[Characteristics of Dissolved Organic Matter in Overlying Water During Algal Bloom Decay]

In this study, indoor simulation experiments were performed to elucidate the effects of migration and transformation of dissolving organic matter (DOM) during the decay of algal blooms. Based on ultraviolet-visible spectra (UV-vis) and excitation-emission matrix spectroscopy (EEMs), spectral characterizations of dissolved organic matter (DOM) in overlying water were evaluated with analyses of the physical and chemical indexes, variation in dissolved organic carbon (DOC), and variation in dissolved inorganic carbon (DIC). Results showed that at the early stage of decay, a large amount of organic matter was released, and dissolved oxygen (DO) decreased sharply. With the extension of reaction time, DOC gradually changed into DIC, which further changed the oxidation-reduction and acid-base characteristics of the water. UV-vis spectra showed that a large amount of DOM was released with high aromaticity and a high degree of humification, and the released DOM was gradually degraded. With the application of parallel factor analysis in excitation-emission matrix spectroscopy (EEM-PARAFAC), three fluorescence components were analyzed:refractory humic-like substances (C1), protein-like tryptophan substances (C2) produced by algae, and fulvic-like substances (C3) related to microbial activities. Most protein-like tryptophan substances were degraded into fulvic-like substances by microorganisms during the decaying process. Heterotrophic microorganisms promoted the release of algae-derived DOM and accelerated the degradation of DOM. The DOM born during algae blooms decaying process was eventually converted into humic-like substance, which was difficult to be degraded. We analyzed correlations of water quality, UV-vis spectrum, and EEMs parameters. Results showed that ORP was positively correlated (P<0.05) with DO. There was a significant negative correlation (P<0.05) between pH and DOC, which was consistent with the trend of the transformation to from DOC to DIC; C1 was positively correlated (P<0.05) with Fn₃₅₅; and C2 was significantly positively correlated (P<0.05) with DOC and Fn₂₈₀; C3 was positively correlated (P<0.05) with FI, BIX and β:α. The variation trend of these spectral parameters was consistent with that of DOM components. In summary, with the analyses of water quality characteristics and spectral characteristics of DOM in overlying water during algae blooms decaying process, it was expected that our results could contribute to the further exploration of the dynamic migration and transformation of lake DOM and the changes of carbon cycling.