| Summary: | Mercury (Hg) is a major contaminant in the Arctic marine ecosystem, with concentrations in marine mammals and Indigenous Peoples frequently exceeding safety thresholds. The key step of Hg bioaccumulation is Hg methylation in the ocean, as the resulting monomethylmercury (MMHg) biomagnifies in the marine food webs. However, little is known about the sources and dynamics of seawater MMHg in the Arctic. In this research, high vertical resolution profiles of total Hg and methylated Hg (MeHg, sum of MMHg and dimethylmercury) were measured, for the first time, in seawater across the Canadian Arctic from the Canada Basin in the west, through the Canadian Arctic Archipelago, to Baffin Bay in the east and reaching Labrador Sea in the North Atlantic Ocean. Whereas total Hg concentrations are lower in the western Canadian Arctic, MeHg is enriched at shallow depths and its peak concentration decreases from west to east. Biological uptake of this subsurface MeHg and subsequent biomagnification can readily explain the regional gradients of biotic Hg in the Canadian Arctic. Seawater MeHg concentrations show significant correlations with nutrients and apparent oxygen utilization, but this does not necessarily support that MeHg is produced in-situ in the water column; instead, further analysis with water masses and N* reveals that the subsurface MeHg is likely originated from the Chukchi Sea sediments and advected within the Upper Halocline Water to the Canadian Arctic. The long-distance transport implies that MeHg in Arctic seawater must have a half-life much longer than previously determined from the seawater incubation approach, which is problematic in estimating Hg methylation and demethylation rates in seawater. Incubation studies with an Arctic copepod (Calanus hyperboreus) show that the microenvironments in copepod guts and fecal pellets are unlikely hotspots for Hg methylation, and that the copepod preferentially bioaccumulates MMHg over inorganic Hg and the main uptake pathway is trophic transfer. This study underlines ...
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