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Trait-mediated biotic interactions within zooplankton, such as inducible defences against predation, are key determinants of community structure and dynamics in lentic ecosystems. The role of such interactions in lotic environments, however, has been rarely investigated, as abiotic constraints related to hydrology are generally thought to impair the occurrence of biotic interactions in rivers. We hypothesize that, under conditions of reduced flow during summer, trait-mediated predator–prey interactions can be significant driving forces in the main channel of a large river, despite the disruptive effect of downstream transport. To verify this assumption, we carried out a field study in the lowland stretch of the Po River (Italy). We analysed temporal patterns of morphometric variation in a population of Brachionus calyciflorus, potentially triggered by the predatory rotifer Asplanchna brightwellii. Our results indicate that the presence of Asplanchna can impact the variability of different morphological traits in B. calyciflorus even under the disturbance effect of turbulence and drift. Asplanchna feeding electivity on several taxa was also investigated, confirming the predator ability to efficiently and selectively feed under lotic conditions. We suggest that complex interactions and trade-offs might occur among life-history traits, predator–prey relationships and physical constraints imposed by flow within zooplankton communities in large rivers.