Environmental Controls of Diffusive and Ebullitive Methane Emissions at a Subdaily Time Scale in the Littoral Zone of a Midlatitude Shallow Lake
T. Taoka, Hiroki Iwata, R. Hirata, Yoshiyuki Takahashi, Yuichi Miyabara, Masayuki Itoh
Abstract
Abstract Environmental controls on methane (CH 4 ) emission from lakes are poorly understood at subdaily time scales due to a lack of continuous data, especially for ebullition. We used a novel technique to partition eddy covariance CH 4 flux observed in the littoral zone of a midlatitude shallow lake in Japan and examined the environmental controls on diffusion and ebullitive CH 4 flux separately at a subdaily time scale in different seasons. Using the high‐frequency data, we investigated how CH 4 accumulation in the water and sediment layers alters the dynamics and environmental controls of fluxes. The contribution of ebullitive flux to total flux was 57% on average. Environmental controls of diffusive and ebullitive fluxes known in the literature were confirmed. We further found that the environmental controls were different in different seasons and suggested that additional consideration of CH 4 accumulation could explain the variability. The transfer of accumulated dissolved CH 4 from the bottom water layer to the surface in summer and the accumulation of dissolved CH 4 under surface ice in winter were suggested to be important for explaining the variability of diffusive flux. In summer, a higher ebullitive flux tended to occur following triggers such as a decrease in hydrostatic pressure. In winter, the impact of triggers was not obvious, and a higher ebullitive flux tended to occur in the morning. We suggested that the low CH 4 production rate in winter slowed the replenishment of bubbles in the sediment, negating the effect of triggers on ebullition.