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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1. 大鳥池の7月下旬のプランクトンの鉛直分布をしらべた.
2, 底棲動物は見られなかったが湖底泥上にDaphnia ambignaの越冬卵が多数採集された.これは後孵化させて確認した.
3. プランクトンは大部分動物プランクトンが占め, その大部分が0-10m.の水層に生活する。
4, 0-10m.間を詳細にしらべると, 表層 (0-1m.) は比較的少く, 2-5m.の層が密度が最も大であった.
5. 採水によって確かめた水の濁っている層と, プランクトンの密度の大なる層とは一致している.

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The Asahi mountain-chain lies on the west side of the Ôu mountain range, the main watershed of the northeastern Japan, and runs parallel to the latter, from north to south. At the southern end of this mountain-chain there is the highest peak of Mt. Daiasahi-dake (1870 m. above the sea) and at the northern end the second higher peak, Mt. Ito-dake (1771 m. above the sea). These mountains are non-volcanic, uplifted blocks and are formed by granitic rocks.
The limnoligical survey of inland waters in the area north of Mt. Ito-dake was carried out by Dr. Masuzo UENO and his collaborators late in July, 1959. With the materials collected in the following waters during this survey, the author studied on the water quality and the diatom flora.
I. Lake Ôtori-ike (the surface area 0.5 km², the max. depth 65 m). Water samples from 0, 10, 20, 30, 40, 50, 63 m, respectively.
II. Three inflowing streams on the southern shore of Lake Otori-ike, namely, Higashizawa, Naka-no-zawa, and Nishi-no-zawa.
III. Bog-lake Misumi-ike. Littoral and pelagic samples.
IV. Two small marshes near the summit of Mt. Ito-dake.
The chemical analyses of the water (Table 1) shows that Lake Otori-ike seems to belong to the oligotrophic type, the bog-lake Misumi-ike and the marshes near the summit of Mt. Ito-dake belong to the dystrophic type.

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Lake Ôtori-ike is a mountain lake lying at an elevation of 963 meters at the northern part of the Asahi mountain range. The plankton flora of this lake is extremely poor in summer, only Dinobryon cylindricum being found.
Bog-lake Misumi-ike lies near Lake Otori-ike, and is shallow and dystrophic type. The shore of this lake is furnished with a zonation of Menianthes trifoliata. The members of phytoplankton of this lake consisted of four species, i. e. Dinobryon cylindricum, Gymnodinium neglectum, Gloeocystis planctonica, and Tetraspora limnetica. The algal flora of the littoral zone is especially rich in desmids which are followed by cyanophycean and euglenoid algae.
Mt. Itô-dake is situated at the southern shore of Lake Ôtori-ike and its top is 1771 meters above the sea. There are two Sphagnum-bogs at its northern slope, lying at an elevation of about 1550 metres. The algae found in these bogs are rich in desmids which are followed by blue-green algae. The majority of the members of algae in these two bogs is also found in some Sphagnum-bogs of similar nature at the alpine and subalpine regions in the central and northern parts of Japan.

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This report deals with the foods eaten by Salvelinus pluvius which were fished in three streams, namely, the Higashi-zawa, Sawagurumi-zawa, and Ôtori-gawa, of which the former two were the inflows and the last is the outflow of LakeÔtoriike, Yamagata Prefecture. The results of analysis of the contents of twenty-nine stomachs showed that the majority of foods eaten were insects, both aquatic and terrestrial. The aquatic foods are composed chiefly of caddis worms and stonefly nymphs, and in particular the nymph ofScopura longa. The terrestrial foods are represented by ants, wasps, and beetles. The fishes (S. pluvius) of the Higashi-zawa and Sawagurumi-zawa took chiefly aquatic foods which made up 52.9-56.9 volume per cent. of foods eaten, while the fishes of the Ôtori-gawa ate chiefly terrestrial foods, 47 volume per cent. of foods eaten (Fig.1). The nematode parasite Spiroptera salvelina FUJITA was found in the stomachs. Its parasitic frequency is the highest in the fishes from the Otori-gawa and is the lowest in those from the Higashi-zawa.

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大鳥三角池は, 大鳥池の北西隅の北西約250m. (直線距離) にあり, 森林に囲まれているので発見し難く, 従って陸水学的な調査研究の行われたことを知らないのである.山形県綜合学術調査会陸水班は, 大鳥池畔滞在中, 7月26日午後ならびに27日午後の2回にわけて, 全員が池畔に臨み, 観測および採集を行った.その結果の概要を次に述べよう. この報文は主として水野が草した.
大鳥三角池は大鳥池北岸, 海抜約1060m.にあり, 森林に囲まれた浅い凹窪地の潴水で, 大鳥池成生時の山崩に際してできたものと思われる, その湖面面積1519m²の長円形の湿原池で, 最大深度は2.8m.である, 湖は赤褐黒色で, 湖水の色度 (メチールオレンジ) は15, 強酸性でpH 4.4である. KMnO₄消費量15 mg/lで腐植質性貧栄養の水を湛えている.沿岸帯はのこ種池沼の特徴種ミツガシワおよびコウホネをめぐらし, 湖岸はミズゴケ湿原よりなる.プランクトンは動物を主体としその主要種は, Acanthodiaptomus pacificus, Daphnia rosea, および, Tropocyolops prasinusである. 湖畔ミズゴケ中には藻類に富み24種が同定せられた.

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