JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, C06004, doi:10.1029/2011JC007737, 2012
Contribution of atmospheric nitrogen deposition
to new production in the nitrogen limited photic
zone of the northern Indian Ocean
Arvind Singh,1,2 Naveen Gandhi,1,3 and R. Ramesh1
Received 4 November 2011; revised 10 April 2012; accepted 25 April 2012; published 6 June 2012.
[1] Primary productivity in the sunlit surface layers of tropical oceans is mostly limited
by the supply of reactive nitrogen (Nr) through upwelling, N2 fixation by diazotrophs,
riverine flux and atmospheric deposition. The relative importance of these processes
varies from region to region. Using recent data on the nitrogen content of aerosols
over the ocean and marine new production in parts of the northern Indian Ocean
for the period 1994–2006 CE, a quantitative assessment of the contribution of
atmospheric deposition to new production in the two biogeochemically different basins
of the northern Indian Ocean, viz., the Arabian Sea and the Bay of Bengal, is presented.
By suitably converting the measured concentrations of aerosol nitrogen into fluxes and
comparing them with 15N tracer-based direct new and primary production measurements, it
is inferred that the contribution of atmospheric deposition to new production in the
northern Indian Ocean could at best be 3%. Our estimate of 1.39 Tg N year1 of Nr flux
into the northern Indian Ocean through aerosols is a step toward significantly reducing
the uncertainty in the global nitrogen budget.
Citation: Singh, A., N. Gandhi, and R. Ramesh (2012), Contribution of atmospheric nitrogen deposition to new production in
the nitrogen limited photic zone of the northern Indian Ocean, J. Geophys. Res., 117, C06004, doi:10.1029/2011JC007737.
1. Introduction
[2] Marine primary production (rate of carbon fixation,
measured in mg C m2 day1) is one of the major sinks of
atmospheric carbon dioxide, thus an important regulator of
the Earth’s climate [Sabine et al., 2004]. The rate of transfer
of fixed carbon from the sunlit layers of the ocean (i.e., the
top 100 m) to the deeper layers is termed export production. This is believed to be approximately equal to the ‘new
production’, i.e., the fraction of primary production sustained by the input of new nutrients into the photic zone,
over annual time scales [Eppley and Peterson, 1979]. The
availability of nutrients, especially reactive nitrogen (Nr),
is known to limit the biological productivity in the surface
ocean in many regions [Broecker, 1974]. As nitrogen is an
essential nutrient for the growth of marine biota, we focus
here on understanding the role of atmospheric deposition
of nitrogen in affecting marine productivity. Regionally
atmospheric deposition may have significant impacts on
marine biogeochemistry and could support up to 30% of
1
Physical Research Laboratory, Ahmedabad, India.
Now at Bermuda Institute of Ocean Sciences, St. Georges, Bermuda.
Now at Indian Institute of Tropical Meteorology, Pune, India.
2
3
Corresponding author: A. Singh, Bermuda Institute of Ocean
Sciences, 17 Biological Station, St. Georges GE01, Bermuda.
([email protected])
Copyright 2012 by the American Geophysical Union.
0148-0227/12/2011JC007737
the new production [Spokes et al., 2000]: on the other hand,
atmospheric Si and P inputs may have only minimal impacts
on regional new production as these inputs are usually
smaller than the upward flux from below the sunlit layer
[Krishnamurthy et al., 2010].
[3] Nitrogen enters the surface ocean through a number of
processes; e.g., N2 fixation, upwelling/eddy diffusion, fertilizer use on land and waste discharge, and atmospheric
deposition by aerosols [Galloway et al., 2004]. Molecular
nitrogen (N2) is most abundant in the Earth’s atmosphere,
but it can only be utilized by a specific group of marine
microorganisms (diazotrophs such as Trichodesmium). Most
other organisms assimilate oxidized and reduced forms of
+
nitrogen, i.e., reactive nitrogen (e.g., NO
3 , NH4 ) [e.g.,
Gandhi et al., 2011a]. Upwelling is an important process
through which new nitrogen is introduced to the surface, but
this occurs seasonally in specific regions of the ocean, e.g.,
in the western Arabian Sea during summer [Ryther et al.,
1966]. Nitrogen inputs from fertilizer use on land and
waste discharge are mostly limited to the coastal region
[Seitzinger et al., 2006].
[4] Nitrogen is lost from the ocean through denitrification
and anaerobic ammonium oxidation (anammox) to the
atmosphere, and burial of organic matter in the sediments
[e.g., Kuypers et al., 2003]. Recently, an imbalance between
the nitrogen loss and gain rates has been observed that suggests nitrogen gain could have been underestimated; other
less well–studied or currently unknown processes may exist
and fix substantial amounts of N2 [Codispoti, 2007]. This
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SINGH ET AL.: ROLE OF N DEPOSITION IN NEW PRODUCTION
prompted us to study one such input processes, i.e., atmospheric deposition to the ocean.
[5] As a consequence of increasing anthropogenic activity
such as deforestation, fossil fuel burning and industrialization, the global nitrogen emissions have increased since the
preindustrial era [Galloway et al., 2004; Duce et al., 2008].
Wet and dry deposition of Nr (mainly nitrate and ammonia)
can stimulate phytoplankton production and change phytoplankton community structure and composition, when not
limited by other nutrients such as iron [Mills et al., 2004;
Baker et al., 2007]. The western Arabian Sea, however,
appears to be at least seasonally iron limited region [Naqvi
et al., 2010] despite the supply of iron to this region through
Arabian dust deposition [e.g., Krishnamurti et al., 1998].
The present study primarily focuses on the aerosol nitrogen
influx and its contribution to new production in the northern
Indian Ocean.
[6] The Arabian Sea and the Bay of Bengal, the two
northern Indian Ocean basins, located in similar latitudes, are
significantly different in many aspects: strong winds during
summer monsoon produce intense upwelling in the northwestern Arabian Sea. Nutrients brought up through upwelling enhance surface productivity. Cooler winds during the
winter monsoon, lead to convective mixing in the northeastern Arabian Sea, which brings up nutrients from the deep,
thus increasing surface productivity [e.g., Prakash and
Ramesh, 2007]. In addition, the Arabian Sea receives nutrients through atmospheric transport from Arabia in the west
and the Thar in the east [Krishnamurti et al., 1998]. These
processes make the Arabian Sea one of the most productive
regions in the world [Madhupratap et al., 1996; Smith, 2001;
Kumar et al., 2004, 2010; Prakash and Ramesh, 2007;
Prakash et al., 2008; Gandhi et al., 2010a]. On the other
hand, a large influx of fresh water stratifies the Bay of Bengal
surface, limiting surface productivity [Kumar et al., 2004;
Singh et al., 2010; Singh and Ramesh, 2011]. The sources of
atmospheric nitrogen deposition at these two sites may be
different, but are believed to be mainly from the developing
nations, India and China [Galloway et al., 2004]. Hence,
these two basins of the northern Indian Ocean provide an
ideal marine environment to study the impact of atmospheric
deposition on the surface ocean biogeochemistry.
[7] The contribution of atmospheric nitrogen deposition to
new production is poorly constrained. Patra et al. [2007]
discussed the relative significance of atmospheric deposition over vertical mixing in maintaining chlorophyll abundances in the northern Indian Ocean. Since atmospheric
input is comparable to or less than new production because
most of the production is otherwise fuelled by recycled
nutrients [Eppley and Peterson, 1979]; a comparison of total
primary production with atmospheric deposition might lead
to erroneously large values [e.g., Srinivas et al., 2011].
Spokes et al. [2000] reported that up to 30% of the new
production in the oligotrophic waters of the northeast
Atlantic could be supported by atmospheric nitrogen inputs
in spring. The northern Indian Ocean lacks any such an
estimate. For example, systematic surface productivity
measurements in the Bay of Bengal became available only
during the last decade. In the open Bay of Bengal [Wiggert
et al., 2006], where the supply of nutrients through riverine
influx and upwelling is not significant, atmospheric deposition could be an important source of nitrogen. Kumar et al.
C06004
[2004], based on limited data, suggested that a significant
part of new production in the Bay of Bengal could be due to
atmospheric deposition. Since then, more data have become
available both on new production and atmospheric deposition in various parts of the northern Indian Ocean [Rastogi,
2005; Prakash, 2008; Gandhi, 2010]. Here we assess the
role of atmospheric nitrogen deposition in contributing to
new production in the northern Indian Ocean.
2. Methods
[8] The measured concentrations of nitrate, nitrite and
ammonium in aerosols are converted into dry and wet
deposition fluxes. Dry deposition occurs when particles
settle under gravity whereas during wet deposition particles
are scavenged by precipitation. Productivity is estimated on
the basis of uptake rates of nitrate, ammonium and urea
during photosynthesis.
2.1. Calculation of Dry and Wet Deposition Fluxes
[9] Nitrate, nitrite, and ammonia concentrations in aerosols collected over the Arabian Sea and the Bay of Bengal
were obtained from the literature [Rhoads et al., 1997;
Krishnamurti et al., 1998; Johansen et al., 1999; Gibb et al.,
1999; Sarin et al., 1999; Bange et al., 2000; Rengarajan and
Sarin, 2004; Rastogi, 2005; Kumar et al., 2008]. Aerosol
measurements cover almost the entire Arabian Sea, whereas
a lesser area is covered in the Bay of Bengal (Figure 1). We
have estimated deposition fluxes from the nitrogen compounds’ concentration in the aerosol. Deposition of nitrogen
occurs through gravitational settling (dry deposition) and
precipitation (wet deposition). The dry deposition flux is
given by
Fd ¼ Vd Cd ;
ð1Þ
where Cd is the measured concentration of the compound of
interest in aerosols, and Vd the particle settling velocity,
which depends on complex interactions of various parameters such as wind speed, particle size, relative humidity, and
sea surface roughness [Duce et al., 1991]; as a result Vd has
large uncertainties. Thus, to simplify estimates of deposition
fluxes, a mean value of Vd accounting for the aerosol size
distribution, is frequently used. Vd of 1.5 cm s1 for nitrate
and 0.05 cm s1 for ammonium were used (because Vd varies
with particle size as does the mean size of particles removing
nitrate and ammonium) in these calculations [Schafer et al.,
1993], as was done earlier by Bange et al. [2000].
[10] The wet deposition flux, is given by
Fw ¼ P S Cd ra 1 rw ;
ð2Þ
where P represents the rain rate, S the scavenging ratio and
ra (1.2 kg m3), and rw (103 kg m3), the densities of air and
water, respectively. S = Cr /Cd, where Cr is the concentration
of the compound of interest in rain. S is 330 and 200 for
nitrate and ammonium, respectively [Duce et al., 1991],
calculated based on mass mixing ratios (i.e., both Cr and Cd
are in kg/kg units) [Barrie, 1985]. Equation (2) was updated
by Bange et al. [2000] after Duce et al. [1991]. For the
estimation of the seasonal wet deposition flux, Bange et al.
[2000] used a constant P, (780 mm year1 for the Arabian
Sea, 2550 mm year1 for the Bay of Bengal) as estimated by
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Figure 1. Aerosol sampling locations in the northern Indian Ocean. Legend: solid triangle, Gibb et al.
[1999]; open triangle, Johansen et al. [1999]; diamond, Rhoads et al. [1997]; cross, Krishnamurti et al.
[1998]; solid circle, Sarin et al. [1999]; solid square, Bange et al. [2000]; open square, Rengarajan and
Sarin [2004]; open circle, Rastogi [2005]; asterisk, Kumar et al. [2008]. See auxiliary material for more
details.
Kumar and Prasad [1997]. High interannual and spatial
variability of rainfall results in large uncertainties in wet
deposition fluxes. To minimize these uncertainties, we estimated the wet deposition flux using rainfall data from the
Global Precipitation Climatology Project (GPCP) (available
at http://disc2.nascom.nasa.gov/Giovanni/tovas/rain.GPCP.2.
shtml, latest accessed on September 5, 2011) for each location at the corresponding time (Tables 1–3; see also auxiliary
material).1 GPCP data have an uncertainty of 7.4% [Xie
et al., 2003]. For calculating the total deposition flux, the
areas of the Arabian Sea and the Bay of Bengal are taken
as 4.93 1012 m2 and 2.93 1012 m2, respectively [Bange
et al., 2000]. We have estimated weighted averages of dry
and wet deposition fluxes as there was a large variation in the
data density and the resulting fluxes (Tables 1 and 2). GPCP
data for the corresponding years are presented in Table 3, wet
and dry deposition fluxes can be reproduced using this data
and nitrogen concentrations from the papers cited in Table 1.
2.2. Calculation of New and Primary Production
[11] The 15N tracer technique [Dugdale and Goering,
1967], besides providing an estimate of primary production,
yields an estimate of new production as well (i.e., nitrate
uptake/total nitrogen uptake). Primary production is estimated by summing the photic zone integrated uptake rates of
nitrate, ammonia and urea and multiplied by the Redfield
ratio (C:N:P :: 106:16:1). Because of simultaneous microbial
processes occurring in the sunlit layer of ocean, variation
1
Auxiliary materials are available in the HTML. doi:10.1029/
2011JC007737.
in Redfield ratio (C:N:P varies from 70:10:1 to 200:27:1)
introduces some error in the total production (carbon uptake
rates) estimates [Arrigo, 2005, and references therein]. Since
we compare only nitrogen uptake rates (based on the 15N
tracer technique) with deposition fluxes, such errors are
avoided here. More details of this technique are discussed by
Kumar and Ramesh [2005]. 15N tracer technique-based new
and primary production data in the northern Indian Ocean
have been obtained from published studies [McCarthy et al.,
1999; Watts et al., 1999; Watts and Owens, 1999; Sambrotto,
2001; Kumar et al., 2004, 2010; Prakash et al., 2008; Gandhi
et al., 2010a; N. Gandhi et al., Primary and new production in
the thermocline ridge region of the southern Indian Ocean:,
submitted to Deep Sea Research, Part I, 2012] (Tables 1 and
2 and Figure 2); as in the case of aerosols, measurements
cover most of the Arabian Sea, but are spatially limited in the
Bay of Bengal. As for the aerosol deposition fluxes, we have
estimated the weighted averages of new and primary production, and the contributions (in %) of aerosol deposition to
new production are calculated as the ratios of the sum of dry
and wet deposition fluxes to the new production (Tables 1
and 2).
[12] The error in the productivity measurements is less
than 10% [Gandhi et al., 2011a], overall error would be
larger due to patchiness of the plankton and seasonal and
interannual variations.
2.3. Errors in the Estimates
[13] Estimates of deposition fluxes are more uncertain
than the new production estimates as the former depends
on several variables, e.g., wind speed, relative humidity and
3 of 11
4 of 11
CAS weighted average
WAS
WAS weighted average
BOB
EAS
EAS weighted average
CAS
CAS weighted average
WAS
WAS weighted average
BOB
BOB weighted average
EAS weighted average
CAS
EAS
BOB weighted average
CAS weighted average
WAS
WAS weighted average
BOB
EAS weighted average
CAS
EAS
Region
Jul–Aug 1995
Aug–Oct 1994
Jul–Aug 1995
Jul–Aug 2008
Jul–Aug 1995
Apr–May 2003
Mar–Apr 1995
Mar–Apr 1995
Mar–Apr 1995
Apr 2006
Jan 1995
Nov–Dec 1994
Jan 2003
Feb–Mar 2003
Feb–Mar 2004
Year
16
22
22
ND
42
42
6.1
44
15
5.0c (6)
5.0
3.7 (11)
4.9 (4)
11 (3)
5.2
4.8 (5)
4.8
30
18
18
7.2
7.2
17 (4)
1
6.3
30
8
26
26
ND
ND
8.7
26b
10
14
8
PP
3.3
3.1 (5)
3.1
5.4 (9)
5.4
3.6c (6)
0.7 (8)
1.9
3.3 (3)
2.4
3.2 (17)
3.2
2.3 (6)
12.7 (6)
7.1 (11)
7.3
2.4 (13)
NP
Year
Feb 1997
Feb 1999
Jan 1996
Nov–Dec 1994
Jan 1996
Feb 1997
Sambrotto [2001]
Feb–Mar 2001
Apr–May 1994
Feb–Mar 1995
Apr 1995
May 1995
May 1995
Mar 1997
Mar–Apr 1998
Apr–May 2006
Jul–Aug 1995
Jul–Aug 1995–96
Jul–Aug 1995
Aug–Oct 1994
Summer Monsoon
Watts et al. [1999]
Sambrotto [2001]
Gandhi et al. (submitted
manuscript, 2012)
Sambrotto [2001]
Kumar et al. [2004]
Sambrotto [2001]
Sambrotto [2001]
Spring Intermonsoon
Sambrotto [2001]
Mar 2001
Gandhi et al. [2010a]
Apr–May 2006
McCarthy et al. [1999]
Watts and Owens [1999]
Winter Monsoon
Kumar et al. [2010]
Jan 1996
Kumar et al. [2010]
Prakash et al. [2008]
References
0.002
0.002
104
0.002
0.02
0.02
ND
0.01
0.01 (17)
0.01
0.01
0.01
0.01 (15)
0.01 (20)
104 (10)
0.01 (21)
0.01
104
0.01
0.01
0.001
0.002
0.03
0.01
0.01
0.001
0.002
0.003
0.01
0.11
0.003
0.05
0.00
0.02
0.03
0.03
0.01
0.02
0.01
0.11
0.00 (10)
0.03 (10)
0.04 (9)
0.02
0.02 (10)
0.02
0.03 (6)
0.04 (6)
0.04
0.01 (6)
0.01 (8)
0.01
0.02 (10)
0.05 (8)
0.04 (9)
0.03 (22)
0.03 (28)
0.02 (27)
0.03 (8)
0.01 (11)
0.03
0.11
WN
0.11 (4)
DN
0.01
0.03
0.03
0.01
0.01
104
ND
0.02
0.02
0.01
0.02
0.02
0.02
0.05
0.07
0.04
0.04
0.02
0.03
0.01
0.04
ND
0.22
0.003
0.08
0.04
0.04
0.05
0.05
0.04
0.06
0.05
0.22
TAD
Table 1. New Production, Primary Production, Total Atmospheric Nitrogen Deposition After Dividing the Northern Indian Ocean Into Four Zonesa
Bange et al. [2000]
Sarin et al. [1999]
Johansen et al. [1999]
Gibb et al. [1999]
Rastogi [2005]
Sarin et al. [1999]
Sarin et al. [1999]
Rhoads et al. [1997]
Bange et al. [2000]
Johansen et al. [1999]
Bange et al. [2000]
Rengarajan and Sarin [2004]
Kumar et al. [2008]
Rastogi [2005]
Kumar et al. [2008]
Rengarajan and Sarin [2004]
Rengarajan and Sarin [2004]
Krishnamurti et al. [1998]
Gibb et al. [1999]
Krishnamurti et al. [1998]
Sarin et al. [1999]
Krishnamurti et al. [1998]
References
0.6
0.2
0.4
1.2
1.1
1.6
1.7
3.0
%C
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SINGH ET AL.: ROLE OF N DEPOSITION IN NEW PRODUCTION
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%C
References
ND
ND
ND
ND
TAD
WN
DN
Kumar et al. [2004]
Sep–Oct 2002
EAS
CAS
WAS
WAS weighted average
BOB
BOB weighted average
Nov 1995
1.5 (15)
1.5
2.6 (9)
2.6
ND
ND
11
11
4
4
McCarthy et al. [1999]
Year
Fall Intermonsoon
References
PP
NP
Year
Region
Table 1. (continued)
a
NP, new production (mmol N m2 day1); PP, primary production (mmol N m2 day1); TAD, total (dry + wet, i.e., DN + WN) atmospheric nitrogen deposition (mmol N m2 day1); %C, percentage contribution
of aerosol to NP, i.e., (TAD/NP) 100%; EAS, Eastern Arabian Sea; CAS, Central Arabian Sea; WAS, Western Arabian Sea; BOB, Bay of Bengal; ND, no data available. Data of new production and atmospheric
deposition are obtained from the cited references; see auxiliary material for more details. Values given in parentheses are number of stations/samples.
b
PP is obtained from Kumar [2004]. Same number of samples is used for primary production to the new production unless otherwise specified. Above fluxes are calculated as described in section 2.1.
c
Weighted average of 6 samples from north and southeastern Arabian Sea.
SINGH ET AL.: ROLE OF N DEPOSITION IN NEW PRODUCTION
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Table 2. New Production, Primary Production, Total Atmospheric
Nitrogen Deposition, in the Northern Indian Oceana
Season
Region
NP
PP
TAD
%C
WM
SIM
SM
FIM
WM
SIM
SM
FIM
Arabian Sea
Arabian Sea
Arabian Sea
Arabian Sea
Bay of Bengal
Bay of Bengal
Bay of Bengal
Bay of Bengal
4.8 (53)
2.4 (22)
5.1 (29)
1.5 (15)
ND
5.4 (9)
ND
2.6 (9)
16.4 (53)
12.8 (20)
22.4 (29)
11 (15)
ND
7.2 (9)
ND
4.0 (9)
0.06 (43)
0.04 (137)
0.02 (52)
ND
0.05 (12)
0.02 (21)
ND
ND
1.2
1.7
0.4
–
–
0.4
–
–
NP, new production (mmol N m2 day1); PP, primary production
(mmol N m2 day1); TAD, total (dry + wet) atmospheric nitrogen
deposition (mmol N m2 day1); %C, percentage contribution of aerosol
to NP, i.e., (TAD/NP) 100%; WM, winter monsoon; SIM, spring
intermonsoon; SM, summer monsoon; FIM, fall intermonsoon; ND, no
data available. Units and other details are the same as those in Table 1.
Values in parentheses are number of stations/samples. NP, PP and TAD
can be reproduced taking the weighted average of the data presented in
Table 1.
a
rainfall, each with associated error. New production estimates
are based on the 15N measurements using a mass spectrometer
that is quite precise, however, uncertainties may arise from
scaling up the estimates to large ocean areas.
[14] If Vd has uncertainty sVd and Cd has sCd then error in
Fd through Fd = Vd Cd is given by
s Fd
¼
Fd
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
sV d 2
sC d 2
þ
:
Vd
Cd
ð3Þ
Estimates of Vd, based on various long-term data sets
and models, have an error 50% [Schafer et al., 1993].
Cd measurements are relatively well constrained and error is
15% (as reported by Kumar et al. [2008]; however
uncertainly could be larger due to high variability in the
atmosphere). Hence the overall uncertainty in dry deposition
flux is 52%, dominated by sVd/Vd. Uncertainly in wet
deposition fluxes are estimated by propagating respective
Table 3. Rainfall Data Used for Wet Deposition Flux Estimates
Over the Different Regions in the Northern Indian Ocean
Date
Region
Rainfall (mm year1)
Aug–Oct 1994
Apr–May 1994
Nov–Dec 1994
Feb–Mar 1995
Apr 1995
May 1995
Jul–Aug 1995
Jan 1996
Jan 1996
Feb 1997
Feb 1997
Mar 1997
Mar–Apr 1998
Feb 1999
Feb–Mar 2001
Mar 2001
Apr–May 2006
CAS
EAS
CAS
CAS
EAS
CAS
CAS/WASa
EAS
CAS/WASa
CAS
BOB
CAS
CAS
BOB
BOB
EAS
EAS
207
50
857
53
718
729
318
767
981
7
299
91
128
919
430
37
1249
a
Sampling locations during Jul–Aug 1995 and Jan 1996 were close by
and hence same rainfall is used for wet deposition flux estimates in CAS
and WAS.
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SINGH ET AL.: ROLE OF N DEPOSITION IN NEW PRODUCTION
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Figure 2. New and primary productivity sample locations in the northern Indian Ocean. Legend:
asterisk, Watts et al. [1999]; open circle, McCarthy et al. [1999]; solid circle, Watts and Owens [1999];
cross, Sambrotto [2001]; diamond, Kumar et al. [2004]; open triangle, Prakash et al. [2008]; open square,
Kumar et al. [2010]; solid triangle, Gandhi et al. [2010a]; solid square, Gandhi et al. (submitted manuscript, 2012). See auxiliary material for more details.
uncertainties in Fw = P S Cd r1
a rw
s Fw
¼
Fw
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
s 2 s 2 s 2
P
S
Cd
þ
þ
;
P
S
Cd
ð4Þ
where sP and sS are uncertainties involved in rainfall and
scavenging ratio, respectively. The above expression is
based on the assumption that there is no error in ra and rw.
Rainfall measurement has an analytical error of 7.4%
[Xie et al., 2003]. S is most uncertain here and varies from
280–480 (excluding an outlier value of 870 reported by Wolf
et al. [1986] from a short-term measurement on Bermuda)
for nitrate and 160–340 for ammonium [Duce et al., 1991].
We adopted the average values reported by Duce et al. [1991],
i.e., 330 and 200 for nitrate and ammonium, respectively.
Hence the maximum uncertainty in sS could be (480–330) 100/330 = 45%. Thus, the uncertainly in wet deposition flux
is 48%. Note that Duce et al. [1991] estimated S from the
ratio of concentrations in rain and simultaneously measured
air concentrations at ground level at various ocean provinces
(but not in the Indian Ocean). Here it is assumed that S has the
same range in northern Indian
Oceanffi as in the other oceans.
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
The overall uncertainty ( sF2w þ sF2d ) in the estimated total
nitrogen deposition flux is 71%, mainly caused by the large
uncertainties in poorly constrained dry depositional velocities
of nitrate and ammonium.
2.4. Zone Classification
[15] We analyze the data into two ways. First (case A), the
northern Indian Ocean is divided into four different zones
(1) Western Arabian Sea, 50 –60 E; (2) Central Arabian
Sea, 60 –70 E; (3) Eastern Arabian Sea, 70 –78 E; and
(4) Bay of Bengal, 78 –95 E, and the results are presented in
Table 1. These boundaries are not well constrained and are
subjected to change (likely to vary by 2 depending on the
sampling area). This division is based on the differences in
biogeochemistry among these basins; e.g., the western Arabian Sea gets nutrients through upwelling during the summer
monsoon, whereas the eastern Arabian Sea, by convective
vertical mixing in the winter [Madhupratap et al., 1996;
Prakash and Ramesh, 2007]. Second (case B), we consider
the Arabian Sea and the Bay of Bengal as two different
basins and list the results in Table 2.
3. Results and Discussion
[16] Data from some simultaneous and nearly co-located
measurements for new production and aerosol concentrations are available in both basins (Figures 1 and 2), but these
were sampled during different cruises. Exactly simultaneous
and co-located measurements of aerosol and ocean productivity are not possible since the standard protocols for measurements of the two are quite different: aerosol samples are
mostly collected while ship is moving (by pumping air over
a large area to increase the collected mass) and productivity
measurements need the ship to be stationary, and are done
over a limited spot. Even if done simultaneously, the aerosols collected may not represent any immediate effect on the
ambient ocean because (1) they may be transported from
elsewhere and (2) their timescales of interaction are different. As far as possible, we match the data of corresponding
locations for deposition flux, primary and new production.
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3.1. Seasonal Variations in New Production
and Atmospheric Deposition Flux
[17] For the first case A, the aerosol deposition flux, new
and total primary production (in mmol N m2 day1) of the
Bay of Bengal and three zones of the Arabian Sea are listed
in Table 1. Seasons are classified as winter (December–
February), spring (March–May), summer (June–August),
and fall (September–November). Aerosol data are not
available for the fall intermonsoon in the Arabian Sea while
in the Bay of Bengal, data for only the winter and spring
intermonsoons are available. Also, there is no aerosol data
for spring in the western Arabian Sea and summer in the
eastern Arabian Sea. New and total primary production
estimations are available for all the seasons in the Arabian
Sea, while they are unavailable for the winter and summer
monsoons in the Bay of Bengal.
[18] In the eastern Arabian Sea, new production was
maximum (7.3 mmol N m2 day1) during the winter
monsoon and minimum (1.9 mmol N m2 day1) during the
spring intermonsoon (Table 1). New production was
somewhat moderate (5.0 mmol N m2 day1) during summer in the eastern Arabian Sea (Table 1). On the other hand,
atmospheric nitrogen deposition was also maximum
(0.22 mmol N m2 day1) during the winter in the eastern
Arabian Sea. The observed higher new production could be
result of higher atmospheric deposition in the eastern Arabian Sea besides prominent winter mixing in the region.
Such higher deposition during the winter monsoon could be
possible due to episodic events which were reported in some
places in the ocean as well [Owens et al., 1992]. However,
we note that reports on episodic events were overemphasized
[Michaels et al., 1993]. Limited data suggest that there was
0.02 mmol N m2 day1 deposition during the spring intermonsoon, however, there is no such data for summer and fall
intermonsoon over the eastern Arabian Sea.
[19] In the central Arabian Sea, new production was significantly higher during the summer (5.2 mmol N m2
day1) than during the spring intermonsoon (3.3 mmol N
m2 day1) and the winter monsoon (2.4 mmol N m2
day1). Atmospheric nitrogen deposition was minimum
(0.01 mmol N m2 day1) in the summer monsoon over
the central Arabian Sea. This is attributable to the winds
blowing from the ocean to the Indian subcontinent during
summer [Prakash and Ramesh, 2007]. Higher primary production in the summer monsoon suggests the importance
of intense upwelling in the Arabian Sea. During the winter
and spring intermonsoon, the central Arabian Sea witnessed
equal amounts (0.04 mmol N m2 day1) of nitrogen
deposition by aerosols (Table 1).
[20] A significant seasonal variation in the new and primary production is observed during 1995 in the western
Arabian Sea with maximum during the summer while minimum during the winter. Paucity of the data did not allow us
to infer seasonal variation in the deposition flux in the
western Arabian Sea (Table 1). Likewise, it is also difficult
to infer the seasonal variability in the productivity and
deposition fluxes in the Bay of Bengal (Table 1).
[21] In the second case B, when the Arabian Sea is considered as a single basin, we observed seasonal variability in
the new production and deposition flux. New and primary
productions were higher during the summer monsoon than
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in other seasons in the Arabian Sea. This could be because of
upwelling, which is induced by the summer Findlater jet
[Findlater, 1969]. In addition, convective mixing could be
responsible for new and primary production to be significantly higher during winter than in spring and fall intermonsoons. Nitrogen atmospheric deposition was higher
during winter than during spring and summer monsoon
(Table 2).
3.2. Spatial Variations in the Productivity
and the Atmospheric Deposition Flux
[22] In the first case A, new production was significantly
higher in the eastern Arabian Sea during the winter monsoon
than in the other three zones (Table 1), attributable to winter
mixing, a well known process that occurs in the western
coast of India [Madhupratap et al., 1996; Gandhi et al.,
2011b]. All the three zones of the Arabian Sea depict comparable new production during the spring intermonsoon in
1995, suggesting no significant spatial variations in new
production, possibly because of a common process (could be
N2 fixation in the presence of diazotrophs) acting all over
the Arabian Sea during this season [Gandhi et al., 2011a].
It was difficult to decipher any significant trend (increasing
or decreasing) from the data set (Table 1).
[23] In the second case B, we observed significant higher
primary production in the Arabian Sea than the Bay of
Bengal; however, new production was comparable. There
was no such significant difference between the deposition
fluxes in the two basins. Deposition flux was higher during
the winter monsoon than in any other season in the Arabian
Sea (Table 2). This might be attributable to the atmospheric
inputs from the Thar desert [Krishnamurti et al., 1998].
[24] Flux estimations mostly pertained to the central Arabian Sea (Figure 1). When the Arabian Sea is considered as a
whole (in the section 3.1) we note that deposition flux is
higher during the winter monsoon (Table 2), and after
dividing the Arabian Sea into three zones we observed
that deposition flux was higher mainly in the eastern Arabian
Sea (Table 1). Winds blowing from the Indian subcontinent during the winter monsoon, might have contributed to
this flux.
3.3. Contribution of Atmospheric Nitrogen Deposition
to New Production in the Northern Indian Ocean
[25] In the case A, higher productivity was observed during the summer monsoon as expected, but the contribution
from deposition flux was less (Table 1). This suggests that
atmospheric deposition was not an important source of
nitrogen to the Arabian Sea, and nitrogen through upwelling
of deeper waters could be the most important source of new
nitrogen to the surface waters during this season. These
observations belong mainly to the central Arabian Sea and
the western Arabian Sea, where intense winds cause
upwelling during this season [Findlater, 1969].
[26] All the three components (atmospheric deposition,
new production, and contribution of the former to the latter,
i.e., 3%) were higher during the winter than during other
seasons in the eastern Arabian Sea (Table 1). On the scale of
global oceans and considering the residence time of nitrogen
(i.e., 1000 years), 3% contribution from atmospheric
nitrogen (being an additional input to the ocean unlike
nitrogen inputs to the photic zone through upwelling) to new
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production could be significant [Duce et al., 2008]. However, in regions such as the Arabian Sea, where seasonal
upwelling prevails, atmospheric nitrogen deposition is
masked by local upwelling of nitrate (Table 1).
[27] In the case B, aerosol contributions are 1.2, 1.7 and
0.4% to the new production in the Arabian Sea during the
winter monsoon, the spring intermonsoon and the summer
monsoon, respectively. Although atmospheric deposition is
the highest during winter monsoon its contribution to new
production is maximum during the spring intermonsoon
because of low new production during the spring, which
could be attributed to the stratification resulting in the low
supply of nutrients from below. Both the atmospheric
deposition and its contribution to new production were
higher during the winter monsoon than in the summer
monsoon, as winds blowing from the land to the ocean
during the winter increase the atmospheric contribution. On
the other hand, both atmospheric deposition and its contribution to new production are low during summer (Table 2)
as most of the nutrients are either upwelled or regenerated
during the summer monsoon [Sambrotto, 2001].
[28] In the Bay of Bengal, simultaneous data for new production and atmospheric nitrogen deposition are only available for the spring intermonsoon (Table 2). New production
during the spring intermonsoon in the Bay of Bengal is
comparable to those during winter and summer monsoons in
the Arabian Sea, while the aerosol contribution is less (0.4%).
Also, upwelling is not a contributor to new production in the
Bay of Bengal [Gauns et al., 2005]. The Bay of Bengal
receives a large volume of river run-off (1.6 1012 m3
year1) [Subramanian, 1993]. It is likely that nitrogen flux
from river run-off could play a significant role for new production in the coastal areas of the Bay of Bengal [Singh and
Ramesh, 2011]. Higher new production could be triggered
by cold core eddy formation during the spring intermonsoon
in open ocean area of the Bay of Bengal [Prasanna Kumar
et al., 2007; Gandhi et al., 2010b].
[29] The contribution of atmospheric deposition to new
production was maximum (1.7%) in the Arabian Sea and
minimum (0.4%) in the Bay of Bengal during the same
season, i.e., the spring intermonsoon; however, the difference between the contributions during the summer monsoon
in the Arabian Sea and during the spring intermonsoon in the
Bay of Bengal is insignificant.
[30] Using NOAA-AVHRR data of duration 1996–2003,
Parameswaran et al. [2008] reported the significant impact
of continental aerosols in the Arabian Sea: during March–
April from the Indian subcontinent and during June–
September from Arabian deserts, while in the Bay of Bengal,
influence of the subcontinent was more during November–
May and minimal during June–September. These observations are consistent with the results presented here.
[31] Due to the limited seasonal and spatial matching of
aerosol and new production observations (Tables 1 and 2), it
is difficult to infer the role of deposition flux in the new and
primary production in the Bay of Bengal. However, it is
likely that there is a minor contribution of atmospheric
deposition to new production in the Bay of Bengal.
3.4. Changes in the Redfield Ratio and Biodiversity
[32] The Redfield ratio quantitatively links the marine carbon,
nitrogen and phosphorus cycles in numerous biogeochemical
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applications. The sum of all biological processes in the ocean
(including nitrogen fixation and denitrification) is believed
to maintain the deep ocean C:N:P ratio at 106:16:1, but it is
well known that N:P ratios vary strongly even within the nondiazotrophic autotrophs [Arrigo, 2005]. Reactive nitrogen flux
in the South Asian region has increased significantly in the
last few decades, and consequently the atmospheric deposition
of reactive anthropogenic nitrogen over the northern Indian
Ocean is among the highest in the world [Duce et al., 2008].
Such deposition fluxes are expected to alter the relative concentrations of marine macronutrients (N:P:Si) available for
phytoplankton uptake, in turn resulting in a change in the
phytoplankton community structure. As discussed by Duce
et al. [2008], phosphorous concentration in the surface ocean
is not significantly affected by anthropogenic activity and hence
nitrogen deposition alone is likely to enhance N:P in the surface layers. Although the impact of atmospheric nitrogen
deposition to marine new production is small, seasonal occurrences of potential dinitrogen fixers (Trichodesmium) in the
Arabian Sea may be greatly affected. Unlike other plankton,
the diazotrophs grow under very different environmental conditions (e.g., in the absence of nitrate and in warmer, stratified
waters) [Karl et al., 2002]. Iron deposition by mineral aerosols,
provided photic zone is not limited in phosphate, may promote
the occurrence of diazotrophs [Mills et al., 2004].
[33] The present study suggests that atmospheric fluxes
have minor contribution to the new production in the northern Indian Ocean as the nitrogen supply through upwelling
and rivers dominate. However, the potential impacts of
atmospheric fluxes in future would be determined by the
changes in anthropogenic activity. The regions, where Nr
deposition shows an increasing trend as estimated by
Galloway et al. [2004], may face larger impacts (positive/
negative). At present, our knowledge is too limited to fix the
time scale of impact of atmospheric nitrogen on oceanic
production [Duce et al., 2008].
3.5. Synthesis of the Available Estimates
[34] We compared the nitrogen deposition flux estimations obtained from earlier work and the present study in the
northern Indian Ocean. Duce et al. [1991] estimated a
nitrogen deposition flux of 2 Tg year1 (FdNO3 = 0.14,
FdNH4 = 0.17, FwNO3 = 0.49, FwNH4 = 1.17 Tg N year1)
over the northern Indian Ocean using data obtained prior to
1984. This flux was dominated by wet ammonium flux.
Duce et al. [1991] concluded that this large ammonium flux
could be due to the source of ammonium from the ocean
itself. Bange et al. [2000] estimated the total nitrogen
deposition (both dry and wet) as 0.84 + 0.74 ≈ 1.58 Tg N
year1 in the Arabian Sea.
[35] Taking into account more data from the recent past,
we have calculated 0.92 Tg N year1 flux into the Arabian
Sea. NO
2 data (wherever available) was also included in this
analysis. These fluxes were dominated by dry deposition of
nitrogen (FdNO3+dNO2+dNH4 = 0.60, FwNO3+wNO2+wNH4 =
0.32 Tg N year1). The nitrogen deposition flux over
the Bay of Bengal is significantly less than that in the
Arabian Sea, i.e., 0.46 Tg N year1 (FdNO3+dNH4 = 0.31,
FwNO3+wNH4 = 0.15 Tg N year1). Hence the total Nr deposition over the northern Indian Ocean is 1.39 Tg N year1.
The fluxes presented here agree well with earlier estimates
within the uncertainties.
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[36] Duce et al. [2008] estimated a global average Nr
deposition flux of 14.3 mmol N m2 year1 (reported value
200 mg N m2 year1). Our estimate of the deposition flux
over the Arabian Sea is 13.4 mmol N m2 year1 (no data
from fall intermonsoon) and 11.3 mmol N m2 year1 (no
data from winter and fall) over the Bay of Bengal. Previous
estimates by Duce et al. [2008] are not significantly different from our estimates, considering the associated
uncertainties. New productivity in the Arabian Sea is
1.46 mol N m2 year1 hence the contribution of Nr
deposition flux to the new productivity is 0.9%. The
contribution of Nr deposition flux to the new productivity
(1.46 mol N m2 year1) in the Bay of Bengal (no data
from summer and fall) resembles that of the Arabian Sea.
Large uncertainties in the deposition flux and paucity of data
preclude the detection of any temporal trends in the deposition
fluxes. Direct sampling of rain over the northern Indian Ocean
could be important to better constrain the present estimates,
as suggested by Baker et al. [2010] for the Atlantic region.
[37] Organic nitrogen in aerosols could be an important
component of Nr in the marine atmosphere [Cape et al.,
2011, and references therein] that can contribute up to 24%
of total soluble nitrogen in the aerosol [Lesworth et al.,
2010]. Unfortunately, no data on the organic nitrogen content of aerosols are currently available for the Arabian Sea.
However, a recent study in the Bay of Bengal reported that
organic and inorganic aerosols combined can support 13%
of the primary production [Srinivas et al., 2011]. This study
indicates that there is a need of measurement of organic
aerosols for better understanding biogeochemical cycle of
nitrogen in the ocean.
3.6. Other Nitrogen Sources to the Arabian Sea
[38] The Arabian Sea loses 60 Tg N year1 (i.e., 40%
of global N loss) through pelagic denitrification and thus has
global importance [Codispoti, 2007, and references therein].
Nitrogen gain into the Arabian Sea through N2 fixation is
15.4 Tg N year1 [Gandhi et al., 2011a] and a minor
contribution (0.06 Tg N year1) from the rivers [Singh and
Ramesh, 2011]. Major source of nitrogen in the Arabian Sea
is via advection of waters from the south, i.e., 38 Tg N
year1 [Bange et al., 2000]. In addition, the present estimates suggest that atmospheric deposition contributes
0.92 Tg N year1. Thus, nitrogen sources are somewhat
comparable to the sinks in the Arabian Sea. However, this
upward revision of nitrogen sources in the Arabian Sea does
not solve the mystery of missing nitrogen in the world ocean
[Codispoti, 2007]. To refer internal nutrient cycling, we have
noted that high new production in the Arabian Sea could be
sustained by an upward flux of nitrate (i.e., 22 Tg N year1
an average value of the nitrate flux reported by Gandhi et al.
[2011b] in their Table 4, and integrated over the Arabian Sea).
3.7. Other Nitrogen Sources to the Bay of Bengal
[39] Riverine fluxes and vertical mixing by eddies are the
other and main sources of nitrogen to the Bay of Bengal
[Prasanna Kumar et al., 2007]. Cyclonic eddies, that occur
in the Bay during October–December, can increase the biological productivity of the Bay by injecting nutrients into the
otherwise oligotrophic waters [Prasanna Kumar et al., 2007;
Gandhi et al., 2010b]. However, the effect is not seen in the
upper surface, restricting the eddy effect to below 20 m
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during the fall and the spring intermonsoons of 2002–2003
[Prasanna Kumar et al., 2007]. Productivity measurements
during the same expedition, however, show higher new
productivity in the Bay of Bengal [Kumar et al., 2004].
Observations during 2003 also did not show high Chlorophyll a or productivity associated with a cyclonic eddy
[Muraleedharan et al., 2007]. Nitrogen inputs through rivers
are most likely utilized in the estuarine and primary productivity in the coastal Bay of Bengal [Singh and Ramesh,
2011]; riverine impact on open ocean biological productivity is negligible [Duce et al., 2008]. Rivers of the Indian
subcontinent contribute 0.38 Tg N year1 to the Bay of
Bengal [Singh and Ramesh, 2011], comparable to the present
estimate of the atmospheric deposition flux (0.46 Tg N
year1) to the region. All these phenomena together suggest
that the atmospheric inputs could be significantly higher
during the above two seasons. However, our analysis based
on the available data on aerosols does not confirm this
(Tables 1 and 2). This also seems to indicate that N2 fixation
and advective flux of nitrogen could be significant in the Bay
of Bengal, where no such measurements have been reported
so far. Unlike the Arabian Sea, nitrogen is not lost from the
oxygen minimum zones of the Bay of Bengal via pelagic
denitrification. Nevertheless, there is 4.1 Tg N year1 loss,
mainly via sedimentary denitrification [Naqvi, 2008]. The Bay
of Bengal still remains under-sampled, hence we cannot infer
whether or not nitrogen fluxes are in balance in this region.
4. Conclusion
[40] There is no significant spatial variation in new productivity in the three zones of the Arabian Sea (i.e., eastern,
central and western) during the spring intermonsoon 1995.
However, a significant seasonal variation in the western
Arabian Sea is observed during 1995. Nitrogen deposition
flux was higher in the winter monsoon in the eastern Arabian
Sea than that in the other parts of the northern Indian Ocean.
New production and deposition flux in the Arabian Sea were
comparable to those in the Bay of Bengal. Aerosols deposit
1.39 Tg nitrogen per year in the northern Indian Ocean,
with a major fraction (67%) in the Arabian Sea. Atmospheric fluxes make a minor contribution to the marine new
productivity (maximum up to 3% in the eastern Arabian
Sea during the winter monsoon). However, this minor contribution is likely to become important in the near future and
play a crucial role in the removal of excess atmospheric CO2
through marine productivity. Nitrogen sources are comparable to sinks in the Arabian Sea, however, no such inference
could be drawn for the Bay of Bengal due to paucity of data.
Assessing N2 fixation by diazotrophs and measurements of
organic aerosols are urgently needed to better constrain
nitrogen inputs into the northern Indian Ocean.
[41] Acknowledgments. We sincerely thank the three anonymous
reviewers and Carmen G. Castro, Associate Editor, whose critical comments and suggestions helped improving the manuscript significantly.
Thanks are due to M. W. Lomas for a critical review of this work.
References
Arrigo, K. R. (2005), Marine microorganisms and global nutrient cycles,
Nature, 437, 349–355, doi:10.1038/nature04159.
Baker, A. R., K. Westo, S. D. Kelly, M. Voss, P. Streu, and J. N. Cape
(2007), Dry and wet deposition of nutrients from the tropical Atlantic
9 of 11
C06004
SINGH ET AL.: ROLE OF N DEPOSITION IN NEW PRODUCTION
atmosphere: Links to primary productivity and nitrogen fixation, Deep
Sea Res., Part I, 54, 1704–1720, doi:10.1016/j.dsr.2007.07.001.
Baker, A. R., T. Lesworth, C. Adams, T. D. Jickells, and L. Ganzeveld
(2010), Estimation of atmospheric nutrient inputs to the Atlantic Ocean
from 50 N to 50 S based on large-scale field sampling: Fixed nitrogen
and dry deposition of phosphorus, Global Biogeochem. Cycles, 24,
GB3006, doi:10.1029/2009GB003634.
Bange, H. W., T. Rixen, A. M. Johansen, R. L. Siefert, R. Ramesh,
V. Ittekkot, M. R. Hoffmann, and M. O. Andreae (2000), A revised
nitrogen budget for Arabian Sea, Global Biogeochem. Cycles, 14(4),
1283–1297, doi:10.1029/1999GB001228.
Barrie, L. A. (1985), Scavenging ratios, wet deposition, and in-cloud oxidation: An application to the oxides of sulphur and nitrogen, J. Geophys.
Res., 90, 5789–5799, doi:10.1029/JD090iD03p05789.
Broecker, W. S. (1974), Chemical Oceanography, 214 pp., Harcourt Brace
Jovanovich, New York.
Cape, J. N., S. E. Cornell, T. D. Jickells, and E. Nemitz (2011), Organic
nitrogen in the atmosphere—Where does it come from? A review of
sources and methods, Atmos. Res., 102, 30–48, doi:10.1016/j.atmosres.
2011.07.009.
Codispoti, L. A. (2007), An oceanic fixed nitrogen sink exceeding 400 Tg
N a1 vs the concept of homeostasis in the fixed-nitrogen inventory,
Biogeosciences, 4, 233–253, doi:10.5194/bg-4-233-2007.
Duce, R. A., et al. (1991), The atmospheric input of trace species to the
world ocean, Global Biogeochem. Cycles, 5, 193–259, doi:10.1029/
91GB01778.
Duce, R. A., et al. (2008), Impacts of atmospheric anthropogenic nitrogen
on the open ocean, Science, 320, 893–897, doi:10.1126/science.1150369.
Dugdale, R. C., and J. J. Goering (1967), Uptake of new and regenerated
forms of nitrogen in primary productivity, Limnol. Oceanogr., 12,
196–206, doi:10.4319/lo.1967.12.2.0196.
Eppley, R. W., and B. J. Peterson (1979), Particulate organic matter
flux and planktonic new production in the deep ocean, Nature, 282,
677–680, doi:10.1038/282677a0.
Findlater, J. (1969), A major low-level air current near the Indian Ocean
during the northern summer, Q. J. R. Meteorol. Soc., 95, 362–380,
doi:10.1002/qj.49709540409.
Galloway, J. N., et al. (2004), Nitrogen cycles: Past, present and future,
Biogeochemistry, 70, 153–226, doi:10.1007/s10533-004-0370-0.
Gandhi, N. (2010), Carbon fixation in the hydrosphere: Quantification in
the Indian region using 13C and 15N isotopes, Ph.D. thesis, 206 pp.,
Mohan Lal Sukhadia Univ., Udaipur, India.
Gandhi, N., R. Ramesh, R. Srivastava, M. S. Sheshshayee, R. M. Dwivedi,
and M. Raman (2010a), Nitrogen uptake rates during spring in the NE
Arabian Sea, Int. J. Oceanogr., 2010, 127493, doi:10.1155/2010/127493.
Gandhi, N., A. Singh, R. Ramesh, and M. S. Sheshshayee (2010b), Nitrogen sources for new production in the NE Indian Ocean, Adv. Geosci.,
24, 55–67.
Gandhi, N., A. Singh, S. Prakash, R. Ramesh, M. Raman, M. S.
Sheshshayee, and S. Shetye (2011a), First direct measurements of N2
fixation during a Trichodesmium bloom in the eastern Arabian Sea,
Global Biogeochem. Cycles, 25, GB4014, doi:10.1029/2010GB003970.
Gandhi, N., R. Ramesh, S. Prakash, and S. Kumar (2011b), Nitrogen
sources for new production in the NE Arabian Sea, J. Sea Res., 65,
265–274, doi:10.1016/j.seares.2010.12.002.
Gauns, M., M. Madhupratap, N. Ramaiah, R. Jyothibabu, Veronica
Fernandes, J. T. Paul, and S. Prasanna Kumar (2005), Comparative
accounts of biological productivity characteristics and estimates of carbon
fluxes in the Arabian Sea and the Bay of Bengal, Deep Sea Res., Part II,
52, 2003–2017, doi:10.1016/j.dsr2.2005.05.009.
Gibb, S. W., R. F. C. Mantoura, and P. S. Liss (1999), Ocean-atmosphere
exchange and atmospheric speciation of ammonia and methylamines in
the region of the NW Arabian Sea, Global Biogeochem. Cycles, 13,
161–178, doi:10.1029/98GB00743.
Johansen, A. M., R. L. Siefert, and M. R. Hoffmann (1999), Chemical characterization of ambient aerosol collected during the southwest monsoon
and intermonsoon seasons over Arabian Sea: Anions and cations, J. Geophys. Res., 104, 26,325–26,347, doi:10.1029/1999JD900405.
Karl, D. M., A. Michaels, B. Bergman, D. G. Capone, E. J. Carpenter,
R. Letelier, F. Lipschultz, H. Paerl, D. Sigman, and L. Stal (2002),
Dinitrogen fixation in the world’s oceans, Biogeochemistry, 57, 47–98,
doi:10.1023/A:1015798105851.
Krishnamurthy, A., J. K. Moore, N. Mahowald, C. Luo, and C. S. Zender
(2010), Impacts of atmospheric nutrient inputs on marine biogeochemistry, J. Geophys. Res., 115, G01006, doi:10.1029/2009JG001115.
Krishnamurti, T. N., B. Jha, J. Prospero, A. Jayaraman, and V. Ramanathann
(1998), Aerosol and pollutant transport and their impact on radiative
forcing over the tropical Indian Ocean during the January–February
1996 pre-INDOEX cruise, Tellus B, 50, 521–542.
C06004
Kumar, A., M. M. Sarin, and A. K. Sudhir (2008), Mineral and anthropogenic aerosols in Arabian Sea–atmospheric boundary layer: Sources
and spatial variability, Atmos. Environ., 42, 5169–5181, doi:10.1016/
j.atmosenv.2008.03.004.
Kumar, M. R. R., and T. G. Prasad (1997), Annual and interannual variation
of precipitation over the tropical Indian Ocean, J. Geophys. Res., 102,
18,519–18,527, doi:10.1029/97JC00979.
Kumar, S. (2004), Biogeochemistry of Nitrogen Isotopes in Northern India
Ocean, Ph.D. thesis, 202 pp., Maharaja Sayajirao Univ. of Baroda, Varodara, India.
Kumar, S., and R. Ramesh (2005), Productivity measurements in the Bay of
Bengal using the 15N tracer: Implications to the global carbon cycle,
Indian J. Mar. Sci., 34(2), 153–162.
Kumar, S., R. Ramesh, S. Sardesai, and M. S. Sheshshayee (2004), High
new production in the Bay of Bengal: Possible causes and implications,
Geophys. Res. Lett., 31, L18304, doi:10.1029/2004GL021005.
Kumar, S., R. Ramesh, R. M. Dwivedi, M. Raman, M. S. Sheshshayee, and
W. D’Souza (2010), Nitrogen uptake in the northeastern Arabian Sea during winter cooling, Int. J. Oceanogr., 2010, 819029, doi:10.1155/2010/
819029.
Kuypers, M. M. M., A. O. Sliekers, G. Lavik, M. Schmid, B. B. Jørgensen,
J. G. Kuenen, J. S. Sinninghe Damst’e, M. Strous, and M. S. M. Jetten
(2003), Anaerobic ammonium oxidation by anammox bacteria in the
Black Sea, Nature, 422, 608–611, doi:10.1038/nature01472.
Lesworth, T., A. R. Baker, and T. Jickells (2010), Aerosol organic nitrogen
over the remote Atlantic Ocean, Atmos. Environ., 44, 1887–1893,
doi:10.1016/j.atmosenv.2010.02.021.
Madhupratap, M., S. Prasanna Kumar, P. M. A. Bhattathiri, M. Dileep
Kumar, S. Raghukumar, K. K. C. Nair, and N. Ramaiah (1996), Mechanism of the biological response to winter cooling in the northeastern Arabian Sea, Nature, 384, 549–552, doi:10.1038/384549a0.
McCarthy, J. J., C. Garside, and L. N. John (1999), Nitrogen dynamics during the Arabian Sea northeast monsoon, Deep Sea Res., Part II, 46,
1623–1664, doi:10.1016/S0967-0645(99)00038-7.
Michaels, A. F., D. A. Siegel, R. I. Johnson, A. H. Knap, and J. N. Galloway
(1993), Episodic inputs of atmospheric nitrogen to the Sargasso Sea: Contributions to new production and phytoplankton blooms, Global Biogeochem. Cycles, 7, 339–351, doi:10.1029/93GB00178.
Mills, M. M., C. Ridame, M. Davey, J. La Roche, and R. J. Geider (2004),
Iron and phosphorus co-limit nitrogen fixation in the eastern tropical
North Atlantic, Nature, 429, 292–294, doi:10.1038/nature02550.
Muraleedharan, K. R., P. Jasmine, C. T. Achuthankutty, C. Revichandran,
P. K. Dineshkumar, P. Anand, and P. Rejomon (2007), Influence of
basin-scale and mesoscale physical processes on biological productivity
in the Bay of Bengal during the summer monsoon, Prog. Oceanogr.,
72(4), 364–383, doi:10.1016/j.pocean.2006.09.012.
Naqvi, S. W. A. (2008), The Indian Ocean, in Nitrogen in the Marine
Environment, edited by D. G. Capone et al., pp. 631–681, Elsevier,
Burlington, Mass., doi:10.1016/B978-0-12-372522-6.00014-1
Naqvi, S. W. A., et al. (2010), The Arabian Sea as a high-nutrient, lowchlorophyll region during the late Southwest Monsoon, Biogeosciences,
7, 2091–2100, doi:10.5194/bg-7-2091-2010.
Owens, N. J. P., J. N. Galloway, and R. A. Duce (1992), Episodic
atmospheric nitrogen deposition to oligotrophic oceans, Nature, 357,
397–399, doi:10.1038/357397a0.
Parameswaran, K., S. K. Nair, and K. Rajeev (2008), Impact of aerosols
from the Asian Continent on the adjoining oceanic environments,
J. Earth Syst. Sci., 117, 83–102, doi:10.1007/s12040-008-0016-z.
Patra, P. K., M. D. Kumar, N. Mahowald, and V. V. S. S. Sarma (2007),
Atmospheric deposition and surface stratification as controls of contrasting chlorophyll abundance in the North Indian Ocean, J. Geophys. Res.,
112, C05029, doi:10.1029/2006JC003885.
Prakash, S. (2008), Role of the ocean in the global carbon cycle, Ph.D.
thesis, 151 pp., Maharaja Sayajirao Univ. of Baroda, Varodara, India.
Prakash, S., and R. Ramesh (2007), Is the Arabian Sea getting more productive?, Curr. Sci., 92, 667–671.
Prakash, S., R. Ramesh, M. S. Sheshshayee, R. M. Dwivedi, and M. Raman
(2008), Quantification of new production during a winter Noctiluca scintillans bloom in the Arabian Sea, Geophys. Res. Lett., 35, L08604,
doi:10.1029/2008GL033819.
Prasanna Kumar, S., M. Nuncio, N. Ramaiah, S. Sardesai, J. Narvekar,
V. Fernandes, and J. T. Paul (2007), Eddy-mediated biological productivity in the Bay of Bengal during fall and spring intermonsoons, Deep Sea
Res., Part I, 54, 1619–1640, doi:10.1016/j.dsr.2007.06.002.
Rastogi, N. (2005), Environmental radionuclides and chemical constituents
in rain and aerosols: Biogeochemical sources and temporal variations,
Ph.D. thesis, 149 pp., Mohan Lal Sukhadia Univ., Udaipur, India.
10 of 11
C06004
SINGH ET AL.: ROLE OF N DEPOSITION IN NEW PRODUCTION
Rengarajan, R., and M. M. Sarin (2004), Atmospheric deposition fluxes of
7
Be, 210Pb and chemical species to the Arabian Sea and Bay of Bengal,
Indian J. Mar. Sci., 33, 56–64.
Rhoads, K. P., P. Kelley, R. R. Dickerson, T. P. Carsey, M. Farmer, D. L.
Savoie, and J. M. Prospero (1997), Composition of the troposphere the
Indian Ocean during the monsoonal transition, J. Geophys. Res., 102,
18,981–18,995, doi:10.1029/97JD01078.
Ryther, J. H., J. R. Hall, A. K. Pease, A. Bakun, and M. M. Jones (1966),
Primary organic production in relation to the chemistry and hydrography
of the western Indian Ocean, Limnol. Oceanogr., 11, 371–380,
doi:10.4319/lo.1966.11.3.0371.
Sabine, C. L., et al. (2004), The oceanic sink for anthropogenic CO2,
Science, 305, 367–371, doi:10.1126/science.1097403.
Sambrotto, R. N. (2001), Nitrogen production in the northern Arabian Sea
during the spring intermonsoon and southwest monsoon seasons, Deep
Sea Res., Part II, 48, 1173–1198, doi:10.1016/S0967-0645(00)00135-1.
Sarin, M. M., R. Rengarajan, and S. Krishnaswami (1999), Aerosol NO
3
and 210Pb distribution over the central-eastern Arabian Sea and their
air-sea deposition fluxes, Tellus B, 51, 749–758.
Schafer, P., H. Kreilein, M. Muller, and G. Gravenhorst (1993), Cycling of
inorganic nitrogen compounds between atmosphere and ocean in tropical
areas off south east Asia, in Monsoon Biogeochemistry, edited by
V. Ittekkot and R. R. Nair, pp. 19–36, Univ. Hamburg, Hamburg, Germany.
Seitzinger, S., J. A. Harrison, J. K. Böhlke, A. F. Bouwman, R. Lowrance,
B. Peterson, C. Tobias, and G. Van Drecht (2006), Denitrification across
landscapes and waterscapes: A synthesis, Ecol. Appl., 16, 2064–2090,
doi:10.1890/1051-0761(2006)016[2064:DALAWA]2.0.CO;2.
Singh, A., and R. Ramesh (2011), Contribution of riverine dissolved inorganic nitrogen flux to new production in the coastal northern Indian
Ocean: An assessment, Int. J. Oceanogr., 2011, 983561, doi:10.1155/
2011/983561.
C06004
Singh, A., R. A. Jani, and R. Ramesh (2010), Spatiotemporal variations of
the d 18O–salinity relation in the northern Indian Ocean, Deep Sea Res.,
Part I, 57, 1422–1431, doi:10.1016/j.dsr.2010.08.002.
Smith, S. L. (2001), Understanding the Arabian Sea: Reflections on the
1994–1996 Arabian Sea Expedition, Deep Sea Res, Part II, 48,
1385–1402.
Spokes, L. J., S. G. Yeatman, S. E. Cornell, and T. M. Jickells (2000),
Nitrogen deposition to the eastern Atlantic Ocean: The importance of
southeasterly flow, Tellus B, 52, 37–49.
Srinivas, B., M. M. Sarin, and V. V. S. S. Sarma (2011), Atmospheric
dry deposition of inorganic and organic nitrogen to the Bay of Bengal:
Impact of continental outflow, Mar. Chem., 127, 170–179, doi:10.1016/
j.marchem.2011.09.002.
Subramanian, V. (1993), Sediment load of Indian Rivers, Curr. Sci., 64,
928–930.
Watts, L. J., and N. J. P. Owens (1999), Nitrogen assimilation and f-ratio in
the northwestern Indian Ocean during an intermonsoon period, Deep Sea
Res., Part II, 46, 725–743, doi:10.1016/S0967-0645(98)00125-8.
Watts, L. J., S. Sathyendranath, C. Caverhill, H. Mass, T. Platt, and N. J. P.
Owens (1999), Modelling new production in the northwest Indian Ocean
region, Mar. Ecol. Prog. Ser., 183, 1–12, doi:10.3354/meps183001.
Wiggert, J. D., R. G. Murtugudde, and J. R. Christian (2006), Annual ecosystem variability in the tropical Indian Ocean: Results of a coupled biophysical ocean general circulation model, Deep Sea Res., Part II, 53,
644–676, doi:10.1016/j.dsr2.2006.01.027.
Wolf, G. T., M. S. Ruthkosky, D. P. Stroup, P. E. Korsog, M. A. Ferman,
G. J. Wendel, and D. H. Stedman (1986), Measurements of SOx, NOx
and aerosol species on Bermuda, Atmos. Environ., 20, 1229–1239.
Xie, P., J. E. Janowiak, P. A. Arkin, R. F. Adler, A. Gruber, R. R. Ferraro,
G. J. Huffman, and S. Curtis (2003), GPCP pentad precipitation analyses:
An experimental dataset based on gauge observations and satellite estimates, J. Clim., 16, 2197–2214, doi:10.1175/2769.1.
11 of 11