Biogeochemistry of Dissolved Organic Matter in the Northwestern South China Sea under the Combined Influence of Coastal Currents and Typhoon Wipha

 Recent studies indicate that typhoons can trigger intense organic matter degradation in coastal areas. Nevertheless, as coastal currents enhance primary production, the balance between organic matter addition and degradation remains unclear, which restricts a comprehensive understanding of the carbon cycle. This study investigated the biogeochemical processes of dissolved organic matter (DOM) in the northwestern South China Sea, which is affected by the coastal current along the western Guangdong coast, before and after the passage of Typhoon Wipha (2019), through measuring DOM-related parameters and applying the three-end-member mixing model. The results demonstrated that in the nearshore, DOM exhibited a significant net addition before the typhoon. This was mainly due to the strong coastal current that facilitated the primary productivity. After the typhoon, DOM levels in coastal waters increased significantly due to greater land-based input, stronger vertical mixing, and higher primary production. However, the net addition of DOM was lower than pre-typhoon, primarily because of enhanced DOM degradation. In the offshore area, the biological activities stimulated by the strong coastal current remained the primary cause of most DOM additions before the typhoon. Nevertheless, after the typhoon, DOM showed net removal, as degradation exceeded production supported by the coastal current, with removal rates of 7% to 17%. This indicates that typhoons accelerate the degradation of DOM in coastal regions, potentially reducing marine carbon storage enhanced by coastal currents, offering insights into how the coastal carbon cycle responds to environmental changes.