Summer carbonate chemistry dynamics in the Southern Yellow Sea and the East China Sea: Regional variations and controls

Abstract

Surface partial pressure of CO2 (pCO2) and pertinent parameters (i.e., pH, total alkalinity, dissolved oxygen, chlorophyll a) were investigated in the southern Yellow Sea (SYS) and the East China Sea (ECS) basing on two surveys conducted in June and August of 2013. The results suggested carbonate chemistry dynamics and related controlling factors were provided with significant temporal and spatial variations in different subregions of these two continental shelf seas. The western of SYS (SYSW) was CO2-undersaturated both in June and August, with the average FCO2 −1.88mmolm−2d−1 and −3.72mmolm−2d−1, respectively. The phytoplankton initiated CO2-absorption and the suspended sediment induced CO2-emission jointly controlled the air–sea CO2 exchange there. The center of SYS (SYSC) also behaved as an obvious CO2 sink (−1.57mmolm−2d−1and −3.99mmolm−2d−1 in June and August, respectively), probably due to elevated TA/DIC ratio and the subsequent effects of spring bloom. As for the Yangtze River estuary (YRE), it changed from an obvious CO2 sink (−1.28mmolm−2d−1) in June into a very weak CO2 source (0.04mmolm−2d−1) in August. This change was probably associated with the rising of seawater temperature and monthly variation of Yangtze River discharge. The inner shelf of ECS (ECSS) experienced obvious air–sea CO2 flux changes during from June (−8.88mmolm−2d−1) to August (−0.36mmolm−2d−1) as well. Biological DIC consumption in the upper layer and DIC regenerated from respiration in the subsurface jointly controlled this pCO2 variation. As a whole, the SYS and ECS acted as an obvious CO2 sink during summer and could absorb atmospheric CO2 with the average air–sea flux (FCO2) −2.68mmolm−2d−1.The summary of air–sea CO2 flux in the ECS and SYS during recent two decades indicated the ECS served as quite a stable CO2 sink, whereas the SYS experienced obvious change. Discharge of Yangtze River and anthropogenic nutrients loading could profoundly affect the variations of pCO2 and FCO2 in future.