Effect of river and floodplain restoration and reconnection on aquatic macroinvertebrates: seasonal responses over time

Innovative floodplain reconnection actions are being implemented in various river systems, with the goal of restoring natural processes that support dynamic floodplains, rivers, and aquatic ecosystems. But how effective are these restoration actions? Aquatic macroinvertebrates respond quickly to flow, substrate, and temperature changes. Therefore, we examined the response of benthic macroinvertebrate assemblages to an intensive river and floodplain restoration project in the South Fork McKenzie River of western Oregon (U.S.A.) for 3 years (spring and fall each year) following implementation. The restoration was expected to increase geomorphic heterogeneity and surface water inundation during low‐flow conditions, thereby increasing biotic diversity. As predicted, the restored area had more surface water at base flow compared to pre‐restoration conditions and, as such, supported two to four times greater macroinvertebrate biomass across the entire wetted area, whereas biomass on a per‐square meter scale did not increase throughout the study. Restoration altered the physical conditions of riffle habitat types and macroinvertebrate community structure, reflecting slower‐water traits. Although restoration did not appear to change habitat conditions in pools or glides, it did increase the number of these features across the active bottomland. Gamma diversity of macroinvertebrate taxa richness was higher in riffles post‐restoration compared to pre‐restoration and the control area. Alpha diversity of macroinvertebrate taxa richness did not increase following restoration, but Shannon diversity measures did increase for both riffles and pools, indicating a more even distribution of taxa biomass. These findings support the hypothesis that river and floodplain restoration can increase macroinvertebrate diversity and overall biomass.