Lake Pavin mixing: New insights from high resolution continuous measurements

Abstract

As a meromictic lake, Lake Pavin mixing is very specific. The chemocline located at about 60 m depth separates the mixolimnion (fully or partially mixed according to the year) and the monimolimnion. Deep layers are geothermally heated and stability is ensured at the bottom of the lake by the increasing dissolved substance concentration. The monimolimnion forms a compartment which has its own specific dynamics but that may interact with the mixolimnion at large time scales. Understanding of physical mixing processes is crucial to study further geochemical processes. Temperature and turbulence were investigated in 2006 and 2007 using continuous measurements, a CTD and a high resolution temperature microstructure profiler (SCAMP). Continuous measurements give the evidence of a sublacustrine spring discharging intermittently into the mixolimnion around 55 m depth. This cold water input was observed using thermistor chains at different depths in 2007. Because of its low saline content, the spring water input rises in the water column by saline convection. The use of a simple conceptual model, representing turbulent diapycnal diffusivity and convection shows its role in maintaining the meromixis characteristic of the lake on the intra-annual time scale. The spring also influences seasonal overturns and thus contributes to establish the depth of the mixolimnion-monimolimnion interface on the interannual time scale. Using SCAMP, vertical dispersion coefficients are estimated by different methods. Vertical dispersion coefficients show a high space and time variability. The use of these data in the geochemical model AQUASIM applied to Lake Pavin shows a variability of model outputs directly depending on mixing inputs and their variability.