Living with constraints - food quality effects on zooplankton

Copepods and daphniids exhibit different grazing patterns, which affect the seston size distribution. I have studied whether the grazing of one guild could be beneficial for the other guild. Daphniids grew faster on seston previous manipulated by copepods than on seston which was not manipulated. Seston manipulated by daphniids was a poor food source for Daphnia and thus, growth declined. When marine copepods were feeding on plankton previously manipulated by copepods their growth potential (RNA:DNA) decreased with increasing copepod densities. In the laboratory I studied the effect of fatty acids and phosphorus (P) on Daphnia magna life-history and stoichiometry. D. magna had low EPA saturation threshold for growth (0.04 µg EPA L-1) and high storage capacity. However for P, daphniids had high requirements but only small storage capacity. P storage could not be utilised, whereas EPA storage could compensate periods of poor food quality.Introduction 3 1 Resources and growth – general limitations 3 2 Food quality 4 3 The study organisms 9 4 Hypotheses 12 5 Thesis outline 13 Chapters I Differential impacts of copepods and cladocerans on lake seston, 17 and resulting effects on zooplankton growth II Impacts of copepods on marine seston, and resulting effects on 35 Calanus finmarchicus RNA:DNA ratios III Resource quality effects on life histories of Daphnia 54 IV Differential impacts of phosphorus and fatty acids on Daphnia 69 growth and reproduction V Discussion 91 Summary 98 Zusammenfassung 101 References 104 Acknowledgement 111 Curriculum vitae 112 Erklärung 11