Page:Niger Delta Ecosystems- the ERA Handbook, 1998.djvu/86

The Freshwater Ecozone 6.4.4 THE LAKES SUB-ECOZONE

Permanent and seasonal lakes occur throughout the natural Fresh-water ecozones, and are sometimes known as aquatic grasslands and herbaceous swamps. They form in any condition where water collects either permanently or temporarily, which is either too deep for swamp forest to develop (such as an ox-bow lake), or too young to have been colonised by Freshwater plants. Fresh-water lagoons come into this category.

#

Permanent lakes are more likely to contain floating plants which obtain oxygen through the stomata in their leaves, and distribute it to the plant body via air canals permeating the stems and roots. Examples are the water-cabbage (Salvina spp.), the water lilies (such as Nymphaea lotus and Nymphaea micranthus) and the floating grass (Vossia cuspidata).

On the edges of the permanent lakes and in seasonal lakes are sedges (Cyperus spp), ferns (e.g. Nephrolepis biserrata), grasses such as Paspalum (Paspalum) and Bamboo. (These species are also found on river margins.)

#

The lake ecosystems are important in animal ecology because they provide tranquil and euphotic (light) conditions for some animals to breed. The larvae of certain insects which develop in water, such as mosquitoes and dragonflies, need such conditions, as does the reproduction of frogs and toads.

The insects and amphibia, young and adult, in turn attract birds.

Such a lake may also serve as a drinking hole and be visited by larger mammals; it may be permanent home for a wide range of other vertebrates (see 3.7) including fish, reptiles such as crocodiles, and even mammals such as hippopotamus.

6.4.5 THE SEASONALLY EXPOSED ALLUVIUM SUB-ECOZONE

Seasonally exposed alluvium is made up of the fresh deposits of silt that appear on the outside of curves of rivers when the water level drops in the dry season. (See figure 4.) These areas always appear opposite areas of riverbank erosion, which is often quite severe (as at Anyama), and represent the formation of new land. This rapid and visible erosion and deposition is evidence of the dynamic and young geological nature of the Delta as a whole.

As farmers know, these sub-ecozones are very fertile; they are intensively cultivated. 84