Influences of benthic boundary-layer flow on feeding rates of ciliates and flagellates at the sediment-water interface

Abstract

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-1 Above u* = 1.0 cm s-1 clearance rates of Cohnilembus sp. and P. magna were constant, whereas the clearance rate of E. minuta was reduced by 40% at u* = 1.5 cm s-1 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* = 0 and 0.9 cm s-1. 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.