<p>Effects of phosphorus enrichment have been well studied in streams with low ambient nutrient levels, while its impact on nutrient-rich systems has been considered negligible. We analyzed the effects of nutrient addition (phosphorus and nitrogen) on the biomass and metabolism of the macrophyte-epiphytic complex and epipelic biofilms in a nutrient-rich Pampean stream (Argentina), applying a before-after control-impact (BACI) experimental design. Samplings were made at control and impacted reaches during pre-enrichment (9&#xa0;months) and enrichment periods (12&#xa0;months). Phosphorus concentration was increased fourfold in the impacted reach relative to the control reach. Net primary production (NPP) and respiration of basal communities were estimated by the chamber incubation method and expressed on an area basis (m²). We observed a low response of macrophyte and epipelic NPP to the enrichment, though the respiration of epipelic biofilms increased in the impacted reach compared to the control reach and to the pre-enrichment period. This response was related to higher bacterial density in both epiphytic and epipelic biofilms in the impacted reach. Nutrient addition in the Pampean stream therefore stimulated heterotrophic processes, likely associated with the high demand of phosphorus by bacteria in the sediments.</p>