Characteristics Of Dissolved Organic Carbon Transport And Carbon Dioxide Exchange At Water-Air Interface In The Upper Yellow River

As a part of global carbon cycle,riverine carbon transport plays an important role in global carbon cycle.Dissolved organic carbon（DOC）is one of the main forms of carbon in river habitats.It has multiple functions such as feedback of river ecosystem evolution and influence of carbon dioxide（CO₂）exchange between water and atmosphere,and is the key link of river carbon transport.With the intensification of human development of rivers,the "river-reservoir" system composed of rivers and reservoirs has become the main form of rivers,which hinders the transport of carbon and becomes a hot area of greenhouse gas emissions.The upper reaches of the Yellow River is located on the edge of the Qinghai-Tibet Plateau and is a typical ecologically fragile area with significant environmental specificity and sensitivity.Due to the large number of planned cascade hydropower stations,a typical "river-reservoir" system has been formed.As water energy utilization gradually advances to the source area,it has a strong impact on the fragile ecosystem,and also significantly changes the DOC transport model and the source-sink pattern of CO₂,thus affecting the river carbon cycle process,Therefore,the research on the transport model of DOC and the characteristics of CO₂ exchange at the water-air interface in the "river-reservoir" system of the upper reaches of the Yellow River not only involves the key research fields of the transport and transformation of river biogenic materials,but also relates to the academic focus of the carbon cycle budget.It is of great significance to understand the role of river carbon transport in the global carbon cycle.In this study,the upper reaches of the Yellow River were divided into source section（Elinghu lake-Tangnaihai）and cascade reservoir section（Longyangxia-Liujiaxia）according to the different degree of cascade development,and the "river-reservoir" composite system was selected as the study area to focus on DOC transport and water-air interface CO₂ exchange,which are the key links of river carbon cycle.It is expected to further understand the role of carbon transport in the "river-reservoir" system of the upper reaches of the Yellow River in the study of global carbon cycle,and to provide scientific basis for the assessment and prediction of the environmental effects of ecological environment protection and hydropower development in the upper reaches of the Yellow River.The main conclusions are as follows:（1）The temporal and spatial distribution characteristics of DOC concentration in the "riverreservoir" system of the upper reaches of the Yellow River were clarified,and the main driving factors of temporal and spatial variation were explained.The distribution of DOC concentration showed spatial differences and seasonal changes,and the vertical DOC concentration along the water depth of Longyangxia Reservoir and Liujiaxia Reservoir in the cascade reservoir section showed seasonal stratification.In the dry period,the DOC content was higher in the cascade reservoir section than in the source section,and showed significant differences.In the flood period,the source section is higher than the cascade reservoir section,the difference is not significant.The average concentration of DOC in the whole river section was higher in the wet season than in the dry period.The change of DOC content in the source region was mainly driven by freezing and thawing cycles of frozen soil,melting of snow mountain glaciers,rainfall and temperature.In the cascade reservoir section,it is mainly driven by factors such as changes in runoff conditions,human regulation,and changes in hydraulic retention time,and the vertical seasonal thermal stratification of the reservoir drives the vertical redistribution of DOC content in each reservoir along the water depth,making it seasonally stratified.At the same time,water temperature,dissolved oxygen,pH,alkalinity and nitrogen,phosphorus content are the main factors affecting DOC content.（2）The DOC components of "river-reservoir" system in the upper reaches of the Yellow River were identified and the possible sources of DOC were analyzed.DOC components in the"river-reservoir" system in the upper reaches of the Yellow River have obvious spatiotemporal heterogeneity.In the dry season,three fluorescent components were identified and classified into two categories:humic-like and tryptophan-like.The mixed source is mainly exogenous input humus,supplemented by endogenous protein,and the endogenous degree of cascade reservoir section is stronger than that of source section.During the flood period,two fluorescent components were identified and classified into one category:humic-like substances.The main source is exogenous input humus,and the source section shows stronger exogenous characteristics than the cascade reservoir section.In general,the fluorescence intensity in the flood period is higher than that in the dry period.Changes in temperature and rainfall conditions caused by seasonal changes in climate and river hydrology,ablation of permafrost active layers,and high-intensity cascade hydropower development may be the main driving factors for their spatial and temporal heterogeneity.（3）The temporal and spatial variation characteristics and driving factors of carbon dioxide partial pressure（pCO₂）in the "river-reservoir" system of the upper reaches of the Yellow River were systematically analyzed.The transformation mechanism of DOC and CO₂ was discussed.The CO₂ exchange flux at the water-air interface was estimated,and the CO₂ source-sink pattern was explained.Driven by the freeze-thaw cycle of the permafrost active layer and the development of cascade reservoirs,the average pCO₂ in the two water periods was higher in the cascade reservoir section than in the source section and higher in the flood period than in the dry period.Driven by water temperature stratification and light conditions,the pCO₂ of each reservoir in the cascade reservoir section exhibited seasonal variations along with water depth.The environmental factors TN,TP,T,DO and DOC were the main influencing factors of pCO₂ distribution and could be used as predictors of pCO₂ in the dry period（R2=0.40 P<0.01）.In the dry period,the FCO₂ in the source section was-112.91 ± 165.94 mmol/（m²·d）,which was a sink of CO₂,and the FCO₂ in the cascade reservoir section was 131.02± 156.77 mmol/（m²·d）,which was a source of CO₂.In the flood period,the FCO₂ in the SR was 686.54±624.33 mmol/（m²·d）,and the FCO₂ in the cascade reservoir section was 466.10±366.67 mmol/（m²·d）.Both the source section and the cascade reservoir section were sinks of CO₂.（4）The change of DOC transport flux in the "river-reservoir" system in the upper reaches of the Yellow River was estimated and analyzed.The interception and transport effect of the"river-reservoir" system was clarified,and the cumulative effect of the DOC transport was explored.Driven by the seasonal variation of river flow and the influence of rainfall,temperature and artificial regulation,the DOC flux transported from the source section of the "river-reservoir"system in the upper reaches of the Yellow River to the cascade reservoirs in the sampling year was 83.82 kt,and the DOC flux transported from the cascade reservoirs to the downstream reaches was 94.45 kt.And the time distribution of DOC flux transported to the downstream of the "river-reservoir" system tends to be flat,and it has the effect of reducing and lagging the peak transport flux of DOC.Longyangxia,and Lijiaxia reservoirs in the cascade reservoir section showed interception effect on DOC transport,Liujiaxia reservoir showed transport effect.Driven by the change of climate and land water storage and the influence of cascade development,the DOC export flux of the "river-reservoir" system in the upper reaches of the Yellow River showed a downward trend from 1980 to 2004,and showed an upward trend after 2004.With the development of cascade reservoirs,the cumulative interception rate of DOC transport in the"river-reservoir" system remained below 5%,showing a logarithmic growth pattern.