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

[摘要] Despite growing evidence that methane ( CH 4 ) 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 CH 4 increases with lake size and should represent the majority of CH 4 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 CH 4 production is closely related to phytoplankton metabolism and is a common feature in five contrasting African lakes. Nevertheless, methanotrophic activity in surface waters and CH 4 emissions to the atmosphere were predominantly fuelled by CH 4 generated in sediments and physically transported to the surface. Indeed, CH 4 bubble dissolution flux and diffusive benthic CH 4 flux were several orders of magnitude higher than CH 4 production in surface waters. Microbial CH 4 consumption dramatically decreased with increasing sunlight intensity, suggesting that the freshwater “ CH 4 paradox” might be also partly explained by photo-inhibition of CH 4 oxidizers in the illuminated zone. Sunlight appeared as an overlooked but important factor determining the CH 4 dynamics in surface waters, directly affecting its production by photoautotrophs and consumption by methanotrophs.

[学科分类] 大气科学