Suspension feeders are common among diverse taxa throughout marine ecosystems. A focus on unifying, physical constraints on particle-capture mechanisms yields insights into functional morphologies as well as predictive understanding of the roles of suspension feeding in trophic and particle dynamics. Common physical constraints of morphology, particle characteristics, and flow environment are found at the level of particle encounter, determining particle availability to suspension feeders. Encounter mechanisms were modeled in terms of rates, contrasting with previous treatments in terms of efficiencies, for cylindrical and spherical morphologies under assumptions of steady, laminar, low appendage Reynolds-number (Re) flow. A minimal clearance rate was predicted for micrometer-sized particles, due to the balance of direct interception and Brownian diffusional encounter. For a variety of protozoans and invertebrates, clearance rates for colloids may greatly exceed those for larger cells in terms of both particle numbers and organic carbon.

Violations of model assumptions were explored empirically. Selective particle encounter was found dependent on Re regime and particle:appendage diameter. Direct-interception rates for an isolated cylinder showed a stronger direct dependence on particle size and a trend of weaker inverse dependence on appendage diameter as Re rose above unity. As particle:appendage diameter rose above 0.1, the dependence of encounter rate on appendage diameter turned from inverse to direct, implying an optimal appendage size for encountering particles of a given size.

Implications of nonuniform flow were studied with planktonic, protozoan suspension feeders in laminar shear fields mimicking turbulence-driven submicroscale shear. While feeding rates of several bacterivorous and herbivorous flagellates and ciliates were unaffected by imposed shear rate, feeding rates of an aloricate choanoflagellate and a heliozoan were directly related to shear rate over an environmentally realistic range. The latter results are likely produced by a direct influence of shear on encounter rates. Spatial and temporal variations in turbulence may affect feeding rates and microbial food-web dynamics in a species-specific manner, with the strongest coupling predicted for slow-swimming and nonmotile protozoans. Despite low body Re, local flow and feeding rates are directly influenced by larger-scale, inertial flow phenomena.