Different circulation history of mercury in aquatic biota from King George Island of the Antarctic

Abstract

To trace the circulation history of aquatic bioavailable Hg in the Antarctic, the species and isotopic compositions of Hg in sediment, Archaeogastropoda (Agas), Neogastropoda (Ngas), and fish collected from King George Island were studied in detail. Positive mass independent fractionation (MIF) was observed and positively correlated with the percentages of methylmercury (MeHg%) in Agas and Ngas, suggesting an effect of MeHg accumulation during trophic transfer on MIF signatures. However, both the ratios of Δ¹⁹⁹Hg/δ²⁰²Hg and Δ¹⁹⁹Hg/Δ²⁰¹Hg indicated different circulation histories of Hg in Agas, Ngas, and fish. The microbial methylation in sediment was the primary source of MeHg in Agas and Ngas (Δ¹⁹⁹Hg/δ²⁰²Hg ∼0, Δ¹⁹⁹Hg/Δ²⁰¹Hg ∼1.00). In contrast, the MeHg in fish (Δ¹⁹⁹Hg/δ²⁰²Hg = 0.55 ± 0.06, Δ¹⁹⁹Hg/Δ²⁰¹Hg = 1.19 ± 0.17) came from the combined sources of residual MeHg which had sunk from the surface water and microbial-methylated MeHg in sediments, and the bioavailable Hg in the sediments contributed to approximately 44% of the total Hg in fish. Subsequently, the Δ¹⁹⁹Hg values of bioavailable MeHg and IHg in sediments were quantitatively calculated, which provided key end-member information for future source apportionment in the Antarctic and other pelagic regions. It was also found that the Hg accumulated in Agas and Ngas had no history of MeHg photo-degradation, indicating that the methylated Hg in benthic zones suffered little photo-degradation and thus presented high bioavailability and environmental risk.

Keywords: Antarctic; Mercury isotope; Methylmercury source; Photo-degradation.