Seasonal variations in air-sea CO2 flux in the Bohai Sea and the temporal relationship between surface and bottom-water carbonate dynamics in a year

The Bohai Sea is the only China seas lacking data-based estimation of annual air-sea CO₂ flux. To fill this gap, we investigated regional differences in the seasonal variation of surface fugacity of CO₂ (ƒCO₂) and air-sea CO₂ fluxes in the Bohai Sea, based on mapping datasets collected from 15 carbonate-surveying cruises between 2011 and 2023. The results showed that air-sea CO₂ exchanges in the Bohai Sea are subject to both spatial and temporal variabilities. To sum up, the Bohai Sea acts as a very weak net CO₂ sinking area with a net uptake rate of 0.05 mol m⁻² yr⁻¹ in the surveyed area. The seasonal variation in ƒCO₂ can be attributed to spring/summer biological drawdown, summer/autumn vertical mixing, temperature variations, and air-sea re-equilibration. As a nutrient-rich ocean margin, the Bohai Sea experiences high primary production in its surface waters in spring, generating plenty of biogenic detritus and contributing to the oxygen shortage and acidification in the bottom waters in summer.