577

Journal of Oceanography, Vol. 58, pp. 577 to 588, 2002

* E-mail address: abek@fra.affrc.go.jp

Copyright © The Oceanographic Society of Japan.

Relationship between Cd and PO4 in the Subtropical Seanear the Ryukyu Islands

KAZUO ABE*

Ishigaki Tropical Station, Seikai National Fisheries Research Institute,Fukai-Ota, Ishigaki, Okinawa 907-0451, Japan

(Received 26 February 2001; in revised form 22 November 2001; accepted 18 December 2001)

The relationship between dissolved cadmium (Cd) and phosphate (PO4) was exam-ined at three stations in the subtropical area near the Ryukyu Islands in May 1999.Preformed PO4 was obtained using the Redfield ratio in order to separate the surfacewater and the other layers in this study area. Almost 0 µM (–0.043 µM to 0.094 µM)was estimated in the layers above 300 m and 250 m at Sts. 1 and 3 and at St. 2, respec-tively. Up to these depths, water was considered to be uniform, and these layers weredefined as the surface water in this study area. In the surface water, the slopes of theregression lines of the Cd–PO4 plot were 0.162, 0.156, and 0.226 (nM/µM) at Sts. 1, 2,and 3, respectively, and these values were much closer to the estimated regeneratedratio of Cd to PO4 from the Apparent Oxygen Utilization (AOU)–Cd/PO4 plots, whichwas 0.197 (nM/µM) in this study area. Below surface layers, the slopes of the Cd–PO4plot changed to 0.371, 0.352, and 0.362 (nM/µM) at Sts. 1, 2, and 3, respectively. Inthe relationships between Cd and PO4, clear deviations or kinks were observed atthree stations at a PO4 concentration of approximately 0.2 µM in the plot, which wasattributable to the discontinuity of surface water and the other layers across the NorthPacific subtropical mode water. In studies of the interaction between surface waterand biogenic particles concerning the Cd/PO4 ratio, separate analyses of seawater(surface water and the other layers) should be carried out to obtain the individualsurface water ratio because the Cd/PO4 ratio in the surface water is expected to differfrom that of the underlying water. Furthermore, the biological fractionation of theseconstituents is based on the surface water ratio.

proximately 0.3–0.36 nM/µM) in the Pacific Ocean (e.g.,Bruland, 1980) have been reported. Boyle (1988) was thefirst to compile the global Cd–PO4 datasets and he founda clear deviation (kink) that suggested discontinuity be-tween surface and deep water where the concentration ofPO4 was 1.3 µM in his plot. However, de Baar et al. (1994)subsequently plotted the Cd–PO4 concentration graph forall of the world’s oceans and developed a broadscatterplot. They demonstrated that the plot used by Boyle(1988) was regionally limited and that there was a pre-sumed shift in the relationship. When Cd concentrationsfrom the surface to the deep water column are plottedagainst PO4, a clear straight line can sometimes be dis-cerned at first glance; however, when the plot is exam-ined in detail, a deviation seems to exist in the relation-ship, especially near the origin (e.g., the plot of the NorthPacific Ocean in figure 2 in de Baar et al., 1994 and fig-ure 9 in Bruland, 1980). As noted above, thebiogeochemical cycle is one of the important factors that

1. IntroductionThe behavior of cadmium (Cd) in the ocean is

strongly correlated with that of phosphate (PO4) and re-cent studies have regarded it as very important in the in-teraction with marine biological activities. It has beenreported that the distribution of Cd in seawater is regu-lated by marine biogeochemical processes, such as anuptake by phytoplankton in surface waters, consequen-tial decomposition of the organic matter produced, andremineralization in deep waters (e.g., Bruland, 1980). Ingeneral, a plot of Cd against PO4 concentration in seawatershows good linearity; however, the slopes of the regres-sion lines between the two elements vary from basin tobasin, and relatively low values (approximately 0.2 nM/µM) in the Atlantic (e.g., Yeats et al., 1995) and high (ap-

Keywords:⋅ Cd,⋅ PO4,⋅ kink,⋅ AOU,⋅ the RyukyuIslands.

578 K. Abe

regulate the relationship between Cd and PO4, and theratios (slope) of the Cd/PO4 plot in surface water aregreatly influenced by biological activities and the bal-ance of the removal and supply of these elements from/tothe surface layer. Although the role of Cd in thephytoplankton life cycle has not been clearly elucidated,substitution for zinc (Zn) in marine phytoplankton and apositive effect of Cd on its growth have been reported(Price and Morel, 1990; Lee and Morel, 1995). Theseauthors explained that Zn is normally one of the compo-nents of a metalloenzyme, Zn-carbonic-anhydrase, whichis very important for phytoplankton growth; however, Cdreplaces Zn as Cd-carbonic-anhydrase under Zn-limitedconditions, and consequently the addition of Cd acceler-ates phytoplankton growth under such conditions. Fur-thermore, the activity of carbonic-anhydrase was en-hanced as pCO2 decreased in ambient seawater (Cullenet al., 1999; Burkhardt et al., 2001). Cullen et al. (1999)also showed that the Cd/P ratio in phytoplankton increasedat low pCO2 and that a very low Cd/P ratio of less than0.1 mmol/mol was found at 800 ppm of pCO2 off centralCalifornia. These results mean that the uptake of Cd andP by the phytoplankton was modulated by the ambientconditions, such as the Zn concentration and pCO2 inseawater. It may be assumed that detailed study of the

biogeochemical process combined with thephytoplanktonic metabolic system will contribute to abetter understanding of the worldwide function and roleof Cd in the ecosystem. Furthermore, such a study mightlead to the possible use of Cd as an effective tool for con-trolling the growth of phytoplankton (primary produc-tion). Nevertheless, in order to gain a more profound un-derstanding of the Cd biogeochemical cycle in relationto PO4, it would be very important to understand the re-lationship between the two constituents in seawater andbiogenic particles, as well as the interaction betweenseawater and particulate matter in relation to Cd–PO4because the composition of the biogenic particulate mat-ter is largely influenced by that of the surface seawater,where marine organisms (phytoplankton) participate inthe growth process (Boyle et al., 1981; Elderfield andRickaby, 2000). Therefore, an understanding of the rela-tionship between Cd and PO4 and establishing a suitableline (a deviation), especially in the surface seawater inthe plot of these two constituents, would be a first stepon the road to a more complete understanding of theseproblems. In this study, the precise relationship in the Cd–PO4 plot was examined in the subtropical sea near theRyukyu Islands (a relatively oligotrophic zone); a previ-ous report had shown a clear linearity in the relationship

Fig. 1. Sampling sites in this study.

Cd and PO4 in the Subtropical Sea 579

between the two constituents near the study sites (Pai andChen, 1994). Briefly, the areas near the study sites aredescribed as follows. The East China Sea is close to thisstudy site and is one of the typical marginal seas. It cov-ers 7.5 × 105 km2 from the continental shelf region, witha water depth of less than 200 m, to the area of the RyukyuIslands. The residence time of shelf water in this sea wasestimated to be 0.8 years (Tsunogai et al., 1997). The EastChina Sea is geographically separated from the NorthPacific by the Ryukyu Islands, and, with regard to thewater exchange between the two seas, the Kerama Gap(with a maximum water depth of about 1,900 m and awidth of 50 km in a 400 m isobath) has played an impor-tant role as the passage connecting the two seas (Nitani,1972; Morinaga et al., 1998). The low saline intermedi-ate water at 700–730 m (originating from the North Pa-cific Intermediate Water (NPIW)) intrudes into theOkinawa Trough through this strait and joins the KuroshioCurrent, which is a typical warm current that enters theEast China Sea at the Yonaguni Canyon, flows along thecontinental margin, and finally goes into the Pacific Oceanat the Tokara Strait (Guo and Morinaga, 1998). TheOkinawa Trough is located between the edge of the con-tinental shelf in the East China Sea and the Ryukyu Is-lands. It covers more than 600 km, with a maximum depthin excess of 2,000 m in the southern part of the basin.

2. Sampling and MethodsWater samples were taken vertically at three stations

from 0 to 2,000 m or 1,600 m in the subtropical sea nearthe Ryukyu Islands (St. 1: 23°00.0′ N, 126°20.0′ E; St. 2:25°12.4′ N, 123°41.8′ E; and St. 3: 25°40.0′ N, 125°15.1′E; water depths, 5,920 m, 2,120 m, and 1,650 m for Sts.1, 2, and 3, respectively) using acid-cleaned rosette-mounted 1.7 l Niskin bottles aboard the R/V Yoko-Maruof the Seikai National Fisheries Research Institute in May1999 (Fig. 1). Station 1 was located in the western NorthPacific, and Sts. 2 and 3, in the Okinawa Trough area.The water samples for dissolved cadmium analysis weretransferred to acid-cleaned polyethylene bottles and fro-zen immediately until analysis in the laboratory. The dis-solved cadmium in the unfiltered samples was concen-trated by the APDC co-precipitation method of Boyle andEdmond (1977) at pH 2 before the determination by AAS(Atomic Absorption Spectrophotometer). The detailedanalytical procedure for Cd can be found in Abe (2001).Temperature and salinity were measured with a Sea BirdSBE9plus CTD. PO4 was determined colorimetrically,with an analytical error of 1.39% at a concentration of1.33 µM (n = 9). Dissolved oxygen was analyzed on boardby Winkler titration, and the Apparent Oxygen Utiliza-tion (AOU) was calculated by subtracting the measuredoxygen content from the saturation value of dissolvedoxygen.

3. Results and Discussion

3.1 Temperature-salinity and vertical distributions of Cd/PO4All the results from the seawater analyses are listed

in Appendix. The main thermocline developed up to ap-proximately 750 m at the three stations, and Fig. 2 presentsthe T-S diagram. CTD data were collected from the sur-face to a water depth of 2,000 m at Sts. 1 and 2 and 1,600m at St. 3. The surface layer seemed to be influenced bythe mixing of the subtropical mode water (Masuzawa,1969) and the continental-shelf coastal water. Salinitymaxima were observed at approximately 150–250 m, andsalinity minima were observed in the intermediate layer(550–700 m), suggesting that the NPIW influenced thelow salinity (Morinaga et al., 1998). Vertical profiles ofdissolved Cd and PO4 from the surface to 2,000 m (1,600m at St. 3) are shown in Fig. 3. The vertical profiles ofdissolved Cd were typical of nutrient PO4: Cd was de-pleted in the surface water and increased in concentra-tion with depth. This distribution pattern broadly agreeswith a previously reported trend in the open oceans (e.g.,Bruland et al., 1978, 1994; Bruland, 1980; Pai and Chen,1994). The maximum concentrations of Cd were 1.07 nM(1,200 m, St. 1), 0.98 nM (1,700 m, St. 2), and 0.99 nM(1,600 m, St. 3), which are quite similar to those in thewestern equatorial Pacific (Abe, 2001) and lower thanthose in the subarctic Pacific (Abe, submitted). Figure 4illustrates a comparison of the vertical profiles of Cd withthe data from Pai and Chen (1994), in which the studysites were much closer to those of this study. As shown inthis figure, similar profiles were generally obtained ineach group (Sts. 1 and 23 in the Pacific Ocean and Sts. 2

Fig. 2. T-S diagram. A small numeral in this figure denotes thewater depth corresponding to each point.

580 K. Abe

and 4531 in the East China Sea) although some disagree-ment in the plot was noted (e.g., approximately 250–700m between Sts. 2 and 4531).

3.2 Preformed PO4 and CdThe relationship between AOU and PO4 is shown in

Fig. 5. The intercepts of the vertical axis indicate pre-formed concentrations of PO4, and the line in this figurecorresponds to the ratio of PO4/AOU, 1:138, as in Kudo

et al. (1996b). Quite similar preformed PO4 of nearly 0µM above 300 m (Sts. 1 and 3) and 250 m (St. 2) and theaverage values of 1.24 µM, 1.08 µM, and 1.02 µM below1,000 m for Sts. 1, 2, and 3, respectively, are shown, dem-onstrating that the regeneration model introduced by Kudoet al. (1996b) could be adapted to this study area. Kudoet al. (1996b) explained that the defined preformed frac-tions between surface and deep water were shown as aresult of the physical mixing of these water masses and

Fig. 3. Vertical distributions of dissolved Cd and PO4.

Fig. 4. Comparison of the Cd data with Pai and Chen (1994). Sts. 1 and 23 in Pai and Chen and Sts. 2 and 4531 in Pai and Chenare plotted in each panel.

Cd and PO4 in the Subtropical Sea 581

were thus considered to be apparent ones, and a similarpattern was also observed in the equatorial Pacific (Abe,2001); however, slightly higher values of 1.19 µM–1.29µM were calculated as preformed PO4 in deep water inthat area. In this study, in regard to preformed PO4, waterlayers could be separated into two, a surface layer andthe other layers across the water depths (250–300 m). Thesurface layer seemed to consist of the same water massabove these depths at each station.

Similarly, to estimate the regenerated and preformedCd concentrations in the surface layer, the relationshipsbetween Cd and AOU above 300 m at Sts. 1 and 3 and250 m at St. 2 are illustrated in Fig. 6. Judging from theresults of preformed PO4, water masses up to depths of300 m at Sts. 1 and 3 and 250 m at St. 2 were consideredto be one layer (surface). In the calculation of the Cd–AOU relationship, the data above 100 m were excludedbecause there were no PO4 data above 100 m correspond-ing to Cd data. The calculated regression lines were asfollows:

Cd = –0.00918 + (0.00154 × 10–3) × AOU (r = 0.998)··· (St. 1) (1)

Cd = 0.00350 + (0.00124 × 10–3) × AOU (r = 0.943)··· (St. 2) (2)

Cd = 0.0107 + (0.00153 × 10–3) × AOU (r = 0.978)··· (St. 3). (3)

In this study, although a good correlation was obtainedbetween Cd and AOU, the universality of the three slopesdoes not seem to be unambiguous due to the smaller

number of points in Fig. 6 and the lack of accuracy in theCd data set caused by the extremely low concentrations.Therefore, the average value was adopted as the slope ofCd–AOU in this study, and the slope obtained was(0.00143 × 10–3) in the layer from 100 m to 300 m. Thus,the molar ratio of the consumed O2 to the regenerated Cdwas 699,000 at the three study sites. This ratio was about40% smaller than those in the equatorial region,1,150,000–1,220,000 (Abe, 2001), and a much smallervalue of 254,000 was also observed in the subarctic sur-face water (Abe, submitted), which indicates that theremineralization rates of Cd vary from ocean to oceanaccording to the oceanographic conditions, including bio-

Fig. 5. Relationship between AOU and PO4 from surface todeep water. The slopes of the straight lines are 7.2 × 10–3

(µM/µM), which correspond to the regeneration ratio ofAOU for PO4 of 1:138. The y-intercept values of the threeupper lines indicate averaged preformed PO4 in water deeperthan 1,000 m.

Fig. 6. Relationship between AOU and Cd in surface water.The three lines are the regression curves for each station.

Fig. 7. Relationship between AOU and Cd from surface to deepwater. The lower line shows the relationship between Cdand AOU of 0.00143 (nM/µM) obtained in the surface layer.The value of the slope in the three upper lines is the sameas that in the surface water. The y-intercept values indicatethe averaged preformed Cd below 1,000 m at each stationwhen the regeneration ratio of Cd to consumed O2 was as-sumed to be the same as it was in surface water.

582 K. Abe

logical activities. In addition, the Redfield ratio definesthe ratio of the regenerated PO4 to the consumed O2 as1:138 (Redfield et al., 1963), so the average ratio of theregenerated Cd to PO4 has been calculated to be 0.197(nM/µM) in this study. Cd data from the surface to thedeep layer are plotted against AOU in Fig. 7 and pre-formed Cd was estimated using the ratio of 699,000 asthe consumed O2 to the regenerated Cd on the assump-tion that this ratio was constant at all depths. The pre-formed Cd was almost 0 nM in the surface layer at Sts. 1and 2, and slightly higher values were calculated at St. 3(averaging 0.018 nM). For preformed Cd below 1,000 m,the average values were 0.679 nM, 0.610 nM, and 0.625nM at Sts. 1, 2, and 3, respectively, showing quite similarconcentrations, although a slightly larger one was ob-served at St. 1. Abe (2001) reported a preformed Cd con-centration of 0.717 nM below 1,000 m in the westernequatorial Pacific, which seems to be higher than thosefound in this study. In this study, the preformed PO4 andCd concentrations in deep water (averaging below 1,000m) were found to be 1.24 µM, 1.08 µM, and 1.02 µM forPO4 and 0.679 nM, 0.610 nM, and 0.625 nM for Cd atSts. 1, 2, and 3, respectively. A preformed fraction itselfindicates an initial concentration when the water mass isexposed to excess oxygen in the surface layer before itbegins to descend. Although one cannot ascertain wherethese water masses with such high concentrations of PO4and Cd were formed, Frew (1995) reported approximately0.4–0.7 nM of Cd and 1.2–2.1 µM of PO4 above 100 m inthe Princess Elizabeth Trough in the Antarctic area; fur-thermore, he predicted concentrations in the AntarcticBottom Water (AABW) of 0.71 nM and 2.16 µM for Cd

and PO4, respectively. A water mass originating in theAntarctic could possibly be modified by mixture, detritalinput, and biological process when it moves northward.Observed concentrations can be separated into two frac-tions, a biologically regenerated one and a preformed one,which is expected from the plot against AOU. Therefore,the percentage of the regenerated fractions of Cd and PO4was calculated by dividing the difference between theobserved and the preformed values by the observed con-centration. Calculated regenerated fractions of both ele-ments are plotted in Fig. 8. At the shallower depth, a highpercentage of the regenerated fractions was calculated(some over 100%, which are thus fictional values), andthis percentage decreased with depth. Below 500 m, quitesimilar values of approximately 35% (Cd) and 60% (PO4)were obtained. In deep water (below 1,000 m), the aver-age percentages were 33% (St. 1), 36% (St. 2), and 35%(St. 3) for Cd and 58% (St. 1), 61% (St. 2), and 63% (St.3) for PO4. The percentage of Cd was much greater thanthat in the deep water of the equatorial Pacific (approxi-mately 21%) (Abe, 2001) and lower than that in the sub-arctic sea (more than 40%, Abe, submitted). For PO4, re-generated fractions generally agreed with those of theequatorial Pacific (Abe, 2001). As noted above, in rela-tion to the percentage of the regenerated and preformedfractions, slightly different patterns could be found up to500 m among the three stations in this study area; how-ever, below 1,000 m, there seemed to be no great differ-ences in the percentage of the regenerated fraction of thethree stations. As a result, Cd and PO4 are expected tobehave very similarly in the deep water at these studysites.

Fig. 8. Vertical distributions of regenerated Cd and PO4 at the three stations.

Cd and PO4 in the Subtropical Sea 583

3.3 Relationship between Cd and PO4The data obtained were classified into two groups:

surface water and other layers, according to the results ofpreformed PO4 at each station (see section above). In thesurface water, organisms (phytoplankton) fractionate theCd and PO4 (Boyle et al., 1981), and culture experimentshave demonstrated this fractionation (Abe and Matsunaga,1988), which is related to the physiological state of thephytoplankton in the growth process (Kudo et al., 1996a).As a result, the ratio of Cd to PO4 in the phytoplanktonshould be considerably affected by the ratio in ambientseawater, and biogenic particulate matter, such asphytoplankton, should influence the vertical transport andbiogeochemical cycling of these elements. Consequently,obtaining the individual ratio (slope) of the surface waterindependently from that of the other layers is very im-portant. In the surface layer (above 300 m at Sts. 1 and 3and 250 m at St. 2), a considerable number of Cd andPO4 concentrations were observed to be under the detec-tion limit. These values were regarded as nearly 0, andthe regression lines were calculated as zero-intercept lin-ear-regression curves as follows:

Cd = (0.162 × 10–3) × PO4 (r = 0.977) ··· (St. 1) (4)

Cd = (0.156 × 10–3) × PO4 (r = 0.652) ··· (St. 2) (5)

Cd = (0.226 × 10–3) × PO4 (r = 0.777) ··· (St. 3). (6)

The slopes obtained in the Cd–PO4 relationship in thesurface layer were 0.162, 0.156, and 0.226 (nM/µM) atSts. 1, 2, and 3, respectively (all data: 0.183). These val-ues are about 50% of those reported in the North Pacific(Bruland et al., 1978, 1994; Bruland, 1980; Knauer andMartin, 1981) and higher than those reported in the Southor equatorial Pacific surface water (Frew and Hunter,1995; Kudo et al., 1996b; Abe, 2001). The calculated ra-tio of regenerated Cd to PO4 (average of the three sta-tions) was 0.197 (refer to Subsection 3.2), which seemsto be similar to the Cd–PO4 slope in the surface layer.Therefore, considering that the preformed fractions werealmost zero, the regeneration process might be a factorthat mainly regulates the ratio of dissolved Cd and PO4in the surface water. However, the calculated values ofthe slopes scattered widely by more than 20%, and moreprecise analysis will be necessary in order to analyzelower concentrations in the surface water. One of the im-portant factors that relate to the ratios of the two con-stituents in the surface water is the composition of thephytoplankton living in that layer. In the Pacific Ocean,generally, in high-latitude areas such as the subarctic zone,the ratios of Cd/PO4 were high, with diatoms being theyear-long dominant phytoplankton species (Motizuki etal., 2001) with high productivity. On the other hand,Furuya et al. (1996) reported that the sum of the relative

abundance of cocolithophorids and dinoflagellates wasabout 80% near the Ryukyu Islands. The difference inthe phytoplankton species might influence the pattern ofuptake and vertical transportation of the two constituentsand, consequently, their biogeochemical cycling. Cd in-corporated into the CaCO3 shell could not regenerate eas-ily and return to the surface water, which is one possiblereason for the depletion of Cd relative to PO4. It is notclear whether the composition of the living phytoplanktonor the ratio of Cd/PO4 in seawater behave preferentiallyand regulate one another. However, in relation to thebiogeochemical cycle, the supply from the deep layer waspossibly important in regulating the correlation of Cd andPO4 in the surface layer in this study area, and the up-take-remineralization rate of the two constituents is con-sidered to be balanced in scales of time and space. In theother (intermediate and deep) layers, the slope of Cd/PO4changed to 0.371, 0.352, and 0.362 (nM/µM) with nega-tive Cd intercepts at Sts. 1, 2, and 3, respectively. Theregression lines are as follows:

Cd = –0.0577 + (0.371 × 10–3) × PO4 (r = 0.997)··· (St. 1) (7)

Cd = –0.0235 + (0.352 × 10–3) × PO4 (r = 0.994)··· (St. 2) (8)

Cd = –0.0326 + (0.362 × 10–3) × PO4 (r = 0.995)··· (St. 3). (9)

A good correlation was obtained in the three regressionlines, and the slopes of the above three lines seem to besimilar and in general agreement with those found for thePacific, compiled by de Baar et al. (1994). The relation-ships between the two constituents at the three stationsare illustrated in Fig. 9, and Fig. 10 shows magnified fig-ures near the origin. As shown in these figures, separatecalculations (surface and the other layers) lead to twostraight lines and deviations (kinks) in each panel; how-ever, few points were plotted in the surface layers, andthe location of the deviation was much closer to the ori-gin. In Fig. 9, one data set in the North Pacific (St. 23 inPai and Chen (1994)) has been plotted in the panel of St.1 in this study. The regression line is illustrated as a bro-ken line. As shown in this figure, the broken line passesnear the origin and, at first glance, it is very difficult toseparate into two lines. However, in this study, the watermass between the surface and other layers was not actu-ally in the same group, and two lines with a kink of ap-proximately 0.2 µM of PO4 can be plotted, although akink was previously reported to exist where the PO4 wasover 1 µM, which can be explained as follows. The firstreport of a distinct break in the relationship at a PO4 con-centration of approximately 1.5 µM in the plot in thenorthwest Indian Ocean was published by Saager et al.

584 K. Abe

Fig. 9. Relationship between dissolved Cd and PO4. The re-gression lines were calculated for two layers, the surfacelayer and other ones, and two lines are shown in each panel.In the upper panel of St. 1, the data of St. 23 in Pai andChen (1994) are also plotted (broken line). This line showsCd = –0.0308 + (0.380 × 10–3) × PO4 (r = 0.993).

Fig. 10. Relationship between dissolved Cd and PO4 in sur-face water. Broken lines are the regression lines for the otherlayers (intermediate and deep water). Dotted areas showareas under the detection limit of the two elements; a de-viation existed at PO4 of approximately 0.2 µM.

(1992), who reported a discontinuity between surface anddeep waters, their Cd/PO4 slopes being 0.15–0.16 (nM/µM) and 0.5–0.87 (nM/µM) in the surface (above theapproximate depth of the thermocline, 100–150 m) anddeep layers, respectively. The low slope of the surfacewater was attributable to the extremely preferential up-take of Cd rather than PO4. In the western equatorial Pa-cific, a clear deviation (kink) in the Cd–PO4 plot was alsoobserved across the thermocline (approximately 150 m),where the PO4 concentration was approximately 1.1 µM(Abe, 2001). Nevertheless, in these two reports the exist-

ence of the thermocline seemed to be one of the factorsthat physically separated the surface and other layers,leading to the occurrence of a break (kink) in the plot ofthe two components. This study found no clearly recog-nizable thermocline between the surface and other lay-ers; however, concerning preformed PO4, water massesshould be classified into two groups, and different slopesin the Cd–PO4 plot were observed across the two layers.One possible factor that caused the discontinuity in theCd–PO4 plot in this study is the influence of the NorthPacific subtropical mode water (NPSTMW). This water,which lies between the seasonal and main thermoclines

Cd and PO4 in the Subtropical Sea 585

in the subtropical North Pacific, has a characteristic tem-perature of 16.5°C and a salinity of 34.75 (Masuzawa,1969). The system by which this mode water is formed isconsidered to relate to the mixed layer in winter (Bingham,1992; Hosoda et al., 2001), so the distribution of the modewater could change from year to year (Taneda et al., 2000;Hanawa and Yoritaka, 2001). Furthermore, a large amountof mode water is carried westward by the westward sub-tropical current, and the influence of this mode waterextends to the spread area. Although the role of the modewater in the formation of the discontinuity in the Cd–PO4plot is not clear, it is possible that the kink point in theplot moves from year to year because the concentrationof Cd and PO4 above the mode water should be fixed ac-cording to the mixing depth in the formation process ofthe mode water. If the degree of the penetration of themode water and the depth were less than those in thisstudy (the study sites located at the western border of themode water shown in Masuzawa (1969)), the kink pointwould approach the origin and would be more difficult todiscover. In this study, the characteristics of the modewater defined in Masuzawa (1969) were observed atdepths of 371 m, 273 m, and 311 m at Sts. 1, 2, and 3,respectively, and the surface water masses defined in thisstudy (above 300 m, 250 m, and 300 m at Sts. 1, 2, and 3,respectively) were just above the depths of the modewater, which suggests that the existence of the NPSTMWpossibly separates the surface water from the other lay-ers, leading to the discontinuity in the plot of Cd–PO4 inthis study area.

Pai and Chen (1994) showed a clear linearity with ahigh correlation coefficient, the fluctuations of Cd–PO4slopes and the intercepts lying in the range of 0.352 to0.374 (nM/µM) and –0.02 to 0.01 (nM) at stations nearthese study sites. The regression curve of all the stationsin their report is:

Cd = –0.00 + (0.366 × 10–3) × PO4 (r = 0.992). (10)

An attempt was made to draw a regression curve usingall the data in this study, and the result is as follows:

Cd = –0.0370 + (0.361 × 10–3) × PO4 (r = 0.997). (11)

A first glance at this curve (Eq. (11)), which is quite simi-lar to that of Pai and Chen (1994) (Eq. (10)) except forthe calculation of a slightly larger negative intercept, sug-gested that the separation of the two layers does not seemto be important. The average slope and intercept of Eqs.(7), (8), and (9) are 0.362 and –0.0379, respectively, andthere are no great differences between these values andthose in the whole regression curve (0.361 and –0.0370for the slope and the intercept, respectively (Eq. (11)).This result implies that the data in a range below approxi-mately 0.2 µM of PO4 (surface water) contribute insig-

nificantly to the slope and intercept of the whole regres-sion curve. This is assumed to occur because the breakpoint in the Cd–PO4 plot exists close to the origin. In thisstudy, the PO4 concentration to which the Redfield ratiocould be adaptable in the PO4–AOU plot extends to ap-proximately 0.3 µM, which corresponds to a water depthof 250–300 m (surface layer). When Cd and PO4 concen-trations are low enough in the defined surface layer, theplotted points in this layer could cluster near the origin.Consequently, a break point approaches the origin, and awhole plot up to the deep water could seem, at first glance,to fall in one straight line. However, even though the breakpoint in the Cd–PO4 plot exists near the origin and seemsto be insignificant in the whole plot from the surface tothe deep layer, the existence or not of a break point shouldbe examined. An approach based on particulate matterand settling particles might provide a considerable amountof important information for solving this problem. In con-clusion, in the relationship between Cd and PO4, a devia-tion (kink) across the surface water and the other layerswas actually observed at a PO4 concentration of approxi-mately 0.2 µM in the plot of the two constituents in thisstudy area, and separate analyses for these two layersshould be considered in order to determine the Cd–PO4relationship in seawater.

4. ConclusionThe objective of this study was to examine the pre-

cise relationship between Cd and PO4, especially in thesurface layer in the subtropical area near the Ryukyu Is-lands (sampling sites were located in the East China Seaand North Pacific Ocean). Among the results, a clear de-viation (kink) was observed at a PO4 concentration ofapproximately 0.2 µM in the Cd–PO4 plot, which sug-gests that two lines should be arranged to indicate a rela-tionship between the two constituents; however, a clearstraight line was drawn near the study sites in a previousreport. The slopes obtained when plotting the surfacewater data at these study sites were about 50% less thanthose previously reported in the North Pacific and higherthan those in the South or equatorial Pacific surface wa-ter. The results of this study suggest that an examinationof the existence of different concentration ratios (slopes)of Cd to PO4 between the surface and other layers in someareas will provide a better understanding of the relation-ship between the two constituents in the ocean, eventhough the break point is found quite close to the origin.

AcknowledgementsI gratefully acknowledge the officers and crew of the

R/V Yoko-Maru of the Seikai National Fisheries ResearchInstitute for their help with the sampling. I also grate-fully acknowledge the assistance of three anonymous re-viewers who helped me improve the manuscript. Part ofthis study was supported by the program for the “Global

586 K. Abe

carbon cycle and related mapping based on satellite im-agery” (GCMAPS), which is funded by the Ministry ofEducation, Science and Technology of Japan.

Appendix(see pp. 587–588)

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Cd and PO4 in the Subtropical Sea 587

Appendix. Seawater analysis results.

Depth Temp. Salinity Density Oxygen PO4 Cd AOU(m) (°C) (ml/l) (µM) (nM) (µM)

St. 10 26.08 34.57 22.67 4.89 nd nd –11.2

10 26.07 34.58 22.70 4.84 nd nd –8.520 25.99 34.63 22.75 4.85 nd nd –8.550 25.39 34.83 23.09 4.69 — nd 0

100 23.56 34.75 23.58 4.93 0.058 nd –4.9150 22.28 34.80 23.99 4.61 0.064 0.014 14.7200 21.00 34.85 24.38 4.60 0.13 — 19.6250 19.06 34.87 24.90 4.67 0.18 0.027 24.1300 17.90 34.82 25.16 4.57 0.26 0.043 33.5350 17.38 34.78 25.34 4.67 0.38 0.061 33.9400 15.83 34.69 25.55 4.74 0.47 0.082 35.7450 14.37 34.56 25.77 4.51 0.74 0.207 56.7500 13.08 34.47 25.96 4.43 0.89 0.329 67.4550 11.40 34.34 26.19 4.07 1.32 0.431 92.9600 9.71 34.26 26.42 3.53 1.75 0.635 127.7700 7.33 34.22 26.76 2.74 2.48 0.854 178.6850 6.03 34.25 27.06 1.80 2.83 1.043 236.2

1000 4.21 34.37 27.27 1.81 3.04 1.032 243.31200 3.45 34.47 27.42 1.87 3.03 1.070 246.41400 2.83 34.52 27.52 2.17 2.99 1.046 237.91700 2.37 34.58 27.60 2.57 2.79 0.972 224.12000 2.04 34.62 27.66 2.92 2.76 0.940 211.2

St. 20 27.70 34.78 22.46 4.73 nd nd –8.0

10 27.23 34.78 22.48 4.79 nd — –10.320 26.09 34.74 22.87 4.94 nd nd –12.950 24.96 34.75 23.16 4.92 nd — –8.0

100 22.93 34.85 23.84 4.61 0.058 0.020 12.9150 20.31 34.90 24.61 4.53 0.19 0.032 27.2200 19.17 34.85 24.86 4.62 0.25 0.042 28.1250 18.18 34.83 25.10 4.63 0.32 0.044 32.1300 16.16 34.71 25.49 4.54 0.55 0.121 46.0350 14.76 34.62 25.74 4.32 0.86 0.261 62.9400 13.42 34.50 25.92 4.15 0.96 0.291 77.7450 11.92 34.41 26.14 3.92 1.01 0.401 96.4500 11.31 34.36 26.22 3.84 1.34 0.493 104.0550 9.80 34.31 26.45 3.48 1.71 0.561 129.0600 8.69 34.31 26.63 3.07 1.96 0.673 154.5700 6.83 34.33 26.92 2.51 2.50 0.901 192.4850 5.09 34.40 27.18 2.14 2.73 0.933 221.4

1000 4.37 34.42 27.28 2.03 2.78 0.944 231.71200 4.04 34.43 27.33 2.00 2.77 0.902 235.71400 3.86 34.45 27.36 2.00 2.87 0.973 237.11700 3.76 34.45 27.38 2.00 2.81 0.975 237.92000 3.75 34.46 27.38 1.99 2.72 0.946 238.4

588 K. Abe

*Less than 0.05 µM of PO4 and 0.010 nM of Cd (3.85 times of the deviation of the blank) are shown as “nd”.

Appendix. (continued).

Depth Temp. Salinity Density Oxygen PO4 Cd AOU(m) (°C) (ml/l) (µM) (nM) (µM)

St. 30 26.07 34.06 22.31 4.93 nd nd –12.5

10 26.08 34.68 22.77 4.88 nd nd –10.720 25.99 34.68 22.79 4.89 nd 0.013 –11.250 25.22 34.72 23.07 4.91 nd 0.017 –8.0

100 23.70 34.78 23.56 4.82 nd 0.015 0.89150 22.01 34.86 24.08 4.53 0.17 0.034 20.1200 19.54 34.85 24.77 4.52 0.31 0.063 32.1250 17.70 34.83 25.21 4.61 0.23 0.065 34.8300 16.65 34.77 25.42 4.62 0.37 — 40.6350 15.35 34.68 25.63 4.43 0.58 0.155 56.7400 14.07 34.56 25.83 4.23 0.91 0.274 71.0450 12.39 34.45 26.09 3.72 1.34 0.465 102.7500 10.37 34.31 26.35 3.67 1.35 0.504 117.0550 9.06 34.30 26.56 3.24 1.49 — 144.6600 8.09 34.36 26.76 2.97 1.98 0.699 162.9700 6.67 34.35 26.97 2.57 2.32 0.815 191.5800 5.31 34.39 27.15 2.18 2.48 0.831 217.9900 4.78 34.40 27.23 2.06 2.66 0.878 227.2

1000 4.45 34.42 27.28 1.98 2.68 0.926 233.51200 4.13 34.43 27.32 1.99 2.70 0.965 235.71400 3.87 34.45 27.36 1.98 2.72 0.969 237.91600 3.82 34.45 27.37 2.07 2.78 0.986 234.4