## 2 Materials and Methods
### Lake Water  
Lake water sampling and analytical methods have been described previously [24,33]. Unfiltered, epilimmetic lake water was sampled two to three times (typically three) per year, between May and September, during most years of the study period. Field crews generally sampled the upper ca. 5 cm of water out of a canoe heading upwind, in the approximate center of the lake, using trace-metal clean sampling protocols described in [21]. Water was collected in a pre-cleaned Teflon(r) PEP (fluorinated ethylene propylene) bottle, preserved by addition of HCl (to a final normality of ca. 0.02 \(N\)), and shipped to the USGS Mercury Research Laboratory (MRL) in Middleton, Wisconsin, for analysis. Detailed descriptions of all the analytical procedures used by the USGS MRL are available at the following website: [[https://wi.water.usgs.gov/mercury-lab/research](https://wi.water.usgs.gov/mercury-lab/research)]([https://wi.water.usgs.gov/mercury-lab/research](https://wi.water.usgs.gov/mercury-lab/research)) (accessed on 1 February 2021) and the descriptions are summarized below. Total Hg determinations in lake water (TH\({}_{\text{gaq}}\)) were determined by U.S.EPA Method 1631 [29]. The USGS MRL typically achieves a daily detection limit (DDL) for total mercury analytical runs of about 0.06 pM and the precision, measured as the relative percent difference (RPD) between analytical duplicates averages 10 percent. Methylmercury (MeHg\({}_{\text{gaq}}\)) samples were analyzed by U.S.EPA Method 1630 ([34,35] with the added advancement (starting in 2006) of including isotope dilution by adding a small known amount (about 30 picograms) of isotopically labeled methylmercury (Me\({}^{200}\)Hg) to each sample, which allows for more accurate measures of sample recovery rates. Water samples for other constituents were also collected from the same lake locations with a 2 m long, 3.2 cm inner diameter PVC tube that integrates a 2 L sample from the upper 2 m of the water column. Samples collected once per summer, typically in July, were analyzed for several major ions including sulfate by White Water Associates, Inc. (Amasa, Michigan) (2006-2013) and CT Laboratories (Baraboo, Wisconsin) (2014-2018). Sample processing, handling, and quality assurance and quality control procedures are described in [PERSON] et al. [36]. For the 2001-2012 time period [24], trends in MeHg\({}_{\text{gaq}}\) conformed reasonably to a linear-regression model for three of the four lakes. However, inspection of data for the longer time (through 2018) revealed nonlinearity at some sites. Therefore, we used locally weighted regression using the LOESS procedure in SAS, again allowing the procedure to select the smoothing parameter by the default Akaike Information Criterion algorithm within the LOESS procedure. To calculate the percent change in concentrations from 2001-2018, we used LOESS-predicted concentrations for arbitrary dates approximately at the beginning and end of the period of data collection (1 July 2001 and 1 July 2018). No slope parameter (and hence, no \(p\)-value for significance) is calculated in LOESS. Relevance of the trends can be assessed by examining the magnitude of change viewed along with the variability of the data; and by examining whether the 95-percent confidence interval for the LOESS smooth at the end of the time period includes, or does not include, the LOESS-predicted value for the start of the time period. Lake water chemistry and lake level data from 2000-2007 are available from the USGS's National Water Information System web retrieval ([[https://doi.org/10.5066/F7P55](https://doi.org/10.5066/F7P55) KJN]([https://doi.org/10.5066/F7P55](https://doi.org/10.5066/F7P55) KJN), accessed on 1 February 2021), for the following USGS site identification numbers: BrownLake (483059092474501); Peary Lake (483129092462001); Ryan Lake (483109092422601); and Shoepack Lake (482951092531601).