Mechanisms of long-term variations in the thermal structure of a warm lake

Analysis of a 39-yr record (1969-2008) of temperature profiles from Lake Kinneret, Israel, revealed that average epilimnion thickness decreased by ~ 1.2 m (~ 3 cm yr^-1), metalimnion thickness decreased by between ~ 1.0 m (December) and ~ 2.0 m (April), and average temperature of the epilimnion increased by, 1°C (~ 0.028°C yr^-1). Average hypolimnetic temperature remained constant (~ 15°C), so that the thermal gradient across the metalimnion increased. The average duration of the stratification period (~ 286 d) remained unchanged. We show that the stratification changes were the result of a decrease in lake level, a slight increase in air temperature during the spring and summer, and a reduction of inflows to the lake. These changes in time affected the stratification pattern in the same direction, i.e., an increase in the lake epilimnetic temperature and an increase in the thermal gradient across the metalimnion. Analysis of the data indicates that the effect of overpumping, which leads to a reduction in lake level, is larger than the effect of changes due to air-temperature increase and the reduction in inflow volumes.