ABSTRACT

Weight ratios of organic carbon to nitrogen in plankton, detritus settled at 25.5 m through 28.5 m water depths, and bottom surface sediments in Lake Kizaki-ko (max. depth: 28.5 m) are 5.7, 10.7 and 15.5 respectively. This suggests that in the lake water the nitrogenous fraction of dead planktonic material is more easily mineralized microbiologically than the carbonaceous fraction. The seasonal observation of the chemical components in the lake water shows the following facts: (1) During the early stage of a stagnation period, nitrogen fixation is generally more active than denitrification. On the contrary, denitrification gradually exceeds nitrogen fixation with the progress of stagnation. (2) At the end of the stagnation period, the amount of denitrified N₂ is considerably large as compared with those of the other mineralized nitrogenous compounds, and the denitrification constitutes the dominant process determining nitrogen metabolism in the lake water. (3) The average ratio of total mineralized carbon to nitrogen at the end of the stagnation period is 3.5, a value considerably smaller than the corresponding value for plankton, 5.7. This fact supports the view that the greater ease for the mineralization of nitrogenous fraction in dead planktonic material as compared with that of carbonaceous fraction causes the increase in the ratio of org.C/org.N of organic detritus from upper water layers towards the bottom. (4) The mineralization rates of carbon and nitrogen in the organic detritus in the lake water are 51%/y and 76%/y respectively.