The southern coast of Korea is a hydrographically complex and highly productive coastal system; however, its seasonal variability of air-sea CO₂ flux and aragonite saturation state (Ω_arag) is yet to be quantitatively resolved. To identify the controls on this variability, we analyzed inorganic carbonate parameter data obtained from four seasonal surveys conducted at approximately 150 stations between February 2022 and August 2023. Surface pCO₂ and aragonite saturation state were calculated using the CO2SYS program. The region acted as a CO₂ sink in winter and a weak source in other seasons, with an annual mean flux of -0.26 mmol m^-2 day^-1. Decomposition of the surface pCO₂ into thermal and non-thermal effects showed a strongly negative non-thermal effect in summer, driven by intensified photosynthesis. Specifically, the non-thermal effect offset approximately 60% of the thermal increase, preventing the region from becoming a strong CO₂ source. In other seasons, physical processes such as vertical mixing outweighed photosynthetic uptake. Seasonal shifts in the relative dominance of diatoms and dinoflagellates modulated this photosynthetic carbon uptake efficiency. Photosynthesis also exerted major control on the aragonite saturation state in this region. Ω_arag exhibited clear seasonality, with winter minima of ~2 in both surface and bottom waters. In summer, strong freshwater input lowered salinity, yet Ω_arag increased, indicating that photosynthesis-driven carbon removal associated with nutrient supply outweighed dilution-driven decreases in carbonate ion availability. Compared with 2010–2013, the 2022–2023 data further revealed a newly emerged negative salinity-Ω_arag relationship in Gwangyang Bay, supporting the dominance of photosynthesis-driven carbon removal over simple dilution. These results demonstrate that photosynthesis-driven carbon removal is a major control on air-sea CO₂ flux and Ω_arag along the southern coast of Korea, with phytoplankton community composition and physical processes jointly shaping their seasonal variability. This framework offers a basis for assessing coastal carbon and carbonate dynamics in productive, freshwater-influenced coastal systems.

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