Direct-interception feeding by marine zooflagellates: the importance of surface and hydrodynamic forces

Abstract

We present a theory of direct-interception feeding by marine 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 complex set of surface-forces uniquely determines prey trajectories about zooflagellate grazers and consequently, clearance rate (volume cleared flag.-1 h-1) and specific clearance rate (volume cleared [volume flag.]-1 h-1). As a first approximation to this general ''Force-Balance'' approach, we utilize a model taken from the filtration literature (Spielman and Goren 1970 [Environ. Sci., Tech. 4: 135-140]) 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 (Ug) of 200 .mu.m s-1, clearance rates (ClrFB) and specific clearance rates (SpClrFB) are predicted to range respectively from 0.13 to 1.8 nl flag.-1 h-1 and from 0.09 to 7.6 .times. 104 h-1 for grazer (Rg) and prey (Rp) radii typical of zooflagellate-picoplankton interations. Analysis shows that ClrFB is roughly proportional to Rp0.8 which strongly contrasts with the Rp2.0 proportionality predicted by a model based on geometric considerations (Fenchel 1982a [Mar. Ecol. Prog. Ser. 8: 211-223], 1984 [in: ''Flows of energy and materials in marine ecosystems'', Plenum Press]). For given zooflagellate and prey size, ClrFB can be up to 10 times greater than Geometric model predictions 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 an order of magnitude. The remaining discrepancies may be 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.