The effects of a sediment flushing on Alpine macroinvertebrate communities

Sediment flushing from dams can help desilting reservoirs and reinstate the longitudinal sediment transfer continuity in rivers, but negative ecological impacts might arise. We monitored the ecological effects of a flushing event conducted from 27th May to 14th June 2019 in the Rienz River, in South Tyrol (NE Italy). Using a multi-habitat scheme approach, we collected macroinvertebrates 10 days before, and then 40 and 74 days after the completion of the operations. We selected seven biological traits—organism size, life cycle duration, mobility (dispersal and locomotion), feeding type, substrate and current velocity preference—to characterize and compare the invertebrate communities before and after the sediment pulse disturbance. Turbidity was recorded continuously for the entire duration of the event. Results indicate that invertebrate assemblages exhibited a general decrease in taxonomic richness and Shannon diversity 40 days after the event, but density and richness recovered over time. Shifts in species composition were observed in post flushing samples, with a reduction in density of sensitive species (Heptageniidae) and shredders. Post-flushing samples were generally characterized by sediment-tolerant taxa, able to cope with the new habitat conditions. Altered taxonomic and functional community composition following the flushing prevented the full functional recovery to pre-disturbance conditions.