Itapaji Reservoir in Ekiti State serves for municipal water supply, artisanal fishing, agricultural and domestic uses, and recently earmarked for irrigation of the adjoining land. There is paucity of information on the limnology of the reservoir, hence this study to provide relevant information on its trophic status for effective water management. Plankton samples (120) were collected with 2L plastic bottle monthly from April, 2013 to March, 2015 at five purposively selected stations along the reservoir. Plankton samples were identified and counted microscopically, using standard identification keys. Species diversity was determined with Shannon-Weiner’s Index. Data were analysed using descriptive statistics, student’s t-test, and ANOVA at α0.05. Among the six classes of phytoplankton encountered, Bacillariophyceae was the most abundant (40.4%). The order of dominance in phytoplankton was Bacillariophyceae> Cyanophyceae> Euglenophyceae> Chlorophyceae> Zygnemataceae and Dinophyceae. All the encountered Phytoplankton taxa showed significant spatial variation. Pollution-indicator phytoplankton (Spirulina, Oscillatoria, Synedra, Euglena, Trachelomonas and Phacus) encountered accounted for 41.9% of phytoplankton population. Zooplankton was dominated by rotifers (38.4 %). Phytoplankton and zooplankton taxa recorded mean species diversities of 1.7±0.3 and 1.1±0.2 respectively. The abundance of pollution-indicator species revealed that Itapaji Reservoir is under pollution stress.
Key Words: Municipal, Pollution-indicator, Bio-diversity, Ecosystem