Fig. 4. CH₄ strongly varies as a function of bottom depth within lakes.

Continuous measurements of dissolved CH₄ concentrations were made in Lakes Victoria, Tanganyika, Albert, Kivu, and Edward, with an equilibrator connected to a laser spectrometer except for Lake Kivu (compilation of 65 discrete samples measured by headspace with a gas chromatograph obtained during five cruises; n indicates the number of measurements). The inset shows the raw data, and the main panel shows the data binned by depth intervals of 10. Dotted lines are the curves fitted to data (table S3). Data bins per depth interval were combined with bathymetry maps to derive CH₄ (and pCO₂; not shown here) values spatially representative of the largest lakes, accounting for the strong horizontal gradients as a function of depth.