Vertical mixing in Überlingersee (Lake Constance) traced by SF6 and heat

Vertical mixing in Uberlingersee is studied by releasing sulfur hexafluoride (SF6) as a tracer at a central hypolimnic depth of 60 m and measuring its subsequent vertical dispersion over a period of three months. The experiment started with a streaky tracer injection of 1 liter gaseous SF6 (STP) in August 1990. At that time the lake showed a typical strong summer stratification which in a weakened form lasts until November. From the SF6 profiles of fifteen surveys at three sampling sites vertical diffusivitiesK z are calculated compensating internal seiche displacement and horizontal tracer loss. Except of the bottom region no sampling site or time period is marked by significant differences in the hypolimnicK z profile. So vertical mixing in the whole Uberlingersee is described by mean diffusivities decreasing from 1.7 cm2/s at 120 m depth to 0.4 cm2/s in 30 m. The minimal value of 0.3 cm2/s in the thermocline region at 20 m depth is only based on observations in autumn. For a strong summer stratification it is certainly lower. The gradient-flux-method for heat was applied to compute a meanK z (T) profile from continuously measured temperature profiles. Significant differences resulting from the two tracers showed, that theK z (T) values are underestimated by up to a factor of 5 if cooling by lateral exchange is neglected. Particularly, internal seiche pumping of colder water from the adjacent Lake Obersee over the separating sill of Mainau into the deep Uberlingersee basin is observed in 1990 from August onward, obviously controlling the heat budget below the sill level.