Methylmercury in Seawater and Its Bioaccumulation in Marine Food Webs of the Canadian Arctic
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Date
2019-03-28
Authors
Wang, Kang
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Abstract
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 the importance of seawater MeHg in controlling Hg
bioaccumulation in Arctic marine food webs, and calls for more studies on processes that
produce and maintain the subsurface seawater MeHg enrichment to better understand the
exposure of MeHg to the Arctic ecosystem and Indigenous Peoples.
Description
Keywords
Methylmercury, Seawater, Canadian Arctic, Bioaccumulation
Citation
Wang, K., Munson, K.M., Beaupré-Laperrière, A., Mucci, A., Macdonald, R.M., Wang, F. (2018). Subsurface seawater methylmercury maximum explains biotic mercury concentrations in the Canadian Arctic. Scientific Reports, 8(1), 14465. https://doi.org/10.1038/s41598-018-32760-0.