Increases in Total and Methylmercury in Zooplankton following Flooding of a Peatland Reservoir

Fish from new reservoirs often have elevated concentrations of methylmercury (MeHg) that they primarily accumulate from food such as zooplankton. The objectives of this research were (i) to determine the effect of reservoir creation on total mercury (THg) and MeHg in zooplankton and (ii) to examine how variations in community structure and water chemistry affect MeHg bioaccumulation by zooplankton. Beginning in June 1992, we measured concentrations of THg and MeHg in zooplankton from an experimental reservoir (L979) and an unmanipulated reference pond (L632). After flooding of L979 in June 1993, mean concentrations of MeHg in zooplankton increased from 32 to >300 ng g-1 dw and THg increased from 87 to >500 ng g-1 dw. Annual fluxes of MeHg through the zooplankton community increased 10−100× after impoundment. MeHg concentra tions in zooplankton, seston, and water were strongly correlated with each other (r > 0.92). Bioaccumulation factors relating MeHg in zooplankton to MeHg in water or seston did not change after impoundment, despite large changes in water chemistry and zooplankton community structure. Concentrations of Hg in zooplankton from Lake 632 did not change dramatically over the 4 years of study.