Mesoscale cyclonic eddies are important dynamical processes regulating the biogeochemical cycling of trace elements in the open ocean, yet their influence on the distribution of methylmercury (MeHg) remains poorly understood. In this study, we measured monomethylmercury (MMHg) and dimethylmercury (DMHg) in a cyclonic eddy in the Kuroshio‑Oyashio Extension in September 2024. MMHg concentrations were higher at the eddy center (0.115 ± 0.067 pM) than at the edge (0.072 ± 0.043 pM) and outside (0.075 ± 0.059 pM). Cyclonic eddy diffusion flux calculations show a higher upward MMHg flux at the center, accelerating vertical exchange across water masses. Water mass analysis revealed that the North Pacific Intermediate Water (NPIW) is the primary MMHg reservoir (0.168 ± 0.063 pM), and the cyclonic eddy promotes MMHg upwelling through Ekman pumping. In addition, dissolved oxygen and nutrients likely promote MMHg formation within the eddy. No significant spatial patterns were observed for DMHg, suggesting that its vertical distribution is mainly controlled by biochemical processes, rather than the cyclonic eddy. The positive correlation between DMHg and MMHg implies that DMHg may derive from further methylation of MMHg. This study demonstrates that cyclonic eddies can govern MMHg distribution through physical transport, providing new insights into oceanic MeHg cycling.

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