Variation in spectral characteristics of dissolved organic matter in inland rivers in various trophic states, and their relationship with phytoplankton

Dissolved organic matter (DOM) plays an important role in aquatic ecosystems. This study aimed to reveal the variation in spectral characteristics of DOM in inland rivers in various trophic states, and understand the relationship between spectral characteristics of DOM and phytoplankton composition in various trophic states. Our investigation was carried out on the Weihe River in Shaanxi Province, China. Three humic-like substances and one protein-like substance were identified from the DOM by examination of three-dimensional (3D) fluorescence combined with parallel factor (PARAFAC) analysis and UV–Vis spectroscopy. The trophic states in this study area ranged from mesotrophic to hyper-eutrophic (37 < Trophic States Index (TSI) < 76). As the trophic states increased, the amount of phytoplankton rose, and the dominant phylum gradually changed from bacillariophyta to cyanophyta and chlorophyta. The UVA humic-like substance C1 (λEx/Em = 250(310)/400 nm) and the protein-like substance C4 (λEx/Em = 240(280)/338), frequently found in pure cultures of various phytoplankton species, were considered to be phytoplankton secretions. As the trophic state increased, the fluorescence intensity of C1 and C4 increased, along with the rising phytoplankton biomass. The UVC humic-like substance C2 (λEx/Em = 260/480 nm), derived from a terrestrial source, did not change significantly along with the increasing trophic state. The humic-like substance C3 (λEx/Em = 270/430 nm), produced by microbial decomposition, increased significantly in individual places as the trophic state increased. We concluded that the trophic state can increase the endogenous proportion of DOM by promoting phytoplankton growth, but that the changes in phytoplankton community affected by trophic state have no direct influence on the spectral characteristics of DOM.