Title

North-south changes of zooplankton community and copepods population along the 110°E line in the Indian sector of
the Southern Ocean during the austral summer

Author(s)Sugioka, Rikuto; Matsuno, Kohei; Takahashi, Keigo D.; Makabe, Ryosuke; Takahashi, Kunio T.; Moteki, Masato;
Odate, Tsuneo; Yamaguchi, Atsushi

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北海道大学水産科学研究彙報, 71(1), 39-50

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2021-08-03

10.14943/bull.fish.71.1.39

http://hdl.handle.net/2115/82343

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bull.fish.71.1.39.pdf

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Hokkaido University Collection of Scholarly and Academic Papers : HUSCAPBull. Fish. Sci. Hokkaido Univ.
71(1), 39-50, 2021.
DOI 10.14943/bull.fish.71.1.39

Sugioka et al. : North-south changes of zooplankton in Southern Ocean

North-south changes of zooplankton community and copepods population alongthe 110°E line in the Indian sector of the Southern Ocean during the austral summer
Rikuto Sugioka1), Kohei Matsuno1,2), Keigo D. Takahashi3), Ryosuke Makabe3,4,5), Kunio T. Takahashi3,4),
Masato Moteki4,5), Tsuneo Odate3,4) and Atsushi Yamaguchi1,2)(Received 22 March 2021, Accepted 1 April 2021)
Abstract   Various zooplankton studies have been conducted in the Indian sector of the Southern Ocean, but studies investigating
spatial changes in both meso- and macro-zooplankton populations are limited. To reveal the north-south change of the mesoand macro-zooplankton community structure and the population structure of large copepods, we investigated zooplankton
communities collected by the Ocean Research Institute (ORI) net along the 110°E transect in the Indian sector of the Southern
Ocean during the austral summer. The zooplankton abundance and biomass were high at the stations north of 60°S and was
dominated by large copepods, especially Calanoides acutus. A cluster analysis identified four zooplankton community groups.
The distribution pattern of the community groups is likely to be related to the frontal system. The population structure of thedominant species of C. acutus, Calanus propinquus and Rhincalanus gigas had a similar tendency ; early copepodite stages wereabundant in the northern region, while late copepodite stages dominated in the southern region. The meridional changes in the
population structure are explained by different reproduction periods in each region related to sea-ice melt timing.
Key words : Antarctic Ocean, Zooplankton community, Copepoda, Population structure

Introduction
In the Indian sector of the Southern Ocean, physical fronts,
which are boundaries of different water masses, tend todevelop because strong ocean currents such as the Antarctic
Circumpolar Current (ACC) are present (Moore et al., 1999 ;Sokolov and Rintoul, 2002). Four fronts are defined by thepotential water temperature and distributed from north tosouth as follows ; the southern branch of the Polar Front (PFS), the northern branch of the Southern Antarctic Circumpolar
Current Front (SACC N), the Southern Boundary (SB) andthe Antarctic Slope Front (ASF) (Sokolov and Rintoul, 2002 ;Aoki et al., 2006). Around the fronts, high primary production is observed due to the input of nutrients from upwelling(Moore et al., 1999 ; de Baar et al., 1995 ; Laubscher et al.,1993). However, because local eddies repeatedly develop
and disappear in this area, ocean currents such as the ACC
often meander, and a sporadic water intrusion occurs between--

different water masses (Chiba et al., 2001). As above, theIndian sector of the Southern Ocean is characterized by fronts
related to high primary production and short term changes in
the hydrographic conditions following local scale current
branches.
Zooplankton have a shorter life cycle and higher turnoverrate between generations than predators such as marine mammals, sea birds and fish in the Southern Ocean. Because
they are close to the base (i.e., primary production) of the foodweb, their response (e.g., growth) to environmental change issensitive (Takahashi et al., 1998). The zooplankton community structure also quickly responds to environmental changes
in this study region (Chiba et al., 2001). Generally, zooplankton communities have different structures in the northand the south ; in northern areas, salps, small euphausiids,
amphipods, copepods and chaetognaths dominate, while insouthern areas, large euphausiids and fish larvae dominate(Hosie and Cochran, 1994 ; Hosie et al., 1997). Because of1)

Faculty/Graduate School of Fisheries Science, Hokkaido University(北海道大学大学院水産科学研究院海洋生物学分野)
2)
Arctic Research Center, Hokkaido University(北海道大学北極域研究センター)
3)
The Graduate University for Advanced Studies, SOKENDAI(総合研究大学院大学)
4)
National Institute of Polar Research(国立極地研究所)
5)
Tokyo University of Marine Science and Technology(東京海洋大学)— 39 —--

Bull. Fish. Sci. Hokkaido Univ. 71(1), 2021

the meandering of ocean currents such as the ACC, zooplankton communities also show longitudinal differences (Hosie etal., 2000 ; Chiba et al., 2001 ; Hosie et al., 2014).
Copepods are the most dominant species in the zooplankton community, and Calanoides acutus, Calanus propinquus,
Metridia gerlachei, Metridia lucens, Paraeuchaeta antarctica
and Rhincalanus gigas are the dominant copepods (Chiba etal., 2001 ; Pane et al., 2004). Among the copepods, species specific distributions are observed ; C. acutus and R.
gigas are approximately ten times more abundant than C. propinquus in the ACC (Bathman et al., 1993) and the Scotia Sea(Voronina, 1975 ; Voronina et al., 1978 ; Vladimirskaya,
1978 ; Marin, 1987 ; Bondarenko and Polonsky, 1990 ;Atkinson, 1991). However, in this study area (i.e., theIndian sector of the Southern Ocean), although many studies
have been conducted using a Continuous Plankton Recorder(CPR) or a North Pacific Standard Net (NORPAC) (Takahashi
et al., 1998 ; Chiba et al., 2001 ; Takahashi et al., 2011 ;Takahashi et al., 2017), few studies have investigated thecommunity structure of meso and macro zooplankton.
Regarding to population structure of copepods, it is informative parameter to understand life cycle of the copepods andpredict the future changes on the species abundance by modelling (e.g., Matsuno et al., 2020). However, in the Indian
sector of the Southern Ocean, the information is limited.
Because of the problem, it is still unclear how relate the environmental factors including sea ice extent to the population
structure.
The purpose of this study was to reveal the north southchanges of the zooplankton community structure, copepod
species composition and population structure along the 110°E
transect in the Indian sector of the Southern Ocean during the
austral summer. To this end, we test two hypotheses : 1)
that the zooplankton community will differ by fronts, 2) thatthe population structure of copepods will associate with sea
ice extent.
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Materials and Methods
Field sampling
The sampling was conducted during a UM 17 09 cruisefrom 31 December 2017 to 22 January 2018 by the training
vessel Umitaka maru of the Tokyo University of Marine Science and Technology. Zooplankton were collected at 9 stations from 59.00 to 64.68°S on the 110°E transect by anoblique tow using an Ocean Research Institute (ORI) net(mouth diameter : 1.6 m ; mesh size : 500 µm) from adepth of 200 m to the surface (Fig. 1, Table 1). COMPACT TD (JFE Advantech Co., Ltd.) was attached to the net
opening to measure the maximum depth. A flow meter(General Oceanics, Inc.) was attached to the center of the net
ring to measure the volume of filtered sea water. Zooplankton samples were preserved in 5% neutral formaldehyde sea---

Fig. 1.

Location of the sampling stations along the 110°E in the
Indian sector of the Southern Ocean during January
2018.

water and taken back to the laboratory. At each station,
CTD (Sea Bird Electronics Inc., SBE 911plus) cast was conducted to measure the temperature, salinity and fluorescence. The seawater was filtered onto a GF/F filter andchlorophyll a (chl. a) was extracted with N,N dimethylformamide and measured with a fluorometer (Turner Designs
Inc., 10 AU 005). For converting fluorescence to chl. aconcentration (mg m－3), we applied the relationship between
measured chl. a concentration and fluorescence (Appendix 1).
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Identification and enumeration
In the laboratory, each sample was divided from 1/16 to
1/256 using a Motoda plankton splitter (Motoda, 1959).
Number of the enumerated individuals were ranged from 176
to 642 (mean 356) per sample. All animals were enumerated at the taxa level under a stereo microscope and weighedwith a precision of 0.01 g using an electron microbalance(AT261, METTLER TOLEDO). Copepods were identified
to the species level and copepodid stage level according to
Bradford Grieve et al. (1999). The mean copepodid stage(MCS) of dominant large sized copepods (C. acutus, C. propinquus and R. gigas) was calculated using the following
equation :--- i 1 i  Ai
6 i 1 Ai
6

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MCS 

where i (1 6 indicates C1 C6) is the copepodid stage, and Ai(ind. m－3) is the abundance of the ith copepodid stage.

— 40 —--

Sugioka et al. : North-south changes of zooplankton in Southern Ocean
Table 1. Summary of ORI net sampling and time since sea-ice melt (days) derived by AMSR2 at each station along the 110°E in the
Indian sector of the Southern Ocean during January 2018.
Posotion

Station

start

C01
KC5
KM4
D01
C03
C04
B01
C05
KC6
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Date & Time (UTC)finish59.00 °S

58.98 °S

110.00 °E
59.99 °S
110.01 °E
60.93 °S
109.97 °E
61.88 °S
109.15 °E
62.04 °S
109.99 °E
63.01 °S
110.00 °E
63.50 °S
110.06 °E
64.00 °S
110.02 °E
64.68 °S
109.77 °E

110.01 °E
59.97 °S
110.03 °E
60.95 °S
109.99 °E
61.90 °S
109.16 °E
62.06 °S
109.98 °E
63.03 °S
109.99 °E
63.51 °S
110.09 °E
64.01 °S
110.04 °E
64.68 °S
109.73 °E

Filtered
water
(m3)

Time sincesea-icemelt (days)

start

finishTowingdepth(m)2018/1/7 14 : 30

2018/1/7 14 : 59

128

2,775

-

2018/1/8 3 : 00

2018/1/8 3 : 28

167

3,907

100

2018/1/8 21 : 10

2018/1/8 21 : 40

169

4,486

68

2018/1/9 10 : 24

2018/1/9 10 : 52

187

4,026

59

2018/1/9 17 : 09

2018/1/9 17 : 40

139

3,141

59

2018/1/10 6 : 25

2018/1/10 6 : 55

167

2,815

51

2018/1/10 13 : 10

2018/1/10 13 : 38

177

3,345

42

2018/1/10 19 : 57

2018/1/10 20 : 25

168

3,568

31

2018/1/11 11 : 11

2018/1/11 11 : 38

142

3,888

25

: not available (no sea ice coverage).

struct the matrix of Bray Curtis similarity. This similarity
was subjected to cluster analysis using the Unweighted Pair
Group Method using Arithmetic mean (UPGMA). This
analysis was produced using Primer7. One way ANOVA
and the Turkey Kramer test were performed in StatView toassess the difference of the abundance among clusters. To
clarify the relationship between the zooplankton community
and the environmental parameters (latitude, temperature,salinity and chl. a concentration), BEST BIOENV was performed using PRIMER v7 software (PRIMER E Ltd.).
To reveal the relationship between MCS and TSM or chl. aconcentration, we tested linear regressions. However, thecomparison between MCS and chl. a concentration could not
be made because there were many lacks on chl. a data.
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Satellite observation
To evaluate the timing of sea ice retreat at each station, the
AMSR2 (Advanced Microwave Scanning Radiometer 2)
standard sea ice concentration (SIC) product was obtained
from the Arctic Data archive System (ADS, https://ads.nipr.
ac.jp/). The AMSR2 data were supplied by the Japan Aerospace Exploration Agency through the ADS, under a partnership between the National Institute of Polar Research and
JAXA. Using the data, we calculated the time since sea ice
melt (TSM). The TSM was defined as the number of theopen water days from the last date when the SIC fell below
20%, prior to the observed annual sea ice minimum acrossthe study region, to the sampling date.
To investigate the spatial and temporal variability of chlorophyll a concentration around the sampling region, ocean
color data from November 2017 to January 2018 were downloaded from the GlobColour project (https://hermes.acri.fr/
index.php). The merged products, calculated with aweighted averaging merging (AVW) method, had a spatial
resolution of 4 km and a temporal resolution of 8 days.
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Statistical analysis
The data on the number of individuals (X ; ind. m－3) per
taxon (in the case of copepods, per species) were logarithm
transformed (Log10 [X+1]) in order to reduce the bias of abundant species, and then the Bray Curtis index was used to con-

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Results
Sea ice extent and surface chlorophyll a-

From satellite observation, TSM was calculated from 25 to
100 days except st. C01 because of no ice coverage though ayear. The TSM showed the longest at st. KC5 and the shortest at st. KC6 (Table 1). For phytoplankton bloom, it isonset from early December around 60°S (i.e., sts. KC5 andKM4) (Fig. 2). Then, the phytoplankton bloom shifted
gradually to southward over time, but the bloom area was notobserved sts. C05 and KC6 during the sampling period (Fig.
2).

— 41 —-

Bull. Fish. Sci. Hokkaido Univ. 71(1), 2021

Fig 2.

Satellite-derived surface chlorophyll a concentration around the 110°E transect in the Indian sector of the Southern Ocean from November
2017 to January 2018.

Hydrography

Abundance, biomass and species composition

The water temperature varied from －1.8 to 3.0°C and washigh at the surface layer and at 200 m, while it was low(≤ 0°C) between 50 and 150 m (Fig. 3A). The salinity
ranged from 33.2 to 35.0 and was low at the surface (Fig.
3B). For north south changes, lower temperature and salinity values were observed in the southern stations (Fig. 3A and
B). Because physical fronts can be defined according to thepotential temperature (cf. Sokolov and Rintoul, 2002 ; Aoki
et al., 2006), four fronts were observed as follows ; PF S was
between st. KC5 and KM4, SACC N was between st. KM4
and D01, SB was between st. B01 and C05 and ASF was
between st. C05 and KC6. A high chl. a concentration wasseen at 40 m, and a massive phytoplankton bloom wasobserved around 62 63°S (Fig. 3C).

In the sampling region, the zooplankton abundance ranged
from 12.99 to 247.90 ind. m－3, and the biomass varied from
21.26 to 346.87 mg m－3 (Fig. 4A and B). Both of thesemeasures were high at the stations north of 60°S (i.e., st. C01
and KC5), while the biomass was high north of 60°S and at
st. KC6, the most southern station. Copepods were the mostdominant taxon, contributing to 68 93% of the total abundance of each zooplankton community (Fig. 4A). Copepods, chaetognaths and amphipods occupied 29 90%, 1 16%
and 1 23% of the total zooplankton biomass, respectively(Fig. 4B).
Copepod abundance was high at st. C01 (126 ind. m－3)
and KC5 (92 ind. m－3), and less than 70 ind. m－3 at thesouthern stations (Fig. 5). At all stations, C. acutus and R.
gigas comprised 21 68% and 13 52% of the total copepods

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— 42 —--

Sugioka et al. : North-south changes of zooplankton in Southern Ocean

higher (about ten times) than those of the other groups (Table
3 and Fig. 6B). Group A was characterized by a high abundance of copepods C. acutus, C. propinquus, Calanus simillimus and R. gigas compared to the other groups.
Chaetognaths and euphausiids had significant differences
between group A and D and between group A and B, respectively (Table 3).
Copepod population structure
The north south changes of the population structure for thedominant species (C. acutus, C. propinquus, M. gerlachei and
R. gigas) varied with the species. For C. acutus and C. propinquus, early copepodid stages (from one (C1) to four (C4))
dominated at the northern stations, while late copepodid
stages (from five (C5) to adults (C6)) dominated at the southern stations (Fig. 7A and B). In the case of M. gerlachei,
only C4 and older stages occurred, but early copepodid stages(C1 C3) were absent at the all stations (Fig. 7C). C5 C6 of
R. gigas occurred frequently at the most northern station (st.
C01), and early stages (C1 C4) were increased at the otherstations (Fig. 7D). Comparing the MCS of the dominant
species and the TSM, a significant negative relationship was
found for only C. acutus (Fig. 8). However for C. propinquus and R. gigas, the relationships were not significant (p =0.082 and 0.893, respectively).
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Discussion
Zooplankton community structure and hydrography

Fig. 3.

Many studies about zooplankton communities have beenreported using CPR and NORPAC in this research area (Takahashi et al., 1998 ; Chiba et al., 2001 ; Takahashi et al.,2011 ; Takahashi et al., 2017). CPR is a sampling device
that is towed from a ship to continuously collect surface (6 8m depth) zooplankton (Takahashi et al., 2011 ; Hosie et al.,2014 ; Takahashi et al., 2017). The NORPAC net canquantitatively collect meso zooplakton by vertical hauls from150 m to the surface (Takahashi et al., 1998 ; Chiba et al.,2001). The ORI net is often used to collect fish larvae ;however, in addition to meso zooplankton, macro zooplankton with a high swimming capability can be caught quantitively because of the high towing speed and the large net
mouth (Cuzin Roudy and Labat, 1992). In a previous study
on zooplankton abundance based on NORPAC (Chiba et al.,2001), Chiba et al. (2001) showed that the abundance, biomass and biomass/abundance ratio were 286 ± 615 ind. m－3,
214 ± 509 mg m－3 and 0.75, respectively. The current
study showed that they were 79.90 ± 106.79 ind. m－3,
122.37 ± 107.09 mg m－3 and 1.53, respectively. In short,
this study found a lower abundance and biomass and a higher
biomass/abundance ratio than the previous research. Thissuggests that the ORI net could collect large and heavy meso
and macro zooplankton individuals quantitively while miss-Meridional changes in temperature (A),salinity (B) and chlorophyll a concentration (C) along the 110°E in the Indian sector of the Southern Ocean during January
2018.

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abundance, respectively (Fig. 5). M. gerlachei wasincreased at southern stations near Antarctica.
Cluster analysis

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From the results of the cluster analysis based on abundance, four zooplankton community groups were identified at
a 61% similarity level (Fig. 6A). Each group was called A(two stations), B (one station), C (three stations) and D (threestations) from the north to the south (Fig. 6C). The BESTBIOENV results showed that latitude and salinity wereselected as the most contributing parameters to the zooplankton clustering (Table 2).
Comparing the abundances between the clustering groups,
the total zooplankton abundances in group A was significantly-— 43 —--

Bull. Fish. Sci. Hokkaido Univ. 71(1), 2021

Fig. 4.

Meridional changes in zooplankton community on abundance (A) and biomass (B) along the 110°E in the Indian sector of
the Southern Ocean during January 2018.

ing small meso zooplankton that could avoid the mesh due tothe large mesh size (500 µm).
In the Indian sector of the Southern Ocean, physical environments such as fronts, divergence and convergence generally divide the zooplankton community structure (Errhif et al.,1997 ; Tanimura et al., 2008). In this study, the community
structure was distinctly classified by PF S and SACC N, butsimilar communities were observed in the area beyond SB
and ASF. This mismatch of distributions between fronts andthe zooplankton community was presumably caused by thehigh similarity between groups C and D (ANOSIM, Global
R=0.593) compared to the low similarity between groups A
and B.
According to Hosie et al. (2000) and Wallis et al. (2016),
zooplankton communities are affected by fronts as well as sea
ice, water temperature and chl. a concentration. However,the BEST BIOENV analysis showed that salinity was theparameter most related to community classification, and therelationship with chl. a concentration was low. In general,
biological production (i.e., chl. a concentration) is known to
be enhanced around the fronts in the Southern Ocean (Laubscher et al., 1993 ; de Baar et al., 1995 ; Moore et al., 1999).
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In contrast, Sokolov and Rintoul (2007) reported that the distribution of chl. a concentration is affected by upwelling ofhigh nutrient deep water due to the seafloor topography andwater currents and is not associated with fronts. Chiba et al.
(2001) revealed that the environmental factors influencing thezooplankton community structure were highly related in theorder of latitude, water temperature, and chl. a concentration. Thus, in this study, the zooplankton community structure was divided with the water mass and had a low
relationship with chl. a concentration in the Indian sector of
the Southern Ocean during the austral summer. The findings
are consistent with previous studies.
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North south change of copepods-

In this study area, large copepods have species specific distributions related with the complicated current and their life
cycle. On abundance, C. acutus and R. gigas occurred tentimes more than C. propinquus in the ACC north of SB (Bathmann et al., 1993) and the Scotia Sea (Voronina, 1975 ; Voronina et al., 1978 ; Vladimirskaya, 1978 ; Marin, 1987 ;Bondarenko and Polonsky, 1990 ; Atkinson, 1991). The
results of the copepod community structure in this study— 44 —-

Sugioka et al. : North-south changes of zooplankton in Southern Ocean

southern stations. Similarly, the species abundance in the
PFZ was approximately 12.93 times greater than those at thesouthern stations of this study. Because the sampling methods (CPR vs. ORI) and the research area (entire Southern
Ocean vs. the Indian sector) were different between this study
and the previous study, these differences may have affected
the change of abundance. Because the results were similar
despite the differences in the sampling methods and theregion, the dominance and high abundance for C. acutus in
the PFZ are interpreted as a general trend in the Indian sectorof the Southern Ocean during the austral summer.
North south change of the population structure for dominant large copepods-

The dominant large copepod C. acutus grow to C5 until thesummer in the north of SB and then descend to the deep layerand diapause using wax ester as an energy source in the winter (Bathmann et al., 1993). After overwintering, an adultfemale is fertilized in the deep layer and spawns in the surface
layer. Hatched nauplius grow to late stages and descend to
the deep layer for diapause and reproduction (Voronina,1978). They reproduce from the spring to the summer, andthe reproductive period is delayed gradually southwards(Voronina, 1978 ; Hagen and Schnack Schiel, 1996). An
adult male stays in the deep layer even in the spring (Marin,
1988).
In this study, early stages (C1 C3) were often observed at
the north stations (from st. KC5 to st. D01), and late stages(C4 C6) dominated at the southern stations. Because of thispopulation change between the north and the south, it isthought that this species reproduced earlier in the north region
than in the south region (Voronina, 1978 ; Hagen and
Schnack Schiel, 1996). From st. C01 to D01, since no adultand many early copepodite stages were observed, they hadfinished reproduction there. Between st. C03 and st. B01,
both adults and early copepodite stages were occurred, inducing they had started reproduction. This species starts to
reproduce with grazing in the surface layer when a phytoplankton bloom occurs (Marin, 1988). In this study period,
because pre matured adult females were observed south of st.
C03 (data not shown) with the phytoplankton bloom, this species presumably continue to reproduce using the sufficient
food. At st. C05 and KC6, only adults and late copepodite
stages were observed with no phytoplankton bloom. Thatis, the population structure of this species is thought to belinked to the timing of phytoplankton bloom.
During the spring, small scale phytoplankton blooms
regionally occur around 60°S, and these blooms shift from thenorth to the south in association with sea ice melt timing(Sokolov and Rintoul, 2007 ; Iida and Odate, 2014). The
similar trend was found in this study by satellite observation.
According to Sedwick and DiTullio (1997), melting sea ice
stimulates phytoplankton production due to the availability of--Fig. 5.

Meridional changes in copepods community on abundance along the 110°E in the
Indian sector of the Southern Ocean during January 2018.

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almost corresponded with the results of previous studies.
The subantarctic species C. simillimus is abundant in the
ACC (Takahashi et al., 2011). However, the copepods werehardly observed in this study, C. simillimus occurred only in
the PFZ. Because C. simillimus was most abundant at thenorthern station in the PFZ, corresponding with previous studies, C. simillimus seems to be distributed in the northern PFZ
despite the lack of quantitative net sampling.
Considering the horizontal change of the copepod community structure, remarkable east west changes in abundance
have been observed compared to north south changes (Hosieet al., 2000 ; Chiba et al., 2001). Hosie et al. (2014)
reported that the copepod abundance was ten times higher in
the north area (PFZ) than in the south area (ACC). In thisstudy, three dominant copepods (C. acutus, C. propinquus andR. gigas) except M. gerlachei were abundant in the PFZ, similar to the results in Hosie et al. (2014). This is because the
distribution of the most dominant species C. acutus was similar in this study and the previous study. According to Hosieet al. (2014), the abundance of C. acutus at the stations in the
PFZ was approximately three times higher than those at the--— 45 —

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Bull. Fish. Sci. Hokkaido Univ. 71(1), 2021

Fig. 6.

Results of cluster analysis based on zooplankton abundance along the 110°E in the Indian sector of the Southern Ocean during January 2018 (A). Four groups (A-D) were identified from Bray-Curtis similarity connected with UPGMA. Meanabundance and species/taxon composition of each group (B). Spatial distribution of the group in the Indian sector of theSouthern Ocean during January 2018 (C).

Table 2. Results of BEST-BIOENV for zooplankton community with environmental parameters (latitude(Lat), temperature (Tem), salinity (Sal), chlorophyll a concentration (Chl)) in the Indian sectorof the Southern Ocean during January 2018.
Number of variables

Best variables combinations using 0-200 m layer data
(Spearman Rank Correlation Coefficient)1

Lat(0.67)
Lat, Sal(0.67)
Lat, Sal, Chl(0.59)
Lat, Tem, Sal, Chl(0.47)

2
3
4

Sal(0.53)
Lat, Chl(0.46)
Lat, Tem, Sal(0.50)

iron from the ice. Thus, the timing of sea ice melt determines the phytoplankton bloom and causes the north southchange of the population structure of C. acutus during thesummer. The strong negative relationship between MCS
and TSM supports this finding.
C. propinquus has a similar life cycle to C. acutus ; theygrow to C5 C6 until the austral summer north of SB andreproduce from the spring to the summer (Bathmann et al.,1993). However, they do not have a diapause phase in thewinter, and all copepodite stages are distributed in the surface
layer (Voronina, 1978 ; Hagen and Schnack Schiel, 1996).
In this study, it is assumed that C. propinquus reproduces inthis study period or after this research because adult females
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Tem, Sal(0.39)

Sal, Chl(0.38)

Lat, Tem(0.38)

occurred at the same dominance as matured individuals atonly two southern stations (st. C05 and KC6). In addition,
given that early copepodite stages dominated the population
at the northern stations, the north south change in the population structure of C. propinquus induced the variation of theirreproduction period similar to C. acutus.
The north south changes had thought to be mostly inducedby the timing of sea ice melt, as mentioned above. However, the relationship between MCS and TSM was not significant for C. propinquus. This was caused by the high MCS(> 5) for C. propinquus observed at st. C05 and KC6 where
phytoplankton bloom was not occurred yet. In other words,
they did not reproduce at the southern stations. Considering— 46 —---

Sugioka et al. : North-south changes of zooplankton in Southern Ocean
Table 3. Comparison of the zooplankton abundance between the groups identified by cluster analysis (cf. Fig.
6A) in the Indian sector of the Southern Ocean during January 2018. Differences between the groups
were tested by one-way ANOVA and post hoc test by Tukey-Kramer. Any groups not connected by
the underlines are significantly different (p < 0.05). Values are mean. Numbers in parenthesis are thenumber of stations belonging to each group.
Abundance (ind m−3)
A (2)

B (1)

C (3)

D (3)

One-way
ANOVA

Aetideus sp.
Calanoides acutus
Calanus propinquus
Calanus simillimus
Ctenocalanus citer
Euchirella rostomagna
Haloptilus oxycephalus
Heterorhabdus sp.
Metridia gerlachei
Metridia lucens
Oithona frigida
Oithona similis
Oncaea antarctica
Paraeuchaeta antarctica
Racovitzanus antarctica
Rhincalanus gigas
Rhincalanus gigas nauplius
Scaphocalanus sp.

0108.86
24.49
1.27
1.05
0.52
0.52
0.52
0.74
0.74
0
0
0.74
2.52
0
18.51
64.92
0

0
17.58
2.4
0.11
0
0
0
0
0
0
0
0.28
0
0
0
1.83
6.51
0

0.11
5.8
1.96
0.23
0
0
0.45
0.27
0.3
0
0
0
0
0.88
0.76
8.71
0.34
0.24

0
7.87
0.52
0
0
0
0.18
0.03
1.66
0
0.03
0
0
0.79
0.27
2.17
0
0.1

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

Amphipoda
Chaetognatha
Cnidaria
Decapoda
Euphausiacea
Ostracoda
Polychaeta
Pteropoda
Total

1.64
14.42
0
0.14
7.03
1.66
0.2
0.2
250.67

0.54
2.6
0.46
0
2.74
0.23
0.54
0.06
35.58

0.22
5.67
0
0
2.4
0.15
0.2
0.35
29.32

0.17
1.41
0
0
0.47
0.33
0.24
0.07
16.29

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

Species

Tukey-Kramer

C
D
D

D
C
B

B
B
C

A
A
A

B

D

C

A

D

B

C

A

D

C

B

A

D

C

B

A

* : p < 0.05, ** : p < 0.01, *** : p < 0.001.

above the results, life cycle of C. propinquus may be related
to the timing of sea ice melt as well as C. acutus.
R. gigas has one or two year generation lengths in eacharea, and it has been reported to be one year in the ACC(Arashkevich, 1978). In the Weddell Sea, the species has a
two year generation length because all copepodite stages
occur during the summer (Marin, 1988). In this study area,
a nauplius was observed at the northern stations (st. C01
D01), and late copepodite stages dominated the population at
st. C01, KM4 and D01. Therefore, it is thought that R. gigas
reproduced actively at the northern stations in this study
period.
The chl. a concentration derived by satellite observation
could not compare with the MCS of the dominant copepods.
Instead of the comparison, TSM showed a significant rela-

-

-

-

-

-

tionship with MCS, and the relationship (i.e., significant ornot) varied with the species. This was caused by differencesof life cycle of the species. In other words, C acutus seems
to have the strongest relationship to sea ice on their life cycle.
In future, if the TSM change, population structure of the species will be the most strongly affected.
Conclusion
This study revealed the structures of the meso and macro
zooplankton community and the population of the dominant
copepods in the Indian sector of the Southern Ocean during
the austral summer of 2018. The zooplankton community
structure had meridional differences associated with oceanic
fronts but a low relationship with chl. a concentration. In
terms of the species composition of copepods, this study— 47 —--

Bull. Fish. Sci. Hokkaido Univ. 71(1), 2021

Fig. 7.

Meridional changes in stage composition for the dominant four copepods on abundance along the 110°E in the Indian sectorof the Southern Ocean during January 2018.

showed the same results as those found in previous reports(e.g., Hosie et al., 2014) ; the most dominant species C. acutus changes to total zooplankton on abundance.
Regarding the population structures of the dominant copepods (C. acutus, C. propinnquus and R. gigas), early copepodite stages were often observed in the northern stations, andlate stages dominated in the southern stations. This meridional change was caused by the north south temporal variationof phytoplankton blooms associated with sea ice melt timing.
This variation affected the reproduction period of these cope-pods, resulting in different population structures between thenorth and the south.
In conclusion, the front is the most important factor restricting on the zooplankton community. While, sea ice extent isa significant driver to changing the population structure for
copepods in the Indian sector of the Southern Ocean during
the austral summer.

--— 48 —-

Sugioka et al. : North-south changes of zooplankton in Southern Ocean

References

Fig. 8.

The relationships between mean copepodite stage (MCS)
for dominant copepods and time since sea-ice melt(TSM) along the 110°E in the Indian sector of the Southern Ocean during January 2018. ** : p < 0.01

Acknowledgements
We are grateful to the captain and crew of the T/VUmitaka maru of TUMSAT for collecting the samples andhydrographic data used in this study. The ADS dataset isarchived and provided by the Arctic Data archive System(ADS), which was developed by the National Institute ofPolar Research (NIPR). Part of this study was supported by
a Grant in Aid for Challenging Research (Pioneering)
20K20573, and Scientific Research 20H03054 (B),
19H03037 (B), 18K14506 (Early Career Scientists),
17H06319 (Scientific Research on Innovative Areas) and
17H01483 (A) from the Japanese Society for the Promotionof Science (JSPS).
---— 49 —

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Appendix 1. (Sugioka et al., 縮尺1/3)— 50 —