Reproductive ecology of the dominant dinoflagellate, <Emphasis Type="Italic">Ceratium fusus</Emphasis>, in coastal area of Sagami Bay,Japan

The seasonal abundance of the dominant dinoflagellate, Ceratium fusus, was investigated from January 2000 to December 2003 in a coastal region of Sagami Bay, Japan. The growth of this species was also examined under laboratory conditions. In Sagami Bay, C. fusus increased significantly from April to September, and decreased from November to February, though it was found at all times through out the observation period. C. fusus increased markedly in September 2001 and August 2003 after heavy rainfalls that produced pycnoclines. Rapid growth was observed over a salinity range of 24 to 30, with the highest specific rate of 0.59 d⁻¹ measured under the following conditions: salinity 27, temperature 24°C, photon irradiance 600 μmol m⁻²s⁻¹. The growth rate of C. fusus increased with increasing irradiance from 58 to 216 μmol m⁻²s⁻¹, plateauing between 216 and 796 μmol m⁻²s⁻¹ under all temperature and salinity treatments (except at a temperature of 12°C). Both field and laboratory experiments indicated that C. fusus has the ability to grow under wide ranges of water temperatures (14–28°C), salinities (20–34), and photon irradiance (50–800 μmol m⁻²s⁻¹); it is also able to grow at low nutrient concentrations. This physiological flexibility ensures that populations persist when bloom conditions come to an end.     

Dynamics of elastically moored floating objects

The two-dimensional problem of wave transformation by, and motions of, moored floating objects is solved numerically as a boundary value problem by direct use of Green's identity formula for a potential function. The cross-sectional shape of the floating object, the bottom configuration and the mooring arrangements may be all arbitrary. For a given incident wave, the three modes of body motion, the wave system and mooring forces are all solved at the same time. A laboratory experiment is conducted to verify the theory. Generally good agreements between the theory and experiments are obtained as long as the viscous damping due to flow separation is small. A numerical experiment indicates that a conventional sluck mooring is to worsen the wave attenuation by a floating breakwater and that a properly arranged elastic mooring can considerably improve the wave attenuation by a floating breakwater.     

Algal Growth Potential and the limiting nutrient in Mikawa Bay

The Algal Growth Potential (AGP) of water samples collected off Gamagori in Mikawa Bay was measured from May 1978 through February 1979, and the limiting nutrient was determined using regression analysis and enrichment bioassays. The surface and bottom water samples had AGP that produced increments of chemical oxygen demand (COD) of 2.1 mg l^–1 and 3.1 mg l^–1, respectively, on average. These values ofCOD correspond to 46% and 97% of the average COD values of the raw water samples at the surface and bottom, respectively. Seasonal changes of AGP showed a close correlation with those of dissolved inorganic nitrogen (DIN) concentration. Enrichment bioassays showed that DIN was the most deficient nutrient. The DIN:phosphate-phosphorus (PO₄ ^3–-P) ratios and DIN: dissolved phosphorus (DP) ratios in the water samples were below the cellular N:P ratios of the natural algal populations. These results suggest that AGP was mainly limited by DIN concentration.     

Interannual fluctuations in recruitment of walleye pollock in the Oyashio region related to environmental changes

The Japanese Pacific walleye pollock (Theragra chalcogramma) stock is the largest stock of this species in Japanese waters. It is a key component of the Oyashio ecosystem. In southern Hokkaido waters, these fish spawn mainly during January and February near the mouth of Funka Bay (FB), and most eggs and larvae are transported into FB. During midsummer juvenile pollock migrate along the southern coast of Hokkaido to a nursery ground on the continental shelf off eastern Hokkaido (Doto area). However, some eggs and larvae are transported southward to the Tohoku region (TR). Transport depends largely on the Oyashio, which generally flows southward along the eastern coasts of Hokkaido and Tohoku. Thus, this stock has two different recruitment routes: FB–Doto and FB–TR. In the 1980s, when the southward flow of the Oyashio was strong, the number of age-2 pollock estimated from a virtual population analysis (VPA) indicated that recruitment to the entire stock remained at a medium level. In the 1990s, when the Oyashio weakened, strong year-classes occurred in 1991, 1994, and 1995, but not in the latter half of the 1990s. Juvenile catches in the TR by commercial fisheries, which can be taken as indices of recruitment level via FB–TR, were high during the 1980s and decreased in the 1990s. Although there was no significant difference in the average number of recruits between the 1980s and the 1990s as estimated from a VPA, the recruitment patterns differed between the two decades. Here, we propose that recruitment routes of this stock shifted in response to environmental changes.     

Experiments of wave-driven soil transport in clay beds

The physical processes of instability and soil mass transport in a soft clay bed by waves were quantitatively reproduced and measured in a laboratory soil-wave tank. Soft clays behave like plasto-elastic materials and dissipate wave energy quickly. Waves destroy clay fabrics gradually and soften clay. Waves induce mass transport of clay in the clay bed. The clay mass transport increases with wave energy. The Froude-Mach similitude is applied to the experimental data to hindcast the soil mass transport caused by Hurricane Camille in 1969.     

Phosphorus and nitrogen cyclings in the pelagic system of hiroshima bay: Results of numerical model simulation

A numerical ecosystem model expressing both phosphorus and nitrogen cyclings in the pelagic system of Hiroshima Bay, Japan, was developed in order to investigate the seasonal variations of these elements and their annual budgets. Based on the geophysical and chemical structures of the bay, Hiroshima Bay was divided into northern and southern boxes, themselves divided into two layers of an upper 5 m and a lower layer according to the stratification of the water column. The model consists of equations representing all relevant physical and biological processes. The results revealed that the internal regeneration of materials was an important source of bioavailable nutrients for phytoplankton growth in the water column. The incorporation of phytoplankton aggregation improved the accuracy of the outputs in comparison to the observed data, especially during the stratified summer season. The results also indicated that phosphorus limitation of phytoplankton growth occurs in the upper layer during summer while light limitation occurs in the lower layer. In addition, physical processes such as diffusion and advection were also important as they ensured that most of regenerated nutrients in the lower layer were transported to the upper layer. Thus, these processes might support the high primary production and the production of oysters that are extensively cultured in this bay. Considering the informative results obtained, the model used in this study provides a sound basis and tool to describe the dynamics of phosphorus and nitrogen cyclings in Hiroshima Bay.     

The chemical composition and the cadmium,chromium and vanadium contents of shall-water deposits in Tokyo Bay

The major elements and the minor elements cadmium, chromium and vanadium in 12 samples of shallow-water deposits collected in Tokyo Bay were studied. Average silica content of the deposits is 53.93%, which is approximate to the average of 54.15% of red clay. Iron (av. 5.67% Fe₂O₃), titanium (av. 0.71% TiO₂), especially manganese (av. 0.87% MnO), are more abundant in the deposits than in the deposits along the entire sea-coasts of Honshu and of Nanao Bay, Japan. The deposits show higher contents of cadmium (av. 0.61 ppm), the chromium (av. 86 ppm) and vanadium (av. 79ppm). About 3 to 17 times cadmium and manganese contents compared with those in the deposits along the sea-coasts of Honshu and of Nanao Bay, Japan was found. Such accumulation, of cadmium and manganese in the deposits is probably caused by artificial contamination rather than of natural origin. No relation was found between the contents of cadmium, chromium and vanadium and those of major elements.     

Effects of fixation and storage on flow cytometric analysis of marine bacteria

Flow cytometry (FCM) is now becoming a routine tool for the enumeration and optical characterization of bacteria in marine environments. We investigated the effects of sample fixation and storage upon flow cytometric determination of marine bacteria. Fixed and unfixed seawater samples were analyzed by FCM immediately aboard ship and/or later in the laboratory, and the appearances of the fluorescence signals and bacterial counts of these samples were compared. Fixation and storage led to the formation of multiple peaks in fluorescence histograms; this was also seen in 22 out of 36 samples frozen in liquid nitrogen. Fixation did not, but storage did induce a decrease of bacterial counts: a rapid decrease during the first 3 days followed by a slower decline. The decline of cell numbers in stored samples was expressed by a regression model. Our studies indicate that precaution is necessary when interpreting the data from fixed and/or stored marine bacterial samples analyzed by FCM. The possibility that the procedure of fixation and storage leads to the appearance of high DNA and low DNA bacterial groups should be considered.