Multiple stressors rewire trophic cascades and ecosystem processes in freshwater systems

Freshwater ecosystems are increasingly exposed to climate warming and diffuse chemical pollution, yet their combined ecological impacts remain poorly understood. Using a full-factorial mesocosm experiment spanning winter and summer, we tested how a mixture of common herbicides and fungicides (metazachlor, S-metolachlor, chloridazon, metribuzin, propiconazole, and terbuthylazine) at environmentally realistic concentrations interacts with +4 °C warming to affect community structure and ecosystem functioning. We found that warming and the pesticide mixture produced complex, non-additive responses across trophic levels in the pelagic food web from phytoplankton to predatory insects and in benthic detritivores. These responses included warming-driven shifts from top-down to bottom-up control, pesticide-induced trophic cascades mediated by altered predator-prey interactions, and joint stressor effects that reduced key ecosystem processes such as nutrient cycling and leaf litter decomposition. Our findings demonstrate that warming modifies not only the ecological effects but also the pathways through which pesticide mixtures can influence communities, with contrasting top-down pathways in winter and bottom-up pathways in summer. These results challenge current frameworks for environmental risk assessment, which typically ignore the combined effects of climate change and chemical pollution, and underscore the urgent need to incorporate multi-stressor dynamics into freshwater conservation under climate change.