Abstract
The partial pressure of CO2(pCO2) in the sea and the air-sea CO2flux in plume waters are subject to interactions among biological production, horizontal advection, and upwelling under wind events. In this study, time series ofpCO2and other biogeochemical parameters in the dynamic Changjiang plume were presented to illuminate the controlling factors ofpCO2and the air-sea CO2flux after a strong south wind event (July 23–24, maximum of 11.2 ms–1). The surfacepCO2decreased by 310 μatm (to 184 μatm) from July 24 to 26. Low-pCO2waters (<200 μatm) were observed in the following 2 days. Corresponding chlorophyllaand dissolved oxygen (DO) increase, and a significant relationship between DO and npCO2indicated that biological uptake drove thepCO2decrease. The salinity of undersaturated-CO2waters 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 CO2had been biologically reduced. Four days after the wind event, the upwelling of high-CO2waters was observed, which increased thepCO2by 428 μatm (up to 584 μatm) within 6 days. Eight days after the onset of upwelling, the surfacepCO2started to decrease (from 661 to 346 μatm within 3 days), which was probably associated with biological production. Regarding the air-sea CO2flux, the carbon sink of the plume was enhanced as the low-pCO2plume 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.
Highlights
The carbon cycles in shelf seas are important components of global carbon cycles
The wind speeds were generally less than 8 m s−1 in our record, except on July 23 and 24, when strong south winds prevailed with a maximum wind speed of 11.2 m s−1 (Figures 1, 2E)
A sharp increase in oxygen and a significant decrease in pressure of CO2 (pCO2) occurred on July 24–26, at which time the maximum surface chlorophyll a (Chl a) concentration was three times than that measured on July 23 (Figure 2D)
Summary
The carbon cycles in shelf seas are important components of global carbon cycles. With 7% of the global ocean surface area, shelf seas take up to 0.05–0.4 Pg C a−1 (Borges et al, 2005; Cai et al, 2006; Chen et al, 2013; Dai et al, 2013; Laruelle et al, 2014), accounting for more than 10% of the global ocean carbon sink. The carbon sink is typically strong in river-dominated margins where large river plumes, such as the Changjiang plume (Zhai and Dai, 2009; Tseng et al, 2014; Guo et al, 2015), Pearl River plume (Zhai et al, 2005; Guo et al, 2009), and Amazon River plume (Ternon et al, 2000), induce significant biological uptake of carbon (Dai et al, 2013; Cao et al, 2020). A. et al, 2008; Guo et al, 2015) due to the influences of plume dynamics (Huang et al, 2013; Tseng et al, 2014), physical mixing, and the advection of water masses (Li et al, 2019), nutrient supply, temperature, etc. The responses of CO2 in plume waters to wind are largely unknown
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