Dynamics of dissolved methane and methane oxidation in dimictic Lake Nojiri during winter

Abstract

Changes in the vertical distribution of dissolved CH4 were monitored during the autumnal lake overturn period in mesotrophic Lake Nojiri, Japan (4.4 km2 in area and 9.4 x 107 m3 in vol). A survey in 1992 revealed that the surface CH4 concentration was highest in December, when the lake overturned. During the following two winters (1993-1994 and 1994-1995) we carried out detailed sampling during the autumnal overturn period. As a result of lake overturn in mid-December, CH4 that had accumulated in the hypolimnion during the stratification period mixed rapidly throughout the water column. Increased CH4 in the epilimnion quickly disappeared after the overturn as a result of the rising of CH4 oxidation activity throughout the water column. The in situ-specific CH4 oxidation rate peaked at 0.274, 0.235, and 1.01 d-1 at 0.5, 20, and 36 m, respectively, during the overturn, and then declined the following month. During this period, the diffusive flux of methane across the air-water interface increased but was not the dominant sink (avg rate of 4.5 kg lake-1 d-1); instead, methane oxidation in the water column was the dominant CH4 sink (avg rate of 67.8 kg lake-1 d-1), removing ~94% of the CH4 during the overturn period. A significant methane flux from the bottom sediments throughout the overturn period was confirmed but decreased gradually as the overturn proceeded. The production of organic carbon as a result of CH4 oxidation in the water column by methanotrophs was comparable in extent to that generated by primary production at that time.