How macrophytes can improve the ecological assessment of rivers in a multiple-pressure context: development of a new diagnostic tool.

To achieve the European Union's water policy objective of "good" ecological status for all water bodies, a number of biotic indices have been developed, some of which are based on macrophyte communities. Despite the initial targets, 60% of water bodies still fail to achieve this status. One of the numerous reasons for this is the difficulty of accurately identifying the real causes of the degradation of water bodies, especially in a context of multiple pressures, so that truly effective restoration measures can be taken. To disentangle the effects of these co-occurring anthropogenic pressures on rivers, we have developed a diagnostic tool based on random forest models to predict the probability of impairment of river macrophyte communities. Thirteen categories of pressure on water quality, including diffuse contaminations by micropollutants (such as Polycyclic Aromatic Hydrocarbons and pesticides), and seven categories of pressure linked to hydromorphological alterations, were taken into account. By modifying local environments, each individual pressure induces changes in the taxonomic composition of macrophyte assemblages and in the adaptations selected within these assemblages. Based on a large set of taxonomic and functional metrics, this diagnostic approach has been illustrated in its ability (i) to monitor these changes at the river reach scale, but also (ii) to highlight clear spatial or temporal trends in the evolution of anthropogenic pressures on the scale of mainland France. The proposed models, based on macrophytes, are ecologically relevant and are promising for identifying water quality and habitat degradation from springs to lowland rivers. We hope that this progress will lead to a better understanding of the causes of river degradation, which will consistently help river managers to make better conservation or restoration decisions.