Uncovering aquatic toxicity as a specific stressor among multiple environmental drivers affecting the functional structure of French stream macroinvertebrate communities

Abstract Since recent years, freshwater ecosystems such as rivers are subjected to increasing pressures from human-induced drivers of changes, including chemical contamination, warming and nutrient enrichment. Among those, toxic chemicals are particularly concerning, as aquatic contamination is strongly increasing but their effects on ecosystem structure and functioning remain poorly understood and difficult to predict. Yet, those are expected to impact freshwater ecological communities, notably through selecting species exhibiting particular biological traits. A growing literature tested predictions on the effects of multiple environmental drivers, including chemicals, on the functional structure of macroinvertebrate communities, but disentangling the respective influences of the different drivers on the observed patterns usually remains challenging. Here, we used multivariate analyses with variation partitioning to estimate the independent and combined effects of toxic contamination, water temperature and nutrient load on the species abundances and biological traits of macroinvertebrate communities in 76 French streams. Toxic contamination was quantified based on the Gammarus feeding inhibition indicator recorded during repeated in situ biotests. We then compared trait responses with expectations from a priori predictions derived from the literature, and looked at the link between taxon turnover and taxonomic and functional diversity metrics using linear models. Our results suggest that, when considered independently, each driver had specific impact on invertebrate taxa and biological traits, with different consequences on community composition and diversity. Notably, we showed significant changes in taxonomic composition linked to toxicity, associated with the reduction of both taxonomical and functional richness, but no significant effect on biological trait composition. In contrast, temperature and moreover nutrient load strongly effected traits, but with variables associations with diversity metrics depending on driver. Finally, when all three drivers were considered in combination, patterns were similar to those observed for temperature, suggesting dominance rather than additive effects between drivers. Our study reveals particular negative impacts of toxicity on macroinvertebrate communities in contrast with other drivers, which highlights the potential risk posed by contaminants for freshwater biodiversity. Competing Interest Statement The authors have declared no competing interest. Footnotes remi.recoura-massaquant{at}inrae.fr, benjamin.alric{at}inrae.fr, jeremy.piffady{at}inrae.fr, olivier.geffard{at}inrae.fr Conflict of Interest: The authors declare no conflict of interest. Data availability statement: Datasets used or generated during this study are available from the French governmental repository (recherche.data.gouv): https://doi.org/10.57745/CZTZWJ R scripts added as supplemental files