Ecological Status of Opa Reservoir, Obafemi Awolowo University, Ile Ife, based on the Abundance and Diversity of its Planktonic Flora

Abstract

A study investigating the spatial and temporal distribution, composition and abundance of plankton in Opa reservoir, Obafemi Awolowo University, Ile-Ife, Southwest Nigeria, was conducted over a period of an annual cycle. The study was undertaken with a view of providing a more recent catalogue of planktonic flora and possibly an update of the reservoir's trophic status. Quantitative net planktons were collected monthly from both the surface and bottom levels at three sampling stations established at the dam site (lacustrine), mid-lake (transition) and upper inflow (riverine) parts of the reservoir. The divisions recorded were Bacillariophyta > Cyanophyta > Chlorophyta > Euglenophyta > Myzozoa > Ochrophyta = Charophyta > Cryptophyta in order of abundance. Vertically, the highest occurrence of species was recorded at the lacustrine bottom station (71 species), while the least occurrence was observed in the transition bottom station (51 species). A total of sixteen plankton species showed significant seasonal variation in abundance during this study period, while only seven species had significant spatial variation (p ≤ 0.05). Higher abundance was observed during the rainy season (170,797,350 Org/m 3 from seventy-two species) than dry season (5,138,400 Org/m 3 from forty-nine species). Notable bio-indicator plankton species recorded were Anabaena circinalis, Anabaena flos-aquae, Microcystis sp., Aphanocapsa litoralis and Microcystis aeruginosa. Some other pollution indicator species recorded were Synedra ulna, Oscillatoria agardhii, Phacus sp., Surirella sp., Closterium sp., Aphanocapsa sp. and Euglena sp. Hence, Opa reservoir is very rich in Bacillariophyta (diatoms), followed by Cyanophyta (bluegreen) and Chlorophyta (green algae), which are known to characterize eutrophic lakes.