has led to the idea that the consequences of nutrient enrichment (i.e., high primary production) can be managed by food web management (e.g., Kitchell, 1992). This idea, sometimes called biomanipulation, was pioneered by Shapiro and his students (e.g., Shapiro et al., 1975).

The role of organic matter in natural waters has been the subject of numerous studies over the past 25 years, and substantial advances have been made in understanding the role of natural dissolved organic matter (NDOM) in controlling light transmission, in affecting the bioavailability of metals, and more recently in stimulating photolysis of synthetic organic compounds (via the production of highly reactive photointermediates such as the hydroxyl radical and singlet oxygen following the absorption of light energy by NDOM). The importance of particulate organic matter (detritus) and NDOM in energy flow within aquatic systems also has become better understood (e.g., Wetzel, 1992), but this is still an area in need of additional research.

Within the past 10 to 15 years, the principles of fluid mechanics have been applied to lakes in increasingly sophisticated ways by engineers and physical scientists, who have created the subdiscipline of environmental fluid mechanics (or environmental hydraulics). This work has helped limnologists understand the dynamics of water flow on a wide range of scales: from microscopic (e.g., nutrient depletion microzones surrounding plankton, and the patchy nature of the microhabitat and its role in maintaining multispecies communities in the face of competition for a common resource) to macroscopic (e.g., design of more efficient aerators and understanding the fate of silt-laden tributaries in reservoirs).

Cycling of major and minor nutrient elements (carbon, nitrogen, phosphorus, sulfur, and minor metals) within lake basins has been a topic of research in mainstream limnology for many decades. Volume 1 of Hutchinson's (1957) Treatise on Limnology gathered and synthesized extensive amounts of both published and unpublished data regarding these cycles in lakes and is still an important resource. Concerns about eutrophication and acid deposition stimulated much additional research on these cycles over the past few decades. Studies on elemental cycling at various scales, from ecosystems to the globe, has become a recognized field of study called biogeochemistry (e.g., Gorham, 1991a; Schlesinger, 1991). In limnological biogeochemistry, lakes are used as models for larger systems (e.g., the oceans) and as convenient systems for the measurement of process rates, fluxes, and storage mechanisms. Such studies are intrinsically interdisciplinary and typically involve elements of chemistry, microbiology, hydrology, and mathematical analysis (modeling).

Finally, the subject of ecosystem energetics has been a major organizing tool in ecology for more than 50 years, since Lindeman published his classic 1942 study of "trophic dynamics"—energy flow through the food web in Cedar Bog Lake. This study was the first important analysis of



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