Diatom assemblages in glacial-fed streams of Italian Western Alps

The rate of glacier retreat has recently increased, with changes in the glacier discharge, river flow and water characteristics, creating specific environmental conditions for highly specialized organisms. We here investigated diatom communities in glacier-fed rivers of the Valle d'Aosta region (NW-Italy), analyzing data collected between 2010 and 2019 for the WFD monitoring programs. We present a complete checklist of the species inhabiting this region, many of which are classified as endangered in the diatom Red List. We found significant differences between sites with different catchment glacier cover (CGC), in terms of taxonomic composition, ecological guilds and abundance of Red List taxa. In particular, we observed a significant increase in species richness with the decrease in CGC, as well as significant differences in terms of taxonomic composition. We highlighted the role of Achnanthidium lineare as an indicator species for streams with the highest glacial influence, together with Hannaea arcus, Eucocconeis laevis and other oligotrophic low profile species that are well adapted to physical stress induced by glacier runoff. On the other hand, streams with lower glacial influence were characterized by motile species mostly belonging to the genera Navicula and Nitzschia, due to the nutrient increase. Didymosphenia geminata was recorded in streams with marginal glacier cover; recently observed blooms indicate that conditions may become increasingly favorable for the spread of this species in Alpine environments, as glaciers retreat. The abundance of Red List diatoms increased significantly with increasing glacier cover suggesting that glacier-fed rivers are niches for the conservation of diatom biodiversity.