a U.S. national park, has no permanent human inhabitants, and the watershed for Siskiwit Lake is wholly forested. Thus, there are no sources of organochlorine compounds in the watershed; long-range atmospheric transport of these compounds is the only logical explanation for their presence in the lake. In Canada, airborne contamination of lakes has resulted in consumption advisories for fish throughout Ontario and in regions as remote as the Yukon Territories (Kidd et al., 1995b) (see Box 3-9).

The presence of metals and synthetic organic compounds is a concern because many of them may cause birth defects, cancer, or immunological and reproductive disorders (Colborn and Clement, 1992; Colborn et al., 1993). Many bioaccumulate to such high levels that they are toxic to the end-members of aquatic food chains. In the case of mercury, rules of the U.S. Food and Drug Administration state that fish cannot be sold if the concentration exceeds 1 part per million (ppm). In Canada, the corresponding threshold for commercial sale is 0.5 ppm; that value is exceeded in many areas, most notably in reservoirs (Rosenberg et al., 1995). The Minnesota Department of Health recommends that pregnant women eat no more than one meal per month of fish containing more than 0.15 ppm of mercury. Minnesota has a lake water standard for mercury concentra

BOX 3-9 LONG-RANGE TRANSPORT OF TOXIC COMPOUNDS TO LAKE LABERGE, YUKON TERRITORIES

Lake Laberge is widely regarded as a symbol of remote northern wilderness as the result of the popular poem "The Cremation of Sam McGee," written by Robert W. Service during the Yukon gold rush. The catchment of the lake is still largely uninhabited except for the city of Whitehorse, above the lake on the Yukon River. The lake is large (200 km2) and served as an important source of protein for local indigenous peoples.

In 1992, the fishery of the lake was closed because of high toxaphene concentrations in the fish. Concentrations of DDT and PCBs also were higher than in other large lakes of the Yukon. Initially, investigators hypothesized that the contamination was caused by surreptitious dumping of toxaphene in the lake. However, detailed studies of toxaphene concentrations and stable isotopes of nitrogen in Laberge and other lakes of the area, which had lower toxaphene concentrations, showed that the higher toxaphene concentrations in Laberge fish may be explained by the fact that food chains were one step longer in Laberge than in other lakes of the area (Kidd et al., 1995a,b). The toxaphene entered the lake via contaminated rainfall from the United States and Eurasia and then accumulated in the fatty tissues of aquatic organisms, with higher levels of toxaphene found at higher levels of the food chain (see Figure 3-3).



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