Currently, there are 3 distinct and conflicting predictions of the dependency on prey size of the clearance rates [(volume cleared of prey) flag.-' time-') of flagellates that feed by the mechanism of direct-interception. Clearance rates for this group of organisms have been predicted to b e 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 freeLiving 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 requlre reexamination.

We present a theory of du-ect-interception feeding by marme zooflagellates based upon fundamental principles of hydrodynamics and physical chemistry. Analysis shows that in the absence of confounding behaviors the balance between fluid drag and a con~plex set of surface-forces uniquely determines prey trajectories about zooflagellate grazers and consequently, clearance rate (volume cleared flag.-' h-') and specific clearance rate (volume cleared [volume flag.]-' h-'). As a first approximation to this general 'Force-Balance' approach, w e utilize a model taken from the filtration literature (Spielman & Goren 1970 [Envlron. Sci., Tech. 4 . 135-1401) in which wall-corrected fluid drag is balanced with the London-van der Waals force (FLondon) Using literature estimates of FLondon, and a standard grazer swimming speed (U,) of 200 pm S -', clearance rates (Clr, ,,) and specific clearance rates (SpClrFB) are predicted to range respectively from 0.13 to 1.8 nl flag.-' h-' and from 0.09 to 7.6 X 104 h-' for grazer (Rg) and prey (R,) radii typical of zooflagellate-picoplankton interactions. Analysis shows that CkFB is roughly proportional to RpO.%which strongly contrasts with the RP2.' proportionality predicted by a model based on geometric considerahons (Fenchel 1982a [Mar. Ecol Prog. Ser 8: 211-2231 'Flows of energy and materials in marine ecosystems', Plenum Press]). For given zooflagellate and prey size, Clr,, can be up to 10 times greater than Geometric model pre&ctions with the greatest disparity between models occurring for relatively large grazers feeding on small prey. ClrFB is generally within a factor of 2 of empirical values in the literature, but in some instances underpredicts by a n order of magnitude. The remaining discrepancies may b e explained by uncertainties in grazer size, swimming speeds and London-van der Waals force. Attempts to incorporate nonspherical shapes, flagellar hydrodynamics, and hydrophobic and steric forces remain viable areas for fine-tuning the model's predictive capabilities