On Wednesday, December 12, 2018, Jim FELIX FAURE, PhD student in biodiversity, ecology, environment at the Centre Alpin de Recherche sur les réseaux Trophiques des Écosystèmes Limniques (CARRTEL), will submit his thesis " Devenir des sols ennoyés sous les réservoirs de barrage. Effects on sedimentation and the ecology of water bodies ".
The defense will take place at 2pm, in Amphithéâtre 1 of the IUT de Chambéry, on the Bourget-du-Lac campus.
Summary of the thesis
On a global scale, dam reservoirs cover around 0.26,106 km2. Their filling transforms terrestrial ecosystems into aquatic ones, and causes major modifications to biogeochemical cycles on both local and global scales. This thesis focuses on the evolution of soils flooded by dam reservoirs and their influence on sediment flux, trophic status and reservoir greenhouse gas emissions. Three sites were studied in order to identify these different aspects: two large mesotrophic reservoirs in the mid-mountains (Sarrans, 1934, Massif Central) and lowlands (Guerlédan, 1931, Brittany), and a small oligotrophic subalpine lake modified for hydroelectric production (Corne, 1976, Northern Alps).
Hydroelectric use leads to a seasonal tidal range, which conditions increased soil erosion in the tidal zone and transfer of these materials to the water body. At Sarrans, soil erosion in the annual drawdown zone accounts for around a third of the total mass of sediment accumulated since impoundment. In the small, modified Corne lake, sediment analysis shows the arrival of an initially organic, then progressively mineral flow from the soils in the tidal zone, and the exhaustion of this flow after around forty years. These material flows should therefore be taken into account when calculating sediment accumulation in reservoirs.
In the area still under water, soils evolve in a hypoxic environment and are gradually covered by sediment. At both Guerlédan and Sarrans, morphological and geochemical analysis of permanently submerged soils reveals dull-colored soils, in which the original pedological horizons can still be distinguished. Their pH has risen in contact with the water mass. The spodic podzol horizons have lost most of their amorphous iron, while their amorphous Al content has changed little. Eighty years after impoundment, these soils have lost between 40% and 50% of their original carbon and nitrogen stock. This carbon flux, if released in the form of CO2 or CH4, can quantitatively explain most of the peak in GHG emissions from reservoirs after impoundment. Carbon flows transferred to the sediment by soil erosion in the tidal zone, and potentially mineralized in the area still under water, must therefore be deducted from the sediment carbon storage balances traditionally attributed to dam reservoirs.
At Sarrans (and probably Guerlédan), compared with the inflows from the tributary and turbines at the reservoir outlet, the transfer of carbon and nitrogen from the soils in the tidal zone to the water body does not appear to be sufficient to fuel a trophic revival following flooding; unless erosion of the soils in the annual tidal zone, and particularly of the organic horizons, takes place rapidly after impoundment. At Lac de la Corne, the paleo-limnological approach indicates that soil carbon fluxes are metabolized by the lake. Microalgal communities show a rise in the trophic status of the lake followed by a progressive depression, thus confirming the "Trophic upsurge" theory.
Dam reservoirs are exceptional sites for measuring the evolution of drowned soils over variable time periods, and provide valuable archives of past soil properties.