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Ecoregion Description


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528: Northern Eastern Rift

Major Habitat Type:

xeric freshwaters and endorheic (closed) basins

Author:

Abebe Getahun, Department of Biology, Addis Ababa University, Addis Ababa, Ethiopia

Countries:

Djibouti; Ethiopia; Somalia

Boundaries:

Numerous highly productive lakes lie in the Eastern Rift valley that cleaves the eastern and western sections of the high altitude Ethiopian dome and extends from the edge of the xeric, Red Sea Coastal ecoregion [527] in the north to Lake Awassa in the south. 

Main rivers or other water bodies:

Two closed basins occur in this section of the Rift Valley: in the south, Lake Awassa basin, consisting of Lake Awassa and the swampy Lake Shallo; and the Oromo lakes (previously known as the Galla lakes) basin in the north, composed of a series of four interconnected lakes (Abijata, Langano, Shala and Zwai). There is also a number of hot springs adjacent to the lakes.

This ecoregion also contains a number of crater lakes (Bishoftu, Aranguade, Hora, Kilotes and Pawlo) located at the northwestern edges of the rift valley around the town of Debrezeit (Mohr 1961), at an altitude of about 1,900 m. These lakes lie in volcanic explosion craters produced about 7,000 years ago. The Awash River, which begins in the Ethiopian Plateau, flows north in the Rift Valley and terminates in Lake Abhe, a closed lake near the border between Ethiopia and Djibouti.

Climate:

The mean annual air temperature of this ecoregion varies with altitude and ranges between 20oC and 22oC (Tudorancea et al. 1999). There is a four-month dry season from November to February and an eight-month rainy season from March to October. The main rains occur between July and September.

Freshwater habitats:

Five major lakes and several rivers lie within this ecoregion. Lake Zwai, the most northerly lake, is located about 160 km south of Addis Ababa at an altitude of 1,840 m. The lake covers 434 km2 and its average depth is 2.5 meters (Balarin 1986). Extensive marshes of Papyrus (Cyperus papyrus) border the lake and produce large numbers of mosquitoes (Anopheles pharoensis, A. mauritianus and Taeniorhynchus uniformis)(Omer-Cooper 1930). The Makki River flows into the lake from the northwest and the River Kattar flows into it from the northeast. The lake’s waters flow out through the River Sucsuci into Lake Abijata.

The next three lakes in the chain are Lake Langano, Lake Abijata, and Lake Shala. Lake Langano, located at an altitude of 1,582 m, covers 241 km2, and has a maximum depth of 47.9 m and a mean depth of 17 m. Salinity is 1.88 g per liter (Wood & Talling 1988). Lake Langano receives most of its water from small rivers that drain from the Arsi Mountains, which make up the eastern wall of the Rift valley. The only outlet from the lake is the Hora Kelo River, which flows into Lake Abijata (Ethiopian Wildlife and Natural History Society 1996). Lake Abijata is found at an altitude of about 1,600 m and is an alkaline lake. The shores slope gradually, and are muddy, with areas of Juncus vegetation. The depth is about 10 m, and the bottom of the lake is sandy. Three rivers feed Lake Abijata: Gogessa, Bulbula, and Hora Kelo. Water from Lake Zwai also flows into Lake Abijata through River Sucsuci. Lake Abijata has no outlet, and it loses its water by evaporation only. Lake Shala, located at 7o28’ N 38o30’E, is at 266 m the deepest of the Ethiopian Rift lakes. It is 28 km long and 15 km wide, and is surrounded primarily by Pleistocene volcanic rocks. The eastern and western shores are covered by lacustrine deposits and Holocene sands, occasionally blackened by obsidian detritus (Mohr 1961). The great depth of Lake Shala may be related to the origin of the basin by intense faulting. Two permanent creeks and several seasonal streams flow into its closed basin.

Lake Awassa has a smaller surface area than Shala and is completely enclosed by faulting. Located at an altitude of 1,680 m, the lake lies south of the other lakes in the ecoregion. It has a surface area of 88 km2, a maximum depth of 22 m, and an average depth of 11 m. Lake Awassa is a polymictic lake. The water is murky and alkaline with a pH of between 8.75 and 9.05 (Tudorancea et al. 1988). Like Lakes Abijata and Shala, it is a terminal lake without any visible outlet. Its main tributary is the Tikur Wuha River, which drains swampy Lake Shallo. An extensive belt of submergent and emergent rooted vegetation, which extends about 150 m offshore, covers the littoral zone.

The flora varies among the different lakes. Three major groups of algae dominate: Chlorophyceae, Cyanophyceae and Diatomophyceae (Tudorancea et al. 1999). The most common emergent plants are Scirpus spp., Typha angustifolia, Paspalidium germinatum, and Phragmites sp. Nymphaea coerulea and Potamogeton spp. are the dominant species of floating and submerged vegetation. Abijata and Shala Lakes lack aquatic macrophytes.

Terrestrial Habitats:

The vegetation to the east and south of Lake Shala is Acacia-Euphorbia savanna. The most common trees are the woodland acacias Acacia etbaica, A. tortilis, and Euphorbia abyssinica, and bushes of Maytenus senegalensis. There is also a rich grass and herb flora (Ethiopian Wildlife and Natural History Society 1996). Beds of bulrushes and sesbania occur where the hot springs and rivers enter the lake, but most of the shore has steep cliffs.

Fish Fauna:

The lakes and streams of the Northern Rift ecoregion support a depauperate freshwater fauna, including only eight fish, with few endemic species. The fish fauna of Lake Awassa consists of two species of Barbus (B. intermedius and B. cf. amphigramma), the North African catfish (Clarias gariepinus), and Oreochromis niloticus. Oreochromis niloticus is abundant in the lakes and rivers of this ecoregion. No fish are recorded from Lake Shala. Despite their depauperate number of species, the lakes of this ecoregion support most of the fish production of Ethiopia.

Description of endemic fishes:

Three endemic fish, Barbus ethiopicus, B. microterolepis, and Garra makiensis, inhabit Lake Zwai and its adjacent rivers.

Other noteworthy fishes:

Introduced species of Tilapia zilli, Clarias gariepinus, and carps are present in Lake Zwai.

Other noteworthy aquatic biotic elements:

Only thirteen aquatic frogs, three aquatic reptiles, twelve aquatic mollusks, and four aquatic mammals live in or adjacent to the freshwater lakes and streams. There are considerable numbers of pelicans, cormorants, ducks, snipe, stilts, egrets, grebes, ibis, herons, gulls, and darters around Lake Zwai. One endemic frog, Bufo langanoensis, lives along the shores of Lake Langano and its tributaries. Lakes Zwai and Langano also harbor hippopotamus (Hippopotamus amphibius). 

During the 1970s and 1980s over 400 species of birds were recorded from the Abijata-Shala National Park. This park is positioned in one of the narrowest parts of the Great Rift Valley, which is a major flyway for both Palaearctic and African migrants, particularly raptors, flamingoes, and other water birds (Ethiopian Wildlife and Natural History Society 1996). Many of these birds stop over to rest and feed within the ecoregion. The shallow waters of Lake Abijatta are remarkably rich in insect life, with large swarms of Corixa in particular, though plankton diversity is low. The zooplankton fauna in the Ethiopian rift lakes is dominated, in terms of biomass, by copepods and cladocerans (Tudorancea et al. 1999).

Justification for delineation:

This ecoregion is defined by the northern lakes of the Ethiopian rift valley and distinguished by lakes with a distinctive fauna when compared to the more southern rift valley lakes of Chamo and Abaya. The fish fauna in the northern lakes appears to have been derived from Awash and associated rivers, while the fish fauna of the latter is Nilo-Sudanic. These faunal affinities may be explained by the tumultuous geologic past of the ecoregion, which was exposed to six volcanic events between the Oligocene and the present (Woldegabriel et al. 1990). Analyses of invertebrate and fish fossils found in the sediments of the lakes indicate that Lakes Zwai, Abijata, Langano and Shala were once united into a single freshwater lake draining northward into the Awash River (Grove et al. 1975; Gasse & Street 1978). The present-day lakes are the result of subsequent tectonic or volcanic activity (Tudorancea et al. 1999).

Level of taxonomic exploration:

Fair. Although considerable baseline information is still lacking, this is one of the most studied ecoregions in Ethiopia. Physical and chemical features of some of the lakes are available in Wood et al. (1978) and Wood and Talling (1988). Zooplankton and phytoplankton studies of Lake Awassa are found in Mengistou and Fernando (1991a; 1991b), Mengistou et al. (1991), and Kifle and Belay (1990). Tudorancea and Harrison (1988), Tudorancea and Zullini (1989) and Tudorancea et al. (1989) studied the benthic communities of some of these lakes. Teferra (1987; Teferra 1988, 1989), Tadesse (1988), Teferra and Fernando (1989), Admassu (1994), and Tudorancea et al. (1988) have studied the biology of Oreochromis niloticus, the dominant fish species in these lakes. Nevertheless, the invertebrate fauna, the phytoplankton, and the macrophytes of many of the lakes have not been identified or studied. Moreover, the biodiversity of the rivers and streams is virtually unknown.

References/sources:

Admassu, D. (1994). "Maturity, fecundity, brood size and sex ratio of Tilapia (Oreochromis niloticus L.) in Lake Awassa" SINET: Ethiopian Journal of Science 17(1) 53-69.

Balarin, J. D. (1986). "National reviews for aquaculture development in Africa, Ethiopia" FAO Fisheries circular 770(9) 120.

Gasse, F.,Street, F. A. (1978). "Late quaternary lake-level fluctuation and environments of the northern rift valley and Afar regions (Ethiopia and Djibouti)" Paleogeography, Paleoclimatology and Paleoecology 24 279-325.

Grove, A. T., Street, F. A., et al. (1975). "Former lake levels and climatic change in the Rift Valley of southern Ethiopia" Geography Journal 141 177-202.

Kifle, D.,Belay, A. (1990). "Seasonal variations in phytoplankton primary production in relation to light and nutrients in Lake Awassa, Ethiopia" Hydrobiologia 196 217-227.

Mengistou, S.,Fernando, C. H. (1991). "Biomass and production of the major dominant crustacean zooplankton in a tropical rift valley lake, Awassa, Ethiopia" Journal of Plankton Research 13 831-851.

Mengistou, S.,Fernando, C. H. (1991). "Seasonality and abundance of the dominant crustacean zooplankton in a tropical rift valley lake, Awassa, Ethiopia" Hydrobiologia 226 137-152.

Mengistou, S.,Fernando, C. H. (1991). "Biomass and production of the major dominant crustacean zooplankton in a tropical rift valley lake, Awassa, Ethiopia" Journal of Plankton Research 13 831-851.

Mohr, P. A. (1961). "The geology, structure, and origin of the Bishoftu explosion craters, Shoa, Ethiopia" Bulletin of the Geophysical Observatory, University College, Addis Ababa 2 65-101.

Omer-Cooper, J. (1930). "Dr. Hugh Scott’s expedition to Abyssinia- A preliminary investigation of the freshwater fauna of Abyssinia" Proc. Zool. Soc. Lond. Part I: 195-207.

Tadesse, Z. (1988). "Studies on some aspects of the biology of Oreochromis niloticus L. (Pisces: Cichlidae) in Lake Zwai, Ethiopia" Unpublished Unpublished MSc thesis. School of Graduate studies, Addis Ababa University: Addis Ababa, Ethiopia

Teferra, G. (1987). "A study on a herbivorous fish (Oreochromis niloticus L.) diet and its quality in two Ethiopian Rift Valley Lakes, Awassa and Zwai" Journal of Fish Biology 30 439-449.

Teferra, G. (1988). "Digestive efficiency and nutrient composition gradient in the gut of Oreochromis niloticus L. in Lake Awassa, Ethiopia" Journal of Fish Biology 33 501-509.

Teferra, G. (1989). "Stomach pH, feeding rhythm and ingestion rate in Oreochromis niloticus L. (Pisces: Cichlidae) in Lake Awassa, Ethiopia" Hydrobiologia 174 43-48.

Teferra, G.,Fernando, C. H. (1989). "The food habits of a herbivorous fish (Oreochromis niloticus L.) in Lake Awassa, Ethiopia" Hydrobiologia 174 195-200.

Tudorancea, C., Baxter, R. M., et al. (1989). "A comparative limnological study of zoobenthic associations in lakes of the Ethiopian rift valley" Archiv für Hydrobiologie/ Supplement 83 121-174.

Tudorancea, C., Fernando, C. H., et al. (1988). "Food and feeding ecology of Oreochromis niloticus (Linnaeus, 1758) juveniles in Lake Awassa (Ethiopia)" Archiv für Hydrobiologie/ Supplement 79 267-289.

Tudorancea, C., GebreMariam, Z., et al. (1999)"Limnology in Ethiopia" In Wetzel, R.G.;Gopal, B. (Ed.). Limnology in developing countries, 2. (pp. 63-118) New Delhi, India: International Association for Limnology.

Tudorancea, C.,Harrison, A. D. (1988). "The benthic communities of the saline lakes Abijata and Shala (Ethiopia)" Hydrobiologia 158 117-123.

Tudorancea, C.,Zullini, A. (1989). "Associations and distribution of benthic nematodes in the Ethiopian rift valley lakes" Hydrobiologia 179 81-96.

Woldegabriel, G., Aronson, J. L., et al. (1990). "Geology, geochronology, and rift basin development in the central sector of the main Ethiopian rift" Geological Society of America Bulletin 102 439-458.

Wood, C. A.,Lovett, R. (1979). "Rainfall reliability in Ethiopia, Tables and maps" SINET: Ethiopian Journal of Science 2 111-120.

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