Microfaunal response to fertilization of an arctic tundra stream

1. As part of a whole-system study, the response of the heterotrophic microfaunal community colonizing artificial substrata (polyfoam units) to fertilization of an arctic tundra stream was followed for 6 weeks during the summer. 2. Dominant heterotrophic microfauna observed included amoebae (approximately 40% of colonizing biomass), rotifers (36% of biomass) and ciliates (25% of biomass). 3. Biomass of heterotrophic microfauna on artificial substrata was not significantly different in a control reach and an experimental reach fertilized with phosphorus (loading rate ten times ambient), but in a reach fertilized with both phosphorus and nitrogen (loading rates ten times ambient) biomass was double that of the control and phosphorus-fertilized reaches. The lack of response in the phosphorus reach was probably due to greater insect grazing as a result of previous phosphorus fertilization of this reach. 4. Abundance of microfauna on epilithic surfaces in the river was higher on rocks from pools than on rocks from riffle areas, but abundance on the artificial substrata was higher than on the natural rocks. 5. The results suggest that microfauna of arctic tundra streams are regulated by grazers and that their importance in transfers among trophic levels is greater in pools than in riffles