Thermal structure and response to long-term climatic changes in Lake Qiandaohu, a deep subtropical reservoir in China

Using the vertical temperature profiles of Lake Qiandaohu from January 2010 to April 2013, we evaluated the monthly and seasonal variations of water temperature and thermocline parameters, and developed empirical models among thermocline depth (TD), thickness (TT), and strength (TS). We also developed empirical models between TD, TT, TS, and surface-water temperature (0-2 m) (T0-2 (m)), and transparency (Secchi disk depth, SDD). Additionally, we assessed the changes in TD, TT, and TS over the past 62 yr, based on our empirical models, air temperature data from 1951 to 2012, and SDD data from 1987 to 2012. Lake Qiandaohu is warm monomictic, with a long period of thermal stratification from April until January, and only a short period of mixing in the winter or spring (February or March). There were significant correlations between SDD and TD (positive), and between SDD and TT (negative). There was a significant negative correlation between T0-2 m and TD during the stratification weakness period (July-February), and a significant positive correlation between T0-2 m and TT for all data, including the stratification formation and weakness periods. Air temperature near the lake rose 1.2 degrees C between 1951 and 2012, corresponding to a 0.8 degrees C increase in T0-2 (m), and a 0.78 m decrease in SDD between 1987 and 2012. The increase in air temperature and the decrease in SDD caused a decrease in TD and an increase in TT, facilitating the thermal stratification and stability of the lake; therefore, climate warming has had a significant effect on the thermal regime of Lake Qiandaohu.