The perturbations of a water column at the deepest part of Lake Onogawa by a local heavy rainfall were analyzed. Mixing throughout the water column (18.8-m deep) was indicated by changes in the distribution of water temperature. This mixing perturbed the hypolimnetic seasonal distributions of dissolved components. All particulate components peaked at 10 m, suggesting a center of inflowing water. Compared with background levels, particulate nitrogen (PN), particulate carbon (PC), particulate phosphorus (PP), and suspended solids (SS) increased from 16-fold for PN to 100-fold for SS. Soluble reactive phosphorus was the only dissolved component that formed a clear maximum at 10 m, as did the particulate components. Assuming that SS consists mainly of mineral particles, SS can be classified into three categories: (1) A rapidly sinking fraction, the main body of the loaded SS, with a sinking rate exceeding 1 m day Ϫ1 and radii exceeding 2-3 µm.(2) A slowly sinking fraction with a sinking speed of about 10 cm day Ϫ1 and radii of 0.7-0.9 µm; this fraction is calculated to be about 4% of the total loaded SS at most. (3) A fraction that was essentially retained in the water column. The maximum estimate of this fraction was 0.5% of the total.

The concentration of dissolved oxygen in waters 0.5-0.6 m above the bottom of Lake Hibara, a dimictic lake, was zero in early spring ranged from 3.75 to 10.1 mg l Ϫ1 . The depth profiles of water temperature suggest that water had not circulated prior to the sample collections of 1994 and 1997, but it had done so in the cases of the other years, suggesting that winter conditions were well preserved in the former years. On the other hand, the dissolved oxygen in the same strata decreased severely in summer. However, more or less titratable amounts of dissolved oxygen still remained (0.01-0.73 mg l Ϫ1 ) at the final stages of summer stratification from 1992 to 1998. These facts show that a completely anoxic condition is not formed in this lake in summer but is sometimes formed in winter. It is interesting to note that in spite of unfavorable winter conditions for oxygen consumption, i.e., shorter duration and lower water temperature, oxygen is exhausted. These facts suggest that ventilation to the depths is much greater in summer than in winter.