Boundaries

The ecoregion includes river drainage areas and lakes of the Black Sea coast in Russia, Georgia, and Turkey from the Sukko rivulet (north of Novorossiysk) to the Yesil Irmak (Yesilirmak) River basin (exclusive). In the north the ecoregion border is contiguous with the Kuban River drainage [428] along the main divide of the Greater Caucasus Range, and in the east by the western slopes of the Central Caucasus and Likhskiy [Likhs K’edi], Meskhetskiy [Meskhet’is K’edi], and Arsianskiy [Arsianis K’edi] divide ranges. In the south it is contiguous with the Upper Tigris and Euphrates ecoregion [442] along the Pontic Range [Dogukaradeniz Daglari], and in the west it borders the Northern Anatolia ecoregion [430].

Topography

Divide ridges (Suramskiy Ridge and others) separate the river network of Transcuacasia into two unequal parts: eastern (large) and western. The basin of the Black Sea includes rivers flowing down the southern slopes of the western part of the Greater Caucasus, Suram Ridge, and Adzharian-Imeretin ridges. These rivers are characterized by large dips and considerable water volume. There are more than 200 rivers and streams, their length varying considerably from less than 1 km to 288 km. Mountain rivers, such as the Bzyb, Kodori, and Inguri, flow through narrow, yet steep dipping rifts throughout much of their length. Only near the confluence with the Black Sea do they spread into the plain, yet still retain their fast flow and character as mountain rivers. Fifty-one percent of the Rioni drainage is situated in a mountain locality. The lower reaches of the Rioni and other rivers of the Colchis lowland and Kakhaber plain are of plain character, and have low flow velocity. In some areas the lowlands are swampy, and have many floodplains and delta lakes.

The Rioni River is the largest river of Western Transcaucsia. The Rioni rises from glaciers on the southern slopes of the Great Caucasus. In the upper section up to Kutaisi the river flows along a wild, nearly inaccessible rift. Downstream from Kutaisi it flows into an extensive swampy lowland that abruptly changes the character of the river’s flow. Here it flows through a meandering channel that is split into branches and tributaries, forming numerous sand islands. In the floodplain there are many saucer lakes. The length of the river is 288 km, and the area of the entire catchment is 13,500 km². As with all rivers of the region, the Rioni is characterized by relatively high water volume. Even though its basin area is 1/14 the size of the Kura, the average annual water discharge of the Rioni is 430 m³/sec, only slightly lower than the Kura.

There are many lakes of different origin throughout the ecoregion. First are lakes such as Ritsa and Amtkelskoye that have been formed by the stemming of rivers by rockslides, and have inflow and outflow with a pronounced current. Next are lakes in the lowland zone that are close to the sea coast. For example, Lake Bebesyr was formed as a result of stemming of the Dzhakoba River 8 km from its mouth. Lake Inkit is a remnant of a sea bay and is situated 330 m from the coast, but is a freshwater lake. Lake Paliastomi, located on the Mengrel lowland near Poti, Georgia, is a shallow lagoon situated in the estuary of the Rioni 2 km from the sea coast, and connected to the Rioni through a stream (now connected by a number of artificial anabranches). The lake water salinity was approximately 13‰ (the lake is being freshened considerably now because of water inflow from the Rioni River).

A characteristic hydrographic feature of the ecoregion is the considerable development of karst phenomena. The limestone-karst region of western Georgia extends as a wide band along the southern slopes of the Greater Caucasus, covering river drainages from the Bzybi to the Rioni. Many rivers passing through the karst zone are lost in limestone cavities and then resurface again in powerful form. For instance, the Shaora River in the drainage of the Rioni River is lost among limestone, but over 2 km downstream it resurfaces from a rocky cave in a form of a wide current named Sharpula. Another example is the Tkvibula River in the drainage of the Inguri River.

Freshwater habitats

Rivers of the southern slopes of the Greater Caucasus originate in the region of glacial and permanent snow, and then fall rapidly along steep slopes and into deep and narrow ravines encumbered by stones and rock fragments. Many of them in the upper reaches cascade like waterfalls. The main characteristic feature of rivers in the alpine zone is the high flow velocity attaining 3—4 m/sec, low temperature, and high oxygen content. Food supply for fishes is poor in terms of quality and quantity.

In the coniferous forest zone rivers flow through deep and narrow ravines with vertical walls and steep channels. This creates high flow velocity that decreases slightly downstream. As the altitude declines in the middle zone the rivers form additional parts, such as small creeks with slow currents, small arms, glides, and rifts. Water temperature rises, oxygen content in water declines, and the diversity and number of aquatic organisms increase.

Downstream in the lower reaches rivers enter the lowland; there they flow in wide valleys and their current is calm. Due to the decrease of flow velocity, transport strength of rivers is weakened. After the spring flood pebble and sand are deposited in the river channel, forming islands, glides, backwaters, and bayous.

The Rioni River carries approximately eight million tons of suspended alluvium annually. During flooding of the plains, turbid waters of the Rioni River deposit a large amount of mud. As a result, the river channel is formed by its own alluvium and is elevated above the adjoining surroundings.

Terrestrial habitats

Based on climatic and geobotanical characteristic features, the ecoregion is subdivided into four vertical zones: 1) the lower zone to an altitude of 600—800 m above sea level, with a humid subtropical climate and dense forests of Colchis type; 2) the middle zone at an altitude from 800 to approximately 1350 m, with a temperate climate and beech-chestnut forests; 3) the upper storey of coniferous forests (fir and spruce are predominant) with more severe climatic conditions; and 4) the alpine zone from approximately 2000 m up to the lower boundary of the nival zone (where it is pronounced), with sparse growth of trees, subalpine, and alpine meadows.

Description of endemic fishes

Riverine fishes of Western Transcaucasia display a notable isolation. The following true fluvial species are endemic to this ecoregion: Turkish brook lamprey (Eudontomyzon lanceolata) (Kux & Steiner 1972), Colchic nase (Chondrostoma colchicum) (Derjugin 1899), Eurasian minnow (Phoxinus colchicus) (Berg 1910), and riverine goby (Neogobius rhodioni) (Vasiljeva & Vasiljev 1994). Near-endemic fluvial species include Rhodeus colchicus (Bogutskaya & Komlev 2001), Colchic khramulya (Capoeta sieboldii) (Steindachner 1864), Anatolian khramulya (C. tinca) (Heckel 1843), Cobitis satunini (Gladkov 1935), and N. constructor (Nordmann 1840). It should be mentioned that the separation among the hydrographic systems of the Caucasian ecoregions, and accordingly the formation of their freshwater fauna, began in the Upper Miocene-Lower Pliocene due to the formation of the Caucasian Island and then Peninsula, connected with the Fore-Asian Massif.

The Turkish brook lamprey (Eudontomyzon lanceolata) (Kux & Steiner 1972) is a poorly known brook lamprey that needs additional study. The assignment of this lamprey from the northeastern Black Sea coast to E. lanceolata is based only on unpublished molecular data of Nick Lang. It is also commonly identified as E. mariae. Its larvae attain a length of 140 mm before metamorphosis (by the fourth to fifth year of life).

The Abrau sprat (Clupeonella abrau) (Maliatsky 1928) is restricted to a single small lake, Lake Abrau, which is a warm and turbid lake, 10 m in depth and 84 m above sea level. However, it is different from the Black Sea sprat (C. cultriventris) in body shape and number of gill rakers. It is a pelagic fish that feeds on crustaceans such as copepods and mysids by following their daily vertical movements. It swims in surface layers at night and descends during daylight hours. It spawns in open water and reaches a maximum age of 2 years. According to some personal communications it is still numerous in the lake.

Rhodeus colchicus (Bogutskaya & Komlev 2001) was described recently based on some morphological differences from the common bitterling (R. amarus). It is interesting that the range of the species is restricted by the southern part of the ecoregion and is quite remote from the range of the common bitterling. This supports the Transcaucasian fauna’s clear isolation from the fauna of the northern coast of the former Paratethis Sea.

The Colchic nase (Chondrostoma colchicum) (Derjugin 1899) is a pure riverine fish that prefers a slow current and pebble bottom overgrown with algae to feeds on. It does not migrate, but spawns in the shallowest areas of streams and rivers. It spawns much earlier than the barbels, from March to early May at temperatures between 12 and 15 °C.

The Eurasian minnow (Phoxinus colchicus) (Berg 1910) is a small minnow with pronounced sexual dimorphism. It prefers small streams with less current. This fish is known by its interrupted range, and is absent entirely in some rivers of the ecoregion (along the coast). For example, in western Georgia it is only present in some parts of the Coruh system, then it is found in Korola to Kintrishi, and further to the northwest it is only in the Kodori and Psou rivers. The species needs to be studied further, and may involve several distinct species.

Cobitis satunini (Gladkov 1935) is a typical spined loach that prefers stagnant to slowly running waters with a sandy or silty bottom. There is presently no data on its biology, and its taxonomy needs further clarification. It is commonly considered conspecific with "Cobitis taenia satunini" from the Kura River (Caspian basin).

Neogobius constructor (Nordmann 1840) and riverine goby (Neogobius rhodioni) (Vasiljeva & Vasiljev 1994) are rare examples of pure riverine gobies that prefer fast current foothill and mountainous streams. They are not sympatric - the former is distributed in coastal rivers in the south from the Bzybskiy Ridge (Psyrtskha to Chorokh), and the latter is distributed north from the Bzybskiy Ridge (rivers of Abkhasia and Russia). They are rather close in biology – both are benthic dwellers; lay adhesive eggs that are deposited on stones; the males guard eggs until hatching; and both feed on a wide variety of invertebrates. Both species had been considered conspecific for quite a long time before N. rhodioni was described. However, the taxonomic status and distribution is still unclear.

Other noteworthy fishes

It is worth mentioning that two species of the genus Capoeta (khramula) are distributed in the ecoregion, showing clear affinities between the Western Transcaucasia and Asia Minor fish fauna. The khramulas are typical inhabitants of slow-flowing and stagnant waters, and feed on algae and detritus (and have a very long intestinal tract).

The Rioni River was known as the last river within the Black Sea basin that had a native population of Atlantic sturgeon (Acipenser sturio). However, there are no reliable data on the fish since the 1990s.

The Caucasian sturgeon (Acipenser colchicus) (more commonly known as A. persicus colchicus, but may represent a distinct species according to Kottelat & Freyhof, in press)  may be almost extinct (or extinct). It was distributed in the southeastern Black Sea basin, Rioni drainage, Georgian and Turkish coast, as well as the lower Danube. It is now only occasionally recorded off the Georgian and Turkish coasts. Individuals of this species (subspecies) may be confused with A. gueldenstaedtii.

Alosa tanaica palaeostomi (Sadowsky 1934) is a semi-anadromous shad that is only distributed in the water bodies of the Kolchis Plain. It spawns in fresh and brackish waters, mainly in Lake Paliastomi. Its real status is poorly known, however, as it has become rare due to construction and alteration of the water regime of Lake Paliastomi.

Ecological phenomena

The ecoregion is characterized by a pronounced vertical zonation in the dispersal of fishes. In the alpine zone, brook trout (riverine form of Salmo trutta labrax) is the only species comprising the fish populations of many mountain lakes where trout can attain high numbers. In the forest zone (down to the foothill zone), apart from trout, other mostly rheophilous species appear, such as the European chub (Squalius cephalus), Barbus, Colchic nase, spirlin (Alburnoides fasciatus), Barbatula, and riverine gobies. The fauna of the foothill zone includes bitterling, both species of loaches, Pontian tube-nose, and monkey gobies. In the lower reaches the fauna differs by the presence of mostly phytophilous species. They are typical inhabitants of slow-flowing rivers and stagnant waters, and many of them are migratory, semi-migratory, or euryhaline (species of the genera Alosa, Esox, Rutilus, Abramis, Scardinius, Petroleuciscus, Gasterosteus, Knipowitschia, Perca, Sander, and Mugilidae family).

It is also worth mentioning that smaller rivers of the ecoregion may vary considerably in species composition due to more or less recent local ecological events, such as drying up, a mudflow, or local pollution. Besides these, the fish fauna in each stream strongly depends on historical exchanges since, at present, exchanges of freshwater species are difficult because of the increased salinity of coastal sea water.

Justification for delineation

For ecoregions of the former Soviet Union, a species/genera/family presence/absence matrix was compiled for a hierarchy of hydrographic units, and cluster analysis and ordination techniques (Primer v.6 statistics software) were employed to assess biotic similarities among hydrographic units and to identify major faunal breaks.

This ecoregion is clearly distinct from the other Caucasian ecoregions by the main divide ridges of the Caucasus. It overlaps the rivers of the Black Sea on the southern side of the Greater Caucasus Range. In terms of paleohistory, the separation is well supported by the isolation from the Ciscaucasian drainage areas and the rivers of the Caspian Transcaucasian basin since the Miocene to Early Pliocene.