Abstract
Although anthropogenic mercury (Hg) releases to the environment have been substantially lowered in the United States and Canada since 1990, concerns remain for contamination in fish from remote lakes and rivers where atmospheric deposition is the predominant source of mercury. How have aquatic ecosystems responded? We report on one of the longest known multimedia data sets for mercury in atmospheric deposition: aqueous total mercury (THgaq), methylmercury (MeHgaq), and sulfate from epilimnetic lake-water samples from four lakes in Voyageurs National Park (VNP) in northern Minnesota; and total mercury (THg) in aquatic biota from the same lakes from 2001–2018. Wet Hg deposition at two regional Mercury Deposition Network sites (Fernberg and Marcell, Minnesota) decreased by an average of 22 percent from 1998–2018; much of the decreases occurred prior to 2009, with relatively flat trends since 2009. In the four VNP lakes, epilimnetic MeHgaq concentrations declined by an average of 44 percent and THgaq by an average of 27 percent. For the three lakes with long-term biomonitoring, temporal patterns in biotic THg concentrations were similar to patterns in MeHgaq concentrations; however, biotic THg concentrations declined significantly in only one lake. Epilimnetic MeHgaq may be responding both to a decline in atmospheric Hg deposition as well as a decline in sulfate deposition, which is an important driver of mercury methylation in the environment. Results from this case study suggest that regional- to continental-scale decreases in both mercury and sulfate emissions have benefitted aquatic resources, even in the face of global increases in mercury emissions.
Highlights
Human activities have considerably increased the amount of mercury in the atmosphere and deposited into aquatic ecosystems [1]
This paper addresses the question: how have aquatic ecosystems responded? Here, we report on one of the longest known paired data sets for mercury in atmospheric deposition; mercury, methylmercury, and key other chemical and physical parameters in several lacustrine systems; and total mercury in aquatic biota from the same lacustrine systems
This paper reports an updated analysis of trends in mercury deposition at two northern Minnesota NADP Mercury Deposition Network (MDN) sites, as well as trends in methylmercury and total mercury concentrations in epilimnetic lake water and in biota from four lakes in a minimally disturbed area within Voyageurs National Park, in northern Minnesota
Summary
Human activities have considerably increased the amount of mercury in the atmosphere and deposited into aquatic ecosystems [1]. Mercury in fish tissues is predominantly in the methylmercury form [3]; factors that control methylmercury production in aquatic ecosystems are of interest Two such factors, relevant to the current paper, are the availability of inorganic mercury and sulfate. Increased mercury loading to aquatic ecosystems results in a proportional increase in methylmercury production and accumulation by aquatic biota [4]. Because sulfate reducing bacteria are important as methylators of mercury [5,6], elevated sulfate deposition can result in increased methylmercury production in aquatic ecosystems [5,7]; it is likely, that methylmercury concentrations in aquatic ecosystems (including fish) are greater than background, preindustrial levels, but greater than one might expect with solely a proportional response to mercury inputs
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