Seasonal variability of sea surface pCO2 and air-sea CO2 flux in a high turbidity coastal ocean in the vicinity of the East China Sea

The sea surface partial pressure of CO 2 ( p CO 2 ) and air-sea carbon flux in estuarine and bay areas, influenced by both natural and anthropogenic factors, remain poorly understood and inadequately assessed. This study, based on seasonal underway observations conducted in 2024, analyzed the seasonal variations in surface seawater p CO 2 and air-sea CO 2 flux in the high-turbidity coastal waters of Zhejiang, including Hangzhou Bay (HZB), Xiangshan Bay (XSB), Sanmen Bay (SMB), and the nearshore waters (NSW). The results indicate that the p CO 2 in the study area ranged from 194 to 739 μatm throughout the year, exhibiting significant spatiotemporal heterogeneity. In HZB, the lowest p CO 2 was observed in winter, averaging 453 μatm, whereas the values in spring and summer were around 600 μatm, with a subsequent decline to 481 μatm in autumn. In XSB, p CO 2 reached its minimum in winter (194 μatm), attributed to vigorous biological activity, and peaked in spring, averaging 639 μatm. In SMB, p CO 2 was relatively lower in autumn and winter (~470 μatm), and higher in spring and summer (~640 μatm). In the NSW, p CO 2 was lower in winter and spring (~445 μatm), and increased to ~510 μatm in summer and autumn. The p CO 2 was predominantly regulated by sea surface temperature and horizontal mixing, while other factors like biological activity also had significant impacts. The annual average CO 2 flux was 6.0±3.7 mmol m -2 d -1 in HZB, 1.2±2.3 mmol m -2 d -1 in XSB, 7.0±3.2 mmol m -2 d -1 in SMB and 5.2±5.9 mmol m -2 d -1 in the NSW. Higher wind speeds in autumn and winter, coupled with elevated the p CO 2 difference between the surface water and the atmosphere (Δ p CO 2 ) in spring and summer, collectively drove the seasonal variations in CO 2 flux. On an annual scale, both the estuarine and bay areas and the nearshore regions functioned as carbon sources.