The effects of external flow on the feeding currents of sessile microorganisms

Microscopic sessile suspension feeders (MSSFs) form an important part of aquatic ecosystems. MSSFs live attached to surfaces and consume bacteria and debris. Their environmental impact is mediated by their clearance rate, which depends on the feeding current that they generate to bring in food. The clearance rate has been hypothesized to be limited by recirculating eddies, though eddies are reduced or eliminated if MSSFs push the water at an angle rather than perpendicular to the surface. Those results, however, considered MSSFs in still water, while they live in water with external flow (i.e. the current in a stream or ocean). We investigate the influence of external flow on the MSSF feeding current. Our calculations show that even very slow external flow  is sufficient to disrupt the eddies around perpendicular MSSFs, providing a constant supply of fresh nutrients. However, the clearance rate decreases for MSSFs in external flow at a range of non-perpendicular orientations due to the formation of eddies that severely restrict feeding. We observe these eddies experimentally for the common suspension feeder Vorticella, and we also find that some MSSFs may be forced into orientations with reduced clearance rates by flows typical of rivers and streams.