Increasing contribution of labile terrestrial organic carbon exacerbates coastal hypoxia

Coastal hypoxia is intimately linked to organic carbon (OC) degradation, yet how OC compositional heterogeneity regulates its fate and oxygen dynamics remains elusive. This study quantifies the component-specific degradation kinetics of particulate organic carbon (POC) during estuarine transport in the Pearl River Estuary (PRE), using dual carbon-isotope (δ¹³C and F¹⁴C) constraints and lipid biomarkers. A four-endmember model (riverine primary production carbon, OC_Rpp; soil OC; fossil OC; and marine OC) reveals that OC_Rpp dominates terrestrial POC (32-85%). Exponential decay modeling reveals that the spatial decay constant decreased sequentially from labile OC_Rpp (0.0495 km^-1) to soil OC (0.0393 km^-1) and refractory fossil OC (0.0110 km^-1), resulting in approximately 90% OC_Rpp loss over the 170 km transport pathway. This selective removal of labile and younger OC fractions resulted in dramatic ¹⁴C aging of up to 15,000 years of residual terrestrial biospheric OC. By integrating decadal datasets into a coupled physical-biogeochemical model, we identify a "lability trap" phenomenon: despite a 35% decline in total terrestrial POC flux from 2006 to 2023, the labile OC_Rpp flux increased by 150%, with its contribution to total oxygen consumption increasing from 10.4% in 2006 to 46.6% in 2023. This shift triggers steeper oxygen depletion near the river mouth, thereby exacerbating the hypoxia. These findings demonstrate that OC lability exerts stronger control over estuarine oxygen budgets and carbon sequestration than bulk quantity. Analogous effects in other large-river systems (e.g., Changjiang River estuary) challenge OC quantity-centric frameworks and provide similar empirical constraints for lability-resolved carbon cycle models.