Towards a multi-metric assessment of river restoration ecological outcomes across projects and contexts

Abstract The assessment of the ecological outcomes of river restoration projects remains a complex undertaking due to the high variability of responses observed. This variability is dependent on a number of factors, including the location of the site under study, the indicators and taxa utilised in the analysis. In this study, a multi-metric and multi-contextual approach was applied to analyse 62 river restoration projects located in France, Germany and the Netherlands. This methodological approach facilitated the characterisation of the responses exhibited by macroinvertebrate communities in response to restoration initiatives, and the subsequent identification of the environmental contexts and the categories of restoration measures that exerted influence on these responses. Utilising log-response ratios based on both taxonomic and functional ecological metrics, a k-means clustering analysis was performed. This analysis yielded two distinct groups of sites: the first exhibiting predominantly positive responses to restoration, and the second demonstrating neutral or marginally negative responses. Positive responses have been shown to be associated with ambitious restoration projects, which are characterised by measures targeting regional-scale processes. These measures include the implementation of projects designed to improve river longitudinal continuity in degraded catchment areas, where the density of obstacles and the intensity of agricultural use are particularly high. Conversely, restoration initiatives targeting local-scale issues within more natural contexts or at high altitude exhibited minimal to no ecological response. This study emphasises the significance of an integrated approach that incorporates various facets (taxonomic and functional) of biodiversity, along with a meticulous examination of the environmental context and project construction, to enhance the evaluation of the ecological benefits of river restoration. The findings of this study may also be applicable in the context of fieldwork, providing practitioners with a framework to inform the design of restoration projects.