webs, termed "biomanipulation," to reduce populations of nuisance algae in situations where nutrient and water inputs cannot be altered sufficiently (Shapiro et al., 1975; Shapiro and Wright, 1984; Shapiro, 1995). Biomanipulation controls algal blooms by maximizing the population of grazing zooplankton. The zooplankton population can be increased by minimizing the population of zooplanktivorous fish by establishing large populations of piscivorous predators. Carpenter et al. (1985) developed a set of specific predictions on the cascading impacts of shifting populations of large piscivores on the primary producers of lakes and tested the predictions through a series of whole-lake experiments (Carpenter and Kitchell, 1993). These experiments supported the basic biomanipulation concept, although some limnologists have criticized it (see Shapiro, 1995). The efficacy of biomanipulation as a management tool is currently being explored in Lake Mendota, Wisconsin, through a collaborative effort involving the University of Wisconsin–Madison and the Wisconsin Department of Natural Resources (Kitchell, 1992).

The response of limnologists to improve understanding of the eutrophication problem is one of the important success stories in ecological science. Key investigations involved both fundamental and applied science. Practical solutions were implemented that reversed the trend of increasing eutrophication in many important lakes, including Lakes Erie, Michigan, and Tahoe. However, many lakes continue to suffer from eutrophication problems in part because of the economic difficulties involved in implementing effective controls on nonpoint nutrient sources. Development of cost-effective strategies for controlling nonpoint sources of nutrients, minimizing their effects, and restoring degraded lakes remains a key research need. Moreover, many fundamental and practical limnological questions remain unanswered or incompletely resolved. For example, limnologists cannot predict which specific assemblage of blue-green algae will result from specific nutrient loadings, nor can they predict which other algae will replace them when nutrient levels are decreased or biomanipulation is used.

Accumulation of Toxic Pollutants

Municipal and industrial waste discharges, urban runoff, and agricultural runoff all contain trace concentrations of toxic compounds (Novotny and Chesters, 1981; Lazaro, 1990). Often present in these discharges are organochlorine compounds, some of which are among the most toxic substances known to humans. Examples include dioxins and PCBs (poly-chlorinated biphenyls), which enter waterways from industrial sources, as well as DDT, toxaphene, and many other pesticides, which enter lakes and streams via runoff from agricultural lands and managed forests.

Originally, people believed that release of toxic compounds in trace

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