Warming-induced methylmercury production undermines the effects of mercury mitigation in a temperate coastal bay

Although the Minamata Convention on Mercury has effectively reduced anthropogenic mercury (Hg) emissions, whether and when a similar reduction will be seen in the risk of Hg to humans and ecosystems remains uncertain. This is largely due to the possible decoupling of methylmercury (MeHg), the predominant neurotoxin, from declining Hg levels under climate change. Here, decade-long observations (2015-2025) in the Jiaozhou Bay-a representative temperate coastal bay with legacy Hg pollution-reveal that warming triggers a substantial increase in internal production of MeHg in coastal sediments, outweighing the benefits of declining total Hg inputs. This increase is driven by the preferential stimulation of methylators carrying hgcAB genes and a warming-induced "substrate pump" that releases sequestered Hg from iron-organic carbon complexes. We find that 1 °C of warming triggers a > 0.01 ng g^-1 d^-1 increase in MeHg production rates, resulting in a 57% rise in MeHg concentrations-an effect potent enough to outweigh a decade of cumulative efforts in Hg emission reductions. These findings indicate that climate warming threatens to convert organic-rich, legacy-contaminated coastal muddy sediments into active MeHg sources, underscoring the necessity to integrate climate adaptation into global Hg management frameworks.