- Finally the soil is raised well above the waterline and is well aerated by the burrows of land crabs. Concurrent with these soil changes, the mangrove vegetation is replaced by dry-land communities.
It takes about a hundred years for fresh alluvial mud to be converted into fresh-water land by mangroves: this is also the average life span of a successful Rhizophora racemosa tree.
However, the dynamics of the Niger Delta landscape itself may interrupt this simple progression. Rivers move across the delta plain and erosion patterns change; young alluvial deposits with tall Rhizophora racemosa may be eroded away by a shifting river course before they have a chance to develop Chicoco soils; a widening estuary (as at Sangana) may erode a spit and remove the protection it offered from the Atlantic breakers to a young mangrove forest developing on the edge of the lagoon behind.
Spit: a strip of sand or shingle projecting from the shore, usually above the water, deposited by the action of the waves (longshore drift).
Lagoon: a body of water with restricted entry to the sea, protected from the full impact of the waves behind a spit, a reef or a sand bar.
The tidal step: a feature of the banks of the mangrove creeks and rivers is a step, noticeable at low tide, between the high and low tide marks. This is caused by the rush of water during the four daily tidal movements, compared with the "quiet" periods at high and low tides: thus the increased velocity of the water "rush hour" causes erosion. See Figure 3.
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.
However the mangrove forests of the BAM are one of the few terrestrial ecosystems (salt flats in temperate zones are another) that also take matter up from the sea. The tide brings in mineral salts, especially phosphate and nitrates, together with detritus and other sediments that become trapped in the mangrove roots and incorporated in the soil-making process. These are vital nutrients as the mangrove soils become dry land.
7.5 FOOD CHAINS OF THE BAM ECOSYSTEM
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.
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. Half of this falls as leaves and dead wood, to be decomposed by fungi and bacteria.
The chemical compounds and detritus products of this decomposition provide nutrients for the plants and animals within the mangrove forest. However, via tidal
