Spatiotemporal patterns in methane flux and gas transfer velocity at low wind speeds: Implications for upscaling studies on small lakes

Lakes contribute significantly to the global natural emissions of methane (CH₄) and carbon dioxide. However, to accurately incorporate them into the continental carbon balance more detailed surveys of lacustrine greenhouse gas emissions are needed, especially in respect to spatiotemporal variability and to how this affects the upscaling of results. We investigated CH₄ flux from a small, wind-shielded lake during 10 field trips over a 14 month period. We show that floating chambers may be used to calibrate the relationship between gas transfer velocity (k) and wind speed at 10 m height (U₁₀) to the local system, in order to obtain more accurate estimates of diffusive CH₄ flux than by applying general models predicting k based on U₁₀. We confirm earlier studies indicating strong within-lake spatial variation in this relationship and in ebullitive CH₄ flux within the lake basin. However, in contrast to the pattern reported in other studies, ebullitive CH₄ flux was highest in the central parts of the lake. Our results indicate positive relationships between k and U₁₀ at very low U₁₀ (0–3 m s⁻¹), which disagrees with earlier suggestions that this relationship may be negligible at low U₁₀ values. We estimate annually averaged open water CH₄ emission from Lake Gerzensee to be 3.6–5.8 mmol m⁻² d⁻¹. Our data suggest that estimates of greenhouse gas emissions from aquatic systems to the atmosphere based on the upscaling of short-term and small-scale measurements can be improved if both spatial and temporal variabilities of emissions are taken into account.