Climate Change and Alpine Rivers

Climate change is rapidly altering the natural features of our rivers, intensifying extreme events such as droughts and floods. Alongside global-scale factors, we find local causes, such as water captation for human purposes. River droughts, in particular, are becoming increasingly common even in the alpine environments. In this contect, a recent national PRIN project (aptly named NOACQUA - Responses of Communities and Ecosystem Processes in Watercourses) carried out by the Alpstream research group in collaboration with UNIPR and UNIFE, has highlighted the potential impact of droughts on river functionality and biological communities inhabiting alpine streams that only recently have been experiencing hydrological intermittence.

In this project, we focused in particular on biodiversity, functionality, and resilience mechanisms of benthic communities. Since spring 2017, biological samplings were conducted in 13 rivers in the Piedmont Alps, where a perennial reach (control) and an intermittent section (disturbed) were identified. In these reaches, two sampling campaigns were carried out, analyzing key environmental parameters and characterizing diatom and benthic macroinvertebrate communities. Additional in-depth campaigns were conducted in three rivers: the Po, Pellice, and Varaita. The findings, summarized in 10 peer-reviewed scientific publications, demonstrated that drought events negatively impact the functionality of alpine lotic systems, altering internal primary productivity (estimated as benthic chlorophyll concentration) and reducing the degradation capacity of terrestrial organic matter. The shift to an intermittent regime leads to biodiversity loss and taxonomic and functional homogenization within alpine communities, which endures as a biological “memory” after the drought. This underscores the lack of resistance and resilience mechanisms of alpine benthic communities in comparison with those colonizing naturally intermittent systems, such as those characterizing the mediterranean area.

Regarding benthic diatoms, it was observed that the hydrological cycle typical of intermittent rivers (i.e. water flow - lentification - drought - rewetting), significantly alters benthic communities both taxonomically and functionally. Moreover, after drought, the recovery time for diatom communities in alpine rivers is significantly longer than what observed in naturally intermittent systems.

The resilience of benthic macroinvertebrates was analyzed by investigating the importance of two potential refuges exploited during drought: the interstitial and the hyporheic zones. For this purpose, three piezometers were installed in the Saluzzo reach of the Po River, an area affected by the seasonal droughts.

Using these instruments, we collected organisms and water samples from August 2017 to August 2019. Analyses, conducted in collaboration with POLITO, UNIPR, and FEM, revealed that:

i) very few organisms are able to use the subsurface environments to survive drought periods;

ii) macroinvertebrate recolonization occurs primarily via drift

To assess the importance of residual pools for macrobenthos recolonization, we carried out an experiment in the FEM’s artificial channels, which furtherly demonstrated that pools play a minor role during the recolonization process, while drift remains the primary mechanism.