Investigating the Effects of Flow Regulation Combining Stream Geomorphology With Macroinvertebrate Ecology: A Case Study in the Orobic Alps (Italy)

Abstract

Mountain streams exhibit hydrogeomorphological heterogeneity, which is further intensified by the presence of hydraulic infrastructure. Despite extensive research on the influence of hydrological conditions and flow regulation on ecology, there is a notable absence of holistic studies investigating stream dynamics from both geomorphologic and ecological perspectives. Our work seeks to address this gap by characterizing stream channels' morphology, sediment grain-size distribution and streambed lithological composition combined with an evaluation of bedload rates and an assessment of macroinvertebrate taxonomic composition and flow preferences. The study was conducted along the longitudinal profiles of two steep headwater streams of the Orobic Alps (Northern Italy), one regulated by reservoirs and the other unregulated. Five reaches were surveyed, with two for each stream, plus one below their confluence. The investigation unveiled a patchwork of geomorphological conditions and bedload rates across the five stream reaches examined. These variations, related to physiographic setting and flow regulation, were associated with differences in the taxonomic composition of macroinvertebrate assemblages and their flow preferences. In the stream regulated by reservoirs, a higher taxonomic richness and abundance of both limnephilic and reophilic taxa were observed compared with the unregulated stream. Below their confluence, the composition of the macroinvertebrate assemblage exhibited marked temporal variability, likely associated with the highly fluctuating hydromorphic conditions. Despite the lack of a detailed hydrological characterization, our study highlights the interconnectedness of geomorphology, bedload rates and macroinvertebrate responses under distinct hydrological conditions, providing complementary insights. Thus, we advocate for interdisciplinary studies to better unravel mountain watercourses dynamics.