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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology Linkages Among Diverse Aquatic Ecosystems: A Neglected Field of Study Eville Gorham Department of Ecology, Evolution, and Behavior University of Minnesota, St. Paul St. Paul, Minnesota SUMMARY This paper discusses the scarcity of studies of linkages (functional couplings) among wetlands, streams, rivers, and small and large lakes. It examines difficulties in the pursuit of such studies and the drawbacks of restricting studies to single ecosystems. Several examples of ecosystem linkages are described, along with examples of important linkages deserving greater consideration in the future. The paper suggests mechanisms for fostering closer ties among students of different kinds of ecosystems in both research and teaching. INTRODUCTION Aquatic scientists commonly combine a variety of scientific approaches—physical, chemical, biological and ecological—in the study of their particular ecosystem, whether it be stream, lake, wetland, or ocean. It has been far less common, even unusual, for these same aquatic scientists to investigate in any detailed way the interactions of the type of ecosystem in which they specialize with other types of aquatic ecosystems, despite the fact that all are linked through the hydrological cycle and in many other ways. Indeed, it is more common for such ecologists to examine interactions with upland ecosystems, for instance, between streams and their valleys (Hynes, 1975; Vannote et al., 1980; Gregory et al., 1991), or with the atmosphere, as in studies of methane emissions from peatlands (Bartlett and Harriss, 1993; Harriss et al., 1993), than to focus on the linkages among streams, wetlands, lakes, and oceans.
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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology Figure 1 illustrates a way of categorizing research studies by both research field and ecosystem type (the atmosphere is added as a source of inputs). A reading of the technical literature indicates that most ecosystem studies proceed horizontally in Figure 1. One important exception is the study of anadromous fish that migrate between the ocean and fresh waters. Another is the study of nutrient cycling, in which atmospheric inputs are followed through the gravitational phase of the hydrological cycle (i.e., vertically in Figure 1) to catchments, streams, lakes, and wetlands (e.g., Bormann and Likens, 1979; Urban and Eisenreich, 1988). Nutrient inputs from rivers to estuaries and oceans have been followed similarly. Given the great success and significance of such nutrient-cycling studies, which are very widely cited in the literature of ecosystem ecology and biogeochemistry, a strong argument can be made for broadening greatly such vertically oriented research on the linkages among aquatic ecosystems to include other aspects of ecosystem function, diverse examples of which are given in the following pages. There are, however, impediments to the pursuit of such studies. DIFFICULTIES IN THE PURSUIT OF INTERECOSYSTEM STUDIES It is difficult for any one person to acquire knowledge of more than one type of ecosystem. Few have expertise in both streams and lakes, or Ecosystems Major Fields of Research Physics Chemistry Organismal Biology Community and Ecosystem Ecology Landscape Ecology Global Studies Atmosphere (deposition) Upland catchments Wetlands Streams and rivers Small lakes Large lakes Oceans FIGURE 1 A matrix illustrating scientific approaches and the different sorts of ecosystems to which they are applied.
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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology combine one of those two with an interest in wetland ecosystems or in oceans. Despite the need for them, generalists in ecosystem studies are rare, just as are field botanists who can identify more than one group among the vascular plants, bryophytes, lichens, and algae. Given the rarity of generalists, team approaches are useful, although these have their own problems of interpersonal relations, differences in the scientific cultures (with their unfortunate ''pecking orders") from which team members spring, leadership, funding, etc. One result of the lack of generalists is the infrequent occurrence of studies focusing on linkages among aquatic ecosystems. An examination of titles (and, in a few cases where the title was insufficient, abstracts) in recent issues of specialist journals revealed the following rations of linkage-oriented papers to the total number: Limnology and Oceanography (1 to 88), Journal of the North American Benthological Society (0 to 70), Wetlands (5 to 83), Journal of Marine Research (0 to 60), and Canadian Journal of Fisheries and Aquatic Sciences (4 to 99). In total, only 2.5 percent of the papers discussed interecosystem studies. This lack of examples means that the topic is seldom brought to the attention of either practicing scientists or students beginning to read technical literature. Indeed, the need for studies of functional couplings among streams, lakes, and wetlands is mentioned seldom, and without detailed consideration, in recent documents discussing the future of freshwater studies (Lewis et al., 1995; Naiman et al., 1995), although Lewis et al. do deplore the fragmentation of freshwater scientists into separate societies dealing with streams (North American Benthological Society), lakes and oceans (American Society of Limnology and Oceanography), and wetlands (Society of Wetland Scientists). Because interecosystem studies are unusual, it may be difficult to attract research funds for them, as is sometimes the case with interdisciplinary studies. Members of review panels usually specialize in one of the major types of aquatic ecosystem, and some of them—consciously or unconsciously—may not be sympathetic to proposals to examine linkages among the various types. The reverse may of course be true of other reviewers; more needs to be known about such reviewer bias in order to find ways of countering it. EXAMPLES OF INTERDISCIPLINARY AND INTERECOSYSTEM STUDIES The nature and importance of interdisciplinary and interecosystem studies can be illustrated most clearly by a series of examples that demonstrate, first, the linkages among disciplines and, second, the functional couplings among different types of inland aquatic ecosystems.
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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology Linkages Among Major Fields of Research Examples of the employment of linked physical, chemical, and biological approaches to the study of aquatic ecosystems are numerous and readily found in any modern textbook on lakes (Wetzel, 1983), streams (Hynes, 1970; Allan, 1995), wetlands (Mitsch and Gosselink, 1993), and oceans (Duxbury and Duxbury, 1994), or in books on individual ecosystems such as those of the Hubbard Brook Experimental Forest (Likens et al., 1977; Bormann and Likens, 1979). The following few examples illustrate their nature and their relevance to the solution of important environmental problems. Example 1. It has long been known (Mortimer, 1956) that physical stratification of shallow, highly productive lakes in summer, by isolating their deeper waters from contact with the atmosphere, is responsible for the severe depletion of oxygen that makes those deeper lakes uninhabitable by many organisms, including a number of fish species and their prey. Likewise, the freezing phase of the annual temperature cycle in northern latitudes may lead to oxygen depletion throughout shallow lakes and therefore to the winterkill that often decimates sport fisheries. Example 2. Cultural eutrophication, involving nuisance algal blooms caused by nutrient enrichment from fertilizers and sewage, is a serious environmental problem. Paleoecological reconstruction of its history usually employs radioisotope dating of sediment cores, combined with the measurement of sedimentation rates for nutrients such as phosphorus and for the plant and animal microfossils and fossilized plant pigments that are indicators of aquatic productivity (Brugam, 1978; Engstrom et al., 1985). Conversely, studies of bryophyte fossils in radiocarbon-dated peat cores (Janssens, 1983) have allowed the construction of profiles of past levels of acidity and water tables in peatlands (Gorham and Janssens, 1992; Janssens et al., 1992) so that we can see if future human activities causing acid deposition from the atmosphere or climatic warming lead to changes that transcend those that have occurred in past decades, centuries, or millennia. Example 3. In studies of the toxic effects of acid deposition from the atmosphere on stream and lake biota—with special reference to sport and commercial fisheries—it has been necessary to consider patterns of air mass movement, amounts of precipitation, the timing of snowmelt, and other physical factors that control the deposition and pathways of strong acids resulting from urban-industrial air pollution. Also involved are studies of geology, soil chemistry, alkalinity generation by biological processes, physiological analysis of gill function in response to high aluminum concentrations, examination of various alterations in biotic communities and their food chains, and research on a host of other biological responses to interacting physical and chemical properties of the environment (Freedman, 1989a; Anonymous, 1990).
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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology Example 4. Recent studies in northern Minnesota (Siegel, 1983, 1988; Glaser et al., 1990) have shown that upwelling of circumneutral ground water rich in calcium bicarbonate is vital to the development of patterned peatlands in northern landscapes. This upwelling water forms distinct fen water tracks that become dominated by sedges and flow through the peatlands in a way that separates them sharply from the ovoid, acid sphagnum bogs that are the other distinctive feature of such landscapes. Because peatlands are an important source and sink for greenhouse gases, and fens and bogs differ in their production and consumption of such gases, it is important to understand the hydrology that controls their development. Fens and bogs also differ greatly in their acidity, and hence in their export of acidity to streams and lakes, which in turn influences plants and animals downstream from these peatlands (see next section). Linkages Among Different Types of Aquatic Ecosystem Studies that link different aquatic ecosystems, although by no means lacking, are much more difficult to find in the literature, as shown previously. The following are several examples of such studies. Example 1. Among the earliest studies to link peatlands and lakes were those that described the temporal linkages involved in the pattern of aquatic succession leading—on a time scale of decades to millennia—from lake to sedge meadow (DeLuc, 1810) and even to raised bog (Aiton, 1811; see also Gorham, 1953). Raymond Lindeman (1941) was one of many later investigators of such phenomena. Such successional studies can be very important in devising programs of ecosystem rehabilitation, which are best carried out from detailed knowledge of how a given ecosystem reached its present state. Example 2. Chemical budgets for lakes, reservoirs, and wetlands necessarily involve measurement of varying flow rates and associated chemical fluxes through inflow and outflow streams (Schindler et al., 1976; Likens et al, 1977; Urban and Eisenreich, 1989; Urban et al., 1990). Such budgets are of vital importance to understanding whether a given aquatic ecosystem can store nutrients or toxins to a significant degree and thus prevent runoff to downstream ecosystems. Such storage may also be a source of later problems if disturbance—for instance, drawdown of a wetland—releases such materials downstream (Bayley et al., 1992). Example 3. Many scientists and engineers concerned with problems of cultural eutrophication have examined the hypothesis that wetlands can act as traps for nutrients such as phosphorus and nitrogen in runoff from adjacent upland soils and thus mitigate nuisance algal blooms in adjacent lakes (Johnston, 1991; Richardson and Craft, 1993). Ecological limitations
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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology on such uses of wetlands have been discussed by Guntenspergen and Stearns (1981). Example 4. Analysis of carbon isotopes has shown that particulate organic detritus eroded from tundra peatlands is a significant source of nutrition for food chains leading to fish and ducks in nearby ponds and lakes (Schell, 1983). This sort of information provides a better understanding of food chain dynamics in freshwater habitats, a matter of particular concern for indigenous peoples and sport fishermen. Example 5. Geochemical study of iron and aluminum in acid Nova Scotian lakes (Urban et al., 1990) has shown that both elements, weathered from upland catchments, are combined with dissolved organic matter (DOM), supplied from nearby peatlands, to form metal humates. Precipitation of such humates from solution may regulate the concentrations of both iron and aluminum, and to a lesser extent DOM, in these acid lakes. DOM concentrations in the lake water are also related to the proportion of the catchment that is covered by peat (Gorham et al., 1986; Kortelainen, 1993a). The importance of such studies is twofold. Dissolved organic matter in these waters consists largely of colored organic (humic) acids that can, like acid rain, acidify fresh waters. Unlike acid rain, however, which releases aluminum from soils and sediments in forms toxic to fish, humic acids form nontoxic complexes with aluminum. Example 6. Studies of diverse catchments in northwestern Ontario (St. Louis et al., 1994) have revealed that wetlands are important sources of methylmercury to downstream aquatic ecosystems. In its methylated form, mercury can bioaccumulate to toxic levels in aquatic food chains, leading ultimately to humans through fish (Minnesota Fish Consumption Advisory, 1994). Example 7. Studies of northern pike, an important North American sport fish, show that it depends on wetlands flooded in spring for spawning and nursery areas. This is undoubtedly true for other organisms. Example 8. The influence of beaver on small streams, ponds, and lakes is profound (Johnston and Naiman, 1990a,b) and can be understood only at the landscape level because these furbearing mammals range over all three types of water body, as well as adjacent uplands. All of these habitats are affected substantially by beaver use, which has an influence that may last as a legacy on the landscape for decades or more after they have left the area. Example 9. Wetlands can have an appreciable influence on the water budgets of streams and rivers. An extreme example is the White Nile, which flows through a vast wetland, the Sudd. As much as half of the water flowing into the Sudd is lost by evapotranspiration from the wetland (Melack, 1992), which must greatly affect all aspects of the river downstream.
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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology DRAWBACKS TO FRAGMENTATION BY ECOSYSTEM The previous examples illustrate clearly the importance of studying functional couplings among aquatic ecosystems as well as to the surrounding uplands; without such studies, our understanding would be seriously inadequate. The nature of various problems of environmental degradation, many of which affect all upland and aquatic ecosystems in one way or another, provides a further and extremely cogent reason for such interecosystem studies, as shown by the following examples: Example 1. If as expected, climate warming becomes a serious environmental problem in the next century (Intergovernmental Panel on Climate Change, Working Group 1, 1992), the Great Plains of North America will likely see a sharp decline in the number of prairie potholes combining wetlands suitable for the breeding of ducks with open water suitable for their feeding. Interpreting changes in these prairie potholes and in their associated biota as they dry out owing to greatly increased drought frequency will require a careful assessment of the linkages among lakes, ponds, and wetlands as they are affected in differing degrees by alterations in ground water hydrology. The prospect for devastating effects on duck populations in the Great Plains—already threatened by human overexploitation—is alarming. Example 2. Studies of lake acidification ignored for a long time the possibility that upstream and marginal peatlands can be a potent source of acid inputs; these studies ascribed lake acidification solely to atmospheric deposition of sulfuric and nitric acids derived from air pollutants. Organic acids from peat bogs (Gorham et al., 1985) have, however, proved to be significant and, in some cases, predominant contributors to lake acidification in areas of Nova Scotia (Gorham et al., 1986; Kerekes et al., 1986; Gorham et al., in review), Norway (Brakke et al., 1987), and Finland (Kortelainen, 1993b). They must, therefore, be taken into account in much of the north temperate zone, where bogs are common features of the landscape. On the one hand, ignoring such natural acid inputs may overemphasize the effects of acid rain caused by human activity, but on the other hand, natural acidification of this kind may predispose some aquatic ecosystems to damage by relatively small inputs of acid rain.1 Example 3. Peats have a great capacity to bind toxaphene and DDT (Rapaport and Eisenreich, 1986), as well as other organochlorine micropollutants. The possibility exists, therefore, that wetlands—by their effective trapping of such materials—are a significant factor in lessening the 1 It should be noted that some clear water lakes on rocks that are not readily weathered can become quite acid through a variety of natural processes (Ford, 1990), though not to the degree brought about by acidic deposition from the atmosphere (Renberg, 1990) or substantial inputs from peat bogs in the catchment (Gorham et al., 1986).
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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology buildup of such toxic pollutants in downstream ecosystems. If severe climate warming comes to pass and leads to drawdown of peatland water tables, it may well be that subsequent peat aeration and oxidation will release these toxins—largely concentrated in the surface peats—downstream. The same may be true of nitrogen and sulfur, stored in these peatlands in large amounts (Gorham, 1994) and liable to be released in acid form by drought and fire (Bayley et al., 1992). Bog mosses and lichens also have a strong affinity for radioactive fallout (Gorham, 1958, 1959; Miettinen, 1969), which could be released similarly. In all three cases there is the potential for ecological damage, but at present we lack sufficient information to evaluate its likely significance. Example 4. Oil pollution is a largely local problem in all sorts of ecosystems from the uplands to the ocean. Many different organisms from diverse habitats are capable of degrading different petroleum compounds (Freedman, 1989b). The search for the most efficient among them for use in bioremediation should encompass the full range of those habitats. In all of these examples, restriction of studies to a single type of ecosystem would also restrict, to a very significant extent, both our thinking about and our understanding of serious environmental problems. IMPORTANT QUESTIONS FOR FUTURE RESEARCH ON THE LINKAGES AMONG AQUATIC ECOSYSTEMS More attention to interecosystem linkages can be justified only if truly important research questions, both fundamental and practical, can be asked about them. The following are a few examples suggested from experience in studying lakes, streams, and wetlands. Individual scientists will be able to add their own examples, and environmental surprises in the years to come will reveal many more cases in which a better understanding of interecosystem linkages would have served us well. Example 1. How does the influence of organic carbon compounds (both particulate and dissolved) on (1) aquatic productivity; (2) acid-base balance; (3) oxidation-reduction potential; (4) trace-metal transport; and (5) emissions of volatile carbon, nitrogen, and sulfur compounds change from the time of its deposition as dead organic matter (forest litter and aquatic plant remains) to the time of its ultimate arrival in the open ocean, having passed through streams, wetlands, small and large lakes, large rivers, and estuaries? All of these items are important elements in the metabolism of aquatic ecosystems, on which sport and commercial fisheries depend, but although we have fitted some parts of the metabolic puzzle together we do not yet have anything close to a complete picture spanning the full range of aquatic ecosystems. Example 2. Do chemical inputs of nutrients and toxins from wetlands have an influence on the biodiversity of receiving streams and lakes? It
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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology is all but certain that they do, but we have far too little information about such matters, which may affect the overall functioning of these aquatic ecosystems and also be important to the success or failure of threatened and endangered species. Example 3. In a broader way, we may ask how riparian wetlands, which are extensions of the littoral zones of streams and rivers, influence the functioning of the actual littoral zones and their adjacent water bodies. Wetzel (1990, 1992) has provided a general account of effects on carbon cycling, but much remains to be done in terms of the cycles of nutrients and toxins and their effects upon species composition, productivity, food chains, etc. The invasion of North American wetlands by the Eurasian exotic purple loosestrife (Thompson et al., 1987), as it affects the functioning of adjacent streams and lakes, is a specific case worthy of further investigation in this context. Also relevant are the movements of organisms such as insects, amphibians, and furbearing mammals to and from uplands, wetlands, and adjacent littoral areas of streams and lakes. Example 4. In a related question, what is the influence of streams and lakes on the plant and animal communities of adjacent wetland and upland ecosystems? This is an important matter that concerns the design of buffer zones around lakes and the management of riparian zones. Example 5. How will effects of global warming on wetlands influence ecosystem structure and function in the streams and lakes that receive inputs from them (for instance, of nutrients such as nitrogen and phosphorus, acidifying agents such as organic (humic) acids and sulfuric acid, or stored metal and organic pollutants)? Meyer and Pulliam (1992) provide some interesting insights that suggest future research directions. Example 6. An extremely important problem in landscape ecology (Risser, 1987), as well as in regional and global studies, is that of scaling up from purely local studies of individual ecosystems to develop understanding at the landscape or higher level. It is here that interactions among aquatic (and upland) ecosystems are of major importance, because they must be taken into account at each successive step in the scaling process if we are to understand adequately, for instance, nutrient cycling and the processing of toxic contaminants, as well as their influence on species composition, productivity, carbon storage, etc.—all vital components in the functioning and management of the biosphere. In many cases, processes of ecosystem interaction will be nonlinear and may involve threshold and stochastic phenomena, making it necessary to study them over long periods for adequate understanding (Likens, 1989). For instance, consider the influence of peatlands on small lakes in the same catchment and receiving drainage from them. The studies of Gorham et al. (1986) indicate that the influence of such peatlands on the concentrations of colored organic acids in the lakes increases in nonlinear fashion with the ratio of peatland area to lake area. Even a very small proportion
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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology of peatland in a catchment leads to a rapid increase in lake acidity, but the rate of increase slows steadily as the proportion of peatlands in the catchment rises. As an example of a threshold phenomenon, peatlands may shift rather suddenly—over a period of decades to centuries and after millennia of relative stability—from circumneutral sedge meadows to strongly acid moss bogs as the level of upward peat accumulation reaches a threshold that shuts off supplies of bases from the mineral soil, so that acid-loving Sphagnum mosses can invade the sedge communities (Gorham and Janssens, 1992). The consequences for the acidity of downstream waters, and hence for their plants and animals, can be considerable. Elements of the answers to all these questions undoubtedly exist in technical literature, but more active research on interecosystem processes would help bring them into focus, filling many gaps in our present information. MECHANISMS FOR FOSTERING CLOSER TIES AMONG STUDENTS OF DIFFERENT AQUATIC ECOSYSTEMS If scientists are to be encouraged to study functional couplings among the diverse types of aquatic ecosystems, increased attention to both mechanisms and incentives for bringing them together will be required. That attention must focus not only on research itself, but also on professional recognition of the importance of generalist ecosystem studies and on the teaching of future generations of aquatic scientists. Research The best way to encourage research on ecosystem linkages is to provide research and training grants explicitly for that purpose. Foundations may perhaps be more willing to encourage innovative ideas for such studies, but many federal agencies are engaged in the study of environmental problems that, as noted earlier, involve a variety of interecosystem processes. The National Science Foundation (NSF), without such a problem-oriented mandate, might also be approached to begin a modest pioneer program on ecosystem linkages in the same way that it began the Long-Term Ecological Research Program under the Ecosystem Studies Program. Alternatively, it could be suggested for inclusion in the new NSF-Environmental Protection Agency partnership for environmental research. Another way to foster research is to hold workshops and symposia devoted to bringing together the scientists who study different kinds of aquatic ecosystems. Such meetings may be sponsored by scientific societies, government agencies, or private foundations; scientific journals—societal or private—frequently assist the process by publishing their proceedings. Gordon Conferences, the Dahlem Conferences, and North Atlantic
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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology Treaty Organization Advanced Institutes are excellent examples. Scientific societies can also hold joint meetings to bring their members together, as happened at Edmonton recently with the American Society of Limnology and Oceanography and the Society of Wetland Scientists. It is not enough, however, merely to bring scientists from different backgrounds together; they must also be brought to focus directly in individual sessions on the functional couplings among wetlands, streams, lakes, and oceans rather than simply to compare ecosystem processes such as net primary production, food chain dynamics, cycling of nutrients and toxins, and emissions of trace gases, valuable as such environmental and biotic comparisons may be. Professional recognition for ecosystem generalists who study functional couplings is also important. If they are regarded merely as dilettantes, their careers are likely to suffer in departments staffed largely by specialists. Teaching Students can be recruited more effectively for graduate studies that deal with linkages among aquatic ecosystems (and with upland ecosystems and the atmosphere) if they learn about them as undergraduates. Such teaching should not be left too late or students will already have become restricted in their studies, and barriers will have arisen to an interecosystem approach. Under what rubric such an approach can be taught most effectively remains to be seen; if a few examples are presented in courses strongly focused primarily on one of the above-mentioned disciplines, they are unlikely to achieve their purpose. Courses on ecosystem ecology and biogeochemistry may be the best places to proselytize for interecosystem studies, especially if they are team-taught by specialists in the different types of ecosystems who are committed to a focus on linkages in both teaching and research. Support for curriculum development could be sought from education-oriented private foundations and from the recently expanded Education Program of the National Science Foundation. Organizational support for teaching the functional couplings among ecosystems (and with the atmosphere) might come most readily from the linkage of specialist departments into interdisciplinary programs, centers, institutes, etc. The Watershed Science Program of Utah State University is an example, linking groups in fisheries and wildlife, forest resources, geography and earth resources, and range science. Such programs, however, may not always have the assured staffing and line-item budgets required for long-term commitment, and the allegiance of program faculty (and often their graduate students) to their home departments may make such programs less than ideal. Departments that are organized specifically
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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology for interdisciplinary studies of the environment—as, for instance, at the University of Virginia where the Department of Environmental Sciences encompasses four tracks (ecology, geology, hydrology, and atmospheric sciences)—probably provide the optimal model, with their own line-item and supply budgets. CONCLUSION Given the fundamental scientific and practical economic importance of linkages among aquatic (and other) ecosystems and the present difficulties of studying them, a concerted effort to foster research on them is badly needed. Initiatives should therefore be developed at this time, in both teaching and research, to attract scientists to their study. A broader graduate training, developing thorough familiarity with more than one type of ecosystem, would be greatly conducive to thinking about the functional couplings among ecosystems. ACKNOWLEDGMENTS I thank Pat Brezonik for assigning me this topic. Jackie MacDonald and my fellow committee members provided many helpful comments and suggestions, as did Jesse Ford and Susan Galatowitsch. REFERENCES Aiton, W. 1811. Treatise on the Origin, Qualities, and Cultivation of Moss-Earth, with Direction for Converting It into Manure. Ayr, Scotland: Wilson and Paul. Allan, J. D. 1995. Stream Ecology: Structure and Function of Running Waters. London: Chapman and Hall. Anonymous. 1990. Acid Deposition: State of Science and Technology. Washington, D.C.: National Acid Precipitation Assessment Program. Bartlett, K. B., and R. C. Harriss. 1993. Review and assessment of methane emissions from wetlands. Chemosphere 26:261–320. Bayley, S. E., D. W. Schindler, B. R. Parker, M. P. Stainton, and K. G. Beaty. 1992. Effects of forest fire and drought on acidity of a base-poor boreal forest stream: Similarities between climatic warming and acidic precipitation. Biogeochemistry 17:191–204. Bormann, F. H., and G. E. Likens. 1979. Pattern and Process in a Forested Ecosystem. Berlin: Springer-Verlag. Brakke, D. F., A. Henriksen, and S. A. Norton. 1987. The relative importance of acidity sources for humic lakes in Norway. Nature 329:432–434. Brugam, R. B. 1978. Human disturbance and the historical development of Linsley Pond. Ecology 59:19–36. DeLuc, J. A. 1810. Geological Travels, Vol. 1. London: Rivington. Duxbury, A. C., and A. B. Duxbury. 1994. An Introduction to the World's Oceans, 4th ed. Dubuque, Ia.: W. C. Brown. Engstrom, D. R., E. B. Swain, and J. C. Kingston. 1985. A paleolimnological record of
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Representative terms from entire chapter: