Web Release Date: September 6,
Mercury in Soils, Lakes, and Fish in Voyageurs National Park (Minnesota): Importance of Atmospheric Deposition and Ecosystem Factors
University of Wisconsin-La Crosse, River Studies Center, La Crosse, Wisconsin 54601, U.S. Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, Wisconsin 54603, Chemistry Department, Gustavus Adolphus College, St. Peter, Minnesota 56082, U.S. Geological Survey, Water Science Center of Minnesota, Mounds View, Minnesota 55112, St. Croix Watershed Research Station, Marine on St. Croix, Minnesota 55047, U.S. Geological Survey, National Center, Reston, Virginia 20192, and Metropolitan Council Environmental Services, St. Paul, Minnesota 55106
Received for review April 5, 2006
Revised manuscript received July 17, 2006
Accepted July 24, 2006
Abstract:
Concentrations of methylmercury in game fish from many interior lakes in Voyageurs National Park (MN, U.S.A.) substantially exceed criteria for the protection of human health. We assessed the importance of atmospheric and geologic sources of mercury to interior lakes and watersheds within the Park and identified ecosystem factors associated with variation in methylmercury contamination of lacustrine food webs. Geologic sources of mercury were small, based on analyses of underlying bedrock and C-horizon soils, and nearly all mercury in the O- and A-horizon soils was derived from atmospheric deposition. Analyses of dated sediment cores from five lakes showed that most (63% ± 13%) of the mercury accumulated in lake sediments during the 1900s was from anthropogenic sources. Contamination of food webs was assessed by analysis of whole, 1-year-old yellow perch (Perca flavescens), a regionally important prey fish. The concentrations of total mercury in yellow perch and of methylmercury in lake water varied substantially among lakes, reflecting the influence of ecosystem processes and variables that affect the microbial production and abundance of methylmercury. Models developed with the information-theoretic approach (Akaike Information Criteria) identified lake water pH, dissolved sulfate, and total organic carbon (an indicator of wetland influence) as factors influencing methylmercury concentrations in lake water and fish. We conclude that nearly all of the mercury in fish in this seemingly pristine landscape was derived from atmospheric deposition, that most of this bioaccumulated mercury was from anthropogenic sources, and that both watershed and lacustrine factors exert important controls on the bioaccumulation of methylmercury.
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