Variation in Spectral Characteristics of Dissolved Organic Matter and Its Relationship with Phytoplankton of Eutrophic Shallow Lakes in Spring and Summer

The compositional characteristics of dissolved organic matter (DOM) have important implications for lake water quality and aquatic ecology. Seasonal changes of dissolved organic matter (DOM) as well as phytoplankton abundance and composition in Shahu Lake from April to July were characterized by three-dimensional fluorescence spectroscopy (3DEEMs) combined with parallel factor (PARAFAC) analysis. The relationship between the response of components of the DOM and phytoplankton abundance were explored via Pearson correlation and redundancy analysis (RDA) in the overlying water. The results showed that the DOM was composed mainly of tryptophan-like (C2+C4), fulvic-acid-like (C3), humic-acid-like (C1), and tyrosine-like (C5) compounds that accounted for 44.47%, 20.18%, 20.04%, and 15.31%, respectively, of the DOM. The DOM was derived from both endogenous and terrestrial sources. With seasonal changes, endogenous DOM produced by phytoplankton growth and metabolism gradually increased. In spring and summer (April-July), Chl-a concentrations were significantly correlated with C3 (p < 0.01) and C5 (p < 0.05). The concentration of protein-like fractions (C2+C4, C5) were correlated with Cyanobacteria abundance, and the concentrations of humic-like component content (C1, C3) were correlated with the abundance of Xanthophyta, Chlorophyta, and Cryptophytes. Overall, phytoplankton density and Chl-a content increased by 125% and 197%, respectively, and the abundance of C3 and C5 in the DOM increased by 7.7% and 22.15% in parallel. Thus, seasonal phytoplankton growth had an important influence on the composition of the DOM.