Diatoms in very-shallow pools of the site of community importance Danta di Cadore Mires (south-eastern Alps), and the potential contribution of these habitats to diatom biodiversity conservation

Mires are very selective habitats, and the colonisation by specifically adapted organisms makes them very important both for nature conservation and environmental education. They have, however, been threatened by different types of impacts. Our objective was to study diatoms in stations representative of the main typologies in the Site of Community Importance IT3230060 "Danta Mires", also to produce data for a management plan in the frame of a Life Project. Five shallow pools within different mountain mires were sampled in summer 2005 and morphologically and physicochemically characterized. The sampled sites cover a rather wide pH and TDS content gradient with very low nitrate and silica values. Diatoms were found to dominate in terms of numbers of taxa, followed by desmids, cyanoprokaryotes and filamentous green algae. In total, 86 diatom taxa, belonging to 35 genera, were found. As expected for acidic, low-mineralization waters the genera with the highest number of taxa were Eunotia and Pinnularia, followed by Encyonema, Cymbopleura, Gomphonema, and Nitzschia. The most frequent and abundant taxa were: Frustulia crassinervia, Nitzschia acidoclinata, Kobayasiella micropunctata, Encyonema neogracile, Brachysira brebissonii, Nitzschia perminuta, and Encyonema lunatum. Taxa abundant only in one or few sites were: Eunotia paludosa, Staurosira spinarum, Encyonopsis cesatii, and Achnanthidium minutissimum. Several rare taxa of special interest for their distribution and/or rarity could be found (e.g. Chamaepinnularia schauppiana, Cymbopleura incertiformis var. linearis, C. subapiculata, Eunotia lapponica, Microfissurata paludosa). On average, 476 17 taxa per site were found. The highest number (26) was found in a sediment sample taken in a site with special hydrochemical characteristics, and the minimum (11 taxa) in the sediment sample of the demolition materials dump mire. Diatom communities allowed characterising the mire pools studied as oligotrophic, acidic environments often affected by desiccation. On the basis of the autecology and distribution of the most frequent and abundant taxa, pH and hydroperiod appeared to be the most relevant environmental determinants. A striking result, also in view of eventual management consequences, was the very high proportion (72%) of rare and Red List taxa (including also species of the categories "Threatened with extinction", "Severely endangered", and "Endangered"). To preserve diatom biodiversity, these mires must be sheltered from uses other than recreational ones, and abrupt changes in hydrological regime and increases in nutrient input must be prevented.