Ecological stoichiometry and the evolution of plankton communities

Nutrients such as nitrogen and phosphorus are required for the growth and survival of all organisms, and limit the productivity of many ecosystems. Ecological stoichiometry is the study of the elemental composition of organisms, and focuses on how variation in nutritional requirements shapes interactions between species. In this thesis, I focus on selective grazing by zooplankton on nutritious phytoplankton prey, and examine several mathematical models of these tiny aquatic plants and herbivores. My analysis reveals an important dilemma faced by phytoplankton, which became known as the competition-predation trade-off. Phytoplankton species that are more efficient in nutrient uptake will not only be better competitors, but will also be tastier prey for their zooplankton predators. This trade-off favors the evolution of less palatable phytoplankton in the presence of selective zooplankton, and allows the coexistence of specialists in nutrient uptake and grazing avoidance. Selective grazing on nutritious prey is predicted to have a substantial impact on various aspects of plankton communities, including the dynamical stability of their populations (chapters 2–4), the nitrogen and phosphorus content of phytoplankton cells (chapter 3), the individual size of phytoplankton and zooplankton (chapter 4), and the water depth at which plankton thrive (chapter 5). These models are relatively simple and based on sound biological assumptions, and can therefore be readily tested in laboratory experiments. The body of theory developed in this thesis can also be incorporated in more comprehensive biogeochemical models, which will help to advance our broader understanding of aquatic ecosystems.