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East African mangroves

Highly productive nurseries for fish and prawns, eastern African mangroves significantly enhance the biodiversity of surrounding marine habitats while providing vital habitat for migratory birds, marine turtles, dugongs and porpoises. The most developed mangroves in this ecoregion extend as far as 50 km inland, with canopy heights up to 30 m. However, Eastern African mangroves are threatened in many areas by overuse and conversion by a growing human population that utilizes the mangroves for rice farming, shrimp aquaculture, and for construction materials and the timber trade.

  • Scientific Code
    (AT1402)
  • Ecoregion Category
    Afrotropical
  • Size
    5,800 square miles
  • Status
    Critical/Endangered
  • Habitats

Description
Location and General Description
Eastern African mangroves encompass mangrove areas found in Mozambique, Tanzania, Kenya, and Somalia. The dominant climatic influences on most of the region are the seasonal wind patterns, associated with the Northeast monsoon (NEM) and the Southeast monsoon (SEM), which blow towards the mainland from the northeast and southeast at different times of year, and which affect the movement of the major coastal currents. The SEM, which occurs between April and October, is associated with periods of heavier rainfall as well as stronger wind and waves. South of Malindi, in Kenya, the region is humid tropical. To the north, it is Sahelian (Ruwa 1993). A major tropical current also hits the East African coast perpendicularly around the Tanzania-Moçambique border and then divides north and south.

Because of their high productivity and the availability of sheltered areas needed by juvenile fish and other marine organisms, Eastern African mangroves along open coasts enhance biodiversity in neighboring habitats such as coral reefs, which are nutrient poor, and which, in turn, shelter the mangroves from harsh wave action. The large areas of mangrove found at river mouths, especially in the deltas of larger rivers, provide important habitat for migratory birds. Because of their ecological functions, these mangroves are important to many animals not exclusively associated with mangrove areas, including the dugong, several species of marine turtle and different species of porpoise.

Annual rainfall averages range between 750 and 1,500 mm, with the highest found in southern Kenya and northern Tanzania. Maximum tidal amplitudes range from 3.2 m in Tanzania to 3.5 m in Kenya and 5.6 m in Mozambique (Spalding et al. 1997). Most of the Kenyan coastline, which has only two major rivers, is semi-arid, but additional freshwater is derived from submarine groundwater discharge, which creates conditions suitable for mangroves.

There are two general categories of mangroves: those found in fringe communities along the open coastline, and creek mangroves, which are found at river mouths. Fringe mangroves often indicate the presence of groundwater discharge sufficient to dramatically lower salinity levels, as are found at Mida Creek and the Lamu Archipelago (Ruwa 1993). Mangroves found at river mouths have greater patterns of zonation among species, because the tides, and thus the mangroves, reach farther inland. The most developed mangroves are found between the Beira and the Save Rivers in Mozambique, where they extend up to 50 km inland with canopy heights of up to 30 m (Spalding et al. 1997). The major river drainages with the most extensive mangroves are the Tana and Sabaki in Kenya, the Rufiji and Ruvuma in Tanzania, and the Zambezi and Limpopo in Mozambique. In much of central and northern Mozambique, mangrove development is inhibited because of greater physical disturbances: it lies within the "cyclone zone," an area found in the southern hemisphere between 10° S and 30° S. At the northern end, in Somalia, an upwelling of cold water inhibits mangrove development.

Although it has less extensive mangrove areas, Eastern Africa has a greater diversity of mangrove species than West Africa. The species-composition also varies, with those of Eastern Africa related to species around the Indian Ocean while those of West Africa are similar to those of the Americas. Eight species of mangroves are found throughout the region, the distribution of which is primarily determined by salinity gradients, depth of water table, and the soil’s pH and oxygen content. Avicennia marina is associated with sandy soils, Rhizopora mucronata with muddy soils along rivers and creeks, Ceriops tagal with dry areas, Bruguiera gymnorrhiza with wet areas, and Lumnitzera racemosa and Xylocarpus granatum with the landward fringe, where they also indicate the transition to brackish water (Chapman 1977; Diop 1993; Commision of European Communities 1992). Sonneratia alba is the pioneer species found on open coasts, with Heritiera littoralis and Bruguiera often found behind it.

Biodiversity Features
This mangrove ecoregion includes the two largest stands of mangrove on the East African coast (Zambezi and Rufiji Delta systems). These two sites, in addition to many smaller mangrove areas are of greatest importance for migratory birds, feeding and breeding sea turtles, small remnant populations of dugong, and for their vital role as nursery areas for marine animals (especially shrimps).

Sea turtles that visit mangroves of this ecoregion are green turtle (Chelonia mydas, EN), hawksbill turtle (Eretmochelys imbricata, CR), and olive ridley (Lepidochelys olivacea, EN). These species also nest around the river mouths of some of the larger mangrove stands in the ecoregion, particularly in northern Kenya, southern Tanzania and Mozambique (Hughes and Hughes 1992). One bird species is considered near-endemic to this ecoregion, the mangrove kingfisher (Halcyon senegaloides). The dugong (Dugong dugon), although better associated with sea grass beds, is a rare marine mammal that is facing extinction.

The mangrove forest of the Rufiji Delta is any important site for migratory wetland birds, such as curlew sandpiper (Calidris ferruginea), little stint (Calidris minuta), crab plover (Dromas ardeola), roseate tern (Sterna dougallii) and Caspian tern (Hydroprogne caspia) (Bregnballe et al. 1990; Semesi 1993). It is also habitat for Nile crocodiles (Crocodilus niloticus), hippopotamus (Hippotamus amphibius), Sykes monkey (Cercopithicus mitis) and otter (Lutra maculicollis).

The Bazaruto Archipelago in Mozambique provides an example of mangroves found in conjunction with a variety of other habitats: coral reefs, seagrass beds, rocky intertidal areas, and dune forest. These mangroves, combined with the other habitats, support a large number of threatened and endangered turtles and marine mammals, and are particularly important for the survival of probably the last viable population of dugongs in Eastern Africa. The Zambezi delta in Mozambique supports a huge prawn fishery just offshore and in the vicinity of the mangrove stands there are many large sharks, a humpback whale nursery, notable populations of porpoises and the mangroves themselves provide an important stopover site for migratory wetland birds.

Current Status
Estimates of existing mangrove area in the region range from 2,555 km2 to 7,211 km2 (Spalding et al. 1997). The most extensive areas of mangroves are found in the Rufiji River Delta in Tanzania and the Zambezi River Delta in Mozambique. Protected areas containing mangroves include Mafia Island Marine Park, Jozani National Park and Sadaani Game Reserve in Tanzania; Watamu Marine National Park and Ras Tenewi Marine National Park in Kenya; and Bazaruto Marine National Park, Ilhas da Inhaca e dos Portugueses Faunal Reserve, Marromeu Game Reserve, Pomene Game Reserve, and Maputo Game Reserve in Mozambique.

Other large mangrove stands in Tanzania and Kenya have been designated as forest reserves, and are managed by special mangrove units within the Forestry Division, which in Tanzania have developed and are implementing management plans (Gaudian et al. 1995, Semesi 1993). However, these plans may not be adequate given the complexity of mangrove ecosystems, and the links to distant headwater areas of watersheds, which are the source of many of the impacts (Semesi 1993; Kulindwa et al. 1998). Although large areas such as Kiunga National Marine Reserve in Kenya are protected, an important mangrove area that is not protected is the Tana River Delta (Hughes and Hughes 1992). Somalia lacks an organized system of protected areas. However, in 1990 its coastal habitats were judged to be the most extensive and well preserved in Africa (Stuart et al. 1990), although harvesting of mangroves and their conversion to charcoal for sale in the Middle East has been ongoing.

Types and Severity of Threats
There is a long history of exploitation of the mangroves of this ecoregion. This goes back at least to the last century when the coastal area of eastern Africa was controlled by Arabic sultans from Zanzibar. Mangrove wood was a prized resource that the sultans harvested and used for building on Zanzibar, and exported to the Middle East. The trade of mangrove timber was taken over by the Germans in 1893, and continues today with larger towns along the coast of East Africa and also with the Middle East.

Areas of mangrove are still being lost through conversion to rice paddies, saltpans, aquaculture, and urbanization. Mangroves also receive untreated wastes discharged to rivers, as well as oil and industrial pollution in some places, silt from erosion, and pesticides contained in runoff. Large numbers of mangroves trees are also removed from surviving stands as timber for housing construction as well as for fuelwood used domestically and for smoking fish (Semesi 1993). Damming rivers also threatens mangroves by increasing salinity during the dry season. Mangrove destruction may also lead to increased siltation of coral reefs and coastal erosion.

Justification of Ecoregion Delineation
The mangroves of East Africa are biogeographically related to those along the west coast of Madagascar and South Africa. This ecoregion comprises mangroves stands within the tropical latitudes in East Africa, influenced by the South Equatorial Current and East African Coastal Current (EACC), which turns into the Mozambique Current. The northern boundary is where the EACC meets the Somali Current, and the southern boundary is where the Agulhas Current intersects the Mozambique Current.

References
Bregnballe, T., Halberg, K., Hansen, L.N., Petersen, I.K. & Thorup, O. 1990. Ornithological winter surveys on the coast of Tanzania 1988-89. ICBP Study Report 43, ICBP Cambridge.

Chapman, V.J. 1977. Africa B. The Remainder of Africa. In: Chapman, V.J. ed. Ecosystems of the World 1: Wet Coastal Ecosystems. Elsevier Scientific Publishing Company, New York.

Commission of the European Communities, Directorate-General for Development. 1992. Mangroves of Africa and Madagascar. Brussels, Luxembourg: Office for Official Publications of the European Communities.

Diop, E.S. (ed.) 1993. Conservation and Sustainable Utilization of Mangrove Forests in Latin America and Africa Regions. Part II – Africa. International Society for Mangrove Ecosystems and Coastal marine Project of UNESCO. Mangrove Ecosystems Technical Reports volume 3. ISSN 0919-2646.

Gaudian, G., Koyo, A. & Wells, S. 1995. Marine Region 12: East Africa. In: Kelleher, G., Bleakley, C. and Wells, S. (eds) 1995. A Global Representative System of Marine Protected Areas. Volume III, Central Indian Ocean, Arabian Seas, East Africa and East Asian Seas. Great Barrier Reef Marine Park Authority, The World Bank, The World Conservation Union (IUCN). Published by The World Bank Environment Department, Washington D.C.

Hughes R.H. & Hughes, J.S. 1992. A Directory of African Wetlands. IUCN, Gland Switzerland and Cambridge UK/ UNEP, Nairobi, Kenya/ WCMC, Cambridge, UK.

Kulindwa, K., Sosovele, H. & Mgaya, Y.D. 1998. Socio-economic Root Causes of the Loss of Biodiversity: The Case of Mangrove Forests in the three Deltas of Rufiji, Ruvu and Wami in Tanzania. WWF, Washington.

Semesi, A.K. 1993. Mangrove Ecosystems of Tanzania. In Diop (ed) 1993.

Spalding, M.D., Blasco, F. & Field, C.D. (eds) 1997. World Mangrove Atlas. The International Society for Mangrove Ecosystems, Okinawa, Japan.

Stuart, S. N., R. J. Adams and M. D. Jenkins. 1990. Biodiversity in Sub-Saharan Africa and its Islands: Conservation, Management and Sustainable Use. Occasional Papers of the IUCN Species Survival Commission No.6. IUCN, Gland, Switzerland.

Prepared by: Sylvia Tognetti
Reviewed by: In progress

 

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