Long-Term Trends in Regional Wet Mercury Deposition and Lacustrine Mercury Concentrations in Four Lakes in Voyageurs National Park

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.

Mercury sulfur initiative

“…In addition to a changing climate, the Upper Great Lakes region has experienced and will continue to experience other major trajectories of environmental change. Perhaps most directly relevant to toxic metal accumulation, and in response to emissions reductions associated with the Clean Air Act (1977 and 1990 Amendments), the atmospheric deposition of mercury has generally decreased (Brigham et al 2021), although not at all Mercury Deposition Network sites in northern Minnesota. In theory, reduced atmospheric deposition of mercury should reduce the amount of methylmercury entering food chains, but in practice, mercury toxicity and transport are much more strongly associated with food web transfer.…”

Geographic Drivers of Mercury Entry into Aquatic Food Webs Revealed by Mercury Stable Isotopes in Dragonfly Larvae

“…Prior to microbial methylation and bioaccumulation, Hg enters an ecosystem from a variety of pathways, the most common of which in remote areas is atmospheric deposition . While wet and dry Hg deposition occurs across the landscape, recent research suggests that dry deposition through direct assimilation to vascular plants, as well as canopy throughfall, contributes up to 75% of the total inorganic Hg loading in forested locations worldwide. − However, wet deposition is still a major Hg delivery mechanism for many iconic U.S. ecosystems such as the Great Lakes and the Florida Everglades. , Little is known about the relative contributions of Hg from the different delivery mechanisms to freshwater food webs, which can result in a disconnect between observed atmospheric Hg trends and biological burdens. , For example, concentrations of wet atmospheric Hg deposition have decreased over time across most of the United States, , but in many ecosystems, biological Hg concentrations remain elevated , or historical declines have stagnated . Given the ubiquity of atmospheric Hg deposition as well as the high potential for Hg methylation in undeveloped waterbodies (e.g., wetlands, forested canopies), the risk of bioaccumulation is still prevalent even in the most remote food webs. , …”

Predicting sport fish mercury contamination in heavily managed reservoirs: Implications for human and ecological health

“…Consumption of contaminated sport fish (with Hg incorporated in their muscle tissue) is the major method of transfer of aquatic Hg to humans [ 1 , 2 ]. However, fish Hg concentrations can be inherently variable due to several factors, and changes in contributions of Hg to the environment do not necessarily translate directly to changes in fish Hg concentrations because of complex biogeochemical interactions [ 8 , 9 ]. Thus, Hg deposition information alone cannot be used to infer fish Hg concentrations across the landscape, and extensive, large-scale monitoring programs can be cost prohibitive.…”