Effects of food type on the life history of Daphnia clones from lakes differing in trophic state. I. Daphnia galeata feeding on Scenedesmus and Oscillatoria

1. Effects of the green alga Scenedesmus obliquus and the filamentous cyanobacterium Oscillatoria limnetica as food sources on the growth and reproduction of nine Daphnia galeata clones were studied. A high concentration of these two algae species was fed to Daphnia clones collected from four lakes with varying trophic conditions in a life history experiment. 2. Although Oscillatoria is regarded as a low quality food for Daphnia because of its filamentous morphology and suspected nutritional inadequacy, the cyanobacterium sustained both body growth and reproduction in D. galeata. Growth and reproduction were, however, lower and the onset of reproduction was delayed on a diet of Oscillatoria compared to a diet of Scenedesmus. The intrinsic growth rate, r, for each D. galeata clone was positive on a diet of Oscillatoria, ranging from 0.06 to 0.16 mm day(-1), whereas the intrinsic growth rates on a diet of Scenedesmus ranged from 0.27 to 0.34 mm day(-1). 3. The degree of growth depression on the Oscillatoria diet varied among clones. Clonal responses were correlated with body size; clones with larger body size decreased their growth and reproduction more on a diet of Oscillatoria. The responses of different clones from the same lake to the two diets resembled each other, irrespective of geography or trophic state of the lake. [KEYWORDS: Aphanizomenon-flos-aquae; fresh-water zooplankton; blue-green-algae; resource depression; seasonal succession; size selection; pulex; reproduction; magna; cladocerans