A Comparative Study Of Carbon Flux Changes At The Water-Air Interface Of Typical Reservoirs In The Yangtze River Basin

Reservoir greenhouse gas source and sink changes are a complex process,and multi-dimensional and multi-variable changes lead to great uncertainty in the greenhouse gas source and sink changes.It is of great scientific significance to reveal the characteristics and mechanisms of greenhouse gas sources and sinks in reservoirs,and to scientifically understand the contribution of human activities of dam storage to global climate change.In order to grasp the distribution characteristics of greenhouse gas carbon fluxes in typical reservoirs in the upper reaches of the Yangtze River,and to clarify the influencing factors of CO₂ and CH₄ concentrations in the reservoirs,this paper selects five small dams in the upper reaches of the Yangtze River Basin,Xiluodu,Xiangjiaba,Three Gorges and Shizitan.Two typical reservoirs with different nutritional statuses were used as the research objects.The static box method and the thin boundary layer（TBL）model method were used to quantitatively estimate the pre-flood（early May to early June）,flood season（mid-July to early September）,and post-flood（From mid-November to the end of December）CO₂ and CH₄ fluxes at the water-air interface of the reservoir,analyze the temporal and spatial characteristics of CO₂ and CH₄ concentration in the surface water and the diffusion fluxes of CO₂ and CH₄ at the water-air interface,and study the main environmental parameters for the reservoir CO₂ ,CH₄ For the influence of CH₄ concentration,the value of the gas mass transfer coefficient k at the water-gas interface of the reservoir was estimated using the existing empirical formula,and a k model suitable for this type of reservoir was established.The main research conclusions of this paper are as follows:（1）The CO₂ concentration range of typical reservoirs in the Yangtze River Basin is 8.46～92.74μmol·L^-1,which is the highest in the Three Gorges Reservoir,and the lowest in the Xiaoba secondary reservoir.Reservoir CH₄ concentration range is0.011～1.143μmol·L^-1.The CH₄ concentration is the highest in the Three Gorges Reservoir,and the lowest in the Xiluodu Reservoir.In the year,there were significant changes in different periods,with the overall flood season being the highest and lower after the flood.The CO₂ flux at the water-gas interface ranges from-5.72～93.85mmol·（m²·d）^-1,with an average value of 24.81±16.12 mmol·（m²·d）^-1,which is lower than the global average CO₂ of reservoirs,and the overall performance It is the“source”of atmospheric CO₂ ,the highest in the Three Gorges Reservoir,and the lowest in the Xiaoba secondary reservoir;The CH₄ flux at the water-gas interface is 0.005～1.094mmol·（m²·d）^-1,with an average value of 0.120±0.182 mmol·（m²·d）^-1.Compared with the global typical reservoir,the CH₄ flux The whole is at a relatively low level.It is the“source”of atmospheric CH₄ throughout the year,the highest in the Three Gorges Reservoir,and the lowest in Xiluodu Reservoir,which changes significantly in different periods of the year,and the overall flood season is higher than that after the flood season.（2）The water temperature of typical reservoirs in the Yangtze River Basin is between 8.5～30.4℃,and the concentrations of DO,DOC,DTN and DTP are between:2.89～20.11,0.31～4.87,1.45～4.09,0.006～0.117 mg·L^-1,Chl-a concentration is between0.42～78.22μg·L^-1.Reservoir CO₂ concentration is significantly negatively correlated with DO and Chl-a.Chl-a characterizes the primary productivity of water.The higher the Chl-a concentration,the greater the photosynthesis of phytoplankton,resulting in the reduction of CO₂ concentration.It has a significant effect on CO₂ ;CO₂ concentration is significantly positively correlated with DTN and DTP.An increase in the concentration of nitrogen and phosphorus in the water can improve the primary productivity of aquatic plants,and increase the respiration of the water,causing the CO₂ concentration in the water to rise.The concentration of CH₄ in the reservoir is significantly positively correlated with DOC and DTP.DOC provides a reaction substrate for the production of CH₄ .Phosphorus input can provide a substrate for aquatic activities,accelerate the respiration of aquatic organisms,reduce the DO of the water,and form an anaerobic environment.As a result,the methanogenesis is promoted and the concentration of CH₄ in the water is increased.However,CH₄ is continuously oxidized by DO during the upward diffusion process and is significantly affected by water depth.Therefore,CH₄ concentration is significantly negatively correlated with water depth.（3）The carbon flux at the water-gas interface depends on the difference in greenhouse gas concentration and the value of the gas mass transfer coefficient k.The difference in greenhouse gas concentration represents the direction and intensity of gas exchange at the water-gas interface.The magnitude of the exchange depends on the gas mass transfer coefficient k,which is critical for determining the source-sink relationship of greenhouse gases in aquatic ecosystems.The k₆₀₀of the five typical case reservoirs ranged from 6.9～42.9 cm·h^-1(mean:24.1±7.1 cm·h^-1),of which the k₆₀₀of the Xiaoba,Xiluodu,Xiangjiaba,Three Gorges and Shizitan reservoirs The ranges are:6.9～29.0,16.2～30.9,16.2～33.1,15.5～39.5,11.4～42.9 cm·h^-1.The k₆₀₀values of CO₂ and CH₄ are similar,but on the whole,the k₆₀₀value of CO₂ is slightly higher than that of CH₄ .The k₆₀₀values of CO₂ and CH₄ both show that the flood season is about twice that after the flood.The CO₂ and CH₄ flux values calculated by the established k₆₀₀model are compared with the CO₂ and CH₄ flux values calculated by the static box method,and it is found that the coefficients of variation CV of the CO₂ and CH₄ flux values calculated by the two are more than 90%.In this paper,through sampling and monitoring analysis of typical reservoirs in the Yangtze River Basin,the monitoring results as a whole"source"of atmospheric CO₂ and CH₄ ,are of great significance for estimating the greenhouse gas flux and annual net emissions of the reservoir.Research remains the key to future greenhouse gas research in reservoirs.