How does floodplain restoration of a highly anthropized river enhance aquatic biodiversity? Lessons from a decade of macroinvertebrate and fish recolonization

Side arm engineering is a common restoration practice used to rehabilitate the lateral dimension of anthropized rivers. However, benefits for aquatic biodiversity in these restored areas remain highly uncertain, and the lack of high-resolution temporal monitoring limits our understanding of mechanisms constraining recolonization. This study investigated ten years of ecological succession within invertebrate and fish communities in a 5-km-long created channel in the floodplain of the highly anthropized and biologically invaded Rhine River (France). More specifically, we examined colonization patterns over time through biodiversity indices, taxonomic composition and functional attributes of communities. During the early stage of succession, we observed that dispersal capacities played a key role in invertebrate colonization, in contrast to fish, which showed a high similarity in composition with the main Rhine channel. After ten years, we observed low gains in richness, not in diversity and evenness, alongside important divergences in both invertebrate and fish community composition. Invertebrate taxa were functionally more adapted to lateral habitats. This pattern was not observed for fish, which instead displayed higher feeding specialization to bottom feeding. Surprisingly, all structural and functional changes in communities were essentially driven by native species, with exotic species contributing only additively to the overall invertebrate and fish community trajectories. We conclude that floodplain reconnection of a highly anthropized and invaded river promotes the diversification of lateral habitats for scarce native invertebrates of the main channel and their predators (e.g., benthivorous fish) within a decade, the minimum timespan required to capture a mature successional stage. • Engineered side arms promote the establishment of species associated to floodplain. • Structural and functional changes in communities were driven by native species. • Common diversity indices appear inappropriate to assess the restoration benefits. • A reliable assessment of created habitats requires at least ten years of monitoring.