ABSTRACT-High mercury concentrations in fish of some Minnesota lakes and rivers were first noted in 1971. Major anthropogenic sources of mercury to Minnesota rivers generally have been identified and controlled, but it has been difficult to identify mercury sources to remote northeastern Minnesota lakes containing fish with elevated mercury levels. Analysis of sediments from these lakes suggests that mercury deposition has increased by a factor of about 3.5 since white settlement in the state. Because the increase is spatially constant across northeastern Minnesota, atmospheric pollution appears to be responsible for the increase. Given that fish mercury varies considerably among lakes, local environmental processes apparently control the conversion of inorganic mercury into the methylmercury found in fish. Mercury levels in Minnesota fish are related to wa~er chemistry, which in turn is influenced by watershed geology. Fish from lakes in limestonerich watersheds are less likely to have high mercury levels than lakes in low-alkalinity regions. lakes with colored water are more likely to contain fish with high mercury levels. Trend analysis shows that mercury levels in fish increased significantly between the 1930s and 1980s (comparison between museum specimens and recent samples) and between the 1970s and 1980s. Rates of increase are highest in low-alkalinity lakes
