Human-driven spatial and temporal contrasts in dissolved organic matter from submarine groundwater discharge and river runoff to Zhanjiang Bay, northern South China sea

Accurately estimating coastal carbon budgets requires a comprehensive understanding of dissolved organic matter (DOM) inputs from both groundwater discharge and river runoff, particularly in small, human-impacted tropical watersheds. This study provides a representative watershed surrounding Zhanjiang Bay in the northern South China Sea to broaden current knowledge in this field. By characterizing the spatiotemporal dynamics of water quality parameters, dissolved organic carbon (DOC), and both chromophoric and fluorescent DOM (CDOM and FDOM), we identified key patterns in DOM composition, sources, fluxes, and human-induced shifts. DOM in groundwater and river water around Zhanjiang Bay exhibited pronounced spatial heterogeneity but limited seasonal variation. Principal component and Cluster analyses identified three distinct groups. Cluster 1 represents shallow, oxygen-rich groundwater systems from cultivated land and artificial surfaces, with low DOM and microbially processed features. Cluster 2 comprises low-oxygen groundwater from grassland and wetland areas, marked by elevated DOC and humic-like FDOM, reflecting stronger terrestrial and anthropogenic inputs. Cluster 3 includes three rivers from urbanized catchments, enriched in DOC and protein-like FDOM from both anthropogenic and biological sources. Despite comparable water volumes, groundwater contributed less labile DOM flux than rivers. Riverine DOM showed a shift toward DOC-rich but CDOM-poor signature under human influence, deviating from natural tropical patterns and becoming similar to those of anthropogenically impacted rivers in northern China, indicating reduced stability and long-term carbon storage potential in marginal seas.