Introduction**  
Human activities have considerably increased the amount of mercury in the atmosphere and deposited into aquatic ecosystems [1]. Elevated mercury levels in fish have resulted in widespread fish-consumption advisories across the United States (U.S.), North America, and globally to protect human health [2], and also pose ecotoxicological risk to piscivorous wildlife [3]. High fish-mercury levels can occur anywhere, including remote, relatively pristine ecosystems where the predominant source of mercury is atmospheric deposition. Although deposited primarily as inorganic mercury, within aquatic ecosystems some fraction of the total mercury load is converted to methylmercury [3]. Mercury in fish tissues is predominantly in the methylmercury form [3]; thus, 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]. Furthermore, because sulfate reducing bacteria are important as methylators of mercury [5; 6], elevated sulfate deposition also can result in increased methylmercury production in aquatic ecosystems [5; 7]; it is likely, therefore, that methylmercury concentrations in aquatic ecosystems (including fish) are not only greater than background, preindustrial levels, but greater than one might expect with solely a proportional response to mercury inputs. With an increased understanding of the global extent of atmospherically driven mercury contamination in aquatic ecosystems, the U.S. and Canada have undertaken significant efforts to reduce anthropogenic mercury emissions. Lake-coring evidence indicates that mercury inputs to remote, mid-continental lakes peaked in the 1960s-1970s and declined subsequently [8]. More recently, emissions reporting has shown substantial declines in U.S. mercury emissions of approximately 87 percent from 1990-2017 [9], with the largest decreases occurring in the mid 1990's following regulations limiting emissions from hospital and municipal waste incinerators and chlor-alkali facilities; similarly large declines in mercury emissions occurred in Canada over the same time frame [10] (Table 1). Perhaps equally beneficial, from a methylmercury production and bioaccumulation perspective, efforts to control acid precipitation during the 1970s and 1980s led to large decreases in emission and deposition of sulfur oxides across the U.S. since 1970 [11]. Consistent with this continental-scale trend, sulfate deposition within the Voyage National Park region has declined substantially. Data from the National Atmospheric Deposition Program's (NADP) site MN16 (Marcell, Minnesota) show annual wet sulfate deposition declining from about 10 kg ha\({}^{-1}\) in 1980, to about 6 kg ha\({}^{-1}\) in 2000, to about 2.4 kg ha\({}^{-1}\) in 2018 (Figure S1). [PERSON] et al. [12] have previously attributed declining fish-mercury levels in lakes at lise Royale National Park (Lake Superior, USA) to declines in sulfate loading. Although North American mercury emissions have declined sharply (Table 1 and [13]), and emissions have declined in several other regions as well [13], industrial development and associated increases in mercury emissions--particularly in Asia--have led to a net global increase in anthropogenic mercury emissions of 1.8 percent per year [14]. The relative importance of North American versus global emissions to local deposition in northern Minnesota is unclear. A recent modeling study found that North American emission reductions have a more pronounced effect on wet mercury deposition reductions in the eastern U.S., where there was a greater concentration of sources, than in comparatively remote northern Minnesota [15], but that effort did not focus on ecosystem responses.