Journal of Oceanography (2022) 78:89–101
https://doi.org/10.1007/s10872-021-00630-5

ORIGINAL ARTICLE

Spatial distribution of the protist community in the southern partof the Okhotsk Sea off Hokkaido during summer
Yusuke Hamao1· Kohei Matsuno1,2 · Yoko Mitani3,4 · Atsushi Yamaguchi1,2Received: 20 July 2021 / Revised: 20 December 2021 / Accepted: 22 December 2021 / Published online: 27 January 2022
© The Author(s) 2022

Abstract
The southern part of the Okhotsk Sea off Hokkaido has rich fishery resources supported by high primary production. The
phytoplankton community has been described by several studies based on size-fractionated chlorophyll a concentrationand primary production; however, the details of the species composition of nano- and micro-protist communities have notbeen well investigated. We evaluated the spatial distribution and relationship with hydrography of nano- and micro-protist
communities in the southern region of the Okhotsk Sea off Hokkaido during the summer of 2019. Forty-two species ofdiatoms from 19 genera, dinoflagellates from 7 genera, tintinnid ciliates, oligotrich ciliates, and 1 genus of silicoflagellates
were identified. Their spatial distribution showed distinct differences among the taxa. Cluster analysis based on the protist
cell density identified six groups with geographically well-distinguished distributions. Diatom-dominant communities were
distributed in the Cold Water Belt and Soya warm current along the coastal area of Hokkaido. Nanoflagellate-dominatedcommunities were distributed in the offshore area. Some harmful species of diatoms (Pseudonitzschia pseudodelicatissima)and dinoflagellates (Alexandrium spp.) were dominant in the Cold Water Belt off Wakkanai. We suggest that the upwellingof cold water could increase the abundance of harmful species, with immanent negative impacts on marine ecosystems or
fishery resources. In summary, the species composition and abundance of the summer protist community was strongly related
to different water masses in the southern part of the Okhotsk Sea off Hokkaido.
Keywords Diatoms · Dinoflagellates · Ciliates · Nanoflagellates · Okhotsk Sea · Cold Water Belt · Harmful algae

1 Introduction
The southern part of the Okhotsk Sea off Hokkaido is known
as one of the important fishing grounds in Japan due to abundant fishery resources, such as scallops and salmon (Kasai
et al. 2017; Sakurai 2011; Shiomoto et al. 2018). At higher
trophic levels (sea birds and marine mammals), this region* Yusuke Hamao
hamayu-5362@eis.hokudai.ac.jp
1

Faculty/Graduate School of Fisheries Sciences,
Hokkaido University, 3‑1‑1 Minato‑cho, Hakodate,
Hokkaido 041‑8611, Japan

2

Arctic Research Center, Hokkaido University, Kita‑21
Nishi‑11 Kita‑ku, Sapporo, Hokkaido 001‑0021, Japan

3

Field Science Center for Northern Biosphere, Hokkaido
University, 20‑5 Benten‑Cho, Hakodate, Hokkaido 040‑0051,
Japan

4

Present Address: Wildlife Research Center, Kyoto University,
2‑24 Tanaka‑Sekiden‑cho, Sakyo, Kyoto 606‑8203, Japanis also an important foraging habitat (Kazama et al. 2018).
In summer, this area is characterized by three different water
masses: the warm and saline Soya Warm Current (SWC) distributed in the coastal region, the less-saline Fresh Surface
Okhotsk Sea Water (FSOSW) observed in the surface layerof the offshore region, and low-temperature Intermediate
Cold Water (ICW) distributed in the lower layer (Takizawa
1982). In addition, the Cold Water Belt (CWB) is distributed
between the SWC and FSOSW during summer and autumn
along the southern part of the Okhotsk Sea off Hokkaido(Ishizu et al. 2006, 2008; Kuma et al. 2014; Mitsudera et al.
2011). Because the water in the CWB is composed of a mixture of high iron concentration SWC and upwelling highnutrient ICW (Ishizu et al. 2008), high primary production isobserved even in summer, resulting in rich fishery resources(Mustapha et al. 2009) and much prey for sea birds (e.g.,short-tailed shearwaters and northern fulmar) on the coast of
the Okhotsk Sea in Hokkaido (Tanaka and Kajihara 1979).
The high primary production in this area during the seaice-free seasons has been revealed by previous studies based

13

Vol.:(0123456789)

90

on size-fractionated chlorophyll a concentration (Chl. a),and the 13C isotope-labeling method in incubation experiments (Kasai et al. 2010; Kasai and Hirakawa 2015; Shiomoto 2011; Shiomoto et al. 2018). From July to October,assemblages of the larger size fraction (> 10 µm) dominate
more in coastal regions than in the offshore regions (Kasai
et al. 2010; Kasai and Hirakawa 2015). Near the Shiretoko
Peninsula, in late spring to early summer and in late summer to early autumn, the proportion of the larger fraction(> 10 µm) is high on the western side of the peninsula,whereas the proportion of the smaller fraction (< 2 µm)
is high on the eastern side (Shiomoto 2011). In terms ofspecies composition of nano- and micro-protists, harmful
dinoflagellates and diatoms are described in the Okhotsk
Sea (e.g., Shimada et al. 2010; Kasai and Hirakawa 2015).
Many investigations have been performed on Alexandriumtamarense, among the harmful dinoflagellates, as it causes
the accumulation of paralytic shellfish toxin (PST) (e.g.,
Shimada et al. 2010; Shimada 2017). In diatoms, changes
in species composition between warm and cold waters havebeen revealed (Kasai and Hirakawa 2015). Spring diatom
blooms are composed mainly of Thalassiosira nordenskioeldii, Thalassiosira gravida, Thalassiosira eccentrica, andChaetoceros subsecundus in coastal surface water (Sorokin
and Sorokin 1999). In the winter season Odontella aurita
and Thalassiosira spp. are dominant species (McMinn et al.
2008). However, in contrast to the availability of information
about Chl. a and primary productivity, the species composition of nano- and micro-protists during summer is not welldescribed, and spatial variation in nano- and micro-protists,
which are also important as primary producers, is not wellunderstood.
In the present study we investigated the spatial distribution of nano- and micro-protist communities off Wakkanai

Fig. 1  Locations of the stationsin the southern Okhotsk Sea
from 27 June to 1 July 2019.
The numbers indicate station ID

13

Y. Hamao et al.

and around the Shiretoko Peninsula during summer to clarify
their relationship with the hydrological environment. In particular, we focused on the CWB off Wakkanai and a comparison of the east and west sides of the Shiretoko Peninsula, which possess the characteristic features of this area, toevaluate the effects of physical and chemical features of theenvironment on the nano- and micro-protist communities.

2 Materials and methods
Conductivity–Temperature–Depth (CTD; Sea-Bird SBE911 plus) casts were conducted at 28 stations (located at
44–46° N, 142–146° E) in the southern part of the Okhotsk
Sea off Hokkaido between June 27 and July 1, 2019, on
T/S Oshoro-Maru (Fig. 1, Supplementary Table 1). Linetransects were established along each of the eight sets ofstations; we termed the transects L1–L8 in sequence fromthe northwestern to the southeastern parts (Fig. 2a). Water
samples (1 L) were collected from the surface and the layerof maximum Chl. a (range of 10–35 m in depth) using aplastic bucket and Niskin rosette water sampler mounted on
the CTD frame. The samples were fixed with 1% glutaraldehyde immediately. Seawater samples for measurement ofChl. a and nutrient contents were collected from the samelayers as the fixed samples. Water samples (250 mL) were
filtered using Whatman GF/F filters for the measurement ofChl. a. After filtration, the filters were immersed in 6 mL
of N,N-dimethylformamide under dark conditions for 24 h,and then, Chl. a concentration was measured using a fluorometer (10-AU, Tuner Designs, Inc.) (Holm-Hansen et al.
1965). The water samples for nutrient analyses were frozen and stored at − 80 °C onboard. Nutrient concentrations(nitrate, nitrite, phosphate, and silicate) were measured usingSpatial distribution of the protist community in the southern part of the Okhotsk Sea off Hokkaido…

91

Fig. 2  Eight transects (L1–L8) in the southern Okhotsk Sea from June 27 to July 1, 2019. a Cross-sectional distribution of temperature, salinity,and fluorescence at each transect. b Solid triangles at the bottom indicate sampling stations at each transect

an autoanalyzer (QuAAtro 2HR, BL-TEC) in a land-basedlaboratory.
In the land-based laboratory the fixed samples (1 L)
were stored on a flat table for more than 1 day to allow
the phytoplankton cells to settle at the bottom of the bottle. Afterwards, the samples were concentrated to 20 mL
using a siphon. Subsamples (500 µL) of the concentrated
sample were mounted on a glass slide using a micropipette. Phytoplankton cells were identified and enumeratedusing an inverted microscope (Eclipse-TE200, Nikon) at
40–600 × magnification. Diatoms, dinoflagellates, andsilicoflagellates were identified to the lowest possible levels (species or genus) according to the methods proposed
by Hasle and Syvertsen (1997), Horner (2002), and Karen(1997). Tintinnid ciliates and oligotrich ciliates were counted
separately. Nanoplankton (size range was 4.8–12.6 µm) werealso counted, but their cell densities might have been underestimated due to their small size. All of the nanoflagellates
were classified as autotrophic/mixotrophic/heterotrophic,
because we did not use epifluorescence microscopy to check
whether the cells had chloroplasts. Cell counting was performed with a maximum of 300 cells per subsample. After

13

92

Y. Hamao et al.

counting, we calculated the abundances of each taxon in thesample (cells ­L−1).
The abundances (X: cells L­ −1) for all of the taxa were
1/4
fourth root transformed (X ) prior to the statistical analyses
to reduce any bias in the estimations of abundance (Quinn
and Keough 2002). Similarities between the nano- andmicro-protist samples were calculated using the Bray–Curtis
similarity index, and then, a dendrogram was created using
the unweighted pair group method using the arithmetic mean
(UPGMA). Nonmetric multidimensional scaling (NMDS)
ordination was performed to distribute the groups onto a
two-dimensional map. Pearson regression analyses were
performed using environmental data (temperature; salinity;
sampling depth; fluorescence; and Chl. a, nitrate + nitrite,
phosphate, and silicate concentrations) as dependent variables to clarify the significance of their relationships to thecluster groups. Cluster analysis and NMDS were performed
using PRIMERv7 (PRIMER-E Ltd.). Intergroup differences
between each of nano- and micro-protists (abundance ofeach species or taxon) and hydrography data were tested
using one-way ANOVA and Tukey–Kramer post hoc test
using Statview (SAS Institute Inc.).

3 Results
3.1 Spatial distribution in hydrography
Along L2, high temperatures (> 10 °C) were observed at
depths of < 20 m on the inshore side, whereas low temperatures (2 °C) were observed at depths of > 20 m on theoffshore side (Fig. 2). Similarly, cold water (< 2 °C) was
distributed on the offshore sides of L3, L4, and L5. However, at L6–L8 located on the eastern side of the Shiretoko
Peninsula, warm water above 10 °C was generally observed
at depths of < 20 m, but no cold water prevailed. Salinity
showed a similar distribution to temperature (Fig. 2). On
the coastal side of L2, high salinity (33.6–34.2) was foundthroughout the water column. Low salinity (32.2–32.4)
was observed at a depth of < 30 m on the offshore side.
The cold water in L3–L5 showed slightly low salinity at
approximately 32.4. At L6–L8, salinity was higher than thaton the western side (L3–L5). Fluorescence was extremely
low through the study area, but relatively high values wereobserved at depths of 0–30 m in L2–L4, especially at thesurface in the middle area along L2.
The CWB was indicated by observations of the horizontal
distribution of temperature being 2–3 °C lower (observed inpatches at the surface) compared to surrounding water; theCWB was observed at Stations S8 and S15 off Wakkanai,
which demonstrated high Chl. a concentration (Fig. 3). The
CWB separated warm and saline water in the inshore areafrom cold and fresh water in the offshore area. In addition,

13

Fig. 3  Horizontal distribution of chlorophyll a and nutrients at thesurface (left panel) and chlorophyll a maximum depth (right panel) in
the southern Okhotsk Sea from June 27 to July 1, 2019highly saline water was distributed off Wakkanai and aroundthe Shiretoko Peninsula. The concentrations of nitrate andnitrite ranged from 0.11 to 11.64 µmol ­L−1 at the surface
and in the maximum Chl. a layer (Fig. 3). Almost all surface water showed low concentrations of nitrate and nitrite
compared to the maximum Chl. a layer. In the maximum
Chl. a layer, the highest concentrations of nitrate and nitrite
were observed at Station B3 in the western side of theSpatial distribution of the protist community in the southern part of the Okhotsk Sea off Hokkaido…

93

Shiretoko Peninsula. By contrast, extremely low concentrations of nitrate and nitrite (close to zero) were observedin the coastal areas off Wakkanai and on the central andeastern sides of the Shiretoko Peninsula. Phosphate and silicate concentrations ranged from 0.09 to 1.37 µmol ­L−1 and1.34 to 20.14 µmol ­L−1, respectively, and exhibited horizontal patterns similar to those of nitrate and nitrite. Chl.
a ranged from 0.53 to 7.63 µg ­L−1 and showed a similar
distribution between the surface and the maximum Chl. alayer; the highest value was observed at Station S15 in the
CWB off Wakkanai, and relatively high concentrations wereobserved in the western and central areas of offshore Shiretoko Peninsula.

3.2 Spatial distribution of the protists
Diatoms and nanoflagellates were dominant among theprotists throughout the study area in terms of cell density­ −1) of diatoms(Fig. 4). High abundances (> 5.0 × ­104 cells L
were observed in the CWB (at Stations S8 and S15). Aroundthe Shiretoko Peninsula, diatom abundance was high on thewestern side and central areas (> 5.0 × ­104 cells L­ −1) andlow on the eastern side in both 0 m and maximum Chl. alayers. The total abundance of dinoflagellates ranged from
3.3 × ­102 to 6.25 × ­104 cells ­L−1, and their distribution pattern contrasted to the distribution of diatoms. Off Wakkanai, the abundance of dinoflagellates was higher offshore(> 1.0 × ­104 cells ­L−1) and lower near the coast. Around theShiretoko Peninsula, their abundance was low on the western side and high (> 1.0 × ­104 cells ­L−1) on the central andeastern sides. The total ciliate abundance was lower thanthose of diatoms and dinoflagellates, ranging from 2.45 × ­102
to 8.93 × ­103 cells ­L−1, and distributed at similar densities
throughout the study area. The nanoflagellate abundance
ranged from 3.27 × ­103 to 8.93 × ­104 cells ­L−1 and nanoflagellates was the most dominant taxon at several stations. Highabundances, above 1.0 × ­104 cells ­L−1, were observed in bothlayers in most areas but were slightly lower at Stations S7and S17 off Wakkanai.

3.3 Relationship between protist communityand hydrography
In this study, 42 species of diatoms in 19 genera, 7 genera of dinoflagellates, tintinnid ciliates, oligotrich ciliates,and 1 genus of silicoflagellates were observed (Table 1).
The cluster analysis using abundance separated the protist communities into six groups (A–F) at 57% similarity(Fig. 5a). Group A, distributed off Wakkanai (Fig. 5c), had
the lowest total abundance (3.86 × ­104 cells ­L−1) and was
dominated by dinoflagellates (Fig. 5b), especially Alexandrium spp. (Table 1). Group B was mostly distributed offshore around the Shiretoko Peninsula and was dominated by

Fig. 4  Horizontal distributions of diatoms, dinoflagellates, and nanoflagellates in abundance at the surface (left panel) and chlorophyll amaximum depth (right panel) in the southern Okhotsk Sea from June
27 to July 1, 2019

nanoflagellates. Dictyocha spp. and unidentified dinoflagellates were the characteristic species in this group. GroupC occurred locally near the Shiretoko Peninsula and was
dominated by nanoflagellates with no characteristic species. Group D, distributed mostly on the surface from L3
to around the Shiretoko Peninsula, was characterized byabundant nanoflagellates and low abundances in several species of Chaetoceros spp. Group E was locally distributed
off Wakkanai and was dominated by diatoms. Some speciesof Chaetoceros spp., Pseudonitzschia pseudodelicatissima
and Pleurosigma normanii were characteristic of this group(Table 1). Group F, which occurred widely from off Wakkanai to the eastern side of the Shiretoko Peninsula, showed the

13

13

Diatoms
Asterionellopsis glacialis
Asterionellopsis kariana
Bacteriastrum delicatulum
Bacteriastrum furcatum
Bacteriastrum hyalinum
Chaetoceros affinis
Chaetoceros atlanticus
Chaetoceros concavicornis
Chaetoceros convolutus
Chaetoceros criophilus
Chaetoceros curvisetus
Chaetoceros danicus
Chaetoceros debilis
Chaetoceros decipiens
Chaetoceros diadema
Chaetoceros didymus
Chaetoceros furcellatus
Chaetoceros laciniosus
Chaetoceros radicans
Chaetoceros socialis
Chaetoceros teres
Corethron hystrix
Coscinodiscus centralis
Cylindrotheca closterium
Dactyliosolen fragilissimus
Eucampia zodiacus
Fragilariopsis atlantica
Fragilariopsis pseudonana
Guinardia flaccida
Leptocylindrus danicus
Leptocylindrus minimus
Merosira arctica
Navicula spp.
Paralia sulcata
Pleurosigma normanii

Species

0
0
0
0
29
0
0
0
2288
0
0
0
0
29
0
0
0
147
0
0
0
40
0
137
55
0
0
0
0
0
93
0
16
0
0

A (5)
0
0
0
0
0
123
0
290
979
0
0
0
0
0
0
0
0
0
0
0
0
1
0
5
0
0
0
22
0
222
65
0
0
4
5

B (11)
0
0
0
0
0
143
0
143
0
0
0
106
0
0
573
0
0
96
0
0
0
153
0
354
141
0
0
106
0
430
0
0
176
0
0

C (2)

Mean abundance (cell ­L−1)

0
0
0
0
0
1834
22
266
2419
0
0
36
0
47
259
6
201
2527
184
2139
0
943
0
82
141
15
0
0
0
519
0
0
0
0
3

D (14)
00000
12,569
0
0
0
0
0
858
1348
245
1151
0
5584
6154
0
25,105
0
0
0
819
0
0
0
0
0
3599
0
0
0
0
490

E (2)
384
14
62
6
0
11,577
0
212
1538
5
12
629
1738
1000
1269
941
1146
12,623905
16,345
273
992
28
1096
1605
249
5
84
135
5506
1595
142
24
0
81

F (21)
NS
NS
NS
NS
NS
***
NS
NS
NS
NS
NS
**
**
NS
*
**
***
***
NS
***
NS
NS
NS
*
*
NS
NS
NS
NS
***
NS
NS
NS
NS
***

One-way
ANOVA

B2

Not detected

Not detected

D2

B2

C2

D2

B2

C1, 2
C2

D2

D
D2

A1, 2
A2

A2

C1, 2

D2

2

D2

C2

D2

C2

B2

Tukey–Kramer test

F2

E1, 2

E1, 2
D2

F1

D2
F2
E1, 2

F1

F1

Table 1  Comparison of nano- and micro-protist abundance among the groups identified using cluster analysis (cf. Fig. 5) in the southern Okhotsk Sea from June 27 to July 1, 2019

E1

F1

F1
F1

E1

F1
E1
F1

E1

E1

94
Y. Hamao et al.

0
0
0
0
347
0
0
0
2064
134
21
34
8
0
24
103
19,97820,302
1390
1916
3306
992
52,596
79,261

11,057
0
0
152
1024
14
0
4820
17,066
515
3810
4325
224
14,072
38,562

B (11)

0
14
0
0
28
0
0
0
2875

A (5)

863
2001
2864
20
45,20863,437

126
0
0
0
96
0
0
9980
10,201

50
0
525
0
2068
78
0
0
5143

C (2)

Mean abundance (cell ­L−1)

1121
1600
2721
226
44,07464,495

373
50
13
39
197
104
0
4424
5202

278
42
0
0
304
0
0
4
12,271

D (14)

328
0
328
0
11,928113,522

18,663
0
0
0
1234
0
0
0
19,8971234
21,391
0
0
0
0
823
0
81,368

E (2)

1156
1755
2910
27
23,735
119,314

6523
22
0
133
348
340
150
9381
16,896

7321
1867
254
46
3488
0
215
335
75,744F (21)

NS
*
NS
**
***
**

***
NS
NS
NS
**
*
NS
*
NS

*
***
NS
NS
***
***
NS
NS
***

One-way
ANOVA

E2
A2

F2
F2
D2

C2
A2
C2

A1, 2

D2

F1, 2

D2
A2

C2

D2

D2

B2

C2

C2

D2

A2

A2

A2

B2

Not detected

Not detected

Tukey–Kramer test

A2
D1
B1, 2

B1, 2

C1, 2

F1, 2
B2

F1, 2

D2

B2

D2

D2
C1
E1, 2

C1, 2

F1,2

A1
D1, 2

A1

F1

C1, 2

F2

B1
B1
F1

A1

B1

E1
F1

E1

E1

F1

E1

***: p < 0.001**: p < 0.01*: p < 0.05

NS: not significant

Differences among the groups were tested using one-way ANOVA and post hoc Tukey–Kramer tests. Any groups not connected by the superscript numbers on the groups were significantly different (p < 0.05). Values are means. Numbers in parentheses are the number of stations belonging to each group

Pseudo-nitzschia fraudulenta
Pseudo-nitzschia pseudodelicatissima
Pseudo-nitzschia pungens
Skeletonema costatum
Thalassionema nitzschioides
Thalassiosira eccentrica
Thalassiosira nordenskioeldii
Thalassiosira pacifica
Total diatoms
Dinoflagellates
Alexandrium spp.
Ceratium fusus
Ceratium spp.
Dinophysis spp.
Prorocentrum spp.
Protoperidinium spp.
Pyrophacus spp.
Unidentified dinoflagellates
Total dinoflagellates
Ciliates
Tintinnid ciliates
Oligotrich ciliates
Total Ciliates
Dictyocha spp.
Nanoflagellates
Total

Species

Table 1  (continued)

Spatial distribution of the protist community in the southern part of the Okhotsk Sea off Hokkaido…
95

13

96

Y. Hamao et al.

Fig. 5  Results of cluster analysis based on nano- and micro-protist
abundance in the southern Okhotsk Sea from June 27 to July 1, 2019.
a Six groups (A–F) were identified using the Bray–Curtis similarity
connected with UPGMA. Mean abundance and species/taxon composition of each group b Horizontal distribution of the six groups in thesouthern Okhotsk Sea during June 27 and July 1, 2019. c Nonmetricmultidimensional scaling plots of the six groups. d The direction andlength of the lines indicate the relationship to groups and the strengthof the relationship. Dep.: depth, Flu.: fluorescence, Chl. : chlorophylla, Sal.: salinity.

highest abundance (1.19 × ­105 cells ­L−1) and was dominatedby diatoms. Chaetoceros spp., Dactyliosolen fragilissimus,
and Leptocylindrus dannicus were characteristic species ofdiatoms in this group.
Each group was clearly separated in the NMDS plot
(Fig. 5d). The environmental parameters that showed significant relationships (p < 0.05) with the protist community
included sampling depth; salinity; fluorescence; and Chl.
a, silicate, phosphate, and nitrate + nitrite concentrations(Fig. 5d). One-way ANOVA showed that the environmental factors that differed among the groups were salinity andnitrate + nitrite and silicate concentrations (Table 2). Salinity

and nutrient concentrations showed opposite trends. Groups
C and F had high salinity and low nutrient concentrations,whereas groups B, D, and E had low salinity and high nutrient concentrations.
The protist groups are shown on a T–S diagram to
depict the relationships between the protist communities
and water mass categories (Fig. 6). We categorized thewater mass off Wakkanai and around the Shiretoko Peninsula during summer as SWC (temperature = 7–20 °C,
salinity = 33.6–34.3), FSOSW (temperature ≤ 18 °C,
salinity ≤ 32.5) or ICW (temperature = − 1.8 to 2 °C,
salinity = 32.8–33.4) according to Takizawa (1982).

13

8.2 ± 2.49
32.2 ± 0.11
1.2 ± 0.34
3.3 ± 2.08
0.36 ± 0.24
5.7 ± 2.08

A (5)GroupsC (2)9.2 ± 0.31
33.2 ± 0.05
1.1 ± 0.32
1.7 ± 1.40
0.37 ± 0.11
4.7 ± 1.77

B (11)
8.4 ± 2.80
32.7 ± 0.36
1.5 ± 0.47
4.4 ± 3.48
0.46 ± 0.28
10.2 ± 6.40E (2)6.9 ± 0.78
33.1 ± 0.27
3.4 ± 1.03
5.4 ± 2.59
0.5 ± 0.14
9.2 ± 1.90

D (14)
9.2 ± 2.99
32.6 ± 0.27
1.3 ± 0.41
8.7 ± 2.45
0.44 ± 0.42
8.4 ± 1.81

10.3 ± 2.35
33.3 ± 0.43
2.5 ± 2.01
1.1 ± 1.86
0.21 ± 0.19
3.5 ± 0.58

F (21)
NS
***
*
***
NS
*

One-way
ANOVA

A1, 2

F2

D1,2

B1, 2

A1,2

C1, 2

E2, 3

B2, 3

D2, 3
A3
Not detected
F2
C2

Tukey–Kramer test

E1, 2

E1, 2

C1, 2

B1

D1

F1

***: p < 0.001**: p < 0.01*: p < 0.05

NS: not significant

Differences among the groups were tested using one-way ANOVA and post hoc Tukey–Kramer tests. Any groups not connected by the superscript numbers on the groups were significantly different (p < 0.05). Values are means (± SD). Numbers in parentheses are the number of stations belonging to each group

Temperature (°C)
Salinity
Chl. a (µg L­ −1)
NO2 + ­NO3 (µmol ­L−1)
PO4 (µmol ­L−1)
SiO2 (µmol ­L−1)

Parameters

Table 2  Comparison of the environmental factors among the groups identified using cluster analysis (cf. Fig. 5) in the southern Okhotsk Sea from June 27 to July 1, 2019.

Spatial distribution of the protist community in the southern part of the Okhotsk Sea off Hokkaido…
97

Fig. 6  Temperature–salinity diagram showing the six groups identified from the Bray–Curtis similarity (cf. Figure 5a) around the Wakkanai (a) and Shiretoko Peninsulas (b) (cf. Figure 1) in the southern
Okhotsk Sea from June 27 to July 1, 2019. Circle and triangle frames
for each symbol indicate the water was collected from the surface orchlorophyll a maximum depth, respectively. SCW: Soya Warm Current, FSOSW: Fresh Surface Okhotsk Sea Water, ICW: Intermediate
Cold Water (cf. Takizawa et al. 1982).

Off Wakkanai, Group A plotted below 32.5 on salinity,
which indicated that the group was strongly influenced
by FSOSW. Group F was under the SWC with a salinity
markedly above 33.6, except at a few stations (Fig. 6a).
Group E, locally distributed off Wakkanai, was plottedin the middle of groups A and F. Around the Shiretoko
Peninsula, some of groups B and D were plotted below
32.5 on salinity and categorized in the FSOSW (Fig. 6b).
However, most of groups C and F were in mixed temperatures between 8 and 14 °C and mixed salinities between
32.5 and 33.6.

13

98

4 Discussion
Through the sampling region, nano- and micro-protist communities were roughly divided into two groups (Fig. 5a andb) with clear spatial patterns; a diatom-dominated group(including group E and F) in coastal areas and a nanoflagellates-dominated group (including groups A, B, C and D)
in offshore areas (Fig. 5c). The diatom-dominated groupmainly occurred in the water mass categorized as SWC
and some occurred in the mixed area between SWC andFSOSW, whereas the nanoflagellates-dominated group was
mainly seen in FSOSW (Fig. 6). This division was strongly
related to several hydrographic features (Fig. 5d). The diatom-dominated group was located at high Chl. a and low
nutrient concentration (NMDS plot, Fig. 5d) suggesting thatdiatoms were able to grow in the season previous to oursampling using the nutrients then available. By contrast,
the nanoflagellates-dominated group occurred alongside
relatively high nutrients (Table 2 and Fig. 5d), but where
the large-sized phytoplankton abundance was low. This
mis-matched relationship may be due to low iron concentration in the offshore waters (Kudo et al. 2011; Kuma et al.
2014). According to Kudo et al. (2011), iron concentration
is low in the FSOSW in this area during summer. In addition, compared to large phytoplankton, small phytoplankton
are more able to grow under low iron concentrations due totheir low iron requirement (Brand et al. 1983; Sunda andHuntsman 1995; Timmermans et al. 2001). Thus, the difference in iron requirement of nanoflagellates and diatoms
could induce the dominance of nanoflagellates in offshore
areas, where iron concentrations are low (Kudo et al. 2011;
Kuma et al. 2014). In the southern part of the Okhotsk Sea
off Hokkaido during summer we propose that the differenthydrographic features of the water masses (including theirnutrients and iron concentrations) influence the distributionof protist communities.

4.1 Effect of the CWB on the protist community
off Wakkanai
Off Wakkanai, the protist communities were clearly
divided between coastal and offshore regions; groups E
and F with high diatom abundance were observed along
the coast, whereas group A with the lowest abundance
was seen offshore (Fig. 5b and c). The CWB off Wakkanai is colder and with a higher Chl. a distribution during
summer (Ishizu et al. 2006; Mitsudera et al. 2011). GroupE occurred in waters that were slightly cooler and withhigher Chl. a than those, where groups A and F occurred(Table 2 and Fig. 6), so group E in this study could be
categorized as a CWB community.

13

Y. Hamao et al.

There are currently two hypotheses regarding the mechanism of formation of the CWB: one is that the cold water atthe bottom in the SWC is continuously vertically advectedto the surface (Ishizu et al. 2006), and the other is that thecold water at the bottom near the Soya Strait upwells andthen horizontally advects off Wakkanai (Iida et al. 2018;
Mitsudera et al. 2011). In the former case, the upwellingcold water is considered to be the ICW that exists below the
SWC and FSOSW during the summer. According to Ishizu
et al. (2008), the water in the CWB is not composed of a
specific water mass and is described as consisting of a mixture of SWC, FSOSW, and ICW. By contrast, in the latter
case, the upwelling cold water is considered to be Japan Sea
Intermediate Water (JSIW). Although the physical processes
involved in the formation of the CWB are still under discussion, both the ICW and the JSIW have been reported to havehigh nutrient concentrations (Kuma et al. 2014; Maita andToya 1986). In this study, higher nitrate, phosphate, and silicate concentrations were observed in Station S8 than thoseobserved for the surrounding waters (Fig. 3); this may havebeen caused by upwelling from the bottom layer.
Iron plays an important role in the metabolic processesof growth, photosynthesis, respiration, and nitrogen assimilation in marine phytoplankton (Behrenfeld and Milligan
2013; Twining and Baines 2013). Iron-limitation of phytoplankton growth would be expected to occur in coastal
upwelling areas (Aguilar-Islas et al. 2007; Bruland et al.
2001; Gerringa et al. 2012; Hutchins et al. 2002; Sedwick
et al. 2011). In this study area, dissolved iron concentrations
are low in the FSOSW (< 1 nM) and ICW (0.5 nM) but high(> 1 nM) in the SWC during spring and summer, becauseof the supply of iron from the Asian continent via atmospheric dust and from rivers (Kudo et al. 2011; Kuma et al.
2014). Kudo et al. (2011) conducted incubation experiments
to investigate the effects of these nutrients and iron concentrations on phytoplankton in the SWC, FSOSW, and ICW.
They reported that large phytoplankton (> 10 µm, composed
mainly of diatoms) dominated in the case of mixed ICW and
SCW. Thus, the mixture of low nutrient, high iron concentration SWC and high nutrient, low iron concentration ICW
provided suitable conditions for diatom growth, eventuallyforming the diatom-dominated group E.
As mentioned above, the CWB provides suitable conditions for diatom growth; thus, not only primary production
but also production at higher trophic levels can be increasedin the CWB (Mustapha et al. 2009). Group F showed highdiversity in species composition, whereas group E wasdominated by some harmful species. In particular, Pseudonitzschia pseudodelicatissima and Alexandrium spp. were,respectively, 11 and 2.9 times higher in group E than ingroup F. Since P. pseudodelicatissima has domoic acid incells (Bates et al. 2018; Moschandreou et al. 2010), the grazing activity of copepods ceases for a while after grazing on it

99

Spatial distribution of the protist community in the southern part of the Okhotsk Sea off Hokkaido…

(Tammikehto et al. 2012), which means that primary production cannot reach higher trophic levels. As an example of thenegative impact, more than 100 sea birds (brown pelicans)died in 5 days, because they fed on mackerel contaminatedby domoic acid-producing diatoms (Pseudonitzschia spp.)(Beltran et al. 1997). In addition, Alexandrium spp., including A. tamarense and A. catenella, cause paralytic shellfish
toxin accumulation (PST). Therefore, the water in the CWB
has potentially negative impacts on both marine ecosystems
and fishery resources both directly and indirectly.

cysts (5–21 °C) (Anderson 1998; Shimada 2017). Moreover,A. tamarense blooms near the CWB originate from Aniva
Bay on southern Sakhalin Island (Orlova et al. 2004) andare transported along the front with the SWC (Oguma et al.
2011). Although we did not identify Alexandrium spp. to thespecies level in this study, the abundance and distributionof Alexandrium spp. were similar to those reported earlier,which suggests that the high temperature of the bottom layer
due to the warm SWC and no nutrient limitation may facilitate the growth of Alexandrium spp.

4.2 High abundance distribution of Alexandrium
spp. off Wakkanai

4.3 Influence of the SWC on the protist community
around Shiretoko Peninsula

The abundance of Alexandrium spp. was high in groups A
and E off Wakkanai (Table 1). As mentioned above, group
E was categorized as a CWB community, whereas group A
had the lowest salinity and belonged to FSOSW. The area
off Wakkanai is known as one of the important scallop fishing grounds of the world (Kosaka and Ito 2006), but PST
occurs approximately once every 4 years and causes economic losses (Shimada et al. 2012). Due to the importanceof A. tamarense as a cause of PST, it has been the subjectof many investigations (e.g., by Shimada et al. (2010) andShimada (2017)). The abundance of A. tamarense in late
July was highest in the FSOSW, and A. tamarense blooms
existed slightly offshore of the CWB (Miyazono and Minoda
1990; Shimada et al. 2012; Shimada 2017). These previously reported distribution patterns were similar to thoseseen herein. The growth of A. tamarense vegetative cells is
limited when < 0.12 µM phosphate is available (Yamamoto
and Tarutani 1999). A. tamarense is thought to be able toutilize nutrients in the lower layers by diel vertical migration(Lewis et al. 2006; MacIntyre et al. 1997). Lack of nutrientswas unlikely to restrict the growth of A. tamarense in theregions, where groups A and E were observed, since phosphate concentrations were above 0.12 µM at the surface andat the layer of maximum Chl. a.
Alexandrium tamarense is known to have a life cycle inwhich the cysts (hypnozygotes, formed by sexual reproduction) germinate and produce planomeiocytes. Then, nursing
cells asexually divide and proliferate by meiosis. Again, they
produce motile planozygotes by sexual reproduction, andthey become settling cysts and are dormant in the sediment(Ishikawa and Imai 2011; Shimada 2017). In the surface
water of the Okhotsk Sea during summer, A. tamarense
increases in “warm years” when the SWC is dominant anddecreases in “cold years” when the ICW is dominant. Thisis because the cysts in the bottom sediment may germinate
during the dominance of the SWC (Shimada 2017). Since
the warm SWC flows as a submerged current near the bottom
during spring (Aota 1975), the bottom temperature increases
to the optimal temperature for germination of A. tamarense

In the Okhotsk Sea facing northeastern Hokkaido the warmand saline SWC flows along the coast, and the distribution of
the SWC expands with increasing current speed from spring
to summer (Aota 1975; Takizawa 1982). Numerical models
have shown that the SWC on the north side of the Shiretoko Peninsula does not directly enter the east side of thepeninsula (Mori et al. 2010) and that the flow path changes
depending on the wind direction (Sato et al. 2006). In addition, based on field observations, the SWC flows into thesubsurface layer (above 150 m depth, Oguma et al. 2007) on
the east side of the peninsula (Sato et al. 2006). The diatomdominated group F was widely distributed along the coastal
area off Wakkanai and the eastern Shiretoko Peninsula in thisstudy, in a distribution similar to that of the SWC (Fig. 5c).
Specifically, group F was concentrated near the coast in thewestern and central areas of the Shiretoko Peninsula, and it
occurred in the Chl. a maximum layer on the east side of thepeninsula (Fig. 5c). Thus, since the distribution of group F
coincided with the route of the SWC (Mori et al. 2010; Satoet al. 2006; Takizawa 1982), group F could be defined as an
SWC community in this study.

4.4 East–west comparison across the Shiretoko
Peninsula
In this study, differences in the protist community wereobserved between the eastern and western sides of the Shiretoko Peninsula; the diatom-dominated group F was seen on thewestern side, but on the eastern side, nanoflagellate-dominatedgroups B and D also occurred along with group F. As an example, a more detailed comparison at Station B1 (on the westside of the peninsula) shows that the contributions of diatoms
and nanoflagellates to total abundance were 56% and 21%,respectively. By contrast, at Station F1 on the east side of thepeninsula, the contributions of diatoms and nanoflagellates
to total abundance were 0.9% and 82%, respectively. Even
though our study did not report on < 2 µm phytoplankton, theeast–west differences that we observed in the protist community were consistent with those of a previous study (Shiomoto

13

100

2011). According to Shiomoto (2011), since there was no clear
difference in nutrient concentrations on either side of the peninsula, the west–east difference is potentially due to the strong
predation pressure by zooplankton on large phytoplankton
and weak predation pressure on small phytoplankton on theeastern side. However, in the current study we observed thatthe amounts of nutrients on the eastern side were significantly
lower than those on the western side, especially in the zone ofChl. a maximum, which differs from the results of the previous study. This distribution of nutrients can be explained by
the presence of the high-nutrient ICW (Maita and Toya 1986),
observed at > 20 m depth in the western (L4) part of the peninsula. In general, it is well known that large phytoplankton
require higher nutrient concentrations for growth than small
phytoplankton (Sarthou et al. 2005). Therefore, we proposethat the east–west difference in nutrient concentrations induced
the differences in the protist community that we observed inthis study.

5 Conclusion
There were distinct patterns in the spatial distribution of the
protist community in the southern part of the Okhotsk Sea
off Hokkaido. Diatoms dominated in the coastal areas, where
the SWC was dominant or near the upwelling that develops
the CWB. By contrast, nanoflagellates were dominant in theoffshore areas. The toxic dinoflagellate Alexandrium spp.
was abundant in the CWB and FSOSW. Thus, the distribution of the protists was largely governed by the different
water masses in the southern part of the Okhotsk Sea off
Hokkaido during summer.
Supplementary Information The online version contains supplementary material available at https://d​ oi.o​ rg/1​ 0.1​ 007/s​ 10872-0​ 21-0​ 0630-5.
Acknowledgements We thank the captain, officers, crew, and researchers on board the T/S Oshoro-Maru, Hokkaido University, for theirgreat efforts during the field sampling. We would like to thank Dr.
Tomohiro Nakamura for providing valuable comments about thephysical background of the Okhotsk Sea. This research was supportedby the Environment Research and Technology Development Fund(JPMEERF20214002) of the Environmental Restoration and Conservation Agency of Japan. This work was also conducted by the Arctic
Challenge for Sustainability II (ArCS II) project (JPMXD1420318865).
In addition, this work was partly supported by Grants-in-Aid for Challenging Research (Pioneering) JP20K20573 and Scientific Research
JP20H03054 (B), JP19H03037 (B), JP21H02263 (B), JP21H03583 (B)and JP17H01483 (A) from the Japanese Society for the Promotion of
Science (JSPS).
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as longas you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
were made. The images or other third party material in this article areincluded in the article's Creative Commons licence, unless indicated13

Y. Hamao et al.
otherwise in a credit line to the material. If material is not included in
the article's Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you willneed to obtain permission directly from the copyright holder. To view acopy of this licence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

References
Aguilar-Islas AM, Hurst MP, Buck KN, Sohst B, Smith GJ, Lohan
MC, Bruland KW (2007) Micro- and macronutrients in the southeastern Bering Sea: insight into iron-replete and iron-depleted
regimes. Prog Oceanogr 73:99–126
Anderson DM (1998) Physiology and bloom dynamics of toxic Alexandrium species, with emphasis on life cycle transitions. NATOASI Ser G41:30–47
Aota M (1975) Studies on the Soya Warm current. Low Temp Sci
A33:151–172 (In Japanese with English summary)Bates SS, Hubbard KA, Lundholm N, Montresor M, Leaw CP (2018)
Pseudo-nitzschia, Nitzschia, and domoic acid: new research since2011. Harmful Algae 79:3–43
Behrenfeld MJ, Milligan AJ (2013) Photophysiological expressions ofiron stress in phytoplankton. Annu Rev Mar Sci 5:217–246
Beltrán AS, Palafox-Uribe M, Grajales-Montiel J, Cruz-Villacorta A,
Ochoa JL (1997) Sea bird mortality at Cabo San Lucas, Mexico: evidence that toxic diatom blooms are spreading. Toxicon
35:447–453
Brand LE, Sunda WG, Guillard RRL (1983) Limitation of marine
phytoplankton reproductive rates by zinc, manganese, and iron.
Limnol Oceanogr 28:1182–1198
Bruland KW, Rue EL, Smith GJ (2001) Iron and macronutrients in
California coastal upwelling regimes: implications for diatom
blooms. Limnol Oceanogr 46:1661–1674
Gerringa LJA, Alderkamp AC, Laan P, Thuroczy CE, DeBaar HJW,
Mills MM, van Haren H, Arrigo KR (2012) Iron from melting
glaciers fuels the phytoplankton blooms in Amundsen Sea (Southern Ocean): iron biogeochemistry. Deep Sea Res II 71–76:16–31
Hasle GR, Syvertsen EE (1997) Marine diatoms. In: Tomas CR (ed)Identifying marine phytoplankton. Academic Press, San Diego,
pp 5–385
Holm-Hansen O, Lorenzen CJ, Holmes RW, Strickland JDH (1965)
Fluorometric determination of chlorophyll. ICES J Mar Sci
30:3–15
Horner RA (2002) A taxonomic guide to some common marine phytoplankton. Biopress Limited, England, p 195
Hutchins DA, Hare CE, Weaver RS, Zhang Y, Firme F, DiTullio GR,
Alm MB, Riseman SF, Maucher JM, Geesey ME, Trick CG,
Smith GJ, Rue EL, Conn J, Bruland KW (2002) Phytoplankton
iron limitation in the Humboldt current and Peru upwelling. Limnol Oceanogr 47:997–1011
Iida H, Isoda Y, Kobayashi N, Horio K (2018) Observations and modelexperiments of the diurnal eddy of the Cold Water Belt along theoffshore side of the Soya Warm Current. Oceanogr Jpn 27:155–
174 (In Japanese with English abstract)
Ishikawa A, Imai I (2011) Diversity of life cycles and life cycle strategies of microalgal flagellates. Bull Plankton Soc Jpn 58:60–64 (InJapanese with English abstract)
Ishizu M, Kitade Y, Matsuyama M (2006) Formation mechanism of theCold-Water Belt formed off the Soya Warm Current. J Oceanogr
62:457–471
Ishizu M, Kitade Y, Matsuyama M (2008) Characteristics of thecold-water belt formed off Soya Warm Current. J Geophys Res
113:C12010. https://​doi.​org/​10.​1029/​2008J​C0047​86

Spatial distribution of the protist community in the southern part of the Okhotsk Sea off Hokkaido…
Karen AS (1997) Marine Dinoflagellates. In: Tomas CR (ed) Identifying marine phytoplankton. Academic Press, San Diego, pp
387–584
Kasai H, Hirakawa K (2015) Seasonal changes of primary productionin the south-western Okhotsk Sea off Hokkaido, Japan during theice-free period. Plankton Benthos Res 10:178–186
Kasai H, Nakano Y, Ono T, Tsuda A (2010) Seasonal change of oceanographic conditions and chlorophyll a vertical distribution in thesouthwestern Okhotsk Sea during the non-iced season. J Oceanogr
66:13–26
Kasai H, Nagata R, Murai K, Katakura S, Tateyama K, Hamaoka S(2017) Seasonal change in oceanographic environments and theinfluence of interannual variation in the timing of sea-ice retreat
on chlorophyll a concentration in the coastal water of northeastern
Hokkaido along the Okhotsk Sea. Bull Coast Oceanogr 54:181–
192 (In Japanese with English summary)
Kazama K, Nishizawa B, Tsukamoto S, Gonzalez JE, Kazama MT,
Watanuki Y (2018) Male and female Black-tailed Gulls Larus
crassirostris feed on the same prey species but use different feeding habitats. J Ornithol 159:923–934
Kosaka Y, Ito H (2006) Chapter 22 Japan. In: Shumway SE, Parson
GJ (eds) Scallops: biology, ecology and aquaculture. Elsevier,
Amsterdam, pp 1093–1141
Kudo I, Aya F, Takata H, Kobayashi N (2011) Oceanographic structure and biological productivity in the coastal area of Okhotsk
Sea. Bull Coast Oceanogr 49:13–21 (In Japanese with English
abstract)
Kuma K, Sasayama R, Hioki N, Morita Y, Isoda Y, Hirawake T, Imai
K, Aramaki T, Nakamura T, Nishioka J, Ebuchi N (2014) Chemical evidence for the origin of the cold water belt along the northeastern coast of Hokkaido. J Oceanogr 70:377–387
Lewis NI, Xu W, Jericho SK, Kreuzer HJ, Jericho MH, Cembella AD(2006) Swimming speed of three species of Alexandrium (Dinophyceae) as determined by digital inline holography. Phycologia
45:61–70
MacIntyre JG, Cullen JJ, Cembella AD (1997) Vertical migration,
nutrition and toxicity in the dinoflagellate Alexandrium tamarense. Mar Ecol Prog Ser 148:201–216
Maita Y, Toya K (1986) Characteristics on the distribution and composition of nutrients in subarctic regions. Bull Jpn Soc Fish Oceanogr 50:105–113 (In Japanese with English abstract)
McMinn A, Hattori H, Hirawake T, Iwamoto A (2008) Preliminary
investigation of Okhotsk Sea ice algae; taxonomic compositionand photosynthetic activity. Polar Biol 31:1011–1015
Mitsudera H, Uchimoto K, Nakamura T (2011) Physical oceanographyof Soya Warm Current: mechanisms of the Cold Belt formation
off the Soya Warm Current. Bull Coast Oceanogr 49:3–12 (InJapanese with English abstract)
Miyazono A, Minoda T (1990) Regional distribution of dinoflagellate
population off the Coasts of Hokkaido in the Japan and Okhotsk
Seas from spring to autumn, 1983. J Oceanogr Soc Jpn 46:96–106
Mori F, Isoda Y, Abe S, Kobayashi N, Yahaba H, Isogai Y (2010)
Seasonal variations of surface flow patterns in the Nemuro Strait.
Oceanogr Jpn 19:89–110 (In Japanese with English abstract)
Moschandreou KK, Papaefthimiou D, Katikou P, Kalopesa E, Panou
A, Nikolaidis G (2010) Morphology, phylogeny and toxin analysisof Pseudo-nitzschia pseudodelicatissima (Bacillariophyceae) isolated from the Thermaikos Gulf, Greece. Phycologia 49:260–273
Mustapha MA, Saito S, Lihan T (2009) Satellite-measured seasonalvariations in primary production in the scallop-farming region of
the Okhotsk Sea. ICES J Mar Sci 66:1557–1569
Oguma S, Kawasaki Y, Azumaya T (2007) Water mass variation process in the Nemuro Strait during spring and autumn. Oceanogr Jpn
16:361–374 (In Japanese with English abstract)
Oguma S, Ono T, Watanabe YW, Kasai H, Watanabe S, Nomura D,
Mitsudera H (2011) Flux of low salinity water from Aniva Bay

101(Sakhalin Island) to the southern Okhotsk Sea. Est Coast Shelf
Sci 91:24–32
Orlova TY, Selina MS, Stonik IV (2004) Species structure of plankton
microalgae on the coast of the Sea of Okhotsk on Sakhalin Island.
Russ J Mar Biol 30:77–86
Quinn GP, Keough MJ (2002) Experimental design and date analysis
for biologists. Cambridge University Press, New York, p 557
Sakurai Y (2011) Conservation of biodiversity and sustainable fisheries
in the coastal marine ecosystems of the Shiretoko World Natural
Heritage Site. Bull Coast Oceanogr 48:139–147 (In Japanesewith English abstract)
Sarthou G, Timmermans KR, Blain S, Treguer P (2005) Growth
physiology and fate of diatoms in the ocean: a review. J Sea Res
53:25–42
Sato M, Kono T, Kawasaki Y (2006) Inflow of Soya Warm Current
induced by wind in the northern Nemuro Strait in summer. Oceanogr Jpn 16:455–470 (In Japanese with English abstract)
Sedwick PN, Marsay CM, Sohst BM, Aguilar-Islas AM, Lohan MC,
Long MC, Arrigo KR, Dunbar RB, Saito MA, Smith WO, DiTullio GR (2011) Early season depletion of dissolved iron in theRoss Sea polynya: implications for iron dynamics on the Antartic
continental shelf. J Geophys Res 116:C12019. https://​doi.​org/​10.​1029/​2010J​C0065​53
Shimada H, Sawada M, Kuribayashi T, Nakata A, Miyazono A, Asami
H (2010) Spatial distribution of the toxic dinoflagellate Alexandrium tamarense in summer in the Okhotsk Sea off Hokkaido.
Plankton Benthos Res 5:1–10
Shimada H, Sawada M, Tanaka I, Asami H, Fukamachi Y (2012) A
method for predicting the occurrence of paralytic shellfish poisoning along the coast of Hokkaido in the Okhotsk Sea in summer.
Fish Sci 78:865–877
Shimada H (2017) Biological oceanographic study on method forpredicting the occurrence of paralytic shellfish toxin along the
Okhotsk Sea coast off Hokkaido. PhD thesis, Hokkaido University, Graduate School of Fisheries Sciences. https://​doi.​org/​10.​14943/​docto​ral.​r7027 (In Japanese with English summary)
Shiomoto A (2011) Phytoplankton biomass and production in the
coastal area of the Shiretoko Peninsula during late spring to early
autumn: comparison between the Okhotsk Sea and the Nemuro
Straight. Bull Coast Oceanogr 49:37–48 (In Japanese with English abstract)
Shiomoto A, Fujimoto Y, Mimura N, Sasaki A, Itoi D, Imasato S,
Takahashi N, Takenaka Y, Fujita T (2018) Seasonal variations ofchlorophyll a and environmental factors in the coastal area of the
Okhotsk Sea. Nippon Suisan Gakkaishi 84:241–253
Sorokin YI, Sorokin PY (1999) Production in the Sea of Okhotsk. J
Plankton Res 21:201–230
Sunda WG, Huntsman SA (1995) Iron uptake and growth limitation in
oceanic and coastal phytoplankton. Mar Chem 50:189–206
Takizawa T (1982) Characteristics of the Soya Warm Current in the
Okhotsk Sea. J Oceanogr Soc Jpn 38:281–292
Tammilehto A, Nielsen TG, Krock B, Moller EF, Lundholm N (2012)
Calanus spp. —vectors for the biotoxin, domoic acid, in the Arctic
marine ecosystem? Harmful Algae 20:165–174
Tanaka Y, Kajihara T (1979) The distribution of Fulmaris glacialis andPuffinus tenuirostris in the North Pacific and Okhotsk Sea during
the summer. J Yamashina Inst Ornithol 11:79–86
Timmermans KR, Gerringa LJA, De Baar HJW, Van der Wagt B, Veldhuis MJW, De Jong JTM, Croot PL (2001) Growth rates of large
and small Southern Ocean diatoms in relation to availability ofiron in natural seawater. Limnol Oceanogr 46:260–266
Twining BS, Baines SB (2013) The trace metal composition of marine
phytoplankton. Annu Rev Mar Sci 5:191–215
Yamamoto T, Tarutani K (1999) Growth and phosphate uptake kinetics of the toxic dinoflagellate Alexandrium tamarense from Hiroshima Bay in the Seto Inland Sea, Japan. Phycol Res 47:27–32

13