Monomethylmercury Isotopes Reveal a Hidden Biomagnification Pathway in Marine Ecosystems and Their Environmental Implications

Monomethylmercury (MMHg) is a potent neurotoxin to which humans are exposed via fish consumption. However, the relative importance of planktonic and benthic biomagnification pathways to fish MMHg concentrations in marine food webs is challenging to quantify. Here, we apply compound-specific isotope analysis (CSIA) of Hg to identify fish MMHg biomagnification pathways across nearshore bay (NB), marine continental shelf (MCS), and pelagic ocean (PO) regions. We observe significant differences in Δ¹⁹⁹Hg between MMHg and total mercury (THg), highlighting the limitations of using THg isotopes to resolve MMHg dynamics in the environment. In NB fish, Δ¹⁹⁹Hg of MMHg closely matches that of benthic invertebrates, while in MCS and PO fish, it aligns with phytoplankton. According to the MMHg isotope binary mixing model, about 85% of MMHg in NB fish derives from the benthic biomagnification pathway, whereas over 90% of MMHg in MCS and PO fish originates from seawater-phytoplankton trophic transfer. These findings reveal that the benthic biomagnification pathway in near-shore regions has been underestimated in previous models, leading to potential uncertainties in evaluating marine Hg cycling and human exposure risks. This study highlights the importance of the benthic biomagnification pathway in coastal environments and demonstrates the potential of the CSIA of Hg for investigating MMHg biomagnification pathways in marine food webs, which provides new insights for global Hg pollution management under the Minamata Convention.