Effects of hydrological changes on the sources and optical properties of dissolved organic matter in wetlands of the middle reaches of the Yellow River.

Dissolved organic matter (DOM) in riverine wetlands is the core carrier of the carbon cycle. However, the dynamics of DOM in large riverine wetlands subject to anthropogenic hydrological regulation remain poorly understood, and the differences in DOM composition and sources among hydrological periods are still unclear. In this study, water samples were collected from the riparian wetland in the Wuzhi section of the middle reaches of the Yellow River during the dry season (DS), normal season (NS), water-sediment regulation scheme (WSRS), and wet season (WS). Ultraviolet-visible (UV-vis) absorption spectroscopy and excitation-emission matrix (EEM) spectroscopy, combined with parallel factor analysis (PARAFAC), were used to characterize the composition and sources of DOM across hydrological periods. Two-dimensional correlation spectroscopy (2D-COS) was applied to reveal the ordered pattern of DOM fluorescent components along the flow direction. The results showed that DOM content, composition, and sources differed among periods. DOC concentration was highest during the WSRS, followed by that during the WS, and was lower during the DS and NS. Three components were identified. Humic-like components C1 and C3 peaked in the WS, whereas the protein-like component C2 peaked in the DS. Within the same season, the water-sediment regulation event caused a significant change in the fluorescence intensity of C2. Spectral parameters indicated that DOM in the WS may have been influenced by terrestrial and autochthonous sources, while autochthonous contributions appeared to be stronger in the other periods. 2D-COS analysis revealed an ordered pattern of DOM fluorescent components along the flow direction, following the sequence protein-like → microbial humic-like → terrestrial humic-like components. This pattern may be influenced by multiple processes including tributary input, agricultural runoff, and sediment release. This study helps to address the limited understanding of DOM dynamics in wetlands under hydrological disturbance and provides baseline data for interpreting the compositional changes and along flow variation patterns of DOM in riparian wetlands across hydrological periods.