Page:Niger Delta Ecosystems- the ERA Handbook, 1998.djvu/91

The Brackish Ecozone Mangrove forests therefore developed along the more sheltered parts of the African coasts, where freshwater from lagoons and rivers mixes with seawater.

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 * Mangroves tolerate, rather than prefer, brackish water. There are only a few mangrove species, and they are not all closely related; what they have in common is the ability to tolerate brackish water. It must be stressed that mangroves do not require brackish conditions; they actually prefer lower salt concentrations, but compete badly with other species. However their ability to thrive in brackish, waterlogged conditions is unique amongst the tropical plants, and gives them the competitive edge to overrun these coastal areas.
 * Mangroves are pioneers. Mangrove forests represent pioneer communities, colonising new alluvial deposits and helping to form new land. Once in place, mangrove roots trap alluvial deposits and organic matter; this material, in addition to the biomass of the mangrove trees themselves, eventually creates a rich organic soil rising above the high tide mark. From then on, freshwater plant species can begin to invade the ecosystem and the weaker mangrove species are displaced.
 * Mangroves will not develop on bare sand. They need at least a layer of silty or sandy clay. Because alluvial mud and the subsequent mangrove soils develop in waterlogged anaerobic conditions in a brackish-water regime, they are acid sulphate soils or 'sulphaquepts' (see 4.5.5). They range from the younger Cat Clays, recently deposited alluvial muds with little organic matter, to the peaty Chicoco soils that have a high organic matter content as a result of the mat of roots that have developed.

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Mangrove forests do occur on the East coast of Africa and in the Red Sea, but the most extensive African mangrove forests extend from Lake Nokoue, in the Benin Republic, to the Gabon Estuary. Along the coasts of Benin Republic and Southwest Nigeria, between the Imo and Calabar/Cross River Estuaries and in the Cameroon Estuary, they have been greatly disturbed.

The Niger Delta mangrove forest is itself disturbed as a result of urban and industrial development around Warri, in the Forcados Estuary and particularly around Port Harcourt and along the Bonny River, where invasion by Nypa Palm is a problem. However it remains the largest discrete mangrove forest in Africa, covering about 5000 kms.

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In terms of species composition the mangrove forests of the West African coast, including Nigeria, are the same as those on the tropical American shores on the other side of the Atlantic, thousands of miles to the West. Yet those of East Africa are more similar to the Asiatic mangrove forests. This is because the cold waters off the southern tips of Africa and South America limit the drift of mangrove seeds in or out of the Atlantic (while they are able to move between the Indian and Pacific oceans more comfortably).

7.3 PLANT SPECIES COMPOSITION OF THE NIGERIAN MANGROVES

West African coastal mangrove forests, from Senegal through Nigeria to Angola, all have the same plant species composition. Three species of Red mangrove live in the deeper 89