Effects of food quantity and quality on the life history of Daphnia lumholtzi in Mwanza Gulf (Lake Victoria, Tanzania).

Until the 1950s, large-bodied calanoids and cladocerans dominated the zooplankton community of Lake Victoria, whereas cyclopoid copepods only comprised 10% of microcrustaceans. From the 1960's onwards, cyclopoid copepods increased to 70-90% of zooplankton and cladocerans, now dominated by small species, decreased to ca. 5%. Concomitantly phytoplankton biomass increased and shifted from dominance of diatoms to Cyanobacteria, which were hypothesized to be of less nutritional quality, causing the shift in zooplankton. We investigated whether the natural assemblage of Cyanobacteria in Mwanza Gulf negatively affected growth and fecundity of cladocerans. In 2010-2011, we performed life-history experiments with the cladoceran Daphnia lumholtzi, feeding it natural seston from Mwanza Gulf from three different locations. A laboratory-strain of the green alga Scenedesmus obliquus, proven to be high-quality food, was used as a control. Growth of D. lumholtzi in the rainy season and at one station in the dry season was just as high as in the control treatment. If there were negative effects of natural seston these were small. Although the evidence is circumstantial, this suggests that Cyanobacteria and/or their detritus could have been better food than expected and that food quality is not limiting the growth of D. lumholtzi in L. Victoria.