Regulation of air-sea CO2 flux and aragonite saturation state in coral reef ecosystems along the eastern coast of Hainan Island, China.

Coral reefs are highly sensitive to environmental stressors and human disturbance, leading to significant variability in seawater carbonate parameters, including partial pressure of CO2 (pCO2) and aragonite saturation state (Ωarag). Although coral reefs extensively occupy the eastern coast of Hainan Island, existing research is insufficient for a comprehensive understanding of their carbonate dynamics and acidification status. In this study, we characterize carbonate chemistry across six coral reef ecosystems along the east Hainan coast. The observed ranges for sea surface pCO2 (342-584 μatm), air-sea CO2 flux (-0.65-2.28 mmol C m-2 d-1), and Ωarag (2.64-3.77) exhibit substantial regional variability, which correlates with wide ranges of dissolved inorganic carbon (DIC, 1848-1968 μmol kg-1) and total alkalinity (TA, 2170-2285 μmol kg-1). A quantitative analysis indicates that the distributions of these carbonate parameters are primarily influenced by the upwelling of CO2-enriched subsurface waters. Additional factors such as coastal riverine plumes, sea surface CO2 outgassing, and biological carbon metabolism significantly shape the overall carbonate chemistry. Correlation analyses suggest that dissolved oxygen (DO) and soluble reactive phosphorus (SRP) serve as indicators of biological organic metabolism, while salinity and turbidity reflect inorganic carbon metabolism. Currently, there appears to be no immediate risk from acidification affecting calcified organisms within these environments, however, they may face substantial threats from seawater acidification by the end of this century due to global warming effects, alongside elevated atmospheric CO2 concentrations as well as regional disturbances arising from both biological organic and inorganic carbon metabolism.