Methane paradox in tropical lakes? Sedimentary fluxes rather than pelagic production in oxic conditions sustain methanotrophy and emissions to the atmosphere

Abstract

Abstract. Despite growing evidence that methane (CH4) formation could also occur in well-oxygenated surface fresh waters, its significance at the ecosystem scale is uncertain. Empirical models based on data gathered at high latitude predict that the contribution of oxic CH4 increases with lake size and should represent the majority of CH4 emissions in large lakes. However, such predictive models could not directly apply to tropical lakes, which differ from their temperate counterparts in some fundamental characteristics, such as year-round elevated water temperature. We conducted stable-isotope tracer experiments, which revealed that oxic CH4 production is closely related to phytoplankton metabolism and is a common feature in five contrasting African lakes. Nevertheless, methanotrophic activity in surface waters and CH4 emissions to the atmosphere were predominantly fuelled by CH4 generated in sediments and physically transported to the surface. Indeed, CH4 bubble dissolution flux and diffusive benthic CH4 flux were several orders of magnitude higher than CH4 production in surface waters. Microbial CH4 consumption dramatically decreased with increasing sunlight intensity, suggesting that the freshwater “CH4 paradox” might be also partly explained by photo-inhibition of CH4 oxidizers in the illuminated zone. Sunlight appeared as an overlooked but important factor determining the CH4 dynamics in surface waters, directly affecting its production by photoautotrophs and consumption by methanotrophs.

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

“…For CH 4 , no relation was observed with other measured variables. The positive relation of CH 4 and Chl-a should be interpreted as resulting from the dependence of CH 4 in surface waters from sedimentary CH 4 fluxes to the water column ( 22 ) that were higher in shallow and productive systems (fig. S9), while in stratified deep systems, the CH 4 removal by MOX led to low-surface CH 4 ( 49 ).…”

“…Together, our conclusion that CO 2 emission estimates from tropical lakes are lower than those from previous studies remains valid. Daytime diffusive CH 4 emissions could be higher than nighttime values, possibly because of the photoinhibition of MOX ( 22 ), leading to higher CH 4 concentrations during daytime (also based on dusk-dawn comparisons of CH 4 in Lake Victoria; fig. S14) and higher daytime wind speed and k (fig.…”

“…Such a relation has been previously interpreted by some studies as resulting from the production of CH 4 in aerobic conditions (19), either directly (20) or indirectly (21) linked to phytoplankton metabolism. Although production of CH 4 in aerobic conditions linked to phytoplankton metabolism was shown to occur in five African lakes, the related input flux to the water column was orders of magnitude lower than sedimentary CH 4 inputs to the water column (22). In nonhumic lakes, high phytoplankton biomass also led to low CO 2 values that, combined with high CH 4 , also resulted in an overall positive relation between the CH 4 :CO 2 ratio and Chl-a (Fig.…”

“…P was measured during 2-hour incubations along a gradient of light intensity using 13 C-HCO 3 − as a tracer, as described in detail elsewhere (22). For Lake Mai Ndombe, P was calculated with a statistical model using Secchi disk depth and Chl-a concentration developed for the Congo River network (17).…”

Half of global methane emissions come from highly variable aquatic ecosystem sources

“…As a result of global warming, increased methane emissions are expected across biomes and latitudes because of the higher activities of methanogenic archaea at elevated temperatures relative to methanotrophic microorganisms 27,28 . However, not only archaea but also saprotrophic fungi and cyanobacteria can produced methane under various environmental conditions [29][30][31] .…”