Abstract
Mercury (Hg) is a contaminant of major concern in Arctic marine ecosystems. Decades of Hg observations in marine biota from across the Canadian Arctic show generally higher concentrations in the west than in the east. Various hypotheses have attributed this longitudinal biotic Hg gradient to regional differences in atmospheric or terrestrial inputs of inorganic Hg, but it is methylmercury (MeHg) that accumulates and biomagnifies in marine biota. Here, we present high-resolution vertical profiles of total Hg and MeHg in seawater along a transect from the Canada Basin, across the Canadian Arctic Archipelago (CAA) and Baffin Bay, and into the Labrador Sea. Total Hg concentrations are lower in the western Arctic, opposing the biotic Hg distributions. In contrast, MeHg exhibits a distinctive subsurface maximum at shallow depths of 100–300 m, with its peak concentration decreasing eastwards. As this subsurface MeHg maximum lies within the habitat of zooplankton and other lower trophic-level biota, biological uptake of subsurface MeHg and subsequent biomagnification readily explains the biotic Hg concentration gradient. Understanding the risk of MeHg to the Arctic marine ecosystem and Indigenous Peoples will thus require an elucidation of the processes that generate and maintain this subsurface MeHg maximum.
Highlights
Monitoring data collected during the past four decades have shown Hg concentrations in Canadian Arctic marine mammals to be highly elevated, frequently exceeding toxicity thresholds[1,2]
Most hypotheses attribute higher marine biotic Hg concentrations in the western Canadian Arctic to elevated inputs of inorganic Hg to these regions. These inputs include (1) atmospheric deposition of anthropogenic Hg from Asian sources[3], which is enhanced locally by atmospheric mercury depletion events (AMDEs) during polar sunrise[7]; (2) riverine Hg input from the Mackenzie River[8,9], which may be enhanced by tundra uptake of atmospheric elemental Hg10 [and permafrost thawing[11]; and (3) a naturally high geological background of Hg12
Total Hg concentrations show a distinctive longitudinal gradient along the transect (Fig. 1b, Table 1), with concentrations increasing from the Canada Basin and the western Canadian Arctic Archipelago (CAA) (1.76 ± 1.15 pM) to the eastern CAA and Baffin Bay (2.62 ± 1.97 pM)
Summary
Occur in coastal regions and most of the deposited Hg is re-emitted to the atmosphere before snow melts[22], limiting its transfer into the ocean. A notable difference is that the subsurface MeHg peak occurs at a much shallower depth in the western Canadian Arctic (100–300 m), in agreement with recent reports from the Beaufort Sea[15] and the central Arctic Ocean[16]. This MeHg maximum occurs at shallow depths that are just below the surface productive layer (see Fig. S2), which may enhance MeHg availability to organisms at the base of the marine food webs[16]. Understanding the risk of MeHg to the Arctic marine ecosystem and Indigenous Peoples will require an elucidation of the processes that generate and maintain the subsurface seawater MeHg maximum
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