Influences of turbulence on suspension feeding by planktonic protozoa: Experiments in laminar shear

Laminar shear below the turbulence microscale can create relative motion between protozoan suspension feeders and their prey, potentially influencing encounter and retention. Theory suggests that ingestion rate may rise sigmoidally with increasing turbulence strength, although interference with feeding mechanisms might occur at some turbulence levels. We measured rates of feeding on fluorescently labeled prey at con-centrations below feeding saturation in a survey of cultured bacterivorous and herbivorous flagellates, ciliates, and a helioflagellate over a wide range of shear rates produced in rotating Couette flows. Shears of 0.1-10 s-l (corresponding to moderate to extremely strong marine and estuarine turbulence) enhanced clearance rates by the aloricate choanoflagellate, Monosiga sp., up to 2.7 x the mean rate in still water. Shears of l-10 s- l enhanced clearance rates by the helioflagellate, Ciliophrys marina, up to 7.0 x the mean in still water. The data are consistent with a sigmoidal response to increasing shear rate. In contrast, clearance rates of the tintinnid, Helicostomella sp., were suppressed at 10 s-l to as low as 0.42 x the still-water rate. Several other flagellates and ciliates (Paraphysomonas sp., two unidentified chrysomonads, Diaphanoeca grandis, Favella sp., and an unidentified heterotrich) showed no significant effects. We hypothesize that the protozoa most susceptible to an influence of turbulence are nonmotile (e.g. Heliozoa, Foraminifera, Radiolaria) or are wea