Combined effects of climate warming and environmentally 1 relevant concentrations of pharmaceutical active compounds on a 2 freshwater community 3

Predicting the combined effects of stressors on ecosystems is a pressing concern due to increasing anthropogenic pressures on the Earth’s biota. Climate warming and chemical pollution are two major stressors affecting freshwater biota. The latter is becoming more common due to the widespread use of mixtures of chemical compounds and the low removal efficiency of water treatment plants, especially in the case of pharmaceutical active compounds (PhACs). To evaluate the effects of warming and PhACs, we conducted a full factorial experiment (with/without PhAC mixture and with/without warming) in heated outdoor mesocosms with a common community of macroinvertebrates and plankton taxa from different trophic levels. The experiment was conducted twice, in winter and in summer. We repeatedly measured PhAC concentrations, environmental parameters, zooplankton density, and aquatic insect emergence. The summer experiment showed much stronger effects of stressors, dominated by temperature effects, on community structure and temporal dynamics of individual taxa. Both warming and PhACs altered invertebrate community composition in the summer experiment, with contrasting effects observed between insect emergence and phytoplankton and zooplankton responses. Our results suggest that PhACs at environmentally relevant concentrations can alter the effects of climate warming on freshwater biota, with individual-level responses such as delayed or accelerated development and altered phenology of key taxa.