Trophic controls on thermal adaptation of phytoplankton size and stoichiometry

Temperature impacts physiological function, driving evolutionary adaptation in physiological traits that influence ecosystem properties. Temperature also impacts ecological rates, altering the strength of trophic and competitive interactions. Yet, possible effects of temperature-driven shifts in ecological interactions on physiological adaptation are unclear. We investigate how ecological interactions shape thermal adaptation of phytoplankton cell size, a trait that affects macromolecular composition, Phosphorus:Carbon ratio, competitive ability, and grazing susceptibility. We identify evolutionarily stable strategies in a nutrient-phytoplankton-zooplankton system. We find that trophic interactions strongly impact the evolutionarily stable cell size across temperatures. Without zooplankton, cell size and P:C ratio declines monotonically with temperature. With zooplankton, cell size and P:C ratio varies unimodally with temperature, due to temperature-dependent shifts in the grazer’s capacity to ease nutrient competition by controlling phytoplankton. Size-selective grazing does not qualitatively alter this result but facilitates diversification. We conclude that ecological interactions play critical roles in physiological adaptation to warming.