Carbon Stable Isotope Characteristics Of Dissolved Inorganic Carbon Of Cascade Reservoirs At The Upper Reaches Of Xijiang River

Dissolved inorganic carbon（DIC）plays a key role in the biogeochemistry of water ecosystems and is an important component of the carbon budget.The development of hydropower has changed the DIC cycle in natural rivers,with rivers dammed up to form reservoirs,which are gradually shifting to"lake-type"reservoirs.Ecosystem evolution has enhanced biogeochemical cycling and further influenced the carbon cycle in water bodies.To budget the global carbon cycle and predict changes in the carbon cycle more accurately,it is important to identify the sources of carbon that have an impact on river systems.Therefore,it is essential to identify DIC sources and understand their biogeochemical cycles.In this paper,the Wanfenghu Reservoir in the Nanpan River,the Guangzhao Reservoir in the Beipan River,and the Longtan Reservoir in the Hongshui River in the karst mountain area in southwest China were selected for research.The main sources of DIC and their influences in three reservoirs were investigated by analyzing the physicochemical parameters,DIC,andδ¹³C_DIC characteristics to understand how carbon in the reservoirs is controlled by processes within the river and evolves.The following conclusions were reached through research:1.Wanfenghu Reservoir:（1）In the epilimnion:due to dilution effects,the lowest values of EC,pCO₂,and DIC occur in the summer high runoff stage.At the water column:Eh and NO₃^--N did not change significantly with water depth in summer,while other indicators changed significantly and had the greatest variation in the thermocline.T,pH,and Eh all decreased with increasing water depth in both seasons,with pCO₂ being the opposite.The DIC was negatively correlated with pH,Eh,and positively correlated with EC and pCO₂ on the water column in both seasons.（2）DIC andδ¹³C_DIC had significant spatio-temporal differences,with a summer DIC of 2.66 mmol·L^-1 to 4.9 mmol·L^-1,while the winter water column was well mixed vertically and the DIC range was narrowed to 3.38 mmol·L^-1 to 4.52 mmol·L^-1.In summer,δ¹³C_DIC was-7.71‰to-1.38‰,indicating that carbonate mineral dissolution was dominant.In winter,δ¹³C_DIC was-16.93‰to-9.44‰,and the input of CO₂ from biogenic sources and organic matter mineralization were the main source of DIC.During the thermal stratification of the reservoir,DIC andδ¹³C_DICIC in the summer epilimnion were positively correlated,and the gradient of change between DIC andδ¹³C_DIC was greatest in the thermocline.In both seasons,DIC andδ¹³C_DIC were negatively correlated on the water column and in the winter epilimnion,but in winter DIC andδ¹³C_DIC had no clear trend with water depth.2.Guangzhao Reservoir:（1）The DIC ranged from 1.80 mmol·L^-11 to5.02 mmol·L^-1,while theδ¹³C_DIC was-7.45‰to-1.26‰.DIC was positively correlated with EC and TA,and negatively correlated with water temperature.Theδ¹³C_DIC in the epilimnion was negatively correlated with TA,EC,and DIC,whileδ¹³C_DIC was positively proportional to EC,pCO₂,TA and DIC on the water column.（2）From upstream to downstream,indicators in the epilimnion changed obviously along the river.TA,EC,SIc,and DIC were generally decreasing,while theδ¹³C_DIC was gradually increasing,which was significantly affected by carbonate mineral dissolution.On the water column,thermal and chemical stratification had a significant effect on the carbon cycle.The decomposition of organic matter released a large amount of CO₂ in deep water,which caused DIC,pCO₂,andδ¹³C_DIC to increase gradually with water depth.The DIC andδ¹³C_DIC exhibited significant spatial heterogeneity across the water column.The HCO₃^-from the weathering of carbonate mineral was the main source of DIC in the summer.3.Longtan Reservoir:（1）The main influences and sources of DIC were not the same in both seasons.The summer DIC andδ¹³C_DIC ranged from 2.04 mmol·L^-11 to 4.12 mmol·L^-1 and-5.60‰to 0.04‰,respectively;in winter,the DIC ranged from 3.33 mmol·L^-1 to 4.61 mmol·L^-1,while theδ¹³C_DIC was-15.90‰^-9.12‰.In summer,the dilution effect significantly reduced the DIC concentration,and the thermal stratification of the water column made the difference of DIC on the water column significant,while in winter,the difference was lower due to the mixing effect.（2）Theδ¹³C_DICIC was significantly more positive in summer than in winter,and the strong weathering of carbonate with large HCO₃^-input was the main source of DIC.DIC andδ¹³C_DIC were negatively correlated in winter,with most of the DIC coming from soil CO₂ from biogenic sources and CO₂ from organic matter respiration.In winter,thermal stratification of the water column disappeared and the mixing process resulted in the upwelling of carbonaceous water with lowerδ¹³C at the bottom,which mixed with the epilimnion resulting in a lowerδ¹³C_DIC than in summer.