## 4 Discussion  
Wet Hg deposition at two regional MDN sites (Fernberg and Marcell, Minnesota) declined by an average of 22 percent from 1998-2018, with much of the decline occurring prior to 2010. In the four lakes, epilimnetic MeHgaq concentrations declined by an average  
Figure 8: Total mercury (THg) in age-1 yellow perch versus time in (**A**) Brown Lake, (**B**) Peary Lake, and (**C**) Ryan Lake. Solid symbols represent geometric mean concentrations from yellow perch, open symbols indicate that yellow perch concentrations are derived from dragonfly larvae using the linear regression shown in Figure 7. Shaded area shows the 95-percent confidence interval. [Smooth lines are locally weighted regression. Smoothing parameters: (**A**) Brown Lake = 0.75; (**B**) Peary Lake = 0.83; and (**C**) Ryan Lake = 0.76. Gray shading indicates the 95-percent confidence interval. of 44 percent and THg\({}_{\text{aq}}\) by an average of 27 percent. Although the magnitude of trend in some lakes was small, it is noteworthy that for all the lakes both MeHg\({}_{\text{aq}}\) and THg\({}_{\text{aq}}\) show declines for the 2001-2018 time period, including the latter part of that period when wet Hg deposition rates leveled off, suggesting a lag related to watershed inputs. Epilimmetic MeHg\({}_{\text{aq}}\) 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. The long-term reduction in epilimmetic sulfate concentrations in the lakes also reflects declines in sulfate deposition, as has been observed elsewhere [43]. This observation is a good example of the importance of collecting data on other known key factors (for example, sulfate) that control mercury cycling in the environment when the goal is to accurately attribute the drivers of change. Environmental mercury data sets that include long-term monitoring of multiple media (atmosphere, surface water, and biota) in a relatively small area are exceedingly rare. As such, the opportunities to assess baselines and trends in mercury levels in aquatic ecosystems, especially in the lead-up to expected globally driven emissions changes from the Minamata Treaty [16], are likewise uncommon. Previously, it was hypothesized that inflowing water from a lake upstream from Brown Lake (Oslo Lake), which yielded relatively high concentrations of MeHg\({}_{\text{aq}}\) in a 2001-2002 survey of small lakes in Voyageurs National Park [21], could be responsible for the increase in MeHg\({}_{\text{aq}}\) in Brown Lake during the 2001-2012 time period. The apparent trend reversal, i.e., the decline in MeHg\({}_{\text{aq}}\) in Brown Lake that occurred starting around 2010, coupled with the correlation between lake level and MeHg\({}_{\text{aq}}\), supports the hypothesis. Higher observed MeHg\({}_{\text{aq}}\) concentrations coincident in time with higher lake levels is consistent with the generally held conceptual understanding from the mercury literature that wetter conditions and cyclical inundation and draining of low-lying areas (such as wetlands) leads to increased MeHg\({}_{\text{aq}}\) production within a lake's watershed, irrespective of loading from upstream lakes. However, the remaining lakes in this study showed no correlation between MeHg\({}_{\text{aq}}\) and lake level. This null finding is in contrast to the coherence of water level and MeHg in water and fish observed for lakes in northern Wisconsin [44; 45]. However, it should be noted that the ecological setting in northern Wisconsin is quite different from the Voyageurs National Park region, especially in regard to hydrology. The Northern Highlands of northern Wisconsin are characterized by high permeability due to deep outwash sands and gravel that yield poorly integrated surface drainages. Our study area, in contrast, is more of a classical boreal system with shallow soils overlying bedrock and highly integrated flow systems. As such, the lack of concurrence between findings of [PERSON] et al.