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Kawasaki, Japan

Tachibana A.,Tokyo University of Marine Science and Technology | Ishimaru T.,Tokyo University of Marine Science and Technology | Itoh H.,Suidosha Co. | Yoshida Y.,Yokohama Port and Airport Technology Investigation Office
Journal of Oceanography | Year: 2013

Seasonal change in the meso-sized copepod community structure in the central part of Tokyo Bay was investigated from January 2006 to December 2008. Three seasonal community groups were detected, and seasonal shifts of these communities are explained by life history characteristics of indicator species and seasonal changes in the hydrographical environment. In the winter-spring community, Acartia omorii and Centropages abdominalis dominate because of high growth rates at low temperature. A shift to the early summer community is caused by a diapause of Ce. abdominalis as resting eggs and an increase in the growth and egg-production rate of Pseudodiaptomus marinus at high temperature. A shift to the summer-fall community is caused by a diapause of A. omorii at hypoxic and high temperature conditions and an increase of Temora turbinata, Paracalanus parvus and other oceanic species by an enhancement of the estuary circulation. Then, the community returns to a winter-spring one by the recovery of A. omorii and Ce. abdominalis with low temperatures and oxygenation of bottom water and by the disappearance of oceanic warm-water species at low temperature. Seasonal community shifts occurred almost regularly, but the shift from a winter-spring community to an early summer one occurred 1 month early in 2007 when the water temperature was warmer than in other years. © 2013 The Oceanographic Society of Japan and Springer Japan. Source

Itoh H.,Suidosha Co. | Nishida S.,University of Tokyo
Journal of Natural History | Year: 2015

The spatiotemporal distributions of the planktonic copepod communities in Tokyo Bay in 1986–1987, when it was in a more eutrophicated condition than at present, were examined using zooplankton samples collected by vertical hauls of Kitahara’s quantitative net (mesh opening, 0.1 mm) from 19 stations in July, August, October, December 1986 and February 1987. The total abundance of copepods ranged from 3 × 104 to 2750 × 104 ind. m−2 with peaks in July and August and a remarkable decrease in October. Oithona davisae dominated most samples with the subdominant species Acartia omorii in the inner and central areas in December and February, and Paracalanus parvus s.l. in the outer area in October, December and February. The copepod communities in the inner and central areas were classified into 2–4 groups in July, August and October, whereas they mostly comprised a single group in December and February. These differences in distribution pattern might be related to seasonal changes in environmental factors such as river discharge, intrusion of high-salinity outer-bay water, and hypoxic water. In comparison with the community in July 1948, the area of dominance of O. davisae had expanded to the whole bay and A. omorii had decreased in the inner area in July 1986, while other copepods such as P. parvus s.l. and Microsetella norvegica had decreased in the whole bay. © 2015 Taylor & Francis. Source

Itoh H.,Toho University | Itoh H.,Suidosha Co. | Aoki N.,Chiyoda Corporation
Bulletin of the Plankton Society of Japan | Year: 2010

The seasonal occurrence of planktonic copepods was investigated by vertical hauls of a plankton net (mouth diameter: 36cm; mesh opening: 100 μm) and observations of resting eggs in bottom mud at a fixed station in the innermost part of Tokyo Bay during 1990-92. Fourteen species of Calanoida, 16 species of Cyclopoida and five species of Harpacticoida occurred in the study period. Number of species increased from summer to autumn by inflow of species living outside of the bay. The total abundance of copepodids ranged between 76.3 × 10 2-53.3 × 10 4 inds. m -3 with a peak in July in each year. The most dominant species, Oithona davisae, comprised more than 90% of the adult copepods in summer and fewer than 80% in autumnspring with reduction of its population and expansion of other copepod's populations. The seasonal mean-abundance was high in winter for Acartia omorii, Centropages abdominalis and Danielssenia typica, in summer for Oithona davisae and Oncaea waldemari and in autumn for Paracalanus parvus s.l, Parvocalanus crassirostris, Oithona simplex and Microsetella norvegica. The resting eggs of A. omorii, C. abdominalis, Centropages tenuiremis and Labidocera rotunda were found in the bottom mud from the study site, suggesting that these four calanoids maintain their population as resting-eggs in the seasons when their planktonic populations disappear from the water column, as known in other inlet waters of Japan. The occurrence of epibenthic copepods and resting eggs suggest that the shallow waters where oxygen depletion is relatively weak as in the study site, may play a vital role in sustaining the copepod fauna in Tokyo Bay. The mean abundance of O. davisae in July in the present study was more than five times as much as that obtained in the inner part of the bay in 1948. An estimation on the long-term fluctuation of abundance of this species based on the observed data of temperature, salinity and transparency (converted to chlorophyll-a) since 1950 and a multiple regression formula for the relationships between the copepods' abundance and these environmental factors obtained in the present study period shows that abundance of O. davisae reached more than twice the present level in the 1960s and decreased rapidly in the 1980s. © The Plankton Society of Japan 2010. Source

Miyamoto H.,University of Tokyo | Miyamoto H.,Tohoku National Fisheries Research Institute | Kotori M.,7 48 1 Kurokawa Cho | Itoh H.,Toho University | And 2 more authors.
Journal of Plankton Research | Year: 2014

We analyzed the species composition of pelagic chaetognaths in the Indo-Pacific region as reported in published papers to demonstrate geographic variation in their species diversity and its relationships with environmental variables. The resulting latitudinal pattern in species richness (SR) was asymmetric to the equator, with a peak at 35°N in the Kuroshio area and a gradual decrease to a minimum at ∼40°S. Shannon's diversity index (H′) showed a similar pattern, except that it plateaued between 20°N and 35°S. Regression analyses and structural equation models (SEMs) showed that temperature was the most important explanatory variable, both for SR and H′. The SEM also demonstrated that chlorophyll a concentration and mixed-layer depth were the second most effective variables for SR and H′, respectively. On the basis of these results, models were constructed to estimate geographical variation in SR and H′. The estimated SR was high in coastal waters in tropical to subtropical areas, while H′ was high at midlatitudes in the open ocean. The present data highlight the importance of the coastal waters of Southeast Asia and the Indian Ocean, but also indicate that a gap still exists in this area, which necessitates further data mining as well as basic research. © 2014 The Author 2014. Source

Itoh H.,Suidosha Co. | Nakata K.,Fisheries Research Agency | Sasaki K.,Nishi 23 | Ichikawa T.,Japan National Research Institute of Fisheries Science | Hidaka K.,Japan National Research Institute of Fisheries Science
Plankton and Benthos Research | Year: 2014

Species composition and vertical distribution of oncaeid copepods, which are potentially important prey for juvenile fish, were investigated in the Kuroshio Extension region, the NW Pacific, in April, August, November 1998 and February 2001. Samples were collected from 8 discrete layers in the epipelagic zone (0–200 m depth) using MOCNESS (0.064 mm mesh) during both day and night. Thirty-five oncaeid species were identified. ‘Oncaea’ (s.l.) zernovi and Spinoncaea ivlevi were numerically the dominant species comprising 20.0–48.2% and 15.2–26.8%, respectively, of adult oncaeid copepods in the epipelagic zone. Cluster analysis on all samples revealed that these were separated into three groups with discrete vertical ranges; the first one appearing in the 0–50 m depth surface layer in April and August and consisting mainly of Oncaea (s. str.), the second one located in the deepest layer and composed mostly of ‘O.’ zernovi and S. ivlevi with some mesopelagic species, and the third one located above the second one and having intermediate species composition. Species-specific vertical distributions indicate that most oncaeid populations shifted downward from August to November, when the thermocline remarkably descended. However, most Oncaea spp. did not show a downward shift with the thermocline, and were positively correlated to appendicularian abundances, suggesting that appendicularian houses, known to be oncaeid habitats and to provide food, were a possible factor affecting their vertical distribution. Niche partitioning, allowing coexistence of congeners, might be explained by differences in body size and distribution layers in Oncaea and by differences in distribution layer in Triconia. © The Plankton Society of Japan. Source

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