The Characteristics Of Thermal Stratification And Its Influencing Factors In Deep Lake And Shallow Lake

Lake thermal stratification and thermodynamic circulation are important factors that restrict the physical and chemical processes(e.g.,dissolved oxygen distribution,sediment release),and the surface and bottom layers of water flow and convection.It is the most important physical process of the lake,which has an important role on biological metabolism and material decomposition,profound impact on the lake's ecosystem,and affect the lake biological production and ecosystem evolution of the important indicators.Therefore,a comprehensive understanding on lake thermodynamic characteristics has great significance.However,at present,study on the thermodynamic characteristics of lakes is still relatively scarce,and further research is needed.In order to understand the thermodynamic characteristics of lakes and their influencing factors,this paper focuses on the thermodynamic characteristics of deep lakes and shallow lakes,using high frequency observation data in Qiandao Lake and Taihu Lake to analyzes the long-term thermal stratification processes,daily thermal stratification processes,temporal distribution characteristics and main influencing factor.The main conclusions are as following:(1)The research results showed that the water temperature of Qiandao Lake gradually increased from February to August,and then gradually decreased from October 2014 to September 2015.The lake water isothermally was found in winter and there is thermal stratification in spring,summer and autumn.During the period,the annual thermal stratification process was divided into four stages,the mixing period: from January 28 to March 27;the formation period: from the end of March to the end of June;the stable period:from early July to mid-September;the depression period: from mid to late September to the end of January.The stratification start time was March 28,and the end time was January 27,and the thermal stratification lasted for 310 days.There was a significant positive correlation between temperature and surface water temperature(0-2 m),which was significantly correlated with middle water temperature(19-21 m)and lowest with water temperature(39-41 m).The main factors influencing the thermodynamic process of water were the water temperature and transparency.The surface water temperature(0-2 m)had a significant negative correlation with the thermocline depth,and had a significant positive correlation with the thermocline thickness,and had no significant correlation with the thermocline intensity.(2)The thermal stratification parameters of Qiandao Lake were calculated based on the vertical distribution of the physical and chemical parameters collected from 60 sampling sites covering the whole Lake Qiandaohu in July 2014 and May 2015,and then the spatial and temporal distribution characteristics and main influencing factors were analyzed.The resultsshowed that the vertical distribution of water temperature in Lake Qiandaohu showed a positive temperature distribution,and there were different degrees of thermal stratification for Lake Qiandaohu in spring and summer.Notable increasing trends were recorded for the depth,thickness and strength of themocline from the upstream Xin'anjiang river mouth to the downstream open waters,and the trends were pronounced in July than that in May.Our results further suggested that water temperature,transparency and water depth were the main factors affecting the thermocline depth.Significant and negative linear relationships between the thermocline depth and water temperature at the surface layer(0-2 m),but positive linear relationships between the thermocline depth and transparency,and between the thermocline depth and water depth were found in summer with the stable thermal stratification.(3)The annual variation of thermal stratification in Taihu Lake in 2015 is the most obvious in summer,followed by autumn.In the spring and winter,the Taihu Lake is basically in mixed state.According to the monitoring of Taihu Lake in June and July 2016,we can know that the days of thermal stratification in Taihu Lake in June and July 2016 was 4 days and 16 days respectively.The duration of the daily thermal stratification is 2-10 hours in June,more than 10 hours in July.There were differences in the appeared time of extreme temperature in different depths of Taihu Lake.The appeared time of extreme temperature in the water temperature from bottom layer is later than that in surface layer.(4)Through the monitoring of the artificial high-frequency vertical profile of Taihu Lake in July 22,2016,the results showed that the trend of the thermocline depth was opposite to the thermocline thickness,which was consistent with the trend of the thermocline strength.The vertical distribution of water temperature displayed that the daily variation of the water temperature from surface layer in Taihu Lake was the largest,followed by other layers,the bottom being the smallest.The main influencing factors of the daily stratification in Taihu Lake were temperature,wind speed,solar radiation and algal blooms.The temperature had a positive effect on the daily stratification in Taihu Lake.The wind speed had a reverse effect on the daily stratification in Taihu Lake.The presence of daily stratification in Taihu Lake will hinder the transmission of dissolved oxygen from the surface layer to the bottom layer of water.The vertical temperature difference will increases,which lead to the content of dissolved oxygen of bottom layer reduced accordingly.