Fig. 2. Lake morphology controls CH₄, N₂O, and CO₂ in lacustrine surface waters.

Surface water–dissolved CH₄ concentration (A and B), pCO₂ (C and D), N₂O saturation level (%N₂O) (E and F), and Chl-a concentration (G and H) in 24 African tropical lakes versus lake surface area and average depth. For lakes where multiple measurements were made, the symbol shows the median (n indicates the number of samplings, detailed in table S7). Insets show data binned (median) by classes of surface area or depth. Horizontal dotted lines indicate the atmospheric equilibrium of the three gases, additionally for CO₂ two average estimates for tropical (4, 7) and global lakes (1). Solid lines are fits to the data (table S3) from which humic lakes were excluded for pCO₂ and %N₂O. Data of pCO₂ in Lake Malawi were obtained by another group but with a comparable high-quality method (equilibrator coupled to an infrared CO₂ analyzer) (48). CO₂ and %N₂O data in humic lakes were clustered but did not show a pattern with lake surface and mean depth, so the median was used to upscale the values at continental scale. CO₂ and %N₂O in nonhumic lakes were positively related to mean depth and these relations were used to scale the values at continental scale. CH₄ was negatively related to mean depth, irrespective of the lake type, and this relation was used to scale values at continental scale.