Freshwater Ecoregions of the World
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417: Upper Danube

Author

Jennifer Hales

Reviewers

Jürg Bloesch, International Association for Danube Research (IAD) and Alumnus Swiss Federal Institute of Aquatic Science and Technology (EAWAG)

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Biodiversity
Species Richness
# of Endemic Species
Threats
Description

Major Habitat Type

Temperate upland rivers

Countries

Austria, Croatia, Czech Republic, Germany, Hungary, Italy, Poland, Slovakia, Slovenia, Switzerland, Slovakia

Boundaries

This ecoregion covers the basin and sub-basins of the Upper Danube and parts of the Middle Danube to the confluence of the Drava River. The ecoregion spans approximately 248,000 km2 across the countries of Germany, Austria, Czech Republic, Slovakia, Hungary, Slovenia, Croatia, and a small segment of Serbia. It is bordered to the west by the Eastern Alps and Black Forest Mountains and to the north by the Bohemian Forest Mountains (also known in Czech as Šumava), and to the northeast by the Western Carpathians.

Note that the WWF-US Upper Danube [417] and Dniester-Lower Danube [418] freshwater ecoregions (based on Abell et al. 2008) differ from European ecoregions (based on Illies 1978) as used in the EU Water Framework Directive (EEA 2002). The Upper Danube [417] overlaps with EU ecoregions 4 (Alps), 9 (Central highlands) and 11 (Hungarian lowlands).

Drainages flowing into

Black Sea

Main rivers or other water bodies

The main rivers in this ecoregion include the Danube and its largest tributaries based on mean discharge: Inn, Drava, Váh, and Morava (Sommerwerk et al. 2009). Other smaller tributaries include the Lech, Isar, Marcal, and Sío.  The largest lakes in the ecoregion are Balaton and Neusiedl. Lake Balaton in Hungary is the largest lake in Central Europe and Lake Neusiedl is the largest steppe lake in Central Europe. Other notable alpine and subalpine lakes include Ammersee, Chiemsee, Starnberger See, Attersee, and Traunsee.

Topography

This mountainous ecoregion was formerly part of the Paratethys, an extension of the Tethys which existed prior to the Mediterranean Sea (Liepolt 1967). The Eastern Alps (max. altitudes 3900-4052 m a.s.l.), Black Forest Mountains (max. altitudes 1164-1493 m a.s.l.), Bohemian Forest Mountains (max. altitudes 800-1456 m a.s.l.), and Western Carpathians (max. altitudes 2500-2655 m a.s.l.) drain toward the Black Sea. The mountains consist mostly of crystalline metamorphic bedrock as well as limestone bedrock. Alluvial sediments dominate river valleys (Sommerwerk et al. 2009). Elevations range from 75 m to more than 4052 m at Piz Bernina in the Alps (Hijmans et al. 2004).

Climate

The climate is diverse, ranging from an Atlantic climate in the west with mild winters and high, evenly distributed precipitation; a Continental climate to the east with lower precipitation and cold winters; a Mediterranean climate in the Drava basin; and an Alpine climate at high elevations (Sommerwerk et al. 2009). Mean annual precipitation ranges from 500 mm in the plains to more than 2000 mm in the Alps, with peaks in July. Mean annual temperatures range from -5 ºC at alpine peaks to 11 ºC in the lower Middle Danube sub-basin (Hijmans et al. 2004).

Freshwater habitats

The Danube (2826 km) is the second longest river in Europe after the Volga, and is one of the world’s most important, presently encompassing 19 countries. The river rises from the confluence of the Breg and Brigach rivers in the Black Forest Mountains. From here, the Danube flows through gorges and plains, splitting into channels separated by backwaters, side arms, and forested floodplains (Wohl 2010). The upper course of the Danube has a subalpine character with a high flow velocity and low temperatures; the flow regime is pluvial-nival. It is also influenced by alpine tributaries with a glacial-nival flow regime, resulting in a balanced flow regime with a moderate maximum between April and June (WWF 1999). In terms of biota, the river zonation (Illies 1961; Illies & Botosaneanu 1963) and fish zonation (Huet 1949) play an important role to classify typical stretch-specific fish and benthos assemblages. The mountainous rhithral, encompassing the trout and grayling regions, mostly hosts salmonids and lotic gravel-bound benthos, above all insect larvae, while the lowland potamal encompassing the barbel, bream and ruff-flounder regions hosts mostly cyprinids and lentic benthos adapted to sandy-muddy sediments, such as worms (Oligochaeta) and midges (Chironomids). Near Donaueschingen (Immendingen) there is the remarkable “Donauversickerung” (exfiltration of the Danube) in a karst region where the Danube dries out for more than 250 days per year and looses its water to the River Rhine Basin (emerging in the Aach pothole source with a yield of about 8 m3/s).

The Morava River is a lowland river that originates in the Jeseníky Mountains and crosses the Vienna basin before reaching its confluence with the Danube near Bratislava. Its average annual discharge is 110 m/s3. The Morava experiences a bimodal flood regime with the first peak occurring in early spring and the second in early summer when the Danube backs up into the lower Morava (Sommerwerk et al. 2009).

The Middle Danube extends from Bratislava to the Iron Gate dams on the border of Romania and Serbia. This ecoregion encompasses the upper section of the Middle Danube to the Drava confluence, while the lower section falls within ecoregion 418. The Middle Danube has more of a plains character with meanders and branches as it flows through an internal delta. Its flow regime and biogeochemistry also differs from the Upper Danube where alluvial deposits prevail. The largest tributaries flowing into this section include the Váh on the left bank and Drava on the right bank. The Váh rises in the Carpathian Mountains, flowing through long narrow valleys. The average annual discharge is 190 m/s3 with peak discharge occurring during March and April due to snowmelt. The Drava, in contrast, rises in the Southern Alps. Through its course it changes from an alpine river to a braided and meandering lowland river. Two flood peaks occur along this river, the first in May and June and the second in late autumn. Floods are common, fluctuating between 5 and 6 meters in the lower river. Mean annual discharge is 541 m/s3 (Sommerwerk et al. 2009).

Important features of the Upper Danube ecoregion include alpine and subalpine lakes, which are predominantly oligotrophic and are characterized by their transparency (Dokulil 2005). Some of the most notable include the glacial lakes Ammersee (47.6 km2, 1775 x 106 m3), Chiemsee (82.2 km2, 2053 x 106 m3), Starnberger See (56.4 km2, 2998 x 106 m3), Attersee (45.6 km2, 3945 x 106 m3), and Traunsee (25.6 km2, 2303 x 106 m3).

The Upper Danube is highly altered by dams, navigation channels and bank constructions for flood protection. The alpine tributaries are heavily used for hydropower. Remaining floodplains along the Danube in this ecoregion are being restored (e.g. between Neuburg and Ingolstadt, Donau Auen near Vienna, Gemenc floodplains in Hungary). Important wetlands include the floodplains of the Morava River and former floodplains of Mokrady dolního Podyjí, which is characterized by oxbow lakes, pools, and riverine forests (IUCN 1993). There are also peatlands, marshes, marshy meadows, soda lakes, swamp forests, bogs, and fish ponds. Peatlands cover an extensive share of the ecoregion, including areas like Ocsa and Sumava (Wetlands International 2005). The Lower Drava features outstanding near natural floodplains with high biodiversity, including the Kopacki rit at the Danube confluence.

Terrestrial Habitats

Topographic and climatic heterogeneity distinguish several biogeographical regions: Alpine, Continental and Pannonian. Terrestrial ecoregions are further divided into the Western European broadleaf forests, Alps conifer and mixed forests, Central European mixed forests, Pannonian mixed forests, and Carpathian montane forests (WWF 2001). The Western European broadleaf forests of the Bohemian and Black Forest mountains are characterized by lowland to altimontane beech and mixed beech forests. The richest floral diversity occurs in the Alps conifer and mixed forests (Davis et al. 1994). Deciduous forests include species like pedunculate oak (Quercus robur), sessile oak (Q. petraea), and downy oak (Q. pubescens). Mountain areas are composed of mixed beech (Fagus sylvatica), silver fir (Abies alba), and pure spruce (Picea abies), as well as peat bogs and meadows. In the alpine zone dominant species include juniper (Juniperus communis subsp. alpina) scrub and dwarf ericaceous shrubs (Davis et al. 1994). The Pannonian mixed forests occupy much of the eastern part of the ecoregion in the Pannonian basin. Species include pedunculate oak, sessile oak, and turkey oak (Q. cerris), as well as mixed oak-hornbeam forests, steppes, and floodplain vegetation (Bohn et al. 2000).

Fish Fauna

The Danube historically has acted as a refuge as well as a corridor for migrations and recolonizations between the Alpine and Mediterranean regions in the west and Ponto-Caspian and central Asian biogeographic regions in the east. This has resulted in rich freshwater biodiversity across the basin [ecoregions 417, 418] (Sommerwerk et al. 2009). The Upper Danube ecoregion contains around 60 species, nearly half that of the Dniester - Lower Danube ecoregion [418]. Most of the species are in the families Cyprinidae (47%), Salmonidae (17%), Percidae (13.5%), and Cobitidae (7%).

Sturgeons as “living fossils” are the flagship fishes of the Danube and almost extinct except the sterlet (Acipenser ruthenus).  The potamodromous sterlet is the last sturgeon species remaining in the Upper Danube. Its populations are supported by restocking programs from hatcheries. Other Acipenseridae, like the potamodromous/anadromous Russian sturgeon (A. gueldenstaedtii), the anadromous stellate sturgeon (A. stellatus) and beluga (Huso huso), and the potamodromous ship sturgeon (A. nudiventris), have been extirpated in the Upper Danube due to overfishing, river modification, and habitat loss (Bloesch et al. 2005). Rare catches of A. nudiventris and the potamodromous form of A. gueldenstaedtii have been recently reported for the upper section of the Middle Danube (Guti 2006). All migratory sturgeon species are highly endangered in the Middle and Lower Danube and their major tributaries Sava, Drava and Tisza. The Atlantic sturgeon (A. sturio) is extinct in the Danube Basin.

Species restricted to the Danube basin [ecoregions 417, 418] include the Danube whitefin gudgeon (Romanogobio vladykovi), stone gudgeon (R.uranoscopus), perlfisch (Rutilus meidingerii), cactus roach (R. virgo), yellow pope (Gymnocephalus schraetser), streber (Zingel streber), zingel (Z. zingel), Danubian brook lamprey (Eudontomyzon vladykovi), huchen (Hucho hucho), and European mudminnow (Umbra krameri). Other species include the Bulgarian golden loach (Sabanejewia bulgarica), brown trout (Salmo trutta), gudgeon (Gobio gobio), Crucian carp (Carassius carassius), and European grayling (Thymallus thymallus), which is one of the key reference species in the ecoregion (Kottelat & Freyhof 2007). Alpine charr (Salvelinus umbla) is restricted to alpine and subalpine lakes in both the Upper Danube and Central and Western Europe [404] ecoregions. The bleak (Alburnus alburnus) is the only known species to occur across the entire length of the Danube (Sommerwerk et al. 2009).

Description of endemic fishes

The Upper Danube is known for its salmonids, including five endemic Coregonus species and one endemic Salvelinus species. Some of the endemics occur in the ecoregion’s alpine and subalpine lakes. These include the Reinanke (Coregonus atterensis) in Lake Attersee and possibly Lake Mondsee, Ammersee Kilch (C. bavaricus) in Lake Attersee and possibly Lake Wolfgangsee, Riedling (C. danneri) in Lake Traunsee, Schwebrenke (C. hoferi) in Lake Chiemsee, Renke (C. renke), and Ammersee Saibling (Salvelinus evasus) in Lake Ammersee (Kottelat & Freyhof 2007). Some species, such as the Perlfisch (Rutilis meidingeri), spawns in lake tributaries or streams connecting the lakes.

Other noteworthy fishes

Many species have experienced significant declines throughout the ecoregion. Notable species include the globally threatened huchen (Hucho hucho), Ammersee Kilch (Coregonus bavaricus), Schwebrenke (C. hoferi), and European mudminnow (Umbra krameri) (IUCN 2009). The Mayforelle (Salmo schiefermuelleri) was once found in the sub-alpine lakes of this ecoregion, but is now extinct. The huchen (Hucho hucho), once widespread throughout the Upper Danube, now occupies only 10% of its former range (Wohl 2010). Economically important species include carp (Cyprinus carpio), pike (Esox lucius), pikeperch (Sander lucioperca), Volga pikeperch (S. volgensis), asp (Apsius aspius), tench (Tinca tinca), and European catfish (Silurus glanis) (Sommerwerk et al. 2009).

Other noteworthy aquatic biotic elements

The rivers, lakes, and wetlands in this ecoregion provide staging, breeding, and wintering areas for migrating and resident birds. Some species include white-tailed eagle (Haliaeetus albicilla), fish eagle (Pandion haliaetus), black-throated diver (Gavia arctica), long tailed duck (Clangula hyemalis), white heron (Egretta alba), black-headed gull (Larus ridibundus), and black-tailed godwit (Limosa limosa). Newly expanding cormorant (Phalacrocorax carbo sinensis and Ph. pygmaeus) populations may decimate fish populations as these birds are key predators. There are also populations of Eurasian otter (Lutra lutra) and water shrews (Neomys fodiens and N. anomalus) (Sommerwerk et al. 2009).

Special ecological relevance should also be given to the macrophytes (Janauer et al. 2003) and the macroinvertebrates (Sommerwerk et al. 2009). There are no endemic reptiles, but one important endemic amphibian is the the Danube crested newt (Triturus dobrogicus), with its western subpopulation located at least partly in this ecoregion (Puky et al. 2005; IUCN 2009). 

Ecological phenomena

Globalization of transport and the Rhine-Main-Danube canal for navigation have increased the introduction of exotic and/or invasive species, which commonly compete successfully with native species. A good example in the subalpine/ alpine regions is the spread of artificially bred and stocked rainbow trout (Oncorhynchus mykiss), which diminished the native brown trout (Salmo trutta fario) considerably. Furthermore, common established exotic/allochthonous fish species in this ecoregion are represented by the topmouth gudgeon (Pseudorasbora parva), brown bullhead (Ameiurus nebulosus), Prussian carp (Carassius gibelio), and pumpkinseed (Lepomis gibbosus).

Evolutionary phenomena

At the beginning of the post-glacial times, the Carpathian Basin, i.e. the eastern part of this ecoregion, may have acted as a meeting point of several migration routes (Varga 2010). Having a barrier function as well as providing refuges for certain cold-tolerant e.g. amphibian and reptile taxa such as the common lizard (Zootoca vivipara), which evolved peculiar karyotypic characters in Austria and northwestern Hungary (Surget-Groba et al. 2007).

The Middle Danube and the Carpathians feature the Ponto-Caspian fauna representing significant relict and endemic species. This ecoregion is also rich in fossils. A recent example of Alpine whitefish (Coregonus) radiation is given by Hudson et al. (2010). 

Justification for delineation

Northern and eastern European ecoregions were delineated through a top-down process using major basins as a starting point and incorporating traditionally recognized zoogeographic patterns where appropriate (Abell et al. 2008; Balon et al. 1986; Kottelat & Freyhof 2007). Although this ecoregion shares much of the same ichthyofauna with the Central and Western Europe ecoregion [404] to the north, it contains several genera (e.g. Zingel and Eudontomyzon) that occur minimally, if at all, in ecoregion 404 (M. Kottelat pers. comm. Jan 16, 2006).

Level of taxonomic exploration

The overall taxonomic exploration in the Upper Danube can be rated “good” for flora and fauna, including all hierarchical taxonomic levels from algae to mammals.

 

References

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