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Tokyo Bay is surrounded by a growing urban metropolis and has experienced varying levels of water quality over time. Vertical profiles of nutrients and chlorophyll a (Chl a) were obtained monthly from 1989 to 2015 at two sampling stations located in the center of Tokyo Bay. From 2006 to 2015, profiles of dissolved organic nitrogen (DON) and phosphorus were also obtained. Mann–Kendall trend analysis was used to establish the significance of the long-term trends for all parameters. Water-column integrated nutrient concentrations showed significant decreases at both stations, except for integrated phosphate concentration at one station. Water-column integrated Chl a concentration also decreased, indicating that active nutrient consumption due to an increase in primary production was not responsible for the declining nutrient concentrations. Rather, the decreases reflect the reduced loading of nutrients into the bay due to the implementation of advanced wastewater treatment techniques. Phosphate concentrations in the surface waters of Tokyo Bay were frequently below detection limits during spring and summer after 2000. During the 10-yr period following 2006, dissolved organic phosphorus concentrations decreased significantly at the surface at both stations, whereas DON concentrations did not change significantly. The ratios of total dissolved nitrogen to total dissolved phosphorus in the surface waters were much higher than the Redfield ratio for both stations, and these ratios increased significantly during the most recent decade. Continued declines in total dissolved phosphorus concentration could strengthen phosphorus limitation of primary production in Tokyo Bay.

A part of the radiocaesium from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident was emitted as glassy, water-resistant caesium-bearing microparticles (CsMPs). Here, we isolated and investigated seven CsMPs from marine particulate matter and sediment. From the elemental composition, the 134 Cs/ 137 Cs activity ratio, and the 137 Cs activity per unit volume results, we inferred that the five CsMPs collected from particulate matter were emitted from Unit 2 of the FDNPP, whereas the two CsMPs collected from marine sediment were possibly emitted from Unit 3, as suggested by (i) the presence of calcium and absence of zinc and (ii) the direction of the atmospheric plume during the radionuclide emission event from Unit 3. The presence of CsMPs can cause overestimation of the solid–water distribution coefficient of Cs in marine sediments and particulate matter and a high apparent radiocaesium concentration factor for marine biota. CsMPs emitted from Unit 2, which were collected from the estuary of a river that flowed through a highly contaminated area, may have been deposited on land and then transported by the river. By contrast, CsMPs emitted from Unit 3 were possibly transported eastward by the wind and deposited directly onto the ocean surface.

Many common and abundant copepods in coastal areas adapt to complex environments through diapause (physiological dormancy) by laying resting eggs. However, the response mechanisms of these species’ phenology to environmental changes is poorly understood, especially the influence of both climatic and anthropogenic changes. Using monthly time series data for zooplankton in Tokyo Bay, Japan, from 1981 to 2010, we investigated phenological and quantitative changes in the dominant species Acartia omorii, Centropages abdominalis, and Labidocera rotunda in response to climatic and hydrographic variation in this semienclosed eutrophic coastal area. The first two species produce resting eggs in the warm season and the latter in the cold season. Decreasing abundance and increasing resting egg periods were observed for A. omorii and C. abdominalis, and the reverse for L. rotunda. Time series analysis revealed a possible mechanism driving these phenologic shifts. Wintertime warming of the water column associated with climate variability driven by the Pacific Decadal Oscillation index, followed by rapid development of summertime stratification and the formation of hypoxic bottom water layers, were responsible for the early initiation and delayed termination of the resting egg period for cold-water species. The warming trend in Tokyo Bay, possibly induced by inflow of warm oceanic water due to anthropogenic factors, caused these phenologic changes in the resting period and subsequently affected zooplankton abundance. These results suggest that the multiple stressors caused by climate change and other anthropogenic impacts can affect regional zooplankton communities in coastal areas.

The red macroalga Agarophyton chilensis is a well-known producer of eicosanoids such as hydroxyeicosatetraenoic acids, but the alga produces almost no prostaglandins, unlike the closely related A. vermiculophyllum. This indicates that the related two algae would have different enzyme systems or substrate composition. To carry out more in-depth discussions on the metabolic pathway of eicosanoids between the two algae, we investigated the characteristics of glycerolipids, which are the substrates of eicosanoids production, of A. chilensis and compared them to the reported values of A. vermiculophyllum. In A. chilensis, monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG), and phosphatidylcholine (PC) were the major lipid classes and accounted for 44.4% of the total lipid extract. The predominant fatty acids were arachidonic acid (20:4n-6), an eicosanoids precursor, and palmitic acid (16:0). The 20:4n-6 content was extremely high in MGDG and PC (>70%), and the 16:0 content was extremely high in DGDG and SQDG (>40%). A chiral-phase HPLC analysis showed that fatty acids were esterified at the sn-1 and sn-2 positions of those lipids. The glycerolipid molecular species were determined by reversed-phase HPLC–ESI–MS analysis. The main glycerolipid molecular species were 20:4n-6/20:4n-6 (sn-1/sn-2) for MGDG (63.8%) and PC (48.2%), 20:4n-6/16:0 for DGDG (71.1%) and SQDG (29.4%). These lipid characteristics of A. chilensis were almost the same as those of A. vermiculophyllum. Hence, the differences of the eicosanoids producing ability between the two algae would not be due to the difference of substrate composition but the difference of enzyme system.

After the Fukushima Daiichi Nuclear Power Plant accident in March 2011, concentrations of cesium isotopes ( 133 Cs, 134 Cs, and 137 Cs) were measured in zooplankton collected in the Pacific off the east coast of Japan from May 2012 to February 2015. The time series of the data exhibited sporadic 137 Cs concentration peaks in zooplankton. In addition, the atom ratio of 137 Cs/ 133 Cs in zooplankton was consistently high compared to that in ambient seawater throughout the sampling period. These phenomena cannot be explained fully by the bioaccumulation of 137 Cs in zooplankton via ambient seawater intake, the inclusion of resuspended sediment in the plankton sample, or the taxonomic composition of the plankton. Autoradiography revealed highly radioactive particles within zooplankton samples, which could be the main factor underlying the sporadic appearance of high 137 Cs concentrations in zooplankton as well as the higher ratio of 137 Cs/ 133 Cs in zooplankton than in seawater.

The composition of the phytoplankton community in Tokyo Bay was analyzed using samples collected monthly at 0 and 5 m depths from 2003 to 2017, along with environmental factors. Three diatom genera— Skeletonema , Thalassiosira , and Pseudo-nitzschia —dominated over the study period. The sum of the three genera accounted for 75% of the total phytoplankton cell density. Skeletonema was the most abundant taxon, of which the cell density reached > 10 4  cells mL −1 . A pattern of seasonal change in the community composition was detected, and the varying compositions were grouped into three seasons: a mixing season (November–February), an early stratified season (March–June), and a late stratified season (July–October). The mixing season was characterized by high nutrient content (dissolved inorganic nitrogen > 30 µM) and low temperature (< 17 °C), and the late stratified season was characterized by high temperature (> 22 °C) and low salinity (< 28 at the surface). In a modified seasonal Mann–Kendall test, we did not find any increasing or decreasing trend of three diatom genera in the analysis for whole months. However, the test applied to each seasonal group revealed a significant increasing trend of Pseudo-nitzschia abundance in the late stratified season at 0 m depth. In contrast, Skeletonema abundance showed a decreasing trend at a 5 m depth in the same season. The succession of the phytoplankton community was more obvious when the yearly trend was analyzed for each season.

Little is known about the rate of increase of coastal seawater p CO 2 ( p CO 2 sea ), despite its necessity for assessing future oceanic CO 2 uptake capacity. We examined temporal changes in p CO 2 sea in central Sagami Bay during 2001–2009. Weekly p CO 2 sea was reconstructed using time series of particulate organic carbon isotope delta (POC-δ 13 C) of settling particles at 150 m from moored sediment-trap experiments. For p CO 2 sea estimation, an empirical relationship between suspended POC-δ 13 C and aqueous CO 2 concentration from repeat ship observations in 2007–2008 was applied to the trapped POC-δ 13 C. Air–sea CO 2 flux was calculated using the air–sea p CO 2 difference with gas transfer velocity. Estimated Bay p CO 2 sea varied by 190 μatm (mean 294 μatm) and was mostly below atmospheric p CO 2 ( p CO 2 air ). The mean oceanic CO 2 uptake was 82 mg m −2 d −1 , suggesting that Sagami Bay is an efficient sink for atmospheric CO 2 . Meanwhile, carbon sequestration to the mesopelagic layer by particulate carbon export accounted for 60–75% of the CO 2 uptake, with the rest likely removed horizontally via surface water exchange. The p CO 2 sea showed an increasing trend of + 3.9 µatm y −1 , approximately twice that of p CO 2 air , and the two converged. Concurrently, a decreasing trend in POC export flux and an increasing trend in nitrogen isotope delta of trapped particles were found. Particularly, a large summer p CO 2 sea increasing rate (+ 4.9 µatm y −1 ) was observed accompanied by POC concentration decreasing, which resulted in a decrease in CO 2 uptake over time. Long-term summer nutrient depletion and reduced primary production may increase p CO 2 sea in the Bay.

Recent global environmental changes such as an increase in sea surface temperature (SST) are likely to impact primary productivity of phytoplankton in the Southern Ocean. However, models to estimate net primary production using satellite data use SST and uncertain estimation of chlorophyll a (chl-a) concentration. A primary productivity model for satellite ocean color data from the Southern Ocean, which is based on the light absorption coefficient of phytoplankton to reduce uncertainties of sea surface chl-a estimations and bias in optimal values of chl-a normalized productivity derived from SST, has been developed. The new model was able to estimate net primary productivity in the water column (PP eu) without dependency on temperature when in the range of −2 to 25°C, and it explained 51% of the observed variability in PP eu with a root mean square error (RMSE) of 0.15. Application of the model revealed that the SST dependent model has overestimated PP eu in warmer waters around the Subtropical Front, and underestimated PP eu in colder waters poleward of the Sub-Antarctic Front. This absorption-based primary productivity model contributes to a study of the relationship among spatio-temporal variations in the physical environment, and biogeochemical cycles in the Southern Ocean.

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.

The horizontal and vertical distributions of fish were examined off Lützow-Holm Bay in the Indian Ocean sector of the Southern Ocean during midnight sun in January 2005. Fish were sampled from six discrete depth layers (0–2,000 m). The most abundant fish in layers from the surface to 200 m were larval stages of Electrona antarctica and Notolepis coatsi . In layers from 200 to 2,000 m, fish assemblages were relatively uniform among all stations and were dominated by E. antarctica (juvenile–subadult), Cyclothone microdon , and Bathylagus antarcticus . Cluster analysis revealed three epipelagic communities related to water temperature and salinity. An ontogenetic habitat shift to deeper layers was apparent for E. antarctica , N. coatsi , and B. antarcticus . Preferences for warm waters were observed in E. antarctica (larvae) and N. coatsi (preflexion to flexion larvae), although they were distributed across a broad range of temperature and salinity in epipelagic zones.