Understanding the processes that regulate food chain length in nature is a classic theme in ecology. Two factors expected to explain variation in food chain length are resource availability (productivity) and environmental stress. We examined the impact of anthropogenic activities, both a source of stress and productivity, on lotic food webs using two simple food web descriptors based on stable nitrogen isotopes (δ15N). We used 1) trophic position of small fish and 2) slopes of δ15N-size class relationships in the invertebrate community as variables related to food chain length. Trophic positions and δ15N-size slopes changed significantly across the 23 study sites selected to vary in productivity and impact from industrial activities (trophic position=0.4 to 1.9; slopes=−1 to +2.8). Fish trophic position was not affected by productivity (chlorophyll a) or environmental stress (number of factories on the watershed) and was not modulated by the proportion of predatory invertebrates in the food web leading to fish. This lack of relationship suggests that variation in trophic position is likely influenced by fish feeding behaviour and is therefore not a good integrator of changes in food chain length induced by anthropogenic activities. δ15N-size slopes significantly increased with productivity and decreased with stress, suggesting that trophic organization by size within invertebrate communities is affected by these factors. These slopes also tended to be greater when the proportion of predators in the community increased in larger size classes. These results suggest that when productivity is high and stress is low, proportions of higher δ15N-value predators in lotic food webs tend to increase, contributing to the lengthening of food chains. They also suggest that δ15N-size relationships of invertebrate communities may be used as indicators of food web structure alteration by human activities.