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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2020-142
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-142
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 05 May 2020

Submitted as: research article | 05 May 2020

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This preprint is currently under review for the journal BG.

Methane paradox in tropical lakes? Sedimentary fluxes rather than water column production in oxic waters sustain methanotrophy and emissions to the atmosphere

Cédric Morana1,2, Steven Bouillon1, Vimac Nolla-Ardèvol1, Fleur A. E. Roland2, William Okello3, Jean-Pierre Descy2, Angela Nankabirwa3, Erina Nabafu3, Dirk Springael1, and Alberto V. Borges2 Cédric Morana et al.
  • 1Department of Earth & Environmental Sciences, KU Leuven, Belgium
  • 2Chemical Oceanography Unit, Université de Liège, Belgium
  • 3Limnology Unit, National Fisheries Resources Research Institute, Uganda

Abstract. Despite growing evidence that methane (CH4) formation could also occur in well-oxygenated surface freshwaters, 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, measured 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.

Cédric Morana et al.

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Short summary
A growing body of studies challenges the paradigm that methane (CH4) production occurs only under anaerobic conditions. Our field experiments revealed that oxic CH4 production is closely related to phytoplankton metabolism, and is indeed a common feature in 5 contrasting African lakes. Nevertheless, we found that methanotrophic activity in surface waters and CH4 emissions to the atmosphere were predominantly fueled by CH4 generated in sediments and physically transported to the surface.
A growing body of studies challenges the paradigm that methane (CH4) production occurs only...
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