The carbon cycle of Quebec boreal reservoirs investigated by elemental compositions and isotopic values

Dissolved and particulate organic matter (POM) of three Quebec boreal reservoirs of different ages (Laforge-1, 7 years; Robert-Bourassa, 25 years and Cabonga, 70 years at the time of sampling) and sets of lakes from the same watersheds was analyzed using organic carbon concentrations, C/N and C/P elemental composition, δ13C and δ15N isotopic values. The reservoirs are characterized by lower dissolved organic carbon concentrations with lower C/N ratios and by lower δ13C and higher δ15N in POM. They contain more autochthonous dissolved organic matter and less terrigenous organic matter than the lakes. Some of those characteristics are more pronounced in the younger than in the older reservoirs. The differences can be attributed to two causes: (1) more extended degradation of terrigenous organic matter, caused by an increase in residence time; and (2) differences in food web structure resulting from the phenomenon known as trophic upsurge, in newly flooded reservoirs. The results indicate that some effects of reservoir creation on the carbon cycle are short term perturbations, others however long term features of those reservoirs. The implications of these findings for CO2 emissions from reservoirs are discussed.