Author(s): Lakew Wondimu Abhachire

Abstract: The physico-chemical parameters of the water and phytoplankton of the Koka Reservoir and Awash River were investigated for a period of one year. The water temperature in the study sites showed temporal variations which followed the local atmospheric temperature during the wet and dry season. The pH of the river water was comparatively lower than that of reservoir due to the influx of sewage and industrial pollutants. The dissolved oxygen and free carbon dioxide in the reservoir water varied considerably due to the occurrence of algal bloom and flooding of the catchment area during rainy season. The phytoplankton constituted mainly of green algae and diatoms in the riverside site and their abundance was controlled by the massive growth of the macrophyte, Eicchornia crassipes especially during the dry season. In the reservoir the diatoms and green algae predominated after the cessation of the rain. However, during the high temperature months the blue green algae predominated in the shallow near shore regions in the reservoir. The possible reasons for the alteration in physico-chemical and biological features in the two study sites are discussed in this paper.

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