Copper (Cu) exposure can increase leaf-associated fungal biomass, an important food component for leaf-shredding macroinvertebrates. To test if this positive nutritional effect supports the physiological fitness of these animals and to assess its importance compared to waterborne toxicity, we performed a 24-day-bioassay in combination with a 2×2 factorial design using the amphipod shredder Gammarus fossarum and a field-relevant Cu concentration of 25 μg/L (n = 65). Waterborne toxicity was negligible, while gammarids fed leaves exposed to Cu during microbial colonization exhibited a near-significant impairment in growth (∼30%) and a significantly reduced lipid content (∼20%). These effects appear to be governed by dietary uptake of Cu, which accumulated in leaves as well as gammarids and likely overrode the positive nutritional effect of the increased fungal biomass. Our results suggest that for adsorptive freshwater contaminants dietary uptake should be evaluated already during the registration process to safeguard the integrity of detritus-based ecosystems.