Contract grant sponsor: Austrian Agency for International Cooperation in Education and Research (OeAD-GmbH) as part of W.O. PhD studies. Supporting grant number: 874/1090. Sponsor: British Ecological Society (BES). Supporting grant number: A/4173-1. Sponsor: International Foundation for sciences (IFS). Seasonal development of cyanobacteria and microcystin production in Ugandan freshwater lakes OKELLO William 1 ，KURMAYER Rainer 2 1 National Fisheries Resources Research Institute (NaFIRRI), Plot No. 39/45 Nile Crescent, P.O. Box 343, Jinja, Uganda 2 Institute of Limnology, Austrian Academy of Sciences, Mondseestrasse 9, 5310 Mondsee, Austria Abstract: This study pioneered investigation on seasonal development of phytoplankton and potential microcystin (MC)-producing cyanobacteria and MC concentrations in freshwater lakes in Uganda. During one year (May 2007-April 2008) chemical and physical characteristics, the phytoplankton composition and the MC concentrations were recorded monthly in a hypertrophic crater lake (Lake Saka), in shallow eutrophic lakes (Lakes Mburo, George, Edward), and in Lake Victoria (Murchison Bay, Napoleon Gulf). Throughout the study period, cyanobacteria (composed of the genera Anabaena, Aphanocapsa, Chroococcus, Cylindrospermopsis, Microcystis, Planktolyngbya, Planktothrix) dominated and always contributed >50% to total phytoplankton biovolume. All samples from all sampling sites were found to contain MC. Samples from Lake Saka had the maximum MC concentration (10 μg L -1 ) in July 2007. The minimum concentration (0.02 μg L -1 ) was recorded in Lake George (Kahendero) in the months of May 07, June 07, January 08 and April 08. At the sampling sites in the other three lakes intermediate MC concentrations (0.1 - 2.5 μg MC-LR eq. L -1 ) were recorded. For all sampling sites highly significant positive linear relationships between the total MC concentration and Microcystis cell numbers were obtained. Relating the total MC concentrations to Microcystis cells revealed a >100-fold variation in the average MC contents per cell between lakes. While Microcystis from Lake George showed the lowest MC cell quotas (0.03-1.24 fg cell -1 ) Microcystis from Lake Saka consistently showed maximum MC cell contents (14-144 fg cell -1 ). It is concluded that at all sites MC production is due to the occurrence of Microcystis, however between sites the populations differ consistently and independently of the season in their average MC content per cell. Keywords: Blue-green algae, Cyanotoxins, Microcystis, Phytoplankton, Water monitoring 1. Introduction During the last decades cyanobacteria in freshwater have been of general awareness due to their ability to produce various hepatotoxic and neurotoxic substances. It is generally agreed that the hepatotoxic microcystin (MCs) are probably the most abundant toxins produced by cyanobacteria in freshwater (1). While the number of taxa that has been found to produce MCs is constantly increasing, the MC-producing genera that are of major importance in phytoplankton have already been identified during the nineties: Anabaena, Microcystis, and Planktothrix (2, 3). In a recent paper, we could show that cyanobacteria contribute significantly to the phytoplankton of freshwater lakes in Uganda while other algal groups like diatoms, green algae, and cryptomonads are of a relatively minor importance (4). We further concluded that in Uganda the genus Microcystis is favoured under more shallow, eutrophic conditions which is in correspondence to the general theory on how abiotic and biotic factors govern phytoplankton associations that have been defined since 1984 (5). In contrast to Microcystis other genera known to produce MCs were found in lower abundance, e.g. Anabaena was abundant in Lake Victoria and one of the Crater Lakes while Planktothrix was not observed. However, in this earlier study we were unable to monitor the phytoplankton community during different seasons. This issue is of relevance as usually dry seasons with precipitation minima and wet seasons with maximum precipitation have been correlated with changes in phytoplankton composition. Usually precipitation varies annually from 30 mm to 132 mm in Kasese, western Uganda and 60 mm to 184 mm in