Ciliate protozoans as links in freshwater planktonic food chains

Abstract

THE potential importance of bacteria in freshwater planktonic food webs is recognised¹, but the mechanisms of their incorporation are not fully known. Most planktonic bacteria are not attached to particles and are less than 1 µm (refs 2–4) in size—a range inefficiently filtered by crustacean zooplankton⁵. Daphnia and Diaptomus feed at lower rates on free living bacteria than on algae^6,7. Bacterial food sources have been used with varying degrees of success to culture zooplankton^8–11. These variations may be due to the unnatural range of concentrations used and the uncontrolled effects of different culture media, laboratory strains and the degree of accessibility to the zooplankton (because of cell clumping and sedimentation). Ciliate protozoan microzooplankton may be important intermediaries which transform ultrafine organic matter, including bacteria, into a particle size range readily available to crustacean zooplankton. Here we present evidence that ciliates are cropped by crustacean zooplankton in the natural environment. We have also measured feeding, filtering, and assimilation rates of Daphnia magna on both a large and small ciliate in the laboratory and have observed feeding behaviour directly. Dissolved organic carbon, bacteria, ciliate protozoans and crustacean zooplankton may represent a significant pathway of carbon flow in the freshwater plankton.