Poor direct exploitation of terrestrial particulate organic material from peat layers by Daphnia galeata

Terrestrial organic material (t-OM) can subsidize lake food webs indirectly via incorporation of dissolved t-OM by bacteria and subsequent transfer to higher trophic levels or directly through metazoan consumption of particulate t-OM (t-POM). We tested the effects of peat layer t-POM on Daphnia galeata performance. A pure t-POM diet could not sustain survival, growth, and reproduction of D. galeata. Mixtures of heterotrophic bacteria (Pseudomonas sp.) and phytoplankton (Rhodomonas lacustris) gave higher survival, growth, and reproduction than mixtures of t-POM and Rhodomonas. Daphnids performed best when feeding on pure Rhodomonas diets. Quantification of phosphorus (P) and essential biochemicals (i.e., fatty acids) revealed that Rhodomonas had the highest amounts of all these components. Pseudomonas, while rich in P, contained few essential fatty acids, and t-POM had low concentrations of both P and fatty acids. We therefore suggest that the poor food quality of t-POM in our experiment was due to its suboptimal mineral and biochemical composition and that a substantial proportion of high-quality phytoplankton is necessary to sustain zooplankton biomass.