On Wednesday, December 12, 2018, Jim FELIX FAURE, PhD student in biodiversity, ecology, and environment atthe Alpine Center for Research on Trophic Networks in Limnic Ecosystems (CARRTEL), will defend his thesis entitled " The fate of soils flooded by dam reservoirs. Effects on sedimentation and the ecology of water bodies."
The defense will take place at 2 p.m. in Amphitheater 1 of the IUT in Chambéry, on the Bourget-du-Lac campus.
Thesis summary
Globally, dam reservoirs cover approximately 0.26 million square kilometers. Filling these reservoirs transforms terrestrial ecosystems into aquatic ecosystems and causes major changes in biogeochemical cycles at both the local and global levels. This thesis focuses on the evolution of soils flooded by dam reservoirs and the influence of these soils on sediment flows, trophic status, and greenhouse gas emissions from reservoirs. Three sites were studied in order to identify these different aspects: two large mesotrophic reservoirs in the mid-mountains (Sarrans, 1934, Massif Central) and on the plains (Guerlédan, 1931, Brittany) and a small oligotrophic subalpine lake modified for hydroelectric production (Corne, 1976, Northern Alps).
Hydroelectric use leads to seasonal tidal action, which causes increased soil erosion in the tidal zone and a transfer of these materials to the water body. At Sarrans, soil erosion in the annual tidal zone contributes to about one-third of the total sediment mass accumulated since the reservoir was filled. In the small modified lake of Corne, sediment analysis shows the arrival of a flow that is initially organic and then gradually becomes mineral, originating from the soils of the tidal zone, and the depletion of this flow after about 40 years. These material flows should therefore be taken into account when calculating sediment accumulation in reservoirs.
In the area that is always under water, the soils evolve in a hypoxic environment and are gradually covered with sediments. In Guerlédan, as in Sarrans, morphological and geochemical analysis of permanently submerged soils shows dull-colored soils, but in which the original soil horizons can still be distinguished. Their pH has risen on contact with the water mass. The spodic horizons of podzols have lost most of their amorphous iron, while their amorphous Al content has changed little. Eighty years after flooding, these soils have lost between 40% and 50% of their original carbon and nitrogen stocks. This carbon flux, if released in the form of CO2 or CH4, can quantitatively explain most of the peak GHG emissions from reservoirs after flooding. The carbon flows transferred to the sediment by erosion of the soils in the tidal zone, and potentially mineralized in the zone that is always under water, must therefore be deducted from the sedimentary carbon storage balances traditionally attributed to dam reservoirs.
At Sarrans (and probably Guerlédan), compared to the inflows from the tributary and turbined at the outlet of the reservoir, the transfer of carbon and nitrogen from the soils of the tidal zone to the water body does not seem sufficient to fuel a trophic rebound following impoundment; unless soil erosion in the annual tidal zone, particularly of organic horizons, occurs rapidly after flooding. At Lac de la Corne, the paleolimnological approach indicates that soil carbon flows are metabolized by the lake. Microalgal communities show an increase in the lake's trophic status followed by a gradual decline, thus confirming the "trophic upsurge" theory.
Dam reservoirs are exceptional sites for measuring the evolution of flooded soils over varying periods of time and provide valuable archives of past soil properties.