Effects of intermittence on Alpine streams ecosystems: hyporheic zone and resilience

Over the last few years, a significant portion of rivers in Northern Italy have been progressively shifting from naturally perennial to intermittent systems, with prolonged dry phases occurring in summer and also in winter . However, the aquatic communities of Alpine rivers lack strategies and adaptations to survive such hydrological and physico-chemical changes. Aim of this study is to explore the impact of intermittence in previously-perennial Alpine stream reaches, considering biodiversity, taxonomic and functional aspects. In detail, we aimed to assess the role of the hyporheic zone in increasing the resilience of these aquatic systems to drought. The hyporheic habitat was monitored in 2017 in two sites (one perennial and one intermittent) in the upper course of the Po River (Val Po, Cuneo province) with piezometers installed in the riverbed, reaching -1 m (both sites) and -3 m depth (intermittent site). Sampling was conducted from July to March 2018 at short time intervals to follow the strongest drought recorded in the area from the 1800s. Sampling was resumed at both stations in January 2018 following the onset of winter precipitations, and conducted at short time intervals during the rewetting phase, and monthly after both stations were permanently submersed. Relative and total abundances of hyporheic and benthic invertebrates changed differently in the two stations during the different phases of the hydrological cycle, suggesting that the hyporheic and benthic communities respond quickly to the onset of the drought. As expected, where water is permanent, the hyporheic communities are abundant; conversely, increasingly harshness (i.e, reduction of the water table) affects hyporheic communities and increases the use of the hyporheic habitat as a refuge by benthic taxa