Lake Issyk-Kul is the largest in the mountains of Central Asia and is one of the
world's deepest. The lake was formed in the lower Carboniferous and, being located in a
tectonically active region, experienced frequent level changes caused by tectonic
movements and associated regional climate change and the varying regime of glaciers
(Tsigelnaya, 1995). Extensive archaeological and historical data confirm that numerous
transgressions, often accompanied by an overflow, occurred between the 2nd century BC and
the 1st century AD, about the 4th-6th, 7th-8th centuries, and between the second half of
the 18th and the first half of the 19th centuries. Regression phases followed the
transgressions, most notably during the 14th and the 16th centuries when the lake level
was lower than now. The 20th century was a period of falling lake level. This reduction is
caused primarily by natural variability of water flow in the lake tributaries which is
enhanced by water abstraction for economic needs (Ratkovich, 1993; Tsigelnaya, 1995). With
respect to the hydrochemistry of the Issyk-Kul, there are a number of features which this
lake does not share with other large lakes. In particular, its open part is characterized
by a uniform horizontal and vertical distribution of the main ions, which indicates a good
water exchange between various areas of the lake and the development of a deep circulation
(Tsigelnaya, 1995). The mean salinity of the Issyk-Kul is 5 ppt which puts it at the
fresh-water end of saline lakes. Apparently in the early stages of lake development, its
water was fresh but now it has entered a period of gradual salt accumulation and
increasing sulphate concentrations. This indicates a transition from the hydrocarbonate
sulphate to the sulphate type (Tsigelnaya, 1995).
The Issyk-Kul drains a high mountainous catchment with low biotic production and the
annual river discharge is low in comparison with the lake volume. The discharge to volume
ratio is only 0.2 per cent for the Issyk-Kul while for the Balkhash it is 13.4 per cent
(Tarasov, 1961). As a result, the supply of nutrients and organic matter is low and the
Issyk-Kul is a typical mountainous oligotrophic lake with low biological productivity. It
has only twenty two fish species, ten of which have been introduced into the lake, and the
mean fish catch is 1.9 kg ha-1. This is marginally lower than in the other
large unproductive lakes, such as Baikal, Ladoga, and Onega, and much lower than in the
Balkhash, Sevan, and Chudskoe, where it varies between 7 and 30 kg ha-1
(Tsigelnaya, 1995).
