Journal of Plankton Research Vol.21 no.11 pp.2019–2035, 1999
Size-fractionated primary production studies in the vicinity of the
Subtropical Front and an adjacent warm-core eddy south of Africa
in austral winter
P.W.Froneman, C.D.McQuaid and R.K.Laubscher
Southern Ocean Group, Department of Zoology and Entomology, Rhodes
University, Box 94, Grahamstown, 6140, South Africa
Abstract. Results are presented of size-fractionated primary production studies conducted in the
vicinity of the Subtropical Front (STF), an adjacent warm-core eddy, and in Sub-antarctic waters
during the third South African Antarctic Marine Ecosystem Study (SAAMES III) in austral winter
(June/July) 1993. Throughout the investigation, total chlorophyll (Chl a) biomass and production
were dominated by small nano- and picophytoplankton. No distinct patterns in total Chl a were
evident. At stations (n = 7) occupied in the vicinity of the STF, total integrated biomass values ranged
from 31 to 53 mg Chl a m–2. In the vicinity of the eddy, integrated biomass at the eddy edge (n = 3)
ranged from 24 to 54 mg Chl a m–2 and from 32 to 43 mg Chl a m–2 in the eddy (n = 2). At the station
occupied in the Sub-antarctic waters, total integrated biomass was 43 mg Chl a m–2. Total daily integrated production was highest at stations occupied in the vicinity of the STF and at the eddy edge.
Here, total integrated production ranged from 150 to 423 mg C m–2 day–1 and from 244 to 326
mg C m–2 day–1, respectively. In the eddy centre, total integrated production varied between 134 and
156 mg C m–2 day–1. At the station occupied in the Sub-antarctic waters, the lowest integrated production (141 mg C m–2 day–1) during the entire survey was recorded. Availability of macronutrients did
not appear to limit total production. However, the low silicate concentrations during the survey may
account for the predominance of small nano- and picophytoplankton. Differences in production rates
between the eddy edge and eddy core were related to water column stability. In contrast, at stations
occupied in the vicinity of the STF, the control of phytoplankton production appears to be related to
several processes, including water column stability and, possibly, iron availability.
Introduction
The Southern Ocean, with its large expanse of cold sea water, is potentially a huge
sink for atmospheric CO2 (Siegenthaler and Sarmiento, 1993). Recent efforts
have attempted to model the physical factors governing the sequestering capabilities of the ocean using the diffusibility of CO2 through the sea–air interface,
CO2 solubility in relation to sea surface temperatures and the movement of ocean
currents (Siegenthaler and Sarmiento, 1993). The biological component involved
in the sequestration of atmospheric CO2, the so-called ‘biological pump’, is,
however, more difficult to model as the distribution of living organisms in the
ocean demonstrated a high degree of spatial/temporal variability (Longhurst,
1991). It is well documented that frontal regions in the Southern Ocean are generally characterized by elevated biological activity in austral summer; few studies
have been conducted in winter (Laubscher et al., 1993; Froneman et al., 1995;
Bradford-Grieve et al., 1997). Central to the carbon cycle of the oceans is the
process of photosynthesis by phytoplankton (Siegenthaler and Sarmiento, 1993;
Falkowski et al., 1998; Legendre and Michaud, 1998). Understanding the
processes that control phytoplankton production and seasonal variability in the
Southern Ocean is, therefore, of central importance in understanding the role of
the ocean in the global carbon cycle.
© Oxford University Press 1999
2019
P.W.Froneman, C.D.McQuaid and R.K.Laubscher
The Subtropical Front (STF) is one of the major oceanic fronts of the Southern Ocean and separates Sub-antarctic waters in the south from Subtropical
waters in the north (Lutjeharms and Valentine, 1984, 1988; Tomczak and
Godfrey, 1994). The front is characterized by a sharp gradient in temperature and
salinity, and as a consequence represents an important biogeographical barrier to
the distribution of phytoplankton, zooplankton (Deacon, 1982; Froneman et al.,
1995, 1997a; Pakhomov and Perissinotto, 1997) and seabirds (Abrams, 1985;
Pakhomov and McQuaid, 1996). The region of the front typically exhibits chlorophyll (Chl) biomass enhancement (Allanson et al., 1981; Sullivan et al., 1993;
Weeks and Shillington, 1994; Froneman et al., 1995, 1997a). Several hypotheses
have been proposed to account for the elevated chlorophyll concentrations
recorded in the vicinity of the front, including passive transport (Lutjeharms and
Walters, 1985; Franks, 1992) and increased in situ production resulting from localized water column stability (Laubscher et al., 1993; Bradford Grieve et al., 1997).
The interaction of the Agulhas Retroflection Current (ARC) with the northern boundary of the STF in the region south of Africa results in the formation
and shedding of eddies (Lutjeharms and Valentine, 1988; Duncombe Rae, 1991).
These eddies subsequently move southwards across the STF, transporting heat,
salt and nutrients into the surrounding environment (Duncombe Rae, 1991; van
Ballegooyen et al., 1994). Biological studies conducted in the region of these
eddies have shown that they are also important in transferring plankton
communities across the strong biogeographical barrier represented by the STF
(Deacon, 1982; Froneman et al., 1997b; Pakhomov and Perissinotto, 1997). In
addition, the eddies are characterized by elevated primary production resulting
from the increased thermal stability at the warm-core eddy edge (Dower and
Lucas, 1993). Van Ballegooyen et al. (1994) has shown that up to nine rings may
be shed from the ARC annually, suggesting that the increased phytoplankton
production associated with the eddies may contribute significantly to the regional
production.
Studies conducted in the region of the STF south of Africa during austral
summer have shown that the front is characterized by elevated primary production (Dower and Lucas, 1993; Laubscher et al., 1993). Little information on the
seasonal trends in production in the region of the STF are, however, available.
Here we present the results of size-fractionated primary production studies
conducted in the region of the STF and an adjacent warm-core eddy shed from
the Agulhas Retroflection in late austral (June/July) winter 1993.
Method
The data were collected aboard the MV SA ‘Agulhas’ during the third cruise of
the South African Antarctic Marine Ecosystem Study (SAAMES III) conducted
to the region of the STF and across a warm-core eddy shed from the Agulhas
Retroflection in June–July 1993 (Figure 1). Water samples were collected with a
12 3 8 l Niskin bottle rosette from depths corresponding to 100, 50, 25, 10, 5 and
1% subsurface light levels. Light depths were determined at 08:00 h each morning
using a Li Cor spherical quantum sensor.
2020
Size-fractionated primary production
Fig. 1. Position of primary production stations occupied in the vicinity of the Subtropical Front and
an adjacent warm-core eddy during the third South African Antarctic Marine Ecosystem Study
(SAAMES III) conducted in June–July 1993. STF, Subtropical Front.
Macronutrient concentrations were measured on board using a Technicon II
Autoanalyser following the methods of Strickland and Parsons (1968) and
Mostert (1983). Unfortunately, data on the concentrations of the macronutrients
(nitrite, nitrate and ammonia) are only available for the surface waters (± 5 m
depth). However, data collected during a previous study conducted in the region
south of Africa showed a good correlation between surface macronutrient
concentrations and those found in the upper water column (R.K.Laubscher,
personal communication). Dissolved inorganic carbon was calculated using the
potentiometric titration method of Almgren et al. (1983).
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P.W.Froneman, C.D.McQuaid and R.K.Laubscher
Chlorophyll a and phaeopigments were extracted in 90% acetone and calculated from fluorescence readings on a Turner Model III fluorometer (Parsons et
al., 1984) calibrated with pure chlorophyll a (Sigma). Three size classes of phytoplankton, micro- (200–20 µm), nano- (20–2.0 µm) and pico- (2.0–0.45 µm), were
gently separated by serial filtration (vacuum pressure < 5 cmHg).
Conductivity, temperature and pressure measurements were made using a Neil
Brown Mk III CTD attached to a rosette sampler. Unfortunately, due to technical problems, no density data were available. Mixed-layer depths were, as a
consequence, assumed to correspond to the top of the thermocline. Thermocline
depths were obtained from CTD temperature traces. Daily surface photosynthetically active radiation (PAR; 400–700 nm) was measured using a Li Cor
4p spherical quantum sensor (LI-1935A). A Li Cor Data Logger (LI-1000)
logged 10–15 min means throughout the day. Estimates of size-fractionated
phytoplankton production rates were carried out following the JGOFS protocol
(JGOFS Report No. 6, 1990). Replicate 250 ml aliquots from each specified light
level were collected in polycarbonate bottles. All manipulations were carried out
under low-light conditions to prevent light shock. 14C (Amersham) was added to
each polycarbonate bottle to give a specific activity of 25 µCi ml–1. Non-specific
14C uptake by the primary producers and possible organic 14C contamination
present in the 14C stock solution were accounted for by removing and immediately acidifying 1 ml aliquots from the 50 and 10% light depths. Samples were
incubated under simulated light conditions in an incubator cooled with surface
waters.
Production rates were extrapolated to 24 h using ambient light data. Areal
productivity and chlorophyll a (extending to 1% of surface irradiance, corresponding to depth ranging between 73 and 86 m) were obtained by trapezoidal
integration. Photosynthetic capacities of the phytoplankton were calculated by
dividing the production rate by the corresponding chlorophyll a concentration.
Results
Photosynthetically active radiation
Daily PAR fluctuated widely during the investigation. Generally, the lowest
values (range 10.5–30.7 mol m–2 day–1) were recorded at stations associated with
the eddy and in the Sub-antarctic waters (Table I). Exceptions were recorded at
Stations C90 and C101 in the vicinity of the STF where the daily flux was <18 mol
m–2 day–1 (Table I). At stations occupied in the region of the STF, daily flux
ranged from 16.2 to 62.5 mol m–2 day–1 (Table I).
Thermocline depth
A detailed description of the physical oceanography during the survey is
discussed elsewhere (Lutjeharms et al., 1994). At stations occupied within the
region of the STF, a well-developed thermocline was observed in the upper water
column (Figure 2a). Exceptions were recorded at Stations C104 and C109 where
no thermocline was observed in the upper 150 m of the water column (Figure 2a).
2022
18.8
13.7
11.4
11.8
10.8
15.5
11.6
13.7
Subtropical Front
C84
03/07
C90
05/07
C95
06/07
C101
07/07
C104
08/07
C106
09/07
C109
10/07
Sub-antarctic waters
C55
27/06
PAR, photosynthetically active radiation (mol
16.1
17.0
Warm-core ring
C43
25/06
C49
26/06
Temp (°C)
10.7
12.5
12.8
Date
Warm-core edge
C35
24/06
C67
28/06
C79
02/07
Station
m–2
day
35.09
35.51
35.10
34.47
34.73
34.53
34.74
34.75
35.39
35.80
34.59
34.95
34.93
–1);
7.2
8.3
10.3
11.6
8.1
13.2
10.7
6.2
6.1
9.8
10.1
8.7
6.6
Wind (m s–1)
Dm, mixed-layer depth (m).
22.9
62.5
47.9
16.2
17.3
24.8
47.3
27.4
28.6
30.0
30.7
10.5
35.0
Salinity (p.s.u.) PAR
119
84
65
81
69
84
73
91
118
107
87
61
58
Dm
5.7
1.3
1.6
7.5
6.6
8.6
3.1
6.9
4.4
3.2
10.9
5.5
7.6
0.4
0.4
0.4
0.7
0.6
0.6
0.3
0.4
0.5
0.4
1.5
0.5
0.6
2.9
4.2
4.9
3.2
3.1
2.6
6.5
5.0
6.5
5.0
3.9
3.5
3.9
38.1
33.4
48.4
28.5
38.4
38.2
37.9
52.9
32.9
53.3
28.5
54.2
41.3
Surface nutrient (µM) Integrated Chl a
——————————– (mg Chl a m–2)
NO3
PO4
SiO2
Table I. Physicochemical parameters at production stations occupied in the region of the Subtropical Front and a warm-core eddy during SAAMES III in late
austral winter 1993
Size-fractionated primary production
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P.W.Froneman, C.D.McQuaid and R.K.Laubscher
At the station occupied in the Sub-antarctic waters (Station C55), the thermocline depth exceeded the 1% light depth. Thermoclines in the warm-core ring
(Stations C43 and C49) were poorly developed and the upper water column
appeared well mixed. At these stations, the mixed-layer depth also exceeded the
1% light depth (Figure 2b). In contrast, at stations occupied at the edge of the
eddy (Stations C35, C67 and C79), the upper water column was thermally stratified (Figure 2b). Here, the 1% light depth more or less corresponded to the depth
of the thermocline (Figure 2b).
Surface nutrients
Surface macronutrient concentrations in the region of investigation are shown in
Table I. Generally, surface concentrations of nitrate were highest at stations occupied in the region of the warm-core edge and in the region of the STF. Here,
nitrate concentrations ranged between 1.3 and 10.9 µM l–1. Exceptions were
recorded at Stations C84 and C90 located in the STF, where the lowest concentrations of nitrate during the entire cruise were recorded Outside these regions,
surface concentrations of nitrate were <6 µM l –1 (Table I). Phosphate concentrations were highest (range 0.5–1.5 µM l –1) at stations occupied at the warmcore edge. At the remaining stations, phosphate concentrations ranged from 0.3
to 0.7 µM l–1 (Table I). Silicate concentrations were generally highest at stations
Fig. 2. Temperature profiles at primary production stations occupied in (a) the region of the Subtropical Front and (b) across a warm-core ring in late austral winter (June/July) 1993. The 1% light
depth is indicated by a horizontal bar.
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Size-fractionated primary production
in the warm-core eddy (range 5.0–6.5 µM l–1) and lowest in the Subantarctic
waters (2.9 µM l–1). At stations in the STF, silicate concentrations ranged from
2.9 to 6.5 µM l–1, and between 3.5 and 3.9 µM l–1 at stations located at the edge
of the warm-core eddy (Table I).
Integrated chlorophyll a
Total integrated biomass (depths ranging from 83 to 96 m) throughout the survey
showed no clear pattern (Figure 3). At stations located in the vicinity of the STF,
total integrated biomass ranged from 28.51 to 52.99 mg Chl a m–2. At stations
located at the periphery of the eddy and in the eddy, total biomass values ranged
from 32.89 and 45.27 mg Chl a m–2, and between 23.60 and 54.18 mg Chl a m–2,
respectively (Figure 3). In the Sub-antarctic waters, total integrated biomass was
38.12 mg Chl a m–2 (Figure 3).
Throughout the study, total integrated biomass, as suggested by Chl a, was
dominated by picophytoplankton which comprised up to 60% (range 35–61%)
of the total. An exception was recorded at Station C90 (STF) where microphytoplankton dominated integrated Chl a (Figure 3). Picophytoplankton integrated biomass varied between 14.60 and 31.37 mg Chl a m–2 at the STF, between
12.49 and 23.22 mg Chl a m–2 at the eddy edge, and between 23.09 and 27.11 mg
Fig. 3. Integrated chlorophyll a biomass to 1% light level at production stations occupied during the
third South African Antarctic Marine Ecosystem Study (SAAMES III) conducted to the region of
the Subtropical Front in June–July 1993. SAW, Sub-antarctic waters; STF, Subtropical Front; WCE,
warm-core edge; WCR, warm-core ring.
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P.W.Froneman, C.D.McQuaid and R.K.Laubscher
Chl a m–2 in the eddy. Nanophytoplankton was the second most important
contributor to total integrated biomass, comprising between 27 and 38% of the
total. Nanophytoplankton concentrations during the survey ranged from 10.48 to
20.97 mg Chl a m–2. Generally, the integrated microphytoplankton biomass was
<6 mg Chl a m–2 or <10% of the total integrated biomass (Figure 3).
Chlorophyll a profiles for the three size fractions generally demonstrated the
same pattern throughout the study area (Figure 4). At stations occupied in the
warm-core eddy (Stations C43 and C49) and in the Sub-antarctic waters (Station
C55), Chl a was evenly distributed within the euphotic zone (Figure 4). At
stations located at the ring edge, maximum Chl a concentrations were recorded
at depths <1% surface irradiance. Below this depth, a decrease in Chl a concentrations was observed (Figure 4). In the region of the STF, Chl a concentrations
were highest above the 5% light depth. Below the 5% surface irradiance depth,
a dramatic decrease in Chl a was observed. Exceptions were recorded at Stations
C106 and C109 (Figure 4). At Station C106, Chl a concentrations increased with
depth, while at Station C109, the decease in Chl a coincided with 1% light depth
(Figure 4).
Integrated production
Total integrated primary production (depths corresponding to between 68 and
82 m) during the study was highest at stations occupied in the vicinity of the STF
and eddy edge (Figure 5). At STF stations, daily integrated production varied
from 154.1 to 422.3 mg C m–2 day–1, and between 239.2 and 332.8 mg C m–2 day–1
at the eddy edge. Within the eddy, total integrated production ranged from 133.8
to 156.1 mg C m–2 day–1. At the Sub-antarctic station, total integrated production
was 141.4 mg C m–2 day–1 (Figure 5).
With the exception of stations along the periphery of the warm-core eddy and
Station C84, located in the region of the STF, picophytoplankton were identified
as the most important contributors to total production (Figure 5). At the periphery of the eddy, nanophytoplankton were the most important contributors to
total production, while at Station C84 microphytoplankton dominated total
production (Figure 5). Integrated picophytoplankton and nanophytoplankton
production during the survey ranged from 73.4 to 256.3 mg C m–2 day–1 and from
51.8 to 180.2 mg C m–2 day–1, respectively. Microphytoplankton integrated
production ranged from 9.0 to 91.2 mg C m–2 day–1, equivalent to <20% of total
production (Figure 5).
Depth profiles of production within the three size fractions demonstrated a
distinct pattern (Figure 6). At stations occupied in the ring (Stations C43 and
C49) and the Sub-antarctic waters (Station C55), production was uniform
throughout the euphotic zone (Figure 6). In contrast, at stations occupied at the
ring edge (C35, C67 and C79), maximum production was recorded at light depths
>50% of surface irradiation. Within the region of the STF, maximum production
within the three size fractions generally occurred at light depths >50% of the
surface irradiation. An exception was recorded at Station C106, where production appeared uniform throughout the water (up to 60 m depth) (Figure 6).
2026
Size-fractionated primary production
Fig. 4. Size-fractionated chlorophyll a profiles at production stations occupied in the region of the
Subtropical Front and across a warm-core ring in late austral winter (June/July) 1993. 1–3, warm-core
edge; 4–5, warm-core eddy; 6–12, Subtropical Front; 13, Sub-antarctic waters.
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P.W.Froneman, C.D.McQuaid and R.K.Laubscher
Fig. 5. Size-fractionated integrated daily phytoplankton production to 1% light level at production
stations occupied during the third South African Antarctic Marine Ecosystem Study (SAAMES III)
conducted to the region of the Subtropical Front in June–July 1993. SAW, Sub-antarctic waters; STF,
Subtropical Front; WCE, warm-core edge; WCR, warm-core ring. Pico, <2 µm; nano, 2–20 µm; micro,
>20 µm.
Photosynthetic capacities
Throughout the investigation, the photosynthetic capacity values (production per
unit chlorophyll at each depth) of the three fractions combined were <1.0 mg C
(mg Chl a) h–1 (Figure 7). Again, a distinct pattern in the photosynthetic capacities was evident during the study. At stations occupied in the Sub-antarctic waters
and in the eddy ring, the photosynthetic capacity values were uniform throughout the upper water column (Figure 7). Here, the values were always <0.3 mg C
(mg Chl a) h–1. In contrast, at stations occupied at the ring edge, maximum photosynthetic capacities were recorded at light depths >25% of the surface irradiation.
Here, the values were >0.5 mg C (mg Chl a) h–1. Below the 25% light depth,
however, the values decreased dramatically. At stations occupied within the STF,
maximum photosynthetic capacity values were generally recorded in waters
corresponding to depths <50% of the surface irradiance. An exception was
recorded at Station C90 where photosynthetic capacities were uniform throughout the water column (Figure 7).
Discussion
The Southern Ocean is a major surface repository of macronutrients, an important region of deep water formation, and a major sink for dissolved carbon dioxide
2028
Size-fractionated primary production
Fig. 6. Size-fractionated production profiles at production stations occupied during the third South
African Antarctic Marine Ecosystem Study (SAAMES III) conducted to the region of the Subtropical Front and across a warm-core eddy in austral winter (June/July) 1993. 1–3, warm-core edge; 4–5,
warm-core eddy; 6–12, Subtropical Front; 13, Sub-antarctic waters.
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P.W.Froneman, C.D.McQuaid and R.K.Laubscher
Fig. 7. Photosynthetic capacities of phytoplankton assemblages at production stations occupied in the
vicinity of the Subtropical Front and across a warm-core eddy during the third South African Antarctic Marine Ecosystem Study (SAAMES III) in austral winter (June/July) 1993. 1–3, warm-core edge;
4–5, warm-core eddy; 6–12, Subtropical Front; 13, Sub-antarctic waters.
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Size-fractionated primary production
(Siegenthaler and Sarmiento, 1993). As a consequence, the role of the Southern
Ocean in the global carbon cycle is the subject of intensive investigation. Despite
its potential importance in the carbon cycle, the Southern Ocean is the basin with
the greatest uncertainty on source/sink behaviour (Attwood and Monteiro, 1994).
This is mainly due to the poor spatial/temporal resolution of published studies
and a poor understanding of the contribution of total primary production to the
different subsystems (Jacques and Fukuchi, 1994). Primary production studies in
regions characterized by elevated production, such as oceanic fronts, are, therefore, of particular interest. Here, we present the results of size-fractionated
primary production studies conducted in the vicinity of one of the major oceanic
frontal systems of the Southern Ocean, the STF during austral winter 1993.
The estimates of total integrated phytoplankton production in the region of the
STF (range from 153.2 to 421.8 mg C m–2 day–1; Figure 5) are in agreement with
several previous studies conducted in the region of the front south of Africa
(Allanson et al., 1981; Dower and Lucas, 1993; Froneman et al., in review) and
other regions (Laubscher et al., 1993; Bradford-Grieve et al., 1997; Clementson et
al., 1998) in different seasons (see Table II). The absence of a seasonal signal is
supported by satellite composite data which show that total biomass at the front
shows no clear seasonality and alternates between periods of high and low concentration (Weeks and Shillington, 1994). While it is evident that water column stability accounts for part of this variance, e.g. Station 101 was characterized by high
water column stability and production (Figures 2a and 5), it is evident that other
factors also limit production. For example, Station C95 was characterized by high
water column stability, but relatively low production (Figures 2a and 5). BradfordGrieve et al. (1997) suggested that production in the region of the STF in spring
near New Zealand may have been limited by the availability of dissolved reactive
silicate. However, Dower and Lucas (1993), in a study conducted south of Africa
in April, suggested that when small cells dominated total production, the concentrations of macronutrients in the region of the front were not limiting total production. As the macronutrient concentrations recorded during this study are in the
same range as those presented in Dower and Lucas (1993), and small cells dominated, total production was apparently not limited by macronutrient availability.
It is possible, however, that the low concentrations of silicate during the study may
account for the predominance of the small nano- and picophytoplankton. Of
particular interest are Stations C84 and C106, which were characterized by the
Table II. Comparative results of seasonal primary production studies conducted in different sectors
of the Southern Ocean in the region of the Subtropical Front
Author
Region
Season
Production rates (mg C m–2 day–1)
Dower and Lucas, 1993
Laubscher et al., 1993
Bradford-Grieve et al., 1997
Bradford-Grieve et al., 1997
Clementson et al., 1998
This study
South of Africa
South of Africa
Pacific sector
Pacific sector
Australasian sector
South of Africa
Autumn
Summer
Summer
Winter
Summer
Winter
±404
±320
977–995
249–275
325
244–326
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P.W.Froneman, C.D.McQuaid and R.K.Laubscher
highest production rates of microphytoplankton during the entire study (Figure 5).
These stations were located at the northern boundary of the STF in close proximity to the ARC (Figure 1). Since the ARC waters have passed close to the
African continent, it is possible that they would contain relatively high concentrations of the trace metal, iron. The transfer of iron-rich waters from the ARC to
the STF could result from interactions between the fronts which result in variability in current flow and cross-frontal mixing (Lutjeharms and Valentine, 1988;
Duncombe Rae, 1991). The higher concentrations of iron would stimulate growth
of the larger microphytoplankton (Martin et al., 1990; de Baar et al., 1995). Unfortunately, we do not have any direct evidence to support this hypothesis.
During this study, integrated primary production in the eddy centre (range from
140.5 to 158.7 mg C m–2 day–1) was less than at the eddy edge (range 239.2–332.0
mg C m–2 day–1; Figure 5). The differences in production between the two regions
are consistent with previous studies conducted in the vicinity of warm-core eddy
shed from the Agulhas Retroflection (Dower and Lucas, 1993). Indeed, a similar
pattern has been observed in the vicinity of eddies located in the Gulf Stream and
Eastern Australian Current (Tranter et al., 1980; Smith and Baker, 1985). Dower
and Lucas (1993) suggested that nutrient concentrations in the vicinity of a warmcore eddy south of Africa were not responsible for the differences in production
between the eddy edge and eddy ring. Our concentrations of macronutrients in
the vicinity of the eddy are in the same range as those reported in Dower and
Lucas (1993), suggesting that the differences in production recorded during this
study were due to other factors. Dower and Lucas (1993) suggested that water
column stability accounted for the differences in production between the eddy
edge and eddy core. Within the ring centre, heat loss to the atmosphere causes
deep convective mixing, while at the eddy edge thermal stability resulted in high
water column stability (Dower and Lucas, 1993). Water column profiles during this
study showed that the upper water column in the eddy centre appeared well mixed,
while at the ring edge clear thermal stratification was observed (Figure 2b). The
difference in hydrological conditions between the two regions is clearly reflected
in the Chl a, production and assimilation profiles. At stations occupying the ring
centre, the values were uniformly distributed throughout the euphotic zone,
suggesting a high degree of mixing within the euphotic zone (Figures 4 and 7). In
contrast, at the eddy edge, the values were highest in the upper surface waters,
indicating water column stability (Figures 4 and 7). The apparent discrepancy
between biomass and production at the ring edge and ring centre can probably be
related to grazing by zooplankton. Grazing studies conducted in parallel to the
production studies showed that the grazing impact was highest at the edge of the
eddy (Froneman and Perissinotto, 1996).
Apart from the production rates recorded in the ring centre, the lowest production was recorded at Station C55 in the Sub-antarctic waters (Figure 4). Here,
total integrated production was estimated at 152 mg C m–2 day–1. It is well documented that Sub-antarctic waters are relatively nutrient rich, suggesting that the
low production rates recorded during this study were not the result of nutrient
availability (Laubscher et al., 1993). The water column profile at this station
showed that the upper water column was well mixed. The extent of the vertical
2032
Size-fractionated primary production
mixing is evident from the distribution of Chl a, which was uniform throughout
the euphotic zone (Figure 5). Here again, it appears that total production was
limited by water column stability. The estimates of production during this study
are lower than those reported in the literature for the same region during austral
summer (Allanson et al., 1981; Laubscher et al., 1993). These facts suggest a
seasonal influence. It is well documented that wind stress in the Sub-antarctic
Zone is stronger in winter than summer (Weeks and Shillington, 1994). As a
consequence of the higher wind activity during winter, thermal stratification
would be broken down, with a resultant increase in the mixing of the upper water
column. Under these conditions, production would be limited as phytoplankton
cells would be subject to a changing light environment (Lewis et al., 1984; Fogg,
1991). The increase in mixing would also account for the predominance of small
cells in the Sub-antarctic waters (Fogg, 1991).
Comparisons can be made between the results obtained during this study and
other regions of the STF. Although the production rates measured during this
study are comparable to those obtained in the region of the STF east of New
Zealand in winter (Bradford-Grieve et al., 1997), the contribution of the various
size fractions to total production differed significantly. Small cells dominated total
production during this study, while in the region of New Zealand, total production
was dominated by microphytoplankton (Bradford-Grieve et al., 1997). Differences
in the results are likely related to hydrology. Indeed, a study conducted in the
region of the STF south of Africa and in the mid Atlantic Ocean showed marked
differences in the hydrology of the STF between the different sectors (Barange
et al., 1998). At face value, the results obtained during this study are comparable
to the findings conducted in the NE subarctic Pacific region in winter (Boyd et al.,
1995). Both regions are characterized by macronutrient concentrations in excess
of phytoplankton growth requirements. Similarly, total phytoplankton production
in winter in both regions appears to be limited by light (Boyd et al., 1995). Despite
these similarities, significant seasonal differences in the factors controlling primary
production exist between the two regions. In the present study, growth rates of
phytoplankton were comparable to summer values (Table II), suggesting that the
factors controlling primary production do not differ seasonally. In contrast, phytoplankton production values in the NE subarctic Pacific region are elevated in
summer (Boyd et al., 1995). These facts suggest that the factors controlling primary
production in the NE subarctic Pacific region differ seasonally.
In conclusion, the results of this investigation showed that the region of the STF
and the periphery of the eddy are characterized by elevated primary production in
winter. Although we do not have any data on the role of physical processes in
sequestering atmospheric CO2 during the study, with the biological data collected,
we can provide some insight into the role of the biological pump in sequestering
atmospheric CO2 during the period of investigation. The efficiency of the biological
pump is critically dependant on the magnitude of primary production and the
subsequent partitioning between the various size classes of herbivorous zooplankton which determines the rate of vertical carbon flux (Longhurst, 1991; Falkowski
et al., 1998; Legendre and Michaud, 1998). Grazing studies conducted in parallel
to this size-fractionated production study demonstrated that protozooplankton
2033
P.W.Froneman, C.D.McQuaid and R.K.Laubscher
and mesozooplankton represented the main sink for daily primary production
(Froneman and Perissinotto, 1996; Pakhomov and Perissinotto, 1997). This partitioning appears largely to be determined by the size structure of the phytoplankton
assemblages which were dominated by small phytoplankton cells (Fortier et al.,
1994; Froneman et al., 1997b). It is well documented that the meso- and protozooplankton retain carbon in the surface waters (Fortier et al., 1994), suggesting that
little carbon was exported to depth. As a consequence, it appears that the
biological pump was relatively inefficient during the period of investigation.
Acknowledgements
We would like to thank the Department of Environmental Affairs and Tourism,
South Africa, and Rhodes University for providing funds and facilities for this
study. Our thanks also go to the officers and crew of the SA ‘Agulhas’ for their
invaluable assistance at sea.
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Received on May 10, 1999; accepted on June 9, 1999
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