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Complex Response to Decline in Atmospheric Deposition of Mercury

Areal photograph of lakes in Voyageurs National Park, Minnesota
Voyageurs National Park (VNP), a pristine setting with abundant lakes, wetlands, and streams situated on granitic bedrock, is located near northern Minnesota's border with Canada. Long-term studies at VNP have revealed trends in mercury concentration in precipitation, surface water, and fish. Photo credit: David P. Krabbenhoft, USGS.
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U.S. Geological Survey (USGS) scientists found that mercury concentrations in shallow waters and methylmercury (MeHg) concentrations in fish in four lakes in Voyageurs National Park, Minnesota, were not consistent with decreases in the wet atmospheric deposition of Hg  recorded at nearby monitoring stations for over a decade. Methylmercury is a toxic form of mercury (Hg) that accumulates and biomagnifies in aquatic food webs. Some of the Hg entering aquatic ecosystems is converted to MeHg. Among other things, sulfate levels have been related to the conversion of Hg to MeHg.

Records indicate decreases in the annual wet deposition of mercury and sulfate at two adjacent precipitation monitoring sites from 1998 to 2012 (mean decreases of 32 and 48 percent, respectively). While two of the four lakes studied showed consistent decreases in Hg in water and in MeHg in small yellow perch between 2001 and 2012 (mean decreases of 46.5 and 34.5 percent, respectively), a third lake showed significant increases in both Hg in water and MeHg in fish, and a fourth lake showed no consistent trend during the same period. The fourth lake, however, was known to be disturbed by forest fire and changes in shoreline due to beaver activity, both of which could affect Hg methylation.

An increasing number of monitoring sites around the Nation have documented declining trends in mercury deposition over the past decade. This study, however, demonstrates that ecosystem responses can be complex, local factors within the watershed may be important, and responses in ecosystem mercury levels may require longer time periods than those observed in this study.

Other reports from this collaborative, long-term effort (Weiner and others, 2006, and Woodruff and others, 2009) are listed below. This study was supported by the USGS Toxic Substances Hydrology Program, Minnesota Pollution Control Agency, National Park Service, U.S. Environmental Protection Agency, National Park Service/USGS Water Quality Partnership Program, and University of Wisconsin-La Crosse.

References

Brigham, M.E., Sandheinrich, M.B., Gay, D.A., Maki, R.P., Krabbenhoft, D.P., and Wiener, J.G., 2014, Lacustrine responses to decreasing wet mercury deposition rates--Results from a case study in northern Minnesota: Environmental Science and Technology, doi:10.1021/es500301a (Advanced Web release).

Wiener, J.G., Knights, B.C., Sandheinrich, M.B., Jeremiason, J.D., Brigham, M.E., Engstrom, D.R., Woodruff, L.G., Cannon, W.F., and Balogh, S.J., 2006, Mercury in soils, lakes, and fish in Voyageurs National Park (Minnesota)–Importance of atmospheric deposition and ecosystem factors: Environmental Science and Technology, v. 40, no. 20, p. 6261-6268, doi:10.1021/es060822h.

Woodruff, L.G., Sandheinrich, M., Brigham, M.E., and Cannon, W.F., 2009, Impact of wildfire on levels of mercury in forested watershed systems–Voyageurs National Park, Minnesota: U.S. Geological Survey Scientific Investigations Report 2009-5151, 19 p.

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Created on June 5, 2014.