Deep-Sea Research II 48 (2001) 2155}2171
Annual cycles of nutrients and oxygen in the upper layers of the
North Atlantic Ocean
Ferial Louanchi, Raymond G. Najjar*
Department of Meteorology, The Pennsylvania State University, 503 Walker Building, University Park, PA 16802, USA
Abstract
Monthly climatologies of nutrients and oxygen in the upper North Atlantic Ocean are analyzed and used
to estimate the rates of spring}summer new production and remineralization. The annual cycle of surface
nutrients is characterized by a spring}summer biological drawdown and a fall}winter increase due to vertical
mixing. The drawdown is accompanied by maximum oxygen supersaturations. In the subsurface layer
(100}200 m), the decrease of oxygen concentrations during the spring}summer period re#ects the remineralization of organic matter. The spring}summer drawdown of surface nutrients implies new production rates
that are high in the high latitudes (around 40 g C/m on average), and low in the low and temperate latitudes
(about 15 g C/m on average). In the subsurface layer, respiration rates are computed from the oxygen
concentration decrease during the spring}summer period. Results show that most of the new production in
the low latitudes, and half or less of it in the high latitudes, is remineralized above 200 m. The spring}summer
new production in the North Atlantic Ocean is estimated to be 1.3 Pg C. Considering that the spring}summer
production that is not mineralized in the top 200 m of the ocean is mainly composed of particulate organic
matter the spring}summer export at 200 m is estimated to be between 0.4 and 0.6 Pg C for the whole North
Atlantic Ocean. 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
According to the Intergovernmental Panel on Climate Change, the CO increase in the
atmosphere during the past century has had a `discernable in#uencea on the Earth's climate
(Houghton et al., 1996). Thus, a prerequisite for successful climate prediction is a sound prediction
* Corresponding author. Tel.: 1-814-863-8752; fax: 1-814-865-3663.
E-mail addresses: [email protected] (F. Louanchi), [email protected] (R.G. Najjar).
0967-0645/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0 9 6 7 - 0 6 4 5 ( 0 0 ) 0 0 1 8 5 - 5
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F. Louanchi, R.G. Najjar / Deep-Sea Research II 48 (2001) 2155}2171
of atmospheric CO . The latter is made di$cult because the current budget of atmospheric CO is
not well understood. Part of this uncertainty is due to an incomplete knowledge of carbon #uxes
within the ocean and across the air}sea interface (Tans et al., 1990). In this context, the biological
pump in the ocean plays a key role in the global carbon budget by transporting organic carbon in
particulate and dissolved forms from the surface ocean to the deep ocean.
Oxygen, nitrate and phosphate are excellent candidates for understanding the cycling of organic
carbon in the ocean.The distributions of these tracers largely re#ect photosynthesis and respiration
in the water column. Red"eld et al. (1963) showed that the relative abundance of C, N and P in
particulate organic matter is relatively "xed, suggesting that the rates of biological cycling of these
elements also occur in "xed ratios. Though only selected organisms use silicon * unlike C, N and
P, which are processed by all organisms * this element is of particular importance because it can
limit the production of diatoms, which are thought to mediate a large fraction of carbon export
from surface waters (Dugdale and Goering, 1967). Many studies have used seasonal changes in
nutrients to estimate the rates of new production in speci"c areas and for speci"c periods (see
synthesis by Minas and Minas, 1992; Minas and Codispoti, 1993). Oxygen and CO variations also
have been used for the same purpose, as well as for estimates of remineralization (Jenkins and
Goldman, 1985; Oudot, 1989; Spitzer and Jenkins, 1989; Hansell and Carlson, 1998; Najjar and
Keeling, 2000; Ono et al., 2001).
Here, we present an analysis of seasonal nutrient (Louanchi and Najjar, 2000) and oxygen
(Najjar and Keeling, 1997) climatologies for the North Atlantic Ocean and apply established
techniques for estimating new production and remineralization. The North Atlantic Ocean is of
particular interest because it is a strong seasonal sink of atmospheric CO , due to the high primary
productivity occurring during spring time (Taylor et al., 1991; Watson et al., 1991). It also appears
to be a strong sink for anthropogenic CO (Gruber, 1996). Because the North Atlantic is the most
sampled ocean basin (Levitus et al., 1993; Conkright et al., 1994; Levitus and Boyer, 1994a; Levitus
and Boyer, 1994b), it provides an excellent opportunity for an extensive study of nutrient and
oxygen annual cycles and to quantify new production, shallow remineralization and carbon export
from the upper ocean.
2. Techniques and data
We have created global phosphate, nitrate and silicate monthly climatologies in the upper 500 m
of the ocean by using the data from the World Ocean Atlas 1998 (Conkright et al., 1998), produced
by the Ocean Climate Laboratory at the National Oceanographic Data Center (NODC)
(Louanchi and Najjar, 2000). The monthly oxygen anomaly (excess over saturation) climatology
used here is from Najjar and Keeling (1997). Fig. 1 shows that the data density for phosphate in the
North Atlantic, the most frequently measured nutrient, is very high. For silicate and nitrate, the
density is about 50 and 15% of phosphate, respectively. Therefore, phosphate maps are probably
more reliable than nitrate or silicate ones. Here, we present a brief description of
the methods used to create the maps. Full details are given in Louanchi and Najjar (2000).
We used the same methodology as Najjar and Keeling (1997) to create monthly nutrient maps,
except that nutrient}temperature relationships were used to "lter and extrapolate the nutrient
data. We found that nutrient}temperature relationships for the upper 500 m in the NODC data set
F. Louanchi, R.G. Najjar / Deep-Sea Research II 48 (2001) 2155}2171
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Fig. 1. Distribution of (a) phosphate, (b) nitrate, and (c) silicate pro"les left after "ltering NODC data in the North
Atlantic Ocean. Also shown are the outlines of the regions over which the averages have been made.
varied regionally, so we partitioned the global ocean into eight regions, one of which was the North
Atlantic (north of 153N), and de"ned a best-"t curve to the data for each region.
To remove unrepresentative and erroneous data, we deleted all observations that were more
than a de"ned deviation from the best-"t curve for each region. The deviations were determined
subjectively and allowed to vary linearly with temperature, being larger in colder waters (about two
to three standard deviations) than in warmer waters (about one standard deviation).
Vertical and horizontal spacing of the data are the top 14 NODC standard levels (0, 10, 20, 30,
50, 75, 100, 125, 150, 200, 250, 300, 400 and 500 m) and the equal-area grid described in Najjar and
Keeling (1997), respectively. Vertical interpolation was performed on each pro"le using the
monotonic scheme of Ste!en (1990). The horizontal grid is 23 in latitude and variable in longitude
(23 at the equator to 1203 at the poles). For each depth level, an average was computed for each grid
point containing an observation. As in Najjar and Keeling (1997), we did this for each month, and
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Table 1
Some characteristics of nutrient}temperature relationships in the North Atlantic Ocean. (From Louanchi and Najjar,
2000)
Phosphate
Silicate
Nitrate
Number of pro"les
r
Average distance to "t (mol/l)
51005
26064
8419
0.54
0.45
0.60
0.25
3.3
4.3
then we also used these binned data to create three- and "ve-month moving averages. To "ll in
unde"ned values of the "ve-month moving average "elds, the nutrient}temperature relationships
described above and the temperature climatology of Levitus and Boyer (1994b) were used. Where
data density allowed it, the map obtained was "lled with the three-month moving averages or,
preferably, one-month averages. Following Najjar and Keeling (1997), distance-weighted averaging
with a 1000 km Cressman function was used to smooth the monthly "elds. A "nal procedure to
remove outliers was to compare the blended "elds with those produced using the nutrient}temperature relationships alone, which we call regression "elds. If the two di!ered by more
than the interval used for the "ltering at a given grid point, then the grid point in the blended "eld
was replaced by the corresponding grid point in the regression "eld. The "elds were then smoothed
one "nal time using the Cressman function.
Table 1 shows that the r values for the best-"t curves are rather low in the North Atlantic
Ocean. However, as the North Atlantic is highly sampled, the use of the best "ts to extrapolate the
data was minimal. We therefore expect that the averaged distance to the "t listed in Table 1 is an
upper-bound estimate of the error in the maps themselves.
3. Results and discussion
3.1. Surface seasonal changes
Fig. 2 shows the seasonal variations of the oxygen anomaly and the nutrients in North Atlantic
surface waters. Temperature and salinity also are shown to provide the physical context. Seasurface temperature increases and sea-surface salinity decreases from winter to summer over most
of the North Atlantic Ocean, but the variations are most pronounced in the western Atlantic,
particularly around 453N, where air}sea #uxes of heat and fresh water are most seasonal (Esbensen
and Kushnir, 1981; Schmitt et al., 1989). The geochemical components follow the seasonal
variation expected: the oxygen anomaly increases from winter to summer, consistent with an
increase in temperature and high phytoplankton production. The drawdown of nutrients is also
consistent with the high production during this period. From winter to summer, the equatorial area
shows a decrease of temperature and oxygen anomaly and an increase of salinity and nutrients,
suggesting that the upwelling intensity is higher in summer than in winter.
Seasonal changes of nutrients and the oxygen anomaly are generally high in the subpolar area
and the Labrador Sea, consistent with the high productivity of these regions (Taylor et al., 1991;
F. Louanchi, R.G. Najjar / Deep-Sea Research II 48 (2001) 2155}2171
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Fig. 2. Seasonal di!erences of surface (a) temperature, (b) salinity, (c) oxygen anomaly, (d) phosphate, (e) nitrate, and (f)
silicate in the North Atlantic. Di!erences are the average of July, August and September (JAS) minus the average of
January, February and March (JFM).
Platt et al., 1991; Sathyendranath et al., 1995). Large seasonal nutrient variations have been
reported previously in the subarctic Atlantic. Near Iceland, Takahashi et al. (1993) showed seasonal
di!erences (as de"ned here) of 0.5, 9 and 5 mol/l for phosphate, nitrate and silicate, respectively,
consistent with our results in terms of phosphate and silicate but 50% larger than the seasonal
di!erence of nitrate we "nd. In the area of the North Atlantic Bloom Experiment, the seasonal
di!erences we "nd are 50% lower than what is reported by Takahashi et al. (1993). The North
Atlantic subtropical gyre has a low seasonal signal in nutrients, though somewhat higher than what
is observed at the time-series station near Bermuda (Michaels et al., 1994; Bates et al., 1996). The
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biogeochemical signature of the northwest African upwelling is not seen in our maps despite the
fact that high primary productivity has been reported for that area (Minas and Codispoti, 1993).
Our interpolation routine most likely smoothes out the expected features in this and other localized
upwelling regions. Such spatial and temporal smoothing also may explain why seasonality is
underestimated in the subarctic region and overestimated near Bermuda.
3.2. Regional vertical seasonal variations
Vertical distributions of the geochemical data between 0 and 300 m were averaged over broad
areas of the North Atlantic and are shown in Figs. 3 and 4. The areas are: the Arctic Ocean (north
of 603N except the Labrador Sea), the Labrador Sea, the subarctic area (40}603N), the northwest
Fig. 3. Average vertical distributions of (a, e, i) phosphate, (b, f, j) nitrate, (c, g, k) silicate and (d, h, l) oxygen
concentrations in winter (average of January, February and March, small rectangle) and summer (average of July, August
and September, circles) between 0 and 300 m in the Arctic Ocean, the Labrador Sea and the subarctic area.
F. Louanchi, R.G. Najjar / Deep-Sea Research II 48 (2001) 2155}2171
2161
Fig. 4. Same as Fig. 3, but for the subtropical area, the northwest African upwelling area and the Gulf of Guinea.
African upwelling area (10}303N and 15}303W), the Gulf of Guinea (Equator to 103N, 303W to
153E), and the subtropical gyre (Equator to 403N except the northwest African upwelling area and
the Gulf of Guinea) (Fig. 1).
In the Arctic Ocean, the Labrador Sea and the subarctic area, pro"les for all geochemical
components are almost constant in winter, consistent with deep convection during that time
(Fig. 3). During summer, lower concentrations of nutrients are found in the upper 100 m of the
ocean, consistent with high spring}summer biological uptake. The seasonal surface oxygen concentration change is negligible (Fig. 3d), presumably due to the opposing e!ects of photosynthesis and
warming. In the Arctic Ocean, the Labrador Sea and the subarctic area, phosphate concentrations
are lower in August than in February between 100 and 300 m. This feature is also observed for NO
in the Arctic. These are unexpected features because remineralization in the seasonal thermocline is
expected to result in an increase of nutrients from winter to summer. The expected signature of
water column remineralization and opal dissolution are seen in the subsurface silicate increase and
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oxygen decrease, respectively, from winter to summer. We do not have an explanation for the
anomalous subsurface behavior of phosphate and nitrate.
The subtropical area is characterized by lower nutrient and oxygen concentrations than the
subpolar area (Fig. 4a}d). In the northwest African upwelling and subtropical areas, there is a small
drawdown of nutrients and a slight oxygen increase, on average, from winter to summer in the
upper 50 m of the ocean (Fig. 4a}h). Below this depth, nitrate and silicate increase slightly and
phosphate decreases slightly from winter to summer. During the same period, oxygen decreases in
the subtropical area and increases in the northwest African upwelling. The Gulf of Guinea area
does not show changes at any depth (Fig. 4i}l).
3.3. Annual cycles of oxygen and nutrients in the North Atlantic
The annual cycles of temperature, salinity, oxygen anomaly and nutrients in the Arctic Ocean,
the Labrador Sea and the subpolar gyre at the sea surface (averaged between 0 and 20 m) and 125 m
(averaged between 100 and 150 m) are presented in Figs. 5 and 6, respectively. The three areas have
very similar annual cycles. Surface nutrient concentrations peak in January}March when surface
waters are coldest, the mixed layer is the deepest and surface waters are mixed with nutrient-rich
subsurface waters (Fig. 5c}e). As the mixed-layer shoals, and the surface layer warms up, a drawdown of nutrients and an increase in oxygen anomaly are observed, consistent with high rates of
photosynthesis. Minimum nutrient concentrations are found in the July}September period. Oxygen anomalies peak in late spring/early summer for the three areas and start to decrease throughout the summer due to outgassing. Mixing with O -depleted subsurface waters favors this trend
through the fall.
At 125 m, seasonal changes in nutrients are not clear (Fig. 6c}e). In the Labrador Sea, a strong
decrease of Silicate of about 7 mol/l is observed from August to September (Fig. 6e). We believe
that uncertainty in the data or the extrapolation methodology for Silicate is responsible for this
unrealistic signal because the relationship between Silicate and temperature in the North Atlantic
is not as well de"ned as in other areas of the world ocean. Oxygen anomalies have a clear seasonal
trend peaking in April}May due to vertical mixing and decreasing through August}September
(Fig. 6e) due to remineralization (Najjar and Keeling, 1997).
The annual cycles of temperature, salinity, oxygen anomaly and nutrients in the subtropical gyre
and the upwelling areas at sea surface (average between 0 and 20 m) and at 125 m (average between
100 and 150 m) are presented in Figs. 7 and 8, respectively. Seasonal variations of surface nutrients
in the low latitudes are generally low. In the Gulf of Guinea, a semi-annual cycle is found similar to
what is observed in the North Indian Ocean due to the monsoons (Najjar and Keeling, 1997). Early
studies have demonstrated the existence of this semi-annual cycle, with two periods of upwelling
and hence two periods of production (Codispoti, 1981). This feature is perceptible in the temperature, phosphate and oxygen anomaly annual cycles (Fig. 7a, c and f), but less clear in the nitrate
and silicate cycles. The maxima are found in February and August. The strongest drawdown of
nutrients is found between August and November. The increase of nutrients from April}May to
August is accompanied by a decrease of the sea surface temperature, implying that the upwelling
fully develops during that period. The other period of upwelling seems to be around November}January but is not as clear as the summer upwelling in terms of the temperature signal. At
125 m, seasonal changes of nutrients and oxygen are not clear (Fig. 8d and e). In the northwest
F. Louanchi, R.G. Najjar / Deep-Sea Research II 48 (2001) 2155}2171
2163
Fig. 5. Annual cycles of (a) temperature, (b) salinity, (c) phosphate, (d) nitrate, (e) silicate and (f) oxygen anomaly and
in the Arctic Ocean (solid line), the Labrador Sea (dotted line) and the subarctic gyre (dashed line) averaged between 0
and 20 m.
African upwelling region, the decreasing trend of oxygen anomaly might be due to both respiration
and dilution by upwelled O -depleted waters.
An estimate of new production for the North Atlantic Ocean can be made by integrating
spring}summer phosphate, nitrate and silicate drawdowns over the upper 100 m. A lower bound is
obtained considering the July}August}September average vertical integral minus the January}February}March vertical integral, and a higher bound from the maximum vertical integral
minus the minimum vertical integral. These calculations are performed for each grid box and
integrated over the area of the six aforementioned regions. Results are presented in Table 2.
Integrated seasonal drawdowns of nutrients allow us to estimate regional surface Red"eld ratios.
In the Arctic Ocean and the Labrador Sea, the N/P ratio is higher than the standard Red"eld
et al. (1963) ratio, possibly due to a smoothing of the seasonal variation of phosphate by the spatial
extrapolation used. The drawdown of silicate is generally lower than that of nitrate, leading to
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Fig. 6. Annual cycles of (a) temperature, (b) salinity, (c) phosphate, (d) nitrate, (e) silicate and (f) oxygen anomaly in
the Arctic Ocean (solid line), the Labrador Sea (dotted line) and the subarctic gyre (dashed line), averaged between 100
and 150 m.
a Si/N ratio of 0.6. Production in the high latitudes is partly supported by diatoms, which are
thought to have a N/Si uptake ratio of 1 (Richards, 1958). The di!erence is probably made by
other phytoplankton species for which silicate is useless. This discrepancy also may re#ect the
di!erences in recycling of N and Si. The low N/P ratios compared to standard Red"eld ratio
found in most of the tropics and subtropics may be due to the highest density of phosphate data
compared to that of nitrate (Table 1). This ratio also may re#ect non-Red"eldian processes such as
lateral advection or nitrogen "xation, as has been suggested in the Bermuda area (Michaels et al.,
1993).
The spring}summer biogenic silica production we found in the high latitudes, 5}11 Tmol/period,
is comparable to the estimate of 14 Tmol/yr for the global North subpolar and polar areas obtained
from a recent modeling study (Gnanadesikan, 1999). Our estimate in the subtropical Atlantic,
4.3 Tmol/period on average, is larger than what is proposed by this author. However, the rate of
F. Louanchi, R.G. Najjar / Deep-Sea Research II 48 (2001) 2155}2171
2165
Fig. 7. Annual cycles of (a) temperature, (b) salinity, (c) phosphate, (d) nitrate, (e) silicate and (f) oxygen anomaly in the
Gulf of Guinea (solid line), the northwest African upwelling area (dotted line) and the subtropical gyre (dashed line)
averaged between 0 and 20 m.
production would be about 0.9 mmol/m/d, which is close to what was observed in the Sargasso
Sea (Nelson et al., 1995).
3.4. New production and remineralization estimates in the North Atlantic
During spring}summer, the observed surface nutrient drawdowns should be roughly equivalent
to new production because vertical mixing is low. In subsurface waters, the decrease in oxygen
anomaly as strati"cation occurs can directly be related to remineralization because vertical mixing
is low and the exchange with the surface layer is reduced at that time. Hence, a complete estimate of
new production and aphotic zone respiration can be made for the spring}summer period in the
North Atlantic.
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Fig. 8. Annual cycles of (a) temperature, (b) salinity, (c) phosphate, (d) nitrate, (e) silicate and (f) oxygen anomaly in the
Gulf of Guinea (solid line), the northwest African upwelling area (dotted line) and the subtropical gyre (dashed line),
averaged between 100 and 150 m.
To estimate new production in carbon units for each region, we average the spring}summer
phosphate drawdowns shown in Table 2 and multiply them by a C/P ratio of 106/1 (Red"eld et al.,
1963). The remineralization rates integrated between 100 and 200 m are computed from the oxygen
anomaly decrease. The lower bound is obtained by considering the di!erence between JFM and
JAS vertical integrals, whereas the higher bound is the di!erence between the maximum and
minimum vertical integrals. Remineralization rate is given in carbon units using a C/!O ratio
of 106/138. Results for each area are shown in Table 3.
The new production values are generally high in the Arctic Ocean, the subpolar gyre and the
Labrador Sea, ranging from 34 to 47 g C/m/period. These results are lower than observations
from the North Atlantic Bloom Experiment site, which shows higher nitrate and phosphate
seasonal drawdowns than our maps (Martin et al., 1993; Sambrotto et al., 1993). On a basin-wide
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Table 2
Seasonal drawdown of phosphate (P}P), nitrate (P}N) and silicate (P}Si) integrated over the top 100 m of the North
Atlantic Ocean and integrated over the six regions de"ned in the text. Units for seasonal drawdown are Tmol (10 mol)
of nutrient per period. The lower values correspond to JAS minus JFM drawdown, whereas the higher values correspond
to the seasonal range for each nutrient
P}P
P}N
P}Si
N/P
Si/N
Arctic Ocean
Labrador Sea
Subpolar gyre
Subtropical gyre
NWA upwelling
Gulf of Guinea
0.20}0.47
0.03}0.07
0.10}0.38
0.15}0.54
0.02}0.06
0.02}0.07
4.3}8.1
0.8}0.9
2.2}3.9
1.3}4.4
0.3}0.6
0.3}0.8
2.5}5.3
0.3}0.8
2.3}4.9
1.8}6.8
0.2}0.8
0.2}0.8
18.5
17.0
12.9
8.2
11.7
10.5
0.6
0.6
1.2
1.5
1.3
1.0
North Atlantic
0.52}1.59
9.1}18.7
7.2}19.4
12.8
1.15
Table 3
Spring}summer new production integrated over the top 100 m of the ocean (NP-PO ) calculated from seasonal
phosphate drawdown and remineralization integrated between 100 and 200 m (R-O ) calculated from seasonal oxygen
anomaly decrease for the six regions de"ned in the text. New production and respiration estimates are averages of the
higher and lower bounds (see text). Primary production estimates are integrated from February to August and are
derived from global monthly maps of Antoine et al. (1996). The units for regional production and remineralization are
g C/m per period. North Atlantic production and remineralization are given in g C/m per period and Pg C (10 g) per
period
NP-PO
R-O
%R/NP
P.P.
f-ratio
Arctic Ocean
Labrador Sea
Subpolar gyre
Subtropical gyre
NWA upwelling
Gulf of Guinea
37.9
46.7
34.6
17.3
14.2
18.0
12.6
19.6
16.8
11.5
*
*
33
42
49
66
*
*
93
100
102
72
82
92
0.41
0.47
0.34
0.24
0.17
0.20
North Atlantic
(g C/m per period)
(Pg C per period)
25.1
1.34
13.0
0.70
52
52
84
4.5
0.29
0.29
scale, our new production results in the subarctic gyre, the Labrador Sea and the Arctic Ocean
agree well with the range of 18}60 g C/m/yr estimated from ocean-color data in temperate and
high latitudes (Campbell and Aarup, 1992) and with the range of 18}74 g C/m/period estimated
between 30 and 603N using air}sea O #uxes (Najjar and Keeling, 2000). In these areas, remineral
ization between 100 and 200 m is about 30 to 50% of the new production from April to September,
implying that less than half of the surface (0}100 m) new production is respired between 100 and
200 m.
The low and temperate latitudes are characterized by low new production rates ranging from 14
to 18 g C/m/period. Although new production rates as high as 25 g C/m/yr have been reported for
the Sargasso Sea (Spitzer and Jenkins, 1989), the averages we found from our calculation agree with
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earlier studies based on models and data, which found new production ranging from 2 to 15 g
C/m/yr in the subtropical gyre (Schlitzer, 1989; Michaels et al., 1994; Williams and Follows, 1998).
In the northwest African upwelling, higher values of new production were reported by Minas and
Codispoti (1993). As our calculation is only based on seasonal changes in nutrients, we probably
miss that part of new production supported by lateral advection processes. Moreover, it has been
suggested that nitrogen "xation enhances new production in the Bermuda area (Michaels et al.,
1993; Gruber et al., 1998), and our computation of new production does not include such a process.
Therefore, our new production estimate is a lower bound. Remineralization rates are found to be of
the same order of magnitude as surface production rates implying that most of the production is
respired above 200 m. The rate of shallow mineralization found in the subtropical gyre agrees with
that found from a recent analysis of oxygen and dissolved inorganic carbon in the Sargasso Sea
(Ono et al., 2001). In the Gulf of Guinea and the northwest African upwelling area, seasonal
movement of the pycnocline (Najjar and Keeling, 1997) prevents us from an accurate estimate of
the respiration.
Primary production (PP in Table 3) from Antoine et al. (1996) is averaged for the February}August period, allowing us to estimate the f-ratio for each area (Table 3). The computed f-ratio,
varying from 0.17 in the low latitudes to 0.47 in the Labrador Sea, agrees generally well with early
studies (Minas and Codispoti, 1993; Martin et al., 1993). This analysis provides an independent
support for the assertion of Laws et al. (2000) that f-ratio decreases with increasing temperature.
Using the new production and respiration rates calculated from nutrient and oxygen seasonal
changes, an export of carbon can be derived for the whole North Atlantic Ocean below 200 m
during spring and summer. If it is assumed that the production that is not respired is completely
exported below 200 m, and if this value is integrated over the considered area, one would arrive at
a net sink of carbon of 0.6}0.7 Pg C/period for the whole North Atlantic. This value represents
a maximum since the production of organic matter includes both particulate and dissolved pools,
the latter accumulating in the surface waters during summer (Williams, 1995). Hansell and Carlson
(1998) estimated that dissolved organic matter (DOM) represents about 17% of the total
spring}summer net community production. This would reduce the total North Atlantic sink of
carbon to about 0.4 Pg C/period. However, it has been shown that DOM participates in the carbon
export to the ocean interior in about the same proportion as the sinking organic particles (Carlson
et al., 1994; Ducklow et al., 1995). Moreover, the partitioning of organic carbon between dissolved
and particulate phases varies in time and space (Carlson et al., 1998). Hence, it is di$cult to
estimate the uncertainty related to the DOM pool in the carbon export. These considerations lead
us to evaluate a carbon export below 200 m lying between 0.4 and 0.6 Pg C in the North Atlantic
during the spring}summer period.
4. Conclusions
Annual cycles of oxygen and nutrients have been analyzed in the upper 200 m of the North
Atlantic ocean. The high latitudes have clear annual nutrient cycles, with a drawdown in
spring}summer due to phytoplankton blooms and an increase from summer to winter due to
vertical mixing. The temperate and low latitudes show generally lower availability of nutrients in
the surface layer than in the high latitudes, and hence lower seasonal variations in both nutrients
F. Louanchi, R.G. Najjar / Deep-Sea Research II 48 (2001) 2155}2171
2169
and oxygen. A remarkable feature is the semi-annual cycle evidenced by the surface phosphate and
oxygen data in the Gulf of Guinea, which is due to monsoon-driven upwelling.
For each area, during the spring}summer period, surface nutrient and subsurface oxygen
decreases allow us to estimate production and remineralization rates in the upper ocean of the
North Atlantic. The spring}summer drawdown of nutrients, due to the biological uptake, is used to
estimate new production ranging from 15 g C/m/period for the low latitudes to 40 g C/m/period
for the high latitudes. These estimates agree generally well with those reported in the literature for
the corresponding areas. New production estimated from the seasonal drawdown in nutrients is
about 1.3 Pg C/period for the North Atlantic, which is 12}37% of the range of the global new
production estimates reported by Ducklow (1995). We found that in the subtropical regions, most
of the new production is respired above 200 m, whereas in the high latitudes less than 50% on
average is respired above that depth. Assuming that the unmineralized organic matter is exported
below 200 m by either particle sinking or vertical mixing, would bring a net spring}summer carbon
sink of 0.5 Pg C for the whole North Atlantic Ocean. This estimate represents about 3}5% of the
total annual new production currently admitted (Ducklow, 1995; Falkowski et al., 1998).
Acknowledgements
We thank Dr. David Antoine (LPCM- France) for making his primary productivity monthly
maps available to us. We are grateful to two anonymous reviewers for their comments which
helped to make the manuscript clearer and enhanced our interpretation. This work was supported
by NASA funding NAG5-6451 through the US-JGOFS program.
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