Niger Delta Ecosystems: the ERA Handbook/What is the Niger Delta?
4 WHAT IS THE NIGER DELTA?
- A Unique Ecosystem
- Local Climate
- Natural Vegetation
- Natural Animal Communities
- The Natural Ecosystem
4.1 A UNIQUE ECOSYSTEM
The Niger Delta is unique by virtue of its size (nearly 26,000 km2 – see 11.2), its location and its origins.
Although it is situated in one of the wettest places on earth, it is fed by a river that passes largely through Sahel and dry savannah landscapes - geologically, some of the oldest on earth.
Considering the other continents, there are also few comparisons lying within the tropics. The Ganges Delta is only just within the tropics, but the Ganges Plain itself, to the North, is largely fed by the young Himalayan mountains. The large South American rivers reach the sea through flooded valleys rather than deltas; the Orinoco has a delta at latitude 10 degrees north, but its catchment is very different, lying within the Andes mountains and then a humid plain. Similar ecosystems to the Niger Delta might be expected in some of the Southeast Asian deltas such as the Mekong or the Fly (in New Guinea). However these river systems are shorter than the Niger/Benue and again run through humid plains rather than dry Savannah.
Some of the smaller deltas along the West African coast are similar, but they largely drain humid biomes. The Volta is an exception; Map 3 shows its large catchment area in the dry Savannah hinterland. However it discharges in the Dahomey Gap, an area of low rainfall, and could never have been a similar ecozone to the Niger Delta. (Human activity has also markedly changed the Volta delta, now being rapidly lost to coastal erosion as the Volta Dam traps most of the river's sediment load.)
Map 1 shows the Niger Delta in relation to the rest of Africa. It is a huge fan of land, fed by deposition from the Niger-Benue river system as it spreads out to reach the Atlantic Ocean on the coast of Nigeria. The Niger Delta lies just North of the equator, facing the Southeast trade winds in one of the areas of highest rainfall.
Map 1 also shows the West African biomes and how the forest biome relates to rainfall. The Dahomey Gap, an area of savannah country on the Ghana coast, separates the two distinct areas or sub-regions of high rainfall forest biomes.
In biogeographical terms, the Delta lies in the West African sub-region of the Afrotropical region. The following sections describe the principal parameters of the Delta in its physical geography and as an ecosystem.
Map 3A shows the geological setting of the Niger Delta. As with any landscape, it has undergone vast changes and continues to develop over time.
It is important to understand that there have been two distinct Niger Deltas over recent geological time. In Tertiary times, the sea level was at least 30m higher than it is today. Therefore the second delta, laid down in Quarternary times, does not simply lie on top of the Tertiary deposits that formed the first Delta. Rather, the Niger river cut a wide flood plain between Yenegoa and Onitsha on its way to the more distant sea. This sudden drop in level is still very evident today, particularly in Port Harcourt. Here, a cliff rises straight up for 10 metres above the 20-metre deep Bonny River, making an ideal site for the busy port.
During the Tertiary period, a number of basins on the perimeters of the continents filled up with marine sediments; over immense periods of time these have produced rich deposits of oil. One such basin, known to geologists as the Benue Basin, formed much of what are now the Niger and Benue river basins. It extends inland from the Niger Delta and from Mount Cameroon as far West as present-day Cotonou.
The more recent Quarternary deposits form a dynamic landscape. The rivers meander to form oxbow lakes or to 'capture' the flow of other rivers, which may then dry up; they may form levees, or over geological time they may swell and erode these away again.
Rivers to the East and West of the Niger do contribute to the Delta, but they are relatively short. By far the majority of the Quarternary deposits originate from the Niger/Benue system itself. However at various times, the river system has shifted and changed and different branches have carried more or less water. Today, the Forcados and the Nun are the largest, but within the past 75,000 years the Brass River has been more important than it is now. At one time the Sombreiro/New Calabar/Bonny systems will have been more major branches of the Niger. Features in the Anyama district illustrate past changes in river activity (see Map 8); at one time the Kolo Creek carried most water. Then local uplift, caused by deeper earth movements, allowed the Ekole Creek to take over: in turn its own head waters were captured by the Nun.
Such localised uplift is in tension with the depression caused by the weight of deposition. This interrelationship complicates the local geology as explained in Chapter 14. Another example is the Sombreiro-Ogoni terrace plain, which is several metres higher than the rest of the old coastal terrace (the Sombreiro-Warri delta plain). Again, this feature is most dramatically visible in Port Harcourt, where the old town drops suddenly to the Nembe Waterside.
The sea level continued to rise and fall during the lower (or older) Quarternary period, further shaping the changing Delta. Examples can be seen at Forcados and Bonny. At one time a sudden drop in level caused the rivers to rapidly erode new paths through deposits to the receding coastline. When the sea rose again it flooded these valleys to form the deep, wide estuaries of today.
4.3 LOCAL CLIMATE
The tropical hot monsoon climate of the Niger Delta characterises the area around the Bight of Biafra, including the lower Cross River Basin and the Southwesterly slopes of Mount Cameroon. This climate is a function of location.
The Niger Delta straddles latitude 5 degrees North of the equator. It extends into the Gulf of Guinea, dividing the Bight of Benin (to the West) from the Bight of Biafra (to the East). Except in the Northeast, where it rises to 10-15m., most of the Delta is less than 6m above sea level; it shares the monsoon climate of the surrounding Bights.
Mean annual rainfall is always high, although it varies within the Delta. Between 1948 and 1980, average rainfalls ranged from just below 2000mm. at Port Harcourt to over 4000mm. on the Southwest coast (because of the Northeast tradewinds) and around Bonny (because of the concave nature of the coast here). The Delta has such high rainfall levels because it is so near to the Atlantic, the source of the moisture; the high equatorial temperatures cause it to evaporate from the ocean surface and rise, to be carried Northwest by the Southeast trade winds. Funnelled up the Bight of Biafra, when these winds hit the Adamawa massif and rise upwards, the cooled water vapour condenses and falls as rain on the Niger Delta and on the Cross and Sanga river basins, making the area one of the wettest on earth.
Heavy rains commence in February and continue through to November, with peaks around July and September (a little earlier in Warri and a little later in Port Harcourt); relative humidity is over 80% throughout the year. However, temperature is moderated by the high cloud cover and proximity to the sea; the mean temperature as measured at Brass ranges from 24 to 27 degrees Celsius.
The hydrology of the Niger Delta has to be understood in terms of two water types; freshwater, either 'white' or 'black'; and brackish water, which is salty (although less salty than seawater).
4.4.1 FRESHWATER – 'WHITE-WATER'
Water coming into the Delta from the Niger/Benue river system is termed 'white-water'. It is so called because the sediment it brings down from the interior makes it very cloudy. As the rivers branch and lose velocity, this sediment is dropped and over time has formed the Delta itself.
Most white-water discharges into the Forcardos and the Nun-Ecole river systems (see Map 3B), but some also reaches the Orashi/Brass system, especially during the annual floods. Then the Niger itself spreads across to the Orashi, below Onitsha. These rivers all peak in October when the whole Niger/Benue is carrying down its maximum load, and flooding is severe.
Significant sediment loads are also carried by the Osse (or 'Ovia') river that runs from the Yoruba Uplands into the Benin Estuary, and by the smaller Imo river system.
4.4.2 FRESHWATER - 'BLACK-WATER'
However, because of the very high rainfall, a number of river systems (primarily the New Calabar, Sombreiro and Warri systems) rise within the Niger Delta itself. They discharge 'black-water', so called because of the organic acids it carries in solution. The water is darkened but remains clear, as it carries little or no sediment.
In contrast to the white-water bearing rivers that arise in the North and drain into the Delta, there is a much less pronounced seasonal peak flow in the locally fed blackwater rivers. This is because the freshwater ecosystems of the Delta act as a sponge, regulating flow by soaking up water in the wet season and releasing it slowly through the dryer months.
Where black-water discharges into a white-water river, there may be a visually dramatic contrast, as where the local Okomu river flows into the Osse.
4.4.3 BRACKISH WATER
A major factor in the hydrology of the Niger Delta is the intrusion of seawater, making water in much of the area slightly salty, or brackish. The influence of seawater extends up to 60km inland, from northeast of Sapele (on the Benin river), northeast of Warri, and as far as Choba on the New Calabar river. It extends rather less far along the coastline, for just under 20 km between the Sangana and the Forcardos.
As freshwater comes down the river to meet seawater, there is a gradient of salinity. The concentration of salts is in a constant state of flux; both daily, according to the tides, and seasonally, according to levels of freshwater discharge. At the peak of the white-water discharge in October, freshwater extends out into the Atlantic; flying over the Niger Delta at that time of year, the silt-laden waters are an impressive and slightly disturbing sight. However in January, with low volumes of freshwater in the rivers, brackish water briefly intrudes into freshwater systems. During this time, tidal movements have a greater influence on water flow than the river currents.
It is no coincidence that the world's first great civilisations developed where there were especially fertile soils; soils are the basis of human economic activity. An understanding of soils goes a long way towards understanding human ecology.
4.5.1 SOIL PROFILES
Soils can be described in terms of their 'profile'; that is, a description of a vertical section from the surface down to the parent material. This can variously be achieved by simply digging a hole, by taking out a plug sample with an auger, or as is often the case, by looking at the sides of a borrow-pit. Usually a series of layers can be seen. They are known as 'horizons'.
The figure 5 and the explanation below describe a generalised soil in terms of four horizons, working down from the surface. Many of the Niger Delta soils are far from 'typical', but here are the relevant terms nonetheless.
#The 'A' horizon
Also known as the eluvial topsoil, dominated by eluviation - the downward movement of nutrients. Within this horizon there may be four further layers.
On the surface lies the litter of fallen plants; then partially decomposed litter, plant roots and dead animals; below this is a layer of fully decomposed organic matter, stained black or brown with the nutrient-rich end product of decomposition called humus; at the bottom is a lighter coloured layer from which nutrients have been leached, or carried down by drainage water, to the lower B horizon.
#The 'B' horizon
The B horizon is the illuvial topsoil, dominated by illuviation - the acquisition of nutrients carried down by drainage water from higher layers. This is why it is generally darker in colour than the lowest layer of the 'A' horizon.
#The 'C' horizon
This is the subsoil, made up mostly of broken-down parent material and some elements of the topsoil.
#The 'D' horizon
This is the underlying parent material itself, which may for example be rock, gravel, or sand deposits.
This is a generalised description. In reality, some soils show very clear layering while others vary more gradually. An entire soil profile may be a few centimetres in depth, or extend for tens of metres.
4.5.2 TYPICAL TROPICAL MONSOON SOIL
Typical tropical monsoon soils, as found throughout the Delta, have a very shallow A horizon, because decomposition is so rapid. However, the underlying B horizon may be very deep, because the parent material is breaking down rapidly. The B horizon may vary in colour from deep red to white, depending on the proportion and chemical state of the iron in the soil.
Under natural conditions, leached nutrients in this B horizon are taken up very rapidly by tree roots so that the soil is often inherently poor. On anything steeper than a gentle slope, this soil is very easily eroded.
4.5.3 SOILS, VEGETATION AND OXYGEN
Soils are important to humankind because we depend upon the plants which grow in them for our survival. We depend on the ability of plants to trap sunlight energy in the synthesis of carbohydrates, the start of our foodchain; and on the oxygen they produce in this process that we need to make use of this stored energy.
But plants need some of this oxygen, too. While it is the green leaves of plants above the surface of the ground that receive and process energy from the sun, a great deal of a plant's activity occurs below the surface. With a few notable exceptions, such as the water hyacinth, plants depend upon the soil for their anchorage, nutrition, water supply and for their oxygen.
An understanding of soil conditions, particularly in wet areas like the Niger Delta, must include the realisation that plants can drown just as we do, if the soil surrounding them is so waterlogged that they cannot take up oxygen through their roots. Particularly where water is not moving through the soil, the oxygen supply is limited and even the best-adapted plants may suffer.
4.5.4 GEOGRAPHICAL FACTORS AFFECTING SOILS
Human use of the soil resources of the Delta has further increased the variety of soil types. However, Delta soils have all developed in relation to the following geographical factors:
Material brought down from the ancient landscapes of the West African interior by the River Niger are not rich in nutrients. These sediments are largely made up of quartz and kaolinite, plus some iron oxides. Quartz, or silicon dioxide, is one of the most common minerals in the earth's crust and is more or less inert in chemical terms. Kaolinite, a silicate clay, is also relatively unreactive; its molecular structure makes it less prone to swelling and shrinking than other clays.
Material brought down from the volcanic rock of the Adamawa massif by the river Benue (see Map 3) is richer in nutrients, but forms a far smaller proportion of the parent material.
This is generally low and flat; the gully erosion so common further north is not a problem here. However topography still has an effect, and even the small undulations of the Delta bring about substantial variations in soil conditions within a small area, known as soil catenas. For example, even the very slightly undulating landscapes at Akassa and Ogoni show at least three noticeable soil conditions as follows:
- on top of a ridge or hill are the eluvial soils, which tend to be comparatively coarse and well oxidated because material is always being moved downhill
- soils on the hill slopes are colluvial; they both gain and lose material. Further down the slope they tend to be both deeper and less well oxidated
- material accumulates at the bottom of slopes to form illuvial soils; even less well oxidated, they are still potentially fertile if kept from continuous waterlogging.
This is generally a direct function of topography. In the Delta, the natural soils form in an environment of flooding and high water-tables that is subject to both freshwater and brackish water regimes. These conditions severely limit the downward movement of dissolved nutrients. Soils are also flooded with water for long periods, so that chemical reactions occur in low-oxygen or 'reduced' conditions.
#Climate and Vegetation
The tropical climate of the Niger Delta naturally has a direct bearing on the soils; it ensures that the climax vegetation is forest, that biological activity and biomass is high, and that nutrient uptake and recycling are both rapid.
4.5.5 COMMON SOIL TYPES OF THE NIGER DELTA
Two general tendencies are to be remembered in understanding soil types in the Delta.
- Firstly, that waterlogging tends to lead to reduction (loss of oxygen); and
- Secondly, that the better-drained the soil, the greater the leaching of nutrients.
#Shallow, poorly drained soils - Inceptisol Aquepts
Inceptisol Aquept soils cover the greater part of the Niger Delta. They are young, poorly drained and shallow, and form only one layer or 'horizon'. (This is why they are also sometimes called 'Camibisols'.) The terms 'Inceptisol' and 'Aquept' are used by the U.S. taxonomic system; 'Inceptisol' is a general term for the young soils of the humid regions of the earth. The term 'Aquept' refers to soils that have been waterlogged for long enough to become reduced.
They are characterised by a blue, grey and/or green colouration, coming from the reduced iron in the soil. Where these soils are drained and exposed to the air through seasonal drying, or by human activity, this reduced or ferrous form of iron is oxidised into the ferric form. This shows in the soil as streaks of red. Soils that have been waterlogged and then oxidised in this way may then be known as 'Gley' or 'Hydromorphic' soils.
#Acid Sulphate Soils – Sulphaquepts
Sulphaquepts, or Acid Sulphate Soils, typically occur in the brackish conditions of Delta mangrove areas .....where sulphate ions, carried by inundating seawater, are reduced to hydrogen sulphide under anaerobic conditions in sediments high in organic matter. Hydrogen sulphide then reacts with iron compounds present in the soil-forming pyrite (FeS2). When these deposits are exposed to the air and the soil is low in calcium carbonate, FeS2 is oxidised into ferric sulphate and free sulphuric acid, producing pH values in the order of 2 and 3 [extremely acidic]. The ferric sulphate ions are further hydrolised into straw-coloured jarosite (basic ferric sulphate material) and accumulated in the soil, giving it characteristic bright yellow mottles. (Sanchez, 1976)
In the Okoroba-Nembe district these soils are of three main types:
- Cat-clays: the more recently deposited alluvial muds. When exposed to the air, they oxidise and release bubbles of hydrogen sulphide, producing the typical rotten-egg smell of mangroves at low tide.
- Chicoco Soil: older, peaty clay soils, formed over time from the thick mat of mangrove secondary roots.
- Saline Sandy Soils: containing far less organic matter and far fewer nutrients, being about 75% sand. Ogonny describes them as having been developed by the erosion of sandy deposits, presumably the beach-ridge barriers and the Sombreiro-Warri delta plain remnants near Degema, Buguma and Port Harcourt, as well as the remnants of the coastal terrace near Ogoni and Andonni.
#Udic Oxisoils - the better drained freshwater soils
On the higher ground of the Delta, such as the old coastal terrace, the higher levees and the ridges of the barrier islands, soils are better drained and downward movement of water is not impeded by an impervious layer. Soils here are described as Udic Oxisols (U.S. taxonomy).
The profiles of oxisols are easily visible in the borrow-pits and gas pipeline trenches at Botam-Tai in Ogoni. They generally have deep B horizons consisting of kaolinite, iron oxides and quartz. They are generally red or yellow, with a good and uniform structure but of low fertility.
'Udic' describes the soils typical of tropical rainforests. They form in hot, rainy climates where there is an almost consistent downward movement of water through the soil horizons, and where there are large amounts of added biomass. In natural rainforests this gives a high surface concentration of humus. However in the Niger Delta, where sand is a major parent material and where heavy rain causes leaching for much of the year, the Udic Oxisols are more sandy and depend on biomass being returned to the soil to maintain fertility.
#Higher levee soils
These are closer to Oxisols than Inceptisols, inasmuch as they are not waterlogged. However they are not deep, and high water tables limit the leaching of nutrients.
ERA define them as 'young shallow oxisols'; the soils become exhausted by agriculture, as do the Udic oxisols.
4.5.6 THE INFLUENCE OF HUMAN ACTIVITY ON SOILS
The soils described above are the natural soils of the Niger Delta; that is, before their alteration by human activity.
However, an important feature of the Oxisols is their dependence on massive additions of added biomass. Being sandy, they depend on humus to hold the sand together and give structure, as well as to maintain fertility. When forest is cleared for agriculture, the soils cultivated and the remaining biomass burnt before fallow years, humus content declines rapidly and is not adequately replaced. Nutrients and clays are leached, and poor sandy soils are the result.
This problem is less severe on the levees, because the high water table stops nutrients being leached away so rapidly. However it is severe on deep soils such as those found on the Ogoni Plain.
4.6 NATURAL VEGETATION
Soils and vegetation are develop together. For instance, the stilt-rooted or Rhizophora mangroves colonise newly deposited mud; eventually they create a humus-rich soil made up largely of a thick mat of their own decomposing secondary roots.
The 'natural vegetation' of the Niger Delta, the climax vegetation, if unaffected by modern human activity, and is described in later chapters. However, we must be aware of two ecological principles.
4.6.1 SPECIES DIVERSITY IS HIGHER IN THE TROPICS
For any given altitude and rainfall level, there will be a greater diversity of species in the tropics than in the temperate zones. There are four reasons for this:
- Climatic Stability - the tropics have not experienced recent devastating glaciations;
- Faster Evolution - at higher temperatures, generation times are shorter;
- Longer Growing Seasons - plants in the tropics are not required to spend large amounts of energy simply surviving the adverse seasons, but can 'afford' to expend more energy on new ways to compete;
- Greater Variability of Habitats - this naturally encourages the evolution of a greater variety of plant species.
4.6.2 CLIMAX VEGETATION VARIES WITH RAINFALL AND DRAINAGE
For any given conditions of latitude, altitude and maturity, the climax vegetation and its bioactivity both vary with rainfall levels and are constrained by the level of drainage.
The West African coast itself is a good example of the effect of rainfall levels; Southern Ghana is at the same latitude and altitude as Southern Nigeria, yet the climax vegetation is savannah. This is because it lies in the 'Dahomey Gap' where far less rain falls.
Drainage is just as important, however. Total bioactivity generally increases towards the equator, towards sea level, and with increasing rainfall; the highest bioactivities on earth are found in the tropical rainforests. The Niger Delta lies in such a biome, but the restricted drainage means that bioactivity here is somewhat lower than in other West African rainforests. (The area of highest bioactivity on earth is actually just to the West of the Delta, on the Cross River State border with Cameroon.)
4.7 NATURAL ANIMAL COMMUNITIES
In any given location, the bioactivity of the vegetation in turn determines local bioactivity of animals. In the case of the Niger Delta, for example, there are 134 freshwater and brackish water fish species, as compared with 192 for the entire continent of Europe.
The fact that animals can move complicates the assessment of their population levels and of species diversity in a given area. For example, they may feed in one ecozone and sleep in another; they may also evade observers. Plants can be immediately and directly observed and counted. However a rapid survey of animal communities has to rely partly on past experience, using vegetation as an indicator of which animals may be likely to live in or visit an area, and also on local knowledge.
People living in a locality will generally know most about the animals they hunt and/or that impede their farming activities—the 'pests'. These tend to be the vertebrates, and especially the mammals, because they are closest to humans.
However, it is important to remember that all the vertebrates together make up less than 5% of the animal species. In terms of overall bioactivity the other 95% are far more important, in particular the Arthropods which include the class of animals with the most species of all—the insects.
The larger vertebrates, such as pangolins or parrots, may appear to be most significant because they appeal to sentiment or aesthetics. However the extinction of an as-yet unnamed insect may be of greater impact; every animal plays a role in its ecosystem, and any species loss will have an effect.
However, while the large animals should not monopolise our attention, their condition is an indicator of the overall health of an ecosystem. Their reduction may be the first obvious warning that something is wrong. Hunting pressure may often be the reason for a crash in numbers, but the prime reason is habitat degradation or loss; this is why effective and sustainable conservation efforts cannot be targeted at single species in isolation.
4.8 THE NATURAL ECOSYSTEM
In answering this Chapter's question 'What is the Niger Delta?', we are defining the natural ecosystem of the Niger Delta as a series of ecozones and sub-ecozones, within the Biome that is the West and Central African Tropical Rainforest.
Because it is the most immediately obvious manifestation to an observer, ecozones and their ecosystems are often described only in terms of their vegetation. However, as the preceding sections have emphasised, an ecosystem is the product of vegetation, animal communities and, most particularly in the case of the Niger delta, the soils and soil-water regime.
ERA definitions and abbreviations are used as the terminology for this book; they may overlap or differ with other descriptions (none of which are definitive) but are consistent and relevant to our concerns.
Map 4 shows the distribution of the ecozones and sub-ecozones, as listed below.
|THE NATURAL ECOZONES OF THE NIGER DELTA|
Each is considered in detail in the following chapters. However it must be stressed that at this stage, we are still considering the natural ecosystems of these zones, as they would be if humankind were still playing out natural roles within them. Only then will the modern ecosystems as affected by modern human activities will then be understood and considered.