c h l o r o p h y l l
2
and n i t r a t e a t t h e s u r f a c e f o r map
i n c r e a s e d as
u p w e l l ing s t r e n g t h increased.
4.
Abundance o f copepod n a u p l i i
i n c r e a s e d d r a m a t i c a l l y d u r i n g downwell-
10 ( s t r o n g u p w e l l i n g ) sampled on
i n g , c o n s t i t u t i n g one o f t h e s t r o n g e s t
6-7 May 1983.
c o n t r a s t s observed.
206 The c o e f f i c i e n t o f v a r i a t i o n i n abundances ( s t a n d a r d d e v i a t i o n as a p e r c e n t a g e o f t h e mean) i s a s i m p l e way o f a s s e s s i n g v a r i a b i l i t y i n a s e t o f samples, and i n t h e case o f t h e maps i s u s e f u l as a c r i t e r i o n f o r comparing t h e d i f f e r e n t u p w e l l i n g s i t u a t i o n s and a l s o i n a s s e s s i n g d i e l v a r i a b i l i t y w i t h i n a g i v e n map.
O f t h e samples c o l l e c t e d a t n i g h t , t h e t r e n d was f o r v a r i a t i o n t o be
l e s s w i t h i n f r o n t a l zones t h a n o u t s i d e ( 2 0 o u t o f 26 cases; T a b l e 2).
Abundance
o f Calanus p a c i f i c u s was l e s s v a r i a b l e w i t h i n f r o n t a l zones compared w i t h o u t s i d e i n a l l cases ( T a b l e 2). One would expect h i g h e r v a r i a b i l i t y a t t h e s u r f a c e a t n i g h t t h a n d u r i n g t h e day owing t o t h e v a r i o u s p a t t e r n s o f d i e l v e r t i c a l m i g r a t i o n p o s s i b l e f o r d i f f e r e n t taxa.
C.
W i t h t h e e x c e p t i o n o f O i t h o n a spp.,
p a c i f i c u s C3, and
p.
parvus
C4, t h i s was t r u e i n t h e f r o n t a l zones d u r i n g t h e d o w n w e l l i n g e p i s o d e ( T a b l e
3).
D u r i n g moderate (and s t r o n g ) u p w e l l i n g , however, v a r i a t i o n was always
( g e n e r a l l y ) g r e a t e r i n t h e f r o n t a l zones i n daytime, s u g g e s t i n g t h a t a d v e c t i v e e f f e c t s predominated o v e r b i o l o g i c a l e f f e c t s ( d i e l m i g r a t i o n ) i n d e t e r m i n i n g variability.
F o r b o t h a p a s s i v e organism, such as a weakly swimming n a u p l i u s ,
and a more a c t i v e organism, such as t h e o n t o g e n e t i c a l l y m i g r a t i n g
34"30
34'30'
\
14
- 15
A P R I L 1983
340h
F i g . 5A.
19- 20 APRIL 1983
34ON
Abundance o f copepodid
F i g . 58.
Abundance o f copepodid s t a g e V
s t a g e V o f Calanus p a c i f i c u s a t t h e
o f Calanus p a c i f i c u s a t t h e s u r f a c e
s u r f a c e d u r i n g moderate u p w e l l i n g .
d u r i n g downwelling.
Note t h a t t h i s
r e s t i n g s t a g e i s found n e a r s h o r e o n l y d u r i n g u p w e l l i n g b u t i s found o f f s h o r e d u r i ng downwell ing.
207
TABLE 3 C o e f f i c i e n t s o f v a r i a t i o n o f samples c o l l e c t e d i n f r o n t a l areas. ~
Taxon
Downwell ingt
Copepod naupl ii Euphausiid n a u p l i i Appendi c u l a r i a Oithona spp. Paracal anus p a r v u s Calanus p a c i f i c u s Calanus p a c i f i c u s
Mode r a t e u p w e l l ing
c2
c3
c4 c5 Paracal anus p a r v u s
c4 c5
N 17.63 N 125.66 N 57.93 D 91.30 N 56.59 D 87.60 N 159.85 D 107.40 N 169.39 N 200.25 N 131.49 D 152.73 N 75.51
> D
> 12.76 > D
> 64.10 > D
> 40.87 > N
> 49.53 > D
> 43.84 > N
> 81.21 > D
> 109.03
D 65.13 D 156.91 O 200.00 D 91.21 D 93.92 D 200.58 D 200.01
> N
> 97.24 > D
> 109.84 > D
> 153.53 > D
> 105.16 > N
> 120.15 > D
> 65.02
D 160.93 D 141.40 D 71.13
~~
Strong u p w e l l ing
> N
D > N 113.66 > 97.41
> 40.42
*
> N
*
> 97.03 > N
N 144.27 D 114.38 N 83.23 N 170.74
> 56.95 > N
> 1.75 > N
> 57.74 > N
> 146.08 > N
> 16.66
* * * * * *
D 202.19 D 115.38 D 89.19 N 120.04
> N
> 92.76 > N
> 68.50 > N
> 52.91
> D
> 129.45 > N
> 46.16 > D
> 66.27 > D
> 129.82
* * * * * *
> N
> 173.19 > N
> 16.29 > N
> 87.50 > D > 100.31
'N denotes n i g h t , D denotes day. *None captured. TABLE 4 C o e f f i c i e n t s o f v a r i a t i o n o f samples c o l l e c t e d a t n i g h t . Upwell ing c o n d i t i o n Taxon Copepod naupl ii Euphausi i d naupl ii Appendicul a r i a Oithona spp. Paracal anus p a r v u s Calanus p a c i f i c u s
I n Front SUP' > MUP > 97.41 > 40.42 * DWN > * 125.66 SUP > DWN > 144.27 > 57.93 MUP > SUP > 49.53 > 46.16 SUP > MUP > 83.23 > 57.74 SUP > MUP > 170.74 > 146.08
Out o f F r o n t DWN > 17.63 MUP > 97.03 MUP > 56.95 DWN > 1.75 DWN > 56.59 DWN > 81.21
SUP 51.15 SUP 253.39 SUP 151.28 SUP 77.07 SUP 68.15 DUN 228.85
> MUP > DWN > 47.26 > 15.85 > DUN > MUP > 162.16 > 151.52 > MUP > DNW > 139.59 > 45.96 > DWN > MUP
> 73.26 > 55.82 > MUP > DWN > 65.73 > 25.17 > MUP > SUP
> 172.78 > 134.52
'SUP denotes s t r o n g u p w e l l ing , MUP denotes moderate' u p w e l l ing , and DUN denotes downwell ing. *None c a p t u r e d d u r i n g s t r o n g u p w e l l ing.
208 Calanus p a c i f i c u s , t h e l e a s t v a r i a b i l i t y i n f r o n t a l zones was observed d u r i n g d o w n w e l l i n g ( T a b l e 4).
The most v a r i a b i l i t y was o f t e n , b u t n o t always, observed
i n s t r o n g u p w e l l i n g ( 9 o u t o f 11 cases; T a b l e 4).
I n general, increasing
s t r e n g t h o f u p w e l l ing was a s s o c i a t e d w i t h i n c r e a s e d v a r i a b i 1 i t y i n abundance o f z o o p l a n k t o n c a p t u r e d a t n i g h t , b o t h i n f r o n t a l zones and o u t s i d e . I n daytime, v a r i a b i l i t y i n g e n e r a l was a l s o l e a s t i n t h e f r o n t a l zones d u r i n g Areas o u t s i d e t h e
downwell ing and g r e a t e s t d u r i n g moderate u p w e l l ing (Tab1 e 5).
f r o n t a l zones showed g r e a t e s t v a r i a t i o n d u r i n g s t r o n g u p w e l l i n g ( T a b l e s 4 and 5), s u g g e s t i n g t h a t o v e r a l l i n t h e P t . Conception area, d i e 1 m i g r a t i o n i s not a very important c o n t r i b u t o r t o v a r i a t i o n i n surface populations r e l a t i v e t o t h e i m p o r t a n c e o f t h e s t r e n g t h o f a d v e c t i o n a s s o c i a t e d w i t h u p w e l l i n g episodes. Abundances w i t h i n f r o n t s and o u t s i d e were compared s t a t i s t i c a l l y u s i n g l o g t r a n s f o r m e d data.
A t night during
moderate u p w e l l ing , copepod naupl ii , euphausi i d naupl ii, and appendicul a r i a were s i g n i f i c a n t l y more abundant i n f r o n t a l zones t h a n o u t s i d e ( t 2 5 = 2.291; P< 0.05, = 3.263;
P < 0.05,
t 2 5 = 4.115,
P
5
0.05).
and t 2 5
A t night during
s t r o n g u p w e l l ing , copepod naupl ii were s i g n i f i c a n t l y more abundant i n f r o n t a l
P
zones t h a n o u t s i d e ( t i 0 = 2.300; 0.05). 34O30'
5
A t n i g h t d u r i n g downwelling,
appendicul a r i a and t o t a l Paracal anus p a r v u s were s i g n i f i c a n t l y more abundant o u t s i d e f r o n t a l zones t h a n t h e y were i n s i d e ( t 2 2 = 3.034; a
. a a
a
a
a
I
I
121ow
P 5 0.05).
P
5
0.05;
O i t h o n a spp.
t21 = and
t o t a l Calanus p a c i f i c u s showed no s t a t i s t i c a l l y s i g n i f i c a n t differences at
6 M A Y 1983
340 N
4.171;
I 20° 30'W
night. A f i n a l c o n s i d e r a t i o n i s whether t h e
rank o r d e r i n g o f t a x a v a r i e s w i t h r e s p e c t t o f r o n t a l and n o n f r o n t a l zones Fig. 5C.
Abundance o f copepodid
Stage V o f Calanus p a c i f i c u s
and a c c o r d i n g t o s t r e n g t h o f u p w e l l i n g . Rank d i f f e r e n c e s between f r o n t a l and
a t t h e surface during strong
n o n f r o n t a l zones showed g r e a t e s t con-
u p w e l l ing.
t r a s t i n t h e d o w n w e l l i n g s i t u a t i o n and
209 TABLE 5 C o e f f i c i e n t s o f v a r i a t i o n o f samples c o l l e c t e d i n t h e day. Upwell ing c o n d i t i o n Taxon
I n Front
Copepod naupl ii Euphausi i d naupl ii Appendicul a r i a Oithona spp. Paracal anus p a r v u s Calanus p a c i f i c u s
Out F r o n t
SUP' > MUP > DWN 113.66 > 65.13 > 12.76 * MUP > DWN * 156.91 > 64.10 MUP > SUP > OWN 200.00 > 129.45 > 40.87 SUP > DWN > MUP 114.38 > 91.30 > 91.21 MUP > SUP > DWN 93.92 > 66.27 > 43.84 MUP > SUP > DWN 200.58 > 129.82 > 87.60
SUP > MUP > DWN 110.19 > 61.06 > 14.35 * MUP > DWN * 264.54 > 96.43 MUP > SUP > DWN 168.38 > 116.72 > 64.28 SUP > DWN > MUP 54.29 > 47.22 > 39.51 SUP > DWN > MUP 47.94 > 41.00 > 40.65 DNW > MUP > SUP 179.23 > 178.44 > 160.65
'SUP denotes s t r o n g u p w e l l ing , MUP denotes moderate u p w e l l ing , and DWN denotes downwell ing. *None c a p t u r e d d u r i ng s t r o n g u p w e l l ing. l e a s t c o n t r a s t i n t h e s t r o n g u p w e l l i n g s i t u a t i o n ( T a b l e 6). w i t h i n t h e f r o n t a l zone a r e compared,
When ranks o f t a x a
g r e a t e s t d i f f e r e n c e was between downwell-
i n g and moderate u p w e l l i n g and t h e l e a s t d i f f e r e n c e was between moderate u p w e l l i n g and s t r o n g u p w e l l i n g ( T a b l e 6).
I n a l l s i t u a t i o n s copepod n a u p l i i were
ranked f i r s t , and w i t h i n t h e f r o n t a l zones, copepod n a u p l i i , O i t h o n a spp. and a p p e n d i c u l a r i a comprised t h e t o p t h r e e t a x a r e g a r d l e s s o f u p w e l l i n g s t r e n g t h ( T a b l e 6).
D u r i n g s t r o n g u p w e l l i n g , no Calocalanus spp. were c a p t u r e d a t a l l ;
e u p h a u s i i d n a u p l i i were n o t c a p t u r e d w i t h i n t h e f r o n t s ; and no Mecynocera c l a u s i were c a p t u r e d o u t s i d e t h e f r o n t s ( T a b l e 6).
Except f o r copepodid s t a g e I, no
Calanus p a c i f i c u s were found o u t s i d e t h e f r o n t a l zones d u r i n g s t r o n g u p w e l l i n g ( T a b l e 6).
The g r e a t e s t d i f f e r e n c e s i n rank o r d e r i n f r o n t s between moderate
u p w e l l i n g and s t r o n g u p w e l l i n g were i n Oncaea spp.
( h i g h e r i n moderate u p w e l l -
i n g ) and Calanus p a c i f i c u s s t a g e C I ( h i g h e r i n s t r o n g u p w e l l i n g ) ; between downwe1 1ing and s t r o n g u p w e l l ing g r e a t e s t d i f f e r e n c e s were i n Paracal anus p a r v u s stages C I I / I I I and C I V ( h i g h e r i n s t r o n g u p w e l l i n g ) .
The average rank o f
Calanus p a c i f i c u s , when a l l copepodid stages were p r e s e n t , was h i g h e s t i n s t r o n g u p w e l l i n g and l o w e s t i n moderate u p w e l l i n g .
DISCUSSION The n e a r s h o r e zone i n f l u e n c e d by c o a s t a l u p w e l l i n g g e n e r a l l y has been i g n o r e d i n p r e v i o u s analyses o f z o o p l a n k t o n i n t h e C a l i - f o r n i a C u r r e n t and c o a s t a l areas o f f southern C a l i f o r n i a .
I n so doing, e a r l i e r s t u d i e s i d e n t i f i e d o f f s h o r e l o c a -
t i o n s o f i n c r e a s e d biomass and found r e a s o n a b l e mechanisms t o e x p l a i n t h e
TABLE 6 Rank o r d e r o f zooplankton t a x a I n f r o n t a l r e g l o n s and o u t s l d e d u r l n g t h r e e d l f f e r e n t u p w e l l l n g reglmes.
I
DOWNWELLING In
ank
1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16 17 18 19 20 21
22
-
-
In
- -
-
,
, ~
-
HODERATE UFWELLING
out
copepod naup I I 1 copepod naup I I I Append I cu l a r la Append 1 cu I a r la Olthona spp. Olthona spp. 00 euphauslld n a u p l l l P. parvus ++ euphausl I d calyptopes p. p a r v u s C 5 00 P. parvus++ , euphauslld n a u p l l l P. parvus C5 , spp. Oncaea spp. C. p a c l f l c u s C4 00 Clausocalanus tt* euphauslld calyptopes C. p a c l f l c u s C1* P. parvus C2t3 Corycaeus ' C. p a c i f l c u s C5 Mecynocera P. p a r v u s C 4 C. p a c l f l c u s C4 ' A c a r t l a spp. C. p a c l f l c u s *+C2 p a c l f i c u s c3 C. p a c l f l c u s C3 Mecynocera P. p a r v u s *+C4 C. p a c i f i c u s ' C5 A c a r t l a spp. C. p a c l f l c u s C l P. p a r v u s *+C4 Ciausocalanus Chaetognatha C. p a c t f i c u s 2 . p a c l f l c u s C2 Calocalanus spp. C. p a c l f t c u s Chaetognatha
B
-
c.
, ~I 1
B B
N w 0
out
copepod naup I I I Olthona spp. Appendlcularla*+
copepod naup I I I Olthona spp.
P. parvus C2+3 P. parvus C5
-P. parvus C4
I--
-
I c.
B
B*t
cz
P. P.
parvus C2+3 parvus c5
P.
parvus
$3
euphauslld calyptopes P. parvus C4
euphauslld n a u p l l l * t euphauslld calyptopes 00 P. parvus ++ C. p a c l f l c u s C5 C. p a c l f l c u s C4 Calocalanus Corycaeus Mecynocera Chaetognatha C. p a c l f l c u s A c a r t l a spp. Ciausocalanus C. p a c l f l c u s C3 C. p a c l f l c u s C l pacificus
-
STRONG UPWELLING
2.
I
p a c l f l c u s C5 Append I cu l a r la Mecynocera c l a u s l CD Clausocalanus +t Corycaeus C. p a c l f l c u s C4 euphauslld n a u p l l l C. p a c l f l c u s C1 Chaetognatha A c a r t l a spp. p a c i f i c u s $3 Calocalanus C. p a c l f l c u s C3 p a c t t t c u s cz
-
c.
-
I c.
Largest d l f f e r e n c e s I n rank I n f r o n t a l zone versus o u t of f r o n t a l zone (>6 u n l t s ) . 'Dlfterence s l g n l f l c a n t a t P < 0.05.
In
out
copepod naupl I I Olthona spp. Appendlcularla* P. p a r v u s C5 P. parvus C4 P. parvus C2+3 C. p a c l f l c u g C1 P. p a r v u s 00 C. p a c l f l c u s tt C. p a c l f l c u s C2 Corycaeus Mecynocera A c a r t l a spp. euphausl I d calyptopes C. p a c l f l c u s C. p a c l f l c u s Oncaea spp. Clausocalanus C. p a c l f l c u s C3 Chaetognatha
-
copepod naup I I I Olthona spp. P. C4 euphauslld n a u p l l l p. parvus c5 P. parvus C2+3
-
-C. p a c l f l c u s C1
-
Append1cu l a r l a Corycaeus spp. 00 P. pervus ++ A c a r t l a spp. Ctausocalanus $3 euphausl I d calyptopes Chaetognatha
-
-
B
-
-
211
TABLE 7 Occurrence o f copepodid stage V (CV) o f Calanus p a c i f i c u s w i t h i n t e n k i l o m e t e r s o f t h e coast. Map Number
Upwell ing State
Date
Number o f Samples
Percent age C o n t a i n i n g CV's ~~
1 2
3 4 5 7 10
April 6 April 9 A p r i l 11 A p r i l 14-15 A p r i l 19-20 A p r i l 24-25 May 6-7
Non-upwel 1 ing Non-upwell ing Moderate upwell ing Moderate upwell ing Downwell ing Non-upwell ing Strong upwell ing
observations (Bernal and McGowan, 1981; Chelton,
9
0
15
20 33 42
6 12 8 20 12
1982).
0 55 92
We have i d e n t i f i e d a
nearshore source t h a t may a l s o c o n t r i b u t e t o t h e biomass peak observed 100-200 km from shore and t h a t has c o n s i d e r a b l e s m a l l - s c a l e s p a t i a l v a r i a t i o n .
All of
our observations suggest a conceptual model i n which f r o n t a l zones a r e s i t e s o f highest secondary p r o d u c t i o n and i n which t h e u p w e l l i n g process a c t i v e l y i n t r o duces a l a t e subadult h e r b i v o r e i n t o t h e s u r f a c e layer.
Any ideas about how t h e
nearshore p e l a g i c ecosystem f u n c t i o n s a r e somewhat d i f f i c u l t t o t e s t because o f t h e three-dimensional
d i s t r i b u t i o n and seasonal appearance o f Calanus p a c i f i c u s
(and perhaps o t h e r h e r b i v o r e s as w e l l ) combined w i t h a complicated near-surface c i r c u l a t ion (Davis and Regi er, 1984; Brink, 1983). The importance o f t h e u p w e l l i n g process combined w i t h o n t o g e n e t i c upward m i g r a t i o n o f copepodid stage V o f Calanus p a c i f i c u s i s obvious i n t h e seven maps t h a t have been analyzed ( T a b l e 7).
When t h e s t a t e o f u p w e l l i n g i s assigned an
a r b i t r a r y number, one f o r downwelling through f o u r f o r s t r o n g u p w e l l i n g , t h e c o r r e l a t i o n c o e f f i c i e n t between u p w e l l i n g s t r e n g t h and t h e percentage o f nearshore samples c o n t a i n i n g copepodid stage V i s r2 = 0.844
(n=7).
During s t r o n g
upwelling, 92% o f samples w i t h i n 10 km o f shore contained stage CV, w h i l e d u r i n g downwelling none d i d (Table 7).
Although a seasonal e f f e c t cannot be completely
ruled out as a p o s s i b l e c o n t r i b u t o r t o t h i s s t r i k i n g c o n t r a s t (more animals were m i g r a t i n g upward a t t h e l a t e r date), t h e v a r i a t i o n i n u p w e l l i n g s t r e n g t h and t h e persistence o f t h e t r e n d over t h e f i v e weeks o f t h e study suggest t h a t u p w e l l i n g does p l a y a major r o l e i n b r i n g i n g t h e s e animals t o t h e s u r f a c e nearshore. Deeper water e i t h e r i n t h e Santa Barbara Channel o r n o r t h o f P t . Conception can be thought o f as c o n t a i n i n g a seed p o p u l a t i o n o f stage CV Calanus p a c i f i c u s , i n diapause perhaps from t h e p r e v i o u s u p w e l l i n g seasoo.
A combination o f upward
m i g r a t i o n and u p w e l l i n g processes i n t r o d u c e t h e s e CV's i n t o t h e s u r f a c e l a y e r where they m o l t and become r e p r o d u c t i v e l y a c t i v e .
N a u p l i i r e s u l t i n g from t h e
reproduction a r e aggregated i n f r o n t a l zones where c o n c e n t r a t i o n s o f food a r e
212 e l e v a t e d also.
The f r o n t a l zones associated w i t h u p w e l l i n g thereby become s i t e s
o f h i g h secondary p r o d u c t i o n compared w i t h surrounding waters. Balances i n p o p u l a t i o n dynamics cannot be achieved l o c a l l y i n t h i s u p w e l l i n g area because o f v a r i a b i l i t y i n sources o f water f o r u p w e l l i n g ( A t k i n s o n e t al., 1986) and i n t h e t r a j e c t o r i e s o f water a t t h e s u r f a c e (Davis and Regier, 1984). During t h e s t r o n g u p w e l l i n g event discussed here, s u r f a c e c u r r e n t
-
following
d r i f t e r s covered t h e d i s t a n c e from P t . Conception n e a r l y t o Santa Cruz I s l a n d (34ON, 120OW o r approximately 106 km) i n approximately 3 days.
Thus, any cope-
p o d i d stage V's s u r f a c i n g d u r i n g t h i s event would have tended t o be e n t r a i n e d i n t h e d i r e c t i o n o f t h e Santa Barbara Channel. During downwelling, on t h e o t h e r hand, net s u r f a c e f l o w tended t o be l e s s ( s u r f a c e d r i f t e r s tended t o t r a v e r s e small l o o p s ) i n t h e P t . Conception
-
Pt.
Argue1 1o area, v a r i abi 1 it y was reduced and abundance o f copepod naupl ii increased d r a m a t i c a l l y .
F r o n t a l zones, however, were s t i l l t h e areas i n which
p o t e n t i a l secondary p r o d u c t i o n was highest.
The l i f e h i s t o r y o f Calanus
p a c i f i c u s , i t s r e l a t i o n s h i p t o u p w e l l i n g c i r c u l a t i o n and t h e p o t e n t i a l l y dominant r o l e o f f r o n t a l zones i n f o c u s i n g secondary (and p r i m a r y ) production, require considerable additional exploration.
all
It seems t h a t Calanus p a c i f i c u s ,
which dominates t h e crustacean p l a n k t o n d u r i n g t h e u p w e l l i n g season, i s " i n j e c t e d " i n t o t h e s u r f a c e l a y e r d u r i n g u p w e l l i n g where i t s secondary product i o n may be achieved p r i m a r i l y i n f r o n t a l zones. ACKNOWLEDGMENTS We thank K.
Devonald and t h e c a p t a i n and crew o f R.V.
t ance a t sea i n c o l 1e c t i ng t h e zoopl ankton samples.
done by E.M.
New Horizon f o r a s s i s -
Laboratory analyses were
Schwarting, and her thorough and p r o f e s s i o n a l h e l p i s g r a t e f u l l y
acknowledged.
The f i e l d work was supported by N.S.F.
grant OCE-82-15228 and t h e
1 a b o r a t o r y analyses by Department o f Energy g r a n t DE-AC02-76CH00016 and NSF g r a n t OCE-85-07438.
The manuscript was prepared w i t h f i n a n a c i a1 support from
DOE g r a n t DE-AC02-76CH00016 and N.S.F.
g r a n t OCE-8507438.
LITERATURE CITED Alldredge, A.L., B.H. Robison, A. Fleminger, J.J. Torres, J.M. King, and W.M. Hammer, 1984. D i r e c t sampling and i n s i t u o b s e r v a t i o n o f a p e r s i s t e n t copeMar. pod aggregation i n t h e mesopelagic zone o f t h e Santa Barbara basin. Biol., 80: 75-81. Atkinson, L.P., K.H. B r i n k , R. Davis, B.H. Jones, T. Paluszkiewicz, and D. S t u a r t , 1986. Mesoscale v a r i a b i l i t y i n t h e v i c i n i t y o f P o i n t s Conception and J. Geophys. Res., A r g u e l l o d u r i n g April-May 1983: t h e OPUS 83 experiment. su bmi t t ed Bernal, P.A., 1981. A review o f t h e low-frequency response o f t h e p e l a g i c ecosystem i n t h e C a l i f o r n i a Current. C a l i f . Coop. Oceanic Fish. Invest. Rep., 22: 49-62. Bernal, P.A. and J.A. McGowan, 1981. Advection and u p w e l l i n g i n t h e C a l i f o r n i a Current. I n : F.A. Richards ( E d i t o r ) , Coastal Upwelling. American Geophysic a l Union, Washington, D.C., pp. 381-399.
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213 Binet, 0. and E. Suisse de S a i n t e C l a i r e , 1975. Le copepode p l a n k t o n i q u e Calanoides c a r i n a t u s r e p a r t i t i o n e t c y c l e b i o l o g i q u e au l a r g e de l a Cote , s e r i e s Oceanographique, 13: 15-30. d ' I v o i r e . Cahiers O.R.S.T.O.M. Boyd, C.M. and S.L. Smith, 1983. Plankton, u p w e l l i n g , and c o a s t a l l y t r a p p e d waves o f f Peru. Deep-sea Res., 30: 723-742. Brink, K.H., 1983. The near-surface dynamics o f c o a s t a l upwelling. Prog. Oceanog. , 12, 223-257. Brink, K.H. and R.D. Muench, 1986. C i r c u l a t i o n i n t h e P o i n t Conception-Santa Barbara channel region. J. Geophys. Res., i n press. 1982. Large-scale response o f t h e C a l i f o r n i a Current t o f o r c i n g Chelton, D.B., by t h e wind s t r e s s c u r l . C a l i f . Coop. Oceanic Fish. Invest. Rep., 23: 130-148. Cox, J.L., L.R. Haury and J.J. Simpson, 1982. S p a t i a l p a t t e r n s o f g r a z i n g - r e l a t e d parameters i n C a l i f o r n i a c o a s t a l s u r f a c e waters, J u l y 1979. J. Mar. Res., 40: 1127-1153. f o l l o w i n g d r i f t e r s i n OPUS-83. Davis, R.E. and L. Regier, 1984. Current Scripps I n s t i t u t i o n o f Oceanography Ref. No. 84-12, 41 pp. Honegger, 1977. D i u r n a l v e r t i c a l m i g r a t i o n : adaptive Enright, J.T. and H.-W. s i g n i f i c a n c e and t i m i n g . P a r t 2. Test o f t h e model: d e t a i l s o f t i m i n g . Limnol. Oceanogr. , 22: 873-886. F i e d l e r , P.C., 1984. S a t e l l i t e observations o f t h e 1982-1983 E l Niho along t h e U.S. P a c i f i c coast. Science, 224: 1251-1254. Fleminger, A., 1964. D i s t r i b u t i o n a l a t l a s o f c a l a n o i d copepods i n t h e C a l i f o r n i a Current region. P a r t I. C a l i f . Coop. Oceanic Fish. Invest., A t l a s 2, 313 pp. Haury, L.R., 1976. A comparison o f zooplankton p a t t e r n s i n t h e C a l i f o r n i a Current and North P a c i f i c Central Gyre. Mar. Biol., 37: 159-167. K.H. B r i n k , D. Blasco and L.P. Atkinson, 1986. Asymmetric Jones, B.H., d i s t r i b u t i o n s o f phytoplankton associated w i t h a c o a s t a l u p w e l l i n g center. Cont. S h e l f Res., submitted. Longhurst, A.R. , 1967. V e r t i c a l d i s t r i b u t i o n o f zooplankton i n r e l a t i o n t o t h e eastern P a c i f i c oxygen minimum. Deep-sea Res. , 14: 51-63. and E.R. Brooks, 1970. P r o d u c t i o n o f t h e p l a n k t o n i c copepod Mullin, M.M. Calanus he1 olandicus. B u l l . Scripps. I n s t . Ocean., 17: 89-103. P a f m r d . Grazing and i n g e s t i o n r a t e s o f n a u p l i i , copepodids and a d u l t s o f t h e marine p l a n k t o n i c copepod Cal anus he1 go1 andicus.' Mar. Biol.. 11: 286-298. Paluszkiewicz, T., W. Chandler, A. Grindle, and L. Atkinson, 1984. OPUS: P r e l i m i n a r y Hydrographic Data Report. Skidaway I n s t i t u t e o f Oceanography, Savannah. 218 pp. Smith, S.L., 1982. The northwestern I n d i a n Ocean d u r i n g t h e monsoons o f 1979: d i s t r i b u t i o n , abundance and f e e d i n g o f zooplankton. Deep-sea Res. , 29: 1331-1353. S t a r , J.L. and M.M. M u l l i n , 1981. Zooplanktonic assemblages i n t h r e e areas o f t h e North P a c i f i c as revealed by continuous h o r i z o n t a l t r a n s e c t s . Deep-sea Res. , 28A: 1303-1322. Wickett, W.P., 1967. Ekman t r a n s p o r t and zooplankton c o n c e n t r a t i o n s i n t h e J. Fish. Res. Bd. Canada, 24: 581-594. North P a c i f i c Ocean.
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215
OBSERVATIONS OF FINESTRUCTURE FORMED I N A CONTINENTAL SHELF FRONT (SOUTHEASTERN BERING SEA)
L.K.
COACHMAN
ABSTRACT Observations of f i n e s t r u c t u r e formed i n t h e mid-shelf f r o n t o f a broad c o n t i n e n t a l s h e l f sea p r o v i d e unique d a t a f o r e l u c i d a t i n g t h e f o r m a t i o n and subsequent behavior o f d i s t i n c t water p a r c e l s c r e a t e d by p a r t i a l mixing o f shelf bottom w a t e r and b a s i n water.
The measurements i n c l u d e one week o f
hourly v e r t i c a l c u r r e n t p r o f i l e s and g r i d s o f CTD s t a t i o n s .
The p a r t i a l
mixtures are i d e n t i f i e d by a c o l d t e m p e r a t u r e s i q n a l from t h e s h e l f bottom water.
They are e x t r u d e d seaward from t h e mixing zone by p r e s s u r e q r a d i e n t s
along i s o p y c n a l s i n t o t h e graded d e n s i t y s t r u c t u r e o f w a t e r columns t o seaward. I f t h e mixtures have s u f f i c i e n t i n i t i a l volume, t h e y c o l l a p s e i n t o " b l i n i " (Russian pancakes) of o r d e r 10 t o 2 0 m t h i c k n e s s and 3 t o 5 km d i a m e t e r , t h e horizontal s c a l e determined by t h e Rossby r a d i u s , and m a i n t a i n a d i s t i n c t dynamical i d e n t i t y a f t e r l e a v i n g t h e f r o n t a l zone.
Salt-finqer a c t i v i t y i s
associated with some o f t h e i n i t i a l m i x t u r e s b u t a p p e a r s t o be a t r a n s i e n t phenomenon.
I f t h e ocean environment t o seaward h a s low e n e r a y ( a s on t h e
Bering s h e l f ) t h e l a r g e r " b l i n i " can have l i f e t i m e s of approximately one month.
INTRODUCTION
Continental s h e l f seas o f mid- and h i g h e r l a t i t u d e s c o n t a i n semi-permanent f r o n t s (Fig. 1). The f r o n t s are zones of enhanced h o r i z o n t a l p r o p e r t y g r a d i e n t s denoting t h e b o u n d a r i e s between water masses d i f f e r i n g i n T/S p r o p e r t i e s and/or water column s t r u c t u r e .
Within t h e s e f r o n t s , t h e water masses a r e i n t e r a c t i n q
" l a t e r a l l y " , and a c o m o n p r o d u c t o f t h e i r i n t e r a c t i o n i s t h e f o r m a t i o n o f finestructure f e a t u res .
F i n e s t r u c t u r e , c o r r e l a t e d i n v e r s i o n s i n T/S on v e r t i c a l
s c a l e s o f 1 m t o 100 m (Munk, 1981) due t o an i n t e r l e a v i n q and l a y e r i n q o f t w o water masses w i t h n e a r l y t h e same d e n s i t i e s , are common f e a t u r e s o f a l l o c e a n i c f r o n t a l zones ( s e e , e . g . ,
Williams, 1981).
Roden, 1974; Gordon e t a l . , 1977; Horne, 1978a;
The i n t e r l e a v i n g o f two water masses j u x t a p o s e d l a t e r a l l y on
v e r t i c a l s c a l e s s i g n i f i c a n t l y l a r g e r t h a n m i c r o s t r u c t u r e scales ( < 1 m v e r t i c a l dimension) a p p e a r s t o be a predominant mode o f t h e i r i n t e r a c t i o n ( J o y c e , 1 9 7 7 ) ; t h i s p r e f e r e n c e f o r a dominant scale l a r g e r t h a n m i c r o s t r u c t u r e h a s been demonstrated i n t h e e l e g a n t l a b o r a t o r y e x p e r i m e n t s ?f Turner (1978) and Ruddick and Turner (1979).
216
I. "TYPICAL"
ZOOL
E I I-
MID-LATITUDE
SHELF
'
IT. EXTRA-WIDE
SHELF (e.g.BERING S E A )
A SHELFBREAK FRONT B TIDAL FRONT C MID-SHELF FRONT
\
ISOPLETHS
5 FINESTRUCTURE FORMATION
Fiq. 1. Schematic c r o s s - s e c t i o n s of a " t y p i c a l " m i d - l a t i t u d e s h e l f (I) and one of t h e four World Ocean s h e l v e s 5 t o 1 0 times broader (II), showing t h e semipermanent f r o n t s . The extra-wide shelves c o n t a i n a t h i r d f r o n t i n a d d i t i o n t o t h e s h e l f b r e a k and i n n e r t i d a l f r o n t s a t mid-shelf, which i s an important s i t e of f i n e s t r u c t u r e formation.
The importance of t h e f i n e s t r u c t u r e formed i n t h e s h e l f f r o n t s i s t h a t they modulate t o an important e x t e n t t h e c r o s s - s h e l f exchanges of p r o p e r t i e s ( e . g . , n u t r i e n t s ; Voorhis e t a l . ,
1976; Horne, 197833; Houghton and Marra, 1983) and,
where they a r e p r e s e n t , enhance t h e l a r g e r - s c a l e v e r t i c a l ' (Voorhis e t a l . ,
1976; Coachman and Walsh, 1981).
f l u x e s of p r o p e r t i e s
The l a t t e r e f f e c t i s due i n
p a r t t o a s m a l l e r - s c a l e s h e a r i n g of t h e p r o p e r t y f i e l d s by t h e i n t r u s i v e l a y e r i n g , and sometimes t o t h e presence of "double-diffusive" processes
(Turner,
1978) which may be a s s o c i a t e d with t h e upper and lower boundaries of t h e i n t r u s i v e l a y e r s (Gregg, 1975; G a r g e t t , 1976). The f i n e s t r u c t u r e a r e p a r t i a l mixtures of t h e juxtaposed water masses which escape t h e formation a r e a b e f o r e complete mixing, t h u s r e t a i n i n g t h e i r i d e n t i t y
as s e p a r a t e water p a r c e l s .
These p a r c e l s a r e 3-dimensional, but t h e only
dimension known with any c e r t a i n t y i s t h e v e r t i c a l ; modern CTDs provide a c c u r a t e r e s o l u t i o n of T/S down t o b e t t e r than 1 m. shape and e x t e n t ?
What i s t h e i r h o r i z o n t a l
Are they ribbon o r s h e e t - l i k e ? o r rounder, more l i k e
'No d i s t i n c t i o n i s made i n t h i s paper between " v e r t i c a l " and "cross-isopycnal" when considering q u a s i - v e r t i c a l d i f f u s i o n . Where t h i s phenomenon i s considered (e.g., s e c t i o n 5) isopycnals a r e q u a s i - h o r i z o n t a l and t h e r e i s l i t t l e d i s t i n c t i o n between then.
217 pancakes?
Not only a r e t h e shapes of t h e s e f e a t u r e s not known i n d e t a i l ,
n e i t h e r a r e important a s p e c t s of t h e i r behavior, such a s r e l a t i v e motion, l i f e t i m e , and e f f e c t on f l u x e s .
The problem i s t h a t t h e t y p i c a l f i n e s t r u c t u r e
have small h o r i z o n t a l s c a l e s ( o r d e r of a few k i l o m e t e r s ) and a r e t o o v a r i a b l e s p a t i a l l y and temporally t o be resolved by o r d i n a r y oceanographic sampling strategy. The l a t e s t information providing d e t a i l s of t h e s e f e a t u r e s f o r s h e l f sea f r o n t s comes from t h e study o f Houghton and Marra (1983).
They occupied
"microlines" of s t a t i o n s (1 km s t a t i o n spacing) a c r o s s t h e s h e l f b r e a k f r o n t of t h e New York Bight ( N Y B ) , wherethe NYB"co1d pool" water (Houghton, e t a l . , 1982) e x t r u d e s a s a nose-like f e a t u r e -10 m t o 40 m t h i c k i n t o c o n t i n e n t a l slope water ( c f . Voorhis, e t a l . , 1976; Posmentier and Houghton, 1978, 1981; Welch, 1981).
This i s a zone of a c t i v e f i n e s t r u c t u r e formation (Fig. 1, I ) .
Houqhton and Marra reported 5 t o 1 0 m s c a l e f i n e s t r u c t u r e t o be p r e v a l e n t i n two regions, along t h e "nose" of t h e c o l d pool e x t r u s i o n and on i t s underside.
The
f e a t u r e s n e a r t h e "nose" were n o t coherent over d i s t a n c e s of 1 km, but t h i s i s a region of h o r i z o n t a l s h e a r of t h e s l o p e c u r r e n t ( a s suggested by t h e f l o a t measurements of Voorhis e t a l . , 1976 and t h e c u r r e n t meter d a t a referenced by Houghton and Marra (1983)). In c o n t r a s t , t h e f e a t u r e s under t h e c o l d e x t r u s i o n had h o r i z o n t a l coherences of about 5 km.
Likewise, s i m i l a r f i n e s t r u c t u r e
associated with a s l o p e water i n t r u s i o n above t h e cold pool appeared t o be coherent alongshore over 6-10 km and had c r o s s - s h e l f dimensions o f a few kilometers.
These r e s u l t s suggest t h a t not only i s f i n e s t r u c t u r e o c c u r r i n g on a
hierarchy of v e r t i c a l s c a l e s , t h e i n t e r l e a v i n g s a l s o e x h i b i t a h i e r a r c h y of horizontal scal es.
This i s implied i n t h e models o f i n t r u s i o n s o f Toole and
Georgi (1981), whose work i d e n t i f i e d a number of parameters important t o s e t t i n g v e r t i c a l s c a l e s , such as e f f e c t i v e v e r t i c a l s a l t d i f f u s i v i t y and f l u x r a t i o of h e a t t o s a l t , v e r t i c a l d e n s i t y g r a d i e n t and l a t e r a l s a l i n i t y g r a d i e n t , a s w e l l a s t h e C o r i o l i s a c c e l e r a t i o n f o r d e f i n i n g t h e along f r o n t s c a l e . Houghton and Marra (1983) a l s o mapped t h e d e n s i t y r a t i o R = aAT/6AS [where P -1 ( a p / a T ) , 6 = p - ' ( a p / a s ) , and AT and AS a r e T and S d i f f e r e n c e s over a
a = -p
v e r t i c a l i n t e r v a l - - t h e y used 5-10 m], on t h e i r d e t a i l e d "microline" t o show t h a t s a l t f i n g e r i n g , t h e a r e a s where 1 . 0 < Rp < 1 . 5 , was a c t i v e i n t h e same a r e a s where t h e 5-10 m f i n e s t r u c t u r e was p r e v a l e n t , v i z . o f f t h e nose of t h e c o l d pool and on i t s underside.
One f i n e s t r u c t u r e i n t r u s i o n under t h e cold pool had
a v e r t i c a l dimension of -4 m , on-off s h e l f dimension o f -5 km, and a d e n s i t y
-1.2-1.4, from which they e s t i m a t e d a l i f e t i m e of P T h i s i s t h e same l i f e t i m e as estimated f o r o t h e r s i m i l a r f e a t u r e s , -1 1-2 days f o r a -10 m t h i c k l a y e r with AT " 2-C and AS - 0 . 5 p kg by
r a t i o on t h e lower s i d e of R T
2
2 days.
e.g.,
Posmentier and Houghton (1978), who noted f u r t h e r t h a t without double-diffusive f l u x enhancement l i f e t i m e s would be considerably longer.
218 The extra-wide Bering Sea s h e l f c o n t a i n s a "cold pool" of water over t h e c e n t r a l s h e l f , f o r which t h e seaward boundary i s t h e middle f r o n t (Fig. 1, 11). This f r o n t i s a prominent s i t e of f i n e s t r u c t u r e formation (Coachman and Charnell, 1979; Kinder and Schumacher, 1981); t h e f i n e s t r u c t u r e are p a r t i a l mixtures of c o l d pool water with warmer, more s a l i n e Bering Sea b a s i n water moving landward a s a bottom l a y e r a c r o s s t h e o u t e r s h e l f .
The only r e p o r t providing any d e t a i l
on t h i s f i n e s t r u c t u r e i s Coachman and Charnell (1979). s h e l f s e c t i o n of f i v e s t a t i o n s a t -25 km i n t e r v a l s .
They analyzed a cross-
Two a d j a c e n t s t a t i o n s
( s t a s . 60, 61; Coachman and C h a r n e l l , 1979, F i g s . 19-21) showed a complex h i e r archy of f i n e s t r u c t u r e very much l i k e t h a t of a s t a t i o n from t h e NYB (Posmentier and Houghton, 1978, Fig. 3 ) w i t h n i n e d i s t i n c t i v e l a y e r s o f a l t e r n a t i n g w a r m , s a l i n e with c o l d , less s a l i n e ; t h e r e was one l a r g e c o l d e x t r u s i o n between 60 and 80 m depths w i t h AT
-
-
and t h e l a y e r s , 4 t o 10 m t h i c k -1 w i t h 0.2 < AT < 0.6OC and 0.2 < AS < 0.5 gm kg , were d i s t r i b u t e d between 30 m and 90 m depths.
4°C and AS
0 . 5 gm kg-',
Because t h e s t a t i o n s showed such p r e c i s e s i m i l a r i t y i n
l a y e r i n g , Coachman and Charnell concluded t h e f e a t u r e s were s h e e t o r ribbon-like with an on-offshore dimension >25 km.
However, t h i s may have been an unusual
c a s e or an i n c o r r e c t i n t e r p r e t a t i o n ; i n many repeated s e c t i o n s of a s t a n d a r d c r o s s - s h e l f s e c t i o n d u r i n g t h e PROBES p r o j e c t w i t h s t a t i o n s spaced a t 25 km f i n e s t r u c t u r e f e a t u r e s could n o t be c o r r e l a t e d between a d j a c e n t s t a t i o n s (Coachman, 1985); it t h u s seems l i k e l y t h a t t h e u s u a l c r o s s - s h e l f s c a l e f o r i n d i v i d u a l f i n e s t r u c t u r e f e a t u r e s i s <25 km.
Two o b s e r v a t i o n s o f Coachman and Charnell (1979) suggest t h a t doubled i f f u s i v e p r o c e s s e s are a s s o c i a t e d with t h e f i n e s t r u c t u r e .
One was t h e downward
progression of t h e T/S c o r r e l a t i o n a t t h e i r stas. 6 0 , 61 which showed a s e r i e s of cyclonic whorls which, as analyzed by Posmentier and Houghton (19781, a r e c h a r a c t e r i s t i c of a more r a p i d v e r t i c a l h e a t t h a n s a l t f l u x .
The o t h e r was t h a t
t h e i r l a y e r s d e f i n i t e l y i n c r e a s e d i n d e n s i t y i n a bulk s e n s e , by about 0.2 going off-shore.
Ut,
Where double-diffusion i s a c t i v e v e r t i c a l f l u x e s a r e dominated
by t h e s a l t - f i n g e r i n g phenomenon--thus
t h e buoyancy f l u x i s downward i n t o c o l d ,
less s a l i n e i n t r u s i o n s which tend t o g a i n d e n s i t y and s i n k as t h e y move away from t h e i r formation a r e a , and t h e o p p o s i t e i s t r u e f o r warm, s a l i n e i n t r u s i o n s (Turner, 1978).
In t h e Bering Sea case c o l d pool e x t r u s i o n s dominate t h e f i n e -
s t r u c t u r e , and t h e observed i n c r e a s e i n d e n s i t y i s compatible with doublediffusion. Thus t h e r e appears t o be c o n s i d e r a b l e s i m i l a r i t y between t h e N e w York Bight and s o u t h e a s t e r n Bering Sea s h e l v e s i n t h e n a t u r e of t h e l a t e r a l w a t e r mass i n t e r a c t i o n between c o l d pool s h e l f w a t e r and s l o p e w a t e r .
The gl;oss i n t e r -
a c t i o n i s an e x t r u s i o n of c o l d pool water c r e a t i n g a l a r g e f i n e s t r u c t u r e some t e n s of meters t h i c k , c o l d e r and less s a l i n e than t h e s l o p e water.
Super-
imposed, and p a r t i c u l a r l y along t h e upper and lower s i d e s and a t t h e nose of
t h e l a r g e r f i n e s t r u c t u r e , a r e many i n t e r l e a v i n g s of 2 t o 1 0 m v e r t i c a l s c a l e . With t h e s e i n t r u s i o n s t h e r e may be a s s o c i a t e d double-diffusive phenomena.
The
h o r i z o n t a l s c a l e s of t h e s e predominant f i n e s t r u c t u r e are a few km, with l i f e times perhaps on t h e o r d e r of a few days. There a r e d i f f e r e n c e s between t h e regimes within which t h e f i n e s t r u c t u r e d i s p e r s e s , as w e l l a s many unanswered q u e s t i o n s about t h e i r d i s s i p a t i o n and e f f e c t on h o r i z o n t a l and v e r t i c a l f l u x e s . t h e flow f i e l d .
A major d i f f e r e n c e i s t h e n a t u r e of
In t h e NYB t h e f i n e s t r u c t u r e e x t r u d e s i n t o a modest c u r r e n t ;
t h i s undoubtedly i s r e s p o n s i b l e f o r an along-shelf suggested by Voorhis e t a l . unknown.
e l o n g a t i o n of t h e f e a t u r e s a s
(1976), b u t i t s r o l e i n t h e i r d i s s i p a t i o n is
In t h e Bering Sea t h e flow f i e l d i s overwhelmingly t i d a l , with only
slow (<5 cm/s) along- and a c r o s s - s h e l f low-frequency c u r r e n t s (Schumacher and Kinder, 1983), which r a i s e s another q u e s t i o n .
A c o l d e r mid-depth
l a y e r (-50 t o
'80 m) e x i s t s a c r o s s t h e e n t i r e o u t e r s h e l f >lo0 km from t h e middle f r o n t f o r which t h e source i s t h e cold pool water: a l s o , s i g n i f i c a n t f i n e s t r u c t u r e i s frequently encountered i n t h e o u t e r p a r t of t h e o u t e r s h e l f (Coachman and Charnell, 1979; Kinder and Schumacher, 1981: Coachman, 1 9 8 5 ) , so t h e q u e s t i o n
i s , i f t h e l i f e t i m e s of t h e t y p i c a l f i n e s t r u c t u r e f e a t u r e s i s a few days, how weeks a t
do t h e s e f e a t u r e s s u r v i v e t h e t r a n s i t o f t h e o u t e r s h e l f , a t r i p of '3 5 cm/s?
In June-July
1982 a study was undertaken i n t h e s o u t h e a s t e r n Bering Sea with
t h e R/V Alpha Helix s p e c i f i c a l l y t o i n v e s t i g a t e t h e f i n e s t r u c t u r e formed i n t h e middle f r o n t .
In a d d i t i o n t o a N e i l Brown CTD, t h e study employed a v e r t i c a l -
p r o f i l i n g c u r r e n t meter system (Cyclesonde:
Van Leer e t a l . ,
1974).
We
encountered s t r o n g f i n e s t r u c t u r e and watched i t s behavior f o r s e v e r a l days. This paper r e p o r t s t h e r e s u l t s of t h e o b s e r v a t i o n s and a n a l y s e s , which provide new d a t a toward understanding t h e s e important f e a t u r e s o f oceanic mixing.
THE EXPERIMENT
The l o c a t i o n of t h e experiment was chosen t o coincide with t h e s t a n d a r d cross-shelf
s e c t i o n e s t a b l i s h e d f o r t h e PROBES s t u d i e s i n 1978 (Goering and
McRoy, 1981) and which has been occupied l i t e r a l l y dozens of times s i n c e , providing c o n s i d e r a b l e background on t h e g e n e r a l hydrography. the l o c a t i o n s of s t a t i o n s 4-14 of t h e PROBES s t a n d a r d s e c t i o n .
Figure 2 shows Also shown a r e
t h e l o c a t i o n s of two c u r r e n t meter moorings s t r a d d l i n g t h e middle f r o n t ( P R - 1 , PR-2)
f o r which records a r e a v a i l a b l e from two d i f f e r e n t p e r i o d s :
March-April
one month i n
1980, analyzed by Coachman (1982), and 492 months Feb-June
1981,
f o r which t h e records a r e a s y e t only p a r t i a l l y analyzed (Coachman, 1985); t h u s t h e r e was also background on t h e flow f i e l d near t h e s i t e of t h e f i n e s t r u c t u r e formation.
Moorings w e r e i n p l a c e f o r t h e experiment, b u t , u n f o r t u n a t e l y , t h e
important mooring PR-1 w a s never recovered.
F i g . 2 . F i n e s t r u c t u r e experiment o v e r t h e o u t e r p a r t of t h e s o u t h e a s t e r n B e r i n g Sea s h e l f . S t a t i o n s 4-14 ( s o l i d c i r c l e s ) a r e t h e s t a n d a r d PROBES l i n e ; PR-1 and PR-2 c u r r e n t meter mooring l o c a t i o n s p r e v i o u s l y o c c u p i e d ; CYC-1 and CYC-2 t h e Cyclesonde deployment l o c a t i o n s ; and s t a s . 113-124 t h e s e c t i o n a l o n q Hachure i n d i c a t e s t h e m i d d l e f r o n t zone o f f i n e s t r u c t u r e t h e 110 m i s o b a t h . formation.
The g e n e r a l scheme was t o r u n t h e PROBES s e c t i o n t o d e t e r m i n e t h e l o c a t i o n o f t h e m i d d l e f r o n t and t h e T/S c h a r a c t e r o f t h e b a s i c i n t e r a c t i n q w a t e r masses i n J u n e , 1982, t h e n d e p l o y t h e Cyclesonde i n a l o c a t i o n w i t h s t r o n q f i n e s t r u c 1 t u r e development. The f i r s t deployment, f o r 4 / 2 d a y s , w a s c l o s e t o PR-1 (CYC-1 i n F i g .
2 ) ; it w a s t h e n moved one s t a t i o n s p a c i n g o n s h o r e (CYC-2)
where
a f u l l 7 d a y s o f 1 p e r h o u r v e r t i c a l p r o f i l e s o f h o r i z o n t a l c u r r e n t s and t e m p e r a t u r e were o b t a i n e d .
F o r c a l i b r a t i o n p u r p o s e s , CTDs were t a k e n a s c l o s e
as w e d a r e d t o t h e Cyclesonde t i m e d t o a v e r t i c a l p r o f i l e : 19 d u r i n g CYC-2.
7 d u r i n g CYC-1 and
A l s o d u r i n g t h e s e c o n d deployment t h r e e g r i d s o f c l o s e l y
s p a c e d (3-5 km) CTD s t a t i o n s s u r r o u n d i n g CYC-2 were o c c u p i e d t o d e f i n e t h e h o r i z o n t a l s t r u c t u r e of t h e major f i n e s t r u c t u r e .
A t t h e c l o s e of t h e e x p e r i m e n t
a s e c t i o n p a r a l l e l t o t h e f a c e o f t h e f r o n t a l o n g t h e 110 m i s o b a t h ( s t a s . 113-124 i n F i g . 2 ) w a s t a k e n t o p r o v i d e d a t a on t h e a l o n g - f r o n t n a t u r e of t h e finestructure intrusions. The CTD w a s a N e i l Brown, w i t h a c c u r a c i e s b e t t e r t h a n 0 . 0 1 u n i t s i n b o t h T and S .
S a l i n i t i e s and d e n s i t i e s w e r e c a l c u l a t e d u s i n g t h e I n t e r n a t i o n a l
Oceanoqraphic T a b l e s r e c e n t l y a d o p t e d by UNESCO.
w a s on f i n e s t r u c t u r e , t h e
S and T d a t a
Because t h e f o c u s o f t h e work
were a v e r a g e d o v e r 1 - m i n t e r v a l s i n t h e
vertical. P o s i t i o n s were d e t e r m i n e d u s i n g Loran-C, i n t h e s o u t h e a s t e r n Bering Sea.
which p r o v i d e s e x c e l l e n t r e s o l u t i o n
221 The Cyclesonde was model PCM-1.5 from Marine P r o f i l e s , I n c . , Miami, F L , equipped with Savonius r o t o r s (providing a redundancy i n speed measurement), a compass sensing o r i e n t a t i o n t o within +3O, a temperature probe (estimated accuracy * O . O l " C ) ,
and a c o n d u c t i v i t y c e l l from which t h e d a t a u n f o r t u n a t e l y
The instrument a l s o records p r e s s u r e and tilt from t h e h o r i z o n t a l .
are useless.
We made a modification t o t h e standard instrument:
with t h e instrument runninq
continuously t a p e l e n g t h l i m i t s t h e endurance t o <2 days a t t h e f a s t e s t samplinq r a t e , so a switch and t i m e r were introduced which s t a r t e d t a p e t r a n s p o r t a t t h e beginning of each p r o f i l e , then stopped t h e t a p e a t a s e l e c t e d time i n t e r v a l later, e.g.,
With hourly p r o f i l e s of 15 min d u r a t i o n and t h e
15 o r 30 min.
f a s t e s t scan r a t e a f u l l 7 days of measurements were achieved a t CYC-2. The Cyclesonde speed d a t a were processed a s follows. of t h e instrument was about 1 0 cm s
The mean v e r t i c a l speed
-1, b u t v a r i e d a l i t t l e between p r o f i l e s
because i t a c t u a l l y " f l i e s " i n t h e water and t h e r e f o r e i t s v e r t i c a l speed depends on h o r i z o n t a l c u r r e n t speed.
The beginnings and ends of each p r o f i l e
were estimated from t h e p r e s s u r e and tilt d a t a , and then t h e t o t a l number of scans a t 2-sec i n t e r v a l s allowed e s t i m a t i o n of t h e mean v e r t i c a l speed; t h i s was subtracted v e c t o r i c a l l y from t h e recorded speeds t o g i v e h o r i z o n t a l speeds. The 2-sec sampling r a t e meant t h a t only about 4 t o 6 scans were obtained i n each v e r t i c a l meter t r a v e r s e d , so t h e U and V v a l u e s were smoothed with a 3-pt running mean and averaged over 2-m
intervals.
Thus, t h e v e r t i c a l shear d a t a
a r e inadequate f o r examining m i c r o s t r u c t u r e s c a l e phenomena (Sl m ) but s u i t a b l e f o r t h e f i n e s t r u c t u r e which i s t h e focus of t h e study ( c f . Evans, 1982).
RESULTS
1.
Large-scale i n t e r a c t i o n Temperature and s a l i n i t y s e c t i o n s f o r t h e i n i t i a l occupation of t h e PROBES
l i n e a r e shown i n Fig. 3. finestructure: 3.8'C
of T of AT
-
E x c e l l e n t c o n d i t i o n s e x i s t e d f o r t h e observation of
t h e s h e l f water of T
and S
4 O C and AS
-
32.8 gm kg
-1
-
O°C and S
-
31.7 gm kg
-1
and b a s i n water
provided temperature and s a l i n i t y c o n t r a s t s
1 gm kg-'.
The middle f r o n t l i e s between s t a s . 8 and 1 0 and f i n e s t r u c t u r e formation i s clearly v i s i b l e , e s p eci al l y i n t h e T section. very much l i k e t h a t i n t h e NYB i n summer:
The l a y e r i n g of i n t r u s i o n s looks
a warm, more s a l i n e l a y e r aimed
onshore between -35-55 m ar.d a c o l d e x t r u s i o n beneath, between -55-80 m.
There
i s a l s o a t h i n n e r c o l d l a y e r - 5 m t h i c k between t h e major warm l a y e r and t h e surface mixed l a y e r .
These f e a t u r e s show n i c e l y i n t h e T/S c o r r e l a t i o n s of t h e
s t a t i o n s (Fig. 4 ) ; from s t a . 1 0 seaward t o s t a . 5 t h e curves each show two temperature minima with t h e maximum from t h e warm, s a l i n e i n t r u s i o n between. These general f e a t u r e s of t h e v e r t i c a l water mass d i s t r i b u t i o n s a r e s i m i l a r across t h e o u t e r s h e l f t o beyond s t a . 4 , b u t i n d e t a i l t h e a d j a c e n t s t a t i o n s do
222 PROBES LINES o4
5
6
7
8
9
10
11
12
13
14
20 40 60 80 100
120
TEMPERATURE,'C 2 0 - 2 1 JUNE 1982
140 I
I
L
Fig. 3 . Temperature (upper) and s a l i n i t y (lower) s e c t i o n s along t h e PROBES l i n e a t t h e beginning of t h e experiment. The middle f r o n t encompasses s t a s . 9 and 1 0 , w i t h i n which can be seen two c o l d e r , l e s s s a l i n e l a y e r s Note t h e e x t r u d i n g seaward with a w a r m e r , more s a l i n e l a y e r between. suggestion f a r t h e r off-shore of i s o l a t e d f i n e s t r u c t u r e p a r c e l s .
not look very much a l i k e .
Looking a l s o a t t h e c r o s s - s e c t i o n s
(Fig. 3 ) , t h e
f i n e s t r u c t u r e does not appear t o be s h e e t s and l a y e r s of long l a t e r a l dimens i o n s a s was suggested from t h e s e c t i o n a n a l y z e d b y Coachman and Charnell (1979), b u t r a t h e r of numerous i s o l a t e d f e a t u r e s with h o r i z o n t a l dimensions l e s s than one s t a t i o n spacing ( ~ 2 5km). The T/S c o r r e l a t i o n s (Fig. 4 ) show another r e s u l t corresponding with t h a t r e p o r t e d by Coachman and Charnell (1979):
t h e lower c o l d e x t r u s i o n composed of
f i n e s t r u c t u r e , which c r e a t e s t h e temperature minimum i n mid-water column a c r o s s t h e o u t e r s h e l f t o seaward of s t a . 4 , i n c r e a s e s i n d e n s i t y by >0.2
u
t'
Thus,
t h e f i n e s t r u c t u r e f e a t u r e s must on t h e average be l o s i n g buoyancy i n t h e i r > l o 0 km t r a n s i t of t h e o u t e r s h e l f . A l l t h e f i n e s t r u c t u r e r e p r e s e n t s mixtures of t h r e e b a s i c waters ( c f . F i g . 3 ) :
a s u r f a c e w a t e r which covers t h e whole system down t o 20-30 m depths, t h e c o l d , deeper s h e l f water i n s h o r e , and t h e w a r m , s a l i n e b a s i n water below -80 m i n t h e
223
5
32
32.5
33
SALINITY, qm kq-' Fig. 4.
T/S curves f o r s t a t i o n s 4-13 o f t h e PROBES l i n e , 20-21 June 1982.
outer p a r t of t h e s e c t i o n .
The t h r e e a r e arranged s p a t i a l l y such t h a t only t h e
shelf bottom water mixes with both o f t h e others--a
q u a s i - l a t e r a l mixing with
basin water a c r o s s t h e zone of t h e middle f r o n t , and v e r t i c a l mixing with surface water i n t h e more landward p a r t of t h e middle f r o n t .
I t i s t h e very
cold (
The s h e l f - s u r f a c e water mixtures a r e a l i t t l e
l e s s dense and c r e a t e t h e upper of t h e two cold f i n e s t r u c t u r e e x t r u s i o n s , while the lower, denser e x t r u s i o n s a r e products of t h e s h e l f - b a s i n bottom waters mixing " l a t e r a l l y " w i t h i n t h e middle f r o n t .
This hypothesis i s d e p i c t e d
schematically i n t h e T/S plane i n Fig. 5.
2.
Cyclesonde o b s e r v a t i o n s
The Cyclesonde w a s deployed i n two l o c a t i o n s : CYC-1 was mid-way between 1 stas. 7 and 8 f o r 4 / 2 days, and CYC-2 mid-way between stas. 8 and 9 f o r 7 days (Fig. 2 ) .
The instrument p r o f i l e d every hour on t h e hour, a t CYC-1 between
20 and 106 m and a t CYC-2
between 28 and 85 m.
The vector-average c u r r e n t over t h e water column measured a t CYC-1 i s shown i n Fig. 6 i n t h e form of a p r o g r e s s i v e v e c t o r diagram ( P M ) .
W e see t h e
dominant motion was a semi-diurnal t i d a l s i g n a l (with a l a r g e i n e q u a l i t y ) -1 As t h e superimposed on a q u i t e steady set almost due n o r t h a t 4.2 cm s
.
onshore d i r e c t i o n i s 043O (along t h e o r d i n a t e i n Fig. 7 ) , t h e flow was both
224 OUTER DOMAIN --
-
CROSS ISOPYCNAL MIXING ABETTED BY E STRUCTURE LAYERING
MIDDLE FRONT -M I X E D INTRUSIONS ALONG ISOPYCNALS
A$,/
@PYCNOCLlNE
MIXTURES
@BOTTOM WATER M I X T U R E S
CENTRAL DOMAIN MIXING ALONG ISOPYCNALS
'D
1 31.5
x
I
32
1
I
32.5
33
SALINITY, grn kg-'
F i g . 5. S c h e m a t i c d i a g r a m i n t h e T/S p l a n e o f t h e mixing w i t h i n t h e m i d d l e f r o n t which creates t h e two d i s t i n c t c o l d , less s a l i n e f i n e s t r u c t u r e : the s h a l l o w e r e x t r u s i o n s are s h e l f b o t t o m w a t e r / s u r f a c e l a y e r m i x t u r e s and t h e d e e p e r e x t r u s i o n s ':lateral" m i x i n g s o f s h e l f / b a s i n w a t e r .
c o n v e r g i n g i n t h e m i d d l e f r o n t and f l o w i n g n o r t h w e s t p a r a l l e l w i t h t h e b a t h y m e t r y ; t h i s is a u s u a l f l o w c o n d i t i o n i n t h e o u t e r s h e l f domain ( c f . Coachman, 1 9 8 5 ) . The f l o w r e c o r d e d a t CYC-2
i s shown i n F i g . 7 i n t h e same f o r m a t .
The same
dominance o f a t i d a l s i g n a l i s e v i d e n t , though it a p p e a r s t h a t t h e s e m i - d i u r n a l component was somewhat s t r o n g e r . t h e week:
A l s o , t h e low-frequency
f l o w changed d u r i n g
f o r t h e f i r s t t h r e e d a y s it w a s d i r e c t l y o n s h o r e a t - 4 c m s-l ( t h e
f i r s t day o f r e c o r d -5.8 c m s-'),
b u t t h e n backed t o t h e n o r t h , t h a t i s , t u r n e d
more p a r a l l e l t o t h e b a t h y m e t r y , and d e c e l e r a t e d t o < 2 c m s - l . T e m p e r a t u r e i s t h e v a r i a b l e by which f i n e s t r u c t u r e f e a t u r e s c a n b e p o s i t i v e l y i d e n t i f i e d , p a r t i c u l a r l y t h e deeper cold ex t rusi ons t h a t a r e mixtures of s h e l f bottom water (-0°C) and b a s i n water ( - 3 . 8 " C ) .
T e m p e r a t u r e p r o f i l e s a t CYC-1
showed some maxima and minima i n d i c a t i v e o f f i n e s t r u c t u r e , p a r t i c u l a r l y a t e n d e n c y f o r a minimum between 50 and 65 m. c o n t r a s t w a s <0.5'C,
But t h e g r e a t e s t t e m p e r a t u r e
so t h e f i n e s t r u c t u r e f e a t u r e s w e r e n o t o b v i o u s and
amenable t o more d e t a i l e d a n a l y s i s .
225
/ +\,
\
//
/
CYC- 1 MEAN 28- 106 in
0
1
2
3
4 I
KILOMETERS
Fig. 6. The vector-mean flow over t h e measured water column (28-106 m) a t CYC-1, f o r t h e p e r i o d 23 June 2300 t o 27 June 1600, presented a s a PVD of t h e hourly v a l u e s . Tic marks a r e a t 6-hour i n t e r v a l s .
I n c o n t r a s t , t h e CYC-2 r e c o r d s contained o b s e r v a t i o n s of a very w e l l developed f i n e s t r u c t u r e , a s can be seen i n t h e time-history p l o t of t h e temperature f i e l d ( F i g . 8 ) .
A cold c o r e , 15 t o 20 m t h i c k c e n t e r e d a t 50 t o
55 m depth, with temperature c o l d e r by > l 0 C above o r below appeared periodically a t the station.
I t remained over t h e s t a t i o n t y p i c a l l y 3 t o 6
hours, then a b r u p t l y vanished only t o reappear -6 hours l a t e r , a timing s t r o n g l y suggestive of t i d a l a c t i v i t y . be q u i t e s h a r p - - i t
The h o r i z o n t a l edge w a s observed t o
'
would on occasion completely appear o r d i s a p p e a r between
two p r o f i l e s , which i n d i c a t e s a h o r i z o n t a l temperature g r a d i e n t l i k e t h a t observed along t h e edge of t h e NYB c o l d e x t r u s i o n , >1.SoC/km.
For t h e f i r s t
t h r e e days of record t h e c o r e temperatures of t h e f e a t u r e were c o l d , < 2 . S ° C , but toward t h e end of t h e f o u r t h day ( J D 182) t h e cold s i g n a l became progress i v e l y l e s s i n t e n s e , and on t h e l a s t two days t h e f e a t u r e had n e a r l y vanished from t h e Cyclesonde l o c a t i o n , with only a weak i n d i c a t i o n o f cold toward t h e ends of J D 184 and J D 185. These o b s e r v a t i o n s can be explained i n t h e following way.
The s t r o n g l y
developed c o l d l a y e r i s a p a r t i a l mixture of bottom s h e l f water and b a s i n water formed i n t h e middle f r o n t j u s t landward of C k - 2 , ward i n t h e form of a f i n e s t r u c t u r e .
and which h a s extruded sea-
The f e a t u r e has a d i s c r e t e l a t e r a l shape
226
CYC-2 MEAN 28-85m
JD179
06
Fig. 7.
As i n Fig.
6 , b u t f o r CYC-2,
2 8 June 0600 t o 5 J u l y 0600.
with a f a i r l y s h a r p boundary, a t l e a s t along p a r t of i t s p e r i m e t e r , and i s being advected about by t h e c u r r e n t s .
A s t h e flow f i e l d i s predominantly
o s c i l l a t o r y a t t i d a l p e r i o d s , t h e f i n e s t r u c t u r e i s being advected back and f o r t h a c r o s s t h e Cyclesonde l o c a t i o n , reappearing a t approximately 12-hour intervals.
The f i n e s t r u c t u r e has a very c o l d c o r e , b u t i s somewhat w a r m e r
towards i t s perimeter;
sometimes only an edge of t h e f e a t u r e reaches t h e CYC-2
l o c a t i o n , p a r t i c u l a r l y toward t h e end of t h e record p e r i o d when t h e n e t advection toward t h e n o r t h (Fig. 7 ) has moved t h e core of t h e f i n e s t r u c t u r e "out of reach" of CYC-2. We now focus o u r a t t e n t i o n on t h i s d i s t i n c t i v e f e a t u r e of l a t e r a l water mass i n t e r a c t i o n , f o r which t h e r e a r e considerable water mass and h o r i z o n t a l flow f i e l d d a t a .
227 JD 480
JD 179
24
02
04
06
g:
;" JD 181 4 7 3:
16
(8
20
22
JD 182
24
02
04
1
I
06
08
10
I4
12
36
tE
30 40
50
60
0
t 80 t
L
CTD STATIONS
I
JD 183
JO 184
99
CTD STATIONS
125
CTD STATIONS
F i g . 8. T i m e - h i s t o r y o f t e m p e r a t u r e (C) r e c o r d e d by t h e Cyclesonde a t CYC-2. CTD s t a t i o n s t a k e n a t CYC-2 are i n d i c a t e d b e n e a t h ; T ' s <3OC are hachured.
3.
The f i n e s t r u c t u r e
3.1.
Spatial reconstruction
The f i r s t o b j e c t i v e i s t o map t h e s i z e and s h a p e o f t h e f i n e s t r u c t u r e . During t h e c o u r s e o f t h e Cyclesonde deployment 19 CTD s t a t i o n s were made v e r y c l o s e t o CYC-2,
t i m e d t o a p r o f i l e t i m e , w i t h t h e o b j e c t o f s a m p l i n g t h e same
w a t e r mass w i t h t h e h i g h e r a c c u r a c y i n s t r u m e n t : F i g . 8.
s t a t i o n times a r e i n d i c a t e d i n
The T/S c o r r e l a t i o n s f o r s i x s t a t i o n s which sampled f i v e s u c c e s s i v e
228
2 .o 32.05
32.10
I 32.15
I
32.20
I
32.25
1
32.30
SALINITY, gm kg-I F i q . 9. T/S c o r r e l a t i o n s f o r s t a t i o n s taken through f i v e s u c c e s s i v e re-appearances of t h e cold core a t CYC-2 ( c f . Fig. 8 ) . Sigma-5 r e p r e s e n t s t h e i n_s_ i t u d e n s i t y a t p = 55 db, and t h e successive re-appearances a r e of t h e same f e a t u r e with t h e temperature minimum on t h e a = 25.88 s u r f a c e .
re-appearances o f t h e cold core ( c f . Fig. 8) a r e p l o t t e d i n Fig. 9.
W e see
t h a t t h e "core" l i e s p r e c i s e l y along t h e same d e n s i t y curve [ i n s i t u d e n s i t i e s a t a p r e s s u r e of 55 db, SrT,P The conclusion i s t h a t t h e successive
a r e used i n t h i s a n a l y s i s ; t h u s , a5 r e p r e s e n t s p t h e approximate mean depth of t h e c o r e ] . re-appearances a t CYC-2
a r e of t h e same f i n e s t r u c t u r e f e a t u r e - - i t
i s improbable
t h a t s e p a r a t e , d i s t i n c t p a r t i a l mixings of bottom s h e l f and b a s i n water would produce f e a t u r e s with p r e c i s e l y t h e same core d e n s i t i e s . The next s t e p i s t o attempt t o d e s c r i b e t h e s p a t i a l e x t e n t of t h e f i n e structure.
A t t h r e e times during t h e deployment week box-like g r i d s of CTD
s t a t i o n s were occupied a s r a p i d l y a s p o s s i b l e around CYC-2
(Table 1 ) .
The temperature i n t h e d e n s i t y band 25.87 < a < 25.89 g i v e s a d i s t i n c t i v e d e s c r i p t i o n of t h e f e a t u r e .
A s CHASE provides t h e d e n s e s t a r e a l coverage,
t h e s e s t a t i o n s were dead-reckoned t o J D 182, 0200, u s i n q t h e m e a n v a l u e s of c u r r e n t between 50 and 56 m depths observed by t h e Cyclesonde.
The flow f i e l d
229 TABLE 1
Box e x p e r i m e n t s
Name
Date/Time
Stas
Nominal S p a c i n g (n m i l e s )
Box I
J D 180
48-57
4.5
72-95
3
99-111
5
CHASE BOX
I1
0300-1100 J D 182 0200-1230 J D 183-1800 J D 184-0630
I
I
I
10,
166-W
50'
I
Fig. 1 0 . Temperature on t h e a = 25.88 s u r f a c e , t h e d e n s i t y o f t h e c o l d c o r e o f f i n e s t r u c t u r e , c o n s t r u c t e d from t h e CTD s t a t i o n s o f CHASE ( d o t s ) dead-reckoned Hourly l o c a t i o n s of w a t e r p a r c e l s d e t e r m i n e d from a PVD of t h e t o JD 182/0200. mean 50-56 m c u r r e n t s measured a t CYC-2 were a l s o dead-reckoned t o J D 182/0200, c r e a t i n g t h e f i n e d o t t e d l i n e ( c f . F i g . ll), and t h e 2 . 5 , 3 , and 4OC i s o t h e r m s along t h i s t r a c k were i n t e r p r e t e d from F i g . 8 . On t h i s u s u r f a c e e x i s t e d a c o l d f e a t u r e (<3OC) -20 x 20 km i n e x t e n t which i n c l u d e d 3 c o l d c o r e s (<2.5OC) and one w a r m c o r e (>4OC).
was n o t s t r o n g d u r i n g CHASE ( c f . F i g . 7 )
<1km.
SO
maximum s t a t i o n d i s p l a c e m e n t w a s
The dead-reckoned s t a t i o n l o c a t i o n s a r e shown i n F i g . 10.
A d d i t i o n a l s p a t i a l d e f i n i t i o n o f p a r t of t h e T f i e l d w a s o b t a i n e d as f o l l o w s . The t r a c k of water motion i n d i c a t e d by t h e PVD diagram o f mean 50-56 m c u r r e n t s was dead-reckoned
t o J D 182/0200--this
by t h e f i n e - d o t t e d l i n e .
r e c o n s t r u c t e d p a t h i s shown i n F i g . 1 0
Then from t h e t i m e - h i s t o r y
o f t e m p e r a t u r e a t CYC-2
( F i g . 8 ) a p p r o x i m a t e t i m e s of a p p e a r a n c e of s p e c i f i c t e m p e r a t u r e v a l u e s ( e . g . ,
230 4OC, 3'C,
2.5OC, coldest T) were estimated and plotted along the reconstructed
PVD track. The reconstructed temperature field on
(I
= 25.88 is shown in Fig. 10.
Unfortunately, the plan view of the finestructure features is not very precise; I was misled in the experimental design by my previous experience (viz. Coachman and Charnell, 1979) which had suggested horizontal coherence of the features of at least '10 km, while the actual horizontal scale turns out to be $5 km.
But one finestructure core is quite well defined when the extrapo-
lated Cyclesonde data are included. A more detailed reconstruction is shown in Fig. 11.
It was an - 3 km ovoid of T < 2.5'C
04'
02'
166OW
centered, at JD 182/0200, just
56'
58'
E
06
14'
04
12'
6"
I
8'
t86/06 I
I
04'
I
I
02'
I
I
I
1660~
I
58'
I
I
56'
Fig. 11. Detailed reconstruction of the minimum temperature between 50 and 56 m along the Cyclesonde track. The straight segments creating the light line are hourly 50-56 m flow displacements (cf. Fig. 7) dead-reckoned to JD 182/0200 (sta. 72); numbered dots are dead-reckoned CTD stations. The isotherms near the Cyclesonde track were interpreted from the time-history of T at CYC-2 (Fig. 8). The cold finestructure seen repeatedly by the Cyclesonde was an - 3 km ovoid.
north of CYC-2.
It had a very sharp horizontal temperature gradient along its
western edge, >1.5OC km-l, which accounts for the extremely abrupt appearances and disappearances of very cold temperature at CYC-2 (Fiq. 8). Hereafter I refer to this well-defined cold core of the finestructure as We note that not all the appearances of were due to CC.
<2.5OC
cc.
temperature at the Cyclesonde
The initial appearance, at JD 179/0600 (Fig. 81, was not
connected with CC but a separate cold core, which by JD 182/0200 was located 10 km north of CYC-2.
Likewise, the cold appearance at JD 184/0200 was a
separate feature which, at JD 182/0200, lay 5 km SSE of CYC-2 (cf. Fig. 11). These two cold cores were never adequately sampled by either the Cyclesonde nor CTD stations, so only their presence within the larger cold finestructure field can be noted. 68, 83.
CYC-2.
Thus, the CTD stations which sampled CC were 57, 62, 64,
[In Fig. 8 sta. 60 is shown as coinciding with the appearance of CC at However, its T profile did not match that of the Cyclesonde; it was
apparently taken too far away from CYC-2 to be sampling the same water mass.] To obtain a broader view of the cold finestructure field we can use all CTD stations taken within the general vicinity of CYC-2, including those of BOX I and BOX 11, but to do so requires the assumption that time changes are very small compared to the spatial variability. This is the classical oceanographic sampling dilemma.
However, the estimates obtained on vertical diffusion (see
below), the most probable mechanism for "eroding" the temperature minimum, suggest only very slow warming and a possible long lifetime for these larger isolated water parcels.
This is consistent with the contemporary discovery in
the World Ocean of numerous isolated "lens" of water retaining their anomalous characteristics for months and located thousands of kilometers from their sources (see, e.g., Lindstrom and Taft, 1985; Riser et al., 1985). With the above caveat in mind, Fig.
12
shows the temperature field on
u = 25.88 constructed from all stations and the Cyclesonde track (not shown) dead-reckoned to JD 182/0200.
This is a less accurate presentation because of
possible horizontal shear in the flow field, and maximum station displacement was 10 km; nevertheless, the result provides further insight into the spatial characteristics of the finestructure.
The horizontal field appears to be
composed as a hierarchy, with a large, cold feature, approximately 15 x 25 km, encompassing four much smaller and colder cores of order 3 to 5 km extent, and,
in this case, also including a warm core of approximately the same dimension. In Fig. 12, the small core including stas. 54, 94 is pictured as being distinct from the core measured at sta. 105.
Fig. 13 shows the T/S diagram of
these stations. Station 105 shows one very "active" cold layer, with a density inversion, centered on a = 25.88, while the core - 8 km farther north represented by stas. 54, 94 shows a two-layered cold structure, an indication
232 10'
166OW
50'
I
I
I
0
.-.---_
40'
0
/-\\ /'
. TEMPERATURE, O C ON c =25.88
10'
166OW
50'
40'
Temperature on t h e u = 25.88 s u r f a c e c o n s t r u c t e d from a l l s t a t i o n s t o J D 182/0200. Some CTD s t a t i o n s a r e numbered, t h e s e c t i o n s of Figs. 1 4 , 1 5 i n d i c a t e d , and T values given f o r f o u r cold and one warm core. In h o r i z o n t a l space t h e f i n e s t r u c t u r e shows a h i e r a r c h i c a l s t r u c t u r e , with a l a r g e ( o r d e r 20 km) c o l d a r e a encompassing f o u r s m a l l e r ( o r d e r 3-5 km) cold c o r e s and one warm core. Fig. 12.
(BOX I , I I , CHASE) and Cyclesonde d a t a dead-reckoned
of t h e cold on u = 25.88 b u t a more s t r o n g l y developed l a y e r beneath on
u
= 25.90.
Data a r e inadequate f o r determining more e x a c t l y t h e r e l a t i o n s h i p
between t h e s e water masses, b u t they a r e d e f i n i t e l y d i f f e r e n t cores. I n t h e v e r t i c a l dimension t h e f i n e s t r u c t u r e c o r e s average between 1 0 and 2 0 m t h i c k n e s s , t a k i n g t h e 3'C
isotherm a s a boundary.
from t h e measurements a t CYC-2
(Fig. 8 ) and i n t h e two s e c t i o n s constructed
This can be seen both
a c r o s s t h e f e a t u r e (Figs. 1 4 , 1 5 ; see F i g . 12 f o r l o c a t i o n s ) . p a r t of t h e c o r e s it i s a l i t t l e thicker--maximum s e p a r a t i o n was 19 m ( J D 181/1000, Fig. 8 ) .
measured 3'
In t h e c o l d e s t isotherm
The edge of t h e f e a t u r e tends t o
be b l u n t r a t h e r than t a p e r e d , and p a r t i c u l a r l y so along i t s west and southwest side--when
t h e c o l d c o r e appeared a t CYC-2
it was t y p i c a l l y 8-10 m t h i c k .
When t h e o r i e n t a t i o n t o t h e s h e l f topography i s considered ( s e e Fig. 1 2 ) , t h e r e
i s t h e impression t h a t t h e off-shore s i d e of t h e f i n e s t r u c t u r e t e n d s t o be hl-rnt-r thnn t h o onqhorp
side.
233
3.5 -
V W
3.0 -
3
k IL
W
a E W
+ 2.5 -
2 .o
I
I
I
I
Fig. 13. T/S diagram of s t a t i o n s o f two cold c o r e s . The more s o u t h e r l y one a t s t a . 105 ( c f . Fig. 12) had a s i n g l e c o l d l a y e r on u = 25.88, while t h e core -8 km f a r t h e r n o r t h was double l a y e r e d , on 25.88 and 25.90. STAT 10N S
30
4c
E 5c 8 I-
h 6C n 7c
TEMPERATURE, "C
8C
+5km--!
\
Fig. 1 4 . Temperature s e c t i o n a c r o s s t h e f i n e s t r u c t u r e f e a t u r e i n c l u d i n g CC (see Fig. 1 2 ) . The d e n s i t y s u r f a c e s i n t h e f i n e s t r u c t u r e a r e e l e v a t e d 1 0 m compared with t h o s e o u t s i d e t h e f e a t u r e .
234
Fig. 15. Temperature s e c t i o n a c r o s s t h e f i n e s t r u c t u r e ( s e e Fig. 12), b u t i n c l u d i n g t h e warm core.
The d e n s i t y s u r f a c e s i n t h e g e n e r a l f i n e s t r u c t u r e f e a t u r e with i t s c o r e s a r e e l e v a t e d about 10 m above t h e i r depth i n t h e water columns onshore and offshore of t h e f e a t u r e .
This i s shown i n Figs. 1 4 , 15 by t h e isopycnal u = 25.88;
in
t h e p l a n view of t h e depth of t h e isopycnal (Fig. 16) we see t h e depth range
DEPTH (rn) OF U = 2 5 . 8 8 10'
166O W
50'
4 0'
Fig. 16. Depth of t h e u = 25.88 isopycnal s u r f a c e ( s t a t i o n l o c a t i o n s deadreckoned a s i n Fig. 1 2 ) . Notice how t h e a r e a of 52-55 m depths c o i n c i d e s with t h e g e n e r a l f i n e s t r u c t u r e f e a t u r e . The even shallower band on t h e very South i s a t o t a l l y d i f f e r e n t water mass.
235 52-55 m e s s e n t i a l l y coincides w i t h t h e f e a t u r e , while on-shore and off-shore depths of t h i s s u r f a c e a r e 260 m. time, i . e . ,
i n t h e along-shelf
South and s o u t h e a s t of t h e f e a t u r e a t t h i s
direction t o the east, the
u
= 25.88 s u r f a c e l a y
even shallower, <50 m , with t o t a l l y d i f f e r e n t water c h a r a c t e r i s t i c s ( T
S
-
32.4; c f . Figs. 9 , 13).
"
4.5OC,
I t i s a l s o n o t a b l e t h a t t h e w a r m c o r e imbedded i n
t h e g e n e r a l f i n e s t r u c t u r e f e a t u r e a l s o has e l e v a t e d d e n s i t y s u r f a c e s (Fig. 1 5 ) .
3.2
F i e l d of motion The f i n e s t r u c t u r e , i n a d d i t i o n t o being swept around by t h e t i d a l c u r r e n t s ,
was being slowly advected n o r t h e r l y with t h e s u b t i d a l flow (Fig. 7 ) .
The
dynamic topography of 55/110 db (Fig. 17) i n d i c a t e s a "w s e t with a c a l c u l a t e d
-
50'OYN. HEIGHTS I
55/110 db
I
I
I .
I
I
Fig. 1 7 . Dynamic topography (dyn m ) of 55/110 db, constructed from a l l C and W i n d i c a t e l o c a t i o n s of t h e cold s t a t i o n s dead-reckoned t o J D 182/0200. and warm c o r e s ( c f . Fig. 1 2 ) . The dynamic h e i g h t d i f f e r e n c e a c r o s s t h e g e n e r a l f e a t u r e AD = 0.006 i n 20 km i s e q u i v a l e n t t o 2 . 4 c m / s .
speed of 2.4 c m s-',
i n agreement with t h e s u b t i d a l set measured a t CYC-2.
Thus, t h e o v e r a l l f i e l d seems t o have been e s s e n t i a l l y i n geostrophic balance, but t h e presence of t h e f i n e s t r u c t u r e c r e a t e s d i s t o r t i o n s i n t h e b a r o c l i n i c f i e l d which a r e n o t well resolved by t h e d a t a . We now examine more c l o s e l y t h e motion a s s o d a t e d with CC.
Figure 18 shows
vectors of t h e motion of t h e core l a y e r of CC (50-56 m) r e l a t i v e t o t h a t i n t h e
236
Fiq. 18. Vectors of motion of t h e c o r e l a y e r (50-56 m) r e l a t i v e t o t h a t i n the water column 10 m above f o r s e l e c t e d p o s i t i o n s along t h e r e c o n s t r u c t e d Cyclesonde t r a c k , superimposed on t h e minimum temperature f i e l d . This d e p i c t i o n of r e l a t i v e motion suggests a clockwise r o t a t i o n of CC a t 2-3 cm/s with a 1-11/2km r a d i u s of curvature.
water column 1 0 m shallower f o r s e l e c t e d p o s i t i o n s along t h e r e c o n s t r u c t e d Cyclesonde t r a c k .
The v e c t o r s i n d i c a t e a d e f i n i t e sense of clockwise r o t a t i o n
of t h e core r e l a t i v e t o t h e water column, with a speed of '2
cm s
-1
.
Geo-
s t r o p h i c flow (over 100 db) was c a l c u l a t e d f o r t h e s e c t i o n c r o s s i n g t h e core (Fig. 19; c f . Figs. 1 2 , 1 4 ) .
Geostrophy i n d i c a t e s a banded s t r u c t u r e t o t h e
flow f i e l d , with a l i t t l e s t r o n g e r NW flow a l t e r n a t i n g with weak SE flow a t the s c a l e of t h e s t a t i o n spacing ( - 3 km). (50 t o 60 m ) ,
But r i g h t a t t h e l e v e l of t h e cold core
i n d i c a t e d by t h e 2.5OC isotherm from t h e temperature s e c t i o n
(Fig. 1 4 ) , t h e flow between s t a s . 57 and 90 i s - 2 t o 3 cm s
-1
l e s s toward t h e
NW than above o r below, and between s t a s . 83 and 57 it i s t h e same magnitude
less t o t h e SE.
This i s a clockwise motion of t h e core r e l a t i v e t o t h e water
column above and below of - 2 t o 3 cm s-l, i n agreement with t h e d i r e c t measurements.
The Rossby number (R
motion i s q u i t e s m a l l ,
0
-0.1.
= U/fr,
where r = r a d i u s ) f o r t h i s r e l a t i v e
I t t h u s appears t h a t CC was r o t a t i n g clockwise i n
quasi-geostrophic balance r e l a t i v e t o t h e l a r g e r - s c a l e flow f i e l d .
237 STATIONS
Fig. 19. Geostrophic flow (over 100 db) throuqh t h e s e c t i o n c r o s s i n g t h e f i n e s t r u c t u r e including CC ( s e e Figs. 1 2 , 1 4 ) . Speeds i n cm/s, p l u s toward t h e northwest. The d o t t e d l i n e is t h e 2.5OC isotherm from t h e temperature s e c t i o n (Fig. 1 4 ) .
Shear i n t h e water column i n r e l a t i o n t o t h e cold core f e a t u r e s was examined
i n more d e t a i l .
From t h e Cyclesonde d a t a , temperature was used t o d e f i n e cores
as t h e l a y e r encompassinq water with T < 0.3OC above t h e minimum observed value. The shear was determined a s S = (U2
+ V2)1/2 where t h e
U's and V ' s a r e t h e
differences i n t h e U and V v e l o c i t y components over a 6-m i n t e r v a l .
For 54
p r o f i l e s which r e g i s t e r e d a minimum T < 3"C, S w a s determined immediately above and below t h e core and a c r o s s t h e core i t s e l f .
The r e s u l t s a r e p l o t t e d a s
functions of core temperature i n Fig. 2 0 . F i r s t , we note t h a t i n g e n e r a l t h e s h e a r s a r e n o t s t r o n q .
the upper and lower i n t e r f a c e s of t h e c o r e s
(-
The values a c r o s s
1 ~ 1 0 -s-l) ~ correspond t o those
associated with thermohaline s t e p s under t h e Mediterranean outflow (Simpson e t a l . , 1979), b u t a r e about one-half o r d e r s m a l l e r than r e p o r t e d from t h e base of t h e mixed l a y e r i n t h e A t l a n t i c (Evans, 1982). The shear a c r o s s t h e core f o r a l l c a s e s w a s s i g n i f i c a n t l y less
-2 -1 than t h e s h e a r i n t h e i n t e r f a c e s above ( S = 0 . 5 0 ~ 1 0 s ) -2 -1 and below (S = 0 . 6 2 ~ 1 0 s ) : t h e l a t t e r two values d i f f e r s i g n i f i c a n t l y a t (S = 0 . 2 8 ~ 1 0 - s-') ~
the 98% l e v e l .
When only t h e c o l d e s t c o r e s a r e considered, t h e c o n t r a s t s a r e
f o r a l l c o r e s with T < 2.5"C (n = 2 7 ) , core s h e a r = -2 -1 -2 -1 0.23~10-~ s-l, S(above) = 0 . 5 4 ~ 1 0 s and S(be1ow) = 0 . 6 9 ~ 1 0 s
even g r e a t e r :
.
In s p i t e of t h e l a r g e s c a t t e r i n t h e d a t a of Fig. 2 0 , t h e r e appear t o be trends i n t h e d i s t r i b u t i o n of water column s h e a r with r e s p e c t t o t h e f i n e structure.
The t r e n d s a r e suggested by least squares f i t s t o t h e values from
above, below, and w i t h i n cores.
I n very c o l d - c o r e s s h e a r i s l e s s than above
and below ( a s noted above), b u t f o r warmer cores t h e v a l u e s appear t o converge.
238 1.5
-
A ABOVE
X
X
0
A
X
CORE CORE BELOW CORE
A N I
9
-
X
1.0
X
X
X X
x x
X
x x
A
0
A
v)
LL
a
W
I 0.5
In
- 2.4, D
0
i 3
I
2.1
2.2
10
2.3
O
Ir
A 0
8 0
0
0
I
2.5 2.6 2.7 CORE TEMPERATURE, "C
2.8
-
2.9
I 3.0
Fig. 20. Shears above, a c r o s s , and below f i n e s t r u c t u r s c o l d c o r e s with T < 3OC, from 54 Cyclesonde p r o f i l e s . Shear given a s S = [ ( A U ) + (AV)'I1/', calculated over 6-m i n t e r v a l s . The t r e n d l i n e s were f i t t e d by l e a s t squares. When f i n e s t r u c t u r e c o r e s a r e newly developed, s h e a r s a c r o s s them a r e much less than above o r below; when not w e l l developed ( o r have been eroded) water column s h e a r s a r e more evenly d i s t r i b u t e d with r e s p e c t t o t h e f i n e s t r u c t u r e f e a t u r e .
The value of minimum temperature must t o some degree r e f l e c t t h e age of t h e features--those
with t h e c o l d e s t temperatures a r e much more l i k e l y t o be more
r e c e n t l y formed than t h o s e of h i g h e r minimum temperature. cold (T < 3'C)
The only source of
is t h e c e n t r a l s h e l f bottom water, and subsequently they can
only become warmer.
These "younger" f e a t u r e s s e e m t o be moving r e l a t i v e t o t h e
water column as a s o l i d body, with l i t t l e s h e a r a c r o s s ( o r w i t h i n ) them, while t h e r e l a t i v e motions a r e much g r e a t e r and concentrated i n t h e i n t e r f a c e s above and below t h e f e a t u r e , p a r t i c u l a r l y below ( t h e double-diffusive
interface).
On
"aging", t h e s h e a r s above and below d e f i n i t e l y tend t o decrease while t h a t a c r o s s t h e c o r e i n c r e a s e s (as shown by t h e t r e n d l i n e s i n F i g . 2 0 , f i t t e d by l e a s t squares).
Thus, as d i f f u s i o n a c t s t o reduce t h e temperature (and s a l t )
c o n t r a s t of t h e f i n e s t r u c t u r e f e a t u r e s , they a l s o tend t o l o s e t h e i r i d e n t i t y a s d i s t i n c t dynamical f e a t u r e s i n t h e water column.
4.
Formation of t h e f i n e s t r u c t u r e W e a r e now i n a p o s i t i o n t o d i s c u s s formation of t h e f i n e s t r u c t u r e .
Maxworthy (1984) r e c e n t l y p r e s e n t e d a conceptual model f o r mixing a t t i d a l f r o n t s which considered t h e ambient s t r a t i f i c a t i o n ( F i g . 2 1 , l e f t ) .
When t h e
mixing of s t r a t i f i e d f l u i d columns i s l o c a l i z e d i n space, mixed products w i l l escape from t h e mixing region i n t o t h e s t r a t i f i e d region a s i n t r u s i o n s along t h e appropriate density surfaces.
These may be a s a s i n g l e g r a v i t y c u r r e n t
(Fig. 21b) o r m u l t i p l e f i n g e r s (Fig. 2 1 c ) .
When t h e s t r a t i f i c a t i o n i s weak
and/or t h e mixing i s s t r o n q , t h e zone w i l l extend t o t h e s u r f a c e c r e a t i n g a
239
PROBES ?
b
B
STATION5 9
l!
10
WIND-MIXED LAYER
I
I20
MAXWORTHY'S
CONCEPTUAL MODEL
5 L O S k WATER
TIDALLY-MIXED LAYER
APPLICATION TO
MID-SHELF
FRONT
Fiq. 21. Maxworthy's (1984) conceptual model of f i n e s t r u c t u r e formation ( l e f t ) and i t s a p p l i c a t i o n t o f i n e s t r u c t u r e formation i n t h e middle f r o n t of t h e Bering Sea.
surface " f r o n t " (Fig. 21b); otherwise t h e mixing zone w i l l remain subsurface, c r e a t i n g a " f r o n t " extending upward from t h e bottom over p a r t of t h e water column (Fig. 21a).
The l i m i t of v e r t i c a l e x t e n t w i l l be t h e l a r g e s t eddies
produced by t h e t i d a l c u r r e n t k i n e t i c energy t h a t can overturn t h e s t r a t i f i c a t i o n (Fig. 21d). Maxworthy's conceptual model appears t o f i t p r e c i s e l y t h e o b s e r v a t i o n s of f i n e s t r u c t u r e formation i n t h e middle f r o n t o f t h e Bering Sea s h e l f , and i n Fig. 2 1 , r i g h t , t h e model i s s c a l e d t o t h e Bering Sea s h e l f s i t u a t i o n .
The
cold bottom w a t e r of t h e c e n t r a l s h e l f domain i n s h o r e of PROBES s t a t i o n 11 i s being extruded by t h e p r e s s u r e f i e l d i n t o t h e graded d e n s i t y s t r u c t u r e of t h e
240 o u t e r s h e l f domain. s u r f a c e wind-mixed
I t s d e n s i t y i s a l i t t l e g r e a t e r t h a n t h e waters o f t h e
l a y e r but s l i g h t l y l e s s t h a n t h o s e of t h e warmer, more
s a l i n e slope waters working t h e i r way onshore a s a bottom l a y e r a c r o s s t h e o u t e r s h e l f domain ( c f . Coachman and Charnell, 1979; Coachman and Walsh, 1981); theref o r e t h e e x t r u s i o n i s i n t h e g e n e r a l depth range -40 t o 70 m. The t i d a l c u r r e n t s encounter t h e somewhat more s t e e p l y s l o p i n g bottom near 100 m water depths; between PROBES s t a t i o n s 8 and 9 t h e bottom s l o p e i s 3 times g r e a t e r than elsewhere i n t h e c e n t r a l and o u t e r domains, and t h e water columns s h o r t e n b y - 2 0 % g o i n g onshore.
Turbulent k i n e t i c energy from t h e t i d a l c u r r e n t s
inshore from s t a t i o n 9 extends much f a r t h e r up t h e water column c r e a t i n g p a r t i a l mixtures o f t h e "cold pool" water with t h e w a r m e r , more s a l i n e s l o p e water. The mixtures extrude seaward i n t o t h e o u t e r s h e l f ambient s t r a t i f i c a t i o n as t h e finestructure features. The reasonableness of t h i s formation p r o c e s s f o r t h e Bering Sea case w a s i n v e s t i q a t e d by comparing t h e i n c r e a s e of p o t e n t i a l energy i n water columns c o n t a i n i n g f i n e s t r u c t u r e with t h e k i n e t i c energy a v a i l a b l e from t h e t i d a l The p o t e n t i a l energy r e l a t i v e t o t h e mixed s t a t e i s (Simpson and
currents.
Hunter, 1974) : 0
V =
Vh
=
1
-
(p
p)
gzdz
-h 0
1 pdz, where
For a v e r t i c a l p i s d e n s i t y and h t h e column depth. -h l y mixed system V = 0 , and becomes i n c r e a s i n g l y negative f o r more s t a b l e
with
= l/h
stratification.
A s t h e f i n e s t r u c t u r e was a l l observed deeper than 40 m , t h i s
was taken a s t h e r e f e r e n c e depth and a number of s t a t i o n s e x h i b i t i n g well developed c o l d c o r e s were i n t e g r a t e d t o bottom (-110 m) ; f o r t h e s e
7=
-15 J m-3.
Nearby s t a t i o n s with no evidence of f i n e s t r u c t u r e were s i m i l a r l y averaqed, and -3 V = -30 J m I t t h u s appears t h a t formation of t h e f i n e s t r u c t u r e c a l l s f o r
-
.
an i n c r e a s e i n p o t e n t i a l energy p e r u n i t volume of -15 J m-3. The r a t e o f decay of energy p e r u n i t mass from t i d a l c u r r e n t s i s (Pingree and G r i f f i t h s , 1978) :
where C i s t h e drag c o e f f i c i e n t and U t h e t i d a l c u r r e n t speed; p e r u n i t Volume D t h i s is:
241 When m u l t i p l i e d by t i m e , t h i s e x p r e s s i o n g i v e s an estimate o f t h e amount o f k i n e t i c energy p e r u n i t volume which h a s gone i n t o m i x i n g t h e water column of l e n g t h h. C
D
Taking a water column o f h = 60 m (40 m t o 100 m d e p t h s ) and
= 3.5 x
( V i n c e n t and Harvey, 1 9 7 6 ) , T a b l e 2 w a s p r e p a r e d showing t h e
amount of KE expended on mixinq f o r v a r i o u s t i m e p e r i o d s and s p e e d s of t i d a l
TABLE 2
Energy p e r u n i t volume* from t i d a l c u r r e n t s f o r v a r i o u s s p e e d s and d u r a t i o n s
A t . hours
u,
an s
-1
4
5
20
6.7
8.4
10.1
25
13.1
16.4
19.7
30
22
28
34
CD = 3 . 5 ~ 1 0 - ~h; 3
*KE =
PCD+
currents.
I>
6
= 60 m
A t , J/m
3
The r e s u l t s s u g g e s t t h a t t i d a l c u r r e n t s 225 c m s-l s u s t a i n e d f o r
p e r i o d s >4 h o u r s c a n p r o v i d e an amount of e n e r g y f o r mixing which e q u a l s o r exceeds t h e p o t e n t i a l e n e r g y i n v o l v e d i n t h e o b s e r v e d f i n e s t r u c t u r e f o r m a t i o n . The Cyclesonde c u r r e n t measurements and moored c u r r e n t meter r e c o r d s from t h e -1 same l o c a t i o n (Coachman, 1982) show t h a t i n c i d e n c e s of U > 25 c m s for p e r i o d s >4 h r are common, o c c u r r i n g on t h e a v e r a g e about e v e r y second t i d a l cycle. Maxworthy (op. c i t . ) went on t o p o i n t o u t t h a t on i n i t i a l f o r m a t i o n r o t a t i o n i s n o t important because s t r a t i f i c a t i o n dominates, b u t following c o l l a p s e of t h e mixed f l u i d i n t o a f i n e s t r u c t u r e i n t r u s i o n t h e i n t e r n a l Rossby number R
= Nb/ZwL
,
where N = V l i s a l l f r e q u e n c y , b t h e t h i c k n e s s
and
L t h e l e n g t h of t h e i n t r u -
s i o n , w i l l r a p i d l y approach 1 and r o t a t i o n w i l l become e q u a l l y i m p o r t a n t .
Our
o b s e r v a t i o n s s u g g e s t t h a t t h i s i s e x a c t l y what o c c u r s ; i n f a c t , t h e h o r i z o n t a l s c a l i n g of t h e " c o l d core" f i n e s t r u c t u r e seems t o b e s e t by C o r i o l i s a c t i n q
on t h e motion o f t h e f i n e s t r u c t u r e i n t r u s i o n s i n t h e s p e c i f i c d e n s i t y g r a d i e n t s of t h e water columns o f t h e o u t e r domain.
F o r s t a . 57 which d e f i n e s t h e core -2 -1 of t h e CC ( F i g s . 11, 1 2 , 1 4 ) , N = 1.12 x 1 0 s , b = 1 9 m , and t h e i n t e r n a l Rossby r a d i u s i s -1.8 km.
T h i s i s n o t o n l y t h e r a d i u s of t h i s c o l d core, b u t
approximately t h a t o f t h e o t h e r t h r e e c o l d cores as w e l l ( c f . F i g . 1 2 ) .
242 To summarize f i n e s t r u c t u r e formation i n t h e mid-shelf
f r o n t of t h e Bering
o u t e r s h e l f water columns a r e s t r a t i f i e d w i t h d e n s i t y i n c r e a s i n g downward
Sea:
t o a warm, more s a l i n e s l o p e water being “pumped“ landward a c r o s s t h e s h e l f , while c e n t r a l s h e l f columns c o n t a i n a t h i c k lower l a y e r o f c o l d e r , l e s s s a l i n e water extruding seaward. isobath.
These water masses are juxtaposed a t about t h e 100 m
A t t h e s e depths t h e bottom s l o p e i s a l i t t l e s t e e p e r than elsewhere,
and on t h e shallower inshore s i d e t i d a l c u r r e n t energy becomes s u f f i c i e n t much of t h e time t o c r e a t e mixtures of t h e two water masses t o shallower depths (-40 m ) ;
t h u s -100 m c o n s t i t u t e s t h e boundary o f t h e d i s c r e t e mixing zone of
Maxworthy‘s model.
The p a r t i a l w a t e r mixtures c o n s t i t u t i n g f i n e s t r u c t u r e
c o l l a p s e and extrude seaward i n i t i a l l y a s g r a v i t y c u r r e n t s , b u t q u i c k l y begin r o t a t i n g clockwise under t h e i n f l u e n c e of C o r i o l i s ’ a c c e l e r a t i o n .
The balance
of C o r i o l i s f o r c e w i t h t h e p r e s s u r e g r a d i e n t f o r c e o f t h e “ f o r e i g n “ water mass i n t h e ambient s t r a t i f i c a t i o n s e t s t h e scale s i z e of t h e s e more o r less c i r c u l a r “ b l i n i ” (Russian pancakes w i t h an a s p e c t r a t i o o f -1:200) which continue t o r o t a t e i n quasi-geostrophic balance slowly with r e s p e c t t o t h e water column f o r some time a f t e r t h e i r formation while being advected about by t h e ambient flow f i e l d .
5.
Diffusion and l i f e t i m e Following t h e i r formation and escape from t h e mixing zone i n t o t h e s t r a t i -
f i e d water columns of t h e o u t e r s h e l f domain, decay of t h e f e a t u r e s w i l l be p r i m a r i l y through v e r t i c a l d i f f u s i o n f i l l i n g i n t h e temperature and s a l i n i t y minima. The f i n e s t r u c t u r e a r e c o l d e r and l e s s s a l i n e t h a n t h e ambient w a t e r , so t h e i n t e r f a c e above i s t h e “ s a l t f i n g e r ” i n t e r f a c e and t h a t below t h e “doublediffusive” interface.
S u b s t a n t i a l evidence has now accumulated t h a t double-
d i f f u s i v e convection ( a more r a p i d f l u x o f h e a t than s a l t ) p l a y s a s i g n i f i c a n t r o l e i n ocean mixing; i n t h e t r a n s i t i o n downward from warm, s a l i n e t o c o l d e r , l e s s s a l i n e water t h e d i f f e r e n t i a l d i f f u s i o n can l e a d t o t h e development of salt-finger activity.
Schmitt (1979, 1981) showed t h a t l i t t l e s a l t - f i n g e r
a c t i v i t y occurs u n l e s s R
P R drops below 1.6 t o 1.7. P
< 2 , and t h a t vigorous a c t i v i t y i s p r e s e n t when
The p o s s i b i l i t y of s a l t - f i n g e r a c t i v i t y a s s o c i a t e d with t h e f i n e s t r u c t u r e was i n v e s t i g a t e d by c a l c u l a t i n g R
P
over 3 t o 5-m i n t e r v a l s of s t e e p e s t T and S
g r a d i e n t s above t h e core f o r a l l CTD s t a t i o n s e x h i b i t i n g a well-defined T minimum (n = 2 9 ) .
The v a l u e s a r e p l o t t e d i n Fig. 22 a s f u n c t i o n s of c o r e
temperature ( A ) and s t a b i l i t y N
2
One-half of t h e s t a t i o n s had R 0.93.
(B).
P
< 1 . 7 , with a minimum observed value of
In s p i t e o f t h e wide range of observed r a t i o s , t h e r e i s a d e f i n i t e t r e n d
243
2.5 -
2.0
-
0 .
1.5W
8
1.0-
>
P
a--;-
-----_-_----.. . . 0 -. '
-___------
h
. I
I
a 0.5
1.7
I
I
3.0v)
W
2.5-
2.0
-
1.5
-
1.0
-
0.51
I
I
I
1.o
0.5
STABILITY,
r2
1.5
I
2.0
Fig. 22. Upper i n t e r f a c e d e n s i t y r a t i o s ( R p ) f o r a l l CTD s t a t i o n s showinq pronounced T and S minima i n d i c a t i v e of f i n e s t r u c t u r e , p l o t t e d a s f u n c t i o n s of core temperature (A) and s t a b i l i t y N2 (B). The broken l i n e s i n ( A ) a r e averages over increments of core temperature. Values i n c o l d core CC ( s t a s . 5 7 , 6 2 , 6 4 , 68, 83) a r e c i r c l e d .
of i n c r e a s i n g R ' s with i n c r e a s i n g core temperature (Fig. 2 2 A ) ; t h e c o l d e r P ("younger") cores show a tendency f o r having v a l u e s of R
i n t e r f a c e compared with warmer c o r e s , where
5P
-f
2.
P
< 2 i n t h e i r upper
But d e f i n i t e l y not a l l
cores e x h i b i t evidence of s a l t f i n g e r i n g , only some of them.
The d e n s i t y
r a t i o s a r e w e l l c o r r e l a t e d w i t h s t a b i l i t y of t h e upper i n t e r f a c e (Fig. 2 2 B ) , with t h e p o s s i b i l i t y of s a l t - f i n g e r i n g a c t i v i t y only when t h e s t a b i l i t y i s very -1 low (N < 1 ~ 1 0 -s ~ 1 . I t t h u s appears t h a t it i s t h e degree o f l o c a l i n s i t u s t r a t i f i c a t i o n i n t o which a f i n e s t r u c t u r e i n t r u d e s t h a t determines whether o r not t h e r e w i l l be a c t i v e "double d i f f u s i o n " a s s o c i a t e d w i t h it.
I f t h e newly-
formed mixed p a r c e l e n t e r s a s t r o n g d e n s i t y g r a d i e n t , t h e p o s s i b i l i t y i s l e s s , Also, e r o s i o n of a c o r e minimum over t i m e tends t o i n c r e a s e
and v i c e versa.
the s t a b i l i t y of t h e upper ( s a l t f i n g e r ) i n t e r f a c e , which adds t o t h e good c o r r e l a t i o n of R
P
with s t a b i l i t y .
244
In Fig. 22, the stations of CC are identified separately, and their R ' s P range between 1.5 and 1.9, values on the margin for salt-finger activity; their T/S curves (Fig. 9) also do not show any marked instabilities.
It thus appears
that this cold core finestructure was not the site of active salt-fingering. The only station of the ensemble that did show a marked density inversion was sta. 105 (Fig. 13).
Vertical profiles of T, S, and in situ density through the
minima are shown in Fig. 23.
Over the depth interval 4 8 to 50 m R P
=
1.00, and
IN SlTU DENSITY,CT 25.65 I
32.0 I
25.75 I
I
SALINITY, 32.4 I
I
25.85
I
25.95
gm kg-' 32.2 I
I
J
TEMPERATURE, "C 2.0
2.5
3.0
3.5
4.0
STA. 105
Fig. 23. Vertical profiles of T, S , and in situ density ( u ) for sta. 105, the only station of the ensemble (n=29) showing marked density inversions. The R = 1.00 and the "kinks" in the S (and u) curves are suggestive of active s%lt-finger activity in the upper interface of the finestructure.
from the profiles one can visualize excess salt moving downward near the bottom of the thermocline, distorting (temporarily) the S and density curves.
It
appears that some of the finestructure formed in the middle front has doublediffusive activity (salt-fingering in the upper interface) but by no means all of the features do. The question of possible Kelvin-Helmholtz instability of the interfaces, which could lead to a growth of internal waves and enhanced vertical mixing, is 2 2 assessed through calculation of the gradient Richardson number Ri = N / S
.
When Ri < 0.25 the possibility of dynamic instability exists.
The temperature
profiles for all CTD stations taken at CYC-2 which sampled a cold 3inestructure between 50 m and 60 m were matched with their corresponding Cyclesonde T
245 p r o f i l e s ; t h e s i x s t a t i o n s t h a t showed c l o s e correspondence a r e p l o t t e d i n Fig. 24 ( t h e o t h e r s t a t i o n s do n o t correspond because of t h e l a r g e h o r i z o n t a l f i e l d g r a d i e n t s and our r e l u c t a n c e t o t a k e t h e CTD s t a t i o n s p r e c i s e l y a t
All t h e s t a t i o n s a r e w i t h i n CC o r on i t s edge ( s t a . 72). 2 p l o t t e d are N , Sz , and R i c a l c u l a t e d over 2-m i n t e r v a l s . CYC-2).
RI
5 . qm k d '
32.0
32.2
0.25
32.4
4 50
0
'
\\
70
3
10
3
10
K
',..
\.,
M)
,..." '
1
1 1 1 1 LOG to R, -I 0 1
w
T(CYC)"i
',..-
T
'..S
80
-
CIC PROFILE UOYEO DOWN 2m
STA. 68 Ri
S. qm kc' 32.0 32.2 32.4
0.25
70
80
crc
PROFILE Y O V E O DOWN 2m
-
1
1 1 1 1
STA. 72
5. qm k d ' 32.0 32.2 32.4
0
CIC PROFILE YOVFO DOWN Zm
STA. 83
2
4
Also
246
5. qm kq-'
32.0
32.2
324
50
00 CIC P R O F ~ L E M O V E D DOWN zm
-
-
STA. 57
R,
5. qm kq-'
32.0
32.2
32.4
5 . qm h<'
32.0
32.2
32.4
0.25
1
3
10
-R,
0.25
1
3
10
LOG 10 RI
0
2
4
-I
0
I
CIC PROFlLF M O Y E O DOWN Zrn
STA. 64
Fig. 24A,B. V e r t i c a l p r o f i l e s of T, S, N', ,'S and R i f o r t h e six s t a t i o n s with f i n e s t r u c t u r e taken a t CYC-2 f o r which t h e T p r o f i l e s matched, i n d i c a t i n g t h a t t h e Cyclesonde and CTD were sampling t h e same water mass. R i ' s <0.25 a r e rare, and do n o t seem t o be a s s o c i a t e d w i t h t h e main f i n e s t r u c t u r e b u t w i t h smaller-scale s t r u c t u r e above and below.
Across t h e well-developed c o l d c o r e t h e s t a b i l i t i e s a r e not l a r g e , b u t t h e s h e a r s a r e even less, l e a d i n g t o l a r g e v a l u e s of R i .
Only above and below t h e
f e a t u r e a r e t h e r e o c c a s i o n a l l a r g e v a l u e s of s h e a r , and t h e s e d o n ' t seem t o be a s s o c i a t e d with t h e upper and lower i n t e r f a c e s of t h e c o r e b u t r a t h e r with s t e p s t r u c t u r e ( s t a s . 62, 64) o r much s m a l l e r s c a l e secondary f i n e s t r u c t u r e
247 ( s t a . 68).
Even so, v a l u e s of R i 50.25 a r e r a r e .
When t h e c o l d c o r e i s less
well developed, e i t h e r a t i t s edge ( s t a s . 72, 8 3 ) , o r p o s s i b l y "older" (sta. 831, s h e a r s a r e more uniformly d i s p e r s e d through t h e water column (cf. Fig. 20); again R i ' s 50.25 a r e seldom seen.
W e conclude t h a t dynamic
i n s t a b i l i t i e s s i g n i f i e d by s m a l l g r a d i e n t Richardson numbers a r e n o t an important aspect of s t r o n g l y developed c o l d c o r e f i n e s t r u c t u r e . W e can make an estimate of t h e v e r t i c a l d i f f u s i o n a s s o c i a t e d with CC,
probably t h e major mechanism by which t h e T and S minima are reduced over time.
With a c o n s t a n t eddy c o e f f i c i e n t 'K
( $ = T , S ) , simple v e r t i c a l
diffusion i s approximately
where s u b s c r i p t s U and L r e f e r t o t h e upper and lower i n t e r f a c e s o f t h e f i n e structure minima.
Five CTD sts. ( 5 7 , 62, 64, 68, 83) over t h e course of two
days r e g i s t e r e d s t r o n g minima i n CC, and t h e i r minimum v a l u e s of T and S on a = 25.88 a r e p l o t t e d i n Fig. 25.
S t a t i o n 64 d i f f e r e d from t h e o t h e r s i n t h a t
CTD STATIONS IN COLD CORE 62
57
f
t
68
64
t
83
t
$.
0 TEMPERATURE X SALINITY
To 32.12
2.6
X
k
Y
> 32.K k
W 2.4
z
d 32.08
rr
3
l-
a rr
w 2.2 5 Q
W
x
-0 ,
-
2
I-
2.o
I
12
JD 1 8 0
I
1
00
JD181
I
I
42
'
I
00
I
1
92
JD182
Fig. 25. Minimum v a l u e s o f T,S on u = 25.88 a t 5 s t a t i o n s i n f i n e s t r u c t u r e CC, as functions of time. S t a . 64 showed c o l d e r and less s a l i n e v a l u e s on a s l i g h t l y less dense s u r f a c e . S t a . 68 (and probably a l s o 83) were n o t i n t h e "core" of t h e f e a t u r e ( c f . Fig. 11). The t r e n d l i n e s suggest maximum r a t e s of heating and s a l t i n g .
colder and less s a l i n e v a l u e s were r e g i s t e r e d on a s l i g h t l y less dense s u r f a c e , a r e f l e c t i o n of s u b t l e t i e s i n t h e mixing and i n t e r l e a v i n g process g i v i n g rise t o considerable v a r i a b i l i t y i n t h e small-scale f e a t u r e s of f i n e s t r u c t u r e
248
(cf. also Fig. 13).
Station 68 clearly was not in the "core" of CC.
Likewise,
it is also probable that sta. 83 was not actually in the "core", given its location at the extreme southern tip (Fig. 11).
Therefore by including
sta. 83 values we get an estimate of maximum rates of warming and salting, as -6 OC s -1 and 0.19~10-~ gm kg-' s -1, respectively. shown in Fig. 25, of 1.9~10 Mean curvatures of T,S at the upper and lower boundaries of the minima were calculated over 4, 6, and 8 m intervals for the four stations. Then, using the diffusion equation, values of KT = Ks = 0.20 m2 s-l were obtained. If the slightly elevated values of T,S at sta. 62 vis-a-vis sta. 57 represent more accurately the time change over 18 hours, then the rates are less. Using 2 these rates and curvatures for the two stations gives KT = 0.12 cm s-l and 2 Ks = 0.08 cm s ' . The fact that the eddy coefficients for heat and salt flux are essentially the same reinforces the previous conclusion that CC was not the site of vigorous double-diffusive activity. We can use the eddy coefficients to estimate the possible lifetime of CC. To blend in with the ambient T profile, the core temperature of CC must increase by >1.S0C.
But as the core "fills", the curvatures in the upper and
lower interfaces will decrease.
Using a combined upper plus lower curvature
value one-half that of the initial cold core (0.4~10-~ 'C cm-2) gives lifetimes 2 -1 between 22 and 43 days for K T t s of 0.2 and 0.1 cm s Thus, it appears that
.
the lifetimes of the coldest finestructure formed in the middle front that do not have vigorous double-diffusive activity associated with them is the order of a month. It has been postulated (Coachman and Charnell, 1979) that the mid-depth layer (between the surface wind-mixed and bottom tidally-mixed layers) across the outer shelf contains very little mixing energy, which permits the persistence of the cold finestructure.
In confirmation, comparison was made of the
mean water column shear in the interval 40 m located 25 km farther seaward (Fig. 2).
-
82 m between CYC-2 and CYC-1
Shears were calculated over 3-m
intervals of the hourly profiles for a similar flow day at each mooring, with the following results: -2 -1 0.57 2 0.37 x 10 s -1 = 0.73 2 0.46 X S
CYC-1, JD 175 (Fig. 6):
S =
CYC-2, JD 180 (Fig. 7):
S
The mid-layer shear is indeed less in the outer shelf away from the mid-shelf front.
Thus the data support the hypothesis that the lifetimes of many of the
cold finestructure features is sufficiently long, in a low-energy environment, for them to transit the outer shelf and cause the small temperature minima between 50 and 100 m frequently observed at stations taken near the shelfbreak.
CONCLUDING REMARKS
This paper r e p o r t s unique d a t a from f i n e s t r u c t u r e f e a t u r e s formed i n a s h e l f sea f r o n t .
Equipped with a good CTD and p r o f i l i n g c u r r e n t meter, and a i d e d by
luck, we obtained new information f o r d e s c r i b i n g t h e v e r t i c a l and h o r i z o n t a l nature of t h e s e p a r t i a l w a t e r m a s s mixtures and i n t e r p r e t i n g t h e i r formation and subsequent behavior. The f i n e s t r u c t u r e formed i n mid-latitude
s h e l f s e a f r o n t a l zones where t h e
generally c o l d e r , l e s s s a l i n e s h e l f water i s mixing " l a t e r a l l y " with warmer, more s a l i n e b a s i n water appears t o be g e n e r i c a l l y s i m i l a r , whether it i s formed i n t h e s h e l f b r e a k f r o n t of " t y p i c a l " s h e l v e s (New York Bight) o r t h e mid-shelf
f r o n t of t h e extra-wide s h e l v e s (Bering Sea).
Off-shore d i r e c t e d
pressure g r a d i e n t s push t h e c o l d e r , l e s s s a l i n e s h e l f water seaward i n t o t h e graded d e n s i t y s t r u c t u r e of t h e b a s i n water, and t h e f i n e s t r u c t u r e , p a r t i a l l y mixed products o f t h e g e n e r a l "lateral" i n t e r a c t i o n p r o c e s s , a r e found on t h e seaward s i d e of t h e f r o n t a l mixing zone. The c r e a t i o n o f t h e p a r t i a l l y - m i x e d f i n e s t r u c t u r e r e q u i r e s a s p a t i a l d i s c o n t i n u i t y i n t h e mixing p r o c e s s .
The major energy f o r mixing water masses on
these s h e l v e s comes from t i d a l c u r r e n t s , which s t i r t h e water columns upwards from t h e bottom over some t e n s of meters.
P a r t i a l mixtures of s h e l f and b a s i n
water form where both water masses a r e within t h e t i d a l mixing zone, with t y p i c a l l y t h e c o l d e r , l e s s s a l i n e s h e l f water over t h e warm, more s a l i n e b a s i n water; t h u s it i s r e a l l y a v e r t i c a l mixing on a l o c a l i z e d s c a l e .
The s p a t i a l
d i s c o n t i n u i t y i s caused by changes i n depth going a c r o s s t h e s h e l f ; on " t y p i c a l " s h e l v e s where t i d a l c u r r e n t a c t i o n becomes s t r o n g enough and can "reach" f a r enough up t h e water column t o involve both water masses i s c l o s e t o the shelfbreak.
On t h e Bering Sea s h e l f , t h e o u t e r s h e l f i s t o o deep ( - 1 2 0 m)
f o r t h i s t o happen, and it occurs where t h e bottom s h o a l s t o < l o 0 m. argument i s r e v e r s i b l e :
The
t h e l o c a t i o n of t h e f r o n t a l zone i n d i c a t i v e of t h e
boundary between b a s i c s h e l f water ( f i l l i n g t h e s h e l f s e a beneath a s u r f a c e l a y e r d i r e c t l y a f f e c t e d by m a s s and energy exchange) and b a s i n water i s determined by t h i s process. The f i n e s t r u c t u r e s a r e found only on t h e seaward s i d e of t h e f r o n t s , f o r two reasons.
(1) The p r e s s u r e g r a d i e n t s a r e i n g e n e r a l d i r e c t e d o f f s h o r e , s o
t h a t t h e f i n e s t r u c t u r e s a f t e r c r e a t i o n ( p a r t i a l mixing) a r e on t h e average pressed seaward, and ( 2 ) t o s u r v i v e f o r any l e n g t h of time t h e f e a t u r e s must take up p o s i t i o n s i n water columns w i t h much l e s s mixing energy, and t h i s i s only p o s s i b l e on t h e seaward (deeper) s i d e of t h e f r o n t s . Maxworthy's (1984) model appears t o d e s c r i b e p r e c i s e l y - t h e formation process, and from h i s model and o u r r e s u l t s w e can hypothesize f u r t h e r . During some phases of some t i d a l c y c l e s , s h e l f and b a s i n w a t e r s are approp r i a t e l y layered and s t i r r i n g from t h e t i d a l c u r r e n t a c t i v i t y c r e a t e s some
250 volume of p a r t i a l mixture; from t h e T/S curves ( F i g . 4) t h e l a r g e r f e a t u r e s ( e . g . , CC) a r e perhaps 50 t o 75% s h e l f water, with a t o t a l volume o f o r d e r 6 3 10 m ( F i g s . 11, 1 4 ) . The l a r g e r volumes would be c r e a t e d when t i d a l excursions a r e f a i r l y long, perhaps > 3 k m , and probably i n t h e onshelf d i r e c t i o n ( f l o o d ) when water column motion i s a g a i n s t t h e g e n e r a l o f f s h o r e pressure gradient.
This i s a common occurrence ( c f . Figs. 6 , 7 ) .
Then
during ebb t h e mixtures c o l l a p s e and a r e pushed from t h e seaward s i d e of t h e mixing zone, some a c t u a l l y escaping t o produce t h e f i n e s t r u c t u r e s . The f i n e s t r u c t u r e s a r e observed on a h i e r a r c h y of s c a l e s , both v e r t i c a l and horizontal.
I n t h e v e r t i c a l , Coachman and Charnel1 (1979) counted s e v e r a l
hundred f i n e s t r u c t u r e s a t 104 s t a t i o n s spread a c r o s s t h e o u t e r Bering Sea s h e l f d u r i n g summer, 1976.
The f e a t u r e s ranged from 1 m up t o 25 m v e r t i c a l dimen-
s i o n , w i t h a d e f i n i t e i n c r e a s e i n numbers with decrease i n t h i c k n e s s .
In the
h o r i z o n t a l , t h i s study has documented a l a r g e r cold ( t h e s i g n a t u r e of t h e p a r t i a l l y - m i x e d water) f i e l d of o r d e r 2 0 km a c r o s s w i t h i n which were f o u r d i s c r e t e c o l d e r c o r e s o f o r d e r 3 km.
We can now hypothesize t h e r e i s a cut-off
s c a l e s i z e , a c e r t a i n volume of partially-mixed
water, such t h a t when it
c o l l a p s e s a s a d e n s i t y c u r r e n t along i t s isopycnal i n t o t h e graded d e n s i t y s t r u c t u r e t o seaward, i t i s s u f f i c i e n t l y l a r g e t o maintain i t s e l f a s a s e p a r a t e e n t i t y a g a i n s t t h e subsequent ambient mixing.
These become d i s t i n c t dynamic
f e a t u r e s , more o r less round because t h e y r o t a t e r e l a t i v e t o t h e w a t e r column e s s e n t i a l l y as s o l i d bodies and t h e i r s c a l e s i z e i s s e t by t h e a p p r o p r i a t e i n t e r n a l Rossby r a d i u s .
These form t h e cold c o r e s .
Once formed a s s e p a r a t e
e n t i t i e s , they erode only very slowly because s h e a r s i n t h e water column away from t h e f r o n t a r e small, and w i t h i n t h e f i n e s t r u c t u r e a r e even s m a l l e r .
We
n o t e t h i s w i l l not be t h e case f o r f i n e s t r u c t u r e s formed i n t h e more " t y p i c a l " s h e l f s e a f r o n t s (Fig. 1, I ) where t h e mid-layers o f t h e o f f s h o r e water columns are more e n e r g e t i c .
Off t h e NYB, f o r example, t h e presence of h o r i -
z o n t a l s h e a r d i s t o r t s t h e f e a t u r e s i n t h e along-shelf
d i r e c t i o n and t h e i r
l i f e t i m e s aremuch s h o r t e r than i n t h e o u t e r domain of t h e Bering Sea. The p a r t i a l mixtures c r e a t i n g f i n e s t r u c t u r e s m a l l e r t h a n t h e cut-off volume a r e t r a n s i e n t and erode r e l a t i v e l y r a p i d l y .
I n a r e a s and a t times when l o t s of
t h e s e mixtures a r e formed, t h e i r decay c o n t r i b u t e s t o c r e a t i o n of a l a r g e r , g e n e r a l l y c o l d e r mid-layer, study.
a s observed surrounding t h e c o l d c o r e s d u r i n g t h i s
The e r o s i o n of t h e c o l d c o r e s themselves from t h e i r t o p and bottom
s u r f a c e s and around t h e i r p e r i m e t e r s a l s o c o n t r i b u t e s t o t h e c r e a t i o n of t h e l a r g e r c o l d area. The p o s s i b l e l i f e t i m e s of t h e l a r g e r f i n e s t r u c t u r e , which t a k e on a d i s c r e t e dynamic i d e n t i t y , appears t o be q u i t e long ( o r d e r of a month) because t h e i r decay i s b a s i c a l l y l i m i t e d t o simple d i f f u s i o n i n t h e v e r t i c a l which i s q u i t e slow.
There w i l l a l s o be some slow e r o s i o n of t h e p e r i m e t e r by t h e formation
251 of smaller f i n e s t r u c t u r e s through mixing of t h e core and ambient waters. Double d i f f u s i v e phenomena, which enhances v e r t i c a l d i f f u s i o n and markedly reduces t h e l i f e t i m e s of t h e "bliny", does appear t o be q u i t e p r e v a l e n t i n t h e regions of f i n e s t r u c t u r e .
I n t h i s study about one-half of t h e c a l c u l a t e d
d e n s i t y r a t i o s suggested s a l t - f i n g e r i n g ,
and s u b s t a n t i a l r e g i o n s of low R ' s P
were found i n t h e NYB f i n e s t r u c t u r e formation region.
But t h e occurrence of
double-diffusive phenomena depends on t h e s t r e n g t h of t h e l o c a l d e n s i t y gradient; t h e lower t h e g r a d i e n t , t h e more l i k e l y t h e occurrence, and v i c e versa.
So a s t h e f i n e s t r u c t u r e erodes, t h e l o c a l d e n s i t y g r a d i e n t s i n c r e a s e
and t h e double d i f f u s i v e a c t i v i t y i s damped o u t .
Thus many o f t h e s m a l l e r
f i n e s t r u c t u r e s may be r a p i d l y mixed away through t h i s e f f e c t , b u t t h e l a r g e r p a r c e l s , bigger than t h e cut-off volume, a r e only reduced somewhat i n t h i c k ness i f double d i f f u s i o n i s p r e s e n t i n i t i a l l y .
The presence of i n s t a b i l i t i e s ,
which could a l s o cause more r a p i d e r o s i o n of t h e f e a t u r e s i s d e f i n i t e l y not indicated.
Even though t h e d e n s i t y g r a d i e n t s a s s o c i a t e d with t h e f e a t u r e s a r e
l o w , s h e a r s a r e even less, and Richardson numbers approaching 0.25 a r e r a r e . The f i n e s t r u c t u r e " b l i n i " w i l l a l s o be eroded t o some e x t e n t around t h e i r The n a t u r e of t h i s e r o s i o n i s probably through t h e formation of
perimeters.
much s m a l l e r f i n e s t r u c t u r e (compare t h e h i e r a r c h i c a l concept of Joyce, 19771, where t h e water masses involved a r e t h e core water and t h e ambient water of t h e adjacent s t r a t i f i c a t i o n , and t h e energy f o r mixing i s t h e l o c a l s h e a r . This e f f e c t would continue throughout t h e i r l i f e t i m e s , i . e . , the o u t e r s h e l f domain.
while they t r a n s i t
I t seems probable t h a t t r a n s i e n t s a l t - f i n g e r a c t i v i t y
would be a s s o c i a t e d with t h i s p r o c e s s , which could be r e s p o n s i b l e f o r t h e observed bulk i n c r e a s e i n d e n s i t y of t h e T and S minima a c r o s s t h e o u t e r s h e l f . The f i n e s t r u c t u r e s formed i n t h e mid-shelf
f r o n t of t h e Bering Sea i n f l u e n c e
t h e water mass p r o p e r t i e s and s t r u c t u r e a c r o s s t h e whole o u t e r s h e l f domain t o t h e s h e l f b r e a k more than 100 km seaward ( c f . Fig. 2 ) .
Water columns i n t h i s
domain e x h i b i t a temperature minimum a t mid-depths, between -50 and 100 m ( c f . Fig. 4 ) .
A t t h e c l o s e of t h e study w e occupied a 165 km CTD s e c t i o n along
the 110 m i s o b a t h (see Fig. 2 f o r l o c a t i o n ) , and t h e temperatures a r e p l o t t e d i n Fig. 26. observed.
Westward from s t a t i o n 1 2 1 a number of l a r g e r f i n e s t r u c t u r e s were The conclusion i s t h a t t h e minimum temperature l a y e r r e s u l t s
because of a s u f f i c i e n t production of l a r g e r f i n e s t r u c t u r e s whose l i f e t i m e s a r e long enough t o s u r v i v e t r a n s i t of t h e o u t e r domain b e f o r e being completely dissipated.
These f e a t u r e s a l s o t r a n s p o r t seaward any o t h e r p r o p e r t i e s
involved i n t h e mixing, f o r example, h i g h e r l e v e l s o f n u t r i e n t s than a r e normal f o r t h e mid-depth l a y e r .
Fig. 27 shows t h e NO3 d i s t r ' i b u t i o n a c r o s s CC.
The
source f o r high v a l u e s of NOj i s t h e bottom b a s i n water, and c l e a r l y t h i s n i t r a t e has been mixed i n t o t h e f i n e s t r u c t u r e e l e v a t i n g i t s v a l u e s vis-a-vis This the ambient w a t e r , and w i l l be t r a n s p o r t e d by t h e f i n e s t r u c t u r e .
252
I TEMP MINIMA IN
WATER COLUMN:^
2
2 4
4
2
4
5
4
5
5
8
TEMP. SECTION ALONG 110 m . 3 - 4$11 Fig. 26. Temperature s e c t i o n along t h e 11U m i s o b a t h , t h e seaward s i d e of t h e middle f r o n t (see F i g . 2 f o r l o c a t i o n ) . T ' s c o l d e r than 2.5"C are hachured. Numerous l a r g e f i n e s t r u c t u r e were observed west of s t a . 122. STATIONS
Fig. 27. N i t r a t e d i s t r i b u t i o n across f i n e s t r u c t u r e CC ( c f . Fig. 1 4 ) . N i t r a t e from t h e deeper b a s i n water has been mixed up i n t o t h e f i n e s t r u c t u r e d u r i n g i t s formation.
phenomenon accounts f o r t h e enhanced h o r i z o n t a l and v e r t i c a l f l u x e s of n i t r a t e i n mid-depth l a y e r s observed i n t h e o u t e r domain of t h e Bering Sea (Coachman and Walsh, 1981).
253 ACKNOWLEDGEMENTS Support f o r PROBES w a s provided by t h e National Science Foundation, Division of P o l a r Programs, which a l s o supported t h e f i n e s t r u c t u r e study, and the f i n a l stage of manuscript p r e p a r a t i o n w a s i n p a r t supported by t h e s a m e organization under t h e ISHTAR program.
I am indebted t o my c o l l e a g u e s i n
these programs, a f l o a t and ashore, f o r continuing congenial and f r u i t f u l scientific collaboration.
I a m a l s o indebted t o S. R i s e r f o r a c r i t i c a l
evaluation of t h e manuscript.
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Cont. S h e l f . Res. 1:
Coachman, L.K. 1985 Circulation, water masses, and f l u x e s on t h e s o u t h e a s t e r n Bering Sea s h e l f . Cont. Shelf R e s . ( i n p r e s s ) . Coachman, L.K. and R.L. Charnel1 1979 On l a t e r a l water mass i n t e r a c t i o n - a case study, B r i s t o l Bay, Alaska. J. Phys. Oceanogr. 9: 278-297. Coachman, L.K. and J.J. Walsh 1981 A d i f f u s i o n model of c r o s s - s h e l f exchange of n u t r i e n t s i n t h e s o u t h e a s t e r n Bering Sea. Deep-sea Res. 28A: 819-846. Cresswell, G.H. 1967 Quasi-synoptic monthly hydrography o f t h e t r a n s i t i o n region between c o a s t a l and slope water south of Cape Cod, Mass. WHO1 Rep. 67-35. Evans, D.L. 1981 Velocity shear i n a thermohaline staircase.
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1982 Evans, D.L. Observations of small-scale s h e a r and d e n s i t y s t r u c t u r e i n t h e ocean. Deep-sea Res. 29: 581-595. Gargett, A.E. 1976 An i n v e s t i g a t i o n of t h e occurrence o f oceanic t u r b u l e n c e with r e s p e c t t o f i n e s t r u c t u r e . J. Phys. Oceanogr. 6: 139-156. Goering, J.J. and C.P. McRoy 1981 A synopsis of PROBES. EOS 62(44) : 730-731. Gordon, A.L. and F. Aikman I11 1981 S a l i n i t y maximum i n t h e pycnocline of t h e Middle A t l a n t i c Bight. Oceanogr. 26: 123-130. Gordon, A.L., D.T. Georgi and H.W. Taylor 1977 Antarctic p o l a r f r o n t zone i n t h e Western S c o t i a Sea J. Phys. Oceanogr. 7: 309-328.
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Gregg, M.C. 1975 Microstructure and intrusions in the California Current. 5: 253-278.
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Horne, E.P.W. 1978a Interleaving at the subsurface front in the slope water off Nova Scotia. J. Geophys. Res. 83: 3659-3671. Horne, E.P.W. 197813 Physical aspects of the Nova Scotian shelfbreak fronts. Chap. 7, pp. 59-68 in Oceanic Fronts in Coastal Processes, M.J. Bowman and W.E. Esaias, eds. Berlin/Heidelberg/New York: Springer-Verlag. Houghton, R.W. and J. Marra 1983 Physical/biological structure and exchange across the thermohaline shelf/ slope front in the New York Bight. J. Geophys. Res. 88: 4467-4481. Houghton, R.W., R. Schlitz, R.C. Beardsley, B. Butman and J.L. Chamberlain 1982 The Middle Atlantic Bight cold pool: evolution of the temperature structure during summer 1979. J. Phys. Oceanogr. 12: 1019-1029. Joyce, T.M. 1977 A note on the lateral mixing of water masses.
J. Phys. Oceanogr. 7: 626-629.
Kinder, T.H. and J.D. Schumacher 1981 Hydrographic structure over the continental shelf of the southeastern Bering Sea. In The Eastern Bering Sea: Oceanography and Rescurces, vol. 1 , D.W. Hood and J.A. Calder, eds. Seattle: Univ. Washington Press, pp. 31-52. Lindstrom, E.J. and B.A. Taft 1985 Small water transporting eddies: an analysis of statistical outliers in the POLYMODE Local Dynamics Experiment hydrographic data. J. Phys. Oceanogr. (in press). Maxworthy, T. 1984 On the formation of tidal mixing fronts. (unpublishedmanuscript).
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Mooers, C.N.K, C.N. Flagg and W.C. Boicourt 1978 Prograde and retrograde fronts. Chap. 6, pp. 43-58 in Oceanic Fronts in Coastal Processes, M.J. Bowman and W.E. Esaias, eds. Berlin/Heidelberg/ New York: Springer-Verlag. Munk, W. 1981 Internal waves and small-scale processes. Chap. 9, pp. 264-291 in Evolution of Physical Oceanography, B.A. Warren and C. Wunsch, eds. Cambridge and London: The MIT Press. Pingree, R.D. and D.K. Griffiths 1978 Tidal fronts on the shelf seas around the British Isles. J. Geophys. Res. 83: 4615-4622. Posmentier, E.S. and R.W. Houghton 1978 Fine structure instabilities induced by double diffusion in the shelf/slope water front. J. Geophys. Res. 83: 5135-5138. Posmentier, E.S. and R.W. Houghton 1981 Springtime evolution of the New England shelf break front. 86: 4253-4259.
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255 Riser, S.C., W.B. Owens, H . T . Rossby and C.C. Ebbesmeyer 1985 The s t r u c t u r e , dynamics, and o r i g i n o f a small-scale l e n s o f water i n t h e Western N o r t h A t l a n t i c t h e r m o c l i n e . J . Phys. Oceanogr. ( i n p r e s s ) . 1974 Roden, G . I . Thennohaline s t r u c t u r e , f r o n t s , and a i r - s e a e n e r g y exchange o f t h e t r a d e J . Phys. Oceanogr. 4: 168-182. wind r e g i o n e a s t o f H a w a i i . Ruddick, B.R. and J . S . T u r n e r 1979 The v e r t i c a l l e n g t h s c a l e o f d o u b l e - d i f f u s i v e i n t r u s i o n s . 903-913. 1979 S c h m i t t , R.W. The growth r a t e of s u p e r - c r i t i c a l
salt fingers.
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S c h m i t t , R.W. 1981 Form o f t h e t e m p e r a t u r e - s a l i n i t y r e l a t i o n s h i p i n C e n t r a l Water: e v i d e n c e f o r J. Phys. Oceanogr. 11: 1015-1026. d o u b l e - d i f f u s i v e mixing. Schumacher, J . D . and T.H. K i n d e r 1983 Low-frequency c u r r e n t r e g i m e s o v e r t h e B e r i n g Sea s h e l f . 13: 607-623.
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Simpson, J . H . and J . R . Hunter 1974 F r o n t s i n t h e I r i s h Sea. N a t u r e 250: 404-406. Norris and J . S t r a t f o r d 1979 Simpson, J . H . , M.R. H o w e , N.C.G. V e l o c i t y s h e a r i n t h e s t e p s below t h e M e d i t e r r a n e a n o u t f l o w . 26: 1381-1386. S t e r n , M.E. 1967 Lateral mixing o f w a t e r masses.
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Toole, J . M . and D.T. G e o r g i 1 9 8 1 On t h e dynamic- and e f f e c t s of d o u b l e - d i f f u s i v e l y d r i v e n i n t r u s i o n s . Prog. Oceanogr. 10: 123-145.
Turner, J.S.
1978 Double-diffusive 2887-2901.
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Van Leer, J . C . , W. Diiing, R. E r a t h , E . Kennelly and A . S p e i d e l 1974 The Cyclesonde: an u n a t t e n d e d v e r t i c a l p r o f i l e r f o r s c a l a r and v e c t o r q u a n t i t i e s i n t h e u p p e r ocean. Deep-sea R e s . 21: 385-400. Vincent, C.E. and J . G . Harvey 1976 Roughness l e n g t h i n t h e t u r b u l e n t Ekman l a y e r above t h e sea b e d . 2 2 : M75-M81.
Mar. Geol.
D.C. Webb and R.C. M i l l a r d 1976 Voorhis, A . D . , C u r r e n t s t r u c t u r e and m i x i n g i n t h e s h e l f - s l o p e water f r o n t s o u t h o f New England. J . Geophys. R e s . 81: 3695-3708.
Welch, C.S. 1981 Mid-level i n t r u s i o n s a t t h e c o n t i n e n t a l s h e l f edge. 86: 11,013-11,019.
J . Geophys. R e s .
W i l l i a m s , A . J . I11 1981 The r o l e o f d o u b l e d i f f u s i o n i n a Gulf Stream f r o n t a l i n t r u s i o n . J. Geophys. R e s . 86: 1917-1928.
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251
SOME ASPECTS OF THE LIGURO-PROVENCAL FRONTAL ECOHYDRODYNAMICS
J-H. HECQ*,
J-M. BOUQUEGNEAU~, s. DJENIDI,
A. GOFFART""
M. FRANKIGNOULLE,
and M. LICOT
Department o f Oceanography U n i v e r s i t y o f L i e g e
x Chercheur Q u a l i f i e FNRS X*
Chercheur I R S I A
INTRODUCTION I n t h e o l i g o t r o p h i c sea w a t e r , t h e b i o l o g i c a l p r o d u c t i o n i s m a i n l y i n f l u e n c e d by zones o f d i s c o n t i n u i t y such as f r o n t s and t h e r m o c l i n e s . I n t h e Mediterranean Sea, t h e f r o n t a t t h e edge o f t h e c o n t i n e n t a l s h e l f s e p a r a t e s general and c o a s t a l c i r c u l a t i o n s , l i m i t i n g l a t e r a l d i f f u s i o n . The v e r t i c a l motions a s s o c i a t e d w i t h t h i s f r o n t b r i n g n u t r i e n t s t o t h e s u r f a c e , i n c r e a s i n g t h e b i o l o g i c a l p r o d u c t i v i t y (PRIEUR, 1981). The produced b i o l o g i c a l m a t e r i a l i s exported off-shore p e r p e n d i c u l a r l y t o t h e f r o n t . Since a few y e a r s , i n t e r d i s c i p l i n a r y s t u d i e s have been l e d i n t h e C o r s i c a n area o f t h e L i g u r o - P r o v e n c a l f r o n t b y t h e U n i v e r s i t y o f L i e g e a t STARES0 Station (Calvi
-
Corsica).
These s t u d i e s a r e i n t e g r a t e d i n an i n t e r n a t i o n a l program w i t h t h e c o l l a b o ration o f
L. PRIEUR ( S t a t i o n z o o l o g i q u e o f V i l l e f r a n c h e - s u r - M e r ,
France).
General c i r c u l a t i o n The dynamics o f t h e M e d i t e r r a n e a n Sea i s m a i n l y dominated by t h e g e n e r a l and t h e l o c a l a t m o s p h e r i c f o r c i n g s . Due t o t h e c l i m a t o l o g y o f t h e M e d i t e r r a n e a n area, t h e w a t e r b a l a n c e i s n e g a t i v e , t h e e v a p o r a t i o n b e i n g more i m p o r t a n t t h a n t h e p r e c i p i t a t i o n s and t h e r i v e r s u p p l y . I n t h i s c o n c e n t r a t i o n b a s i n , t h e d e f i c i t i s compensated f o r b y an i m p o r t a n t w a t e r f l u x t h r o u g h t h e G i b r a l t a r S t r a i t (LACOMBE, i973; BETHOUX, 1980). The A t l a n t i c s u r f a c e l a y e r f l o w s eastwards a l o n g t h e North' A f r i c a n c o a s t , forming t h e so c a l l e d A f r i c a n c u r r e n t . I n t h e Western p a r t o f t h e M e d i t e r r a n e a n l a r g e c y c l o n i c c i r c u l a t i o n s a r e developed i n t h e T h y r r h e n i a n Sea and i n t h e Liguro-Provencal b a s i n ( f i g u r e 1).
258
F i g . 1 : Summer s u p e r f i c i a l c i r c u l a t i o n ( a f t e r SCHMIDT and NIELSEN) The Summer h e a t i n g o f t h e s u r f a c e l a y e r l e a d s t o t h e f o r m a t i o n o f a seasonnal t h e r m o c l i n e . D u r i n g t h e w i n t e r , under t h e e f f e c t o f d r y and c o l d c o n t i n e n t a l winds ( M i s t r a l , Tramontane, . . . ) and o f t h e d i f f e r e n c e o f temper a t u r e between a i r and sea, t h e e v a p o r a t i o n and h e a t t r a n s f e r f r o m t h e sea t o t h e a i r become v e r y i m p o r t a n t . Hence t h e d e n s i t y o f t h e s u r f a c e l a y e r i n c r e a s e s , i n d u c i n g a b a r o c l i n i c i n s t a b i l i t y . The consequent v e r t i c a l m i x i n g and c o n v e c t i o n g i v e r i s e t o deep w a t e r s . T h i s phenomenon o f deep w a t e r f o r m a t i o n i s observed i n t h e North-West M e d i t e r r a n e a n (GASCARD, 1978), i n t h e L e v a n t i n b a s i n and i n t h e A d r i a t i c Sea. The w a t e r formed d u r i n g t h e w i n t e r i n t h e L e v a n t i n b a s i n c r o s s e s t h e S i c i l i a n S t r a i t and spreads i n t h e Western b a s i n , d e s c r i b i n g c y c l o n i c c i r c u l a t i o n s of a s o - c a l l e d i n t e r m e d i a t e w a t e r . So, t h e M e d i t e r r a n e a n Sea seems t o be a t h r e e l a y e r system : a s u r f a c e l a y e r o f A t l a n t i c w a t e r (- 0 d i a t e l e v a n t i n e l a y e r (- 100
-
100 m) c o l d and l i t t l e s a l i n e , an i n t e r m e 500 m) warmer and more s a l t e d and a t y p i c a l l y
m e d i t e r r a n e a n deep l a y e r ( u n d e r 600 m) c o l d e r and l e s s s a l t e d . Above t h e summer t h e r m o c l i n e , when i t e x i s t s , t h e upper l a y e r o f t h e sea has h i g h e r t e m p e r a t u r e and s a l i n i t y t h a n t h e A t l a n t i c w a t e r .
-
There i s n o d o u b t t h a t t h i s g e n e r a l scheme o f c i r c u l a t i o n i s h i g h l y p e r t u r b e d and l o c a l l y m o d i f i e d , a t l e a s t i n t h e s u r f a c e l a y e r , b y t h e t r a n s i e n t wind events. From t h i s g e n e r a l s u r v e y o b t a i n e d by means o f c l a s s i c a l f i e - l d measurements, one emphasizes t h e i n t e n s e c y c l o n i c g y r e i n t h e L i g u r o - P r o v e n c a l b a s i n c a r r y i n g
259
along s u r f a c e and s u b s u r f a c e w a t e r s . Since a few y e a r s , t h e new p o s s i b i l i t i e s o f f e r e d b y t h e development o f remote s e n s i n g t e c h n i q u e s has a l l o w e d a b e t t e r m o n i t o r i n g o f o c e a n i c phenomena l i k e t h e f r o n t a l s t r u c t u r e s i n t h e Western M e d i t e r r a n e a n Sea. The e x a m i n a t i o n o f a one y e a r s e r i e s o f i n f r a r e d thermographies p r o v i d e d by t h e Centre de M e t e o r o l o g i e S p a t i a l e (Lannion, France) shows f o r i n s t a n c e t h e seasonnal v a r i a b i l i t y o f t h e f r o n t s e p a r a t i n g t h e d i f f e r e n t w a t e r masses i n the Liguro-Provencal basin ( f i g u r e 2). S p r i n g images do n o t p r o v i d e much i n f o r m a t i o n because o f t h e weakness o f t h e s u r f a c e t e m p e r a t u r e g r a d i e n t s due t o t h e s t r o n g m i x i n g o f t h e s u r f a c e l a y e r by t h e c o l d winds d u r i n g t h e w i n t e r .
18-24/02/83
M
d
, *
*
.
I
E
F i g . 2 : NOAA s a t e l l i t e i n f r a r e d thermographies 5howing s u r f a c e t e m p e r a t u r e gradient() ( f r o m C e n t r e de M e t e o r o l o g i e s p a t i a l e , Lannion, France)
260
The h e a t i n g o f t h e s u r f a c e water d u r i n g t h e l a t e s p r i n g p r o g r e s s i v e l y e s t a b l i s h e s a v e r t i c a l s t r a t i f i c a t i o n . The consequent seasonnal thermocline i s a f f e c t e d by t h e water motions which d i s t u r b i t , g i v i n g r i s e t o h o r i z o n t a l temperature g r a d i e n t s v i s i b l e form s a t e l l i t e s i n t h e form o f more o r l e s s marked f r o n t s . The most i m p o r t a n t f r o n t f o r us, i n r e l a t i o n w i t h t h e c y c l o n i c c i r c u l a t i o n i n t h e Liguro-Provenqal basin, separates dense water o f t h e c o l d core from the warm water running around i t . The w i d t h o f t h e warm water s t r i p i s v a r i a b l e , t h e f r o n t being sometimes f i r m l y s i t u a t e d i n t h e c o a s t a l zone, l i k e i t i s t h e case o f f N o r t h Corsica from C a l v i t o Cape Corse. I n s p i t e o f i t s v a r i a b i l i t y i n space and i n time, t h e thermal f r o n t associated w i t h t h i s c y c l o n i c l o o p i s p e r s i s t e n t throughout t h e summer and f a l l . However, d u r i n g t h e autumn, i t begins t o weaken and t o present i n s t a b i l i t i e s . The w i n t e r m e t e o r o l o g i c a l f o r c i n g removes t h e seasonnal thermocline and consequently the whole t y p i c a l summer f r o n t s . Then t h e w i n t e r and s p r i n g thermographies do n o t show t h e Liguro-Provencal gyre, although i t i s present i n w i n t e r as shown i n f i g u r e 3. During t h e s e seasons t h e d e n s i t y f i e l d appears as m a i n l y c o n t r o l l e d by t h e s a l i n i t y .
F i g . 3 : Dynamical topoqraphy i n t h e North-West Mediterranean duping t h e w i n t e r ( f r o m BOSCALS DE REAL).
261
Hydrological data The study of hydrological d a t a (temperature, s a l i n i t y ) collected in March, July and October 1984, d u r i n g several oceanographic cruises across the Ligure-Provencal front (Corsican a r e a ) , provides a more detailed picture of water masses distribution and seasonal fluctuations. These campaigns have been carried out on board of the "Recteur Dubuisson", the oceanographic ship of the University of Liege a t Calvi (Corsica). Ten stations have been selected (figure 4) on the Calvi-Nice axis, from Calvi ( s t a t i o n n.1) t o 30 nautical miles offshore ( s t a t i o n n"10).
I
Fig. 4 : Position of sampling s t a t i o n s Temperature and s a l i n i t y measurements have been carried o u t a t every ten stations, from the surface t o 200 meters. The isotherms, isohalines and isopycnals distributions are presented. I n March, a strong gradient of s a l i n i t y separates coastal waters (S < 38.2 o / o o ) from offshore waters (figure 5 A ) (S > 38.4 The d i s t r i bution o f isotherms shows t h a t the upper layers are not r e a l l y homothermal. Figure 58 shows t h a t colder waters ( T < 12.9.C) are merely situated beneath the haline gradient while warmer waters ( T > 12.9"C) are situated above i t , near the coast O/..).
262
. . . .
I
100-
FI
T "c MARCH
... ...
IS
~
-
1
.. .. .. ..
,
o MARCH
1200
*
.. .. .. .. ... ...
1
...1
W A (mg/m3)
120 5
1
*
*
*
'
MARCH
-
Fig. 5 : MARS 1984. D i s t r i b u t i o n o f i s o h a l i n e s (A), isotherms ( B ) , isopycnals ( C ) and chlorophyll A concentration ( D ) across Corsican f r o n t region from t h e c o a s t ( s t 1 ) t o 30 miles offshore ( s t 10) and from s u r f a c e t o 200 meters deep ( H E C Q e t a l , 1905).
263 D e n s i t y d i s t r i b u t i o n ( f i g u r e 5C) shows t h a t t h e t h e r m o h a l i n e f r o n t s e p a r a t e d two a r e a s :
-
Offshore s t a t i o n s w i t h waters o f h i g h density, c h a r a c t e r i s t i c o f
L e v a n t i n e o r i g i n ; t h i s r e g i o n i s u n s t r a t i f i e d and seems t o be a d i v e r g e n c e area,
-
Onshore s t a t i o n s w i t h l e s s dense w a t e r s , c h a r a c t e r i s t i c o f A t l a n t i c
o r i g i n . T h i s r e g i o n i s l i t t l e s t r a t i f i e d w i t h a tendance o f convergence c l o s e t o t h e g r a d i e n t . I n t h a t p e r i o d , t h e f r o n t crosses t h e sea s u r f a c e a b o u t t 1 5 m i l e s o f f t h e c o a s t and t h e i s o p y c n a l s s l a n t w i t h a 1.6 % s l o p e f r o m t h e f r o n t a r e a t o t h e c o a s t (HECQ e t a l , 1985). I n June, a v e r t i c a l s t r a t i f i c a t i o n i s i n i t i a t e d ( f i g u r e 68) w i t h t h e h e a t i n g o f upper l a y e r s . T h i s v e r t i c a l g r a d i e n t o f t e m p e r a t u r e masks t h e h o r i z o n t a l g r a d i e n t a t l e a s t i n t h e 75 upper meters. From t h e p o i n t o f view o f t h e s a l i n i t y ( f i g u r e 6A), t h e h a l i n e g r a d i e n t s e p a r a t i n g o f f s h o r e and c o a s t a l w a t e r s i n more i m p o r t a n t below 50 m t h a n above. The d e n s i t y diagram ( f i g u r e 6C) summarizes t h e w a t e r masses d i s t r i b u t i o n . I n t h e upper l a y e r ( f r o m t h e s u r f a c e t o t h e 50 m d e p t h ) , t h e d i s t r i b u t i o n o f i s o p y c n a l s i s a p p r o x i m a t e l y h o r i z o n t a l . Below 75 m, b o t h a c o a s t a l s t r a t i f i e d a r e a and an o f f s h o r e u n s t r a t i f i e d zone w i t h h i g h d e n s i t y v a l u e s a r e s t i l l observed. T h i s suggests t h a t t h e d i v e r g e n c e does n o t r e a c h t h e s u r f a c e b u t o n l y a f f e c t s t h e w a t e r s be1 ow t h e t h e r m o c l ine.
I n October ( f i g u r e 7), t h e d i s t r i b u t i o n o f i s o h a l i n e s i s q u i t e s i m i l a r t o t h e s i t u a t i o n i n June and two r e g i o n s a r e s e p a r a t e d by a f r o n t a l d i s c o n t i n u i t y . The i n c r e a s e o f i s o t h e r m a l and i s o p y c n a l s l o p e suggests t h e o n s e t o f a destabilization. The " i n s i t u " measurements c o n f i r m f a i r l y w e l l t h e e x i s t e n c e o f d i f f e r e n t w a t e r masses as d e s c r i b e d above. The w a t e r s i t u a t e d between t h e f r o n t and t h e C o r s i c a n c o a s t has t h e c h a r a c t e r i s t i c s o f an A t l a n t i c w a t e r . The w a t e r s i t u a t e d beyond t h e f r o n t has t h e c h a r a c t e r i s t i c s o f an i n t e r m e d i a t e L e v a n t i n e w a t e r . I n w i n t e r and a t t h e b e g i n i n g o f s p r i n g , s a l i n i t y i n f l u e n c e s t h e most t h e s l o p e of i s o p y c n a l s ( f r o m o f f s h o r e areas t o t h e c o a s t ) . I n t h a t p e r i o d , divergences r e a c h s u r f a c e l a y e r s . I n c o n t r a r y , d u r i n g summer, s u r f a c e t e m p e r a t u r e i n f l u e n c e s h o r i z o n t a l s t r a t i f i c a t i o n o f t h e upper l a y e r s and f r o n t a l d i v e r g e n c e s seem t o r e a c h o n l y w a t e r l a y e r s below 100 m e t e r s . I n a d d i t i o n t o t h e measurements o f s a l i n i t y and temperature, t o t a l a l k a l i n i t y a n a l y s e s have been c a r r i e d o u t on t h e same samples. T o t a l a l k a l i n i t y i s independent o f b i o l o g i c a l processes as f i r as r e a c t i o n s i n v o l v i n g carbonates exchanges a r e n e g l i g i b l e (FRANKIGNOULLE and BOUQUEGNEAU, 1985)
264
1%JmE
B 2
0
0
-
r
F”......i n
.....
Fig. 6 :
J U N E 1984. D i s t r i b u t i o n o f i s o h a l i n e s ( A ) , isotherms ( B ) , isopycnals (C) and c h l o r o p y l l A c o n c e n t r a t i o n s (D) across Corsican f r o n t r e g i o n from t h e coast ( s t 1) t o 30 m i l e s o f f s h o r e ( s t 10) and from surface t o 200 meters deep (HECQ e t a l , 1985).
265
J.,
,
.
1% OCTOBER
,
,
,
,
,
/j .. ... ...
2
Fig. 7 : OCTOBER 1984. D i s t r i b u t i o n of i s o h a l i n e s ( A ) , isotherms ( B ) , isopycnals ( C ) and chlorophyll A concentrations (D) across Corsican f r o n t region from t h e c o a s t ( s t 1) t o 30 miles offshore f s t 10) and from s u r f a c e t o 200 meters deep (HECQ e t a1 , 1985).
266
and we b e l i e v e t h a t t h i s parameter can be used t o d i s c u s s t h e o r i g i n and movements o f w a t e r masses : deep waters are more a l k a l i n e than s u r f a c e ones. F i g u r e 8 shows t h e r e s u l t o b t a i n e d i n March, June and October 1984. I n March, t h e thermohaline f r o n t separates l e s s a l k a l i n e onshore waters
(< 2.60 m Eq. L - l ) from a l k a l i n e r o f f s h o r e ones (> 2.65 m Eq. L - l ) . A d i v e r gence o f o f f s h o r e waters and a convergence o f onshore ones a r e c l e a r l y suggested a l o n g t h e isopycnal d i s c o n t i n u i t y . I n June, once again: t h e h a l i n e f r o n t separates more and l e s s a l k a l i n e waters. A divergence phenomenon appears i n o f f s h o r e waters b u t t h e maxima o f a l k a l i n i t y a r e found j u s t beneath t h e t h e r m o c l i n e : t h e r e i s a divergence a t s t a t i o n 7 up t o 50 meters where t h e p y c n o c l i n e has been detected; a t s t a t i o n 10, a t 40 meters depth, t h e h i g h e s t observed a l k a l i n i t y value i s observed. I n October, t h e waters a r e r e l a t i v e l y more homogeneous and no i m p o r t a n t divergence o r convergence a r e suggested. The f r o n t a l d i s c o n t i n u i t y i s t i l l apparent and a g r a d i e n t o f a l k a l i n i t y remains b o t h from c o a s t a l t o o f f s h o r e waters and from t h e s u r f a c e t o t h e bottom. These r e s u l t s f i t w e l l w i t h t h e o t h e r h y d r o l o g i c a l d a t a d e s c r i b e d above. Phytoplankton d a t a I n v e s t i g a t i o n s performed i n s p r i n g have shown a s p a t i a l h e t e r o g e n e i t y i n c h l o r o p h y l l d i s t r i b u t i o n i n t h e L i g u r i a n Sea (JACQUES e t a l , 1973). Moreover, i n t h e Bay o f C a l v i , v e r t i c a l phytoplankton d i s t r i b u t i o n e x h i b i t s a seasonal e v o l u t i o n r e l a t e d t o t h e thermal s t r u c t u r e o f t h e water column (HECQ e t a l , 1981, 1985;
LEGENDRE, 1981).
Oceanographic d a t a c o l l e c t e d i n t h e area o f t h e f r o n t d u r i n g 1984 have l e a d t o an accurate p i c t u r e o f t h e spatio-temporal
phytoplankton d i s t r i b u t i o n .
The area i n v e s t i g a t e d and t h e techniques have been d e s c r i b e d e a r l i e r . C h l o r o p h y l l A has been analysed on water c o l l e c t e d a t t w e l v e d i f f e r e n t depths ( f r o m s u r f a c e t o 200 m) ( a c c o r d i n g t o STRICKLAND and PARSONS, 1968). I n March, c h l o r o p h y l l A c o n c e n t r a t i o n f r o n t a l area : more than 0.3 mgr. c h l . A/m
( f i g u r e 5D) i s i m p o r t a n t i n t h e 3 from s t a t i o n 2 t o s t a t i o n 6,
where t h e amount o f n u t r i e n t s i s h i g h . 3 The maximal c o n c e n t r a t i o n s a r e recorded a t s t a t i o n 4 (> 0.5 mgr. c h l . A/m ) . A t t h i s s t a t i o n , t h e l i v i n g phytoplankton i s found deeper than e u p h o t i c depth
and t h e v e r t i c a l d i s t r i b u t i o n o f c h l o r o p h y l l A suggests a downwards t r a n s p o r t o f p h y t o p l a n k t o n a l o n g i s o p y c n a l s i n t h e area o f t h e f r o n t ; e.g. a t s t a t i o n 4, l i v i n g c h l o r o p h y l l c o n t e n t s t i l l reaches 0.4 mg/m3 a t 100 meters depth.
N
Fig. 8 : Distribution of a l k a l i n i t y i s o l i n e s across Corsican f r o n t a l region (number of stations.depth i n meters) i n March, June and October 1984. Alkalinity has been determined by electrochemical t i t r a t i o n according t o GRAN (1952)
m
-2
268
During t h a t period, the maximum of primary production i s observed a t the level of the front (figure 9) (from 0 t o 25 m depth : 50 t o 70 mg C m-3 0-l) and i s associated t o high chlorophyll A concentrations. In the other hand primary productivity (production per unit biomass) reaches a maximum a t tbe same s t a t i o n s b u t only just beneath the surface (200 mg C. mg chl A - l B-'). The biomass distribution being l i k e a plume (figure 5D), we can conclude that phytoplankton i s produced near the surface between s t a t i o n s 3 and 4 (LICOT, 1985), and i s carried along the isopycnals, i n relation w i t h the convergence associated with the frontal system. In June (figure 6D), when the value of the isopycnals slope i s smaller than t h a t found i n March (0.5 % below the s t r a t i f i e d layer) - maximal phytoplankton biomasses are observed j u s t below the s t r a t i f i e d layer (< 50 meters). The chlorophyll distribution seems t o follow the general slope of isopycnals 3 ( u t 28.4 - 28.6). The highest concentrations (> 0.4 mgr. chl. A/m ) are observed i n the open sea a t s t a t i o n 10, a t 50 meter depth where a l k a l i n i t y and density data show a divergence (figure 8) supplying an important nutrient concentration (LICOT, 1985). During that period, accumulations of chlorophyll A a r e situated i n the lowest level of the thermocline. A t t h a t level, the l i g h t intensity i s \reduced b u t s u f f i c i e n t l y high nutrients concentrations are present. Primary production i n June i s a l s o maximum a t the coast and offshore a t the level of the seasonal thermocline. A t the level of the haline f r o n t , primary production remains important (> 25 mg C-3 D-') despite a poor algal biomass. The productivity p r o f i l e are quite d i f f e r e n t : coastal s t a t i o n s (1 - 3) w i t h high productivity (800 - 1000 mg. C mg chl A - l , D - l ) are separated from offshore stations of poor productivity (< 200 mg. C mg chl A - l D - l ) by an horizontal gradient corresponding t o the thermohaline front. In October, a destabilization begins between 10 and 15 miles away from the coast ( s t a t i o n s 3 to 5 ) , while in offshore areas (stations 8 t o l o ) , the distribution of maximal chlorophyll concentrations (> 0.7 mgr. chl A/m 3 a t station 9) i s similar t o that observed in June. However, in the coastal area, chlorophyll A contents show a completely d i f f e r e n t pattern which i s c h a r a c t e r i s t i c of that period : a maximum of chlorophyll A (> 0.8 mg chl A/m 3 ) i s found below 100 m depth. Probably this accumulation of chlorophyll below the euphotic layer corresponds t o the general trend of the coastal upper waters t o destabilize. With the breakdown of the s t r a t i f i c a t i o n , a return t o the winter conditions i s i n i t i a t e d . I n October, highest productions and productivities a r e found near the coast and a t the level of thermohaline front.
10 0
10
-
20-
3040-
SO
Fig. 9 : Distribution Of primary production (mgC . I I I - ~ .0 - l ) (A) and primary productivity (mgC. mgChl A - l . D - l ) Liguro-ProvenGal front in 1984 (LICOT, 1985).
( B ) across
h3 Q,
W
270
CONCLUSIONS
Seasonal hydro1 og cal studies rea ized d u r i n g 1984 show the presence of d i s t i n c t water masses separated by fronts and thermoclines. A t these boundaries vertical movements occur as evidenced by temperature, s a l i n i t y and a l k a l i n i t y distributions. High a l k a l i n i t y values characterize deep waters. This parameter has appeared t o be an original and useful tool t o study the water masses and movements. The hydrological structures are relatively constant from year t o year (LICOT, 1985) and t h e i r evolution throughout the year can be summarized as follows :
- I n winter when the external forcing favours the mixing, an important haline front separates the coastal more s t r a t i f i e d l i g h t pool from the waters of intermediate origin, nutrient-rich and undergoing high vertical mi x i ng .
- I n spring and summer, with surface heating a vertical s t r a t i f i c a t i o n i s induced, followed by the establishment of the seasonal thermocline, hence conducting t o an increased s t a b i l i t y . Hivernal water masses sink and the intermediate waters can not reach the surface. Destabilizing factors (such as winds, cold a i r masses) can locally generate divergences with r i s e of cold waters. A l t h o u g h vertical mixing i s reduced by the presence o f the thermocline, a shoaling of the isopycnals from the coast t o the open sea is y e t observed during t h a t period. - A t the approach of the winter the thermal balance between the sea and the atmosphere reverses, leading t o a destabilization of the water
column and t o the breakdown of the seasonal thermocline. Vertical mixing i s enhanced and intermediate water masses can r i s e u p t o the sea-surface. The description of the chlorophyllian pigments distribution i n the upper 1 ayers a1 ong the transect i s presented. In spring, maxima of phytoplankton are found on the thermohaline front. In summer (June t o October), chlorophyll maxima are situated below the s t r a t i f i e d layer which slopes down from offshore t o the coast. The impact of frontal dynamics on primary production i s well emphasized in our data.
271 ACKNOWLEDGEMENTS The authors acknowledge t h e h e l p o f Captain X. BRUNEAU and t h e crewmembers o f R.V.
"Recteur Dubuisson" an'd D. BAY, c o - d i r e c t o r o f the
S t a t i o n STARESO.
REFERENCES Bethoux, J.P., 1980, Mean water f l u x e s accross s e c t i o n s i n t h e Mediterranean sea evaluated on the b a s i s o f water and s a l t budgets and o f observed s a l i n i t i e s , Oceanologica Acta, 3,1, 79-88. F r a n k i g n o u l l e , M. and Bouquegneau, J.M., 1985, Ecohydrodynamic study o f Liguro-Provencal f r o n t (Corsican Area), I V sea water COP system data, Proc. 1 s t Cong. Oceanol Belg. Acad. Sci , 4 - 6 March 1985 ( i n p r e s s ) .
.
.
Gascard, J.C., 1978, Mediterranean deep water formation. B a r o c l i n i c i n s t a b i l i t y and oceanic eddies, Oceanologica Acta, 4, 3, 315-330. Gran, G., 1952, Determination o f t h e e q u i v a l e n t p o i n t i n p o t e n t i o m e t r i c t i t r a t i o n s - p a r t I 1 , I n t . Congress Anal. Chem., 77 pp, 661-671. Hecq, J.H., Gaspar, A . and Dauby, P., 1981, C a r a c t e r i s t i q u e s 6cologiques e t biochimiques de l'@cosysteme planctonique en b a i e de C a l v i (Corse) , B u l l . SOC. Roy. S c i . Liege, 50, 440-445. Hecq, J.H., L i c o t , M., G o f f a r t , A., Mouchet, A., F r a n k i g n o u l l e , M., Bouquegneau, J.M., Disteche, A., Godeaux, 3. e t Nihoul, J.C.J., 1985, Ecohydrodynamical study o f t h e Liguro-Provencal f r o n t ( C o r s i c a ) , I. H y d r o l o g i c a l Data, Proc. 1 s t Cong. Oceanol. Belg. Acad. Sci., 4-6 March 1985 ( i n press). Jacques, G., Minas, H. J., Minas, M. e t N i v a l , P., 1973, I n f l u e n c e des c o n d i t i o n s h i v e r n a l e s s u r l e s productions phyto- e t zooplanctoniques en Mediterranee nord-occidentale, 11, Biomasse e t productionphytoplanctoniques, Mar. Bio1.,23, 251-265. Lacombe, H., 1973, A p e r p s u r 1 ' a p p o r t 1 1 'oceanographie physique des recherches recentes en Mediterranee, B u l l . E t . Commun. Medit., 7, 5-25. Legendre, L., 1981, Hydrodynamic c o n t r o l o f marine phytoplankton p r o d u c t i o n : the paradox o f s t a b i l i t y , Ecohydrodynamics : Proceedings o f t h e 12th I n t . Liege Colloquium on Ocean Hydrodynamics, Ed. J.C.J.Nihou1, E l s e v i e r Oceanogr.
32, 191-207. L i c o t , M., 1985, Etude ecohydrodynamique du f r o n t Liguro-ProvenGal au l a r g e de l a Corse, R e l a t i o n e n t r e l'hydrodynamique, l e s parametres physicochimiques e t l a p r o d u c t i o n p r i m a i r e , P.h. Thesis U n i v e r s i t y o f Liege, 131 pp. P r i e u r , L., 1981, Heterogeneite s p a t i o - t e m p o r e l l e dans l e bassin LiguroProvencal, Rapp. Comm. I n t . Mer Medit., 27, 6, 177-179. S t r i c k l a n d , J.D.H. e t Parsons, T.R., 1968, A manual o f sea water a n a l y s i s , B u l l . Fish. Res. Bd Can., 125, 1-311.
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273
PLANKTON DISTRIBUTIONS AND PROCESSES I N THE BALTIC BOUNDARY Z O N E S
M.
Kahrul,
5 . NBmmann',
M.
Simm
1
,
0
and K .
V i baste'
' I n s t i t u t e o f Thermophysics and E l e c t r o p h y s c s , T a l l i n n 200031 ( U S S R ) 'Institute (USSR )
o f Zoology and Botany,
Vanemuise S t .
1,
Pa d i s k i S t .
21,
T a r t u 202400
ABSTRACT
Due to their estuarine origin, the brackish water masses of the Baltic Sea result from complicated mixing processes. As a result, boundary zones span the whole spectrum of space scales in both the vertical and horizontal directions. The ecological effects of these boundary zones are exemplified by the results of recent surveys. Evidence of a striking increase in primary productivity in an offshore front, elevated levels of chlorophyll and phytoplankton biomass in the thermocline, offshore and in coastal boundary areas, at the periphery of synoptic eddies, and of a higher standing stock of zooplankton in or adjacent to frontal boundaries has been obtained. Flow-through particle counting has revealed abrupt changes in the size distribution of phytoplankton populations indicating different stages of the vernal bloom. It is stressed that the frequency of the boundary effects is poorly known in both space and time, thus the assessment of their overall significance is not possible at present.
INTRODUCTION The B a l t i c Sea may b e r e g a r d e d a s a l a r g e " o v e r m i x e d " (Shaffer,
f e r e n t w a t e r masses.
A v a r i e t y o f m i x i n g p r o c e s s e s a r e i n v o l v e d on
the whole spectrum o f scales. diffusion,
As t h e m i x i n g i s n o t a s m o o t h ,
a whole spectrum o f boundary zones,
t a l and v e r t i c a l , the v e r t i c a l , cline.
estuary
1 9 7 9 ) . I t s b r a c k i s h w a t e r s r e s u l t from m i x i n g between d i f -
i.e.
i s being generated.
Fickian
both i n the horizon-
Most p e r s i s t e n t a r e t h o s e i n
the seasonal thermocline
and t h e permanent h a l o -
As b o t h a r e s t r o n g p y c n o c l i n e s a n d r e s t r i c t t h e e x t e n t o f
v e r t i c a l mixing (Kullenberg,
19821, t h e y e x e r c i s e a p r o f o u n d i n f l u -
ence o n t h e e c o l o g y o f t h e B a l t i c ( e . g . the horizontal,
Jansson e t a l . ,
1984).
In
t h e boundary zones a r e as u b i q u i t o u s t h o u g h n o t so
274
persistent. Sharp boundaries in larger scales are commonly known as fronts. These transition zones between different water masses are often characterized by the accumulation o f biomass and the increased biological activity (e.9. Holligan, 1 9 8 1 1 , and are supposed to be important for the feeding and survival o f larval and juvenile fish ( l l e s and Sinclair, 1 9 8 2 ) . The ecological significance o f boundary zones in the Baltic, apart from the restrictions to the vertical mixing by the pycnoclines, is virtually unknown. Compared to better studied oceanic shelf analogs (Holligan, 1 9 8 1 ; Marra et al., 1 9 8 3 ; Fasham et al., 1 9 8 5 ) , the fronts in the Baltic are expected to be smaller in scales and less persistent in time. Their shorter life-time and intermit-
tent occurence makes their investigation extreme1.y difficult. From almost any good-quality satellite image o f the Baltic ( s e e Horstmann, 1 9 8 3 ; Gidhagen, 1 9 8 4 ) it i s readily evident that the whole Baltic is tightly packed with eddy-like features and has more o r less distinct boundaries in between (Fig.1). Taken singularly, each boundary zone may have little ecological impact. However, to assess the impact o f the whole pattern o f locally increased gradients and boundary zones i s a challenging and important task. Up to now only single ecological studies o f fronts have been made in the Baltic (Kahru et al., 1 9 8 4 ) . It i s our general feeling that due to the diversity o f h y d r o l o g i c a l / h y d r o b i o l o g i c a l situations and the complexity o f interactions, a statistically large number o f cases must be analysed before any reliable assessment o f the overall ecological significance o f fronts and other boundaries in the Baltic can be made. Here we report some results o f our biologically oriented studies from both the offshore and coastal areas of the Baltic. METHODS Vertical profilinq Vertical distributions o f temperature, salinity, density, and chlorophyll
a
fluorescence were obtained by a complex o f a Neil Brown Mark I11 CTD and a submersible fluorometer. As fluorometer first a "Variosens" (Frungel and Koch, 1 9 7 6 ) and later a "EOS" manufactured by Elektro Optik Juan F. Suarez (Henstedt-Ulzburg, FRG) and described by Astheimer and Haardt ( 1 9 8 4 ) were used. The signals were interfaced to a computer and processed on-line to equispaced ( A z = 5 0 cm) vertical profiles. D i s c r e t e " me a s u r e men t s Water samples were drawn from Niskin samplers and used for on-
275
F i g . 1. C Z C S i m a g e ( c h a n n e l 3 ) o f t h e w e s t e r n B a l t i c Sea. A g g r e g a t i o n s o f t h e blue-green algae serve as t r a c e r s o f t h e near-surface c u r r e n t f i e l d . W e l l - d e f i n e d e d d i e s a r e v i s i b l e s o u t h and west o f the Bornholm i s l a n d .
board analyses o f phosphate, counts.
the extracted chlorophyll
silicate,
2, n i t r a t e , n i t r i t e ,
p r i m a r y p r o d u c t i o n and f o r l a t e r p h y t o p l a n k t o n
Not a l l t h e p a r a m e t e r s were measured on e v e r y s u r v e y .
Car-
b o n f i x a t i o n t e s t s w e r e made i n a c o n t r o l l e d t e m p e r a t u r e a n d l i g h t bath using the
I4C
teqhnique.
The p o t e n t i a l p r i m a r y p r o d u c t i o n was
measured by i n c u b a t i n g v i a l s w i t h 120 m l o f u n f i l t e r e d seawater i n duplicate a t a saturating l i g h t intensity. taken by v e r t i c a l net
(mesh s i z e
= 0.09
w a t e r c o l u m n o r t h r o u g h t h e u p p e r 10-m
Zooplankton samples were
mm) h a u l s t h r o u g h t h e w h o l e layer.
P h y t o p l a n k t o n samples
were processed w i t h t h e Utermtihl method (Utermtihl, t a i l s o n t h e m e t h o d s a r e f o u n d i n Kahrcr e t a l .
1 9 5 8 ) . More de-
(1984).
276 Underway s a m p l i n q S i n c e 1 9 8 5 a new s y s t e m h a s b e e n d e v e l o p e d , m e r s i b l e pump a t 5 m d e p t h , a n a l y z e r H i a c / R o y c o PC-320 fluorometer,
a bubble trap, (Pugh,
1978),
c o n s i s t i n g o f a sub-
an o n - l i n e
a T u r n e r D e s i g n s 10-005R
l o g g e r w i t h f l e x i b l e d i s k storage.
and a c o m p u t e r / d a t a
P a r t i c l e s i n 12 s i z e classes w i t h the equivalent 1000 micrometers were counted.
p a r t i c l e size
diameter
The t i m e i n t e r v a l b e t w e e n t h e f u l l
c y c l e s o f m e a s u r e m e n t s was 1 m i n .
Depending on t h e s h i p speed t h e
c o r r e s p o n d i n g s p a c e i n t e r v a l was b e t w e e n 1 8 0 a n d 4 0 0 m. ously the near-surface Near-shore
from 1 t o
Simultane-
t e m p e r a t u r e and c o n d u c t i v i t y were recorded.
surveys
C o a s t a l a r e a s were v i s i t e d w i t h s m a l l e r v e s s e l s and s i m p l e r equipment
(NBmmann,
o r a t o r y analyses.
1985),
l e a v i n g most o f t h e samples t o l a t e r lab-
I n addition,
a l g a l a s s a y s w e r e made w i t h a c u l -
t u r e o f Scendesmus b r a s i l i e n s i s t o a s s e s s t h e c o n c e n t r a t i o n o f nut r i e n t s i n t h e brackish water, p l a n k t o n (see K a l l q v i s t ,
d i r e c t l y u t i l i z a b l e by t h e phyto-
1973).
The p r i n c i p a l c o m p o n e n t a n a l y s i s was u s e d t o d i s c o v e r s p a t i a l p a t t e r n s i n m u l t i v a r i a t e d a t a o f p h y t o p l a n k t o n numbers, c o n c e n t r a t i o n s and o t h e r e n v i r o n m e n t a l v a r i a b l e s .
nutrient
The s p e c i e s - a b u n -
d a n c e c u r v e s w e r e c a l c u l a t e d f o r t h e e m p i r i c a l p h y t o p l a n k t o n abundances t o d i s c o v e r s h i f t s i n t h e dominance p a t t e r n w i t h i n t h e phytop l a n k t o n assemblages
(see L e v i c h ,
1980).
RESULTS Vertical structure The s u b s u r f a c e c h l o r o p h y l l maximum i s a c o n s p i c u o u s many o c e a n i c a n d s h e l f a r e a s ( C u l l e n ,
1982).
feature o f
I n the Baltic,
a close
a s s o c i a t i o n b e t w e e n t h e c h l o r o p h y l l maximum l a y e r s a n d t h e d e p t h s o f t h e l o c a l maxima i n t h e B r u n t - V a i s a l a ed (Kahru, sity
1981).
f r e q u e n c y has been a s c e r t a i n -
I n r e l a t i o n t o t h e v e r t i c a l d i s t r i b u t i o n o f den-
(or, temperature)
f o u r phenomenological types o f t h e chloro-
p h y l l maximum l a y e r s may b e d i s t i n g u i s h e d well-mixed
(Fig.2).
I n case o f a
u p p e r l a y e r a n d b l o o m c o n d i t i o n s t h e t y p e H 1 maximum may
extend t o the surface.
However,
t y p e H 1 may a l s o b e a s u b s u r f a c e
maximum i n a n i n t e r n a l homogeneous l a y e r .
Type H2,
i.e.
a well-de-
f i n e d c h l o r o p h y l l maximum w i t h o u t a n a d j a c e n t v e r t i c a l d e n s i t y g r a dient,
i s most p r o b a b l y caused by t h e a c t i v e a g g r e g a t i o n o f v e r t i -
c a l l y migrating phytoplankton.
D u r i n g t h e summer t h e r m a l s t r a t i f i -
c a t i o n t h e c h l o r o p h y l l maxima a r e m o s t commonly a s s o c i a t e d w i t h t h e
277
H1
GI
G2
I F i g . 2. P h e n o m e n o l o g i c a l t y p o l o g y o f t h e c h l o r o p h y l maxima ( c o n tinuous curves) i n r e l a t i o n t o the v e r t i c a l density structure (dott e d c u r v e ) . The c h l o r o p h y l l maxima r e s i d e e i t h e r i n a v e r t i c a l l y h o mogeneous r e g i o n ( t y p e s H ) o r i n a g r a d i e n t r e g i o n types G ) . Both types are f u r t h e r d i f f e r e n t i a t e d i n r e l a t i o n t o the association w i t h an extremum i n t h e v e r t i c a l d e n s i t y g r a d i e n t . ypes 1 a r e bounded a t l e a s t o n one s i d e by a d e n s i t y extremum Type H 1 may a l so be bounded f r o m above. T y p e s - 2 a r e n o t . b o u n d e d by extrema i n t h e density gradient.
vertical density gradient,
b e l o n g i n g e i t h e r t o t y p e G 1 ( w i t h an ex-
tremum i n t h e v e r t i c a l d e n s i t y g r a d i e n t )
o r t y p e G2 ( w i t h o u t a n ap-
p a r e n t extremum i n t h e v e r t i c a l d e n s i t y g r a d i e n t ) . s i t i o n s b e t w e e n t h e t y p e s do o c c u r . t i o n s i n August,
However,
tran-
I n t h e t y p i c a l summer c o n d i -
1979 a l l t h e 62 v e r t i c a l p r o f i l e s observed had
c h l o r o p h y l l maxima w i t h i n t h e i n t e r v a l o f 4 m f r o m t h e n e a r e s t l o c a l maximum i n t h e B r u n t - V a i s a l a
frequency
t i o n between t h e two depths o f 1.3
with the standard devia-
m (Kahru,
1981).
I t should be
n o t e d t h a t t h e c h l o r o p h y l l maxima c o n s i d e r e d h e r e h a v e t h e v e r t i c a l s c a l e o f a few m e t e r s and a r e r e p r o d u c e d on c o n s e q u t i v e v e r t i c a l profiles.
I n t h i s respect they d i f f e r
cussed by A s t h e i m e r and H a a r d t
(1984).
from t h e "micropatches"
dis-
As we h a v e n o r a t e m e a s u r e -
ments f o r t h e B a l t i c c h l o r o p h y l l maxima,
no f i r m c o n n e c t i o n s be-
tween t h e p h e n o m e n o l o g i c a l t y p e s and t h e f u n c t i o n a l t y p e s o f C u l l e n (1982) can be e s t a b l i s h e d . I n o r n e a r h o r i z o n t a l f r o n t s t h e v e r t i c a l s t r u c t u r e i s much m o r e complicated,
showing v a r i o u s i n t r u s i o n s and i n t e r l e a v i n g s
The c o n v e r s i o n o f f l u o r e s c e n c e t o c h l o r o p h y l l
a
(Fig.
3).
concentrations i n
l a y e r s o f d i f f e r e n t o r i g i n and w i t h d i f f e r e n t p h y t o p l a n k t o n populat i o n s may b e p r o b l e m a t i c a l d u e t o v a r i a t i o n s i n t h e f l u o r e s c e n c e yield.
However,
i n most cases w i t h s u f f i c i e n t r a n g e o f d a t a p o i n t s ,
the c a l i b r a t i o n regressions Aitsam,
(Fig.
4 ) a r e q u i t e r e l i a b l e ( K a h r u and
1985).
Another problem,
r e l a t e d t o t h e p o t e n t i a l s o f remote sensing t o
e s t i m a t e t h e d i s t r i b u t i o n s o f c h l o r o p h y l l and p r i m a r y p r o d u c t i o n , . i s t h e c o r r e l a t i o n s t r e n g t h between t h e near-surface and t h e p h o t i c z o n e i n t e g r a l c h l o r o p h y l l . ' O n t i c a l p r o f i l e s from t h e B a l t i c (Kahru,
chlorophyll
t h e b a s i s o f 570 v e r -
1 9 8 5 ) i t h a s b e e n shown t h a t
218
10
.-
20
-.
30
--
40
-.
t
EiUHVEYS S D K P K E C=-EI.IE;3+ iZl.lEi7F' H= O .EiH@ -. -
t
7
/-
t
t t
t
20 I' 0 I N 5 I T U FLLIURESCENCE/
GO R E L . UNITS
F i g . 4 . C a l i b r a t i o n regression o f e x t r a c t e d c h l o r o p h y l l 5 from disc r e t e w a t e r samples versus i n s i t u f l u o r e s c e n c e f o r sur-veys i n J u l y August, 1982.
279 only i n case o f a well-mixed concentration
upper l a y e r serves t h e near-surface
as a good e s t i m a t e o f i n case of
t h e o t h e r hand,
a stratified
t i o n between t h e n e a r - s u r f a c e low.
Moreover,
t h e p h o t i c zone c h l o r o p h y l l .
On
photic layer the correla-
and t h e p h o t i c zone c h l o r o p h y l l i s
i t has been documented t h a t i f t h e r e a r e s t a t i o n s
w i t h a s t r o n g s u b s u r f a c e maximum a n d w i t h a m o r e o r less u n i f o r m vertical distribution
(Fig.
51, o r ift h e p h o t i c z o n e d e p t h c h a n g e s
p r i m a r i l y due t o n o n - c h l o r o p h y l l o u s the near-surface
material,
t h e r e l a t i o n between
and t h e p h o t i c zone c h l o r o p h y l l i s i n v e r s e .
This
can be a s o u r c e o f e r r o r f o r t h e r e m o t e e s t i m a t i o n s o f c h l o r o p h y l l and p r i m a r y p r o d u c t i v i t y .
..-
.... ,
I
...
LI'.
IU
w Q
F i g . 5. Sequence o f v e r t i c a l c h l o r o p h y l l p r o f i l e s along a trans e c t l i n e w i t h a space s t e p o f 2.5 n a u t i c a l m i l e s ( G o t l a n d b a s i n ,
-3
.
e n s u i n g p r o f i l e i s o f f s e t 3 mg m The s h i f t i n t h e v e r t i c a l d i s t r i b u t i o n f r o m a s t r o n g s u b s u r f a c e maximum t o a more o r less u n i f o r m v e r t i c a l d i s t r i b u t i o n g a v e r i s e t o a n i n v e r s e r e l a t i o n s h i p between t h e s u r f a c e and t h e p h o t i c zone c h l o r o p h y l l ( o n l y t h e c e n t r a l p a r t o f t h e t r a n s e c t i s shown).
6 Aug 1 9 8 2 ) . E a c h
O f f s h o r e b o u n d a r i e s a s s o c i a t e d w i t h f r o n t s and e d d i e s Our f i r s t b i o l o g i c a l f r o n t s t u d y i n t h e s o u t h - e a s t e r n b a s i n i n 1982 y i e l d e d r e w a r d i n g r e s u l t s ( K a h r u e t a l . , 30-m t h i c k w a t e r
Gotland
1984).
A
mass w i t h a n o m a l o u s l y l o w s a l i n i t y e x t e n d e d v e r t i -
c a l l y across t h e h o r i z o n t a l l y uniform thermocline
(Fig.
f r o n t was p r i m a r i l y a s a l i n i t y f r o n t a n d n o t a d e n s i t y the i s o p y c n a l s i n t h e t o p 10-14 m were i n c l i n e d ) .
6). front
The (only
I n the top layer
the h i g h e r s a l i n i t y w a t e r had p r o t r u d e d o n t o t h e l o w - s a l i n i t y water.
I t was h y p o t h e s i z e d t h a t a f t e r t h e i n i t i s 1 c o n v e r g e n c e o f t h e t w o water masses,
a v e r t i c a l s t r e t c h i n g o f t h e t o p l a y e r was p r o d u c e d ,
the t o p i s o t h e r m s b e i n g l i f t e d up,
and a c y c l o n i c
c i r c u l a t i o n es-
280
0
E
15
I I-
n
w 0
30
I a
I
!
3
s
7
3
! I
13
IS
1
17
Fig. 6. (upper panel) Schematic description o f the frontal structure and of the proposed mechanism for frontal upwelling (Gotland basin, 6 Aug 1 9 8 2 ) . Left from the dashed curve: the low-salinity anomaly; continuous curves: isopycnals ( o r isotherms); arrows: cyclonic circulation; stippled area: supposed deformation o f the free surface. (lower panel) Horizontal distribution o f the potential primary pro- 3 -1 duction ( m g C m h ) near the surface ( 5 ) and in the thermocline ( T )
281 tablished.
The l i m i t e d u p w e l l i n g o f
the upper thermocline water
suggested t o be r e s p o n s i b l e f o r t h e 7 - f o l d
was
increase i n the poten-
t i a l primary p r o d u c t i v i t y adjacent t o the s a l i n i t y interface.
The
p r o d u c t i v i t y v a l u e s were h i g h e r i n t h e h i g h e r - s a l i n i t y s i d e o f t h e f r o n t a n d l e v e l l e d down f u r t . h e r
away
from t h e f r o n t .
The s h a r p t h e r -
m o c l i n e where t h e u p w e l l e d w a t e r s c o u l d o r i g i n a t e f r o m o b v i o u s l y accumulated s i n k i n g organic matter which released n u t r i e n t s .
The
changes i n p r i m a r y p r o d u c t i v i t y were p r i m a r i l y caused by changes i n
the a s s i m i l a t i o n numbers and n o t i n t h e biomass as t h e c h l o r o p h y l l c o n t e n t h a d o n l y a s l i g h t maximum a d j a c e n t t o t h e f r o n t a l z o n e . ever,
How-
t h e z o o p l a n k t o n b i o m a s s more t h a n d o u b l e d i n t h e h i g h e r - s a l i -
n i t y side o f the front. A n o t h e r t y p e o f f r o n t s was u n d e r o b s e r v a t i o n i n t h e B o r n h o l m b a s i n i n t h e summer o f 1 9 8 4 . ered twice
( 2 and 6 A u g u s t )
I n a 4-day
i n t e r v a l t h e t r a n s e c t was c o v -
and a g a i n a f t e r 4 days p r o f i l i n g s were
made o n t h e r e c t a n g u l a r g r i d o f s t a t i o n s
(Fig.
7).
While both o f
. F i g . 7. Study a r e a i n t h e Bornholm b a s i n i n August, 1984. S t a t i o n s p a c i n g a l o n g t h e t r a n s e c t and on t h e g r i d i s 2 . 5 n a u t i c a l m i l e s .
282
CHL 0-60m
Fig. 8. Vertical distributions o f density (sigma-t units) and chlorophyll g (mg m-3) on the transect, 6 Aug 1984. Arrows point to the frontal structures labeled I and 11. An eddy-like deformation of the thermocline ( E ) and two bottom-mixed homogeneous water masses ( H ) near shallow banks are labeled. Note the chlorophyll maximum in the low-salinity side o f front I and another maximum above one o f the bottom-mixing areas. The contour interval is 0.5 for chlorophyll, 0.25 for the sigma-t range 4.5-6.5 and 1.0 for the sigma-t range 7-10.
283 the f r o n t a l s t r u c t u r e s found changes i n s a l i n i t y , frontal structures
(Fig.
8)
were p r i m a r i l y caused by
t h e y were a l s o d i s t i n c t d e n s i t y f r o n t s .
-
( o r m e a n d e r e d ) i n t h e NU d i r e c t i o n ( b o t h f r o n t s 4-day
interval,
The
were n o t s t a t i o n a r y i n space b u t were a d v e c t e d
front
I - 5 km i n t h e 2 n d 4 - d a y
1 0 km i n t h e 1 s t
interval).
The o t h e r
d i s t r i b u t i o n s revealed several features r e l a t e d t o the f r o n t a l structures. front
The c h l o r o p h y l l maximum was b o u n d t o t h e l o w - s a l i n i t y
I on b o t h o f t h e t r a n s e c t s .
tions of
t h e n o n m i g r a t i n g z o o p l a n k t o n most c o n s p i c u o u s l y
t o the both frontal
edge o f
The t w o maxima i n t h e d i s t r i b u -
structures (Fig.
9A).
responded
The d i s t r i b u t i o n p a t t e r n
o f copepods c o u l d have been s i m i l a r u n l e s s t h e u p p e r l a y e r samples
were overmasked by t h e i r d i e 1 v e r t i c a l m i g r a t i o n . cies i n the frontal
The d o m i n a n t s p e -
maxima w e r e B o s m i n a c o r e q o n i m a r i t i m a
c l a d o c e r a n s a n d S y n c h a e t a monopus f o r t h e r o t i f e r s . c o n c e n t r a t i o n s were q u i t e v a r i a b l e ; tween t h e s t a s . (Fig.
96).
6-10
however,
The n u t r i e n t
t h e n i t r a t e maximum b e -
may b e a s s o c i a t e d w i t h t h e f r o n t a l r e g i o n
The p h o s p h a t e maximum b e t w e e n t h e s t a s .
seems t o b e c h a r a c t e r i s t i c o f t h e w a t e r front
for the
1 0 - 1 8 ( n o t shown)
mass o n t h e SE s i d e o f
I . A l t h o u g h t h e a n a l y t i c a l e r r o r v a r i a n c e o f t h e n u t r i e n t de-
t e r m i n a t i o n s m i g h t be c o n s i d e r a b l e ,
t h e space r e s o l u t i o n o f 2.5
n.
m i l e s is o b v i o u s l y t o o b i g t o r e s o l v e t h e i n t e n s e s m a l l e r s c a l e variability
( s e e Hansen,
1985).
The same was p r o b a b l y t r u e o f t h e
primary p r o d u c t i v i t y d i s t r i b u t i o n s .
The p r o d u c t i v i t y v a l u e s w e r e
h i g h l y v a r i a b l e a n d d i d n o t show o b v i o u s c o n n e c t i o n s w i t h t h e o t h e r variables.
T h i s may p a r t l y b e a t t r i b u t e d t o t h e d o m i n a n c e o f t h e
c y a n o b a c t e r i a (Aphanizomenon,
Microcystis,
Nodularia)
forming l a r g e
a g g r e g a t e s and i n c r e a s i n g t h e r e t h r o u g h t h e sample v a r i a n c e . Unfortunately,
no c u r r e n t measurements i n t h e f r o n t s were a v a i l -
able b u t i n general divergence.
f r o n t s a r e known a s s i t e s o f a c t i v e c o n v e r g e n c e /
The i n c r e a s e d a b u n d a n c e o f z o o p l a n k t o n i n t h e f r o n t a l
regions might have r e s u l t e d from t h e f l o w convergence i n t h e f r o n t s . The o r g a n i s m s w h i c h c a n m a i n t a i n t h e i r p r e f e r r e d d e p t h c a n e a s i l y accumulate i n convergence zones mechanism,
however,
d i d n o t work
( O l s o n and Backus,
abundant i n t h e r e g i o n between t h e two f r o n t s . increased gradients i n the across-front means m e r e l y 2 - d i m e n s i o n a l
The same
A l t h o u g h f r o n t s have
direction,
t h e y a r e by no
( v e r t i c a l and a c r o s s - f r o n t )
T h i s was s u b s t a n t i a t e d b y m a p p i n g t h e t e m p e r a t u r e , chlorophyll
1985).
f o r i c h t i o p l a n k t o n w h i c h was m o r e
d i s t r i b u t i o n s on t h e 2-dimensional
s a l i n i t y and
g r i d (Fig.
Changes i n t h e l o c a t i o n o f t h e f r o n t a l b o u n d a r y , meandering or advection o f t h e f r o n t ,
phenomena.
10).
i n t e r p r e t e d as t h e
c o u l d e a s i l y be f o l l o w e d i n
284
I
8
R
a
Y 3
2
Fig. 9. Horizontal distributions along the transect through the B o r n holm basin, 6 Aug 1984. ( A ) Salinity ( o / o o ) at the depths o f 5 m ( 5 ) and 1 0 m ( 0 1 , biomass (mg m - ' ) of the cladocerans ( L ) and rotifers ( R ) in the upper 10-m layer. ( 8 ) Surface (lm) concentrations o f nitrate ( N , p M ) , chlorophyll g a s extracted from water samples ( C ) , and the mean chlorophyl-1 g concentration in the top 10-m layer a s measured fluorometrically ( + , mg rn-3)
285
Fig. 10. H o r i z o n t a l d i s t r i b u t i o n s o n t h e 10 x 10 n. m i l e grid ( o r the position s e e Fig. 7): n e a r - s u r f a c e ( 5 m) s a l i n i t y ( ' d o ) and t h e top 10-m c h l o r o p h y l l c o n c e n t r a t i o n ( m g m-'). the t h e r m o h a l i n e structure. H o w e v e r , t h e b i o l o g i c a l a n d c h e m i c a distributions had u n d e r g o n e d r a s t i c c h a n g e s b e t w e e n t h e s u r v e y s
As the d o m i n a n t c u r r e n t s o u g h t t o b e i n t h e along-front d i r e c t i o n s , w e suggest t h a t t h e p r i n c i p a l s o u r c e o f v a r i a t i o n s b e t w e e n t h e s u b s e quent s u r v e y s w a s t h e a d v e c t i o n o f v a r i o u s s c a l e p a t c h e s a c r o s s t h e station grid. T h i s c o u l d a c c o u n t for t h e l a c k o f r e s e m b l a n c e b e t w e e n the c o n s e q u e n t s u r v e y s for t h e c h l o r o p h y l l (Figs. 8 a n d 10) a n d n u trient d i s t r i b u t i o n s . We h a v e p r e s e n t e d e v i d e n c e t h a t t h e s y n o p t i c s c a l e e d d i e s c o n -
tribute s i g n i f i c a n t l y t o t h e d i s t r i b u t i o n s o f b i o l o g i c a l and c h e m i cal v a r i a b l e s i n t h e B a l t i c ( K a h r u e t al., 1 9 8 1 , 1 9 8 2 ; Aitsam e t al., 1984). E d d i e s may d e v e l o p from f r o n t a l b o u n d a r i e s ( P i n g r e e e t al., 1979) and c a n t h e m s e l v e s p r o d u c e s m a l l e r s c a l e f r o n t s by a c c e n t u a t ing t h e e x i s t i n g gradients. The s y n o p t i c s c a l e e d d i e s c o n t r i b u t e probably e x t e n s i v e l y t o t h e v e r t i c a l flux o f n u t r i e n t s i n t h e i n t e r i o r o f t h e B a l t i c ( K a h r u , 1982). H o w e v e r , t h e e f f e c t o f a n eddy field o n t h e n u t r i e n t flux i s h i g h l y d e p e n d e n t o n t h e v e r t i c a l thermohaline a n d n u t r i e n t s t r a t i f i c a t i o n a s w e l l a s o n t h e e n e r g y a v a i l able t o o v e r c o m e t h e buoyancy forces. C o n s e q u e n t l y , s o m e e d d i e s simply t r a n s p o r t n u t r i e n t a n d c h l o r o p h y l l a n o m a l i e s o v e r l a r g e d i s tances (Aitsam e t al., 1984). Apart from l i g t i n g o r l o w e r i n g t h e pycnoclines ( a n d , p o s s i b l y , n u t r i c l i n e ) in eddy c e n t e r s , t h e e d d i e s
286
influence the mixing i n t h e i r periphery,
probably owing t o t h e i n -
t e n s i f i e d shear zones a t t h e eddy b o u n d a r i e s .
Hansen) showing c o h e r e n t p a t c h - l i k e
11 i s a r e s u l t o f
Fig.
a j o i n t s u r v e y w i t h t h e I n s t i t u t f u r Meereskunde,
K i e l (Dr.
H.P.
anomalies i n t h e hydrochemical
REL. DYN. TOPOGR. 50-3 d h
F i g . 11. H o r i z o n t a l d i s t r i b u t i o n s o n a 1 0 x 1 0 n . m i l e s a r e a i n t h e B o r n h o l m b a s i n , 2 1 - 2 2 J u n e 1 9 8 2 . The p h o s p h a t e a n d s i l i c a l e d a t a w e r e m e a s u r e d b y Dr. H . P . Hansen ( I f M , K i e l ) .
p a r a m e t e r s and c h l o r o p h y l l d i s t r i b u t i o n .
The a n o m a l y w i t h maxima i n
t h e n u t r i e n t c o n c e n t r a t i o n s a n d m i n i m a i n pH a n d o x y g e n was l o c a t e d i n t h e eddy p e r i p h e r y w i t h maximal g r a d i e n t s i n t h e g e o s t r o p h i c rents.
The c h l o r o p h y l l p a t c h i n t h e u p p e r 30-m
e x a c t l y above t h e n u t r i e n t p a t c h a t 40-60
cur-
l a y e r was l o c a t e d
m depths.
Some o f t h e d i f -
f e r e n c e s b e t w e e n t h e c o r r e s p o n d i n g c o n t o u r maps a r e n o t d u e t o d i f f e r e n t d i s t r i b u t i o n s b u t t o d i f f e r e n t s a m p l i n g schemes.
Whereas t h e
CTD and c h l o r o p h y l l p r o f i l e s were t a k e n a t t h e g r i d p o i n t s w i t h a 2.5
n.
miles step,
t h e h y d r o c h e m i c a l d a t a were o b t a i n e d by h o r i z o n -
t a l scanning along the p a r a l l e l g r i d l i n e s . Offshore
fronts
a l s o d e l i m i t d i f f e r e n t plankton communities or
seasonal succession stages. the flow-through
T h i s has been o b s e r v e d w i t h t h e u s e o f
measurement system.
As m o s t o f t h e d a t a a r e s t i l l
o n l y one example w i l l b e p r e s e n t e d .
under s c r u t i n y ,
A sharp s a l i n i t y
f r o n t was o b s e r v e d d u r i n g t h e s p r i n g b l o o m n e a r t h e o p e n i n g t o t h e Gulf
of
Finland.
T o g e t h e r w i t h t h e d r o p i n s a l i n i t y an a b r u p t change
i n t h e p a r t i c l e s i z e s t r u c t u r e was o b s e r v e d ( F i g . c h a n n e l 7 v a l u e s r e m a i n e d o n t h e same l e v e l , w e l l as t h e fluorescence
12).
While the
c h a n n e l s 8 and 9 as
showed a s h a r p i n c r e a s e .
I t was p r o v e d l a t -
e r t h a t t h e c h a n g e s w e r e d u e t o t h e i n c r e a s e d mean s i z e o f t h e d i a t o m Achnanthes
The l o n g e r c h a i n s o f A .
taeniata "chains".
c o l o n i e s were a s s i g n e d b y t h e c o u n t e r Together w i t h t h e o t h e r changes ( T a b l e rophyll,
taeniata
i n t o the higher s i z e classes.
11, e . g .
the increased chlo-
pheopigment and p r i m a r y p r o d u c t i o n l e v e l s ,
phytoplankton
abundance and t h e i n c r e a s e d numbers o f t h e d e c a y i n g cells o f t h e c o l d w a t e r s p e c i e s M e l o s i r a a r c t i c a and N i t z s c h i a f r i q i d a t h i s i n d i c a t e s t h a t t h e f r o n t marked a boundary between d i f f e r e n t success i o n a l s t a g e s o f t h e v e r n a l bloom. boundary e f f e c t s ,
e.g.
W h e t h e r t h e r e w e r e some s p e c i f i c
e l e v a t e d v a l u e s compared t o t h e b o t h s i d e s ,
i s n o t c l e a r b u t t h e w a t e r s o f t h e G u l f o f F i n l a n d were c e r t a i n l y i n a more advanced s t a g e .
TABLE 1 Near-surface
( 5 m) h y d r o g r a p h i c d a t a a n d p h y t o p l a n k t o n p a r a m e t e r s
for s t a t i o n s r e p r e s e n t a t i v e o f t h e e i t h e r s i d e o f t h e f r o n t and adjacent t o the f r o n t
(cf.
Fig.
12). F
T5 7.33 Salinity (o/oo) 0.42 PO - P ( p M ) C h f o r o p h y l l a (mg m-3) 5.47 1.25 P h a e o p i g m e n t ( - ,, - ) Light-saturated primary -1 -3 21.0 p r o d u c t i o n (mg C m h T o t a l p h y t o p l a n k t o n abundance ( s o l i t a r y c e l l s and c o l o n i e s / r n l ) 4 9 0 A c h n a n t h e s t a e n i a t a ( c o l o n i e s / m l ) 225 Mean e o u i v a l e n t d i a m e t e r o f (A. t a e n i a t a c o l o n i e s ( p m ) 41.9
T6
6.77 0.48 14.28 2.16
6.85 0.40 12.68
48.1
48.5
808 388 45.7
1.80
596 339 48.3
F i g . 1 2 . N e a r - s u r f a c e ( 5 m ) h o r i z o n t a l d i s t r i b u t i o n s o f c h l o r o p h y l l f l u o r e s c e n c e and P a r t i c l e c o n c e n t r a t i o n s i n t h e e q u i v a l e n t d i a m e t e r r a n g e s 2 8 - 4 2 , 4 2 - 7 3 a n d 7 3 - 1 0 5 pm The i n t e r v a l b e t w e e n p o i n t s T5 a n d T6 i s 5 0 n . m i l e s . W a t e r s a m p l e s w e r e t a k e n f r o m p o i n t s T 5 , F , T 6 . The f u l l s c a l e i s e q u i v a l e n t t o 1 2 0 p a r t i c l e s / m l .
289 B o u n d a r y e f f e c t s i n t h e Moonsund a r e a Altogether
6 s u r v e y s w e r e made i n t h e s t u d y a r e a
May,
J u l y and O c t o b e r ,
man,
1985).
1 9 8 2 a n d May,
(Fig.
J u l y and November,
13) i n
1 9 8 3 (N6m-
I n t h e space o f t h e f i r s t two p r i n c i p a l components a
b o u n d a r y a r e a b e t w e e n t h e w a t e r s o f t h e M a t s a l u Bay a n d t h e S o e l a Strait i s distinguished of
13,
(Figs.
14).
The b o u n d a r y a r e a i s a s i t e
s t r o n g v e r t i c a l s t i r r i n g due t o a s t r o n g ,
over s h a l l o w a r e a s , masses.
fluctuating current
and o f l a t e r a l exchange between d i f f e r e n t w a t e r i n i t s usual
The b o u n d a r y d i s c u s s e d h e r e i s n o t a f r o n t
sense b u t r a t h e r an a r e a o f c o n t a c t b e t w e e n d i f f e r e n t c o m m u n i t i e s and w a t e r m a s s e s ,
and has a s c a l e o f a b o u t
meters ( t h e water
transparency)
t o the east
15),
(Fig.
t h e boundary zone,
1 0 km.
W h i l e some p a r a -
decreased q r a d u a l l y
from t h e west
s e v e r a l o t h e r s s h o w e d p r o n o u n c e d maxima i n
e.g.
the nutrients,
p l a n k t o n biomass and s p e c i e s r i c h n e s s . o f the spring phytoplankton
(Fig.
a l g a l assay values,
phyto-
The s p e c i e s - a b u n d a n c e
curves
1 6 ) show t h a t w h i l e t h e e u t r o p h i c
M a t s a l u Bay c o m m u n i t y i s s t r o n g l y d o m i n a t e d b y a s i n g l e s p e c i e s (Diatoma elongatum v.
tenuis),
and t h e o l i g o t r o p h i c community has
v e r y l o n g a b u n d a n c e s w i t h a l o w number o f s p e c i e s ,
t h e boundary
p h y t o p l a n k t o n community has a r i c h v a r i e t y o f r e l a t i v e l y u n i f o r m l y d i s t r i b u t e d species.
The r e s u l t s o f t h e 6 s u r v e y s v a r i e d t o some
e x t e n t b u t a t l e a s t some o f t h e b o u n d a r y e f f e c t s m e n t i o n e d w e r e p r e s e n t on a l l t h e s u r v e y s .
DISCUSSION U s i n g r e s u l t s f r o m o u r r e c e n t s t u d i e s we h a v e p r e s e n t e d e x a m p l e s o f various physical-biological
c o u p l i n g s between t h e s y n o p t i c s c a l e
and t h e s m a l l e r s c a l e d i s t r i b u t i o n s o f v a r i a b l e s
i n t h e B a l t i c Sea.
We m u s t a d m i t t h a t o u r a b i l i t i e s t o i n t e r p r e t e v a r i o u s d e t a i l s i n these d i s t r i b u t i o n s tion,
transport
i n terms o f t h e dynamics ( i - e . ,
t h e i r genera-
and d e c a y ) a r e l i m i t e d and ambiquous.
Although the
boundary zones descri.bed h e r e i n have o f t e n been c h a r a c t e r i s e d by elevated values o f p l a n k t o n c o n c e n t r a t i o n s and/or
rates o f related
processes with respect t o t h e surrounding waters,
t h i s i s by no
means a l w a y s t h e c a s e .
As r i g h t l y n o t e d b y R i c h a r d s o n e t a l .
(1985),
there i s a n a t u r a l tendency t o p r e f e r e n t i a l l y r e p o r t those occasions w i t h s i g n i f i c a n t boundary e f f e c t s , w i t h no a p p a r e n t d i f f e r e n c e s .
and t o " f o r g e t "
t h e cases
We h a v e d e m o n s t r a t e d t h a t t h e e c o l o g -
i c a l s y s t e m i n t h e B a l t i c c a n a c t i v e l y res-pond t o t h e p h y s i c a l forcings,
such as t h e s y n o p t i c eddies,
b e l i e v e t h a t much o f t h e v a r i a b i l i t y
f r o n t a l dynamics,
etc.
We
i n t h e response stems from
290
F i g . 1 3 . Study a r e a and h o r i z o n t a l p a t t e r n o f bberesores o f t h e f i r s t p r i n c i p a l c o m p o n e n t i n May, 1 9 8 3 .
Loadings
A
8
i2
4l-n 2 3
1 .
.
0
2
i
.
-2
. 8 9 1 - depth 2 3 -
temperature Secchi depth 4 - n i t r a t e concentration 5 - t o t a l phosphorus 6 - t o t a l number o f p h y t o p l a n k t o n 7 - number o f Cyanophyta 8 - number o f D i a t o m e a 9 - number o f P y r r h o p h y t a
1
-2
0
2
1
1 0 - t o t a l p h y t o p l a n k t o n biomass 11 - b i o m a s s o f D i a t o m e a 1 2 - number o f s p e c i e s
F i g . 1 4 . ( A ) S t r u c t u r e o f t h e f i r s t t w o p r i n c i p a l components w i t h t h e i n g r e d i e n t v a r i a b l e s l i s t e d below; ( B ) O r d i n a t i o n o f s t a t i o n s i n t h e space o f t h e f i r s t two components
2 91
No 3 Algal assay
10 of species Ph y topl. biomass
remp.
Secchi d e p t h I
1
2
3
4
5
6
7
8
S t . number
F i g . 15. H o r i z o n t a l p r o f i l e s o f s o m e b i o l o g i c a l , c h e m i c a l a n d p h y s i c a l v a r i a b l e s a c r o s s t h e M o o n s u n d b o u n d a r y a r e a i n M a y , 1983.
N
-
nuvbpr o f
i n d i v i d u a l s 0f-r s n r c i e s
Fig. 16. S p e c i e s - a b u n d a n c e c u r v e s o f t h r e e p h y t o p l a n k t o n c o m m u n i t i e s : - S o e l a S t r a i t , B - b o u n d a r y a r e a , a n d C - M a t s a l u Bay.
A
292 t h e i n t r i c a t e hydrodynamics o f t h a t i n t h e face o f
the forcings
themselves.
the ubiquitous occurrence of
I t seems
f r o n t s and o t h e r
b o u n d a r y phenomena i n t h e B a l t i c a n d i n t h e f a c e o f t h e r e s u l t s f r o m t h i s and o t h e r a r e a s ,
i t i s n o t t h e q u e s t i o n any more w h e t h e r t h e
b o u n d a r i e s have b i o l o g i c a l e f f e c t s b u t what i s t h e d e n s i t y o f o c c u r r e n c e i n space and t i m e o f t h e b o u n d a r i e s w i t h c e r t a i n c h a r a c t e r i s tics.
The n e x t t a s k t h e n s h o u l d b e t h e d e t e r m i n a t i o n o f t h e o v e r a l l
i m p o r t a n c e o f t h e B a l t i c boundary zones t o i t s e c o l o g y , fisheries,
etc.
t o the local
A m a j o r t a s k f o r t h e coming y e a r s would be t o accu-
m u l a t e r e p r e s e n t a t i v e s t a t i s t i c s on b i o l o g i c a l e f f e c t s o f f r o n t s a n d o t h e r b o u n d a r y phenomena,
and t o r e v e a l t h e mechanisms w h i c h
govern t h e b i o l o g i c a l response t o t h e p h y s i c a l p l i s h t h i s task,
forcing.
To accom-
a m a j o r improvement i n t h e s a m p l i n g s t r a t e g y and
techniques should take place.
First,
mote s e n s i n g ( s e e Campell and E s a i a s , i n s i t u m e t h o d s s h o u l d b e made.
ena s h o u l d be s t u d i e d i n t h e i r a t i o n t i l l t h e i r decay.
more e x t e n s i v e use o f t h e re1985) and h i g h r e s o l u t i o n
The t e m p o r a l d y n a m i c s o f t h e phenomf u l l dynamics,
i.e.
from t h e i r gener-
I n a system which i s n o t a t e q u i l i b r i u m ,
t h e t e m p o r a l sequence o f e v e n t s i s c r u c i a l
(Harris,
1983).
t h e c o r r e l a t i o n s between p l a n k t o n i c and e n v i r o n m e n t a l
Therefore
v a r i a b l e s as
m e a s u r e d a t a s i n g l e i n s t a n t n e e d n o t y i e l d much i n s i g h t .
It i s also
evident t h a t t h e v a r i a b i l i t y a t t h e l e v e l o f i n t e g r a l parameters such as biomass o r i n v i v o
fluorescence obscures t h e i n h e r e n t v a r i -
a b i l i t y o f a p l a n k t o n community a t t h e l e v e l o f
populations.
a t t h e l e v e l o f p o p u l a t i o n s or even t h e i r d e v e l o p m e n t a l the s i g n i f i c a n t i n t e r a c t i o n s take place. high-resolution,
flow-through
It i s
stages t h a t
lde h o p e t h a t t h e u s e o f t h e
p a r t i c l e c o u n t i n g system w i t h t h e ac-
companying measurements w i l l p r o v i d e u s e f u l i n s i g h t s i n t h e n e a r future.
Acknowledqements. among whom J.
M.
Puvi,
helpful.
T.
Ide a r e g r e a t l y i n d e b t e d t o a number o f p e r s o n s
J. L i l l e s , J. L o k k ,
Elken,
I. K o t t a ,
L.
Kaarma.n,
A.
Randveer,
H.
S a l a n d i and A .
Raid,
The C Z C S i m a g e o n F i g .
Horstmann ( I n s t i t u t
S i h t have been most
1 was k i n d l y s u p p l i e d b y D r .
f u r Meereskunde,
U.
Kiel).
REFERENCES A i t s a m , A . , Hansen, H.P., E l k e n , J., K a h r u , M . , L a a n e m e t s , J., P a j u s t e , M., P a v e l s o n , J. a n d T a l p s e p p , L . , 1 9 8 4 . P h y s i c a l a n d c h e m i c a l v a r i a b i l i t y o f t h e B a l t i c Sea: a j o i n t e x p e r i m e n t i n t h e G o t l a n d b a s i n . C o n t . S h e l f Res., 3: 2 9 1 - 3 1 0 .
293 A s t h e i m e r , H. a n d H a a r d t , H . , 1 9 8 4 . S m a l l - s c a l e p a t c h i n e s s o f t h e c h l o r o p h y l l - f l u o r e s c e n c e i n t h e sea: a s p e c t s o f i n s t r u m e n t a t i o n , d a t a p r o c e s s i n g , a n d i n t e r p r e t a t i o n . M a r . E c o l . P r o g . Ser., 1 5 : 233-245. C a m p b e l l , J.W. a n d E s a i a s , W . E . , 1985. S p a t i a l p a t t e r n s i n temperat u r e and c h l o r o p h y l l on N a n t u c k e t S h o a l s f r o m a i r b o r n e remote s e n s i n g d a t a , May 7 - 9 , 1 9 8 1 . J . M a r . R e s . , 4 3 : 1 3 9 - 1 6 1 . Cullen, J.J., 1 9 8 2 . The d e e p c h l o r o p h y l l maximum: c o m p a r i n g v e r t i c a l p r o f i l e s o f c h l o r o p h y l l 2 . Can. J. F i s h A q u a t . S c i . , 39: 791-803. P l a t t , T., I r w i n , B. a n d J o n e s , K . , 1985. F a c t o r s Fasham, M . J . R . , a f f e c t i n g t h e s p a t i a l p a t t e r n o f t h e d e e p c h l o r o p h y l l maximum i n t h e r e g i o n o f t h e A z o r e s f r o n t . P r o g . Oceanog., 1 4 : 129-165. F r u n g e l , F. and Koch, C . , 1976. P r a c t i c a l e x p e r i e n c e w i t h t h e V a r i o sens equipment i n m e a s u r i n g c h l o r o p h y l l c o n c e n t r a t i o n s and f l u o r e s c e n t t r a c e r s u b s t a n c e s , l i k e R h o d a m i n e , F l u o r e s c e i n , a n d some new s u b s t a n c e s . I E E E J . Ocean. E n g n . , 1: 2 1 - 3 2 . Gidhagen, L . , 1984. C o a s t a l u p w e l l i n g i n t h e B a l t i c - a p r e s e n t a t i o n o f s a t e l l i t e and i n s i t u measurements o f sea s u r f a c e t e m p e r a t u r e s i n d i c a t i n g c o a s t a l u p w e l l i n g . P a r t s I , 11. S M H I - r e p o r t s RHO-37, N o r r k o p i n g , 40+60 p p . Hansen, H.P., 1 9 8 4 . The s i g n i f i c a n c e o f d i s c r e t e B a l t i c n u t r i e n t samples d i s c u s s e d on t h e b a s i s o f c o n t i n u o u s measurements. D. h y drogr. Z., 37: 245-258. 1983. Mixed l a y e r p h y s i c s and p h y t o p l a n k t o n p o p u l a H a r r i s , G.P., t i o n s : s t u d i e s i n e q u i l i b r i u m and n o n - e q u i l i b r i u m e c o l o g y . Prog. P h y c o l . Res., 2: 1-52. H o l l i g a n , P.M., 1981. B i o l o g i c a l i m p l i c a t i o n s o f f r o n t s on t h e n o r t h west European c o n t i n e n t a l s h e l f . P h i l . Trans. R. SOC. Lond., A302: 5 4 7 - 5 6 2 . Horstmann, U., 1983. D i s t r i b u t i o n p a t t e r n s o f t e m p e r a t u r e and w a t e r c o l o u r i n t h e B a l t i c Sea a s r e c o r d e d i n s a t e l l i t e i m a g e s : i n d i c a t o r s f o r p h y t o p l a n k t o n growth. B e r . I n s t . Meeresk. U n i v . K i e l , 1 0 6 (11, 1 4 7 p p . 1982. A t l a n t i c h e r r i n g : s t o c k d i s c r e t e I l e s , J.D. and S i n c l a i r , M., n e s s and abundance. S c i e n c e , 215: 627-633. Jansson, 8.-O., W i l m o t , W . and W u l f f , F . , 1984. C o u p l i n g t h e subs y s t e m s - t h e B a l t i c Sea a s a c a s e s t u d y . I n : M . J . R . Fasham ( E d i t o r ) , F l o w s o f e n e r g y and m a t e r i a l s i n m a r i n e ecosystems. Plenum P r e s s , New Y o r k , p p . 5 4 9 - 5 9 5 . Kahru, M., 1 9 8 1 . R e l a t i o n s b e t w e e n t h e d e p t h s o f c h l o r o p h y l l maxima a n d t h e v e r t i c a l s t r u c t u r e o f d e n s i t y f i e l d i n t h e B a l t i c Sea. Oceanology, 21: 76-79. Kahru, M., 1 9 8 2 . The i n f l u e n c e o f h y d r o d y n a m i c s o n t h e c h l o r o p h y l l f i e l d i n t h e o p e n B a l t i c . I n : J.C.J. N i h o u l ( E d i t o r ) , Hydrodyn a m i c s o f s e m i - e n c l o s e d s e a s . E l s v i e r , Amsterdam, pp. 531-542. 1 9 8 5 . Remote s e n s i n g a n d t h e v e r t i c a l d i s t r i b u t i o n o f Kahru, M., t h e B a l t i c c h l o r o p h y l l . P r o c . 1 4 t h Conf. B a l t i c Oceanographers, Gdynia, i n press. Kahru, M. and A i t s a m , A . , 1985. C h l o r o p h y l l v a r i a b i l i t y i n t h e B a l t i c Sea: a p i t f a l l f o r m o n i t o r i n g . J. Cons. i n t . E x p l o r . M e r . , i n press. Kahru, M., Aitsam, A . and E l k e n , J . , 1981. C o a r s e - s c a l e s p a t i a l s t r u c t u r e o f phytoplankton standing crop i n r e l a t i o n t o hydrog r a p h y i n t h e o p e n B a l t i c Sea. M a r . E c o l . P r o g . Ser., 5 : 3 1 1 - 3 1 8 . A i t s a m , A . and E l k e n , J., 1 9 8 2 . S p a t i o - t e m p o r a l d y n a m i c s Kahru, M., o f c h l o r o p h y l l i n t h e o p e n B a l t i c Sea. J. P l a n k t o n R e s . , 4: 779-790. K a h r u , M . , E l k e n , J., K o t t a , I.,S i m m , M . a n d V i l b a s t e , K . , 1 9 8 4 . P l a n k t o n d i s t r i b u t i o n s and p r o c e s s e s a c r o s s a f r o n t i n t h e open
294 B a l t i c Sea. M a r . E c o l . P r o g . Ser., 2 0 : 1 0 1 - 1 1 1 . Kullenberg, G., 1 9 8 2 . M i x i n g i n t h e B a l t i c Sea a n d i m p l i c a t i o n s f o r t h e e n v i r o n m e n t a l c o n d i t i o n s . I n : J.C.J. N i h o u l ( E d i t o r ) , Hydrodynamics o f s e m i - e n c l o s e d s e a s . E l s v i e r , Amsterdam, p p . 3 9 9 - 4 1 8 . K a l l q v i s t , T., 1 9 7 3 . Use o f a l g a l a s s a y f o r i n v e s t i g a t i n g a b r a c k i s h water area. I n : S.G. Lonnberg ( E d i t o r ) , A l g a l assay o f water p o l l u t i o n r e s e a r c h . Oy K i r j a p a i n o , H e l s i n k i , p p . 1 1 1 - 1 2 4 . Levich, A.P., 1 9 8 0 . The s t r u c t u r e o f e c o l o g i c a l c o m m u n i t i e s ( i n R u s s i a n ) . Moscow S t a t e U n i v e r s i t y , Moscow, 1 8 1 p p . M a r r a , J., H o u g h t o n , R . W . , Boardman, D.C. and Neale, P.J., 1982. V a r i a b i l i t y i n s u r f a c e c h l o r o p h y l l 5 a t a s h e l f - b r e a k f r o n t . J. M a r . Res., 4 0 : 5 7 5 - 5 9 1 . Nbmmann, S . , 1 9 8 5 . M e s o s c a l e s p a t i a l v a r i a b i l i t y o f t h e p h y t o p l a n k t o n d i s t r i b u t i o n i n t h e Moonsund a r e a o f t h e B a l t i c Sea. P r o c . 1 4 t h Conf. B a l t i c Oceanographers, Gdynia, i n p r e s s . O l s o n , D.B. a n d B a c k u s , R . H . , 1 9 8 5 . The c o n c e n t r a t i n g o f o r g a n i s m s a t f r o n t s : a c o l d - w a t e r f i s h and warm-core G u l f Stream r i n g . J. Mar. Res., 4 3 : 1 1 3 - 1 3 7 . P i n g r e e , R.D., H o l l i g a n , P.M. and M a r d e l l , G.T., 1979. Phytoplankton g r o w t h and c y c l o n i c e d d i e s . N a t u r e , Lond., 2 7 8 : 2 4 5 - 2 4 7 . Pugh, P . R . , 1 9 7 8 . The a p p l i c a t i o n o f p a r t i c l e c o u n t i n g t o an u n d e r s t a n d i n g o f t h e s m a l l - s c a l e d i s t r i b u t i o n o f p l a n k t o n . I n : J.H. S t e e l e ( E d i t o r ) , S p a t i a l p a t t e r n i n p l a n k t o n c o m m u n i t i e s . Plenum P r e s s , New Y o r k , p p . 1 1 1 - 1 2 9 . Richardson, K., L a v i n - P e r e g r i n a , M.F., Michelson, E.G. and Simpson, J.H., 1985. Seasonal d i s t r i b u t i o n o f c h l o r o p h y l l 2 i n r e l a t i o n t o p h y s i c a l s t r u c t u r e i n t h e w e s t e r n I r i s h Sea. O c e a n o l . A c t a , 8: 7 7 - 8 6 . Shaffer, G., 1 9 7 9 . On t h e p h o s p h o r u s a n d o x y g e n d y n a m i c s o f t h e B a l t i c Sea. C o n t r . A s k o L a b . U n i v . S t o c k h o l m , No. 2 6 , 9 0 p p . 1 9 5 8 . Zur Vervollkommnung d e r q u a n t i t a t i v e n PhytoU t e r m o h l , H., p l a n k t o n - M e t h o d i k . M i t t . I n t . Ver. T h e o r . Angew. L i m n o l . , 1 - 3 8 .
295
THE FU3I.X OF THE LOOP CURRENT IN THE GJLF OF MMICO FIloNTS D.A.
SALAS de LMXJ and M.A. MNRFAL-GCMEZ
CCNACYT, Consejo AEicional de Ciencia y rnnologia,
&im
CHER, Universite de Li%e, B-5 S a t Tilman, B 4 0 0 0 Liwe, Belgium
m w The role of the Lmp Current on,the fronts of the Gulf of Mexico is studied using the results of the Monreal-Ghez and Salas de -6n (1965) Gulf of mica numerical model. The horizontal thermal structure can be depicted with the pycm l i n e ananaly. Assdng that biological processes take place i n the frontal areas, the thermal boundaries a t the cyclonic and anticyclonic eddies, as w e l l as the Loop Current have an effect on the chlorophyll-a concentration i n the Gulf of Mexico.' IwlTmUCTIoN
Differences between the waters of the Gulf of Mexico arad waters of the Imp Current are rather mall, showing a slightly horizontal thermal gradient
(Ichiye, 1962). Nevertheless, the anticyclonic and cyclonic rings separated
fmn the Imp Current appear as warm salty bodies which advect into the Mexican Current, balancing the s a l t and heat budget of the western Gulf (Elliot, 1982). Fronts associated to surfacing of cooler waters with high levels of phytoplankton pignent and the path of the warm Lmp Current, with low concentration of phytoplankton pigment were observed by Yentsch (1984) from Coastal Zone Color Scanner (CZCS) images of the west Florida coast. I n the Bay of Campeche, a high biological productivity has been observed (Licea-
N a n , 1977) as w e l l as on the west Florida continental shelf. Using E a r t h &sources Technology Satellite (ERTS) and shipboard measurments, Maul (1975) depicted the Lcop Current and denonstrated that, the cyclonic burdaries are associated with fronts and charges i n chlorophyll-a concentration a d temperature.
U s i n g the above consideration, the pycmcline m l y , and the cyclonic
and anticyclonic eddy boudaries, w i l l be related to the behavior of the major frontal zones i n the
mlf
of Mexico.
DISUJSSION The role of the Lcop Current and the detached eddies i n the G u l f of Mexico fmntqenesis can be inferred fran the numerical model developed by Monreal&ez
and Salas de A n (1985). I n t h i s model, the driving mechanism i s a j e t
through the Yucatan Channel and the Florida S t r a t . The Lmp Current generated only w i t h the Yucatan and Florida j e t show lenses coinciding with the observed
296
Fig. la. : Results of the l i n e a r b a r c c l i n i c model. P y c m c l i n e height a n m l y i n m.
297
Fig. lb. : Results of the l i n e a r baroclinic model. Fycnocline height a n m l y i n m.
298
thermal structures (Fig. 1 a, b ) .
Assuming t h a t phytoplankton patches r e f l e c t the density o r thermal f i e l d (Yentsch, 1984), t h e influence of eddies i n the spatial patterns of grmth can be explained i n terms of what can be observed i n Fig. 2. The anticyclonic eddies with a diving pycnocline are associated to warm rings i n which l i g h t e r waters are located a t the center and heavier waters a t t h e external border of
the eddies. The Loop Current, t h e anticyclonic and t h e cyclonic eddies repres e n t d i f f e r e n t w a t e r masses, t h e last one colder than the others. The boundary between the Lmp Current and the anticyclonic ad. the cyclonic eddies w i l l be an a c t i v e f r o n t a l zone. The anticycloniceddies and the k o p Current located i n the middle of the Gulf of Mexico w i l l m r k as a natural bourdary f o r the phytoplanktonic growth. On the other hard, w i t h a cyclonic c i r c u l a t i o n i n the west of Florida, the Texas-Louissiana s h e l f , and the Bay of Campeche, one w i l l expect high concentration of phytoplankton.
---
Fig. 2. : Geostrophic relationships i n the Loop Current arad cyclonic eddies. The boundary of the cyclonic eddies w i t h t h e Loop Current is a zone of frontcgenesis. One can imagine three d i f f e r e n t mechanisms interacting between the Loop
Current and the cyclonic eddies. The f i r s t one c o r r e s p n d s to a divergence zone i n which t h e laver layer i s surfacing. The second one represents a convergence zone produced by the interaction of the cyclonic transport i n t o the Loop Current when the d e n s i t i e s are almost similar. The last mechanism i s similar to t h e second one, but i n this case the density of the Loop Current
w a t e r s are markedly l i g h t e r than eddy waters. H e r e the Loop Current waters w i l l be pushed up, and f r o n t a l conditions w i l l r e s u l t (Fig. 3 ) .
299
Fig. 3 . : Interacting mechanisms between the Lcop Current and t h e cyclonic eddies. The f i r s t one correspond to a divergence zone and the secord ard t h i r d ones to a convergence zone. By ccmparing transport predicted by the &el
and d i s t r i b u t i o n of m e a n
concentration of organisms observed in the Bay of Campche by Licea-Duran (1977), one can see high concentrations in the northeast Bay of Campxhe f o r
the m n t h of September coinciding with a w e l l defined
cyclonic eddy and a high
transport i n t h e Yucatan Channel. This e f f e c t can be explained i n terms of mechanism number 3 showed i n Fig. 3 . Finally associating Icop Current and cyclonic eddies, one can identify the s d - p e m e n t f r o n t a l zones i n the area. Fig. 4 gives an a m p l e f o r the month of October. coNculs1oN
Despite the simplifying hypothesis of the M n r e a l a z and Salas de Leon (1985) model, many features of the Gulf of Mexico fronts can be i n f e r r d fran
the limp Current dynmics. I n p a r t i c u l a r t h e strong anticyclonic eddy i n the middle of the Gulf appears as a natural bax-dary f o r the phytoplanktonic growth
Areas of possible f r o n t a l a c t i v i t y can be deduced i n zones of convergence and divergence between the Icop Current and the cyclonic eddies.
300
Fig. 4. : Semi-permanent front induced by the Loop Current. Numbers 1 , 2 and 3 represent the mechanisms shmed in Fig. 3. Numbers 4 represent a sensible area to frontal activity induced by a cyclonic or anticyclonic eddy and a closed boundary. REFERENCES Elliot, B.A., 1982. Anticyclonic rings in the Gulf of Mexico. J. Phys. Oceanogr. 12 : 1292-1309. Ichiye, T., 1962. Circulation and water mass distribution in the Gulf of Mexico. Geofis. Int. Mex., 2 : 47-76. Licea-Duran, S., 1977. Variacion estacional del fitoplancton de la Bahia de Campeche, Mexico (1971-1972). FA0 Fish. Report, 200 : 253-273. Maul, G.A., 1975. An evaluation of the use of the earth resources thecnology satellite for observing Ocean current boundaries in the Gulf Stream. NOAA Technical Report E m . 335-AOML 18, 125 pp. Monreal-Gomez, M.A. and D.A. Salas de Leon, 1985. Barotropic and baroclinic modes in the Gulf of Mexico. (To be published) On Proc. Symp. on Oceanology, Bruxelles, 10 pp. Yentsch, C.S., 1984. Satellite representation of features of Ocean circulation indicated by CZCS colorimetry. In J.C.J. Nihoul (Editor), Remote Sensing of Shelf Sea Hydrcdynamics. Elsevier Oceanography Series, 38. Amsterdam, pp. 337-354.
301
PRELIMINARY STUDY OF A FRONT IN T H E BAY OF CAMPECHE, MEXICO S.P.R. Czitrom, F. Ruiz, M.A. Alatorre and A.R. Padilla Instituto de Ciencias del Mar y Limnologia, Apartado Postal 7 0 - 3 0 5 , 0 4 5 1 0 MCxico D.F.,
Ciudad Universitaria, U.N.A.M.,
MEXICO.
ABSTRACT Data collected during three surveys of the Bay of Campeche are used to describe the thermohaline field at different times of the year. During one of these surveys, a river discharge induced front and associated stratification were observed. A one dimensional model of vertical mixing i s used to analyse the effect of wind on the frontal stratification. It i s likely that layering will survive a typical storm for the area while the front appears to have greater persistence during windy weather than the associated stratification. Results from a computer model of the residual circulation suggest that attention must be payed t o the general circulation of the Gulf of Mexico i n the design of future data collection and in its interpretation. INTRODUCTION The Bay of Campeche in the Gulf of Mexico much
human
economic
activity
is
carried
is an area where
out.
Fishing
grounds
are highly exploited, substantial quantities of industrial wastes are dumped into the rivers that discharge in the area a n d , more recently, offshore oil exploration and scale has economy.
played
an
increasing
The physical
therefore
be
studied
part
processes to
exploitation on a in
financing
occurring
provide
the
in
the
grand
Mexican
these waters must
framework
within
which
a
rational exploitation of the area can be carried out. In this context, the study of fronts - which may be defined as
the boundaries
chemical relevance.
or
biological
It has
congregate near other
authors
between been
masses
characteristics known for many
fronts in (see
water
the
Murray,
of
different
-
acquires
a
special
years that fish tend
seas of Japan
1975;
physical,
Klemas,
(Uda, 1 9 3 8 )
1980)
have
to
while
observed
that the size and distribution of oil slicks and other contaminants can be strongly affected by the presen'ce of fronts. Sightings of a sharp boundary between river discharge and oceanic waters along which bands of floating debris accumulate
302
have been made in the Bay of Campeche (I. Emilsson, L . personal the
communication).
Mexican
oilwell
This
shoreline
blowout
in
from
1979.
front may greater
The
have helped
damage
results
during
presented
in
Lizarraga, to
protect
the
Ixtoc-1
this
paper
are a preliminary study of fronts in the Bay of Campeche a s part of a longer term effort to understand their dynamics.
THE DATA Three surveys of the area shown in fig. in September 1977, June Niskin
bottles fitted
1978 and April 1984.
with
reversing
1 were carried out In the first two,
thermometers were
used
to
take samples for the determination of salinity and density. In the third survey, a Neil Brown CTD system gave readings of conductivity and temperature in the water column a t the stations marked with
dots
in
fig.
1.
Also,
continuous measurements of
surface
salinity and temperature were made while the ship steamed between stations. A satellite positioning system w a s used f o r navigation.
0
9s
92'
20
9
b
20°
199
93-
9 2-
9
D
Figure 1 . Bathymetry of the area of study based on soundings taken during the April 1984 survey. The transect refe'rred to in the text i s marked with a heavy line a d the station positions with dots. T h e rivers Grijalva, Usumacinta and San Pedro y San Pablo are marked G R , US and PyP respectively.
303
Figure 2 shows contour maps of surface salinity, temperature and sigma-t during the CTD survey of April 1984. A marked front characterized by intensified gradients can be seen in the surface density field which i s clearly the result of a frontal structure in the distribution of salinity. No frontal structure w a s apparent in
the
miles
temperature
field.
from the coast and
The
front
was
25 nautical
observed
ran approximately parallel
to
it for
at least 70 n m , turning landwards off the western inlet to the Terminos Lagoon. The band of strongest surface gradients had a width of about 5 nm within which a change of 1 salinity unit and
1 sigma-t
unit occurred.
this band
T h e location o f
between
stations was pinpointed using the continuous surface measurements of salinity and temperature. Contours of perpendicular
salinity, temperature and
sigma-t
a
on
section
to the coast which i s typical of the western half
3. T h e section i s marked with of vertical stratification can be distinguished. O n e i s associated to the front and lies in shallow water to the south of it. The other o n e , apparently of the grid
a
heavy
can be seen i n fig.
line
in
fig.
1.
Two
areas
not related to the front, lies below 30 m this study
we
have
focused
our
and to the north.
interest in
upper 3 0
the
m
In of
water where the front and associated stratification are confined. In the eastern half of the grid, the water column exhibited little or no layering. The strength of stratification for a surface layer 30 m thick was estimated by computing
@=
0
where
l / h I - h ( P - p ) g z dz
P
llh
1 opd, p
h
-h Simpson et al., 1977) for all CTD casts. Here I.s density, is depth and z is the vertical coordinate positive upwards.
@
can
=
(see
be
interpreted
vertically =
mix
a
(Czitrom,
@ s / @
as
the
stratified
amount column
1982) was
also
of of
work
per
water.
computed
m3
The for
needed
to
parameter
Rs
all
stations.
Here,
0s
0
l/hJ-h
=
and T and be
( P (T,S)
interpreted
as
the
to
vertically
Thus
is
the
haline
F(T,S))gz
dz
T
where
=
l/h
J-:
S are temperature and salinity respectively.
required Rs
-
amount mix
proportion
stratification.
of
only of
work
per
the .observed
density
m’
that that
(?s
would
salinity
layering
T dz can be
profile.
is due
to
3 04
93"
920 I
91' 20°
I!
I
I
I
I'
I!
F i g u r e 2. I s o l i n e s o f a ) S a l i n i t y , b ) T e m p e r a t u r e and'c) S i g m a - t at t h e s u r f a c e i n t h e a r e a d u r i n g t h e A p r i l 1984 s u r v e y . A d e n s i t y front is clearly the result of the salinity distribution.
305
F i g u r e 3. I s o l i n e s o f a ) S a l i n i t y , b ) T e m p e r a t u r e a n d c ) S i g m a - t o n t h e t r a n s e c t m a r k e d o n f i g . 1.
3 06
Figure
4
shows
the
distribution of
@ for a surface layer
3 0 m thick during the survey of April 1 9 8 4 . Most of the bay exhi-
bited little or no layering except for an area immediately landward (
of
0> 5
the
front
(compare
with
nearly
80% of
Joules/m3 ) ,
to the salinity
profile
(Rs
=
0.79
fig. the f
2).
At
these
stations
stratification w a s
0.09)
which points to the
river discharge a s the main stratifying agent. At stations marked "*" in fig. 4 , stratification below m
was
deep).
nearly
all thermal ( R s
those marked
At
f
0.06 f o r
" o " , significant salinity
30
a layer 1 0 0 m layering worked
-0.39 f 0 . 1 2 for a layer 1 0 0 Overturning of the water column a t these stations was
against vertical m deep).
0.00
=
due
stability
(Rs
=
prevented b y the temperature profile.
I'
. @I \
I
0
@
2u
i. \
I
I'
a
computed for a layer 3 0 m deep. 4 . Distribution of 1s the amount of work per m needed to vertically mix the water A t stations where @ > 5 Joules/m3 , R s = 0.79 f 0 . 0 9 .
~~~~~. Rs
is the proportion of denslty stratification that is due to salinity layering. Stations marked "*" and " o " had va'lues of R s o f 0.00 f 0.06 arid - 0 . 3 9 f 0 . 1 2 respectively for a layer 100 m deep.
307
During the September 1977 and June 1978 surveys of the area, the thermohaline structure was entirely different to that described above. In 1977, no haline front and associated stratification was present. Further offshore ( 4 0 nm), the thermocline struck the surface forming a strong thermal front with associated stratification seaward of it. In June 1978 there was some indication of a haline front displaced westwards about 60 nm o f where it was observed in April 1984. DISCUSSION The above observations show that the haline front and associated stratification are not permanent features in the shallow waters of the Bay of Campeche. Rough estimates of the persistence of stratification during storms o f various intensities can be made using a model .given by Simpson et al. (1978) which describes the time variation of
Q) for the Irish Sea. This model considers the changes in stratification induced at a column of water by surface heat input, wind stirring and tidal mixing. If we assume that the wind mixing constants given b y
these authors apply to the Bay of Campeche,
we can estimate the rate of change of
a) due
to this agent.
where is the wind mixing efficiency coefficient ( 0 . 0 2 3 ) , 3 fAis the density of air (1.216 kg/m ) , Ks is the surface drag coefficient (0.00215)
x
8 and 6 is
the ratio of the wind induced
surface current to wind speed ( 0 . 0 3 ) . 1W13 is the time average of the wind speed cubed and h is depth. During a typical storm in the area, 1 W I 3 is likely to be of the order of 600 m 3/sec3 as
-
computed using wind speed and direction data taken during a storm in December 1982 (unpublished data). Assuming h is 3 0 m , eq. -3 1 yields a value of 3 Joules m day -1 Since the observed values
8
.
-3
of associated to the front were of about 3 0 Joules m , it would take such a storm a minimum of 10 days to destroy layering. Typical storms in this area last between 3 and 5 days s o that stratification associated to a front may be expected to survive such a storm. During a hurricane, however, l W I 3 may be expected t o reach 7000 m3/sec3 a s computed using data published by Martin ( 1 9 8 2 ) s o that eq. 1 yields 36 Joules m F 3 day-'. Such a hurricane could destroy the stratification associated to a front in less than one day.
308 The destruction of stratification however does not necessarily imply the dissappearance of t h e front. This may be inferred from of a frontal structure in fig. 5 , which shows the
the presence
distribution of depth averaged density during the survey of April 1984.
Further work would
information
on
the
time
have to be carried out to obtain more scales of
evolution
of
the
front
and
associated phenomenae. 0
E 28
92O
93O I
I
I 9'
Figure 5 . Distribution of depth averaged density in the area during the April 1 9 8 4 survey. The presence of strong gradients in this figure indicates that the front may still be present even if the water column were vertically mixed b y a storm (compare with fig. 2.c).
It would
appear
that
the haline
front
is a
consequence of
discharge into the Bay of Campeche by the various rivers in the area. Fig. 6 shows averages over more than 10 years of mean monthly discharges into the bay for some of these rivers (S.R.H., 1969;
haline
S.R.H.,
1976).
It i s interesting
front was clearly
present
to note
i n April o f
that while
1 9 8 4 at
the
the time
of lowest discharge, it was not apparent i n the area in September 1 9 7 7 when rivers were probably at their highest level. One of the aims of future work in the area would be to study the mecha-
309
nisms
of
frontogenesis
and
the
relationship,
if
any,
between
the amount of river discharge and its evolution.
9 m3
xI0 10
9 8
7 6
I
\US"MACINTA
I
5
4 3
2
I IAN IFEB h AR IAPR h A Y 'JUN IJUL 'AUG 'SEP b C T 'NOV lDEC Figure 6. Mean monthly discharges averaged years for rivers in the area of study.
over
'
more
than
10
The rivers Papaloapan and Coatzacoalcos, some 150 nm to the west of the area of study, discharge comparable volumes of water into
the
Gulf
of
Mexico
(see
6).
fig.
It
remains
to
be
seen
whether these rivers also generate fronts and, if s o , whether they form part of a continuous feature extending from the ,Terminos Lagoon or if they form separate frontal systems. At other fronts of similar spatial scales, the density structure
induces
a
circulation
at
the -surface which
converges
on
the front line where foam and other floating objects are trapped (see
Garvine,
1974).
Whether
such
a
circulation i s present
at
310
the
front
in the Bay
of Campeche has more than a academic in-
terest only. This information would be valuable to decision makers during an emergency during the
if indeed the floating hydrocarbons spilled
the Ixtoc-1 oilwell by
coast
the
modellers would
blowout were inhibited from reaching
circulation
associated
to
a
front.
Computer
have to make use of this information in the pre-
diction of the behaviour o f future oil spills. Being
such a
shallow area which
is not
sheltered
from the
deep ocean in the Gulf of Mexico, it i s likely that the evolution of
the front, and
in general the
Bay of Campeche, i s highly
thermohaline structure in the
vulnerable
to physical processes oc-
curring at the surface and at the landward and seaward boundaries. Evidence Mexico
that
the changing residual circulation in the Gulf of
throughout
the
year
may
influence
the evolution of
the
front is provided b y a computer model of the residual circulation driven by wind and b y the Yucatan current (Salas & Monreal, this volume).
The
circulation patterns
predicted
by
this model
show
strong currents along the coast of the Bay of Campeche in September
and
weak
ones
in April.
These results would
be
consistent
with the observations presented in this study if the river fresh water were swept away b y the strong residual currents in September despite the high level of discharge while being allowed to induce a
well
are
developed
weak
and
frontal
river
structure
discharge
careful consideration must the
design
of
is
in
low.
April This
when
the
example
currents
shows
that
be given to this type of process in
future data
collection
for
the
study
of
fronts
in the Bay of Campeche while highlighting the potential usefulness of
computer models
data
acquisition
for
could
the
interpretation
also
of
these data.
assist .computer simulators
in
This the
calibration of their models. ACKNOWLEDGEMENTS We
wish
to
thank
the
members
of
CINVESTAV,
MCrida.
for
allowing us to share their ship time. This research w a s sponsored by the Consejo Nacional d e Ciencia y Tecnologia (CONACYT),
MCxico
with grant No. PCMABNA-005112. Contribution No.408 of the y Limnologia, U.N.A.M., Mbxico.
Instituto
de
Ciencias
del
Mar
311
REFERENCES Czitrom, S.P.R., 1 9 8 2 . Density Stratification and a n Associated Front in Liverpool Bay. Ph.D. Thesis, University College of North Wales, Bangor, U.K. Garvine, R.W., 1 9 7 4 . Dynamics of Small-Scale Oceanic Fronts. Journal of Physical Oceanography, Vol. 4 , No. 4 , pp. 5 5 7 569.
Klemas, V., 1 9 8 0 . Remote sensing of coastal fronts and their effects on oil dispersion. International Journal o f Remote Sensing, Vol. 1 , No. 1 , pp. 1 1 - 2 8 . Martin, J., 1 9 8 2 . Mixed-layer simulation of buoy observations taken during hurricane "Eloise". Journal o f Geophysical Research, 8 7 C , pp. 4 0 9 - 4 2 7 . Murray, S.P., 1 9 7 5 . Wind and Current Effects on Large-Scale Oil slicks. Proceedings of the Seventh Annual Offshore Technology Conference, Houston, Texas, May 5 - 8 , 1 9 7 5 . p p . 5 2 3 - 5 3 3 . S.R.H., 1 9 6 9 . Boletin Hidrolbgico No. 3 8 de la Secretaria de Recursos HidrAulicos, Mexico. S.R.H., 1 9 7 6 . Atlas del Agua de la Rephblica Mexicana, Secretaria de Recursos HidrAulicos, Mexico. Simpson, J.H., D.H. Hughes and N.G.C. Morris, 1 9 7 7 . The Relation of seasonal Stratification to Tidal Mixing on the Continental Shelf. In: M. Angel (Ed.), A Voyage of Discovery. Pergamon Press, Oxford, pp. 3 2 7 - 3 4 0 . Simpson, J.H., C.M. Allen and N.G.C. Morris, 1 9 7 8 . Fronts on the Continental Shelf. Journal of Geophysical Research, Vol. 8 3 , NO. C 9 , pp. 4 6 0 7 - 4 6 1 4 . Uda, M., 1 9 3 8 . Researches o n "siome" or current rip in the seas and oceans. Geophysical Magazine, Vol. 1 1 , No. 4 , p p . 3 0 7 372.
This Page Intentionally Left Blank
313
THE
INTERACTION OF PHYSICAL AND BIOLOGICAL PROCESSES I N A MODEL OF THE VERTICAL
DISTRIBUTION OF PHYTOPLANKTON UNDER STRATIFICATION TAYLOR, J . R . W .
A.H.
HARRIS a n d J . A I K E N
I n s t i t u t e f o r Marine E n v i r o n m e n t a l R e s e a r c h P r o s p e c t P l a c e , The Hoe Plymouth, PL1 3 D H , UK.
ABSTRACT A t i m e - d e p e n d e n t a d v e c t i o n - d i f f u s i o n model of t h e v e r t i c a l d i s t r i b u t i o n of p h y t o p l a n k t o n and a s i n g l e n u t r i e n t i n a s t r a t i f i e d hydrodynamic r e g i m e is described. I t s p r o p e r t i e s u n d e r summer c o n d i t i o n s t y p i c a l o f a t e m p e r a t e s h e l f s e a or a d e e p o c e a n s i t u a t i o n are a n a l y s e d . I n t h e s h e l f c a s e t h e system t e n d s r a p i d l y to a s t e a d y s t a t e with a t h e r m o c l i n e peak o f p h y t o p l a n k t o n . The model a l l o w s f o r p a r t i a l n u t r i e n t r e c y c l i n g ; t h e e f f e c t of t h i s is found t o h a v e a g e n e r a l l y a n t i s y m m e t r i c Increasing recycling r e l a t i o n t o t h a t o f n u t r i e n t u p t a k e by p h y t o p l a n k t o n . i n c r e a s e s s t e a d y s t a t e p h y t o p l a n k t o n c o n c e n t r a t i o n s b u t h a s n e g l i g i b l e e f f e c t on nutrient, increasing uptake reduces steady-state n u t r i e n t but leaves phytoplankton unaltered. A s e i t h e r n u t r i e n t u p t a k e or r e c y c l i n g is r e d u c e d a to oscillate appears. E x p l i c i t r e l a t i o n s for steady state tendency c o n c e n t r a t i o n s and c o n d i t i o n s for o s c i l l a t i o n s are d e r i v e d for a s i m p l i f i e d model o f p h y t o p l a n k t o n o v e r a n i n f i n i t e n u t r i e n t p o o l . I n c r e a s e d i n c i d e n t l i g h t is found t o r e d u c e s u r f a c e p h y t o p l a n k t o n , whose n u t r i e n t s u p p l y is pre-empted by i n c r e a s e d g r o w t h i n t h e t h e r m o c l i n e . This e f f e c t is d e m o n s t r a t e d i n a s i m p l e t w o - l a y e r model, and t h e d e t e r m i n a n t s o f v e r t i c a l d i s t r i b u t i o n d i s c u s s e d . The d i u r n a l l i g h t c y c l e is shown t o be e q u i v a l e n t t o a r e d u c t i o n i n mean l i g h t i n t e n s i t y . Day-night l i g h t v a r i a t i o n is o n l y weakly r e f l e c t e d i n t h e p h y t o p l a n k t o n b u t p r o d u c e s a r o u n d 25% n u t r i e n t v a r i a t i o n D i u r n a l v a r i a t i o n i n t u r b u l e n c e d u e t o r e d u c e d day time i n t h e mixed l a y e r . c o n v e c t i o n has n e g l i g i b l e e f f e c t o n s t e a d y s t a t e mean l e v e l s , b u t somewhat enhances t h i s day-night n u t r i e n t v a r i a t i o n .
INTRODUCTION
abundance
The
sinking,
of p h y t o p l a n k t o n a t a n y d e p t h c h a n g e s i n r e s p o n s e t o
s u c h as l a t e r a l a d v e c t i o n . phytoplankton since
growth,
d i f f u s i o n , n u t r i e n t l i m i t a t i o n and g r a z i n g , a s w e l l a s o t h e r p r o c e s s e s was
The e a r l i e s t a t t e m p t t o model v e r t i c a l p r o f i l e s
t h a t of R i l e y
g g.
( 1 9 4 9 ) i n t h e Western North
of
Atlantic,
w h i c h , ' n u m e r i c a l m o d e l s w i t h i n c r e a s i n g d e g r e e s of c o m p l e x i t y h a v e
been
c o n s t r u c t e d , some o f t h e s e b e i n g d i s c u s s e d and compared by P a t t e n ( 1 9 6 8 ) , S t e e l e a n d Henderson ( 1 9 7 6 ) , and Fasham sented model
by in
g g.
P l a t t , Denman and J a s s b y ( 1 9 7 7 1 , S t e e l e and M u l l i n ( 1 9 7 7 )
(1983).
Current l i m i t s t o t h i s development a r e perhaps repre-
t h e work of Woods and Onken (1982) who h a v e d e v e l o p e d which a l a r g e number of i n d i v i d u a l p h y t o p l a n k t e r s a r e
d i u r n a l l y varying,
v e r t i c a l l y s t r a t i f i e d sea;
and P a c e
g g.
a
Lagrangian
tracked
in
a
( 1 9 8 4 ) who h a v e
314 produced a continental shelf food web without stratification that is composed of
17 state variables. Such models
may describe or predict phytoplankton changes, but their
very
complexity may impede understanding of the way biological and physical processes interact
in
stratified systems, a perception which motivates
the
continuing
theoretical analyses of simple systems. Criminale and Winter (1974) examined the dynamical Riley
behaviour of small perturbations from equilibrium in
the model of
S &. (1949), and Criminale (1980) has recently extended this analysis to
include a current and a current-shear.
These studies investigated the vertical
distribution of a single phytoplankton species without considering the dynamics of nutrients, grazers or other phytoplankton species. Roughgarden (1978) and Tilman
Peterson (1975). son
Following earlier work by
S & (19781, Powell and Richer-
(1985) have examined the competition between two species of
for two nutrients using simple models based on
phytoplankton
Michaelis-Menten
expressions.
Their models included nutrient recycling (without any nutrient l o s s ) but did not consider vertical variation. extremes
phytoplankton and Jamart %
The present paper is situated between these two a
that it treats a one dimensional stratified system with
in
&.
The model used is similar to
a single nutrient.
(1979) and Fasham
&. (1983).
single
those of
However, in the present case,
the model has an open bottom so that cells can sink and be returned as nutrient, nutrient recycling may be imperfect, and daily variation of convectional mixing (Woods, 1980; Woods and Onken. 1982) is included. An understanding of the roles played In
by recycling and diurnal changes are objectives of
particular,
this
investigation.
the daily variation in the turbulence of the mixed layer is a
factor that has not been studied in simple models. We concentrate on the mid-summer situation for a stably stratified shelf sea, typified
by
thermocline mixed
a
shallow
(251111mixed
surface layer, and
(temperature gradient up to 1'C.m-';
bottom layer (typical depth loom).
a
5m thick),
strong, narrow
above a
tidally
These conditions are typical of
the
central Celtic Sea in mid-summer, where the upper mixed layer characteristically exhibits nutrient depletion and a peak concentration of phytoplankton
(z. 3 mg
m-3) occurs in the thermocline (Joint and Pomroy, 1984. Aiken 1985).
Such areas
have
laterally homogenous vertical structures of temperature and
chlorophyll
concentration over distances of 50km (Aiken, 1985), it might be inferred that a quasi-steady state balance exists between the biological and physical processes. In
the model there is no attempt to simulate the development of
cycle
nutrient system. dynamically. typical of mixed
the
seasonal
but attention is focused on the dynamics of the coupled phytoplankton and With this restriction it is unnecessary to treat the grazers
By way of a contrast to the shelf case, we consider a situation, the summer conditions in a temperate ocean,
in which
layer overlies a broad deep thermocline (eg Williams and
the surface
Hopkins.
1975,
315 1 9 7 6 ) . Our model c o n t a i n s s e v e r a l o f t h e p r o c e s s e s c o n s i d e r e d t o b e i m p o r t a n t i n
more
the
analysis how
b u t is s u f f i c i e n t l y s i m p l e
r e a l i s t i c s i m u l a t i o n of t h e s y s t e m ,
a l l o w s a n a s s e s s m e n t of t h e i r r e l e v a n c e t o its o v e r a l l
t h i s depends on t h e p a r t i c u l a r parameter v a l u e s used.
s t r u c t u r e a n d g e n e r a l b e h a v i o u r of t h e model,
that
dynamics,
and
After o u t l i n i n g
the
t h e s e n s i t i v i t y to its parameters
is e x p l o r e d by b o t h n u m e r i c a l e x p e r i m e n t and t h e o r e t i c a l a n a l y s i s .
MODEL model d e s c r i b e s t h e n u t r i e n t - d e p e n d e n t g r o w t h of a s i n g l e
The
phytoplankton
species,
a l l o w i n g f o r s i n k i n g and v a r i a b l e v e r t i c a l m i x i n g . The s p e c i f i c growth
rate
the
of
p h y t o p l a n k t o n is t a k e n t o b e t h e p r o d u c t
two
of
components,
a
d e p e n d e n c e ( a ) on i n c i d e n t l i g h t (I) and a f u n c t i o n ( @ ) of t h e c o n c e n t r a t i o n ( N )
of
a single nutrient.
The
phytoplankton
direct
respiration,
Michaelis-Menten
subject to a specific loss r a t e (m),
which
regenerates the nutrient
due t o an
with
relation.
or
death
efficiency,
E.
p r o f i l e s of p h y t o p l a n k t o n and n u t r i e n t s a r e c a l c u l a t e d a s f u n c t i o n s o f
Vertical depth
The form u s e d f o r C$ is t h e
are
z
and
time
t.
Phytoplankton
c e l l s sink
at
a
rate
(v)
within
a
hydrodynamic r e g i m e d e f i n e d by a s p a t i a l l y ( a n d on o c c a s i o n t e m p o r a l l y ) v a r i a b l e turbulent
diffusion
coefficient,
K(z,t).
The
changes
in
phytoplankton
c o n c e n t r a t i o n ( P ) and n u t r i e n t c o n c e n t r a t i o n a r e t h e n :
Boundary model.
c o n d i t i o n s a t t h e s u r f a c e and b o t t o m a r e a l s o needed t o c o n s t r a i n
the
A s t h e r e c a n be no f l u x a c r o s s t h e s e a - s u r f a c e :
K(z,t)nP/Dz
= VP
and
K(z,t);iN/>z
must h o l d a t t h e s u r f a c e .
=
0
(2.3)
A t t h e b o t t o m P ( z , t ) and N ( z , t ) a r e a s s i g n e d c o n s t a n t
v a l u e s , s i n c e t h i s allows p h y t o p l a n k t o n t o s e d i m e n t t o t h e sea-bed and n u t r i e n t s to
r e t u r n t o t h e w a t e r column from below.
phytoplankton
However,
i f a t a n y time a flow
up from t h e b o t t o m s h o u l d o c c u r t h e f l u x of p h y t o p l a n k t o n a t
of
the
sea-bed is s e t t o z e r o . S p e c i f i c Growth R a t e The light
s p e c i f i c growth r a t e f o r e x c e s s n u t r i e n t , i n t e n s i t y I a t a n y d e p t h by t h e ' t a n h '
a(I),
was o b t a i n e d from
the
f o r m u l a o f J a s s b y and P l a t t ( 1 9 7 6 )
and P l a t t and J a s s b y ( 1 9 7 6 ) which t a k e s a c c o u n t of p h o t o s a t u r a t i o n a t h i g h l i g h t intensities:
316 a(I)
=
a. P m t a n h (a’I/Pm)
(2.4)
The l i g h t i n t e n s i t y , I , is t h e i r r a d i a n c e (Wm-2) i n t h e p h o t o s y n t h e t i c a l l y a c t i v e r e g i o n (400-700 nm), a’ is t h e i n i t i a l s l o p e o f t h e l i g h t s a t u r a t i o n curve
( 0 . 1 2 mg C (mg
mg C (mg
Chi)-'
h-l ) .
d a y s (a(I) = 0.036 h-’) The
surface
approximated
light
Chl1-l
and pm is t h e a s s i m i l a t i o n number
hdl W-’m2)
The c o n s t a n t a.
(2.5
was chosen t o g i v e a d o u b l i n g time o f 0.8
when I is 170 Wm-2. i n t e n s i t y was e i t h e r k e p t c o n s t a n t a t 170 Wm-2
or
on t h e p o s i t i v e h a l f o f a d i u r n a l s i n e wave o f a m p l i t u d e 534
which h a s t h e same d a i l y a v e r a g e .
The l i g h t i r r a d i a n c e p r o f i l e was
else Wm-2
determined
from
with the attenuation coefficient,
t i o n ( P i n mg ( C h l m-3)
A
K,
b e i n g dependent on p h y t o p l a n k t o n c o n c e n t r a -
and g i v e n by:
c a r b o n t o c h l o r o p h y l l r a t i o o f 40 was used s o t h a t ,
be n i t r o g e n , t h e c o n v e r s i o n f a c t o r Y is 0.503 mM N (mg
assuming t h e n u t r i e n t t o
Chi)-'.
e x t e n t o f r e d u c t i o n i n growth r a t e by n u t r i e n t l i m i t a t i o n is
The
determined
by a Michaelis-Menten r e l a t i o n :
$(N)
=
(2.7)
N/(N+v) 0 . 2 and 0.8 m M N m-3
Values of 0.05,
were used f o r t h e h a l f - s a t u r a t i o n
c o n s t a n t , v, independent o f d e p t h (MacIsaac and Dugdale, 1 9 6 9 ) . Loss, R e c y c l i n g and S i n k i n g The s p e c i f i c l o s s r a t e ,
m,
i n c l u d e s t h e m o r t a l i t y o f c e l l s due t o d e a t h and
g r a z i n g and t h e l o s s due t o r e s p i r a t i o n .
Two c o n s t a n t v a l u e s a r e used f o r
specific
m o r t a l i t y 0.05 o r 0.2 x
and t h e r e s p i r a t i o n r a t e was t a k e n t o
be 0.04 x
s-l ( i e 0.043,
l i n g efficiency, this
E,
s-l
0.173 and 0.035 d a y - l ,
the
r e s p e c t i v e l y ) . The recyc-
a l l o w s f o r t h e p o s s i b i l i t y t h a t n o t a l l o f t h e n u t r i e n t from
l o s s o f c e l l s is r e t u r n e d t o s o l u t i o n .
used f o r t h e f r a c t i o n r e t u r n e d :
E = 0
Three c o n s t a n t v a l u e s
(no recycling),
f =
0.2 o r
E =
have
been
1 (perfect
recycling). Although t h e s i n k i n g s p e e d o f p h y t o p l a n k t o n c e l l s depends on s p e c i e s , time of year,
ambient n u t r i e n t c o n c e n t r a t i o n e t c . . s u c h v a r i a t i o n s i n s i n k i n g speed a r e
p r o b a b l y n o t l a r g e i n t h e i r e f f e c t i n most cases and a c o n s t a n t v a l u e o f
cm
s-1 (0.86 m day-’ ) was used t h r o u g h o u t .
Turbulent Diffusion The
surface
mixed l a y e r was t a k e n t o b e 25m deep and i n t h e a b s e n c e of
di-
317 u r n a l changes t h e t u r b u l e n t d i f f u s i o n c o e f f i c i e n t ,
K , was a c o n s t a n t 80 cm2 s-l
c.(1983)
for
an
a r e a o f t h e C e l t i c Sea when t h e t h e r m o c l i n e was becoming e s t a b l i s h e d b u t
is
in
agreement w i t h v a l u e s r e p o r t e d e l s e w h e r e
within it.
Sundby,
T h i s is h i g h e r t h a n t h e v a l u e s computed by Fasham
James,
1971;
1977;
However, t u r b u l e n t mixing i n t h e s u r f a c e l a y e r f o l l o w s a d a i l y
1983).
variation
(Kullenberg,
(Woods,
1980,
1985) w h i l e mixing i n t h e immediate v i c i n i t y
of
the
s u r f a c e is by wind and wave a c t i o n and is c o n t i n u o u s ,
below t h i s t h e c o n v e c t i o n
which
To e x p l o r e t h e e f f e c t
o c c u r s a t n i g h t is t h e main s o u r c e o f mixing.
layer in
of
i n some i n s t a n c e s K was dropped t o 1 cm2s-' i n t h e lower h a l f o f t h e mixed
this,
d u r i n g t h e day.
Below t h e mixed l a y e r no d i u r n a l v a r i a t i o n was i n c l u d e d
the turbulent diffusion coefficient.
a
used:
I n t h i s r e g i o n two p r o f i l e s o f K were
' s h e l f ' case when a v a l u e o f 0.1 cm2 s-l
was used between 25 and 30
and
a v a l u e o f 100 cm2 s-l was used i n a t i d a l l y mixed l a y e r between 30
the
bottom,
a n ' o c e a n i c ' c a s e where K was s e t a t 1 em2 s-l
and
below t h e mixed l a y e r .
at a l l
m
m
and
depths
These c a s e s a r e i n g e n e r a l agreement w i t h estimates
of
0.05 t o 1 em2 s-l f o r t h e t h e r m o c l i n e (James, 1977; King and Devol, 1979; J a s s b y
and
Powell,
1975)
and o f l o 2 t o l o 3 cm2 s-l from bottom t i d a l mixing
(James,
1977 or u s i n g e q u a t i o n 4 o f P i n g r e e and G r i f f i t h s , 1 9 7 7 ) . Numerical S o l u t i o n I f t h e v a l u e s o f P ( z , t ) and N ( z , t ) a r e d e f i n e d on a v e r t i c a l g r i d o f J p o i n t s from j
=
1,2,
.....,J
w i t h s p a c i n g Az,
e q u a t i o n s ( 2 . 1 ) and ( 2 . 2 ) can b e a p p r o x i -
mated by t h e f i n i t e d i f f e r e n c e e q u a t i o n s : (N?+l-
Nn)/At J
=
(P;"-
Pn)/At J
= an
-Y an J
J
+
Nn'l J
Pn/(Nn J J
+
v ) + cYmPn
J
Nn+l Pn / ( N Y + v ) -m P i J J
[KY++
(P:+l
-
v (Pn J
- Pn) - KY-L (Pn J 2 J
-
-
Pn J-1
)/A2
P y - l ) ] / ( 2 A ~2 ) (2.8)
pn, J At.
are t h e v a l u e s a t the j t h g r i d - p o i n t a f t e r t h e n t h time-step o f l e n g t h The growth term has been t r e a t e d i m p l i c i t l y and t h e Crank - Nicholson
Nn J
scheme is used f o r t h e d i f f u s i o n terms. the
The growth r a t e s , an
3'
are calculated at
same g r i d - p o i n t s a s PnJ and NnJ w h i l e t h e d i f f u s i o n c o e f f i c i e n t s
Kn J+1/2
are
318 at
evaluated solved,
points
incorporating
mid-way between t h e g r i d p o i n t s . t h e boundary c o n d i t i o n s ,
m a t r i x a l g o r i t h m ( e g Richtmyer and Morton,
by means o f
1957,
equations
These
the
were
tri-diagonal
p 1 8 9 ) , u s i n g d e p t h increment
o f 1 metre and a 5 minute t i m e - s t e p . RESULTS WITHOUT D I U R N A L CHANGES Each
r u n of t h e model was f o r a p e r i o d w i t h a s h a l l o w (25 m ) mixed l a y e r
which t h e n u t r i e n t was i n i t i a l l y a l r e a d y c o n s i d e r a b l y d e p l e t e d ( N ( z , O )
~ n - ~ ) . Beneath thermocline
below t h a t .
exhibiting
ocean,
and
the
From t h e s e i n i t i a l c o n d i t i o n s steady
f u r t h e r n u t r i e n t d e p l e t i o n i n t h e s u r f a c e mixed l a y e r
peak p h y t o p l a n k t o n c o n c e n t r a t i o n i n t h e t h e r m o c l i n e ( F i g . phytoplankton
and
a
T h e mixed l a y e r
1).
concentrations achieved i n t h e s h e l f c a s e a r e higher than in
a
t h e peak i n t h e t h e r m o c l i n e l a r g e r b u t n a r r o w e r ,
s t r u c t u r e which is f r e q u e n t l y o b s e r v e d . the
in
The i n i t i a l c o n c e n t r a t i o n o f phytopla-
p h y t o p l a n k t o n and n u t r i e n t c o n c e n t r a t i o n s g e n e r a l l y t e n d towards a
state
more
3 mM N mF3
i n i t i a l c o n c e n t r a t i o n s were h i g h e r ;
P(z.0) was 0.1 mg C h l md3 a t a l l d e p t h s .
nkton, both
this,
and 6 mM N m-3
in
0.lmM N
=
the
difference
in
The s h e l f t u r b u l e n c e p r o f i l e produces a
r a p i d approach t o t h e s t e a d y s t a t e ( w i t h i n a b o u t 50 d a y s ) t h a n is found in ocean case ( F i g .
between
the
development initial
2),
which may be a t t r i b u t a b l e t o t h e
greater
i n i t i a l and f i n a l n u t r i e n t p r o f i l e i n t h e l a t t e r .
disparity
I n most
cases
t o w a r d s t h e s t e a d y s t a t e i n t h e mixed l a y e r is c h a r a c t e r i s e d b y
overshoot
of p h y t o p l a n k t o n c o n c e n t r a t i o n i n r e s p o n s e t o
the
an
somewhat
h i g h i n i t i a l n u t r i e n t c o n c e n t r a t i o n s , f o l l o w e d by its more or less damped r e t u r n towards the s t e a d y state. because
the
The r i s e towards t h e o v e r s h o o t is l a r g e l y e x p o n e n t i a l
n u t r i e n t l e v e l is a l m o s t unchanged.
S i n c e the r a t e
of
increase
OCEAN 0
-
Phytoplankton
20 -
20
', Nutrient
40-
0
Phytoplankton (rng Chllrn3)
2
4
02
04
',6
06
8
10
08
10
Phytoplankton (mq C h l / m 3 1
Fig. 1 V e r t i c a l p r o f i l e s o f p h y t o p l a n k t o n and n u t r i e n t a f t e r 1Oc days under s h e l f o r o c e a n i c t u r b u l e n c e c o n d i t i o n s (see t e x t ) , w i t h w = 0 . 2 mM p e r c u b i c m, m = 0.078 p e r day and E = 0.2.
Shell Phyloplanhlon Imq Chllm3 1
5 )2
04 15 08
10
.. : 3
08
20-
25-
rn
30-
0
35
-
5-
03 10 -
-
15
? 20-
6
0 03 01
-
25
-
30
I “
300
35
-
20
10
30
40
M
60
70
I
80
90
l , , , , , , , , , > , , , , , , , , J
IW
10
20
30
40
Ua”J
M
60
70
80
90
100
Ua”l
Fig.2 The t e m p o r a l d e v e l o p m e n t t o w a r d s t h e p r o f i l e s of F i g . 1 from t h e i n i t i a l c o n d i t i o n s d e s c r i b e d i n t h e t e x t ( v = 0 . 2 mM p e r c u b i c m , m = 0.078 p e r d a y , E = 0.2) depends
on
t h e d i f f e r e n c e between growth and l o s s e s ( i n c l u d i n g
diffusion
and
s i n k i n g ) , a small c h a n g e e i t h e r i n g r o w t h o r m o r t a l i t y is a m p l i f i e d i n t h e r j t e . After
an i n i t i a l s l i g h t iqcrease,
phytoplankton
uptake,
r e f l e c t i n g upward d i f f u s i o n u n b a l a Jed by
changes i n t h e dissolved n u t r i e n t concentration
mixed l a y e r m i r r o r t h o s e o f t h e p h y t o p l a n k t o n ; temporarily
the
c’
the t o t a l nutrient concentration
i n c r e a s e s w h i l e upward d i f f u s i o n o f n u t r i e n t s is g r e a t e r t h a n down-
wards loss o f phytoplankton. t h e r a t e of t h e i n i t i a l i n c r e a s e w i l l be d e t e r m i n e d l a r g e l y
Clearly net
by
the
g r o w t h r a t e of t h e p h y t o p l a n k t o n a n d so c a n b e m a r k e d l y raised by r e d u c t i o n
i n l o s s (in) o r t h e M i c h a e l i s c o n s t a n t ( v ) ; ( F i g s . 3 and 5 ) .
Although t h e s t e a d y
s t a t e n u t r i e n t c o n c e n t r a t i o n a l s o shows a marked p o s i t i v e r e l a t i o n t o v ( F i g s and
5).
plankton
levels,
Conversely,
seems
3
t h i s is n o t mirrored by a c o r r e s p o n d i n g d e c l i n e i n s t e a d y s t a t e phytowhich a r e r e l a t i v e l y i n s e n s i t i v e t o t h e
and e q u a l l y c o n t r a r y t o n a i v e e x p e c t a t i o n s ,
largely
Michaelis
constant.
r e c y c l i n g of
i r r e l e v a n t t o its f i n a l c o n c e n t r a t i o n ( F i g .
nutrient
3 ) b u t when
e f f i c i e n t it may c o n s i d e r a b l y i n c r e a s e t h e c o n c e n t r a t i o n o f p h y t o p l a n k t o n t h i s supports (Fig.
3).
Thus, for d i f f e r e n t r e a s o n s ,
highly which
increasing the recycling
e f f i c i e n c y ( E ) and r e d u c i n g t h e M i c h a e l i s c o n s t a n t ( v ) b o t h i n c r e a s e t h e p r o p o r -
320
"r-1
S h e l l S u r t a c e P h y t o p l a n k t o n [mq C h l / m 3 ]
Shell S u r l a c e Dissolved Nutrient 1mMolesIm
o.B
3l
001
0001
10.
-
50 Days
0
0
100
100
50 Days
0
0.2
0.0
50
100
50
0
0
100
100
50
Days
Days
Days
1 .o
0.0
0.2
-Recycling
R e c y c l i n g eltic8ency-
0
100
50 Days
-
1.o
elliciency
Fig. 3 E f f e c t s o f t h e Michaelis c o n s t a n t , recycling e f f i c i e n c y and l o s s r a t e ( d a s h e d l i n e s , m = 0.078 p e r d a y ; c o n t i n u o u s l i n e s , m = 0.208 p e r day) on the c a l c u l a t e d t e m p o r a l d e v e l o p m e n t o f s u r f a c e p h y t o p l a n k t o n and n u t r i e n t under shelf conditions. N u m e r i c a l v a l u e s w i t h i n e a c h s u b - f i g u r e i n d i c a t e decay times a n d o s c i l l a t i o n p e r i o d s ( d a y s ) c a l c u l a t e d u s i n g t h e s i n g l e - l a y e r approximation (see text). Arrows i n d i c a t e s t e a d y - s t a t e v a l u e s e x p e c t e d u n d e r t h e same approximation (parameter v a l u e s as Fig. 6 ) t i o n o f t h e n u t r i e n t i n t h e mixed l a y e r which is l o c k e d i n t h e p h y t o p l a n k t o n , an effect
which,
Another
is l e s s marked i n t h e t h e r m o c l i n e
although present,
f e a t u r e which may b e s e e n i n F i g .
recycling
efficiency,
as
the
3 is t h a t ,
(Fig.
u n d e r c o n d i t i o n s of
n e t g r o w t h r a t e (a@-m) d e c l i n e s and
hence
b i o l o g i c a l r e s p o n s e time of t h e s y s t e m i n c r e a s e s r e l a t i v e t o its f i x e d
4).
low the
physical
c h a r a c t e r i s t i c s , a tendency t o o s c i l l a t e appears. The s o u r c e of much of t h i s b e h a v i o u r , (simple view
though
of
d i f f i c u l t t o d i s c e r n i n t h e f u l l model
i t i s ) is e l u c i d a t e d by c o n s i d e r a t i o n of a y e t
more
A s t h e t h e r m o c l i n e a n d mixed l a y e r show similar
reality.
schmatic responses,
c o n s i d e r , a s a model, a l a y e r of p h y t o p l a n k t o n g r o w i n g o v e r a p o o l of n u t r i e n t s . Suppose
the
layer
phytoplankton a r e
ii
is o f t h i c k n e s s h and t h e c o n c e n t r a t i o n s
and
v a l u e s below t h e l a y e r .
P,
respectively,
with
simple
B = a/v a n d 8 = 0 .
approximation
turbulent
nutrient
and
E q u a t i o n s ( 2 . 1 ) and ( 2 . 2 ) may b e a p p r o x i m a t e d by:-
aN/(N+v) h a s been a p p r o x i m a t e d i n t h e s e e q u a t i o n s by BN occasions i < < v ,
of
iio, Fo b e i n g t h e corresponding
to
+
8 and,
s i n c e on
most
The l a s t term i n e a c h e q u a t i o n r e p r e s e n t s a
vertical diffusion.
The c o e f f i c i e n t k - w i l l
be
d i f f u s i o n c o e f f i c i e n t i n t h e t h e r m o c l i n e d i v i d e d by t h e s q u a r e of
a p p r o p r i a t e v e r t i c a l d i s t a n c e ( o f t h e o r d e r of h ) .
I f P0
the an
is n e g l i g i b l e compared
321 S h e l l P r o ~ l o r l i o n0 1 Nulrienl Dissolved a t Ihe Surface
06 50 Days
0 Days
Days
0.0
0.2
Days
1.o
Recycling e l l l c l e n c y
100
50
100
0
Days
Days
-
0.2
0.0
1.0
Recycling ell~ctency
4 The p r o p o r t i o n s of t h e t o t a l n u t r i e n t d i s s o l v e d a t t h e s u r f a c e and i n t h e t h e r m o c l i n e i n t h e s i t u a t i o n s d e p i c t e d i n F i g . 3.
Fig.
P,
to
t h e s t e a d y - s t a t e s o l u t i o n s t o (3.1) and ( 3 . 2 ) are:-
N = (m
v/h
+
-
k(No P = hY(BN A
P
or In
N)
0
-
0) /B
-
v/h
-
v/h
+
k/h
(3.3)
Em)
-
hBY (m
these
-
k/h
.
-
k(Bii0
=
though
+
+
m +
.
equations
N
k/h
-
does
0)
+
(3.4)
Em)
n o t depend
on
increases with
recycling
the
o n l y a p p e a r s i n t h e e q u a t i o n f o r dG/dt.
E
efficiency,
P
s o t h a t t h e p r o p o r t i o n o f t h e t o t a l n u t r i e n t s t h a t is d i s s o l -
E,
decreases w i t h i n c r e a s i n g r e c y c l i n g e f f i c i e n c y as i n F i g .
ved
even
The p h y t o p l a n k t o n a b u n d a n c e
The r a t i o of
4.
d i s s o l v e d n u t r i e n t s t o t h e n u t r i e n t s i n t h e p h y t o p l a n k t o n is:
.
-N
=
h(m + v / h + k/h
YP
k(Bio
-
m
-
Em)
(m
-
v/h
-
+
v/h
+
k/h
-
8)
(3.5)
k/h + 0 )
a n e x p r e s s i o n w h i c h shows t h a t t h e r e l a t i v e d e c l i n e of d i s s o l v e d n u t r i e n t s
with
i n c r e a s e d r e c y c l i n g e f f i c i e n c y is d u e t o a b a l a n c e of t h e t h r e e p r o c e s s e s :
cell
loss,
r e c y c l i n g a n d upward d i f f u s i o n of n u t r i e n t s .
Further, since a reduction
i n t h e M i c h a e l i s c o n s t a n t is e q u i v a l e n t t o a n i n c r e a s e i n b o t h 6 and 0 t h i s w i l l
also reduce t h e ratio. T h e p r e d i c t i o n s of e q u a t i o n s (3.3) a n d ( 3 . 4 ) a r e shown o n F i g . 3.
agreement
is g e n e r a l l y c l o s e ,
in particular,
The
t h e i n s e n s i t i v i t y of t h e
phyto-
p l a n k t o n a b u n d a n c e t o t h e Michaelis-Menten p a r a m e t e r is r e p r o d u c e d . The steady
temporal state
behaviour
of t h i s s i n g l e - l a y e r model i n t h e v i c i n i t y
a l s o r e p r o d u c e s t h e c h a n g e s i n F i g . 3.
I f P and
. N
are
of
its
322
q r;Tj
l l o 'o .. o m
5 0 . 2
O 'M 01
0.05
I 0
a
--_
-..
0I
I
0
100
50
0
50
100
50
0
100
rl.,ys
Days
Days
DdVS
DAYS
0.0
0.2
1.o
0.0
0.2
-R e c y c l m g
n lVr
-
1.o
-R e c y c l i n q e l l i c i e n c y
elllclencv
Fig. 5 The d e p e n d e n c e of t h e t e m p o r a l d e v e l o p m e n t of s u r f a c e p h y t o p l a n k t o n and n u t r i e n t o n n u t r i e n t u p t a k e , r e c y c l i n g and p h y t o p l a n k t o n l o s s i n a n o c e a n i c t u r b u l e n c e r e g i m e ( d a s h e d l i n e s , m = 0.078 p e r d a y ; s o l i d l i n e s , m = 0.208 per day). equilibrium
- , .
s o l u t i o n s of ( 3 . 1 ) and ( 3 . 2 ) ,
P
=
P
+
p and
i= i
+
the
n then in
v i c i n i t y of t h e s t e a d y - s t a t e : dp/dt
=
(BN
+
8
-
,.
m - v/h - k/h)p
BPn
+
(3.6) (3.7)
When s o l u t i o n s p
AeAt, n
=
and A may b e e i t h e r
=
g r o w i n g or d e c a y i n g waves. defined
time
T h e s e s o l u t i o n s are e x p r e s s e d on F i g . 3 as a decay-
2n/(real
as
BeAt are s o u g h t , A is g i v e n by a q u a d r a t i c e q u a t i o n
real, r e p r e s e n t i n g g r o w t h or d e c a y , o r complex, r e p r e s e n t i n g
2 n / ( i m a g i n a r y p a r t of A ) .
part
of A ) and
an
oscillation
period
which
p a r a m e t e r v a l u e s where these o c c u r r e d i n t h e f u l l n u m e r i c a l model, p r e d i c t e d p e r i o d s t e n d t o b e too s h o r t .
although the
There is g e n e r a l a g r e e m e n t between t h e
d e c a y - t i m e a n d t h e time t o r e a c h e q u i l i b r i u m a f t e r t h e i n i t i a l i n c r e a s e ( a t
start
the
c o n d i t i o n s a r e too f a r from e q u i l i b r i u m f o r ( 3 . 6 ) and ( 3 . 7 ) t o be a p p l i c -
able); and
is
O s c i l l a t i n g s o l u t i o n s are p r e d i c t e d i n t h e r e g i o n of
i n p a r t i c u l a r , v e r y slow d e c a y is p r e d i c t e d a t h i g h r e c y c l i n g e f f i c i e n c y
t h i s is where t h e s t e a d y - s t a t e
is b a r e l y r e a c h e d .
T h e s i m p l e model
(3.6)
and ( 3 . 7 ) t h e r e f o r e allows t h e r a n g e of p a r a m e t e r s g i v i n g r i s e t o o s c i l l a t i o n t o be determined (Fig. 6 ) . If
Po is n e g l i g i b l e ,
t h e c o e f f i c i e n t of p d i s a p p e a r s from ( 3 . 6 ) and h is
a
s o l u t i o n of: +
A(k/h
+
BY;)
+
B k ( i o - N)/h
=
0
(3.8)
323 C y c l e s Per D e c a y l i m e
Fig. 6 T h e e f f e c t s of t h e M i c h a e l i s c o n s t a n t ( u s i n g B = a / v , see t e x t ) a n d p h y t o p l a n k t o n l o s s r a t e o n t h e d y n a m i c s of t h e s i n g l e - l a y e r model of n u t r i e n t d e p e n d e n t g r o w t h (Eq: 3.1 a n d 3.2) a = 0.583 p e r d a y , e = 0 , E = 0 . 2 , Po = 0.1 mg C h l p e r c u b i c m. No = 6 mM p e r c u b i c m , k = 0.102m p e r d a y , h = 2%. O s c i l l a t i n g s o l u t i o n s t o (3. 8) o c c u r i f a n d o n l y i f
The
right
hand s i d e r e p r e s e n t s t h e v e r t i c a l d i f f u s i o n
i n d e p e n d e n t of r e c y c l i n g e f f i c i e n c y .
However,
a n d so t h e p r o p e n s i t y t o o s c i l l a t e is g r e a t e r .
-
No,
of
increasing
nutrients, E
and
is
3.4)
r a i s e s P (Eq.
On t h e o t h e r h a n d , low v a l u e s of
as occur in the oceanic case, tend t o eliminate o s c i l l a t i o n s (Fig. 5).
RESPONSE TO LIGHT INTENSITY
As
expected,
a
reduction
i n c r e a s e of t h e p h y t o p l a n k t o n . cribed
in
the
steady-state change while
i n l i g h t intensity reduces t h e i n i t i a l However,
previous section,
phytoplankton
unlike t h e parameter variations
which a l l p r o d u c e d similar
equilibrium
two
concentration of phytoplankton i n t h e
of des-
in
changes
i n t h e t h e r m o c l i n e t o t h o s e i n t h e mixed
i n l i g h t i n t e n s i t y causes o p p o s i t e changes i n t h e the
rate
the
layer,
regions.
a
Thus,
thermocline
in-
creases a s t h e l i g h t i n t e n s i t y is r a i s e d ( F i g . 7 ) t h e c o n c e n t r a t i o n i n t h e m i x e d layer declines. declines
with
T h e c o n c e n t r a t i o n of d i s s o l v e d n u t r i e n t i n t h e mixed l a y e r a l s o i n c r e a s i n g l i g h t a n d so i t a p p e a r s t h a t ,
trates below t h e mixed l a y e r ,
when more l i g h t
pene-
t h e r e is more g r o w t h a t t h e s e d e p t h s r e d u c i n g t h e
s u p p l y of n u t r i e n t s t o t h e s u r f a c e l a y e r .
T h i s process can be thought
a n a l o g y w i t h i n t e r n a l w a v e s , a s a n i n t e r n a l mode of t h e s y s t e m .
of,
by
The process can
b e d e s c r i b e d i f our e a r l i e r a n a l y s i s is e x t e n d e d t o c o n s i d e r two l a y e r s : a mixed l a y e r with n u t r i e n t and phytoplnnkton c o n c e n t r a t i o n s i n which t h e s e v a l u e s are t h e bottom l a y e r a r e then:-
No
kT a n d PT, -
a n d Po ( = O ) .
respectively.
NM a n d PM a n d a
thermocline
The c o r r e s p o n d i n g v a l u e s i n
T h e e q u a t i o n s r e l a t i n g t h e s e v a r i a b l e s are
324 TWO-LAYER ANALYSIS
2.00
3 K
c
6P
0.025
0.50
.
a NM
0.25
100 Light (W/rn2)
0.001
300
200
Mixed layer specific growth
Light ( W l r n 2 1
rate ( 1 0 - ~ 1 ~ 1
Fig. 7 The dependence of phytoplankton (dashed lines) and nutrient (solid lines) concentrations after 100 days on light incident at the surface ( u = 0.2 mM per cubic m, m = 0.078 per day, E = 0.2). Values at 27.5111 and 36.5111depth are representative of the thermocline in shelf and ocean respectively. The right-hand plot was calculated using the two-layer model (Eq. 4.1 to 4.41, with parameters adjusted to provide general agreement with the shelf case (a.. = 0.618 per day, N = 3 mM per cubic m, kl = 0.216111per day, k2 = 0.173m perl'day, hM= 25m, hT= 5mT? dPM/dt
=
(BMNM
diM/dt
=
-Y(BMNM
=
-Y[BTNT
-
dNT/dt
+
-
-
-
-
m
-
+
OM
-
m)PM
+
BT
-
m1PT
BM
V
/hM) PM
-
+
(iT-
+
k2
+
kl(NO
-
k2
-
(PT -
-
PM)/hM
(4.1)
iM)/hM
- -NT)/hT
(4.2)
-
-
k2 (NT
-
-
NM)/hT
(4.4)
The coefficient k2 represents diffusion between the mixed layer (depth hM) and the thermocline, k, between the thermocline (thickness hT) and the bottom layer. In each layer the Michaelis-Menten relation has been linearised about appropriate to the layer. =
BT
Although the mixed layer the values
B
M
=
a
point
aM/u and OM
0 can be used as before, in the thermocline:
aTCNTo/(NTo
=
+
v)l
2
where NTO is a typical nutrient concentration in the thermocline. Therefore. if aT
=
aM, BT is smaller than 8,
and BT is larger than BM reflecting
linearity of the Michealis-Menten relation.
the non-
325
Generally, NTO effectively zero.
will
be
BT
sufficiently large compared to v that
will
be
Under these circumstances a solution is readily available;
its properties are encapsulated by the following relations:
pM
1 7
=
sls3(NO-NM)/[(s1+s2)(BHNH+BM-Em)
+
q s ( e -Em)/(m-e 2 3
T
+q
T 3
)I (4.9)
1 , .
PM/PT
In
(m
=
- eT
+
q )/q 3 2
(4.10)
these, specific transfer terms have been defined by
sl=kl/hT, s2=k2/hT,
S3=k2/hM, ql=(v+k2)/hM, q2=(v+k2)/hT and q3=q2+sl. Equations 4.8 to 4.10 have been
simplified by the recognition that NM may be neglected in comparison with
As with the single layer analysis. in the mixed layer the nutrient concenNT. tration depends on the recycling efficiency and the phytoplankton concentration increases with
E
decreases with
E.
so that the proportion of the total nutrients that is dissolved
The proportion of nutrient dissolved behaves similarly in the
thermocline (Eq. 4.9) and this is reflected in the numerical results (Fig. 4). Increasing the surface light intensity will make a and hence BM, larger
in these equations. ,.
proportion was
on
eM
eT
and
O M and BT are all increased in the
.
..
same
.
both NM and NT are decreased and the ratio of P M to PT declines, as in the numerical model. Note that the ratio of PM to PT does not
found
depend
When BM,
E
or No.
In equations (4.6) to (4.9) only the non-linearity of
Michaelis-Menten relationship
has been included, there is no need
of
the
self-
shading o r light saturation although these will cause some modification of process.
assumptions, for which have
the
This response to variations in a is not the result of our simplifying it applies to the steady-state solutions of (4.1)
can be determined numerically (Fig 7). been evaluated using equation (4.5) and aT
to
(4.4)
In these calculations BT and =
eT
aM/4 to allow for the reduced
,.
growth due to lower light levels in the thermocline. The total phytoplankton abundance, reasing
light.
declines slightly with dechMPM + %PT, However in the full numerical model the total phytoplankton in
the mixed layer and thermocline increases as the light is reduced and the vertical decrease occur.
only
if
integration includes the population below the thermocline does a
326 EFFECT OF D I U R N A L V A R I A T I O N There are two major p h y s i c a l s o u r c e s o f d a i l y v a r i a t i o n : of
irradiance
easily
and t h e c y c l e o f c o n v e c t i v e mixing.
examined
using
t h e day-night c y c l e
are
These p r o c e s s e s
t h e s h e l f c a s e where a s t e a d y
state
is
more
most
rapidly
acheived. Diurnally varying l i g h t The d a i l y c y c l e o f l i g h t c o n c e n t r a t e s t h e i r r a d i a n c e i n t o i n t e r v a l s o f higher i n t e n s i t y , and some o f t h i s r a d i a t i o n is t h e n n o t u t i l i s e d b e c a u s e phytoplankton growth is r e l a t e d t o l i g h t i n t e n s i t y by a law o f d i m i n i s h i n g r e t u r n s .
For t h i s
a d i u r n a l l y varying incident l i g h t h a s an e f f e c t s i m i l a r t o a
reduction
reason
i n mean l i g h t l e v e l . plankton
T h a t is, t h e r e is a n i n c r e a s e i n b o t h s t e a d y s t a t e phyto-
and n u t r i e n t i n t h e mixed l a y e r and a d e c l i n e i n t h e t h e r m o c l i n e peaks
o f p h y t o p l a n k t o n ( T a b l e 1 , column 2 ; c f . Fig. 7 ) . for
the
surface
The e f f e c t is most pronounced
n u t r i e n t c o n c e n t r a t i o n which is a l m o s t
trebled
whereas
change i n p h y t o p l a n k t o n is o n l y s l i g h t ( e q u i v a l e n t t o a change i n mean l i g h t 10
- 20%).
amplification
However, of
the
the of
a marked s l o w i n g o f t h e i n i t i a l i n c r e a s e is produced
by
small r e d u c t i o n i n growth r a t e i n t h e
in
manner
noted
s e c t i o n 2. P h y t o p l a n k t o n is i n mg Chl p e r c u b i c m Table 1 E f f e c t of t h e d i u r n a l cycle. and n u t r i e n t is i n mM p e r c u b i c m. Values g i v e n a r e f o r t h e s h e l f c a s e a t t h e end o f 100 days. E f f e c t s on t h e d a i l y a v e r a g e s a r e e x p r e s s e d a s d e v i a t i o n s from t h e averages without a d a i l y cycle. Mean w i t h no c y c l e Surface phytoplankton Surface nutrient Thermocline p h y t o p l a n k t o n Thermocline n u t r i e n t
0.67 0.028 0.95 1.960
D A I L Y AVERAGES Additive e f f e c t s Light Turbulence L i g h t and cycle cycle turbulence
0.02 0.043 -0.08 0.145
-0.02 -0.004 0.01 0.030
0.00 0.040
0.08 0.175
PERCENTAGE INCREASE FROM NIGHT TO DAY Light Turbulence L i g h t and cycle cycle turbulence
Surface phytoplankton (day) Surface n u t r i e n t (day) Thermocline p h y t o p l a n k t o n ( d a y ) Thermocline n u t r i e n t ( d a y )
6 -23 14 -2
-2 -37 0 0
3 -38 12 -2
TIME TO FIRST MAXIMUM OR M I N I M U M (DAYS) Light Turbulence L i g h t and No d a i l y cycle cycle cycle turbulence S u r f a c e p h y t o p l a n k t o n max S u r f a c e n u t r i e n t min
8.75 13.5
19.0 24.0
8.75 13.5
18.5 211.5
327 In
addition
to such e f f e c t s averaged over t h e
concentrations
vary
only
noticably higher a t night, the
time-scale
daily
cycle,
s l i g h t l y diurnally but n u t r i e n t
phytoplankton
concentrations
a r o u n d 30% i n t h e example shown i n T a b l e 1 .
for vertical
m i x i n g between
the surface
and t h e
are
Because
thermocline
( - ( m i x e d l a y e r d e p t h ) 2 / K ) is a t l e a s t o n e d a y , t h e d a y - n i g h t c y c l e d o e s n o t show the
the
c o u p l i n g between t h e t h e r m o c l i n e and mixed l a y e r which is
equilibrium
response t o changing l i g h t . Diurnally varying turbulence During
t h e daytime,
m i x i n g i n t h e lower p a r t o f t h e mixed l a y e r is
greatly
r e d u c e d b e c a u s e of t h e a b s e n c e of c o n v e c t i o n ; t h i s r e d u c e s t h e flow of n u t r i e n t s up
through
t h e t h e r m o c l i n e and may a f f e c t t h e downward f l u x
of
phytoplankton
c e l l s . A s a r e s u l t , t h e e f f e c t o f t h e d a i l y c y c l e of t u r b u l e n c e is t o r e d u c e t h e daily
c o n c e n t r a t i o n of b o t h p h y t o p l a n k t o n and n u t r i e n t a t t h e
average
(Table
column
1,
2).
About h a l f of t h i s c h a n g e a p p e a r s
reduction
in
t h e mean t u r b u l e n c e of t h e mixed
abundance
is
s l i g h t l y h i g h e r a t n i g h t t h a n i n t h e day ( T a b l e 1 ) which
of
result
by day.
to
a
phytoplankton is
is
the an
T h e r e is n e a r l y t w i c e a s much s u r f a c e n u t r i e n t a t n i g h t a s
lm jL-- r
t h e day.
sponse
Surface
d i f f u s i o n from t h e l o w e r p a r t o f t h e mixed l a y e r where t h e r e
enhancement in
layer.
surface
attributable
time
Adding a d i u r n a l v a r i a t i o n i n t u r b u l e n c e d i d n o t a f f e c t
of
t h e model,
t h e times of t h e f i r s t p h y t o p l a n k t o n
the
re-
maximum
and
n u t r i e n t minimum r e m a i n i n g unchanged ( T a b l e 1 ) .
Shell N u l r i e n l
Shell Phyloplanklon
0
E
. ....... ........
D
I
E
0 001
:z
r m c
~~
~
-. 0 10
> -
sc Y
~~
j F = L = .......... = ? L Y I
c I
a 5
......
100
- 0 3
r
Z
a c 0 10
001
0
25
50 Days
15
1w
0
25
50
75
100
Days
Fig. 8 The e f f e c t s of d i u r n a l l y v a r y i n g t u r b u l e n c e and l i g h t on t h e t e m p o r a l e v o l u t i o n of p h y t o p l a n k t o n a n d n u t r i e n t c o n c e n t r a t i o n s a t t h e s u r f a c e and i n t h e The d a s h e d l i n e is u n d e r thermocline, subject t o a s h e l f turbulence p r o f i l e . c o n s t a n t c o n d i t i o n s . F o r t h e s o l i d l i n e t h e s y s t e m was s u b j e c t t o t h e same mean s u r f a c e l i g h t , b u t c o n c e n t r a t e d i n t h e p o s i t i v e p a r t of a d i u r n a l s i n e wave; t u r b u l e n c e was s i m u l t a n e o u s l y r e d u c e d i n t h e lower h a l f of t h e mixed l a y e r d u r i n g t h e d a y . I n e a c h case t h e p a r a m e t e r v a l u e s of F i g s . 1 and 2 were u s e d .
D i u r n a l l y v a r y i n g l i g h t and t u r b u l e n c e When
combined,
additive
e f f e c t s of t h e c y c l e s o f l i g h t
the
and
turbulence
appear
t h e i r e f f e c t s on t h e p h y t o p l a n k t o n c a n c e l a t t h e s u r f a c e
and
o n l y t h e i n c r e a s e i n n u t r i e n t s due t o l i g h t v a r i a t i o n .
Particularly
leaving noticable
is t h e r e s u l t i n g i n c r e a s e i n t h e a m p l i t u d e of t h e d a y - n i g h t c y c l e i n t h e s u r f a c e nutrient.
The
o v e r a l l e f f e c t o f t h e c y c l e o f day and n i g h t is t o d e c r e a s e the
average proportion of n u t r i e n t incorporated i n the phytoplankton (Fig. 8 ) . DISCUSSION The
phytoplankton
and n u t r i e n t i n t h e s h e l f case reach
l i b r i u m w i t h i n 10-20 d a y s ,
c a s e a l s o seems t o be moving towards a n e q u i l i b r i u m . patterns
generally
will
a
dynamical
even i n t h e p r e s e n c e of d i u r n a l f o r c i n g .
equi-
The ocean
While f l u c t u a t i n g weather
p r e v e n t c o n d i t i o n s i n t h e sea b e i n g s t a b l e
for
this
l e n g t h o f t i m e , t h e system w i l l p r o b a b l y be d i s p l a c e d from e q u i l i b r i u m less than i n i t i a l c o n d i t i o n s used h e r e ,
the
s o t h a t many p r o p e r t i e s o f t h e s t e a d y
state
may be r e l e v a n t t o c o n d i t o n s i n t h e sea. The a p p r o a c h t o t h e s t e a d y s t a t e sometimes i n v o l v e s o s c i l l a t i o n s ; these being induced near
a n imbalance between t h e p r o c e s s e s o f growth
by
nutrient
s u p p l y by d i f f u s i o n ( c . f eq.
3.9).
and
the
Such o s c i l l a t i o n s r e p r e s e n t
and
the
e q u i l i b r i u m r e s p o n s e o f L o t k a - V o l t e r r a s y s t e m s ( e g May,
present
in
the
two-species,
two-nutrient
decay, 1974),
and
were
of
Powell
and
c o m p e t i t i o n model
R i c h e r s o n ( 1 9 8 5 ) , a l t h o u g h w i t h much l o n g e r p e r i o d s t h a n g i v e n h e r e (100 days or longer)
.
When t h e system h a s r e a c h e d a s t e a d y s t a t e t h e t o t a l n u t r i e n t c o n t e n t of l a y e r is m a i n t a i n e d by a b a l a n c e o f t h r e e f l o w s :
mixed
p o r t of phytoplankton c e l l s , upward that
t h e l o s s d u r i n g t h e i r incomplete r e c y l i n g and
the
W e have shown
d i f f u s i o n of dissolved n u t r i e n t through t h e thermocline. varying
the
t h e n e t downward t r a n s -
t h e e f f i c i e n c y of r e c y c l i n g h a s n e g l i g i b l e e f f e c t on
the
upward
n u t r i e n t f l u x and merely r e d i s t r i b u t e s t h e two l o s s e s . l i m i t a t i o n o f t h e p r e s e n t model may b e t h a t t h e r e c y c l i n g o c c u r s i n s t a n t a -
A
neously,
a l t h o u g h the model d o e s a l l o w p h y t o p l a n k t o n t o l e a v e a t t h e bottom and
return as nutrient.
Any time d e l a y is l i k e l y t o b e most i m p o r t a n t when i t l e a d s
t o a transport of t h e n u t r i e n t s . tion
can
abundance the
model
I n p a r t i c u l a r , z o o p l a n k t o n g r a z i n g and excre-
r e s u l t in t h e v e r t i c a l t r a n s f e r of
nutrients.
If
the
zooplankton
is o n l y weakly a f f e c t e d by changes i n t h e p h y t o p l a n k t o n c o n c e n t r a t i o n
r e s u l t s w i l l s t i l l apply,
f o r t h e e f f e c t c a n be t r e a t e d
by
making
adjustments t o t h e d i f f u s i o n rate o r t h e r e c y c l i n g e f f i c i e n c y . Because t h e i r r a d i a n c e is d e p t h d e p e n d e n t , causes
a
higher
thermocline influence
each of
order
changing t h e - l i g h t
r e s p o n s e o f t h e system i n which t h e
behave d i f f e r e n t l y .
mixed
The two-layer a n a l y s i s shows
intensity layer
and
that
the
growth i n t h e t h e r m o c l i n e on t h e f l u x o f n u t r i e n t s a c r o s s
it
iS
329 sufficient to explain the model results, without considering such processes as self-shading. of a
However, the addition of self-shading introduces the possibility
positive feedback which
producing surface blooms. reduction
may be
The day
-
important under
certain conditions,
night cycle of light is equivalent
to
a
in light intensity and so the response of the model to changing light
is important in considering the effects of a diurnal cycle.
Diurnal model
light variations do not qualitatively change the results from the
and the quantitative effects are slight.
turbulence
Including the daily
cycle of
tends to cancel even these so that it may not always be necessary to
consider diurnal variations in models of this type.
Only in the nutrient or in
the transient response is the absence of a diurnal variation likely to be at all noticeable.
Day-night differences in phytoplankton abundance were negligible in
agreement with Woods and Onken (1982) even if the doubling time was reduced an order of magnitude. tended to cancel. it
The effects due to the light and turbulence cycles again
The daily signal was stronger in the surface nutrient where
was almost a factor of two,
case.
Of
the processes reinforcing each other
in
this
course, some of the biological processes not treated might make the
daily cycle more important (eg die1 migration of grazers), buffering
by
but then the general
systems of phytoplankton physiology (eg photoadaptation and
reservoirs) could lessen its importance.
Inclusion of additional trophic levels
may change the properties of the system we have described. simplified models
nutrient
Further analysis of
can be used to explore the extent to which
such additions
define the limits of applicability of our system. REFERENCES The Undulating Oceanographic Recorder Mark 2, a multiple Aiken. J., 1985. oceanographic sampler for mapping and modelling the biophysical marine environment. In: Zirino, A. (ed), Mapping Strategies in Chemical Oceanography. American Chemical Society, 209, pp 315-332. Criminale, W.O.. 1980. Effects of mean current and stability on depth distribution of marine phytoplankton. J. Math. Biol.. 10: 33-51. Criminale, W.O. and Winter, D.F., 1974. The stability of steady-state depth distribution of marine phytoplankton. Am. Nat.. 108: 679-687. Fasham, M.J.R., Holligan, P.M. and Pugh, P.R.. 1983. The spatial and temporal development of th.e spring phytoplankton bloom in the Celtic Sea, April 1979. Prog. Oceanog., 12: 87-145. Jamart, B.M., Winter, D.F. and Banse, K., 1979. Sensitivity analysis of a mathematical model of phytoplankton growth and nutrient distribution in the Pacific Ocean of the northwestern U.S. coast. J. Plankt. Res., 1: 267-290 James, I.D.. 1977. A model of the annual cycle of temperature in a frontal region of the Celtic Sea. Est. Coastal Mar. Sci., 5: 339-353. Jassby, A.D. and Platt, T., 1976. Mathematical formulation of the relationship between photosynthesis and light for phytoplankton. Limnol. Oceanogr.. 21: 540-547. Jassby, A.D. and Powell, T., 1975. Vertical patterns of eddy diffusion during stratification in Castle Lake, California: Limnol. Oceanogr., 20: 530-542. Joint, I.R. and Pomroy, A.J., 1983. Production of picoplankton and small nanoplankton in the Celtic Sea. Mar. Biol., 77: 19-27.
330 King, F.D. and Devol, A.H., 1979. Estimates o f v e r t i c a l e d d y d i f f u s i o n t h r o u g h t h e t h e r m o c l i n e from p h y t o p l a n k t o n n i t r a t e u p t a k e r a t e s i n t h e mixed l a y e r o f t h e e a s t e r n t r o p i c a l P a c i f i c . Limnol. G c e a n o g r . , 24: 645-651. Kullenberg, G., 1971. Vertical d i f f u s i o n i n s h a l l o w w a t e r s . T e l l u s , 23: 129-
175. MacIsaac, J . J . and Dugdale, R . C . , 1969. The k i n e t i c s o f n i t r a t e a n d ammonium u p t a k e by n a t u r a l p o p u l a t i o n s of m a r i n e p h y t o p l a n k t o n . Deep-sea Res., 16:
45-57. May, R . M . , 1973. S t a b i l i t y and c o m p l e x i t y i n model e c o s y s t e m s . Princeton, P r i n c e t o n U n i v e r s i t y Press, 265pp. Pace, M.L., Glasser, J . E . and Pomeroy, L . R . , 1984. A s i m u l a t i o n a n a l y s i s o f c o n t i n e n t a l s h e l f f o o d webs. Mar. B i o l . , 82: 47-63. P a t t e n , B.C., 1968. M a t h e m a t i c a l models o f p l a n k t o n p r o d u c t i o n . I n t . Revue. g e s . H y d r o b i o l . , 53:357-408. P e t e r s o n , R., 1975. The p a r a d o x of t h e p l a n k t o n : a n e q u i l i b r i u m h y p o t h e s i s . Am. Nat., 109: 35-49. Pingree, R.D. and G r i f f i t h s , D . K . , 1977. The b o t t o m mixed l a y e r o n t h e c o n t i n e n t a l s h e l f . E s t . C o a s t . Mar. S c i . , 5: 399-413. P l a t t , T., Denman, K.L. and J a s s b y , A.D., 1977. M o d e l l i n g t h e p r o d u c t i v i t y of phytoplankton. I n : G o l d b e r g , E . D . , McCase, I . N . , G ' B r i e n , J . J . and S t e e l e , J.H. ( E d i t o r s ) , The S e a Vol. 6 : Marine M o d e l l i n g , J o h n Wiley and S o n s , New York, London, Sydney, T o r o n t o , pp.857-890. P l a t t , T . and J a s s b y , A . D . , 1976. The r e l a t i o n s h i p between p h o t o s y n t h e s i s and J . Phycol., l i g h t f o r n a t u r a l assemblages of c o a s t a l marine phytoplankton. 1 2 : 421-430. P o w e l l , T and R i c h e r s o n , P . J . , 1985. Temporal v a r i a t i o n , s p a t i a l h e t e r o g e n e i t y , and c o m p e t i t i o n f o r r e s o u r c e s i n p l a n k t o n s y s t e m s : a t h e o r e t i c a l model. Am. Nat., 125: 431-4611. Richtmyer, R.D. and Morton, K . W . , 1957. D i f f e r e n c e methods f o r i n i t i a l - v a l u e p r o b l e m s . I n t e r s c i e n c e p u b l i s h e r s , N e w York, London Sydney., 405 pp. R i l e y , G.A., Stommel, H . and Bumpus, D.F., 1949. Q u a n t i t a t i v q e c o l o g y o f t h e B u l l . Bingham Gceanogr. C o l l . , 12: plankton o f t h e western North A t l a n t i c .
1-169. Roughgarden, J . , 1978. I n f l u e n c e of c o m p e t i t i o n on p a t c h i n e s s i n a random e n v i r o n m e n t . T h e o r . P o p u l . B i o l . , 14: 185-203. Steele, J.H. and H e n d e r s o n , E.W., 1976. S i m u l a t i o n o f v e r t i c a l s t r u c t u r e i n a p l a n k t o n e c o s y s t e m . S c o t . F i s h . Res. Rep., No. 5. S t e e l e , J . H . and M u l l i n , M . M . , 1977. Z o o p l a n k t o n dynamics. I n : G o l d b e r g , E.D., McCave, I . N . , O'Brien, J . J . and S t e e l e , J . H . ( E d i t o r s ) , The S e a Vol. 6: M a r i n e M o d e l l i n g , J o h n Wiley and S o n s , N e w York, London, Sydney, T o r o n t o , pp.
857-890. Sundby, S . , 1983.. A o n e - d i m e n s i o n a l model f o r t h e v e r t i c a l d i s t r i b u t i o n of p e l a g i c f i s h e g g s i n t h e mixed l a y e r . Deep-sea Res., 30: 645-661. T i l m a n , D . , Kilham, S.S. and Kilham, P . , 1982. P h y t o p l a n k t o n community e c o l o g y : t h e r o l e o f l i m i t i n g n u t r i e n t s . Ann. Rev., E c o l . , S y s t . . 13: 349-373. Williams, R. and H o p k i n s , C . C . , 1975. S a m p l i n g a t o c e a n w e a t h e r s t a t i o n I N D I A i n 1973. A n n a l e s B i o l o g i q u e s , 30: 60-62. W i l l i a m s , R. and Hopkins, C.C.,1976. Sampling a t ocean weather s t a t i o n I N D I A i n 1974. A n n a l e s B i o l o g i q u e s , 31 : 56-60. Woods, J . D . , 1980. D i u r n a l and s e a s o n a l v a r i a t i o n o f c o n v e c t i o n i n t h e windmixed l a y e r o f t h e o c e a n . Q u a r t . J . , R. Met. SOC., 106:379-394. Woods, J . D . , 1985. The p h y s i c s o f t h e r m o c l i n e v e n t i l a t i o n . I n : N i h o u l , J . C . J . (Ed.), Coupled o c e a n - a t m o s p h e r e models. E l s e v i e r Oceanography S e r i e s , 40, Amsterdam, O x f o r d , N e w York, Tokyo, pp 543-590. Woods, J . D . and Gnken, R . , 1982. D i u r n a l v a r i a t i o n and p r i m a r y p r o d u c t i o n i n t h e o c e a n - p r e l i m i n a r y r e s u l t s o f a L a g r a n g i a n e n s e m b l e model. J. Plankt. Res., 4 : 735-756.
331
E S T I M A T E S OF T H E N I T R O G E N FLUX R E Q U I R E D FOR THE MAINTENANCE OF SUBSURFACE CHLOROPHYLL M A X I M A ON THE AGULHAS BANH.
R.A.
CARTER,
BARTLETT a n d V . P .
P.D.
SWART
N a t i o n a l Research I n s t i t u t e f o r Oceanology, Stellenbosch,
Box 3 2 0 ,
7600 RSA
ABSTRACl The w a t e r s o v e r l y i n g t h e A g u l h a s Bank a r e d o m i n a t e d b y s t r o n g t h e r m a l s t r a t i f i c a t i o n i n t h e a u s t r a l summer. Chlorophyll distributions associated w i t h the stratification show w e l l dev e l o p e d maxima a t t h e b a s e o f t h e t h e r m o c l i n e . Nitrate-nitrogen p r o f i l e s g e n e r a l l y show t h a t t h e c h l o r o p h y l l maxima a r e s i t u a t e d i n t h e r e g i n n o f t h e w a t e r column where n i t r a t e - n i t r o g e n concentrations are l i m i t i n g f o r phytoplankton growth. Estimates o f mesozooplankton g r a z i n g and t h e p a r t i t i o n o f p h y t o p l a n k t o n production t o grazing, decomposer pathways, etc., indicate a p h y t o p l a n k t o n p r o d u c t i o n r a t e o f c. 6 3 0 m g C / m e t r e s q u a r e d / d a y a s b e i n g n e c e s s a r y t o m a i n t a i n t h e o b s e r v e d c h l o r o p h y l l maxima. T h i s e s t i m a t e c o n v e r t s t o a n i t r o g e n r e q u i r e m e n t o f 8 . 2 mM N / m e t r e squared/day. 24% o f t h i s r e q u i r e m e n t c a n b e s u p p l i e d b y z o o plankton excretion. The r e m a i n d e r ( 6 . 2 mM N / m e t r e s q u a r e d / d a y ) c a n b e s u p p l i e d b y v e r t i c a l f l u x f r o m t h e b o t t n m w a t e r s , if t h e conditions pertaining i n the s t r a t i f i e d waters o f the English Channel a p p l y and t h e d i f f e r e n c e s i n n i t r a t e - n i t r o g e n c o n c e n t r a t i o n g r a d i e n t s i n t h e n i t r a c l i n e s i n E n g l i s h Channel and Agulhas Bank w a t e r s a r e t a k e n i n t o a c c o u n t . Additional phytoplankton g r a z i n g by o t h e r s i z e f r a c t i o n s i n t h e z o o p l a n k t o n and m i c r o nekton w i l l require e i t h e r increased v e r t i c a l d i f f u s i o n rates o r t u r b u l e n t breakdown o f t h e t h e r m o c l i n e t o meet t h e i n c r e a s e d n i t r o g e n demand.
INTRODUCTION The
Agulhas
tinental continent the
shelf (Fig.
waters
Bank
i s
area
situated
1).
extensive,
Surveys
overlying
the
s t r a t i f i e d i n summer a n d Decker,
south
Shannon, Bank
the
(eg.
tion
between
zooplankton
the
1973;
sparse
c h l o r o p h y l l maxima d e v e l o p
ated
the
(eg.
face
Shannon e t a l .
irregularly
o f
Agulhas
area
clines.
De
an
Brown,
(1984)
subsurface populations,
south
1966)
are
consub-
h a v e shown
strongly
1981)
indicates
have discussed chlorophyll the
maxima,
intense
that
the
subsur-
the
thermo-
likely
connec-
their
associ-
w i t h
the
that
thermally
plankton l i t e r a t u r e from
i n association
and
shaped African
spawning
by
the
332 anchovy,
Engraulis capensis,
t h a t o c c u r s o n t h e A g u l h a s Bank ( c f .
1975)).
E n g r a u l i s mordax o f f s o u t h e r n C a l i f o r n i a ( L a s k e r ,
I
C A P E
PROVINCE
26'E
20%
I
F i g . 1. Map s h o w i n g t h e l o c a t i o n a n d d i m e n s i o n s o f t h e A g u l h a s Bank, s o u t h o f S o u t h A f r i c a . Depths a r e in metres. The d a s h e d l i n e indicates the p o s i t i o n o f the transect i l l u s t r a t e d i n Fig. 2; t h e c l o s e d c i r c l e shows t h e s t a t i o n l o c a t i o n f o r w h i c h d a t a a r e p r e s e n t e d i n F i g . 4 and t h e s t a r i n d i c a t e s t h e l o c a t i o n o f t h e m e a s u r e m e n t s made b y Tromp a n d H o r s t m a n ( 1 9 7 9 ) . Recent surveys
by
have confirmed t h e
t h e R.V.
c i a t e d subsurface c h l o r o p h y l l sentative from
a
February,
1984.
has a p a r a l l e l
transect
their
ture,
3
vertical
location
large spatial
out
west
see
sections
Fig.
taken
made
1)
in
extent.
of
Cape
Agulhas
This
of
latter
these
(Fig.
1)
point
structures, indicating
t h r e e t o f i v e months i n a g e o g r a p h i c a l l y r e s t r i c t e d
a r e a (Tromp and Horstman, Fig.
(for
of
Bank
and asso-
2 i l l u s t r a t e s repre-
Fig.
form
i n the temporal persistence
carried
l i f e spans o f
maxima.
the
Agulhas
thermal s t r a t i f i c a t i o n
Features o f note are the strengths o f the struc-
t u r e s as w e l l as
research
in
distributions
cross-shelf
M e i r i n g NaudE! o n t h e
existence o f
depicts
chlorophyll
typical
5
1979). summer
vertical
and n i t r a t e - n i t r o g e n
profiles
o f
tempera-
from t h e Agulhas
Bank.
S i m i l a r t o n o r t h e r n hemisphere temperate c o n t i n e n t a l s h e l f waters
333
t r a n s e c t on t h e Fig. 2. Wertica1 sections from a cross-shelf A: temperature; 8: c h l o r o p h y l l 5 A g u l h a s B a n k , F e b r u a r , y , 1984. (units are p g / l )
(e.g.
1982:
Cullen,
Holligan
maxima a r e s i t u a t e d i n t h e Fig.
3b,
tions. and
c)
a1.,1984a,
features
the
consequently
suggest
chlorophyll
b),
the
i n nitrate-nitrogen
t h e maximum g r a d i e n t
These
above
et
thermocline but generally
that
maxima
nitrogen supply
to
phytoplankton
may
the
be
(e.g.
concentra-
production
nitrogen
phytoplankton i n
c u m s t a n c e s h a s r e c e i v e d some a t t e n t i o n .
chlorophyll above
limited these
in and
cir-
C u l l e n and Eppley (1981)
showed were
that
southern
dominated
by
(Holligan e t al. cally
Californian
subsurface
dinoflagellates
(as
are
chlorophyll of
Gulf
maxima
Maine
maxima
1984a)) and t h a t t h e s e organisms m i g r a t e d v e r t i -
t o replenish c e l l nutrient levels.
Holligan e t al.
(1984~)
d e m o n s t r a t e d t h a t C e l t i c Sea c h l o r o p h y l l m a x i m a were d o m i n a t e d b y naked
flagellates
nutrient differ
from
diatoms
the
(De
generally
above
Decker,
that
vertical
The
case
in
Bank
that
Carter,
supplied
a
therefore
they
are
be
maxima
dominated
data),
and
concentrations.
either
their
chlorophyll
unpublished
chlorophyll
must
diffusion
Agulhas
examples
1973;
a t t a i n higher
in this
supply
and
requirements.
by
also
Nitrogen
through
vertical
d i f f u s i o n o r by t u r b u l e n t d i s r u p t i o n o f t h e t h e r m o c l i n e . As a p r e l i m i n a r y
to further
r e s e a r c h on t h e dynamics o f
Agul-
h a s Bank c h l o r o p h y l l maxima h e r e we c a l c u l a t e t h e n i t r o g e n s u p p l y rates
required
distributions
to
maintain
i n the
face
observed of
a s s o c i a t e d mesozooplankton.
phytoplankton
estimated
grazing
pressure
by
the
T h i s method i s used because
of
the
i n t h e a r e a and
paucity o f r e l i a b l e primary production estimates
Our major purpose i s
t h e l o n g e r time base t h e approach provides. t o determine whether
(chlorophyll)
thermocline
invoked t o provide nitrogen a t
d i s r u p t i o n processes
the
required r a t e
to
have
the
t o be
euphotic
zone.
DATA
i s based on t h e
The c a l c u l a t i o n d e s c r i b e d b e l o w tributions
of
phytoplankton
depicted i n Fig.
in
collected
4.
the
(as
Plettenberg
1984.
Chlorophyll
a
acetone
extracts
filtered
of
calibrated
against
tion)
well
as
(Strickland
and
Parsons,
(dry
weight;
plankton
dry
same
of
pure
were
zooplankton
during
measured
1966),
those
zooplankton
and
converted
species to
Corpora-
samples
were
submersible
pump
larger
the
than
phyto-
105
pm
biomass estimates
identification.
carbon
90%
determinations
used f o r
being taken f o r
April, on
fluorometer
(Sigma
Mesozooplankton (2000 1 / m i n )
dis-
i s based were
1)
(Fig.
chlorophyll
depths as Only
were
and
spectrophotometric
subsamples
Lovegrove,
2)
figure
11 s a m p l e s o n a T u r n e r
a high yield
the
weights
region
1972).
samples.
were c o l l e c t e d w i t h
Bay
this
phaeopigments
parallel
o b t a i n e d b y means o f
plankton pigment
and
extracts
as
system operated a t
chlorophyll
The d a t a o n w h i c h
vertical
equivalents
Zooby
335 0
5
10
I: 1:
15
do
c:
0
5
10
15
10
I.
Y ?
0
20
10
0
20
10
F i n . 3 . V e r t i c a l o r o f i l e s m e a s u r e d o n t h e A q u l h a s Bank d u r i n s February, 1984, showing t y p i c a l d i s t r i b u t i o n s i n s h a l l o w ( A ) ; m o d e r a t e l y d e e p ( 8 ) and deep m i x e d l a y e r ( C ) c o n d i t i o n s .
m u l t i p l y i n g by a f a c t o r o f 0,45 subsamples ranges
by
were
fractionated
filtration
to
into
through
105-297
appropriate
1979).
Mostert
(1983)
on
Both sets
filtered
of
p m size
p m and >297 meshes.
The
nitrate-
4 were measured
c o n c e n t r a t i o n s shown i n F i g .
and a m m o n i a - n i t r o g e n according
(Hutchings,
samples
on
an
onboard
Technicon a u t o a n a l y z e r system.
CALCULATION
The c a l c u l a t i o n i s b a s e d on t h e f o l l o w i n g t w o a s s u m p t i o n s :
1)
Phytoplankton
negligible
production
relative
to
below
rates at
the
chlorophyll
and above
the
maximum
chlorophyll
i s
maxi-
mum.
2)
Steady-state
transport
conditions
apply,
i.e.
there
The c o n s e q u e n c e s o f t h e s e a s s u m p t i o n s a r e vorous
i s
no
isopycnal
o f e i t h e r p h y t o p l a n k t o n o r n i t r o g e n i n t o t h e area.
zooplankton
i n the
water
column
are
that
a l l
dependent
the herbion
phyto-
p l a n k t o n p r o d u c t i o n a t a n d a b o v e t h e c h l o r o p h y l l maximum a n d t h a t the o n l y source o f
'new'
n i t r o g e n t o t h e euphotic zone i s d i f f u s -
336 I -1
F i g . 4. V e r t i c a l p r o f i l e s m e a s u r e d o n t h e A g u l h a s Bank i n A p r i l , 1984. A: Temperature, n i t r a t e - and ammonia-nitrogen; 8: Plankton. i o n or a d v e c t i o n f r o m below. The
i n t e g r a t e d combined presented i n Fig.
profile
4
p l a n k t o n was d o m i n a t e d b y natus,
zooplankton i s 494
rngC/metre
the Calanoid
Paracalanus parvus,
biomass
Paracalanus
estimate
squared.
for The
the zoo-
copepods C a l a n o i d e s c a r i -
sp.
and
Centropages
bra-
A l l o f t h e s e s p e c i e s were r e p o r t e d f r o m t h e a r e a by De
chiatus. Decker
(1973).
Carter
versus
temperature
(T)
(1983) for
measured
similar
sized
respiration species
rates
from
(R)
southern
Eenguela k e l p beds and o b t a i n e d r e g r e s s i o n s o f
R ( I 02/mgDW/hr)
+
= 3,45
0,17T
f o r copepods i n t h e s i z e r a n g e 105-297 p m ,
R( for
I OZ/mgDW/hr) copepods
gressions
and
r e y ~ (1979) ranges
of
= 3,11
larger those to
+
0,16T
t h a n 2 9 7 prn. for
Carter
(1983)
production/respiration
calculate
copepods.
and
consumption
Applying
these
rates rates
used given
for
and
the
the
these by
re-
Humph-
t w c
size
appropriate
337 temperature used
to
corrections
construct
to
Fig.
sumption estimate o f
the
4
two
size
yields
a
126 m g C / m e t r e
range
biomass
zooplankton
estimates
community
con-
squared/day.
R e c e n t a n a l y s e s h a v e i n d i c a t e d t h a t o n l y some 2 0 - 3 0 % o f p h y t o plankton production i s
channelled
through
g o i n g t h r o u g h decomposer p r o c e s s e s Peterson, of
this
I f a figure of
1983).
calculation
the
net
z o o p l a n k t o n r a t i o n i s 630
t o
the
squared)
mgC/metre
a
chlorophyll
for
squared/day
required
orophyll
ratio
plankton
community
i t
i s
of
55,85,
assumed
o c c u r s a t and
i s
doubling
above
that the
most
satisfy
According
(207 m g / m e t r e 3.0
number o f
u s i n g a measured c a r b o n / c h l -
equivalent
rate
rest
1984~;
to
squared/day.
concentration
which,
the
the purposes
t h i s implies a phytoplankton assimilation
mgC/mgChla/metre
above,
20% i s assumed
photosynthesis
the
integrated
zooplankton;
H o l l i g a n e t al.,
(e.9.
of
to
an
0,05/day.
of
the
maximum;
phyto-
as
phytoplankton
a
chlorophyll
overall But,
stated
production
the
integrated
c h l o r o p h y l l c o n c e n t r a t i o n f o r t h i s s e c t i o n o f t h e w a t e r column i s 52 m g / m e t r e becomes metre
squared and c o n s e q u e n t l y
C,22/day
squared/day.
C a r t e r (1983) and
and
values
the
nitrogen requirement
for
t h e r e f o r e 8,2 mM N/metre Holligan e t al.
for
the
high,
the
maximum
assimi-
W i l l -
(eg.
the
English
t h e Agulhas
excretion
rate
This i s equivalent
sampled The
n=12).
production
this
of
to
rate
i s
rement o f t h e phytoplankton.
Bank,
species.
24% o f Thus
phytoplankton
was of
included i n
19,1% o f
from Fig.
is- to
body
4 yields
a
squared/
nitrogen requi-
76% o r 6 , 2 mM N / m e t r e of
range
Generalizing t h i s
the estimated
flux
a
Paracalanus
2 mM a m m o n i a - N / m e t r e
c.
day m u s t b e s u p p l i e d b y v e r t i c a l
for
Channel.
an e x c r e t i o n r a t e
t o t h e zooplankton biomass e s t i m a t e d e r i v e d
the
(sd=C,15,
estimated
( 1 9 8 4 ~ ) measured e x c r e t i o n r a t e s
being obtained f o r
i f
phytoplankton
squared/day.
a s p e c i e s i m p o r t a n t on
thermocline
whilst
6,07
1 9 8 4 was
t h e above
t h e i r s u i t e o f measurements;
day.
rate
rngC/mgChla/
extraordinarily
theoretical
ratio
d i f f e r e n t s i z e d copepods i n
population
doubling
12,l
1983).
o n t h e A g u l h a s Bank i n A p r i l ,
nitrogen/day
not
Benguela k e l p beds,
i s well within
The mean c a r b o n / n i t r o g e n
parvus,
are
required
number
r e p o r t i n g p h y t o p l a n k t o n d o u b l i n g r a t e s between 0,23
l a t i o n number
of
the
assimilation
These
i n southern
1,48/day
iams e t a l . ,
the
squared/
n i t r o g e n from below maintain
e p i s o d i c or p e r i o d i c d i s r u p t i o n o f t h e t h e r m o c l i n e .
itself
the
without
338 Holligan
et
(1984~1, i n
al.
umns i n t h e C e l t i c
2 , 3 mM N / m e t r e bottom
tion
the
This
Agulhas
in
gradients
diffusion
rate
(Fick's
Law),
Agulhas
Bank
far
i s
than
less
However,
in
nitracline al.
a t t a i n c.
0,75
pM
to
the
basis
a
that
vertical
required
concentra-
Channel
4 i s c.
compared t o
this
English
i n Fig.
proportional
squared/day
rate o f
calculated
(1984c),
apparent
waters
the
nitrate-nitrogen
a proportionally higher
On
Channel. N/metre
i s
i n Holligan e t
displayed
whereas t h e g r a d i e n t
col-
water
on t h e b a s i s o f t h e h e a t f l u x i n t o the
Bank. the
stratified
calculated a vertical diffusion
squared/day
waters.
flux for
Sea,
thermally
2
waters,
pM N/metre
concentration
rate
i s
diffusion
gradient
e x p e c t e d for
thus
calculated
As
N/metre.
for
the
rate
of
English
6 mM
c.
c a n b e c a l c u l a t e d f o r s t r a t i f i e d A g u l h a s Bank
waters which i s c l o s e t o t h e r a t e r e q u i r e d by the phytoplankton. Therefore, and t h e
when v i e w e d
analyses
recycling appears
by
to
be
of
the
that
only
et
zooplankton
just
al.
and
sufficient
to
4 ) . However,
p h y l l maximum ( F i g . fact
i n the l i g h t o f
Holligan
plankton rate.
thus
eg.
Consequently
disruption
of
this
the
the
c),
nitrogen the
nitrogen diffusion
observed
and
chloro-
c o n c l u s i o n i s based on t h e
w i l l
required
higher v e r t i c a l thermocline
calculations
and
was
used
in
the
L a r g e r z o o p l a n k t o n ( 500
f i s h larvae,
increasing
above b
vertical maintain
mesozooplankton biomass
t i o n o f the zooplankton r a t i o n . also micronekton,
the
(1984a,
also
g r a z e on
phytoplankton rates or
diffusion associated
estima-
pm)
and
t h e phytoproduction turbulent
enrichment
of
the
w a t e r s a b o v e t h e c h l o r o p h y l l maximum m u s t t h u s b e i n v o k e d t o meet t h e h i g h e r n i t r o g e n demand. double
diffusion,
19821,
storms
upwelling
Processes t h a t (e.g.
Swart,
o r b r e a k i n g i n t e r n a l waves.
can o p e r a t e here are
1983;
Schumann e t
These
al.,
phenonema are
currently being investigated.
REFERENCES B r o w n , P.C., 1980. P h y t o p l a n k t o n p r o d u c t i o n s t u d i e s i n t h e c o a s t a l w a t e r s o f f t h e Cape P e n i n s u l a , S o u t h A f r i c a . M.Sc t h e s i s , U n i v e r s i t y o f Cape Town, 9 8 p p . B r o w n , P.C., 1981. P e l a g i c p h y t o p l a n k t o n , p r i m a r y p r o d u c t i o n and n u t r i e n t s u p p l y i n t h e s o u t h e r n Benguela r e g i o n . Trans. roy. Soc. S. A f r i . , 4 4 : 347-356. 1983. The r o l e o f p l a n k t o n a n d m i c r o n e k t o n i n c a r C a r t e r , R.A., bon f l o w t h r o u g h a s o u t h e r n B e n g u e l a k e l p bed. Ph.D t h e s i s , U n i v e r s i t y o f Cape Town, 174pp.
339 C u l l e n , J.J., 1 9 8 2 . The d e e p c h l o r o p h y l l maximum: Comparing vertical profiles o f chlorophyll Can. J. F i s h . A q u a t . Sci., 39: 791-803. C u l l e n , J . J . a n d E p p l e y , R.W., 1981. C h l o r o p h y l l maximum l a y e r s o f t h e S o u t h e r n C a l i f o r n i a B i g h t and p o s s i b l e mechanisms o f t h e i r f o r m a t i o n and m a i n t e n a n c e . Oceanol. A c t a , 4 : 23-32. 1973. A g u l h a s Bank P l a n k t o n . I n : 8. Z e i t s c h e l De D e c k e r , A.H., ( E d i t o r ) , B i o l o g y o f t h e I n d i a n Ocean. E c o l o g i c a l s t u d i e s . A n a l y s i s a n d s y n t h e s i s , v o l . 3 , S p r i n g e r V e r l a g , B e r l i n , pp. 189-219. H o l l i g a n , P.M., B a l c h , W.M. a n d Y e n t s c h , C.M., 1 9 8 4 a . The s i g n i f i c a n c e o f s u b s u r f a c e c h l o r o p h y l l , n i t r i t e a n d ammonia maxima i n r e l a t i o n t o n i t r o g e n f o r phytoplankton growth i n s t r a t i f i e d w a t e r s o f t h e G u l f o f M a i n e . J. Mar. Res., 4 2 : 1 0 5 2 - 1 0 7 3 . H o l l i g a n , P.M., H a r r i s , R.P., Newell, R.C., H a r b o u r , D.S., Head, R.N., L i n l e y , E.A.S., Lucas, M.I., T r a n t e r , P.R.G. a n d Weekl e y , C.M., 1984b. V e r t i c a l d i s t r i b u t i o n a n d p a r t i t i o n i n g o f o r g a n i c c a r b o n i n m i x e d , f r o n t a l and s t r a t i f i e d w a t e r s o f t h e E n g l i s h Channel. Mar. E c o l . Prog. S e r . , 14: 111-127. H o l l i g a n , P.M., W i l l i a m s , P.J.LeB., P u r d i e , D. a n d H a r r i s , R.P., 1 9 8 4 ~ . P h o t o s y n t h e s i s , r e s p i r a t i o n and n i t r o g e n s u p p l y o f p l a n k t o n p o p u l a t i o n s i n s t r a t i f i e d , f r o n t a l and t i d a l l y m i x e d s h e l f w a t e r s . Mar. E c o l . P r o g . S e r . 1 7 : 201-213. H u m p h r e y s , W.E., 1979. P r o d u c t i o n and r e s p i r a t i o n i n a n i m a l p o p u l a t i o n s . J. A n i m a l E c o l . , 4 8 : 427-453. H u t c h i n g s , L., 1979. Z o o p l a n k t o n o f t h e Cape P e n i n s u l a u p w e l l i n g region. Ph.D t h e s i s , U n i v e r s i t y o f Cape Town, 2 4 0 p p . L a s k e r , R., 1975. F i e l d c r i t e r i a f o r s u r v i v a l o f a n c h o v y l a r v a e : t h e r e l a t i o n b e t w e e n i n s h o r e c h l o r o p h y l l maximum l a y e r s a n d s u c c e s s f u l f i r s t f e e d i n g . F i s h B u l l . (U.S.), 73: 453-462. Lovegrove, T., 1966. The d e t e r m i n a t i o n o f t h e d r y w e i g h t o f p l a n k t o n and t h e e f f e c t s o f v a r i o u s f a c t o r s on t h e v a l u e s H. Barnes ( E d i t o r ) , Contemporary S t u d i e s i n obtained. In: M a r i n e S c i e n c e . George A l l e n and Unwin L t d . , pp 429-467. M o s t e r t , S.A., 1983. P r o c e d u r e s used i n S o u t h A f r i c a f o r t h e automatic photometric determination o f micronutrients i n s e a w a t e r . S. A f r . J. m a r . S c i . , 1: 189-198. P e t e r s o n , B.J., 1983. S y n t h e s i s o f c a r b o n s t o c k s a n d f l o w s i n t h e o p e n o c e a n m i x e d l a y e r . N A T O Doc. No. 1873A-03/28/83, 1Opp. Schumann, E . H . , P e r r i n s , L.A. and H u n t e r , I . T . , 1982. U p w e l l i n g a l o n g t h e s o u t h c o a s t o f t h e Cape P r o v i n c e , S o u t h A f r i c a . S. A f r . J. S c i . , 78: 238-242. S h a n n o n , L.U., 1966. H y d r o l o g y o f t h e s o u t h a n d w e s t c o a s t s o f S o u t h A f r i c a . I n v e s t . Rep. D i v . S e a f i s h . S. A f r . , 58: 52pp. Shannon, L.V., H u t c h i n g s , L . , B a i l e y , G.W. and S h e l t o n , P.A., 1984. S p a t i a l a n d t e m p o r a l d i s t r i b u t i o n o f c h l o r o p h y l l i n S o u t h e r n A f r i c a n w a t e r s as deduced f r o m s h i p and s a t e l l i t e m e a s u r e m e n t s a n d t h e i r i m p l i c a t i o n s f o r p e l a g i c f i s h e r i e s . S. A f r . J. m a r . S c i . , 2: 109-130. S t r i c k l a n d , J.D.H. and P a r s o n s , T.R., 1972. A p r a c t i c l h a n d b o o k o f s e a w a t e r a n a l y s i s . 2 n d E d i t i o n , B u l l . F i s h . Res. Ed. Canada, 167: 310pp. S w a r t , V.P., 1983. I n f l u e n c e o f t h e A g u l h a s C u r r e n t o n t h e A g u l h a s Bank. S. A f r . J. S c i . , 7 9 : 160-161. Tromp, B. a n d H o r s t m a n , D., 1979. Cape s o u t h c o a s t u p w e l l i n g m e a s u r e m e n t s . U n p u b l i s h e d MS., Sea F i s h e r i e s R e s e a r c h I n s t i t u t e , 44pp. W i l l i a m s , P.LeB., Heinemann, K.R., M a r r ' a , J. a n d P u r d i e , D . A . , 1983. C o m p a r i s o n s o f 14C a n d 02 m e a s u r e m e n t s o f p h y t o p l a n k t o n p r o d u c t i o n i n o l i g o t r o p h i c w a t e r s . N a t u r e , 305: 49-50.
a.
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341
MODELLING THE T I M E DEPENDENT PHOTOADAPTATION OF PHYTOPLANKTON TO FLUCTUATING
IIGHT*
KENNETH L. DENMAN' and JOHN MARRA2 ' I n s t i t u t e o f Ocean Sciences,
P.0.Box
6000, Sidney B.C.
Canada, V8L 4B2
'Lamont-Doherty G e o l o g i c a l Observatory o f Columbia U n i v e r s i t y , Palisades, N. Y., U.S.A. 10964
ABSTRACT P h y t o p l a n k t o n i n t h e p y c n o c l i n e and i n t h e upper m i x i n g l a y e r o f t h e ocean are s u b j e c t e d t o v e r t i c a l displacements o v e r a range o f t i m e s c a l e s c h a r a c t e r i s t i c o f f l u i d motions r e s u l t i n g f r o m i n t e r n a l waves and t u r b u l e n t processes. Because underwater l i g h t decreases a p p r o x i m a t e l y e x p o n e n t i a l l y w i t h depth, t h e p h y t o p l a n k t o n t h u s e x p e r i e n c e v a r i a t i o n s i n i r r a d i a n c e o v e r t h e same range o f time scales. I n general, because o f p h o t o a d a p t a t i o n . t h e i n s t a n t a n e o u s r a t e o f p h o t o s y n t h e s i s does n o t depend o n l y on t h e i n s t a n t a n e o u s i r r a d i a n c e b u t r a t h e r on some f u n c t i o n o f t h e c e l l s ' l i g h t h i s t o r y . We p r e s e n t a l i n e a r response model f o r p h o t o a d a p t a t i o n o f t h e r a t e o f p h o t o s y n t h e s i s t o v a r y i n g i r r a d i a n c e . Results o f l a b o r a t o r y experiments under c o n t r o l l e d l i g h t i n g a r e used t o d e t e r mine t i m e s c a l e s f o r p h o t o a d a p t a t i o n and t o e v a l u a t e f u n c t i o n a l r e l a t i o n s h i p s and c o e f f i c i e n t s f o r t h e model. The model i s used t o s i m u l a t e t h e g e n e r a t i o n o f p h o t o s y n t h e s i s - i r r a d i a n c e curves from i n c u b a t i o n experiments, and i t i s t e s t e d w i t h d a t a f r o m a c u l t u r e grown i n a greenhouse under n a t u r a l s u n l i g h t . F o r an i d e a l i z e d case, t h e f r e q u e n c y response f u n c t i o n f o r a d a p t a t i o n t o i n t e r n a l wave displacements i s c a l c u l a t e d . Waves w i t h p e r i o d s equal t o o r s h o r t e r t h a n t h e a d a p t i v e response t i m e a l l o w l i t t l e a d a p t a t i o n . INTRODUCTION Successful s u r v i v a l and p h o t o s y n t h e t i c p r o d u c t i o n o f p h y t o p l a n k t o n r e q u i r e t h a t t h e y f i n d and s t a y i n r e g i o n s o f t h e ocean w i t h b o t h s u f f i c i e n t l i g h t and s u f f i c i e n t nutrients.
T h i s a d m i t t e d l y s i m p l i s t i c view o f p h y t o p l a n k t o n dynamics
emphasizes t h e importance t o p r i m a r y p r o d u c t i o n o f l i g h t a v a i l a b i l i t y and u t i l i z a t i o n by t h e c e l l s .
S i g n i f i c a n t r e s e a r c h e f f o r t has gone i n t o u n d e r s t a n d i n g
and q u a n t i f y i n g t h e f u n c t i o n a l dependence o f t h e r a t e o f p h o t o s y n t h e t i c product i o n P on a v a i l a b l e l i g h t I (e.g. so-called
Platt.
Gallegos and H a r r i s o n ,
1980).
The
'P-I c u r v e s ' t h a t r e s u l t a r e u s u a l l y o b t a i n e d f r o m 14C i n c u b a t i o n
experiments where samples o f l i v i n g p h y t o p l a n k t o n a r e i n o c u l a t e d w i t h 14C,
and
s p l i t i n t o subsamples which a r e i n c u b a t e d s i m u l t a n e o u s l y a t d i f f e r e n t c o n s t a n t i r r a d i a n c e s f o r a s e t t i m e p e r i o d ( 1 h t o 1 day, u s u a l l y s e v e r a l h).
The n e t
carbon uptake by t h e c e l l s i s determined and d i v i d e d by t h e i n c u b a t i o n t i m e t o g i v e an average r a t e o f p h o t o s y n t h e t i c carbon p r o d u c t i o n a t each l i g h t i n t e n sity.
*
The i n d i v i d u a l d a t a v a l u e s a r e t h e n p l o t t e d , u s u a l l y g i v i n g a r e l a t i v e l y
L-DGO c o n t r i b u t i o n no. 3391.
342 I n g e n e r a l , P - I curves a r e n o t r e p e a t a b l e between e x p e r i -
smooth P - I curve.
ments. and d e t e r m i n i n g t h e sources o f t h i s v a r i a b l i t y i s a r e s e a r c h problem currently o f great interest. T h a t one s o u r c e o f t h i s v a r i a b i l i t y may be t h e a d a p t a t i o n o f t h e c e l l s ' phy-
.
s i o l o g y t o t h e v a r y i n g i r r a d i a n c e i t s e l f was c l e a r l y demonstrated by H a r r i s and L o t t (1973) and H a r r i s (1973).
They showed f r o m s h o r t t e r m p h o t o s y n t h e t i c oxy-
gen p r o d u c t i o n r a t e measurements w i t h l a b o r a t o r y c u l t u r e s and f i e l d samples t h a t the 'instantaneous'
r a t e v a l u e s p r e s e n t e d on a P - I phase p l o t f o r a f u l l day d i d
n o t t r a c e o u t a s i n g l e P - I curve.
There was h y s t e r e s i s :
a c l o s e d c u r v e was
drawn o u t w i t h h i g h r a t e s i n t h e morning, a midday d e p r e s s i o n , and v a l u e s i n t h e a f t e r n o o n r e c o v e r i n g b u t l o w e r t h a n a t t h e same i r r a d i a n c e s observed i n t h e morning.
The e x t e n t o f t h e h y s t e r e s i s v a r i e d w i t h season and w i t h t h e degree of These f i n d i n g s were r e p e a t e d and extend-
c l o u d i n e s s o v e r t h e p r e v i o u s few days. ed by Marra (1978a,
b).
Nore r e c e n t s t u d i e s by M a r r a (1980b). M a t l i c k (1980).
R i v k i n e t al.
Lewis, C u l l e n and P l a t t (1984). ted),
(1982).
F a l k o w s k i (1980. 1983). P r 5 z e 1 i n and
S l a g s t a d (1982).
Post e t al.
(1984).
Lewis and Smith (1983).
Geider and P l a t t ( s u b m i t -
and o t h e r s have e s t a b l i s h e d t h a t t h i s p h o t o a d a p t a t i o n o f t h e p h y t o p l a n k t o n
t o d i f f e r e n t p r e c o n d i t i o n i n g o r l i g h t h i s t o r i e s occurs i n a v a r i e t y o f d i f f e r e n t forms and a v a r i e t y o f d i f f e r e n t t i m e s c a l e s .
I n addition,
several o f these
s t u d i e s have been s u c c e s s f u l i n m o d e l l i n g t h e a d a p t i v e responses by f i r s t o r d e r reaction kinetics,
s u g g e s t i n g a p o s s i b l e f o r m f o r a more d e t a i l e d model.
MODEL FORMULATION L i n e a r response model P r e v i o u s work shows t h a t t h e f u n c t i o n a l r e l a t i o n s h i p between t h e r a t e o f p r i m a r y p r o d u c t i o n and t h e i r r a d i a n c e ( t h e P - I c u r v e ) i s c l e a r l y n o n l i n e a r and n o t unique.
However, t h e c o e f f i c i e n t s t h a t d e s c r i b e t h e P - I c u r v e may them-
selves adapt t o i r r a d i a n c e c o n d i t i o n s i n a l i n e a r fashion.
Hence, we w i l l s t a r t
w i t h t h e assumption t h a t each c o e f f i c i e n t obeys t h e p r i n c i p l e s o f l i n e a r systems t h e o r y and can be d e s c r i b e d by a l i n e a r response model as o u t l i n e d i n , f o r example, J e n k i n s and Watts (1968). s t r a i g h t forward,
A p a r t f r o m t h e s o l u t i o n s b e i n g known and
o t h e r advantages a r e t h a t s o l u t i o n s w i t h d i f f e r e n t response
t i m e s can be superposed and t h a t each c o e f f i c i e n t can have independent response times, We assume t h a t t h e p h y t o p l a n k t o n have two l i m i t i n g P - I curves, a f u l l y d a r k
adapted c u r v e ( s u b s c r i p t D) and a f u l l y h i g h l i g h t adapted c u r v e ( s u b s c r i p t L), such t h a t t h e i n s t a n t a n e o u s r a t e o f p r o d u c t i o n would always f a l l between t h e s e two curves.
Furthermore,
t h e degree o f a d a p t a t i o n o f t h e c e l l s a t - a n y t i m e from
t h e d a r k adapted c u r v e t o t h e l i g h t adapted c u r v e i s a l i n e a r f u n c t i o n o f r e c e n t ' i n h i b i t i n g ' irradiances.
I f A ( t ) i s a t i m e dependent c o e f f i c i e n t o f t h e c u r v e
343 d e s c r i b i n g t h e f u n c t i o n a l dependence on i r r a d i a n c e I ( t ) o f t h e i n s t a n t a n e o u s photosynthesis r a t e P(t),
A ( t ) = Y ( t ) AL
+
(1
-
then,
Y(t))
a f t e r J e n k i n s and Watts (1968).
AD = Y ( t ) ( A L - AD)
The degree o f a d a p t a t i o n Y ( t )
of A(t)
+
AD.
f r o m c u r v e D t o c u r v e L i s g i v e n by t h e
convolution i n t e g r a l 0
Y(t)
X ( t - t s )dt,.
= Jh(ts) 0
F o r m a l l y , h ( t ) i s t h e response o f t h e l i n e a r system t o an impulse f u n c t i o n i n p u t , and X ( t ) .
the input function,
i s t h e i n h i b i t i n g o r adaptive c a p a b i l i t y o f
a g i v e n i r r a d i a n c e I(t). Marra (1980b).
F a l k o w s k i (1980.
1983) and o t h e r s have
used w i t h some success f i r s t o r d e r k i n e t i c s t o f i t d a t a f r o m p h o t o a d a p t a t i o n experiments, s u g g e s t i n g a s i m p l e e x p o n e n t i a l response f u n c t i o n f o r m f o r h ( t ) : 1 -t/T, h(t) = - e T I n particular, f u n c t i o n s Y,
(3) f o r a step f u n c t i o n i n X(t).
we g e t Y ( t )
= 1 - e-t/T.
The
h, X and T a r e s p e c i f i c t o t h e c o e f f i c i e n t A and would be so
s u b s c r i p t e d when more t h a n one c o e f f i c i e n t was b e i n g modelled. t h e r e can be m u l t i p l e
response t i m e s T f o r each c o e f f i c i e n t ,
I n addition, f o r example, one
w i t h a v a l u e o f 1 o r 2 h and one w i t h a v a l u e o f s e v e r a l days. B e f o r e t h e a d a p t a t i o n model d e s c r i b e d by e q u a t i o n s ( 1 ) - ( 3 )
can be o f use, we
must choose e x p l i c i t f u n c t i o n a l forms f o r t h e i n s t a n t a n e o u s r a t e o f photos y n t h e t i c p r o d u c t i o n P ( t ) and t h e i n h i b i t i n g o r a d a p t i v e c a p a b i l i t y X ( I ( t ) )
of a
g i v e n i r r a d i a n c e . and a v a l u e f o r t h e a d a p t a t i o n response t i m e T t h a t s c a l e s t h e e x p o n e n t i a l response.
S i n c e t h e experiments t o be used measured P ( t ) r a t h e r
t h a n A ( t ) (e.g.
1978a. b).
Marra,
s i m p l e r f u n c t i o n a l forms t h a t can be i n v e r t e d
t o solve f o r t h e c o e f f i c i e n t s A ( t ) are desired. Choice o f f u n c t i o n a l forms S u b s t a n t i a l e f f o r t has been expended on f i t t i n g v a r i o u s f u n c t i o n a l forms o f t h e P - I c u r v e t o e x p e r i m e n t a l data, e.g. 1981; and G a l l e g o s and P l a t t .
1981.
P l a t t e t al..
1980; Lederman and T e t t ,
These curves do n o t r e p r e s e n t i n s t a n t a n e o u s
r e l a t i o n s h i p s s i n c e t h e p r o d u c t i v i t y r a t e s o b t a i n e d a r e t i m e averages f r o m 14C i n c u b a t i o n e x p e r i m e n t s l a s t i n g f r o m about 112 t o s e v e r a l h. s h a l l assume s i m i l a r forms f o r t h e i n s t a n t a n e o u s P - I curves.
Nevertheless,
we
We s h a l l use two
e x p r e s s i o n s t h a t f i t t h e i n c u b a t i o n d a t a w e l l where p h o t o i n h i b i t i o n does n o t occur:
344
where p a ( t ) = P s ( t ) Pg/a.
f o r c o n s t a n t s l o p e a t I = 0 and p b ( t ) = P u f o r c o n s t a n t I k =
The two cases o f c o n s t a n t i n i t i a l s l o p e and c o n s t a n t I k u s u a l l y w i l l o n l y
y i e l d small d i f f e r e n c e s i n the r e s u l t s .
The l a t t e r case r e p r e s e n t s t h e s i t u -
a t i o n o f t e n found w i t h f i t t e d c o e f f i c i e n t s from a l a r g e number o f i n c u b a t i o n s t h a t t h e c o e f f i c i e n t s a and Ps a r e
w i t h n a t u r a l assemblages o f p h y t o p l a n k t o n :
p o s i t i v e l y c o r r e l a t e d implying t h a t t h e i r r a t i o I k i s roughly constant. Expressions ( 4 ) and ( 5 ) a r e p l o t t e d i n Fig.
1.
I Fig. 1. Expressions f o r t h e i n s t a n t a n e o u s r a t e o f p r o d u c t i o n P as a f u n c t i o n o f i r r a d i a n c e I. We have made two i m p l i c i t assumptions:
(i) the c o e f f i c i e n t A ( t )
i n f a c t t h e asymptotic r a t e o f production Ps(t), p h o t o i n h i b i t i o n f a c t o r b. p r i n c i p l e o f course,
i n ( 1 ) i s now
and (ii)t h e r e i s no e x p l i c i t
Rather, a d a p t a t i o n occurs o n l y by v a r y i n g P,(t).
In
t h e o t h e r c o e f f i c i e n t s a and b c o u l d a l s o vary.
There i s l i t t l e i n f o r m a t i o n on t h e i n h i b i t i n g o r a d a p t i v e c a p a b i l i t y o f a g i v e n i r r a d i a n c e I. o t h e r t h a n t h e assumptions t h a t h i g h l i g h t somehow damages t h e c e l l u l a r p h o t o s y n t h e t i c a p p a r a t u s a t a r a t e f a s t e r t h a n r e c o v e r y processes o c c u r b u t t h a t a t l o w e r l i g h t r e c o v e r y processes may keep pace w i t h photodestruction.
For mathematical expediency,
we t r y two e x p r e s s i o n s f o r X(1). one
345 w i t h a d a p t i v e i n h i b i t i o n i n c r e a s i n g f r o m I = 0 and one w i t h a t h r e s h o l d f o r i n h i b i t i o n a t I b (see P l a t t and Gallegos,
The e x p r e s s i o n s a r e p l o t t e d i n Fig.
2:
1980):
t h e sharpness of t h e o n s e t o f i n h i b i t i o n
can be i n c r e a s e d by i n c r e a s i n g t h e exponent i n (6) and (7).
We s h a l l choose I b
( w h i c h a l s o s c a l e s t h e r a t e o f i n c r e a s e o f X ) f r o m e x p e r i m e n t a l data.
'r
.a
1
Fig. 2. E x p r e s s i o n s f o r t h e i n h i b i t i n g o r a d a p t i n g c a p a b i l i t y X(1) as a f u n c t i o n o f ir r a d i ance. Once a l l t h e r e q u i r e d f u n c t i o n s and c o e f f i c i e n t s a r e e s t i m a t e d , t h e model i s s o l v e d i n t h e f o l l o w i n g manner.
A t each t i m e t, t h e degree o f a d a p t a t i o n Y ( t )
i s c a l c u l a t e d f r o m ( 2 ) . f o r each v a r y i n g c o e f f i c i e n t based on t h e h i s t o r y o f i n h i b i t i n g i r r a d i a n c e r e c e i v e d by t h e c e l l s X(1) ( c a l c u l a t e d f r o m ( 6 ) o r (7)). The i n s t a n t a n e o u s v a l u e o f each a d a p t i n g c o e f f i c i e n t i s c a l c u l a t e d f r o m ( 1 ) and the instantaneous r a t e o f photosynthetic production i s c a l c u l a t e d from (4) o r
(5).
Given an a p p r o p r i a t e i n i t i a l c o n d i t i o n , t h e c o n v o l u t i o n i n t e g r a l (2) need
o n l y be c a l c u l a t e d back one t i m e s t e p and t h e p r e v i o u s v a l u e a d j u s t e d by a constant f a c t o r .
For s p e c i a l s i m p l e cases (e.g.
can be s o l v e d e x a c t l y as a f u n c t i o n o f time.
constant irradiance),
-
t h e model
346 Estimation o f c o e f f i c i e n t s from experiment Marra (1978b.
1980a) c a r r i e d o u t l a b o r a t o r y e x p e r i m e n t s w i t h p h y t o p l a n k t o n
c u l t u r e s grown under c o n t r o l l e d l i g h t c o n d i t i o n s t h a t p r o v i d e d a t a s u i t a b l e f o r t e s t i n g t h e a d a p t a t i o n model. (12 D:
l i g h t cycle
Pure c u l t u r e s were s u b j e c t e d t o a 12-h dark: 12-h
12 L). where t h e l i g h t was c o n s t a n t d u r i n g each day b u t t h e
i n t e n s i t y was s e t randomly a t one o f s e v e r a l l e v e l s between 60 and 1500 pE m-‘ s-l
o f p h o t o s y n t h e t i c a l l y a c t i v e r a d i a t i o n (PAR).
The r a t e o f p h o t o s y n t h e t i c
oxygen p r o d u c t i o n was measured a t 0.5 h i n t e r v a l s w i t h an oxygen e l e c t r o d e .
The
r e s u l t s shown i n F i g . 3 f o r t h e d i a t o m L a u d e r i a b o r e a l i s were s i m i l a r f o r s e v e r a l species: f a l l i n g off
i n i t i a l h i g h oxygen p r o d u c t i o n a t h i g h l i g h t i n t e n s i t i e s
r a p i d l y t o r o u g h l y c o n s t a n t low v a l u e s a f t e r s e v e r a l hours, and
s m a l l changes i n p r o d u c t i o n a t low l i g h t i n t e n s i t i e s .
The e f f e c t was n o t
s t r i c t l y a d i u r n a l f e a t u r e as s i m i l a r c u r v e s were o b t a i n e d by v a r y i n g t h e s t a r t t i m e by s e v e r a l hours.
The s u p p r e s s i o n a t h i g h l i g h t i n t e n s i t i e s may be t h e
r e s u l t o f i n c r e a s e d r e s p i r a t i o n w i t h exposure t o h i g h l i g h t ( F a l k o w s k i , Dubinsky and Wyman, 1985).
n CI
U
a I
LLI + a
U
+ 1500 870
-
-0-
495
-*- 295 -a-
150
-0-
60
Z
l-
0
0 3
n 0 U
a TIME (h)
F i g . 3. Measurements o f oxygen p r o d u c t i o n r a t e P ( i n pg-at.
02 h - l c e l l - ’ )
a g a i n s t t i m e a t v a r i o u s c o n s t a n t i r r a d i a n c e s ( i n pE m-’ s-’ PAR), a f t e r 12 h o f dark conditioning. Redrawn f r o m Marra, 1978b (we have c o r r e c t e d u n i t s f o r P).
347
ao-
60
a
8
40
A
p(0.125h)
0
P(6.125h)
20
I
I
1000
1500
IRRADIANCE , I Fig. 4. ( a ) Data f r o m F i g . 3 r e p l o t t e d a g a i n s t i r r a d i a n c e f o r two times. The s o l i d l i n e s a r e e s t i m a t e d P - I c u r v e s f o r e q u i l i b r i u m d a r k adapted P D ( I ) , and high l i g h t adapted P L ( I ) , c e l l s . (b) Estimate o f X ( I ) , t h e adapting c a p a b i l i t y o f a g i v e n i r r a d i a n c e , f r o m t h e f r a c t i o n a l d i s t a n c e between P D ( I ) and P L ( I ) o f the s o l i d c i r c l e s i n panel ( a ) . The c u r v e i n f i g .
3 f o r t h e h i g h e s t i r r a d i a n c e approximates t h e a d a p t a t i o n
from f u l l y d a r k adapted c e l l s t o f u l l y l i g h t adapted c e l l s .
The o t h e r c u r v e s
represent t h e a d a p t a t i o n f r o m d a r k adapted c e l l s t o c e l l s adapted t o some i n t e r mediate l i g h t i n t e n s i t i e s .
I n F i g . 4 we have r e p l o t t e d t h e oxygen p r o d u c t i o n
rates a g a i n s t i r r a d i a n c e , f o r t h e i n i t i a l sample t i m e ( t = 0.125 h ) and t h e f i n a l common t i m e ( t = 6.125 h).
( 4 ) ) f o r d a r k adapted c e l l s , P D ( I ) ,
The two c u r v e s a r e e s t i m a t e d P - I curves ( u s i n g . and f o r h i g h l i g h t adapted c e l l s ,
PL(I).
Curve P L ( I ) would o c c u r f o r c e l l s t h a t were s u b j e c t e d t o h i y h i r r a d i a n c e s such
348 t h a t X(1)
FZ
1 f o r s u f f i c i e n t time t h a t Y ( X ( 1 ) )
t = 6.125
h i n F i g . 4 a r e between t h e two c u r v e s because t h e c e l l s had adapted
t o l o w e r i r r a d i a n c e s such t h a t X(1)
b i t i n g s t r e n g t h o f an i r r a d i a n c e estimate
1.
<
h between P D ( I ) and P L ( I )
a t t = 6.125
of X(I)
I.
1.
The p r o d u c t i o n r a t e s a t
The f r a c t i o n o f t h e p r o d u c t i o n r a t e s
i s an a p p r o x i m a t i o n t o X(1).
the inhi-
I n t h e b o t t o m panel we have p l o t t e d an
u s i n g ( 6 ) f r o m t h e d a t a i n t h e t o p panel.
T h i s c u r v e w i l l be
used i n t h e model.
n
A
a I
n 4-1
J
a
U
TIME (h) Fig. 5. L o g - l i n e a r p l o t o f d a t a fromi F i g . 3 ( f o r I = 1500). The a s y m p t o t i c p r o d u c t i o n r a t e a t l a r g e t i m e was t a k e n as P, = PL = 4 a c c o r d i n g t o F i g . 2a. U n i t s a r e as b e f o r e .
The a d a p t a t i o n t i m e T f o r t h e d a t a o f F i g . 3 can now be e s t i m a t e d . sponse f o r a s t e p f u n c t i o n i n c r e a s e i n
I (and X ) f r o m 0 i s Y ( t )
where x2 i s t h e v a l u e o f X a f t e r t h e s t e p i n c r e a s e i n I .
P,(t)
i n either (4) or
expression
For l a r g e
( 5 ) , and s u D s t i t u t i n g i n ( 1 ) where A ( t )
Rearranging. t a k i n g l o g a r i t h i i s .
= x2(1
and s e t t i n g x i = 1 f o r l a r g e
=
The r e -
- e -t/T)
I , P ( t ) ->
Ps(t) = P(t)
I , we g e t t h e
349 Thus a p l o t o f l o g ( P ( t ) l i n e w i t h s l o p e -1/T tation.
- PL) as a f u n c t i o n o f t i m e t s h o u l d y i e l d a s t r a i g h t
i f t h i s model i s a r e a s o n a b l e r e p r e s e n t a t i o n o f t h e adap-
We have p l o t t e d t h e s e d a t a f o r I = 1500 pE m-'
i n F i g . 5.
s-l
The
c u r v e i s r o u g h l y l i n e a r w i t h a s l o p e t h a t g i v e s a response t i m e e s t i m a t e o f T = 1.1 h.
Beyond a t i m e t = 4 h. t h e p r o d u c t i o n r a t e s were so near P1 t h a t t h e r e
was a l a r g e s c a t t e r on t h e l o g a r i t h m i c p l o t . From t h e d a t a o f M a r r a ' s (1978b,
1980a) 12 D: 12 L experiments, we have been
a b l e t o e s t i m a t e a l l t h e c o e f f i c i e n t s r e q u i r e d t o complete t h e l i n e a r a d a p t a t i o n model f o r t h o s e data.
The model s h o u l d now be a b l e t o s i m u l a t e t h o s e data.
SOLUTIONS FOR BATCH CULTURE DATA L i g h t - D a r k Step F u n c t i o n s The a n a l y t i c s o l u t i o n (8) t o t h e model f o r a s t e p f u n c t i o n i n i r r a d i a n c e can be extended t o t h e g e n e r a l case o f a s t e p a t t i m e t = 0 f r o m any c o n s t a n t prec o n d i t i o n i n g i r r a d i a n c e I 1 t o a n o t h e r c o n s t a n t i r r a d i a n c e i r r a d i a n c e 12. s o l u t i o n f o r t h e a d a p t a t i o n o f P,(t)
where x1 = X ( I 1 ) and x2 = X ( I 2 ) .
The
is
E q u a t i o n (10) can t h e n be s u b s t i t u t e d i n t o (4)
o r (5). a r e shown i n Fig. 6.
The r e s u l t s f r o m (10) f o r M a r r a ' s e x p e r i m e n t s ( F i g . 3 ) We used ( 4 ) w i t h c o n s t a n t IK and ( 6 ) w i t h I b = 200 pE m-' t h e model has c a p t u r e d t h e main f e a t u r e s o f t h e data.
s-'.
Qualitatively,
I t does n o t d u p l i c a t e t h e
slow i n c r e a s e ( o r i n i t i a l p l a t e a u ) o f t h e r a t e o f p r o d u c t i o n a t t h e lower l i g h t intensities.
T h a t would p r o b a b l y r e q u i r e s e p a r a t e a d a p t a t i o n o f t h e s l o p e a
w i t h a d i f f e r e n t response t i m e and a d i f f e r e n t f u n c t i o n a l f o r m f o r X(1). C l e a r l y f r o m F i g . 3.
i n t h e usual experiments t o o b t a i n P-I
t i o n can o c c u r d u r i n g t h e i n c u b a t i o n .
r e l a t i o n s , adapta-
Each e s t i m a t e o f p r o d u c t i o n i s t h e n o n l y
a t i m e average f o r t h e d u r a t i o n o f t h e e x p e r i m e n t and i s i n g e n e r a l a poor e s t i m a t e o f t h e i n s t a n t a n e o u s r a t e a t any t i m e d u r i n g t h e i n c u b a t i o n . i n t e g r a t i n g a l o n g t h e curves i n Fig. 3. Marra (1978b) s i m u l a t e d P - I
By curves f r o m
i n c u b a t i o n s by c a l c u l a t i n g t h e average p r o d u c t i o n r a t e a t each i r r a a i a n c e f o r s e v e r a l d i f f e r e n t i n c u b a t i o n t i m e s Ti:
350
r
-----
.......
----
-
1500 870 495 295 150 60
TIME (h) Fig. 6. Model o u t p u t f o r t h e s e r i e s o f a d a p t i v e t i m e c o u r s e d a t a i n F i g . 3.
>
n '
I Fig. 7. Model s i m u l a t i o n o f P-I c u r v e s o b t a i n e d f r o m i n c u b a t i o n s w i t h d i f f e r e n t v a l u e s o f t h e r a t i o o f i n c u b a t i o n t i m e t o P, a d a p t i v e response t i m e Ti/T.
351 The r e s u l t i n g P a v - I
c u r v e s showed t h a t p h o t o i n h i b i t i o n a t h i g h l i g h t i n t e n s i t i e s
i n P - I c u r v e s o b t a i n e d f r o m i n c u b a t i o n s may be l a r g e l y a f u n c t i o n o f l e n g t h o f incubation.
We have s i m u l a t e d t h e s e r e s u l t s u s i n g ( 1 1 ) f o r a v a r i e t y o f d i f f e r (where we have used e x p r e s s i o n s ( 5 ) and (7)).
e n t r a t i o s o f Ti/T
The r e s u l t s
p l o t t e d i n F i g . 7 show an a p p a r e n t p h o t o i n h i b i t i o n e f f e c t t h a t i n c r e a s e s as t h e i n c u b a t i o n t i m e Ti
P.,
becomes l a r g e r t h a n T, t h e response t i m e f o r a d a p t a t i o n o f
The o n s e t o f t h e a p p a r e n t p h o t o i n h i b i t i o n i s equal t o IB. t h e t h r e s h o l d f o r
t h e i n h i b i t i n g a b i l i t y o f i r r a d i a n c e i n (7).
This result, t h a t i n h i b i t i o n i n
P-I c u r v e s can o c c u r i n a model w i t h no e x p l i c i t i n h i b i t i o n , i s n o t a g e n e r a l result.
F o r a d i f f e r e n t s p e c i e s o f p h y t o p l a n k t o n whose l o n g t e r m a d a p t a t i o n was
such t h a t exposure t o h i g h l i g h t a f t e r l o w l i g h t c o n d i t . i o n i n g r e s u l t e d i n upward a d a p t a t i o n o f P,
Lewis and Smith (1983) observed t h e g r e a t e s t p h o t o i n h i b i t i o n
w i t h t h e s h o r t e s t i n c u b a t i o n time.
To o b t a i n such a r e s u l t w i t h o u r model, we
would have t o i n c l u d e an e x p l i c i t p h o t o i n h i b i t i o n t e r m t h a t i t s e l f was capable o f adaptation. Diurnal cycles The model s h o u l d s i m u l a t e t h e r e s u l t s o f d i u r n a l t i m e c o u r s e s t u d i e s (e.g. H a r r i s , 1973; H a r r i s and L o t t ,
1973; Marra, 1978a) t h a t documented t h e e x i s t e n c e
o f p h o t o a d a p t a t i o n b o t h i n l a b o r a t o r y c u l t u r e s and i n samples r e c e n t l y t a k e n We s h o u l d e x p e c t t h e model t o s i m u l a t e b o t h t h e midday
from l a k e s o r t h e ocean.
depression i n p r o d u c t i o n r a t e and t h e h y s t e r e s i s i n t h e P - I phase p l o t s r e s u l t i n g f r o m t h e response t i m e r e q u i r e d f o r a d a p t a t i o n .
I n F i g . 8 we show t h e
r e s u l t s f o r model parameters s i m i l a r t o t h o s e used t o reproduce t h e asymmetry
6
12
18
TIME OF DAY ( h )
I (t)
Fig. 8. Response o f a d a p t i v e model t o d i u r n a l i r r a d i a n c e c y c l e . ( a ) Time course o f i r r a d i a n c e I and p r o d u c t i o n r a t e P. ( b ) Phase p l o t o f P - I showing h y s t e r e s i s r e s u l t i n g f r o m midday and a f t e r n o o n depression.
S o l i d t r i a n g l e denotes noon.
352 r e s u l t i n g f r o m t h e i n c o m p l e t e r e c o v e r y o f t h e p h o t o s y n t h e t i c apparatus f o i l o w i n g t h e suppression caused by t h e h i g h i r r a d i a n c e s a t midday. S i m u l a t i o n o f 'Greenhouse' d a t a Marra and Heinemann (1982) r e p o r t e d on a s e r i e s o f t i m e course measurements of oxygen p r o d u c t i o n by b a t c h c u l t u r e s o f p h y t o p l a n k t o n g r o w i n g i n a window They found t h a t t h e P - I phase
greenhouse under n a t u r a l c y c l e s o f i r r a d i a n c e .
p l o t s were r o u g h l y l i n e a r on shady days w i t h peak i r r a d i a n c e s l e s s t h a n about 200 pE m-'
s-l
b u t t h a t s i g n i f i c a n t h y s t e r e s i s o c c u r r e d on days w i t h peak
i r r a d i a n c e s g r e a t e r t h a n 400
pE ind2 s-'.
Marra,
Heinemann and L a n d r i a u (1985)
were a b l e t o s i m u l a t e t h e low i r r a d i a n c e days f r o m c o n s t a n t d a i l y P - I curves, b u t o n l y a s t a t i s t i c a l model w i t h t i m e v a r y i n g c o e f f i c i e n t s c o u l d f i t t h e data f o r days o f v a r i a b l e i r r a d i a n c e (Neale and iilarra. 1985). One o f t h e greenhouse experiments (Greenhouse I V ) c o n s i s t e d o f a 2 week time s e r i e s w i t h t h e same d i a t o m species L a u d e r i a b o r e a l i s t h a t was used i n t h e 12 12 L experiments t h a t we used t o develop o u r model.
D:
We r a n o u r model on these
d a t a w i t h t h e same a d a p t a t i o n t i m e T = 1.1 h b u t w i t h curves e s t i m a t e d t o be the upper and lower envelopes about t h e d a t a on a P - I phase p l o t ( f i g . 1980a; o r f i g .
l a i n Meale and Marra, 1985).
5 i n Marra,
As b e f o r e , o n l y t h e asymptote Ps
was a l l o w e d t o adapt t o t h e v a r y i n g l i g h t h i s t o r y .
Results are presented i n
Fig. 9 f o r 30-31 October 1979, p a r t i a l l y sunny days w i t h peak i r r a d i a n c e s o f about 950 pE m-'
s-'.
The main v a r i a t i o n s i n i n s t a n t a n e o u s p r o d u c t i o n r a t e are
reproduced e x c e p t f o r an o v e r s h o o t i n t h e e a r l y morning r i s e i n p r o d u c t i o n rate. Since t h e P - I curves were e s t i m a t e d i n d i r e c t l y from data, and t h e response time and model c h a r a c t e r i s t i c s were determined f r o m d a t a on t h e same species b u t 3 y e a r s e a r l i e r under d i f f e r e n t c o n d i t i o n s o f growth, good.
t h e e x t e n t o f agreement i s
It i s e n t i r e l y p o s s i b l e t h a t o t h e r c o e f f i c i e n t s ( i n i t i a l s l o p e a o r i n -
h i b i t i o n b) were a l s o undergoing a d a p t a t i o n t o t h e h i g h l y v a r i a b l e i r r a d i a n c e . EFFECTS
OF
INTERNAL WAVES
No a d a p t a t i o n I n coastal regions, phytoplankton a r e o f t e n displaced v e r t i c a l l y 10's o f meters by p a s s i n g i n t e r n a l waves (e.g. 1983).
Denman and Herman, 1978; Haury e t al.,
Because o f t h e a p p r o x i m a t e l y e x p o n e n t i a l decay w i t h depth o f i r r a d i a n c e
below t h e sea surface,
t h o s e v e r t i c a l displacements t r a n s l a t e i n t o f l u c t u a t i o n s
i n t h e l i g h t i n t e n s i t y r e c e i v e d b y c e l l s l o c a t e d on c o n s t a n t d e n s i t y surfaces. These f l u c t u a t i o n s can be g r e a t e r t h a n a f a c t o r o f 100 (Haury e t al.);
for
example,
w i t h a v e r t i c a l displacement o f 10 m and an attenuance c o e f f i c i e n t o f
0.3 m-l,
t h e i r r a d i a n c e changes by a f a c t o r o f about 20.
353 1000 r
6
8
10
12
14
16
16
6
8
10
12
14
16
16
TIME OF DAY (h) Fig. 9. lilodel r e s u l t s and d a t a f r o m c u l t u r e s grown i n a greenhouse (Marra and Heinemann. 1982) f o r ( a ) 30 and ( b ) 31 October, 1979. The model was r u n w i t h u n i t s as b e f o r e . f u n c t i o n s ( 4 ) and (6) w i t h Ps = 35, a = 0.3 and I b = 200;
I f t h e i r r a d i a n c e f l u c t u a t i o n s e x p e r i e n c e d by c e l l s were s t r i c t l y s i n u s o i d a l , t h e n e g a t i v e c u r v a t u r e o f t h e P - I c u r v e s would cause t h e average r a t e o f photos y n t h e t i c p r o d u c t i o n t o be l e s s t h a n i n t h e absence of i n t e r n a l waves.
However,
t h e i r r a d i a n c e c y c l e e x p e r i e n c e d by c e l l s u s u a l l y w i l l n o t be s i n u s o i d a l because o f n o n l i n e a r i t i e s i n b o t h t h e shape o f t h e i n t e r n a l waves and t h e decay o f i r r a d i a n c e w i t h depth.
Consider t h e i r r a d i a n c e I ( Z ) on a d e n s i t y s u r f a c e b e i n g
d i s p l a c e d v e r t i c a l l y by i n t e r n a l waves i n s t a n t a n e o u s l y a t a d e p t h Z ( t ) .
F o r PAR
r a d i a t i o n , a s i n g l e a t t e n u a n c e c o e f f i c i e n t c i s a good a p p r o x i m a t i o n
where I, i s t h e PAR i r r a d i a n c e j u s t below t h e sea surface.
I n t e r n a l waves'near
t h e sea s u r f a c e o f t e n have f l a t t e n e d c r e s t s (e.g.
P h i l l i p s , 1977)
s i m i l a r t o t h a t p l o t t e d i n F i g . 10a.
fig.
5.1.
Depending on t h e degree o f t h a t f l a t t e n i n g
354
(a) 0
-
-E
I
1
I
I
I
I
1
I
5
I F
a
w
0
10-
15-
(b)
0
a
[I [I
TIME F i g . 10. ( a ) I d e a l i z e d t i m e s e r i e s f o r t h e d e p t h Z ( t ) o f a p h y t o p l a n k t e r l o c a t e d on an i s o p y c n a l s u r f a c e b e i n g d i s p l a c e d v e r t i c a l l y by a n e a r - s u r f a c e i n t e r n a l wave. ( b ) P o s s i b l e forms f o r t h e i r r a d i a n c e f l u c t u a t i o n s e x p e r i e n c e d by t h e c e l l i n a l i g h t f i e l d d e c a y i n g e x p o n e n t i a l l y w i t h depth. and on t h e a m p l i t u d e o f t h e i n t e r n a l wave r e l a t i v e t o t h e a t t e n u a n c e l e n g t h c- 1 , t h e i r r a d i a n c e f l u c t u a t i o n s e x p e r i e n c e d by c e l l s on a d e n s i t y s u r f a c e a t d e p t h
Z ( t ) may b e r o u g h l y s i n u s o i d a l o r skewed towards e i t h e r t h e h i g h e r o r l o w e r v a l u e s as d e p i c t e d by t h e t h r e e curves i n F i g . lob. For a p u r e l y s i n u s o i d a l i n t e r n a l wave,
t h e e x p o n e n t i a l dependence o f i r r a -
d i a n c e w i t h d e p t h ( 1 2 ) causes t h e average i r r a d i a n c e e x p e r i e n c e d by a phytop l a n k t e r t o be g r e a t e r t h a n i n t h e absence o f t h e wave.
Below some c r i t i c a l
d e p t h (where t h e l i n e a r p o r t i o n o f t h e P - I c u r v e p e r t a i n s and t h e n o n l i n e a r i t y i n t h e i r r a d i a n c e p r o f i l e dominates), i n c r e a s e t h e average p r o d u c t i o n (G.
t h e i n t e r n a l wave can t h e n a c t u a l l y Holloway, pers.
comm.).
We have found
examples o f t h i s e f f e c t b u t no g e n e r a l e x p r e s s i o n f o r t h e c r i t i c a l depth.
355 Adaptation present I n t e r n a l waves e x i s t a t p e r i o d s from t h e l o c a l i n e r t i a l p e r i o d (17 h a t 45' l a t ) t o t h e buoyancy p e r i o d ( o f o r d e r 5 min i n t h e upper t h e r m o c l i n e ) .
Because
t h e response t i m e T f o r t h e t y p e o f p h o t o a d a p t a t i o n examined h e r e i s 1-3 h, we expect m i n i m a l a d a p t a t i o n t o l i g h t f l u c t u a t i o n s caused by i n t e r n a l waves w i t h p e r i o d s much s h o r t e r t h a n T and s u b s t a n t i a l a d a p t a t i o n t o f l u c t u a t i o n s a t p e r i o d s much g r e a t e r t h a n T.
To t e s t t h i s h y p o t h e s i s w i t h o u r model, we p r e s e n t
an i d e a l i z e d case f o r which we can o b t a i n an a n a l y t i c s o l u t i o n t h a t i l l u s t r a t e s t h e n a t u r e o f a d a p t a t i o n t o f l u c t u a t i o n s a t i n t e r n a l wave f r e q u e n c i e s .
Fig. 11. I d e a l i z e d case where s i n u s o i d a l i r r a d i a n c e I r e s u l t s i n an a d a p t i n g c a p a b i l i t y X(1) a l s o a p p r o x i m a t e l y s i n u s o i d a l i n time. Consider t h e case where t h e i r r a d i a n c e i m p i n g i n g on c e l l s b e i n g d i s p l a c e d up and down by i n t e r n a l waves i s g i v e n by
i.e.
we approximate t h e m i d d l e c u r v e i n Fig.
10b w i t h a s i n u s o i d .
Now l e t t h e
i n h i b i t i n g s t r e n g t h o f t h a t i r r a d i a n c e a l s o be approximated by a s i n u s o i d
X ( t ) = g1 i.e.
+
g2 cos 27rft.
(14)
t h e f l u c t u a t i n g i r r a d i a n c e i s on t h e l i n e a r p a r t o f t h e X(1) c u r v e as
d e p i c t e d i n Fig. 11.
For o u r model w i t h P s ( t )
f u n c t i o n i s ( J e n k i n s and Watts,
1968)
a d a p t i n g , t h e s t e p response
356
where cp(f) = -atan
(2?rfT).
As b e f o r e we s u b s t i t u t e (15) i n t o ( 1 ) f o r P s ( t )
which i s then s u b s t i t u t e d i n t o ( 5 ) f o r t h e instantaneous r a t e o f production P(t). To g e t t h e mean r a t e o f p r o d u c t i o n f o r any f r e q u e n c y f, we must average P ( t ) The s o l u t i o n c o n s i s t s o f two p a r t s :
o v e r a complete p e r i o d l / f . t e r m P,
where
and an a d a p t i n g t e r m Pa.
Io(z)
=
; i=
e -
‘OS
+
The non-adapting
a non-adapting
t e r m has t h e f o r m
dQ i s a m o d i f i e d Bessel f u n c t i o n o f z e r o o r d e r
(p. 376, Abramowitz and Stegun. 1965).
Io(z)
i s a nionotonically increasing
f u n c t i o n o f z w i t h I o ( 0 ) = 1. The a d a p t i n g term,
i n which we a r e most i n t e r e s t e d , can a l s o be expressed i n
terms o f a m o d i f i e d Bessel f u n c t i o n
;J=
= -
I1(z) = -Ii(-z)
cos
e
‘OS
€IdQ
and has t h e f o r m
Pa
=
gz ( P o
-
PL) 11(-a Rz/PD) G ( f ) cos cp(f) e -a R1/PD
where
G ( f ) cos cp(f) =
F o r PD > PL,
‘OS
z?rfT
(lab)
[l + ( 2 7 r f r n
we g e t Pa < 0; t h a t i s . f o r o u r riiodel parameters, t h e a d a p t a t i o n t o
i n t e r n a l waves reduces t h e average r a t e o f p h o t o s y n t h e t i c p r o d u c t i o n . more,
Pa
-
0 when f >> 1/T.
Further-
so t h e r e d u c t i o n o n l y o c c u r s f o r i n t e r n a l waves
w i t h p e r i o d s l o n g compared t o T.
L.ie p l o t (18b) as a f u n c t i o n o f f T i n F i g . 12.
For i n t e r n a l wave p e r i o d s j u s t t w i c e t h e a d a p t a t i o n response t i m e T, Pa i s reduced t o about 0.1 o f i t s v a l u e when f << T-’.
For T x 1 h. t h e r e w i l l be
s i g n i f i c a n t a d a p t a t i o n t o i n t e r n a l waves a t t h e s e m i d i u r n a l t i d a l frequency, b u t a t f r e q u e n c i e s approaching t h e buoyancy f r e q u e n c y (1-10 cph). w i l l be n e g l i g i b l e .
t h e adaptation
.
357
fT Fig. 12. T r a n s f e r f u n c t i o n f o r a d a p t a t i o n t o i r r a d i a n c e f l u c t u a t i o n s induced by i n t e r n a l waves, p l o t t e d as a f u n c t i o n o f i n t e r n a l wave f r e q u e n c y f s c a l e d by t h e a d a p t i v e response t i m e T ( = 1.1 h f o r t h e d a t a o f Marra, 1978b). The r e d u c t i o n o f average p r o d u c t i o n i n f l u c t u a t i n g l i g h t and t h e l a c k o f a d a p t a t i o n a t h i g h f r e q u e n c i e s a r e c o n t r a r y t o t h e r e s u l t s o f Walsh and Legendre (1983) and o t h e r s .
They found t h e c o e f f i c i e n t s a and P,
f l u c t u a t i n g l i g h t i n c e r t a i n cases.
However,
t o i n c r e a s e under
t h e i r e x p e r i m e n t s were w i t h f r e
quencies o f l i g h t f l u c t u a t i o n o f 1-10 Hz, a t which o t h e r b i o p h y s i c a l processes a s s o c i a t e d w i t h l i g h t s t i m u l a t i o n may be o p e r a t i n g (e.g. Powell,
Abbott,
R i c h e r s o n and
1982), r a t h e r t h a n t h e t y p e o f a d a p t a t i o n addressed here.
FUTURE WORK We have shown t h a t a l i n e a r f i r s t o r d e r e x p o n e n t i a l response model o f t h e a d a p t a t i o n o f p h o t o s y n t h e t i c r a t e reproduces t h e e s s e n t i a l f e a t u r e s i n d a t a o b t a i n e d f r o m b a t c h c u l t u r e s o f p h y t o p l a n k t o n grown under n a t u r a l and a r t i f i c i a l lighting.
The model d i d n o t i n c l u d e an e x p l i c i t p h o t o i n h i b i t i o n term; o n l y one
c o e f f i c i e n t , t h e a s y m p t o t i c h i g h i r r a d i a n c e r a t e o f p r o d u c t i o n P, adapt.
was a l l o w e d t o
Small i n c r e a s e s i n P ( t ) a t low i r r a d i a n c e s a f t e r d a r k p r e c o n d i t i o n i n g
were n o t reproduced. Because t h e model i s l i n e a r ,
i t can be extended such t h a t each c o e f f i c i e n t
can adapt s e p a r a t e l y and can have m u l t i p l e t i m e s c a l e s o f a d a p t a t i o n . t i m e i n c u b a t i o n d a t a ( o f t h e t y p e p r e s e n t e d i n Lewis e t al., Smith,
Short
1984; and Lewis and
1983) g i v i n g t h e a d a p t i v e response s e p a r a t e l y f o r each c o e f f i c i e n t i n t h e
P - I c u r v e s a r e r e q u i r e d t o e x t e n d t h e model i n t h i s manner. f o r X(1).
The f u n c t i o n a l f o r m
t h e a d a p t i n g c a p a b i l i t y o f a g i v e n i r r a d i a n c e was chosen f r o m s c a n t y
data; s p e c i f i c e x p e r i m e n t s must b e d e s i g n e d t o d e t e r m i n e i t s form.
358 I t takes l i t t l e s o l a r r a d i a t i o n t o s t a b i l i z e t h e upper ocean except under extreme c o n d i t i o n s (Denman and Gargett,
1983: Woods and Barkmann, submitted).
Thus, a d a p t a t i o n t o v e r t i c a l displacements by i n t e r n a l waves may be o f more general importance than a d a p t a t i o n t o v e r t i c a l displacements caused by t u r b u l e n t v e r t i c a l mixing.
To t h a t end, we should extend t h e s i m u l a t i o n o f t h e adaptation
o f p h y t o p l a n k t o n on i n t e r n a l waves over a range o f combinations o f wave shapes and i r r a d i a n c e p r o f i l e s , and we should perform f u r t h e r l a b o r a t o r y experiments w i t h programmed f l u c t u a t i n g l i g h t s . ACKNOWLEDGEMENTS We thank Paul Falkowski, Marlon Lewis, Pat Neale and Trevor P l a t t f o r h e l p f u l discussions.
Howard Freeland, Greg Hol loway, and Richard Thomson c o n t r i b u t e d t o
t h e i n t e r n a l wave a n a l y s i s .
REFERENCES Abbott, M.R., Richerson, P.J. and Powell, T.M., 1982. I n s i t u responsB o f phytop l a n k t o n fluorescence t o r a p i d v a r i a t i o n s i n l i g h t . Limnol. Oceanogr., 27: 21 8-22 5. Abramowitz. M. and Stegun. I.A., 1965. Handbook o f Mathematical Functions. Dover, New York, 1046 pp. Denman, K.L. and Gargett. A.E., 1983. Time and space s c a l e s o f v e r t i c a l m i x i n g and a d v e c t i o n o f phytoplankton i n t h e upper ocean. Limnol. Oceanogr., 28: 801-815. 1978. Space-time s t r u c t u r e o f a c o n t i n e n t a l s h e l f Denman, K.L. and Herman, A.W., ecosystem measured by a towed p o r p o i s i n g v e h i c l e . J. Mar. Res.. 36: 693-714. Falkowski, P.G., 1980. Light-shade a d a p t a t i o n i n marine phytoplankton. I n : P.G. Falkowski ( E d i t o r ) , Primary P r o d u c t i v i t y i n t h e Sea. Plenum, New York. pp. 99-1 19. Fa1 kowski, P.G., 1983. Light-shade a d a p t a t i o n and v e r t i c a l m i x i n g o f marine phytoplankton: a comparative f i e l d study. J. Mar. Res., 41: 215-237. Falkowski, P.G., Dubinsky, Z. and Wyrnan. K., 1985. Growth-irradiance r e l a t i o n s h i p s i n phytoplankton. Limnol. Oceanogr., 30: 311-321. Gal legos, C. L. and P l a t t , T., 1981. Photosynthesis measurements on n a t u r a l p o p u l a t i o n s o f phytoplankton: numerical a n a l y s i s . In: T. P l a t t ( E d i t o r ) , P h y s i o l o g i c a l Bases o f Phytoplankton Ecology, Can. B u l l . Fish. Aquat. Sci.. 210: 103-112. Geider, R. and P l a t t . T., I n prep. A m e c h a n i s t i c model o f photoadaptation i n microalgae. H a r r i s , G.P.. 1973. Die1 and annual c y c l e s o f n e t p l a n k t o n photosynthesis i n Lake Ontario. J. Fish. Res. Board Can., 30: 1779-1787. Harris, G.P. and L o t t , J.N., 1973. L i g h t i n t e n s i t y and p h o t o s y n t h e t i c r a t e s i n phytoplankton. J. Fish. Res. Board Can., 30: 1771-1778. Haury. L.R., Wiebe, P.H.. O r r , M.H. and Briscoe, M.G., 1983. T i d a l l y generated high-frequency i n t e r n a l wave packets and t h e i r e f f e c t s on p l a n k t o n i n Massachusetts Bay. J. Mar. Res., 41: 65-112. 1968. S p e c t r a l A n a l y s i s and I t s A p p l i c a t i o n s . Jenkins, G.M. and Watts, D.G., Holden-Day, San Francisco. 525 pp. Lederman, T.C. and T e t t . P., 1981. Problems i n m o d e l l i n g t h e photosynthesis - l i g h t r e l a t i o n s h i p . Bot. Mar. 24: 125-134.
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Lewis, ibl. R., Cullen, J. J. and P l a t t , T.. 1984. R e l a t i o n s h i p s between v e r t i c a l m i x i n g and photoadaptation o f phytoplankton: s i m i l a r i t y c r i t e r i a . dar. Ecol. Prog. Ser., 15: 141-149. 1983. A small volume, short-incubation-time method Lewis, M.R. and Smith, J.C., f o r measurement o f photosynthesis as a f u n c t i o n o f i n c i d e n t i r r a d i a n c e . ijlar. Ecol. Prog. Ser., 13: 99-102. Marra, J.. 1978a. E f f e c t o f short-term v a r i a t i o n s i n l i g h t i n t e n s i t y on photos y n t h e s i s of a marine p h y t o p l a n k t e r : a l a b o r a t o r y s i m u l a t i o n study. Mar. Biol., 46: 191-202. Marra, J., 1978b. Phytoplankton p h o t o s y n t h e t i c response t o v e r t i c a l movement i n a mixed l a y e r . Mar. Biol., 46: 203-208. Marra, J., i980a. V e r t i c a l m i x i n g and p r i m a r y production. I n : P.G. Falkowski ( E d i t o r ) , Primary P r o d u c t i v i t y i n t h e Sea. Plenum, New York, pp. 121-137. Marra, J., 19806. Time course o f l i g h t i n t e n s i t y a d a p t a t i o n i n a marine diatom. Mar. B i o l . Lett., 1: 175-183. Marra, J. and Heinemann. K., 1982. Photosynthesis response by phytoplankton t o s u n l i g h t v a r i a b i l i t y . Limnol. Oceanogr., 27: 1141-1153. Marra, J.. Heinemann. K. and Landriau, Jr., G., 1985. Observed and p r e d i c t e d measurements o f photosynthesis i n a phytoplankton c u l t u r e exposed t o n a t u r a l i r r a d i a n c e . Mar. Ecol. Prog. Ser., 24: 43-50. Neale, P.J. and Marra, J., 1985. Short-term v a r i a t i o n o f Pmax under n a t u r a l i r r a d i a n c e c o n d i t i o n s : a model and i t s i m p l i c a t i o n s . Mar. Ecol. Prog. Ser., i n press. P h i l l i p s , O.M., 1977. The Dynamics o f t h e Upper Ocean. Cambridge U n i v e r s i t y Press, Cambridge, 336 pp. 1980. P h o t o i n h i b i t i o n o f photoP l a t t , T., Gallegos. C.L. and Harrison, W.G., s y n t h e s i s i n n a t u r a l assemblages o f marine phytoplankton. J. Mar. Res.. 38: 687-701. P l a t t , T. and Gallegos, C.L., 1980. M o d e l l i n g p r i m a r y production. I n : P.G. Falkowski ( E d i t o r ) , Primary P r o d u c t i v i t y i n t h e Sea. Plenum, New York. pp. 339-362. Dubinsky, Z., Wyman, K. and Falkowski, P.G.. 1984. K i n e t i c s o f Post, A.F.. l i g h t - i n t e n s i t y a d a p t a t i o n i n a marine p l a n k t o n i c diatom. Mar. B i o l . , 83: 231 -238. PrSzelin, B.B. and M a t l i c k , H.A., 1980. Time-course o f photoadaptation i n t h e photosynthesis-irradiance r e l a t i o n s h i p o f a d i n o f l a g e l l a t e e x h i b i t i n g photos y n t h e t i c p e r i o d i c i t y . Mar. Biol., 58: 85-96. Rivkin. R.B., S e l i g e r , H.H.. S w i f t , E. and Biggley. W.H., 1982. Light-shade a d a p t a t i o n by t h e oceanic d i n o f l a g e l l a t e s P y r o c y s t i s n o c t i l u c a and p. f u s i f o r m i s . Mar. Biol.. 68: 181-191. Slaqstad. D.. 1982. A model o f phytoplankton qrowth - effect; o f v e r t i c a l m i x i n q and a d a p t a t i o n t o l i g h t . Model: Ident. Control, 3: 111-130. Walsh. P. and Legendre, L., 1983. Photosynthesis o f n a t u r a l phytoplankton under h i g h frequency l i g h t f l u c t u a t i o n s s i m u l a t i n g those induced by sea s u r f a c e waves. Limnol. Oceanogr., 28: 688-697. Woods, J.D. and Barkmann, W., Submitted. The response o f t h e upper ocean t o s o l a r heating. I: The mixed layer. Quart. J. R. Met. SOC.
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361
THE EFFECTS OF THE BROAD SPECTRUM OF PHYSICAL ACTIVITY ON THE BIOLOGICAL PROCESSES IN THE CHESAPEAKE BAY
A. BRANDT', c. c. H. H. SELI G E R ~ ,M. A. TYLER3 'Johns Hopkins University, Applied Physics Laboratory, Laurel, Maryland 20707, USA 'McCollum Pratt Institute and Dept. of Biology, Johns Hopkins University, Baltimore, Maryland 21218, USA 3College of Marine Studies, University of Delaware, Newark, Delaware, 19711 (currently on leave at National Science Foundation, Washington, D.C.)
SARA BUN^,
ABSTRACT A multidisciplinary program to investigate the bio-physical processes and interactions in estuarine environments has been initiated. Significant effects on the biological food chains can re'sult from physical activity on virtually all scales -- from seasonal circulatio and stratification patterns to the localized small-scale internal wave and turbulent mixing activity. The focus of initial efforts has been on the small-scale dynamic processes in the Chesapeake Bay. Simultaneous biological, physical, and chemical measurements have illustrated the nature of these processes in the Chesapeake Bay and bring to light the key role they play in the total system ecology. INTRODUCTION The Chesapeake Bay, one of the world's most productive estuaries, has suffered a substantial decline in several of its noteworthy biological species. In particular, the striped bass population has declined to the point where the State of Maryland has prohibited commercial and recreational fishing, and the oyster survival rate has fallen to its lowest level in many years (Seliger, et al., 1985). A recently completed U.S. Environmental Protection Agency study (EPA, 1982) has documented the declining state of the Bay and identified the general source of the problems: nutrient overenrichment due to urban and farm runoff. However, it is not possible to identify the specific links in the food chain that were detrimentally affected or to precisely evaluate the effects of the increased volume of anoxic water that was evident from the historical data. Thus, it is not possible to assess the potential effects of proposed nutrient control techniques nor even to effectively separate anthropogenic effects from natural effects.
The difficulty in assessing the specific.nature of the Bay's problems results from the complexity of the estuarine processes. The relevant processes are clearly multidisciplinary in nature: physical, chemical, and biological (EPA, 1983). Moreover, these processes operate over a broad range of scales: from annual variations, in which differing physical and chemical properties of the water (e.g., temperature and dissolved oxygen levels) can affect annual spawning success rates to local subtidal* processes such as the turbulent mixing within the water column, which directly affects plankton photosynthesis and feeding. To address estuarine processes in general, and the issues facing the Chesapeake Bay in particular, a multidisciplinary research team has been established. The long-term objectives of our team effort are to investigate the characteristics of the small-scale (subtidal) dynamic physical processes, to quantify the effects of the physical processes on biological activity and productivity, and to gain a fundamental understanding of the processes resulting 'in the development and evolution of anoxic water (Officer, et al., 1984; Seliger, et al., 1985). The multidisciplinary team effort has focused on a series of experiments in the Chesapeake Bay in which simultaneous measurements of the physical, chemical, and biological processes were made at frequencies appropriate to the subtidal variability. These field experiments were conducted during May 1984 and May 1985.t May is a critical month in the Chesapeake Bay: because of the increased stratification resulting from the spring runoff and the rapidly increasing, spring temperature, the plankton biomass evolution is rapidly increasing and the rate of increase of anoxia is at a maximum. The test scenario for these studies was to anchor the research vessel at a location of interest so that frequent or continuous measurements could be made throughout a complete tidal cycle. The locations were selected on the basis of the local bathymetery, the historical phytoplankton distribution, and the anticipated dissolved oxygen levels. The locations are shown in Figure 1. A description of the instruments used for the multidisciplinary measurements at each station is presented in Table 1.
*In this report "subtidal" refers to processes having periods less than the tidal period. +The 1985 test was conducted immediately following the LiBge colloquium and, as the data are still being reduced, will not be extensively discussed herein.
363
Fig. 1
Sampling stations for Chesapeake Bay experiments.
The purpose of this paper is to provide an overview of the broad range of physical processes acting on the biological material in this estuarine environment. Emphasis will be on the small-scale, subtidal processes and on phytoplankton ecology. These field studies provide a unique look at subtidal processes and will be used to illustrate the nature of these processes, rather than to
364 Table 1 Instrumentation for multidisciplinary Chesapeake Bay studies
Sampling frequency
Resolution
Continuous vertical profiles C.T.D.
30 rnin
10 cm
Dissolved oxygen
30 rnin
0.1 ppm
In vivo fluorescence
30 rnin
1 pg1-l CH L A
Current velocity, direction
30 rnin
1 cm 5-1
Phytoplankton species concentrations
2h
Zero = < 3 mi-1
Chlorophyll a extraction
2h
0.1 pgI-lCHL A
Nutrient concentrations
2h
0.5 pM
Dissolved oxygen
2h
0.1 ppm
Acoustic echosounder
Continuous
0.1 m, particles> 1 mm
Thermistor (10) - Conductivity (6)chain
Continuous
0.1OC; 0.1 mmho; 0.5 m
Microbiology optical imaging
Continuous
0.5 mm
Discrete vertical profiles
.
Time series measurements
provide a comprehensive picture of the Bay ecology. The following section will review the physical processes extant in the Chesapeake Bay and illustrate the ubiquity and surprising strength of the high-frequency internal wave field. The next section will illustrate, using results from our multidisciplinary studies, the major effects of the physical dynamics on key biological processes. The final section will discuss some wider implications of the subtidal processes and their variability. PHYSICAL DYNAMICS OF THE CHESAPEAKE BAY ESTUARY The Chesapeake Bay is classified as a partially mixed, salt-wedge estuary which is vertically stratified during the spring and summer and well mixed during the winter. Over the years, the Chesapeake Bay and its tributary rivers have been extensively studied, most notably from the physical point of view by Pritchard and co-workers (e.g.: Pritchard 1954, 1956). Scientific studies of the Bay's
biological and physical processes and even anoxia date back to the early part of the century (Newcombe and Horne, 1938). The overriding emphasis of these studies has been on the mean, tidally-averaged processes, which are critical to the water circulation and to chemical and biological transport. It is only in recent years that the instrumentation necessary to precisely measure the subtidal processes has become available. With such measurements, the direct role of these small-scale, subtidal processes on the biological activity is beginning to come to light. Processes and Scales The physical processes affecting the stratification, or degree of vertical mixing, in an estuary can be classified on the basis of three time scales: (1) seasonal processes, ( 2 ) short-term processes, and (3) very short-period, small-scale mixing processes. Within the first class there are two principal forces working to affect the degree of stratification: solar heating and fresh water inflows. On the short-term, subseasonal time scales, the principal forces are wind, spring-neap tidal variations, the tidal variations themselves, long-period internal waves, cross-bay seiching, and diurnal variations in heat flux. Finally, in the third class, are the very short-period internal waves and convective and turbulent mixing processes. Each of these temporal classes of physical phenomena plays a direct role in developing, maintaining, or destroying the degree of vertical stratification. In the Chesapeake Bay, the principal driving force on the seasonal time scale is the fresh water inflow, primarily from the Susquehanna River. This river influx establishes the gravitational estuarine circulation and, hence, the efficacy of the subsurface transport pathway (Tyler and Seliger, 1978), which is so important to the biological communities. The degree of vertical mixing has important consequences for the biological communities in the Bay, especially with respect to the development and maintenance of anoxic conditions. When the halocline is strong enough to suppress vertical mixing, there is an effective isolation of the deeper layer from both the downward mixing of dissolved oxygen and the subsurface transport of more oxygenated water from the mouth of the Bay. Thus, on longer time-scales, factors that control the degree of vertical stratification are also important to the primary biological processes associated with anoxic conditions.
366
The dynamics of the mixing processes themselves, however, have important effects at much shorter time-scales.
At time-scales of
hours to weeks, there are a number of physical mechanisms that can impart significant energy to the processes that result in mixing. The largest energy source is, of course, tidal, but other mechanisms such as wind forcing, internal waves, and seiching can also be important. The actual dynamics of these mechanisms are only now being studied as other than a mean, averaged effect: see for example, Partch and Smith (1978). Such studies have shown that for a strongly two-layered system, significant vertical exchange at the interface may occur during limited portions of the tidal cycle and is often related to the breaking of internal waves. Stratification in the Chesapeake Bay Development of Tidally-Averaged Properties.
The evolution of
the tidally-averaged stratification in the Chesapeake Bay is illustrated by the sequence of Bay transect profiles shown in Figure 2.
Here the increasing strength of the pycnocline during
the spring months is quite evident.
This strong layering results
from the spring runoff, mostly input at the head of the Bay, and the counterbalancing, denser salt wedge flowing upstream along the bottom. This process typically culminates in early June with a strong salinity gradient and an associated oxycline, which separates hypoxic and eventually anoxic water in the lower depths from the oxygenated surface water, as shown in Figure 3. In 1984 the river runoff was very strong, resulting in a rather high degree of stratification, an associated high degree of internal wave activity, and an extensive volume of anoxic water (Seliger, et al., 1985). In contrast, 1985 was a relatively weak runoff year and evidenced a somewhat weaker stratification and a lower level of internal wave and mixing activity, albeit still significant with regard to effects on the biological processes. Subtidal Variability.
Although the variations in tidal current
during the course of a tidal cycle are well understood, the variations in the mean stratification during the course of a tidal cycle and the high frequency, internal wave activity have not often been considered. Prior to the present studies, only limited measurements of internal. wave activity in the Chesapeake Bay or similar environments had been made (Clarke, et.al., 1983; Sarabun, 1980). In the current Chesapeake Bay investigation, considerable variations in the properties of the mean water column were evident. These
367
0
5
5 Q
6
a 16 24 32 40 300
260
220
140
100
60
20
0 -20
Distance from mouth of bay (km)
Fig. 2
Evolution of pycnocline stratification - Chesapeake Bay - Spring 1984.
variations are illustrated in Figure 4, which shows data obtained during a 26-hour period ( 2 tidal cycles) at a mid-Bay station. The axial current contours shown in Figure 4a illustrate the typical ebb-flood cycle during the second tidal period; even this process may not always conform to the standard pattern, as illustrated by the pervasive flooding below ~8 m depth during the entire first cycle. The salinity contours shown in Figure 4b illustrate the variability of the water column properties resulting from tidal forcing and internal wave activity.
368
Distance from mouth of bay (krn)
Fig. 3
Salinity and DO in Chesapeake Bay at peak stratification.
The Brunt-Vaisala (BV) frequency is a measure of the degree of stratification and, in a two-layered system, a measure of the strength of the interfacial layer. The BV frequency is defined by
where p is the fluid density, and z is the depth. Figure 4c illustrates the BV frequency variation throughout the tidal cycle. The strong density gradient, characterized by the high BV values, forms an interfacial region that tends to inhibit the vertical transport of plankton and oxygen. An additional illustration of the marked changes that occur during a tidal cycle is presented in Figure 5. Here the contrast between the smooth gradient and step-like temperature profiles during the respective flood and ebb portions of the tidal cycle is quite evident.
369
5
-E 5 10
B
n
15
5 1 P
d
1
20
Fig. 4
I
I
I
I
I
I
I
I
I
I
I
I
Time series of vertical distributions of (a) axial tidal currents (in centimeters per second), (b) salinity (in parts per thousand) and (c) BV frequency for the 26-hour station occupied on May 30, 1984, just south of the Chesapeake Bay Bridge.
370
I
I
I
I
1-
C Bay 1984 Sta 858
13
Fig. 5
I
I
14
15
I I I 16 17 18 Temperature ("C)
I 19
Temperature profile variation during a tidal cycle.
High-Frequency, Subtidal Processes - Internal Waves. The multidisciplinary field studies conducted in the spring of 1984 and 1985 were focused primarily on subtidal processes. During the cruises, the research vessels occupied a series of stations at fixed locations along the axis of the mid-Bay. Each station was occupied for one to two tidal cycles (1.13 to 26 hours). The
371
physical oceanographic instruments deployed to measure the highfrequency activity were a 200-kHz narrow-beam acoustic echosounder and a 10-element, high-frequency-response thermistor chain array (Table 1). The echosounder and thermistor chain data obtained during the experiments illustrate the presence of strong, high-frequency internal-wave and mixing activity in the central portion of the water column. As an illustration of those observations, Figure 6
Fig. 6
Time series of gray-scale encoded acoustic backscatter intensity versus depth, taken at a station just below the Chesapeake Bay Bridge on May 31, 1984.
shows a grey-scale encoded plot of acoustic backscatter intensity for a short portion of the 26-hour period. A train of high-frequency (1- to 2-minute period) internal waves is clearly evident, with the largest waves attaining a peak-to-peak waveheights of 6 to 7 meters. Between the crests of the largest waves are clouds of acoustic scatterers that have apparently been collected by convergences in the wave's velocity field. During this period, the acoustic scatterers were small zooplankton (copepods), which accumulated in the pycnocline along with the algae. Figure 7 is a thermistor chain record taken at about the same time as the echosounder record in Figure 6. It also shows the large amplitude internal waves. In these chain data, two wave packets are evident a s periodic, wave-like changes in temperature. Associated with the 1- to 2-minute waves are bursts of high-frequency turbulent fluctuations with periods on the order of seconds. The upper portion of the thermistor chain was in an isothermal region
372
0940
0945
Fig. 7
0950
0955 Time (h)
1000
1010
1005
Time series of temperature for the thermistor chain for May 31, 1984, taken just below the Chesapeake Bay Bridge. Thermistors are separated by 0.5 meter. Note the two wave packets between time periods 0945-0955 and 1002-1009.
during this period (see Figure 5 ) so that oscillations in the water column cannot be seen by the upper thermistors. These data were obtained during the same time period covered by the contour plots shown in Figure 4 , where the existence of the high-frequency activity is masked by the sparseness of even the hourly samples. In Figure 8, thermistor data from a different location in the Bay also show evidence of a broad range of physical activity, including long-period internal waves (20- to 30-minute periods), high-frequency internal waves having ~ 1 -to 2-minute periods, and
3
I
I
I'
I
I
I
Time (h) Fig. 8
Time series of temperature for the thermistor chain from May 24, 1984, taken in mid-Bay just south of Tilghman Island. Thermistors are separated by 0.5 meter.
373
turbulent motions with fluctuations measured in seconds. (Note that the temperature scale in Figure 8 is 1.4 times larger than that in Figure 7.) The power spectra (power spectral density vs. frequency) for two individual thermistor records (30-minute segments) are shown in Figure 9 . Most apparent is the dominant peak (evident on both traces) centered at 1.100 s, corresponding to the visually dominant oscillation on the chain records shown in Figures 7 and 8. The higher overall level of the spectral distribution at Station 8 5 8 , Figure 9 , reflects the high level of wave activity present at that location. For these data the maximum water column Brunt-Vaisala frequency is about 60 cyc/hr, or 0.016 H z . The Brunt-Vaisala frequency determines the high frequency limit of wave activity. The spectral slope at lower frequencies is 21.8, which is in general agreement with the accepted value for moored measurements of internal-wave displacements (Garrett and Munk, 1 9 7 9 ) . The -4 fall-off at higher frequencies is considerably higher ( _ Z W ) , while the leveling off and rise at the highest frequencies, f > 0.05 Hz, is indicative of a high degree of turbulent activity, perhaps due to local wave overturning. (Note that for this experiment the thermistor response time was 1.7 ms, while the overall data resolution and actual digitization rate was 10 H z . ) An attempt was made to correlate the level of internal-wave activity with the tidal phase to test the hypothesis that the internal waves were generated by the action of the ebb tidal current flowing over local variations in the bathymetery. Figure 10 shows a plot of the water-column-averaged wave intensity for 30-minute data segments centered at the indicated times, for two different Bay locations. The tidal phase corresponding to each data segment is indicated by E for ebb, F for flood, and no mark for tidal nodes. As no correlation is apparent, a more complex wave generation process may be at work; its resolution will have to await further, more comprehensive data. These spring 1984 data graphically illustrate the energetic nature of high-frequency activity present in the Bay. While the mean stratification is an important seasonal controlling factor in the Bay, much of the vertical mixing results from phenomena with considerably shorter time-scales. It follows, therefore, that any realistic attempt to understand the Bay's biological and chemical processes must be carried out in close conjunction with the study of the physical transport and mixing processes.
374 10
I
t
I
E
10'
10-
lo-'
lo-'
lo-'
-4
I
1
I
10-3
10-2
0.1
Frequency
Fig. 9
(Hz)
Power spectral density of temperature fluctuations at two stations.
375
0.6
1
1
I
-
---
F 0.5 -
?
1
I
I
Sta 858 May 30-31, 1984 Sta 834 May 24, 1984
0.4 -
F
m m
Y
In -0
m
.m
, E
OO
2
4
6
8
10
12
1
Time ( h ) , arbitrary origin
Fig. 10
Water column averaged wave intensity.
PHYSICAL EFFECTS ON BIOLOGICAL PROCESSES The physical processes described above can have direct and often profound effects on the biological activity.
In general, physical
processes will have a direct effect on the biological processes that are of a comparable scale. An example is the effect of fluid turbulence on plankton: turbulent motions can result in patchy plankton distributions and increased motions of individual plankters leading to enhanced food availability. Physical processes of scales larger than the biological species will result in mean or integral effects. The transport of phytoplankton through the estuary is an example. The interactions at various scales are illustrated schematically in Figure 11.
Examples of these processes
376
System ecology
Watershed properties
Spawning patterns
Stream flow
Benthic distributions
Biological transport
Physical transport Circulation patterns
Mean transport
Mean stratification
Local patchiness
Nutrient, oxygen, sediment distribution
Diurnal variations
Small-scale dynamics
Localized bio-effects
Convective mixing
5
Distribution-feeding Photosynthesis-exposure
Internal waves
A-Primary productivity
Fig. 11
Bio-physical interactions.
resulting from our studies in the Chesapeake Bay will be discussed in this section. Some of these effects are seen to be quite dramatic and heretofore underestimated, if not unanticipated. Circulation and Transport The tidally-averaged circulation patterns, driven by the Bay stratification (see Figures 2 and 3 ) , affect the transport of the non-motile Bay species. This integral effect on the transport of phytoplankton, particularly Prorocentrum, has been studied
377
Fig. 12
Prorocentrum transport pathway in the Chesapeake Bay.
extensively by Tyler and Seliger (1978, 1981). Figure 12 illustrates the up-Bay transport of phytoplankton by the net inflow of ocean water. At the sharply shallowing up-Bay station, ~ 2 5 0km from the Bay mouth, the Prorocentrum are mixed into the surface water and result in visible "red-tides". In 1984 the stratification was sufficient to inhibit this vertical advection despite the intense internal wave and turbulent mixing illustrated in Figures 6 and 7. Internal Wave Effects The high-frequency internal waves, shown in Figures 6 through 8, also have significant direct effects on the plankton distributions. The most evident and dramatic effect is the vertical oscillation of the plankton layers induced by the high-frequency, internal waves. This effect can be seen directly in Figure 6, as the backscattering signal arises primarily from the zooplankton layer. In the 1984 experiment, internal-wave-induced oscillations of 5 to 7 meters were observed frequently, while waves of 2 to 4 meters were a common occurrence in the up-Bay stations. In 1985, because of the
318
difF.-;ing mean stratification, the maximum wave heights observed were '1.4 meters. One direct result of these internal-wave oscillations on plankton layers is the increased exposure to the higher light levels present nearer the surface, which results in an increased potential for photosynthetic activity. For example, if an exponential decrease in light intensity with depth is assumed (Tyler and Preisendorfer, 1962), then the ratio of light intensity at two depths will be given by
where Az is the difference between the upper, u , and lower 2 , depths, and k is the extinction coefficient. For the relatively turbid Bay water, a typical value of k is 1 m-l (Kirk, 1983). This represents a l/e loss of light in 1 meter. Equatiqn (2) then indicates that the light intensity due to the internal wave activity, centered at the mean level of the plankton layer", would increase by factors of 4.5, 12, and 3 3 for internal wave heights of 3 , 5, and 7 meters, respectively. Moreover, this variation can take place over periods as short as 100 seconds! The potential of such activity for increasing the net photosynthesis, especially of a nominally trapped "deep" layer is evident. In addition, this level of wave activity should have a profound effect on the local mixing and plankton-food exposure. This latter effect is even more pronounced in the local convergence zones between wave crests. The large backscatter regions (zooplankton masses) visible on the right of Figure 6 result from the internalwave-induced velocity field in which the vertical wave oscillations induce local horizontal currents and thus orbital fluid motions that converge between wave crests. Suspended material (plankton and sediment) in the wave field will be transported along these orbital streamlines and will tend to accumulate at the velocity nodes - convergence zones. The Pycnocline Boundary Evident throughout the present studies in the highly-stratified environment is the observation that there is a confluence of physical, biological, and chemical processes at the pycnocline. *Using
4
of the peak-to-peak wave heights.
379
The strength of many estuarine pycnoclines in general, and that in the Chesapeake Bay in particular, is markedly greater than in the deep ocean, as illustrated in Figures 2 through 5. This strong gradient region is a region of intense internal wave and turbulent mixing activity. In addition, the stratification interface acts as a physical boundary, limiting biological and chemical transport. The distribution and physical trapping of the phytoplankton mass is illustrated in Figure 13, which shows the distributions of salinity* and in vivo fluorescence (IVF) during one of the 26-hour stations. Vertical profiles illustrate this process even more vividly. Figure 14 shows that the dissolved oxygen (DO) is trapped below the maximum density gradient, uT, and that the biomass, IVF and P. minimum counts, are trapped below the density gradient and above the zero DO level. Further evidence of this trapping effect is shown in Figure 15, from an earlier cruise. Here a vertical profile of organism distributions shows that bacteria, Prorocentrum, and the mysid shrimp Neomysis Americana have accumulated below the pycnocline and just above the anoxic layer. At this time tows at 2.15 meters produced larval anchovy, and reports by numerous fishing boats in the areas indicated that bluefish were being caught at 1.30 feet (9 meters), approximately the depth of the pycnocline. These data illustrate the extreme degree to which life-supporting conditions can be confined to a very narrow, subpycnocline band during strong anoxic conditions. FURTHER IMPLICATIONS OF HIGH FREQUENCY PROCESSES Sampling Strategies As indicated, the biological systems in the Chesapeake Bay can be strongly influenced by physical transport and mixing processes.
In particular, the physical dynamics play a major role in determining the spatial and temporal distributions of biological activity. On a long-term mean basis, the estuarine circulation, coupled with the physiology of the organisms, gives rise to a strong layering in the vertical and intermittency in the horizontal and temporal dimensions. This interaction means that sampling strategies must take account of these factors in order to monitor properly the seasonal developmen and distribution of the biological organisms.
For example, Figure 13
*This plot is a non-smoothed version of Figure 4b.
I
I
I 3
1800
1
I 2200
0200
I 0600
I
I
1
I
I
I 1000
I 1400
Time (h) Fig. 13
I
Temporal evolution of (a) salinity and (b) in vivo fluorescence.
381
0
I
I
I
I
I
I
I
I
I
Station 858- 30 V 84 2045 H. fS.6.F.)
TI I
I
I
1
I
I I I
I
X
I K
I I
I
1 I I I I
Fig. 14
1
I
2
4
I 6
I 8
Simultaneous vertical profiles of Bay parameters.
shows clearly how sampling at a limited number of pre-set depths might easily miss a rather thin layer of Prorocentrum at mid-depth. In the presence of short period internal waves as shown in Figures 6 and 7 , with crest-to-trough heights of up to 2 5 percent of the total water column depth, a water sample drawn from a depth of 8 meters could be in the pycnocline, above it, or below it -- all within the space of less than 2 minutes. In addition, there is the problem of when (or where horizontally) to sample. The high-frequency, strongly-nonlinear, internal waves have two further compounding effects apart from the periodic vertical displacement of layers. The periodic velocity field associated with the internal waves gives rise to convergence zones between the crests which, as seen in Figure 6 , result in the concentration of scatterers between the crests. Thus, if one were to sample in a region clearly above the greatest vertical displacemen. of the pycnocline, one must still contend with the spatial/temporal
382 O#
I
30
I
-
Species '
Scale factor
10-3 m1-1 20 pm lo-* 1 - l 5 mm X ~O-~ITII-~ 0.5prn X
0 Bacteria
1
0
Size
x
w Prorocentrum
A Neomysis americana
40
I
I
I
1
I
5
10
15
3
Counts ( X scale factor)
Fig. 15
Vertical distribution of acoustic scatterers in the lower Chesapeake Bay in late spring. The pycnocline is at 14 meters.
intermittency engendered by the internal-wave velocity field. Secondly, periodic breaking of the internal waves also occurs, resulting in actual physical exchange across the pycnocline. Evolution of Anoxic Water One of the principal reasons for concern with nutrient levels in the Bay (EPA, 1982), particularly nitrogen and phosphorus, is the strong historical correlation between an increase in these nutrient levels and a decrease in dissolved oxygen in the deeper areas of the Bay and the resulting effects on the Bay food chains (Seliger, et al., 1985). One mechanism postulated for this correlation is that the increase in algae growth due to the increase in nutrients ultimately results in the consumption of large amounts of dissolved oxygen when the algae die and decay (Officer, et al. 1984). The trapping of this oxygen depleted water below the pycnocline, as
383 discussed above, can thus be a significant factor in the viability of critical stages of the Bay food chains. Thus, the decrease in dissolved oxygen levels below the pycnocline could be a determining factor for the observed decline in the harvest levels of several commercially important species. SUMMARY The present series of studies has brought to light several significant, multidisciplinary effects. In particular, we have found that large-amplitude (up to 25 percent of total water column depth), short-period internal waves are prevalent in the midChesapeake Bay region. Indeed, it has been shown that significant physical activity exists at all scales in the Chesapeake Bay and that biological processes are affected at all scales. Particularly important are the direct effects of small-scale physical processes on the biological activity. These physical processes inolude: internal wave oscillations, local convergence zones, and the strong pycnocline interface. Acknowledgements J. R. Austin is gratefully acknowledged for the inspiration and continuing support for the multi-institute, multidisciplinary effort. We also express our thanks to our co-investigators J. A. Boggs, W. H. Biggley, R. B. Biggs, and to W. R. Drummond, J. E. Hopkins, G. D. Smith, and C. J. Vogt, who played key roles in our effort. REFERENCES Clarke, T. L., Crynock, V. F., and Proni, J. R., (1983), "Simultaneouls Acoustic Doppler and Backscatter Observations of Estuarine Internal Waves," EOS Trans. Am. Geophys. Union, 64, 1022. EPA, (1983) Fundamental Research on Estuaries: The Importance of an Interdisciplinary Approach, National Research Council, National Academy Press, Washington, D.C. EPA, '(1982) Chesapeake Bay Program Technical Studies: A Synthesis, U . S . Environmental Protection Aqency, Washinqton, D.C. Garrett, C. and Munk, W., (1979), 'Internal Waves in the Ocean," Ann. Rev. Fluid Mech., Annual Reviews, Inc., 11, 339-69. Kirk, J. T. 0 . (1983), Light and Photosynthesis in Aquatic Ecosystems Cambridge University Press, Cambridge. Newcombe, C.L., and Horne, W. A. (1938) "Oxygen-Poor Waters of the Chesapeake Bay," Science, 88, 80-81. Officer, C. B., Biggs, R. B., Taft, J. L., Cronin, L. E., Tyler, M. A and Boynton W. R., (1984) "Chesapeake Bay Anoxia: Origin, Develop ment, and Significance," Science, 223, 22-27. Partch. E. N. and Smith, J. D., (1978),"Time Dependent Mixinq in a Salt Wedge Estuary,"-Est. Coast, Mar. Sci., 5; 3-19. Pritchard. D. W.., .(19541, "A Studv of the Salt Balance in a Coastal Plain Estuary," J. Marine Res., 13, 133-144. I
.
3 84
Pritchard. D. W... (1956). . . . "The Dvnamic Structure of a Coastal Plain estuary," J. Marine Res., Is, 33-42. Sarabun, C. C., ( 1 9 8 0 ) , "Structure and Formation of Delaware Bay Fronts," Ph.D. Thesis, College of Marine Studies, University-of Delaware. Seliger, H. H., Boggs, J. A., and Biggley, W. H., (1985), "Catastrophic Anoxia in the Chesapeake Bay in 1984," Science, 228, 70-73. Tyler, J. E., and Preisendorfer, R. W. (1962) "Light," Ch. 8 in The Sea, Vol. 1, John Wiley and Sons, New York. Tyler, M. A. and Seliger, H. H., (1978), "Annual Subsurface Transport of a Red Tide Dinoflasellate to its Bloom Area: Water Circulation Patterns and Organism-Distributions in the Chesapeake Bay, " Limnol. Oceanog., 3, 227-246. Tvler, M. A. and Seliqer, H. H., (1981). "Selection for a Red Tide Organism: Physiological Responses to the Physical Environment ," Limnol. Oceanog., 26, 310-324.
-
385
ASPECTS OF THE KOFTHERV BERING SEA ECOHMRODYNAMICS Jacques C . J . NIHOUL Qw,
university of Liege (Belgium)
INTRWUCTTION The Northem Bering Sea i s a r e l a t i v e l y shallow basin limited by the Bering S t r a i t t o t h e north and S t Lawrence Island to the south (Fig. 1 ) . The flaw passing through t h e Bering S t r a i t , f r a n the P a c i f i c Ocean t o t h e Artic Ocean, penetrates the Northern Bering Sea through the S t r a i t of Anadyr, t o the west of S t Lawrence Island, and by the S t r a i t of Shpanberq, to t h e east. More than 60 % of the mean northward transport of water through t h e Bering S t r a i t is
derived from the "Anadyr Stream", a subsidiary of the Bering Slope Current which flows arounl t h e coasts of the Gulf of Amdyr, f o l l m i n g the 60-70 isobaths, to the Wadyr S t r a i t and the wstern part of the Shpanberg S t r a i t (Coachman e t a l , 1975). The proportion of t h a t stream which goes through the S t r a i t of Anadyr o r s k i r t s S t Lawrence Island, as w e l l as t h e orientation, with respect t o the S t r a i t ' s a x i s , and seasonal variations of the entering flow, is l i k e l y t o have a strong influence on the subsequent deploywrit of ihat flow i n the Northern Bering Sea and i n the Chukchi Sea. Observations suggest that the Anadyr strean is the main source of nutrients
and biological productivity i n the Northern Bering Sea (Walsh e t a l , 1985). In preliminary studies f o r the ISHTAR Research Project (e.g. Walsh e t a l , 1985), a series of numerical s h l a t i o n s were performed, w i t h 2D barotropic a d 3D baroclinic m a t h ~ t i c a l m o d e l s ,to test this hypothesis and determine i f the residual c i r c u l a t i o n pattern i n the Northern Bering Sea w a s iladeed canpatible with observed biological d a t a (Walsh and D i e t e r l e , 1986; N i h l e t a l , 1986). me r e s u l t s of these exploratory sirmlations confirm the general t r d of the Anadyr Stream to spread to the east a f t e r passing the Anadyr S t r a i t : t h e nutrient r i c h Anadyr waters deploying eastwards and progressively fostering biological productivity in the whole basin (Fig. 2, 3, 4 ) . Studies of t h e year-to-year v a r i a b i l i t y of the flow p a t t e r n reveal however the existence of occasionally strong secondary flows i n the form of eastwards propagating interleaving layers of f r o n t a l origin. These layers may contribute significantly to t h e cross-stream diffusion of nutrients and subsequent
386
Fig. 1. The Northern Bering d &&chi seas including Berm Strait. b i o l q i c a l activities. I t is shown i n the follming that the main features of these layers can be
explained by a simple mdel of barmlinic instabilities predicting length scales ard time scales i n excellent agreawnt w i t h the observations. A scenario of the Northern Bering Sea Ecohydrcdynmics, including the
general circulation pattern ard the local frontal secondary flows, is then
presented as a working hypothesis t o be tested by field surveys ard mathenat i c a l models.
387
NON-LINEAR 30 MODEL GHER U N I V E R S I T Y OF L I E G E
r' CAPE LISBURNE
Fig. 2. Current pattern i n the top layer of the Northern Berinq Sea for a total flow through the Bering S t r a i t of 1.8 106m3s-l .
3 88
N O N - L I N E A R 30 MODEL GHER
UNIVERSITY
OF L I E G E
Fig. 3 . Current pattern i n t h e middle layer of the Northern Berina Sea for a total flow through the Bering S t r a i t of 1.8 106m3s-l .
389
NON-LINEAR 30 MODEL CHER UNIVERSITY OF L I E G E
~~
~~~
~
~
~~~
~
~~
Fig. 4. Current pattern in the bottcn layer of the Northern Berinq Sea for a total flow through the Bering Strait of 1.8 1061x3s- l .
390 The dateline erqocline
The marine system is characterized by f a i r l y well-defined "spectral windaus",
i.e. domains of length-scales (inversely, wave-numbers) and t k scales (inversely, frequencies) associated with i d e n t i f i e d phenaoena.
These windows
may correspond t o eigenm=des of the system (internal waves, i n e r t i a l oscillations, Rossby waves, E l N S i o
... )
o r external forcinq (annual o r daily varia-
t i o n s of insolation, tides, storms, atxmsphere climate changes
... )
(e.9.
bbnin e t a1 1977, Nihoul 1985). In general, ti= scales and length scales are related and it is customary t o associate high frequencies and high wave numbers, s m a l l frequencies and
smll wave nurrJ3ers although the association may be d i f f e r e n t for eiqenmdes and forced o s c i l l a t i o n s . The t r a n s f e r of energy between windows is effected by non-linear interac-
tions. Chemical and ecoloqical interaction processes can also be characterized by s p e c i f i c t k scales and the comparison between these t k scales and those of hydrodynamic p h e n m a indicates which processes are actually i n corrpetition i n the sea (Nihoul 1984, E m and Pawel 1984). Obviously, a t hydrodynamic scales much smaller than interaction scales, very l i t t l e interaction takes place over time of significant hydrodynamic changes and basically the constituents are transported and dispersed passively by the sea. ckl the other hand, hydrodynamic processes with t i r e scales much l a r g e r than interaction scales scarcely a f f e c t the dynamics of interac-
t i o n s over any time of i n t e r e s t .
wesoscale, synopticscde and seasonalscale processes i n the 10-4- 10- s-
1
range of frequencies form what one tends t o c a l l now the "weather of the sea" while longer tim scale phenaoena a f f e c t both the oceanic and the atmspheric
climates. Independently of c l i m t e problems, the year-to-year
v a r i a b i l i t y of the
m r i n e system, associated w i t h globalscale pmcesses, m y be an important el-nt
i n forecasting the reserves (population dynamics) and permissible
hauls of the basic comrcial f i s h ( N i h o u l 1985).
Evidence of year-teyear v a r i a b i l i t y of the Berina Sea i s qiven by Coachmn e t a1 (1975) and Aagaard e t
al (1985). Although year-to-year v a r i a b i l i t y is a climatic e f f e c t , one of its min consequences is a d f i c a t i o n of the typical sea-ather ptterns. This
may r e s u l t from changes i n the general oceanic circulation and enerqy trans-
ports o r (and) from similar changes i n the atmsphere with differences i n typical atxmspheric-weather patterns over the m r i n e area which i s being investigated.
391 Thus, some exceptional years, rather inprtant m d i f i c a t i o n s w i l l be obser-
ved in mso- and synoptic scale processes characteristic of a p a r t i c u l a r time of the year and these w i l l e n t a i l changes i n the flaw f i e l d and i n many chemical and biological processes which d e p d on transport and dispersion. A t y p i c a l i l l u s t r a t i o n of this p h e n m o n can perhaps be found in a sequence of s i x mte sensing photqraphs taken on June 18, June 19, July 15, July 16, August 1 and August 30, 1984. These @otogra@x shaw a marked plume of cold water, originating near Cape Chukotski on the Soviet Coast and spreading i n the Northern Bering Sea under t h e e f f e c t of the general Northward c i r c u l a t i o n and lateral i n s t a b i l i t i e s , eddies and extrusions progressing tawards t h e East (Fig. 5).
Fig. 5. Thermal h g e of the Northern Bering Sea f o r 16 July 1984. (Cold w a t e r i n w h i t e ) Considering the climatological wind f i e l d f o r this tim of the year, t h i s
plume could be due to a rather intense upwelling bringing, to the surface, cold bottom water with high n u t r i e n t concentrations.
392 The extruding layers which have typically a width of the order of 10 km,
i n t h e e a r l y stages of developrent, widen progressively as they flaw eastwards, spreading the n u t r i e n t r i c h water over the Northern Bering Sea.
As shown in the next section, the f o m t i o n of such layers can be explained by a barcclinic i n s t a b i l i t y of the f r o n t a l edge of t h e cold plum.
176'
168'
172' I
I
I
1
I
160'
164' I
I
1
I
71'
70
69
68
67
66
65
64
63
92 I70° L
..
.
,
-
166'
162'
.
Fig. 6. Distribution of temperature O C a t 5 m observed by "Brown Bear", 26 July-28 August 1960 ( f m Fleming and Hegqarty, 1966).
393
Evidence of similar f r o n t s , i n the region and qeneral d i r e c t i o n of the dateline i n the Northern Bering Sea can be found i n several f i e l d surveys (e.g. Ccachmm e t al 1975, f i g . 6 ) but t h e i r i n t e n s i t y can be hiahly variable and, i n t h i s respect, t h e 1984 sumer s i t u a t i o n m y have been exceptional*. Nevertheless the d a t e l i n e f r o n t , when it occurs, c o n s t i t u t e s an extremely e f f i c i e n t "ergocline" and t h e cross-front transport by the extruding layers
is equivalent t o a r a t h e r intense lateral mixing, extending the reuion of b i o l q i c a l production and determining t o a larue extent the munt of organic
matter which i s ultimately transporter t o the chukchi Sea and further. A simple mdel of baroclinic i n s t a b i l i t y
The problem of barcclinic s t a b i l i t y has been extensively studied and many
d e l s , w i t h various degrees of sophistication and n w r i c a l s k i l l , can be found i n the l i t e r a t u r e (e.g. Eady 1949, Stone 1966, 1970, 1971, Tang 1971). In a simple form, appropriate t o the d a t e l i n e ergocline i n t h e Northern Bering Sea, the problem can be described as the determination of the condit i o n s of i n s t a b i l i t y of a depth-dependent horizontal current, flowinq parallel t o constant buoyancy surfaces i n a region of s i g n i f i c a n t buoyancy gradients The f a s t e s t growing r r d e of the l i n e a r s t a b i l i t y problem s i v e s
(Fig. 6 ) .
rise to the observed extruding layers and its length-scale sets the width of these layers i n t h e i n i t i a l stages of developwnt. The basic equations applicable to t h i s problem are t h e inviscid Boussinesq
equations, viz. 0.y = 0
5
where
y
b
g
=
-
i s t h e velocity, u
P
O
,
g
f
t h e Coriolis frequency,
the acceleration of gravity,
p
b
t h e buoyancy,
the density
(po
its
constant reference value ) ,and where igx3
q = Po
++ Only tworemote s e n s i n g p h o t ~ a p h s ~ r e a v a i l a b l e a n d c l e a r e n o u a h f o r o t h e r y e a r s : 18July 1982 and13June1983. I f one goeslmking f o r i t , o n e cansee, onthem, traces of thesamecoastalupwellinq, r e s u l t i n u i n a s m a l l , q u i c k l y d i f f u s e d p l m carried a1ongintotheNorthernBeringSeabut theevent i s ofnocomparable m r t a n c e and may be smeared i n the aeneralwatercirculation pattern.
394
(,e3 is the u n i t vector along t h e vertical axis pointing upwards). For t h e purpose of this study, takinq i n t o account that the length-scale of t h e p e r t u r b a t i o n is much smaller than t h e characteristic scale of v a r i a t i o n
of the basic flow, one may assume that the latter is h o r i z o n t a l l y uniform and extending t o i n f i n i t y .
The buoyancy and v e l o c i t y f i e l d s , i n the unperturbed
state, are then qiven by
-Vbo
-
=
m2 9,
+
n2 ,e3
i.e. (7)
where
n
and m
are, respectively, the " v e r t i c a l " and "horizontal"
..
Brunt-VaiGla frequencies , i e
and where
U
i s a constant of i n t e g r a t i o n representinq an eventual regional
l a r g e scale flow d i r e c t e d along t h e f r o n t (i.e. with the approxirration made i n the Northern Bering Sea, parallel t o the coast). Assuming constant depth h
and constant values of
m
and
d e f i n e a length-scale L
=
m2hf-*
and the followinq non-dimensional v a r i a b l e s and parmters
n,
one can
3 95
where denotes a perturbation of the basic state. The non-dimensional form of the Boussinesq equations for the perturbation may then be written, in the quasi-hydrostatic approximation,
zao+ ( 6 + z ) - +a vu
-
ax
2 at ,
(s + z ) aa --u++w ax
- -an ay
= 0
(26)
'Ihe prturbaticm are assumed to be mall a d to have time and space dependences of the form o(z)exp [ i (as + 6y - wt) J where w denotes a complex frequency (instability occurs when the imaginary part of w is 'positive) and where is the appropriate amplitude, function of z . Subsitutinq in eqs (22) - (26) and solvinq for W(z) (the amplitude of W), one obtains
where 5 = (Yz
-
w
+
as
belongs to the class of Hem's equations. It must be solved subject to the boundary conditions Q. (27)
W=O W=O
at at
z = O
z = 1
'Ihe boundary value prablem set by e q s . ( 2 8 ) , (29) and (30)leads to a general conplex dispersion relation of the form
396
where
wr
and wi
are respectively the real and i n q i n e r y parts of w
.
Separating the real and imaginery parts of eq. (31) and eliminatina wr
- as,
as a function of a,y and r. The perturbation f o r which wi is maximum has the largest growth rate and generates the observed cross-erqo-
one finds
wi
c l i n e secondary flows. I n the Northern Berinq Sea, observations (e.g. Coachman e t a1 1975, Sanbrotto 1984) indicate that
hence
For values of the Richardson ncnnber of that order, one can shm (e.g. Stone 1966, 1970, Happel et a1 1986) that the
maxirmpll
growth rate is obtained f o r
The typical wave-length of the f a s t e s t growing perturbation i s then given bY
i.e., taking
h
50 m
,
X'LlOIan
i n agreemnt with the observations. The secondary flow pattern of eastward extruding layers i n the Northern
Bering Sea m y thus presumably be a t t r i b u t e d t o the baroclinic i n s t a b i l i t y of
the dateline ergocline which is formed, i n well-defined environmental conditions, a t the edge of a p l m of cold upwlled water passina throuuh the Anadyr S t r a i t .
397 A scenario f o r the Northern Berinq Sea Ekohydrcdynamics
It is n m believed that n u t r i e n t s are e s s e n t i a l l y brought t o the Northern
Bering Sea by the Anadyr Stream and that b i o l q i c a l production i n t h e area is closely related to t h e deployment and resisence t i m e there of Anadyr Stream waters, i n d i f f e r e n t e n v i r o m n t a l conditions. The occasional occurence of a marked u p e l l i n g plume swept along i n t h e Northern Bering Sea a s an unstable f r o n t a l current and the subsequent develop-
mt of extrudinq layers, flowing eastwards, contribute, i n exactly the same way, t o the lateral diffusion of the n u t r i e n t s and one m y arcge t h a t t h e productivity of t h e N o r t h e r n Bering S e a dependsonthe i n t e n s i t y and the variab i l i t y of both the primary and secondary flows. It is illuminating, i n t h i s respect, to examine the r e s u l t s of t h e Second
Soviet-American Expedition i n the Bering Sea, 27 June
-
31 July 1984, a period
characterized, as pointed out before, by a Ell-marked erqocline event (Sambrotto 1984). The expedition had selected four p l y q o n s of observations : three of which were more or less d i s t r i b u t e d a l o n a the an ad^ Stream. While Polygon I upstream w a s characterized by r e l a t i v e l y hiqh surface tmpratures ( > 6OC) arid
high n u t r i e n t concentrations, increasing w i t h depth, t h e transect between Polycjons I1 and I11 showed a decrease i n surface temperature dawn to 2"C, a depletion of n u t r i e n t s i n surface waters due to a c t i v e phytoplankton production, high concentrations of m n i a i n the bottom layers indicating a c t i v e
microbial degradation of organic matter (as c o n f i d by microbioloaical studies) and high rates of organic sedimentation. The additional observation of a l a r g e r sea b i r d s ' population a t Polygon I11 suggest that the fox3 chain
has completely developed along the Anadyr Strem. before it penetrates the Northern Bering Sea and the follcwing ecohydrodynamic scenario appears q u i t e plausible. Marine productivity in the N o r t h e s t e r n Bering and Chukchi Seas is achieved
i n two successive phases : one downstream of t h e shelf-break and the other
downstream of St. Lawrence Island. The f i r s t involves intense primary production along the "Anadyr stream", w i t h sedimentation of orqanic mtter, both south of Cape Chukotski and S t . Lawrence Island, and, generally speaking, i n the outer-lagoon of the slowly revolving w a t e r i n t h e Gulf of Anadyr's secon-
dary g y r e f l o w
(Fia.
7). The second phase develops north of t h e Anadyr S t r a i t ,
spreadingto the eastern side of the Northern Bering Sea to a variable extent, due to i n t e r m u a l changes i n w i d forcing.
The eastward excursion of nutrient
enriched w a t e r is then a function, not only of the i n t e n s i t y a d direction of t h e inflowing current from the m l f of Anadyr, but also of the i n t e n s i t y of t h e
coastal u p e l l i n g , the resultinqplmdevelopnent, and f r o n t a l i n s t a b i l i t i e s .
398
Fig. 7. The d i s t r i b u t i o n of organic carbon ( % dw) i n the surface sediments of t h e Bering-mukchi Seas (lqalsh e t a1 1985).
above can be nothing mre than darina hypotheses. A mre thorough investigation requires mre data (and i n particular, more remote sensing data and meteorological information) and the d e v e l o p nwt of m t h e m t i c a l models grming to f u l l three-dhmsional maturity. ?his w i l l hopefully be the program of the second phase of the Imm A t t h i s stage, the scenario described
project.
399
Acknmledgwnts The author is indebted to the National Science Foundation for its support
in the smpe of the first phase of the 1SHT.W project. REFERENCES
Aagaard, K., Roach, A.T. ard Schumcher, J.D., 1984. On the wind-driven variability of the flow through the Berinq Strait. J. Geophys. Res., (in press). COachlMn, L.K., Aagaard, K. and Tripp, R.B., 1975. Berinq Strait. The re+@ nal physical oceanography. Univ. of Mashington Press, 172 pp. D e m , K.L. and Powell, Th. M., 1984. Effects of physical processes on planktonic ecosystem in the coastal ocean. Oceanogr. Mar. Biol. Ann. Rev., 22 : 125-168.
Eady, E.T., 1949. Long waves and cyclone waves. Tellus, 1: 33-52. Fleminu, R.H. ard Heggarty, D., 1966. Oceanoqraphy of the southeastern Chukchi Sea. In: Environment of the Cape Thoqxon %?@on, Alaska US Atomic Energy Cormn., Div. of Tech. Information: 697-754. Happel, J.J., Nihoul, J.C.J. and Deleersnijder,E., 1986. Som properties of the H e m equation with application to the study of baroclinic instability. To be published. Heun, K., 1889. Math. Annalen, 33: 161-179. Monim, A.S., Kamenkovich, V.M. ard K o r t , V.G., 1977. Variability of the oceans. Wiley - Interscience Publ., N.Y., 241 pp. Nihoul, J.C.J., 1985. Perspective in marine ncdellina. JWJ, Ispra, 404 pp. Nihoul, J.C.J., Waleffe, F. and Djenidi, S., 1986. A 3Dnmrical model of the Northern Bering Sea. Enviromtal Software, 1: 1-7. Sambrotto, R.N., 1984. Cruise report of the second Soviet-Amrican m i t i o n in the Bering Sea aboard the R/V Akadkc Korolev, 27 June - 31 July 1984. Unpublished report. Stone, P.H., 1966. On non-geostrophic baroclinic stability. J. Atrms. Sci., 23: 390-400.
Stone, P.H., 1970. On non-geostrophic baroclinic stability : Part 11. J. Atmos. Sci., 27: 721-726. Stone, P.H., 1971. Baroclinic stability under non-hydrostatic conditions. J. Fluid Mech., 45: 659-671. Tang, Ch.M., 1971. The stability of continuous baroclinic mdels with planetary vorticity gradient. Tellus, 23: 285-294. Walsh, J.J. , Blackburn, T.H., Coachnun, L.K., &ring, J.J., McRoy, C.P., Nihoul, J.C.J., Parker, P.L., Sprinqer, A.M., Tripp, R.B., whitledqe, T.E. and Wirick, C.D., 1985. The role of the Bering Strait in carbon/nitroyen fluxes of polar marine ecosystems. Proceedinqs Fairbanks Conf. on Marine Living Systems of the Far North, May 1985. To be published. Walsh, J.J. and Dieterle, J., 1986. Simulation analysis of plankton dynamics in the Northern &ring Sea. In: J.C.J. Plihoul (FAitor), Ilarine Interfaces Fcohydrdynamics. (Elsevier Cceanqraphy Series, 42) Elsevier, Amsterdam (thisv o l i m ) .
This Page Intentionally Left Blank
401
SIMULATION ANALYSIS OF PLANKTON DYNAMICS IN THE NORTHERN BERING SEA
J. WALSH and D. A. DIETERLE Department of Marine Science, University of South Florida, St. Petersburg, Florida 33701 U.S.A. J.
INTRODUCTION "She noticed a curious appearance in the air: it puzzled her very much at first, but after watching it a minute or two, she made it out to be a grin, and said to herself 'It's the Cheshire Cat: now I shall have somebody to talk to!' and she went on. 'Would you tell me please, which way I ought to go from here?' 'That depends a good deal on where you want to get to,' said the Cat. I-so long as I get somewhere,' Alice added as an explanation. 'Oh, you're sure to do that' said the Cat, 'if you only walk long enough' and this time it vanished quite slowly, beginning with the end of the tail, and ending with a grin, which remained some time after the rest of it had gone." (Carroll, 1865)
.... .... ....
Reconstruction of a Cheshire Cat from just its grin, like the description of a marine ecosystem deduced from a few shipboard measurements, is an appropriate objective of simulation analysis.
Simulation models, like isolated
current meter data, nutrient measurements, or plankton tows only provide an accurate reconstruction, however, if the object of the study is already known.
If one "walks long enough" in an iterative process of model confron-
tation with hopefully unaliased measurements (Walsh, 1972), a reasonable, if not unique, description of an unknown Cheshire Cat, i.e., natural system, emerges from this process. As part of a multidisciplinary study of the fate of dissolved carbon and nitrogen in the northern Bering Sea within the ISHTAR (Inner SHelf Transfer And Recycling) program, the present simulation analysis represents a first attempt to describe the plankton dynamics of this shelf ecosystem, similar to initial attempts to constrain the bounds of biological interactions within an upwelling ecosystem (Walsh and Dugdale, 1971). After four cruises of the RIV Alpha Helix, Discoverer, and Akademik Korolov to the Bering/Chukchi Seas during July-August 1982-84, the vague outlines of the grin of this high-latitude Cheshire Cat became visible. During the 1983-84 survey cruises of the Alpha Helix and Akademik Korolev, for example, biological rate measurements had been made of 14C and 15N 'uptake
402
by phytoplankton, of the abundance and species distribution of macrozooplankton, of the grazing rates and abundance of micro-zooplankton, of the abundance and production of bacterioplankton by 15N dilution techniques, of the oxygen consumption, 15N and 35S turnover by micro-benthos and meiobenthos, of macro-benthos abundance and their release of recycled nitrogen compounds, all in relation to the distributions of temperature, salinity, nutrients, chlorophyll, particulate carbon, and particulate nitrogen. These initial data have been discussed by Walsh
&.
(1986a) and are summarized Alaska Coastal Water is a warmer,
in the two carbon budgets of Figure 1.
less saline water type, derived from a mixture of Yukon River water and Bering shelf water between Bering Strait and Shpanberg Strait, to the east of St. Lawrence Island, while Anadyr Stream Water is the colder, more saline shelf water between Bering Strait and Anadyr Strait, to the west of St. Lawrence Island.
I PLANKTON
PRIMARY
--=I 285.0
83.0_
e.g.
ZOOPHAGOUS FISH, e.8. CAPELIN AND
4.2
0 . 6 -
I
HERRING
PSEUDO
PRODUCERS
o.4 0.1
*
AVIAN APEX PREDATOR, e.8. MURRESAND KITTEWAKES
~~
e.g.
PREDATOR.
MEIOFAUNA
DIATOMS
POOL A ^
Figure 1.
I
La."
2.5
1
POLYCHAETES
2.1
PREDATOR, e.g
0.1
KING CRAB
The annual carbon flow (g C m -2 yr-') within food webs of the Anadyr Stream Water (upper value) and of Alaska Coastal Water (lower value).
403 Of the two sources of "new" nitrogen, i.e., nitrate, to the northern Bering Sea from the Yukon River and from the shelf-break (Coachman and Walsh, 1981) south of St. Lawrence Island, the riverine input apparently leads to at An least five-fold less primary production in Alaska Coastal Water (Fig. 1). additional input of terrestrial detrital carbon from freshwater discharge is also inferred in our budget for this part of the northern Bering Sea, in contrast to the more productive communities of the Anadyr Stream Water. Although the distances from the shelf-break of the northwestern Bering Sea to the mouth of the Yukon River and to Anadyr Strait, on the eastern and western sides of St. Lawrence Island, are about the same, nitrate is evidently not stripped, en route from deep to shallow water on the western side of the Bering Sea, within the Anadyr Stream Water.
7:
70
65
60
55
175OE
Figure 2.
175O
165O
155OW
A chlorophyll composite (ug k-') of the surface distribution of phytoplankton biomass within the BeringfChukchi Seas during June-August 1978-84.
404
Local upwelling in and north of Anadyr Strait may intensify the supply of nitrate in this shelf region as well, accounting for part of the higher primary production of Anadyr Stream Water (Fig. 1 ) . Approximately 40% of the fixed carbon of algal photosynthesis was evidently not consumed by zooplankton, bacterioplankton, and benthos within Anadyr Stream Water, compared to complete utilization of marine and terrestrial carbon in Alaska Coastal Water.
Consequently, a large signal of unconsumed algal biomass (Fig. 2) and
&.. 1984) was found on the western side of Bering productivity (Sambrotto Strait, with the input of phyto-detritus to the downstream sediments of the Chukchi Sea (Walsh % &., 1985) presumably derived from this food web, not from that of Alaska Stream Water. Based on these few, preliminary field measurements of the biological rates and previous estimates of the flow field (Coachman % g . , 1975). we were able to assign, however, only mean spatially-averaged fluxes between the state variables of the annual carbon budgets in Figure 1. could have been written for each variable, i.e.,
State equations
zooplankton, detritus pool,
and microplankton, and sol.ved numerically for their steady-state values at one point of a spatially homogeneous Bering Sea as a relatively trivial exercise.
With the meager biological data set available to us from these and
other surveys (50-100 rate measurements and <200 surface chlorophyll determinations over 6 years--see Fig. 2 ) , we decided instead to construct a depthaveraged model of the flow, nitrate, and chlorophyll fields for exploration of the spatial consequences of both the carbon budgets (Fig. 1) and possible interannual changes of the physical habitat, i.e., a variation of 0.3-1.2 Sv northward transport through Bering Strait (Aagaard
G.,
1985).
METHODS Accordingly, we used a depth-integrated form of the Navier-Stokes and continuity equations, ignoring tidal forces, of
au = -gH as + F" - BX + fV at ax
- aats = aaxv , a ayv jz=O
where U = Z=-H ud and V =
(3)
-%!
are the horizontal transports in the x and y
direction from the sea surface to the bottom, -H; where u and'v are the
405
horizontal velocities; where g is the acceleration of gravity and
s
is the
sea surface elevation in the term for the pressure force of the slope of the sea surface; where F and B are the wind and bottom stress components of the terms for the frictional forces in the x and y directions; and f is the -4 Coriolis parameterization, assumed to be a constant of 1 . 3 2 x 10 at 65" N. These are the linearized, barotropic equations of motion, since the terms for the acceleration of momentum and for the density-driven buoyancy force of the horizontal pressure gradient have been deleted.
x x
au av The interior solutions of u and v were obtained by assuming = = 0. i.e., dividing U and V by the local depth, -H, which was entered in the model as a digitized form of the bottom topography (Fig. 3 ) .
At the upstream open
62 -
Figure 3 .
Bottom topography (m) of a numerical model of the Bering/Chukchi Seas.
406
boundaries of the model (Fig. 4 ) across the Anadyr and Shpanberg Straits,
V was proscribed by assuming that of the total transport of the 3 cases ( 0 . 3 , 0.6, and 1.2 Sv) through Bering Strait, 60% passes through Anadyr Strait and 40% through Shpanberg Strait, while s was then determined here from eq. ( 3 ) . At t h e downstream open boundary, U and s were functions of both adjacent,
interior solutions and the phase velocity,
6, i.e.,
a radiative boundary
condition (Orlanski, 1976) in which only outward energy flux occurs, without significant distortion of the solutions of eqs. (1)-(3).
At the land bound-
aries, U = V = 0 such that there were 1825 active grid points (i,j) of 10 km spacing in these simulations (Fig. 4).
64-
62-
I
1
Figure
I
'
,
1
Eulerian gri-, with 10 km spacing, of
t
2
I
r
numerical mo !l.
At the bottom boundary, a mean, linearized bottom stress was used over two 2 adjacent grid points, i.e., instead of the usual BX = Cpu and By = Cpv2, the formulation was
407
where
P is the density of sea water, C is the drag coefficient ( s 2 x -1 At the surface boundary, and a is assumed to be a constant of 10 cm sec
.
no wind stress was applied, i.e.,
FX
=
Py
=
0, in the three cases of varying
transport through the Bering Strait. One could think of the three circdation patterns of 0 . 3 , 0.6, and 1.2 Sv. however, as the flow response under varying conditions of northerly or southerly wind forcing. These equations were solved using a finite difference technique (Platzman, 1972) in which the state variables were discretized over space on a Richardson lattice in a staggered manner.
The time integration of equatlons
(1-3) was thus carried out by the following forward difference scheme:
sn+l = sn i,j i,j
-
At
z
n+l (Ui+l,j
-
un+l i,j
-
At
n+l ('i,j+l
- 'n+I) i,j
At time n+l in the numerical solution, eq. ( 6 ) is computed for the entire spatial grid (Fig. 4 ) , then eq. (7) and then eq. (8); that is, only one time level is stored for any variable at each of the 1825 grid points. The time step, At, for this difference scheme of a two-dimensional grid must satisfy the requirement that At
408
-2
, and Hmax = 50 m, yields At must be less than 5 minutes; we used a 3-minute time step in the simulations. Upon solution of eqs. (1)-(3), the barotropic velocities, u and v. were
g = 10 m sec
then entered in the non-linear state equations for nitrate, N, in units of ug-at NO3 Q-l, and phytoplankton, P, in units of ug chlorophyll Q-l,
where K
and K are numerical diffusion coefficients (Hirt, 1968) derived Y from the finite-difference approximation of eqs. (9)-(10), and where a is the
growth rate (hr-') a
=
d(1.43
of the phytoplankton expressed by
sin 0.2618 t)(N/q
+ N)
(11)
in which d = 0.025 hr-I to allow a maximum daily growth rate of 0.43 day-' over 12 hours of daylight, while the sine function is set to zero at night,
0 then as well, and (N/q + N) is a Michaelis-Menton expression for nitrate limitation of the algal growth, with the half-saturation constant, q, -1 (Walsh, 1975). The other parameters of taken to be 1.5 ug-at NO3 R eq. (10) are b = 2, the assumed conversion ratio of chlorophyll/particulate nitrogen within the phytoplankton and w is their sinking velocity, 10 m -1 day Since there is no term in eqs. (9)-(10) for light limitation of phytoplankton growth with depth, P and N are not really averages over the i.e., a
=
.
water column, but can be considered representative of nitrate and chlorophyll concentrations within the first attenuation depth of the water column, i.e., 1-10 m, where photosynthesis is maximal. The algal sinking rate, w, of eq. (10) is divided by the local depth, H, -1 -1 hr ) are the same as those such that the units of thj.s loss term (ug chl ?.! of the growth, diffusion, and advective terms.
The implicit vertical balance
of chlorophyll fluxes is thus taken to be the influx at the surface, which is zero, and the outflux at the bottom of the water column to the sediments,
where P near the bottom is the same as P near the surface, i.e., a homoge-1 neous distribution, aP/az = 0. The selection of w = 10 m day , instead of -1 sinking rates of %l m day measured in the laboratory, will be discussed with respect to the simulation results. The upstream boundary conditions of nitrate and chlorophyll across Anadyr and Shpanberg Straits were taken from previous cruises in 1969-84,,e.g.. Fig. 2, during which similar longitudinal gradients of both variables were
409
found, decreasing from west to east. We assumed values of nitrate to be -1 10 vg-at NOg 1-l in the middle of Anadyr Strait and 1 vg-at NO3 1 near the
middle of Shpanberg Strait. At these boundaries, values of chlorophyll were 6 vg chl 2-l in the middle of Anadyr Strait and 1 ug chl .t-l in Shpanberg
Strait. At the air and land boundaries, P and N were both taken to be zero, i.e., no influx of nutrients from either rainfall or the Yukon River.
The
initial conditions of nitrate and chlorophyll within the interior of the grid (Fig. 4) were N = P = 0 in simulation cases where growth and sinking were not considered (Figs. 8 , 9 , 1 7 ) . The initial conditions were N = N + P/b from the previous solutions of the advective/dispersive cases, while P was set to 0 . 5 ug chl L - l ,
when other cases of growth and sinking were subsequently studied. Finally, at the downstream boundary conditions of N and P between
Capes Dezhneva and Hope, an alternative method to the radiative boundary condition was used to solve eqs. ( 9 ) - ( 1 0 ) , that of upstream finite differences (Molenkamp, 1968; Roache, 1 9 7 2 ) , which introduces the numerical diffusion estimated by Kx and K Y' RESULTS
Currents To specify the relative importance of the upstream boundary conditions,
i.e., far field currents and previous biological interactions, on the solution of eqs. ( 1 ) - ( 3 ) and ( 9 ) - ( 1 0 ) ,
a series of cases were run with different
terms of these equations set to zero. With no forces acting on the interior flow field of the 0 . 6 Sv case, for example, there is no setup of the surface elevation, such that eq. (3) becomes
o = - au + - av
ax
ay
and eqs. ( 1 ) - ( 2 ) are not relevant, resulting in currents which neither follow the bottom topography. nor accelerate downstream, except within the constricted area of Bering Strait, with a maximum flow of 4 5 . 6 cm sec-' (Fig. 5 ) . Inclusion of the Coriolis, pressure, and frictional forces of eqs. (1)-(2), acting on the boundary currents which enter the model's domain in the 0.6 Sv case, generated horizontal sea level gradients in both the x
-
a"at = and y directions. At steady state, i.e., as at = at = 0, the largest gradient, "JZO cm/300 km, was between Anadyr Stream Water, off the Siberian peninsula, and Alaska Stream Water, off the mouth of the Yukon River -5 3 ' -2 (Fig. 6 ) . Such a sea-surface slope of 0 . 0 7 x 10 , and g = 10 cm sec -2 -4 would lead to a fluid acceleration of %7 x 10 cm sec
, compared to
,
%l x
410
64
62LHYER MEAN V E L O C I T Y I C M / S E C l CUNTINIJITY 10.6 S V I
I
Figure 5 .
,
'
Continuity solution of a depth-averaged flow field (cm sec-l) with 0 . 6 Sv transport through Bering Strait.
-2
cm sec
-1
accelerations of the flow from either a 7 m sec
wind, exert-
5
ing stress at the sea surface for one day (10 sec), or from the usual tidal forces. Over one day, with no frictional losses at the bottom, a current of 70 cm sec-' would develop from such an acceleration of the fluid in this
0 . 6 Sv case of the model.
Since eqs. ( 1 ) - ( 3 ) are linear, the current velocities for the 0 . 3 Sv and 1.2 SV cases are half and double, respectively, of the 0.6 -1
speeds in Bering Strait were 3 6 . 5 cm sec
Sv case. Maximum -1
for the 0.3 Sv case, 72.9 cm sec
for the 0 . 6 Sv case (Fig. 7 ) . compared to the continuity solution of 4 5 . 6 cm -1
sec
for the same case (Fig. 5 ) , and 142.7 cm sec-l for the 1.2 Sv case (Fig. 1 6 ) . A combination of the Coriolis correction for a rotating earth coordinate system, and the acceleration of the boundary currents induced by the horizontal pressure gradient, led to flow along the isobaths (Fig. 7),
in
B
2-
-
N
(0-
1
-
N c-
68-
ID *
54-
-0-10
I
ID N
SER ELEVRTION I C M ) STEROY FLOW 10.6 SVI
I
a
Figure
-2S+
Figure 7.
,
/
1
,
,
,
1
1
The depth-averaged f l o w f i e l d (cm s e c - l ) a c i r c u l a t i o n sub-model w i t h h o r i z o n t a l p r e s s u r e , bottom f r i c t i o n , and C o r i o l i s f o r c e s a f t e r 240 h o u r s i n t h e 0.6 Sv c a s e
411
The s p a t i a l d i s t r i b u t i o n o f s e a l e v e l e l e v a t i o n (cm) i n a flow f i e l d w i t h 0.6 Sv t r a n s p o r t through Bering S t r a i t .
-10-25
4YER MERN VELOCIIfICM/SECl STER@Y 10.6 SVI
412 sharp contrast to the simple continuity case (Fig. 5).
In this model, the
steady flow pattern can be simplistically described as the implied vorticity balance of eqs. ( 1 ) - ( 3 ) , in which the torque imparted to the water column by the bottom stress is compensated by the cross-isobath movement of water to conserve angular momentum. Algal growth Within the second flow field of the 0.6 Sv case (Fig. 7),
deletion of the
growth and sinking terms of eqs. ( 9 ) - ( 1 0 ) represents an analysis of the dispersion of nitrate (Fig. 8 ) and chlorophyll (Fig. 9 ) from their upstream boundaries.
In this situation, the distribution of chlorophyll north of
St. Lawrence Island is determined only by previous population growth south of Anadyr and Shpanberg Straits and the currents flowing north to Bering Strait (Fig. 7).
This simple simulated chlorophyll field (Fig. 9 ) exhibited an
east-west gradient between Siberia and Alaska of thc same magnitude displayed by a composite of the surface chlorophyll measurements taken over 6 years (Fig. 2).
The northward termination of the 4 pg !L-l isopleth of chlorophyll
in the model (Fig. 9) was also similar to the historical data base (Fig. 2) for the Chukchi Sea (Hillman, 19841..
With the algal population growth rates
of a0.6-1.0 day-' observed during the 1983-84 ISHTAR cruises between St. Lawrence Island and Bering Strait, however, an assumption of no in situ growth is, of course, unrealistic. Addition of a modest 0 . 4 3 day-' growth term in eqs. (9)-(10)
led to a50Z
2
depletion of the nitrate stocks north of St. Lawrence Island (Fig. 10).
situ growth of algal populations north of St. Lawrence Island, together with phytoplankton biomass passing through Anadyr and Shpanberg Straits, resulted in more than 12 ug chl !L-l within the model, west of both Northeast Cape on St. Lawrence Island and Cape Wales on the Alaskan mainland (Fig. 1 1 ) .
Such
an extensive accumulation of phytoplankton standing stocks was not observed during short cruises in June-August, between 1978 to 1984 (Fig. 2 ) , suggesting that either grazing losses (Fig. 1) or sinking losses must have a significant impact on the primary production of the Bering-Chukchi seas. Sinking loss The large biomass (Stoker, 1981) and ingestion demands of the benthos, as well as the possible export of carbon within Anadyr Stream Water (Fig. 11, led us to parameterize all phytoplankton losses within the sinking term, w/H P, of this simulation model (Walsh, 1983). Recent field evidence suggests rapid sinking rates of 20-40 m day-' for spring blooms within the
62
62-
CHLOROPHYLL STEAOY FLOW 10.6 SV1
NITROGEN STERDY FLOW 10.6 S V 1 1 : 0 DRYS 1
_ I
Figure 8.
I
,
r I
I
I
I
,
I
I
1
"
The steady-state spatial distribution of nitrate (mg-at m - 3 ) , with neither growth nor sinking, in the 0.6 Sv case.
Figure 9
The steady-state spatial distribution of chlorophyll (mg m-3). with neither growth nor sinking, in the 0.6 Sv case.
-E9
- b9
E4-
, I
7
-i9
NITROGEN
STEROY FLOW 10.6
ORCPHYLL Y FLOW 10.6 SVI
svi
7 DRYS
=
T
I
F i g u r e 10.
The spatial distribution of nitrate (mg-at m-3) after 7 days of growth, with no sinking, in the 0.6 Sv case.
Figure 11.
7 DAYS
,
,
,
,
,
I
'
The s p a t i a l distribution of chlorophyll (mg m-3) after 7 days of growth, with no sinking, in the 0.6 Sv case.
415 mid-Atlantic Bight (Walsh, % Sea (Bodungen
g., 1981).
c.,
g . , 198hb; Walsh
% 1986c) and the Baltic A choice of a modest sinking rate of 10 m day-1
f o r w in eq. ( l o ) , however, allowed sufficient phytoplankton losses that the
nitrate stocks formed a spatial pattern after 7 days (Fig. 12), which was similar to the case with no algal growth (Fig. 8).
After 14 days of simu-
lated time. however, the nitrate stocks were again depleted by ~ 5 0 %(Fig. 14) of those at 7 days (Fig. 1 2 ) , but with a significant difference between this 14-day result and that spatial nitrate pattern derived from the growth, but no sinking case (Fig. 10)--halfway
between St. Lawrence Island and Bering
Strait a minimum in the nitrate field (Fig. 14) developed after 14 days of water movement (Fig. 7) from Anadyr Strait to Bering Strait. -1
At an average flow of 15-20 cm sec
within the 0.6 Sv case, a phyto-
plankton cell would take ".lo-14 days to be moved from Anadyr Strait to about 200 km downstream towards Bering Strait, where the nitrate minimum was found
after 14 days of simulated time (Fig. 14).
Within the first 4 days of such a
phytoplankton trajectory, the chlorophyll biomass derived from primary production south of Anadyr and Shpanberg Straits (Fig. 9) would have sunk out of -1
a 40 m water column at a sinking rate of 10 m day
.
Removal of the previ-
ously grown 6-8 ug chl Q-l of algal biomass would deepen the euphotic zone, making available more light in the water column for initiation of primary production north of St. Lawrence Island. After sinking losses were imposed for 7 days of simulated time, for example, 2'
ug chl Q-l were found over most of the model's domain (Fig. 13).
in sharp contrast to the growth only case of Figure 11.
The algal population
from the Anadyr Strait boundary condition has sunk out of the model, but t' local populations at the interior grid points have not had enough time to significantly increase their biomass within just one week. population growth rate of only 0.43 day-', it would take at least 6-7.5
With a maximal
o r a doubling time of s1.5
days,
days, with "0 losses, for an exponential
increase of the local populations t o occur from the 0.5 ug chl 1-l initial condition to biomass levels of 8-16 ug chl I-'.
These high levels of chloro-
phyll biomass were found after 14 days of simulated time (Fig. 15), however, in the region of the nitrate minimum (Fig. 14).
These simulation results
suggest that the accumulated algal biomass, grown south of St. Lawrence Island, sinks out between this island and Bering Strait, whereas that increment of algal biomass grown north of Anadyr and Shpanberg Straits is likely to sink out within the Chukchi Sea.
416
I
W U
I
I
N I
NITROGEN T
STEROY FLOW 10.6 SV1 = 7 DAYS IWS-10 H/Ol L
L
z g
u s
u o u
.rl
o o m
C M w w .
4
N
9
a
The spatial distribution of nitrate (mg-at m-3) after 7 days of growth and sinking in the 0.6 Sv case.
The spatial distribution of chlorophyll (mg m-3) after 7 days of growth and sinking in the 0.6 Sv case.
,
62-
62-
T
CHLOROPHYLL STEADY FLOW 18.6 S V I 14 DRYS lWS=l0 H/01
F i g u r e 15.
The s p a t i a l d i s t r i b u t i o n of chlorophy (mg m-3) a f t e r 14 days of growth and s i n k i n g i n t h e 0.6 Sv case.
417
The s p a t i a l d i s t r i b u t i o n of n i t r a t e (mg-at m-3) a f t e r 14 days of growth s i n k i n g i n t h e 0.6 Sv case.
a m
F i g u r e 14.
=
418 Habitat variability A change in northward transport of water through the Bering Strait might
alter the sites of the two depocenters of algal carbon suggested by our model. To explore the role of physical variability, interannual or seasonal, in determining the intensity of the sources for these depocenters, the southern one, fed from the boundary algal input, and the northern one, fed from in situ algal production downstream of St. Lawrence Island, we considIn addition to the 0.6 Sv transport cases discussed
ered three flow regimes.
above, we simulated biochemical interactions within both a weaker flow field
of 0 . 3 Sv and a stronger one of 1.2 Sv. Only the results of the chlorophyll fields within the 1.2 Sv flow regime Under a situation of neither growth nor
(Fig. 16) are presented here.
64-
62LAYER MEAN VELOCITYICH/SECl STERDY (1.2 S V I
1
,
I
,
I
,
1
s
t
,
I
'
I
'
r
Figure 16. The depth-averaged flow field (cm sec-') of a circulation submodel with horizontal pressure, bottom friction, and Coriolis forces after 240 hours in the 1.2 Sv case.
419 sinking, the spatial pattern of chlorophyll in the 1.2 Sv case (Fig. 17) was the same as the 0.6 Sv case (Fig. 9).
Although the advective/diffusive gains
of chlorophyll were larger at each grid point in the 1.6 Sv case, the losses were also larger, such that the net result was no change in the local abundance of algal biomass.
Of course, the nutrient patterns remained the
same as well. In the situation of growth and dispersion, but no sinking, within the faster flow field, the chlorophyll plume of Anadyr Stream Water extended 1.50 km farther north (Fig. 18) than before (Fig. 11).
However, there was no
change in the lateral displacement of algal biomass to the east (Figs. 11,
18).
The slower flow field of the 0 . 3 Sv case led to 1.50 km less penetration
north of the chlorophyll plume of the Anadvr Stream Water than that within the 0.6 Sv regime (Fig. 11).
Thus, we found about 100 km total displacement
along the Siberian coast of algal populations, derived from the boundary condition, during possible iriterdiinual or seasonal variations of 0.3-1.2 Sv flow north through the Bering Strait. The results of our sinking loss experiment within the 1.2 Sv flow field suggest, however, that the algal populations grown north of St. Lawrence Island did not shift their locations in response to a change in the transport.
After 7 (Fig. 19) and 14 (Fig. L O ) days within the 1.2 Sv growth and
sinking case, the chlorophyll isopleths of the faster flow regime were still 1.50 km farther north off the Siberian coast than those within the 0.6 Sv flow field (Figs. 13 and 15).
Within the shelf region directly nortll of
St. Lawrence Islmd, however, there was little change in the strength of the currents between the 0.6 Sv (Fig. 7) and 1.2 Sv (Fig. 16) cases. Consequently, the 12 pg X - l
isopleth of chlorophyll remained in the same
location after 14 days of both flow regimes (Figs. 15, 20).
J t continued to
be positioned between the 4 0 m isobaths on the Siberian and Alaskan sides of the Ghirikov Basin, north of St. Lawrence Island, i.e., where the smallest daily sinking loss (0.25 day-')
occurred in both flow regimes.
The
implication of these simulation runs is that one might expect the algal sources for the southern depocenter LO migrate north or south, while the sources for the northern depocenter might remain fixed, during interannual or seasonal changes in the flow field linking the Bering and Cliukchi Seas. Verification An example of a seasonal departure from the apparent "summer" steady-state system, implied by the meager June-August data sets collected during 1-2 week cruises in 1978-84 (Fig. 2), was obtained on the first leg (June 28-July 10)
420 I
L
n
-
I r l M
l
m a e
$24 m c r
arl
$4
l n M l n
u 0 .d
c c 0 r o c
62-
CHLOROPHYLL STEROY FLOW (1.2 SV1 1 = 7 ORYS lWS:10 M/D1
62-
CHLOROPHYLL STEROY FLOW 11.2 S V I
T
= 14 ORYS IWS-10 M / O l
r
Figure 19.
The spatial distribution of chlorophyll (mg U I - ~ )after 7 days of growth and sinking in the 1.2 Sv case.
Figure 20.
The spatial distribution of chlorophyll (mg m-3) after 14 days of growth and sinking in the 1.2 Sv case.
422
of the 1985 ISHTAR field experiment. ice was still
so
A s the result of a "late" spring, pack-
abundant between St. Lawrence Island and Norton Sound that
Leg 1 of the 1985 field experiment had to be postponed for two weeks, thus representing a "spring snapshot'' in July 1985 of this system rather than the usual "summer snapshots" of previous months of July. Consequently, surface (Fig. 21) and bottom (Fig. 22) maps of nitrate, measured at 54 stations (Fig. 21) on 4-9 July 1985. depict >20 pg-at NO3 .9
174 l
Figure 21.
172 ~
170 l
168 ~
"
166 ~
-1
throughout the column of
164 '
162 l
160 '
The spatial distribution of nitrate (mg-at m-3) within surface waters of the Bering Sea during 4-9 July 1985. with respect to the locations of the moored fluorometers (a) and of hydrographic stations ( 0 ) .
423
Anadyr Stream Water in a section taken along the International Dateline from Anadyr Strait to Bering Strait.
Less than 15 ug-at NOj
surface in the same region on 12-17 July 1984 (Walsh
?,!
-1
were found at the
e.,1986a),
similar
to previous observations during June 1969 (Husby and Hufford, 1969) and simulated in the above model, e.g..
Fig. 8.
As in previous years, however, cO.5 ug-at NO3 Q
-1
were found throughout
the water column of Alaska Coastal Water, with a monotonic west-east decline of nitrate in bottom water (Fig. 22).
The surface nitrate pattern contained
sharper horizontal gradients (Fig. 21) than the bottom nitrate map, with c0.5
Vg-at NO3 Q-l
found west and north of St. Lawrence Island as well.
68
66
64
62
Figure 2 2 .
The spatial distribution of nitrate (mg-at m-3) within bottom waters of the Bering Sea during 4-9 July 1985.
The
424
surface chlorophyll map (Fig. 23) was the mirror image of the surface nitrate pattern, with a downstream increase of algal biomass as the nitrate stocks declined, reminiscent of similar plankton dynamics of coastal upwelling areas off Peru, Baja California, and Northwest Africa (Walsh, 1983). At this time of year in 1985, there was no surface signature of an input of phytoplankton from the boundaries of Anadyr and Shpanberg Straits (Fig. 23).
The bottom chlorophyll map (Fig. 2 4 ) provided a striking contrast
to the surface one, however, with a tenfold higher algal biomass found south of St. Lawrence Island and within the two straits. With low chlorophyll
Figure 23.
The spatial distribution of chlorophyll (mg m-3) within surface waters of the Bering Sea during 4-9 July 1985.
425
content of the euphotic zone in these regions, the near-bottom algal biomass must reflect eitl :r pr-vious production events or an algal source farther to the south and west of St. Lawrence Island. During July 1984, for example, >40 Pg chl 11-l were found in the middle of the water column within Anadyr Strait, yet 1-2 orders of magnitude less algal biomass were found here in July 1985. Farther north, between the 40 m isobaths and the Diomede Islands, 5-10 ug chl L-l were found throughout the water column in July 1985 (Figs. 23-24), indicative of intense in situ production and similar to observations of
Figure 24.
-3 The spatial distribution of chlorophyll (mg m ) within bottom waters of the Bering Sea during 4-9 July 1985.
426
previous years (Fig. 2).
During the second leg of the 1985 ISHTAR field
experiment in the Chukchi Sea, A 0 pg chl !?.-l
were found in the middle of the
water column, just north of Bering Strait, implying that the source of carbon for the second depocenter had not shifted location between this and previous years. We do not yet have available the current meter data for June 1985, but our simple simulation model would suggest an "atypical" flow pattern had occurred within and south of Anadyr Strait in response to the late spring of 1985.
This latest ISHTAR field experiment will, of course, provide addi-
tional verification data for the models and an opportunity to upgrade them. CONCLUSIONS Any theory can be improved, of course, and this present model is no exception. We intend to increase 1) the vertical resolution (Nihoul, 1984), 2) the bio-chemical complexity (Walsh
g &.,
1981), and 3) the accuracy of
particle trajectories (Walsh and McRoy, 1985) of our models as the ISHTAR field experiments generate nore data, taken at higher temporal and spatial frequencies.
Figures 21-24 are examples of the data base that will even-
tually be available for additional simulation analyses. days during 4-9 July 1985, instead of 6 years (Fig. 2),
Taken over only 5 these shipboard
surface and bottom maps of nitrate and chlorophyll provide insight into the seasonal, interannual, and vertical complexities of this Arctic ecosystem. Furthermore, during the first leg of the 1985 ISHTAR field experiment, nine current meter moorings were recovered from a collaborative SAI deployment the previous October 1984, while ten more current meter noorings were installed for retrieval in October 1985.
Thus, year-long current meter
records will be available from Anadyr, Shpanberg, and Bering Straits to update our circulation sub-models at the end of this ISHTAR field experiment. At four of the ISHTAR current meter moorings (@ of Fig. 21), seven internally recording fluorometers (Whitledge and Wirick, 1986) were deployed as well, with s i x located along the date-line and one in Shpanberg Strait as the result of the above simulation analysis. Upon recovery of these fluorometers, we will also have time series of chlorophyll fluctuations at 15 minute intervals within the euphotic and aphotic zones of the northern Bering Sea for %3 months to justify the proposed increased resolution of the models, i.e., a sufficient data base for verification of the simulation output before the next field experiment (Walsh, 1972). appeared
As Alice noticed, however, "in another minute the whole head
.... The Cat seemed to think that there was enough of it now in
sight, and no more of it appeared"; reconstruction of a natural system is thus a stepwise process.
Our next simulation analysis will consist of a
421
three-dimensional model of more biological state variables, when the reduced data set justifies the additional computation of a multi-layered resolution of the vertical structure of the water column.
Considering the progress over
the last decade of remote sensing, of in situ instrumentation, and of the speed and capacity of digital computers, we are confident that, in the next decade, reconstruction of a number of local marine ecosystems will occur, providing the basis for eventual networking of simulation models on a global scale. ACKNOWLEDGEMENTS This research was mainly funded by the Division of Polar Programs, National Science Foundation, as part of the ISIITAR program.
Additional
support for our computer facility was provided by funds from the Department of Energy and the Natjonal Aeronautics and Space Administration. REFERENCES Aagaard, K., Roach, A. T. and Shumacher, J. D., 1965. On the wind-driven variability of the flow through Bering Strait. J. Geophys. Res. (In press). Bodungen, B., Brockel, K., Smetacek. V. and Zeitzschel, R.. 1981. Growth and sedimentation of the phytoplankton sprfng bloom in the Bornholm Sea (Baltic Sea). Kiel. Meer. Son., 5: 460-490. Carroll, L., 1865. Alice's Adventures in Wonderland. MacMillan, 164 pp. Coachman, L. K., Aagaard, R. and Tripp, R. B., 1975. Bering Strait: The Regional Oceanography. University of Washington Press, Seattle, 172 pp. Coachman, L. K. and Walsh, J. J., 1981. A diffusion model of cross-shelf exchange of nutrients in the Bering Sea. Deep-sea Res., 28: 819-837. Hillman, S. R., 1964. Near-shore chlorophyll concentrations in the Chukchi Sea. Polar Record, 22: 182-186. Hirt, C. W., 1968. Heuristic stability theory for finite-difference equations. J. Computat. Phys., 2: 339-355. Husby, D. M. and Hufford, G. L., 1969. Oceanographic investigation of the northern Bering Sea and Bering Strait, 8-21 June 1969. U. S. Coast Guard Oceanogr. Rep. No. 42, CG 373-42, Washington, D. C. Molenkamp, C. R., 1968. Accuracy of finite-difference methods applied to the advection equation. J . Appl. Meteor., 7: 160-167. Nihoul, J. J., 1984. A three-dimensional general marine circulation model in a remote sensing perspective. Ann. Geophys., 2: 433-442. Orlanski, I., 1976. A simple boundary condition for unbounded hyperbolic flows. J. Computat. Phys.. 21: 251-259. Platzman, G. W., 1972. Two dimensional free oscillations in natural basins. J. Phys. Oceanogr., 2: 117-136. Roache, P. J.. 1972. On artificial viscosity. J. Computat. Phys., 10: 169-184.
Sambrotto, R. N., Goering, J. J. and McRoy, C. P., 1964. Large yearly production of phytoplankton in the western Bering Strait. Science, 225: 1147-1150.
Stoker, S., 1981. Benthic invertebrate macrofauna of the eastern Bering/ Chukchi continental shelf. In: D. W. Hood and J. A. Calder (Editors), The Eastern Bering Sea Shelf: Oceanography and Resources. University of Washington Press, Seattle, pp. 1069-1091.
428 Walsh, J . J., 1972. Implications of a systems approach to oceanography. Science, 176: 969-975. Walsh, J. J., 1975. A spatial simulation model of the Peru upwelling ecosystem. Deep-sea Res., 22: 201-236. Walsh, J. J., 1983. Death in the sea: enigmatic phytoplankton losses. Prog. Oceanogr., 12: 1-86. Walsh, J . J. and Dugdale, R. C., 1971. A simulation model for the nitrogen flow in the Peruvian upwelling system. Inv. Pesq., 35: 309-330. Walsh, J. J., Rowe, G. T., Iverson, R. L. and McRoy, C. P., 1981. Biological export of shelf carbon is a neglected sink of the global C02 cycle. Nature, 291: 196-201. Walsh, J. J. and McRoy, C. P., 1985. Ecosystem analysis in the southeastern Bering Sea. Cont. Shelf Res. (In press). Walsh, J. J., Premuzic, E. T., Gaffney, J. S., Rowe, G. T., Harbottle, G . , Stoenner, R. W., Balsam, W. L., Betzer, P. R. and Macko, S. A., 1985. Organic storage of C02 on the continental slope off the mid-Atlantic Bight, the Southeastern Bering Sea, and the Peru coast. Deep-sea Res., 32: 853-883. Walsh, J. J., McRoy, C. P., Blackburn, T. H., Coachman, L. K., Goering, J. J., Nihoul, J . J., Parker, P. L., Springer, A. M., Tripp, R. B., Whitledge, T. E. and Wirick, C. D., 1986a. The role of Bering Strait in the carbodnitrogen fluxes of polar marine ecosystems. In: L. Rey and V. Alexander (Editors), Marine Living Systems of the Far North. McMillan Co. (In press). Walsh, J. J., Dieterle, D. A. and Esaias, W. E . , 1986b. Satellite detection of export of the 1979 spring bloom from the mid-Atlantic Bight. Deep-sea Res. (Revised). Walsh, J. J.; Wirick, C. D., Pietrefesa, L. J., Whitledge, T. E. and Hoge, F. E., 1986c. High frequency sampling of the 1984 spring bloom within the mid-Atlantic Bight: synoptic ship-board, aircraft, and in situ perspectives of the SEEP-I experiment. Con. Shelf Res. (Submitted). Whitledge, T. E. and Wirick, C. D., 1986. Development of a moored in situ fluorometer for phytoplankton studies. M. J. Bowman, C. M. Yentsch and W. J. Petersen (Editors), Tidal Mixing and Plankton Dynamics. Springer-Verlag (In press).
429
THE TERRESTRIAL-MARINE INTERFACE: MODELLING NITROGEN TRANSFORMATIONS DURING ITS TRANSFER THROUGH THE SCHELDT R I V E R SYSTEM AND ITS ESTUARINE ZONE G. BILLEN, C. LANCELOT, E. DEBECKER and P. SERVAIS
Groupe de M i cr o b io lo g i e d e s M il ie u x Aquatiques U n i v e r s i t y of B r u s s e l s , 50 av, F. R o o s e v e l t, B-1050 B r u x e l l e s (Belgium)
ABSTRACT A budget of n i t r o g e n t r a n s f o r m a t i o n s d u r i n g i t s t r a n s f e r through t h e r i v e r system and t h e e s t u a r y o f t h e S c h e l d t shows t h a t as much a s 70% of t o t a l N d i s ch ar g ed i n t o t h e r i v e r system i s e l i m i n a t e d b e f o r e r e a c h i n g t h e sea. D e n i t r i f i c a t i o n and, i n d i r e c t l y , primary p r o d u ct i o n ap p ear a s s i g n i f i c a n t p r o c e s s e s i n t h i s e l i m i n a t i o n . The f a c t o r s c o n t r o l l i n g t h e i n t e n s i t y of t h e s e p r o c e s s e s , b o t h i n t h e r i v e r system and i n t h e e s t u a r y , are i d e n t i f i e d . Th i s a l l o w s mathematical m o d e l li n g o f t h e s e p r o c e s s e s and p r e d i c t i o n o f t h e impact o f large scale waste water p u r i f i c a t i o n p o l i c i e s on t h e i n p u t o f n i t r o g e n i n t o t h e B el g i an c o a s t a l zone. INTRODUCTION
For t h e n eed s o f a r a t i o n a l management o f t h e q u a l i t y o f coastal waters models r e l a t i n g human a c t i v i t i e s ( a g r i c u l t u r a l , domestic and i n d u s t r i a l )
to
t h e e c o l o g i c a l working of coastal ecosytems must be e s t a b l i s h e d . I n such models, n o t o n l y t h e e s t u a r i n e zones, b u t a l s o t h e whole r i v e r system, a c r o s s which most d i s c h a r g e s from human a c t i v i t i e s f l o w b e f o r e t h e y r each t h e sea, c a n be viewed as a n i n t e r f a c e where i n t e n s e b i o l o g i c a l and physico-chemical p r o c e s s e s o ccu r , r e s u l t i n g substances discharges
in
p a r t i a l e l i m i n a t i o n o r i m m o b i l i zat i o n o f t h e
It is w i t h t h i s concern i n mind t h a t t h e p r e s e n t s t u d y o f n i t r o g e n t r a n s f e r s through t h e r i v e r system and e s t u a r y o f t h e S c h e l d t , was undertaken. I t had t h e f o l l o w i n g purposes: (i)
To
establish
a
budget
of
n i t r o g e n t r a n s f o r m a t i o n p r o c e s s e s d u r i n g its
t r a n s i t from a g r i c u l t u r a l s o i l s of t h e S c h e l d t watershed t o t h e Bel g i an c o a s t a l zone. ( i i ) To u n d er s t a n d t h e mechanisms c o n t r o l l i n g t h e i n t e n s i t y of t h e most i m p o r t a n t p r o c e s s e s in v o lv e d i n t h i s budget. ( i i i ) To b u i l d up a model o f n i t r o g e n (and car b o n )
b eh av i o u r
in
the
river
system and i n t h e e s t u a r i n e zone i n o r d e r t o relate d i s c h a r g e i n t o t h e r i v e r s and o u t p u t s i n t o t h e sea. T h i s model is i n t e n d e d t o p r e d i c t t h e e f f e c t o f
430 d i f f e r e n t waste water p u r i f i c a t i o n p o l i c i e s on t h e c o a s t a l areas. The r e s u l t s of t h e f i r s t s t e p of t h i s s t u d y , i.e. t h e budget o f n i t r o g e n t r a n s f o r m a t i o n s i n t h e S c h e l d t watershed and e s t u a r y , h as been published e ls ewh er e ( B i l l e n e t a l , 1986). I t is summarized by Fi g . 1 which allows identification of transformations.
the
main
biological
cattle dom. *dust,. waste waste
France
processes
i n v o l v ed
in
n i t r o g en
cattle da ind waste waste
Sea
F i g . 1 : Ni t r o g en budget f o r t h e S c h e l d t r i v e r system and e s t u a r y (from B i l l e n e t a l , 1986). Fluxes are e x p r e s s e d i n l o 3 TN/yr. Fi g u r es i n b r a c k e t s have been e v a l u a t e d by d i f f e r e n c e .
D e n i t r i f i c a t i o n i n t h e r i v e r system e l i m i n a t e s more t h a n 60% o f t h e t o t a l n i t r o g e n l o ad . Uptake o f n i t r o g e n by primary p r o d u cer s p l a y s o n l y a minor d i r e c t role i n t h e budget, r e p r e s e n t i n g l e s s t h an 8% o f t o t a l i n p u t s . Primary p r o d u c t i o n is however of i n d i r e c t importance for t h e n i t r o g e n c y c l e , because o f i t s r o l e as a s o u r c e of r a p i d l y b i o d e g r ad ab l e o r g a n i c carbon.This is of p a r t i c u l a r s i g n i f i c a n c e i n t h e upper e s t u a r i n e s e c t i o n o f t h e S c h e l d t , where de cay i n g fresh water phytoplankton c o n t r i b u t e s t o s u s t a i n h i g h h e t e r o t r o p h i c anaerobic conditions a c t i v i t i e s which r e s u l t i n t h e e s t a b l i s h m e n t of f a v o u r a b l e t o d e n i t r i f i c a t i o n i n t h e water column. Th i s p r o cess is r e s p o n s i b l e for t h e e l i m i n a t i o n o f 40% o f t h e n i t r o g e n l oad e n t e r i n g t h e e s t u a r i n e zone. Thus, as a r e s u l t o f d e n i t r i f i c a t i o n i n r i v e r and i n t h e upper e s t u a r y , o n l y 28% o f t h e t o t a l n i t r o g e n d i s c h a r g e d i n t o t h e S c h e l d t r i v e r system does a c t u a l l y r each t h e sea. I t is t h e r e f o r e l i k e l y t h a t any m o d i f i c a t i o n of t h e f a c t o r s a f f e c t i n g d e n i t r i f i c a t i o n i n t h e h y d r o g r ap h i cal network, e.g. as a
431 r e s u l t of a large s c a l e p r o g r a m o f waste water p u r i f i c a t i o n a f f e c t t h e i n p u t o f n i t r o g e n i n t o t h e c o a s t a l areas.
could
deeply
I n t h i s paper, we t r y t o i d e n t i f y t h e f a c t o r s c o n t r o l l i n g t h e processes o f d e n i t r i f i c a t i o n and p r i m a r y p r o d u c t i o n , b o t h i n t h e r i v e r s y s t e m and i n t h e estuary.
For
each
of
these
processes
mathematical
sub-models
e s t a b l i s h e d and t h e i r c o h e r e n c y w i l l b e c h e c k e d a g a i n s t i n s i t u Together,
these
sub-models
water p u r i f i c a t i o n s c e n a r i o on coastal z o n e .
allow the
will
be
observations.
t o p r e d i c t t h e e f f e c t of a n e x t r e m e waste discharge
of
nitrogen
to
the
Belgian
DENITRIFICATION I N RIVERS
A s a n a n a e r o b i c p r o c e s s , d e n i t r i f i c a t i o n c a n o c c u r e i t h e r i n t h e water column, when d e p r i v e d o f oxygen o r w i t h i n t h e s e d i m e n t s , below t h e o x i c l a y e r .
waters are n o t u n u s u a l i n t h e S c h e l d t h y d r o g r a p h i c a l n e t w o r k , water. The e x t e n t o f a n a e r o b i c z o n e s i s however h i g h l y v a r i a b l e , and d i f f i c u l t t o e v a l u a t e a t t h e scale o f t h e e n t i r e r i v e r s y s t e m . B e n t h i c d e n i t r i f i c a t i o n on t h e o t h e r h a n d , i s more
Anoxic
surface
r e c e i v i n g large a m o u n t s o f u n t r e a t e d waste
w i d e s p r e a d and a s e m i - e m p i r i c a l s t u d y of its d e p e n d e n c e on t h e o r g a n i c matter c o n t e n t of t h e s e d i m e n t s w i l l a l l o w e v a l u a t i o n o f i t s r a t e a t t h e scale o f t h e whole h y d r o g r a p h i c a l network. C o n t r o l o f b e n t i c d e n i t r i f i c a t i o n b~ o r g a n i c matter c o n t e n t o f s e d i m e n t s A c c o r d i n g t o t h e d i a g e n e t i c model o f V a n d e r b o r g h t a n d B i l l e n (19751, t h e f l u x of n i t r a t e across 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 ( F N O 3 ) i s g i v e n by:
where
kn
is t h e r a t e of n i t r i f i c a t i o n i n t h e o x i c l a y e r o f t h e sediments;
is t h e depth of t h e oxic l a y e r ;
zn
is t h e first o r d e r c o n s t a n t o f d e n i t r i c a t i o n
kd
CoNo3 Di
i s t h e c o n c e n t r a t i o n o f n i t r a t e i n t h e o v e r l y i n g water i s t h e d i f f u s i o n c o e f f i c i e n t i n t h e p u r e water.
The d e p t h of t h e o x i c l a y e r , z n , is g i v e n by
2 . D i . zn = F002
i s t h e oxygen c o n c e n t r a t i o n i n t h e o v e r l y i n g water
where
Fo 02
is t h e f l u x of oxygen across t h e s e d i m e n t water interface
432
The f l u x o f oxygen, F o 02
FoO2 = where
, can
be e x p r e s s e d as
Ro2
. zn
02
is t h e z e r o o r d e r rate o f oxygen consumption by t h e sediments
R
Combining ( 2 ) and ( 3 ) y i e l d s
F'
= 02
J 2
. D i . C o o 2 . Ro',
(4)
Direct measurements show t h a t t h e rate o f oxygen
consumption
by
sediment
samples c a n be r e l a t e d t o o r g a n i c matter c o n t e n t and t e m p e r a t u r e ( F i g . 2 ) .
Kc
f 10,
c
-d I-
8'
a 10 % organic matter
a. F i g . 2:
b.
10
20
30
temperature. 'C
a . R e l a t i o n s h i p observed between t h e r a t e of oxygen consumption by
sediment sample ( a t 5'C)
and o r g a n i c matter c o n t e n t .
b . Temperature dependence of oxygen consumption r a t RO2 (mmole/m3.h) = 6 0 8 . o r g a n i c matter % .2 T - 5 * C
10°C
Combining
the
empirical
r e l a t i o n s h i p i l l u s t r a t e d i n Fig. 2 w i t h r e l a t i o n function of the organic
(4) allows c a l c u l a t i o n o f t h e f l u x of oxygen as a
matter c o n t e n t o f t h e s e d i m e n t s . C a l c u l a t e d v a l u e s compared w e l l w i t h e x p e r i m e n t a l d e t e r m i n a t i o n s performed i n v a r i o u s B e l g i a n r i v e r s f o l l o w i n g t h e procedure d e s c r i b e d by Dessery e t a1 (1982). F i g u r e 3, provided D i is a d j u s t e d -6
-
-5
i n t h e range 10 10 cm2/sec. The r a t e o f d e n i t r i f i c a t i o n , kd, as e x p e r i m e n t a l y d e t e r m i n a t e d by s h o r t
term
kinetics
of
nitrate
comsumption
a f t e r n i t r a t e a d d i t i o n t o a sediment
sample, was a l s o found t o be r e l a t e d t o t h e o r g a n i c matter c o n t e n t ( F i g . 4). The rate o f n i t r i f i c a t i o n , kn, was determined i n a few r i v e r sediinents a c c o r d i n g t o t h e method d e s c r i b e d by Hansen e t a1 (1981). Values i n t h e
range
433
-
0 300 mmoles/m .h were recorded. Such rates can be shown t o have only a n e g l i g e a b l e e f f e c t on t h e f l u x o f n i t r a t e as c a l c u l a t e d by r e l a t i o n ( 1 ) .
5
0
10
4
15
%organic matter
F i g . 3 : R e l a t i o n s h i p between t h e f l u x of oxygen a c r o s s t h e sed i m en t - w at er i n t e r f a c e and t h e o r a n i c matter c o n t e n t of t h e sed i m en t s, c a l c u l a t e d f o r D i = 10-6cm2sec-' ( ) and 10-5cm2sec-' (---), a t 5 and 20°C. Experimental d a t a are from Edwards and P o l l y (1965) own measurements i n B e l g i a n r i v e r s ( 0 ) .
( 0 )
and from o u r
R e l a t i o n s ( l ) , ( 2 ) and ( 4 ) , a l o n g w i t h t h e e m p i r i c a l r e l a t i o n s h i p s and kd t o o r g a n i c matter c o n t e n t , a l l o w c a l c u l a t i o n o f t h e f l u x relating R of n i t r a t e consumed by r i v e r sediments. These c a l c u l a t i o n s compare w e l l w i t h d i r e c t d e t e r m i n a t i o n c a r r i e d on i n a r a n g e o f B e l g i a n r i v e r s ( f i g . 5 ) . Ev al u at i o n o f b e n t h i c and water column d e n i t r i f i c a t i o n a t t h e scale of river system The semi-empirical model d e s c r i b e d above a l l o w s t o e v a l u a t e b e n t h i c d e n i t r i f i c a t i o n provided t h e t o t a l area o f r i v e r bottom and t h e o r g a n i c c o n t e n t o f t h e se d i m e n ts are known. The former i n f o r m at i o n can be o b t a i n e d by means o f t h e geomorphological a n a l y s i s of r i v e r sytems i n t r o d u ced by Horton (1945). I n t h i s a n a l y s i s , each r i v e r is a f f e c t e d by an o r d e r . Small r i v e r s w i t h o u t t r i b u t a r i e s are z e r o o r d e r , r i v e r s having only
-
t r i b u t a r i e s of order (n 1 ) are c a l l e d o r d e r ( n ) . On b a s i s o f t h e a n a l y s i s of small r e p r e s e n t a t i v e sub-watersheds, i t is th en p o s s i b l e t o deduce g e n e r a l r e l a t i o n s h i p s between o r d e r on t h e one hand, and t h e number o f t r i b u t a r i e s ,
434
% organic matter F i g . 4 : R e l a t i o n s h i p observed between t h e f i r s t o r d e r c o n s t a n t of d e n i t r i f i c a t i o n ( k d ) of r i v e r s e d i m e n t samples and t h e o r g a n i c m a t t e r c o n t e n t . T - 20 kd(h-') = a ( X OM - 0 . 5 ) ' 2 10 w i t h a i n t h e range 0.45 - 0.1 (mean 0 . 3 ) .
.
-
% organic matter F i g . 5 : R e l a t i o n s h i p between t h e f l u x of n i t r a t e consumption by r i v e r sediments and t h e i r o r g a n i c matter c o n t e n t , a s c a l c u l a t e d by t h e model developed i n t h e t e x t , f o r t h e f o l l o w i n g v a l u e s of t h e p a r a m e t e r s : D i = 10-5cm2/sec C o o 2 = 350 !JM
-
Co
Nos
= 400
pM
Temperature = 20°C ( a , c , d ) o r 5°C ( b ) = 0.3 ( a , b ) , 0.45 ( c ) o r 0. 1 ( d ) Dots r e p r e s e n t s d i r e c t measurements i n B e l g i a n r i v e r s
435
t h e i r mean l e n g t h , t h e area o f t h e i r watershed and t h e i r width, on
the
other
hand. Table
1
summarises
the
results
of
t h i s analysis for the Scheldt r i v e r t h e Belgian
system. On b a s i s o f t h e s e d a t a t h e t o t a l area of r i v e r bottom i n
S c h e l d t watershed l i m i t e d a t Rupelmonde can be e s t i m a t e d a t 55 km2.
TABLE 1
Horton a n a l y s i s of t h e S c h e l d t watershed Number of tributaries
Order
Mean length
Mean width
T o t a l area of r i v e r bottom
Area of bottom/ total
area of watershed
1
1000 340 110 32 10
2 3
4 5 b
3
7
1
The
organic
(km)
(m)
( km2)
1.8 4.5 11 26 60 140 330
0.9 1.8 4.1 9-2
1.b 2.8 5.0 7.7 10 17 86
17 41 260
*
( km */km )
73 128 229 353 472 794
-
matter c o n t e n t o f r i v e r s e d i m e n t s depends on t h e " d e n s i t y " o f
o r g a n i c d i s c h a r g e s , i.e. domestic,
industrial
t h e r a t i o between t h e amount o f o r g a n i c matter (from or a g r i c u l t u r a l o r i g i n ) discharged i n t o a r i v e r s e c t i o n
and t h e bottom area o f t h i s s e c t i o n . A v a i l a b l e d a t a r e l a t i v e t o s e v e r a l B e l g i a n d i s t r i c t s are r e p o r t e d i n Fig. 6. I n s p i t e o f a r a t h e r large d i s p e r i s o n i t shows a t r e n d o f i n c r e a s i n g mean o r g a n i c matter c o n t e n t w i t h i n c r e a s i n g d e n s i t y of a i s c h a r g e
.
/ F i g . 6 : Observed r e l a t i o n s h i p be tween t h e mean orp.anic m a t t e r c o n t e n t of r i v e r sediments and d e n s i t y of ornanic disc h a r g e for several Belgian d i s t r i c t s .
0
0
' 300k
50 density of organic dischaw, TC/km?day
436
On b a s i s o f these r e l a t i o n s h i p s , t h e rate of d e n i t r i f i c a t i o n i n t h e se d i m en t s h a s been e v a l u a t e d , as shown i n Tab l e 2, for t h e h y d r o g r ap h i cal network of each d i s t r i c t from t h e B e l g i a n S c h e l d t watershed. A t o t a l v a l u e of 13-22 lo3 TN/yr is o b t a i n e d . T h i s r e p r e s e n t s 25-401 o f t o t a l e l i m i n a t i o n o f n i t r o g e n i n t h e r i v e r system as e v a l u a t e d by d i f f e r e n c e i n t h e n i t r o g e n budget The e s t i m a t i o n non accounted f o r by d e n i t r i f i c a t i o n i n t h e (Fi g . 1). se d i m en t s must probably be e x p l a i n e d by d e n i t r i f i c a t i o n i n t h e water column o f anaerobic r i v e r s . TABLE 2
E v a l u a t i o n o f b e n t h i c d e n i t r i f i c a t i o n i n t h e h y d r o g r ap h i cal network o f t h e d i s t r i c t s from t h e S c h e l d t watershed District
Mean 02
Ma1i n e s
Turhout Bruxelles Leuven Nivelles Halle-Vilvorde Kortijk Ro u l er s Tielt Aalst Audenarde Termonde S t Niklaas(2/3) Gent Ath Mons Soignies Tournai Pious cr o n Hasselt Maaseik Total
Mean NO,
Mean r i v e r bottom
Pm
m r
km
0 0 0 0
400 400 670 400 500 670 500 550 430 670 550 400 400 400 400 400 400 400 460 400 400
4.56 2.82 0.34 2.42 2.27 1.96 1.59 0.57 0.68 0.98 2.67 14.75 8 2.26 1.02 1.22 1.08 2.45 0.21 1.89 1.23
sat 0
sat sat sat 0 0 0 0 0 sat
sat sat sat sat sat
sat
%
Nitrate f l u x (TN/yr)
organic matter
55
12 16.5 19 6.1 5.6 17 12 19 9 17.5 4 5 5 20 8 15 7.5 8.8 18 16 12.5
20° c
5"c
2636 2268 530 681 390 2723 504 279 148 1403 646 3337 1810 2216 188 365 189 487 82 593 321
1567 1348 315 405 218 1619 294 164 85 834 384 1984 1076 1317 107 213 107 279 48 347 187
-
-
22000
13000
PHYTOPLANKTON GROWTH I N THE RIVER SYSTEM AND THE ESTUARY Dependence of p h y to p l a n k t o n g r o w th on e n v ir o n m en t al factors The v a r i a t i o n s of a phytoplankton community Phy i n a r i v e r o r e s t u a r i n e sys t em are t h e r e s u l t a n t o f b o t h b i o l o g i c a l and hydrodynamical p r o c e s s e s i.e. growth and m o r t a l i t y on t h e one hand and d i l u t i o n o r d i s p e r s i o n l i n k e d t o t h e hydrodynamic o f t h e system on t h e o t h e r hand. The e v o l u t i o n e q u a t i o n can be w r i t t e n :
437
dy = XPhy
(5)
+ (p - d ) Phy
dt
xis t h e hydrodynamical
o p e r a t o r which w i l l be d i s c u s s e d below f o r t h e case o f r i v e r and e s t u a r y r e s p e c t i v e l y .
where
is t h e growth r a t e is a f i r s t o r d e r m o r t a l i t y r a t e i n c l u d i n g g r a z i n g by
d
h er b i vo r o u s zooplankton, spontaneous l y s i s and s ed i m e n ta ti o n . The growth rate ,p ,depends on b o t h a v a i l a b l e l i g h t i n t e n s i t y and t em p er at u r e. Major n u t r i e n t c o n c e n t r a t i o n s are always s a t u r a t i n g i n Bel g i an r i v e r s and do n o t c o n t r o l p h y to la n k to n growth. The c l a s s i c a l Vollenweider r e l a t i o n s h i p (Vollenweider, 1965) s i m p l i f i e d for
n o n - p h o t o i n h i b i te d
communities
was used t o e x p r e s s t h e c o n t r o l by l i g h t
i n t e n s i t y on t h e growth rate. T h is i n t u r n was e s t i m a t e d by t h e i n t e g r a t i o n o f t h i s e q u a t i o n on t h e v a r i a t i o n s o f a v a i l a b l e l i g h t i n t e n s i t y w i t h t h e time and depth:
is the incident photosynthetically available l i g h t
where I.
i n t e n s i t y ( r a n g e 400-700 nm)
X h
is t h e photoperiod is t h e depth
rl
is t h e l i g h t a t t e n u a t i o n c o e f f i c i e n t and are p h y s i o l o g i c a l p a r a m e t er s which c h a r a c t e r i z e t h e l i g h t - p h o t o s y n t h e s i s curve.
Pmax
Determinatiom o f pmax
)
the
growth
rate
at
saturating
ligh
intensity,
y i e l d e d v a l u e s o f a b o u t 0.3 h- a t 2OoC i n r i v e r s and ab o u t 0.5 h- i n t h e downstream p a r t o f t h e S c h e l d t e s t u a r y . The dependence on t em p er at u r e can be e xp r es s ed by a Clloof 2 . Temperature i t s e l f v a r i e s s e a s o n a l l y a c c o r d in g t o : T
I
15
-
9 cos
W
(t
-
30)
Ik t h e s a t u r a t i n g l i g h t i n t e n s i t y l e v e l , was found t o be p h y s i o l o g i c a l l y a d j u s t e d t o t h e amount o f l i g h t energy a v a i l a b l e f o r t h e water column ( F i g .
7).
438
0
0
0
0
0
-
0
1
0
Phar/r) , J/cm2.day.m-'
1
500
F i g . 7 : R e l a t i o n s h i p o b s e r v e d b e t w e e n Ik and t h e a v a i l a b l e l i g h t e n e r g y f o r t h e w a t e r column, P h a r / n
n
t h e l i g h t a t t e n u a t i o n c o e f f i c i e n t , d e p e n d s b o t h on t h e c o n t e n t o f d e t r i t a l s u s p e n d e d matter and on t h e s e l f - s h a d i n g o f p h y t o p l a n k t o n i t s e l f :
n
=
ndet
+
rl
PhY
on t h e suspended matter c o n t e n t o f water ( F i g . 8a) de t was e s t a b l i s h e d d u r i n g p e r i o d s w i t h o u t i m p o r t a n t p h y t o p l a n k t o n biomass. From t h i s knowledge, s e l f - s h a d i n g by p h y t o p l a n k t o n c o u l d b e e s t i m a t e d for f l o w e r i n g The dependence o f n
p e r i o d s . Its c o n t r o l by p h y t o p l a n k t o n i c biomass is g i v e n by f i g u r e 8b.
Fig. 8 :
a. b.
R e l a t i o n s h i p between n and s u s p e n d e d m a t t e r R e l a t i o n s h i p b e t w e e n qdet and p h y t o p l a n k t o n b i o m a s s
PhY
439 Incident photosynthetically available l i g h t
intensities
and
photoperiods
were c a l c u l a t e d from i n t e g r a t e d measurements o f t o t a l i n c i d e n t s o l a r r a d i a t i o n c o l l e c t e d e v e r y 30 min a t Uccle by t h e " I n s t i t u t Royal M e t e o r o l o g i q u e de Belgique). The m o r t a l i t y r a t e , d , experimentally determined. Previous
observations
in
is
a
other
composit
parameter
difficult
to
be
freshwater a q u a t i c systems ( B i l l e n et a l ,
1983) g i v e estimates a r o u n d 0.005 h - l . Modelling phytoplankton growth i n t h e r i v e r system S m i t h e t a1 ( 1 9 8 4 ) h a v e shown t h a t i n t h e case o f a r i v e r t h e hydrodynamical o p e r a t o r i n e q u a t i o n ( 5 ) is r e d u c e d t o a d i l u t i o n f a c t o r d e p e n d i n g on
x
p l u v i o s i t y , d r a i n a g e c o n d i t i o n s and w a t e r s h e d shape. I t c a n b e w r i t t e n
x = i ds A
where
dx i s t h e wetted s e c t i o n o f t h e r i v e r and
A
dQ dx
is t h e l o n g i t u d i n a l g r a d i e n t o f discharge,
owing t o t h e i n p u t s o f water by t h e a f f l u e n t s , s u p p o s e d t o b e d e v o i d e d o f biomass.
It
has
been
shown
elsewhere
et
(Billen
al,
1985; Debecker e t a l , i n
p r e p a r a t i o n ) t h a t i n small r i v e r s ( w i t h Horton o r d e r less t h a n 0.61, the d i l u t i o n f a c t o r is a l w a y s h i g h e r t h a n t h e n e t g r o w t h rate o f p h y t o p l a n k t o n . In
these
rivers,
development
therefore
only
macrophytes
are
able
to
develop.
The
of a s i g n i f i c a n t p h y t o p l a n k t o n p o p u l a t i o n is r e s t r i c t e d t o r i v e r s
w i t h o r d e r 6 o r h i g h e r , i.e.,
i n t h e case
of
the
Scheldt
estuary,
in
the
S c h e l d t i t s e l f and t h e Dyle-Rupel. F o r a r i v e r o f g i v e n geometry, e q u a t i o n ( 5 ) a l l o w s c a l c u l a t i o n o f phytop l a n k t o n biomass r e a c h e d a t a f i x e d o b s e r v a t i o n s t a t i o n , a f t e r t h e t r a v e l o f t h e water m a s s e s from t h e s o u r c e , where p h y t o p l a n k t o n i s s u p p o s e d t o b e p r e s e n t a t a low b u t non z e r o c o n c e n t r a t i o n . The d i l u t i o n term is c a l c u l a t e d a c c o r d i n g t o r e l a t i o n ( 6 ) . The w e t t e d s e c t i o n is t h e p r o d u c t o f mean w i d t h (W) and mean d e p t h (HI. The f o r m e r is c a l c u l a t e d from t h e f o l l o w i n g e m p i r i c a l r e l a t i o n s h i p w i t h w a t e r s h e d area,
(SW), found f o r B e l g i a n r i v e r s .
w(m)
=
0.8
/Ti7
The mean d e p t h is c a l c u l a t e d from MaFlning's f o r m u l a : H(m)
=
(9.n ) 5 ' 3 w
. Js
(7)
440 where
is t h e s l o p e o f t h e r i v e r t h e Matlning c o e f f i c i e n t a d j u s t e d t o 0.07.
S and n
t e r m s i n r e l a t i o n ( 6 ) is e v a l u a t e d from t h e s e a s o n a l v a r i a t i o n s of dx d i s c h a r g e a t t h e o u t l e t o f t h e r i v e r , assuming t h a t d i s c h a r g e i n c r e a s e s The
l i n e a r l y w i t h d i s t a n c e from t h e source. Figure 9 presents the results of these calculations for the Scheldt r i v e r a t S t Amands (km 220). Comparison w i t h o b s e r v e d d a t a of p h y t o p l a n k t o n biomass
shows
that
are
phytoplankton
the
major
trends
satisfactorily
of
simulated
the by
seasonal this,
yet
variations
of
oversimplified,
model.
*0°1
F i g . 9 : Observed a n d s i m u l a t e d s e a s o n a l v a r i a t i o n s of p h y t o p l a n k t o n b i o m a s s i n t h e S c h e l d t a t St Amands (km 2 2 0 ) .
Modelling phytoplankton growth i n t h e e s t u a r y T r a n s p o r t a n d d i s p e r s i o n p r o c e s s e s i n t h e e s t u a r y r e s u l t b o t h from a resid u a l downwards c i r c u l a t i o n , l i n k e d t o t h e f r e s h w a t e r d i s c h a r g e and from
flood
and e b b c u r r e n t s l i n k e d t o t h e t i d e s . I t is p o s s i b l e t o d e s c r i b e t h e r e s u l t a n t e f f e c t o f t h e s e complex h y d r o d y n a m i c a l p r o c e s s e s by a s i m p l i f i e d model t a k i n g i n t o a c c o u n t a d v e c t i o n by currents. The o p e r a t o r
x
the
residual
discharge
e q u a t i o n ( 5 ) is t h u s w r i t t e n :
and
dispersion
by
tide
441 where
is the freshwater dicharge
Q
A(x)
is t h e w e t t e d s e c t i o n which obeys t h e f o l l o w i n g relationship: A(x) ( m 2 ) = 151350.10
Ds
-
0.02 x (km)
is an a p p a r e n t mixing c o e f f i c i e n t a d j u s t e d on b a s i s of e x p e r i m e n t a l l y observed s a l i n i t y p r o f i l e s :
Ds(m2/h) = 1 0 ' 10
+ 4500
+
22500
. Q(m3/sec)
. Q(m3/sec)
for x < 75 k 3 for x 80 km
Phytoplankton i n t h e e s t u a r y , c o n s i s t s i n two d i s t i n c t p o p u l a t i o n s , adapted
respectively
to
f r e s h and seawater. The dependence o f t h e i r maximum
growth r a t e on s a l i n i t y (S) is e x p e r i m e n t a l l y relationships:
shown
to
obey
the
following
f o r t h e freshwater population -(
pmax(S) = umax
.e
s - 1 7 2
51
f o r t h e sea water p o p u l a t i o n
T h e i r m o r t a l i t y r a t e , d, was supposed t o b e i n c r e a s e d by a factor 3 above 5 respectively.
g C1-11 and below 10 g C l - / l
Equation ( 5 ) a l l o w s c a l c u l a t i o n o f t h e dynamics o f t h e s e two p o p u l a t i o n s . The s e a s o n a l v a r i a t i o n s o f phytoplankton biomass a t S t Amands s i m u l a t e d by t h e r i v e r system model d e s c r i b e d above, are used as t h e upstream l i m i t c o n d i t i o n f o r f r e s h w a t e r p o p u l a t i o n . The seawater p o p u l a t i o n i s supposed t o be a b s e n t a t t h i s l i m i t . Conversely, t h e s e a s o n a l v a r i a t i o n s o f phytoplankton biomass observed i n t h e Belgian c o a s t a l zone ( J o i r i s e t a l , 1982) i s used as t h e downstream l i m i t c o n d i t i o n , where f r e s h w a t e r phytoplankton is set a t zero. For s e t t i n g t h e v a l u e o f t h e e x t i n c t i o n c o e f f i c i e n t
,a
complete
of t h e suspended matter i n t h e e s t u a r y s h o u l d be n e c e s s a r y . I n s t e a d , e m p i r i c a l r e l a t i o n s h i p s between suspended matter mes, deduced from t h e o b s e r v a t i o n o f a l a r g e number o f l o n g i t u d i n a l p r o f i l e s , were used:
model
mes(mg/l) = 4000
. 1/Q(m3/sec)
mes(mg/l) = 60
= 0.6 mes(mg/l) = 60
Q ( m 3 / s e c ) + 30
i n t h e Antwerp zone
i
f o r Q < 50 m3/sec f o r Q > 50 m3/sec i n t h e mixing zone i n t h e downstream'zone
442
F i g u r e 10 shows some o f l o n g i t u d i n a l p r o f i l e s o f p h y t o p l a n k t o n biomass s i m u l a t e d by t h i s model f o r t h e y e a r 1983. A v a i l a b l e e x p e r i m e n t a l o b s e r v a t i o n s
are also p l o t t e d , and show t h a t t h e s i m u l a t i o n r e p r o d u c e s t h e m a j o r t r e n d s and l e v e l s of b o t h t h e s p a t i a l and t e m p o r a l p h y t o p l a n k t o n v a r i a t i o n s w i t h i n t h e estuary. F i g u r e 11 shows t h e s i m u l a t i o n of t h e s e a s o n a l v a r i a t i o n s of biomass and p r i m a r y p r o d u c t i o n a t Antwerp (km 80) and Doe1 (km 60).
"1
April 1983
7
0
distance to the s8a. km
F i g . 10 : S i m u l a t e d and o b s e r v e d l o n g i t u d i n a l p r o f i l e s o f p h y t o p l a n k t o n biomass i n t h e S c h e l d t e s t u a r y i n 1983.
443
m
J
F M A M J
J A S O N D
a.
b. Fig. 1 1 : Observed and calculated seasonal variations of phytoplankton biomass (a) and primary production (b) at Antwerp ( * ) and Doe1 (o), in 1983.
NITROGEN TRANSFORMATIONS IN THE SCHELDT ESTUARY
-Control
of heterotrophic activity & organic matter. Organic matter in natural water is mostly supplied under the form of
macromolecular biopolymers which have to be hydrolysed before they can be taken up by microorganisms.This exoenzymatic hydrolysis is thought to represent the limiting step in the microbial utilization of organic matter and obeys a Michaelis-Menten kinetics (Somville and Billen, 1983; Billen, 1984; Fontigny et al, in press). It is customary to consider that organic matter in natural waters is present under at least two classes with different biodegradabilities with different succeptibilities of (Westrich, 1983; Garber, 1984) i.e. exoenzymatic hydrolysis. Because exoenzymes in natural waters are generally attached to the external envelopes of bacteria and are not subject to regulation (Vives-Rego et al, 1984; Fontigny et al, submitted), the concentration of exoenzymes is directly proportional to the bacterial biomass.
444 The p r o c e s s o f m i c r o b i a l o r g a n i c matter d e g r a d a t i o n is t h u s r e p r e s e n t e d by t h e f o l l o w i n g system o f e q u a t i o n s :
dB=
H1
-dB
where Hi
and H Z
r e p r e s e n t t h e c o n c e n t r a t i o n o f each class o f o r g a n i c matter
and P and e2
,t h e i r r a t e o f s u p p l y t h e i r maximum r a t e o f exoenzymatic h y d r o l y s i s
elmax
K 1H
B
+
e’max H2
+ K2H ) B
+
P
(elmax
H2
dt
max and K2H the half-saturation constant of their hydrolysis i s t h e b a c t e r i a l biomass
is t h e growth y i e l d c o n s t a n t , i.e. t h e r a t i o o f b a c t e r i a l biomass formed t o t h e t o t a l o r g a n i c matter m e t a b o l iz e d .
Y
is g e n e r a l l y c l o s e t o 0.3 i n n i t r o g e n non l i m i t e d environments ( L a n c e l o t and B i l l e n , 1985; S e r v a i s
and B i l l e n , i n p r e s s ) . is the f i r s t order mortality constant of bacteria, ( S e r v a i s e t a l , 1985). which is c l o s e t o 0.01 h
d
The k i n e t i c p a r a m e te r s o f biopolymers h y d r o l y s i s were determined by a d j u s t m en t on t h e r e s u l t s o f a b a t c h experiment i n which t h e b a c t e r i a l utilization of the o r g a n i c matter from S c h e l d t water, u n t r e a t e d or c o n c e n t r a t e d by u l t r a f i l t r a t i o n , was followed f o r 30 d ay s ( B i l l e n 1983). The f o l l o w i n g v a l u e s o f t h e p a r a m e te rs a t 2OoC were o b t ai n ed : elmax K1H
et
al,
= 0.4 h-’ = 1
mg C / 1 1
e2max
K2H
=
0.06 h-
=
13 m g C / 1
The dependence o f e ‘210 Of
2.
max
on t e m p e r a tu r e was supposed t o be c h a r a c t e r i z e d by a
Organic matter is s u p p l i e d by d i s c h a r g e o f domestic, a g r i c u l t u r a l o r i n d u s t r i a l waste ( T a b le 3 ) and by m o r t a l i t y o f phytoplankton ( c a l c u l a t e d by t h e phytoplankton model).
445
It w i l l b e c o n s i d e r e d t h a t o r g a n i c matter from waste is made o f 70% r a p i d l y u s a b l e o r g a n i c matter (HI ), and 30% more s l o w l y d e g r a d a b l e o r g a n i c matter (H2 ). For o r g a n i c matter s u p p l i e d by phytoplankton l y s i s , t h e s e p r o p o r t i o n s are 80 and 20% r e s p e c t i v e l y . TABLE 3 Domestic, a g r i c u l t u r a l and i n d u s t r i a l d i s c h a r g e o f o r g a n i c matter and n i t r o g e n i n t h e S c h e l d t e s t u a r y (from d a t a quoted i n B i l l e n e t a l , i n press).
Domestic wastes t o t a l p o p u l a t i o n : 1 130 000 inhab. e x i s t i n g p u r i f i c a t i o n c a p a c i t y (1982): organic C discharge: t o t a l N Discharge:
980 500 inhab. equiv.
34 TC/day 9 TN/day
I n d u s t r i a l wastes Total discharge before purification:
lgO TC/day 45 TN/day
% p u r i f i c a t i o n (1982)
80%
organic C discharge: t o t a l N discharge:
30 TC/day 26 TN/day
A g r i c u l t u r a l wastes ( c a t t l e waste produced i n e x c e s s o f s p r e a d i n g c a p a c i t y ) 84 TC/day organic C discharge: t o t a l N discharge: 44 TN/day
Redox p r o c e s s e s
and
transformations
For modelling t h e e f f e c t of h e t e r o t r o p h i c a c t i v i t y on water q u a l i t y ( i n c l u d i n g ammonium and n i t r a t e c o n c e n t r a t i o n s ) , t h e whole redox b a l a n c e
must
be c o n s i d e r e d . The p r i n c i p l e s o f t h i s m o d e l l i n g h a s been d e s c r i b e d i n d e t a i l s by B i l l e n and Smitz, 1976. The redox s t a t e o f t h e water is c h a r a c t e r i z e d by a f u n c t i o n F involved in d e f i n e d as t h e weighted sum o f a l l o x i d a n t s respiration:
microbial
F(meq/l) = 4 [ 0 2 1 + 2 [MnOz] + 8 [NO31 + 1 [Fe(OH)3] + 8 [ S O L , ]
This
variable
is
subject
to
hydrodynamical
transport
consumption by o r g a n o t r o p h i c a c t i v i t y ( a s c a l c u l a t e d by above),
to
production
the
processes, model
by p h o t o s y n t h e s i s ( a s c a l c u l a t e d by t h e phytoplankton
model, t a k i n g i n t o a c c o u n t a p h o t o s y n t h e t i c q u o t i e n t of 1.25 moles 0, /mole (Williams
et
al,
to
described
1979))
and
to
C
s u p p l y by r e a e r a t i o n ( p r o p o r t i o n a l t o t h e
446
a
ox gen s a t u r a t i o n d e f i c i t , w i t h
Y
h- / d e p t h (m).
The
-
F
2 at -
reaeration
coefficient
equal
to
0.1
in
1:
Org. Act ( m e q / l ) + Prim Prod (meq/l) + Reaer.
of
value
F
is
known
at
(8)
t h e upstream ( s e e Fig. 1 2 ) and downstream
(225.556 meq/l) l i m i t s o f t h e model.
A00
.--m
-B
I
No;
e
"i c
9,
,200
200
8 0 2
0
0
0
.
-200
-200
so;-7
18
discharge, m3/s
a.
Fig.
F ,meq/I.
b.
12 : a. E m p i r i c a l r e l a t i o n s h i o observed a t Rupelmonde between redox poten-
t i a l and d i s c h a r g e b. T h e o r e t i c a l r e l a t i o n s h i p between t h e F f u n c t i o n (weighted sum of o x i d a n t s ) and redox p o t e n t i a l f o r t h e composition of water a t Rupelmonde (Redox t i t r a t i o n c u r v e )
The l i n k between F v a l u e and redox p o t e n t i a l is r e p r e s e n t e d by a t o t h a t shown i n figure 12b for t h e case o f I t depends on t h e t o t a l c o n c e n t r a t i o n o f each redox c o u p l e s i n v o l v e d , which must be c a l c u l a t e d a t e a c h p o i n t o f t h e l o n g i t u d i n a l p r o f i l e . f i l t r a t i o n curve similar
Rupelmonde.
++
,
Fe(OH), / Fe For Mn02 /Mn is assumed t o be c o n s e r v a t i v e
++
.
and SOY /HSThe t o t a l c o n c e n t r a t i o n ( p o s s i b l e l o s s e s by s e d i m e n t a t i o n o f
species are n e g l e c t e d ) . For m i n e r a l n i t r o g e n , e s t a b l i s h e d a t e a c h p o i n t , t a k i n g i n t o account:
-
-
a
complete
solid
balance
is
i n p u t o f ammonium n i t r o g e n by water d i s c h a r g e s m i n e r a l i z a t i o n o f organic n i t r o g e n l i n k e d t o organotrophic a c t i v i t y d e n i t r i f i c a t i o n r e p r e s e n t i n g t h e t o t a l o f organotrophic a c t i v i t y n i t r a t e s are t h e b e s t o x i d a n t s p r e s e n t i n t h e water.
when
44 I
The r e l a t i o n s h i p between F and Eh d e s c r i b e d i n f i g u r e 12b r e s u l t s from an e x t e r n a l thermodynamical e q u i l i b r i u m h y p o t h e s i s . Owing t o t h e slow development o f n i t r i f y i n g b a c t e r i a , t h i s h y p o t h e s i s can l e a d t o o v e r e s t i m a t e t h e r a t e o f n i t r i f i c a t i o n . An e m p i r i c a l l i m i t o f t h e rate of n i t r i f i c a t i o n is t h e r e f o r e i n t r o d u c e d w i t h i n t h e model. T h i s l i m i t i s s e t
to
1
mole/l.h
on
basis
of
calculation
of
e x p e r i m e n t a l o b s e r v a t i o n s ( S o m v i l l e , 1978). Longitudinal ammonium denitrification
and
nitrate
distribution
A few examples o f observed l o n g i t u d i n a l p r o f i l e s o f concentration
in
and
ammonium
and
nitrate
t h e S c h e l d t e s t u a r y are shown i n f i g u r e 13. F i g u r e 14 shows
t h e p r o f i l e s s i m u l a t e d w i t h t h e model d e s c r i b e d above f o r t h e y e a r 1983.
=b March 1983
March 1978
0 100
1
50
0
Awi
October1983
October 1978
\NH;
distance. krn F i g . 13: Observed l o n g i t u d i n a l p r o f i l e s of ammonium and n i t r a t e concentrationsin the Scheldt estuary
distance. km
-
F i g . 14: C a l c u l a t e d l o n g i t u d i n a l p r o f i l e s of ammonium and n i t r a t e concentrations i n the Scheldt estuary i n 1983
448
F i g u r e 15 p r e s e n t s t h e s e a s o n a l v a r i a t i o n s o f n i t r o g e n e x p o r t a t i o n i n t o t h e sea and o f n i t r o g e n i m p o r t a t i o n i n t o t h e e s t u a r i n e zone from t h e r i v e r system and waste discharges. The d i f f e r e n c e between b o t h c u r v e s r e p r e s e n t s It c a n be e v a l u a t e d t o 14 l o 3 TN/yr i n e l i m i n a t i o n by d e n i t r i f i c a t i o n . r e a s o n a b l e agreement w i t h t h e e s t i m a t i o n o f 20 lo3 TN/yr based on t h e N budget of the estuary (see f i g . 1).
200-
,m,
0
‘1 100I-
04
. .
, , , , , , , , , J F M A M J J A S O N D ’
F i g . 15: C a l c u l a t e d s e a s o n a l v a r i a t i o n s of t h e n i t r o g e n f l u x e s imported t o t h e e s t u a r i n e zone from t h e r i v e r system and t h e d i s c h a r g e s i n t h e Antwerp r e g i o n and e x p o r t e d from t h e e s t u a r y t o t h e s e a .
SIMULATION OF WASTE WATER PURIFICATION SCENARIO Owing t o t h e dominant r o l e o f d e n i t r i f i c a t i o n i n t h e N budget o f t h e Scheldt
watershed,
any change i n t h e f a c t o r s c o n t r o l l i n g t h i s p r o c e s s can be
t h o u g h t t o have a profound e f f e c t on t h e n i t r o g e n o u t p u t i n t o
the
sea.
The
sub-models d e s c r i b e d i n t h i s paper a l l o w t o p r e d i c t t h e s e effects. A s a n exampple, w e w i l l p r e s e n t h e r e t h e s i m u l a t i o n of t h e s i t u a t i o n which would r e s u l t from a l a r g e scale program o f primary and secondary p u r i f i c a t i o n o f domestic, i n d u s t r i a l
and
agricultural
wastes,
eliminating
90% o f
the
o r g a n i c l o a d . It is w e l l known (Bund and S t r a u b , 1980) t h a t t h i s kind o f waste
water t r e a t m e n t d o e s n o t r e t a i n more t h a n a b o u t 30% o f t h e n i t r o g e n l o a d . Such a program would t h e r e f o r e r e s u l t i n a c o n s i d e r a b l e improvement o f t h e q u a l i t y of t h e r i v e r system b u t o n l y i n a small r e d u c t i o n o f n i t r o g e n i n p u t s . On t h e c o n t r a r y , d i s a p p e a r a n c e o f a n a e r o b i c r e a c h e s o f r i v e r s and r e d u c t i o n o f t h e o r g a n i c matter c o n t e n t o f t h e s e d i m e n t s could l e a d t o a s e v e r e r e d u c t i o n o f d e n i t r i f i c a t i o n b o t h i n t h e r i v e r system and t h e e s t u a r y , t h u s r e s u l t i n g i n
449
i n c r e a s e d i n p u t s i n t o t h e sea. Using t h e model d e s c r i b e d i n t h e f i r s t p a r t o f t h i s p ap er , d e n i t r i f i c a t i o n i n r i v e r s ed i m e n ts i n t h e p u r i f i c a t i o n s c e n a r i o co n si d er ed h e r e , can be estimated t o 2 3 l o 3 TN/yr. C o n s id e r i n g i n a d d i t i o n t h a t no more d e n i t r i f i c a t i o n would o c c u r i n t h e water column o f t h e r i v e r system, t h e i n p u t
-
o f n i t r o g e n i n t o t h e e s t u a r i n e zone would b e i n c r e a s e d by a t l e a s t a f a c t o r 2 (Compare f i g . 1 and f i g . 16). The redox model o f t h e S c h e l d t e s t u a r y , r u n f o r these new s e t o f l i m i t c o n d i t i o n s , y i e l d s t h e r e s u l t s shown i n f i g u r e 17. D e n i t r i f i c a t i o n i n t h e e s t u a r y would becomes n e g l i g e a b l e and t h e o u t p u t i n t o t h e sea would i n c r e a s e d from 27 l o 3 TN/yr i n t h e p r e s e n t s i t u a t i o n t o 70 l o 3 TN/yr ( F i g . 16). T h i s s i m u l a t i o n , however, d o e s n o t t a k e i n t o acco u n t a p o s s i b l e change i n t h e growth c o n d i t i o n s o f phytoplankton owing t o r e d u c t i o n o f suspended matter. I n s p i t e o f t h i s l i m i t a t i o n , i t c a n b e concluded t h a t g e n e r a l i z e d waste water p u r i f i c a t i o n w i t h o u t t e r t i a r y t r e a t m e n t s h o ul d t h u s p a r a d o x i c a l l y r e s u l t i n an i n c r e a s e o f n i t r o g e n d i s c h a r g e i n t o c o a s t a l water, i n c r e a s i n g t h e r i s k s o f eutrophication.
F i g . 16: Simulated n i t r o g e n budget f o r t h e S c h e l d t r i v e r system and e s t u a r y w i t h a 90% r e d u c t i o n of t h e o r g a n i c l o ad w i t h o u t t e r t i a r y t r e a t m e n t .
450
n
x)o.
. . .
0, . 100
,
.
,
,
0
50
distance. km
F i g . 1 7 : S i m u l a t e d l o n g i t u d i n a l p r o f i l e s of n i t r a t e and ammonium c o n c e n t r a t i o n s i n t h e S c h e l d t e s t u a r y f o r t h e s c e n a r i o of 90% r e d u c t i o n of t h e o r g a n i c l o a d i n t h e whole w a t e r s h e d w i t h o u t t e r t i a r y t r e a t m e n t . ACKNOWLEDGMENT T h i s p a p e r p r e s e n t t h e s y n t h e s i s of s e v e r a l y e a r s s t u d i e s of t h e S c h e l d t and
its
ENV-522-B),
watershed. the
These
Ministry
studies
of
Management U n i t ) , t h e M i n i s t r y C o n c e r t e e e n Oceanographie).
Public
of
were
supported
Health
Scientific
by
the
EEC
(contract
( c o n v e n t i o n w i t h t h e North Sea Policy
(Action
de
Recherche
G . B i l l e n is Research A s s o c i a t e o f t h e Fonds National Belge de l a Recherche
Scientifique.
451 REFERENCES Smitz, J., Somville, M and Wollast, R. 1976. Degradation de la Billen, G., matiere organique et processus d'oxydo-reduction dans l'estuaire de 1'Escaut. In: Wollast, R. and Nihoul, J.C.J. (Editors). Modele Mathematique de la Mer du Nord. Rapport de Synthese. Ministere de la Politique Scientifique Bruxelles, Vol. 10, pp 102-152. Billen, G. Dessery, S., Lancelot, C. Meybeck, M and Somville, M. 1983. Suivi et Modelisation de l'amelioration de la qualite de l'eau dans le bassin de storage de Mery s/Oise. 2d Rapport d'avancement. Compagnie Generale des Eaux, Paris, 80 pp. Billen, G., Lancelot, C., Servais, P., Somville, M. and Vives Rego, J. 1983. Etablissement d'un modele predictif de la qualite de l'eau de l'estuaire de 1'Escaut. Rapport final. Unite de Gestion du Modele Mathematique de la Mer du Nord. Ministere de la Sante Publique, Bruxelles, 67 pp. Billen, G. 1984. Heterotrophic utilization and regeneration of nitrogen. In: P.J. LeB, (Editors). Heterotrophic activity in Hobbie, J.E. and Williams the sea. Plenum Press. New York, pp 313-355. Billen, G., Debecker, E., Lancelot, C., Mathot, S., Servais, P. and Stainier, E. 1985. Etude des processus de transfert, d'immobilisation et de transformation de l'azote dans son cheminement depuis les sols agricoles jusqu'a la mer. Rapport de synthese.Contrat ENV-522(B). Commission des Communautes Europeennes, Bruxelles, 101 pp. Billen, G., Somville, M. Debecker, E. and Servais P. 1986. A nitrogen budget of the Scheldt hydrographical basin. Neth. J. Sea Res. in press. Bond, R. G. and Straub, C. P. 1980. Handbook of environmental control. IV Wastewater treatment and disposal. CRC Press. Dessery, S., Billen, G., Meybeck, M. and Cavelier, C. 1982. Evaluation et modelisation des echanges d'azote a travers l'interface eau-sediments dans le bassin de Mery-s/Oise. J. Franc. Hydrobiol., 13: 215-235. Edwards, R. W. and Rolley, H. L. J. 1965. Oxygen consumption of river muds. J. Ecol. 53: 1-22. Garber, J. M. 1984. Laboratory study of nitrogen and phosphorus remineralisation during the decomposition of coastal plankton and seston. Est. Coast. Shelf Sci., 18: 685-702. Hansen, J. I. K., Henriksen, K. and Blackburn, T. H. 1981. Seasonal distribution of nitrifying bacteria and rates of nitrification in coastal marine sediments. Microb. Ecol. 7: 297-304. Horton, R . E. 1945. Erosional development of streams and their drainage basins: hydrophysical approach to quantitative morphology. Geol. SOC. Am. Bull. 56: 275-370. Joiris, C., Billen, G., Lancelot, C., Daro, M.H., Mommaerts, J.P., Bertels, A., Bossicart, M., Nijs, J. 1982. A budget of carbon cycling in the Belgian coastal zone: relative roles of zooplankton, bacterioplankton and benthos in te utilization of primary production.Neth. J. Sea Res. 16: 260-275. Lancelot, C. and Billen, G. 1985. Carbon Nitrogen relationships in nutrient metabolism of coastal marine ecosystems. Adv. Aqu. Microbiol., 3, in press. Servais, P., Billen, G. and Vives Rego, J. 1985. Rate of Bacterial mortality in aquatic environments. Appl. Env. Microbiol., 49: 1448-1454. Servais, P., Billen, G. and Hascoet, M.C. 1986. Determination of the biodegradable fraction of dissolved organic matter in waters. Water Res., in pess. Smitz, J., Descy, J.P., Everbecq, E., Servais, P. and Billen, G. 1985. Etude ecologique de la Haute Meuse et modelisation du fonctionnement de l'ecosysteme aquatique. Rapport final. Ministere de la Region Wallonne pour l'Eau, 1'Environnement et la Vie Rurale. Namur, 250 pp.
,
452
So m v i l l e, M. 1978. A method f o r t h e measurement of' n i t r i f i c a t i o n rates i n water. Water Res. 12: 843-848. Vanderborght, J.P. and B i l l e n , G. 1975. Vertical d i s t r i b u t i o n o f n i t r a t e i n water of marine s e d i m en t s with nitrification and interstitial d e n i t r i f i c a t i o n . Limnol. Oceanogr. 20: 953-961. Vives Rego, J., B i l l e n , C., F o n t ig n y , A. and So m v i l l e, M. 1985. F r ee and a t t a c h e d p r o t e o l y t i c a c t i v i t y i n water environments. Mar. Ecol. Progr. S e r . 21: 245-249. Vollenweider, R. A. 1965. C a l c u l a t i o n models o f p h o t o s y n t h e s i s d e p t h c u r v e s an some i m p l i c a t i o n s r e g a r d i n g day rate estimates i n primary production measurements. In: Goldman, C. R. ( E d i t o r ) . Primary p r o d u ct i o n i n a q u a t i c environments. U n i v e r s i t y of C a l i f o r n i a Press. W es t r i ch , J . T. 1983. The consequences and c o n t r o l s of b a c t e r i a l s u l p h a t e r e d u c t i o n i n marine sediments. Ph. D. Thesis., Yale U n i v e r s i t y , USA. W i l ams, P.J. LeB, Raine, R. C. and Bryan, J.R.. 1979. Agreement between t h e "C and oxygen methods o f measuring phytoplankton p r o d u ct i o n : r eas s es s m en t of t h e p h o t o s y n t h e t i c q u o t i e n t . Oceanol. Acta, 2: 411-416.
453
MOBILIZATION OF MAJOR AND TRACE ELEMENTS AT THE WATER-SEDIMENT INTERFACE I N THE BELGIAN COASTAL AREA AND THE SCHELDT ESTUARY
W. BAEYENSl, G. GILLAIN',
M. HOENIG3 AND F. DEHAIRS'
' A n a l y t i c a l Chemistry, V r i j e U n i v e r s i t e i t Brussel, 1050 Brussel (Belgium) 'Laboratoire
d'Oceanologie,
U n i v e r s i t e de LiGge, 4000 Liege (Belgium)
3 1 n s t i t u t de Recherches Chimiques,
M i n i s t e r e de 1 ' A g r i c u l t u r e ,
1980 Tervuren
(Belgium)
ABSTRACT I n a s h a l l o w c o a s t a l area such as t h e B e l g i a n c o a s t a l waters, a q u i t e l a r g e amount o f suspended m a t t e r reaches t h e bottom where a p a r t o f i t i s m o b i l i z e d and r e l e a s e d i n t o t h e o v e r l y i n g water. H e t e r o t r o p h i c b a c t e r i a l degradation o f POM ( P a r t i c u l a t e Organic M a t t e r ) , which i s e s s e n t i a l l y l i m i t e d t o t h e f i r s t cent i m e t e r s o f t h e sediments, i s t h e process which i s r e s p o n s i b l e f o r t h a t m o b i l i z a t i o n . The presence o r absence of d i s s o l v e d i r o n and manganese depends on t h e redox s t a t e , anoxic o r o x i c , r e s p e c t i v e l y , o f t h e sediment. The behaviour o f s t r o n t i u m i n t h e f i r s t centimeters seems t o be almost independent o f POM degradat.ion as w e l l as redox c o n d i t i o n s . The percentages o f excess'Cu, Zn and Cd r e m o b i l i z e d d u r i n g t h e t r a n s i t i o n from suspended m a t t e r t o t h e f i r s t c e n t i m e t e r s o f t h e bottom sediments a r e very h i g h (100, 80 and 70% r e s p e c t i v e l y ) and independent o f t h e t y p e o f sediment, suggesting POM i s t h e i r c a r r i e r . Pb i s more r e l u c t a n t t o m o b i l i z a t i o n and shows a h i g h e r m o b i l i z a t i o n percentage i n anoxic sediments, suggesting t h a t d i s s o l u t i o n o f Fe and Mn phases a l s o r e l e a s e s a s i g n i f i c a n t p a r t o f t h e s o l i d Pb content. E p i b e n t h i c f l u x e s o f Cd, Cu, Pb and Zn have been estimated i n two d i f f e r e n t ways : ( 1 ) by c o n s i d e r i n g a s i m i l a r behaviour between t h e metal and POM f o r which r e m o b i l i s a t i o n r a t e s a r e known and ( 2 ) by u s i n g v e r t i c a l pore water d a t a p r o f i l e s . Both estimates agree f a i r l y w e l l f o r Cu, Zn and Cd, b u t t h e y d i f f e r s i g n i f i c a n t l y f o r Pb. 1. INTRODUCTION The Southern B i g h t o f t h e N o r t h Sea i s a p r o d u c t i v e ecosystem w i t h a d a i l y 2 p r i m a r y p r o d u c t i o n f r o m 10 t o 280 mg N/m .day (an annual p r i m a r y p r o d u c t i o n o f 2 2 25 g N/m .Y) i n t h e c o a s t a l area, and f r o m 10 t o 154 mg N/m .day (annual 20.5 g 2 N/m .Y) i n t h e o f f s h o r e area (Baeyens e t al., 1983 ; Momaerts e t al., 1984 ; Baeyens e t al.,
1984). Near t h e B e l g i a n c o a s t o n l y a small f r a c t i o n o f t h i s o r -
ganic m a t t e r i s grazed d i r e c t l y by zooplankton : most o f i t i s m i n e r a l i z e d by b a c t e r i a . Since water depth i n t h i s a r e a . i s o n l y 10-30m,
a significant part of
t h e o r g a n i c m a t t e r sedimentates i n c e r t a i n d e p o s i t i o n areas and i s m i n e r a l i z e d i n t h e sea f l o o r . Many mud areas,
o r g a n i c a l l y enriched,
occur i n t h e n o r t h e a s t o f t h e Belgian
454 c o a s t a l area. These a n o x i c mud d e p o s i t s , o f t e n p o l l u t e d by heavy m e t a l s have nematods as t h e o n l y s u r v i v i n g metazoans ( H e i p e t a l . ,
1984). Coarse sands and
even g r a v e l s w i t h a v e r y l o w o r g a n i c m a t t e r c o n t e n t and a v e r y d i v e r s e infauna o c c u r more o f f s h o r e . The o r g a n i c m a t t e r s t o c k and m i n e r a l i z a t i o n r a t e i n t h e sediments a r e essent i a l elements t o u n d e r s t a n d t h e f l u x e s o f m a j o r and t r a c e elements a t t h e water/ sediment i n t e r f a c e . I n sandy sediments, a t l e a s t i n t h e upper l a y e r s o n l y oxygen i s used as t h e t e r m i n a l e l e c t r o n a c c e p t o r i n t h e o x y d a t i o n process.
Elements
bound t o POM ( P a r t i c u l a t e Organic M a t t e r ) may be t r a n s f e r r e d t o t h e d i s s o l v e d phase and t h e breakdown o f DOM ( D i s s o l v e d Organic M a t t e r ) may change t h e speciat i o n o f t h e d i s s o l v e d species,
b u t t h e geochemical b e h a v i o u r i s l e s s complex
t h a n i n muddy sediments. I n muds, a whole s e r i e s o f e l e c t r o n a c c e p t o r s w i l l succeed each o t h e r when t h e r e d o x p o t e n t i a l goes down: oxygen, manganese, n i t r a t e , i r o n and s u l p h a t e . Some o f t h e r e a c t i o n s i n v o l v e d h e r e w i l l change t h e pH as w e l l . Many compounds w i l l disappear, o t h e r w i l l be produced when t h e Eh-pH cond i t i o n s a r e m o d i f i e d : as an example,
t h e r e d u c t i o n o f s u l p h a t e t o s u l p h i d e at
v e r y l o w r e d o x p o t e n t i a l s w i l l f o r m h a r d l y s o l u b l e Hg, Cd,.
.. s u l p h i d e s .
I n t h i s paper some e f f e c t s o f b a c t e r i a l metabolism on t h e d i s t r i b u t i o n o f maj o r and t r a c e elements i n d i f f e r e n t t y p e s o f c o a s t a l sediments a r e discussed. I n a d d i t i o n e p i b e n t h i c f l u x e s of t r a c e elements have been e s t i m a t e d i n t h e coastal a r e a and t h e S c h e l d t e s t u a r y .
2. SAMPLING AND ANALYSIS The c o r i n g s t a t i o n s i n t h e c o a s t a l zone a r e shown on F i g u r e 1A. Bottom sediment p r o p e r t i e s a t s t a t i o n s 33,
34 a r e q u i t e d i f f e r e n t f r o m t h e s e a t s t a t i o n s
1149, 1150 and 1151 as i s i n d i c a t e d on F i g u r e s 16 and 1C. A t t h e l a t t e r s t a t i o n s t h e f i n e sand f r a c t i o n ( < 74 p m) ranges f r o m 50 t o 100% and t h e o r g a n i c carbon c o n t e n t f r o m 8-16%, w h i l e a t t h e s t a t i o n s 33 and 34 b o t h parameters a r e much l o wer. The c o r i n g s t a t i o n s i n t h e S c h e l d t e s t u a r y (2,7,12,15,18,25)
a r e shown i n F i -
g u r e 1D. S t a t i o n s 2 and 7 (downstream) a r e c h a r a c t e r i z e d by a dominant
sand
f r a c t i o n . S t a t i o n s 12 t o 18 a r e s i t u a t e d i n t h e s e d i m e n t a t i o n zone : t h e s e d i ments a r e r i c h i n o r g a n i c m a t t t e r . A t s t a t i o n 25 ( t h e most upstream) t h e bottom has a c l a y s t r u c t u r e (Boom c l a y ) . The sediment c o r e s were sampled by d i v e r s i n s e r t i n g p l e x i g l a s s t u b e s i n t o t h e sediments. A f t e r e x t r u s i o n o f t h e c o r e s , t h e y were s e c t i o n e d i n t o 2 cm i n t e r v a l s . Pore w a t e r s were expressed under n i t r o g e n p r e s s u r e u s i n g a t e f l o n made squeezing d e v i c e o f t h e Rheeburg t y p e (Rheeburg,
1967).
Suspended m a t t e r was
c o l l e c t e d by f i l t r a t i o n on M i l l i p o r e 0.45 P m f i l t e r s and by c e n t r i f u g a t i o n . Sed i m e n t i n g p a r t i c l e s were c o l l e c t e d w i t h sediment t r a p s , s i m i l a r i n d e s i g n t o t h e t y p e d e s c r i b e d by Z e i t z s c h e l e t a l . ,
(1978), b u t f o r s i n g l e sample c o l l e c t i o n .
455
A
34
+
NL
F i g u r e s : 1 A - c o r i n g s t a t i o n s i n t h e c o a s t a l zone ; 1 8 - d i s t r i b u t i o n of f i n e sand f r a c t i o n s ( < 74
p
m ) ; 1 C - d i s t r i b u t i o n of o r g a n i c carbon c o n t e n t of s u p e r f i c i a l
sediment as measured by t h e w e i g h t l o s s a t 550°C
(Wollast,
1976) ; 1 0 - c o r i n g
s t a t i o n s i n t h e Scheldt estuary. I n s o l u t i o n Fe, Mn, S r and P were analyzed by I C P ( I L Z O O O ) , NO3- w i t h an aut o - a n a l y z e r ( T e c h n i c o n ) , Zn, Cd, Pb and Cu w i t h DPASV ( B r u k e r E 310). I n t h e sol i d phase Fe, Mn, S r , A1 were analyzed by I C P ( a f t e r f u s i o n w i t h L i B 0 2 ) , Cd, Pb, Cu and Zn by FAAS o r GFAAS ( a f t e r HF/HC1/HN03 m i n e r a l i z a t i o n ) , and N,C-inorganic and C-organic w i t h a C , H, N-analyzer ( P e r k i n - E l m e r ) .
3. PARTICULATE ORGANIC MATTER (POM) DEGRADATION Watercol umn B i o d e g r a d a t i o n of h i g h m o l e c u l a r w e i g h t o r g a n i c m a t e r i a l i s a t w o - s t e p p r o cess w i t h r a t e l i m i t i n g exoenzymatic h y d r o l y s i s i n t e r v e n i n g b e f o r e b a c t e r i a l upt a k e o f amino a c i d s . B i l l e n e t al.,
(1980) have demonstrated t h a t t h e u t i l i z a -
t i o n r a t e o f amino a c i d s by h e t e r o t r o p h i c b a c t e r i a d u r i n g t h e s p r i n g bloom var i e d between 0.1 and 4% h - l i n t h e B e l g i a n c o a s t a l area. Given t h e observed con-
456 c e n t r a t i o n o f prominent amino a c i d s i n sea water (0.23-0.92
mmol/l),
and t h e i r
average N-content (17.6 mg N/mmol ) , t h e depth i n t e g r a t e d c o n c e n t r a t i o n o f n i t r o 2 gen i n t h e amino a c i d s ranges f r o m 60 t o 240 mg N/m ( t h e average water depth i s 15 m). The p e l a g i c r e m i n e r a l i z a t i o n f l u x o f n i t r o g e n can t h u s v a r y between 1.4 and 230 mg N/m 2 .day.
However a h i g h r e m i n e r a l i z a t i o n f l u x i s observed o n l y a t 2 t h e end o f t h e s p r i n g bloom ( 3 g N/m .month) w h i l e i t i s almost i n s i g n i f i c a n t a t o t h e r p e r i o d s (Baeyens e t a l . ,
1984).
The d e g r a d a t i o n o f o r g a n i c m a t t e r i n t h e c o a s t a l waters has no i n f l u e n c e on t h e redox p o t e n t i a l o f t h e system. Oxygen i s t h e o n l y e l e c t r o n acceptor i n v o l v e d and i s always abundantly present.
This i s n o t always t h e case i n t h e upstream
p a r t o f t h e Scheldt e s t u a r y ( s t a t i o n s 12 t o 25). Oxygen i s o f t e n exhausted and o t h e r e l e c t r o n acceptors such as mangenese and i r o n are then used. The r e d u c t i o n o f these elements w i l l a l s o l e a d t o t h e i r d i s s o l u t i o n . Sediments
A good e s t i m a t e o f p r o t e i n degradation i n t h e sediments i s t h e ammonification r a t e . I n t h e sediments o f t h e Southern B i g h t o f t h e North Sea, B i l l e n (1976) has s t u d i e d t h e ammonification r a t e as a f u n c t i o n o f o r g a n i c m a t t e r content, oxydants, etc..
. His
depth,
main c o n c l u s i o n s can be summarized as f o l l o w s :
( 1 ) The ammonification r a t e observed i n t h e f i r s t 5 t o 8 cm of N o r t h Sea sediments ranges f r o m 0.2
t o 3.10-6
p
3
moles N/cm . s .
These r a t e s a r e n o t
s u b s t a n t i a l l y d i f f e r e n t f o r muddy and sandy sediments. Seasonal v a r i a t i o n s , any,
if
cannot be recognized i n t h e broad spectrum o f values c r e a t e d by l o c a l ,
s p a t i a l v a r i a t i o n s . The r e m i n e r a l i z a t i o n f l u x i n t h e upper sediment l a y e r , i n t e 2 g r a t e d over a month, t h u s ranges f r o m 0.36 t o 8.7 g N/m .month. These values are o f t h e same magnitude as t h e maximal p e l a g i c r e m i n e r a l i z a t i o n f l u x a t t h e end o f t h e s p r i n g bloom, b u t t h e y are much h i g h e r t h a n t h e p e l a g i c f l u x e s i n t h e o t h e r periods. ( 2 ) I n o r g a n i c r i c h sediments, exhaustyon o f oxydants may s e v e r e l y reduce t h e
u t i l i z a t i o n o f h y d r o l y s i s products f r o m o r g a n i c macromolecules.
These s m a l l e r
m e t a b o l i t e s ( p r o t e i n s and amino sugars) accumulate and l i m i t t h e i r own produc-
I n anaerobic c o n d i t i o n s ( s u l p h a t e exhaus3 t e d ) , ammonification i s slower than 0 . 0 5 ~ 1 0 - ~p moles N/cm . s .
t i o n by i n h i b i t i o n o f t h e exoenzymes.
( 3 ) I n o r g a n i c medium and poor sediments, exhaustion o f o r g a n i c m a t t e r w i l l s t o p t h e p r o d u c t i o n o f d i s s o l v e d n i t r o g e n . However, i n t h e deeper l a y e r s o f most o f these sediments, o r g a n i c m a t t e r i s n o t exhausted. I t i s t h u s s u r p r i s i n g t o see t h a t t h e ammonification r a t e i s t h e r e much lower t h a n i n t h e superf i c i a l l a y e r . The most probable e x p l a n a t i o n i s t h a t t h e f r e s h l y deposited organ i c m a t t e r i n t h e upper l a y e r i s much e a s i l y hydrolyzed by exoenzymes than' t h e aged o r g a n i c m a t t e r i n t h e deeper l a y e r s , which i s ( 1 " ) more d e p l e t e d i n e a s i l y
457 degradable n i t r o g e n compounds such as p r o t e i n s and (2") more p r o t e c t e d t o exoenz y m a t i c a t t a c k due t o c o m p l e x a t i o n w i t h m i n e r a l s . ( 4 ) The t u r n o v e r t i m e o f ammonia i s s h o r t w i t h r e s p e c t t o t h e p e r i o d
o f varia-
t i o n o f t h e environmental conditions. Therefore i t i s reasonable t o consider t h e ammonia p r o f i l e s as s t a t i o n a r y . The a d j u s t m e n t o f t h e s t a t i o n a r y e q u a t i o n describing
t h e ammonia b e h a v i o u r i n t h e sediments ( B i l l e n , 1976) on t h e experimen-
t a l l y o b t a i n e d v e r t i c a l ammonia p r o f i l e s y i e l d a d i s p e r s i o n c o e f f i c i e n t r a n g i n g
2
f r o m 0.5 t o Z . O X ~ O - ~cm / s .
These v a l u e s a r e s u b s t a n t i a l l y h i g h e r t h a n b i o t u r b a -
t i o n and m o l e c u l a r d i f f u s i o n c o e f f i c i e n t s ,
and suggest a t u r b u l e n t d i f f u s i v e
c o n t r o l as i s l i k e l y o t o c c u r i n t h e s h a l l o w b e l g i a n c o a s t a l environment, s t r o n g l y a f f e c t e d by t i d e s . 4. METALS MOBILIZATION Major elements D e g r a d a t i o n o f POM i n t h e
watercolumn does n o t i n f l u e n c e t h e b e h a v i o u r o f
i r o n and manganese as l o n g as a e r o b i c c o n d i t i o n s p r e v a i l .
I n t h e upstream p a r t
o f t h e S c h e l d t e s t u a r y t h e s e c o n d i t i o n s a r e no l o n g e r f u l f i l l e d ;
b o t h elements
serve as e l e c t r o n a c c e p t o r s i n t h e d e g r a d a t i o n process o f POM and appear i n t h e d i s s o l v e d phase. I n t h e downstream a r e a where oxygen reappears t h e y p r e c i p i t a t e and a r e t r a n s p o r t e d f u r t h e r seawards w i t h t h e remainder o f t h e suspended matter. C o n s i d e r i n g t h e b e h a v i o u r o f Fe and Mn i n t h e sediments, t w o sediment t y p e s can be d i s t i n g u i s h e d .
I n t h e t o p l a y e r s o f t h e sandy c o r e s ( s t a t i o n s 33 and 341,
t h e r a t i o o f Fe and Mn t o A1 i s h i g h r e l a t i v e t o t h e s o i l r e f e r e n c e value, i n d i c a t i n g an excess s i t u a t i o n . T h i s r a t i o decreases w i t h depth, e n d i n g i n a no excess s i t u a t i o n . No d i s s o l v e d i r o n o r manganese i s observed i n t h e upper l a y e r s . I n t h e deeper l a y e r s where oxygen i s exhausted,
a low dissolved concentration
may be found. I n t h e muddy c o r e s (M1149 t o 1151) l o c a t e d more i n s h o r e , no excess Fe o r Mn i s observed i n t h e s o l i d phase ( F i g u r e 2 ) . They b o t h appear r a p i d l y i n t h e d i s s o l v e d phase as oxygen i s a l m o s t i m m e d i a t e l y exhausted.
I n fact, the dis-
s o l v e d and s o l i d phase p r o f i l e s appear t o be complementary. S i n c e t h e s e elements a r e e a s i l y r e o x y d i z e d when oxygen i s p r e s e n t , t h e i r f l u x e s o u t o f t h e sediment may be o n l y a s m a l l f r a c t i o n o f t h a t which i s produced. Sundby and S i l v e r b e r g (1984) f o u n d i n t h e i r s t u d y t h a t most o f t h e d i s s o l v e d manganese produced (7187%) does n o t escape i n t o t h e watercolumn b u t p r e c i p i t a t e w i t h i n t h e sediment. S t r o n t i u m which i s n o t d i r e c t l y i n v o l v e d i n t h e o x y d a t i o n process o f
POM
shows a near c o n s e r v a t i v e b e h a v i o u r ; o n l y some s m a l l d i s s o l u t i o n and p r e c i p i t a t i o n o c c u r s w h i c h seems t o be c o r r e l a t e d w i t h t h e b e h a v i o u r o f t h e o t h e r e l e ments.
P a r t . Pb ( - - )
0.012
0.5
0.0140.4
0
1
0
0
6
2
10
16
20
40
60
(cm)
4
8
I
12 POC
F i g u r e 2 : V e r t i c a l p r o f i l e s o f Mn/A1, ( m g / l ) , Fe ( m g / l ) , P b (
II
Fe/A1,
0iss.Mn
10
20
Diss.Fe
POC (mg/g) and P b ( v 9/91 i n t h e s o l i d phase ; Mn
g / l ) i n t h e d i s s o l v e d phase (Core M1149).
30 I
Diss. Pb ( - )
459
Trace elements I n o x i c environments m i c r o b i a l d e g r a d a t i o n o f POM i s a m a j o r p r o d u c t i o n p r o cess o f d i s s o l v e d t r a c e m e t a l s .
I n a n o x i c environments d i s s o l u t i o n o f i r o n and
manganese oxydes o r hydroxydes may a l s o r e l e a s e adsorbed and i n c o r p o r a t e d t r a c e m e t a l s . S i n c e t h e d e q r a d a t i o n r a t e and t h e r e s i d e n c e t i m e o f POM i n t h e s e d i m e n t s a r e much h i g h e r t h a n i n t h e watercolumn, much l o w e r c o n c e n t r a t i o n s o f POM and a s s o c i a t e d m e t a l s ( s o l i d phase) can be expected t h e r e . I n F i g u r e 3 we compare excess m e t a l c o m p o s i t i o n between suspended m a t t e r ( t o t a l suspended m a t t e r and sediment t r a p m a t e r i a l ) and sediments.
Particle size
e f f e c t s , such as d i l u t i o n by q u a r t z g r a i n s , a r e c o r r e c t e d f o r by c o n s i d e r i n g t h e excess t r a c e m e t a l amounts r e l a t i v e t o A l . T h i s A1 i s assumed t o be e s s e n t i a l l y a s s o c i a t e d w i t h t h e s m a l l - s i z e d a l u m i n o s i l i c a t e f r a c t i o n and t o behave i n a cons e r v a t i v e way. From F i g u r e 3 we see t h a t f o r Cu, Zn and Cd more t h a n 70% o f t h e r e l a t i v e c o n t e n t s i n suspended m a t t e r have disappeared i n t h e f i r s t c e n t i m e t e r o f t h e o x i c sediments. F o r Pb t h i s i s o n l y about 30%. I n a n o x i c sediments changes i n r e d o x p o t e n t i a l o r pH may a l s o r e m o b i l i z e met a l s a s s o c i a t e d w i t h r e d o x s e n s i t i v e p a r t i c u l a t e phases. While Cu, Zn and Cd a r e m o b i l i z e d t o s i m i l a r degrees,
b o t h i n o x i c and a n o x i c sediments,
Pb i s n o t .
In
a n o x i c sediments Pb r e m o b i l i z a t i o n amounts t o about 50% ( o f t h e amount i n suspended m a t t e r ) as compared t o o n l y 30% i n o x i c sediments. I n t h e deeper l a y e r s m o b i l i z a t i o n r e l a t i v e t o suspended m a t t e r c o m p o s i t i o n , i n c r e a s e s o n l y o f a f u r t h e r 7 t o 11%, i n agreement w i t h t h e d i s c u s s e d l o w e r r a t e o f a m m o n i f i c a t i o n i n t h e s e l a y e r s . The c o m p l e m e n t a r i t y between t h e d i s s o l v e d and s o l i d phase p r o f i l e s o f Pb i n t h e muddy c o r e M1149 ( F i g u r e 2 ) i l l u s t r a t e s t h e e f f e c t o f remobi1iz a t i o n . We a l s o c a l c u l a t e d changes i n excess metal c o m p o s i t i o n r e l a t i v e t o aluminium from suspended m a t t e r i n a sediment t r a p t o sediments.
The m o b i l i z a t i o n percen-
t a g e s a r e s i m i l a r t o t h e s e of t o t a l suspended m a t t e r f o r Cu, s l i g h t l y l o w e r f o r Zn and Pb and s l i g h t l y h i g h e r f o r Cd.
5. EPIBENTHIC FLUXES OF METALS Coastal a r e a E p i b e n t h i c f l u x e s have been e s t i m a t e d i n t w o d i f f e r e n t ways. Since t h e c y c l i n g o f POM and t h e r a t i o m e t a l t o POM i n t h e s o l i d phase a r e known, m e t a l s f l u x e s a t t h e water/sediment
i n t e r f a c e can be deduced under t h e
c o n d i t i o n t h a t POM and m e t a l s behave s i m i l a r l y .
T h i s assumption i s r e a l i s t i c : i n
t h e p r e v i o u s paragraph we b r o u g h t e v i d e n c e t h a t POM d e g r a d a t i o n i n t h e f i r s t c e n t i m e t e r s o f t h e sediments w i l l l i b e r a t e most o f t h e excess m e t a l i n t h e s o l i d phase i n sandy as w e l l as muddy sediments.
The sedimentary o u t f l u x o f POM has
.
c.
been based on t h e a m m o n i f i c a t i o n r a t e i n t h e sediments and equal a b o u t 22 g N/m'.Y
or
460
F i g u r e 3 : Change i n excess metal composition r e l a t i v e t o aluminium f r o m suspended m a t t e r t o sediments : t h e B e l g i a n c o a s t a l zone.
,PI ’:. .,....;..,.: ,. ... :::
///////////////////////////
O X I C SEDIMENTS
At Elem.
-
Susp. M a t t e r
A N O X I C SEDIMENTS
1 cm
At
Sed. Trap
Elem.
( % remobilized)
Zn
82
70
cu
100
Cd
71
Pb
31
-
Susp. M a t t e r
1 cm Sed. Trap
( % remobilized)
Zn
79
65
100
cu
100
100
77
Cd
71
77
23
Pb
54
48
OXIC
m At
-
15 cm
At
-
5 cm
Zn
89
81
Zn
88
80
cu
100
100
cu
100
100
Pb
40
36
Pb
65
61
***
461 2 160 g C/m . Y (POM) The r a t i o metal t o POM i n suspended m a t t e r i n t h e c o a s t a l area i s g i v e n i n Table 1. The r e m o b i l i z a t i o n o f POM i n t h e sediments i s
.
almost 100 %. For t h e t r a c e metals i t v a r i e s f r o m 100 (Cu) t o 43 (Pb) as r e p o r t e d i n F i g u r e 3. The metal o u t f l u x e s shown i n Table 1 a r e thus based on t h e POM o u t f l u x and t h e r a t i o o f metal t o POM i n t h e o u t f l u x ( t h i s r a t i o equals t h e r a t i o i n t h e suspended m a t t e r m u l t i p l i e d by t h e r e m o b i l i z a t i o n percentage i n t h e sediments). TABLE 1
3
R a t i o o f metal t o POM i n suspended m a t t e r (10 R )
I
I I
I Hg 0.080
Zn 2.95
Cd 0.055
Pb 1.21
cu 0.67
I I
R e m o b i l i z a t i o n percentage o f metals from t h e s o l i d phase i n t h e sediments
I
' I
I
96 Hg
Zn 80
Cd 71
Pb 43
cu 100
I I
Epi b e n t h i c metals f l u x e s (tons/kmL. Y )
I
I
I I
Hg 0.012
Zn 0.38
Cd 0.0062
Pb 0.082
cu 0.11
I I
The second approach i s based on v e r t i c a l pore w a t e r d a t a p r o f i l e s . As an example, such v e r t i c a l p r o f i l e s observed i n a muddy core (51"18'20"N-2°58'40"E) are shown i n F i g u r e 4. From these p r o f i l e s c o n c e n t r a t i o n g r a d i e n t s a t t h e water/sediment
i n t e r f a c e can be i n f e r r e d .
These g r a d i e n t s a r e g i v e n i n Table 2.
These c o n c e n t r a t i o n s r e f e r t o t h e l i q u i d phase and should be c o r r e c t e d f o r t h e p o r o s i t y o f t h e sediment. A t t h e t o p o f t h e sediment t h i s p o r o s i t y i s 0.5 i n t h e sandy cores (33 and 34) and 0.8 i n t h e muddy cores (1149 t o 1151). The 2 see p a r t 3, has been e s t i m a t e d a t cm / s . The
d i f f u s i o n c o e f f i c i e n t (DS),
r e s u l t i n g o u t f l u x e s can then be c a l c u l a t e d according t o :
J = p DS dC/dz
J
= o u t f l u x (ng/cm2.s)
; p = porosity ;
(1)
DS= d i f f u s i o n c o e f f i c i e n t (cm2 / s ) ;
3 C = c o n c e n t r a t i o n (ng/cm ) ; z = depth (cm). The Zn and Cd o u t f l u x e s based on POM r e m o b i l i z a t i o n seem t o compare t o those obtained f r o m pore water d a t a p r o f i l e s . For Cu t h e agreement i s w i t h i n a f a c t o r o f 3. However,for Pb t h e r e e x i s t s an o r d e r o f magnitude d i f f e r e n c e , w i t h
t h e va-
l u e based on pore water p r o f i l e s on t h e low s i d e . The r e s u l t s o b t a i n e d w i t h t h e two d i f f e r e n t approaches suggest t h a t Cd, Zn and Cu may have a sedimentation and m o b i l i z a t i o n behaviour which i s c l o s e l y r e l a t e d t o t h a t o f o r g a n i c m a t t e r . For
462
0 "
z n
Q
m
Q LL
L
u)
E a 0
n n
U 0
E
N
I
0
0
0)
L 0 V
E
3
'0 h U
c .r
L 3 m
aJ
W c,
L
c, VI
.r
.r
m .v
7
V
0
c V
aJ
+ c
L
m
c,
.r
0
c
n
L
0
v-
.r
1 1 Y 0
2 I
0 v-
I
cu' 0
!A
.r
463
Pb t h e r e m o b i l i z a t i o n f l u x as based on Pb/POM r a t i o s i s 10 times l a r g e r t h a n t h e c a l c u l a t e d d i f f u s i v e o u t f l u x . This i n d i c a t e s t h a t Pb i f bound t o POM i s more r e l u c t a n t t o m o b i l i z a t i o n o r r a t h e r t h a t Pb i s associated m a i n l y w i t h o t h e r c a r r i e r s than POM. P a r t i c u l a t e Mn and Fe phases are l i k e l y i n v o l v e d as evidenced above by t h e increased r e m o b i l i z a t i o n o f Pb i n anoxic sediments. This i s c o n f i r med a l s o by our r e c e n t d a t a f o r N o r t h Sea suspended m a t t e r showing s i g n i f i c a n t p o s i t i v e r e l a t i o n s h i p s between Pb and Fe, Mn and A l - s i l i c a t e phases. TABLE 2
I
I I
Concentration g r a d i e n t s on t h e water/sediment i n t e r f a c e 4 (ng/cm 1
I
I
I
II ‘;ieIII 1149 1150 1151
I I I I I
I I I
Zn 9.6 5.0 21.0 28.7
I
I I I I I I
Cd 0.68 0.27 0.13 0.35
I
I I I I I I
Pb 0.22 0.20 0.43 n.m.
1 1 I I I I
Cu 5.7 0.58 0.52 1.37
1
I I I
i
I I I I I
!
I
Sedimentary metal o u t f l u x e s
I
i
I Core 33 1149 1150 1151 Mean
I Zn 48 40 168 230 122
I
Pb 1.1 1.6 3.5
Cd 3.4 2.1 1 .o 2.8 2.3
cu 28 4.7 4.2 11 12
2.1
I I
2 Sedimentary metal o u t f l u x e s (Tons/km . Y )
i
I I I 1 I I I
I
I I
I I
i
I n.m.
0.44
I I
i
0.0084
I I 0.0077
i
I 1
i
0.044
I I
i
= n o t measured
Scheldt e s t u a r y Sedimentation f l u x e s i n t h e e s t u a r y a r e v e r y d i f f i c u l t t o e s t i m a t e due t o t h e h i g h t i d a l energy d i s s i p a t e d i n t h e system; sediments are p e r i o d i c a l l y deposited and eroded i n most areas, p a r t i c u l a r l y i n t h e zone o f t h e t u r b i d i t y maximum (between 50 and 80 km from t h e mouth) (Baeyens e t a l . , b i d i t y maximum t h e r i v e r i s a narrow channel
1981). Upstream o f t h e t u r -
i n c i s e d i n t h e Boom c l a y and
c u r r e n t s a r e v e r y h i g h so t h a t accumulation o f suspended m a t t e r a t t h e bottom i s almost prevented. The f l u x o f suspended m a t t e r and hence p a r t i c u l a t e metals from t h e upstream zone i n t o t h e area o f t u r b i d i t y maximum o r sedimentation can thus be e s t i m a t e d
in a f a i r l y c o r r e c t manner. The same can be done a t t h e downstream
464 boundary o f t h e sedimentation zone where we f i n d t h e p a r t i c l e s which escaped from t h e t r a p p i n g zone,
mostly the f i n e r material.
The sedimentation r a t e of
p a r t i c u l a t e metals i n t h e sedimentation zone can thus be estimated based on t h e i r f l u x e s a t t h e up- and downstream boundaries (Table 3 ) . I n a d d i t i o n t h e r e m o b i l i z a t i o n percentages o f metals f r o m t h e s o l i d phase can be deduced f r o m t h e i r c o n c e n t r a t i o n s i n suspended and bottom sediments (Table 3 ) . These f r a c t i o n s are n o t c o r r e c t e d f o r t h e s o i l component and perhaps t h e r e f o r e lower remob i l i z a t i o n percentages are obtained i n t h e e s t u a r y than i n t h e c o a s t a l zone, exc e p t f o r Pb which shows t h e r e m o b i l i z a t i o n percentage f o r muddy sediments i n the sea. The r e d o x p o t e n t i a l i n t h e Scheldt e s t u a r i n e sediments i s a l s o very low,
SO
t h a t a s i g n i f i c a n t l e a d r e l e a s e may be due t o t h e d i s s o l u t i o n o f i r o n and manganese oxydes and hydroxides. Using t h e sedimentation r a t e and t h e r e m o b i l i z a t i o n percentage an e p i b e n t h i c f l u x can be c a l c u l a t e d . These o u t f l u x e s are i n the Scheldt e s t u a r y f r o m 5 t o 50 times h i g h e r than i n t h e c o a s t a l zone (Table 3 ) . TABLE 3
I
1 I I
I Sedimentation r a t e s i n t h e sedimentation zone o f t h e Scheldt e s t u a r y (Tons/km 2 .Y) I
I
\
I
Zn 2.5
I
1 I
j
I
\ I
I
I
Cd 0.092
I
1 1
Pb 0.74
1 1
I
Cu 0.96
i
I
cd 0.06
i
I
Pb 0.42
i I
cu 0.74
i
I
1
I
I
R e m o b i l i z a t i o n percentage f r o m t h e s o l i d phase i n t h e sediments
Zn 1.6
I
I
Hg 0.0166
I
I
I
1 1
I I
Hg
-
1
I
i
I
Based on t h e pore water d a t a p r o f i l e s , we can c a l c u l a t e t h e sedimentary outf l u x e s o f metals i n t h e same way as described i n t h e previous s e c t i o n . For lack o f a b e t t e r e s t i m a t e o f t h e t u r b u l e n t d i f f u s i o n c o e f f i c i e n t i n t h e area we adop-4 2 cm / s ) . This value
t e d t h e c o e f f i c i e n t estimated f o r t h e c o a s t a l area ( D S = 10
i s , however, probably t o o low. Applying equation ( 1 ) t o t h e Scheldt pore water d a t a y i e l d t h e e p i b e n t h i c f l u x e s given i n Table 4. These f l u x e s are a l l comparab l e w i t h i n an order o f magnitude w i t h those d e r i v e d from t h e sedimentation r a t e s , except f o r Pb which i s t h i r t e e n times lower.
465
TABLE 4
I
I I I
Concentration g r a d i e n t s a t t h e water/sediment i n t e r f a c e
I
i
i n t h e Scheldt e s t u a r y (ng/cm4)
I Core 12 15 15-1 18 18-1 18-2 25 Mean
I
i
Zn 56.9 17.7 n.m. n.m. 2.7 8.7 35.1 24.2
i
I
i
1 Cd 0.30 0.13 0.02 1.2 0.12 0.03 n.m. 0.30
i
Pb 2.3 -0.24 0.20 2.0 0.49 0.40 2.6 1.11
I
cu 10.0 3.8 1.8 2.9 7.5 6.6 3.4 5.1
I
E p i b e n t h i c metals f l u x e s
I
i
I
I
I
Zn
1 - 5 2 ' 110 ng/cm s I
I
ITons/km2.Y
I
I
194 0.71
I I
I
I
I
Cd
Pb
2.4
8.88
0.0088
0.032
I
I I I I I
Cu 40.8 0.15
I
n.m.
= n o t measured
CONCLUSIONS The h e t e r o t r o p h i c b a c t e r i a l degradation o f POM takes e s s e n t i a l l y p l a c e i n t h e f i r s t c e n t i m e t e r s o f t h e bottom sediments,
zone wherein t h e major f r a c t i o n o f
t h e excess t r a c e metal amount r e l a t i v e t o A1 i s a l s o l i b e r a t e d f r o m t h e s o l i d phase. The s i m i l a r i t y between t h e Cu, Zn and Cd sedimentary o u t f l u x e s based on POM r e m o b i l i z a t i o n and those o b t a i n e d f r o m pore water d a t a p r o f i l e s on t h e one
hand, and t h e f a c t t h a t d i f f e r e n t redox p o t e n t i a l c o n d i t i o n s seem n o t t o have a major e f f e c t on t h e m o b i l i z a t i o n o f t h e t h r e e metals on t h e o t h e r hand, suggest t h a t these metals a r e associated w i t h POM. For Pb, o t h e r c a r r i e r s a r e a l s o i n volved : most l i k e l y Mn and Fe phases as i s evidenced by t h e increased r e m o b i l i s a t i o n o f Pb i n anoxic sediments.
ACKNOWLEDGEMENTS This research was m a i n l y sponsored by t h e M i n i s t r y o f S c i e n t i f i c P o l i c y as a p a r t o f Geconcerteerde A c t i e s Oceanologie. t h e EEC ( c o n t r a c t ENV-766-B).
A d d i t i o n a l support was p r o v i d e d by
466
REFERENCES Baeyens W., Adam Y., Mommaerts J.P. and P i c h o t G., 1981. Numerical s i m u l a t i o n s o f s a l i n i t y , t u r b i d i t y and sediment accummulation i n t h e Scheldt estuary. I n : J.C.J. Nihoul ( E d i t o r ) , Ecohydrodynamics, E l s e v i e r , Amsterdam, 319-332. 1983. N i t r o g e n c y c l e s i n a Baeyens W., Goeyens L., Dehairs F. and Decadt G., c o a s t a l and an open sea zone o f f t h e B e l g i a n c o a s t . Trans. Am. Geophys. Un., EOS, 64, 52, 1024 ( A b s t r a c t ) . Baeyens W., Mommaerts J.P., Goeyens L., Dehairs F., Dedeurwaerder H. and Decadt G., 1984. Dynamic p a t t e r n s o f d i s s o l v e d n i t r o g e n i n t h e Southern B i g h t o f t h e North Sea. Estuarine, Coastal and S h e l f Science, 18, 499-510. B i l l e n G., 1976. Etude ecologique des t r a n s f o r m a t i o n s de l ' a z o t e dans l e s sediments marins. Ph. D. Thesis, U n i v e r s i t e L i b r e de B r u x e l l l e s , Brussels (Belgium), 266 pp. Bi l e n G., J o i r i s C., Wijnant J. and G i l l a i n G., 1980. Concentration and microb i o l o g i c a l u t i l i z a t i o n o f small o r g a n i c molecules i n t h e Scheldt estuary, t h e B e l g i a n c o a s t a l zone o f t h e N o r t h Sea and t h e E n g l i s h Channel. Estuarine, Coastal and S h e l f Science, 2, 279-294. He p C., Herman R. and Vinkx M., 1984. V a r i a b i l i t y and p r o d u c t i v i t y o f meiobent h o s i n t h e Southern B i g h t o f t h e North Sea. Rapp. Pv.; Reun. Cons. I n t . Exp l o r . Mer, 183, 51-56. Mommaerts J.P., P i c h o t G., Ozer J., Adam Y. and Baeyens W., 1984. N i t r o g e n cyc l i n g and budget i n B e l g i a n c o a s t a l waters : N o r t h Sea areas w i t h and w i t h o u t r i v e r i n p u t s . Rapp. P.-v. Reun. Cons. I n t . Explor. Mer, 183, 57-69. Rheeburg W.S., 1967. An improved i n t e r s t i t i a l water sampler. Limnol. Oceanogr., 12, 163-170. Sundby B. and S i l v e r b e r g N., 1985. Manganese f l u x e s i n t h e b e n t h i c boundary l a y e r . Limnol. Oceanogr., 30 (21, 372-381. W o l l a s t R., 1976. P r o p r i e t e s physico-chirniques des sediments e t des suspensions de l a Mer du Nord. In: J.C.J. Nihoul e t F. Gullentops ( E d i t o r s ) , Sedimentolog i e , Vol. 4 o f t h e F i n a l Report o f t h e P r o j e c t Sea. M i n i s t r y o f S c i e n t i f i c P o l i c y , Brussels. Z e i t s c h e l B., Diekmann P. and Uhlmann L., 1978. A new m u l t i sample sediment t r a p Mar. B i o l . , 45, 285-288.
***
461
SEASONAL NUTRIENT SUPPLY TO COASTAL WATERS L. RYDBERG and J. SUNDBERG Department of Oceanography, University of Gothenburg, Box 4038, S - 4 0 0 40 Gothenburg (Sweden)
ABSTRACT Monthly nutrient and salinity observations have been undertaken during the years 1 9 8 2 - 1 9 8 5 in the southeastern part of the Kattegat on the Swedish west coast. Two minor embayments in that area, the Laholm bay and the Skalderviken receive large amounts of inorganic nitrogen from a couple of small rivers. Oxygen deficit occurs as a frequent feature in the deep water outside the bays. The observations have been used to calculate bimonthly mean concentrations of total nitrogen, total phophorus, inorganic nitrogen and phosphate within three different water masses; one defined as local surface water within the Laholm bay, one as Kattegat surface water, outside the bay and the third as Kattegat deep water. Using the observed landbased supply of nutrients, the deep water supply, calculated from entrainment theory and the measured nutrient gradients, we have determined the exchange of water and nutrients between the Laholm bay surface water and the Kattegat surface water. We have also calculated the "net assimilation" of inorganic nitrogen and phosphate within the Laholm bay. In an earlier report, we did similar calculations on an annual mean basis. Here, we have done a separation between a winter period from November to February when the assimilation is low, and the rest of the year, when most of the primary production occurs. From March to October, approximately 3 / 4 of the inorganic nitrogen supply to the local water is of land based origin while the phosphate supply is dominated by entrainment from the deep water. 2/3 of the total inorganic nitrogen supply is assimilated by the primary production within the local water. The rest may be found as a loss from the local water to the Kattegat, which occurs mainly during the winter, but also during the spring, when the supply of phosphate is frequently too small for the primary production. The supply of phosphate to the bay seems to be low for the rest of the season as well, and the IN/IP ratio based on the external nutrient supply is well above the Redfield ratio. Still, however, the surface water concentrations of inorganic nutrients points towards nitrogen as the limiting nutrient for the main part of the productive season, indicating a more effective internal regeneration of phophorus and/or denitrification at the bottom.
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SKAGE R R A K
Fig 1 . Map over the Kattegat. Stations are marked with black dots.
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INTRODUCTION Intensive algal blooms and thedestruction ofbottom fisherydue to oxygen deficit have been seen occasionally in the southeastern Kattegat (fig 1 ) over the last fifteen years(Rosenberg,1985).Severe oxygen deficit and bottom death was observed both in 1980 and 1981. Since then there has been a gradual recovery of the deep bottoms and the fishery. Oxygen measurements have also indicated somewhat better conditions (Fig. 2). The waters in this area are characterized by a strong vertical salinity stratification, which is caused by the outflow of low salinity water from the Baltic Sea and an inflow of high salinity water of oceanic origin at deeper levels. The surface water has a salinity between 12 - 25 , and a thickness of approximately 15 m. The deep water salinity is normally between 32 - 34, and has its origin in the Skagerrak. The halocline depth is strongly influenced by the outflow from the Baltic, but it varies also due to local winds which redistribute the surface waters and induce mixing, mainly as upward entrainment. In this part of the Kattegat, the depth is generally less than 30 m. The varying halocline depth thus implies that the deep water volume sometimes becomes small or even disappears, at least in the Laholm bay where the depth does not exceed 20 m.
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Fig 2. Oxygen concentrations within the deep water of the southeastern Kattegat from 1981 to 1985. The curve shows a mean value for the stations K5, K7 and K9, 1 m above the bottom (mean depth, 27 m, mean salinity, 33.1). The characteristic minimum value for each station individually has been 1 ml/l, while the inner stations have had even lower concentrations when deep water has existed at those stations.
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The phytoplankton production in the Kattegat begins with a springbloom, usually in early March ( 1 - 2 gC/m2 day), followed by a rather stable summer production of the order of 0 . 4 gC/m2 day. The production is normally lowafter the end of October. The annual primary production seems to be just above 1 0 0 gC/m2 year (Aertebjerg et al., 1 9 8 1 ) . The production in the outer part of the Laholm bay has been measured since 1 9 8 1 by Edler (pers.comm2 who has found a value of the order 1 5 0 gC/m2 year, thus a little higher than in the open Kattegat. The Department of Oceanography at the University of Gothenburg started an intensive field program in the southeastern Kattegat in February 1 9 8 2 , with emphasis on the situation in the Laholm bay. The bay recieves a large nutrient load, especially inorganic nitrogen through the rivers Lagan and Nissan (Fig. 3c). The purpose was to determine to what extent the local supply of nutrients influenced the severe oxygen conditions within the area, and if so, whether a decrease in the nitrogen supply from land (which origins from fertilizers used in the agricultural districts) could contribute to better oxygen conditions. Since the beginning, we have carried out approximately one cruise per month to the area. The field program, which includes hydrography and nutrient chemistry at 15 stations will be continued until the end of 1 9 8 5 . Parallel1 to our programme, the local government is running a monthly follow-up of the landbased input of nutrients to the area (Fleischer et al, 1 9 8 5 ) , and biologists from the universities of Lund and Gothenburg are studying, among other things, the development of the phytoplankton and zooplankton production and the various species within the area. In 1 9 8 4 we made a first rough estimate of the relative importance of the local landbased nitrogen supply, by doing a comparison with the supply of deep water nitrogen. That approach was based on yearly mean values, determined from 22 surveys (Rydberg and Sundberg, 1985). Today, we have carried out another year of monthly observations and will now make a next step forward by using bimonthly mean nutrient concentrations to determine the seasonal (winter/summer) nutrient fluxes,and the phytoplankton nutrient assimilation within the bay. In the future, we shall put large efforts into studying the
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relation between nutrient supply and oxygen consumption. This implies that we must follow the variability of the parameters involved on a monthly time scale, or even shorter. Whether this will be sucessful or not is still an open question and also whether one should try a "real time description" or whether one should use a "mean quantity description" as we do here.
OBSERVATIONS AND METHODS The monthly programme included CTD-profiling (Neil Brown, MK 111) and discrete sampling with a rosette water sampler (Cen. Oceanics). At each of approximately 15 stations (of which the results from 1 1 stations are used here, see Fig. 1) between 2 and 6 water samples were taken for analyses of salinity, oxygen, nitrate, nitrite, ammonium (IN = I(nitrite + nitrate + ammonium), total nitrogen (TN), phosphate (IP) and total phophorus (TP). For the calculations made here, we shall make use of data from 2 9 expeditions during the period from February 1 9 8 2 to December 1 9 8 4 . The time spacing appears from Fig. 3a. More details concerning observations and methods are given in Rydberg ( 1 9 8 5 ) .
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To give a general view of the time and space variability of the observed parameters, we have done as follows: Spatial mean concentrations of IN (IP) and TN (TP) within the surface water of the Laholm bay and the Skalderviken have been determined for each cruise separately (Figs. 3a, b and 4 a , b). These are based on the eight inner stations, i.e. 1 1 - 1 6 , 4 , 6 and 7 . The surface water is defined as.the water between 0-10 m , or when the halocline is shallower than 1 0 m (has happened once) as the water with salinities S<30. These "local" mean values can be compared with the corresponding mean values for the Kattegat surface water (formed by addition of the values at the stations K 5 - K 9 ) , which are also shown in the same figures. To get a detailed picture of the spatial variations between the various stations, we have also determined the annual mean surface water concentrations of IN, TN, IP and TP for each station individually (Figs. 3c, d and 4 c , d).
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Fig 3a, b. Mean surface water concentrations of IN and TN, within the Laholm bay and the Skalderviken(-) and within the Kattegat (-----) . The mean values are determined for the eight inner stations (11-16, 4 - 7 ) and for the three outer stations K5 - K9. Surface water has been defined as the water in the interval 0-10 m. The dots ( o , * ) refers to observations performed by other institutes.
We have furthermore calculated bimonthly surface water mean concentrations of IN, TN, IP and TP for the Laholm bay (stns 1 1 16, with half weight for the stns 1 1 and 16) and for the Kattegat stations K7-K9 (Figs. 5 a - d, and Table 1). Finally, we have determined the bimonthly mean concentrations for the same parameters Im above the bottom at the stations K5 - K9, which are the only stations where high salinity deep water has always been present (Figs. 5 a - d) The monthly observations of nutrient concentrations (IN, TN, IP and TP) in the rivers, the freshwater supply and the municipal sewage, measured by the local government, were Used to calculate the
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bimonthly land based supply of nutrients (and freshwater) to the Laholrn bay. These are seen from Figs. 6 a - c and also from Table 1. In earlier reports we have done a comparison between the land based supply of nutrients and the deep water supply, where the later was calculated from the observed nutrient concentrations multiplied by a "theoretically" determined entrainment transport. In Fleisch'er et al., 1985, for example, we made use of the observed bimonthly mean depth of the halocline (depth to isohaline 25),h, , the observed mean salinity difference, AS between the surface water (see above) and the deep water and observations of the long time mean wind at the Danish weather station Christianso.
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Fig 4a, b. Mean surface water concentrations of IP and TP, within the Laholm bay and the Skalderviken(-) and within the Kattegat (-----) . The mean values are determined for the eight inner stations ( 1 1 - 1 6 , 4 - 7 ) and for the three outer stations K5 - K9. Surface water has been defined as the water in the interval 0-10 m. The dots ( o , * ) refers to observations performed by other institutes.
The entrainment velocity, w, was then calculated according to the formulas;
where u* is the friction velocity, g is the gravity constant and W, the wind velocity. fJ = 8 * lo-' relates the salinity to the sea water density, while the expression (c Q~ ) / Q = (1.25 * l o - ' ) * , Stigebrandt, 1983, determined a mean value for the constant ma = 1 , within the Kattegat, implying a mean entraiment velocity of approximately 2 5 cm/day. w is lower in the southern Kattegat due to a stronger vertical salinity gradient. In this report, the bimonthly entrainment transport, qo = w * A (where A is the halocline area within the bay) was corrected, with just marginal changes, for true windspeed at Spodsbjerg, Denmark.
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TABLE 1 . Bimonthly mean concentrations (C, (uM)) of TN, IN, TP and IP within the Laholm bay surface water (L), the Kattegat surface water ( 1 ) and the Kattegat deep water (01, and3bimonthly supply of nutrients ( R , (g/s)) and freshwater ( q t , (m / s ) ) to the Laholm bay ( 1 9 8 2 - 8 4 ) . Jan - Feb CTNL CTN 1 CTNO CINL CINI CINO CTPL CTP 1 CTPO CIPL CIPl CIPO RTN RIN RTP RIP qP
25.77 23.92 23.42 10.59 8.16 0.72 1.17 1.13 1.35 0.71 0.71 1 .oo 234.1 133.2 5.01 2.00 206.0
Mar - Awr 20.73 19.23 21.97 1.41 0.55 11.12 0.76 0.76 1.49 0.07 0.07 0.91 197.9 108.1 5.35 2.14 174.0
May- Jun 17.69 16.26 22.62 0.34 0.22 12.97 0.45 0.48 1.53 0.04 0.03 1.07 110.7 49.5 3.20 1.28 88.0
Jul - Aua 21.57 20.08 24.43 0.20 0.22 0.27 0.64 0.59 1.33 0.08 0.06 1.02 58.6 22.1 1.82 0.73 45.0
Sep - Oct 22.34 20.61 24.32 0.76 0.31 9.90 0.82 0.70 1.67 0.16 0.13 1.12 106.6 43.9 3.14 1.26 94.0
Nov Dec 25.61 21.41 24.50 4.50 3.77 8.86 0.94 0.93 1.42 0.42 0.48 1.06 205.9 103.2 4.19 1.68 165.0
These observations were kindly given to us by Stigebrandt and refer to the same period ( 1 9 8 2 - 1 9 8 4 ) as the other quantities. The result is shown in Fig 6 c. We like to stress that the calculation of the entrainment transport is one uncertain point in this report. There are large variabilities in the halocline depth and in the salinities as well. Sometimes there is no deep water at all in the Laholm bay, implying zero deep water transport , and it has not been possible for us to cover these variabilities in a sufficient way. In the report mentioned before (Rydberg and Sundberg, 1 9 8 5 ) , we did an alternative approach, where we treated the entrainment transport as an unknown, which was determined by using one more equation than we shall do here. Comments The surface water concentrations of IN and IP in the Kattegat (Figs. 5a, c) follow an annual cycle with maximum values of 8 . 2 uM ( 0 . 7 uM for IP) during January/February, a rapid decrease after the springbloom in March and thereafter very low concentrations
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Fig 4c, d. Annual mean deviation, AC of IP and TP from a spatial mean concentration, C within the Kattegat surface water. This mean value (CIP = 0.236 uM, CTP = =.778 u M ) is determined from the observations at the stations K5 - K9. The results are based on 29 expeditions from Feb 1982 - Nov 1984. The local land based supply of TP is indicated by arrows.
during the summer. There is weak increase in the IP concentrations during the summer, which can be found both in our measurements and in those by the Danish Agency of Environmental Protection during the years 1975-1978 (see also Rydberg and Sundberg, 1984). A rapid increase in IP and IN occurs from October when the primary production decreases. The concentrations of IN are higher within the Laholm bay than outside, especially during the winter months when we also expect the assimilation to be small. The difference indicates the large local supply of IN. During the summer period, the concentrations of IN are the same as in the Kattegat, thus indicating a rapid assimilation of the local supply (in fact, even the stations nearest to the river mouths, which are not shown here,
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Fig 5 a - d. Bimonthly mean concentrations of IN, TN, IP, and TP within the surface water of the Laholm bay (the stations 1 1 - 1 6 with half weight for 1 1 and 1 6 , ) , within the surface water of the Kattegat (the stations K7 and K9, - - - - - 1 , and in the deep water of the Kattegat (the stations K5, K7 and K9, -.-.-. - I .
show very low IN concentrations). The landbased IP supply, on the other hand, is small. Consequently, the corresponding gradients in IP are also small. The importance of the local nutrient supply is easily seen also from the surface water concentrations of TN and TP, shown in Figs. S b and 5 d. There is a horizontal gradient in TN, but in this case as a year round feature, which is exepected as TN is nearly (see discussion) conservative. The gradient in TP, on the other hand, is r e l a t i v e l y weak. The d e c r e a s e i n TN and T P c o n c e n t r a t i o n s during the spring follows the decrease in IN and IP and is of the same order, but goes slower. The phase lag is of theorderof lmonth which seems to indicate that the spring bloom sinks to the bottom within that time. If that interpretation is correct, the phytoplankton settling velocity is of the order of 1 5 m / month. The
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Fig 6a, b. Bimonthly, landbased supply of IN and TN, IP and TP, respectively, to the Laholm bay. The supply follows the freshwater input (compare Fig. 6c), and the calculations are based on monthly observations during the years 1 9 8 2 - 1 9 8 4 .
Fig 6c. Bimonthly supply of fres water, q, (measured) and deep water, qo (calculated, see text) to the Laholm bay surface water.
later increase in both TN and TP concentrations may indicate an ecological succession, where the external nutrient supply is conserved in the surface water by the living organisms. The Figs. 5 a - d also show the deep water nutrient concentrations. These are influenced by the water exchange with the rest of the Kattegat deep water but also by the local biochemical processes. There are no large variations in these components during the year, and except for the increased concentrations of IN after the spring bloom, it is difficult to observe even the slightest coupling to what occurs in the surface water. A comparison between the deep water oxygen concentrations (Fig. 2 ) and the deep water IN and IP concentrations also shows that we should not expect to
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find a simple consumption/production ratio on the time scales studied here. The strong decrease in oxygen concentrations during the summer is actually accompanied by a weak decrease in IN concentrations and nearly constant IP concentrations. The decrease may indicate that denitrification at the bottom is an important sink for nitrogen. Still however, the concentrations of IN and IP are rather high in this area ( 1 1 (1.0for IPIuM), compared to the mean values for Kattegat deep water (6 (0.5 for 1P)uM). The landbased supplies of nutrients are also subject to strong seasonal variability (Figs. 6a, b). These are actually dominated by the variations in the river supply (Fig. 6c), while the concentrations in the rivers are nearly constant throughout the year. The entrainment transport, q o , also shown in Fig. 6c, is almost constant, although the windstress is much larger during the winter. A larger windstress means a rapidly increasing entrainment velocity, but at the same time a deepening halocline and, for the Laholm bay, a decreased halocline area and thus a suppressed entrainment transport. A comparison between the Figs. 5a and 5c indicate a (weak) phosphorus limitation within the bay (but not within the Kattegat) during the early spring, and a clear nitrogen limitation from June/July and towards the autumn. This is in accordance with the size of the surface water nutrient pool before the spring bloom (IN/IP is nearly 16 (by atoms) within the bay, but a large extra supply of IN is added during the bloom; IN/IP is 12 in the Kattegat, also with an extra addition of IN - but relatively smaller). The gradually increasing concentrations of IP (Fig. 5c) during the later part o f the season is due to a decreasing IN/IP ratio in the deep water (more effective P regeneration ? ) which can also be seen from the Figs. 5a and 5c.
A SIMPLE BOX
MODEL
We will now make use of the observations in a simple box model consisting of three water masses; Laholm bay surface water (indexed L), identified by the spatial mean surface water concentrations at the stations 1 1 - 1 6 (with half weight for stns 1 1 and d
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C r N i= 19.20 ( 2 2 36 I phi C r p r = 0.66 ( 1 . 0 3 ) CIN, = 0.36 ( 5 . 1 0 )
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7777Fig 7. A box model for the Laholm bay and the southeastern Kattegat, indicating the observed summer and (winter) mean concentrations and fluxes within and between each of three water masses defined in the text. There are no physical borders involved in this model.
SUP 61 (1091
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F i g 8 . Observed and calculated fluxes and assimilation of IN (g/s) according to the model in fig 7.
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16), Kattegat surface water ( I ) , identified by the mean surface water concentrations at the stations K7 and K9 and Kattegat deep water ( O ) , finally, by the mean concentrations 1 m above the bottom at the stations K5 - K9. The model will be used to determine the exchange of water between the surface waters of the Laholm bay and the Kattegat and the assimilation of nutrients within the bay. The model is shown in Fig. 7, where we have inserted the winter (November - February) and the summer (March - October) mean concentrations of IN, TN, IP, TP and s, for each of the three boxes. The model also indicates the size of the landbased nutrient fluxes (RTN, RIN,. . . . . . ) and the volume fluxes (ql, . . . . ) between the boxes, which are defined as follows;
q1 qf qo
diffusive volume flow between box L) and 1 ) - doubly directed fresh-water supply, advective volume flow from box L) net upward entrainment flow from box 0 ) to box L)
We implicitely assume that the Kattegat boxes are infinite and include an effective horizontal mixing. This implies that the fluxes of properties from the Laholm bay does not influence the properties in the other boxes. We may now write down the following equations for the conservation of TN and IN within the bay; ql(CTNL-CTN1) + qfCTNL - q,(CTNO-CTNL) - RTN gl(CINL-CIN1) + qiCINL - qo(CINO-CINL) - RIN
=
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where CTNL (RTN) is the concentration (landbased supply) of TN within (to) the Laholm bay surface water, and so on. PIN finally is meant to be the planktonic assimilation of IN within the bay, but may as well be a global term, which contains all other sources and sinks for IN . We thus assume that TN is conservative, which is not self-evident. Unfortunately, the outflow of low salinity Baltic water through the Sound causes larger salinity gradients perpendicular to the coast (due to the earth's rotation) than does the local freshwater supply. This implies that there are problems to use the salinity as a tracer. Similar problems occur with the
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phosphorus components. These various difficulties will be brought up in the discussion. By inserting summer and winter mean values, taken from Table 1 , (weighted for the number of observations during each two month period) it is now possible to calculate the assimilation of IN, PIN and the water exchange, q,. The results are seen from Fig. 8.
DISCUSSION During the period from March to October, the supply of IN to the bay is dominated by the transport from land, which is 6 1 gfs, while the supply from the deep water is 2 0 gfs (Fig. 8). The diffusive outflow to the Kattegat is 29 gfs and the net assimilation 5 2 gfs (corresponding to a mean net assimilation of 1 mMfm2 day, assuming an area of the bay of 300 km2. We did not discuss any physical borders for the bay water earlier). The flux of IN to the Kattegat waters, mainly occurs in March and April, when the landbased supply is large, and there is still a gradient between the bay and the Kattegat waters ( s e e Fig. 6 ) . The supply of IN from land is larger during the winter than during the summer, while the deep water supply is approximately similar. The assimilation is expected to be low during these months, however, but it is seen from Fig. 8 that the calculations indicate a surprisingly large net uptake of 2 6 gfs. This seems too high. Contrary, IN (like TN) should behave almost like a conservative substance during these months. If we assume that this is the case, we may use Eq. lb (with the term PIN excluded) to determine the water exchange, q, , during the winter. q, then becomes 5000 m3 f s , which is larger than the value determined from eq. la (3600 m3 f s ) and also marginally larger than the summer exchange, 4 6 0 0 m3 f s . It seems more convenient to have a larger water exchange during the winter, and thus we believe that the value for q,, determined from the TN equation is too small. One reason could be that the observations of TN are too few to give a correct horizontal gradient during the winter; we do have some problems with the observations of TN. Approximately every twentieth value seems wrong (which means too high), and for
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unexplainable reasons we have accepted only values below 4 0 uM. Still however, just very few incorrect values at the stns K7 and K9 may be enough to give a change in q
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of the order 1000 m3 f s
.
On the other hand there are other sources of errors, which will be taken up in the forthcoming discussion. One possibility is that the supply of IN to the surface water of the bay is underestimated, due to a mineralization of some of the externally supplied TN. The mineralization of the landbased organic nitrogen supply is probably a slow process, compared to the water exchange, which occurs within a week. A qualified guess is that no more than 1 0 % of the organic nitrogen supply (approximately 5 0 % of the external TN supply) is mineralized within a week, which implies a contribution of the order < 6 g/s during the summer. The atmospheric supply of IN was also left outside the equations. The term is small but not quite negligible ( < 1 0 g/s IN). The net supply of IN may thus be underestimated with < I 5 g/s (and thus, the net assimilation as well). A net loss of TN from the surface water occurs due to a sinking primary production, but also due to sinking organic matter derived from land. The sedimentation must be most important during the spring bloom, and carpets of dead phytoplankton and organic matter have also been found on the bottom . A maximum TN content of nearly 1000 tons was observed in these areas in April 1 9 8 4 (Hakansson and Floderus, 1 9 8 5 ) . The net uptake of IN during the springbloom is of the order of 500 tons (based on the surface water nutrient pool including a limited external supply during the springbloom period). It does not seem unreasonable to assume that the springbloom, as a whole, represents a loss of TN from the bay waters. Only SO % of the bloom will go to the bottom locally,however, as a result of the low sinkinq rate (a phytoplankton orqanism or cell reachesthe bottomof thebay i n 2 weeks) compared tothe retention time for the surface water ( 1 week). If we put this assumption into eq. la,itrepresentsasinkfor TN with mostly 12 g / s (and a 15 %’ decrease in q 1 1 . This is not much compared to the landbased supply, which is above 100 g / s , but the loss could be a bit higher due to sedimentation of organic nitrogen during the rest of the season. The sedimentation has been the subject of many discussions and observations in the bay, with changing success however.
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In the calculations based on yearly mean values (Rydberg and Sundberg, 1985) we determined an assimilation ratio IN/IP (by atoms) of 14. This result was obtained using an equation for IP, equivalent to the one for IN (eq. Ib). Since then we have added observations from another year, and also made a division into a summer and a winter period. These changes influenced the phosphate gradients in such a way, that if we do the same calculations today, we get a very small net assimilation of only 2 . 8 g/s during the summer. There is also a loss of IP from the local waters during the summer of 2 . 8 g/s, which seems curious. One would expect the supply of phosphate to the surface water to be very effectively assimilated, due to the large supply and the rapid uptake of IN. The IN/IP supply ratio is approximately 30 during the summer and nearly 100 for the landbased supply alone. For the moment, the use of IP (and TP as well) in the budget calculations seems as hazardous as using salinity. The "signal" from land is simply too weak, while the more important supply from the deep water cannot be accurately measured. We may note however, that, it seems reasonable to have substantial contribution to the local waters from mineralization of organic phosphorus from land (the TP supply is approximately 4 g / s , and organic phosphorus seems more readily mineralized than does organic nitrogen). This contribution implies a substantial increase to the assimilation value given above. To summarize, we see that even though we have really put large efforts into observations of the nutrient conditions, the results are still not good enough to do "marine modelling". On the other hand, we have got a good insight in the distribution of the local nutrient supply and its importance relative to the contributions from the "sea". The equations give us a possibility to calculate changes in the nitrogen conditions due to a local decrease in nitrogen supply (which could be done, as the origin of this nitrogen is the heavy farming districts on land). Such calculations are of limited value, however as long as we cannot present a direct coupling to the deep water oxygen consumption, which in turn requires a model based on the monthly variations (at least). It probably also requires that internal processes such as remineralization within the surface, deep and bottom waters (occuring on timescales which are short compared to those consi-
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dered here) and denitrification be considered. Here we have only investigated the external inputs. To manage these coming problems, we have carried out a series of intensive observations during shorter periods where the aim is to observe processes like oxygen consumption, nutrient assimilation and remineralization etc., on the timescales of the processes themselves, i.e. from days to hours.
REFERENCES Aertebjerg, G., Jacobsen, T., Gargas, E. and Buch, E., 1981. The Belt Project. Evaluation of the physical, chemical and biological measurements. The National Agency for Environmental Protection, Copenhagen, Denmark, 122 pp. Fleischer, S . , Rydberg, L., Stibe, L. and Sundberg, J . , 1985. Temporal variations in the nutrient transport to the Laholm bay (in Swedish with english abstract). Vatten, 41:29-35 HAkansson, L. and Floderus, S., 1985. Bottom lenses and nutrient dynamics in the Laholm bay (in Swedish with english abstract). Vatten, 41: 20-28 Miljostyrelsen, 1984. Iltsvind i de danske farvand (with english summary) The National Agency for Environmental Protection, Copenhagen, Denmark. Rosenberg, R., 1985. Eutrophication - the future marine coastal nuisance?. Mar. Poll. Bull., 16(6): 227-231. Rydberg, L. and Sundberg, J . , 1984. On the supply of nutrients to the Kattegat. Rep. no 44, Oceanografiska institutionen, Goteborgs universitet, Box 4038, S-400 40, Gothenburg, Sweden. 17pp Rydberg, L. and Sundberg, J., 1985. External nutrient supply to coastal waters. A comparison between different sources. Rep. Journ. Mar. Res. Inst., Reykjavik, Island (in print). Rydberg, L., 1985. Some observations of nutrient fluxes through the coastal zone. I.C.E.S. C.M. no 62, 1984 ( to appear in Rapp.P.-v. Reun. Cons. int. Explor. Mer.)
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LARVAL SETTLEMENT OF SOFT-SEDIMENT INVERTEBRATES: ANALYSIS UF NEAR-BOTTOM VELOCITY PROFILES*
SOME PREDICTIONS BASED ON AN
CHERYL ANN BUTMAM** Ocean Engineering Department, Woods Hole Oceanographic I n s t i t u t i o n , Woods Hole, Massachusetts
02543 (USA)
MSTKACT During settlement, p l a n k t o n i c l a r v a e may a c t i v e l y s e l e c t h a b i t a t s , they may be p a s s i v e l y deposited onto t h e seabed, o r b o t h processes may apply, b u t f o r d i f f e r e n t s p a t i a l o r temporal scales o r f o r d i f f e r e n t f l o w regimes. Proposing r e a l i s t i c s e t t l e m e n t scenerios i n v o l v i n g b o t h passive d e p o s i t i o n and a c t i v e h a b i t a t s e l e c t i o n can p r o f i t from a p r i o r i analyses o f near-bed f l o w c h a r a c t e r i s t i c s r e l a t i v e t o known aspects o f l a r v a l b i o l o g y (i.e., swim speeds and f a l l v e l o c i t i e s ) . Toward t h i s end, smooth-turbulent v e l o c i t y p r o f i l e s were c a l c u l a t e d f o r everyday t i d a l f l o w s a t a shallow s u b t i d a l study s i t e , where continuous near-bed f l o w measurements were a v a i l a b l e . V e l o c i t y p r o f i l e s were c o n s t r u c t e d f o r a r e a l i s t i c range o f f l o w c o n d i t i o n s . Rough-turbulent f l o w p r o f i l e s a l s o were c a l c u l a t e d , assuming storm waves p e r i o d i c a l l y are s u f f i c i e n t t o resuspend sediments and make a r i p p l e d seabed. Under most f l o w c o n d i t i o n s analyzed, mean f l o w speeds exceed maximum l a r v a l swim speeds, even t o w i t h i n t e n t h s of m i l l i m e t e r s from t h e bed. I n t h e smooth-turbulent flows, l a r v a e g e n e r a l l y would encounter no opposed v e l o c i t y i f they swam v e r t i c a l l y i n the viscous sublayer, t o h e i g h t s of about 0.25-cm above t h e bed. I n r o u g h - t u r b u l e n t flows, eddies r e g u l a r l y penetrate t o w i t h i n t e n t h s o f m i l l i m e t e r s o f the bed, so l a r v a e would experience eddy v e l o c i t i e s w i t h components i n a l l d i r e c t i o n s very c l o s e t o the bed. It i s concluded t h a t , a t l e a s t a t t h i s study s i t e , l a r v a e probably do n o t search f o r p r e f e r r e d h a b i t a t s by h o r i z o n t a l swimning. Larvae may swim v e r t i c a l l y down t o t e s t the s u b s t r a t e and then swim v e r t i c a l l y up t o be advected downstream. However, i t a l s o i s noted t h a t measured l a r v a l swim speeds and f a l l v e l o c i t i e s a r e about the same order-of-magnitude, so a t best, l a r v a e may o n l y be a b l e t o m a i n t a i n p o s i t i o n when swimning v e r t i c a l l y . INTKOOUCTIUN I n temperate l a t i t u d e s , most i n f a u n a l organisms have p l a n k t o n i c l a r v a e t h a t e v e n t u a l l y s e t t l e onto t h e seabed and become benthic a d u l t s .
Larval s e t t l e m e n t
s i t e s may be a c t i v e l y s e l e c t e d by larvae, l a r v a e may be p a s s i v e l y deposited o n t o t h e seabed, o r b o t h processes may operate b u t on d i f f e r e n t temporal o r s p a t i a l scales.
There i s support i n t h e l i t e r a t u r e f o r b o t h a c t i v e s e l e c t i o n and
p a s s i v e deposition;
however, hydrodynamical c o n d i t i o n s i n t h e f i e l d t h a t may
p e r m i t e i t h e r process have n o t been determined.
I n the present study, some
r e a l i s t i c bottom boundary-layer f l o w p r o f i l e s a r e constructed, based on p h y s i c a l measurements from a s p e c i f i c f i e l d study s i t e .
Characteristics o f the f l o w
* C o n t r i b u t i o n number 6046 from Woods Hole Oceanographic I n s t i t u t i o n **Previously p u b l i s h e d as Cheryl Ann Hannan
488
very c l o s e t o (i.e.,
5 m i l l i m e t e r s o f ) t h e seabed a r e analyzed r e l a t i v e t o
p e r t i n e n t dspeccs o f l a r v a l b i o l o y y (e.y., velocities).
measured swim speeds and f a l l
Based on these r e s u l t s , i n s i g h t can be gained r e g a r d i n g f l o w
p r o f i l e s t h d t would p e r m i t a c t i v e s e l e c t i o n and f l o w p r o f i l e s where l a r v a e would oe ddvected and d e p o s i t e d l i k e passive p a r t i c l e s .
I n a d d i t i o n , r e s u l t s o f the
nedr-bed f l o w d n a l y s i s i n d i c a t e how t h e l a r v a e may a c t u a l l y move between habitats i n the f i e l d ,
thereby s u y g e s t i n g reasonable s e l e c t i o n mechanisms f o r f u t u r e study.
Aceive h a b i t a t s e l e c t i o n by
d
v a r i e t y o f soft-sediment invertebrate larvae
and meiofauna has been demonstrated a t very small s p a t i a l s c a l e s ( m i l l i m e t e r s t o c e n t i m e t e r s ) i n s t i l l -water l a b o r a t o r y experiments (e.g., and Campbell, 1972; Scheltema, 1974; Strathmann,
1978).
see reviews by Meadows Active selection also
i s s t r o n g l y suggested from r e s u l t s o f f i e l d experiments (e.g., v l i l l i a i n s , 1980; Gallagher e t al., of c e n t i m e t e r s t o 2U meters).
O l i v e r , 1979;
1983) conducted a t l a r g e r s p a t i a l scales (tens
Experiments performed i n c o n t r o l l e d l a b o r a t o r y
f l o w reyimes t h a t mimic s p e c i f i c f i e l d environments a r e r e q u i r e d , however, t o detendine hydrodynamic c o n d i t i o n s t h a t would p e r m i t a c t i v e s e l e c t i o n i n t h e f i e l d and t o s p e c i f y t h e s p a t i a l s c a l e s i n v o l v e d . s p e c i f i c rnechanisms whereby l a r v a e p e r c e i v e i n f o r m a t i o n about a v a i l a b l e h d D i t d t s and then s e l e c t a p a r t i c u l d r l o c a t i o n f o r s e t t l e m e n t a r e p o o r l y unuersl;oud
dnd a r e p r i m d r i l y s p e c u l a t i v e f o r s o f t - s u b s t r a t e i n v e r t e b r a t e s ( b u t
see L r i s p ' s [1974] and B u r k e ' s [1983] reviews o f t h e h a r d - s u b s t r a t e l i t e r a t u r e on t h i s t o p i c ) .
Ubservations o f some l a r v a l species d u r i n g s e t t l e m e n t i n s t i l l
water i n d i c a t e t h a t t h e organisms must c o n t a c t a s u r f a c e t o p e r c e i v e a s p e c i f i c cue (e.j.,
Milson, 1968; Caldwell, 1972; Cameron and Hinegardner,
1974;
tickelbdrger, 1977) and, more r e c e n t l y , Suer and P h i l l i p s (1983) demonstrated t h a t t h e ChelliCdl f a c t o r promoting metamorphosis o f t h e i r s o f t - s u b s t r a t e study organisin was e f f e c t i v e o n l y i f i t was absorbed o n t o a s o l i d surface. " t d c t i l e chemical sense,''
Thus, the
c o i n e d by C r i s p and Meadows (1963) t o d e s c r i b e t h e
process o f chemoreception i n b a r n a c l e c y p r i d s , a l s o may a p p l y t o t h e s e t t l e m e n t o f spft-substrate larvae. p o t e n t i a l h a b i t a t s (i.e.,
I n f o r m a t i o n on t h e way l a r v a e may move between by swimming, hopping, c r a w l i n g , o r by b e i n g p a s s i v e l y
d i s t r i b u t e d ) d u r i n g s e l e c t i o n i n moving f l u i d i s scant, b e i n g l i m i t e d t o some e d r l y o b s e r v a t i o n s o f s e t t l i n g polychaete 1 arvae ( Whi t l e g g e , 1890, c i t e d i n Irrdy, 1Y74; d i l s o n , 1948, 1958; b u t see t h e q u a n t i t a t i v e work on b a r n a c l e c y p r i d s by Crisp, 1955; C r i s p and Meadows, 1962). U n t i l r e c e n t l y , o n l y a handful o f researchers ( i n c l u d i n g P r a t t , 1953; Baygerman, 1953; Fager, 1964; Moore, 1975; T y l e r and Banner, 1977) considered pdssive d e p o s i t i o n o f l a r v a e as a r e a l i s t i c a l t e r n a t i v e hypothesis t o a c t i v e selection.
I n r e c e n t experiments on t h e r o l e o f p h y s i c a l processes i n s i n k i n g ,
s e t t l e i m i t and r e c r u i t m e n t o f 1 a r v a l i n f a u n a o r meiofauna, hydrodynamic nu1 1
489 hypotheses g e n e r a l l y c o u l d n o t be r e j e c t e d . fluid-dyndiiiicdl
These s t u d i e s showed t h a t , from
c o n s i d e r a t i o n s , i t i s p o s s i b l e t o account f o r p a t t e r n s o f
c e r t a i n organism d i s t r i b u t i o n s by passive accumulation (Eckman, 1979, 1983; Hojue and i 4 i l l e r , 1981). passive s i n k i n g (Hannan, 1984a, b ) and passive resuspension and t r a n s p o r t (Palmer and Gust, 1985; b u t see a l s o Grant, 1981). The r e s u l t s s t i p u l a t e t h a t near-bed flow processes must be added t o t h e l i s t o f p o t e n t i a l f a c t o r s c o n t r o l 1 i n g t h e p o p u l a t i o n dynamics of soft-sediment organisms. A c t i v e h d b i t a t s e l e c t i o n and passive d e p o s i t i o n need n o t be m u t u a l l y e x c l u s i ve a1 t e r n a t i ve hypotheses t o account f o r p a t t e r n s o f 1a r v a l s e t t l e m e n t . f o r exdmple, hydrodynaliiicdl processes may s o r t and d i s t r i b u t e l a r v a e over r e l a t i v e l y l a r g e areas (meters t o tens of k i l o m e t e r s ) of t h e seabed, j u s t as sediilients arc! s o r t e d and d i s t r i b u t e d .
Then, once l a r v a e have been i n i t i a l l y
d e p o s i t e d i n a p a r t i c u l d r sedimentary environment, they may r e d i s t r i b u t e a t s l n a l l e r s p d t i a l scales ( m i l l i m e t e r s t o c e n t i m e t e r s ) by a c t i v e l y choosing a prdfdrred microhabitat.
A v a r i e t y of o t h e r scenerios a r e p o s s i b l e where passive
d e p o s i t i o n dnd dCtiVe s e l e c t i o n operate a t d i f f e r e n t s p a t i a l o r temporal scales. Lonsiderable i n s i g h t i n t o t h e p l a u s i b i l i t y of each scenerio can be oordineu itirouyh dn a n a l y s i s of v e l o c i t y p r o f i l e s t h a t are l i k e l y t o occur c l o s e t o t h e seaoed i n h a b i t a t s where l a r v a e s e t t l e . The mean f l o w speed a t a given n e i y h t above t h e oed sets, f o r example, t h e r e q u i r e d swim speed f o r a l a r v a t o e f f e c t i v e l y maneuver i n a p l a n e p a r a l l e l t o t h e mean f l o w and a l s o s e t s t h e n o r i z o n r d l distdncc!
d
l a r v d would be advected i f o n l y passive s i n k i n g occurred.
I n t h e p r e s e n t study, near-bed v e l o c i t y p r o f i l e s are c a l c u l a t e d f o r a s p e c i f i c sofr-sedinient environment, where experiments w i t h s e t t l i n g l a r v a e have oeen conducted s i n c e 1980 (see Hannan, 1984a, b). flows
dC
S u f f i c i e n t data on near-bed
t h i s s i t e arc! a v a i l a b l e t o p e r m i t p r o f i l e c a l c u l a t i o n s f o r a r e a l i s t i c
ranye o f f l o w c o n d i t i o n s .
The r e s u l t i n g p r o f i l e s are c o n s t r a i n e d by t h e
dssulliptioiis u n d e r l y i n g t h e c a l c u l a t i o n s (see PROFILE CALCULATIONS AND RESULTS), and thus, they may o r may n o t commonly occur a t t h e study s i t e . p r o f i l e s showti here a r 2 meant o n l y t o be i l l u s t r a t i v e .
However, t h e
They r e p r e s e n t a f i r s t
attempt a t g a i n i n g q u a n t i t a t i v e i n s i g h t r e g a r d i n g t h e o r d e r - o f m a g n i t u d e o f f l o w speeds p o t e n t i a l l y encountered by a l a r v a as i t y e t s c l o s e r and c l o s e r t o t h e seabed.
I n a d d i t i o n , these p r o f i l e s can be modeled i n a l a b o r a t o r y flume,
a1 lowing experinlental t e s t s o f t h e hypotheses yenerated from t h i s study. STUUY S I T E AND FLOW MEASUREMENTS
Study s i t e d e s c r i p t i o n and surface c i r c u l a t i o n The f i e l d study s i t e ( F i y . 11, S t a t i o n 35 (from Sanders e t al., l o c d t e d i n Buzzards Bay, Massachusetts (USA) i n 15 m o f water. priinarily
drc!
1980), i s
Bottom sediments
medium sand (251J-50Uvm), p e r i o d i c a l l y o v e r l a i n w i t h a mud veneer.
490
71.W
F i g . 1. Map of Southeastern Massachusetts (from Sanders e t al., 1980) showing l o c d t i o n of duzrards day, on t h e western border o f Cape Cod. The l o c a t i o n o f S t a t i o n 35 i s i n d i c a t e d by an a s t e r i s k . m i u e r s et
dl.
(1981))c h a r a c t e r i z e d t h e sediments as "moderately w e l l t o p o o r l y
sorted,'' based on monthly samples o f t h e top 4-cm o f sediment f o r one year; sedimri'c composition was 0.5-6.7
percent gravel ( > 2 mm), 59-90 p e r c e n t sand
( 0 3 ~ u r- L mm), and 10-37 p e r c e n t mud ( s i l t
+
c l a y , < 63 urn) d u r i n g t h i s time.
Previous d e s c r i p t i o n s o f the surface c i r c u l a t i o n o f Buzzards B a y have presuiiied t h d t c u r r e n t s wsre p r i m a r i l y t i d a l (e.g.,
Redfield, 19531, b u t u n t i l
r e c e n t l y (see below), few f l o w ilieasurements were made.
Because t h e main a x i s o f
t h e bdy i s o r i e n t e d northeast/southwest (see Fig. 11, t i d a l c u r r e n t s g e n e r a l l y dre o r i e n t e d d l o i i j t h i s a x i s .
I n some areas o f t h e bay, however, t h e r e i s a
s l i y h t tendency f o r a counterclockwise gyre i n t h e surface c i r c u l a t i o n of dur.zards day.
Surfdce t i d a l c u r r e n t s g e n e r a l l y are weak, r a r e l y exceeding
50 cm/sec, and a r e s l i y h t l y s t r o n g e r and of l o n g e r d u r a t i o n d u r i n g t h e f l o o d tndn w r i n g t h e eDb t i d e . J u r i n j t h e sumrner, when l a r v a e are s e t t l i n g , t h e p r e v a i l i n g winds are from t h e southwest as a r e s u l t o f t h e Bermuda high-pressure system l y i n g t o t h e southeast o f Cape Cod.
Winds are s t r o n g e s t i n t h e afternoon, when l o c a l
491
seaweezes auyment t h e p r e v a i l i n g southwesterly winds. fro111 t h e southwest experience t h e l o n g e s t fetch, can reacn h e i y h t s o f 1-1.2 m i n 2-3 hrs.
A t S t a t i o n 35, winds
so l o c a l seas a t t h e study s i t e
However, under these non-storm
c o n d i t i o n s , l o c d l l y yenerated wind waves i n t h e bay a r e f e t c h - l i m i t e d t o and r d r e l y p e n e t r a t e t o t h e bottom a t t h e study s i t e .
BUZZARDS BAY Sta 35
ii,
EAST-WEST
10
- I
-10;
5-10
-20 -25
4 sec
The e n t i r e bay g e n e r a l l y
?:j -15
-
c 1-15
23
24
25
26
27
1:
JUL 1982
25 20 15
NORTH-SOUTH
1
15
10
10
5
5
0
0
-5
-5
2 -10
-10;
-15
-15
-20 -25
2 1: 23
24
25
26
27
JUL 1982 2300
-
2250
I
2300
PRESSURE
2250
I2200
2200
5 21 50
. 2150 5
;2100
.2100
W
2050
2050 2000
4?3
I
I
24
I
25
I
26
27
c 2000
2 g
JUL 1982
f i y . 2. P l o t s o f t h e east-west and north-south components t o t h e near-bottom c u r r e n t s ( 0 . 5 4 above t h e bed) and near-bottom pressure a t S t a t i o n 35 d u r i n g a l a r v d e experiment (see Hannan, 1984a, b ) from 7/23/82 through 7/27/82. The values p l o t t e d a r e e d i t e d one-hour averages d u r i n g t h e i n t e r v a l .
492
i s vdrtically s t r d t i f i e d d u r i n g the summer (Rosenfeld e t a l . , 1984) due t o surfdce heating. L3ecause of variations in bottom topography, r e l a t i v e l y cold wdter can p e r s i s t a t depth i n the south and southwestern portions of the bay; t h i s cruss-bay temperature gradient may r e s u l t i n weak density-driven flows d u r i n g the summer (M.D.
Grant, personal communication).
Idedr-bottolii f 1ow iJeasurements Ouring larvde experirnents by the author i n the surfliner of 1982 (see Hannan, 1 ~ d 4 a ,D ) , Ur. Bradford Butman (U.S. Geological Survey, Woods Hole) deployed a buttoill-iiioorzd tripod instrument system t o continuously measure near-bottom flows. The tripod system (described in Butman and Folger, 1979) has instruments f o r iliedsuring current speed dnd direction, pressure, l i g h t transmission
BUZZARDS BAY Sta 35 $22
22
a
+
ia 23
JUL
24
25
26
27
+
1982 25 20 15 10
-
ri
5a v, '0
;.:
;:;
pvEv
> 1.0 0.0 v, a 23
:
1.0 >
0.0
24
25
26
27
a
JUL
1982
Fig. 3. Plots of near-bottom water temperate ("Cl, current speed. (0.5-m above t h e Ded) and pressure standdrd deviation ("PSDEV") a t Station 35 d u r i n g a larvae experiment (see Hannan, 1984a, b ) from 7/23/82 t h r o u g h 7/27/82. The values plucted dre edited one-hour averayes d u r i n g the interval.
493 INTERVAL 9 / 2 0 / B 2
(5 M Y S )
INTERVAL 7/27/82 (4 DAYS)
30W
g25W
0
8 20-
LL
a a 3 0
B
00
15-
z W 3
B lo=
25
LL [L
i
4
M
16
8
24
INTERVAL 9/21/82 (I DAY)
5-
OOc INTERVAL 9/15/82 ( I DAY)
a
5 1
0
r
4
F
1- I I
I 8
12
16
20
24
INTERVAL 9/22/82 ( I DAY)
1
L 16
20
24
FLOW SPEED (cm/sec) I.0-meters ABOVE THE BOTTOM
1 1 12 16 20 FLOW SPEED (crn/sec) 1.0-meters 8
24
ABOVE THE BOTTOM
Fig. 4. Flow speed-frequency histograms f o r currents measured 1.0-m above the bottorli a t Station 35 during f i v e different intervals when larvae experiments were conducted (see Hannan, 1984a, b ) . Intervals are i d e n t i f i e d on the graphs oy the date t h a t they ended; interval duration, i n days, also i s shown. Average values f o r the edited "burst" measurements are plotted f o r a l l intervals except 7/27/82, where measurements taken a t the midpoint of the 3.75min intervals are plotted.
494 and tenperature, and a l s o i s equipped w i t h a camera t h a t takes bottom ptiotoyraphs.
Savonius r o t o r s f o r measuriny c u r r e n t speed are l o c a t e d 0.5- and
1.-ni above t h e seabed; small vanes a r e mounted below each r o t o r f o r sensing current direction.
Currents and pressure were sampled i n two ways (see Butman
and F o l y r r , 1979); an average measurement was made over a 3.75-min i n t e r v a l and a " o u r s t " o f ineasureinents were taken i n t h e middle o f t h i s i n t e r v a l ( 2 4 b u r s t measurements were taken a t 2-sec i n t e r v a l s ) .
The c u r r e n t speed and pressure
ilirasurerilents r e p o r t e d here u s u a l l y are from t h e 3.75-min currenL d i r e c t i o n s are from t h e b u r s t samples.
averages and t h e
L i g h t transmission and
ternperdture were sampled o n l y a t t h e m i d p o i n t o f each 3 . 7 5 4 1 1 i n t e r v a l .
Bottom
photoyraphs w2re taken every hour. The nedr-bed f l o w ilaasurements i n d i c a t e t h a t bottom f l o w s a t S t a t i o n 35 are p r i m d r i l y t i d a l l y driven (Fiy. 2).
The semidiurnal p e r i o d i c i t y t y p i c a l of t i d e s
a t t h i s l a t i t u d e can be seen i n t h e pressure record.
As w i t h t h e surface
c u r r e n t s , t h e f l o w s a r e o r i e n t e d p r i m a r i l y n o r t h - s o u t h and t h e r e i s l i t t l e f l o w east-west, i n d i c d t i n j t h a t t h e t i d a l f l o w s t r a v e r s e approximately t h e l o n g a x i s o f tluzzards 8ay (see F i y . 11, a t l e a s t near the c o a s t where f l o w s a r e p o l a r i z e d by he shore.
dear-bottom c u r r e n t speed o s c i 11ates between approximately a minimum and indximuiii v a l u e t w i c e d a i l y ( F i y . 31, as expected f o r these t i d a l l y d r i v e n flows. However, Decduse o t h e r p h y s i c a l phenomena (e.g., d e n s i t y - d r i v e n and wind-driven c u r r e n t s ) a l s o c o n t r i b u t e t o t h e flows, c u r r e n t speeds do n o t always go t o z e w dnu t h e curves are n o t smooth.
P e r i o d i c a l l y , s u r f a c e storm a c t i v i t y was
d e t e c t e d i n t h e near-bottom f l o w s a t S t a t i o n 35 (e.y.,
see peak i n PSDEV on
7/Ls/dL i n F i y . 3 ) ; such s t r o n y s u r f a c e winds cause t h e r e g u l a r l y o s c i l l a t i n g t i d a l flows t o d e v i a t e s u b s t a n t i a l l y .
Near-bottom water temperature v a r i e d
l i t c l e un t h e short-term,
b u t y r a d u a l l y cooled about 5 ° C between 7/27/82 and
9/22/84 (Hannan, 1984b).
Flow speed 1.O-m
d i,idXiillUrli
above t h e bed v a r i e d between z e r o and
of 22 cm/sec d u r i n g t h e summer and e a r l y f a l l o f 1982 ( F i g . 4);
however, u s u a l l y o n l y a maximum o f 16 cm/sec was reached. b i ~ l t ) ( H LUtbCKIPTIUI4
UF BUUI4DAKY-LAYEK FLOWS OVER SOFT SEDIMENTS
As water f l o w s over t h e seabed, a r e g i o n o f shear ( t h e slope o f t h e v e l o c i t y p r o f i l e , du/dz, where
u
= t h e h o r i z o n t a l v e l o c i t y component and z =
the
p e r p e n d i c u l a r d i s t a n c e from t h e surface; r e f e r t o Fig. 5) develops as a r e s u l t o f t h e r e t d r d i n y e f f e c t o f t h e boundary on t h e f l o w .
This r e g i o n o f shear near
t h e bed i s r e f e r r e d t o as a "boundary l a y e r " . The mean v e l o c i t y p r o f i l e i s c o n s t r d i n e d by c o n d i t i o n s a t each end o f t h e boundary l a y e r : u = 0 a t z = 0 ( t h e " n o - s l i p " c o n d i t i o n a t t h e boundary) and u = U ( t h e free-stream v e l o c i t y a t z = 6 ) ( t h e bounddry-layer t h i c k n e s s ) (see F i g . 5). The shape o f t h e v e l o c i t y
495
Z
U U
Fig. 5. Diayralr o f a t u r b u l e n t bottom boundary l a y e r p l o t t e d on a l i n e a r scale f o r OOLII axes, showing t h e r e l a t i v e p o s i t i o n s of t h e viscous sublayer, t h e l o g ldy?r, and t h e l o y - d e f i c i t l a y e r . p r o f i l e v d r i e s depending on f l o w p r o p e r t i e s (e.y.,
t h e f l o w Reynolds number, t h e
bdckjround turbulence, and a c c e l e r a t i o n s ) , f l u i d p r o p e r t i e s (e.9. induced by tewperdturr, c n a r a c t 2 r i s t i c s (e.g.,
, stratification
s a l i n i t y , and suspended sediments) and boundary t h 2 bed roughness and t h e cohesiveness of sediments).
The bounddry-] ayer t h i c k n e s s depends on t h e boundary shear s t r e s s and i n v e r s e l y on t h e f o r c i n y frequency f o r t h e flow, velocity (&/p density),
K
, where
7
KU*/U,
where u* i s t h e bottom shear
i s t h e bottom shear s t r e s s and p i s t h e f l u i d
i s von I(annan's c o n s t a n t o f 0.4,
and u i s 2n/P (where P i s t h e
p e r i o d of t h e f l o w ) . deCdU$e l a r v a l s e t t l e m e n t takes place i n s i d e t h e bottom boundary l a y e r , i t i s i n s t r u c t 1 ve t o b r i e f l y review r e l e v a n t c h a r a c t e r i s t i c s o f boundary l a y e r s t h a t
m y fond o v e r s o f t sediments under simple, steady-flow c o n d i t i o n s (see a l s o t h e r e c e n t d i s c u s s i o n by IJowell and Jumars, 1984). V e l o c i t y p r o f i l e s which may occur a t t h e stucly s i t e then are c a l c u l a t e d (see PROFILE CALCULATIONS AND RESULTS), based on 0 0 t h f i e l d data and assumptions about p r o f i l e c h a r a c t e r i s t i c s
.
The
f o l l o w i n g d i s c u s s i o n i s somewhat i d e a l i z e d , f o r t h e sake o f posing l o g i c a l p r e d i c t i o n s concerning t h e r o l e of hydrodynamical processes i n l a r v a l s e t t l e m n t ; f o r t h i s modest y o a l , t h e i d e a l i z a t i o n does n o t s i g n i f i c a n t l y a f f e c t For r e c e n t reviews o f t h e s t a t e - o f - t h e - a r t i n t n e outcollie o f t h i s study. geopqysicdl oounddry-layer flows, see dowell (1983) and Grant and Madsen (1986).
496
General c h a r a c t e r i s t i c s o f boundary l a y e r s T h i s d i s c u s s i o n considers steady, u n i f o r m f l o w o v e r a bottom which i s u n i f o n over l a r g e h o r i z o n t a l distances, r e l a t i v e t o the h e i g h t o f f the bed.
I n theory,
t h e bottom boundary l a y e r may be l a m i n a r o r t u r b u l e n t , depending on t h e r e l a t i v e importance o f viscous versus i n e r t i a l f o r c e s i n the flow, as c h a r a c t e r i z e d by t h e f l o w Reynolds number, Ref = LU/V (where L = t h e c h a r a c t e r i s t i c l e n g t h scale f o r t h e f l o w , U = t h e c h a r a c t e r i s t i c reference v e l o c i t y o f the flow, and v = t h e kinematic v i s c o s i t y o f the f l u i d ) .
Laminar
boundary l a y e r s occur a t low Ref where t u r b u l e n t f l u c t u a t i o n s are r e l a t i v e l y unimportant.
Laminar boundary l a y e r s have pronounced stream-wise s t a b i l i t y ; any
d i s t u r b a n c e t o t h e l a y e r w i l l be q u i c k l y d i s s i p a t e d by v i s c o s i t y downstream, r e s t o r i n g the p r o f i l e t o t h e predisturbance case.
Thus, o n l y h o r i z o n t a l
v e l o c i t i e s a r e present i n l a m i n a r boundary l a y e r s i n steady, u n i f o r m flows. Here, v e l o c i t i e s have b o t h a mean
Turbulent boundary l a y e r s occur a t h i g h Ref.
and a f l u c t u a t i n g component; f l u c t u a t i o n s a r e due t o t u r b u l e n t eddies, which can have v e l o c i t y components i n a l l d i r e c t i o n s .
T r a n s f e r o f mass and momentum w i t h i n
t h e l a y e r occurs due t o products o f coherent v e l o c i t y f l u c t u a t i o n s associated w i t h these eddies. t h e bed.
Near t h e bottom, the e n e r g e t i c eddies s c a l e w i t h h e i g h t above
The t u r b u l e n c e i s produced by t h e product o f v e r t i c a l shear and
Reynolds s t r e s s due t o t h e presence o f the boundary. The Ref i s a good p r e d i c t o r o f l a m i n a r o r t u r b u l e n t boundary l a y e r s f o r f l o w s over smooth f l a t p l a t e s , b u t o t h e r f a c t o r s are important t o t h i s p r e d i c t i o n i n ocean f l o w s t r a v e l i n g over sediments o r bumpy seabeds. may be generated i n the f l o w by a source away from the bed (e.g.,
Turbulence
wave breaking)
o r t u r b u l e n c e may be " t r i p p e d " a t t h e seabed by a r e l a t i v e l y l a r g e f l o w disturbance on the bottom. F o r t u r b u l e n t flows, t h e roughness Reynolds number, Re,
= u,kb/v
(where kb = t h e hydrodynamic bed roughness scale), i s a
b e t t e r p r e d i c t o r o f bottom boundary l a y e r c h a r a c t e r i s t i c s .
However, turbulence
i s such a p e r v a s i v e f e a t u r e o f ocean f l o w s t h a t even i f l o c a l Re* a r e i n t h e l a m i n a r range, t h e f l o w s o f t e n a r e t u r b u l e n t (see Yaglom, 1979). I n essence, l a m i n a r boundary l a y e r s are r a r e i n the ocean. T u r b u l e n t boundary l a y e r s Turbulent f l o w s a r e c l a s s i f i e d as smooth, rough, o r t r a n s i t i o n a l (e.g., S c h l i c h t i n g , 19791, depending on Re* o f t h e f l o w . o f the boundary, viscous f o r c e s dominate the flow.
see
I n t h e imnediate v i c i n i t y A pronounced viscous sublayer
(see Fig. 5) may develop i n t h e case o f f l o w o v e r hydrodynamically smooth bottoms (e.g.,
see Eckelmann, 1974) o c c u r r i n g a t r e l a t i v e l y l o w Re.,
viscous sublayer has c h a r a c t e r i s t i c s o f l a m i n a r boundary l a y e r s .
The Over
hydrodynamically rough bottoms, v i s c o s i t y s t i l l a c t s a t t h e boundary, b u t no
497
d i s t i n c t well-behaved sublayer forms conparable t o t h e smooth case and eddies may p e n e t r a t e t o w i t h i n t e n t h s o f m i l l i m e t e r s o f t h e bed; thus, i n r o u g h - t u r b u l e n t f l o w t h e v e l o c i t y s t r u c t u r e c l o s e t o t h e bed i s complicated (e.g.,
see Yaglom, 1979) and n o t well-known.
F o r i n t e r m e d i a t e Re,
t r a n s i t i o n a l f l o w occurs, w i t h c h a r a c t e r i s t i c s i n t e r m e d i a t e between smooth- and F o r pipes, f l o w s are shown t o be smooth-turbulent f o r Re,
rough-turbulent.
and r o u g h - t u r b u l e n t f o r Re,
< 5
> 70 (see S c h l i c h t i n g , 1979); f o r open-channel o r
geophysical flows, these values may be more l i k e 3.5 and 100, r e s p e c t i v e l y (e.g.,
see review o f Nowell and Jumars, 1984).
Based on e m p i r i c a l s t u d i e s and s c a l i n g arguments (see Clauser, 19561, t u r b u l e n t boundary l a y e r s i n t h e l a b o r a t o r y can be d i v i d e d i n t o t h r e e regions Adjacent t o the boundary, i n the viscous s u b l a y e r ( f o r
( r e f e r t o F i g . 5).
smooth-turbulent f l o w s ) , v e l o c i t y v a r i e s l i n e a r l y w i t h d i s t a n c e from t h e boundary according t o u/u* = region.
U*Z/V,
t h e s c a l i n g parameters f o r t h i s f l o w
The o u t e r r e g i o n o f f l o w i s c a l l e d the l o g - d e f i c i t l a y e r because the
d e f i c i t velocity,
i s l o g a r i t h m i c a l l y r e l a t e d t o z/a.
(u-U)/u,,
Between these
two l a y e r s (and o v e r l a p p i n g w i t h the l o w e r p o r t i o n o f the l o g - d e f i c i t l a y e r ) i s t h e l o g l a y e r , a major f e a t u r e o f steady, u n i f o r m flows.
The v e l o c i t y p r o f i l e
i n t h e l o g l a y e r i s described by:
(where B = t h e e m p i r i c a l l y d e f i n e d constant o f i n t e g r a t i o n ) .
The v e l o c i t y s c a l e
o f eddies (i.e., t h e root-mean-square o f t h e v e l o c i t y f l u c t u a t i o n s ) i n t h e l o g l a y e r i s about 10 percent o f the free-stream v e l o c i t y , U (see Hinze, 1975). F o r smooth-turbulent f l o w s , t h e shape o f t h e v e l o c i t y p r o f i l e i n t h e l o g l a y e r depends on u* and depends on ,u,
v
U.
F o r f u l l y rough-turbulent flows, t h e v e l o c i t y p r o f i l e
and bed geometry.
From e m p i r i c a l s t u d i e s o f smooth-turbulent
p i p e f l o w s (see S c h l i c h t i n g , 19791, the l o w e r l i m i t o f the l o g l a y e r i s approximated by 11.6u/u, 5.Uv/u,.
and t h e upper l i m i t o f t h e viscous sublayer by
Between these heights, t h e r e i s a complicated wake l a y e r t h a t cannot
be described simply.
I n channel f l o w s and geophysical boundary l a y e r s , the wake
region may be l a r g e r (see reviews o f Nowell, 1983; Grant and Madsen, 1986). Ocean bottom boundary l a y e r s T y p i c a l oceanic bottom boundary l a y e r s vary between smooth-turbulent and f u l l y rough-turbulent.
F o r example, t h e d e t a i l e d v e l o c i t y p r o f i l e s measured i n
a l a b o r a t o r y flume by Grant e t a l . (1982) over an area o f u n i f o r m i n t e r t i d a l sands taken from B a r n s t a b l e Harbor, Massachusetts, t y p i f i e d a c l a s s i c a l smooth-turbulent boundaty l a y e r .
Other examples i n c l u d e the p r o f i l e s measured
i n t h e l a b o r a t o r y f l m e s t u d i e s o f Nowell and Church (19791, Nowell e t a l . (19811,
498
Lckman e t al. (19811, Eckman (1983) and see also the review of Jumars and Nowell (lSb4j. In the ocean, smooth-turbulent profiles were measured by Chriss and Calawell (19&2), transitional by Grant e t al. (19851, and rough-turbulent profiles by Lross ana bowel1 (19831, Grant e t a l . (1984); many other examples e x i s t . Note t h a t , a t a given study s i t e , a flow can be smooth-turbulent under one tlow condition ana rough-turbulent under another condition, f o r example, due t o chanyes i n zo (a parameterization of the bed roughness length scale, k b ) o r i n other sediment properties caused by bioturbation, sediment transport or beufonli aevelopment (see Grant and kadsen, 1979, 1982). PRUFILt CALCULATIONS At& KESULTS Profiles of current speed w i t h i n the log layer a t a s i t e can be calculated, g i v e n the following assunptions. (1) There i s quasi-steady, uniform, neutrally s t r d t i f i e d tlow over the bed. ( 2 ) The bed i s uniform over large horizontal uistances, relative t o the height above the bed of the calculated velocities. ( 5 ) bottom rouyhness is small, compared t o the boundary-layer thickness. In aaaition, informdtion must be available on velocities occurring a t some height above the beu within the log layer and on bottom roughness characteristics. These assunptions periodically are met a t Station 35; for example, d u r i n g flood o r ebb t i u e when near-bed flows are only t i d a l l y driven and there are no compl i c a t i ons f rom w i nd-dri ven ci rcul a t i on, density -dri ven ci rcul a t i on o r surfdw WdVeS. Thus, the profiles calculated here accurately represent nearbeu flow conditions only a certain percentage of the time. The rest of the t i n e , the velocity profiles resulting from unsteady o r non-uniform flows are imposea on the steady-flow case (e.y., the log layer p r o f i l e ) , so composite profiles of flows t h a t would be measured over the bed a r e d i f f i c u l t t o predict ( f o r a discussion of these features, see Grant and Madsen, 1986). Some of these cotliplicatea bounaary-layer flows have been modeled (e.g., Smith and McLean, 1977; (Irant and Madsen, 1979, 19821, b u t such calculations are not necessary for the ti rst-oraer approach of this paper (see GENERAL DESCRIPTION OF BOUNDARYLAYEK FLUWb..
.).
Smooth-turbul e n t profiles hinoth-turbblent velocity profiles were calculated f o r everyday flow conditions a t the study site. Smooth-turbulent p r o f i l e s were indicated by the estisiatea ranye i n he* [see Table 1) f o r the range of measured near-bed flows ( F i y . 4), by the observed seabed roughness and because of the preliminary results of detailed velocity measurements near the bed, made by Dr. William D . Grant (khuIj. Current speed and direction were measured over a.6-hr period, d u r i n y non-stom conuitions a t Station 35 i n October 1982, using four vertically
499
TABLE 1 Parameter values f o r v e l o c i t y p r o f i l e s shown i n Figs. 6, 7 and 8. a
*
u50
U
(cm/sec)
(cm/sec)
CDb (x
zO
(cmx
(0.1) (6)
Re:
1
(cm)
Smooth-Turbulent A. B.
C.
15.3 9.8 4.6
60 0.40 0.20
15.3 15.3
0.98 0.82
1.53 1.66 1.89
1.8 2.8 5.6
1.2 0.8 0.3
165 111 55
100 33
2 94 74
270 226
Rough-Turbul e n t D.
E.
4.14 2.90
at150 = u a t z = 50 cm. bFor z = 50 cm. cFor smooth-turbulent flow, kb = 200 um and f o r rough-turbulent flow, kb = ( % ) ( Z o ) . stacked acoustic-time-travel
c u r r e n t meters (described i n Grant e t a1
., 1984)
mounted a t distances o f approximately 30-, 50-, 100- and 200cm above the bed. From these d i r e c t f l o w measurements, i f t h e v e l o c i t y p r o f i l e i s logarithmic, i t i s possible t o estimate u* from equation (1) using t h e p r o f i l e technique (Grant e t al., 19841, since U*/K i s given by t h e slope o f t h e v e l o c i t y p r o f i l e . Thus, i t i s possible t o c a l c u l a t e Re* t o determine ifflows are smooth-turbulent, rough-turbulent, o r t r a n s i t i o n a l . The p r e l i m i n a r y r e s u l t s i n d i c a t e t h a t , during non-storm conditions, t h e f l o w i s smooth-turbulent t o t r a n s i t i o n a l (W.D. Grant, personal communication). For smooth-turbulent l a b o r a t o r y pipe flows, e m p i r i c a l r e s u l t s show t h a t t h e general l o g - l a y e r equation given i n ( 1 ) has t h e s p e c i f i c form of:
(see Schlichting, 1979); note t h a t t h e constant d i f f e r s s l i g h t l y f o r channel flows and geophysical flows. To c a l c u l a t e a p r o f i l e from t h i s r e l a t i o n s h i p r e q u i r e s estimates o f V, u ( z ) measured i n s i d e t h e l o g layer, and u*; also, some i t e r a t i o n i s necessary.
2
For a l l c a l c u l a t i o n s , v = 0.01 cm /sec was used.
To choose u ( z ) r e q u i r e s an estimate o f t h e thickness o f t h e l o g l a y e r .
This
thickness can be approximated by (0.10)(6) (Clauser, 1956; Grant and Madsen, 19861, where 6 = t h e boundary-layer thickness. For a t i d a l flow, 6 = KU*/U,
12 hr, i n where u = t h e t i d a l frequency (2r/P, where P i s t h e t i d a l p e r i o d o f t h i s c a s e ) . For l o g - l a y e r thicknesses estimated here (see Table 11, v e l o c i t i e s measured a t 0.5m above t h e bed w i l l always be i n t h e l o g l a y e r ; f o r slower flows a t t h e s i t e , measurements a t z = 1.0 m may be above t h e l o g l a y e r . To be
500
conservative, uSL) (i.e.y
u a t z = 50 cm) was used i n c a l c u l a t i o n s here.
d i f f e r e n c e i n mean v e l o c i t i e s measured a t z = 0.5 m and
z
The
= 1.0 m was
c o n s i s t e n t l y between 1 and 2 cm/sec (6. Butman, personal comnunication), and f o r z = SU ciii, t h e range o f v e l o c i t i e s (4.6 t o 15.3 cm/sec, see Table 1)
r.nus,
used t o c a l c u l a t e p r o f i l e s here seems reasonable based on t h e f l o w measurements dT;
z
= 1.U showri i n F i g . 4.
The choice o f values f o r u* needed t o c a l c u l a t e smooth-turbulent p r o f i l e s wds c u n s t r d i n e d by t h e requirement t h a t CD, t h e bottom drag c o e f f i c i e n t ( 6" = U , / U ~ ) ~i r u s t be about 1 x 1 t o~ Z~X ~ O -( ~t y p i c a l values measured f o r siilouth-turbulent f l o w s ) .
Smie i t e r a t i o n was r e q u i r e d t o o b t a i n t h e values
F i g . 6. Smooth-turbulent v e l o c i t y p r o f i l e s on a semi-log p l o t , c a l c u l a t e d f o r a rdnge o f near-bottom f l o w speeds measured a t S t a t i o n 35. Parameter values are l i s t e d i n Table 1. The l o g l a y e r i s t h e s t r a i g h t - l i n e p o r t i o n of each curve. delow t h i s , t h e dashed curves i n d i c a t e approximately t h e r e g i o n o f t h e wake l a y e r , where v e l o c i t i e s a r e d i f f i c u l t t o estimate (see GENERAL DESCRIPTION OF dUUIJUAKY-LAYkK FLUIJS 1.
...
501
l i s t e d i n Table 1. To determine Ke* f o r the profiles, k b = 200 I m was used f o r the smothturbulent case (see Table 1). This value was chosen because, while sediments a t the study s i t e are primarily sands (250-500 pin, see STUDY SITE AND FLOW ~~lt~bCldtiw~dTb), surface sediments are heavily pelletized by the dominant infaunal organism, i4edioinastus ambiseta (a small polychaete worm). This organism occurs in dDuiiddnces of up t o 2x10 5 per square meter (Sanders e t a1 1980); i t feeds below the sediment surface and deposits discrete cylindrical fecal p e l l e t s ( ” eru pm x 2 W pm) on the sediment surface. Note, however, t h a t k b as large as 830 pm s t i l l would r e s u l t i n Re, < 5.0 f o r even the l a r g e s t value o f u* l i s t e d in Table 1 f o r smooth-turbulent flows. Sriiooth-turbulent velocity profiles are shown in Fig. 6 f o r z between 0.001 and 1U cm. Also shown by a horizontal dashed l i n e on the figure, i s the approximate s i z e (3UU urn) of a s e t t l i n g polychaete larva; however, i n temperate l a t i t u d e s , s e t t l i n y larvae can vary i n s i z e by approximately an order-ofindjniude (from 1UU t o 1001) pin). delow t h i s height, a 300 pm larva would n o t have room t o maneuver i n a flow by horizontal swimming. I t would be s i t t i n g on tne bottom o r crawling dlonj the bed and, a t most, the flows would cause i t t o roll. For d more detailed look a t velocities very near the bed (i.e., a t
.,
l.ok 1
PI
-
Maximum Swim Speed of Polychaete Larvae
E
s
‘ 1
I
A
Approximate Size
f
of Settling I-orvae
I
I I I I
0.1
0.2
0.3
0.4
0.6 0.7 0.8 0.9 HORIZONTAL FLOW SPEED (cm/sec)
0.5
1 0 .
1.1
1.2
Fig. 7. Srtiooth-turbulent velocity profiles in the viscous sublayer on a semi-log p l o t , calculated f o r a range of near-bottom flow speeds measured a t Stdrion 35. Parameter values are l i s t e d in Table 1.
13 .
502
distances r e l e v a n t t o s e t t l i n g l a r v a e ) , t h e same smooth-turbulent p r o f i l e s (see Taole 11 are p l o t t e d f o r z between 0.001 and 0.2 cm i n Fig. 7. I n addition t o t h e approximate s i z e o f s e t t l i n y l a r v a e ( h o r i z o n t a l dashed 1 ine), t h e maximum ioedsured swim speed o f a polychaete l a r v a ( f r o m t h e review o f Chia e t a1 1984)
.,
i s shown as a v e r t i c a l dashed l i n e on t h e figure. Flow speeds t o t h e r i g h t o f t h i s l i n e would advect larvae; larvae m a y e f f e c t i v e l y maneuver by h o r i z o n t a l swirnminy i n flows t o t h e l e f t of t h i s l i n e . Thus, l a r v a e would be expected t o e f f e c t i v e l y rmneuver by h o r i z o n t a l swimning o n l y f o r flows o c c u r r i n g i n t h e upper l e f t - h a n d quadrant o f t h e f i g u r e . The r e s u l t s i n d i c a t e t h a t l a r v a e can h o r i z o n t a l l y swim o n l y i n t h e slowest p r o f i l e p l o t t e d (see C i n F i g . 7) and o n l y t o a h e i g h t o f about O.l-cm above the m o v e t h i s h e i y h t i n p r o f i l e C and f o r v e l o c i t i e s a t z > 300 pm i n p r o f i l e s A and 6, l a r v a e e s s e n t i a l l y would be advected by t h e flow. The v e l o c i t i e s p l o t t e d i n Fig. 7 a l l l i e w i t h i n t h e viscous sublayer (see F i g . 61,
bed.
and tnus, mean flow components occur o n l y i n t h e stream-wise ( h o r i z o n t a l ) direccion.
vlhile they are being advected h o r i z o n t a l l y , l a r v a e c o u l d s t i l l swim v e r t i c a l l y t o h e i y h t s of a t l e a s t 0.1-m above t h e bed and face no opposed velocity.
However, even i n smooth-turbulent flows, t h e viscous sublayer
p d r i o d i c a l l y i s s u b j e c t t o t u r b u l e n t eddy p e n e t r a t i o n so v e r t i c a l v e l o c i t i e s , o f - t h e - o r d e r ( d . l ) ( U ) , c o u l d be present from time-to-time. Kouyh-turbulent p r o f i l e s In d u d i t i o n t o the snlooth-turbulent case f o r everyday flows, i t i s possible t o c o n s t r u c t rough-turbulent p r o f i l e s a t t h e s i t e f o r c o n d i t i o n s f o l l o w i n g a It i s observed a t I l i d J O r s t o m w i t h s u f f i c i e n t bottom s t r e s s t o move sediments. t h e s i t e t h a t storm winds o r i e n t e d down t h e l o n g a x i s o f t h e bay (see Fig. 1) jenerdte s u f f i c i e n t bottom s t r e s s t o cause r i p p l e s t o form on t h e seabed. A f t e r t h e storm, t h e r i p p l e s p e r s i s t u n t i l they are o b l i t e r a t e d by b e n t h i c b i o l o g i c a l processes. Because r i p p l e s s e t a much l a r g e r bottom roughness scale than g r a i n roughness o r f e c a l p e l l e t roughness, rough-turbulent f l o w s can r e s u l t f o r the sdiiie r d n j e o f everyday f o r c i n y c o n d i t i o n s t h a t produced smooth-turbulent p r o f i l e s f o r t h e non-rippled bed. Two rough-turbulent p r o f i l e s were c a l c u l a t e d here (see Table 11, using h, o f 0.5 and 0.15 cm. cases, a r i p p l e steepness ( h / l , where 1 = t h e distance between r i p p l e c r e s t s ) of U.2 was used. This corresponds t o t h e maximum r i p p l e steepness = 15.3 cm/sec and two d i f f e r e n t r i p p l e heights,
In
Dotti
ooserved under waves (see Grant and Madsen, 1982).
For a r i p p l e d bed, t h e
bottoio roughness parameter, zo, can be estimated by zo = h ( h / l ) (Grant e t al., 19841, so zo = U.1 respectively
.
cm and 0.03 cm for the 0.5-cm and'0.15-cm tall ripples,
503
F o r rough-turbulent flow, e m p i r i c a l r e s u l t s show t h a t t h e general l o g - l a y e r q u d t i o n y i v e n i n (1) has t h e form:
Again, u* can be c a l c u l a t e d once zo and a reference value f o r u ( z ) are Known. F o r the same u ( z ) used i n t h e smooth-turbulent case, CD i s expected t o CD > Z X ~ O - (see ~ Table 1). oe h i g h a r f o r r o u g h - t u r b u l e n t flows, i.e., The two rough-turbulent p r o f i l e s are p l o t t e d , along w i t h a smooth-turbulent p r o f i l e , f o r t h e same us,
0.OJ' 0
'
'
2
i n Fig. 8.
'
The slopes o f t h e curves f o r t h e l o g
' ' ' ' ' I ' ' ' 4 6 8 10 12 HORIZONTAL FLOW SPEED (crn/sec)
'
14
'
'
16
F i g . 8. Two rough-turbulent v e l o c i t y p r o f i l e s and a smooth-turbulent p r o f i l e , a l l haviny t h e same u u, b u t d i f f e r e n t values o f u* and z (see Table 1). H l o g l a y e r i s known a c c u r a t e l y describe a rough-turbupent p r o f i l e a t disrdnces > (lUU)(z ) f r o m t h e bed, and i s a reasonable p r e d i c t o r f o r distances oecueen (lO)(zo) an! (li)O)(z 1. Thus, on t h e f i g u r e , t h e rough-turbulent p r o f i l e curves are dashed beyow (lO)(zo), i n d i c a t i n g t h a t v e l o c i t i e s i n t h i s r e y i o n may be described by some o t h e r wake f u n c t i o n . The dashed p o r t i o n of t h e smooth-turbulent p r o f i l e represents t h e wake l a y e r (see Fig. 6 ) .
50
504
l a y e r d r e srdidller i n t h e rough-turbulent cases, than i n t h e smooth-turbulent Thus, a t
CdSe.
d
given h e i y h t above t h e bed (below z = 50 cm), v e l o c i t i e s are
lower i n t h e r o u g h - t u r b u l e n t flows.
This simply r e f l e c t s t h e f a c t t h a t , i n
r o u q h - t u r b u l e n t flows, eddies c l o s e t o t h e bed are m i x i n g low-momentum f l u i d near t h e beu w i t h higher-momentum f l u i d away from t h e bed so t h a t near-bed mean v e l o c i t i e s are lower, r e l a t i v e t o t h e smooth-turbulent case. The r o u g h - t u r b u l e n t p r o f i l e s i n t e r c e p t t h e o r d i n a t e a t zo and, i n these cases, zo i s g r e a t e r than o r equal t o t h e approximate s i z e o f s e t t l i n g l a r v a e
( s e e Fig. 8 ) .
Thus, i t appears t h a t i n r o u g h - t u r b u l e n t f l o w s l a r v a e may have a
l o t o f v e r t i c a l d i s t a n c e t o maneuver by h o r i z o n t a l swimming b e f o r e f l o w speeds redLh
d
v d l u e t h a t the oryanisms cannot swim a g a i n s t .
However, i t i s i m p o r t a n t
t o r e a l i z e t h d t zo i s a roughness parameter, r e f l e c t i n g where t h e flow e t t e c t i v e l y yoes t o zero.
The boundary l a y e r a c t u a l l y can a t t a c h anywhere
Detween t h e trough and t h e c r e s t o f t h e r i p p l e s ; i n f a c t , i n t e r n a l boundary l a y e r s w i t h a i f f e r e n t p r o f i l e c h a r a c t e r i s t i c s form i n t h i s complicated f l o w region c l o s e t o t h e seabed (see c a p t i o n t o Fig. 8).
Depending on where t h e
l d r v a i s s i t u a t e d r e l a t i v e t o t h e roughness elements, t h e animal c o u l d experience r e l a t i v e l y h i g h o r low v e l o c i t i e s .
For example, very low f l o w s
y e n e r d l l y woula b e expected i n the l e e o f a r i p p l e c r e s t i n a steady flow,
but
euuies d l s o can be shea from these c r e s t s . tven though, f o r a yiven z, mean h o r i z o n t a l v e l o c i t i e s are l o w e r i n t h e r o u g h - t u r b u l e n t flows than i n t h e smooth-turbulent f l o w p l o t t e d i n Fig. 8, r u u i e s r e g u l a r l y redch t o w i t h i n t e n t h s o f m i l l i m e t e r s o f t h e seabed i n r o u g h - t u r b u l e n t flows (see LENEhAL DESCRIPTION OF BOUKDARY-LAYER FLOWS
...1.
As
p r e v i u u s l y ilirtitioned, t h e most e n e r y e t i c eddies i n t h e f l o w have v e l o c i t i e s o f about lC, percent o f t h e free-stream v e l o c i t y , U.
15.5
CIII/S~C
(at z =
SU
chi)
For example, i f U was
f o r t h e f l o w s i n Fig. 8, then eddy v e l o c i t i e s are a
fiiaxirduni o t 1.5 cm/sec a t t h i s l e v e l above t h e bed, e a s i l y exceeding values r e q l r i r e d t o p r o h i b i t e f f e c t i v e s w i m i n g by a l a r v a i n 3 d i r e c t i o n (see k i y . 6); eday v e l o c i t i e s c l o s e r t o t h e bed are s m a l l e r and l e s s energetic. I n surmary, unaer most f l o w c o n d i t i o n s analyzed here, h o r i z o n t a l f l o w speeds exceed maximum l a r v a l swim speeas even t o w i t h i n one body diameter o f t h e op,anisai
troni the seabed.
I f l a r v a e a c t i v e l y maneuver i n a flow, then v e r t i c a l
swimminy t o y e t i n t o h i g h e r o r lower h o r i z o n t a l f l o w s seems most l i k e l y .
These
k i n d s o t behdviors o f t e n have been proposed f o r p l a n k t o n i c organisms i n t h e water column, f o r example, t o account f o r v e r t i c a l m i g r a t i o n s o f copepods (see reviews by Lonyhurst, 1976; Pearre, 1975; and a l s o t h e r e c e n t c o l l e c t i o n of papers i n Angel and O'Brien,
1564). M i l e i k o v s k y (1973) a l s o proposed t h a t
" h i gti" v e r t i c a l skirii speeds o f soft-sediment i n v e r t e b r a t e 1arvae m a y account f o r t h e i r r e t e n t i o n i n near-shore and e s t u a r i n e waters;
retention of, especially,
505
crustacean and bivalve larvae i n estuaries by active vertical movements of the organisriis has a buryeoning l i t e r a t u r e (e.y., see symposium on this subject i n Kennedy, 19d2). However, previous t o the present study, the relative effectiveness of horizontal versus vertical swimiing f o r organisms in flows very close t o the seabed has never been investigated quantitatively. Ucher cdlculations As with orhrr p a r t i c l e s , larvae have mass so they always are sinking through tne water a t a speed specific t o t h e i r s i z e , shape and density. Fall velocities of dnesthetized polychaete 1 arvae wen? measured d i r e c t l y by the author (see
Hannan 1984a, b ) and span about an order-ofinagnitude,
from 0.01 t o 0.3 cm/sec,
-" t
t
u
s
0.1 -
w
> -I -I
z
l9 O
Y
0
A A
0.01 0.01
1
1
1
1
1
I
I
1
1
0.I
Fiy. 9, Helationship between f a l l velocity and s i z e f o r polychaete larvae tested in the study of Hannan (1984a, b ) . The length of the organisms, a f t e r tney were anesthetized, i s plotted ayainst t h e i r measured f a l l velocity. Fall velocities w2re measured in two different s e t t l i n g chambers f o r two different groups of larvde, indicated by the closed versus the open symbols. The different syirbol s represent different anesthetizing treatments, b u t there w5re iio siynificdnt differences in f a l l velocity t h a t could be attributed t o treatiiierit (see Hannan, 1984b). The crosses represent mean f a l l velocity (f.S D ) dild irean lengtti (*SU) f o r the two groups of organism tested. Details of the methods d r e yiven in Hannan (1984b).
506
roughly i n c r e a s i n g w i t h i n c r e a s i n g body s i z e ( F i g . 9).
It i s i n t e r e s t i n g t h a t
i t i i s rdnge o v e r l a p s t h e range o f measured swim speeds f o r polychaete l a r v a e (u.05 t o U.52 crn/sec,
see review o f Chia e t al.,
1984).
Thus, even when l a r v a e
ar2 s w i I t i i n j v e r t i c d l l y i t i s p o s s i b l e t h a t they are capable o n l y o f standing still! The p r e v i o u s analyses have focused on how v e r y near-bottom f l o w v e l o c i t i e s mdy l i m i t o r a l l o w a c t i v e l a r v a l movements near t h e bed.
IJ~K a r t h e o t h e r extreme.
It a l s o i s f r u i t f u l t o
Assuming t h a t l a r v a e o n l y s i n k toward t h e bed l i k e
passive p a r t i c l e s (see Hannan, 1984a, b), I have c a l c u l a t e d t h e h o r i z o n t a l disrdnce they woula b e c a r r i e d by s p e c i f i c f l o w s b e f o r e reaching t h e bottom, y i v e n v a r i o u s s t a r t i n g h e i g h t s above t h e bed (see Fig. 10).
These r e s u l t s are
u s r t u l , f o r exdsiple, i n p r e d i c t i n y distances between h a b i t a t s where l a r v a e would oe a b l e t o t e s t t h e substrate.
Once deposited, i f l a r v a e are c a r r i e d t o a
c e r t d i n d i s t d n c e m o v e the bed (i.e.,
by resuspension o r by v e r t i c a l swimning),
then p r o f i l e ChdrdCteriStiCS determine t h e h o r i z o n t a l advection d i s t a n c e (i.e.,
were rtie n e x t t e s t l o c a t i o n would b e ) from t h a t h e i g h t .
F o r t h e flows
/I
zm t
30 20
m
a
rn
F
I
0
(3
m
W
I
5:: n
a
3 F
(L
0.01 0.0 I
0
x .." VECTED (cm) WHILE FALLING A T RATE OF o.Iocm/sec
Fig. 111. H o r i z o n t a l distances t h a t p a s s i v e l y s i n k i n g l a r v a e would be advected y i v r r i vdriocrs s t d r t i n j h e i y h t s above t h e bed. Larval f a l l v e l o c i t i e s were taken as U . l cm/sec (see F i g . 9) and h o r i z o n t a l v e l o c i t i e s f o r v a r i o u s s c d r i i n g h e i g h t s above the bed w e E taken from smooth-turbulent p r o f i l e s A and C i n Fig. 6.
507
considered, l a r v a e may be c a r r i e d from t e n t h s o f m i l l i m e t e r s t o meters, f o r s t d r r i n j h d i y h t s up t o 3 cm ( o r about 100 body diameters o f a 300 urn l a r v a ) aDove t h e bed.
Ift h i s wzre t h e mechanism o f h a b i t a t s e l e c t i o n by l a r v a e , then
tiiese s p d t i a l scales apply. UI>UJSSION ANU CONCLUSIONS
t l i o l o y i s t s have ooserved and q u a n t i f i e d s w i m i n g i n p l a n k t o n i c organisms f o r over n a l f
d
c e n t u r y (see review o f Chia e t al.,
N i t h t h e e x c e p t i o n of
1984).
v e r c i c d l i l i i y r d t i o n s , i t i s u s u a l l y assumed t h a t t h e organisms have l i t t l e c o n t r o l o f t h e i r p o s i t i o n i n t h e water column through swimming, s i n c e h o r i z o n t a l I n fact, flow speeds y r e a t l y exceed t h e i r swim speeds (Mileikovsky, 1973). cechnically t h i s d i s t i n q u i s h e s between plankton, " t h e d r i f t e r s " , and nekton,
"rne s w i i i m r s " (see tiardy, 1965).
Likewise, l a r v a l d i s p e r s a l i n t h e p l a n k t o n
u s u a l l y i s assumed t o be p h y s i c a l l y c o n t r o l l e d (e.g., doicourr, 1982; Levin, 1983).
see Scheltema,
1971;
However, u n t i l r e c e n t l y , s e t t l e m e n t o f l a r v a e
onto t h e seabed was assumed t o be b i o l o y i c a l l y c o n t r o l l e d , through a c t i v e nduicdt s e l e c t i o n by the animals (see reviews c i t e d i n INTHODUCTION).
An
underlying assumption t o t h i s t e n e t i s t h a t t h e organisms can e x e r t some c o n t r o l wdr
t h e i r p o s i t i o n c l o s e t o t h e seabed, i n o r d e r t o s e l e c t h a b i t a t s .
precise mechanisms i n v o l v e d (e.y.,
The
h o r i z o n t a l o r v e r t i c a l swimming, hopping o r
c r d w l i n j J have never been c l e a r , b u t f o r an organism t o choose a h a b i t a t , i t seems necessdry f o r t h e animal t o be able t o peruse t h e a v a i l a b l e s i t e s ( b u t see
also r h e " t h r e s h o l d s t i m u l u s " hypothesis o f Doyle, 1975, 1976). It seems reasonable t o expect t h a t t h e r e w i l l be some l i m i t i n g h e i g h t above cne sedoed, below wnich f l o w speeds would be low enough t o a l l o w organisms t o Ilianeuver e f f e c t i v e l y .
This f o l l o w s from t h e " n o - s l i p c o n d i t i o n " i n f l u i d The r a t e a t which
dyiiditiics; f l o w s p e d must yo t o zero a t t h e boundary. v e l o c i t y decreases w i t h d i s t a n c e from t h e bed (i.e.,
t h e shape o f t h e v e l o c i t y
p r v f i l e l i l e t e m i n e s t h i s " l i m i t i n y maneuvering h e i g h t " .
I f l a r v a e are choosing
n a o i t d t s by swiliiminy dround near t h e bed, i t should be p o s s i b l e t o c o n s t r a i n the I i ~ f l i t i n iiidiieuvering j h e i y h t , f o r a given mean-stream flow.
This was a goal o f
the present study. The snlooth-turbulent v e l o c i t y p r o f i l e s c o n s t r u c t e d here f o r everyday f l o w conditions a t S t a t i o n 35 i n Buzzards Bay, Massachusetts (see Fig. 1) i n d i c a t e cndc
only the slowest f l o w iiodeled ( p r o f i l e C i n Figs. 6 and 7) would p e n i t
e f f e c t i v e h o r i z o n t a l swimming by l a r v a e near t h e bed. liiiiititij
I n t h i s case, t h e
rliarieuveriny l i e i y h t i s about 0.1 cm ( o r about t h r e e body diameters o f a
NU-vm l a r v a ) , f o r a maximum swim speed o f 0.5 cm/sec.
This f l o w p r o f i l e was
consrructed f o r a measured f l o w speed o f 4.6 cm/sec a t z = 50 cm.
During f i v e
larvde experiments i n t h e summer and e a r l y f a l l o f 1982, f l o w s < 6.0 cm/sec a t
508
z = 1UU cili o c c u r r e d from 24.4 t o 56.8 (mean = 43.2)
p e r c e n t of t h e t i m e (see
F i 3 . 41.
These a r e t h e flows f o r which l a r v a e would b e expected t o e f f e c t i v e l y
mneuver,
dt
z = lulr
crli
l e a s t t o h e i g h t s o f 0.1-cm
above t h e bed, because f l o w s measured a t
were 1 t o 2 cei/sec f a s t e r than those measured a t z = 50 cm (B.
butman, personal coanuni c a t i o n ) . These r e s u l t s subyest t h a t d u r i n y about 4U p e r c e n t o f t h e t i d a l c y c l e a t t h i s s i t e , i t i s p h y s i c a l l y r e a l i s t i c f o r l a r v a e t o swim around n e a r t h e bed, exploriny available habitats f o r settlement.
However, f o r about h a l f o f these
1-lows ( t h o s e between 4 and 6 cm/sec,
see Fig. 41, l a r v a l maneuvering would be
contiriea t o u i s t a n c e s of o n l y U.1-cm
( o r t h r e e body diameters o f a 300-pm l a r v a )
above t h e bed.
Ubviously, t h e l a r y e r t h e s e t t l i n g l a r v a , t h e s m a l l e r t h e
rlianeuveriny tieibht.
As mentioned e a r l i e r (see PKUFILE CALCULATIONS AND
ktblrLTS), l d r v d e would encounter no opposed v e l o c i t y i f they swam v e r t i c a l l y t o heibtits o f
dbVUt
U.25 cni above t h e bed, w i t h i n t h e viscous sublayer, i n a l l o f
t h e siliooth-turbulent p r o f i l e s p l o t t e d (see F i g . 6 ) . w i t h v e l o c i t i e s on-the-oruer-of
However, t u r b u l e n t eddies,
1 ciii/sec o r l e s s and w i t h components i n a l l
u i r e c t i o n s , d r e expected above t h e sublayer and p e r i o d i c a l l y even i n s i d e t h e subloyet-, ilidkiny l a r v a l nianeuveriny i n
any d i r e c t i o n d i f f i c u l t .
Kouyh-turbulent flows d r e expected, a p r i o r i , t o have r e l a t i v e l y l o w e r mean v e l o c i t i e s c l o s e t o the bed than siiiooth-turbulent f l o w s w i t h t h e same meanstream v e l o c i t y , due t o t u r b u l e n t m i x i n y n e a r t h e bed. t h e LdSeS iliuoeleu
here ( s e e Fig. 8).
T h i s was demonstrated i n
The advantages i n c u r r e d by l o w e r mean
v e l o c i t i e s n e d r t h e bed may n o t outweigh t h e disadvantages o f increased eddy p e r i e t r d t i o n t o w i t h i n l d r v a l -body diameters o f t h e bed, however, s i n c e l a r v a e woulu coristaritlq experience f l u c t u d t i n g eddy v e l o c i t i e s i n a l l f l o w d i r e c t i o n s . bri-the-cittier-hdnd,
1 arvde rliay f i n d some r e f u y e i n t h e microtopography i n
s l o w - f l o w regioris b e h i n d f l o w obstacles (e.y.,
see Eckman 1979, 1983).
The p r o f i l e s c a l c u l a t e d here a r e f o r quasi-steady ( i .e.,
c u r r e n t ) boundary
l a y e r s , w i t h o u t c o n s i d e r a t i o n o f p o s s i b l e e f f e c t s o f s u r f a c e waves.
A1 though
r a v e - g i n e r d t e d v e l o c i t i e s do n o t reach t h e bottom a t t h e study s i t e discussed tiere, wave e f f e c t s on t h e bed a r e p r e v a l e n t i n many common c o a s t a l h a b i t a t s
where l a r v a e s e t t l e . l a y e r s can form,
Where wave e f f e c t s extend t o t h e seabed, wave boundary The combined e f f e c t s
i n a d d i t i o n t o c u r r e n t boundary l a y e r s .
o f wave anu c u r r e n t boundary l a y e r s on near-bed v e l o c i t y p r o f i l e s and sediment t r a n s p o r t d r e uiscussed i n Grant and Madsen (1979,
1982).
I n general, wave
bounudry l a y e r s are t h i n n e r than c u r r e n t boundary l a y e r s and h i g h e r s t r e s s e s occur c l o s e r t o t h e seabed i n t h e wave boundary l a y e r .
These h i g h e r near-bed
v e l o c i t i e s have obvious i n i p l i c a t i o n s t o l a r v a l s e t t l e m e n t . A c o n c l u s i o n o f t h i s study i s t h a t , a t l e a s t a t t h e study s i t e modeled,
l a r v d e probably do n o t search t o r p r e f e r r e d h a b i t a t s by a c t i v e h o r i z o n t a l
509
swimning near t h e bed, s i n c e t h e b u l k o f t h e f l o w s modeled would n o t p e r m i t such searches even f o r maximum measured l a r v a l swim speeds.
The l a r v a e may swim
v e r t i c a l l y i n smooth-turbulent flows, going down t o t e s t t h e s u b s t r a t e and up t o be advected t o another s i t e downstream.
It i s c u r i o u s t h a t measured swim speeds
and f a l l v e l o c i t i e s o f polychaete l a r v a e a r e t h e same order-of-magnitude, suggesting t h a t l a r v a e may only be a b l e t o m a i n t a i n p o s i t i o n i n the water column while swimning up; measurements of swim speeds and f a l l v e l o c i t i e s f o r t h e same i n d i v i d u a l are r e q u i r e d t o t e s t t h i s hypothesis. An e s t i m a t e was made here of advection distances between s u b s t r a t e t e s t s by a l a r v a t h a t used, f o r example, t h e " b a l l o o n i s t technique" ( c o i n e d by P.A. Jumars, personal comnunication) where an organism swims o r i s l i f t e d up o f f t h e bottom, i s advected w i t h the f l o w and then p a s s i v e l y s i n k s t o a new s i t e downstream.
Using t h i s technique,
l a r v a e c o u l d t e s t s u b s t r a t e s separated by
scales o f m i l l i m e t e r s t o meters, depending on t h e i r s t a r t i n g h e i g h t above the bed (see Fig. 10). This f i r s t attempt t o determine near-bed f l o w v e l o c i t i e s r e l a t i v e t o aspects
of l a r v a l s e t t l e m e n t b i o l o g y has suggested some r e a l i s t i c flow-regime dependent settlement mechanisms m e r i t i n g f u r t h e r study. The c a l c u l a t i o n s were n e c e s s a r i l y idealized. i n sane cases, b u t t h e i d e a l i z a t i o n s do n o t s i g n i f i c a n t l y a f f e c t t h e outcome o f the study. The modeled f i e l d p r o f i l e s represent f i r s t - o r d e r - t y p e solutions f o r t h e purposes o f hypothesis development; d i r e c t measurements a r e needed t o t e s t the ideas presented here. The analyses suggest marly important areas f o r f u t u r e research on l a r v a l b i o l o g y ( i . e . , directions,
q u a n t i f y i n g swim speeds and
f a l l v e l o c i t i e s , and e x c u r s i o n h e i g h t s above t & bed d u r i n g
searches) and on l a r v a l ecology d u r i n g s e t t l e m e n t ( i . e .
, quantifying
habitat
selection f o r a r e a l i s t i c range o f f i e l d f l o w s modeled i n a l a b o r a t o r y flume). ACKNOWLEDGEMENTS
I thank B. Butman f o r making the p h y s i c a l measurements i n the f i e l d and P. Shoukimas f o r processing these data. I am very g r a t e f u l f o r t h e i n v a l u a b l e t u t o r i n g and advice given t o me by W.D.
Grant on boundary-layer flows.
G. McManamin s k i l l f u l l y typed t h e manuscript on very s h o r t n o t i c e , f o r which I
should be shot. The w r i t i n g was supported by a g r a n t from the PEW Menorial Trust and NSF OCE-85000875; ongoing f i e l d s t u d i e s on aspects o f near-bottom flows, sediment t r a n s p o r t and b e n t h i c b i o l o g y i n Buzzards Bay a r e funded by the
W.H.O.I.
Sea Grant Program (R/P-21) NOAA Contract No. NA84AA-D-00033 and t h e
Coastal Research Center a t W.H.O.I.
510
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TURBIDITY AND COHESIVE SEDIMENT DYNAMICS
Emnanuel Partheniades, Professor Department o f Engineering Sciences, U n i v e r s i t y o f F l o r i d a , Gainesville, F l o r i d a , U.S.A.
ABSTRACT T h i s review paper summarizes the most important recent advances on cohesive sediment f l o c c u l a t i o n , deposition and resuspension and ways o f a p p l i c a t i o n i n t u r b i d i t y c o n t r o l . Equations f o r t h e degree and r a t e s o f deposition and resuspension are presented together w i t h a simp1 i f i e d model o f cohesive sediment-flow i n t e r a c t i o n which explains the observed phenomena. Factors a f f e c t i n g deposition and resuspens i o n are also discussed. INTRODUCTION T u r b i d i t y i n natural water bodies amounts t o a p a r t i a l o r complete blockage o f l i g h t transmission w i t h serious environmental impacts t h e most d i r e c t and important o f which i s the i n t e r f e r e n c e w i t h the photosynthetic process o f plankton product i o n . I n coastal and estuarine waters t u r b i d i t y i s caused p r i m a r i l y by f i n e sediment ( s i l t and c l a y ) i n suspension. These sediments may a l s o cause severe p o l l u t i o n o f the benthic l a y e r s and marine feeding grounds by adsorption o f dissolved p o l l u tants, such as heavy minerals, on t h e surfaces o f suspended p a r t i c l e s and subsequent f l o c c u l a t i o n and deposition. The muddy layer, thus formed, may c o n t a i n a p o l l u t a n t concentration much higher than t h a t i m the water. I n a d d i t i o n t o the environmental p o l \ u t i o n , f i n e sediments c o n s t i t u t e the primary source o f shoaling i n e s t u a r i n e waterways and c e r t a i n harbors as a r e s u l t o f which expensive dredging maintenance i s required. The annual c o s t o f such maintenance i n t h e United States only approaches t h e h a l f - a - b i l l i o n d o l l a r mark (Partheniades, 1979). Fine sediments i n e s t u a r i e s may be o f marine o r a l l u v i a l o r i g i n o r may be introduced during dredging and f i l l i n g operations.
They c o n s t i t u t e t h e main bed material i n drowned r i v e r valley-type
e s t u a r i e s formed a f t e r the i c e m e l t (Pritchard, 1967). The need f o r a r a t i o n a l c o n t r o l o f shoaling and t u r b i d i t y motivated extensive research during the l a s t t h i r t y years on t h e fundamental and a p p l i e d aspects o f cohesive sediment dynamics as w e l l as on the engineering and rheologic p r o p e r t i e s o f estuarine sediments.
The to-date acquired knowledge, although n o t y e t complete,
c o n t r i b u t e d t o a substancial understanding o f t h e processes o f f l o c c u l a t i o n , transport, deposition and resuspension o f f i n e cohesive sediments.
This knowledge can
516
already be incorporated i n mathematical o r h y b r i d models f o r the p r e d i c t i o n o f shoaling zones and shoaling rates.
O f p a r t i c u l a r i n t e r e s t t o the p r e d i c t i o n and
c o n t r o l o f t u r b i d i t y are the i n i t i a t i o n , degree and r a t e s o f deposition o f suspended sediment and the i n i t i a t i o n , degree and r a t e s o f resuspension o f deposited cohesive sediment beds. discussed.
The most recent advances i n these areas are herewith summarized and
THE PROCESS OF FLOCCULATION
F i n e sediments are composed o f p a r t i c l e s small enough and o f s p e c i f i c area high enough f o r the surface physico-chemical forces t o become dominant.
The nature of
these forces and the f a c t o r s a f f e c t i n g them have been summarized and discussed elsewhere (Corps o f Engineers 1960, Partheniades 1962, 1965, 1973, 1979, and Partheniades and Paaswell 1970).
I n d i v i d u a l p a r t i c l e s have a t h i n p l a t e o r needle
shape w i t h dimensions ranging from a f r a c t i o n o f one micron t o a few microns.
When
dispersed i n water the f i n e r p a r t i c l e s , s p e c i f i c a l l y t h e ones w i t h an average diameter o f l e s s than 2 microns, can be maintained i n d e f i n i t e l y i n supension by the Brownian motion o f water p a r t i c l e s , w h i l e even a r e l a t i v e l y very small degree o f a g i t a t i o n i s s u f f i c i e n t t o keep i n suspension the l a r g e r range o f the e n t i r e f i n e sediment population. A homogeneous dispersion o f p a r t i c l e s small enough t h a t no measurable deposition takes place w i t h i n a l o n g time p e r i o d i s defined by van Olphen (Van Olphen, 1963) as " c o l o i d a l s o l u t i o n " o r llsolll; otherwise i t i s defined as a "suspension". Most comnonly, however, both cases are r e f e r r e d t o as " c l a y suspensions"
.
Modern e l e c t r o n microscopy and X-ray diffraction techniques have revealed t h a t c l a y s are composed e s s e n t i a l l y o f one o r more members o f a small group o f c l a y minerals w i t h a predominently c r y s t a l i n e s t r u c t u r e whereby the atoms are arranged i n d e f i n i t e geometric patterns.
Argi 11aceous (clayey) material can be considered as
made up o f a number o f these c l a y minerals stacked on each other i n the form o f a sheet o f layered s t r u c t u r e so t h a t c l a y p a r t i c l e s are shaped e i t h e r as a book o r s h e e t - l i k e u n i t o r as a bundle o f needles, tubes and f i b e r s . Clay p a r t i c l e s dispersed i n water may be brought s u f f i c i e n t l y close together, e i t h e r by Brownian motion o r by ambient turbulence, f o r the physico-chemical forces t o i n t e r a c t . A f t e r c o l l i s i o n they e i t h e r separate o r c l i n g t o each other. Under c e r t a i n c o n d i t i o n s and s p e c i f i c a l l y i n the presence o f dissolved s a l t s , the n e t e f f e c t o f the i n t e r p a r t i c l e physico-chemical forces i s a t t r a c t i o n .
Colliding
p a r t i c l e s then s t i c k together forming agglomerates which grow q u i c k l y t o sizes s u f f i c i e n t l y l a r g e for deposition. The phenomenon i s known as " f l o c c u l a t i o n " , the agglomerates are defined as 'lflocs" and the sol o r suspension i s c a l l e d " f l o c c u l a ted" o r unstable (Van Olphen, 1963).
I n a f l o c c u l a t e d suspension the f l o c r a t h e r
517 than the i n d i v i d u a l c l a y p a r t i c l e becomes the basic s e t t l i n g u n i t .
The f l o c s i z e
d i s t r i b u t i o n and, t h e r e f o r e , t h e d e p o s i t i o n r a t e s depend on both t h e i n t e r p a r t i c l e physico-chemical
forces and the f l o w c o n d i t i o n s .
The same physico-chemical f o r c e s These forces are
p r o v i d e t h e main r e s i s t a n c e t o e r o s i o n o f deposited cohesive beds.
f u n c t i o n s o f t h e c l a y mineralogy, t h e i o n s d i s s o l v e d i n t h e water and t h e s t r e s s history.
I n a d d i t i o n , the f o l l o w i n g f a c t o r s may a f f e c t f l o c u l a t i o n , d e p o s i t i o n and
resuspension:
(1) suspended sediment c o n s e n t r a t i o n as i t a f f e c t s the frequency o f
c o l l i s i o n o f suspended p a r t i c l e s and f l o c s ;
( 2 ) pH, which has been found t o a f f e c t
t h e e l e c t r o n e g a t i v i t y o f the s o i l c o l l o i d s ;
( 3 ) temperature,
f o r which t h e r e i s
limited e v i d e n c e t h a t it e n h a n c e s f l o c c u l a t i o n a n d increases deposition rates; ( 3 ) d i s s o l v e d organic matter, which promotes f l o c c u l a t i o n on t h e one hand by p r o v i d i n g a d d i t i o n a l l i n k s between i n o r g a n i c p a r t i c l e s w h i l e on t h e o t h e r hand r e t a r d s s e t t l i n g by reducing t h e o v e r a l l d e n s i t y and t h e e q u i v a l e n t diameter o f t h e f l o c s and ( 5 ) d i s s o l v e d m i n e r a l s when they a f f e c t t h e double-layer thickness o r e n t e r i n some way i n t o a physico-chemical r e a c t i o n .
These f a c t o r s are discussed i n
references (Corps o f Engineers, 1960; Partheniades, 1979) and s u n a r i z e d i n (Partheniades, 1984). To r e l a t e t u r b i d i t y w i t h cohesive sediment dynamics one has t o consider t h e v a r i o u s stages f l o c c u l a t i o n goes through and the dominant f a c t o r s and forces which c o n t r o l t h e growth o f the p a r t i c l e agglomerates. Assuming t h a t the proper physico-chemical c o n d i t i o n s f o r f l o c c u l a t i o n e x i s t , c o l l i d i n g p a r t i c l e s w i l l form f i r s t the basic agglomerate u n i t p r e v i o u s l y d e f i n e d as floc.
I n s t a t i o n a r y o r q u a s i - s t a t i o n a r y waters, p a r t i c l e c o l l i s i o n i s caused by the
Brownian motion only w h i l e i n f l o w i n g waters t h e t u r b u l e n t v e l o c i t i e s f a r outweigh the Brownian motion as a f l o c c u l a t i o n f a c t o r .
. S e t t l i n g f l o c s may c o l l i d e among
themselves o r w i t h o t h e r p a r t i c l e s due t o e i t h e r d i f f e r e n t i a l s e t t l i n g , o r t u r b u l e n t v e l o c i t i e s o r both, thus forming second, t h i r d and higher order aggregates. siinpl i c i t y we s h a l l consider only t h r e e classes o f p a r t i c l e aggregates:
For
t h e primary
f l o c , which i s the basic aggregate w i t h the h i g h e s t density; t h e f l o c aggregates, c o n s i s t i n g o f a number o f f l o c s ; and'the aggregate network, c o n s i s t i n g o f a combinat i o n o f f l o c aggregates.
Networks are normally encountered i n r e c e n t cohesive
sediment deposits and a t the lower depth o f c l a y suspensions s e t t l i n g i n quiescent waters.
Krone i n h i s studies o f t h e r h e l o g i c p r o p e r t i e s o f e s t u a r i n e sediments
(Krone, 1963) estimated the f l oc d e n s i t y f o r seven clayey muds between 1.44 and 1.64 gr/cm3.
This estimate was based on a r h e o l o g i c model f o r the v i s c o s i t y o f cohesive
sediment suspensions and h i s experiments were conducted w i t h a sediment concentrat i o n h i g h e r than 20000 ppm (0.02 gr/cm3) and a water d e n s i t y o f p = 1.025 gr/cm3. T h i s f l o c d e n s i t y range corresponds t o a volume f r a c t i o n o f s o l i d p a r t i c l e s between 0.085 and 0.253 and t o a water volume o f 75% t o 92% o f the t o t a l f l o c volume.
These
518
values are comparable t o those o f c o n s o l i d a t e d recent cohesive sediment deposits i n e s t u a r i n e shoals.
Figure 1 shows a schematic diagram o f aggregate networks which i s
r e p r e s e n t a t i v e o f t h e l o o s e s t s t a t e o f r e c e n t l y deposited f i n e sediments (Partheniades, 1962, 1965). The continuous growth o f f l o c s and f l o c aggregates i s counteracted by t h e stresses developed on the surfaces and w i t h i n t h e s e t t l i n g u n i t s due t o t h e drag f o r c e s exerted by t h e ambient water, t o t h e c o l l i s i o n w i t h o t h e r s e t t l i n g f l o c s and aggregates and t o t h e h i g h near-bed v e l o c i t y gradients, known a l s o as "shear rates".
I n q u a s i - s t a t i o n a r y waters t h e f i r s t two f a c t o r s a r e e s s e n t i a l l y t h e only
s i g n i f i c a n t ones, w h i l e i n moving waters t h e t h i r d f a c t o r , as recent research by t h e w r i t e r and co-workers have shown, (Mehta and Partheniades, 1973, 1975, 1979, 1982 and Partheniades, 1979), i s t h e one which l i m i t s t h e maximum s i z e o f t h e s e t t l i n g E q u a l l y important t o t h e f l o w and/or s e t t l ing-induced shear stresses i s
aggregates.
t h e s t r e n g t h o f t h e f l o c s and o f t h e h i g h e r order agglomerates.
The agglomerate
s t r e n g t h i s determined by t h e magnitude o f t h e i n t e r p a r t i c l e physico-chemical
bonds,
which i n t u r n are f u n c t i o n s o f t h e c l a y mineralogy and t h e chemistry o f t h e water. The s t r e n g t h o f f l o c s o f m o n t m o r i l l o n i t e mud from t h e San Francisco Bay was f i r s t s t u d i e d by Krone (1962) i n a laminar constant shear f l o w f i e l d generated between two mutually r o t a t i n g cylinders.
Considering t h e shear f o r c e developed d u r i n g p a r t i c l e
c o l l i s i o n as the main f a c t o r l i m i t i n g the f l o c size, he d e r i v e d t h e r e l a t i o n s h i p 'cr
=
16 A r 3n max
where
T
=
IJ
du/dy i s t h e u n i f o r m shear s t r e s s i n t h e laminar f l o w f i e l d , r i s the
r a d i u s o f t h e f l o c o r aggregate, A r i s t h e r a d i u s o f t h e c o n t a c t area d u r i n g C o l l i s i o n and
i s t h e shear s t r e n g t h o f t h e f l o c
or
aggregate.
From magnified photo-
graphs o f s e t t l i n g f l o c s and from t h e measured t o t a l shear f o r c e a t t h e i n n e r 2 was estimated t o be about 0.27 N/m This value i s w i t h i n t h e eroding cylinder, T ma x range o f shear s t r e s s f o r a dense bed w i t h a macroscopic shear s t r e n g t h o f
.
0.250 N/m2 i n t h e a u t h o r ' s e a r l y research (Partheniades, 1962, 1965). For constant shear strength, stress
T
t h e f l o c s i z e i s r e l a t e d t o t h e f i e l d shear
through Eq. 1, i.e.
r = 16x A 'mrax r
This increase o f r i n i n v e r s e p r o p o r t i o n t o T has been e x p e r i m e n t a l l y v e r i f i e d by 2 Krone (1962) down t o T = 0.06 dynes/cm Below t h a t shear s t r e s s a r a p i d and
.
e r r a t i c increase o f r was observed, apparently due t o t h e fbrmation o f f l o c aggregates.
519
Fonlure Plane
’
Fig. 1. Schematic p i c t u r e o f a f l o c c u l a t e d cohesive bed a t i t s l o o s e s t s t a t e (Partheniades, 1962, 1965, 1973). The i n t e r p a r t i c l e cohesive bonds are expected t o vary s t a t i s t i c a l l y w i t h i n a wide range. The law o f d i s t r i b u t i o n o f these bonds i s n o t known b u t i t can reasonably be assumed normal, (Partheniades, 1962, 1965). It f o l l o w s t h a t the s t r e n g t h o f the f l o c s and o f the aggregates i s expected t o vary randomly. I f the c o n t r o l l i n g nearbed shear s t r e s s i s constant, the maximum size, r, each aggregate may reach w i l l be a l s o random, as determined by Eq. 2. This aspect w i l l be f u r t h e r discussed i n t h e process o f deposition and resuspension. F l o c s and f l o c aggregates w i l l e v e n t u a l l y approach the near-bed zone where the shear s t r e s s r a t t a i n s i t s maximun value. If
‘max
,ZIiF 3nrr
t h e f l o c o r aggregate w i l l s t i c k t o the bed developing permanent physico-chemical bonds w i t h it; otherwise i t w i l l be broken up and reentrained t o the main f l o w by turbulence. THE STRUCTURE AND PROPERTIES OF THE FLOCS The m i c r o s t r u c t u r e o f c l a y f a b r i c has been under i n v e s t i g a t i o n f o r more than h a l f a century. Terzaghi (1925) and Casagrande (1932) presented a mechanistic model f o r t h e c l a y f a b r i c according t o which numerous s i n g l e c l a y particles are held together by cohesion t o form u n i t s o r c e l l s i n the form o f a honeycombed s t r u c t u r e . A discussion o f the various t h e o r i e s and models on t h e m i c r o s t r u c t u r e o f c l a y f a b r i c i s beyond the purpose o f t h i s paper. A very good and concise sumnary was presented by Quinn (1980).
U n t i l r e c e n t l y the p r e v a i l i n g concept was t h a t the fundamental
520
I n f r e s h water deposits the
u n i t i n any c l a y f a b r i c i s the s i n g l e p a r t i c l e .
dominant f a b r i c was believed t o be more open and o f the cardhouse type w i t h a p r e v a i l i n g edge t o face attachment o f p a r t i c l e s , w h i l e i n s a l t water the c l a y s t r u c t u r e was assumed t o be more random w i t h edge t o edge, face t o face and edge t o face attachments.
More recent i n v e s t i g a t o r s challenged these ideas and claimed t h a t
the basic u n i t o f c l a y f a b r i c i s the "clay packet", t h a t i s a domain o f several clay p a r t i c l e s i n p a r a l l e l array i n the form o f a book.
T h i s arrangement i s r e f e r r e d t o
as " t u r b o s t r a t i c s t r u c t u r e " o r "mu1 t i p l e aggregate p a r t i c l e f a b r i c " and was r e c e n t l y i n v e s t i g a t e d by Quinn under the supervision o f Eades and the author. A scanning e l e c t r o n microscope was used t o examine k a o l i n i t e f l o c s formed i n quiescent water through a freeze-dried process (Quinn, 1980).
The k a o l i n i t e p a r t i c l e s ranged from 1
t o 2 microns i n average diameter. Regardless o f c l a y concentration o r e l e c t r o l y t e content, no " s i n g l e p a r t i c l e behavior" was observed; instead, even i n extremely d i l u t e suspensions, the fundamental u n i t s were m u l t i p l e p a r t i c l e books w h i l e the number o f s i n g l e p a r t i c l e s was r e l a t i v e l y small.
The books appear i n a form o f
steps; they are the product o f feldspar weathering and, although they may weather f u r t h e r , t h i s does n o t necessarily mean dispersion i n t o i n d i v i d u a l p l a t e l e t s .
Flocs
formed i n f r e s h water were found t o become l a r g e r and s t r u c t u r a l l y more complex as t h e c l a y concentration i n suspension increased.
A tendency was, moreover, detected
f o r f r e s h water f l o c s t o form elongated networks made up o f p l a t e l e t s and packets j o i n e d together i n edge-to-face and face-to-face s t a i r - s t e p configurations. The d i f f e r e n c e s between fresh and s a l t water f l o c c u l a t i o n were n o t as d r a s t i c as expected. Apart from a somewhat more open s t r u c t u r e i n f r e s h water f l o c s , t h e r e i s both edge-to-face and face-to-face f l o c c u l a t i o n o f p a r t i c l e s i n e i t h e r case. It c o u l d only be concluded t h a t f r e s h water c l a y s d i s p l a y a tendency t o form open bookhouse-type randomly s t r u c t u r e d f l o c s w i t h edge-to-face
and edge-to-edge packet
j u n c t i o n s w h i l e i n s a l t water a greater tendency f o r more subparallel face-to-face packet j u n c t i o n was detected.
Figures 2 and 3 g i v e a good example o f s a l t and f r e s h
water f l o c s formed a t the lowest suspended c l a y concentration (100 ppm).
The c l a y
packets are c l e a r l y displayed i n both figures; however a s i n g l e c l a y p l a t e l e t can be observed i n t h e f r e s h water f l o c which tends t o produce a more open s t r u c t u r e approaching the t r a d i t i o n a l p i c t u r e o f "cardhouse" s t r u c t u r e . It was observed t h a t t h e lower t h e concentration o f suspended clay, the greater percentage t h e r e was of packets w i t h l e s s than f i v e i n d i v i d u a l p l a t e l e t s per packet w h i l e the number o f i n d i v i d u a l p l a t e l e t s per packet increases w i t h increased concentration o f suspended This i s c l e a r l y demonstrated i n Figs. 4 and 5. I n Fig. 4 an increase o f
clay.
face-to-face-oriented
p a r t i c l e s per packet i s observed w h i l e the packets themselves The packet i n the l e f t center i s i n t e r -
are predominently o r i e n t e d face-to-face.
p r e t e d by Quinn as an example o f a booklet t h a t remained i n t a c t a f t e r i n i t i a l
521
formation v i a weathering o f a feldspar.
I n t h e f r e s h w a t e r f l o c o f Fig. 5, an
i n c r e a s e o f t h e number o f p a r t i c l e p l a t e l e t s p e r packet i s a l s o observed; however, t h e l a t t e r f o l l o w more an open edge-to-edge i n t h e s a l t water f l o c o f Fig. 4.
and edge-to-face
arrangement t h a n t h o s e
Moreover, a s i n g l e p a r t i c l e can be r e a d i l y
observed i n t h e l o w e r r i g h t c o n t r i b u t i n g t o t h e openness o f t h e s t r u c t u r e . F i g u r e 6, shows an example o f a t y p i c a l j o i n t c o n n e c t i n g f l o c s i n f l o c aggregates and aggregate networks (Fig.
1).
F i g u r e 7 shows a n o t h e r f l o c aggregate formed i n
f r e s h w a t e r w h i l e Fig. 8 shows a s i m i l a r f l o c aggregate formed i n s a l t w a t e r under t h e same c o n c e n t r a t i o n o f suspended c l a y as t h e p r e v i o u s two examples.
The g r e a t e r
degree of randomness i n comparison t o t h e f r e s h w a t e r f l o c s i s c l e a r l y observed. F i n a l l y , t h e same s t u d y i n d i c a t e d an i n c r e a s e o f t h e average f l o c s i z e w i t h suspended sediment c o n c e n t r a t i o n (Fig. 9).
T h i s i s , o f course, t o be expected, s i n c e t h e
number o f p a r t i c l e s p e r u n i t volume and, t h e r e f o r e , t h e frequency o f i n t e r p a r t i c l e c o l l i s i o n s a r e expected t o be p r o p o r t i o n a l t o t h e suspended sediment c o n c e n t r a tion.
It f o l l o w s t h a t t h e number o f p a r t i c l e s i n a f l o c and, t h e r e f o r e , t h e f l o c
volume a t a p a r t i c u l a r t i m e s h o u l d a l s o be a p p r o x i m a t e l y p r o p o r t i o n a l t o t h e suspended sediment c o n c e n t r a t i o n .
0.2
gr/e
T h i s was indeed t h e case f o r C = 0.1
and 0.3 g r l a . However, f o r c o n c e n t r a t i o n s h i g h e r t h a n 0.3
volume was found t o i n c r e a s e much more r a p i d l y t h a n C.
gr/&
gr/e the floc
There i s no obvious explana-
t i o n f o r t h i s p e c u l i a r phenomenon a t t h i s time; i t appears, however, t h a t t h e answer s h o u l d be sought i n t h e i n i t i a l f l o c - f o r m i n g s t a g e r a t h e r t h a n i n t h e c o l l i s i o n frequency d u r i n g s e t t l i n g . The e f f e c t o f ambient t u r b u l e n c e and shear s t r e s s e s on t h e s i z e , d e n s i t y and s e t t l i n g v e l o c i t y o f f l o c s was r e c e n t l y i n v e s t i g a t e d by Kusuda e t a1 (1981).
It was
found t h a t b o t h t h e average and maximum f l o c s i z e decrease w i t h i n c r e a s i n g m i x i n g intensity.
The l a t t e r was q u a n t i t a t i v e l y d e f i n e d as
”
where c0 i s t h e r a t e o f energy decay p e r u n i t volume o f f l u i d , and viscosity of the fluid.
u i s t h e dynamic
G i s i n d i c a t i v e o f t h e average r a t e o f energy d i s s i p a t i o n
i n t h e m i x i n g chamber w i t h r o t a t i n g blades and can r e a d i l y be computed.
This mixing
process generates a h i g h l y n o n - u n i f o r m f l o w f i e l d w i t h zones o f h i g h v e l o c i t y g r a d i e n t s and small s c a l e eddies,
such as between t h e blades, and zones o f l o w v e l o c i t y
g r a d i e n t s and l a r g e s c a l e eddies.
The q u e s t i o n a r i s e s as t o which zone dominates
t h e development o f t h e f l o c s , since, due t o t h e c o m p l e x i t y o f t h e f l o w f i e l d i n t h e m i x i n g chamber, t h e e f f e c t o f each zone cannot be separated.
Therefore,
t h e con-
c l u s i o n s reached i n t h e s t u d y i n q u e s t i o n cannot v e r y w e l l be e x t r a p o l t e d t o open channels.
It was found n e x t t h a t when t h e c l a y suspension was f i r s t s u b j e c t e d t o a
522
h i g h agitation rate, Gh, followed by a lower r a t e , G,, the flocs were less dense than the ones formed under the lower agitation r a t e G, only and t h a t the floc density reduced w i t h increasing r a t i o Gh/G, Apparently under constant agitation rate, G,, only primary flocs are formed w i t h density and sizes as dictated by t h a t particular rate, while i n the case of a high mixing r a t e followed by a lower one, the final agglomerates are essentially floc aggregates consisting of groups of primary flocs formed during the f i r s t stage of high mixing rates. Moreover, even for a constant mixing rate the dry weight of flocs was found t o b e proportional t o the 2.5 power of the floc diameter rather than the t h i r d power, which suggests t h a t the average floc density decreases w i t h increasing floc size. Expressions for the s e t t l i n g velocity and the floc density were given from curve f i t t i n g ; however, for the aforementioned reasons, these expressions cannot be generalized.
Fig. 2. S a l t water kaolinite floc a t C = 0.1 g r / t ( Q u i n n , 1980)
Fig. 3. Fresh water kaolinite floc a t C = 0.1 gr/a (Quinn, 1980)
Fig. 4. S a l t water kaolinite floc a t C = 0.4 gr/a ( Q u i n n , 1980)
Fig. 5 Fresh water kaolinite f l o c a t C = 0.4 g r / t ( Q u i n n , 1980)
523
Fig. 6. Fresh water f l o c aggregate a t C = 0.5 g r / i (Quinn, 1980)
Fig. 7. Fresh water f l o c aggregate a t C = 0.5 g r / g (Quinn, 1980)
GRAYS OF CLAY I LITER OF WATER
Fig. 8. S a l t water f l o c aggregate a t C = 0.5 g r / a (Quinn, 1980)
-
Fig. 9 Floc s i z e c l a y concentrat i o n r e l a t i o n s h i p (Quinn, 1980)
DEPOSITION OF FLOCCULATED SUSPENSIONS D e p o s i t i o n s t a r t s as soon as f l o c s grow l a r g e enough t o o b t a i n measurable s e t -
tl i n g v e l o c i t i e s .
I n t h e h y p o t h e t i c a l case o f f l o c c u l a t i o n s t a r t i n g and developing
i n a p e r f e c t l y q u i e s c e n t water environment, t h e Brownian motion c o n s t i t u t e s t h e o n l y cause o f p a r t i c l e c o l l i s i o n .
This would r e s u l t i n r e l a t i v e l y slow f l o c c u l a t i o n
r a t e s s i n c e t h e molecular d i f f u s i v i t y i s o f t h e o r d e r o f k i n e m a t i c v i s c o s i t y , i.e. 3 2 -1 2 I n c o n t r a s t , i n a t y p i c a l case o f an open channel w i t h 10- cm /sec = 10 m /sec. an average v e l o c i t y o f lm/sec,
a Manning's f r i c t i o n c o e f f i c i e n t n ' = 0.025
and a
h y d r a u l i c r a d i u s Rh = 1Om t h e average eddy d i f f u s i v i t y i s o f t h e o r d e r o f 300 cm2/sec.
I n r e a l i t y , however, n a t u r a l water bodies very seldom remain i n an
524
Even i n lakes, tideless
absolutely motionless s t a t e f o r prolonged time periods.
bays and reservoirs, water i n f l o w , winds, density c u r r e n t s and C o r i o l i s acceleration a r e s u f f i c i e n t t o generate a l a r g e scale motion and mixing. Moreover, the chances are t h a t f i n e sediments are s u f f i c i e n t l y f l o c c u l a t e d f o r deposition t o s t a r t before they enter the more stagnant zones o f estuaries. Since deposition c o n s t i t u t e s the basic mechanism f o r e l i m i n a t i o n o r r e d u c t i o n o f sediment-induced t u r b i d i t y , the question a r i s e s as t o what h y d r a u l i c parameters and physico-chemical properties of the sediment-water system c o n t r o l the i n i t i a t i o n , degree and r a t e s o f deposition O f suspended f i n e sediments as w e l l as the i n i t i a t i o n , degree and r a t e s o f resuspension o f s i m i l a r sediment already deposited.
These questions have been the subject of
i n t e n s i v e research by the author and h i s co-workers f o r the p a s t twenty f i v e years and l e d t o a fundamental framework f o r cohesive sediment dynamics (Metha and Partheniades, 1973, 1975, 1979, 1982; Partheniades, 1979, 1984; Partheniades and Kennedy, 1966; Partheniades and Paaswell, 1970).
The most important r e s u l t s o f the
work p e r t a i n i n g p a r t i c u l a r l y t o d e p o s i t i o n are herewith summarized. It should f i r s t be pointed o u t t h a t deposition experiments i n quiescent water m g
reveal some important aspects, such as the e f f e c t o f s a l i n i t y and other physicochemical f a c t o r s on f l o c c u l a t i o n and deposition, b u t bear l i t t l e resemblance t o the s i t u a t i o n i n f l o w i n g water.
I n t h e f i r s t case the only r e s t r i c t i o n t o f l o c growth
are the settling-generated shear stresses on the f l o c s and, possibly t h e stresses due t o f l o c c o l l i s i o n .
A l l aggregates w i l l e v e n t u a l l y deposit and a s e t t l i n g inter-
face i s formed separating t u r b i d water from almost c l e a r water. marks the lower l i m i t o f s e t t l i n g v e l o c i t y o f the flocs.
This i n t e r f a c e
I n f l o w i n g waters the
t u r b u l e n t v e l o c i t y f l u c t u a t i o n s c o n s t i t u t e t h e dominant mechanism o f p a r t i c l e and f l o c c o l l i s i o n . The f l o c and aggregates thus formed are expected t o have sizes and strengths ranging between an upper and a lower l i m i t .
A l l o f them w i l l eventually
approach t h e bed; however, only the ones w i t h s i z e and strength s a t i s f y i n g Eqs. 1 and 2 w i l l reach the bed and w i l l become p a r t o f it. Otherwise, they w i l l be brokee up and reentrained back i n t o the main flow.
It appears, therefore,
that, although the f l o c s and f l o c aggregates a r e formed i n the far-bed f l o w region, where the
shear r a t e s are minimal and the turbulence s t r u c t u r e nearly homogeneous, the degree and r a t e s of deposition w i l l be determined i n a near-bed zone where the shear rate a t t a i n s i t s highest value. Upon contact w i t h the bed, f l o c s and f l o c aggregates w i l l develop bonds w i t h the l a t t e r so t h a t a l i f t f o r c e considerably higher than u#
submerged weight of the s e t t l e d u n i t w i l l be r e q u i r e d f o r resuspension. Moreover, i t has been shown ( B l i n c o and Partheniades, 1971) t h a t t h e t u r b u l e n t v e l o c i t y f l u c t u a t i o n s and the t u r b u l e n t d i f f u s i v i t y i n the far-bed region are functions o f the bed f r i c t i o n and the near bed f l o w conditions. It f o l l o w s t h a t the i n i t i a t i o n , degree and r a t e s o f deposition are expected t o be f u n c t i o n s o f the near bed flow conditions. U n l i k e i n quiescent s e t t l i n g , i n f l o w i n g sediment suspensions no inter-
525
face between t u r b i d and c l e a r water i s formed.
Due t o the low s e t t l i n g v e l o c i t i e s ,
the c o n c e n t r a t i o n gradients o f suspended sediment are very low, provided t h a t other f a c t o r s , such as density s t r a t i f i c a t i o n , are n o t present. The recent research on the subject has been i n determining the r e l a t i o n s h i p s l i n k i n g the i n i t i a t i o n , degree and r a t e s o f deposition t o the near-bed f l o w conditions. From deposition experiments i n quiescent water w i t h a s i l t y - c l a y sediment from t h e Sari Franscisco Bay, Krone (1962) found that, f o r constant i n i t i a l sediment c o n c e n t r a t i o n , substancial f l o c c u l a t i o n takes place f o r a s a l i n i t y o f 1000 ppm.
The median s e t t l i n g v e l o c i t y increases w i t h i n c r e a s i n g s a l i n i t y a t t a i n i n g a maximum for a s a l i n i t y ranging from 5000 ppm ( f o r C = 120 ppm) t o 15000 ppm ( f o r C = 1000 ppml where C i s the concentration o f suspended sediment.
The median s e t t l i n g
v e l o c i t i e s corresponding t o the optimum s a l i n i t y were found t o f o l l o w the law v
S
= a C4 /3
where
a
(5)
i s a constant.
Krone explained the above r e l a t i o n s h i p on the basis o f
K r u y t ' s law f o r the k i n e t i c s o f f l o c c u l a t i o n (Krone, 1962) which reads
where No i s the number o f primary p a r t i c l e s i n the suspension, N,
i s the number o f
f l o c s w i t h an average number o f n primary p a r t i c l e s per f l o c ; tf i s the time o f f l o c c u l a t i o n ; and tc i s the average time between c o l l i s i o n o r the r e c i p r o c a l o f the c o l l i s i o n probability.
For t >> tc Eq. 6 can be approximated by
No/Nn i s equal t o the number o f p a r t i c l e s n per f l o c . I f r i s the f l o c diameter then No/N i s p r o p o r t i o n a l t o r3 and r i s p r o p o r t i o n a l t o (No/Nn)'l3 or to
273 .
(tf/fC)
The s e t t l i n g v e l o c i t y vs then being p r o p o r t i o n a l t o
?,
i s also
The c o ? l i s i o n p r o b a b i l i t y l/tci s expected t o be p r o p o r t i o n a l t o (tf/tc)4/3. p r o p o r t i o n a l t o C; therefore, f o r constant f l o c c u l a t i o n time tf, vs should be p r o p o r t i o n a l t o C4j3, which indeed was the case. Krone attempted next t o f i n d a sediment deposition law i n terms o f the bed shear s t r e s s f o r the San Francisco Bay mud, which c o n s i s t s o f about equal proportions o f s i l t and c l a y .
The c l a y
p o r t i o n i s predomi n e n t l y composed o f montmori 11o n i t e w i t h some i11it e and traces of some f i n e sand and organic matter.
Krone f i r s t gave a c r i t i c a l shear s t r e s s
f o r deposition equal t o 0.6 dynes/cm2 and then he derived the f o l l o w i n g experimental concentration-time laws: For suspended sediment concentration l e s s than 0.3 g r / r
526
"sPrt (8) = Co exp [ h where Co i s the i n i t i a l mass concentration o f sediment; t i s the time; h i s the i s the depth o f flow, pr = 1 T ~ / T & ; T~ i s the bed shear s t r e s s and
-
c
-
" t h r e s h o l d o f rb" f o r deposition being equal t o 0.6 dynes/cm2. t h a t f o r T ~ 0.6 > d y n e s / c d no sediment deposits.
Krone claimed
The term Pr was i n t e r p r e t e d as
t h e " p r o b a b i l i t y o f a p a r t i c l e s t i c k i n g t o the bed". For C > 0.3 gr/a he gave the law log C =
-
(9 1
k l o g t t constant
where k i s a constant depending on T ~ / T ~ . For concentrations 0.3 g r / a < C < 10 g r / a he found a value f o r T$ equal t o 0.8 dynes/cm2 r a t h e r than 0.6 dynes/cm2. The reason f o r the existence o f two d i f f e r e n t laws was a t t r i b u t e d t o the hinderance The l i m i t o f 0.3 g r l a , however, i s too low t o account f o r any d r a s t i c change i n the f l o c c u l a t i o n and deposition process. From Eq. 8 vs was during s e t t l i n g s .
found equal t o 6.6 x 10-4cm/sec which was about 20 percent o f the s e t t l i n g v e l o c i t y i n quiescent water. Equation 8 can be explained by the sediment c o n t i n u i t y equation: mN "sPr
(10)
h
where N i s the number o f p a r t i c l e s o r f l o c s per u n i t volume and m i s the average mass o f each p a r t i c l e o r f l o c .
Obviously mN = C which leads t o Eq. 8 provided
t h a t vs does n o t change w i t h time.
Equation 9 does n o t seem t o f o l l o w any
sediment c o n t i n u i t y model. The author (Partheniades, 1962, 1965, 1979, 1984; Partheniades and Paaswell, 1970) i n v e s t i g a t e d f i r s t the erosion mechanism o f dense and f l o c c u l a t e d cohesive sediment beds and presented a model f o r the erosion process g i v i n g t h e erosion r a t e s as a f u n c t i o n o f the bed shear s t r e s s and the physico-chemical p r o p e r t i e s o f the soil-water system.
H i s f i r s t deposition experiments suggested t h a t no
interchange o f suspended and bed p a r t i c l e s takes place.
This conclusion was con-
firmed i n the f o l l o w i n g twenty years o f research on deposition and resuspension. The observed independent occurence o f erosion and deposition o f cohesive sediments l e d t o separate i n v e s t i g a t i o n s f o r each process notwithstanding the f a c t t h a t they both c o n s t i t u t e two phases o f the same phenomenon. The more recent work s t a r t e d w i t h the study o f the depositional behavior o f f i n e sediment suspensions, f i r s t a t M.I.T. i n 1963 (Partheniades and Kennedy, 1966, Partheniades, Cross and Ayora, 1968) and l a t e r on a t the U n i v e r s i t y o f F l o r i d a from 1968 t o
527
1983 (Mehta and Partheniades, 1973, 1975, 1979, 1982; Partheniades, 1973, 1979, 1984).
The experiments were conducted i n a s p e c i a l apparatus developed f i r s t a t
M.I.T.
and subsequently enlarged and improved a t t h e U n i v e r s i t y o f f l o r i d a .
l a s t one i s p i c t u r e d i s Fig. 10 and o u t l i n e d s c h e m a t i c a l l y i n Fig. 11. component c o n s i s t s o f an annular channel 60 in. (10.2
m) i n mean diameter, 4 in.
(1.525
cm) wide and w i t h a maximum depth o f 18 in.
The
I t s main
(45.8
cm) c o n t a i n i n g t h e water-
sediment mixture, and a v e r t i c a l l y movable annular r i n g p o s i t i o n e d w i t h i n t h e
A simultaneous r o t a t i o n o f t h e
channel and i n c o n t a c t w i t h t h e water surface.
two components i n o p p o s i t e d i r e c t i o n s generates a t u r b u l e n t u n i f o r m f l o w f i e l d f r e e o f f l o c d i s r u p t i n g elements, such as pump blades, r e t u r n p i p e s and d i f fusers, which have t o be present i n conventional l a b o r a t o r y flumes.
The e f f e c t
o f t h e r o t a t i o n - i n d u c e d secondary c u r r e n t s on t h e d e p o s i t i o n has been e l i m i n a t e d by a proper s e l e c t i o n of t h e speeds o f t h e channel and o f t h e r i n g so t h a t t h e sediment deposits u n i f o r m l y accross t h e channel.
A t these o p e r a t i o n a l speeds t h e
bed shear s t r e s s across t h e channel measured by a Preston tube was found t o be The apparatus i s p r o p e r l y i n s t r u -
u n i f o r m (Mehta and Partheniades, 1973, 1975).
mented f o r a d i r e c t and p r e c i s e measurement o f t h e shear stresses a t t h e r i n g and a t t h e channel bottom. F i g u r e 12 shows f o u r t y p i c a l v e l o c i t y p r o f i l e s i n t h e middle p o r t i o n o f t h e channel d i s p l a y i n g two w a l l l a y e r s o f h i g h shear r a t e s and a core segment o f near A p l o t o f t h e near bed
constant v e l o c i t y and near homogeneous turbulence.
v e l o c i t y p r o f i l e s suggested a l o g a r i t h m i c v e l o c i t y d i s t r i b u t i o n law o f t h e form: -=-
u*
logy Yo
where u* i s t h e f r i c t i o n v e l o c i t y a t t h e bed;
K
0.40,
as i n pipes and open
channels; and yo i s t h e d i s t a n c e a t which u = 0 found t o be equal t o about 0.5 mn. F i g u r e 13 shows a t y p i c a l example o f suspended sediment c o n c e n t r a t i o n - t i m e curves i n t h e o u t l i n e d experimental system where C i s t h e c o n c e n t r a t i o n a t t i m e t and Co i s t h e value o f C a t t h e s t a r t o f deposition.
It i s observed t h a t , a f t e r
a s h o r t t r a n s i e n t period, t h e c o n c e n t r a t i o n reaches a constant value h e r e i n def i n e d as " e q u i l i b r i u m c o n c e n t r a t i o n " symbolized by Ceq which decreases w i t h decreasing bed shear s t r e s s , value,
T ~ .
Ceq becomes zero f o r rb
below a c r i t i c a l
T~
F i g u r e 14 shows a c o n c e n t r a t i o n - t i m e p l o t f o r k a o l i n i t e c l a y i n d i s t i l l e d water f o r constant f l o w c o n d i t i o n s b u t v a r i a b l e i n i t i a l c o n c e n t r a t i o n , C o (Partheniades, 1973, 1984 and Partheniades and Kennedy, 1966).
co.
It i s observed t h a t t h e
= Ceq/Co, remains constant and independeq This means t h a t a g i v e n f l o w can m a i n t a i n i n suspension a constant
r e l a t i v e equilibrium concentration, C e n t of
*
528
Fig. 10.
Annular channel and r i n g assembly (Mehta and Partheniades, 1973, 1975).
529
Fig. 11. Schematic o u t l i n e o f annular channel and r i n g assembly (Mehta and Partheniades, 1973, 1975).
6
5
4 7
;;3 h
2
I
o
0
r
10
'
20
30
40
50
60
70
I
80
U (cm/rec)
Fig. 12. Typical v e l o c i t y p r o f i l e s i n t h e annular channel (Mehta and Partheniades, 1973).
530
TIME (Hours)
Fig. 13. T y p i c a l suspended sediment c o n c e n t r a t i o n - t i m e curves f o r c o h e s i v e sediments (Mehta and Partheniades, 1973). 30 20 DEPTH
-
WNCENTRATlON VS. TIYE I6CM.
YEWCITY. 81.0 c M / b E c .
FOR VARIOUS INITIAL CONCENTRATIONS
10
=:a n
;. 2
c
E ' z 0 c z
2
r W
0
1
E d
i.6
TIME AFTER START IN HOURS
Fig. 14. V a r i a t i o n o f suspended sediment w i t h t i m e (Partheniades, P a r t h e n i a d e s and Kennedy, 1966).
1973, 1984 and
531
f r a c t i o n o f a p a r t i c u l a r sediment regardless o f the absolute value o f the concentration.
This conclusion was v e r i f i e d f o r a v a r i e t y o f c l a y types and water It can be concluded t h a t qC:
chemistry.
represents t h a t p a r t o f suspended
sediment which can never form f l o c s w i t h s u f f i c i e n t l y strong bonds t o r e s i s t the near-bed d i s r u p t i v e shear stresses and j o i n the bed. Likewise, the r a t i o ** * = co c /c = 1 represents t h a t p o r t i o n o f the suspended sediment eq 0 ceq t h a t i s able t o form f l o c s w i t h s u f f i c i e n t l y strong bonds t o reach the bed and
-
ceq
-
become p a r t o f it. The existence of the e q u i l i b r i u m concentration precludes any exchange between bed and suspended p a r t i c l e s o r f l ocs during deposition and the material i n suspension a t e q u i l i b r i u m i s composed o f the same p a r t i c l e s .
This
conclusion has been d i r e c t l y v e r i f i e d by gradually r e p l a c i n g the water-sediment m i x t u r e i n the r o t a t i n g channel a t e q u i b l i r i u m w i t h clean water (Partheniades, Cross and Ayora, 1968). Under steady-state conditions, i.e. when e q u i l i b r i u m concentration i s a t t a i n ed, there i s a balance between the downward sediment mass f l u x due t o g r a v i t a t i o n a l s e t t l i n g and the upward d i f f u s s i v e transport, according t o the equation -vC=E
s
where
Y
dC T
(12)
i s the t u r b u l e n t d i f f u s i v i t y i n the y d i r e c t i o n normal t o the bed. For Y t h e zone f a r from t h e bed, where the v e l o c i t y i s nearly constant and the turbuE
lence appears t o be nearly homogeneous, independent o f y.
I n d i c a t i n g by
E
E i s expected t o be almost constant and Y t h a t constant value, i n t e g r a t i o n o f Eq. 12 f o r
constant vs y i e l d s
6
= exp[
-
vS
(y
- a) 3
(13)
where Ca i s the concentration a t the reference p o i n t y = a. concentration d i s t r i b u t i o n p l o t f o r T~ c l o s e l y Eq. 12 w i t h V ~ / E= 0.026 in.- 1
.
For
T,, =
2.95 dynes/cm',
V ~ / Ewas
Figure 15, shows a
= 1.85 dynes/cm2 which s a t i s f i e s q u i t e
equal t o 0.015 in.-'
(5.12 x 10-3cm-1).
E a r l y v e l o c i t y d i s t r i b u t i o n measurements i n an open flume w i t h suspended sediment by the w r i t e r w i t h the help of a s p e c i a l l y designed Prandtl tube, i n d i c a t e d t h a t the v e l o c i t y p r o f i l e s were n o t measurably a f f e c t e d by the suspended sediment even a t concentrations near 10,000 ppm (Partheniades, 1962). This suggests t h a t t h e t u r b u l e n t eddy d i f f u s i v i t y i s unaffected by the suspended sediment a t l e a s t f o r concentrations up t o t h a t l i m i t . Indeed, experiments w i t h i n i t i a l concentration w i t h k a o l i n i t e Co equal t o 7,680 ppm and 16,900 ppm and a t , bed shear stresses o f 1.87 dynes/cm2 and 1.85 dyneslcn? r e s p e c t i v e l y gave values o f 0.022 in.-'
(8.66 x 10-3~m-1) and 0.026 in.-l
V ~ / E equal
(10.24 x 10-3cm-1)
to
respectively,
532
i.e.
a d e v i a t i o n o f t h e o r d e r o f t h e e x p e r i m e n t a l e r r o r (Mehta and Partheniades,
1973).
These c o n c l u s i o n s , however, s h o u l d be c o n s i d e r e d as t e n t a t i v e .
Moreover,
t h e eddy d i f f u s i v i t y as w e l l as t h e o v e r a l l t u r b u l e n c e s t r u c t u r e i s expected t o be a f f e c t e d by t h e suspended sediment f o r c o n c e n t r a t i o n above a c e r t a i n 1 m i t . T h i s phase has n o t been s t u d i e d y e t . uniform v e l o c i t y region
E
The p r e s e n t e d r e s u l t s i n d i c a t e t h a t i n t h e
i s a f u n c t i o n o f t h e bed shear s t r e s s .
a c t u a l s e t t l i n g v e l o c i t y vs can be o b t a i n e d i f anenometer. F i g u r e 16 shows a p l o t o f C
T~~~~
ceq f o r deposition"
i m p l i e d i n e a r l i e r i n v e s t i g a t i o n s (Krone, 1962); a
measurable degree o f d e p o s i t i o n can t a k e p l a c e f o r value o f
T
~
~
~
~
T~
as h i g h as t e n t i m e s t h e
.
The s p e c i f i c l a w f o r t h e degree o f d e p o s i t i o n , sought.
It can be concluded t h a t
and t h a t t h e r e i s no such a t h i n g as a " c r i t i c a l shear
T~
T ~ ,as
as r e p r e s e n t e d by C
F i g u r e 17 shows a n o r m a l - l o g a r i t h m i c p l o t o f C
s i o n a l i z e d excess bed shear s t r e s s " ameter", where
12 t h e
*
which i s t h e l i m i t f o r complete d e p o s i t i o n .
depends o n l y on
From Eq
can be measured by a h o t f i l m
vs t h e bed shear s t r e s s , T ~ , f o r v a r i o u s depths eq An e x t r a p o l a t i o n o f t h e p l o t i n t e r s e c t s t h e a b s c i s s a
and i n i t i a l c o n c e n t r a t i o n s . at
E
*
*
T~
-
*
was n e x t eq ' versus t h e "non-dimen-
eq 1, d e f i n e d as t h e "bed shear s t r e s s p a r -
f o r k a o l i n i t e c l a y i n d i s t i l l e d water.
= Tb'Tbmin' leads t o t h e r e l a t i o n s h i p T~
*
This p l o t
where
-
Here u i s t h e g e o m e t r i c s t a n d a r d d e v i a t i o n ; ( T ~ - 1)50i s t h e g e o m e t r i c mean, Y i.e. t h e v a l u e o f t h e bed shear s t r e s s parameter f o r which 50 p e r c e n t of t h e t o t a l sediment d e p o s i t s ; and w i s a dummy v a r i a b l e .
u = 0.049 and ( T ~ * Y
The d a t a o f Fig. 17 g i v e
= 0.84.
The u n i q u e dependance o f Ce,
*
on
T~
confirms t h e e a r l i e r stated n o t i o n t h a t
d e p o s i t i o n o f c o h e s i v e sediments i s c o n t r o l l e d a t t h e bed w h i l e t h e t u r b u l e n c e i n t h e far-bed region contributes only t o t h e formation o f t h e f l o c s . c l u s i o n was reached f o r t h e e r o s i o n o f c o h e s i v e sediment beds.
The same con-
533
Fig. 15. 1973).
D e p t h - c o n c e n t r a t i o n p r o f i l e a t e q u i l i b r i u m (Mehta and Partheniades,
10 09 08
07 06
" 0 5
c
04
0 3 02 01 0 0
5
1
6
7
0
9
10
ti
Fig. 16. R e l a t i v e e q u i l i b r i u m c o n c e n t r a t i o n versus bed shear s t r e s s (Mehta and Partheniades, 1973). E q u a t i o n s 14 and 15 were s u b s e q u e n t l y extended t o o t h e r c l a y t y p e s and w a t e r qualities.
I n Fig. 18 two s e t s o f d a t a have been p l o t t e d , one f o r a m i x t u r e o f
equal p a r t s o f k a o l i n i t e and San F r a n c i s c o Bay mud i n s a l t w a t e r a t ocean s a l i n i t y ( s e r i e s C j and a n o t h e r f o r San F r a n c i s c o Bay mud o n l y a l s o a t ocean s a l i n i t y
( s e r i e s D), t o g e t h e r w i t h t h e average d o t t e d l i n e f r o m Fig. 17.
The same d a t a
a r e p l o t t e d i n Fig. 19 t o g e t h e r w i t h t h e d a t a o f s e r i e s A and B f o r k a o l i n i t e c l a y i n s a l t w a t e r a t ocean s a l i n i t y f o r depths o f 6 i n . and 9 in.,
*
-
*
-
versus t h e parameter ( T ~ l ) / ( ~ ~l ) s o . experiments a t M.I.T. Ayora,
respectively
The data by t h e a u t h o r e t a1 f r o m
( P a r t h e n i a d e s and Kennedy, 1966; Partheniades, 'Cross and
1968), t h e o n l y d a t a by t h e a u t h o r f r o m h i s aforementioned o r i g i n a l
534 experiments i n an open flume w i t h San Francisco Bay mud (Partheniades, 1965, 1962) and t h e . d a t a by R o s i l l o n and Volkenborn from experiments i n an open flume w i t h water a t ocean s a l i n i t y and sediment from t h e Maracaibo Bay i n Venezuela ( R o s i l l o n and Volkenkborn, 1964) have a l s o been p l o t t e d .
The agreement f o r
sediment-water systems so d r a s t i c a l l y d i f f e r e n t i s indeed remarkable.
It can be
concluded t h a t a l l t h e physico-chemical p r o p e r t i e s o f t h e sediment-water system, which determine t h e d i s t r i b u t i o n o f t h e s i z e and t h e s t r e n g t h o f t h e f l o c s , can
*
be represented by two r e a d i l y determinable parameters:
Fig. 17.
*
T~
-
T~~~~
and ( T ~- l)50.
R e l a t i v e e q u i l i b r i u m c o n c e n t r a t i o n versus bed shear s t r e s s parameter
1 (Mehta and Partheniades, 1973, 1975).
Fig. 18. R e l a t i v e e q u i l i b r i u m c o n c e n t r a t i o n s versus suspensions (Mehta and Partheniades, 1973, 1975).
T*
b -
1 f o r v a r i o u s sediment
535
0 01
01
10
I
*b
Fig. 19. R e l a t i v e e q u i l i b r i u m c o n s e n t r a t i o n s versus ( T - 1 ) / ( T b * v a r i o u s sediment suspensions (Mehta and Partheniades, 1973, 1975). For f o u r d i f f e r e n t sediment-water systems related t o
*
-
1150 f o r
( ~ iappears ~ t~ o be c l~ osely 1)50
through t h e equation
T~~~~
= 4 exp(-1.27
( T ~
-
T
~
~
~
~
)
(16)
with
T~~~~
i n dynes/cm2 (Mehta and Partheniades, 1973).
that
T~~~~
i s r e a l l y t h e o n l y parameter r e p r e s e n t i n g t h e e f f e c t o f t h e p h y s i c o -
It appears, t h e r e f o r e ,
chemical p r o p e r t i e s o f t h e sediment-water system on t h e degree o f d e p o s i t i o n . T h i s c o n c l u s i o n has t o be c o n f i r m e d f o r more cases.
-
and (T;
There appears t o be a c o r r e -
1)50w i t h t h e c a t i o n exchange c a p a c i t y (CEC) o f
l a t i o n o f both
T~~~~
t h e sediment.
However, CEC i s a p r o p e r t y o f t h e sediment o n l y and n o t o f t h e
sediment-water system.
I t s use, t h e r e f o r e ,
f o r t h e e s t i m a t e o f t h e degree o f
deposition i s questionable. The t i m e r a t e s o f d e p o s i t i o n o f t h e d e p o s i t a b l e p a r t o f t h e sediment were i n v e s t i g a t e d next.
I n a c l o s e d system, l i k e t h e e x p e r i m e n t a l s e t - u p used, t h e
d e p o s i t a b l e p a r t o f t h e sediment can be r e p r e s e n t e d by t h e c o n c e n t r a t i o n d i f f e r -
-
*
-
-
= (Co C)/(Co C ) where C, t h e eq suspended sediment c o n c e n t r a t i o n a t t i m e t, r e p r e s e n t s t h e f r a c t i o n o f t h e
ence Co
Ceq.
Therefore, t h e r a t i o C
d e p o s i t a b l e sediment d e p o s i t e d d u r i n g t i m e t a f t e r t h e b e g i n n i n g o f d e p o s i t i o n . F i g u r e s 20a and 20b show two l o g a r i t h m i c - n o r m a l p l o t s o f C* v e r s u s t h e nond i m e n s i o n a l i z e d time, t / t 5 0 ,
f o r l o w and h i g h i n i t i a l c o r i c e n t r a t i o n s , where t50
i s t h e t i m e a t which C* = 0.50.
536
a.
Low i n i t i a l concentration.
b.
High i n i t i a l concentration.
99 98 95
90
eo 70
d 60 5 50 *" 4 0 30
20 10
5 2
0I
Fig. 20.
Deposition r a t e s (Mehta and Partheniades, 1973, 1975).
A l l points f a l l very c l o s e l y on s t r a i g h t l i n e s described by t h e equations
c *= - 1
Jz;;
where
*
J'
--
exp(-
2 5) dw
537 where a2 i s t h e g e o m e t r i c s t a n d a r d d e v i a t i o n .
Therefore, t h e deposition rates
can be expressed by one and t h e same law, a t l e a s t up t o t h e l i m i t o f 20,000 r a t h e r t h a n by t h e t h r e e d i f f e r e n t laws (Eqs.
For i n i t i a l sediment c o n c e n t r a t i o n s above 25,000
(1962).
ppm
7, 8 and 9 ) g i v e n by Krone ppm t h e d e p o s i t i o n
r a t e s s t a r t d e v i a t i n g from Eqs. 16 and 17 (Mehta and Partheniades, 1973).
Sim-
i l a r d e v i a t i o n s and somewhat e r r a t i c b e h a v i o r were a l s o observed i n some cases i n which of
T~
T ~ ,
was s u b s t a n t i l a l y l o w e r t h a n
T
~
~ ~t h a t . f o r l o w v a l u e s It~ appears
t h e d e p o s i t i o n r a t e s depend more and more on t h e s i z e o f aggregates formed
i n t h e main f l o w and t h a t t h e random process o f t h e f o r m a t i o n o f t h e s e aggregates i s r e f l e c t e d i n t h e e r r a t i c nature o f t h e deposition.
Deposition o f k a o l i n i t e
suspensions i n s a l t w a t e r and o f Maracaibo sediments ( R o s i l l o n and Volkenborn, 1964) a l s o f o l l o w e d t h e laws g i v e n by Eqs. 16 and 17.
The same i s t r u e f o r depo-
s i t i o n r a t e s o f San F r a n c i s c o Bay mud o b t a i n e d by Krone (1962) and t h e w r i t e r ( P a r t h e n i a d e s , 1962, 1965) i n flume experiments; however, i n t h e two l a s t cases t 5 0 was much h i g h e r t h a n i n t h e e x p e r i m e n t s i n t h e a n n u l a r channel.
The obvious
e x p l a n a t i o n f o r t h i s d i f f e r e n c e l i e s i n t h e i n t e n s i t y and s t r u c t u r e o f t u r b u l e n c e i n t h e f a r - b e d region.
I n t h e r o t a t i n g c h a n n e l - r i n g system t h e t u r b u l e n c e s t r u c -
t u r e i n t h a t r e g i o n i s d e f i n e d , as a l r e a d y e x p l a i n e d , e x c l u s i v e l y by t h e near bed I n t h e flume experi-
f l o w c o n d i t i o n s and s p e c i f i c a l l y by t h e bed shear s t r e s s .
ments t h e t u r b u l e n c e l e v e l was s i g n i f i c a n t l y i n f l u e n c e d by t h e c o n d i t i o n s i n t h e r e t u r n pipe, where t h e boundary s t r e s s e s were much h i g h e r t h a n t h o s e i n t h e flume, and by t h e t u r b u l e n c e generated by t h e pump blades and t h e d i f f u s e r s . F i g u r e 21 shows an example o f c o r r e l a t i o n o f t h e b a s i c d e p o s i t i o n r a t e parameters a2 and t 5 0 w i t h
*
T~
and i n i t i a l sediment c o n c e n t r a t i o n , C o , from which
t h e f o l l o w i n g general c o n c l u s i o n s have been reached: 1.
The mean d e p o s i t i o n time, t 5 0 , i n c r e a s e s f i r s t w i t h
*
v a l u e s o f T between 1 and 1.5. b* increasing T ~ .
* T~
r e a c h i n g i t s peak f o r
From t h e n on i t decreases w i t h
*
2.
For t h e same ‘ c ~ , t 5 0 seems t o i n c r e a s e w i t h i n c r e a s i n g d e p t h a l t h o u g h t h e r e
3.
The s t a n d a r d d e v i a t i o n ,
i s considerable overlapping. i t s peak, l i k e t50,
*
a2, i n c r e a s e s i n i t i a l l y w i t h i n c r e a s i n g ‘ c ~ r e a c h i n g
*
f o r v a l u e s o f ‘ c ~between 1 and 1.5.
From t h e n on, i t
appears t o remain c o n s t a n t o r t o s l i g h t l y decrease w i t h i n c r e a s i n g
4.
*
T ~ .
a2 seems t o be a f f e c t e d v e r y l i t t l e by t h e d e p t h o f f l o w b u t t h e n a t u r e o f t h a t e f f e c t i s not c l e a r yet. These c o n c l u s i o n s can be e x p l a i n e d as f o l l o w s .
We r e c a l l f i r s t t h a t o2 i s a
measure o f t h e spread o f t h e s e t t l i n g t i m e o f t h e f l o c p o p u l a t i o n , and t50i s t h e mean s e t t l i n g time.
I n d i c a t i n g by h t h e channel depth and by USf t h e s e t t l i n g
v e l o c i t y o f a f l o c f r o m t h e p a r t o f t h e e n t i r e f l o c p o p u l a t i o n which e v e n t u a l l y reaches t h e bed, t h e n t h e t i m e t s g i v e n by t h e r a t i o
538
c o u l d be considered as a measure o f s e t t l i n g time f o r t h a t p a r t i c u l a r f l o c . Figure 22, shows a schematic p r o b a b i l i t y density function,
f(1og t s ) f o r the
l o g a r i t h m o f the s e t t l i n g time, ts. For near zero v e l o c i t i e s the f l o c s are expected t o reach t h e i r maximum s i z e and
tSs0
i t s minimum value, since the f l o c
d i s r u p t i n g stresses are then n e g l i g i b l e and the f l o c s i z e would then depend s o l e l y on the frequency o f p a r t i c l e and f l o c c o l l i s i o n as they s e t t l e t o ,the bed.
Moreover and f o r the same reason, the smaller f l o c s j o i n t r e a d i l y together
t o form l a r g e r u n i t s something t h a t would narrow down the spread o f the d i s t r i b u t i o n and would decrease a2. With i n c r e a s i n g shear s t r e s s more and more f l o c s a r e e l i m i n a t e d from the permanently depositable population s t a r t i n g from the smallest f l o c diameter and increasing gradually.
Therefore, the depositable
population i s composed o f l a r g e r and l a r g e r f l o c s as the bed shear s t r e s s i n creases.
A t the same time the turbulence i n t e n s i t y and the c o e f f i c i e n t o f
v e r t i c a l eddy d i f f u s i v i t y increase thus r e t a r d i n g the eventual deposition o f a depositable f l o c t o the bed.
I n f a c t both Usf and ts i n Eq. 19 are apparent values i n a t u r b u l e n t flow f i e l d . An increase o f the apparent s e t t l i n g time i s r e f l e c t e d i n a s h i f t i n g o f l o g t s 5 0 t o the r i g h t i n Fig. 22.
Thus the s e t t l i n g
time and, therefore, the r a t e s o f deposition, are subjected w i t h increasing
‘ c ~t o
two opposing e f f e c t s : an increase o f the sizes o f f l o c s which eventually s e t t l e and an increase o f the t u r b u l e n t d i f f u s i v i t y . According t o Fig. 21, the d i f f u s i o n e f f e c t appears t o be i n i t i a l l y dominant.
However, t h e s e t t l i n g v e l o c i t y
increases i n p r o p o r t i o n t o the t h i r d power o f the f l o c diameter. value o f
An optimum
i s , therefore, expected t o e x i s t beyond which the weight o f the f l o c s becomes dominant. A t t h a t p o i n t %o reaches i t s highest value w h i l e beyond i t the increase o f Usf w i l l overcompensate the increase o f eddy d i y f u s i v i t y . The T~
n e t r e s u l t i s a decrease o f the o v e r a l l s e t t l i n g time and a s h i f t i n g o f l o g ts50 t o the l e f t . Regarding now the standard deviation, a2, we f i r s t observe t h a t an increase breaks down the weaker f l o c s and f l o c aggregates thus i n c r e a s i n g the spread o f the d i s t r i b u t i o n o f the f l o c sizes. As l o n g as T~ c T~~~~ e v e n t u a l l y a l l
of
T~
f l o c s deposit; however, the spread o f the f l o c s i z e d i s t r i b u t i o n i s expected t o increase w i t h ‘ c ~ ; t h i s i s indeed the case i n Fig. 21. c e n t r a t i o n , Ceq,
When the e q u i l i b r i u m con-
becomes substantial, the spread s t a r t s narrowing down and even-
t u a l l y i t e i t h e r l e v e l s o f f o r even decreases.
Indeed, Fig. 21, as w e l l as o t h e r
\
experiments n o t shown i n t h i s paper, i n d i c a t e a sharp increase o f a2 t o a value * o f T~ between 1 and 1.5 w h i l e f o r values o f T~ above the l i m i t u2 e i t h e r remains constant o r decreases b u t a t a r a t e much smaller than t h e r a t e o f increase f o r
539
* T~
<
1 t o 1.5 (Partheniades, 1984).
log t50
- T~*
It should be noted t h a t t h e peak of t h e
*
curves occurs w i t h i n t h e same range o f
05
10
15
1
23
n
31 33
2 0
T~
.
5370 5280 7680
25
30
TD’
Example o f v a r i a t i o n o f t 5 0 and u2 w i t h Fig. 21. 1973, 1975)
* T~
(Mehta and Partheniades,
f (log tcJ
Fig. 22.
Schematic p r o b a b i l i t y d e n s i t y f u n c t i o n o f l o g t,.
From Eqs. 16 and 17 t h e f o l l o w i n g expression can be d e r i v e d f o r t h e t i m e - r a t e o f deposition:
540
I t should be k e p t i n mind, however, t h a t t h i s e q u a t i o n is v a l i d o n l y f o r a
n o n - d i s p e r s i v e system, i.e.
f o r a f l o w system w i t h o u t l o n g i t u d i n a l c o n c e n t r a t i o n
g r a d i e n t s. The d e p o s i t i o n a l parameters a p p e a r i n g i n Eqs. 14, 15, 1 7 and 18 change as t h e sediment d i s p e r s e s and as i t goes t h r o u g h t h e d e p o s i t i o n - r e s u s p e n s i o n c y c l e s . These changes have been r e c e n t l y i n v e s t i g a t e d by Mehta, t h e a u t h o r and o t h e r s and t h e y r e f l e c t changes i n t h e s i z e and s t r e n g t h o f f l o c s ( D i x i t , Mehta and Partheniades, 1982; Mehta, P a r t h e n i a d e s and McAnnaly, 1982).
A b r i e f summary, o f t h i s
i n v e s t i g a t i o n i s g i v e n by t h e w r i t e r (1984).
A MODEL OF I N T E R A C T I O N BETWEEN FLOW, SUSPENDED SEDIMENT AND BED The summarized e r o s i o n a l and d e p o s i t i o n a l b e h a v i o r o f cohesive sediments appear a t f i r s t g l a n c e t o c o n t r a d i c t t h e hydrodynamic b e h a v i o r o f coarse sediments w h i l e t h e y resemble i n c e r t a i n aspects t h e wash l o a d ( E i n s t e i n , 1950).
A t h e o r e t i c a l model e x p l a i n i n g t h e observed b e h a v i o r o f c o h e s i v e
sediments and l i n k i n g t h e wash l o a d and bed m a t e r i a l l o a d has r e c e n t l y been developed by t h e a u t h o r (1977).
An o u t l i n e of t h e e s s e n t i a l s o f t h a t model i s
h e r e w i t h presented. A c c o r d i n g t o E i n s t e i n ' s o r i g i n a l approach, a near bed sediment g r a i n i n suspension w i l l d e p o s i t i f t h e i n s t a n t a n e o u s hydrodynamic l i f t f o r c e , L, e x e r t e d by t h e f l o w on t h e p a r t i c l e , i s l e s s t h a n t h e submerged w e i g h t o f t h e p a r t i c l e , wb; t h a t i s i f Wb/L
>
1.
L i k e w i s e , a bed p a r t i c l e w i l l be e n t r a i n e d i f L exceeds
t h e summation o f t h e e r o s i o n - r e s i s t i n g f o r c e s .
For coarse g r a i n s E i n s t e i n con-
s i d e r e d t h e submerged w e i g h t o f t h e g r a i n , wb, as t h e o n l y r e s i s t i n g f o r c e .
In
g e n e r a l , however, t h e e n t r a i n m e n t w i l l a l s o be r e s i s t e d by t h e i n t e r p a r t i c l e or i n t e r f l o c f r i c t i o n and i n t e r l o c k i n g f o r c e s and,
i n t h e case o f c o h e s i v e s e d i -
ments, by t h e physico-chemical bonds developed between t h e f l o c s o r p a r t i c l e s and t h e bed.
F r i c t i o n and i n t e r l o c k i n g can be i n c o r p o r a t e d i n wb t h r o u g h a dimen-
sionless coefficient,
B, l a r g e r t h a n u n i t y .
r e p r e s e n t e d by a n e t a t t r a c t i v e f o r c e , bed
(Fig.
23).
The physico-chemical bonds can be
Fa, between t h e p a r t i c l e o r f l o c and t h e
Thus, t h e t o t a l r e s i s t i n g f o r c e i s BWb + Fa and t h e c o n d i t i o n
f o r e r o s i o n becomes:
BWb + Fa
L Next, E i n s t e i n , on t h e b a s i s o f h i s experiments w i t h E l Sammi ( E i n s t e i n and E l Samni, 1949), assumed t h e f o l l o w i n g s t o c h a s t i c form f o r
L:
54 1
I: i s
i n which
t h e mean value o f L and q i s a dimensionless random v a r i a b l e w i t h
mean zero and standard d e v i a t i o n q0 found t o be equal t o spheres.
The p r o b a b i l i t y o f erosion, Pe,
& for
semicircular
then becomes:
w h i l e t h e p r o b a b i l i t y f o r deposit on, Pd, takes t h e form: P
d = Pr (
0 6
'b T -1
)
(24)
It becomes obvious from the above equations t h a t :
n e i t h e r e r o s i o n nor depos t i o n can poss b l y occur; ( 1 ) i f Wb < L < 8Wb + F a ( 2 ) i f L < wb only d e p o s i t i o n occurs; and ( 3 ) i f L > BWb + Fa o n l y e r o s i o n takes place. I n t h e o r i g i n a l E i n s t e i n ' s model L was considered t a k i n g values from
--
t o +-
w h i l e f o l l o w i n g t h e normal d i s t r i b u t i o n law ( E i n s t e i n , 1950; E i n s t e i n and E l Samni, 1949).
It would be more r e a l i s t i c , however, t o assume t h a t L i s l i m i t e d
by an upper bound, Lu, and a lower bound L,. i n t e r v a l s Le L,
L,
-
Ld and Lu
-
L,
The r e l a t i v e magnitude o f the
and t h e r e l a t i v e l o c a t i o n o f the p o i n t s L e y ld,
determine t h e k i n d o f t h e sedimentation process.
We consider f i r s t the s i t u a t i o n where Le
-
Ld > Lu
-
L, which s p e c i f i c a l l y
a p p l i e s t o cohesive sediments, since f o r t h e l a t t e r Fa becomes much l a r g e r than wb and where B a t t a i n s a maximum value due t o the i r r e g u l a r shape o f t h e c l a y p a r t i c l e s and f l o c s . and L,
BWb + Fa.
-
Ld cannot p o s s i b l y l i e between Lu
It f o l l o w s t h a t i n t h i s case simultaneous e r o s i o n and d e p o s i t i o n i s
n o t possible. 1.
The i n t e r v a l then Le
which means t h a t L cannot reach values smaller than wb and l a r g e r than
Lu > Le.
The f o l l o w i n g t h r e e s p e c i a l cases may occur: I n t h i s case e r o s i o n may occur b u t no d e p o s i t i o n since L can
a t t a i n values l a r g e r than Le b u t never smaller than Ld.
T h i s corresponds t o
the case o f r e l a t i v e l y h i g h v e l o c i t i e s .
2.
Ld < L,
< Lu < Le.
I n t h i s case,' as already explained, n e i t h e r e r o s i o n nor
d e p o s i t i o n can p o s s i b l y occur.
The sediment i s simply t r a n s p o r t e d through
the channel, l i k e a wash load, l e a v i n g no t r a c e s on the bed.
3.
L,
< Ld i n which case Lu < Le.
That means t h a t d e p o s i t i o n can occur since
t h e l i f t f o r c e s are s u f f i c i e n t l y low f o r d e p o s i t i o n b u t never h i g h enough f o r erosion.
This case corresponds t o r e l a t i v e l y l o * flows and low bed
s!war stresses.
542
Flow
4L
Fig. 23. Forces due t o f r i c t i o n , (Partheniades, 1977).
i n t e r l o c k i n g , and physico-chemical a t t r a c t i o n
I t f o l l o w s from the above observations t h a t f o r simultaneous erosion and
deposition o f sediment p a r t i c l e s t o take place and f o r the e x i s t e n c e o f a bed l o a d f u n c t i o n (Einstein, 1950) the i n t e r v a l Le the i n t e r v a l L,
-
-
Ld should l i e e n t i r e l y w i t h i n
L.,
It should be noted t h a t as the sediment becomes coarser, the forces Fa
decrease and tend t o zero.
Likewise, w i t h increasing g r a i n s i z e and as the
sediment p a r t i c l e s approach the spherical state, the c o e f f i c i e n t B tends t o u n i t y . This i s the reason why sediments which d i s p l a y t h e behavior o f bed m a t e r i a l load and f o r which a bed l o a d f u n c t i o n e x i s t s , are unexceptionally coarse (Einstein, Anderson, and Johnson, 1940).
RESUSPENSION OF DEPOSITED FINE SEDIMENT
Resuspension o f deposited sediment i s p a r t o f the normal deposition-resuspension cycles i n t i d a l estuaries b u t i t can a l s o be the r e s u l t o f extreme events such as storms, waves and wind d r i v e n currents.
An understanding o f the resu-
spension process o f n a t u r a l l y deposited f l o c s as well as equations f o r the i n i t i ation, degree and r a t e s o f resuspension are n-eeded i n order t o be able t o formul a t e an appropriate sediment source f u n c t i o n i n mathematical models f o r sediment t r a n s p o r t and t o p r e d i c t t u r b i d i t y l e v e l s . The process o f resuspensions has been r e c e n t l y i n v e s t i g a t e d by Mehta and the author (illetha and Partheniades, 1979, 1982). (Partheniades, 1984).
A summary o f t h i s work i s given i n
The discussion o f t h i s research phase i s , here l i m i t e d t o
the s t r u c t u r e and engineering p r o p e r t i e s o f the deposited bed since the l a t t e r i s d i r e c t l y r e l a t e d t o the s t r u c t u r e o f the f l o c aggregates which c o n s t i t u t e the main theme o f t h i s paper. I t was recognized since the author's e a r l y experiments (Partheniades, 1962,
543
1965) t h a t f l o c c u l a t e d c o h e s i v e sediment beds formed by d e p o s i t i o n d i s p l a y an i n c r e a s i n g r e s i s t a n c e t o e r o s i o n w i t h depth ( F i g . 24).
In contrast, a r t i f i c a l l y
p r e p a r e d c o h e s i v e beds o f u n i f o r m c o n s i s t e n c y g e t eroded a t c o n s t a n t r a t e s ( F i g . 25).
0.04 0021,
rb:0.207Nm2
,,
, ,,, , ,, , , ,, , , ,,
,, , ,,
,I-1
' 0 2 4 6 8 I0 12 14 16 182022 24
Fig. 24. R e l a t i v e suspended sediment c o n c e n t r a t i o n versus t i m e f o r a s t r a i f i e d ' k a o l i n i t e bed d e p o s i t e d i n d i s t i l l e d w a t e r and eroded under T~ = 0.207 N/m (Mehta and Partheniades, 1979, 1982).
3.
Fig. 25. Suspended sediment c o n c e n t r a t i o n versus t p e f o r a u n i f o r m k a o l i n i t e (Mehta and P a r t h e n i a d e s , bed i n d i s t i l l e d w a t e r eroded under T~ = 0.413 N/m 1979, 1982). S i n c e t h e r e i s no d e p o s i t i o n and no exchange between bed f l o c s and suspended f l o c s , t h e gradual decrease o f t h e resuspension r a t e s o f n a t u r a l l y d e p o s i t e d sediment and, under c e r t a i n circumstances, t h e e v e n t u a l t e r m i n a t i o n o f s c o u r i n g can be e x p l a i n e d o n l y by an i n c r e a s e o f t h e i n t e r p a r t i c l e c o h e s i v e bonds w i t h depth.
The l a t t e r cannot be a t t r i b u t e d t o t h e overburden c o n s o l i d a t i o n p r e s s u r e
f i r s t because t h a t p r e s s u r e i s e x t r e m e l y low and second, because i f i t were i m p o r t a n t i t would have shown up i n t h e o r i g i n a l u n i f o r m bed o f Fig. 25 where t h e average d e n s i t y was o f t h e o r d e r o f t h e d e n s i t y o f a d e p o s i t e d bed.
The bond
i n c r e a s e i s s i m p l y i n h e r e n t i n t h e i n t e r n a l f l o c s t r u c t u r e as f l o c s s e g r e g a t e
544
during deposition w i t h larger and denser flocs depositing f i r s t t h u s forming a s t r a t i f i e d bed of increasing density and strength w i t h depth. Figures 26 and 27 show the variation of the density and the shear strength of the deposited bed respectively. The f i r s t was determined by a specially designed 2.5 cm diameter netal tube in which bed core samples were frozen by an alcohol-dry ice mixture and by subsequent slicing of tine frozen samples. The shear strength T~ was taken as equal t o the applied shear s t r e s s T~ when the r a t e o f concentration increase of suspended sediment, dC , reaches a near zero value. The increase of the overall bed density with depth r e f l e c t s an increase of the floc density. The l a t t e r can be attributed, t o both a denser arrangement of p a r t i c l e s and packets in each aggregate and t o d larger number of particles per packet, as observed by Quinn (1980). The increase of shear strength of the bed r e f l e c t s the resistance of the p a r t i c l e s and of the various aggregates against removal and reentrainment by the flow-induced drag and l i f t forces (Partheniades, 1977). Indeed, both the submerged weight of an aggregate and the number of the physico-chemical bonds holding i t t o the bed are expected to increase with incredsing size, density and number of units ( p a r t i c l e s and packets) per aggregate. SHEAR
-E
N
r
I-
n
W
n
STRENGTH
T, (Nrn-')
r?sJ 23
F i g . 26. Variation of bed density with
depth (Mehta and Partheniades, 1982).
Figure 27. Variation of bed shear strength with depth (Mehta and Partheniades, 1982).
For a given bed shear s t r e s s , T ~ ,resuspension will proceed a t reducing rates until the total depth of erosion Az will be the one corresponding t o rS = rb in F i g . 27. The instantaneous resuspension rat2s E have been found t o be functions of the excess bed shear s t r e s s T~ - I according to the equation: S
= exp
(
? a
'b
-
' r ~ S
(25)
545
where Eo and a a r e e m p i r i c a l c o e f f i c i e n t s .
The c o n c e n t r a t i o n o f suspended
sediment d u r i n g resuspension can be described by t h e l a w
c=cs
( 1
-
where 6 i s a c o e f f i c i e n t ranging between 0.013 h r - l and 0.038 hr-';
t i s the time
i n hours and Cs i s 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 when any f u r t h e r resuspension has ceased,
.e.
when e r o s i o n has proceeded t o a depth f o r , w h i c h
d i s t r i b u t i o n o f density
ps
is = T ~ .
The
o f t h e deposited bed f o l l o w s t h e law
-5
pS H - z - = c ( 7 ) pS
where
ps
i s t h e average bed d e n s i t y ; H i s t h e bed t h i c k n e s s and c and 5 a r e
dimensionless c o e f f i c i e n t s equal t o 0.794 and 0.288 theniades, 1982). time, Tdc, i.e.
O f s i g n i f i c a n t importance t o resuspension i s t h e consol i d a t i o n
t h e t i m e between t h e completion o f t h e d e p o s i t i o n process and t h e
beginning o f resuspension. strength
T~
= T
r e s p e c t i v e l y (Mehta and Par-
S
Figure 28 shows t h e d i s t r i b u t i o n o f t h e e r o s i v e shear
f o r t h e i n d i c a t e d f o u r values o f Tdc.
It i s obvious t h a t T~
increases s i g n i f i c a n t l y w i t h t i m e p a r t i c u l a r l y f o r t h e f i r s t few hours a f t e r dep o s i t i o n b u t t h i s increase may c o n t i n u e f o r a very l o n g t i m e and i t appears t o be due t o a t h i x o t r o p i c rearrangement o f p a r t i c l e s w i t h i n t h e f l o c c u l a t e d network o f t h e deposited bed.
It i s r e f l e c t e d i n an increase o f 5 and Fa i n Eqs. 21 t o
24.
Fig. 28. V a r i a t i o n o f c r i t i c a l e r o s i o n s t r e s s w i t h depth fpr k a o l i n i t i c bed i n t a p water f o r v a r i o u s consol i d a t i o n times, Tdc (Mehta and Partheniades, 1982).
546 SUMMARY AND CONCLUSIONS T u r b i d i t y due t o f i n e sediment suspensions i n n o n - s t r a t i f i e d n a t u r a l w a t e r bodies i s l i n k e d t o t h e f l o w - i n d u c e d bed shear s t r e s s e s and t o t h e p h y s i c o chemical p r o p e r t i e s o f t h e sediment-water system.
Considering deposition, t h e
e n t i r e f l o w f i e l d can be d i v i d e d i n t o a near-bed r e g i o n o f h i g h shear r a t e s and a f a r - b e d r e g i o n o f l o w shear r a t e s and o f n e a r l y homogeneous t u r b u l e n c e .
The
former determines t h e degree o f d e p o s i t i o n , t h a t i s t h a t p r o p o r t i o n o f t h e e n t i r e sediment which forms f l o c s w i t h s u f f i c i e n t l y h i g h bonds t o s e t t l e on t h e bed, w h i l e t h e l a t t e r c o n t r i b u t e s o n l y t o t h e f o r m a t i o n o f f l o c s and f l o c aggregates.
E q u a t i o n s f o r t h e degree and r a t e s o f d e p o s i t i o n have been developed i n
terms o f t h e bed shear s t r e s s and o t h e r parameters r e p r e s e n t i n g t h e p h y s i c o chemical p r o p e r t i e s o f t h e sediment-water system.
I n n a t u r a l f l o w systems t h e s e
parameters a r e expected t o v a r y as t h e sediment d i s p e r s e s d u r i n g e v e r y d e p o s i tion-resuspension cycle. D e p o s i t e d sediment beds d i s p l a y a s t r o n g degree o f s t r a t i f i c a t i o n w it h an i n c r e a s i n g d e n s i t y and shear s t r e n g t h with depth.
T h i s i s due t o a n a t u r a l p r o -
cess o f f l o c s e g g r e g a t i o n w i t h r e s p e c t t o s i z e , d e n s i t y and s t r e n g t h d u r i n g t h e process o f d e p o s i t i o n .
Indeed, as r e c e n t e l e c t r o n microscope s t u d i e s have shown,
f l o c s a r e composed o f p a c k e t s o f i n d i v i d u a l c l a y p a r t i c l e s i n book-type arrangements.
The denser and l a r g e r f l o c s c o n t a i n l a r g e r p a c k e t s t h u s p o s s e s s i n g h i g h e r
shear s t r e n g t h than s m a l l e r f l o c s .
As a r e s u l t , t h e e r o s i o n r a t e s d i m i n i s h
r a p i d l y as t h e e r o s i o n proceeds and t h e y may s t o p a l t o g e t h e r a t a d e p t h where t h e f l o w - i n d u c e d shear s t r e s s e s become equal t o t h e e r o s i v e shear s t r e n g t h o f t h e bed.
Experimental e q u a t i o n s have been d e r i v e d f o r t h e d i s t r i b u t i o n o f bed den-
s i t y w i t h depth, t h e i n s t a n t a n e o u s e r o s i o n r a t e s and t h e t i m e v a r i a t i o n o f suspended sediment c o n c e n t r a t i o n d u r i n g t h e process o f resuspension.
The c o n s o l -
i d a t i o n time, t h a t i s t h e t i m e between t h e c o m p l e t i o n o f d e p o s i t i o n and t h e b e g i n n i n g o f resuspension, has a s i g n i f i c a n t e f f e c t on t h e e r o s i v e shear s t r e n g t h o f t h e bed and on t h e e r o s i o n r a t e s .
A mathematical model o f s e d i m e n t - f l o w i n t e r a c t i o n has been developed which e x p l a i n s t h e above phenomena and l i n k s t h e b e h a v i o r o f t h e bed m a t e r i a l l o a d and o f t h e wash load.
REFERENCES B1 i n c o , P. H. and Partheniades, E. , "Turbulence C h a r a c e t r t s t i c s i n Free S u r f a c e Flows o v e r Smooth and Rough Boundaries", J o u r n a l o f Hydr. Res., I.A.H.R., Vol. 9, NO. 1, pp. 44-71, 1971. Casagrande, A., "The S t r u c t u r e o f Clay and i t s Importance i n Foundation E n g i n e e r i n g " , C o n t r i c u t i o n s t o S o i l Mechanics, Boston SOC. o f C i v i l Engrs, 1940, p. 72 and J o u r n a l , Boston SOC. o f C i v i l Engrs., Vol. 19, A p r i l , 1932.
547
Committee on T i d a l H y d r a u l i c s , Corps o f Engineers, U. S. Army, " S o i l as a F a c t o r i n S h o a l i n g Processes, a L i t e r a t u r e Review", Tech. B u l l . No. 4, 1960, 47 pages. D i x i t , J. G., Mehta, A. J. and Partheniades, E., " R e d e p o s i t i o n a l P r o p e r t i e s of Cohesive Sediments D e p o s i t e d i n a Long Flume", Report No. UFL/COEL-82/002, Dept. o f Coastal and Oceanographic Engrg. , August, 1982. E i n s t e i n , H. A. "The Bed-Load F u n c t i o n f o r Sediment T r a n s p o r t a t i o n i n Open Channel flows", T e c h n i c a l B u l l e t i n No. 1026, U. S. Department o f A g r i c u l t u r e , Washington, D. C., 1950. " A D i s t i n c t i o n between Bed E i n s t e i n , H. A., Anderson, A. G., and Johnson J. W., Load and Suspended Load i n N a t u r a l Streams", T r a n s a c t i o n s , 'American Geophysical Union, Vol. 21, P a r t 2, 1940, pp. 628-633. E i n s t e i n , H. A., and E l Samni, S. A., "Hydrodynamic Forces on a Rough W a l l " , Review o f Modern Physics, Vol. 21, 1949, pp. 520-524. Krone. R. B.. "Flume S t u d i e s o f t h e T r a n s o o r t o f Sediment i n E s t u a r i a l S h o a l i n q Processes'. F i n a l Report, Hydr. Engrg: and S a n i t a r y Engrg Res. Lab., Univ.. o f C a l i f o r n i a , Berkeley, C a l i f . , 1962. Krone, R. B., "A Study o f R h e o l o g i c P r o p e r t i e s o f E s t u a r i a l Sediments". Final Report No. 63-8 Hydr. Engrg Lab and S a n i t a r y Engrg Res. Lab., U n i v e r s i m C a l i f o r n i a , Berkeley, C a l i f . , 1963. Kusuda, T., Koga, K., Yorozu, H., and M a y a , Y., " D e n s i t y and S e t t l i n g V e l o c i t y o f F l o c s " , Memoirs o f t h e F a c u l t y o f Engrg., Kyushu Univ., Vol. 41, NO. 3, 1981, PP. 259-280. Mehta, A.'J. , and Partheniades, E., " D e p o s i t i o n a l Behavior o f Cohesive Sediments", T e c h n i c a l Report No. 16, Coastal and Oceanographic Engrg Lab., U n i v e r s i t y o f f l o r i d a , G a i n e s v i l l e , fla., March 1973. Mehta, A. J., and Partheniades, E., "An I n v e s t i g a t i o n o f t h e D e p o s i t i o n P r o p e r t i e s o f fl o c c u l a t e d F i n e Sediments", J o u r n a l o f Hydraul i c Research, Vole 12, NO. 4, 1975, pp. 361-381. Mehta, A. J. and Partheniades, E., ' K a o l i n i t e Resuspension P r o p e r t i e s " , Tech. Note, J o u r n a l o f t h e H y d r a u l i c s Div., ASCE, Vol. 104, No. HY4, Proc. Paper 14477, A p r i l , 1979, pp. 409-416. Mehta, A. J., and Partheniades, E., "Resuspension o f D e p o s i t e d Cohesive Sediment Beds", Proceedings, 1 8 t h Coastal E n g i n e e r i n g Conference, V01.2, Cape Town, So. A f r i c a , Nov. 14-19 9 1982, PP* 1569-1588. Mehta, A. J., Partheniades, E. and McAnally, W., "Properties o f Deposited K a o l i n i t e i n a Long Flume", Proceedings, Hydr. Div. Conf. on A p p l i e d Research t o H y d r a u l i c P r a c t i c e , ASCE, Jackson, Miss., Aug, , 1982. Partheniades, E., "A s t u d y o f E r o s i o n and D e p o s i t i o n o f Cohesive S o i l s i n S a l t Water", t h e s i s p r e s e n t e d t o t h e U n i v e r s i t y o f Cal i f o r n ' i a , a t Berkeley, C a l i f . i n 1962, i n p a r t i a l f u l f i l l m e n t o f t h e r e q u i r e m e n t s f o r t h e degree of D o c t o r o f Philosophy. Partheniades, E., " E r o s i o n and D e p o s i t i o n o f Cohesive S o i l s " , J o u r n a l o f t h e H y d r a u l i c s D i v i s i o n , ASCE, Vol. 91, No. HY1, Proc. Paper 4204, Jan., 1 9 6 5s PP. 105-1 39. Partheniades, E., " U n i f i e d View o f Wash Load and Bed M a t e r i a l Load", J o u r n a l of t h e H y d r a u l i c s D i v i s i o n , ASCE, Vol. 103, No. HY9, Proc. Paper 13215, September, 1977; pp: 17557-1 057. E n g i n e e r i n g P r o p e r t i e s o f E s t u a r i n e Sediments", L e c t u r e Partheniades, E., No. 16, N o r t h A t l a n t i c T r e a t y O r g a n i z a t i o n , Advanced Study I n s t i t u t e of E s t u a r y Dynamics, Lisbon, P o r t u g a l , June 1973. Partheniades, E., "Cohesive Sediment T r a n s p o r t Mechanics and E s t u a r i n e S e d i m e n t a t i o n " , L e c t u r e Notes, I n t e r n a t . Course on Sediment T r a n s p o r t i n E s t u a r i n e and Coastal Environment', Poona, I n d i a , Nov. 26-Dec. 15, 1979, p. 164. Partheniades, E., "A Fundamental Framework f o r Cohesive Sediment Dynamics", Proceedings, Cohesive Sediment Dynamics Workshop, Tampa, F l o r i d a , Nov. 12-14, 1984 ( I n p r i n t ) . Partheniades, E., and Kennedy, J. F., " D e p o s i t i o n a l Behavior o f F i n e Sediment i n a T u r b u l e n t F l u i d M o t i o n " , Proceedings, 1 0 t h Conference on Coastal
548
E n g i n e e r i n g , Tokyo, Japan, Vol. 11, Chapt. 41, Sept., 1966, pp. 707-729. Partheniades, E., Cross, R. H., and Ayora, A., " F u r t h e r R e s u l t s on t h e D e p o s i t i o n o f Cohesive Sediments", Proceedings, 1 1 t h Conference on Coastal E n g i n e e r i n g , London, England, Vol. 11, Chapt. 47, Sept., 1968, pp. 723-742. Partheniades, E., and Paaswell, R. E., " E r o d i b i l i t y o f Channels w i t h Cohesive Boundary", J o u r n a l o f t h e H y d r a u l i c s D i v i s i o n , A X E , Vol. 96, No. HY 3, Proc. Paper 71 56, March 1970, pp. 755-771. "What i s an E s t u a r y : P h y s i c a l V i e w p o i n t " , E s t u a r i e s , E d i t e d by P r i t c h a r d , D. W., G. H. L a u f f , Publ. No. 83, Am. Assoc. Adv. o f Science, 1967. Quinn, M. J., "A Scanning E l e c t r o n Microscope Study o f t h e M i c r o s t r u c t u r e o f Dispersed and F l o c c u l a t e d Kaol i n i t e Clay t a k e n o u t o f Suspension", A t h e s i s p r e s e n t e d t o t h e U n i v e r s i t y o f F l o r i d a i n p a r t i a l f u l f i , l l m e n t of t h e Requirement f o r t h e Degree o f Master o f Science, 1980. R o s i l l o n , R., and Volkenborn, C., "Sedimentacibn de M a t e r i a l Cohesiva en Agua Salada", Diploma Thesis, Dept. o f C i v i l Engrg., Univ. o f Z u l i a , Maracaibo, Venezuel a, 1964. Terzaghi , K., "Erdbaumechanik", F. Deuticke, Vienna, A u s t r i a , 1925. Van Olphen, H., "An I n t r o d u c t i o n t o Clay C o l l o i d Chemistry", I n t e r s i c e n c e P u b l i c a t i o n , 1963. APPENDIX
'a
i n s t a n t a n e o u s suspended sediment c o n c e n t r a t i o n reference concentration a t y = a
C*
co - c/co - c
C
CEC ceq
eq c a t i o n exchange c a p a c i t y e q u i l i b r i u m c o n c e n t r a t i o n o f suspended sediment
ceq
** ceq CO CS
E EO
f 0 Fa
i n i t i a l c o n c e n t r a t i o n o f suspended sediment steady s t a t e c o n c e n t r a t i o n when resuspension ceases i n s t a n t a n e o u s resuspension r a t e e m p i r i c a l r e f e r e n c e resuspension r a t e function of ( ) a n e t a t t r a c t i v e physico-chemical f o r c e between a p a r t i c l e o r f l o c and t h e bed
L
mixing i n t e n s i t y depth o f f l o w bed t h i c k n e s s proportional i t y constant l i f t f o r c e on sediment g r a i n o r f l o c
i
average v a l u e o f L
Ld
l i f t force a t the threshold f o r deposition
Le
l i f t force a t the threshold f o r erosion
LQ
l o w e r bound o f 1
LU
upper bound o f
G h H
k
L
,
549
average M S S o f a f l o c Manning's f r i c t i o n c o e f f i c i e n t number o f p a r t i c l e s o r f l o c s p e r u n i t volume number of f l o c s w i t h a number o f n p r i m a r y p a r t i c l e s p e r f l o c
m
n N Nn
number o f p r i m a r y p a r t i c l e s i n t h e suspension
NO
Pr 'e
p r o b a b i l i t y f o r erosion
'd
p r o b a b i l i t y f o r deposition
'r r
probability of
tC
radius o f a f l o c time average t i m e between f l o c c o l l i s i o n
tf
time o f flocculation
t50
mean v a l u e o f d e p o s i t i o n t i m e
t
s e t t l i n g time
tS
ts50 T
mean v a l u e o f ts
Tdc
consolidation time 1o c a l v e l o c i t y f r i c t i o n velocity
U U*
settling velocity o f a floc
"sf
median s e t t l i n g v e l o c i t y o f a sediment suspension
VS
submerged w e i g h t o f sediment p a r t i c l e o r f l o c
'b Y YO Y
d i s t a n c e f r o m t h e o r e t i c a l bed d i s t a n c e from bed a t which u = 0
-
[ l o g b; l)/(T; - 1)501/0y d i s t a n c e below bed s u r f a c e H - z dimension1 ess c o e f f i c i e n t d i m e n s i o n l e s s c o e f f i c i e n t a c c o u n t i n g f o r i n t e r p a r t i c l e f r i c t i o n and i n t e r l o c k i n g ; a l s o exponent r a d i u s o f c o n t a c t area between two f l o c s uniform turbulent d i f f u s i v i t y r a t e o f energy decay p e r u n i t volume o f f l u i d
Z Z'
a
B Ar E E
0
t u r b u l e n t d i f f u s i v i t y a t a d i s t a n c e y from t h e bed dimension1 ess c o e f f i c i e n t normally d i s t r i b u t e d dimensionless l i f t c o e f f i c i e n t
rl
standard d e v i a t i o n o f K
9
5 P pS
= = = = =
rl
Karman's u n i v e r s a l c o n s t a n t dynamic v i s c o s i t y d i m e n s i o n l e s s exponent water density bed d e n s i t y
= average v a l u e o f pS
u2
= standard d e v i a t i o n o f
C*
550 = d i m e n s i o n l e s s exponent
O3 O
= maximum i n t e r p a r t i c l e t e n s i l e f o r c e
O
= standard d e v i a t i o n o f d i s t r i b u t i o n o f C
T
= shear s t r e s s i n general
ma x
Y
Tb
Tbmi n
* eq
= average bed shear s t r e s s
= minimum bed shear s t r e s s below which a l l sediment d e p o s i t s
Tz *
= t h r e s h o l d bed shear s t r e s s a c c o r d i n g t o K r o n e ' s d e f i n i t i o n
'Ib
= Tb"bmi
T
= maximum shear s t r e s s
T
ma x
C
T
w
S
n
= threshold value o f
T~
for the i n i t i a t i o n o f erosion
= e r o s i v e s t r e n g t h o f d e p o s i t e d beds = dummy v a r i a b l e
551
A D R I A 84 A JOINT REMOTE SENSING EXPERIMENT P. SCHLITTENHARDT
Commission o f t h e European Communities, J o i n t Research Centre, I s p r a E s t a b l i s h ment,
21020 I s p r a (Va), ( I t a l y ) .
ABSTRACT A j o i n t experiment has been c a r r i e d o u t i n t h e N o r t h e r n A d r i a t i c Sea t o c a l i b r a t e and e v a l u a t e remotely sensed d a t a and t o c o n t r i b u t e t o t h e i n t e g r a t i o n o f i n s i t u measurements, remote sensing and hydrodynamic m o d e l l i n g . A f t e r a summary o f t h e experiment examples a r e shown o f the a i r b o r n e sensor data, s e a t r u t h measurements and s a t e l l i t e images. INTRODUCTION W i t h i n t h e l a s t few years, t h e a p p l i c a t i o n o f remote sensors f o r s t u d i e s o f ocean-and-coastal p h i c research.
zones have demonstrated i m p o r t a n t c a p a b i l i t i e s f o r oceanograThe J o i n t Research Centre (JRC) o f I s p r a o f t h e European Commu-
n i t i e s s t a r t e d i n 1977 a research a c t i v i t y on t h e use o f Coastal Zone C o l o r Scanner (CZCS) data f o r q u a n t i t a t i v e d e t e r m i n a t i o n o f c h l o r o p h y l l and suspended matter concentration.
D i f f e r e n t a l g o r i t h m s have been developed f o r t h e atmos-
p h e r i c c o r r e c t i o n and i n t e r p r e t a t i o n o f CZCS d a t a and s e r i e s o f b i o - o p t i c a l and atmospheric measurements d a t a s e t s were gathered w i t h i n t h i s research a c t i v i t y . As a r e s u l t o f these s t u d i e s i n the past, t o extend t h e a p p l i c a t i o n o f remote sensing techniques and t o promote t h e c o l l a b o r a t i o n o f t h e i n v o l v e d i n s t i t u t e s and o r g a n i z a t i o n s , the A D R I A 84 experiment was organized. THE EXPERIMENT The ADRIA 84 experiment has been conceived t o a t t a i n several o b j e c t i v e s :
-
a p p l i c a t i o n o b j e c t i v e s i n c l u d e t h e assessment o f remote sensing techniques t o g e t h e r w i t h i n s i t u measurements f o r a q u a n t i t a t i v e i n v e s t i g a t i o n o f dynamica1 processes ;
-
measurement method o b j e c t i v e s i n c l u d e a study o f an a i r b o r n e L i d a r system and i t s comparison w i t h o t h e r remotely sensed data and i n s i t u measurements ;
-
oceanographic o b j e c t i v e s i n c l u d e t e s t and e v a l u a t i o n o f e x i s ’ t i n g hydrodynamic models.
552
This large-scale experiment should c o n t r i b u t e t o t h e i n t e g r a t i o n o f i n s i t u measurements, remote s e n s i n g and hydrodynamic m o d e l l i n g as a new o v e r a l l method f o r oceanographic r e s e a r c h and sea p r o t e c t i o n . REMOTE SENSING - Ocean C o l o u r (CZCS) - t e m p e r a t u r e (NOAA-7 I R ) F i e 1 d measurement ATM/METEO d a t a vertical profiles n o n - v i s i b l e parameters
Model 1 ing - calibration - initiation - operation
-
The e x p e r i m e n t has been c a r r i e d o u t between August 23 and September 7. A l a r g e number o f s c i e n t i f i c teams f r o m n a t i o n a l l a b o r a t o r i e s have p a r t i c i p a t e d .
The e x p e r i m e n t was m a i n l y sponsored b y t h e EEC ( f l i g h t s and measurements), CNR (oceanographic v e s s e l s and p l a t f o r m ) , Regione Emilia-Romagna ( o c e a n o g r a p h i c
v e s s e l ) , Regione F r i u l i - V e n e z i a - G i u l i a ( f l i g h t ) , and DFVLR ( L i d a r f l i g h t ) . +The s e l e c t e d t e s t s i t e , t h e N o r t h e r n p a r t o f t h e A d r i a t i c Sea, i s t h e shallowe s t area o f the Mediterranean.
Large r i v e r r u n - o f f , extended l a g o o n systems
and t h e s e a s o n a l l y v a r i a b l e heat-exchange a t t h e s u r f a c e g i v e r i s e t o r e l a t i v e l y l a r g e l e v e l s and g r a d i e n t s o f p h y t o p l a n k t o n .
The lagoon systems and t h e r i v e r s ,
as w e l l as w a t e r d r a i n e d f r o m h e a v i l y c u l t i v a t e d s u r r o u n d i n g l a n d , a r e respons i b l e f o r t h e p o l l u t a n t s i n these r e g i o n s .
F o r p r o t e c t i o n and s u r v e i l l a n c e
a deeper u n d e r s t a n d i n g i s necessary o f t h e mechanisms o f e v o l u t i o n , p r o p a g a t i o n and t h e i m p a c t o f t h e p o l l u t a n t s . The main a c t i v i t i e s a r e summarized i n T a b l e I, t h e i n v o l v e d i n s t t u t e s a r e l i s t e d i n T a b l e 11. TABLE I
-
Summary o f t h e main a c t i v t i e s
Spacecraft (radiometers) NIMBUS 7 : CZCS NOAA 7 / 8 : AVHRR LANDSAT : TM METEOSAT Airborne instruments I R scanner LL t e l e v i s i o n camera MSS scanner OC R/ SC R PRT 5 L i d a r system
CESNA CESNA DO
-
28
DO - 28
S h i p and p l a t f o r m measurements EOS r a d i o m e t e r PRT 5 r a d i o m e t e r CTO p r o f i l e s Water s a m p l i n g Chlor. fluorescence ( c o n t . ) A1 ga 1 umi n i scence Phytoplankton analyses Particle size distribution Trace elements Mini-Lidar Spectral radiometer
553
-
TABLE I 1
Involved i n s t i t u t e s
I s t i t u t o d i Automazioni N a v a l i del CNR ( V i a l e Causa 18, 1-16141 Genova, Mr. Siccardi) I s t i t u t o d i B i o l o g i a d e l Mare, CNR ( R i v a 7 M a r t i n i 1364/A 1-30122 Venezia, M r . Franco) DFVLR Oberpfaffenhofen (D-8031 Wessling, V. Amman, Chr. Werner) GKSS Forschungszentrum (D-2054 Geestacht, R. D o e r f f e r ) I n s t i t u t f i r Meereskunde (DUsternbrooker Weg 21 D-2300 K i e l , A. Schmi t z - P f e i f f e r ) I s t i t u t o d i Ricerca s u l l e Onde E l e t t r o m a g n e t i c h e ( V i a P a n c i a t i c h i 64 1-50127 Firenze, L.P. P a n t a n i ) I s t i t u t o p e r l o S t u d i o d e l l a Dinamica d e l l e Grandi Masse (Ca' Papadopoli, 1364 San Paolo, 1-30125 Venezia, L. Alberotanza) Regione E m i l i a Romagna, Assessorato Ambiente e D i f e s a del Suolo ( V i a d e i W i l l e 21, 1-40121 Bologna, G. N e s p o l i , L. Montanari) O s s e r v a t o r i o G e o f i s i c o Sperimentale (PO Box 2011 , 1-34016 T r i e s t e , A. M i c h e l a t o ) U n i v e r s i t l d i Firenze, I s t i t u t o B o t a n i c 0 ( V i a M i c h e l l i 1, 1-50127 F i r e n z e , M. I n n a m o r a t i ) U n i v e r s i t a d i Bologna, I s t i t u t o CNR ( V i a P i c h a t B e r t C.R., 1-40127 Bologna, R. Guzzi) U n i v e r s i t a t Oldenburg (Ammerlander Heerstrasse 67, D-2900 Oldenburg, R. Reuter) Regione F r i u l i - V e n e z i a - G i u l i a , D i r e z i o n e Regionale L a v o r i Pubbl i c i , Centro R i levamento I d r o m e t e o r o l o g i c o (Riva N a z a r i o Sauro 8, 1-34124 T r i e s t e , Mr. Verri) U n i v e r s i t a t Regensburg, FB Physik ( U n i v e r s i t a t s t r . 31, D-8400 Regensburg, H. Krause) I s t i t u t o d i F i s i c a d e l l 'Atmosfera, CNR ( P i a z z a l e L. S t u r z o 31, 1-00144 Roma, G. Dalu) S c i e n t i f i c Data ( V i a B o l i s 10, 1-35100 Padova, A. Ongaro) SHIPS R / V B a n n o c k (CNR) - Trieste/Ancona R/V Umberto d'Ancona (CNR) - Venezia - S. G i o r g i o R/V L i t u s (CNR) - Venezia - S. G i o r g i o R / V Daphne (Reg. Emilia-Romagna) - Cesenatico
A1 RBASE -to Venezia/Lido LIPV, AEROCLUB G. A n i e l l o t t o , S. N i c o l l , Venezia-Lido Aeroporto Venezia/Tessera, S e r v i z i o Meteorolbgico TESSERA
EXAMPLES OF MEASUREMENTS The d a t a c o l l e c t e d d u r i n g t h e A D R I A 84 campaign w i l l be e d i t e d i n a j o i n t Data Catalogue.
The examples o f a i r b o r n e sensor d a t a and s e a t r u t h d a t a i s a
f i r s t i n d i c a t i o n o f t h e measurements, w h i l e t h e s a t e l l i t e maps and 1983
-
-
from 1982
r e p r e s e n t t h e t y p i c a l summer s i t u a t i o n i n t h e N o r t h e r n A d r i a t i c Sea.
A i r b o r n e sensors For t h e a i r b o r n e measurements t h r e e a i r c r a f t s have been i n v o l v e d : two DO 28 from DFVLR and one CESNA Skywagon 28 f r o m t h e Regione F r i u l i - V e n e z i a - G i u l i a . O v e r f l i g h t s o f about 2 hour d u r a t i o n have been executed between 11.00 a.m.
1.00 p.m. l o c a l time, on several days d u r i n g t h e experiment. l i n e i s shown i n F i g . 5 t o g e t h e r w i t h t h e s h i p s t a t i o n g r i d .
and
The general f l i g h t
554
F i g . 1 shows t h e c h a r a c t e r i s t i c s o f t h e passive sensors on board o f one a i r p l a n e , w h i l e F i g . 2 presents t h e c h a r a c t e r i s t i c s and p r i n c i p a l way o f o p e r a t i o n o f t h e i n v o l v e d L i d a r system. To p r o v i d e an overview o f t h e s p a t i a l c o l o u r and SST v a r i a t i o n s , t h r e e r e p r e s e n t a t i v e s i g n a l s were s e l e c t e d from t h e OCR, SCR and PRT data, and p l o t t e d a l o n g t h e f l i g h t l i n e s . These s i g n a l s a r e : - GBR t h e Green-to-Blue Radiance R a t i o o f t h e OCR channels 3 ( 5 5 1 nm) and 1 (446 nm), i n d i c a t i v e o f
-
changes i n water c o l o u r ;
SST t h e sea s u r f a c e temperature from t h e PRT-readings ; FLH t h e r e l a t i v e Fluorescence-Line-Height
o f t h e c h l o r o p h y l l fluorescence
peak (682 nm) f r o m t h e SCR channels. F i g . 3 g i v e s an example o f t h i s " p a t t e r n q u i c k l o o k " f o r f l i g h t N o . 3 (30.8. 1984). On t h e presented i n - s h o r e t r a c k t h e r i v e r Po plume i s marked c l e a r l y by t h e GBR s i g n a l and t h e FLH s i g n a l . F i g . 4 shows t h e same s i g n a l s f o r f l i g h t
N O . 4 (31.8.1984).
The d i f f e r e n t p a t t e r n o f these two days i l l u s t r a t e s t h e
*
s h a r t - t e r m changes i n ocean c o l o u r i n t h e Po area over one day. I n F i g . 4 t h e s i g n a l s o f t h i s p a s s i v e sensor are a l s o compared w i t h t h e L i d a r s i g n a l s o f t h e same day and time. The L i d a r system i s working w i t h two p u l s e l a s e r s which make i t p o s s i b l e t o use two e x c i t a t i o n wavelengths s i m u l t a n e o u s l y : one i n t h e uv a t
380 nm, another i n t h e v i s i b l e a t 451 nm. Seven o p t i c a l bands a r e a v a i l a b l e f o r s i g n a l d e t e c t i o n between near uv and t h e r e d p a r t o f t h e spectrum. The average bandwidth i s about 10 nm. I n F i g . 4, t h e f o l l o w i n g s i g n a l s a r e shown :
-
e x c i t a t i o n 451 nm/detection 684 nm
-
e x c i t a t i o n 308 nm/detection 533 nm
-
signal proportional t o the concentration
-
signal proportional t o the concentration
-
signal inversely proportional t o the
o f chlorophyll i n the water ; o f Gelbstoff i n the water ;
-
e x c i t a t i o n 451 nm/detection 533 nm
l i g h t a t t e n u a t i o n c o e f f i c i e n t a t the corresponding wavelength ;
-
e x c i t a t i o n 308 nm/detection 344 nm
-
signal inversely proportional t o the
l i g h t a t t e n u a t i o n c o e f f i c i e n t a t t h e corresponding wavelenght. Ship board measurements Surface parameters can be c o r r e l a t e d w i t h remote sensing data, w h i l e v e r t i c a l h y d r o l o g i c measurements such as s a l i n i t y and temperature a r e necessary f o r hydrodynamic s t u d i e s .
Therefore, the ship-board measurements have been c a r r i e d
o u t u s i n g continuous sampling methods a t t h e sea s u r f a c e as w e l l as v e r t i c a l sampling and p r o f i l e s a t t h e s t a t i o n s . shown i n F i g . 5. areas.
The standard g r i d o f t h e s t a t i o n s i s '
The f o u r i n v o l v e d s h i p s have covered d i f f e r e n t b u t o v e r l a p p i n g
F i g . 6 shows
-
as an example
-
t h e v e r t i c a l d i s t r i b u t i o n o f oxygen,
555
SENSOR FIELD OF VIEW O C R I S C R SKY V I E W
F i g . 1.a.
P a s s i v e a i r b o r n e sensors
556
-
DFVLR OBERPFAFFENHOFFEN V. AMANN
PASSIVE SENSOR CHARACTERISTICS MULTISPECTRAL LINESCANNER (M'S, BENDIX) F.0.V ? SO0 J.F.0.V 2 , s mrad 8 \!IS ( A 1 40-60nm), 2 KIR (Ab30nm) Spectral Bands 1TIR (8-14 Um) SNR 14-235 OCEAN COLOR SCANRADIOMETER (OCR, DFVLR) F.0.V f 50" Angular Resol. 3,6" (60 mrad) 5 V J S ( A A 11-13 nm!, 1 NIR (AA 13nm) Spectral Bands det. by 8Bit Encoding SNR Q
SIX-CHANNEL RADIOMETER (SCR, DFVLR) F.0.V So Spectral Bands 5 RED (10-13nm), 1 NIR ( A A 13nm) SNR > 103 PRECISION RADIATION THERMOMETER (PRT5, BARNES) 2" F.0.V Spectral Band 9,s - 1 1 . 5 p m NEAT 0,lK HASSELBLAD CAMERA (HB) Focal length 40 mm Film Agfa Color S O S , Kodak IR 2 4 4 3 CALJBRATION SOURCES Optical range Ulbricht Sphere, Radiance Standard NBS Blackbodies Thermal range
Fig. 1.b. Passive airborne sensors.
557
LlDAR CHARACTERISTICS lasers emissionlnm pulse energylmJ pulse lengthlns repetition rate receiver detectionlnm bandwidthlnm photomultipliers transient recorder
Laser transmitt%
dye
dye
308 70 12
450 5 8
533 5 8
max 20 Hz Cassegrain f110, f = 4 m
34413801500153316501685 10 EM1 9812,9818 500 MHz, 6 bit
-
field of view
excimer
1
Blue or UV laser b e a m
D. DIEBEL-LANGOHR R. REGTER
Principle of operation of airborne lidar
target area
Fi g. 2. Lidar c h a r a c t e r i s t i c s .
558
SIGNAL ...
Presented signals:
*
- G B R the green-to-blue radiance
I
ratio of the OCR channels
3 (551 nm) and 1 (446nm)
- F L H the height of the chlorophyll fluorescence line from the
SCR
- SST the sea surface temperature from the P R T
GBR
FLH
Flight No. 3
SST 30.8.1984
F i g . 3. S i g n a l s f r o m a i r b o r n e p a s s i v e sensors
-
Flight
N O 3
-
EE W b O b
E B
c)O .. ..
x x
EE r c )
.. ..
mc) d m E E Ax
EE Wc)
8s: .. .. E B x x
EE
r m
:,I
5.. 8.. x x
d
cq
559
W
>
v)
v)
.r
m a 0
+
560
3
15'
10
Section at 44'
GRID OF SHIP STATIONS FROM P . FRANC0 ISTITUTO DI BIOLOGIA DI MARE, VENEZIA
Fig. 5. ADRIA 84.
-
F l i g h t Track
-
Ship S t a t i o n s .
44'40' N
56 1
0
Adria '84
BANNOCK
10
Date: 1984.08.30
?
Loool Time:
20
d
:
18.50
L
a 0 30
D 0 3 5
0
t --
7
Adria '84
t
fj 18
BANNOCK
Date: 1984.08.29
'i
20
0
d
:
Loool Time: 17.48
L
n 0
Ba29 30
F i g . 6. Two v e r t i c a l CTD p r o f i l e s (kms).
562 206 0
21 2
TEMPERATURE 4
-
6
-
8
-
E I In
lb -
23
12
'
21
14
I
w
19
-
18
12
-
74
-
26
-
70
["C] I
10
-
227
-
-
15
--
28
3'
-
34
-
I1
-
7
-
4
-
6
-
30
8
10
-E
12
I I-
16
L
2
l4
1R
0 ?O 2/
(4 26 28
30
37 34
206
21 2
227
-
-
-
-
37.
-
-
0 NO3 IlIIzId
NH3
SALlNITY [Voo]
-
REGIONE E M I L I A ROMAGNA, 6. MONTANARI
F i g . 7. D i s t r i b u t i o n o f t e m p e r a t u r e and s a l i n i t y a t 44"40' N o r t h .
563
l i g h t transmission, s a l i n i t y and temperature f o r the two s t a t i o n s 8035 and 6029 (Bannock). Fig. 7 presents the distribution of temperature and s a l i n i t y The s a l i n i t y distribution derived from as measured i n a section a t 44'40". CTD profiles i s i l l u s t r a t e d in Fig. 8. Sate1 1 i t e images Different s a t e l l i t e data have been acquired within the experiment. A l i s t i n g of the scenes received from the Coastal Zone Color Scanner (CZCS) on NIMBUS-7 and the Advanced Very High Resolution Radiometer (AVHRR) on NOAA-satellite i s given in Table 111. The archived data will be elaborated w i t h i n the next few months. TABLE I11
- Acquired
CZCS and NOAA-7 scenes
Orbit number 840817 18 19 84082 1 22 23 24 840827 28 29 30 31 840901 02 03 840905 840908 840910
czcs
Time GMT
29359 29373 29387 29414 29428 29442 29456 29497
1011 1034 1047 0941 0959 1017 1035 0947
29525
1023
-
-
-
-
-
-
-
29622 29663 29692
1036 0958 1036
-
NOAA-7 Time GMT
0309/1439 - /1427 0244/ 0413/1401 0401/ 1349 0348/1336 0335/ 1324/1506 0323/1453
-
Three s a t e l l i t e images from 1982 and 1983 a r e presented i n Figs. 8,9aand 9b. These images i l l u s t r a t e the typical summer situation of the Northern Adriatic Sea dominated by the Pd r i v e r outflow - a similar situation to the period of the campaign. F i g . 8 i s an example of sea surface temperature in "C derived from AVHRR-data (NOAA-7) of 7 July 1983. The correction of the measured temperature f o r the atmospheric absorption was obtained by an improved s p l i t window technique. F i g s . 9a and 9b are axamples of chlorophyll concentration maps in mg/m 3 derived from CZCS-data (NIMBUS-7). The images have been corrected f o r atmospheric e f f e c t s and are processed t o geographically r e c t i f i e d maps.
564
Fig. 8. Example o f sea sur f ace temperature i n "C derived from AVHRR-data on NOAA-7 s a t e l l i t e . Day : 7.6.1983.
565
3
Fig. 9a. C h l o r o p h y l l c o n c e n t r a t i o n i n mg/m d e r i v e d from CZCS on NIMBUS-7 s a t e l l i t e . a) O r b i t 18121 ; Day 27.05.1982 ; GMT 10.34.
566
3 Fig. 9b. Chlorophyll concentration in mg/m derived from CZCS on NIMBUS-7 s a t e l l i t e . b ) Orbit 18176 ; Day 31.05.1982 ; GMT 9.55.
567
The atmospheric c o r r e c t i o n procedure and the pigment a l g o r i t h m were e l a b o r a t e d e s p e c i a l l y f o r the r a t h e r t u r b i d near c o a s t a l water o f t h e N o r t h e r n A d r i a t i c Sea.
I n F i g . 9a - O r b i t No.18121,
Day 27.05.1982,
GMT 10.34
-
the largest
p a r t o f the area i s c h a r a c t e r i z e d by low c h l o r o p h y l l c o n c e n t r a t i o n ( < O.lmg/m 3 ) b u t near t h e I t a l i a n coast t h e c h l o r o p h y l l c o n c e n t r a t i o n increases t o even more than 10 mg/m3.
F i g . 9b
-
O r b i t No.18176,
h i g h q u a l i t y n e a r l y w i t h o u t any haze.
Day 31.05.82,
GMT 9.55
-
i s o f very
I t g i v e s v e r y good d e t a i l s o f the
P6 r i v e r o u t f l o w and t h e h o r i z o n t a l d i s t r i b u t i o n o f t h e r i v e r water. CONCLUSIONS From t h e f i r s t e v a l u a t i o n o f t h e experimental data, i t i s be i e v e d t h a t t h e main g o a l s o f t h e experiment c o u l d be achieved.
-
The i n s i t u measurements o f p h y s i c a l , biochemical and o p t i c a
water parame-
t e r s can be used f o r a d e s c r i p t i v e a n a l y s i s o f t h e hydrographic s i t u a t i o n as w e l l as a comparison w i t h t h e remotely sensed data.
-
The d i f f e r e n t f l i g h t and s a t e l l i t e d a t a w i l l c o n t r i b u t e t o t h e e v a l u a t i o n
-
The complete data w i l l be s i g n i f i c a n t f o r the c a l i b r a t i o n and v a l i d a t i o n o f
o f passive and a c t i v e sensors. t h e hydrodynamic model. The f u r t h e r e l a b o r a t i o n and i n t e r p r e t a t i o n o f t h e c o l l e c t e d d a t a w i l l cont r i b u t e t o t h e i n t e g r a t i o n o f i n s i t u measurements, remote sensing and hydrodynamic model 1ing
.
REFERENCES Camagni, P. e t a l . , 1983. Marine remote sensing a c t i v i t i e s o f JRC I s p r a . Proc. EARSeL/ESA Symp. Remote Sensing A p p l i c a t i o n s f o r Environment Studies, Brussels, 26-29 A p r i l 1983, 35-49. F e r r a r i , E. e t a l . , 1984. Remote m o n i t o r i n g o f sediments and c h l o r o p h y l l as t r a c e r o f p o l l u t a n t movements i n a mediterranean c o a s t a l area. IGARSS'84 Symp., Strasbourg, 27-30 August 1984, 701-707. Nykjaer, L. and S c h l i t t e n h a r d t , P. and Sturm, B., 1984. Q u a l i t a t i v e and quant i t a t i v e i n t e r p r e t a t i o n o f ocean c o l o r - NIMBUS-7 CZCS imagery o f t h e N o r t h e r n A d r i a t i c Sea f r o m May t o September 1982, JRC t e c h n i c a l note SA/1.04.E2.84.05. S c h l i t t e n h a r d t , P.M. ( e d i t o r ) , 1984. Workshop on Remote Sensing o f Coastal T r a n s p o r t i n t h e N o r t h e r n A d r i a t i c Sea, 11-12 October 1983, S.A. 1.05.E2.85. 03, JRC-Ispra, I t a l y . S c h l i t t e n h a r d t , P.M., 1984. ADRIA 84 a j o i n t remote sensing experiment, Tech. Note No. 1.05 .E2.84.91, JRC- I s p r a , I t a l y . S c h l i t t e n h a r d t , P.M., 1985. ADRIA 84 - a j o i n t remote sensing experiment, 2nd Report : A s h o r t summary o f t h e experiment, Tech. Note No.1.05.E2.85.20, JRC-Ispra, I t a l y .
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569
IDENTIFICATION OF HYDROGRAPHIC FRONTS BY AIRBORNE LIDAR MEASUREMENTS OF GELGSTOFF DISTRIBUTIONS D. DIEBEL-LANGOHR, T. HENGSTERMANN and R. REUTER
U n i v e r s i t a t Oldenburg, Fachbereich P h y s i k , P.0.Box
2503, 2900 Oldenburg
( F e d e r a l Republ i c o f Germany)
ABSTRACT L a s e r remote s e n s i n g ( l i d a r ) a l l o w s t h e measurement o f v a r i o u s h y d r o g r a p h i c parameters o f i n t e r e s t f o r w a t e r q u a l i t y m o n i t o r i n ? and oceanographic r e s e a r c h . Operated f r o m a i r c r a f t a n e a r l y s y n o p t i c i n v e s t i g a t i o n o f extended a r e a s o f t h e sea i s a c h i e v e d w h i c h i s p a r t i c u l a r l y i m p o r t a n t i f h y d r o g r a p h i c c o n d i t i o n s a r e changing r a p i d l y i n t i m e , e.g. due t o t i d e s . Among t h e substances t h a t a r e det e c t a b l e w i t h l i d a r , dissolved organic m a t t e r ( G e l b s t o f f ) gives r i s e t o very dominant f l u o r e s c e n c e s i g n a l s and i s t h u s s e n s i t i v e l y measured i n c o a s t a l w a t e r s where t h e c o n c e n t r a t i o n i s g e n e r a l l y h i g h . Due t o i t s good s t a b i l i t y G e l b s t o f f can be used as a n a t u r a l t r a c e r f o r t h e s t u d y o f t r a n s p o r t and m i x i n g , and f o r t h e i d e n t i f i c a t i o n o f c h a r a c t e r i s t i c w a t e r masses and f r o n t a l systems. R e s u l t s o b t a i n e d d u r i n ? f l i g h t s o v e r t h e N o r t h Sea and t h e n o r t h e r n A d r i a t i c w i t h t h e Oceanographic L i d a r System (OLS) developed a t t h e U n i v e r s i t y of 01 denburg a r e presented.
I NTRODUCTIOM I n t h e p a s t decade t h e use o f remote s e n s i n g methods has become a n i m p o r t a n t t o o l i n oceanoaraphic research. The reason f o r t h i s l i e s i n t h e complex d y namics of t h e ocean where t h e c h a r a c t e r i s t i c l e n g t h s c a l e s o f many r e l e v a n t processes range between 1 0 and 1000 km. The o c c u r r e n c e o f eddies as a g e n e r a l f e a t u r e o f t h e o c e a n i c c i r c u l a t i o n has been v e r i f i e d w i t h remote sensincj. F r o n t s
i n c o a s t a l r e g i o n s produced by r i v e r plumes and thermal processes i n t h e upper w a t e r l a y e r w i t h h i g h g r a d i e n t s o f t h e p h y s i c a l and b i o l o o i c a l parameters have been observed w h i c h a r e changina r a p i d l y i n t i m e due t o t i d e s . C l a s s i c a l methods o f e x p e r i m e n t a l oceanographic r e s e a r c h , i n p a r t i c u l a r t h e c o l l e c t i o n o f d a t a b y use o f s h i p b o a r d i n s t r u m e n t s , do n o t always l e a d t o a n i n f o r m a t i o n r e p r e s e n t a t i v e f o r l a r g e r a r e a s o f t h e sea. An i n t e r p r e t a t i o n o f t h e s e phenomena r e q u i r e s a d d i t i o n a l measurements w i t h s e n s o r systems, f r o m which s y n o p t i c d a t a w i t h h i a h h o r i z o n t a l r e s o l u t i o n can be deduced. The use o f s a t e l l i t e s f o r measurements o f t h e c o l o u r o f t h e sea by means o f scanning o p t i c a l r a d i o m e t e r s has l e d t o remarkable r e s u l t s o v e r t h e open ocean. These spaceborne systems a l l o w a s e n s i t i v e and q u a n t i t a t i v e d e l i n e a t i o n o f t h e b i o l o c j i c a l p r o d u c t i v i t y i n t h e s u r f a c e l a y e r s i n c e t h e w a t e r c o l o u r can be r e -
510
l a t e d t o t h e presence o f p h y t o p l a n k t o n (Gordon e t a l . ,
1983). However, problems
a r i s e f o r t h e i n t e r p r e t a t i o n o f d a t a o b t a i n e d o v e r c o a s t a l w a t e r s where o t h e r substances such as suspended m i n e r a l s and d i s s o l v e d o r g a n i c m a t t e r ( G e l b s t o f f ) o p t i c a l l y compete w i t h p h y t o p l a n k t o n piaments and a l s o i n f l u e n c e t h e w a t e r c o l o u r . Due t o t h i s , and because o f t h e maskinq e f f e c t o f t h e atmosphere which i s r e s p o n s i b l e f o r a b o u t 90 X o f t h e d e t e c t e d s i g n a l ( F i s c h e r , 1984), a q u a n t i t a t i v e e v a l u a t i o n o f t h e s e d i f f e r e n t substances has n o t been a c h i e v e d so f a r . A n o t h e r method o f remote s e n s i n o makes use o f l i d a r systems ( f i g h t
petection
and r a n g i n ? ) i n s t a l l e d i n a i r c r a f t . A c l o s e r d i s t a n c e between t h e i n s t r u m e n t and t h e w a t e r s u r f a c e - t h e f l i g h t h e i g h t b e i n g t y p i c a l l y 100-500 m
-
reduces
a t m o s p h e r i c e f f e c t s on t h e d e t e c t o r s i y a l s i o n i f i c a n t l y and a l l o w s t h e a p p l i c a t i o n o f a n a r t i f i c i a l monochromatic l i g h t s o u r c e , t h e l a s e r . I t w i l l be shown below t h a t l a s e r s p e c t r o s c o p i c methods r e s u l t i n a s p e c i f i c measurement o f c e r t a i n o p t i c a l l y d e t e c t a b l e w a t e r column parameters. Some o f t h e s e q u a n t i t i e s c a n b e d e t e c t e d as v e r t i c a l p r o f i l e s down t o w a t e r depths c o r r e s p o n d i n g t o a b o u t 6 o p t i c a l a t t e n u a t i o n l e n g t h s , i f l a s e r p u l s e s o f a few nanoseconds l e n g t h and a f a s t h i g h r e s o l v i n g s i g n a l r e c e i v e r system a r e u t i l i z e d . T h i s d e p t h p r o f i l i n g c a p a b i l i t y i s d i s c u s s e d i n a s e p a r a t e paper g i v e n i n t h i s volume.
A t a n o p e r a t i o n a l l e v e l , d a t a on t h e s p e c t r a l l i a h t t u r b i d i t y and on t h e c o n c e n t r a t i o n o f p h y t o p l a n k t o n and G e l b s t o f f can be o b t a i n e d . I n p a r t i c u l a r t h e l a t t e r which i s m a i n l y b r o u g h t i n t o t h e sea b y r i v e r r u n - o f f i s s e n s i t i v e l y measured due t o i t s s p e c i f i c f l u o r e s c e n c e and possesses a n o t a b l e s t a b i l i t y w i t h r e s p e c t t o b i o l o g i c a l and chemical d e g r a d a t i o n . I t t h u s r e s p r e s e n t s a conservat i v e p r o p e r t y o f seawater and can be used as a n a t u r a l t r a c e r f o r t h e d e s c r i p t i o n o f c h a r a c t e r i s t i c w a t e r masses and f o r t h e i d e n t i f i c a t i o n o f h y d r o g r a p h i c f r o n t s i n c o a s t a l w a t e r s where t h e c o n c e n t r a t i o n i s g e n e r a l l y h i g h . The oceanographic l i d a r i s t h u s a s e n s o r a l l o w i n g
- an
e x t e n s i o n o f s h i p b o a r d measurements w i t h l i m i t e d h o r i z o n t a l r e s o l u t i o n b y
airborne data o f t h e surface l a y e r obtained over l a r g e r areas,
-
a n e a r l y synoptic c a l i b r a t i o n o f large-scale s a t e l l i t e data along t h e a i r c r a f t f l i g h t t r a c k i n terms o f t h e d i f f e r e n t w a t e r c c m s t i t u e n t s d e t e r m i n i n g t h e w a t e r c o l o u r measured b y t h e s a t e l l i t e ,
-a
s p e c i f i c measurement o f h y d r o g r a p h i c parameters c h a r a c t e r i z i n g f r o n t a l sys-
tems i n c o a s t a l w a t e r s ; t h i s c a n b e done w i t h i n t i m e p e r i o d s t h a t a r e s h o r t compared w i t h t h e t i m e s c a l e s o f dominant c i r c u l a t i o n processes, e.g.
tidal
currents. SPECTROSCOPIC PROPERTIES
OF SEAWATER
I r r a d i a t i o n o f seawater w i t h monochromatic l i g h t l e a d s t o a n e m i s s i o n of secondary l i g h t due t o e l a s t i c and i n e l a s t i c o p t i c a l i n t e r a c t i o n s w i t h t h e w a t e r i t s e l f and w i t h p a r t i c u l a t e and d i s s o l v e d m a t t e r . An e m i s s i o n spectrum o b t a i n e d
571 i n v i t r o f r o m a water sample taken from t h e German B i g h t , Fig. 1, d i s p l a y s s t r u c t u r e s t h a t a r e r e l a t e d t o e l a s t i c s c a t t e r i n g a t w a t e r molecules and hydros o l s , t o i n e l a s t i c w a t e r Raman s c a t t e r i n g , and t o t h e fluorescence o f G e l b s t o f f and c h l o r o p h y l l 5 ( t h e l a t t e r , n o t g i v e n i n t h e f i g u r e , peaking a t 685 nm w i t h a h a l f w i d t h of 20 nm). Since these s t r u c t u r e s a r e s p e c t r a l l y w e l l separated and a r e c o n s t a n t i n shape i n a f i r s t approximation t h e y can be s p e c i f i c a l l y measured by an a p p r o p r i a t e choice o f a few d e t e c t i o n wavelengths. The p a r t i a l jnterference o f G e l b s t o f f f l u o r e s c e n c e w i t h w a t e r Raman , s c a t t e r i n g and c h l o r o phyll
a fluorescence
has t o be taken i n t o account by a d d i t i o n a l d e t e c t i o n
channels nearby these s i g n a l s f o r an i d e n t i f i c a t i o n o f t h e i r t r u e b a s e l i n e .
I
i.0
I
Fig. 1.
Y
2 5
0.8
1
0.6
.Lt
Emission spectrum o f a n a t u r a l water sample taken from t h e German B i g h t . E x c i t a t i o n wavel e n g t h i s 308 nm. The peaks a t 308 and 344 nm a r e due t o e l a s t i c s c a t t e r i n g and water Raman s c a t t e r i n g , r e s p e c t i v e l y . The broad f l u o r e s c e n c e band c e n t r e d a t 420 nm i s due t o G e l b s t o f f . The curve i s c o r r e c t e d f o r t h e s p e c t r a l response
L.
r:
0.4
(0
E 6 4J
0.2
)
Among these s p e c t r a l data, water Raman s c a t t e r i n p represents an i m p o r t a n t i n f o r m a t i o n f o r t h e l i d a r measurina process. The corresponding peak shown i n Fig. 1 i s due t o t h e s t r e t c h i n g v i b r a t i o n o f t h e water molecule b e i n g s h i f t e d by a wavenumber o f 3400 cm-' w i t h r e s p e c t t o t h e e x c i t a t i o n l i n e . The water Raman s c a t t e r e f f i c i e n c y f o l l o w s a l / A 4
l a w w i t h v a r y i n g e x c i t a t i o n wavelength,
w i t h a value o f t h e cross s e c t i o n o f 4.5 ( + .3) hex = 468 nm ( S l u s h e r and Derr, 1975.).
cm2/molecule
sr at
Except f o r a weak temperature dependence
( Y a l r a f e n , 1967), t h e e f f i c i e n c y i s c o n s t a n t w i t h r e s p e c t t o o t h e r thermodynamic qua n t it i e s
.
Besides i n s t r u m e n t a l f a c t o r s as t h e l a s e r i n t e n s i t y , t h e d e t e c t o r s e n s i t i v i t y and t h e f l i a h t h e i g h t , water Raman b a c k s c a t t e r measured w i t h l i d a r i s thus g i v e n by t h e p e n e t r a t i o n depth o f t h e l a s e r beam i n t o t h e water column and t h e a t t e n u a t i o n o f t h e Raman s c a t t e r e d l i g h t on i t s way back t o t h e water surface. As a r e s u l t o f t h e t h e o r y t h e s i g n a l i n t e n s i t y i s p r o p o r t i o n a l t o cex +
CR,
t h e sum
o f t h e l i g h t a t t e n u a t i o n c o e f f i c i e n t s a t t h e l a s e r e x c i t a t i o n and t h e Raman s c a t t e r i n g wavelength (Kung and I t z k a n , 1976). I n case o f h i g h l y t u r b i d waters
512
where m u l t i p l e s c a t t e r i n g a t hydrosols becomes dominant, t h e s i g n a l measured has a tendency t o approach t h e d i f f u s e a t t e n u a t i o n c o e f f i c i e n t s kex + kR (Gordon , 1982). I n a d d i t i o n t o t u r b i d i t y measurements i n t h e s u r f a c e l a y e r , water Raman s c a t t e r i n g serves as a c a l i b r a t i o n s i g n a l f o r n o r m a l i z i n g t h e f l u o r e s c e n c e data o f G e l b s t o f f and c h l o r o p h y l l 2 t o a c o n s t a n t measuring volume. The normalized f l u o r e s c e n c e i s t h e n p r o p o r t i o n a l t o t h e c o n c e n t r a t i o n o f t h e s e substances; concerning c h l o r o p h y l l
a,
t h e dependence o f t h e i n v i v o f l u o r e s c e n c e e f f i c i e n c y
on t h e ambient l i g h t f i e l d must a l s o be considered (Gunther,
1985). The impor-
tance o f t h i s n o r m a l i z a t i o n has been p o i n t e d o u t by several a u t h o r s , e.g.
Hoge
and S w i f t (1982). E r r o r s i n t h e d e t e r m i n a t i o n o f G e l b s t o f f c o n c e n t r a t i o n s r e s u l t i n g f r o m t h e n e c e s s i t y o f measuring i t s f l u o r e s c e n c e and w a t e r Raman s c a t t e r i n o a t d i f f e r e n t d e t e c t i o n wavelengths where t h e l i g h t a t t e n u a t i o n coe f f i c i e n t s and hence t h e water volume under c o n s i d e r a t i o n a r e n o t i d e n t i c a l w i l l be discussed i n t h e f o l l o w i n g s e c t i o n . GELBSTOFF AS AN OPTICAL TRACER SUBSTANCE Absorption The e x i s t e n c e o f d i s s o l v e d o r g a n i c m a t t e r as a common f e a t u r e o f most c o a s t a l areas has been w e l l known f o r a l o n g time. Because o f i t s s t r o n g l i g h t absorpt i o n a t b l u e wavelengths which r e s u l t s i n a y e l l o w c o l o u r o f these waters, K a l l e (1937) c h a r a c t e r i z e d t h i s substance by t h e t e r m G e l b s t o f f . Chemically, G e l b s t o f f i s c l a s s i f i e d as humic substance which describes o r g a n i c macromolec u l e s w i t h h i g h l y v a r y i n g composition t h a t a r e produced d u r i n a t h e decay o f p l a n t s . The major p a r t o f G e l b s t o f f i s o f c o n t i n e n t a l o r i g i n and i s brought i n t o t h e sea by r i v e r discharge. An i n s i t u p r o d u c t i o n due t o p l a n k t o n blooms and b a c t e r i a l growth may be o f relevance i n t h e open ocean, i n c o a s t a l ' w a t e r s such c o n t r i b u t i o n s seem t o be n e g l i g i b l e ( H d j e r s l e v , 1980). Q u a n t i t a t i v e measurements o f G e l b s t o f f i n a b s o l u t e c o n c e n t r a t i o n u n i t s a r e hampered b y t h e d i f f i c u l t y o f e s t a b l i s h i n g a p p r o p r i a t e a n a l y t i c a l methods which can be used on a r o u t i n e b a s i s . The c u r r e n t l y u t i l i z e d procedure r e l a t e s Gelbs t o f f t o i t s l i g h t a b s o r p t i o n a t b l u e wavelengths a f t e r removing p a r t i c u l a t e s by f i l t r a t i o n w i t h a nominal pore w i d t h o f 0.5
urn. Based on u l t r a f i l t r a t i o n o f
B a l t i c water and weighing o f t h e d r y o r g a n i c r e s i d u a l
, Nyquist
(1979) f i n d s an
a b s o r p t i o n c o e f f i c i e n t a = 0.212 m - l a t X = 450 nm f o r a c o n c e n t r a t i o n o f 1 mg/l. However, t h e d e d u c t i o n o f G e l b s t o f f c o n c e n t r a t i o n s f r o m a b s o r p t i o n data i n volves several assumptions: ( i ) f i l t r a t i o n w i t h t h e g i v e n pore s i z e e f f i c i e n t l y separates d i s s o l v e d and suspended absorbing m a t e r i a l . T h i s concerns t h e problem t h a t b a c t e r i a and m i n e r a l p a r t i c l e s , b o t h s l i g h t l y absorbing, a r e n o t completely r e t a i n e d and t h a t t h e l a r g e r o r g a n i c macromolecules show a tendency t o f o r m
573
c o l l o i d s w i t h i n c r e a s i n g s a l i n i t y and/or pH (Haekel, 1982). ( i i ) t h e composition o f G e l b s t o f f i s supposed t o be c o n s t a n t i n d i f f e r e n t areas o f t h e sea i n c l u d i n g b r a c k i s h and freshwater and possesses an i n v a r i a n t s p e c i f i c a b s o r p t i o n spectrum. I t w i l l be shown t h a t t h e a b s o r p t i o n s p e c t r a o f r i v e r and open sea water show
d i s t i n c t d i f f e r e n c e s which m i g h t anain be a t t r i b u t e d t o changes o f chemical parameters. As a r e s u l t o f i n v e s t i g a t i o n s made by v a r i o u s working groups (e.g. 1976; Nyquist, 1979; Bricaud e t a l . ,
Lundgren,
1979) and performed i n . d i f f e r e n t areas o f
t h e ocean, t h e a b s o r p t i o n c o e f f i c i e n t ay o f G e l b s t o f f p u r i f i e d water as t h e r e f e r e n c e medium
-
-
measured w i t h r e s p e c t t o
increases e x p o n e n t i a l l y w i t h decreasing
wavelength i n t h e near UV and b l u e p o r t i o n o f t h e spectrum a c c o r d i n g t o aY
~
exp (-bX)
(1)
w i t h b = 0.014 t 0.0025 nm-'.
Combining t h i s w i t h N y q u i s t ' s r e l a t i o n , i t f o l l o w s
f o r the specific attenuation c o e f f i c i e n t
s Y = .212 exp [-b(A-450)]
= a /n
Y
(2)
w i t h n the Gelbstoff concentration. Data o b t a i n e d w i t h h i g h l y r e s o l v i n g spectrophotometers h a v i n g an o p t i c a l p a t h o f 1 m l e n g t h r e v e a l t h a t t h e spectrum i s composed o f several a b s o r p t i o n bands c o v e r i n g t h e whole v i s i b l e , t h e i r o r i g i n b e i n g a c t u a l l y unknown. Two i n t e n s e o v e r l a p p i n g bands w i t h u n i d e n t i f i e d maxima i n t h e UV y i e l d an approximately exp o n e n t i a l shape a t b l u e wavelengths. T h i s f i n d i n g was f i r s t r e p o r t e d by Diehl and Haardt (1980) f o r B a l t i c Sea water and i s a l s o c h a r a c t e r i s t i c o f t h e North Sea and t h e A d r i a t i c (Fig. 2).
I n c o n t r a s t t o t h i s , t h e a b s o r p t i o n spectrum o f r i v e r water which has been examined m a i n l y f o r t h e A d r i a t i c up t o now, i s almost monotonic, whereby t h e e x p o n e n t i a l c h a r a c t e r i s t i c i n t h e b l u e i s about t h e same as w i t h open sea waters (Fig. 3). The occurrence o f d i s t i n c t a b s o r p t i o n bands i s l e s s obvious here. F1 uorescence Another method f o r measuring G e l b s t o f f , which can be a p p l i e d c o n t i n u o u s l y i n s i t u o r by use o f a i r b o r n e l i d a r , takes advantage o f i t s f l u o r e s c e n c e (Fig. 1). The simultaneous r e g i s t r a t i o n o f water Raman s c a t t e r i n g a l l o w s an a b s o l u t e c a l i b r a t i o n o f t h e data w i t h r e s p e c t t o t h e s e n s i t i v i t y o f t h e instrument. A s e l e c t i o n o f t h e f l u o r e s c e n c e d e t e c t i o n wavelength s u f f i c i e n t l y c l o s e t o t h e Raman s c a t t e r wavelength reduces systematic e r r o r s e f f i c i e n t l y r e s u l t i n g from t h e g e n e r a l l y d i f f e r e n t a t t e n u a t i o n c o e f f i c i e n t s a t t h e d e t e c t i o n wavelengths. As a r e s u l t o f t h e t h e o r y o f t h e l i d a r measuring process discussed i n t h e f o l l o w i n g s e c t i o n , t h e water Raman s c a t t e r i n ? and t h e G e l b s t o f f f l u o r e s c e n c e s i g n a l PR and
574 A
-
FLUREX I 23 04 82 Gelbstoff attenuation research platform Nordsee
E
L
u
01
001
1
____ LOO
500
\
700
600
-
I
I
800
A lnml
F i g . 2. S p e c t r a l a t t e n u a t i o n c o e f f i c i e n t o f a w a t e r sample t a k e n f r o m t h e G e r m n Bight.
ADRIA ' 8 L 29 08 1984 Gelbstoff attenuation Po , Brenta, Adige
101
1
\\
Brenta /,Po
1 iI
Loo
~-
I
500
600
700
800 h l n m l
-
F i g . 3. S p e c t r a l a t t e n u a t i o n c o e f f i c i e n t o f w a t e r samples t a k e n f r o m n o r t h Italian rivers.
575
Py r e c e i v e d f r o m a homogeneous w a t e r column can be w r i t t e n as
nR and n a r e t h e s p e c i f i c e f f i c i e n c i e s o f w a t e r Raman s c a t t e r i n g and Y G e l b s t o f f f l u o r e s c e n c e , n t h e G e l b s t o f f c o n c e n t r a t i o n , cw and c t h e sum o f t h e P l i g h t a t t e n u a t i o n c o e f f i c i e n t s a t t h e l a s e r e x c i t a t i o n and t h e d e t e c t i o n wave-
where
l e n g t h s o f c l e a r w a t e r and p a r t i c u l a t e m a t t e r , and a
t h e sum o f t h e r e s p e c t i v e Y bein? t h e s p e c i f i c Y - n.sY’ s Y G e l b s t o f f a b s o r p t i o n c o e f f i c i e n t , t h e r e l a t i v e e r r o r f o r d e t e r m i n i n g t h e Gelb-
absorption c o e f f i c i e n t s o f Gelbstoff. With a s t o f f concentration w i t h l i d a r i s then
I n F i g . 4 r e s u l t s a r e p l o t t e d as a f u n c t i o n o f t h e G e l b s t o f f c o n c e n t r a t i o n n w i t h t h e p a r t i c l e a t t e n u a t i o n c o e f f i c i e n t a t X = 344 nm as t h e parameter. The w a v e l e n g t h dependence o f c
i s assumed t o b e p r o p o r t i o n a l t o 1/X w h i c h h o l d s f o r P most o c e a n i c h y d r o s o l s . s (1) f o l l o w s an e x p o n e n t i a l l a w a c c o r d i n g t o ( 2 ) . D a t ? Y o f cw a r e t a k e n f r o m t a b l e s o f t h e d i f f u s e c l e a r w a t e r a t t e n u a t i o n c o e f f i c i e t r e p o r t e d b y Smith and Baker (1981). C o r r e l a t i o n between a b s o r p t i o n and f l u o r e s c e n c e F1 uorescence as an a1 t e r n a t i v e t e c h n i q u e f o r t h e e x a m i n a t i o n o f G e l b s t o
F
n e c e s s i t a t e s t h a t b o t h methods l e a d t o c o n s i s t e n t r e s u l t s . T h i s h o l d s f o r t h e A d r i a t i c (Russo, 1983) and t h e N o r t h Sea w i t h o c e a n i c v a l u e s o f s a l i n i t y where a c o r r e l a t i o n c o e f f i c i e n t o f a b s o r p t i o n and f l u o r e s c e n c e o f a b o u t 0.9 i s found. However, t h e c o r r e l a t i o n d r o p s d o w n - t o a b o u t 0.5-0.7 f r e s h w a t e r i n f l u e n c e and f o r r i v e r water.
i n coastal areas w i t h
I t i s s u r p r i s i n g t h a t H d j e r s l e v (1980)
f i n d s no c o r r e l a t i o n t o be p r e s e n t i n t h e B a l t i c . Near t h e Gleser e s t u a r y i n t h e German B i g h t d r a s t i c d e v i a t i o n s o f t h e f l u o r e s c e n c e spectrum compared t o t h e open sea a r e observed ( F i g . 5) which must be due t o a v a r y i n p c o m p o s i t i o n o f G e l b s t o f f as a r e s u l t o f changing chemical c o n d i t i o n s o f t h e w a t e r . Compared t o a b s o r p t i o n measurements performed a t b l u e wavelengths and u s i n g a 1 m p a t h l e n g t h i n s t r u m e n t , t h e f l u o r e s c e n c e t e c h n i q u e w i t h n e a r UV e x c i t a t i o n shows a much b e t t e r l o w e r l i m i t o f s e n s i t i v i t y . I n terms o f a b s o l u t e c o n c e n t r a t i o n u n i t s g i v e n above t h e s e l i m i t s a r e 0.1 and 0.01 mg/l,
respectively.
S t a b i 1 it y Concerning t h e s t a b i l i t y o f G e l b s t o f f as a p r i o r c o n d i t i o n f o r i t s use as a
576
A,. 1251
= 306 n m
A,
z
34Lnm
A,
I
380nm
i
10 1 5 2 0 25
h.. = 308nm A, = 3 L L n m
50 100
105
Fig. 4. R e l a t i v e e r r o r f o r t h e l i d a r measurement o f G e l b s t o f f , eq. ( 4 ) , as a f u n c t i o n o f t h e c o n c e n t r a t i o n w i t h t h e p a r t i c l e a t t e n u a t i o n c o e f f i c i e n t as t h e parameter. E x c i t a t i o n wavelength i s 308 nm, water Raman s c a t t e r i n g wavelength i s 344 nm. The G e l b s t o f f f l u o r e s c e n c e i s d e t e c t e d a t 380 and 500 nm, r e s p e c t i v e l y . The importance o f a fluorescence d e t e c t i o n c l o s e t o t h e e x c i t a t i o n and water Raman s c a t t e r wavelengths i s obvious.
900
400
500
600
F i g . 5. G e l b s t o f f f l u o r e s c e n c e s p e c t r a o f water samples taken from t h e open sea and n e a r t h e shore l i n e (Weser e s t u a r y ) o f t h e German B i g h t . The curves a r e normalized t o i d e n t i c a l f l u o r e s c e n c e maxima t o enable a comparison o f t h e shape o f t h e spectra. The predominance o f f l u o r e s c e n c e a t h i g h e r wavelenoths f o r t h e shore l i n e sample m i g h t be due t o t h e h i g h e r c o n c e n t r a t i o n o f l a r g e o r g a n i c molecules.
577
n a t u r a l t r a c e r substance i t i s g e n e r a l l y s t a t e d t h a t a d e g r a d a t i o n due t o b i o l o g i c a l processes does n o t e x i s t . Haekel (1982) f i n d s an i n v a r i a n t c o n c e n t r a t i o n i n w a t e r samples s t o r e d i n t h e dark. The l i f e t i m e o f G e l b s t o f f w i t h r e s p e c t t o photochemical decay has been examined i n f i r s t e x p e r i m e n t s b y t h e a u t h o r s i n c o l l a b o r a t i o n w i t h D r . R. D o e r f f e r , G U S Forschungszentrum Geesthacht. A dec r e a s e o f t h e c o n c e n t r a t i o n by 50% o v e r a p e r i o d o f 125 days measured w i t h f l u o r e s c e n c e and a b s o r p t i o n meters i s i d e n t i c a l l y found f o r 3 w a t e r samples exposed t o a n i l l u m i n a t i o n w h i c h corresponds t o d a y l i g h t c o n d i t i o n s , compared t o samples s t o r e d i n t h e d a r k . T h i s r e s u l t s i n a good s t a b i l i t y w i t h i n t i m e s c a l e s c h a r a c t e r i s t i c f o r t r a n s p o r t i n t h e German B i g h t and most o t h e r c o a s t a l areas,
i f t h e e f f e c t i v e exposure o f G e l b s t o f f i n t h e w a t e r column t o s u n l i g h t i s t a k e n i n t o account. To summarize t h e c h a r a c t e r i s t i c s o f G e l b s t o f f , i t can be s t a t e d t h a t t h i s m a t e r i a l i s s e n s i t i v e l y i d e n t i f i e d w i t h o p t i c a l methods, i n p a r t i c u l a r w i t h f l u o r e s c e n c e s p e c t r o s c o p y , i n c o a s t a l w a t e r s due t o i t s h i o h c o n c e n t r a t i o n and f l u o r e s c e n c e y i e l d . A c c o r d i n g t o t h e p r e s e n t knowledge i t s s t a b i l i t y i s suff i c i e n t l y h i g h t o provide a n a t u r a l t r a c e r f o r t h e study o f c h a r a c t e r i s t i c w a t e r masses. S i n c e i t s c o n c e n t r a t i o n i s o n l y i n f l u e n c e d by m i x i n g processes G e l b s t o f f t u r n s o u t t o be a b e t t e r c o n s e r v a t i v e parameter t h a n t e m p e r a t u r e i n s h a l l o w c o a s t a l waters. However, a number o f q u e s t i o n s have t o be examined i n detai 1:
-
t h e changes i n t h e c o m p o s i t i o n and t h e o p t i c a l p r o p e r t i e s o f G e l b s t o f f d u r i n g t r a n s p o r t b y r i v e r s t o t h e open sea, t o be t a k e n i n t o a c c o u n t b y a p p r o p r i a t e a l g o r i t h m s a p p l i e d t o f l u o r e s c e n c e and a b s o r p t i o n d a t a ,
-
t h e c o r r e l a t i o n o f a b s o r p t i o n and f l u o r e s c e n c e as a f u n c t i o n o f v a r y i n g s a l i n i t y and pH,
-
t h e r o l e o f a possible i n s i t u production,
-
t h e photochemical decay o f G e l b s t o f f ,
I
a q u a n t i t a t i v e d e t e r m i n a t i o n o f t h e s p e c i f i c f l u o r e s c e n c e e f f i c i e n c y as a f u n c t i o n o f t h e e x c i t a t i o n and e m i s s i o n wavelength.
THE OCEANOGRAPHIC LIDAR Theory o f t h e measuring process The oceanographic l i d a r i s an a c t i v e s e n s o r by w h i c h a n a i r b o r n e p r o b i n g o f o p t i c a l l y d e t e c t a b l e parameters i n t h e upper w a t e r l a y e r s i s achieved. I n i t s b a s i c c o n c e p t t h e l i d a r c o n s i s t s o f a l a s e r e m i t t i n g a t near UV o r v i s i b l e wavelengths where w a t e r shows a good l i g h t t r a n s m i s s i o n and o f a t e l e s c o p e f o r t h e d e t e c t i o n o f l a s e r - i n d u c e d r a d i a t i o n f r o m t h e w a t e r Column. The i n t e n s i t y o f t h e r e c e i v e d l i g h t i s measured a t d e t e c t i o n wavelengths w h i c h a r e c h a r a c t e r i s t i c f o r s c a t t e r i n g and f l u o r e s c e n c e o f t h e substances under i n v e s t i g a t i o n
578
( F i g . 1 ) . To overcome s i g n a l c o n t r i b u t i o n s due t c ; s u n l i ? h t , h i g h power p u l s e l a s e r systems a r e g e n e r a l l y u t i l i z e d whereby t h e background l i g h t can be e l i m i n a t e d by a p p r o p r i a t e . s i g n a l g a t i n g t e c h n i q u e s .
A q u a n t i t a t i v e f o r m u l a t i o n o f t h e measuring process i s d e r i v e d f r o m t h e l i d a r equation (Browell
, 1977)
g i v i n g t h e l a s e r - i n d u c e d s i g n a l dP r e c e i v e d f r o m a
depth i n t e r v a l dz a t depth z: exp
Z
(-jo
c dz')
dP=Arl
dz
(5)
( z+mH)2 w i t h z = 0 a t t h e w a t e r s u r f a c e and z p o s i t i v e downwards, and where q =
q(z,Aex,Aem)
quantum e f f i c i e n c y o f f l u o r e s c e n c e o r w a t e r Raman
scattering, c
sum o f l i g h t a t t e n u a t i o n c o e f f i c i e n t s a t c (z,Aex) + cem(z,Xe,) ex t h e e x c i t a t i o n and t h e e m i s s i o n wavelength,
=
H a i r c r a f t f l i g h t height,
m r e f r a c t i v e index o f water, A i n c l u d e s i n s t r u m e n t a l f a c t o r s , s i g n a l l o s s e s i n t h e atmosphere, and e f f e c t s o f t h e r o u g h w a t e r s u r f a c e on t h e beam p r o p a g a t i o n . The s i g n a l o r i g i n a t i n g f r o m a w a t e r l a y e r a t d e p t h z1 5 z 5 z 2 reads:
= +
A exp ( -
Z1
c dz)
22
-
22
dz
rl
(z+mHj2
Z
=
A e x p ( - Jozl c d z )
0
W i t h w = (z+mH)/mH ( B r o w e l l , 1979) and assuming rl and c t o be c o n s t a n t a t Z 1 I Z < Z
2, t h e i n t e g r a l o v e r t h i s l a y e r i s w r i t t e n as
With t h e f u r t h e r s u b s t i t u t i o n
1
W2
dw=--. mHc +
2 W
exp (-mHcw)
exp (-mHcw) d
2
W
,
579 and a p p r o x i m a t i n g mHc >> 2/w w h i c h i s e q u i v a l e n t t o c >> 2/(z+mH)
and w i l l be
v a l i d i n a l l p r a c t i c a l cases, t h e i n t e g r a t i o n o f ( 7 ) i s s t r a i g h t f o r w a r d :
The g e n e r a l s o l u t i o n o f t h e l i d a r e q u a t i o n ( 6 ) f o r a w a t e r column w i t h a r b i t r a r y s t r a t i f i c a t i o n t h e n reads
and t h e t o t a l s i g n a l i s o b t a i n e d by t h e summation o f a l l c o n t r i b u t i o n s ( 8 ) between z=O and z > > l / c .
p
= - -
A
(mH)‘
F o r a homogeneous w a t e r column i t f o l l o w s
R‘ cR
f o r w a t e r Raman s c a t t e r i n g where vR i s c o n s t a n t , and
p
= - -
A
(mH)‘
‘F cF
f o r f l u o r e s c e n c e o f Gel b s t o f f o r c h l o r o p h y l l
a.
The n o r m a l i z a t i o n o f f l u o r e s -
cence s i g n a l s t o t h e w a t e r Raman s c a t t e r i n g y i e l d s d a t a on t h e c o n c e n t r a t i o n nF o f t h e fluorescing matter,
‘F R‘
=
-‘F / - ‘R R‘
“
nF
(11)
‘F
as o u t l i n e d i n t h e p r e c e d i n g s e c t i o n , eq. ( 3 ) and (4). Instrument d e s c r i p t i o n The Oceanographic L i d a r System (OLS) developed a t t h e U n i v e r s i t y o f Oldenburg has been d e s i g n e d f o r i n s t a l l a t i o n i n Do 28 o r Do 228 res’earch a i r c r a f t . Lasers and t h e d e t e c t o r system a r e mounted on an o p t i c a l t a b l e t o o b t a i n a r i g i d a l i g n m e n t o f t h e o p t i c a l s e t u p ( F i g . 6). System o p e r a t i o n and d a t a p r o c e s s i n g i s done i n - f l i g h t by one o p e r a t o r . The t o t a l system w e i g h t i s 500 kg; t h e payload
580
1
Fig. 6. Optical part of the Oceanographic Lidar System. The position of the telescope is above a bottom batch of the a i r c r a f t for f r e e f i e l d of view.towards the water surface. The ray path of the l a s e r beams a r e shown as broken lines.
capacity of the Do 28 allows a f l i g h t time of 3 hours, t h a t of the Do 228 of 6 hours. A Lambda Physik EMG 101 excimer l a s e r serves as the main l i g h t source with a peak power of 1 0 MW and a pulse length of 12 ns a t a Wavelength of 308 nm. Front and rear o u t p u t of the l a s e r a r e utilized as the l i d a r beam or as the pumping beam f o r a Lambda Physik FL 2001 dye l a s e r w i t h a peak power of 1 MW a n d a pulse length of 6 ns, respectively. The dye l a s e r output wavelength i s s e t a t 450 nm. The maximum pulse repetition r a t e i s 20 Hz. The signal receiver i s a Schmidt-Cassegrain f/10 telescope with a 40 cm aperture diameter. The f i e l d of view i s s e t t o 5 mrad, which corresponds t o the beam divergence of the excimer laser. A t a f l i g h t height of typically 200 m t h i s results in a footprint diameter of 1 m. Dichroic beamsplitters deflect selected spectral ranges of the received l i g h t t o interference and edge f i l t e r s with typical bandwidths of 10 nm, and 10 dynode f a s t photomultipliers EM1 9812 and 9818. The multipliers a r e activated by a gating c i r c u i t f o r time periods which can be s e t between 100 ns and 3 ps following each l a s e r shot t o avoid a nonlinear response due t o sunlight-induced background. The detection wavelengths a r e chosen a t 344, 366, 380, 500, 533, 650 and 685 nm. The channels a t 344 and 533 nm correspond t o the wavelengths of water' Raman scattering w i t h 308 and 450 nm excitation, respectively. Chlorophyll a
581
fluorescence is observed at 685 nm. The 366, 380, 500 and 650 nm channels are used for the measurement of the Gelbstoff fluorescence spectrum and for the identification of the baseline of water Raman scattering and chlorophyll 9 fluorescence (Fig. 1 ) . A calibration of the detection channels with respect to their relative spectral sensitivities is performed on the ground with a 1 mz plate of white teflon irradiated by the excimer laser. This material has the advantage of possessing a fluorescence spectrum covering the whole vi,sible and near U V with an efficiency which corresponds well to the typical intensities of the water column return. Moreover, teflon is a stable material with good resistance against contamination, making it very suitable for use on an airfield. Signal digitization is done with a Biomation Model 6500 transient recorder at a sampling rate of 500 MHz and a resolution of 6 bit. Since the digitizer is a one-channel instrument, 3 photomultipliers selected by the operator are sequentially combined on one signal line and fed to the transient recorder. The system is controlled by a LSI 11/02 microcomputer, by which laser selection and triggering, selection of different detection modes, quick look data output and data storage on floppy discs or magnetic tapes is achieved. A schematic of the signal flow i s shown in Fig. 7.
Fig. 7. Schematic of the signal flow of the Oceanographic Lidar System.
582
EXPERIMENTAL RESULTS North Sea Since 1984 t h e I n s t i t u t e f o r Marine Research, Bremerhaven, t h e German Hydrog r a p h i c O f f i c e and t h e U n i v e r s i t i e s o f Hamburg and Oldenburg have conducted a j o i n t research programme a i m i n g a t a b e t t e r understanding o f f r o n t a l systems and c h a r a c t e r i s t i c water masses o f t h e German B i g h t . The scope o f t h i s p r o j e c t and r e s u l t s o f b i o l o g i c a l and p h y s i c a l i n s i t u o b s e r v a t i o n s a r e d e s c r i b e d by Krause e t a l . (1985) i n a comprehensive paper g i v e n i n t h i s volume. As p a r t o f t h i s p r o j e c t , a i r b o r n e experiments were done i n p a r a l l e l w i t h shipboard i n v e s t i g a t i o n s c a r r i e d o u t by these i n s t i t u t i o n s . A combination o f these data and an i n t e g r a l i n t e r p r e t a t i o n a r e s t i l l i n progress. This concerns a l s o a c a l i b r a t i o n o f t h e l i d a r measurements i n a b s o l u t e u n i t s which a t present n e c e s s i t a t e s a d d i t i o n a l ground t r u t h i n f o r m a t i o n . R e s u l t s a r e thus shown i n r e l a t i v e u n i t s , and o n l y e s t i m a t e s o f t h e substance c o n c e n t r a t i o n s a r e g i v e n which a r e d e r i v e d from a few i n s i t u data. New methods f o r an a b s o l u t e c a l i b r a t i o n o f OLS d a t a w i t h o u t t h e need o f ground t r u t h a r e now under i n v e s t i g a t i o n .
Here we r e p o r t some r e s u l t s o f OLS f l i g h t s which demonstrate t h e c a p a b i l i t y o f l i d a r f o r l a r g e s c a l e m o n i t o r i n g , i n p a r t i c u l a r concerning t h e c h a r a c t e r i z a t i o n o f water masses and t h e i d e n t i f i c a t i o n o f f r o n t s a t t h e water s u r f a c e on t h e b a s i s o f G e l b s t o f f nieasurements. F i g . 8 d i s p l a y s p r o f i l e s o b t a i n e d on a t r a c k from t h e i s l a n d o f Helgoland t o Wilhelmshaven on October 25, 1983. I n t h i s f l i g h t which was t h e f i r s t a i r b o r n e o p e r a t i o n o f OLS, t h e dye l a s e r was e m i t t i n g a t a wavelength o f 450 nm w i t h a r e p e t i t i o n r a t e o f 2 Hz. L i g h t a t t e n u a t i o n i s d e r i v e d from t h e water Ranian s c a t t e r i n g s i g n a l a t 533 nni according t o equ. (10) a f t e r s u b t r a c t i o n o f background fluorescence;
t h e data correspond thus t o c ( 4 5 0 ) + c ( 5 3 3 ) . G e l b s t o f f concentra-
t i o n i s c a l c u l a t e d from t h e f l u o r e s c e n c e measured a t 500 nrn normalized t o t h e a fluoreswater Ranian s c a t t e r i n g , equ. ( 1 1 ) . The same a p p l i e s t o c h l o r o p h y l l cence d e t e c t e d a t 685 nm y i e l d i n g t h e r e s p e c t i v e c o n c e n t r a t i o n .
A c l o s e c o r r e l a t i o n of t h e l i g h t a t t e n u a t i o n c o e f f i c i e n t and o f t h e G e l b s t o f f and c h l a c o n c e n t r a t i o n i s obvious from t h e data. I n t h e open German B i g h t these parameters a r e h o r i z o n t a l l y homogeneous. The sharp peak a t km 32 w i t h a w i d t h o f about 2 km i s found a t t h e Old Elbe V a l l e y and i s probably a t t r i b u t e d t o E l b e r i v e r water. The enhanced values a t t h e end o f t h e t r a c k , km 40
-
45, w i t h a
s t r o n g patchiness p r e s e n t a t small scales, m i g h t be due t o t h e i n f l u e n c e o f t h e Weser r i v e r plume. R e s u l t s o b t a i n e d d u r i n g a f l i g h t on March 25, 1985 along a west-east t r a c k
583 Illghtnumer: 7 L 2 25. I0.83
aate:
t1m;
0
11: 01
20
40
distance R r l
Fig. 8. Results of l i d a r measurements performed on a t r a c k from Helgoland t o Wilhelmshaven, showing t h e l i g h t a t t e n u a t i o n a t blue/green wavelengths, and t h e Gelbstoff ( y s ) and chl a- concentration. F l i g h t height was 200 m , signal r e p e t i t i o n r a t e 2 Hz. on l a t i t u d e 54'00' N , s t a r t i n g a t 7'34' E north of the i s l a n d of Langeoog and ending on t h e shore of the Elbe e s t u a r y , a r e shown in Fig. 9. I n t h i s experiment, t h e excimer l a s e r was operated a t a r e p e t i t i o n r a t e of 10 Hz, i t s emission wavelength being 308 nm. The UV a t t e n u a t i o n c o e f f i c i e n t i s derived from the excimer l a s e r induced water Raman s c a t t e r detected a t 344 nm y i e l d i n g ' c (308) + c(344). Chl g concent r a t i o n i s deduced from i t s fluorescence a t 685 nm; these data show a r a t h e r poor signal-to-noise r a t i o because of t h e e x c i t a t i o n i n near UV which i s not very appropriate f o r the purpose of chl g detection. Concerning Gelbstoff, two p r o f i l e s - a r e shown, t h e upper curve being derived from the fluorescence measured a t 380 nm, the lower one a t 500 nm. According t o t h e statements given above, equ. ( 4 ) and Fig. 4, the fluorescence detection channel c l o s e r t o the e x c i t a t i o n and water Raman s c a t t e r i n g wavelengths y i e l d s more a c c u r a t e data because of t h e d e t e r i o r a t i n g e f f e c t of dissolved and p a r t i c u l a t e matter and t h e i r s p e c t r a l l y varying l i g h t a t t e n u a t i o n . The upper curve w i l l thus r e f l e c t t h e t r u e Gelbstoff concentration w i t h b e t t e r r e l i a b i l i t y , and the importance of a s u i t a b l e choice of the fluorescence detection wavelength i s c l e a r l y seen. A synchronized measurement of airborne and shipboard data was done a t the beginning of the t r a c k , km 0. In situ values of 2.6 vg/l o f chl a and 1.4 mg/l
584
o f G e l b s t o f f were f o u n d w h i c h g i v e s an i d e a o f t h e s c a l i n g o f t h e p r o f i l e s . Compared w i t h t h e d a t a shown i n F i g . 9, a c o r r e l a t i o n o f t h e measured p a r a m e t e r s i s n o t f o u n d here. The c h l $ d i s t r i b u t i o n i s v e r y i r r e g u l a r . The l i g h t a t t e n u a t i o n c o e f f i c i e n t shows an i n c r e a s i n g tendency i n f r o n t of t h e Weser and E l b e mouth as i t has t o be expected, however, no d i s t i n c t s i g n a t u r e s a r e observed.
In c o n t r a s t t o t h i s , G e l b s t o f f d a t a r e v e a l t h e presence o f two w a t e r
masses w i t h a v i r t u a l l y c o n s t a n t c o n c e n t r a t i o n , s e p a r a t e d by a s t r o n g g r a d i e n t a t km 48. The p o o r c o r r e l a t i o n w i t h t h e l i g h t a t t e n u a t i o n i s p r o b a b l y a r e s u l t of t h e c o m p e t i t i n g i n f l u e n c e s o f suspended m i n e r a l s and p h y t o p l a n k t o n on t h i s parameter
.
The assumption t h a t t h e h y d r o g r a p h i c s i t u a t i o n i n t h e s u r f a c e l a y e r i s char a c t e r i z e d by two homogeneous w a t e r masses s e p a r a t e d by a f r o n t o f t h e p h y s i c a l parameters as o u t l i n e d by t h e G e l b s t o f f c o n c e n t r a t i o n , has t o be c o n f i r m e d by
i n s i t u d a t a o f t e m p e r a t u r e and s a l i n i t y .
o:.
0 ,
?* 1 O0
I
F i g . 9. R e s u l t s o f l i d a r measurements performed on a w e s t - e a s t t r a c k on l a t i t u de 54'00' N, s t a r t i n g a t 7'34' E and e n d i n g on t h e s h o r e of t h e E l b e e s t u a r y . The l i g h t a t t e n u a t i o n a t UV wavelengths and t h e G e l b s t o f f ( y s ) and c h l conc e n t r a t i o n s a r e shown. F o r an e x p l a n a t i o n o f t h e two G e l b s t o f f p r o f i l e s see t e x t . F l i g h t h e i g h t was 200 m, s i g n a l r e p e t i t i o n r a t e was 1 0 Hz.
585
A d r i a t i c Sea I n August 984 t h e experiment A O R I A 84 was performed i n t h e n o r t h e r n A d r i a t i c Sea under t h e auspices o f t h e Commission o f t h e European Communities, I S P R A Establishment
The goal o f t h e experimen6 i n which a l a r g e number o f i n s t i t u -
t i o n s have been i n v o l v e d was an i n v e s t i g a t i o n o f t h e complicated hydrodynamic processes p r e s e n t i n t h i s area i n t h e summer p e r i o d , and an examination o f t h e p o t e n t i a l o f a i r b o r n e and s a t e l l i t e remote sensing f o r t h e i r study ( S c h l i t t e n h a r d t , 1984, 1985). The Oceanographic L i d a r System was operated i n t h e p e r i o d o f August 29
-
31
on f l i g h t t r a c k s which were assumed t o c o v e r t h e areas o f h i g h e s t i n t e r e s t : a n o r t h - s o u t h t r a c k on l o n g i t u d e 12"40' E passing nearby t h e Po d e l t a , a second t r a c k on 13'00'
E i n t h e open A d r i a t i c where waters w i t h a l o w e r i n f l u e n c e o f
r i v e r r u n - o f f were expected. F l i g h t h e i g h t was 200 m throughout t h e measurements, and b o t h t h e excimer and t h e dye l a s e r were operated a t a r e p e t i t i o n r a t e o f 2.5 Hz. R e s u l t s o f a f l i g h t on August 30 a r e shown i n F i g . 10a,b.
From t h e excimer
l a s e r induced s i g n a l s , t h e UV l i g h t a t t e n u a t i o n c o e f f i c i e n t c(308) + c(344) and t h e G e l b s t o f f c o n c e n t r a t i o n ( F i g . IOa), from those o f t h e dye l a s e r , t h e
+ c ( 5 3 3 ) and t h e c h l c o n c e n t r a t i o n (450) ( F i g . l o b ) a r e d e r i v e d a c c o r d i n g t o t h e methods d e s c r i b e d i n t h e preceding sections.
blue/green a t t e n u a t i o n c o e f f i c i e n t
c
The Po r i v e r plume w i t h a h i g h v a r i a b i l i t y o f t h e measured data, and t h e e x i s t e n c e o f d i f f e r e n t water masses w i t h i n d i v i d u a l l i g h t a t t e n u a t i o n and substance c o n c e n t r a t i o n a r e obvious from t h e data. These water masses a r e separated by f r o n t s w i t h g r a d i e n t s o f t y p i c a l l y a f a c t o r o f 10 o v e r d i s t a n c e s o f 1 -
3 km.
A comparison w i t h i n s i t u f i n d i n g s o b t a i n e d i n t h e n o r t h e r n p a r t o f t h e f l i g h t t r a c k on 13'00 E y i e l d s t h e f o l l o w i n g data t o be c h a r a c t e r i s t i c f o r t h e = water mass p r e s e n t i n t h a t area: a t t e n u a t i o n c o e f f i c i e n t ( c ( 4 5 5 ) + 0.2 1
-
-
0.3 m-',
Gelbstoff concentration
0.15 - 0.2 mg/l,
chl
2 concentration
2 p g / l . The l o w e r s e n s i t i v i t y l i m i t o f OLS i n l i m p i d waters w i t h low s e d i -
ment l o a d i s thus i n t h e o r d e r o f 0.01 mg/l f o r G e l b s t o f f and
0.1 p g / l f o r
chl a. The hydrographic c o n d i t i o n s on t h e o u t e r t r a c k t u r n o u t t o be v e r y s t a b l e w i t h i n t h e p e r i o d o f i n v e s t i g a t i o n . The s i t u a t i o n found on t h e t r a c k near t h e c o a s t l i n e i s c o n s i d e r a b l y d i f f e r e n t : t h e Po r i v e r plume s c a r c e l y i d e n t i f i e d on August 29 shows a s t r o n g e v o l u t i o n towards t h e open sea on t h e f o l l o w i n g days. F i g . 11 d i s p l a y s t h i s e v o l u t i o n from t h e G e l b s t o f f data, t h e o t h e r measured parameters y i e l d i n g a v e r y s i m i l a r tendency. The f u r t h e r i n t e r p r e t a t i o n o f these data and a comparison w i t h i n s i t u and sat e l l i t e o b s e r v a t i o n s a r e now i n progress. I t i s expected t h a t , i n view o f t h e
IPE
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Fig. 10a. Distribution of the UV attenuation coefficient (left) and the Gelbstoff concentration (right) in the northern Adriatic Sea measured with lidar on August 30, 1984. Scaling of the profiles is identical except for the track on longitude 13"OO' E which is given with higher resolution. Flight height was 200 m, signal repetition rate was 2.5 Hz.
Fig. l o b . D i s t r i b u t i o n o f t h e blue/green a t t e n u a t i o n c o e f f i c i e n t ( l e f t ) and t h e c h l 5 conc e n t r a t i o n ( r i g h t ) i n t h e n o r t h e r n A d r i a t i c Sea measured w i t h l i d a r on Auzust 30, 1984. S c a l i n g of t h e p r o f i l e s i s i d e n t i c a l except f o r t h e t r a c k on l o n g i t u d e 13 00' E which i s given w i t h h i g h e r r e s o l u t i o n . F l i g h t h e i g h t was 200 m, s i g n a l r e p e t i t i o n r a t e was 2.5 Hz.
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F i g . 11. R e s u l t s o f l i d a r measurements o f G e l b s t o f f ( y s ) c o n c e n t r a t i o n s performed i n t h e n o r t h e r n A d r i a t i c on l o n g i t u d e 12'40' E i n t h e p e r i o d o f August 29 31, 1984. G e l b s t o f f data show a d r a s t i c i n c r e a s e i n f r o n t o f t h e Po d e l t a over t h e p e r i o d o f i n v e s t i g a t i o n .
-
enormous number o f data obtained, t h e r e s u l t s o f A D R I A 84 w i l l enable a very d e t a i l e d d e l i n e a t i o n o f t h e hydrographic and b i o l o g i c a l processes i n t h e n o r t h e r n A d r i a t i c Sea.
CONCLUSIONS The o p t i c a l p r o p e r t i e s o f G e l b s t o f f and i t s h i g h c o n c e n t r a t i o n found i n c o a s t a l zones p r e s e n t problems f o r t h e i n v e s t i g a t i o n o f these waters w i t h opt i c a l s a t e l l i t e radiometry. The e v a l u a t i o n o f c h l o r o p h y l l c o n c e n t r a t i o n s from water l e a v i n g r a d i a n c e d a t a measured w i t h these sensors i s hampered by t h e spect r a l l y s e l e c t i v e l i g h t absorption o f Gelbstoff. On t h e o t h e r hand, t h i s m a t e r i a l can be s e n s i t i v e l y measured w i t h l i d a r due t o t h e s p e c i f i c and i n t e n s e f l u o r e s c e n c e emission. T h i s i s demonstrated i n experiments performed i n t h e German B i g h t and t h e A d r i a t i c Sea. Moreover, l i d a r a l l o w s an e v a l u a t i o n o f l i g h t t u r b i d i t y and o f c h l o r o p h y l l c o n c e n t r a t i o n s w i t h o u t d e t e r i o r a t i n g i n t e r f e r e n c e o f G e l b s t o f f o r o t h e r water column c o n s t i t u e n t s .
A combination o f s y n o p t i c l i d a r measurements w i t h s a t e l l i t e imagery r e s u l t s t h u s i n a c a l i b r a t i o n o f s a t e l l i t e d a t a i n terms o f these parameters.
589
Taking i n t o account t h e good s t a b i l i t y , G e l b s t o f f can be u t i l i z e d as a t r a c e r substance f o r t h e i n v e s t i g a t i o n o f c h a r a c t e r i s t i c water masses. I n a d d i t i o n t o t h e i n v e s t i g a t i o n o f dynamic processes, t h i s c h a r a c t e r i s t i c enables a study o f t r a n s p o r t and m i x i n g o f r i v e r water i n c o a s t a l areas.
A q u a n t i t a t i v e d e l i n e a t i o n o f these processes n e c e s s i t a t e s some f u r t h e r i n v e s t i g a t i o n s o f G e l b s t o f f , p a r t i c u l a r l y an e v a l u a t i o n o f more r e l i a b l e data on i t s f l u o r e s c e n c e e f f i c i e n c y and s t a b i l i t y w i t h r e s p e c t t o changing chemical c o n d i t i o n s i n c o a s t a l waters. E f f o r t s w i l l be done i n o r d e r t o e s t a b l i s h procedures f o r an a b s o l u t e c a l i b r a t i o n o f l i d a r data w i t h o u t t h e need o f f u r t h e r ground t r u t h i n f o r m a t i o n . T h i s w i l l y i e l d an i m p o r t a n t improvement o f oceanographic l i d a r remote sensing. ACKNOWLEDGEMENTS The experiments i n t h e German B i g h t were performed i n c o o p e r a t i o n w i t h t h e I n s t i t u t e f o r Marine Research, Bremerhaven. The p r o j e c t A D R I A 84 was i n i t i a t e d , r e a l i z e d and supported by t h e Commission o f t h e European Communities,
ISPRA
Establishment. We w i s h t o express o u r thanks t o P r o f . G. Krause, Bremerhaven, and t o D r . P. S c h l i t t e n h a r d t ,
Ispra.
We a r e g r a t e f u l t o DFVLR Oberpfaffenhofen f o r making a v a i l a b l e t h e research a i r c r a f t . We a r e e s p e c i a l l y i n d e b t e d t o M r . H. F i n k e n z e l l e r and M r . P. Vogel f o r t h e i r comprehensive support and t h e l o g i s t i c a l c o o r d i n a t i o n d u r i n g t h e experiments. The development o f t h e Oceanographic L i d a r System i s f i n a n c e d by
a grant
from t h e Bundesministerium f u r Forschung und Technologie; research on water masses and f r o n t a l systems by a g r a n t from t h e Deutsche Forschungsgemeinschaft.
REFERENCES Bricaud, A., Morel, A. and P r i e u r , L., 1981. Absorption o f d i s s o l v e d o r g a n i c m a t t e r o f t h e sea ( y e l l o w substance) i n t h e UV and v i s i b l e domains. Limnol. Oceanogr., 26: 43-53. B r o w e l l , E.V., 1977. A n a l y s i s o f l a s e r f l u o r o s e n s o r systems f o r remote algae d e t e c t i o n and q u a n t i f i c a t i o n . Report NASA TN D-8447. NASA Langley Research Center, Hampton, 39 pp. D i e h l , P. and Haardt, H., 1980. Measurement o f t h e s p e c t r a l a t t e n u a t i o n t o s u p p o r t b i o l o g i c a l research i n a p l a n k t o n tube experiment. Oceanol. Acta, 3: 89-95. F i s c h e r , J . and GraB1, H., 1984. R a d i a t i v e t r a n s f e r i n an atmosphere - ocean system: an a z i m u t a l l y dependent m a t r i x o p e r a t o r approach. Appl. Opt., 23: 1032-1039. 1982. I n t e r p r e t a t i o n o f a i r b o r n e oceanic l i d a r : e f f e c t s o f mulGordon, H.R., t i p l e s c a t t e r i n g . Appl. Opt., 21: 2996-3000. Clark, D.K., Brown, J.W., Brown, O.B., Evans, R.'H. and Broenkow, Gordon, H.R., W.W., 1983. Phytoplankton pigment c o n c e n t r a t i o n s i n t h e M i d d l e A t l a n t i c B i g h t : comparison of s h i p d e t e r m i n a t i o n s and CZCS estimates. Appl. Opt., 22: 20-36.
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590
Gunther, K.P., 1985. A q u a n t i t a t i v e d e s c r i p t i o n o f t h e c h l o r o p h y l l 2 f l u o r e s cence r e d u c t i o n due t o g l o b a l i r r a d i a t i o n i n t h e s u r f a c e l a y e r . Proc. 17. I n t e r n a t i o n a l L i e g e Colloquium on Ocean Hydrodynamics. Haekel, W., 1982. Untersuchungen z u r Schwermetallbindung durch Huminstoffe i n K s t u a r i e n . PhD t h e s i s , U n i v e r s i t a t K i e l , 147 pp. Hoge, F.E. and S w i f t , R.N., 1982. C e l i n e a t i o n o f e s t u a r i n e f r o n t s i n t h e German B i g h t u s i n g a i r b o r n e l a s e r - induced water Raman b a c k s c a t t e r and f l u o r escence o f water column c o n s t i t u e n t s . I n t . J. Remote Sensing, 3: 475-495. H d j e r s l e v , N.K., 1980. On t h e o r i g i n o f y e l l o w substance i n t h e marine e n v i ronment. I n : S t u d i e s i n p h y s i c a l oceanography, Kdbenhavns U n i v e r s i t e t , I ns t i t u t f o r F y s i s k Oceanografi, Report N r . 42: 39-56. K a l l e , K., 1937. Meereskundliche chemische Untersuchungen n i i t H i l f e des Zeissschen P u l f r i c h Photometers. V I : D i e Bestimmung des N i t r a t s und des Gelbs t o f f s . Ann. Hydr. u. M a r i t . Meteorol., 276-282. Kung, R.T.V. and I t z k a n , I.,1976. Absolute o i l f l u o r e s c e n c e conversion e f f i ciency. Appl. Opt., 15: 409-415. Lundgren, B., 1979. S p e c t r a l t r a n s m i t t a n c e measurements i n t h e B a l t i c . Kdbenhavns U n i v e r s i t e t , I n s t i t u t f o r F y s i s k Oceanografi, Report N r . 30, 38 pp. N y q u i s t , G., 1979. I n v e s t i g a t i o n o f some o p t i c a l p r o p e r t i e s o f seawater w i t h s p e c i a l r e f e r e n c e t o l i g n i n s u l f o n a t e s and humic substances. PhD t h e s i s , Goteborgs U n i v e r s i t e t , 200 pp. 1983. A study o f t h e i n h e r e n t o p t i c a l p r o p e r t i e s o f y e l l o w subRusso, M.C., stance from t h e n o r t h e r n p a r t o f t h e A d r i a t i c Sea. I n : P.M. S c h l i t t e n h a r d t ( E d i t o r ) , Workshop on Remote Sensing o f Coastal T r a n s p o r t i n t h e N o r t h e r n A d r i a t i c Sea, Proceedings, J o i n t Research Centre: 59-74. S c h l i t t e n h a r d t , P., 1984. A D R I A 84. A j o i n t remote sensing experiment. Techn i c a l Note No. 1.05.EZ.84.91. Commission o f t h e European Communities, J o i n t Research Centre, I s p r a , 33 pp. S c h l i t t e n h a r d t , P., 1985. ADRIA 84. A j o i n t remote sensing experiment. 2. Rep o r t : a s h o r t summary o f t h e experiment. Technical Note No. 1.05.EZ.85.20. Commission o f t h e European Communities, J o i n t Research Centre, I s p r a , 58 pp. Slusher, R.B. and Derr, V.E., 1975. Temperature dependence and c r o s s s e c t i o n s of some Stokes and a n t i - S t o k e s Raman l i n e s i n i c e Ih. Appl. Opt., 14: 2116 2120. Smith, R.C. and Baker, K.S., 1981. O p t i c a l p r o p e r t i e s o f t h e c l e a r e s t n a t u r a l waters (200 - 800 nm). Appl. Opt. 20: 177-184. 1967. Raman s p e c t r a l s t u d i e s o f t h e e f f e c t s o f temperature on Walrafen, G.E., water s t r u c t u r e . J. Chem. Phys., 47: 114-126.
591
WATER DEPTH RESOLVED DETERMINATION OF HYDROGRAPHIC PARAMETERS FROM AIRBORNE LIDAR MEASUREMENTS D. DIEBEL-LANGOHR, T. HENGSTERMANN and R. REUTER Universitat Oldenburg, Fachbereich Physik, P.0.Box 2503, 2900 Oldenburg, (Federal Republic of Germany)
ABSTRACT The l i d a r method applied from low flying a i r c r a f t over the ocean has a potent i a l f o r the depth resolved investiaation of hydroaraphic parameters. This method requires l a s e r pulses in the nanosecond range which a r e directed towards the water surface. Fluorescence and Raman s c a t t e r of water column constituents a r e collected by a telescope system and recorded as time resolved signals. They contain information on the depth distribution of these substances. The l i d a r signals depend on the shape of the l a s e r pulse, the fluorescent and scattering decay times of the substances under investigation, t h e i r distribution with depth, and the time response of the instrument, and a r e mathematically described by a convolution of these parameters. Algorithms f o r the evaluation of depth profiles from l i d a r data are presented and applied t o various models of s t r a t i fied surface layers. Time resolved measurements were performed over the northern Adriatic Sea in September 1984 with the Oceanographic L i d a r System (OLS) of the University of Oldenburg. Profiles of the attenuation coefficient obtained in t h i s experiment a r e presented.
INTRODUCTION Since 1963 the potential of l i d a r measurements f o r the derivation of ranae resolved parameters of the atmosphere has been demonstrated. Gaseous constituents a r e identified with the Differential Absorption Lidar (DIAL) (Zuev, 1963; Uchino, 1980); the detection of Mie scattered l i g h t allows the mapping of aerosol structures (Allen, 1972); Rayleigh and Raman scattering i s used f o r temperature profiling (Schwiesow, 1981 ; Arshinov, 1983). One of the crucial problems f o r atmospheric l i d a r measurements i s the large dynamic range of the signals due t o the 1/R2 dependence with distance R (Allen, 1972; Shimizu, 1985; Harms, 1978). In the past years the possi b i 1 i t y of measuring depth resolved hydrographic parameters w i t h l i d a r technique has been studied (Hoge, 1983). Time resolved scattering and fluorescence signals contain information on the vertical d i s t r i bution of the attenuation coefficient and of the concentration of Gelbstoff and chlorophyll a. The dynamic of the signals i s determined here by the exponential signal compression due t o the l i g h t attenuation in the water column whereas f o r f l i g h t heights of typically 100-500 m the 1 / R 2 dependence is l e s s important. The measurement of profiles i s limited t o a few attenuation lengths. To obtain a
592
h i q h d e p t h r e s o l u t i o n w i h i n t h i s ranae l a s e r p u l s e s w i t h a l e n g t h o f o n l y some nanoseconds and a s i g n a l d e t e c t i o n a t t i m e i n t e r v a l s s m a l l compared t o t h e t o t a l s i g n a l d u r a t i o n a r e requ red. A l i d a r r e t u r n s i g n a l depends on t h e shape o f t h e e x c i t i n g l a s e r p u l s e , t h e
a t t e n u a t i o n and t h e substance c o n c e n t r a t i o n s a l o n g t h e l i g h t p a t h , t h e decay f u n c t i o n o f t h e s c a t t e r o r f l u o r e s c e n c e p r o c e s s , and t h e response f u n c t i o n o f t h e l i d a r system. M a t h e m a t i c a l l y t h e d e t e c t e d s i g n a l i s a c o n v o l u t i o n o f a l l t h e s e f u n c t i o n s . An i n t e r p r e t a t i o n o f l i d a r measurements w i t h o i r t f i r s , t p e r f o r m i n g a d e c o n v o l u t i o n r e s u l t s i n a reduced ranae r e s o l u t i o n . T h i s i s n o t problemat i c f o r a t m o s p h e r i c s t u d i e s where a r e s o l u t i o n o f some meters i s g e n e r a l l y s u f f i c i e n t . F o r h y d r o g r a p h i c p r o f i l i n a o f t h e s u r f a c e l a y e r o f t h e ocean where t h e p e n e t r a t i o n d e p t h o f b l u e / q r e e n l a s e r l i a h t i s l i m i t e d t o a b o u t 20 m e t e r s i n most cases, a d e p t h r e s o l u t i o n o f 1 m e t e r o r b e t t e r s h o u l d be achieved. I n t h e f o l l o w i n g , al9orithms a r e presented t o evaluate v e r t i c a l p r o f i l e s o f t h e a t t e n u a t i o n c o e f f i c i e n t and t h e c o n c e n t r a t i o n o f G e l b s t o f f f r o m t i m e r e s o l v e d w a t e r Raman s c a t t e r i n g and G e l b s t o f f f l u o r e s c e n c e s i g n a l s . I t i s n o t necessary t o c o n s i d e r t h e decay f u n c t i o n s s i n c e t h e decay t i m e s o f t h e s e o p t i c a l i n t e r a c t i o n s a r e s h o r t e r t h a n t h e 2 ns t i m e r e s o l u t i o n o f t h e Oceanographic L i d a r System (OLS) o f t h e U n i v e r s i t y o f Oldenburg. The r i s e and f a l l t i m e s o f t h e p h o t o m u l t i p l i e r s used a r e a b o u t 2 ns, t h e r i s e t i m e o f t h e a m p l i f i e r i s l e s s t h a n 3 ns, t h e f a l l t i m e l e s s t h a n 5 ns p e r decade. So i n a f i r s t a p p r o x i m a t i o n t h e l i d a r s i g n a l can be assumed t o be a c o n v o l u t i o n o f t h e l a s e r p u l s e shape and o f a f u n c t i o n c o n t a i n i n g t h e a t t e n u a t i o n c o e f f i c i e n t and t h e c o n c e n t r a t i o n o f Gel b s t o f f .
THEORETICAL BACKGROUND R e l a t i o n between t i m e r e s o l v e d l i d a r s i g n a l s and d e p t h r e s o l v e d h y d r o g r a p h i c parameters F i g . 1 i l l u s t r a t e s t h e p r i n c i p l e l a s e r p u l s e shape P L ( t ) and t h e w a t e r column r e t u r n s i g n a l P ( A , t ) measured a t t h e d e t e c t i o n wavelength A. P(A,t) can i n c l u d e f l u o r e s c e n c e and s c a t t e r i n g c o n t r i b u t i o n s . The w a t e r column i s i n g e n e r a l composed o f w a t e r ' i t s e l f ( i n d e x w), suspended m i n e r a l s ( i n d e x p ) , G e l b s t o f f ( i n d e x y ) , and c h l o r o p h y l l
2
( i n d e x c ) . The s i g n a l P(A,t)
i s d e t e r m i n e d b y t h e i r con-
c e n t r a t i o n s nw, nP y nY, nc, b y t h e f l u o r e s c e n c e and s c a t t e r i n g e f f i c i e n c i e s
$(A),
rlp(A), n Y ( A ) , rlc(A), and b y t h e a t t e n u a t i o n c o e f f i c i e n t c ( h ~ , A ) w h i c h
means h e r e t h e sum o f t h e a t t e n u a t i o n c o e f f i c i e n t s a t t h e l a s e r and a t t h e det e c t i o n wavelength. np, ny, nc, and c ( A ~ , h ) a r e f u n c t i o n s o f t h e w a t e r d e p t h z.
A s i g n a l S(A,z) f r o m d e p t h z i s f i r s t dependent on t h e a t t e n u a t i o n of t h e l a s e r l i g h t and o f t h e l i g h t e x c i t e d a t d e p t h z i n t h e w a t e r column between
0 6 z ' 6 z, and on t h e c o n c e n t r a t i o n s o f t h e f l u o r e s c e n t and s c a t t e r i n g sub-
593
t
- 0
z
water surface
water d e p t h
F i g . 1. I l l u s t r a t i o n o f t h e a b b r e v i a t i o n s used. P L ( t ) l a s e r p u l s e shape, P ( X , t ) l i d a r s i g n a l d e t e c t e d a t wavelength A, z w a t e r depth, nw, np, concent r a t i o n s o f w a t e r , suspended m i n e r a l s , G e l b s t o f f , c h l o r o p h y l l a, :fi~!?j sum o f t h e a t t e n u a t i o n c o e f f i c i e n t s a t t h e l a s e r and a t t h e d e t e c t i o n wavelength. stances a t d e p t h z. The d e p t h f u n c t i o n S(A,z) can be w r i t t e n as ( B r o w e l l , 1977):
where H i s t h e f l i g h t h e i g h t and m t h e r e f r a c t i v e i n d e x o f water. The d e p t h z i s r e l a t e d t o a t i m e t b e i n g t w i c e t h e t i m e t h e l i g h t needs t o pass t h r o u g h a w a t e r l a y e r o f d e p t h z. The t r a n s f o r m a t i o n w i t h t = 2 zm/v ( v l i g h t v e l o c i t y ) y i e l d s :
594
The s i g n a l P ( X , t )
i s composed o f s i g n a l s f r o m a l l w a t e r depths. A t a f i x e d t i m e
t t h e l i d a r system i s n o t o n l y r e c e i v i n g a s i o n a l f r o m a f i x e d d e p t h z b u t f r o m
a water l a y e r w i t h a thickness corresponding t o t h e d u r a t i o n o f t h e l a s e r pulse. T h i s i s m a t h e m a t i c a l l y expressed by a c o n v o l u t i o n o f t h e d e p t h f u n c t i o n S ( A , t ) , and t h e l a s e r p u l s e P L ( t ) :
P(X,t)
--
m
PL(t')
*
S(X,t-t') d t '
S(X,t) = 0 a t t < 0
(3)
a,
Algorithms f o r t h e d e r i v a t i o n o f depth p r o f i l e s The a l g o r i t h m s t o c a l c u l a t e d e p t h p r o f i l e s o f t h e a t t e n u a t i o n c o e f f i c i e n t and t h e substance c o n c e n t r a t i o n s f r o m t h e measurable l i d a r s i g n a l s have t o s t a r t w i t h t h e s o l u t i o n o f t h e c o n v o l u t i o n i n t e g r a l ( 3 ) i n o r d e r t o express S(X,t)
by
t h e q u a n t i t i e s P L ( t ) and P ( X , t ) . One way i s t o p e r f o r m a F o u r i e r a n a l y s i s w h i c h y i e l d s :
where T means t h e F o u r i e r t r a n s f o r m a t i o n o f t h e f u n c t i o n w h i c h f o l l o w s . A l t e r n a t i v e l y t h e l a s e r p u l s e shape P L ( t ) can be approximated by t h e 6-funct i o n 6(0) which would be c o r r e c t f o r an i n f i n i t e l y s h o r t l a s e r p u l s e a t t i m e
t = 0. Then t h e c o n v o l u t i o n i n t e g r a l ( 3 ) can be s o l v e d d i r e c t l y . The r e s u l t o f t h i s Delta pulse analysis i s :
The n e x t s t e p i s t o c a l c u l a t e t h e d e p t h p r o f i l e s from t h e f u n c t i o n s S ( X , t ) w h i c h a r e o b t a i n e d f r o m l i d a r s i g n a l s measured a t d i f f e r e n t d e t e c t i o n wavelengths. As an example t h e e v a l u a t i o n o f t h e a t t e n u a t i o n c o e f f i c i e n t and o f t h e conc e n t r a t i o n o f Gel b s t o f f i s discussed. I n t h e case o f w a t e r Raman s c a t t e r i n g d e t e c t e d a t a wavelength XR where o t h e r s i g n a l c o n t r i b u t i o n s , e.g. gible, (2) simplifies to:
fluorescence o f c h l o r o p h y l l
5
o r Gelbstoff are negli-
595
(6)
W i t h s i g n a l s d e t e c t e d t i m e r e s o l v e d a t t i m e i n t e r v a l s o f l e n g t h A t one can f o r m the quotient
assuming c(hL,hR,t') and (vt/2m)+mH c(hL,hRyt)
2
c(AL,hR,t)
(v(t+At)/2m)+mH.
2
= (2m/vAt)
I n [S(XR,t)
w i t h i n t h e small time i n t e r v a l t I t ' 6 t + A t This y i e l d s f o r the attenuation c o e f f i c i e n t
/ S(h~,t+At)]
The d e p t h f u n c t i o n ( 2 ) i s reduced t o S(hy,t)
- ny(t)
*
-
ny(hy)
+
exp [ - ( v / 2 m ) j L c ( X ~ , X y , t ' ) d t ' l / ( ( ~ t / 2 m ) + r n H ) ~
t 2 0
(9)
0
f o r a G e l b s t o f f f l u o r e s c e n c e s i g n a l i f t h e c o r r e s p o n d i n g e f f i c i e n c y ny i s d o m i n a t i n g a t t h e w a v e l e n g t h Xy. F o r a d e t e c t i o n w a v e l e n g t h h
Y
chosen c l o s e t o
AR t h e a t t e n u a t i o n c o e f f i c i e n t s c(XL,XR) and c ( A ~ , h y ) can be assumed t o be n e a r -
l y equal
, and
(9) can be w r i t t e n as:
and i t f o l l o w s :
Discussion o f t h e algorithms Model s i g n a l s o f w a t e r Raman s c a t t e r i n g and G e l b s t o f f f l u o r e s c e n c e were comp u t e d a c c o r d i n g t o ( 3 ) based on a r e a l i s t i c l a s e r p u l s e shape PL, and on model d e p t h p r o f i l e s o f t h e G e l b s t o f f c o n c e n t r a t i o n ny and o f t h e a t t e n u a t i o n coe f f i c i e n t s c(XL,XR) and c(XL,XY) w h i c h a r e s e t t o b e i d e n t i c a l . The example g i v e n i n F i g s . 2a and 2b shows t h a t t h e p r o f i l e s r e c o n s t r u c t e d by a p p l i c a t i o n o f t h e F o u r i e r a n a l y s i s and t h e D e l t a p u l s e ' a n a l y s i s f i t t h e g i v e n model d e p t h p r o f i l e s w e l l . The d i s a d v a n t a g e o f t h e D e l t a p u l s e a n a l y s i s i s t h a t
i t o n l y a l l o w s a c a l c u l a t i o n o f smoothed v a l u e s o f t h e a t t e n u a t i o n c o e f f i c i e n t and t h e G e l b s t o f f c o n c e n t r a t i o n . T h i s smoothing e f f e c t i s a consequence of t h e
596
2
5
7.5
12
s
17.5
Fig. 2a. Depth p r o f i l e s o f t h e a t t e n u a t i o n c o e f f i c i e n t and t h e G e l b s t o f f concent r a t i o n r e c o n s t r u c t e d w i t h t h e F o u r i e r ana y s i s . f i n i t e h a l f w i d t h o f t h e l a s e r p u l s e which
s approximated by a 6 - f u n c t i o n .
In
t h e case o f t h e F o u r i e r a n a l y s i s t h e agreement o f t h e c a l c u l a t e d p r o f i l e s w i t h t h e model p r o f i l e s i s m a i n l y determined by t h e frequency range used.
A r e a l i s t i c l i d a r system i s r e c o r d i n g siGnals w i t h a ' f i n i t e amplitude r e s o l u t i o n . The h i g h t i m e r e s o l u t i o n which i s necessary f o r t h e d e r i v a t i o n o f depth p r o f i l e s has as consequence a l o w amplitude r e s o l u t i o n . For example t h e OLS det e c t s s i g n a l s a t 2 ns t i m e i n t e r v a l s w i t h an a m p l i t u d e d i g i t i z a t i o n o f o n l y 6 b i t . For examination o f t h e l i m i t e d a m p l i t u d e - a n d t i m e r e s o l u t i o n o f t h e d i g i t i z a t i o n process on t h e q u a l i t y o f t h e r e c o n s t r u c t e d depth p r o f i l e s , t h e model s i g n a l s d e s c r i b e d above a r e d i g i t i z e d w i t h d i f f e r e n t amplitude r e s o l u t i o n s bef o r e a p p l y i n g t h e F o u r i e r and t h e D e l t a p u l s e a n a l y s i s . The t i m e r e s o l u t i o n i s always 2 ns. I n o r d e r t o enhance t h e r e s o l u t i o n a t low s i g n a l l e v e l s t h e u t i l i z a t i o n o f a l o g a r i t h m i c a m p l i f i e r i s assumed which converts t h e e x p o n e n t i a l decay o f P(A,t)
i n t o a l i n e a r one.
It can be demonstrated t h a t f o r a g i v e n accuracy f o r t h e d e r i v e d p r o f i l e s t h e
597
--
Fig. 2b. Depth p r o f i l e s o f t h e a t t e n u a t i o n c o e f f i c i e n t and t h e G e l b s t o f f concent r a t i o n reconstructed w i t h the Delta pulse analysis. F o u r i e r a n a l y s i s n e c e s s i t a t e s a h i g h e r amplitude r e s o l u t i o n than t h e D e l t a p u l s e a n a l y s i s . F i g . 3a and F i g . 3b show t h a t t h e p r o f i l e s o f t h e a t t e n u a t i o n c o e f f i c i e n t and o f t h e G e l b s t o f f c o n c e n t r a t i o n can be computed w i t h an e r r o r o f l e s s than 10% w i t h t h e D e l t a p u l s e a n a l y s i s w i t h 1 0 b i t s i g n a l r e s o l u t i o n compared t o
14 b i t needed f o r t h e F o u r i e r a n a l y s i s . The maximum e r r o r depends on t h e t i m e and t h e a m p l i t u d e r e s o l u t i o n , t h e l o g a r i t h m i c a m p l i f i c a t i o n , and t h e a t t e n u a t i o n c o e f f i c i e n t , and can e a s i l y be c a l culated f o r the Delta pulse analysis.
I n F i g . 4 t h e r e l a t i v e maximum e r r o r f o r
t h e a t t e n u a t i o n c o e f f i c i e n t i s p l o t t e d as f u n c t i o n o f t h e number o f b i t s f o r t h e d i g i t i z a t i o n w i t h d i f f e r e n t values o f t h e a t t e n u a t i o n c o e f f i c i e n t . The t i m e r e s o l u t i o n o f 2 ns and t h e a m p l i f i e r c h a r a c t e r i s t i c s a r e f i x e d . An i m p o r t a n t r e s u l t i s t h a t t h e 6 b i t amplitude r e s o l u t i o n o f t h e OLS i s t o o low f o r a s i n g l e s h o t d e t e r m i n a t i o n o f depth p r o f i l e s o f hydrographic parameters.
598
Fig. 3a. Depth p r o f i l e s o f the a t t e n u a t i o n c o e f f i c i e n t and t h e G e l b s t o f f concent r a t i o n reconstructed w i t h t h e F o u r i e r a n a l y s i s f o r a 14 b i t amplitude reso1u t i o n . EXPERIMENTAL RESULTS I n 1984 f i r s t l i d a r measurements w i t h t h e OLS f o r t h e d e r i v a t i o n o f t h e depth dependent a t t e n u a t i o n c o e f f i c i e n t were c a r r i e d o u t over the northern A d r i a t i c Sea. To use t h e s p e c t r a l range where
water shows t h e highest l i g h t transmission
t h e OLS was working w i t h an e x c i t a t i o n wavelength o f 450 nm r e s u l t i n g i n a water Raman s c a t t e r wavelength o f 533 nm. The s i g n a l s were l o g a r i t h m i c a l l y a m p l i f i e d and recorded a t time i n t e r v a l s o f 2 ns w i t h an amplitude r e s o l u t i o n o f 6 b i t . An averaging over 20 s i g n a l s which corresponds t o an averaging over 0.5 km f l i g h t distance f o r t h e 2 Hz r e p e t i t i o n r a t e used was necessary t o increase t h e amplitude r e s o l u t i o n . The l a s e r pulses
w i t h a peak power o f IMW had a h a l f w i d t h
o f 6 ns which allowed t o apply t h e Delta pulse a n a l y s i s according t o (5).
For
t h e chosen f l i g h t t r a c k on t h e l a t i t u d e o f the Lagoon o f Venice t h e G e l b s t o f f fluorescence c o n t r i b u t i o n a t t h e Raman s c a t t e r i n g wavelength was l e s s than 10% so t h a t t h e approximation ( 6 ) w i t h t h e f o l l o w i n g c a l c u l a t i o n s (7) and ( 8 ) could
599
F i g . 3b. Depth p r o f i l e s o f t h e a t t e n u a t i o n c o e f f i c i e n t and t h e G e l b s t o f f concent r a t i o n r e c o n s t r u c t e d w i t h t h e D e l t a p u l s e a n a l y s i s f o r a 10 b i t a m p l i t u d e r e solution. be used. W i t h t h e d e s c r i b e d equipment, p r o f i l e s o f t h e a t t e n u a t i o n c o e f f i c i e n t w i t h a d e p t h r e s o l u t i o n o f a b o u t 1 m c o u l d be d e r i v e d f r o m t h e
measured s i g n a l s f o r
5-6 a t t e n u a t i o n l e n g t h s w i t h t h e D e l t a p u l s e a n a l y s i s . P r o f i l e s c a l c u l a t e d w i t h t h i s a l g o r i t h m s t a r t a t t h e v a l u e 0 and a r e n o t r e a l i s t i c f o r t h e f i r s t m e t e r o f t h e w a t e r column as a consequence o f t h e a p p r o x i m a t i o n o f t h e r e a l l a s e r p u l s e shape by a &pulse. Along t h e t r a c k f r o m 45"19'N,
12"39'E t o 45"19'N,
p e r f o r m e d o n l y d i f f e r i n g by a t i m e d e l a y
12'57'E
two f l i g h t s were
o f 20 m i n u t e s and n a v i g a t i o n u n c e r -
t a i n t i e s o f a b o u t 200 meters. T h e r e f o r e t h e f i n e s t r u c t u r e s o f t h e d e p t h p r o f i l e s c a n n o t be expected t o be i d e n t i c a l f o r b o t h
f l i g h t s , b u t t h e good c o r -
respondence o f t h e main c h a r a c t e r i s t i c s i s o b v i o u s f r o m
Fig. 5 : except f o r t h e
p r o f i l e s o b t a i n e d i n t h e west t h e a t t e n u a t i o n c o e f f i c i e n t has a s t r o n g maximum
600
C=05 C=O1 C=10 C=03
AC C
Im-’ 1
1 2
1 0
08
06
0 4
02
I
r,
6
8
10
12 14 number of b i t s
Fig. 4. Relative maximum e r r o r f o r t h e a t t e n u a t i o n c o e f f i c i e n t c a l c u l a t e d with the Delta pulse a n a l y s i s a s function of the amplitude resolution f o r d i f f e r e n t values of t h e a t t e n u a t i o n c o e f f i c i e n t . The time resolution i s 2 ns. of 0.6-0.7
m-’
a t a d e p t h of 2 m , f a l l s down t o a broad minimum of 0.2-0.3 m-’
centered around 6 m water d e p t h , and increases t o a value of about 0.5 m-’ f o r g r e a t e r depths. In t h e western p a r t of t h e t r a c k t h e a t t e n u a t i o n c o e f f i c i e n t i s r e l a t i v e l y homogeneous with d e p t h . CONCLUSIONS In October 1985 f l i g h t s with an improved l i d a r equipment were c a r r i e d out over the German Bight. The r e p e t i t i o n r a t e was increased t o 1 0 Hz, and a simultaneous measurement of a logarithmically amplified Gelbstoff fluorescence signal was performed a t a detectionwavelengthclose t o t h e water Raman detection wavelength. Since Gelbstoff fluorescence a t the Raman s c a t t e r wavelength cannot be neglected in t h i s area of t h e North Sea, i t i s necessary t o s u b t r a c t t h a t back-
601
- 24 x.
u 01
:22
c
-
In
0
20
ac
c= e!?
z:
16
1%
GZ
14
12
10
8
6
4
2
0
0 2 4 6 8 10 '214 It depth !m!
'depth
[ml
Fig. 5. P r o f i l e s o f t h e a t t e n u a t i o n c o e f f i c i e n t c = c(450nm) + c(533nm) i n t h e A d r i a t i c Sea, d e r i v e d w i t h t h e D e l t a p u l s e a n a l y s i s . F l i g h t t r a c k from 45"19'N, 12'39'E t o 45"19'N, IZ"57'E. August 31, 1984. ground b e f o r e c a l c u l a t i n g t h e depth p r o f i l e o f t h e a t t e n u a t i o n c o e f f i c i e n t . T h i s i s p o s s i b l e by u s i n g t h e measured G e l b s t o f f fluorescence s i g n a l i f t h e r a t i o o f t h e G e l b s t o f f f l u o r e s c e n c e e f f i c i e n c i e s a t t h e two d e t e c t i o n wavel e n g t h s i s known. Furthermore t h i s s i g n a l w i l l a l l o w t h e e v a l u a t i o n of t h e depth dependent G e l b s t o f f c o n c e n t r a t i o n .
602
ACLNOWLEDGEIVIENTS The experiment A D R I A ‘ 3 4 was i n i t i a t e d and organized by D r . P. S c h l i t t e n h a r d t from t h e European Coiiiiiiunities, I S P R A Establishment. We a r e inclebted t o t h e F l i q h t Department o f t h e DFVLR, Oberpfaffenhofen, f o r t h e a v a i l i b i l i t y o f t h e a i r c r a f t and f o r t h e support d u r i n q A D R I A ‘84. The development o f t h e Oceanographic L i d a r System was f i n a n c e d by a g r a n t from t h e Bundesniinister fiir Forschung und Technologie. Bonn. We a r e g r a t e f u l t.0 o u r colleagues a t t h e U n i v e r s i t y o f Oldenburq, D r . K.P. Gunther,
K. Loquay, R. Zimmerniann
f o r t h e i r support concerning t h e p r e p a r a t i o n
and performance o f t h e experiment. REFERENCES 1972. Laser Radar (LIOAR) f o r Mapping Aerosol S t r u c A l l e n , R.J., Evans, W.E., t u r e s . The Review o f S c i e n t i f i c Instruments, 43 : 1422-1432. Zuev, V.E., M i t e v , V.M., 1983. Atmospheric Arshinov, Yu.F., Bobrovnikov, S.M., temperature measurement u s i n g a pure r o t a t i o n a l Rarnan l i d a r . A p p l i e d O p t i c s , 22 : 2984-2990. B r o w e l l , E.V., 1977. A n a l y s i s o f l a s e r f l u o r e s e n s o r systems f o r remote algae d e t e c t i o n and q u a n t i f i c a t i o n . Report NASA TN 0-8447 : 39 pp. tiarins, J . , Lahmann, W., Weitkamp, C., 1978. Geometrical compression o f l i d a r r e t u r n s i g n a l s . A p p l i e d Optics, 17 : 1131-1135. 1983. A i r b o r n e d e t e c t i o n o f oceanic t u r b i d i t y c e l l Hoge, F.E., S w i f t , R.N., s t r u c t u r e u s i n g depth-resolved laser-induced water Rarnan b a c k s c a t t e r . A p p l i e d Optics, 22 : 3778-3786. Schwiesow, R.L., Lading, L., 1981.Teniperature P r o f i l i n g by R a y l e i g h - S c a t t e r i n g L i d a r . A p p l i e d Optics, 20 : 1972-1379 Shimizu, H., Sasano, Y., Nakane, H., Sugiiiioto, N., Matsui, I . , Takeuchi, N., 1985. Large s c a l e l a s e r r a d a r f o r measuring aerosol d i s t r i b u t i o n over a wide area. A p p l i e d Optics, 24 : 617-626. U c h i n i , O., Waeda, N., Shibata, T., H i r i n o , M., Fujuwara, M., 1980. Measurement o f S t r a t o s p h e r i c V e r t i c a l Ozone D i s t r i b u t i o n w i t h a Xe-C1 L i d a r : E s t i mated I n f l u e n c e o f Aerosols. A p p l i e d Optics, 19 : 4175-4181. Zuev, V.V., Zuev, V.E., rdakushkin, Yu.S., Marichev, V.N., M i t s e l , A.A., 1983. Laser sounding o f atmospheric h u m i d i t y : experiment. A p p l i e d O p t i c s , 22 : 3742-3746.
603
A QUANTITATIVE DESCRIPTION OF THE CHLOROPHYLL A FLUORESCENCE REDUCTION DUE TO GLOBAL IRRADIATION IN THE SURFACE LAYER
K. P. GUNTHEK University of Oldenburg, FB 8-Physics,P.O. Box 2503 D - 2900 Oldenburg (Federal Republic of Germany)
ABSTRACT The measurement of the chlorophyll a fluorescence from low flying aircraft by lidar method allows to monitor p5ytoplankton distribution synoptically as well as their temporal and spatial evolution. The fluorescence signals depend on the water attenuation, the chlorophyll d concentration and on the in vivo fluorescence efficiency, which varies with environmental parameters. Neglecting the influence of nutrient availability, temperature and phytoplankton composition, the global irradiation in the surface layer can result in a reduction of the in vivo fluorescence efficiency up to a factor 4. Based on a comprehensive photochemical model of the photosynthetic apparatus, a quantitative description of the reduction of the in vivo fluorescence efficiency due to photosynthetic active radiation is presented. In situ and lidar measurements were performed. Both data sets show a daily cycle of the in vivo fluorescence efficiency and are compared with the results of the model.
INTRODUCTION The synoptic measurement of chlorophyll distributions in open sea and coastal waters became of great importance during the last decade due to the development of fluorescence lidar systems operated from low flying aircraft (Kim, 1973; Humola et al., 1975; Farmer et al., 1979; Bristow et al., 1981; Hoge and Swift, 1981;Hoge and Swift, 1983;Diebel-Langohret a1.,1983,1985).The fluorescence signals of phytoplankton depend on the chlorophyll d concentration, the in vivo fluorescence efficiency and on the light attenuation of the water. Large fluctuations in the penetration depth of the laser beam due to variations in the concentration of other substances such as suspendend sediments and dissolved organic matter (yellow substance) influence the chlorophyll 3 fluorescence readings even under constant phytoplankton concentration. Bristow et al. (1981) proposed
604
successfully to normalize xhe lidar fluorescence data to the water Raman scattering signal, which depend to a good approximation solely on the optical attenuation depth. The normalized fluorescence data showed a high correlation with a concentration extracted by wet chemistry methods. ground truth chlorophyll Since the beginning of laser remote sensing of chlorophyll a, laboratory tank tests of single species phytoplankton fluorescence excited by laser systems were performed to investigate the in vivo fluorescence efficiency under controlled conditions. In order to differentiate the color groups of phytoplankton found in the sea a multiwavelength laser excitation system was investigated by Mumola et al. (1975). A linear relationship between the extracted chlorophyll 2 concentration and the four wavelength lidar fluorosensor data was found during the log phase growth of single species phytoplankton cultures (Brown et al., 1978, 1981 ). Under nutritional stress, in vivo chlorophyll a fluorescence shows an increase up to a factor of 2-4 (Kiefer, 1973 b,c; Blasco, 1975; Slovacek et al., I
1977).
Moreover, the reduction of the in vivo fluorescence efficiency due to high light is a well known phenomenon since the work of Loftus and Seliger (1975), Kiefer (1978 a,b), Heaney (1978), Vincent (1979) and Abbott et al. (1982). This photoinhibition can be observed clearly in homogeneous water masses with low turbulent mixing and nearly constant chlorophyll 2 concentration profiling the fluorescence and the downwelling irradiance. Thus, for the interpretation of actively remote sensed fluorescence and in situ data in terms of chlorophyll 2 concentrations it is necessary to find a concept to correct the fluorescence efficiency for the impact of environmental factors. Due to the fact that the nutritional concentration cannot be monitored by a remote sensor until now, the main interest of this work was to investigate the influence of light in the surface layer on the fluorescence efficiency. Three different approaches are possible. Laboratory o r in situ data normalized to constant chlorophyll a concentration are analyzed using numerical methods e.g. fit procedures giving an empirical understanding of what happens with the molecular fluorescence efficiency with increasing light. Second, time series of light and fluorescence are investigated by calculating the coherence spectrum and its confidence interval using fast Fourier transformation (Abbott et al., 1982) assuming a linear relationship between the two parameters. Third, based on a comprehensive photochemical model of the photosynthetic apparatus, where the .fluorescence originates, the reduction of the fluorescence efficiency due to photosynthetic active radiation is described in terms of physical rate constants and other photosynthesis related parameters.
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THEORETICAL BACKGROUND According to present knowledge, photosynthesis and in vivo chlorophyll g fluorescence of algae and higher plants occur in the cellular organelles called chloroplasts of typical dimension of 3 - 10 pn. Within this structure, pigments embedded in membranes, called thylakoids, absorb the visible light energy. Common to all oxygen producing algae and higher plants is chorophyll fi , which acts as a light harvesting protein complex and in special chlorophyll 2 protein complexes as reaction center, where the excitation energy is transformed to photochemical energy. Typical absorption bands of in vivo chlorophyll 2 are found in the blue at 440 nm and in the red at 670 nm. Other pigments found in the thyb and g, biliproteins (phycoerythrin, phycocyanin and allolakoids, chlorophyll phycocyanin) and carotinoids (carotenes and xanthophylls) enhance the spectral absorption in the 480 to 660 nm band. The pigment composition i s controlled by environmental conditions as nutritional supply, temperature and the spectral light intensity. Under high light, the cellular chlorophyll 2 content i s reduced compared to low light in less than a generation time (Riper et al., 1979), whereas the carotinoid synthesis i s stimulated to protect the photosynthetic unit by reducing the efficiency of energy transfer to the reaction centers or by photochemical quenching. The excitation energy from the accessory pigments is fed to two different photochemical reaction centers via exciton transfer, a resonant dipol-dipol interaction described by the Forster-mechanism showing a strong dependence on the distance of the two interacting molecules and on the relative orientation of the dipols (Forster, 1948). Calculations show that for efficient energy transfer the typical distance i s less than 10 nm. The typfcal time constant of exciton transfer to the trapping center i s in the picosecond range. At the reaction centers the transformation of excitation energy into photochemical energy'occurs by charge separation. At present, one assumes that the reaction center i s a molecular complex consisting of the chlorophyll molecule accepting the excitation energy and of an electron donor and acceptor, respectively. The electron acceptor of reaction center I 1 i s termed Q (for quencher) and is probably a specialized plastoquinone or phaeophytin molecule (Klimov et al., 1977). The electron donor of reaction center I 1 is the water splitting complex. The photochemical energy from photosystem I 1 i s transported by redox reactions to photosystem I. The electron donor for reaction center I usually i s a plastocyanin molecule, while the electron acceptor i s possibly a bound iron-sulfur protein or a flavoprotein, The photochemical energy
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from photosystem I is brought to the Calvin cycle to drive the biochemical formation of ATP in the cyclic and NADPH in the noncyclic electron transport. Figure 1 shows a schematic diagram of the photosynthetic unit, divided in the light harvesting chlorophyll proteins (LHCP),the antenna pigments of photosystem I and 11, the photochemical reaction centers I , I 1 and the simplified electron transport chain, connecting the reaction centers. Only one substructure of the electron transport chain is shown,the so-called plastoquinone pool, which stores the electrons coming from reaction center 11. The plastoquinone pool has a regu-
/ / / /
? / / / /
/
:i /
!i I
Fig.
1:
+++
+
Schematic diagram of the structure and organization of a photosynthetic unit. Chl a / b - complex : light harvesting chlorophyll proteins with fluorescence emission at 685 nm Chl dI , Chl dII : antenna pigments of photosystem I, I 1 fluorescence emission at 730 nm, 695 nm RZ I , RZ I 1 : reaction center I, I 1 for charge separation : plastoquinone pool PQ : rate constants of fluorescence from photosystem k F I ’ kFI I I , 1 1 , respectively : rate constants o f energy transfer from antenna k T I ’ kTI I ’ ktII to reaction center and vice versa : rate constant of energy transfer from antenna I 1 kT(II + I ) to antenna I , called spillover The dotted line indicates an interaction of the plastoquinone pool with the 1 ight harvesting chlorophyll proteins by enzymatic reactions (phosphorylation of LHCP).
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latory function, concerning the state of the thylakoid membrane (Horton and Black, 1980; Horton et al. 1981). The highly reduced state of plastoquinone induces an enzymatic reaction, the phosphorylation of the light harvesting chlorophyll proteins.This, in turn,changes the structure of the thylakoid membrane (Barber, 1983) and thus the distance between the accessory pigments and the reaction centers. The energy transfer to reaction center I 1 is reduced. Due to the fact that the coupling between reaction center I and the light harvesting chlorophyll proteins is very weak (Andersson and Anderson, 1980), the transfer of: excitation energy to the reaction center I is nearly unaffected. In total, the input of electrons from reaction center I 1 to the plastoquinone pool is reduced while the demand of electrons by reaction center I remains nearly constant. This change of energy transfer affects the in vivo chlorophyll a fluorescence as well as the rate of photosynthesis. The time constant for this reversible process is in the minute range. For the discussion of the in vivo chlorophyll d fluorescence it is important to understand where the observable fluorescence originates. The analysis of fluorescence spectra of chloroplasts of higher plants at 77 K (Goedheer, 1964) and of measuremenLs o f the in vivo fluorescence of algae in the pico- to nanosecond range (Moya and Garcia, 1983; Haehnel et al., 1983) lead to the conclusion that a proteins act in the photosynthetic unit. at least three different chlorophyll The fluorescence emission of the light harvesting chlorophyll proteins has its maximum at 685 nm, while those of the antenna chlorophylls of photosystem I 1 and I are at 695 and 730 nm, respectively (Fig.1). At physiological temperatures the in vivo fluorescence spectra of algae show only one maximum at 685 nm with a bandwidth of about 10 nm and a shoulder at 730 nm, corresponding to the weak fluorescence of the antenna of photosystem I. In general, for the observed chlorophyll a fluorescence FII one can write:
FIl = labs * FII where Iabs is the absorbed light intensity and Q F I I is a variable fluorescence efficiency, depending on environmental parameters. According to the bipartite model of Butler and Kitajima (1975) which incorporates the internal structure and organization of a photosynthetic unit described above, the variable fluorescence efficiency Q F I I can be formulated in the following way: Q FII = fi * + FII * f(AII) . where I3 is the energy distribution parameter describing the relative amount of absorbed light energy going to photosystem 11, 0 F I I is a constant fluorescence efficiency depending on the desexcitation rate constants of the fluorescent antenna chlorophyll molecules and f(AII) a model function describing the connection of photosynthetic units in the thylakoids. The parameter AII represents the relative number of open reaction centers. In the open state the reaction center can @
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accept excitation energy from the antenna for charge separation.The closed state i s termed the oxidized state where excitation energy i s transferred back to the antenna. This process enhances the probability of fluorescence,thermal desactivation and the so-called spillover to photosystem I. The model of Butler and Kitaa fluorescence of jima (1975) can explain the fast increase of the chlorophyll dark adapted cells after the onset of continuous light due to closed reaction centers. The reduction of the fluorescence due to high light, called photoinhibition, is not included in the bipartite model. Introducing an intensity dependent energy distribution parameter O(IphaR) describing the light dependent state of the thylakoid membrane, and an intensity dependent parameter AII(IPhaR), it is possible to model the photoinhibition in a quantitative way. As mentioned above, the membrane state is regulated by the redox state of the plastoquinone pool (Horton and Black, 1980; Allen et al., 1981). In turn the redox state of the plastoquinone is controlled by the amount of light absorbed by the cell.. Assuming R(IphaR) decreases from a maximum level Bmax under low light to a minimum level Omin with increasing light exponentially, an intensity parameter I 1 determines the state of the membrane. In contrast, the light dependence of AII(IPhaR) i s modeled by an exponential decrease with increasing light determined by a light parameter I o , indicating the adaption of the reaction centers according to the growth conditions, e.g. shade or light adapted cells. With these assumptions, confirmed by biochemical and physiological results as well as by inspecting the transient behaviour of the chlorophyll d fluorescence after the onset of continuous light, the so-cailed Kautsky effect (Kautsky and Hirsch, 1931), one can calculate the relative variation of the fluorescence efficiency due to the global irradiation. It is important to note that the light influencing phytoplankton i s restricted t o a wavelength band from 350 nm to 750 nm, the so-called photosynthetic active radiation. In Figure 2, the relative decrease of the fluorescence efficiency with increasing light is shown. The dark value of the fluorescence efficiency 1s set to 1. Calculating the normalized fluorescence with the expanded bipartite model, a second light parameter, the excitation light, had to be introduced. Assuming a short excitation pulse of typical pulse width of some ps, the energy distribution parameter 0 is not influenced by the excitation pulse due to the long time constants observed for phosphorylation of the light harvesting chlorophyll proteins. The impact o f the excitation pulse i s thus restri,cted to the parameter AII. Moreover, the number of open reaction centers is influenced by the photosynthetic active radiation too.
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1
n o r m a l i zed f 1 uorescence
(pu1se 1
1.0
.8
.6 .4
.2 0.0 0
100
200
300
400
500
photosynthetic active r a d i a t i o n CW/qml
Fig. 2: Diagram shows the normalized chlorophyll a fluorescence F depending on the photosynthetic active radiation (PhaRT according to tfle expanded bipartite model. Excitation was modeled by short pulses influencing the reaction centers exclusively. The parameter are choosen in accordance with the results presented in Figure 3.
EXPERIMENTAL RESULTS During the Fluorescence Remote Sensing Experiment, FLUREX '82,continuous ground truth measurements with an in situ fluorometer and an in situ attenuation meter, held at constant water depth, were performed at the research platform NORDSEE to investigate the daily cycle of the fluorescence efficiency due to the impact of photosynthetic active radiation. Additionally, a two-channel fluorescence lidar system was installed at the top of the research platform to monitor the chlorophyll a fluorescence simultaneously.The system,described by Gehlhaar et al. (1981), was modified for the-specific detection of the in vivo chlorophyll a fluorescence at 685 nm and the water Raman scattering at 650 nm. The excitation wavelength of the flashlamp pumped dye laser was adjusted to 532 nm with pulse energies of about 350 mJ. The lidar signals gave the depth integrated chlorophyll fi fluorescence and the effective water turbidity, resulting from the beam attenuation at the laser wavelength and form the diffuse attenuation at the Raman scattering wavelength. Ground truth water samples were collected at regular intervals. Subsamples were used for chlorophyll extraction according
610
to the method of Whitney and Darley (1979). The rest of the water samples was used to determine the total beam attenuation coefficient over the spectral region from 400 nin to 800 nm with a laboratory photometer. The cuvette length was one meter. The detector solid angle was 0.27' with a spectral bandwidth of 5 nm. In a second step, the attenuation due to dissolved organic matter (yellow substance) was determined after filtration with 0.2 pm filters. Bidestilled water was used as reference standard. A first analysis of the fluorescence data shows a high correlation of the Raman corrected fluorescence lidar signals and the in situ fluorescence data over the whole experimental period with a correlation coefficient of -97. Both fluorescence data vary by a factor of 4 in time while the analysis of the extracted chlorophyll a values reveals a relatively constant chlorophyll a concentration, indicating that the in situ and the lidar signals are both affected by environmental factors. To analyse the influence of daylight on the fluorescence efficiency in detail, the continuously recorded in situ fluorescence data are normalized to constant chlorophyll 2 concentration taking into account the result of the analysis of the water samples and of the continuously recorded in situ attenuation data at 670 nm. With a good approximation the influence of yellow substance on the diffuse attenuation coefficient at 670 nm can be neglected due to yellow substance concentrations of less than -74 mg/l. Additionally,the influence of particulate matter on the attenuation was nearly constant in time and less than .lm-'. With these findings the continuous in situ attenuation data can be considered to be proportional to the chlorophyll concentration. Assuming that the in situ fluorescence data are proportional to the chlorophyll a concentration and the fluorescence efficiency, the normalization of the fluorescence readings can be accomplished by dividing the fluorescence data by thqwater and suspended sediment corrected attenuation values. With this approximation, the normalized fluorescence data in relative units can be regarded as a measurement of the fluorescence efficiency. The mean square error of the normalized fluorescence is approximately 18 %. For the four days, April 20 to 23, Figure 3 to 6 show the daily cycle of fluorescence,efficiency together with the daily cycle of the photosynthetic active radiation given by the dotted lines. The solid line shows the result of the expanded photosynthetic model. The fluorescence efficiency is set to 1 during night. The model parameters Rmax, I. and I 1 are fitted by a computer program. For all days, only the parameter Rmin had to be changed from .2 to .3 to describe the observed photoinhibition with a high correlation. On April 20, the weather was calm without clouds. At noon, the maximum value o f the photosynthetic active radiation was about 350 Wm-2 . The fluorescence
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efficiency was reduced to about 60% of the fluorescence efficiency at night. With increasing radiation the fluorescence efficiency decreased and vice versa. For April 21, the mean photosynthetic active radiation was 150 Wm-2 with short clear ups. For this cloudy day, the model parameter Rmin had to increase to .25 for optimizing the theoretical results to the measured data. The fluorescence inhibition of typical 25% to 30% was clearly seen for both data sets. The maximum deviation was in the expected error of the experimental data. On April 22, the weather was partly cloudy with short clear ups similar to April 21, showing a maximum irradiance of about 470 Wm-2 . The fluorescence efficiency showed a daily cycle due to the influence of the photosynthetic active radiation again. Although the maximum irradiance on April 22 was higher compared to April 20, the fluorescence efficiency was reduced to about 50% compared to 60%, suggesting that the short clear ups at noon had minor influence due to an internal regulation mechanism with time constants greater than the shortest increase of the light during this day. The best approximation of the model to the in situ data could be achieved by increasing the parameter Rmin to 0.3. For April 23, the same parameter set as for April 22 was found minimizing the deviation of the model and the measured data. A good correlation between photosynthetic active radiation and fluorescence efficiency was observed until the afternoon. The increase of the global irradiation in the late afternoon was not observed in a comparative decrease of the measured fluorescence efficiency. Due to a nonconsistent increase o f the night values up to 1.2, it was assumed that a systematic error had been introduced.
-
FLUREX’82
52 0
20.4.82
E
p
400
Fig. 3: Daily cycle of the relative chl p fluorescence efficiency and of the photosynthetic active radiation on April 20, 1982. dotted line: measured data; solid line: results of the proposed model For the fluorescence efficiency taking into account the influence of the photosynthetic active radiation
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Fig. 4: Daily cycle of the relative chl a fluorescence efficiency ana of the photosynthetic active.radiation on April 21, 1982. dotted line: measured data; solid line: results of the proposed model for the fluorescence efficiency taking into account the influence of the photosynthetic active radiati on
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Fig. 5: Daily cycle of the relative chl a fluorescence efficiency ana of the photosynthetic active radiation on April 22, 1982. dotted line: measured data; solid line: results of the proposed model for the fluorescence efficiency taking into account the influence of the photosynthetic active radiation
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Fig. 6: Daily cycle of the relative chl a fluorescence efficiency a d of the photosynthetic active radiation on April 23, 1982. dotted line: measured data; solid line: results of the proposed model for the fluorescence efficiency taking into account the influence o f the photosynthetic active radiation
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DISCUSSION For the interpretation of the model parameters introduced to expand the bipartite model of Butler and Kitajima (1975) for a quantitative description of the in vivo fluorescence reduction due to high light a sensitivity study was made with respect to the fluorescence and the photosynthesis efficiency. The parameter Io, describing the light dependence of the state of the reaction center 11, can range from 1 Wm-’ to 30 Wm-‘ giving an error of about 2% with respect to the fluorescence reduction. Regarding the influence of I. on the efficiency of photosynthesis, which can be deduced by the bipartite model, it can be seen, that I. determines the light value for maximum photosynthesis (Fig.7). A comparison with the empirical formula of Steele (1962) for the description of photosynthesis efficiency shows that the free parameter of Steele i s reciprocal related to I. and describes the light value of maximum photosynthesis, too. Additionally, investigations of Ryther and Menzel (1959) confirm that for shade adapted algae the maximum of photosynthesis is typical at 10 Wm-* and for
614
-
normalized rate o f p h o t o s y n t h e s i s
v)
v)
.
IE-18
11-388
Betamin-.2E
Betamax-.BE
M -nodel W-nodal
-
I 0
50
100
photosynthetic
150
200
250
active r a d i a t i o n
CW/qrnl
Fig. 7: Diagram shows normalized rate of photosynthesis according the expanded bipartite model. The dotted line represents the result of the "matrixmodel", intrduced by Butler and Kitajima (1975). In this model, an exchange of excitation energy between photosynthetic units is assumed, in contrast to the "separate package model". The solid line illustrates the results of the "separate package model" for the rate of photosynthesis. For the reduction of the fluorescence efficiency both models give similar results.
sun adapted algae at 70 Wm-2. With ID = 30 Wm-2, the maximum i s in the range observed by Ryther and Menzel indicating that the algae population under investigation.during FLUREX '82 is assumed to be shade adapted. The parameter I, determines the fluorescence reduction due to high light levels. Varying I 1 in the range from 200 Wm-2 to 400 , the normalized fluorescence efficiency changes by about 12% compared to I, = 300 Wm- 2. According to the analysis, the parameter I 1 describes the light adapted state of the membrane and can be related to the threshold level for photoinhibition introduced by Kiefer (1973a), Heaney (1978) and Vincent (1979). The parameter I 1 i s nearly insensitive to the efficiency of photosynthesis. To analyze the influence of Rmin and Rmax on the normalized fluorescence efficiency Fn, an approximation can be given, assuming I 0 and I 1 as constant for the time of investigation:
615
where Iphar is the intensity of the photosynthetic active radiation. The approximation shows, that Fn is determined by the ratio of Rmin to Omax. A variation of Rmin and Rmax with fixed ratio reveals that the approximation of F, compared to the exact evaluation i s within an error of 2%. Analyzing the influence of Omin and Rmax on the efficiency of photosynthesis, one can show that Rmax determines the light level of maximum photosynthesis, while the maximum of photosynthesis is to a first approximation independent of Bmin . Assuming that the ratio Omin to Dmax describes the physiological state of the membrane due to phosphorylation, an increase of this ratio is equivilant to a minor decrease of photoinhibition at high light levels. In the limit of Omax, no photoinhibition is expected according to the expanded bipartite model. The data of Kiefer (1973a, 1973b) and Blasco (1975) as well as own investigations confirm that under nutritional stress the photoinhibition of algae is reduced compared to algae in the exponential growth phase resulting in a higher fluorescence efficiency. Thus, it can be assumed that the ratio Bmin to Rmax depends on the nutrient availability and on environmental stress factors. In conclusion, the four parameters introduced by the proposed model can be reduced to two parameters determining the photoinhibition of the in vivo fluorescence efficiency due to high light. For actively remote sensed chlorophyll 5 fluorescence as well as for in situ fluorescence data it is necessary to correct the readings with respect to the impact o f the photosynthetic active radiation. The daily cycle of the global irradiation influences the in vivo fluorescence efficiency as shown in this paper. The transformation o f in vivo fluorescence data to chlorophyll a values can be optimized introducing the expanded bipartite model for data evaluation. The typical error for data processed by the proposed method is in the range given by standard extraction methods. ACKNOWLEDGEMENTS The experiment at the research platform NORDSEE during FLUREXI82 was financed by a grant from the Bundesministerium fur Forschung und Technologie. REFERENCES Abbott, M.R., Richerson, P.J. and Powell, T.M., 1982. In situ response of phytoplankton fluorescence to rapid variations in light. Limnol. Oceanogr., 27: 218-225. Allen, J.F., Bennett, J., Steinback, K.E. and Arntzen, Ch.J., 1981. Chloroplast protein phosphorylation couples plastoquinone redox state to distribution of
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excitation energy between photosystems. Nature, 291: 25-29. Andersson, 8. and Anderson, J.M., 1980. Lateral heterogeneity in the distribution of chlorophyll-protein complexes of the thylakoid membranes of spinach chloroplasts. BBA, 593: 427-440. Barber, J., 1983. Membrane conformational change due to phosphorylation and the control of energy transfer in photosynthesis. Photobiochem. Photobiophys., 5: 181- 190. Blasco, D., 1975. Variations of the ratio in vivo-fluorescence/chlorophyll and its application to oceanography. Effects of limiting different nutrients, of night and day and dependence on the species under investigation. NASA Technical Translation, TTF-16: 317 pp. Bristow, M., Nielsen, D., Bundy, D. and Furtek, R., 1981. Use,of water Raman emission to correct airborne laser fluorosensor data for effects of water optical attenuation. Appl. Opt., 20: 2889-2906. Brown, C.A., Farmer, F.H., Jarrett, 0. and Staton, W.L., 1978. Laboratory studies of in vivo fluorescence of phytoplankton. Proc. Fourth Joint Conf. Sens. Environ. Pollutants. American Chem. SOC. 782-788. Brown, C.A., Jarrett, 0. and Farmer, F.H., 1981. Laboratory tank studies of single species of phytoplankton using a remote sensing fluorosensor. NASA TP-1821. Butler, W.L. and Kitajima, M., 1975. A tripartite model for chlorophyll fluorescence. In: M. Avron (Editor), Proceedings of the Third International Congress on Photosynthesis. Elsevier, Amsterdam, 13 pp. Diebel-Langohr, D., Gunther, K.P. and Reuter, R.,1983. Lidar applications in remote sensing of ocean properties. Int. Coll. on Spectral Signatures of Objects in Remote Sensing, Conf. Proc., Bordeaux. Diebel-Langohr, D., Gunther, K.P., Hengstermann, Th., Loquay, K., Reuter, R. and Zimmermann, R., 1985. An airborne lidar system for oceanographic measurements. In: Optoelektronik in der Technik, Tagungsberichte LASER 85 - Optoelektronik, Munchen 1. - 5. Juli 1985, Springer Verlag (in press). Farmer, F.H., Brown, C.A., Jarrett, O., Campbell, J.W. and Staton, W.L., 1979. Remote sensing of phytoplankton density and diversity in Narragansett Bay using an airborne fluorosensor. In: Proceedings Thirteenth International Symposium on Remote Sensing of the Environment, 23-27 Apr. 1979.Environmental Research Institute of Michigan, Ann Arbor. 1793-1805. Forster, Th., 1948. Zwischenmolekulare Energiewanderung und Fluoreszenz. Annalen der Physik, 2: 55-75. Gehlhaar, U., Gunther, K.P. and Luther, J., 1981. Compact and highly sensitive fluorescence lidar for oceanographic measurements. Appl. Opt., 20: 3318-3320. Goedheer, J.C., 1964. Fluorescence bands and chlorophyll forms. BBA 88: 304-317. Haehnel, W., Holtzwarth, A.R. and Wendler, J., 1983. Picosecond fluorescence kinetics and energy transfer in the antenna chlorophylls of green algae. Photochem. Photobiol. 37: 435-443. Heaney, S.I., 1978. Some observations on the use of the in vivo fluorescence technique to determine chlorophyll in natural populations and cultures of freshwater phytoplankton. Freshwater Biol. 8: 115-126.. Hoge, F.E. and Swift,R.N., 1981. Airborne simultaneous spectroscopic detection of laser-induced water Raman backscatter and fluorescence from chlorophyll and other natural occuring pigments. Appl. Opt. 20: 3197-3205. Hoge, F.E. and Swift, R.N., 1983. Airborne dual excitation and mapping of phytoplankton photopigments in a Gulf Stream Warm Core Ring. Appl. Opt. 22: 2272-2281. Horton, P. and Black, M.T., 1980. Activation of adenosine 5' triphophate-induced quenching of chlorophyll fluorescence by reduced plastoquinone. The FEBS Lett. 119: basis of state I - state I 1 transitions in chloroplasts. 141- 144. Horton, P., Allen, J.F., Black, M.T. and Bennett, J., 1981. Regulation of phosphorylation of chloroplast membrane polypeptides by redox state of plastoquinone. FEBS Lett., 125: 193-196.
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Kautsky, H. and Hirsch, A., 1931. Neue Versuche zur Kohlensaureassimilation. Naturwissenschaften, 19: 964. Kiefer, D.A., 1973a. Fluorescence properties of natura phytoplankton population. Mar. Biol. 22: 263-269. Kiefer. D.A.. 1973b. ChloroDhvll a fluorescence in marine centric diatoms: responses'of chloroplasts t o - ligfit and nutrient stress. Mar. Biol. 23: 39-46. Kiefer, D.A., 1973c. The in vivo measurements o f chlorophyll by fluorometry. I n : L.H. Stevenson and R.R. Colwell (Editors), The Belle W. Baruch Library in Marine Science: Estuarine Microbial Ecology. University o f South Carolina Press. Kim, H.H., 1973. New algae mapping technique by use of an airborne laser fluorosensor. Appl. Opt., 12: 1454-1458. Klimov, V.V., Klevanik, V.A., Shuvalov, V.A. and Kravsnovsky,A.A., 1977. Reduction of phaeophytin in the primary light reaction of photosystem 11. FEBS Lett. 82: 183-186. Loftus, M.E. and Seliger, H., 1975. Some limitations of the in vivo fluorescence technique. Chesapeake Sci. 16: 79-92. Moya, I . and Garcia, R.,1983. Phase fluorimetric lifetime spectra. I . In algal cells at 77 K. BBA 722: 480-491. Mumola, P.B., Jarrett, 0. and Brown, C.A., 1975. Multiwavelength lidar for remote sensing of chlorophyll a in algae and phytoplankton. The use of lasers for hydrographic studies. NESA-SP 375: 137-145. Riper, D.M., Owens, T.G. and Falkowski, P.G.,1979. Chlorophyll turnover in Skeletonema costatum, a marine plankton diatom. Plant. Physiol. 64: 49-54. Slovacek, R.E. and Hannan, P.T., 1977. In vivo fluorescence determination of phytoplankton chlorophyll a . Limnol. Oceanogr. 22: 919-925. Steele, J.H., 1962. Environmental control of photosynthesis in the sea. Limnol. Oceanogr., 7: 137-150. Vincent, W.F., 1979. Mechanisms of rapid photosynthetic adaption in natural phytoplankton communities. 1.Redistribution of excitation energy between photosystem I and 11. J. Phycol. 15: 429-434. Whitney, D.E. and Darley, W.M., 1979. A method for the determination of chlorophyll a in samples containing degradation products. Limnol. Oceanogr., 24: 183-186.
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Dakar
2241 ( S k n b g a l )
ABS TRAC T Two CZCS p i c t u r e s (11 J a n u a r y 1981 and 1 March 1 9 8 1 ) o f phyt o p l a n k t o n blooms d u r i n g t h e w i n d - i n d u c e d u p w e l l i n g i n S e n e g a l waters a r e p r e s e n t e d . The i n f l u e n c e o f u p w e l l i n g on t h e mesoscal e s p a t i a l v a r i a b i l i t y o f t h e s u r f a c e t e m p e r a t u r e and biomass f i e l d s i s shown. S t r o n g f r o n t a l p h y t o p l a n k t o n f e a t u r e s d o m i n a t e i n t h e f i r s t p e r i o d o f u p w e l l i n g and a more homogeneous bloom i s f o u n d a t t h e e n d o f t h e u p w e l l i n g p e r i o d . We t r y t o r e l a t e t h e i n s t a n t a n e o u s f i e l d s t o t h e f r o n t a l dynamical p r o c e s s e s . INTRODUCTION
E v e r s i n c e t h e Meteor e x p e d i t i o n s i n t h e e a r l y 1 9 5 0 ' s and t h e n t h e CINECA, A.
Von Humbolt a n d J O I N T c r u i s e s , t h e West A f r i -
c a n u p w e l l i n g system h a s been t h e s u b j e c t o f i n t e n s i v e systema-
t i c a l r e s e a r c h ( H a g e n , 1 9 7 4 , Schemainda e t a l . , 1 9 7 5 ) . I n 1 9 8 2 , t h i s c u l m i n a t e d i n t h e p u b l i c a t i o n o f an I.C.E.S.
r e p o r t on t h e
C a n a r y C u r r e n t . L e s s i s known o n t h e West A f r i c a n c o a s t u n d e r t h e 15ON l a t i t u d e
( f i g . 1 ) . S p e c i f i c d a t a o v e r S e n e g a l waters
a r e p r o v i d e d by B e r r i t ( 1 9 6 1 a n d 1 9 6 2 ) , R o s s i g n o l ( 1 9 7 3 1 , R e b e r t ( 1 9 7 4 , 1 9 7 7 , 1 9 8 3 1 , Tour6 ( 1 9 8 2 , 1 9 8 3 ) . I n g e n e r a l , t h e S e n e g a l e s e s y s t e m shows t h e p r o p e r t i e s o f a n u p w e l l l n g s y s t e m r e l a t e d t o t h e t r a d e w i n d s on t h e e a s t e r n bound a r i e s o f o c e a n s ( M i t t e l s t a e d t , 1 9 8 3 ) . The n o r t h e a s t e r n w i n d s t r i g g e r t h e Ekman t r a n s p o r t which r e s u l t s i n u p w e l l i n g o f c o l d and n u t r i e n t r i c h deep w a t e r (mixing o f S o u t h and North A t l a n t i c C e n t r a l Water). A t t h e l a t i t u d e o f S e n e g a l , t h e u p w e l l i n g o c c u r s f r o m December t o March, r e l a t e d t o t h e s o u t h e r n p o s i t i o n o f t h e I n t e r T r o p i c a l Convergence Zone ( " d r y s e a s o n " ) a n d f o r m s a c h a r a c t e r i s t i c c o l d plume ( f i g . 2 ) .
620
10'
CAP VERT lo'
Fig.
1. S i t u a t i o n o f t h e s t u d i e d area on t h e West A f r i c a n c o a s t . Major b a t h y m e t r i c f e a t u r e s o f t h e S e n e g a l e s e c o n t i n e n t a l s h e l f . Note t h e d i f f e r e n c e b e t w e e n t h e d e e p n a r r o w cont i n e n t a l s h e l f t o t h e n o r t h o f D a k a r , a n d t h e b r o a d and s h a l l o w one i n t h e s o u t h . The w i d t h v a r i e s f r o m 30 km a t S t L o u i s , t o 10 km a b o v e t h e Cap t o 100 krn i n f r o n t o f Gambia.
621
Fig.
2 . C u r r e n t s a t 5 rn, f r o m t h e - 2 O t h t o 31st o f March 1974 ( R e b e r t , 1 9 7 4 ) o n t h e S e n e g a l e s e c o n t i n e n t a l s h e l f . Note t h e maximal s o u r c e o f d i v e r g e n c e b e t w e e n i s o b a t h s 2 0 rn a n d 50 rn.
622
P h y t o p l a n k t o n p r o d u c t i v i t y i s g r e a t l y i n c r e a s e d i n t h e upwell i n g zone o f n u t r i e n t - r i c h w a t e r s . Annual p r i m a r y p r o d u c t i o n r e a c h e s 250 g.C m-2
(Schemaida e t a l . , 1 9 7 5 ) . T h i s h i g h p r o d u c t i -
v i t y p l a c e s t h e S e n e g a l s h e l f a s t h e s e c o n d most p r o d u c t i v e r e g i o n , a f t e r t h e Cap B l a n c ( 3 2 5 g.C m-2
year -1).
The u s e f u l n e s s o f v i s i b l e i m a g e r y f o r d e t e r m i n i n g p h y t o p l a n k t o n p r o d u c t i o n i n t h e u p w e l l i n g a r e a s h a s been w e l l d e m o n s t r a t e d on t h e u p w e l l i n g o f f P e r u ( F e l d m a n , 1 9 8 5 1 , on t h e C a 1 , i f o r n i a n c o a s t ( L a u r s e t a l . , 1 9 8 4 , T r a g a n z a e t a l . , 1 9 8 2 ) , on t h e B e n g u e l a c u r r e n t r e g i o n o f f S o u t h A f r i c a (Shannon e t a l . , 1 9 8 4 ) and o v e r Senegal ( S t u r m , 1983 ; V i o l l i e r and Sturm, 1 9 8 4 ) . S i n c e 1 9 8 2 , ORSTOM and t h e C . R . O . D . T . ,
i n c o o p e r a t i o n w i t h ESA, h a v e
e s t a b l i s h e d a c o n t i n u o u s s a t e l l i t e c o v e r a g e o f t h e Gulf of G u i n e a ("LISTAO" o p e r a t i o n , C i t e a u e t a l . ,
1984) i n o r d e r t o
s t u d y t h e n a t u r a l environment o f t h e l i s t a o (main t u n a s p e c i e s ) , u s i n g t h e i n f r a r e d d a t a o f t h e g e o s t a t i o n n a r y s a t e l l i t e METEOS A T . What i s m i s s i n g , h o w e v e r , i s t h e u n d e r s t a n d i n g o f t h e evo-
l u t i o n o f t h e d i s t r i b u t i o n and abundance o f p h y t o p l a n k t o n , f i r s t l i n k o f t h e f o o d c h a i n . The p u r p o s e o f t h i s s t u d y i s t o c l a s s i f y the phytoplankton responses t o t h e upwelling event i n order t o explain the primary production cycle i n t h i s region. s t e p , two i m a g e s o f NIMBUS-7
I n a first
( C o a s t a l Zone C o l o r S c a n n e r ) were
u s e d t o p r o v i d e maps o f p h y t o p l a n k t o n b i o m a s s d u r i n g t h e 1981 upwelling. In a d d i t i o n , t h e C.R.O.D.T.
h a s conducted sea-going
e x p e d i t i o n s over t h e c o n t i n e n t a l s h e l f break s i n c e 1966, provid i n g a n o c e a n o g r a p h i c knowledge o f t h e r e g i o n . I - M A T E R I A L AND METHODS 1.1 - Nimbus-7 C . Z . C . S .
experiment
The C o a s t a l Zone C o l o r S c a n n e r e x p e r i m e n t w a s b u i l t s p e c i f i c a l l y t o p r o v i d e a n e s t i m a t e o f p h y t o p l a n k t o n pigment c o n c e n t r a tions
In
t h e s u r f a c e l a y e r o f t h e o c e a n , b y m e a s u r i n g t h e upwel-
l e d s p e c t r a l radiances i n four channels i n t h e v i s i b l e (443, 5 2 0 , 5 5 0 , 670 n m ) , and one c h a n n e l i n t h e n e a r i n f r a r e d ( 7 5 0 n m ) ; a n i n f r a r e d c h a n n e l ( 1 1 . 5 pm) g i v e s s i m u l t a n e o u s images o f t h e s u p e r f i c i a l temperature ( f i r s t microns) (Hovis e t a l . , 1 9 8 0 ) . Nimbus-7 h a s a c i r c u l a r s u n - s y n c h r o n o u s a s c e n d i n g node o r b i t w i t h a p a t h w i d t h o f 1956 km, s p a t i a l r e s o l u t i o n o f 825 m x 825m and a r e p e a t c y c l e o f 26 d a y s .
623
1.2 - NIMBUS-7 - CZCS data
The two CZCS images presented were recorded on January 11, 1981 at 1250 UT, orbit 11198 and on March 1 , 1981 at 1230 UT or-
bit 11875. They correspond respectively to the beginning of the Senegalese upwelling (January) and to its maximal phase (March), according to Tour6 ( 1 9 8 3 ) and Rebert ( 1 9 8 3 ) . The area studied extends from the Bissagos in the south ( 1 2 O N ,
16OW) to Saint
Louis ( 1 6 O N , 16OW), and out to sea about 2 O o W (see fig. 1 ) . It covers mainly the continental shelf and open waters on a width of 2 ' 1.3
-
of longitude. Processing of CZCS data and chlorophyll algorithm
In order to determine subsurface upwelling radiances from CZCS data, the atmospheric contribution due to scattering and aerosol effects must be removed. The atmospheric corrections were achieved as follows. The absolute reflectance values were calculated from the raw channel data (Viollier, 1 9 8 2 ) , taking into account the decrease of sensitivity of the sensor. The Rayleigh scattering contribution was calculated using an algorithm from Viollier et al. ( 1 9 8 0 ) . The aerosol component is estimated from the channel 4 reflectance at 670 nm (Gordon and Clark, 1 9 8 0 ) . F o r the latter, a negative value was chosen for n , the Angstrom exponent, expressing the exponential dependence between aerosol optical thickness and wavelength. The influence of residual water reflectance at 670 nm was taken into account using an empirical coefficient, v , of the'proportionality between channels 3 and 4. The parameters used in the processing are summarized and discussed in annex I. Pigment concentration maps (chlorophyll a + phaeopigments) were obtained from satellite-derived true water reflectances in the three channels : 443, 520 and 550 nm, using an algorithm proposed by Gordon et al. ( 1 9 8 0 ) . The concentration of "chlorophyll" 3
C (in mg.m- ) is related to the ratio of reflectances R(X) as follows : log C
=
a + b log 443 f o r low values of C , R 550
log C
=
c
+ d log
c<
1 . 5 mg.m-
3
(1)
520 for high values of C , C > ' 1 . 5 mg.m-3 ( 2 ) R 550
The coefficients a, b , c , d were determined empirically from combined measurements of pigment and sea reflectances at 443, 520 and 550 nm (Gordon et al., 1 9 8 0 ) . These values were chosen
624 b e c a u s e t h e y c o r r e s p o n d t o a v e r y b r o a d s p e c t r u m o f water t y p e s a n d c o n c e n t r a t i o n s a t s e a . The CZCS-derived
values represent the
a v e r a g e pigment c o n c e n t r a t i o n s t o a d e p t h o f 1 o p t i c a l a t t e n u a t i o n l e n g t h , which v a r i e s from 17 m t o 1 . 6 m o v e r t h e p i g m e n t c o n c e n t r a t i o n r a n g e o f 0 . 1 - 10 mg.m
-3
( S m i t h and B a k e r , 1 9 7 8 ) .
The e x p e c t e d a c c u r a c y on c h l o r o p h y l l d e t e r m i n a t i o n f r o m CZCS i s log C
2
0 . 5 (Gordon e t a l . ,
1980).
The o b t a i n e d s c e n e s were g e o m e t r i c a l l y remapped on a M e r c a t o r p r o j e c t i o n (GIPSY s o f t w a r e , Belbeoch,
1 9 8 3 ) . For t h e c a l i b r a t i o n
o f s c e n e s , s e e a n n e x I. The i n f r a r e d i m a g e s were u s e d , d e s p i t e n o i s y s c a n l i n e s , due t o d e t e c t i n g e r r o r s . They w e r e e n h a n c e d
s o t h a t sea s u r f a c e t e m p e r a t u r e r a n g e s from w a r m ( b l a c k ) t o c o l d ( w h i t e ) . Land, c l o u d s a n d d u s t w i n d s were masked i n w h i t e . The c h l o r o p h y l l maps were e n h a n c e d s o t h a t c h l o r o p h y l l p o o r w a t e r s (C
<
I m g . ~ n - ~ a) p p e a r i n b l a c k a n d r i c h w a t e r s ( C
>
5 ~ n g . m - ~ ap)
p e a r i n white. The d u s t wind o b s e r v e d o n t h e " c h l o r o p h y l l " image o f t h e 1st o f March 1981 masks o n l y t h e b l u e t o - n e a r
i n f r a r e d c h a n n e l s , and
d o e s n o t a f f e c t t h e i n f r a r e d image. The a e r o s o l s , m a i n l y compos e d o f s a n d d u s t o f l a n d o r i g i n a n d p u s h e d by t r a d e w i n d s o v e r t h e s e a a r e s t r o n g l y r e f l e c t a n t b u t h a v e t h e same t e m p e r a t u r e
a s t h e s e a . On t h e c o n t r a r y , t h e a e r o s o l s o f m a r i t i m e o r i g i n were w e l l e l i m i n a t e d by t h e a t m o s p h e r i c c o r r e c t i o n s .
I1 - RESULTS The C Z C S images from t h e 1 1 t h o f J a n u a r y 8 1 and 1st o f March 8 1 p r o v i d e two d i m e n s i o n a l s y n o p t i c v i e w s af s e a s u r f a c e temper a t u r e a n d s u b s u r f a c e r e f l e c t a n c e s . They show a number o f s u r f a ce-expressed
f e a t u r e s o f t h e u p w e l l i n g dynamics.
11.1 - S e a S u r f a c e T e m p e r a t u r e (SST) f i e l d s (CZCS) 11.1.1 - 11 J a n u a r y 1981 t e m p e r a t u r e map On t h e 1 1 t h o f J a n u a r y ( f i g u r e 3 a ) , t h e SST f i e l d i s m a i n l y composed o f two w a t e r m a s s e s : t h e warmer s t r a t i f i e d water mass b e t w e e n t h e l o n g i t u d e s o f 19OW and 1 8 O W ( t r o p i c a l waters a c c o r ding t o Rossignol,
1 9 7 8 ) a n d t h e c o l d e r mixed water mass p r o d u -
c e d by t h e c o a s t a l u p w e l l i n g .
F r o n t a l i n s t a b i l i t i e s are seen a t
t h e i n t e r f a c e o f t h e two w a t e r masses. The u p w e l l e d waters show t h r e e main p a t t e r n s . The most s t r i k i n g f e a t u r e i s t h e c o l d plume u n d e r t h e Cap V e r t , which c o i n c i d e s w i t h t h e maximal d i v e r g e n c e o f t h e c u r r e n t s ( s e e f i g . 2 ,
625 R e b e r t , 1 9 6 3 ) a n d t h e main r e g i o n o f v e r t i c a l m o t i o n on t h e midd l e o f t h e s h e l f . The w e s t e r n l i m i t o f t h e plume f o l l o w s e x a c t l y t h e 2 0 0 m e t e r s i s o b a t h a t 17'30W
( f i g . 1 ) . I t f o r m s t h e main
f r o n t a l s y s t e m b e t w e e n two o p p o s i n g hydrodynamic f e a t u r e s : t h e u p w e l l i n g d i v e r g e n c e a n d t h e t r o p i c a l waters c o n v e r g e n c e . The s o u t h e r n l i m i t , a t t h e l a t i t u d e o f t h e Gambia ( 1 3 O N ) ,
is
d i s t o r t e d by a l o n g f e a t u r e e x t e n d i n g t o t h e w e s t a n d t h e n t o the north
( 1 3 O N ,
1 8 O W ) .
A wedge o f warmer w a t e r a p p e a r s n e a r t h e
c o a s t i n f r o n t o f M ' b o u r (14ON). N o r t h o f Cap V e r t ( t h e Great C o a s t from Dakar t o S a i n t L o u i s ) , c o l d p l u m e s p e r p e n d i c u l a r t o t h e c o a s t a n d g y r e s o f 100 kms wide a r e v i s i b l e .
One i s o b v i o u s l y t u r n i n g a r o u n d t h e c a p e
showing a c y c l o n i c r o t a t i o n .
The c o l d hammer s h a p e d plume a b o v e
t h e peninsula i s r e l a t e d t o t h e n o r t h e r n upwelling system. I n o f f s h o r e w a t e r s , 200 km f r o m t h e c o a s t a n d a r o u n d 19OW, 15ON, t h e r e a p p e a r i n s t a b i l i t i e s a n d d o u b l e e d d i e s which are c h a r a c t e r i s t i c o f t h e f r o n t a l mechanisms b e t w e e n w a t e r masses o f different density. I n January, the northern p a r t of the continental shelf ( G r e a t C o a s t ) i s c o l d e r t h a n t h e s o u t h e r n o n e . The image o f t h e 1 2 t h o f J a n u a r y , n o t shown h e r e , r e v e a l s t h e same o c e a n o g r a p h i c p a t terns. The m e t e o r o l o g i c a l f i l e o f m e r c h a n t s h i p s SST i n d i c a t e s t h a t t e m p e r a t u r e s o f t h e s o u t h e r n s h e l f show a n o r t h - s o u t h
gradient
f r o m 21OC u n d e r t h e Cap t o 26OC a t t h e B i s s a g o s . 11.1.2
- 1 March 1981 t e m p e r a t u r e map
The main t e m p e r a t u r e f e a t u r e o f t h e 1 March s c e n e ( f i g . 3 b ) i s t h e i n t e n s i f i c a t i o n o f t h e u p w e l l i n g p a t c h u n d e r t h e Cap V e r t . The c o l d plume forms m e a n d e r s d i r e c t e d t o t h e s o u t h a n d p a r a l l e l t o t h e c o a s t . A n a r r o w c e n t r a l v e i n o f c o l d w a t e r marks t h e z o n e o f maximal d i v e r g e n c e b e t w e e n t h e 20 m a n d 50 m i s o b a t h s ( c o m p a r e f i g . 1 a n d 2 ) . The g r a d i e n t b e t w e e n t r o p i c a l a n d u p w e l l i n g waters i s s h a r p e r t h a n i n J a n u a r y and i s i n d e n t e d by t h r e e g y r e s . S i m i l a r g y r e s h a v e b e e n r e c o r d e d by M e t e o s a t d u r i n g f i v e s u c c e s s i v e d a y s i n F e b r u a r y 8 2 ( R o y , 1 9 8 2 ) . They a r e q u a s i permanent d u r i n g t h e upw elling p e r i o d . The e d d i e s o f t h e s o u t h e r n plume a r e s t r i c t l y c y c l o n i c a n d h a v e a d i a m e t e r o f 30 km. The s o u t h e r n g y r e i s a p p a r e n t l y a n t i c y c l o n i c a n d h a s a g r e a t e r d i a m e t e r o f 80 km ( p e r s i s t e n c e o f January
pattern).
626
F i g . 3a. C Z C S image i n t h e t h e r m a l i n f r a r e d o f t h e u p w e l l i n g c o l d plume on J a n u a r y 11, 1981 o v e r S e n e g a l w a t e r s .
S i n c e J a n u a r y , t h e c o l d tongue is expending f u r t h e r t o t h e s o u t h , as i t i s g e n e r a l l y o b s e r v e d from c r u i s e s m e a s u r e m e n t s . A t t h e l a t i t u d e l e v e l o f t h e Saloum (14ON), t h e plume i s de-
t a c h e d from t h e c o a s t which c a u s e s a s e c u n d a r y c o a s t a l f r o n t f i t t i n g t h e 20 meters i s o b a t h .
627
F i g . 3b. CZCS image i n t h e t h e r m a l i n f r a r e d o f t h e u p w e l l i n g c o l d plume on March 1 , 1981 o v e r Senegal waters.
Comparing t h e n o r t h e r n and s o u t h e r n p a r t s o f t h e image, t h e u p w e l l i n g plume i s c o l d e r t h a n t h e Cap V e r t s h e l f w a t e r . The merchant s h i p SST i n d i c a t e s a g r a d i e n t from 18OC under t h e cape t o 2 2 O C a t t h e Bissagos l a t i t u d e .
628
11.2
-
T u r b i d i t y map
The c o a s t a l t u r b i d i t y i s formed by suspended mater a1 re a t e d
or n o t t o p h y t o p l a n k t o n ( o r g a n i c or i n o r g a n i c m a t t e r )
This m a -
t e r i a l i n c r e a s e s t h e r e f l e c t a n c e s o v e r t h e whole v i s i b l e s p e c trum (Morel and P r i e u r , 1 9 8 0 ) , and t h e n i n t h e t h r e e CZCS chann e l s . The CZCS r e f l e c t a n c e a t c h a n n e l 3 (550 nm) c a n be u s e d t o measure t h e t o t a l s e s t o n ( Z b i n d e n , 1 9 8 1 ) .
F i g . 4 . CZCS map o f t o t a l suspended m a t t e r on J a n u a r y 11, 1981 o v e r t h e S e n e g a l e s e c o n t i n e n t a l s h e l f showing d i s t i n c t i ve b a t h y m e t r i c f e a t u r e s w i t h i n t h e 2 0 m i s o b a t h , e s p e c i a l l y i n f r o n t o f Casamance and Gambie.
629
On t h e p i c t u r e o f S e n e g a l , t h e h i g h r e f l e c t a n c e s o b s e r v e d a l o n g t h e c o a s t ( 4 0 km wide bands a t t h e B i s s a g o s l a t i t u d e ) a r e c a u s e d by a r e s u s p e n s i o n o f bottom s e d i m e n t s due t o s w e l l . The l i m i t of the c o a s t a l t u r b i d i t y follows t h e 20 m isobath ; a
s t r o n g c o a s t a l m i x i n g e f f e c t i v e l y o c c u r s between t h e c o a s t and t h i s i s o b a t h . I n t h e s o u t h , t h e r e f l e c t i n g band shows f e a t u r e s which g i v e a good mapping o f t h e submarine t o p o g r a p h y ( s e e f i g . 4 i n J a n u a r y ) . I n t h e n o r t h , t h e most r e f l e c t i n g band a p p e a r s a t
S t L o u i s ( o u t l e t o f t h e S e n e g a l r i v e r ) . These two v e r y h i g h l y r e f l e c t i n g b a n d s c o r r e s p o n d t o h i g h mud c o n t e n t i n t h e b o t t o m s e d i m e n t s (Domain, 19821, r a t h e r t h a n t o a r i v e r r u n - o f f
of part i c l e s (no r a i n fall during t h e dry season). O f f t h i s t u r b i d z o n e , t h e waters have s t r i c l y a n o c e a n i c o r i -
g i n and a r e o n l y i n f l u e n c e d by t h e p h y t o p l a n k t o n c o n t e n t (case 1 w a t e r s , Morel and P r i e u r , 1 9 8 0 ) .
1 1 . 3 - C h l o r o p h y l l a d i s t r i b u t i o n s (CZCS) 11.3.1
-
11 J a n u a r y 1981 c h l o r o p h y l l map
The most i m p r e s s i v e c h a r a c t e r i s t i c o f t h i s image ( s e e f i g . 5 a ) i s i t s c o n s i d e r a b l e s p a t i a l h e t e r o g e n e i t y . The p a t c h e s o f p h y t o p l a n k t o n a r e e l o n g a t e d and form n a r r o w meanders and r i b b o n l i k e f e a t u r e s i n some v e r y s p e c i f i c p l a c e s . Under t h e Cap V e r t t h e crown s h a p e o f t h e bloom i s r e m a r k a b l e . D i s c r e t e p a t c h e s , onl y 5 m i l e s b r o a d and 1 5 m i l e s l o n g , a p p e a r a t t h e w e s t e r n e d g e o f t h e u p w e l l e d plume a t 17.5OoW, which c o r r e s p o n d s t o t h e t h e r -
mal f r o n t w i t h t r o p i c a l w a t e r s . The c e n t e r o f t h e plume i n s i d e the continental shelf is depleted i n chlorophyll. I n t h e n o r t h , t h e e n r i c h m e n t i s inhomogeneous and f o l l o w s t h e p a t t e r n s o f t h e c o l d g y r e s . T h e r e , t h e p a t c h e s a r e l a r g e r and show t h e i r maximal c o n c e n t r a t i o n a t t h e end o f t h e e d d i e s . Some o f f s h o r e p h y t o p l a n k t o n p a t t e r n s a r e a l s o v i s i b l e on t h e ) follow the f r o n t s between w a r m and c o l d waters ( 2 . 5 ~ n g . m - ~ and temperature f e a t u r e s . 3 The g r e a t e r c o n c e n t r a t i o n s ( 5 mg.m ) a r e found i n t h e n o r t h e r n p a r t o f t h e u p w e l l i n g , above -t:?e Cap V e r t , a t t h e o u t l e t o f t h e Gambia r i v e r and i n c o a s t a l a r e a o f f M'bour (14ON). The t u r b i d waters a t t h e s o u t h o f t h e s c e n e a r e a l s o r i c h i n 3 c h l o r o p h y l l ( 4 mg.m ) .
630
F i g . 5 a . C Z C S p h y t o p l a n k t o n pigment c o n c e n t r a t i o n map o v e r Seneg a l w a t e r s on J a n u a r y 11, 1981. P r o c e s s e d u s i n g a l g o r i t h m o f Gordon e t a l . (1980), and r a t i o o f c h a n n e l s 2 ( 5 2 0 nm) and 3 (550 nm). The conc n t r a t i o n s r a n g e from low c o n t e n t w a t e r (C3< 1 mg.m-’) i n black t o high c o n t e n t w a t e r ( C > 5 mg. ) i n w h i t e . The h i g h t u r b i d i t y c o a s t a l band a p p e a r s i n b l a c k ; l a n d , c l o u d s and d u s t s a n d i n w h i t e . Note t h e crown shap.e o f t h e c h l o r o p h y l l e n r i c h m e n t on t h e plume.
11.3.2
-
1 March 1981 chloropey;>-F3p
The c h l b r o p h y l l d i s t r i b u t i o n shows a g r e a t e r homogeneity ( f i g . 5 b ) . Two v e r y l a r g e p a t c h e s o f h i g h c o n c e n t r a t i o n ( 8 ~ n g . r n - ~ a) r e found : t h e s o u t h e r n bloom ( 4 0 m i l e s w i d e ) i s r e j e c t e d on t h e s h e l f . The n o r t h e r n one f o l l o w s t h e v e i n o f c o l d w a t e r . They b o t h form a s t r o n g g r a d i e n t w i t h t h e open w a t e r s (1.5 ~ng.m-~).
631
F i g . 5 b . CZCS p h y t o p l a n k t o n map o v e r S e n e g a l w a t e r s on March 1 , 1 9 8 1 . Same p r o c e s s i n g and same c h l o r o p h y l l s c a l e a s J a n u a r y ( l e g e n d f i g . 5 a ) . Note t h e u n i f o r m a s p e c t o f t h e p h y t o p l a n k t o n bloom.
No c o l o r e d d i e s a r e v i s i b l e e x c e p t one c u r v e d e f f e c t on t h e plume which r e f l e c t s t h e p o s i t i o n o f t h e c o l d c y c l o n i c g y r e . O f f s h o r e , t h e r e i s a n i n t r u s i o n o f r e l a t i v e l y p o o r water f o r ming a " r i v e r " , e x t e n d i n g from t h e s o u t h e a s t t o w a r d s t h e Cap ( 1 . 7 ~ n g . m - ~ ) c, r e a t i n g o f f s h o r e c o l o r f r o n t s , c o r r e s p o n d i n g w i t h
t h e thermal f r o n t s .
632 I11
-
111.1
DISCUSSION
-
What w e know a b o u t t h e u p w e l l i n g mechanism
The h o r i z o n t a l c i r c u l a t i o n o v e r t h e c o n t i n e n t a l s h e l f i s w e l l shown i n f i g u r e 2 ( R e b e r t , 1 9 8 3 ) . The u p w e l l e d waters form a c o l d plume u n d e r t h e p e n i n s u l a . I n s i d e t h e s h e l f , t h e s u r f a c e c u r r e n t s a r e d i r e c t e d s o u t h w a r d and t e n d t o b e p a r a l l e l t o t h e wind d i r e c t i o n . A t t h e s h e l f b r e a k , a s o u t h w a r d s u r f a c e c u r r e n t o f 70 cm.s-'
and a b o t t o m n o r t h w a r d c u r r e n t o f 10 cm.s-'
have
b e e n m e a s u r e d ( R e b e r t , 1 9 8 3 ; T e i s s o n , 1981 : see f i g . 6 ) . Alongs h o r e , Kelvin-type
waves i n d u c e d by wind v a r i a t i o n s p r o p a g a t e
n o r t h w a r d (CrBpon e t a l , 1 9 8 4 ) . O f f s h o r e , t h e main c i r c u l a t i o n
i s d i r e c t e d northward. Due t o t h e s h e l f t o p o g r a p h y ( s e e f i g . 2 1 ,
t h e u p w e l l i n g modes
d i f f e r f r o m n o r t h t o s o u t h ( R e b e r t , 1 9 8 3 ) . To t h e s o u t h o f t h e Cap V e r t , where t h e s h e l f i s s h a l l o w , two c e l l s o f v e r t i c a l c i r c u l a t i o n occur : t h e first c e l l w i l l c r e a t e an offshore f r o n t , where c o l d waters d i v e u n d e r t h e l i g h t e r o n e s ; t h e s e c o n d c e l l
i s c o a s t a l . To t h e n o r t h o f t h e p e n i n s u l a , o n l y one c e l l o f upwelling occurs because of the deepness of the s h e l f . 1 1 1 . 2 - I s t h e e v o l u t i o n o f t h e t h e r m a l f i e l d w e l l shown by CZCS?
A t t h e b e g i n n i n g o f t h e c o l d s e a s o n , t h e u p w e l l i n g upward mo-
t i o n d o e s n o t d i s t u r b t h e s t r a t i f i c a t i o n and a f f e c t s t h e l a y e r u n d e r t h e t h e r m o c l i n e c a u s i n g o n l y a weak c o o l i n g ( T e i s s o n , 1 9 8 3 ) . A t i t s maximal p h a s e , t h e u p w e l l i n g c a u s e s t h e i n t r u s i o n o f t h e t h e r m o c l i n e t o t h e s u r f a c e , which c o o l s t h e whole mixed l a y e r . T h i s e v o l u t i o n i s w e l l shown by t h e CZCS. E f f e c t i v e l y , from J a n u a r y t o March, t h e a s p e c t o f t h e u p w e l l i n g plume c h a n g e d
from a s l i g h t l y c o o l s u r f a c e f i e l d w i t h a weak o f f s h o r e g r a d i e n t t o a c o l d p a t c h w i t h a s h a r p f r o n t a l r e g i o n c h a r a c t e r i z e d by cyc l o n i c g y r e s . The March g y r e s may b e g e n e r a t e d by waves on t h e c o n t i n e n t a l s h e l f , or by i n s t a b i l i t i e s between t h e two w a t e r
masses. The i n t e n s i f i c a t i o n o f t h e v e r t i c a l m o t i o n s i s accompan i e d by a change o f t h e g e n e r a l c i r c u l a t i o n ( f i g . 7 ) . I n Januar y , n o r t h e r n a d v e c t i o n p r o c e s s i s s t r o n g e r t h a n t h e s o u t h e r n upw e l l i n g . I n J a n u a r y 1981, a d v e c t i o n c o u l d be t h e consequence o f 10 d a y s o f e a s t e r l y w i n d s (ASECNA d a t a ) , p u s h i n g t h e n o r t h e r n
c o l d g y r e s t o t h e s o u t h a r o u n d t h e Cap. I n March, a b o v e t h e Cap, c i r c u l a t i o n c h a n g e s as s e e n on t h e
633
ao km
50
10
a) Composante nord-sud
(cmls).
c ) Composante
50
IM .w
t a t - O u e s t ,t iaothenner.
=-
F i g . 6 . C u r r e n t s e c t i o n s of t h e S e n e g a l e s e c o n t i n e n t a l s h e l f a l o n g t h e 14ON t r a n s e c t ( i n T e i s s o n , 1 9 8 3 ) a ) N o r t h - S o u t h component d u r i n g s t r o n g t r a d e s p e r i o d ( 2 6 March 1 9 7 4 ) . b ) N o r t h - S o u t h component d u r i n g weak t r a d e s p e r i o d (11 F e b r u a r y 1 9 7 7 ) ; c ) E a s t - W e s t component a n d i s o t h e r m s ( 1 2 A p r i l 1 9 7 7 ) .
634
Fig. 7. Superficial circulation and repartition of isotherms during the cold season on the Senegalese continental she1 f a ) weak upwelling period (December-January) and advection b) strong upwelling period (February-April) ( f r o m Rebert,
.
1983).
635
March 1981 image. F o r R e b e r t ( 1 9 8 3 ) , t h e r e i s c o n v e r g e n c e of
w a r m waters t o t h e c o a s t due t o westward r o t a t i o n o f w i n d s . T h i s p r o c e s s may a l s o r e s u l t from t h e b l o c k i n g o f t h e u p w e l l i n g by
a Kelvin f r o n t generated at t h e c o a s t a l d i s c o n t i n u i t y of the cap e and p r o p a g a t i n g n o r t h w a r d (CrBpon a t a l . , 1 9 8 4 ) . I n f a c t , n o r t h w e s t e r l y winds have been r e c o r d e d a t S t L o u i s and Dakar d u r i n g t h e 10 d a y s p r e c e d i n g t h e s c e n e , which c o u l d f a v o r t h e p i l i n g - u p hypothesis. The wedge o f w a r m c o a s t a l s h a l l o w w a t e r s ( e s p e c i a l l y s e e n i n March) i s c a u s e d by s o l a r h e a t i n g , w i t h o u t i n f l u e n c e o f upwelling
.
111.2.1
-
R e l a t i o n w i t h t h e c o a s t a l wind measurements
The i n t e n s i f i c a t i o n o f t h e u p w e l l i n g s y s t e m c a n b e r e l a t e d t o t h e c h a n g e o f t h e c o a s t a l winds d i r e c t i o n . The winds on t h e S e n e g a l e s e c o a s t a r e c h a r a c t e r i z e d by a m o d e r a t e i n t e n s i t y ( 2 t o 6 m.s-',
compared t o 10 m . s - '
i n Mauritania). Their d i r e c t i o n
c h a n g e s p r o g r e s s i v e l y from n o r t h e a s t t o n o r t h w e s t between Decemb e r and J u n e ( K i r k and S p e t h , 1 9 8 5 ) . The ASECNA c o a s t a l measurements show t h a t i n J a n u a r y 1981, t h e r e w e r e c o n t i n e n t a l t h e r m i c winds ( 0 t o 90 d e g r e e s ) w i t h a n i n t e n s i t y of 5 m.s-', (330"
-
3 0 " and 330"
a n d i n March predominant m a r i t i m e t r a d e s
-
60") w i t h t h e same i n t e n s i t y . A s t h e tem-
p e r a t u r e r e s p o n s e i s maximal t o t h i s l a s t c a t e g o r y ( P o r t o l a n o , 1 9 8 1 ) , t h e i n t e n s i f i c a t i o n o f t h e u p w e l l i n g between J a n u a r y and March 1981 c a n b e r e l a t e d t o a change i n d i r e c t i o n r a t h e r t h a n t o a n i n c r e a s e o f t h e wind stress. 111.3
-
Is t h e e v o l u t i o n o f t h e c h l o r o p h y l l f i e l d w e l l s e e n by
czcs
?
I n J a n u a r y , t h e C Z C S s c e n e shows t h a t t h e s p a t i a l c h l o r o p h y l l d i s t r i b u t i o n i s p a t c h y and a s s o c i a t e d w i t h t h e f r o n t a l z o n e s . One day l a t e r , ( t h e 1 2 t h o f J a n u a r y s c e n e c o u l d n o t be p r o c e s s e d q u a n t i t a t i v e l y ) t h e e d d i e s p a t t e r n s i n t h e n o r t h a r e unchang e d . Over t h e s h e l f b r e a k , t h e w i d t h o f one o f t h e crown p a t c h e s h a s almost doubled. A s t h e s u r f a c e o f t h e o t h e r p a t c h e s remains unchanged, t h i s i n d i c a t e s t h a t t h e main zone o f , g r o w t h i s w e l l l o c a t e d on t h e f r o n t . I n March, a t t h e c o n t r a r y , t h e p a t c h e s a r e b r o a d i n d i c a t i n g a more g l o b a l e n r i c h m e n t o f t h e c o l d water mass.
636 111.3.1 - R e l a t i o n w i t h t h e s e a - t r u t h s
The s e a - t r u t h s d a t a ( R / V " L a u r e n t Amaro" c r u i s e s from J a n u a r y t o A p r i l 1 9 8 1 ) a l l o w an a p p r o a c h o f t h e s e a s o n a l e v o l u t i o n o f t h e s h e l f waters c h a r a c t e r i s t i c s . Only s e c c h i - d i s k d e p t h s , nut r i e n t s w e r e t a k e n (TourC, 1 9 8 2 ) . The s e c c h i - d i s k d e p t h ( S . D . D . )
r e m a i n s h e t e r o g e n e o u s from
J a n u a r y t o F e b r u a r y 8 1 , showing u s u a l l y minimal v a l u e s (14m) on t h e 200m i s o b a t h . I n A p r i l 8 1 , homogeneously low SDD a r e measur e d on t h e s h e l f . A s h a r p f r o n t a l zone i s formed above th'e s h e l f b r e a k , w i t h v a l u e s v a r y i n g from 9 t o 15m i n a f e w k i l o m e t e r s
(see f i g . 8 ) . A s no r i v e r r u n o f f o c c u r s d u r i n g t h e d r y s e a s o n , t h e SDD are
d i r e c t l y r e l a t e d t o m a r i n e p a r t i c u l a t e matter ( n o d i s s o l v e d m a t t e r from t h e r i v e r s ) . The r e l a t i o n SDD-chl a (TourC, 1 9 8 3 ) var i e s from month t o month, i n d i c a t i n g a s e a s o n a l e v o l u t i o n o f t h e
w a t e r components from s a n d and mud ( b e f o r e December) t o l i v i n g c e l l s ( u n t i l May). Then, t h e e v o l u t i o n o f t h e SDD i s a good i n dex o f t h e p h y t o p l a n k t o n e v o l u t i o n d u r i n g t h e u p w e l l i n g s e a s o n . The SSD v a l u e s i n d i c a t e h i g h c h l o r o p h y l l c o n c e n t r a t i o n (maximum of 12 mg.m-3).
These h i g h v a l u e s c o r r e s p o n d t o t h e moderate c o a s -
t a l c o n c e n t r a t i o n s found a t Dakar f o r t h e l a s t t e n y e a r s ( G a l l a r d o , 1 9 8 1 ) , and t o u s u a l v a l u e s found i n t h e B a i e d e GorCe ( D i a , 1982 ; Tourk, 1 9 8 3 ) . I n c o n c l u s i o n , t h e SSD i n d i c a t e s a n e v o l u t i o n between h e t e r o g e n e o u s p h y t o p l a n k t o n r e p a r t i t i o n i n t h e f i r s t p e r i o d o f t h e upw e l l i n g t o w a r d s a n homogeneous bloom o v e r t h e s h e l f a t t h e end of t h e p e r i o d . T h i s c o i n c i d e s w i t h t h e e v o l u t i o n s h o w n by t h e CZCS c h l o r o p h y l l maps. 111.4
-
P r o c e s s e s o f phytoplankton growth
The p a t c h e s o b s e r v e d i n t h e u p w e l l i n g area r e s u l t from t h e c o m b i n a t i o n o f u p w e l l i n g r a t e , growth r a t e , t h e g r a z i n g p r e s s u r e , t h e s i n k i n g o f c e l l s and n u t r i e n t i n p u t and r e g e n e r a t i o n . T h e u p w e l l i n g p r o v i d e s t h e n u t r i e n t s t o t h e s u r f a c e waters a s t h e o r i g i n o f d e e p waters i s a m i x i n g o f 80 % SACW and 2 0 % NACW ( R e b e r t , 1 9 8 3 ) . The n i t r a t e c o n c e n t r a t i o n s measured i n t h e
B a i e of Gor6e a r e h i g h ( 1 5
atg.1-1)
d u r i n g t h e whole p e r i o d
of upwelling. The r e s i d e n c e t i m e o f t h e r i c h u p w e l l e d waters c a n b e estimat e d . They d e s c r i b e a h e l i c a l motion which t a k e s one month, w i t h
637 17'30W
17'W
14'30N
14"
F i g . 8 . S e c c h i - d i s k d e p t h s ( i n m e t e r s ) , d u r i n g t h e CRODT R / V " L a u r e n t Amaro" c r u i s e f r o m t h e 6 t h t o 1 0 t h o f A p r i l 1 9 8 1 , a f t e r t h e maximal p h a s e o f u p w e l l i n g ( f r o m T o u r & , 1983).
638
one week i n t h e e u p h o t i c zone ( T e i s s o n , 1 9 8 1 ) . T h i s t i m e i s s u f f i c i e n t f o r a bloom developpment s i n c e t h e p h y t o p l a n k t o n growth r a t e t y p i c a l l y v a r i e s from one d i v i s i o n p e r day t o one d i v i s i o n p e r 10 d a y s . The a v a i l a b i l i t y o f l i g h t c a n b e e s t i m a t e d from t h e SDD, s i n c e Z 1 %, d e p t h o f t h e e u p h o t i c zone i s a p p r o x i m a t e l y 2 . 2 x Z 10 %, Z 10 % b e i n g e q u a l t o SDD ( H o j e r s l e v , 1 9 8 1 ) . The c o r r e s p o n d i n g e u p h o t i c zone d e p t h v a r i e s from 8 m a t t h e c o a s t t o , 2 8 m o f f shore. 111.5
-
C o n t r i b u t i o n of t h e CZCS t o t h e u n d e r s t a n d i n g of b i o l o g i c a l processes
I n J a n u a r y 1981, t h e blooms a r e n o t a s s o c i a t e d s t r i c t l y t o t h e c o l d plume. They o c c u r mainly on t h e c o l d s i d e o f t h e s h e l f b r e a k f r o n t . The p h y t o p l a n k t o n p a t c h e s may be a s s o c i a t e d t o nu-
t r i e n t s u p p l i e s highly connected t o v e r t i c a l motions a t t h e s h e l f b r e a k . The s t r i c t s u p e r p o s i t i o n o f c o l d and h i g h n u t r i e n t w a t e r s i s t h e r u l e i n t h e M a u r i t a n i a n u p w e l l i n g ( C o s t e and Minas, 1 9 8 2 ) . T h i s c o u l d n o t b e t h e c a s e h e r e , a t l e a s t i n January. I n t h e plume c e n t e r , s t r o n g mixing may l i m i t t h e p r i m a r y prod u c t i o n . The newly upwelled w a t e r s are o f t e n found d e p l e t e d i n c h l o r o p h y l l ( M o r e l , 1 9 8 2 ) b e c a u s e t h e a l g a l growth i s n o t a c h i e ved y e t or r a t h e r because t h e new upwelled w a t e r s must f i r s t be Ilconditioned" by mixing w i t h s u r f a c e w a t e r s ( B a r b e r e t a l . 1 9 7 1 ) . I n March 1981, t h e blooms o c c u r o v e r a l a r g e p a r t o f t h e c o l d plume, on t h e i n n e r p a r t o f t h e s h e l f . The CZCS ? t u r b i d i t y " map shows t h a t t h e suspended m a t t e r c o n t e n t i s t h e r e low enough t o a l l o w a p r o d u c t i o n . The a l g a e may a l s o a v o i d t h e c o l d waters o f t h e c e n t r a l v e i n . The g r a z i n g p r e s s u r e may a l s o be t o o h i g h on t h e e x t e r n s i d e of t h e f r o n t . The more homogeneous a s p e c t o f t h e bloom may i n d i c a t e a w i d e r d i s t r i b u t i o n and s u p p l y of n u t r i e n t s o v e r t h e s h e l f , which i s c a u s e d by d i f f u s i o n p r o c e s s f o l l o w i n g two months o f v e r t i c a l motion and c o n t i n u o u s i n p u t o f n u t r i e n t s . I n c o n c l u s i o n , we c o u l d s e e t h e m a n i f e s t a t i o n o f two product i v e , s y s t e m s , one young i n J a n u a r y w i t h new produced biomass, and one mature i n March, a t i t s s t a b l e p h a s e . The d i f f e r e n c e between t h e two s y s t e m s o b s e r v e d i n 198.1 c o u l d a l s o be e x p l a i n e d by t h e model o f H i l l and Johnson ( 1 9 7 4 ) . The two c e l l s o f v e r t i c a l c i r c u l a t i o n , c o a s t a l and o f f s h o r e a r e
639 s e p a r a t e d by a c o n v e r g e n c e zone s i t u a t e d j u s t a b o v e t h e 200 m isobath a t t h e s h e l f break.
On t h i s c e l l , d o w n w e l l i n g m o t i o n com-
p e n s a t e t h e v e r t i c a l upward m o t i o n s .
I n January 1981, t h e fron-
t a l bloom o f t h e c o l d plume may c o r r e s p o n d t o t h i s c o n v e r g e n c e zone.
The bloom o f March 1 9 8 1 may t h e n c o r r e s p o n d t o a n accumu-
l a t i o n o f b i o m a s s due t o t h e c o a s t a l u p w e l l i n g c e l l . CONCLUSION The CZCS ( C o a s t a l Zone C o l o r S c a n n e r ) p r o v i d e s new i n f o r m a t i o n on t h e p h y t o p l a n k t o n v a r i a b i l i t y d u r i n g t h e u p w e l l i n g per i o d o v e r t h e S e n e g a l e s e s h e l f . The two s c e n e s o f t h e 1 1 t h o f J a n u a r y and 1st o f March 1981 g i v e two i m a g e s o f v e r y d i s t i n c t p h a s e s o f t h e u p w e l l i n g ( i n f r a r e d s c e n e s ) a n d two c h a r a c t e r i s t i c r e s p o n s e s o f p h y t o p l a n k t o n ( c h l o r o p h y l l maps) t o t h e s e e v e n t s . The g r e a t s p a t i a l r e s o l u t i o n o f t h e s e n s o r a l l o w s a d e t a i l e d d e s c r i p t i o n o f t h e p a t c h i n e s s o f t h e blooms o v e r t h e s h e l f , e x p e c i a l l y a t t h e b e g i n n i n g o f ' t h e u p w e l l i n g when t h e blooms a r e frontal. The CZCS i s t h e o n l y t o o l f o r b i o l o g i s t s t o u n d e r s t a n d t h e f u g i t i v e phenomena r e l a t e d t o h i g h l y l o c a l i z e d or v a r y i n g p r o c e s s e s . The i m a g e s o f o c e a n c o l o r p r o v i d e a n i n f o r m a t i o n which i s g r e a t l y d e p e n d e n t o f t h e i n f r a r e d i n f o r m a t i o n b u t w h i c h complements i t . T h i s work, d e s p i t e i t s p o n c t u a l c o n c l u s i o n s l e t s a p p e a r t h e p o s s i b i l i t y of a c l a s s i f i c a t i o n of t h e phytoplankton responses t o t h e h i g h l y v a r y i n g p r o c e s s u s of u p w e l l i n g : f r o n t a l a t t h e s h e l f b r e a k or more homogeneous on t h e c o a s t , w i t h more a m p l i t u d e o n t h e n o r t h e r n or s o u t h e r n p a r t o f t h e c o n t i n e n t a l s h e l f , t h e p e n i n s u l a o f t h e Cap V e r t m a r k i n g t h e l i m i t b e t w e e n two e c o s y s t e m s . The CZCS t w o - d i m e n s i o n a l
f i e l d s may b r i n g i n f o r m a t i o n
o n t h e mechanisms o f p h y t o p l a n k t o n d e v e l o p m e n t . a n d be u s e f u l i n d e t e r m i n i n g t h e r e g i o n s of g r e a t e r s e n s i t i v i t y of a l g a e populat i o n s t o t h e combination o f physical c o n t r o l l i n g f a c t o r s . AKNOWLEDGEMENTS T h i s s t u d y would n o t h a v e b e e n p o s s i b l e w i t h o u t t h e d a t a b a s e from t h e C e n t r e d e R e c h e r c h e O c k a n o g r a p h i q u e d e Dakar T h i a r o y e .
We t h a n k D r J . P a g e s f o r h i s h e l p f u l comments'. W e a k n o w l e d g e t h e a d v i c e and h e l p of M.
V i o l l i e r f o r a t m o s p h e r i c c o r r e c t i o n s . We
w i s h a l s o t o t h a n k IFREMER f o r t h e d a t a p r o c e s s i n g f a c i l i t i e s ,
640
t h e Antenne ORSTOM a n d t h e C e n t r e d e M k t k o r o l o g i e S p a t i a l e f o r s c i e n t i f i c s u p p o r t , p h o t o g r a p h i c work and f o r a s s i s t a n c e i n t h e preparation o f t h e manuscript. ANNEXE I C A L I B R A T I O N O F CZCS DATA
The r e t r i e v a l o f t h e p i g m e n t
( c h l o r o p h y l l + p h e o p i g m e n t s ) con-
c e n t r a t i o n s f r o m CZCS d a t a h a s b e e n a s u b j e c t o f i n t e n s i v e s t u d i e s u s i n g sea t r u t h s m e a s u r e m e n t s o f sea r e f l e c t a n c e s a n d c h l o r o p h y l l , a d d e d t o m o d e l l i n g o f t h e a t m o s p h e r e l a y e r s (Gordon
e t a l . , 1 9 8 0 ; V i o l l i e r e t a l . , 1 9 8 0 ) . Even i n t h e w o r s t c a s e , t h e c h l o r o p h y l l c o n c e n t r a t i o n c a n be e s t i m a t e d t o w i t h i n a f a c t o r o f 2 (Gordon e t a l . , t h e E n g l i s h Channel
1 9 8 0 ) . A b e t t e r a c c u r a c y w a s found i n
(log C
2 0 . 1 9 ) by H o l l i g a n e t a l . ( 1 9 8 3 ) .
The main unknown i n t h e p r o c e s s i n g i s t h e a e r o s o l c o n t r i b u t i o n t o t h e t o t a l o b s e r v e d r a d i a n c e i n CZCS c h a n n e l s . The a e r o s o l c o n t e n t o f t h e a t m o s p h e r e i s e x t r e m e l y v a r i a b l e i n s p a c e and t i m e , and t h e s c a t t e r i n g o f t h e a e r o s o l s i s n o t p r e dictable a priori. A t t h e l a t i t u d e o f S e n e g a l , t h e main s o u r c e o f a e r o s o l s a d d e d
t o n o r m a l h a z e i s t h e s a n d wind t h a t comes from t h e l a n d d u r i n g t h e t r a d e p e r i o d s . When c h e c k i n g s e r i e s o f M e t e o s a t i m a g e s i n t h e v i s i b l e c h a n n e l o v e r t h i s a r e a from y e a r t o y e a r , one c a n
see t h a t t h e s e d u s t h a z e s o c c u r r e g u l a r l y f r o m December t o . M a r c h ( s p e c i a l l y during the 1980's). The o p t i c a l p r o p e r t i e s o f t h e s e p a r t i c l e s h a v e b e e n m e a s u r e d a l o n g t h e A f r i c a n c o a s t d u r i n g c r u i s e s b e t w e e n Dakar a n d A b i d j a n by C e r f ( 1 9 8 2 ) . The v a l u e s o f t h e Angstrom e x p o n e n t ( e x p r e s s i n g t h e s p e c t r a l dependence o f t h e a e r o s o l t h i c k n e s s ) a r e found v e r y low ( e v e n n
=
0 ) , compared t o t h o s e known a t m i d l a t i t u d e coun-
t r i e s . T h i s i n d i c a t e s a n a t u r a l o r i g i n o f t h e a e r o s o l s and s p e c i f i c p r o p e r t i e s of t h e mixing of m a r i t i m e and c o n t i n e n t a l t r o p i cal airs. D e s p i t e t h e s e u n c e r t a i n t i e s , w e have d e t e r m i n e d q u a n t i t a t i v e l y t h e " c h l o r o p h y l l " c o n c e n t r a t i o n s . The p a r a m e t e r s u s e d f o r t h e processing
o f t h e 2 CZCS s c e n e s w e r e :
- Angstrom e x p o n e n t ( a e r o s o l t u r b i d i t y ) : - 0 . 5
;
- c o e f f i c i e n t o f p r o p o r t i o n a l i t y b e t w e e n c h a n n e l s 3 and 4
v
=
1.5 ;
64 I
- c o e f f i c i e n t s c , d o f e q u a t i o n (2) f o r r e t r i e v a l o f c h l o r o p h y l l c o n c e n t r a t i o n s (Gordon e t a l . , d
=
1980) c
=
0,3 ;
-3.73. The r a t i o o f c h a n n e l s 2 and 3 (520 and 550 nm)
h a s been used b e c a u s e t h e s c e n e i s i n a c o a s t a l r e g i o n , where t h e c h l a c o n c e n t r a t i o n s a r e h i g h . The r e s u l t o f t h e t o t a l p r o c e s s i n g i s shown b e l o w i n t a b l e 1 , where a r e e x p r e s s e d s e a r e f l e c t a n c e s i n t h e t h r e e CZCS c h a n n e l s f o r d i f f e r e n t p i x e l s of t h e image, r e p r e s e n t a t i v e o f t h e d i f f e r e n t w a t e r masses.
L a t itude
CZCS r e f i e c t a n c e s
Chlorophyll
( i n %)
Longitude pixel
a t 440, 520, 550 nm
11 J a n u a r y 81
mg.m
-3
( o r b i t 11198)
1
17.40 W;15
N
0.7
1.1
1.2
5
2
16.40 W , 15.4 N
1.8
3.4
3.8
5
3
19
N
2.1
1.5
1.4
2.5
W , 14
1 March 81
1
17.10 W , 14.1 N
2 3
( o r b i t 11875)
0.6
1.3
1.4
8
16.40 W , 15.4 N
1.5
3.4
3.8
5.4
18.50 W , 14.5 N
3.8
2.8
2.3
1.7
T a b l e 1 : R e f l e c t a n c e s ( i n % ) f o r CZCS i m a g e s o f 11 J a n u a r y 81 and 1 March 81 a n d d e r i v e d x h l a v a l u e s c a l c u l a t e d f r o m e q u a t i o n (2). From t h e s e r e f l e c t a n c e v a l u e s , two t y p e s o f w a t e r s c a n b e e a s i l y d i s t i n g u i s h e d , t h e t u r b i d w a t e r s w i t h a mixed i n f l u e n c e o f r e s u s p e n d e d matter a n d l i v i n g p h y t o p l a n k t o n i c c e l l s ( p i x e l s 2 ) , a n d t h e p u r e c a s e 1 w a t e r s w i t h t h e a l o n e i n f l u e n c e o f phyt o p l a n k t o n b i o m a s s ( p i x e l s 1 , h i g h C ; p i x e l s 3 , low C ) .
Clear
w a t e r p i x e l s were f o u n d o f f s h o r e o u t o f t h e s c e n e s s i n c e t h e g r o w t h o f p h y t o p l a n k t o n o c c u r s w i t h i n t h e 2 0 9 m i l e s band a l o n g t h e Senegalese c o a s t . The CZCS r e f l e c t a n c e v a l u e s a r e c o m p a r a b l e w i t h t h e m e a s u r e m e n t s made d u r i n g t h e G U I D O M n i a n r e g i o n ( M o r e l , 1982).
-
C I N E C A c r u i s e s i n t h e Maurita-
642
REFERENCES Abbott, M.R. and Zion, M.F., 1985. Satellite observations of phytoplankton variability during an upwelling event. Contin. Shelf Res. 4 : 661-680. Barber, R.T., Dugdale, R.C., Mac Isaac, J.J., and Smith, R.L., 1971. Variations in phytoplankton growth associated with the source and conditioning of upwelling water. Invest. Pesq. 35, 171-193. Berrit, G.R., 1962. Contribution 2 la connaissance des variations saisonnieres dans le Golfe de Guinee. Cah. Ockanogr. 14 : 633-643. Citeau, J., Guillot, B. et Lak, R., 1984. Operation LISTAO : Reconnaissance de l'environnement inter-tropical 2 l'aide des satellites Meteosat et Goes-E. Tklkdktection numkro 10. The Canary Current, 1982. Studies of an upwelling system. Rapport et procks verbaux des reunions du Conseil International pour 1'Exploration de la mer. Symposium held in Las Palmas, April 1978, volume 10. Cerf, A., 1980. Atmospheric turbidity over West-Africa. Contributions to Atmospheric physics, 53 : 414-429. Coste, B. and Minas, H.J., 1982. Analyse des facteurs regissant la distribution des sels nutritifs dans la zone de remontke d'eau des c6tes mauritaniennes. Oceanol. Acta, 5 : 315-324. Crkpon, M., Richez, C. and Chartier, M., 1984. Effects of coastline geometry on upwellings. J. Phys. Oceanogr. 14 : 1365-1382. Dia, A., 1984. Observations oceanographiques effectuees en 1982. Doc. Arch. 126 CRODT-ISRA. Domain, F., 1979. Etude des temperatures de la mer au voisinage de Mauritanie et du Sknegal. Tklkdetection 3 , 42p. Domain, F., 1982. Rkpartition de la matiere organiyue de la couverture skdimentaire du plateau continental ouest-africain. Rapp. p-v. Rkun. Cons. int. Explor. Mer., 180 : 339-341. Feldman, G.C., 1985. Variability of the productive habitat in the Eastern Equatorial Pacific.. Proceedings of Symposium on Vertical motion in the Equatorial upper ocean and its effects upon living resources and the atmosphere, Paris, May 1985. Gallardo, Y., 1981. On two marine ecosystems of Senegal separated by a peninsula, in : J.C.J. Nihoul (Ed"itor), Ecohydrodynamics, Elsevier, Amsterdam, pp 141-154. Gordon, H.R., Clark, D.K., Mueller, J.L. and Hovis, Z.A., 1980. Phytoplankton pigments from Nimbus-7 Coastal Zone Color Scanner : comparisons with surface measurements. Science, 210 : 63-66. Gordon, H.R. and Clark, D.K., 1981. Clear water radiances for atmospheric correction of Coastal Zone Color Scanner imagery. Appl. Opt. 20 : 4175-4180. Hagen, E., 1974. A simple scheme of the development of cold water upwelling circulation cell along the Northwest African coast. Beitrage zur Meereskunde 33 : 115-125. Hill, R.B. and Jonhson, J.A., 1974. A theory of upwelling over the shelf break. J. Phys. Oceanogr. 4 : 19-26. Hojerslev, N.K., 1981. The colour of the sea and its relation to surface chlorophyll and depth of the euphotic zone. Eurasep Newsletter n02. Holligan, P., Viollier, M., Dupouy, C. and Aiken, J., 1983. Satellite studies on the distribution of chlorophyll and dinoflagellate blooms in the eastern English channel. Contin. Shelf Res. 2 : 81-96.
643
Hovis, W.A., Clark, D.K., Anderson, F., Austin, R.W., Wilson, W.H., Baker, E.T., Ball, D., Gordon, H.R., Mueller, J.L., El-Sayed, S.Z., Stiirm, B., Wrigley, R.C. and Yentsch, C.S., 1980. Nimbus-7 Coastal Zone Color Scanner : system description and initial imagery. Science, 210 : 60-63. Kirk, A. and Speth, P., 1985. Wind conditions along the coasts of Northwest Africa and Portugal during 1972-79. T. 0. A. N. 30 : 15-16. Laurs, R.M., Fielder, P.C. and Montgomery, D.R., 1984. Albacore tuna catch distributions relative to environmental features observed from satellites. Deep Sea Res. 31 .: 1085-1099. Mittelstaedt, A. and Prieur, L., 1980. Analysis of variations in ocean color. Limnol. Oceanogr. 22 : 709-722. Morel, A., 1982. Optical properties and radiant energy in the waters of the Guinea Dome and the Mauritanian upwelling area in relation to primary production. Rapp. P. -v. R6un. Cons. Explor. Mer., 180 : 94-107. Portolano, P., 1981. Contribution 5. l'etude de l'hydroclimat des cdtes shnhgalaises. Doc. Sci. CRODT-ORSTOM. Rebert, J.P., 1983. Hydrologie et dynamique des eaux du plateau continental senegalais. Doc. Sci. 89. CRODT-ISRA. Rossignol, M., 1973. Contribution A 1'Btude du "Complexe Guinhen" Centre ORSTOM de Cayenne-Ochanogr. Doc. Arch. 17, 143p. Schemainda, R., Nehring, D. and Schulz, S., 1975. Ozeanologische Untersuchungen zum Produktionspotential der nordwestafrikanischen Wasserauftriebsregion 1970-2973. Geod. Geoph. Veroff. 4, 16, 85 p. Shannon, L.V., Mostert, S.A., Walters, N.M. and Anderson, F.P., 1983. Chlorophyll concentrations in the southern Benguela current region as determined by satellite (Nimbus-7 Coastal Zone Color Scanner). J. Plankton Res. 5 : 565-583. Shannon, L.V., Schlittenhardt, P. and Mostert, S.A., 1984. The Nimbus-7 experiment in the Benguela Current Region off Southern Africa, February 1980. 2. Interpretation of imagery and oceanographic implications. J. Geophys. Res. 89 : 4968-4976. Smith, R.C. and Baker, K., 1978. The Bio-optical state of ocean waters and remote sensing. Limnol. Oceanogr. 23 : 247-259. Sturm, B., 1983. Selected topics of Coastal Zone Color Scanner (CZCS) data evaluation. in : A.P. Cracknell (editor), Remote Sensing Applications in Marine Science and Technology, pp 137-167. Teisson, C., 1983. Le phCnom6ne d'upwelling le long des cdtes du Senhgal. Caracthristiques physiques et modelisation. Doc. Arch. 123 CRODT-ISRA. TourC, D., 1982. Observations oc6anographiques effectuhes en 1981. Doc. Arch. 125 CRODT-ISRA. TeurC, D., 1983. Contribution 5 1'Ctude de l'upwelling de la Baie de Gorbe et de ses consequences sur le developpement de la biomasse phytoplanctonique. Th6se de 36me cycle. Paris VI, 151 p. Viollier, M., Tanre, D. and Deschamps, P.Y., 1980. An algorithm f o r remote sensing of water color from space. Boundary Layer Meteorol. 16 : 247-267. Viollier, M., 1982. Radiometric calibration of Coastal Zone Color Scanner on Nimbus-7 : a proposed adjustment. Appl. Opt., 21 : 1142-1145. Viollier, M. and Sturm, B., 1984. CZCS data analysis in turbid coastal water. J. of Geoph. Res. 89, 4977-4985.
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Williams, S . P . , Szajna, E.F. and Hovis, W.A., 1985. Nimbus-7 Coastal Zone Color Scanner (CZCS). Level 2 data product user's guide. NASA Technical Memorandum 86202. Zbinden, R., 1981. L e s suspensions de la Baie du Mont SaintMichel ; ktude microgranulomktrique et radiomktrique. ThGse de 36me cycle. Paris V I , 302p.
Abbreviations ASECNA CRODT
: Agence pour la Skcuritk de la Navigation ACrienne.
czcs
: Coastal Zone Color Scanner.
: Centre de Recherche Ockanographique de Dakar-Thiaroye.
IFREMER : Institut Francais de Recherche pour 1'Exploitation de la Mer. NASA
: National Aeronautics and Space Administration.
ORSTOM
: Institut FranGais de Recherche Scientifique pour le
Dkveloppement en Coopkration.
645
MANGROVE ECOSYSTEM STUDY OF CHAKORIA SUNDERBANS AT CHITTAGONG WITH SPECIAL EMPHASIS ON SHRIMP PONDS BY REMOTE SENSING TECHNIQUES
0. QUADER, M.A.H.
PRAMANIK, F.A. KHAN and F.C. POLCYN
Bangladesh Space Research and Remote Sensing O r g a n i z a t i o n (SPARRSO), Dhaka (Bangladesh)
ABSTRACT Mangrove ecosystem o f Chakoria Sunderbans have been g r e a t l y i n f l u e n c e d by man's a c t i v i t i e s . I n o r d e r t o a l l o w f o r a s u i t a b l e h a b i t a t f o r shrimp c u l t u r e much o f t h i s ecosystem has been a l t e r e d . The o b j e c t i v e o f t h i s s t u d y was t o i d e n t i f y shrimp-ponds and m o n i t o r temporal changes and t h e i r impact on t h e Chakoria Sunderbans Mangrove Ecosystem, u s i n g a e r i a l photography and Landsat MSS d i g i t a l data. The low a l t i t u d e a e r i a l photographs which were taken i n 1975, 1981 and 1983 have been used f o r mapping i n t h i s study. These maps were used t o i d e n t i f y t h e shrimp-ponds and t o assess t h e s p a t i a l coverage and temporal change i n mangrove ecosystems. Several a d d i t i o n a l maps showing shrimp-ponds a l l o t t e d by t h e Government t o t h e p u b l i c and a l s o showing t h e l o c a t i o n s o f r i v e r s , coastl i n e and o t h e r n a t u r a l f e a t u r e s were a l s o used. Landsat CCTs were c o l l e c t e d f o r two dates (March 7, 1976 and December 3, 1980). These tapes were analysed by u s i n g t h e IBM/360 computer and a s o f t w a r e program known as LARSYS, which was developed by t h e Laboratory f o r A p p l i c a t i o n o f Remote Sensing (LARS), Purdue U n i v e r s i t y , USA. The M u l t i t e m p o r a l Landsat MSS d a t a were v e r y u s e f u l i n i d e n t i f y i n g changes i n t h e Mangrove Ecosystem o f Chakoria Sunderbans. But t h e boundaries o f t h e shrimp-ponds a r e n o t d e t e c t a b l e i n MSS d a t a . Because o f t h e improve ground r e s o l u t i o n , t h e use o f Thematic Mapper (TM) d a t a f o r m o n i t o r i n g c o a s t a l mangrove ecosystem o f f e r s many advantages over MSS data. INTRODUCTION The Bangladesh p a r t o f t h e Sunderbans ranks among t h e l a r g e s t mangroves i n t h e w o r l d w i t h a t o t a l area o f 590,000 ha, o f which about 410,000 ha a r e l a n d area. Minor .areas o f mangrove f o r e s t occur near Chittagong. Furthermore mangrove species a r e p l a n t e d e x t e n s i v e l y as c o a s t a l a f f o r e s t a t i o n t o p r o t e c t embankments and new a c c r e t i o n s . I n t h e C h i t t a g o n g d i s t r i c t , t h e mangrove ecosystem has been g r e a t l y i n f l u e n c e d by man's a c t i v i t i e s . I n o r d e r t o a l l o w f o r a suitable
h a b i t a t f o r shrimp c u l t u r e much o f t h i s ecosystem has been a l t e r e d .
The areas occupied by mangrove v e g e t a t i o n have been r e c e n t l y c u t . The o b j e c t i v e o f t h i s study was t o m o n i t o r temporal changes i n t h e Chakoria Sunderbans mangrove ecosystem o f Chittagong d i s t r i c t u s i n g Landsat MSS ( M u l t i s p e c t r a l Scanner) d a t a c o l l e c t e d a t o r b i t a l a l t i t u d e s and a e r i a l photographs t a k e n i n 1975, 1981 and 1983.
646
Study Area The mangrove f o r e s t o f Chakoria Sunderbans has demarcated boundaries surrounded by waterways on a l l sides. Chakoria Sunderbans i s a d e l t a a t t h e mouth o f t h e r i v e r Matamuhuri and i s a low l y i n g s a l i n e swamp. The s o i l w i t h i n t h e mangrove ecosystem c o n s i s t s o f r i c h a l l u v i u m , sand, g r a v e l and o t h e r m a t e r i a l s , deposited by running water. I n t h e f o r e s t s t h e r e are many low l y i n g i s l a n d s which a r e submerged a t h i g h t i d e s and t h e y a r e i n t e r s e c t e d by a system o f canals and creeks. The f o r e s t has been d i v i d e d i n t o two blocks which have been f u r t h e r subdivided i n t o 21 compartments. I n t h e e a r l y 5 0 ’ s these f o r e s t s were stocked w i t h 20 types o f mangrove genus l i k e Heriteria Avicinia Rokinias, Aegiceras, Braguiera, Carpa, Coriops, Soneretia, Eyocaria, Phoenix, Tamarine, Rhizophora, e t c
... Some o f
the species a t t a i n e d a h e i g h t o f 40 t o 50 f e e t .
Recently, t h e c o n d i t i o n o f t h e f o r e s t s has been d e t e r i o r a t i n g and t h e weeds l i k e Cauba kanta and Nunia kanta form dense canopies and occupy t h e areas once dominated by t i m b e r species. Various species o f marine shrimp spawn i n t h e sea waters up t o a depth o f 100 meters b u t t h e i r l a r v a l stages are p l a n k t o n i c and r e q u i r e l e s s s a l i n e waters f o r s u r v i v a l and f u r t h e r development. The young shrimp move i n t h e c o a s t a l r i v e r s , canals, creeks and mangrove swamps w i t h t i d a l c u r r e n t s . The young shrimp remain i n these areas up t o p r e - a d u l t stages o f growth. As t h e s a l i n i t y t o l e r a n c e o f t h e young shrimp increases, t h e y m i g r a t e back t o t h e sea f o r f u r t h e r growth, development o f gonads and spawning. The mangrove f o r e s t o f Chakoria Sunderbans p r o v i d e i d e a l e c o l o g i c a l c o n d i t i o n s f o r e a r l y stages o f marine shrimp and thus c o n s t i t u t e t h e i r nursery ground where t h e y can be trapped and c u l t u r e d i n impoundments. The species o f commercial shrimp which are a v a i l a b l e i n abundance i n t h e i r e a r l y stages o f growth i n t h e canals and creeks o f Chakoria Sunderbans which a r e c u l t u r e d .on a l a r g e s c a l e a r e : g i a n t t i g e r shrimp (Bagda C h i n g r i ) and w h i t e shrimp (Sada i h i n g r i ) . METHODOLOGY Data C o l l e c t i o n Several types o f data have been c o l l e c t e d f o r s t u d y i n g t h e impact o f shrimp c u l t u r i n g on t h e Chakoria Sunderbans Mangrove Ecosystem. These i n c l u d e : low a l t i t u d e a e r i a l photography, thematic map data (maps d i s p l a y i n g t h e l o c a t i o n o f mangrove v e g e t a t i o n ) and Landsat MSS d i g i t a l data. The low a l t i t u d e a e r i a l photography was c o l l e c t e d i n January 1975, December 1981 and December 1983. These b l a c k and w h i t e photographs and c o l o r i n f r a r e d photography were c o l l e c t e d a t a s c a l e o f 1:30,000
and 1:50,000
respectively
over t h e whole c o u n t r y and t h e c o a s t a l areas o f Bangladesh. These photographs were taken t o p r o v i d e i n s i g h t s i n t o t h e l o c a t i o n o f n a t u r a l resources and
647
d i s t r i b u t i o n o f mangrove v e g e t a t i o n i n the r e g i o n . These maps were t r a c e d from a e r i a l photography t o h e l p d e p i c t t h e l o c a t i o n and d i s t r i b u t i o n o f shrimp beds and magrove ecosystems. The t r a c i n g method c o n s i s t e d o f e n l a r g i n g t h e s c a l e o f the photographs t o t h a t o f the base maps and u s i n g a sheet o f mylar t o t r a c e i n f o r m a t i o n from the photos t o t h e map surfaces by mapograph. These maps correspond t o t h e dates o f the a e r i a l photography and were used t o assess t h e s p a t i a l coverage and temporal change i n mangrove ecosystems. Several addi t i o n a l maps (predrawn) were o b t a i n e d from the Bangladesh F i s h e r i e s D i r e c t o r a t e t o show the l o c a t i o n o f shrimp farms a l l o t t e d t o the p r i v a t e entrepreneurs and l o c a t i o n o f r i v e r s , c o a s t l i n e s and o t h e r n a t u r a l f e a t u r e s . Landsat Data Landsat MSS data were c o l l e c t e d f o r two dates (March 2, 1976 and December 3, 1980). The m u l t i t e m p o r a l Landsat MSS d a t a were very u s e f u l i n i d e n t i f y i n g changes i n the mangrove ecosystem o f Chakoria Sunderbans. These d a t a were a l s o e f f e c t i v e i n c a t e g o r i s i n g o r c l a s s i f y i n g landcover f e a t u r e s o t h e r than mangrove vegetation. The m u l t i s p e c t r a l c h a r a c t e r i s t i c o f the data allowed the a n a l y s t t o make d e t a i l e d d e c i s i o n s r e g a r d i n g the type o f landcover found i n t h e area and the temporal v a r i a t i o n i n landcover f e a t u r e s . The MSS d a t a used have f o u r separate s p e c t r a l bands. These bands occupy s e l e c t i v e p o r t i o n s of t h e electromagnetic spectrum (band 4, 5 t o 6 micrometers; band 5, 6 t o 7 micrometers; band 6, 7 t o 8 micrometers and band 7, 8 t o 11 micrometers). Data Analysis Before i n f o r m a t i o n c o u l d be e x t r a c t e d from Landsat MSS data, a number o f computer based a l g o r i t h m s o r processors were implemented t o process the data. These d i g i t a l data processing programs are p a r t o f an i n t e g r a t e d system of software known as LARSYS. They i n c l u d e : CDISPLAY, PICTUREPRINT, CLUSTER, MERGESTATISTICS, SEPARABILITY, CLASSIFYPOINTS, and PRINT RESULTS and are used t o d i s p l a y t h e d a t a and implement v a r i o u s p a t t e r n r e c o g n i t i o n techniques i n o r d e r t o e x t r a c t i n f o r m a t i o n f o r t r a i n i n g c l a s s purposes and f i n a l c l a s s i f i c a t i o n o f landcover f e a t u r e s . RESULTS AND DISCUSSIONS Mangrove ecosystems represent an i m p o r t a n t p a r t o f t h e w o r l d ' s c o a s t a l ecosystems. They p r o v i d e i m p o r t a n t f u n c t i o n s i n p r o t e c t i n g coasts from e x t e n s i v e e r o s i o n , extending and b u i l d i n g i s l a n d s and p r o v i d i n g energy i n p u t i n t o f i s h e r i e s (Odum, 1971). P r i o r t o 1977 the Chakoria Sunderbans ecosystem remained p r i m a r i l y u n a l t e r e d by man's a c t i v i t i e s . However, r e c e n t l y ( a f t e r 1977) considerable change has
648
occurred. These changes are mainly i n the form o f d e s t r u c t i o n o f mangrove f o r e s t s f o r shrimp c u l t u r i n g . I n t e r p r e t a t i o n o f a e r i a l photographs : the t h r e e maps made from a e r i a l photographs were used t o h e l p determine temporal changes i n mangrove ecosystem ( F i g . 1 , 2 , 3 ) . The f i r s t map made from 1975 a e r i a l photographs d e p i c t s considerable f o r e s t cover encompassing n e a r l y a l l o f Chakoria Sunderbans ( F i g . 1). There were no shrimp farms found w i t h i n the mangrove f o r e s t a t t h i s date. The second map produced (1981) showed t h a t considerable change had occured i n the mangrove f o r e s t ( F i g . 2 ) . Approximately 42 shrimp ponds were found w i t h i n t h e c o n f i n e s o f t h e mangrove f o r e s t and p o r t i o n s o f the f o r e s t had been destroyed. E v a l u a t i o n o f the t h i r d map (1983) d e p i c t s even g r e a t e r d e f o r e s t a t i o n o f Chakoria Sunderbans than e v i d e n t i n 1981 ( F i g . 3 ) . A d d i t i o n a l shrimp ponds were a l s o c o n s t r u c t e d s i n c e the 1981 a e r i a l photography was imaged. These maps categorized t h e shrimp ponds i n t o 4 d i f f e r e n t classes, Symbol A are shrimp ponds where shrimp c u l t u r e i s p r e s e n t l y occuring, Symbol B i s where shrimp ponds w i l l be prepared, Symbol C i n d i c a t e s l o c a t i o n s where mangrove f o r e s t has been destroyed f o r shrimp pond a c t i v i t i e s and D represents areas i n which mangrove f o r e s t s t i l l e x i s t i n i t s n a t u r a l s t a t e . I t i s e v i d e n t from t h e maps and t a b l e t h a t shrimp ponds a r e i n c r e a s i n g i n abundance b o t h i n and o u t s i d e o f Chakoria Sunderbans. I n t e r p r e t a t i o n o f a e r i a l photography was based upon t o n a l v a r i a t i o n s and s p a t i a l r e l a t i o n s h i p s . Stereocopic viewing was a l s o h e l p f u l i n i n t e r p r e t a t i o n o f s p a t i a l and temporal changes w i t h i n t h e ecosystems. C l a s s i f i c a t i o n o f Landsat data : c l a s s i f i c a t i o n o f 1976 Landsat data revealed e x t e n s i v e coverage o f mangrove f o r e s t i n t h e Chakoria Sunderbans t i d a l e s t u a r y . Other types o f v e g e t a t i o n and n a t u r a l f e a t u r e s were a l s o present. However, t h e main i n t e r e s t o f mangrove v e g e t a t i o n was dominant. Four d i s t i n c t s p e c t r a l classes o f mangrove were c l a s s i f i e d i n the study area u s i n g t h e March 2, 1976 Landsat data s e t . These s p e c t r a l classes a r e probably r e l a t e d t o the d e n s i t y o f t h e mangrove f o r e s t i n t h e area, however, i t i s p o s s i b l e t h a t t h e d i f f e r e n c e s observed a r e due t o v a r i a t i o n s i n species composition. The l o c a t i o n o f canals and open waterways
was a l s o d e t e c t a b l e . No shrimp ponds
however Gould be i d e n t i f i e d . C l a s s i f i c a t i o n o f 1980 Landsat MSS d a t a showed t h a t considerable mangrove d e f o r e s t a t i o n had occurred throughout along major canals which d i v i d e the areas occupied by mangrove f o r e s t . The l o c a t i o n s of canals and streams were d i f f i c u l t t o i d e n t i f y on t h i s c l a s s i f i c a t i o n . T h i s was due t o d e s t r u c t i o n o f f o r e s t near waterways i n the area. Large d i f f e r e n c e s s p e c t r a l l y between the canals and mangrove v e g e t a t i o n a1 lowed waterways t o be l o c a t e d e a s i l y on c l a s s i f i c a t i o n maps prepared from 1976 Landsat MSS data.
649
650
651
652
CONCLUSIONS The r e s u l t o f mangrove f o r e s t d e s t r u c t i o n i n t h i s a r e a c o u l d have s e r i o u s i m p a c t s such as : ( i ) decreases i n s h r i m p and f i s h p o p u l a t i o n , ( i i ) i n c r e a s e d d i s c h a r g e o f n u t r i e n t s i n a l e s s decomposed f o r m i n t o open ocean w a t e r and ( i ii) t h e d e s t r u c t i o n o f n u r s e r y ground f o r m a r i n e fauna. These problems may be overcome b y p l a n t i n g mangroves on t h e embankments and n e a r t h e s h o r e o f t h e embankments o f shrimps farms. T h i s w i l l e n s u r e r e t e n t i o n o f s u f f i c i e n t mangrove v e g e t a t i o n needed f o r p r o d u c t i o n o f shrimps on a s u s t a i n e d b a s i s , a t t h e same t i m e t h e t r e e s w i l l a c t as s h e l t e r b e l t a g a i n s t c y c l o n e s and t i d a l surges t h a t f r e q u e n t l y o c c u r i n t h e s e a r e a s d u r i n g mnsoons. Remotely sensed d a t a c o l l e c t e d by o r b i t a l p l a t f o r m s o f f e r a d a t a base i n w h i c h measurements o f s p a t i a l and temporal changes i n d e l i c a t e m a r i n e ecosystems can be made. It i s hoped t h a t t h e changes o c c u r r i n g w i t h i n t h e Chakoria Sunderbans mangrove ecosystems can be e f f e c t i v e l y m o n i t o r e d i n t h e f u t u r e u s i n g t h e s e and more advanced d a t a sources. A n a l y s i s o f L a n d s a t MSS d a t a has i n d i c a t e d t h a t d e f o r e s t a t i o n o f C h a k o r i a Sunderbans i s o c c u r r i n g a t a r a p i d r a t e . The i m p l i c a t i o n s o f t h i s o c c u r r e n c e a r e n o t a t t h i s t i m e c l e a r , however a d d i t i o n a l a n a l y s i s o f Landsat MSS d a t a and o t h e r d a t a sources such as SPOT and TM, may p r o v i d e i n f o r m a t i o n on t h e e f f e c t s o f t h e s e a c t i v i t i e s on t h e p l a n t and animal communities o f t h e area. The use o f Thematic Mapper (TM) d a t a f o r m o n i t o r i n g c o a s t a l mangrove ecosystems o f f e r s many p o t e n t i a l advantages o v e r MSS d a t a . The m a j o r advantage o f TM d a t a i s t h e i r a b i l i t y t o d i s c r i m i n a t e v e r y s m a l l d i f f e r e n c e s i n v e g e t a t i v e c o v e r . I f TM d a t a had been used i n t h i s a n a l y s i s i t may have been p o s s i b l e t o d i s c r i m i n a t e s m a l l e r d i f f e r e n c e s i n v e g e t a t i o n p r e s e n t w i t h i n manarove ecosystems.Smal1 changes i n t h e C h a k o r i a Sunderbans ecosystems may have been d e t e c t e d . D i s c r i m i n a t i o n o f i n d i v i d u a l s h r i m p ponds may be a l s o p o s s i b l e u s i n g Thematic Mapper d a t a . TABLE Summary o f t h e a r e a s o f each c a t e g o r y f o r each d a t e i n h e c t a r e
Mangrove F o r e s t Mangrove F o r e s t c u t Shrimp bed under preparation Shrimp ponds Total
1975 (black & white) a e r i a l photographs
1981 (black & white) a e r i a l photo
1983 ( I R a e r i a l photo)
2882.86
1780.15 364.63
1788.73 158.74
90.21 674.25
113.16 980.20
-
I
2882.86
I
2909.24
I
3040.83
653
ACKNOWLEDGEMENTS Deepest a p p r e c i a t i o n i s made t o D r . L o u i s B a r t l o u c i , Technical D i r e c t o r , LARS, Purdue U n i v e r s i t y , USA, M r . Carlos Valenzuela f o r a d v i s i n g and a s s i s t i n g t h e work, M r . James C l i n t h o r n e f o r a s s i s t a n c e i n a n a l y s i n g and p r e p a r i n g t h e r e p o r t m a t e r i a l s , Mrs. Maryleen Kleeper f o r t y p i n g t h e r e p o r t . M r . Abul Hossain
of SPARRSO typed t h e manuscript. We g r a t e f u l l y acknowledge t h e s u p p o r t o f FA0 and SPARRSO t o c a r r y o u t t h e research work i n t h e USA. REFERENCES ADB, F i s h e r i e s , 1980, U t i l i z a t i o n o f Chakoria Sunderbans Mangrove F o r e s t r y f o r Aquaculture. A f e a s i b i l i t y s t u d y r e p o r t o f F i s h e r i e s D i r e c t o r a t e . Bangladesh, Appendix p 5-2, p 1-3 FAO, 1982, Management and U t i l i s a t i o n o f Mangrove i n Asia and t h e P a c i f i c FA0 Environment paper 3. M-08 I S B N 92-5-101221-0, p 6, 82 KLEMAS, V. and BARTLETT, D.S., 1980, Remote Sensing o f Coastal Environments and Resources, Proceedings o f t h e Fourteenth I n t e r n a t i o n a l Symposium on Remote Sensing o f Environment, Ann Arbor, Michigan, p 543-562 ODUM, E.P.,
1971, Fundamentals o f Ecology, W.B.
Sanders Company, P h i l a d e l p h i a
SWAIN, P.H. and D A V I S , S.M., e d i t o r s , 1978, Remote Sensing, The Q u a n t i t a t i v e Approach, McGraw-Hill, I n c . New York
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BIOLOGICAL PROCESSES ASSOCIATED WITH THE PYCNOCLINE AND SURFACE FRONTS I N THE SOUTHEASTERN BERING SEA TERRY E. WHITLEDGE1 and JOHN J. WALSHZ 1Oceanographic Sciences D i v i s i o n , Oceanographic Sciences D i v i s i o n , Brookhaven N a t i o n a l Laboratory, Upton, NY
11973
%epartment o f Marine Science, U n i v e r s i t y o f South F l o r i d a , S t .
Petersburg, FL
33701
INTRODUCTION The f a c t o r s such as l i g h t and n u t r i e n t c o n c e n t r a t i o n conducive t o p r i m a r y p r o d u c t i o n processes i n t h e ocean environment a r e w e l l known i n t h e q u a l i t a t i v e sense b u t o n l y an approximation o f t h e s e e s s e n t i a l i n g r e d i e n t s can b e q u a n t i f i e d e s p e c i a l l y when t h e a b s o l u t e amounts a r e o f t e n n o t very important.
The a v a i l -
a b i l i t y o f n u t r i e n t s and l i g h t a r e s u b j e c t t o a wide range o f environmental f a c t o r s t h a t vary w i d e l y i n b o t h space and time.
It i s u s e f u l t o study an
oceanic area t h a t has h i g h primary p r o d u c t i o n and a small amount o f r e l a t i v e water movement and m i x i n g processes t h a t render b i o l o g i c a l p r o d u c t i o n measure-
I t i s a l s o u s e f u l t o have w e l l d e f i n e d r e g i o n s such as
ments d i f f i c u l t .
s u r f a c e f r o n t s o r pycnoclines which a r e areas o f increased p r o d u c t i o n and can be used t o focus t h e o b s e r v a t i o n s program. The Processes and Resources o f t h e Bering-Sea (PROBES) program on t h e southe a s t e r n B e r i n g Sea s h e l f s t u d i e d t h e wide (-600 km) and f l a t s h e l f which increases t h e d i s t a n c e s between t h e a c t i v e s i t e s o f s h e l f processes t o b e t t e r enable o b s e r v a t i o n s t o be made (McRoy e t al.,
1985).
I n addition, t h i s portion
o f t h e southeastern B e r i n g Sea s h e l f has very small n e t c u r r e n t v e l o c i t i e s ( 4 cm s e c - l ) ( K i n d e r and Schmacher, 1981) and i n t e r n a l waves a r e n o t present. This paper w i l l show t h e p e r s i s t e n c e o f s u r f a c e f r o n t s and s t r e n g t h o f t h e p y c n o c l i n e t h a t i s coupled t o t h e p r i m a r y p r o d u c t i o n and t h e p h y t o p l a n k t o n biomass. Physical s e t t i n g The southeast Bering Sea was sampled f o r f i v e years i n t h e r e g i o n between t h e P r i b i l o f I s l a n d and t h e A l e u t i a n chain.
A standard a c r o s s - s h e l f t r a n s e c t o f 19
s t a t i o n s was occupied n e a r l y f i f t y times between t h e 1500 m and t h e 40 m i s o b a t h s (Fig.
1).
This standard t r a n s e c t crossed t h r e e f r o n t a l regions t h a t
656 59"
580
57"
56O
55"
540
170'
168"
166"
162O
164"
158"
160'
Fig. 1. PROBES sample l o c a t i o n s a c r o s s t h e s o u t h e a s t e r n B e r i n g Sea s h e l f . Approximate l o c a t i o n s f o r t h e o u t e r f r o n t (OF), m i d d l e f r o n t (MF) and i n n e r f r o n t ( I F ) a r e shown as h a t c h e d l i n e s . The t r a c k o f t h e s h i p b o a r d s u r f a c e map i s shown as z i g - z a g a l o n y t h e o u t e r f r o n t . o c c u r r e d on t h e o u t e r s h e l f (170 m), m i d d l e s h e l f (100 m), and t h e i n n e r s h e l f ( 5 0 m).
The mean c u r r e n t v e l o c i t i e s a r e e s t i m a t e d t o b e 5 t o 10 ( s l o p e ) , 1 t o 5
(outer shelf),
<1 ( m i d d l e s h e l f ) , and 1 t o 3 ( i n n e r s h e l f ) cm sec-1 so t h e
c e n t r a l p a r t o f t h e s h e l f between t h e 50 and 100 m i s o b a t h s e x h i b i t v e r y l i t t l e n e t movement and t h e predominant f l o w i s t i d a l (Coachman, 1985). The f r o n t a l r e g i o n s The f r o n t a l r e y i o n s can b e c l e a r l y d e f i n e d by b o t h p h y s i c a l and b i o l o g i c a l measurements.
The most e x t e n s i v e o b s e r v a t i o n t h a t have been c o l l e c t e d which
show t h e l a t e r a l e x t e n t o f a f r o n t was o b t a i n e d by s u r f a c e underway mapping a t the outer front.
The z i g - z a g t r a c k o f t h e s h i p a t n i g h t ( K e l l y e t al.,
was o r i e n t e d a l o n g t h e f r o n t a l r e g i o n ( F i g .
1).
1975)
The a r e a l d i s t r i b u t i o n o f
n i t r a t e ( F i g . 2 ) showed a h i g h e r c o n c e n t r a t i o n a t t h e f r o n t b u t t h e r e was a s u b s t a n t i a l amount o f s t r u c t u r e i n t h e observed f e a t u r e s .
The n i t r a t e and
c h l o r o p h y l l ( F i g . 3) were l o w e r i n t h e s o u t h e a s t where t h e g r a d i e n t s o f b o t h were a l s o s m a l l e r .
The n o n - u n i f o r m d i s t r i b u t i o n s o f n i t r a t e and c h l o r o p h y l l
v a r i e d on a s p a t i a l s c a l e o f about 10 m i l e s a c r o s s t h e f r o n t .
I n t h e northwest
657
SURFACE NITRAT
40'
20'
168"W
40'
20'
167"
40
F i g . 2. The d i s t r i b u t i o n o f s u r f a c e n i t r a t e ( u g - a t 1-1) a l o n g t h e o u t e r f r o n t as d e t e r m i n e d by s h i p b o a r d underway s u r f a c e map i n g a t n i g h t . The h i g h l i g h t e d a r e a has c o n c e n t r a t i o n s y r e a t e r t h a n 6 ug-at 1 -
P.
20'
168"W
40'
20'
20'1 .a
55" 50'
20'
168"
40'
20'
167"
40'
F i g . 3. The d i s t r i b u t i o n o f s u r f a c e c h l o r o p h y l l (ug 1-1) measured by i n v i v o f l u o r e s c e n c e along t h e o u t e r f r o n t as determined by s h i p b o a r d underway s u r f a c e The h i g h l i g h t e d a r e a has 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 6 mapping a t n i g h t . uy 1-1. s e c t i o n o f t h e mapped area t h e c o n c e n t r a t i o n o f n i t r a t e i n c r e a s e d t w o f o l d on e i t h e r s i d e o f t h e f r o n t which c o i n c i d e d w i t h a t w o f o l d decrease o f c h l o r o phyll.
The s c a l e s o f b o t h n i t r a t e and c h l o r o p h y l l f e a t u r e s corresponded c l o s e l y
658
0 60
I20
I I-
a
n W
I-
a
n W
-1 I - 4 JUNE 198;
0
100
200
300
400
500
DISTANCE ( k m l
F i g . 4. The across s h e l f v e r t i c a l d i s t r i b u t i o n o f n i t r a t e (ug-at 1-1) f o r autumn through spring. The h i g h l i g h t e d areas have c o n c e n t r a t i o n g r e a t e r t h a n 15 ug-at 1-1.
659
0
60
5
120
c w
180 240
300
60
5
8
120
1
I I -
w
0
180
240
1-4 JUNL 1980
300 0
50
- 100 E
I50 0 w
200 250
300 0
60
-
F I20 f
z =: I 8 0 240 300
90
180
270
OISlAWCt(kd
360
450
90
I80
270
360
350
DISTLWCC Ikml
F i g . 5. The across s h e l f v e r t i c a l d i s t r i b u t i o n o f ammonium ( u g - a t 1-1) f o r The h i g h l i g h t e d areas have 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 2 s p r i n g t h r o u y h autumn. u y - a t 1-1.
660
w i t h o n l y a s l i g h t d i s p l a c e m e n t o f c h l o r o p h y l l maximum t o t h e n o r t h w e s t f r o m t h e n i t r a t e minimum.
The seasonal changes i n t h e a c r o s s s h e l f d i s t r i b u t i o n o f
n i t r a t e ( F i g . 4) a r e apparent, however, t h e f r o n t a l f e a t u r e s always p r e s e n t w i t h
As t h e n i t r a t e d e c l i n e s i n c o n c e n t r a t i o n t h e same
t h e exception o f winter.
r e l a t i v e d i s t r i b u t i o n p a t t e r n s p e r s i s t from s p r i n g t h r o u g h autumn. The ammonium d i s t r i b u t i o n s a c r o s s t h e s h e l f has an i n t e r e s t i n g p a t t e r n o n l y d u r i n g l a t e s p r i n g and summer when d e g r a d a t i o n processes have s t a r t e d r e c y c l i n g t h e p h y t o p l a n k t o n from t h e s p r i n g bloom.
The m a j o r r e g i o n of,ammonium produc-
t i o n occurs i n t h e m i d d l e domain between t h e i n n e r and m i d d l e f r o n t s ( F i g . 5 ) .
As t h e ammonium c o n c e n t r a t i o n s i n c r e a s e i n t h e b o t t o m l a y e r d u r i n g t h e s p r i n g , t h e b o u n d a r i e s between i n n e r and m i d d l e f r o n t s a r e c l e a r l y d e f i n e d as an extenT h i s mid-depth o f f s h o r e
s i o n o f t h e s u b p y c n o c l i n e w a t e r t o t h e o u t e r domain.
f l o w i s p r o b a b l y t h e b a l a n c i n g e f f e c t o f t h e onshore movement o f b o t t o m w a t e r On t h e o u t e r 200 km o f t h e t r a n s e c t . P r i m a r y p r o d u c t i o n processes r e q u i r e l i g h t which can o n l y b e o b t a i n e d i n t h e upper l a y e r o f t h e w a t e r column so an a n a l y s i s on t h e u p p e r 40 m o f t h e w a t e r column approximates t h e c o n d i t i o n s t h a t e x i s t March t h r o u g h June d u r i n g t h e
I
2
3
4
5
6
7
8
I0
9
6
STATION S I I 12 I3 ,
I
14
15
,
,
I6 ,
I8
17 ,
19
,
,
5
4
-u
3
0,
a
F 2
z
W 4
=
I
0' -I
I-
-2
0
I
I
I
4
I00
200
300
400
500
DISTANCE ( k m l
F i y . 6. The mean t e m p e r a t u r e (OC) i n t h e upper 40 m o f t h e w a t e r column on t h e 3 June 1980. PROBES a c r o s s s h e l f t r a n s e c t o f s t a t i o n s from 24 March
-
66 1
The mean temperature i n t h e upper 40 m o f t h e water
maximum primary production.
6) has t h e h i g h e s t g r a d i e n t across t h e m i d d l e
column across t h e s h e l f (Fig.
f r o n t ( S t a 9-13) i n March, b u t i n May t h e i n n e r f r o n t ( S t a 16-17) and m i d d l e f r o n t ( S t a 8-11) a r e d i s t i n c t l y apparent i n t h e t r a n s e c t s t a t i o n s .
The r e l a t i v e
a c r o s s - s h e l f d i s t r i b u t i o n o f mean temperature remains unchanged through t h e s p r i n g and summer t i m e periods. The c o n c e n t r a t i o n o f n i t r a t e i n t e g r a t e d f o r t h e upper 40 m of t h e water column begins t h e s p r i n g p e r i o d w i t h a u n i f o r m gradi'ent o f decreasing n i t r a t e over t h e o f f s h o r e end o f t h e t r a n s e c t o f approx. 150 km ( S t a 1-8) and a very constant l e v e l o f n i t r a t e f o r t h e n e x t 250 km ( S t a 8-17) w i t h a very sharp d e c l i n e a t t h e i n s h o r e end o f t h e t r a n s e c t ( S t a 17-19) (Fig. 7).
The m i d d l e
p o r t i o n o f t h e t r a n s e c t ( S t a 6-12) centered over t h e m i d d l e f r o n t r e g i o n has a n i n i t i a l r a p i d d e c l i n e i n n i t r a t e c o n c e n t r a t i o n d u r i n y March and A p r i l .
The
i n n e r f r o n t e x h i b i t s a s i m i l a r d e c l i n e i n n i t r a t e which g i v e s t h e t r a n s e c t a bimodal d i s t r i b u t i o n o f n i t r a t e by t h e b e g i n n i n g o f May.
Both areas o f low
n i t r a t e a r e associated w i t h f r o n t a l areas ( m i d d l e and i n n e r ) where t h e h o r i z o n t a l temperature temperature g r a d i e n t s a r e l a r g e s t .
The c h l o r o p h y l l v a l u e shows
STAT IONS
2
I
I000
\
I
3
4
' I '
5
6
7
8
9
I0
II
I2
I3
I
'
'
'
'
'
'
'
'
1415
'
'
16
17
18
19
'
'
'
'
0
0
I00
200
300
400
500
DISTANCE (km)
Fig. 7. The i n t e g r a t e d n i t r a t e c o n c e n t r a t i o n s (mg-at m-2) i n t h e upper 40 m o f t h e water column on t h e PROBES across s h e l f t r a n s e c t o f s t a t i o n s f r a n 24 March 3 June 1980.
-
662 STATIONS I
2
3
4
5
6
7
8
t
W
TE7 5 0 -I
9
I0
I1
I2
13 14 I 5
16
17
19
I8
17 MAY 3 MAY
)r
L
0
4 500 u I a
c W
(L
W
\I
f 250
I I
\ \
0
0
I00
200
300
500
400
DISTANCE Ihm)
Fig. 8. The i n t e g r a t e d e x t r a c t e d c h l o r o p h y l l c o n c e n t r a t i o n s (mg m-2) i n t h e upper 40 m o f t h e water column on t h e PROBES across s h e l f t r a n s e c t of s t a t i o n s froin 24 March - 3 June 1980.
t h e expected i n c r e a s e i n b o t h f r o n t a l r e g i o n s a t t h i s t i m e (Fig. 8). Duriny May, t h e m i d d l e s h e l f r e g i o n ( S t a 12-16) between t h e m i d d l e and i n n e r f r o n t s reaches t h e necessary s t a b i l i t y o f t h e water column,to undergo an e x t r e n e l y r a p i d bloom p e r i o d (Whitledge e t al.,
1985) which d e p l e t e d n i t r a t e
across t h e m i d d l e and i n n e r s h e l f t o l i m i t i n g values and produced c h l o r o p h y l l c o n c e n t r a t i o n s as h i g h as 700 mg m-3 i n t h e 40 m upper water column.
The
oceanic and o u t e r s h e l f l o c a t i o n s ( S t a 1-6) maintained an extremely l a r g e n i t r a t e g r a d i e n t and no p h y t o p l a n k t o n bloom was observed by J u l y . temperature increases i n t h e upper water column was o n l y 1.5"C
The small
on t h e o f f s h o r e
end o f t h e t r a n s e c t w h i l e a 4.OoC t e n p e r a t u r e r i s e occurred near t h e i n n e r front.
The c h l o r o p h y l l produced a t t h e o u t e r f r o n t c o u l d have been eaten by
zooplankton p o p u l a t i o n s as i t was produced and thereby r e g u l a t e d t h e phytoplankt o n p r o d u c t i o n r a t e s (Smith e t al.,
1985; Dagg e t al.,
1982).
The ammonium c o n c e n t r a t i o n s (Fig. 9 ) a r e p r i m a r i l y produced i n t h e bottom 1 dyer be1 ow 40 m by processes consuini ng p h y t o p l a n k t o n p r o d u c t i o n t h a t has s e t t l e d from t h e upper l a y e r .
The l a r g e s t ammonium c o n c e n t r a t i o n s occur i n t h e
m i d d l e s h e l f r e g i o n ( S t a 10-16) i n t h e near bottom environment by s h e l l f i s h o r
663 STATIONS I
400
2
3
4
5
6
7
8
9
I0
II
I2
,
,
I
I
,
,
T
,
T
,
I
I3
14
15 I
I6
17
I
,
I8 I9
,
,
0 DISTANCE Ikm)
Fig. 9. The i n t e g r a t e d ammonium c o n c e n t r a t i o n s (mg-at m-2) i n t h e uper 100 m o f t h e water o r t h e bottom on t h e PROBES across s h e l f t r a n s e c t o f s t a t i o n s from 24 March - 5 August 1980. m i c r o b i o t a w h i l e t h e o u t e r s t a t i o n s ( S t a 1-9) have a probable consumption o f t h e phytoplankton by zooplankton throughout t h e water column (Fig.
10).
The
ammonium observed on t h e o u t e r s t a t i o n s a t 40-60 m has an o r i g i n a t t h e m i d d l e s h e l f and i s probably advected o f f s h o r e
in^ a
narrow 1 ayer.
The pycnocl ine
As s p r i n g approaches a t t h e end o f w i n t e r t h e s u r f a c e l a y e r warms and t h e v e r t i c a l s t a b i l i t y o f t h e water column increases (Fig.
6) and enables t h e phyto-
p l a n k t o n p o p u l a t i o n t o secure enought l i g h t t o i n i t i a t e t h e s p r i n g bloom (Whitledye e t al.,
1985).
I n t h e shallow s h e l f r e g i o n where t h e depth i s 50 m
o r less, t h e s p r i n g bloom may occur b e f o r e s t r a t i f i c a t i o n i s i n i t i a t e d because t h e b o t t m i s a c t i n g as t h e i n t e r f a c e i n s t e a d o f t h e pycnocline.
This e f f e c t
a l s o i n c l u d e s near bottom r e c y c l i n g processes i n t h e mixed l a y e r and would i n c r e a s e t h e r e l a t i v e c o n t r i b u t i o n o f t h e bottom a g r e a t deal over a more t y p i c a l two-1 ayered system.
As t h e s p r i n g progresses and t h e p y c n o c l i n e develops i n t h e m i d d l e and o u t e r s h e l f , t h e phytoplankton bloom becomes s h e l f w i d e u n t i l t h e n i t r o g e n i s depleted i n t h e upper l a y e r .
A t t h i s p o i n t i n t i m e when n u t r i e n t s a r e low, t h e r a t e of
664 N O W W
g = NC)OIOU)
m
-mar-
k
STATIONS
E
--NIO*
oor-wm
O=fiUIO'O%'kE
0
50
100
AMMONIUM (pg -at {-I) 23-25 JUNE 1978
IbO
E -Nm * o kn o
2b0
360
I
4b0
'
AMMONIUM (pg-ott-'l
I50
13-15 JUNE 1979 500
2oo0
100
200
300
400
500
m ~ wc m ~ = C U I ' P O E Y Z~ $ i
50
100
AMMONIUM h - a t 1-'1 11-14 JUNE 1981
I50
200 DISTANCE ( k m )
Fiy. 10. The d i s t r i b u t i o n o f mmonium c o n c e n t r a t i o n s (ug-at 1-1) across t h e s h e l f on t h e PROBES t r a n s e c t i n June o f t h e years 1978-1981. p r i m a r y p r o d u c t i o n i s dependent on t h e q u a n t i t i e s o f n u t r i e n t s a v a i l a b l e i n t h e euphotic zone.
The bottom l a y e r over t h e s h e l f r e c e i v e s a c o n t i n u e d f l u x of
n i t r a t e f r a n t h e s h e l f edge and t h e r e c y c l e d n i t r o g e n regenerated near t h e b o t tom.
Storm events occur every 5 t o 6 days i n t h e Bering Sea and a f t e r t h e
storms t h e upper mixed l a y e r i s 5 t o 10 m deeper and a n e t i n c r e a s e o f n u t r i e n t s i s observed.
Storms o f about a 24-hour d u r a t i o n w i t h wind speeds o f about 10 m
sec-1 o r y r e a t e r a r e r e q u i r e d t o have a s i g n i f i c a n t e f f e c t .
Even e a r l y i n t h e
s p r i n g season w h i l e l a r g e amounts o f n i t r a t e a r e s t i l l present over t h e s h e l f , t h e e f f e c t s o f v e r t i c a l wind m i x i n g can i n c r e a s e t h e n i t r a t e c o n c e n t r a t i o n by 10% (Fig. 11).
One month l a t e r , when t h e n i t r a t e c o n c e n t r a t i o n s a r e v e r y small
i n t h e upper 15 m o f t h e water column, c o n c e n t r a t i o n by 1.5
a wind m i x i n g event increased t h e n i t r a t e
uy-at 1-1 and make t h e mixed l a y e r 5 m deeper.
increases o f 8 and 0.4
Coincident
py-at 1 - 1 o f s i l i c a t e and phosphate a l s o occur.
A series
o f storms over t h e summer c o u l d produce 18 m i x i n g events i n t h r e e months and b r i n y about 700 vg-at 1-1 NO3 i n t o t h e euphotic zone.
The l a r g e ammonium
p r o d u c t i o n near t h e b o t t a n o f t h e m i d d l e s h e l f combined w i t h t h e wind-induced The r a t e of
m i x i n g would enhance t h e v e r t i c a l f l u x o f r e c y c l e d nitrogen.
ammonim n i t r o g e n produced was estimated t o be a mean o f 7.2 mg-at m-2 d - l f o r
665
0 10
20
30
60 70
80
30 40
50
tt I
I
,
I
p,
1
Fig. 11. The c o n c e n t r a t i o n s o f n i t r a t e ( u g - a t 1-1) i n t h e 'water column b e f o r e (0) and a f t e r ( a ) w i n d m i x i n g on a) 22-24 A p r i l and b ) 13-16 May 1979.
666
t h e y e a r s 1979-1981 ( W h i t l e d g e e t al.,
1985).
N i t r i f i c a t i o n o r eventual
m i x i n y / t r a n s p o r t must reduce t h e ambient c o n c e n t r a t i o n s o f ammoni tiin t o u n d e t e c t a b l e l e v e l s i n t h e autumn. B e r i n g Sea p r o d u c t i o n a s s o c i a t e d w i t h s u r f a c e f r o n t s The q u e s t i o n o f whether t h e s u r f a c e f r o n t s o r t h e p y c n o c l i n e have a m a j o r i n f l u e n c e on t h e p r i m a r y p r o d u c t i o n i n t h e B e r i n g Sea can b e d i s c u s s e d w i t h r e s p e c t t o t h e r e l a t i v e n i t r a t e l o s s e s and c h l o r o p h y l l p r o d u c t i o n a t s p e c i f i e d locations.
The maximum u p t a k e o f n i t r a t e observed d u r i n g t h e s p r i n g bloom on
t h e m i d d l e s h e l f was a mean of 28.7 mg-at m-2 d - l f o r 1979-1981 ( W h i t l e d g e e t
1985).
al.,
The n i t r a t e l o s s e s between s t a t i o n s 2 and 9 , s t a t i o n s 5 and 9 , s t a t i o n s 13 and 9, and s t a t i o n s 13 and 18, f o r g i v e n t i m e p e r i o d s , q u a n t i f y t h e d i f f e r e n c e between t h e o u t e r , middle, and i n n e r f r o n t s w i t h t h e domains between t h e f r o n t s (Table 1).
I n most i n s t a n c e s t h e n i t r a t e c o n c e n t r a t i o n s i n t h e n o n - f r o n t a l
TABLE 1 Maximum and Minimum I n t e g r a t e d N i t r a t e i n Upper 40 m Across t h e S h e l f (my-at m-2). Maximum v a l u e s a r e i n b o l d - f a c e type.
2 4 March
3 May
784 ,169
1060,
669 /736\321354
1052 ~45d61~285/420~69P45j41 1297
1 7 May
1118'
\ 87 /244,1
23 May
946,
273-823,
3 June
53p32\6
.
51/302L8
31106\35/105\7
17 /125\120
4-
667
areas d i d n o t change as much as t h e f r o n t a l areas. The l o c a t i o n s o f maximum and minimum c o n c e n t r a t i o n s i n t h e upper l a y e r move 1 o r 2 s t a t i o n s p o s i t i o n s between consecutive o b s e r v a t i o n s d u r i n g t h e s p r i n g season b u t t h e s e changes probably r e s u l t from l a t e r a l movement i n t h e s h e l f responses t o storm events o r o t h e r e x t e r n a l f o r c i n g . o f t e n coincide with t h e previous d i s t r i b u t i o n s .
Subsequent o b s e r v a t i o n s
The most n o t a b l e f e a t u r e i s t h e
pronounced minimum o f n i t r a t e between s t a t i o n s 5 and 13 throughout t h e e n t i r e spring.
The maximum c h l o r o p h y l l c o n c e n t r a t i o n s ( T a b l e 2) c o i n c i d e w i t h t h i s
minimum except near s t a t i o n 15 on 3-17 May.
Both t h e n i t r a t e loss and t h e
c h l o r o p h y l l appearance were t h e l a r g e s t observed.
Since t h e y a r e s u r f a c e
f e a t u r e s , o u t e r and i n n e r f r o n t s a r e apparent i n t h e n i t r a t e and c h l o r o p h y l l distributions.
The m i d d l e f r o n t which d i v i d e s t h e two-layered system fran t h e
r e y i o n w i t h f i n e s c a l e f e a t u r e s between t h e l a y e r s (Coachman e t al.,
1980) shows
most c l e a r l y as t h e o u t e r edge o f t h e l a r g e ammonium c o n c e n t r a t i o n s i n t h e b o t t m l a y e r ( T a b l e 3).
A l a g i n t h e p r o d u c t i o n o f mmonium i n t h e bottom l a y e r
a f t e r t h e i n t e n s e s p r i n g p h y t o p l a n k t o n bloom causes t h e maximun ammonium concent r a t i o n t o appear i n June through August.
TABLE 2 Maximum and Minimum I n t e g r a t e d C h l o r o p h y l l i n Upper 40 m Across t h e Shelf (mg m-2). Maximum values a r e i n b o l d - f a c e type.
28 Oct 11 Feb 24 March 12 A p r i l 22 A p r i l
28 April 3 May 1 7 May 23 May 3 June
668
TABLE 3 Maximum and Minimum I n t e g r a t e d Ammonium i n Upper 40 m Across t h e S h e l f (mg-at m-2). Maximum v a l u e s a r e i n b o l d - f a c e type.
11 Feb
24 March 28 @ r i 1 0 3 May
17 May 23 May 0 3 June
28 June 17 July 0 5 August
28 October
From a comparison o f n i t r a t e d i f f e r e n c e s ( T a b l e 4 ) , i t i s apparent t h a t t h e r e i s a y r a d i e n t o f n i t r a t e concentration across t h e s h e l f i n t h e southeastern B e r i n y Sea between March and June and a t a b l e o f d i f f e r e n c e s c o n f i r m t h a t t h e o u t e r f r o n t has a l a r g e r n i t r a t e c o n c e n t r a t i o n t h a n t h e m i d d l e f r o n t (mean d i f f e r e n c e = 686 ing-at N 2 ) and t h e m i d d l e f r o n t has a h i g h e r n i t r a t e c o n c e n t r a t i o n t h a n t h e i n n e r f r o n t (mean d i f f e r e n c e = 98 mg-at V 2 ) ( T a b l e 4). I n a d d i t i o n , t h e c e n t r a l s h e l f w a t e r r e p r e s e n t e d by s t a t i o n 1 3 has g r e a t e r c o n c e n t r a t i o n s o f n i t r a t e t h a n t h e m i d d l e o r i n n e r f r o n t even a f t e r t h e l a r g e decrease between 3 and 17 May. C h l o r o p h y l l i n c r e a s e s i n t h e upper 40 m o f t h e w a t e r column ( T a b l e 5) were t y p i c a l l y i n v e r s e l y r e l a t e d t o t h e n i t r a t e concentrations,
however, t h e v a r i a n c e
i s even g r e a t e r t h a n n i t r a t e due t o t h e changes i n s t o r m m i x i n g b u t a l s o by s i n k i n g and/or h e r b i v o r e g r a z i n g processes which change t h e v e r t i c a l
669
TABLE 4 D i f f e r e n c e s o f Maximum and Minimum N i t r a t e C o n c e n t r a t i o n s Between F r o n t a l and Values were ca1c.uN o n f r o n t a l L o c a t i o n s o v e r a 40 m S u r f a c e L a y e r (mg-at m-2). l a t e d from maximum and minimum t a b l e o f i n t e g r a t e d n i t r a t e a c r o s s t h e s h e l f . Outer f r o n t middle f r o n t 24 12 22 28 03 17 23 03
39 1 494 520 552 767 1210 943 615
Mar Apr Apr Apr May May May Jun
Outer s h e l f middle f r o n t 192 440 436 412 276
Central s h e l f inner front
67 249 232 309 360 157 103 108
-
1325 146
d i s t r i b u t i o n o f phytoplankton c e l l s .
Central s h e l f middle front
415 296 3 26 421
-
23 6 99 121
As t h e s p r i n g progresses, t h e m i d d l e f r o n t
had more c h l o r o p h y l l t h a n t h e c e n t r a l s h e l f w a t e r b u t t h i s changed m a r k e d l y d u r i n y t h e maximum c h l o r o p h y l l appearance between 3 and 17 May.
By May 17 b o t h
t h e o u t e r s h e l f and t h e c e n t r a l s h e l f had more c h l o r o p h y l l t h a n t h e m i d d l e front. DISCUSSION
The i n t e n s e l y sampled s o u t h e a s t B e r i n g Sea s h e l f d u r i n g t h e PROBES p r o j e c t p r e s e n t s a v e r y good s e t o f d a t a t h a t can be used t o i n v e s t i g a t e t h e s i g n i f i cance o f f r o n t a l r e g i o n s on n u t r i e n t and c h l o r o p h y l l d i s t r i b u t i o n s .
A large
number of r e p e a t e d s t a t i o n s o v e r a f o u r - y e a r p e r i o d a l l o w s f o r a v e r y c o m p l e t e view o f t h e response o f n i t r a t e and c h l o r o p h y l l d i s t r i b u t i o n s around t h e f r o n t s under changing c o n d i t i o n s .
T h i s a n a l y s i s was persued t o d e m o n s t r a t e t h a t t h e
TABLE 5 D i f f e r e n c e s o f Maximum and M i nimum C h l o r o p h y l l C o n c e n t r a t i o n s Between F r o n t a l and N o n f r o n t a l L o c a t i o n s o v e r a 40 m S u r f a c e L a y e r (mg w2). Values were c a l c u l a t e d from maximum and minimum t a b l e o f i n t e g r a t e d c h l o r o p h y l l a c r o s s t h e s h e l f . Outer f r o n t middle f r o n t 24 12 22 28 03 17 23 03
Mar Apr Apr Apr May May May Jun
17 355 452 327 664 264 852 -253
Outer s h e l f middle f r o n t 8 145 494 323 473 -21 56 5 95
Central s h e l f middle f r o n t 21 348 522 409 56 1 -280 650 103
Central s h e l f inner front 20 91 316 300
-
'
- 50 22 1 39 5
670 f r o n t s were always i d e n t i f i a b l e by e i t h e r n u t r i e n t o r c h l o r o p h y l l concentrations.
The mean p r i m a r y p r o d u c t i v i t y r a t e s have been w e l l d e l i n e a t e d u s i n g
PROBES observations and simple models o f p h y t o p l a n k t o n growth (Sambrotto e t al., 1985; Walsh and McRoy, 1985), however, t h e p h y s i c a l s t r u c t u r e o f s u r f a c e f r o n t s and i n t e r a c t i o n w i t h t h e p y c n o c l i n e i s s t i l l r e q u i r e d b e f o r e t h e t h e observed v a r i a b i l i t y i n p r o d u c t i o n can b e simulated.
F u r t h e r work on t h e development o f
such a s i m u l a t i o n model i s t h e next l o g i c a l s t e p i n t h i s work. ACKNOWL EUGME NTS The authors would l i k e t o thank a l l o f t h e s c i e n t i s t s , t e c h n i c i a n s , s t u d e n t s associated w i t h t h e PROBES p r o j e c t . c o l l e c t t h e many f i e l d data.
and
A t r u e j o i n t e f f o r t was needed t o
This research was supported by t h e N a t i o n a l
Science Foundation Grant No. DPP 76-23340 t o t h e U n i v e r s i t y o f Alaska and under t h e auspices o f t h e U n i t e d States Department o f Energy under c o n t r a c t No. UE-AC02-76CH00016 t o Brookhaven N a t i o n a l Laboratory.
H EFERENCES 1980. F r o n t a l Coachman, L.K., T.H. Kinder, J.P. Schumacher, and R.B. Tripp. s y s t m s o f t h e southeastern B e r i n g Sea s h e l f . In: S t r a t i f i e d Flows, 2nd I A H H Symposium, Trondheim, June 1980, T. Carstens and T. McClimans (eds.), Tapir, Trondheim. pp. 917-933. Coachman, L.K. 1385. C i r c u l a t i o n , water masses, and f l u x e s and t r a n s p o r t on t h e southeastern B e r i n g Sea s h e l f . ( i n press). Cont. S h e l f Res. Dayy, M.J., J. V i d a l , T.E. Whitledge, R.L. Iverson, and J.J. Goering. 1982. The feeding, r e s p i r a t i o n , and e x c r e t i o n o f zooplankton i n t h e B e r i n g Sea d u r i n g a s p r i n g bloom. Deep-sea Res. 2: 45-63. Duydale. 1975. Results o f sea s u r f a c e K e l l y , J.C., T.E. Whitledge, and R.C. Limnol. Oceanogr. 20: 784-794. mapping i n t h e Peru Upwell ing System. Kinder, T.H. and J.D. Schumacher. 1981. C i r c u l a t i o n over t h e c o n t i n e n t a l s h e l f of t h e southeastern Bering Sea. I n : The Eastern Bering Sea S h e l f : Oceanography and Resources, Vol. 1, D.W. Hood and.J.A. Calder (eds.), U n i v e r s i t y of Washington, S e a t t l e , pp. 53-75. McRoy, C.P., D.W. Hood, L.K. Coachman, J.J. Walsh, and J.J. Goering. 1985. Processes and Resources o f t h e Bering Sea S h e l f (PROBES): The development and accomplishnents o f t h e p r o j e c t . Cont. S h e l f Res. 4: ( i n press). Sainbrotto, R.N., H.J. Niebauer, J.J. Goe-, and.mverson. 1985. Relat i o n s h i p between v e r t i c a l mixing, n i t r a t e uptake and p h y t o p l a n k t o n growth i n t h e southeast Bering Sea iniddle s h e l f . -Cont. S h e l f Res. ( i n press). 1985. V a r i a t i o n s i n t h e d i s m b u t i o n s , abundance, Smith, S.L. and J. Vidal. and development o f copepods i n t h e southeastern B e r i n g Sea i n 1980 and 1981. Cont. S h e l f Res. 4: ( i n press). Walsh,J.zdTP.-McRoy. 1985. Ecosystem a n a l y s i s i n t h e southeastern B e r i n g Sea. Cont. S h e l f Res. 4: ( i n press). Whitledge, T.E. R d a r K and J.J. Walsh. 1985. Seasonal i n o r g a n i c n i t r o g e n d i s t r i b u t i o n s and dynamics i n t h e southeastern B e r i n g Sea. S h e l f Kes. 4: ( i n press). --
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4:
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