Ecological evolution of recent proglacial lakes in the deglaciating Alps (Cevedale Glacier, Italy)

The Alpine cryosphere is strongly affected by the current climate-driven deglaciation. In this context, one of the most evident effects of glacier retreat is the increase in number and volume of new proglacial lakes, that in the past decades progressively became a frequent feature of the high mountain landscape. Common traits of proglacial lakes are related to the inflow of glacial meltwater and include low water temperature and high amounts of inorganic particles suspended in the water column, which results in high turbidity. These factors shape a harsh and rapidly evolving habitat, with only partially known selective effects on lake biota. While planktonic communities in proglacial lakes have attracted more attention from the scientific community, knowledge on benthic communities remains scattered. Nonetheless, given the low input of allochthonous organic matter from bare proglacial forefields, local periphyton may represent a crucial carbon source fuelling the food webs of glacier-fed lakes. This thesis contributes to the general knowledge of Alpine proglacial lake ecosystems, by integrating observations and modelling of lake hydrodynamic, temperature and turbidity patterns with investigations on benthic biological communities. The synergy between different disciplines allows to provide a broad picture of the dynamics involved in the ecological evolution of proglacial lake ecosystems influenced by the current Alpine deglaciation. For this purpose, a lake cluster formed by the regression of the Cevedale Glacier was investigated over a two-year period, during the ice-free season. Four proglacial lakes were selected, representing the different evolutionary stages of proglacial lake ecosystems, going from one turbid ice-contact lake to intermediate and distal lakes. Physical, chemical and biological components were extensively monitored in the lakes through the installation of temperature, light and electrical conductivity sensors, and by collecting samples for chemical and biological analyses (eDNA and benthic diatoms). The environmental gradients recorded in the study site are mainly linked to the different lake age and the related glacial influence. Physical dynamics were further analysed in one turbid lake, by installing chains of thermistors in the water column, to observe vertical stratification phenomena. These data were used to calibrate a 2-D hydrodynamic model, which allowed to link the lake thermal dynamics to the turbidity patterns. Input data for the model set-up were obtained from in situ measurements of outlet discharges, water level and the concentration of inorganic suspended solids in the lake. The model simulation showed that during periods characterised by warm air temperatures and rare precipitation events, the lake promptly responds developing a daily thermal stratification, which in turn determines the depth of intrusion of the turbid inflow, often entering the lake as an interflow. Therefore, thermal stratification can influence the light penetration in the water column of shallow layers through the control on turbidity patterns. From an ecological perspective, these results indicate that light availability in the littoral area can occur also in periods of sustained glacier turbid inflow, thus determining the presence of stratification-driven windows of opportunity for periphyton growth. This result was consistent with the seasonal evolution of the biomass of littoral photosynthetic communities in the lake, measured as chlorophyll-a and organic content in the biofilm and as benthic diatom density. A peak in photosynthetic biomass was indeed observed in August 2022, suggesting that periphyton growth can be sustained even during periods of maximum turbid inflow from the glacier. Detailed analyses on biological communities indicate a progressive increase in littoral α-diversity from the ice-contact lake to the distal lake, for both eukaryotic (including diatoms) and prokaryotic assemblages. However, taxonomic compositions significantly differed from lake to lake. These results indicate that the progressive loss of glacial influence during the evolution of the studied lakes is related to an increase in α-diversity in littoral communities and to a simultaneous homogenisation of communities, which leads to a decreasing β-diversity. First stages of colonisation (i.e., communities in the most recent lakes), are dominated by psychrophilic, aerophilic, soil-adapted and pioneer species. The selective physical setting of habitat in the Cevedale proglacial lakes, characterised by cold and turbid water inflow and the continuous deposition of high amounts of fine-grained sediment on shores, allows the establishment of specialised diatom communities. One diatom species (i.e., Pinnularia bullacostae) not previously recorded in the European Alps was observed in the Cevedale proglacial lakes. The results of this thesis point out the relevance of proglacial lakes for the Alpine environment: despites strongly dependent on hydrodynamic patterns and characterised by harsh habitats, they can host specialised biodiversity. In a climatic change perspective, proglacial lake habitats are expected to undergo relevant modifications in the upcoming decades, since the projected increase in air temperature and concomitant decrease in summer precipitation could accelerate natural colonisation processes. This will further extend the expected effects of global warming over biodiversity and ecological evolution of Alpine aquatic ecosystems.