Page:Niger Delta Ecosystems- the ERA Handbook, 1998.djvu/218

Human Ecosystems: Sangana in Akassa Akassa is subject to both deposition and erosion, the balance of which determines whether the island (or various parts of it) is in advance or retreat. Deposition is facilitated by sand brought down the tributaries of the Niger River and spread along the coast by the longshore drift. At Akassa the drift is from West to East (a little further West the movement is from West to East as the Northeast current hits the "nose" of the Niger Delta, splitting East and West along the coast). Sand is also distributed back up the Estuaries.

The balance between deposition and erosion is complicated. On the coast, it is determined by the amount of sand being brought down the rivers. In the estuaries, it is also determined by the volume of fresh water being carried by the rivers and by the source of the water. The Nun, for instance carries more water from the upper Niger, and thus more sand, than the Sangana, whose water comes largely from within the Delta.

Thus there has tended to be more erosion pressure on the Southwest end of Akassa ("Sangana Point" on Map 10.) and deposition at the Southeast end forming the spit of Cape Nun. Nonetheless because of the Easterly current, sand brought down the Sangana Estuary has tended to be swept East, counteracting the erosion.

However within the last 50 years the Sangana Estuary has become wider: within this period the local people could shout across the river and an aerial photograph of 1963 shows a Cape Sangana, that is no more.

19.3 SOILS

The sandy Akassa soils are Catenas (see 4.5.5) showing a repetitive pattern from North to South according to the low corrugated topography described above. Typically they graduate from the relatively well-drained top of the sand ridge, through a rising water table, through seasonal swamp, to permanent swamp. However there are variations on this theme: the swamp may be brackish or fresh-water; there may be no permanent swamp; and the soils are older in the North than in the South. In addition there are mangrove soils and coastal strand soils.

The soils on the highest part of the sand ridges are Oxisols (4.5.5), sandy soils, subject to continual leaching because of the high rainfall and therefore dependant upon the rainforest humus for their fertility. But they are younger than the Botem-Tai Oxisols and having a high water table, more shallow with a dark brown, humus and iron rich, subsoil between 0.5 and 1.5m where the leachates have accumulated.

In the troughs between the ridges the soil type depends upon whether or not they are subject to permanent inundation and if they are, if it is by fresh or brackish water. The soils collect material rather than lose it (thus despite forming on sandy deposits they have a higher silt and clay content than the ridge top soils) leaching is absent, being young they have only a single soil horizon, and because they are regularly or temporarily water-logged they are reduced. Thus technically these soils are Inceptisols-Aquepts (4.5.5).

In fresh-water conditions where there is some drainage, there are grey-blue "gley" soils, because of the reduced iron and other ions, with red-brown mottles of oxidised iron.

Mangrove soils occur where the troughs carry tidal creeks, in the brackish-water ecozone in the North of the island and where mangrove trees have colonised the West side of the Nun Estuary which is protected from the prevailing Southwesterly winds and currents. These are acid sulphate soils formed where sulphate in seawater reacts with iron in the soils to form pyrites which when oxidised forms sulphates (Hydrogen sulphate gives the rotten egg smell of mangrove swamps at low tide) and sulphuric acid. These 216