Organismal stoichiometry and the adaptive advantage of variable nutrient use and production efficiency in Daphnia

Models that describe the ecological stoichiometry of individuals and populations often include a simplifying assumption that nutrient ratios in consumers are constant. However, several experiments suggest that organisms can alter their internal stoichiometry under nutrient poor conditions. These experiments also suggest that the production efficiency (biomass produced per unit ingested) of limiting nutrients increases when food quality is poor. We explored how these two physiological adaptations affected growth and reproduction in Daphnia under low nutrient levels by developing three different dynamic simulation models of an individual daphnid under stoichiometric constraints. All three models were converted into individual-based population models interacting with a process-based algal population model. Calibration of the three individual models demonstrated that the assumption of constant nutrient ratios was unable to replicate experimental observations of Daphnia growth under nutrient limited conditions. The addition of increasing phosphorous production efficiency improved the Daphnia's ability to survive and grow, but still not within reported ranges. The addition of variable stoichiometry in the form of increasing nutrient use efficiency yielded an individual daphnid model that could be calibrated across reported food quantity/quality gradients. Further, the model accurately simulated the demonstrated ability of Daphnia to overcome the limitation of poor food quality and eventually suppress the algal population in a high-energy, low-nutrient system. The model accurately predicted the timing of the peak of the Daphnia population as well as the equilibrium biomass levels for both populations reported in a recent experiment. Explorations of model behavior further demonstrated that the adaptations under consideration increase total system P cycling which increases total system production.