Effects of Flash Flood Events on Freshwater Communities in the Alpine Stream Corsaglia (Cuneo Province, Italy)

Alpine streams are increasingly exposed to multiple disturbances, including extreme flood events, expected to become more frequent under climate change, and the expansion of small hydropower plants (SHPs). Understanding the resilience of aquatic communities and the ability of biomonitoring tools to detect disturbance-driven changes is essential for ecological assessment. We conducted an eleven-year monitoring programme (2014–2024) in the Corsaglia Stream (Northwestern Italy), comprising 16 sampling campaigns before, during, and after two flood events and SHP construction. Macroinvertebrate and fish communities were analysed using taxonomic, functional, and temporal beta-diversity metrics. Macroinvertebrates were assessed using the nationally standardised STAR_ICMi biomonitoring index and the recently developed Flow-T index. Macroinvertebrate assemblages showed high resilience, recovering taxonomic richness while maintaining “Good ecological status” despite severe flood-induced collapse. Recovery followed a nestedness-to-turnover trajectory, indicating recolonization from refugia rather than community replacement. Flow-T detected transient functional changes not captured by STAR_ICMi. In contrast, fish communities exhibited persistent structural changes, with reduced abundance, marked shifts in species composition, and slower recovery of native salmonids and European bullhead (Cottus gobio) under combined flood and hydropower impacts. These findings show that integrating taxonomic, functional, and temporal approaches improves ecological assessment beyond single-index biomonitoring approaches such as STAR_ICMi alone.