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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).