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Chapter 19 Future of Research in Coastal Lagoons
Coastal Lagoon Processes edited by B. ylerfve (Elsevier Oceanography Series, 60) Q 1994 Elsevier Science Publishers B.V. All rights reserved
553
Chapter 19
Future of Research in Coastal Lagoons H. Postma Netherlands Institute for Sea Research, P. 0.Box 59, 1790 AB Den Burg, Texel, The Netherlands
Most probably the number of scientists active in nearshore research, including small water bodies like lagoons, is an order of magnitude larger than in 'blue ocean' research. This situation in the first place reflects the fact that every individual coastal system has its own characteristics which require a separate study and, at the same time, that generalization ofresults to other systems is more difficult. Other reasons are the relatively great economic importanceattached to coastal areas and the, mostly negative, effects of human interference in nearshore processes. Nevertheless, it is to be expected that the rapidly increasing knowledge of many separate systems will lead to more efforts to synthesize this knowledge. Some aspects which may assist these efforts are described in this chapter. One positive development must be mentioned at the beginning: the number ofjournals and books devoted to coastal research is rapidly increasing. Exchange of information is obviously essential for a subject where one might easily be satisfied with the description of one system or a small group of systems. The present volume derives its value first of all from its synthetic character. Methodologies A synthesis of coastal processes requires common methodologies and common problems. We start with a discussion of methodologies because progress in techniques of measurements is essential for a better understanding of coastal processes. A large number of methods - physical, geological, chemical and biological - has now been developed making accurate measurements of parameters possible. Of the physical methods must be mentioned new ways to measure movements of water masses, salinity and temperature patterns, and optical properties, partly by means of remote sensing. These have increased our insight into so-called fronts generated by the tides or by density differences and have greatly improved our knowledge of mixing and exchange between bodies of water. Applied to lagoons, they provide better and more information
554
Future of Research in Coastal Lagoons
on water transport pathways, including that of river water, and on residence (flushing) times. Intercomparison of lagoons of different sizes and shapes under various climate and tidal regimes will yield a numerical framework allowing predictions of changes where lagoons are modified. One of the possible effects of greenhouse warming that will be felt in coastal waters is a change, presumably a rise, in sea level. Whether this will indeed happen is at present uncertain, but the possibility itself is now already generating much research. Obviously, in a very shallow water body a change of only one or two decimeters will be important. The measurement of mean sea level, tidal amplitudes and extreme water levels will, therefore, be intensified. Technically, continuous registration of water level is not new and hundreds of tide gauges have already been active over long periods. New equipment has been installed in several places under the auspices of the World Ocean Climate Experiment (WOCE). Moreover, sea level measurement from space has become a distinct possibility. The main problem will be to continue such measurements over long periods. The same holds for the measurement of accompanying effects such as changes in temperature, fresh water supply, wave attack and ocean circulation. In the case of geological methods we are concerned with properties of sediments, the mechanics of sediment movements and measurements of sediment transport. Development of new methodology to progress beyond the descriptive phase is probably more urgent in this field than in any other. Notwithstanding considerable research efforts, our understanding of erosion and deposition processes is still rather poor. Results of laboratory experiments cannot easily be transferred to natural conditions where waves and currents act simultaneously. Moreover, extreme events such as spring tides, storms and hurricanes, and even scouring by ice exert a great influence on sediment movement. Remote sensing techniques are certainly very useful for the determination of turbidity patterns; even more important, however, is the development of instruments which can measure sediment behavior in situ autonomously and continuously under all circumstances. In the very shallow water of lagoons, the role of the sea bottom as a reservoir for and modifier of dissolved and particulate matter is predominant. Very accurate profiling is essential for estimating exchange with the overlying water and for learning the history of a sediment deposit, and techniques are now available to sample very thin slices in sediment cores. Several sensitive methods are now available to determine the age and thus the rates of deposition of sediment layers. Perhaps the development of chemical methods is the most spectacular aspect of new methodology. This development has very closely followed the successes of analytical chemistry. Several substances now measured rou-
H.Postma
555
tinely could not even be shown to be present a few years ago. As a result, in nearshore waters, a new branch of science is born which has as its subject the modification of chemical compounds in the transition zone between river water and the sea. Research focuses on the behavior of trace elements such as metals and also on man-made substances with a polluting potential. New methods have also greatly improved the determination of biogenic compounds. This holds for the classical nutrients: accurate measurements of, for example, nitrogen species have been developed quite recently as well as for organic substances. For the latter we are still in the phase of classification which is probably as far as one can go from a chemical point of view. The almost complete mixture of living organic matter and its organic waste products, so characteristic for coastal waters, and the use and reuse of waste products by living organisms, defies our understanding of such details of food chains. How long has the discussion about the possible use of dissolved organic matter as food now been going on? How much of the particulate organic matter in the sea bottom is really refractory? Progress in the development of biological methods has been relatively slow: there is not much automation possible in the determination of (marine) organisms. Nevertheless, methods for estimating numbers and biomass of smaller species such as phyto- and zooplankton have been improved considerably. Also, new analytical methods are available now for the determination of productivity and mineralization. These methods have, in addition, greatly benefited from new insights into micro-biological processes. For greater water depths, devices have been developed which automatically and continuously sample and analyze a number of biologically important parameters and directly observe animal behavior without disturbance. This is, among other things, important for studies of feeding behavior and for better estimates of bioturbation. These devices could with some adaptation also be used in coastal lagoons. Large enclosures (mesocosms), preferably consisting of a water column in contact with a sediment layer, have already proved to be very useful for the understanding of coastal ecosystems since biological and chemical parameters can, to some degree, be controlled. However, the number of sites where they have been used is rather small. More measurements, especially in tropical waters, are needed. Problems in Lagoon Research
In thinking about what should be given priority in coastal lagoon research it is not exactly possible to make a strict distinction between lagoons and other shallow water bodies with restricted connection to the ocean. Much can be learned from comparison of the whole spectrum of almost completely closed, so-called choked lagoons, to leaky lagoons or other open water bodies (estuaries and bays).
556
Future of Research in Coastal Lagoons
A condition for being considered a lagoon could be that land and ocean influence are of the same magnitude. Land influence does not have to be restricted to supply of river water, but could also include atmospheric input andor a salt and heat balance different from the open sea. It could also be stipulated that the residence time of water in the system should be so long that possible non-conservative behavior of elements becomes apparent. An even more strict condition would be that autonomic plankton communities are developed, but this would probably exclude too many water bodies. It is also difficult to make a clear distinction between pristine lagoons and those influenced by man’s activities. One reason is that the first type has become very rare. Not only do modifications in a lagoon itself have to be taken into account, but also those in adjacent land areas and in the ocean. A second reason is that obviously the most complete investigations are being made into modified and economically important lagoons. Finally, as a third reason, much can be learned about fundamental processes in lagoons from studies of lagoons under stress. Problems and processes which, in my opinion, will receive intensified attention in future research can be classified as follows: (1) morphology and stability under a regime of changing sea level and sediment supply; (2) biogeochemical processes in ecosystems under stress from, among other things, increasing eutrophication; and (3) transport pathways through and biogeochemical processes in lagoons. Morphology and Stability
We are well aware today of the relative fragility of sedimentary structures since so many examples of beach and sand barrier erosion have now been studied. It seems that on a global scale many more coastlines are now retreating than growing. One would like to know whether perhaps a certain optimum in natural coastal development has been passed. It would not be surprising, geologically speaking, if after the period of the last few thousand years that favored sand supply to the coastal zone -a gradually decreasing speed of sea-level rise - small forces causing losses from the system have become dominant. Such forces, e.g. longshore drifk, have always been active, but may in the past have been overruled by onshore transport. Human activities, however, are now in the first place responsible for coastal erosion. These are: excavation of sand in rivers, beaches and in lagoons themselves, deepening of navigation channels, trapping of mud and sand in inland reservoirs, and coastal subsidence by extraction of water and fossil fuel. Accelerated rise of sea level by greenhouse warming may become important in the future. For lagoons the consequences are a weakening of sand barriers, an increase in water depth, sometimes an increase in tidal range and, consequently, an acceleration of water exchange. These effects may result in a
H. Postma
557
more frequent flooding of marshes and a decrease in size of intertidal flats. Moreover, regulation of rivers, including building of reservoirs, cause profound changes in volume and periodicity of water and mud supply. The consequences of morphological changes for water movements can now largely be predicted by hydrodynamic modelling. These changes also have a n influence on the various ecosystems in lagoons and their interrelationships: marshes, sea grasses, intertidal and pelagic communities. Even more important are direct modifications of morphology. The most widespread of these is the reclamation and otherwise elimination of marshlands and mangroves. Lagoons are generally considered sinks of sand carried inward to adjust for submergence. Obviously, this process will come to a n end. Mud supply may similarly be affected, as is apparent in several deltas, but there are mostly multiple sources for mud which are not all exhausted at the same time. Moreover, residual transport of mud is preferably directed onshore by tidal pumping or water density differences and, after having settled for some time, mud consolidates and is removed with more difficultly than sand. It can to some degree take over the role of sand as infill where the latter material is not available. Whether this will happen depends largely on the strength of waves and currents. Our knowledge of fine grained sediment transport is still poor in at least two main respects: firstly, where there are several sources which are difficult to distinguish in the mixture. More refined methods to distinguish between clay types will improve the situation. Secondly, deposition and erosion are very much influenced by biological and chemical processes such as bioturbation and particle aggregation. More studies of these processes are required and comparison of different lagoons will be very useful. Remote sensing of turbidity patterns is also very helpful. Before leaving the subject we must again point out that only longterm measurements can give insight into sediment transport in lagoons. In the regular pattern, sand is moved chiefly during spring tides, but more important are extreme events like storms and river floods and, in cold climates, ice scraping. Under these conditions lagoons may change from importers to exporters of sediment, vice versa, and the old pattern may be re-established only after a long time interval or not at all.
Ecosystems Under Stress Coastal areas are characterized by a high productivity due to a combination of favorable factors. There is a rich supply of nutrients from the land and, especially in ocean upwelling areas, from the ocean. There is a short cycle of production and mineralization between the shallow bottom and the overlying water and the euphotic zone extends almost or completely to the bottom. In addition to phytoplankton, benthic flora takes care of part of primary production.
558
Future of Research in Coastal Lagoons
In most lagoons mineralization exceeds production of organic matter. In healthy lagoons aerobic conditions in the water column are present, however, by the actions of winds and tides which take care of sufficient oxygen supply. Sediment deposits are mostly anaerobic closely below the surface. Ecosystems can nowadays be successfully modelled. These models allow prediction of the direction of future changes caused by natural processes but especially by man s exploitation and input of waste products. Refinement of ecosystem models and construction of such models for different types of lagoons will be one of the primary tasks of future research. Ecosystem models can be used to indicate admissible limits of eutrophication. Undesirable plankton blooms or anaerobic conditions can in this way be avoided or, if they occur, be remedied. This means in most cases a prevention of excess nutrient input. Composition of plant communities, however, does not only depend on absolute concentrations of nutrients, but also on nutrient ratio’s. Decrease of one nutrient relative to another may change this composition in an undesirable direction. More attention to such effects will have to be given in the future. Most lagoons are ecocomplexes, containing more than one ecosystem: wetlands, marshes, sea grass fields, intertidal flats and pelagic systems. These may have to be considered separately and subsequently be integrated. Wetlands and marshes are oRen the most productive parts, exporting organic matter to the others. They are also the most threatened types. Loss of the marsh by reclamation or otherwise can turn a lagoon from a system exporting into one of importing organic matter. Loss of tidal flat areas by subsidence or rise of sea level may be another important modifying factor. The change of a lagoon from export to import has consequences for the adjacent shelf. The matter of export or import has for this reason already been discussed intensively in the past, a discussion which will certainly continue, a.0. because of present interest in the storage of excess carbon from fossil fuel. Lagoon populations are under considerable natural stress because of the great variability of such basic factors as temperatures, salinity, strength of waves and currents, ice cover in winter, and sometimes bad connections with the open sea. The effects on (animal) populations have been studied for a long time and much is known about adaptation to extreme conditions by benthic animals, avoidance of extremes by seasonal migrations to offshore waters and impoverishment by isolation. An important question is whether the ratio between net supply of organic matter by local primary production plus eventual import of organic matter from outside on the one hand and net secondary production by zooplankton, benthos and fish on the other in lagoons is also higher or lower than in other marine systems. The answer seems to a large degree to depend on food quality; imported organic detritus has a lower food value than fresh phytoplankton and is perhaps mainly used by bacteria.
H.Postma
559
Stresses by input of wastes including heat and salt, and by over-exploitation cause a decrease of species. Studies on biodiversity will have to be intensified since it is one of the factors determining stability of biological communities. Another development to be considered in this respect is the increase of mariculture which claims ever increasing areas in lagoons at the expense of original populations. Transport and Pathways of Materials
Coastal lagoons, like other coastal systems, are important passages from land to ocean for biogenic elements such as nutrients and carbon compounds, for heavy metals and for man-made materials among which (chlorinated) hydrocarbons. During passage, elements and compounds are recycled, settle for a shorter or longer time in lagoons and finally depart unchanged or in a modified form. Relatively little is known about residence times of various substances. These may vary between the flushing time of a lagoon, mostly only a few days of weeks, and permanent deposition. Longer residence times are caused by adherence to particles. Obviously for every substance this time will be different. Residence times are important characteristics, especially for pollutants, as yardsticks of accumulation and will require more attention in future studies. The biogeochemistry of elements in coastal waters has been studied extensively for over a century. In the preceding paragraph a few words have already been devoted to the question of nutrient ratios. "he oceanic Redfield ratio for N:P of 15 (in atoms) is not valid for lagoons; marsh vegetation, for example, has a lower nitrogen content than phytoplankton since the latter is richer in protein. Organic waste, on the other hand, mostly has a relatively high N/P ratio. Where modern purification plants are installed, the ratio becomes higher when phosphate is removed separately. Disproportionality between N and P tends to increase by production of phytoplankton which, also in lagoons, closely follows the Redfield ratio. Little is still known about the relative importance for the nitrogen balance of nitrification and denitrification processes, although lagoon deposits are good sites for these processes. Another biogenic element to be considered is silica. In lagoons this element is used by phytoplankton diatoms, benthic diatoms and marsh plants. After fixation silica is in most environments very slowly or not at all redissolved. This is not so in lagoons since dissolution can take place rapidly in anaerobic sediment, especially at elevated temperature. As a result, lagoons will often be exporters of dissolved silica to the open sea, adding to the amounts being brought to the sea by rivers. Nevertheless, since silica, contrary to phosphorus and nitrogen, is not enriched by input of waste water, the concentration is relatively low in many eutrophic lagoons. The mass balance of silica requires further investigation.
560
Future of Research in Coastal Lagoons
The last biologically important element t o be discussed here is carbon. Inorganic carbon is hardly ever a limiting factor for plant growth in lagoons, except perhaps in dense algal mats. The ratio of carbon in organic matter to the other biogenic elements is greatly variable, even for phytoplankton. Many lagoons export considerable amounts of dissolved organic carbon. It is, therefore, difficult to estimate a mass balance for carbon by analogy with the nutrients. However, much carbon is fixed permanently in lagoon deposits as refractory carbon. Below the surface layer of a lagoon sediment this type represents most of these carbon present, except in carbonate deposits. Extra quantities are slowly added not only to eutrophic lagoons but also to other coastal systems and the question has been put forward if these amounts have any significance for the fixation of fossil carbon. A further study of the amounts of organic carbon sidetracked from the main carbon cycle in different types of lagoons is worth while. A few words must be added about the cycles of metals in lagoons. A main characteristic is their accumulation in fine-grained materials and in living and dead organic matter. This property prolongs residence times and causes concentrations in suspended sediments and deposits to be higher than in both the increasing materials and in this adjacent open sea Potentially toxic metals as copper, cadmium, mercury and tin have, therefore, received much attention. Dissolution takes place, among others, in anaerobic sediment layers and by decomposition of binding organic matter. The redissolved matter may combine with dissolved organic matter and thus escape to the open sea. Details of these processes need further study. Whether metals cause damage to ecosystems depends on extra input above natural levels, concentrations in situ, degree of toxicity and speciation. The latter sometimes increases toxicity, as for example is the case for mercury, but can also make metals less poisonous, as in the case for copper. These effects have been well documented. In the majority of lagoons, however, we are badly informed about the effectiveness of metal binding and, consequently, about recovery times after the input of extra amounts have been terminated. With the exception of a few now classical cases, metals do not immediately cause visible damage, but in lagoons where the annual input continues to surpass the natural supply, such damage may become only apparent after a prolonged period. More harm than by metals, which after all are natural compounds of ecosystems and in small concentrations often essential, is done by artificial compounds such as chlorinated hydrocarbons. These belong to a category, which in any concentration has to be looked at with great suspicion. Like metals they may remain for a long time in the environment. When used as pesticides, they are often sprayed over large areas so that sources are difficult to locate. Fortunately, our knowledge of their behavior has been
H. Postma
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growing rapidly in recent years, so that protection is primarily a matter of good management. A closing remark for this paragraph must be that extreme water movements -hurricanes, winter storms, ice movements and even tsunamis etc. - tend to put the brakes on the building up of concentrations of all elements, in water as well as in sediments. Thus, to the negative effects of such events, at least one positive effect has to be added. Conclusion
I n the preceding chapters some gaps in our knowledge have been indicated, which in my opinion deserves attention in future research programs. These gaps are described in greater detail in the separate chapters of this book. Since our knowledge is very unevenly spread over the globe, a future main task is to study neglected lagoons. These are not only located in tropical areas, which are mostly well accessible, but also in remote areas as for example those around the Arctic. Other still neglected systems are hypersaline lagoons. It is probable that a global approach will yield new principles on the functioning of lagoons and their ecosystems which can not yet be predicted.
553
Chapter 19
Future of Research in Coastal Lagoons H. Postma Netherlands Institute for Sea Research, P. 0.Box 59, 1790 AB Den Burg, Texel, The Netherlands
Most probably the number of scientists active in nearshore research, including small water bodies like lagoons, is an order of magnitude larger than in 'blue ocean' research. This situation in the first place reflects the fact that every individual coastal system has its own characteristics which require a separate study and, at the same time, that generalization ofresults to other systems is more difficult. Other reasons are the relatively great economic importanceattached to coastal areas and the, mostly negative, effects of human interference in nearshore processes. Nevertheless, it is to be expected that the rapidly increasing knowledge of many separate systems will lead to more efforts to synthesize this knowledge. Some aspects which may assist these efforts are described in this chapter. One positive development must be mentioned at the beginning: the number ofjournals and books devoted to coastal research is rapidly increasing. Exchange of information is obviously essential for a subject where one might easily be satisfied with the description of one system or a small group of systems. The present volume derives its value first of all from its synthetic character. Methodologies A synthesis of coastal processes requires common methodologies and common problems. We start with a discussion of methodologies because progress in techniques of measurements is essential for a better understanding of coastal processes. A large number of methods - physical, geological, chemical and biological - has now been developed making accurate measurements of parameters possible. Of the physical methods must be mentioned new ways to measure movements of water masses, salinity and temperature patterns, and optical properties, partly by means of remote sensing. These have increased our insight into so-called fronts generated by the tides or by density differences and have greatly improved our knowledge of mixing and exchange between bodies of water. Applied to lagoons, they provide better and more information
554
Future of Research in Coastal Lagoons
on water transport pathways, including that of river water, and on residence (flushing) times. Intercomparison of lagoons of different sizes and shapes under various climate and tidal regimes will yield a numerical framework allowing predictions of changes where lagoons are modified. One of the possible effects of greenhouse warming that will be felt in coastal waters is a change, presumably a rise, in sea level. Whether this will indeed happen is at present uncertain, but the possibility itself is now already generating much research. Obviously, in a very shallow water body a change of only one or two decimeters will be important. The measurement of mean sea level, tidal amplitudes and extreme water levels will, therefore, be intensified. Technically, continuous registration of water level is not new and hundreds of tide gauges have already been active over long periods. New equipment has been installed in several places under the auspices of the World Ocean Climate Experiment (WOCE). Moreover, sea level measurement from space has become a distinct possibility. The main problem will be to continue such measurements over long periods. The same holds for the measurement of accompanying effects such as changes in temperature, fresh water supply, wave attack and ocean circulation. In the case of geological methods we are concerned with properties of sediments, the mechanics of sediment movements and measurements of sediment transport. Development of new methodology to progress beyond the descriptive phase is probably more urgent in this field than in any other. Notwithstanding considerable research efforts, our understanding of erosion and deposition processes is still rather poor. Results of laboratory experiments cannot easily be transferred to natural conditions where waves and currents act simultaneously. Moreover, extreme events such as spring tides, storms and hurricanes, and even scouring by ice exert a great influence on sediment movement. Remote sensing techniques are certainly very useful for the determination of turbidity patterns; even more important, however, is the development of instruments which can measure sediment behavior in situ autonomously and continuously under all circumstances. In the very shallow water of lagoons, the role of the sea bottom as a reservoir for and modifier of dissolved and particulate matter is predominant. Very accurate profiling is essential for estimating exchange with the overlying water and for learning the history of a sediment deposit, and techniques are now available to sample very thin slices in sediment cores. Several sensitive methods are now available to determine the age and thus the rates of deposition of sediment layers. Perhaps the development of chemical methods is the most spectacular aspect of new methodology. This development has very closely followed the successes of analytical chemistry. Several substances now measured rou-
H.Postma
555
tinely could not even be shown to be present a few years ago. As a result, in nearshore waters, a new branch of science is born which has as its subject the modification of chemical compounds in the transition zone between river water and the sea. Research focuses on the behavior of trace elements such as metals and also on man-made substances with a polluting potential. New methods have also greatly improved the determination of biogenic compounds. This holds for the classical nutrients: accurate measurements of, for example, nitrogen species have been developed quite recently as well as for organic substances. For the latter we are still in the phase of classification which is probably as far as one can go from a chemical point of view. The almost complete mixture of living organic matter and its organic waste products, so characteristic for coastal waters, and the use and reuse of waste products by living organisms, defies our understanding of such details of food chains. How long has the discussion about the possible use of dissolved organic matter as food now been going on? How much of the particulate organic matter in the sea bottom is really refractory? Progress in the development of biological methods has been relatively slow: there is not much automation possible in the determination of (marine) organisms. Nevertheless, methods for estimating numbers and biomass of smaller species such as phyto- and zooplankton have been improved considerably. Also, new analytical methods are available now for the determination of productivity and mineralization. These methods have, in addition, greatly benefited from new insights into micro-biological processes. For greater water depths, devices have been developed which automatically and continuously sample and analyze a number of biologically important parameters and directly observe animal behavior without disturbance. This is, among other things, important for studies of feeding behavior and for better estimates of bioturbation. These devices could with some adaptation also be used in coastal lagoons. Large enclosures (mesocosms), preferably consisting of a water column in contact with a sediment layer, have already proved to be very useful for the understanding of coastal ecosystems since biological and chemical parameters can, to some degree, be controlled. However, the number of sites where they have been used is rather small. More measurements, especially in tropical waters, are needed. Problems in Lagoon Research
In thinking about what should be given priority in coastal lagoon research it is not exactly possible to make a strict distinction between lagoons and other shallow water bodies with restricted connection to the ocean. Much can be learned from comparison of the whole spectrum of almost completely closed, so-called choked lagoons, to leaky lagoons or other open water bodies (estuaries and bays).
556
Future of Research in Coastal Lagoons
A condition for being considered a lagoon could be that land and ocean influence are of the same magnitude. Land influence does not have to be restricted to supply of river water, but could also include atmospheric input andor a salt and heat balance different from the open sea. It could also be stipulated that the residence time of water in the system should be so long that possible non-conservative behavior of elements becomes apparent. An even more strict condition would be that autonomic plankton communities are developed, but this would probably exclude too many water bodies. It is also difficult to make a clear distinction between pristine lagoons and those influenced by man’s activities. One reason is that the first type has become very rare. Not only do modifications in a lagoon itself have to be taken into account, but also those in adjacent land areas and in the ocean. A second reason is that obviously the most complete investigations are being made into modified and economically important lagoons. Finally, as a third reason, much can be learned about fundamental processes in lagoons from studies of lagoons under stress. Problems and processes which, in my opinion, will receive intensified attention in future research can be classified as follows: (1) morphology and stability under a regime of changing sea level and sediment supply; (2) biogeochemical processes in ecosystems under stress from, among other things, increasing eutrophication; and (3) transport pathways through and biogeochemical processes in lagoons. Morphology and Stability
We are well aware today of the relative fragility of sedimentary structures since so many examples of beach and sand barrier erosion have now been studied. It seems that on a global scale many more coastlines are now retreating than growing. One would like to know whether perhaps a certain optimum in natural coastal development has been passed. It would not be surprising, geologically speaking, if after the period of the last few thousand years that favored sand supply to the coastal zone -a gradually decreasing speed of sea-level rise - small forces causing losses from the system have become dominant. Such forces, e.g. longshore drifk, have always been active, but may in the past have been overruled by onshore transport. Human activities, however, are now in the first place responsible for coastal erosion. These are: excavation of sand in rivers, beaches and in lagoons themselves, deepening of navigation channels, trapping of mud and sand in inland reservoirs, and coastal subsidence by extraction of water and fossil fuel. Accelerated rise of sea level by greenhouse warming may become important in the future. For lagoons the consequences are a weakening of sand barriers, an increase in water depth, sometimes an increase in tidal range and, consequently, an acceleration of water exchange. These effects may result in a
H. Postma
557
more frequent flooding of marshes and a decrease in size of intertidal flats. Moreover, regulation of rivers, including building of reservoirs, cause profound changes in volume and periodicity of water and mud supply. The consequences of morphological changes for water movements can now largely be predicted by hydrodynamic modelling. These changes also have a n influence on the various ecosystems in lagoons and their interrelationships: marshes, sea grasses, intertidal and pelagic communities. Even more important are direct modifications of morphology. The most widespread of these is the reclamation and otherwise elimination of marshlands and mangroves. Lagoons are generally considered sinks of sand carried inward to adjust for submergence. Obviously, this process will come to a n end. Mud supply may similarly be affected, as is apparent in several deltas, but there are mostly multiple sources for mud which are not all exhausted at the same time. Moreover, residual transport of mud is preferably directed onshore by tidal pumping or water density differences and, after having settled for some time, mud consolidates and is removed with more difficultly than sand. It can to some degree take over the role of sand as infill where the latter material is not available. Whether this will happen depends largely on the strength of waves and currents. Our knowledge of fine grained sediment transport is still poor in at least two main respects: firstly, where there are several sources which are difficult to distinguish in the mixture. More refined methods to distinguish between clay types will improve the situation. Secondly, deposition and erosion are very much influenced by biological and chemical processes such as bioturbation and particle aggregation. More studies of these processes are required and comparison of different lagoons will be very useful. Remote sensing of turbidity patterns is also very helpful. Before leaving the subject we must again point out that only longterm measurements can give insight into sediment transport in lagoons. In the regular pattern, sand is moved chiefly during spring tides, but more important are extreme events like storms and river floods and, in cold climates, ice scraping. Under these conditions lagoons may change from importers to exporters of sediment, vice versa, and the old pattern may be re-established only after a long time interval or not at all.
Ecosystems Under Stress Coastal areas are characterized by a high productivity due to a combination of favorable factors. There is a rich supply of nutrients from the land and, especially in ocean upwelling areas, from the ocean. There is a short cycle of production and mineralization between the shallow bottom and the overlying water and the euphotic zone extends almost or completely to the bottom. In addition to phytoplankton, benthic flora takes care of part of primary production.
558
Future of Research in Coastal Lagoons
In most lagoons mineralization exceeds production of organic matter. In healthy lagoons aerobic conditions in the water column are present, however, by the actions of winds and tides which take care of sufficient oxygen supply. Sediment deposits are mostly anaerobic closely below the surface. Ecosystems can nowadays be successfully modelled. These models allow prediction of the direction of future changes caused by natural processes but especially by man s exploitation and input of waste products. Refinement of ecosystem models and construction of such models for different types of lagoons will be one of the primary tasks of future research. Ecosystem models can be used to indicate admissible limits of eutrophication. Undesirable plankton blooms or anaerobic conditions can in this way be avoided or, if they occur, be remedied. This means in most cases a prevention of excess nutrient input. Composition of plant communities, however, does not only depend on absolute concentrations of nutrients, but also on nutrient ratio’s. Decrease of one nutrient relative to another may change this composition in an undesirable direction. More attention to such effects will have to be given in the future. Most lagoons are ecocomplexes, containing more than one ecosystem: wetlands, marshes, sea grass fields, intertidal flats and pelagic systems. These may have to be considered separately and subsequently be integrated. Wetlands and marshes are oRen the most productive parts, exporting organic matter to the others. They are also the most threatened types. Loss of the marsh by reclamation or otherwise can turn a lagoon from a system exporting into one of importing organic matter. Loss of tidal flat areas by subsidence or rise of sea level may be another important modifying factor. The change of a lagoon from export to import has consequences for the adjacent shelf. The matter of export or import has for this reason already been discussed intensively in the past, a discussion which will certainly continue, a.0. because of present interest in the storage of excess carbon from fossil fuel. Lagoon populations are under considerable natural stress because of the great variability of such basic factors as temperatures, salinity, strength of waves and currents, ice cover in winter, and sometimes bad connections with the open sea. The effects on (animal) populations have been studied for a long time and much is known about adaptation to extreme conditions by benthic animals, avoidance of extremes by seasonal migrations to offshore waters and impoverishment by isolation. An important question is whether the ratio between net supply of organic matter by local primary production plus eventual import of organic matter from outside on the one hand and net secondary production by zooplankton, benthos and fish on the other in lagoons is also higher or lower than in other marine systems. The answer seems to a large degree to depend on food quality; imported organic detritus has a lower food value than fresh phytoplankton and is perhaps mainly used by bacteria.
H.Postma
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Stresses by input of wastes including heat and salt, and by over-exploitation cause a decrease of species. Studies on biodiversity will have to be intensified since it is one of the factors determining stability of biological communities. Another development to be considered in this respect is the increase of mariculture which claims ever increasing areas in lagoons at the expense of original populations. Transport and Pathways of Materials
Coastal lagoons, like other coastal systems, are important passages from land to ocean for biogenic elements such as nutrients and carbon compounds, for heavy metals and for man-made materials among which (chlorinated) hydrocarbons. During passage, elements and compounds are recycled, settle for a shorter or longer time in lagoons and finally depart unchanged or in a modified form. Relatively little is known about residence times of various substances. These may vary between the flushing time of a lagoon, mostly only a few days of weeks, and permanent deposition. Longer residence times are caused by adherence to particles. Obviously for every substance this time will be different. Residence times are important characteristics, especially for pollutants, as yardsticks of accumulation and will require more attention in future studies. The biogeochemistry of elements in coastal waters has been studied extensively for over a century. In the preceding paragraph a few words have already been devoted to the question of nutrient ratios. "he oceanic Redfield ratio for N:P of 15 (in atoms) is not valid for lagoons; marsh vegetation, for example, has a lower nitrogen content than phytoplankton since the latter is richer in protein. Organic waste, on the other hand, mostly has a relatively high N/P ratio. Where modern purification plants are installed, the ratio becomes higher when phosphate is removed separately. Disproportionality between N and P tends to increase by production of phytoplankton which, also in lagoons, closely follows the Redfield ratio. Little is still known about the relative importance for the nitrogen balance of nitrification and denitrification processes, although lagoon deposits are good sites for these processes. Another biogenic element to be considered is silica. In lagoons this element is used by phytoplankton diatoms, benthic diatoms and marsh plants. After fixation silica is in most environments very slowly or not at all redissolved. This is not so in lagoons since dissolution can take place rapidly in anaerobic sediment, especially at elevated temperature. As a result, lagoons will often be exporters of dissolved silica to the open sea, adding to the amounts being brought to the sea by rivers. Nevertheless, since silica, contrary to phosphorus and nitrogen, is not enriched by input of waste water, the concentration is relatively low in many eutrophic lagoons. The mass balance of silica requires further investigation.
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The last biologically important element t o be discussed here is carbon. Inorganic carbon is hardly ever a limiting factor for plant growth in lagoons, except perhaps in dense algal mats. The ratio of carbon in organic matter to the other biogenic elements is greatly variable, even for phytoplankton. Many lagoons export considerable amounts of dissolved organic carbon. It is, therefore, difficult to estimate a mass balance for carbon by analogy with the nutrients. However, much carbon is fixed permanently in lagoon deposits as refractory carbon. Below the surface layer of a lagoon sediment this type represents most of these carbon present, except in carbonate deposits. Extra quantities are slowly added not only to eutrophic lagoons but also to other coastal systems and the question has been put forward if these amounts have any significance for the fixation of fossil carbon. A further study of the amounts of organic carbon sidetracked from the main carbon cycle in different types of lagoons is worth while. A few words must be added about the cycles of metals in lagoons. A main characteristic is their accumulation in fine-grained materials and in living and dead organic matter. This property prolongs residence times and causes concentrations in suspended sediments and deposits to be higher than in both the increasing materials and in this adjacent open sea Potentially toxic metals as copper, cadmium, mercury and tin have, therefore, received much attention. Dissolution takes place, among others, in anaerobic sediment layers and by decomposition of binding organic matter. The redissolved matter may combine with dissolved organic matter and thus escape to the open sea. Details of these processes need further study. Whether metals cause damage to ecosystems depends on extra input above natural levels, concentrations in situ, degree of toxicity and speciation. The latter sometimes increases toxicity, as for example is the case for mercury, but can also make metals less poisonous, as in the case for copper. These effects have been well documented. In the majority of lagoons, however, we are badly informed about the effectiveness of metal binding and, consequently, about recovery times after the input of extra amounts have been terminated. With the exception of a few now classical cases, metals do not immediately cause visible damage, but in lagoons where the annual input continues to surpass the natural supply, such damage may become only apparent after a prolonged period. More harm than by metals, which after all are natural compounds of ecosystems and in small concentrations often essential, is done by artificial compounds such as chlorinated hydrocarbons. These belong to a category, which in any concentration has to be looked at with great suspicion. Like metals they may remain for a long time in the environment. When used as pesticides, they are often sprayed over large areas so that sources are difficult to locate. Fortunately, our knowledge of their behavior has been
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growing rapidly in recent years, so that protection is primarily a matter of good management. A closing remark for this paragraph must be that extreme water movements -hurricanes, winter storms, ice movements and even tsunamis etc. - tend to put the brakes on the building up of concentrations of all elements, in water as well as in sediments. Thus, to the negative effects of such events, at least one positive effect has to be added. Conclusion
I n the preceding chapters some gaps in our knowledge have been indicated, which in my opinion deserves attention in future research programs. These gaps are described in greater detail in the separate chapters of this book. Since our knowledge is very unevenly spread over the globe, a future main task is to study neglected lagoons. These are not only located in tropical areas, which are mostly well accessible, but also in remote areas as for example those around the Arctic. Other still neglected systems are hypersaline lagoons. It is probable that a global approach will yield new principles on the functioning of lagoons and their ecosystems which can not yet be predicted.