Abstract: Human exposure to monomethylmercury (MMHg) through seafood consumption and associated health risks is a global concern. We investigate the potential sources and processes of MMHg in coastal and freshwater seafood through combing of δ¹³C, δ¹⁵N, and specific Hg (including MMHg and inorganic Hg (IHg)) isotopes. Coastal and freshwater species exhibit different isotope vales of C, N and MMHg. Geographical differences between δ²⁰²Hg_MMHg and Δ¹⁹⁹Hg_MMHg in seafood may be attribute to ocean MMHg input in to the coastal ecosystem. Coastal and freshwater seafood have consistent slope of Δ¹⁹⁹Hg_MMHg/δ²⁰²Hg_MMHg, suggesting that MMHg in those organisms undergo a similar photodegradation process. The Δ¹⁹⁹Hg_MMHg values in coastal fish (mean ± 1SD: 1.15‰ ± 0.39‰) are significantly higher than those in coastal shellfish (0.67‰ ± 0.67‰) (p < 0.05), which indicates diverse MMHg absorption pools of coastal fish and shellfish. The extremely high values of Δ¹⁹⁹Hg_IHg in coastal and freshwater aquatic organisms could be explained by the in vivo demethylation of MMHg and reabsorption of IHg by aquatic organisms. We found a significant positive correlation between δ¹⁵N and Δ¹⁹⁹Hg_IHg, which may indicate that aquatic species in various TLs may have different ability of MMHg demethylation. Our results showed that combining multiple stable isotope analysis can give an overall profile of aquatic MMHg biogeochemical cycle and deserve to pay more attentions in future.

Keywords: Aquatic organisms; Monomethylmercury; C isotope; N isotope; Specific Hg isotope