Trophic Interactions Influence Thermal Adaptation of Phytoplankton Size and Stoichiometry

AbstractUnderstanding how temperature affects adaptation of cell size is challenging because cell size mediates numerous physiological and ecological trade-offs. While physiological mechanisms can lead to decreases in cell size with warming (the temperature-size rule [TSR]), it is unclear how ecological processes (competition, predation) combine to modify the TSR. Here, we evaluate how ecological interactions affect thermal adaptation of phytoplankton cell size. We perform an eco-evolutionary analysis of a nutrient-phytoplankton-zooplankton model. The model assumes that phytoplankton experience size-dependent constraints on resource allocation that cause small cells to sacrifice investment in growth machinery, thereby reducing maximum growth rate but increasing competitive ability. We find that trophic interactions strongly impact the evolutionarily stable cell size across temperatures. Without zooplankton, cell size declines monotonically with temperature, consistent with the TSR. With zooplankton, cell size varies unimodally with temperature, due to temperature-dependent shifts in the grazer's capacity to ease nutrient competition by controlling phytoplankton biomass. Size-selective grazing does not qualitatively alter this result but can facilitate coexistence of phytoplankton via a competition-predation resistance trade-off. Trophic interactions therefore can produce temperature-size responses in phytoplankton that differ qualitatively from the canonical TSR, and an understanding of how temperature affects cell size is incomplete without this ecological component.