The distribution of Fe in the Australian sector of the Southern Ocean

Deep-Sea Research I 47 (2000) 55}84

The distribution of Fe in the Australian sector of the Southern Ocean Y. Sohrin!,*, S. Iwamoto", M. Matsui", H. Obata#, E. Nakayama#, K. Suzuki$, N. Handa$, M. Ishii% !Faculty of Technology, Kanazawa University, 2-40-20 Kodatsuno, Kanazawa 920-8667, Japan "Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan #Department of Ecosystem Studies, School of Environmental Science, University of Shiga Prefecture, Hikone, Shiga 522-0057, Japan $Institute for Hydrospheric-Atmospheric Sciences, Nagoya University, Chikusa-ku, Nagoya 464-8601, Japan %Geochemical Research Department, Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan Received 4 May 1998; received in revised form 22 December 1998; accepted 10 March 1999

Abstract The vertical pro"les of labile Fe in seawater in the Australian sector of the Southern Ocean (50}653S along 1403E) were obtained in the austral spring 1994/1995 by concentration with 8-hydroxyquinoline immobilized #uoride containing metal alkoxide glass (MAF-8HQ) followed by determination with chemiluminescence on board. While the concentrations of Fe were low in the surface water column (0.14$0.12 nM, n"97 for 0}100 m) and increased in deep water at all stations, the pro"les varied depending on the latitude. The Fe concentrations in intermediate and deep water were 0.30$0.12 (n"36 for 500}2000 m) between the Antarctic Front (AF) and Antarctic Divergence (AD), and steeply increased south of the AD. The high Fe region ((1.5 nM) extended northwards along the continental slope and coincided with the Antarctic Bottom Water. Another Fe maximum in deep water (&1.2 nM) was located on the north side of the Paci"c Antarctic Ridge. The Fe concentrations showed a minimum (&0.3 nM) at 3000 m depth at 623S, which was almost coincident with the maximum of SiO 2 (&125 lM) in the Lower Circumpolar Deep Water. There were some patchy Fe maximums in shallow water between the AF and AD, which did not coincide with the maximums of PO and 4 NO in the Upper Circumpolar Deep Water. These results indicate that the distribution of Fe is 3 nutrient-like but strongly in#uenced by local sources and water circulation. The Fe : PO ratios in the surface water column ((3]10~4) were lower than the critical 4 value for a phytoplankton bloom (1]10~3) proposed by de Baar et al. (1990, Marine Ecology Progress Series 65, 105}122). Although the concentrations of Fe did not correlate with those of

* Corresponding author. Tel.: #81-76-234-4792; fax: #81-76-234-4800. E-mail address: [email protected] (Y. Sohrin) 0967-0637/99/$ - see front matter ( 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 3 7 ( 9 9 ) 0 0 0 4 9 - 7

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Y. Sohrin et al. / Deep-Sea Research I 47 (2000) 55}84

chlorophyll a, they positively correlated with the indices of primary productivity (growth rate, productivity by simulated in situ 13C method and monthly integrated net community production). It seems that phytoplankton su!ered Fe stress and that Fe was a limiting factor of the primary production. However, it is likely that there were some co-limiting factors, such as grazing at the ice edge and depletion of SiO and critical-depth/mixed-depth relationship in the Permanently 2 Open Ocean Zone and Polar Front Zone. ( 1999 Elsevier Science Ltd. All rights reserved. Keywords: Iron; The Southern Ocean; Seawater; Phytoplankton; Primary productivity; Net community production

1. Introduction Iron is the fourth most abundant element in the continental crust (Taylor, 1964) and is extensively used by modern technology. On the other hand, the Fe concentration in seawater is only at the nM level (Bruland et al., 1994; Gordon et al., 1982,1997; Landing and Bruland, 1987; Martin and Gordon, 1988; Martin et al., 1989,1993; Saager et al., 1989; Wu and Luther III, 1994,1996). Since highly elaborate techniques are necessary to avoid contamination during sampling and analysis and to detect the traces of Fe, data on the oceanic distribution of Fe are still limited. Recently, the Moss Landing Marine Laboratory group, which is leading the study of the marine chemistry of Fe, has compiled its data set (Johnson et al., 1997a). The data set consists of 30 open ocean concentration pro"les, which are mostly from the eastern Paci"c and include several pro"les in the North Atlantic and Southern Oceans (the Drake Passage and the Ross Sea). The dissolved Fe pro"les have a uniform shape with a nutrient-like pro"le at each station. There is no inter-ocean fractionation as there is for nitrate (NO ), phosphate (PO ) and silicate (SiO ). Nutrient elements generally 3 4 2 have oceanic residence times on the order of 103(q(105 yr (Whit"eld and Turner, 1987). The residence time of Fe is estimated to be on the order of 100 yr (Bruland et al., 1994; Landing and Bruland, 1987), which is much shorter than the oceanic stirring time (103 yr). These observations have led Johnson et al. (1997a) to the following model: Iron is taken up by organisms in surface water followed by remineralization from sinking organic matter. In deep water, although scavenging removes Fe, the concentration is maintained at &0.6 nM by apparent solubility. The chemistry of Fe is very complex, and its dissolved form and solubility have not been ascertained. Thermodynamically, Fe should be trivalent in oxygenated seawater, although transient Fe(II) species may be produced by photochemical reduction in surface water (Johnson et al., 1994; Miller et al., 1995). Calculations suggest that the Fe(III) species are Fe(OH)` and Fe(OH) (Turner et al., 1981). However, reported 2 3 values of solubility for Fe(OH) vary widely from 0.15 to 10 nmol kg~1 (Byrne and 3 Kester, 1976; Kuma et al., 1992; Millero et al., 1995; Motekaitis and Martell, 1987). Discrimination between dissolved and particulate Fe is operationally based on "ltration with 0.2}0.45 lm pore size "ters, but its physicochemical validity is uncertain. Colloidal species may contribute to both fractions. Labile Fe, de"ned by complexation with 8-hydroxyquinoline, was found to be distributed not only in the

Y. Sohrin et al. / Deep-Sea Research I 47 (2000) 55}84

57

dissolved fraction but also in the particulate fraction (Wells and Mayer, 1991). Conversely, some of the dissolved Fe was non-labile, and therefore probably not available to phytoplankton. Judging from the results of cathodic stripping voltammetry, more than 99% of dissolved Fe may be organically complexed (van den Berg, 1995; Gledhill and van den Berg, 1994; Rue and Bruland, 1995; Wu and Luther III, 1995). Kuma et al. (1996) and Johnson et al. (1997a,b) have suggested that natural organic ligands control the dissolved concentration of Fe in seawater. The distribution of Fe is especially intriguing in high-nutrient low-chlorophyll (HNLC) regions, such as the Southern Ocean. It has been proposed that the HNLC condition is caused by the de"ciency of Fe (Martin, 1992; Martin and Fitzwater, 1988; Martin et al., 1990a). Iron is essential in the electron-transport chains in photosynthesis and respiration, the reduction of nitrate and nitrite, the nitrogen "xation (Lippard and Berg, 1994; Raven, 1988) and the synthesis route of chlorophyll (Chereskin and Castelfranco, 1982). Several Fe enrichment experiments have shown increased growth of phytoplankton in subarctic, equatorial Paci"c and Antarctic HNLC waters (for example of the Antarctic: Helbling et al., 1991; van Leeuwe et al., 1997; Scharek et al., 1997; Takeda, 1998). Aeolian dust of continental weathering products is probably a dominant source of Fe for the pelagic photic zone (Duce and Tindale, 1991; Moore et al., 1984). The aeolian Fe #ux is low in the HNLC regions. Recent studies in the Southern and equatorial Paci"c Oceans have shown that Fe supply through oceanic jets and upwelling can cause local pulses of enhanced primary productivity (de Baar et al., 1995; Coale et al., 1996; Landry et al., 1997; Smetacek et al., 1997). However, data on Fe distribution in the Southern Ocean are almost entirely limited to the Atlantic sector, Weddell Sea and Ross Sea ( de Baar et al., 1995; Johnson et al., 1997a; LoK scher et al., 1997; Martin et al., 1990b; Nolting et al., 1991; Westerlund and OG hman, 1991), and reports are scarce in the vast Indian and Paci"c sectors (Nolting et al., 1998; Sarthou et al., 1997; Sedwick et al., 1997). Here we report the distribution of labile Fe in the Australian sector observed during the austral spring 1994/1995. Emphasis is placed on the sectional distribution of Fe along &1403E that extends from 503S to 653S throughout the water column and the relationship between Fe and indices of biological production particularly around the Antarctic Divergence (AD).

2. Hydrography During our cruise, the Subantarctic Front (SAF), the Antarctic Front (AF) and the AD were located at about 503S, 543S and 64.43S, respectively (Fig. 1). The Seasonal Ice Zone (SIZ) extended to &623S. The ice edge was observed at 64.83S on December 19, 1994, and retreated to 65.53S on January 21, 1995. In December, there were two divergences, at 64.43S and 64.93S, characterized by fronts of temperature and salinity at &100 m depth. Near these divergences, surface currents were complex, and hydrographic properties changed abruptly in a short distance. In January, while the northern divergence was observed at nearly the same position, the southern divergence had moved to 65.33S.

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Y. Sohrin et al. / Deep-Sea Research I 47 (2000) 55}84

Fig. 1. Sampling stations on the KH-94-4 cruise of R.V. Hakuho Maru: h, stations during Leg 2 (December 13, 1994}January 4, 1995); L, stations during Leg 3 (January 9}28, 1995). SAF, Subantarctic Front; AF, Antarctic Front; AD, Antarctic Divergence; PAR, Paci"c Antarctic Ridge; SIZ, Seasonal Ice Zone.

The ¹}S diagrams for some stations are shown in Fig. 2. Cold subsurface water ((!1.73C; S"34.1}34.4) exists south of 643S at 30}200 m depth. This is a remainder of winter mixed layer water (Gordon and Huber, 1990). This water spreads northward, mixing with surface water and the Circumpolar Deep Water (CDW), sinks at the AF and forms the Antarctic Intermediate Water (AAIW) (Sievers and Nowlin, 1984; Whitworth and Nowlin, 1987). At Stn. 25, the AAIW reaches to about 1000 m depth. The Antarctic Bottom Water (AABW), which is characterized by low temperature ((!0.63C), low salinity (&34.6) and high oxygen ('5.7 ml l~1), is observed along the continental slope of Antarctica. From the results of the deployment of current moorings, it has been suggested that the bottom water originates from high density water, which is formed at a polynya over the eastern continental shelf and sinks while #owing westwards (Wakatsuchi, personal communication). The CDW

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59

Fig. 2. T}S diagrams: h, Stn. 11; L, Stn. 22; £, Stn. 32; n, Stn. 25.

overlying the AABW is formed by mixing with the North Atlantic Deep Water (NADW), AAIW and AABW. The Upper Circumpolar Deep Water (UCDW) is characterized by a temperature maximum and oxygen minimum, and the Lower Circumpolar Deep Water (LCDW) is characterized by a salinity maximum. The lower part of the LCDW contains high concentrations of SiO (Edmond et al., 1979). The 2 CDW circulates eastwards, forced by the Antarctic Circumpolar Current (ACC), and wells up at the AD.

3. Methods 3.1. Determination of Fe Samples were collected during Leg 2 (December 13, 1994}January 4, 1995) and Leg 3 (January 9}28, 1995) on the KH-94-4 cruise of the R.V. Hakuho Maru. Our sampling stations were located nearly along the 1403E meridian as shown in Fig. 1. Much ship-time was spent for observation around the AD. On Leg 2, Stns. 13 and 11 were located north of and within the AD, respectively, and Stn. 14 was located at the ice edge. On Leg 3, Stn. 40 was located north of the AD, and Stns. 43 and 45 were located at the ice edge. Seawater samples for the determination of Fe were taken with a CTD carousel suspended on titanium armored cable. Two kinds of Niskin samplers were mounted on the carousel. One was a conventional type, in which an inner spring was replaced by a silicon tube, and the other was a lever action type. The interior of the

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Y. Sohrin et al. / Deep-Sea Research I 47 (2000) 55}84

samplers was coated with Te#on and precleaned with detergent and 0.3 M HCl and rinsed with water deionized using a Milli-Q system. Surface water samples were collected with a pump system, which was constructed of Tygon tubing and a bellows pump. The #ow line of the pump system was free from metallic parts and precleaned in the way that the Niskin samplers were. Seawater was transferred from the samplers to low-density polyethylene bottles on deck. A silicon tube and bell (Nalgene) was used to avoid contamination with airborne particles. Immediately, the samples were brought into a clean room laboratory (class 100) on the vessel and acidi"ed to pH 3.2 with formic acid-ammonium formate bu!er. The concentration of Fe was determined with an automated shipboard analytical system developed by Obata et al. (1993). The sample was passed through a column of 8-hydroxyquinoline immobilized #uoride containing metal alkoxide glass (MAF8HQ). Collected Fe was eluted with 0.3 M HCl, and the eluent was mixed with luminol, NH OH and H O solutions. The intensity of chemiluminescence was 4 2 2 measured in a #ow-through cell, and the Fe concentration was calculated using a calibration curve based on the peak height. All apparatus used were precleaned as stated previously (Obata et al., 1993). The standard solutions for calibration were prepared using surface seawater collected from the equatorial Paci"c, in which Fe had been thoroughly exhausted by biological activity. The calibration curve was parabolic with a small intercept. The calibration was conducted when the reagent solutions were renewed, and the blank and standard solutions were introduced after every &10 samples to monitor the change in calibration. The accuracy of our method was ascertained by analysis of reference seawater supplied by the National Research Council of Canada (Obata et al., 1993). From repeated determinations of seawater samples, the average precision (1 SD) in nM unit was 0.05 for [Fe])0.1 (n"24), 0.04 for 0.1([Fe])0.5 (n"77), and 0.08 for 0.5([Fe] (n"18). Therefore, 0.15 nM (3 SD) was employed as the lower limit of determination. For the analysis of samples collected using plural samplers at the same time and position, the variations were within the range of precision. Therefore, it seems that there were no di!erences in contamination among the Niskin samplers. All data for Fe as well as hydrographic properties are listed in Table 1. Because of improper usage of the pump system, four surface water samples may have been contaminated. Three deep water samples out of 267 collected with Niskin samplers may have been contaminated. These data are marked in Table 1 and excluded from the discussion below. The concentrations in not detected samples were assumed to be 0.05 nM in data processing. 3.2. Indices of biological productivity Each chlorophyll a (Chl a) sample was collected on a Whatman GF/F "lter, extracted into dimethylformamide and determined by #uorometry on board. For pigment analysis using high-performance liquid chromatography (HPLC), surface seawater (5 l) was "ltered through a GF/F "lter, which was immediately transferred into an amber vial and stored under nitrogen at !853C until analysis. The sample was sonicated in cold 90% acetone/water (v/v) under subdued light. Pigments were

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Table 1 Data on hydrography, nutrients, Chl a, and labile Fe PO 4 (lM)

Chl a (lg l~1)

Stn. 11 (64340@S, 140301@E; Depth 2943 m; 19-12-1994 13:00}15:00) B 0 33.598 !0.900 8.06 49.0 34.9 N 9 33.709 !0.993 7.97 51.5 35.4 LA 18 33.847 !1.476 6.80 52.5 35.7 N 29 33.933 !1.580 6.21 54.3 36.5 LA 47 34.145 !1.668 5.26 60.0 38.0 LA 70 34.218 !1.507 4.72 65.0 39.1 LA 96 34.290 !1.221 4.76 70.0 39.6 N 121 34.330 !1.752 4.79 71.7 38.7 LA 146 34.395 !1.158 4.82 78.7 39.4 N 169 34.410 !1.459 4.93 80.8 37.9 LA 197 34.487 !0.589 5.03 88.5 39.9 LA 294 34.609 0.427 5.22 99.4 40.4 LA 498 34.709 1.106 5.34 110.6 40.6 N 738 34.708 0.927 4.40 120.3 41.0 LA 985 34.706 0.755 127.2 41.3 N 1232 34.697 0.542 132.0 41.7 LA 1479 34.690 0.353 134.1 41.7 N 1724 34.674 0.105 133.3 41.7 N 1968 34.676 !0.071 131.5 41.7 N 2214 34.674 !0.201 130.4 41.2 N 2463 34.675 !0.311 129.4 41.1 LA 2707 34.669 !0.465 129.3 41.0 N 2860 34.653 !0.622 118.3 40.9

2.31 2.33 2.40 2.47 2.60 2.61 2.65 2.62 2.67 2.62 2.69 2.76 2.71 2.75 2.74 2.80 2.88 2.83 2.79 2.79 2.79 2.74 2.73

0.13 0.15 0.19 0.23 0.70 0.60 0.20 0.07 0.03 0.01 0.01 0.01

Stn. 11 BCTD-1 (64340@S, 139357@E; Depth 2982 m; 20-12-1994 23:06-23:38) B, P 0 0.000 8.05 47.4 33.4 LA 11 33.610 !1.083 8.03 47.6 33.5 LA 19 33.645 !1.212 7.98 48.2 33.7 LA 29 33.779 !1.498 7.87 49.4 34.2 LA 50 34.087 !1.723 7.10 58.5 36.5 LA 73 34.178 !1.677 61.7 37.6 LA 98 34.279 !1.313 6.59 66.4 38.0 LA 123 34.392 !0.719 6.16 72.9 38.4 LA 147 34.461 !0.332 6.04 79.2 38.6 LA 171 34.510 !0.052 5.65 87.2 38.7 LA 196 34.558 0.217 5.40 91.1 40.6 LA 498 34.709 1.128 4.77 108.7 38.6

2.34 2.29 2.29 2.23 2.39 2.38 2.50 2.55 2.57 2.59 2.57 2.57

0.12 0.15 0.14 0.19 0.44 0.34 0.12 0.06 0.03 0.01 0.01

0.48" ND ND ND ND ND ND 0.18 ND 0.22 0.28 0.30

Stn. 13 BCTD-2 (64320@S, 139359@E; Depth 3455 m; 23-12-1994 13:05}14:03) B, P 0 !0.100 7.98 37.9 27.8 LA 10 33.562 !0.199 8.03 38.5 28.6 LA 20 33.709 !0.949 7.97 39.3 28.4 LA 30 33.887 !1.216 7.37 45.6 29.9 LA 50 34.207 !0.614 6.61 53.5 31.8

1.92 1.98 2.01 2.12 2.28

0.05 0.06 0.08 0.14 0.44

0.26 ND ND 0.27 ND

Sampler

Depth (m)

Salinity

Pot. T (3C)

Oxygen (ml l~1)

SiO 2 (lM)

NO 3 (lM)

Fe (nM)

ND! ND 0.25 0.23 ND ND 0.39 0.29 0.48 0.48 0.40 0.39 0.45 0.47 0.45 0.41 0.49 0.45 0.50 0.44 0.89 1.43

(continued on next page)

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Table 1 (continued) Sampler

LA LA LA LA LA LA LA

Depth (m)

Salinity

74 99 124 148 173 198 495

34.430 34.573 34.598 34.622 34.636 34.655 34.709

Pot. T (3C) 0.742 1.585 1.714 1.782 1.821 1.909 1.703

Oxygen (ml l~1)

SiO 2 (lM)

NO 3 (lM)

PO 4 (lM)

Chl a (lg l~1)

Fe (nM)

4.48 4.26 4.16 4.44 4.28 4.24 4.46

70.7 74.2 76.0 77.0 78.2 77.9 83.7

35.2 35.3 35.4 35.1 34.9 34.3 32.6

2.48 2.57 2.55 2.52 2.49 2.48 2.34

0.47 0.09 0.04 0.03 0.01 0.01

ND ND 0.27 ND 0.16 ND 0.28

1.91 1.92

0.05 0.06

1.97 2.15

0.09 0.10

2.34

0.23

2.54 2.55 2.54 2.52

0.23 0.09 0.06 0.02

2.50

0.01

2.47

0.01

0.18 ND ND ND ND ND ND ND ND ND ND 0.48 0.17 0.16 0.15 0.22 ND 0.17 0.18 0.15 0.18 0.19 0.22 0.15

Stn. 13 BCTD-3 (64321@S, 139358@E; Depth 3432 m; 24-12-1994 00:02}01:05) B, P 0 0.400 7.98 39.7 28.1 N 9 33.597 !0.336 8.00 40.2 28.5 LA 10 33.594 !0.308 N 10 33.593 !0.361 LA 20 33.608 !0.626 7.83 41.4 29.0 LA 30 34.002 !1.174 7.20 49.9 30.6 N 50 34.364 !0.156 LA 50 34.353 !0.090 5.85 63.3 33.2 N 74 34.559 1.366 LA 75 34.557 1.352 4.42 74.5 35.7 LA 101 34.612 1.697 4.15 78.0 35.6 LA 123 34.631 1.758 4.12 78.3 35.7 LA 149 34.650 1.812 4.10 80.2 35.0 N 149 34.650 1.812 LA 172 34.662 1.824 4.17 80.8 35.0 N 173 34.662 1.824 LA 198 34.673 1.813 LA 200 34.673 1.806 4.24 82.1 34.9 N 200 34.673 1.804 N 200 34.673 1.803 N 502 34.729 1.664 N 502 34.729 1.664 LA 503 34.729 1.663 4.45 87.4 33.0 N 503 34.729 1.665 Stn. 13 BCTD-4 (64322@S, 140300@E; Depth 3421 m; 24-12-1994 23:03}01:32) B, P 0 33.612 !0.100 7.97 38.2 28.1 N 8 33.613 !0.234 7.94 38.0 28.2 LA 18 33.868 !0.529 7.90 39.4 28.6 N 30 33.999 !1.374 7.84 40.1 29.0 LA 49 34.120 !1.581 7.35 46.1 30.3 LA 73 34.391 0.300 5.56 61.7 33.7 LA 98 34.500 0.911 4.81 69.2 34.6 N 123 34.594 1.594 4.30 74.4 35.2 LA 147 34.649 1.531 4.33 76.1 35.3 N 171 34.650 1.689 4.28 76.5 35.3 LA 196 34.674 1.807 4.23 77.2 34.4 LA 296 34.694 1.642 4.36 78.9 33.9 LA 494 34.720 1.521 4.49 83.0 32.9

2.33

1.98 1.94 2.00 2.03 2.18 2.45 2.53 2.56 2.53 2.57 2.47 2.20 2.40

0.06 0.05 0.10 0.12 0.19 0.56 0.10 0.04 0.02 0.01 0.01 0.01

2.91" ND ND 0.25 ND ND 0.21 0.17 0.15 ND ND ND 0.18

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Sampler

Depth (m)

Salinity

Pot. T (3C)

Oxygen (ml l~1)

SiO 2 (lM)

NO 3 (lM)

PO 4 (lM)

N LA N N N N N LA N N N

739 987 1233 1725 1970 2214 2459 2705 2949 3192 3353

34.739 34.728 34.718 34.693 34.689 34.682 34.681 34.673 34.674 34.671 34.660

1.382 1.120 0.942 0.528 0.295 0.127 !0.016 !0.174 !0.289 !0.409 !0.499

4.65 4.44 4.74 4.99 4.95 5.07 5.13 5.40 5.39 5.34 5.64

89.3 97.1 102.6 110.5 114.5 115.3 116.2 109.1 112.2 107.4 98.6

32.8 32.9 33.2 34.3 33.8 34.2 34.4 33.9 34.2 34.0 33.7

2.37 2.36 2.37 2.41 2.47 2.45 2.45 2.41 2.43 2.42 2.36

Stn. 14 (65306@S, 140300@E; Depth 2947 m; 26-12-1994 06:55}07:38) B, P 0 !0.900 7.84 44.4 29.1 LA 9 33.739 !1.246 7.83 44.6 28.4 LA 19 34.042 !1.665 7.36 48.8 29.5 LA 29 34.235 !1.767 7.23 51.4 30.4 LA 49 34.274 !1.788 7.23 54.0 30.5 LA 73 34.305 !1.811 7.16 55.3 30.6 LA 99 34.328 !1.803 7.19 56.9 32.1 LA 172 34.389 !1.594 7.01 62.6 32.9 LA 197 34.417 !1.443 6.76 65.7 32.7 N 246 34.463 !1.016 N 297 34.516 !0.487 6.02 73.4 32.2 LA 498 34.696 0.899 4.84 92.0 32.6 Stn. 20 (64301@S, 139357@E; Depth 3690 m; 27-12-1994 06:45}10:29) B, N 0 33.961 1.100 8.06 25.0 25.3 N 8 33.850 0.782 8.16 23.1 25.1 LA 18 33.925 0.335 8.19 21.4 25.4 N 28 33.962 !0.201 8.21 22.6 26.6 LA 49 34.231 !0.260 6.17 51.2 32.7 LA 73 34.503 1.430 4.28 66.1 36.0 LA 99 34.547 1.759 4.13 70.0 36.3 N 122 34.609 1.870 4.03 71.9 35.7 LA 147 34.628 1.931 4.07 73.4 35.3 N 174 34.649 1.940 4.06 73.8 35.2 LA 195 34.663 1.912 4.10 74.7 34.9 N 295 34.702 1.884 4.21 77.2 33.4 LA 493 34.723 1.747 80.1 32.3 N 738 34.740 1.521 4.57 86.0 32.0 LA 985 34.735 1.282 4.65 92.7 32.0 N 1230 34.724 1.014 4.70 100.7 33.1 LA 1477 34.711 0.789 4.75 106.3 32.9 N 1722 34.704 0.585 111.6 33.3 N 1970 34.694 0.387 4.88 114.9 33.5 N 2459 34.676 0.047 5.12 116.8 33.8

Chl a (lg l~1)

Fe (nM) 0.16 0.17 0.16 0.34 0.34 0.43 0.46 0.81 0.58 0.65 0.79

1.94 1.85 1.92 2.13 2.10 2.11 2.09 2.14 2.16

0.39 0.37 0.51 0.48 0.77 0.27 0.10 0.01 0.01

2.27 2.32

0.01

1.67 1.60 1.65 1.78 2.41 2.65 2.65 2.61 2.57 2.53 2.51 2.42 2.36 2.31 2.32 2.36 2.39 2.42 2.43 2.43

0.18 0.45 0.69 1.07 0.49 0.05 0.01 0.01 0.01 0.01 0.01 0.00

0.65" 0.18 0.17 ND 0.24 0.21 0.23 0.33 0.34 0.27 0.28 0.28 0.15 ND ND ND 0.22 0.30 0.16 0.18 0.24 0.26 0.28

(continued on next page)

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Table 1 (continued) Chl a (lg l~1)

Sampler

Depth (m)

Salinity

Pot. T (3C)

Oxygen (ml l~1)

SiO 2 (lM)

NO 3 (lM)

PO 4 (lM)

N LA N N N

2703 2948 3191 3437 3601

34.678 34.674 34.676 34.666 34.650

!0.066 !0.193 !0.324 !0.434 !0.631

5.24 5.31

114.7 116.8 111.6 108.6 96.7

33.8 33.5 33.5 33.1 32.7

2.42 2.42 2.40 2.39 2.34

Stn. 22 (62302@S, 140302@E; Depth 4217 m; 28-12-1994 03:06}05:54) B, P 0 33.760 1.600 7.92 22.7 25.4 N 9 33.755 1.222 7.39 22.2 25.6 LA 19 33.763 1.207 8.02 21.9 25.6 N 29 33.772 0.853 8.30 21.9 26.3 LA 50 33.841 !0.285 8.39 23.7 27.3 LA 73 33.944 !1.159 8.02 31.2 29.7 LA 99 34.040 !0.997 7.25 38.7 32.3 N 123 34.242 0.234 5.61 52.6 35.4 LA 147 34.379 1.570 4.72 61.4 37.3 N 172 34.440 1.870 4.30 66.8 37.7 LA 197 34.495 2.010 4.12 69.7 37.4 N 297 34.581 2.049 4.00 75.4 36.7 LA 494 34.672 2.058 4.10 79.4 34.9 N 740 34.723 1.898 4.32 82.6 33.8 LA 987 34.740 1.713 4.48 86.8 33.2 N 1232 34.742 1.495 4.58 92.8 32.7 LA 1430 34.734 1.270 4.65 98.7 33.0 N 1725 34.726 1.066 4.71 104.1 33.5 N 1969 34.717 0.864 4.82 114.2 33.7 N 2459 34.697 0.456 4.90 119.3 34.3 N 2949 34.683 0.101 5.05 124.0 34.6 LA 3436 34.679 !0.127 5.28 125.1 34.4 N 3680 34.675 !0.280 5.33 120.5 34.2 N 3921 34.672 !0.360 5.44 116.0 34.4 N 4153 34.671 !0.426 5.48 114.4 34.2

1.70 1.71 1.71 1.73 1.83 2.11 2.27 2.51 2.64 2.67 2.66 2.59 2.46 2.35 2.31 2.30 2.32 2.34 2.36 2.42 2.43 2.43 2.41 2.41 2.40

0.10 0.16 0.18 0.42 0.99 0.60 0.11 0.05 0.02 0.01 0.01

Stn. 25 (50303@S, 142347@E; Depth 3991 m; 31-12-1994 12:40}15:58) B 0 34.368 9.100 6.48 3.2 15.8 N 10 34.449 8.991 6.53 3.1 15.8 LA 20 34.450 8.975 6.51 3.2 15.7 N 29 34.446 8.931 6.49 3.1 16.1 LA 72 34.467 8.840 6.53 3.1 16.2 LA 99 34.512 8.812 6.37 2.8 15.9 N 123 34.627 8.832 6.16 2.8 16.0 LA 149 34.587 8.594 6.25 3.2 16.9 N 173 34.551 8.362 6.20 3.8 17.9 LA 199 34.442 7.696 6.42 3.9 18.5 N 295 34.357 7.013 6.44 4.5 20.1 LA 488 34.236 5.849 6.19 7.5 24.1 N 737 34.324 4.761 5.01 22.5 31.1 LA 985 34.392 3.649 4.44 43.2 34.4

1.18 1.18 1.18 1.19 1.19 1.19 1.19 1.23 1.29 1.37 1.45 1.67 2.15 2.42

0.13 0.12 0.13 0.12 0.13 0.14 0.04 0.03 0.02 0.01 0.01

5.51 5.77

Fe (nM)

0.34

1.15

0.81" ND ND ND ND ND ND ND ND ND ND ND ND 0.20 0.15 0.20 0.42 0.23 0.38 0.27 0.26 0.37 0.39 0.50 0.63

ND ND ND ND ND ND ND 0.30 0.19 ND ND 0.25 0.43

Y. Sohrin et al. / Deep-Sea Research I 47 (2000) 55}84

Sampler

Depth (m)

Salinity

N LA N N N N LA N N N

1230 1478 1723 1967 2218 2464 2951 3439 3684 3927

34.451 34.557 34.651 34.704 34.732 34.747 34.733 34.722 34.715 34.716

Pot. T (3C)

65

Oxygen (ml l~1)

SiO 2 (lM)

NO 3 (lM)

PO 4 (lM)

4.21 4.02 4.09 4.17 4.34 4.46 4.52 4.64 4.69 4.69

55.1 64.7 71.0 74.1 77.8 81.0 101.4 110.2 114.1 113.8

36.5 36.2 34.8 34.2 33.2 32.7 33.8 34.0 34.7 34.4

2.54 2.54 2.46 2.38 2.31 2.28 2.37 2.39 2.41 2.42

Stn. 32 (56300@S, 143341@E; Depth 3435 m; 12-1-1995 04:58}07:31) B, P 0 33.851 3.500 7.55 1.1 25.5 LA 10 33.847 3.380 7.57 1.0 25.4 LA 22 33.847 3.124 7.68 1.3 25.3 N 30 33.843 3.100 7.60 1.4 25.3 LA 38 33.848 3.054 7.66 1.5 25.3 LA 49 33.848 2.983 7.62 1.5 25.0 LA 72 33.892 1.268 7.73 11.5 27.5 N 101 33.916 0.491 7.59 19.6 29.0 LA 122 33.926 !0.124 7.79 21.4 29.5 N 148 34.046 0.858 6.71 29.7 32.4 LA 199 34.269 1.921 5.10 47.5 36.0 N 299 34.407 2.138 4.34 60.4 37.3 LA 496 34.565 2.238 3.99 69.8 36.7 N 735 34.665 2.146 4.06 71.8 35.0 LA 988 34.716 2.012 4.28 76.8 33.3 N 1234 34.742 1.821 4.50 80.0 32.4 LA 1478 34.741 1.627 4.45 81.6 32.6 N 1726 34.737 1.398 4.59 88.2 32.7 N 1968 34.730 1.155 4.68 94.6 33.1 N 2216 34.720 0.925 4.73 94.7 33.4 N 2462 34.710 0.715 4.78 103.6 33.9 LA 2707 34.702 0.524 4.83 112.6 33.8 N 2952 34.695 0.343 5.01 113.2 34.2 N 3195 34.691 0.229 4.98 118.3 34.3 N 3410 34.684 0.221 4.98 124.2 34.4

1.62 1.70 1.64 1.62 1.66 1.62 2.02 2.12 2.17 2.30 2.55 2.65 2.61 2.47 2.36 2.35 2.31 2.29 2.33 2.36 2.38 2.40 2.42 2.44 2.46

Stn. 34 (60301@S, 142300@E; Depth 4248 m; 13-1-1995 06:02}09:20) B 0 33.851 3.400 7.58 1.6 25.1 N 10 33.847 3.270 7.62 1.5 25.1 N 21 33.848 3.235 7.68 1.0 25.0 N 30 33.851 3.163 7.74 0.9 24.9 LA 40 33.859 2.875 7.75 1.8 27.8 LA 50 33.879 1.719 7.81 6.4 29.2

1.52 1.54 1.56 1.57 1.68 1.95

2.800 2.524 2.353 2.211 2.035 1.843 1.260 0.926 0.756 0.720

Chl a (lg l~1)

Fe (nM) 0.39 0.42 0.36 0.38 0.54 0.55 1.12 0.98 0.87 1.16

0.47 0.53 0.54 0.67 0.53 0.75 0.09 0.07 0.03 0.02 0.02

0.46 0.48 0.54 0.68 1.16 0.98

0.27 ND 0.20 0.28 0.22 0.20 ND ND 0.88" 0.34 0.26 0.32 0.26 0.20 0.25 0.28 0.25 0.35 0.40 0.52 0.62 0.64 0.53 0.41 4.89!

0.17 0.16 0.37 0.17 0.38

(continued on next page)

66

Y. Sohrin et al. / Deep-Sea Research I 47 (2000) 55}84

Table 1 (continued) Sampler

Depth (m)

Salinity

Pot. T (3C)

Oxygen (ml l~1)

SiO 2 (lM)

NO 3 (lM)

PO 4 (lM)

Chl a (lg l~1)

Fe (nM)

LA N LA N LA N LA N LA N LA N N N N LA N N N

73 100 122 150 199 296 495 743 987 1234 1480 1701 1971 2463 2952 3439 3682 3925 4154

33.894 33.906 33.927 34.032 34.286 34.442 34.591 34.677 34.721 34.740 34.741 34.735 34.724 34.708 34.686 34.682 34.680 34.678 34.674

0.953 0.378 0.151 0.484 1.858 2.189 2.246 2.157 1.997 1.810 1.573 1.348 1.099 0.696 0.312 0.046 !0.027 !0.099 !0.235

7.72 7.75 7.72 6.85 5.01 4.25 4.01 4.15 4.32 4.49 4.53 4.62 4.69 4.77 4.93 5.09 5.16 5.19 5.29

15.1 19.3 22.1 30.4 49.9 61.4 69.6 73.5 76.4 80.3 85.4

28.0 28.4 29.5 32.0 36.2 37.0 35.7 34.1 32.8 32.1 32.1

2.11 2.13 2.16 2.31 2.62 2.67 2.57 2.46 2.36 2.31 2.30

0.35 0.20 0.09 0.04 0.01 0.00

ND ND ND ND 0.26 0.22 0.49 0.26 0.36 0.29 0.29 0.31 0.39 0.57 0.55 0.42 0.62 0.31 0.39

Stn. 40 (64310@S, 140340@E; Depth 3651 m; 15-1-1995 05:17}11:34) B, P 0 33.727 3.300 7.97 16.8 22.7 LA 10 33.754 2.397 8.19 16.8 23.1 N 20 33.880 0.996 8.85 16.6 23.2 N 30 33.982 !0.458 8.27 33.9 27.7 LA 40 34.041 !0.981 7.84 43.0 29.4 LA 50 34.134 !1.322 7.26 52.2 31.4 LA 75 34.309 !0.777 6.42 63.8 33.5 N 99 34.464 0.159 5.39 75.0 35.2 LA 123 34.542 0.754 4.94 80.4 35.8 N 148 34.590 1.096 4.66 84.6 35.9 LA 200 34.630 1.208 4.61 86.6 35.3 N 297 34.682 1.392 4.56 89.2 35.1 LA 495 34.729 1.468 4.58 94.8 34.4 LA 742 34.732 1.253 4.63 98.2 34.0 LA 989 34.728 1.031 105.9 34.5 LA 1235 34.701 0.731 4.86 111.3 34.9 LA 1480 34.711 0.664 4.78 119.9 35.5 N 1750 34.698 0.419 4.90 124.2 35.9 LA 1972 34.692 0.260 4.99 127.4 35.9 N 2218 34.688 0.107 5.08 128.3 35.9 LA 2461 34.684 !0.001 5.21 128.1 35.5 N 2707 34.680 !0.106 5.20 131.7 35.2 LA 2950 34.679 !0.230 5.34 121.2 35.1 N 3193 34.677 !0.355 5.48 116.7 34.7 LA 3436 34.661 !0.550 5.68 105.9 34.6 N 3486 34.657 !0.601 5.78 105.5 34.6 LA 3563 34.655 !0.630 5.86 105.1 34.9

1.16 1.20 1.29 1.85 2.06 2.24 2.36 2.46 2.49 2.50 2.46 2.41 2.35 2.30 2.35 2.38 2.41 2.42 2.44 2.44 2.43 2.44 2.42 2.41 2.39 2.38 2.40

0.95 0.95 1.95 1.41 1.04 0.46 0.22 0.09 0.03 0.02 0.01 0.01

ND ND ND 0.16 ND ND ND 0.20 0.28 ND ND 0.34 0.28 0.43 0.42 0.59 0.41 1.36" 0.68 0.56 0.58 0.91 0.71 0.71 1.44

Y. Sohrin et al. / Deep-Sea Research I 47 (2000) 55}84

67

NO 3 (lM)

PO 4 (lM)

Chl a (lg l~1)

Fe (nM)

27.2 26.6 27.3 28.9 31.1 31.7 31.6 31.9 31.8 32.1 32.1 33.3 33.5 33.7 33.7 33.7 33.7 34.1 34.6 34.3

1.79 1.80 1.81 2.02 2.23 2.27 2.26 2.27 2.26 2.27 2.28 2.38 2.39 2.40 2.40 2.40 2.40 2.43 2.45 2.41

0.40 0.47 0.47 0.37 0.41 0.44 0.32 0.17 0.11 0.01 0.01 0.01

0.43 0.28 ND 0.31 0.15 0.42 0.21 0.26 0.33 0.29 0.51 0.34 0.39 0.60 0.52 0.80 0.82

Stn. 45 (65330@S, 140326@E; Depth 1317 m; 21-1-1995 13:37}15:04) B, P 0 33.369 !1.200 8.06 45.5 27.1 N 9 33.550 !1.156 8.00 46.1 27.5 LA 20 33.669 !0.682 7.93 47.0 27.6 N 30 33.711 !0.668 7.93 47.0 27.9 LA 40 34.245 !1.584 7.53 55.7 16.4 LA 49 34.304 !1.707 7.51 57.6 30.6 LA 74 34.338 !1.793 7.40 60.0 31.3 N 98 34.350 !1.816 7.36 60.6 31.5 LA 122 34.358 !1.813 7.34 62.3 31.6 N 147 34.364 !1.804 7.42 63.2 31.5 LA 198 34.392 !1.736 7.14 65.3 31.8 N 297 34.512 !0.695 6.18 78.1 32.6 LA 495 34.667 0.528 5.07 95.5 33.1 N 741 34.682 0.364 4.97 106.1 33.9 LA 988 34.675 0.062 5.17 108.7 34.0 N 1085 34.656 !0.305 5.54 98.8 33.5 LA 1183 34.659 !0.383 5.58 97.7 33.6 N 1233 34.649 !0.513 5.73 95.3 33.5

1.85 1.91 1.92 1.94 2.25 2.33 2.38 2.37 2.35 2.35 2.41 2.50 2.55 2.59 2.60 2.55 2.54 2.51

0.28 0.35 0.33 0.37 0.58 0.72 0.31 0.20 0.09 0.04 0.02 0.01

Sampler

Depth (m)

Salinity

Pot. T (3C)

Oxygen (ml l~1)

SiO 2 (lM)

Stn. 43 (65331@S, 139349@E; Depth 1612 m; 20-1-1995 9:17}15:07) B, P 0 33.466 !0.500 7.90 43.3 LA 10 33.462 !0.574 7.92 44.5 N 19 33.464 !0.575 7.93 44.0 N 29 33.825 !0.750 7.60 47.9 LA 40 34.211 !1.637 7.21 54.1 LA 50 34.265 !1.655 7.14 56.9 LA 72 34.313 !1.732 7.22 59.7 N 98 34.341 !1.754 7.01 60.8 LA 124 34.353 !1.789 7.31 63.3 N 150 34.366 !1.633 7.16 65.0 LA 196 34.393 !1.620 7.20 68.5 N 297 34.569 !0.100 5.69 82.1 LA 499 34.668 0.481 5.14 96.7 LA 740 34.675 0.100 5.13 98.0 LA 986 34.683 0.055 5.09 99.6 LA 1231 34.660 !0.312 5.54 99.6 LA 1382 34.640 !0.584 5.91 102.0 LA 1479 34.638 !0.610 5.87 109.6 LA 1479 34.639 !0.616 5.90 118.0 LA 1530 34.636 !0.635 5.93 115.8

Note: B, bucket; P, pump; N, conventional Niskin; LA, lever action type. !Less than the lower limits of determination. "Probably contaminated.

0.68 0.80

0.30 0.18 0.34 0.15 0.22 0.67 0.37 0.43 0.43 0.54 0.50 0.51 0.36 0.53 0.58 0.65 0.77 0.76

68

Y. Sohrin et al. / Deep-Sea Research I 47 (2000) 55}84

extracted for 4 h in the dark at !403C and subjected to determination by HPLC (Suzuki et al., 1997). Multiple regression analysis of Chl a and accessory pigment concentrations was conducted to estimate the contribution of algal classes to the total phytoplankton crop. The pigments used were fucoxanthin for diatoms, 19@-hexanoyloxyfucoxanthin for prymnesiophytes (haptophytes), peridinin for dino#agellates and 19@-butanoyloxyfucoxanthin for chrysophytes. The F-test showed that the obtained multiple regression equation was statistically signi"cant (p(0.01). Simulated in situ incubation using surface seawater with added 13C was carried out for 24 h on deck for productivity measurement. The seawater samples (0.6 l) were collected with the lever action samplers and maintained in acid-cleaned polycarbonate bottles. The incubated seawater was "ltered through a GF/F "lter and stored in the way that the samples for HPLC pigment analysis were. After the "lter was fumed over HCl to remove inorganic carbon, the contents of organic carbon and 13C in the particulate matter were determined by a mass spectrometer with an elemental analyzer. Production rate was calculated according to the method of Hama et al. (1983). Total dissolved inorganic carbon (DIC) and CO partial pressure in surface 2 seawater were measured every 0.75 h during the cruise. Seawater pumped continuously from the vessel's bottom was introduced into an automated coulometric DIC analysis system (Ishii et al., 1998). The reference seawater, which was traceable to the certi"ed reference material provided by Dr. A.G. Dickson (Scripps Institution of Oceanography), was analyzed in each run. Carbon dioxide-in-air (1%) was introduced into the system every 4.5 h to monitor the e$ciency of the coulometer. Vertical pro"les of total DIC ((500 m) were also obtained using subsamples from Niskin samplers. Monthly integrated net community production (NCP), which is the di!erence between primary production and respiration by all the autotrophic and heterotrophic organisms present through a water column (Minas et al., 1986), in the SIZ was estimated from the di!erence in the total DIC normalized to S"34 between Legs 2 and 3. The details of the calculation have been reported elsewhere (Ishii et al., 1998).

4. Results and discussion 4.1. Labile Fe species The determined Fe is a chemically labile species, which is in un"ltered seawater and reacts with 8-hydroxyquinoline on MAF-8HQ at pH 3.2. It may include dissolved and some colloidal or particulate species. Wells et al. (1991) reported that the labile fraction of total Fe, de"ned by extraction with 8-hydroxyquinoline at pH 6 in 1 h, correlates with the availability of Fe to marine phytoplankton. Our labile Fe species also may be available to phytoplankton, but we have yet no data to support the contention. In most previous studies, the `dissolveda values of Fe were determined after "ltration with 0.2}0.45 lm and months of acidi"cation to pH(2. It should be noted that the Fe species determined by our method is not identical with that determined by previous workers.

Y. Sohrin et al. / Deep-Sea Research I 47 (2000) 55}84

69

Fig. 3. Vertical pro"les of labile Fe: n, collected with conventional Niskin samplers; L, collected with lever action type samplers. Error bars show the standard deviation of multiple determinations.

4.2. Distribution of labile Fe in deep water Vertical pro"les of labile Fe at selected stations are shown in Fig. 3. While the concentrations of Fe were low in surface water and increased in deep water at all stations, the pro"les varied substantially depending on the latitude. Strong correlation was not observed between the Fe concentrations and the other hydrographic properties at each station south of Stn. 32. The correlation coe$cients throughout the water column were 0.47}0.82 between Fe and SiO , 0.15}0.65 between Fe and NO , and 2 3 0.15}0.73 between Fe and PO . 4 Section plots for labile Fe, oxygen, SiO and PO are shown in Fig. 4. The Fe 2 4 concentrations in intermediate and deep water were 0.30$0.12 (n"36 for 500}2000 m) between the AF and AD. The Fe concentrations steeply increased south of 64.43S. The maximum concentrations of SiO (&130 lM), PO (&2.9 lM) and 2 4 NO (&42 lM) were observed at the AD. The source of these elements is probably the 3 sediments over the continental shelf and slope. Particles resuspended and dissolved species released from the sediments well up due to the winter mixing on sea-ice formation (Gordon and Huber, 1990) and the upwelling at the AD. The high Fe region ((1.5 nM) extended northward along the continental slope. This coincided with the AABW of low-temperature and high-oxygen. The concentrations of PO and SiO in 4 2

70

Y. Sohrin et al. / Deep-Sea Research I 47 (2000) 55}84

the AABW were lower than those in the overlying LCDW north of 64.43S. The analysis of chloro#uorocarbon in seawater has shown that the distribution of CFC-11 and CFC-12 was similar to that of Fe (Watanabe et al., 1996). The ratio of CFC-11 and CFC-12 in the bottom water was nearly equal to that in surface water, which suggests this bottom water was formed within the last several decades. The distribution of Fe was not uniform in the CDW. The maximum (&1.2 nM) was observed at 3000 m depth at 503S, and the minimum (&0.3 nM) was observed at 3000 m depth at 623S. The Fe minimum was almost coincident with the maximum of SiO (&125 lM) at 3000}3500 m depth at 56}623S. The distribution of PO was more 2 4 uniform, and its trend was similar to that of SiO . The CDW, on average, circles 2

Fig. 4. Section plots through a water column along the &1403E meridian: (a) labile Fe (nM); (b) oxygen (ml l~1); (c) SiO (lM); (d) PO (lM). The data of Stns. 25, 32, 34, 22, 13, 11 and 45 were used for the plots. The 2 4 bold line simply connects the bottoms of the sampling stations and does not represent the real topography.

Y. Sohrin et al. / Deep-Sea Research I 47 (2000) 55}84

71

Fig. 4. (continued).

several times around the Antarctic Continent. During the circulation, the LCDW enhances the contents of SiO that are remineralized from biogenic particles (Edmond 2 et al., 1979). In the case of Fe, it seems that scavenging loss prevailed over the remineralization gain. The Fe maximum is located on the north side of the Paci"c Antarctic Ridge (PAR). The high concentrations of Fe may be transported from the Indian Ocean or added at the Ridge. It has been reported that the Fe contents are high in the sediments over the PAR as well as those over the East Paci"c Rise (BostroK m et al., 1973). Iron may be derived from hydrothermal origin through direct emission of the #uids from active vents or resuspension of the metalliferous sediments over the ridge. Maximums of more than 4 nM dissolved Fe have been observed on the Atlantic Antarctic Ridge (LoK scher et al., 1997).

72

Y. Sohrin et al. / Deep-Sea Research I 47 (2000) 55}84

The labile Fe concentrations in this area are slightly lower than dissolved deep concentrations of Fe in the Drake Passage (0.4}0.8 nM, Martin et al., 1990b), in the Atlantic sector (0.4}2.8 nM, LoK scher et al., 1997; 2}8 nM, Nolting et al., 1991) and in the Weddell Sea (averaging 1.2 nM, Westerlund and OG hman, 1991). Since it is possible that these di!erences result from the di!erent pretreatments and methods for determination of Fe, we need further investigation to make clear the situation. 4.3. Distribution of labile Fe in shallow water The average labile Fe concentration in the surface water column was 0.14$ 0.12 nM (n"97 for 0}100 m). This is similar to the dissolved Fe concentrations in surface waters of the Drake Passage (0.16 nM, Martin et al., 1990b) and the Southern ACC in the Atlantic sector (0.17}0.49 nM, LoK scher et al., 1997). Sedwick et al. (1997) measured dissolved and total-dissolvable Fe in seawater from the upper water column ()350 m) at stations along 1403E during January 1995. Their stations at 533S and 503S are close to our Stns. 32 and 25, and the dissolved Fe concentrations (0.1}0.4 nM) are really comparable to our results. Section pro"les of labile Fe, temperature, dissolved oxygen and PO in shallow 4 water ((200 m) are shown in Fig. 5. The Fe concentrations in shallow water also steeply increased south of 64.43S. While salinity, SiO , NO and PO were at 2 3 4 a maximum around the AD, the maximum of Fe (&0.6 nM) was observed at the ice edge. The water containing high Fe ('0.4 nM) was characterized by low-temperature ((!13C) and high-oxygen ('5 ml l~1). In surface water ((50 m), the concentration of Fe was relatively low, which is probably attributable to uptake by phytoplankton. The distribution of Fe at the AD was irregular and mosaic. At Stns. 11 and 13, seawater sampling was repeated at nearly the same position in order to investigate the daily variation of biological activity. The results for Fe largely varied beyond the precision of our method. Since the variation of Fe reached to 500 m depth, it should not have been caused by biological activity. The distribution of Fe at the AD is probably a!ected by complex eddies and frontal structure as well as the melting of icebergs (de Baar et al., 1995) and patchy distribution of organisms. North of the AD, while the concentrations of Fe were normally low ((0.2 nM), maximums were observed at 30}50 m depth at 60}563S (&0.4 nM) and 150}300 m depth of 56}503S (&0.3 nM). These maximums did not coincide with cold water. It has been reported that the UCDW is characterized by maximums of PO and NO 4 3 and a minimum of oxygen (Sievers and Nowlin, 1984; Whitworth and Nowlin, 1987). They are attributed to oxidative decomposition of organic matter. The maximum zone of PO (&2.5 lM) was observed at 75}500 m depth at 64}563S; its center became 4 deeper toward the north. Some Fe may be remineralized from the organic matter as well as PO . However, the distribution of Fe was patchier than that of PO , and the 4 4 maximums of Fe did not coincide with those of PO . Another possible mechanism for 4 Fe maximums is lateral transport by the ACC. One candidate for an Fe source may be the Kerguelen Plateau, which lies &5000 km west of our stations. Very high concentrations of dissolved Fe (&66 nM) have been observed on the shelf of South Orkney Islands (Nolting et al., 1991). Sarthou et al. (1997) observed total dissolvable

Y. Sohrin et al. / Deep-Sea Research I 47 (2000) 55}84

73

Fig. 5. Section plots in a shallow water column (0}200 m) along the &1403E meridian: (a) labile Fe (nM); (b) temperature (3C); (c) oxygen (ml l~1); (d) PO (lM). The data of Stns. 25, 32, 34, 22, 20, 13, 11 and 45 were 4 used for the plots.

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Fe up to 4.8 nmol kg~1 to the southwest of the Kerguelen Islands. Reductive dissolution from shelf sediments was suggested as a major source for Fe. Although the eastward mean speed of the ACC is &0.01 m s~1 (Broecker and Peng, 1982), the AF, which is associated with strong current jets with velocities reaching 1 m s~1, runs around the Kerguelen Plateau. A large amount of mesoscale eddy variability within the ACC (Nowlin and Klinck, 1986) may contribute to the transport of Fe. A long scale lateral transport of Fe in subsurface water has been reported for the equatorial Paci"c (Coale et al., 1996; Gordon et al., 1997). In this case, Fe originating from the New Guinea Platform is transported &7500 km by the equatorial undercurrent over a time scale of &3 months, forming a subsurface maximum on the transect along 1403W. 4.4. Factors controlling the distribution of Fe The vertical pro"les of labile Fe in the present area (Figs. 3}5) show surface depletion, which implies that Fe is taken up by organisms in surface water and released from sinking particles. The labile Fe concentrations in intermediate and deep water between the AF and AD are 0.30$0.12 (n"36 for 500}2000 m), which are signi"cantly lower than dissolved Fe concentrations obtained in other oceanic regimes by Johnson et al. (1997a). Their new measurements in deep water during the Southern Ocean JGOFS program of 0.3}0.5 nM are much like our values (Johnson, personal communication). He showed that their and our data (0}2000 m) are consistent with their model, assuming annual carbon export is &0.5 mol C m~2 yr~1. The labile Fe concentration sharply increases south of the AD, and the high Fe region extends northwards along the continental slope. Another deep Fe maximum is located over the PAR, and shallow Fe patches are observed between the AF and AD. These Fe maximums are presumably superimposed on the nutrient-like pro"les through local sources and water circulation. The probable Fe sources are organic-rich sediments over shelves and slopes, metalliferous sediments over the mid-oceanic ridge, and hydrothermal vents. In addition, the Fe concentration is reduced in the LCDW, which may be attributed to extensive scavenging during the circulation of the ACC. Thus, there is much yet to be done before we understand the mechanism controlling the distribution of Fe. 4.5. Distribution of plankton The high concentrations of Chl a ('1 lg l~1) were observed only at 40 m depth at 603S (Stn. 34) and 20}40 m at 643S (Stns. 20 and 40). Chl a was patchily distributed at the AD. Temperature, salinity, in vivo #uorescence and particle concentrations of surface water were measured every minute during the cruise using the AMEMBO system. Mesoscale #uorescence maximums were irregularly observed between the AF and AD. Our stations for vertical samples did not gather in the regions where the #uorescence was exceptionally high or low. The HPLC pigment analysis of surface seawater was conducted on Leg 3. Fig. 6 shows latitudinal distribution of algal classes as estimated by the pigment analysis. Diatoms were dominant (65}24%), and

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75

Fig. 6. Contribution of fucoxanthin-containing diatoms (j), 19@-hexanoyloxyfucoxanthin-containing prymnesiophytes (L), peridinin-containing dino#agellates (n), and 19@-butanoyloxyfucoxanthin-containing chrysophytes (£) and other algal classes (h) to total Chl a biomass in surface water along &1403E during Leg 3.

prymnesiophytes were second dominant (35}11%) south of 543S. The percentage of diatoms decreased and that of prymnesiophytes increased south of 643S. Dino#agellates (10}6%) and chrysophytes (19}4%) were minor at these stations. North of 503S, the percentage of diatoms decreased and that of others increased. At Stn. 28 (47.63S, 147.53E), the percentage was 9% for diatoms, 49% for prymnesiophytes, 12% for dino#agellates and 22% for chrysophytes. The dominant taxa of zooplankton di!ered between the stations around the AD (Nishida et al., 1996). Salps were predominant at Stn. 13, while no salp occurred and copepods, chaetognaths and pteropods predominated at Stn. 11. At Stn. 43, the taxonomic composition was similar to that of Stn. 11, but euphausiids were more abundant than pteropods. 4.6. Relationship between Fe and indices of biological production Fig. 7 shows the distribution of integrated concentrations in the surface water column ((100 m) for labile Fe, Chl a, SiO , NO and PO along &1403E. The Fe 2 3 4 concentration was integrated between 10 and 100 m, since some 0 m samples were contaminated with Fe. The integrated values for Fe were high for 56}603S, where the subsurface maximums were observed, and increased southwards beyond the AD. There was no clear relationship between the values of Fe and Chl a. The values for the major nutrients also did not correlate with those for Chl a. The mean concentration ratio of Fe : PO through the surface water column was 0.1}2]10~4. 4 The concentrations of labile Fe are plotted against the concentrations of major nutrients for three stations in Fig. 8. At Stn. 13, since regression lines have positive intercepts on the abscissas, it seems that Fe was depleted earlier than the other nutrients by phytoplankton. This trend was common among stations south of 623S. However, at Stn. 40, which was occupied about one month later at nearly the same position as Stn. 13, the concentrations of SiO , NO and PO in surface water 2 3 4 decreased by &21, &5 and &0.5 lM, respectively, although the Fe concentrations were low as before. The integrated value of Chl a through the water column (0}100 m) at Stn. 40 was 73 mg m~2, which was signi"cantly higher than that of 24 mg m~2 at

76

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Fig. 7. Distribution of the integrated concentrations in the surface water column (10}100 m for labile Fe and 0}100 m for the others) along &1403E: h, stations during Leg 2; L, stations during Leg 3.

Stn. 13. These results suggest that moderate growth of phytoplankton decreased the concentrations of the major nutrients, while Fe remained at low concentration. For the stations north of Stn. 32, SiO may be exhausted earlier than Fe, since Fe vs SiO 2 2 plots have positive intercepts on the ordinates. de Baar et al. (1997) have reported that Fe stress tends to in#uence the major chemical constituents of the phytoplankton cell in the order Chl a'C*N'P. Low Fe concentration ()0.7 nM) hampers assimilation of nitrate, leading to lower N : P ratios in the biomass of phytoplankton than the standard Red"eld ratio of 16. This in turn causes higher N : P ratios in ambient surface water and lower ratios in the export of plankton matter into underlying waters. In surface waters at Stns. 34 and 40, the NO : PO ratio increased to 16}20. On the other hand, the NO : PO ratio in deep 3 4 3 4 water (*480 m) was 14.3$0.3 (n"107) except for the data of Stn. 45, which was 13.1$0.1 (n"6). The former value is equal to that observed for the Atlantic sector

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Fig. 8. The relationship between labile Fe and major nutrients.

(de Baar et al., 1997). The concentrations of NO in the upper water column ((300 m) 3 are plotted against those of PO in Fig. 9. Except for the data of Stns. 34, 40 and 25, 4 the regression line is NO "12.5 PO #4.1 (r"0.90 and n"126). 3 4 For the data of Stns. 34 and 40,

(1)

NO "9.8 PO #10.1 (r"0.94 and n"24). (2) 3 4 The slope was 12.6 in the deep water ('175 m) of the Atlantic sector, and decreased to 4.4 in the shallow water ((300 m) of anomalous Fragilariopsis kerguelensis bloom stations (de Baar et al., 1997). These unusual relationships between NO and PO may 3 4 suggest Fe stress encountered by a phytoplankton community. However, at Stn. 25, where the Fe concentrations in shallow water were also low, the regression line is NO "15.5 PO !2.3 (r"0.98 and n"11) 3 4 which is similar to the global trend.

(3)

78

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Fig. 9. The relationship between NO and PO in the upper water column ((300 m): L, data for Stns. 34 3 4 and 40; n, data for Stn. 25; v, the remaining data. The numbers of the regression lines correspond to those in the text.

Table 2 Average concentrations of labile Fe (0}10 m), phytoplankton growth rates, microzooplankton grazing rates and primary productivity in surface water Stn.

Fe (nM)

34 40 13 11 43

0.17 0.05" 0.13" 0.05" 0.36

Growth rates! (d~1)

Grazing rates! (d~1)

Primary productivity (lg C l~1 d~1) 6.8$0.2 6.1$0.3

!0.02 0.22 0.66

0.01 0.05 0.69

9.1$0.2

!After Tsuda and Kawaguchi (1997). "ND values on Table 1 were assumed to be 0.05 M. Data of Stns. 13 BCTD-2, 3 and 4 were used for calculation of the average at Stn. 13, and data of Stns. 11 and 11 BCTD-1 were used for Stn. 11.

Table 2 shows the average concentration of labile Fe (0}10 m), phytoplankton growth rates, microzooplankton grazing rates and 13C primary productivity in surface water around the AD. The phytoplankton growth rates and microzooplankton grazing rates were measured by the dilution method (Tsuda and Kawaguchi, 1997). The growth rate was high at the ice edge (0.66 d~1), where the Fe concentration was high. This value was about half of the maximum observed in the Antarctic

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Table 3 Monthly integrated net community production (NCP) in the SIZ estimated from the di!erence in normalized total DIC in the surface water between Legs 2 and 3 Zone

NCP (g C m~2 month~1)

Thickness of the SW (m)

623S}633S 633S}643S 643S}64.53S 64.53S}653S 653S}

4.8 2.0 4.8 8.1 5.2

60 50 40 40 40

Contribution of air-sea CO #ux has not been taken into account yet. 2

Peninsula area in the same season, where Chl a concentration exceeded 2 lg l~1. The microzooplankton grazing rate was also high at the ice edge (0.69 d~1), which was about two times higher than the maximum observed in the Antarctic Peninsula area. The 13C primary productivity was also signi"cantly higher at the ice edge station (9.1$0.1 lg C l~1 d~1) than that at the north of the AD (6.1$0.3 lg C l~1 d~1). Total DIC in surface water normalized to S"34 was higher south of 64.43S (2165$6 lmol kg~1) than that in the outer SIZ (2148$6 lmol kg~1 between 64.4 and 623S) in December. The normalized total DIC decreased by &15 lmol kg~1 on the average in January south of 643S, while the decrease between 64 and 623S was only &5 lmol kg~1 and cannot be regarded as statistically signi"cant. The temporal decrease in total DIC is probably due to the biological consumption of DIC, similarly to that is observed in the Indian sector (Ishii et al., 1998). Based on the decrease in DIC, we evaluated the monthly integrated NCP in the SIZ, of which results are shown in Table 3. The monthly integrated NCP is signi"cantly higher south of the AD, where the integrated concentration of Fe in the surface water column is high (Fig. 7). Jennings et al. (1984) estimated the integrated NCP in the Weddell Sea in summer (60}90 d) to be 20 g C m~2. In the Indian sector, Minas and Minas (1992) have reported that the seasonal integrated NCP along the section from 403S to 623S was 30 g C m~2. Ishii et al. (1998) have reported the seasonal integrated NCP between 303E and 1503E south of 633S to be 10}48 g C m~2. The present data are comparable to these. Very high integrated NCP (101 g C m~2) has been observed by Karl et al. (1991) in the Gerlache Strait near the Antarctic Peninsula during the December to January spring bloom period, where high dissolved Fe in surface water (7.4 nM) has been reported (Martin et al., 1990b). 4.7. Factors limiting primary production It has been recognized that a latitudinal and concentric zonation gives a realistic idea of the dynamics of nutrients and plankton in the Southern Ocean. TreH guer and Jacques (1992) have suggested that four major sub-systems should be considered south of the SAF: the very productive Coastal and Continental Shelf Zone (CCSZ),

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the SIZ, the well-mixed Permanently Open Ocean Zone (POOZ) and the Polar Front Zone (PFZ). Our stations between the ice edge and 623S belong to the SIZ, Stns. 34 and 32 belong to POOZ, and Stn. 25 is located at the north rim of the PFZ. In the SIZ, the AD is a remarkable boundary for the distribution of labile Fe and major nutrients. South of the AD, the concentrations in surface water were more than 43 lM for SiO , 26 lM for NO and 1.7 lM for PO . A Sharp pycnocline was 2 3 4 observed at &40 m depth. These conditions are probably favorable to a phytoplankton bloom. Although the Fe concentrations were elevated toward the ice edge, the Fe : PO ratio in seawater was still less than 3]10~4. Sunda and Huntsman 4 (1995,1997) have demonstrated that cellular Fe : C ratios of phytoplankton are highly variable depending on the accessibility of Fe to the cell. The cellular Fe : C and therefore Fe : P ratios can not be used as Red"eld type certainties. We merely point out that the observed Fe : PO ratio was lower than the blooming condition 4 (1]10~3) proposed by de Baar et al. (1990). Fe was depleted earlier than major nutrients (Fig. 8). The distribution of primary productivity was positively correlated with that of Fe (Fig. 7; Table 2). While the NCP value was also increased south of the AD, the value for 64.5}653S was higher than that for '653S (Table 3), which was reverse to the trend of Fe stock. This may be caused by the di!erence in time after the retreat of sea-ice. It is likely that Fe in surface water for 64.5}653S had been extensively taken up by phytoplankton. The unusual relationship between NO and 3 PO in seawater, especially at Stn. 40, where the Chl a concentration was high 4 ('1 lg l~1), may imply physiological limitation to a phytoplankton community by Fe. Takeda (1998) has performed Fe enrichment bottle incubation experiments using surface water collected from Stn. 40. In Fe added samples (Fe"1.2}1.3 nM), the doubling rate of Chl a increased by 1.6 (10% light intensity)}3.3 times and NO 3 consumption increased by 4 (10% light intensity)}3.2 times compared with controls (Fe"0.16}0.20 nM). Microscopic examinations showed substantial growth of largesize diatoms (Chaetoceros spp. and Nitzschia spp.) in Fe-enriched bottles compared with the control. These results suggest that the primary productivity of the diatoms was probably limited by Fe. Small-size phytoplankton also may have su!ered Fe stress. However, it is likely that high grazing activity of microzooplankton was also a major limiting factor for the standing crop of pico- and nanophytoplankton (Table 2). The north region of the SIZ seems more unfavorable for phytoplankton, since the concentrations of Fe and SiO were lower (Fig. 7). The SiO concentrations in surface 2 2 water further decreased to (2 lM in the POOZ. The concentration ratio in surface water was &1]10~4 for Fe : PO and &1 for SiO : PO at Stns. 32 and 34. 4 2 4 Assuming the diatom Si : P ratio of 20 (Takeda, 1998), SiO is also insu$cient to 2 diatoms. This can be ascertained by the SiO vs. Fe plot in Fig. 8. During our cruise in 2 January, diatoms dominated the Chl a biomass (Fig. 6). Wright et al. (1996) performed pigment HPLC analysis between 653S, 883E and 453S, 1423E during March 1987. According to their data, diatoms were dominant in the SIZ but decreased north of 603S. In the POOZ, haptophytes were dominant and diatoms were a minor fraction. It is likely that the phytoplankton composition changes with season because of lack of SiO . Moreover, sigma-t pro"les exhibited weak strati"cation in the POOZ. The 2

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81

di!erence in sigma-t between 2 and 100 m depths was less than 0.3 at Stns. 32 and 34. It appears that the critical-depth/mixed-depth relationship also can limit the biomass of phytoplankton (Mitchell et al., 1991). While signi"cant increases in phytoplankton biomass have been reported in the PFZ (Bathmann et al., 1997; Laubscher et al., 1993), the Chl a concentration was low at Stn. 25. This station would have been unfavorable for phytoplankton growth, since Fe and SiO were depleted. Moreover, water strati"cation was very weak, since the 2 di!erence in sigma-t between 2 and 100 m depths was 0.07. In conclusion, the labile Fe concentrations in the euphotic zone were low, and phytoplankton was probably under Fe stress throughout the present area. South of the AD, however, the Fe concentrations were elevated, and moderately high primary productivity and phytoplankton growth rates were observed. These results indicate the signi"cance of Fe for the growth of phytoplankton in the Southern Ocean.

Acknowledgements We are grateful to Profs. Kouichi Kawaguchi and Makoto Terazaki who not only gave us the opportunity to join this cruise onboard the R.V. Hakuho Maru, but supported us all the way through. We thank the crew, o$cers and scientists aboard. We wish to thank Mr. Hiroshi Hasumoto for the skilled operation of the CTD and Dr. Shuichi Watanabe for the analysis of nutrients. We are grateful for discussions with Drs. Masaaki Wakatsuchi, Mikio Naganobu, Fukashi Fukui, Shuichi Watanabe, Shigenobu Takeda, Atsushi Tsuda, Rumi Fukuda and Hiroshi Ogawa. We also appreciate valuable comments from Dr. Kenneth S. Johnson and an anonymous reviewer. This study was partly supported by research grants from Kyoto University and Nissan Science Foundation to Y. S.

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