THE DARK SIDE OF THE ICE: GLACIOLOGICAL AND BIOLOGICAL ASPECTS OF SUPRAGLACIAL DEBRIS

This research deals with a multi-disciplinary analysis of the supraglacial debris. Debris significantly influences the evolution of glacier surface, its energy balance, and the carbon fluxes and storage. In this work through a multi-disciplinary approach, we focused on this dark side of the glaciers from two different points of view: the glaciological and the biological one. In the last decades, the large majority of glaciers, including those on the Italian Alps, showed a great increase of supraglacial debris cover. The analyses performed on aerial and UAVs imagery on a wide glacierized sector of Italy, highlighted that the debris-covered area doubled in the period 2003–2012, reaching an increase up to 30.10% of the whole glacier area. However, these changes in surface features, fed by an increased availability of debris, occurred with different patches, according to the physical properties of the bedrocks hosting the glaciers. This suggests that further studies are needed to quantify the occurrence and distribution of supraglacial debris on all the Italian glaciation. The ice albedo and, consequently, the energy balance of glaciers not only are affected by the presence or absence of a thick and continuous debris cover on the glacier surface, but also by the amount and distribution of the fine and sparse debris and dust that discontinuously cover glaciers. Sparse debris is thus important for determining the evolution of ice bodies, but its quantification is arduous, as the availability of high-resolution imagery, both from satellite and UAV, is mandatory. In this work we showed that the processing of an UAV image of the glacier through a segmentation approach allows describing ice features at a small-scale, including the distribution of fine debris. Moreover, we found evidence of darkening phenomena due to an increased amount of fine and sparse debris on the surface of glaciers. The darkening of glaciers is probably favoring organisms living in the supraglacial debris; however, organisms can promote glacier darkening because they produce dark matter (e.g. humic substances) and are themselves part of the dark debris quantified in glaciological analyses. A positive feedback seems therefore to occur on glacier surface, promoting the increase of supraglacial debris. The analyses of the life on supraglacial debris indicates that a glacier cannot be considered as an isolated environment, although it has different characteristics than the surrounding areas. Nematodes and Rotifers, for instance, can diffusely colonize supraglacial debris only in the presence of allochthonous organic matter, which represents the main source of organic carbon for these organisms in supraglacial environments where primary producers are scarce. Moreover, the study of bacterial communities in snow highlighted a possible contribution of organisms transported from the area where the air masses originated, as well as a non-negligible input of local air bacteria, maybe due to the deposition of local particulate during snowfall. This strong relation between glacier and ice-marginal environments is observable also from the bacterial community of the cryoconite holes. Indeed, we showed that ice-marginal environments may act as sources of bacteria for these micro habitats, but differences in environmental conditions limit the number of bacterial strains that may survive in them. At the same time, cryoconite holes host some organisms that were not found in any ice-marginal environment we sampled, thus suggesting that some bacteria may reach cryoconite from distant sources. These bacterial communities of cryoconite holes have a wide temporal evolution throughout an ablation season, with autotrophic Cyanobacteria populations dominating communities after snow melt, and heterotrophic populations increasing in abundance later in the season. The complex bacterial communities that inhabit glacier surface have large impacts on biogeochemical processes, in particular on the carbon cycle. In fact, we provided evidence for the occurrence in these environments of metabolic pathways that differ from those of oxygenic phototrophs and the respiration of heterotrophic organisms beforehand described on glacier surface. Indeed, we observed high abundance of heterotrophic anoxygenic phototrophs, suggesting that light might supplement the energy needed by the organisms permitting them to use some organic molecules as carbon sources. Furthermore, these communities could produce CO2 also by the oxidation of CO, which may be produced by photodegradation of organic matter present in the cryoconite. Finally, we investigated the fate of contaminants on the glaciers surface assessing a key role of the bacteria in the chlorpyrifos degradation. In summary, the results presented in this PhD thesis improved our knowledge of the supraglacial debris, its components and its evolution. The double view on the glacier system, both glaciological and biological, permits a deeper description of the mutual relations between bio and geo components.