Boundaries

This ecoregion encompasses basins of the Dniester and Danube rivers. It shares its western and southern borders with the following ecoregions: Upper Danube [417], Dalmatia [419], Albania [420], Vardar [422], and Thrace [423]. The ecoregion also shares borders with Central and Western Europe [404] and Dnieper (South Bug) [425] to the north. The northeastern border runs along the Dniester-Dnieper divide [part of 425] and the Podolskaya Vozvyshennost Hills, whereas the northwestern border touches the Ceski Les and Shumava ridges. The ecoregion also encompasses the Stara Planina Mountains, the main divide between the Danube drainage area (in former Yugoslavia) and the nearby rivers flowing into the Adriatic Sea, and the Danube deltaic area and coastal lakes at Konstanza (Romania).

Freshwater habitats

The Danube River has a very high water volume, with a mean annual flow of 6400 m³ per second into its estuary. Melting mountain snow, heavy rains, and groundwater feed the river, and floods occur during warmer seasons (February until August). The Danube is particularly shallow in September and October, before a potential freezing-over in January and February. Freezing does not regularly occur every year.

During the winter and warmer months, the Prut River experiences frequent flooding due to heavy rain. The Prut River freezes for only a short time—about 50 days—in January and February.

The Dniester River can be divided into three parts based on flood conditions, water regime, and physical-geographic characteristics: (1) from the river’s Carpathian source to the village of Kosuets in the Soroksky District, (2) the middle portion in Podol from the village of Kosuets to the town of Dubesar, and (3) the lower reaches from Dubesar to the estuary. 

The Carpathian region makes up a large part of the Dniester River system, encompassing approximately 50% of the river flow. Frequent torrential rain leads to numerous flooding events, and the river cuts through limestone ridges and reefs in its upper reaches. The Dniester River is lined by extremely thick and steep (nearly vertical) limestone rocks, which are eroded into stony deposits where the river meets the stone. The Podol region has little impact on the Dniester River regime. The area’s uneven, silt-sand or clay-like river bottom allows underground water to accumulate. The lower region of the Dniester River experiences low annual precipitation, and has a weak hydrographic network in a divided plain with a low slope. The river widens for tens of kilometers, and forms a number of meanders and floodplain water bodies.

Turbidity and water mineralization vary depending on the region’s rates of erosion and physical-geographical features. Within the limits of the mixed forest zone in the north and southwest of the ecoregion, average annual turbidity stays between 20-50 g/m3 (occasionally rising to 200-300 g/m3), and mineralization rarely exceeds 200 mg/l. In the forest-steppe zone, turbidity reaches 100-250 g/m3 (or up to 500 g/m3 within the limits of the Podolsk upland), and mineralization exceeds 600-1000 mg/l.

Along the coasts of the Black Sea are a large number of limans (i.e. lakes or estuaries formed at river mouths), including the Dniester liman in the estuary of the Dniester River, a group of Odessa limans, (Khadzhibeiskiy, Kuyalnitskiy, Tiligulskiy, Berezanskiy), Dnieper – Bug liman, and others. Many of them are salt (mineral) lakes with thick deposits of silt. The limans have various connections with the sea; some of them are isolated from the sea by sand spits, whereas others retain a connection with the sea through narrow straights ("girlas"). Limans that are situated in estuaries of large rivers have an open outlet into the sea and a free water exchange with it. Limans of the Black Sea coast are believed to be a consequence of the postglacial or recent subsidence of the Russian continental platform. The Dniester liman is shallow (approximately 350 km² in area), and is connected with the sea by two anabranches ("girlas").

Historically, the deltaic and liman systems include numerous interconnected water bodies, both fresh and brackish, as well as wetland areas. The diversity of biotopes created a wide spectrum of ecological conditions and habitats that supported high biological productivity as well as high diversity of flora and fauna. Along the northern shore of the Black Sea, because the water has such a low salinity, freshwater fish enter saline areas and are able to migrate between the sea and rivers – behaviors not associated with these same species elsewhere in Europe. 

Hydrography and salinity of many lakes and limans of the ecoregion have been changed due to human activity. For example, the Dniester Liman was freshened from the Danube via a canal, and Kagul Lake in the Danube delta is completely regulated by a locked canal.

Terrestrial habitats

The terrestrial ecoregions comprising the Dniester-Lower Danube ecoregion include Pannonian mixed forests, Dinnaric Mountains mixed forests, Balkan mixed forests, Carpathian montane forests, Central European mixed forests, East European forest steppe, and Pontic steppe. Durmast oaks tend to grow in carbonaceous areas along the banks (formed in interspaces between the rocks) of the Dniester River. Forest plantations—comprised of poplar, willow, and oak trees—have been planted in the face of widespread forest loss. The floodplain—with a silt-clay ground cover—is dominated by bogs and dense, moisture-loving plants. The Danube River basin includes high mountain chains, large plains, sand dunes, forested or marshy wetlands, and karst landscapes.

Description of endemic fishes

The ecoregion is home to one endemic (monotypic) genus, Romanichthys, and a number of endemic species such as Balkan loach (Cobitis elongata), Save sculpin (Cottus metae), sculpin (Cottus transsylvanicus), Carpathian lamprey (Eudontomyzon danfordi), Gasterosteus crenobiontus, asprete (Romanichthys valsanicola), Danube delta gudgeon (Romanogobio antipai), Banat sand gudgeon (R. banaticus), Romanian golden loach (Sabanejewia romanica), Golden spined loach (S. vallachica), and Petzea rudd (Scardinius racovitzai).

Asprete (Romanichthys valsanicola) is endemic to the Danube River drainage in Romania—mainly in the upper reaches of the Arges River, and its tributaries (the Vâlsan and Râul Doamnei rivers).  Though adjacent basins require further research, the species appears restricted to a 1 km-long area of the upper Vâlsan River. This nocturnal bottom dweller inhabits clear, cold, and fast running mountain streams, and likes to hide under stones.  Deforestation, road and dam construction, and stone exploitation have driven the species near extinction. In 1965, the construction of a dam on the Arges River caused the disappearance of the fishes. Damming, stone mining, overfishing, and pollution are major threats, and the species is currently the focus of an EU Life program.

Carpathian lamprey (Eudontomyzon danfordi), a predatory freshwater resident lamprey, inhabits the Danube drainage (Tisza and Timis systems) in Romania, Slovakia, Ukraine, and Hungary. It inhabits piedmont and montane zones in clear, well-oxygenated brooks. Adults feed on live and dead fish, and die after spawning.

Danube delta gudgeon (Romanogobio antipai) is distributed in the Lower Danube River (from its confluence with the Arges River to the downstream delta) throughout Romania and Ukraine. The (likely extinct) species prefers the deep waters of sandy-bottomed rivers.

Banat sand gudgeon (Romanogobio banaticus) inhabits the Timis, Tiza, Nera, Bega, Caras, Crisul Alb and Crisul Negru rivers of the Danube drainage. The species prefers fast-flowing sandy-bottomed rivers in the lower stretches of large and middle-sized tributaries. The species is locally threatened by development and organic pollution.

Bulgarian golden loach (Sabanejewia bulgarica) inhabits the middle and lower reaches of the Danube River, and is found near Bratislava in the lowest reaches of the river’s tributaries.  It also prefers deep water.

Romanian golden loach (Sabanejewia romanica) prefers sandy Romanian streams with clean water, and moderate to fast current. The species is found in the Danube basin, and the Arges, Olt, Jui, Mures and Tapolitza rivers.

Golden spined loach (Sabanejewia vallachica) is restricted to the Lalomita River and tributaries of the lower Siret River in the Danube drainage system. The species prefers sandy streams with clean water and moderate to fast currents.

Petzea rudd (Scardinius racovitzai) lives in a 10 000 m² region close to Oradea in Western Romania. The critically endangered species inhabits densely vegetated, shallow, and muddy hot springs.

Near-endemic species (for the entire Danube drainage area) mostly inhabit the upper reaches of the Danube River or rivers of the Baltic basin.  Some of these include Barbus waleckii, Cobitis elongatoides, Danubian brook lamprey (Eudontomyzon vladykovi),  Danube ruffe (Gymnocephalus baloni), schraetzer (G. schraetser), huchen (Hucho hucho), percarina (Percarina demidoffii), Kessler’s gudgeon (Romanogobio kesslerii), Danubian longbarbel gudgeon (R. uranoscopus), R. vladykovi, Rutilus virgo, Bulgarian golden loach (Sabanejewia bulgarica), mudminnow (Umbra krameri), streber (Zingel streber), and zingel (Z. zingel).

Balkan loach (Cobitis elongata) is also found in the following Danube drainage systems: the Nera River (Romania), Sava, Kupa (Slovenia, Croatia), Morava (Yugoslavia), Vit and Iantra (Bulgaria).  The species prefers moderate to fast-flowing shallow rivers (at least 6-15 m wide), with sandy or rocky banks and shores, and submerged vegetation.

Save sculpin (Cottus metae) inhabits the upper Save drainage in Slovenia. The species may inhabit parts of the upper Danube basin as well.

Cottus transsylvanicus was recently found in the upper reaches of the Arges River in Romania. The species loves cold, rushing rivers, and may be also present in other Danube tributaries. Some scientists believe sculpins were isolated in many of the lower Danube River’s tributaries during glacial periods.  Several additional species might remain undiscovered in this area.

Percarina (Percarina demidoffii) is a species locally distributed in marine waters of low (1-8‰) salinity and some limans. It differs from P. maeotica by scale pattern and coloration. Though it was thought to enter freshwater sections of lower reaches of rivers, it has never been reliably recorded from fresh waters. The species may now be threatened by increased salinity of the sea and reduction of suitable habitats; there have been no records of the fish in the last few years.

Streber (Zingel streber) resides in the Danube and Dniester river drainages. The rare species prefers to remain in the main courses of small to large-sized rivers, in stretches with strong currents and stony substrate.

Zingel (Zingel zingel) also inhabits the Danube and Dniester river drainages, alongside Z. Streber.

Other noteworthy fishes

The Umbridae family has one genus in Europe (Umbra krameri). North America has two (U. limi, U. pygmaea), as well as one monotypic genus (Novumbra hubbsi). The Umbridae family contains distinctive, small fishes with short snouts, teeth rooted in jaws, fins without spines, a rounded caudal fin, and no adipose fin.  Their pelvic fins are situated in the middle of their bodies, whereas the dorsal and anal fins are located behind the pelvic fin region.

The only species in the European portion of the ecoregion—European mudminnow (Umbra krameri)—inhabits the Danube drainage from Vienna to the delta and lower reaches of Dneister drainage. The species prefers densely vegetated water bodies, usually in small ditches, oxbows, backwaters and shallow lakes. Mudminnows are vulnerable to encroachment, but are still fairly common in some localities. 

The ecoregion is also home to some endemic subspecies such as Carpathian gudgeon (Gobio gobio carpathicus) and Vit sculpin (Cottus gobio haemusi). Carpathian gudgeon inhabits the Tiza River in the Danube drainage. Further research is needed for its taxonomic status and exact distribution. Vit sculpin is only known to inhabit the Danube basin in Bulgaria, although it may occur in the Beli Vit River and other lower Danube tributaries. It was recently re-established as a distinct species.

Ecological phenomena

The Danube basins and Dniester River support an extremely diverse fauna in terms of ecological peculiarities. Species include fishes that migrate long distances, as well as anadromous species and resident swamp-dwellers. Ripe individuals of Acipenser, Huso, and Alosa species enter the lower reaches of the Danube, where they still have some spawning sites, and their offspring return to the delta and liman. The conditions in the ecoregion are favorable for such euryhaline species that inhabit both fresh and saline waters of up to 20-25‰ salinity.

Justification for delineation

The Danube and Dniester drainages are a part of a single paleo-hydrographic unit, and represent the largest river system in Western Pontic basin. The basin is not highly impacted by glacial events, and contains heterogenous fauna of ancient pre-Pleistocene fishes.  Most of these fishes are of Danubian-Mediterranean-Mesopotanian descent, as well as many post-glacial immigrant species.

Level of taxonomic exploration

Fair: some nominal gobiid forms/subspecies from Benthophilus and Neogobius genera need revision as well as status and synonymies of migratory Alosa herrings.