PREY-SIZE DEPENDENCY OF GRAZING BY FREE-LIVING MARINE FLAGELLATES

Currently, there are 3 distinct and conflicting predictions of the dependency on prey size of the clearance rates [(volume cleared of prey) flag.-1 time-1) of flagellates that feed by the mechanism of direct-interception. Clearance rates for this group of organisms have been predicted to be proportional roughly to either the first, second or third powers of prey radius indicating either weakly, moderately or strongly size-selective grazing behaviors, respectively. These predicted relationships were tested experimentally for cultured and natural flagellate populations fed mixtures of large and small fluorescently labelled prey (either heat-killed bacteria or latex microspheres). Evidence from 7 experiments indicated that flagellate feeding is only weakly size-selective; that is, clearance rate varies, roughly, in direct proportion to prey radius. These results provide preliminary support of a prey capture model that is based entirely on the interaction of hydrodynamic forces and surface forces arising between free-living marine flagellates and picoplankton-size prey. The extent to which small size provides a refuge from grazing mortality among the picoplankton, and the potential for a significant population of dormant bacteria, is probably less than previously believed; consequently the role of the most minute bacteria in pelagic food webs may require reexamination.