Previous research has documented the effects of both herbivory and ambient nutrient concentrations on the composition and stoichiometric characteristics of periphyton communities, however little has been done to explore how these top-down and bottom-up processes interact to affect the cycling of multiple elements. In this study, algal biofilms were reared over a 30 day period in three stream-side once-through artificial channels fed by Fox Creek, an undisturbed, N-limited tributary of the South Fork Eel River, Mendocino, California. Ammonium enrichment treatments were applied to two of the channels with target concentrations 100 and 200 µg/L above ambient concentration. Half of each flume was exposed to a dominant invertebrate grazer. The effect of experimental treatments on nutrient demand was observed as uptake of NH₄ and SRP by the biofilms in enclosed chamber experiments. Algal biomass was significantly higher in the nutrient enriched flumes and biomass decreased significantly under grazer treatments. Total NH₄ and SRP uptake will increase under enrichment conditions and decrease under herbivore treatments, while mass specific uptake may increase under grazing pressure. Significant changes in the epilithic community composition are expected to result from grazer treatments.