Dynamics of vertical mixing in the hypolimnion of a deep lake: Lake Geneva

Abstract

Based on 6 yr of high-resolution temperature profiles taken monthly in the 300-m-deep Lake Geneva (Lac Leman), we examine the hypothesis that vertical mixing is the dominant mixing mechanism in the hypolimnion. Using the flux gradient method, we estimate vertical turbulent mixing coefficients K z for periods of 1 month, the summer warming season, and over the multiannual continuous heating trend observed in the lake. The hypolimnion can be divided into two distinct layers: above 90 m and below 90 m. A correlation between K z and the Brunt-Vaisala frequency N 2 is found in the upper hypolimnion down to ∼ 90 m, indicating that wind-induced vertical mixing is dominant there. Once a thermocline has developed at a depth of ∼20 m the heat gain below 90 m becomes very small, and stratification there is stronger than in the bottom layers of shallow lakes. In the deep hypolimnion we observed large-scale temperature inversions and intrusions, suggesting that mixing is noncontinuous and cannot be adequately described by vertical mixing alone.