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

Abstract

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, d 13 C and d 15 N isotopic values. The reservoirs are characterized by lower dissolved organic carbon concentrations with lower C/N ratios and by lower d 13 C and higher d 15 N 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 differ- ences 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.