## 4 Discussion
Owing to long-term atmospheric deposition of anthropogenic mercury and sulfate, lakes in Voyageurs National Park, and regionally, surely have elevated methylmercury levels in both water and biota, relative to pre-industrial conditions. It is encouraging, however, that declines in anthropogenic mercury and sulfur emissions have translated to declines in wet mercury and sulfate deposition, which in turn appear to have resulted in declines in methylmercury contamination in lake ecosystems within the park. The relatively large MeHg\({}_{\text{aq}}\) declines, in comparison to declines in THg\({}_{\text{aq}}\), are consistent with the notion that MeHg\({}_{\text{aq}}\) levels are influenced by both anthropogenic mercury as well as anthropogenic sulfate deposition. It is worth noting that the response of lake ecosystems to decreased mercury inputs is expected to include both a rapid component owing to direct deposition to the lake surface and a slow component driven as previously deposited mercury slowly re-equilibrates from wetland and upland soils [50]. Lastly, as emphasized previously, there are relatively few published long-term, multimedia data sets that include atmospheric mercury-deposition monitoring coupled with methylmercury and total mercury in lake water and mercury in lake biota. This is particularly important for undisturbed settings where methylmercury production and bio-accumulation are largely governed by natural processing of atmospheric pollutant loads. [PERSON] et al. [44] reported trends for aqueous total mercury and methylmercury and biotic mercury for two lakes in northern Wisconsin (Little Rock Lake, 1988-2017 and Trout Bog, 1999-2017) that are relatively close (ca. 275 km southeast of VNP), yet the two studies yielded trend analyses that are notably different. This variability in temporal trends is consistent with the overarching influence that within-lake and watershed bio-geochemistry can have on mercury transport and methylmercury production, potentially decoupling them from trends in mercury loading. This highlights the importance of considering the context of each ecosystem and supports the notion that recovery from many decades of sustained mercury emissions is unlikely to be a linear process. Data sets like the one used in this study, while rare, will serve as critically important baselines for executing effectiveness evaluations associated with the post-Minamata-Treaty implementation. Although more extensive networks have been proposed to monitor ecosystem responses to controls on anthropogenic mercury emissions [51], in the absence of such programs leading up to the global change expected from the Minamata Treaty, researchers might better coordinate small-scale, long-term research efforts so that broader-scale assessments can be made. The following are available online at [[https://www.mdpi.com/2076-341](https://www.mdpi.com/2076-341)]([https://www.mdpi.com/2076-341](https://www.mdpi.com/2076-341)) 7/11/4/1879/s1, Table S1: Outlier samples identified by regression analysis of log[Hgprecip] versus logPrecip, seasonal terms, and time. Figure S1: Annual wet sulfate deposition for the National Atmospheric Deposition Network site at Marcell, Minnesota. Figure S2: Precipitation volume trend plots for National Atmospheric Deposition Network/Mercury Deposition Network sites Marcell, Minnesota (MN16) and Fernberg, Minnesota (MN18). M.E.B. analyzed precipitation and lake-water chemistry data; C.A.E.-S. analyzed the biotic data. [PERSON] contributed to water chemistry methods and quality assurance of the water chemistry data. All authors contributed to preparation of the manuscript. All authors have read and agreed to the published version of the manuscript. Preparation of this manuscript was funded by the National Park Service (NPS). National Atmospheric Deposition Network/Mercury Deposition Network sites were supported by the U.S. Forest Service, U.S. Environmental Protection Agency, and the Minnesota Pollution Control Agency (MPCA).