Thermal stratification dynamics in a large and deep subtropical reservoir revealed by high-frequency buoy data.

We measure the thermal stratification dynamics in Lake Qiandaohu, China, a deep subtropical reservoir, to better understand the mixing mechanism and its response to lake warming. A high-frequency monitoring buoy dataset from February 2016 to October 2017 is used to evaluate variations in the water temperature profile, Schmidt stability (SS) and thermocline parameters, such as the thermocline depth (TD), bottom depth (TB), thickness (TT), and strength (TS), and elucidate the potential effects of thermal stratification on the lake's ecosystem. High-frequency observation data demonstrate that the lake's thermal-stratification cycle can be divided into three stages: formation, stationary and weakening periods. Consequently, a significant positive correlation between the TB and TT during the formation period and a significant negative correlation between the TD and TT are found during the stationary and weakening periods. Additionally, strong positive correlations exist among the TS, TT and SS for all the data. Our data indicated that an increase in the air temperature caused the surface water temperature, TT, TS and SS to increase. Furthermore, thermal stratification affected the vertical distribution of dissolved oxygen and expanded the area of the hypoxic-anoxic zone. The incomplete mixing of the water from December 2016 to February 2017 because of the high air temperature, which was 2.49 °C higher than the mean air temperature of 1966-2015 (6.44 °C), created the hypoxia hypolimnion from March to May 2017. Under the background of global warming, the thermal stratification of Lake Qiandaohu will likely intensify and further significantly affect the lake's ecosystem.