Oxidation of methane in the oxic surface layer of a deep lake sediment (Lake Constance)

Oxygen and methane metabolism were measured using intact sediment cores taken from the profundal (147 m depth) of Lake Constance. Vertical O2 profiles were determined with O2 microelectrodes. Oxygen penetrated into the sediment to a depth of about 1.5–2.5 mm. The potential O2 consumption rates did not differ significantly between various sampling dates and sampling sites on the deep lake floor. Dissolved CH4 increased linearly between 2 and 20 cm depth resulting linearly between 2 and 10 cm depth resulting in a diffusive flux of about 369 μmol CH4 m−2 d−1 into the oxic sediment surface layer as calculated form Fick's law. Activities of methanogenesis were measured in slurried sediment subcores. Integration of these activities over 2–10 cm depth indicated a total production of 1400 μmol CH4 m−2 d−1. Incubation of intact sediment cores overlaid with O2-containing hypolimnetic water resulted in a flux of about 35 μmol CH4 m−2 d−1 out of the sediment into the water. However, as soon as dissolved O2 had decreased to less than about 18 μM O2, the CH4 flux abruptly increased to about 480 μmol CH4 m−2 d−1. This anaerobic CH4 flux was similar to the CH4 production estimated from the vertical distribution of dissolved CH4, but was much higher than the CH4 flux measured under aerobic conditions. Therefore, about 93% of the produced CH4 must have been oxidized within the oxic sediment surface layer by aerobic methanotrophic bacteria which consumed about > 9% of the O2 flux into the sediment.