Journal of Oceanography
Vol. 50, pp. 515 to 529. 1994
Kinetics of Nitrate and Ammonium Uptake by the Natural
Populations of Marine Phytoplankton in the Surface Water
of the Oyashio Region during Spring and Summer
AKIHIRO SHIOMOTO1, KATSUYUKI SASAKI2, TORU SHIMODA2 and SATSUKI MATSUMURA1
1National
Research Institute of Far Seas Fisheries, 7-1, Orido 5 chome, Shimizu-shi, Shizuoka 424, Japan
2National Research Institute of Fisheries Science,
Fukuura 2 chome, Kanazawa-ku, Yokohama-shi, Kanagawa 236, Japan
(Received 12 February 1993; in revised form 21 April 1994; accepted 11 May 1994)
The maximum uptake rate (ρmax) and affinity constant (KS) for nitrate and ammonium were estimated in the surface water of offshore Oyashio in May (spring)
and September (summer), 1990. The average ρmax/Chl. a for ammonium was 2.1
times larger than that of nitrate in both seasons. The average ρmax/Chl. a for both
nitrogens were 3.5 times larger in summer than in spring. Water temperature and
size composition of phytoplankton population were related to the seasonal difference
in the ρmax/Chl. a. Phytoplankton population showed high affinity for both nitrogens
in the spring and summer. In addition, the contribution of “new” production to
total production was estimated by ρmax [ρmax – NO3/(ρmax – NO3 + ρmax – NH4 )]. The
spring value was in the range of 0.26 to 0.45 (mean ± SD = 0.35 ± 0.092), and the
values in spring bloom were especially a little over 0.4. The summer value was in
the range of 0.30 to 0.37 (0.34 ± 0.04).
1. Introduction
Nitrogenous nutrient is one of the most important environmental factors that regulate
primary production. Numerous experiments of nitrogen uptake by the phytoplankton population
have been carried out in many areas during various seasons in the sea (Dugdale and Goering,
1967; McCarthy et al., 1977; Olson, 1980; Garside, 1981; Glibert et al., 1982; Glibert, 1982;
Harrison et al., 1982; Kanda et al., 1985a). Nitrogen uptake generally can be expressed by the
Michaelis-Menten equation (MacIsaac and Dugdale, 1969). Parameters known as the maximum
uptake rate (ρmax) and the half-saturation constant (KS) are calculated according to the equations.
The parameter ρmax reflects the uptake capacity of phytoplankton, while KS is a measure of the
affinity for the given substrate (nitrate, ammonium etc.). Therefore, the determination of these
parameters contributes to clarification of the effects of nutrients on primary production.
The Oyashio region is located in the northwestern Pacific Ocean. Some measurements of
primary production have been conducted in the offshore and neritic waters (Saijo and Ichimura,
1960; Aruga and Monsi, 1962; Aruga et al., 1968; Takahashi and Ichimura, 1972; Taniguchi and
Kawamura, 1972; Taguchi et al., 1977; Maita and Yanada, 1978; Nishihama and Kawamada,
1979; Maita and Odate, 1988). The area is one of the highest primary productivity regions in the
world’s oceans (Koblentz-Mishke et al., 1970). Primary production, furthermore, is divided into
two parts, “new” production and “regenerated” production (Dugdale and Goering, 1967). It is
essential to estimate the contribution of “new” production to total production for determining the
flow of materials to higher trophic levels. The f-ratio is a useful indicator for the contribution of
“new” production (Eppley and Peterson, 1979). This ratio is calculated by in situ rates of nitrate
516
A. Shiomoto et al.
and ammonium uptake obtained by an uptake experiment using 15N labeled nitrogens.
In the Oyashio region, however, only a few nitrogen uptake rates have been measured
(Kanda et al., 1988). This region is generally eutrophic, but nutrients are depleted at the surface
during the summer (after the spring bloom) (Tanaka et al., 1991). We carried out several concentration dependency experiments in the spring and summer seasons, in which the ambient
nutrient conditions are marked differently, and determined the ρmax and KS of nitrate and ammonium uptake. In this paper, we attempted to clarify the characteristics of nitrate and
ammonium uptake in the seasons. In addition, we estimated the contribution of “new” production
to total production using ρmax instead of in situ rates of nitrogen uptake, because the ambient
concentration of nitrogens were so close to or under the detection limit that we could not obtain
in situ rates.
2. Materials and Methods
This study was carried out in the offshore Oyashio during a cruise of the R/V “Soyo Maru
(494.38 t)”, the National Research Institute of Fisheries Science, in May 1990 (spring), and
during a cruise of the R/V “Shunyo Maru (393.44 t)”, the National Research Institute of Far Seas
Fisheries, in September 1990 (summer) (Fig. 1). Samples of surface seawater were collected with
a submersible pump (Ebara Type DVSN). These samples were sieved through a 200 µm mesh
screen to remove large zooplankton and then used in uptake experiments and analyses of their
various chemical components. Seawater collections were accomplished between 8 and 10
o’clock in the morning, and the incubations were started within one hour.
The samples (2-1) were dispensed into 2 liter poly carbonate bottles and enriched by the
addition of either Na15NO3 (99.6 atom% 15N) to a final concentration of 1 to 30 µmol l–1 at Stas.
1, 2 and 4, and 0.25 to 2 µmol l–1 at Sta. 3, or 15NH4Cl (99.8 atom% 15N) to a final concentration
of 0.125 to 2 µmol l–1 for samples collected at all stations in the spring. Samples taken during the
summer were enriched by the addition of either Na15NO3 to a final concentration of 0.5 to 8
Fig. 1.
Nitrogen Uptake in the Oyashio Region
517
µmol l–1, or 15NH4 Cl to a final concentration of 0.125 to 4 µmol l–1 at all stations. Incubations were
conducted under simulated surface conditions, being cooled with near-surface seawater for 2–
3 h. After incubation, the particulate matter was collected on precombusted 47 mm Whatman
GF/F filters (450°C for 4 h) with gentle suction, then rinsed with 3.5% NaCl solution. The filters
were immediately frozen and preserved for later isotope analyses on land. They were completely
dried in a vacuum desiccator. The isotopic ratios 15N to 14N were then determined by a quadrupole
mass-spectrometer (Nichiden Anelva TE360). Particulate organic nitrogen were determined
simultaneously. The nitrogen uptake rate (µmol l–1 h–1, denoted as ρ) was computed according
to the equations described by Dugdale and Wilkerson (1986). The photosynthetic rate was
furthermore simultaneously measured using the 13C method (Hama et al., 1983).
Surface temperature was measured using thermometer and salinity, and was determined
with an Auto-Lab salinometer. Subsurface temperature and salinity were measured with a Neil
Brown Mark III CTD system. Chlorophyll a (Chl. a) was determined by fluorometry after extraction using 90% acetone (Parsons et al., 1984). Water samples for nutrient determinations
were stored frozen and analyzed on land according to Parsons et al. (1984). On-deck photon flux
was monitored every fifty minutes with a Li Cor Model 1000 flat quantum sensor during the
spring and about every three hours during the summer.
3. Results
3.1 Outline of the study area
The physical, chemical and biological environmental parameters of the surface water at each
station are summarized in Table 1. Temperatures were between 5 and 9°C in the spring and 16
and 20°C in the summer; that is, about 10°C higher in the summer than in spring. Salinity at Sta.
7 was about 33.6, while other stations had values of less than 33.4. All stations except Sta. 7 were
typical of Oyashio water, while Sta. 7 was affected by warm water (Ohtani, 1971; Kawai, 1972).
Fine days were frequent; no rainy days occurred during the sampling periods. The mean light
intensity during the incubation periods was in the range of 500 to 1,500 µEin m–2 s–1, except for
Sta. 7 (259 µEin m–2 s–1). Nitrate relatively abundant at Stas. 1, 2 and 4 (5–10 µmol l–1) in the
spring, but was depleted at Sta. 3. Nitrate was less than about 1 µmol l–1 at all stations in the
summer. Ammonium was between 0.2 and 0.7 µmol l–1 in the spring, and less than the detection
limit in the summer. Chl. a concentration varied among the stations, being relatively high at Stas.
2 and 3 in the spring. During the summer, Chl. a values were almost constant among stations, and
nearly equal to the lowest spring value.
Vertical profiles of σt and concentrations of nitrate and ammonium shallower than 100 m are
shown in Fig. 2. The σt increased with depth at all stations. The pycnocline was between 40 and
50 m at Stas. 1 and 3, between 30 and 40 m at Sta. 2 and between 10 and 30 m at Sta. 4, even though
the pycnoclines in the spring were not as sharp as those in the summer. Developed pycnoclines
were found between depths of 10 and 30 m at Stas. 5 and 6, but not at Sta. 7 in summer. The
pycnoclines developed in the depths shallower than the lower limit of the euphotic zone (1% light
level) at Stas. 5 and 6. Nitrate concentration was nearly constant shallower than 30 m and
increased according to depth in the spring. In the summer, it was depleted at the surface, but
increased at depths beyond 10 m, locating around the upper limit of pycnocline in summer.
3.2 Concentration dependency experiments
The results of concentration dependency experiments are shown in Fig. 3. In spring, the
518
A. Shiomoto et al.
Fig. 2.
uptake rates of nitrate at Stas. 1 and 4 and those of ammonium at Stas. 1, 2 and 3 were nearly
constant, while other uptake rates conformed to the Michaelis-Menten equation. During the
summer, nitrate uptake rates at Stas. 5 and 6 were nearly constant, while other uptake rates
conformed to the Michaelis-Menten equation. All uptake rates, except for the nitrate uptake at
Sta. 2, were saturated within the range of the injected substrate concentration.
The parameters ρmax and KS were calculated from the regression line fitted to a plot of S/ρ
versus substrate concentration (S) (e.g. Wright and Hobbie, 1966). Furthermore, ρmax/Chl. a was
calculated to compare the maximum uptake rate of the phytoplankton population at the various
stations. These parameters are summarized in Table 2. ρmax for both nitrate and ammonium have
a tendency to be higher in summer than in spring, but significant differences were not found
(t-test: p < 0.05). In contrast, the ρmax/Chl. a for the two nitrogens were obviously higher in
summer than in spring. The average values of both nitrogens were 3.5 times larger in the former
season than in the latter. In addition, the average ρmax/Chl. a for ammonium were 2.1 times larger
than those for nitrate.
All values of KS except for one (nitrate at Sta. 2) were less than 0.6 µmol l–1, and less than
or close to the ambient substrate concentration (see Table 1). These results reveal that the affinity
of the phytoplankton population for substrates was high in both the spring and summer seasons,
with no significant difference between the two. The nitrate KS at Sta. 2 was much greater than
other values found in this study and those reported in the literature (e.g. MacIsaac and Dugdale,
1969; Hattori, 1982). This Sta. 2 value was found in the unsaturated uptake, even at maximum
substrate concentration of 40 µmol l–1 (Fig. 3). Presently, there is no explanation for this very
6.9
33.386
866
6.6
0.6
2.5
Temperature (°C)
Salinity
Light (µEin m –2 s –1 )*
NO3 – (µmol l –1 )
NH4 + (µmol l –1 )
Chl. a (µg l –1 )
2
5.1
33.075
1581
10.2
0.7
4.4
*Mean value during incubation period.
1
Station
Spring
6.0
32.896
1189
0.1
0.2
6.0
3
8.7
33.337
498
5.9
0.5
1.0
4
16.0
32.688
1205
0.5
0
1.3
5
17.9
32.465
869
1.4
0
1.3
Summer
6
Table 1. Summary of physical, chemical and biological environmental factors of the surface water.
19.6
33.593
259
0
0
1.5
7
Nitrogen Uptake in the Oyashio Region
519
NO3 –
NH4 +
NO3 –
NH4 +
NO3 –
NH4 +
NO3 –
NH4 +
NO3 –
NH4 +
0.00394
0.0110
0.00158
0.00442
–0.010
0.14
4
5
0.998
0.983
0.197
0.0788
1
3
0.0139
0.0205
0.00232
0.00342
0.16
–0.0041
5
5
0.998
0.998
0.241
0.0402
Spring
0.0114
0.0142
0.00220
0.00324
28.5
0.025
4
4
0.985
0.998
0.180
0.0409
2
n: Number of data used in calculating ρmax and KS.
r: Correlation coefficient of regression line obtained by a plot of S/ρ versus substrate (S).
ρC (µmol l –1 h –1 )
ρC /Chl. a (µmol µgChl. a–1 h –1 )
r
n
K S (µmol l –1 )
ρmax /Chl. a (µmol µgChl. a–1 h –1 )
ρmax (µmol l –1 h –1 )
Station
0.00515
0.0130
0.00505
0.0127
0.46
0.60
4
5
0.996
0.990
0.135
0.132
4
0.00870
0.0193
0.00669
0.0148
–0.20
0.023
5
5
0.996
0.998
0.302
0.232
5
6
0.0142
0.0255
0.0114
0.0204
0.27
0.025
4
5
0.999
0.999
0.466
0.358
Summer
Table 2. Comparison of ρmax, ρmax /Chl. a and KS for nitrate and ammonium, ρC and ρC/Chl. a of the surface
water.
0.0163
0.0383
0.0111
0.0261
0.16
0.11
5
5
0.999
0.996
0.303
0.202
7
520
A. Shiomoto et al.
Nitrogen Uptake in the Oyashio Region
521
Fig. 3.
large value, and further observations should be conducted in spring season to resolve the
problem. The negative values probably resulted from the very low KS value for nitrogen uptake
and/or the measurement error caused by freezing of seawater sample. In addition, both ρC
(photosynthetic rate) and ρC/Chl. a were obviously higher in summer than in spring, similar to
the nitrogen uptake (Table 2).
3.3 The contribution of “new” production to total production calculated by ρmax
The f-ratio [ρNO3/(ρNO3 + ρNH4)] has been used as an indicator for the contribution of
“new” production to total production (Eppley and Peterson, 1979). This ratio can be estimated
using the uptake rates of nitrate and ammonium at the ambient concentrations calculated by the
equations for rectangular hyperbolas obtained from the concentration dependency experiments
shown in Fig. 3. However, we could not calculate the in situ rates of nitrate and ammonium uptake
1
0.26
Station
New/Total
0.45
2
Spring
0.40
3
0.28
4
0.31
5
0.36
Summer
6
0.30
7
Table 3. The contribution of “new” production to total production calculated by ρmax at the surface water.
See the text for detail.
522
A. Shiomoto et al.
Nitrogen Uptake in the Oyashio Region
523
in the case that the ambient concentrations of nitrogens were close to or under the detection limit.
Furthermore, the preservation of water samples by freezing resulted in uncertainties of ambient
concentrations. Thereby, in this paper, we estimated the contribution of “new” production to total
production using the ρmax for nitrate and ammonium [ρmax – NO3/(ρmax – NO3 + ρmax – NH4 )]. The
results are listed in Table 3. The value was in the range of 0.26 to 0.45 in the spring. The values
at Stas. 2 and 3 were almost equal and relatively high, while those at Stas. 1 and 4 were almost
equal and relatively low. The mean value at the former stations was 0.43, and that at the latter
stations was 0.27. The former value was 1.5 times larger than the latter value. The mean value
(±SD) was 0.35 ± 0.092 for all spring data. The summer value was in the range of 0.30 to 0.36,
and within the range of the spring values. The difference among stations was smaller in summer
than in spring, and the mean value (±SD) was 0.32 ± 0.032 in the summer. The mean values in
both seasons were not significantly different.
4. Discussion
There is a little information about nitrogen uptake in the Oyashio region. It is valuable to
elucidate the position where the maximum uptake rate (ρmax/Chl. a) for nitrate and ammonium
of the phytoplankton population in this study takes in the maximum uptake rates reported in the
North Pacific Ocean. Therefore, we compared our results with the results in the literature (Table
4). In Auke Bay which is under a eutrophic condition, all the ρmax/Chl. a for nitrate were high
during the spring bloom period, while many low values were observed during the pre- and
postbloom period (Kanda et al., 1989). The values in our spring study were several times smaller
than the lower values during the non-bloom period and one order of magnitude smaller than the
higher values during the non-bloom period and the values during the bloom period in Auke Bay.
The ρmax /Chl. a for nitrate on the coastal side of Kuroshio was about twice as large as that on the
oceanic side (Shiomoto, A., unpubl. data). Auke Bay is a bay in the northern North Pacific Ocean,
and our study field is the offshore region. The ρmax/Chl. a for nitrate may be larger in coastal
region than in offshore region. Furthermore, we encountered a spring bloom at Stas. 2 and 3 in
this study, but the bloom at the stations was not under peak conditions (see below). The ρmax/Chl.
a for nitrate may be very low during the spring season, except for the prime time of bloom in the
offshore Oyashio. The mean values of ρmax /Chl. a for nitrate in our summer study were within
the range of previously reported values in summer under an oligotrophic condition (Kanda et al.,
1985b; Sahlsten, 1987; Shiomoto and Maita, 1990; Shiomoto, A., unpubl. data). However, the
values were located at a relatively low level. Most of the ρmax/Chl. a for ammonium in our spring
study were smaller than the values in spring under a eutrophic condition (Kanda et al., 1988, 1989).
The values in our summer study were smaller than the values in summer under an oligotrophic
condition (Kanda et al., 1985b; Sahlsten, 1987; Shiomoto and Maita, 1990; Shiomoto, A.,
unpubl. data) but nearly equal to the values in summer under a eutrophic condition (Kanda et al.,
1988, 1989). From the comparison of our results with the results in the literature, the ρmax/Chl.
a for both nitrogens in this study were located at a relatively low level in the North Pacific, and
the ρmax/Chl. a for nitrate in our spring study may have been particularly low.
The ρmax/Chl. a for nitrate and ammonium were obviously higher in summer than in spring
(Table 2). High ρmax /Chl. a for both nitrogens are observed at a high temperature (Kanda et al.,
1985a) and small-size phytoplankton is generally associated with a higher uptake rate of nitrogen
than large-size phytoplankton (Rönner et al., 1983; Koike et al., 1986; LeBouteiller, 1986; Probyn,
1990). The temperature was higher in summer than in spring (Table 1) and large-size phytoplankton
is abundant during the spring bloom in the offshore Oyashio whereas small-size phytoplankton
ND
0.00685
–0.0293
0.0239
–0.0397
0.00734
–0.0203
0.00158
–0.00505
(0.00279)b
0.0123
0.0201
0.0236
0.0414
ND
0.0199
Mesotrophic Region (1 < NO 3 – < 3 µmol l –1 )
Summer
North Pacific Ocean
Transitional
0.0319
0.0116
0.0117
0.0166
0.0120
0.00324
–0.0127
(0.00595)b
0.00791 a
ND
0.0254
Summer
Around Hachijo Is. Inside the upwelling
North Pacific Ocean
Subarctic
Subarctic
Offshore Oyashio
postbloom (NO 3 – depletion)
bloom (NO3 – non-depletion)
Eutrophic Region (NO3 – > 3 µmol l –1 )
Spring
Oceanic Oyashio
Auke Bay, Alaska
prebloom (NO3 – non-depletion)
ρmax – NH 4
(µmol µgChl. a–1 h –1 )
ρmax – NO 3
17.9
1.3
15.7
14.3
4.07
0.1
–10.2
15
–26
3
–16
<3 c
NH4 +
0.3
0.2
0.5
0.08
0
–1.4
ND
ND
ND
0.63
(µmol l –1 )
NO3 –
Shiomoto and Maita (1990)
Kanda et al. (1985b)
Shiomoto and Maita (1990)
This study (spring study)
Kanda et al. (1988)
Kanda et al. (1989)
References
Table 4. Maximum uptake rates of nitrate (ρmax – NO3/Chl. a) and ammonium (ρmax – NH4/Chl. a) and
concentrations of nitrate and ammonium at surface waters in the North Pacific Ocean.
524
A. Shiomoto et al.
0.0594
0.00554
–0.0532
0.0419
0.0610
0.0148
–0.0261
(0.0204)b
0.0130
0.00731
0.00669
–0.0114
(0.00973)b
ND
0.00178
–0.00956
0.0831
0.247
0.0496
0.00599
–0.0120
0.0126
0.024
0.0141
0.00339
–0.00658
a: Maybe maximum. b: Mean value. c: Most of values <0.5 µmol l–1.
Winter
Subtropical Central gyre
North Pacific Ocean
Oligotrophic Region (NO3 – < 1 µmol l –1 )
Summer
Around Hachijo Is. Outside the upwelling
Central Pacific Gyre
North Pacific Ocean Subtropical
Off Enshu-nada
Coastal side of Kuroshio
Oceanic side of Kuroshio
Offshore Oyashio
Winter
North Pacific Ocean
(0–40°N)
ND
<1
0
0
0.2
–0.7
0.18
<0.1
0.13
1.31
–3.22
0.06
ND
0
0
0
0.08
<0.1
0.2
ND
Eppley et al. (1973)
Kanda et al. (1985a)
This study (summer study)
Kanda et al. (1985b)
Sahlsten (1987)
Shiomoto and Maita (1990)
Shiomoto (unpubl.)
Kanda et al. (1985a)
Nitrogen Uptake in the Oyashio Region
525
526
A. Shiomoto et al.
is abundant during summer (Ogishima, 1991; Taguchi et al., 1992). Consequently, the seasonal
difference of the ρmax /Chl. a for both nitrogens reflected the seasonal patterns in temperature and
size composition of the phytoplankton population. Further observations should be conducted in
the Oyashio region.
The month of May marks the occurrence of spring bloom in the waters of the offshore
Oyashio (Ogishima, 1991). Values of Chl. a at Stas. 2 and 3 were nearly equal to the value during
the bloom, while nitrate was abundant at Sta. 2 and depleted at Sta. 3 (Table 1). The cell numbers
of diatoms at the surface were 1–2 × 105 cells l–1 at Stas. 2 and 3 (Nakata, pers. comm.). These
values are nearly equal to the values found during the spring bloom in the coastal region (Nakata,
1982; Odate, 1987). On the other hand, Chl. a was not very high and nitrate was abundant at Stas.
1 and 4 (Table 1). The cell numbers of diatoms at the surface at both stations were less than 104
cells l–1 (Nakata, pers. comm.). From these results, it was judged that spring bloom of diatoms
occurred just before the observation at Sta. 2, and was in the latter stage at Sta. 3.
Nitrate uptake was unsaturated even at very high substrate concentration (about 40
µmol l–1) just after the onset of the bloom (Sta. 2) (Fig. 3a). In contrast, nitrate uptake was
saturated at low substrate concentrations (about 1–2 µmol l–1) in the latter stage of the bloom (Sta.
3). These results reveal that the affinity for nitrate of the phytoplankton population increased as
the spring bloom progressed. Cheatoceros community was dominant diatom at the surface at
Stas. 2 and 3, and small species of Cheatoceros (C. radicans etc.) were particularly abundant at
Sta. 3 (Nakata, pers. comm.). The affinity for nitrate of phytoplankton tends to decrease with cell
size (Malone, 1980). The temporal change of the affinity for nitrate appears to be an adaptation
of the phytoplankton (diatoms) to the decrease of nitrate concentration during bloom and/or the
succession of large-size diatoms to small-size ones. The same succession was also observed in
the coastal region (Nakata, 1982; Odate, 1987). The change of nitrate concentration may be
related to the succession of diatoms during spring bloom in the Oyashio region.
Nitrate is a very important source of nitrogen for primary production, because it leads the
net increase of the entire ecosystem, as well as standing crops of phytoplankton (Dugdale and
Goering, 1967). The contribution of nitrate to the primary production frequently exceeds 50%
during bloom (e.g. Olson, 1980; Paasche and Kristiansen, 1982). The contribution was a little
over 40% at Stas. 2 and 3 where spring bloom occurred. This is not inconsistent with past
information.
The f-ratios were a little over 0.4 during spring season in the northeastern Pacific Ocean, but
decreased to 0.1–0.2 during summer (Miller et al., 1988). On the contrary, the contribution of
“new” production to total production calculated by ρmax in the summer season was more than 0.3
(Table 3). f-ratio is generally larger than 0.3 in eutrophic regions and mostly smaller than 0.1 in
oligotrophic regions (Eppley and Peterson, 1979; Olson, 1980; Collos and Slawyk, 1986; Murray
et al., 1989; Shiomoto and Maita, 1990). The summertime values shown in Table 3 fall under the
category of the f-ratio reported in eutrophic region, although surface waters during summer are
under oligotrophic conditions in the Oyashio region (Tanaka et al., 1991; Fig. 2). Relatively high
concentrations of Chl. a were also observed in the summer (Table 1), although such higher values
of Chl. a have not been frequently observed in the season (Saijo and Ichimura, 1960; Aruga and
Monsi, 1962; Ogishima, 1991; Taguchi et al., 1992). This supports the high contribution of
“new” production in the summer. Nitrate existed richly just below the pycnocline (10–30 m), i.e.
at subsurface (Fig. 2). It is expected that an abundant upward input of nitrate occurs by wind stress
as well as by diffusion. Mixing of water was confined to a layer shallower than the euphotic zone
in the summer (Fig. 2). Thereby the nutrients supplied to the surface layers are probably
Nitrogen Uptake in the Oyashio Region
527
efficiently utilized by phytoplankton communities. The characteristics of the environmental
factors, i.e. abundant nutrients at subsurface and shallower pycnocline, probably add much to the
high contribution of “new” production to total production in summertime. Finally, we discussed
“new” production using maximum uptake rate in this paper; so we should fill the gap between
the contribution of “new” production estimated by in situ uptake rate ( f-ratio) and by ρmax in future
studies.
Acknowledgements
We wish to thank the captains and crew of the R/V “Soyo Maru” and “Shunyo Maru” for their
sample collections. We appreciate Dr. T. Saino of the Ocean Research Institute, University of
Tokyo, for kindly permitting the use of the mass spectrometer. We are also grateful to K. Nakata
of National Research Institute of Fisheries Science, for her communication assistance regarding
phytoplankton cell number.
References
Aruga, Y. and M. Monsi (1962): Primary production in the northwestern part of the Pacific off Honshu, Japan. J.
Oceanogr. Soc. Japan, 18, 37–46.
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