Trophic interactions are a key element to understand food web processes and ecosystem functioning. In aquatic ecosystems, many essential nutrients are supplied primarily by primary producer like phytoplankton. Phytoplankton is grazed by primary consumers, which in turn are hunted by predators. Energy and nutrient transfer, as well as inter-specific interactions along this trophic cascade play important roles in the dynamics of food webs. However, many species evolved defences to protect themselves against predation. Additionally, defences can be inducible instead of constitutive to reduce costs that may come along with the formation of these defences. Such induced defences such as morphological modifications and increased reproduction may imply changing demands of resources that have to be provided by primary producers. Thus, we hypothesize that the phytoplankton community has a major impact on the dynamic between higher trophic level, such as predator-prey interactions, by potentially modulating the defence responses of primary consumers.
Understanding trophic interactions is crucial for assessing energy and nutrient fluxes within food webs and to get further insight into food web processes. But food web processes are affected by predator-prey relationships, that influence population dynamics. Thus, research of nutritional ecology as well as predator-prey-interactions are strongly interlaced. As important primary consumers, Daphnia species strongly influence food web processes, such as energy and nutrient transfer from primary producer to higher trophic level. Thanks to its high phenotypic plasticity, Daphnia is able to cope with changing environmental conditions using a broad variety of mechanisms, such as the expression of diverse induced defences and adjusted resource allocation. Inducible defences as well as the influence of nutrient availability, especially essential lipids, have been investigated intensively. However, only a few studies show that food quantity influences induced defences in Daphnia, with ambiguous results. Additionally, there is no study exploring the effect of food quality on defence induction.
In this project, I will combine nutritional ecology and research of predator-prey interactions in cooperation with the group of Dr. D. Martin-Creuzburg of the Limnological Institute of the University of Konstanz. The main aim of my project is to improve our capability of projecting consequences of changes in phytoplankton communities for the freshwater keystone species of Daphnia. Therefore, we will focusing on the effects of food quantity and quality mediated by essential lipids on inducible defences in Daphnia, that are triggered by different predator types (invertebrate vs. vertebrate). Therefore, I will comparatively assess the combined effects of carbon and essential lipids on D. magna and D. longicephala at the trait and whole animal performance level and assess potentially increasing requirements and costs for induced defences in D. magna.
I will evaluate how the total carbon availability and essential lipids (i.e. cholesterol and EPA) influences Daphnia's defence expression and resource allocation, and how the different defence traits are affected not only by single changes, but also by combined effects of the nutrient balance. By exploring these changes and assessing nutritional costs for inducible defences, we will be able to assess a more comprehensive picture of how induced defences in Daphnia are affected by the phytoplankton community. This will contribute to our knowledge of trophic interactions and to our understanding of food web processes.