Dynamics of Greenhouse Gases (CH4 and CO2) in Meromictic Lake Burgsee, Germany

Extraordinary amounts of greenhouse gases can be stored within the monimolimnion of meromictic lakes, that is, in the water body which is excluded from mixing events. Lake Burgsee (Thuringia, Germany) is a shallow (depth <5 m) lake with a approximately 24 m deep sinkhole, which is fed by underground brine sources and has formed such a monimolimnion. We investigated the carbon dioxide and methane dynamics in this meromictic lake, from production potentials in the sediment via concentrations in the monimolimnion and mixolimnion to emissions to the atmosphere. In the monimolimnion, we found one of the highest methane concentrations (up to >5 mmol L-1) ever reported for a natural freshwater lake, while carbon dioxide concentrations in the water and methane production rates in the sediments were rather ordinary and within the range of holomictic eutrophic lakes. At the thermocline, gas concentrations accumulated to approximately 100 mu mol L-1 CH4 and 80-230 mu mol L-1 CO2. Estimated fluxes to the atmosphere reached considerable 3.5 mmol CH4 m(-2) d(-1) and 1.5 mmol CO2 m(-2) d(-1) above the sinkhole and 0.8 mmol CH4 m(-2) d(-1) and 0.4 mmol CO2 m(-2) d(-1) above the near-by shallow lake center in 2018. Our results demonstrate that lakes in natural brine areas may provide significant storages and releases of greenhouse gases and require further investigation.