Combining taxonomic, functional and biomonitoring metrics to evaluate the recovery of alpine stream macroinvertebrate communities after an extreme flood

Floods are usually recognized as a natural disturbance of lotic ecosystems shaping riverine communities. Although benthic macroinvertebrates display morphological and behavioural adaptations to cope with flood-related conditions, the diversity and density of these organisms are generally reduced after a flood. Understanding the mechanisms and timing of the post-flood recovery of macroinvertebrate communities assumes a key importance in aquatic ecology, but our current knowledge is limited by the restricted number of studies as well as the metrics used to evaluate the success in recovery. In this study, the temporal recovery of macroinvertebrate communities in the Anza River (northwestern Italy) after an extreme flood was evaluated by analysing a multifaceted set of taxonomic, functional, and biomonitoring metrics. The taxonomic composition of macroinvertebrate communities changed over time along with a significant increment in the percentage of fine sediment in the substrate immediately after the flood. Overall, richness and density metrics significantly declined after the flood but, within 9 months, they approached or even exceeded the before-flood values. Functional richness and functional evenness, instead, decreased over time and, after nine months, did not recover to the before-flood values. Although the considered biomonitoring indices were significantly reduced by the flood, they differed in the post-flood recovery outcome. Since floods are expected to be more frequent in the next future due to climate change, the results of this study provide evidence on which metrics drive the post-flood recovery of macroinvertebrate communities with potential insights for disentangling the impacts of natural and anthropogenic pressures on river ecosystems.