Dual Carbon Isotope Tracing Of Dissolved Organic Carbon Sources In Natural Reservoirs And Artificial Lakes In Guiyang

Reservoir is an important part of the terrestrial water environment,which has frequent material exchange with the terrestrial ecosystem.Soil carbon carried by water flow stored in reservoir sediments,and reservoirs participate in the global carbon cycle through sediment storage and CO₂ release.The study of dissolved organic carbon（DOC）source will help to understand the role of reservoir as"carbon source"and"carbon sink",and reveal the influence of natural environment and human activities on carbon behavior.In this study,water profile samples and possible source endmembers were collected from different types of reservoirs in karst area（open-Aha Reservoir and closed-Lake Guanshan）in winter and summer.Basic water quality parameters,DOC content,DOC double carbon isotope value(δ¹³C-?¹⁴C)and source endmember double carbon isotope value were determined.The temporal and spatial distribution characteristics of DOC content and carbon isotope were explored,and the contribution values of different endmembers were quantified by using the isotope multi-endmember mixed model with double carbon isotope values,so as to analyze the DOC migration and transformation process of different types of reservoirs,providing scientific data for exploring the sources of reservoir DOC in karst areas.The main conclusions are as follows:1、The average DOC content in Aha Reservoir is 3.3±0.27 mg·L^-1 in winter and5.39±0.39 mg·L^-1 in summer,and the surface DOC content is higher in winter.The average DOC content in Lake Guanshan is 4.16±0.47 mg·L^-1 in winter and 6.29±0.47mg·L^-1 in summer.The DOC content in Aha Reservoir increases significantly in summer compared with winter.The total DOC content in Lake Guanshan was higher than that in Aha Reservoir.Besides the terrestrial input,the algae production in the lake may have a greater contribution.2、The averageδ¹³C_DOC of surface,middle and bottom water in Aha Reservoir in winter was-28.18±0.31‰,-28.15±0.38‰and-28.22±0.21‰,respectively,and the averageδ¹³C_DOC of the whole reservoir was-28.18±0.31‰.The averageδ¹³C_DOC of Lake Guanshan was-30.63±0.55‰in winter and-30.46±0.53‰in summer.The waterδ¹³C_DOC of Aha Reservoir and Lake Guanshan was uniformly distributed,and there was no significant difference at different sampling points and depths.Theδ¹³C_DOC of Lake Guanshan was lower than that of Aha Reservoir,which may be affected by the respiration of aquatic organisms.The average of surface,middle and bottom water?¹⁴C_DOC in winter of Aha Reservoir was-87.9±6.41‰,-86.72±8.45‰,-87.02±9.99‰,and the overall average of?¹⁴C_DOC was-87.24±8.42‰,the average value of?¹⁴C_DOCwas-131.59±11.36‰in summer in Aha Reservoir.The average?¹⁴C_DOC of Lake Guanshan was 4.23±10.24‰in winter and-14.25±8.33‰in summer.The?¹⁴C_DOC of Aha Reservoir in summer was more negative than that in winter,and the erosion capacity of the basin was strengthened in summer,so the old carbon from terrestrial sources was easier to enter the reservoir.3、The total water DOC ¹⁴C age of Aha Reservoir was 888±200 years,and that of Lake Guanshan was 162±92 years on average.The DOC ¹⁴C age of Aha Reservoir was higher than that of Lake Guanshan,and the old carbon stored in Aha Reservoir was mostly ancient carbon.In addition to the old carbon eroded by the river into the reservoir,there is also the re-release of old carbon in the sediments.The DOC ¹⁴C age of Lake Guanshan was young and basically modern carbon.4、The source endmembers of DOC in Aha Reservoir can be divided into terrigenous source,algae source,sediment source,vegetation source and soil source.In winter,terrigenous sources,algae sources,sediment sources,vegetation sources and soil sources contributed 40%,2%,11%,39%and 8%to DOC,respectively.In summer,the contribution rates was 28%,14%,13%,33%and 12%,respectively.The DOC source endmembers of Lake Guanshan can be divided into terrigenous source,algal source,vegetation source and soil source.The contribution rates of terrestrial sources,algae sources,vegetation sources and soil sources to DOC in winter were 36%,40%,21%and 3%respectively,and were 29%,28%,14%and 29%respectively in summer.The DOC of Aha Reservoir mainly comes from terrigenous sources and vegetation sources,while the DOC of Lake Guanshan was greatly affected by terrigenous sources and algae sources,and the contribution of algae sources was more prominent in winter when terrigenous source input decreases.