Seasonal Variations in Surface CO2 Partial Pressure and Air–Sea CO2 Flux in the Lower Pearl River Estuary Based on High-Resolution Long-Time-Series Buoy Observations

The estuarine carbon cycle represents a highly dynamic biogeochemical process, with carbon source–sink patterns shifting dramatically under variable natural forcing and anthropogenic disturbances. Most previous investigations in the Pearl River Estuary (PRE) have depended on instantaneous observations, which cannot resolve the continuous high-frequency variability in surface water partial pressure of carbon dioxide (pCO2). Based on one year (2019) of high-resolution time-series data from a moored buoy in the lower PRE, this study explored the dominant mechanisms controlling seasonal variations in pCO2 and air–sea CO2 fluxes. In spring and summer, substantial nutrient inputs from Pearl River runoff fueled vigorous phytoplankton growth and intense biological CO2 uptake, lowering the monthly mean pCO2 to an annual minimum of 270 μatm in June, with an extreme low of 76 μatm. From July to September, reduced river discharge and rising sea surface temperature (SST) drove a pronounced pCO2 rebound, reaching an annual peak of 404 μatm in September. In winter, the sea surface temperature (SST) and the trend of pCO2temp largely explained the overall variations in pCO2, indicating the SST was the most controlling factor, and mixing process brought CO2-rich subsurface water upward. The contributions of driving factors exhibited distinct seasonal differentiation: biological uptake and runoff dominated in spring, temperature and runoff–biological coupling co-dominated in summer and autumn, and temperature served as the absolute dominant factor in winter, collectively constituting a complete seasonal cycle of controlling processes. Overall, the lower PRE functioned as a year-round atmospheric CO2 sink, with a mean flux of −2.46 ± 2.95 mmol m−2 d−1. This study elucidated the complex carbon cycling mechanism in the lower PRE, reduced uncertainty in regional carbon flux estimation, and offered a scientific foundation for estuarine carbon budget accounting.