In order to assess the risk of radiocesium (134Cs, 137Cs) in ecosystem of Tokyo Bay after the Fukushima Daiichi Nuclear Power Station (FDNPS) accident, we investigated radiocesium concentrations of Tokyo Bay sediment and of Japanese whiting Sillago japonica living in Tokyo Bay. The surveys were conducted once a year during from August 2012 to July 2016. The sediment samples were taken from two points, North side and South side, off the coast of Kisarazu-city, Chiba. The ranges of radiocesium concentration in South side sediment were higher than those in North side sediment. Time dependent decrease in radiocesium concentrations were observed in the sediments. The ecological half-lives of radiocesium were estimated at 1.3 years of 134Cs, 4.0 years of 137Cs, in North side sediment, and at 1.1 years of 134Cs, 2.4 years of 137Cs, in South side sediment. Only a little differences in radiocesium/TOC ratio of the sediment were observed between North side and South side. It is considered that TOC plays an important role as a radiocesium binding substance in the sediment on this study area. The ranges of radiocesium concentration in Japanese whiting were <0.041～0.27 Bq/kg-wet of 134Cs and 0.11～1.4 Bq/kg-wet of 137Cs. A correlation between radiocesium level and body size, the estimated age, was not shown in the Japanese whiting. We recognized that the radiocesium concentrations in Japanese whiting living in Tokyo Bay are in safe level as a food. Ecological half-lives of radiocesium in the Japanese whiting were estimated at 3.7 years of 134Cs and 16 years of 137Cs. Concentration of radiocesium in the Japanese whiting was one or two orders of magnitude higher than that in the sea water, and an order of magnitude lower than that in the sediment. Major diets of Japanese whiting are benthic animals, which feed on organic matters of sediment. From these facts, it is considered that radiocesium in the Japanese whiting is mainly transferred from the sediment.
The wind stress is generally expressed using the drag coefficient CD with the wind speed at a height of 10 m above the sea surface, U10. However, there is considerable disagreement in the observed values of CD. To develop a model of CD, measurements of wind stress are necessary, and the wind stress needs to be calculated using the eddy-correlation method, which measures the horizontal and vertical wind components. The wind stress measurement is limited to fixed installations due to the effect of the platform on the wind flow. Therefore, a numerical simulation is a better method to select installation locations where the wind stress can be measured with high precision, by excluding the effect of the platform. As the first approach, we investigated the application of numerical simulations to high-precision observations of flow around the Hiratsuka observation tower of the University of Tokyo using CFD (Computational Fluid Dynamics). We found that the wind velocity obtained by the numerical simulations tended to be similar to the measured wind speed values. In addition, the flow visualization showed the effect of the observation tower on the wind flow. As a result, the ratio of the effect of the Hiratsuka Tower has ranges of 5%–10% for heights of 4.1 m–2.2 m, from the center of the top of the tower. Therefore, numerical simulations have an ability to determinate the position of the high-precision observations in the wind stress measurements.
In an inner bay ecosystem, benthic macrofauna (macrobenthos) plays such important roles as purifier of the sea water and as food source for higher trophic levels predators. The ecosystem model is an effective method to predict the standing stock of macrobenthos that fluctuated drastically associated with dynamics of hypoxic water in the inner bay. Most of the models to date, however, are involved in some shortcomings. First, categorization of feeding type of macrobenthos in the numerical models does not reflect the hypoxic tolerance of species. Secondly, neither cumulative effects of water temperature nor those of hypoxia are taken into consideration in formulating the mortality rate against hypoxia. In addition, recovery of standing stock of macrobenthos due to recruitment of the young population is not considered. In this study, the feeding type of macrobenthos on the numerical models is redefined according to the hypoxic tolerance of each species. The mathematical expression of mortality rate under the hypoxic condition is also renewed by means of the improved Oxygen-deficient Sensitivity Index (iOSI), which is calculated from dissolved oxygen (DO) concentration and water temperature. Moreover recruitment is given by the amount of young population estimated from field observation and DO concentration in the bottom water. The standing stock of macrobenthos predicted by the improved ecosystem model accurately reproduced observed value in Mikawa Bay, Jun. to Nov in 2014. In addition, the improved ecosystem model makes it possible to predict changes in the structure of macrobenthic communities that linked to the dynamics of hypoxia. The model is expected to serve as effective and practical tool for various environmental policies and measures.
Conservation and creation of habitats in the coastal area are set as a new target of “Basic Program for the Conservation of the Environment of the Seto Inland Sea”. In addition, when the impacts of a landfill are unavoidable, it is required to conduct appropriate compensation measures based on the result of quantitative evaluation. The quantification of ecosystem services which lost by a landfill is desirable for selecting the most effective measure. However, it is difficult to figure out quantitatively a whole variety of ecosystem services. In this report, we try to quantify the biomass around harbor structures, using a simple evaluation method of biomass carbon. On the gentle slope revetment and the adjacent seabed of the artificial island built in the western Seto Inland Sea, we compare the biomass carbon per unit area for each of three layers on the revetment and for each of habitat foundations. Moreover, in the case of conducting a landfill project with and without an eco-friendly structure in this area, we estimate the annual biological production using the biomass carbon and the amount of increase in biomass carbon.
As seaborne trade has greatly increased in recent years, it becomes more difficult to secure crew of ships. Therefore, it is an important issue how to realize unmanned robot ships which can automatically navigate without collisions even in congested waters. Although Rolls-Royce is planning to build a remotely controlled ship in 2020, standard control technology for unmanned ships has not been developed yet. Therefore an automatic collision avoidance system is discussed by carrying out not only computer simulations but also model experiments prior to the tests using actual vessels. For this purpose, the authors built an experimental system for the validation of automatic collision avoidance algorithm. In this paper, model experiments using multiple ships conducted at Marine Dynamics Basin at National Research Institute of Fisheries Engineering are introduced. Through comparisons with numerical simulations which the same algorithm for collision avoidance is implemented, it is found that there is a discrepancy in occurrence of collision in extremely congested situation.