Mercury isotope variations within the marine food web of Chinese Bohai Sea: Implications for mercury sources and biogeochemical cycling

Mercury (Hg) speciation and isotopic compositions in a large-scale food web and seawater from Chinese Bohai Sea were analyzed to investigate methylmercury (MeHg) sources and Hg cycling. The biota showed similar to 5 parts per thousand variation in mass dependent fractionation (MDF, -4.57 to 0.53 parts per thousand in delta Hg-202) and mostly positive odd-isotope mass independent fractionation (odd-MIF, -0.01 to 1.21 parts per thousand in Delta Hg-199). Both MDF and odd-MIF in coastal biota showed significant correlations with their trophic levels and MeHg fractions, likely reflecting a preferential trophic transfer of MeHg with higher delta Hg-202 and Delta Hg-199 than inorganic Hg. The MDF and odd-MIF of biota were largely affected by their feeding habits and living territories, and MeHg in pelagic food web was more photodegraded than in coastal food web (21-31% vs. 9-11%). From the Hg isotope signatures of pelagic biota and extrapolated coastal MeHg, we suggest that MeHg in the food webs was likely derived from sediments. Interestingly, we observed complementary even-MIF (mainly negative Delta Hg-200 of -0.36 to 0.08 parts per thousand and positive Delta Hg-204 of -0.05 to 0.82 parts per thousand) in the biota and a significant linear slope of -0.5 for Delta Hg-200/Delta Hg-204. This leads us to speculate that atmospheric Hg-0 is an important source to bioaccumulated MeHg, although the exact source-receptor relationships need further investigation.