ELSEVIER Marine Chemistry 5 l(1995) 223-237

Cadmium-phosphorus cycling at the subtropical convergence south of New Zealand

Russell D. Frew ‘, Keith A. Hunter Department of Chemistry, University of Otago, Box 56, Dunedin, New Zealand

Received 21 September 1994; accepted 22 June 1995

Abstract

The water column distributions of cadmium (Cd) and phosphorus (P) in the region of the subtropical convergence (STC) south of New Zealand have been studied. Mixed layer Cd concentrations were extremely low throughout the study area, varying from 0.012 to 0.059 nmol kg- ‘. In subantarctic waters south of the STC, deep-water Cd concentrations were intermediate between those of the North Atlantic and the North Pacific oceans, consistent with deep-water circulation. Cd varied linearly with P with a slope of 0.35 X 10m3 mol Cd/mol P, but waters in the upper 1000 m were significantly depleted in Cd compared to temperate waters (zero-P intercept at - 0.23 nmol Cd kg- ’ >. The lowest Cd concentrations were observed in subtropical waters north of the STC. Here the Cd-P slope was only 0.08 X 10m3 mol Cd/mol P, indicating that the Cd-P properties in this region are dominated by mixing of low-Cd, high-P subantarctic water across the STC. Freshwater sources of Cd and P were found to be negligible. This work implies that the northern region of the Southern Ocean plays a major role in the global Cd-P relationship. Two mechanisms are suggested: (1) the ventilation of low-Cd-P subantarctic water to intermediate depth through Antarctic Intermediate Water formation, and (2) the remineralisation of low-Cd-P detritus from biota produced in waters formed at the STC.

1. Introduction analogue since the ratio of Cd and P concentrations

It is now well-established that the geographical distribution of cadmium (Cd) in the deep ocean is

similar to that of the labile micronutrient phosphorus (P> (e.g. Boyle et al., 1976; Bruland et al., 1978;

Bruland, 1980; Bruland and Franks, 1983; Hunter and Ho, 1991; Frew and Hunter, 1992). Thus vertical

profiles of Cd show nutrient-like depletion in surface waters and enrichment at depth. Early work tended

to suggest that Cd could be regarded as a close P

in different regions of the ocean exhibited little

variation, particularly when comparing North At- lantic and North Pacific data (Boyle et al., 1976; Bruland, 1980; Bruland and Franks, 1983). This

would imply that Cd is taken up by surface-layer phytoplankton in a relatively constant Redfield type ratio (Redfield et al., 1963) with respect to carbon

and the micronutrients. Whether the relationship arises from specific mechanism(s) of cellular incor- poration (Walsh and Hunter, 19921, or by more

’ Present address: School of Environmental Sciences, Univer- sity of East Anglia, Norwich NR4 7TJ, UK. E-mail: r.frew@uea.ac.uk.

adventitious associations such as scavenging has yet to be determined.

In deep waters of the open ocean, the concentra- tion of Cd at a given value of P is consistent within

0304-4203/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SD1 0304-4203(95)00057-7

224 R.D. Frew, K.A. Hunter/Marine Chemistry 51 (199s) 223-237

1.2

0.6

j-f, , j

0 1 2 3 4

[PI (pmol kg-‘)

Fig. 1. Cd vs. P for open ocean samples below the mixed layer.

Data were compiled by Boyle (1988) with the addition of Antarc-

tic results (Martin et al., 1990). Dashed lines, drawn to emphasise

the non-linearity of the global data, comprise the least-squares

regression lines through the data having P concentration greater

than or less than 1.3 pmol kg-‘, respectively.

about +7%. However, there is a significant slope change in the relationship at P N 1.3 pmol kg-’ (Boyle, 1988). Below P N 1.3 pmol kg- ’ (dominated

by Atlantic and upper ocean data), the Cd-P rela- tionship has a slope of about 0.21 nmol Cd/pm01 P, passing only slightly below the origin at zero P (Fig. 1). Above P N 1.3 pmol kg- ’ (dominated by South

Atlantic and Pacific data), the Cd-P relationship has a slope of about 0.40 nmol Cd/pm01 P and has a zero P intercept at Cd w - 0.25 nmol kg-‘. Much lower Cd-P ratios have been found in nutrient-rich waters from high latitudes (Danielsson and Wester- lund, 1983; Jones and Murray, 1984; Danielsson et al., 198.5; Martin et al., 1990; Nolting et al., 1991; Westerlund and ehman, 1991; Nolting and de Baar, 1994) and upwelling regions (Jannasch, 1990). Frew and Hunter (1992) suggested that the kink may be caused by the injection of Cd-depleted, high-latitude,

Southern Ocean waters into intermediate depths of

the global ocean. Very low and variable Cd-P ratios have been

found in surface waters, marginal basins and conti-

nental shelf waters (e.g. Danielsson and Westerlund, 1983; Kremling and Pohl, 1989; Pohl et al., 1993).

Boyle et al. (1981) demonstrated there are significant

differences between the surface distributions of Cd

and P, and comments that determining the mecha- nisms producing these differences will contribute

significantly to understanding of the biological up-

take and recycling of trace metals.

The majority of reliable Cd data have been ob-

tained from lower latitudes of the Northern Hemi-

sphere, there are relatively few (until recently) data available for the important Southern Ocean where the high concentrations of nutrients in surface waters

present conditions that are quite different from most temperate regions. Here we present the complete data set from the limited study described by Frew

and Hunter (1992) including 13 further stations, which focuses on the influence of Cd-depleted sub-

antarctic waters on subtropical waters in the subtrop- ical convergence (STC) zone south of New Zealand. The cycling of Cd and P is discussed in conjunction with the hydrology of the region.

2. Cruise track and study area

In May 1989, a joint New Zealand/Australian cruise was conducted on the Commonwealth Scien- tific and Industrial Research Organisation (CSIRO) research vessel, R/V Franklin. This cruise com- prised extensive sampling of the water column at a

position over the Puyseger Trench (stations 1 l-14) and two additional legs (Fig. 2). The first of these legs consisted of a southerly transect from the mouth of Doubtful Sound, down the Fiordland Coast and

into the head of the Solander Trough. The second (Foveaux Strait leg) comprised a transect from the western approach of Foveaux Strait eastward through the Strait to a position (station 55) at the shelf break.

The Puyseger Trench, with a depth of up to 6000 m, is the main topographical feature of the region studied. Cold Antarctic Intermediate water (AAIW) is uplifted in the water column as the current encoun- ters the continental shelf at the Puyseger Trench, and

R.D. Flew, KA. Hunter/Marine Chemistry 51 (1995) 223-237 225

Doubtful Sound

49 I I I I I 1 L

164 166 166 170”E 172

Fig. 2. Map of R/V Franklin cruise track showing locations and

numbers of sampling stations in this study.

nearby Solander Trough. This current is moving northeast and flows around the bottom of the conti-

nental shelf and up the eastern coast to the Chatham Rise (Heath, 1981).

The hydrological properties of the water masses in this region at the time of this cruise have been previously described (Vincent et al., 1991; Butler et al., 1992). Warm subtropical water from the Tasman current is diverted northwards up the West coast of New Zealand (Westland Current), and south around Stewart Island and through Foveaux Strait. The southward flowing subtropical water is modified by freshwater and mixes with subantarctic water at the STC to form the Southland Current. Southland Cur-

rent water extends through Foveaux Strait and con- siderably south of Stewart Island. Once through Foveaux Strait, the Southland Current is located on the continental shelf and slope off the east coast of the South Island of New Zealand being bounded on

the coastal side by low-salinity, shallow, coastal water and on the seaward side by Subantarctic Sur- face Water (SASW; Heath, 1972).

3. Sampling and analysis

The Franklin was equipped with two hydro- graphic winches, one with 6000 m of conducting

cable for a CTD-Rosette sampler. This was fitted with 8-l Niskin sample bottles having an internal Teflon coating, Teflon valves and silicone rubber

inner springs. On the second hydrographic winch was 1850 m of Kevlar hydroline for sampling with

2.5-l Teflon-coated General Oceanics Go-FLO bot-

tles. Once deployed, these bottles were activated

serially by solid PVC messengers.

The Franklin was also equipped with a Class-100

clean laboratory van within which all samples were

withdrawn from the bottles. Samples were stored in

l-l conventional polyethylene (CPE) bottles that had been acid-cleaned as described in Ahlers et al. (1991). After cleaning sample bottles were filled with a 1%

solution of ultrapure quartz distilled (Kuehnen et al., 1972) HCl and doubly bagged in polyethylene for

storage, the inner bag also being acid-cleaned.

Samples from all depths down to 4500 m were taken from the trace metal Niskin bottles deployed

on the CTD-Rosette. The bottles were first removed from the rosette and mounted on a rack at the

entrance of the clean room. A stainless steel clamp was attached to hold the endcaps firmly in a sealed position. Teflon tubing for pressurising the bottle

with filtered compressed air and for transferring the sample into the clean room were then attached to the bottles. This arrangement allowed sub-samples to be withdrawn from inside the clean room, optionally with in-line pressure filtration of the sample. Sub- samples from the Niskin bottles used in the present work were transferred directly without filtration into

l-l CPE bottles, CSIRO scientists collected filtered as well as unfiltered samples. Sub-samples from the Go-FLO bottles were collected by taking the Go-FLO into the clean room and withdrawing samples di-

rectly, without filtering, into the sub-sample bottles as above.

Because of the limited length of the Kevlar hydro-

line, all samples at depths greater than 1700 m were obtained with the Niskin/Rosette system. To check the integrity of the samples collected using the Niskin

bottles, replicate samples were taken using both Go-

FLO and Niskin bottles deployed close together on a hydrocast with the Kevlar line. In addition, a number of common depths were sampled with both systems to check on the reliability of the Niskin/Rosette system. The majority of the samples collected were split into two replicates, one analyzed in the present

226 R.D. Frew. K.A. Hunter/Marine Chemistry 51 (1995) 223-237

work and one analyzed by Dr. Dennis Mackey of CSIRO. Thus the joint study provided an opportunity to cross-check both sampling methods and methods

of analysis. Unfortunately, this situation is still com-

paratively rare in spite of the well-known difficulties

associated with trace metal measurements. The re- sults of this intercalibration exercise will be pre-

sented in detail elsewhere, but showed quantitative

agreement between both methods of analysis and both methods of sampling ( ? 4%).

Cd was analyzed within a few weeks of the cruise

by graphite furnace atomic absorption spectropho- tometry (GFAAS) after solvent extraction using a

method adapted from that of Danielsson et al. (1982)

by substituting chloroform for Freon-l 1 as the ex-

tracting solvent. All sample manipulations up to the point of GFAAS were carried out in a Class-100

clean laboratory. Water samples were acidified the

day before analysis with 2 ml of 10 M ultrapure HCl per litre of sample to desorb metal ions from the plastic surfaces of the bottle. The sample aliquot taken for extraction was generally 100 g. For some shallow waters where the Cd content was expected

to be very low, a 200 g aliquot of sample was extracted. Duplicate aliquots were analyzed for every sample. The chelating agent solution contained 1% w/u of ammonium pyrrolidine dithiocarbamate (APDC) and was stabilised by the addition of 1% w/u of ammonium acetate buffer solution. The solu-

tion was prepared freshly for use and purified by extraction with two volumes of chloroform. All chlo- roform used was purified by double distillation fol- lowed by 4 times extraction with 20% u/u ultrapure HCl. The seawater was buffered with a solution of purified ammonium acetate made up so that, on addition of a 1 ml aliquot to 100 g of acidified water sample, a final pH of 4.8 was achieved.

For extraction, an aliquot of the seawater sample

was weighed into a 250 ml Teflon separating funnel and 1 ml each of the ammonium acetate and APDC solutions were added per 100 g of sample. This was extracted twice using 10 ml aliquots of ultrapure chloroform which were combined in a 25 ml polyethylene vial. The chelated metals were then back extracted by adding 50 FL of 16 M ultrapure HNO, to the vial and shaking vigorously for 2 min. Following this the acid was diluted by adding 1.95 ml of ultrapure water to the vial which was shaken

and left for the phases to separate. Aliquots were removed directly from the supematant acid layer

using a micropipette for GFAAS analysis. Standard

operating conditions, including background correc- tion and a L’vov platform, were used.

Matrix effects were checked by standard addition

analysis. No matrix effects were found and so con-

centrations were determined by comparison with

standard solutions prepared by dilution of a 1000 mg

Cd 1-l stock solution with 0.8 M HNO, solution. A recovery efficiency of 92 + 4% was determined by

standard addition analysis of previously extracted

seawater. Analytical blanks, determined for every analytical run by the extraction of both ultrapure

water and pre-extracted seawater samples, were typi- cally less than 0.0006 nmol kg-‘. The limit of

detection based on twice the standard deviation of all

blanks, was 0.0005 nmol kg-‘. All results presented below are corrected for analytical blanks and recov- ery efficiency. In cases where the results for dupli-

cate samples did not agree, samples were re-analyzed in duplicate.

Because the samples for this work were not fil- tered, the Cd results refer to total dissolvable Cd. This will include the truly dissolved fraction plus any Cd leached from particulate matter by the acidi- fication of the sample and/or the extracting ligands. However, the CSIRO Cd measurements for samples

collected during the present study showed no differ- ence between filtered and non-filtered replicate sam- ples (Dennis Mackey, pers. commun., 1990). Thus valid comparisons can still be made with published results for dissolved (i.e. filtered) samples from the open ocean.

The Cd concentration in the freshwater input into the region was investigated in separate expeditions to Doubtful Sound and the Waiau, Aparima and Oreti

rivers (Fig. 2). Samples for Cd were collected by hand dipping of acid-cleaned sample bottles as de-

scribed in Ahlers et al. (1991). Cd concentrations were determined as for seawater samples.

3.1. Nutrient analysis

The concentrations of phosphate, silicate, nitrate, and nitrite were determined on board using a 5 channel auto-analyzer system. Samples were drawn from the sampling bottles, stored in the dark at 4°C

R.D. Frew, K.A. Hunter/Marine Chemistry 51 (1995) 223-237 227

and analyzed, unfiltered, within 6 h. Salinity and

temperature data were obtained using a Neil Brown CTD system attached to the rosette. The standard CTD cast was 750 m being the maximum depth for

operation of the insitu fluorometer, this instrument was removed for occasional deeper stations. The

CTD-Rosette system was deployed immediately prior

to each Kevlar hydrocast.

The New Zealand Oceanographic Institute (NZOI) research vessel Rapuhia was undertaking a study in

the region at the same time as Franklin providing an

opportunity for intercalibration of methods. On Ra-

puhia samples were filtered through GF/F filters

and then analyzed for nitrate plus nitrite. The remain-

ing filtrate was stored frozen until analysis l-2

months later for reactive phosphorus and silicate. A station was selected where the two ships sam-

pled concurrently about 200 m apart. The nitrate plus nitrite results were in close agreement but silica and phosphorus values were much higher on Franklin.

Since the nitrate plus nitrite results are in good

agreement, this discrepancy probably reflects the effects of freezing and storage on Rapuhia rather than the difference between filtered (Rapuhia) and

unfiltered (Franklin) samples. Because this work depends heavily on good P

data for later discussions on Cd-P ratios further investigation of the validity of the Franklin P results is required. Station F22 in Doubtful Sound was

deliberately close to station 11 of a previous study undertaken six months earlier by University of Otago researchers. In this earlier study reactive phosphate concentrations were determined on fresh samples by standard calorimetric techniques (Murphy and Riley, 1962) using a shore-based spectrophotometer (Kim et al., 1990). The concentrations of P in the salt wedge layer of the two stations were very similar

(mean difference F22 - Station 11 = - 0.02 pmol kg-‘). Station F55 sampled water masses exten- sively studied by Hawke (1988, 1989), and the P results are in close agreement.

A further check is a comparison with results from nearby GEOSECS stations. In Fig. 3 the Franklin

results are plotted with data from GEOSECS station 293. While the upper 1000 m of both stations agree very well, Franklin P are about 0.2 pmol kg-’

higher than GEOSECS in the deep and bottom waters. This apparent discrepancy between the Franklin and

1000

E 2000

5 8 n 3000

4000

I I

0 1 2

[PI (pmol kg-‘)

Fig, 3. Vertical profiles of P for GFDSEB station 293 (52“35’S, 178”2’W, l ), Franklin Puyseger Trench station (48’05’S, 164’30’E; 0) and Scorpio station (43”S, 164’E; 0).

GEOSECS is undoubtably due to the stations being in different ocean basins. When Franklin P data are compared with Scorpio data from the Tasman Basin (Fig. 3) the profiles are almost identical throughout the water column.

Thus, contrary to the assertions of de Baar et al. (1994), there is no reason to doubt the P data from the Franklin cruise, they are fully consistent with all high-quality data from the region.

4. Results and discussion

The results of the Cd and nutrient analyses are presented in Table 1.

4.1. Puyseger trench

Stations 1 l- 14 were located at the same site over the Puyseger Trench (Fig. 2). The salinity profile (Fig. 4a) clear1 y s h ows the salinity maximum of the

228 R.D. Frew, K.A. Hunter/Marine Chemistry 51 (1995) 223-237

Table 1 Cadmium and nutrient results from Franklin cruise

Station Depth [Cdl PI [NO,1 tSi1 Cm) (nM) ( @I) ( /.LM) ( /LM)

11 700 0.34 1.81 NK 500 0.26 -

600 0.29 1.30

300 0.10 0.97 200 0.06 0.85

75 0.04 0.71 NK 25 0.04 0.71

12

13

14

22

4450 0.64 2.41 35.2 134.8 NR4000 0.65 2.41 34.9 134.8 NR 3500 0.62 2.41 35.0 132.9 NR 3000 0.64 2.37 34.3 122.9 NR 2500 0.61 2.35 34.0 111.1 NR 2250 0.58 2.38 34.2 105.1 NR 2000 0.62 2.43 34.7 97.2 NR 1750 0.63 2.47 35.4 89.2 NR 1500 0.62 2.49 35.7 79.3 NR 1200 0.52 2.32 33.4 46.4 NR 1000 0.48 2.18 31.9 33.7 NR 750 0.37 1.87 27.1 19.0

1600 0.64 2.43 35.2 77.3 NK 1590 0.63 2.38 34.9 69.4

1200 0.56 2.23 32.5 36.1 NK 1190 0.53 2.21 32.2 35.9

1000 0.46 1.99 29.4 24.6 NK 990 0.45 1.99 29.4 24.4 NK 790 0.33 1.72 24.8 14.1

1600 0.65 2.44 36.1 89.3 1300 0.63 2.33 34.2 49.6 1100 0.59 2.22 32.8 36.7 900 0.40 2.00 29.8 24.8 400 0.21 1.37 18.5 6.2 200 0.08 1.07 14.4 4.8 100 0.06 0.82 10.0 2.8 50 0.02 0.68 7.6 1.8 25 0.03 0.70 7.6 1.8

400 0.052

350 0.067 300 0.054 250 0.054 200 0.058 150 0.048 100 0.033 50 0.022 25 0.011 0 0.013

0.93 0.93 0.88 0.63 0.81 0.8 1 0.70 0.39 0.39 -

26.4 -

17.7

12.7 9.9 7.8

7.6

9.9 9.9 9.5 5.9 8.7 8.5 6.7 2.4 1.6

17.3 -

5.8 4.2

3.0

2.0 2.0

9.8

6.1 5.7 4.1 4.5 3.9 2.2 2.3 5.3 _

R.D. Frew. KA. Hunter/Marine Chemistry 51 (1995) 223-237 229

Table 1 (continued)

Station Depth [Cdl PI [NO,1 LSil Cm) (nM) (@I) ( FM) ( FM)

38 300 0.083

200 0.046

150 0.032

100 0.022

50 0.015

25 0.026

41 50 0.03 0.41 3.1 2.0

25 0.02 0.41 3.1 2.0

44 30

15

0.02

0.02

0.47

0.45

3.7

3.7

2.2

2.2

46 16

41 17

28

0.029 0.47 4.8 2.6

0.042

0.048

0.64 6.9 2.6 -

49 21 0.059 0.69 7.4 2.6

17 0.056 0.69 7.4 2.6

51 50 0.050 0.75 a.2 2.6

22 0.042 0.75 8.2 2.6

53 100 0.052 0.78 9.4 3.2

50 0.047 0.78 9.4 3.2

25 0.044 0.78 9.4 3.2

55 NR 800 0.50 2.02 31.3 23.4

NR600 0.39 1.77 26.4 13.9

NR 400 0.32 1.47 22.4 7.7

NR 200 0.28 1.39 20.5 5.9

NR 100 0.05 1.06 16.0 2.5

21 100 0.04

50 0.02

25 0.02

31 100 0.02

50 0.01

25 0.01

0.61 5.3

0.31 1.2

0.30 1.0

0.37 2.3

0.36 2.0

0.34 1.9

2.7

1.5

1.5

1.5

2.1

2.1

33 100 0.025 0.56 5.6 2.6

50 0.014 0.40 2.5 1.9

25 0.012 0.36 2.0 2.1

0.75

0.67

0.56

0.53

0.5 1

12.1 4.7

a.2 3.2

7.2 2.7

5.8 2.0

5.2 2.0

5.2 2.2

The sampling system used for each sample is indicated with the depth of the sample. “NK” denotes Niskin sampling bottle deployed on

Kevlar hydroline while “NR” denotes Niskin sampling bottle deployed on the CTD/Rosette. AlI other samples were collected with

GGFUJ bottles deployed on the Kevlar hydroline.

230 R.D. FEW, K.A. Hunter/Marine Chemistry 51 11995) 223-237

Salinity

34.4 34.6 34.8

Temperature (“C)

2.5 5.0 7.5 10.0

12.5

I(C) ’ ’ I ,

’ SASW ’ I -

G‘ 10.0 -

e

z 7.5 - 3 % t 5.0 - AAIW

E”

: 2.5 -

POW o.or , I , I I I I -.

34.2 34.4 34.6 34.0 35.0

Salinity

Fig. 4. Hydrographic CTD data for the Puyseger Trench station at 48’05’S, 164“30’E in subantarctic water. (a) Salinity vs. depth. (b) Temperature vs. depth. (c) Temperature-salinity diagram (with water masses).

Subantarctic Upper Water (SAUW, z = 140 m, S = 34.8) and minimum of the low-salinity (S = 34.4) AAIW centred at a depth z = 1000 m. Temperature (Fig. 4b) decreased steadily down to about z = 1300

m and less steeply from there to z = 3200 m. The deep water column was relatively uniform below this

depth with respect to both T and S. The surface mixed layer was approximately 120 m deep overly- ing SAUW. This SAUW water has a southward component of motion which accounts for its higher temperature and salinity than the water further south (Pickard and Emery, 1990). The T-S diagram (Fig. 4c) is characteristic of five body mixing with SASW

overlying SAUW and a core of AAIW. Below these are Warm Deep Waters (WDW) of Atlantic Deep Water origin and Pacific Deep Water (PDW) pro-

duced by mixing of Antarctic Bottom Water (AABW)

with the overlying WDW. The core of the AAIW is located at T = 5OC, S = 34.4 which agrees favourably

with the T-S properties of South Pacific Central Water as defined by Sverdrup et al. (1942).

The vertical profiles of Cd and P (Fig. 5a,b)

showed the classic features of deep ocean water at

high latitude. P had a large surface-water concentra-

tion (0.7 pmol kg- ’ ) while Cd was depleted to near zero (0.03 nmol kg-’ ). The concentrations of both

species increased with depth to maxima of [P] = 2.49 pmol kg-’ and [Cd] = 0.65 nmol kg-’ at about

z = 1500 m. From this depth, the concentrations of both P and Cd were nearly constant down to z = 4500

m. The mean Cd concentration for surface waters

was 0.037 nmol kg-‘, in the same range as found in the Indian Ocean (Danielsson, 1980;) but lower than

most results for North Pacific waters (Boyle et al.,

1976; Jones and Murray, 1984; Matsunaga and Abe, 1985). Bruland (1980) found very low Cd concentra- tions (0.002 nmol kg-’ ) at North Pacific olig-

atrophic gyre stations. However, surface P was also very low in this region (0.06 pmol kg-‘) relative to the Puyseger Trench (0.71 pmol kg-’ ). The low Cd

concentration in Puyseger Trench surface waters in the presence of significant P is consistent with other

[Cdl (nmol kg-‘). [PI (pmol kg-‘)

loo0

T 2000 V

5

0 D 3000

4000

0.2 0.4 0.6 0.5 1.0 1.5 2.0 2.5

0

0

0

0

0

0

0

0 0

0

0

0

0

0

I I I L I

Fig. 5. Vertical profiles of (a) Cd and (b) P for the Puyseger Trench station at 48”05’S, 164”30’E in subantarctic water (stations 11-14; Table 1).

R.D. Frew, K.A. Hunter/Marine Chemistry 51 (1995) 223-237 231

recent studies in polar waters (Martin et al., 1990; Nolting et al., 1991; Nolting and de Baar, 1994).

The deep-water Cd concentration at the Puyseger Trench station (0.65 nmol kg- ’ ) is consistent with the established global trend of increasing Cd concen- tration in the direction of the main deep-water circu- lation. Previous investigations have found Cd con- centrations in deep water of 0.22 nmol kg- ’ for the northeast Atlantic (Danielsson et al., 1985), 0.57 nmol kg- ’ for the Indian Ocean (Danielsson, 1980; Morley et al., 1993), 0.75 nmol kg-’ in the northeast Tasman (Hunter and Ho, 1991), and 1.1 nmol kg-’ for the north Pacific (Bruland, 1980).

4.2. Stations 27-38

This leg was from station 27, outside the entrance to Doubtful Sound, down the southwest coast and then eastward to the western approach of Foveaux Strait (Fig. 2). Station 38 was at the western ap- proach to Foveaux Strait. As described earlier, the hydrology of this region is dominated by the conver- gence of the warm Tasman current and the cold subantarctic water at the STC.

The temperature and salinity sections for this western transect (Butler et al., 1992, fig. 13a,b) show the progressive mixing of the cooler, less saline subantarctic water with warm more saline subtropi- cal waters. Salinities in the upper 150 m of stations 27-33 reflect freshwater input from Fiordland with station 27 being particularly influenced by input from Doubtful Sound. The temperature section shows decreasing mixed layer ( z = 0- 150 m) temperatures from station 27 to 38. Station 38 is well mixed with T and S characteristics intermediate between the subantarctic waters of the Puyseger Trench (Fig. 4) and the subtropical waters of station 27. This modi- fied sub-tropical water forms the Southland Current (Vincent et al., 1991).

The position of the STC between station 33 and 38, was readily apparent from the P results (Fig. 6b). Hawke (1989) was able to similarly identify the STC from P concentrations off the Otago coast, northeast of this study region. The concentrations of P in the surface mixed layer increased significantly along the transect from station 27 to station 38 across the STC. Surface-water concentrations north of the STC were relatively low (0.35 pmol kg-’ ) typical of subtropi-

100

E 5 200

e 0

300

[Cdl (nmol kg-‘). [PI (pmol kg-‘)

0.03 0.06 0.09 I I

K

’ (al

\

i \

\ \

\ ‘\ .

Fig. 6. Vertical profiles of (a) Cd and (b) P for Leg 1, stations 27 (01, 31 CO), 33 (0) and 38 (W). The bottom depths are indicated.

cal water, at station 38 they were typical of sub- antarctic water, e.g. as observed in the Puyseger Trench and consistent with uplifting. The general position of the STC indicated by the P data was confirmed by sea surface temperature satellite im- agery (Vincent et al., 1991; Butler et al., 1992). Unlike P, Cd concentrations did not increase signifi- cantly along this transect and were consistently low in the surface layer (0.012-0.025 nmol kg-’ ). The Cd concentration profiles (Fig. 6a) at station 38 showed the same trend as the micronutrients, i.e. very low in the surface and increasing with depth. It is unclear whether the slightly high Cd value found for 25 m at this station is genuine or a contamination artifact.

4.3. Foveaux Strait

The final leg of the cruise (Fig. 2) was a transect from the western approach of Foveaux Strait (station 41), through the Strait to the continental shelf break (station 55). The major part of this transect is through the vertically well-mixed water of the Southland Current (modified subtropical water). The final sta- tion of this leg (station 55) was off the continental shelf and over subantarctic water.

The temperature and salinity section (Butler et al., 1992, fig. 16a,b) shows the waters through Foveaux

232 R.D. Frew, K.A. Hunter/Marine Chemistry 51 (1995) 223-237

(a) Cd (nmol kg.‘)

Distance along transect (km)

(b) Phmol kg.‘) 0.76

loo0 50 100 150 200

Distance along transect (km)

Fig. 7. Sectional profiles of (a) Cd and (b) P for Leg 2 (Foveaux

Strait), stations 41, 44, 46, 47, 49, 51 and 53.

Strait to be vertically well mixed (the salinity range was S = 34.63-34.75). There was no vertical strati- fication of either salinity or temperature. The major

decrease in both temperature (from 13.6 to 10.9”C) and salinity (from S = 34.75 to 34.63) from station 41 to the adjacent station 44 is due to mixing of both subantarctic and neritic water with the subtropical water as the attained salinity is less than that of either subtropical or subantarctic water. Subsequent increases in salinity and decreases in temperature through Foveaux Strait (stations 44-53) are at-

tributed to further modification of the Southland Current by intrusion of subantarctic water (Vincent et al., 1991).

The concentrations of P (Fig. 7b) increased steadily from station 41 through to station 53 (from 0.41 to 0.78 pmol kg- ’ ) while Si also increased (from 2.0 to 3.2 pmol kg - ’ , Table 1). These trends

are similar to those observed in the previous leg from stations 27-38. The principal freshwater input

to this region comes from fiords in Fiordland of which Doubtful Sound is the dominant source (Toebes and Palmer, 1969). However, data of Kim et al. (1990) show that this freshwater contains too little

P ( < 0.05 pmol kg- ’ > to have a significant effect on the P composition of Foveaux Strait water. The

main cause of increasing P and Si along this leg is

the increasing influence of subantarctic water

(Vincent et al., 1991).

The concentration of Cd (Fig. 7a) increased along

the transect from station 41 (0.022 nmol kg-’ ) to a

maximum at station 49 (0.059 nmol kg-‘) and then

decreased slightly to 0.05 nmol kg-’ at station 55. This trend is different to that of the micronutrients,

the Cd concentrations at station 53 are not signifi- cantly different to that at station 47. Subantarctic

waters, while high in nutrients, are strongly depleted in Cd. Thus mixing of water across the STC into

Foveaux Strait is expected to bring about an increase in nutrient concentration but a decrease in Cd con-

centration. This conclusion is supported by the Cd-P relationships discussed below.

The salinity and temperature profiles at station 55 (Fig. 8a,b) show a relatively warm fresh (S = 34.6, T = 9.2) surface layer of Southland Current water. This well mixed surface layer is about 120 m deep and overlies much saltier Subantarctic Mode Water (SAMW) formed south of the STC in winter (Butler

et al., 1992). The surface P concentration at this station was the

highest value found in the surface waters of this study (1.1 pmol kg- ’ ). The concentration profile of P exhibited a gradual increase with depth (Fig. 9b). Cd concentrations were extremely low in the mixed layer with a sudden increase at about z = 200 m followed by a gradual increase with depth thereafter. The concentrations of each in the depth range z = 200-800 m ([PI = 1.39-2.02 pmol kg- ’ ; [Cd] = 0.28 to 0.50 nmol kg-’ ) are consistent with those

found in the depth range z = 400-1000 m at the Puyseger Trench. This is consistent with an upward movement of AAIW as it encounters the Puyseger Trench and Snares Depression (Butler et al., 1992).

4.4. Riverine cadmium supply to Foveaux Strait

The increase in Cd concentrations observed along the Foveaux Strait transect indicates there is a small

R.D. Frew, K.A. Hunter/Marine Chemistry 51 (1995) 223-237 233

Fig. 8.

Salinity Temperature (“C)

34.3 34.4 5 6 7 8 9

(0)

200 -

400 -

600 -

800 -

10

E(c) ’ I I I

34.2 34.3 34.4 34.5

Salinity

Hydrographic CTD data for station 55. (a) Salinity vs.

depth. (b) Temperature vs. depth.

200 0 0

0 0

0 0

800 0 0

[Cdl (nmol kg-‘)

0.2 0.4 0.6 I I

’ 0 (a) 0

[PI (pmol kg-‘)

0.5 1.0 1.5 2.0 2.5 1 I I I I

0 (b) 0

1 I I I I I 1 1

Fig. 9. Vertical profiles of (a) Cd and (b) P for station 55.

localised source of Cd, and possibly micronutrients,

in Foveaux Strait. Elevated Cd concentrations in shelf waters have been observed in previous studies and attributed to “continental sources” which in-

cludes river-borne Cd and sedimentary sources. It is usually very hard to resolve these two sources (Bru-

land and Franks, 1983; Boyle .et al., 1984), unless

one of the sources is well constrained. Kremling and

Hydes (1988) reported similarly elevated Cd levels in shelf waters around the British Isles which they

attributed to sedimentary regeneration. In Foveaux Strait possible point sources are

river-borne Cd. The three rivers (Waiau, Aparima

and Oreti) discharging into the region are illustrated

in Fig. 2. The influence of this freshwater input can be seen in the salinity section of Foveaux Strait

which showed a significant drop in S from station 41 to 44 (Vincent et al., 1991; Butler et al., 1992).

There is also a significant freshwater input prior to

Foveaux Strait discharging from Doubtful Sound. Cd concentration results and mean flow rates

(Young and Foster, 1986) of these rivers and from the Doubtful Sound (Kim et al., 1990) are presented in Table 2. The highest Cd concentration found was 0.063 nmol kg-’ for the Waiau River, the other

concentrations being, 0.017 nmol kg-’ for Aparima, 0.013 nmol kg-’ for Oreti, and 0.002 nmol kg-’ for

Doubtful Sound. All these values are extremely low,

and consistent with data reported for uncontaminated freshwater systems. Ahlers and Hunter (1989) re- ported 0.06-0.20 nmol kg-’ for the Manuherikia River, New Zealand. Values reported for other rela- tively unperturbed environments include the Amazon (0.05-0.10 nmol kg-i, Boyle et al., 1982; 0.05 nmol kg-i, Grant et al., 19821, Yantze ( < 30 nmol kg-’ 1, and Orinoco (< 30 nmol kg-‘, Grant et al., 1982).

Table 2

Cadmium concentration and mean flowrate of freshwater for

Doubtful Sound and three Southland rivers

River Annual mean flow 3 -I (m s )

[Cdl (nmol kg-”

Doubtful Sound 400 0.002 f 0.001

Waiau 437 0.063 f 0.004

Aparima 20 0.017*0.004

Oreti 46 0.013 f 0.005

234 R.D. Frew, K.A. Hunter/Marine Chemistry 51 (1995) 223-237

Erel et al. (1991) found 0.1 nmol kg-’ Cd in a remote mountain stream.

While representative transport of Cd cannot be derived from this single measurement of the Cd concentration of each river water (Ahlers and Hunter, 19891, it serves to indicate the background input from this source. The Cd concentrations found in the freshwater discharging into the region, even for the Waiau River, are far too low to account for the increase in [Cd] (from 0.02 to 0.06 nmol kg-’ ) found through Foveaux Strait. The much larger con- centration gradient between the freshwater and sub- tropical water for Si compared to Cd (and P> is not reflected in the relative increases in the concentra- tions of these species in Foveaux Strait waters. Vin- cent et al. (1991) measured ammonium and urea and found an increase in these highly labile nitrogen substrates toward the sediments through Foveaux Strait. These bottom gradients imply fast rates of reduced nitrogen release by regeneration in the sedi- ments relative to rates of diffusion and advection in the overlying water column. Hence the source of increased Cd concentrations found in Foveaux Strait is most likely due to the regeneration of organic matter in the sediment.

4.5. Cadmium /phosphate relationships

Stations 11-14 and 55 were over subantarctic waters to the south of the STC. The combined plot of Cd vs. P for these stations gave a straight line having the least-squares regression equation:

[Cd] (nmolkgg’) = 0.35[P] ( pmolkgg') - 0.23

( r2 = 0.96)

The slope of this Cd-P diagram is similar to those found in deep South Atlantic and North Pacific waters and the results for the deepest samples ap- proach values typical of these oceanic regions. How- ever, samples throughout the mixed layer and most of the AAIW layer ( z = 0- 1500 m) are significantly depleted in Cd with respect to P relative to studies in temperate waters. The very low concentrations of Cd with high P concentrations in the surface mixed layer at station 55 are a result of the mixing processes leading to the formation of the Southland Current. Here Cd-and P-depleted subtropical water mixes with

1.0

0.8

- ‘r

y” 0.6

z E c

_ 0.4

0.2

0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0

[PI (pmol kg-‘)

Fig. 10. Cd-P diagram for subantarctic waters of the present study

(stations 1 l-14, 55; 0) compared with data for the Drake Pas-

sage (60’46’S, 63’26’W, Martin et al., 1990; A), the Scotia and

Weddell Seas (57%0.1’S, 48’23.9’W. 61”59.7’S, 48”59.6’W; Nolt-

ing et al., 1991; 0 ). The second-order least-squares regression

line for all these data is shown (solid line).

P-rich, Cd-depleted SASW to produce low-Cd, high- P Southland Current water.

Recent data for Antarctic waters (Martin et al., 1990; Nolting et al., 1991; Nolting and de Baar, 1994) have shown a similar near-surface depletion in Cd relative to P as the subantarctic waters of the present study. The upward curvature of these results (Fig. 10) shows that, in these waters, Cd has a deeper regeneration cycle than P. While there is similarity in the Cd-P properties of both Antarctic (56-64’S, Martin et al., 1990) and subantarctic (48”S, this study) waters, there are insufficient Southern Ocean data available to determine whether this anomalously low Cd is a general property of high- nutrient surface waters south of the STC.

The Cd-P properties of all stations north of the STC including Foveaux Strait (Stations 27-53, Fig. 2) were found to be quite different from those south

R.D. Frew, K.A. Hunter/Marine Chemistry 51 (1995) 223-237 235

0.8

0.6 A -i

y"

s OO 0

00 0

8O

0.0 0.5 1.0 1.5 2.0 2.5 3.0

[PI (pmol kg-‘)

Fig. 11. Cd-P diagram for waters north of the STC (stations

27-53; A ). Results for the Puyseger Trench and station 55 of this

study are plotted for comparison.

of the STC. The combined Cd vs. P diagram for these stations (Fig. 11) showed a straight line rela-

tionship having the least-squares regression equation:

[Cd] (nmolkg-‘) = O.O79[P] ( pmolkgg') - 0.010

Frew and Hunter (1992) discussed the signifi- cance of this low-Cd subantarctic waters on the

global Cd-P relationship by ventilation of intermedi-

ate depths through formation of Antarctic Intermedi-

ate waters. Also of global significance is the effect on the biological control of the Cd-P relationship. The STC extends around the globe at approximately 40% and marks the northernmost extent of sub-

antarctic water. Mixing of the surface waters at the STC and through mesoscale circulation features

(eddies) forms an environment that sustains a large amount of Southern Ocean productivity (Roether et

al., 1993; Yoshimori and Kishi, 1994). This study has found such waters to be extremely low in Cd relative to P hence the biota growing in these waters would be expected to also have a low Cd-P ratio. The regeneration of the resulting detritus will have the effect of decreasing the insitu Cd-P ratio, espe- cially at intermediate depth where maximum regen- eration occurs.

( r2 = 0.79) 5. Summary

This Cd-P slope is substantially lower than values Cd concentrations throughout the study area were

typical of temperate waters (0.25-0.35 nmol extremely low for a near-shore region and underline

Cd/pm01 P>, in particular the same subtropical South its pristine nature with respect to Cd. The phe-

Pacific waters further north at 35-40”s (Hunter and nomenon of low Cd concentrations in the presence

Ho, 1991). Other workers have noted that behaviour of high nutrient concentrations is quite unusual. Con-

of Cd and P in coastal, shelf or estuarine waters is ventional wisdom would suggest that in regions of quite different from that of the open ocean. This has upwelling, high surface-water nutrient concentrations

been attributed to the predominance of riverine input that promote productivity should be accompanied by for Cd or the decoupling of the Cd-P relationship high concentrations of biologically-active trace met-

through breakdown of the controlling mechanism. als such as Cd. However, in the region south of New Here the riverine input of Cd is insignificant and we Zealand, the influx of high-nutrient water across the see that Cd is still significantly coupled with P STC actually brings about a significant decrease in though the relationship is quite different to that Cd relative to P, to levels that are among the lowest found in the open ocean. observed anywhere in the ocean.

As discussed above, surface waters south of the STC have anomalously low Cd concentrations, im- plying that the very low Cd-P ratios found just north of the STC result from the mixing of Cd-depleted, P-rich subantarctic waters across the STC into

Foveaux Strait and the coastal waters around the southwest of Fiordland. Data from within Doubtful

Sound (Kim et al., 1990) are also presented in Fig.

11 and show that these same low Cd-P properties

extend into the Fiord.

236 R.D. Frew. K.A. Hunter/Marine Chemistry 51 (1995) 223-237

Cd concentrations in deep water sampled over the Puyseger Trench at 48% are intermediate between deep Atlantic and deep Pacific concentrations, con- sistent with deep-water transport. However, in the

high-nutrient subantarctic waters south of the STC,

Cd is strongly depleted, relative to P, in the upper

1000 m of the water column compared to temperate ocean waters. Mixing of this Cd-depleted subantarc-

tic water across the STC gives rise to a Cd-P ratio

for subtropical waters that is a factor of 4 lower than

those typical of this water type elsewhere. The per-

sistence of these anomalous Cd-P properties into the waters of Doubtful Sound and the Southland Current suggests the STC is the principal source of P im-

ported into this region. This work suggests that the northern region of the

Southern Ocean plays a major role in the global

Cd-P relationship. Two mechanisms are suggested: (1) the ventilation of low Cd-P subantarctic water to

intermediate depth through Antarctic Intermediate

Water formation, and (2) the remineralisation of low Cd-P detritus from biota produced in waters formed at the STC. It would be interesting to determine changes in Cd-P relationships in a transect from south of the ACC northwards to where the behaviour apparently becomes typical of temperate waters (Hunter and Ho, 1991).

Acknowledgements

We are grateful to the ships crew and scientific personnel on R/V Franklin cruise FRO789 for their assistance. We also thank Professors Dennis Burton and Ed Boyle for their constructive comments on the

manuscript.

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