No abstract

The study assessed the physico-chemical characteristics of sediments from Owella reservoir in Osun State, Nigeria with the primary aim of creating a baseline information in aspect of limnology and also to further assess influenced by anthropogenic activities as well as characteristics that were affected mostly by natural sources in the water body. Twenty sampling stations were established along the reservoir representing its three different sections and two zones for a period of two years. On the average, the reservoir sediment was mostly sand, clayey-sand in texture, acidic in chemical reaction, low in nutrients and moderate in organic matter. Although, nutrients were identified to be mostly of anthropogenic origin, while alkali metals and alkaline earth metals were from both anthropogenic and natural sources. The overall order of dominance of the exchangeable cations was: Mg2+ > Ca2+ > H+ > Al3+ > K+ >Na+ and anions in the order PO34- > SO42− > NO3−> Cl−. Significantly difference (p < 0.05) were observed in the physico-chemical characteristics of the sediment across the two zones (littoral and open water) of the reservoir and horizontal variation along the three basin sections (upstream, mid-basin and downstream) and over the two seasons of the year were mostly not significant (p > 0.05). The overall mean values obtained for particle size distribution along the reservoir were in the following order: sand (7.31%) > Silt (43%) > clay (57%). Surface run-offs are major contaminant in Nigeria waterbodies and through geochemical weathering of the top soils. The results showed low concentration of cations, anions and nutrients compound in the sediment of Owella reservoir which suggests low influence of anthropogenic activities on the sediment quality.

An appraisal of the current ecological status of Asejire Reservoir was carried out using the reservoir phytoplankton composition and community structure. Monthly sampling was carried out in nine selected sites for twelve months. Data were analysed using descriptive statistics, analysis of variance, Shannon-Weiner and Evenness indices. One hundred and fifty-four phytoplankton taxa were identified. Taxa dominance was in the following order: Bacillariophycea>Chlorophycea>Charophycea>Cyanophycea>Euglenophycea>Ochrophycea>Dinophyc eaa. Microcystis sp. were the most abundant species followed by Anabaena sp. and Closterium sp. The riverine zone accounted for 50.8% of the total phytoplankton population. One-way analysis of variance between the zones indicated that there was a significant difference (F=11.41, df=2, p=0.0000146) in the spatial distribution among the stations. Most of the recorded phytoplankton species are cosmopolitan with the presence of the following species: Staurastrum, Closterium, Cosmarium, Anabaena, and Oscillatoria. The presence of some pollution indicator species is a cause of concern and the need to ensure holistic and effective monitoring measure is put in place to safeguard the reservoir.