pollutants such as heavy metals, pesticides, polychlorinated biphenyls, and sulfuric and nitric acids (the damaging components of acid rain) to even the most remote environments (Czuczwa et al., 1984). For example, fish in some subarctic and high-alpine lakes contain toxaphene at levels above health standards (Kidd et al., 1995). Subsistence fisheries for aboriginal people are threatened in many areas of the far North (Lockhart, 1995). Pollutants affecting these remote areas are transported by air from the United States and Eurasia (Barrie, 1986; Lockhart, 1995). The contaminants concentrate in fatty tissues of organisms high on the food chain, in some cases rendering top predators 100,000 to 10 million times more contaminated than the rainwater that delivers the contaminants (Schindler et al., 1995).

• Introduction of exotic species and loss of native species: Exotic species introduced to a water body either on purpose or accidentally can decimate native species and significantly alter the aquatic food web. For example, nonnative plants such as water hyacinth, hydrilla, and Eurasian watermilfoil have spread to thousands of acres of U.S. lakes. The uncontrolled growth of these plants, some of which were introduced by the aquarium industry and others because they were regarded as visually attractive, interferes with swimming, clogs canals and drainage outlets, and alters the aquatic food web (National Research Council, 1992). Zebra mussels, introduced to the Great Lakes in 1986 from ship ballast water, are threatening the survival of important commercial fish and native clams and mussels (Roberts, 1990). In the Boundary Waters Canoe Area wilderness in northeastern Minnesota, walleyes and smallmouth bass at one time were stocked in some lakes, where they have taken hold, sometimes to the detriment of native species (Friends of the Boundary Waters Wilderness, 1992). Scientists have made similar observations in western mountain lakes: 80 percent of alpine lakes in the western United States have been stocked with nonnative species (Bahls, 1992). In western Canada, 20 percent of lakes in the mountain national parks have been stocked (Donald, 1987). Research has shown that these exotic species have altered natural food webs dramatically (Lamontagne and Schindler, 1994; Leavitt et al., 1994; Paul and Schindler, in press).

Because of these and other problems, eliminating point-source discharges of pollutants to water bodies, while necessary, is insufficient to prevent further degradation of damaged aquatic ecosystems. Researchers have attempted to estimate the net effect of reducing point-source discharges, with discouraging results. For example, EPA researchers simulated point-source releases of contaminants along the 630,000 miles of larger U.S. rivers and streams that receive 85 percent of total point-source discharges in the United States and modeled the effects of decreases in discharges of biological oxygen-demanding materials, total suspended