Methane and carbon dioxide emissions from tropical reservoirs:: Significance of downstream rivers

Methane (CH4) and carbon dioxide (CO2) concentrations and water-air fluxes were measured in three tropical reservoirs and their respective rivers downstream of the dams. From reservoirs, CH4 and CO2 flux were in the range of 3 +/- 2 and 254 +/- 392mmol. m(-2).d(-1), respectively. Rivers downstream of dams were significantly enriched in CH4 and CO2 originating from reservoir hypolimnions. From rivers, CH4 and CO2 flux were in the range of 60 +/- 38 and 859 +/- 400 mmol. m(-2).d(-1), respectively. Despite their relatively small surfaces, rivers downstream of dams accounted for a significant fraction ( 9 - 33% for CH4 and 7 - 25% for CO2) of the emissions across the reservoir surfaces classically taken into account for reservoirs. A significant fraction of CH4 appeared to degas at the vicinity of the dam ( turbines and spillways), although it could not be quantified.