Optics And Thermal Characteristics Of Wuliangsuhai During The Frozen Period And The Primary Productivity In The Water Under Ice

Lakes,located at high latitudes and high altitudes,experience an freezing and thawing process every year.During the freezing period,the water is affected by the temperature,and freezes from top to bottom,finally forms an ice sheet covering the entire lake.Compared with the ice-free lakes,the ice cover,as the most significant external change in the freezing period of the frozen lake,has an important impact on the original physical,chemical and ecological environment of the lake:With the growth of the ice sheet,nutrients such as nitrogen and phosphorus in the lake will migrate from the ice to the water,and the concentration of nutrients in the water will increase greatly;due to the existence of the ice sheet,the most direct material and energy exchange between the lake and the atmosphere is isolated,so that the dissolved oxygen in the lake water cannot be recharged by the atmosphere.At the same time,the ice cover changes the transmission of light to the water,affecting the light field environment of the water under the ice.All of these have a significant impact on the operation of the lake ecosystem during the frozen period.During the freezing period,the lack of oxygen in the water under the ice may cause fish and other animals to die.During the thawing in the next year,the large amount of nutrients stored in the water under the ice can easily cause large-scale rapid growth of phytoplankton,which may cause the outbreak of "water bloom" and other phenomena.As the global climate continues to warm,issues such as the environment and ecology of lakes located in high latitude are attracting more and more attention.Based on these,this paper takes the typical seasonal frozen lake Wuliangsuhai in the cold and arid regions of northern China as the research object.the optical characteristics and thermal characteristics of the Wuliangsuhai during the frozen period were analyzed by using the high-frequency monitoring data in the field.the primary productivity calculation model was improved,the primary productivity in the water under the ice was calculated and analyzed based on the high-frequency data of dissolved oxygen concentration.to provide a theoretical basis for lake ice closure research,water environmental protection and eutrophication control,the results of the study are critical for the management and the assessment of the frozen lake and the impact of the environment and climate change on the lake.Based on the research in this paper,the following results are mainly obtained:Firstly,the optical properties of Wuliangsuhai during the frozen period were analyzed.During the frozen period,due to the influence of the ice,the transmittance in the water under ice is 10-11%,and the transmittance near the water-sediment interface is 5-6%.During snow accumulation period,the transmittance in the water under the ice drops to about 1%,and the transmittance near the water-sediment interface is less than 1%.During snow-free period,the albedo of the ice is about 45%,while during snow-covered period,the albedo above the ice sheet is about 81%.The reflection at the water_sediment interface is about 1.3%-4%of the incident at the interface.The results show that the existence of ice affects the transmission of light to the water under the ice,and because of the existence of the snow on the ice,the reflection on the ice surface is enhanced significantly,while the incidence of water under the ice is greatly reduced.If there is no snow on the ice surface,the total incidence on the ice surface can penetrate the entire lake water body of Wuliangsuhai to reach the sediment layer at the bottom of the lake.The sediment layer has a deep absorption effect on photosynthetically active radiation,reaching more than 95%,the photosynthetically active radiation in the water under ice is mainly derived from primary radiation,and the contribution of secondary reflection is negligible.Secondly,the thermal characteristics of Wuliangsuhai during the frozen period were analyzed.The ice temperature,the temperature of the water under the ice and the heat flux in the ice were analyzed.The ice temperature and the temperature of the water under the ice during the frozen period showed a clear upward trend from the surface layer to the bottom layer.The temperature was clearly stratified in both the ice and the water under the ice,and there was a diurnal variation pattern.When there is snow on the ice,the temperature of the ice and the temperature of the water under the ice will be affected.The vertical heat transfer flux in the ice of Wuliangsuhai is mainly affected by temperature and ice thickness.the heat flux in ice decreases with the increase of ice thickness,and the heat transfer flux of surface ice is greatly affected by temperature.The results show that solar radiation has a significant effect on ice temperature and water temperature under ice.this effect is significant on diurnal and seasonal time scales.the existence of snow on the ice affects ice temperature and water temperature by affecting incidence.Finally,the evolution characteristics of dissolved oxygen in the water under the ice were analyzed,and based on this,the primary productivity in the water under the ice was calculated,and the influencing factors were analyzed.There is also a significant diurnal variation pattern and stratification phenomenon in the dissolved oxygen concentration in the water under the ice in Wuliangsuhai during the frozen period.As the depth increases,the dissolved oxygen concentration gradually decreases.Due to the influence of snowfall,the water-sediment interface may reach an anaerobic state,and as the snow slowly dissolves,the dissolved oxygen concentration slowly rises.During the ablation period of Wuliangsuhai,the ice-melting water will have a short-term dilution effect on the dissolved oxygen concentration in the upper water layer.Along with the enhancement of photosynthetically active radiation,the dissolved oxygen concentration in the water under the ice increases remarkably.When the photosynthetically active radiation is stronger thal 20Wm-2,the correlation between photosynthetically active radiation and dissolved oxygen is extremely strong(R2=0.97).Compared with the lower layer of water,the concentration of dissolved oxygen in the upper layer is more easily affected by photosynthetically active radiation.Water temperature is also one of the main influencing factors of dissolved oxygen concentration in the water under the ice.The lower the water temperature,the higher the concentration of dissolved oxygen in the water.The temperature of sediment-water interface in Wuliangsuhai is higher during the frozen period,which is close to the average temperature of the region.the suitability of temperature supports the decomposition in water and sediment,which consumes oxygen.There was an obvious diurnal variation pattern in the primary productivity(total primary productivity and net primary productivity)of Wuliangsuhai during the frozen period.Enhanced with the enhancement of photosynthetically active radiation(R2=0.776),and during the ablation period,the primary productivity in the water under the ice is significantly enhanced.During the snowfall,the primary productivity decreased significantly and reached a negative value.As the thickness of the snow decreased,the primary productivity gradually increased to a positive value,when the dissolved oxygen output was greater than the dissolved oxygen consumption.Due to the influence of snowfall,the respiration in the water under the ice briefly decreases sharply,but it will rise slowly.During the ablation phase,the respiration in the water under the ice is significantly enhanced.Overall,snowfall has less impact on the respiration in the water under the ice,and the time influenced is shorter.During the whole frozen period,the photosynthesis in the water under the ice had been ongoing,even during the period the photosynthetically active radiation in the water under the ice was below 0.2 Wm-2,the photosynthesis in the water under the ice continued.The changes of photosynthetically active radiation causes the change of water temperature under the ice,which affects the breathing in the water under the ice.