Coupling of anthropogenic activities and natural factors on dissolved organic matter: Insights from coastal urban rivers in southern China

Accelerated urbanization has reshaped the biogeochemistry of dissolved organic matter (DOM) in urban rivers, yet the interplay between anthropogenic and natural influences on riverine DOM remains unclear. This study utilized absorbance and fluorescence spectroscopy coupled with parallel factor analysis (PARAFAC), to explore the spatiotemporal distribution of DOM in twenty coastal rivers located in a highly urbanized area. Findings highlight increased DOM abundance, especially the protein-like component (C2: 1.48 RU in urban areas and 0.24 RU in suburban areas), in the rivers of urban areas, contrasted by seasonal shifts of chromophoric DOM (CDOM) and humic-like DOM in the rivers of suburban areas (a(350): 3.18 m(-1) in the wet season and 1.23 m(-1) in the dry season). Tidal mixing's role in DOM distribution is evidenced by diurnal salinity changes and its inverse relationship with DOM abundance. Notably, the Shenzhen River's unique DOM composition (a higher abundance of humic-like DOM and a lower abundance of protein-like DOM) suggests agricultural impacts. Strong correlations (R-2 values reaching up to 0.87) between spectral DOM indicators and chemical oxygen demand (COD) underscore the great potential of spectral methods for water quality monitoring. Overall, this study illuminates the coupled effects of urbanization and natural factors on riverine DOM, offering insights for the biogeochemical cycling of DOM and water quality management in urban rivers, with implications for similar urbanized regions globally.