Nick Ashton-Jones, with Oronto Douglas and Susi Arnott

“ Generally the sea is the final sink for organic and inorganic materials being washed off the land. In open rivers and estuaries, mangrove forests lose dissolved nutrients, detritus and semi-decomposed litter to the sea; in lagoons and more protected areas, nutrients are washed out with the tide. ”

“ Mature individuals can grow to this size if they are the pioneers on fresh deposits of soft mud on the riverine or lagoon-side margins of the sub-ecozone. However, later plants, developing from seedlings that have started out on mature mangrove sites, may not grow above 4 metres, as competition and impoverished soils prevent them from fully developing their root systems. It is therefore usual to find a vigorous outer fringe of mangrove forest where new silt is being deposited, with a lower shrubby tangle of smaller trees on the older soil inland. ”

“ At the top end of this food chain, the dominant animals of the BAM ecozone are crustaceans, molluscs and fish. Compared with the neighbouring inshore and freshwater ecosystems, higher animal species diversity is low; comparatively few animals are fulltime residents of the mangrove forest. However, within the ecozone, greatest diversity is found in protected areas where there is strong ebb and flow of tides ”

“ Rhizophora racemosa is the predominant species; an arched tangle of R. racemosa stilt roots gives the Niger Delta mangrove forest its characteristic appearance.
An ordinary root system would be unable to support a large tree on such swampy ground. R. racemosa roots do not penetrate the soil to any great depth; instead, they divide into a mass of thick rootlets immediately below the surface. The tree therefore stands upon a system of arches entirely supported by a thick, felt-like raft of its own making.
Larger trees also put out rubbery adventitious roots which drop down over the water. ”

“ BAM ecosystems have very low biodiversity and biomass compared to the adjacent FAM ecosystems. For example, in the Okoroba-Nembe district there is mile after mile of Rhizophora racemosa and nothing else. But despite this, mangrove forests provide a refuge and breeding ground for large populations of aquatic animals which depend on them for part or all of their life cycles. ”

“ Growing in the most shallow inner zone of the mangrove forest, Avicennia species are also characterised by root type. Their stubby and vertical pneumatophore or 'breathing' roots grow upwards from submerged lateral roots; they rise above the reduced mud and water to obtain oxygen. Mature stands of Avicennia trees are widely spaced and straight, rising above a carpet of pneumatophores. ”

“ Similarly, few birds spend their entire life in the ecozone. Some nest in the BAM, such as the Hammerkop; many others visit the ecozone to scavenge and to feed on insects or fish. Some of the most common are heron, egret, kingfishers, sunbirds, weaverbirds, swallows, African Grey Parrots and Fishing Owls. ”

“ Small fin-fish feed both directly on the detritus and upon the polychaete worms. Their faeces are in turn a substrate for more microorganisms. These small fin-fish are food for larger carnivorous and omnivorous fish, which are in turn food for even larger fish and for birds. Large zooplankton feed on the larvae of crabs and other arthropods. ”

“ Even the most developed BAM ecosystems still have a relatively low biomass of about 150 tonnes of dry matter per hectare. However the turnover or productivity of organic matter is high—up to 15 tonnes/ha per year under favourable conditions. ”