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UP: Umweltphysik

UP 9: Poster: Hydrosph
äre

UP 9.4: Poster

Dienstag, 14. März 2006, 14:00–16:00, C

Environmental tracers indicate exceptional mixing event in Lake Lugano, Switzerland — •Christian P. Holzner¹, Werner Aeschbach-Hertig², Marco Simona³, and Rolf Kipfer^1,4 — ¹Water Resources and Drinking Water, Eawag, Switzerland — ²Institute of Environmental Physics, University of Heidelberg — ³Ufficio Protezione e Depurazione Acque, Bellinzona, Switzerland — ⁴Institute of Isotope Geochemistry and Mineral Resources, ETHZ, Switzerland

The deep northern basin of Lake Lugano (z_max = 288 m, mean residence time ∼14 yrs) showed a permanent stratification over the last few decades due to strong eutrophication. Seasonal mixing only reached down to 60 - 100 m water depth and the deep water below was stagnant and anoxic. In winter 1999/2000 first signs of a decrease of the stratification stability were observed. The very long, cold and windy winter of 2004/2005 finally lead to a “complete overturn” of the northern basin of Lake Lugano, i.e. to a massive deep water exchange and drastic changes in the distribution of oxygen and nutrients in the water column.

Lake Lugano was monitored using noble gases and other environmental tracers (like SF₆) since 1990. Here we compare measurements from 2005 (shortly after the mixing event) with earlier data. Helium concentrations decreased in the deep water because terrigenic ⁴He and tritiogenic ³He that had accumulated in the deep water were transfered to the shallower water and partly released to the atmosphere. SF₆ increased close to atmospheric equilibrium concentrations in the whole lake. Therefore, environmental tracers clearly indicate a considerable deep water renewal and gas exchange in winter 2004/2005.