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Ecoregion Description
565: Lake Rukwa
Major Habitat Type:
large lakes
Author:
Ashley Brown and Robin Abell, Conservation Science Program, World Wildlife Fund-US, Washington, DC, USA
Reviewers:
Tim Davenport, Wildlife Conservation Society, Mbeya, Tanzania
Countries:
Tanzania; Zambia
Boundaries:
Lake Rukwa is a saline lake of over 85,000 km2 located in the Rift Valley. The lake lies within the southwest corner of Tanzania, east of Lake Tanganyika’s [542] southern tip. The Lake Rukwa ecoregion, comprised of the lake and its catchment, is bordered in the south east by the Mbeya Range, in the west by the slopes of the Ufipa escarpment and Mbizi (up to 2,664 m asl), and in the north-east by rocky cliffs and rolling hills that reach as high as 1,707 m at Mount Sange (Hughes & Hughes 1992; Baker & Baker 2001; Baker & Baker 2002).
Drainages flowing into:
The lake is endorheic, with no external drainage (Hughes & Hughes 1992; Seegers 1996).
Main rivers or other water bodies:
There are several large tributaries to the lake. The Lupa, Chambua, and Songwe Rivers drain the Mbeya Range and flow into the lake from the south, the Rungwa feeds the lake in the north, and the Momba River flows in from the west. In addition, there are several ephemeral rivers that flow into the lake during the wet season.
Topography:
Lake Rukwa lies in a rift within the Western Rift Valley, and the lake itself sits at 790 m asl.
Climate:
The climate of the ecoregion is tropical and wet. There is one rainy season with most precipitation falling from October to April, although the Ufipa Highlands also experience rains in May and October (very rarely). Average annual rainfall ranges from about 650 mm in the south of the basin to about 900 mm in the north to about 2,500 mm in the Ufipa Highlands (Seegers 1996). In the southern portion the mean annual temperature is 21 oC, with a mean maximum in the warmest month of about 28 oC and a mean minimum in the coolest month of 12.7 oC (Hughes & Hughes 1992). Surface water temperatures of the lake range from a minimum of about 20 oC to a maximum of about 35 oC (Seegers 1996).
Freshwater habitats:
At high water, there is a single lake, but at lower levels the lake splits into two basins that differ in both size and depth. The water levels are currently high (Baker & Baker 2002), although there is evidence that the level is once again falling. The lakebed slopes toward the east, resulting in greatest depths on the eastern shore. The south basin has maximum depths reaching about 10-15 m. The north basin is shallower, and it occasionally dries completely. The water depth of the swamp barrier between the two basins is usually 1 m or less (Hughes & Hughes 1992; Seegers 1996).
The lake stretches lengthwise for about 165 km, with widths of 37 km in the north basin and a maximum width of 48 km near the middle of the lake (Seegers 1996). Along the northern and western shores of the lake lie extensive wetlands that include both permanent swamps and temporary floodplains. The presence of dead or dying trees under water is evidence that these areas were flooded relatively recently (Seegers 1996). At the deltas of the Luika, Songwe, Momba, and Chambue Rivers (south lake), and Kavu and Rungwa Rivers (north lake), there are also large swampy areas (Seegers 1996). The waters of the lake have high levels of sodium and high alkalinity (pH between 8.0 and 9.0) (Seegers 1996). The less saline swamps that surround the lake contain Cyperus papyrus and Phragmites mauritianus, among other species. C. papyrus can also be found along with Oryza sp., Leersia sp., Vossia cuspidata and Typha sp. in deeper waters of the lake.
Terrestrial Habitats:
Floodplain vegetation surrounding Lake Rukwa is primarily grassland, with short grasses dominated by the salt-tolerant Diplachne fusca, Sporobolus spicata, and S. robustus (Hughes & Hughes 1992). Gallery forests of woodland, with many Acacia species, grow along the tributaries that feed the lake and along the lake’s edges (Hughes & Hughes 1992).
Fish Fauna:
In total, there are about 60 fish species in the ecoregion, with nearly one-third endemic. A variety of habitats are available to fish in the lake and its tributaries. Small fish, including species of the Aplocheilichthys and Haplochromis genera, young and semi-adult Barbus, and Chelaethiops rukwaensis, inhabit the aquatic macrophytes on the lake margins (Seegers 1996). Swampy deltas from the Luika, Songwe, and Chambue Rivers provide habitat for another species assemblage that includes Oreochromis and Tilapia species. These two genera are also the most important commercially fished species of Lake Rukwa. The rocky habitat of the eastern shore appears to be inhabited by Labeo cylindricus, though in general these rocky shores host fewer fish species than other habitats in the lake. The Piti River contains typical riverine species including Amphilius jacksonii, Leptoglanis rotundiceps, and Chiloglanis trilobatus (Seegers 1996).
Description of endemic fishes:
This ecoregion hosts an aquatic fauna that includes endemic fish species flocks of the cichlid genus Haplochromis and the catfish genus Chiloglanis (family Mochokidae). Both flocks have six known species (Seegers 1996).
Other noteworthy aquatic biotic elements:
Large numbers of non-breeding wetland birds are known to occur at the lake, although many of these records are from the 1950s (Vesey-FitzGerald & Beesley 1960). Species include Pelecanus onocrotalus, Plegadis falcinellus, Plectropterus gambensis, Chlidonias leucopterus, and Rynchops flavirostris (Baker & Baker 2001). A colony of about 40,000 great white pelicans (Pelecanus onocrotalus) occurs on Lake Rukwa and about 20,000 white-winged terns (Chlidonias leucopterus) were observed at Lake Rukwa in January 1995 (Finlayson & Moser 1991; Wetlands International 2002).
Other aquatic species of interest in the ecoregion include the marsh mongoose (Atilax paludinosus), spot-necked otter (Lutra maculicollis), African clawless otter (Aonyx capensis), and hippopotamus (Hippopotamus amphibius). The Nile crocodile (Crocodylus niloticus) may occur in high densities as evidenced by the continuing high annual rate of human mortality attributed to crocodiles.
Justification for delineation:
This ecoregion is defined by the Lake Rukwa basin with its two endemic fish species flocks. Recent connections existed between the Rukwa basin and both the Malagarasi system [543] and the Chambeshi (which drains to Lake Bangweulu [544]). There is also evidence of connections between the headwaters of the Manda and Kalambo Rivers, which presently drain into Lake Tanganyika, and the Mfwizi River of the Rukwa drainage (Seegers 1996).
Level of taxonomic exploration:
Good. Important historical references for this ecoregion include Ricardo (1939), Verheyen (1939), Vesey-FitzGerald and Beesley (1960) and Vesey-FitzGerald (1964)(Ricardo 1939; Verheyen 1939; Vesey-FitzGerald and Beesley 1960; Vesey-FitzGerald 1964). The fish species of the lake are well known with Seeger’s (1996) volume on the fish of the Lake Rukwa basin. Additionally, technical reports available at the Regional Natural Resources Office in Mbeya, such as Davenport (2000) review the status of the basin’s ecosystems and provide comprehensive species lists of flora and fauna of the area(Davenport 2000).
References/sources:
Baker, N. E.,Baker, E. M. (2001)"Tanzania" In Fishpool, L.D.C.;Evans, M.I. (Ed.). Important bird areas in Africa and associated islands: Priority sites for conservation. (pp. 897-945) Newbury and Cambridge, UK: Pisces Publications and BirdLife International (Birdlife Conservation Series No. 11).
Baker, N. E.,Baker, E. M. (2002) "Important Bird Areas in Tanzania: A first inventory". Dar es Salaam, Tanzania. Wildlife Conservation Society of Tanzania.
Davenport, T. R. B. (2000) "Lake Rukwa Basin Integrated Project Unpublished technical report for 1999-2000". Mbeya, Tanzania. WCS / CIC / Game Division.
Finlayson, M.,Moser, M. (1991). "Wetlands" Oxford, UK: Facts on File.
Hughes, R. H.,Hughes, J. S. (1992). "A directory of African wetlands" Gland, Switzerland, Nairobi, Kenya, and Cambridge, UK: IUCN, UNEP, and WCMC.
Ricardo, C. K. (1939). "The fishes of Lake Rukwa" Journal of the Linnean Society of London 40(275) 625-657.
Seegers, L. (1996). "The fishes of the Lake Rukwa drainage, Annales du Musée royal de l’Afrique Central, Sciences Zoologiques, 287" Tervuren, Belgium:
Verheyen, R. (1939). "Notes sur la faune ornithologique de l’Afrique Central" Bull. Mus. Roy. Hist. Nat. Belgique XV 61.
Vesey-FitzGerald, D. (1964). "Mammals of the Rukwa valley" Tanganyika Notes and Records 62 1-12.
Vesey-FitzGerald, D.,Beesley, J. S. S. (1960). "An annotated list of the birds of the Rukwa Valley" Tanganyika Notes and Records 54 91-110.
Wetlands, International (2002) "Ramsar Sites Database: A directory of wetlands of international importance" <http://www.wetlands.org/RDB/Ramsar_Dir/_COUNTRIES.htm>(2003)
