A simple method based on physico-chemical property of sediment to estimate the similarity of tidal flat as benthic habitat was proposed. In the study, six tidal flats extending on river mouth in the Ariake Bay (Chikugo river mouth, Shiota river mouth, Rokkaku river mouth, Kikuchi river mouth, and Isahaya Bay) could be classified into five groups from results of the cluster analyses of sediment properties (particle-size distribution, water content and sulphide), and multidimensional scaling (MDS). The classified sediment groups showed the characteristic biota on biomass and species composition, indicating that biota in tidal flat will be estimated by sediment property.
We used a numerical model (the bioaccumulation model) to analyze the radioactivity concentrations of radioactive cesium in organisms living in sea areas, focusing specifically on Japanese whiting (Sillago japonica) in Tokyo Bay. The radioactivity concentrations of radioactive cesium in Japanese whiting in the period between March 11, 2011 and August 31, 2013 were spatiotemporally analyzed using the model. The concentration of 134Cs in Japanese whiting obtained using the model was 0.18 Bq · kg–1 on August 20, 2011, and this concentration then gradually decreased. The concentration of 137Cs in the whiting was 0.36 Bq · kg–1 on April 25, 2013, and this concentration remained essentially unchanged afterwards. These two tendencies agreed with the experimental measurements obtained in the present study. Therefore, the model is useful for spatiotemporal analysis of radioactivity concentrations of radioactive cesium in Japanese whiting living in Tokyo Bay. The committed effective dose—calculated based on the radioactivity concentration of 137Cs in Japanese whiting obtained using the model—was 0.03 mSv, which is smaller than the allowable internal exposure dose due to food consumption specified by the Ministry of Health, Labour and Welfare.
Construction of offshore wind farms with high-power turbines affect on marine ecosystems in shallow waters. Cetaceans, especially coastal dolphins and porpoises, are the key species in its ecosystem and need to be assessed properly. In these couple of decades, passive acoustic monitoring (PAM) is getting popular for cetacean census. They produce distinctive and species/family specific sounds, which can be used to monitor presence of animals. Here, we introduce the PAM survey for coastal dolphins and porpoises using stereo event recording system (A-tag, MMT INC., Saitama, Japan). The stereo hydrophones are most sensitive at 70 and 130–140 kHz, respectively. This leads us to discriminate dolphins (Delphinidae) or porpoises (Phocoenidae) because porpoises produce narrower band higher frequency sound than dolphins. Independent sound source in bearing angles, which is calculated from time-of-arrival difference of same signal recorded in two hydrophones of the A-tag, is used as the proxy of number of existing animals. Fixed or stationed A-tag records local movements of phonating dolphins or porpoises, which was related with the tidal current and time of a day or a year in most cases. Towed A-tag from a moving platform such as ship reveals highly concentrated area of dolphins or porpoises and seasonal change of distributions on transect lines of thousands kilometers. For setting A-tag and downloading the data, Logger Tools system (MMT INC., Saitama, Japan) is used on PC. And Igor Pro softwere (WaveMetrics, Inc., Oregon, USA) is used for data analysis to exclude false positives that were generated by background noise such as eco-sounder of ship and snapping shrimp sound.
The governments and the industry are developing major programs to produce gases economically for commercial use. The safety record of the industry, in general, is remarkable with respect to deepwater exploration. Nevertheless, as the production increases the risks also increase. Recognizing this, the development of tools for contingency and emergency planning and impact assessment have become an integral part of the exploration programs. In the United States, some of the oil and gas platforms are in the hurricane zone. When the exploration and production reaches the full scale, some of the rigs in Japan may be in the areas vulnerable to typhoons. These factors further heighten the need for contingency and emergency planning. A computer model that can predict the fate and transport of gasses and hydrates accidentally released is needed for dealing with emergencies, contingency planning, and impact assessment. The water depths to many of these gas resources easily exceed 1,000 m. At these depths, the high pressure and cold temperatures combine to convert the gases (if they are already in gas form) to hydrates, which are mixture of gas and water with a consistency like frazil ice. The conversion to hydrates is a physical process that is totally reversible. The gas hydrates are lighter than water and hence move upward. When they reach areas that are of lower pressure in regions nearer to the water surface they dissociate and get converted to gas. Hydrates and gases dissolve in water. When gas is released, initially, the dynamics become important and the behavior is like a jet or plume. As they get dispersed the dynamics of the plume becomes unimportant, but gases and hydrates are still subjected to advection and dispersion. This report presents a model MEGADEEP that is complete with the above mentioned dynamic and non-dynamic behavior of the plume. The model considers the complete thermodynamics and the hydrodynamics of the plume. It also includes the hydrate formation and dissociation kinetics and dissolution. The model can simulate the conversion of gas to hydrate and vice versa as the gases travel through the water column. MEGADEEP is a new model that is developed to predict the behavior of gas and gas hydrates released in deepwater. It has many processes that previously have never been modeled such as bubble break up and coalescence in gas plumes in the deep water and multiple size gas bubbles in the plume phase. Previous models were developed for oil and gas blowouts with emphasis on oil fate. The present model MEGADEEP focuses on gas fate and transport and makes comprehensive simulations from the point of release in the deep until all gas and hydrate are dissolved in water or until they reach the surface. This report contains the model formulation and the simulation of several possible scenarios.
The air-sea CO2 gas transfer velocity is generally expressed as a function only of the wind speed U10. However, there exists considerable disagreement among the observed values. The disagreement is especially large in the context of the different sea surface conditions (wind wave growth and swell etc.). Consequently, many models of the air-sea CO2 gas transfer velocity are proposed by field and laboratory experiments. In this study, we evaluate the estimated global air-sea CO2 gas flux using the different some air-sea CO2 gas transfer velocity models (field experiment model, laboratory experiment model and hybrid model considering wave breaking). The 6-hourly wind speed and mean period of wind and wave data sets by ECMWF were used. The maximum difference of annual global air-sea CO2 gas flux was 0.76 PgC/year. The annual global air-sea CO2 gas flux of each laboratory experiment models were the smallest value, and each hybrid models were near value to each field experiment models. The difference of each model in low latitude is large, same as the difference in middle latitude. This shows that the difference of the result of each model in low latitude is significant for the estimation of air-sea CO2 gas flux.
Drag coefficient is an important parameter in order to correctly estimate the air-sea momentum flux. However, the parametrization of the drag coefficient hasn’t been established due to the variation in the field data. Instead, a number of drag coefficient model formulae have been proposed, even though almost all these models haven’t discussed the extreme wind speed range. With regards to such models, it is unclear how the drag coefficient changes in the extreme wind speed range as the wind speed increased. In this study, we investigated the effect of the drag coefficient models concerning the air-sea momentum flux in the extreme wind range on a global scale, comparing the difference in the drag coefficient models between Charnock (1955) and Takagaki et al. (2012). Interestingly, the former model didn’t discuss the extreme wind speed range while the latter one considered it. We have found that the difference of the models in the annual global air-sea momentum flux was small because the occurrence frequency of strong wind was approximately 1% with a wind speed of 20 m/s or more. However, we also discovered that the difference of the models was shown in the middle latitude where the annual mean air-sea momentum flux was large. In addition, the estimated data showed that the difference of the models in the drag coefficient was large in the extreme wind speed range and that the largest difference became 23% with a wind speed of 35 m/s or more. These results clearly show that the difference of the two models concerning the drag coefficient has a significant impact on the estimation of a regional air-sea momentum flux in an extreme wind speed range such as that seen in a tropical cyclone environment.