Zooplankton Composiiion and Seasonal Dynamics in Lake Ziway. Ethiopia

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Addis Ababa University


The species compositilln and seasonal dynamics (abundance) of zooplankton in Lake Ziway (a tropical lake) was studied from f1arch 19,H to f1arc, h 19UB. Bimonthly vertical haul zooplankton samples were taken from 3 stations (2 stations in the limnetic zone and 1 in the littoral zone) using a 61[)Jill mesh net. The 1 imnet ic community was domi nated by eycl opoi d cupepods \'Ihil e the 1 ittoral community by the Hotifera. Ca 1 anoi d copepods and ffarpacticoids did not appear in the samples collected. The Cyclopoid copepod species identified and counted in each sample included f1icrocyclops vilricans and Nesocyclops ~equatorialis. 6frocyclops species was found in only three of the sampling periods. Diaphanosoma excisum and Alona davidi constituted the cladoceran population of the lake. The Rot ifera were domi nant in terms of thf? number of speci es. Out of the 13 species of Rotifera identified, the numerically dominant species was Brachionus angularis and the least ilbundant species were ~. falcatus and Lecana luna which were encountered in (lnly two and one sampling periods respect i ve ly. r~i nor groups of zooplankton recorded inc 1 uded Chi ronomi d larvae, the Ostracod Cypria javana, an unidentified nematod and an unidentified i1rthropod larva. 1111 major zooplankton groups and species identified shol'lcd significant seasonal variation in their abundance. Copepods and cladocerans both at group and spec i es 1 eve 1 s hO~led max i mum abundance du ri n9 the I'let sea son and minimum abundance during t.he dry season. The Roti fera in general peaked after the rainy season. t,1aximum contribution to the annual total zoop 1 ankon abundance came from eyc 1 opoi d zoop 1 ankters. The abundance pattern of the zooplankton of the lake are discussed in relation to suggested environmental factors.