Evidence for regulation of monomethyl mercury by nitrate in a seasonally stratified, eutrophic lake.

The accumulation of monomethyl mercury (CH 3 Hg + ) in aquatic ecosystems is a redox sensitive process that is accelerated under sulfate-reducing conditions. While nitrate (NO 3 - ) reduction is energetically favored over sulfate reduction, the influence of NO 3 - on the accumulation of CH 3 Hg + has not been reported in the literature. We examined temporal and vertical patterns in redox constituents and CH 3 Hg + concentrations in the hypolimnion of a dimictic lake, Onondaga Lake, prior to and following increases in NO 3 - inputs. Detailed water-column profiles and a long-term record revealed marked decreases in the accumulation of CH 3 Hg + in the anoxic hypolimnion coinciding with long-term decreases in the deposition of organic matter coupled with recent increases in NO 3 - concentrations. CH 3 Hg + concentrations in the hypolimnion were substantially abated when NO 3 - was present above the sediment—water interface. A decrease in the peak hypolimnetic mass of CH 3 Hg + and shortening ofthe period of elevated CH 3 Hg + concentrations resulted in more than a 50% decline in the accumulated CH 3 Hg + . NO3 - regulation of CH 3 Hg + accumulation may be a widespread phenomenon in oxygen-limited freshwater and terrestrial environments, and could have an important, not previously recognized, effect on the biogeochemistry of mercury.