Variations And Mechanism Of Thermal Stratification In A Eutrophic Lake

Among several influential factors,the geographical position and depth of a lake determine its thermal structure.Thermal stratification accelerates the bloom of cyanobacteria,especially in eutrophic water bodies.In temperate zones,shallow lakes show significant differences in thermal stratification compared to deep lakes.Here,the variation in thermal stratification in Lake Taihu,a super eutrophic,shallow lake,is studied systematically.(1)Corresponding to the seasonal variation of solar radiation,the thermal stratification in Lake Taihu is strong and lasting in summer,stronger in spring than winter,and the weakest in fall.Lake Taihu is a warm polymictic lake whose thermal stratification varies in short cycles of one day to a few days.The thermal stratification in Lake Taihu has a shallow depth in the upper region and strong temperature gradient,the maximum of which exceeds 5 °C m-1 The water temperature in the entire layer changes in a relatively consistent manner.Therefore,compared to a deep lake at similar latitude,the thermal stratification in Lake Taihu exhibits fewer seasonal differences,but the wide variation in the short-term becomes important.Shallow polymictic lakes share the characteristic of diurnal mixing.But differences on the duration and frequency of long-lasting thermal stratification are still found in these lakes,which may result from the differences of local climate,lake depth and fetch.(2)Quantifying the strength of thermal stratification,Mixed Layer Depth(MLD)is widely used.Here Kara's optimal definition and the traditional difference criterion are compared in the level of ?T =0.8,0.5 and 0.2 ?.It was found that Kara's method performs poorly on processing the shallow inversion layer in Lake Taihu,because the MLD changes without continuity and stability.However,this problem does not exist in the traditional difference criterion.As for ?T,the MLD shows how thermal stratification develops and disappears when?T ?0.2 ? by performing some weak stratifications that doesn't appear when 0.5 or 0.8 ? is chosen.Therefore,the traditional difference criterion with ?T =0.2 ? is applied to calculate MLD in Lake Taihu.(3)A prominent response of thermal stratification to weather conditions is found,being controlled by the stratifying effect of solar radiation and the mixing effect of wind disturbance.Other than the diurnal stratification and convection,the rePresentative responses of thermal stratification to these two factors with contrary effects are also discussed.When solar radiation increases,stronger wind is required to prevent the lake from becoming stratified.A daily average wind speed greater than 6 m s-1 can maintain the mixed state in Lake Taihu.(4)Wind-induced convection is detected during thermal stratification.Due to lack of solar radiation,convection occurs more easily in nighttime than in daytime.Convection occurs frequently in fall and winter,whereas long-lasting and stable stratification causes fewer convections in summer.