The Seasonal Effect of Coastal Current and Upwelling on the Dynamics of Dissolved Organic Matter in the Northwestern South China Sea

Abstract Previous studies have primarily focused on the impact of human activities on coastal dissolved organic matter (DOM) cycles, while neglecting the role of dynamic processes. This limits our comprehensive understanding of the dynamic processes of coastal DOM, particularly in coastal waters affected by multiple dynamic processes. In this study, the dynamics of DOM in the northwestern South China Sea (SCS) were investigated using DOM‐related parameters and a three end‐member mixing model, which accounted for the effects of coastal currents and upwelling during the spring, summer, and winter seasons. Our results demonstrated that the net addition of DOM was predominantly observed in the nearshore during all seasons, owing to strong coastal currents that induced phytoplankton production and particle desorption. However, owing to weaker coastal currents and upwelling in spring and winter, DOM removal induced by decomposition was observed in the western offshore area. This observation can be attributed to the abundance of microorganisms in spring and increased vertical water mixing in winter. Conversely, in summer, stronger upwelling provided more bio‐refractory DOM, limiting its decomposition, whereas stronger coastal currents stimulated productivity and particle desorption, resulting in the net addition of DOM during this period. In the eastern offshore area, DOM decomposition was the primary mechanism for eliminating DOM owing to weak or absent coastal currents and upwelling. Our study revealed that DOM decomposition is widespread in the coastal regions of the northwestern SCS and that stronger coastal currents and upwelling facilitate the net addition of DOM.