Big Soda Lake (Nevada). 3. Pelagic methanogenesis and anaerobic methane oxidation

In situ rates of methanogenesis and methane oxidation were measured in meromictic Big Soda Lake. Methane production was measured by the accumulation of methane in the headspaces of anaerobically sealed water samples; radiotracer was used to follow methane oxidation. Nearly all the methane oxidation occurred in the anoxic zones of the lake. Rates of anaerobic oxidation exceeded production at all depths studied in both the mixolimnion (2–6 vs. 0.1–1 nmol liter⁻¹ d⁻¹) and monimolimnion (49–85 vs. 1.6–12 nmol liter⁻¹ d⁻¹) of the lake. Thus, a net consumption of methane equivalent to 1.36 mmol m⁻² d⁻¹ occurred in the anoxic water column. Anaerobic methane oxidation had a first-order rate constant of 8.1±0.5 × 10⁻⁴ d⁻¹, and activity was eliminated by filter sterilization. However, in situ methane oxidation was of insufficient magnitude to cause a noticeable decrease of ambient dissolved methane levels over an incubation period of 97 h.