Study On The Dynamics And Metabolic Rate Of Dissolved Oxygen In Wuliangsuhai During The Ice-coverd Period

More than half of the lakes in the world have obvious icing processes.The ice layer covers the lake surface and isolates the lake water from the air and cannot complete atmospheric reoxygenation.At the same time,the ice layer greatly reduces or even isolates the solar radiation entering the sub-glacial water body and inhibits the photosynthesis of phytoplankton and submerged plants.Under the action of continuous respiration,chemical action and the oxygen consumption of sediment,the frozen lakes often present the state of continuous decrease of dissolved oxygen or hypoxia,which will worsen the water quality and threaten the ecological security of water,especially the eutrophic lakes,especially in eutrophic lakes,this process will be more severe.The distribution of the main seasonal frozen areas in China is highly coincident with the arid and semi-arid areas.For example,in the northwest of Inner Mongolia and the Qinghai-Tibet Plateau,the climatic conditions in winter are significantly different from those in previous studies.It is still unclear how the oxygen environment of lakes changes under freezing conditions and what factors restrict it.Based on these understanding,the paper takes Wuliangsuhai Lake in Inner Mongolia as an example to carry out in-situ temperature and oxygen observation experiments and sediment oxygen consumption experiments of two typical shallow lakes in cold and arid areas in winter to analyze the dynamic changes of temperature and oxygen in lakes and their main influencing factors,and after calculating the rate of dissolved oxygen metabolism in the water body,the mechanism discusses the influence of weather,ice and snow conditions on the dynamics of dissolved oxygen.The main research results are as follows:（1）During the freezing period,both water temperature and dissolved oxygen have obvious diurnal changes.The daily variation range of water temperature at different depths is between 0.1-3.5℃,the daily variation range of water temperature decreases and then increases with increasing depth;During periods of non-snow cover,the daily variation range of dissolved oxygen at different depths is between 0.3-6.5 mg L^-1.Studies have shown that water temperature is not only affected by solar radiation,but also ice temperature and mud temperature.On the whole,the upper water temperature has a strong temperature inversion structure,and the middle and lower water temperature has a weak or nearly isothermal structure,the dissolved oxygen concentration in the water gradually decreases as the depth increases.During the entire freezing period,the total amount of dissolved oxygen in the water body decreased first and then increased,which is closely related to the intensity of photosynthetically active radiation（R²=0.73）.（2）A model for the migration of dissolved oxygen at the ice-water interface during the freezing and melting of lake ice has been developed.During the freezing period,the dynamic changes of dissolved oxygen caused by the freezing and melting of lake ice account for about2%of the photosynthetic oxygen production or respiratory oxygen consumption.However,during the entire growth period of the lake ice,freezing oxygen can increase the dissolved oxygen concentration of the water under the ice by 3.5-6 mg L^-1as a whole.By introducing an oxygen migration model in the freezing and thawing process,a model for calculating the rate of dissolved oxygen metabolism in lake ecosystems was established.Through this model,the photosynthetic oxygen production rate 0.053-5.32 mg O₂L^-1d^-1and the respiratory oxygen consumption rate 0.06-6.24 mg O₂L^-1d^-1of the lake during the freezing period were calculated;There was a significant positive correlation between the respiratory oxygen consumption rate（R²=0.78）.The analysis shows that the photosynthetic active radiation intensity is the dominant factor of photosynthesis（R²=0.77）;the oxygen consumption test results of sediments show that the oxygen consumption of sediments accounts for 25%-40%of the total oxygen consumption of ecosystem respiration.（3）Snow will increase the albedo of the lake surface and decrease the transmittance and transmitted radiation intensity.During the period when there is no snow on the lake ice,the albedo is between 0.30 and 0.37.When the lake is covered with snow,the albedo can reach up to 0.87;the transmittance of the lake ice before snowfall is 0.27,and the photosynthetic oxygen production rate is 2.28 mg O₂L^-1d^-1;After snowfall,the transmittance drops to 0.004and the photosynthetic oxygen production rate drops to 0.31 mg O₂L^-1d^-1.With the melting of snow and lake ice,the intensity of transmitted radiation in the water and the photosynthesis of plants began to increase.Based on the above understanding,this paper proposes two ways in which the ice and snow process affects the dynamics of dissolved oxygen in subglacial lakes:First,the effect of lake ice freezing and oxygenation and thawing dilution;Second,the thickness of snow and ice layer and its optical properties control the solar radiation transmission process.These research conclusions help us better understand the dynamic changes of dissolved oxygen in seasonal frozen lakes.