Sources and Fates of Mercury and Methylmercury in Wisconsin Lakes (Chapter 1.12)
The mercury cycle in seven northern Wisconsin seepage lakes was characterized by high atmospheric influx. removal by sedimentation and evasive efflux, and by in-lake transformation of Hg to biologically sequestered methyl-Hg species. Direct depositional Hg loading from the atmosphere to lakes was roughly 10 !lg/m2/year, with rain and snow the principal delivery vectors. Annual atmospheric Hg deposition exceeded estimated fish bioaccumulation by a factor of roughly 10. The atmospheric Hg influx was roughly balanced by losses to sediments and the return of volatile HgO to the atmosphere. 1be relative importance of sedimentation and gaseous evasion as Hg loss terms varied from lake to lake, with sedimentation/evasion ratios ranging from 9: I to I: I in the seven lakes studied. Residence times for Hg varied from roughly 125 to 300 days in these lakes. Methyl-Hg in these lakes also had an atmospheric source, estimated to re roughly I\% of the total Hg inputs. Although the direct atmospheric deposition and sediment accumulation of methyl-Hg roughly balanced. the atmospheric influx of methyl-Hg was much lower than annual rates of methyl-Hg bioaccumulation. Unless the recycling efficiency of methyl-Hg was extraordinarily high, in situ production was an important source of methyl-Hg species. Most of the methyl-Hg in these lakes was stored in fish tissue. Assuming fish production of 30\%/year, the annual bioaccumulation of methyl-Hg exceeded sediment accumulation by a factor of 6 to 7. No dimethyl-Hg has been observed in any Wisconsin lake. The distribution of Hg species in the study lakes was characterized by very dilute pools that varied seasonally and spatially. Waterborne Hg species had concentrations in the picomolar to femtomolar range, with parts per million to parts per billion concentrations in sediment and organisms. Average waterborne Hg and methyl-Hg concentrations correlated negatively with lakewater pH and positively with DOC. Seasonal cycles involved decreasing concentrations under ice cover, followed by build-up during summer. Epilimnetic concentrations ranged from 1 to 3 ng/L Hg and 0.05 lo 0.5 ng/L melhyl-Hg. Higher mercury concentrations were observed at depth in stratified lakes (Hg \textgreater45 ng!L and melhyl-Hg \textgreater 10 ng/L) and Hg maxima were observed near microbial layers in the watercolumn. In anoxic, sulfidic plankton layers, \textgreater50\% of the Hg may be in the methyl-Hg form (vs. S to 15\% in the epilimnion). Methyl-Hg was biomagnified in the foodchain of Little Rock Lake, but there was evidence that nonmethyl-Hg species became more dilute at higher trophic levels. The bioconcentration factor for methyl-Hg increased by threefold for each trophic level, approaching l 07 in fish. The Hg in fish was almost all methylated (\textgreater95\%), while the Hg in sediments was primarily nonmethyl-Hg (\textgreater97\%). Since most methyl-Hg in the study lakes appeared to be sequestered by fish biomass, fish contamination could be significantly enhanced by small increases in net rates of methyl-Hg production, recycling, or loading.
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