Phytoplankton community structure and environment in the Kenyan waters of Lake Victoria

1 Phytoplankton species composition, numerical abundance, spatial distribution and total biomass measured as chlorophyll a concentration were studied in relation to environmental factors in September 1994 (dry season) and March 1995 (rainy season), respectively, in the Kenyan waters of Lake Victoria; 103 species were recorded. 2 Blue-green algae (Cyanophyceae) were most diverse, followed by diatoms (Bacillariophyceae), green algae (Chlorophyceae) and dinoflagellates (Dinophyceae). 3 Twinspan separated the phytoplankton communities in the Nyanza Gulf and those in the open lake during both seasons. During the dry season, the Nyanza Gulf was strongly dominated by blue-greens, while diatoms dominated in the open lake. During the rainy season, blue-greens remained dominant in the Nyanza Gulf although the number of species found was lower than during the dry season; in the open lake, blue-greens replaced diatoms as the dominant group and there were more species than in the dry season. 4 Canonical correspondence analysis indicated that the phytoplankton species distribution was significantly correlated with turbidity during the dry season and with SiO2 during the rainy season. Chlorophyll a concentrations ranging from 2.0 to 71.5 mg m-3 in the dry season and 2.0–17.2 mg m-3 in the rainy season confirm earlier reports of increasing phytoplankton biomass in Lake Victoria since the 1960s.