Speciation of mercury in the waters of the Weddell, Amundsen and Ross Seas (Southern Ocean)

Despite the distance from large anthropogenic emission sources, toxic mercury is transported via the atmosphere and oceans to the Southern Ocean. Seawater samples were collected at selected stations and were analysed for total mercury (HgT) (8 stations), dissolved gaseous mercury (DGM) (62 stations) and methylmercury (12 stations) during winter (Weddell Sea), spring (Weddell Sea) and summer (Amundsen and Ross Seas) in the Southern Ocean. The HgT distribution in water columns was found to not vary significantly with depth. In the Weddell Sea the average column concentration was higher in spring (2.6 ± 1.3 pM, 2 stations) than in winter (2.0 ± 1.0 pM, 6 stations). We hypothesize that the seasonal HgT increase is due to atmospheric deposition of particulate Hg(II) formed during atmospheric mercury depletion events (AMDEs), as well as the addition of inorganic mercury species from melting sea ice and snow. Furthermore, HgT concentrations found in this study were significantly higher than previously measured in the Southern Ocean (Cossa et al., 2011), which was hypothesized to be due to seasonal variations in atmospheric deposition.

The average water column DGM concentration in the Weddell Sea was 454 ± 254 fM in winter and 384 ± 239 fM in spring. The lowest average DGM concentration was found in summer in the Amundsen and Ross Seas (299 ± 137 fM). The highest observed concentration in winter was hypothesized to be caused by the larger sea ice coverage, which is known to reduce the evasion of Hg(0) from the sea surface.

The average monomethylmercury (MMHg) concentration in the Weddell Sea was 60 ± 30 fM in winter (6 stations) and 95 ± 85 fM in spring (2 stations), showing no significant seasonal difference. In the Amundsen and Ross Seas the summer average concentration of MeHg (MMHg and dimethylmercury; DMHg) was 135 ± 189 fM (4 stations). The highest MeHg concentration was found in modified circumpolar deep water, which is known to have high primary production.

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