The bottom fauna of the rapids of the Black Volta River in Ghana

Summary

The increase in abundance and biomass of the bottom fauna in the Black Volta River rapids seems to be closely related to increasing rate of water flow. These differences are due mainly to the increasing numbers of Cheumatopsyche larvae, a web spinning caddisfly. At higher current speeds more food is provided for this omnivorous species, and this seems to determine its density on the substratum. In a very slow current which prevents the use of webs for food catching, this genus is absent.

The abundance and standing crop of bottom fauna at the end of dry season can be considered as the highest for the whole year. The presence of several instars of the commonest species guarantees almost continuous emergence and recolonization of the substratum during the dry season. With the onset of heavy rains and floods the communities found in the rapids are disturbed, and eventually completely destroyed. This is because the substrate is carried away by the swift flow of the river. Recolonization can be expected after the rains when the water level is low and the substratum again stabilized.

The fauna of rapids is a community which can be expected to be almost completely destroyed when it becomes permanently flooded in any man-made lake. Some organisms, however, such as the larvae of Ecnomus and Cricotopus may establish themselves again in a habitat physically and chemically akin to that of rapids; such a habitat is for example the bark of flooded trees exposed to wave action and surrounded by water well saturated with oxygen.