Yutaka Isoda

Temporal and Spatial Variability of Phytoplankton Pigment Concentrations in the Japan Sea Derived from CZCS Images

Temporal and spatial variability of phytoplankton pigment concentrations in the Japan Sea are described, using monthly mean composite images of the Coastal Zone Color Scanner (CZCS). In order to describe the seasonal changes of pigment concentration from the results of the empirical orthogonal function (EOF) analysis, we selected four areas in the south Japan Sea. The pigment concentrations in these areas show remarkable seasonal variations. Two annual blooms appear in spring and fall. The spring bloom starts in the Japan Sea in February and March, when critical depth (CRD) becomes equal to mixed layer depth (MLD). The spring bloom in the southern areas (April) occurs one month in advance of that in the northern areas (May). This indicates that the pigment concentrations in the southern areas may increase rapidly in comparison with the northern areas since the water temperature increases faster in spring in the southern than in the northern areas. The fall bloom appears first in the southwest region, then in the southeast and northeast regions, finally appearing in the northwest region. Fall bloom appears in November and December when MLD becomes equal to CRD. The fall bloom can be explained by deepening of MLD in the Japan Sea. The pigment concentrations in winter are higher than those in summer. The low pigment concentrations dominate in summer.

Lateral variation of 134Cs and 137Cs concentrations in surface seawater in and around the Japan Sea after the Fukushima Dai-ichi Nuclear Power Plant accident.

A total of 82 surface seawater samples was collected in the Japan Sea and the southwestern Okhotsk Sea before and after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Analysis of (134)Cs and (137)Cs concentrations using low-background γ-spectrometry revealed that the (137)Cs concentration of the samples collected in June 2011 was 1.5-2.8mBq/L, which is approximately 1-2 times higher than the pre-accident (137)Cs level, while the (134)Cs concentration was less than detectable to 1mBq/L. In addition to (134)Cs being clearly detected (∼1mBq/L), (137)Cs concentration in water samples from the northeastern Japan Sea (2-2.8mBq/L) was also higher than that from the coast in the southwestern Japan Sea (∼1.5mBq/L). These higher concentrations in the northeastern Japan Sea could be ascribed to the atmospheric transport of nuclides from the FDNPP as aerosols and subsequent transport and dilution after delivery to the sea surface.

Control on dissolved iron concentrations in deep waters in the western North Pacific: Iron(III) hydroxide solubility

[1]The vertical profiles of nutrient, AOU (apparent oxygen utilization), and Fe(III) hydroxide solubility (Fe(III) solubility) in the western North Pacific commonly show that their concentrations increased with depth below the surface mixed layer and have strong gradients around the North Pacific Intermediate Water (NPIW) in a potential density (s q ) range of 26.7‐27.0. They had the maximum values at as q range of 27.0‐27.5, and then rapidly decreased with depth at highers q than 27.5. The similarity of their profiles versuss q suggests that they are controlled by similar processes with an intrinsic timescale such as deep-ocean circulation. The vertical profiles of dissolved Fe were also characterized by mid-depth maxima and, subsequently, a slight decrease with depth, which is markedly similar to nutrient and Fe(III) solubility depth profiles. This implies that the major source of dissolved Fe in the deep ocean is release during the remineralization of biogenic organic matter. In the present study, we attempted to confirm that the dissolved Fe depth profiles are controlled by the sinking particulate organic matter (POM), the production of dissolved Fe from POM during carbon remineralization, the scavenging of dissolved Fe and the temporal fixed Fe(III) solubility depth profile. Using the production rate (a )o f dissolved Fe (1/a =5 ‐10 years), the calculated depth profile of dissolved Fe is in remarkable agreement with the observed profile of dissolved Fe with mid-depth maxima. Therefore we concluded that dissolved Fe concentrations in deep ocean waters are controlled primary by the Fe(III) solubility. INDEXTERMS:4805 Oceanography: Biological and Chemical: Biogeochemical cycles (1615); 4807 Oceanography: Biological and Chemical: Chemical speciation and complexation; 4842 Oceanography: Biological and Chemical: Modeling; 4875 Oceanography: Biological and Chemical: Trace elements;KEYWORDS:iron(III) hydroxide solubility, dissolved iron, deep water, western North Pacific Ocean Citation:Kuma, K., Y. Isoda, and S. Nakabayashi, Control on dissolved iron concentrations in deep waters in the western North Pacific: Iron(III) hydroxide solubility,J. Geophys. Res.,108(C9), 3289, doi:10.1029/2002JC001481, 2003.

Modeling of the spring bloom in Funka Bay, Japan

Abstract Effects of vertical stability and concentration of nutrient on the diatom bloom in Funka Bay, south Hokkaido, in spring were investigated using one-dimensional and three-dimensional ecosystem models. In the model, six compartments: two for phytoplankton (diatom and dinoflagellate), two for nutrients (silicate and nitrate), one for zooplankton and one for detritus were considered. Vertical stability depended on the net heat flux through the sea surface. Calculated results are compared with the observational results for 1981 (Tsunogai and Watanabe, Journal of the Oceanographical Society of Japan 39 (1983) 231–239). This comparison shows that the rapid spring diatom bloom corresponds to the timing when the net heat flux through the sea surface changes from cooling to heating. This result suggests that the stability of the water column due to warming play an important role in the onset of the diatom bloom in Funka Bay. In addition, the limiting factor of the diatom bloom during the spring bloom is not the silicate, but nitrate. The model can reproduce the observed data by changing the nutrient uptake rate. This suggests a dramatic increase in the assimilation rate of silicate after the consumption of nitrate.

228Ra/226Ra ratio and 7Be concentration in the Sea of Japan as indicators for water transport: comparison with migration pattern of Fukushima Dai-ichi NPP-derived 134Cs and 137Cs

To assess the migration patterns of radiocesium emitted from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), we analyzed (228)Ra/(226)Ra ratios and (7)Be concentrations and compared them with (134)Cs and (137)Cs concentrations in seawater samples collected within the Sea of Japan before and after the FDNPP accident (i.e., during the period 2007-2012) using low-background γ-spectrometry. The (228)Ra/(226)Ra ratios in surface waters exhibited lateral and seasonal variations, reflecting the flow patterns of surface water. This indicates the transport patterns of the FDNPP-derived radiocesium by surface water. Cosmogenic (7)Be (half-life: 53.3 d) exhibited markedly high concentrations (5-10 mBq/L) at depths shallower than 50 m, with concentrations decreasing steeply (0.2-2 mBq/L) at depths of 50-250 m. The distribution of (7)Be concentrations suggests that the downward delivery of the FDNPP-derived radiocesium to below 50 m depth was negligible for a few months prior to its removal from the Sea of Japan.

A Late Winter Hydrography in Hidaka Bay, South of Hokkaido, Japan

Hydrographic observations in Hidaka Bay, south of Hokkaido, Japan were carried out in late winter 1996 and 1997 to examine the spatial distributions and circulation features of two different water masses, i.e., Coastal Oyashio Water (COW) and Tsugaru Warm Water (TWW), and their modifications. It is known that COW is mostly composed of cold and low-salinity water of the melted drift ice coming from the Okhotsk Sea and flows into Hidaka Bay from winter to spring and TWW with high-salinity continuously supplies from the Tsugaru Strait to the North Pacific. Cold surface mixed layers (<26.2σθ, 0–100 m depth) were found mainly over the shelf slope, confirming that anti-clockwise flow of COW was formed. TWW was relatively high in salinity and low in potential vorticity, and had some patch-like water masses with a temperature and salinity maximum in the limited area in the further offshore at the deeper density levels of 26.6–26.8σθ. The fine structure of vertical temperature and salinity profiles appeared between TWW and COW is an indication of enhanced vertical mixing (double-diffusive mixing), as inferred from the estimated Turner angles. At a mouth of the Tsugaru Strait in late winter 1997, a significant thermohaline front between TWW and the modified COW was formed and a main path of TWW spreaded south along the Sanriku coast, probably as the bottom controlled flow. Hence, the patch-like TWW observed in late winter is isolated from the Tsugaru Warm Current and then rapidly modified due to a diapycnal mixing.

Interaction of a warm eddy with the coastal current at the eastern boundary area in the Tsushima Current region

Abstract The Tsushima Current region in the Japan Sea is one of the eddy-rich areas. According to Isoda (1994) ( Journal of Oceanography , 50 , 1–15), when an eastward moving warm eddy reached its eastern boundary area, two or three mesoscale eddies frequently evolved and gradually disappeared into the coastal current. Such process associated with the eddy-current interaction are examined numerically using a reduced gravity model on the f-plane. Once a warm (anticyclonic) eddy influences the coastal current, the following two kinds of meanders evolve. In the nonlinear regime, an original eddy moves onshore and becomes part of the seaward meander. Furthermore, this eddy excites another small seaward meander or a new eddy at its northern part due to the vorticity control of the distorted coastal current. After that, a newly formed northern eddy travels downstream with an advective speed of the coastal current, while the potential vorticity of an original eddy becomes to lose within a larger potential vorticity anomaly onshore side of the coastal current.

Vertical profiles of Fukushima Dai-ichi NPP-derived radiocesium concentrations in the waters of the southwestern Okhotsk Sea (2011–2017)

We examined the vertical 134Cs and 137Cs concentration profiles in the southwestern Okhotsk Sea in 2011, 2013, and 2017. In June 2011, atmospheric deposition-derived 134Cs from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) was detected at depths of 0-200 m (0.06-0.6 mBq/L). In July 2013, 134Cs detected at depths of 100-200 m (∼0.05 mBq/L) was ascribed to the transport of low-level 134Cs-contaminated water and/or the convection of radioactive depositions (<0.03 mBq/L at depths of 0-50 m). In July 2017, 134Cs was detected in water samples at depths above 300 m (0.03-0.05 mBq/L), and the inventory, decay-corrected to the FDNPP accident date, exhibited its maximum value (85 Bq/m2) during this period. Combining temperature-salinity data with the concentrations of global fallout-derived 137Cs led to a plausible explanation for this observation, which is a consequence of re-entry of FDNPP-derived radiocesium through the Kuril Strait from the northwestern North Pacific Ocean to the Okhotsk Sea and subsequent mixing with the south Okhotsk subsurface layer until 2017.

Temperature Minima in the Gulf of Alaska in the Summer from 1994 to 2002

Suga et al. (2003)は180°断面で観測される亜表層の水温極小水の起源がベーリング海西部の冬季混合層にあることを示した。アラスカ湾においても水温極小が存在しており, この水塊の存在は水深100～150mに形成される強い塩分(密度)躍層によって保たれている。このアラスカ湾の水温極小水の起源を明らかにするために, 9年間(1994-2002年)継続して行なわれた145°W断面の水温・塩分資料を解析した。その結果, 180°断面と145°W断面で観測される水温極小の水塊特性の違いが明らかになった。次に, 冬季海面水温と145°W断面で観測された水温極小のコア水温の経年変動解析から, 145°W断面の水温極小はアラスカ湾付近における冬季の大気海洋相互作用により, その場で形成されている可能性を示した。145°W断面では上述の水温極小以外に, 塩分躍層以深にもう一つの水温極小がみられた。この水温極小は26.7σθよりも重い密度帯で微細な水温逆転構造をもち, 西向きの流れ場の中に多く存在する傾向がある。