Journal of Oceanography, Vol. 60, pp. 93 to 117, 2004
Nutrient and Plankton Dynamics in the NE and NW Gyres
of the Subarctic Pacific Ocean
P AUL J. HARRISON1*, FRANK A. WHITNEY 2, A TSUSHI TSUDA3, HIROAKI SAITO 4
and K AZUAKI TADOKORO5
1
AMCE Program, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong
Institute of Ocean Sciences, Fisheries and Oceans Canada, Sidney, BC, Canada V8L 4B2
3
Oceanic Research Institute, University of Tokyo, Nakano-ku, Tokyo 164-8639, Japan
4
Tohoku National Fisheries Research Institute, Shiogama, Miyagi 985-0001, Japan
5
Frontier Research System for Global Change,
Showamachi Kanazawa-ku, Yokahama, Kanagawa 236-0001, Japan
2
(Received 3 October 2003; in revised form 25 November 2003; accepted 26 November 2003)
The western subarctic gyre (WSG) and the eastern Alaska Grye (AG) on each side of
the subarctic North Pacific, have many similarities. In both gyres, macronutrients
are generally high and chl is low, and hence both gyres are High Nitrate, Low Chlorophyll (HNLC) regions. Despite the general similarities between these two gyres, there
are many important differences. The time series station established at Stn KNOT on
the southwest edge of the WSG and two in situ mesoscale iron enrichment experiments at each of the gyres has provided more information on iron concentrations, the
dual role of iron and silicate limitation and seasonal cycles in the gyres. There is more
seasonality in many parameters at Stn KNOT than at Stn P. There is an increase in
Chl and primary productivity at Stn KNOT in May followed by increased iron limitation in summer. Low DIC:NO3 ratios and high Si:NO3 ratios in the WSG, indicate
lower calcification and higher diatom production than at Stn P. The sources of iron
for these areas are still not clear, but horizontal transport of iron rich coastal water
and vertical transport could be important sources at certain times of the year in addition to dust input. Satellite images show that chl-rich coastal waters occasionally extend to the vicinity of Stn KNOT and therefore Stn KNOT may not always represent
conditions in the main part of the WSG. This review focuses mainly on a comparison
of Stn KNOT and Stn P, two time series stations on the edge of two very large gyres.
At present, we have a limited understanding of the temporal and spatial variability
within each of these large gyres.
Keywords:
⋅ Subarctic Pacific,
⋅ Stn P,
⋅ Stn KNOT,
⋅ nutrients,
⋅ phytoplankton,
⋅ iron,
⋅ HNLC,
⋅ primary productivity,
⋅ silicate,
⋅ gyre.
with a large gyre on each side; the Alaskan Gyre (AG) is
on the east side and the smaller Western Subarctic Gyre
(WSG) is on the west side (Fig. 1). There is another gyre
in the Bering Sea, but the properties of this gyre will not
be covered in this review.
There are two extensive time series stations associated with each of the gyres. Station Papa (also referred to
as Stn P and Ocean Station Papa (OSP)) is located at 50°N
and 145°W in 4200 m of water on the southeast edge of
the AG (Fig. 1). There is also a time series conducted
along a transect running from the British Columbia coast
to Stn P, called Line P, with frequently sampled stations
at P4, P12, P16, P20 and P26 (Stn P) (Fig. 1). This transect
from the productive BC coast to the HNLC northeast Pacific at Stn P has provided valuable information and
insights into the interaction between the coast and the open
1. Introduction
The subarctic North Pacific is a large complex area
that can be divided into 12 provinces (Wong et al., 2002b)
and is one of the three large HNLC (high nitrate, low chlorophyll) regions of the world. The relatively high precipitation in this region results in a relatively strong, shallow halocline at 100–120 m and multiple thermoclines in
spring and summer also present a barrier to vertical exchange (Denman and Gargett, 1988). Nutrients in the deep
water are the highest in the global ocean because the North
Pacific is at the end of the abyssal circulation. The North
Pacific spans about 10,000 km in an east-west direction
* Corresponding author. E-mail: [email protected]
Copyright © The Oceanographic Society of Japan.
93
Fig. 1. General circulation in the Subarctic North Pacific showing the Alaska Gyre, Western Subarctic Gyre, Stn P and Line P.
Double arrows are intense boundary currents. The Subarctic Boundary separates the subarctic Pacific region to the north from
the subtropical Pacific region to the south. EKC = East Kamchatka Current; OY = Oyashio; SAC = Subarctic Current; AC =
Alaska Current; KR = Kuroshio; KEX = Kuroshio Extension; TWC = Tsushima Warm Current; SAF = Subarctic Front;
SAB = Subarctic Boundary; KBF = Kuroshio Bifurcation Front; KNOT = Stn KNOT and SEEDS = mesoscale iron enrichment site in 2002.
ocean. It is worth noting that there are surprisingly few
data available from the central area of the AG. On the
western side, Stn KNOT (Kyodo North Pacific Time Series, 44°N and 155°E) was sampled almost monthly from
June 1998 until Feb. 2000 and this data set has provided
a time series for the WSG (Fig. 1). There is no coastal to
open ocean transect time series on the western side, analogous to Line P on the eastern side, although some measurements have been made along Line A in the Oyashio
region (Saito et al., 2002).
Papers summarizing results from the Canadian
JGOFS research in the NE Subarctic Pacific can be found
mainly in a special volume of Deep-Sea Research, Vol.
46 (Boyd et al., 1999b) and many papers from the Japanese JGOFS research have appeared in another special
volume of Deep-Sea Research, Vol. 49 (Saino et al.,
2002). A comparison of the ecosystem dynamics in the
eastern and western gyres of the subarctic Pacific appeared in the special volume of Progress in Oceanography (Vol. 43 No. 2–4, 1999). A brief summary of the Stn
KNOT and Stn P time series is provided by Karl et al.
(2003). The nutrients and phytoplankton dynamics in the
WSG and AG have previously been reviewed (Harrison
et al., 1999; Harrison, 2002), however, there have been
some important new data from the time series at Stn
KNOT at the southwest edge of the WSG and from two
in situ mesoscale iron enrichment experiments near each
of the two gyres (Tsuda et al., 2003a; Boyd et al., 2003).
These new data provide more information on the iron
concentrations, the dual role of iron and silicon limitation, and the top down control of phytoplankton by
microzooplankton and mesozooplankton. In addition, it
is now clear that these gyres are strongly influenced by
94
P. J. Harrison et al.
inputs from coastal regions that may explain some of the
nutrient and plankton dynamics observed in these gyres
and at Stns KNOT and Papa.
2. Interactions between the Gyres and the Coast
The general circulation of the subarctic North Pacific is shown in Fig. 1. The subarctic boundary is defined by the Subarctic Front which is characterized as a
region of relatively high temperature and salinity gradients (Yuan and Talley, 1996), extending from ~40°N off
the coast of Japan, westward to ~148°W and beyond (Fig.
1). The AG is a large cyclonic (anti-clockwise) upwelling
gyre that is bounded on the south by the eastward-flowing slow (~10 cm s–1) Subarctic Current that bifurcates
into the northward flowing Alaska Current and the southward flowing California Current. East of the bifurcation,
surface waters tend to have a mix of subarctic (lower salinity) and subtropical (nutrient poor) characteristics
(Whitney et al., 1998). On the east, the gyre is bounded
by the Alaska Current and on the north and west, by the
strong (>30 cm s –1 ) southwestward-flowing Alaska
Stream Current along the continental slope of Alaska
(Dodimead et al., 1963; Favorite et al., 1976; Bograd et
al., 1999) (Fig. 1). The Alaska Stream Current connects
with the Subarctic Current near 170°W, completing the
circulation of the gyre. Bograd et al. (1999) analyzed the
trajectories of satellite-tracked drifters drogued within the
mixed layer (15 m) and below the pycnocline (120 m).
Mean drifter transit time at the surface to complete one
cycle in the AG was about 1.5 to 2 years.
The region southwest of the Queen Charlotte Islands
(~51°N, 134°W) is an area where eddies form in the winter due to tidal rectification (Thomson and Wilson, 1987;
Thomson and Gower, 1998) (Fig. 1). These eddies are
called Haida eddies and they move slowly westward,
sometimes in the direction of Stn P (Crawford, 2002;
Crawford et al., 2002). Due to their large volume (~100
km in diameter and ~500 m deep), they transport substantial amounts of high nutrient and high iron coastal
water offshore (Whitney and Robert, 2002). They remain
intact for 1 to 3 years at which time they are no longer
distinctly recognizable with TOPEX/POSEIDON
altimetry. Further north along the southeast Alaskan coast,
more eddies are formed, possibly due to instabilities in
the coastal currents (Crawford et al., 2000). These eddies will either move westward into the open ocean or
northward into the Alaskan Stream. Eddies formed along
the eastern boundary of North America transport coastal
water and organisms hundreds of kilometers away from
the coast into open ocean, a process which does not occur along the Asian shore of the North Pacific.
The physical oceanography off the coast of Japan is
complex and is influenced by the western boundary currents of the Kuroshio and Oyashio (Fig. 1). The Kuroshio
is a subtropical warm water current that connects to the
Kuroshio Extension and propagates warm core rings. The
Oyashio is a southwestward-flowing cold water current
(Fig. 1).
In winter, nutrient-rich, surface water is transported
by the East Kamchatka Current and this water mixes with
cold water from the Sea of Okhotsk (Nakamura et al.,
2000a, b; Aramaki et al., 2001; Wong et al., 2002b) (Fig.
1). This mixed water forms a cold-core eddy south of
Bussol’ Strait (44–46°N and 151–154°E) (Yasuda et al.,
2000; Rogachev, 2000), and its position varies which influences the path of the Oyashio and the biological productivity of the area (Rogachev, 2000; Murata et al., 2002;
Kusakabe et al., 2002). Warm core rings that separate from
the Kurioshio Extension, move along the Japan Trench
and occasionally along the Kuril-Kamchatka Trench and
this is another source of the anti-cyclonic eddy which may
be intensified by the water transported through Bussol’
Strait (Yasuda et al., 2000; Itoh and Sugimoto, 2001).
The Oyashio usually separates into two branches; the
Oyashio first branch flows along Hokkaido and the second branch flows to the south (Fig. 1). The southern end
of the Oyashio first branch is the southeastern edge of
the western subarctic circulation (Sekine, 1988). The
Oyashio turns eastward off Honshu at around 38–40°N
and it mixes with warm saline subtropical Kurioshio water with various mixing ratios (Kono, 1997; Yasuda et al.,
2002; Hiroe et al., 2002). The physical characteristics of
the Oyashio-Kuroshio interfrontal region are quite complex and various fronts are formed here, such as the
Subarctic Front (temperature front), Subarctic Boundary
(salinity front), and the Kurishio Bifurcation Front
(Yasuda, 2003; Fig. 1). The area between the Subarctic
Front and the Subarctic Boundary is referred to as the
Transition Domain (Favorite et al., 1976).
Stn KNOT is located in the southwestern part of the
WSG (Ohtani, 1989; Kono, 1997; Kono and Kawasaki,
1997; Imai et al., 2002) and occasional intrusions of transition domain water have been observed (Tsurushima et
al., 2002). Satellite images show that a chlorophyll-rich
coastal plume occasionally extended to the vicinity of Stn
KNOT (Sasaoka et al., 2002). The extension of coastal
water close to Stn KNOT was also observed by ship observations (Imai et al., 2002). This suggests that coastal
waters may influence chemical and biological parameters
that are measured at Stn KNOT and therefore Stn KNOT
may not be a good representation of conditions in the
WSG. There are also some clear seasonal N-S gradients
in nutrients at Stn KNOT due to the advance of the
Oyashio Water to the south in winter and its retreat to the
north in summer (the N-S migration of the Oyashio Front)
(Tsurushima et al., 2002). Hence there is considerable
variability at Stn KNOT due to variability in the large
scale water circulation patterns which may not occur in
the main part of the WSG.
3. Physical and Chemical Characteristics
Key physical and chemical parameters at Stn P have
been summarized by Whitney and Freeland (1999; Fig.
2). While the MLD at Stn P and Stn KNOT were calculated differently, a comparison found that there was no
significant difference between the two methods. The
mixed layer depth (MLD) is much shallower in summer
(~40 m) compared to winter (90–120 m) because the average wind speed decreases from 12 to 7 m s–1 in summer, the surface layer sigma-t decreases from 25.7 to 24.5
and the surface layer temperature increases from 6 to
12°C. All three parameters act together to decrease the
MLD by increasing stratification. The stratification and
increased light and temperature in the summer increases
primary productivity from 300 to about 600 mg C m–2d–1
and there is a subsequent drawdown in nitrate from 16 to
about 9 µM NO3 and silicate from 24 to 12.5 µM. Ammonium is very low (from undetectable to 0.5 µ M) and
can reach 1 µM just below the euphotic zone in summer
(Whitney, unplubl. data). Urea is low (<0.5 µM) and variable (Varela and Harrison, 1999a). Even though primary
productivity increases during the summer, there is no clear
seasonal cycle in Chl. During the weathership era from
1964 to 1976, frequent sampling showed that Chl remained in the 0.2 to 0.4 µg L–1 range throughout the year
with occasional periodic increases >1 µg L–1 and several
examples of >2 µg L –1 (see figure 5 in Harrison et al.,
1999). At least one of these periods of high chlorophyll
(1972) coincided with silicate depletion which was
thought to arise from elevated iron inputs (Wong and
Matear, 1999).
Nutrient and Plankton Dynamics in the NE and NW Gyres of the Subarctic Pacific Ocean
95
20
40
60
80
15
33.5
10
33.0
5
32.5
0
32.0
Jan Feb Ma Apr Ma Jun Jul Aug Sep Oct Nov Dec
r
y
25
50
20
40
15
30
10
20
5
10
)
34.0
-3
120
20
Salinity (psu)
100
0
0
Silicic acid (mmol m
Nitrate (mmol m
-3
)
o
Temperature ( C)
Mixed Layer Depth (m)
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
0
Jan Feb Ma Apr Ma Jun Jul Aug Sep Oct Nov Dec
Fig. 3. Time series of mixed layer depth, temperature, salinity
nitrate and silicic acid concentrations at Stn KNOT.
Fig. 2. A) Average annual cycles of mixed layer depth, and
surface layer sigma-t, 1956–97; B) monthly averaged surface temperature, salinity and wind speed from 1971–80;
and C) surface nitrate (monthly averages from 1969–81)
and silicate (monthly averages from 1974–81 at Stn P (from
Whitney and Freeland, 1999).
In comparison, at Stn KNOT, the MLD ranges from
80–100 m in winter to <10 m in summer and even <5 m
in Aug. 1998 and 1999 (Tsurushima et al., 2002) (Fig. 3).
The mean temperature in the MLD ranged from 16°C in
Sept. 1998 to 2°C in Feb. This 13°C range in temperature
is in contrast to the 6°C range at Stn P. The mean salinity
over the MLD ranged from 32.5 in Oct. to >33.2 in May/
June. The change in sigma-t was mainly due to the large
change in temperature, rather than the small change in
salinity, and consequently the pycnocline is relatively
weak in summer compared to Stn P.
In winter at Stn KNOT, nitrate was 18 to 19.6 µM
and was drawn down to about 5 µM in August, while silicate ranged from >30 µM in winter to <10 µM in summer
(Fig. 3). Therefore these nutrients did not appear to be
limiting to phytoplankton growth during spring and summer, unless there are episodic short-lived drawdown
events that were not captured by the monthly sampling.
A comparison of the two surveys of late summer surface nutrients across the southern edge of these two gyres
96
P. J. Harrison et al.
(148 to 180°E and 46.5 to 45°N for WSG and 125 to
157°W and 48.5 to 53°N for the AG) revealed the contrast between the two gyres. The western survey crossed
the Oyashio and extended through the WSG, while the
Line P section crossed a coastal area of N depletion and
extended to an area south of the center of the AG. Generally, the nutrient concentrations are much higher in the
WSG than the AG, both nearshore and in the gyres (Fig.
4). Off the Kuril Islands (~150°E) in the WSG, the southward-flowing Oyashio is cool and nutrient-rich (Fig. 4).
Further from shore, nutrient levels decrease and then
slowly increase to 18 µM nitrate and 30 µM silicate at
~170°E, representative of WSG waters in August (Wong
et al., 2002b; Fig. 4). The AG is generally much poorer
in nutrients during summer. Nutrients are supplied to the
BC continental shelf via upwelling (Whitney and Welch,
2002) and riverine inputs (Whitney et al., 2003), but a
broad transitional domain between the coast and the
HNLC region is seasonally nitrate depleted and frequently
smaller regions also become silicate-limited (Fig. 4), probably when strong stratification shoals the mixed layer or
iron supply is increased (Whitney et al., 2003). HNLC
waters at Stn P typically contain as much as 8 µM nitrate
and 15 µ M silicate in summer (Whitney and Freeland,
1999). Therefore nitrogen and silicon are seldom limiting during the summer at Stn P, although periods of silicate depletion have been observed in the 1970s (Wong
and Matear, 1999).
In winter, mixing extends to about 80–100 m at Stn
Tully, Sep 1994
Vinogradov, Aug 1992
35
35
NO3
25
20
20
15
15
10
10
5
5
0
0
148
152
156
160 164 168
Longitude (E)
172
176
157
180
33.6
18
18
33.4
16
16
14
14
32.8
12
32.6
10
32.4
T (C)
33.0
153
149
145 141 137 133
Longitude (W)
129
125
33.6
S
T
33.2
Salinity
NO3 and Si (uML)
30
Si
25
33.4
33.2
33
12
32.8
10
32.6
8
8
32.0
6
6
31.8
4
4
32.4
32.2
32.2
148
152
156
160 164 168
Longitude (E)
172
176
Salinity
NO3 and Si (uM)
30
32
31.8
157
180
153
149
145
141
137
133
129
125
Longitude (W)
Fig. 4. Surface nitrate, silicate, salinity and temperature in late summer on a section across the Western Subarctic Gyre at 45 to
46.5°N (R/V Vinogradov, Aug 1992) and across the Alaskan Gyre at 48.5 to 52.5°N (Tully, Sept 1994).
Si (uM)
Sigma-t
25
26
0
27
20
40
60
NO3 (uM)
80
100
0
0
50
50
50
100
100
100
150
150
150
200
Depth (m)
0
Depth (m)
Depth (m)
24
0
200
250
250
300
300
350
350
400
400
10
20
30
40
200
250
300
WSG Aug 92
OSP Sep 94
OSP Feb 95
350
400
Fig. 5. Vertical profiles of sigma-t, silicic acid and nitrate during winter and late summer at Stn P and in the Western Subarctic
Gyre (51°N, 165°E). The winter supply of macronutrients is considerably higher in the WSG because concentrations are
higher in waters underlying the mixed layer.
KNOT (Tsurushima et al., 2002) and about 90 to 120 m
at Stn P (Whitney and Freeland, 1999). In the WSG, higher
nutrient levels occur in winter because subsurface waters
are much richer in nutrients (Fig. 5) compared to AG.
For example, at 100 m during summer, WSG waters contained 31 µM NO3 and 56 µM Si (August 1992), compared with Stn P which had nutrient concentrations of 15
µM NO3 and 22 µM Si (September 1994). Higher nutrient levels are consistent over the upper few hundred meters in the WSG, even though the vertical profiles of density of the eastern and western gyre waters are similar.
Several decades ago, it was unclear what factors limited phytoplankton growth in the WSG and the AG since
macronutrients were never limiting. However, the use of
trace metal clean techniques have been employed during
the last two decades and iron has been confirmed to limit
primary productivity in carboy experiments onboard ship
(Boyd et al., 1996; Boyd and Harrison, 1999) and in
mesoscale iron enrichment studies in the two gyres (Tsuda
et al., 2003a; Boyd et al., 2003).
Since Fe limits phytoplankton growth especially in
summer in these two gyres, there has been considerable
interest in the various sources of iron. Fe is supplied from
below (winter mixing, diffusion), by horizontal transport
(recirculation, eddies) and from above (dust). However,
the magnitude of various iron sources has been the subject of considerable discussion (Boyd et al., 1998b;
Johnson et al., 2003). Fe typically behaves much like Si
in the upper 500 m of the N Pacific, since it is stripped
from the upper ocean by phytoplankton and remineralized
as detritus sinks into the ocean. In the eastern subarctic
Pacific, it has also been shown that mesoscale eddies
Nutrient and Plankton Dynamics in the NE and NW Gyres of the Subarctic Pacific Ocean
97
Table 1. Relative supplies of nutrients to the mixed layer from below. Dissolved Fe/Si ratios are obtained from linear regressions
of data in the upper 600 to 800 m (R2 > 0.9 in all cases) and axis intercepts (nM Fe).
Stn
Lat (°N)
Long (°W)
Sfc Fe
(nM)
Si:NO3
(200 m)
Fe/Si
(nM Fe/uM Si)
P4
P12
P16
P20
P
Z4
Z9
Z14
Z19
Haida
KNOT
48.65
48.97
49.28
49.57
50.0
52.65
55.0
57.5
59.46
49.28
44
126.67
130.67
134.67
138.67
145
145
145
145
145
134.67
155°E
0.12
0.05
0.02
0.02
0.03
0.04
0.04
0.04
0.29
0.08
0.15
1.43
1.47
1.64
1.79
1.92
1.82
1.94
1.97
1.63
1.29
2.1
0.016
0.0049
0.0043
0.0053
0.0045
Insufficient data
0.0065
0.0067
N.L., 0.010 at 200 m
N.L., 0.018 at 200 m
0.0085*
Ref., Date, Max depth
2.
2.
2.
2.
2.
2.
2.
2.
2.
1.
3.
June 1998 to 600 m
June 1998 to 800 m
June 1998 to 800 m
June 1998 to 600 m
June 1998 to 800 m
June 1998 to 100 m
June 1998 to 800 m
June 1998 to 800 m
June 1998
Sept 1998
May 2000/July 1991
N.L. = non-linear.
1. Nishioka et al. (2001); 2. Johnson et al. (2003); 3. Nishioka et al. (2003a); Whitney (unpublished data).
*Nishioka et al. (2003a) did not quote nutrient data. Nutrients collected from the same location in 1991, over a depth of 150
to 600 m were used to generate this regression.
transports substantial amounts of iron from coastal areas, well into the Gulf of Alaska (Johnson et al., 2003).
They measured total leachable Fe concentrations (Fe that
is leached by microwave digestion of seawater at pH 2)
of over 10 nM and dissolved Fe levels of >1 nM in young
Haida eddies. While these eddies can transport coastal
water with high Fe into the Gulf of Alaska, it is less clear
that these eddies actually reach Stn P which has been previously suggested as a way to explain some of the low
silicate concentrations that have been observed, especially
in the 1970s (Wong and Matear, 1999). Surface drifters
show that relatively iron-rich coastal waters from the
Alaskan shelf can reach Stn P within several months
(Bograd et al., 1999), but this coastal water may be severely diluted by the time it reaches Stn P.
Dust transport has also been inferred to be an important source of Fe to the upper ocean (Boyd et al., 1998b).
Major sources of air borne particles include desiccated
soils in China (Gobi Desert), forest fires in Asia, Alaska
and BC, and volcanoes in Alaska and Kamchatka. Bishop
et al. (2002) suggested that increased POC levels measured by their profiling optical instrument were the result of Fe input from the Gobi Desert. However, they do
not account for the atypical drift pattern of the profiler
that suggests it may have been influenced by mesoscale
eddy circulation.
Fe supply to the western Pacific is higher than the
eastern Pacific for several reasons (Nishioka et al., 2003).
The dust source from the Gobi Desert is closer to the WSG
than the AG and prevailing winds favour transport into
the ocean (Boyd et al., 1998b). In addition, circulation
patterns carry water away from coastal areas near
98
P. J. Harrison et al.
Kamchatka and Japan to the WSG within a few months
via the eastward-flowing Oyashio (Fig. 1). In contrast,
some of the water reaching Stn P came from the coast via
the Alaska Current, the Alaska Stream Current and the
Subarctic Current (Fig. 1) has not been in contact with
land for ~2 years assuming surface current speeds of <10
cm s–1 and a gyre circumference of over 4000 km (travel
time ~500 days).
A greater supply of macronutrients (nitrate and silicate) in the WSG, and a substantially higher seasonal
nutrient utilization in the WSG (14–22 µM nitrate) as
shown in Table 1, compared with the AG (8–10 µM seasonal nitrate utilization; Wong et al., 2002a, b) suggests
a greater supply of Fe to the WSG. Prevailing currents
carry water away from the Asian coast, and since land is
a major source of Fe for oceanic waters, one would expect higher Fe levels in the WSG compared with the AG
(Table 1). Nishioka et al. (2001, 2003) and Johnson et al.
(2003) have collaborated in collecting an extensive Fe
data set in the AG and they have confirmed that Fe levels
in these HNLC waters in this region are extremely low
(~0.05 nM), while surface mixed layer iron concentrations are similar or higher at Stn KNOT (0.15 nM; Table
1). Recently, Fujishima et al. (2001) found that the dissolved iron concentration in the deep water (>200 m)
southwest of the WSG was ~1 nM which is about twice
as high as the AG in summer. Nishioka et al. (2003) found
that particulate iron was significantly higher in the surface mixed layer at Stn KNOT, compared to Stn P. The
increase in dissolved iron with depth from the subsurface
to intermediate water, was larger at western stations than
at Stn P, and the estimated iron flux from below the sur-
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
0
20
40
Euphotic Zone Depth (m)
60
Stn P
80
100
0
20
40
KNOT
60
80
100
0
20
40
Oyashio
60
80
100
Fig. 6. Seasonal change in the photic zone depth (1% light
depth, but 0.9% light depth for KNOT) in the Alaskan Gyre
(Stn P), the Western Subarctic Gyre (KNOT) and the
Oyashio region. The photic zone depth in the Alaskan Gyre
was calculated from the Secchi depth (WODC) using the
relationship between the attenuation coefficient (k: m–1) and
Secchi depth (SD: m) where k = 1.45/SD (Walker, 1980).
Photic zone depths in the Western Subarctic Gyre are from
Imai et al., 2002 and in the Oyashio they are calculated
from the Secchi depths (circles, Saito et al., 1998; Kasai et
al., 2001), or measured by an underwater Biospherical
PRR600 or PUV500 radiometer (diamonds, Saito, unpubl.
data).
face is several times higher at Stn KNOT than at Stn P
(Nishioka, pers. comm).
Iron and macronutrient supply ratios are summarized
for the AG and Stn KNOT (Table 1). Both the impoverishment of iron in surface waters and the relatively low
levels of Fe per unit Si are evident in the AG at stations
distant from the continental slope. Using linear regressions to estimate the supply ratios of Fe (nM) to Si (µ M),
oceanic waters in AG consistently contain 0.004 to 0.007
nM Fe/µM Si in the upper 600 to 800 m. This is in agreement with the results of Martin et al. (1989) who found
ratios of 0.0047 nM Fe/ µM Si in the North Pacific. Only
near continental slopes do these ratios increase. However,
at Stn KNOT, this ratio resembles the more coastal waters of the BC and Alaskan coasts. In addition to higher
Si/Fe ratios at Stn KNOT, silicate is also in relatively
higher supply than nitrate. Waters mixed towards the surface in the WSG will supply relatively more iron and silicate.
At Stn KNOT the photic zone showed little seasonal
variation except during brief bloom periods in May and
ranged from 44 m in Oct. to 62 m in June (Fig. 6; Imai et
al., 2002). This is in contrast to Stn P where the photic
zone is more variable being shallower (30–50 m) in summer and deeper (60–80 m) in winter (Harrison et al., 1999;
Sherry et al., 1999), than the WSG. The Oyashio region
showed a clear seasonal cycle with the photic zone being
shallower in spring and autumn due to increased
phytoplankter biomass, compared to summer and winter
(Fig. 6).
4. Phytoplankton Biomass and Community Structure
At Stn KNOT chlorophyll concentrations varied seasonally and frequently increased to >1 µg L–1 in summer
(Imai et al., 2002). It was surprising that the minimum
Chl (<0.5 µg L–1) was observed in late summer and not
in winter when Chl was ~0.6 µg L–1. The integrated Chl
showed little seasonal variation with winter values of
about 30–40 mg Chl m–2 and late spring values of about
25 to 50 mg Chl m–2 in May (Imai et al., 2002). The smallest size fraction (<2 µ m) and the middle size fraction (2–
10 µm) accounted for 41–76% and 13–38% respectively
of the total chlorophyll. These size fractions are discussed
in more detail by Liu et al. (2004). In contrast, at Stn P,
Chl is low (0.3 to 0.6 mg m–3) and shows little seasonal
variation. Vertical profiles show little variation in Chl
within the photic zone (Wong et al., 1995; Boyd et al.,
1995; Boyd and Harrison, 1999). Integrated Chl ranges
from 22.5 to 24.5 mg m–2. The small size fraction (<5
µm) dominated the total Chl in all seasons and especially
in winter. The 5–20 µm size fraction generally increased
in spring and summer, but it was not usually greater than
the <5 µm fraction (Boyd and Harrison, 1999).
In the WSG at Stn KNOT, there was a clear spring
increase in diatoms with cell abundance of phytoplankton
(>10 um) reaching about 11 × 10 8 cells m–2. In contrast,
the lowest abundance occurred in summer followed by
fall and winter with 0.8, 1.4 and 2.7 × 108 cells m–2 respectively. It was surprising that cells were more abundant in winter than summer and fall. However only cells
>10 um were counted and this would explain the relatively high Chl in summer but low cell abundance since
the abundant picoplankton were not counted (Mochizuki
et al., 2002). Liu et al. (2004) reported that the >2 um
size fraction made up >50% and up to 75% of the total
Chl in winter/spring and summer/fall respectively. Among
the picoplankton, picoeukaryotes and Synechococcus were
equally abundant, but the former was more important in
terms of cell carbon. Diatoms numerically dominated the
phytoplankton assemblage in all four seasons (62–91%
of total cell abundance) (Mochizuki et al., 2002). Centrics
were always more abundant than the pennates, especially
in the winter (Shiomoto and Asami, 1999). The genus
Thalassiosira dominated all year round with T. pacifica
and T. nordenskioeldii decreasing from spring to fall and
T. oestrupii and T. eccentrica increasing from early summer to fall (Table 2). Other large diatoms such as
Nutrient and Plankton Dynamics in the NE and NW Gyres of the Subarctic Pacific Ocean
99
100
P. J. Harrison et al.
1
Mochizuki et al. (2002); 2Clemons and Miller (1984); 3Booth et al. (1993); 4Taylor and Waters (1982); 5Takahashi (1986, 1987); 6Putland (unpubl. results 1997–1998)
(February, June and September).
Table 2. Typical phytoplankton in the WSG and AG. Note changes in diatom nomenclature over time (Neodenticulum vs.
Denticulopsis; Corethron species). The nomenclature used by authors has been retained.
Table 3. Summary of the estimated body size, prosome length, life cycle and downward migration of major populations of
Neocalanus flemingeri in each area of the subarctic Pacific. Downward migration of Neocalanus plumchrus is provided for
comparison.
ND = not determined.
1
Tsuda et al. (2001); 2Geinrikh (2002); 3Goldblatt et al. (1999); 4Mackas et al. (1998).
Corethron criophilum, Chaetoceros concavicornis,
Ch. debilis and Coscinodiscus granii were periodically
abundant. Resting spores of Chaetoceros occurred all year
and all resting spore formers were neritic Chaetoceros
species. In the pennate diatoms, Neodenticula seminae
and Pseudo-nitzschia spp. were always dominant and
Thalassionema nitzschiodes increased sporadically in Sept
and Fragilariopsis spp. was in low abundance in the fall
and spring.
The abundance of dinoflagellates increased in summer and fall when the total cell abundance was relatively
low (Mochizuki et al., 2002). The combined abundance
of Prorocentrum spp. and Gymnodinium was relatively
high throughout the year. Silicoflagellate abundance was
extremely low.
At a station near Stn KNOT (43.5°N, 155°E on July
27, 1997), coccolithophore abundance was low (2050 ×
10 3 cells m –2, 0–50 m integration) and three species occurred (Emiliania huxleyi, Neosphaera coccolithomorpha,
and Calyptrolithophora papilifera) (Hattori et al., 2003).
While coccolithophore abundance was low in the WSG
in June/July, they were more abundant in the transition
domain at >170°E. The low abundance of
coccolithophores near the WSG agrees with the low Corg/
Cinorg flux ratio observed by Honda et al. (2002) and the
relatively low percentage of CaCO3 production relative
to organic production as observed in sediment trap samples (Wong et al., 1999).
There are only a few detailed taxonomic data for Stn
P that cover all the seasons and span several years (Table
2). Early studies revealed that small phytoflagellates such
as prasinophytes and prymesiophytes were dominant with
diatoms comprising a secondary component (Taylor and
Waters, 1982; Booth et al., 1993). Recent
chemotaxonomic pigment analysis confirmed these early
observations and found that pelagophytes and green algae (prasinophytes and chlorophytes) were also present
(Suzuki et al., 2002b). It appears that the diatom assemblage is dominated by centrics such as Proboscia (formerly Rhizosolenia) alata, Corethron criophylum, and
Chaetoceros atlanticus, and pennates such as
Neodenticulum seminae, Pseudo-nitzschia spp. and
Thalassiothrix longissima (Table 2). Note that Mochizuki
et al. (2002) have used the diatom assemblage collected
by sediment traps at Stn P (data from Takahashi, 1986,
1987) to represent the surface diatom assemblage. Centric diatoms are not commonly seen in sediment traps
(Takahashi, 1987), suggesting that their frustules are not
robust enough to survive to depth, or they are fragmented
by grazers in the upper ocean. Consequently, centric diatoms at Stn P are under represented in Mochizuki et al.’s
(2002) comparison with the WSG.
Coccolithophores can be periodically abundant at Stn
P (up to 750,000 cells L–1, Putland et al., 2003), especially in late spring in the surface waters at Stn P
(Crawford et al., 2003). Wong et al. (2002a) found elevated uptake of carbon relative to nitrate and suggested
calcification by coccolithophores was responsible.
Coccolithophores at Stn P have previously been underestimated because of cell preservation solutions that are
acidic and hence the coccoliths dissolve and this makes
coccolithophores difficult to recognize.
5. Zooplankton Biomass and Community Structure
During this decade, our understanding of the lower
trophic levels, especially mesozooplankton, of the North
Pacific has progressed significantly due to JGOFS and
GLOBEC related studies. The large suspension-feeding
copepods (Neocalanus spp. and Eucalanus bungii) occupy
the epipelagic layer in summer and the mesopelagic layer
Nutrient and Plankton Dynamics in the NE and NW Gyres of the Subarctic Pacific Ocean
101
in winter due to their ontogenetic vertical migration
(Mackas and Tsuda, 1999; Kobari and Ikeda, 1999), and
they are important food for pelagic and mesopelagic organisms (Odate, 1994; Moku et al., 2000; Yamamura et
al., 2002). Neocalanus flemingeri was established as a
new species in 1988 (Miller, 1988), and the life histories
of three species of Neocalanus in the eastern North pacific were re-examined (Miller and Clemons, 1988). These
studies revealed that the three Neocalanus species (N.
cristatus, N. flemingeri and N. plumchrus) have an annual life cycle with dormancy as adults or pre-adult stages
in the deep layers from summer to winter, and their offspring rise to the surface or subsurface layers for growth
in spring (Table 3). Segregation of the growing season
between N. flemingeri and N. plumchrus, and the vertical
segregation between surface grazers (N. plumchrus and
N. flemingeri) and subsurface grazers (N. cristatus and
Eucalanus bungii) are also characteristic of life cycles of
these copepods (Mackas et al., 1993). In the western
subarctic Pacific and Bering Sea, the life cycles of these
copepods are basically identical to those in the eastern
Pacific (Tsuda et al., 1999; Kobari and Ikeda, 1999, 2001a,
2001b; Geinrikh, 2002), however, there are some differences. Firstly, N. flemingeri with a biennial life cycle is
found in the Sea of Okhotsk, the Japan Sea and the
Oyashio region (Miller and Terazaki, 1989; Tsuda et al.,
1999). In the Oyashio region, both annual and biennial
life cycles were found with a difference in the body sizes
(Tsuda et al., 1999). Kobari and Ikeda (2001a) suggested
that the bimodal size distribution of N. flemingeri in the
Oyashio region come from sexual dimorphism. However,
Tsuda et al. (2001) found the bimodal size distribution
even in the adult stage and strongly suggested that large
N. flemingeri with a biennial life cycle in the Oyashio
region are advected from the Sea of Okhotsk. Secondly,
body sizes vary significantly among the basins in all species of Neocalanus (Table 3). Generally, the body sizes
are smaller in the individuals in the open ocean than those
in the marginal seas and the slope waters. If we compare
both gyres, the individuals in the WSG are larger than
those in the AG (Tsuda et al., 2001). These body size variations are well correlated with habitat temperature (Kobari
et al., 2003b). Thirdly, the timing of the life cycles also
varies locally. The period of downward migration of N.
flemingeri and N. plumchrus is somewhat later in the
western Bering Sea (Geinrikh, 2002) compared with the
AG and Oyashio region (Miller and Clemons, 1988; Tsuda
et al., 1999). Moreover, in the coastal region of the eastern subarctic Pacific, the downward migration of N.
plumchrus was estimated to occur as early as April to May
(Goldblatt et al., 1999). E. bungii also showed local variation in the life cycle depending on the timing of the primary production maximum (Tsuda et al., 2003b).
More distinct local variation in the life cycle is ob-
102
P. J. Harrison et al.
served in Eucalanus bungii than in Neocalanus species.
In the Oyashio region, maturation and spawning of E.
bungii takes place in April and May, and the first
copepodites appear abundantly in June which is two
months earlier than those of the open ocean population
(Miller et al., 1984; Tsuda et al., 2003b). However, we
have to be aware of the decadal variation in the developmental timing of these copepods. The peak of the
zooplankton biomass (5th copepodite of N. plumchrus
makes up half of the premigrant population) at Stn P was
mid to late July in the early 1970s, and was as early as
mid-May in the 1990s (Mackas et al., 1998). When we
compared the developmental timing of N. plumchrus between the original description (Miller and Clemons, 1988)
and that of the Oyashio region (Tsuda et al., 1999), the
timing is almost identical. Life cycles in the Oyashio region were studied using samples collected in the late
1990s, and if we compare the timing of N. plumchrus in
the 1990s at Stn P, N. plumchrus in the AG developed
one to two months earlier than in the Oyashio region.
In addition to the studies of large suspension-feeding copepods, the life cycle of other zooplankton species
has been intensively studied in the western subarctic Pacific. These studies include chaetognaths, Sagitta elegans
(Nishiuchi et al., 1997; Kotori, 1999), amphipods,
Cyphocaris challengeri and Primno abyssalis (Yamada
and Ikeda, 2000; Yamada et al., 2003), ostracods,
Discoconchoecia pseudodiscophora, Orthoconchoecia
haddoni, and Metaconchoecia skogsbergi (Kaeriyama and
Ikeda, 2003) and copepods, Gaidius variabilis,
Pleuromamma scutulata, Heterorhabdus tanneri,
Paraeuchaeta elongate, Scolecithricella minor,
Pseudocalanus minutus and P. newmani (Ozaki and Ikeda,
1998, 1999; Yamaguchi et al., 1998, 1999; Yamaguchi
and Ikeda, 2000a, b). These studies clearly showed that
the majority of zooplankton species have annual life cycles. However some species (some populations of N.
flemingeri, E. bungii, S. elegans and G. variabilis) show
a longer life cycle and a small number of small copepods
show multiple generations per year (M. pacifica, S.
minor, and P. newmani). Some of the results from life
cycle analysis were put into an ecological-physical coupled model, and the effects of ontogenetic migration by
dominant mesozooplankton on phytoplankton community
were modeled (Kishi et al., 2001).
Mesozooplankton grazing on phytoplankton had been
assumed as a cause of stable and low phytoplankton standing stocks in the open ocean of the subarctic Pacific
(Heinrikh, 1962). However, iron as the limiting nutrient
and microzooplankton grazing on pico- and
nanophytoplankton have been shown to be major factors
responsible for HNLC waters (Martin and Fitzwater, 1988;
Cullen, 1995). The in situ measurements of grazing rates
of mesozooplankton suggested that their grazing rates are
60N
Latitude
<200m
50N
40N
140E
160E
180
160W
140W
120W
Longitude
3
5
7
9
11
13
Carbon Export (gC m -2)
15
Fig. 7. Estimated export flux by mesozooplankton ontogenetic vertical migration in the subarctic North Pacific, based on the wet
weight distribution of mesozooplankton in summer (Sugimoto and Tadokoro, 1997) and the export flux estimated in the
Oyashio area (Kobari et al., 2003a). Shallow area was excluded in the calculation.
too low to graze down the phytoplankton production in
both gyres (Dagg, 1993a; Tsuda and Sugisaki, 1994). The
bottle incubation of the HNLC waters with iron and
mesozooplankton additions suggested that five times
greater mesozooplankton biomass is needed to graze down
the iron-stimulated diatom growth (Boyd et al., 1999a).
The role of mesozooplankton as primary consumers and
their slow numerical response was confirmed by a
mesoscale iron enrichment experiment in the western
subarctic Pacific (Tsuda et al., 2003a, c). The main food
sources of dominant suspension-feeding copepods are
microzooplankton and sinking particles in the open ocean
(Dagg, 1993b; Gifford, 1993). In the coastal areas and
ocean margins where large-sized phytoplankton such as
diatoms are available, they mainly feed on phytoplankton
(Dagg et al., 1982; Kobari et al., 2003a). Ecosystem models also showed mesozooplankton grazing significantly
influenced microphytoplankton dynamics in these regions
(Saito et al., 2002; Yoshie et al., 2003). These facts suggest that suspension-feeding copepods opportunistically
feed on suitable sized particles according to the abundance of the particles, and the season. It is also important
to note that these copepods showed relatively high growth
rates (0.15 d–1) even in the HNLC region (Miller and
Nielsen, 1988).
Trophic relationships of the ecosystem have been also
been revealed by the long-term monitoring of the ecosystems. Brodeur and Ware (1992) demonstrated that
mesozooplankton biomass increased between late 1950’s
and 1980’s accompanying the increase in salmon stocks
in the Gulf of Alaska. Miller et al. (1992) showed that
the annual variation of body length of N. plumchrus and
N. flemingeri was negatively correlated with new production at Stn P. Moreover, developmental timing was earlier from 1970s to 1990s possibly due to a slight warming of the surface water (Mackas et al., 1998). In some
restricted areas of the North Pacific, there is top-down
control of mesozooplankton. In the subarctic current area
of the central North Pacific, there are clear biennial fluctuations of mesozooplankton and phytoplankton stand-
ing stocks (Shiomoto et al., 1997; Sugimoto and
Tadokoro, 1997, 1998) due to the trophic cascade from
the biennial fluctuation of pink salmon stocks. In odd
years, pink salmon abundance is high, zooplankton are
low and phytoplankton biomass is high. The body length
of Neocalanus copepods also showed a biennial pattern
in this area, being longer in odd years and shorter in even
years (Kobari et al., 2001).
Mesozooplankton in the high latitude oceans have
been studied not only as primary or secondary consumers, but also as transporters of organic materials to the
deep ocean through vertical migration (Longhurst et al.,
1990; Bradford-Grieve et al., 2001). Ontogenetic seasonal
migration is the diagnostic characteristic of the life cycle
of mesozooplankton in the subpolar and polar oceans
(Conover, 1988). Mesozooplankton feed and grow in the
surface layer during the productive season, and sink to
the mesopelagic layer (>500 m) with accumulated organic
matter in late summer/fall. These processes contribute to
the export flux in two ways: one is transportation of organic matter through the ontogenetic vertical migration
and subsequent respiration and predation by higher trophic
organisms in the mesopelagic layer. Another one is fecal
pellet production during the growth season in the surface
layer. Generally, the flux of sinking particles such as fecal
pellets and aggregates decreases exponentially with depth
(e.g. Martin et al., 1993). However, vertical migration
can be assumed as very efficient transportation of organic
matter, because there is no bacterial attack or predation
by detrivores. More importantly, ontogenetic migrators
might escape from the sediment traps, and even when they
are captured, researchers have been eliminating these organisms as swimmers. Kobari et al. (2003a) estimated
the contribution of Neocalanus spp. to the export flux in
the Oyashio area from the life cycle analysis as 6.5 g C
m–2y–1 at 1000 m. The estimated organic carbon flux by
sediment traps in the western North Pacific was 2.3 g C
m–2y–1 (calculated from Table 3, Honda et al., 2002).
These values suggest that vital transportation by
Neocalanus copepods is much larger than the detritus flux.
Nutrient and Plankton Dynamics in the NE and NW Gyres of the Subarctic Pacific Ocean
103
Table 4. Long-term averages of wet weight of surface mesozooplankton (0–150 m) during summer, and estimated export flux by
defecation and ontogenetic migration of Neocalanus copepods in the subarctic Pacific (40–60°N). Biomass data are modified
from Sugimoto and Tadokoro (1997). The export flux was estimated from the export flux in the Oyashio region (Kobari et al.,
2003a) and the relative biomass of each area in the Oyashio region.
Eastern Pacific
Western Pacific
Bering Sea
Sea of Okhotsk
Area of observations
(10 6 km2 )
Averaged wet weight
(g m– 2 )
Total export by Neocalanus
(10 1 3 g Cy – 1 )
6.90
3.45
1.32
1.05
34.6
50.6
67.7
66.8
4.43
3.23
1.66
1.31
Kobari et al. (2003a) also estimated the vital export by
Neocalanus copepods using the simple average of the wet
weight from the North Pacific (Sugimoto and Tadokoro,
1997) and the export flux from the Oyashio area. We calculated the export flux in each 1° × 1° grid from the wet
weight distribution from the modified version of Sugimoto
and Tadokoro (1997), and integrated it for the subarctic
Pacific (Fig. 7; Table 4). The estimated flux was 1.06 ×
10 14 g Cy –1 and is comparable to the value of 1.7 × 1014 g
Cy –1 for Neocalanus tonsus in the Southern Ocean (Bradford-Grieve et al., 2001). These facts suggest that ontogenetic migrators in the subpolar and polar region export a
globally significant amount of carbon to the deep ocean.
Moreover, the contributions of the marginal seas (the Sea
of Okhotsk and the Bering Sea) are significant in spite of
their relatively small areas (Table 4).
6. Rate Processes and Controlling Factors
6.1 Nutrient uptake rates and nutrient ratios
At Stn P, phytoplankton use NH4 preferentially (55%)
over urea (24%) and nitrate (21%) for growth without
any apparent seasonal trend in this preference (Fig. 8;
Varela and Harrison, 1999a). The seasonally averaged,
depth-integrated f-ratio was 0.21 and if urea is excluded
from the calculation (as is the typical case), the f-ratio
was 0.36. The first measurements of size-fractionated N
substrates were conducted by Varela (1997).
Phytoplankton <2 µm consumed two-thirds of the regenerated nitrogen (NH 4 and urea). Nitrate uptake represents
only 20% of the total N taken up by these small cells and
laboratory experiments suggest that nitrate uptake may
be inhibited by ambient NH4 concentrations, even though
NH 4 is low (0.17 to 0.54 µ M) (Varela and Harrison,
1999b). For the large phytoplankton, iron limitation reduces the utilization of nitrate since iron is a component
of nitrate and nitrite reductases (Fig. 8; Milligan and
Harrison, 2000). Similar size-fractionated nitrogen uptake rates have not been conducted for the WSG.
Iron limitation of phytoplankton growth has been
repeatedly confirmed in both gyres during the last few
104
P. J. Harrison et al.
years. At Stn P, on-deck carboy experiments have clearly
shown that large (>18 µm) mainly pennate diatoms (e.g.
Pseudonitzschia sp.) grow when 2 nM Fe is added (Boyd
et al., 1996). Iron stress has also been confirmed by biophysical indicators (Fv/Fm fluorescence ratio, Boyd et
al., 1998a; Suzuki et al., 2002a) and biochemical markers such as the production of flavodoxin (LaRoche et al.,
1996). However, during winter, iron and light may be colimiting (Maldonado et al., 1999). It is still unclear
whether small cells are iron-limited. Boyd et al. (1996)
saw no increase in the small size fraction, but they may
have been consumed by microzooplankton present in the
carboy experiments. In the IronEx II study, small cells
were observed to be moderately Fe-limited (CavenderBares et al., 1999). Further study is needed to resolve
whether small phytoplankton are Fe-limited at Stn P. In
the WSG, iron fertilization increased gross growth rates
of ultraphytoplankton (<5 µm) demonstrating that Fe limited the growth of ultraphytoplankton (Saito et al., 2003).
Iron limitation affects many cellular processes including chlorophyll synthesis, nitrate and silicate utilization, growth rates, and sinking rates. Recently, there has
been considerable interest in ambient nutrient ratios and
nutrient utilization ratios, particularly Si:N ratios (Takeda,
1998). Under iron limitation, silicate utilization increases
or NO3 utilization decreases and therefore the Si:N ratio
increases to 2 or 3, compared to the normal ratio of about
1:1 Si:N (Brzezinski, 1985). In the NE subarctic Pacific,
Whitney et al. (2003) showed that Si limitation (rather
than nitrate) may co-occur with Fe limitation, or Si limitation may be induced with increased Fe inputs. This suggestion of increased Fe inputs enhancing Si drawdown
may explain three years in the 1970s at Stn P when Si
concentrations declined to limiting concentrations (Wong
and Matear, 1999).
The drawdown ratio of Si:NO 3 observed at Stn P is
~1.5 and therefore supply ratios of <1.5 Si:NO3 may lead
to Si limitation (Whitney and Freeland, 1999). The Fe:C
ratio is ~2.4 µM/M for larger (diatom) cells (Maldonado
and Price, 1999) and the Si:C ratio is ~0.15 (Brzezinski,
1985). These ratios yield a Fe:Si ratio for diatoms of ~5
50
Si (uM)
40
30
Si limited
20
HNLC
Nitrate limited
10
Coastal
0
0
5
10
15
Nitrate (uM)
20
25
30
54
B rit is h
Colu m bia
OS P
52
50
µM/M (0.005 nM/µM Si). These ratios can be used to
evaluate which nutrient can become limiting in various
regions of the subarctic N Pacific. Data from the AG area
has been used to define three regions based on surface
nutrients in the summer (Whitney et al., 1998). The three
regions are: a coastal region that is Fe rich, NO3-depleted
because of high Si input from rivers (e.g. Stns P4, Z19,
and a Haida eddy); a transitional region which may become either Si or N limited depending on the supply ratios from below; and a HNLC region where Fe:Si ratios
are low (e.g. Stns P20, P, Z4, Z9, and Z14) (Table 1; Fig.
9).
There is a tendency for HNLC waters in the AG to
become Si-limited under certain conditions (Whitney et
al., 2003). During a summer survey in 2002, they found
strong vertical stratification and an unusually thin mixed
layer that increased the mixed layer light levels and may
have reduced vertical diffusion. When the Fe supply is
increased or Fe requirements of phytoplankton are reduced due to higher incident light levels (Maldonado et
al., 1999) then diatom growth may be favored and the
silicate pump (Dugdale et al., 1995) preferentially removes Si (compared with N which is recycled more
quickly), leading to Si depletion.
At Stn P, Fe levels (vs. Si) have been observed to
change over periods of a few months (Fig. 10). Nutrient
supply and utilization varied strongly during the 1998–
99 El Niño/La Niña cycle (Whitney and Welch, 2002). In
1998, Si levels were exceptionally low along much of Line
P in summer, whereas there was little evidence of Si uptake during 1999 (Fig. 10). During this strong oscillation, Fe levels at Stn P varied significantly in the upper
1000 m (Nishioka et al., 2001). In June 1999, Fe concentrations did not regress linearly with Si from 400 to 1000
m suggesting a fresh input of Fe into deep waters. Since
there is no evidence of this input near the surface, the
deep water signal may be showing stronger recirculation
of coastal waters from Alaska. In June 1998, the linear
Latitude (N)
Fig. 8. Uptake of three nitrogen sources (NO 3, NH 4, urea) by
two different size fractions of phytoplankton. Average rates
for all stations along Line P and all seasons during 1992–
94 (from Varela, 1997; Varela and Harrison, 1999a).
Line P
48
146
144
142
140
138
136
134
132
130
128
126
124
122
Longitude (W)
Fig. 9. Silicic acid versus nitrate showing that coastal waters
tend to become N-limited while HNLC waters tend towards
Si limitation. Lower figure shows coastal waters and HNLC
waters in relation to N and Si-limited regions on a transect
from the coast to Stn P (OSP) in September 2002.
regression intersects the x-axis suggesting that Fe was
limiting phytoplankton, not Si, at this time (Fig. 10).
Current evidence suggests that either dissolved iron or
silicate can be the limiting nutrient for phytoplankton at
Stn P.
Iron is probably the limiting micronutrient in most
parts of the WSG, even though it receives a larger aeolian
dust flux compared with the AG (Duce and Tindale, 1991;
Nishioka et al., 2003; Tsuda et al., 2003a). Although the
iron dissolution ratio of aeolian dust is unknown, a model
study suggested that iron may be moderately limiting even
in the Oyashio region (Fung et al., 2000). In the WSG,
except for the coastal and the Oyashio regions, the iron
concentration during summer is <0.1 nM in surface waters (Nishioka et al., 2003). Dissolved iron concentrations
in the WSG are similar to the AG, but the particulate iron
concentration (>0.2 µm) is higher in the WSG, although
the bioavailability of particulate iron is still unknown.
The WSG is a HNLC region during the summer probably
due to iron limitation. Nitrate depletion (<1 µM) occurs
only in the region with temperatures >17–20°C and HNLC
waters eastward of 148°E, although the southwestern
position of the HNLC waters is variable. In the eastern
part, iron-limited HNLC waters were also observed at
Station KNOT and the SEEDS site (47–48°N, 163–
165°E). At Stn KNOT, macronutrients were not depleted
throughout the year, even though a small spring increase
in Chl was observed (Imai et al., 2002). Saito et al. (2002)
Nutrient and Plankton Dynamics in the NE and NW Gyres of the Subarctic Pacific Ocean
105
1.4
1.2
Jun-99
Dissolved Fe (nmol/kg)
1.0
0.0062x + 0.12
Feb 99
0.8
0.006x + 0.066
Jul 87
0.6
0.0044x - 0.039
Jun 98
0.4
0.2
0.0
0
20
40
60
80
100
120
140
160
Silicate (umo/L)
Fig. 10. Dissolved iron vs. silicate during various seasons and
El Niño/La Niña years showing maximum and minimum
linear regressions (slope and intercept) in units of µM Fe/
M Si over the 1998/99 period (data from Nishioka et al.,
2001 and Martin et al., 1989).
also reported that part of the Oyashio region changed from
a “blooming region” to a HNLC region during summer.
They suggested that part of the WSG is a HNLC region
in the summer, but with a small spring bloom during May.
Satellite images also reveal that phytoplankton blooms
occur in the coastal waters off the Kuril Islands and the
Kamchatka Peninsula ca. 300–500 km westward of the
WSG (Sasaoka et al., 2002). This distance of the coastal
bloom from the WSG is similar to the nutrient-depleted
region along the coast of Canada and Alaska. On the other
hand, in the central part of the WSG at the SEEDS site,
the spring bloom did not occur due to iron limitation. This
suggests that even in the western part of the WSG closer
to the coast, iron limitation most likely prevents complete consumption of macronutrients. The most important source of iron appears to be coastal waters, not aeolian
dust.
Recently there has been interest in measuring
particulate inorganic carbon (PIC) and particulate organic
carbon (POC) production by an acidification/trapping
protocol (Crawford et al., 2003). PIC production is an
indicator of the degree of calcification taking place by
coccolithophores. They found the PIC:POC ratio ranged
from 0.04 to 0.1, consistent with the lower end of values
reported for shallow sediment traps at OSP (Wong et al.,
1999; Wong and Crawford, 2002). When Crawford et al.
(2003) added Fe to on-deck incubations of Stn P water,
the ratio doubled, indicating that coccolithophores were
stimulated by Fe. Martin et al. (1989) also observed an
increase in the abundance of certain coccolithophores at
some stations in response to a Fe addition. However, Lam
et al. (2001) found no increase in the PIC:POC ratio at
Stn P when Fe was added and coccolithophores were ob106
P. J. Harrison et al.
served to be abundant. Muggli and Harrison (1996) found
that the coccolithophore, Emiliania huxleyi had a relatively low Fe requirement compared with diatoms (Muggli
et al., 1996; Muggli and Harrison, 1997). The addition of
Zn caused a small but significant increase in the total
chlorophyll and the increase was mainly in the small cells
(Crawford et al., 2003). Fukuda et al. (2000) also suggested that zinc may be a secondary factor influencing
phytoplankton growth in the subarctic North Pacific.
6.2 Primary productivity
At Stn P, primary productivity has been measured
over the last 40 years and there has been an increase in
the annual productivity by ~3 times (Fig. 11A). It is difficult to determine whether this is real or a result of adopting the “trace metal clean technique” in the mid 1980s
(Fitzwater et al., 1982), or due to the regime shift that
occurred in 1976–77 (Brodeur and Ware, 1992). Early
measurements between 1960–1976 estimated annual productivity to be ~60 g C m–2y–1. In the mid-1980s to early
1990s, Welschmeyer et al. (1993) and Wong et al. (1995)
reported estimates of 170 and 140 g C m–2y–1 respectively
(Fig. 11A). Between 1992–97, the Canadian JGOFS group
sampled Stn P three times per year and reported estimates
of 215 g C m–2y–1. Their elevated estimate was due to the
two times higher rates in spring and summer. This annual
primary productivity is quite high considering that Stn P
is a HNLC region.
At Stn P, there is low seasonal variation with spring/
summer values about two times higher than winter values (Wong et al., 1995; Boyd and Harrison, 1999). It is
somewhat surprising that there is no clear correlation
between primary productivity and irradiance
(Welschmeyer et al., 1993; Boyd and Harrison, 1999),
although Fe limitation would prevent cells from responding to increased irradiance in spring, summer and fall and
the relatively shallow MLD in winter may render cells
only moderately light-limited. Size fractionated primary
productivity studies indicated that small (<5 µm) cells
dominated the primary productivity and these cells utilized primarily regenerated nutrients. (Boyd and Harrison,
1999; Varela and Harrison, 1999a).
There is a marked contrast between Stn P and Stn
KNOT. In spring at Stn KNOT, the MLD is relatively
shallow (~40 m in May; Fig. 3), the photic zone is ~50
m, chlorophyll is high (40 mg m–2). There is a large nitrate drawdown (high f-ratio) and primary productivity
reaches a seasonal peak of ~500 mg C m–2d–1 (Fig. 11B;
Mochizuki et al., 2002; see their table 4). This peak in
seasonal activity and peak cell abundance indicates a
spring diatom increase (Mochizuki et al., 2002).
Mochizuki et al. (2002) found that many neritic species
were abundant in the spring bloom, and in particular, cells
and resting spores of neritic diatoms such as Chaetoceros
A
Fig. 11. A) Primary productivity at Stn P for the seasonal average during spring, summer, fall and winter (hatched area).
A = annual primary production from McAllister (1972);
B = annual primary production of Wong et al. (1995); C =
annual primary production of Welshmeyer et al. (1993)
based on SUPER cruises, S1 to S6 and L = Lorenzen data
(from Wong et al., 1995). B) Primary productivity at Stn
KNOT for the period 1998–2000. The bars represent the
average for spring (May), summer (June–August), autumn
(September–November) and winter (December–February)
(from Imai et al., 2002).
spp. They speculated that these neritic diatoms were transported from coastal regions via the East Kamchatka Current to Stn KNOT to depth. At Stn KNOT, they suggest
that there is likely to be greater carbon export of these
spring bloom diatoms to depth, compared to Stn P where
very small cells are dominant in spring. This suggestion
is supported by the order of magnitude decline in cell
abundance and diatom cell numbers from spring to summer at Stn KNOT (Mochizuki et al., 2002; see their table
4). In contrast, in spring at Stn P, the MLD is 25 to 50 m
(Whitney and Welch, 2002), the photic zone is ~50–80
m, chlorophyll is low ~25 mg m–2, but primary productivity is surprisingly high (415–850 mg C m–2d–1).
During summer at Stn KNOT, the MLD is very shallow (<20 m; Fig. 3), the photic zone is ~50 m, the chlorophyll is similar to spring (~38 mg m–2), but primary productivity is about half that in spring (220 mg C m–2d–1;
Fig. 11B). This marked decrease in primary productivity
is likely due to Fe limitation since macronutrient concentrations are not limiting. Again, in contrast at Stn P, the
spring MLD is shallow ~40 m (Fig. 2), the photic zone is
variable (20–60 m), chlorophyll is low (~25 mg m–2), but
productivity reaches a seasonal peak of 470–850 mg C
m–2d–1. The reason for the higher productivity at Stn P is
not clear, but it may be due to the somewhat deeper MLD
than at Stn KNOT that is still within the photic zone (Imai
et al., 2002).
In winter at Stn KNOT, the MLD is ~70–100 m and
the photic zone is ~60 m (Figs. 3 and 6), and primary
productivity is 10-fold lower than in spring (Fig. 11B).
There is a significant correlation between irradiance and
primary productivity (Imai et al., 2002; see their figure
6A) and therefore, the cells are clearly light-limited since
they are mixed below the 1% light depth. The very cold
water temperatures (~3°C; Fig. 3) at Stn KNOT give rise
to low photosynthetic rates, in addition to low light
(Shiomoto et al., 1998). At Stn P, the photic zone is also
shallower than the MLD and the cells are not very lightlimited and according to Maldonado et al. (1999) the cells
are probably iron and light co-limited. The water temperatures are twice as high as at Stn KNOT (6 vs. 3°C)
but the higher water temperatures alone can not account
for the six times higher primary productivity at Stn P vs.
KNOT (300 vs. 50 mg C m–2d–1; Imai et al., 2002). Perhaps vertical mixing is also a factor (Odata and Furuya,
1995).
7.
The Biological Pump: Response to Environmental Variability
7.1 Alaska gyre
Harrison et al. (1999) proposed a simple food chain
conceptual model for Stn P with two nitrogen sources and
two size fractions of phytoplankton and zooplankton (Fig.
12). A similar arrangement has been used in a model by
Peña (2003). The conceptual model shows that small
phytoplankton (<5 µ m) are composed of mainly
prasinophytes and prymnesiophytes which grow at 0.2 to
0.9 d–1 in spring and summer. Their nitrogen requirements
are met by regenerated nutrients, and nitrate only accounts
for 25% of the total N uptake and hence the f-ratio is low
~0.25 (Varela and Harrison, 1999a). The use of regenerated nutrients by small phytoplankton reduces their iron
requirements somewhat. Coccolithophores such as
Emiliania huxleyi may not be iron-limited (Muggli and
Harrison, 1996), and other small phytoplankton in this
size fraction are not limited (Boyd and Harrison, 1999)
Nutrient and Plankton Dynamics in the NE and NW Gyres of the Subarctic Pacific Ocean
107
Fig. 12. Simple food chain at Stn P, showing bottom up control
of large phytoplankton by episodic Fe input and subsequent
utilization of NO3 and Si (OH)4 and top down control of
small phytoplankton by microzooplankton grazing which
produce recycled NH 4 and urea that are used by the small
phytoplankton (modified from Harrison et al., 1999).
or slightly Fe-limited (Crawford et al., 2003). The biomass
of the small phytoplankton is kept relatively constant by
a variety of micrograzers (protozooans and flagellates 20–
200 um) whose growth rate is similar to the small
phytoplankton (Landry et al., 1993; Strom et al., 1993;
Boyd and Harrison, 1999).
The microzooplankton obtained 58% of their ingestion from bacteria probably because bacteria were more
abundant than picoplankton such as Synechococcus, rather
than autotrophic picoplankton (Rivkin et al., 1999). The
biomass of the microzooplankton shows little seasonality
(Strom et al., 2000). Therefore the nearly 3-fold increase
in primary productivity from winter to summer is passed
through the microzooplankton to provide part of the large
5 to 10-fold increase in biomass of the rapidly growing
copepodites in the mesozooplankton in May/June (Landry
et al., 1993; Strom et al., 2000). The mesozooplankton
(>200 µm) are composed mainly of large copepods such
as Neocalanus sp. and they are opportunistic filter-feeding omnivores rather than herbivores in HNLC regions
where chl is low (Dagg, 1993a; Goldblatt et al., 1999).
This food chain of small phytoplankton growing on regenerated N and being consumed and controlled by
microzooplankton, which in turn are consumed by
mesozooplankton, is the usual food chain for Stn P. However, when there is an increase in the iron concentration,
large phytoplankton (>10 µm), such as various pennates
(e.g. Pseudo-nitzschia spp. and Thalassiothrix spp.) and
centrics such as (Proboscia formerly Rhizosolenia), and
Chaetoceros sp. grow up. These large diatoms consume
nitrate and silicate until nutrient resources limit their
growth. Thus the ecosystem can quickly change from top
down control by microzooplankton grazers to bottom up
control by an episodic increase in iron concentration. If
108
P. J. Harrison et al.
the increase in Fe concentration is small, then Fe will be
limiting and as demonstrated in laboratory experiments
(Muggli and Harrison, 1996). Fe limitation increases the
sinking rates of these large diatoms (Muggli et al., 1996).
In terms of the biological pump, episodic increases in Fe,
turns on the biological pump and the export of carbon
probably increases due to the sinking of the large diatoms.
It is possible to test whether the ecosystem functions
according to the conceptual model in Fig. 12 by using
large manipulations or by observing natural perturbations/
variability. At Stn P, a mesoscale Fe enrichment was conducted in July 2002 as part of the Canadian and Japanese
SOLAS (Surface Ocean Lower Atmosphere Study) programs. Generally, the ecosystem response followed the
model prediction when iron was added (Boyd et al., 2003).
Large pennates and centric diatoms grew up and a small
percentage of this biomass sank to depth. In this experiment, two infusions of Fe resulted in the diatom bloom
becoming Fe and silicate-limited as has been observed in
the equatorial Pacific (Dugdale and Wilkerson, 1998) and
the Southern Ocean (Nelson et al., 2002). This was initially surprising because based on the initial silicate (12–
14 µM) and nitrate (8–10 µM) and a 1.5 Si:N ratio, one
would expect that N should have become limiting based
on the 1:1 Si:N ratio for most diatoms (Brzezinski, 1985).
However, the fact that N is recycled more rapidly than
silicate, and a change in phytoplankton Si:N utilization
ratios under Fe stress would explain why silicate becomes
limiting before nitrogen (Whitney et al., 2003).
This response of the ecosystem to the mesoscale addition of Fe is similar to other observations where it is
speculated that Fe was added by natural events. For example, in the 1970s, there were three years when silicate
was drawn down to limiting concentrations (Wong and
Matear, 1999). We now suspect that iron concentrations
were elevated due to either recirculation of waters from
the Alaskan coast or transport of coastal waters by a Haida
eddy carrying Fe-rich water. Whitney et al. (2003) have
found that eddies tend to become Si-limited since they
initially have high Fe:Si ratios (Table 1). The dominant
process appears to be the more efficient recycling of N
compared to Si whenever Fe concentrations are elevated
(e.g. eddies, dust inputs, deeper winter mixing, and
recirculation from the Alaskan coast). Therefore in the
AG and eastern subarctic Pacific, in general, carbon export may be enhanced during times of Si uptake that is
stimulated by a natural addition of Fe from various
sources. Therefore, Si limitation, rather than N limitation, accompanies inputs of Fe in the AG.
Mean monthly particle flux at 3800 m reached a minimum in Feb. (38 mg m–2d–1) and a maximum in summer
(150 mg m–2d–1). The average particle flux at 1000 and
3800 m is 52 and 32 g m–2y–1 which is about 20% of the
surface primary productivity (Wong et al., 1999). The
largest flux occurred in 1982/83 during the large El Niño
event and 90% of the flux was opal (Wong and Matear,
1999). Assuming the mean primary productivity is 589
mg C m–2d–1 and C export is 18.2 mg C m–2d–1 during the
summer, the e-ratio (the ratio of C export flux to primary
productivity) is 0.03, which is in the same range as other
HNLC areas (Karl et al., 2003).
There is now considerable evidence that a regime
shift occurred in 1976 in the eastern subarctic Pacific
(Brodeur and Ware, 1992). In addition to the two times
increase in zooplankton biomass, Freeland et al. (1997)
have suggested that the winter mixed layer depth has decreased from 125 m (1956–1975 avg.) to 112 m (1976–
1991 avg.) due to warming and freshening of the surface
layer. If the main source of iron is dust, the dust input
would occur in a smaller volume (shallower MLD) and
this could increase the iron concentration. However, a
shallower mixed layer could result in less macronutrients
and Fe being mixed to the surface. This could increase
the degree of Fe limitation, assuming that some of the
iron required for summer phytoplankton growth does
come from vertical mixing, and nitrate and silicate winter values should be lower. In fact, Whitney and Freeland
(1999) observed that nitrate and silicate winter surface
concentrations decreased by 2.5 and 3.6 µM respectively
and the Si:N utilization ratio from Feb to Sept decreased
from 1.08 to 0.92 due mainly to a decrease in silicate
uptake. This suggests that there may be fewer diatoms
now than before 1976.
During strong El Niño events, subtropical waters
move northwards to the southern edge of the gyre. Measurements along Line P show that this enhanced northward
transport of warm salty waters decreases winter mixing
and consequently decreases the supply of nutrients to the
photic zone (Whitney et al., 1998; Whitney and Freeland,
1999). In addition, there is proportionally less Si in the
subtropical waters that moved northwards in 1998 and
consequently the typical zone of low nitrate water that
extends out from the coast, increases in width and is accompanied by low silicate (Whitney and Welch, 2002).
Therefore, the effects of stratification on nutrient concentrations were especially obvious during the 1997–98
El Niño when surface water temperatures were warmer.
The El Niño also affected the mesozooplankton community structure and there was an increase in the abundance
of mid-California neritic and oceanic species (Mackas and
Galbraith, 2002). The particulate inorganic carbon nearly
doubled during El Niño years (Wong et al., 1998), while
there was only a small increase in particulate organic carbon (Wong and Crawford, 2002). Therefore, the carbonate part of the biological pump, and the coccolithophorid
growth are enhanced during El Nino in the eastern
subarctic Pacific.
7.2 Western subarctic gyre
Does the WSG follow the conceptual model proposed
by Harrison et al. (1999)? Generally the ecosystem in the
WSG does follow the model, although the time series at
Stn KNOT ran for a relatively short period (about 2 years)
compared to Stn P. Therefore internannual variability is
not well characterized. The food chain consisting of small
phytoplankton being grazed by microzooplankton appears
to be the dominant food chain especially in summer when
productivity is low (Liu et al., 2002a). In spring, the availability of more Fe at Stn KNOT than at Stn P, due to dust
input, coastal water transport offshore and winter mixing, gives an increase in Chl (>1 µg L–1) and productivity (525 mg C m–2d–1) (Imai et al., 2002). This small spring
increase in diatoms indicates that the biological pump has
been partially activated by higher Fe concentrations and
this is likely to be a period of higher carbon flux with
little nitrogen recycling, but data are required to confirm
this suggestion. In contrast, Stn P must rely on random
episodic inputs of Fe and hence there is a lack of
seasonality that is obvious at Stn KNOT with its higher
spring biomass and productivity, followed by a 50 to 75%
reduction in productivity in summer and fall (Imai et al.,
2002). However, CZCS satellite data indicates a late autumn increase in Chl in the WSG (Banse and English,
1999). Therefore Stn KNOT is somewhat less controlled
by iron, especially in spring, than Stn P, and there is a
seasonal succession of phytoplankton, from spring diatoms to summer picoplankton (Liu et al., 2002b, 2004)
that is not evident at Stn P.
On-deck iron enrichment studies have been conducted in the WSG. In October 1993, 1 nM Fe was added
to surface samples, the initial nitrate concentration of 10
µM was depleted in 5 days and a large increase in Chl
occurred (from 1.3 to 5 µg chl L–1) with all size classes
of phytoplankton increasing (Takeda, 1998). The large
size class was dominated by the centric diatom
Thalassiosira sp. and not by pennates and centric diatoms as found at Stn P.
In July 2001 a mesoscale iron enrichment was conducted near the centre of the WSG, northeast of Stn KNOT
and the increase in Chl was several times larger than any
other mesoscale iron enrichment experiment (Tsuda et al.,
2003a). A 3 nM Fe addition resulted in a shift in
phytoplankton species from pennates such as Pseudonitzschia turgidula to a centric diatom, Chaetoceros
debilis that had a very high growth rate of up to 2.6
doublings/day, despite the 9.5°C water temperature. This
is in contrast to Stn P (Boyd and Harrison, 1999; Boyd et
al., 2003), the equatorial Pacific (Coale et al., 1996) and
the Southern Ocean (Boyd et al., 2000), where iron additions stimulated a mixture of centric and pennates (Boyd
et al., 2003). Therefore, the starting phytoplankton species composition strongly influences the ecosystem’s re-
Nutrient and Plankton Dynamics in the NE and NW Gyres of the Subarctic Pacific Ocean
109
sponse (phytoplankton growth, aggregation of cells and
carbon export) to an increase in iron supply. The initial
concentration of macronutrients was 34 and 18 µM silicate and nitrate respectively (1.9 Si:N ratio) and by day
13, silicate and nitrate were both 4 µ M (2.1 Si:N
drawdown ratio). It is likely that Si would have been depleted before NO 3 at the end of the drawdown. These results are similar to Stn P where Si is drawn down much
faster than N.
The results from a ship-of-opportunity monitoring
program across the North Pacific have provided further
evidence of the differences between the eastern and western subarctic Pacific (Wong et al., 2002b). In the east,
elevated DIC:NO3 ratios and low silicate:nitrate ratios
were due to high CaCO 3 production and low production
of diatoms. The percent of CaCO3 production to organic
carbon production was up to 70–75% in the eastern
subarctic Pacific. In contrast, in the west, low DIC:NO3
ratios and high silicate:nitrate ratios suggested low calcification and high diatom production. This is consistent
with higher diatom abundance in the photic zone
(Obayashi et al., 2001) and higher opal fluxes in the water column (Noriki et al., 1999; Honda et al., 2002). Annual production in the AG estimated from seasonal
drawdown of nitrate at the surface was 76–83 g C m –2y–1
(Wong et al., 2002a), similar to estimates using carbon
isotopes (Boyd and Harrison, 1999) which yields a high
f-ratio and suggests an ecosystem less dependent on recycled nutrients. In the WSG, Tsurushima et al. (2002)
also measured net community production in the surface
mixed layer in summer estimated from the change in nutrients, and found that it ranged from 250–600 mg C
m –2 d–1 , again in good agreement with carbon isotope
measurements (Imai et al., 2002).
8. Future Directions
Duce and Tindale (1991) and others have shown a
longitudinal dust gradient across the North Pacific with
the western Pacific receiving an order of magnitude more
dust than the eastern side, due to the closer proximity to
the main dust source, the Gobi Desert. Since dust is a
source of iron (although the degree of the bioavailability
of the Fe is presently not well known), this has lead to
the longitudinal iron gradient hypothesis that may explain
some of the differences that have been observed between
the eastern and western gyres. A number of observations
appear to support the iron gradient hypothesis.
If part of the iron that is associated with dust is biologically available, then one can assume that the main
dust input that occurs in spring is like a natural iron fertilization event. During May there is a peak in chlorophyll
and primary productivity at Stn KNOT. This peak in chlorophyll (1–2 µg chl L –1) is dominated by centric diatoms
that generally are known to have high iron requirements.
110
P. J. Harrison et al.
In contrast, in the AG, where the dust/iron input is an
order of magnitude less than the WSG, there is little or
no seasonality in chlorophyll and the two times increase
in primary productivity in summer appears to be due to
an increase in light availability. The phytoplankton
biomass is dominated by small prasinophytes and
prymnesiophytes, some of which are known to have low
iron requirements. Diatoms (mainly pennates) only become dominant when Fe is added, and this was confirmed
in the recent mesoscale iron enrichment experiment at Stn
P.
Is the increase in diatoms in May at Stn KNOT due
to dust or other factors such as light, temperature and the
MLD, or the input of iron-rich coastal water? In order to
resolve this question, a cruise should be conducted to measure the response of the WSG ecosystem and many other
biogeochemical parameters during the peak input of dust/
iron in spring. Capitalizing on this natural fertilization
event (spring dust event) may be more informative in
understanding the natural biogeochemistry of iron input
from dust than conducting mesoscale iron enrichment
experiments, since it would be a direct test of the iron/
dust hypothesis.
9. Conclusions
During the last few years, mesoscale iron enrichment
experiments were conducted in each gyre and the Stn
KNOT time series was established in the WSG. Results
from these studies have brought several important advances in our understanding of the ecosystems in the WSG
and the AG. However, it is important to note that both
Stn KNOT and Stn P are located near the edge of the WSG
and AG respectively and at present we assume that they
represent conditions in the main part of each gyre. We
require more data on the temporal and spatial variability
within the main part of each of these large gyres if we are
to make valued comparisons between these two gyres.
While this paper compares the WSG and the AG, the real
comparison is focused on two time series stations at the
edge of two very large gyres.
1) Silicate plays an important regulatory role of
phytoplankton growth along with iron in the AG. If a large
episodic input of Fe occurs at Stn P, the pennate diatoms
that grow up are likely to become limited by silicate, rather
than nitrogen. This is likely due to factors such as the
more rapid recycling of nitrogen compared to silicon in
the photic zone. At present, it is unclear if Si plays a similar role in the WGS, although results from the 13 day
mesoscale Fe enrichment at the SEEDS site indicate that
Si may have become limiting, if the experiment was followed longer.
2) Seasonal nutrient drawdown ratios revealed further differences between these two gyres. In the AG, elevated DIC:NO3 ratios and low silicate:nitrate ratios were
likely associated with high CaCO3 production and low
diatom production in surface waters. In contrast, the WSG
exhibits low DIC:NO3 ratios and higher silicate to nitrate
ratios, probably due to low calcification and high diatom
production. These observations are consistent with the
greater percentage of diatoms in the phytoplankton assemblage and higher opal fluxes in the water column.
3) Despite the general similarities (e.g. both are
HNLC regions) in the WSG and the AG, there are many
important differences. The WSG has higher nutrient concentrations (e.g. nitrate ranges from 10–23 µM and silicate ranges from 10–40 µM in the WSG vs. 6–17 µM for
nitrate and 8–22 µM for silicate in the AG). Iron concentrations are several times higher in the subsurface waters
of the WSG than the AG, probably due to the closer proximity to the source of dust from the Gobi Desert and the
transport of Fe-rich coastal water offshore by the Oyashio.
The photic zone in the WSG showed little seasonal variation (ranging from 45 to 60 m), while the photic zone at
Stn P is shallower in summer (30–50 m) and deeper in
winter (60–80 m). The mixed layer depth (MLD) is <40
m in summer and 90–120 m in winter at Stn P, while Stn
KNOT has a shallower MLD in summer (often <10 m)
and also shallower in winter (80–100 m). While chl concentrations are low in both gyres, chl in the WSG is about
two times higher than Stn P (0.5–1.5 vs. 0.3–0.5 µg L–1).
Most of the chl is in the small size fraction (<5 µm) and
is dominated by prymnesiophytes and prasinophytes in
the AG. Diatoms normally make up only a very small fraction of the chl (<10%) in AG, but they make up a larger
fraction in the WSG, especially in May when a small diatom bloom may occur (>1 µg chl L–1). Despite the lower
chl at Stn P, the annual primary productivity is about two
times higher (90 vs. 140 to 215 g C m–2y–1), however,
nitrate drawdown in surface waters is considerably higher
in the WSG (avg. for 1995–2000 being 12.7 µM in AG,
compared to 16.7 µM in the WSG) and assuming nutrient
uptake over similar depth ranges, this leads to a higher
new production rate and f-ratio in the WSG.
4) Previously, dust was considered to be the most
important source of iron input into these gyres. Recently,
attention has been drawn to horizontal transport of ironrich coastal water offshore. Neritic diatom species are frequently observed at Stn KNOT, suggesting offshore transport by the Oyashio. Coastal water could be transported
to Stn P by eddies or by the cyclonic circulation of the
AG. In each case, the transit time is nearly two years,
compared to a few months in the case of the Oyashio transporting water to Stn KNOT.
5) Mesoscale iron enrichment experiments demonstrated clear iron limitation in both gyres, however centric diatoms (Thalassiosira in an Oct experiment and
Chaetoceros debilis in a July in situ iron addition) became dominant in the WSG, compared to pennate dia-
toms (mainly Pseudo-nitzschia spp.) and some centrics
becoming dominant in the AG. A mesoscale iron enrichment at Stn P revealed that these waters became Si-limted
when Fe was added. Therefore silicate plays an important role when elevated Fe concentrations occur at Stn P
due to natural transport processes associated with
mesoscale eddies, recirculation from the Alaskan coast,
dust storms, or forest fire smoke.
6) Mesozooplankton communities in both gyres are
dominated by the same species, ontogenetic migrating
copepods. These copepods mainly feed on
microzooplankton in the open ocean gyres, but they will
feed on diatoms when they are abundant. The life cycles
of Neocalanus spp. are consistent in both gyres, although
N. flemingeri showed a biennual life cycle in marginal
seas (the Sea of Okhotsk and Japan Sea). In contrast, twomonths earlier upward migration, spawning, and shorter
life span by Eucalanus bungii has been observed in the
edge of the WSG. The biomass of mesozooplankton is
higher in the ocean margin, and somewhat higher in WSG
than AG. The body sizes of dominant copepods are also
larger in WSG than the AG. These differences of
mesozooplankton characteristics are considered to be due
to the greater availability of diatoms in WSG.
7) While many parameters in the AG are more constant than the WSG, which shows clear seasonality, El
Niño/La Niña cycles provide marked interannual variations. One may view these natural variations as “natural
experiments” that provide information on the response
of the ecosystem. For example, in the AG, calcification
and new production, increase during the El Nino years
but at present we do not know what mechanisms are responsible for these observations.
Acknowledgements
A. Bychkov and C. S. Wong made the 1992 survey
of the WSG possible and they kindly provided the data.
J. Putland, K. Denman and A. Pena kindly made
phytoplankton results from Station P available. Thanks
also to K. Johnson for providing iron data which is not
yet published.
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