Lurling, M., M. De Lange
Abstract:
The effects of Daphnia infochemicals on morphology and biochemical composition of Scenedesmus was studied, influencing Daphnia life history parameters. Three species of Scenedesmus (S. acutus, S. obliquus and S. subspicatus) were tested for Daphnia induced colony formation. Subsequently life history experiments with Daphnia magna and D. cucullata were performed with unicellular and colonial S. acutus. Life history parameters of both Daphnia species were influenced by the used food type. Feeding on colonial Scenedesmus resulted in longer generation times and interclutch durations. The number of newborns was smaller in all clutches when animals were fed with colonies compared to unicells. However, size at maturity and survival were not different between animals reared on the two food types. The intrinsic rate of population increase (r) was significantly lower for daphnids feeding on colonies; r was reduced from 0.22 to 0.15 d^-1 and from 0.31 to 0.30 d^-1 for D. cucullata and D. magna, respectively. A short time grazing experiment with 3 size classes of Scenedesmus revealed lower clearance rates for D. cucullata and juvenile D. magna when animals were allowed to graze on large sized Scenedesmus. No differences in total lipid, protein and carbohydrate content between the unicellular and colonial ecomorphs of the three Scenedesmus species was found. However, fatty acid concentration and composition was changed. Total fatty acids as % of freeze dry-weight were increased in colonies. The ratio of mono-unsaturated and poly-unsaturated fatty acids and the ratio w3/w6 fatty acids decreased in colonies, while the ratio of total saturated and unsaturated fatty acids remained constant. The negative influence of colonial Scenedesmus on population growth rate of Daphnia is mainly determined by algal morphology than biochemical composition.
(Dept. Water Quality Managment & Aquatic Ecology, Ritzema Bosweg 32a, Agricultural University Wageningen, The Netherlands)
*******