Past Pollution Can Shape Future Climatic Vulnerability: Fungicide Exposure Alters Detritivore Performance During Heatwaves and Recovery

ABSTRACT Natural populations are increasingly exposed to multiple stressors, such as chemical pollution and climate‐change‐driven heatwaves. While it has become clear that we need to account for stressor interactions, it is less well understood how sequential exposure to different stressors influences organismal responses as well as their potential for recovery when conditions turn benign. In this study, we tested how exposure to chemical pollution (here a widely used fungicide, Tebuconazole) affects thermal resilience and post‐stress recovery in juvenile Asellus aquaticus , a functionally important freshwater detritivore. In a standardized laboratory experiment, we exposed juvenile isopods first to low levels (50 μg/L) of Tebuconazole or clean water (0 μg/L), followed by either the same or alternative chemical treatment in the presence or absence of a heatwave (22°C), before being returned to clean water and benign temperature (17°C) to assess potential for recovery of individuals from different stress histories. The stressors had weak effects on survival, but clear context‐dependent effects on sublethal performance measures. Specifically, fungicide exposure reduced food consumption, growth and pigmentation and these effects were stronger with longer exposure. Within Phase 2, heatwave exposure alone did not further worsen these effects relative to the corresponding no‐heatwave groups with the same prior exposure history, but individuals exposed simultaneously to fungicide and heat stress in Phase 2 showed a clear synergistic reduction in food consumption, which persisted during the recovery phase. Pigmentation responses were more complex and should be interpreted cautiously: while fungicide‐exposed individuals tended to be lighter on average, some stress‐exposed groups increased pigmentation relative to controls during the recovery phase, particularly after heatwave exposure. Taken together, our results reveal complex context dependency of stressor interactions and show that pollution history can make organisms more vulnerable to other stressors, such as climate change, even after conditions improve. Understanding such sequential stressor dynamics is essential for predicting organismal and ecosystem responses to global change.