Artificial circulation potentially causes the succession of a phytoplankton community through complex mechanisms in hydraulic and ecological prosseses. In order to clarify the effect of tolerance against prolonged darkness on the community succession, the dark survival was studied for three phytoplankton taxa, Microcystis aeruginosa (Cyanobacteria), Scenedesmus quadricauda (Chlorophyceae), and Melosira ambigua (Bacillariophyceae). The period of darkness was varied by 5 cases, such as 5days, 10days, 15days, 20days and the control. Thereafter, all samples were reilluminated.
In the cases of more than 10days of darkness M. aeruginosa decreased more markedly with the darkness period, and became only 1% of the initial cell number in 20day darkness period. Whereas, S. quadricauda and M. ambigua remained their biomass even with the 20day dark period. After restarting the light-dark cycle, however, all the three species similarly increased exponentially and reached the maximum biomass levels. These results suggest that the different tolerance against prolonged darkness may cause the succession of phytoplankton community in the process of artificial circulation.

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The rates of sulfate reduction and methane production in surface sediment were surveyed with some environmental factors at four different sites in the inner part of Tokyo Bay from May to October, 1990. Dissolved oxygen in bottom water was almost depleted in August and September at the sites where the bottom has been dredged down, while at the sites of natural sea bed there was mostly more than 1mg O₂·l^-1. The sulfate reduction rate was measured by a radiotracer method. The rate was much higher at the dredged sites compared with the sites of natural bed. The sulfate reduction rate per area at the dredged site increased from May and reached the highest rate, 4.2 mmol m^-2·d^-1 in September. On the other hand, at the sites of natural bed the rate was nearly constant and less than 0.8 mmol m^-2·d^-1. The relatively high rate per area of methane production was observed only at the dredged site and its seasonal change was very similar to that of sulfate reduction. These showed that sulfate reduction and methane production were active in the sediments where the bottom water was apt to be stagnant and thereby was deoxygenated.

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Zizania latifolia is profitable for nutrients uptake and nutrient elimination from enclosed system, serving as food for migrating swans. Experiments in the laboratory scale were conducted to evaluate the inorganic nitrogen uptake by Zizania latifolia with the changing inorganic nitrogen concentration and its ratio. In order to eliminate the effects of soil adsorption, experiments were carried out on water culture. The highest rate of inorganic nitrogen (ITN) uptake was obtained when the NH₄-N and NO₃-N concentrations were the same in November. The uptake rate of NH₄-N by Zizania latifolia was higher than that of NO₃-N. It was also known that the uptake rate of ITN during April and July was higher than that of ITN during August and November. The highest rate of NH₄-N uptake was acquired when initial NH₄-N concentration was 21mg·l^-1. But, when the initial NH₄-N concentration was 28mg·l^-1, the rate of NH₄-N uptake was declined. The relationship between ITN concentrations and the rate of ITN uptake was obtained as the following: Y=1.03X⁄(6.8+X)

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In order to predict nutrient fluxes in Hakata bay, we have developed an ecosystem model named the Sediment-Water Interaction Model (SWIM). The model, consisting of two sub-models with hydrodynamic and biological models, simulates the circulation process of nutrient between water column and sediment, such as nutrient regeneration from sediments as well as ecological structures on the growth of phytoplankton and zooplankton. This model was applied to prevent eutrophication in Hakata Bay, located in western Japan. The calculated results of the tidal currents by the hydrodynamic model showed good agreement with the observed currents. Moreover, SWIM simulated reasonably well the seasonal variations of water quality, and reproduced spatial heterogeneity of water quality in the bay, observed in the field. According to the simulation of phosphorus circulation at the head of the bay, it was predicted that the regeneration process of phosphorus across the sediment-water interface had a strong influence on the water quality of the bay.

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For comprehensive hazard assessment of environmental water samples with lower toxicity, simple bioassay using three human cell lines in combination with brief vacuum condensation have been developed. Based on the toxicity recovery test using 2 model compounds, it was shown that loss of the toxicity during the condensation process was negligible.
Then this bioassay system has been applied to environmental water samples from Lake Kasumigaura, Ayase River and Shinano River. With the condensation process, the cytotoxicity of the samples can be detected much more sensitively. The samples from Lake Kasumigaura showed the highest cytotoxicity in August and showed spatial variations depending on sampling points. In case of river water samples, higher cytotoxicity was associated with higher osmolarity. No significant correlation between the cytotoxicity and water quality data except osmolarity was observed.
These results indicate that this bioassay system is quite useful for toxicity assessment of environmental water with lower toxicity. However, to integrate this kind of indices in environmental management process, systematic and continuous accumulation of bioassay data on water samples with authentic water quality data must be essential.

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The water quality of carp (Nishiki-goi) breeding ponds was investigated. The water quality was summarized as follows: pH 6.5-7.5; hardness 60-135mg·l^-1; PO₄^3- and NO₃-N less than 10mg·l^-1.
An immobilized denitrifying photosynthetic bacterium which was equipped in a circulating filtration system was utilized for the improvement of water quality concerning carp breeding aquariums. Among the immobilization carriers tested, porous ceramic immobilization showed relatively good effects on water quality improvement, such as COD and nitrate removals. For about one month, such effects were maintained and the water quality was kept within the good water quality range for carp breeder ponds.

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