The physicochemical water condition and zooplankton fauna of the two main inflows of Aiba Reservoir were assessed over an annual hydrological cycle (May 2013–March 2014). The concentrations of total solids (TS) and total suspended solids (TSS) in the two streams were unusually higher in the dry season for typical inland waters of Nigeria and showed a decrease from the upper reaches towards the lower reaches (reservoir’s inlet). Dissolved oxygen, , and recorded their highest concentrations at the reservoir’s inlet. A total of 37 species of zooplankton were recorded in the study, comprising 5 species of Protozoa, 14 species of Rotifera, 10 species of Copepoda, 4 species of Ostracoda, and 4 species of Insecta. Aiba stream recorded higher number of zooplankton species and abundance than Onikan stream. While number of zooplankton species and abundance showed an increase from the upper reaches to the lower reaches of both streams, species diversity and equitability generally showed a decrease. Correlation and regression analysis suggests that the concentration of TS and TSS played a major role in determining the zooplankton community structure of the streams. concentrations as well as the community structures of zooplankton faunas of the two streams were indicative of a polluted freshwater system with unstable habitat structure. 1. Introduction Throughout human history, water has played a dual role as a life-giving liquid and as a resource for waste disposal. Without careful management, these two uses can conflict, potentially dangerously. In the developed world, water pollution reached crisis proportions around the mid-twentieth century but is now increasingly controlled. In developing countries however, poor water quality remains a major health threat. Also, while economic growth in developed countries has helped to bring about improvements in water quality, many developing countries are finding it difficult to follow this trend [1]. Ensuring that water quality in aquatic environments remains within natural ranges is essential for sustaining viable, abundant, and diverse communities of organisms. Degradation of water quality erodes the availability of water for humans and ecosystem and decreases species diversity and abundance of resident communities. The changes in environmental quality can be associated with changes in water quality parameters such as sediment load, nutrient concentration, temperature, dissolved oxygen levels, and pH [2]. Freshwater zooplankton in the tropics comprises predominantly the rotifers, cladocerans, copepods, and occasionally
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