Comparison of carbonate parameters and air–sea CO<Subscript>2</Subscript> flux in the southern Yellow Sea and East China Sea during spring and summer of 2011

In this work, we examined the carbonate parameters, i.e. total alkalinity (TA), pH, and partial pressure of CO₂ (pCO₂), and the air–sea CO₂ flux (FCO₂) in the continental shelves of the southern Yellow Sea (SYS) and East China Sea (ECS), based on two field surveys conducted in April and August of 2011. Surface pCO₂ showed significant spatial variations, ranging from 246 to 686 µatm in spring (average ± standard deviation = 379 ± 95 µatm) and from 178 to 680 µatm in summer (384 ± 114 µatm). During the spring cruise, the central SYS (pCO₂ < 240 µatm) and the Changjiang estuary (pCO₂ < 300 µatm) were under-saturated with CO₂, while the southern SYS and the southwestern ECS were supersaturated (pCO₂ = 420–680 µatm). In summer, however, the CO₂-supersaturated waters (pCO₂ = 380–680 µatm) occupied a relatively wide area, including the nearshore of the SYS and the Changjiang estuary, whereas pCO₂-deficient water (pCO₂ = 220–380 µatm) was observed only at the offshore ECS. In general, the entire SYS and ECS area behaved as a sustained CO₂ sink, with average FCO₂ of ?3.9 and ?2.1 mmol m^?2 d^?1 in spring and summer, respectively. Phytoplankton production was the driving force for CO₂ absorption, especially during the spring cruise. In addition, we found that typical water mixing processes and decomposition of terrestrial material were responsible for the release of CO₂ in three turbidity maximum regions.