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.