AME 13:161-175 (1997)  -  doi:10.3354/ame013161

Experiments, using natural plankton collected from a eutrophic site in Saginaw Bay, Lake Huron (USA) and from a hypereutrophic wetland of southern Lake Erie (USA), were conducted to test the hypothesis that bacterivory can control aquatic nitrification rates. The dynamics of nitrogen and protists in these experiments revealed a consistent pattern: an increase in concentrations of nitrates due to oxidation of NH₄⁺ always followed the collapse of bacterivorous nanoplankton populations. This collapse was, in turn, caused by predation pressure of larger ciliates and metazooplankton. Experiments, using enrichment batch cultures maintained at near-ambient concentrations of NH₄⁺, indicated that bacterivorous protists can inhibit nitrification directly by reducing bacterial numbers and indirectly by promoting bacterial aggregation. The latter experiments also suggest that feeding strategies of microbial grazers, e.g. suspension-feeding Spumella sp. versus surface-feeding Bodo saltans, may determine their grazing impacts on nitrifiers. Finally, ingestion rates of fluorescently labeled nitrifying bacteria (FLNB) by the natural planktonic assemblage from Saginaw Bay demonstrated that nanoflagellates were able to efficiently prey on low concentrations of FLNB. Our study suggests that previously neglected trophic factors may be of potential importance for mediating nitrification rates in the pelagic environment.

Nitrification · Protozoa · Zooplankton · Bacterivory · Predation