Combined effects of climate warming and pharmaceuticals on a tri-trophic freshwater food web

Multiple anthropogenic stressors influence the functioning of ponds and lakes, but their combined effects are often little understood. We ran two mesocosm experiments to evaluate the effects of warming (+4°C above ambient) and environmentally relevant concentrations of a mixture of commonly used pharmaceuticals, an emerging class of chemical contaminants, on tri-trophic food webs representative of pelagic communities in ponds and other small standing waters. We quantified the main and interactive effects of warming and pharmaceuticals on each trophic level and attributed them to the direct effects of both stressors and the indirect effects arising through biotic interactions. Warming and pharmaceuticals had stronger effects in the summer experiment, altering zooplankton community composition and causing delayed or accelerated emergence of top insect predators. In summer, both stressors and top predators reduced filter-feeding zooplankton biomass, while warming and pharmaceuticals had opposing effects on phytoplankton. In the winter experiment, the effects were much weaker and primarily limited to a positive effect of warming on phytoplankton biomass. Overall, we show that pharmaceuticals can exacerbate the effects of climate warming in freshwater ecosystems, especially during the warm season. Our results demonstrate the utility of community-level studies across different seasons for the risk assessment of multiple emerging stressors in freshwater ecosystems. ### Competing Interest Statement The authors have declared no competing interest.