The Benthos Study of Lake Awasa

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Addis Ababa University


Exploratory benthic studies "ere c'BTrieQ m;t'thi'0ugl19Ut Lf\Ke A"ORll at 17 stations for 10 months. Bottom samples ".ire colle"t.ed by using an Ekman grab and the fauna of the macrophytes from the littoral region was sampled using a fine standard hand net. For determination of total organic matter of the profundal mud an oven and a muffle furnace "ere used. C .. mmunities of benthic fauna '{ere reported from the Lake in relation to the types of bottom substrates and aquatic macrophytes. The profundal mud '{as analysed with respect to its total organic matter and texture. Conditions in the mouth of Tikur Huha (the main inflOlf) as a small control area was compared to the main lake ,rith regard to its chemical, vegetations and bottom types. The fauna of open littoral, weed beds, the mouth of Tikur lfuha and seasonally flooded areas "ere identified and their distribution in the lake ;T/l,S mapped. The ostracods "ere found to be the most numerous of' 2 j all benthic conununities (mean number 48,751/m ). Other benthic crustaceans such as cyclopoid copepods and Cladocera 'Iere also reported. Chironomids "ere also found to be among the most dominant benthicOther mncroinvertebrates including' Epheme~CiP:tqd (.GHRr:i.-1. aridSCl0;oQ.J.l spp), Odonata nymphs (Anisoptera and Zygoptera); H6t'e>'optera, C('leopt~ra, caddis fly larvae (mainly Leptoceridae), gastropod snails and some others 1(ere mainly found from the roacrophyte zone. Some parasitic and semi-- parasitic forms 1(el'l' also collected. In the littoral zone fauna numbers decreased ;Iith increasing depth. In the profundal l'egion, benthos was absent. This seems to be due to the flocculant nature of the bottom mud. Hater level fluctuations did not seem to affect faunal distribution. Seasonal variations of the benthic organisms of the lake 1'ere tested for their significance using students t-test and most ~f the benthos were fOlmd to be 1m affected by ';,et and dry seasons. Only the mayfly Cloeon spp and the chironomids Pr.2,cla(}ius spp "el'e found to be significantly affected by seasonal variations. forms. 2,( genel'a of chironomids vere reported from the lake. ,!'1ilodorum spp. seem to be the dominant littoral midges in the vegetation. Clad"!,anytarsus and Procladius species are the dominant bottom midges in the mud belm{ the vegetation and under open \fater.