Coastal Upwelling Enhances Carbon Sources in the Changjiang Plume‐Impacted Shelf During Summer

Abstract River plume‐impacted shelf marginal seas exhibit strong carbon sequestration potential due to their high biological productivity. However, frequent coastal upwelling events complicate the carbon source‐sink dynamics because of the competing effects on seawater partial pressure of carbon dioxide ( p CO 2 ): the upwelling of dissolved inorganic carbon (DIC)‐rich deep waters initially elevate surface p CO 2 , while subsequent biological uptake lowers it. As a case study, we used a novel wave‐driven profiler to obtain high‐resolution vertical profiles in the Changjiang plume‐impacted shelf area (CPS) and to investigate upwelling‐induced variability in p CO 2 and carbon source‐sink dynamics. The observations were conducted near Gouqi Island, where coastal upwelling frequently occurs. Based on a p CO 2 mass balance model, we found that biological processes (contributing 30.6% to p CO 2 increase) and physical transport (contributing 21.2% to p CO 2 decrease) jointly dominated hourly mixed layer p CO 2 variability in the study area. Importantly, we found that α SBW (shelf bottom water fraction) served as a good quantitative proxy for upwelling intensity, with each 1% increase in α SBW associated with a 6‐μatm increase in mixed layer. Given the significantly higher mean α SBW values during 20–22 August (34 ± 5%) than 28–30 August (11 ± 7%), we defined the former as the upwelling period and the latter as the post‐upwelling period. The air‐sea CO 2 flux () during the upwelling period (24.04 ± 16.24 mmol m −2 d −1 ) was significantly higher than post‐upwelling period (1.25 ± 0.98 mmol m −2 d −1 ). These findings provide new mechanistic insights into how coastal upwelling regulates carbon source‐sink dynamics in large river‐dominated shelf seas and highlight its importance for improving predictions of carbon sequestration potential in marginal seas.