Mass balance studies of mercury and methyl mercury in small temperate/boreal lakes of the Northern hemisphere
Mass balance studies in Wisconsin, Canada, and Sweden indicate that atmospheric deposition is the principal source of Hg to many lakes, whether delivered directly or indirectly via terrestrial runoff. The reported depositional flux varies 10-fold among these northern regions (3 to 30g Hg/km2/y). Watershed export rates vary from about 1 to 6 g Hg/km2/y, indicating that a substantial fraction of the atmospheric load is retained by terrestrial catchments. Hg residence times vary from 150 to 300 days in small Wisconsin lakes. Losses occur through sedimentation, outflow, and gaseous evasion. Environmental factors such as pH and dissolved humic matter (DHM) effect the budgets, concentrations, and speciation of Hg in lakewaters. Sedimentation is favored by low pH and low DHM, gaseous evasion is favored by high pH, and high MeHg concentrations are favored by high DHM and low pH. Multiple regression models containing DHM and pH explained 85\% to 90\% of the variability in waterborne Hg (0.2 to 4.8 ng/L) and methyl mercury (MeHg: 0.04 to 2.2 ng/L) among WI lakes ranging in pH from 4.5 to 8 and in organic carbon from 1 to 22 mg/L. It is unclear whether DHM affects MeHg production in lakes or simply reflects co-transport with organic C from riparian wetland. Northern wetlands export 0.1 to 1 g MeHg/km2/y along with 10 to 100 kg C/ha/y (versus \textless0.05 g MeHg/km2/y from uplands and 0.03 to 0.3 g MeHg/km2/y deposited atmospherically). In Little Rock Lake, WI, pelagic fish are the largest reservoir for MeHg while organic sediments are the largest Hg(II) reservoir. Biotic MeHg concentrations increase 2-fold to 4-fold with increasing trophic level, but MeHg turnover rates are highest at low trophic levels. We estimate turnover rates of 5 g MeHg/km2/y by pelagic microorganisms versus 0. 5 g MeHg/km2/y by pelagic fish. These high turnover rates may indicate that in situ rates of MeHg formation and destruction are correspondingly high.