The stress-gradient hypothesis predicts that biotic interactions within food webs are context dependent, since environmental stressors can attenuate consumer–prey interactions. Yet, how heavy metal pollution infuences the impacts of predatory fsh on ecosystem structure is unknown. This study was conducted in the Osor stream (Spain), which features a metal (mainly Zn) pollution gradient. We aimed to determine how the responses of benthic communities to the presence and absence of predatory fsh interact with environmental stress and to test whether the top-down control of top predators is context dependent. To address these questions, periphyton biomass and macroinvertebrate densities were determined throughout an exclosure/ enclosure mesocosm experiment using the Mediterranean barbel (Barbus meridionalis) as a top predator. The monitoring study showed that metal accumulation in periphyton and macroinvertebrates refected patterns observed in water. The mesocosm study showed that fsh predation efects on larval chironomids were not context-dependent and that periphyton biomass was markedly lower in the presence of fsh regardless of metal pollution levels. This strong top-down control on periphytic algae was attributed to the foraging behaviour of fsh causing bioturbation. In contrast, the top predator removal revealed grazer-periphyton interactions, which were mediated by heavy metal pollution. That is, periphyton beneftted from a lower grazing pressure in the metal-polluted sites. Together, our results suggest that the top–down control by fshes depends more on functional traits (e.g. feeding behaviour) than on feeding guild, and demonstrate the capacity of top predators to modify anthropogenic stressor efects on stream food-web structure.