Methane sources and sinks in Lake Kivu
Unique worldwide, Lake Kivu stores enormous amounts of CH4 and CO2. A recent study reported that CH4 concentrations in the lake have increased by up to 15% in the last 30 years and that accumulation at this rate could lead to catastrophic outgassing by similar to 2100. This study investigates the present-day CH4 formation and oxidation in Lake Kivu. Analyses of C-14 and C-13 in CH4 and potential carbon sources revealed that below 260 m, an unusually high similar to 65% of the CH4 originates either from reduction of geogenic CO2 with mostly geogenic H-2 or from direct inflows of geogenic CH4. Aerobic CH4 oxidation, performed by close relatives of type X CH4-oxidizing bacteria, is the main process preventing CH4 from escaping to the atmosphere. Anaerobic CH4 oxidation, carried out by CH4-oxidizing archaea in the SO42- -reducing zone, was also detected but is limited by the availability of sulfate. Changes in C-14(CH4) and C-13(CH4) since the 1970s suggest that the amount of CH4 produced from degrading organic material has increased due to higher accumulation of organic matter. This, as well as the sudden onset of carbonates in the 1960s, has previously been explained by three environmental changes: (1) introduction of nonnative fish, (2) amplified subaquatic inflows following hydrological changes, and (3) increased external inputs due to the fast growing population. The resulting enhancement of primary production and organic matter sedimentation likely caused CH4 to increase. However, given the large proportion of old CH4 carbon, we cannot exclude an increased inflow of geogenic H-2 or CH4.