Spatiotemporal Patterns and Environmental Controls of Dissolved Organic Matter in Coastal Rivers of the Bohai Rim, China

Dissolved organic matter (DOM) in coastal rivers plays a key role in land–ocean carbon cycling, yet its spatiotemporal dynamics drivers remain poorly understood. This study investigated five typical rivers across the Bohai Rim, China, using ultraviolet–visible absorption, fluorescence spectroscopy, and stable carbon isotope analysis. Fluorescence indices and dual-proxy source characterization indicated that autochthonous DOM, mainly from phytoplankton and microbial metabolism, dominated the bulk DOM pool, whereas terrestrial C3 plant inputs served as secondary but spatially variable sources. Among all measured environmental variables, water temperature exhibited relatively strong correlations with DOM optical properties, while salinity showed limited influence. Mantel tests and partial least squares path modeling further suggested that water temperature indirectly shaped temporal variations in DOM by modulating phytoplankton biomass and diversity, which in turn promoted the production of low-aromaticity autochthonous DOM. This cascading framework synthesizes thermal, nutrient, and microbial controls on fluvial DOM, improving understanding of regional carbon budgets and providing scientific support for integrated management of the Bohai coastal zone.