Oceanus Volume 19, Number 5, Fall 1976 Oceanus Volume 19, Number 5, Fall 1976 William H. MacLeish, Editor Johanna Price and Paul R. Ryan, Associate Editors Elizabeth Fricke, Editorial Assistant Editorial Advisory Board Robert A. Frosch, Associate Director for Applied Oceanography, Woods Hole Oceanographic Institution Edward D. Goldberg, Professor of Chemistry, Scripps Institution of Oceanography Richard L. Haedrich, Associate Scientist, Department of Biology, Woods Hole Oceanographic Institution John A. Knauss, Dean of the Graduate School of Oceanography, University of Rhode Island Allan R. Robinson, Gordon McKay Professor of Geophysical Fluid Dynamics, Harvard University David A. Ross, Associate Scientist, Department of Geology and Geophysics, Woods Hole Oceanographic Institution John G. Sclater, Associate Professor, Department of Earth and Planetary Sciences, Massachusetts Institute of Technology Allyn C. Vine, Senior Scientist, Department of Geology and Geophysics, Woods Hole Oceanographic Institution Woods Hole Oceanographic Institution Charles F. Adams, Chairman, Board of Trustees Paul M. Fye, President and Director Townsend Hornor, President of the Associates Note to Librarians This issue, Fall 1976, is Number 5 of Volume 19. The Winter 1977 issue will be Number 1 of Volume 20, and thereafter, the quarterly issues of each volume will appear within the calendar year. l-'dilorial correspondence: Oceanus, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543. Telephone (617) 548-1400. Subscription correspondence: All subscription and single-copy orders and change-of-address information should be addressed to Oceanus, 2401 Revere Beach Parkway, Everett, Mass. 02149. Urgent subscription matters should be referred to our editorial office, listed above. Subscription rates: one year, $8; two years, $14. Subscribers outside the U.S. or Canada please add $2 per year handling charge; checks accompanying foreign orders must be payable in U.S. currency and drawn on a U.S. bank. Single copy price, $2.25; forty percent discount on single copy orders of five or more. When sending change of address, please include mailing label. Claims for missing numbers will not be honored later than 3 months after publication; foreign claims, 5 months. Postmaster: Please send Form 3579 to Oceanus, Woods Hole, Massachusetts 02543. Contents PHYSICAL OCEANOGRAPHY OF ESTUARIES by Charles B. Officer The soundness of many future environmental decisions rests on a more complete understanding of the physics, chemistry, geology, and biology of estuaries. Q GREEN BORDERS OF THE SEA by Ivan Valiela and Susan Vince The reduction of food-producing coastal wetlands through building development threatens shell and fin fish, wildlife and wildfowl, not to mention our aesthetic heritage. POLL UTION HIS TOR Y OF ESTUARINE SEDIMENTS by Edward D. Goldberg Ttie flow rates of pollutants entering an estuary-sewer effluents, industrial discharges, atmospheric fallout, dumping, and releases from ships- can be measured in sediment deposits and such recordings can be of great use to marine management, BARRIER BEACHES OF THE EAST COAST by Paul J. Godfrey Tfie survival of most estuaries depends on the natural functioning of barrier beaches, which in turn are threatened by man 's indiscriminate development. 27 NUTRIENTS IN ESTUARIES by John E. Hobble While estuaries usually contain high concentrations of various nutrients, it is phosphorus and nitrogen that control the growth ofestuarine plants. A1 PHYSIOLOGICAL ADAPTATIONS OF ESTUARINE ANIMALS by Winona B. Vernberg and F. John Vemberg Salinity, temperature, and stress variations are the key elements that affect the life-and-death cycle of the diverse animal life in estuaries. FISHES AND ESTUARIES by R. L. Haedrich and C. A. S. Hall Man takes his greatest harvest of seafoods from the estuarine environment, with five of our six most important commercial fish species dependent on these areas. CONSTITUTIONAL ISSUES AND ESTUARINE MANAGEMENT by John S. Bant a Tlie constitutional arguments against estuarine management touch on a number of issues that center on often emotional reasoning about private property and restrictions on its development. INDEX The cover photograph is of the Sandy Neck area of Cape Cod, Massachusetts. It was taken by Anita Brosius. Copyright © 1976 by Woods Hole Oceanographic Institution. Published quarterly by Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543. Second-class postage paid at Woods Hole, Massachusetts, and additional mailing points. NOTICE TO SUBSCRIBERS Rate Increase Rising costs have forced us to raise prices for subscriptions entered or renewed after January first, 1977. The new domestic rates will be $10 for one year, $18 for two. There will continue to be a $2 per year handling charge for foreign subscriptions. Subscribers wishing to do so may renew at present rates ($8 and $14) by sending a check payable to Woods Hole Oceanographic Institution to the editorial office address listed below. These orders must be received at Woods Hole prior to January first. Special Student Rate Beginning January first, 1977, students at all levels may enter or renew subscriptions at the original Oceanus rates ($8 for one year, $14 for two). These special rates are available only through application to our editorial offices at Woods Hole, Mass. 02543. Attention Teachers We offer a 40 percent discount on bulk orders of five or more copies of each current issue - that comes to only $1.35 a copy at present rates, $1.65 at those announced above. The same discount applies to back copies ($1.80 at bulk rates-and back copy prices will not go up in January) and to one-year subscriptions for class adoption ($4.80 per subscription at present rates, $6.00 after January). Bulk orders should be sent to our editorial office at the address listed above, accompanied by a check payable to Woods Hole Oceanographic Institution. ' •9 ' ' » * ..' V '• » V-" *• *' °* - Ph/sicd Oceanography of Estuaries by Charles B. Officer Estuaries are of immense societal importance, far beyond their relatively limited geographical extent. They are frequently areas of high fertility and large phytoplankton populations. The zooplankton of estuaries can be characterized in much the same terms as the phytoplankton; it is volumetrically abundant but often limited as to species composition. One of the more common and significant features of estuaries is the relatively large spring and summer zooplankton populations. This is a result of a high level of primary production. It also reflects the addition of numerous larvae of benthic invertebrates. It has been estimated that 60 to 80 percent of the commercial marine fisheries resources depend on estuaries for part or all of their life cycle. Man also concentrates much of his activities adjacent to the estuarine environment. Apollo photograph of Chesapeake Bay, Norfolk, Hampton Roads, and Cape Charles area taken by 70mm Hasselblad camera aboard spacecraft that crew Thomas Stafford, Donald Slayton, and Vance Brand docked with Soyuz counterpart on July 17, 19 75. (Courtesy NASA) In the United States a third of the population lives and works close to estuaries. Of the 10 largest metropolitan areas in the world, 7 border estuarine areas (New York, Tokyo, London, Shanghai, Buenos Aires, Osaka, and Los Angeles). Of the 66 largest cities in the world, 39 are in coastal areas. A knowledge and understanding of estuarine circulation and mixing is essential to the consideration of many of the problems that presently confront investigators looking at the chemical and biological aspects of water quality and ecology and the geological aspects of sediment transport and distribution. Our present understanding of the hydrodynamics of estuaries has evolved over the years through contributions from individuals with varying backgrounds in geophysics, physical oceanography, biology, chemistry, and civil engineering. Some of these contributions have corne from purely scientific investigations, while others have been related to rather specific engineering problems. Recently, there has been an increased emphasis on understanding the circulation and mixing processes in estuaries, and this has been directly related to the existing and potential future pollution problems of estuaries. Some indication of the increased interest in the geophysics of estuaries has been the recent occasion of two professional symposia on the subject. One, entitled Geophysics, Estuaries, and the Environment, was held at the American Association for the Advancement of Science meeting in February 1976 under the auspices of the Geophysics Research Board of the National Academy of Sciences. The other, entitled Transport Processes in Estuarine Environments, was held at the University of South Carolina under the auspices of their Institute for Coastal Research. The scientific papers from both symposia will be published in book form. One of the general conclusions from both symposia was that we simply do not have an adequate scientific understanding of the physics, chemistry, geology, and biology of estuaries necessary for many of the environmental decisions that have to be made. We cannot predict with certainty the environmental effects of some changes: we do not even know the physical processes involved in others. Perhaps it was not that important to have this understanding in the past, and the faults of decisions made in good faith with the best of available knowledge could be tolerated. Perhaps this will not be the case for the future. Estuarine Hydrodynamics The term estuary has varying shades of meaning. From a standard English dictionary we have that an estuary is a wide mouth of a river where its current meets the sea and is influenced by the tides, or alternatively, that it is an inlet or arm of the sea. It is derived from the Latin aestuarium, which, in turn, is derived from the Latin aestus. As referred to the ocean the term aestus meant to the Romans the undulating motion of the sea, a swell or a surge, and later, as their explorations carried them beyond the Mediterranean, the ebb and flow of the sea, or the tide. The term aestuarium meant a place where the aestus was observable. For geophysicists, physical oceanographers, and civil engineers, the definition of an estuary given some several years ago by D. W. Pritchard is usually followed. This is that an estuary is a semienclosed body of water having a free connection with the open sea and within which the seawater is measurably diluted with fresh water derived from land drainage. Traditionally the term estuary has been applied to the lower reaches of a river into which the seawater intrudes and mixes with fresh water draining seaward from the land. The term has been extended to include bays, inlets, gulfs, and sounds into which several rivers empty and in which the mixing of fresh and salt water occurs. We prefer this broader definition of an estuary. As has been pointed out and elaborated by B. H. Ketchum, H. Stommel, and others, we are fortunate in having a natural tracer in estuaries. This is the fresh water flow into the estuary. A convenient measure of the fresh water fraction at any location can be given simply in terms of the observed salinity as referred to the normal ocean salinity of the coastal waters into which the estuary empties. From the observed vertical salinity distribution we can distinguish various estuarine conditions. We define a well-mixed condition as one in which there is essentially no variation in the salinity in a vertical column. We can also distinguish a stratified condition with a halocline between the upper and lower portions of the water column of nearly constant salinity. We define a weakly, or partially, stratified condition as one in which there is a change of salinity of only a few parts per thousand from surface to bottom, and a strongly, or highly, stratified condition as one in which there is a change in salinity of several parts per thousand from surface to bottom. We can also distinguish a condition in which there is an interface between two different water types. We define an arrested salt wedge as one in which there is a stable salt wedge underlying a strong, fresh water flow above, and a fjord entrainment type flow as one in which there is a relatively stagnant deep water mass overlain by a thin river runoff flow. It is important to appreciate that these are descriptive terms only and cannot, in general, be applied to an estuary as a whole; for any given estuary may show well-mixed or stratified conditions, for example, as a function of longitudinal distance along the estuary, season of the year, or even in some cases phase of the tidal cycle. One of the better understood aspects of estuarine hydrodynamics is that of the longitudinal circulation and mixing characteristics for well-mixed and stratified estuaries as averaged over a tidal cycle. The driving forces for the circulation are the longitudinal surface slope force, acting in a downestuary direction, and the longitudinal density gradient force, related to the longitudinal salinity gradient, acting in an upestuary direction. These two driving forces are balanced by the internal and bottom frictional forces, and there may in some cases be an important contribution from a third driving force related to the wind stress at the surface. The surface slope force is constant as a function of depth, and the density gradient force increases essentially linearly as a function of depth. For the condition in which there is a small river runoff flow, which is common for many estuaries, the net effect is, then, that the surface slope force will be dominant in the upper portion of the water column, producing a net circulation flow downestuary, and that the density gradient force will be dominant in the lower portion of the water column, producing a net circulation flow upestuary. An example of this type of circulation is shown in Figure 1 for the Mersey Estuary. The longitudinal current velocity and salinity values are tidal averaged for a given station for four different measurement periods. The vertical coordinate is depth scaled to the total water depth. Although this type of longitudinal density gradient flow is important, it is hardly an end in itself in understanding estuarine circulation. For larger estuaries, in particular, lateral effects, estuarine geometry, and atmospheric pressure and wind stress effects become important. In a paper presented at the South Carolina symposium, Pritchard discussed current records taken in Chesapeake Bay over an extended period of time of a year. He noted that, as averaged over a tidal cycle, one can observe times of reverse gradient flows in which the flow at the surface is upestuary and that near the bottom downestuary, times of storage flows in which the flow at all depths is either in an upestuary or downestuary direction, and times of three-layered flows in which the flow near the surface and the bottom are in one direction and that at mid-depth in the other direction in addition to times of the normal density gradient flow. Certainly one conclusion from these observations is that we have much to learn. The previous two paragraphs have concerned the mean motion as averaged over a tidal cycle. The tidal motions, themselves, are of great importance in most well-mixed and stratified estuaries for the mixing and ultimate longitudinal dispersion of potential pollutants and in the determination of bottom and internal frictional effects. In terms of a simplified longitudinal dispersion coefficient, there are two contributing effects, one from the tidal mixing itself and the other from the net circulation, or velocity shear, effect of the mean motion. For estuaries in which Velocity cm/sec -10 -5 o 5 10 15 O- 30 June -2 July, 1959 A- 28-29 July, I960 + - 6-8 July, 1959 7 - I August, I960 Salinity %0 27 MERSEY ESTUARY 28 h 0.6 -O-30June-July, 1959 +\ Figure 1. Mean profiles of velocity and salinity over a tidal period for the Mersey Estuary. (After K. F. Bowden, 1963, Int. J. Air and Water Pollut., 7:343-46, Fig. 2.) FRIERFJORD DENSITY DlFF.(p-Po) kg/m5 SalinityfO/oo) T (°C 0 2 4 6 8 10 12 14 161820 05 1015202530 02 4 6 8 1012 1' K. _ { * ^ I \ ? 10 - X h- N ^ , \ I'° i ^ | o. Q. Q_ £ UJ Q UJ 0 Figure 2. Observed stratification, Frier fjord, October 26, 1966. (After T. Carstens, 1970, American Society of Civil Engineers, J. Waterways and Harbor Div., vol. 96, WW1, pp. 97-104, Fig. 1.) there is substantial tidal motion, the tidal mixing contribution will usually be dominant. For conditions in which an interface exists between two different water types, simple two- layered hydraulic theory can describe several of the basic observables. Beyond this and in consideration of interfacial stability and entrainment and mixing across the interface, or steep gradient, the physical understanding and theoretical description become considerably more complex. An example of summer conditions in a Norwegian fjord is shown in Figure 2. The interface is quite sharp, showing a change in salinity of several parts per thousand over a vertical distance of a fraction of a meter. One possible method of investigating estuarine interfaces is that shown in Figure 3. This is a high-frequency acoustic reflection record of the salt wedge interface in the Ishikari River; the type of record is much the same as a conventional echo sounder record of the bottom. Some question can be raised as to whether the acoustic reflection is returned from the impedance contrast across the fresh to salt water interface or from detritus or bubbles accumulated on the interface. In either case it would appear to be an interesting technique that might be used for investigating the dynamics of estuarine interfaces and hydrodynamic flow conditions. Applications Estuarine hydrodynamics finds application in the description of water quality parameters such as biochemical oxygen demand and dissolved oxygen, nutrient cycle relations, and coliform bacteria distribution; in the description of the geological parameters of suspended sediment and particulate matter distribution and transport and bottom BOUNDARY OF SALT AND FRESH WATER ST.IV . TVirrl RIVER SURFACE RIVER BED | -SECOND ECHO OF BED ' 0 RIVER MOUTH ST. I Figure 3. A longitudinal profile of a salt wedge in the mouth of the Ishikari River, July 22, 1964. (After H. Eukushima, M. Kashiwamura, and I. Yakuwa, 1966, American Society of Civil Engineers, Proceedings of the Tenth Conference on Coastal Engineering, pp. 1435-47, Fig. 10.) 6 •••••••••• e* 2 4 6 8 10 12 14 16 18 20 23 24 36 28 30 Model Section Number Figure 4. Computed and observed dissolved oxygen profiles for the Delaware Estuary, June-August, 1964. Dots represent individual grab samples. (After R. V. Tliomann, 1972, Systems analysis and water quality management, McGraw-Hill, New York, Fig. 7-7.) sediment transport; and in the description of the biological parameters of phytoplankton, zooplankton, and fish eggs and larvae distributions and blooms. For the first, the substances are miscible and the hydrodynamic equations can be applied directly with appropriate chemical and biological reaction and interaction effects included. For the second, the substances are not miscible with the estuarine waters; other physical processes become involved, which complicate a quantitative description. For the third, essentially only a qualitative, or statistical, description can be given. One of the more successful applications has been in the description of dissolved oxygen and coliform bacteria distributions and, to a lesser extent, nitrogen cycle relations for various estuaries. These descriptions, for the most part, have been made for cross-sectional and tidal-averaged conditions under steady state. Coupled linear differential equations are used. Each equation is in the form of a one-dimensional, longitudinal equation with a net advection term, a dispersion term, including both tidal mixing and net circulation effects, and various reaction terms for the decay or build-up of the quantity considered and its feedforward and feedback contributions. Solutions are usually given through finite difference numerical calculations, with a computer, that permit the important inclusions of changes in geometry, coefficient values, and pollutant and fresh water source contributions along the estuary. Sometimes the dispersion, reaeration, and reaction coefficients are determined from separate experiments; sometimes they are determined by adjustment to give a best fit of the model to the field observations. An example of a comparison of the numerical model calculations and the field observations for dissolved oxygen along an extended length of 135 kilometers for the Delaware Estuary is shown in Figure 4. Although on the average the comparison is quite good, this figure also raises indirectly questions as to the relative importance of tidal, transient, vertical, and lateral effects in describing the scatter in the field observations. Let us continue, now, with a consideration of the application of estuarine hydrodynamics to geology. One of the more important indices as to bottom sediment transport is the direction of the tidal-averaged bottom current. For the middle to lower reaches of a well-mixed or stratified estuary, longitudinal density gradient flow will usually be dominant, and the net bottom current will be in an upestuary direction. For the upper reaches of an estuary and its associated tributary river, the longitudinal salinity gradient effect will be negligible, and the flow will usually be riverine in character and downestuary at all depths. We would then expect that the region of sediment accumulation would be at the null zone between these two flow conditions. In the gross this is indeed the case. An example for the Savannah River is shown in Figure 5. The flow at the surface is predominantly downestuary. That at the bottom is predominantly downestuary from the upper end of the harbor to about station 130, and predominantly upestuary from this station to the harbor entrance. The region of shoaling is in the vicinity of zero net bottom motion. Another related geological aspect is that of the suspended particulate matter distribution in an estuary. One of the more interesting aspects of such distributions is the turbidity maximum, which exists in the upper reaches of a number of estuaries. The turbidity maximum is related, at least in part, to a local resuspension of bottom sediments by tidal 75 c o TD J O 50 25 -Surface Channel shoaling very heavy in this reach 1 Mean tide river Q = 5670 cps I I I I I I I I 75 50 e o J ^n I T3 O 93 113 133 153 173 1000 ft channel stations 25 193 Figure 5. Relation between normal surface and bottom flow in Savannah Harbor. (After H. B. Simmons, 1966, Estuary and coastline hydrodynamics, McGraw-Hill, New York, pp. 673-90, Fig. 16.9.) action in the accumulation zone. An example for the northern reach of San Francisco Bay is shown in Figure 6. The magnitude of this turbidity maximum increases with the increased, winter suspended particle input; and its position varies in response to the tributary river discharge. A third hydrodynamically related geologic effect is the dense static suspensions of fine cohesive sediments that sometimes occur in estuaries and form a distinct layer above the normal bottom. These suspensions are variously referred to as fluid mud, fluff, and flocculent layers; and the particle concentrations are quite high— of the order of 100,000 milligrams per liter or more with corresponding densities of 1.1-1.4 grams per milliliter. They are somewhat ephemeral in character. They move with the bottom current, and they are sometimes present or absent depending on the phase of the tidal cycle or the neap to spring tidal variations. Where observed, they have been found ESTUARY RIVER SUSPENDED PARTICLES !5-»5 Ml «5-90 ^ 90-200 >200 JAN JUL| JAN (\ JULg JAN SAN FRANCISCO BAY 300 ISO 0 SUSPENDED PARTICLE] SALINITY CONTOURS (%„) «0 60 80 DISTANCE FROM GOLDEN GATE (km) JAN JULg JAN JUL| JAN 20OO IOOO DISCHARGE Im'/ltc) Figure 6. Seasonal distribution of properties in the northern reach of San Francisco Bay, 1961-63: (A) Suspended particle concentration in the estuary; (B) suspended particle concentration in Sacramento River; (C) salinity at 1 meter depth in the estuary compared with location of suspended particle and plankton maxima and approximate location of nontidal current null zone, solid black lines; (D) combined discharge from Sacramento and San Joaquin rivers. (After D. H. Peterson, T. J. Conomos, W. W. Broenkow, and E. P. Scrivani, 1975, in Estuarine Research, ed. L. E. Cronin, vol. I, Academic Press, New York, pp. 153-87, Fig. 9.) to be quite high in heavy metal content and are apparently largely anerobic, both phenomena being of potential environmental significance. These layers are readily distinguishable on a conventional echo sounder record, and an example for the Severn Estuary is shown in Figure 7. With regard to the interrelations and dependence of estuarine biota on hydrodynamic effects, we enter a more complex arena. Much of the scientific investigation in the past in estuaries has been directly related to the biota. This will probably be true for the future, but perhaps with more emphasis on physical, chemical, and geological interrelated biological effects rather than discipline- oriented biological descriptions. Here the discussion is limited to two examples where hydrodynamics appears to play an important role in the description of the biota. The null zone, which occurs in the upper reaches of some estuaries between the riverine and density gradient type flow conditions, is also the region of maximum residence time for the passage of a water particle from its tributary river to the ocean. It becomes an important region for phenomena that are time-dependent for their generation. It can be argued, then, that the null zone will permit the highest in situ summer phytoplankton growth, or bloom, through reproduction. An example that this dependence does exist is shown in Panel C of Figure 6 for San Francisco Bay. Here the summer plankton maximum corresponds well with the location of the null zone, as does the winter turbidity maximum. A few years ago E. L. Bousfield, in a paper in the Journal of the Fisheries Research Board of Canada (12:342-61), described the general oceanography and biota of the Miramichi Estuary. A feature of particular interest is the effect of the estuary net circulation, or density gradient controlled, flow on the distribution of the larval stages of a brackish water barnacle Balanus improvisus during its 18-day planktonic life period. The first three nauplius stages, found mainly in the upper portion of the water column, are transported successively seaward. The fourth and fifth stages, found near mid-depths, are concentrated near the mouth of the estuary. And the sixth-stage nauplius and the cyprid, found mainly in the lower portion of the water column, are carried progressively landward toward the head of the estuary. It would 8 Fluid mud with undulating surface Stratified settled mud with undulating surface V> f- Figure 7. A 30-kilohertz echo sounder record showing fluid mud and settled mud in the Severn Estuary. (After R. Kirby and W. R. Parker, 1974, Dock and Harbour Authority, 54:423-24, Fig. 1.) appear then that, at least in this case, the ecology of such biota is dependent not only on the environment of their adult habitat but also on the environment of the estuary as a whole. Charles B. Officer is an adjunct professor of earth sciences at Dartmouth College and a partner in Marine Environmental Services. He also is a guest investigator at the Woods Hole Oceanographic Institution. Suggested Readings Dyer, K. R. 1973. Estuaries: A physical introduction. London: Wiley. Ippen, A. T., ed. 1966. Estuary and coastline hydrodynamics. New York: McGraw-Hill. Kullenberg, G., and J. W. Talbot, eds. 1974. Physical processes responsible for dispersal of pollutants in the sea. International Council for the Exploration of the Sea, Rapports, 167:1-259. Nelson, B. W., ed. 1973. Environmental framework of coastal plain estuaries. Geological Society of America, Memoir 133. Officer, C. B. 1976. Physical oceanography of estuaries and associated coastal waters. New York: Wiley. Thomann, R. V. 1972. Systems analysis and water quality management. New York: McGraw-Hill. Ward, G. H., and W. H. Espey, eds. 1971. Estuarine modeling: An assessment, No. 16070 DZV 02/71, Environmental Protection Agency, Washington, D.C. Spartina alterniflora or typical salt marsh cord grass found from Nova Scotia to Texas. (Paul J. Godfrey) The margins of the world's oceans and estuaries are largely bordered by a narrow green ribbon of vegetation. For geographical and economic reasons, these coastal strips have been severely stressed and altered over the centuries. The variety of benefits provided by wetlands has only recently become well documented. Unless society has a better understanding of the losses incurred in destroying wetlands, this historical pattern will continue. Wetlands are of two types: salt marshes or mangrove swamps (Figure 1). The plants found in salt marshes are mainly grasses, while mangrove swamps are dominated by various species of mangrove trees. In temperate latitudes, Spartina grasses are the most common, while most mangrove vegetation consists of members of the genera Rhizophora, Avicennia, and others. Only a few types of plants have evolved to live in this border between sea and land. Marsh and mangrove plants grow in oxygen-poor mud and peat intermittently submerged by seawater. The plants have a variety of adaptations to solve these environmental problems, such as specialized glands to excrete salt. Marsh grasses have a system of conduits carrying air to the part of the plant growing underground. Mangroves have air-containing vessels in their roots as well as aerial prop roots, or pneumatophores, to accomplish the same purpose. Marshes and mangroves are distributed all over the world and either are associated with estuaries or are found behind island bars that provide protection from the open sea (Figure 2). No one has satisfactorily explained the striking degree to which the boundary between marshes and mangroves parallels latitude on all continents. Mangroves cover much of the coasts between 30°N and 30°S, and marshes take over north and south of these latitudes (Figure 1 ). In a number of places- southern Brazil, Florida, northern New Zealand- mixtures of both types of vegetation appear. Where mangroves and grasses grow together, mangroves can outgrow and shade grasses. In the absence of mangroves, well-developed marshes would be found nearer the Equator. Spartina grasses grow on Brazilian coasts as well as in temperate North America, but the Brazilian stands occur where the mud has not yet been colonized by mangroves. The explanation for the sharp boundary may require only an accounting of why mangroves cannot grow beyond the tropics. There is some speculation that mangrove seedlings cannot survive freezing temperatures. No experimental evidence is available, however, and in some localities where mangroves are found, freezing does occur. 10 Pressure on the Wetlands Concern with mosquito-borne diseases has historically colored attitudes toward coastal wetlands. Sixteenth-century Spaniards made a point of founding Buenos Aires ("good airs") on a site where winds blew the presumed "bad air" (mala aria is the Italian origin of our word for the mosquito-borne fever) of coastal marshes seaward. As it turned out, the place was not well situated, since neither the local tribes nor the mosquitos were kind to the settlers. Health hazards are one thing, economics quite another. Marshes and mangroves often flourish where human settlements are likely to prosper- protected sites at the junctures of river and sea transportation routes. The result has been widespread filling and the disappearance of wetlands. The expansion of Boston over the marshy estuary of the Charles River and Boston Harbor is a good example (Figure 3). Another is the development of Venice since A.D. 600 over the marshy lagoons and sand bars at the head of the Adriatic. This took place despite the continued policy of the Venetian administration against altering the lagoon. In the mid-thirteenth century, a dam was built across the low-lying, marshy Amstel River, starting the long struggle between settlement and tidal water that has produced present-day Amsterdam. Another reason for the reduction of coastal wetlands is that these areas can be used for agricultural and industrial purposes because of the fertile soils and normally low land prices. In Java, Sumatra, the Philippines, and Taiwan, mangroves have been cleared to produce systems of carefully managed tidal ponds fringed with mangrove levees. Fish and prawn are cultured in such ponds, and production is among the highest recorded. More damaging practices have been used in Sri Lanka and Mozambique, where reclaimed swamps are now coconut groves. In Thailand and Mozambique erstwhile mangrove areas have become shallow Figure 1. World distribution of well-established salt marshes and mangroves. Both habitats are more widespread, since we used only records from the literature and our own experience. Many places,such as Alaska, the Arabian Peninsula, Baja California, and northern and western Eurasia,are not well represented in the references. Notice, however, the clear demarcation between marshes and mangroves. 11 evaporation ponds for the production of salt. Also in Thailand and elsewhere in Southeast Asia, extensive acreage of estuarine mangrove has been virtually eliminated by the harvesting of mangrove timber for firewood and charcoal. In Malaya and a few other countries, recently developed management practices permit balanced exploitation of swamps, but elsewhere the decimation continues. One of the most dramatic episodes in European agrarian history was the struggle for use of the rich soils of wetlands. There is no better example than the well-known diked polders- reclaimed lowlands— of Holland and Belgium. Records of dikes in what is now the Netherlands go back as early as A.D. 1018. In Flanders, poldering was first recorded in A.D. 1111-1115. The construction of polders was in its heyday in the early 1600s and again in the 1800s, spurred by the increased price of grain in Amsterdam and other markets. The polders provided both protection from the sea and needed agricultural land, powerful imperatives in earlier times. Today the balance between the benefits of agriculture and of fisheries in the North Sea is more difficult to evaluate. Flood control no longer requires diking. Additionally, research is underscoring the economic importance of marshes, tidal flats, and inlets, such as the Waddensea in Holland. The Waddensea is now the primary nursery for North Sea fish and the only nursery area available in the North Sea for brown shrimp, a major crop. The current annual return of fish that depend on the Waddensea nurseries is reported to be Figure 2. Air view of a Cape Cod salt marsh. The entire marsh is drained by the single channel in lower center and a barrier beach protects the marsh. No major fresh-water streams enter this marsh, but elsewhere the major channel is often a river or estuary. 12 more than $400 million. Nevertheless, society continues to intrude. Petrochemical complexes, power stations, and extensive new dikes are being planned along the coasts of the Netherlands and West Germany. Pipelines carry oil and gas from the North Sea through the Waddensea, while pollutants from the Rhine River are transported northward into this tidal flat. In the Wash, a very large tidal area in Great Britain, 15 percent of sandbanks and mud flats will be converted into fresh-water lagoons by 2020 through the building of banks 14 meters high. The fresh water is needed to satisfy the projected requirements of southeastern England. All these activities, some critical to human welfare, some not so important, affect the wetlands in which they take place (Figure 4). There have been some successful attempts in North Carolina, the Netherlands, San Francisco Bay, and elsewhere to construct marshes by planting marsh grasses on dredge spoils and on eroding mud flats. In Guyana, Spartina braziliensis has been planted on mud flats for coastal protection. Once the stands of these grasses are established, mangrove seedlings are planted, and the mangroves eventually replace the grasses. With these exceptions, the interaction between people and marshes has resulted in reduced marsh acreage. In Connecticut, Upland South Bay I836 Upland South Bay Figure 3. Back Bay, Boston, from 1814 to the present. In 1814, the main part of Boston was situated in the northeast, separated by the Neck from the mainland, where most of the marshlands were located. By 1836, filling was in place to support railroad tracks over the Roxbury flats. The main dam, built of stone and earth fill, was completed in 1821 and used tidal energy to run mills. The reduction of tidal flushing in the Back Bay Basin, the large area in the right center of the map, led to stagnation and stench. Filling continued westward from the Common and northward from Tremont Street. Today, the shore of South Bay has moved to the southwest, all the marshes are gone, and a little water in the Fenway remains from the Muddy River marsh. 1976 Commonwealth Ave Fenway i/}* Garden Common | t | Salt Marsh | | Water | | Upland 13 • Figure 4. Industrial development over a salt marsh in Saugus, Massachusetts. The photo was taken from a solid waste disposal area (foreground) that now covers a substantial part of the marsh. 50 percent of the original marshes have been destroyed, and of the 14,000 acres remaining in 1969, about 200 acres have been filled each year. Not all examples are as striking as Connecticut's, but the trends are similar. Because of the continued threat to marshes and mangroves, it is important to consider the consequences of such widespread reduction in acreage. Properties of Wetlands We have already mentioned coastal wetlands as nurseries for commercially important fish. In the southeastern United States, 60 percent of the species use tidal marshes as nursery grounds. A variety of wildfowl and mammals also rear young in marshes and mangroves. The Department of the Interior estimates that in good years up to 200,000 ducks can be produced in northeastern coastal marshes, while 700,000 may be raised in the southern coastal marshes. Many migratory ducks, geese, and other waterfowl use coastal wetlands as resting stations and feeding grounds during their seasonal movements. Other species, such as bluefish or flounder, may make transient use of marshes for feeding, overwintering, or as nurseries (see page 55). A major portion of the catch of oysters, clams, scallops, eels, alewives, smelt, and other food species is taken directly from salt marshes in this country. In Connecticut, the shellfish industry earned $20 million annually from 1900 to 1920. More recently, the annual income has dropped to $1.5 million, presumably due to reduction of spawning beds. In the small Massachusetts town of Falmouth (population 20,000), where our laboratories are located, shellfish valued at $151,900 were harvested in 1975 from the local marshes and inlets. Shellfish harvests, and therefore marsh conservation, are of importance to the economy of coastal towns. A further property of coastal wetlands is their ability to scrub contaminants from tidal water. Since most marshes are in estuaries or have ground water flowing into them, the ability to retain contaminants may prevent further transport of pollutants to the sea. For example, researchers in Fiji and Hawaii have found that inorganic nitrogen released by a waste treatment plant upstream in a mangrove estuary was reduced by 56 percent by the time the contaminated water entered the sea after passing through mangrove swamps (Table 1). These results are indirect, since river sediments might trap nutrients even if the swamps were not present. We have conducted experiments involving the addition of sewage-based fertilizer to marsh plants in Cape Cod and found that both nutrients and metals were retained (Tables 2, 3). The data showed that the retainment of nutrients was high and depended on the amount added. Retainment of metals varied with the kind of metal but was especially high in the case of copper, iron, and lead. Marshes and mangroves, then, seem to have the ability to retain nutrients and heavy metals. We also have evidence for retention of chlorinated hydrocarbons and perhaps pathogens brought into marshes by contamination. The major component in contaminant retention appears to be the mud, the surfaces of whose finely divided particles provide sites for adsorption. Nitrogen retainment by marshes is especially important, since large inputs of this element can lead to algal blooms in coastal water, as happened in Moriches Bay in Long Island, New York, where agricultural wastes created extreme nitrogen enrichments. Nitrogen entering wetland systems follows two major routes. The mud below the surface layer bears little oxygen and thus favors 14 Table 1. Reduction of nitrate (NO,-N) and ammonium (NH.-N) from a sewage treatment plant after passage through a mangrove estuary. The plant is 2.5 kilometers upstream from the river mouth and the river is lined with mangrove swamps. (After Nedwell, D. B. 1975. Inorganic nitrogen metabolism in a eutrophicated tropical mangrove estuary. Water Res. 9:221-31 ) Amounts of Inorganic Nitrogen (milligram atoms nitrogen per day x 10 ) Output from treatment plant Net export from river Decrease due to passage through mangrove estuary Mean NO3-N 1.30* 0.77 41 30 0.12 0.10 19 NH4-N 0.80 1.20 54 63 1.10 0.42 72 DIN** 2.10 1.20 43 56 1.41 0.42 69 The two values are from two separate sampling dates, ic Dissolved inorganic nitrogen. Apparently nitrite (NO-,-N) was very low and was not considered. Table 2. Retention of ammonium (NH.-N) and phosphate (PO.-P) by salt marsh plots. The nutrients were provided at two dosages of sewage- based fertilizer. Amount of fertilizer added (grams per square meter per week) Percentage of added nutrients retained in marsh NH4-N P04-P Plot 1 25.2 79.7 91.1 Plot 2 8.4 95.6 84.1 Table 3. Retention of metals by two kinds of salt marsh habitats within experimental plots. The metals were provided by experimental additions of sewage-based fertilizer. Low marsh is dominated by Spartina alterniflora, while S. patens and Distichlis spicata are the main grasses in high marsh. (Data compiled by A. Bourg) Percentage of added metal retained by marsh Metal Low marsh High marsh Copper 60 100 Iron 80 100 Lead 55 100 Manganese 55 60 Nickel 45 65 Zinc 20 45 Cadmium 20 35 Chromium 20 50 15 the activity of bacteria that convert inorganic nitrogen into nitrogen gas. This process of denitrification is probably the primary way that eutrophic waters are scrubbed of nitrogen by liberation of nitrogen gas into the atmosphere. The second major pathway for the remaining nitrogen is into sediments and then plant production. Plant productivity in marshes is among the highest in the world, equalling extensively managed agricultural areas. Since the growth of marsh grasses is limited by the supply of nitrogen, the more nitrogen that is scrubbed from tidal water by the marsh, the more grass can grow. Little of the plant production is consumed by animals while the plants are still alive. Instead, mangrove leaves fall onto the water where tidal flow eventually moves them to deeper waters. In marshes, the grasses die and become food for bacteria and fungi, which in time are consumed by a variety of animals. Reduction in size of the grass particles takes place, facilitating their being flushed out of the marsh. Through this export wetlands provide organic matter for the numerous detritus feeders, such as shell and fin fish found in coastal waters. Given this high concentration of nutrients and organic particles, it is natural that experimental work in estuarine aquaculture of shellfish should be undertaken. Initial systems employing dead mangrove branches or cut squares of old rubber tires threaded on a line as settlement surfaces for oysters work well within mangrove swamps in Cuba and Puerto Rico. Marketable shellfish can be harvested within 6 to 8 months, with the advantage of very low technological requirements (Figure 5). The shellfish wardens of Falmouth and neighboring Bourne have had success in rearing oysters and clams in tidal channels within salt marshes. The Cost of Reduction Coastal fisheries, migratory birds, natural treatment of contaminated waters, potential sites for aquacultural development— these are some of the areas affected by reduction of coastal wetlands. Others have to do with stabilization of shorelines, protection and repair from storm damage through natural re-establishment of marsh plants-even the possibility, borne out by our retainment studies, that marshes could be incorporated in tertiary treatment systems for disposal of sewage wastes. We have not discussed the aesthetic aspects of wetlands, since these intangibles are difficult to gauge in the consideration of the value of marshes. Prosaic dollars-and-cents arguments are likely to be more convincing. Attempts to put a price on a unit of marshland have been made, using some of the wetland functions mentioned earlier. Based on the fisheries, waste treatment properties, and aquaculture potential, researchers in Georgia came up with a very speculative annual return of $4150 per acre of marshland. This more or less means that in the absence of marshes, society would have to provide that amount of money to accomplish what the marshes are actually doing. It is very hard, however, to make the economic argument for wetlands compelling, since coastal wetlands can rarely compete with other economic incentives. For example, in the Hackensack Meadowlands of New Jersey, one acre of marsh may be worth $100,000 to industrial developers. Similarly, the Waddensea wetland cannot offset the massive economic and political interests involved in oil production and transport. Yet other social values must enter the equation. Onshore jobs related to coastal fisheries or ways of life based on the yield of wetlands are examples. Others are the aesthetic benefit of open greenbelts in a crowded world and the irreplaceable nature of a food- producing wetland in a time of costly and scarce food supply. Perhaps we are arriving at a time when these factors should be recognized and considered in the management of resources. In most coastal areas of the United States, wetlands can be owned by individuals, yet the functions and properties of wetlands-and their importance to society-are such that common ownership and management for the common good Figure 5. A red mangrove f Rhizophora mangle,/ swamp in Sierra Leone. Tlie tidal range is shown by the heig/it of oysters /'Ostrea tulipay settled on the roots and trunks of the trees. (A. G. Humes) 16 Sidney E. King's painting of Jamestown, Virginia, as it may have appeared a few years after settlement and before widespread filling and the disappearance of wetlands. (Courtesy National Park Service) would seem to make better sense. It is hard to see how the average individual owner of a parcel of wetland, if he behaves in an economically rational fashion, could be interested in marsh conservation. Except for aesthetic aspects, the owner's main interest would be to build his dock or house or to sell to a developer, not the welfare of coastal fisheries. Clearly, there are exceptions. Many owners of private marshland do work to maintain the integrity of their property. Certainly, conservation easements, such as those enacted in Massachusetts, are helpful in redressing the economic disadvantage of maintaining wetlands in their original state. We are merely arguing that the political and economic system in the United States does not naturally provide incentives for conservation. Our experience in Massachusetts, a state with very good wetland laws, is that the priority of private ownership is so established that little can be done against the parcel- by-parcel destruction of marshlands. The one sure solution seems to be municipal or state purchase of wetlands. Of course, this runs headlong into political and economic interests and is an issue that can and should be decided by the voting public. In the tropics mangroves are faring worse than temperate latitude marshes. In most less developed countries, little room is allowed for long- term considerations, which by necessity take second place to attempts to improve the local economy. Awareness of environmental problems seems to be rising in less developed countries, however. In time, reasonable management policies, comprising conservation as well as aquacultural and other uses, may appear. It is not clear, though, that such measures will be enacted before wetlands have largely disappeared. Ivan Valiela is an associate professor and Susan Vince is a graduate student in the Boston University Marine Program, Marine Biological laboratory. Woods Hole. Suggested Readings Lugo, A. E., and S. C. Snedaker. 1974. The ecology of mangroves. Ann. Rev. Ecol. and Syst. 5:39-64. MacNae, W. 1968. A general account of the fauna and flora of mangrove swamps and forests of the Indo-West Pacific region. Adv. Mar. Biol. 6:73-270. Teal, J., and M. Teal. 1969. Life and death of the salt marsh. Boston: Atlantic-Little, Brown. 17 Pollution History of Estuarine Sediments by Edward D. Goldberg ~ -•« vi' Industrial discharge in 1973 polluted this salt marsh at Middleton, Rhode Island. The law now requires every industry to obtain a permit regulating its discharges. (Hope Alexander / Courtesy Environmental Protection Agency/ Docutneria) Many forces can change the structure and composition of the estuarine environment. In addition to natural events like hurricanes, earthquakes, and typhoons, there are the agricultural, social, and industrial activities of man that can introduce new substances to this system where river and ocean waters mix. Some of these substances, or pollutants, are accommodated in the sediments; others are transported to the open ocean. The flow rates of the pollutants entering the estuary may be recorded in its deposits, and such records may be of great use to those responsible for the management of this marine resource. In addition to describing present-day and past entries of these materials to an estuary, in principle we may be able to predict future ones with a knowledge of present and past productions and estimates of future production, for a given pollutant. What are the characteristics of pollutants that may be recorded in estuarine sediments? First of all, the material must have a persistence in the environment. Radioactive substances with short half-lives, organic materials easily degraded by microbial action, or chemically unstable molecules may leave no record of their presence in the coastal system. On the other hand, stable materials, in either dissolved or particle form, that are rapidly removed to the sediments provide favorable case studies. Estuaries display high levels of biological activity, so that pollutants may be transported to 18 the sea floor in fecal matter, organic remains of organisms, and skeletal materials. Thus, highly reactive elements like lead, copper, and zinc, and materials taken up by living organisms like DDT, PCBs, and petroleum components, are examples of pollutants whose historical entries to estuaries have been measured. Not all sediments faithfully record the flow rates of pollutants. Those low in organic matter may maintain in their upper strata a community of organisms whose burrowing activities can distort or perhaps even destroy the pollution record. On the other hand, deposits with high levels of organic matter, which are often lacking in dissolved oxygen (anoxic) as a result of the oxidation of organic materials by bacteria, do not support such burrowing organisms and can therefore provide a reliable pollution history. One indication of the absence of animal activity (bioturbation) is the presence of annual or semiannual sediment layers of different compositions; that is, more plant remains enter the sediments in the spring blooms than during the less productive seasons of the year. Geochronology In order to ascertain the amount of a pollutant entering a deposit per unit time, the age of the strata (its geochronology) must be determined. Time assignments can be accomplished in several ways, including radiometry, stratigraphy, and physical evidence from dated events such as hurricanes and typhoons. The reliability of age assignments can be enhanced by the use of two independent techniques of geochronology, where possible. The natural radioactive nuclide most frequently used in dating estuarine sediments is lead-210 (Pb-210). Its half-life of 22.4 years allows periods of a century or so to be studied in the sedimentary column inasmuch as its decay over four such half-lives can usually be measured. Pb-210 is separated from its parent in the uranium-238 series, radium-226 (Ra-226), through diffusion of the rare gas nuclide radon-222 (Rn-222) from crustal rocks to the atmosphere, or through sorption on solid phases precipitating to the sediments without significant amounts of Ra-226. In the former case the Pb-210 is brought to the marine system in rain or with dry fallout. Radionuclides produced by man as a consequence of nuclear weapons testing have been used successfully to place time frames in estuarine sediments. Radioactive debris such as the fission products strontium-90 and cesium-137, induced activities such as cobalt-60 and zinc-65, and fuel remnants such as the transuranics plutonium-239 and americium-241 can in principle be utilized (Figure 1). This radioactivity is introduced in large part to the stratosphere from surface and atmospheric tests where it remains for about a year before returning to the earth's surface. Following periods of intense testing in the late 1950s and early 1960s, fallout maxima of radioactive debris were observed in 1959 and 1963. Estuaries received direct fallout of this radioactivity as well as that which fell on the earth's crustal rocks and soils and was subsequently remobilized to the marine environment by winds or rivers. Thus, depth profiles of plutonium, for example, in estuarine sediments often show increasing amounts in strata deposited from 1954 to the present time. The first measurable quantities are usually in the 1954 200 100 1954 1959 1964 1969 1974 CT> ^£. \ e Q. •o O V (\J 8 Q 8 12 16 20 24 28 32 36 -- I960 -- 1950 --— 1940 -- 1920 -- 1900 -- 1800 Santa Monica Basin Figure 3. Lead concentrations in sediments from the Santa Monica Basin off Southern California. Increased levels appear in strata deposited after 1940. (After Chow et ai, 1973. Copyright 1973 by the American Association for the Advancement of Science) (Figure 3) as well as by the isotopic composition of the lead. This anthropogenic lead first became evident in the 1940s and has a unique isotopic composition compared to that of lead deposited before its extensive use as a gasoline additive. Studies on the fluxes to basins as a function of distance from the coastline show a fallout with respect to the square of the radius, indicating the intensive impacts are nearshore. Hurricane Agnes in Chesapeake Bay Of the larger estuaries on the eastern coast of the United States, Chesapeake Bay is one of the most frequently studied because it is greatly affected by millions of residents of Maryland, Delaware, Virginia, and the District of Columbia. In addition, the tropical storm Agnes caused more sediment to be discharged during one week in June 1972 than during the past several decades (Schubel, 1974). Is hurricane Agnes recorded in the sediments? Did this catastrophe destroy or alter the pollution records in Chesapeake sediments? In collaboration with scientists from Johns Hopkins University, we have sought answers to these questions through an examination of four box cores to depths of 70 centimeters taken in 1975 (Figure 4). Two samples were taken near the outfall of the Susquehanna River: one, close to the outfall of the Potomac River; and one, midway between the two discharge areas. Through attempts to establish time frames for these deposits and through chemical, mineralogical, and physical analyses of the sediments, natural and pollution phenomena were found to be recorded. The first 20 to 30 centimeters of the two southerly cores and the entire lengths of the two northerly cores were essentially uniform in all properties. There were no decreases in the concentrations of the radioactive species, like Pb-210, which should have shown an exponential decrease with depth, had there been continuous accumulations. Herein is the record of the hurricane. On the other hand, the two southerly cores, at depths from about 20 to approximately 70 centimeters, did have time frames that could be developed with Pb-210 or Pu-239+240 geochronologies. In these cases, there were increases over the past several decades in the concentrations of such heavy-metal pollutants as lead, copper, zinc, and nickel. The anoxic nature of the cores had preserved the pollution history. Perhaps of even greater interest was the observation that the two southerly cores recorded different intensities of pollution, presumably reflecting entries from two different river systems. The sediment adjacent to the Potomac received smaller anthropogenic fluxes of heavy metals, as well as their natural fluxes, than did the more northerly deposit, which received its solids primarily from the Susquehanna. In addition, the intensities of the fluxes were less than those from the Susquehanna, which is in accord with the sense that the Susquehanna delivers around 90 percent of the sedimentary solids to Chesapeake Bay. Records of Fossil Fuel Combustion The amounts of material released to the environment from the combustion of fossil fuels, coal, oils, and lignite, are comparable to those transported in rivers during the weathering processes. Burning introduces the substances to the atmosphere, where washout and dry fallout processes can return them to the earth's surface. Such elements as arsenic, mercury, cadmium, tin, antimony, lead, zinc, thallium, silver, and bismuth, whose compounds are often relatively volatile, may be preferentially mobilized. Since the principal sites of fossil fuel combustion are in the Northern 22 38° - 37° - Figure 4. Locations of four box cores taken in Chesapeake Bay in 1975. Hemisphere, the impacts on coastal marine waters are most evident at these latitudes. Scientists in Germany (Erlenkeuser et al., 1974) have studied the sediments of the western Baltic coast and have found that cadmium, lead, zinc, and copper are enriched in sediments recently deposited. Concentrations of other metals such as iron, manganese, nickel, and cobalt are unchanged throughout the lengths of the cores. The time frame in these deposits was introduced with carbon-14. The enrichment of heavy metals in the sediments is associated with coal burning (coal combustion potentially mobilizes 70-200 times more of these metals than oil burning) on the basis of the relative contents of the metals in coal and in sediments and on the amounts of coal burned. The measured values in the upper levels of the sediment corresponded to a mixture of 93 percent sediment and 7 percent coal ash, where the unaltered composition of the sedimentary components was based on those of prehistoric strata. The anthropogenic input of heavy metals then corresponded to the European coal production, and presumably utilization, over the past several centuries (Figure 5). Records of Industrial Activity The metal composition in sediments of the Puget Sound estuarine system have been perturbed by two major human activities: a copper smelter and a chlor-alkali plant. The former discharged large amounts of arsenic and antimony; the latter, substantial quantities of mercury. Where natural levels of arsenic ranged between 3 and 15 parts per million and of antimony between 0.3 and 1.0 parts per million in the deep sediment, the values in contaminated surface strata were as high as 10,000 parts per million. On the basis of the geographical distributions of these materials, transport was attributed to both wind and water (Creceliuset al., 1975). The mercury levels of the surface sediments adjacent to the chlor-alkali plant were elevated a thousandfold over the natural background values during the years in which the element was discharged from this industrial activity. The pollutant and natural mercury are associated with easily oxidizable organic matter. On the other hand, naturally occurring arsenic and antimony are bound generally to extractable iron and aluminum compounds, whereas these two elements in polluted zones are nonextractable and appear bound in rather stable chemical forms. Thus, whereas arsenic and antimony mobilized by man are relatively inert in the deposits, the mercury can be mobilized in European coal production I06 tons/y 200 400 600 800 1000 Anthropogenic input of heavy metals mg/m^/y 0 40 80 120 160 1950- 1900- 1850- 1800- 1750-1 Cool Figure 5. European coal production compared with the anthropogenic flow of heavy metals into Baltic Sea sediments. (After Erlenkeuser et al., 1974) 23 principle by changes in the oxidation-reduction conditions. DDT and PCBs Sewage outfalls can deliver to the coastal environments toxic substances that can alter the makeup of plant and animal communities. For example, until 1972 fairly large quantities of DDT were released into the Los Angeles municipal-waste system. Horn and his colleagues (1974) estimated that 19 metric tons were discharged in 1971. Atmospheric fallout and agricultural runoff were estimated to add several metric tons per year. The history of these releases is well recorded in the coastal sediments, where measurable quantities of DDT degradation products appear in the strata deposited after 1952 (Figure 6). Similarly, the industrially used polychlorinated biphenyls (PCBs) have entered coastal waters primarily via sewer .c 'tt> ^ 150 T3 0) JC Q "o 100 Q J o Q. d 2 50 o> Q. in i_ o 3 0 Background, lOppb 1945 1950 1955 I960 1965 Year o> a> ^ 150 T3 0) co «- 100 o ° Q- O S. 50 o 3 o Background, 30 ppb 1920 1935 1940 1955 I960 Year Figure 6. Deposition of DDE and PCB in dated sediments of the Santa Barbara Basin. (After Horn et al., 1974. Copyright 1974 by the American Association for the Advancement of Science) outfalls (for 1970-71, 10 metric tons per year) as compared to surface runoff (0.25 metric tons per year for the same time period). PCBs were first noted in the 1945 strata. These inputs have been associated with the population decreases of the California brown pelican off Anacapa Island and with reproductive failures in sea lions through premature births and abortions. The victimized birds laid eggs with thin shells that were liable to fracture and breakage. Many of the eggs collapsed during the incubation period. High levels of DDT and its metabolites were found in the birds. Similarly, there were enrichments of both DDT, and its metabolites, and PCBs in the premature sea lion pups and their mothers. However, there were elevated concentrations of other chemicals, like cadmium, bromine, selenium, and mercury, that may have contributed to the situation. Possibly, the problem had natural, not man-instigated, causes. Since the early 1970s, inputs of DDT and PCBs have been markedly decreased due to restrictions on their use. Radioactive Wastes The management of radioactive wastes from nuclear reactors and from nuclear reprocessing plants is a major concern in most national energy programs. Since many installations are located in estuarine areas and discharge low-level wastes to the coastal waters, an understanding of the behavior of artificial radionuclides is essential (Oceanus, Fall 1974). The transuranic elements (all elements heavier than uranium), especially plutonium, have been the object of several recent studies, primarily because of the question of their effects on public health. The most extensive investigations have been carried out in the area of the northeast Irish Sea where there are nuclear installations, including a reprocessing plant at Windscale, England. Measurements of plutonium in seawater, human foods (such as fish and shellfish, and the seaweed Porphyra), and sediments done by Hetherington and his colleagues (1975) indicate that 96 percent of the introduced plutonium is removed to the sedimentary column in the immediate vicinity of the outfall. This finding establishes the importance of the sedimentary column in the future behaviors of this transuranic. The plutonium concentrations in surface sediments relative to the overlying filtered waters, to distances of slightly over 100 kilometers from the outfall, are related to the mineralogy and the size distributions of the particles. There were no remarkable enrichments of plutonium in the sediments closest to the outfall, which led the investigators to suggest that a large fraction of the 24 •Vi- V. ' , ,n 3PW&* Water from a pulp mill circulating in Bellingham Bay, Puget Sound, Washington, 1973. (Doug Wilson, Courtesy Environmental Protection Agency I Documeria) plutonium is being removed from the vicinity altogether. Concentrations of plutonium in estuarine sediments in the area appeared to decrease exponentially with depth to distances of about 24 centimeters. No time frame was introduced into the sedimentary strata. However, there is a possibility that the sedimentation rate is too low to account for the plutonium at these greater depths. There may be a reworking biologically of these sediments with a resultant downward displacement of plutonium. However, of greater importance is the estimate that a major fraction of the plutonium discharged from the Windscale operation is at present associated with the seabed. This induction is of great importance in accessing the longer-term consequences of the radioactive discharges to man and to marine ecosystems. Fate of Pollutants Not all of the pollutants that enter an estuary are retained in its sediments. Some enter the open ocean. There are only a few investigations that have addressed themselves to the question of what percentage of a given substance entering an estuarine system leaves that system. Perhaps the most detailed study to date is that of Windom (1975) on the Savannah River salt-marsh estuary. Windom compared the fluxes of metals carried into the Savannah system by nine rivers with the amounts precipitating annually to the sediments. The differences were taken to be the metal inputs to the open ocean system. Of the five metals studied- iron, manganese, cadmium, copper, and mercury— the last three could have entered the estuary as a consequence of man's activities. Essentially all of the dissolved iron precipitated in the estuary, while practically all of the dissolved copper and cadmium went to the open ocean. The net flux of dissolved mercury through the estuary was greater than its river input, suggesting that there must have been some release of the metal that was adsorbed to the particulate matter. The particulate matter, carried by the rivers, and its adsorbed materials stayed within the estuary. Although this conclusion may be specific only to this rather unusual salt-marsh system, the general method of determining which substances the Savannah estuary retains, and in what amounts, may be applicable to other areas. 25 Conclusion The zone where the river meets the ocean is one of man's most important resources. And yet human activities are changing the form and composition of estuarine sediments. The history of some of these changes is recorded in the sediments. Challenges to or permutations in the integrity of marine ecosystems may be more subtle and hence difficult to detect. Work from the CEPEX project (Controlled Ecosystem Pollution Experiment), sponsored by the U.S. Office of the International Decade of Ocean Exploration (Oceanus, Fall 1974), suggests that there may be a predictable succession of species in a community subjected to an environmental stress, be it copper, mercury, or petroleum. There is usually a weakest species, the first to fall victim to the alteration of the environment. This situation has also been observed in terrestrial communities. Estuaries adjacent to industrialized societies are subjected to a variety of stresses— in sewer effluents, industrial discharges, atmospheric fallout, dumping, and accidental releases from ships. If it is the case that many stresses act in a similar way on a population, it is conceivable that serious alterations to community structure can take place. Such permutations may even extend to the adjoining coastal oceans. Clearly, the possibility of such an occurrence requires serious assessment. The estuarine zone has become more and more attractive to marine chemists, because it is the site of some of the most dramatic chemical reactions in the ocean system. Many of these reactions are a consequence of river-ocean interaction or of the often high biological productivity. Each estuary probably is a chemical entity unto itself, unique because of the particular terrestrial and marine chemistries that give rise to it. Man's alterations, as well as natural processes, deserve further study in order to manage properly these important zones. Edward D. Goldberg is professor of chemistry at Scripps Institution of Oceanography, University of California at San Diego. References Chow, T. J., K. W. Bruland, K. Bertine, A. Soutar, M. Koide, and E. D. Goldberg. 1973. Lead pollution: records in Southern California coastal sediments. Science 181:551-52. Crecelius, E. A., M. H. Bothner, and R. Carpenter. 1975. Geochemistries of arsenic, antimony, mercury, and related elements in sediments of Puget Sound. Environ. Sci. and Technol. 9:325-33. Erlenkeuser, H., E. Suess, and H. Wilkomm. 1974. Industrialization affects heavy metal and carbon isotope concentrations in recent Baltic Sea sediments. Geochim. Cosmochim. Acta 38:832^2. Hetherington, J. A., D. F. Jefferies, and M. B. Lovett. 1975. Some investigation into the behavior of plutonium in the marine environment. In Impacts of nuclear releases into the aquatic environment, pp. 193-212. Vienna: International Atomic Energy Agency. Horn, W., R. W. Risebrough, A. Soutar, and D. R. Young. 1974. Deposition of DDE and polychlorinated biphenyls in dated sediments of the Santa Barbara Basin. Science 184:1197-99. Huntzicker, J. J., S. K. Friedlander, and C. I. Davidson. 1975. Material balance for automobile-emitted lead in Los Angeles Basin. Environ. Sci. and Technol. 9:448-57. Schubel, J. R. 1974. Effects of tropical storm Agnes on the suspended solids of the northern Chesapeake Bay. In Suspended solids in water, ed. R. J. Gibbs, pp. 113-32. New York: Plenum. Windom, H. L. 1975. Heavy metal fluxes through salt-marsh estuaries. In Estuarine Research, ed. L. E. Cronin, vol. 1, pp. 137-52. New York: Academic Press. 26 Barrier Beaches of the East Coast by Paul J. Godfrey 1 A critical part of many estuarine systems is the long finger of land that makes up the seaward boundary of the estuary and protects it from the direct onslaught of the sea. These low-lying strips of land are barrier beaches. The usual definition of an estuary is a zone of transition where fresh water flowing to the ocean mixes with salt water. The seaward end of the estuary is often a large bay, sound, or lagoon connected to the sea by means of inlets, particularly on the East Coast of North America. The land on either side of the inlet is a barrier beach. The existence of most estuaries depends on the presence of barrier beaches on the ocean side. This is particularly true for bay-or-lagoon type estuaries. These beaches, as the name implies, protect the estuarine environment from waves and currents that would otherwise demolish the delicate ecological mechanisms in the waters behind the barrier. What happens on the barrier beach can have a direct effect on the conditions in the estuary. Barrier beaches are dynamic, continually changing structures; under present conditions, they are migrating landward. The mechanisms of migration Aerial view of Cape Lookout National Seashore, North Carolina, with its diamond patterned lighthouse. This marks the southern end of the Outer Banks, one of the most extensive barrier island systems in the world. Core Banks sweeps northeastward to Cape Hatteras-beyond the horizon; Barden's Inlet separates Core Banks from Shackle ford Banks on the left. The shallow and highly productive estuarine waters of Core Sound separate the Banks from the mainland. The eastward migration of Barden's Inlet, artificially maintained by dredging, is threatening the survival of the lighthouse, a National Historical Landmark, built in 1859. 27 GLACIAL DEPOSITS Figure 1. A Northeastern barrier spit created by the erosion of glacial deposits and the transport of sand by littoral drift from right to left. Behind is a lagoon and river estuary draining to the sea through the inlet. *»•* r\0* Figure 2. Barrier islands and inlets found along the central and southeastern United States coast. have an effect on the aquatic environments behind the barrier, the places into which the barrier beach moves. Nearly all estuarine systems have barrier beaches associated with them. Only the very large estuaries, such as the Chesapeake and Delaware systems, lack barriers where they open into the ocean; and even the large systems have barrier beaches along portions of the estuary. Barrier beaches are rather common structures wherever waves erode sediments and the waves are of sufficient force to carry these sediments along a beach to a place of deposition. Barriers are therefore the product of wave action and erosion in one region and deposition in another. They also are the result of storm, tides, and wind— all features of the land/ocean interface. In addition to protecting estuaries, the barrier beach system is of considerable importance to the whole coastline, particularly if the coast is low. The barriers protect the mainland from severe wave attack during storms, and slow down the floods from storm surges. They create protected harbors where coastal towns can develop. The survival of estuaries depends, at least in part, on the survival and natural functioning of barrier beaches. Already, many beaches have been so modified by development that they can no longer respond to the oceanic forces that dictate the terms of survival. It is ironic that the very section of lagoonal estuary most critical to survival is that portion which is often the first to be developed— the ocean beach. Various conservation programs are underway to halt, or at least slow down, the indiscriminate development that has marred so many barrier beaches. Structure of a Barrier Beach The term "barrier beach" includes barrier spits and barrier islands. A barrier spit (Figure 1) is a long finger of land, consisting of an ocean beach, dunes, and a marsh on the back side. It extends out from the end of a shoreline, which was the original source of sediments making up the spit. In most cases, the spit will still be attached to the source. It grows out across the mouth of the estuary or bay as waves erode the sediments on the updrift side of the bay and the sand is transported in the general direction of the average wave approach by littoral drift. Thus, the spit grows downdrift: the direction of the growth depends on the average wave angle. Along most of the East Coast, this direction is either south or west. If a spit is breached by an inlet, or series of inlets, then barrier islands are formed. The barrier island is usually much like a spit, although in some cases much larger and wider (Figure 2). But not all barrier islands are derived from spits. It is important to note major distinctions between various types of barrier islands. Large barrier islands have spits flanking either end of the island and extending into the inlets. The physiographic structure of a barrier consists of intertidal ocean beaches, dunes, barrier flats, intertidal salt marshes and flats (Figure 3). The arrangement of these parts is fairly predictable, although the exact proportion of one part compared to another can vary considerably. Some barriers have very wide beaches, others very narrow. Certain barriers consist mostly of sand flats and a few dunes, while others are mainly composed of dunes. The intertidal areas behind a barrier may feature extensive marshes while on others the marshes may be greatly reduced. In general, the zones of a 28 "typical barrier beach" can be described as follows: Intertidal Ocean Beach What most people call "the beach" is the highly variable sand and gravel structure on which ocean waves lose their energy. It is a habitat in which the sand is constantly in motion and is the first line of defense of the estuarine barrier. The ocean beach consists of three parts: the nearshore, foreshore, and backshore. The nearshore contains two sand ridges that function in the sand budget of the beach: the inner bar, not exposed at low tide, and the "ridge"' that is exposed. Between each rise are depressions— the "trough" between the inner bar and the ridge, and the "runnel" between the ridge and the foreshore. At low tide, the runnel carries wave swash that comes over the ridge back to the deeper water. The beginning of the foreshore is marked by the "step," a wave scarp cut at low tide, and rises to the berm crest, usually the highest part of the beach. The backshore slopes back from the berm crest to the dunes. The shape of the beach and the width of the berm at any given time depend on the wave energy regime. During low energy periods, primarily in summer, the beach is wide and steep as waves move sand from the inner bar to the foreshore. During high wave energy periods, the waves move sand from the beach to deeper water and the beach becomes narrow and slopes gently. This pattern of erosion and deposition is typical of summer and winter cycles, or stormy versus fair weather periods at any time of year. Because the beach is in nearly constant motion, sessile plants and animals cannot survive in this habitat, particularly when wave energies are high. Instead, beach organisms are adapted to constantly changing conditions and consist of animals that can roll around in the sand and rebury themselves, such as mole crabs and coquina clams. A distinct community of microscopic animals and unicellular algae lives between the grains of sand on the beach. Bacteria play a major role in the beach ecosystem by breaking down the vast quantities of detritus that wash ashore, mostly algae from the sea and eelgrass and cord grass from the estuaries behind the beach. Drift lines deposited on the beach by high tides are often precursors of new dune lines in addition to providing organic detritus for the beach ecosystem. Fragments of dune plants washed ashore with the drift are buried by blowing sand and regenerate the following spring. Besides beach grass and dusty miller, which are perennials, seeds of annual plants wash ashore and germinate on the beach, where they survive until a storm washes them away or they reach the end of the growing season. Dunes The dunes on a barrier beach are the estuary's primary defense against flooding by storm- driven tides, although a certain amount of this flooding is beneficial. The k'ey to extensive dune LSI' LAGOON SEDIMENTS DUNE DRIFT BEDDING LINES ^•*-ii*-^ STEP SHOREFACE SEDIMENTS Figure 3. Tfie basic physiographic and ecological zones of a typical barrier beach. 77/ SALT MARSHES 100m Figure 5. Evidence ofoverwash retreat on Core Bank, N. C. Figure 5a: Aerial view of the Codds Creek area in April, 1976, showing the berm with scattered dunes, the predominance of overwash fans, barrier flats with grasslands and scattered shrub thickets that grade into the high marsh (light tones), and the low salt marsh (dark tones). Figure 5b: Map of changes at Codds Creek taken from aerial photographs of 1 939 and 1 96 7. Overwashes during the late 1 950s and early 1960s filled in areas that were once marsh and open water, and portions of the fill became highly productive salt marshes. 32 Formation of Salt Marshes Salt marshes can form behind barrier beaches in four ways: invasion of uplands as the sea level rises, sediment buildup from river outflow, overwash, and inlet dynamics. Of these four, the most common processes along most of the East Coast are overwash and inlet dynamics, the latter being most significant in many areas. If a major overwash moves sand completely across a barrier beach into the lagoon, the sediment dropped into the water will soon be colonized by a new salt marsh. In some cases, older marshes are buried by the sand, but new and often more productive marshes form at the lagoon's edge (Figure 5). As a spit grows, the early stages are dominated by overwashes that regularly move sand from the ocean to the bay. The sand brought in from the beach is colonized by Spartina grass and the growing marsh follows the spit. Once dunes start to form, the overwash slows down. This process eventually becomes a relatively rare event until the dunes begin breaking down from retreat of the shoreline. Major overwashes that occur after a barrier system is created move additional sediment to the backside and a fringe of marsh grows into the bay. Of considerable importance to the barrier/ estuary system are the inlets that provide a means for marine organisms to move in and out of the estuary and for organic matter produced in the estuary to move seaward. The inlets also are a sediment trap. Sand moving along the beach is carried into an inlet where it is deposited in large shoals. When the inlet eventually closes, as most temporary storm created inlets do, these shoals, depending on their elevation, become salt marshes or underwater grass beds: these communities thus occupy large areas of formerly open water (Figure 6). A look at marsh islands behind barrier beaches all along the East Coast shows the pattern of flood tide deltas that mark the sites of old' inlets. As inlets open and close, the development of new marshes and the erosion of older marshes along the sides of the inlet become part of the mechanism that rejuvenates aging marsh systems behind many barriers. In some cases, salt marshes are absent from the bay side of the barrier. The reasons for these conditions are unclear but seem related to distance from an inlet, recent overwash, wave conditions, or drastic changes in bay salinity. There is evidence that marshes behind barrier beaches, especially older ones, can be eroded away by continual wave action in a large bay. Such sites of erosion are invariably a long way from existing or Shoals, potential surface for marsh or underwater vegetation salt marsh grassland dunes berm 1 Km Inlet position before closure, January 1971 Inlet position October 1958 Figure 6. Changes at Drum Inlet, North Carolina, showing formation of new salt marshes on tidal deltas. Cores s/?ow the composition of a shoal and the presence of beach shelf from littoral drift into the inlet. (After Godfrey and Godfrey, 1 974) 33 SAND DUNES INTERTIDAL MARSH HIGH MARSH UPLANDS Figure 7. Sandy Neck, Barnstable, Mass., showing the growth of a spit across an open bay with subsequent marsh development behind. If breached by an inlet, such a barrier could become an island. even recent inlets. Also, sections protected by a major natural or artificial dune system that has stopped overwash often lack marshes on the bay side, or have very old, eroding high marshes. In these cases, excessive protection from inlet formation and overwash is detrimental to the over- all estuarine system just as would be too frequent overwashing or inlet formation. Some barrier beaches are so open to wave attack on their back side that marshes cannot develop in the open water and only appear where protected by small barrier beaches attached to the main barrier. Monomoy Island in Massachusetts is a good example of this condition. Some lagoons, such as Laguna Madre behind Padre Island in Texas, develop such high salinities that even the salt tolerant Spartinas cannot grow. In all these cases, the back side of the barrier consists of open sand beaches. Formation of Barrier Beaches The formation of barriers and the estuaries they protect is the subject of considerable controversy among coastal geologists. The two most commonly discussed theories are the spit theory and the drowned beach ridge theory. A third idea combines the two into a sensible compromise. A fourth relates to the reworking of sediments brought down to the sea by rivers. The spit method calls for the growth of a long sand finger off a headland of eroding sediments. As the headland erodes, the spit grows across open water, enclosing a lagoon that develops estuarine conditions: Nauset Beach and Sandy Neck on Cape Cod are good examples of spits (Figure 7). The spit keeps growing in the direction of the littoral drift as long as the source of sand holds out and the spit does not strike land on the other side. In some cases, barrier spits have grown completely across a lagoon and cut it off from the sea, although inlets can form periodically and result in some drainage to the ocean. Such completely closed lagoons of often rather fresh water are sometimes called "salt ponds" and are common features on Nantucket and Martha's Vineyard in Massachusetts. When a spit is breached by inlets, it becomes a barrier island or a series of islands. Monomoy Island is a good example of a barrier spit that became a barrier island when breached by an inlet. Fire Island in New York appears to have developed, at least initially, by the growth of a spit from glacial deposits to the east on Long Island. As the shoreline retreats with the rising sea, so, too, do the spits, even though they are being supplied with sand from an eroding source. According to the second theory (Figure 8), dune ridges formed along the shore 5 to 6000 years ago as a result of erosion of coastal plain and river sediments, during a time when sea level rise had slowed down from a previously rapid climb following the melting of the ice sheets. Evidence suggests that this slower rate of rise has continued to the present (about 0.3 meters per 100 years). Once the dune ridges formed on what was then part of the mainland, the lowlands behind were flooded and became the broad, shallow lagoons that characterize our southeast coast. With the continuously rising sea, the dune ridges were pushed back to their present positions and are still retreating. These dune ridges became the barrier islands we see today, although they have been reworked many times. The evidence to support this theory comes from sediments beneath the lagoons, which are like mainland deposits rather than marine (as they would be behind spits that grew across open water) and from 34 Figure 8. 77z43~ in sediment ( g PC>43~/g dry sediment) n 0 0.72 + 0.03 -1.0 7 0.5 0.73 t 0.02 -0.4 4 1.0 0.90 + 0.07 +0.6 8 2.5 0.89 + 0.05 +7.6 8 4.3 0.87 + 0.002 + 11.0 8 8.4 1.61 ± 0.22 +30.9 3 43 60 • Figure 4. The distribution of total phosphorus and oxygen in a section along the axis of the Gulf of Venezuela (the depth is in meters). (After Red field et al, 1 963) poorly understood but has the potential of being much more important than the physical-chemical processes. One way this process operates is by the filtering action of molluscs, such as clams and oysters. These can filter clay particles from the water and produce pellets and flakes that then behave like sand grains. Other particulate matter, and its associated nutrients, is also removed at such a rapid rate that an oyster bed can completely cover itself with deposited sediment in 36 days. This is eight times the amount of sediment deposited by gravity alone. Oysters filter a surprisingly large quantity of water each week; estimates vary from 70 to 300 liters. Clearly oysters and other filter feeders in estuaries can completely process the whole volume of the estuary in a few days or weeks. Another biodeposition process is the trapping of particulate matter by large rooted plants. Mangroves provide protection from wind and currents so that particles rapidly settle out. Marshes also act as giant filters of particulate matter both mechanically and as sites for many filter feeders (for example, mussels). In addition, the marsh vegetation stabilizes the sediments so that they are not resuspended at every tidal change (Odum, 1970). The importance of this process is hard to gauge, but it is known that the onset of severe silting of many English harbors coincided with the filling and diking of the marshes. Large amounts of nutrients are taken up by the photosynthetic organisms of estuaries such as the rooted plants, the attached algae, the phytoplankton algae, and the sediment algae. The most easily seen of these are the large plants such as the salt marsh grass Spartina and the eelgrass Zostera. These plants take up nutrients only from the sediments and thus do not compete with the algae directly. However, they do create conditions favorable for biodeposition and also tie up a tremendous amount of nutrients in the plant tissue. Thus, the annual production by Spartina of 700 grams of carbon per square meter in a Georgia salt marsh will incorporate 1 1 grams of nitrogen per square meter and 2 grams of phosphorus. The underground roots and rhizomes may contain twice this amount. Other estuaries, such as that near Beaufort, North Carolina (Table 3), will have the submerged plants as the main producer (here it is eelgrass). Phytoplankton algae are not abundant in many estuaries because of rapid flushing and the turbid waters. Yet they may be the most important food for zooplankton and invertebrate larvae. In very large estuaries, such as Chesapeake Bay, there is adequate time for these algae to develop, and annual primary production may reach several hundreds of grams of carbon per square meter. All the algae in an estuary are extremely effective in removing nutrients, even from nutrient- poor water. Their main competitors may well be the bacteria, which need to remove both nitrogen and phosphorus from the water in order to decompose organic matter from higher plants. In one experiment (Thayer, 1974), where bacteria and algae competed for nutrients in the presence of chopped Spartina, the bacteria won and prevented algal growth. Another test (W. G. Harrison, personal communication) showed that most of the phosphorus was taken up by organisms, likely bacteria, that passed through a 1 micron filter. Nitrate and ammonia, on the other hand, were taken up mostly by larger organisms. Nutrient Cycling A molecule of nitrogen or phosphorus may pass through organisms and back into a pool of inorganic ions a number of times before leaving the estuary. Table 3. Organic carbon production (grams of carbon per square meter per year) in salt marshes and adjacent estuaries at Sapelo Island, Georgia, and near Beaufort, North Carolina. (After Cooper, 1974; Williams, 1973) Georgia salt marsh (g c/m2/yr) Beaufort shallow estuary (g c/m2/yr) Salt marsh 700 256 Submerged plants Attached micro algae — 300 75 Mud algae 420 Phytoplankton 66 44 Water Paniculate 14,000 Phosphate 19,000 Dissolved Organic 6,000 39,000 Particulate Phosphate ° Food " Modiolus Population Pseudofeces 4700 Body Shell Liquor 25,000 I 1,000 1,200 37,200 Mortality 21 Gametes 1 1 Dissolved Organic 23 Phosphate 260 Feces 460 Figure 5. Diagram of phosphorus flow through the mussel population. Values for the water and the mussel population are in micrograms of phosphorus per square meter per day. Ttie flux rates of phosphorus in food and pseudofeces are calculated values necessary to balance the other, measured flux rates. (After Kuenzler, 1 961) Tliis cycling may actually control the rate of primary production to a greater extent than the absolute concentrations of nitrogen or phosphorus. A detailed study of the quantity and movement of phosphorus through a population of mussels (Modiolus) in a salt marsh (Figure 5) revealed that the animals actually cycled as much phosphorus as did the plants. The mass of the mussels, however, was miniscule compared to that of the plants. The major ecological importance of the mussels in the nutrient cycle was to remove large amounts of particulate matter from the water and to deposit most of this on the marsh surface as pseudofeces. Plants with roots appear to take up nutrients from the sediments and release some of them to the water. The movements of phosphorus within Zostera, illustrated in Figure 6, are complex, and phosphorus appears to move in all possible directions. Much of the phosphorus that is taken into the plant by the roots is lost from the leaves. This "phosphorus pump" can provide all of the phosphorus needed by the phytoplankton in relatively shallow estuaries. Nitrogen also cycles in estuaries, but we know only a few details. In part, this is a result of a lack of a suitable tool-there is no usable radioactive isotope of nitrogen. The over-all picture is a decrease in the inorganic forms of nitrogen Seawater (25jjg P/liter) 5.41 I 8.61 7.22 Leaves I 48- 6.89 I 1.39 0.66 Roots a Rhizomes 1.31 - 820 I I 074 _*_ Interstitial Water (25pg P/liter) Seawater (25pg P/liter) 6871 I 8.61 7.22 Leaves 1.48 - 4 1 87.50 139 0.66 Roots S Rhizomes 1.31 - 104.14 074 _*_ Interstitial Water (2000 vg P/liter) Figure 6. Calculated daily phosphorus flux through I gram dry weight of eelgrass. Left: Uniform dissolved reactive phosphorus concentration in water. Right: Phosphate gradient similar to the natural environment. Units are in micrograms of phosphorus per gram of plant per day. (After McRoy et al, 1972) 45 as the water moves through the estuary (Table 4); this is similar to the events in the river. Nitrate is almost completely removed from solution, while ammonia recycles rapidly. In the Pamlico River Estuary, the urea (another product of decomposition) is completely turned over every day during the summer, but during the winter the turnover time is 200 days. Ammonia in the same estuary may cycle even faster since the algal photosynthesis during a single day in August required 23 1 metric tons of nitrogen while only 5 metric tons of nitrate nitrogen and 100 metric tons of ammonia nitrogen were present. This implies that ammonia may have a turnover time of half a day. Table 4. Total inputs and outputs of nutrients as N or P to the Pamlico River, August 1971, through July 1972, in tonnes. (After Hobbie et al., 1975) P04 NO3 NH4 Input Output Net gain to estuary 715 459 2804 1425 795 744 + 256 + 1379 +51 V \ \ Obtaining "meter-square" samples from an intertidal oyster bar (top), and (below) a large oyster bar exposed at low tide. Intertidal oysters are found in the warm waters of southeastern states but are restricted to deeper water in northern regions. (William Lang) 46 Conclusions Estuaries usually contain high concentrations of nutrients. For the most part, this is a result of high concentrations of nutrients in the inflowing fresh water, but it may also be due to erosion of older marine sediments. The intimate contact of the sediments with the shallow estuarine waters is another extremely important feature. When organic matter decays at the top of the sediments, nitrogen and phosphorus are made available to the organisms in these shallow waters. Estuaries also have ways to prevent nutrients from being washed out to sea. One is the upstream movement of bottom water rich in nutrients; another is the sedimentation and coagulation of particulate matter. Both the circulation of water through the upper layers of the sediment and the resuspension of sediments into the water column serve to move regenerated nutrients from the sediments to the water. Also, both eelgrass and Spartina pump phosphorus from the sediments into the water column. The high concentrations of nutrients entering estuaries and the mechanisms and processes that retain these nutrients within estuaries provide a rich nutrient environment for the growth of plants. These are mostly rooted plants and algae attached to the sediments or to large plant stems, because phytoplankton are usually washed out of estuaries before large blooms can develop. Man's activities have fertilized the rivers and streams feeding many estuaries. Yet, this increase in nutrients has not changed the organisms present in most estuaries. In part this is because estuaries are naturally rich in nutrients. Also, blooms of phytoplankton algae never get the chance to develop in the same way that they would in fertilized lakes or reservoirs. Only when the natural circulation of estuaries is extremely slow or is interfered with will detrimental algal blooms develop. For example, the severe algal blooms in Moriches Bay and Great South Bay, Long Island (Ryther and Dunston, 1971), were caused by a combination of nutrient input from duck farms and a slow circulation of the estuary (reaction time of one month). Thus, estuaries have a number of mechanisms to conserve and utilize available nutrients. Estuaries can also deal with quantities of nutrients that would destroy the ecological balance of a lake or reservoir. John E. Hobbie is a senior scientist at the Ecosystems Center, the Marine Biological Laboratory, Woods Hole. References Alberts, J., R. Harriss, H. Mattraw, and A. Hanke. 1970. Studies on the geochemistry and hydrography of the Charlotte Harbor Estuary, Florida. Progress Report No. 2. Mote Marine Laboratory, Sarasota and Placida, Florida. Carpenter, J. H., D. W. Pritchard, and R. C. Whaley. 1969. Observations on the eutrophication and nutrient cycles in some coastal plain estuaries. In Eutrophication: Causes, consequences, and correctives, pp. 210-21. Washington, D.C.: National Academy of Sciences. Cooper, A. W. 1974. Salt marshes. In Coastal ecological systems of the United States, ed. H. T. Odum, B. J. Copeland, and E. A. McMahan, vol. 2, pp. 55-98. Washington, D.C.: The Conservation Foundation. Edzwald, J. K., J. B. Upchurch, and C. R. O'Melia. 1974. Coagulation in estuaries. Environm. Sci. Tech. 8:58-63. Hobbie, J. E., B. J. Copeland, and W. G. Harrison. 1975. Sources and fates of nutrients of the Pamlico River Estuary, North Carolina. In Estuarine research, ed. L. E. Cronin, vol. 1, pp. 287-302. New York: Academic Press. Jaworski, N. A., E. W. Lear, Jr., and O. Villa. 1972. Nutrient management in the Potomac Estuary. In Nutrients and eutrophication, ed. G. E. Likens, pp. 246-73. Special Symposia, vol. 1 , American Society of Limnology and Oceanography. Kuenzler, E. J. 1 96 1 . Phosphorus budget of a mussel population. Limnol. Oceanogr. 6:400-15. McRoy, C. P., R. J. Barsdate, and M. Nebert. 1972. Phosphorus cycling in an eelgrass (Zostera marina L.) ecosystem. Limnol. Oceanogr. 17:58-67. Odum, W. E. 1970. Insidious alteration of the estuarine environment. Trans. Amer. Fish. Soc. 99:836-47. Pomeroy, L. R., E. E. Smith, and C. M. Grant. 1965. The exchange of phosphate between estuarine water and sediments. Limnol. Oceanogr. 10:167-72. Redfield, A. C., B. H. Ketchum, and F. A. Richards. 1963. The influence of organisms on the composition of sea-water. In The Sea, ed. M. N. Hill, vol. 2, pp. 26-77. New York: Wiley. Ryther, J. H., and W. M. Dunstan. 1971. Nitrogen, phosphorus and eutrophication in the coastal marine environment. Science 171:1008-12. Thayer, G. W. 1974. Identity and regulation of nutrients limiting phytoplankton production in the shallow estuaries near Beaufort, N. C. Oecologia 14:75-92. Upchurch, J. B., J. K. Edzwald, and C. R. O'Melia. 1974. Phosphates in sediments of Pamlico Estuary. Environm. Sci. Tech. 8:56-58. Vollenweider, R. A. 1968. Scientific fundamentals of the eutrophication of lakes and flowing waters, with particular reference to nitrogen and phosphorus as factors in eutrophication. Tech. Rept. DAS/CSI/68.27 to the Organization for Economic Cooperation and Development, Paris. Williams, R. B. 1973. Nutrient levels and primary productivity in the estuary. In Proceedings of the coastal marsh and estuary management symposium, ed. R. H. Chabreck, pp. 58-89. Baton Rouge: Louisiana State University. 47 ft **m • of Estuarine Animals by Wmhna B. Vernberg and F. John. Vernberg The estuarine environment is one in which water from a fresh water system mixes with seawater in a semienclosed region. As a result, one of its chief characteristics is a marked fluctuation in salinity. The amount of salinity change varies with the amount of fresh water runoff, and this may change on a temporal and seasonal basis. The amount of tidal fluctuation also affects the range in salinity flux. There are other fluctuating environmental factors that estuarine organisms also must face. Vast amounts of silt borne by fresh water runoff and deposited in the estuary form extensive tidal flats. The water bordering these tidal flats tends to be shallow, and temperature variations may be extremely pronounced during one tidal cycle or may be seasonally controlled by temperature changes in the open sea. Amounts of dissolved oxygen also may change markedly throughout a 24-hour cycle as a function of temperature and phytoplankton production. There are large amounts of organic material carried in with the fresh water, and estuarine waters generally are turbid, foreclosing the option of using vision in locating food, finding a suitable habitat, or seeking a mate. To the natural stresses common to estuarine existence we must also add those man-induced stresses— pollutants, thermal 48 rock Sturges / Courtesy .\'cw England Rirer Basins 'ommissionl additions, dredging, and filling— that have now become a part of the "normal" stresses of many present day estuarine systems. Within this dynamic region lives a mixture of animals who have evolved the diverse physiological mechanisms needed to cope with fluctuating environmental conditions. To further complicate life in an estuary, a high degree of variability in environmental factors exists between different parts of an estuary. Thus animals inhabitating a region near the ocean endure a different environmental complex than those near the source of fresh water, while those in the water column experience different stresses than animals in the intertidal mud flats. Not only does environmental heterogeneity exist, but the various stages in the life cycle of a species may occupy different partitions of the estuary. This extreme variability of habitats and species is a delightful challenge to biologists concerned with understanding physiological adaptation. One of the most fundamental and intriguing questions that can be posed is how it is possible for one group of animals to thrive under conditions that would be intolerable for another group. The physiological adaptations of a tropical zone species. for example, enable them to survive constant high temperatures that would quickly prove fatal to a species found in Arctic waters. Similarly, species living in estuaries are also well adapted to an environment that would be very stressful to fresh water or open ocean organisms. When encountering a stressful situation, an animal has two alternatives: it can migrate to a more favorable environment, or it can remain and survive, depending on its adaptive capacity. If conditions are too stressful, the animal dies. However, not all stresses are of a nonliving origin, for organisms must compete intraspecifically and interspecifically for resources, such as space and food. The dynamic interaction of the animal and its environment is represented in Figure 1. For each species there is a certain sector of the total range of expression of an abiotic environmental factor that is compatible with life. At either end of this gradient, there is a point beyond which an organism cannot survive. The broad middle sector that is compatible with life is called the zone of compatibility; the region at either end of this zone is the lethal zone (Figure 2). The dividing line between these two zones is not simply determined since numerous intra- and extra- Ecological Niche Figure 1. Interaction of an organism and its environment. /After I'crnherx and I'crnberg. 1971 49 2 50 - Zone of compatibility Upper incipient lethal point Lower incipient lethal point Low High Environmental gradient Figure 2. The zones of resistance (lethal zone) and compatibility. (After Vernberg and Vernberg, 1972) organismic factors are involved, such as nutritional state and the possible interaction of numerous physical parameters. Differences in organismic response may reflect an altered physiological state (acclimation) due to exposure to a given set of environmental conditions. When the environment changes again, the animal adjusts by altering its physiological response. However, the range of these environmentally induced changes is determined by the organism's genetic composition and is not limitless. Different species have different genetic constitutions. Thus, estuarine species exhibit a wide range of physiological responses. For ease of presentation, the physiological diversity exhibited by estuarine animals will be discussed in the following two major sections: lethal zone and zone of compatibility. Lethal Zone Recently the alarm over pollution has heightened interest on the lethal effect of toxic substances on the estuarine biota (see page 18). But normal environmental factors also play a regulatory role on the distribution and abundance of organisms by disruption of their functional processes. As salinity decreases, the number of marine animals decreases; a salinity range of 5-8° /oo (parts per thousand) is apparently the critical salinity boundary between fresh water and marine faunas. A number of physiological processes appear to be involved; growth is impaired or stunted, reproduction is inhibited, and nerve conduction patterns are altered. In addition, the body fluid concentration of many brackish water animals can be reduced to a salinity of about 5°/oo before serious damage results. Below this salinity, distortion of cellular electrochemical properties occurs and the tissue albumin fraction undergoes marked changes. Typically, animals from the higher intertidal zone can withstand greater temperature extremes than those species living subtidally. For example, the oyster living in the intertidal zone survives higher temperatures than does the subtidal bay scallop. Of additional interest is that the response of the oyster can shift, depending on the previous thermal level of exposure, whereas the scallop's response is less labile. Thus, the thermal limits of the oyster can change with the season, an adaptive response for an organism that, by its sessile nature, cannot migrate when "environmental times" are difficult. Temperature changes have been shown to alter the biochemical pathways of certain animals, a change that apparently has energetic value. Not only do differences exist between species, but the responses of different stages in the life history of one species vary. For example, the adult fiddler crabs commonly found in marshes or intertidal beaches tolerate both higher and lower temperatures than do the larval stages that live in the water column. Numerous examples demonstrate that other environmental factors, including oxygen, tidal action, sediment type, and species competition, may increase mortality levels. Although many studies have dealt with one of these factors at a time, the natural environmental complex obviously consists of all of these factors, and various combinations of factors may fluctuate at the same time. Although not as thoroughly studied, the results of multiple factor interaction investigation indicate typically that multiple stresses decrease the viability of organisms over that determined when only one factor is being stressful. Unfortunately, the physiological bases for this environmentally induced disruption of the functional integrity of organisms are poorly known. Zone of Compatibility Regulation of Water and Ions The body fluids of many animals living in the open ocean have the same osmotic content as the surrounding seawater, and since this environment remains relatively constant, these animals do not have to face the problem of water balance. Estuarine animals, on the other hand, must have some physiological means to adjust to alternating high 50 and low salinities. Otherwise, their tissues and cells would absorb a great amount of water when they encounter low-salinity water. One of two strategies, then, may be adopted by estuarine animals: they either have the capability to maintain an optimum osmoconcentration in their body fluids regardless of the external environment, or their tissues and cells have the ability to tolerate dilution. Generally, animals are classified as being either osmoconformers or osmoregulators. An osmoconforming animal is one that does not regulate the osmotic concentration of its extracellular body fluid when that of the external environment changes. Instead, the concentration of this fluid conforms to that of the external environment. In contrast, the osmoconcentration of the extracellular fluid of an osmoregulating animal remains relatively constant when the external environmental fluid fluctuates in osmotic properties. Animals in estuarine waters typically have the ability to hyperosmoregulate in low-salinity waters. Those which are semiterrestrial or live in areas subjected to rains, such as salt marshes and mangrove swamps, are capable of both hyper- and hypo-osmoregulating. That is, when salinities are low, they are able to maintain their extracellular body fluid at a higher osmotic concentration than the surrounding waters; when salinities are exceptionally high, they are able to maintain their body fluid at a lower osmotic concentration. One osmoregulating mechanism that is commonly found is the walling off of the organism from the external environment, thereby preventing excess water and ion exchange. As a general rule, estuarine organisms tend to have body surfaces that show a marked decrease in permeability over those of oceanic ones. To gain impermeability, estuarine animals may show increased calcium deposits in the exoskeleton or have an increased number of mucous glands: they may also have morphological adaptations, such as opercula, to protect and isolate respiratory surfaces from adverse environmental conditions. Animals living in fluctuating salinities often are confronted with an external milieu that is hypotonic to their body fluid. Under these conditions, the body fluids of the animal would tend to lose ions and/or gain water. The kidney or renal organ is of great value in both volume regulation and salt retention. There are some exceptions, but generally animals living in regions where they are exposed to low salinity tend to have more renal units, a greater total surface for glomerular filtration, Shallow-water oysters being collected by long-handled tongs in a salt marsh creek. (William Lang) and longer and more highly differentiated renal tubules that function for salt adsorption. The gills also play an important role in salt excretion and/or uptake. Osmoregulating organisms may differ in the method of maintaining their osmotic concentration. Some polychaetes when exposed to low salinity are capable of active uptake of Na and Cl~, and the gills of the Chinese wool-handed crab (Eriocheir sinensis) can remove Na from an external medium with a Na concentration of 8 millimole, even when the internal Na concentration is 300 millimole. Sharks have a relatively high blood osmoconcentration by maintaining a relatively high blood concentration of urea. Other species rely on changes in the amount of free amino acids in cells to maintain osmoconcentration. Organic substances, such as glucose, may also be used by animals to increase or maintain osmotic concentrations rather than relying solely on inorganic electrolytes. Although osmotic regulation is important, the selective regulation of specific ions is essential. Protoplasmic integrity cannot be maintained in the absence or excess of certain ions. For example, Ca++ added to low-salinity water that is ordinarily lethal will enable animals to survive. Feeding In contrast to many pelagic systems, the primary producers in estuaries-and particularly in shallow, warm-water ones-include not only phytoplankton but also the marsh grasses, sea grasses, reeds, and other marsh vegetation. When this plant material dies, it is decomposed and becomes a part of the sediment as detritus. As a result, food chains in estuaries are based on this rich source of organic 51 detritus derived from these various plants. Feeding habits of the benthic estuarine organisms can be correlated with the hydrodynamic characteristics of their environment. In areas of slow currents where the mean diameter of sediments is less than 0.09 millimeter, most animals are detritus feeders. In areas where mean grain size exceeds 0.09 millimeter, filter feeding dominates, while predation is common where mean sediment size is greater than 0.15 millimeter. Among the larger estuarine consumers, most are omniverous. Generally, they are able to utilize alternate foods from time-to-time and from place-to-place, depending on the stage in their life cycle and food availability. Thus, these animals can rely on detritus, a wide variety of bottom animals, and fishes. One of the most successful estuarine fish, the striped mullet (Mugil cephalis), illustrates well some of the aspects of estuarine nutrition. This species feeds on mosquito larvae, copepods, and other zooplankton until it attains a length of about 30 millimeters. Then it either feeds on detritus by sucking up the surface layers of mud, or browses on micro-algae attached to submerged surfaces. Sediment-feeding fish may take up small mouthfuls at random or may skim along the bottom, lips barely touching the sediment, and suck up the top layer. Both types of feeders strain a small quantity bf the sediment by the pharyngeal filter and expel undesired material. Browsing animals nibble the attached algae, digest suitable food, and expel the remainder. Respiratory Adaptations To function successfully in a fluctuating estuarine environment, animals depend on functional metabolic machinery. Thus, it is not surprising to see striking and diverse types of adaptations. Reduced salinity may increase metabolic rate (rate of oxygen consumption) in some animals, while in others it remains unchanged or is decreased. The basic mechanisms that dictate these responses are unclear. Variability in response to oxygen content by various species can generally be correlated with the animal's mode of life. For example, the bottom- dwelling toadfish (Opsanus tau) can continue respiring and extracting oxygen from the surrounding water until almost all oxygen is removed, while, in contrast, an active surface- dwelling fish, the mackerel (Scomber scombrus), fails to withdraw oxygen when the oxygen content drops below 70 millimeters of mercury. In many animals, when the oxygen content decreases, the rate of oxygen consumption drops, but in other species the rate is unaltered. If the rate is unaltered, the animal must be able to remove more oxygen from the water even when the external oxygen tension is decreased. Various physiological ploys may be used to accomplish this, such as increasing the amount of water passing over the respiratory surface per unit time, improving the efficiency of removing a higher amount of oxygen from the water, or changing the amount and/or type of respiratory pigment. Prolonged exposure to a low but nonlethal oxygen tension may cause profound changes in the animal's metabolic performance, that is, a shift in biochemical pathways and/or the elaboration of new isozymes. When the tide is out, the intertidal oyster closes its shell and shifts from an aerobic to an anaerobic metabolic system. In contrast, the fiddler crab (genus Uca) emerges from its burrow when the water recedes and shifts from anaerobic to aerobic metabolism. Thus, two intertidal species respond to tidal changes in a different manner. In general, those species living in oxygen-poor water differ from animals in oxygen-rich water in respect to their respiratory pigments. These are chemical entities that have a reversible affinity for oxygen and are found in blood, body fluids, and/or selective tissues. The respiratory pigments of animals from low- A female fiddler crab /'Uca minax,). Males are readily distinguished by having one large daw. (William Lang) 52 oxygen environments are more oxygen sensitive and will become saturated at lower oxygen concentrations than pigments of animals from high- oxygen areas, an obvious adaptive advantage enabling an organism to acquire oxygen from the water and transport it to the cells. The amount of respiratory pigment per unit volume tends to be higher in animals with a high rate of locomotor activity. Perception of Environment Animals living in the intertidal areas bordering an estuary will use vision in many life-supporting activities. In fiddler crabs, visual cues are very important during periods of low tide in moving about over the beach to different micro- environments to feed, reproduce, wet their gills, escape from predators, or release larvae. Many of these activities have directional components that are adaptive, and there is evidence that these crabs can use sun position and planes of polarized light as well as landmarks in carrying out their daily activities. However, animals living in the sediment- ladened water column of an estuary cannot depend on visual cues as readily as can either intertidal zone or open ocean forms. Some estuarine animals are able to utilize tidal salinity changes to their advantage in seeking a suitable environment using rheotactic and chemoreceptive modalities. The European eel, (Angiiilla vulgaris) illustrates well how such orientation is accomplished. After hatching in the tropical waters of the Sargasso Sea, the young larvae slowly migrate to inland waters along the coast of Europe and Africa. In the autumn of their third year, they metamorphose from willow leaf-shaped larvae into transparent elvers and become distributed along the European and North African coasts. The elvers are able to move into the estuaries in these areas with the tides by staying in the surface waters during incoming flood tides and then descending to bottom waters during ebb tides. By doing this they are able to avoid being washed out to sea. Day and night this pattern continues until they are in the estuary, and once there with the ebb tide flowing over them, they can distinguish between the overhead water masses. When a water mass containing inland water passes over them, they orient themselves by heading into the current and then make their way to inland waters to complete their development. The animals apparently are able to distinguish the specific odor of inland water by chemoreception, for inland water proved to be a very strong attractant, but the eels showed no response when exposed to tap water. There is evidence that other animals that breed in the sea but spend a part of their life cycle in the estuary utilize similar mechanisms for making their way to and from the open ocean. Because visual display would be of limited value in estuarine waters, some fish rely primarily on audition for communication. The male toadfish (Opsanus tail}, for example, has a well-defined spawning ritual. He first seeks a suitable nesting site, such as a tin can or a large empty shell, and stands guard. When he is ready to spawn, he emits a characteristic boat whistle call, which in turn acts as a stimulus to attract females that are ready to lay eggs. After the female lays a clutch of eggs, she leaves the nest, but the male remains until the young are free-swimming. If other fish approach the nest while the male toadfish is standing guard, he will emit a series of aggressive grunt sounds presumably to cause the intruders to leave the nest area. Population Continuity The animals living in the intertidal zone tend to lead a semiterrestrial existence as adults, but the larvae of most species live in the water columns within the estuary. Thus, the reproductive cycle must be geared to insure release of gametes into a favorable watery environment. Most mobile species migrate into a favorable environment to release their larvae, while the more sessile species must time the release of their reproductive products to coincide with periods when the adults are submerged. Since subtidal animals are always covered with water, when and where their larvae are released depends in large part on larval tolerance to estuarine conditions. For both intertidal and subtidal organisms to survive as a species, then, the larvae must be released when temperatures are not too extreme, salinities are optimal, and food is available. How are the reproductive cycles of these organisms timed to insure that the larvae are released into a favorable environment? One very important factor is temperature. In some animals, 53 spawning is initiated by a thermal change alone, while in others rising temperature and lengthening photoperiod interact to influence the reproductive cycle. Reduced salinity reduces the reproductive potential in many estuarine species. Mobile species will move into higher-salinity waters to breed and spawn, while sessile animals tend to breed during seasons of the year when salinities are highest. There are some species, however, in which low salinity acts as a breeding stimulus, as illustrated by a number of species of fish in Indian estuaries. The peak of breeding activity in these fish occurs during the monsoon season, when salinities are at their lowest levels. As is true of many marine animals, the reproduction of some estuarine animals is timed to coincide precisely with certain phases of the moon. Epidemic swarming has been linked to lunar periodicity in a number of species. The crab Cardisoma guanhumi offers a good example of this phenomenon. This crab lives intertidally and along drainage ditches on the south Florida coast, and it must return to the sea to spawn. The spawning season extends from late June to early December, and spawning occurs in sharp peaks near the time of full moon and in lesser peaks at new moon periods. Egg-bearing (ovigerous) females appear simultaneously, moving toward the sea, reaching the highest concentration between one night before the full moon and right after; then the spawning periods stops abruptly. These, then, are some of the ways that estuarine animals are able to cope with their unique environments. Winona B. Vernberg is program director for environmental health studies in the School of Public Health and Public Health Professor of Biology, Department of Biology, University of South Carolina. F. John Vernberg is director of the Belle W. Baruch Institute for Marine Biology and Coastal Research and is the Baruch Professor of Marine Ecology, Department of Biology, University of South Carolina. Publication 161 from the Belle W. Baruch Institute for Marine Biology and Coastal Research. Suggested Readings Bliss, D. E., and L. H. Mantel, eds. 1968. Terrestrial adaptations in Crustacea. Am. Soc. Zool. 8:307-685. Lent, C., ed. Adaptations of intertidal organisms. Am. Soc. Zool. 9:211-419. Remane, A., and C. Schlieper. 1971. Biology of brackish water. New York: Wiley-Interscience. Vernberg, F. J., ed. 1975. Physiological adaptation to the environment. New York: Intext Educational Publishers. Vernberg, W. B., and F. J. Vernberg. 1972. Environmental physiology of marine animals. New York: Springer-Verlag. 54 (•Fishes and Estuaries by R. L. HtmlriHi and C. A. S. Few areas of the earth support such large stocks of fish as do estuaries. Perhaps only African savannas and oceanic upwelling regions support similar concentrations of vertebrate biomass (Table 1). Studies of the total biotic energy fixed by plants and used in these places indicate that, in general, estuaries have the greatest availability of food of any region in the world (Odum, 1971 ; Teal and Teal, 1969; Woodwell, Rich, and Hall, 1973). These conditions have made estuaries rich feeding grounds for fishes, and it is also here that man takes his greatest harvest of sea foods. Five of our six most important commercial fish species are dependent in some way on estuaries (Smith, 1966). Scientists have been impressed by the degree to which these regions are "stressful." The many abrupt changes in temperature, salinity, and chemical and oxygen concentrations over seasonal, daily, and tidal cycles imply a high degree of physiological stress on estuarine organisms. Many studies have shown that the physiological cost of adaptation to such conditions is high. Why should areas of such a high degree of stress have so many fish? And why should some estuarine fish groups, such as salmon, evolve life histories involving long- distance migrations that appear to increase their exposure to stress? Striped Boss (Roccus saxatilis} Bluefish (Pomatomus saltatrix} Sampling Problems The first problem in attempting to answer these questions is getting a good sample. Unlike clams and diatoms, fish are too clever and too mobile to be passively captured in most sampling gear. No one piece of gear is fully adequate. The seines suitable for smelt do not retain sand eels. Trawls are fine for winter flounder and juveniles of some fish, but not for eels, which must be trapped. Mummichogs lend themselves to trapping, but then the eels in the trap eat them. Weakfish and bluefish are perhaps best angled for. For most sampling procedures, the sample is, at best, only semiquantitative. However, there are several ways that one can get, albeit with considerable effort, reasonably good quantitative estimates. One way is to catch a large number of fish, tag them, and then derive population estimates according to the number of tagged fish that are recaptured. Tagging is doubly useful. Not only does it allow population estimates to be made, it also provides a way of tracing the movements of the fishes. But tagging is not always an easy matter. Large fishes, such as striped bass, white perch, and salmon, are not difficult to tag, and because these are important game fish, tags can be recovered without too much effort by enlisting the aid of sportsmen. Small, delicate fishes, such as silversides and young alewives, may be almost impossible to tag. Even if they could be, a 55 considerable recapture program would be necessary. Where physical tagging is impossible, indirect statistical methods can be used, such as following year-classes or using egg-and-larva surveys. Such approaches can be used only for those few species that show little migratory tendency and that are sampled readily in all growth stages throughout the year. We know relatively little about the lives of estuarine fishes, but we do know enough to say that hardly any fit this requirement. Sonar offers some hope for extensive and accurate surveys of estuarine fish. Some day it may be possible to have computer memories of sonar "fingerprints" for different species and online print-outs of species-by-species biomass. However, many technical problems remain, and it will be a long time before any such system is routine. One recently "rediscovered" method holds promise for giving quantitative estimates in some estuarine environments. This is the drop net, conceptually similar to the quadrate used by field botanists (Figure 1). A horizontal, chain-bordered net is dropped randomly or according to some sampling plan, and the fish that are caught are identified, counted, and weighed. The method works well in estuaries that are less than 2 meters deep with an uncluttered bottom of sand or mud. There are problems, however, associated with the use of the drop net. For example, fish may be either attracted or repelled by the net's frame. We have found that in turbid environments, where there seems to be little or no gear selectivity, the drop net gives an accurate estimate of biomass. We caught virtually no fish in a standard otter trawl used in areas where Table 1. Biomass in certain ecosystems. we measured 50 or more grams per square meter of fish with the drop net. The Fish in an Estuary Assuming the sampling problems are solved, what types of fish will be found in the nets? Phylogenetically, many important marine groups are represented in the estuarine assemblage of fishes. The New England assemblage is dominated (relative to their contribution to the total fauna) by salmoniforms (trouts and smelt), atheriniforms (silversides and mummichogs), and gasterosteiforms (sticklebacks). These are the groups whose basic adaptations are to an estuarine existence. Otherwise dominant marine groups— the gadiforms (cods), clupeiforms (herrings), anguilliforms (eels), and perciforms (basses, perch, and other spiny-rayed fishes)— are represented in the estuary by only a relatively few species. In these groups, it is some specific adaptation, not an adaptation of the group as a whole, that allows the fish to thrive in the estuary. In many cases it is difficult to characterize a fish as "estuarine" or "marine." Tagging studies show that certain fishes may range quite far from the estuary where they spend at least a part of their lives. The movements of salmon are among the most extensive. The oceanwide wanderings of the five species of Pacific salmon have been known for some time, but only recently has it become apparent that the Atlantic salmon moves quite far as well. Results of tagging indicate that fish from both European and North American rivers congregate in the rich waters off Greenland, where they feed and fatten before Group Area Grams per square meter Birds Moose Humans Fishes Fishes Fishes Fishes Large mammals Fishes Fishes Fishes Fishes Fishes Anchovy New Hampshire forest Isle Royale, Michigan United States Unpolluted rivers Georges Bank Atlantic salmon river, Matamek, Quebec Narragansett Bay Central and East African grasslands Gulf of Mexico Flax Pond (Long Island) Estuary (annual average) California kelp bed Bermuda coral reef in summer Narragansett Bay salt marsh embayment Peruvian upwelling in autumn 0.04 0.7 0.9 1-5 1.6-7.4 2.1-17.8 3.2 3.5-23.6 5.6-31.6 24.0 33.2-37.6 59.3 69.2 216.7 56 1/4 MESH NYLON NET SLOTTED CORNER POSTS, WITH RING a RELEASE-PIN TRIGGER MECHANISM CHAIN BORDER PURSING ROPE AND RINGS 2"x4"x IOMETER BOARDS PULL TO SPRING NET 55 GALLON DRUM IN 2"x4" FRAME NET SPRUNG WEIGHTED BORDER OF NET -LEAD DEADMAN >ULL TO PURSE Figure 1. A drop net used in estuarine sampling. returning to home rivers to spawn. To encompass these apparent oceanic excursions and at the same time to keep in mind the essentially estuarine focus of such fishes, McHugh (1967) introduced the interesting notion of an extended estuarine zone. The definition of this zone depends not on man's terrestrial and topographic viewpoint, where the estuary extends hardly beyond the capes that guard a river's meeting with the sea, but rather on the fish's aquatic and hydrographic viewpoint. Salinity is the important characteristic. The flow of rivers in some places, most markedly the North Pacific, and an excess of precipitation over evaporation in others result in vast shallow lenses of low-salinity water overlying water of more oceanic characteristics. McHugh's chart of the offshore estuarine zones, as bounded by the 33.5°/oo (parts per thousand) isohaline shows that these have considerable extent (Figure 2). The largest offshore estuary includes almost the entire North Pacific north of about 40°N, where an estuarine analogy had earlier been argued by Tully and Barber (1960). The Northwest Atlantic Estuary extends from Cape Hatteras to Labrador. The boundaries and even the appropriateness of the extended estuaries can be argued, but the concept of a seaward extension of the estuary must remain an important one for fishes. Many species for which the conventional estuary is an important habitat at some stage in their lives are found well offshore at other times. The salmons, again, are the most extreme examples. The list of New England fishes would have to include, in addition to Atlantic salmon, the sturgeon, sea lamprey, American eel, shads and sea herring, weakfish, butterfish, bluefish, and winter flounder. Figure 2. Offshore estuarine zones of the world bounded by the 33.5°/oo (parts per thousand) isohaline. These shallow lenses (shaded areas) overlie water of higher salinity. (After McHugh, 1967) 57 An Atlantic salmon on its upriver spawning migration in the Mirarnichi River, New Brunswick, Canada. (John Gibson) Another group of fishes is more closely associated with the estuary proper, and its members are rarely found more than a few miles from shore. In New England, such fishes are the smelt, salters (sea-run trout), sticklebacks, pipefish, silversides, mummichogs, tomcod, kingfish, striped bass, and white perch. The list is not large. Even with the species of the extended estuary added, the total is only about 10 percent of the total marine fish fauna of the New England coasts and shelf (Bigelow and Schroeder, 1953). Nonetheless, members of this group have considerable economic importance and are particularly sought by both commercial and sport fishermen. Physiological Aspects of Estuarine Adaptation The great range in temperature and in salinity that can be encountered in the estuary could place considerable physiological stress on a fish living there. Despite the ability of most fish to move about and adjust their local environment, some degree of temperature tolerance and osmoregulatory ability would be required to cope successfully. Such physiological adaptations are among the most important, if not the major ones, that an estuarine fish must possess. The predominantly estuarine species belong to groups that are tolerant of wide ranges in temperature and salinity. The salmon and their relatives, many of which move from salt to fresh water to spawn, have the ability to adjust rapidly to great changes in salinity, as do sticklebacks and killifishes (mummichogs). Salt balance in these species is achieved through a generally low permeability of the body surfaces and marked activity of the kidneys and salt glands in the gills. Near-freezing conditions are met by the winter flounder through a seasonal change in the blood serum chemistry, which depresses the freezing point. In the Hudson River, striped bass move into relatively fresh water to overwinter. The cold, saltier water of shallow bays, with a depressed freezing point, has the potential to freeze the fishes' blood. Winter flounder, striped bass, tomcod, and others have individually adapted to estuarine conditions, although the group to which each belongs tends on the whole to be less tolerant of extremes. We do not know how great is the energetic cost for an estuarine fish to maintain the complicated biochemical, physiological, and behavioral arsenal it requires to exist in the estuary, but it cannot be inconsiderable. The Natural History of Estuarine Fishes A broad spectrum of ecological roles is played by fishes in estuaries. Most of the information available on feeding has been summarized by de Sylva (1975), and his list of references is surprisingly short. However, even in the New England estuaries that contain few fish species, many patterns of feeding are found. Mummichogs feed on detritus and small invertebrates, which they hunt in marsh channels and in the Spartina grass. Schools of silversides move over the more sandy areas preying on small crustaceans in the water. Winter flounder feed omnivorously on the bottom, but concentrate on worms, isopods, and small clams. Smelt feed up in the water column on sand shrimp and on small fishes, not infrequently their own young. Eels, too, feed on small fishes but presumably nearer the bottom and generally at night. Young alewives and menhaden strain zooplankton as they swim. Carnivores, such as bluefish, are active during the day, feeding on herrings and silversides, while striped bass prey on smaller fishes, eels, and seaworms, especially in the evening and at night. The estuary is a spawning site for only some of the fishes found there. In New England, the winter flounder lays large clusters of adhesive, nonbuoyant eggs during the coldest part of the year-winter into early spring. The tomcod lays large numbers of nonbuoyant eggs somewhat earlier, from late fall into winter. The production of sticky, nonbuoyant eggs must be a particular adaptation to keep the eggs within the estuary, for the closest relatives of both these fishes, which live offshore, lay pelagic eggs. Mummichogs spawn in spring, when large schools crowd into small tidal creeks. The males become very brightly colored and ardently court the females. The large eggs are laid in shallow water, where they stick to rocks and plants on the bottom. The silverside waits until a warmer part of 58 the year, late spring and summer, to spawn over sandy bottoms and near the base of the Spartina, where the abundant eggs adhere with long sticky filaments. Sticklebacks do not rely on great numbers of eggs to insure survival. Instead, the male stickleback builds a small nest of plant material during spring and into summer. After an elaborate courtship, a few sticky eggs are spawned within the nest. The male guards them until hatching and even for some time thereafter. The pipefish goes the stickleback one better. In this species, the eggs, laid in the summer, are brooded by the male in a special pouch on the abdomen. In all these species, incubation is fairly rapid and ranges from about a week in silversides and the stickleback to about three weeks in tomcod. A number of important estuarine fishes spawn not in the estuary, but in the fresh water upstream. Of New England fishes, these include the smelt, alewives and shad, sea-run trout and salmon, and the striped bass. Smelt are the earliest spawners, ascending rivers and brooks in late winter and early spring, the coldest time of the year. They generally run only a short distance into fresh water, sometimes just above the tideline, although in some large river systems they may run more than a hundred miles from the sea. The eggs are laid in large sticky mats on the bottom in running water. Immediately after hatching, in about two weeks, the young drop down to the estuary. When the water has warmed a bit in the spring, and when the shadbush blooms, the alewives begin their spawning run. These fish require fairly still water to lay their eggs, and thus ascend only rivers or brooks that drain from ponds. The eggs are laid in masses that sink to the bottom and adhere there until hatching, about a week later. Like the young smelt, small alewives move down to the estuary fairly soon. Trouts and Atlantic salmon move into fresh water generally in the spring and early summer but do not spawn until fall. They may run for long distances, well up into the cold, well-oxygenated headwaters. These fish bury their eggs in gravel bars, where the eggs overwinter to hatch in the spring. The young spend two to three years in the river before drifting down to the sea. A prime example of this fresh-water spawning pattern, called anadromy, is found in the striped bass, a popular sport and commercial fish. Stripers are rarely caught more than 10 kilometers from shore, and estuaries are essential for their reproduction and early life history. Stripers apparently require some 100 kilometers of free- flowing fresh and/or brackish water to successfully spawn, and as a result, they spawn only in the largest estuaries. The Hudson, Delaware, and the Chesapeake systems have traditionally been important spawning areas for the stripers, although dams and pollution have eliminated Delaware spawning. Fish spawned from these estuaries migrate northward during the summer to support extensive coastal fisheries in New York and New England. On the West Coast, where the striper was introduced in 1879, the Sacramento River is a spawning center. Talbot (1966) reviewed the life history of the striped bass and documented the crucial role that the estuarine ecosystem plays. The following account of striped bass in the Hudson River is based on the work of Talbot and others, as well as on analyses developed at Oak Ridge National Laboratory as part of environmental impact studies for Hudson River power plants. The fish spawn in late May or early June above the salt water (Figure 3). The semibuoyant eggs and larvae appear to spend some days or weeks very near the bottom of the river, drifting little as the bottom currents are not strong. By about two weeks of age the young bass are found throughout the water column and begin to drift downstream. As the fish become strong enough to swim feebly, they begin to migrate vertically, moving toward the bottom during the daytime and to the surface at night. Once within the influence of the estuarine salt wedge, the young fish are swept seaward during the night in the relatively fresh surface water, and upstream during the day when nearer the bottom. At about six weeks, the fish are able to maintain their position by swimming against the current, and at this time the majority of the fish are found in shallow regions in Haverstraw Bay and the Tappan Zee, the broad bays below Peekskill, New York, downstream to the New Jersey state line. A smaller number are found along the shoreline. The daily migration pattern of the two-to- six-week-old fish appears to be a critical factor. Without this pattern, the young fish would be too weak to swim against the seaward-flowing surface waters and would be swept to sea if they remained in the main currents. If they remained entirely on the bottom, they would be moved upstream above their nursery grounds. The salt wedge is normally associated with the biotically richest regions of estuaries, and fish that exhibit a pattern of diurnal vertical migration would be concentrated in the region of highest available food supplies (Figure 4). As many other anadromous fish also show this pattern, it seems clear that selection has taken place for such a behavioral pattern in the young of many estuarine fish species. Pearcy (1962) has 59 Freshwater Estuarme Marine Adults -Feed ing Ground o Eggs -Spawning at brackish /freshwater interface ^"7^3 Larvae-^ yNursery Area 'j-^fo Young Figure 3. Ttie striped bass, an important estitarine species, is a semianadromous fish, moving from saline water to, or almost to, fresh water to spawn. It usually spawns near the interface of fresh and low salinity water. In the estuary the eggs and larvae drift downstream, and the developing fish feed throughout the system until they reach maturity and repeat the cycle. (After Cronin, L. E., and A. J. Mansueti. 1971. The biology of the estuary. In A Symposium on the Biological Significance of Estuaries, Sport Fishing Institute, Washington, D.C.) shown a similar migration pattern tor larval flounders, and the salt wedge circulation pattern is probably also important for young Pacific salmon. Less is known in detail about the estuarine fishes that spawn in the sea. Butterfish and bluefish spawn well offshore, but the distribution of their eggs has only recently been documented by the National Marine Fisheries Service. The young quickly make an appearance in the estuary where they feed and grow. The eel is the apparent long- distance champion. Its eggs are unknown, but the smallest larvae first appear in the southeastern Sargasso Sea, where they spend a year drifting in this great oceanic gyre. In the spring, young elvers move into the estuaries. Some remain there, but others continue to migrate far up rivers and brooks. The eel makes most of its growth either in estuaries or in fresh water. In the fall, some, but not all, of the adults migrate down and out to sea, presumably to their unknown, remote spawning rendezvous. Regardless of the spawning pattern, the time at which many juvenile fishes reach the estuary is closely coincident with periods of maximum food production. The fish may be evolutionary programmed, so to speak, to take advantage of pulses in food just at the stage in their life when they are growing the fastest. For example, our analysis of Pacific salmon indicates that the young enter coastal waters at just the right time to catch the large, and predictable, pulse in zooplankton. The zooplankton pulse, in turn, follows the spring phytoplankton bloom. Most of the fishes that spend their juvenile periods in estuaries do not stay there as adults. This is particularly true for the shallow regions that HIGH FOOD NURSERY AREA Figure 4. Spawning patterns and the salt wedge in an estuary. The graph indicates mean primary production across the region. 60 are most important as nursery areas. Perhaps the shallower regions are uncomfortable for large fish, but we have often wondered whether there is a more fundamental reason why large fish do not "cheat" by feeding more heavily in shallow nursery areas. Adult salmon are found far at sea and younger salmon nearer to shore. Juvenile snapper blues are familiar residents of shallow estuaries, but the adult bluefish tend to be found further offshore where potential food is more limited. Many possible explanations come to mind, of course, but there may be selection pressures to allow the younger fish free access to the most productive estuaries. A few generalities emerge from these brief considerations of natural history. A variety of feeding patterns is present in the estuary, which means that the food resource in general is shared among the fish species. The estuary seems most important, not as a spawning area, but as a feeding and nursery area for the young of many species, regardless of where they spawn. For those species that do spawn in or near the estuary, the production of large numbers of adherent demersal eggs that develop and hatch in a very short time appears to be the rule. Direct competition is avoided by the use of different areas and times for spawning. The observation that dominance by a few species tends to be the rule in estuarine fish communities follows from the large spawning aggregations and the great numbers of young produced. In New England, two to four species make up 80 percent or more by numbers of the fishes found in an estuary. A Hypothesis of Seasonal Use It is clear that fishes make varied demands on estuaries. The pressure of these demands varies throughout the year, particularly for species with seasonal patterns of reproduction and growth. Seasonal comings and goings further enhance a picture of seasonal change. We have argued (Haedrich, 1975) that seasonal change in the local fish community should be quite marked in the healthiest temperate estuaries. Seasonality allows multiple use, and evolution and co-adaptation within the community permit the greatest possible number of species to share the resources. The estuarine environment from which the most species can benefit— the estuary with the highest annual species diversity— should be the one where a good seasonal turnover occurs. Where the environment is rendered less equitable— for example, by pollution— the number of species which can use that area will be reduced. Gradually the more sensitive species will drop out, leaving only the most hardy. Those that remain will be those basically adapted to the greatest extremes of an inhospitable climate. Such fishes, presumably, would be ones already attuned to estuarine life. What little comparative data there are seem to bear out this working hypothesis. In Woods Hole Harbor, a relatively unpolluted area, 38 species were taken in trawl samples made during the course of a year (Figure 5). There was considerable turnover both in species present and in relative abundances from season to season. Percentage similarity (an index that takes the presence and abundance of species into account; it ranges from 0 percent where there is no overlap to 100 percent where overlap is total) from spring to summer was 16 percent; from summer to fall, 52 percent; from fall to winter, 34 percent; and from winter to spring, 69 percent. Four species accounted for 80 percent of all individuals. Lynn-Saugus Harbor, Massachusetts, a polluted area where 93 percent of the clam flats were closed because of gross contamination, had only 21 species trawled throughout the year. The lowest value of similarity between seasons was from fall to winter, where it was still quite high at 75 percent. From summer to fall it was 86 percent. A single species comprised more than 80 percent of all individuals. That F