Potential carbon fixation via methane oxidation in well-oxygenated riverbed gravels

Due to a combination of local methanogenesis and high background concentrations in the groundwater, water in the River Lambourn is 51 times supersaturated with methane (162 nmol CH₄ L^-1). Pore-water concentrations of methane in the gravels of the riverbed were much lower throughout the year (71 nmol CH₄ L^-1), suggesting significant methane oxidation. To investigate the potential for methane oxidation as a novel chemosynthetic source of carbon to the food web, we made simultaneous measurements, in laboratory chambers, of primary production, respiration, and methane oxidation associated with the gravels. Biomass-specific net primary production was up to 2.7 mmol O₂ mg^-1 chlorophyll (Chl) h^-1 and was similarly high for respiration (2.7 mmol O₂ mg^-1 Chl h^-1). We also found active methane (CH4) oxidation with the rate increasing in proportion to concentration. At the maximum rate of 0.18 mmol CH₄ mg^-1 Chl h^-1 and a growth efficiency of 0.8, net carbon fixation via methane oxidation was equivalent to 6% of the carbon fixed via net photosynthetic primary production. However, production via methane oxidation could be proportionately much greater under the shade of the profuse instream or riparian vegetation, deep in the gravels, and especially during winter, when light is limiting (< 25 mmol quanta m^-2 s^-1).