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 14month period. We show that floating chambers may be used to calibrate the relationship between gas transfer velocity (k) and wind speed at 10m height (U-10) to the local system, in order to obtain more accurate estimates of diffusive CH4 flux than by applying general models predicting k based on U-10. 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 U-10 at very low U-10 (0-3ms(-1)), which disagrees with earlier suggestions that this relationship may be negligible at low U-10 values. We estimate annually averaged open water CH4 emission from Lake Gerzensee to be 3.6-5.8mmolm(-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.