GEOPHYSICAL
RESEARCH LETTERS, VOL. 27, NO. 7, PAGES 1013-1016, APRIL 1,200O
Atmospheric intertropical convergence impacts surface ocean
carbon and nitrogen biogeochemistry in the western tropical
Pacific
L
Dennis A. Hansel1
Bermuda Biological Station for Research, Inc., St. Georges, GE-01, Bermuda
Richard A. Feely
Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Seattle
Abstract: Concentrations of inorganic and organic carbon
and organic nitrogen, as well as other hydrographic and biogeochemical variables, were measured in the oligotrophic
waters of the western South Pacific Ocean (5-35”s; 17O”W).
With those data, we assessthe impact of the intertropical convergences of the western tropical Pacific Ocean on surface
ocean biogeochemistry. Low salinity, oligotrophic waters of
the western tropical Pacific underlying regions of high net
precipitation are characterized by concentrations of organic
carbon and nitrogen that are elevated relative to higher salinity (lower net precipitation) zones. We hypothesize that water
column stratification, forced by high net precipitation, favors
enhanced rates of Nz fixation, with resultant elevation of organic carbon and nitrogen concentrations in the region. An
effect of anthropogenic ocean warming and the associated increasedhydrologic cycle in the tropical Pacific may be to alter
the biogeochemical cycling of carbon and nitrogen through
enhancement of N2 fixation.
1. Introduction
Intertropical convergence zones (ITCZ) are major components of the marine hydrologic cycle. Convergence of moisture-laden trade winds in these zones results in persistent,
deep atmospheric convection and precipitation (Merrill,
1989). The ITCZ of the equatorial North Pacific is the most
important of the convergences in the Pacific Ocean. It is normally visible as a band of clouds north of and parallel to the
equator between 2” and 12%J,lying across the entire breadth
of the Pacific Ocean. Over the vast warm pool of the western
equatorial Pacific the ITCZ intersects the South Pacific Convergence Zone (SPCZ), which extends to the southeast over
the South Pacific Ocean.
High rates of precipitation over ocean locales increase
stratification of the water column and introduce biologically
limiting elements such as iron and nitrogen, which can
strongly affect marine primary productivity. The influence of
the atmospheric convergence zones on surface ocean biogeochemistry, such as organic carbon and nitrogen concentrations, remains unknown. Investigations on the factors controlling organic nitrogen concentrations in the oligotrophic
ocean have highlighted the roles of transport from upwelling
Copyright 2000 by the American Geophysical Union.
Papernumber 1999GLOO2376.
0094-8276/00/1999GL002376$05.00
zones (Hansel1 and Waterhouse, 1997) and of nitrogen fixation. Karl et al. (1995) suggestedthat increases in organic nitrogen stocks in the subtropical North Pacific were due to decreased upper-ocean mixing driven by an ENS0 event and a
consequent increase in abundance and activity of nitrogen
fixing organisms. The stabilizing effects of net precipitation
in the tropical Pacific may similarly favor such biogeochemical processes. In this paper we investigate the impact of the
SPCZ on carbon and nitrogen stocks in the oligotrophic westem South Pacific and discuss the role of climate change in the
tropical Pacific in accentuating the impact.
2. Experimental
As part of the U.S. NOAA Ocean Atmosphere Carbon Exchange Study (OACES) samples for total inorganic carbon
(TC02), total organic carbon (TOC), total organic nitrogen
(TON), sea surface partial pressure of CO2 (pCO3 and other
hydrographic variables were collected in the western South
Pacific along 17OOW(World Ocean Circulation Experiment
Line P15S; March-April 1996) (Fig. 1). Hydrographic stations were occupied at 0.5O intervals, with lower sampling
frequency for some of the variables reported here. TOC,
TC02 chlorophyll a and pCOz were determined at sea using
described methods (Hansel1 et al., 1997; Feely et al., 1995;
Lamb et al., 1997; Wanninkhof et al., 1993). TON was determined ashore on frozen samples using an UV oxidation
method (Hansel1and Waterhouse, 1997).
3. Results and Discussiod
In this report we focus on the oligotrophic zone sampled on
the section, including relatively arid regions of the South Pacific subtropical gyre and regions of high net precipitation
underlying the atmospheric convergences (Fig. 1). Regions
of high net precipitation are characterized .by relatively low
surface layer salinity. A zone of low salinity was centered
near 15OSwhile a zone of elevated salinity (due to net evaporation) -was centered to the south near 30’S (Fig. 2a). Mean
concentrations of TON and TOC were elevated in the low salinity waters (5.4 * 0.3 PM and 81.7 + 2.8 PM, respectively)
relative to values of 4.8 &-0.4 PM and 74.2 + 2.5 PM in the
arid region (Table 1). The mean salinity normalized TCO;!
concentration in the low salinity zone (1930 + 40 pmole kg-‘)
was 45 pmole kg-’ lower than the mean values in the high salinity waters (1975 & 19 pmole kg-‘). pC02 was depressedto
below atmospheric levels in the low salinity zone, making the
1013
HANSELL AND FEELY: ATMOSPHERIC CONVERGENCE AND OCEAN CARBON
1014
20N
30s
2
130E
155E
180E
155w
13ow
Longitude
Figure 1. Distribution of annualnet precipitation (m y-‘) in the westernPacific along with the location of the cruise
transect(data from Joseyet al., 1998).
region a sink for atmospheric COa. Chlorophyll a concentrations were higher by a factor of 2 in the low salinity region.
Nitrogen is limiting in oligotrophic waters becausea net effect of primary production is to move surface nitrogen to the
deep ocean as sinking biogenic particles. Oligotrophic re6
4
I
+
a
-30
-20
-10
gions with elevated organic N concentrations require, then,
new sources of N to maintain the elevated stocks. The elevated TON values cannot be attributed to equatorially upwelled nitrate transported as TON to the oligotrophic regions
because the nutrient enriched equatorial Pacific waters are
subducted below, rather than into, the surface warm pool (Lukas and Lindstrom, 1991). Three alternative mechanisms for
producing and maintaining elevated TON in regions of low
salinity are: 1) atmospheric deposition of inorganic and organic nitrogen; *2) upward turbulent flux of nitrate into the
euphotic zone and; 3) elevated rates of N2 fixation, with enhancement due to increased water column stability and/or
deposition of trace elements with dust.
In the region of the SPCZ, the wet deposition of nitrate is
low (1 pmole N03--N mm2d-’ has been reported for Somoa;
'34
Table 1. Surface Ocean Hydrographic and Biogeochemical
Characteristics.
90~~2012050
Net Precipitation(m y-l)
*Salinity
*Temperature(“C)
T80
c>
0
I- 70
b
60
-30
20
1
-10
1900
Latitude
Figure 2. Upper water column distributions of TON, salinity,
TOC and nTC02 in the oligotrophic sector along 17O”W. All
values are upper 50 m means. a) TON (PM N) and salinity,
an$ b) TOC (PM C) and salinity normalized TC02 @moles C
Zones of low salinity indicate regions experiencing
kg )
precipitation in excessof evaporation.
*TOC (PM)
*nTCOz (pmoles kg-‘)
_.PC@ (clatm)
*TON (PM)
*Chlorophyll a (pg 1-l)
Maximum dNOJaz
(p,mol 1-l m-l)
Depth of maximum
10-20”s
25-35”s
cl.5
’ 34.81 I!I 0.20
29.0 I!I 0.7
81.7 * 2.8
1930f40
319 * 17
5.4 I!I 0.3
0.04 -I 0.02
0.08 III 0.04
. =o
35.72 1’10.10
23.3 311.5
74.2 312.5
1975 z!I19
359 c?I12
4.8 III 0.4
0.02 zk0.01
0.07 Ilk0.02
156 I!I 22
192+34
NOj- gradient(m)
*upper 50 m means(*SD). nTC02 concentrationsare
normalizedto a salinity of 35. Maximum vertical gradients
anddepthsfor nitratearethe zonalmeansin the nitracline.
\
HANSELL AND FEELY: ATMOSPHERIC CONVERGENCE AND OCEAN CARBON
Pszenny et al., 1982). Rates for the wet deposition of organic
nitrogen and ammonium as well as dry deposition of all nitrogen forms remain unmeasured so these could more than double the rate due to nitrate deposition alone (Cornell et al.,
1995). Whether or not such low deposition rates could account for the elevated TON values is debatable. The western
Sargasso Sea, another oligotrophic region receiving relatively
high rainfall, is perhaps the best studied oceanic region in
terms of the atmospheric deposition of nitrogen and it’s contribution to ocean biogeochemistry. Wet deposition of nitrogen near Bermuda (Knap et al., 1986) has been reported to be
16-80 pmol N me2d-‘, but even this relatively high rate is
thought to contribute little to the local nitrogen cycle
(Michaels et al., 1993). Given such findings for the Sargasso
Sea, atmospheric deposition of nitrogen is unlikely to be the
primary cause of elevated TON in the western Pacific.
The upward turbulent flux of nitrate from depth is another
potential source of the elevated surface nitrogen. The vertical
gradient of nitrate into the euphotic zone is, however, weak
and of similar value and depth as found in the lower TON
zone (Table 1). The existence of a highly stratified isothermal
barrier layer (Lukas and Lindstrom, 1991) in the low salinity
zone will further restrict the diffusive flux of nitrate to the
surface.
We hypothesize that N2 fixation is the primary source of
elevated organic nitrogen concentrations in the western Pacific. Several findings support this hypothesis. At the time of
the cruise, Trichodesmium spp. biomass was present to levels
of 5 pg 1-i in the low salinity zone (F. Chavez, pers. comm.).
Indeed, high concentrations of diazotrophs (Carpenter, 1983)
as well as low 6% values in suspendedparticulate organic
nitrogen (PON; Saino and Hattori, 1987) are characteristic of
the western tropical Pacific. In the southwest Pacific, a persistent surface enrichment of Trichodesmium, extending from
northeast of New Caledonia to 25’S at the dateline, has been
delineated with remote sensing (Dupouy et al., 2000). Such
enhanced surface pigment concentrations are present during
January-March (Murtugudde et al., 1999), along the axis of
the SPCZ. Finally, the process of N2 fixation releases significant amounts of dissolved organic nitrogen (DON) to the water column (Glibert and Bronk, 1994). Elevated DON concentrations are characteristic of Trichodesmium blooms in the
subtropical North Pacific (Karl et al., 1997).
If N2 fixation is the source of elevated TON and TOC values then the controls on this process should be evident in the
physical environment. Water column stability imparted by
the low density surface layer may play the dominant role in
favoring N2 fixation under the SPCZ. Other potential stimulants of N2 fixation, such as dust and associatedtrace element
input, have low rates of deposition in the region (Prosper0 et
al., 1989). Highly stable surface waters are characteristic of
the isothermal barrier layer in the western Pacific and this
may be a central requirement in creating conditions favorable
to Trichodesmium growth in the region. Similarly, periods of
increased stability in the subtropical North ‘Pacific associated
with El Nino events have been reported to result in increased
abundance and activity of N2-fixing organisms (Karl et al.,
1995).
Elevated TON and TOC concentrations are likely to be
found throughout the region of high net precipitation in the
western tropical Pacific, which in turn can be tracked by the
distribution of low surface salinity. The area1extent of the
low salinity, western Pacific warm pool broadly covers the
1015
distribution of low 6%PON (Saino and Hattori, 1987), as
well as the distribution of elevated diazotroph concentrations
(Carpenter, 1983), the putative source of the elevated organic
matter. If this scenario is correct then the location of enhanced N2 fixation must track the location of the warm pool
as it migrates with ENS0 conditions. During El Nino, a
warm, fresh mixed layer accompanied by an underlying barrier layer develops in the central and eastern equatorial Pacific
in association with increased precipitation (Ando and
McPhaden, 1997). Conversely, during La Nina the- warm
pool is confined to the far western Pacific.
The impact of high precipitation on N2 fixation rates
should be realized not only in the western tropical Pacific but
in other tropical regions as well. These include the equatorial
North Atlantic Ocean, the Indonesian archipelago and the
western Indian Ocean. All of these oligotrophic regions experience high net precipitation (Xie and Arkin, 1997). TON
determinations in these waters are few, but an exception is the
northeastern equatorial Atlantic Ocean where precipitation
(Xie and Arkin, 1997) reaches >5 m y-i. The area of high
precipitation overlaps ocean waters characterized by abundant
Trichodesmium populations (6,500 trichomes rn-‘) and elevated organic nitrogen concentrations (>9 PM; Vidal et al.,
1999). This finding supports the hypothesis that high rates of
net precipitation and the stratification it imparts stimulates N2
fixation in tropical oligotrophic waters.
An effect of anthropogenic ocean warming and climate
change in the tropical Pacific may be to alter the biogeochemical cycling of carbon and nitrogen through enhancement of N2 fixation. Results from modeling (Graham, 1995)
and observational (Flohn et al., 1992) studies are consistent in
suggesting an increasingly active tropical hydrologic cycle
that is driven by increasing tropical ocean temperatures. Increasesin precipitation over the tropical Pacific, in particular,
have been noted (Graham et al., 1994; Nitta and Yamada,
1989). Increased precipitation and surface layer stratification
may amplify the impact of N2 fixation on the regional N cycle, with ramifications on the transport of fixed N to the
higher, oligotrophic latitudes. The contributions to the oceanic cycling of carbon and nitrogen of these vast ocean regions of high net precipitation, and the impacts of changes in
rates of the tropical hydrologic cycle, will make for compelling modeling and field studies.
Acknowledgments. The assistanceof the officers and crew of
NOAA Ship Discoverer is gratefully acknowledged. We thank
Marilyn Roberts, Paula Hansell, Rachel Parsons and Christine Pequignet for their analytical support during the cruises and in our
home laboratories. Support for DAH came from NOAA Award
NA56GP0207 and NSF Grant OCE-972609. This research was supported by the NOAA Climate and Global Change Program as part of
the Global Carbon Cycle program (Contract Numbers GCC-95-627B
and GC-99-220 to RAF). We thank Dr. Lisa Dilling of the NOAA
Office of Global Programs for her efforts in the coordination oft this
project. Contribution Numbers 2089 from the Pacific Marine Environmental Laboratory and 1549 from the Bermuda Biological Station
for Research,
Inc. have been given to this manuscript.
-.
References
Ando, K. & McPhaden, M.J., Variability of surface layer hydrography in the tropical Pacific Ocean, 1’ of Geophys. Res., 102,
23,063-23,078, 1997.
Carpenter, E.J., Nitrogen fixation by the marine Oscillatoria (Trichodesmium) in the world’s oceans, in Nitrogen in the Marine Environment, edited by E.J. Carpenter & D.G. Capone, pp. 65-104,
Academic Press, New York, 1983.
1016
HANSELL AND FEELY: ATMOSPHERIC CONVERGENCE AND OCEAN CARBON
Cornell, S., Rendell, A. & Jickells, T., Atmospheric inputs of dissolved organic nitrogen to the oceans, Nature, 376, 243-246,
1995.
Dupouy, C., Neveux, J., Subramaniam, A., Mulholland, M.R., Montoya, J.P., Campbell, L., Carpenter, E.J. & Capone, D.G., Satellite
captures Trichodesmium blooms in the southwestern tropical Pacific. EOS, 81(2), 13, 2000.
Feely, R.A., Wanninkhof, R., Costa, C.E., Murphy, P.P., Lamb, M.F.
& Steckley, M.D., CO2 distributions in the equatorial Pacific during the 1991-1992 ENS0 event, Deep-Sea Res. II, 42, 365-386,
1995.
Flohn, H., Kapala, A., Knoche, H.R. & Machel H., Water vapour as
an amplifier of the greenhouse effect: new aspects, Meteorol. Z.,
I, 122-138, 1992.
Glibert, P.M. & Bronk, D.A., Release of dissolved organic nitrogen
by marine diazotrophic cyanobacteria, Trichodesmium spp., Appl.
Environ. Microbial., 60,3996-4000, 1994.
Graham, N.E., Simulation of recent global temperature trends, Science, 267,666-671, 1995.
Graham, N.E., Bamett, T.P., Wilde, R., Ponater, M. & Shubert, S.,
On the roles of tropical and midlatitude SSTs in forcing interannual to interdecadal variability in the winter Northern Hemisphere
circulation, J. Clim., 7, 1416-1441, 1994.
Hansell, D.A., Carlson, C.A., Bates, N. & Poisson, A., Horizontal
and vertical removal of organic carbon in the equatorial Pacific
Ocean: a mass balance assessment, Deep-Sea Res. 11, 44, 2 1152130,1997.
Hansell, D.A. & Waterhouse, T.Y., Controls on the distribution of
organic carbon and nitrogen in the eastern Pacific Ocean, DeepSea Res. I, 44,843-857, 1997.
Josey, S. A., Kent, E. C. & Taylor, P. K., The Southampton Oceanography Centre (SOC) Ocean - Atmosphere Heat, Momentum and
Freshwater Flux Atlas, Southampton Oceanography Centre Report No. 6, 30 pp. & figs., 1998.
Karl, D.M., Letelier, R., Hebel, D., Tupas, L., Dore, J., Christian, J.
& Winn, C., Ecosystem changes in the North Pacific subtropical
gyre attributed to the 1991-92 El Nino, Nature, 3 73, 230-234,
1995.
Karl, D.M., Letelier, R., Tupas, L., Dore, J., Christian, J. & Hebel,
D., The role of nitrogen fixation in biogeochemical cycling in the
subtropical North Pacific Ocean, Nature, 388,533-538, 1997.
Knap, A.H., Jickells, T., Pszenny, A. & Galloway, J., Significance of
atmospheric-derived fixed nitrogen on productivity in the Sargasso Sea,Nature, 320, 158-160, 1986.
Lamb, M.F., Bullister, J.L., Feely, R.A., Johnson, G.C., Wisegarver,
D.P., Taft, B., Wanninkhof, R., McTaggart, K.E., Krogslund,
K.A., Mordy, C., Hargreaves, K., Greeley, D., Lantry, T., Chen,
H., Huss, B., Millero, F.J., Byrne, R.H., Hansell, D.A., Chavez,
F.P., Quay, PD., Guenther, P.R., Zhang, J.-Z., Gardner, W.,
Richardson, M.J. & Peng, T.-H., Chemical and hydrographic
measurements in the eastern Pact@ during the CGC94 expedition
(WOCE section PI8), NOAA Data Report ERL PMEL-61a
(PB97-158075), 235 pp., 1997.
Lukas, R. & Lindstrom, E., The mixed layer of the western equatorial
Pacific Ocean, J. of Geophys.Res., 96,3343-3357, 1991.
Merrill, J.T., Atmospheric long-range transport to the Pacific Ocean,
Chem.1Oceanogr., 10, 15-50, 1989.
Michaels, A.F., Siegel, D.A., Johnson, R.J., Knap, A.H. & Galloway,
J.N., Episodic inputs of atmospheric nitrogen to the Sargasso Sea:
contributions to new production and phytoplankton blooms,
Global Biogeochem. Cycles, 7,339-35 1, 1993.
Murtugudde, R.G., Signor& S.R., Christian, J.R., Busalacchi, A.J.,
McClain, C.R. & Picaut, J., Ocean color variability of the tropical
Indo-Pacific basin observed by SeaWiFS during 1997-1998. J. of
Geophys. Rex, 104, 1835I-18366, 1999.
Nitta, T. & Yamada, S., Recent warming of tropical sea surface temperature and its relationship to the northern hemisphere circulation, J Meteorol. Sot. Jpn., 67, 375-383, 1989.
Prospero, J.M., Uematsu, M. & Savoie, D.L., Mineral aerosol transport to the Pacific Ocean, Chem. Oceanogr., 10,188-218, 1989.
Pszenny, A.A.P., MacIntyre, F. & Duce, R.A., Seasalt and the acidity
of marine rain on the windward coast of Samoa, Geophys. Res.
Lett., 9, 75 I-754, 1982.
Saino, T. & Hattori, A., Geographical variation of the water column
distribution of suspendedparticulate organic nitrogen and its 15N
natural abundance in the Pacific and its marginal seas, Deep-Sea
Res., 34, 807-827, 1987.
Vidal, M., Duarte, CM. & Agusti, S., Dissolved organic nitrogen and
phosphorus pools and fluxes in the central Atlantic Ocean, Limnol. Oceanogr., 44, 106-115, 1999.
Wanninkhof, R. & Thoning, K., Measurement of COz in surface water using continuous and discrete sampling methods, Mar. Chem.,
44, 189-205, 1993.
Xie, P. & Arkin, P.A., Global precipitation: a 17-year monthly analysis based on guage observations, satellite estimates, and numerical
model outputs, Bull. Amer. Meteorol. Sot., 78,2539-2558, 1997.
D. Hansell, Bermuda Biological Station for Research, Inc., 17
Biological Lane, St. Georges, GE-01, Berrnuda. (e-mail:
[email protected])
R. Feely, Pacific Marine Environmental Laboratory, National
Oceanic and Atmospheric Administration, Bin C15700,7600 Sand
Point Way, Seattle, WA 98 115. (e-mail: [email protected])
(Received June 9, 1999; acceptedJanuary 18,200O.)