Diatoms in springs of the Alps: spring types, environmental determinants, and substratum

Spring habitats are highly diverse and have a mosaic microhabitat structure. They are endangered by diffuse exploitation as drinking-water resources, an impact likely to increase with climate change. Diatoms were sampled from stones and bryophytes in 110 nonthermal, near-natural springs in the southeastern Alps (Trentino) and 16 carbonate springs in the Dolomiti Bellunesi National Park. Fuzzy clustering identified 6 assemblages in the main types of springs. Carbonate rheocrenes hosted Achnanthidium spp., Gomphonema elegantissimum, and Nitzschia fonticola. Several of these species are rheophilic. This group was divided into 4 subassemblages defined by decreasing A. lineare and increasing A. pyrenaicum with increasing flow. Carbonate rheocrenes with shading or moderate NO3- enrichment contained a majority of sciaphilic and NO3-tolerant taxa: Cocconeis taxa, Amphora spp., Caloneis fontinalis, Reimeria spp., and Eunotia arcubus. Well-buffered siliceous rheocrenes supported Diatoma spp., Eunotia minor, Encyonema minutum, Navicula exilis, and Planothidium lanceolatum. Many of these species are heliophilic and rheophilic. Carbonate rheocrenes with lower conductivities or seasonal desiccation contained typical taxa of unstable environments: Diadesmis spp., Planothidium frequentissimum, Meridion circulare, and Achnanthidium dolomiticum. Carbonate rock-face seepages and some tufa springs supported xerotolerant diatom species with a preference for higher conductivities: Encyonopsis spp., Delicata spp., Gomphonema lateripunctatum, Denticula spp., and Cymbopleura spp. Siliceous seepages and pool springs, some very-low alkalinity rheocrenes, and 1 iron spring were characterized by acidophilous mire taxa, such as Eunotia spp., Frustulia crassinervia, and Tabellaria flocculosa, and very-low-alkalinity indicators, such as Psammothidium acidoclinatum. The other iron springs harbored species-poor assemblages with low numbers of cells. Many diatom species showed a significant preference for stones or bryophytes. Epibryon had higher richness and diversity than epilithon, and mean diversity did not differ among the most frequent bryophyte species. Bryophytes are quick and easy to sample and are proposed as the target substratum for diatom-based assessments of springs.