Effect of increased hypolimnion water temperature on stability of thermal profiles in stratified lakes

The results demonstrated a distinct increase in temperature during the summer stagnation in the water layer below 7 meters of depth in the experimental part of the lake. There, the temperature of near-bottom water increased by 5.6⁰C from June to September, while in the control part was just 0.4⁰C. Higher temperatures of the hypolimnion waters meant that the thermal profiles were less stable. Cooler waters at the bottm were removed through the pipe and replaced with warmer waters, which were previously found in the upper hypolimnion or in the metalimnion. Studies on the hourly temperature variation in vertical profiles showed that the increase in temperature in the experimental part was very dynamic. In 5 days, despite evident thermal stratification (the thermal gradient 4.0⁰C/m), the temperature at the bottom in this part of the lake increased by 0.3⁰C, while in the control remained at the same level. Such considerable warming of the hypolimnion waters and interference with natural thermal systems may have significant influence on the duration of summer stagnation, the rate of metabolic processes and, consequently, the rate of oxygen consumption. Keywords: water temperature, stratified lakes, hypolimnetic withdrawal, lake restoration;