Changes from a Sink of Atmospheric CO2 to a Source with Governing Factors in CO2 Uptake of the East China Sea Shelf in Summer

Surface partial pressure of CO₂ (pCO₂), temperature (T), salinity (S), nutrients, and chlorophyll a (Chl-a) were measured in the East China Sea (ECS) in summer (June and August) 2003. Surface pCO₂ ranged from 109.1 to 435.3 μatm (average 297.3 ± 56.2 matm, n = 1113) in June and from 235.0 to 467.2 μatm (average 373.3 ± 33.2 μatm, n=1624) in August. There was a significant increase ca. 26% in pCO₂ values within 2 months so that the ΔpCO₂ changed from negative to positive. The pCO₂ changes in Kuroshio and Taiwan Current Warm Waters (S > 33.2) were mainly controlled by sea surface temperature (SST) and the ΔpCO₂ equals zero at 27 °C (pCO_2air = pCO_2w). Furthermore, the pCO₂ data generally decreased exponentially with Chl-a increases in Changjing Diluted Water (CDW) , except for coastal upwelled waters. The results showed the summer pCO₂ distribution was associated with changes in water masses. From June to August 2003, the CDW area coverage was actually decreased by ~80%, resulting in less biological effect and a SST increase. Consequently, the air-sea CO₂ flux shifted from a sink of atmospheric CO₂ (June to July) to a source (August) and its average flux is -1.8±2.7 mmole m^-2 d^-1 in the ECS in summer. Further, the empirical regression relationships were established according to SST and Chl-a data as major controlling variables. The pCO₂ data calculated by using remotely sensed SST and Chl-a data well correlated with the underway pCO₂ data. It demonstrated again the shift of CO₂ uptake in summer in the ECS from a sink to a source due to changes in the CDW area associated with the discharge alteration.