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 influences the impacts of predatory fish 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 fish 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 reflected patterns observed in water. The mesocosm study showed that fish predation effects on larval chironomids were not context-dependent and that periphyton biomass was markedly lower in the presence of fish regardless of metal pollution levels. This strong top-down control on periphytic algae was attributed to the foraging behaviour of fish causing bioturbation. In contrast, the top predator removal revealed grazer-periphyton interactions, which were mediated by heavy metal pollution. That is, periphyton benefitted from a lower grazing pressure in the metal-polluted sites. Together, our results suggest that the top–down control by fishes depends more on functional traits (e.g. feeding behaviour) than on feeding guild, and demonstrate the capacity of top predators to modify anthropogenic stressor effects on stream food-web structure.

压力梯度假说预测食物网内的生物相互作用取决于环境，因为环境压力源可以减弱消费者与猎物的相互作用。然而，重金属污染如何影响捕食性鱼类对生态系统结构的影响尚不清楚。这项研究是在 Osor 溪流（西班牙）进行的，该溪流具有金属（主要是 Zn）污染梯度。我们的目的是确定底栖生物群落对掠食性鱼类存在和不存在的反应如何与环境压力相互作用，并测试对顶级掠食者的自上而下控制是否依赖于环境。为了解决这些问题，在使用地中海触须（Barbus meridionalis) 作为顶级捕食者。监测研究表明，附生生物和大型无脊椎动物中的金属积累反映了在水中观察到的模式。中宇宙研究表明，鱼类对摇蚊幼虫的捕食影响与环境无关，并且无论金属污染水平如何，在鱼类存在的情况下，附生生物的生物量都显着降低。这种对附生藻类的强烈自上而下控制归因于鱼类的觅食行为导致生物扰动。相比之下，顶部捕食者的去除揭示了食草动物与附生生物的相互作用，这是由重金属污染介导的。也就是说，附生生物受益于金属污染场地较低的放牧压力。总之，我们的结果表明，鱼类的自上而下控制更多地取决于功能特征（例如摄食行为）而不是摄食行会，