Reducing phosphorus (P) loading to rivers is seen as a key mitigation measure to improve aquatic ecology and control excessive algal growth, as P is widely assumed to be the limiting nutrient in most rivers. Nutrient enrichment experiments using within-river flume mesocosms were conducted in the oligotrophic River Rede, to determine how periphyton accrual was affected by increasing P concentrations. Increasing the soluble reactive phosphorus (SRP) concentration from the ambient concentration of 15 µg L-1 to concentrations ranging from 30 µg L-1 to 130 µg L-1 had no significant effect of periphyton growth rate, demonstrating that the periphyton was not P limited, even in this nutrient poor river. However, at SRP concentrations greater than 100 µg L-1, diatom communities shifted to species that were more tolerant of higher nutrient concentrations. Elemental analysis showed that there was a positive linear relationship between biofilm P content and the SRP concentration in the overlying water. This ability to store P suggests that periphyton growth is being limited by a secondary factor (such as nitrogen (N)) and may provide a mechanism by which future periodic increases in N concentration may stimulate periphyton growth. Flow velocity, light, and invertebrate grazing pressure also have important roles in controlling periphyton biomass in the River Rede.