Effects of hydropower management on the sediment composition and metabolism of a small Alpine lake

The ecological equilibrium of water reservoirs may differ from that of natural lakes. We questioned this difference by analysing the sediments of a small oligotrophic Alpine lake, whose management was modified for hydroelectric production since 1976. Corne Lake is formed by a shallow depression connected to a deep depression. The hydropower management induced water level fluctuations (+2 m in summer; a 8 m in winter) that emptied the shallow depression during the winter months and promoted the erosion of littoral soils and tributary channel sediment and the sedimentation in the deep depression. The sediment of the original lake was a low-density organic mud. The sediment composition varied according to 3 phases, which chronology is debated. During a first phase we measured an increase in the ratio of Diatom/Chrysophycea and bioavailable P, as well as a decrease in the C/N ratio and bulk radiocarbon age of the sediment, suggesting a trophic surge. A second phase was characterised by a high rate of mineral sedimentation, an increase of benthic diatom genera in the deep depression of the lake and acidophilic diatoms in the shallow depression. In the third phase covering the last upper cm of the cores, the sediment tended to return to its initial composition, but the algae community differed from its initial state. We suggest that the management of Alpine lakes as reservoirs induce long-Term ecological changes in relation to water level fluctuations and littoral habitats degradation.