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High concentrations of phosphate ion precipitate with carbonates by simple coprecipitation as the pH of marl lake water is increased. High pH microenvironments associated with photosynthesis probably induce phosphate‐carbonate precipitation, which in turn functions as one of several indirect density‐dependent population controls.

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Changes in the vertical distribution of dissolved CH, were monitored during the autumnal lake overturn period in mesotrophic Lake Nojiri, Japan (4.4 km* in area and 9.4 X 10' m3 in vol). A survey in 1992 revealed that the surface CH, 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, CH, that had accumulated in the hypolimnion during the stratification period mixed rapidly throughout the water column. Increased CH, in the epilimnion quickly disappeared after the overturn as a result of the rising of CH, oxidation activity throughout the water column. The in situ-specific CH, oxidation rate peaked at 0.274, 0.235, and 1.01 d-' 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-' d-l); instead, methane oxidation in the water column was the dominant CH, sink (avg rate of 67.8 kg lake-' d-l), removing -94% of the CH, 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 CH, oxidation in the water column by methanotrophs was comparable in extent to that generated by primary production at that time.

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