Biogeosciences, 17, 2441–2452, 2020
https://doi.org/10.5194/bg-17-2441-2020© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Phytoplankton growth and consumption by microzooplankton
stimulated by turbulent nitrate flux suggest rapid trophic
transfer in the oligotrophic Kuroshio
Toru Kobari1 , Taiga Honma2 , Daisuke Hasegawa3 , Naoki Yoshie4 , Eisuke Tsutsumi5 , Takeshi Matsuno6 ,Takeyoshi Nagai7 , Takeru Kanayama2 , Fukutaro Karu2 , Koji Suzuki8 , Takahiro Tanaka3 , Xinyu Guo4 , Gen Kume1 ,Ayako Nishina1 , and Hirohiko Nakamura1
1 Aquatic Sciences, Faculty of Fisheries, Kagoshima University, 4-50-20 Shimoarata,

Kagoshima, Kagoshima 890-0056, Japan
2 Aquatic Sciences, Graduate School of Fisheries, Kagoshima University, 4-50-20 Shimoarata,
Kagoshima, Kagoshima 890-0056, Japan
3 Tohoku National Fisheries Research Institute, Japan Fisheries Research and Education Agency, 3-27-5 Shinhama-cho,
Shiogama, Miyagi 985-0001, Japan
4 Center for Marine Environmental Studies, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan
5 Atmosphere and Ocean Research Institute, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan
6 Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan
7 Department of Ocean Sciences, Tokyo University of Marine Science and Technology, 4-5-7 Konan Minato-ku,
Tokyo 108-8477, Japan
8 Faculty of Environmental Earth Science, Hokkaido University, North 10 West 5 Kita-ku,
Sapporo, Hokkaido 060-0810, Japan
Correspondence: Toru Kobari (kobari@fish.kagoshima-u.ac.jp)
Received: 20 September 2019 – Discussion started: 7 October 2019
Revised: 17 March 2020 – Accepted: 29 March 2020 – Published: 6 May 2020

Abstract. The Kuroshio Current has been thought to be biologically unproductive because of its oligotrophic conditions and low plankton standing stocks. Even though vulnerable life stages of major foraging fishes risk being entrapped by frontal eddies and meanders and encountering
low food availability, they have life cycle strategies that include growing and recruiting around the Kuroshio Current.
Here we report that phytoplankton growth and consumption
by microzooplankton are stimulated by turbulent nitrate flux
amplified by the Kuroshio Current. Oceanographic observations demonstrate that the Kuroshio Current topographically enhances significant turbulent mixing and nitrate influx
to the euphotic zone. Graduated nutrient enrichment experiments show that growth rates of phytoplankton and microheterotroph communities were stimulated within the range ofthe turbulent nitrate flux. Results of dilution experiments imply significant microzooplankton grazing on phytoplankton.

We propose that these rapid and systematic trophodynamics
enhance biological productivity in the Kuroshio.

1

Introduction

The Kuroshio Current is the western boundary current of
the North Pacific Subtropical Gyre (Qiu, 2001; Hu et al.,2015). The Kuroshio enters the East China Sea from theeast of Taiwan and flows along the continental slope until
it passes through the Tokara Strait into the western North
Pacific (Fig. 1a). The Kuroshio has been thought to be biologically unproductive because ambient nutrient concentrations and plankton standing stocks in its waters are low(Guo, 1991; Hirota, 1995). In spite of such seemingly unproductive conditions, the Kuroshio in the East China Sea(ECS-Kuroshio) is adjacent to major spawning and nursery

Published by Copernicus Publications on behalf of the European Geosciences Union.

2442

T. Kobari et al.: Phytoplankton growth and consumption by microzooplankton

grounds of foraging species such as sardines (Watanabe etal., 1996), jack mackerel (Sassa et al., 2008), chub mackerel(Sassa and Tsukamoto, 2010), and common squid (Bower etal., 1999). Indeed, good fishing grounds have been found forvarious fishes and squid near the Kuroshio, and the catchesfrom those grounds account for more than half of the total catch in Japanese waters (Saito, 2019). It is risky, however, for highly vulnerable early life stages of many foraging species to grow and recruit in the oligotrophic and unproductive waters of the ECS-Kuroshio (hereafter called the“Kuroshio Paradox”; Saito, 2019), even if the warm temperatures of the Kuroshio Current can enhance cellular metabolic
processes and thereby stimulate growth. Conventional wisdom is that survival of these early stages is supported by
the high plankton productivity on the continental shelf and in
the Kuroshio front (Nakata et al., 1995). However, these areas of high productivity are limited in extent and spatiotemporally highly variable because the Kuroshio Current often
meanders (Nakata and Hidaka, 2003). Coastal water masses
are sometimes entrapped and transported into the Kuroshio
and to more pelagic sites (Nakamura et al., 2006; Kobari etal., 2019). Use of waters in the vicinity of the oligotrophic
Kuroshio as a nursery and feeding ground would therefore
appear to be a risky strategy, unless there is a mechanismthat enhances biological production in the Kuroshio.
There is increasing information about the community
structure of phytoplankton and zooplankton in the Kuroshio.
Phytoplankton standing stocks in the Kuroshio consist
mainly of picoplankton and nanoplankton, and the predominant components are haptophytes, diatoms, and unicellular cyanobacteria like Prochlorococcus and Synechococcus(T. Hasegawa et al., 2019; Endo and Suzuki, 2019). Heterotrophic bacteria and calanoid copepods contribute to heterotrophic biomass in the Kuroshio, whereas microzooplankton biomass is relatively small (Kobari et al., 2019). Based ona mass balance model, Kobari et al. (2019) have concluded
that mesozooplankton standing stocks in the Kuroshio aresupported by microautotrophs and nanoautotrophs and microzooplankton. However, we have little understanding of
how biogeochemical processes and trophodynamics support
the plankton community in the Kuroshio.
In recent years, some mechanisms that supply nutrients tothe oligotrophic waters of the Kuroshio have been identified.
The Kuroshio nutrient stream characterized by an intense
core of nutrient flux at subsurface contributes substantially to
productivity in the euphotic zone of the Kuroshio in a manner similar to the contribution of the nutrient stream along the
Gulf Stream (Komatsu and Hiroe, 2019). Turbulence aroundthe Kuroshio appears to be an important mechanism that supplies nutrients via upward movement of deeper waters into
the Kuroshio (Nagai et al., 2019a). Frontal disturbances also
contribute to the supply of nutrients into the euphotic zoneof the Kuroshio (Kuroda, 2019). Moreover, the island masseffect produced by the Kuroshio Current as it flows over thebottom topography of the Japanese archipelago induces an
Biogeosciences, 17, 2441–2452, 2020

upward supply of nutrients (Hasegawa, 2019). These nutrient supplies have been hypothesized to stimulate biologicalproductivity in the Kuroshio. Within the wide path of theKuroshio, the supply of nutrients by these mechanisms canbe particularly efficacious around the Tokara Strait becauseof the extensive frontal disturbances (Nakamura et al., 2006)
and strong turbulence (Tsutsumi et al., 2017; Nagai et al.,2017, 2019b) in that area.
Here we report evidence of phytoplankton productivity and subsequent microzooplankton grazing stimulated byturbulence-induced nitrate fluxes in the Kuroshio Current.
Oceanographic observations demonstrated a substantial nitrate flux caused by turbulent mixing in the Tokara Strait of
the ECS-Kuroshio. Nutrient-amended bottle incubation experiments showed that the growth rates of phytoplankton,
microheterotrophs, and the grazing rates of microzooplankton on phytoplankton were elevated within the area impacted
by this turbulence-induced nitrate flux.
2
2.1

Materials and methods
Onboard observations and experiments

All oceanographic observations and bottle incubations weredone in the Kuroshio Current where it passes throughthe Tokara Strait. Samplings for nitrate concentrations andmeasurements of turbulent diffusivity were conducted at
14 stations along two transects across the Kuroshio Current (Fig. 1a) during cruises of the T/S Kagoshima-maru in
November 2015.
The nitrate profiles were measured with a nitrate sensor (Deep SUNA V2) attached to an SBE 911plus
conductivity–temperature–depth (CTD) system (Sea-Bird
Electronics). Turbulent diffusivity was estimated from microstructure measurements made with a microstructure profiler (TurboMAP-L, JFE Advantech Co., Ltd.) and the equations of Osborn (1980). The profiler was deployed immediately after each CTD cast at the same station. The nitratesensor was calibrated with measured nitrate concentrations(Fig. S1 in the Supplement). Because the precision of the nitrate sensor in this study was 0.37 mmol m−3 (estimated by
D. Hasegawa et al., 2019), if we had calculated the verticalnitrate gradient from the raw data, then the noise level would
have been too high to resolve the normal background nitrate
stratification of O (10−1 mmol m−4 ). We therefore needed toaverage the sensor data vertically to reduce the level of noise.
The bin-averaged vertical gradient of the sensor data can be
written as follows:s
Cri − Cri−1
2w
∂Cs
∼
±P
,
(1)
∂z
1z
1z3 f
where Cs is the nitrate concentration reported by the sensor,
Cr is the real concentration, w is the average vertical deployment speed of the sensor, f is the sampling frequency, andwww.biogeosciences.net/17/2441/2020/

T. Kobari et al.: Phytoplankton growth and consumption by microzooplankton

2443

Figure 1. Locations for oceanographic observations and onboard experiments in the Kuroshio Current of the East China Sea (ECS-Kuroshio).
(a) Oceanographic observations by Deep SUNA V2 and TurboMAP-L (yellow lines). (b) Onboard experiments for phytoplankton andmicrozooplankton growth (EXPa : red and blue circles) and for microzooplankton grazing (EXPb : yellow circles). EXPa was conducted in
the upstream (blue circles) and downstream Kuroshio (red circles) in the Tokara Strait. Current directions and velocities (arrows) are shown
as monthly means during November 2016. Bottom depth (m) is indicated as colored contours.

1z is the average bin size. In this study f = 1 Hz and w =0.5 m s−1 . The second term on the right side of Eq. (1) indicates the expected precision of the bin-averaged vertical gradient of nitrate (see the detailed discussions in D. Hasegawa
et al., 2019). In this study, we set 1z = 10 m to resolve thelikely vertical gradient with an expected imprecision of O
(10−2 mmol m−4 ).
A total of 16 nitrate and turbulent diffusivity profiles wereaveraged at the stations that were studied during the KG1515
cruise of the T/S Kagoshima-maru across the Kuroshio path.
The profiles of the gradients of the averaged nitrate concentrations and averaged turbulent diffusivity were then multiplied at each depth to calculate the average turbulent nitrate
fluxes. Both parameters were binned and averaged within
10 m intervals. The vertical gradient of the averaged nitrate profile (CNO3 ) and the averaged vertical diffusivity (Kz )were then multiplied at each depth (z) to estimate the areaaveraged vertical turbulent nitrate flux (FNO3 ) as follows:
FNO3 = −Kz × (∂CNO3 /∂z).

(2)

In recent years, there has been a lively discussion about theimportance of the diapycnal advective flux associated withthe diffusive flux (e.g., Du et al., 2017). However, in thepresent study, we assumed that the important nutrient flux
was the flux across the base of the euphotic zone, not theflux through the pycnocline, which can be broken down by
turbulent mixing. In addition, because our study area included frontal regions, unlike the South China Sea where theKuroshio flows over seamounts, density fluctuations could
www.biogeosciences.net/17/2441/2020/

have been caused not only by turbulent mixing but also by
advection and the movement of fronts. Accordingly, we focused our analysis on the vertical turbulent nutrient flux using
Cartesian coordinates rather than on the diapycnal flux using
isopycnal coordinates.
We performed two different types of bottle incubations.
For phytoplankton and microheterotrophs, growth rates in response to in situ nitrate fluxes induced by turbulent mixing
were estimated using bottle incubations with nutrient gradients (EXPa ) at eight stations in November 2016 and November 2017. To estimate microzooplankton grazing rates on
phytoplankton, dilution experiments (EXPb ) following themethodology of Landry and Hasset (1982) were performed
at eight stations in November 2017 (Fig. 1b, Table 1).
2.2

Experimental setup

Seawater samples for all experiments were obtained using 2.5 L Niskin-X bottles attached to a CTD profiler andcarousel multisampling system (CTD-CMS: SBE 911plus,
Sea-Bird Electronics). The samples were transferred by gravity filtration using a silicon tube with a nylon filter (0.1 mm
mesh opening) into the incubation bottles for EXPa and
EXPb .
The EXPa experiment was performed using duplicate
2.3 L polycarbonate bottles without added nutrients and witha mixture of nitrate (NaNO3 ) and phosphate (KH2 PO4 )in an atomic N : P ratio of 15 : 1. The nitrate concentrations were either 0 (control), 0.05, 0.15, 0.5, 0.75, 1.5, or
5 µmol L−1 . If the turbulent nitrate influx at the subsurface
Biogeosciences, 17, 2441–2452, 2020

2444

T. Kobari et al.: Phytoplankton growth and consumption by microzooplankton

Table 1. Location and environmental conditions at the stations in the ECS-Kuroshio where gradient enrichment (EXPa ) and dilution experiments (EXPb ) were conducted. Depth: sampling depth (m) of water samples for each experiment. WT: mean water temperature during
the experiments (◦ C). NUTso : nutrient concentrations (µmol L−1 ) at the beginning of each experiment. CHLo : chlorophyll a concentration(µgCHL L−1 ) at the beginning of the experiments. MiZo : microheterotroph standing stock at the beginning of each experiment (µgC L−1 ).
DL: below the detection limit.
StationLocation

Date

Year

Depth

Longitude

Latitude30◦ 110 N
29◦ 500 N
29◦ 530 N
29◦ 510 N
29◦ 340 N
30◦ 060 N
30◦ 240 N
31◦ 240 N

129◦ 41.00 E
129◦ 08.40 E
129◦ 22.40 E
129◦ 57.20 E
128◦ 26.30 E
130◦ 11.90 E
131◦ 23.60 E
132◦ 29.20 E

13 Nov
13 Nov
14 Nov
14 Nov
12 Nov
14 Nov
15 Nov
16 Nov

2016
2016
2016
2016
2017
2017
2017
2017

68
75
81
91
50
105
115
90

30◦ 100 N
30◦ 100 N
30◦ 110 N30◦ 000 N
30◦ 000 N
29◦ 190 N
29◦ 260 N
29◦ 090 N

129◦ 17.50 E
129◦ 17.50 E
129◦ 17.20 E
129◦ 15.10 E
129◦ 15.00 E
129◦ 09.40 E
129◦ 12.40 E
129◦ 00.00 E

3 Nov
7 Nov
7 Nov
3 Nov
7 Nov
6 Nov
6 Nov
6 Nov

2017
2017
2017
2017
2017
2017
2017
2017

13
95
34
12
110
76
71
105

WT

NUTso

CHLo

MiZo

NO3 + NO2

PO4

26.1
26.2
25.1
26.1
25.6
24.8
25.5
25.0

DL
DL
0.21
0.26
0.18
0.57
1.82
0.16

0.02
0.01
0.04
0.07
DL
0.02
0.12
DL

0.34
0.41
0.35
0.44
0.31
0.52
0.15
0.27

0.19
0.27
0.15
0.12
0.23
0.79
0.34
0.55

25.5
25.5
25.3
25.4
25.7
25.6
25.6
25.6

0.10
DL
0.02
DL
1.61
DL
0.03
0.11

0.03
DL
0.01
0.02
0.11
0.02
0.01
0.02

0.23
0.16
0.24
0.16
0.14
0.29
0.21
0.20

0.12
0.15
0.05
0.13
0.04
0.22
0.17
0.15

EXPa
C02
C03
F01
G01
K02
K05
K08
K11
EXPb
A05a
A05b
A05c
A06a
A06b
A08a
A08b
A09a

chlorophyll maximum observed in the Tokara Strait (O:0.788 mmol m−2 d−1 ; see Results) were continued for 5.3 dwhile the Kuroshio Current (0.33 m s−1 , Zhu et al., 2017)
passed through the Tokara Strait (150 km), then the phytoplankton in a 10 m thick layer could have consumed nitrateequivalent to a nitrate enrichment of 0.41 µmol L−1 .
The EXPb experiment was conducted using triplicate 1.2 L
polycarbonate bottles with microzooplankton as grazers andinvolved dilutions of the microzooplankton standing stocks
in the original water samples so that the concentrationsof microzooplankton equaled 1, 0.6, 0.3, or 0.1 times theconcentration in the undiluted water. These treatment bottles were enriched with 3 µmol L−1 nitrate (NaNO3 ) and
0.2 µmol L−1 phosphate (KH2 PO4 ) to promote phytoplankton growth. In addition, to evaluate nutrient limitation of phytoplankton growth, extra triplicate undiluted bottles were incubated without nutrient amendments.
All incubation bottles were soaked in 10 % HCl and rinsed
with surface seawater at each station before use (Landry etal., 1995). All experimental bottles were incubated for 72 h
for EXPa and 24 h for EXPb in a water bath with running surface seawater for temperature control and were covered bynylon mesh screening (i.e., screening with 5 mm openings)
to reduce irradiance to 75 % of the surface irradiance. Phytoplankton growth in the incubation bottles might have been an

Biogeosciences, 17, 2441–2452, 2020

overestimate of in situ growth because subsurface irradiance
was lower than the irradiance in the incubation bottles.
2.3

Sample analysis

Chlorophyll a concentrations were determined at the beginning and end of the EXPa and EXPb incubations. Subsamples of 500–1000 mL were filtered through a nylon
mesh (11 µm mesh opening: Millipore NY1104700) and a
glass fiber filter (2 µm: Whatman GM / F; 0.7 µm: Whatman
GF / F) for EXPa and through a glass fiber filter (GF / F)
for EXPb at a pressure less than 20 kPa. Photosynthetic pigments were extracted overnight in N,N-dimethylformamide
at −20 ◦ C in the dark, and the chlorophyll a concentrations
were determined with a fluorometer (Turner Designs 10AUor TD700). Size fractions were defined as pico for chlorophyll in phytoplankton smaller than 2 µm, nano for chlorophyll in phytoplankton between 2 and 11 µm in size, and micro for chlorophyll in phytoplankton larger than 11 µm.
Microsized heterotrophs in the incubation bottles at the
beginning of EXPa and EXPb were examined. Subsamplesof 500 mL were collected and fixed with 3 % acid Lugol’ssolution. We identified and counted three taxonomic groupsof the microheterotroph community (naked ciliates, tintinnids, and copepod nauplii) with an inverted microscope (Leica Leitz DMRD). Some marine planktonic ciliates and flagwww.biogeosciences.net/17/2441/2020/

T. Kobari et al.: Phytoplankton growth and consumption by microzooplankton

2445

ellates are known to be mixotrophs (Gaines and Elbrächter,1987), but we assumed naked ciliates and tintinnids to be heterotrophic in the present study. The sizes of cells or of individuals were measured, the biovolume was computed basedon geometric shape, and the carbon content was estimatedusing conversion equations (Put and Stoecker, 1989; Verityand Langdon, 1984; Parsons et al., 1984).
2.4

Rate calculations

Apparent growth rates (g: d−1 ) in the incubation bottles
of EXPa and EXPb were calculated from size-fractionatedchlorophyll a concentrations (µg L−1 ) or standing stocks(µg C L−1 ) of microheterotroph groups identified at the beginning (Co ) and end (Ct ) of the incubations period (t: d):g = [ln(Ct ) − ln(Co )]/t,(3)

where Ct in the incubation bottles of EXPb can be calculatedusing the following equation (Landry et al., 1995):
Ct = Co × exp[(gmax − m) × t],(4)

where gmax and m are the maximum growth rate of sizefractionated phytoplankton (d−1 ) and their mortality rateby microzooplankton grazing (d−1 ), respectively. The maximum growth rate (gmax ) and mortality rate were determinedwith a linear regression of the apparent growth rate (g)against dilution factors (X):g = gmax − mX,(5)where m is the mortality rate in the undiluted water (X = 1).
All parameters derived from EXPa and EXPb are listed inTables 2 and 3.
2.5

Data analysis

To quantify the sensitivity of phytoplankton growth rates tonutrient supply rates, we calculated the slopes of linear regressions of growth rates for the size-fractionated chlorophyll a concentrations versus the logarithms of the enriched
nitrate concentrations. We then computed the Pearson correlation coefficient of these slopes to nitrate plus nitrite andphosphate concentrations and microzooplankton biomass atthe beginning of each incubation. A one-way analysis of variance (ANOVA) with a post hoc Tukey honestly significant
difference test was used to compare maximum growth rates,
mortality rates, and net growth rates among the three size
fractions.
3
3.1

Results
Oceanographic observations

Turbulent diffusivity and nitrate concentrations were measured in order to estimate the vertical turbulent nitrate flux
www.biogeosciences.net/17/2441/2020/

Figure 2. Vertical profiles of environmental conditions in the
Kuroshio Current. (a) Nitrate gradient curve (orange) and chlorophyll a concentrations (green) measured with a nitrate sensor (Deep
SUNA V2) attached to an SBE 911plus CTD system. (b) Turbulent
diffusivity measured with a TurboMAP-L (blue). (c) Calculated turbulent nitrate fluxes (red) in the ECS-Kuroshio. The shaded areas
are the 95 % confidence intervals obtained by a bootstrap process.

along the transects across the Kuroshio Current in the Tokara
Strait, where a shallow ridge lies in the path of the Kuroshio.
We obtained 16 pairs of vertical profiles of turbulent diffusivity and nitrate concentrations and estimated the averages
and 95 % confidence intervals of the vertical profiles. The
averaged chlorophyll a profile (Fig. 2a), which was recordedwith a light-emitting diode fluorometer on a TurboMAP-L
profiler, revealed a subsurface chlorophyll maximum (SCM)
at 60 m, which was almost coincident with a sharp increase in
the nitrate concentration (i.e., the top of the nitracline). Vertical diffusivity of O (10−4 m2 s−1 , Fig. 2b) was higher at 70 m
than at depths of 80–130 m. Just below the SCM peak, relatively high nitrate concentrations and vertical diffusivity induced vertical turbulent nitrate fluxes of O (1 mmol m−2 d−1 ,Fig. 2c).
3.2

Gradient-enrichment experiments (EXPa )To evaluate how the turbulent nitrate fluxes measured in the
Tokara Strait increased the standing stocks of phytoplankton
and microheterotrophs in the Kuroshio, we conducted bottle incubations of the phytoplankton and microheterotroph
communities enriched with different nutrient concentrations(EXPa ). The total chlorophyll a concentrations at the beginning of EXPa averaged among the duplicate samples ranged
from 0.15 to 0.52 µg L−1 (Table 1). The picofractions andnanofractions accounted for more than 80 % of the total
chlorophyll a (Fig. 3a). All size-fractionated chlorophyll adeclined or changed little toward the end of the incubations
Biogeosciences, 17, 2441–2452, 2020

2446

T. Kobari et al.: Phytoplankton growth and consumption by microzooplankton

Table 2. Phytoplankton growth rate (d−1 ) derived from the gradient-enrichment experiments (EXPa ) in the ECS-Kuroshio. Enriched nitrateconcentrations (µmol L−1 ) are shown at the top of each column. A and B: duplicate bottles. Pico: chlorophyll smaller than 2 µm. Nano:
chlorophyll between 2 and 11 µm. Micro: chlorophyll larger than 11 µm.
Station

0

0.05

0.15

0.5

0.75

1.5

5

A

B

A

B

A

B

A

B

A

B

A

B

A

B

−0.108−0.1160.150
0.062−0.305−0.1470.348−0.062−0.116−0.1180.159
0.051−0.2820.027
0.266−0.036−0.089−0.0730.332
0.135−0.2050.007
0.350−0.105−0.082−0.0780.277
0.089−0.265−0.0530.315−0.0920.019−0.0040.282
0.163−0.113
0.037
0.333
0.043

−0.073−0.0080.344
0.108−0.3050.084
0.407−0.081

0.470
0.453
0.445
0.438
0.264
0.329
0.361
0.193

0.426
0.426
0.495
0.477
0.295
0.176
0.185
0.179

0.422
0.588
0.511
0.795
0.119
0.263
0.448
0.514

0.441
0.706
0.497
0.736
0.097
0.168
0.416
0.390

0.686
0.780
0.490
0.828
0.422
0.645
0.377
0.765

0.798
0.892
0.385
0.969
0.652
0.716
0.468
0.730

0.796
0.862
0.372
0.861
0.831
0.792
0.403
0.469

0.556
0.906
0.467
0.781
0.669
0.701
0.417
0.558

−0.479−0.275−0.244−0.304−0.321−0.3890.353−0.138−0.260−0.261−0.154−0.172−0.149−0.3180.244−0.088−0.208−0.211−0.286−0.313−0.384−0.6800.508−0.257−0.409−0.257−0.092−0.189−0.152−0.5460.472−0.243−0.297−0.080−0.025−0.1650.022−0.2670.455−0.134−0.345−0.2060.101−0.1170.035−0.3940.436−0.293−0.0500.113
0.182−0.0630.223−0.484
0.406
0.073

0.144
0.031
0.050
−0.1780.251−0.248
0.397
0.026

0.173
0.247
0.148
0.100
−0.027−0.407
0.473
0.175

0.151
0.192
0.039
0.001
−0.135−0.458
0.369
0.201

0.249
0.363
0.015
0.286
0.433
0.053
0.408
0.296

0.333
0.355
0.056
0.325
0.229
−0.034
0.546
0.312

0.330
0.288
0.104
0.369
0.559
0.102
0.380
0.434

0.264
0.256
0.105
0.053
0.523
0.196
0.384
0.501

−0.383−0.202−0.0710.019−0.245−0.0870.032−0.197−0.188−0.258−0.091−0.061−0.2530.031
0.055−0.216−0.186−0.259−0.0540.051−0.2570.014−0.013−0.194−0.199−0.282−0.032−0.032−0.275−0.0270.228−0.146−0.119−0.1430.050
0.019−0.2430.103
0.262−0.046−0.162−0.1600.129
0.008−0.2300.157
0.201−0.071

0.188
0.017
0.205
0.156
−0.0460.057
0.240−0.0050.143−0.019
0.144
0.162
0.010
0.261
0.069
0.033

0.162
0.148
0.216
0.323
−0.067
0.130
0.262
0.163

0.241
0.191
0.141
0.188
−0.101
0.339
0.281
0.076

0.257
0.194
0.170
0.338
0.065
0.316
0.177
0.236

0.291
0.248
0.134
0.308
−0.030
0.255
0.284
0.049

0.377
0.230
0.031
0.344
0.203
0.368
0.222
0.092

0.205
0.300
0.172
0.366
0.089
0.404
0.327
0.179

Micro
C02
C03
F01
G01
K02
K05
K08
K11
Nano
C02
C03
F01
G01
K02
K05
K08
K11
Pico
C02
C03
F01
G01
K02
K05
K08
K11

at nitrate enrichments < 0.15 µmol L−1 , but it increased at enrichments > 0.5 µmol L−1 .
At the beginning of the incubations, microheterotroph
standing stocks averaged among the duplicate samples
ranged from 0.12 to 0.79 µg C L−1 (Table 1). Naked ciliates
accounted for 51 %–96 % of the microheterotrophic biomass
in terms of carbon at the beginning of the incubations. Copepod nauplii were the second greatest contributor to the microheterotroph biomass because of their low abundance but
large individual body mass; tintinnid ciliates were a minor
component of the microheterotroph biomass. The standing
stocks of all taxonomic groups in the microheterotroph category increased with increasing nitrate enrichment (Fig. 3b),but the patterns of increase in response to nutrient enrichment
were less clear than was the case for the size-fractionatedchlorophyll a concentrations.
Based on the changes of the standing stocks between the
beginning and end of the incubations, we investigated thegrowth rates of the chlorophyll and microheterotrophs. The
growth rates of all size-fractionated chlorophyll increased at
higher concentrations of added nitrate (Fig. 4a). Growth rates
were negative or close to zero for all size fractions at nitrate

Biogeosciences, 17, 2441–2452, 2020

enrichments < 0.15 µmol L−1 . However, the growth rates ofthe picosized and microsized chlorophyll were positive at nitrate enrichments > 0.5 µmol L−1 , which were nearly equivalent to the concentrations associated with the turbulent nitrate fluxes observed in the Tokara Strait (see Sect. 2.2). Because microheterotroph growth rates varied among stations,the response of microheterotroph growth to the nutrient enrichments was ambiguous (Fig. 4b). Growth rates were positive for copepod nauplii at all nitrate enrichments and werehigher for naked and tintinnid ciliates at higher nitrate enrichments. Thus, the standing stocks of phytoplankton andmicroheterotrophs were likely increased by additions of nitrate within the range of fluxes measured in the Tokara Strait.
The slope of a linear regression of the growth rates of thesize-fractionated chlorophyll and the logarithms of the nitrate enrichments for each incubation provided a metric of
the sensitivity of phytoplankton growth rates to nutrient supplies. The steeper slopes at some stations in the upstream
Kuroshio in the Tokara Strait compared to the slopes at otherstations (Fig. S2) suggested that the apparent phytoplankton
growth rates varied with nutrient concentrations or predatory impacts at the beginning of the incubations. To deter-

www.biogeosciences.net/17/2441/2020/

p< 0.01< 0.01< 0.01< 0.01< 0.01< 0.01< 0.01< 0.01
0.757
0.901
0.875
0.541
0.750
0.856
0.693
0.760
0.492
0.113
0.122
0.550
−0.7140.009
0.488−0.052
0.199−0.232−0.0740.347−0.735−0.113−0.123−0.324
1.059
1.073
0.828
0.941
−0.052
0.765
0.937
0.526
0.045
0.107
0.201
0.339
0.065
0.181
0.529
−0.2350.059−0.030−0.269−0.315−0.038−0.537−0.620−0.474
0.962
0.583
0.500
0.597
−0.037
0.564
1.122
0.022
0.913
0.477
0.313
0.232
−0.118
0.401
0.553
0.353
0.181
−0.239−0.0660.341−0.629−0.1500.430−0.082−0.267−0.505−0.1830.247−0.789−0.072−0.259−0.182
1.345
1.050
1.192
1.270
0.247
1.289
1.068
0.734
1.181
1.354
1.298
0.865
0.947
1.448
0.652
0.894
0.681
0.279
0.190
0.646
−1.0200.035
0.509−0.0260.415−0.109−0.0250.440−0.869−0.097−0.040−0.399
0.283
0.931
0.501
0.179
0.648
0.434
0.370
0.488
A05a
A05b
A05c
A06a
A06b
A08a
A08b
A09a

0.887
1.106
0.647
0.814
−0.398
0.458
0.846
0.417

gmax
gen
go
gmax
Station

m

2447

0.619
1.051
0.752
0.381
0.711
0.775
0.471
0.640

r2
gen
go
gmax
go
m

gen

gmax

m

go

gen

m

Total
Micro
Nano
Pico

Table 3. Parameters derived from the dilution experiments (EXPb ) in the ECS-Kuroshio. gmax : maximum growth rate (d−1 ). m: mortality rate by microzooplankton grazing (d−1 ). go :net growth rate measured in the nonenriched and nondiluted bottles (d−1 ). gen : net growth rate measured in the enriched and nondiluted bottles (d−1 ). r 2 : coefficient of determination
defined from the linear regression of the apparent growth rate of total chlorophyll a concentrations against dilution factors. p: p value. Pico: chlorophyll smaller than 2 µm. Nano:
chlorophyll between 2 and 11 µm. Micro: chlorophyll larger than 11 µm. Total: total chlorophyll from pico to micro.

T. Kobari et al.: Phytoplankton growth and consumption by microzooplankton

www.biogeosciences.net/17/2441/2020/

Figure 3. Changes in phytoplankton and microsized heterotroph
standing stocks during the gradient-enrichment experiments(EXPa ). (a) Size-fractionated chlorophyll a concentrations (CHL).
(b) Microheterotroph standing stocks (MiZ). To : at the beginningof the gradient-enrichment experiments. 0: no enrichment. 0.05 to
5.0 µmol L−1 : enrichment. Box-and-whisker diagram at each nitrate concentration was compiled from the results conducted at theeight stations. Box represents first (bottom), second (bar), and third(top) quartiles, and cross marks are the average values. Whiskers indicate minimum and maximum values, and circles are outliers. Pico:chlorophyll smaller than 2 µm (yellow). Nano: chlorophyll between
2 and 11 µm (green). Micro: chlorophyll larger than 11 µm (red).
NC: naked ciliates (light blue). TC: tintinnid ciliates (light green).
CN: copepod nauplii (light pink).

mine whether growth rates of the size-fractionated chlorophyll might have varied with initial nutrient concentrations(bottom-up control) or predator biomasses (top-down control) at the beginning of the experiments, we compared theslopes to the nitrate plus nitrite concentrations (Fig. 5a),phosphate concentrations (Fig. 5b), and microheterotroph
biomasses (Fig. 5c) in the ambient seawater without enrichment. No significant correlation was found between the microheterotrophic biomass and the rate of change of any sizefractionated chlorophyll. In contrast, the fact that there wasa negative correlation between the slopes for all size fractions and the nitrate plus nitrite or phosphate concentrations
indicated that the stimulation of the phytoplankton growth
rates by nutrients was greater for all chlorophyll size fractions under more oligotrophic conditions. Thus, the variations of phytoplankton growth rates were likely associatedwith nutrient concentrations at the beginning of the incubations.
Biogeosciences, 17, 2441–2452, 2020

2448

T. Kobari et al.: Phytoplankton growth and consumption by microzooplankton
rates (gmax ). For all size-fractionated chlorophyll, the factthat go was lower than gen (Fig. 7) indicated that net growth0
rates were limited by nutrients. The values of gen and gen
were comparable; i.e., there was no significant difference between the two (Welch’s t test). These results implied that the
gen of all size-fractionated chlorophyll could balance microzooplankton grazing mortality by growing at the maximum
rate. In the case of the nanochlorophyll, the net growth rates
were a bit low because the mortality rates due to microzooplankton grazing exceeded the maximum growth rates.

4

Figure 4. Changes in phytoplankton and microsized heterotroph
growth rates in response to nitrate enrichments in the gradientenrichment experiments (EXPa ). (a) Growth rates (g: d−1 ) of
size-fractionated chlorophyll. (b) Microheterotroph growth rates (g:d−1 ). 0: no enrichment. 0.05 to 5.0 µmol L−1 : enrichment. Boxand-whisker diagram at each nitrate concentration is based on the
results conducted at the eight stations. The symbols have the same
meaning as in Fig. 3.

3.3

Dilution experiments (EXPb )To evaluate how much each size-fractionated phytoplankton population was removed by microzooplankton grazing, we conducted dilution experiments concurrently withthe gradient-enrichment experiments. The maximum growth
rates (i.e., the intercepts of the regressions corresponding to
X = 0 in Eq. 5) were relatively high for the nanochlorophyll(Fig. 6a), but the differences were insignificant among thethree size fractions (ANOVA, p > 0.05). These results indicated that the growth potential in the absence of microzooplankton grazing was similar for the nanosized chlorophyll compared with the picofractions and microfractions.
In contrast, the slopes of the regressions are the mortality
rates due to microzooplankton grazing, and the fact that they
were significantly higher for the nanochlorophyll versus the
picochlorophyll and microchlorophyll (ANOVA + Tukey,
p < 0.05) indicated that the microzooplankton preferentiallygrazed on the nanochlorophyll.
To evaluate the impact of microzooplankton grazing on
phytoplankton growth, we compared three different net
growth rates: the observed net growth rates without enrichment (go ), the net growth rates with enrichment (gen ) in the0 ) estimated byundiluted bottles, and the net growth rates (gensubtracting the mortality rate (m) from the maximum growth
Biogeosciences, 17, 2441–2452, 2020

Discussion

The Kuroshio Current impinges on numerous shallow ridges
with small islands and seamounts in the Tokara Strait. Several studies have pointed out that those steep topographic features stir and modify the water column through upwelling(Hasegawa et al., 2004, 2008) and turbulent mixing (Tsutsumi et al., 2017; Nagai et al., 2017). Compared with the turbulent nitrate fluxes reported in previous studies, the fluxes
observed in the Tokara Strait were 1 order of magnitude
higher than those reported in the Kuroshio Extension front(Kaneko et al., 2012, 2013; Nagai et al., 2017), which isgreater than those at other oceanic sites, and equivalent tothose at coastal sites (Cyr et al., 2015). The turbulent nitrate
flux in the downstream Kuroshio Current near the Tokara
Strait is similar in magnitude to our estimates (Nagai et al.,2019b). Because the Kuroshio Current runs steadily throughthe Tokara Strait, this nutrient supply induced by turbulent
diffusivity is considered to be one of the mechanisms thatenhance phytoplankton productivity, even under oligotrophic
conditions in the Kuroshio Current.
Despite the large turbulent nitrate flux (O: 1mmol m−2 d−1 ), the chlorophyll a concentrations in
the area of the Tokara Strait impacted by the Kuroshio
Current were as low as the values reported from nearby areasof the Kuroshio (Kobari et al., 2018, 2019) and oceanic sites
in the North Pacific Ocean (Calbet and Landry, 2004). Basedon the gradient-enrichment experiments, standing stocks
and the growth rates of all size-fractionated phytoplankton increased at nitrate enrichments above 0.5 µmol L−1 ,which were equivalent to the concentrations produced by
the observed turbulent nitrate flux. These results suggestthat phytoplankton standing stocks and growth rates arestimulated by the magnitude of the observed turbulent nitrateflux.
In global comparisons, microzooplankton grazing has asignificant impact on phytoplankton, particularly at oceanic
sites (Calbet and Landry, 2004). Microzooplankton standing stocks in the Kuroshio Current as it passes through theTokara Strait are lower than those on the continental shelf ofthe ECS (Chen et al., 2003) and might be removed by mesozooplankton predation (Kobari et al., 2019). The low microzooplankton standing stocks in the Kuroshio Current imply
www.biogeosciences.net/17/2441/2020/

T. Kobari et al.: Phytoplankton growth and consumption by microzooplankton

2449

Figure 5. Correlation of the regression slopes of phytoplankton growth rates to nutrient concentrations and microsized heterotroph biomass atthe beginning of the gradient-enrichment experiments (EXPa ). (a) Regression slopes of the size-fractionated phytoplankton growth versus theconcentrations of nitrate (NO3 ) plus nitrite (NO2 ). (b) Regression slopes of the size-fractionated phytoplankton growth versus the phosphateconcentrations (PO4 ). (c) Regression slopes of the size-fractionated phytoplankton growth versus the microheterotroph biomass (MiZ). r:
Pearson correlation coefficient. Pico: chlorophyll smaller than 2 µm. Nano: chlorophyll between 2 and 11 µm. Micro: chlorophyll larger than
11 µm. ∗ : p < 0.05. ns: not significant.

Figure 6. Comparisons of phytoplankton growth and mortality rates among the three size-fractionated chlorophyll derived from the dilution
experiments (EXPb ). (a) Maximum growth rates (gmax ). (b) Mortality rates by microzooplankton grazing (m). Box-and-whisker diagramat each nitrate concentration was compiled from the results conducted at the eight stations. Box represents first (bottom), second (bar) andthird (top) quartiles, and cross marks are the average values. Whiskers indicate minimum and maximum values, and circles are outliers.
Asterisk means significant difference among the three size fractions (ANOVA + Tukey, p < 0.05). Pico: chlorophyll smaller than 2 µm.
Nano: chlorophyll between 2 and 11 µm. Micro: chlorophyll larger than 11 µm.

low microzooplankton grazing on phytoplankton. However,the dilution experiments demonstrated that phytoplanktonmortality by microzooplankton grazing was high and equivalent to 41 %–122 % of the maximum growth rates of thephytoplankton, based on the ratio of the mortality rate to
the maximum growth rates of total chlorophyll a (Table 2).
Indeed, phytoplankton could likely balance microzooplankton grazing mortality by growing at maximum rates, particularly in the case of the nanophytoplankton (Fig. 7). These
results from concurrently conducted experiments suggested
that phytoplankton standing stocks are stimulated by turbulent nitrate fluxes and are then quickly removed by microzooplankton grazing, particularly in the case of nanophytoplankton. Taking into account the size range of prey for ciliates (Pierce and Turner, 1992) and copepod nauplii (Uyewww.biogeosciences.net/17/2441/2020/and Kasahara, 1983), microzooplankton grazing could be amajor reason why phytoplankton do not attain high growth
rates and standing stocks, even when their growth potential ishigh and they are sensitive to nutrient enrichments. The rapidtransfer of the elevated phytoplankton production to microzooplankton might thus be a possible explanation for the low
chlorophyll concentrations, even when there are large turbulent nitrate fluxes in the Kuroshio Current.
The standing stocks and growth rates of all microheterotrophs were relatively high in the higher nitrate enrichments, but the patterns of increase were less clear thanin the case of the phytoplankton. This difference was probably due to the large variations in the microheterotroph standing stocks among stations (Table 1) and slower growth thanphytoplankton. Indeed, the lack of clarity of this pattern was
Biogeosciences, 17, 2441–2452, 2020

2450

T. Kobari et al.: Phytoplankton growth and consumption by microzooplankton

Table 4. Parameters derived from relationship of phytoplankton growth rates against logarithmically transformed concentrations of enriched
nitrate in the gradient-enrichment experiments (EXPa ). Slope: sensitivity of phytoplankton growth rate to logarithmically transformed concentrations of enriched nitrate. Intercept: growth potential at the low nitrate concentration. r 2 : coefficient of determination defined fromthe linear regression of growth rate of size-fractionated chlorophyll a concentrations against logarithmically transformed concentrations ofenriched nitrate. Pico: chlorophyll smaller than 2 µm. Nano: chlorophyll between 2 and 11 µm. Micro: chlorophyll larger than 11 µm.
Pico

Nano

StationSlope

Intercept

r2

C02
C03
F01
G01
K02
K05
K08
K11

0.281
0.295
0.074
0.203
0.213
0.188
0.070
0.167

0.178
0.121
0.129
0.243
−0.014
0.251
0.231
0.077

0.848
0.922
0.317
0.866
0.883
0.772
0.242
0.750

Slope
0.370
0.308
0.120
0.272
0.364
0.355
−0.0380.394

Figure 7. Comparisons of phytoplankton net growth derived fromthe dilution experiments (EXPb ) among the three different methods. go : observed net growth rates without enrichment in the nondiluted bottles. gen : observed net growth rates with enrichment in the
0 : estimated net growth rates subtracting thenondiluted bottles. gen
mortality rates (m) from the maximum growth rates (gmax ). Boxand-whisker diagram at each nitrate concentration was compiled
from the results conducted at the eight stations. Asterisk means significant difference between go and gen (Welch’s t test, p < 0.05).
The symbols have the same meaning as in Fig. 6.

remarkable for the copepod nauplii because of their relatively slow growth rates, lower abundance in the bottles, andlarger individual body masses. In contrast, intraguild predation within microheterotroph communities might be another explanation for the ambiguous pattern of their standing
stocks and growth rates. The growth rates of copepod nauplii
were always higher than those of naked ciliates, especiallywhen there was no or little nitrate supplied. The ratio of meanequivalent spherical diameter of body mass for copepod nauplii (88 µm) and naked ciliates (16 µm) was estimated to be
5 : 1, which is much different from a typical predator–preymass ratio (i.e., 18 : 1, Hansen et al., 1994). Thus, it is unlikely that intraguild predation of copepod nauplii on naked
ciliates would happen in the bottles. More importantly to the
Biogeosciences, 17, 2441–2452, 2020

Micro

Intercept

r2

Slope

Intercept

r2

0.131
0.177
0.067
0.085
0.233
−0.165
0.426
0.201

0.831
0.830
0.420
0.688
0.726
0.729
0.213
0.943

0.458
0.560
0.077
0.448
0.531
0.419
0.045
0.403

0.492
0.611
0.430
0.657
0.353
0.439
0.386
0.409

0.846
0.914
0.368
0.817
0.872
0.843
0.162
0.744

ambiguous pattern of the growth of the microheterotrophs,
the results from the concurrently conducted experiments implied that phytoplankton productivity was stimulated by the
turbulent nitrate flux and the phytoplankton rapidly grazed
by microzooplankton, but standing stocks and growth ratesof microheterotrophs were not elevated during 3 d in the
Kuroshio Current. An increase in microheterotroph standing
stocks and their trophic transfer to mesozooplankton might
have been apparent further downstream in the Kuroshio Current.
There is increasing evidence that turbulence-induced nutrient fluxes promote phytoplankton growth in the open
ocean (Kaneko et al., 2013; Nagai et al., 2017, 2019b). However, there is no experimental documentation for a responseof the phytoplankton community to this nutrient supply or ofsubsequent trophic transfer in a planktonic food web. In thetropical and subtropical oceans, microzooplankton grazinghas been thought to be a major source of phytoplankton mortality and has been shown to account for more than 75 % ofphytoplankton daily growth (Calbet and Landry, 2004). Furthermore, strong trophic linkages are well known between
microbes and metazoans through microzooplankton (Calbet
and Landry, 1999; Calbet et al., 2001; Calbet and Saiz, 2005;
Kobari et al., 2010). Our study has provided the first experimental evidence that phytoplankton standing stocks andgrowth rates are stimulated by turbulent nutrient fluxes andare rapidly grazed by microzooplankton. These results imply
that biological productivity may be underestimated becauseof the apparently low nutrient concentrations and low phytoplankton biomass in the Kuroshio. Because of the strong turbulence amplified by the Kuroshio Current, phytoplankton
productivity stimulated by nutrient fluxes and rapid trophic
transfer to microzooplankton are likely to happen in the
Tokara Strait and downstream. We therefore propose that undocumented biological productivity in the Kuroshio is sustained by these rapid and systematic trophodynamics. Suchwww.biogeosciences.net/17/2441/2020/

T. Kobari et al.: Phytoplankton growth and consumption by microzooplankton
undocumented biological production, elevated by the rapid
and systematic trophodynamics, may provide a good supplyof food for the vulnerable stages of foraging fishes aroundthe Kuroshio and thus explain part of the Kuroshio Paradox.

Data availability. Data used in this paper are available athttps://doi.org/10.6084/m9.figshare.12198486 (Kobari, 2020).

Supplement. The supplement related to this article is available online at: https://doi.org/10.5194/bg-17-2441-2020-supplement.

Author contributions. ToK, DH, and NY conceived and designed
the oceanographic observations and experiments. DH, HN, AN, ET,
TM, and TN performed the oceanographic observations and turbulence measurements. ToK, TH, TaK, and FK performed the onboard
experiments. ToK, TH, TaK, FK, NY, and KS analyzed the samples
and data of the onboard experiments. DH and TT analyzed the dataof oceanographic observations and turbulence measurements. ToK,
GK, HN, and XG organized the research cruises.

Competing interests. The authors declare that they have no conflictof interest.

Acknowledgements. We thank the captains and crew of the T/S
Kagoshima-maru for their help in oceanographic observations andsample collections.

Financial support. This research has been supported by the Japan
Society for the Promotion of Science (grant nos. 17K00522,
18H04920, and 15H05817) and the Ministry of Education, Culture,
Sports, Science, and Technology in Japan (the Study of Kuroshio
Ecosystem Dynamics for Sustainable Fisheries) (grant no. JPMXD0511102330).

Review statement. This paper was edited by Minhan Dai and reviewed by Zhiyu Liu, Chih-hao Hsieh, and Naomi Harada.

References
Bower, J. R., Nakamura, Y., Mori, K., Yamamoto, J., Isoda, Y., andSakurai, Y.: Distribution of Todarodes pacificus (Cephalopoda:
Ommastrephidae) paralarvae near the Kuroshio off southern
Kyushu, Japan, Mar. Biol., 135, 99–106, 1999.
Calbet, A. and Landry, M. R.: Mesozooplankton influences on the
microbial food web: Direct and indirect trophic interactions in
the oligotrophic open-ocean, Limnol. Oceanogr., 44, 1370–1380,
1999.
Calbet, A. and Landry, M. R.: Nunnery S. Bacteria-flagellate interactions in the microbial food web of the oligotrophic subtropicalNorth Pacific, Aquat. Microb. Ecol., 23, 283–292, 2001.

www.biogeosciences.net/17/2441/2020/

2451

Calbet, A. and Landry, M. R.: Phytoplankton growth, microzooplankton grazing, and carbon cycling in marine systems, Limnol.
Oceanogr., 49, 51–57, 2004.
Calbet, A. and Saiz, E.: The ciliate-copepod link in marine ecosystems, Aquat. Microb. Ecol., 38, 157–167, 2005.
Chen, C. C., Shiah, F. K., Gong, G. C., and Chiang, K. P.: Planktonic community respiration in the East China Sea: importanceof microbial consumption of organic carbon, Deep-Sea Res. Pt.
II, 50, 1311–1325, 2003.
Cyr, F., Bourgault, D., and Galbraith, P. S.: Gosselin M. Turbulent
nitrate fluxes in the Lower St. Lawrence Estuary, Canada, J. Geophys. Res., 120, 2308–2330, 2015.
Du, C., Liu, Z., Kao, S.-J., and Dai, M.: Diapycnal fluxes of nutrients in an oligotrophic oceanic regime: the South China Sea,
Geophys. Res. Lett., 44, 11510–11518, 2017.
Endo, H. and Suzuki, K.: Spatial variations in community structureof haptophytes across the Kuroshio front in the Tokara Strait,
in: Kuroshio Current, Physical, Biogeochemical and Ecosystem
Dynamics, edited by: Nagai, T., Saito, H., Suzuki, K., and Takahashi, M., Geophysical Monograph 243, John Wiley & Sons,
Hoboken, 207–221, 2019.
Gaines, G. and Elbrächter. M.: Heterotrophic nutrition, in: The biology of dinoflagellates, edited by: Taylor, F. J. R., Blackwell,
Oxford, 224–268, 1987.
Guo, Y. J.: The Kuroshio, Part II. Primary production and phytoplankton, Oceanogr, Mar. Bio. Ann. Rev., 29, 155–189, 1991.
Hansen, B., Bjørnsen, P. K., and Hansen, P. J.: The size ratio between planktonic predators and their prey, Limnol. Oceanogr.,39, 395–403, 1994.
Hasegawa, D.: Island Mass Effect, in: Kuroshio Current, Physical,
Biogeochemical and Ecosystem Dynamics, edited by: Nagai, T.,
Saito, H., Suzuki, K., and Takahashi, M., Geophysical Monograph 243, John Wiley & Sons, Hoboken, 163–174, 2019.
Hasegawa, D., Yamazaki, H., Lueck, R. G., and Seuront, L.: How
islands stir and fertilize the upper ocean, Geophys. Res. Lett., 31,
L16303, https://doi.org/10.1029/2004GL020143, 2004.
Hasegawa, D., Yamazaki, H., Ishimaru, T., Nagashima, H., andKoike, Y.: Apparent phytoplankton bloom due to island mass effect, J. Mar. Syst., 69, 238–246, 2008.
Hasegawa, D., Tanaka, T., Matsuno, T., Senjyu, T., Tsutsumi, E.,
Nakamura, H., Nishina, A., Kobari, T., Yoshie, N., Guo, X., Nagai, T., Okunishi, T., and Yasuda, I.: Measuring the vertical turbulent nitrate flux using sensors, Bull. Coast. Oceanogr., 57, 59–64,
2019.
Hasegawa, T., Kitajima S., and Kiyomoto Y.: Phytoplankton distribution in the Kuroshio region of the southern East China Sea inearly spring, in: Kuroshio Current, Physical, Biogeochemical andEcosystem Dynamics, edited by: Nagai, T., Saito, H., Suzuki, K.,
and Takahashi, M., Geophysical Monograph 243, John Wiley &Sons, Hoboken, 191–205, 2019.
Hirota, Y.: The Kuroshio, Part III. Zooplankton, Oceanogr. Mar.
Biol., 33, 151–220, 1995.
Hu, D., Wu, L., Cai, W., Gupta, A. S., Ganachaud, A., Qiu, B., Gordon, A. L., Lin, X., Chen, Z., Hu, S., Wang, G., Wang, Q., Sprintall, J., Qu, T., Kashino, Y., Wang, F., William, S., and Kessler, W.
S.: Pacific western boundary currents and their roles in climate,
Nature, 522, 299–308, 2015.
Kaneko, H., Yasuda, I., Komatsu, K., and Itoh, S.: Observations of
the structure of turbulent mixing across the Kuroshio, Geophys.

Biogeosciences, 17, 2441–2452, 2020

2452

T. Kobari et al.: Phytoplankton growth and consumption by microzooplankton

Res. Lett., 39, L15602, https://doi.org/10.1029/2012GL052419,
2012.
Kaneko, H., Yasuda, I., Komatsu, K., and Itoh, S.: Observations ofvertical turbulent nitrate flux across the Kuroshio, Geophys. Res.
Lett., 40, 3123–3127, 2013.
Kobari,T.:
Biogeosci17_Kobari.zip,
Dataset,figshare,https://doi.org/10.6084/m9.figshare.12198486.v1, 2020.
Kobari, T., Mitsui, K., Ota, T., Ichinomiya, M., and Gomi, Y.:
Response of heterotrophic bacteria to the spring phytoplankton
bloom in the Oyashio region, Deep-Sea Res. Pt. II, 57, 1671–
1678, 2010.
Kobari, T., Makihara, W., Kawafuchi, T., Sato, K., and Kume,
G.: Geographic variability in taxonomic composition, standing stock, and productivity of the mesozooplankton community around the Kuroshio Current in the East China Sea, Fish.
Oceanogr., 27, 336–350, 2018.
Kobari, T., Kobari, Y., Miyamoto, H., Okazaki, Y., Kume, G.,
Kondo, R., and Habano, A.: Variability in taxonomic composition, standing stock and productivity of the plankton community
in the Kuroshio and its neighboring waters, in: Kuroshio Current,
Physical, Biogeochemical and Ecosystem Dynamics, edited by:
Nagai, T., Saito, H., Suzuki, K., and Takahashi, M., Geophysical
Monograph 243, John Wiley & Sons, Hoboken, 223–243, 2019.
Komatsu, K. and Hiroe, Y.: Structure and impact of the Kuroshio
nutrient stream, in: Kuroshio Current, Physical, Biogeochemical
and Ecosystem Dynamics, edited by: Nagai, T., Saito, H., Suzuki,
K., and Takahashi, M., Geophysical Monograph 243, John Wiley& Sons, Hoboken, 85–104, 2019.
Kuroda, H.: The Kuroshio-induced nutrient supply in the shelf andsloe region off the southern coast of Japan, in: Kuroshio Current,
Physical, Biogeochemical and Ecosystem Dynamics, edited by:
Nagai, T., Saito, H., Suzuki, K., and Takahashi, M., Geophysical
Monograph 243, John Wiley & Sons, Hoboken, 137–146, 2019.
Landry, M. R. and Hasset, R. P.: Estimating the grazing impact ofmarine micro-zooplankton, Mar. Biol., 67, 283–288, 1982.
Landry, M. R., Kirshtein, J., and Constantinou, J.: A refined dilution technique for measuring the community grazing impact of
microzooplankton with experimental tests in the central equatorial Pacific, Mar. Ecol.-Prog. Ser., 120, 53–63, 1995.
Nagai, T., Hasegawa, D., Tanaka, T., Nakamura, H., Tsutsumi, E.,
Inoue, R., and Yamashiro, T.: First evidence of coherent bandsof strong turbulent layers associated with high-wavenumber
internal-wave shear in the upstream Kuroshio, Sci. Rep., 7,14555, https://doi.org/10.1038/s41598-017-15167-1, 2017.
Nagai, T., Clayton, S., and Uchiyama, Y.: Multiscale routes to supply nutrients through the Kuroshio nutrient stream, in: Kuroshio
Current, Physical, Biogeochemical and Ecosystem Dynamics,edited by: Nagai, T., Saito, H., Suzuki, K., and Takahashi, M.,
Geophysical Monograph 243, John Wiley & Sons, Hoboken,
105–125, 2019a.
Nagai, T., Durán, G. S., Otero, D. A., Mori, Y., Yoshie,N., Ohgi, K., Hasegawa, D., Nishina, A., and Kobari, T.:
How the Kuroshio Current delivers nutrients to sunlit layers on the continental shelves with aid of near-internal
waves and turbulence, Geophys. Res. Lett., 46, 6726–6735,
https://doi.org/10.1029/2019GL082680, 2019b.
Nakamura, H., Yamashiro, T., Nishina, A., and Ichikawa,
H.: Time frequency variability of Kuroshio meanders

Biogeosciences, 17, 2441–2452, 2020

in Tokara Strait, Geophys. Res. Lett., 33, L21605,
https://doi.org/10.1029/2006GL027516, 2006.
Nakata, K. and Hidaka, K.: Decadal-scale variability in the
Kuroshio marine ecosystem in winter, Fish. Oceanogr., 12, 234–
244, 2003.
Nakata, K., Zenitani, H., and Inagake, D.: Differences in food availability for Japanese sardine larvae between the frontal region andthe waters on the offshore side of Kuroshio, Fish. Oceanogr., 4,
68–79, 1995.
Osborn, T.: Estimates of the local rate of vertical diffusion from
dissipation measurements, J. Phys. Oceanogr., 10, 83–89, 1980.
Parsons, T. R., Takahashi, M., an Hargrave, B.: Biological oceanographic processes, Pergamon Press, Oxford, 1984.
Pierce, R. W. and Turner, J. T.: Ecology of planktonic ciliates inmarine food webs, Rev. Aquat. Sci., 6, 139–181, 1992.
Putt, M. and Stoecker, D. K.: An experimentally determined carbon:volume ration for marine “oligotrichous” ciliates from estuarine and coastal waters, Limnol. Oceanogr., 34, 1097–1103,
1989.
Qiu, B.: Kuroshio and Oyashio Currents, in: Encyclopedia of Ocean
Sciences, edited by: Steele, J. H., Academic Press, New York,358–369, 2001.
Saito, H.: The Kuroshio: its recognition, scientific activities andemerging issues, in: Kuroshio Current, Physical, Biogeochemical
and Ecosystem Dynamics, edited by: Nagai, T., Saito, H., Suzuki,
K., and Takahashi, M., Geophysical Monograph 243, John Wiley& Sons, Hoboken, 1–11, 2019.
Sassa, C. and Tsukamoto, Y.: Distribution and growth of Scomber
japonicus and S. australasicus larvae in the southern East China
Sea in response to oceanographic conditions, Mar. Ecol.-Prog.
Ser., 419, 185–199, 2010.
Sassa, C., Tsukamoto, Y., Nishiuchi, K., and Konishi, Y.: Spawning
ground and larval transport processes of jack mackerel Tranchurus japonicas in the shelf-break region of the southern East China
Sea, Cont. Shelf. Res., 28, 2574–2583, 2008.
Tsutsumi, E., Matsuno, T., Lien, R. C., Nakamura, H., Senjyu,
T., and Guo, X.: Turbulent mixing within the Kuroshio in
the Tokara Strait, J. Geophys. Res.-Oceans, 122, 7082–7094,
https://doi.org/10.1002/2017JC013049, 2017.
Uye, S. I. and Kasahara, S.: Grazing of various developmental
stages of Pseudodiaptomus marinus (Copepoda: Calanoida) on
naturally occurring particles, Bull. Plankton Soc. Japan, 30, 147–
158, 1983.
Verity, P. G. and Langdon, C.: Relationships between lorica volume,carbon, nitrogen and ATP content of tintinnids in Narragansett
Bay, J. Plankton Res., 6, 859–868, 1984.
Watanabe, Y., Zenitani, H., and Kimura, R.: Offshore expansion of
spawning of the Japanese sardine, Sardinops melanostictus, andits implication for egg and larval survival, Can. J. Fish. Aquat.
Sci., 53, 55–61, 1996.
Zhu, X. H., Nakamura, H., Dong, M., Nishina, A., and Yamashiro,
T.: Tidal currents and Kuroshio transport variations in the Tokara
Strait estimated from ferryboat ADCP data, J. Geophys. Res.,
122, 2120–2142, 2017.

www.biogeosciences.net/17/2441/2020/