Diel variation of CH₄ emission fluxes in a small artificial lake: Toward more accurate methods of observation

Lakes are significant sources in global methane (CH₄) budgets. However, estimations of the magnitude of global CH₄ emissions from lakes may be highly biased owing to the uncertainties in data originating from observation times, methods, and parameterizations of the gas transfer velocity (k). Here, we conducted continuous 48-hour measurements of CH₄ fluxes using the floating chamber method seasonally at Lake Baihua, a small reservoir in southwestern China, and compared the results with estimates derived from boundary layer models. Results showed that there was a weak dependency of k on wind speed, indicating that wind speed was not the major factor regulating gas exchange in such small lakes. It is thus concluded that the wind speed-dependent boundary layer model method is not suitable for CH₄ flux observations in small and medium-sized lake, and that the floating chamber method is recommended for use instead. The measured CH₄ fluxes displayed remarkably diurnal patterns, therefore the use of single observations to represent daily average values comes with unacceptably large uncertainties. A reasonable alternative is averaging observations made at sunrise and at sunset to represent daily values, which has a much smaller uncertainty (ranging from 0.8% to 13.6%). The coincident peaks of CH₄ and chlorophyll concentrations in the subsurface indicate that CH₄ originated mainly from aerobic methanogenesis. Solar radiation is likely one of the major factors regulating CH₄ production and emissions in the lake through enhancing CH₄ production, inhibiting CH₄ oxidation, and probably changing hydrodynamics conditions. Therefore, irradiation should be taken into consideration as a key factor in observing CH₄ fluxes in lakes. As sampling times are limited, observations during both sunny and cloudy weather should be proportionally included. This is the first time, to the best of our knowledge, that solar radiation has been proposed as a key driver of CH₄ emissions from lakes.