DYNAMICS OF DISSOLVED METHANE AND METHANE OXIDATION IN DIMICTIC LAKE NOJIRI DURING WINTER

Changes in the vertical distribution of dissolved CH4 were monitored d uring the autumnal lake overturn period in mesotrophic Lake Nojiri, Ja pan (4.4 km(2) in area and 9.4 x 10(7) m(3) in vol). a survey in 1992 revealed that the surface CH4 concentration was highest in December, w hen the lake overturned. During the following two winters (1993-1994 a nd 1994-1995) we carried out detailed sampling during the autumnal ove rturn period. As a result of lake overturn in mid-December, CH4 that h ad accumulated in the hypolimnion during the stratification period mix ed rapidly throughout the water column. Increased CH4 in the epilimnio n quickly disappeared after the overturn as a result of the rising of CH4 oxidation activity throughout the water column. The in situ-specif ic CH4 oxidation rate peaked at 0.274, 0.235, and 1.01 d(-1) at 0.5, 2 0, and 36 m, respectively, during the overturn, and then declined the following month. During this period, the diffusive flux of methane acr oss the air-water interface increased but was not the dominant sink (a vg rate of 4.5 kg lake(-1) d(-1)); instead, methane oxidation in the w ater column was the dominant CH4 sink (avg rate of 67.8 kg lake(-1) d( -1)), removing similar to 94% of the CH4 during the overturn period. A significant methane flux from the bottom sediments throughout the ove rturn period was confirmed but decreased gradually as the overturn pro ceeded. 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.