Seasonal variations in phytoplankton populations in Ogelube Lake, a small natural West African Lake

Water samples were collected on a monthly basis at three locations along the north–south axis of the lake at 1 m depth intervals for 1 year. A sedimentation technique was used for microscopic examination of the samples. The monthly mean and seasonal phytoplankton densities were calculated. The original data were square‐root transformed, and analysis of variance performed with SPSS version 10.0. The significant treatment effects were determined using Fisher's least significance difference at the 5% probability level. The mean phytoplankton population per millilitre was very high for the entire sampling period. There was a phytoplankton bloom formation in the early rainy season, with the mean total phytoplankton population density for the rainy season (April–October) being significantly higher than that of for the dry season (November–March). A population increase was observed during the late part of the dry season to the early part of the rainy season. During the rainy season, Chlorophyceae (mostly desmids) were most abundant, followed by Cyanobacteria, Bacillariophyceae, Euglenophyceae, Dinophyceae, Cryptophyceae, Chrysophyceae and Xanthophyceae, in decreasing order of abundance. This order changed slightly in the dry season, when there was relative abundance of Bacillariophyceae over Cyanophyceae, and Dinophyceae over Euglenophyceae. The low population of Euglenophyceae indicates that the organic pollution is still low, with the predominance of desmids, indicating an oligotrophic lake condition.