Diatom monitors of close-to-pristine, very-low alkalinity habitats: Three new Eunotia species from springs in Nature Parks of the south-eastern Alps

On the basis of LM and SEM observations as well as of environmental data, three species of the predominantly-acidophilousdiatom genus Eunotia (E. cisalpina, E. fallacoides, E. insubrica) are described as new to science, and characterized from theecological point of view. The three new species were not distinguished from other similar ones in the past but can be identified at theLM by careful observation of diacritical characters such as valve outline (in particular the shape of the endings), dimensions, andstria density. One of the three new species (E. insubrica) presents a very peculiar ultrastructural feature, i.e. well-developedpseudoseptum-like structures at the apices. The three new species were pointed out by detailed analyses of materials sampled in thesouth-eastern Alps. It is put forward that diatom investigations adopting high-resolution taxonomy are particularly opportune inhabitats such as those colonized by the three new species. The three new species were found to be characteristic of low-alkalinity,naturally slightly-acidic, strictly-oligotrophic, high-mountain springs and lakes. Efficient desiccation-resistance strategies,apparently widespread in the genus Eunotia, might be supposed also for the three new species, that were found to thrive also in lowdischargesprings and in the shallow littoral waters of lakes showing seasonal or exploitation-enhanced water-level fluctuations. Thediatom assemblages of these aquatic habitats possess a high species richness, and include many rare and Red List species, and arethus of prominent importance for diatom biodiversity conservation. These relatively-pristine aquatic habitats on siliceous bedrocksare however highly sensitive, and endangered because of diffuse pollution. Climate- and environmental-change driven increasedrelease of organic acids, and contamination by airborne inorganic acids and nutrients are all potential threats to these still highintegritybut fragile systems. The detailed characterization of their diatom assemblages, that are excellent monitors of acid-base andnutrient status is thus of the foremost importance. In particular the distinction between naturally-acidic and acidified systems is arelevant topic that might benefit from detailed knowledge of taxonomy and ecology of both communities and indicative taxa of thediatom assemblages.