Temporal and spatial variation of pCO2 in the coastal waters of the Pearl river estuary in winter and summer and its influencing factors: A model study.

Clarifying the complex physical-chemical-biological interactions is essential for evaluating the partial pressure of carbon dioxide (pCO2) in coastal waters. In this paper, a high-resolution three-dimensional coupled physical-biogeochemical model was established for the northern South China Sea (NSCS). The spatiotemporal distribution of the pCO2 in the coastal waters of the Pearl River estuary (CWPRE) was simulated, and its controlling factors were analyzed. Compared with in-situ observations and satellite data, the model accurately reproduced the changes in temperature, salinity, nutrients, chlorophyll, and air-sea CO2 flux (FCO2) in the NSCS. The results show that sea surface temperature played a critical role in the wintertime spatiotemporal distribution of pCO2, with pCO2 in coastal waters first decreasing and then increasing. In summer, non-temperature effects regulate pCO2 in coastal waters. Regression analyses indicated that the spatiotemporal distribution of summer pCO2 in the coastal waters was significantly influenced by chlorophyll and water stratification. A budget analysis of dissolved inorganic carbon (DIC) in the coastal waters indicates that the pCO2 in the spring-summer transition period was lowered due to enhanced biological activity. During the seasonal transition from summer to autumn, biological activity decreased while vertical mixing rates increased. Meanwhile, horizontal advection reduces total alkalinity in the coastal area and significantly contributed to the oscillating rise in surface pCO2. In the CWPRE, the air-sea CO2 fluxes show a strong sink for CO2 in winter with a flux of -4.02 ± 0.15 mmol m-2 d-1 and a weak sink in summer with a flux of -1.17 ± 0.33 mmol m-2 d-1.