by EPA, 39 states provided trophic classification on a total of 15,514 lakes (U.S. EPA, 1990b). About 30 percent of the surveyed lakes were classified as eutrophic or hypereutrophic, and 23 percent were mesotrophic. Trophic conditions were unknown in about 30 percent of the lakes included in the survey. In some cases, a lake is eutrophic simply as a result of natural circumstances (e.g., ecoregional characteristics), but nonpoint pollution from agricultural and urban run-off is the cause of use impairment from excess nutrients in most lakes.

The trophic status of the North American Great Lakes, including Great Bear Lake and Great Slave Lake, was summarized by Robertson and Scavia (1984). They concluded that Lakes Ontario and Erie are eutrophic and that Green Bay (Lake Michigan), Saginaw Bay (Lake Huron), and the Lake Erie western basin are highly eutrophic. The other lakes are mesotrophic or oligotrophic.

Canada has the largest acreage of lakes in the world, and a complete inventory, much less an assessment of their trophic states, is not available at this time. Most of them are thought to be oligotrophic, and in terms of raw numbers, the great majority of Canadian lakes lie in wilderness or undeveloped forests. Nonetheless, many lakes in agricultural areas of southern Canada have water quality problems resulting from excessive nutrients, and recreational developments have led to impaired water quality in some lakes located within driving distance of major urban areas such as Toronto. A small sample of 130 Canadian lakes found 16 of them to be eutrophic (Janus and Vollenweider, 1981).

Summary reports (e.g., Vollenweider and Kerekes, 1981; Forsberg, 1987) show that eutrophication problems are widespread throughout Europe. Reports of this nature do not exist for other continents, but accounts of extensive soil erosion and massive siltation of reservoirs everywhere, coupled with the absence of wastewater treatment in many areas (Brown and Wolf, 1984; Postel, 1985), suggest that water bodies worldwide are affected by excessive biological production and its consequences. Rapid in-filling of major impoundments in Third World nations is particularly troubling in view of their needs for irrigation water, potable supplies, and flood control. Deforestation and cultivation of marginal lands are causing soil losses at rates that will fill some impoundments in these countries in 5 to 20 years (Brown and Wolf, 1984).

ACIDIFICATION

The National Surface Water Survey (NSWS), a major survey of lakes and streams in acid-sensitive regions of the United States, was



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