Greenhouse gas emissions from African lakes are no longer a blind spot

Abstract

Natural lakes are thought to be globally important sources of greenhouse gases (CO₂, CH₄, and N₂O) to the atmosphere although nearly no data have been previously reported from Africa. We collected CO₂, CH₄, and N₂O data in 24 African lakes that accounted for 49% of total lacustrine surface area of the African continent and covered a wide range of morphology and productivity. The surface water concentrations of dissolved CO₂ were much lower than values attributed in current literature to tropical lakes and lower than in boreal systems because of a higher productivity. In contrast, surface water-dissolved CH₄ concentrations were generally higher than in boreal systems. The lowest CO₂ and the highest CH₄ concentrations were observed in the more shallow and productive lakes. Emissions of CO₂ may likely have been substantially overestimated by a factor between 9 and 18 in African lakes and between 6 and 26 in pan-tropical lakes.

Fig. 1.. Location of the 24 sampled African lakes, plus Lake Malawi (48).

This dataset of CO₂, CH₄, and N₂O in surface waters covers a wide range of morphological conditions (surface area and depth), water column physical structure, catchment land cover, and lake productivity. Map indicates cover by savannah, forest, and flooded forest (from lightest to darkest shade of green).

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.

Fig. 3.. CO₂ levels are highly variable among African lakes and patterns depend on primary production.

Surface water pCO₂ in several African tropical lakes versus pelagic aquatic primary production (A), Chl-a concentration (B), oxygen saturation level (%O₂) (C), cyanobacteria abundance (D), carbon stable isotope composition of dissolved inorganic carbon (δ¹³C-DIC) (E), DOC (F), water temperature (G), and CDOM SR (H). Dotted lines and solid lines as in Fig. 2. The black dotted line gives the relation of pCO₂ versus DOC from a global dataset (2), while the black dotted line in the inset gives the relation of pCO₂ versus temperature from a dataset of tropical lakes (9). For lakes where multiple measurements were made, the symbol shows the median (n indicates the number of pCO₂ measurements).

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.

Fig. 5.. Net ecosystem autotrophy drives a CO₂ sink in most of the studied African lakes.

Variations in several African tropical lakes of the air-water gradient of the pCO₂ (ΔpCO₂) as a function of the ratio of aquatic pelagic primary production (P) and community respiration (R) (A) and the air-water CO₂ flux (FCO₂) as a function of NEP (B). P/R > 1 and NEP > 0 correspond to net autotrophy at the community level and is, in most cases, associated with being a sink of atmospheric CO₂ (ΔpCO₂ < 0, FCO₂ < 0). For lakes where multiple measurements were made, the symbol shows the median (n indicates the number of measurements).

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).

Fig. 7.. CH₄ is higher and CO₂ is lower in African lakes than in boreal lakes.

The ratio of dissolved CH₄ and CO₂ concentrations (A to C), dissolved CH₄ (D to F), CO₂ (G to I) concentrations, and N₂O saturation levels (%N₂O) (J to L) in surface waters of several African tropical lakes compared to paired CO₂ and CH₄ measurements (n = 224) in lakes located in the boreal climatic zone compiled from literature in (26) located in Finland, Canada, Siberia (latitude, >60°N) and %N₂O in Finland (50) sorted into three size classes (0.1 to 1 km², 1 to 10 km², and >10 km²). Surface water temperature in boreal lakes was 9.8°C (median) versus 25.8°C in African lakes. Box indicates the median; bars indicate the first and third quartile. n indicates the number of data points.

Fig. 8.. Allochthonous versus autochthonous origin of DOC in African lakes depends on type (humic versus nonhumic).

DOC versus Chl-a (A) concentration and versus cyanobacteria abundance (B) in surface waters of several African tropical lakes. 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).

Fig. 9.. Large contribution to CO₂, CH₄, and N₂O emissions from very large lakes.

Air-water flux of CO₂ (FCO₂), CH₄ (FCH₄ diffusive), and N₂O (FN₂O) integrated for African tropical (A, C, and E) and pan-tropical lakes (B, D, and F) that were classified into humic and nonhumic, as well as surface area (G and H) and number of lakes per size classes (I and J) from HydroLAKES (29), except for Lake Chad for which the more recent and realistic surface area was applied (30).