Dynamics of dissolved methane and methane oxidation in dimictic Lake Nojiri during winter
Limnol. Oceanogr., 43(1), 1998, 10-17 | DOI: 10.4319/lo.1998.43.1.0010
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.