Methane paradox in tropical lakes? Sedimentary fluxes rather than pelagic production in oxic conditions sustain methanotrophy and emissions to the atmosphere
2020
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.
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