The partial recovery of Lake Erie is one of the major success stories of the Clean Water Act, but at the same time the lake's problems are not over. The lake continues to receive excess phosphorus from agricultural runoff, and recycling of phosphorus that has accumulated in the bottom sediments continues. Although states surrounding the Great Lakes report that the presence of excess phosphorus and other elemental nutrients is now a major concern along less than 3 percent of Great Lakes shoreline, the presence of toxic compounds such as polychlorinated biphenyls remains a concern and has resulted in advisories against consuming Lake Erie fish (EPA, 1994). Furthermore, invasions of exotic species such as zebra mussels threaten to disrupt the lake's food web and its production of game fish (see Chapter 3).
can convey large quantities of nitrogen, phosphorus, and organic matter that stimulate excess growth of algae and oxygen-consuming bacteria. This process, known as cultural eutrophication, ultimately results in loss of water clarity, loss of oxygen in bottom waters, and a shift in the food web from valuable game fish to less desirable species. High concentrations of pesticides also may be present in agricultural runoff; fish kills are commonly reported to insurance companies or the EPA when major storms follow pesticide applications (National Research Council, 1992). Storm runoff from urban areas also can transport high concentrations of pollutants such as lawn chemicals, metals, automotive oil and grease, and bacteria from animal wastes. Both agricultural and urban runoff contain sediment that can carry adsorbed contaminants and smother fish habitats and spawning areas.
Alteration of natural hydrology: Throughout history, but especially in the twentieth century, humans have manipulated water bodies and their surrounding watersheds to serve purposes such as providing power, supplying water for irrigation, and regulating water flows to allow farming and building on floodplains. As a result, half of the wetlands, which once helped to cleanse the water flowing into rivers and lakes, are now gone from the lower 48 states (see, for example, Box 1-3) (National Research Council, 1995). The more than 80,000 dams in the United States (Frederick, 1991) alter downstream flow patterns in ways that can jeopardize the survival of important fish species. For example, 19 major dams and 100 smaller power projects have been built along the Columbia River in Washington and Oregon since the 1930s; the resulting habitat modifications have contributed to the extinction of 106 stocks of Pacific salmon that once spawned in the river (McGinnis, 1994). The EPA (1994) estimates that hydrologic, habitat, and flow modifications prevent desired uses of 36 percent of U.S. lakes and reservoirs and 7 percent of rivers and streams.
Atmospheric transport of pollutants: Atmospheric circulation can transport