Abstract
Lakes are considered the second largest natural source of atmospheric methane (CH4). However, current estimates are still uncertain and do not account for diel variability of CH4 emissions. In this study, we performed high-resolution measurements of CH4 flux from several lakes, using an automated and sensor-based flux measurement approach (in total 4,580 measurements), and demonstrated a clear and consistent diel lake CH4 flux pattern during stratification and mixing periods. The maximum of CH4 flux were always noted between 10:00 and 16:00, whereas lower CH4 fluxes typically occurred during the nighttime (00:00-04:00). Regardless of the lake, CH4 emissions were on an average 2.4 higher during the day compared to the nighttime. Fluxes were higher during daytime on nearly 80% of the days. Accordingly, estimates and extrapolations based on daytime measurements only most likely result in overestimated fluxes, and consideration of diel variability is critical to properly assess the total lake CH4 flux, representing a key component of the global CH4 budget. Hence, based on a combination of our data and additional literature information considering diel variability across latitudes, we discuss ways to derive a diel variability correction factor for previous measurements made during daytime only.
Highlights
Lakes are considered the second largest natural source of atmospheric methane (CH4)
We performed lake CH4 flux measurements resolving diel variability in four lakes with different physicochemical characteristics, which are situated along a climate gradient from the northern temperate to northern boreal zone (SI Appendix, Table S1 and Fig. S1)
The use of automated flux chambers [11] together with additional manual measurements resulted in a comprehensive dataset of CH4 emissions with a mean temporal resolution of 2 h
Summary
Estimates and extrapolations based on daytime measurements only most likely result in overestimated fluxes, and consideration of diel variability is critical to properly assess the total lake CH4 flux, representing a key component of the global CH4 budget. Ebullition is highly dependent on CH4 production in the sediments and physical factors triggering the bubble release (e.g., variation in atmospheric and hydrostatic pressure) Both diffusive flux and ebullition can be under strong influence of physical forcing, via e.g., pressure, wind, temperature, precipitation or radiation, which can operate on very short time scales (minutes to hours) and, thereby, potentially lead to a great CH4 flux variability within one diel cycle. Our findings were compared with available literature to evaluate possible key drivers and implications of the observed diel patterns This information was used to discuss ways to better consider diel variability when using former data and designing future measurements
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