河口—海湾连续体是陆源溶解有机碳向海洋输送和转化的重要区域，认识溶解有机碳在其中的生物可利用性及微生物降解特征，对于理解陆源有机碳在向海输送过程中的归趋与转化机制具有重要意义。以九龙江口—厦门湾连续体为研究对象，于2025 年11 月沿盐度梯度（2.4~32.0）设置9 个采样站位，开展为期60 d 的溶解有机碳降解培养实验，并结合光谱分析、一级降解动力学模型和端元模型分析，研究溶解有机碳组分沿盐度梯度的转化特征。原位调查发现，溶解有机碳、有色溶解有机物及荧光溶解有机物含量随盐度升高而降低。培养实验结果表明，生物可降解溶解有机碳占溶解有机碳的比例由低盐端的19.1%~21.8% 降至河口混合区盐度27.4 站位的最低值8.6%，进入海湾区后回升至10.7%~12.9%；荧光溶解有机物中的类蛋白组分C2 在盐度2.4~22.0 降低，且降幅逐渐减小，而盐度高于22.0 站位则呈现增加现象。上述结果表明，溶解有机碳生物可利用性和组成沿盐度梯度呈复杂变化。对4 个不同盐度站位（分别为2.4、13.9、22.0 和31.5）的溶解有机碳降解动力学分析显示，溶解有机碳一级降解速率常数为0.0330~0.1010 d−1，沿盐度梯度呈现先升高后降低趋势，其中盐度13.9 站位最高；溶解有机碳、难降解溶解有机碳和C2 在河口低盐区至混合区明显正偏离保守混合线，在低盐度区域呈现明显表观净添加特征，表明低盐度区域是高效的有机碳转化区域。综上，表明河口—海湾连续体是溶解有机碳生物转化的活跃且复杂区域，对输送至近岸水体的溶解有机碳组成、生物可利用性发挥了重要的调控作用。

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.