Boundaries

The vast swamps of the Sudd are the primary feature of the Upper Nile ecoregion, which is situated mainly in Sudan with smaller areas in the Democratic Republic of Congo, Uganda, and Ethiopia. The ecoregion encompasses the basin of the White Nile River; its major tributaries, the Sobat River and Bahr el Ghazal; Lake Albert; and Lake Albert’s main influent, the Semliki River. The point at which the White Nile joins with the Blue Nile marks the northernmost border of the ecoregion (Rzóska 1974; Dumont 1986; Hughes & Hughes 1992).

Topography

The southern and highest upstream part of this ecoregion is situated in the Rift Valley. This area is rugged, with mountain peaks over 4,600 m. Further to the north, the topography changes dramatically where the Albert Nile flows into the shallow depression of the Sudd. The depression ranges in elevation between 420 to 380 m asl and stretches for about 600 km from end to end. Most of the depression is flat with a gradient of 0.01% or less, and it is underlain by clayey soils (Food and Agriculture Organization 1997).

Freshwater habitats

The dynamic Sudd wetland, whose size varies substantially in response to seasonal and inter-annual changes in water input, contains a diversity of habitats and supports a rich aquatic and terrestrial fauna (Rzóska 1974). Water entering the Sudd swamps drains from the hills of the Nile-Congo watershed divide, the escarpment of the Uganda Plateau, the Imatong Mountains, and the Ethiopian Plateau.

High flows in inflowing rivers peak between September and November, with flow volumes lowest between April and June. Widespread flooding occurs from July to November from the combined effects of rains and spillover from the rising rivers (Rzóska 1974). More than half the water entering the swamps is lost through evaporation and evapotranspiration within the swamps (Food and Agriculture Organization 1997). Because greater inflows appear to correlate with greater losses, the outflow from the swamps into the Bahr el Jabal is relatively constant. This outflow ultimately contributes about half of the White Nile’s flow; the remainder comes from the Sobat River (Murakami 1995).

The floodplain ecosystem supports a variety of plant species with a succession from those adapted to mesic environments to those adapted to more xeric environments. Moving from the interior of the swamps, the floral zones of the Sudd grade outward from open-water and submerged vegetation of the river-lake, to floating fringe vegetation, to seasonally flooded grasslands, to rain-fed grasslands, and finally to floodplain woodlands (Hickley & Bailey 1987). Cyperus papyrus is dominant at the riverine fringe and in the wettest swamps andcovers about 3,900 km² within the Sudd. According to Howell et al. (1988), Cyperus papyrus forms a fringe along the Bahr el Jabal that is up to 30 km broad in the south, declining to 50 m in the north, and disappearing completely east of Wath Wang Kech. Associated species are few, but those that are present are climbers and tend to be most common at channel margins where there is more light. Species include Coccinia grandis, Cayratia ibuensis, Luffa cylindrical, Zehneria minutiflora, Vigna luteola, and the fern Cyclosorus interruptus. Phragmites and Typha domingensis swamps are present behind the papyrus stands and cover about 13,600 km². They are most extensive in the central and northern parts of the Sudd away from the main river channels, but they have been little studied due to their inaccessible nature. There is also an open water area of about 1,500 km² that has floating and submerged aquatic plants. Flooding may be too deep in these areas to allow successful colonization by Typha (Howell et al. 1988).

Seasonally river-flooded grassland covers about 16,200 km² and seasonal floodplains up to 25 km wide exist on both sides of the main swamps. Wild rice (Oryza longistaminata) and Echinochloa pyramidalis grasslands dominate these seasonal floodplains. Oryza longistaminata is perennial and provides high-quality grazing for much of the year, although in dry years the re-growth after burning is sparse. Echinochloa pyramidalis, on the other hand, produces some re-growth in the dry season and thus provides important year-round pastures. The river-flooded grasslands, referred to as toic by the Dinka tribe, are important to them as dry season pasturelands.

Terrestrial habitats

Beyond the floodplain are rain-flooded grasslands that cover about 20,000 km²(Robertson 2001). These are dominated by Hyparrhenia rufa and are the major local source of thatching material. Mixed woodlands of Acacia seyal, Ziziphus mauritiana, Combretum fragrans, and Balanites aegypticaca border the grasslands (Denny 1991). Of these species the most extensive are Acacia seyal (5,400 km²) and Balanites aegypticaca (5,300 km²) (Howell et al. 1988).

Description of endemic fishes

Of the 16 endemic species within the ecoregion, Lake Albert provides habitat for seven. These are Lates macrophthalmus, Haplochromis loati, Thoracochromis albertianus, T. avium T. bullatus, T. mahagiensis,and Neobola bredoi (Lowe-McConnell 1987).

Ecological phenomena

The floodplains of the Sudd are globally outstanding for their congregations of waterbirds, with about 60 species known and a maximum of nearly three million birds recorded at one time (Robertson 2001).

Justification for delineation

This ecoregion is defined mainly by the Sudd swamp and its basin, which support an assemblage of floodplain-adapted species. It encompasses the basin of the White Nile River; its major tributaries, the Sobat River and Bahr el Ghazal; Lake Albert; and Lake Albert’s main influent, the Semliki River. The ecoregion excludes, however, lakes Victoria, Kyoga, and Edward, and the Victoria Nile, all of which feed directly or indirectly into Lake Albert. Lake Albert is included within this ecoregion due to its Nilotic riverine ichthyofauna, distinct from the lacustrine fauna of the Lakes Kivu, Edward, George, and Victoria [857] ecoregion.

Level of taxonomic exploration

Fair