Interaction between seawater carbon dioxide dynamics and stratification in shallow coastal waters: A preliminary study based on a weekly validated three-dimensional ecological model

Shallow coastal waters (SCWs) have attracted wide attention in recent years due to their strong carbon sequestration capacity. However, the complex carbon dioxide (CO₂) dynamics in the water column makes it difficult to estimate the air–water CO₂ fluxes (F_CO2) accurately. We developed a numerical model of CO₂ dynamics in water based on field measurements for a typical stratified semi-enclosed shallow bay: the Yatsushiro Sea, Japan. The developed model showed an excellent ability to reproduce the stratification and CO₂ dynamics of the Yatsushiro Sea. Through numerical model simulations, we analyzed the annual CO₂ dynamics in the Yatsushiro Sea in 2018. The results show that the effect of stratification on the CO₂ dynamics in seawater varies greatly depending on the distance from the estuary and the period. In the estuarine region, stratification manifests itself throughout the year by promoting the maintenance of a high partial pressure of CO₂ (pCO₂) in surface waters, resulting in surface pCO₂ being higher than atmospheric pCO₂ for up to 40 days during the flood period (average surface pCO₂ of 539.94 µatm). In contrast, in areas farther from the estuary, stratification mainly acts to promote the maintenance of high pCO₂ in surface waters during periods of high freshwater influence. Then changes to a lower surface pCO₂ before the freshwater influence leads towards complete dissipation. Finally, we estimated the F_CO2 of the Yatsushiro Sea in 2018, and the results showed that the Yatsushiro Sea was a sink area for atmospheric CO₂ in 2018 (−1.70 mmol/m²/day).