The Freshwater Ecozone
The Fresh-water ecozone is significant in another way: together with the Brackishwater ecozone it is the last terrestrial sink for the drainage of much of Nigeria and a very large chunk of West Africa. Such systems generally reflect the ecosystems that they drain, and concentrate all the problems of those ecosystems. Just as measuring the sediment load of a river gives an indication of erosion upstream, so assessing the chemical compounds carried in a river indicates pollution inputs. The Niger Delta does not exist in isolation from the rest of the river ecosystem; in fact it suffers the most from any upstream misuse of the ecosystem.
Thus, some of the preconceptions about lowland tropical rainforest (see 5.3) must be reconsidered for the case of the freshwater ecozone.
- Firstly, extensive parts of the palm swamp forest are naturally impenetrable, where waterlogging limits the growth of tall, thickly crowned trees so that light is available for densely growing palms. The subsequent thickets of raffia and oil palm can be impossible to navigate.
- Secondly, seasonal swamp soils may be very rich in nutrients, again because of limited drainage (a high water table means lower flows of water through the system and therefore less leaching).
Figure 4. describes the freshwater ecozone and its sub-ecozones in diagrammatic form. Note the following points:
- The two levees are the two sides of a white-water river valley. Over time, the river has moved from right to left and is now eroding the left bank and depositing sand and silt on the right bank.
- The levees remain above water all year; in terms of topography, their soils are eluvial and colluvial (see 4.5.4); these are well-drained but shallow and may be described as 'young shallow oxisols' (see 4.5.5).
- The flood plain is under shallow water during the peak flow of the river. At these times, the soil has Illuvial characteristics. However, during the driest part of the year (December to March) when the flood recedes, the soils experience Eluvial and Colluvial processes as water drains down through them.
- The most recent silts are building up on the left flank of the young deposits; these are only exposed during the driest months when the river is at its lowest.
- The swamp to the far left of the diagram is not uniform, but is made up of permanent and seasonal patches. Permanent swamp is below the water table all year round, while seasonal swamp is actually only submerged at the very height of the wet season but remains waterlogged for the rest of the year. A black-water river may run through the swamp.
- The swamp soils are inceptisol aquepts (see 4.5.5), but there is not a sharp cut-off from the levee oxisols; from the levee towards the swamp, as the water table comes nearer and nearer to the surface, are transitional soils.
- Between the flood plain and the right-hand levee are shown a seasonal flood-river and an earthworm ridge (see 6.4.3). These are just two of the features that may complicate a section of the real FAM ecozone.
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