Abstract. In the present study, we measured independently CH4 ebullition and diffusion in the footprint of an eddy covariance system (EC) measuring CH4 emissions in the Nam Theun 2 Reservoir, a recently impounded (2008) subtropical hydroelectric reservoir located in the Lao People's Democratic Republic (PDR), Southeast Asia. The EC fluxes were very consistent with the sum of the two terms measured independently (diffusive fluxes + ebullition = EC fluxes), indicating that the EC system picked up both diffusive fluxes and ebullition from the reservoir. We showed a diurnal bimodal pattern of CH4 emissions anti-correlated with atmospheric pressure. During daytime, a large atmospheric pressure drop triggers CH4 ebullition (up to 100 mmol m−2 d−1), whereas at night, a more moderate peak of CH4 emissions was recorded. As a consequence, fluxes during daytime were twice as high as during nighttime. Additionally, more than 4800 discrete measurements of CH4 ebullition were performed at a weekly/fortnightly frequency, covering water depths ranging from 0.4 to 16 m and various types of flooded ecosystems. Methane ebullition varies significantly seasonally and depends mostly on water level change during the warm dry season, whereas no relationship was observed during the cold dry season. On average, ebullition was 8.5 ± 10.5 mmol m−2 d−1 and ranged from 0 to 201.7 mmol m−2 d−1. An artificial neural network (ANN) model could explain up to 46% of seasonal variability of ebullition by considering total static pressure (the sum of hydrostatic and atmospheric pressure), variations in the total static pressure, and bottom temperature as controlling factors. This model allowed extrapolation of CH4 ebullition on the reservoir scale and performance of gap filling over four years. Our results clearly showed a very high seasonality: 50% of the yearly CH4 ebullition occurs within four months of the warm dry season. Overall, ebullition contributed 60–80% of total emissions from the surface of the reservoir (disregarding downstream emissions), suggesting that ebullition is a major pathway in young hydroelectric reservoirs in the tropics.

-A fish population monitoring was carried out in the reservoir and tributaries of the hydropower project Nam Theun 2 (Lao PDR) at the end of the warm and dry seasons and at the end of the warm and wet seasons between 2008 and 2013. The study focused on three aquatic systems (river upstream, transition area, and reservoir) and different drowned habitats (dense/light forests and agricultural soils) associated or not with the presence of villages. The reservoir fish population was expected to follow three distinct phases, which should be respectively a rapid increase in biomass and abundance (trophic upsurge) followed by a decline and a stabilization of the population. This succession appeared to be different to the commonly observed scenario in terms of biomass, abundance and taxonomic richness compared to other temperate and tropical reservoirs. The indicator species (H. macrolepidota and P. carinatus) did not show changes in their size range since impoundment. The presence of structured habitats (e.g. inundated forest) favoured the fish population in terms of biomass and abundance. The fish population in the transition area and the river upstream improved quickly after the impoundment and presented higher taxonomic richness compared to the reservoir. Finally, the transition area appeared to be the most productive area within the Nam Theun 2 Reservoir especially if it remained outside of fishing activities. Key words -fish population, impoundment, neotropical reservoir, habitatsRésumé -Un suivi des populations piscicoles a été réalisé au sein du réservoir hydro-élec-trique de Nam Theun 2 (Laos) et de ses affluents à la fin de la saison chaude et sèche et à la fin de la saison chaude et humide de 2008 à 2013. Trois systèmes aquatiques ont été étu-diés (rivière amont, zone de transition et réservoir) ainsi que différents habitats inondés (forêt dense/clairsemée, sols agricoles) associés ou non à la présence de villages. Il était attendu

Inland waters in general and specifically freshwater reservoirs are recognized as source of CH 4 to the atmosphere. Although the diffusion at the air-water interface is the most studied pathway, its spatial and temporal variations are poorly documented.We measured fortnightly CH 4 concentrations and physico-chemical parameters at a surface area representative of less than 1 % of the total reservoir surface. We highly recommend measurements of diffusive fluxes around water intakes in order to evaluate if such results can be generalized. 25can only be captured by high frequency monitoring. Spatial heterogeneity of CH 4 emissions at the surface of reservoirs is also very high. It mostly depends on the spatial vari-11352 Abstract Introduction Conclusions References Tables twice the volume of the reservoir (3908 Mm 3 ). A continuous flow of 2 m 3 s −1 (and occa-11353 Abstract Introduction Conclusions ReferencesTables 20 mercial gas standards (10, 100 and 1010 ppmv, Air Liquid "crystal" standards) were injected after analysis of every 10 samples for calibration. Duplicate injection of samples showed reproducibility better than 5 %. Abstract IntroductionConclusions References Tables and Borges et al. (2004) and (2) k 600 determined in area with comparable hydrology/hydrodynamics. Abstract Introduction Conclusions ReferencesTables season, the highest aerobic oxidation rate was observed in 2012. Abstract Introduction Conclusions ReferencesTables hydroelectric project (Lao PDR) and the associated environmental monitoring programmes, Hydroécol. Appl.,

Abstract. Inland waters in general and freshwater reservoirs specifically are recognized as a source of CH4 into the atmosphere. Although the diffusion at the air–water interface is the most studied pathway, its spatial and temporal variations are poorly documented. We measured temperature and O2 and CH4 concentrations every 2 weeks for 3.5 years at nine stations in a subtropical monomictic reservoir which was flooded in 2008 (Nam Theun 2 Reservoir, Lao PDR). Based on these results, we quantified CH4 storage in the water column and diffusive fluxes from June 2009 to December 2012. We compared diffusive emissions with ebullition from Deshmukh et al. (2014) and aerobic methane oxidation and downstream emissions from Deshmukh et al. (2016). In this monomictic reservoir, the seasonal variations of CH4 concentration and storage were highly dependent on the thermal stratification. Hypolimnic CH4 concentration and CH4 storage reached their maximum in the warm dry season (WD) when the reservoir was stratified. Concentration and storage decreased during the warm wet (WW) season and reached its minimum after the reservoir overturned in the cool dry (CD) season. The sharp decreases in CH4 storage were concomitant with extreme diffusive fluxes (up to 200 mmol m−2 d−1). These sporadic emissions occurred mostly in the inflow region in the WW season and during overturn in the CD season in the area of the reservoir that has the highest CH4 storage. Although they corresponded to less than 10 % of the observations, these extreme CH4 emissions (> 5 mmol m−2 d−1) contributed up to 50 % of total annual emissions by diffusion. During the transition between the WD and WW seasons, a new emission hotspot was identified upstream of the water intake where diffusive fluxes peaked at 600 mmol m−2 d−1 in 2010 down to 200 mmol m−2 d−1 in 2012. The hotspot was attributed to the mixing induced by the water intakes (artificial mixing). Emissions from this area contributed 15–25 % to total annual emissions, although they occur in a surface area representative of less than 1 % of the total reservoir surface. We highly recommend measurements of diffusive fluxes around water intakes in order to evaluate whether such results can be generalized.