Mapping Ghost Springs: Strategic Ecohydrogeological Zones for Diatom-Based Monitoring and Conservation in the Northern Apennines

Springs are biodiversity hotspots and climate refugia, yet they remain among the most overlooked and threatened freshwater ecosystems, largely absent from regional monitoring programs. In the Emilia-Romagna Region (Northern Apennines, Italy), where more than 8400 springs are documented but most persist as unmonitored “ghost springs”, we developed and tested an ecohydrogeological framework to support their long-term monitoring and conservation. Twenty-five springs were selected as representatives of five strategic zones, which we identify, defined by aquifer type (Ophiolites, Turbiditic Sandstones, Shallow Marine Sandstones, Triassic Gypsum, and lowland Fontanili) and characterized through integrated hydrogeological, hydrochemical, and biological sampling. Benthic diatoms (393 species in 69 genera) growing on three different substrates (epibryon, epilithon and epipelon) per spring, vegetation, and physical and chemical parameters were analyzed. Diatom assemblage composition differed significantly among zones (PERMANOVA, p < 0.001), structured primarily by electrical conductivity between sites (community level) and by discharge regime between substrates (microhabitat level); the proportion of aerial diatoms (88 species) proved a useful indicator of discharge fluctuations, inferable from a single biological sample. Threatened taxa (41% of evaluated species) concentrated in the ridge zones. Although calibrated in the Northern Apennines, the five-zone framework offers a biologically validated and transferable tool to other regions for spring conservation as sentinel ecosystems under climate change and anthropogenic exploitation.