Seasonal Variation of Air‐Sea CO2 Flux and Contribution of Biological Processes to Carbon Source/Sink in a Large River‐Dominated Shelf Sea

Abstract Productive shelf seas are often considered hotspots for absorbing atmospheric CO 2 . However, the contribution of biological processes to carbon source or sink remains unclear in global large river‐dominated shelf seas. As a case study, we applied a three end‐members mixing model to quantify the contribution of biological processes (including the release of CO 2 from organic matter degradation by microbes and CO 2 uptake by phytoplankton) to the variations of air‐sea CO 2 flux (F) in the Changjiang plume‐impacted shelf area (CPS). We also established a 1‐D mass budget model to quantitatively assess the factors controlling of the partial pressure of carbon dioxide ( p CO 2 ) during seasonal transitions. Our results showed that CPS acted as a source of atmospheric CO 2 in summer (2.0 ± 13.9 mmol m −2 d −1 ) and autumn (5.4 ± 8.6 mmol m −2 d −1 ), but as a sink in winter (−6.8 ± 9.4 mmol m −2 d −1 ) and spring (−1.0 ± 4.8 mmol m −2 d −1 ). Biological processes significantly influence p CO 2 variability. It is worth noting that during the winter‐to‐spring transition, biological processes contributed 36% (the largest among the four transition periods) to the p CO 2 decrease. In the CPS, the air‐sea CO 2 flux caused by biological processes in winter, spring, summer, and autumn are 2.6 ± 4.1, −15.1 ± 22.0, 2.2 ± 8.2, and −2.0 ± 4.9 mmol m −2 d −1 , respectively, with relative contributions of biological processes to F of 13.5%, −53.9%, 5.0%, and −11.0%, respectively. These findings suggest biological uptake of dissolved inorganic carbon in global continental shelf seas significantly enhance carbon sequestration and contribute to mitigating global warming.