Changes in food quality of the green alga Scenedesmus induced by Daphnia infochemicals: biochemical composition and morphology

1. The effects of Daphnia infochemicals on the morphology and biochemical composition of Scenedesmus were studied and subsequently their influence on Daphnia life history. Three species of Scenedesmus (S. acutus, S. obliquus and S. subspicatus) were tested for Daphnia‐induced colony formation. Life history experiments were performed with Daphnia cucullata and D. magna feeding on unicellular or colonial S. acutus. 2. Colony formation was promoted when S. acutus and S. obliquus were exposed to filtered water from a D. magna culture. S. subspicatus did not form colonies when exposed to culture water, but showed some colony formation when exposed to live D. magna. 3. No clear differences were found in total lipid, protein or carbohydrate content between the unicellular and colonial ecomorphs of the three Scenedesmus species. However, fatty acid (FA) concentration and composition were changed. Total FA as a percentage of frozen dry weight (DW) were increased in colonies. The ratio of monounsaturated (MUFA) and polyunsaturated fatty acids (PUFA) and the ratio of ω3 : ω6 fatty acids decreased in colonies. 4. A short‐term grazing experiment with three size classes of Scenedesmus revealed lower clearance rates for small Daphnia when feeding on large colonial Scenedesmus. 5. Life history parameters of both Daphnia species were influenced by food type. The intrinsic rate of population increase (r) was significantly lower for daphnids feeding on colonies. 6. The negative influence of colonial Scenedesmus on population growth of Daphnia seems to be determined by algal morphology rather than biochemical composition.