Responses of Macroinvertebrate Communities to Hydromorphological Restoration of Headwater Streams in Brittany

Indices of fauna communities, including macroinvertebrates, have been widely used as indicators of environmental changes in streams with great success. However, in the evaluations of in-stream restorations, results from the deployment of macroinvertebrate community indices as bio-indicators have been inconclusive, with scanty evidence for success. This study aims to determine if in headwater streams, and particularly according to the type of in-stream restoration studied (i.e., suppression of concrete pipe), bio-indicators based on macroinvertebrate can be relevant. We monitored three headwater stream reaches where artificial structures constituting hydromorphological impairments to the streams were removed. We collected macroinvertebrate samples from impacted stream sections and control sites established along the streams. Samples were collected before and after the restoration activities, in a before-after-control-impact (BACI) study design. We used two macroinvertebrate-based multimetric tools (I2M2 and ERA) to evaluate the ecological status of the streams, based on macroinvertebrate community indices, and to identify the relative contributions of watershed anthropic pressures to the ecological status. We found that the removal of the artificial structures and the restoration of natural flow were successful in reducing clogging. However, only taxonomic richness showed a positive significant change. The results indicate that the presence of confounding factors, not addressed by the restoration works, may have been responsible for this minimal success in biodiversity recovery. Although the multimetric assessment tools were able to differentiate between streams and disentangle the effects of multiple pressures contributing to degradation in the streams, they were limited to a level below the watershed scale. Our results showed that for a better outcome for macroinvertebrate biodiversity improvement, methods of in-stream restorations must consider the multiple pressures contributing to the degradation of fauna communities in watersheds