Niger Delta Ecosystems: the ERA Handbook/The Natural Estuaries & In-shore Waters of the Niger Delta
9 THE NATURAL ESTUARIES AND IN-SHORE WATERS OF THE NIGER DELTA
- Ecosystem Dynamics of the Natural Estuaries and Inshore Waters
- Plants and Animals of the Estuaries and Inshore Waters
In comparison with land and rivers, very little is known about the Estuaries and In-shore Waters as an ecozone or as natural ecosystems. This is because they are far less accessible to human beings.
What we know about the ecozone is confined to what we learn of it from the shoreline; by passing over the top of it, as swimmers or in boats; by randomly pulling life out of it by fishing; by depth sounding and by very rare and clumsy deep-sea diving and dredging. This is like learning about the forest by skimming its canopy in a light plane and occasionally parachuting out.
This situation is especially difficult for ecologists. For example, depth sounding and dredging indicate the physical topography and geological makeup of the seabed, and may also allow a survey of species diversity of the seabed. However this tells us very little about how it works as a living system or of interactions with the wider environment, any more than a contour map and a bucket full of mixed soil horizons tells us about terrestrial soil surface systems.
The water of this ecozone has a salinity that is slightly less than offshore seawater. However salinity changes over a daily cycle, with the ebb and flow of the tides, and also seasonally according to rainfall, river flow and currents. Moreover, at any given time, salinity will be different in different parts of the ecozone: swimmers notice this as they swim from warm salty seawater to much cooler freshwater flowing into it, at low tide when the rivers flush out.
Mangroves form the prime vegetation of the estuaries and form an ecotone between the natural estuary/inshore water ecosystems and the natural freshwater ecosystems (see chapter 6.). Similarly, the estuaries/inshore waters ecosystems form an ecotone between the sea itself and the brackish-water ecosystems further inland.
The natural Estuaries and Inshore Waters ecozone covers roughly a quarter of the whole surface area of the Niger Delta. If the Delta is seen as the drainage 'sink' for much of Nigeria and other parts of West Africa, via the Niger River system, then the estuaries and inshore waters ecosystem is like a series of drainage holes through which everything passes on its way to the deeper sea.
9.2 ECOSYSTEM DYNAMICS OF THE ESTUARIES AND INSHORE WATERS
The physical environment varies over time and space in terms of salinity, acidity, nutrient status and light, to name just the major parameters.
For example, the salinity gradient from freshwater high up the estuary to seawater at or beyond the mouth changes over time. The gradient 'contour lines' are squeezed inland as seawater pushes up into the estuary with each high tide (there are two a day here); these contours also shift further out to sea during the annual wet season, when higher volumes of freshwater are coming downriver. These changes in turn affect the movements of fish and their life cycles, because different fish have different salinity needs and tolerances.
98% of the sediment load discharged into the estuaries and inshore waters from the Niger Delta is quartz-sand; most of the silt borne down through the drainage system is trapped in the mangrove forests, and most of what escapes is then swept into the deep offshore canyons by long-shore drift.
Acidity (or pH), turbidity (cloudiness due to suspended sediment) and nutrient levels tend to be lower in estuaries fed by black-water and when river discharge is low.
High turbidity results in low light levels. Together with the low nutrient content of much of the discharged water, this results in a low primary production (i.e. of plant biomass), at between 100-150 mg per cubic metre (A.A. Amadi). However, overall bioactivity is high because of the relationship of the estuary/inshore waters ecosystem with the highly bioactive mangrove ecosystems adjacent to it. The simplest facet of this relationship is that many of the estuary and inshore animals feed on the plants and smaller animals of the mangrove ecosystem.
9.3 PLANTS AND ANIMALS OF THE ESTUARIES AND INSHORE WATERS
Light-loving phytoplankton are primitive marine organisms that live by photosynthesis (see 4.5.3). Their activity tends to be low because turbidity results in low light penetration; however this is sometimes offset by the higher nutrient inputs during the wet season, when organic matter is discharged from the rivers.
Levels of the zooplankton which feed on the primary production of phytoplankton will naturally also be low.
Seaweed may be red, green or brown algae. Turbidity also limits their presence and activity, but nitrogen-fixing oligotrophic blue-green algae are common. Together with detritus of mangrove forest systems, these are the basis of marine food chains.
Molluscs largely belong to the brackish-water ecosystems. However, although the Crustaceans (shrimps, primarily the Pink Shrimp Penaeus notali, crabs, and lobsters) all depend to some degree on mangrove detritus for food they properly belong to the estuary/inshore waters ecozone.
Fish are the dominant vertebrate class and again express the great bio-diversity, bioactivity and biomass of the Niger Delta as compared to temperate waters.
B.S. Moses describes 72 species of saltwater fish in Akwa Ibom, Cross River and Rivers States.
The following 34 species have been documented as being caught in the Inshore waters of the Niger Delta.
|Ilisha africana||Herring of Bonga|
|Pellonula leonensis||Guinean sprat|
|TRICHIURIDAE||Trichiurus lepterus||Ribbon fish|
|PERIOPHTHALAMIDAE||Periophthalmus koelreuteri||Mudskipper |
Mammals found in the natural estuary/inshore waters ecozone include Hippopotamus, Manatee and/or Dugong, depending on salinity, Dolphins and (although rarely) Whales.
The water, open spaces and abundance of food in the form of fish attract a wide range of birds including Herons, Cormorants, Egrets, Ibises, the Greenshank, the Redshank and Terns.