Composition, structure, and distribution of diatom assemblages in Mediterranean thermal spring ecotones affected by natural and human disturbances

Springs are unique aquatic environments that significantly contribute to regional and global biodiversity but are also susceptible to disturbances because of their small size. Mediterranean thermal springs remain poorly studied, and little is known about how different environmental and disturbance factors affect their aquatic communities. In this study, we analyzed diatom assemblages from six thermal springs of Sardinia (Italy), both isolated and connected to river systems, their relationship with environmental variables, ecotonal properties (i.e., transition of springs with terrestrial and/or river environments) and primary sources of disturbance. A total of 196 species (65 genera) were found from two substrates (cobbles and surface sediments). Overall, low discharge and ecotonal properties, influenced by natural and human disturbances (temporary flooding, water abstraction and recreational activities), contribute to form heterogeneous diatom assemblages composed of aquatic-aerial, aerial and planktonic species. Springs connected to rivers and that are more disturbed showed a higher presence of planktonic and habitat generalist species and a higher species richness, diversity and evenness than more isolated springs. By contrast, the latter seems to harbor smaller pools of taxa with crenophilous species and species with a preference for less disturbed habitats or with more restricted geographical distribution. Water temperature, “river influence,” discharge, orthophosphates and dissolved inorganic nitrogen played a major role in shaping diatom assemblages. Our results indicate that diatoms can provide helpful information about the ecological functioning of thermal springs and can be an effective tool to track their environmental changes related to the increasing natural and human pressures in the Mediterranean area.