Impact of food quality on aquatic consumers : Behavioral and physiological adjustments

Food quantity and quality together determine growth rates of consumers and the utilisation efficiencies of available resources in aquatic and terrestrial ecosystems. The effect of food quality on the performance of consumers is dependent on both, its direct influence on ingestion and assimilation rates, and on the behavioural and physiological adjustments of consumers to their food environment. The main target of this thesis was to investigate the nature and scope of behavioural and physiological adjustments in consumers and assess the resulting consequences for consumers’ fitness and ecosystem-wide nutrient flows. In paper I, we investigated the extent of elemental homeostasis across several taxonomic groups of planktonic herbivores. We found that adjustments in elemental ratios (C:N:P) in body tissues are an important physiological response of heterotrophic flagellates, but that in ciliates and multi-cellular organisms C:N:P ratios varied much less than in their algal prey. Hence, alternative regulatory mechanisms determine the reactions of metazoan zooplankton to decreases in food quality. In paper II, we developed a theoretical model to explore regulation in behaviour and digestive physiology of consumers to changes in the food environment. Our results demonstrate that feeding and digestion of consumers are determined by trade-offs between benefits and costs of investments in these processes. We revealed that the flexibility in consumers’ behaviour and physiology had strong influences on assimilation rates and efficiencies and thereby affected growth rates and a wide range of ecosystem functions. In paper III, we investigated the scope and consequences of adjustments in feeding and assimilation rates of copepods exposed to different diets. An important finding was that consumers can use resources, which are available in surplus, to increase the uptake of a limiting nutrient. Such nutrient interconversion led to co-limitation, the simultaneous limitation of copepods by two different nutrients. Finally, in paper IV, we aimed to test the effect of food quality on population dynamics in the field. We investigated zooplankton populations in tropical soda-lakes, an environment with a surplus of planktonic food sources that thus provides an ideal setting for investigations of food quality. However, we found that the hatching of resting eggs from lake sediments was the main driver of zooplankton bloom formation resulting in non-cyclical dynamics that were not related to food quality. These findings contributed to our understanding under which circumstance and by which mechanisms food quality affects the performance of consumers. My results highlight that food quality has not only direct effects on consumers’ growth but also triggers behavioral and physiological responses in consumers to maximize their fitness.At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: Manuscript.