This report presents the results of an experiment conducted in artificial streams in which the growth of benthic algae was studied in relation to the phosphorus and nitrogen concentrations in the water. The goal of the study was to determine whether benthic algal biomass in the upper Athabasca River is limited by the availability of phosphorus or nitrogen. The results include the taxonomic, growth, and biomass responses by periphytic algae to phosphate additions of up to 50 micrograms per liter. The report also develops a model describing change in relative peak biomass with phosphorus addition, using data from the Athabasca experiments. Comparisons are made with a similarly derived model using results from experiments on the South Thompson River