Study On The Multi-media Existence Pattern And Driving Mechanism Of GHGs In Lake Wuliangsuhai

Lakes are an important source of atmospheric carbon dioxide（CO₂）,methane（CH₄）and nitrous oxide（N₂O）emissions,especially eutrophic shallow-water lakes in cold regions,the obvious seasonal climatic differences make the concentration and flux of greenhouse gases（GHGs）in lake waters have significant spatial and temporal variability characteristics.Seasonal frozen lakes in the north have a freezing period of up to 4-5 months.Due to the existence of the ice sheet,greenhouse gases gradually accumulate in the water under the ice and are concentrated and rapidly discharged at the ice off.Only a few carbon assessment studies of lakes include the ice cover period.With the development of climate change and global warming,the study of greenhouse gases in frozen lakes has received more and more attention.In this thesis,Lake Wuliangsuhai,a shallow eutrophic lake in the cold and arid region,was taken as the research object,carried out seven consecutive sampling surveys at 20sampling points in the open water area of the whole lake from January 2020 to January 2021.The concentrations of CO₂,CH₄ and N₂O in water and ice were measured by headspace equilibrium-gas chromatography and vacuum bag melting method,respectively.The thin boundary layer model method was used to calculate the diffusion flux of greenhouse gas water-air interface.To clarify the spatial and temporal variability of the dissolved concentration and diffusion fluxes of greenhouse gases in Lake Wuliangsuhai,and the driving mechanism of the impacts of ice sealing and eutrophication on the greenhouse gas dynamics,and to estimate the total annual diffusion emission of greenhouse gases in Lake Wuliangngsuhai and determine the warming effect of greenhouse gases in Lake Wuliangngsuhai,with a view to providing scientific and technological information for the study of greenhouse gas dynamics of the lakes and the regional and global carbon budgets.The study aims to provide scientific basis and data support for the study of lake greenhouse gas dynamics,regional and global carbon budget,climate change response and water environment protection.Based on this study,the main results are as follows:（1）The temporal and spatial differences in the dissolved concentrations of greenhouse gases GHGs in the water column of the Lake Wuliangsuhai were significant.Temporally,the concentrations of CO₂ and N₂O in the subglacial water body were significantly higher than those in the ice free,and the cumulative effect under the ice was obvious,with a gradual upward trend in the concentration of N₂O in the ice free period.There was no significant difference in the concentration of CH₄ in the water body under the ice compared with that in the ice free period,and the cumulative effect under the ice was weak.Spatially,N₂O concentration in the water body gradually decreased from the inlet at the northwest end of the lake to the outlet at the south end,CO₂concentration was higher in the northern part of the lake than in the southern part as a whole,and CH₄concentration was high in the northeast part of the lake in winter,while it was high in the southern part of the lake in summer and fall,which may be due to the distribution of aquatic plants and lateral diffusion and transport of gases.For greenhouse gas concentrations in lake ice bodies,CO₂:surface ice>bottom ice,CH₄ and N₂O:surface ice<bottom ice.GHGs concentrations in the ice body（in the growth direction of ice thickness）under ice sealing followed the same trend as the distribution of bubble content and nutrient concentration in the ice body.（2）Lake Wuliangsuhai is the source of atmospheric greenhouse gases between water-air interface,with CO₂ and N₂O coexisting as source-sinks in the ice-free period.For CO₂and CH₄,fluxes were significantly higher in summer than in autumn（CO₂:15.9±18.6mmol/m²/d and 4.1±18.5 mmol/m²/d,CH₄:5.7±4.2 mmol/m²/d and 1.3±0.9 mmol/m²/d）,and the fluxes of N₂O were higher in autumn（3.5±6.7μmol/m²/d）compared to summer（2.6±8.5μmol/m²/d）was higher.Comparison with the results of other water ecosystems showed that the CO₂ and N₂O fluxes of greenhouse gases in the Lake Wuliangsuhai were low compared with most lakes.Compared with 310 Chinese lakes in the review article,the average CH₄ flux was higher than 75%of the lakes.（3）The combined trophic index TLI of the Lake Wuliangsuhai was 57.1±5.5throughout the study period,with the highest eutrophication in January 2020,which was attributed to snow cover on the ice surface.Both GHGs concentrations and TLI were positively correlated,indicating that both eutrophication contributed to GHGs emissions.RDA analysis of GHGs concentrations and fluxes in relation to environmental variables in each season indicated that p H was the most critical factor influencing CO₂ concentrations and fluxes in summer,and SD was the most critical factor influencing CH₄ concentrations and fluxes in summer,with an explanatory rate of 42%.DIC and DO were the most critical factors DIC and DO were the key factors affecting CH₄ concentration and flux in autumn,and the explanation rate of DIC reached 51.3%.EC was the most critical factor affecting N₂O concentration and flux,and the explanation rate reached 57%,and EC indirectly affected the production of N₂O in the lake by affecting the transport and transformation process of N element in the water body.（4）The greenhouse gas emissions from the Lake Wuliangsuhai are characterized by accumulation during the ice cover period and coexistence of sources and sinks and slow release during the ice-free period.The total annual emissions of CO₂,CH₄ and N₂O in the Lake Wuliangsuhai were estimated to be 12.0×10³ t,1.9×10³ t and 2.0 t,respectively(the units in the text are 12.0 Gg CO₂ yr^-1,1.9 Gg CH₄ yr^-1 and 2.0 Ton N₂O yr^-1),indicating that the eutrophication of eutrophication of shallow lakes in cold and arid zones lake C is mainly emitted in the form of CO₂.The accumulation of GHGs during the the freezing period accounted for 57%,22%and 35%of the annual emissions,respectively,indicating that the freezing period is a critical period for GHGs emissions.CH₄ contributed the most to the total greenhouse effect of GHGs,which accounting for 83%of the in the Lake Wuliangsuhai.