In natural water regions, organic matter in the bottom sediments is transformed by microbiological activities at a high rate in its surface layer. In anoxic sediments, ammonium nitrogen generated from organic nitrogenous compounds is accumulated. In shallow-water regions, where the temperature of the bottom sediments shows a seasonal variation, the concentration of ammonium nitrogen in the sediments changes periodically through a year, as the microbiological activity is, in general, influenced by the temperature.
At four stations set up in the south basin of Lake Biwa, the vertical distributions of ammonium nitrogen in the interstitial water of the bottom sediments were observed over one year from 1977 to 1978. The vertical distributions of particulate organic nitrogen and the viable count of ammonifying bacteria, which are responsible for the formation of ammonia in the bottom sediments, were also measured. At each station, the concentration of ammonium nitrogen in the interstitial water was high in summer and low in winter, and the seasonal variation was marked in the subsurface layer of the sediments. At all stations, the highest concentrations in the year were observed in autumn, while the depth at which it appeared varied from station to station.

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In shallow-water regions of natural environments, the concentration of ammonium nitrogen in the bottom sediments shows a seasonal variation : change of sedimentation rate, change of mass transfer rate at interface might also be the reasons for the seasonal change of ammonium concentration.
The net generation rate of ammonium nitrogen in the bottom sediments and the release rate of ammonium nitrogen from the bottom sediments into the overlying water were estimated by applying the mathematical model, described previously (KAMIYAMA, 1978), to the vertical distributions of ammonium nitrogen in the interstitial water of the bottom sediments in Mikawa Bay and in the south basin of Lake Biwa, and the following results were obtained : the generation rate was greater in the upper layer of the sediments, especially in summer; negative rates were calculated for a definite period in the subsurface layer of the sediments, which suggests the possibility of denitrification occurring in sediments where redox potential showed a seasonal fluctuation; the release rate of ammonium nitrogen from the sediments into the overlying water changed seasonally, increasing in summer and decreasing in winter, and sometimes showed a negative rate (absorption) in the south basin of Lake Biwa.

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Seasonal fluctuation of dissolved organic matter and chlorophyll-a was measured in Lake Nakanuma, a small eutrophic lake in Ibaragi Prefecture during the period from spring turnover to summer stratification. The maximum concentrations of dissolved organic carbon (DOC) and dissolved carbohydrates (DCHO) were observed four and two weeks after that of chlorophyll-a, respectively. Dissolved organic matters rich in amino acids were accumulated in deep water under anaerobic conditions. Throughout the experimental period, only about 20% of DOC could be attributed to DCHO and DAA carbon, but a larger fraction remained to be identified.

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In this paper the author deals with the diatom from the Yodo estuary of Osaka, Japan. The samples were taken from the surface of the stone at four stations near the river mouth of the River Yodo on October 12, 1977.
In all 49 taxa of diatoms were identified, of which 37 were alive and 12 were observed only in cleaned material. These 12 taxa of the empty or dead frustules were mainly composed of oligo or mesohalobous forms, and they seem to be allochthonous in origin. Living diatoms were composed of marine or brackish water forms.
Arranging the genera in order of the greater number of species, variety and form, the Navicula-Amphora-Nitzschia or Navicula-Nitzschia-Amphora type, was recognized. This may be one of the characteristics of the epilithic diatom flora of the Yodo estuary.
The common living diatoms found in the lowest course of the Yodo are as follows : Licmophora gracilis var. anglica, Achnanthes kuwaitensis, A. pseudogroenlandica, Pleurosigma barbadense, P. sp. I, Navicula agnita, N. diserta, N. hyalosirella, N. normalis, N. tripunctata var. shizonemoides, N. sp. I, III, Amphora angusta, A. polita, A. sp. I, II, Nitzschia frustulum and N. inconspicua.

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Minami-Daito and Kita-Daito Islands are raised atoll reefs consisted of marginal hills (height; about 50 m above sea level) and central lowland (height; about 1 or 2 m a.s.l.). The islands would be more raised in the last ice-age than at present, and many ponds in the central lowland are considered to be originated from dolines or uvalas formed in that age (Fig. 2).
Lake sediment is mainly composed of humus and a clay layer is found in the lowest part. After NAKAO (1977), result of carbon dating of wood pieces taken just above the clay layer was 8, 560 Y. B. P, for Amida-ike in Minami-Daito Island (12.6 m under the lake bottom and 20.1 m under the water surface) and 7, 550 Y. B. P. for Aka-ike in Kita-Daito Island (17.8 m under the bottom and 19.8 m under the water surface). This means that dolines and uvalas formed in the last ice-age had submerged after it and created the ponds.
In Minami-Daito Island, flat and marshy area (L4) is found in the south-west part of the central lowland, and isolated hills and dolines (Fig. 3) are found in the north-east part of the lowland (L3). L3 surface may be the original landform during the ice-age, and in the south-west part, it would be submerged under L4 surface which consists of recent deposit (Fig. 4). Most of ponds distribute on L4 surface, but several lakes in Minami-Daito Island situate on L3 surface. In Kita-Daito Island, L4 surface is narrow and ponds are situated on L3 surface.
Distribution of electric conductivity of lake water and shallow groundwater is shown in Fig. 5, where the values for lake water is low as compared with that for groundwater. This means that sea-salt intrusion into the central lowland, especially into lakes, may be small and intermittently.

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Sampling efficiency of two chironomids, Chironomus plumosus and Tokunagayusurika akamusi, was tested with five bottom mud samplers, of which three were a grab type and two were a core type. The capacity of these samplers was as follows : (A) a standard Ekman-Birge grab, 17cm×225cm²; (B) a modified Ekman-Binge grab, 22cm×225cm²; (C) a tall box type grab. 40cm×213cm²; (D) a cylindrical core sampler, 70cm×12cm²; and (E) a square core sampler 70cm×28cm².
Since the chironomids distributed at various depths in the mud, the samplers capable of taking mud from the deeper layers were more efficient. The most efficient sampler was the tall box type grab, followed by the square core sampler. The cylindrical core sampler was not necessarily efficient in collecting these chironomid larvae, though mud was taken from the deeper layers, suggesting that the core diameter was a critical in collection of the chironomid larvae.

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