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

Abstract

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 FeSO 4 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.