Moderate CO2 sink due to phytoplankton bloom following a typhoon passage over the East China Sea

In September 2017, following passage of Typhoon Talim, daily satellite imagery captured a large area (∼300-km long and ∼160-km wide) of high concentration of chlorophyll a (Chl a) that persisted for at least 8 days on the East China Sea (ECS) shelf. Ship-based measurements were conducted 5–6 days after the passage of Talim. Results showed that surface seawater within the area of high Chl a concentration had features of high salinity, low temperature, and low partial pressure of CO2 (pCO2) in comparison with surrounding seawater. The high Chl a concentration reflected a phytoplankton bloom triggered by upwelling nutrient-rich deeper water attributable to the passage of Typhoon Talim. The change in surface pCO2 in the area of high Chl a concentration was explained by the combined effects of upwelling deeper water and subsequent biological production. The air–sea CO2 flux in the area of high Chl a concentration was −2.8 ± 0.4 mmol m−2 d−1 in the post-typhoon stage, which turned the study area from near equilibrium (0.3 ± 0.3 mmol m−2 d−1 in the surrounding water) to a moderate CO2 sink. This phytoplankton bloom absorbed 0.013 Tg C during the 8 days following the passage of Talim, effectively canceling out 28% of the typhoon-induced CO2 efflux during typhoon passage across the ECS. Therefore, the effect of typhoons on air–sea CO2 flux in the post-typhoon stage cannot be ignored. •Surface water had low pCO2 and high Chl a after a typhoon passage.•Surface pCO2 drawdown was due to phytoplankton bloom lasting at least 8 days.•Typhoon could turn the ECS shelf from near equilibrium to a moderate CO2 sink.