Oxic methane (CH₄) production (OMP) occurs in diverse oxygenated surface waters worldwide. However, phytoplankton-driven OMP in natural marine environments remains poorly documented. During a research cruise in the highly productive southern East China Sea, we measured OMP by incubating phytoplankton-rich surface waters, and found that CH₄ production was positively correlated with chlorophyll a concentration and primary production, and that natural phytoplankton communities predominated by diatoms led to higher CH₄ production. OMP ranged between 0.9−2.1 mg CH₄ g Chl a⁻¹ h⁻¹, indicating that 0.02−0.06 % of the photosynthetically fixed CO₂ could be released as CH₄. Measurements of the phytoplankton-free filtrate demonstrated a negligible contribution to OMP by heterotrophs, substantiating that phytoplankton are contributing to the CH₄ oversaturation in the coastal oxic layer of this regions. Moreover, high OMP in the photic zone partially counterbalances photosynthetic CO₂ sequestration by phytoplankton and should be accounted for in assessing fluxes of greenhouse gases.

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