quantities caused little harm to the environment because concentrations in water were very low—usually in the part per billion, trillion, or even quadrillion range. However, such views did not account for biomagnification in aquatic food chains (Gilbertson, 1988; Peakall, 1993). Many limnological studies have shown that in the passage up food chains, organochlorine compounds can accumulate in predatory fish or fish-eating birds and mammals at concentrations up to 10 million times greater than those in lake water. Even in very remote areas, the concentrations of organochlorine compounds in fish and marine mammals can be high enough to require that consumption by humans be limited.

Much of the runoff of toxic pollutants is episodic in nature, yet past research has focused mostly on long-term ambient conditions. Limnologists are developing better approaches for capturing episodic pulses, identifying their consequences, and determining the synergistic effects of multiple contaminants and sources (Davies, 1991; Herricks et al., 1994). Limnologists also continue to study long-term, sublethal effects of toxic compounds on organisms and populations; many management decisions are based on lethal doses, whereas the true long-term consequences may result from more subtle effects. Finally, limnologists are advancing knowledge of the effects of toxic compounds that have accumulated in the sediments of rivers and lakes, but much more needs to be learned about the long-term effects of these compounds. It is known, however, that some pesticides have subtle, even intergenerational effects on endocrine systems (Colborn and Clement, 1992; Colborn et al., 1993).

In summary, limnologists, along with other water scientists, have a long history of identifying the sources, routes, and consequences of pollutants from urban, industrial, and agricultural areas. Limnologists have been and will continue to be instrumental in identifying the linkages among waste discharges, runoff, water quality, and ecosystem functioning (see Box 3-7) Their involvement in developing solutions to the problems of nonpoint-source pollution from runoff will be especially critical as these problems increase.

Acid Rain and Toxic Air Pollutants

Acid rain has been known as a localized problem resulting from industrial development since the middle of the nineteenth century; in this century it has become recognized as a broad regional to global environmental problem, with major occurrences in Europe, eastern North America, the former Soviet Union, and more recently in East Asia. It is a consequence of two major human activities. One is the use of fossil fuel. For example, many sources of coal contain substantial amounts of sulfur, which upon combustion is released to the atmosphere as sulfur dioxide (SO2). In addition, the high-temperature combustion of all fossil fuels,

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