To investigate and compare the trophic structures of hydrothermal vent communities at different depths, the carbon and nitrogen stable isotope ratios of dominant species were analyzed in the communities at Myojin Knoll (1,220–1,360 m) in the Izu-Ogasawara Arc and the Nikko Seamount (380–550 m) in the North Mariana Arc. At the Myojin knoll, a barnacle, Ashinkailepas seepiophila had high δ15N value, which suggested to be derived from photosynthetic products. Similarly, at Nikko Seamount, a polynoid polychaete, Gandulfus yunohana, a xanthid crab, Alvinocaaris sp., and Symphurus thermophilus had high δ15N values. These animals, too, are suggested to consume photosynthetic products. The range of δ13C values at Myojin Knoll was wider than that at Nikko Seamount. The difference between the ranges of these values suggest that different chemical environments between the two vent sites influence the components and distribution of chemoautotrophic bacteria found at each site, leading to different faunal compositions.
The vertical distribution and seasonal variation of pelagic chaetognaths was investigated in Sagami Bay, based on stratified zooplankton samples from the upper 1,400 m. The chaetognaths were most abundant in the 100–150 m layer in January and May 2005, whereas they were concentrated in the upper 50 m in the other months. Among the 28 species identified, Zonosagitta nagae had the highest mean standing stock, followed by Flaccisagitta enflata and Eukrohnia hamata. Cluster analysis based on species composition and density separated chaetognath communities into four groups (Groups A–D). While the distribution of Group C was unclear due to their rare occurrence, the other groups were more closely associated with depth than with season. The epipelagic group (Group A) was further divided into four sub-groups, which were related to seasonal hydrographic variation. The mesopelagic group (Group B) was mainly composed of samples from the 150–400 m layer, although Group A, in which the epipelagic species Z. nagae dominated, was distributed in this layer from May to July. Below 400 m, all samples were included in the bathypelagic group (Group D). In this group, Eukrohnia hamata was dominant with larger standing stocks than in other tropical-temperate waters, suggesting that intrusions of subarctic water drive the large standing stock of this species. Combined, these observations suggest that the seasonal and vertical patterns of the chaetognath community in Sagami Bay are influenced by hydrographic changes in the epipelagic layer and the submerged subarctic water in the mesopelagic layer.
Aulacoseira ambigua abundance and filament length were measured weekly during spring and autumn bloom periods in Trout Lake, Wisconsin, USA. In addition, several chemical and biological variables thought to influence A. ambigua growth were assessed. Results of the field-based observations were complimented with controlled laboratory experiments to evaluate the effects of phosphorus availability on A. ambigua growth. Relative to the autumn bloom period, A. ambigua colonies were generally larger and more abundant in spring prior to the termination of the bloom in June. Chlorophyll-a concentrations indicated that other phytoplankton were also more abundant during the spring bloom. Batch culture experiments indicated that increased phosphorus availability during the spring bloom contributed to the seasonal increase in A. ambigua filament length and abundance. Increase in A. ambigua filament length in response to increased phosphorus availability is discussed as a mechanism that may increase nutrient removal through sedimentation and subsequently decrease the efficiency of nutrient regeneration in higher productivity lakes.
To evaluate zooplankton interannual and latitudinal changes, Optical Plankton Counter analyses were made on preserved net zooplankton samples collected by NORPAC net from 0–150 m at 35°N–51°N stations along 180° in the central North Pacific during early–mid June 1981–2000. The mean numerical abundance of total zooplankton for the 20 years varied latitudinally from 19,200 to 84,300 ind. m-2 but the differences between the three oceanic domains were not significant. However, highly significant latitudinal changes were observed in the mean zooplankton biomass, which ranged from 1.44 to 13.2 mg dry mass m-2 with higher values in the Transitional Domain (TR) than in the Subarctic and Subtropical Domains. The high biomass in the TR was caused by the dominance of large-sized zooplankton with equivalent spherical diameters (ESD) of 2–4 mm, regarded to consist mainly of Neocalanus spp. C5. Both the slope and intercept of the Normalized Biomass Size Spectrum also showed significant latitudinal changes with a moderate slope and low intercept in the TR due to the dominance of large zooplankton with 2–4 mm ESD in biomass. In contrast to these large latitudinal changes, only limited interannual variations were observed for zooplankton abundance and biomass in the central North Pacific during the study period.
The diversity and short-term changes in the protistan microplankton community from April to June 2006 in Sagami Bay were revealed by 18S rRNA gene clone analysis. A total of 1,076 clones consisted of 68 phylotypes of dinoflagellates, 96 phylotypes of diatoms and 27 phylotypes of other protists affiliated with Ciliophora, Prymnesiomonada, Chlorophyta, Cercozoa, Chytridiomycota, and Heterokonta. Approximately half of all dinoflagellate phylotypes were affiliated with the following genera: Ceratium, Gonyaulax, Gymnodinium, Gyrodinium, Lepidodinium, Neoceratium, Prorocentrum, and Woloszynskia. The other half was classified into seven uncultured groups. These dinoflagellate clones were mostly detected in May, in contrast to the diatom clones, which were detected frequently throughout the study period. Diatoms were diverse and consisted of 14 genera and three uncultured groups. The genera Discostella, Thalassiosira and Skeletonema were dominant in April, May and June, respectively. Species richness (number of phylotypes) and diversity (Shannon-Weiner) of the whole protistan microplankton community were highest in May. This is the first example of a comprehensive molecular biological analysis of protistan microplankton community structure, and the results clearly showed a dynamic shift in the protistan community in coastal waters from April to June in Sagami Bay. The results of a direct comparison between the clone analysis and microscopic observations indicated that the clone analysis had the great advantage of enabling identification of plankton that were morphologically indistinguishable, and to reveal detailed information on the biodiversity of protistan microplankton. This advancement in molecular biological analysis will assist in our understanding of the biodiversity of protistan microplankton.
The jack-knife shrimp, Haliporoides sibogae, is an unutilized fisheries resources with a population widely distributed on the continental slope in the East China Sea, southwest of Nagasaki. The growth and longevity of this shrimp were investigated based on 3,733 specimens collected by monthly beam-trawl sampling between May 2006 and March 2009. The spawning season of the H. sibogae population southwest of Nagasaki was from November to March, and the main spawning season was in January and February. The longevity of H. sibogae was estimated to be about 38 months and the growth coefficient of this shrimp population was found to be less than half that of another two populations in Japanese waters. The secondary sexual characters of H. sibogae females were also confirmed.
The aim of this study is to evaluate regional and interannual changes in abundance, biomass and body size of the hydromedusa Aglantha digitale in the subarctic Pacific. A. digitale was sampled by 0–150 m vertical tows using a 100 μm mesh-size NORPAC net at stations from 41°30'N to 49°30'N along the 165°E line (western subarctic Pacific) and from 39°00'N to 53°30'N along the 165°W line (eastern subarctic Pacific) during the summers of 2003–2006. The mean numerical abundances during the course of the study in the west and east were 32.9 (range, 0–368) and 169.2 (0–768) ind. m-2, respectively, and those of biomass were 26.3 (0–264) and 69.1 (0–418) mg DM m-2, respectively. The abundance and biomass showed no significant north-south or year-to-year differences within each transect but they were significantly greater in the east in some years. The body size was characterized with the dominance of small-sized individuals in the east. The higher abundance and biomass, and smaller body size in the east were probably due to sampling during or just after their reproduction in the east.
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