The role of environmental variables in structuring epiphytic and epilithic diatom assemblages in springs and streams of the Dolomiti Bellunesi National Park (south-eastern Alps)

Diatom assemblages were investigated in 16 springs and fi ve streams of the Dolomiti Bellunesi National Park (south-eastern Alps, Italy), by collecting individual samples of the main substrata (stones, bryophytes, surface sediment), and measuring an extensive number of morphological, physical, and chemical variables. An empirical grouping of these aquatic ecosystems, based mainly on morphological features and physical parameters, mostly agreed with groupings produced by non-metric multidimensional scaling. Signifi cant differences among the diatom assemblages were found among the groups identifi ed; however, these differences were more signifi cant in the epilithic than in the epiphytic assemblages. Epiphytic and epilithic diatoms differed signifi cantly according to: a) the identity of several species, b) the relative abundance of some other species, c) the Bray-Curtis distance between the assemblages, and d) taxa richness and diversity index. However, on studying species composition by canonical analysis, with reference to environmental variables (here called macrohabitat) on the one hand, and the type of substrata (stones and mosses) on the other, it was shown that the macrohabitat was responsible for more than 40 % of the variability among diatom assemblages, whereas the microhabitat (i.e. different substrata) accounted for only 3 %. Multivariate analyses (CCA) showed that the environmental factors most infl uential in structuring diatom assemblages were substrate particle size (infl uenced in turn by current velocity), permanence of fl ow, shade, nitrates, and total phosphorus. Characteristic groups of species were identifi ed in the different morphological and hydrological types of springs and streams. Stream sites were often affected to a certain extent by organic pollution, even within the protected area, and consequently colonized by cosmopolitan, eutraphentic or tolerant taxa. In springs, in addition to common and widely distributed species, many poorly known, rare, or Red List species were found, in particular among the less abundant taxa. Our study therefore highlights the important role of springs as valuable habitats for the conservation of aquatic biodiversity.