Deepwater dynamics and spatial heterogeneity observed in the deep hypolimnion of a large lake (Lake Geneva)

In many deep lakes, global warming is weakening wintertime convective cooling, thus reducing the occurrence of complete vertical overturning. At the same time, our understanding of the deepwater dynamics in deep lakes remains elusive, mainly because spatiotemporally resolved in situ measurements are lacking. We address this knowledge gap by exploring the dynamics in the deep hypolimnion of Lake Geneva (max. depth 309 m) by means of extensive field observations.

Due to its great depth and the mild central European climate, Lake Geneva remains weakly stratified during most years, with complete convective overturning only occurring during severely cold winters. Recent studies show that three-dimensional (3D) transport processes, such as cold-water density currents, coastal upwelling, and wind-driven interbasin exchange significantly impact the dynamics in Lake Geneva’s deep hypolimnion, contributing to its ventilation.

From February to July 2021, five moorings equipped with Acoustic Doppler Current Profiles (ADCPs), current meters, thermistors and Dissolved Oxygen (DO) loggers were deployed at different locations and depths across the ~300-m deep central plateau (~12 km × 6 km) of Lake Geneva. One mooring remained in place until December 2021.

The nearly year-long measurements show frequent, large temperature peaks of ~0.03-0.15°C that last ~1-10 d at ~300-m depth, indicating significant isotherm tilting and downward transport of warmer waters, corresponding to vertical excursions of ~30-80 m. During those events, near-bottom DO levels temporarily increase by ~1-2 mg l^-1. Furthermore, the different mooring sites reveal large spatial heterogeneity across the 300-m deep plateau, both in the magnitude and temporal variability of the observed peaks.

From February to December 2021, a mean warming of ~0.07°C was observed at 300-m depth. Over longer periods, a “saw-tooth” pattern was previously found in deep lakes, which consists of continuous warming over several years, interrupted by sudden cooling during particularly cold winters (not the case during our campaign). In contrast, mean DO levels at 300-m depth first increase during spring, stagnate in early summer, and then gradually decrease until late fall/early winter.

Rotary spectra of the current velocities in the deepest layers show a broad peak in the clockwise-rotating component close to, but below the local inertial period (~16.5 h), in agreement with recent findings of dominant clockwise-rotating inertial currents in Lake Geneva’s deep hypolimnion. However, rotary wavelet analysis further reveals that the broad peak in the clockwise spectra is composed of several distinct frequency bands, concentrating mainly at ~16 h and ~12-14 h. The latter is close to the internal Poincaré wave period, as reported in the literature, indicating that both inertial currents and near-inertial internal waves are important.

Altogether, these preliminary results demonstrate that Lake Geneva’s deep hypolimnion is surprisingly energetic and characterized by a strong spatial heterogeneity that can only be explained by large-scale 3D flow features, challenging the classic one-dimensional concept of deepwater renewal in large, deep lakes. In the next step, a validated 3D hydrodynamic model will be used to further investigate the observed temperature/DO peaks and trends, the ever-present oscillating currents, as well as determine the origin of these processes.