The efficiency of carbon transfer from primary producers to higher trophic levels is an important factor that determines the trophic structure of aquatic food webs. A decoupling of primary and secondary production at the phytoplankton--zooplankton interface can be attributed to the nutritional inadequacy of phytoplankton species. Recently, we have shown that the low carbon-transfer efficiency from cyanobacteria to the keystone genus Daphnia is caused by the lack of sterols in cyanobacteria. In the present thesis, further evidence is provided for the significance of dietary sterols for the performance of the herbivore Daphnia. A shortage in dietary sterols had serious consequences for a variety of life history traits which points to a strong impact of dietary sterols on population dynamics of the herbivorous grazer. Cholesterol, which is of major importance in arthropod physiology, is hardly represented in plant material. Instead, eukaryotic phytoplankton contains a great variety of phytosterols that differ from cholesterol in their chemical structure. Therefore, herbivorous grazers have to use the sterols found in their diet to synthesize cholesterol. In this thesis, I showed that dietary sterols differ in their adequacy to support Daphnia growth and that structural differences of dietary sterols can have pronounced effects on life history traits of Daphnia. In the field, the carbon that is fixed by primary producers, can be modified prior to ingestion by herbivorous crustaceans. I showed here that heterotrophic flagellates and ciliates as intermediary grazers can serve as a trophic link between picocyanobacteria- and Daphnia-production, thereby upgrading the nutritional value of a picocyanobacterial food source. For heterotrophic flagellates, this trophic upgrading is primarily due to the addition of sterols to the dietary carbon. In contrast, ciliates presumably lack the ability to synthesize sterols de novo. Instead, they synthesize the pentacyclic triterpenoid alcohol tetrahymanol and related compounds that provide functional equivalents for sterols as stabilizers of cell membranes. Data presented here suggest that these sterol-surrogates replace sterols as membrane reinforcers also in crustacean tissues, leading to the observed trophic upgrading of the sterol-free picocyanobacterial diet by ciliates. Despite the production of tetrahymanol and related compounds, ciliates were found to be less nutritious for daphnids than many algae. Here, I presented data showing that the ciliates' food quality for Daphnia is significantly enhanced when the ciliates were previously fed on sterol-supplemented albumin beads. Hence, the nutritional value of ciliates for Daphnia is determined by the sterol availability in the ciliates' diet. Ciliates feeding on a prokaryotic food source cannot rely on a dietary source of sterols and, therefore, will be nutritionally inadequate for daphnids, even though the production of tetrahymanol and related compounds might partly compensate for the sterol deficiency.