Effects of a river restoration project along the Old Rhine River (France-Germany): Response of macroinvertebrate communities

The rise of restoration projects on large rivers is a response to the increasing human-induced pressures on these ecosystems. Despite this, there is a relative lack of data documenting restoration success using macroinvertebrate communities in such environments, with those existing frequently producing contrasting results. Here, we examined post-restoration responses of macroinvertebrates following a unique experimental restoration approach based on controlled bank erosion and artificial groyne implementation, initiated in 2013 on the Old Rhine River (France-Germany). We investigated how macroinvertebrate communities have responded to restoration-induced variations in three main abiotic parameters, i.e. water depth, flow velocity and substrate type, by comparing the restored section with unrestored ones. The Eco-hydro-morphological index (EHMID), a modified version of a hydro-morphological diversity index, showed a gain in mesohabitat heterogeneity along the whole site. Newly created mesohabitats with low flow velocity and finer substrate were dissimilar to those along the rest of the Old Rhine channel, favouring burrowing taxa such as Odonata. The presence of such insect larvae was related to the post-restoration emergence of typical alluvial terrestrial-aquatic border connectivity, and the rise in macrophytes over time. On the whole site, changes in composition or in functional profile diversity were highly related to the high degree of mesohabitat heterogeneity from the restored section, which would persist as long as groynes remain. The main inter-annual effect concerned the decrease in invasive taxa abundance that also varied according to any changes in fluvial forms. Our findings confirmed that macroinvertebrate responses are highly influenced by hydrological events and are dependent on the study-scale monitoring, clearly putting forward fine-scale hydromorphological gradients. Biological results from this restoration project should approach those obtained in smaller rivers restored using deflectors, suggesting a potential application of the hydraulic law of similarities. However, the accuracy of biological prediction using said application is limited by the distance from source populations, biological invasions and internal river dynamics.