Spatiotemporal Distribution Characteristics Of Dissolved Organic Matter And Microbial Community In Typical Surface Water Of Hangzhou

Dissolved organic matter（DOM）and microorganisms are indispensable components of surface water ecosystems.They are important in facilitating material transformations and energy flows,and in maintaining the health and balance of aquatic ecosystems.To reveal the changing patterns of DOM and microorganisms in urban surface water ecosystems,so as to better remediate and manage the environmental problems of water bodies.In this study,the typical surface waters of Hangzhou region（Qiantang River,Jiuxi River,Beijing-Hangzhou Canal,West Lake,Xixi River,and Shangtang River）were selected for the study,and surface water samples were collected during the dry（February2022）,normal（October 2021）,and wet（June 2022）seasons,respectively.The main water quality pollution indicators in the study area were investigated by principal component analysis;the composition,source,and spatial-temporal variation characteristics of DOM in surface water were analyzed by excitation-emission-matrix spectra combined with parallel factor analysis,and the correlation between fluorescence components of DOM and water quality indicators was explored;the compositional structure,spatial-temporal distribution characteristics,and main functions of microbial communities were investigated based on high-throughput sequencing technology,and the main environmental factors affecting the structure of microbial communities were explored using correlation analysis.In addition,the effect of denitrifying bacteria on the removal of NO₃^--N from raw river water was also investigated.The main results are as follows:（1）The pollution of surface water environment in the Hangzhou region exhibited different characteristics at different times of the year.The water environment was mainly polluted with elemental N during the dry season,and the concentration of TN in all surface waters did not meet the V surface water standard,which was caused by NO₃^--N pollution,while the content of TN,NH₄⁺-N,NO₃^--N,COD and DOC increased significantly during the dry season.In contrast,during the normal season,elemental P pollution was more dominant and TP concentrations in all surface waters did not meet the class IV surface water standard.The aquatic environment was relatively good during the wet season.In terms of spatial distribution,the Qiantang River,Jiuxi River,Beijing-Hangzhou Canal,Xixi River,and Shangtang River were mainly polluted by N and P elements,and West Lake was mainly polluted by N elements and organic matter.The principal component analysis showed that NO₃^--N,TN,TP,and DOC were the main environmental indicators influencing the water quality differences,indicating that the surface waters in the Hangzhou region were mainly polluted by NO₃^--N,elemental P pollutants,and organic matter.The comprehensive assessment showed that the water quality of the Nine Streams and the Beijing-Hangzhou Canal was worse and the overall quality of each surface water was worse during the dry season.Therefore,protecting the aquatic environment and reducing the input of nitrogen,phosphorus,and organic matter,especially through better management and control during the dry season,were key to maintaining the health and balance of the aquatic ecosystem.（2）The DOM in surface waters of the Hangzhou region was mainly composed of tryptophan-like fraction C1,tyrosine-like fraction C2,marine humic-like fraction C3,and fulvic acid-like fraction C4.Among them,protein-like fractions C1 and C2 were the main components.The DOM was mainly of endogenous origin,with strong authigenic characteristics and weak humus characteristics.DOM concentrations showed a temporal trend of dry season>normal season>wet season;Spatially,the concentration of DOM was highest in the Qiantang River,mainly due to the influence of humus input from marine sources during the normal season,while the concentration of DOM was lowest in the Jiuxi River.Correlation analysis revealed that tryptophan-like fraction C1 and complexion-like fraction C2 were homologous in the environment and showed significant correlations with DOC,NH₄⁺-N,and EC,suggesting that these two fractions had potential environmental indicator uses.Therefore,an in-depth understanding of the composition and spatial and temporal characteristics of DOM was of critical importance for maintaining and monitoring the health and balance of the aquatic environment.（3）In the water ecosystem of the Hangzhou region,the changes in microbial community structure had temporal and spatial differences.In terms of time,the abundance and diversity of microbial communities were significantly higher during the normal season than those during the dry season;in terms of space,the abundance and diversity of microbial communities were generally higher in the Beijing-Hangzhou Canal,Jiuxi River,and Shangtang River than those in the Qiantang River,West Lake,and Xixi River.At the phylum level,the mainly dominant bacteria of the microbial community included Proteobacteria,Bacteroidetes,Cyanobacteria,and Actinobacteria.At the genus level,there were 16 dominant microorganisms in the dry season and 18 in the shallow season,which both contained Pseudomonas.The study also identified 10 dominant functional groups,including nitrate-reducing microorganisms,and correlation analyses showed that NO₃^--N,TN,and DOC were key factors influencing the structure of the microbial community.In particular,the content of Pseudomonas was significantly positive correlation with NO₃^--N concentration.These results suggested that the Pseudomonas contained bacteria with denitrification functions.In addition,there was a significant correlation between the fluorescence fractions of DOM and the structure of the microbial community.These findings provided important theoretical support for the study of the structure and function of microbial communities in surface waters.（4）Pseudomonas stutzeri ZH-14 showed excellent performance in the study of nitrogen removal from raw water in the Jiuxi River.After 36 h of treatment,the NO₃^--N concentration was reduced to a minimum and the NO₃^--N removal rate reached 61.75%,which was a significant removal effect.Meanwhile,no significant accumulation of NO₂^--N and NH₄⁺-N was observed.These results indicated that Pseudomonas stutzeri ZH-14 has a good denitrification effect in practical application and can be used as a potentially efficient denitrifying microorganism to improve water quality and reduce nitrogen pollution,which was of great significance for water pollution control and water resource protection.