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

The accumulation of monomethyl mercury (CH₃Hg⁺) in aquatic ecosystems is a redox sensitive process that is accelerated under sulfate-reducing conditions. While nitrate (NO₃^-) reduction is energetically favored over sulfate reduction, the influence of NO₃^- on the accumulation of CH₃Hg⁺ has not been reported in the literature. We examined temporal and vertical patterns in redox constituents and CH₃Hg⁺ concentrations in the hypolimnion of a dimictic lake, Onondaga Lake, prior to and following increases in NO₃^- inputs. Detailed water-column profiles and a long-term record revealed marked decreases in the accumulation of CH ₃Hg⁺ in the anoxic hypolimnion coinciding with long-term decreases in the deposition of organic matter coupled with recent increases in NO₃^- concentrations. CH₃Hg⁺ 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 CH₃Hg⁺ and shortening of the period of elevated CH₃Hg⁺ concentrations resulted in more than a 50% decline in the accumulated CH₃Hg ⁺. NO₃^- regulation of CH₃Hg ⁺ 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.