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Abstract
Phytoplankton abundance and composition in relation to physico-chemical parameters were investigated from September 2005 to October 2007 at 51 stations of various depths in the nearshore, intermediate and deep offshore waters of Lake Victoria. Shallow nearshore waters had the highest abundance of phytoplankton (mean 2.12 × 108 ind. l−1), compared to the intermediate (1.96 × 108 ind. l−1) and deep offshore waters (7.59 × 107 ind. l−1). Bacillariophyta was the most abundant group (48.17% of total phytoplankton) and was uniformly distributed in all waters, followed by Cyanobacteria (33.33%), which decreased with distance offshore. Chlorophyta, the third highest in abundance (15.5%), increased with distance offshore. A total of 92 phytoplankton species were recorded. The most diverse phyla were Chlorophyta with 37 species, of which Staurastrum sp. and Siphonctosiphon polymorphus were the most abundant species, followed by Bacillariophyta with 25 species and Cyanobacteria with 21 species. In general, the phytoplankton community was dominated by the less nutritious species Nitzschia acicularis (Bacillariophyta), Lyngbya circumcreta and Microcystis flos aquae (Cyanobacteria). These findings contrast with those of most studies from before 1990, but agree with many thereafter. Eutrophication, affecting availability of nutrients etc., and pollution, affecting transparency and water pH, were the forces responsible for the observed spatial and temporal variability in phytoplankton abundance patterns and composition.
