Methane emission from a Swiss lake: A full year cycle
Résumé
The build-up of methane has been monitored occasionally over three years and monthly during a full yearís cycle in Lake Rotsee (Lucerne, Switzerland). Concentrations and carbon isotopic composition of methane has been used to describe the sources and losses of methane in the water column. High methane concentrations (up to 1mM) were measured in fall (over several years) in the anoxic water layer. Aerobic and anaerobic methane oxidation in the water column later in the year could be shown. Methane oxidation rates were highest in the interface between oxic and anoxic water layers around 8- 10 m depth. The profile of carbon isotopic composition of methane showed strong indications for methane oxidation at the same depth. Anaerobic methane oxidizers ANME-1 and ANME-2 and aerobic methane oxidizing bacteria (MOB) were detected using FISH at the interface. Trying to sequence the responsible organisms, however, was only successful for the aerobic methanotrophs detecting Methylomonas and Methylobacter. A search for mcrA, the gene for anaerobic methane oxidation, will reveal whether this process is really occurring. An estimation of methane emissions to the atmosphere during stratification showed an average flux of 6.7 mg/m2/d. However, during lake turnover in October to December direct eddy covariance flux measurements above the lake surface showed a higher emission of 16 mg/m2/d.