Major Habitat Type
Ethiopia, Kenya, Sudan
Encompassing much more than the immediate Lake Turkana environs, the Lake Turkana ecoregion reaches north to include Lakes Abaya and Chamo, as well as the headwaters of the Omo Riverin southwestern Ethiopia.
Main rivers or other water bodies
Lake Turkana is the largest lake in the eastern portion of the Rift Valley and the fourth largest lake by volume in Africa (Beadle 1981). Lying in a low closed basin at approximately 365 m asl, the lake is situated primarily in northwestern Kenya, with only its northernmost end, the Omo Delta, inside Ethiopia. Of the twelve principal rivers that feed Lake Turkana, the River Omo is its only perennial tributary, supplying over 90% of the lake’s inflow (Beadle 1981). The Omo River drains the southwestern portion of the Ethiopian Massif and flows through the Rift Valley into Lake Turkana. Of the seasonal rivers that flow into the lake, the Turkwell and Kerio Rivers are the largest contributors and enter the lake along its western edge and in its southern half (Hughes & Hughes 1992).
Lake Abayaand LakeChamoare located in the northeastern portion of the ecoregion. Five major rivers feed Lake Abaya, the most important of which is the Bilate. During the rainy season, overspill from Lake Abaya is carried to Lake Chamo via the Ualo River (Hughes & Hughes 1992).
The climate of northwestern Kenya is hot, arid, and relatively stable throughout the year. The mean monthly maximum air temperature ranges from 31-33° C. Strong winds with a marked diurnal cycle blow from the south and southeast with little seasonality (Butzer 1971; Ferguson & Harbott 1982). The warmest and driest months are October through January. The period from April through August is somewhat cooler with the highest likelihood of rain, although droughts occur on average every 6-7 years (Nicholson 1982; Kolding 1992). Less than 200 mm of rain falls in northwestern Kenya each year, and its occurrence is extremely unpredictable (Hughes & Hughes 1992).
The Ethiopian Rift Valley has a mean annual rainfall of 600 mm/year, receiving at least fifty percent of the precipitation between July and September. The western foothills of the Ethiopian Rift escarpment receive as much as 800-1,000 mm of rainfall per year. This heavy rainfall causes the Omo River to flood (June through September), bringing nutrient rich waters into Lake Turkana (Beadle 1981).In the far north of the ecoregion, near Addis Ababa, Ethiopia, the mean annual precipitation is 1,302 mm/year (Hughes & Hughes 1992).
Lake Turkana is 260 km long, with an average width of 30 km, a mean depth of 31 m, and a maximum depth of 114 m. It has an area of approximately 7,560 km² and a volume of 237 km³ (Coulter et al. 1986). With no surface outlet, the water budget of the lake is a balance between river and groundwater inflow and evaporation. Evaporation rates are high, at around 2.3-2.8 m/yr. An influx of about 19 km3/yr is required to keep lake levels steady, and high inter- and intra-annual fluctuations in water level occur as a function of the rainfall in distant upland Ethiopia (Kolding 1992). Generally, the lake level fluctuates annually with an amplitude of about 1-1.5 m, but it also undergoes considerable long-term variations that exceed those of any other lake of natural origin (Butzer 1971). The mean retention time of water in the lake is a short 12.5 years (Kolding 1992).
The salinity of Lake Turkana is higher than that of any other large African lake. This is due to the fact that the lake has no outlet, and that it has contracted in volume over the last 7,500 years. Very recent volcanic activity in the basin has also contributed to the high salinity of the lake (Beadle 1981). Water samples taken between 1931 and 1975 record salinity as ranging from 1.7-2.7o/oo (Hughes & Hughes 1992; Spigel & Coulter 1996). The mean conductivity is about 3500 µS/cm, with an estimated rise of about 0.45 µS/cm/yr (Ferguson & Harbott 1982). Due to the volcanic origin of the catchment area the water chemistry is dominated by sodium (more than 95% of the cations by weight) and bicarbonate, which generate a high alkalinity (pH » 9.3).The seasonal inflow of water, coupled with strong diurnal wind patterns, keep the waters of Lake Turkana productive and well mixed. In fact, the oxygen content of Turkana’s deepest water is never less than 70% (Beadle 1981).
The most common emergent plants are the grasses Paspalidium geminatum and Sporobolus spicatus, with extensive beds of Potamogeton occurring in shallow bays (Hughes & Hughes 1992). The waters of Abaya and Chamo contain numerous submerged plants, such as Ceratophyllum demersum, Hydrocotyle sp., and Potamogeton spp., as well as floating plants like Lemna gibba, Nymphaea spp., and Ottelia ulvifolia (Hughes & Hughes 1992).
The evergreen bush and woodland of the Ethiopian Massif grade into deciduous bush in the Rift Valley. Extensive seasonal floodplains exist along the Omo River Delta, at the northern tip of Lake Turkana. Gallery forests of Acacia elatior, Balanites aegyptiaca, and Hyphaena coriacea grow along Lake Turkana's tributaries (Beadle 1981; Hughes & Hughes 1992). Swampy savanna forests of Acacia and Ficus species line the shores of Lake Abaya and species of Typha and Phragmites are common along the banks of both Lakes Abaya and Chamo.
Lake Turkana is unique among the larger lakes of the eastern Rift Valley in that its aquatic fauna is dominated by Nilotic riverine species, rather than by species of the cichlid family (Lowe-McConnell 1993). Compared to other large African lakes, Turkana has relatively low fish species richness, providing habitat for about 50 species, 11 of which are endemic. According to Hopson (1982)(Hopson 1982), four fish communities live in the main lake: a littoral assemblage, an inshore assemblage, an offshore demersal assemblage, and a pelagic assemblage. Spawning migrations of fish are synchronized with the ecoregion’s seasonal flooding, which occurs from June through September. During this time, various fish species migrate up the Omo River (Hydrocynus forskalii, Alestes baremoze, Citharinus citharus, Distichodus niloticus, Barbus bynni) and other ephemeral affluents (Brycinus nurse, Labeo horie, Clarias gariepinus, Synodontis schall) to breed, for periods of both long and short duration (Beadle 1981; Hopson 1982; Lévêque 1997).
Description of endemic fishes
The endemic species nearly all live in the offshore demersal or pelagic zone (Lowe-McConnell 1987). Endemic chiclids include three halpochromine species adapted for deep water: Haplochromis macconneli, H. rudolfianus, and H. turkanae. Other species endemic to Lake Turkana include Barbus turkanae, Brycinus ferox, B. minutus, Labeo brunellii, Lates longispinis and Neobola stellae.
Other noteworthy aquatic biotic elements
Lake Turkana is an important site for waterbirds with up to 220,000 congregants having been recorded at one time and 84 waterbird species, including 34 Palearctic migrants, known from the lake (Bennun & Njoroge 1999). Over 100,000 Calidris minuta have been recorded at the lake, in addition to smaller congregations of other non-breeding waterbirds (Pelecanus rufescens, Phoenicopterus ruber, Vanellus spinosus, Charadrius hiaticula, C. asiaticus, C. pecuarius) (Bennun & Njoroge 1999). Bird species present near Lake Abaya include Anhinga rufa, Bubulcus ibis, Casmerodius albus, Egretta garzetta, Haliaeetus vocifer, and Scotopelia peli (Hughes & Hughes 1992). Other aquatic animals in the ecoregion include Hippopotamus amphibius, Crocodylus spp., and an endemic freshwater turtle, the recently discovered and imperiled Turkana mud turtle (Pelusios broadleyi) (Hughes & Hughes 1992; Expert Center for Taxonomic Identification 2000). Lakes Abaya and Chamo support notably large populations of Crocodylus niloticus and Hippopotamus amphibius (Hughes & Hughes 1992). Three species of frog are endemic to the ecoregion (Bufo chappuisi, B. turkanae and Phrynobatrachus zavattarii).
Justification for delineation
The Lake Turkana ecoregion is defined by the basins of lakes Turkana, Abaya and Chamo and the Omo River basin. Fish species in the ecoregion are mainly of Sudanian origin, providing evidence of a previous connection to the Sobat and the Nile Rivers (Beadle 1981). For example, the Nilotic species, Nile tilapia (Oreochromis niloticus), Bagrus domac, and Nile perch (Lates niloticus) are abundant and common in lakes Turkana, Abaya, and Chamo (Hughes & Hughes 1992). The Omo River basin also has several fish species in common with Lakes Turkana, Abaya, and Chamo. The Turkana basin was formed from tectonic movement during the early Miocene. Evidence suggests that Lake Turkana was once part of a larger body of water that included present day Lake Baringo (south) and the Lotikipi Plains (west). In addition, a connection with the Nile and its tributary, the Sobat River, may have existed more than once during particularly wet periods of the Pleistocene, with the most recent connection occurring not more than 7,000 years BP (Beadle 1981; Dgebuadze et al. 1994)
Level of taxonomic exploration