CO₂ dynamics and sequestration potential in high-nutrient, low-chlorophyll bays: A case study of Yueqing Bay

Bays, as transitional zones in the land-sea continuum, exhibit fluctuating sea-air CO₂ fluxes influenced by climate change and human activities. The role of eutrophic bays as CO₂ sinks is debated, highlighting the need to understand CO₂ dynamics and controlling factors. This research employs the subtropical semi-enclosed Yueqing Bay as a case to investigate the dynamics of pCO₂ and sea-air CO₂ flux, as well as the carbon sink potential in high-nutrient, low-chlorophyll (HNLC) bays through high-resolution underway surveys. Although eutrophic bays worldwide typically function as atmospheric CO₂ sinks on an annual scale, the high concentration of suspended particulate matter (SPM) from the Oujiang River, combined with sediment resuspension and other processes, inhibits primary production, thereby reducing CO₂ sequestration. As a result, Yueqing Bay acts as a net atmospheric CO₂ source in August and November, with fluxes of 6.22 ± 8.79 mmol m^-2 d^-1 and 0.53 ± 0.19 mmol m^-2 d^-1, respectively, and an average flux of 1.23 ± 1.04 mol m^-2 yr^-1. However, acting as nutrient reservoirs, the underutilized nutrients in Yueqing Bay partially contribute to algal blooms, which in turn enhance CO₂ absorption at the Bay Mouth through seawater exchange. Therefore, HNLC bays like Yueqing Bay demonstrate spatial redistribution of CO₂ sink function due to hydrodynamic and biogeochemical processes, offering new insights into the role of bay ecosystems in the carbon cycle.