Primary productivity coupled to oxic methane production in coastal waters of southern China

Oxic methane (CH 4 ) 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 CH 4 production was positively correlated with chlorophyll a concentration and primary production, and that natural phytoplankton communities predominated by diatoms led to higher CH 4 production. Oxic methane production ranged between 0.9 and 2.1 mg CH 4 g Chl a −1 h −1 , indicating that 0.02–0.06% of the photosynthetically fixed CO 2 could be released as CH 4 . Measurements of the phytoplankton‐free filtrate demonstrated a negligible contribution to OMP by heterotrophs, substantiating that phytoplankton are contributing to the CH 4 oversaturation in the coastal oxic layer of this region. Moreover, high OMP in the photic zone partially counterbalances photosynthetic CO 2 sequestration by phytoplankton and should be accounted for in assessing fluxes of greenhouse gases.