Environmental determinants of aerobic methane oxidation in a tropical river network.

• Large variability of CH 4 oxidation rates in a tropical river network. • Temperature response of CH 4 oxidation exceeded that of CH 4 = production. • A model for CH 4 oxidation is developed for tropical fluvial systems. • Stark contrast in the response of CH 4 oxidation to oxygen between rivers and lakes. • CH 4 emission may increase with temperature and organic pollution in Asian rivers. Aerobic methane oxidation (MOX) significantly reduces methane (CH 4) emissions from inland water bodies and is, therefore, an important determinant of global CH 4 budget. Yet, the magnitude and controls of MOX rates in rivers – a quantitatively significant natural source of atmospheric CH 4 – are poorly constrained. Here, we conducted a series of incubation experiments to understand the magnitude and environmental controls of MOX rates in tropical fluvial systems. We observed a large variability in MOX rate (0.03 - 3.45 μmol l-1d-1) shaped by a suit of environmental variables. Accordingly, we developed an empirical model for MOX that incorporate key environmental drivers, including temperature, CH 4 , total phosphorus, and dissolved oxygen (O 2) concentrations, based on the results of our incubation experiments. We show that temperature dependency of MOX (activation energy: 0.66 ± 0.18 eV) is lower than that of sediment methanogenesis (0.71 ± 0.21 eV) in the studied tropical fluvial network. Furthermore, we observed a non-linear relationship between O 2 concentration and MOX, with the highest MOX rate occuring ∼135 μmol O 2 l-1, above or below this "optimal O 2 " concentration, MOX rate shows a gradual decline. Together, our results suggest that the relatively lower temperature response of MOX compared to methanogenesis along with the projected decrease of O 2 concentration due to organic pollution may cause elevated CH 4 emission from tropical southeast Asian rivers. Since estimation of CH 4 oxidation is often neglected in routine CH 4 monitoring programs, the model developed here may help to integrate MOX rate into process-based models for fluvial CH 4 budget. [Display omitted] [ABSTRACT FROM AUTHOR]