Trophic coupling in the microbial food web of a eutrophic shallow lake (Lake Visvijver, Belgium)

Two fractionation experiments were carried out to study trophic interactions in the microbial food web in the shallow eutrophic lake Visvijver. Despite an order of magnitude difference in biomass of bacteria, heterotrophic nanoflagellates (HNF) and ciliates between the two experiments, the trophic interactions observed were similar. Bacteria were consumed by HNF and oligotrich ciliates, oligotrich ciliates fed on HNF as well as bacteria and cyclopoid copepod nauplii preyed on the oligotrich ciliates. While cyclopoid copepods had no effect on any component of the microbial food web, Daphnia fed on bacteria and HNF. No or a weak and slow trophic cascade occurred from oligotrich ciliates to HNF and bacteria. The lack of a pronounced trophic cascade was ascribed to the importance of the omnivorous oligotrich ciliates in the microbial food web. In both experiments, removal of trophic levels by size fractionation resulted in changes in the identity of the dominant bacterivores, but this had little effect on total bacterial grazing losses. Bacterial community composition (monitored in the second experiment using DGGE analysis), on the contrary, was more strongly influenced by changes in the identity of the dominant bacterivores, suggesting the presence of a 'cryptic trophic cascade'.