Phytoplankton of Lake Kivu

This chapter reviews taxonomic composition, biomass, production and nutrient limitation of the phytoplankton of Lake Kivu. Present Lake Kivu phytoplankton is dominated by cyanobacteria – mainly Synechococcus spp. and thin filaments of Planktolyngbya limnetica – and by pennate diatoms, among which Nitzschia bacata and Fragi-laria danica are dominant. Important seasonal shifts occur, with cy-anobacteria developing more in the rainy season, and the diatoms in the dry season. Other groups present are cryptophytes, chrysophytes, chlorophytes and dinoflagellates. According to a survey conducted in the period 2002-2008, the composition of the phytoplankton as-semblage was quasi homogeneous among lake basins. The mean eu-photic depth varied between 17 and 20 m, and the increase in the ra-tio between mixed layer depth : euphotic depth to about 2 in the dry season may have selected for diatoms and cryptophytes, which tend-ed to present their maximal development in this season, when cya-nobacteria slightly decreased. Mean chlorophyll a concentration was 2.03 – 2.24 depending on the lake basin, and the mean daily primary production was 0.62 g C m-2 d-1 (range, 0.14-1.92), i.e. in the same range as in other large oligotrophic East African Rift lakes. Seston elemental ratios indicated a moderate P-deficiency during the dry, mixed season and a severe P limitation during part of the rainy, strat-ified season; the C:N ratio indicated a moderate N limitation throughout the year. Nutrient addition assays pointed to a direct N-limitation and co-limitation by P during rainy seasons and P or N limitation during dry seasons depending on the year. Thus, phyto-plankton ecology in Lake Kivu does not differ from that of other Rift lakes, where seasonal variations result in a trade-off between low light-high nutrient supply and high light-low nutrient supply. Phytoplankton production in Lake Kivu is also similar to that of oth-er Rift lakes, and nutrient limitation of phytoplankton growth may occur as a result of variable availability of N and P, as in Lakes Tan-ganyika and Malawi, even though the extent of P limitation seems greater in Lake Kivu.