Anthropogenic water and sediment regulation drives a refractory terrestrial carbon pulse to the Yellow River estuary

Rivers play an important role in land-to-ocean organic carbon (OC) transport, yet how intensifying human activities alter the composition and reactivity of riverine OC remains poorly understood, hindering predictions of terrestrial OC fate in marine environments. The Yellow River's Water and Sediment Regulation Scheme (WSRS) represents a major anthropogenic disturbance that dramatically alters carbon transport. Here, we characterized particulate and dissolved organic carbon (POC, DOC) contents, stable carbon isotope of POC, and optical properties of dissolved organic matter (DOM) and base-extracted particulate organic matter (POM) in the Yellow River under natural and anthropogenic hydrological variability, to explore the dynamic linkages between POC and DOC pools. Results show that sediment regulation triggered marked increases in POC and DOC contents, accompanied by enhanced aromaticity and humification of both carbon pools. Significant correlations between optical indices of POM and DOM revealed a tight compositional linkage between particulate and dissolved humic substances. The concurrent humic enrichment during sediment regulation could be primarily attributed to the resuspension and release of refractory OM that has previously accumulated within reservoir sediments, probably augmented by the subsequent transformation of resuspended POM into refractory DOM during transit. In 2024, the WSRS exported approximately 60% of the annual POC flux and 30% of the annual DOC flux to estuary and coastal seas. These findings highlight that the WSRS not only amplified short-term carbon export but fundamentally reshaped the composition and reactivity of riverine OC delivered to coastal systems, with important implications for carbon cycling along the estuary-coastal sea continuum.