AUTHOR=Li Dewang , Ni Xiaobo , Wang Kui , Zeng Dingyong , Wang Bin , Jin Haiyan , Li Hongliang , Zhou Feng , Huang Daji , Chen Jianfang TITLE=Biological CO2 Uptake and Upwelling Regulate the Air-Sea CO2 Flux in the Changjiang Plume Under South Winds in Summer JOURNAL=Frontiers in Marine Science VOLUME=8 YEAR=2021 URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2021.709783 DOI=10.3389/fmars.2021.709783 ISSN=2296-7745 ABSTRACT=

The partial pressure of CO₂ (pCO₂) in the sea and the air-sea CO₂ flux in plume waters are subject to interactions among biological production, horizontal advection, and upwelling under wind events. In this study, time series of pCO₂ and other biogeochemical parameters in the dynamic Changjiang plume were presented to illuminate the controlling factors of pCO₂ and the air-sea CO₂ flux after a strong south wind event (July 23–24, maximum of 11.2 ms^–1). The surface pCO₂ decreased by 310 μatm (to 184 μatm) from July 24 to 26. Low-pCO₂ waters (<200 μatm) were observed in the following 2 days. Corresponding chlorophyll a and dissolved oxygen (DO) increase, and a significant relationship between DO and npCO₂ indicated that biological uptake drove the pCO₂ decrease. The salinity of undersaturated-CO₂ waters decreased by 3.57 (from 25.03 to 21.46) within 2 days (July 27–28), suggesting the offshore advection of plume waters in which CO₂ had been biologically reduced. Four days after the wind event, the upwelling of high-CO₂ waters was observed, which increased the pCO₂ by 428 μatm (up to 584 μatm) within 6 days. Eight days after the onset of upwelling, the surface pCO₂ started to decrease (from 661 to 346 μatm within 3 days), which was probably associated with biological production. Regarding the air-sea CO₂ flux, the carbon sink of the plume was enhanced as the low-pCO₂ plume waters were pushed offshore under the south winds. In its initial stage, the subsequent upwelling made the surface waters act as a carbon source to the atmosphere. However, the surface waters became a carbon sink again after a week of upwelling. Such short-term air-sea carbon fluxes driven by wind have likely occurred in other dynamic coastal waters and have probably induced significant uncertainties in flux estimations.