Oceanography and Marine Biology, Vol. 35 PDF
Preview Oceanography and Marine Biology, Vol. 35
OCEANOGRAPHY and MARINE BIOLOGY AN ANNUAL REVIEW Volume 35 OCEANOGRAPHY and MARINE BIOLOGY AN ANNUAL REVIEW Volume 35 Editors A.D.Ansell R.N.Gibson Margaret Barnes The Dunstaffnage Marine Laboratory Oban, Argyll, Scotland Founded by Harold Barnes © A.D.Ansell, R.N.Gibson and Margaret Barnes, 1997 This book is copyright under the Berne Convention. No reproduction without permission. All rights reserved. First published in 1997 by UCL Press UCL Press Limited 1 Gunpowder Square London EC4A 3DE UK and 1900 Frost Road, Suite 101 Bristol Pennsylvania 19007–1598 USA This edition published in the Taylor & Francis e-Library, 2005. “To purchase your own copy of this or any of Taylor & Francis or Routledge’s collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk.” The name of University College London (UCL) is a registered trade mark used by UCL Press with the consent of the owner. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. Library of Congress Cataloging-in-Publication Data are available. ISBN 0-203-50172-1 Master e-book ISBN ISBN 0-203-80996-3 (Adobe eReader Format) ISBN: 1-85728-716-9 HB CONTENTS Preface v Physical processes in inverse estuarine systems 1 Mohammed I.El-Sabh, Than H.Augn & Tad.S.Murty Light absorption by phytoplankton and the vertical light attenuation: ecological and physiological 65 significance Ferdinand Schanz, Peter Senn & Zvy Dubinsky Lingulodinium polyedrum (Gonyaulux polyedra) a blooming dinoflagellate 89 Jane Lewis & Richard Hallett The role of tidal marshes in the ecology of estuarine nekton 1 54 Ronald T.Kneib The hyperbenthos 2 12 Jan MeesMalcolm B.Jones Dormancy in the free-living copepod orders Cyclopoida, Calanoida and Harpacticoid 2 45 Judith Williams-Howze Polychaete reproductive patterns, life cycles and life histories: an overview 3 05 Adriana Giangrande Caloric values of marine invertebrates with an emphasis on the soft parts of marine bivalves 4 07 J.J.Beukema The Hellenic Seas: physics, chemistry, biology and fisheries 4 36 K.I.Stergiou, E.D.Christou, D.Georgopoulos, A.Zenetos & C.Souvermezoglou Author index 5 85 Systematic index 6 25 Subject index 6 42 PREFACE This thirty-fifth Volume of the Annual Review sees another change in our publishing arrangements. During 1996, UCL Press became part of the Taylor & Francis Group who now take over responsibility for publication and distribution. This has meant the loss of most of the staff at UCL Press with whom we had worked closely since they took over from Aberdeen University Press in 1992. We are grateful for all the hard work they put into producing the Annual Review during that time and we wish them well. Happily, UCL Press will retain its identity within the Taylor & Francis Group and there should be no change in the format. Miss Marjorie Leith, of Leith Freelance Editorial Services, who has had a long association with the Review, continues to be responsible for copy editing. Hopefully, our contributors will see no ill effects from the change, and we look forward to a long association with Taylor & Francis. It is a pleasure again to acknowledge the willing cooperation and patience of our contributors, and their usually prompt replies to editorial queries. Volume 35 contains a broad mix of subjects, ranging from physical oceanography, through phytoplankton ecology and invertebrate biology to fisheries, with contributors from nine countries. Interest in the series continues to be maintained among both potential contributors and our readership, despite the many pressures on their time that most marine scientists face and for that we remain grateful. PHYSICAL PROCESSES IN INVERSE ESTUARINE SYSTEMS MOHAMMED I.EL-SABH,1 THAN H.AUNG2 & TAD. S.MURTY3 Abstract An inverse, or negative, estuary is a semi-enclosed sea or embayment within which loss of fresh water is more than the gain by runoff and precipitation combined. The tide-averaged internal circulation comprises an outflow of saline water near the bottom and an inflow of less saline water near the surface. Inverse estuaries are not uncommon features of the world’s coastal environment and one can distinguish two subsets according to geographical location. The first subset includes those estuaries in which freshwater removal is the result of an excess of evaporation over precipitation. These mostly occur in low latitudes of both hemispheres and are associated with arid conditions (e.g. the Red Sea, the Mediterranean, the Arabian Gulf, the Adriatic and the South Australia gulfs). The second subset covers those estuarine systems in which freshwater removal is achieved by freezing and the production of sea ice, which involves rejection of salt from the ice crystal lattice and hence increases the salinity of the underlying fluid. This type is therefore confined to high latitudes and principally the polar regions (e.g. the Weddell Sea, the Ross Sea and the Beaufort Sea). Although there have been many observational and numerical studies of the dynamics of classical positive estuaries since Pritchard’s pioneering work in the early 1950s, inverse estuaries have received relatively little attention. This paper reviews available literature relating to physical processes in several inverse estuarine systems and provides a framework for better understanding their characteristics and dynamics, hence providing another perspective on estuarine processes in general. Although thermohaline circulation generated by excessive evaporation is common to large-scale inverse estuarine systems such as the Mediterranean Sea, the Red Sea, and the relatively smaller embayments such as the South Australia gulfs, there is a significant difference in the dynamics of these two types. The greater depth and weaker tides associated with the former type render frictional effects insignificant in comparison with geostrophic and buoyancy effects. Consequently, thermohaline effects are manifest as a process consisting of the sinking of the saline dense water near Oceanography and Marine Biology: an Annual Review 1997, 35, 1–69 © A.D.Ansell, R.N.Gibson and Margaret Barnes, Editors UCL Press 1 Groupe de recherche en environnement côtier (GREC), Département d’océanographie, Université du Québec à Rimouski, 300, allée des Ursulines, Rimouski (Québec), Canada, G5L 3 A1 2 National Tidal Facility, The Flinders University of South Australia, GPO Box 2100, Adelaide, S.A.-5001, Australia 3 W.F.Baird & Associates Coastal Engineers Ltd., 1145 Hunt Club Road, Suite 1, Ottawa, Ontario, Canada, K1V OY3 2 MOHAMMED I.EL-SABH ET AL. the head and the release of this dense water as a bottom flow below a less dense compensatory inflow. As a result of greater tidal range and shallowness in the South Australia gulfs, the role of frictional effects is significant and the circulation is more complex. Because of the efficiency of vertical mixing due to tides during a large part of the spring-neap cycle, the saline dense water formed near the head by evaporation does not give rise to a clear flow separation constituting a bottom outflow and a surface inflow. The peculiar nature of the interaction among thermohaline circulation, geostrophy, boundary effects, and tidal dissipation were found in the South Australia gulfs. Generally, in classical estuaries, the primary circulation of wind and tidal mixing is a rapid diffuser of salt and pollutants. In inverse estuaries, the secondary circulation associated with thermohaline processes has significant implications for estuarine mass transport. Very little has been done to study the circulation hydrodynamics in the second type of inverse estuaries, namely those of the polar latitudes. The inverse nature of these estuaries is usually mentioned in passing, especially in connection with bottom water formation. We do not know if the important processes of geostrophic adjustment and penetration of saltwater wedges, which play an important role in the hydrodynamics of inverse estuaries of the lower latitudes, are relevant for the polar latitude estuaries. For future work, basic observational programmes coupled with a process- orientated modelling approach would be more useful for inverse estuaries of the polar type before detailed numerical models are developed. Introduction According to Pritchard (1952) and Dyer (1973), a classical (or positive) estuary (Fig. 1A) is a semi-enclosed marginal sea or embayment within which sea water is measur-ably diluted by fresh water. An inverse (or negative) estuary (Fig. 1B) is one in which salt water is concentrated by the removal of fresh water. There are two subsets of inverse estuaries depending upon how the freshwater removal is achieved. In the first subset, excess of evaporation over precipitation is the cause of the freshwater removal. Such estuaries are located in the low latitudes of both hemispheres on the globe. Another requirement is that arid conditions prevail and usually such conditions are associated with the high pressure regions of the general atmospheric circulation. Examples of this type include the Red Sea, the Gulf of Suez, the Mediterranean Sea, the Adriatic Sea, the Arabian Gulf, and, in the Southern hemisphere, the South Australia gulfs and Shark Bay (Fig. 2). In the second subset of inverse estuaries, fresh water is removed through freezing and the occurrence of sea ice. Naturally, estuaries of this type are situated in the higher latitudes on the globe, and principally the polar regions. During the formation of sea ice, salt is rejected from the ice crystal lattice (Foster 1968, Lake & Lewis 1970), which increases the salinity of the underlying water. According to Nunes Vaz et al. (1990) estuaries of the second type are the places where the inverse estuarine processes are directly responsible for producing most of the world’s deep ocean water masses. Examples of this type include the Weddell Sea, the Ross Sea and the Beaufort Sea (Fig. 2). An estuary can exhibit both positive and negative characteristics at the same time in different parts, depending upon the balance between evaporation and freshwater inflow (Wolanski 1986). Also, some estuaries may have inverse characteristics during part of the year and show positive behaviour in other seasons. Seasonal precipitation (for lower latitude or first subset inverse estuaries) or summer melting of sea ice (for polar latitude or second subset inverse estuaries) increases the freshwater inflow, which could change an inverse estuary temporarily into a positive or classical estuary. Although there are many similarities in the dynamics of positive and inverse estuaries, studies of the hydrodynamics of inverse estuaries offer certain insights. In classical estuaries, the buoyancy flux, which is PHYSICAL PROCESSES IN INVERSE ESTUARINE SYSTEMS 3 Figure 1 Schematic illustration of the circulation in (A) classical (positive), (B) inverse (negative) estuarine systems (modified from Tomczak & Godfrey 1994). Figure 2 General map showing location of inverse estuarine systems discussed in the present paper. positive, arises at point sources. In addition, there is a significant variation with time of runoff from land. Because of these two factors, frontal regions occur predominantly in classical estuaries (Nunes Vaz et al. 1990). On the other hand, in inverse estuaries, the buoyancy flux, which is negative, is approximately uniformly distributed throughout the estuary. Hence salinity fronts within an inverse estuary cannot be attributed to buoyancy flux effects. This means that study of the hydrodynamics of inverse estuaries helps 4 MOHAMMED I.EL-SABH ET AL. us to understand the advective processes. Furthermore, since the net flux of mass in an inverse estuary is directed towards the head of the water body, contaminants will tend to congregate towards the head, rather than move into the ocean such as occurs in a classical estuary. The interaction of fresh and salt water provides a circulation of water and mixing processes that are driven by the density difference between the two waters. The density of sea water generally depends on both the salinity and temperature, but in estuaries the salinity range is large and the temperature range is comparatively small. Consequently, temperature has a relatively small influence on the density. General estuarine circulation may be simply defined as the mixing of fresh and saline water in the estuary itself and in the shelf sea that causes a long-term surface seaward flow and bottom opposite flow in a classical estuarine system. In an ideal classical estuary where friction, tides and winds are not taken into account, relatively fresh and less dense river runoff flows in at the head of the gulf and spreads out over the sea water underneath. The subsurface saline water forms a wedge with its thin head directed upstream. This very simplified feature may be modified by several factors. If friction between surface flow and bottom flow is considered, it causes velocity shear at the interface. Water is entrained (dragged up from below) and mixed with the surface layer. The newly mixed water will not sink because its reduced salinity makes it less dense. Consequently, shelf water flows into the classical estuary along the bottom to replace the surface seaward flow. If tidal current is taken into account, tidal flow causes turbulence (random movements) throughout the water column under consideration and it increases mixing between the two layers. As a result, more saline water is transferred from the subsurface to the surface layer and some fresh water from the surface also mixes downward. In general, salinity decreases towards the head of a classical estuary. If wind effect is considered, the circulation pattern is more complicated due to the wind driven currents at the surface layer, the relative shallowness of the estuaries and the Ekman layer depth. If the area of interest is wide enough when compared with the Rossby radius, the Coriolis effect will also be involved (this will be discussed in more detail in the forthcoming sections). However, there is a net upstream flow in the lower layer to replace saline water lost from the system and a net seaward flow in the surface layer, removing both fresh and saline water from the estuary. In inverse estuaries, a very similar type of circulation takes place, the only difference being that the flow directions are opposite (i.e. bottom: dense outflow; surface: less dense inflow). Because of this opposite directional flow, estuaries of this type are appropriately called inverse estuaries or negative estuaries. Usually tides dominate current systems in most estuaries. Where tidal effects are strong, waters in estuaries tend to be less stratified. Waters may be mixed almost completely from top to bottom most of the time in a tidal cycle. To observe estuarine circulation, it is necessary to measure currents over several tidal cycles. When averaged over many tidal cycles, tidal currents cancel one another, leaving a non-tidal or estuarine circulation. Long-term average currents in the surface layers and the bottom layers would indicate a net inflow and a net outflow respectively. The most fundamental of estuarine characteristics is the longitudinal density gradient that, regardless of estuarine type, derives a circulation in the longitudinal-vertical plane, variously known as the estuarine baroclinic or gravitational circulation. Varying degrees of intensity of turbulence can change the estuarine character from highly stratified to vertically mixed types. Since baroclinic circulations are efficient mechanisms for dispersion, turbulence can be considered to reduce the dispersive potential of an estuary (Bowden 1977). The strength of the gravitational circulation, however, is not specified simply by the magnitude of the density gradient in a given estuary. It depends, in a non-linear relationship, on the intensity of ambient turbulence. In highly turbulent conditions, much of the momentum imparted to the circulation by virtue of the horizontal density gradient is diffused vertically. This makes the circulation much slower than
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