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Zonal variability in primary production and nitrogen uptake ratesin the southwestern Indian Ocean and the Southern Ocean
Naveen Gandhi a,b,n, R. Ramesh a,A.H. Laskar a, M.S. Sheshshayee c, Suhas Shetye d, N. Anilkumar d,Shramik M. Patil d, Rahul Mohan d
a Geosciences Division, Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, Indiab Present address: Centre for Climate Change Research, Indian Institute of Tropical Meteorology, Pashan, Pune 411 008, Indiac Department of Crop Physiology, University of Agricultural Sciences, GKVK Campus, Bangalore 560 065, Indiad Natioanl Centre for Antarctic and Ocean Research, Headland Sada, Vasco-da-Gama, Goa-403 804, India
a r t i c l e i n f o
Article history:
Received 26 March 2011
Received in revised form
23 February 2012
Accepted 7 May 2012Available online 19 May 2012
Keywords:
The Southern Ocean
Zonal variability
Light
New production13C15N
a b s t r a c t
Hydrographic parameters along with the primary and new production measurements were carried out
during the austral summer, 2009, in the southwestern Indian Ocean and Indian sector of the Southern
Ocean (SO). The production varies from 185 to 4900 mg C m2 d1 in different zones of SO. The zonal
variations in production accompany variations in SST, salinity and nutrients. Further, the new
production (0.3 to 4.1 mmol N m2 d1) covaries with the overall production, while the uptake of
reduced forms of nitrogen (both NH4and urea) show opposite trends. In the NO3limiting environment
(north of subtropical convergence), NH4uptake dominates the total regenerated production, whereas,
urea uptake dominates the regenerated production under Si, light and micronutrient (e.g., Fe) limiting
conditions (found between the subtropical convergence and Antarctica). On the basis of the C and N
uptake data, the studied region can be divided into five zones (from the south to the north)viz., located
between (i) the Antarctic continent and the polar front (Antarctic zone; ANZ), (ii) the polar and
subantarctic fronts (SAF) (Polar frontal zone; PFZ), (iii) SAF and Agulhas Retroflection fronts (ARF)
(South Subtropical front; SSTF), (iv) subtropical frontal zone (STFZ), and (v) ARF and the north
subtropical front (Subtropical zone; STZ). Except at SSTF, regenerated production dominates in allthe zones. From the south to the north, this could be due to different reasons e.g., light, grazing by
zooplankton, supply of key micronutrients (probably Fe), Si-limitation, or NO3-limitation. In the
absence of such limitations, the maximum possible f-ratio in SO could be as high as 0.7870.12 and
under such conditions the region could export most of the total production to the deep. Supply of
micronutrients through the Agulhas return current and from the Crozet Island supports the higher chl
a, C uptake and new production at the 481E transect relative to the 57.51E transect. The C:N assimilation
ratio is found to be 5.64, marginally lower than the canonical Redfield ratio. This slight difference is
likely due to the variation in the composition of phytoplankton and NO 3-limitation in some zones.
A comparison with earlier results shows that seasonal and spatial variations in f-ratios in these zones
are much higher than its inter-annual variability.
& 2012 Elsevier Ltd. All rights reserved.
1. Introduction
The Southern Ocean (SO) includes the southernmost waters of
the world ocean, south of 601S surrounding Antarctica. It differs
from the other oceans in that its northern boundary does not
adjoin a landmass, but merges into the Atlantic, Indian and Pacific
Oceans. The water mass characteristics of over 50% of the world
ocean by volume are due to processes that occur within SO. It acts
as a physical link between the Pacific, Indian and Atlantic oceans,
exchanging heat and momentum, and is the major source for thedensest deep water in the global ocean (e.g., Matear and Hirst,
1999;Caldeira and Duffy, 2000).
1.1. Circumpolar frontal zones
In SO, zonal variations in specific water properties have been
used to classify regions whose edges are defined by fronts, where
there are rapid changes in water properties that occur over a short
distance (Gordon et al., 1977). SO is comprised of several oceanic
frontal systems, viz., the North Subtropical Front (NSTF), Agulhas
Retroflection Front (ARF), South Subtropical Front (SSTF), Sub-
antarctic Front (SAF) and Polar Front (PF) (Orsi et al., 1995;Belkin
Contents lists available at SciVerse ScienceDirect
journal homepage: www.elsevier.com/locate/dsri
Deep-Sea Research I
0967-0637/$- see front matter& 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dsr.2012.05.003
n Corresponding author. Tel.: 91 20 2590 4453.
E-mail address: [email protected] (N. Gandhi).
Deep-Sea Research I 67 (2012) 3243
http://www.elsevier.com/locate/dsrihttp://www.elsevier.com/locate/dsrihttp://localhost/var/www/apps/conversion/tmp/scratch_7/dx.doi.org/10.1016/j.dsr.2012.05.003mailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_7/dx.doi.org/10.1016/j.dsr.2012.05.003http://localhost/var/www/apps/conversion/tmp/scratch_7/dx.doi.org/10.1016/j.dsr.2012.05.003mailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_7/dx.doi.org/10.1016/j.dsr.2012.05.003http://localhost/var/www/apps/conversion/tmp/scratch_7/dx.doi.org/10.1016/j.dsr.2012.05.003http://localhost/var/www/apps/conversion/tmp/scratch_7/dx.doi.org/10.1016/j.dsr.2012.05.003http://www.elsevier.com/locate/dsrihttp://www.elsevier.com/locate/dsri -
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and Gordon, 1996). The southern ACC front (SACCF) is the third
circumpolar front south of the PF (Orsi et al., 1995). North of the
ACC is the subtropical convergence or subtropical front (STF),
usually found between 351S and 451S, where the average Sea
Surface Temperature (SST) changes from about 12 to 78 1C and
salinity decreases from greater than 34.9 to 34.6 or less (e.g.,
Srivastava et al., 2007). The Antarctic Convergence is approxi-
mately 200 km south of the Polar Front.
In the Indian Ocean sector, the Agulhas Return Front (ARF)marks the southern extent of the subtropical gyre (Fig. 1) and is
formed from the retroflection of the Agulhas Current south of
Africa. The Agulhas Return Front is identified in surface water as
far east as 561E(Holliday and Read, 1998). These frontal systems
subdivide the sector into zones with different biogeochemical
characteristics,viz., the Subtropical Zone (STZ), Subtropical Fron-
tal Zone (STFZ), Subantarctic Zone (SAZ) and Polar Frontal Zone
(PFZ). A fourth zone, the Continental Zone, and the westward
flowing AntarcticCoastal (or Polar) Current are located even
farther poleward, between the Southern Front and the Antarctic
continent. SST poleward of 651S is about 1.0 1C(Deacon, 1984).
1.2. Previous studies on primary production in SO
Primary production in SO is highly variable spatially and
temporarily (Sullivan et al., 1993). During spring and summer,
phytoplankton blooms are frequently observed in coastal regions,
near the ice edge, in polynas, and at the frontal zones. Despite the
high abundance of major nutrients in surface waters, the main
body of the Antarctic Circumpolar Current is characterized by low
levels of biomass and primary production, typically o0.5 mg chl
am3 and 300 mg C m2 d1, respectively (e.g., Holm-Hansen
et al., 1977; El-Sayed, 1978; Sakshaug and Holm-Hansen, 1984;
Holm-Hansen and Mitchell, 1991).
In the Indian Sector of SO, different zones show wide ranges of
production during the austral summer (e.g., Jasmine et al., 2009).
The STZ, and NSTF show low production ( 200 mg C m2 d1).
The production in STFZ, ARF, SAF, and SAZ lies in the range
300400 mg C m2 d1. The SSTF shows the highest production
(4900 mg C m2 d1) while the lowest is found at the SPF and
PFZ (o200 mg C m2 d1). Despite high concentrations of N and
P throughout the year, the regions south of SSTF exhibit low
production. Therefore, these regions of SO, like the subarctic and
equatorial Pacific, are termed as high-nutrient, low chlorophyll
(HNLC) regions (e.g.,El-Sayed, 1984).
Despite the climatic importance of SO, its unique ecosystem
and associated resources, new production measurements in theregion are sparse in both space and time. A summary of NO3, NH4and urea uptake rates measured earlier using 15N tracer techni-
ques is given inTable 1.Slawyk (1979)was the first to use 13C and15N tracers to measure nitrogen and carbon uptake rates around
the Kerguelen Island, SO. The specific uptake rate of NO3 was
related to temperature and no light-limitation effect over C and N
uptake rates was observed (Slawyk, 1979).Mengesha et al. (1998)
found oligotrophic conditions, with a large spatial variation of
phytoplankton biomass and community structure, in the Indian
sector of SO. During spring, the specific and absolute uptake rates
of NO3dominate over NH4and urea uptakes. They also concluded
that seasonal shift in N uptake regime can occur with, as well as
without, marked changes in community structure. To fully under-
stand the functioning of its ecosystem, more such data from
different seasons and different parts of SO are needed. The
present study is an attempt to add to this database and gain
some knowledge on the zonal and meridional variability of C and
N uptake rates in this region.
2. Materials and methods
Sampling was done in SO mainly along the 57.51E (referred as
Transect A henceforth) and 481E (Transect B henceforth) long-
itudes to detect meridional and zonal variations in the physical,
chemical and biological parameters during the austral summer
(FebApr, 2009). SST and salinity were measured at every one-
degree interval. SST was measured using a bucket thermometer.
Salinity (in units of psu) measurements were performed on-board
Fig. 1. Monthly mean (a) sea surface temperature (SST; obtained from MODIS-aqua) and (b) chlorophyll (obtained from SeaWiFS) for the period Feb to April 2009 is
plotted with sampling locations (divided in two Transects A and B) in the different frontal zones of SO. For details see text. Fronts are redrawn from Holliday and Read
(1998).
N. Gandhi et al. / Deep-Sea Research I 67 (2012) 3243 33
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using an Autosol with an external precision of 0.001. The vertical
profiles of temperature and salinity for the sampling locations
were obtained using a CTD-Seabird Electronics Sea sat, SBE 911
Plus, USA. Joint Global Ocean Flux Study (JGOFS) protocol
(UNESCO, 1994) was followed for the estimation of new and
regenerated production using the 15N tracer. Total primary
production was estimated using 13C tracer, following the proce-
dure ofSlawyk et al. (1977). Total primary production, new and
regenerated production measurements were made at 11 locations
(Fig. 1). Out of these, five were along Transect A (57.51E), and six
along Transect B that included four along 481E, one near the
Antarctic coast and the last was just outside the exclusive economic
zone (EEZ) of the Crozet Island (Fig. 1). Water samples were
collected using a CTD rosette fitted with Niskin bottles. Six sampling
depths were chosen to cover the euphotic zone with light intensity
corresponding to 100, 80, 64, 20, 5 and 1% of the surface irradiance.
A Biospherical, photosynthetically-active-radiation (PAR) sensor
(QSP-2300 S/N 70102) attached to a portable conductivity, tem-
perature, and density (CTD) sensor system (SBE 19plus) was used for
the measurement of irradiance levels.
Individual water samples were collected in pre-washed poly-carbonate bottles (Nalgene, USA) for NO3(2 L volume), NH4 (2 L)
and urea (1 L) enrichment experiments, each in duplicate. Gen-
erally, carbon tracer experiments were carried out together with
the NO3 tracer experiments. Samples were also collected at each
station for blank corrections for each tracer (for more details see
Gandhi et al., 2010a,2011a).
Prior to incubation at 10.00 h local time, tracers containing 99
atom% 15N (Na15NO3, 15NH4Cl and
15NH2CO15NH2, procured
from SigmaAldrich, USA) and 13C (NaH13CO3, procured from
Cambridge Isotope Laboratories, Inc. USA) were added to the
bottles. NO3 (Na15NO3) tracer was added at less than 10% of the
ambient NO3 concentration. Ambient NH4 and urea could not be
measured because of logistic reasons; so very small, constant
amounts of NH4 (15NH4Cl) and urea (15NH2CO15NH2) tracerswere added to a final concentration of 0.01 mM. Slawyk (1979)
found NH4 concentrations between 0.1 and 1.05mM in Indian
sector of SO. Assuming similar concentrations for urea also, the
added tracer concentrations of NH4 and urea in this study were
low (o10% of the ambient value). A constant amount
(to a final concentration of 0.2 mM) of carbon tracer was added
with the NO3 tracer.
After the addition of tracers, incubation was performed for
four hours as per the JGOFS protocol (UNESCO, 1994). To simulate
the irradiance at the depths from which samples derived, well-
calibrated neutral density filters were put on the sample bottles.
Subsequently sample bottles covered with neutral density filters
were kept in a big plastic tub on the deck and seawater from a
depth of 6 m was circulated to regulate the temperature during
the incubation from 10:00 to 14.00 h local time at each station.
Immediately after the incubation, samples were transferred to the
shipboard laboratory for filtration and were kept wrapped in a
thick black cloth and in dark until the filtration was over.
All samples were filtered in dark, sequentially through precombusted
(4 h at 400 1C) 47 mm diameter and 0.7 mm pore size Whatmann
GF/F filters. Samples were filtered under low vacuum (o70 mm Hg)
using a manifold filtration unit and vacuum pump (procured from
Millipore, USA), within one and a half hour after the incubation.
Filters were dried in an oven at 50 1C overnight and stored for
isotopic analysis on mass spectrometer. For the blank correction,
zero time enrichment was estimated during incubations. For this,
the same concentrations of isotopically enriched tracers as in
samples, were added to the individual blank samples. Immediately
after the addition, the blank samples were filtered and dried for
isotopic analysis (Gandhi et al., 2010b).
A CarloErba elemental analyzer interfaced via Conflo III to a
Finnigan Delta Plus mass spectrometer was used to measure
particulate organic nitrogen (PON) and carbon (POC) and atom%15N (or 13C) in the filters. For nitrogen, calibrated in-house casein
and international standards (NH4)2SO4 (IAEA-N-2) and KNO3(IAEA-NO-3) were used for checking the external precision
(Gandhi et al., 2011a). While for carbon, calibrated in-house
starch and international standard ANU sucrose were used.
The external precisions of the measurements were consistently
better than 0.5%. The maximum differences in the duplicate
mass-spectrometric measurements of PON and POC were found
to be less than 10%. The coefficients of variation in atom% 15N and
atom% 13C measurement were less than 1%.
For the calculation of nitrogen uptake rates, we use the
equation of Dugdale and Wilkerson (1986). The specific uptake
rate (N taken up per unit particulate N) is calculated based on the
isotope ratio of sample measured at the end of the incubation,
V
N
t 15
Nxs=15
Nenr15
NNnt 1
where, 15Nxsis atom% excess in sample after incubation,15Nenris
atom% 15N in the initially labelled fraction, t is the incubation
time, 15NN is natural abundance of 15N. The uptake rate rt
N
(N taken up in concentration unit) is calculated using VtN and
PON at the end of incubation (PONt),
rNt
VNt nPONt 2
The total N-uptake rate was the sum of nitrate, ammonium
and urea uptake rates. Depth-integrated uptake rates were
calculated by trapezoidal integration. New production was con-
sidered equivalent to NO3 uptake rate and regenerated produc-
tion, equivalent to the sum of NH4 and urea uptake rates; f-ratio
(Eppley and Peterson, 1979) was the ratio of new production to
Table 1
Summary of uptake rates (in the unit of mmol N m 2 d1) of NO3(rNO3), NH4(rNH4) and Urea (rUrea) from the different regions of the Southern Ocean measured using15N tracer technique. Some places only surface uptake rates are given, for which units are specified in the footnote.
Region Season qNO3 qNH4 qUrea Reference
Antarctic polar front Summer 0.912.5 12.254.8 0.710.8 Sambrotto and Mace (2000)
Scotia/Weddell Sea NovDec 0.26.7 NA NA Goeyens et al. (1991)
Scotia/Weddell Sea Winter 0.32.5 0.79.7 NA Cota et al. (1992)
Weddell Seaa NovJan 22910 9110 NA Semeneh et al. (1998)
Prydz Baya JanMar 2761 1185 NA Semeneh et al. (1998)Indian sectora JanMar 22715 39716 NA Semeneh et al. (1998)
Indian sectora Mar 0.41.4 NA NA Slawyk 1979
Indian sectora Spring 0.3100 1030 622 Mengesha et al. (1998)
Indian sectora Summer 522 433 NA Mengesha et al. (1998)
Indian sector Summer 0.97.7 0.53.3 0.21.9 Prakash et al. (2012)
Pacific sector Spring 0.97.6 2.13.0 0.51.8 Savoye et al. (2004)
a Surface uptake rates (mmol m3 d1), NA-data not available.
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42.51S towards the south. Similarly, along Transect A, the surface
salinity decreases by 0.5 psu per degree latitude between 42.51S
and 47.51S, towards the south. As in SST, the differences noticed
here are: (i) decline in the surface salinity is much sharper on
transect B than on A, and (ii) the higher gradient in the surface
salinity is shifted a little southwards on transect A relative to
transect B. Such large variability in SST and salinity is the specialfeature of the polar front. An abrupt increase in salinity and SST
towards the north near 401S is associated with the subtropical
convergence, where the sub-Antarctic water meets the warm sub-
tropical water. The differences related to the SST and salinity
variations across the STF along the monitored transects (A and B)
suggest latitudinal variation of the boundary of the subtropical
convergence at different longitudinal regions in the Indian Sector
(Fig. 2). Contour plots of temperature for transect A is shown in
Fig. 3a. Closer lines in temperature contours represent abrupt
changes across fronts. An increase in salinity at surface as well as
in subsurface waters is found at the location (441S, 651E) north of
the polar front (3b). Temperature based mixed layer depth (MLD)
varies between 3065 m and 25110 m along transects A and B,
respectively (Table 2). Effect of variations in temperature and
salinity across SO over nutrients, phytoplankton and assimilations
of C and N is discussed in the following sections.
3.1.2. Nutrients
Nutrient concentrations for the sampling locations are shown
in Fig. 4. All nutrients are high to the south of the polar front,which is a common feature of the region ( Knox, 1970;El-Sayed,
1978). Surface NO2varies from below detection limit to 0.1 mM at
transect A, while it varies from below detection limit to 0.3 mM at
transect B. Similarly, surface concentrations of NO3 varies from
0.1 to 422 mM and from 0.1 to 426 mM at transect A and B,
respectively. At transect A, surface concentrations of PO4and SiO4vary from 0.1 to 1.6 mM and 0.1 to 428 mM, respectively. Surface
PO4 and SiO4 vary from 0.2 to 1.6 and 0.1 to 434 mM, respec-
tively, at transect B. NO3, PO4 and SiO4 concentrations observed
here are comparable to those reported byJasmine et al. (2009)for
summer 2008. At the subtropical convergence, while NO3and PO4remain significantly higher (Fig. 4), NO2 and SiO4 are quite low,
reflecting their assimilation and polymerization by diatoms and
silico-flagellates. Along transect A, south of the polar front, both
Fig. 2. Latitudinal variation of (a) sea surface temperature ( 1C; SST) and (b) salinity (psu) along 57.51E (Transect A) and 481E (Transect B). (psu practical salinity unit)
shown with different frontal zones in SO. For details see text.
Fig. 3. Vertical sections of (a) temperature (1C) and (b) salinity contours along (57.51E) transect A. Different fronts are visible where temperature and salinity show steep
meridional gradients (i.e., where contours are closest together).
N. Gandhi et al. / Deep-Sea Research I 67 (2012) 324336
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NO2 and NO3 are high and decrease towards north. Higher
concentrations of NO2 and NO3indicate underutilization of these
substrates by phytoplankton probably due to the lack of micro-
nutrients (e.g., Fe) (e.g.,Martin et al., 1991;Prakash et al., 2010) or
high grazing pressure in the region (Froneman et al., 2000),
particularly in SAZ and SAF.
It is widely believed that Fe-limitation is the main cause for
the observed low chlorophyll and higher NO3 in the region,
particularly in SAZ and SAF (e.g., Martin et al., 1991). A similar
scenario is observed along transect B. The only difference is that
relatively higher NO2and NO3values are found on transect B than
on A (Fig. 4) (e.g., 561S, 57.51E vs. 541S, 481E). Similar to NO2 and
NO3profiles, along transect A, south of the polar front, both PO4and
SiO4are high and decrease towards the north (Fig. 4). Here also, the
limitation of Fe could be the reason for the observed higher PO4and
SiO4 towards the south. Data on transect B also show a similar
pattern as on transect A. Here too, the only difference observed is
that relatively higher PO4and SiO4are found on transect B than on
A(Fig. 4) (e.g., 561S, 57.51E vs. 541S, 481E).
While looking at nutrient ratios (N:P, N:Si and Si:P), N-limited
conditions (N:Po10 and N:Sio1; Levasseur and Therriault,
1987) are observed in the regions north of 391S at transect A
Fig. 4. Vertical sections of nutrients (NO2, NO3, PO4 and SiO4) concentrations (mM) along transects A and B.
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The ANZ that includes SO-S4, SO-S5, SO-S6 and SO-S7, also exhibits
very low productivity ( 190 mg C m2 d1; n 4), while the STFZ
that includes SO-S2, and SO-S10, and PFZ that includes SO-S3 and
SOS8, show moderate production of 215mg C m2 d1 and
340 mg C m2 d1, respectively.
The observed variations in the productivity for different zones
are consistent with the earlier observations nearby, of Jasmine
et al. (2009)along the 451E in 2004. Mixing of subtropical waters
and subantarctic bottom waters takes place at the SSTF, whichenhances the nutrient levels and the productivity in this region.
Odate and Fukuchi (1995)also found higher chl a here. Nutrient
limitation at the STZ results in low production, while light could
be the reason for low production at ANZ (Jasmine et al., 2009).
The region PFZ, south of the SSTF, shows low production despite
higher N and P in the surface. Grazing by zooplankton and
probably Fe-limitation probably limits production in this zone
(Jasmine et al., 2009). The region STFZ, north of the SSTF, mostly
shows characteristics of subtropical water (low nutrients) and
low production. Although the subtropical convergence shows
higher production than other regions of SO, it is still not among
the higher productive zones of worlds oceans. This could be due
to limitation of Si during the late summer, evidenced by low N:Si
and Si:P.
3.3.2. Nitrogen uptake rate
3.3.2.1. New production. Euphotic zone integrated new production,
shown inFig. 8a and b, varies from 0.62.4 mmol N m
2
d
1
(withan average of 1.3 mmol N m2 d1) on transect A. A significant
higher new production is also observed near the Antarctic coast
(SO-S6). New production on transect B varies between 0.34.1 mmol
N m2 d1 (with an average of 1.7 mmol N m2 d1). The average
new production on transect B is higher that on A. This is in line with
the higher chl a and C uptake rates at transect B relative to A.
Further, it shows the importance of the Agulhas return current and
the Crozet Island on new production, which is a measure of export
production on annual time scales.
Our values are lower than those reported for the Antarctic polar
front (0.912.5 mmol N m2 d1; Sambrotto and Mace, 2000) and
for the Pacific sector (0.97.6 mmol N m2 d1; Savoye et al.,
2004). Among the five zones, the highest new production is found
to be 4.1 mmol N m2 d1 for the SSTF and the lowest value
(0.4 mmol N m2 d1) is associated with the STZ (Fig. 9).
C uptake rates are also the highest and the lowest at the SSTF
and STZ, respectively (Fig. 7). Such higher new production values
associated with the front have also been reported bySambrotto
and Mace (2000)andSavoye et al. (2004). New production of the
SSTF is similar to the reported bySavoye et al. (2004)in the same
zone. The ANZ and PFZ show moderate new production despite
limiting factors such as light (for the former) and probably Fe
(for the latter). Lower NO3 levels are the main cause for the
observed very low new production at the STFZ and STZ regions.
3.3.2.2. Regenerated production. Euphotic zone integrated NH4uptake rate varies from 0.81.6 mmol N m2 d1 (with an
average of 1.1 mmol N m2
d1
) along transect A, and between0.81.5 mmol N m2 d1 (with an average of 1.1 mmol N m2
d1) along transect B (Fig. 8 c and d). This indicates that no
significant longitudinal variation exists for the NH4 uptake rates,
unlike in the case of new production. Instead, NH4uptake shows a
latitudinal variation, it decreases towards south along both
transects. Overall, NH4 uptake is 20% and 40% lower than that
new production along transects A and B, respectively (Fig. 8c and
d). The values are much lower than those reported for the
Antarctic Polar Front (12.254.8 mmol N m2 d1; Sambrotto
and Mace, 2000), for the Pacific sector (2.13.0 mmol N m2
d1; Savoye et al., 2004), and for the Scotia/Weddell Sea
(0.79.7 mmol N m2 d1;Cota et al., 1992).
The highest NH4 uptake rate is found to be 1.5 mmol
N m2
d1
for the STZ and the lowest (0.8 mmol N m2
d1
),
Fig. 7. Average carbon uptake rates in different zones (STZ, STFZ, SSTF, PFZ and
ANZ; see text for the details) of SO. Number of sampling locations (n) for the
respective zones are shown on the x -axis. Errors associated with the values are
1-sigma standard deviation.
Fig. 6. Euphotic zone integrated carbon uptake rates at different sampling locations along (a) Transect A and (b) Transect B. Errors associated with the values are 1-sigma
standard deviation.
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associated with the ANZ (Fig. 9). An increase is observed in NH4uptake rate from zone 1 to 5 (Fig. 9).
Euphotic zone integrated urea uptake rate varies from0.91.8 mmol N m2 d1 (with an average of 1.3 mmol N m2 d1)
along transect A (Fig. 8e). It varies between 0.92.2 mmol N m2
d1 (with an average of 1.3 mmol N m2 d1) along transect B
(Fig. 8f). This indicates that no significant longitudinal variation
exists in the urea uptake rate, as in the case of NH4 uptake. Urea
uptake also shows a latitudinal variation but unlike NH4 uptake, it
increases towards the south along transect A (Fig. 8e). However,
though SO-S6 shows the lowest urea uptake, it shows the highest
NH4 uptake. Along transect B, urea uptake rates are comparable at
all locations, except at SO-S9. Overall, urea uptake rates are
comparable to new production and higher than NH4 uptake along
both transects. This shows the importance of urea as a potential
nitrogen source for phytoplankton in the region despite the ample
NO3 concentrations, particularly in the south of SSTF. Urea uptake
rates presented here are comparable to the values reported for the
Pacific Sector (0.51.8 mmol N m2 d1;Savoye et al., 2004).
The total regenerated production (the sum of NH4 and ureauptake rates) varies from 2.32.7 and 1.83.3 mmol N m2 d1
along transects A and B, respectively. The highest total regener-
ated production (3.3 mmol N m2 d1) is associated with the
SSTF, which also shows the highest new production. This could
be due to the presence of smaller phytoplankton under Si-
limitation. The regenerated production varies from 2.0 to
2.6 mmol N m2 d1 in the other four zones with an increasing
trend from north to south. The regenerated production values are
slightly higher than new production over SO.
Small size phytoplankton confers a competitive advantage for
nutrients at low concentrations (Leynaert et al., 2004). On the
other hand, as the smaller phytoplankton are susceptible to
grazing by micro-zooplankton (Raven, 1986), their growth pro-
motes production that is based on regenerated nutrients and
Fig. 8. Euphotic zone integrated NO3, NH4and Urea uptake rates at different sampling locations along (a) Transect A and (b) Transect B. Errors associated with the values
are 1-sigma standard deviation.
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hence limits carbon export from the sunlit layers. Lower surface
Fe concentrations in the south of SSTF during late summer are
reported byChever et al. (2010). Jasmine et al. (2009) reported
that zooplankton grazing can consume as high as 89% of the daily
primary production in some areas of SO. In addition, they also
reported that most of the region south of SSTF dominated by
small phytoplankton during late summer in 2004. Similar condi-
tions could be possible in the Indian sector during late summer
2009, which promote growth of small phytoplankton. This leads
to a higher grazing/recycling rate and hence the higher urea based
production in the south of SSTF.
Oligotrophic condition is found to be the main reason for the
dominance of small phytoplankton in the north of SSTF (Jasmine
et al., 2009). In the present study too, such conditions appear tobe the main factors for the low new production and high
regenerated production in the north of SSTF.
3.3.2.3. f-ratio. Euphotic zone integrated f-ratio varies from 0.2 to
0.5 with the highest value at the location SO-S3 along transect A,
while the f-ratio varies from 0.1 to 0.6, with the highest value at
SO-S7 along transect B. The location near the Antarctic coast
(SO-S6) also shows a higher f-ratio (0.6). These values are
significantly lower than those reported by Collos and Slawyk
(1986) for the Indian Sector along 66.51E, as high as 0.98 from a
station at 431S. The SSTF exhibits the highest f-ratio (0.6); the
lowest value, 0.1, is observed over STZ. The PFZ and SAZ show
similarf-ratios (0.5), while the STFZ shows a very low (0.2)f-ratio.Different compositions of plankton populations (species and size)
at these zones could also be a factor for the observed variation in
thef-ratios (Savoye et al., 2004). Their physiological characteristics,
such as preference for NO3or NH4, could vary considerably, even
within the groups, which would probably lead to such differences.
As discussed earlier, unavailability of NO3could be the reason for
the observed high regenerated production and hence low f-ratios
in the STZ and STFZ. Further, growth of nano-phytoplankton likely
due to Fe-limitation and grazing pressure at PFZ and light-
limitation at ANZ lead to low f-ratios. Despite the supply of
ample nutrients, subtropical convergence shows moderately
highf-ratio, and is much lower than that reported byCollos and
Slawyk (1986). Values reported here are comparable to those of
the Pacific (0.50.6; Savoye et al., 2004) and the Indian sectors
(0.20.6; Mengesha et al., 1998) of SO. The reason for moderatef-ratio could be due to the Si-limitation. Si-limitation is known to
play an important role in regulating nitrate uptake rates (Sambrotto
and Mace, 2000), which results in low f-ratios. Dominance of small
phytoplankton under such conditions also results in lowf-ratios and
provides upper bound for f-ratios and drawdown of CO2 from the
atmosphere. Nevertheless, average f-ratio (0.5) indicates that SO is
capable of significant new/export production.
To detect any temporal and/or seasonal variation in thef-ratiosin different zones, we have plotted the average f-ratios from these
zones obtained from earlier studies (see Fig. 10). PNZ and ANZ
show higherf-ratios during austral spring, which could be due to
the supply of nutrients, including iron by the melting of ice during
spring (Sambrotto and Mace, 2000). No significant variations in
f-ratios have been seen in these zones in austral summers of
different years (Fig. 10), except in STFZ. As the spatial extent of
sampling was limited during the present andPrakash et al. (2012)
studies in STFZ, it may not be reasonable to detect trends in this
zone. Overall, the comparison shows that the f-ratios vary
spatially and significantly during different seasons.
The plot of total N uptake (TN; on x-axis) versus nitrate uptake
(NP; ony-axis) reveals a very significant correlation between the
two: NP(0.7870.12) TN(1.5070.47) (r20.83, significant at
0.01 level:Fig. 11). The slope of the regression line suggests that
the maximum possible value of f-ratio for this zone is 0.78 in
summer. This shows that although the production over a large
region of SO is quite low, thef-ratio is moderately high. Therefore,
the region is capable of exporting upto 78% of the total production
to the deep under favorable conditions (e.g., supply of Fe and
other limiting micronutrients). The x-intercept of the regression
line is the minimum amount of regenerated production in the total
absence of extraneous nitrate supply (Kumar et al., 2004), which is
1.5 mmol N m2 d1 (equivalently 100 mg C m2 d1; using the
average observed C:N of 5.64 (seeFig. 12)).
3.3.2.4. C:N assimilation ratio. Fig. 12 shows the plot of total N
uptake (TN; on x-axis) versus carbon uptake (TC; on y-axis) atdifferent depths at all the sampling locations. The ratio of carbon
uptake to the nitrogen uptake (C:N assimilation ratio) varies from
o1 to 410, large variation from the canonical Redfield ratio
(C:N 6.6), at different depths and locations. There is no trend in
the ratio either with depth or laterally. However, the plot reveals
a very significant correlation between the two: TC (5.6470.22)
Fig. 9. Average nitrogen (NO3, NH4 and Urea) uptake rates in different zones
(STZ, STFZ, SSTF, PFZ and ANZ; see text for the details) of SO. Number of sampling
locations in the respective zones are marked on the x-axis. Errors associated with
the values are 1-sigma standard deviation.
Fig. 10. A comparison of average f-ratios (the ratio of uptake rate of NO3 to the
total N uptake rate) obtained during different studies in different zones (STZ, STFZ,
SSTF, PFZ, ANZ; see text for the details) of the Indian sector of SO.
N. Gandhi et al. / Deep-Sea Research I 67 (2012) 3243 41
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NG thank the Director, IITM, Pune. This is also NCAOR contribu-
tion No. 18/2012.
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