Diffusive methane fluxes from Negro, Solimoes and Madeira rivers and fringing lakes in the Amazon basin

The first extensive set of measurements of methane concentrations and fluxes for the Negro River and its major tributaries combined with complementary data for the Solimoes and Madeira rivers and several tributaries are presented and their temporal and spatial variations examined. Fluxes were measured using floating chambers, and dissolved CH4 concentrations were measured by the headspace technique. In the Solimoes basin, tributaries had higher fluxes when water levels were low; no statistical difference among periods for lakes and the main stem river was observed. In the Negro basin, rivers had higher fluxes with greater variations among rivers during high water than during low water based on fluxes calculated from the concentration gradient and modelled gas transfer coefficients. We estimate a regional methane emission of 0.31 Tg C yr(-1) for large river channels in the lowland Amazon basin.