sewage treatment plants); they include various diffuse pollution sources, as well as a variety of physical stresses that directly or indirectly affect aquatic habitats. Limnologists, working with other aquatic scientists, have been involved in developing ways to optimize the preservation of inland waters:
Watershed management: Maintaining and improving the quality of surface waters will require consideration of how the wide variety of human activities on the land surrounding water bodies (their watersheds) affects water quality. Limnologists have been involved in demonstrating how land-use changes alter the yields of chemical constituents and sediments to water bodies, and they should be instrumental in devising scientifically based watershed management plans.
Wetland preservation: Substantial losses of wetlands have occurred in the past. Recognizing the valuable role of wetlands in providing habitats for important species, reducing flood peaks, and filtering runoff before it enters lakes and rivers, the United States has developed a policy intended to prevent further net loss of wetlands. Research by limnologists will be essential for developing a workable way to implement this policy.
Control of cultural eutrophication: Eutrophication—the input of excess nutrients (especially phosphorus and nitrogen) to lakes and rivers from sewage, agricultural fertilizers, and other sources—results in the development of large masses of algae and (often) large aquatic plants. The algae decrease water clarity, deplete deep-water oxygen to the degree that important species of fish cannot survive, and may create taste, odor, and toxicity problems. Work by limnologists has been and will continue to be central to developing plans to control nutrient inputs in order to restore ecosystems as large as Lake Erie and as small as farm ponds.
Reservoir management: More than 80,000 dams exist in North America. Limnologists, working with hydraulic engineers, hydrologists, and fisheries biologists, need to play key roles in developing the scientific understanding necessary to manage the impoundments created by dams and to optimize dam operations to preserve water quality below the dams.
Study of the effects of global warming: Nations worldwide have been discussing strategies to mitigate the impacts of global warming. Limnological studies can shed light on how global warming—which could alter water supply, water temperatures, and related habitat factors—may affect aquatic ecosystems.
Evaluation of toxic pollutants: The release of toxic substances in trace quantities once was thought to cause little harm to the environment, but aquatic scientists have shown repeatedly that such substances can accumulate in fish and fish-eating birds to levels up to 10 million times greater than those in lake water, resulting in health advisories against