Methylmercury Cycling in High Arctic Wetland Ponds: Sources and Sinks

The sources of methylmercury (MeHg; the toxic form of mercury that is biomagnified through foodwebs) to Arctic freshwater organisms have not been clearly identified. We used a mass balance approach to quantify MeHg production in two wetland ponds in the Lake Hazen region of northern Ellesmere Island, NU, in the Canadian High Arctic and to evaluate the importance of these systems as sources of MeHg to Arctic foodwebs. We show that internal production (1.8–40 ng MeHg m^–2 d^–1) is a much larger source of MeHg than external inputs from direct atmospheric deposition (0.029–0.051 ng MeHg m^–2 d^–1), as expected. Furthermore, MeHg cycling in these systems is dominated by Hg­(II) methylation and MeHg photodemethylation (2.0–33 ng MeHg m^–2 d^–1), which is a sink for a large proportion of the MeHg produced by Hg­(II) methylation in these ponds. We also show that MeHg production in the two study ponds is comparable to what has previously been measured in numerous more southerly systems known to be important MeHg sources, such as temperate wetlands and lakes, demonstrating that wetland ponds in the High Arctic are important sources of MeHg to local aquatic foodwebs

Mercury Export to the Arctic Ocean from the Mackenzie River, Canada

Circumpolar rivers, including the Mackenzie River in Canada, are sources of the contaminant mercury (Hg) to the Arctic Ocean, but few Hg export studies exist for these rivers. During the 2007–2010 freshet and open water seasons, we collected river water upstream and downstream of the Mackenzie River delta to quantify total mercury (THg) and methylmercury (MeHg) concentrations and export. Upstream of the delta, flow-weighted mean concentrations of bulk THg and MeHg were 14.6 ± 6.2 ng L^–1 and 0.081 ± 0.045 ng L^–1, respectively. Only 11–13% and 44–51% of bulk THg and MeHg export was in the dissolved form. Using concentration–discharge relationships, we calculated bulk THg and MeHg export into the delta of 2300–4200 kg yr^–1 and 15–23 kg yr^–1 over the course of the study. Discharge is not presently known in channels exiting the delta, so we assessed differences in river Hg concentrations upstream and downstream of the delta to estimate its influence on Hg export to the ocean. Bulk THg and MeHg concentrations decreased 19% and 11% through the delta, likely because of particle settling and other processes in the floodplain. These results suggest that northern deltas may be important accumulators of river Hg in their floodplains before export to the Arctic Ocean