Comparison of sinking particles in the upper 200 m between subarctic station K2 and subtropical station S1 based on drifting sediment trap experiments

Drifting sediment trap experiments were conducted during various seasons to elucidate the characteristics of particles sinking through the upper 200 m of the water column in the western Pacific at subarctic station K2 and subtropical station S1. The sinking particle flux increased when primary productivity was high at each station, during June–July at K2 and during February at S1. Biogenic opal (Opal) and CaCO₃ were the major components of the fluxes at K2 and S1, respectively. Contrary to the expectation of a high flux at the eutrophic station K2 and low flux at the oligotrophic station S1, the annual average organic carbon fluxes at 100 m were comparable at both stations: 62.7 mg C m^?2 day^?1 at K2 and 56.1 mg C m^?2 day^?1 at S1. The similarity of the fluxes was perhaps a reflection of the unexpectedly high primary production at S1. At K2, the organic carbon export ratio (organic carbon flux/primary productivity) was significantly and negatively correlated with primary production and tended to decrease with depth. The magnitude of the rate of attenuation of the organic carbon flux with depth was larger at S1 than at K2. This rate of attenuation tended to decrease and increase with primary production at K2 and S1, respectively. The explanation for these patterns may be that the flux of labile organic carbon at relatively shallow depths decreased with increasing primary production at K2, and zooplankton grazing pressure increased with increasing primary productivity at S1.     

Sex ratio and reproductive activity of benthic copepods in bathyal Sagami Bay (1430 m), central Japan

Sex ratios and reproductive activity of benthic copepod assemblages were investigated at the bathyal site (depth 1430 m) in Sagami Bay, central Japan. The ratio of adult females to adult males was approximately 3.5:1, significantly different from 1:1, although this parameter did not show a seasonal pattern. On the other hand, the percentage of ovigerous females among adult females and the ratio of nauplii to total copepods appeared to fluctuate seasonally in 1997 and 1998. Statistical tests, however, could not detect significant difference in either parameter. We discuss the possibility that the reproductive activity of copepods was enhanced by the increased supply of fresh phytodetritus to the sea floor.     

<Emphasis Type="Italic">In situ</Emphasis> Enclosure Experiment Using a Benthic Chamber System to Assess the Effect of High Concentration of CO<Subscript>2</Subscript> on Deep-Sea Benthic Communities

In order to evaluate the environmental impact associated with sequestration of carbon dioxide in the deep sea, a free fall type field experimental device, the benthic chamber, was developed. In situ experiments to expose deep-sea communities to elevated concentrations of carbon dioxide (average of 20,000 ppm, 5,000 ppm and control) were carried out using this device 3 times, viz., in the winter of 2002 and in the spring and the summer of 2003, in the Kumano Trough at a depth of 2,000 m. In the long-term experiments (about two weeks in winter of 2002 and summer of 2003), the abundance of meiobenthos declined whereas that of bacteria increased under the condition of 20,000 ppm carbon dioxide compared with the control. Among meiofauna, the abundance of foraminifers at the same concentration of carbon dioxide became less than the control even in the short-term (3 days in spring of 2003) experiment, suggesting that organisms with a calcium carbonate exoskeleton are more sensitive to the raised concentration of carbon dioxide. The respiration rate of the benthic community exposed to 20,000 ppm was lower in the early stage of the experiment than in the latter half, whereas it was opposite under the condition of 5,000 ppm. The increase of biological activity in the 20,000 ppm exposure group is probably due to an increase of bacteria adapted to high carbon dioxide concentrations. The present results suggest that the influence of carbon dioxide on the deep-sea benthic ecosystem does not follow a simple, linear relationship with concentration.     

Ecology of deep-sea meiobenthos in the western Pacific

I am deeply honored to have been awarded the 1988 Okada Prize of the Oceanographical Society of Japan. The present paper reviews my previous works regarding the ecology of deep-sea meiobenthos in the western Pacific area. The outline cen be summarized as follows:

Temporal variation in mass fluxes and the major components of sinking particles in Sagami Bay, off Japan

Sinking particles were collected using time-series sediment traps deployed at 350 and 20 mab at Site SB (34° 58.5’N, 139° 20.9’E, 1544 m depth) near the center of Sagami Bay, off Japan with high time resolutions of 5-8 days (March 1997 to August 1998) and 3-4.5 days (March 1998 to August 1998), respectively. The major components (CaCO₃, OM, opal, and clay) of these sinking particles and surface bottom sediments were determined using a stepwise leaching method combined with gravimetry. Average total mass fluxes were 1480, 5560 and 3068 mg/m²/year at 350 mab, at 20 mab, and in the surface sediments, respectively, indicating an enhanced collection of sinking particles at 20 mab. Clay was the dominant component and biogenic components (opal+OM+CaCO₃) were dominated mainly by opal and secondly by OM. On average, opal and CaCO₃ contents decreased gradually as clay content increased with increasing depth from 350 mab-20 mab and in the surface sediments, indicating dissolution of opal and CaCO₃ through sinking, rebound, resuspension or sedimentation processes. Thirteen total mass flux peaks at 17--40-day intervals were observed at 350 mab during the period from March 1997 to August 1998 except for winter, while eight peaks were observed at 20 mab for the period from March 1998 to August 1998. Two types of total mass peaks can be distinguished: one with a clear increase in biogenic flux (opal+OM+CaCO₃) and little or no increase in clay flux and termed a bloom type (B-type), and the other with a clear increase in clay flux, little increase in biogenic flux and termed a resuspension type (R-type). Some R-type peaks, but not all, coincided with total mass flux peaks observed at the mouth of Tokyo Bay and suggested the possibility of the effect of particulate materials transported from Tokyo Bay to site SB. The enormously large peak observed at 20 mab in late May 1998 and that at 350 mab in early June 1998 were considered to be due to some physical perturbations from an earthquake swarm near site SB during the period from April to June 1998. The 17--40-day periodicity was associated clearly with the change in biogenic flux dominated by opal flux and is thought to reflect the periodicity of biological productivity dominated by diatoms in the euphotic zone of Sagami Bay.     

Statistical assessment of load model accuracy for steel grid-reinforced soil walls

The focus of this paper is on quantitative evaluation of four different methods that use closed-form equations to calculate the nominal load in steel grid-reinforced soil walls under operational (end of construction) conditions. The four methods are the Coherent Gravity Method used in the UK, the AASHTO Simplified Method (USA), the PWRC Method used in Japan and the Simplified Stiffness Method. The accuracy of the methods is quantified based on analysis of bias statistics where bias is the ratio of measured to predicted (nominal) load. A large database of 113 measured reinforcement loads collected from 11 instrumented field walls is used in the study. For walls constructed with frictional soils, the Coherent Gravity Method and PWRC Method were the least accurate. The AASHTO Simplified Method demonstrated better accuracy and the Simplified Stiffness Method was the most accurate of all methods examined. The Coherent Gravity Method and the updated Simplified Stiffness Method for steel grid walls in the current study have the advantage that they can be used with soils that have a dependable soil cohesive strength component. However, the accuracy of the Simplified Stiffness Method was much better for all soil types based on bias analyses.

     

PSO-based stability analysis of unreinforced and reinforced soil slopes using non-circular slip surface

Slope stability analysis is one of the most intricate problems of geotechnical engineering because it is mathematically difficult to search the critical slip surface of earth slopes with complex strata owing to the involved multimodal function optimization problem. At present, a minimum factor of safety for a non-circular slip surface in a uniform and unreinforced earth slope can be calculated using several methods; however, for a reinforced soil slope, it cannot be easily calculated because of the additional effect of the reinforcement. One efficient method to search the critical slip surface is particle swarm optimization (PSO). PSO can solve complex non-differentiable problems, and its increasing ease of use has facilitated its application to multimodal function optimization problems in a variety of fields. However, the recommended PSO parameters to calculate the safety factors of unreinforced and reinforced soil slopes, namely the inertia and local and global best solution weighting coefficients, have not been sufficiently investigated. Moreover, the computational efficiency of PSO for safety factor calculation, including computational accuracy and time, has not been clarified. To calculate the unreinforced and reinforced soil slope safety factors, this study considers force and moment equilibriums, including the tensile force of the reinforcement. Firstly, the computational efficiency of the calculation process by PSO was shown to increase the maximum number of slip surface nodes in the calculation of the safety factor. Then, an analysis was carried out to investigate the safety factor sensitivity to the PSO parameters. Based on this analysis, appropriate PSO parameters for the safety factor calculation of unreinforced and reinforced soil slopes were proposed.