Abstract
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 (TSICR) with respect to Rotifer abundance occurred in Transition surface, followed by Riverine surface and the least occurred in Transition bottom portion. The mean TSICR value obtained was 65.20, indicative of hyper-eutrophic, while the mean TSICL 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 TSICR showed the lake as eutrophic, tending towards becoming hyper-eutrophic, which could speed up the aging of the lake.
Highlights
Zooplankton are passive drifters, moving with water currents, yet well adapted for their mode of life, can withstand diverse levels of environmental changes in physicochemical water quality, thereby useful for measuring the status of their environment (Paterson, 2001; Imoobe, 2011; Akindele and Adeniyi, 2013)
The zooplankton assemblage often influences energy flow through classical food chain, nutrient cycling and community population dynamics within a reservoir ecosystem. This ecological niche has made them key actors in top down grazing effect on the bottom up forces which play pivotal roles in bio-manipulation for lake restoration purposes as reported by Carpenter and Kitchell (1993). Despite this enormous role played by zooplankton in waterbodies, their distribution has been reported to be affected by factors such as the hydrologic regime of the waterbody (Casanova and Henry, 2004), physical and chemical variables (Sarkar and Chaudhary, 1999; Arimoro and Oganah, 2010), drainage density, sinuosity ratio and stream frequency (Akindele and Adeniyi, 2013), hydrological characteristics (Mitsch and Gosselink, 2000)
Study area Opa reservoir is sited between latitude 07030'N to 07031'N and longitude 004031'40''E to 004 032'45''E, within the Obafemi Awolowo University community, IleIfe, Southwestern Nigeria (Fawole and Arawomo, 2000)
Summary
Zooplankton are passive drifters, moving with water currents, yet well adapted for their mode of life, can withstand diverse levels of environmental changes in physicochemical water quality, thereby useful for measuring the status of their environment (Paterson, 2001; Imoobe, 2011; Akindele and Adeniyi, 2013). The zooplankton assemblage often influences energy flow through classical food chain, nutrient cycling and community population dynamics within a reservoir ecosystem. This ecological niche has made them key actors in top down grazing effect (trophic cascade) on the bottom up forces which play pivotal roles in bio-manipulation for lake restoration purposes as reported by Carpenter and Kitchell (1993). The study would reveal the present ecological and trophic status of the reservoir
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