This study investigates the carbonate system in the northern South China Sea shelf off the Pearl River estuary during the spring of 2023. Contrary to the typical distribution pattern observed in river-dominated coast where dissolved inorganic carbon (DIC) increases offshore, field observations revealed higher DIC in inshore waters (> 1980 µmol kg − 1 ) than in offshore seawaters (< 1970 µmol kg − 1 ), with DIC in the coastal zone being 20.9±8.8 µmol kg − 1 higher than that offshore. An end-member mixing model indicated that the high DIC coastal water was primarily attributed to mixing with the remnant high-DIC southward winter China Coastal Current water. In addition, biological processes and air-sea CO 2 exchange also played important roles. Two representative regions were examined: the inshore high-DIC region and the offshore low-DIC region. In the inshore high-DIC region, biological processes and air-sea CO₂ exchange increased DIC by 11.6 ± 3.0 (relative to air-sea CO 2 equilibrium) and 1.1 ± 7.0 µmol kg − 1 (relative to conservative mixing), respectively. In the offshore low-DIC region, biological processes decreased DIC by 5.1 ± 3.5 µmol kg − 1 , whereas air-sea CO 2 exchange increased DIC by 9.9 ± 2.8 µmol kg − 1 . Overall, this study highlights the dominant role of the cross-seasonal influence of the remnant water of the coastal current, as well as the secondary but significant contributions of biological activity and air-sea CO 2 exchange to the DIC distribution in coastal regions.
Lei et al. (2026) studied this question.