Ecological shifts of stream ecosystems in a deglaciating area of the European Alps

This thesis provides a contribution to the knowledge on the effects of deglaciation on alpine stream ecosystems, taking into account also the hydroecological influence of thawing permafrost and paraglacial features. With a focus on the European Alps, a review is provided on the climate changes and shifts in the cryosphere (snow, glaciers, permafrost), the related changes in hydrology, geomorphic processes and the physical and chemical habitat of alpine river networks, and the consequent shifts in stream communities and food webs. A conceptual model is provided to summarize the complex interactions and the cascading effects triggered by deglaciation on hydrology, habitat and biota of alpine streams, that can be useful for educational purposes and to help the scientific community to contextualize these issues to other alpine areas. Deglaciation induces homogenisation of river networks, loss of biodiversity, and shifts in primary and secondary production, functional diversity and food webs. The scarce published studies on streams influenced by permafrost provide hints on the role of thawing rock glaciers (i.e. evident form of mountain permafrost) in shaping the ecology of freshwaters, and reveal important research gaps. To increase the knowledge on this topic, different alpine streams fed by waters of different origin were selected in two subcatchments (Zay, Solda) of a deglaciating area of the Central Italian Alps (Solda Valley), and their habitat conditions and benthic invertebrate communities were investigated over a two-year period. Rock glacier-fed streams could be distinguished from those fed by glaciers, groundwater and those of mixed origin because of their constantly clear and very cold waters, stable channels, and high concentrations of ions and trace elements that increased as summer progressed. Furthermore, the Zay rock glacier strongly influenced the glacier-fed stream through an intense export of solutes, which become progressively more relevant towards the end of summer. This influence was also due to the contribution of a proglacial lake and a moraine body, that both strongly decreased the glacial influence along the glacier-fed stream before its confluence with the rock glacier outflow. The wide range of habitat conditions revealed to strongly influence the benthic invertebrate communities in the study area. Channels with groundwater (krenal) and mixed (glacio-rhithral) exhibited a higher taxa richness and diversity. Peaks of abundance and biomass in the catchment were recorded just downstream the talus body, in the upper glacio-rhithral channel. Chironomidae from the cold-adapted genus Diamesa were dominant in the proglacial sections (upper kryal) of the glacier-fed streams. The proglacial lake, the moraine body and the rock glacial tributary at Zay contributed to the amelioration of the environmental features of the glacier-fed stream (lower kryal), boosting high invertebrate biomass and abundance and causing shifts in the community composition (e.g. increased Orthocladiinae and other Diamesinae chironomids, abundant Trichoptera). The two rock glacial communities differed considerably between each other. In fact, the community of the Zay rock glacial stream was partially influenced by the seepage of glacier waters, and resembled those of the surrounding lower kryal. On the contrary, the Solda rock glacial stream, detached from any glacier influence, hosted a rich and diverse community which resembled those of glacio-rhithral and krenal, even though with a higher abundance of Diamesa. Overall, the results of this thesis showed that in the advanced phases of glacier retreat, paraglacial landforms and permafrost can increasingly contribute to the riverscape diversity and shape the ecology of river networks. Because of their unique environmental settings, rock glacial streams should be considered a distinct alpine stream habitat, acting in deglaciating catchments as stepping stones that enhance the upstream colonisation of non-glacial communities following glacier retreat. At the same time, they might represent cold refugia for cold-stenothermal and/or typically glacial taxa when glaciers will be disappeared, because of the slower thawing rate of rock glacier ice. In this context, the presence of Diamesa kryal specialist species in rock glacial streams deserves further investigation, in order to understand the potential conservation value that these habitats may have in buffering the β-diversity reduction which is predicted in alpine areas as a consequence of glacier loss.