Long-Term Monitoring Reveals Carbon Cycle Dynamics in Bohai Sea

The Bohai Sea, a semi-enclosed marginal sea under intense anthropogenic pressure, represents a critical yet dynamic component of the regional carbon cycle. However, understanding its fine-scale carbon dynamics has been limited by a lack of long-term, high-frequency observations. This study deployed a moored buoy system in the central Bohai Sea from September 2020 to December 2021 to obtain continuous, high-temporal-resolution measurements of surface seawater pCO2 and key ancillary environmental parameters. An annual stepwise linear regression model (R2 = 0.793, p< 0.001) revealed that pCO2 variability is governed by a balance between biological drawdown (chlorophyll-a concentration, 16.76% relative contribution) and thermal forcing (SST, 27.69%). Seasonal sub-regressions demonstrated a systematic transition: biological control prevailed in autumn and summer (R2 = 0.922), physical control dominated in winter (R2 = 0.787), and spring exhibited low predictability (R2 = 0.318), implicating unmeasured processes such as vertical mixing. An event-scale composite analysis of eight independent storm events identified a distinct two-phase pCO2 response: an initial decline at the wind peak (from 354.7 to 333.7 μatm) due to enhanced solubility, followed by a delayed rise peaking four days later (355.8 μatm, net increase of 22.1 μatm), concurrent with SST cooling (2.62 °C) and SSS increase (0.08 psu), consistent with the hypothesized wind-driven entrainment of carbon-enriched subsurface waters. The air-sea CO2 flux regression confirmed that the pCO2 gradient is the fundamental driver. These findings establish a mechanistic framework for the carbon sink dynamics of the central Bohai Sea, demonstrating that high-frequency autonomous observations are essential for capturing event-scale processes that control coastal carbon cycling—insights that are inaccessible to traditional shipboard surveys and satellite retrievals.