SEASONALITY AND SPATIAL-DISTRIBUTION OF THE OSTRACODS OF LAKE ZWAI, ETHIOPIA (CRUSTACEA, OSTRACODA)

1. Quantitative benthic samples were collected montly between June 1984 and May 1985 from seven stations at various depths in Lake Zwai, one of the Ethiopian Rift Valley lakes, situated in the northern part of the Rift Valley. The most common species of ostracods were Limnocythere thomasi thomasi Martens (a subspecies endemic to Lake Zwai), Gomphocythere angulata Lowndes (common in, but restricted to, East Africa) and Darwinula stevensoni (Brady & Robertson) (cosmopolitan). 2. Absolute densities of all species varied considerably over the year. In all stations, there were population peaks during the two dry seasons, followed by as yet unexplained population crashes during the short and the long rains. 3. The three common species had different habitat preferences. Darwinula stevensoni avoids parts of the lake where temperatures rise high and both G. angulata and L. thomasi had a preference for sheltered habitats close to vegetation and/or for a specific grain size of the sediment (300-350-mu-m).