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

2019-10-10 376

Mercury (Hg) concentration in upper ocean (< 1000 m) is ~2.5 times of pre-industrial level. The elevated inorganic Hg (IHg) is expected to increase the production of methylmercury (MeHg, the most toxic Hg form) in the oceans, because IHg can be transformed into MeHg through microbial and chemical methylation. The produced MeHg could be bio-accumulated by organisms of lower trophic levels and further bio-magnified by several orders of magnitudes along marine food chains, which would increase human exposure risk to MeHg. Better knowledge on MeHg sources of food webs and Hg cycling in coastal regions is the key to mitigate human exposure to MeHg via seafood consumption.

Here, we comprehensively investigated a large-scale food web, seawater, and sediments from Chinese Bohai Sea, and analyzed the speciation and isotope compositions of Hg along with ecological tracers such as carbon and nitrogen isotopes. Our results demonstrate that bioaccumulation and trophic transfer of MeHg in food webs unlikely fractionate Hg isotopes, and the observed enrichment of heavier and odd Hg isotopes during trophic transfer was likely induced by the preferential trophic transfer of MeHg with higher Hg isotopes than IHg. Based on the Hg isotope fractionation theory obtained from control experiments on IHg methylation and MeHg photo-degradation processes, we calculated that MeHg in pelagic food web was more photodegraded than coastal food web (21-31% vs. 9-11%), and suggested that MeHg in the Bohai food web was likely derived from marine sediments. Interestingly, we observed complementary even-isotope mass independent fractionation signatures in the biota, highlighting the possibility of atmospheric Hg0 incorporation into food webs.

Meng, M.; Sun, R. Y.; Liu, H. W.; Yu, B.; Yin, Y. G.; Hu, L. G.; Chen, J. B.; Shi, J. B. *; Jiang, G. B. Mercury isotope variations within the marine food web of Chinese Bohai Sea: Implications for mercury sources and biogeochemical cycling. Journal of Hazardous Materials. 2019, doi: 10.1016/j.jhazmat.2019.121379.

Access via link: https://doi.org/10.1016/j.jhazmat.2019.121379

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