Significance: Protection of freshwater ecosystems from organic pollutants is important to preserve biodiversity and the goods they provide to society, such as clean drinking water and recreation. Organic chemicals have been shown to adversely impact freshwater ecosystems in local and regional studies. Nevertheless, due to paucity of studies on larger spatial scales, it remains unknown how widespread the risk from organic chemicals is. For the first time, to our knowledge, we provide strong evidence that chemicals threaten the ecological integrity and consequently the biodiversity of almost half of the water bodies on a continental scale, based on the analysis of governmental monitoring data from 4,000 European sites. Due to limitations associated with the monitoring programs, our results are likely to underestimate the actual risks.

Summary: 1. Trait‐based approaches applied to community ecology have led to a considerable advance in understanding the effect of environmental filters on species assembly. Although plant traits are known to vary both between and within species, little is known about the role of intraspecific trait variability in the non‐random assembly mechanisms controlling the coexistence of species, including habitat filtering and niche differentiation. 2. We investigate the role of intraspecific variability in three key functional traits – specific leaf area (SLA), leaf dry matter content (LDMC) and height – in structuring grassland communities distributed along a flooding gradient. We quantified the contribution of intraspecific variability relative to interspecific differences in the trait–gradient relationship, and we used a null model approach to detect patterns of habitat filtering and niche differentiation, with and without intraspecific variability. 3. Community mean SLA and height varied significantly along the flooding gradient and intraspecific variability accounted for 44% and 32%, respectively, of these trait–gradient relationships. LDMC did not vary along the gradient, with and without accounting for intraspecific variability. Our null model approach revealed significant patterns of habitat filtering and niche differentiation for SLA and height, but not for LDMC. More strikingly, considering intraspecific trait variability greatly increased the detection of habitat filtering and was necessary to detect niche differentiation processes. 4. Synthesis. Our study provides evidence for a strong role of intraspecific trait variability in community assembly. Our findings suggest that intraspecific trait variability promotes species coexistence, by enabling species to pass through both abiotic and biotic filters. We argue that community ecology would benefit from more attention to intraspecific variability.

Multiple stressors constitute a serious threat to aquatic ecosystems, particularly in the Mediterranean region where water scarcity is likely to interact with other anthropogenic stressors. Biological traits potentially allow the unravelling of the effects of multiple stressors. However, thus far, trait-based approaches have failed to fully deliver on their promise and still lack strong predictive power when multiple stressors are present. We aimed to quantify specific community tolerances against six anthropogenic stressors and investigate the responses of the underlying macroinvertebrate biological traits and their combinations. We built and calibrated boosted regression tree models to predict community tolerances using multiple biological traits with a priori hypotheses regarding their individual responses to specific stressors. We analysed the combinations of traits underlying community tolerance and the effect of trait association on this tolerance. Our results validated the following three hypotheses: (i) the community tolerance models efficiently and robustly related trait combinations to stressor intensities and, to a lesser extent, to stressors related to the presence of dams and insecticides; (ii) the effects of traits on community tolerance not only depended on trait identity but also on the trait associations emerging at the community level from the co-occurrence of different traits in species; and (iii) the community tolerances and the underlying trait combinations were specific to the different stressors. This study takes a further step towards predictive tools in community ecology that consider combinations and associations of traits as the basis of stressor tolerance. Additionally, the community tolerance concept has potential application to help stream managers in the decision process regarding management options.