Spatial - temporal variability of phytoplankton abundance and species composition in Lake Victoria, Kenya: implication for water quality management

Abstract

Study of phytoplankton abundance and species composition in Lake Victoria, Kenya showed a marked difference between the main lake and the semi-closed Nyanza (Winam) Gulf. Phytoplankton biomass and species composition showed both spatial and seasonal variation. In the main lake the highest chlorophyll concentration was during the short stratification season (August to September) but in the gulf it was after the short and long rainy seasons. Deep mixing depth (aprox. 30 m) in the main lake imposes light limitation to algal growth leading to the observed lower chlorophyll in the main lake, despite the higher soluble reactive phosphorus (SRP) in the main lake compared to the gulf. Blue green algae were the most abundant algal species, contributing between 45 and 65% of total abundance. Cylindriospermopsis africana, a nitrogen fixing and potentially highly toxic species was present in the gulf during the dry season but was absent during the wet season and in the main lake the nitrogen fixing Anabaena sporoides was present in high numbers during mixing period, which can be attributed to low availability of dissolved inorganic nitrogen during lake mixing. Rusinga channel, the transition area between the Nyanza Gulf and the main lake, had high diatom abundance and showed a different phytoplankton species composition compared to the other lake zones. The current dominance of cyanobacteria (blue-green algae) in the lake, and especially bloom forming and potentially toxic species calls for urgent management of pollution loading into the lake in order to stem further degradation of water quality and help in the restoration of Lake Ecosystem.