Abstract
Methane emissions from lakes and reservoirs are a major natural source in the global budget of atmospheric CH4. A large fraction of these emissions are due to diffusive transport of CH4 from surface waters to the atmosphere. It was suggested recently that CH4 production in the oxic surface waters is required to compensate for diffusive CH4 emissions from lakes. In contrast, we demonstrate here that typical diffusive CH4-fluxes from sediments in shallow water zones, Fsed,S, suffice to explain CH4 emissions to the atmosphere. Our analysis is based on the combination of an exceptional data set on surface concentrations of CH4 with a mass balance model of CH4 that is focused on the surface mixed layer and considers CH4-fluxes from sediments, lateral transport, gas exchange with the atmosphere, and includes temperature dependencies of sediment fluxes and gas exchange. Fsed,S not only explains observed surface CH4 concentrations but also concentration differences between shallow and open water zones, and the seasonal variability of emissions and lateral concentration distributions. Hence, our results support the hypothesis that diffusive fluxes from shallow sediments and not oxic methanogenesis are the main source of the CH4 in the surface waters and the CH4 emitted from lakes and reservoirs.
Highlights
Our results support the hypothesis that diffusive fluxes from shallow sediments and not oxic methanogenesis are the main source of the CH4 in the surface waters and the CH4 emitted from lakes and reservoirs
It has been claimed that oxic CH4 production is a major source of CH4 in the surface waters of lakes[9] and that production of CH4 in oxic surface waters is required to compensate the loss of CH4 due to diffusive emissions[30]
Note that the temperature dependence of Fsed,S can be described well using an exponential function with an exponent of 0.122 °C−1 (R2 = 0.46, df = 29, p < 0.001) which closely agrees with the temperature dependence of CH4 emissions from surface waters of lakes[18]
Summary
Our results support the hypothesis that diffusive fluxes from shallow sediments and not oxic methanogenesis are the main source of the CH4 in the surface waters and the CH4 emitted from lakes and reservoirs. The main pathways of CH4 emissions from lakes are ebullition of CH4-rich gas bubbles released from oversaturated sediments and diffusive exchange of CH4 between water and atmosphere at the lake surface. Surface water concentrations of CH4 are spatially not homogeneous[19,22] but typically enriched in shallow water zones[3,23,27,28]. Other studies have explained the comparatively high concentrations of CH4 in surface waters of lakes by lateral transport of CH4-rich waters from shallow water zones[23,27,28]. In shallow water zones CH4 concentrations can be enriched due to mobilization of pore water during www.nature.com/scientificreports/
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
