conditions in sediments high in organic matter. The H₂S reacts with iron compounds in the soil to form pyrite (FeS₂) which when exposed to the air oxidises to form sulphate and free sulphuric acid.
In the Okoroba-Nembe district these soils are of two types. First Cat-clays, the more recently deposited alluvial mud which on exposure to the air oxidises to release bubbles of H₂S - the typical rotten-egg smell of mangroves at low tide. Second, Chicoco soils, the older peaty clay soils formed by the thick mat of Rhizophora mangrove rootlets, and also subject to high oxidisation and deterioration upon exposure.
20.3.3 FRESH-WATER/BRACKISH-WATER SOILS
Soils subject to both brackish and fresh-water regimes occur were mangrove soils have extended above the salt water level and, to a lesser extent, where fresh-water soils are subject to brackish water during the dry season. Both soils show similarities to their mangrove and fresh-water counterparts but tend to be silty clays with a high acidity, the former being higher in organic matter.
20.4 THE NATURAL ECOSYSTEMS
The natural ecosystems of the Okoroba-Nembe district climax as alluvial tropical rainforests. Rainforests which would have been ecologically youthful in relation to the lowland tropical rainforests which are the natural climax inland beyond the Delta. There are two natural ecozones in the district: the Brackish (mangrove) and the Fresh-water ecozones; and the ecotone between them.
20.4.1 NATURAL BRACKISH-WATER ECOSYSTEMS
The natural mangrove forest ecosystems form on water-logged alluvial deposits where the water regime is brackish, between the high and low tide marks, in areas protected from the battering of the Atlantic waves. They are pioneer ecosystems because alluvium and organic matter caught by the mangrove roots, in addition to the biomass created by the mangrove trees themselves, creates their own medium and they literally push out the land into the lagoons, creeks and rivers.
The dominant mangrove species is the Red Mangrove, Rhizophora racemosa with its characteristic stilt and aerial roots. On newly colonised mud the trees can grow up to 40m, but the second generation will be much smaller, such is the species selfcompetition. This gives the characteristic landscape around Nembe where great belts of trees billow up above the flat landscape on the inside curves of the wide rivers.
The natural mangrove ecosystem is low in biodiversity (in the Nembe district, there is mile after mile of stilt-rooted R. racemosa and nothing else) and biomass. But despite this, the mangroves provide a refuge and breeding ground for large populations of aquatic animals which depend on it during part of or for the whole of their life cycles.
20.4.2 NATURAL FRESH-WATER ECOSYSTEMS
The natural fresh-water forest ecozone is made up of three ecosystems defined by the drainage regime of the three fresh-water soils already described. Intermediate conditions exist, for instance on natural or man-made ridges running through the back swamps and in swamps that dry out at the very peak of the dry season.
