Study On Transport Characteristics And Flux Of Dissolved Organic Carbon Under The Influence Of Cascade Development In The Upper Reaches Of The Yellow River

There is a fragile ecosystem in the upper reaches of the Yellow River with great environmental specificity and sensitivity.Cascade development will change the river structure,affect the river material and energy transport and ecosystem structure function to a certain extent.Dissolved organic carbon（DOC）,as an important biogenic substance,indicates the transport and transformation process of river organic carbon.Therefore,it is of great significance to study the distribution characteristics,component sources and transport flux of DOC in cascade reservoirs in the upper reaches of the Yellow River.In this study,the typical cascade reserv oirs in the upper reaches of the Yellow River in China were selected as the research objects.The surface water and stratified water in front of the dam were sampled in winter（November 2019）and summer（July 2020）,and the field environmental data of the reservoirs were monitored.Based on the analysis of water environmental indicators,the components and sources of DOC in reservoirs were analyzed by using the method of three-dimension al fluorescence spectrum combined with parallel factor model,and then the transport flux of DOC in reservoirs was calculated to quantitatively evaluate the effects of accompanying transport and interception and retention of DOC in reservoirs.Specific conclusions are as follows:（1）Th e distribution rules of reservoir water environment index and DOC is analyzed.The hydro-chemical type is mainly bicarbonate-carbonate Ca group water.The values of DO,T and ORP in summer are higher than those in winter,while the values of EC in winter are higher than those in summer,and there is no obvious difference in pH value.COD concentration in winter is obviously higher than that in summer(7.44±3.43 mg·L^-1 in summer and 14.38±1.72 mg·L^-1 in winter).There is no obvious difference between TN and NH₄+-N concentration,and the retention effect of reservoir on nitrogen is limited.There are obvious differences in TP concentration,and the retention effect of phosphorus in reservoirs is relatively strong.DOC in Long-Liu reach is higher in summer than in winter(6.24±0.87 mg·L^-1 in summer and 3.70±0.51 mg·L^-1 in winter),and higher behind the dam than in front of the dam.DOC in stratified water of LYX,LJX,LJXX fluctuated and decreased,while DOC in LXW and GBX fluctuated and increased.Thermocline hinders the material and energy exchange between the upper and lower layers of the reservoir,and changes the DOC concentration level.（2）Clarify the component characteristics of DOC in river reach,and analyze the source distribution of DOC in reservoir.Three components in summer and one in winter are humus-like substances,while two in winter are protein-like substances.Total fluorescence intensity is higher in summer than in winter（1.66×10⁵ a.u.in summer and 0.56 × 10⁵ a.u.in winter）.The source of DOC in both seasons are exogenous and the combination of internal and external sources.There are differences in humus-like substances and protein-like substances.In summer,the land source contribution is great and the humification degree is high.In winter,water bodies have less contribution from autogenous sources.（3）The transport flux of DOC in river reach is estimated,and the transport/interception effect of reservoir is revealed.In a complete hydrological year,the total input and output of DOC reaches are 258.53 kt and 192.23 kt,with a total interception rate of 25.6%.LYX,GBX and LJXX have the effect of intercepting and detaining DOC,which is the "sink" of DOC in river-reservoir system.LXW and LJX have the effect of transporting DOC with flow,which is the "source" of DDC in river-reservoir system,and the cascade reservoirs in Long-Liu reach are the "sink" of DOC.