| The aim of this thesis is to contribute to the study of greenhouse gas emissions and the carbon cycle of boreal reservoirs. The central question is the extent to which the degradation of terrigeneous organic matter originating in the drainage basin contributes to greenhouse gas emissions from reservoirs. Those are frequently attributed exclusively to the degradation of organic matter in flooded soils. A carbon cycle model is devised in order to quantify terrigeneous carbon fluxes in a large hydroelectric complex. This model builds on the knowledge and data acquired by our research team in the course of 10 years of investigation of boreal reservoirs. The field work for this thesis was conducted on three boreal reservoirs of different ages in Quebec (Laforge-1, Robert-Bourassa and Cabonga) and on natural lakes in their drainage basin. The analysis of elemental C/N and C/P ratios, of delta13C and delta15N isotopic signatures and of lignin biomarkers completed by physico-chemical water-column measures was used in order to characterize and quantify all size fractions of aquatic organic matter. The use of reverse osmosis made it possible to sample the entirety of dissolved organic matter, unlike previous studies. Organic matter concentrations and geochemical signatures reveal that a fraction of terrigeneous organic matter sufficient to account for measured CO2 emissions is mineralized in the reservoirs. This results in lower concentrations of terrigeneous organic matter and a higher degree of degradation in reservoirs than in lakes. Compared to freshly prepared forest soil leachates, both lake and reservoir organic matter appears highly degraded. Trough the analysis of elemental ratios and isotopic signatures, differences between lakes and reservoirs, in particular for the youngest reservoirs, could be identified. Those differences are consistent with the trophic upsurge phenomenon characterizing young reservoirs, but can also arise from morphological and hydrodynamic features of the reservoirs. The carbon cycle model, spanning the entire La Grande hydroelectric complex for simulation periods between 50 and 100 years yields a coherent picture of terrigeneous organic carbon fluxes and concentrations. This model can serve as a tool for the evaluation of the total change in continental-atmospheric carbon fluxes as a result of reservoir creation. Such a model can be used as an instrument in the assessment of carbon sources and sinks pertaining to land-use and land-use change in the context of national carbon accounting.; Keywords: greenhouse gas emissions, hydroelectric reservoirs, organic matter, lignin biomarkers, carbon cycle modeling. |
