Deep-water renewal in Lake Issyk-Kul driven by differential cooling

During two field campaigns on Lake Issyk-Kul in March and August 2001, 179 high-resolution conductivitytemperature-depth profiles were measured, along with profiles of dissolved oxygen and light transmission. On the basis of this extensive data set, we investigated the large-scale vertical advective processes responsible for deepwater renewal in Lake Issyk-Kul. At some locations sampled in March, variable peak structures in the tracer profiles were observed that indicate horizontally localized intrusions. These intrusions have their origin in density plumes that propagate along the bottom of the channels in the eastern shelf region. The cold dense water at the bottom of the channels is most likely generated by differential cooling of the water in the shallow shelf regions during winter. During summer, vertical advective processes appear to be limited to the upper 200 m, where upwelling in the open water leads to a doming structure. The August data support the results of earlier investigations that suggested that dynamic forcing by a basinwide gyre is responsible for the upwelling in the open water. Lake Issyk-Kul, which is located at an altitude of 1,606 m in the Tien Chan mountains of northeast Kyrgyzstan, is one of the largest (1740 km3) and deepest (668 m) lakes in the world. It is a closed basin lake and is slightly saline, with 6 g kg21 of dissolved ionic substances (Tsigelnaya 1995). Despite its depth, the lake has large, shallow shelf regions, predominantly in the east and west (Fig. 1a). Twenty-seven percent of the lake (by area) is shallower than 50 m. During previous periods of low lake level, the area of the lake was therefore considerably less than it is now. The river valleys that were cut into the land surrounding the lake at this time were later submerged as the lake level rose and are now easily recognizable on the bathymetric map of Lake Issyk-Kul (Fig. 1a) as channels cut into what are now the shallow regions of the lake (Tsigelnaya 1995). More than 50% of the water volume of the lake is located below 200 m depth. Hence, the exchange between surface and deep water is an important factor in determining the distribution of dissolved substances in the lake and must be taken into account in assessing the ecological impact of pollutants and nutrients possibly introduced by spills (associ1 To whom correspondence should be addressed. Present address: University of Konstanz, Limnological Institute, Mainaustr. 225, D78457 Konstanz, Germany ([email protected]). 2 Present address: Department of Applied Aquatic Ecology, Lake Research Center, Swiss Federal Institute of Environmental Science and Technology, Kastanienbaum, Switzerland. 3 Also affiliated with Isotope Geology, Department of Earth Sciences, Swiss Federal Institute of Science and Technology, Zurich,