Lake Ótori-ke, in latitude 38°22' N., lies at an elevation of 963 ± 3 m to the northern side of Mt. Ito-dake, the northernmost peak of the Asahi mountain range in Yamagata Prefecture. lt is noticeable that this deep mountain lake, having the greatest depth of 65 m, was formed behind a landslide dam which happened in the geological past. The present-day lake, which occupies 0.342 km² in superficies (when 966 m, 0.406 km²), is situated at the bottom of a small drainage basin of granitic rocks. The lake is fed by three short streams of melted water at the southern shore, and the lake water discharges over the top of the dam at its north-eastern corner and runs down as a rapid. Its volume is 1.1 (or 1.23) × 10⁷ m³. It is natural that such a body of water lying in the igneous rock basin and in the subalpine climatic conditions is oligotrophic in nature.
The lake appeared, when the writer visited there in the end of July, 1959, to be dark green and turbid, probably due to the heavy rain in the preceding days. Transparencies were smaller than 2 m. The probable shortness of the daily period of insolation due to the situation of the lake, which is surrounded by steep mountains, may cause the decrease of heat income to the lake. The surface water did not exceed 21°C at the end of July. Sharp but unstable thermal stratification developed at the layers below the surface. This seemed to facilitate heat transport into deep water, where temperatures were observed above 4°C. The oxygen dissolved in the surface water was in an amount of 8.46 mg/l, and its diminution in the deep water was rather great. The water showed acid reaction, probably owing to the acidic igneous rocks of the basin ; its pH values were 6.4 in the surface water and gradually became lower toward the bottom, at which the pH was 5.8. The chemical analyses for some major and minor constituents suggest that this lake has rather dilute water, as is discussed in a separate paper. Besides the surface water, there existed slightly turbid water at the layers below 20 m, suggesting the presence of heterogenous water mass, for which the chemical analyses showed the increase of the amounts of organic matter and nitrogenous compounds. Phosphorus was not found throughout the bottom.
There were found deposits of soft dark grey ooze on the bottom of the limited area deeper than 60 m, where a considerable amount of fragments of fallen leaves from the surrounding forests of the lake was present. No bottom-inhabiting animals were found, but a considerable number of ephippial eggs occurred on the surface of mud. These eggs, it was determined in hatching experiments, belonged to Daphnia living in the lake. In the bottom ooze there were found 28 species of diatoms, among which Cyclotella Meneghiniana Meneghiniana and C. stelligera were dominant.
The plankton is characterized by the occurrence of a few species of animals, among which Conochilus unicornis is the most abundant. The others are crusta-ceans, namely Holopedinm gibberum, Cyclops vicinus, Acanthodiaptomus pacificus and Daphnia ambigua. They were, however, small in quantity, and were concentrated in the upper layers, shallower than 10 m. The primary productivity (phytoplankton), too, was extremely small in quantity, only a small number of Dinobryon cylindricum occurring in the end of July. TheDaphnia found in this lake is peculiar in having a very short shell-spine, and is identical with the races inhabiting the lakes of northeastern Japan (including Hokkaido). It is presumably a peculiar race of thelongispina group, and is identical withDaphnia ambigua Scourfield.

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