Periphyton biomass response to changing phosphorus concentrations in a nutrient-impacted river: a new methodology for phosphorus target setting

Nutrient modification experiments were conducted in streamside flumes to determine the concentration at which P limits algal growth in the mesotrophic River Frome, Dorset, UK. The soluble reactive P (SRP) concentration in each flume was either increased (by P addition), decreased (by precipitating P with iron(II) sulphate solution), or left unaltered (control), producing SRP concentrations ranging from 32 to 420 µ·L–1. Increasing the ambient SRP concentration did not increase epilithic algal growth, showing that the River Frome was not P limited at 109 µ SRP·L–1. In the P-stripped flumes, algal biomass declined as the SRP concentration fell below ~90 µ·L–1, with a 60% biomass reduction at <40 µ SRP·L–1. Phosphorus-diffusing periphytometers deployed in the P-stripped flumes confirmed that reduced rates of algal growth were due to P limitation rather than a physical effect of FeSO4 addition. The ~90 µ·L–1 maximum P-limiting concentration is likely to be similar for comparable nutrient-impacted rivers. This iron-stripping approach expands the existing river nutrient-enrichment methodology so that it can be used in nutrient-impacted rivers and should allow catchment managers to produce knowledge-based P reduction targets prior to introducing remediation.