Abstract：The estuary-bay continuum is a critical region for the transport and transformation of terrestrial Dissolved Organic Carbon (DOC) from land to the ocean. Understanding the bioavailability and microbial degradation characteristics of DOC within this continuum is essential for elucidating the fate and transformation of terrestrial organic carbon during its transport to marine environments. In this study, the Jiulong River Estuary-Xiamen Bay continuum was investigated, with nine sampling stations established along a salinity gradient (2.4~32.0) in November 2025. A 60-day DOC degradation incubation experiment was conducted, combined with spectroscopic analyses, a first-order degradation kinetic model, and an end-member mixing model to investigate the transformation characteristics of DOC components along the salinity gradient. In situ observations showed that the concentrations of DOC, chromophoric dissolved organic matter, and fluorescent dissolved organic matter (FDOM) decreased with increasing salinity. Incubation experiments showed that the proportion of biodegradable dissolved organic carbon (BDOC) in DOC decreased from 19.1%~21.8% at the low-salinity end to a minimum of 8.6% at the estuarine mixing station with a salinity of 27.4 and then increased to 10.7%~12.9% in the bay. The protein-like FDOM component C2 decreased over the salinity range of 2.4~22.0, with the magnitude of its decline gradually decreasing, whereas C2 increased at stations with salinities above 22.0. These results indicate complex variations in DOC bioavailability and composition along the salinity gradient. Analysis of DOC degradation kinetics at four stations with different salinities (2.4, 13.9, 22.0, and 31.5) showed that the first-order DOC degradation rate constants ranged from 0.0330 to 0.1010 d-1, increasing initially and then decreasing along the salinity gradient, with the highest value observed at the station with a salinity of 13.9. DOC, refractory dissolved organic carbon (RDOC), and C2 showed pronounced positive deviations from conservative mixing in the low-salinity to estuarine mixing zone, indicating apparent net addition in the low-salinity region and suggesting that this region is a highly active zone of organic carbon transformation. Overall, this study demonstrates that the estuary-bay continuum is an active and complex region of DOC biotransformation and plays an important role in regulating the composition and bioavailability of DOC transported to coastal waters.