Japan Sea Proper Water Research Articles

Revisit the Upper Portion of the Japan Sea Proper Water: A Recent Structural Change and Freshening in the Formation Area

AbstractLong‐term transition in the deep water of the Japan Sea was investigated to understand how oceans respond to climate change. A basin‐scale density decrease in the upper portion of the Japan Sea Proper Water (UJSPW) by 0.01 σθ was detected from the period 1964–1985 (Period I) to that of 2001–2019 (Period II) using Argo‐ and ship‐based data sets. The density decrease was mainly due to a temperature increase of 0.3°C. Based on the pycnostad density, the UJSPW was redefined as the water mass of 27.30–27.33 σθ, along with the core density of 27.30–27.31 σθ. The dissolved oxygen (DO) and thickness in the UJSPW showed a notable decrease between the periods, along with the isopycnal surfaces flattening. Warming down to 700 m and freshening in the upper 300 m were observed in the winter UJSPW formation area in the northwestern Japan Sea. An influence of the hemispheric‐scale regime shift occurred in 1988/1989 on the UJSPW warming was suggested, along with the long‐term trend in the winter air temperature. The freshening in the formation area was attributable to the increase in winter precipitation in the Amur River basin and the decrease in winter sea‐ice extents in the Okhotsk and northern Japan Seas. The intensified stratification in the UJSPW formation area brought a decrease in the amount of the UJSPW formation through the prevention of winter deep convection, which resulted in the thinner thickness and lower DO in the UJSPW in Period II, along with the temperature increase.

Ocean Dynamics and Methane Plume Activity in Tatar Strait, Far Eastern Federal District, Russia as Revealed by Seawater Chemistry, Hydroacoustics, and Noble Gas Isotopes

This investigation presents methane, noble gas isotopes, CTD, and stable isotopic data for water samples collected in Niskin bottles at Tatar Strait during the spring seasons of 2015 and 2019 onboard the Russian R/V Akademik M.A. Lavrentyev. The results are compared to previous research carried out in 1999 in a nearby portion of the Strait and demonstrate that salinity and temperature can change appreciably. The CTD data from 1999 shows warm surface waters underlain by cold subsurface waters. In contrast, the 2015 data show the CTD data that show warm temperatures and high salinity extending down from the surface well into intermediate waters, while the 2019 data show cold surface waters underlain by even colder subsurface waters. CTD data collected above active gas plume sites along Sakhalin Island’s western shore show no substantial difference in temperature or salinity from the non-plume sites, and the methane concentrations at all of the measured sites are significantly above saturation, even in the shallow waters. Hydroacoustic data also suggest the presence of free gas and gas hydrate–coated methane bubbles from the seafloor at least to the base of upper intermediate waters. All of the intermediate and deep Japan Sea Proper waters in Tatar Strait still retain tritiogenic3He, similar to that observed throughout much of the Japan Sea, indicating limited vertical exchange between these layers and surface waters. An analysis of the δ13C of dissolved inorganic carbon in the seawater shows that positive values are limited to surface waters and that the waters become progressively more depleted in13C with depth. The results are consistent with research over the past several decades which has shown that ventilation of intermediate and deep Japan Sea Proper water is somewhat limited, while both the temperature and salinity of surface and subsurface water layers within the strait are sensitive to the balance between cold, less saline waters contributed by the Amur River/Primorye Current from the north and warm, saline waters contributed by the Tsushima Current from the south.

Temporal variability of plutonium in surface waters of the Sea of Japan

Long-term temporal variations of plutonium in Sea of Japan (SOJ) surface waters have been examined with the aim to better understand its behavior during several decades. The first observation is that 239,240Pu activity concentrations in surface waters of the SOJ during 1977–2019 were 6.5 ± 4.7 mBq m−3 in average, and 5.1 mBq m−3 as the median, whereas 137Cs and 90Sr activity concentrations decreased with time, except of the perturbation due to the 2011 Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Another observation is that sporadic high 239,240Pu activity concentrations occurred in the east Japan Basin, ranging from 1 to 39 mBq m−3. The spatial distribution of 239,240Pu activity concentrations in surface waters revealed that high 239,240Pu levels (>20 mBq m−3) occurred in 1994 in the northern SOJ, which was considered to be due to winter convection. To elucidate factors controlling the temporal variability of surface 239,240Pu levels in the SOJ, a relationship between surface 239,240Pu activity concentrations and vertical diffusion coefficients was examined. The results revealed that this relationship could be classified into two groups: one group did not show a change with increasing diffusion coefficient, while the other group showed a positive correlation. The vertical 239,240Pu distribution in SOJ waters suggests that the high surface 239,240Pu levels occurred due to the upwelling of cyclonic eddy. The rapid recycling of deeper plutonium occurred in the SOJ due to deep winter convection and upwelling associated with cyclonic eddy. The plutonium levels in the SOJ have been found to be sensitive to climate changes. Warming of the SOJ may cause a reduction of winter convection and eddy activity as a result of increasing sea surface temperature. This leads to a decline of recirculation of plutonium and other bioavailable elements from Japan Sea Proper Water (JSPW) to surface water layers. Plutonium would be, therefore, an important indicator of biogeochemical processes in the marine environment, helping to assess climate change impacts on marine ecological systems.

Changes in Mid-Depth Water Mass Ventilation in the Japan Sea Deduced From Long-Term Spatiotemporal Variations of Warming Trends

The influence of global warming on mid-depth water mass ventilation in the Japan Sea was investigated using both Argo-based and ship-based hydrographic datasets. The Argo-based dataset of the entire Japan Sea area revealed a warming trend during the past two decades in the upper portion of the Japan Sea Proper Water (UJSPW), which lies at intermediate depths from just under the main thermocline to approximately 1000 m. The warming rates in the southern Japan Sea are generally greater than those in the northern sea by a factor of 2–3. Long-term hydrographic data obtained over the last five decades in the northeast and southeast of the sea revealed that higher warming rates in the southern sea began from 2008, although a significant warming in both northern and southern seas was initiated from the late 1980s. A stagnation in the UJSPW formation from the late 1980s was suggested by a positive shift in the winter sea surface temperature in its formation region and a decreasing trend in dissolved oxygen concentration during the 1990s. In addition, a vertical multi-box model demonstrated that an imbalance between the heating from the upper layer and the cold water supply from its source region induces a warming in the UJSPW. We conclude that a significant change in the mid-depth water mass ventilation occurred in the entire Japan Sea in the late 1980s due to a stagnation in the UJSPW formation. Subsequently, a modest event in the mid-depth water mass ventilation have occurred since 2008. The higher warming rates in the southern sea than those in the northern sea in the event suggest a reduction in cold UJSPW supply to the southern sea from its formation region.

Acidification and Deoxygenation of the Northwestern Japan/East Sea

Seasonal hypoxia in the bottom waters of the Peter the Great Bay (PGB) of the Japan/East Sea (JES) occurs in summer. Using the empirical relationship between dissolved oxygen (DO) and pH obtained for hypoxic conditions and available historical DO data, acidification rates were estimated. Carefully sampled time-series observations from the northwestern part of the JES, carried out from 1999 to 2014 along the 132°20′ E and 134°00′ E longitudes, were chosen to determine the interannual variability of the sea’s hydrochemical parameters (DO, pH, and TA—the total alkalinity phosphates, nitrate, and silicates). To limit the effects of seasonal and spatial variability, only data obtained in the warm period were used. Additionally, all data from depths shallower than 500 m were discarded because they are affected by high natural variability, mostly due to strong mesoscale dynamic structures. Our results demonstrated that the pH and DO concentrations measured in the Upper Japan Sea Proper Water (750 m), Lower Japan Sea Proper Water (1250, 1750, 2250 m), and Bottom Water (3000 m) have been decreasing in recent years. On the other hand, calculated normalized dissolved inorganic carbon (NDIC), CO2 partial pressure (pCO2), and measured nutrient concentrations have been increasing. Maximum rates of acidification and deoxygenation are occurring at around 750 m. The annual rate of increase of pCO2 in the water exceeds the atmospheric rate more than 2-fold at a depth of 750 m. The observed variability of the hydrochemical properties can be explained by the combination of the slowdown ventilation of the vertical water column and eutrophication. However, the results obtained here are valid for the subpolar region of the JES, not for the whole sea. The synchronization of the deoxygenation of the open part of the JES and PGB has been found.

Long-term trends of oxygen concentration in the waters in bank and shelves of the Southern Japan Sea

While multiple studies have investigated oxygen decrease in Japan Sea Proper Water (JSPW; > 300 m in depth), oxygen variation in continental slope and shelf waters (< 300 m) must also be investigated in order to assess its socioecological impacts. In this study, historical oxygen data in the waters of three continental shelves and a bank of Japan Sea, off-Awashima area (AW), Wakasa Bay (WB), East of Tsushima Straight (ETS), and Yamato Bank (YB), were collected and analyzed to assess temporal variation of oxygen in each region from 1960 to 2000s. Significant decreasing trends of oxygen were detected in the waters below 150 m depth in WB and YB, and below 300 m in AW, in the summer season. In winter, a decreasing trend of oxygen was detected throughout the water column from 300 m to the sea surface in WB and YB. In ETS, a deoxygenation trend was detected throughout the water column from the bottom to the sea surface in the summer season, while no trend was detected in winter. The results suggested that oxygen decreases in AW, WB, and YB were the consequence of the upward propagation of the deoxygenation signal from JSPW, while that of ETS was caused by horizontal propagation of deoxygenation signal from the East China Sea. Assuming that the observed trend will continue in future, it is predicted that part of the water in Tsushima Strait area will reach the general sublethal threshold of oxygen (134 μmol kg−1) by the end of this century.

Paleoceanographic insights on meridional ventilation variations in the Japan Sea since the Last Glacial Maximum: A radiolarian assemblage perspective

The Japan Sea is ideal for investigating deep water formation due to its unique topography and hydrography. However, because of the scarcity of reliable indicator and high-resolution ventilation records, the driving mechanisms behind ventilation changes in the entire Japan Sea since the Last Glacial Maximum (LGM) remain elusive. In this study, we analyze the radiolarian assemblage in three sediment cores from the southwestern, central, and northwestern Japan Sea to reveal ventilation variations over the last 25 ka. Our results suggest that weak ventilation was a widespread phenomenon in the Japan Sea during the LGM. This is inferred by the low relative abundances of species belonging into the Japan Sea Proper Water (JSPW) Assemblage, which are indicative of deep water ventilation in the Japan Sea. The JSPW Assemblage have gradually become the predominant species in studied cores from the last deglaciation to the early Holocene, suggesting an abrupt transition from oxygen poorer to oxygen richer conditions. All investigated regions show re-ventilation during the deglaciation, albeit at different times. The JSPW ventilation in the central Japan Sea started during the 19 ka Melt-water Pulse (MWP) and remained in an interglacial mode after MWP-1A (~14.7 ka), presumably due to the opening of the Tsushima Strait and intrusion of the Tsushima Warm Current (TWC). An abrupt increase in productive shallow water export during the Bølling-Allerød (B/A) was likely the key factor causing poorly oxygenated deep-water conditions in the southwestern Japan Sea. In contrast, the weakening ventilation in the northwestern Japan Sea during the B/A and Younger Dryas was probably caused by the blocking effect of the sea ice.

Drastic hydrographic changes inferred from radiolarian assemblages in the central Japan Sea since the Last Glacial Maximum

Assessing modern radiolarian distribution in the marginal seas is essential for understanding its relationship to hydrographic and environmental variables, upon which subsequent interpretation of past radiolarian records can be made reliably. Here, we combine published radiolarian assemblages in 69 surface sediments from the Japan Sea with those obtained from 15 new surface sediments in this study to determine the modern distribution of radiolarians. The Tsushima Warm Current (TWC) taxa are only found in the south of the Subpolar Front. The results of redundancy analysis confirm that the temperature at 10 m depth (temperature10m) is the main factor controlling the spatial variability of TWC taxa. The water depth plays a key role in affecting the abundances of Japan Sea Proper Water (JSPW) Assemblage including Larcopyle buetschlii Dreyer (adult), Cycladophora davisiana Ehrenberg, and Actinomma group. The JSPW Assemblage can be used as a reliable indicator of ventilation changes in the Japan Sea. Based on the newly developed TWC taxa and JSPW Assemblage, we further reconstructed the water column structure in the central Japan Sea over the past 25 ka. There was weak ventilation in the central Japan Sea even during the Last Glacial Maximum (LGM), although a distinct decrease in the TWC inflow resulted in surface-water freshening and water column stratification. After the LGM, the TWC taxa together with JSPW Assemblage resumed in the central Japan Sea at the 19 ka meltwater pulse (MWP). The MWP-1A was clearly marked by an enhanced intrusion of the TWC. Synchronously, JSPW was ventilated deeply down after ~14.6 ka attaining to the similar level of the Holocene as a result of changing sea surface conditions. Along with the continuing sea-level rise, the matured TWC entered the Japan Sea at 7 ka and since then the modern hydrographic condition has been established. We suggest that the changes in the sea level and TWC strength predominantly determined the surface hydrography and deep ventilation in the central Japan Sea since 25 ka, and the TWC might have notably affected the Japan Sea since the Bølling-Allerød.

Radiolarian Assemblages in Surface Sediments of the Japan Sea

Abstract. Radiolarian assemblages in 69 surface sediment samples from the Japan Sea were moderately diversified, consisting of about 100 taxa in total, although only a few species accounted for a large proportion of most assemblages. First, the assemblages were often dominated by deep-dwelling species: Cycladophora davisiana, Actinomma leptodermum, A. boreale, A. langii, and adult forms of Larcopyle buetschlii. These species were restricted to great depths and were characteristic of the cold, oxygen-rich deep water that fills deep basins of this marginal sea, the so-called Japan Sea Proper Water. Second, although the observed Japan Sea assemblages included some subtropical elements, such as the Dictyocoryne and Euchitonia groups, Didymocyrtis tetrathalamus and the Tetrapyle octacantha group, many of the major temperate and subarctic elements of the North Pacific were essentially excluded. The semi-enclosed topography of the Japan Sea is most likely responsible for the dominance of certain subtropical surf...

Transitional changes in microfossil assemblages in the Japan Sea from the Late Pliocene to Early Pleistocene related to global climatic and local tectonic events

Many micropaleontological studies based on data from on-land sections, oil wells, and deep-sea drilling cores have provided important information about environmental changes in the Japan Sea that are related to the global climate and the local tectonics of the Japanese Islands. Here, major changes in the microfossil assemblages during the Late Pliocene to Early Pleistocene are reviewed. Late Pliocene (3.5–2.7 Ma) surface-water assemblages were characterized mainly by cold–temperate planktonic flora and fauna (nannofossils, diatoms, radiolarians, and planktonic foraminifera), suggesting that nutrient-rich North Pacific surface waters entered the Japan Sea via northern straits. The common occurrence of Pacific-type deep-water radiolarians during this period also suggests that deep water from the North Pacific entered the Japan Sea via the northern straits, indicating a sill depth >500 m. A weak warm-water influence is recognized along the Japanese coast, suggesting a small inflow of warm water via a southern strait. Nannofossil and sublittoral ostracod assemblages record an abrupt cooling event at 2.75 Ma that correlates with the onset of the Northern Hemisphere glaciation. Subsequently, cold intermediate- and deep-water assemblages of ostracods and radiolarians increased in abundance, suggesting active ventilation and the formation of the Japan Sea Proper Water, associated with a strengthened winter monsoon. Pacific-type deep-water radiolarians also disappeared around 2.75 Ma, which is attributed to the intermittent occurrence of deep anoxic environments and limited migration from the North Pacific, resulting from the near-closure or shallowing of the northern strait by a eustatic fall in sea level and tectonic uplift of northeastern Japan. A notable reduction in primary productivity from 2.3 to 1.3 Ma also suggests that the nutrient supply from the North Pacific was restricted by the near-closure of the northern strait. An increase in the abundance of subtropical surface fauna suggests that the inflow of the Tsushima Warm Current into the Japan Sea via a southern strait began at 1.7 Ma. The opening of the southern strait may have occurred after the subsidence of southwestern Japan.

船舶搭載型CTD・ADCPによる富山湾の海洋環境調査

Toyama Bay, a semi-enclosed water area on the northern coast of Japan with an average water depth at 600 m, is known to have three primary water masses: the coastal surface layer, Tsushima Warm Current water and Japan Sea proper water. The present study describes the oceanographic characteristics of the bay based on the oceanographic observation programs, which include temperature and salinity with a CTD (Conductivity, Temperature and Depth profiler) system and current velocities with shipmounted ADCP (Acoustic Doppler Current Profiler), onboard, Toyama National College of Technology (TNCT) training ship, Wakashio-maru, from 2004 to 2012. By the ADCP observations at 2010, 2011 and 2012, the authors observed the main path of Tsushima Warm Current and an anticlockwise eddy with its diameters of about 20-30 nautical miles in the inner parts of the bay. The authors compared these ADCP data with a numerical simulation results for ocean current and satellite remote sensing data for the Chlorophyll-a concentration.

隠岐トラフ周辺海域及び近接陸域における表層堆積物の元素分布：特に日本海固有水のシルト質および粘土質堆積物への強い影響

隠岐トラフ周辺海域及び近接陸域における表層堆積物の元素分布：特に日本海固有水のシルト質および粘土質堆積物への強い影響

Aulographis japonica sp. nov. (Phaeodaria, Aulacanthida, Aulacanthidae), an abundant zooplankton in the deep sea of the Sea of Japan

Zooplankton samples from the deep water of the Sea of Japan often contain yellowish semitransparent spheres (1.0–1.5 mm in diameter). We recognized these spheres as a single phaeodarian species (Cercozoa, Rhizaria) and described them as Aulographis japonica sp. nov. (family Aulacanthidae) in this paper. This species has a high abundance in the Japan Sea Proper Water (JSPW) and occasionally higher biomass than that of copepods. Molecular analysis based on 18S SSU rDNA revealed that Aulacantha scolymantha, which belongs to the same family as A. japonica, is closer to Aulosphaera trigonopa and Protocystis spp., which belong to different orders, than to the present species. The distribution of A. japonica is apparently restricted to low temperature water. Its biomass was the highest in the uppermost layer of JSPW, and this phaeodarian species was the second most important zooplankton below 250 m depth in terms of biomass among the total zooplankton groups. This is probably due to its generalist type of feeding. Considering its large biomass, A. japonica possibly plays an important role in matter cycles within the Sea of Japan.

Box model analysis of the long-term dissolved oxygen variation in the Japan Sea Proper Water

Box model analysis of the long-term dissolved oxygen variation in the Japan Sea Proper Water

Has the upper portion of the Japan Sea Proper Water formation really been enhancing?

Weakening of the upper portion of the Japan Sea Proper Water formation was detected based on the dissolved oxygen concentration and potential temperature from 1970 to 2004, unlike the suggestion of previous studies that it has been enhanced since the 1970s. The revealed warming and decreasing trends in DO concentration throughout the water column indicate that the conveyor belt system in the Japan Sea is still slowing down even in the intermediate layers, though a relatively active dissolved oxygen supply into the intermediate layers is suggested. This fact leads us to modify the previous concept about the water mass formation; though a relatively active formation of the intermediate water mass occurs, the deep water formation is not counterbalanced by the intermediate water formation, and the overall Japan Sea Proper Water formation has been stagnating since the 1970s.

Interdecadal oscillations in the Japan Sea proper water related to the arctic oscillation

The time series of dissolved oxygen (DO) concentration in the Japan Sea Proper Water (JSPW) shows clear interdecadal oscillation with about 20-year periodicity since the 1970s. This interdecadal variation in the DO concentration is positively correlated with the Arctic Oscillation (AO) index; the DO concentration tends to be high during a positive AO phase when the East Asian winter monsoon (EAWM) is relatively weak, which is considered to be an unfavorable condition for the JSPW formation. To clarify the discrepancy, the cause of the interdecadal oscillation was investigated using the wind at Wajima and sea surface temperature (SST) in the JSPW formation area. The cold-air outbreaks determined from the wind at Wajima are synchronized with the interdecadal oscillation of the AO; during a positive AO phase the cold-air outbreaks over the Japan Sea are more active, as reported by Isobe and Beardsley (2007). Since the SST in the JSPW formation area is negatively correlated with cold-air outbreaks, the activity of cold-air outbreaks is more important to the JSPW formation than the EAWM, at least on an interdecadal timescale. Significant correlations of the indicator of low pressure migration with the AO and cold-air outbreaks confirm that atmospheric disturbances move frequently into the Japan Sea from the East China Sea in a positive AO phase. A detailed examination of cold-air outbreaks revealed that the passing of atmospheric low pressures temporarily enhances the east-west pressure gradient over the Japan Sea and effectively brings cold air into the JSPW formation area.

Early Pleistocene evolution of the Japan Sea Intermediate Water

AbstractThe Early Pleistocene fossil record of the Japan Sea reveals that the planktonic foraminifer Globorotalia inflata succeeded in expanding its range from the East China Sea to the Japan Sea during interglacial stages of the Early Pleistocene; however, the species is only found in surface sediment samples from the southwestern‐most part of the sea. Based on the ecology of G. inflata and the water mass structure of the Japan Sea, it appears that its geographical distribution is mainly controlled by the low temperature of the intermediate water, which is caused by the cool‐water mass of Japan Sea Proper Water. The temporal and spatial patterns of G. inflata occurrence indicate that the intermediate water at 200 m depth during interglacial stages between 1.46 and 1.3 Ma was warmest during the Quaternary. This warm intermediate water probably resulted from the influence of the low‐salinity nature of the Tsushima Current due to the large inflow of East China Sea Coastal Water. After 1.3 Ma, the intermediate water at 200 m depth became cooler in a step‐wise manner. This cooling can be explained by an intensification of thermohaline circulation in the Japan Sea associated with strengthening of the East Asian winter monsoon and an increase in salinity due to enlargement of the southern strait. Copyright © 2009 John Wiley &amp; Sons, Ltd.

上越沖海底メタン湧出地域の現世底生有孔虫群集

Benthic foraminiferal assemblages of surface sediments were studied in the methane seepage area in the eastern margin of the Japan Sea to identify the effects of methane seep on foraminiferal distribution. Samples were collected from three areas, each including a methane-seep site. Their water depths range from 882 m to 1006 m in the Japan Sea Proper Water. At the methane-seep sites, there were significantly fewer living (stained) foraminifera than at other sites, and ratio of agglutinated foraminifera is small, showing the dominance of calcareous forms. Planktonic foraminifera are very abundant at seep sites. These suggest higher alkalinity at the sediment-water interface of the seep sites. Cassidulina norvangi, Pseudoparrella takayanagii and Bolivina decussata increase while Trochammina pygmaea, Thalmannammina parkerae, Lagenammina tublata, Reophax cf. dentaliniformis, Saccammina huanghaiensis, etc., decrease at the seep sites. These species are usually common in the Japan Sea Proper Water. Their changes in abundance seem, in part, to relate to alkalinity because the former taxa are calcareous foraminifera and the latter are all agglutinated. There are fewer Bolivina pacifica, Stainforthia fusiformis, Nonionella globosa, and Globobulimina auriculata in the living population at the seep sites, although they are abundant in the total (living + dead) population. These taxa are well-known infauna and can survive in oxygen-depleted environments. This discrepancy may be explained by the mode of methane seepage; the present seeping is not severer than in the near past and is transient at each seep site. The effects of the present methane seeps extend only to less than 40-50 m from each methane-seep site.

Phytoplankton photosynthetic parameters and primary production in Japan Sea and the East China Sea: Toward improving primary production models

Vertical variations in the photosynthesis–irradiance (P–E) parameters and their effects on the primary production (PP) were examined in four different characteristic coastal and offshore areas in Japan Sea and the East China Sea, during a stratified season. In Japan Sea, no significant regional variations were observed in the P–E parameters at a depth of 10m between the coastal and the offshore areas. The daily water column PP (∑PP(Total)) was lower in the offshore area than in the coastal area due to the lower surface chlorophyll a (Chl a) in the offshore area and reductions in Pmax* and α* in the subsurface chlorophyll maximum (SCM) being influenced by the cold Japan Sea Proper Water existing below the euphotic zone. In the East China Sea, all the P–E parameters showed depth variations in both the shelf and Kuroshio waters. Regional variations were observed in the vertical distributions of Chl a and PP; within the surface layer, they were higher in the shelf water than in the Kuroshio water. Within the subsurface layer, this trend was reversed. In the entire water column, these regional variations cancelled each other, and no significant variation was observed in ∑PP(Total) between the two areas. ∑PP(Total) was underestimated when the wavelength dependency of the P–E parameters was ignored in all the four areas (by 21–35% on average). The error in ∑PP(Total) increased in comparison to those obtained by ignoring the depth variations in the P–E parameters (−25% to −39%) and the depth variations in both the P–E parameters and Chl a (−25% to −55%). The present study reveals the effects of vertical variations in the P–E parameters and spectral dependency of photosynthesis on the PP in Japan Sea and the East China Sea, and their relationships with hydrographic conditions such as the water column stratification, influences of Japan Sea Proper Water, and Kuroshio Current.

Transportation of organic matter to the sea floor by carrion falls of the giant jellyfish Nemopilema nomurai in the Sea of Japan

The fate of the giant jellyfish Nemopilema nomurai after death was examined in the southwest Sea of Japan. The density of dead jellyfish was greater than that of live animals. The dead animals are heavier than the Japan Sea Proper Water which occurs deeper than 200 m, suggesting that dead jellyfish sink to the sea floor. The sea floor survey, conducted with a towed video tape recorder (VTR) monitoring system between mid September and mid October, observed a total of 138 jellyfish during 28 of 29 operations. The density of carrion ranged between 0.2 and 5.1 individuals/1,000 m2 (mean ± SE = 1.1 ± 0.2). Ophiuroids occurred abundantly at 23 jellyfish carcasses and a sea anemone was observed attached to five carcasses. The VTR surveys confirmed that carrion sinks to the sea floor not only during the winter, the normal end of life for medusae, but also during the fall. A trap survey baited with medusae was also employed, and four different species were sampled with the traps: the snow crab (Chionoecetes opilio), a shrimp (Pandalopsis japonica), the ivory shell (Buccinum striatissimum) and an ophiuroid (Ophiura sarsii). Much of the trap bait remained (49–68% weight-mean = 60.3%) during the 23 h soak-time, and the reduction in weight was greater than that observed by bacterial decomposition, suggesting benthic animals consume dead organisms. The present study indicates that dead N. nomurai sink to the sea floor continuously and were subsequently consumed by benthic scavengers.