Effect of sporadic destratification, seasonal overturn, and artificial mixing on CH<sub>4</sub> emissions from a subtropical hydroelectric reservoir

Abstract

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.

Carbon dioxide emissions from the flat bottom and shallow Nam Theun 2 Reservoir: drawdown area as a neglected pathway to the atmosphere

“…The dynamics of CO 2 in the NT2R is highly dependent on the hydrology and hydrodynamics of the reservoir as it has already been described for CH 4 (Guérin et al, 2016). During the warm seasons (WD and WW) when the water column is thermally stratified, the vertical profiles of CO 2 concentration in the water column are similar to profiles obtained in other tropical or subtropical reservoirs (Abril et al, 2005;Guérin et al, 2006;Chanudet et al, 2011;Kemenes et al, 2011), but also boreal reservoirs (Demarty et al, 2011). The high concentrations measured in the hypolimnion suggest that the main source of CO 2 is located at the bottom and very likely in the flooded soils, vegetation and sediments, whereas the decrease in CO 2 toward the surface suggest both consumption by primary production and/or loss to the atmosphere (Galy-Lacaux et al, 1997b;St Louis et al, 2000;Abril et al, 2005;Guérin et al, 2008;De Junet et al, 2009;Barros et al, 2011;Chanudet et al, 2011;Teodoru et al, 2011).…”

“…This assumption is reinforced by the fact that during the same sampling, hot moments of CH 4 emissions that should have occurred at the same time were captured (Guérin et al, 2016). As observed in most tropical and subtropical reservoirs, the higher concentrations were observed during the warm seasons due to long residence time of water and warmer conditions enhancing CO 2 build-up (Abril et al, 2005;Chanudet et al, 2011;Kemenes et al, 2011), whereas the lowest were found after reservoir overturn when the water outgassed (Chanudet et al, 2011). A significant shift in the carbon biogeochemical cycling occurred in the reservoir in 2013 with the reservoir water surface becoming a CO 2 sink during the WD season and the beginning of the WW season (from March to August).…”

“…CO 2 emissions are higher in tropical reservoirs than in temperate and boreal ones, a latitudinal difference attributed to the enhancement of OM degradation with temperature (Barros et al, 2011;Marotta et al, 2014;Yvon-Durocher et al, 2014). Emissions occur through diffusion at the air-water interface of the reservoir and from rivers downstream of dams (Abril et al, 2005;Guérin et al, 2006;Kemenes et al, 2011). At the surface of reservoirs, it is well known that emissions significantly vary spatially and temporally.…”

The global methane budget 2000–2012

“…Methane emissions from fresh waters and estuaries occur through a number of pathways including (1) continuous or episodic diffusive flux across water surfaces, (2) ebullition flux from sediments, (3) flux mediated through the aerenchyma of emergent aquatic macrophytes (plant transport) in littoral environments, and also for reservoirs, (4) degassing of CH 4 in the turbines, and (5) elevated diffusive emissions in rivers downstream of the turbines especially if water through the turbines is supplied from anoxic CH 4 -rich water layers in the reservoir (Bastviken et al, 2004;Guérin et al, 2006Guérin et al, , 2016. It is very rare that complete emission budgets include all these types of fluxes.…”

First Assessment of Inorganic Nitrogen Deposition Budget Following the Impoundment of a Subtropical Hydroelectric Reservoir (Nam Theun 2, Lao PDR)

“…This is a transbasin hydroelectric reservoir that diverts water from the Nam Theun River (a Mekong tributary) to the Xe Bangfai River (another Mekong tributary). The reservoir was described in detail in Descloux et al (), and the carbon cycle and especially the greenhouse gas emissions were monitored since impoundment (Chanudet, Guédant, et al, ; Chanudet, Smits, et al, ; Deshmukh et al, , ; Guérin et al, ; Serça et al, ). The reservoir surface fluctuated from 140 km 2 (525.5 m above sea level) to around 490 km 2 at the full water level (538 m above sea level) between 2009 and 2013.…”