A stable-carbon-isotope-based constraint of bulk particulate organic carbon dynamics and budgets in the Yellow Sea: Combining field surveys and isotope fractionation modeling

The isotopic composition of POC (δ¹³C_POC) serves as a critical tracer for marine carbon dynamics. Its traditional applications usually assume a fixed δ¹³C value for marine phytoplankton (δ¹³C_phyto) of − 20 ± 1 ‰, overlooking spatiotemporal variabilities in phytoplankton carbon isotope fractionation (ε_P). This study quantifies ε_P-mediated δ¹³C_POC variations in the Yellow Sea – a temperate shelf sea characterized by seasonal stratification, intense diatom blooms, and terrestrial inputs – by coupling isotope fractionation models (passive CO₂aq diffusion vs. active HCO₃^– transport) with multi-season δ¹³C measurements of Dissolved Inorganic Carbon (DIC) and δ¹³C_POC. During algal bloom periods, δ¹³C_POC values were relatively heavier (−20 ± 1 ‰), driven by active HCO₃^– uptake under low CO₂aq concentrations ([CO₂aq]). Strong agreement between δ¹³C_POC and modeled δ¹³C_phyto values confirmed compositional similarity between POC and algal biomass, with ${\epsilon }_P$ exhibiting significant covariation with [CO₂aq]. In non-bloom periods, lighter δ¹³C_POC values (from − 26 to − 24 ‰) dominated surface layers, with even more depleted signatures (<−26 ‰) occurred in the Deep Chlorophyll Maximum (DCM) and middle layers. Although detritus inputs caused deviations of δ¹³C_POC from modeled δ¹³C_phyto values, temperature-dependent correlations still revealed ${\epsilon }_P$-related isotopic dynamics in POC. This research underscores the necessity of using environment-specific δ¹³C_phyto values to refine POC budget estimates, reducing flux uncertainties by 20–30 % and minimizing isotopic errors to < 1 ‰ across different timescales. This work establishes a framework for incorporating δ¹³C_phyto plasticity into coastal carbon models, resolving long-standing paradoxes of isotopically light POC (<−26 ‰) in marine-dominated systems and advancing high-resolution carbon flux estimates in marginal seas.