Cells of Navicula teneroidesHUSTEDT were cultivated in Petri dishes for some months. Some valves were carrying semiconical membranous covers at on e or both poles of the valve interior. The shadow of the cover was vaguely seen by transmission electron microscope. The cover was clearly seen by scanning electron microscope. The existence of those covers may be peculier to this diatom at present. Some type-specimens of it mounted by HUSTEDT himself offer similar shadows in the light microscopic photographs.

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We collected the total of 43 attached algal communities from the inshore and offshore areas of Lake Biwa during the period from June 20 to July 12,1988, and computed the DAIpo mark at each site from the species composition of the diatom assemblage in the algal community.
From the results, we could recognize an unexpected tendency for the distribution of DAIpo marks to be larger in inshore sites than offshore sites. We want to present some discussion on our findings to this phenomenon in this paper.

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Lake Mashuu, in Hokkaido, is well-known to be the cleanest lake in Japan. However, the species composition of organisms inhabiting this lake requires much clarification, notably with regard to diatoms, which have not previously been described.
The present paper describes an investigation of the diatom species com position of two samples of algae attached to submerged stones located in the shoreline of the lake in the surface water layer. Samples were obtained in August 1985 and 1987 and were found to comprise a total of 74 diatom taxa, among which two were new varieties.
Diatom assemblage index of organic water pollution (DAIpo) was calculated based on the species component in each sample, and was found to be 90.4 (1985)and 91.9 (1987).

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Even in the ecological studies, the correct identification of the species is of basic importance in the work. In order to get an accurate interpretation of water quality, the counting of diatoms using scanning electron microscopy was employed for three Nitzschia species similar to each other and difficult to differentiate. As a result, the use of scanning electron microscopy as a necessary complement of light microscopy diatom examination is recommended.

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In order to observe each component of a single frustule in detail without fail, a new method named ‘Bleaching Method’ of gently loosening and cleaning single diatom frustule using bleaching agents on the market or sodium hypochlorite solution (NaClO) is described. The method enables one to clean and loosen frustule simultaneously but before applying this method a single raw frustule must be sucked up from strewed diatom mass under the microscope with a glass capillary.

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This study examined the diatom assemblages in the surface sediments of two brackish lagoons, Lake Shinji and Lake Nakaumi. The results of this paper are summerized, as follows.
(1) There are distinct difference in composition of diatom assemblages between the two lakes. In Lake Shinji, Cyclotella caspia is dominant, and share s more than 80%. On the contrary, in Lake Nakaumi Cyclotella caspia, Thalas s ionema nitzschioides, Thalassiosira spp. and Fragilaria flavovirens appear together. Paralia sulcata, one of typical diatom species, which commonl y appears in inner bays and brackish lagoons, occurs very rarely only in Lake Nakaumi.
(2) Skeletonema costatum and Chaetoceros sp., which are dominant species in the living planktonic diatom floras of these lakes (AKiyAmA1,976-1985; Date,1976-1985), are not found in sediments. The diatom valves of those species are broken easily soon after their death and hardly deposit in lake floors.
(3) Most of fresh water diatoms which are brought into lakes by rivers are deposited within 2km apart from mouths of rivers. However some species (for example, Synedra ulna) are sometimes brought far from mouths of rivers.

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Lake Teganuma is the most polluted lake in Japan and consists of two parts, Lake Honteganuma and Lake Simoteganuma. Both are extremely shallow hypertrophic lakes. Samples taken from the two lakes from May 1986 to April 1987were examined by TEM and LM to assess the planktonic diatoms. The dominant genera were Cyclotella, Stephanodiscus and Thalassiosira. There were some difference of species composition and seasonality between the two lakes. The common dominant species were Cyclotella stelligera var. pseudostelligera f. woltereckii and Stephanodiscus hantzschii f. tenuis. Cyclotella meneghiniana was abundant in fall in Honteganuma. Simoteganuma had the summer peak of Cyclotella meduanae.

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Planktonic diatoms from Lake Oike, Tsugaru-Juniko, Aomori Prefecture were investigated from August 1986 to June 1988. One hundred and four taxa from 24genera were recorded. The relative abundance of each taxon was measured. Seven taxa had the relative abundance of more than 5 %,22 taxa did one ranging between 1 % and 5 %, and 75 taxa did one of less than 1 %. Asterionella formosa var. formosa, Fragilaria crotonensis var. crotonensis and Aulacosira granulata var. angustissima were major diatoms in this lake. A. formosa var. formosa, F. crotonensis var. crotonensis and A. granulata var. angustissima were dominant from February to June, from June to October and from October to December, respectively. Tabellaria fenestrata which had been abundant in this lake in October tober 1952 (KAWAMURA 1956) did not appear in the present study.

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In this study,474 samples were obtained in 58 rivers in the Hokuriku District from 1974 to 1988. Using these 474 samples, the structures of epilithic diatom assemblages were investigated. The main points to be examined in this study are as follows.
1) I n each sample of epilithic diatom assemblage, the first four taxa with the highest values of relative abundance were chosen. Those 4 taxa were labeled 1st,2nd,3rd,4th grade species according to the degrees of the values of relative abundance. All diatom assemblages can be classified as different types of assemblage by the combination of the four species most abundant in each assemblage.
2) A pollution spectr um was made using 55 1st grade species. This pollution spectrum was compared with that of WATANABeEt al. (1988) which mainly showed. the degrees of water pollution on the Pacific side of Japan, also using 55 dominant species. In H okuriku District the number of taxa of Indifferent species was greater than those of Saproxenous, or Saprophilous species. Twenty-five taxa were common in these two pollution spectra. Most of them were about equal in the range of appearance, though several taxa showed different tendency. This fact seems to suggest that the water quality of rivers in Hokuriku District is slightly different from that of rivers on the Pacific side of Japan.
3) The first grade species in each diatom assemblage can be regarded as representing the contact-type of the assemblage. Three different types of contact to the substrate were identified (I) Total contact, (II) Point contact and (III)Stalk contact-types.
In clean water areas both I and II types were found to be about equal in number, but in polluted water areas the number of I type was by far the greatest.

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[in Japanese]

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