Regional Studies in Marine Science 69 (2024) 103322

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Regional Studies in Marine Sciencejournal homepage: www.elsevier.com/locate/rsma

Seasonal variation in microplankton communities in Suttsu, Hokkaido,
northern Japan, from 2020 to 2022
Kyosei Morimoto a, *, Yusuke Hamao a, Tomoyasu Yamazaki b, Shoko Tatamisashi c,
Masahide Wakita c, Kohei Matsuno a, daFaculty/Graduate School of Fisheries Sciences, Hokkaido University, 3-1-1 Minato-cho, Hakodate, Hokkaido 041-0821, Japan
Shellfish Museum of Rankoshi, 1401, Minatocho, Rankoshi, Hokkaido 048-1341, Japan
Mutsu Institute for Oceanography, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 690 Kitasekine Sekine, Mutsu, Aomori 035-0022, Japand
Arctic Research Centre, Hokkaido University, Kita-21 Nishi-11 Kita-ku, Sapporo, Hokkaido 001-0021, Japanbc

A R T I C L E I N F O

A B S T R A C T

Keywords:
Diatoms
Euglenoids
Warm-water species
Seasonal change
Subpolar Front Current

Southwestern Hokkaido faces intricate physiological and chemical changes. Subsequently, changes in lower
trophic levels are of concern, but fully understanding the relationship between microplankton and hydrography
in this region is required. To examine the relationship between the microplankton community (diatoms, di­noflagellates, ciliates, and euglenoids) and environmental factors in Suttsu, Hokkaido, water samples werecollected at 4–15-day intervals from August 2020 to August 2022 at Yokoma Port. Diatoms were dominant in the
study area, with an apparent seasonal change in species composition. Lower phytoplankton cell density wasobserved in the second year than in the first year during autumn and winter because of low temperatures andlight intensity in the second year. A significant decrease in the number of attached diatoms (Navicula spp.) was
observed during the winter of the second year, possibly due to calm conditions (low tide and wind speed), which
prevent the detachment of weakly attached species. Large blooms of euglenoids (Eutreptiella gymnastica andE. marina) in April and May were caused by the exclusive use of nutrients from the inflow of river water during
the snowmelt season. Warm-water species sporadically occurred, suggesting their transportation by the Tsushima
Warm Current. Temperature, nutrients, and light intensity primarily control microplankton communities, whichvary seasonally. These findings led to the prediction of lower trophic levels due to the impact of warm water
inflow.

1. Introduction
Phytoplanktons are crucial organisms responsible for primary pro­duction in the ocean. Microsized phytoplanktons are mainly composedof diatoms, dinoflagellates, and euglenoids. Diatoms are classified asplanktonic diatoms and benthic diatoms based on habitat differences.
Planktonic diatoms are microalgae that are observed prominently in
inner bays, coastal and offshore areas, and some species have chainforming during spring bloom (Itakura, 2001). Benthic diatoms are ageneral term for diatoms that grow by attaching to various substrates in
the water. Solitary cell is well observed with the ability of moving on thesubstrates. There is no clear boundary between planktonic diatoms andbenthic diatoms, and some species have both floating and attached lives(Kawamura, 1994). In addition, diatoms are divided into two taxonomic
groups: centric and pennate. Most planktonic diatoms are centric, andmost benthic diatoms are pennate (Kawamura, 1994). Diatoms have arapid growth rate, with a maximum growth rate of µmax = 1.5 d− 1 whenlight conditions and nutrient concentrations are favorable (Sarthou
et al., 2005). In addition, many diatoms form resting spores and cansurvive in environments that are unfavorable for growth (Itakura et al.,
1992; Ishii et al., 2011; Matsubara, 2017). Euglenoids are widelydistributed from freshwater to the ocean due to their high salinity
tolerance, such as marine euglenoids Eutreptiella gymnastica (Lee andLim, 2006; Xu et al., 2012). They can also grow in various temperatures(Xu et al., 2012). In Mexico, several hundred fish died from anoxia
caused by Eutreptiella marina blooms (Cortés-Lara et al., 2010). In
southern Hokkaido, a red tide occurrence of Eutreptiella sp. has beenreported at Matsumae port (Shimada, 2021).
The Japan Sea of western Hokkaido is known to be a fishery for
herring, Japanese flying squid, and walleye pollock (Nakata and Tanaka,* Corresponding author.
E-mail address: ryoha19morimoto@eis.hokudai.ac.jp (K. Morimoto).
https://doi.org/10.1016/j.rsma.2023.103322
Received 24 July 2023; Received in revised form 29 November 2023; Accepted 4 December 2023
Available online 5 December 2023
2352-4855/© 2023 Elsevier B.V. All rights reserved.

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2002). Suttsu is located in the southwestern Hokkaido, facing the Sea ofJapan, and the surrounding ocean area is characterized by low nutrientsand productivity (Adachi and Osawa, 2005). In addition to the above
species, scallops and oysters are caught in the waters surrounding
Suttsu. (Hokkaido Research Organization: http://www.fishexp.hro.or.
jp/marinedb/internetdb/fishdb/fish_all.asp). This area is influenced
by the Subpolar Front Current (SFC), which diverges from the Tsushima
Warm Current. Seasonal variation in current speed of the Tsushima
Warm Current, which flows around this western Hokkaido, is smaller(0.25 Sv) than the Tsugaru Warm Current (0.30 Sv) (Nakata and Tanaka,
2002; Nishida et al., 2003; Yabe et al., 2021). However, the Tsushima
Warm Current in the Tsushima Strait has recently been increasing (Kidaet al., 2021), followed by an increase in the SFC, which could change theoceanic environment around Hokkaido. In addition, in recent years, it isreported that sea surface temperatures in the waters surrounding Japan
Sea have been rising. (e.g. https://www.jma.go.jp/jma/en/Activities/
cc.html). As one example, ocean warming, acidification, and deoxy­
genation due to global change-driven may cause serious damage tocalcifiers in Oshoro Bay, which also facing Japan Sea of the western
Hokkaido (Fujii et al., 2021). In contrast, Suttsu is seasonally affected by
freshwater inflow from rivers (the Shiribetsu and Shubuto rivers). For
example, a rapid decrease in salinity and nutrient supply in theneighboring Ishikari Bay occurs with increased river water inflow in
April during the snowmelt season (Adachi and Osawa, 2005). In addi­tion, in Funka Bay, on the Pacific side of Hokkaido, nutrients in the riverwater support new phytoplankton production (Yoshimura and Kudo,
2003). Therefore, river water inflow is also an essential factor affecting
phytoplankton composition and cell density.
Information on phytoplankton in the western side (Sea of Japan) of
Hokkaido includes long-term monthly observations in Oshoro Bay that
concluded that centric diatoms are dominant and that a little relation­
ship exists between regime shifts and phytoplankton cell density (Fukui
et al., 2010). The appearance of the harmful dinoflagellate Karenia
mikimotoi (Shimada et al., 2016; Kakumu et al., 2018) and the north­ward expansion of the distribution area of Heterocapsa circularisquama(Kondo et al., 2012) have been reported in Hakodate Bay, potentiallyowing to an increase in the Tsushima Warm Current. However, thecomprehensive dynamics of micro-sized phytoplankton communities
are still unknown because of methodological issues (100 µm mesh net
collection, monthly frequency) and the investigation of only specificspecies (in many cases, harmful species). A detailed analysis of therelationship between phytoplankton and hydrography is required toaccurately evaluate the effects of changing environmental conditions(increased SFC) on marine ecosystems including fisheries production.

Fig. 1. Location of the sampling station at the Yokoma Port in southern Hokkaido. The red circle indicates a fixed station for water sampling, and the yellow star
indicates a station for monitoring atmospheric conditions (temperature, rainfall, and wind). Light blue stars indicate the observatory of each river.
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This study aimed to clarify seasonal changes in micro-sized phyto­plankton and environmental factors by conducting high-frequency
fixed-point observations over 2 years at Yokoma Port in Suttsu, Hok­kaido. Furthermore, we examined the relationship between micro-sized
phytoplankton and environmental factors and discussed annual changes
in phytoplankton, the mechanism of spring bloom formation, and theoccurrence of warm-water species.

salinity, DIN [=nitrate+nitrite+ammonium], silicate, phosphate, airtemperature, total daily precipitation, mean daily wind speed, tide level,and daylight hours) were analyzed using decision trees. Because celldensities were low for warm-water species, they were converted topresence/absence data and used in the analyses. These analyses were
performed using the packages "rpart" and "partykit" in R (version 4.1.2,
R Development Core Team, 2021).
Based on seasonal changes in the phytoplankton community basedon cluster analysis and hydrography, this study defined August to
October 2020 and September to October 2021 as autumn, November
2020 to March 2021 and November 2021 to March 2022 as winter, April
to May 2021 and 2022 as spring, and June to August 2021 and 2022 assummer (Supplementary Fig. 1, Supplementary Tables 1 and 2). The
Wilcoxon rank-sum test was used to identify annual changes in envi­
ronmental factors during each season using StatView (SAS InstituteInc.).

2. Materials and methods
2.1. Field sampling
A total of 80 samples were collected at the Yokoma Port of Suttsu,
Hokkaido (48.85◦ N, 140.33◦ E) at 4–15-day intervals from August 25,
2020, to August 15, 2022 (Fig. 1). Water samples were collected fromthe sea surface using plastic buckets. For salinity and nutrients (nitrate,
nitrite, ammonium, phosphate, and silicate), water samples were stored
in brown glass bottles and polyethylene bottles at 4 and − 20 ◦ C until
analyzed, respectively. For the phytoplankton samples, 500 mL ofseawater was placed in a plastic bottle, and a fixative was added
immediately. Note that we used two fixatives because of the availabilityof instruments: Lugol acid (final concentration 1%) from August 25,
2020, to January 11, 2022, and glutaraldehyde (final concentration 1%)
from January 25, 2022, to August 15, 2022. Sea surface temperature(SST) was measured during water sampling. The study was conducted
over a two-year period, with the first year defined as August
2020–August 2021 and the second year as September 2021–August2022.
The average daily air temperature, total daily precipitation, average
daily wind speed, maximum wind direction, tide level, and daylight
hours were obtained from the Japan Meteorological Agency website at
the Suttsu Meteorological Station (https://www.jma.go.jp/jma/index.
html). Data on the flow rate of the Shiribetsu River were obtained
from the Ministry of Land, Infrastructure, Transport, and Tourism’s
Hydrologic and Hydrochemical Quality Database at the Nakoma Ob­
servatory, which is located in the middle reaches of the Shiribetsu River(http://www1.river.go.jp). Data on the flow rate of the Shubuto Riverwas obtained from Hokkaido website at the Nakoma Observatory, which
is located in the downstream area of the Shubuto River (https://www.
constr-dept-hokkaido.jp/ks/ikb/iji/ryuuryou/).

3. Results
3.1. Environmental factors
SST ranged from 1.2 to 25.5◦ C and was high in August and low in
February (Fig. 2). Salinity ranged from 22.1 to 34.0 and decreased
rapidly from March to April in 2021 and 2022 and slightly decreased in
winter and early summer (Fig. 2). DIN ranged from 0.56 to 12.2 µM,
which tended to decrease during summer and increase during autumn
and spring. Phosphate ranged from 0.061 to 1.18 µM, increased in
March 2021 and 2022, and decreased during the summers (Fig. 2). Sil­
icate ranged from 1.9 to 94.0 µM, and increased in March–April and
May–June 2021, then decreased at 10–20 µM during the summer(Fig. 2). The DIN:P ratio exhibited fluctuations but no evident seasonal
trend, with a distinct increase in November 2020 and January 2021. The
Si:P ratio increased sharply in June in both years. Air temperatures weregenerally high in summer and low in winter, with a temporary decreasein August 2021 (Fig. 3a). The precipitation was high from summer toautumn (Fig. 3a). The wind speed showed high variability, and the di­rection was evidently northwest in winter and southeast in summer(Fig. 3b, c). The flow rate of the Shiribetsu River increased annuallyfrom March to April, and the flow rate of the Shubuto River increased
annually in April (Fig. 3d, e).

2.2. Sample analysis

3.2. Micro-sized plankton communities

Salinity and nutrients were measured using a salinometer (Model
8400B AUTOSAL, Guildline Instruments Ltd.) and a continuous flow
analyzer (QuAAtro, BL TEC K.K.), respectively (Wakita et al., 2021).
In the laboratory, fixed samples (500 mL) were stored on a stone
table for more than 1 day to allow the phytoplankton cells to settle at thebottom of the bottle. Subsequently, 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. Microsized phyto­plankton cells were identified and enumerated using an inverted mi­
croscope (ECLIPSE Ts2R, Nikon) at 40–600 × magnification. According
to Fukuyo et al. (1997), Hasle and Syvertsen (1997), and Steidinger,
Targen (1997), diatoms, dinoflagellates, and silicoflagellates wereidentified to the lowest possible levels (species or genus). Tinnid andoligotrich ciliates were counted separately. In addition to species andgenus identification, the diatoms were classified as planktonic, benthic,
and warm-water species. Cells were counted with a maximum of 300
cells per sample. After counting, we calculated the cell density (cells
mL− 1).

Regarding cell density, diatoms (1.30–152 cells mL− 1), di­noflagellates (0–231 cells mL− 1), ciliates (0–156 cells mL− 1), and eu­
glenoids (0–4921 cells mL− 1) were observed (Fig. 4a). Diatoms were themost dominant taxon among the four taxonomic groups, accounting foran average of 62.4% of the total microplankton cell density during thestudy period. Dinoflagellate and ciliate cell densities showed apparent
seasonal variations, with high densities in spring and summer. Eugle­
noid cell densities were low in winter and high in spring, particularly
from April to May, when a large euglenoid bloom was observed (Fig. 4a).
Seasonal changes in the species composition were observed. Navicula
spp. were dominant in winter (from November 2020 to the end of March
2021), and euglenoids were dominant in April 2021 (Fig. 4b). During
summer, dinoflagellates were dominant. Centric diatoms dominated inautumn, and euglenoids dominated in spring (Fig. 4b).
Planktonic, benthic, and warm-water diatom species increased in thesummer and decreased in the winter (Fig. 5). Sporadic increases inplanktonic diatoms Chaetoceros spp. were observed in the area in August
to October 2020, June to July 2021, September to October 2021, March
2022, and July to August 2022 (Fig. 4b). The warm-water species
Chaetoceros didymus was observed from August to September 2020.
Bacteriastrum delicatulum increased rapidly from April to June 2021.
Decision tree analysis showed that planktonic species had higher celldensities at ≥ 17.6 ◦ C and < 0.207 µM in phosphate (Fig. 6a). Benthic

2.3. Data analysis
The relationships between the cell density of planktonic, benthic,
and warm-water diatom species and environmental factors (SST,3

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Regional Studies in Marine Science 69 (2024) 103322

Fig. 2. Seasonal changes in environmental parameters surface temperature, salinity, nutrients (DIN, PO₄3⁻, and Si(OH)4), DIN/P ratio, and Si/P ratio at the Yokoma
Port from August 2020 to August 2022.

species had higher cell densities at < 0.8 h in daylight hours and< 6.68 µM DIN (Fig. 6b). Warm-water species occurred at SST ≥ 4.5 ◦ Cand ≥ 17.3 µM in silicate or at SST ≥ 4.5 ◦ C, < 17.3 µM in silicate and< 0.146 µM in phosphate (Fig. 6c).
Annual changes in phytoplankton cell density and environmental
factors were compared for each season (Supplementary Fig. 1 andSupplementary Tables 1 and 2). In autumn, significantly lower celldensities were observed in planktonic diatoms (p < 0.01), warm diatoms(p < 0.05), and dinoflagellates (p < 0.01) in the second year (2021) thanin the first year (2020), associated with lower SST (p < 0.01), higher
nitrate (p < 0.05), higher DIN (p < 0.05), higher phosphate (p < 0.05),
higher silicate (p < 0.05), lower air temperature (p < 0.01), and lower
wind speed (p < 0.05) (Table 1). In winter, the cell densities of benthic
diatoms (p < 0.01) and ciliates (p < 0.05) were significantly lower thanthose in the second year, with lower SST (p < 0.05), higher nitrate(p < 0.01), higher nitrite (p < 0.05), higher ammonium (p < 0.05),
higher phosphate (p < 0.01), and lower tide levels (p < 0.05). No sig­
nificant differences in the environmental factors were found during
spring. In summer, benthic diatoms and dinoflagellates had significantly

lower cell densities (p < 0.01) and higher air temperatures (p < 0.05) in
the first year than in the second year (Table 1).
4. Discussion
4.1. Environmental features at the sampling site
The Tsushima Warm Current can be classified into four main types:
the Subpolar Front Current (SFC), Offshore North Current (ONC),
Offshore South Current (OSC), and Coastal Current (CC) (Yabe et al.,2021). The SFC flows along the Japan Sea coast of Hokkaido (Yabe et al.,2021). The SFC has minimal seasonal variation, with the majority (83%)
flowing northward along Hokkaido and the remaining 16% flowing into
the Tsugaru Strait (Yabe et al., 2021). Yokoma Port, the site of this study,
is located on the Japan Sea coast of Hokkaido and is expected to receive
SFC inflow throughout the year.
Nutrient concentrations in the sea around southern Hokkaido
showed a maximum (6 µM nitrate and nitrite, 0.4 µM phosphate, and8 µM silicate) in March, and after the phytoplankton bloom in April,
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Fig. 3. Seasonal changes in atmospheric condition (air temperature, daily amount of rainfall, daily average wind speed, and direction) (a-c) were refereed at theSuttsu Station from August 2020 to August 2022. Seasonal changes on the discharge of Shubuto River (d) at the Shubuto River Flow Observatory and Shiribetsu River(e) at the Nakoma Observatory in the last 5 years. Note the X-axis was different between atmospheric data and river discharge.

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with a decrease in salinity in April 2021 and 2022. During the same
period, an increase in air temperature and low precipitation suggested
snowmelt water inflow decreased salinity in Yokoma Port during April.
Adachi and Osawa (2005) reported that rivers supply DIN, and verticalmixing in Ishikari Bay supplies phosphate during winter. DIN andphosphate concentrations at Yokoma Port showed maximum values
from the end of March to early April when the river water increased.
This suggests that the nutrients were supplied by rivers near the sam­pling site. Specifically, the environmental conditions around Yokoma
Port were strongly influenced by river water.
The Tsushima Warm Current is a water mass with a salinity ≥ 33.7
(Higaki et al., 2008), which is higher than that around Yokoma Port.
Salinity exceeding 33.7 was observed from the end of July to
mid-September 2021 and at the end of June 2022. Nutrient concentra­
tions in the coastal surface layer during July and September in the
Tsushima Warm Current were reported to be 1–5 µM for DIN, 0.1 µM forphosphate, and 3–7 µM for silicate (Ikeuchi et al., 1998). Nutrient con­centrations (DIN 0.97–4.77 µM, phosphate 0.06–0.37 µM, silicate
10.3–17.9 µM) in the Yokoma Port from July-September 2021 were at
the same level, excluding silicate, as the previously reported Tsushima
Warm Current water. This suggests that the temporal inflow of the
Tsushima Warm Current occurred from July to September 2021.
4.2. Annual changes in phytoplankton and their factors
Diatoms are classified based on their ecological characteristics into
planktonic diatoms, which live a planktonic life, and benthic diatoms,
which adhere to the substrate. Comparing the results of decision trees
between planktonic and benthic diatoms, SST was the most importantfactor to increase cell density for planktonic diatoms, but daylight hour
was chosen as the factor for benthic diatoms. This difference could
indicate their physiological differences, however the detail difference
should be discussed individually based on incubation experimentbecause the boundary between the categories (planktonic vs benthic) is
not clear (Kawamura, 1994).
Interannual changes in phytoplankton and environmental factors
during autumn and winter, including lower phytoplankton cell density,
lower SST, and higher nutrient levels, were observed in the second year.
Decision tree analysis indicated that planktonic diatoms had the lowest
cell densities when the temperature was below 17.6 ◦ C and the numberof daylight hours was below 9.35 h. In the second autumn (Septem­
ber–October 2021), as the average SST (17.1 ◦ C) and daylight hours(6.37 h) were consistent with the conditions, phytoplankton growth was
limited by low SST and light conditions (Bissinger et al., 2008; Shikata
et al., 2010), and the resulting planktonic diatoms decreased. Nutrientswere higher in the second year, possibly due to reduced phytoplankton
growth and suppressed nutrient absorption. On the other hand, sporadic
increases in planktonic diatoms Chaetoceros spp. were observed fromspring to fall during the study period. Most of these periods, with the
exception of March 2022, had phosphate concentrations of about 0.2 µM
and water temperatures of about 18 ◦ C or higher. Based on the results ofthe decision tree analysis, cell densities were highest when phosphate
was < 0.207 µM and water temperature was > 17.55 ◦ C, suggesting thatthe period of increased Chaetoceros spp. was a favorable environment forthis species.
In winter, benthic diatoms decreased significantly in the second year.
Decision tree analysis indicated that benthic diatoms decreased when
daylight hours were between 0.8 and 6.95 h and SST was above 4.4 ◦ C.
However, both of these environmental factors in the winter of the firstand second years were within the conditions (0.8–6.95 daylight hours
and ≥ 4.4 ◦ C SST). The tide level should be discussed as an alternative
factor because it was significantly lower in the second year than in thefirst year. The wind speed was also lower in the second year (but thedifference was not significant). These results suggest that the physio­
logical disturbances driven by wind and waves may weaken during thesecond year. Note that this is the tide level at the sampling time and not

Fig. 4. Seasonal changes in microphytoplankton cell density and composition(a) and species composition (b) at the Yokoma Port from August 2020 to August2022. * : Centric diatoms, * *: Pennate diatoms.

nitrate and nitrite concentrations were 1–2 μM, phosphate was< 0.1 μM, and silicate was as low as 4 μM (Adachi and Osawa, 2005). In
the offshore southern Hokkaido, seasonal variation from winter tosummer in DIN and DIP are 0–6.6 µM and 0–0.64 µM, respectively(Kuribayashi et al., 2014). In the Oshoro Bay, DIN, DIP and silicate range
0.02–29.84 µM, 0.04–0.49 µM and 1.00–22.12 µM (Agboola et al.,2010). In this study, the maximum values of nitrate and nitrite (11.5 µM
in 2021 and 9.68 µM in 2022) and phosphate (1.18 µM in 2021 and
0.83 µM in 2022) levels were observed during March in each year, andthe minimum values of nitrate and nitrite (0.44 µM in 2021 and 0.32 µM
in 2022) and phosphate (0.061 µM in 2021 and 0.11 µM in 2022) levels
were observed during the spring and summer seasons. The silicate
content in the present study remained in the range of 10–20 µM during
spring and summer, which was higher than that in the offshore region
around southern Hokkaido. Compared the variation ranges in nutrientswith previous results, the present results showed a similar trend tocoastal region rather than offshore region. This regional difference may
be attributed to the nutrient inflow from the river (Yoshimura and Kudo,
2003).
The DIN:P ratio was generally below 16 as compared to the Redfieldratio, with the lowest values occurring from the end of April to early May(Adachi and Osawa, 2005). This suggests that DIN may be a limiting
factor for phytoplankton growth in the waters around the Yokoma Port.
The eastern area of Ishikari Bay is affected by water supplied by the
Ishikari River, and salinity was below 20 at the surface in April, when
discharge water increased due to snowmelt (Adachi and Osawa, 2005).
In our case, the Shiribetsu River and Shubuto River flow increasedaround April, which was observed in Suttsu Bay, and was associated6

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Regional Studies in Marine Science 69 (2024) 103322

Fig. 5. Seasonal changes in cell density of planktonic and benthic species (a) and warm-water species (b) at the Yokoma Port from August 2020 to August 2022.

the wave height. Among benthic diatoms, Navicula spp. were signifi­
cantly reduced in the second year. This species has a weaker attachment
to the substrate than other benthic diatoms (i.e., Cocconeis spp.)(Kawamura, 1998). Therefore, it is possible that this species detachedfrom the substrate and was suspended in the water under normal con­ditions. However, in the second year, detachment was difficult under
low-tide and low-wind-intensity conditions, resulting in low cell density.

from 10 to 30 (Xu et al., 2012) and 15–40 (Lee and Lim, 2006).
Therefore, this species can proliferate in environments in which the
inflow of river water reduces salinity. A harmful bloom of E. marina(2.21 × 103 cells mL− 1) was reported in the southern part of Banderas
Bay, Jalisco, Mexico, where hundreds of dead fish were found (Corté­s-Lara et al., 2010). It is considered that this is due to oxygen depletion in
the water and that blooms of this species do not directly affect humanhealth (Cortés-Lara et al., 2010). The euglenoid bloom in Suttsu (2.0 ×
103 cells mL− 1) was comparable to the reported bloom in the southernpart of Banderas Bay. The red tide of Euglenoids has been reported insouthern Hokkaido (Shimada, 2021), so fish harm and death may becaused in the waters in southern Hokkaido, including Yokoma port.
Regarding dominant species during spring bloom, majority studies
reports diatoms around Hokkaido. In Oshoro Bay, Chaetoceros spp.,
Coscinodiscus spp. and Thalassionema spp. are composed (Kanomata andKotori, 2000; Fukui et al., 2010). In Funka Bay, Thalassionema spp.,
Thalassiosira spp. and Chaetoceros spp. are dominant (Nakata, 1982;
Shinada et al., 1999). These previous studies were based on once- or
twice-monthly observations and may have missed short-term changes in
Eutreptiella spp. In Funka Bay, salinity at the center and mouth of the bay
was about 32.0 at low salinity, which was higher than in spring Suttsu

4.3. Euglenoid blooms in spring
Large euglenoid blooms (mean cell density: 2.0 × 103 cells mL− 1)
were identified in four samples during April 2021 and April–May 2022.
There are two species of marine euglenoids: Eutreptiella gymnastica andE. marina. E. marina also occurred from spring to summer in previous
studies conducted in the Sea of Marmara, an inland sea connecting theBlack Sea and the Mediterranean Sea, where blooms with cell densities
reaching 30.2 × 103 cells mL− 1 were observed in spring (Taş, 2019). The
DIN:P ratio decreased from April to May when euglenoid blooms were
found at Yokoma Port, and there was a negative correlation between the
DIN:P ratio and E. marina cell density (r = − 0.21) (Taş, 2019).
E. gymnastica has salinity tolerance and can survive at salinities ranging
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Regional Studies in Marine Science 69 (2024) 103322(Nakata, 1982). Therefore, it is possible that phytoplankton other thanEutreptiella spp. did not dominate because they were not limited by low
salinity and were able to proliferate.
4.4. Appearance trends of warm-water species
As for the occurrence of warm-water species, Chaetoceros didymus
appeared in August-September and October 2020 but was not observed
until the spring of the following year. Since the optimal temperature forthe growth of warm-water species is 16–27 ◦ C (Creswell, 2010), SST< 16 ◦ C during mid-October 2020 restricted the growth of thewarm-water species C. didymus. As an important life strategy, many
species in the genus Chaetoceros form resting stage cells and germinate
even after 12 months at 4 ◦ C (Matsubara, 2017). Among the warm-water
species, C. didymus, C. coarctatus, and C. eibenii have been identified, andC. didymus forms resting spores (Ishii et al., 2011). Therefore, C. didymus
may have overwintered as a resting-stage cell in the study area. How­ever, germination experiments of resting stages of Chaetoceros species
suggest that it takes 10–13 days to germinate at 15 ◦ C and 11 h of light,
and more time is needed at lower temperatures (Matsubara, 2017). This
study observed > 15 ◦ C SST after May–June, with average daylight
hours ranging from 6.6 to 10.4 h in spring and summer. Since phyto­plankton cell density also increases around May–June, the growth ofother phytoplankton (cold-water species) begins earlier than thegermination of this species, even if the resting-stage cells of C. didymus
survive until the following year.
Bacteriastrum delicatulum increased rapidly from April to June 2021.
In the northern Adriatic Sea, a higher cell density of B. delicatulum (119cells mL− 1) was observed from June to August (Godrijan et al., 2012),
consistent with our Yokoma Port results. In addition, the cell density ofB. furcatum, which is classified in the same genus as B. delicatulum, waspositively correlated with PO4 concentration (Bosak et al., 2016).
Phosphate was at its maximum level in April at Yokoma Port, suggestingthat B. delicatulum could grow by taking advantage of the abundant
phosphate present in this study. However, no warm-water species wereobserved in July–August 2021. This may be due to a decrease in nutri­ents caused by dinoflagellate blooms during the same period, which may
have prevented the growth of warm-water species.
In the study area, warm-water species occurred sporadically, withsignificant seasonal variations in their species composition. The decision
tree indicated that ≥ 4.5 ◦ C SST was the most critical factor within thecondition in all seasons except winter. Furthermore, as mentioned
above, the seasonal variation in the Tsushima Warm Current is minimal(Yabe et al., 2021). Considering these facts, the warm-water species
possibly had not been established in the area, but rather that a cell wastransported by the Tsushima Warm Current. The establishment ofwarm-water species in the area is considered to be difficult because of
the short period (4 months from June to September) at optimal tem­
perature (16–27 ◦ C) and the challenging competitive situation for
acquiring daylight and nutrients with other phytoplankton (cold-water
diatoms, dinoflagellates, and euglenoids) during the nutrient-rich spring
season.
5. Conclusion
Diatoms were mainly dominant at Yokoma Port in Suttsu, Hokkaido,
and their species composition showed evident seasonal changes. As anannual change, phytoplankton cell densities in the autumn and winter ofthe second year were significantly lower than those of the same period in
the first year. This may be due to the low SST and daylight hours, which
prevent phytoplankton growth. A significant decrease in the number ofbenthic diatoms was also observed during the winter of the second year,
suggesting that calm sea conditions (low tide levels and wind speed)decreased the detachment of benthic diatoms (Navicula spp.). Largeeuglenoid blooms were observed yearly from April to May. This may be
because of their high salinity tolerance and ability to exclusively utilize

Fig. 6. Decision trees of planktonic (a), benthic (b), and warm-water species (c)in diatoms at the Yokoma Port from August 2020 to August 2022. Warm-water
species were analyzed as presence /absence data.

8

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Regional Studies in Marine Science 69 (2024) 103322

Table 1Comparison of environmental factors between the years in each season at the Yokoma Port from August 2020 to August 2022. Differences between the groups were
tested by Wilcoxon signed rank test. Values are mean ( ± S.D.). Wilco.: Wilcoxon test. * : p < 0.05, * *: p < 0.01, * * * : p < 0.001.
Parameters

Benthic diatoms(cells mL− 1)
Planktonic
diatoms(cells mL− 1)
Warm-water
diatoms(cells mL− 1)
Dinoflagellates(cells mL− 1)
Euglenoids(cells mL− 1)
Ciliates (cells
mL− 1)SST (◦ C)
SSS
NO3- (µM)
NO-2 (µM)
NH4 (µM)
DIN (µM)
PO34 (µM)
Si(OH)4 (µM)
DIN/P
Si/P
Air temperate(◦ C)
Wind speed (ms− 1 )
Rainfall (mm)Tide level (cm)Daylight hours(h)AutumnWinter

1st year(2020)

2nd year(2021)

6.78± 6.5968.4± 33.77.93± 7.5811.1± 13.2

4.56± 5.120.22± 0.6153.2± 66.2
3.87± 5.5018.4± 26.6
20.6± 2.7533.2± 0.441.37± 0.720.18± 0.05
1.41± 0.652.96± 1.250.12± 0.0314.8± 4.8523.8± 7.14129± 54.018.0± 4.505.24± 1.972.17± 5.1528.9± 6.744.77± 3.534.15± 2.812.80± 2.641.83± 2.6917.1± 4.1532.9± 0.623.21± 1.420.22± 0.101.31± 0.544.73± 1.540.30± 0.0921.9± 4.51
16.4± 5.4979.2± 25.7
13.7± 3.972.85± 0.990.05± 0.1524.9± 4.85
6.37± 2.56Wilco.

SpringSummer

1st year(2020)

2nd year(2021)

Wilco.

1st year(2021)

2nd year(2022)

36.5± 28.31.20± 1.334.21± 3.039.52± 17.9**37.3± 42.124.5± 24.5*

3.77± 9.910.78± 1.12**8.81± 26.01.77± 3.3114.8± 32.66.09± 3.4832.0± 2.47
4.66± 2.600.31± 0.161.47± 0.866.45± 2.760.36± 0.2221.6± 18.5
20.7± 10.665.7± 44.40.74± 4.634.37± 1.432.18± 2.10
11.9± 8.121.75± 2.850.29± 0.17
2.09± 4.878.51± 8.874.64± 1.5332.0± 1.656.04± 1.440.23± 0.050.89± 0.327.15± 1.570.42± 0.1527.7± 12.717.8± 4.1569.4± 36.1-0.32± 2.823.72± 1.402.46± 3.307.54± 5.832.05± 2.90

**

**

*

*
*
*

**
*

*
*

**
*
*

**

*

nutrients from the inflow of river water during the snowmelt season.
Some euglenoids caused fish death at cell densities similar in scale tothose of the blooms observed in this study, which is of concern in thevicinity of Suttsu Bay. Although several warm-water species wereobserved, they occurred sporadically in the area, suggesting that they
were transported by the Tsushima Warm Current rather than by estab­lished or resting-stage cells. This study clarified the relationship be­
tween seasonal and annual changes in the phytoplankton communityand environmental conditions in Suttsu. These findings will help predict
the impact of climate change on warm water inflow areas. Suitableconditions can be established for warm-water species if sea surface
temperatures increase due to global warming and the optimal watertemperatures are prolonged. Monitoring the environment (temperatureand nutrients) and phytoplankton communities should continue toevaluate the ecosystem in the Anthropocene climate.

Wilco.

1st year(2021)

2nd year(2022)

Wilco.

14.4± 8.395.97± 5.21

18.5± 8.2017.9± 23.03.68± 2.1150.2± 55.1*0.07± 0.070.00± 0.000.68± 1.300.20± 0.3820.3± 21.81028± 19470.97± 0.999.90± 1.9430.9± 0.75
1.46± 0.680.26± 0.271.00± 0.982.72± 1.540.26± 0.13
23.5± 6.20
11.0± 3.92119± 66.27.96± 2.633.88± 2.330.90± 1.8013.8± 5.158.04± 4.120.55± 0.15
985
± 858
0.28
± 0.1910.5± 0.5231.2± 2.450.75± 0.640.09± 0.030.25± 0.06
1.09± 0.620.30± 0.1625.0± 19.6
3.66± 0.6873.0± 22.911.2± 1.933.23± 0.870.17± 0.2414.7± 3.6810.4± 3.0862.2± 34.95.43± 7.810.63± 0.86
19.9± 3.1932.2± 2.141.59± 1.140.13± 0.050.86± 0.562.58± 1.440.19± 0.1025.5± 18.1
14.4± 4.55
171
± 106
17.9
± 3.274.73± 1.580.05± 0.15
25.5± 2.698.05± 2.7610.4± 11.92.24± 3.288.73± 10.521.0± 1.9332.4± 1.001.65± 0.970.11± 0.040.74± 0.332.50± 1.190.22± 0.11
23.6± 12.1
12.3± 5.44121± 83.020.8± 3.643.45± 0.944.25± 9.2525.4± 6.566.60± 4.26**

Formal analysis, Funding acquisition, Methodology. Matsuno Kohei:
Conceptualization, Methodology, Supervision, Visualization. Yamazaki
Tomoyasu: Investigation. Tatamisashi Shoko: Formal analysis,
Methodology.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Data Availability
Data will be made available on request.
Acknowledgments

CRediT authorship contribution statement

This work was supported by the Japan Society for the Promotion of
Science (JSPS) KAKENHI [grant numbers JP22H00374 (A) andJP21H02263 (B)].

Morimoto Kyosei: Conceptualization, Investigation, Methodology,
Visualization, Writing – original draft, Writing – review & editing.
Hamao Yusuke: Investigation, Methodology. Wakita Masahide:9

K. Morimoto et al.

Regional Studies in Marine Science 69 (2024) 103322

Author statement

Kakumu, A., Morita, K., Shimada, H., Yamagichi, A., Imai, I., 2018. First detection of thenoxious red tide dinoflagellate Karenia mikimotoi and bloom dynamics in 2015 and2016 in Hakodate Bay, Hokkaido, northern Japan. Bull. Plankton Soc. Jpn. 65, 1–11.
Kanomata, I., Kotori, M., 2000. Long-term changes in net-collected phytoplankton
assemblage during spring blooms in Oshoro Bay, Hokkaido, Japan. Bul. Plankton
Soc. Jpn. 47, 93–100.
Kawamura, T., 1994. Taxonomy and ecology of marine benthic diatoms. Mar. Fouling10, 7–25.
Kawamura, T., 1998. Benthic diatoms: ecology and the role in the animal communities.
Sess. Org. 15, 15–22.
Kida, S., Takayama, K., Sasaki, Y.N., Matsuura, H., Hirose, N., 2021. Increasing trend inJapan Sea throughflow transport. J. Oceanogr. 77, 145–153.
Kondo, S., Nakao, R., Iwataki, M., Sakamoto, S., Itakura, S., Matsuyama, Y., Nagasaki, K.,
2012. Heterocapsa circularisquama coming up north－Mass mortality of Pacificoysters due to its blooming at Lake Kamo in Sado Island, japan－. Nippon Suisan
Gakkaishi 78, 719–725 (in Japanese with English abstract).
Kuribayashi, T., Abe, T., Montani, S., 2014. Nutritional status of seaweed communities
along the west coast of the Japan Sea off Hokkaido, Japan, from monitoring data anddetecting δ15N Records in Saccharina Specimens. Bull. Mar. Sci. 52, 75–81.
Lee, C., Lim, W., 2006. Variation of harmful algal blooms in Masan-Chinhae Bay. Sci.
Asia 32, 51–56.
Matsubara, T., 2017. Influences of water temperature and photoperiod on germination/
rejuvenation and growth of marine diatoms. Bull. Plankton Soc. Jpn. 64, 45–49.
Nakata, A., Tanaka, I., 2002. Seasonal and year-to-year variability of baroclinic volumetransport of the Tsushima Warm Current, off the west of Hokkaido. Sci. Rep.
Hokkaido Fish. Exp. Station 63, 1–8.
Nakata, K., 1982. Species composition of phytoplankton community of Funka Bay in the
Spring Bloom, 1981. Bull. Jpn. Soc. Fish. Oceanogr. 41, 23–37.
Nishida, Y., Kanomata, I., Tanaka, I., Sato, S., Takahashi, S., Matsubara, H., 2003.
Seasonal and interannual variations of the volume transport through the Tsugaru
Strait. Oceanogr. Jpn. 12, 487–499 (in Japanese with English abstract).
Sarthou, G., Timmermans, K.R., Blain, S., Tréguer, P., 2005. Growth physiology and fateof diatoms in the ocean: a review. J. Sea Res. 53, 25–42.
Shikata, T., Sakurada, K., Jomoto, Y., Onji, M., Yoshida, M., Ohwada, K., 2010. Effect oftemperature, salinity and light irradiance on phytoplankton growth in the Yatsushiro
Sea. Nippon Suisan Gakkaishi 76, 34–45 (in Japanese with English abstract).
Shimada, H., 2021. Long-term fluctuation of red tide and shellfish toxin along the coastof Hokkaido (Review). Sci. Rep. Hokkaido Fish. Res. Inst. 100, 1–12.
Shimada, H., Kanamori, M., Yoshida, H., Imai, I., 2016. First record of red tide due to theharmful dinoflagellate Karenia mikimotoi in Hakodate Bay, southern Hokkaido, in
autumn 2015. Nippon Suisan Gakkaishi 82, 934–938 (in Japanese with English
abstract).
Shinada, A., Shiga, N., Ban, S., 1999. Structure and magnitude of diatom spring bloom in
Funka Bay, southwestern Hokkaido, Japan, as influenced by the intrusion of Coastal
Oyashio Water. Plankton Biol. Ecol. 46, 24–29.
Steidinger, K.A., Targen, K., 1997. Dinoflagellates. In: Tomas, C.R. (Ed.), Identifying
Marine Phytoplankton. Academic Press, San Diego, pp. 387–584.
Taş, S., 2019. Microalgal blooms in a eutrophic estuary (Golden Horn, Sea of Marmara)following a remediation effort. Bot. Mar. 62, 537–547.
Wakita, M., Sasaki, K., Nagano, A., Abe, H., Tanaka, T., Nagano, K., Sugie, K., Kaneko, H.,
Kimoto, K., Okunishi, T., Takada, M., Yoshino, J., Watanabe, S., 2021. Rapid
Reduction of pH and CaCO3 saturation state in the Tsugaru Strait by the intensified
Tsugaru Warm Current during 2012–2019. Geophys. Res. Lett. 48, e2020GL091332.
Xu, N., Pang, S., Shan, T., Liu, F., Zhao, X., Gao, S., 2012. Molecular identification andculture trials of Eutreptiella gymnastica (Eutreptiales, Euglenophyceae). Chin. J.
Oceanol. Limnol. 30, 446–455.
Yabe, I., Kawaguchi, Y., Wagawa, T., Fujio, S., 2021. Anatomical study of Tsushimawarm current system: determination of principal pathways and its variation. Prog.
Oceano 194, 102590.
Yoshimura, T., Kudo, I., 2003. Riverine nutrient loading and their impact on primary
production in Funka Bay, Japan. J. Oceano 12, 185–193 (in Japanese with English
abstract).

KM and KM designed the study; TY preformed field sampling; KM,
YH, MW and ST analyzed the samples; KM and KM analyzed the data;
and KM and KM wrote the paper with contributions from all authors.
Appendix A. Supporting information
Supplementary data associated with this article can be found in the
online version at doi:10.1016/j.rsma.2023.103322.
References
Adachi, K., Osawa, Y., 2005. Variation in nutrient concentrations in the coastal water ofsouthwestern Hokkaido. Proc. Civ. Eng. Ocean 21, 635–639 (in Japanese withEnglish abstract).
Agboola, J.I., Uchimiya, M., Kudo, I., Kido, K., Osawa, M., 2010. Dynamics of pelagic
variables in two contrasting coastal systems in the western Hokkaido coast off Otaru
port, Japan. Estuar., Coast. Shelf Sci. 86, 477–484.
Bissinger, J.E., Montagnes, D.J., harples, J., Atkinson, D., 2008. Predicting marine
phytoplankton maximum growth rates from temperature: improving on the Eppley
curve using quantile regression. Limnol. Oceanogr. 53, 487–493.
Bosak, S., Godrijan, J., Šilović, T., 2016. Dynamics of the marine planktonic diatom
family Chaetocerotaceae in a Mediterranean coastal zone. Estuar. Coast. Shelf Sci.
180, 69–81.
Cortés-Lara, M.C., Cortes-Altamirano, R., Alonso-Rodríguez, R., Cupul-Magana, A.L.,
2010. Eutreptiella marina (Euglenophyceae) bloom causes significant fish kills in
Banderas Bay, Jalisco, México. Harmful Algae News: IOC Newsl. Toxic. Algae Algal
Blooms 42, 12–13.
Creswell, L., 2010. Phytoplankton culture for aquaculture feed. Southern Regional
Aquaculture Center.
Fujii, M., Takao, S., Yamaka, T., Akamatsu, T., Fujita, Y., Wakita, M., Yamamoto, A.,
Ono, T., 2021. Continuous monitoring and future projection of ocean warming,
acidification, and deoxygenation on the subarctic coast of Hokkaido, Japan. Front.
Mar. Sci. 8, 590020.
Fukui, D., Kitatsuji, S., Ikeda, T., Shiga, N., Yamaguchi, A., 2010. Long-term changes in
the abundance and community structure of net-phytoplankton in Oshoro Bay,
Hokkaido. Bull. Plankton Soc. Jpn. 57, 30–40 (in Japanese with English abstract).
Fukuyo, Y., Inoue, H., Takayama, H., 1997. Division dinophyta. An Illustrated Guide to
Marine Plankton in Japan. Tokai University Press (Ed), pp. 31–146.
Godrijan, J., Marić, D., Imešek, M., Janeković, I., Schweikert, M., Pfannkuchen, M.,
2012. Diversity, occurrence, and habitats of the diatom genus Bacteriastrum(Bacillariophyta) in the northern Adriatic Sea, with the description of B. jadranum sp.
nov. Bot. Mar. 55, 415–426.
Hasle, G.R., Syvertsen, E.E., 1997. Marine diatoms. In: Tomas, C.R. (Ed.), Identifying
Marine Phytoplankton. Academic Press, San Diego, pp. 5–385.
Higaki, N., Isoda, Y., Isogai, Y., Yahaba, H., 2008. Seasonal variations of water system
distributions and flow patterns off the west coast of Hokkaido. J. Oceanogr. 17,
223–240 (in Japanese with English abstract).
Ikeuchi, H., Kamizono, M., Sugino, K., 1998. Distributions of nutrient and plankton in the
Genkai-nada and the Eastern Tsushima Channel. Bull. Fukuoka Fish. Mar. Technol.
Res. Cent. 8, 97–106.
Ishii, K.I., Iwataki, M., Matsuoka, K., Imai, I., 2011. Proposal of identification criteria forresting spores of Chaetoceros species (Bacillariophyceae) from a temperate coastal
sea. Phycologia 50, 351–362.
Itakura, S., 2001. Ecological role of the resting stage cells of planktonic diatoms. Bull.
Plankton Soc. Jpn. 48, 101–103.
Itakura, S., Imai, I., Itoh, K., 1992. Morphology and rejuvenation of Skeletonema costatum(Bacillariophyceae) resting cells from the bottom sediments of Hiroshima Bay, theSeto Inland Sea, Japan. Bull. Plankton Soc. Jpn. 38, 135–145.

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