Study On Spectral Analysis Of The Effect Of Cyanobacterial Degradation On The Occurrence Of Dissolved Organic Matter In Lake Water

Algae outbreaks,as a worldwide concern of water environment,are related to residents’perception,water environment safety and drinking water safety.Metabolic exudation or cell autolysis during the life cycle of algae can produce algal-derived organic matter,which is abundant in the water body during the outbreak of algal blooms,and occupies a relatively high proportion of natural organic matter.In recent years,with the rapid development of the river basin economy,the eutrophication of Chaohu Lake has been intensified,the cyanobacteria blooms frequently occurred in the lake,and the water ecological environment has seriously deteriorated.In this study,the overlying water samples and sediment samples of West Chaohu Lake were collected in the field to analyze the temporal variation characteristics of nutrients in Chaohu Lake water.Through indoor simulated water-algae and water-algae-sediment experiments,and UV-Vis spectroscopy and three-dimensional fluorescence spectroscopy were used as the main analysis methods,this paper systematically studied the effect of cyanobacterial degradation on the occurrence of dissolved organic matter in water.The main research results are as follows:The TP content in the surface water collected monthly in West Chaohu Lake was0.03～0.18 mg/L,with an annual average value of 0.07 mg/L;TN content was 0.26～3.71mg/L,with an annual average value of 1.67 mg/L;L,the annual average value is 28.5μg/L;ammonia nitrogen is 0.02～0.69 mg/L;the permanganate index content is 2.4～8.5 mg/L,the annual average value is 3.64 mg/L.The content of COD_Mn and TP in the water body is generally relatively high in summer and autumn,and low in spring and winter.The nitrogen and phosphorus nutrients are negatively correlated with chlorophyll a in the water body.The growth of algae in the water body will affect the nutrient content in the lake.The spectral analysis of dissolved organic matter showed that the DOM content,aromaticity and humification degree in the interstitial water were higher than those in the overlying water.The three-dimensional fluorescence spectra of water samples were analyzed by EEM-PARAFAC,and the fluorescent components of DOM in the overlying water and interstitial water were all microbial-derived humus-like substances（C1）,protein-like substances（C2）and humic substances-like substances（C3）.Three-dimensional fluorescence characteristic parameter analysis showed that the DOM in the overlying water and interstitial water was mainly contributed by autogenic sources,and the microbial activity had a great influence on the composition and content of DOM.The water-algae two-phase simulation experiment(α₃₅₅,SUVA₂₅₄,S_R,E2/E3parameters of UV-Vis spectrum)shows that the degradation of cyanobacteria at the beginning of the experiment leads to the increase of aromaticity and macromolecular substances in the overlying water,with the photodegradation and microbial degradation of cyanobacteria.The occurrence of macromolecular organic matter in the overlying water is decomposed into small molecular substances,resulting in a significant change in the DOM content in the water.EEM-PARAFAC analysis showed that the fluorescent components of the overlying water were microbial-derived humus（C1）,protein-like（C2）and terrestrial humus（C3）.During the simulation experiment,the average DOC concentration of the overlying water samples was 19.66 mg/L（range:8.82～26.12 mg/L）,the DOC concentration showed an upward trend and reached a peak value of 26.12 mg/L on the 16th day of the experiment,and then showed a downward trend until the end of the experiment,from 26.12mg/L to 21.05 mg/L.The initial increase of L,DOC content may be related to the growth of cyanobacteria at the beginning of the experiment.In the middle and later stages of the experiment,with the decline and sedimentation of cyanobacteria,the content of DOM in the water body was degraded by light and microorganisms,and the adsorption and sedimentation of water particles decreased.During the experiment,the C1 component of dissolved organic matter increased from 44%to 64%,the C2 component decreased from 17%to 8%,and the C3 component decreased from 39%to 28%,indicating that tryptophan-like proteins are easily degraded.At the same time,microbial-derived humus can be released during the process of microbial decomposition of humus-like substances.The three-phase simulation experiment of water-algae-sediment(theα₃₅₅,SUVA₂₅₄,S_R,E2/E3 parameters of UV-Vis spectrum)showed that the aromaticity and humification degree of overlying water and interstitial water were enhanced.During the degradation period of the three-phase system,the average DOC concentration of the overlying water samples was22.19 mg/L（concentration range:11.76～48.54 mg/L）.The DOC concentration showed a maximum value at the beginning of the simulation and showed a significant decreasing trend,which may be related to the stronger microbial action in the presence of sediment and the rapid death of cyanobacteria in the early stage.The initial concentration of DOC in interstitial water decreased from 26.83 mg/L to the final value of 12.14 mg/L.On the 16th day of the experiment,it showed a slight increase and then continued to decline.The DOC concentrations in the overlying water and interstitial water were similar,indicating that algal degradation significantly increased the occurrence of organic matter in water and affected the flux of sediment-water interface.EEM-PARAFAC analysis showed that in the process of photodegradation and microbial degradation,the fluorescent components of DOM in overlying water and interstitial water were microbial-derived humus-like（C1）,humic-like（C2）and protein-like（C3）.It has obvious and strong humus characteristics,while the DOM in interstitial water is mainly contributed by autogenic sources and has obvious humus characteristics;with the increase of experimental time,the proportion of humic substances began to increase,while the protein-like substances decreased significantly.Among them,the proportion of C1 increased by 29%,and the proportion of C2 increased by 9%,indicating that protein-like substances are easily converted into other small molecular substances by biological utilization,and microorganisms will metabolize to produce microbial-derived humus substances;with the experiment proceeding,the overlying water and the proportion of tryptophan-like protein substances in the interstitial water decreased,and the proportion of humus substances increased;among them,the proportion of microbial-derived humus increased the most,which was consistent with the analysis results of field experiments.