water

Article Behavior of Water Mass Beneath the Tsushima Warm Current in the Sea

Xiaorong Fang 1,2,*, Yutaka Isoda 2, Isao Kudo 2, Takafumi Aramaki 3, Keiri Imai 4 and Naoto Ebuchi 5 1 Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai 519082, Guangdong, 2 Faculty of Fisheries Sciences, University, Hakodate 041-8611, Hokkaido, Japan; isoda@fish.hokudai.ac.jp (Y.I.); ikudo@fish.hokudai.ac.jp (I.K.) 3 Center for Global Environmental Research, National Institute for Environmental Studies, Tsukuba, Ibaraki 305-8506, Japan; [email protected] 4 School of Fisheries Sciences, Training Ship “Oshoro-maru”, Hokkaido University, Hakodate 041-8611, Hokkaido, Japan; imai@fish.hokudai.ac.jp 5 Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan; [email protected] * Correspondence: [email protected]

 Received: 7 July 2020; Accepted: 29 July 2020; Published: 3 August 2020 

Abstract: To better understand the behavior of water mass beneath the Tsushima Warm Current (TWC), we use the vertical cross-sections of potential temperature, salinity and dissolved oxygen in the Japan Sea obtained by the T/V Oshoro Maru of the Hokkaido University during 8–29 June in 2011 to analyze its origins and variations. The results show that the potential temperature and salinity beneath the TWC varies little, but the dissolved oxygen varies largely with the geographical location. There are two deep water masses with different dissolved oxygen content below the TWC. One is on the coastal side with the low dissolved oxygen, and the other is on the offshore side with the high dissolved oxygen. It is inferred that the former one is relatively old water and the latter is the new water. By using the phosphate (PO4) and the apparent oxygen utilization (AOU) relationship, 0 we calculate the PO4 (preformed PO4) as a water mass tracer. These results suggest that the water masses beneath the TWC with high and low dissolved oxygen originate from the same surface water mass in the central Japan Sea.

Keywords: Japan Sea; Tsushima Warm Current; apparent oxygen utilization; preformed PO4

1. Introduction The Japan Sea is one of the marginal seas in East Asia, which can significantly impact on regional oceanic general circulation and climate equilibrium. Mixed with the seawater in the , the high-temperature and high-salinity water originating from the Kuroshio enters the Japan Sea through the Tsushima/Korean Strait at the depth less than about 200 m. After that, this warm water of the Tsushima Warm Current (TWC) flows northeastward and its northern boundary near 40 ◦N is called as a polar front. Most of TWC waters flow into the North Pacific through the shallower than 100–150 m, and the rest of them enter the Okhotsk Sea through the Soya Strait (Figure1). As the mean depth of the Japan Sea is over 1000 m, the water in other seas (the North Pacific and the Okhotsk Sea) cannot exchange with the deep water in the Japan Sea. It is well known that the surface water in the Japan Sea is affected by strong winter cooling in the northern Japan Sea, so that the dense cold water sinks into the deep ocean through vertical convections. Therefore, the seawater in the Japan Sea at the depth of 500–3500 m has many unique characteristics, and that’s why it is named “Japan Sea

Water 2020, 12, 2184; doi:10.3390/w12082184 www.mdpi.com/journal/water Water 2020, 12, 2184 2 of 16

Water 2020, 12, 2184 2 of 17 Proper Water (JSPW)” [1]. The temperature and salinity of the deep seawater in the Japan Sea vary little,the whichJapan Sea range vary from little, 0 towhich 1 ◦C range and from from 34.06 0 to 1 to °C 34.08, and from respectively. 34.06 to 34.08, In addition, respectively. the concentration In addition, of 1 dissolvedthe concentration oxygen (O of2) dissolved in the deep oxygen sea is (O more2) in than the 4.5deep mL sea L −is ,more which than is much 4.5 mL higher L−1, which than those is much in the Northhigher Pacific than deepthose waters.in the North Pacific deep waters.

Figure 1. Bottom topography and hydrographic stations used in the study. Figure 1. Bottom topography and hydrographic stations used in the study. In this study, we focus on the intermediate water beneath the Tsushima Warm Water (TWW). In this study, we focus on the intermediate water beneath the Tsushima Warm Water (TWW). The distributions of salinity and O2 around the intermediate water have very special features, Talley et al. The distributions of salinity and O2 around the intermediate water have very special features, Talley (2006)et al. [ 2(2006)] reviewed [2] reviewed the water the mass wate classificationr mass classification and its origin and its (Table origin1) based(Table on 1) thebased salinity on the distribution salinity fromdistribution the ocean from observations the ocean observations in the Japan in Sea the inJapan summer, Sea in summer, 1999. Their 1999. study Their demonstrates study demonstrates that the TWWthat flowsthe TWW from flows the Tsushima from the Tsushima Strait with Strait a salinity with maximuma salinity maximum (Smax) at a(S depthmax) at ofa depth 150 m. of The 150 Japanm. SeaThe Intermediate Japan Sea Intermediate Water (JSIW) Water appears (JSIW) in the appear lowers partin the of TWWlower withpart of the TWW salinity with minimum the salinity (Sm in). Itsminimum origin is (S frommin). Its the origin surface is from low the salinity surface water low salinity in the subarcticwater in the region subarctic (north region of the (north polar of front).the Belowpolar the front). JSIW, Below there the is aJSIW, Smax therecalled is “High-Salinity a Smax called "High-Salinity Intermediate Intermediate Water (HSIW)” Water (also (HSIW)" called “Upper(also Japancalled Sea "Upper Proper Japan Water”) Sea [ 3Proper] near theWater") depth [3] of near 500m. the Figure depth2 a–dof 500m. show Figure the typical 2a–d example show the for typical vertical cross-sectionsexample for ofvertical potential cross-sections temperature of potential (θ), potential temperature density ( θσ),θ ),potential salinity density (S) and dissolved(σθ), salinity oxygen (S) and (O 2) alongdissolved the regular oxygen observation (O2) along linethe regular SM by theobservation Maizuru line Observatory SM by the in Maizuru January Observatory of 2011 (the in locations January are shownof 2011 by (the symbol locationsN in Figureare shown1). Red by solidsymbol line, ▲ redin Figure broken 1). line Red and solid blue line, broken red broken line denote line andσθ blueof 27.0,

Water 2020, 12, 2184 3 of 16

Water 2020, 12, 2184 3 of 17

Smax and Smin, respectively. The TWW with high temperature and high salinity (>5 ◦C and >34 psu, broken line denote σθ of 27.0, Smax and Smin, respectively. The TWW with high temperature and high respectively) occupies the surface layer as the coastal-trapped density-driven flow. So, the Smin layer salinitygradually (>5 deepens ℃ and from>34 psu, the orespectively)ffshore side tooccupies the coast the along surface the bottomlayer as boundary the coastal-trapped of the TWW, density- while a driven flow. So, the Smin layer gradually deepens from the offshore side to the coast along the bottom Smax of HSIW exists near 500m. Such distributions of the Smin and Smax layers suggest the existence of boundary of the TWW, while a Smax of HSIW exists near 500m. Such distributions of the Smin and Smax intermediate intrusion flow, while the distribution of dissolved O2 suggests the costal upwelling flow layers suggest the existence of intermediate intrusion flow, while the distribution of dissolved O2 driven by deep water with low dissolved O2. suggests the costal upwelling flow driven by deep water with low dissolved O2. Table 1. Water Masses and Structures, Identifying Characteristics, and Source. Table 1. Water Masses and Structures, Identifying Characteristics, and Source. Water Mass Distinguishing Characteristic Source Water Mass DistinguishingVertical salinity Characteristic maximum in Tsushima StraitSource inflow, local Tsushima Warm Water (TWW) Verticalupper salinity 150 maximum m in upper 150 Tsushimaevaporation Strait inflow, and subduction local Tsushima Warm Water (TWW) m evaporationSubduction and ofsubduction fresh subpolar Vertical salinity minimum in Japan/East Sea Intermediate Water (JSIW) Subductionwater southward of fresh subpolar across thewater Japan/East Sea Intermediate Water Verticalupper salinity ocean minimum in upper southwardSubpolar across Front the Subpolar (JSIW) ocean FrontConvective cooling of High-Salinity Intermediate Water (HSIW, Vertical salinity maximum Tsushima Warm Water in the High-SalinityUpper Japan Intermediate Sea Proper Water Water, UJSPW) between 200–500 m Convective cooling of Tsushima Vertical salinity maximum between northeast subpolar gyre (HSIW, Upper Japan Sea Proper Warm Water in the northeast 200–500Vertically m homogeneous water Water, UJSPW) subpolarConvective gyre cooling of water in Japan Sea Proper Water between the salinity maximum Vertically homogeneous water between Convectivethe northern cooling subpolar of water gyre in the Japan Sea Proper Water and bottom layer the salinity maximum and bottom layer northern subpolar gyre

Figure 2. Vertical cross-sections of (a) potential temperature(θ), (b) potential density (σθ), (c) salinity Figure 2. Vertical cross-sections of (a) potential temperature(θ), (b) potential density (σθ), (c) salinity (S), (d) dissolved oxygen (O2) in the Japan Sea along line SM. Red solid line, red broken line and blue (S), (d) dissolved oxygen (O2) in the Japan Sea along line SM. Red solid line, red broken line and blue broken line indicate σθ of 27.0, salinity maximum (Smax) and minimum (Smin). broken line indicate σθ of 27.0, salinity maximum (Smax) and minimum (Smin). The water mass beneath the Tsushima Warm Current referring to the JPIW (characterized by The water mass beneath the Tsushima Warm Current referring to the JPIW (characterized by Smin) and the slightly deeper water under JPIW is the water mass of interest in this study. In earlier Smin) and the slightly deeper water under JPIW is the water mass of interest in this study. In earlier researches, the intermediate sea water of the Japan Sea seemed to be an uniform cold water with the constant temperature/salinity of 1 °C/34.1. But the recent high-precision CTD (Conductivity,

Water 2020, 12, 2184 4 of 16 researches, the intermediate sea water of the Japan Sea seemed to be an uniform cold water with the constant temperature/salinity of 1 ◦C/34.1. But the recent high-precision CTD (Conductivity, Temperature, and Depth) observations have already revealed that the water beneath the TWC is formed by multiple water masses from different origins. Some previous studies [4,5] estimated the origin of the intermediate water mass beneath the TWC by using observed temperature, salinity and density data or numerical simulations. However, these results are inconsistent and the origin of the water mass beneath the TWC formed by winter sea surface cooling is still undetermined. On the other hand, many studies [6–8] used chemical tracers such as the preformed phosphate 0 0 (PO4 ) to deduce the origin of water mass. Since PO4 is of the conservative property, it can be widely used as a water mass tracer, especially in the sea areas where the T and S changes slightly, such as the intermediate water of the Japan Sea. Therefore, the purpose of this study is to clarify the behavior of water mass beneath the TWC based on the discussion of original water by using the performed PO4 data in the Japan Sea for the first time. The main conclusions of this study can help to further understand the behavior of water mass beneath the TWC and provide some observation evidences for developing corresponding parameterization schemes in ocean models. The remaining part of this paper is organized as follows. The data and method are described in Section2. The path of TWC in June of 2011, the relationships among T, S and O 2 and the PO4 regeneration with reference to AOU are shown in Section3. Finally, the discussions and summary are given in Sections4 and5.

2. Data and Method

2.1. CTD, XBT, O2 and PO4 Observations Comprehensive hydrographic observations by T/V Oshoro Maru of the Hokkaido University were carried out in the Japan Sea during the period of June 8–29, 2011. We selected a total of six observation lines crossing the northeastward flow of the TWC shown in Figure1. These lines are consisted of several CTD surveys denoted by open circles. The figure in all lines shows the serial numbers of the observations from the Japanese coast to the offshore area (for example, the first observation point in line A is named A1). Line A can get the inflow structure of the TWC through the Tsushima/Korean Strait. Lines B and C can show the coastal-trapped density-driven flow of TWC. Lines D and E respectively locate south and north of the Tsugaru Strait, which is one of the outflows exits of the TWC. Line F is set to capture the seasonally transient flow of the TWC during early summer. The solid circles between observation lines are XBT (Expendable Bathy Thermograph) observation points to examine the spatial distribution of the TWC. In addition, the regular observation data from the Japan Meteorological Agency (JMA, Tokyo, Japan) are also used (https: //www.data.jma.go.jp/gmd/kaiyou/db/vessel_obs/data-report/html/ship/ship.php.) As mentioned in the introduction, line SM (Syunpu Maru ship) consisted of 14 CTD points (closed triangles) were carried out in January of 2011. To take PO4 value of deep water mass for reference, AS2806 (double circle, a ship number) data at the abyssal bottom of the Japan Sea in December of 2011 was selected. The vertical distributions of θ, S and dissolved O2 down to the maximum depth of 300 db were measured with the CTD system. The dissolved O2 and PO4 data were calibrated by using water-sample data taken at all standard depths (0, 10, 20, 30, 50, 75, 100, 125, 150, 200, 250 and 300 db) of CTD stations.

2.2. AOU and OUR Analyses Water masses are often identified by their temperature, salinity and density. However, it is very difficult for the water beneath the TWC with nearly uniform temperature/salinity of 1 ◦C/34.1. Thus, we need other stable conservative tracers such as nutrients or oxygen content to clarify the mixing 0 behavior of different water masses. Many previous studies have used the relationship of PO4 -AOU to re-build the oceanic general circulation or infer the mixing water masses from different origins. 0 Since PO4 is the conservative property and thus it can be used as a water mass tracer. In general, Water 2020, 12, 2184 5 of 16

the decomposition and regeneration process of PO4 consumes the dissolved O2 in seawater. This degree of consumption is expressed by the AOU or O2% (degree of oxygen saturation), which are respectively defined as follows: AOU = O * O (1) 2 − 2 O % = 100 O / O * (2) 2 × 2 2 where O2 is observed by a CTD equipped O2 sensor and O2* (saturation value) is calculated 1 from given T and S (see AppendixA). Here, the unit of AOU is converted from mL L − 1 3 to µM(1 mL L− = 10 /22.391 = 44.661 µM) for the convenience of the following description of AOU-PO4 relationship. Since the low O2 water is formed by the decomposition of particulate organic matter, particularly in the spring season when primary production is high in the surface layer, the waters a few weeks ago may become old and anoxic. Not such a short-term transition process, the steady state of oxygen concentration in the ocean would be maintained by the balance of oxygen supply and consumption. The supply to the intermediate and the deep layers of the Japan Sea (water depth from 500 to 3500 m) comes from the low temperature (0 to 1 ◦C) water masses formed by active vertical mixing of the northern sea surface in winter before the spring bloom. Therefore, it can be easily assumed that the O2 supplied to the intermediate and deep layer water masses is almost the same as the saturated oxygen content (O2*) determined by the TS (temperature and salinity) value in the surface mixed layer. Thus, the downwelling water preserving the TS value at the time of mixing sinks into the intermediate and the deep layers of the Japan Sea. Due to organic matter decomposition, the oxygen gradually goes down as respiration proceeds in the subsurface layers. The OUR (oxygen utilization rate) is usually considered to represent the subsurface oxygen consumption [9,10]. The OUR in the subsurface water is estimated by dividing the change in AOU along a given distance on an isopycnal surface by the change in water age (see Equation (3)).

OUR = dAOU/dt = d(O * O )/dt (3) 2 − 2 The water age (t in Equation (3)) is the time elapsed since the oxygen concentration at equilibrium with air when age is set to zero. In this paper, based on the assumption of that the OUR in the intermediate and deep layer of the Japan Sea, i.e., below the photic layer, is a constant; we can discuss the seawater age and estimate the preformed PO4 by O2 and AOU.

2.3. Preformed PO4 Analyses Redfield et al. (1963) [11] established the stoichiometric equation for average phytoplankton composition and its respiratory decomposition as follows:

(CH O) (NH ) H PO + 138O 106CO + 16HNO + H PO + 122H O (4) 2 106 3 16 3 4 2→ 2 3 3 4 2

The excess PO4 originates from PO4 in the surface water assuming to be saturated with oxygen. 0 0 Redfield et al. (1963) [11] defined this excess PO4 as “preformed PO4 (PO4 )”. This PO4 can be calculated by the following equation.

PO 0 = PO m AOU/138 (5) 4 4 − m where PO4 denotes the measured PO4 concentration and all units are µM. We can discuss the origin 0 of water masses with different dissolved O2, based on the relationship between PO4 and dissolved O2 (or AOU). Considering on a global scale, due to the subsidence of seawater in the polar region where the water temperature is relatively low, PO4 before subsidence (preformed PO4) is indeed a function of water temperature, i.e., lower water temperature tends to have higher PO4. However, the scale of the Japan Sea is much smaller than that of the ocean and the deep water of the Japan Sea is almost uniform Water 2020, 12, 2184 6 of 16

Water 2020, 12, 2184 6 of 17 seawater (JSPW) with a water temperature of less than 1 ◦C (salinity is almost indistinguishable), as mentionedBesides, in we the noticed introduction. that from Hence, some itprevious is very studies difficult [11–13], to discuss the theRedfield origin [11] of theratio water is sensitive mass by usingto observations TS data as a in tracer. sea area As shownand water in this depth study, (see the Appendix estimated B). preformed Deviation POof the4 is P/O particularly2 ratio from eff ectivethe in theRedfield case ofratio the may Japan affect Sea the to estimated speculate PO the40 advection value. However, and the if mixinga certain process P/O2 ratio beneath different the from Tsushima the Warmtraditional waters. Redfield ratio is maintained in the sea area, it is considered that PO40 can be treated as an absoluteBesides, value we (conservative noticed that fromcomponent) some previous with a certain studies bias. [11 –13], the Redfield [11] ratio is sensitive to observations in sea area and water depth (see AppendixB). Deviation of the P /O2 ratio from the 3. Results 0 Redfield ratio may affect the estimated PO4 value. However, if a certain P/O2 ratio different from the traditional Redfield ratio is maintained in the sea area, it is considered that PO 0 can be treated as an 3.1. The Path of TWC in June, 2011 4 absolute value (conservative component) with a certain bias. The TWC has a complex flow path and discontinuous shape. However, in the flow path of the 3. ResultsTsushima/Korean Strait and near the Tsugaru Strait is relatively stable. CTD and XBT observation data are used to show the horizontal distributions of temperature in Figures 3 and 4. The Straight 3.1.lines The represent Path of TWC the CTD in June, observation 2011 lines of A-F, and the rests of black dot are XBT observations. TheFigure TWC 3 hasshows a complex the horizontal flow path distributions and discontinuous of temperature shape. around However, the Tsushima/Korean in the flow path Strait of the Tsushimaat the depths/Korean of Strait50 m and and 100 near m. the In this Tsugaru map we Strait can is only relatively see the stable. isotherm CTD from and the XBT Tsushima/Korean observation data Strait to the Oki Islands waters, because the east Korean Peninsula water has not been measured. We are used to show the horizontal distributions of temperature in Figures3 and4. The Straight lines can see the meandering path of TWC on the shelf (the depth less than 200 m). The meandering wave- represent the CTD observation lines of A-F, and the rests of black dot are XBT observations. length is about 200 km and a significant cold eddy lies to the west of the Oki Islands.

FigureFigure 3. 3.Horizontal Horizontal distributions distributions of of temperature temperature around around the theTsushima Tsushima/Korean/Korean Strait,Strait, atat thethe depthsdepths of 50of m 50 and m 100and m.100 m.

FigureFigure3 shows 4 shows the the horizontal horizontal distributions distributions of of temperature temperature around at the depths the Tsushima of 100 m/Korean and 200 Strait m in at thethe depths sea area of 50 from m andthe 100Tsugaru m. In Strait this mapto the we west can of only Hokkaido. see the The isotherm TWC fromstructure the Tsushimais broken /atKorean the Straitdepth to of the 200 Oki m around Islands the waters, Tsugaru because Strait. theIsotherms east Korean in the sea Peninsula near the 138° water E, has40° N not is very been intensive, measured. Wethe can horizontal see the meandering temperature pathgradient of TWC of the on north-so the shelfuth (thewater depth corresponds less than to 200the pole m). Thefront. meandering A strong wave-lengthflow exits at is the about high 200 water km temperature and a significant region cold alon eddyg the liesright to side the of west the ofisotherm. the Oki This Islands. implies that theFigure main 4waters shows of the TWC horizontal flow out distributionsto the North Pacific of temperature through Tsugaru at the depths Strait. The of 100 rest m of and them 200 seem m in theto sea flow area northward from the Tsugaruas the shallower Strait to the TWC, west and of Hokkaido.then a small The meandering TWC structure path is is broken formed at west the depth of of 200Hokkaido. m around the Tsugaru Strait. Isotherms in the sea near the 138◦ E, 40◦ N is very intensive, the horizontal temperature gradient of the north-south water corresponds to the pole front. A strong flow exits at the high water temperature region along the right side of the isotherm. This implies that the main waters of TWC flow out to the North Pacific through Tsugaru Strait. The rest of them seem to flow northward as the shallower TWC, and then a small meandering path is formed west of Hokkaido.

Water 2020, 12, 2184 7 of 16 WaterWater 2020 2020, 12, ,12 2184, 2184 7 of7 of17 17

Figure 4. The Horizontal distributions of temperature at the depths of 100 m and 200 min the sea area FigureFigure 4. 4.The The Horizontal Horizontal distributions distributions of oftemperature temperature at atthe the depths depths of of100 100 m mand and 200 200 min min the the sea sea area area from the Tsugaru Strait to the west of Hokkaido. fromfrom the the Tsugaru Tsugaru Strait Strait to tothe the west west of ofHokkaido. Hokkaido. Figure5 shows the horizontal distribution of temperature at the depths of 100 m and 200 m FigureFigure 5 shows5 shows the the horizontal horizontal distribution distribution of of temperature temperature at atthe the depths depths of of 100 100 m m and and 200 200 m m in in in June of 2011, referred to the quick maps from Japan Sea National Fisheries Research Institute JuneJune of of 2011, 2011, referred referred to to the the quick quick maps maps from from Japan Japan Sea Sea National National Fisheries Fisheries Research Research Institute Institute (http://jsnfri.fra.affrc.go.jp/, Yokohama, Japan). First, many eddies and the large meander of TWC (http://jsnfri.fra.affrc.go.jp/,(http://jsnfri.fra.affrc.go.jp/, Yokohama, Yokohama, Japan). Japan). Firs First, manyt, many eddies eddies and and the the large large meander meander of of TWC TWC can can can be seen from Figure5. Moreover, a small meander can be seen on the horizontal distribution of bebe seen seen from from Figure Figure 5. 5.Moreover, Moreover, a asmall small meander meander can can be be seen seen on on the the hori horizontalzontal distribution distribution of of temperature at the depth of 100 m, and there is a cold eddy west of the Oki islands. A significant temperaturetemperature at atthe the depth depth of of 100 100 m, m, and and there there is isa colda cold eddy eddy west west of of the the Oki Oki islands. islands. A Asignificant significant offshore TWC is found flowing from the north of the Oki islands to the west of the Tsugaru Strait, offshoreoffshore TWC TWC is isfound found flowing flowing from from the the north north of of the the Oki Oki islands islands to to the the west west of of the the Tsugaru Tsugaru Strait, Strait, forming a large s-shaped path west of the Noto peninsula. This meander includes a noticeable warm formingforming a largea large s-shaped s-shaped path path west west of of the the Noto Noto peninsula. peninsula. This This meander meander includes includes a noticeablea noticeable warm warm eddy, which can be captured by our observation of line B. eddy,eddy, which which can can be be captured captured by by our our observation observation of of line line B. B.

Figure 5. A quick map of the horizontal distributions of temperature at the depths of 100 m and 200 m. FigureFigure 5. 5.A Aquick quick map map of ofthe the horizontal horizontal distributions distributions of oftemperature temperature at atthe the depths depths of of100 100 m mand and 200 200 m.m. 3.2. Relationships Among θ, S and O2

3.2. Relationships Among θ, S and O2 3.2. RelationshipsBefore the description Among θ, S forand the O2 vertical distributions of θ, S and dissolved O2, the relationships among them are analyzed by using all CTD data obtained in our cruise. Dissolved Oxygen versus Before the description for the vertical distributions of θ, S and dissolved O2, the relationships Before the description for the vertical distributions of θ, S and dissolved O2, the relationships amongamong them them are are analyzed analyzed by by using using all all CTD CTD data data ob obtainedtained in in our our cruise. cruise. Dissolved Dissolved Oxygen Oxygen versus versus

Water 2020, 12, 2184 8 of 17 salinity and potential temperature relations (Figure 6) for all CTD observations taken in summer of 2011 are examined because dissolved oxygen plays an important role in identifying water masses in the Japan Sea. Figure 6a,c are the θ-S diagrams as the function of potential density σθ and O2%. Figure 6b,d show the relationship between θ and O2 and that between O2% and O2, respectively. When the density is greater than 27.0, i.e., the water beneath the TWC, O2* varies in a small range of 7.3–8.0 mL L−1, and theWater S 2020is nearly, 12, 2184 constant around 34.0 psu (Figure 6a,c). On the other hand, the TWC water with8 ofthe 16 density lower than 27.0 shows the S with a wide range from 33.0 to 34.6 psu (Figure 6a). For convenience, water masses are divided into three categories with S-values of 34.15 and 33.95 psu. In eachsalinity figure, and high-S potential (more temperature than 34.15 relations psu, the (Figure TWC 6core) for water), all CTD low-S observations (less than taken 33.95 in psu, summer diluted of surface2011 are water) examined and because mid-S dissolved(water beneath oxygen the plays TW anC) important are drawn role by in identifyingred, blue and water black masses dots, in respectively.the Japan Sea.

Figure 6. Relationships of (a,c) θ-S, (b)O2-θ and (d)O2-O2%. Slashes in Figure6a,c are potential Figure 6. Relationships of (a and c) θ-S, (b) O2-θ and (d) O2-O2%. Slashes in Figure 6a,c are potential density σθ and O2%, respectively. density σθ and O2%, respectively.

Figure6a,c are the θ-S diagrams as the function of potential density σθ and O2%. Figure6b,d show Comparing the θ-S diagram (Figure 6a) with the θ-O2 diagram (Figure 6b), the warm waters the relationship between θ and O2 and that between O2% and O2, respectively. When the density is with high-S and low-S correspond to low-O2 and high-O2, respectively. The cold waters beneath1 the greater than 27.0, i.e., the water beneath the TWC, O2* varies in a small range of 7.3–8.0 mL L− , and the TWCS is nearly (σθ >27.1) constant with around almost 34.0constant psu (Figure S-value6a,c). (34.0–34.1) On the show other hand,O2 with the a TWCwide waterrange with(4.8–6.7 the mL density L−1). Distinguishlower than 27.0different shows water the masses S with abe wideneath range the TWC from by 33.0 S will to 34.6be very psu difficult. (Figure6 So,a). it For would convenience, be better towater use O masses2 data arerather divided than S into data three in order categories to investigate with S-values the behaviors of 34.15 of and the 33.95 water psu. mass In beneath each figure, the TWC.high-S The (more O2%-O than2 diagram 34.15 psu, (Figure the TWC 6d) coreshows water), that O low-S2 increases (less than in proportion 33.95 psu, to diluted O2%, and surface with water) such aand relationship mid-S (water the upper-level beneath the and TWC) lower-leve are drawnl water by red, masses blue can and be black clearly dots, divided. respectively. The O2% of lower- level water mass is relatively low (60–80%) in comparison with that of upper-level water (80–100%). Comparing the θ-S diagram (Figure6a) with the θ-O2 diagram (Figure6b), the warm waters The upper blue and red dots stand for low salinity and high salt salinity water of Tsushima Warm with high-S and low-S correspond to low-O2 and high-O2, respectively. The cold waters beneath the Current. The black dots show an expansion of dissolved oxygen, and the green circle indicates water1 TWC (σθ >27.1) with almost constant S-value (34.0–34.1) show O2 with a wide range (4.8–6.7 mL L− ). ofDistinguish 27.0 of density. different The water dashed masses line indicates beneath theits slope. TWC byThat S will is to be say, very there diffi existscult. So,a linear it would relationship be better between dissolved oxygen saturation and dissolved oxygen concentration when the density is near to use O2 data rather than S data in order to investigate the behaviors of the water mass beneath 27.0. Its linear gradient α is about 13%/(mL L−1): the TWC. The O2%-O2 diagram (Figure6d) shows that O 2 increases in proportion to O2%, and with such a relationship the upper-levelslope-α = and (100%) lower-level / (7.3mL water L−1) ~13%/(mL masses can L−1 be) clearly divided. The O(6)2% of lower-level water mass is relatively low (60–80%) in comparison with that of upper-level water (80–100%). The upper blue and red dots stand for low salinity and high salt salinity water of Tsushima Warm Current. The black dots show an expansion of dissolved oxygen, and the green circle indicates water of 27.0 of density. The dashed line indicates its slope. That is to say, there exists a linear relationship between dissolved oxygen saturation and dissolved oxygen concentration when the 1 density is near 27.0. Its linear gradient α is about 13%/(mL L− ):

1 1 slope-α = (100%)/(7.3 mL L− ) ~13%/(mL L− ) (6) Water 2020, 12, 2184 9 of 16 therefore, we infer that the new or old water mass beneath the TWC can be easily detected with dissolved O2 distributions.

3.3. Vertical Cross-Sections of Water Properties Along Lines A to F

The vertical cross-sections of θ, σθ, S, O2 and O2% along lines A to F are shown in Figures7 and8. The right side in each diagram is the Japanese coast. As for line A, the vertical profiles of each property at Stn. A8 (thick solid line) are also drawn to show a more detailed structure of the bottom water Waterintruding 2020, 12 from, 2184 the Japan Sea to the Tsushima/Korean Strait. 10 of 17

θ σ FigureFigure 7. 7.VerticalVertical cross-sections cross-sections of ofθ, σ,θ, θS,, S,O2 O and2 and O2% O2 along% along line line (A) A and and (B B) inin JuneJune, 2011. 2011.

The vertical homogeneity with high S and low dissolved O2 at the mid-depths of 50–140 db at Stn. A8 shows the characteristic of TWW (Figure7A). The TWW extends to the bottom layer in the shallower eastern Strait, but its homogeneity becomes somewhat weaker. The other homogeneous water is found at the bottom layer below the TWW at Stn. A8. It has been considered that the origin of this bottom water is the JSPW, and hence the existence of vertical circulation like the estuarine circulation has been inferred on the Korean side of the Strait. In comparison with the TWW, the bottom 1 water mass is colder (3.4 ◦C) with lower S (34.03 psu) and higher dissolved O2 (5.7 mL L− ). Besides, O2% of the bottom water (77%) is slightly larger than that of the TWW (74–76%). Line B captures a noticeable warm eddy at Stn. B4–B8. Its core water is warmer (10 ◦C), but with 1 lower S (34.1 psu) and higher dissolved O2 (5.6 mL L− ), compared with the surrounding waters. Therefore, it is inferred that this warm eddy may be influenced by the vertical mixing due to the previous winter cooling. The TWW with warm temperature (>10 ◦C), high S (>34.3 psu) and low 1 dissolved O2 (<5 mL L− ), which are the similar properties observed in line A, is confirmed at the depth of less than 200 m on the Japanese coastal side. Then, we can find another water with low 1 dissolved O2 (<5 mL L− ) and low dissolved O2%(<74%) beneath the TWW.

Figure 8. Vertical cross-sections of θ, σθ, S, O2 and O2% along lines (C) to (F) in June, 2011.

Water 2020, 12, 2184 10 of 17

Water 2020Figure, 12, 2184 7. Vertical cross-sections of θ, σθ, S, O2 and O2% along line (A) and (B) in June, 2011. 10 of 16

Figure 8. Vertical cross-sections of θ, σ , S, O and O % along lines C to F in June 2011. Figure 8. Vertical cross-sections of θ, σθ, S,θ O2 and2 O2% 2along lines (C) to (F) in June, 2011.

The vertical cross-sections of water properties along both lines C and D, located south of the Tsugaru Strait, have very similar characteristics. The TWW with warm temperature (>10 ◦C), high S 1 (>34.2 psu) and low dissolved O2 (<5.5 mL L− ) is trapped to the Japanese coastal side. There are two deep water masses with different dissolved O2 and O2% beneath the TWW, although laterally they have the almost same values of θ (2–5 ◦C), σθ (27–27.2) and S (<34.1 psu). One is the water with 1 relatively low dissolved O2 (<6 mL L− ) and low dissolved O2%(<80%) on the coastal side, the other is the water with high dissolved O2 and high dissolved O2% on the offshore side. Such geographical distribution of two water masses with different dissolved O2 is also seen in line E north of the Tsugaru Strait, but the TWW with high S (>34.1 psu) largely shifts to the offshore side (Stn. E4–E5). In line F west of Hokkaido, the TWW with high S and low dissolved O2 is barely recognized. In comparison with all the southern lines of A–E, the thickness of warm water tends to be shallower. Then, a warm eddy (or a small meander) seen at Stn. F4–F7 can be only characterized by θ-structure, and the coastal trapped warm water seen at Stn. F1–F3 is with relative low S (<34.0 psu). Low 1 dissolved O2 (<5.5 mL L− ) and low dissolved O2%(<74%), which corresponds to the coastal deep water observed in lines C–E and is uniformly distributed below the depth of 200–250 db. In order to more closely examine the water mass distributions beneath the TWW, the vertical profiles of dissolved O2 and O2% along line A and the cross-sections along lines B to E where the depth is represented by σθ are shown in Figure9. In the cross-sections from lines B to E, it is found that high dissolved O2 (high dissolved O2%) and low dissolved O2 (low dissolved O2%) patches stand side by side along the iso-pycnals of denser than 27.0. The bottom water with the σθ denser than 27.1 at Stn. A8 seems to have the middle values of these two patches. Such patches with the σθ denser than 27.0 in 1 line F stand one behind another, and another patch with remarkable high dissolved O2 (>6 mL L− ) and high dissolved O2%(>90%) also appears in a layer with the σθ of 27–26.5. This suggests that the vertical mixing of shallower subsurface layer had considerably occurred during the previous winter time. Water 2020, 12, 2184 11 of 17

In order to more closely examine the water mass distributions beneath the TWW, the vertical profiles of dissolved O2 and O2% along line A and the cross-sections along lines B to E where the depth is represented by σθ are shown in Figure 9. In the cross-sections from lines B to E, it is found that high dissolved O2 (high dissolved O2%) and low dissolved O2 (low dissolved O2%) patches stand side by side along the iso-pycnals of denser than 27.0. The bottom water with the σθ denser than 27.1 at Stn. A8 seems to have the middle values of these two patches. Such patches with the σθ denser than 27.0 in line F stand one behind another, and another patch with remarkable high dissolved O2 (>6 mL L−1) and high dissolved O2% (>90%) also appears in a layer with the σθ of 27–26.5. This suggests that theWater vertical2020, 12 ,mixing 2184 of shallower subsurface layer had considerably occurred during the previous11 of 16 winter time.

Figure 9. The vertical profile of O2 and O2% along line A, and the vertical cross-sections of O2 and Figure 9. The vertical profile of O2 and O2% along line (A), and the vertical cross-sections of O2 and O2% along lines B to F where the depth is represented by σθ. O2% along lines (B) to (F) where the depth is represented by σθ.

3.4. Inference of Original Waters Beneath the TWC Using the Preformed PO4 3.4. Inference of Original Waters Beneath the TWC Using the Preformed PO4 It is well known that chemical elements, e.g., NO3, NO2, NH3, PO4, SiO2, Cd and so on, can be It is well known that chemical elements, e.g., NO3, NO2, NH3, PO4, SiO2, Cd and so on, can be used as an indicator to trace the ocean circulation. Among these elements, PO4 is considered to have a used as an indicator to trace the ocean circulation. Among these elements, PO4 is considered to have good correlation with the seawater AOU, because the ratio of PO4/O2 well satisfies Redfield’s equation a good correlation with the seawater AOU, because the ratio of PO4/O2 well satisfies Redfield’s (PO4:AOU = 1:138). Therefore, if there is no mixing among the water masses, the PO4 value of original equation (PO4:AOU = 1:138). Therefore,0 if there is no mixing among the water masses, the PO4 value water, i.e., preformed PO4 (PO4 ), can be estimated from Equation (4) by using the measured PO4 of originalm water, i.e., preformed PO4 (PO40), can be estimated from Equation (4) by using the (PO4 ) and AOU. Actually, lots of related researches apply this PO4-AOU relationship in the Pacific measuredOcean to reconstructPO4 (PO4m) theand ocean AOU. circulation Actually, lots pattern of related and to researches infer a consequence apply this ofPO mixing4-AOU betweenrelationship two in the Pacific Ocean to reconstruct the ocean circulation pattern and to infer a consequence of mixing water masses with different original ratios of PO4-AOU [6–8]. Chen et al. (1996) [13] firstly calculated between two water masses with different original ratios of PO4-AOU [6–8]. Chen et al. (1996) [13] the preformed PO4 in the Japan Sea and explained that the preformed PO4 is a function of potential firstly calculated the preformed PO4 in the Japan Sea and explained that the preformed PO4 is a temperature. The present analysis using PO4 -AOU relationship in the Japan Sea is the first time to functionunderstand of potential the behavior temperature. of the water The masspresent beneath analysis the using TWC. PO4 -AOU relationship in the Japan Sea is the first time to understand the behavior of the water mass beneath the TWC. At first, let’s present the standard PO4-AOU relationship unaffected by the TWC. Figure 10a shows At first,m let’s present0 the standard PO4-AOU relationship unaffected by the TWC. Figure 10a the PO4 -AOU and PO4 -AOU plots for all samples from subsurface (100 db) to bottom (3000 db) showsat AS2806 the PO in December4m-AOU and of PO 2011.40-AOU This observationplots for all samples point locates from insubsurface the deepest (100 basin db) into thebottom Japan (3000 Sea db)(Japan at AS2806 Basin) and in December north of the of TWC’s2011. This northern observat boundaryion point (polar locates front). in the Higher-AOU deepest basin (>130 inµ theM) regionJapan Sea (Japan Basin) and north of the TWC’s northemrn boundary (polar front). Higher-AOU (>130 μM) in the deep layer (>500 db) shows a slope of PO4 -AOU almost equal to the Redfield ratio (1:138) and region in the deep 0layer (>500 db) shows a slope of PO4m -AOU almost equal to the Redfield ratio a zero-slope of PO4 -AOU (the constant value of 0.8–0.85 µM), while lower-AOU region in the upper (1:138) and a zero-slope of PO40 -AOUm (the constant value of 0.8–0.85 μM), while lower-AOU region layer (<500 db) shows a steeper PO4 -AOU slope (1:100) than the slope of deeper water (>500 db). m 0 Both of PO4 and PO4 for the upper layer, however, clearly exhibit a good linear correlation with AOU, and the same intercept of about 0.5 µM at AOU = 0 µM. These relations imply that the deep 0 0 m layer with the constant PO4 is related with the JSPW, and the gradual increase of PO4 (or PO4 ) would be produced by the apparent conservative mixing of two water masses—the in-situ surface 0 water with PO4 of 0.5 µM and the JSPW with high PO4 of 0.8–0.85 µM. Water 2020, 12, 2184 12 of 16 Water 2020, 12, 2184 13 of 17

m 0 Figure 10. (a) The PO4 -AOU and PO4 -AOU plots for all samples from subsurface (100 db) to bottom Figure 10. (a) The PO4m-AOU and PO40-AOU plots for all samples from subsurface (100 db) to bottom (3000 db) at AS2806 in December of 2011; (b) the annual mean distribution map of sea surface PO4 in (3000 db) at AS2806 in Decemberm of 2011; (b)0 the annual mean distribution map of sea surface PO4 in the Japan Sea; (c,d) the PO4 -AOU and PO4 -AOU plots for the upper layer (<300 db), including all the Japan Sea; (c–d)the PO4m-AOU and PO40-AOU plots for the upper layer (<300 db), including all samplessamples in lines in lines A to A F.to F.

4.These Discussions end-members can be found in the annual-mean distribution map of sea surface PO4 in the Japan Sea (Figure 10b), which is referred from the report of [14]. Generally, the surface PO4 in To further discuss and elucidate the dynamic mechanism of the water mass beneath the TWC, the northern half region of the Japan Sea is higher than that in the southern TWC region. And the the schematic diagram for the behavior of water mass along the north-south cross-section in the Japan 0 µ µ PO4 Sea= 0.8–0.85 is shown inM Figureof JSPW 11. The agrees upper with layer the water surface and deep PO4 water> 0.8 mixingM in theoccurs northern at intermediate coastal depth area of the Japan(between Sea. Such Smax a layer distribution and Smin layer) is consistent and bottom with (about the origin 3000 db), of JSPW, respectively. which In is formedthis figure, by the the depth sinking of northernabout surface 500 db corresponds cooling water to the in salinity-maximum winter. The site locationlayer and ofalso AS2806 the upper-limit is between depth the of contour JSPW with lines of 0.4 µalmostM and constant 0.6 µM onPO4 this0 (0.8–0.85 map. This μM). value The JSPW is not originates contradictory from the to thesinking estimated surface POwater4 of with 0.5 µPOM4 for> the surface0.8 waterμM. The at AS2806.upper and lower limits of the intruding intermediate water beneath the TWC are characterized by the Smin and Smax layers, respectively. We infer that the intermediate waters with The PO4-AOU relationship in the TWC region is discussed on the basis of the above information different dissolved O2 are consisted of the same water mmass originating from0 the sinking surface for original water of JSPW. Figure 10c,d show the PO4 -AOU and PO4 -AOU plots for the upper water with PO4 of 0.4–0.7 μM, which seems to form at somewhere north of the TWC. layer (<300 db) including all samples in lines A to F, respectively. Different symbols are used to The distribution of dissolved oxygen denotes that seawater mixing occurs in the middle depth. represent the points in different lines, and the PO4-AOU relationship with the water mass beneath As the seawater flowing to the shore side, the dissolved O2 of this intermediate water is gradually σ the TWCconsumed ( θ > due27.1 to) isthe in plankton red color. respiration Our observations during the inintrusion, June were and carriedhence the out water after with the springhigh/low bloom, so bothdissolved of surface O2 appears PO4 and on AOUthe offshore/coastal at all stations side, show respec nearlytively. zero Such value. behavior For comparison, of the intruding a linear water line is givenaccompanied to assume thewith vertical the dissolved-O mixing2 betweenconsumption the can surface explain water the reason (PO4 =whyAOU the “falsehood”= 0 µM) and coastal the JSPW m 0 (PO4upwelling= 1.8 µM of andthe deep PO4 water= 0.8–0.85 with lowµM dissolved at AOU O=2 130is seenµM) in by Figure blue 2d: color. In the intermediate water, Although the data is scattered, the ratio of PO4/AOU for the upper layer (σθ < 27.1) almost coincide with the blue line which suggests the vertical mixing (Figure 10c,d). The data in upstream lines (lines A and B with symbols of and , respectively) seem to be more scattered. Probably, these waters • # Water 2020, 12, 2184 13 of 16 near the Tsushima Strait may be influenced by the lateral advection of different water masses from the East China Sea. These upstream waters (Lines A,B) are mixed with the TWC, the North Cold Current and the water from East China Sea, so a two end-member mixing liner line between the 0 surface water and the JSPW cannot coincide its variations. On the other hand, PO4 of waters beneath the TWC (σθ > 27.1) shows an almost constant value of 0.4–0.7 µM at all the stations (Figure 10d), i.e., m the PO4 /AOU ratio exhibits nearly equal to the Redfield ratio (Figure 10c). It is found that the sea 0 surface water with PO4 of 0.4–0.7 µM is distributed around the central sea area north of the TWC (Figure 10b). These results suggest that the origin of water beneath the TWC is the same water mass. The water mass is probably formed at somewhere sea surface area between northern coastal area and the north of the TWC, and gradually intrudes beneath the TWC from the offshore side.

4. Discussions ToWater further 2020, 12, discuss2184 and elucidate the dynamic mechanism of the water mass beneath14 the of 17 TWC, the schematic diagram for the behavior of water mass along the north-south cross-section in the Japan there is a new water of high dissolved oxygen on the offshore side, and an old water of low dissolved Sea isoxygen shown on in the Figure inshore 11. side, The simultaneously. upper layer water The and“falsehood” deep water upwelling mixing of occursthe low atdissolved intermediate oxygen depth (betweenalong S themax coast,layer which and S ismin actuallylayer) caused and bottom by the (aboutconsumption 3000 db), of dissolved respectively. oxygen In during this figure, the lateral the depth aboutadvection 500 db correspondsof the water mass to thebeneath salinity-maximum the TWC. layer and also the upper-limit depth of JSPW 0 with almostIn this constant investigation, PO4 (0.8–0.85 the mainµ findingM). The is JSPWthat we originates successfully from deduce the sinking the origin surface of the waterwater with 0 PO4 >beneath0.8 µM. the The TWC upper using and the PO lower4 as limitsa biogeochemical of the intruding tracer, intermediateespecially for the water intermediate beneath thewater TWC of are the Japan Sea where the T and S changes are very small. However, the primary shortage is that the characterized by the Smin and Smax layers, respectively. We infer that the intermediate waters with annual-mean distribution map of sea surface PO4 in the Japan Sea may impact on our results. In the different dissolved O2 are consisted of the same water mass originating from the sinking surface water future, the efforts should be made to obtain more precise observations of annual-mean distribution with PO4 of 0.4–0.7 µM, which seems to form at somewhere north of the TWC. of sea surface PO4 for exploring this issue.

Figure 11. Schematic diagram for the behavior of the water mass beneath the TWC. Figure 11. Schematic diagram for the behavior of the water mass beneath the TWC. The distribution of dissolved oxygen denotes that seawater mixing occurs in the middle depth. 5. Summary As the seawater flowing to the shore side, the dissolved O2 of this intermediate water is gradually consumedThe due objective to the of plankton this study respiration is to examine during the behavior the intrusion, of water and mass hence beneath the the water TWC with through high /low the spatial distributions of observed dissolved O2 and PO4 in the Japan Sea. The precise PO4-AOU dissolved O2 appears on the offshore/coastal side, respectively. Such behavior of the intruding water relationship is a useful tool to better understand the original water and the mixing process between accompanied with the dissolved-O2 consumption can explain the reason why the “falsehood” coastal two water masses. Because the PO40 is the conservative property or constant for different water upwelling of the deep water with low dissolved O is seen in Figure2d: In the intermediate water, masses, which is widely used as a tracer for studying2 water mass movement. Thus, it was examined therein is the a new Japan water Sea for of highboth dissolvedthe upper oxygenlayer and on deep the layer offshore to provide side, and a better an old understanding water of low of dissolved the oxygenwater on behaviors the inshore beneath side, simultaneously.the TWW. A previous The “falsehood” linearity in the upwelling relationship of the was low plotted dissolved in these oxygen figures. The intercept of the vertical axis indicates measured phosphate or preformed phosphate, the line correspond to the ratio of PO4/AOU. Kudo et al. (1996) [7] explain that the defined preformed fractions between surface and deep water were shown as a result of the physical mixing of these water masses. It can be inferred that PO4 in intermediate water was originated from the central surface waters in the Japan Sea, and the PO4 in bottom water was generated from the Japan Sea surface northern waters. In the future, we hope to further explore this behavior of water mass beneath of the TWC in ocean dynamics. Also we will simulate this behavior using some advanced numerical models.

Author Contributions: For research articles with several authors, a short paragraph specifying their individual contributions must be provided. The following statements should be used “Conceptualization, Y.I.; methodology, Y.I., I.K.; software, X.F.; validation, X.F. and I.K.; formal analysis, Y.I., I.K.; investigation, X.F., K.I.,

Water 2020, 12, 2184 14 of 16 along the coast, which is actually caused by the consumption of dissolved oxygen during the lateral advection of the water mass beneath the TWC. In this investigation, the main finding is that we successfully deduce the origin of the water 0 beneath the TWC using the PO4 as a biogeochemical tracer, especially for the intermediate water of the Japan Sea where the T and S changes are very small. However, the primary shortage is that the annual-mean distribution map of sea surface PO4 in the Japan Sea may impact on our results. In the future, the efforts should be made to obtain more precise observations of annual-mean distribution of sea surface PO4 for exploring this issue.

5. Summary The objective of this study is to examine the behavior of water mass beneath the TWC through the spatial distributions of observed dissolved O2 and PO4 in the Japan Sea. The precise PO4-AOU relationship is a useful tool to better understand the original water and the mixing process between 0 two water masses. Because the PO4 is the conservative property or constant for different water masses, which is widely used as a tracer for studying water mass movement. Thus, it was examined in the Japan Sea for both the upper layer and deep layer to provide a better understanding of the water behaviors beneath the TWW. A previous linearity in the relationship was plotted in these figures. The intercept of the vertical axis indicates measured phosphate or preformed phosphate, the line correspond to the ratio of PO4/AOU. Kudo et al. (1996) [7] explain that the defined preformed fractions between surface and deep water were shown as a result of the physical mixing of these water masses. It can be inferred that PO4 in intermediate water was originated from the central surface waters in the Japan Sea, and the PO4 in bottom water was generated from the Japan Sea surface northern waters. In the future, we hope to further explore this behavior of water mass beneath of the TWC in ocean dynamics. Also we will simulate this behavior using some advanced numerical models.

Author Contributions: Conceptualization, Y.I.; methodology, Y.I., I.K.; software, X.F.; validation, X.F. and I.K.; formal analysis, Y.I., I.K.; investigation, X.F., K.I., Y.I., I.K., and T.A.; resources, Y.I., N.E., I.K., T.A.; data curation, X.F.; writing—original draft preparation, X.F.; writing—review and editing, Y.I.; visualization, X.F.; supervision, Y.I.; project administration, Y.I., I.K.; funding acquisition, Y.I., X.F. All authors have read and agreed to the published version of the manuscript. Funding: A part of this work was supported by the Environment Research and Technology Development Fund of the Ministry of the Environment, Japan (A-1002). Acknowledgments: The data used in the study were obtained from the observations by T/V Oshoro Maru of the Hokkaido University. We wish to thank all the crew members on the T/V Oshoro Maru for their hard works. We would also like to thank the reviewers for their thoughtful suggestions. Conflicts of Interest: The authors declare no conflict of interest.

Appendix A

* The saturation of dissolved oxygen (O2 ) is calculated with the potential temperature T and salinity S as follows:       100 100 100 2 lnO∗ = A1 + A2 + A3 ln + A4 + S (B1 + B2(T/100) + B3(T/100) ) (A1) 2 T T T · where T is the absolute temperature (T = 273.15 + t (◦C), Kelvin) and S is salinity. The coefficients A and B are listed in Table A1.

Table A1. Coefficients A and B for calculating the dissolved oxygen saturation.

1 1 Coefficient A (mL L− ) Coefficient B (mL L− ) A1 = 173.4292 − B1 = 0.033096 A2 = 249.6339 − B2 = 0.014259 A3 = 143.3483 B3 = 0.001700 A4 = 21.8492 − − Water 2020, 12, 2184 15 of 16

Appendix B The Redfield et al. (1963) [11] equation for the production (or respiration) of average marine plankton has been used in oceanography for deriving dynamically passive conservative tracers (e.g., ‘NO’ and ‘PO’ [15,16]). The traditional P/O2 Redfield ratio of 1:138, based on planktonic decomposition studies [17], was inferred by effectively assuming an organic matter stoichiometric composition. However, recent studies have shown different ratios of this traditional value (Table A2). Takahashi et al. (1985) [18] examined the correlated changes in nutrients on isopycnals in the thermocline and concluded that the actual P/O2 ratio there was at a higher value of 172. Broecker et al. (1985) [16] and Peng and Broecker (1987) [19] extended this finding by asserting that the P/O2 ratio was near 175 not only in the thermocline, but at all depths throughout the entire world ocean. In contrast, Minster and Boulahdid (1987) [20] and Boulahdid and Minster (1989) [21], using the same data, suggested that the P/O2 ratio was near 172 in the thermocline, and decreased with depth to approximately 115 in all oceans. Chen et al. (1996) [22] firstly calculated the preformed PO4 in the Japan Sea and reported that the Redfield P/O2 ratio in the East Asian Marginal Sea (including the Japan Sea) was approximately 1:121 7. ± As each study contradicts to a previous one, we feel that the Redfield ratio is sensitive to observations in sea area and water depth. As Chen (1996) [13] pointed out, deviation of the P/O2 ratio 0 from the Redfield ratio may affect the estimated PO4 value. However, if a certain P/O2 ratio different 0 from the traditional Redfield ratio was maintained in the sea area, it is considered that PO4 can be treated as an absolute value (conservative component) with a certain bias. In a word, according to these different ratios, we think this ratio is a fixed value with a certain bias from the traditional ratios. 0 Thus, we use an averaged traditional ratio of 1:138 to estimate PO4 as a tracer in this study.

Table A2. Stoichiometric Redfield ratios for consumption of O2 and production of P during respiration in the ocean. All values are relative to a phosphorus value of 1.0.

Source P O2 Area Depth Redfield et al., 1963 [11] 1.0 138 Atlantic Ocean in ocean deep waters Takahashi et al., 1985 [18] 1.0 172 Atlantic and Indian ocean in the deep ocean Broecker et al., 1985 [16]; 1.0 175 entire world ocean all depths Peng and Broecker, 1987 [19] Minster and Boulahdid, 1987 [20]; decreased with depth to 1.0 172 in all oceans Boulahdid and Minster, 1989 [21] approximately 115 Anderson and Sarmiento, 1994 [12] 1.0 170 10 Pacific basins between 400 and 4000 m depth ± Anderson, 1995 [23] 1.0 140 160 − marginal seas of the Chen et al., 1996 [22] 1.0 196 27 ± northwest Pacific beneath the thermocline in 1:125 5 for waters deeper Chen et al., 1996, GRL [13] 1.0 121 7 ± ± Japan Sea than 2000 m Kortzinger et al., 2001 [24] 1.0 165 15 surface ocean surface ocean ± Hedges et al., 2002 [25] 1.0 154 global ocean

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