The independent and interactive effects of resource availability and predation on Daphnia life history and stoichiometry

Human-induced ecological change impacts multiple components of the aquatic food web simultaneously. It is critical to understand how anthropogenic changes in top-down forces like predator introduction and bottom-up forces like nutrient enrichment affect predator-prey dynamics, species interactions, and resource availability in lakes. I measured the quantitative interaction of these factors and its effects on nutrient cycling, algal elemental composition, and Daphnia life history and stoichiometry in a controlled laboratory experiment, a comparative field study, and a lake mesocosm experiment.;In the laboratory experiment, predator kairomones had a significant effect on Daphnia life history regardless of food quality, but the effect of food quality (algal carbon:phosphorus) on Daphnia life history depended on predator presence/absence and identity. Food quality mediated plasticity in Daphnia life history traits in the presence of Chaoborus, but not fish. In contrast, both resource availability and predation affected Daphnia stoichiometry.;The laboratory experiment also addressed the interaction of genetic and environmental factors, demonstrating that interclonal variation in intergenic spacer length and predator cues significantly affected Daphnia life history regardless of food quality. However, phosphorus availability mediated plasticity in life history traits to differing degrees depending on genotype (clone) and environment (predator).;In the field comparison, differences in nutrient availability, food quality, and Daphnia life history and stoichiometry in an alewife lake compared to a no alewife lake indicate that resource availability and predation interact significantly, including an important seasonal component. Mid-level induced responses to predation modulate the impact of these factors as the life history response of Daphnia appears to affect food chain stoichiometry.;The mesocosm experiment demonstrated significant effects of alewife density on nutrient availability, resource quality, and zooplankton community structure—indicating that alewife restoration efforts have important implications for lake ecosystems. Results also identified a threshold density (2 fish/m2) at which alewife effects on resource availability and zooplankton community structure start to level off, indicating potential saturation.;Together, these studies explore direct and indirect interactions of predation and resource availability and their effects on Daphnia stoichiometry, life history, and population dynamics, creating a clear description of the interaction of top-down and bottom-up forces in aquatic ecosystems.