Are hydroelectric reservoirs significant sources of greenhouse gases

Estimates suggest that, per unit of energy produced, greenhouse-gas flux to the atmosphere from some hydroelectric reservoirs may be significant compared to greenhouse-gas emission by fossil-fuelled electricity generation. Greenhouse gases (CO2 and CH4) are produced during bacterial decomposition of flooded peat and forest biomass. The amount emitted will be positively related to the area flooded. Early data from hydroelectric reservoirs in northern Canada support this hypothesis. Our hypothesis is based primarily on two of our past studies which show that both upland forests and peatlands are sites of intense microbial decomposition and greenhouse-gas production when they become covered with water. During the summer of 1992, the first preliminary data were obtained that support our hypothesis. At 12 sampling locations on the LaGrande II-BoydSakami Reservoir complex in northern Quebec, both the CO2 and CH4 were found to be evading to the atmosphere. CO2 concentrations were 2-3 times above atmospheric equilibrium at all sampling sites. This is in contrast to two large lakes, Nipigon and Superior, where CO2 was being absorbed from the atmosphere throughout the ice-free season. Surface CH4 concentrations were 0.05-1.1 μmol L-1 with most sites having concentrations higher than in natural, stratified Canadian shield lakes. Further measurements are required to determine annual fluxes. (19 refs., 3 figs., 2 tabs.)