Given the lack of knowledge about the functioning of high-altitude aquatic ecosystems, it has become urgent to characterize the relationship between hydraulic conditions and aquatic organisms, particularly invertebrates, in order to model and predict the response of alpine rivers to hydrological alterations.

In recent decades, rapid hydrological changes have occurred in alpine catchments due to the combined effects of climate change and water abstraction, with major implications on aquatic biodiversity. While the diminution of seasonal snow cover reduces meltwater flow in spring, glacier shrinkage leads to a significant decrease in annual glacier runoff, especially in summer. The reduction of the meltwater contribution to streamflow at both the stream and the catchment scales also modifies stream environmental conditions (e.g. turbidity, temperature). Quantifying the relative effects of all environmental variables on aquatic communities is required to predict the ecosystem responses to hydrological alterations. To date, no study examined simultaneously the effects of physico-chemistry, hydraulics, and hydrology on the distribution of alpine communities, in particular macroinvertebrates. Impacts of flow alterations can be mitigated by implementing environmental flows, defined as the quantity, timing and quality of water flows required to sustain river ecosystems while satisfying societal needs.

Among existing tools, hydraulic habitat models allow predicting changes in habitat suitability for aquatic species under different flow scenarios. They couple hydraulic models of stream reaches with hydraulic preference models relating the abundance of species to microhabitat hydraulics. Initially developed for fish, hydraulic preference models have never been adapted to alpine, often fishless, streams. It is urgent to develop new hydraulic preference models on alpine species to assess the impact of flow alterations on alpine stream ecosystems. This thesis aims to: (1) identify the main environmental drivers structuring alpine macroinvertebrate communities at the catchment scale, (2) develop hydraulic preference models on key macroinvertebrate species, (3) apply the new alpine hydraulic habitat models to a case study of a small hydropower plant in the Alps. In a first study based on 66 stream sites within three alpine catchments with different glacial influences, models performed on 23 taxa indicated that high turbidity had significant negative effect in 83% of models, and high flow velocity and summer flow had significant (mainly negative) effect in 43% of models. In a second study based on 30 microhabitats samples in five alpine streams, we developed hydraulic preference models for 41 taxa. Half of them had significant response to shear stress and flow velocity, with a good degree of transferability among all types of streams. At least 23% of the most abundant species were rheophilic in alpine streams, thereby threatened by flow reduction, including the glacial stream specialists which will be also affected by glacier retreat. In a third study based on 13 sampling dates (2018-2021) upstream and downstream the intake of a small hydropower plant located at 1,500 m a.s.l., we applied the new alpine hydraulic habitat models, and showed that changes in hydraulic habitats partly explained the lower macroinvertebrate abundances observed in the by-passed reach. Understanding the relationships between macroinvertebrate spatial distribution and environmental variables at both catchment and microhabitat scales helps to predict how communities will be affected by habitat alterations, and thus allow a better management of water resources in the mountains to mitigate the impacts on the alpine biodiversity.

References :

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Cite the thesis

Juliette Becquet. Impacts des altérations hydrologiques sur les macroinvertébrés des cours d'eau alpins. Biodiversité. Université Claude Bernard - Lyon I, 2022. Français. ⟨NNT : 2022LYO10050⟩. ⟨tel-04047007⟩

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