Ecoregion Description

526: Lake Tana

Major Habitat Type:

montane freshwaters

Author:

Michele Thieme and Ashley Brown, World Wildlife Fund-US

Reviewers:

Leo Nagelkerke, Wageningen University Wageningen, The Netherlands

Countries:

Ethiopia

Boundaries:

Lake Tana, a lake in the highlands of Ethiopia lies in the north of Ethiopia and is the source of the Blue Nile. The Blue Nile descends from Lake Tana to Tissisat Falls (c. 40 m high), effectively isolating the lake’s freshwater fauna from the rest of the Nile. 

Main rivers or other water bodies:

Lake Tana was formed by a volcanic blockage that reversed the previously north-flowing river system (Beadle 1981). The total area of the Lake Tana basin is 16,500 km² and the lake itself covers about 3,150 km². Numerous seasonal streams and four perennial rivers feed the lake, while only one—the Blue Nile—leaves it (Nagelkerke 1997).

Topography:

The lake is situated in the highlands of Ethiopia at about 1,800 m, and experiences a tropical highland climate.

Climate:

Air temperatures range widely, between 7 ^oC to 31^oC, whereas water temperatures stay relatively mild, normally between 18^oC and 26^oC (Nagelkerke 1997). The dry season lasts from October/November to May/June with maximum monthly rainfall (up to 500 mm/month) in July. Annual rainfall in the vicinity of the lake averages 1315 mm/year, but evaporation is higher at about 1,800 mm/year (Burgis & Symoens 1987).

Freshwater habitats:

Because evaporation exceeds rainfall, the hydrology of this shallow lake depends largely on the local climate (Burgis & Symoens 1987). Lake level varies depending on seasonal rains. The average difference between the lowest lake level (May-June) and the highest (September-October) is 1.5 m (Nagelkerke 1997). The lake has a mean depth of 8 m, and is well mixed due to relatively strong winds in the evenings (Nagelkerke 1997). The water of the lake is clear, and in places at the lake bottom volcanic peaks form reefs (Sibbing et al. 1998). Cyperus papyrus and other Cyperus spp. line the shores of the lake (Beadle 1981).

Terrestrial Habitats:

Fish Fauna:

The isolation of the lake from all but inflowing rivers has led to a highly endemic freshwater biota. Fish species in the lake are most closely related to those of the Nilo-Soudanian biogeographic region. Lake Tana hosts the only extended cyprinid species flock in Africa. The only other known flock, in the Philippines’ Lake Lanao, has been decimated by introduced species.  Fifteen species of large barbs have been described from Lake Tana (Nagelkerke 1997; Nagelkerke & Sibbing 1998; Nagelkerke & Sibbing 2000). The species flock is believed to be less advanced in its evolution than Lake Lanao’s cyprinid flock (Mina et al. 1996). Eight of the large barbs are piscivorous, and Barbus humilis and the newly described small species, Barbus tanapelagius, are thought to be the major prey species (De Graaf et al. 2000). It is likely that the Lake Tana barbs evolved from one ancestral species that probably resembled Barbus intermedius (Nagelkerke 1997).

Description of endemic fishes:

About 70% of the fish species in this highland lake are endemic, including eighteen endemic cyprinids. The fifteen large barb species of the subgenus Barbus (Labeobarbus) and the 3-4 small barb species of the subgenus Barbus (Enteromius) are currently under revision (Nagelkerke, Agricultural University, Wageningen, Netherlands, pers. comm.). The tilapia (Oreochromis niloticus) of Lake Tana belongs to a widespread species but is described as an endemic subspecies, Oreochromis niloticus tana (Seyoum & Kornfield 1992). The only river loach (family Balitoridae) known from Africa, Nemacheilus abyssinicus, was described from Lake Tana in 1902 and rediscovered in 1992 in the lake and in the upper Omo River (Dgebuadze et al. 1994).

Other noteworthy fishes:

The large catfish, Clarias gariepinus, widespread throughout Africa, also lives in the lake and forms an important part of the fishery.

Other noteworthy aquatic biotic elements:

The invertebrate fauna is relatively depauperate. Fifteen species of molluscs, dominated by the Planorbidae family, have been described, including one endemic. An endemic freshwater sponge, Makedia tanensis, has recently been discovered in the lake. The sponge is small (specimens found were up to about 2 cm), white, and of an encrusting form belonging to a monotypic genus (Manconi et al. 1999).

A high diversity of wetland birds also lives by the lake, including the piscivorous little grebe (Tachybaptus ruficollis), great white pelican (Pelecanus onocrotalus), great and African comorants (Phalacrocorax carbo and P. africanus), and darter (Anhinga rufa). Many Paleartic migrant waterbirds also depend on the lake as feeding and resting grounds.

Evolutionary phenomena:

The radiation of cyprinid species in Lake Tana is globally recognized for its evolutionary importance (Thieme et al. 2005). The only one other extended cyprinid species flock in the world is found in the Philippines’ Lake Lanao.

Justification for delineation:

The ecoregion is isolated from the rest of the Nile by Tissisat Falls (c. 40 m high); and this isolation has played a role in the evolution of the endemic fauna of the lake.  This ecoregion is distinguished by the only extended cyprinid fish species flock in Africa; at present fifteen species of large barbs have been described from this endemic flock. 

Level of taxonomic exploration:

Fair

References/sources:

Beadle, L. C. (1981). "The inland waters of tropical Africa" England: Longman Group Limited.

Burgis, M. J.,Symoens, J. J. (1987). "African wetlands and shallow water bodies" Paris, France: ORSTOM.

De Graaf, M., Dejen, E., et al. (2000). "Barbus tanapelagius, a new species from Lake Tana (Ethiopia): its morphology and ecology" Environmental Biology of Fishes 59(1) 1-9.

Dgebuadze, Y. Y., Golubstov, A. S., et al. (1994). "Four fish species new to the Omo-Turkana basin, with comments on the distribution of Nemacheilus abyssinicus in Ethiopia" Hydrobiologia 286 125-128.

Manconi, R., Cubeddu, T., et al. (1999). "African freshwater sponges: Makedia tanensis gen. et sp. Nov. From Lake Tana, Ethiopia" Memoirs of the Queensland Museum 44 361-367.

Mina, M. V., Mironovsky, A. N., et al. (1996). "Lake Tana large barbs: Phenetics, growth and diversification" Journal of Fish Biology 48(3) 383-404.

Nagelkerke, L. (1997). "The barbs of Lake Tana, Ethiopia: Morphological diversity and its implications for taxonomy, trophic resource partitioning, and fisheries" Wageningen, The Netherlands: Special edition, Agricultural University.

Nagelkerke, L. A. J.,Sibbing, F. A. (1998). "The ‘Barbus’ intermedius species flock of Lake Tana (Ethiopia): I. the ecological and evolutionary significance of morphological diversity" Italian Journal of Zoology 65 (suppl.) 3-7.

Nagelkerke, L. A. J.,Sibbing, F. A. (1997)"A revision of the large barbs (Barbus spp., Cyprinidae, Teleostei) of Lake Tana (Ethiopia), with a description of a new species, Barbus osseensis" In Nagelkerke, L.A.J. (Ed.). The barbs of Lake Tana, Ethiopia: morphological diversity and its implications for taxonomy, trophic resource partitioning, and fisheries. (pp. 179-214) Wageningen, The Netherlands: Wageningen Agricultural University.

Nagelkerke, L. A., Mina, M. V., et al. (1995). "In Lake Tana, a unique fish fauna needs protection" BioScience 45(11) 772-775.

Seyoum, S.,Kornfield, I. (1992). "Identification of the subspecies of Oreochromis niloticus (Pisces: Cichlidae) using restriction endonuclease analysis of mitochondrial DNA" Aquaculture 102(1-2) 29-42.

Sibbing, F. A., Nagelkerke, L. A. J., et al. (1998). "Speciation of endemic Lake Tana barbs (Cyprinidae, Ethiopia) driven by trophic resource partitioning; a molecular and ecomorphological approach" Aquatic Ecology 32 217-227.

Thieme, M. L., Abell, R., et al. (2005). "Freshwater Ecoregions of Africa and Madagascar: A Conservation Assessment" Washington, D.C., USA: Island Press.