The impact of predator exposure and anthropogenic stressors on insect emergence in stream ecosystems

Emergence of aquatic insects is a vital ecological process influencing aquatic ecosystems. Predators can modify this process by inducing behavioural and developmental changes in prey species. However, these natural biotic interactions are increasingly disrupted by anthropogenic changes. Anthropogenic stressors, like freshwater salinisation and warming, pose a significant threat to lotic ecosystems worldwide, with effects that may persist even after the stressors are removed. Our study investigated the effects of fish predator exposure ( Gasterosteus aculeatus and Cottus rhenanus ), increased salinity (ambient vs +136 mg/L NaCl) and increased temperature (ambient vs +3.4 °C) on insect emergence in a controlled stream mesocosm experiment ( ExStream system). We monitored the number and size of emerging insects over a 14-day stressor and recovery phase. Predator exposure and elevated salinity reduced the number of emerging insects and resulted in smaller Nematocera emerging. Increased temperature led to reduced body sizes of emerging Nematocera and increased the number of emerging individuals when interacting with increased salinity, predator exposure or both. Notably, the effects of predator exposure were diminished under increased salinity and temperature, although salinity alone reduced the impact on insect size. The stressors did not cause any lasting effect on insect emergence during the recovery phase, except tendentially the size. Our findings highlight that anthropogenic stressors can alter insect emergence and reduce the impact of fish predation, with the magnitude of these effects likely dependent on the intensity and duration of the stressor input.