The Study Of Carbon And Sources Of CO₂ In Typical Reseroivrs Of The Upper Yangtze Basin,China

Dam storage will cause major changes in the carbon cycle of river ecosystems.Different types of reservoirs have different ways and degrees of changes in the hydrodynamic environment caused by damming and interception,and the corresponding changes in the composition of the material transported in the basin.At present,there is a lack of in-depth understanding and comparison of the evolution of the water environment of different types of reservoirs in the same river basin.The upper reaches of the Yangtze River have a wide coverage area,numerous reservoirs,and different characteristics.Therefore,this study selects five typical case reservoirs（Three Gorges Reservoir）in the upper reaches of the Yangtze River in three time periods before the flood（May to June 2019）,flood season（July to September 2019）,and after the flood（November to December 2019）.,Jinsha River Cascade Reservoir,Youyang Second Reservoir,Shizitan Reservoir,Gaoyang Lake)surface water samples were taken to analyze particulate carbon（PC）,dissolved organic carbon（DOC）,dissolved inorganic carbon（DIC）and water carbon dioxide Temporal and spatial distribution characteristics and influencing factors of pressure p（CO₂）;and its source was analyzed by stable isotope technique.The paper obtained the following main conclusions:（1）The PC concentration in the five typical case reservoirs showed seasonal variation the flood>the pre-flood>after the flood,and the PC concentration in the high eutrophic level water bodies（2.13mg/L in Lion beach reservoir,2.06mg/L in Gaoyang Lake）was significantly higher than the low eutrophic level water bodies（0.32mg/L in Three Gorges Reservoir,0.26mg/L in Jinsha River cascade Reservoir,0.32mg/L in Youyang Secondary Reservoir）,and the DOC seasonal variation was similar to the PC,and the nutrient level in the water bodies was also positively related to the DOC concentration.（2）Before the flood,the DIC concentrations of the Three Gorges Reservoir and the Jinsha River cascade reservoir were 36.89±1.70mg/L,24.32±1.94mg/L;during the flood,they were 27.87±2.67mg/L,16.45±2.69mg/L;after the flood,they were:16.17±0.61mg/L,16.38±0.61mg/L.They all show the characteristics of the pre-flood>the flood>after the flood seasonal changes.The average values of p（CO₂）in the Three Gorges Reservoir,Jinsha River cascade Reservoir,and Youyang Secondary Reservoir were 964±159ppm,674±103ppm,and 520±62ppm,respectively,and they all showed the characteristics of the flood>the pre-flood>after the flood seasonal changes;and The average values of p（CO₂）in Lion beach reservoir and Gaoyang Lake in eutrophic state were 647±462ppm and 885±549ppm,respectively,and they all showed the characteristics of seasonal changes after the flood>the pre-flood>the flood.（3）Correlation analysis of various carbons in the water and various environmental factors,and the results are obtained:For PC,total particulate matter（TPM）,chlorophyll a（Chl-a）,and water depth have a greater impact,while the flow rate,The correlation between factors such as water surface width is not significant enough.TPM is the main carrier of PC,and the two are closely related.The stronger the primary productivity of the water body,the more autogenous particulate matter is produced.In the water area with greater water depth,the surface water particles are more likely to settle.For DOC,the impact of various environmental factors on it is similar to that of PC.As far as DIC is concerned,although flow rate,p H,DO,and Chl-a are significantly correlated with it in a certain period,there are no environmental factors in absolute influence.Analysis of the factors affecting the surface water CO₂found that p H,DO,Chl-a had a greater impact.The main biochemical process of its characterization is that the primary productivity of water,algae,etc.,under photosynthesis,absorbs CO₂in the water and generates a large amount of oxygen,resulting in an increase in the p H of the water.This process is most obvious in the flood season in summer.（4）Through the SIAR model for traceability analysis of particulate organic matter,the typical sources of POC in the upper Yangtze River are mainly PL,soil and C3 plants.Before the flood season,the main source is autochthonous,but during the flood season,due to the increase of rainfall,the dry and steep river bank is conducive to the occurrence of bank soil erosion,and after the flood season,the water level rises,so that the fallout zone is submerged,a large amount of land-based organic matter into the water body,and other reasons,POM begins to be allochthonous.（5）Traceability analysis of DIC in surface water bodies:typical reservoir DIC sources in the upper Yangtze River are dominated by carbonate rock weathering,and the contribution rate of soil CO₂increases during the flood season due to the increased intensity of shoreline scouring.Compared to tributary reservoirs（57.18%）,large reservoirs in the Yangtze River mainstem have a greater contribution of carbonate rock weathering（69.82%）due to the wide distribution of rocks around the channel.The carbon isotopes of p（CO₂）in the surface waters of the reservoir showed pre-flood>mid-flood>post-flood time characteristics,except for the period when the maximum is the mid-flood of the Three Gorges Reservoir.For large reservoirs such as Three Gorges Reservoir,the contribution of allochthonous DIC to CO₂in surface water is increased due to the increase of rainfall during the flood season,which is dominated by the large reservoirs in the process of river and reservoir changes.