New insight into mercury sources and trophic transfer in the Bohai Sea food web using carbon, nitrogen, and mercury stable isotopes.

Ocean serves as a major reservoir and pathway for mercury (Hg) in the global environment; however, the bioaccumulation and trophic transfer behavior of mercury species in marine food web remain poorly understood. Here, we investigated the trophic transfer processes and identified key drivers of inorganic mercury (IHg) and methylmercury (MeHg) in a marine food web in the Bohai Sea. Carbon (δ13C) and nitrogen (δ15N) isotopic data revealed distinct feeding ecologies among biota species. Trophic magnification factor (TMF) analysis indicated biomagnification of MeHg (1.48, 95% CI = 1.10 - 1.97), driven by efficient trophic transfer, whereas IHg exhibited biodilution (0.58, 95% CI = 0.37 - 0.88). A Bayesian MixSIAR model showed that shrimp was the most important dietary source for crab, cephalopoda, and benthic, demersal, and pelagic fishes (contribution: 27.6% - 47.5%). A three-endmember mixing model based on mercury isotopes (δ202Hg, Δ199Hg, and Δ200Hg) quantified the relative contributions of four sources/processes: direct anthropogenic emissions, MeHg demethylation, atmospheric particulate bound mercury deposition, and other sources. Structural equation model (SEM) further identified biota species, body weight, dietary habit, trophic level, and regional pollution level as crucial drivers influencing MeHg bioaccumulation in fish. Our findings highlight the combined effects of environmental factors, biological growth characteristics, and food web ecology on the trophic transfer of mercury species in the marine food web, providing valuable insights into how variations in anthropogenic emissions may influence mercury dynamics in marine ecosystems under current and future environmental scenarios.