Boundaries

From the Cahora Bassa Dam, which forms the upper limit of the ecoregion, the Lower Zambezi River flows southeasterly for 593 km through Mozambique and to the Indian Ocean (Davies 1986). This ecoregion includes the Zambezi below Cahora Bassa and the lower Shire River and falls largely within Mozambique, although it extends slightly into Malawi, Zambia and Zimbabwe. Faunistically, the Lower Zambezi is a crossroads, containing elements of the Middle Zambezi, of the eastern coastal rivers and of the Malawai region, in addition to brackish and marine species.

Freshwater habitats

From Cahora Bassa Dam down to Lupata Gorge, 70 km downstream of Tete, the Zambezi River generally follows a clearly defined channel. From below the gorge to the sea, the river is broad, often consisting of many anastomosing channels with shifting sandbanks. The Zambezi Delta starts at Mopeia, some 120 km from the coast, where the Rio Cuacula splits from the main channel and flows to the east towards Quelimane, and the main Zambezi flows to the southeast.

Large amounts of sediment that the Zambezi River formerly distributed over its floodplains in the Lower Zambezi ecoregion now accumulate behind Kariba and Cahora Bassa Dams leading to changes in the morphology of the river downstream (Davies et al. 2000). However, tributaries below the dams, such as the Shire, still bring in large quantities of sediment during the rains.

The Zambezi Delta, with its associated woodlands, savanna, mangroves, and coastal dunes, is a regionally important area for conservation. Its wetlands form a mosaic of various woodland types on rather unstructured sandy substrates and the southern portion around Marromeu is an area of major significance for wetland biodiversity with an extensive area of papyrus, lagoons, aquatic grasslands, and mangroves (Timberlake 2000; Timberlake et al. 2000). Müller et al. (2000) classified the vegetation in seven groups: forest/woodland, grassland, mangrove, savanna/palm savanna, wetland, dunes/beach, and fallow fields. A total of 445 taxa were sampled in the delta, of which 84 were classified as wetland plants.

Terrestrial habitats

The soils in the area are largely kaolinitic and inland vegetation is relatively undifferentiated with Colophospermum mopane and Adansonia digitata being common (Davies 1986). The alluvial delta region is a mosaic of distinct vegetation types with 13 land cover categories recognised (Beilfuss et al. 2000) and extensive mangrove forests are present along the coast (Davies 1986).

Description of endemic fishes

The Ruo River, a tributary of the Lower Shire, has a unique relict fauna above the 60 m high Zoa Falls (Ecoregion [843]). Apart from those in the Ruo there are no known Lower Zambezi endemics.

Justification for delineation

The Lower Zambezi ecoregion begins at the base of the Cahora Bassa rapids and extends downstream to include the delta. From the Lupata Gorge downstream, the river is broad and often has anastomosing channels with shifting sandbanks (Timberlake 2000). The origins of the Zambezi system are complex, but it is likely that the Upper Zambezi was once separate and probably joined the Limpopo (Skelton 1994). It was captured by the current lower Zambezi during the mid-Pleistocene when the middle Zambezi cut back through the Batoka Gorge, a long (>100 km), east-west fissure through basalt, at the head of which is Victoria Falls, where erosion through similar limestone-filled fissures continues (Davies 1986). The Victoria Falls is presently the western boundary of the middle Zambezi system, separating it from the upper Zambezi. Evidence from the distributions of fish (Jackson 1986) and other freshwater animals such as the Odonata (Pinhey 1978) support this hypothesis about the Zambezi system. A striking example is the occurrence of nine Synodontis species, seven of which are found only in the Upper Zambezi and the other two only in the middle and lower sections. The fish fauna of the Lower Zambezi has several Zambian/Congo elements not found in the Upper Zambezi, such as two Distichodus species, Mormyrops anguilloides, Mormyrus longirostris, Heterobranchus longifilis, Malapterurus shirensis, and Protopterus annectens. Such species may have reached the Lower Zambezi partly through a northern link and partly by movement through the Middle Zambezi. The Lower Zambezi fauna contains all the elements of the Middle Zambezi, some east coast species typically found in floodplains, some species characteristic of the Malawi region, and some marine species (Tweddle & Willoughby 1979; Skelton 1994; Marshall 2000b). This ecoregion is also distinguished by the Zambezi delta, with its convergence of freshwater and marine fauna, extensive mangroves, and floodplain habitats.

Level of taxonomic exploration

Poor. Research in the Lower Zambezi has been very limited. After the major surveys of Peters in the 1840s (Peters 1868), few investigations were undertaken until recently. The Lower Shire River began to receive attention from the early 1970s because of its fisheries importance (Tweddle et al. 1995). The 1999 survey of the Zambezi system, edited and compiled by Timberlake , represents a major step forward in understanding the Zambezi system. Further research is needed in all aspects of the ecoregion’s biodiversity and conservation, and recommendations are made by the various authors in Timberlake (2000).