Heterotrophic protists are integral to sedimentary food webs, but influences on their activities are poorly understood, especially the role of benthic boundary-layer flow. Effects of flow on ingestion rates were measured for bacterivorous protists at the sediment surface in a flume. Results from four common, benthic suspension-feeding ciliates were species specific. Mean clearance rates of the scuticociliates Cohnilembus sp. and Paranophrys magna and the hypotrich Euplotes minuta increased by factors of two to three as friction velocity (u*) increased from 0 to 1.0 cm s
Ϫ1. Above u* ϭ 1.0 cm s Ϫ1 , clearance rates of Cohnilembus sp. and P. magna were constant, whereas the clearance rate of E. minuta was reduced by 40% at u* ϭ 1.5 cm s Ϫ1 . E. minuta thus revealed an optimal flow regime for feeding. In contrast, the mean clearance rate of the scuticociliate, Cyclidium sp., was unrelated to u*. In experiments with sediment cores from a subtidal, silty sand site, the mean clearance rate of the bacterivorous ciliate community of the sediment-water interface increased by a factor of five between u* ϭ 0 and 0.9 cm s
Ϫ1. The effects of flow were likely due to enhanced advection of prey to the ciliates. In contrast, the clearance rate of the nanoflagellate community in sediment cores was unrelated to u*. Tidal currents at the field site were estimated to increase the ciliate community's daily integrated feeding by a factor of 3.2 compared to still water. Epibenthic ciliates create a dynamic link between planktonic and benthic food webs, mediated strongly by benthic boundary-layer flow.Heterotrophic protists play important roles in marine sedimentary food webs as consumers, facilitators of organicmatter decomposition, and as links to higher trophic levels (Fenchel 1987;Patterson et al. 1989; Epstein 1997a,b). An understanding of the factors that influence feeding rates, growth rates, and trophic interactions of benthic heterotrophic protists, however, is at an early stage of development, especially when compared to knowledge of planktonic protistan ecology (Fenchel 1987;Caron and Finlay 1994). Some of the clearest influences on distributions and activities of 1 Previously Cheryl Ann Butman. Present address: Department of Biology, University of California Los Angeles, Los Angeles, California 90095-1606.
AcknowledgmentsWe thank N. Trowbridge, J. Sisson, and M. Gould for help in the field and/or lab; the director of the Rinehart Coastal Research Center (WHOI) for use of facilities; W. McGillis and J. S. Fries for assisting with particle-imaging velocimetry; J. Grant for advice on calibrating the hot film; D. Caron for providing a culture of H. halodurans; and two anonymous reviewers for comments that improved the manuscript.