Research On The Succession Law Of Organic Matter And Bacterial Community In A Water Source Reservoir In South China

The aim of this study was to explore the succession patterns of organic matter and bacterial communities under natural conditions in a water source reservoir in the south,as well as the interaction between the components of dissolved organic matter（DOM）and bacterial communities.In this study,stratified sampling was conducted at the deepest point of a water source reservoir in South China from April 2021 to July 2022.According to the mixing degree of water,the whole year was divided into two periods,the layering period and the mixing period.The physical and chemical indicators of the water in the two periods were monitored.At the same time,high-throughput sequencing was used,combined with three-dimensional fluorescence spectroscopy-parallel factor method（EEM-PARAFAC）and ultraviolet-visible absorption spectroscopy（UV-Vis spectral）method,Continuous temporal and spatial monitoring of microbial populations and dissolved organic matter characteristics in water bodies was conducted to provide relevant data for water quality improvement in water source reservoirs.The main research contents include:To explore the characteristics of annual water quality changes in water source reservoirs;Determine the composition characteristics,physical and chemical characteristics,sources,and seasonal variation patterns of DOM in the source water of the reservoir;Exploring the seasonal changes in bacterial communities in water bodies and exploring the main environmental factors that affect the seasonal evolution of bacterial communities;Exploring the interaction between DOM components and bacterial communities.The main conclusions of this paper are as follows:（1）The reservoir thermal stratification period in this study is longer（March to October）,and the mixing period is shorter（November to February of the next year）,showing a typical subtropical single-cycle mixed mode.DO in the reservoir presents seasonal stratification,and DO in the water body presents the distribution characteristics of high surface layer and low bottom layer.Chlorophyll a and p H showed seasonal distribution of high and low level in the stratification period,and the seasonal anoxia of bottom water caused by thermal stratification led to the highest level of nutrients in the bottom water;There is no significant difference in nutrient levels among different water layers during the mixed period.（2）The spectral analysis results show that the DOM components in the water body are divided into two types of humic components（C1,C2）and one type of protein component（C3）.During the stratification period,the vertical differences in the DOM components are significant,and the C2 and C1 components show the same trend of change,showing a higher variation characteristic in the middle layer of the bottom layer than in the surface layer.In mid August to September,the fluorescence intensity of C3components in the bottom layer increased to the highest,while the vertical differences among the three components during the mixing period were relatively weak.The fluorescence spectrum and ultraviolet-visible absorption spectra indicate that the proportion of newly generated DOM in water bodies throughout the year is relatively high,and the relative molecular weight of DOM throughout the year is relatively high,small molecule DOM is preferentially degraded,while large molecule DOM gradually accumulates,resulting in a higher molecular weight of DOM in water bodies.（3）The 3851 OTUs obtained through high-throughput analysis of water samples belong to 55 phyla,142 classes,328 orders,518 families,941 genera,and 1759species.During the layering period,the dominant bacteria in the water body are Proteobacteria,Actinobacteriota,Bacteroidota and Cyanobacteria.;In the mixed period,the dominant bacteria in the water body are Proteobacteria,Actinobacteriota,Bacteroidota and Firmicutes.The bacterial community has obvious seasonal differences,and the proportion of bacteria with aerobic denitrification potential in the mixed period of water body increases.The redundancy analysis results show that the main influencing factors of bacterial community in the stratified period are COD_Mn,TP,DO,DOC and Fe,and WT,Chl-a,p H and Mn are also environmental factors affecting bacterial community composition;NH₃,Fe,and WT are the main environmental factors affecting the distribution of bacterial communities during the mixed period.（4）The microbial diversity and abundance of bacterial communities are negatively correlated with environmental factors such as DO,Chl-a,and COD_Mn,while the species richness during stratification and mixing periods is at a lower level.Network analysis showed that the humus components of DOM are mainly degraded or produced by Alphaproteobartia,Gammaproteobacteria and Actinobacteria,and tryptophan components are degraded or produced by Gammaproteobacteria,Actinobacteriota and Cyanobacteriia.