| The neurotoxin methylmercury (MeHg) is found in some Arctic freshwater fish and marine mammals at concentrations that may be harmful to Northern Peoples consuming these animals. The key process resulting in mercury (Fig) contamination of organisms is the methylation of inorganic Hg(II) to MeHg, which is much more readily bioaccumulated and biomagnified than Hg(II). Thus, the objective of this research was to determine the sources of MeHg to Arctic freshwater and marine ecosystems by identifying the processes controlling the production and cycling of this toxin. Three aspects of the Hg biogeochemical cycle were examined using enriched Hg stable-isotope tracers: MeHg photodemethylation in lakes, MeHg cycling in wetlands, and Hg(II) methylation in marine waters. The role of light quality in MeHg photodemethylation was experimentally determined, demonstrating that ultraviolet radiation (UV) and not visible light, as has been previously suggested, is the principal driver of this process. We propose a simple model to quantify whole-lake MeHg photodemethylation fluxes, based on experimentally determined photodemethylation rate constants for the different radiation wavebands (UV-B, UV-A and visible), MeHg concentration, and attenuation of solar radiation in the water column. We used a mass-balance approach, quantifying atmospheric inputs, losses from photodemethylation and accumulation in the water column pool, to demonstrate that MeHg production in two different High Arctic wetlands ponds (1.8-40 ng m-2 d-1) is comparable or greater than what has been measured in freshwater ecosystems in temperate latitudes. MeHg production in pond sediments was controlled by Hg(II) availability and the potential rate of Hg(II) methylation. MeHg concentration in the water column of surveyed ponds was positively correlated to MeHg concentration in sediments and proxies of anaerobic microbial activity, but inversely correlated to UV-A exposure, indicating that the cycle of MeHg in these ponds is driven by Hg(II) methylation in sediments and MeHg photodemethylation in the water column. Production of MeHg and dimethylmercury in seawater was detected at all sites sampled in the Canadian Arctic Archipelago. Using measured rate constants of Hg(I1) methylation and MeHg demethylation in a simple model, we conclude that water column methylation accounts for ∼50% of MeHg occurring in Arctic marine waters. |
