Spatial and temporal variability of supraglacial algae on an Alpine glacier (Forni Glacier, Italy)

The assessment of algal biodiversity in mountain glaciers is timely and critical. In this study, we evaluated the biodiversity of algae on an alpine glacier and their spatio-temporal variability in seven supraglacial habitats (snow, bare ice, bédières, water of cryoconite holes, dirt cones, sparse sediment, and cryoconite) over two years. Biovolume, diversity, and community structure were investigated using microscopy observations, providing a comprehensive quantitative and qualitative assessment of the algae dynamics. Algal communities were highly spatially and temporally variable. The community dynamics at the glacier scale were complex and structured by diverse ecological processes. The difference in species richness explained the major part (82.5%) of the variation in algal community composition among habitats. The snow was characterized by the higher local contribution to beta diversity (LCBD) values and by high replacement values. This habitat typically supported unique combinations of species and contributed greatly to glacier biodiversity (with 6 associated indicator taxa). Dirt cones, sparse sediment, and cryoconite communities showed high rates of species replacement, while the communities of bare ice, bédières, and water of cryoconite holes showed lower species richness. Bare ice was characterized by the lowest richness, due to the large dominance of Mesotaenium berggrenii, which bloomed at the end of the ablation period (biovolume 6 to 14-fold higher at late summer 2023 than in the other dates). Under warmer conditions, the forecasted decrease of snow and ice mass may remove the habitats that contribute most to the total biodiversity of the glacier, affecting the overall community dynamics and impacting the surrounding environments that benefit from the glacier's resources.