activities caused accumulations of toxic metals in biota of Lake Coeur d'Alene, Idaho.

Long-range atmospheric transport from widespread sources is blamed for high body burdens of mercury (Hg) in the fish of many otherwise pristine lakes in forested regions of the Upper Midwest (Henning, 1989; Swain and Helwig, 1989). The problem in these states is more pronounced in low-alkalinity (acid-sensitive) lakes, but levels of bioaccumulation are not closely correlated with water pH. Mercury contamination of fish is at least indirectly related to acidic deposition in that fossil fuel burning by power plants contributes to both problems. The accumulation of mercury varies widely among different species of fish; biomagnification proceeds as mercury moves through the food web, and top carnivores such as walleye have the highest body burdens. Within a given species, body burdens increase with size (and age) of the fish.

Several states routinely issue consumption advisories related to mercury contamination of fish in lakes, and there is much concern about the economic impacts of these advisories on sport fishing in the affected regions. The nature of the advisories varies from state to state, and depending on the level of contamination, the advisories may recommend that a certain size range and species of fish not be eaten at all or that consumption be limited to one meal per week or per month. Problems caused by mercury in lakes are not limited to human consumption of contaminated fish; wildlife whose diet includes fish are also at risk. Body burdens of mercury in piscivorous loons in northern Minnesota are high enough to cause acute toxicity and may explain some incidents of loon mortality (Swain and Helwig, 1989).

Contamination problems involving organochlorine compounds such as pesticides and PCBs have been induced in lakes by all three types of sources for chemical stress (point sources, nonpoint watershed sources, and long-range atmospheric transport). High levels of PCBs in fish of the lower Great Lakes are attributed to general, widespread use of these chemicals from the 1920s to the 1970s, but localized cases of sediment contamination can usually be traced to one or a few specific industrial operations. For example, severe contamination of sediments in Waukegan harbor (Lake Michigan) occured as the result of disposal practices by one manufacturer. At the other extreme, high body burdens of PCBs are found in some large lake trout in Lake Superior (at levels sufficient to cause a consumption advisory), in spite of the fact that the lake has only minor point sources and nonpoint watershed sources of PCBs. Atmospheric transport (on scales of hundreds or even thousands of miles) is the



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