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 inclu ded amoebae (approximately 40% of colonizing biomass), rotifers (36% o f 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 p hosphorus and nitrogen (loading rates ten times ambient) biomass was d ouble 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 reac h. 4. Abundance of microfauna on epilithic surfaces in the river was h igher on rocks from pools than on rocks from riffle areas, but abundan ce on the artificial substrata was higher than on the natural rocks. 5 . The results suggest that microfauna of arctic tundra streams are reg ulated by grazers and that their importance in transfers among trophic levels is greater in pools than in riffles.