Temporal variation of dissolved methane in a subtropical mesoscale eddy during a phytoplankton bloom in the southwest Pacific Ocean

• Surface stratification positively influences the levels of oceanic methane.
• Increased methane was associated with a DMSP-containing Phaeocystis/Synechococcus bloom.
• Depleted δ13CCH4 in the MXL suggested in situ biogenic methane production.
• Particle export, zooplankton grazing and eddy dynamics influenced higher methane.
• Methylotrophy was a potential pathway for methane production.

A multi-disciplinary examination of the drivers of dissolved methane was carried out during a phytoplankton bloom located in a subtropical mesoscale eddy. This investigation related temporal signals in methane concentrations with other biophysical and biogeochemical parameters in the upper waters (<300 m) of the southwest Pacific Ocean. In the surface mixed layer, methane supersaturation increased and δ13CCH4 became more depleted coincident with increases in particulate dimethylsulfoniopropionate (DMSPp) and succession from the diatom Asterionellopsis glacialis to the nanoflagellate Phaeocystis globosa and the cyanobacterium Synechococcus sp. In situ methane production was calculated in a surface mixed layer methane budget that incorporated sea-to-air exchange and vertical diffusion. Methane concentrations increased in and below the mixed layer when the export of biogenic particles increased. Increased grazing of microbes by microzooplankton may have contributed to particle recycling (rich in organic carbon and DMSP) and increased the potential for methanogenesis. Phytoplankton species composition and biomass in different bloom phases, and eddy dynamics, were important determinants of methane saturation and emission, and the potential implications for methane are considered for the future surface ocean.