[1] The emissions of carbon dioxide (CO 2 ) and methane (CH 4 ) from the Petit Saut hydroelectric reservoir (Sinnamary River, French Guiana) to the atmosphere were quantified for 10 years since impounding in 1994. Diffusive emissions from the reservoir surface were computed from direct flux measurements in 1994, 1995, and 2003 and from surface concentrations monitoring. Bubbling emissions, which occur only at water depths lower than 10 m, were interpolated from funnel measurements in 1994, 1997, and 2003. Degassing at the outlet of the dam downstream of the turbines was calculated from the difference in gas concentrations upstream and downstream of the dam and the turbined discharge. Diffusive emissions from the Sinnamary tidal river and estuary were quantified from direct flux measurements in 2003 and concentrations monitoring. Total carbon emissions were 0.37 ± 0.01 Mt yr À1 C (CO 2 emissions, 0.30 ± 0.02; CH 4 emissions, 0.07 ± 0.01) the first 3 years after impounding (1994)(1995)(1996) and then decreased to 0.12 ± 0.01 Mt yr À1 C (CO 2 , 0.10 ± 0.01; CH 4 , 0.016 ± 0.006) since 2000. On average over the 10 years, 61% of the CO 2 emissions occurred by diffusion from the reservoir surface, 31% from the estuary, 7% by degassing at the outlet of the dam, and a negligible fraction by bubbling. CH 4 diffusion and bubbling from the reservoir surface were predominant (40% and 44%, respectively) only the first year after impounding. Since 1995, degassing at an aerating weir downstream of the turbines has become the major pathway for CH 4 emissions, reaching 70% of the total CH 4 flux. In 2003, river carbon inputs were balanced by carbon outputs to the ocean and were about 3 times lower than the atmospheric flux, which suggests that 10 years after impounding, the flooded terrestrial carbon is still the predominant contributor to the gaseous emissions. In 10 years, about 22% of the 10 Mt C flooded was lost to the atmosphere. Our results confirm the significance of greenhouse gas emissions from tropical reservoir but stress the importance of: (1) considering all the gas pathways upstream and downstream of the dams and (2) taking into account the reservoir age when upscaling emissions rates at the global scale.

No abstract

International audienceWe have measured simultaneously the methane (CH4) and carbon dioxide (CO2) surface concentrations and water–air fluxes by floating chambers (FC) in the Petit-Saut Reservoir (French Guiana) and its tidal river (Sinnamary River) downstream of the dam, during the two field experiments in wet (May 2003) and dry season (December 2003). The eddy covariance (EC) technique was also used for CO2 fluxes on the lake. The comparison of fluxes obtained by FC and EC showed little discrepancies mainly due to differences in measurements durations which resulted in different average wind speeds. When comparing the gas transfer velocity (k600) for a given wind speed, both methods gave similar results. On the lake and excluding rainy events, we obtained an exponential relationship between k600 and U10, with a significant intercept at 1.7 cm h− 1, probably due to thermal effects. Gas transfer velocity was also positively related to rainfall rates reaching 26.5 cm h−1 for a rainfall rate of 36 mm h− 1. During a 24-h experiment in dry season, rainfall accounted for as much as 25% of the k600. In the river downstream of the dam, k600 values were 3 to 4 times higher than on the lake, and followed a linear relationship with U10

International audienceMethane (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 +/- 392 mmol.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

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.