Land Use Cover and Flow Condition Affect the Spatial Distribution Characteristics of Fluorescent Dissolved Organic Matter in the Yongding River

Dissolved organic matter (DOM) is involved in many biogeochemical processes and plays an important role in aquatic ecosystems. This study integrated three-dimensional fluorescence excitation–emission matrix (EEM), fluorescence regional integration (FRI), and parallel factor analysis (PARAFAC) to better understand the distribution and component characteristics of DOM in the Yongding River and explore the response of DOM to natural and anthropogenic activities. The results showed that the relative abundance of fulvic-like materials of DOM in the river was the highest, with an average of 68.64%. PARAFAC identified three fluorescent components, namely, C1 (microbial humic-like components), C2 (terrestrial humic-like components), and C3 (protein-like components), and their changes with flow confirmed that the riverine DOM was generally influenced by microbial sources and terrestrial inputs. The upper reaches showed strong autochthonous characteristics and a high humification degree of DOM due to a fast flow rate, while the middle reaches showed biological or aquatic bacterial origin due to a moderate flow rate. The lower reaches of the river showed characteristics of biological and bacterial origin, most strongly influenced by human activities. The findings can help provide a basis for identifying DOM characteristics in the Yongding River basin and understanding the geochemical cycle of DOM at a regional scale.