Tropical saline-alkaline lakes as a major source of greenhouse gases: evidence from lake Sonachi, Kenya

Lake Sonachi is a small (0.18 km2), endorheic, crater soda lake located within the Eastern Rift Valley, 90 km NW of Nairobi (central Kenya). It lacks surface inflows or outflows; its water balance is maintained by local precipitation in the crater catchment (~680 mm annual average) and subsurface inflow from the nearby freshwater Lake Naivasha, with water losses only due to evaporation (~1,870 mm per year). Despite its shallowness (5 m depth), the lake is meromictic, being characterized by (i) an upper water layer (mixolimnion; TDS around 8.6 g L-1), affected by diel stratification and mixing, and (ii) a deep and persistent layer (monimolimnion; TDS up to 19 g L-1) that does not participate in vertical mixing. The mixolimnion and monimolimnion are separated by a chemocline at 3.5-4 m depth. The relatively high amount of dissolved organic carbon in the monimolimnion (up to 500 ppm), resulting from the degradation of abundant organic matter stored in the bottom sediments, contributes to the chemical stratification of the water column (biogenic meromixis). The balance between photosynthetic production and microbial consumption largely regulates the dissolved gas concentrations in the water column. Microbial respiration in the anoxic deep layer leads to a significant storage of CH4 (up to 615 μmol L-1; δ13C-CH4 values ≤-60 ‰ vs. V-PDB), as well as CO2. In the mixolimnion, aerobic metabolism consumes oxygen. The occurrence of biogenic CH4 production, even at shallow oxic conditions, was indicated by large CH4 concentrations (~150 μmol L-1), associated with δ13CCH 4 values down to -57 ‰ vs. V-PDB, measured in the shallowest water layers. Epilimnetic waters were undersaturated with CO2 as a consequence and strikingly oversaturated with CH4. Lake Sonachi thus behaves as a sink of CO2, rather than an emitter (as commonly observed for active volcanic lakes), while it can be regarded as a huge emitter of CH4 relative to freshwaters. The estimated water-air CH4 exchange flux (up to ~3.65 g C m-2 d-1), calculated according to the thin boundary layer (TBL) model, is, to the best of our knowledge, the largest diffusive flux ever measured from a lake. The high CH4 emission from Lake Sonachi was probably caused by its shallowness and high perimeter to volume ratio, favoring microbial respiration and resulting in limited CH4 oxidation efficiency. It is worth noting that our estimation is probably conservative, as loss of CH4 through bubbling was not estimated. However, our theoretical simulations indicate that ebullition along the water column might increasingly contribute to CH4 emissions from the lake as a consequence of current and future temperature increases.