Monthly and seasonal variations in the surface carbonate system and air–sea CO₂ flux of the Yellow Sea

Surface carbonate chemistry in the Yellow Sea was investigated based on discrete seawater samples collected from 2017 to 2020 at the Socheongcho Ocean Research Station (S-ORS; 37.423°N, 124.738°E). Records of carbon parameters, including seawater CO₂ partial pressure (pCO₂), revealed considerable seasonal variations, with amplitudes comparable to those observed across the western part of the Yellow Sea. The study site acted as a modest sink (−0.13 mol C m⁻² yr⁻¹) for atmospheric CO₂. Biological processes (primary production and respiration) and physical conditions (temperature and degree of stratification) determined seawater pCO₂, which fluctuated on an intraseasonal timescale between oversaturated and undersaturated with respect to atmospheric pCO₂. Variation in pCO₂ was significant in summer, depending on the biological carbon drawdown and tidal mixing-induced upwelling (increased pCO₂ up to ~1000 μatm). The intraseasonal variability in seawater pCO₂ may bias estimated air–sea CO₂ fluxes, if measurements with a coarser (seasonal) time resolution are used.