Boundaries

The ecoregion is waterless in the north, so its border with the Volga Delta - Northern Caspian Drainages [453] in the west and Aral Sea Drainages [629] to the east is rather formal. The ecoregion boundary in the north follows the geomorphology of the landscape. From Mangyshlak [Manghyshlaq] Bay the border runs along the northern spurs of the Severnyy Aktau Ridge, and further north the border runs along the northern border of the Ust’ Urt [Ustirt] Plateau along the edge of the Sor Karatuley ("sor" means a dried bay, may be marshy or just a shallow bay). The border with  the Aral Sea Drainages [629] runs along the political border between Uzbekistan (Republic of Karakalpakstan) and Turkmenistan. The western part of the ecoregion includes water bodies of the Uzboi Plain (an abandoned watercourse) below the Sarakamysh [Sariqamish Kuli = Sarygamysh Koli] Depression, which belongs to the Aral Sea Drainages ecoregion [629]. From here the border runs southwest to the Atrek drainage area (the Atrek River and Gorgan River basins are included in this ecoregion). In the southwest the border curves, thus excluding rivers of the northern slope of Kopetdag Ridge. In the south, the ecoregion is bordered by the El’ Burs Mountains.

Topography

Four-fifths of the ecoregion is approximately 500 m above sea level. The highest point is Airybaba peak (3139 m) and the lowest place is the Akdzhakaya Hollow in the Kara Kum Desert (81 m below sea level). Lowland areas along the Caspian Sea coast are often more than 20 m below sea level.

Freshwater habitats

The rivers of the ecoregion are characterized by all of the features of rivers formed in arid regions, such as low flow, uneven regime, and considerable fluctuations in the intensity of flow in the off-season. The remaining surface water sources are torrential, with a short period of snow and rain in March – April. River waters are predominantly turbid and shallow.

The Uzboi (Western Uzboi) is an ancient valley with a chain of brackish hollows and lakes in the deserts of northwestern Turkmenistan. It stretches from the Sarykamyshskaya Hollow up to the Caspian Sea, bounding the northwestern sand Kara Kum Desert. The length of the river is about 550 km, and the length of the river channel is 775 km. It has typical features of the channel of the former anabranch, where in ancient times the waters of the Amu Darya partially flowed (feeding Lake Sarykamyshskoye) into the Caspian Sea. Sections of the dried anabranch are occupied by salt marshes and saline lakes, and sometimes also by fresh water bodies (due to subterranean feeding). Freshwater lakes include Yaskhan, Togolok, and Topiatan. At present it has been established that the existence of these lakes is maintained by fresh waters accumulating in lenses beneath sand dunes, which are supplied by the flow running down the Bolshoi Balkhan and Western Kopet Dagh montains. The flow of fresh subterranean waters is sometimes so significant that in spite of evaporation (about 2000 mm year^-1), lake water is used by the local population, and also the flume (sluice) Yaskhan-Balkhanabad is supplied.

Downstream from the Yaskhan the character of the Uzboi is gradually changing. Crossing the Balkhan corridor from the northeast its channel cuts into extensive sand areas and salt marshes, eventually fusing completely with the latter. Of particular interest is the salt marsh Kel-Kor, which lies approximately level with the Caspian Sea, having been a bay of the latter quite recently.

The Atrek is a river in Iran and Turkmenistan. Its approximate length is 669 km, and its drainage is 27,300 km². The average intensity of flow near the urban settlement Kizyl-Atrek is 9.2 m³ s^-1. In the lower reaches the Atrek is divided into several anabranches. Water reaches the Caspian Sea only during floods; during the rest of the year it is used for irrigation. The major flow occurs during rainfall and melting snow. The average annual intensity of flow of the Atrek River is 8.6 m³ s^-1. It has since declined, and is at present approximately 5 m³ s^-1. The regime of distribution of flow is extremely uneven. During the year there may be five or more floods accompanied by torrential floods. The irregularity of flow is, to a large extent, regulated by a network of reservoirs. A tributary of the Atrek River is the Sumbar River. Its length is 245 km, and its basin is 8300 km². The average multi-year intensity of flow of the river in the estuary is 0.93 m³ s^-1.

The Gorgan River is 251 km in length, and lies mostly in the Gorgan Plain. It is regulated by the Vashmgir Dam and its reservoir; the water is used for irrigation.

Terrestrial habitats

A large part of the Ustyurt Plateau is covered by vegetation that is transitional from the subzone of the northern (wormwood-saltwort) deserts to the subzone of the southern (ephemer-wormwood) deserts. Atmogenic forms of relief, clayey flat spaces, extensive dry hollows, and dry channels of ancient and recent temporary streams are widespread throughout the ecoregion.

Description of endemic fishes

There are no strict endemic species in the Turan Plain.

Other noteworthy fishes

Despite the lack of strict endemic species, there are a couple of subspecies that are restricted to this ecoregion. Rutilus rutilus uzboicus represents a morphologically derived form of the roach (R. rutilus) that only occurs in Lake Top’yatan in the Uzboi Plain. It inhabits the lake since the time when there was a water connection through the ancient Lake Sarakamysh, which received water from the Amu Darya River and the Caspian Sea. It is not quite clear which species/subspecies it is closer to: R. caspicus or R. rutilus aralensis. The recent state of R. rutilus uzboicus needs further studies because water from the lake is used for industrial purposes.

The bream (Abramis brama) from Lake Yaskhan (Uzboi Plain) is represented by a peculiar dwarf form. Similar to Rutilus rutilus uzboicus, it represents a remnant from the time when there was a connection through the ancient Lake Sarakamysh, which received water from the Amu Darya River and the Caspian Sea.

Hyrcanogobius bergi is well-known from the Western-Northern Caspian Sea, especially near the mouths of the Volga, Ural, and Emba rivers. However, Ragimov (1977) reports this species from off Gasan Kuli at 37°30’N in Turkmenistan, and Shakirova and Sukhanova (1994) record this species from the Atrek lakes in Turkmenistan. Thus, it may also exist in this ecoregion. Miller and Pinchuk in Miller (2004) map this species from Gorgan Bay and adjacent waters, but this has not been confirmed by specimens.

Justification for delineation

This ecoregion, with few water bodies and rivers, represents the most arid part of the Transcaspian area. The arid rivers are characterized by low flow, uneven regime, and considerable fluctuations in the intensity of flow in the off-season. The fauna of inland water bodies is impoverished and mainly contains species that migrate or occasionally enter from the sea.

Level of taxonomic exploration

Good