Deep-Sea Research II 94 (2013) 22–30

Contents lists available at ScienceDirect

Deep-Sea Research II
journal homepage: www.elsevier.com/locate/dsr2

Distribution of viable diatom resting stage cells in bottom sedimentsof the eastern Bering Sea shelf
Chiko Tsukazaki a,n, Ken-Ichiro Ishii b, Rui Saito a, Kohei Matsuno a, Atsushi Yamaguchi a,
Ichiro Imai a,nab

Graduate School of Fisheries Sciences, Hokkaido University, 3-1-1 Minato-cho, Hakodate, Hokkaido 041-8611, Japan
Graduate School of Agriculture, Kyoto University, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan

art ic l e i nf o

a b s t r a c t

Available online 19 March 2013

Information on diatom resting stages is fundamentally important to understanding the population
dynamics of diatoms including bloom formation. The distribution of viable diatom resting stage cells inbottom sediments of the eastern Bering Sea in July 2009 was investigated by the most probable number(MPN) method. The abundances of diatom resting stage cells ranged from 1.7  103 to 1.2 
106 MPN cells cm−3 wet sediment, comparable to those in shallow eutrophic areas where diatom blooms
frequently occur. Common species during the spring phytoplankton bloom in the eastern Bering Sea
were also dominant in sediments as resting stage cells. It should be noted that relatively high numbers of
ice algae species, especially ribbon-shaped chain forming pennate diatoms, were found in the sediments.
The life cycle strategy using resting stage cells allows planktonic and ice algal species to survive
unfavorable environmental conditions such as the dark winter season, and potentially contribute to form
blooms of several types (subsurface of ice, ice edge, plankton) through vertical mixing.
& 2013 Elsevier Ltd. All rights reserved.

Keywords:
DiatomResting stage cells
Resting spore
Resting cellIce algae
Phytoplankton bloom
Overwintering
Life cycle strategy

1. Introduction
Diatoms are important primary producers within marine ecosystems, and contribute to efﬁcient primary production. Many
diatom species are large and often form colonies in chains. Diatom
blooms occur extensively in the spring and fall along the coastal
areas of middle to high latitudes. It is well known that many
diatoms, especially coastal planktonic species, form resting stage
cells associated with unfavorable conditions such as low nitrogenconcentration and low light intensity (Durbin, 1978; Garrison, 1984;
Hargraves and French, 1975; Hollibaugh et al., 1981; McQuoid andHobson, 1996). Vegetative cells of these diatom species temporarily
disappear from the water column, and then reappear and bloom
again after some period. The life cycle including resting stages
ensures they survive variable coastal environments, leading todomination among phytoplankton populations.
High abundances of diatom resting stage cells (∼106 MPN
cells cm−3 wet sediment) have been reported from bottom sediments of temperate coastal areas such as the Seto Inland Sea,
Japan (Imai et al., 1990; Itakura et al., 1997). The distribution ofresting stage cells in sediments is likely related to the distributionof phytoplankton populations in the water column (Pitcher, 1990).

nCorresponding author. Tel.: þ81 138 40 5541; fax: þ 81 138 40 5542.
E-mail addresses: tskzkuaai@gmail.com (C. Tsukazaki),imai1ro@ﬁsh.hokudai.ac.jp (I. Imai).
0967-0645/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dsr2.2013.03.020

Viable resting stage cells potentially affect the occurrence ofautochthonous plankton species in the water column (Itakura
et al., 1997). Therefore information, for example distribution andabundance, on resting stage cells is fundamentally important indetermining the spatial distribution of diatoms population
dynamics and species succession (McQuoid, 2002).
The eastern Bering Sea shelf is the widest continental sea shelfoutside the Arctic. The wide shallow shelf, more than 500 km
wide, is seasonally covered by sea ice in all years. This is a region
well known for high productivity of upper trophic level organisms
including crabs, ﬁsh, birds, and mammals (McRoy et al., 1986). The
annual increase in primary production usually begins with thegrowth of ice algae on the underside of sea ice (e.g. Acnanthes
taeniata, Fragilaria striatula, Fragilariopsis cylindrus, Fragilariopsis
oceanica; Saito and Taniguchi, 1978), followed by a phytoplankton
bloom in the water column in the ice front zone (e.g. Thalassiosira
gravida, Thalassiosira hyalina, Thalassiosira nordenskioeldii; Saito andTaniguchi, 1978), and the conventional spring bloom in the watercolumn upon thermal stratiﬁcation (e.g. Chaetoceros convolutus,
Chaetoceros debilis, Chaetoceros furcellatus, Chaetoceros diadema ,dinoﬂagellates; McRoy and Goering, 1976; Saito and Taniguchi, 1978 ).
When the ice retreats early in late winter, the open water bloom
begins in late spring in the stratiﬁed water columns due to solar
heating without ice associated spring bloom (Eslinger and Iverson,
2001; Hunt et al., 2002; Stabeno et al., 1998, 2001).
The southeastern Bering Sea shelf is separated into distinguishable hydrographic domains by three fronts, and these domains
have different circulation features with distinct temperature,

C. Tsukazaki et al. / Deep-Sea Research II 94 (2013) 22–30

salinity and stratiﬁcation properties (Coachman et al., 1980;
Coachman, 1986). In the coastal shelf domain (CSD: o50 m isobath), the water column tends to be vertically homogeneous dueto the overlapping of wind and tidal mixing energies. The middle
shelf domain (MSD: 50 oH o100 m isobaths) water column isgenerally homogeneous during fall and winter. Throughout latespring and summer, the surface wind mixed layer is separated
from the tidally mixed bottom layer by a pronounced thermocline.
The outer shelf domain (OSD: 100 oH o170 m isobaths) is a zoneof little or no mixing energy characterized by persistent ﬁne
structuring of properties between the surface wind mixed layer
and tidally mixed bottom layer. The shelf break front separating
shelf water from basin water along the shelf edge zone, the Bering
Slope Current (Kinder et al., 1975; Schumacher and Reed 1992), is
an important physical feature (Springer et al., 1996).
The life cycle of diatoms, including resting stages, is consideredessential to their survival and dominance over the Bering Sea shelf,
similar to the situation of temperate coastal areas where there is amutually intimate interaction between water column and sea
bottom. In this manuscript, we report on the distribution of viable
resting stage cells of diatoms in the eastern Bering Sea shelf bottom
sediments, and discuss the dynamics of diatom populations.

23
170°E

170°W

150°W
68°N

62°N

54°N

64°N

20

19

21

18

50

17
100
50

60°N

22

200

16
15
13 14

2. Materials and methods

12
7

Sampling was conducted at 22 stations in the eastern Bering
Sea shelf from 8 to 15 July 2009 during a cruise OS202 of the T/SOshoro-Maru of Hokkaido University (Fig. 1). A CTD cast was madeat each station to measure water column temperature, salinity,
density and collect discrete samples for nutrient analysis. Major
nutrient concentrations were measured with a Technicon autoanalyzer basically employing the methods reported by Parsons
et al. (1984) and Matsunaga et al. (1990). Sediment sampling was
carried out at 17 stations using a Smith-McIntyre grab sampler or agravity core sampler. The top 3 cm of sediment core was extruded
and stored in darkness at 2 1C for more than 3 months for the
purpose of eliminating vegetative cells . Sediment samples wereanalyzed following the procedure of the most probable number(MPN) method (Imai et al., 1984, 1990) to estimate the abundanceof viable resting stage cells in sediments. Homogenized sediment
samples were suspended in sterile ﬁltered seawater at a concentration of 0.1 g wet weight mL−1 (100 dilution). This 100 dilution ofsuspension was used to prepare a 10−1–10−6 dilution series with
modiﬁed SWM-3 culture medium (Chen et al., 1969; Imai et al.,1996a). Five aliquots (1 mL) of each dilution series were incubatedusing tissue culture microplates under an illumination of 30 μmol photons m−2 s−1 and a 14:10 h light:dark cycle at a temperature of
5 1С on the assumption of representing the sea surface environmental conditions. Appearance of vegetative cells was measured
for each well of the microplates, and species or taxonomic groups
identiﬁed using inverted microscopy every 3 days until the end of
incubation at 7–14 days. Wells in which vegetative cells wereidentiﬁed were scored as positive. MPN of viable diatom restingstage cells (MPN cells g−1 wet sediment) was estimated according
to statistical tables (Itoh and Imai, 1987; Throndsen, 1978), based
upon the number of positive scores in the ﬁve wells of eachdilution. MPN per cubic centimeter of wet sediment was calculated with the apparent speciﬁc gravity of wet sediment determined according to Kamiyama (1996).
Water samples were collected from the sea surface at eachstation and from several depths at St. 11 and St. 18 for phytoplankton and chlorophyll a analyses. Water samples were sequentially ﬁltered through 20 μm mesh, a membrane ﬁlter (2 μm) and a
GF/F ﬁlter, then analyzed for chlorophyll a using a Turner Designs
ﬂuorometer (Suzuki and Ishimaru, 1990). Phytoplankton samples

6

11 10

8
5

56°N

9
4

3
2
1

176°W

170°W

164°W

Fig. 1. Sampling stations in the eastern Bering Sea. ●: stations for collection of
sediment core and surface water, : sediment core, ○: surface water, : water
samples from several depths.

were preserved with glutaraldehyde at a ﬁnal concentration of 1%
and then settled and concentrated to ten to twenty fold. Appropriate aliquots (0.2–1 mL) of concentrated samples were transferred to a slide glass and phytoplankton cells counted using
an inverted microscope. Species were further identiﬁed using alight microscope at 1000  magniﬁcation and a scanning electron
microscope.

3. Results
3.1. Hydrography
According to the theory of Coachman et al. (1980), stations
were divided into three distinct depth domains. Thermoclines
were generally found at 20–30 m in the MSD during the cruise(Fig. 2A). The pycnocline was exceptionally clear at stations with
about 20 m depth in the Saint Laurence Island Polynya region(Smith et al., 1990) (Fig. 2B). These stations were separated fromother MSD stations by characteristics of bottom temperature.
Surface nutrients were depleted over the eastern Bering Sea shelfduring the cruise (Table 1).
3.2. Distribution of phytoplanktonHigh surface chlorophyll a concentrations (St. 5; 6.1 μg L−1, St.
7; 4.7 μg L−1) and phytoplankton cell abundances (6.1  105–
1.8  106 cells L−1) were patchy at stations along the shelf edge.
The dominant species was Pseudo-nitzschia cf. delicatissima (St. 5; 85%,
St. 7; 45%).

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C. Tsukazaki et al. / Deep-Sea Research II 94 (2013) 22–30

Temperature (°C)

Depth (m)Density (σt)

Temp
Sal

Depth (m)

Temp
Sal

Salinity
Fig. 2. Vertical proﬁles of temperature and salinity at station 11 (A-1) and station 18 (B-1), and density proﬁles at station 11 (A-2) and station 18 (B-2) in water columnsof the eastern Bering Sea during the cruise in July 2009.

Table 1
Surface nutrient (NO3 þ NO2,SiO2, PO4) concentration (μM) in the Bering Sea in July
2009. Stations were divided into four domains, OSD (outer shelf domain:
100 oHo 170 m isobaths), MSD (middle shelf domain: 50 o Ho 100 m isobaths),
CSD (coastal shelf domain: o 50 m isobaths) and SLIP (St. Lawrence Island Polynya
region).
St. Domain Depth(m)

Day (July2009)

NO3 þ NO2
(μM)

SiO2
(μM)

PO4
(μM)

1
2
3
4
5
6
7
8
9
10
11
12
13
14
22
15
16
17
18
19
20
21

8
8
9
9
9
9
10
10
10
10
10
11
11
11
15
11
11
12
13
13
13
14

14.25
1.74
0.58
0.40
1.62
6.28
0.37
0.09
0.10
0.08
0.05
0.05
0.09
0.08
0.03
0.06
0.08
0.09
0.11
0.03
0.06
0.31

28.34
13.20
10.35
7.58
20.85
24.77
16.92
2.03
1.83
2.42
1.30
1.30
1.71
2.36
1.06
1.59
2.42
4.36
2.12
3.18
5.71
4.06

1.25
0.55
0.50
0.29
0.59
0.88
0.29
0.22
0.16
0.41
0.15
0.15
0.57
0.41
0.18
0.48
0.48
0.57
0.32
0.43
0.37
0.42

OSD
OSD
OSD
OSD
OSD
OSD
OSD
OSD
MSD
MSD
MSD
MSD
MSD
MSD
MSD
CSD
CSD
SLIP
SLIP
SLIP
SLIP
SLIP

135
121
113
133
136
220
102
115
84
70
75
77
70
64
70
54
45
55
45
53
74
68

In the southern region (St. 11; MSD), there was a maximum inchlorophyll a concentration (1.62 μg L−1) and cell abundance(1.1  105 cells L−1) above the pycnocline (Fig. 3). The small phytoplankton size fractions (o20 μm) contributed to high chlorophyll aconcentrations, and were dominated by small dinoﬂagellates andmicroﬂagellates (78–92%). Diatoms dominated below the pycnocline,

but chlorophyll a concentrations and cell abundances were relatively
low. In the northern region (St. 18; Polynya region), there was a
maximum of chlorophyll a concentration (1.3 μg L−1) and cell abundance (3.8  105 cells L−1) below the pycnocline dominated by large(420 μm) diatoms (71–95%). At this station, dinoﬂagellates andmicroﬂagellates dominated in the abundance in the upper layer but
chlorophyll a concentrations and cell abundances were relatively low.
Fig. 4 illustrates the vertical distributions of diatoms at St. 11
and St. 18. Vegetative cells of Chaetoceros spp. were observed in
the upper layer, and resting spores of Chaetoceros spp., especially
C. furcellatus, were abundant in the lower layer of both stations.
The diatom species characteristically found in the water column
were Paralia sulcata at St. 11, and T. nordenskioeldii and Porosira
glacialis at St. 18.

3.3. Abundances of resting stage cells in sediments estimated by the
MPN method
The numbers of resting stage cells in bottom sediments
estimated by the MPN method were in the range of 1.7  103(St. 1) to 1.2  106 (St. 20) MPN cells cm−3 wet sediment (Fig. 5).
Eighteen centric diatom taxa and 4 pennate diatom taxa wereobserved as vegetative cells after the incubation for the MPN
method (Table 2). Almost all species were already reported as
species found in the Bering Sea (Goering and Iverson, 1981;
Motoda and Minoda, 1974; Saito and Taniguchi, 1978;
Schandelmeier and Alexander, 1981). Dinoﬂagellate cysts werealso estimated (∼1.2  103 MPN cells cm−3 wet sediment). The
highest concentrations were found south of St. Lawrence Island(St. 20). The number of viable cells ranged from 6.1  104 to
9.2  105 MPN cells cm−3 wet sediment (average 2.4  105) in the
MSD and CSD. There were small numbers of resting stage cells in
the OSD and shelf edges where the bottom depths were greater

C. Tsukazaki et al. / Deep-Sea Research II 94 (2013) 22–30

Composition (%)

Depth (m)Depth (m)Composition (%)

25

Total chl a ( μg L-1 )0.7 - 2μm

2μm - 20μm

Abundance ( x 104 cells L-1 )20μm<

Diatoms

Dinoflagellates

Flagellates

Unidentified

Fig. 3. Vertical proﬁles of chlorophyll a concentrations (μg L−1) of three size fractions at station 11 (A-1) and station 18 (B-1), and cell abundances (  104 cells L−1) andtaxonomic group composition of phytoplankton at station 11 (A-2) and station 18 (B-2) in water columns of the eastern Bering Sea in July 2009.

than 100 m, and the numbers ranged from 1.7  103 to
6.8  104 MPN cells cm−3 wet sediment (average 2.1  104).
Fig. 6 shows the dominant diatom species of viable resting stage
cells in sediments. The numbers of resting stage cells in the southern
region and south of St. Lawrence Island are summarized in Table 2.
Chaetoceros spp. (∼1.3  105 MPN cells cm−3 wet sediment), Thalassiosira spp. (∼1.8  105 MPN cells cm−3 wet sediment) and Attheya longicornis (∼8.7  105 MPN cells cm−3 wet sediment) were the dominanttaxa. C. diadema (∼6.8  104 MPN cells cm−3 wet sediment), Chaetoceros socialis (∼6.3  104 MPN cells cm−3 wet sediment) and C. furcellatus (∼2.4  104 MPN cells cm−3 wet sediment) were abundantspecies of the genus Chaetoceros. T. nordenskioeldii (∼1.8  105 MPN
cells cm−3 wet sediment) and T. gravida (∼5.5  104 MPN cells cm−3
wet sediment) were abundant species of the genus Thalassiosira. P.
sulcata was detected at high abundances in the southern region(∼6.3  104 MPN cells cm−3 wet sediment). Skeletonema spp. andOdontella spp. were also identiﬁed but the abundances were relatively
low. Other species with high abundances were the pennate diatoms ofribbon-shaped colony forming species, F. cylindrus, F. oceanica, Fragilaria cf. capucina, Fragilaria oblonga, Pauliera taeniata (∼1.1  105 MPN
cells cm−3 wet sediment) and solitary Navicula spp. (∼3.6  104 MPN
cells cm−3 wet sediment).

4. Discussion
4.1. Phase of phytoplankton populations during the study cruise
Phytoplankton populations were changing from a phase dominated by diatoms to one dominated by ﬂagellates during thissampling period. The phase transition was observed above thepycnocline and low latitudes (St. 11). Surface nutrient concentrations were depleted when surface stratiﬁcation developed, andchlorophyll concentrations declined to low levels over the eastern
Bering Sea shelf (Whitledge et al., 1986). Observed phytoplankton
assemblages suggest that the study period was near the end of a
diatom bloom. High abundances of Chaetoceros spp. resting spores
were observed in the lower layer whereas the vegetative cellabundances were low in the upper layer (Fig. 4). This result
strongly suggests that diatom resting stage cells would haveformed before the cruise and had been sinking near the end of
the bloom.
It is well known that there is a current of water originating at
the Bering Sea shelf edge that is rich in nutrients and plankton(Springer et al., 1996). During this cruise, relatively high chlorophyll a concentrations and phytoplankton (diatoms) cell abundances were observed only at the shelf edge, suggesting thecontribution to primary productivity in summer season.
4.2. Distribution of resting stage cells in the bottom sediments
The viable resting stage cells in sediments are potentially
important seed banks for planktonic blooms. The population sizeof resting stage cells distributed in the eastern Bering Sea shelf
was comparable to that distributed in the Seto Inland Sea of Japan(∼106 MPN cells cm−3 wet sediment: Imai et al., 1990; Itakura
et al., 1997, 1999), and that in the coastal Swedish fjord(∼5.7  104 MPN cells g−1 wet sediment: McQuoid et al. 2002).
The study area of the Seto Inland Sea and coastal Swedish fjord aremostly inshore areas (shallower than depth of 30 m) where

26

C. Tsukazaki et al. / Deep-Sea Research II 94 (2013) 22–30

64°N

Depth (m)Composition (%)

50

100

50

60°N

< 5 x 103

Abundance ( x 104 cells L-1 )5 x 103 ~ 104
Composition (%)
104 ~ 5 x 104
5 x 104 ~ 10556°N
105 ~ 5 x 105
Depth (m)5 x 105 <

176°W

170°W

164°W

Fig. 5. Spatial distribution of total diatom resting stage cells (MPN cells cm−3 wet
sediment) in surface sediments (top 3 cm depth) of the study area. Stations without
circle indicate no data.

Abundance ( x 104 cells L-1)
Chaetoceros concavicornis

Chaetoceros spp.

Chaetoceros sp. resting spore

Cylindrotheca closterium

Fragilariopsis spp.

Paralia sulcata

Porosira glacialis

Pseudo-nitzschia spp.

Thalassiosira nordenskioeldii

Thalassiosira spp.
OthersFig. 4. Vertical distributions of cell abundance (  104 cells L−1) and species
composition of the dominant diatoms in water columns at station 11 (A) andstation 18 (B).

intercommunication of diatoms between water column and sea
bottom is capable and frequent. Although the study area of theBering Sea shelf was offshore and a deeper depth (average 83 m),
the diatom resting stage cells also appeared abundantly in
sediments.
Plankton biomass largely inﬂuences the population size of the
diatom resting stage cells (Itakura et al., 1999; McQuoid, 2002).
High abundances of resting stage cells in sediments are believed toreﬂect dense bloom of diatoms predominant among the plankton

community on the Bering Sea shelf. There is a relationshipbetween the dominant phytoplankton species in the water columns and resting stage cells in sediments. Similarities in species
composition of the resting stages and typical plankton populations
reveal that resting stage cells were formed locally after the blooms
in the water columns.
Viable resting stage cells of Skeletonema spp. were mostabundant in the bottom sediments of the Seto Inland Sea andcoastal Swedish Fjord, but less so over the Bering Sea shelf.
Skeletonema spp. usually occurs in high abundance in eutrophic
to extremely eutrophic regions (Yamada et al., 1980); therefore, thesupply of resting stage cells from the water column is presumably
low in the Bering Sea where the degree of anthropogenic eutrophication is limited.
P. sulcata grows primarily at the sea bottom of shallow areas,
but this species is sometimes found with high cell abundances in
the water column (Abrantes, 1988; Hobson and McQuoid, 1997)
presumably due to physical forces such as strong winds or tidal
mixings (Blasco et al., 1980; Oh and Koh, 1995). In this study, P.
sulcata cells were abundant in bottom sediments at MSD and CSD,
and the lower water column at St. 11 (MSD). Cells of P. sulcata werealso distributed at the surface of CSD in the study period. This factsuggests that cells were transported from bottom to surface layer
by physical forces such as storms and strong tidal action at thebottom.
Chaetoceros concavicornis and Ch. convolutus, both dominant
species in summer in the Bering Sea, were found in the water

C. Tsukazaki et al. / Deep-Sea Research II 94 (2013) 22–30

27

Table 2Abundance ranges and means (MPN cells cm−3 wet sediment) of diatom resting stage cells and dinoﬂagellate cysts in bottom sediments of the study area.
SpeciesClass Bacillariophyceae
Order Centrales
Actinoptychus senarius
Attheya longicornis
Total Chaetoceros spp.
Chaetoceros curvisetus
Chaetoceros diadema
Chaetoceros didymus
Chaetoceros mitra
Chaetoceros socialis
Chaetoceros furcellatus
Chaetoceros laciniosus
Chaetoceros debilis
Other Chaetoceros spp.
Detonula spp.
Skeletonema spp.
Total Thalassiosira spp.
Thalassiosira nordenskioeldii
Thalassiosira gravida
Other Thalassiosira spp.
Paralia sulcata
Porosira glacialis
Odontella spp.
Delphineis spp.
Navicula spp.
Pennate species (ribbon forming)
Fragilaria spp.
Fragilariopsis spp.
Pauliera taeniata
Class Dinophyceae
Total

Southeastern Bering Sea

St. Lawrence Island Polynya region

min–max

mean

min–max

N. D.∼2.6  103
N. D.∼5.6  105
N. D.∼6.9  104
N. D.
N. D.∼3.3  104
N. D.∼2.8  103
N. D.∼1.0  104
N. D.∼2.6  104
N. D.∼2.4  104
N. D.∼1.6  103
N. D.∼3.2  103
N. D.∼7.1  102
N. D.∼1.0  104
N. D.∼6.9  103
N. D.∼1.1  105
N. D.∼7.4  104
N. D.∼5.5  104
N. D.∼7.7  103
N. D.∼6.3  104
N. D.∼3.3  103
N. D.∼1.6  103
N. D.∼6.0  103
N. D.∼1.8  104
N. D.∼9.6  104
N. D.∼2.3  104
N. D.∼9.4  104
N. D.
N. D.∼2.8  102
1.74  103–9.2  105

2.9  102
5.0  104
1.6  104

N. D.
7.2  102–8.7  105
3.1  103–1.3  105
N. D.∼2.5  103
N. D.∼6.8  104
N. D.∼2.7  103
N. D.
3.1  103–6.3  104
N. D.∼1.7  103
N. D.
N. D.
N. D.∼5.1  104
N. D.∼1.9  104
N. D.∼3.2  102
N. D.∼1.8  105
N. D.∼1.8  105
N. D.∼3.7  104
N. D.
N. D.∼3.6  102
N. D.
N. D.∼2.7  103
N. D.
N. D.∼3.6  104
3.6  102–1.1  105
N. D.
N. D.∼1.1  105
N. D.∼3.6  102
N. D.
1.0  104–1.2  106

5.0  103
2.7  102
8.0  102
5.7  103
3.9  103
1.2  102
2.5  102
5.4  101
1.6  103
1.4  103
1.7  104
7.4  103
9.1  103
7.2  102
1.1  104
5.4  102
4.1  1028.2  102
2.1  104
1.9  104
1.1  104
1.7  104
1.5  102
1.4  105mean

2.3  105
7.5  104
6.3  102
2.6  104
6.9  102
3.4  104
4.4  102

1.3  104
4.9  103
8.1  101
9.0  104
7.7  104
1.3  104
1.7  102
9.6  102
1.2  104
2.8  104
2.8  104
9.1  101
4.9  105

N. D. : not detected.

samples during this study. However, we could not detect theappearance of them after incubation of the sediments with the
MPN treatment. These Chaetoceros species appear to have a life
strategy different from those forming resting stage cells.
There is no report of resting stage cells for Pseudo-nitzschia spp.
Although Pseudo-nitzschia spp. were dominant at the shelf edge
during the study period, they are presumably unable to overwinterin the Bering Sea without resting stages due to poor light
conditions. It is considered that they are allochthonous populations transported by the current from the southern areas to theBering Sea.
There are several pennate diatom species forming resting stage
cells (McQuoid and Hobson, 1996). The pennate species found in
sediments in high abundance were ribbon-shaped colony forming
species (F. cylindrus, F. oceanica, Fragilaria cf. capunica, P. taeniata)
and solitary Navicula spp. They frequently comprise the ice algae
populations. These species are known to grow at the subsurface ofsea ice in late winter to early spring in the Bering Sea (Saito andTaniguchi, 1978; Schandelmeier and Alexander, 1981; Goering and
Iverson, 1981). Benthic Delphyneis species were also observed in
the sediments, but already attaching to the substrata such as sandgrains with several cells before incubation. Resembling the centric
diatom species, ice algae species form resting stage cells, and theysurvive in bottom sediments after sinking from sea ice melted
during the warm season and then serve as a source of seed stock
for ice algae was suggested to be sediments (Horner and Schrader,1982). The present study conﬁrmed the existence of resting stage
cells of ice algae species in the bottom sediments during summer
season. The resting stage cells presumably play an important role
as a seed population in developing large biomass of ice algae when
they were resuspended and incorporated into the ice duringfreeze-up.

Attheya sp. were usually found in the plankton associated with
Chaetoceros species (Crawford et al., 1994) and from undersurfaceof sea ice (Melnikov et al., 2002; Schandelmeier and Alexander,
1981; von Quillfeldt, 2000; Werner et al., 2007). They possibly
contribute to primary productivity as plankton and also as ice
algae. High abundances of resting stage cells in sediments indicate
the importance of the genus Attheya as primary producers in the
Bering Sea.
4.3. Contribution of resting stage cells to bloom formation
Germination and rejuvenation of diatom resting stage cells are
generally affected by light (Hargraves and French 1983; Hollibaugh
et al., 1981; Imai et al., 1996b), and hence the resting stage cells inthe sea bottom need to be resuspended to the euphotic zone
where they can germinate or rejuvenate and resultant vegetative
cells commence to grow (Itakura et al., 1997). In regions of
upwelling, the prevalence of resting spores of the genus Chaetoceros in newly upwelled waters (Abrantes, 1988; Pitcher, 1990)
indicate that upwelling events have enough energy to resuspend
the resting stage cells from the benthos into the water column(Pitcher et al., 1991). In the Funka Bay of Japan, the resting sporesof Chaetoceros spp. in the bottom layer were brought back into theeuphotic zone during the vertical mixing of the water column(Odate and Maita, 1990), implying that vertical mixing also has
sufﬁcient energy to resuspend the resting stage cells into thewater column. It was reported that artiﬁcial stirring of stable watercolumns led to larger cell size diatom blooms in marine environment (Eppley et al., 1978).
In a study of the Bering Sea (Sambrotto et al., 1986; Eslinger andIverson, 2001), the spring phytoplankton bloom onset occurred
when the water column was vertically mixed to the sea bottom,

28

C. Tsukazaki et al. / Deep-Sea Research II 94 (2013) 22–30

No data

62°N

60°N

Attheya spp.
Chaetoceros diadema
Chaetoceros socialis

58°N

Chaetoceros furcellatus
Other Chaetoceros spp.
Paralia sulcata
No data

No data

Porosira glacialis
Thalassiosira gravida
56°N

No dataThalassiosira
nordenskioeldii
Navicula spp.

No dataPennate species(ribbon-shaped formings)
Others
174°W

170°W

166°W

Fig. 6. Species composition of the diatom resting stage cells observed in surface sediments (top 3 cm depth) of the study area detected with the most probable number(MPN) method. The stations without circle indicate no data.

and chlorophyll maxima were associated with the shoaling of themixed layer. Blooms often tend to appear suddenly because
vegetative cells originated from resting stage cells are supposed
to divide rapidly with the organic matter already stored in the cells(Shakshaug, 1989). Hence, the trigger of the spring bloom was not
only supply of inorganic nutrients, but also probably the resuspension of resting stage cells into the euphotic layer.
As is the case in temperate seas, the seasonal decrease inatmospheric temperature and increase turbulence enhance thevertical mixing in fall and winter in the Bering Sea shelf. When thesea surface was covered with ice, wind driven mixing ceases but
the convective mixing layer deepens to a depth about 50 100 m in
response to brine drainage of freezing ice (Ikeya and Kawanobe,
2002; Ohtani, 1969, 1973; Sullivan et al., 1988). Upwelling plumesof circulation cell have also been observed at the ice edges in the
Bering Sea (Alexander and Niebauer, 1981; Okkonen and Niebauer,
1995). These physical processes have the potential to mix restingstage cells from the benthos to the water column. A cold watermass (−1.7 to 01 C) remained under the pycnocline in the southernpart of the St. Lawrence Island Polynya and MSD during the study
period suggesting that the sea ice possibly formed in that region
and the vertical convection reached the benthos.
Fig. 7 gives a schematic representation of the life cycles of ice
algae and phytoplankton in the eastern Bering Sea shelf. Hornerand Schrader (1982) reported that microalgae were present in thesea ice as soon as it formed in the fall and generally were scatteredthroughout the ice thickness. In the winter period, resting stage

cells of ribbon-shaped pennate species and solitary Navicula spp.
would be mixed to the water column from the bottom by
convection. If those species are incorporated or attached to thesea ice, they would develop in ice algae. Similarities in composition of benthic and ice algae species (Matheke and Horner, 1974)
support this hypothesis. As an alternate explanation, plankton
species could survive the winter period as resting stage cells andbe resuspended in the water column the following spring andbloom, when they are successfully trapped to the euphotic layer.
Resting stage cells probably are resuspended non-selectively, but
the germination/ rejuvenation in response to favorable light andtemperature conditions ( French and Hargraves, 1985; Hollibaugh
et al., 1981;Imai et al., 1996b) will affect vegetative growth, and soinitiation and composition of bloom.
In a past study, benthic P. sulcata cells existed in the sea ice(Schandelmeier and Alexander, 1981), suggesting that cells suspended from the sea bottom were incorporated into ice. Resting
spores of C. socialis and F. oceanica were also found in the ice (vonQuillfeldt, 1997). There is a strong possibility that the resting stage
cells of plankton species are incorporated into the sea ice together
with ice algae species. Those resting stage cells of plankton species
could germinate in the upper layer when the sea ice is melting,and probably contribute to the ice edge bloom accompanied with
ice algae species. When the water column is vertically mixed tothe sea bottom by the process of wind and tidal mixing, the restingstages of planktonic species probably contribute to the open water
bloom after the water has stabilized.

C. Tsukazaki et al. / Deep-Sea Research II 94 (2013) 22–30

Oct

Nov

Dec

Jan

Feb

Mar

Apr

inner-ice bloom

wind-driven mixing

tidal mixing

convective mixing accelerated
by brine drinageMay

ice edge bloom

Jun

29

Jul

Aug

Sep

open water
phytoplankton bloom

stratification
resting stage
cells

vegetative cells

Fig. 7. Schematic representation of the life cycles of ice algae and planktonic diatoms in the eastern Bering Sea shelf based on the present study and partially following
Coachman et al. (1980), Sullivan et al. (1988) and Ikeya and Kawanobe (2002).

Under conditions that favor extensive spatial and temporalextent of sea ice, more resting stage cells could be incorporated
into the ice as seed populations and contribute to a rich bloom. In
contrast, when sea ice has limited spatial and temporal extent,
there is insufﬁcient light to support a bloom (Hunt et al., 2002)
and a smaller seed population to contribute to a bloom. Thus there
is a possibility that year to year variations in ice extent andduration have large effects in the scale and period of the spring
blooms in the Bering Sea.

5. Conclusion
Diatom species of plankton and ice algae employ resting stages
as survival strategies in the Bering Sea as is the case of the restingstages of many diatoms in temperate waters. It is strongly
suggested that resting stage cells play a crucially important rolein overwintering of diatoms in the Bering Sea, where vegetative
cells are exposed to extremely severe conditions (low temperature, sea ice formation, extreme light limitation) in winter, and insupplying a seed populations for spring blooms through germination and/or rejuvenation in water columns and sea ice.

Acknowledgments
We are grateful to Professor Kenshi Kuma (Graduate School ofEnvironmental Science, Hokkaido University), Associate Professor
Jun Yamamoto (Graduate School of Environmental Science, Hokkaido University), and to the captain, ofﬁcers and crew of T/SOshoro-Maru of Hokkaido University for cooperation in sampling.
We also thank Professor Kenshi Kuma (Graduate School of Fisheries Sciences, Hokkaido University) and members of his laboratory for supplying the environmental data.

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