Relative specific growth rates of periphytic diatoms were investigated as a time‐integrated measure of stream enrichment in the South Brook, Canterbury, New Zealand. The control site was expected to show phosphorus (P) limitation of growth rates with dissolved reactive P (DRP) of 6–10 μg I−1; and the impact site, downstream of a discharge, was expected to show no growth limitation by P with DRP > 150 μg I−1. Specific growth rates (μ) were generally high at the upstream site (μ = 0.45–0.59 div d−1) but the relative specific growth rates indicated that the cellular level kinetics were not limited by nutrient availability (i.e., μ: μmax‐P> 0.8). Laboratory and field bioassays were carried out and all corroborated this interpretation, indicating that the discharge should have no effect on the growth rates of the downstream communities. As predicted, no significant differences occurred in growth between sites. It is concluded that relative specific growth rates could be useful to assist with impact monitoring of nutrient‐enriched discharges.