Trends in Regional Wet Mercury Deposition and Lacustrine Mercury Concentrations in Four Lakes in Voyageurs National Park—An Update

Widespread occurrence of elevated mercury levels in fish that inhabit inland aquatic ecosystems, even in areas where atmospheric deposition is effectively the only source of mercury, has prompted controls on mercury emissions. Anthropogenic mercury (Hg) emissions to the atmosphere have been substantially lowered in the United States and Canada since 1990. How have aquatic ecosystems responded? A previous analysis reported decreases in wet Hg deposition in northeastern Minnesota from 1998-2012, and mixed trends in Hg and methylmercury (MeHg) in lake water and fish from four remote lakes within Voyageurs National Park from 2001-2012 (Brigham, M.E. and others, 2014, Environmental Science & Technology, vol. 48, pp. 6115-6123. DOI: 10.1021/es500301a).

Here, we report updated trends for the same study area for monitoring through 2018. Wet Hg deposition at two regional Mercury Deposition Network sites (Fernberg and Marcell, MN http://nadp.slh.wisc.edu/mdn/) declined by an average of 22.5 percent from 1998-2018, with much of the decline occurring prior to 2011. In the four remote lakes, epilimnetic MeHg concentrations declined by an average of 42 percent and total Hg by an average of 27 percent. Although the magnitude of trend in some lakes was small, it is noteworthy that for all the lakes both MeHg and total Hg show declines for the 2001-2018 time period. Epilimnetic MeHg may be responding both to a decline in atmospheric Hg deposition as well as a decline in sulfate deposition, which is an important co-driver of Hg methylation in the environment. Results from this case study suggest that regional- to continental-scale decreases in both Hg and sulfate emissions have benefitted aquatic resources.