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We investigated how the nutrient status of Matsushima Bay was affected when a wastewater plant was destroyed by a tsunami in March 2011. The nutrient concentrations in the seawater and the treated water from the wastewater plant increased just after the tsunami, but decreased again to pre-tsunami levels after 2013. The amount of untreated water that passed through the wastewater plant decreased just after the tsunami. It was estimated that approximately 40.7 × 103 m3/day of the treated water from the wastewater plant was discharged to Matsushima Bay; therefore, the quantity of effluent from the wastewater plant was less than one percent of the water inflow from Takagi River and Sendai Bay (which are outside Matsushima Bay) to Matsushima Bay. The nutrient concentrations of seawater in Sendai Bay were lower than those in Matsushima Bay. The results suggest that nutrient concentrations in Matsushima Bay after the tsunami did not increase because any untreated or poorly treated effluent was easily diluted by the river flow and the inflow of seawater. Many people were concerned about eutrophication, therefore, because of the decreased functioning of the wastewater plant in Matsushima Bay. Marked eutrophication in the bay was not observed after August 2011.

The Pacific saury (Cololabis saira) is a fish of commercial importance subject to unpredictable epidemics of infection by Pennella sp., a parasitic copepod. We analyzed the distribution of fish presenting with newly attached Pennella to determine the region and season in which Pennella first infect the fish. Pacific sauries migrate northward in the spring to spend the summer in the subarctic region, successively crossing the Subarctic Boundary (SAB) and the Subarctic Front (SAF). The fish then return to subtropical waters in the fall and overwinter there. Pacific sauries infected with Pennella were observed on both the north and south sides of the SAF from May to December. Newly attached Pennella, however, were observed mainly to the south of the SAF during the northward migration of the fish in May and June, and only to the south of the SAB during the southward migration in November and December. These results indicate that the intermediate host or hosts of Pennella inhabit the region south of the SAF, with infection of Pacific saury occurring during late fall and spring. Such information may assist in identifying the intermediate host(s).

Although numerous metagenome, amplicon sequencing-based studies have been conducted to date to characterize marine microbial communities, relatively few have employed full metagenome shotgun sequencing to obtain a broader picture of the functional features of these marine microbial communities. Moreover, most of these studies only performed sporadic sampling, which is insufficient to understand an ecosystem comprehensively. In this study, we regularly conducted seawater sampling along the northeastern Pacific coast of Japan between March 2012 and May 2016. We collected 213 seawater samples and prepared size-based fractions to generate 454 subsets of samples for shotgun metagenome sequencing and analysis. We also determined the sequences of 16S rRNA (n = 111) and 18S rRNA (n = 47) gene amplicons from smaller sample subsets. We thereafter developed the Ocean Monitoring Database for time-series metagenomic data (http://marine-meta.healthscience.sci.waseda.ac.jp/omd/), which provides a three-dimensional bird’s-eye view of the data. This database includes results of digital DNA chip analysis, a novel method for estimating ocean characteristics such as water temperature from metagenomic data. Furthermore, we developed a novel classification method that includes more information about viruses than that acquired using BLAST. We further report the discovery of a large number of previously overlooked (TAG)n repeat sequences in the genomes of marine microbes. We predict that the availability of this time-series database will lead to major discoveries in marine microbiome research.

The taxonomic compositions of marine prokaryotic communities are known to follow seasonal cycles, but functional metagenomic insights into this seasonality is still limited. We analyzed a total of 22 metagenomes collected at 11 time points over a 14-month period from two sites in Sendai Bay, Japan to obtain seasonal snapshots of predicted functional profiles of the non-cyanobacterial prokaryotic community. Along with taxonomic composition, functional gene composition varied seasonally and was related to chlorophyll a concentration, water temperature, and salinity. Spring phytoplankton bloom stimulated increased abundances of putative genes that encode enzymes in amino acid metabolism pathways. Several groups of functional genes, including those related to signal transduction and cellular communication, increased in abundance during the mid- to post-bloom period, which seemed to be associated with a particle-attached lifestyle. Alternatively, genes in carbon metabolism pathways were generally more abundant in the low chlorophyll a period than the bloom period. These results indicate that changes in trophic condition associated with seasonal phytoplankton succession altered the community function of prokaryotes. Our findings on seasonal changes of predicted function provide fundamental information for future research on the mechanisms that shape marine microbial communities.

Yield from fisheries is a tangible benefit of ecosystem services and sustaining or restoring a fish stock level to achieve a maximum sustainable yield (MSY). Snow crab (Chionoecetes opilio) off Tohoku has been managed by a total allowable catch since 1996, although their abundance has not increased even after 2011, when fishing pressure rapidly decreased because of the Great East Japan Earthquake. This implies that their biological characteristics, such as recruits, natural mortality coefficient (M), and terminal molting probabilities (p), might have changed. We developed “just another state-space stock assessment model (JASAM)” to estimate the MSY of the snow crab off Tohoku, Japan, considering interannual variations inMandp. The multi-model inference revealed thatMincreased from 0.2 in 1997 to 0.59 in 2018, although it was not different among the instars, sex, nor terminal molt of crabs. The parameterpalso increased by 1.34–2.46 times depending on the instar growth stages from 1997 to 2018. We estimated the MSYs in three scenarios, which drastically changed ifMandpwere set as they were in the past or at the current values estimated from this study. This result indicated that the MSY of snow crab would also be time-varying based on their time-varying biological characteristics.

To address various fisheries science problems around Japan, the Japan Fisheries Research and Education Agency (FRA) has developed an ocean forecast system by combining an ocean circulation model based on the Regional Ocean Modeling System (ROMS) with three-dimensional variational analysis schemes. This system, which is called FRA-ROMS, is a basic and essential tool for the systematic conduct of fisheries science. The main aim of FRA-ROMS is to realistically simulate mesoscale variations over the Kuroshio-Oyashio region. Here, in situ oceanographic and satellite data were assimilated into FRA-ROMS using a weekly time window. We first examined the reproducibility through comparison with several oceanographic datasets with an Eulerian reference frame. FRA-ROMS was able to reproduce representative features of mesoscale variations such as the position of the Kuroshio path, variability of the Kuroshio Extension, and southward intrusions of the Oyashio. Second, using a Lagrangian reference frame, we estimated position errors between ocean drifters and particles passively transported by simulated currents, because particle tracking is an essential technique used in applications of reanalysis products to fisheries science. Finally, we summarize recent and ongoing fisheries studies that use FRA-ROMS and mention several new developments and enhancements that will be implemented in the near future.

Pre-fishing season (June and July) Pacific saury Cololabis saira occur offshore, east of Japan, before migrating west to nearshore waters where they are exploited by Japanese fishers in the autumn (September–November). To forecast the location of these fishing grounds we develop a migration model using oceanographic (temperature and current velocity) and fisheries (pre-fishing Pacific saury distribution obtained from stock assessment surveys) data, and migration characteristics determined from fishery data analysis. We speculate that Pacific saury migrate seasonally, first north, staying within a certain temperature zone from May to July, then west, remaining within a preferred but gradually increasing temperature zone. We tune our model using hindcast calculation to reproduce actual fishing grounds around Japan. In July 2018 we forecast the locations of early Pacific saury fishing grounds from August to September. Our forecast fishing grounds were subsequently validated by actual positions of the fishing grounds. Our model successfully forecast the locations of early fishing grounds along the Kuril Islands archipelago, and forecast particles that reached an offshore region roughly predicted offshore fishing grounds. This model also identified Pacific saury migration route trajectories in detail, and that these two fishing grounds in 2018 were formed from geographically separated pre-fishing season shoals via different migration routes. We believe that applying our model will improve the efficiency of the Pacific saury fishery and fleet operation through savings in vessel fuel and time spent searching for shoals.

We investigated distribution patterns of a doliolid (Dolioletta gegenbauri) bloom in relation to the physical environment using a video plankton recorder in the Oyashio–Kuroshio mixed water region. Using 12 km transects, doliolid blooms were encountered at a horizontal scale of about 2–3 km, which corresponds to submesoscale physical events. Doliolids were also consistently encountered in the subsurface layer above the pycnocline in warmer (> 14oC) and higher-salinity (> 34) water masses, and seawater density was the most critical factor affecting distribution depth. Compared to previous studies, the density and biomass of the blooms observed in this study (77 mgC m−3 and 4600 inds m−3) were highest in the open ocean. Bloom formation consisted of two phases; first, the seeding population of a nurse stage increased rapidly to 2000 inds m−3 by asexual reproduction, followed by asexual production of phorozooids. Estimated population clearance rates revealed that these dense patches could potentially sweep the surrounding water within 2–3 d. The incidence of exhausted and shrunken zooids was significantly correlated with patch density, suggesting that mortality was due to overgrazing. Shrunken doliolids appeared to sink below the pycnocline, corresponding to 8–17% of the particulate organic carbon flux at 150 m. Hydromedusae, pelagic polycheates, and sapphirinid copepods preyed on the doliolids. These results indicate that doliolids, which were seeded by populations originating from the Kuroshio, formed dense blooms in response to submesoscale physical events and would alter the sinking particle properties (i.e., biological pump) and the epipelagic food web structure through their grazing and mortality.

We investigated the interdecadal trend of Oyashio velocity and transport during 1993–2011 based mainly on linear trend analysis of altimetry and in situ temperature-salinity data from a monitoring line (“A-line”) off the southeastern Hokkaido coast. Significant trends of increasing sea level were detected on the continental slope, north of the Kuril-Kamchatka Trench. Sea level anomaly data revealed a localized clockwise circulation centered near the trench, the suggestion being that the strength of the Oyashio on the slope and the offshore return flow had decreased. The Oyashio mainstream seemed to have shifted from a nearshore to an offshore path. Steric heights estimated from the A-line data exhibited an increasing trend north of the trench, where 50–80% of the increase was determined by halosteric components attributable to a trend of decreasing salinity in the subsurface. The trend of decreasing salinity was related to downward displacement of isohaline/isopycnal surfaces. The largest displacement was above the trench. Horizontal pressure gradients associated with southwestward flows on the slope were weakened. The Oyashio transport decreased by 8.9 Sv (106 m3 s−1) in 19 years. A mesoscale eddy analysis revealed that clockwise eddies appeared more frequently in recent years near the trench around the A-line and could decrease the Oyashio transport. A baroclinic, long Rossby-wave model also predicted that a large-scale baroclinic response to the wind stress could weaken the Oyashio velocity in the upper layer. Dynamical linkage between the localized eddies and large-scale response remains to be clarified in future work.

We investigated the elevated chlorophyll-a (Chl-a) zone found along the coast in winter in Sendai Bay, Japan, using hydrographic observations and a one-dimensional ecosystem model. Chlorophyll-a distribution was vertically homogeneous with a horizontal gradient and could be approximated as a power function of bottom depth; Chl-a concentration drastically increased with decreasing bottom depth, despite temperature and salinity being almost vertically and horizontally homogeneous. The observed results revealed significant correlations among Chl-a and nutrients concentrations proportional to the Redfield ratio. Diatoms accounted for more than 99% of the detected total cells, indicating the occurrence of a diato m bloom. A one-dimensional ecosystem model, which incorporated vertical mixing and the self-shading effect of phytoplankton, revealed that bottom depth was responsible for the occurrence of the bloom during the convective season in coastal area where vertical mixing reached the bottom and that there existed the critical bottom depth where the integrated Chl-a in the water column remained constant. A bloom could occur where the bottom depth is shallower than the critical bottom depth, not when the depth of the mixed layer is shallower than the classical critical depth and the stratification is established. From the observational and model results, it is suggested that the diatom bloom was induced by oceanic water intrusion, which transported nutrients to the bay and the elevated Chl-a zone was formed within a month after the intrusion.

After the adoption of the Aichi Target, data accumulation and evaluation regarding biodiversity have progressed rapidly. The use of ecologically and biologically significant areas (EBSAs) criteria to evaluate important areas enables the identification of effective and prioritized areas for ecosystem management. This includes strategic environmental assessment and discussions aimed at establishing protected marine areas based on scientific data. This paper reviews previous and current ideas as well as the methods used, for the identification of EBSAs. In particular, the following issues are addressed: problems associated with different types of marine ecosystems in the Japanese Archipelago, such as seagrass and seaweed beds, coral reefs, offshore pelagic plankton, and deep-sea benthic ecosystems; and problems associated with the integration of multiple criteria that are not totally exclusive. Several candidate variables accounting for each of the 7 criteria used to identify ecologically important areas are presented. Data availability is the most important criterion that allowed for the comprehensive evaluation of different types of ecosystems in the same localities. In particular, for coastal ecosystems such as seagrass, seaweed beds, and coral reefs, it is possible to carry out broad spatial comparisons using variables representing most of these 7 criteria. Regarding methods for the quantitative evaluation of each criterion and their integration, application of these methods to kelp forest ecosystems in Hokkaido, Northern Japan is presented as a case study.

The hydrographic structure of a quasi-stationary jet separated from the Kuroshio Extension, a phenomenon that possibly leads to deepening of the winter mixed layer, is revealed via intensive observations using a conductivity–temperature–depth profiler and a ship-mounted acoustic Doppler current profiler (ADCP) carried out in September 2009. Data collected from profiling floats set to drift isopycnal layers and time series of sea surface buoys and satellite altimeters were also analyzed to understand the continuity, water characteristics, and time variation of the jet. Although the flow field was complex due to disturbances such as energetic eddies, the jet transported subtropical water to the downstream (northeastern) observation region in a layer shallower than 400 dbar and its highest velocity was concentrated in the mid- and deep layers (≤0.30 m s−1 at the sea surface and ≥0.05 m s−1 at 800-m depth). The velocity axes of the jet detected from the satellite data corresponded to those detected through analysis of the ADCP data, and the intensity of the jet at the sea surface varies over interannual-to-decadal time scales. Part of the interannual-to-decadal variation in the velocity field of the jet is controlled by the dynamic state of the Kuroshio Extension and is correlated with that of the Kuroshio Extension latitude (linear correlation coefficient r ~ 0.67). The relationship between these variations can be interpreted qualitatively as being responsible for the inertial streamers that separate from the crests of meanders of the Kuroshio Extension to the jet region due to the large steering effect.

We investigated predatory behavior of sapphirinid copepods on doliolids around the Kuroshio Extension at stations experiencing blooms of Dolioletta gegenbauri. Onboard observations showed that adult Sapphirina nigromaculata was an active predator of doliolids, with a preference for internal tissues. When entering a doliolid body cavity, sapphirinids left a characteristic bite mark around the fringe of oral and atrial aperture or hole on the tunic of the doliolid. In situ observations with a video plankton recorder (VPR) revealed that association between sapphirinids and doliolids was common in the field. Adult sapphirinids and doliolids exhibiting the characteristic evidence of an attack (bite mark or hole) were found in sediment traps at a depth of 50 m, indicating that the association between these taxa was due to predation. Early copepodites, which were not observed in sedimenttrap samples, appeared in the VPR observations to have a semi-parasitic phase when they attached themselves to nurse chains. The maximal daily ration of sapphirinids estimated by onboard experiments ranged between 29% and 37% of their body carbon weight. Although the mean predation effect by sapphirinids on the doliolid population biomass was only 0.7% d21, sapphirinids potentially had a greater effect on doliolid abundance at the termination of doliolid blooms. Some of the attacked doliolids were discarded by the sapphirinids and contributed to the sinking flux below 150 m, the importance of which as a source of detritus likely increased with depth. Sapphirinids, despite their relatively low abundance in the water column, play a specific role in driving community succession and biogeochemical cycling.

We calculated the mass balance of major congeners of dioxins derived from chloronitrophen (CNP) and pentachlorophenol (PCP) found in Sendai Bay, in order to characterize the present status of marine pollution caused by dioxins and enable the establishment of appropriate countermeasures against dioxin contamination. According to published statistics, about 58 000 t of CNP and about 22 000 t of PCP were shipped to Miyagi Prefecture over a 40-year period, and based on the these amounts of CNP and PCP, we estimated that 30.7 t of 1,3,6,8-tetrachlorinated dibenzo-p-dioxin (1,3,6,8-TeCDD), 1,3,7,9-TeCDD, and octa-chlorodibenzo-p-dioxin (OCDD) were introduced into the terrestrial environment as impurities contained in both pesticides. Mass balance analysis results show that most of these dioxins remained in the terrestrial environment and were not transported by runoff into Sendai Bay. Moreover, degradation mainly reduced the amount of dioxins in the terrestrial environment, instead of runoff or volatilization. Although large amounts of CNP and PCP were shipped to Miyagi Prefecture, only 0.8% of the total quantity of 1,3,6,8-TeCDD, 1,3,7,9-TeCDD, and OCDD contained in pesticides that were applied to paddy fields reached Sendai Bay via the discharge of major rivers. Moreover, most dioxins transported to the bay either flowed offshore or settled at the bottom of the bay. And there was very little bioaccumulation in marine organisms, particularly commercially valuable species.

In order to characterize how bacterial communities are propagated over spatial scales in a coastal area, the bacterial community composition was examined along with a transect line set in a bay at an expiry of spring phytoplankton bloom. Four distinctive bacterial communities were found within the bay by a fingerprinting method of 16S rRNA gene amplicons. The most widely distributed one was distributed in the surface and middle layers at whole area of the bay. The water was characterized by low inorganic nutrients concentration and high bacterial abundance, suggesting that the bacterial community had been developed in the bloom. Pyrosequencing analyses of the gene amplicons indicated that Rhodobacteriaceae and Flavobacteriaceae were abundant in the bacterial community, though the most abundant bacterial taxon was SAR11. The second group was distributed in the bottom water at the coastal side of the bay where considerably high Chl. a concentration was observed, probably because of the sedimentation of phytoplankton bloom. The community diversity was high and Alteromonadaceae, Saprospiraceae, and some families of Actinobacter existed more in this community than the others. The third group was distributed in the deep water near the border with the outside of the bay. The ratio of SAR11 was the highest in this community; besides, Burkholderianceae and Rhodospilliraceae existed in relatively high abundances. Another bacterial community having intermediate characters was observed in the middle to bottom layers around a central part of the bay where vertical water mixing was observed. These findings suggest that spatially different bacterial communities were formed under the influences of phytoplankton bloom and/or hydrographic events such as oceanic seawater intrusion of the bay.

Large quantities of radioactive materials were released into the air and the ocean as a result of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, caused by the 2011 Tohoku earthquake and the subsequent major tsunami off the Pacific coast. There is much concern about radioactive contamination in both the watershed of the Abukuma River, which flows through Fukushima Prefecture, and its estuary, where it discharges into the sea in Miyagi Prefecture. We investigated radioactive cesium dynamics using mixing diagrams obtained from hydrographic observations of the Abukuma River Estuary. Particulate radioactive cesium dominates the cesium load in the river, whereas the dissolved form dominates in the sea. As the salinity increased from

鉛直的・空間的に密に測定されたAOUのデータからの線形回帰, あるいは表層と底層の栄養塩濃度の線形補間によって栄養塩現存量を算出し, その季節変動を明らかにした。下層のDIN現存量は, 1,500～2,600tの範囲で変動し, 夏季に高く, 冬季に低かった。上層のDIN現存量は, 下層に比べ, 夏季は低いものの冬季はほぼ同じであった。DIPの現存量はDINよりも明瞭な季節変動をし, 冬季には上下層とも200tであるのに対し, 夏季には上層は400t, 下層では800tにまで増加した。現存量の時間変化量と海水交換による栄養塩変化量の差から見積もった生物・化学的要因による栄養塩の変化量は, 物理的要因(海水交換)による栄養塩変化量よりも大きかった。夏季の上層では, クロロフィル濃度の増加と対応してDIN・DIPが減少する場合があるものの, 両者が完全に同調するわけではない。夏季の下層における生物化学的要因によるDIP変化量は正であり, 溶出の影響が大きいことが示された。一方で, DIPの吸着や, 一年を通じて脱窒が起こっていることもAOUとレッドフィールド比から示された。

間欠的な外海水進入を伊勢湾におけるCTD・ADCP・係留観測によって研究した。外海水は15cm s-1にも達する速度で海底に沿って進入し,湾内底層を低温,高塩化させた。外海水の進入は約2日間にわたって続き,もともと湾内下層にあった水塊を中居に押し上げた。残差流によるボリュームフラツクスから,湾内での湧昇速度を見積もると,底層での湧昇速度は5.9m d-1に達し,エスチュアリー循環流による湧昇速度の10倍以上となった。このような外海水進入は,ほぼ毎年発生している可能性が示唆された。外海水進入に伴う湧昇は,夏季の湾内庭層にある高濃度の栄養塩を有光層にすみやかに輸送し,高濃度の亜表層クロロフィル極大の形成に寄与している。