Methane Emissions from a Small Wind Shielded Lake Determined by

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Methane Emissions from a Small Wind Shielded Lake Determined by Eddy Covariance, Flux Chambers, Anchored Funnels, and Boundary Model Calculations: A Comparison Carsten J. Schubert,#,* Torsten Diem,#,§ and Werner Eugster‡ #

Swiss Federal Institute of Aquatic Science and Technology (Eawag), Department of Surface WatersResearch and Management, CH-6047 Kastanienbaum, Switzerland, ‡ ETH Zurich, Institute of Agricultural Sciences, CH-8092 Zurich, Switzerland S Supporting Information *

ABSTRACT: Lakes are large sources of methane, held to be responsible for 18% of the radiative forcing, to the atmosphere. Periods of lake overturn (during fall/winter) are short and therefore difficult to capture with field campaigns but potentially one of the most important periods for methane emissions. We studied methane emissions using four different methods, including eddy covariance measurements, floating chambers, anchored funnels, and boundary model calculations. Whereas the first three methods agreed rather well, boundary model estimates were 5−30 times lower leading to a strong underestimation of methane fluxes from aquatic systems. These results show the importance of ebullition as the most important flux pathway and the need for continuous measurements with a large footprint covering also shallow parts of lakes. Although fluxes were high, on average 4 mmol m−2 d−1 during the overturn period, water column microbial methane oxidation removed 75% of the methane and only 25% of potential emissions were released to the atmosphere. Hence, this study illustrates second the importance of considering methane oxidation when estimating the flux of methane from lakes during overturn periods.

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season overturn. Eddy-covariance flux measurements7,8 have so far not been used in natural aquatic systems to estimate methane emissions. They are advantageous since they capture both diffusive flux (normally estimated from surface water− methane concentrations) and ebullition (normally determined using bubble capturing funnels). It is furthermore the only method that allows continuous measurements over longer time periods (here 2.5 months). In small lakes, methane stored in the hypolimnion, which can be emitted to the atmosphere during lake turnover, may contribute up to 45% to the methane budget of such lakes.9 This suggests a potential for large emissions during late season overturn. Here we critically evaluate this methane emission potential with measurements from Rotsee, a small holomictic lake (lakes which mix at least once a year10) in Switzerland that

INTRODUCTION Atmospheric methane concentrations have increased from 0.800 ppm before industrialization to a level of around 1.875 ppm today1 and held responsible for 18% of the radiative forcing.2 Whereas the biggest natural methane emissions stem from wetlands, it was only recently suggested that a substantial partbetween 6 and 16% of natural methane emissions might originate from lakes and other freshwater systems.3 Conservative estimates assume that small lakes with surface areas smaller than 1 km2 account for only one-third of the total emissions from lakes and freshwater systems. This has now been called into question by more recent estimates of the global number of lakes,4 which substantially exceeds previous estimates,5 and the findings that small lakes have higher methane fluxes per unit area than larger lakes.6 Consequently, the amount of methane emitted from small lakes (