Downstream drift of invertebrates in a river in southern Ghana

Abstract

Summary The results of quantitative sampling of the drifting invertebrates from a riffle area in the upper reaches of the Pawmpawm River in the forest of Ghana are presented. The observations were made once a month during 14 months, April 1970 to May 1971, and also on a daily basis for 3 weeks. Changes in numbers and composition of the animals drifting were examined in relation to the volume of water flowing and the current; there appeared to be two distinct drift phenomena, ‘true’ and ‘background’ drift. The animals belonging to each of these two components are discussed in relation to their response to light, to diurnal fluctuations in total drift and to the phases of the moon. There was a peak drift activity after sunset in those animals which are affected by light, e.g. Centroptilum, Austrocaenis, Chironomidae, Simulium . It is concluded that drift may be simply a function of the activity patterns of the insects.