ABSTRACT
Reservoirs are an important source of atmospheric methane (CH4), a potent greenhouse gas. The Mekong, one of the largest Asian rivers, has been heavily dammed and can be a potential hotspot for CH4 emissions. While low diffusive CH4 flux was previously reported from cascading reservoirs in the Upper Mekong, the contribution of ebullition (bubbling) remained unexplored. To better constrain the magnitude and drivers of ebullition from these reservoirs, automated bubble traps were deployed in four reservoirs, allowing for continuous measurement of the ebullitive flux with high temporal resolution for a period of six months. To characterize the spatial variability of CH4 fluxes mediated by ebullition and diffusion, whole-reservoir surveys were conducted using a scientific echo sounder for bubble observations together with a gas equilibrator for mapping dissolved CH4 concentration in surface water from which diffusive fluxes were estimated. Potential production and anaerobic oxidation rates of CH4 were estimated in laboratory incubations of sediment cores collected near the bubble trap deployment sites. The CH4 production potential in sediments increased strongly along the reservoir cascade, with mostly minor reduction by anaerobic oxidation. Surface CH4 fluxes, in contrast, showed high spatial variability in both ebullitive and diffusive pathways (ranging 0.05-44 and 1.8-6.4 mg m-2 d-1, respectively, among all reservoirs). Ebullitive fluxes were about one order of magnitude higher than diffusive fluxes and remained significant at sites as deep as 30-45 m. The repeated spatial pattern of ebullition (higher fluxes at the dam area than in upstream sections) suggests the possible control of emission rates by sediment transport and deposition.
