Insights into the dynamics of the deep hypolimnion of Lake Geneva as revealed by long-term temperature, oxygen, and current measurements

In order to identify or shed light on dominant long-term processes of the deep hypolimnion of Lake Geneva (309 m depth), time series of temperature and horizontal currents and profiles of temperature and oxygen, taken for over a decade in the deepest part of the lake, were analyzed. The focus was on the summer stratification period (May to October). During that period, temperatures near the bottom always increased quasi-linearly with the same gradient and small amplitude variability. Vertical mean temperature gradients in the lowest 60 m of the water column remained nearly constant over the years and were comparable to those of the deep ocean. Mean current velocity at the deepest point was 3.0 +/- 0.5 cm s(-1) and ever-present inertial motions clearly dominated currents. Oxygen decreased quasi-linearly and eutrophication appeared not to affect this rate of decrease. It is suggested that as in the deep ocean, breaking internal waves and friction from decaying inertial motion occurring in the deep hypolimnion strongly contribute to turbulent mixing in that layer. The climate change-induced long-term warming trend seen in the upper layers of the lake was not detected in the deep hypolimnion. Instead, year-to-year climate change-induced variability strongly affected the deep hypolimnion. During cooling in cold winters, lateral advection contributed more to temperature decrease and oxygen renewal in the deep hypolimnion than vertical convection. The present analysis has shown that the dynamics of this layer are highly three-dimensional and that the processes occurring therein cannot be correctly described by traditional one-dimensional concepts.