Title

Comparison of the vertical distribution of pelagic copepod abundance, biomass and community structure between the
Atlantic and Pacific sectors of the Arctic Ocean

Author(s)Abe, Yoshiyuki; Hildebrandt, Nicole; Matsuno, Kohei; Niehoff, Barbara; Yamaguchi, Atsushi

Citation

北海道大学水産科学研究彙報, 70(1), 91-102

Issue Date

2020-08-24

Doc URL

http://hdl.handle.net/2115/79120

Type

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bulletin (article)

bull.fish.70.1.91.pdf

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Hokkaido University Collection of Scholarly and Academic Papers : HUSCAPBull. Fish. Sci. Hokkaido Univ.
70(1), 91-102, 2020.
DOI 10.14943/bull.fish.70.1.91

ABE et al. : Vertical distribution of copepods in the Arctic Ocean

Comparison of the vertical distribution of pelagic copepod abundance,
biomass and community structure between the Atlantic andPacific sectors of the Arctic Ocean
Yoshiyuki Abe1), Nicole Hildebrandt2), Kohei Matsuno3,4), Barbara Niehoff2)
and Atsushi Yamaguchi3,4)(Received 30 April 2020, Accepted 27 May 2020)
Abstract   We compared copepod abundance, biomass and community structure in the Atlantic sector (Fram Strait) and Pacific sector(Canada Basin) of the Arctic Ocean by using vertical stratified zooplankton samples collected from 0-1,000 m or 0-1,500 m during daytime in summer. Abundance and biomass were high at the surface layer and decreased with increasing depth in bothregions. Abundance and biomass in the surface layer in the Fram Strait were 6-18 times higher than those in the Canada Basin.
According to cluster analysis, the copepod communities were separated into three groups that varied vertically and included both
locations. Regional differences were marked for Calanus finmarchicus, which was distributed only in the Fram Strait. In the
Canada Basin, Spinocalanidae composed 56.1% of calanoid copepod abundance for the bathypelagic groups and also contributed
7.5% of abundance throughout the water column. The dominance of Spinocalanidae may reflect the oligotrophic environment
in the Pacific sector of the Arctic Ocean.
Key words : Copepods, Fram Strait, Canada Basin, Vertical distribution, Arctic Ocean, Calanus finmarchicus, Spinocalanidae

Introduction
Recently, atmospheric warming in the Arctic area has progressed 2 3 times faster than the global scale (Trenberth et al.,2007). Reduced sea ice coverage in summer has also beenreported for the Arctic Ocean (Duarte et al., 2012 ; Naam,2012). The reduction of sea ice is reported to be greater forthe regions where the Arctic Ocean connects with the Atlantic
and Pacific Oceans (Shimada et al. 2001, 2006 ; Stroeve etal., 2007 ; Comiso et al., 2008 ; Markus et al., 2009). The
evaluation of the effects of sea ice reduction on marine ecosystems is an important and urgent issue in biological oceanography (Wassmann et al., 2015 ; Hunt et al., 2016).
Consequently, studies of marine zooplankton, especiallycopepod predominant components, in the Arctic Ocean
where it connects to the Atlantic and Pacific Oceans are ofprime importance. For the Pacific sector in the Arctic
Ocean, the transport of Pacific copepods through the Bering
Strait has increased in recent years (Matsuno et al., 2011,2016 ; Nelson et al., 2014 ; Ershova et al., 2016). More--over, reproductive success (egg production and hatching) of
the transported Pacific copepod Neocalanus flemingeri in the
Arctic Ocean has been reported (Matsuno et al., 2015). In
the Atlantic sector of the Arctic Ocean, the transported zooplankton biomass from the Atlantic Ocean is reported to be
2 3 times higher than that transported through the Pacific sector (Wassmann et al., 2015). Reproductive success of thetransported Atlantic copepod Calanus finmarchicus in the
Atlantic sector of the Arctic Ocean has also been reported(Gluchowska et al., 2017 ; Weydmann et al., 2018). These
reports are important since they evaluate changes in copepod
populations in each connecting region of the Arctic Ocean.
However, information on differences in pelagic copepod community structure and vertical distributions in the Atlantic andPacific sectors of the Arctic Ocean are scarce, which preventsan accurate evaluation of future forecasts of biological oceanography in each region (Wassmann et al., 2015 ; Hunt et al.,2016).
In the present study, we evaluate copepod community
structure from the surface to deep layers (down to 1,500 m) in1)

Office for Enhancing Institutional Capacity, Hokkaido University(北海道大学大学力強化推進本部)
2)
Helmholtz Centre for Polar and Marine Research, Alfred Wegener Institute(ドイツ連邦共和国アルフレッドウェゲナー極地海洋研究所)
3)
Faculty of Fisheries Sciences, Hokkaido University(北海道大学大学院水産科学研究院海洋生物学分野)
4)
Arctic Research Center, Hokkaido University(北海道大学北極域研究センター)— 91 —-

Bull. Fish. Sci. Hokkaido Univ. 70(1), 2020

the Atlantic and Pacific sectors of the Arctic Ocean. In addition to species identifications and enumeration, copepod biomass was quantified by measuring body length and applying
length mass regressions. Species diversity and similarity
analyses were used to evaluate patterns of geographical andvertical change in the copepod community. The obtained
results were compared with those in the Arctic Ocean andtheir adjacent seas, and the characteristics in the Atlantic andPacific sectors were clarified. Finally, we discuss future
prospects for the copepod community under global warming
conditions in the Atlantic and Pacific sectors of the Arctic
Ocean.
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Field sampling
Zooplankton sampling in the Atlantic sector was conducted
at one station (78º50′N, 1º59′W) in the Fram Strait on 1 July
2011. Samples were collected from 0 50, 50 200, 200 500,
500 1,000 and 1,000 1,500 m by vertical haul of a Multi Net(150 µm mesh, 0.25 m2 mouth opening, Hydro Bios Inc.
Kiel) during daytime (17 : 59 18 : 47 local time) (Fig. 1). In
the Pacific sector, zooplankton sampling was performed at
one station (73º48′N, 159º58′W) in the Canada Basin on 27
September 2013. Samples were collected from 0 100, 100
250, 250 500 and 500 1,000 m by vertical haul of a Vertical
Multiple Plankton Sampler (VMPS : Terazaki and Tomatsu,
1997 ; 62 µm mesh size, 0.25 m2 mouth opening, Tsurumi
Seiki Co., Ltd. Yokohama) during daytime (06 : 22 06 : 38
local time). At each station, temperature and salinity were--Samples and data analysis
In the land laboratory, enumeration and species identification of copepods were performed under a stereomicroscope.
Calanoid copepods were identified at the species level, andthe total length (TL) were measured by an ocular micrometerwith a precision of 0.1 mm. The dry mass of calanoid copepods was estimated from TL using the following allometricequation :--where DM is individual dry mass (µg DM ind.−1) and TL is
total length (µm) (Mizdalski, 1988). DM was converted tocarbon mass, assuming the carbon content of copepods is
44.7% of the DM (Båmstedt, 1986). For Harpacticoida, weused the following allometric equation :Log10 CM = 1.950 Log10 TL − 5.577

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Log10 DM = 2.546 Log10 TL − 6.697

Material and methods-

measured by CTD (SBE 911plus, Sea Bird Electronics Ltd.,
Washington). After collection, zooplankton samples wereimmediately preserved with 4% buffered formalin.

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where CM is the individual carbon mass (µg C ind.−1) and TL
is the total length (µm) (Uye et al., 2002).
For Cyclopoida, the dry mass was estimated by the following equation :Log10 DM = 2.163 log10 TL − 6.207

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where DM is the individual dry mass (µg DM ind.−1) and TL
is the total length (µm) (Kaneko, 2005). The carbon content
of Cyclopoida was assumed to be 42.5% of DM (James andWilkinson, 1988).
For Poecilostomatoida, we used the following allometricequation :Log10 DM = 2.895 log10 TL − 7.993
where DM is in the individual dry mass (µg DM ind.−1) and
TL is the total length (µm) (Nishibe, 2005). The carbon content of Poecilostomatoida was assumed to be 53% of DM(Nishibe and Ikeda, 2008).
For each sample, the species diversity index (H′) was calculated :H ′ = −∑ ni / Ni × ln ni / Ni

Fig. 1.

Location of sampling stations in the Fram Strait (Atlantic
sector of the Arctic Ocean) and in the Canada Basin
(Pacific sector of the Arctic Ocean). For details of samplings, see the text.

where ni is the abundance (ind. m−2) of each species and Ni is
the total abundance at the ith layer (Shannon and Weaver,1949).
To quantitatively compare the vertical distributions, thedepths where 50% of the population resided (50% distributed
layer : D50%, Pennak, 1943) were calculated for each species. D25% and D75% were also calculated.
To evaluate geographical and vertical changes in the copepod community, cluster analysis based on copepod abundance
was performed. Abundance data (X : ind. m−2) for eachspecies were log transformed (log10[X+1]). Then, similarities between samples were calculated using the Bray Curtis--— 92 —

ABE et al. : Vertical distribution of copepods in the Arctic Ocean

similarity index (Bray and Curtis, 1957). To group samples,
similarity indices were coupled with hierarchical agglomerative clustering using a complete linkage method (UPGMA :Unweighted Pair Group Method using Arithmetic mean)(Field et al., 1982). Nonmetric Multi Dimensional Scaling(NMDS) ordination was executed to delineate the sample
groups on a two dimensional map. All analyses were performed with PRIMER v7 software (Primer e Ltd., Albany,
New Zealand).
---Results
Hydrography
The temperature varied between −1.03 and 5.56ºC in the
Fram Strait (Fig. 2A). Low temperature (−1.03 0ºC) was
observed from the surface to 26 m, while the temperature
increased below that layer and reached 5.56ºC at 53 m.
Below this peak, the temperature decreased with increasing
depth. Salinity ranged between 32.92 and 35.16, and highsalinity (> 35) was observed below 50 m and was stablebelow that layer. Dissolved oxygen was highest (8.47 mlL−1) at the surface layer and remained at a high value of 6 ml
L−1 throughout the water column. Chl. a varied between 0.02
and 1.18 mg m−3 and peaked at a depth of 15 m.
For the Canada Basin, the temperature varied between−1.61 and 0.79ºC (Fig. 2B) and showed little change withdepth. Salinity ranged between 32.92 and 35.16. Salinity
was low (< 30) at 0 40 m, which is characteristic of Polar
Surface Water at that layer. Salinity gradually increased
with increasing depth and reached a peak below a depth of
500 m, similar to the value (> 35) in the Fram Strait. Dissolved oxygen was relatively high near the surface layer,
peaked (8.90 ml L−1) at 30 m, and subsequently decreased
with increasing depth down to 200 m, below which it was
stable. Chl. a ranged between 0.04 and 3.24 mg m−3 andreached a subsurface maximum at 36 m.
Comparison of hydrography between the sectors revealed
greater differences, especially in temperature and salinity.
Thus, surface salinity was lower in the Canada Basin than in
the Fram Strait, while the Fram Strait was warmer than the
Canada Basin through a wide range between 25 and 1,000 m.
--

Copepods

Fig. 2.

In the Fram Strait, both copepod abundance and biomass
were highest at the surface 0 50 m (3,132 ind. m−3 and 18.6
mg C m−3) and decreased with increasing depth (Fig. 3A).
Species diversity (H′) was low at the surface layer (0.70), washigh at 200 500 m (1.74) and remained high below that layer.
In the Canada Basin, copepod abundance was high at 0 50 m
(518 ind. m−3), while high biomass was at 100 250 m (1.3 mg
C m−3) (Fig. 3B). Species diversity was high even in thesurface layer, and the highest value (2.27) occurred at 100
250 m.
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Vertical distribution of temperature, salinity, dissolved
oxygen and Chl. a in the Fram Strait (A) and the Canada
Basin (B). Depth ranges of sampling layers are shownin the numbers in the right columns. Note that thedepth scale is in log-scale.

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For copepod composition by abundance, Cyclopoidaaccounted for 66% at the surface, while the proportion of Calanoida increased at deeper layers in the Fram Strait (Fig. 4A).
On the other hand, in the Canada Basin, Cyclopoidaaccounted for 55% at the surface, but Poecilostomatoida— 93 —

Bull. Fish. Sci. Hokkaido Univ. 70(1), 2020

Fig. 3.

Vertical distribution of copepod abundance, biomass and species diversity indices (H′) based on their abundance data in the
Fram strait (A) and Canada Basin (B).

dominated at 65% 68% below 250 m (Fig. 4B). For copepod composition by biomass, Calanoida predominated (78­99%) throughout the water column in both regions (Fig. 4C,
4D).
The results of cluster analyses and NMDS analyses areshown in Fig. 5. According to the cluster analyses, the copepod community was separated into three groups at 78% Bray
Curtis dissimilarity. The copepod communities wereseparated vertically rather than geographically. Thus, we
named the groups from shallow to deep : epipelagic, mesopelagic and bathypelagic. The epipelagic group contained
samples from 0 50 and 50 200 m in the Fram Strait and
0 100 m in the Canada Basin. The mesopelagic group
included samples at 200 500 and 500 1,000 m in the Fram
Strait and 100 250 and 250 500 m in the Canada Basin.
The bathypelagic group contained the deepest samples in bothregions.
The standing stock and vertical distribution (50% distribution depth (D50%) and distribution core (D25% D75%)) of copepods in the Fram Strait and in the Canada Basin are shown in
Fig. 6. Most of the dominant species were well corresponded with the two regions. The dominant species was
Oithona spp. (D50% : 32 63 m) in the epipelagic group.
Oncaea spp. (D50% : 319 324 m) and Microcalanus spp. (D50% :245 542 m) dominated the mesopelagic group. Regional
differences were marked for Calanus finmarchicus, which
was distributed in the epipelagic layer (D50% : 49 m) only in
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the Fram Strait. Spinocalanus spp. was common to bothregions and was the characteristic species in the bathypelagicgroup.
Vertical changes in the family composition of calanoid
copepods in terms of abundance and biomass are shown in
Fig. 7. In abundance, Calanidae dominated the epipelagicgroup in both regions (reached 93.0% in the Fram Strait and58.3% in the Canada Basin). Clausocalanidae including
Microcalanus spp. dominated the mesopelagic group in bothregions (composed 53.0% in the Fram Strait and 63.6% in the
Canada Basin). For the bathypelagic group, Clausocalanidae dominated in the Fram Strait (63.1%), while Spinocalanidae dominated in the Canada Basin (56.1%).
Concerning biomass, Calanidae dominated throughout thewater column in both regions (composed >60.0% in the Fram
Strait and >60.9% in the Canada Basin). The predominanceof Calanidae was marked at 0 50 m in the Fram Strait (98.6%).
In the mesopelagic group, the compositions of Aetideidae,
Heterorhabditidae and Metridinidae varied in both regions.
In the bathypelagic group, Spinocalanidae (19.4%) was dominant in the Canada Basin.
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Discussion

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Methodological note
We applied different mesh size sampling gears in eachregion (Fram Strait : 150 µm, Canada Basin : 62 µm). The— 94 —

ABE et al. : Vertical distribution of copepods in the Arctic Ocean

Fig. 4.

Vertical changes in composition of four orders (Calanoida, Cyclopoida, Harpacticoida and Poecilostomatoida) of copepods
in abundance (A), (B) and biomass (C), (D) in the Fram Strait (left) and Canada Basin (right).

bias due to the use of different mesh sizes should be considered. The use of smaller mesh sizes has been associatedwith an increased composition of small copepods (Cyclopoidain the epipelagic layer and Poecilostomatoida in the meso
and bathypelagic layer), especially in abundance (Böttger
Schnack, 1996 ; Yamaguchi et al., 2002 ; Homma et al.,2011). In the present study, the composition of Poecilostomatoida in terms of abundance was 0.2 33.6% in the Fram
Strait using a 150 µm mesh net but 19.5 67.8% in the Canada
Basin using a 62 µm mesh net (Fig. 4A, B). In terms ofbiomass, since the individual mass of small Poecilostomatoida was low, large body sized Calanoida predominated in
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both regions (Fig. 4C, D). In terms of depth composition,
Homma et al. (2011) compared the composition in terms ofabundance at different layers (epipelagic layer : 0 250 m ;mesopelagic layer : 250 1,000 m ; bathypelagic layer :1,000 3,000 m) by applying different mesh sizes. Whileabundance increased with the use of smaller mesh sizes, few
effects were detected for the depth composition in terms ofcopepod abundance (Homma et al., 2011).
The sampling period also varied between the two regions (1July in the Fram Strait and 27 September in the Canada
Basin). Both periods were in summer, which corresponds tothe period after sea ice melting in both regions. Consistent— 95 —---

Bull. Fish. Sci. Hokkaido Univ. 70(1), 2020

Fig. 5.

Results of the cluster analysis based on zooplankton
abundance using Bray-Curtis similarity (A). Twodimensional representation of nonmetric multi-dimensional scaling plots (B). The dotted line in (A) andcircles in (B) indicate percentage similarity at 78%.
Fig. 6.

with this study, studies of summer zooplankton distribution
down to greater depths in the Arctic Ocean have used timeintervals ranging from one month (Mumm, 1993 ; Auel andHagen, 2002 ; Kosobokova and Hopcroft, 2010) to threemonths (Mumm et al., 1998 ; Kosobokova and Hirche,
2000). Thus, the difference in sampling periods in this study(ca. three months) is within the ranges of previous studies.
As mentioned above, the applied mesh size and sampling
period differed between locations. Due to differences inmesh size, the abundance of small copepods such as Cyclopoida and Poecilostomatoida would be biased. For biomass,
the effect of different mesh sizes would be minor because of
the low individual mass of the small copepods. For differences in sampling period, since both locations were sampled
in the same summer season, little effect would be expected forthe vertical distribution and community structure of copepods.

Standing stock and vertical distribution of copepods inthe Fram Strait (A) and Canada Basin (B). For eachspecies or taxon, the upper solid circles indicate abundance (ind. m−2 : water column), and the lower open
circles indicate 50% distribution depth (D50%). The vertical bars indicate depth ranges where 25% (D25%) and
75% (D75%) of the population are distributed. The
dashed lines indicate boundaries of copepod communities identified from the cluster analysis (cf. Fig. 5).

Regional comparison
In terms of regional differences, it is notable that bothabundance and biomass at the surface layer were 6 18 times
higher in the Fram Strait than in the Canada Basin (Fig.
3). Considering differences in mesh size (coarse 150 µm in
the Fram Strait), these results suggest that extremely highabundances of copepods occurred at the surface layer of the
Fram Strait. With respect to regional differences in the zooplankton community in the Arctic Ocean, Kosobokova and— 96 —-

ABE et al. : Vertical distribution of copepods in the Arctic Ocean

Fig. 7.

Vertical changes in the composition of families of calanoid copepods in terms of abundance (A, B) and biomass (C, D) in the
Fram Strait (left) and Canada Basin (right). Dashed lines indicate the boundaries of the copepod community identified
from the cluster analysis (cf. Fig. 5).

Hirche (2000) revealed that the distribution of the Atlantic
copepod C. finmarchicus was restricted to east of theLomonosov Ridge, while the abundance and biomass of various zooplankton taxa (appendicularians, chaetognaths andostracods) decreased in the western basins, i.e., the Canada
and Makarov Basins, due to the elimination of Atlantic waterin these regions. In the present study, C. finmarchicus did
not occur in the Canada Basin (Fig. 6). The presence/
absence of transported copepods from the Atlantic Ocean may
induce regional differences in the abundance and biomass ofcopepods at the surface layer. Thus, transported copepods
from the Atlantic Ocean would provide high abundance andbiomass at the surface layer of the Fram Strait.
The species diversity of copepods is known to be high inthe meso and bathypelagic zones of oceans worldwide (Roe,1988 ; Scotto di Carlo et al., 1984 ; Richter, 1994 ; Yama-guchi et al., 2002 ; Homma et al., 2010). In this study, species diversity in the Fram Strait peaked at the mesopelagic
layer (Fig. 3A). However, in the Canada Basin, species
diversity was even high at the surface layer (Fig. 3B). Thismay be due to the cold water temperature throughout thewater column in the Canada Basin (Fig. 2B). Deep sea
copepods are adapted to cold water temperatures, allowing
them to extend their vertical distribution to the surface layer.
The occurrence of deep sea copepods near surface layer may
elevate species diversity in this layer. This pattern was confirmed by the D50% data of the copepods. Seven species had
D50% above a depth of 250 m in the Fram Strait, while eleven
species had D50% above 250 m in the Canada Basin (Fig. 6).
In addition to these species numbers, in the Fram Strait, thepredominance of Atlantic C. finmarchicus may reduce species
diversity at the surface layer, and species diversity showed a— 97 —--

Bull. Fish. Sci. Hokkaido Univ. 70(1), 2020

peak at 200 500 m.
Despite regional differences in abundance and species
diversity, the copepod communities were separated into threegroups that varied vertically at both geographical locations(Fig. 5). For copepods in the Arctic Ocean, it has beenreported that vertical changes in abundance, biomass and species composition are more prominent than regional changes(Kosobokova and Hirche, 2000 ; Auel and Hagen, 2002 ;Kosobokova and Hopcroft, 2010 ; Kosobokova et al., 2011).
Within the 0 1,500 m water column, the zooplankton community was classified into four groups in the central Arctic
Ocean : 0 50, 50 200, 200 1,000, 1,000 1,500 m (Auel andHagen, 2002). Similar classification of the copepod community into three groups (0 300, 300 1,000 and 1,000 3,000m) was reported in the Greenland Sea (Richter, 1994).
Although the depth ranges differed slightly, the vertical classification of the copepod community in this study is similar tothese reports (Fig. 5). These observations suggest that vertical classifications are present for the copepod community
throughout the Arctic Ocean.
While the community structures were similar between thetwo regions, prominent differences in the calanoid copepod
community were observed, with a high composition of Spinocalanidae in the deep layer of the Canada Basin. Spinocalanidae composed 56.1% of the abundance in thebathypelagic groups and also contributed 7.5% of calanoid
copepod abundance throughout the water column in the Canada Basin (Fig. 7). For the calanoid copepod community
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below 200 m depth, Spinocalanidae was reported to be thesecond most dominant family following Clausocalanidae inthe central Arctic Basin and the Canada Basin (Table 1).
Spinocalanidae has been reported to compose 10 17% of calanoid copepod abundance below 200 m in the Canada Basinand the central Arctic Basin (Auel and Hagen, 2002 ; Kosobokova and Hopcroft, 2010).
On the other hand, the dominant copepod families below
200 m were Clausocalanidae, Calanidae and Metridinidae in
the Fram Strait and Greenland Sea (Table 1). The dominance of these families is similar to that in the western andnorthern North Pacific and the Bering Sea (Table 1). In allof these regions, the composition of Spinocalanidae isreported to be extremely low, with values as low as 0.1 0.4%
(Yamaguchi et al., 2002 ; Homma et al., 2011). These facts
suggest that the composition of Spinocalanidae in the Pacificsector of the Arctic Ocean, including the Canada Basin, ismuch higher than in other oceanic regions.
Interestingly, dominance of Spinocalanidae in the calanoid
copepod community has been reported for Pacific basins,
including the Canada Basin and the Makarov Basin (Table 1).
Both the biomass and composition of the Atlantic species
decreased dramatically through the Nansen Basin to theAmundsen Basin along with a decreasing effect of the Atlantic water (Wassmann et al., 2015 ; Kosobokova and Hirche,
2000). The boundary between Atlantic and the Pacific expatriates in the Arctic Ocean is reported to be the Lomonosov
Ridge (Bluhm et al., 2015). The dominance of Spinocalani--

Table 1. Regional comparison of copepod abundance, biomass and dominant calanoid families below 200 m in the Arctic Ocean and theiradjacent seas.
Sampling gear

Mesh size(μm)

Abundance(ind. m–2)

Canada Basin(0-3,000)

Multi net

150

50,176

1,340

Clausocalanidae
Spinocalanidae
Calanidae

Kosobokova and Hopcroft 2010

Canada Basin(0-1,000)

VMPS

62

28,635664

Clausocalanidae
Spinocalanidae
Calanidae

This study

Central Arctic Ocean(0-1,500)

Multi net

150

39,327

Clausocalanidae
Spinocalanidae
Calanidae

Auel and Hagen 2002

Fram Strait(0-3,000)

Multi net

150

187,189

2,488

Clausocalanidae
Calanidae
Metridinidae

This study

Greenland Sea(0-4,000)

Multi net

150

728,160

6,754

Clausocalanidae
Calanidae
Metridinidae

Richter 1994

Western subarctic Pacific(0-3,000)

VMPS

62

735,350

9,096

Clausocalanidae
Metridinidae
Eucalanidae

Yamaguchi et al. 2002

North Pacific(0-3,000)

VMPS

62

284,664

6,576

Clausocalanidae
Metridinidae
Calanidae

Homma et al. 2011

Bering Sea(0-3,000)

VMPS

62

213,413

4,341

Clausocalanidae
Metridinidae
Aetideidae

Homma et al. 2011

Study area
(depth range, m)Biomass Dominant families in deep sea(mg C m–2)
(below 200 m)— 98 —

ReferenceABE et al. : Vertical distribution of copepods in the Arctic Ocean

dae in the Pacific sector in the Arctic Ocean is related to thetypical oligotrophic environment with no effect of Atlantic
water in these regions.
Spinocalanidae is small in body size (0.6 2.2 mm for adultfemales ; Damkaer, 1975 ; Razouls et al., 2005 2020).
Due to the difficulty of morphological identification by microscopic observation, few studies of Spinocalanidae are available. Recently, by applying molecular identification
techniques, 28 morphospecies of Spinocalanidae were discriminated, while 39 species were detected using DNA
sequence analyses and 42 using proteomic fingerprinting(Bode et al., 2017). Spinocalanidae is suggested be polyphyletic and has an important role in material cycling in themeso and bathypelagic layer (Bode et al., 2017). Among
dominant Spinocalanidae in the Canada Basin, Spinocalanus
antarcticus is an omnivorous detritus feeder with a looped
shape mid gut that is 1.7 1.9 times longer than its body length(Kosobokova et al., 2002). This looped gut permits thedigestion of amorphous detrital ball and mineral particles andimproves assimilation efficiency in food limited bathy andabyssopelagic realms (Kosobokova et al., 2002). Spinocalanidae is also reported to be adapted to the oxygen
minimum layer and has important role in material cycling of
the mesopelagic zone in the oligotrophic Arabian Sea (Wishner et al., 2008).
Because of the high stratification of ice melt water at thesurface, the Arctic Basin is an oligotrophic environment, andsize of phytoplankton is small, with limited primary production (Nishino et al., 2011). Moreover, in the Canada Basin,
the occurrence of the anticyclonic circulation called the Beaufort Gyre increases stratification via the accumulation of lesssaline ice melt water at the surface and deep nutricline, resulting in dominance of small phytoplankton and low primary
production (Nishino et al., 2011). Sediment trap experiments have revealed that the amount of sinking flux in the
Canada Basin is similar for the oligotrophic subtropical North
Pacific and is mainly composed of mineral particles (Onodera
et al., 2009). Thus, food resources for deep sea copepods in
the Pacific sector of the Arctic Ocean are expected to be lower
than in other oceanic regions. Under this resource limited
oligotrophic environment of the deep basin of the Arctic
Ocean, Spinocalanidae may have an advantage in terms of theadaptation of the specialized digestive system (looped gut) of
this family. Thus, the dominance of Spinocalanidae, especially in the Canada Basin may be related to low primary productivity in the Pacific sector of the Arctic Basin.
It also should be noted that both the abundance and biomass of total copepods through the water column in the
Pacific sector of the Arctic Ocean were somewhat lower thanin the other oceans. The abundance was 28,635 50,176
ind. m−2 in the Pacific sector but 187,189 735,350 ind. m−2 inthe other locations (Table 1). Biomass was 664 1,340
mg C m−2 in the Pacific Sector but 2,488 9,096 mg C m−2 in
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the other locations (Table 1). These facts suggest that low
primary productivity in the oligotrophic Pacific sector may
induce low zooplankton standing stock throughout the watercolumn.
Future prospects
The Arctic Ocean ecosystem is undergoing changes in its
adjacent connecting regions (Wassmann et al., 2015 ; Hunt etal., 2016). The reduction of sea ice is greatest in the Pacificsector of the Arctic Ocean (Stroeve et al., 2007 ; Comiso etal., 2008 ; Markus et al., 2009). Future prospects in the
Canada Basin include low primary production due to theintensification of anticyclonic circulation, enhanced stratification via accumulation of less saline ice melt water at the surface and deepening of the nutricline (Nishino et al., 2011).
In fact, the size of dominant phytoplankton has reportedly
changed from nano size to pico size (Li et al., 2009). As aresult of low productivity, zooplankton standing stocks are
low throughout the water column in the Canada Basin (Kosobokova and Hopcroft, 2010 ; Matsuno et al., 2012). In the
future, anticyclonic circulation will enhance the reduction ofsea ice, resulting in low primary production and reduced vertical material flux to the deep. Under these conditions, dominance of Spinocalanidae, which is adapted to oligotrophic
environments, may be expected in the meso and bathypelagic layer in the Pacific sector of the Arctic Ocean in the
future.
In the Atlantic sector of the Arctic Ocean, water advection
from the Norwegian Sea through the Fram Strait accounts fora large part of the ocean current and has a large physical andbiological impact on the Arctic Ocean (Cottier et al., 2005 ;Nilsen et al., 2006 ; Hunt et al., 2016). The amount of zooplankton biomass transported from the Atlantic Ocean isreported to be two to three times higher than that from the
Pacific Ocean (Wassmann et al., 2015). Since 1990,
increases in the temperature of water passing through theFram Strait have been reported (Turrell et al., 2003 ; Holliday
et al., 2008). According to long term monitoring, the abundance of C. finmarchicus is higher in warm years (>6ºC SST
at the Atlantic sector in the Arctic Ocean) (Turrell et al.,2003 ; Holliday et al., 2008). Among Atlantic species, C.
finmarchicus is the most important and accounted for 55% ofcopepod biomass throughout the water column at Fram Strait
in the present study. This composition is comparable to thevalue of 40% of zooplankton biomass reported NE of the
Svalbard Islands (Kosobokova, 2012). As the temperatureof Atlantic water increases, the biomass of advected C. finmarchicus is expected to increase in the Atlantic sector of the
Arctic Ocean. C. finmarchicus is an important prey of theAtlantic herring (Clupea harengus) and the Atlantic mackerel(Scomber scombrus) (Marshall and Orr, 1972 ; Prokopchuk
and Sentyabov, 2006). Occurrence of more C. finmarchicus
in the Atlantic sector of the Arctic Ocean may modify the— 99 —----

Bull. Fish. Sci. Hokkaido Univ. 70(1), 2020

northward feeding migration route of these fishes and inducechanges in the marine ecosystem.
Thus, as a future prospect, the copepod community is
expected to change in both the Atlantic and Pacific sectors of
the Arctic Ocean. The pattern of change is expected to vary
between the two regions. Thus, in the Pacific sector of the
Arctic Ocean, due to greater oligotrophy, dominance of Spinocalanidae at the deep layer is expected. On the other
hand, because of warming of the intruding Atlantic water,
dominance of C. finmarchicus at the surface layer is expected
for the Atlantic sector in the Arctic Ocean.
Acknowledgements
We thank the captains, officers, crews and researchers on
board the R/V Polarstern, AWI and the R/V Mirai, JAMSTEC, for their great efforts during field sampling. Part ofthis study was supported by a Grant in Aid for Scientific
Research 17H01483 (A), 18K14506 (Early Career Scientists),
19H03037 (B), 20K20573 (Pioneering) and 20H03054 (B)from the Japanese Society for the Promotion of Science(JSPS). This work was conducted within the Arctic Challenge for Sustainability (ArCS II) project.
--

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