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 CO₂. 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 CO₂ from organic matter degradation by microbes and CO₂ uptake by phytoplankton) to the variations of air-sea CO₂ 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 (pCO₂) during seasonal transitions. Our results showed that CPS acted as a source of atmospheric CO₂ 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 pCO₂ 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 pCO₂ decrease. In the CPS, the air-sea CO₂ 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.