The northwestern South China Sea is a complex marginal sea where physical forcings and biogeochemical processes interact intricately, yet the mechanisms regulating coastal carbon dioxide (CO₂) dynamics remain insufficiently understood. Here, we performed high-resolution underway measurements of surface partial pressure of CO₂ (pCO₂) and collected discrete carbonate samples along/across the inner and middle shelves. The observation data reveal an overall highly heterogeneous environment where river plumes, phytoplankton blooms, and upwelling coexisted. The estimated CO₂ emission is 4.4±7.1 mmol m^-2 d^-1 on the western shelf. The strong southwesterly winds drove Beibu Gulf water eastward through the Qiongzhou Strait, which is opposite to the normal direction of West-Guangdong Coastal Current. Concurrently, the upwelling-favorable winds brought cold, saline, and CO₂-rich subsurface waters to the surface off the west coast of Leizhou Peninsula. The coincidence of low oxygen, pH, high pCO₂ and turbidity bottom waters overlying the seafloor indicates the large oxygen demand of sediment or suspended particles. The diagnostic analyses using excess dissolved inorganic carbon (DIC) and apparent oxygen utilization reveal that aerobic respiration of organic matter is the dominant driver of subsurface carbon and oxygen anomalies, and the external DIC input is mainly from local rivers. Further, a box model simulated with realistic rates/parameterizations demonstrates that net community production, background source water properties, and air-sea gas exchange collectively regulate the dynamic spatio-temporal variability of CO₂ and oxygen. This study can shed light on CO₂ flux evaluation and the related controlling mechanism analysis in tropical coastal seas, providing a critical baseline for assessing future ecosystem responses to changing monsoon conditions.

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