Study On The Mechanism Of Carbon Transport And Transformation In Karst Damed River

Rivers are important conduit to transport terrestrial mutrients to the ocean and play an important role in the global carbon cycle.However,with the rapid development of the global economy since the 19th century,the demand for clean energy has led to the damming of global rivers for power generation,which has become an important way to solve the energy demand.River damming changes the original hydrological regime and river biogeochemical processes seriously.After damming,the hydrodynamic condition slows down and performed many negative effects such as the longer hydraulic retention time（HRT）,the relative stable water temperature in the lentic area,the intercepted biologic elements,and the increasing nutrient load.In addition,lots of reservoirs show obvious limnology in the lentic area.Previous studies have not comprehensively considered the influence of multiple factors.Thus,it is difficult to understand the environmental effects of river cascade damming and the influence mechanism of carbon cycle,especially the migration and transformation of different carbon forms in cascade reservoirs under hydrodynamic conditions.To better explore the cascade damming effect on the carbon cycle in karst rivers（Wujiang River）,we collected the inflow,reservoir and outflow water samples from 8reservoirs with different HRT in January（Winter）,April（Spring）,July（Summer）and October（Autumn）of 2017.In addition,to better understand the influence mechanism and environmental effects of the damming effect on PIC transport and transformation,we collected water samples,suspended particulate matter and sediment samples in the Hongjiadu river-reservoir system in August 2018.Water samples were collected and analysis for basic hydrochemical parameters and isotopic data,including water temperature（WT）,p H,dissolved oxygen（DO）and electrical conductivity（Ec）,major ions,DIC,PIC,DOC,δ¹³C_DIC,δ¹³C_PIC,Δ¹⁴C_DICandΔ¹⁴C_PIC.The PIC samples were analyzed by scanning electron microscopy to observe its morphologic charateristic.The SEM results,hydrochemical and isotopic data can help to explore the migration and transformation process of inorganic carbon in karst river reservoir system.The main conclusions are as follows:（1）The damming effect on basic water chemical parameters:the basic water quality parameters（WT,PH,DO,Chl-a,TDS,etc.）have significant spatial and temporal heterogeneity in the Wujiang cascade reservoirs.Water quality parameters in the river area are similar to those in other karst rivers,and the parameters of p H,DO and TDS are maintained at a high level.However,there is marked difference in reservoir area and outflow area.In the warm season（April and July）,WT,p H,DO and Chl-a of reservoir surface water in the Hongjiadu,Hongfeng,Dongfeng and Wujiangdu with significant thermal stratification are higher than the inflow and released water,and the values decrease markedly with the increase of profile depth and remain stable in the hypolimnion.However,the water parameters are relatively stable and similar to natural rivers in the daily regulated reservoirs of Suofengying and Pengshui.In the cold season（January and October）,with the weakened thermal stratification,the variation of water quality parameters decreased markedly and the water parameters in the reservoir profiles tended to be mixed.The above results indicate that artificial regulation and seasonal alternation are the important factors affecting the water chemical parameters of river reservoir system.（2）The damming effect on chemical weathering rate estimation of carbonate rock:The weathering rate estimation and comparative study of the reservoirs lentic and outflow area in Wujiang cascade reservoirs（Multi-year regulation:Hongjiadu reservoir;Seasonal regulation:Wujiangdu reservoir;Daily regulation:Suofengying reservoir）showed that the annual variation range of weathering rate of the above 3reservoirs was consistent with the order of HRT:Hongjiadu Reservoir（-1.7%～15.4%）>Wujiangdu Reservoir（-0.3%～3.4%）>Suofengying Reservoir（-5.6%～-2.4%）.The results suggest that artificial regulation affect the river hydrochemical characteristics,leading to the difference of hydrochemical distribution in different reservoirs.However,the effect of reservoirs was often neglected in the previous studies on river basin weathering.In the future,the reservoir effect should be taken into account in the evaluation of rock weathering rate to improve the accuracy of the calculation results.（3）The damming effect on DIC migration:similar to other karst rivers,the major cations(Ca²⁺and Mg²⁺)and anions(HCO₃^-and SO₄^2-)suggest that river water chemical composition is mainly influenced by carbonate rock weathering.However,due to river damming,there are seasonal variations in different regions of the river reservoir system.The values ofδ¹³C_DICin the reservoir area were positive and decrease with depth profile obviously.The results showed that DIC in different regions of the river reservoir system was jointly affected by phytoplancton photosynthesis,carbonate precipitation and dissolution,organic matter degradation,CO₂outgassing and other biogeochemical processes.To explore the reservoir effect index in the cascade reservoirs:the PLS model suggest that air temperature and HRT are the most important parameters to affect DIC andδ¹³C_DIC.The good relationship of Ta/HRT andΔ[DIC]andΔ[δ¹³C_DIC]further indicate that DIC concentrations andδ¹³C_DICmainly depend on the variation of HRT and air temperature in the Wujiang cascade reservoirs.The ratio of Ta/HRT can be used to better show the influence degree of river damming effect on the DIC chemical behavior.（4）The source and controlled mechanism of PIC and DIC in the river reservoir system:δ¹³C_DIC,δ¹³C_PIC,Δ¹⁴C_DIC,Δ¹⁴C_PIC,water chemistry and SEM results suggest that the source of the PIC in the reservoir area has completely changed.There is a significant autochthonous carbonate precipitation effect in the epilimnion and 91%of the PIC is from the autochthonous effect.In the released water,about 49%of DIC from autochthonous PIC,indicating that damming induced autochthonous precipitation and accelerated its dissolution,making autochthonous PIC act as an important composition in the karst river reservoir system.However,this has not been paid enough attention in previous studies.（5）Cascade damming effect on CO₂emissions and indicators:Combined CO₂flux data of this study and other studies in the worldwide（dam height over 15m）,we found that that there is significant correlation of the reservoir effect index R_i（HRT/NWL）and CO₂flux andδ¹³C_CO2.Moreover,R_iranging from 0 to 1,compared with HRT,R_ican help to calculate the reservoir surface CO₂flux.In conclusion,the study on carbon migration and transformation processes and control mechanism in karst cascade reservoirs can provide reference for reservoir management to consider HRT and nutrient reduction in future planning and management measures.In addition,addressing these objectives will improve the knowledge base underpinning how to manage reservoirs to minimize the environmental impact,hence providing cleaner energy.