Gaps in our understanding of marine ecosystems prevent us from accurately predicting how climate change will affect plankton communities. We hypothesize that climate-driven shifts in predator community composition will alter the biomass distribution and productivity of lower trophic levels through top-down control of plankton size structure. Warm and cool climate phases are favorable for different assemblages of planktonic predators; temperature-driven changes in these assemblages may alter the relative abundances of predators that are generalists or specialists with respect to prey size selectivity. We studied the effects of size-selective feeding on lower trophic levels using a continuous-size nutrient-phytoplankton-zooplankton (NPZ) model. We varied both the mean predator-prey size ratio and the trophic niche breadth (SD of log prey size distribution) of zooplankton predators. At equilibrium, model ecosystems with generalist predators had more plankton biomass, lower diversity (number of size classes), fewer top predators, and steeper and gappier biomass spectra than ecosystems with specialist predators. Temperature-driven shifts in the dominant predator assemblage may alter plankton community structure and productivity, thus predicting the effects of climate change on marine ecosystems will require knowledge of plankton responses to environmental forcing, particularly by those predators that operate at the extremes of size-selective feeding.