Biodiversity, the present ecological state of the Aral Sea and its impact on future development

Abstract

The Aral sea used to be the fourth largest lake in the world. Its catchment area is huge, two main rivers (Amu Darya and Syr Darya) feed the lake. The balance of hydrological regime changed drastically after 1960 due to regulation of both main rivers and diversion of water for agricultural irrigation and intense cotton production. Salinity increased and most of invertebrate and fish species disappeared. A significant drop of water level has been recorded in the past 20 years and Aral Lake is presently divided into a small northern lake basin and a larger south basin. Kokaral dam construction resulted in increased water level and decreased salinity. Many invertebrate species reappeared in Small Aral and fish returned from Syr Darya river. Ecological situation in Large Aral is different, eastern part of this basin is completely dried out. The data on salinity levels, some chemical characteristics and above all the data about zooplankton, zoobenthos and fish in Small Aral have been recorded and presented in the article. Salinity ranges between 1 and 8 g/L, the lowest is near the river inlet. Five species of zooplankton (Keratella quadrata, Brachionus plicatilis, Evadne anonyx, Calanipeda aquaedulcis, Cyclops vicinus) and rotifers from the genus Synchaeta are very abundant, ten species are less numerous and seven summer species very rare. Different zoobenthos species are present, but only four abundant (Hediste diversicolor, Chironomus plumosus, Syndosmya segmentum and Cyprideis torosa). Zoobenthos mainly consist of Polychaeta, Mollusca, Crustacea and Diptera. The highest diversity was found near the Kokaral dam. Many fish species are commercially important: 14 of them are abundant, including endemic bream Abramis brama orientalis, Chalcalburnus chalcoides aralensis, carp Cyprinus carpio aralensis, and Aral roach Rutilus rutilus aralensis. White-eye bream Abramis sapa aralensis, silver carp Hypophtalmichthys molitrix, orfe Leuciscus idus oxianus, and snakehead Channa argus warpachowskii are less numerous. Aral barbel Barbus brachycephalus brachycephalus and Turkestan barbel Barbus capito conocephalus remain very rare. It can be concluded that significant positive changes occurred after Kokaral dam construction. Particularly, biocenoses and the Aral lake environment have been improved and fisheries returned. Today Kazakhstan Government is discussing an idea to improve this dam and dike and we support this discussion and advise to make it higher. All can lead to improve ecological state of the Small Aral.