Greenhouse Gas Dynamics And Its Influencing Mechanism In Shallow Lake Ulansuhai

Lakes are an important source of greenhouse gas emissions to the atmosphere,and the eutrophication they face increases the uncertainty in the estimation of greenhouse gas emissions from lakes,especially in eutrophic shallow lakes.Therefore,clarifying the impact of eutrophication on GHG dynamics can help the accuracy of regional carbon budgets and can also provide data to support the control of carbon in the management of eutrophic lakes.In this study,we selected the eutrophic shallow lake Ulansuhai in the arid region as the research area,set up seven sampling sites in different areas,and investigated the greenhouse gas dynamics at the water-air interface and sediment-water interface of the Lake Ulansuhai and its influence mechanism through field campaigns and laboratory experiments.The main results showed as follows:（1）The surface CO₂ partial pressure（p CO₂）in Lake Ulansuhai showed obvious spatial and temporal differences.The p CO₂ varied from205.6 to 2042.1μatm,with a mean value of 655.8±225.0μatm,which was higher than the atmospheric CO₂ equilibrium partial pressure,and nearly85%of the samples were supersaturated,with unsaturated samples found mainly in July and October;spatially the adjacent pier area with higher primary productivity had the lowest p CO₂.The p CH₄ varied from 5.3 to1754.4μatm,with a mean value of 125.7±20.4μatm,which was 66 times that of the atmospheric CH₄equilibrium concentration.The ice cover in January resulted in significantly higher p CH₄ than that in other months,and in July p CH₄ was also at higher levels;spatially p CH₄ was significantly lower in the inlet area than at other sampling area.（2）The fluxes of CO₂（FCO₂）and CH₄（FCH₄）at the water-air interface in Lake Ulansuhai showed significant spatial and temporal variability.FCO₂ varied from 32.6 to 285.3 mmol m^-2 d^-1,with a mean value of 22.1±7.1 mmol m^-2 d^-1,with January accounting for 63.3%of total emissions over the four months,while July had the lowest FCO₂ due to higher productivity.The mean value of FCH₄ was 5.4±0.9 mmol m^-2d^-1,with the highest FCH₄ in July accounting for 73.1%of the total flux in the four months,while the lowest in April accounted for only 1.2%;spatially the lowest value was in the inlet area and the highest value in the heavily polluted adjacent pier area;Lake Ulansuhai emit 1422.5 g C（CO₂-eq）m^-2 per year,of which CH₄ contributes 75.1%and with the degree of eutrophication increasing,the proportion of CH₄ contribution increased,indicating that CH₄ emissions are more important for climate change in eutrophic lakes.（3）The pCO₂ and p CH₄ in the surface waters of the Lake Ulansuhai are influenced by several water environment indicators.The results of multiple stepwise regression analysis showed that p H,TN,NO₃^--N,DIC,TP,Chl-a and DOC were the main controlling factors for p CO₂,explaining61.6%of the variation in p CO₂;the main controlling factors for p CH₄ were Bio,WD,NO₃^--N,TN,and TP in the water column,explaining 46.3%of the variation in p CH₄.Both FCO₂ and FCH₄ were significantly influenced by p CO₂ and p CH₄,and FCH₄ also had a significant negative correlation with air pressure.（4）The porewater CO₂ and CH₄ concentrations in Lake Ulansuhai showed significant spatio-temporal and depth differences.CO₂concentrations varied from 16.3 to 4905.0μmol L^-1 with an overall mean of 877.8±31.0μmol L^-1 and CH₄ concentrations varied from 0.16 to2297.1μmol L^-1 with a mean of 689.2±45.0μmol L^-1.Both CO₂ and CH₄concentrations were lowest in January and highest in July,and both concentrations increased with depth,spatially,the highest and lowest values of CO₂ were at adjacent pier area and inlet area,but for CH₄ the highest and lowest values were at aquiculture area and northeast area.（5）Diffusive fluxes at the sediment-water interface both showed significant temporal and spatial variability.CO₂ diffusive fluxes(F_S-WCO₂)varied from-1.65 to 5208.95μmol m^-2 d^-1 with an overall mean of 887.34±123.71μmol m^-2 d^-1;CH₄ diffusive fluxes(F_S-WCH₄)varied from-2.39to 2271.65μmol m^-2 d^-1,with a mean value of 607.13±68.04μmol m^-2 d^-1.Temporally,both F_S-WCO₂ and F_S-WCH₄ were lowest and highest in January and July,respectively;spatially the lowest fluxes were in the inlet area;F_S-WCO₂ and F_S-WCH₄ were 1/25 and 1/10 of the greenhouse gas fluxes at the water-air interface,respectively.（6）The porewater CO₂and CH₄ concentrations were regulated by nutrients in the porewater during the study period.The results of the multiple stepwise regression analysis showed that the main explanatory factors for the spatio-temporal and depth variation in CO₂ concentration were NO₂^--N,DOC,DTN and DTP,which explained 39.7%of the variation in CO₂ concentration;the main explanatory factors for the porewater CH₄ concentration were NO₂^--N,NO₃^--N,DOC and DTN,which explained 33.8%of the variation in CH₄ concentration.The diffusive fluxes at the sediment-water interface(F_S-WCO₂ and F_S-WCH₄)were influenced by the concentration of dissolved gases in the surface sediment,water quality parameters,and regression analysis showed that ST,DOC,NH₄⁺-N and NO₂^--N were the main controlling factors on F_S-WCO₂（R²=0.639）,while the main explanatory factors for F_S-WCH₄ were ST and DOC（R²=0.526）.（7）The overall trophic state of the Ulansuhai is eutrophic,with significant spatial and temporal variation in eutrophication levels,which is an important factor influencing greenhouse gas emissions.Eutrophication can significantly decreased CO₂ emissions from the lake but increased CH₄emissions,resulting in a CH₄ to CO₂ emission ratio of approximately 2 in the GWP,a process facilitated by sediment-water interface diffusion fluxes.Eutrophic shallow lakes in arid regions are an important source of CH₄,and eutrophication changes the pattern of variable greenhouse gas emissions.The results of the study provide data to support the control of carbon in the management of the lake.In summery,this study elucidated the influence mechanism of the GHG dynamics at the water-air interface and sediment-water interface of the shallow eutrophic lake Ulansuhai,and clarified the emission patterns of CO₂ and CH₄ in the eutrophic lake,as well as the promoting effect of the water-interface greenhouse gas flux on the water-air interface flux,which is of great significance to the scientific management of eutrophic lakes.