## 4 Discussion
[44; 45] and our study is not surprising. Trends in fish-tissue THg concentrations moderately tracked MeHg\({}_{\text{aq}}\) or THg\({}_{\text{aq}}\) for Brown and Peary Lakes but not Ryan Lake. Whereas MeHg\({}_{\text{aq}}\) concentrations often correspond to biological mercury uptake and accumulation in many water bodies, there can be substantial variability in the efficiency of transfer into and through food webs due to the context dependence of site-specific bio-geochemical and ecological drivers. Also, whereas MeHg\({}_{\text{aq}}\) or THg\({}_{\text{aq}}\) is an instantaneous measure of conditions, biological tissues integrate exposure over much longer time periods, including the Odonates that are generally several years old. This disparity can complicate interpretations of the effectiveness of decreasing mercury emissions and deposition. In addition, for boreal-like settings, the connectivity to terrestrial soils and their legacy accumulation of decades of mercury deposition is well understood; however, the internal time lags of how long this large pool of mercury will continue to yield meaningful amounts of mercury to downstream aquatic ecosystems remains unknown. This finding does not imply that declining mercury emissions and deposition (and subsequent MeHg\({}_{\text{aq}}\) production) offer limited benefits for mercury risk reduction. Instead, it emphasizes the need to interpret long-term environmental mercury data sets in the context of a complex set of pathways and processes that control mercury cycling in the environment, and the need for multimedia (deposition, water, and biota)and multiconstituent data (beyond just mercury and methylmercury measurements) for effective trend analysis for mercury. 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. The reductions in atmospheric pollutant loading may be of considerable benefit to human and ecosystem health, considering that mercury-based fish-consumption advisories are in place for all lakes of Voyageurs National Park and many lakes in the region, and northern pike (_Eosox lucius_) mercury levels in park lakes have exceeded thresholds for detrimental effects to fish reproduction [46; 47]. A number of MDN sites across North America had substantial declines in wet mercury deposition from the late 1990s through early 2000s, followed by a leveling-off and in some cases increase in Hg deposition starting around 2010 [31]. The two northern Minnesota sites appear to fit this broader geographic pattern. Trend analysis by locally weighted regression showed a relatively sharp decline in the period from 1998 to about 2010, followed by a leveling-off of deposition rates. Although the MDN has a data review and quality assurance program in place, the data-screening procedure employed herein identified a small number of extreme outlier mercury concentrations that appeared unreasonable. Because extreme outliers for an individual sample can bias annual wet-deposition calculations, our screening procedure (or similar ones) warrants further consideration. The larger declines in epilimnetic MeHg\({}_{\text{aq}}\), compared to epilimnetic THg\({}_{\text{aq}}\) is likely driven by both decline in wet Hg deposition (and thus in-lake THg\({}_{\text{aq}}\)), as well as declines in sulfate deposition. As noted previously [24], in response to emission controls related to the Clean Air Act, sulfate deposition has declined dramatically in northern Minnesota, as well as more broadly across North America [11; 48]. Other research has shown that adding sulfate to wetlands greatly increases methylmercury production [7], whereas decreased sulfate loading results in decreased methylmercury production [49].