In the summer of 1963 a limnological survey was made of the three small mountain lakes, Goshiki-numa, Naka-numa and Okure-numa, at Shizu in Yamagata prefecture. These lakes lie at the southern foot of the Gassan Volcano at elevations between 700 and 900 meters. All of them are small and shallow, their basins being not larger than 2.5 ha. in superficies and not exceeding 6 m. at their greatest depth. The morphometric data, bathymetric maps, and the data of the vertical distribution of temperature, the pH and dissolved oxygen in the lake waters, are shown in the Tables 1-3 and 8.
It was noticed that the plankton of these lakes was very scanty both in the number of species in its composition as well as in individual numbers. The components of plankton are restricted only to Bosmina longirostris, Chydorus sphaericus and one or two species of Cyclopoida. No plankton algae occurred, but a large amount of filamentous algae especially Spirogyra was found in the plankton samples. Among the bottom-dwelling animals, oligochaete worms are dominant in Goshiki-numa and Naka-mama ; there were as many as 4, 800 individuals per square meter of the bottom in the former lake, while in Ôkurenoma there occurred a few individuals of Tendipes larvae of the pluntosus type.
It was noticeable that these lake basins except Ôkure-numa were filled with somewhat turbid water which appeared dark green or greenish blue with a milky tinge when the lake surface received sun-light. This abnormal colour reminds us of the disharmonious nature of those lakes, like that of some inorganic strong acid water lakes which are distributed in the volcanic districts of Japan. The lakes under consideration, however, have the water of weak acid reaction, their pH values showing higher than 6. 0. Such a disharmonious condition is due to the inlet waters derived from a copper mine which had operated since 1958 and closed in July 1963. The water flowing out of the mine was, after stagnating for a while in a settling basin (Takishita-numa), running into Naka-numa and Goshiki-numa where it had an effect upon the water as well as the flora and fauna. The amounts of copper dissolved in the lake waters in the summer of 1963 were <2-≤15μg/l in Goshiki-numa and <2-≤20μg/l in Naka-numa; those of zinc were <9-≤85μg/1 in the former lake and ≤23-≤45 pg/I in the latter. Uncontaminated Japanese lakes, such as, Lake Yogo-ko north of Lake Biwa-ko contains 0.7μg Cu/l and 2. 1 pg Zn/l in its water (MORITA 1955); and so the amounts of copper and zinc in the Shizu lakes under consideration may be said to be so large as to have a toxic effect upon microscopic plants and animals. Only a few species of crustaceans have survived, all of which are tolerable to such extreme habitats. The absence of minute plankton algae in Goshikinuma and Naka-numa is probably due to the toxicity of these metal ions. As the operation of the mine has been discontinued since 1963, the inflow of water containing copper and zinc into the lakes has so decreased that its influence upon the flora and fauna seems to be gradually disappearing. There are, however, other sources supplying metal ions to lake water, that is to the bottom mud and to the larger aquatic plants, by means of which copper and zinc are certainly deposited in considerable concentrations. The disharmonious conditions in these lakes will therefore remain for a fairly long period of time.

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Ecological studies on the microbial populations were carried out in the Shiga Lake Group, Nagano Prefecture. The lakes were largely divided into the acidic lakes (pH 4.7-5.1) and the neutral ones (pH 6.6-7.4).
The fungi were counted 0-10/10cc in the surface water and 7-80/g in the bottom mud. The fungus populations of the bottom mud of the Shiga Lake Group were strikingly characterized by the predominance of the genus Penicillium and Pullularia Pullulans.
The bacteria were counted 2.5-15×10³/cc in the surface water and 0.7-4.2×10⁶/g in the bottom muds. They were abundant in the water as well as in the bottom muds of the neutral lakes.
Denitrifying bacteria and Clostridium sp. were distributed almost all the lakes, while nitrate-forming bacteria, sulfur-oxidizing bacteria and aerobic cellulose-decomposing bacteria were only found in the neutral lakes.
According to the microbial populations, the lakes of the Shiga lakes were divided into the following two groups.
Harmonic type : Kidoike, Maruike
Disharmonous type : Misumaike, Kaminokoike, Shimonokoike, Nagaike, Hasuike, Ichinuma
The microbial populations were rich in the harmonic type than that of the disharmonous ones.

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The plankton and algae (Diatomaceae and Desmidiaceae) of Lake Kahoku-gata were studied by the author on 23 September 1958. Lake Kahoku-gata is the largest brackish water lagoon in the Hokuriku District, lying in the area about long. 136°41' E, lat. 36°40' N and at 0.8m. above sea level. It has a superficies of 26 sq. km., 10.0 km. long, 4.2 km. in maximum breadth, with the length of shore line 26 km. Its greatest depth 4 m. is found in the northern part of the lake basin off the mouth of the Asano-gawa river.
The results obtained in the survey are as follows (the number of species in paranthesis shows that of common species found in brackish water):
1. In the surface water, the pH values range from 7.0 to 7.7 and the chlorinity from 0.27 to 0.65 g/l. These values show less alkaline and lower chlorinities than MASHIKO's data obtained in 1949, in which he registered the pH 8.6-8.8 and the chlorinity 1.45-8.45 g/l in the surface and 11.8-13.9 g/l in 3 m. deep.
2. The plankters occurred are as many as 22 species, of which 17 are phytoplankters and the remaining 5 (4) zooplankters. Diatoms are the most common in the former, and both Pseudodiaptomus inopinus and Paracyclopina nana are the most abundant in the latter. Both of these zooplankters are the representative species in Japanese brackish waters, where they are widely distributed and are very common.
3. 54 forms of diatoms and one species of desmid were found in this lake. They are classified as follows:
a) 16 (6) species of plankton diatoms ; b) 30 (14) species of diatom shells found on the surface of the bottom mud ; c) 37 (17) species of diatoms and one species of desmid found on the larger aquatic plants.
The frequency of occurrence of the dominant and subdominant species is shown in average precentage in Table 4. The diatom communities consist chiefly of three brackish water species, namely, Coscinodiscus lacustris, Achnanthes brevipes and Achnanthes brevipes var. intermedia.
According to the ecological types of diatoms deviced for brackish water species, Coscinodiscus lacustris belongs to a "Halophil" designated by HUSTEDT (1930), and Achnanthes brevipes and A. brevipes var. intermedia to a"Mesosaprob" by KOLBE (1927) or β-Mesosaprob" by BUDDE (1931). In this respect, the writer's results obtained in Lake Kahoku-gata seems to agree with the ecological types of these species proposed by the European workers, HUSTEDT and KOLBE.

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