Behavior of a Medium-Sized Basin Connected to a Large Lake

Abstract

Regular measurements—extending over about one year—of water temperature, of electric conductivity, and of the concentration of the stable isotope helium-3 are analyzed with regard to the lateral exchange of water between the main part of Lake Constance—the Obersee—and its smaller neighboring basin, the Überlingersee. In the absence of a significant density stratification (as occurs in late winter and early spring), different wind speeds over the two basins cause different vertical mixing leading to horizontal concentration gradients between the basins. Wind-induced or density-driven horizontal currents are not strong enough to completely reduce the gradients. In contrast, during periods of strong vertical stratification (summer and autumn), internal waves extending along the whole lake cause periodical horizontal currents and mixing and thus lead to homogeneous concentration distributions along the horizontal axis. The seasonal variation of horizontal gradients couples the nutrient balance of Überlingersee to the chemistry of the main lake.