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.
Highlights
Emissions from inland waters are an important component of the global methane (CH4) budget (Bastviken et al, 2011), in particular from tropical latitudes (Borges et al, 2015)
The results from the stable-isotope labelling experiment highlight that only a minimal fraction of the CH4 produced under aerobic conditions originated from acetate in contrast with several earlier studies (Bogard et al, 2014; Donis et al, 2017), which proposed, based on the apparent fractionation factor of δ13C–CH4, that acetoclastic methanogenesis linked to phytoplankton production of organic matter would be the dominant biochemical pathway of pelagic CH4 production in oxic fresh waters
Our results support the study of Bizic et al (2020) and suggest that epilimnetic CH4 production in well-oxygenated conditions was related to dissolved inorganic carbon (DIC) fixation by photosynthesis (Fig. 3) and correlated to primary production (Fig. 4a) and N2 fixation (Fig. 4b)
Summary
Emissions from inland waters are an important component of the global methane (CH4) budget (Bastviken et al, 2011), in particular from tropical latitudes (Borges et al, 2015). While progress has been made in evaluating the CH4 emission rates, much less attention has been given to the underlying microbial production (methanogenesis) and loss (methane oxidation) processes. Because most methanogens are considered to be strict anaerobes, and net vertical diffusion of CH4 from anoxic bottom waters is often negligible (Bastviken et al, 2003), physical processes of CH4 transport from shallow sediments are usually invoked to explain patterns of local CH4 concentration maxima in surface waters (Fernández et al, 2016; Peeters et al, 2019; Martinez-Cruz et al, 2020). It has been shown that Cyanobacteria (Bizicet al., 2020) and widespread marine
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