Abstract
Microzooplankton are the main consumers of marine phytoplankton. Intrinsic traits of phytoplankton can reduce grazing mortality, directly influencing phytoplankton population dynamics, food web ecology and biogeochemical cycling. We examined the impact of calcification in mediating the functional grazing response of three heterotrophic dinoflagellates, on the coccolithophore, Emiliania huxleyi. A variety of parameters, including predator grazing and growth rates, were examined over a 24-48 h period, at one to five prey concentrations for five isolates of E. huxleyi that fell along a gradient of calcification states. Significant differences in ingestion and clearance rate were strain-specific, and no apparent trends were observed in relation to calcification. However, predators had, on average, a 60% slower growth rate on calcified strains relative to naked strains; furthermore, gross growth efficiency was reduced when ingesting calcified strains. A growth rate model demonstrated a positive feedback from grazing interactions whereby decreased predator growth rate on calcified strains resulted in the accumulation of E. huxleyi. This study highlights the complexity involved in understanding the role of prey phenotype on grazing rates, and emphasizes the importance in considering morphological traits when deciphering predator-prey interactions in the plankton.