The present study investigated the ecological status of Opa reservoir, Obafemi Awolowo University, Ile Ife, Nigeria, based on the spatial and temporal variations in abundance and distribution of zooplankton. 72 samples were collected monthly with a quantitative net zooplankton from both the surface and bottom levels in three stations established at the dam site (Lacustrine), mid-lake (Transition) and inflow (Riverine) over a period of an annual cycle. A total of fifty-four (54) species were recorded from the reservoir comprising Rotifera (57.41%) > Arthropoda (33.33%) > Protozoa (5.56%) > Ciliophora (1.85%) = Cnidaria (1.85%), in the order of abundance. The least number of species (47) was recorded at the Transition station, while the highest number of species (49) occurred at the Lacustrine zone. Of all the zooplankton recorded, four species had significant spatial variation, while nine displayed seasonal variations during the study period (p ≤ 0.05). The highest species richness was observed in Transition surface station (4.18), followed by Lacustrine surface station (3.80) and Riverine surface station (3.23). Shannon's index showed that zooplankton species were more diverse during the rainy season than dry season. The highest Trophic State Index (TSI CR ) with respect to Rotifer abundance occurred in Transition surface, followed by Riverine surface and the least occurred in Transition bottom portion. The mean TSI CR value obtained was 65.20, indicative of hyper-eutrophic, while the mean TSI CL value with respect to cyclopoida-calanoida obtained was 58.07 also revealing eutrophic status of the study area. Opa reservoir comprises mainly Rotifers and its TSI CR showed the lake as eutrophic, tending towards becoming hyper-eutrophic, which could speed up the aging of the lake.

Study to determine the ecological status of Opa reservoir, Ile Ife Nigeria was conducted based on the plankton's diversity and physicochemical characteristics of the reservoir. Water samples for the analyses were collected from three established investigated stations on the reservoir for a period of one year between November 2012 and October 2013. Most of the physicochemical parameters' result were within recommended limits of the NESREA for aquatic life, but some heavy metals (Ni and Cr) were above permissible limits. One hundred and thirty-six (136) plankton species belonging to thirteen taxa were encountered whose total abundance and distribution were influenced greatly by dissolved oxygen, salinity and light obstruction parameters. Notable, recorded species at the riverine (Cryptomonas ovata, Branchionus angularis and Cyclops) revealed the unacceptable quality of the reservoir's influx. Besides, the observed low species diversity index, at all zones of the reservoir investigated, revealed dominance by few species that characterized eutrophicated waters. Furthermore, the presence of plankton pollution tolerant species (Anabaena sp., Navicula sp. and Peridinium sp.) also suggested a minimal level of organic pollution in the reservoir. The water qualities of Opa reservoir at the Lacustrine zone (76.14) and Transition zone (67.89) were of occasional threatened while Riverine zone (63.57) was frequently impaired according to the CCME Water Quality Index.

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.