Biogeochemical processes in a small Kenyan Rift Valley soda lake (Lake Sonachi): implications for methane emissions

Biogeochemical sampling was carried out at Sonachi Crater Lake, a small (0.18 km2) soda lake of volcanic origin located within the Eastern Rift Valley, 90 km NW of Nairobi (central Kenya), immediately south-west of lake Naivasha. The lake is protected from winds by steep crater walls and the water level is regulated by rainfall, very limited runoff, evaporation and groundwater inflow. Despite its shallowness (max depth approx. 5 to 7 m), Lake Sonachi shows a permanent strong chemical stratification with a meromictic structure consisting of (i) an upper water layer (mixolimnion; EC around 9.8 mS cm-1), affected by diel stratification and mixing, and (ii) a deeper and persistent layer (monimolimnion; EC up to 20 mS cm-1). Meromixis is reinforced by high amount of organic solutes accumulated at the bottom (up to 500 ppm of dissolved organic carbon) related to leaching and degradation of abundant organic matter stored in sediments. Dissolved gas concentrations along the water column are largely regulated by biological processes, i.e. by a balance between photosynthetic production and microbial consumption. Aerobic metabolism consumes oxygen in the mixolimnion, whilst microbial respiration in the anoxic deep layer leads to a significant accumulation of ΣS2- and CH4, as well as CO2. The high pH (~9.5) limits CO2 concentration in bottom waters, where CH4 is abundant (up to 615 µmol L-1). δ13C-CH4 values ≤-60 ‰ vs. V-PDB point to the occurrence of methanogenic activity even in the presence of relatively high SO42- concentrations. The coexistence of methanogens with sulphate reduction processes is probably favoured by the large availability of accumulating organic matter. Moreover, in the aerobic epilimnion, high CH4 concentrations (~150 µmol L-1) suggest that oxidation is counteracted by biogenic CH4 production even in oxic conditions, as confirmed by a sharp decrease in δ13C-CH4 values. Concentrations of dissolved CH4 measured in lake’s epilimnetic waters are several orders of magnitude higher than those expected for equilibrium with atmosphere. The estimated water-air CH4 exchange flux, up to ~3.65 g C m-2 d-1, is possibly the largest diffusive flux ever measured from a lake, according to available literature. Moreover, our simulations indicate that ebullition along the water column might further contribute to CH4 emissions from the lake, a factor that might become increasingly relevant because of current and future potential temperature increase.