The methylmercury cycle in Little Rock Lake during experimental acidification and recovery

Watras, C. J., K. A. Morrison, O. Regnell, T. K. Kratz

Limnol. Oceanogr., 51(1), 2006, 257-270 | DOI: 10.4319/lo.2006.51.1.0257

ABSTRACT: The cycle of waterborne methylmercury (meHg) in Little Rock Lake is characterized by a period of accumulation during summertime (when the lake is warm and open to the atmosphere) and a period of decline during winter (when the lake is sealed by ice). We followed this cycle for 16 yr, during which time the lake was acidified with H2SO4 and then allowed to recover naturally as part of a long-term field experiment on acidic rain. Mass balance was used to quantify meHg sources and sinks during acidification and recovery. Although year-to-year variability in the summertime accumulation of meHg was high during both acidified and de-acidified years (C.V. = 0.7 and 0.5, respectively), on average 65% more meHg accumulated in the water column during acidification. Most of the meHg mass accumulated in the anoxic hypolimnion (>70%), even though the hypolimnion constituted <5% of the lake volume. In hypolimnetic waters, we observed a direct correlation between the maximum meHg concentration and the sulfate deficit for each year (r2 = 0.5-0.9) and a direct correlation between meHg and sulfide concentrations (r2 = 0.7). Sulfide was directly related to dissolved organic carbon at concentrations between 300 and 600 µmol L-1 carbon (C). Seasonal changes in waterborne Hg(II), meHg, and sulfate reduction covaried with the atmospheric deposition of Hg(II) and SO42-. Across all years, the interaction term [SO42- x Hg(II)] explained 70% of the variation in the meHg accumulation rate during summer. These results indicate that meHg production was co-mediated by several simultaneously occurring processes that affect the supply of Hg(II) substrate to the anoxic hypolimnion and the activity of methylating bacteria that are present there. They imply that meHg levels in lakes may respond to future changes in atmospheric Hg deposition in a rapid but complex way, modulated by environmental variables that can interact synergistically with Hg(II) supply. Such variables include sulfate in acid rain, organic carbon in terrestrial runoff, and temperature.

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