2012 - Zonal Variability in Primary Production and Nitrogen Uptake Rates in the Southwestern Indian...

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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
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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.

N. Gandhi et al. / Deep-Sea Research I 67 (2012) 324334


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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).



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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.



10/12

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.



11/12


12/12

NG thank the Director, IITM, Pune. This is also NCAOR contribu-

tion No. 18/2012.

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