Temporal trends of organic carbon and its response to coastal phytoplankton community shifts in human-impacted large-river estuaries: insights from the Yellow River Estuary.

Global estuaries and coastal areas are experiencing exacerbated eutrophication and altered carbon cycling due to intensive human interventions. However, the mechanisms linking watershed-specific human activities to coastal carbon-nutrient coupling and shifts in the phytoplankton community require further investigation. In this study, we employ a multi-proxy framework integrating bulk organic carbon (OC) indicators (total OC (TOC), δC), molecular biomarkers (brassicasterol (B), dinosterol (D)), and inter-watershed comparative analysis to identify centennial-scale anthropogenic perturbations in the ecological evolution of China's large-river estuaries. Based on the reconstructed sedimentary OC (SOC) records from the Yellow River Estuary (YRE), alongside datasets on water discharge, sediment, and nutrient loading, our study revealed that the temporal trends of terrestrial and marine organic carbon (OC and OC) were coupled with riverine input and human hydrological interventions. Molecular evidence further demonstrated an increase in coastal phytoplankton productivity, which was attributed to enhanced riverine nutrient fluxes, particularly dissolved inorganic nitrogen (DIN). Moreover, changes in the B/(B + D) ratio potentially reflected shifts in phytoplankton community composition in recent decades, characterized by a decline in diatom dominance in response to altered nutrient structure. Similarly, the Yangtze and Pearl River estuaries have experienced comparable ecological transformations, yet exhibit variable trends driven by distinct socioeconomic developments and region-specific nutrient dynamics. Our findings on carbon burial processes and their responses to rapid shifts in phytoplankton communities in large-river estuaries provide critical insights for developing adaptive coastal management strategies under intense human impacts.