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

Abstract

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 . 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, 1984 [in: '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 ' Contribution No.