Methane concentrations and estimated fluxes from Pacific Northwest rivers

Abstract

The concentrations of methane (CH₄) were measured in several Pacific Northwest rivers and flux rates of CH₄ to the atmosphere were estimated. These rivers were supersaturated almost everywhere, with typical CH₄ concentrations in the surface waters between 10 and 500 nM. Even though stream variability was very high, CH₄ concentrations tended to increase significantly in deeper, slower moving sections of the rivers. In more turbulent sections of the rivers, the accumulated CH₄ was rapidly evaded to the atmosphere resulting in high flux rates. Fluxes were estimated with two models (eddy cell and large eddy) and by direct measurement of CH₄ loss within specific river reaches. The flux rates encountered during this study ranged from 0 to 317 mg CH₄ m⁻² d⁻¹. Microbial CH₄ oxidation accounted for about 25% of the total CH₄ lost from the Columbia River but was insignificant in the more turbulent Wenatchee River.