Analysis Of Source And Component Characteristic Of Dissolved Organic Matter In Karst Reservoir Water-A Case Study In Wulixia Reservoir Of Guangxi Province

Global warming and global climate change are becoming more and more concerned by the scientific community.Investigation on the process and mechanism of carbon cycle,especially research on carbon sink and the technology of carbon storage which devoted to reduce the concentration of atmosphere CO₂,is still one of the core issues of global change research in this century.“Karst carbon sink”is very significant for China southwest karst area,under the effect of karst process promote,it can fix large flux of CO₂ from atmosphere.But the“carbon sink effect”was doubted by curl and other experts,they considered that karst geological process is a carbon migration process but not carbon sink.In recent years,to response this doubt,scientists have found that aquatic plants can absorb DIC dissolved in water and form organic matter through photosynthesis.When aquatic plants rot and decompose,some of them can be stored in water in the form of organic carbon such as DOM.DOM is the largest organic carbon pool in the world,and is an important component of the global carbon cycle system.The karst area in China is as high as3.34 million km² and there are many damming reservoirs.What are the characteristics and sources of the water bodies of DOM reservoirs in karst areas?The molecular structure and composition of DOM are very complex and the sources are very wide,which makes it difficult to study the characteristics of DOM in water.In recent years,3D fluorescence spectroscopy combined with parallel factor analysis?PFA?has been widely used in DOM analysis because of its high sensitivity,accurate identification and no damage to the original structure of the sample.The geochemistry characteristics of DOM in water can be qualitatively and quantitatively analyzed,and the composition information of DOM can be effectively revealed.In this study,the Wuli Gorge Reservoir in Guangxi was sampled on December 2016,April 2017 and June 2017,respectively.The composition and source characteristics of were analyzed by studying the hydrochemical characteristics of the reservoir and combining with three-dimensional fluorescence spectroscopy,in order to provide scientific basis for the study of DOM and the understanding of carbon cycle characteristics of karst reservoir.Research results show that the water physical and chemical parameters in water plankton in the study area,precipitation of dilution and rock weathering,show the obvious time characteristic.Overall water temperature presented the trend of June>December>April;Such as photosynthesis,water transparency and aquatic organisms under the influence of present a summer slightly higher than the other two sampling period characteristic;The water chemistry type of Wulixia reservoir was HCO3-Ca-Mg,the nutrient status of the water body was medium nutrient type,and the water maintained a good quality.DOC accounts for about 92%of TOC in the study area,the seasonal distribution of TOC and DOC concentration presentthat:December>June>April,and DOC concentration showed a decrease trend from surface to the bottom in the vertical direction effected by various environmental factors.The mean of CDOM absorption coefficient a?355?in the study area is 0.52 m^-1,which is at a low level.Due to the enhanced photosynthesis of aquatic plankton,the activity of bacteria in the water body is active,the primary productivity of phytoplankton is large,and the concentration of DOM in water increases.The value of a?355?in summer is slightly higher than that in other sampling periods.CDOM feature parameters:relatively high S_275²95 value and M value indicate that the dissolved organic matter in the research area is dominated by small molecular weight substances,with low humic acid share and more rich acid.Low levels of SUVA254 and E253/E203 values of aromatic weak dissolved water in the study area oforganic matter under the direction of the contribution of endogenous substances on dissolved organic matter,containing more benzenering aliphatic non polar functional groups on the phenyl ring,containing carboxy,carbonyl and few polar functional groups indicated that the endogenous strong organic matter in the water body of the study area is stronger.In this study,there is a good correlation between DIC and a?355?,indicating that DIC transformed from organic plankton to photosynthesis in karst water has a certain contribution to the formation of DOM in water.A total of 3 fluorescence components?C1,C2,C3?with the maximum of single or multiple excitation and emission wavelengths are identified in the study area by the PARAFAC model.Among them,C1 belongs to the fulvic acid substance,the C2 belongs to the humic acid substances,C1 and C2,which usually represent the more difficult to degrade DOM,and C3 belongs to the protein like substance?Tryptophan?,reflecting tryptophan derived from algae and microorganism degradation.The 3 components in December,the corresponding excitation and emission wavelengths are C1?250,325/425 nm?,C2?260,370/470 nm?and C3?260/370 nm?,and the excitation and emission wavelengths of the 3 components in June are C1?250,300/390 nm?,C2?270,350/450 nm?and C3?280/340 nm?.A comprehensive analysis of the DOM fluorescence spectrum parameters of water in the study area showed that the higher BIX,?:?,FI values and lower HIX values had a good indication for the source of DOM in the study area.Comparing the fluorescence parameters of different lake DOM that the study area of water source of DOM is influenced by terrigenous matter and endogenous substances,but endogenous characteristics of strong,indicating that the microbial and algae is the main source of these endogenous substances should be dissolved organic matter in water in the study area,and through the photosynthesis of dissolved inorganic carbon into organic carbon and influence on DOM.