Phytoplankton communities in the Bering Sea and adjacent seas

Vertical distribution of phytoplankton in early warming season in the eastern Bering Sea and adjacent sea areas was investigated. In the surface layer which was under the influence of newly melted sea ice in the shelf water region of the Bering Sea in May, remarkably dense populations ofThalassiosira hyalina andT. nordenskiöldii and relatively large populations ofFragilaria andNavicula occupied large part of phytoplankton community. In June, although theThalassiosira populations sunk into the bottom layer and withered, a certain part of theFragilaria-Navicula populations was still suspended in subsurface layer. Thus,Fragilaria-Navicula were the leading components of the June community in the shelf region.In the Bering Basin region, no dense phytoplankton populations were developed until a shallow thermocline was established. In June when the shallow thermocline developed near shelf edge,Thalassiosira decipiens burst out. As the shallow thermocline extended from near shelf to central part of the Basin region with surface warming, the areas of blooming also shifted from near shelf to the central part.Contribution No. 73 from the Research Institute of North Pacific Fisheries, Hokkaido University.     

Subducting seamounts and deformation of overriding forearc wedges around Japan

Two subducting seamounts under inner trench slopes have been identified around Japan on the basis of magnetic anomalies, morphology and geological structure. The first one is located under the foot of the inner trench slope at the junction between the Japan Trench and the Kuril Trench. Another one occurs beneath the slope slightly seaward of the Tosabae (the basement high at the trench slope break along the Nankai Trough off Shikoku). The magnetic anomalies of seamount origin are accompanied by the characteristic morphology of a forearc wedge i.e., a swell landward and a depression seaward. The seamounts beneath the inner trench slopes have preserved magnetization showing reasonably consistent directions, which suggests that the subducting seamounts have kept roughly their original shapes. The morphology of the forearc wedge can be explained by a subducting seamount on the oceanic crust pushing the forearc material forward and upward. Deformation of the forearc wedge by the subducting seamount extends to the forearc basin. The seamounts are stronger and less deformable than the inner slope material and are not offscraped onto inner trench slopes.

Two other examples of deformed inner trench slopes around Japan which can be explained by subduction of topographic highs are presented. One example is a depression on the foot of the inner trench slope northeast of the junction between the Kyushu-Palau Ridge and the Nankai Trough. Another one is an area of complex morphology of the inner trench slope along the Japan Trench around the Daiichi-Kashima Seamount.     

ACTIVE system for monitoring volcanic activity: A case study of the Izu-Oshima Volcano,Central Japan

A system is proposed for the monitoring of changes in the underground structure of an active volcano over time by applying a transient electromagnetic method. The monitoring system is named ACTIVE, which stands for Array of Controlled Transient-electromagnetics for Imaging Volcano Edifice. The system consists of a transmitter dipole used to generate a controlled transient electromagnetic (EM) field and an array of receivers used to measure the vertical component of the transient magnetic field at various distances, with automatic operation of both units. In order to verify the performance of the proposed system, numerical and field experiments were carried out by application of the system to the Izu-Oshima volcano, where a remarkable change in the apparent DC resistivity over time had been detected in association with the eruption in 1986.     

Early diagenesis of fatty acids in lacustrine sediments—II. A statistical approach to changes in fatty acid composition from recent sediments and some source materials

The percentage composition and total amounts of fatty acids from plankton, live and dead plants (waterweeds and higher plants) and a forest soil, considered as possible origins of autochthonous and allochthonous organic materials in lacustrine sediments, have been determined by gas-liquid chromatography. Statistical analyses were carried out by computing distance index of the fatty acid composition between these source materials and the uppermost sediment from Lake Suwa, and the composition diversity index (CDI) of fatty acids in the recent sediments from Lake Suwa.The distance index indicated that the fatty acid composition in the uppermost sediment is similar to those in diatoms, Zooplankton and forest soil, and the values for dead leaves were smaller than for live ones. It suggests that the autolysis and/or decomposition processes prior to deposition onto the top sediment makes the fatty acid composition of organic debris similar to the uppermost sediment. The CPI in recent sediments varied irregularly, whereas the CDI increased during early diagenesis to yield a value comparable to ancient sediments.     

Dynamical response to supernova-induced gas removal in spiral galaxies with dark matter halo

We investigate the dynamical response, in terms of disc size and rotation velocity, to mass loss by supernovae in the evolution of spiral galaxies. A thin baryonic disc having the Kuzmin density profile embedded in a spherical dark matter halo having a density profile proposed by Navarro, Frenk & White is considered. For the purpose of comparison, we also consider the homogeneous and   r ⁻¹  profiles for dark matter in a truncated spherical halo. Assuming for simplicity that the dark matter distribution is not affected by mass-loss from discs and the change of baryonic disc matter distribution is homologous, we evaluate the effects of dynamical response in the resulting discs. We found that the dynamical response only for an adiabatic approximation of mass-loss can simultaneously account for the rotation velocity and disc size as observed particularly in dwarf spiral galaxies, thus reproducing the Tully–Fisher relation and the size versus magnitude relation over the full range of magnitude. Furthermore, we found that the mean specific angular momentum in discs after the mass-loss becomes larger than that before the mass-loss, suggesting that the mass-loss would most likely occur from the central disc region where the specific angular momentum is low.     

Distributions of polycyclic aromatic hydrocarbons and alkylated polycyclic aromatic hydrocarbons in Osaka Bay,Japan

Contaminations in sediments by polycyclic aromatic hydrocarbons (PAHs) and alkylated PAHs were investigated at 44 sites in Osaka Bay, Japan. Concentrations of total PAHs and alkylated PAHs were in the range 6.40–7800 ng/g dry weights and 13.7–1700 ng/g dry weights, respectively. The PAH concentrations tended to be higher along the shoreline in the vicinities of big ports, industrialized areas, and densely populated regions such as the cities of Osaka and Kobe. The major sources appeared to be pyrogenic or both pyrogenic and petrogenic at most of the sites. PAH concentrations were remarkably high at a site near Kobe, where the concentrations of dibenzo(a,h)anthracene and benzo(g,h,i)perylene exceeded the effects-range-medium concentration and eight PAHs were above the corresponding effects-range-low concentrations. Those PAHs may have been derived from the great fire associated with the large earthquake in 1995.     

Simulated distribution and ecotoxicity-based assessment of chemically-dispersed oil in Tokyo Bay

To assess risks of chemically-dispersed oil to marine organisms, oil concentrations in the water were simulated using a hypothetical spill accident in Tokyo Bay. Simulated oil concentrations were then compared with the short-term no-observed effect concentration (NOEC), 0.01 mg/L, obtained through toxicity tests using marine diatoms, amphipod and fish. Area of oil concentrations higher than the NOEC were compared with respect to use and non-use of dispersant. Results of the simulation show relatively faster dispersion near the mouth of the bay compared to its inner sections which is basically related to its stronger water currents. Interestingly, in the inner bay, a large area of chemically-dispersed oil has concentrations higher than the NOEC. It seems emulsifying oil by dispersant increases oil concentrations, which could lead to higher toxicity to aquatic organisms. When stronger winds occur, however, the difference in toxic areas between use and non-use of dispersant is quite small.