Biological CO2 Uptake and Upwelling Regulate the Air-Sea CO2 Flux in the Changjiang Plume Under South Winds in Summer

Abstract

The partial pressure of CO 2 ( p CO 2 ) in the sea and the air-sea CO 2 flux in plume waters are subject to interactions among biological production, horizontal advection, and upwelling under wind events. In this study, time series of p CO 2 and other biogeochemical parameters in the dynamic Changjiang plume were presented to illuminate the controlling factors of p CO 2 and the air-sea CO 2 flux after a strong south wind event (July 23–24, maximum of 11.2 ms –1 ). The surface p CO 2 decreased by 310 μatm (to 184 μatm) from July 24 to 26. Low- p CO 2 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 n p CO 2 indicated that biological uptake drove the p CO 2 decrease. The salinity of undersaturated-CO 2 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 CO 2 had been biologically reduced. Four days after the wind event, the upwelling of high-CO 2 waters was observed, which increased the p CO 2 by 428 μatm (up to 584 μatm) within 6 days. Eight days after the onset of upwelling, the surface p CO 2 started to decrease (from 661 to 346 μatm within 3 days), which was probably associated with biological production. Regarding the air-sea CO 2 flux, the carbon sink of the plume was enhanced as the low- p CO 2 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.

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