Seasonal changes of numbers and species composition of phyto- and zoo-plankters in fertilized and non-fertilized paddy fields were surveyed from June to September of 1982 at Saitama Agricultural Experiment Station, Kumagaya, Saitama Prefecture. Both fields have been irrigated with a small channel from the River Arakawa, and experiments on the effects of fertilizers on the growth of rice plants have been made since 1944. The former field was fertilized with about 12 g N and 4 g P m^-2 in June, whereas the latter was not. In any sampling, concentrations of NH₄-N and PO₄-P, and numbers of phyto- and zoo-plankters were higher in the fertilized field than the non-fertilized one. The following sequence of the dominant groups was recognized in the non- fertilized field : Bacillariophyceae - Cyanophyceae - Bacillariophyceae Cyanophyceae ; in the fertilized one, the sequence was Bacillariophyceae - Cyanophyceae - Chlorophyceae - Chlorophyceae Bacillariophyceae - Bacillariophyceae. The latter was characterized by the appearance of Chlorophyceae (mainly Scenedesmus sp.) in summer. Seasonal changes of the dominant zooplankters are no difference between the two fields and were divided into the following three periods : Protozoa, Crustacea and Rotifera period.

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Seasonal changes and vertical distribution of Uroglena americana, which is known to cause fishy odor of tap waters, were surveyed in Lakes Chuzenji-ko, Yuno-ko, and the mouth of the Yukawa River, which connects the two lakes, during 1983 to 1985. The results were as follows (1) Remarkable increases in cell number of U. americana were observed from the end of May to the middle of June in Lake Yuno-ko and from the end of June to the middle of July in Lake Chuzenji-ko. The fishy odor of tap water, where Lake Chuzenji-ko was its source, occurred for the same period as that in blooming of U. americana in 1983. (2) The maximum cell numbers of U. americana were observed at surface layer in Lake Yuno-ko and at the depth of about 10 m in Lake Chuzenji-ko, where the river water flowed into the same water temperature layer during May to July, suggesting that most of U. americana might flow into Lake Chuzenji-ko through the Yukawa River and grow there. (3) Since Lake Yuno-ko has sufficient nutrient levels for growth of U. americana and the water temperature from June to October is nearly 15 C, a condition favoring growth of U. americana, Lake Yuno-ko would have supplied U. americana for Lake Chuzenji-ko for long period.

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The effects of closed mine-effluent on the freshwater snail, Semisulcospira bensoni, were studied in the Sasu River in the Tsushima Islands. Concentrations of Cd, Pb, and Zn were consistently higher in soft parts of the snails from the Sasu River than in those from the Se River as a control river in the Tsushima. The same trend was observed for the concentrations in the shells. The snails collected at the lower reaches of the Se River were placed in cages at the lower reaches of the Sasu River and of the Se River itself for a month. The mortalities of the snails thus placed were significantly higher in the Sasu River than in the Se River. Though remarkably high concentrations of heavy metals in the attached substance mainly composed of algae were detected in the Sasu River, those in the Se River seemed to be as low as in other non-polluted rivers. The heavy metals were not detected in the water of the Se River, while a certain amount of heavy metals was found in those of the Sasu River. The Zn concentration was relatively high. These results suggest that S. bensoni is a useful monitoring organism for the assessment of heavy metals pollution, and the snails may be affected by heavy metals through the food (attached algae) as well as by the ambient water.

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The unsteady state two-dimensional hydrodynamic dispersion model is used to simulate the seawater intrusion into confined coastal aquifers in response to the tide and to the sudden change in discharge of groundwater in the upstream region.
The main results are as follows : 1) Salt water intrudes most inland at the mean sea level in the ebb tide stage, as in the case of the fresh-salt water interface model by KISHI and INOUCHI (1984), and the transition zone from fresh water to salt water becomes widest at that sea level. 2) The movement of isocontours of concentration induced by the sudden change in groundwater discharge retreats faster than it advances and that movement is comparable with the fresh-salt water interface movement computed by the interface model of KAKINUMA et al. (1984). 3) The concentration distribution in response to the tide computed with higher values of the storage coefficient of the confined aquifer is somewhat different from the one obtained by using PINDER and GRAY'S (1977) approximation to the mass conservation equation. This difference, however, is negligible in field situations, with lower values of the storage coefficient.

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The Japanese form of Daphnia obtusa Kurz, 1874 emend. Scourfield, 1942 is described. This taxon was recorded as a variety of D. pulex (De Geer) in Japan by UÉNO (1927), but has not been definitely described in the Japanese literature since SCOURFIELD's revision (1942). In Toyama Prefecture, D. obtusa occurs only in Yadoya-ike Pond, a small-shallow and turbid pond, from early March to early May.

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Jesogammarus spinopulps M0RIN0 (Anisogammaridae) is one of the freshwater amphipods endemic to Japan. The life history and reproduction of this species were studied in a small lowland pond in Tokyo. J. spinopulps bred in winter and the life cycle was annual. The growth rate of juveniles was retarded from May to August in the field, and the laboratory experiment indicated that it was partly due to high water temperature (>25°C). The high temperature (25°C) accelerated only the growth of newborn juveniles, but repressed the growth rate of larger juveniles, maturity and survival. Juveniles began to develop sexual characteristics from October, and males became significantly larger than females prior to maturity. Reproduction occurred in a cold season from late December to early May, and a female yielded three or four clutches. The reproduction of J. spinopulps is the typical “many small egg” type as compared to other lotic and marine species of similar size. Such a reproductive type may be advantageous in environments where anisogammarids are exposed to harmful high temperature in summer and constrained to breed within a limited period.

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An improved Ekman-Birge bottom mud sampler was devised for sampling undisturbed bottom sediment core. The sediment and the overlying water were taken in a transparent plastic cylinder (diameter, 105 mm, height, 300 mm) equipped in the grab box. The grab was driven by an additional weight of 5 kg into sediment. In sitn tests for determining the vertical profiles of chemicals and bacterial populations in the overlying water and sediment proved successful for sampling undisturbed sediment. A large volume of undisturbed sediment for chemical and bacteriological studies was obtained in one trial.

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A large-celled phototrophic bacterium, which densely populates the O₂-H₂S interface of Lake Kaiike (Kamikoshiki Island), vigorously produced H₂ during the growth. The H₂ production was intimately associated with photosynthesis. In situ H₂ production and N₂ fixation at the bacterial plate (November 1986) were quite low. But when the same sample was incubated after supply with neutralized Na₂S 9H₂O solution under the light, it showed elevated rates of both processes.

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A dialysis culture chamber was constructed and used to determine cellular N/C and P/C ratios and in situ growth rate of individual phytoplankton species. The chamber was made of acrylic tubing and membrane filters of 47 mm in diameter for the diffusion boundary. A much higher permeation rate of dissolved ion into the chamber was obtained when the Nuclepore filters (0.2 and 3.0μm porosity) were used as the boundary, compared to the case when the cellophane membmane was used. Six species of planktonic algae were separatey incubated in the chamber equipped with 1.0μm porosity Nuclepore filters and suspended in a eutrophic pond for four days. The species specific growth rate and the cellular N/C and P/C ratios of these algal populations were obtained. The use of the dialysis culture chamber was thought to be effective in studying chemical composition of the species population in the fields. A few comments on the field use of dialysis culture chambers were made.

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