Characteristics And Degradation Of Dissolved Organic Matter In Rivers Under The Background Of Urbanization In The Three Gorges Reservoir Area

Human activities are affecting and changing the global water resources and aquatic ecosystems.The urbanization process has affected the species composition of river ecosystems,the transparency of water bodies,and biogeochemical functions.DOM has many important functions in aquatic ecosystems:such as O₂ and CO₂ cycles,primary productivity,photosynthesis,and gas-liquid exchange.The composition and structural characteristics of the riverine DOM not only be used as an indicator to reflect the impact of urbanization and other human activities on the river ecosystem,but also significantly affect the carbon cycle process.In recent years,the sources and components,Spatio-temporal variations,degradation and biotransformation of riverine DOM have been widely concerned and studied.As China's urbanization process continues to accelerate,urbanization has significantly affected the river water quality and the composition,structure and sources of DOM.The Yangtze River is the largest river in Asia and the third largest river in the world,and the aquatic ecosystem situation in the Three Gorges Reservoir area has a direct impact on the ecological environment,social economy in the midstream and downstream areas of the Yangtze River.At present,there are few reports on the impact of urbanization on riverine DOM in the Three Gorges Reservoir Area.In this study,typical rivers with different degrees of urbanization in the Three Gorges Reservoir Area are taken as the research objectes,through field sampling,on-site monitoring,experimental analysis,and control experiments,combined with multi-disciplinary theories of ecology,biogeochemistry and so on,applied dissolved organic carbon content measurement,ultraviolet-visible absorption spectrum?UV-Vis?and excitation-emission three-dimensional fluorescence spectrum?EEMs?measurement and multiple statistical analysis,to study the composition,spatio-temporal dynamics,the source and migration,and the bioavailability of DOM under the background of urbanization in the Three Gorges Reservoir Area,thus to investigate the regulating factors of biodegradation and the spatio-temporal variation in river DOM.The main findings are as follows:?1?Spatial distribution of river DOM Characteristics under the background of urbanization in the Three Gorges Reservoir areaThe spatial distribution of riverine DOM under the background of urbanization in the Three Gorges Reservoir area is affected by the level of basin urbanization.As the degree of urbanization increases,the DOC and CDOM concentration of the rivers increase,and the spatial variations of DOC content and CDOM concentration in the rivers are significantly positively correlated?p<0.05?.The DOC content and CDOM concentration in the rivers increase with the increase of urbanization along the flow direction.The higher degree of basin urbanization,the lower degree of humification(SUVA₂₅₄)and aromatization(SUVA₂₈₀),the fewer hydrophobic components(SUVA₂₆₀),and the higher relative concentration of protein-like proteins[Fn?280?]of riverine DOM.The correlation between the degree of humification(SUVA₂₅₄)and the hydrophobic components(SUVA₂₆₀)is extremely significant?p<0.01?.The degree of humification and hydrophobic components of riverine DOM decrease with the increase of urbanization level along the flow direction.?2?Dry and wet seasonal variation of river DOM characteristics under the background of urbanization in the Three Gorges Reservoir AreaUnder the background of urbanization in the Three Gorges Reservoir Area,the riverine DOM characteristics also have significant spatio-temporal changes in dry and wet seasons.Taohuaxi,Nanhe and Pulihe rivers with different urbanization levels in the Three Gorges Reservoir Area,the river DOC contents and CDOM concentrations?p>0.05?were higher in wet season?April and August?than in dry season?January and November?,and the proportion of CDOM?CDOM/DOCmeans??p>0.05?was higher in dry season than wet season.The ratio of fulvic acid?E3/E4 mean?,endogenous in organic matter?E2/E4 mean?,and DOM molecular weight?p<0.05?of riverine DOM with different levels of urbanization are all higher in wet season than dry season.The abundance of protein-like substances[Fn?280?]?p>0.05?of DOM in each river are higher in wet season than dry season.Whether dry or wet seasons,the DOC content,CDOM concentration,and relative abundance of protein-like substances in the rivers of the Three Gorges Reservoir Area are consistent with the level of basin urbanization,the order is Taohuaxi>Nanhe>Pulihe River,and the DOC content and CDOM concentration gradually increase with the urbanization degree along the flow direction.In dry and wet seasons,the proportion of CDOM?CDOM/DOC?,degree of humification or aromatic content(SUVA₂₅₄)and hydro-phobic component content(SUVA₂₆₀)are Pulihe>Nanhe>Taohuaxi,which are opposite to the urbanization degree.In the rivers,the contribution of microbial metabolism in humus sources?FI mean?are higher in dry season than wet season.There was no significant difference between the humification degree?HIX?and the contribution of authigenic DOM?BIX?of DOM in each river in the dry and wet seasons?p>0.05?.?3?The spatial-temporal pattern of river DOM in spring and summer under the background of urbanization in the Three Gorges Reservoir AreaThe Three Gorges Reservoir area is affected by light,flow,and plankton activities.Under the urbanization background,the riverine DOM is more active in spring and summer.Characteristics of riverine DOM,such as content,composition and source,spatial and temporal patterns in spring and summer are:In Taohuaxi river and Pulihe river with different levels of urbanization,the DOC content?p>0.05?and the relative molecular mass of DOM?p<0.01?are higher in summer?August?than in spring?April?.However,the concentration of CDOM?p<0.01?,the proportion of CDOM?CDOM/DOC??p<0.01?,the humic or aromatic content(SUVA₂₅₄)?p<0.01?,the content of hydrophobic components(SUVA₂₆₀)?p<0.01?,endogenous level?E2/E4??p<0.05?,and fulvic acid proportion?E3/E4??p<0.01?are all higher in spring?April?than in summer?August?.The humification degree?HIX?of DOM are higher in spring than in summer,but the relative concentration of protein-like substances[Fn?280?]and the contribution of authigenic DOM?BIX?were higher in summer than spring,the seasonal variations are not significant.In the high-urbanized Taohuaxi river and the low-urbanized Pulihe river,the range of FI means in spring and summer is1.6945¹.7571,indicating that the humus are affected by terrestrial input and plankton or microorganisms,and mainly from endogenous sources.The BIX means in spring and summer are 0.83⁰.92,river DOM have the moderate autogenetic feature.?4?Source of river DOM under the background of urbanization in the Three Gorges Reservoir AreaUnder the background of urbanization in the Three Gorges Reservoir area,the FI mean values of the dry and wet seasons in the Taohuaxi,Nanhe and Pulihe rivers range from 1.7288 to 1.8221,the humus mainly originated from both microorganisms and terrestrial sources,and is mainly microbial metabolism.Taohuaxi,Nanhe and Puli rivers,the BIX values ranging from 0.76 to 0.99 in wet season and 0.79 to 0.96 in dry season.The BIX values in dry and wet season are less than 1,both internal and external sources affected riverine DOM.Under the background of urbanization,the HIX values of rivers in the dry and rainy seasons range from 1.58 to 6.22,and the river DOM has weak humus characteristics.?5?Microbial degradation of river DOM under the background of urbanization in the Three Gorges Reservoir AreaThe urbanization process of the river basin significantly affects microbial utiltilization of river DOM in the Three Gorges Reservoir Area.The results show that,the bioavailability of DOM in the high-urbanized Taohuaxi River(BDOC means:1.42±.42 means:iversmg.L^-1)is higher than that of the low-urbanized Pulihe River(BDOC means:0.96 meansernized t mg.L^-1).In the same season?summer,autumn?and the same culture temperature?20?,30??,the average BDOC of Taohuaxi was higher than that of Pulihe River under the same conditions.At each season and culture temperature,the BDOC values of the downstream samples of Taohuaxi and Pulihe River are higher than that of the upstream samples.The Three Gorges Reservoir Area in the context of urbanization,after the samples of Taohuaxi and Pulihe Rivers were cultured in various conditions for 42 days,the CDOM concentrations decreased,the DOM molecular weight increased,the humic or aromatic content(SUVA₂₅₄)increased,the hydrophobic component(SUVA₂₆₀)increased,the endogenous?E2/E4?of DOM increased,and the proportion of fulvic acid?E3/E4?increased.Both seasonal changes and culture temperature have an effect on the biodegradation of DOM.Under the background of urbanization in the Three Gorges Reservoir Area,the riverine DOC biodegradation?%BDOC?is higher in summer?August?than autumn?November?.During the microbial degradation of DOM,increasing the cultivation temperature?30??from a lower temperature?20??enhance the riverine DOC biodegradation?%BDOC?in the Three Gorges Reservoir Area,In addition,the humic or aromaticity abundance(SUVA₂₅₄)and the hydrophobic component contents(SUVA₂₆₀)of DOM under biodegradation were increased.