¹Section of Ecology, Department of Biology, University of Turku, FIN-20014 Turku, Finland
²Section of Hydrobiology, Department of Biological and Environmental Sciences, University of Jyväskylä, P.O. Box 35 (YAC), FIN-40351 Jyväskylä, Finland

^*Corresponding author: Fax: +358-2-3336550, E-mail: kristiina.vuorio@utu.fi

Abstract

  The spatial distribution of phytoplankton and picocyanobacteria was studied in Lake Tanganyika in relation to environmental factors. Sampling was conducted within three weeks during the wet season of 1998 when the water column stratification was most stable. The secchi depth varied between 11 and 16 m, except off the river Malagarasi (3 m). The depth of the euphotic zone was 33 to 56 m. Altogether, 218 phytoplankton taxa were identified. Their total biomass varied between 13 and 88 mg m^-3 fresh weight. Picocyanobacteria were present at very high densities (104 to 6 x 105 cells ml^-1). The influence of the river Malagarasi appeared to spread along the water surface because of the lower density of the river water. The phytoplankton species number and biomass were particularly high near the river mouth. Excluding that area, there were no statistically significant differences in the species composition among study sites. Nevertheless, differences in water column mixing, indicated by variable temperature gradients, seemed to affect the horizontal distribution of at least some phytoplankton species.