24th November 2014.
Recent research published in Nature Communications has shed new light on aquatic methane dynamics by demonstrating that methanogenesis occurs at the ecosystem-scale in oxic lake waters.
Despite being a recognised mechanism of methane production, the significance of methanogenesis in oxic waters has yet to be determined. The researchers, from the Université du Québec à Montréal, used floating mesocosms that were closed at the bottom but open to the atmosphere, thus excluding non-pelagic methane sources.
During the 28-day experiment, dissolved methane concentrations increased in the mesocosms, and water/air fluxes ranged from 0.07 to 0.36 mmol m-2 day-1. Furthermore, methane production was higher in mesocosms treated with nutrient enrichment, and lower in those treated with dissolved organic carbon (DOC). The difference in methane concentrations and fluxes between treatments was linked to algal dynamics, specifically both gross primary production and net ecosystem production. Analysis also suggested that the methane was produced through acetoclastic methanogenesis. Considering the lake as a whole, mean fluxes from the control (i.e. not-treated) mesocosms accounted for approximately 20% of diffusive fluxes measured during summer, and were of a similar value to ebullitive fluxes.
The authors stress that this is not a missing measurement from aquatic carbon budgets, as this component is captured during standard flux measurements. However, they point out that “incorporating the origin of the methane that outfluxes to the atmosphere from these lakes is critical to our understanding of the regulation of these fluxes and our capacity to predict their future change. In this regard, there are potentially large global implications for algal-driven oxic-water methanogenesis.” Additionally, because methane dynamics differed in response to nutrient and DOC treatments, it is likely that aquatic methane emissions will be affected by environmental changes.
Oxic water column methanogenesis as a major component of aquatic CH4 fluxes. 2014. Bogard, M.J., del Giorgio, P.A., Boutet, L., et al. Nature Communications, DOI: 10.1038/ncomms6350.