In future, freshwaters in Japan are likely to become acidified by increasing acid deposition in eastern Asia. In order to predict the long-term acidification of freshwaters, it is necessary to evaluate acid neutralization mechanism in Japanese watersheds.
In this paper, an estimation method for acid-neutralization by chemical weathering is proposed, and applied to acidified and non-acidified watersheds in order to compare acid-neutralization mechanism. The principal results are :
(1) Japanese non-acidified watersheds receive approximately the same amount of acidic deposition as an acidified watershed in U.S.A..
(2) In Japanese non-acidified watersheds, acid deposition is neutralized by chemical weathering of primary minerals. On the other hand, in the acidified watershed, chemical weathering rates of primary minerals are much smaller than those in Japanese non-acidified watersheds. Therefore, acid deposition is not neutralized in this watershed.
(3) Typical properties of watersheds vulnerable to acidification are :
1) Dominant primary minerals in soil and bedrock are stable against acid deposition.
2) Soil thickness (weathering zone) is relatively small.
3) The ratio of chemical weathering ratio to acidic deposition is relatively small.

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The objective of this study is to investigate the shading effect of the deposition on leaves on the survival of eelgrass to clarify the factors responsible for the loss of eelgrass habitat in an eutrophic coastal zone with small disturbance by flow and waves in Hiroshima Bay. We investigated environmental conditions of a healthy eelgrass bed as a reference site and a site located outside of the bed with no vegetation. Also, we transplanted eelgrass into these sites and observed their survival.
The transplanted eelgrass into the unvegetated site was difficult to survive, whereas those in the vegetated site could survive. The poor survival in the unvegetated site was due to the high levels of suspended solids deposited on the leaves of eelgrass. The higher deposit resulted in 64% decrease in the light intensity on leaves. The shading resulted in the decrease in the daily periods of irradiance-saturated photosynthesis required for eelgrass growth in the unvegetated site clown to 68-19% of that in the vegetated site.
Contents of silt and accumulation of mud in the unvegetated site was higher than that in vegetated site. It is most probable, therefore, that effect of flow in the unvegetated site was lower and not enough to prevent the deposition in leaves.

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Analyses on the stochastic characteristics of inflow and transformation of water quality in a eutrophic reservoir are carried out based on water quality measurements in the reservoir. The reservoir under study has an anaerobic and eutrophic water near the bottom layer throughout a year. Water body of the bottom layer shows high electric conductivity and has never been exchanged in the last few years. The measurements of water quality have been carried out every month. The structures of the obtained time series are analyzed, regarding them as stochastic process. It is found that the time series of COD and T-N are considered to be the AR(1) process. We also attempt to apply the linear time invariant system to the transformation of water quality in the reservoir.

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The purpose of this study is to clarify the effects of flow of overlaying seawater on the soil structure in tidal flat. A natural-like man-made tidal flat and a typical man-made one, both located in a deep inlet an island of in Hiroshima Bay, were selected in this study. Bacterial population, organic matter content and silt content of the soil in the natural-like man-made tidal flat were higher than those in the man-made ones. The shear stress by seawater flow on the sediments of natural and constructed tidal flat was compared.
The amounts of sediment trap and seawater flow rate in the typical man-made tidal flat were 4 times and 2 times, respectively, as high as these in the natural-like man-made tidal flat. The shear stress to the surface of the typical man-made tidal flat was higher than that of the natural-like man-made tidal flat. The frequencies of the shear stress higher than the critical value for silt transport in the typical man-made tidal flat were 1 to 17 times higher than that in the natural-like man-made one. These results indicate that the difference in the soil structure between the two tidal flat was due to the difference in the shear stress caused by seawater flow. Furthermore, the effects of the seawater flow on other tidal flats in Hiroshima Bay were investigated. The results also indicated that the profiles of shear stesses in man-made tidal flats were higher than these in natural ones. It is probable that the seawater flow plays an important role in determining soil structure in tidal flats.

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Time dependent developments of in situ spatial distributions of NH₄⁺- and NO₂^-- oxidizing bacterial populations in domestic wastewater biofilms and in synthetic nitrifying biofilms were investigated by fluorescent in situ hybridization (FISH) with a set of 16S rRNA-targeted oligonucleotide probes. NH₄⁺-oxidizing bacterial populations were classified into three groups according to the probe specificity, and population dynamics of these groups was correlated with the biofilm performance. In situ hybridization indicated that Nitrosomonas spp. (excluding probe NEU stained NH₄⁺-oxidizing bacteria : i.e., N. marina-lineage, N. europaea-lineage, N. eutropha, and N. halophila) and Nitrospira-like bacteria were the numerically dominant species in the domestic wastewater biofilms. However, probe NEU stained NH₄⁺-oxidizing bacteria became dominant populations in the synthetic nitrifying biofilms (which were initially cultured with the primary settling tank effluent) after switching to the synthetic media. This population shift might be attributed to the accumulation of NO₂^--N and relatively higher loads of NH₄⁺-N which accelerate the growth of N. europaea-lineage and N. eutropha. This evidence indirectly supported that N. europaea has been most commonly isolated and studied in most of the previous researches. These observations have considerable significance to our understanding of microbial nitrification process in wastewater treatment processes and in natural environment.

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Filtration experiments were conducted using the double layer membrane consisted of the microfiltration membrane and the activated carbon membrane. Such separation properties as the filtration rate and the rejection were examined using silicon dioxide suspensions containing phenol. The double layer membrane was found to be quite efficient for the separation of the particulate suspensions including the dissolved organic matter. The silicon dioxide particles were rejected by the sieve effects of the microfiltration membrane, whereas phenol was rejected by the adsorption effects of the activated carbon membrane. High phenol concentration caused the reduction of the phenol rejection as filtration proceeded. Although the filtration rate increased with the increase of the filtration pressure, the phenol rejection in the incipient stage of filtration became relatively small under high pressure conditions. It was also shown that the adsorption rate obtained in filtration experiments was significantly larger than that in static adsorption experiments.

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