Hypolimnetic density currents traced by sulphur hexafluoride (SF 6 ) Jürg W. Schlatter*, Alfred Wüest, and Dieter M. Imboden Environmental Physics, Swiss Federal Institute of Environmental Science and Technology (EAWAG) and Swiss Federal Institute of Technology (ETH), CH-8600 Dübendorf, Switzerland Key words: Lake Lucerne, sulphur hexafluoride, lakes, density-driven flow, inter- basin exchange, differential mixing, tracer. ABSTRACT The artificial tracer sulphur hexafluoride (SF 6 ) has been used to study the density-driven deep water exchange between two sill-separated basins of Lake Lucerne, Gersauersee and Urnersee. The sources of the density gradients between the two basins are (1) salinity differences between the major inlets due to the different geology of their drainage areas, and (2) temperature dif- ferences due to spatial variation of wind forcing. Wind speeds are generally larger in Urnersee, especially in spring during the so-called Föhn events, when winds blow from the south. In contrast, Gersauersee is protected form these winds. In spring 1989, a total of 630 g of SF 6 was released at 80 to 120 m depth in the small Treib Basin located between Urnersee and Gersauersee. During about 100 days the distribution of SF 6 in the lake was determined by gaschromatography. Two models are used to quantify the exchange flow, (1) a one-box mass balance model for SF 6 in the deep part of Treib Basin, and (2) a one-dimensional diffusion/advection model describing the tem- poral and vertical temperature variation in Urnersee. According to the first model, the flow into the deep hypolimnion of Urnersee, decreases from 21 · 10 6 m 3 · d –1 at the end of March to about 8 · 10 6 m 3 · d –1 in late April. The second model yields similar flow rates. The decrease of the flow rate during spring, confirmed by both approaches, is consistent (1) with the decreasing strength of the density gradient above the sill during spring and early summer, and (2) with hydrographic information collected in Lake Lucerne during other years. 1. Introduction In fall 1985, physical investigations were carried out in Lake Lucerne (Switzerland) to study the simultaneous influence of river and wind induced mixing (Wüest et al., 1988). Since the wind exposure of two of the several basins, both about 200 m deep, is significantly different, Lake Lucerne appeared to be an ideal object for such a study. The wind-exposed upstream basin, Urnersee (Fig. 1), is known for its year- long rather large oxygen concentration in the whole water column, while the adja- * Present Address: Swiss Federal Office of Metrology (OFMET), CH-3084 Wabern-Bern, Switzerland. Aquat.sci.59 (1997) 225 – 242 1015-1621/97/030225-18 $ 1.50+0.20/0 © Birkhäuser Verlag, Basel, 1997 Aquatic Sciences