Chemical correlates of Hg and methyl-Hg in NOrthern Wisconsin lake waters under ice cover
Total and dissolved concentrations of Hg and methyl-Hg (MeHg) were determined in the surface waters of 19 northern Wisconsin lakes under ice-cover when differences due to temperature, hydrology, productivity, and atmospheric exchange were minimal. Measured concentrations ranged from 0.3 to 5.3 ng/L for HgT and from 0.01 to 2.8 ng/L for MeHgT. Dissolved species comprised 30\% to 95\% of the HgT and MeHgT. MeHg was strongly correlated with Hg for both total and dissolved fractions. Thirteen ancillary constituents were measured in conjunction with the Hg determinations (pH, DOC, DIC, DO2, conductivity, suspended particulate matter (SPM), Ca, Mg, Mn, Fe, Na, SO4, Cl). Simple linear regressions indicated that DOC explained 87\% of the variability in HgT and 79\% of the variability in MeHgT. Of the other measured variables, pH, DO2, Fe and Mn showed weak but significant simple correlations with Hg and MeHg (@ p \textless 0.05). Multiple regression models containing two independent variables, (DOC and pH), explained 92\% of the variability in HgT and 83\% of the variability in MeHgT. Models containing DOC alone fit the dissolved Hg data well. We conclude that organic carbon concentrations have a strong effect on the concentrations of Hg and MeHg in these lakewaters.