Temporal and spatial variations in methane emissions from the littoral zone of a shallow mid-latitude lake with steady methane bubble emission areas

Abstract

Temporal and spatial variations in methane (CH4) emissions from the littoral zone of a shallow mid-latitude lake, Lake Suwa, were examined using the eddy covariance technique. The observation data were supplemented with dissolved CH4 measurement and incubation experiments of lake sediment and water. This lake had areas with steady bubble CH4 emissions, shaping the spatial variation in CH4 flux with strong spot emissions. In the other areas, CH4 was emitted via diffusion and episodic ebullition. CH4 emissions from this area had a clear seasonal variation, reflecting temperature variations, with a maximum monthly mean emission of 0.39 µmolm−2s−1 in August. The higher emission in summer was due to higher CH4 production in the sediment and the resulting higher dissolved CH4 concentration in the surface water. The steady bubble CH4 emission was seasonally controlled by the total hydrostatic pressure, with higher emission under lower pressure, suggesting regulation of the amount of bubbles emitted from the lake bottom by pressure. Isotope analysis revealed that this CH4 was produced biologically. This natural gas seepage was estimated to emit 1.9 Mg of CH4 annually. Compared with other eddy covariance studies over inland waters, CH4 emissions from Lake Suwa may be large for the latitude. This may be related to eutrophication, which enhances CH4 production.