Macroinvertebrate Assemblages Downstream of Dams Show No Sign of Impairment Despite Flow Regulation and Thermal Alteration

Flow regulation by dams produced downstream effects, including geomorphological, hydrological and thermal alterations. We aimed to analyse the alterations of the thermal regime caused by hypolimnetic releases of regulated flow compared to the unregulated thermal regime, and the effects of both thermal and flow regime alterations on macroinvertebrate communities. Stream temperature was monitored over three-years in three rivers with contrasting flow regimes, all located in the Mediterranean ecoregion. Results show that regulated rivers present an inverted flow regime with high discharges in summer, colder water from February to September arising maximum differences of 7.4 & DEG;C in July and presenting a delayed thermal maximum of 2 to 2.5 months compared with the unregulated river. Given the relevant hydrological and thermal alterations found in both regulated rivers, unexpectedly no differences were found between regulated and unregulated rivers when analysing macroinvertebrate richness and diversity indexes. The taxonomic response is significantly different among the three rivers in summer and was mostly explained by summer indicators, with the maximum water temperature of July being present in all models. An indirect driver that exacerbates the thermal differences found between regulated and unregulated rivers would be the geomorphological configuration of the rivers reciprocally interacting with the riparian vegetation. The monitoring of stream temperature is a crucial activity needed to implement since this parameter together with water flow shapes the distribution of aquatic biota in river ecosystems.& COPY; 2023 European Regional Centre for Ecohydrology of the Polish Academy of Sciences. Published by Elsevier B.V. All rights reserved.