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

Morana, Cédric; Bouillon, Steven; Nolla-Ardèvol, Vimac; Roland, Fleur A. E.; Okello, William; Descy, Jean-Pierre; Nankabirwa, Angela; Nabafu, Erina; Springael, Dirk; Borges, Alberto V.

Despite growing evidence that methane (inline-formulaCH4) 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 inline-formulaCH4 increases with lake size and should represent the majority of inline-formulaCH4 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 inline-formulaCH4 production is closely related to phytoplankton metabolism and is a common feature in five contrasting African lakes. Nevertheless, methanotrophic activity in surface waters and inline-formulaCH4 emissions to the atmosphere were predominantly fuelled by inline-formulaCH4 generated in sediments and physically transported to the surface. Indeed, inline-formulaCH4 bubble dissolution flux and diffusive benthic inline-formulaCH4 flux were several orders of magnitude higher than inline-formulaCH4 production in surface waters. Microbial inline-formulaCH4 consumption dramatically decreased with increasing sunlight intensity, suggesting that the freshwater “inline-formulaCH4 paradox” might be also partly explained by photo-inhibition of inline-formulaCH4 oxidizers in the illuminated zone. Sunlight appeared as an overlooked but important factor determining the inline-formulaCH4 dynamics in surface waters, directly affecting its production by photoautotrophs and consumption by methanotrophs.

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Morana, Cédric / Bouillon, Steven / Nolla-Ardèvol, Vimac / et al: Methane paradox in tropical lakes? Sedimentary fluxes rather than pelagic production in oxic conditions sustain methanotrophy and emissions to the atmosphere. 2020. Copernicus Publications.

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