Hiroshi Hasumoto

Dissolved rare earth elements and hydrography in the Sulu Sea

Because most Southeast Asian basins are flushed rapidly by waters from the western Pacific, the effects of respiration and silica dissolution within those basins are hardly discernible based on general hydrographic and geochemical observations (Broecker et al., 1987). However, a different situation is expected for the Sulu Sea because its deep water is isolated from other deep sea basins by shallow sills of ,400 m depth. Hence, any heterogeneity of elemental distributions within the basin must be ascribed to vertical physical and biogeochemical processes. We have obtained detailed vertical profiles of dissolved rare earth elements (REEs) together with conductivity-temperature-depth (CTD) and hydrographic measurements in the Sulu Sea during the 1996 -97 Hakuho-Maru cruise. Vertical profiles clearly indicate that the REE(III)s are enriched in the deep water due to regeneration and are involved in vertical biogeochemical cycling. The light REE(III)s (La, Pr, and Nd) showed a subsurface maximum at 500 m and a minimum;1500 m in depth. None of the hydrographic properties or nutrients shows such features; therefore, the REE(III)s and their elemental ratios may be useful as tracers of water masses. The relationship of REE(III)s versus dissolved Si suggests that the REE(III)s are regenerated in a delayed fashion relative to Si, and it is likely that their dissolution largely occurs at the bottom interface. The mean residence time of the deep water is estimated to be 300 6 150 yr. Consequently, the calculated benthic flux of REE(III)s becomes roughly comparable with the river and atmospheric fluxes. Because of oxidation to Ce(IV), the Ce profile shows a decrease from ;6 pmol/kg at the surface to a constant value at 3.9 pmol/kg below 1500 m, unlike the other REEs. The REEs in the Sulu Sea are enriched in the middle REEs and Ce relative to those of the North Pacific Deep Water and must derive from local sources around the Sulu Sea. Copyright

Seamounts, new moon and eel spawning: The search for the spawning site of the Japanese eel

After analyzing all the collection data for larvae of the Japanese eel, Anguilla japonica, in the western North Pacific, we found that the spawning site of this species appears to be near three seamounts in the West Mariana Ridge, 2000–3000 km away from their freshwater habitats. These seamounts are located in the westward flow of the North Equatorial Current and are hypothesized to provide cues for migrating silver eels and to serve as possible aggregation sites for spawning. Back-calculated birth dates based on otolith microstructure of leptocephali indicate that the Japanese eel does not spawn continuously during the long spawning season from April to November, but is synchronized to spawn periodically once a month during new moon. This lunar periodicity of spawning and the seamount spawning hypothesis are new developments in the millennium-old mystery of eel spawning that has fascinated naturalists since the time of Aristotle.

Hydrothermal plumes at the Rodriguez triple junction, Indian ridge

Water column anomalies of light transmission, Mn, Fe, Al and CH4 concentrations were searched in the central, southeastern and southwestern Indian Ridge segments centered on the Rodriguez Triple Junction (RTJ) (∼25°32′S, ∼70°02′E), for the purpose of locating hydrothermally active areas, in July to August 1993. We found an active zone in the central Indian Ridge segment (25°18–20′S) approximately 12 miles north of the RTJ, where significant hydrothermal plumes were observed at 2,200–2,400 m depth. Intensive tow-yo observations using a CTD rosette multi-sampling system equipped with a transmissometer revealed that the plumes show temporal as well as spatial variations. Discrete water samples within the plumes were enriched in Mn, Fe, and CH4, with maximum concentrations of 9.8 nM, 40.2 nM and 3.3 nM, respectively. Judging from the spatial and chemical characteristics of the plumes, especially from transmission anomalies andC/H4Mn ratios, we speculate that the hydrothermal venting site might be not in the rift valley but on the eastern off-axis zone, several miles distant from the rift valley.

OCTOPUS, an octo parameter underwater sensor, for use in biological oceanography studies

A realtime monitoring system for observing physical, biological, and optical oceanographic parameters in shallow waters was developed. It consists of a CTD with oxygen sensor, anin situ fluorometer, anin situ transmissometer, and anin situ quanta meter together with an on-deck quanta meter. The system is also equipped with a water sampling device. Water sampling can be done during the upward cast on the basis of depth profiles of temperature, salinity, oxygen,in situ fluorescence, transmittance, and underwater irradiance taken during the downward cast.

Phytoplankton in the subtropical convergence during the austral summer: community structure and growth activity

Two kinds of phytoplankton communities were identified at the Subtropical Convergence (STC) at 150°E during the austral summer of 1983–1984: a sub-Antartic diatom-dominated population (>90% in terms of carbon abundance) and a northern population with less diatom dominance (about 50%). Spatial distribution of these populations were closely associated with the hydrography. The mixing of the two occured at the surface in a horizontal scale of <16 km. The northern warm and saline water mass intruded southward into the subsurface layer (30–70 m), and the colder and less saline sub-Antarctic water overlay it, producing a temperature inversion and an associated shallower pycnocline at the central part of the STC. The inversion confined the sub-Antarctic population to the shallower mixed layer which was rich in nitrate and phosphate, resulting in increased chlorophyll a and primary production..

Distribution of the subsurface chlorophyll maximum and its nutrient-light environment in and around the Euroshio off Japan

Continuous distribution of the subsurface chlorophyll maximum (SCM) was confirmed in the Kuroshio and neighbouring areas in observations at 15 to 40 km intervals. Chlorophyll amounts occurring in and immediately around the SCM constituted 60 to 80% of the total chlorophyll in the water column above the 1% light level. The SCM zone received 1 to 10% of the surface irradiance at its center and contained sufficient macronutrients to support approximately one doubling of the existing phytoplankton biomass at most stations. There were several stations where there were higher nutrient concentrations that would support more than one doubling of the existing biomass around the SCM zone, and this was interpreted as resulting from uplift of the SCM zone due to upwelling.

Microzooplankton standing crop in the western subtropical pacific off the Bonin Islands in winter, 1980

Quantitative data on the vertical distribution and biomass of microzooplankton are presented for the western subtropical Pacific west of the Bonin Islands in winter. Microzooplankton other than foraminifera and radiolarians showed similar vertical distribution to chlorophylla. Among microzooplankters, naked ciliates, tintinnids and copepod nauplii were dominant components both in number and volume. Naked ciliates were the most dominant, comprising 82.2 % and 47.7 % of the total microzooplankton by number and volume, respectively. Copepod nauplii, occupying less than 10 % by number, accounted for 35.7 % of the volume of all microzooplankton. Tintinnids contributed less to the microzooplankton standing crop, 12.6 % of the total by number and 17.3 % by volume. Calculated microzooplankton wet weight varied from 0.84 to 1.80 g m−2 and corresponded to 9.9–18.1 % of net zooplankton weight. The relative abundance of microzooplankton to net zooplankton in this study is comparable to that previously reported in the tropical and subtropical Pacific Ocean, but a little higher than in oceanic subarctic areas.

Distribution of Leptocephali and Other Fish Larvae around the Tanegashima Island, with Special Reference to the Coastal Migration of Anguilla japonica

種子島沿岸での、1991年11月(広島大学豊潮丸)および12月(東京大学淡青丸)の採集により、計242尾のウナギ目レプトケファルス幼生と計5520尾の他目の仔稚魚が採集された。ニホンウナギの幼生は採集されなかった。採集されたレプトケファレス幼生の54.5%をギンアナゴ Gnathophis nystromi nystremi が占めた。ギンアナゴ幼生は種子島の西岸および南岸の水深100-10mの測点で、水深や昼夜を間わず採集された。ギンアナゴ幼生、および他目の仔稚魚の中で優先したネズミギス Gonorynchus abbreviatus・ソコイワシ Bathyltagus ochotensis・ハダカイワシ科 Myctophidae spp. は、いずれも通常は沖合い水域に出現する魚種である。当該海域の水温・塩分濃度は表層から底層まで極めて均一であり、しかも同じ時期の黒潮の水温・塩分濃度とよく一致した。以上より、種子島の西岸と南岸には黒潮水が直接流れ込むことが明らかとなった。沖合い性の仔稚魚は黒潮によって運ばれ、種子島沿岸に達するものと推察される。同時に行なわれた、種子島西海岸長浜なぎさにおける採集では多数のニホンウナギのシラスが採集された。