Changes in community composition determine recovery trajectories from multiple agricultural stressors in freshwater ecosystems

Pesticides have been identified worldwide as a threat for aquatic biodiversity due to their widespread use in agriculture and their capacity to reach freshwater ecosystems. Very little is known about the consequences of pesticide mixtures targeting different organism groups on community dynamics. Especially, how horizontal changes within one trophic level are propagated vertically across the food web has been rarely investigated. To get insight on the effects of pesticide mixtures on community dynamics, we performed a mesocosm experiment manipulating three common agricultural stressors: chlorpyrifos (an insecticide), diuron (an herbicide) and nutrients. The results of this study show that all stressors had significant effects on community composition, species richness and abundance. However, recovery trajectories and ecosystem functioning effects largely depended on the type of stressor as well as on post-disturbance trophic and non-trophic interactions. Effects of pesticides were generally recovered by the end of the experimental period when considering abundance, whereas community composition further departed from control systems. High nutrient loads led to a shift in community composition characterized by high taxa dominance and lower species richness, which in turn contributed to increased total organism abundance and reduced recovery times to pesticide exposure. We found interactions between the tested stressors to be significant only few times at the community level, while interactions were more common at the population level. Our findings indicate that management of freshwater ecosystems should consider pre-disturbance community composition and long-term changes in interactions across different organism groups to set effective protection measures.