Page:Niger Delta Ecosystems- the ERA Handbook, 1998.djvu/100

Sand-Barrier Islands

physically trapping sediment and therefore reducing the amount of sand available for deposition downstream.

The formation by of the Niger Delta sand-barrier islands is clearly being influenced both by forest clearance in the Niger/Benue river basin and by dam construction on the Niger. However it is not always clear which is having the greater effect. On Brass Island, for instance, there are obvious indications of beach formation facing the Atlantic, yet the estuary clearly suffers from quite severe erosion.

Estuarine Erosion: Once an estuary has begun to be widened by river erosion (see above), it becomes more vulnerable to erosion by the sea so that there is an exponential increase in estuarine erosion. Conversely, in a form of positive feedback, estuaries where deposition predominates are then increasingly protected from erosion by the sea. Map 3B shows that the most open sections of the Niger Delta coast are those not fed by the main white-water or sediment-bearing rivers; the East of the Delta is opening up to the destructive forces of the Atlantic, because less sediment is available in the deposition/erosion equation. Conversely the Western Niger Delta, well fed with sediment by the Forcados and Nun river systems, is much better protected, although there is evidence of substantial erosion in the Sangana estuary.

8.3 THE NATURAL ECOSYSTEMS

The natural ecosystems of the sand-barrier islands are specifically determined by their proximity to the sea, their high water tables, the sandy parent materials and their topographical character as low corrugated plains. Figures 3A show a typical sand barrier island cross-section and plan.

The interior of a sand-barrier island is essentially a catena landscape, with a high water table. It is bounded by mangrove swamps inland and Beach Strand on the seaside.

Four sub-ecozones can be distinguished:

• ridge-top tropical rainforest,
• freshwater swamp forest between the ridges;
• brackish-water swamp forest or mangrove; and
• beach strand.

This last is the only ecozone unique to the sand barrier islands and will be described in more detail; the first three are essentially similar to ecozones we have previously considered and thus are only briefly explained below.

8.3.1 TROPICAL RAINFOREST ON THE SANDY RIDGES

Because of the way sand barrier islands evolve over time, the sandy ridges can be described as 'fossil beaches'. Their shallow oxisol soils (see 4.5) undergo very little leaching because of the high water table, and nutrients tend to collect just above it (at about 1m. down). The ridges can therefore support a narrow band of tropical rainforest, similar to the levee forest of the FAM. As the ridge falls away and the water table gets

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