Quantifying the ecological effects of water abstraction in Alpine streams through flume simulations

Water abstraction by small hydropower plants is a growing human pressure on Alpine stream ecosystems. Using a set of seminatural streamside flumes, we investigated the response of benthic invertebrate communities to a 50% experimental reduction of the natural discharge. We collected EPT taxa from upstream (control) and downstream (flow reduction) sections of the flumes before starting the simulations, and after 1 and 3 weeks of simulated flow reduction, while also collecting invertebrates drifting from each section every 24–48 h. Despite an initial (i.e., before simulation) difference in benthic densities and diversity between controls and treatments, these metrics changed and differed over time, with the reduced flow section becoming more diverse and with less individuals. Taxonomic composition differed between flow types, with a short-term (1 week after the beginning of flow reduction) increase of taxonomic heterogeneity. There was no evidence that flow reduction affected functional diversity and homogenized functional composition of the communities. Drift composition differed between flow types with drift rates and richness lower under reduced flow. Functional dispersion was always significantly lower in the drift collected in low-flow conditions where drift propensity was significantly higher for animals preferring medium to fast currents, living temporarily attached to the substrate and feeding as passive filterers; drift propensity was slightly lower for organisms moving as crawlers. Hence, short-term taxonomic and functional changes in benthic invertebrate communities with reduction in flow are likely related to behavioural responses such as drift, thus providing insight on the potential mechanisms underlying community-level effects of water abstraction.