Deciphering the composition and bioavailability of dissolved organic matter in rivers discharging into a temperate eutrophic bay.

The riverine transport of dissolved organic matter (DOM) is a crucial link between terrestrial and marine carbon pools, yet studies on the composition and fate of DOM in medium- and small-sized rivers remain limited, hindering a comprehensive understanding of the role of riverine input in coastal carbon cycling. This study focused on rivers discharging into a temperate eutrophic bay in the South Yellow Sea, Haizhou Bay (HZB), and assessed the spatial-temporal variations in the concentrations, composition, and bioavailability of DOM through field investigations and laboratory incubation experiments. The results indicated that the composition of DOM in rivers within the study area exhibited significant seasonal variations. The concentrations of chromophoric DOM (CDOM), terrestrial and microbial humic-like fluorescent DOM (FDOM-C1 and C2) in summer were 2.60, 2.65, and 1.98 times higher than those in winter, respectively, attributed to the enhanced flushing of terrestrial organic matter with larger molecular weight and aromaticity during the flood season. The 28-day microbial incubation revealed that the bioavailability of dissolved organic carbon (DOC) in river water was 23.10 ± 4.63% in winter, approximately 3.14 times that in summer (7.36 ± 2.55%), which was closely related to seasonal differences in DOM composition. The annual DOC input flux from rivers into the HZB was 2.26 × 104 t a-1, with refractory DOC accounting for 87.17%. These findings demonstrated that medium- and small-sized rivers, despite their relatively low discharge, serve as significant conduits of recalcitrant carbon to coastal waters, highlighting the critical role in coastal carbon sequestration.