Study On Distribution Characteristics And Traceability Of Organic Matter In Erhai Lake Based On Multi Technology

In recent years,the water quality of Erhai Lake has shown obvious periodic fluctuation.After the engineering measures of pollution control into the lake,the indicators such as chemical oxygen demand（COD）still exceed the standard from time to time.Based on this,it is very necessary and urgent to carry out accurate and quantitative traceability research on organic pollutants in Erhai Lake.In this study,Erhai Lake is divided into Nearshore Area and lake center area,and its hydrochemical indexes and three-dimensional fluorescence spectral parameters are monitored for a long time in monthly units;In the wet season and dry season,the dissolved organic matter（DOM）in water body was analyzed by means of stable isotope,C/N ratio and three-dimensional fluorescence spectrum,drawing concentration distribution map and the source was analyzed quantitatively by parallel factor analysis（PARAFAC）and Iso Source model;According to the sedimentation rate of Erhai Lake,the sediment samples were stratified and traced to the source.The main conclusions are as follows:（1）Hydrochemical indexes such as total organic carbon（TOC）,total nitrogen（TN）and C/N ratio fluctuate significantly seasonally in the year.From spring to winter,TOC shows the characteristics of"increase to decrease to increase",TN value shows the characteristics of increasing in spring and autumn,decreasing in summer and winter,and large degree of data dispersion.The trend of C/N is the same as that of TOC.Its value is small in wet season and increases in dry season,which reveals the change of the source of organic matter in water.There is a very significant correlation between the corresponding parameters in the Nearshore Area and the central area of the lake,indicating that the whole lake of Erhai is evenly mixed and the regional characteristics of the water area are not very obvious.（2）Three dimensional fluorescence indexes such as fluorescence index（FI）,humification index（HIX）and autogenous source index（BIX）fluctuate periodically within a year.From the analysis of FI,the mean value lies between the characteristic values of water and land,indicating that it is jointly affected by internal and external sources in the year,but the proportion of the two varies with months;The average values of HIX and BIX in each month fluctuate continuously during the year,but the fluctuation trend is just the opposite.The three indexes corresponding to the Nearshore Area and the central area of the lake have（extremely）significant positive correlation,which indicates that the regional fluorescence characteristics of the lake are consistent;There is a significant negative correlation between HIX and BIX,which is consistent with the index characteristics.（3）DOM in Erhai water body mainly comes from algae in wet season.The concentrations of TOC and TN in northern gulf were slightly lower than those in central and southern Lake areas,the range of C/N,δ¹³C andδ¹⁵N in Nearshore Area and Lake Center Area is 3.253-10.082,7.480-8.471;-27.469‰--25.303‰,-26.317‰--26.248‰;5.109‰-9.255‰,5.150‰-6.928‰.From the analysis of three-dimensional fluorescence index,the fluorescence parameters accord with the characteristics of weak humus and strong newly generated authigenic source,which indicates that DOM in water is affected by endogenous pollutants in a large proportion in the water period.Combined with the end source data and Iso Source model,the DOM source of water body in wet season is mainly algae,with a contribution rate of 79.6%.The contribution rates of submerged plants,C3 plants,C4 plants and soil organic matter are 14%,3%,2%and1.4%respectively.This shows that after entering the water bloom outbreak period,the process of algal mass reproduction and life metabolic activities is the main source of DOM in water body,which eventually leads to the periodic increase of COD.（4）During the dry season of Erhai Lake,DOM sources are diverse,and C3 plants,algae and soil organic matter all contribute greatly.The range of C/N、δ¹³C andδ¹⁵N in Nearshore Area and Lake Center Area is 8.785-14.593,9.811-10.531;-27.789‰--25.946‰,-27.335‰--26.467‰;4.977‰-8.1314‰,4.809‰-6.973‰.From the analysis of three-dimensional fluorescence parameters,various indexes in dry season obviously move to the direction of characterizing the range of terrigenous eigenvalues.Combined with the end source data and isosource model,the DOM of water in dry season mainly comes from C3 plants,algae and soil organic matter,accounting for44.6%,28.3%and 24%respectively.Submerged plants and C4 plants also have a weak contribution,with contribution rates of 2.8%and 0.3%respectively.（5）The source of organic matter in Erhai Lake sediments is greatly affected by the region,and varies with the depth.Compare TOC,TN content,C/N,δ¹³C andδ¹⁵N,the value range of each point in the Nearshore area is larger,and the change with depth is more obvious,indicating that the main sources of sediment organic matter in the Nearshore Area are obviously different in different historical periods.The above indicators have a very significant negative correlation with depth.There was a significant correlation between the three parameters of C/N,δ¹³C andδ¹⁵N.Quantitative source analysis shows that the sources of organic matter at different depths in the central area of the lake are relatively unified,mainly endogenous or mixed water and land sources,while the sources of organic matter at various points in the Nearshore Area are diverse and change significantly with depth,which shows that the main sources of sediment organic matter are different in different historical periods,which is related to the nutritional status of the lake water body,hydrometeorological conditions,surrounding land use types and industrial changes.