Phosphorus enrichment of the oligotrophic River Rede (Northumberland, UK) has no effect on periphyton growth rate

Reducing phosphorus (P) loading to rivers is seen as a key mitigation measure to improve aquatic ecology and control excessive algal growth because 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 mu g L-1 to concentrations ranging from 30 to 130 mu g L-1 had no significant effect of periphyton growth rate, demonstrating that the periphyton was not P limited, even in this nutrient-poor river. At SRP concentrations >100 mu g L-1, however, diatom communities shifted to species that were more tolerant of higher nutrient concentrations. Elemental analysis showed 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) and may provide a mechanism by which future periodic increases in nitrogen concentration may stimulate periphyton growth. Flow velocity, light, and invertebrate grazing pressure also have important roles in controlling periphyton biomass in the River Rede.