Philippe Gosse

Carbon dioxide and methane emissions and the carbon budget of a 10-year old tropical reservoir (Petit Saut, French Guiana)

emissions, 0.07 ± 0.01) the first 3 years after impounding (1994–1996) and then decreased to 0.12 ± 0.01 Mt yr 1 C( CO2, 0.10 ± 0.01; CH4, 0.016 ± 0.006) since 2000. On average over the 10 years, 61% of the CO2 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. CH4 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 CH4 emissions, reaching 70% of the total CH4 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.

Gaseous emissions and oxygen consumption in hydroelectric dams: A case study in French Guyana

Methane, carbon dioxide, and hydrogen sulfide emissions from the hydroelectric dam of Petit Saut on the Sinnamary River in French Guyana have been measured over a 2 year period. Since the beginning of the reservoir filling (January 1994), 300 km 2 of tropical forest have been submerged. Emissions of CH 4 by diffusion and by bubbling into the atmosphere or by degassing of the water released into the river, as well as the stock of dissolved gases in the lake, and their temporal evolutions were determined. Maximum emissions of 800 t CH per day were reached in February 1995, corresponding to dissolved CH 4 concentrations of 14 mg 4 L -1 in the water column. The biological oxidation of methane results in a strong oxygen consumption in lake and river waters. Total emissions of CH 4 and CO 2 from January 1994 to December 1995 were calculated from the whole data set, which also allows us to calculate the total carbon loss since reservoir filling. About 10% of the carbon stored in soil and vegetation was released in gaseous form within 2 years.

Long‐term greenhouse gas emissions from hydroelectric reservoirs in tropical forest regions

The objective of this work is to quantify long-term emissions of two major greenhouse gases, CO2 and CH4, produced by the decomposition of the flooded organic matter in tropical artificial reservoirs. In a previous paper [Galy-Lacaux et al., 1997], gas emissions from the tropical reservoir of Petit Saut (French Guiana) were quantified over the first two years after impounding. This work presents emission fluxes and distributions of dissolved methane and carbon dioxide measured in the reservoir of Petit Saut over three and a half years, since the beginning of impounding (1994) and during operation (1995–1997). To assess long term emissions, an experimental campaign was conducted on four hydroelectric reservoirs (Taabo, Buyo, and Ayame I and II) built between 1960 and 1980 in the Ivory Coast. Average dissolved CH4 concentration in the water column of the Petit Saut reservoir first increased, up to a maximum of 14 mg L−1, in May 1995. Then the time course of dissolved CH4 over the three and a half year period, showed periodical variations. These changes were related to changes in the inlet water flow and the residence time of water in the reservoir. In the older African reservoirs, average dissolved methane concentrations were lower and ranged between 0.20 and 0.32 mg L−1. The whole data set allows us to propose an analytical algorithm in order to predict the time course of dissolved CH4 concentration in the Petit Saut reservoir. Temporal variations of total CH4 and CO2 emissions from the reservoir over three and a half years were extrapolated with this algorithm to calculate long term carbon losses. Over a 20-year period the estimated carbon losses in the form of CO2 and CH4 were dominated by the outlet fluxes of dissolved gases (2160 ± 400 Gg (C)), and they correspond to a total net carbon loss of 3.2 Tg (C). The contribution of the Petit Saut reservoir to greenhouse gas emission, over 20 years, is estimated to be 66 ± 20 Tg of CO2 equivalent (56 Tg as CH4 and 9.7 Tg as CO2).

Bacterial ecology of a young equatorial hydroelectric reservoir (Petit Saut, French Guiana)

This work aims at studying the microbial and physical-chemical changes occurring in an equatorial hydroelectric reservoir Petit Saut (French Guiana), over three years, from the time it first filled.Since filling in January 1994, almost the whole water column has remained anoxic, with high concentrations of reduced elements (CH4, iron sulphides, H2S) originating from degradation of the submerged primary forest. These elements deoxygenated the water drained from the dam, as they were biologically and chemically oxidized in the river. Two major physiological guilds of microorganisms occurred. They showed a characteristic stratified arrangement in the lake, below and above an oxycline. Since flooding, we observed a cyclic development of phototrophic bacteria, assumed to be sulphur-oxidizers below the oxycline. The growth of this physiological guild seems to be strongly influenced by sulphide production and by the alternation between rainy and dry seasons. At the oxycline, the population of methane-oxidizing bacteria did not vary in the same way as the green sulphur bacteria. After a lag-phase, which was probably due to inhibition by excessive illumination in the upper water column in 1994, they developed and completely stopped diffusive methane emission in February 1995. The development of both bacterial guilds is directly correlated to changes in the level of the corresponding reduced elements from the bottom and dissolved in the lake water. The decreasing production of CH4 and sulphides induces a progressive extinction of methanotrophic bacteria and phototrophic sulphur-oxidizing bacteria. Meanwhile, the water is becoming clearer and algal populations proliferate in the upper water column, constituting an autochtonous source of organic carbon. Slowly, this aquatic ecosystem is reaching a low-activity equilibrium level related to reduced element production. It is evolving from a system ruled by an anaerobic organotrophic metabolism to an ecosystem governed by phototrophy.

Impact of Methane Oxidation in Tropical Reservoirs on Greenhouse Gases Fluxes and Water Quality

This chapter presents a summary of water quality data (physico-chemical) from 10 years of measurements in the Petit Saut hydroelectric reservoir in French Guiana. Methane oxidation in and downstream of the reservoir are of particular interest. In the first part of the paper we discuss both the primary factors influencing the water quality and the patterns of stratification, methane production and oxidation in the reservoir. Secondly, we present data of methane emissions and oxidation downstream of the dam. We demonstrate that the oxidation of the dissolved CH4 was a major oxygen consumer downstream of the dam. The results indicate that the aerating weir built in the plant outlet canal guarantees the minimum regulatory concentration of 2 mg·L−1 of dissolved oxygen as delineated by the scientific community of Petit Saut, following observations of the resistance to hypoxia in a tropical environment. This long term database, which helped in detecting changes over time (dissolved gases concentrations, CH4 oxidation velocity) will be used to improve the models developed to simulate both water quality and greenhouse gas emissions in a tropical reservoir environment.

Evolution of physico-chemical water quality and methane emissions in the tropical hydroelectric reservoir of Petit Saut (French Guiana)

Hydroelectric dam at Petit Saut The Petit Saut dam is located on the Sinnamary River in French Guiana. lt was constructed by the National Centre for Hydroelectric Equipment (CNEH) of Electricité de France (EDF). January 1994 marked the beginning of the filling phase. At full supply level (35 m in depth), the reservoir stretches across approximately 350 km of wet rainforest left uncleared. The total storage capacity is 3.5 X 10 m (useful storage between 35 and 31.5 m in depth, the minimum operating levels). The installed hydroelectric power system yields 116 MW from four groups of turbines (29 MW each). The average flow o f the Sinnamary River is 267 m s-. The retention time is approximately 5 months (StsSAK!AN 1997). In 1995, an aerating weir was built in the plant oudet canal in order to improve air-water gas exchanges and to increase the oxygen content in the Sinnamary river (GALY-LACAUX 1996, GossE 1994, GossE & GREGOIRE 1997).