Department of Biology, University of Turku, FIN-20014 Turku, Finland. E-mail: kristiina.vuorio@utu.fi

Spatial distribution of phytoplankton in Lake Tanganyika

During the wet season of March to April 1998, 23 study locations were sampled from the surface to a depth of 60 metres at 10 metre intervals of Lake Tanganyika for spatial distribution of phytoplankton. The total phytoplankton biomass varied between 13.4 and 87.6 mg m-3 fresh weight. Although most taxa identified were present in the whole lake, the most species rich samples were off the Malagarasi river mouth. The most abundant cyanobacterial species, Chroococcus cf. catenatus Komárek & Novelo, and Anabaena sp., were commonly found in the southern part of the lake. Chroococcus cf. mipitanensis (Wolosynska) Geitler were more abundant in the north. The occurrence of Anabaena sp. was also associated with high particulate N to P ratio. Cyanobacteria, which can grow under high UV radiation, and chlorophytes, Kirchneriella cf. mayori (G.S. West) Kom.-Legn., Kirchneriella sp. and Lagerheimia subsalsa Lemm., dominated the upper part of the water column. Diatoms, Achnanthes spp., Nitzschia spp. and Nitzschia cf. actinastrioides (Lemm.) van Goor, which have low ability to migrate in the water column and are well adapted to low light, were found most abundantly in deeper water. The phytoplankton community off the Malagarasi river mouth differed clearly from other study locations and the river water seemed to spread offshore close to the surface. The differences in phytoplankton distribution between the main basins could be explained by depth but not by geographical location or particulate nutrient concentrations. Differences in temperature gradients showed dissimilarities in water column mixing between study locations, which might have had an influence on the horizontal distribution of phytoplankton.