Rapid algal growth in the water purification process often causes problems such as turbidity leak. We investigated whether the application of ultraviolet (UV) radiation (254 nm) was effective in controlling algal growth. The objective of this study is to investigate the effect of UV irradiation on some isolated algae at a conventional UV dose (50-200 mJ cm^-2) or a high UV dose (500-1000 mJ cm^-2) . Three algae that obstruct water purification, that is, diatoms of Cyclotella, green algae of Dictyosphaerium, and flagellate of Cryptomonas, were selected and cultivated. As a result, it was observed that the cell concentration was reduced or maintained during one week after conventional UV dose irradiation in all algae. Dictyosphaerium exhibited higher resistance or faster growth compared with Cyclotella and Cryptomonas. Moreover, it was observed that algal cells exposed to a high UV dose were simultaneously broken, and the cell number was reduced in the cases of Cryptomonas and Dictyosphaerium. On the other hand, for Cyclotella, the cell content was changed but the shape of the cell was not. In conclusion, a conventional UV dose inhibited algal growth and a high UV dose injured cellular contents directly and induced cytolysis in some algal species.

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The objective of this study is to clarify the characteristics of natural Zostera bed recovery and verify autonomous Zostera restoration by backfilling depression (old navigation channel) after the disappearance of beds caused by typhoon attacks, on the coast of Iwakuni. After the disappearance, first, Zostera beds, which consisted of Zostera japonica Aschers. et Graebn., appeared in tidal flats at the mouth of Imazu River, and then Zostera beds recovered rapidly, with Zostera marina L. recolonization. The backfilled area had optimum light intensity, sand movement and sediment for Z. marina in three or four years. Not only vegetative shoots of Z. marina and Z. japonica, but also flowering shoots of Z. marina appeared at the backfilled area without transplanting or seeding. Z. marina and Z. japonica tend to increase in density and coverage, respectively, and Zostera distribution patterns of the backfilled area were almost the same as those of a natural habitat. These results suggested that the recovery of Zostera beds on the coast of Iwakuni began with the recolonization of Z. japonica in tidal flats at the mouth of Imazu River, followed by the rapid recovery of Zostera beds by Z. marina recolonization. In addition, it was considered possible to restore self-sustaining Zostera beds by backfilling depression.

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The ground temperature of the Wajiro tidal flat rises up to 40 °C in summer, and this high-temperature condition becomes one of the mortality factors of Manila clams. It was noted in a previous study that Manila clams die at 35 °C or higher, but there are few findings on the percentage of surviving individuals being related to the exposure time to a high-temperature condition. Hence, we carried out a high-temperature exposure experiment of clams, and formulated a survival rate as a function of temperature and exposure time. The formula derived from the experiment in the laboratory and the observation results of temperature and density of clams in the field was indicated the possibility that high-temperature condition was one of the survival factors of clams. It was considered to be valuable as a formula to quantitatively estimate the decrease in a clam population in relation to temperature and time.

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The toxicity of heavy metals would be changed by changes in the water chemistry such as the concentration of salts and dissolved organic carbon (DOC) in river water around an estuary basin. Recently, the development of bioassay based on marine biology has been attracting much attention because of a surge of interest in marine pollution caused by various human activities. In this study, the influence of salinity changes on the toxicity of heavy metals is explained using the Biotic Ligand Model (BLM) . The copper-spiked waters of Sagami River and Tsurumi River, which have slightly high DOC and considerably high salinity, were used in the application of the BLM to the prediction of the toxicity of copper under the condition of much interaction among Cu, salinity and DOC. The toxicity of copper chloride depended on the salinity condition, and the prediction of the toxicity of copper by using the BLM was possible at the salinity of 0.028 mol L^-1～0.14 mol L^-1. Additionally, the copper toxicity was exactly determined by using the copper activity measured with a copper-selective ion electrode under various values of DOC.

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A Ceriodaphnia dubia chronic toxicity test of wastewater with five dilutions has been proposed for wastewater management by the National Institute for Environment Studies. In contrast, a convenient and economical daphnia magna acute toxicity test has also been proposed for wastewater management. In this study, the Daphnia magna acute toxicity test was conducted using wastewater samples from five factories, with and without the addition of inorganic salts and pH adjusted to 5.8 or 8.6. Furthermore, the toxicity of five pure chemicals dissolved in dechlorinated tap water and a solution of inorganic salts was tested. It was demonstrated that in many cases, the inhibition decreased with the addition of salts and was lower in the basic region than in the neutral and acidic regions. From these results, the effect of discharging wastewater after dilution in the water environment with various inorganic salt components could not be evaluated by the proposed dilution test. Thus, a method for managing wastewater discharge was proposed based on the acute toxicity of the undiluted wastewater to Daphnia magna.

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Pseudokirchneriella subcapitata growth inhibition test of wastewater, which was diluted in five stages by mixing of 5%–80% wastewater with 20%–95% OECD medium, was proposed as a wastewater management tool by a research project of the Ministry of the Environment. In this study, the growth inhibition test was conducted in wastewater samples from five factories. Mixtures of 80% wastewater with 20% OECD medium, mixtures of 80% wastewater with 1/50 seawater salts and 20% OECD medium, and 100% wastewater samples after adding a concentrated OECD medium for it to become 100% OECD medium composition were tested. Four types of compounds were tested in this manner. As a result, the inhibition varies greatly in the wastewater from four factories depending on the coexistent inorganic salts. Furthermore, the 20% OECD medium samples show higher inhibition than the 100% OECD medium samples for zinc and copper, but equal or lower inhibition than those for DBS and aniline. Because the concentration and composition of inorganic salts change in the proposed dilution test, the relationship between the dilution rate and inhibition is unclear. Therefore, for wastewater management purposes, the test should be performed in undiluted wastewater by adding a small amount of concentrated OECD medium so it to be 100% OECD medium composition.

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Sand capping using granulated coal ash was carried out for the entire surface of the Hosoioki borrow pit in Lake Nakaumi. The water quality in the borrow pit was measured, and the release rates of nutrients and hydrogen sulfide were determined by the benthic chamber method to estimate the effect of sand capping. NH₄-N and PO₄-P concentrations of overlying water at the center were reduced after the sand capping; however, the nutrient concentration still changed greatly in the summer. H₂S concentrations differed locally, but a declining tendency of the concentration was confirmed in comparison with before sand capping. The mean release rates of NH₄-N, PO₄-P and H₂S were 267 mgN m^-2 day^-1, 35.0 mgP m^-2 day^-1 and 227 mgS m^-2 day^-1, respectively. These rates were reduced by approximately 14%, 22% and 88%, respectively, compared with before sand capping. Since nutrients and H₂S were contained in the pore water of sediment, their existence is the cause of decline in the release-suppressing effect. The nutrient-release-suppressing effect was low. On the other hand, the H₂S-release-suppressing effect was high and continued for around 2 years under the condition of sedimentation on the granulated coal ash.

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