Seasonal variation in the diversity and abundance of phytoplankton in a small African tropical reservoir

Phytoplankton species composition and seasonal changes were investigated in the shallow reservoir of Adzope. Taxonomic composition, diversity and abundance of phytoplankton were studied at 4 stations from May 2008 to February 2009, in relation to physical and chemical factors (temperature, conductivity, dissolved oxygen, transparency, pH, nutrients) and climatic factor (rainfall). The phytoplankton comprised 144 taxa, mainly Chlorophyta (29.45% of taxa), Euglenophyta (29.45%) Bacillariophyta (23.97%) and Cyanobacteria (10.27%). Phytoplankton density values were greatest during the transition season (short rainy season), lower during the dry seasons, and extremely lower during the long rainy season (mean value 356 105cells/l, 44 105 cells/l and 35 105 cells/l respectively). Abundance was dominated by Anabaena constricta Szafer (Geitler) and Microcystis aeruginosa (Kutz.) Kutz. The diversity index varied between 2.1 bits/cells in long rainy season and 4.6 bits/cells in short dry season. The redundancy analysis (RDA) demonstrated a separation between the long rainy season and the other seasons due to the influence of the flood pulse.   Key words: Phytoplankton, shallow reservoir, species dominance, West Africa.