Excess nitrate and nitrogen fixation in the North...

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Marine Chemistry 84 (2004) 243–265

Excess nitrate and nitrogen fixation in the North Atlantic Ocean

Dennis A. Hansella,*, Nicholas R. Batesb, Donald B. Olsona

aRosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, USAbBermuda Biological Station for Research, Inc., St. Georges, GE 01 Bermuda

Received 20 September 2002; received in revised form 7 August 2003; accepted 8 August 2003

Abstract

The process of nitrogen fixation in the subtropical North Atlantic has received considerable study over the last few

decades. The findings have highlighted a large discrepancy in estimates for the locations and rates of nitrogen fixation when

results from biological techniques are compared to geochemical techniques. Here, we evaluated the distribution and rates of

excess nitrate development in the North Atlantic using World Ocean Circulation Experiment (WOCE) nutrient data. These

data indicate that excess nitrate development is largely confined to depths of f 150–400 m in the region of 15–25jN by

25–75jW, an area considerably smaller than that employed by Gruber and Sarmiento [Glob. Biogeochem. Cycles 11 (1997)

235] (10–50jN by 10–90jW) to estimate rates of nitrogen fixation in the North Atlantic. The areally integrated nitrogen

fixation rate for the subtropical North Atlantic was 0.045 mol N m� 2 year� 1, or 62% of the geochemical estimate by Gruber

and Sarmiento [Glob. Biogeochem. Cycles 11 (1997) 235]. The regional rate of fixation was 3.1�1011 mol N year� 1 (1.5–

4.6� 1011 mol N year� 1 given a 50% uncertainty), similar to rates expected from biological measures of fixation, but only

15% of the areal rate estimated by Gruber and Sarmiento [Glob. Biogeochem. Cycles 11 (1997) 235]. An accurate

assessment of the region over which excess nitrate accumulates is critical to the estimate of nitrogen fixation, but remains

prone to large uncertainty because of the gaps in spatial coverage. Additional survey work in the North Atlantic must be

done to lessen the uncertainty. With this work, we reduce the differences between the biological and geochemical rate

estimates, and describe a conceptual model for the location and dynamics of nitrate excess development in the North

Atlantic.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Nitrate; Phosphate; N/P ratio; Subtropical North Atlantic; Nitrogen fixation

1. Introduction

Nitrogen is the limiting nutrient for phytoplankton

growth and export production in many regions of the

global ocean (Codispoti, 1989). The biological pump

removes N from the euphotic zone with high efficiency.

0304-4203/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.marchem.2003.08.004

* Corresponding author. Tel.: +1-305-361-4078.

E-mail address: [email protected] (D.A. Hansell).

The N-bearing, sinking biogenic particles carry carbon

away from the surface ocean, thus sequestering carbon

into the ocean interior and away from the atmosphere

on time scales of years to centuries. The oceanic

nitrogen budget is not static, but set by the balance of

sources and sinks. Fixed nitrogen is lost from the

oceans during the process of denitrification. In order

to maintain the global budget, sources of new nitrogen

are required. These sources include wet and dry depo-

sition of N from the atmosphere, riverine inputs of

D.A. Hansell et al. / Marine Chemistry 84 (2004) 243–265244

inorganic and organic N, and N2 fixation by marine

diazotrophs.

The new N supply most difficult to accurately

quantify is N2 fixation by pelagic phytoplankton

such as Trichodesmium. The global N2 fixation rate

in the ocean has been recently estimated at

8F 3� 1012 mol N year� 1, with c 2� 1012 mol

N year� 1 fixed in the North Atlantic Ocean (Gruber

and Sarmiento, 1997, hereafter GS97). A similar

global rate has been estimated by Lee et al. (2002)

who evaluated net carbon production in nitrate-

depleted tropical and subtropical waters. The GS97

rate of N2 fixation could remove c 5� 1013 mol C

year� 1 from the upper ocean, or 10% of marine

new production estimated by Chavez and Togg-

weiler (1995). Because of the high N/P ratio of

marine diazotrophs, mineralization of these sinking

particles below the euphotic zone can result in

nitrate concentrations in excess of those predicted

from phosphate concentrations and N/P element

stoichiometry (e.g., Redfield et al., 1963; Takahashi

et al., 1985; Anderson and Sarmiento, 1994). N2

fixation was invoked by Fanning (1987, 1992) to

explain high N/P ratios, for example, in the ther-

mocline waters of the western North Atlantic sub-

tropical gyre (or Sargasso Sea). More recently, the

parameter N* was introduced by Michaels et al.

(1996) and Gruber and Sarmiento (1997) to indicate

the degree to which the nitrate concentration is in

excess of that expected from the remineralization of

phosphate at stoichiometries of 16:1 (Redfield et al.,

1963). N* spatial distributions (and concentration

gradients) in the main thermoclines of the world’s

oceans broadly reflect the global distribution of N2

fixation and denitrification. Elevated N* values

indicate a history of net additions of N relative to

P, while low values indicate net removal of N due

to denitrification.

Over the past decade, both geochemical and direct

biological techniques have been employed to deter-

mine the distribution and rates of N2 fixation in the

North Atlantic. Based on depth integrated excess

nitrate (N*) and rates of thermocline ventilation,

annual rates of N2 fixation in the subtropical gyre

of the North Atlantic have been calculated to lie in

the range of 72 mmol N m� 2 year� 1 (GS97) to

133–230 mmol N m� 2 year� 1 (Michaels et al.,

1996). The region of the N* maximum was located

in the western Sargasso Sea, largely between 20jand 40jN in the waters of the main thermocline

(between 200 and 600 m).

These geochemical estimates of N2 fixation in the

western Sargasso Sea (Michaels et al., 1996; GS97)

are several fold higher than most direct biological

measurements of N2 fixation by dominant diazotro-

phic phytoplankton (i.e., Trichodesmium spp.) in the

overlying water. Carpenter (1983) reviewed studies

of directly determined N2 fixation in the entire

Atlantic and found relatively low rates of 0.25–

2.26 mmol N m� 2 year� 1. Capone et al. (1997),

summarizing work in the North Atlantic over the

past few decades, reported a broad range of N2

fixation rates, from 0.51 mmol N m� 2 year� 1 in

the NW Sargasso Sea to 101 mmol N m� 2 year� 1

in the NE Caribbean Sea. In the SW North Atlantic,

Carpenter et al. (1999) reported elevated nitrogen

fixation rates of 3 mmol N m� 2 day� 1 in an

extensive bloom of a colonial diatom containing an

N2-fixing cyanobacterial endosymbiont. They esti-

mated that 4�1010 mol N could be added to the

euphotic zone by the N2 fixers over the lifetime of

the bloom (10 days). More recently, Orcutt et al.

(2001) measured low mean annual rates of N2

fixation (15 mmol N m� 2 year� 1) at the Bermuda

Atlantic Time-series Study (BATS) site. They found

that the surface waters of the western Sargasso Sea

near Bermuda have relatively low densities of Tri-

chodesmium colonies and trichomes, but seasonally

elevated biomass was found during the stratified

summer months in some, but not all, years. South

of Bermuda (26jN) the fixation rate was higher at 34

mmol N m� 2 year� 1.

Except for the single very high rate reported by

Capone et al. (1997) in the NE Caribbean, the N2

fixation rates determined using direct methods are

considerably lower (0.25–34 mmol N m� 2 year� 1)

than the geochemical estimates of 72–230 mmol N

m� 2 year� 1 (Michaels et al., 1996; GS97). It appears

that, even with interannual variability, the biologically

measured fixation rates do not account for more than

20% of the minimum rate determined geochemically.

Some of the biological rates reported may be under-

estimates of the true rates due to undersampling of the

complete suite of diazotrophs present (Zehr et al.,

2001), but the extent of the underestimate is not

known.

D.A. Hansell et al. / Marine Chemistry 84 (2004) 243–265 245

The goals of this paper are to develop an improved

understanding of the spatial distribution and produc-

tion of the excess nitrate found in the subtropical gyre

of the North Atlantic. Our approach was to quantify

geochemically the rate of excess nitrogen production

using end-member analyses and isopycnal mixing

considerations. This approach narrows the discrepancy

between geochemical and biological estimates of ni-

trogen fixation rates. The temporal variability for

excess nitrate in the Sargasso Sea is large. An analysis

of that variability can be found in Bates and Hansell

(2003).

Fig. 1. (A) Locations and designations of WOCE sections and

stations evaluated. Locations of mixed water regions (MWR) I and

II are shown. (B) Schematic of major currents in the region under

study (AC, Azores Current; GC, Guyana Current; NBC, North

Brazil Current; SEC, South Equatorial Current; NECC, North

Equatorial Counter Current; NEC, North Equatorial Current; GSE,

Gulf Stream Extension; NAC, North Atlantic Current).

2. Data analysis

2.1. Data sources

In this analysis, we investigated the distribution

and rates of formation of excess nitrate in the North

Atlantic. Data employed for end-member analysis,

mixing and excess nitrate formation were taken from

the World Ocean Circulation Experiment (WOCE)

hydrographic and nutrient data collections main-

tained by the World Hydrographic Program Data

Archive Center (http://www-ocean.tamu.edu/WOCE/

Progress/data.html). North Atlantic WOCE sections

(and dates of occupation) evaluated are AR01 (Feb-

ruary 1998), A02 (November 1992), A03 (October

1993), A16N (August 1988), A16C (April 1989),

A17N (March 1994), A20 (July 1997), A22 (August

1997) and A24 (June 1997). Section locations are

shown in Fig. 1A. Nutrient data from A03 showed

unreasonably high station-to-station variability, verti-

cal variability and variability in deep water values,

so these data were not used in this analysis. Phos-

phate data are not available on AR01 from c 21.5jto 45.5jW, so our analysis of nutrient distributions is

absent that portion of the line. Salinity data from the

Reid/Mantalya data set (Reid, 1994) were employed

for evaluating subtropical water mixing in the North

Atlantic. Water mass ages were determined from

WOCE North Atlantic pCFC-12 data (e.g., Doney

et al., 1997) using standard solubility considerations

(Warner and Weiss, 1985) and time-series data of the

atmospheric pCFC-12 transient (Walker et al., 2000).

It was assumed that thermocline waters at the out-

crop were in equilibrium with the present atmo-

sphere. Most figures shown here were constructed

with Ocean Data View (Schlitzer, 2002).

We used the concentrations of excess nitrate

(DINxs) as a tracer, where

DINxs ¼ N � 16P ð1Þ

and N is the concentration (Amol kg� 1 N) of

nitrate (plus nitrite where available) and P is the

http:\\www-ocean.tamu.edu\WOCE\Progress\data.html


concentration of soluble reactive phosphate (Amol

kg� 1 P). DINxs is the excess in nitrate concentra-

tion relative to that expected from Redfield et al.

(1963) stoichiometry. The coefficient 16 is the

Redfield ratio of N/P, as confirmed by Takahashi

et al. (1985) and Anderson and Sarmiento (1994).

The coefficient was likewise employed by GS97 in

their evaluation of N* distributions. The GS97

definition of N* is:

N* ¼ ðN � 16P þ 2:90Þ � 0:87 ð2Þ

The constant 2.90 and the 0.87 multiplier were

used in the N* equation to force the global mean

N* value to be zero. More recently, the equation

has been modified with removal of the multiplier

0.87 (Deutsch et al., 2001). The relationship be-

tween N*, as modified by Deutsch et al. (2001),

and DINxs is

N* ¼ DINxs þ 2:9 ð3Þ

giving a constant offset of 2.9 Amol kg� 1 N

between the two variables. We elected to use the

variable DINxs instead of N* since we focused on a

single ocean basin and hence did not require the

constant to force the global mean N* value to be

zero (GS97). Given the constant offset, spatial

gradients of the excess nitrate, whether calculated

as N* or DINxs, would be identical.

2.2. Water mass composition determination

In this paper, the distribution of DINxs was calcu-

lated primarily for the tropical and subtropical North

Atlantic. From the distributions and water mass ages,

we estimated the rate of DINxs development, as well

as identified regions of DINxs decrease due to water

mass mixing. In order to account for binary mixing in

modifying the distribution of DINxs in the subtropical

gyre, the two end-member compositions were deter-

mined for rh surfaces 26.0, 26.5 and 27.0. These

potential density surfaces encompass the part of the

water column showing excess nitrate development in

the North Atlantic.

The formation of upper thermocline waters in the

North Atlantic subtropical gyre involve water mass

formation and subduction in the North Atlantic,

then mixing with South Atlantic water advected

across the equator as part of the global thermoha-

line circulation (Stommel, 1965; Schmitz and

Richardson, 1991). The thermocline can be ana-

lyzed using a two component potential temperature

(h)-salinity mixing calculation. The h-salinity dia-

gram from the Reid/Mantalya data set (Reid, 1994)

and the distribution of temperature on density

surfaces from the WOCE data are shown in Fig.

2. The main feature in the h/S curve is the

pronounced salinity maxima to the right of the

figure. This defines the Subtropical Underwater

(STUW), a water mass formed in the eastern

subtropical gyre that is then mixed with southern

hemisphere water and deeper thermocline waters as

it is carried around the subtropical gyre. If waters in

the STUW along the right hand side of the h/Sdiagram (labeled NC for northern component) are

defined as 100% STUW on density surfaces, and

the left hand side of the cloud is defined as the

southern component end-member (i.e., 0% STUW),

then the other points in the h/S diagram can be

quantified as a percentage of the component formed

in the gyre interior (% STUW). The Reid data were

used to produce a map of the percentage of STUW

formed entirely in the North Atlantic and found on

the 26.5 potential density surface (Fig. 3). This

analysis identifies a region in the eastern limb of

the gyre as the source of newly formed gyre water.

This water formed in the gyre is mixed with

southern component water to create the mixed

waters of the western gyre. Potential temperature

on rh density surfaces (Fig. 2B) was used to

compute water mass fractions in the mixed water

regions (discussed below).

The northern components of the three potential

density surfaces considered here were taken from

WOCE section A16N (Table 1). The northern

component of the rh 27.0 surface was taken from

a region with low apparent oxygen utilization

(AOU, Fig. 4) and low pCFC-12 age lying between

42j and 45jN (Fig. 5). This zone is a few degrees

south of the 46jN winter outcrop position for this

density surface (Jenkins, 1987; Robbins et al.,

2000). The northern component of the rh 26.5

surface was determined in the region of maximum

subtropical water content (Fig. 3), low AOU (Fig.

4) and minimum water mass pCFC-12 age (Fig. 5),

Fig. 2. (A) Potential temperature (jC)/salinity diagram from the Reid/Mantalya data set (Reid, 1994). The two spline fits for gyre end points

(STUW, or northern component (NC) water, and southern component (SC) water) were used to produce the chart of percent North Atlantic

subtropical gyre water over the basin shown in Fig. 3. (B) Scatterplot of potential temperature (h, jC) and sigma theta (rh, kg/m3) in the North

Atlantic (0–60jN) from WOCE data. At each potential density, the coldest and warmest waters are SC and NC, respectively. Mixing of these

waters largely occurs along isopycnal surfaces, so temperatures intermediate to NC and SC indicate degree of mixing.


Fig. 3. Distribution of subtropical underwater formed entirely in the North Atlantic, as indicated by percent fraction of the water, on the rh 26.5

surface. Values >90% indicate the region of formation.


broadly centered near 28jN, 25jW. The outcrop

position for this potential density layer is near

33jN in the northeast Atlantic (Jenkins, 1987).

For the rh 26.0 surface, the region of 19–24jN(Table 1) exhibited low apparent oxygen utilization

(AOU; Table 2) as well as young pCFC-12 age

(Fig. 5) and thus was taken as representative of

northern component water on that density surface.

For the southern component, we chose equatorial

waters taken to be representative of southern hemi-

sphere water transiting into the North Atlantic (Fig.

1B). The southern component waters were evaluat-

ed in the North Equatorial Counter Current using

data from WOCE section A17N (1–5.5jN; Fig.

1B), with the rh 26.0 surface near 110 m, the rh

26.5 surface near 150 m and rh 27.0 near 370 m.

We also evaluated waters of the Guyana Current

(Fig. 1B) as the southern component end-member,

but found that where the WOCE sections (A17N

and A20) crossed the current, salinity and potential

temperature already showed significant mixing of

southern and northern component waters. The

waters of the North Equatorial Counter Current still

retained the low salinities and low temperatures

expected of southern component waters (Fig. 2,

Table 2). As these waters have the same source

as the Guyana Current (i.e., the North Brazil

Current; Fig. 1B), they are reasonable representa-

tives of southern component water prior to mixing

with the northern waters.

Water mass composition was determined using

regression analyses (Fig. 6). WOCE CTD data were

Fig. 4. Distribution of AOU (Amol kg� 1) on rh surfaces (A) 26.5

and (B) 27.0.

Table 1

WOCE sections employed for generating regressions to determine

concentrations of variables on rh 26.0, 26.5 and 27.0 in the northern

and southern component end-members and the mixed water regions

under evaluation

Northern

component

Southern

component

MWRI MWRII

rh 26.0

WOCE

section

A16N A17N – A22

Latitude 19–24jN 1–5.5jN – 19–24jNrh 25.75–26.25 25.75–26.25 – 25.8–26.3

rh 26.5

WOCE

section

A16N A17N – A22

Latitude 26–30jN 1–5.5jN – 19–24jNrh 26.4–26.7 26.3–26.7 – 26.45–26.8

rh 27.0

WOCE

section

A16N A17N A16N A22

Latitude 42–45jN 1–5.5jN 20.5–24jN 19–24jNrh 26.9–27.2 26.9–27.25 26.75–27.25 26.8–27.2

Horizontal (latitudinal) and vertical (rh) ranges of data selected for

each region are shown.


used for determining potential temperature at specific

potential densities, while WOCE bottle data were

employed for nutrient and other determinations (i.e.,

nitrate, phosphate, silicate, AOU, pCFC-12 age). In

each region (Table 1), rh was plotted against the

relevant variables. The rh range bounding each po-

tential density surface (rh 26.0, 26.5 and 27.0) that

showed linearity with the variable of interest in

property/property plots (e.g., Fig. 6) was characterized

by linear regression. The regression equations were

used to estimate property values for the variables at rh

26.0, 26.5 and 27.0 in each region. End-member

compositions determined by this protocol are shown

in Table 2.

Specific regions of the gyre evaluated for DINxs

development were designated mixed water regions I

and II (MWR I and II; Fig. 1A). The highest DINxs

values on rh 27.0 were located in MWR I. It was also

the region taken as representative of northern compo-

nent water on rh 26.0. MWR II was chosen for

analysis because it had elevated DINxs values on rh

26.0 and it served for analysis of DINxs development

across the North Atlantic on rh 26.5.

The waters at MWR I and II are a mix of

northern and southern components so it was neces-

sary to first calculate the preformed values of the

variables of interest. Preformed values are the con-

centrations expected from mixing of the end-mem-

bers alone. Mixing proportions of the northern and

southern components in the mixed water regions

were estimated with the presumption that the waters

on a density horizon can be approximated by binary

mixing. To calculate preformed values we followed

the protocol of Takahashi et al. (1985), employing

potential temperature as the conservative tracer.

Potential temperatures in the northern and southern

end-members are separated by c 7–10 jC on the

density surfaces evaluated here (Fig. 2B). The

fraction of the northern ( fn) and southern ( fs)


Fig. 5. Distribution of pCFC12 ages in years on rh surfaces (A) 26.0, (B) 26.5 and (C) 27.0.


component end-members in the mixed water region

were obtained from

fs ¼ ðhOn � hÞ=ðhOn � hOs Þ ð4Þ

and

fn ¼ 1� fs ð5Þ

where hnO and hs

O are the potential temperatures of

the northern and southern components, respectively,

and h is potential temperature measured in the

region being evaluated. The preformed concentra-

tions (or age) in the mixed water region were

obtained by

CO ¼ fnCOn þ fsC

Os ð6Þ

where CnO and Cs

O are the preformed quantities in

the northern and southern end-members, respective-

ly. Values for CnO and Cs

O were computed using the

rh- property regressions established (e.g., Fig. 6).

Measured and preformed concentrations in MWR I

and II are reported in Table 2. The differences

between preformed and measured values are a

measure of the concentration changes due to pro-

cesses other than mixing, i.e., nitrogen fixation and

the biogeochemical processes of interest.

The largest contributor to error in estimating the

modification of water properties by mixing is the

local heterogeneity of water masses due to mesoscale

and sub-mesoscale spatial variability. The two com-

ponent mixing analysis is particularly robust in the

present case since we are considering a water mass

formed in the North Atlantic subtropical gyre and its

modification through the admixture of southern

hemisphere water that is well characterized in the

North Brazil Current. Errors in the estimation of

properties expected from mixing alone were assessed

using the root mean square variability in regional

observations (measured values). These errors were

then propagated through the equations for determin-

Table 2

Potential temperature, composition and ages of end-members and mixed water regions I and II on rh 26.0, 26.5 and 27.0

Northern Southern MWR II

component componentMeasured Preformeda Differenceb

rh 26.0—mixing into MWR II between northern (at MWR I) (87F 0.3%)c and southern (13%)c components

h (jC) 21.4F 0.1 17.8F 0.2 20.9F 0.2 – –

Silicate (Amol kg� 1) 0.2F 0.2 3.6F 1.2 0.5F 0.1 0.7F 0.4 � 0.2F 0.3

Nitrate (Amol kg� 1) 0.9F 0.9 8.3F 3.6 1.4F 0.7 1.9F 1.3 � 0.5F 0.6

Phosphate (Amol kg� 1) 0.06F 0.05 0.6F 0.2 0.06F 0.04 0.14F 0.1 � 0.1F 0.1

AOU (Amol kg� 1) 8.1F11.0 62.3F 27.1 27.6F 6.0 15.4F 13.3 12.2F 0.7

pCFC-12 age (years) 1.3F 2.9 11.9F 1.5 2.4F 0.9 2.7F 2.7 � 0.3F 1.8

DINxs (Amol kg� 1) � 0.1F 0.3 � 0.2F 1.1 0.4F 0.2 � 0.3F 0.4 0.7F 0.2

Northern Southern MWR II

component componentMeasured Preformed Difference

rh 26.5—mixing into MWR II between northern (90F 0.3%) and southern (10%) components

h (jC) 18.0F 0.2 14.0F 0.1 17.6F 0.1 – –

Silicate (Amol kg� 1) 0.6F 0.1 7.7F 0.4 1.6F 0.1 1.3F 0.2 0.3F 0.1

Nitrate (Amol kg� 1) 2.3F 0.6 17.2F 1.6 5.7F 0.5 3.8F 0.8 1.9F 0.3

Phosphate (Amol kg� 1) 0.1F 0.0 1.2F 0.1 0.3F 0.0 0.2F 0.1 0.1F 0.1

AOU (Amol kg� 1) 16.0F 5.3 108.5F 14.7 42.5F 4.9 25.2F 6.6 17.2F 1.7

pCFC-12 age (years) 2.7F 1.0 13.9F 1.5 9.0F 2.1 3.8F 1.1 5.2F 1.0

DINxs (Amol kg� 1) 0.7F 0.6 � 1.9F 0.3 0.9F 0.1 0.4F 0.6 0.5F 0.5

Northern Southern MWR I


rh 27.0—mixing into MWR I between northern (102F 1%) and southern (� 2%) components

h (jC) 12.7F 0.1 8.7F 0.2 12.8F 0.1 – –




AOU (Amol kg� 1) 18.4F 4.8 155.0F 8.3 98.4F 11.4 18.4F 6.8 80.0F 4.6


DINxs (Amol kg� 1) 0.4F 0.8 � 1.8F 0.5 1.2F 0.4 0.4F 0.9 0.9F 0.5

MWR I Southern MWR II


rh 27.0—mixing into MWR II between MWR I (81F1%) and southern (19%) component

h (jC) 12.8F 0.1 8.7F 0.2 12.0F 0.5 – –




AOU (Amol kg� 1) 98.4F 11.4 155.0F 8.3 124.7F 11 108.9F 13.3 15.8F 2.3


DINxs (Amol kg� 1) 1.3F 0.4 � 1.7F 0.5 0.3F 0.3 0.7F 0.1 � 0.3F 0.2

Changes in composition on rh 27.0 between MWR I and MWR II, with preformed values expected from mixing between southern component

waters and those of MWR I, are also considered.a Preformed values in the mixed waters are those expected from mixing of the northern and southern component waters.b Differences between measured and preformed concentrations are due to processes other than mixing.c Percentages of the northern and southern components in the mixed water regions considered.



ing preformed concentrations and rates using product

and quotient rules given in Rade and Westergren

(1992). The assumption was that the largest uncer-

tainty was in determining the end points, due to local

variability at those sites. The errors increase for very

young waters and those with concentrations approach-

ing the level of the regional variance.

3. Results and discussion

3.1. Distribution of DINxs

Contoured sections of DINxs on three meridional

sections (WOCE lines A16, A20 and A22) are shown

in Fig. 7. The highest concentrations of DINxs lie

between rh surfaces 26 and 27, with the southern

regions showing the greatest accumulations. The

horizontal distributions of DINxs on the rh 26, 26.5

and 27 surfaces are shown in Fig. 8. The zero isoline

indicates the locations of Redfield ratios of N and P,

whereas values >0 Amol kg� 1 indicate the presence of

excess nitrate relative to phosphate. On the rh 26

surface, the most elevated DINxs values (>0.5 Amol

kg� 1) were located in the southwestern North Atlan-

tic. The highest DINxs values on the two other density

surfaces were largely in the southeastern sector of the

North Atlantic basin, where values reached >1 and

>1.5 Amol kg� 1 on the rh 26.5 and 27.0 surfaces,

respectively (Fig. 8). The lowest values on all surfaces

were in the northeast of the basin and in the equatorial

region, reflective of low DINxs northern and southern

components, respectively.

The DINxs distributions shown in Fig. 8 are based

on section occupations that took place over a 10-year

period, so some temporal variations exist that are not

shown in the distribution maps. For example, DINxs

values on the 26.5 surface, west of 60jWon the AR01

line, were >1 Amol kg� 1 during the February 1998

occupation, as shown in Fig. 8B. Values on A22

(occupied August 1997), at the crossover point with

Fig. 6. Representative data regressions employed to determine end-

member and mixed water region compositions on the potential

density surfaces of interest. Data shown are potential temperature (h,jC) and potential density (rh, kg/m

3) at 26.5 from the region of (A)

the northern component, (B) MWR II and (C) the southern

component.

Fig. 7. Contours of DINxs (Amol kg� 1) on WOCE sections (A) A16, (B) A20 and (C) A22.


AR01, were < 1 Amol kg� 1, or as much as 0.6 Amol

kg� 1 less than the values found during AR01. Tem-

poral variability of DINxs in the Sargasso Sea is rapid,

with large changes occurring over seasonal time scales

(Bates and Hansell, 2003). We chose to map the

elevated values present on AR01, but the meridional

extent of those elevated values at the time of the

AR01 occupation is unknown.

Gyre scale circulation patterns, the mixing propor-

tions of low DINxs northern and southern component

waters, in situ DINxs development, and the vertical

sloping of the isopycnals across the basin combine to

control the distributions of DINxs. Taking the rh 26.5

surface (Fig. 8B) as an example for the impact of

these processes, the northern limb of the gyre under-

goes mixing with low-DINxs northern water as it

flows to the east (Line A, Fig. 9). During southward

flow (Line B, Fig. 9), DINxs builds to its highest levels

in the gyre. The southern limb of the gyre (Line C,

Fig. 9) experiences a net decrease in DINxs by dilution

with low-DINxs southern component water.

A distinctive difference in the DINxs distributions

is the southwestern location of the DINxs maximum

on the shallowest potential density surface (rh 26.0)

Fig. 8. Contoured distributions of DINxs (Amol kg� 1) on rh surfaces

(A) 26.0, (B) 26.5 and (C) 27.0.

Fig. 9. Schematic of subtropical gyre circulation, with inputs of SC

and NC waters (the latter introduced vertically in the region

identified by the dashed oval). Circulation streamlines overlay the

contoured distribution of DINxs on the rh 26.5 surface. Lines A, B

and C are discussed in the text.

Fig. 10. Sigma theta in the upper 1000 m across the North Atlantic

at 24.5jN (WOCE section AR01).


versus the southeastern locations of maxima for the

deeper surfaces. A difference in the DINxs distribu-

tions (Fig. 8) between the 26.5 and 27.0 density

surfaces emerges as the gyre circulation moves the

high DINxs waters from east to west on the southern

gyre limb (Fig. 8B,C). DINxs on rh 26.5 decreases by

f 0.25 Amol kg� 1 across the basin, while on the rh

27.0 surface DINxs decreases by >1 Amol kg� 1.

The distribution of density across the North Atlantic

shows that the three surfaces considered here are

relatively shallow in the eastern basin and deepen in

the western basin (Fig. 10). We suggest that if the

process responsible for DINxs development is occur-

ring somewhat uniformly across the basin, and if the

depth of DINxs development across the basin is similar,

then the unequal distributions of DINxs shown in Fig.

Fig. 12. Distribution of the shallowest depths where DINxs = 0

Amol kg� 1.


8 would be expected due to deepening of the isopyc-

nals. We can test for the depths of incipient DINxs

development by evaluating the distribution of the depth

where DINxs = 0 Amol kg� 1 in the North Atlantic.

Since DINxs in the surface mixed layer is generally

< 0 Amol kg� 1, while DINxs at the depth of the DINxs

maximum is normally >0.5 Amol kg� 1, the transition

from negative to positive DINxs (i.e., the depth where

DINxs = 0 Amol kg� 1) is a useful indicator of the depth

at which DINxs begins to develop. DINxs increases to

>0 Amol kg� 1 at depths generally >150m (Figs. 11 and

12) across the entirety of the gyre. Because the iso-

pycnals are relatively shallow in the east (Fig. 10),

development of DINxs in that part of the basin will be

weak on the shallowest surfaces (rh 26.0 was < 150 m

deep) and stronger on the deeper two surfaces (rh 26.5

and 27.0 at 150–450 m deep). In the west, the rh 26.0

surface deepens to intersect the depths of DINxs pro-

duction, while the rh 27.0 surface may lie below the

depth of high DINxs production, and therefore be more

strongly impacted by dilution with low DINxs southern

component water. The effect of sloping isopycnals and

dilution is depicted schematically in Fig. 13. The result

of deepening of density surfaces from east to west (Fig.

10), the observed basin wide uniformity in the depths of

DINxs development (Fig. 12), and dilution by low

Fig. 11. Depth profiles of DINxs at WOCE stations in the area

bounded by 15–30jN across sections A22 (66jW), A20 (52jW)

and A16N (27jW).

DINxs southern component waters in the west (Fig.

9), would be unequal east–west DINxs distributions

and gradients on the rh 26.0, 26.5 and 27.0 surfaces, as

seen in Fig. 8.

3.2. Development of DINxs

It appears that DINxs develops most strongly on the

rh 26.0 surface during transit from the southeast to the

southwest portions of the gyre (Fig. 8A), and on the

rh 26.5 and 27.0 surfaces during transit from the

northeast (near sites most impacted by northern com-

ponent ventilation) to the southeast of the basin (Figs.

8B,C and 9). On the deeper two surfaces, with

subsequent flow to the west, DINxs is apparently

diluted with southern component water, with the

strongest dilution effect seen on the rh 27.0 surface.

The regions of DINxs development and dilution can

be further studied by plotting DINxs against a variable

such as AOU. The AOU maxima (Fig. 4) are located

in the circulation shadow zone (Luyten et al., 1983)

off NW Africa (these waters are not included in the

following analysis of DINxs/AOU). Low AOU in

recently ventilated northern component water (NE

basin) and high AOU in older southern component

water (equatorial region) are evident (Fig. 4). Proper-

ty/property plots of DINxs and AOU show low DINxs

values both at low AOU (northern component water)

and at high AOU (southern component water) (Figs.

14 and 15). DINxs builds on the rh 26.5 surface

between AOU 15 and 50 Amol kg� 1, holds relatively

Fig. 13. Conceptual model depicting development and distribution of DINxs on three rh surfaces on a zonal section in the subtropical North

Atlantic. The relative rates and depths of DINxs formation are indicated by the horizontally aligned gray scale figure, with darkest gray

indicating the highest formation rate. Arrows indicate the contributions of northern (from the northeast) and southern (from the southwest)

component waters, which carry low DINxs values. The highest DINxs values on the rh 26.0 surface are in the west (Fig. 8), where that isopycnal

deepens to intersect the depths of high DINxs production. In the east, the rh 26.0 surface is too shallow to experience strong DINxs accumulation.

The highest DINxs values on both the rh 26.5 and 27.0 surfaces, however, are in the eastern North Atlantic, where these isopycnal surfaces

intersect the depths of maximum DINxs production. As the waters at these densities are transported with the gyre circulation to the west and the

density surfaces deepen, the shallower density surface (rh 26.5) continues to experience DINxs accumulation, but the deeper density surface lies

below the depths where DINxs development is important. DINxs on this deepest surface will decrease as it is diluted with low DINxs southern

component water. On the rh 26.5 surface, dilution with southern component water occurs as well, but DINxs development continues so the

values remain elevated.


constant at values < 1 Amol kg� 1 to AOU of 100

Amol kg� 1, then decreases as southern component

water becomes quantitatively more important (Fig.

14). DINxs is less well correlated with AOU develop-

ment (AOU< 60 Amol kg� 1) on the rh 27.0 surface

(Fig. 15). Dilution with low DINxs southern compo-

nent water is strong across the basin (note decreasing

DINxs with increasing AOU).

Rates of DINxs development were determined after

correcting for the effects of mixing between the end-

members. End-member compositions are given in

Table 2. Also shown in Table 2 are the preformed

and measured values in the mixed water regions. The

regions evaluated for DINxs development (MWR I

and II; Fig. 1A) were taken from the areas of maxi-

mum DINxs along the A16N section (for rh 27.0) and

from the southern Sargasso Sea along the A22 section

(for rh 26.0 and 26.5, where DINxs development is

strong, and for rh 27.0, where dilution with southern

component water is strong). DINxs development on rh

26.5 was evaluated at MWR II alone.

The measured concentrations of most variables in

MWR II on rh 26.0 were less than predicted by

conservative mixing of the two end-members, result-

ing in negative changes in concentrations (Table 2).

This potential density surface was at depths of 100 m

(at MWR I and southern component sites; Table 1) to

200 m (MWR II), so non-conservative concentration

decreases could be due to biological utilization of the

nutrients while in the euphotic zone. AOU increased,

however, suggestive of a net heterotrophic system.

Because two end-member mixing resulted in negative

production rates for several of the variables, we chose

to assume single end-member mixing (i.e., no other

mixing end-members involved in transport and mix-

ing) on this surface (from MWR I, the northern

component of rh 26.0, to MWR II). The rates of

change in the variables are given in Table 3. DINxs

production at 0.43 Amol kg� 1 year� 1 was very

similar to that of GS97 (dDN*/dDs in their Table 2)

who determined a rate of 0.41 Amol kg� 1 year� 1 on

rh 25.9. Adjustment of the GS97 estimate for the

Deutsch et al. (2001) modification of the N* equation

would increase their rate to 0.47 Amol kg� 1 year� 1.

During flow from the northeast gyre to the south-

western Sargasso Sea on rh 26.5 (to MWR II; or Lines

B andC in Fig. 9) DINxs increased at a rate of 0.09 Amol

kg� 1 year� 1 (Table 3). Nitrate increased at 0.37 Amol

kg� 1 year� 1, with most of this accompanied by an

increase in the phosphate concentrations at the Redfield

ratio. The rate of DINxs change can be compared to the

rate estimated by GS97 for that surface (using interpo-

lation of their estimates on bounding surfaces and

adjusted for the Deutsch et al. (2001) modification of

the N* equation). The corrected GS97 rate of N*

development was 0.15 Amol kg� 1 year� 1, or 65%

Fig. 14. (Upper) Plot of AOU (Amol kg� 1) vs. DINxs (Amol kg� 1)

on the rh surface 26.5. Data are from the rh range 26.4 <rh< 26.5,

on the hydrographic sections shown in the map (lower plot).

Northern component (data from north of Line A on the map;

AOU< 25 Amol kg� 1 and DINxs < 0.3 Amol kg� 1) and southern

component (south of Line B; high AOU and low DINxs) data are

plotted within the concavities of Lines A and B in the data plot,

respectively. AOU develops most strongly between Lines A and B,

the region of the subtropical gyre circulation, commensurate with

c 0.5 Amol kg� 1 increase in DINxs.

Fig. 15. (Upper) Plot of AOU (Amol kg� 1) vs. DINxs (Amol kg� 1)

on the rh surface 27.0. Data are from the rh range 26.9 < rh < 27.1,

on the hydrographic sections shown in the map (lower plot). Data

from the ellipse A in the data plot are from ellipse A in the map.

Data from ellipse B are from the region encompassed by ellipse B

on the map. Strong DINxs accumulation in region A is not well

correlated to AOU development. Between regions A and B, DINxs

decreases in part due to mixing.


higher than the DINxs rate of change estimated here.

The oxygen utilization rate (OUR; estimated from the

gradient in AOU divided by the gradient in age) of 3.33

Amol kg� 1 year� 1 on the rh 26.5 surface is somewhat

low compared to earlier estimates. Jenkins (1987)

estimated OUR in the b-triangle area to be 8.9 Amol

kg� 1 year� 1, while Sarmiento et al. (1990) estimated a

rate for the broader subtropical gyre to be 4.1 Amol

kg� 1 year� 1.

On rh 27.0, the rate of change in DINxs during

circulation from the northeast to the southeast sector

of the gyre (MWR I; Line B in Fig. 9) was 0.04 Amol

kg� 1 year� 1 or 53% of the (adjusted) rate estimated

by GS97. During flow from the east to the west along

the southern limb of the gyre (from MWR I to MWR

II; Line C in Fig. 9), the rate of DINxs change was

negative at � 0.10 Amol kg� 1 year� 1. The other

nutrient variables increased at rates more or less

similarly on both Lines B and C, but DINxs showed

Table 3

Estimated rates of change in composition (Amol kg� 1 year� 1) along

path of flow after correction for mixing between end-members

Rate DO2/DProperty

ratio

(A) MWR II on rh 26.0a

Silicate 0.228F 0.407 � 71.3

Nitrate 0.374F 1.185 � 43.4

Phosphate � 0.003F 0.089 5413

OUR 16.25F 10.63 –

DINxs 0.434F 0.259 –

(B) MWR II on rh 26.5b

Silicate 0.056F 0.030 � 59

Nitrate 0.369F 0.128 � 9

Phosphate 0.017F 0.023 � 196

OUR 3.330F 0.963 –

DINxs 0.091F 0.115 –

(C) MWR I on rh 27.0c

Silicate 0.179F 0.028 � 20

Nitrate 0.436F 0.042 � 8

Phosphate 0.025F 0.043 � 147

OUR 3.670F 0.413 –

DINxs 0.039F 0.025 –

(D) MWR II on rh 27.0d

Silicate 0.159F 0.183 � 28

Nitrate 0.173F 0.097 � 26

Phosphate 0.017F 0.183 � 263

OUR 4.471F 2.463 –

DINxs � 0.096F 0.091 –

a Rates between northern component and MWR II on rh 26.0.

Single end-member mixing was assumed on the rh 26 surface; two

end-members mixing was assumed on the other surfaces.b Rates between northern component and MWR II on rh 26.5.c Rates between northern component and MWR I on rh 27.0.d Rates between MWR I and MWR II on rh 27.0.

Table 4

Rates of change (Amol kg� 1 year� 1) assuming binary and single

end-member mixing, as well as the ratio of the rates

Mixing Ratioa

Binary Single Not corrected/

corrected

(A) MWR II on rh 26.5

Silicate 0.06 0.16 2.8

Nitrate 0.37 0.54 1.5

Phosphate 0.03 0.03 1.9

OUR 3.33 4.10 1.2

DINxs 0.09 0.07 0.7

(B) MWR I on rh 27.0

Silicate 0.18 0.18 1.0

Nitrate 4.36 4.36 1.0

Phosphate 0.02 0.03 1.1

OUR 3.67 3.67 1.0

DINxs 0.04 0.04 1.0

(C) MWR II on rh 27.0

Silicate 0.16 0.48 3.0

Nitrate 0.17 0.56 3.2

Phosphate 0.02 0.05 2.7

OUR 4.47 5.20 1.2

DINxs � 0.10 � 0.20 2.1

a Calculated prior to rounding of rates.


a strong negative growth in the southern portion of the

gyre that cannot be explained by mixing alone. The

OUR on rh 27.0 ranged from 3.67 to 4.47 Amol kg� 1

year� 1 for the eastern and southern portions of the

gyre, somewhat lower than the OUR of 6 Amol kg� 1

year� 1 determined by Doney and Bullister (1992) in

the eastern North Atlantic on that surface.

Property/property ratios give some insight as to

which variables (nitrate or phosphate) may have

caused the negative DINxs production on rh 27.0

(Table 3). In the eastern gyre (Line B, Fig. 8), the

DO2/DNO3� had a narrow range of � 8.4 to � 9.0 on

the two potential density surfaces, but in the southern

section (Line C) the ratio increased three-fold (to

� 26) relative to Line B. The DO2/DPO4� 3 ratio had

a narrow range in the east (� 147 to � 196) and

increased by approximately one half to � 263 in the

southern gyre. Takahashi et al. (1985) reported DO2/

DNO3� and DO2/DPO4

� 3 ratios on rh 27.0 in the North

Atlantic of � 9.4 and � 165, respectively, in agree-

ment with those we report in the eastern gyre. Both

phosphate and nitrate increased in the southern gyre at

rates lower than expected given the oxygen utiliza-

tion, but of the two properties nitrate growth was the

weakest relative to OUR. The true cause of the DINxs

concentration decrease is unknown.

With the exception of the rh 26 surface, the rates

given in Table 3 have been corrected for binary

mixing of northern and southern components in the

gyre. On the rh 26 surface, northern component water

strongly dominates in this analysis (i.e., at MWR I,

where the northern component made up essentially

100% of the water) so single end-member mixing is

reasonable. On deeper surfaces production rates as-

suming single end-member mixing overestimate the

rates corrected for binary mixing by up to three-fold

(Table 4). GS97 did not correct for binary mixing on


the two potential density surfaces we have considered,

and this difference in treatment contributes to the

differences in the rates estimated.

A second difference between the calculations here

and by GS97 is the choice of age tracers. There exists

an offset between water mass ages estimated by

pCFC-12 age (used here) and the 3H/3He used by

GS97. Doney et al. (1997) reported a relatively

constant, positive offset in the CFC ages in the

thermocline (older CFC age compared to the 3H/3He

age), and a non-linear offset at 3H/3He ages >12.5

years, with much younger 3H/3He ages relative to

CFC ages. The rates of DINxs development on the rh

26.0 and 26.5 surfaces are unaffected by the offset

because the ages in MWR II are < 9 years (Table 2),

so both tracers should give similar age gradients. On

the deeper isopycnal, however, the offset is significant

and nonlinear, thus affecting calculated rates. In

MWR I on the rh 27.0 surface, the pCFC-12 age of

22 years (Table 2) is approximately equal to a 3H/3He

age of 15 years, estimated from the age offset in Fig.

8A of Doney et al. (1997). Using the younger 3H/3He

age on the rh 27.0 surface rather than the pCFC-12

age would raise the DINxs production rate by f 50%.

This difference can account for the discrepancy in

rates on the rh 27.0 surface estimated here with that

by GS97. van Aken (2001) suggests that the main

cause for the offset in age between the two tracers is

that the surface layer water can remain undersaturated

with respect to the atmospheric CFC concentrations,

causing an underestimate in the OUR. He recommen-

ded an upward revision of CFC-derived estimates of

OUR in the main thermocline by a factor of 1.3.

Applying this factor to the DINxs production rates on

rh 27.0 in Table 3 would reduce the difference with

the estimates of GS97.

Finally, variability in the strength, timing and

location of nitrogen fixation in the North Atlantic is

likely reflected by interannual or decadal changes in

the location and gradients of excess nitrate. Bates and

Hansell (2003) have demonstrated strong temporal

variability in DINxs in the Sargasso Sea, which

correlates well with the North Atlantic Oscillation

(NAO). Likewise, as indicated above, DINxs values

at the cross over point of AR01 and A22 varied by as

much as 0.6 Amol kg� 1, suggesting temporal vari-

ability in that region. GS97 employed data collected

from several oceanographic programs (such as the

Geochemical Sections [GEOSECS] program and

Transient Tracers in the Ocean [TTO]) taking place

over the course of c 2 earlier decades. The WOCE

program data employed in this analysis were collected

later, between 1988 and 1998, so the offset in time of

collections may be important.

3.3. Regional rates of nitrogen fixation

GS97 calculated the rate of nitrogen fixation in the

North Atlantic by applying their rates of N* produc-

tion to the volumes of the isopycnal layers of interest.

The rates of N* development were adjusted for the

effect of elevated N/P ratios in the nitrogen-rich

organic matter produced by diazotrophic organisms

(taken to be 125), thus providing estimates of regional

N2 fixation (with application of Eq. (14) from GS97).

We used the GS97 equations for calculating rates, so

if there is an error in the assumption about the ratio,

then our rates are likewise affected. Analyses are

required to accurately confirm the appropriate ratios

to be used in the rate calculations.

The region over which GS97 applied their volumet-

ric rates was 10–50jN and 10–90jW, an area bound-

ing the subtropical gyre and a portion of the Gulf of

Mexico. We also estimated the volumes bounding the

rh 26.0, 26.5 and 27.0 surfaces where DINxs develop-

ment was evident (Table 5). We caution that WOCE

coverage of the North Atlantic is inadequate to provide

high confidence in the actual areas over which the rates

should be applied, particularly for the rh 27.0 surface.

Our findings, however, do not support the application

of the DINxs development rates to the entirety of the

subtropical gyre as was done in GS97; we found that

the areas where DINxs development takes place were

regional and not gyre wide (Fig. 8). This difference in

regional distribution of excess nitrate development

results in a large difference in estimated regional rates

of nitrogen fixation.

Volumes of the 26.0, 26.5 and 27.0 potential

density intervals were calculated from the areas

bounded by 10–25jN, 25–75jW; 15–25jN, 25–

75jW; and, 15–40jN, 20–40jW, respectively. Se-

lection of the regions over which excess nitrate

develops on density surfaces is of central importance

to the resulting regional estimates of nitrogen fixation.

We used the distributions depicted in Figs. 8, 14 and

15) to guide the selections. Fig. 8A provided the

Table 5

Estimated rates of change of DINxs and rates of nitrification of nitrogen-rich organic material from diazotrophic organisms (N-rich Nitr.) on

three isopycnal surfaces in the North Atlantic

Potential

density

interval

Volumea

(1014 m3)

DDINxs/Dage

(Amol kg� 1 year� 1)

DDINxs/Dage

(1011 mol year� 1)

N-rich Nitr.b

(Amol kg� 1 year� 1)

N-rich Nitr.

(1011 mol year� 1)

GSV/Vc GSN/Nc

25.9–26.1 1.0 0.434F 0.259 0.43 0.580F 0.350 0.58 5.1 4.6

26.4–26.6 2.6 0.091F 0.115 0.24 0.123F 0.155 0.34 6.9 9.1

26.9–27.1 5.1 0.039F 0.025 0.20 0.052F 0.034 0.26 6.9 11.5

a Volumes of the 26.0, 26.5 and 27.0 potential density intervals are from the areas bounded by 10–25jN, 25–75jW; 15–25jN, 25–75jW;

and 15–40jN, 20–40jW, respectively.b Calculated using Eq. (14) of GS97 and assuming zero denitrification.c GSV/V and GSN/N are the ratios of the GS97 volumes and nitrification rates of nitrogen-rich organic material to volumes and rates

determined in this analysis. The GS97 volumes and rates were recalculated for the potential density intervals employed here.


rationale for the choice of region for the rh 26.0

surface. Fig. 8B demonstrates that excess nitrate

development on the rh 26.5 surface is spread widely

across the gyre; hence, the wide region chosen (25–

75jW). Figs. 8C and 15 demonstrate that excess

nitrate development on the deepest density surface

considered here was largely restricted to the eastern

portion of the gyre. The westernmost boundary of the

box defining the rh 27.0 surface lies in a particularly

data sparse region between the A20 and A16 sections.

Given the paucity of data between those widely

spaced lines (i.e., recall the gap in nutrient data on

AR01), we chose the approximate midpoint between

them (40jW) as the western boundary.

The volumes applied here are five to seven times

smaller (Table 5) than the volumes employed by

GS97 over the same potential density intervals (we

do not include the Gulf of Mexico). Our estimates for

the rates of nitrification of nitrogen-rich organic

matter (assuming that nitrogen fixation is the primary

source of the nitrate excess) were 4.6–11.5 times less

than the rates estimated by GS97 because our DINxs

production rates and the volumes over which produc-

tion occurred were smaller (Table 5). The sum of the

rates of nitrification of nitrogen enriched material

estimated here for the North Atlantic subtropical gyre

is 13.5% of the GS97 rate summed over the same

three potential density intervals. Applying this per-

centage to the total rate of nitrogen fixation estimated

by GS97 for the North Atlantic Ocean (23� 1011 mol

N year� 1) results in our areally integrated estimate of

3.1�1011 mol N year� 1. An assigned 50% uncer-

tainty in our choice of areas under which excess

nitrate accumulation occurs gives a range of 1.5–

4.6� 1011 mol N year� 1. This range, determined here

from DINxs spatial variability, is comparable to the

range by Bates and Hansell (2003) based on an

analysis of DINxs temporal variability (0.9 –

4.6� 1011 mol N year� 1). With most of the DINxs

development occurring in the area 15–25jN by 25–

75jW (6.83� 1012 m2), the mean nitrogen fixation

rate is 0.045 mol N m� 2 year� 1, or 62% of the GS97

estimate.

We do not know if N2 fixation is the sole source of

excess nitrate in the subtropical North Atlantic. GS97

corrected for atmospheric deposition of N to the North

Atlantic using the Duce et al. (1991) estimate of 11

mmol N m� 2 year� 1, thus reducing their estimate to

20� 1011 mol N year� 1. Prospero et al. (1996)

estimated a higher rate of 17–20 mmol N m� 2

year� 1, which includes wet and dry deposition of

inorganic and organic N. If this N accumulates at the

depths of the nitrate excess maximum in the areas

considered here, then our rate of N2 fixation would be

an overestimate.

A brief note is included here on a unique region of

high DINxs on potential density surfaces greater than

rh 27.0. Throughout most of the North Atlantic,

DINxs values at depths greater than the base of the

main thermocline are < 0 Amol kg� 1, indicating a

nitrate deficiency relative to phosphate. An exception

exists in the depth range of 1000–4000 m on the A16

line, centered near 38jN (data not shown), where

elevated values for DINxs (>0.5 Amol kg� 1) were

present. These high DINxs values are associated with

elevated salinity, indicative of the Mediterranean Sea

outflow. GS97 plots of N* on rh 27.1 also show

evidence of this potential Mediterranean source of


excess N. Kress and Herut (2001) reported N/P ratios

in the deep water and the Levantine Intermediate

Water of the eastern Mediterranean Sea to be >24

and >18, respectively. Outflow of similarly high N/P

water from the Mediterranean Sea would result in the

DINxs distribution found at >1000 m on section A16.

4. Concluding statements

Geochemical and direct biological estimates of

nitrogen fixation in the tropical and subtropical North

Atlantic converge in this analysis. Our areal rate (0.045

mol N m� 2 year� 1) is three times the mean rate of

nitrogen fixation measured by Orcutt et al. (2001) at

31jN in the western Sargasso Sea near Bermuda,

supporting the finding that most of the N2 fixation

occurs at latitudes south of Bermuda. Our rate is

consistent with the global mean rate of N2 fixation

for the tropical ocean of 0.039 mol m� 2 year� 1 and

twice the rate of 0.026 mol m� 2 year� 1 measured

between 14jN and 22jN in the SW North Atlantic

(Capone et al., 1997). The Capone et al. (1997) global

mean rate of tropical nitrogen fixation, applied over the

area in which we found excess nitrate in the North

Atlantic (6.83� 1012 m2), gives a nitrogen fixation rate

of 2.7� 1011 mol N year� 1. Given the uncertainties in

both estimates, our rate based on geochemical methods

(3.1�1011mol N year� 1) is indistinguishable from the

rate derived from biological measurements.

The distributions of N* in GS97 suggested that

most of the N2 fixation in the Sargasso Sea was in the

west, including the waters around Bermuda. The low

rates measured at the BATS site by Orcutt et al.

(2001), as well as the absence of seasonal variability

in the isotopic composition of DON there (Knapp and

Sigman, 2003), challenge those findings. Lipschultz

and Owens (1996) listed several inconsistencies be-

tween observations of excess nitrate at the BATS site

and measured rates of nitrogen fixation, all suggesting

a weak linkage between N* and nitrogen fixation at

the site. Hansell and Carlson (2001) reported summer

time TON concentrations of 4–5 Amol kg� 1 at BATS,

values that are both relatively low and static. Such

characteristics are not expected if N2 fixation is a

dominant process there. Karl et al. (1992), for exam-

ple, reported a 3.1-fold increase in DON in the

presence of a Trichodesmium spp. bloom near Station

ALOHA in the North Pacific. Because Hansell and

Carlson (2001) found no substantive summer time

increases in TON, they suggested that N2 fixation

rates were unlikely to be as high at the BATS site as

indicated by the GS97 findings. Instead, if N2 fixation

was responsible for elevated N/P ratios in the main

thermocline of the Sargasso Sea, then the process

must take place south of Bermuda and the excess

nitrate transported with the gyre circulation. The

results presented here support that hypothesis.

Here, we showed particularly high excess nitrate

concentrations to the south of Bermuda and in the

eastern portion of the gyre. If the high excess nitrate

signals are truly indicative of high nitrogen fixation

rates in those locales, then it is intriguing to consider

that the regional imprint of nitrogen fixation is largely

aligned with the track of dust transport and deposition

in the North Atlantic (Ginoux et al, 2001). Bates and

Hansell (2003) proposed a link between the temporal

variability of DINxs in the western Sargasso Sea and

variations in dust inputs that were tied to the North

Atlantic Oscillation. Mahaffey et al. (2003) likewise

suggested that biogeochemical proxies for nitrogen

fixation (i.e., high excess nitrate in the thermocline,

cyanobacterial phytopigments and stable isotope sig-

natures), particularly high between 26jN and 32jNon 20jW, are coincident with enhanced atmospheric

dust deposition. Their findings, like those reported

here, indicate that nitrogen fixation may occur at high

rates at mid latitudes in the eastern basin where dust

inputs are high. Support for this comes from Kahler et

al. (personal communication) who measured both

high N2 fixation rates (up to 4.5 mmol N m� 2 day� 1)

) and high rates of DINxs accumulation (0.5 mmol N

m� 2 day� 1) in the thermocline along 30jW (15–

35jN). Tyrrell et al. (2003) reported that high Tricho-

desmium spp. abundance in the eastern North Atlantic

was correlated with shallow mixed layer depths and

high iron deposition. Interestingly, they also found

the highest abundances between 0jN and 15jN in

late summer and more moderate abundances to 30jN.The distributions of the diazotrophs were spatially

offset from the distribution of the excess nitrate

shown here.

Our findings for the distribution and production of

excess nitrate in the North Atlantic have similarities

and dissimilarities to the GS97 results. We concur that

the northern and southern components contribute


water with low excess nitrate, and that nitrate excess

develops below the mixed layer in the subtropical

North Atlantic. The GS97 findings suggest, though,

that N* develops uniformly on the isopycnal surfaces,

regardless of location or depth in the gyre. Our

contrasting findings indicate that excess nitrate devel-

ops at depths of 150–400 m, and so it is only when an

isopycnal lies at those depths will it gain excess nitrate

(Fig. 10). Where an isopycnal such as rh 27.0 is

deeper than 400 m, the surface does not gain addi-

tional excess nitrate, thus restricting the areas in which

that surface should be included for estimating the

regional rate of N2 fixation. Finally, we cannot explain

the differences in the distributions of excess nitrate as

shown by GS97 and in this analysis. It may be that

data quality was different, with GS97 using data sets

from the 1970s, the 1980s and the early 1990s, and

having to apply corrections to some of the data.

Themost important cause for the difference between

our estimate of areally integrated nitrogen fixation in

the North Atlantic and that of GS97 is the area over

which the process is thought to occur. Our regional

estimates for N2 fixation retain uncertainty because the

WOCE sections we used, while providing coverage

adequate to roughly outline the regions of DINxs

development, were inadequate to closely define the

spatial boundaries for estimating volumes of the po-

tential density intervals. The DINxs distributions shown

in Fig. 8 are consistent with gyre circulation (e.g., Fig.

9), accounting for the inflow of DINxs depleted north-

ern and southern components and the accumulation of

DINxs signal over similar depth ranges, as expected for

remineralization of common sinking particles. Not

quantified are processes other than N2 fixation, and

listed by Fanning (1992) andGS97, that may contribute

to the development of excess nitrate in the subtropical

Sargasso Sea. We have reduced the differences be-

tween biological and geochemical estimates of N2

fixation. If additional processes contribute to the geo-

chemical signal, the gap may be reduced further. We

conclude that N2 fixation is an important part of the

nitrogen cycle in the subtropical North Atlantic, but it

occurs at considerably lower rates than reported in

some of the recent literature where it was evaluated

using geochemical data.

If the rate of nitrogen fixation in the North Atlantic

is indeed considerably lower than estimated by GS97,

then recently considered global marine nitrogen budg-

ets are impacted. Gruber and Sarmiento (2002) esti-

mated that present day sources of new nitrogen to the

global ocean (nitrogen fixation, riverine and atmo-

spheric inputs), summing to 17�1012 mol N year� 1,

are in approximate balance with the global marine rate

of denitrification (the sink term for fixed nitrogen).

Codispoti et al. (2001), however, reports a very large

imbalance between fixed nitrogen sources and sinks in

the ocean, with sinks (i.e., denitrification) exceeding

sources by a factor of two. Both the Gruber and

Sarmiento (2002) and the Codispoti et al. (2001)

budgets rely on the GS97 global estimate of nitrogen

fixation, and that estimate is highly reliant on the rates

GS97 reported for the North Atlantic. If the global

marine nitrogen fixation rate is less than suggested by

GS97, then the imbalance reported by Codispoti et al.

(2001) increases and the balanced budget suggested

by Gruber and Sarmiento (2002) is challenged.

Acknowledgements

Our deepest appreciation goes to the numerous

P.I.’s, scientists, technicians and ship’s officers and

crews who completed the enormous task of occupying

and analyzing the WOCE sections and samples. The

products of their efforts will resound through our

science for many years to come. Rana Fine and Debra

Willey calculated water ages and provided plots from

the North Atlantic, and we thank them for this help.

Geoff Daniels is thanked for calculating volume data

for water masses of interest in the Atlantic. Mick

Follows joined our early discussions on the Sargasso

Sea and participated in our seagoing activities. Niki

Gruber, Fred Lipschultz and Associate Editor Mark

Altabet suggested several improvements on the

manuscript. We thank an anonymous reviewer for

extensive comments as well. This work was supported

by the National Science Foundation. DAH and NRB

were supported by NSF OCE-0196143. DBO was

supported by NSF OCE-9981500.

Associate editor: Dr. Mark Altabet.

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