Diffusional encounter of submicrometer particles and small cells by suspension feeders

Models of Brownian diffusion and direct interception were used to predict encounter rates with nonmotile particles for geometries typical of planktonic flagellates, heliozoans, and passively suspension‐feeding invertebrates. The sum of encounter rates (normalized to concentration, thus dimensionally identical to clearance rates) by diffusion and interception has a minimum at particles of bacterial size or slightly larger, with encounter rates for colloids and viruses that are 1–21 times those for bacteria. The ratio of diffusion to interception rates exceeds by 4–6 orders of magnitude the analogous ratio of aerosol‐filtration efficiency indices for cylindrical collectors. These findings should extend to individual filter elements of many other active suspension feeders as well. Absolute encounter rates predicted for particles at environmental concentrations suggest that nonliving, submicrometer particles can make a large contribution to the encounter of particulate organic carbon for protozoan suspension feeders and that these particles as well as bacteria could be important for benthic, passive suspension feeders in low‐velocity regimes. When comparing predictions from encounter‐rate models with measured capture or ingestion rates, the models may serve as null hypotheses against which to judge retention efficiencies and selective ingestion.