Niger Delta Ecosystems: the ERA Handbook/The Natural Sand-Barrier Islands of the Niger Delta
8 THE NATURAL SAND-BARRIER ISLANDS OF THE NIGER DELTA
- The Formation of the Sand Barrier Islands
- The Balance Between Deposition and Erosion
- The Natural Ecosystems
- The Beach Strand Sub-Ecozones
- Animal Communities of the Sand Barrier Islands
8.1 THE FORMATION OF THE SAND BARRIER ISLANDS
The Sand-barrier Islands form a 200km. chain along the Niger Delta coast, protecting the mangrove forests from the battering of the Atlantic Ocean. Without the barrier islands there would be merely a bay of low, crumbling cliffs along the boundary of the old coastal plain; they are both a guardian and a product of the Niger Delta itself.
Eroded sand, borne by the Niger/Benue river systems eventually reaches the sea. However it may then be driven back towards the shore by wind and waves, and may collect in deposits along the shallow bed of the continental shelf.
In this case, sand brought down to the ocean by the Niger/Benue is carried back to the 'nose' of the Delta by the Guinea current (see Maps 1 and 3B). Longshore drift spreads it to the East and West, where it joins sediment contributed by lesser river systems such as the Imo and the Osse (Ovia).
The sand spreads as a band of islands along the coast. They are mostly 'true islands', completely surrounded by water, be it the Atlantic itself or the water of lagoons and of the creeks that twine their way around islands formed further inshore. Where they form in the mouth of an estuary, they may be called sand bars.
Sand thus thrown up by the Atlantic often forms a high beach, behind which streams running from inland may form a lagoon. Eventually the beach is higher than the level of the highest tide, and is colonised by vegetation that further stabilises the new island.
This continuing process forms a low corrugated plain, no more than a few metres above sea level. Sandy ridges running parallel to the coast line alternate with troughs containing swamps, small rivers or, on the estuarine edges of the islands, tidal creeks. (Plan and Section diagrams of a sand-barrier island are given in Figures 3A.)
8.2 THE BALANCE BETWEEN DEPOSITION AND EROSION
Long-shore drift involves erosion and deposition of sediment. Formation of new islands depends on the amount of sand deposited being greater than the amount of erosion.
This balance between deposition and erosion is affected by long-term geological factors. Over shorter time periods, variations in rainfall and in human activities will differentially affect different parts of individual islands and different sub-ecozones. The two human activities that have the most profound impact on this balance are forest clearance and the construction of dams.
Clearing an area of forest vastly reduces the amount of vegetation protecting the soil from rainfall damage. It also largely removes the 'sponge' that moderates the release of rainwater into river systems. The result is increased erosion and increased sediment loads carried by rivers, and therefore an increase in the amount of sediment available for deposition. There may also be an increased risk of flooding, for two main reasons.
- Firstly, because the forest 'sponge' is no longer acting, rainfall runs straight off the land into streams and rivers. These may now disappear during the dry season, but break their banks or overflow in the rainy or wet season.
- Secondly, especially where river flow is sluggish, some of the increased sediment load may settle out on the riverbed. As the riverbed rises, the volume available within the banks for the actual flow of water is reduced and again the risk of overflow is increased.
In many areas of the world, including parts of China, India and Europe, forests have been cleared and entire landscapes devoted to agriculture for hundreds or even thousands of years. In some places, earth walls or dykes have been built to contain floods, and sometimes riverbeds are now actually higher than the surrounding land. When such rivers break their banks, the floods can be catastrophic and may not only destroy fields and buildings but cause the deaths of thousands of people.
However, just as forest clearance can accelerate erosion and increase the sediment load of a river system, the construction of dams often reduces this load by 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.
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 even nearer to the surface, this rainforest merges into seasonal and then permanent swamp forest.
However, the islands and ridges all vary; ridges may be wide enough to support a relatively wide plain of broad-leaved rainforest, or so narrow that the palm forests of the FAM on either side almost meet.
8.3.2 FRESH-WATER ECOSYSTEMS OF THE TROUGHS
Similarly, troughs between ridges may be wide enough to carry a permanent or seasonal lake. However if the trough is very narrow, the broad-leaved rainforest of the ridges either side may make it too dark for any palms to become established.
8.3.3 BRACKISH-WATER ECOSYSTEMS
To the North of the model island illustrated in Fig. 3A, mangrove forest has colonised the brackish-water and sheltered fringe of the island, inland. Mangrove forests are also thriving on the East and Southeast edges of the island, where they are protected from the prevailing south-westerly winds and waves by the sand spit that has developed as a result of long-shore drift. The West side of the island is exposed to stronger winds and currents; nonetheless, a salt-water creek exploring a wider than normal trough has attracted an Avicennia mangrove forest to the protected interior.
8.4 THE BEACH STRAND SUB-ECOZONES
Within the Niger Delta, the Beach Strand ecosystems are unique to the Sand Barrier Islands. The beach strand sub-ecozone can be broken down into three bands: the beach that is exposed at low tide, the beach ridge that has risen above the high tide mark; and littoral forest inland.
8.4.1 THE BEACH
The beach itself, about which very little is known, is obviously it is constantly being changed by the sea. Crabs and Polychate worms live under its surface.
8.4.2 THE BEACH RIDGE
The beach-ridge is a pioneer ecosystem containing a narrow range of plants that can withstand the salty winds blowing off the Atlantic. These include the convolvulus, Ipomea pes-carpea, and the shrubs Hibiscus tiliaceus and Conocarpus erectus, all of which have thick waxy leaves. The soils are inceptisols (see 4.5.5); young, but nonetheless exhibiting a surface horizon which enables larger plants to establish themselves.
8.4.3 THE LITTORAL FOREST
The Littoral Forest is a development of the Beach Ridge ecosystem. The same types of thick waxy leafed plants are present, but more as a dense scrub forest which graduates into small trees that protect plant species more characteristic of the interior. This forest in turn forms a protective barrier against windborne salt for the interior tropical rainforest beyond.
8.5 ANIMAL COMMUNITIES OF THE SAND BARRIER ISLANDS
The sand-barrier islands are a natural ecological microcosm of the Niger Delta as a whole, and the natural diversity of animals species is wide.
Surrounding water and the mangroves are a barrier to many larger mammals. However, animals such as Elephants and Chimpanzees are recorded in the modern ecosystems of some of the islands.