My PhD thesis focuses on carbon storage and accumulation in lake sediments. Carbon in sediments is an important part of the carbon cycle in aquatic ecosystems, particularly in lakes. Our project aims to establish a robust method to estimate C storage in lakes and to propose model(s) that allow us to extrapolate this storage to the regional scale. In the first chapter of my thesis, I develop a methodology to calculate a fine-resolution bathymetry of sediment thickness for 13 lakes in the Eastmain region (Québec). In this region, basin morphology was the best predictor of C storage in the sediments. The second chapter of my thesis examines long-term C storage across 4 regions, 3 of which are in Québec, the other in a boreal region of Sweden. Our results suggest that C storage is higher in boreal than in temperate regions. However, C storage in Swedish lakes is more similar to that of Quebec Laurantians lakes than Quebec boreal lakes. There were regional differences in which lake properties could be used to predict C storage, but across these 4 regions, the dynamic ratio was a consistently good predictor of C storage. Finally, in the third chapter, I compare C accumulation rates at different temporal scales: short-term POC sinking flux, medium-term centennial C accumulation, and long-term millennial C accumulation in the Eastmain region. The C accumulation rate diminishes as the temporal scale increases, but the magnitude of this accumulation rate is heavily dependent on whether it was derived from a single core or from values integrated over the whole basin. The average amount of sinking POC that gets incorporated into long-term sediment storage (burial efficiency) is around 22% in these systems. Interestingly, when C storage in the sediment is calculated as the POC sinking flux minus C lost via benthic respiration, we get values that strongly resemble the long-term C storage values measured in this study. Although C storage in the sediments represents only 4 to 8% of outgoing CO2 fluxes to the atmosphere, it remains an important long-term sink in the C cycle of aquatic systems.
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