In order to define the effect of nutrient enrichment on trophic webs in an Andean mountain stream we performed an experiment using stable isotope ratios (δ 15 N and δ 13 C) to analyze different trophic compartments: 1) basal level: CPOM and biofilm; 2) primary consumers -macroinvertebrates: collector-gatherers (Heterelmis sp., Thraulodes sp. and Tricorythodes sp.), and collectorfilterers (Simulium sp.); 3) predators -fish (Oncorhynchus mykiss and Trichomycterus bogotensis).The average fractionation of nitrogen among the primary consumers with respect to CPOM was 4.7 ‰, and 1.7 ‰ with respect to biofilm.Predators incremented their δ 15 N signal by 5.9 % with respect to primary consumers.A depletion of δ 15 N was observed in Impact with respect to Control reach after fertilization in different compartments (biofilm, Heterelmis, Simulium and Tricorythodes), while depletion was not significant for top predators.In most cases, the δ 13 C signal of biofilm overlapped with that of primary consumers, but a clear enrichment was observed with respect to CPOM.Macroinvertebrate gut contents showed fine detritus to be their most abundant food, and that in general there were no changes in diet as a consequence of nutrient enrichment.The only exception was Heterelmis, who increased its consumption in the Impact reach.