It has been unclear which functions of submerged macrophytes contribute to the change in chlorophyll a (Chl.a) and the microalgal community. In this study, we used a submerged macrophyte, Potamogeton pusillus, and modeled four kinds of functions of the macrophyte on the basis of the surveys in mesocosms and laboratory examinations. An ecosystem model taking into consideration the functions of a submerged macrophyte was developed. The model could simulate the change in several indexes of water quality in mesocosms. Results of simulation using the developed model indicated that allelopathy had little effect in decreasing the level of Chl.a, but caused a decrease in the biomass of cyanobacteria and an increase in that of diatoms. The combination of two functions, allelopathy and the increase in the number of individual of cladoceran by the submerged macrophyte, suggested enhanced predation of microalgae by the cladoceran owing to the decrease in the biomass of cyanobacteria and the relative increase in the biomasses of diatoms and chlorophytes by allelopathy, promoting the decrease in the level of Chl.a. The allelopathy of submerged macrophytes might have direct and indirect effects on microbial communities.

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In order to assess the concentrations of radioactive substances in river basins contaminated by such substances released through the Fukushima Daiichi nuclear power plant accident in March 2011, a watershed model was developed. The model, improved on the basis of the AIST-SHANEL software, can estimate indexes such as the river water concentration and deposition amount of radioactive cesium in river basins after the accident using a submodel for estimating suspended solids and the radioactive cesium load in runoff water. In this study, we carried out some case studies by applying the model to the Abukuma River basin. Two major results were obtained. The model was valid for the estimation of the concentration of radioactive cesium, and the radioactive cesium load in the runoff water was quite low. In particular, for cesium 137, its natural elimination rate is low owing to its long half-life. As a result, the deposition amount of radioactive cesium in river basins would only negligibly decrease. Therefore, we suggest a need for the comprehensive examination of all measures of reducing radioactive cesium including decontamination. The model is useful for evaluating the temporal-spatial concentrations of and the impact of control policies for radioactive substances in river basins associated with nuclear accidents.

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The relationship between lakeshore configuration and siltation was studied in the east coast of the northern basin of Lake Biwa. The data of a survey report on siltation in the littoral zone of the northern basin of Lake Biwa in Shiga Prefecture were used in the analysis. The organic matter and silt contents of the bottom sediment increased at the observation sites with a steep lakeshore slope of over 5%. Furthermore, siltation was enhanced at the offshore area of sites with a large slope change. Acoustic doppler current profilers (ADCP) were set at the observation sites with heavy siltation (Chomeiji) and light siltation (Echi River). The result showed that there was no difference in the average, maximum, and minimum flow speed between the two sites. However, the water flow at Chomeiji was oscillated, whereas at Echi River was unidirectional. Thus, it was suggested that the oscillating flow contributes to siltation by preventing the transport of fine particles such as silt into the offshore zone.

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Soil percolation experiments using aggregated soil of volcanic ash (Akadama) were demonstrated for phosphorus removal from water streams, in which the phosphate removal performance of soil columns and the adsorption capacity of Akadama soil were evaluated through short-term and long-term operations over seven years. The result showed that the removal performance strongly depended on both the empty bed contact time and the thickness of the soil layer. The adsorption capacity test revealed that the cumulative phosphorus adsorption mass was in the range of 0.5-1.5 gP•kg^-1, when the phosphate concentration in the effluent began to increase rapidly. A value equivalent to 10% of the phosphate adsorption coefficient was regarded to be the purification capacity of the soil and was suggested to be an effective index for evaluating the lifetime of Akadama soil in facility planning. The regression line obtained from the relationship between effluent phosphorus concentration and cumulative phosphorus adsorption mass may be applicable to the estimation of phosphorus adsorption mass from the observed phosphorus concentration in the effluent. Moreover, cumulative phosphorus adsorption mass could be estimated from water-quality monitoring data, because the cumulative phosphorus removal mass calculated from water-quality monitoring data was in accord with the cumulative phosphorus adsorption mass to some extent.

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In Hiyoshi Reservoir (Kyoto Prefecture, Japan), two submergible aerators were installed in 1997 and have been used for the elimination of the anaerobic layer in the deep reservoir. In 2009, one of these aerators was modified as a trial, to circulate water in the shallow layer of the reservoir, so as to release spent air (oxygen-poor air) from the top of the aerator into the shallow layer. As a performance evaluation test, a field study of shallow layer circulation was conducted during the summer in 2010, under the condition of a strong thermocline existing at a depth of around 5 m. In the field test, water temperature distribution was measured within a 20 m radius around the aerator and at water depths of 0 to 20 m, during both the operation and idling of the aerator. Secondly, the velocity distribution of radial flow was measured around the aerator to clarify the water entrainment due to the air plume. As a result, it was confirmed that the thermocline was destroyed within a 10-13 m radius during operation, a horizontal cold flow (intrusion) was observed at the depth of the bottom, a cold dome resulted from thermocline destruction, and a newly developed air parcel shredder enhanced the water entrainment effect.

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