Thermal stratification and its influence factors in a large-sized and shallow Lake Taihu

Thermal stratification is an important factor controlling nutrient cycling and phytoplanton production in the aquatic ecosystem.To highlight the stratification characteristic of water temperature and its influence factors in large-sized and shallow lakes,the annual variation of stratification of water temperature was investigated in Lake Taihu using the thermal chain.The analysis of 6-layer water temperature data at the 2.5-m water depth shows that the vertical difference of water temperature in Lake Taihu is generally ranged between 0 and 1℃,but short-term ones also could reach high values between 1 and 4℃,even over 4℃ under the extreme condition.Furthermore,the vertical temperature difference in Lake Taihu varies with seasons,months and days,and the corresponding thermocline depth also changes.The many factors such as solar radiation,air temperature,wind speed,water density,specific heat and cyanobacterial bloom and others impact on thermal stratification.With the increase of solar radiation intensity,the vertical profile of water temperature rises at first and then dips.The maximum temperature appears when the peak solar radiation value of about 700 W/m² occurs.The thermal stratification weakens as the wind speed increases.However, the stratification will be stabilized when the wind speed exceeds a threshold value of 4m/s.This is especially true in summer and autumn seasons.The quicker the change in air temperature is,the larger the difference in water temperature.The inversion layer may occur during cold winder seasons due to the variation of water density and specific heat. The period of strongest stratification occurs in summer due to cyanobacterial blooms.The study indicates that transient thermal stratification can be found in a large-sized shallow lake that is subjected to frequent hydrodynamic disturbances.The stratification characteristics of water temperature are affected by meteorological conditions.The study provides insight into the internal nutrient release process and highlights the mechanism in cyanobacterial blooms.It also provides valuable information for creating the lake ecological model.