Effect of sporadic destratification, seasonal overturn, and artificial mixing on CH₄ emissions from a subtropical hydroelectric reservoir

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.