Grazing pressure is independent of prey size in a generalist herbivorous protist: Insights from experimental temperature gradients

Abstract

Grazing by herbivorous protists contributes to structuring plankton communities through its effect on the growth, biomass, and competitiveness of prey organisms and also impacts the transfer of primary production towards higher trophic levels. Previous evidence shows that heterotrophic processes (grazing rates, g) are more sensitive to temperature than autotrophic ones (phytoplankton growth rates, μ) and also that small cells tend to be more heavily predated than larger ones; however, it remains unresolved how the interplay between changes in temperature and cell size modulates grazing pressure (i.e., g:μ ratio). We addressed this problem by conducting an experiment with four phytoplankton populations, from pico- to microphytoplankton, over a 12 °C gradient and in the presence/absence of a generalist herbivorous protist, Oxyrrhis marina. We found that highest g rates coincided with highest μ rates, which corresponded to intermediate cell sizes. There were no significant differences in either μ or g between the smallest and largest cell sizes considered. The g:μ ratio was largely independent of cell size and C:N ratios, and its thermal dependence was low although species-specific differences were large. We suggest that the similar g:μ found could be the consequence that the energetic demand imposed by rising temperatures would be a more important issue than the mechanical constriction to ingestion derived from prey cell size. Despite the difficulty of quantifying μ and g in natural planktonic communities, we suggest that the g:μ ratio is a key response variable to evaluate thermal sensitivity of food webs because it gives a more integrative view of trophic functioning than both rates separately.