Marine planktonic copepods are an ecologically important group with high species richness and abundance. Here, we propose a new metagenetic approach for revealing the community structure of marine planktonic copepods using 454 pyrosequencing of nuclear large subunit ribosomal DNA. We determined an appropriate similarity threshold for clustering pyrosequencing data into molecular operational taxonomic units (MOTUs) using an artificial community containing 33 morphologically identified species. The 99% similarity threshold had high species-level resolution for MOTU clustering but overestimated species richness. The artificial community was appropriately clustered into MOTUs at 97% similarity, with little inflation in MOTU numbers and with relatively high species-level resolution. The number of sequence reads of each MOTU was correlated with dry weight of that taxon, suggesting that sequence reads could be used as a proxy for biomass. Next, we applied the method to field-collected samples, and the results corresponded reasonably well with morphological analysis of these communities. Numbers of MOTUs were well correlated with species richness at 97% similarity, and large numbers of sequence reads were generally observed in MOTUs derived from species with large biomass. Further, MOTUs were successfully classified into taxonomic groups at the family level at 97% similarity; similar patterns of species richness and biomass were revealed within families with metagenetic and morphological analyses. At the 99% similarity threshold, MOTUs with high proportions of sequence reads were identified as biomass-dominant species in each field-collected sample. The metagenetic approach reported here can be an effective tool for rapid and comprehensive assessment of copepod community structure.

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Enormous quantities of radionuclides were released into the ocean via both atmospheric deposition and direct release as a result of the Fukushima Dai-ichi Nuclear Power Plant (FNPP) accident. This study discusses the southward dispersion of FNPP-derived radioactive cesium (Cs) in subsurface waters. The southernmost point where we found the FNPP-derived (134)Cs (1.5-6.8 Bq m(-3)) was 18 °N, 135 °E, in September 2012. The potential density at the subsurface peaks of (134)Cs (100-500 m) and the increased water column inventories of (137)Cs between 0 and 500 m after the winter of 2011-2012 suggested that the main water mass containing FNPP-derived radioactive Cs was the North Pacific Subtropical Mode Water (NPSTMW), formed as a result of winter convection. We estimated the amount of (134)Cs in core waters of the western part of the NPSTMW to be 0.99 PBq (decay-corrected on 11 March 2011). This accounts for 9.0% of the (134)Cs released from the FNPP, with our estimation revealing that a considerable amount of FNPP-derived radioactive Cs has been transported to the subtropical region by the formation and circulation of the mode water.

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Synechococcus is widely distributed in the world's ocean surfaces, and is often found in sediment traps. However, its distribution and ecological importance have not been well studied in meso-and bathypelagic waters. We measured Synechococcus abundance in the Suruga Bay (central Japan) and the subtropical NW Pacific. Synechococcus abundance at depths of 200 m and below varied from 2.4 to 190 cells ml -1 , but was in proportion to the surface abundance, suggesting transport of epipelagic populations to greater depths. Surprisingly, Synechococcus was evenly distributed from 200 m down to 1420 m (Suruga Bay) or to 2000 m (subtropical NW Pacific), regardless of season. The contribution of deep Synechococcus to the total population was highest in spring in the Suruga Bay (36 to 77%), and lowest in summer in the Suruga Bay (1 to 9%) and in the subtropical NW Pacific (4 and 10%). These results suggest effective transport of Synechococcus cells down the water column during productive seasons by attachment to large particles and limited transport under oligotrophic and stratified conditions. Deep Synechococcus abundance decreased from fall to winter in the Suruga Bay, though in filtered deep seawater it did not significantly decrease for 30 d in the dark, and it increased in a light/dark cycle. Our investigations show that the standing stock of Synechococcus has been significantly underestimated in previous studies of epipelagic waters conducted during productive seasons and that Synechococcus seems to be grazed and to contribute to biogeochemical cycles in the dark ocean. KEY WORDS: Synechococcus · Dark ocean · Vertical distribution · Vertical exportResale or republication not permitted without written consent of the publisher ability to utilize organic substrates (Zubkov et al. 2003, Zubkov & Tarran 2005, suggesting that Synechococcus can survive in darkness (Cottrell & Kirchman 2009).These earlier studies led us to examine how Synechococcus is dispersed throughout the dark ocean. The purpose of the present study is to clarify the abundance and distribution of Synechococcus in the dark ocean and to make inferences about the ecological and biogeochemical roles of the organism. Surprisingly, at depths of 200 m and below, Synechococcus was evenly distributed regardless of the season or area studied. These deep populations accounted for up to 77% of total Synechococcus population in the entire water column. MATERIALS AND METHODSSampling. Seawater was collected in Niskin bottles mounted on a CTD-carousel sampler at Station (Stn) 2 (34°51' N, 138°37' E, Suruga Bay) and Stn C2700 (27°00' N, 138°00' E, subtropical NW Pacific), during cruises on board the RV 'Suruga-maru' (Shizuoka Prefectural Research Institute of Fisheries) and RV 'Soyomaru' (National Research Institute of Fisheries Science, Fisheries Research Agency). The sampling location and hydrological conditions are shown in Table 1. Stn 2 is located near the Suruga Trough, at the center of the Suruga Bay, and Stn C2700 is a time-series station on O-Line (S...