Abstract

This study determined the taxonomic composition of the phytoplankton flora of Esa-Odo Reservoir in Osun State, Southwestern Nigeria; it examined the spatial and seasonal variations in the phytoplankton population and production capacity of the reservoir. These were with a view to providing baseline information on the reservoir, its trophic status, and the eutrophication process. Three sampling stations (Stations 1, 2, and 3) were established at the inflow of the reservoir, the mid basin portion, and the outflow very close to the dam site along the horizontal axis of the reservoir. Field survey of the three sampling stations was conducted four times over the period of study from February to October 2017, covering both dry and rainy seasons. Water samples were collected and preserved for net and total phytoplankton analysis in the laboratory using a light compound microscope. The recorded phytoplankton species were measured, photographed and enumerated. Data collected were subjected to appropriate descriptive inferential (ANOVA) and multivariate statistics. The results showed that a total of one hundred and forty three (143) species of phytoplankton recorded belonged to sixty four (64) genera, forty (40) families, twenty-eight (28) Orders, ten (10) classes and seven (7) Divisions of Algae. Cyanobacteria were the phytoplankton group found in the highest percentage (48%) of occurrence in total phytoplankton population, while Ochrophyta had the lowest percentage (1%) occurrence. Bacillariophyta had the highest abundance of species and were the dominant genera across the upper, middle, and lower basins of the reservoir. The study concluded that Esa-Odo reservoir contained a high diversity of phytoplankton including some species which served as indicators of water quality and trophic status, indicating the water body as oligotrophic with low nutrient content.