Leakage from Traditional Irrigation Systems and Groundwater Upwelling Buffer the Impact of Heavy Streamflow Diversion on Benthic Biodiversity

Headwater streams are biodiversity-rich yet highly vulnerable ecosystems, especially in Mediterranean mountain regions where hydrological regimes are increasingly altered by irrigation and climate change. We assessed the effects of flow reduction by traditional irrigation weirs on benthic macroinvertebrate communities in five headwaters within the National–Natural Park of Sierra Nevada. We sampled upstream (reference) and downstream (flow-impacted) sites of the weirs in spring and autumn. We also quantified stream physicochemical characteristics and macroinvertebrates, and evaluated changes in their community structure. Flow reduction downstream of the abstraction points was severe (76–98% of discharge in upstream reaches), yet downstream reaches remained without significant water chemical alteration, and flow — although minimal — was perennial through the year due to seepage from ditches and upwelling groundwater. Macroinvertebrate density was substantially reduced downstream, particularly in spring (≈ 50% lower). While local (α) and interlocal (β) diversities and community composition did not differ significantly between reach types, flow reduction consistently impaired regional (γ) diversity. Moreover, taxa richness of collectors and several biotic indices — based on Ephemeroptera, Plecoptera, and Trichoptera — significantly decreased in flow-impacted sites. These findings support incorporating EPT-based metrics to enhance the sensitivity of ecological quality assessments in mountain rivers. Overall, maintaining hydrological connectivity through leaky traditional irrigation systems appears pivotal for mitigating further losses of local biodiversity, particularly in impacted reaches that lack groundwater inflow. Furthermore, since even moderate local diversity losses can accumulate to reduce regional diversity, preserving as many stream reaches as possible with natural, or modified flow compatible with biodiversity conservation, is mandatory, particularly in biodiversity hotspots.