Estuarine air–sea CO₂ exchange is an important component of the global carbon cycle, yet its long-term variability remains poorly constrained in rapidly urbanizing estuaries. Using observations from ten research cruises conducted between 2010 and 2019, this study investigates the spatial distribution and interannual variability of surface water partial pressure of CO₂ (pCO₂) and air–sea CO₂ fluxes (CO₂ flux hereafter) in the Pearl River Estuary (PRE). A pronounced river-to-ocean gradient was consistently observed. River-dominated upstream regions acted as persistent source of atmospheric CO₂, whereas pCO₂ and CO₂ fluxes decreased seaward with increasing salinity, and offshore waters approached atmospheric equilibrium or occasionally behaved as weak CO₂ sinks. Interannual variability differed substantially among regions. Upstream reaches exhibited the largest temporal fluctuations and the greatest rates of pCO₂ increase, reflecting strong riverine influence. The river–ocean mixing zone showed marked seasonal dependence, with greater variability during high-discharge periods than during winter. In contrast, the outer estuary exhibited relatively stable temporal behavior with gradual increases broadly consistent with rising atmospheric CO₂. Correlation and regression analyses indicate that river discharge, salinity-driven mixing, sea surface temperature, and biological processes jointly regulate pCO₂ variability, although their relative importance varies along the estuarine gradient. These results reveal that long-term carbon dynamics in the Pearl River Estuary are spatially heterogeneous rather than uniformly changing across the estuary. The study provides an updated assessment of air–sea CO₂ exchange during 2010–2019 and improves our understanding of carbon cycling in subtropical estuaries under increasing anthropogenic and climatic influences.
 

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