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Environmental problems of Northern Eurasia

The Aral Sea

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Lake Changes

Significant human impact on the biota of the Aral Sea predates the changes wrought by declining inflows because introduction of aquatic species to the lake, both deliberate and accidental, date from the late 1920s (Glazovsky, 1995b). This human-induced increase in aquatic biodiversity went into reverse following the changes to lake hydrology induced by the increased off-takes of water from the Amudarya and Syrdarya for crop irrigation. The average water level, surface area, and volume of the Aral Sea steadily declined from the 1960s as average salinity rose. Over the period 1960-90, the average water level fell by more than 15 m. The surface area and volume of the Aral in 1990 was reduced to about one-third of the 1960 values as salinity increased by a factor of three over the period (Table 22.3).

Hydrological parameters of the Aral Sea

Table 22.3 Hydrological parameters of the Aral Sea (1960-2000)

In 1989-90, the declining water levels meant that the Aral became split in two to produce a 'Big Aral' in the main lake basin and a 'Little Aral' to the north (Figure 22.4).

Changes in the surface area of the Aral Sea

Fig. 22.4 Changes in the surface area of the Aral Sea. After UNEP (1992a)

These abrupt changes in hydrology have had a significant impact on biodiversity in the Aral Sea (Williams and Aladin, 1991). The mid-1960s saw the first negative effects of higher salinity on fish when salinity increased to 12-14 per cent in shallow coastal areas, adversely affecting fresh-water fish spawning grounds. Salinity in the open parts of the sea reached similar levels between 1971 and 1975, causing most of the fresh- and brackish-water species to perish, and completely inhibiting reproduction among the large majority of all fish species. During the period 1976-85, the ichthyofauna were relatively stable at salinities of 14-20 per cent and the everyhaline species increased in numbers.

During the second half of the 1980s, mean salinity exceeded 23 per cent and numbers of zoobenthos, zoo-plankton, and phytoplankton fell. Just five species offish remained - the bullhead, flounder, sprat, stickleback, and atherina - from twenty species present in 1960. Many algae also disappeared and only one species of shield plant survived, while green algae species and benthic diatoms increased greatly. In 1960, eleven species of ostracod were known to inhabit the Aral Sea, some of which must have arrived from the Caspian during a previous phase of high water level. The human-induced increase in salinity has extirpated ten of these ostracod species, so that just one species survived in the mid-1990s (Boomer et al., 1996).

The physical and ecological degradation of the Aral has had several negative effects on local economies. Transportation routes across the lake have been severely affected, and the ports at Muynak on the southern shore and Aralsk to the north have become defunct as water levels have receded to leave the harbours dry and the towns landlocked. Commercial fishing in the Aral has also declined rapidly. The annual Aral fish catch fell from 40 000 tonnes in 1962 to half that tonnage by 1967. In 1975, landings barely reached 3000 tonnes and by 1980 the Aral Sea commercial fishery had ceased to function (Glantz et al., 1993; Letolle and Mainguet, 1993).

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