Effects of resuspension and mixing on population dynamics and trophic interactions in a model benthic microbial food web

The effects of resuspension and mixing on the population dynamics and trophic interactions in a simple benthic microbial food web were studied during 3 plankton wheel experiments of 9 to 15 d duration. The food web consisted of a mixed bacterial assemblage, the heterotrophic flagellate Bodo designis, the ciliate Euplotes balteatus and the rhizopod Vannella platypodia. The diatom Amphora coffeaeformis was included to maintain oxygen concentrations, B. designis, E. balteatus, V. platypodia and A. coffeaeformis are hereafter referred to by genus name alone. Population dynamics in 500 ml microcosms on rotating and non-rotating plankton wheels were compared, Final abundances of Amphora increased in suspension in all experiments. Resuspension increased initial growth rates and final abundances of Euplotes and Vannella during a whole-community experiment and a community subset experiment during which resuspension effects with and without the top predator Euplotes were compared. During the community subset experiment, suspended Bodo grew faster and reached higher final abundances than non-resuspended Bodo when Euplotes was absent, but experienced higher loss to Euplotes when the ciliate was present. Individual consumption rates of Bodo by Euplotes were estimated to be higher in suspension (5.5 vs 3.6 Bodo Euplotes(-1) h(-1)), which could not be explained by higher abundance of Bodo alone. Similar but non-significant trends were found for bacteria. During a third experiment, Euplotes did not benefit from resuspension at low food concentrations. Our results show that resuspension and mixing can enhance population growth of autotrophic and heterotrophic protists, and that the trophic coupling between flagellates and ciliates, and possibly between other microbial food web components, can become closer in suspension when the food supply is sufficient. This suggests that subsidiary energy input by resuspension and mixing may increase the biomass turnover in benthic microbial food webs.