Satellite estimation of air-sea CO2 flux in marine aquaculture areas: a case study of Lianjiang County, China

Coastal mariculture areas exhibit high spatiotemporal heterogeneity in carbon source–sink dynamics due to the interaction of natural processes and human activities. A systematic understanding of the spatiotemporal variability and controlling mechanisms of seawater CO₂ partial pressure (pCO₂) and  air–sea CO2 flux (FCO2) remains limited here. Lianjiang County, a leading county in China in terms of fishery output over the past five years, was selected as the study area. Based on Sentinel‑2 reflectance data, we developed a 10 m‑resolution sea surface pCO₂ retrieval model using the XGBoost algorithm. The model demonstrated high accuracy, with a coefficient of determination (R²) of 0.87 and a root‑mean‑square error (RMSE) of 6.89 μatm. Sea surface pCO₂ in the 2024–2025 mariculture period (autumn to spring) witnessed a “higher in warm seasons, lower in cold seasons” pattern, driven by both thermal dynamic and biological processes. Meanwhile, the retrieved pCO₂ exhibited a distinct “mariculture signal”: Compared with ambient seawater, Macroalgae culture areas maintained lower pCO₂ due to strong photosynthesis, whereas shellfish farming areas showed elevated pCO₂ from respiration and calcification; In co-cultivation areas of shellfish and algae, pCO₂ falls between those of their respective monocultures. Although the study area functioned as a weak source of atmospheric CO₂, this study reveals the potential of mariculture as a marine carbon dioxide removal (mCDR) strategy. By reducing background CO₂ efflux, macroalgae cultivation enhances carbon sequestration. In contrast, while shellfish farming elevates CO₂ emissions, integrated aquaculture provides a compensatory mechanism that partially offsets these emissions, reconciling economic value with climate objectives. This work provides a high-resolution remote-sensing approach for monitoring sea surface pCO₂ in coastal mariculture areas, clarifies the joint regulation of carbon dynamics by natural and farming processes, and offers a scientific basis for carbon cycle management in mariculture zones.