Large lakes and reservoirs are used as public water supplies; the American Water Works Association (Achtermann, 1989) estimates that 68 percent of the water used for domestic purposes by the 600 largest utilities (>50,000 customers) comes from impounded surface waters (natural lakes and man-made reservoirs). For simplicity, in this chapter the term lake refers both to natural impoundments and to man-made reservoirs. The five Great Lakes alone supply domestic water to some 24 million Americans. Lakes provide many other economic benefits to society and are used for such diverse purposes as commercial fishing, transportation, irrigation, and dilution of wastewater effluents. Not all of these uses are compatible. The use of lakes as receptacles for wastewater obviously is likely to impair their usefulness as water supplies and recreational resources, but more subtle incompatibilities also exist. For example, the production of warmwater game fish is enhanced by increasing nutrient levels, at least up to a point, but swimmers prefer water to be as clear (hence, unproductive) as possible.
Lake ecosystems are subject to stress from a wide range of human activities within their watersheds and along their shorelines and from the variety of ways that humans use them. These stresses often have caused significant impairment of lake quality. Six major classes of stresses have been important in degrading the quality of U.S. lakes in recent decades:
excessive inputs of nutrient and organic matter, leading to eutrophication;
hydrologic and physical changes such as water-level stabilization;
siltation from inadequate erosion control in agricultural and mining activities;
introduction of exotic species;
acidification from atmospheric sources and acid mine drainage; and
contamination by toxic (or potentially toxic) metals such as mercury and organic compounds such as PCBs and pesticides.
In addition, chemical stresses to lakes can be categorized according to source as (1) point sources (such as municipal wastewater), which generally are the easiest to identify and control; (2) nonpoint or diffuse