The benthic and weed-bed faunas of Lake Awasa (Rift Valley,Ethiopia)

The community structures of the benthic and weed-bed faunas of Lake Awasa, Ethiopia Rift Valley, are described and related to the various types of substratum present. L. Awasa is non-saline (conductivity 860 µS cm^-1), and lies in a shallow basin with maximum depths of 21–22 m. It supports an important commercial fishery mainly based on the tilapia Oreochromis niloticus.The large weed beds of the lake extend out to the 4 m. isobath in most places; they consist mainly of the grass Paspalidium geminatum. Below the 6–8 m isobaths the bottom is covered with a deep layer of flocculant mud, with organic matter constituting about one third of the dry weight. No benthic animals are able to colonize this mud. In shallower regions the bottom consists of hard sand covered with a thin layer of organic detritus, except for a narrow region around the 6 m isobath where the sand is covered with a layer of ostracod shells up to 25 cm deep. All this firmer bottom is colonized by a benthic community consisting mainly of cyclopoids, ostracods, Limnodrilus and chironomids; there are no prosobranch or bivalve molluscs. The weed-bed community consists mainly of Cladocera, ostracods, Ephemeroptera, Heteroptera, chironomids and pulmonate molluscs. These communities are compared with those of other Afro-tropical lakes, and are shown to have much in common with that of Lake Chad (Dejoux, 1983), specially the chironomids (Dejoux, 1976) but L. Awasa has fewer species, the most noticeable absentees being the decapod Crustacea.