2000 - Phytoplankton, Nutrients and Hydrography in the Frontal Zone Between the Southwest Indian Subtropical Gyre and the Southern Ocean

8/11/2019 2000 - Phytoplankton, Nutrients and Hydrography in the Frontal Zone Between the Southwest Indian Subtropical

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*Corresponding author.

E-mail address:[email protected] (J.F. Read).

Also at University of Cape Town, Rondebosch, Cape Town, South Africa.

Deep-Sea Research I 47 (2000) 2341}2368

Phytoplankton, nutrients and hydrographyin the frontal zone between the Southwest

Indian Subtropical gyre and the Southern Ocean

J.F. Read*, M.I. Lucas, S.E. Holley, R.T. PollardSouthampton Oceanography Centre, Empress Dock, Southampton, SO14 3ZH, UK

Received 21 December 1998; received in revised form 12 January 2000; accepted 14 March 2000

Abstract

A survey was made of the Southwest Indian Ocean frontal region between 30 and 503E

containing the Agulhas Return, Subtropical and Subantarctic Fronts. From CTD, SeaSoar and

extracted samples the distribution of nitrate, silicate and chlorophyll a is shown to be strongly

linked to the front and water mass structure, varying zonally and meridionally. Surface

chlorophyll a concentrations were low to the north and south leaving a band of elevated

chlorophyll between the Subtropical and Subantarctic Fronts. The low concentration of

chlorophyllato the north, in Subtropical Water, was clearly due to nitrate limitation. Between

the Subtropical and Subantarctic Fronts, where the chlorophyll aconcentrations were highest,

the surface layer showed silicate depletion limiting diatom growth. South of the Subantarctic

Front there were deep extending, low concentrations of chlorophyll a, but despite plentiful

supplies of macro-nutrients and a well-strati"ed surface layer, high concentrations of chlorophyll

awere absent. Changes from west to east were associated with the meandering of the SouthernOcean Fronts, especially the Subtropical Front, and their strength and proximity to each other.

Concentrations of chlorophyllapeaked where the Agulhas Return, Subtropical and Subantarctic

Fronts were in close proximity. Combined frontal structures appear to have particularly

pronounced vertical stability and are associated with enhanced upwelling of nutrients and

leakage of nutrients across the front. Light levels are high within the shallow stable layer. Such

conditions are clearly favourable for biological growth and support the development of

larger-celled phytoplankton communities. 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Hydrography; Nutrients; Phytoplankton; Oceanic Fronts; Southwest Indian Ocean; 35}453S,30}503E

0967-0637/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.

PII: S 0 9 6 7 - 0 6 3 7 ( 0 0 ) 0 0 0 2 1 - 2


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1. Introduction

In the Subtropical gyres, low primary productivity is associated with very low and

limiting concentrations of nutrients. However, high concentrations of nutrients (N, P,

Si) are available in several oceanic regions, including the eastern Equatorial Paci"cand the Southern Ocean, where biomass and primary productivity remain much

lower than expected, the high nutrient low chlorophyll (HNLC) `paradoxa (Bath-

mann et al., 2000). A particular feature of such systems is that small-celled pico-

and nanoplanktonic phyto-#agellates dominate to the virtual exclusion of diatoms

(Detmer and Bathmann, 1997). This has profound implications for the global carbon

dioxide (CO

) budget, in which diatoms play a particularly important role (Smetacek,

1998). Explanations proposed to account for HNLC conditions have included low

light, deep seasonal mixing, cold temperatures, grazing control, and, more recently,iron limitation (Martin et al., 1990; Cullen, 1991; Nelson and Smith, 1991; de Baar et

al., 1995; Bathmann et al., 2000). The oceanic N : P and N : Si nutrient ratios are also

important determinants of speci"c phytoplankton assemblages which govern biolo-

gically driven ocean}atmosphere exchanges of CO

(Sommer, 1994a, b; TreHguer et al.,

1995; Hutchins and Bruland, 1998). Some uni"cation of these concepts in terms of

co-limitation of iron and light (Sunda and Huntsman, 1997) and silica limitation of

`newaproduction (Dugdale et al., 1995) have created further vigorous debate over the

HNLC `paradoxa.

To the south of Africa, the close juxtaposition of the Agulhas Current, the AgulhasReturn Current (ARC), the Subtropical Front (STF) and the Subantarctic Front

(SAF) creates one of the most energetic and important hydrographic regions of the

world oceans. It is also a region of complex biogeochemistry, phytoplankton distribu-

tion and productivity associated with the transition between subtropical and sub-

antarctic domains (Bathmann et al., 2000). Such an environment provides a unique

location in which to explore the relationships between nutrient and hydrographic

controls of phytoplankton distribution and productivity.

SWINDEX, the Southwest Indian Ocean Experiment, took place in January}

February 1995. The survey was designed to cross the ARC, STF and SAF several

times between the Southwest Indian Subtropical gyre and the Southern Ocean

(Fig. 1). In doing so, we also encountered a northern branch of the Antarctic Polar

Front. The primary survey tools were `SeaSoara, an undulating shallow-pro"ling (to

500 m) CTD (Pollard, 1986) and conventional full-depth CTDs. A simultaneous

acoustic Doppler current pro"ler (ADCP) survey of the frontal region provided

current velocities. Chlorophyll a, #uorescence, the macro-nutrients nitrate (plus ni-

trite), phosphate and silicate and in-coming irradiation were also measured.

The importance of our survey in the Southwest Indian Ocean is that it adds to theHNLC debate through an increased understanding of the zonal hydrography of the

fronts in this dynamic region. As the survey covers a transitional region from a low

nutrient, low chlorophyll environment to a HNLC environment, longitudinal and

zonal comparisons allow us to shed further light on the mechanisms that govern the

HNLC condition. For example, what accounts for the common observation of

elevated chlorophyll concentrations and primary production associated with the STF

2342 J.F. Read et al./Deep-Sea Research I 47 (2000) 2341}2368


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Fig. 1. SWINDEX CTD stations (*) from RRS Discovery cruise 213, 6 Jan}21 Feb 1995, and SeaSoartransects (A}B and C}D) with the topography of the Southwest Indian Ocean (200, 2000 and 4000 m

isobaths). Stations 12722 and 12765 shown in Fig. 6 are circled.

(Dower and Lucas, 1993; Laubscher et al., 1993; Weeks and Shillington, 1994, 1996)

when immediately to the south, nutrient concentrations appear to be more favour-

able? Why is the surface chlorophyll a signature to the west or east of the Agulhas

retro#ection weaker? Signi"cant productivity occurs only when the ARC is in close

and in#uential proximity to the STF *but why? Is the mechanism for this one of

nutrient, or light, availability determined by special hydrographic circumstances?

Answers to these questions will contribute to our understanding of phytoplanktonic

regulation of global CO

budgets in this important region.

2. Background

An extensive review of Southern Ocean fronts between the Greenwich Meridianand Tasmania is given by Belkin and Gordon (1996). Here we summarise brie#y some

salient features of the region of the SWINDEX survey.

The ARC, just to the north of the STF, is an extension to the Agulhas Current

following its retro#ection between 163E and 203E (Lutjeharms and Van Ballegooyen,

1988). The Agulhas Front of the ARC (ARF) has steeper density gradients than

any other front in the Southern Ocean. Its average width of only 96 km covers

J.F. Read et al./Deep-Sea Research I 47 (2000) 2341}2368 2343


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a temperature range of 21}15.73C (Lutjeharms and Valentine, 1984). It is optically

clear, warm (18}253C) but nutrient impoverished and has an observed southern limit

of 403S (Lutjeharms and Valentine, 1984). At times, it can compress closely onto the

STF over distances of 500 km, making these two features sometimes di$cult to

distinguish (Read and Pollard, 1993). Because of the occasional proximity of the ARCto the STF, this region can be marked by extremely strong temperature gradients of

up to 13C km.

The STF to the south of Africa forms the poleward boundary of warm, salty, surface

waters of the South Atlantic subtropical gyre. Lutjeharms and Valentine (1984) note

that its mean latitude is 41340S, although there is considerable variability in its

north}south position. Whitworth and Nowlin (1987) describe the STF as a surface

feature characterised by a southward temperature decrease of 43C, from 14 to 103C,

and a southward salinity gradient of 34.9}

34.4. It de"

nes the boundary betweenSubtropical Surface Water and Subantarctic Surface Water. Lutjeharms and Valen-

tine (1984) give the mean temperature range across the STF as 17.9 to 10.63C. Salinity

decreases are variable but fall by at least 0.5 in the range 35.5}35.6 to 34.3}34.6

(Lutjeharms, 1985; Lutjeharms et al., 1993). South of Africa, the STF is a shallow

feature of little more than 300 m in depth although its downstream shape is deter-

mined by the bottom topography (Weeks and Shillington, 1996).

The SAF is generally taken to be the northern boundary of the Polar Frontal Zone,

marking the transition from antarctic to subantarctic waters (Whitworth and

Nowlin, 1987). The strength and location of the front vary widely aroundthe Southern Ocean. South of Africa the SAF is less prominent than the STF and

Polar Front (PF). Lutjeharms and Valentine (1984) give the mean latitude at

46323S with a mean middle temperature of 7.03C. It is generally associated with an

increase in surface nitrate and phosphate concentrations and a peak in biomass.

However, the latter is less pronounced than seen at the PF to the south, or the STF to

the north.

3. The survey

SWINDEX surveyed the region 35}463S, 28}493E. The cruise started and ended

at Durban, South Africa, from where the RRS Discovery sailed on 6 January

and docked on 21 February, 1995. A total of 106 full-depth CTD and multi-sampler

stations were completed in a series of sections that crossed the major fronts

around the northern side of the Del Cano Rise and across the Southwest Indian Ridge

(Fig. 1). SeaSoar was towed along two sections (AB and CD in Fig. 1), which were

duplicated by station sampling to allow measurements of subsurface nutrient andchlorophyll parameters for comparison with and for calibration of the detailed

SeaSoar data.

Data were collected both by continuously recording instruments and by individual

sample analysis. Conductivity, temperature, pressure and #uorescence were recorded

either by a full depth NBIS Mark IIIc CTD on individual casts or by the undulating

towed SeaSoar, which carried a NBIS Mark III shallow CTD with an additional FSI



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conductivity sensor and a #uorometer. SeaSoar data were collected over the upper

500 m of the water column. Individual CTD stations were worked to the sea bed, but

only the upper water data are used here. Salinity calibrations were obtained for both

deep and shallow CTD instruments by analysis of individual samples drawn at depth

and from the surface. Data from the deep instrument is believed to be good to 0.002(Pollard and Read, 1995).

Upper ocean currents were monitored by a hull-mounted RDI 150 kHz ADCP.

Calibration was e!ected by controlled steaming over a known course together

with Trimble GPS navigation and S.G. Brown gyro-compass with heading

corrections determined from Ashtech 3D-GPS (Pollard and Read, 1989; King and

Cooper, 1993; Gri$ths, 1993). Currents are believed to be more accurate than 0.23 and

10 cm s.

The full-depth CTD was mounted within a multisampler rosette carrying 24

10 lNiskin bottles. Over the upper 500 m, samples were drawn at nominal depths of 500,

400, 300, 200, 100, 75, 50, 25 and 10 m. Samples were analysed for salinity, dissolved

oxygen, nutrient concentrations and #uorescent pigment content. Further samples

taken from the ship's seawater supply, pumped from a depth of 5 m, were analysed for

salinity and #uorometric pigments for SeaSoar calibration. Salinity was obtained by

comparison with IAPSO standard seawater with a Guildline autosal salinometer.

Dissolved oxygen was determined by Winkler titration with amperometric endpoint

detection. The nutrients nitrate plus nitrite, phosphate and silicate, were determined

on a Chemlab autoanalyser. Chlorophyll a and phaeopigments were analysed#uorometrically with a Turner Designs#uorometer model 10-000R. Surface samples

and a few select station pro"les were fractionated into(200 m and(20 m size

classes. Full details of data acquisition, methods and calibration techniques are given

by Pollard and Read (1995).

The data presented here were collected over a period of 35 days. This is su$ciently

near-synoptic to allow us to map the parameters while recognising that some shifts

in frontal positions will have taken place during the course of the survey. Repeat

stations and sections provide an estimate of the potential errors in doing this.

Comparison of temperature/salinity relationships between CTD and SeaSoar

data show that in regions of weak currents changes in water mass structure occurred

over distances up to 10 km. Stronger currents were associated with greater variability

and water mass structures had moved by an average of 11 km and a maximum of

50 km.

Maps were obtained by interpolating CTD data from a chosen depth onto a 30 km

square grid using a bivariate function. Values for each grid point were approximated

by proportionately weighting all the data within a 500 km radius and calculating

a normalised mean. The resulting maps were compared to hand-drawn contours toensure the method produced realistic results.

Sections were produced by linearly interpolating full-depth CTD data onto stan-

dard depths, or for the undulating SeaSoar data, by averaging onto standard depths.

The latter was possible with the large volume of close-spaced data provided by Sea-

Soar. With data on standard, regular, grid spacing, sections were contoured by linear

interpolation between grid points.



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4. Results

Our presentation of the results is divided into four sections. We describe the spatial

distribution of parameters, "rst with maps generated from discrete samples and CTD

data, then with vertical sections that focus on the two seasoar transects. More detaileddescriptions of the vertically integrated chlorophyll are followed by analysis of the

nutrient ratios. In the "nal section we discuss our interpretation of the relationships

between phytoplankton (chlorophyll a), macro-nutrients and the dynamics of the

region.

4.1. Surface structure from the CTD survey

Data from the shallowest CTD rosette bottle sample are mapped to show thespatial distribution of potential temperature, salinity, nitrate#nitrite, silicate and

chlorophyll a (Fig. 2). Samples were drawn from depths between 6 and 20 m. A few

bottles mis"red, leaving 83 samples. Fractionated chlorophyll (Fig. 2f ) was obtained

by analysis of pumped sea water samples taken during CTD stations. These were all

from the same 5 m depth. The total chlorophyllafrom the 5 m samples was compared

with the surface CTD samples. Only minor di!erences in concentration were found (of

order 0.1 mg m), and the spatial distribution of chlorophyll a was near identical.

The CTD samples are used here to provide direct correlation with the nutrient

samples.Potential temperature shows the general hydrographic structure during the survey

(Fig. 2a). The ARF is marked by the enhanced horizontal temperature gradients

between 18 and 19.53C. Although the general trend of the front is west to east, the

front forms two large meanders with southern extensions centred at about 37 and

453E. The STF is marked by a strong temperature gradient between 14 and 15.53C.

The location of the SAF is less clear than that of the fronts to the north. From both

sub-surface and surface structure, it was found to lie between 43 and 443S, centred

more or less along the 10.53C isotherm. The drop in temperature and increase in sili-

cate (Fig. 2d) in the southeast corner of the survey (453S, 473E) indicate the in#uence

of the Antarctic Circumpolar Current although there was no major frontal feature to

represent the PF.

The ARF, STF and SAF converged from west to east across the survey area,

forming a very pronounced feature at about 433S, 453E. The ARF then separated and

turned to the north in a large meander, leaving the Subtropical and Subantarctic

Fronts to continue to the east in a strong uni"ed front.

The fronts, as identi"ed from potential temperature, are superimposed on the other

parameters as a spatial reference to the features. Nitrate (#nitrite) values were lessthan 0.2 mol lin Subtropical Waters (Fig. 2c). The ARF was not apparent since its

nitrate content (derived from tropical regions) is indistinguishable from that of other

Subtropical Waters. Nitrate increased southwards with sharp changes in concentra-

tion across the Subtropical (1}5mol l) and Subantarctic (10}14 mol l) Fronts.

The highest nitrate values (about 20 mol l) occurred at the extreme Southeastern

end of the survey area.



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Fig. 2. Maps derived from samples from 5}20 m depth: (a) potential temperature, the 10.53, 14.53 and

18.53C isotherms are highlighted to represent the Subantarctic (SAF), Subtropical (STF) and Agulhas

Return (ARF) Fronts and overlaid on the other maps, (b) salinity, (c) nitrite plus nitrate, (d) silicate,

(e) chlorophylla, (f) fractionated chlorophyll a netplankton (200}20m) distribution.

The distribution of phosphate is essentially the same as that of nitrate so is not

shown. Some di!erences occur giving rise to deviations from the N : P Red"eld ratio

(Fig. 3).

In contrast, the spatial distribution of silicate (Fig. 2d) is noticeably di!erent from

that of nitrate. Subtropical Water to the north of the ARF exhibits low concentrations

(up to 2.2 mol l). But between the ARF and STF silicate is depleted (less than



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Fig. 3. Potential temperature vs. (a) chlorophyll a, (b) nitrite plus nitrate, (c) silicate. Inset on (b)

nitrate : phosphate. Samples as in Fig. 2.

1.5 mol l), co-incident with nitrate depletion. However, in the modi"ed Sub-

antarctic Water region between the STF and SAF, where nitrate exceeds 5 mol l,

silicate remains depleted. Only in the convergent Southeastern region do surface

silicate concentrations increase to 3 mol l. South of the SAF, silicate concentra-

tions are higher, peaking in two areas, an eddy-like feature centred at about 43.53S,

453E and along the eastern end of the southern survey line. In both these locations

concentrations of silicate reach 4.4 mol lor more.

Between the fronts are a number of cold and warm core eddy-like features. These

show up more clearly in salinity (Fig. 2b) than temperature, as the latter is more

readily modi"ed by air}sea interactions. One eddy, J, with a clear surface signature

lies between the STF and ARF. It apparently originated from the STF to the west of

the survey region and its core has a temperature (about 17.53C) and salinity (34.90)

typical of Subtropical Water modi"ed by Subantarctic Water. The temperature

exhibits evidence of warming since the eddy detached from the STF, but the salinityremains more or less unmodi"ed. In contrast, the feature K, at 41.53S, 473E, is colder

(about 173C) but less fresh (35.15) although clearly still of subtropical origin. However,

the structure is on the edge of the survey area, and it is not clear whether this is an

eddy or a meander in one of the fronts of the region. Neither of these features exhibits

marked chlorophyll or nutrient signatures. However, another eddy to the north of the

ARF at 383S, 463E (L; 193C, 35.10), also of western STF origin, is marked by distinctly



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elevated chlorophyll concentrations (about 0.6 mg m), low silicate (less than

0.2 mol l) and slightly higher nitrate concentrations (0.4}0.6 mol l) than the

surrounding Subtropical Waters. Immediately to the south of this eddy is a meander,

M, in the ARF which encloses a pool of warmer, salty water (19.53C, 35.60) at 403S,

453E. It is marked by elevated silicate but very low chlorophyll concentrations. Thesecharacteristics show that the water originated from the north of the ARF.

Highest surface chlorophylla concentrations, in the range 0.5}0.8 mg m, (Fig. 2e),

are found between the STF and SAF, reaching a maximum (1.25 mg m) at the

strongest convergence. To the northeast, within the region dominated by Subtropical

nutrient-depleted water, there is evidence of a patch of higher chlorophyll water

associated with eddy `Laas described above. A map of size fractionated chlorophyll

distribution (Fig. 2f) shows that larger netplanktonic diatoms (200}20 m) are con-

"ned to discrete patches (up to 0.13 mg m

) within the relatively high chlorophyllSTF frontal region. However, this fraction represents only about 12}15% of the total

biomass for peak concentrations. Everywhere, therefore, nanoplankton (less than

20 m) dominate, which may include smaller diatoms, although this fraction is typi-

cally characterised by thecate and athecate #agellates and picoplankton. The overall

scenario, therefore, is a ubiquitous distribution of small cells, and once chlorophyll

concentrations exceed about 0.3 mg m, there is an emergence of larger cells,

probably diatoms.

The relationships between chlorophyll, nitrate and silicate for surface waters are

emphasised by plotting these parameters for the entire survey region against temper-ature (Fig. 3). Maximum chlorophyll a concentrations fall between 10}173C and

coincide with minimum silicate values but non-limiting nitrate concentrations charac-

teristic of modi"ed Subantarctic Water between the STF and SAF. In warmer

Subtropical Waters (warmer than 173C), low chlorophyll a is associated with min-

imum nitrate but with silicate concentrations of up to 2 mol l. However, in cold

Subantarctic Waters (colder than 103C), low chlorophyll a is associated with max-

imum nitrate and silicate concentrations. This illustrates the HNLC paradox. The

N : P ratio (N"20.36 P } 2.48; r"0.85; n"78) for surface waters (inset Fig. 3b)

shows that nitrate is very slightly limiting relative to phosphate at low concentrations

and that there is some excess of nitrate over the Red"eld ratio of 16 : 1.

4.2. Vertical structure shown by SeaSoar, CTD and discrete sample data

Two SeaSoar sections were completed during the SWINDEX survey. The same

lines were also worked with full-depth CTD pro"les and sampled for nutrients and

chlorophyll (as well as ADCP).

The "rst SeaSoar section (Figs. 4 and 5) was worked from 44.53S, 44.13E northeastto 38.83S, 48.53E. It crossed the convergent STF/SAF front at approximately 42.53S,

453E (line AB in Fig. 1) and sampled the chlorophyll peak seen on the CTD data map

(Fig. 2e). Potential temperature and ADCP vectors show the strength and structure of

the convergent STF/SAF front (Fig. 4). They also show a broad band of northward

#ow separated from Subtropical Water by a second frontal feature, the ARF, at about

39.53S. Comparison of the vertical section with the maps of potential temperature and



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Fig. 4. SeaSoar section AB (48E) showing ADCP vectors from 101 m rotated through 223 as indicated:

(a) potential temperature; (b) #uorescence calibrated to chlorophylla; (c) Brunt}VaKisaKlaK frequency. Labell-

ing indicates the Agulhas Return Front (ARF), converged Subtropical/Subantactic Front (STF/SAF) and

features K and M from Fig. 2.

salinity (Fig. 2) shows that the northward #ow is due to a large meander in the ARF

that runs almost parallel to the SeaSoar section. The data imply that immediately to

the west of the section the convergent front was even more marked with the addition



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Fig. 5. CTD section AB (48E) showing photosynthetically active radiation: (a) chlorophylla and sample

points; (b) nitrite plus nitrate; (c) silicate. Fronts as indicated in Fig. 4.

of the ARF. However, the ARF clearly crosses the SeaSoar section at the northern

end. These two fronts, the ARF and STF/SAF, e!ectively divide the section into

three di!erent regimes from north to south: (a) Subtropical, (b) Agulhas and



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(c) Subantarctic. The shallow patch of warm water at 42.03S is an extension of the

meander (M in Fig. 2b) in the ARF.

The other parameters mirror these divisions. Only low levels of #uorescence

are seen north of the ARF and the maximum is deep, 70}80 m. The lack of visible

#uorescence in surface waters can probably be ascribed to daylight quenching duringthis part of the transect (compare with PAR in Fig. 5). Similarly, at the south end of

the section there is a subsurface #uorescence maximum covering the range 20}80 m,

but extending at lower concentrations to as much as 120 m. Conspicuously, there is

a marked absence of #uorescence in the top 20 m of this region despite nighttime

coverage. Within the intermediate zone of Agulhas water,#uorescence values are both

signi"cantly higher (up to about 1.5 mg m) and extend to the surface in places,

concurrent with nighttime. However, the general subsurface maximum of about

1.0 mg m

occurs primarily at 40 m depth.The Brunt}VaK isaK laK frequency (a measure of the vertical strati"cation Fig. 4c)

has maxima at two depths throughout the section. North of the converged STF/SAF

front, the deeper maximum (at 80}90 m) represents the base of the winter mixed layer,

and the shallower maximum at 30}40 m is associated with summer strati"cation.

However, to the south, in Subantarctic Water, the bottom of the winter mixed layer is

marked by a temperature minimum of 43C, here barely seen at 200 m. The change in

temperature is compensated by the salinity gradient so there is no associated pycno-

cline. Thus, both strati"cation maxima (60}80 m and at 20 m) are the result of

seasonal e!ects: wind mixing and solar heating, where the latter is more recent. Theformation of the shallower pycnocline at 20 m is likely to be assisted by the northward

Ekman #ux, capping the surface layer with water of a more southerly origin.

The 0.6 mg m #uorescence contour (from Fig. 4b) has been overlaid on the

Brunt-VaK isaK laK frequency plot. This demonstrates that the #uorescence maximum has

a di!erent relationship with the strati"cation north and south of the converged

STF/SAF front. To the north, the #uorescence maximum is associated with the upper

strati"ed layer, which coincides also with the nutricline for both nitrate and silicate

(Fig. 5). South of the front, the chlorophyll maximum lies between the two strati"ed

layers. The distribution south of the STF/SAF probably results from a sequence of

deeper wind mixing followed by surface heating. The well-de"ned southern extent of

the surface #uorescence signal probably results from the eastward currents of the

converged front and the northward Ekman #ow. Daylight quenching of the #uor-

escent signal can be discounted because this part of the section was completed at

night.

The high surface#uorescence signal apparent at about 42.83S is spatially coincident

with the convergent front (STF/SAF), but sits above a single maximum in density

structure at 40 m depth. The gap in #uorescence immediately to the north is asso-ciated with the meander (M in Fig. 2b) evident in the potential temperature section.

The same #uorescence contour has also been overlaid on plots of extracted

chlorophyll a, nitrate and silicate obtained from CTD bottle samples (Fig. 5). Note

that while the bottles sampled some of the main chlorophyll features, much of the

structure was missed and it seems likely that there was considerably more structure

than observed in the nutrient concentration gradients as well.



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Fig. 6. CTD and sample pro"les from: (a) station 12722 in the STF/SAF front on section AB (42.553S

45.383E) and (b) station 12765 from the Subantarctic Zone on section CD (42.153S 39.063E). Station

positions are shown in Fig. 1.

The convergent STF/SAF front is marked by a strong nitrate gradient (Fig. 5b)

from 2 mol l in Subtropical Water and reaching 18mol l in Subantarctic

Water. However, silicate shows only a gradual increase in water shallower than 80 m

(Fig. 5c). A more marked front appears in deeper water where higher concentrations of

both silicate and nitrate are associated with colder water (less than 43C). Despite high

nitrate and adequate surface silicate concentrations (about 3}4 mol l) in water

south of the front, there is little chlorophyll (typically less than 0.4 mg m) associated

with it. The ADCP current vectors show a considerable horizontal shear across the

converged front and dome of nutrients, implying a jet-like feature. In common with

other such features, it may well be that there are signi"cant vertical motions (e.g.

Pollard and Regier, 1990; Allen and Smeed, 1996) which account for both the domingof nutrients and the deeply di!used chlorophyll concentrations.

Modi"ed Subtropical Water between the ARF and the convergent STF/SAF front

exhibits a subsurface chlorophyll maximum at approximately 40 m. Warm surface

Subtropical Water evident to the north of the ARF (Fig. 4) is characterised by deep

(80}160 m) and low chlorophyll concentrations of less than 0.45 mg m (Fig. 5).

Nutrient concentrations in the surface layer were very low.



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Fig. 7. SeaSoar section CD (38E) showing ADCP vectors from 101 m rotated through 413 as indicated:

(a) potential temperature; (b) #uorescence calibrated to chlorophylla; (c) Brunt}VaK isaK laK frequency. Fronts

as indicated in Fig. 4.

Interpolation in creating maps and sections across a narrow convergent front of

steep gradients can mask signi"cant "ne scale detail in hydrographic and biological

variables. The station (12722) worked within the frontal feature at 42.53S is plotted as

a pro"le (Fig. 6a) to show the unusual nature of the surface layer. Low salinity water



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Fig. 8. CTD section CD showing photosynthetically active radiation: (a) chlorophyllawith sample points;

(b) nitrite plus nitrate; (c) silicate. Fronts as indicated in Fig. 4.

extended to 25 m forming a very pronounced pycnocline above several interleaved

layers of warm/salt, cool/fresh water. Temperature and salinity of the surface layer fell

between Subtropical and Subantarctic Water (to the north and south, respectively)



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indicating mixing across the front, whereas the water below 100 m had the character-

istics of Subtropical Water. The e!ect of the Subantarctic component in the surface

layer was to increase the nitrate and silicate concentrations relative to stations to the

north, in Subtropical Water (not shown). Associated with this, chlorophyll concentra-

tions reached a peak of 1.25 mg m.The second SeaSoar section (Figs. 7 and 8) was worked in two parts. The SeaSoar

was deployed at 41.43S, 38.53E and towed southeast to 43.43S, 40.13E. Four CTD

stations were occupied back along the same line. As the section had not intersected the

STF, SeaSoar was re-deployed at 41.53S, 38.63E, and towed northwest to the next

CTD station at 40.93S, 38.13E (line CD on Fig. 1). There is therefore an overlap in

space and a gap in time between 41.4 and 41.53S (as demonstrated by the PAR data in

Fig. 8).

The section is of interest primarily for the physical hydrographic contrasts with the"rst SeaSoar section. As described above, section AB crossed the ARF, STF and SAF

at a strong convergence, giving rise to sharp temperature and nitrate gradients and

current speeds of up to 1 m s. In contrast, these fronts were widely spaced across the

more westerly CD section, with weaker gradients and much weaker current speeds

(see Figs. 2, 4, 5, 7 and 8). Temperature and salinity gradients show that the entire

structure of section CD (over 200 km), between 40.9 and 43.53S, was compressed

within only four SeaSoar pro"les (i.e. over about 16 km) at 42.83S on section AB. So

section CD reveals much more structure in the upper water column between the STF

and SAF.The section crossed at least part of the STF at about 41.53S, as shown by the hori-

zontal gradient in potential temperature (Fig. 7a), which increased from about 143C

to the south to 163C to the north. Throughout the section vertical streamers or

ribbons of alternate warm and cool water occurred. These correlated with interleaving

of fresh and saline water seen as maxima and minima in salinity (not shown). The

structure implies vertical movement as well as both vertical and horizontal mixing.

The #uorescence maxima (Fig. 7b) extend closer to the surface than in section AB

(Fig. 4b), with a deeper, less-intense subsurface maximum to the south. Comparison

with surface data (Fig. 2a) show that the SeaSoar section stopped to the north of the

SAF, yet the #uorescence and Brunt}VaK isaK laK frequency (Fig. 7c) showed the charac-

teristics expected across the SAF. The section, therefore, must have ended very close

to the front. A patch of high surface #uorescence (equivalent to about 1.5 mg m)

appears at the northern end of the section within the STF. Overall, the #uorescence

signal (Fig. 7b) is characterised by both vertical and horizontal patchiness, probably

associated with interleaving water masses as described above.

The Brunt}VaK isaK laK frequency (Fig. 7c) lacked the obvious double structure evident

in section AB (Fig. 4c). There was a pronounced maximum in strati"cation, the depthof which varied between 40 and 70 m. Above this, there was evidence of a much

weaker strati"ed layer in places, e.g. between 41.5 and 42.03S at a depth of about 30 m.

The 0.6 mg m #uorescence contour (from Fig. 7b) overlaid on the Brunt}VaK isaK laK

frequency plot shows that the maximum #uorescence concentration occurred above

the strongest strati"cation, e.g. at 413S. In contrast, to the south the subsurface

#uorescence maximum weakened and deepened along the line of weaker strati"cation.



17/28

Extracted chlorophyll a surface samples (Fig. 8a) yield values of about 0.6}0.7

mg m, which are signi"cantly lower than those found at the convergent front (Fig.

5a). Nutrient distributions also di!er (Fig. 8b and c). Both vertical and horizontal

nutrient gradients are weaker than in the compressed frontal region (Fig. 5). Surface

nitrate and silicate concentrations are also about 50% lower between the STF andSAF, suggesting perhaps that compression of the isopycnals in the east gives rise to

upwelling of nutrient-rich deeper water. Fig. 8 also suggests that the STF is marked by

a signi"cant nitrate gradient, but silicate gradients are associated more with the SAF

and deeper, colder water. The 43C water apparent at 200 m in section AB (Fig. 5)

responsible for elevated silicate concentrations was absent at 200 m in section CD,

where the coolest water was 6}73C and consequently the silicate concentration was

only about 9 mol lat 200 m and less than 1 mol lat the surface.

One pro"

le, from station 12765 (Fig. 6b) is shown to compare the water columnstructure between the STF and SAF with that of the converged STF/SAF feature

further east (Fig. 6a). Temperature and salinity are colder and fresher throughout

the top 200 m. There is a strong pycnocline just below 40 m with interleaving of

fresher and saltier water between 40}120 m indicating mixing of Subantarctic

and Agulhas Water, but the pycnocline is weaker. Surface nutrient concentrations

are very similar while the increase with depth through the intrusions of Subantarctic

Water is greater than in the warmer saltier water of the converged front. But

this is insu$cient to increase the chlorophyll concentrations, which are higher

than in water to north and south, but lower than that in the converged front to theeast. From the data available, it would appear that in this situation, the increased

stability at 40 m at the converged front is the factor promoting greater phytoplankton

growth.

In conclusion, section CD has weaker gradients than section AB and shows ex-

tensive interleaving and mixing of water masses. The separate Subtropical and Sub-

antarctic Fronts are associated with increases in nitrate and silicate respectively,

whereas to the east both increase together at the convergent front. Although section

CD shows vertical strati"cation, it is weaker and lacks the double structure of section

AB. Thus the #uorescence is more patchy, and the pronounced maximum seen on

section CD is absent on AB.

4.3. Integrated chlorophyll distribution

For both sections AB and CD, #uorescence and chlorophyll concentrations were

integrated over the top 200 m, based on both SeaSoar and extracted sample data

(Fig. 9). Since the sample data were used to calibrate SeaSoar #uorescence it is

comforting to see a general correspondence in the magnitudes of the data. It is clearthat the SeaSoar data show much more structure, indicating small scale patchiness.

On section AB (48E) SeaSoar data show a gradual increase from north, south to

41.53S. A sharp dip marks the warm meander `Ma (Fig. 2b). The high chlorophyll

patch associated with the converged STF/SAF is unremarkable. The integrated

peak is no higher than that to north or south. Likewise, south of the SAF

the integrated values are not signi"cantly di!erent from the rest of the section. Section



18/28

Fig. 9. Integrated chlorophyll from CTD and#uorescence from SeaSoar over the top 200 m: (a) section AB

(48 E) and (b) section CD (38E). Fronts and features as indicated in Fig. 4.

CD (38E) shows a similar signal. The patch of high chlorophyll at 413S in the STF

peaks at about the same value as that in the converged STF/SAF (48E). Despite

changes in community structure with latitude, the integrated value averages about

40 mg mexcept at the northern end of AB in Subtropical Water. High, near-surface

chlorophyll concentrations in Agulhas Waters are therefore compensated by the

deeply mixed lower concentrations found in Subantarctic Waters. It appears that

HNLC waters south of the SAF have biomass equal to frontal water and higher than

Subtropical Water. However, the paradox remains in that the macro-nutrient con-

centrations of Subantarctic Waters could support much higher concentrations of

biomass.

4.4. Nutrient ratios

In open ocean waters, Red"eld et al. (1963) and Dugdale and Wilkerson (1998) have

demonstrated that the atomic ratio of occurrence of N : Si : P is typically

16 : 16 : 1, although for the Southern Ocean there are signi"cant departures from

the Red"eld ratio due to anomalies in N : Si : P uptake by diatoms and N : Si

recycling rates (QueHguiner et al., 1997; de Baar et al., 1997). Strong phytoplankton



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Table 1

Nutrient ratios for upper ocean water masses

n 95% con"dence interval for

O!set Slope

Subtropical N"!2.65#2.11Si 172 !3.09}!2.21 1.90}2.31

Agulhas N"!1.65#2.78Si 99 !2.41}!0.89 2.50}3.06

STF/SAF N"5.60#2.83Si 102 4.10}7.09 2.36}3.31

Subantarctic ((40 m) N"11.70#2.12Si 49 10.55}12.86 1.76}2.48

Subantarctic (40}200 m) N"17.00#0.62Si 74 16.49}17.49 0.53}0.68

species-speci"c preferences also exist for di!erent nutrients. Diatoms prefer high Si : N

(Sommer, 1994a, b), blue}green algae prefer low N : P (Howarth, 1988) and dino#agel-

lates prefer low N and low P (Margalef, 1978). In surface waters, N : Si : P is due

largely to uptake by phytoplankton, but in deeper waters, N : Si : P largely re#ects

re-mineralisation processes and can signify di!ering water masses.

We have divided the SWINDEX data into regions, giving N : Si ratios for

Agulhas, Subtropical, modi"ed Subantarctic (between the STF and SAF) and Sub-

antarctic water masses, to examine changes in the nutrient ratios due to biological

activity. We have concentrated on the N : Si ratio because of its implications fordriving the diatom/non-diatom community succession, with implications for food

chains and CO

sequestration.

Discrete CTD nutrient samples between the surface and 200 m were used, corre-

sponding to the maximum depth distribution of extracted chlorophyll. Stations were

grouped into di!erent water mass regions on the basis of their common T/S character-

istics. Straight lines were "tted to each group of nitrate and silicate data using the

orthogonal distance regression algorithm (Boggs et al., 1987, 1989) that is appropriate

when all variables have signi"cant errors (Table 1).

Three major points emerge from the nutrient data and regression analyses. These

concern N : Si ratios, potential nutrient limitation and source water nutrient

concentrations. Firstly, the slope of the regression provides the disappearance ratio of

these two nutrients from the surface waters. We consider this to be equivalent to the

molar ratio of biological uptake assuming that nitrate and silicate re-mineralisation

processes in the upper 200 m are minimal relative to the uptake rates. The coe$cients

(Table 1) were all tested at the 95% con"dence interval. The o!sets were all signi"-

cantly di!erent from zero and the slopes were all signi"cantly di!erent from the 1 : 1

Red"eld ratio of occurrence.The slopes give a disappearance ratio of 2}3 : 1 (N : Si) except in Subantarctic

Water 40}200 m deep. The clear implication is of excess nitrate uptake relative to

silicate uptake everywhere. As this is more than double the 1 : 1 Red"eld ratio, this

further implies that more than half of the nitrate uptake is by phytoplankton not

utilising silicate. This largely precludes diatoms and is therefore indicative of

nitrate utilisation by smaller phyto#agellates not requiring silicate. Size fractionated



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chlorophyll a shows that the smaller size group accounted for 85% or more of the

total phytoplankton biomass.

Secondly, from our data and bearing in mind the N : Si depletion ratios dis-

cussed above, we can also draw some inferences about which of the two nutrients is

likely to be, or potentially becomes, limiting with particular respect to diatom growth.In this context, it is useful to consider the half saturation constants (K

) frequently

used to de"ne limiting nutrient concentrations for diatoms. For a model of diatom

growth in the Southern Ocean, Pondaven et al. (1998) usedK"0.3}0.8 mol lfor

nitrate uptake and K"0.83 mol l for silicate uptake. In surface (5}20 m) Sub-

tropical and Agulhas waters, nitrate concentrations are zero or very close to zero

while silicate concentrations remain within the range 1.3}0.6 mol l. Nitrate is

therefore limiting diatom growth, and the utilisation of the residual silicate and nitrate

accounts for the almost complete absence of diatoms in this region. Between theSubtropical and Subantarctic Fronts, nitrate fell in the range 6}12 mol l and

silicate in the range 0.5}2.0 mol l. Based on K

arguments, silicate is already

limiting optimal diatom growth (

Documents

2000 - Phytoplankton, Nutrients and Hydrography in the Frontal Zone Between the Southwest Indian Subtropical Gyre and the Southern Ocean