Vertical exchange due to horizontal density gradients in lakes; the case of Lake Lucerne

Abstract

Recent field investigations have shown that differential mixing causes a significant density driven exchange between the two eastern basins of Lake Lucerne, Gersauersee and Urnersee. The long term vertical mixing characteristics during the winter mixing period and the influence of the resulting exchange flow on water quality are examined through the analysis of ten years (1964 to 1974) of monthly limnological data. Vertical homogenization of the state variables (temperature, electric conductivity and density) occurred on average once every two years in Urnersee, but not once in Gersauersee where the ten year average winter mixed-layer depth extended to 120 m. In Urnersee intense mixing was evident over the whole water column for every year. In contrast intense mixing in the deeper layers of Gersauersee occurred approximately once every four years. A three layer model shows that theT/κ20 signature of the Gersauersee intermediate water correlates best with the Urnersee deep water confirming the regular occurrence of a density driven exchange. For the lower layers the oxygen consumption rate in Urnersee was approximately twice that for Gersauersee. It appears the exchange flow can act either as a source or sink for the oxygen balance of Urnersee deep water. The larger rate in Urnersee hypolimnion might also be associated with the larger sediment input to this system. Density driven exchange resulting from differential mixing is probably an important contribution to the vertical water replenishment in many lakes.