Environmental factors structuring diatom assemblages in thermo-mineral springs of Sardinia, Italy

Thermal springs provide natural laboratories to test the responses of biological communities to various stressors. To date, little is known about how water temperature and chemistry influence the microalgal assemblages in these ecosystems, especially in the circum-Mediterranean area. In this study, we assessed the environmental drivers of benthic diatom assemblages collected from different substrates in 8 thermo-mineral springs of Sardinia, Italy, with a focus on temperature and conductivity. A total of 205 diatom species (67 genera) were found, of which 12 (8 genera) were centric and 193 (59 genera) were pennate. Composition of diatom assemblages was associated with temperature, pH, conductivity, bicarbonates (HCO3−), and sulfates (SO42−). Multivariate analyses also confirmed differences in species composition, richness, diversity, and evenness of diatom assemblages from groups of springs distinguished by temperature and conductivity gradients. We found higher species richness, diversity, and evenness in hyperthermal (water temperature = 52.7−71.5°C) and medium-mineral springs (conductivity = 590−1193 µS/cm). Our results indicate that thermal springs are not always inhospitable environments for diatoms. Springs with high water temperatures hosted rich and diverse diatom assemblages, likely thanks to their ecotonal nature and a combination of multiple local factors.