Fig. 6. CH₄ levels in surface waters of African lakes are driven by lacustrine productivity.

Dissolved CH₄ concentrations (A) and ratio of dissolved CH₄ and CO₂ concentrations (B) versus Chl-a concentration in surface waters of several African tropical lakes. Horizontal dotted line represents the equilibrium with the atmosphere (top) and the average value for boreal lakes (26). Solid line is a fit to the data (table S3). For lakes where multiple measurements were made, the symbol shows the median (n indicates the number of measurements). For CH₄, no relation was observed with other measured variables. The positive relation of CH₄ and Chl-a should be interpreted as resulting from the dependence of CH₄ in surface waters from sedimentary CH₄ fluxes to the water column (22) that were higher in shallow and productive systems (fig. S9), while in stratified deep systems, the CH₄ removal by MOX led to low-surface CH₄ (49).