United States since 1990, when Congress passed the Clean Air Act Amendments, which call for a decrease in sulfur oxide emissions by about 50 percent by the end of the century. Even with controls on sulfur emissions, long-term atmospheric deposition of nitrate- and ammonia-nitrogen remains an uncontrolled threat to many aquatic (and other) ecosystems. The effects of nitrogen deposition have received far less study than the toxic effects associated with the deposition of sulfate and its associated hydrogen ions, and increased attention to this issue is warranted. Indeed, the EPA recently announced that it would not propose a new acid deposition standard to protect sensitive areas from nitrogen deposition because ''scientific uncertainty" makes it "difficult to determine the appropriate level of a standard or standards at this time" (EPA, 1995). This inaction is of particular importance to individuals in New York who are concerned about protecting vulnerable areas in the Adirondack Mountains (Renner, 1995).

Along with sulfur and nitrogen, fossil fuels contain a variety of toxic metals, among them mercury, lead, and cadmium. Upon combustion these are emitted to the atmosphere on particulate matter that can be transported over long distances. Smelters add to the atmospheric loading of these metals. In addition, the combustion of fossil fuels produces a variety of polycyclic aromatic hydrocarbons that can be transported in the atmosphere by particulate matter. Some of these, such as benzo-a-pyrene (also present in tobacco smoke), are extremely toxic. Other organic micropollutants distributed regionally through the atmosphere are a consequence of the agricultural use of herbicides and insecticides. Still other organic micropollutants (such as dioxins and PCBs) are emitted to the atmosphere—as well as directly to aquatic ecosystems—as a result of industrial activity. Although PCBs and many chlorinated insecticides have been banned from manufacture and use in the United States, the legacy of past use still provides a source of these materials. Some banned insecticides such as DDT are still used in other countries, such as Mexico, and enter U.S. ecosystems as a result of long-range atmospheric transport.

The deleterious effects of these metal and synthetic organic compounds upon inhabitants and users of aquatic ecosystems are less well documented than those of acid rain. However, in the case of mercury and PCBs, warnings have been issued in more than 30 states concerning the consumption by humans of fish from numerous lakes and rivers, including many in the Great Lakes states. Atmospheric transport and deposition is the major source of mercury in these regions (Swain et al., 1992). Eisenreich and coworkers (1979) have shown that the principal mechanism by which PCBs enter Lake Superior is long-range atmospheric transport and deposition by rain, snow, and dryfall. Dioxins and PCBs also have been found in sediments and biota of Siskiwit Lake, an otherwise pristine water body on Isle Royale in Lake Superior (Swackhamer and Hites, 1988). Isle Royale,



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