Impact of water-sediment regulation on the optical characteristics and sources of dissolved organic matter in the Xiaolangdi Reservoir, yellow river

Dissolved organic matter (DOM), serving as a key connection between biogenic carbon and inorganic carbon, exerts significant influence on river ecosystems and the global carbon cycle. Dam construction disrupts the natural state of rivers, altering hydrodynamic conditions and affecting the storage, transformation, and downstream transport of DOM within ecosystems. This study focused on the Xiaolangdi Reservoir of the Yellow River. By analyzing the optical properties, it investigated the sources, compositional structure, and transformation dynamics of DOM during the different stages of water–sediment regulation scheme (WSRS). Employing three–dimensional fluorescence spectroscopy combined with parallel factor analysis, the study revealed four categories of fluorescent substances in the reservoir: UVC humic–like (C1), UVA humic–like (C2), protein–like (tyrosine–like) (C3), and protein–like (tyrosine–like and tryptophan–like) (C4). Fluorescent DOM (FDOM) was dominated by humic–like components in Xiaolangdi Reservoir. Fluorescence intensities of various FDOM components exhibited distinct fluctuations during the different stages of WSRS, reaching their lowest point during the water regulation period (WRP) (2172.43 R.U.). This indicates that high–flow releases exert a dilution effect on reservoir FDOM. WSRS significantly altered DOM molecular characteristics. aCDOM(355) was higher during the WRP (5.27 m⁻¹) and the sediment regulation period (SRP) (5.86 m⁻¹). This indicates that high flow rates and suspended sediment concentration (SSC) promote organic matter release, exerting a significant influence on aCDOM(355). Principal component analysis revealed that DOM during the WRP was primarily sourced from terrestrial inputs, with enhanced endogenous contributions during the post–WSRS. pH and flow rate were key factors influencing FDOM component changes during the WRP, while water temperature and SSC were critical factors during the SRP. The study reveals the transformation patterns of DOM sources under high–intensity anthropogenic hydraulic disturbances. The findings hold profound significance for understanding the impact of reservoir sediment flushing operations on the reservoir carbon cycle.