Reduction of seaweed beds is a serious problem in coastal areas of Japan and worldwide, and the lack of dissolved iron in seawater may contribute to the destruction of this ecologically important habitat. We have previously developed a method for restoring seaweed beds using a mixture of steelmaking slag and composts containing humic substances. Since October 2004, we have been performing field tests on the Shaguma coast in Mashike-cho, Hokkaido, Japan, to confirm the effectiveness of this method. However, thus far, no studies have been conducted to evaluate the effects of the hydrodynamic conditions by calculating the iron distributions in the area of the field tests. In this study, we evaluated the continuous effects of this iron fertilization method in Mashike by comparing the changes in seaweed bed distributions with the analyzed iron concentrations over a 5-year study period and the simulation results of iron distribution in the study area. Our findings demonstrated that the biomass of seaweeds at the fertilized (experimental) site was larger than that at the reference site. We also found that both the analyzed iron concentration and the calculated iron distribution in the area corresponded to the distribution of seaweed beds.

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This study is conducted to analyze the effect of coastal structure to depth of closure variation. Analysis on time series bathymetry data has been applied to determine location of depth of closure. The deviation of bathymetry profile changing is also considered. Furthermore, longshore variation of depth of closure is proposed. The hydrodynamic conditions are simulated using Boussinesq model derived by Peregrine (1967). This model is applied considering its applicability to observe non-linear and dispersion phenomenon while wave propagates to the shoreline. The simulation is carried out under regular wave assumption with 20% wave height in deep area is applied as representative wave. The simulation results are obtained in term of surface water level, bottom velocity in x and y direction and current velocity. The result is utilized to calculate maximum bottom velocity just outside boundary layer. To observe sediment movement along the coast, maximum shear stress is calculated under wave-current combined motion. Dimensionless Shields parameter is also assessed. The simulation results are depicted in spatial map. Furthermore, the effect of coastal structure to depth of closure variation is confirmed using hydrodynamic conditions.

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A tidal flat is an important place for sea environments. In recent years , creation of an artificial tidal flat has increased to restore the lost natural environments by developments in the past. We investigated the environmental characteristics of the artificial lagoon tidal flat in "Wild-Bird-Watching-Park in Tokyo Port" , and compared the characteristics with other tidal flats in Tokyo Bay. Then we measured the water quality in the channels between tidal flats and Tokyo Bay for two tidal cycles at every one or two hours. Form the study, we found that the value of ignition loss is high and benthic polychaeta are dominant species of the tidal flats. And then , we found the nutrientfl uxes of NH4-N and PO4-P pass through the channels were flow out from the tidal flat to Tokyo Bay. We thought that quantity of primary production was low in the artificial tidal flat in Tokyo Port .

Kazuki AOYANAGI, Hitoshi TANAKA, Yutaka FUJITA, Makoto UMEDA and Susumu KANAYAMA 1 学生員 修(工) 東北大学大学院工学研究科(〒980-8579 宮城県仙台市青葉区荒巻字青葉6-6-06) 2 フェロー 工博 東北大学大学院工学研究科(〒980-8579 宮城県仙台市青葉区荒巻字青葉6-6-06)Lake Inawashiro located in the center of Fukushima Prefecture is an acid lake which had sufficient selfpurification mechanism and the water quality was extremely good. In recent years, however, water quality became worse due to pollution from the surrounding area. Previous studies of the authors reported a density current from field observation and also numerical simulation to reproduce the current in the lake. In this study, the numerical model has been improved by introducing the effect of wind to turbidity in order to achieve a higher accuracy. Furthermore, several hypothetical wind conditions are imposed to have a better understanding of this effect. It is found that the density current is highly influenced by the wind velocity. Simulation using relatively weak wind speed shows that the density current occurs at the center of the northern shallow area, whereas by increasing the wind velocity, the density current occurs in the east side of the shallow area.