Title

Vertical Distribution, Community Structure, and Active Carbon Flux of Two Macrozooplankton Taxa : Amphipods andEuphausiids in the Summer of the Western North PacificAuthor(s)Hanamiya, Yurika; Murase, Hiroto; Matsuno, Kohei; Yamaguchi, Atsushi

Citation

北海道大学水産科学研究彙報, 70(1), 77-89

Issue Date

2020-08-24

Doc URL

http://hdl.handle.net/2115/79118

Type

File Information

bulletin (article)

bull.fish.70.1.77.pdf

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Hokkaido University Collection of Scholarly and Academic Papers : HUSCAPBull. Fish. Sci. Hokkaido Univ.
70(1), 77-89, 2020.
DOI 10.14943/bull.fish.70.1.77

HANAMIYA et al. : Vertical distribution of amphipods and euphausiids

Vertical Distribution, Community Structure, and Active Carbon Flux of
Two Macrozooplankton Taxa : Amphipods and Euphausiids in
the Summer of the Western North Pacific
Yurika Hanamiya1,2), Hiroto Murase3,4), Kohei Matsuno1,5) and Atsushi Yamaguchi1,5)(Received 24 April 2020, Accepted 11 May 2020)
Abstract   This study conducted diel vertical migration and active migration flux estimation of macrozooplanktonic amphipods andeuphausiids at 0-250 m water column of the three stations in the western North Pacific during summer. For amphipods, 25 species belonging to 17 genera were identified. Their standing stock was 60-574 ind. m−2 during the daytime and 35-5,228 ind.
m−2 at night-time. For euphausiids, 19 species belonging to 7 genera were identified. The standing stock of euphausiids was
80-382 ind. m−2 and 286-2,156 ind. m−2 during the day and at night, respectively. Feeding impacts during the night were estimated to be 0.19-11.76 mg C m−2 day−1 (amphipods) and 5.12-16.42 mg C m−2 day−1 (euphausiids). Respiration during thedaytime accounted for 0.08-6.38 mg C m−2 day−1 (amphipods) and 2.33-7.70 mg C m−2 day−1 (euphausiids). The feeding
impact and respiratory flux by active migration of macrozooplankton taxa were estimated as 8.34-28.18 mg C m−2 day−1 and3.15-14.08 mg C m−2 day−1, respectively. These values corresponded to 3.7-12.4% of primary production (feeding impact) and
2.4-10.7% of sinking passive flux from the euphotic zone (respiratory flux). Thus, this study emphasizes the importance ofactive flux by macrozooplankton for vertical material flux in the western North Pacific.
Key words : Active carbon flux, Biological pump, Phronima spp., Themisto pacifica, Primno abyssalis, Vibilia gibbosa,Euphausia spp., Nematoscelis spp., Thysanoessa spp., Stylocheiron spp.

Introduction
Vertical material flux in the ocean occurs mainly throughbiological activity and is called a “biological pump”. The
main driving source of a biological pump is the passive sinking flux mediated by gravity ; that is, CO2 dissolved in seawater is converted to a particulate organic material by thephotosynthesis of phytoplankton, which sinks from the ocean
surface and settles in the deep layer (Longhurst, 1991). As acomponent of the biological pump, the biological activity ofzooplankton is also important. Zooplankton feed near thesurface layer of the ocean and enhance vertical material transport by egesting fast sinking faecal pellets (Ducklow et al.,2001). The diel vertical migration (DVM) of zooplankton is
also considered to play an important role in the biologicalpump. Diel migrant zooplankton feed near the surface layer
at night and descend to the deep layer during the day ; then-

their metabolic respiration at the deep layer accelerates vertical material flux (Longhurst et al., 1990). Vertical materialtransport due to the DVM of zooplankton is called active flux,
which contradicts passive flux by gravity. Active flux is an
important process in the quantitative evaluation of a biological pump, and its quantitative measurement is being conducted at various locations (Steinberg et al., 2000 ; Al
Mutairi and Landry, 2001).
Macrozooplanktonic amphipods and euphausiids are
known to be important prey of pelagic fishes (Yamamura etal., 2002 ; Brodeur et al., 2007 ; Uchikawa et al., 2008),
squids (Watanabe et al., 2004), seabirds (Ogi, 1980 ; Ogi etal., 1980 ; Springer et al., 1996, 1999), and baleen whale(Murase et al., 2007 ; Konishi et al., 2009). On the other
hand, they feed on microplankton (euphausiids) and mesozooplankton (amphipods) (Nakagawa et al., 2001 ; Dalpadado
et al., 2008 ; Noyon et al., 2009). Thus, macrozooplankton

1)

Faculty/Graduate School 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)
2)
Present address : AEON Hokkaido Co., Ltd.
(現所属：イオン北海道)
3)
National Research Institute of Far Seas Fisheries, Japan Fisheries Research and Education Agency(国際水産資源研究所)
4)Tokyo University of Marine Science and Technology(現所属：東京海洋大学)
5)
Arctic Research Center, Hokkaido University(北海道大学北極域研究センター)— 77 —-

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

plays a role in the transfer of lower trophic production tohigher trophic organisms in the marine ecosystem. Macrozooplankton is known to perform large DVMs (Thurston,
1976a, 1976b ; Mauchline, 1980). The active flux by the
DVM of macrozooplankton is estimated to reach 71% ofgravity passive flux (Steinberg et al., 2000). These facts
suggest that macrozooplankton have an important role inpelagic food web and material flux in marine ecosystems.
The western North Pacific Ocean is characterized by large
scale phytoplankton blooms occurring in spring, and it is
known to be an area with actively driving biological pumps
due to its high nutrient supply and high new primary production (Longhurst, 1998). Despite their importance, the estimation of active flux by zooplankton DVM in this region is
scarce (Takahashi et al., 2009). For macrozooplanktonic
taxa, the estimation of active flux is only limited in the subtropical area (Al Mutairi and Landry, 2001 ; Hidaka et al.,2001).
In this study, day night MOCNESS samplings were conducted at three stations in the western North Pacific Ocean
during summer, and DVM of macrozooplanktonic amphipods
and euphausiids were observed. For the dominant species,
the body size and weight (wet, dry and ash free) were measured, then the weight body size equations were calculated. Based on the body size measurement and application
equations, the biomass was also estimated. The metabolism(respiration carbon) was estimated by applying an empiricalmodel varied by taxon (Ikeda, 2014). Assuming the assimilation efficiency and gross growth efficiency, the feeding
impact at the near surface layer during the night and the respiration at the deep layer during the daytime were assessed.
Thus, the estimated active flux by the macrozooplankton
DVM was compared with primary production in the same
area, and the role of macrozooplankton in the vertical material
flux was evaluated.
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Material and methods
Field sampling
Samplings were conducted on R/V Shunyo Maru (National
Research Institute of Far Seas Fisheries, Japan Fisheries
Research and Education Agency). Day and night samplingwith oblique tow of MOCNESS (1 m2 opening, 0.33 mm
mesh equipped, Wiebe et al., 1985) were conducted fromeight depths (0 20, 20 40, 40 60, 60 80, 80 100, 100 150,
150 200 and 200 250 m) at three stations (39°N [St. 39N],
41°N [St. 41N] and 44°N [St. 44N]) along the 158°E line in
the western North Pacific during 16 25 June 2003 (Fig.
1). After collection, samples were immediately fixed with
10% borax buffered formalin. For each station, the temperature and salinity were measured by CTD (Seabird Electronics).
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Fig. 1.

Location of the sampling stations (39°N, 41°N and
44°N) along 158°E in the western North Pacific.

Quantification and body size measurement
Based on the sub samples (1/1 to 1/420, which varied with
the number of samples), amphipods and euphausiids were
sorted, and the species were identified and counted. For
amphipods, the body lengths (BL : mm) of six dominant
genera (Cyphocaris, Eupronoe, Phronima, Primno, Themisto
and Vibilia) were measured as the maximal distance between
the tip of head to the distal end of the uropod (or telson for
Cyphocaris spp.) of the straightened body. For euphausiids,
the body lengths (BL : mm) of five dominant genera(Euphausia, Nematoscelis, Stylocheiron, Tessarabrachion andThysanoessa) were measured from the posterior margin of theeye notch to the terminal end of the sixth abdominal segment. All measurements were made to the nearest 0.1 mm
under a dissecting microscope with an eyepiece micrometre
or with a slide gauge.
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Weight measurement
For dominant species, namely, 7 species belonging to 6
genera in amphipods (Cyphocaris challengeri, Eupronoe
minuta, Phronima atlantica, Phronima sedentaria, Primno
abyssalis, Themisto pacifica, Vibilia gibbosa) and 5 species of
5 genera in euphausiids (Euphausia pacifica, Nematoscelis
difficilis, Stylocheiron longicorne, Tessarabrachion oculatum,
Thysanoessa inspinata), a batch of specimens was preparedwith sufficient BL intervals, and wet weight (WW), dryweight (DW) and ash free dry weight (AFDW) were determined. WW was determined for batches of pooled specimens after rinsing the animals briefly in distilled water,
blotting them on a filter paper, and then placing them into a
pre weighed combusted aluminium pan. The specimens
were then dried in a drying oven at 60°C for 5 h to obtain the
DW. To obtain the AFDW, the ash content was determinedby weighing the specimens before and after combustion at
480°C for 5 h. A microbalance (Mettler Toledo, MT5) wasused to weigh to a precision of 1 µg. For each species,
regressions between weights (Y : WW, DW, AFDW, mg) and— 78 —--HANAMIYA et al. : Vertical distribution of amphipods and euphausiids

BL (X : mm) were expressed with :ln R = 18.775 + 0.766 × ln BM −5.256 × 1000/   Temp −0.113 × ln Depth + 0.416 (amphipods)
(3)ln R = 18.775 + 0.766 × ln BM −5.256 × 1000/   Temp −0.113 × ln Depth + 0.697 (euphausiids)
(4)

(1)

b

Y= aX

where a and b are fitted constants. Each of the regressions ispresented as either Appendix 1 (amphipods) or Appendix 2(euphausiids).
Abundance and biomass
Based on the counted number (n) of each species in thesubsamples with split rate (s) and filtering volume (F, m3) of
the sample, the abundance at each sampling layer (N, ind.
m−3) was calculated as follows :
N=

n
s× F

(2)

By multiplying the depth of each sampling layer (20 or 50 m),
the abundance in density (N, ind. m−3) was converted to thestanding stock (ind. m−2). Using BL data and DW BL relationships (Appendix 1 and 2), the individual DWs (mg DW
ind.−1) were calculated, then the individual DW was multiplied with the density or standing stock, and the biomass (mgDW m−3 or mg DW m−2) were calculated.

where ln R is the logarithm (base e) of the respiration rate (µlO2 ind.−1 h−1), ln BM is the logarithm of the individual dry
weight (mg DW ind.−1), Temp is the habitat temperature (K),and ln Depth is the logarithm of the habitat depth (m).
Assuming the assimilation efficiency ([G+R]/F) and thegross growth efficiency (G/F) to be 70% and 30%, respectively (Ikeda and Motoda, 1978), the feeding rate at night(Fnight, mg C ind.−1 d−1) was calculated using the following
equation :Fnight = R × 12 × 0.97 × 1 × 1 × hnight
22.4
0.4 1000-(5)

where 12/22.4 is the weight of carbon (12 g) in 1 mol (22.4 L)
carbon dioxide, 0.97 is the respiratory quotient (CO2/O2,
Gnaiger, 1983), 1/0.4 is the F/R, 10−3 is a unit conversion
from μg to mg, and hnight is the residence time at 0 200 m during the night (h [9 hours time required for DVM]). By
multiplying the individual feeding rate (Fnight) by the nighttime standing stock (Nnight, ind. m−2), the daily feeding rate of
the water column (mg C m−2 d−1) was calculated.
The respiration rate during daytime (Rday : mg C ind.−1 d−1)
was calculated by using the following equation :-Active flux estimation-

To evaluate the active flux by DVM of amphipods andeuphausiids, feeding at the epipelagic zone (≤250 m) during
the night and respiration at the deep sea (>250 m) during thedaytime were estimated. First, the time durations at day andnight depths were calculated. At each sampling station, thesunrise was from 4 : 42 to 4 : 59, and the sunset was from19 : 53 to 20 : 13 ; therefore, the daytime and night time were
calculated as 15 hours and 9 hours, respectively. Next, thetime required for the zooplankton DVM was determined by
the swimming speed and the DVM distance, and the residence time of the zooplankton in each layer was determinedby subtracting it from the above day and night time. In thisstudy, the daytime distribution depth was assumed to be 400m, which corresponded with the reported daytime distribution
depths for each taxon (Yamada and Ikeda, 2000 ; Yamada etal., 2002, 2004 ; Taki, 2008). The distribution layer at night
was determined by the distribution centre (D50%, Pennak,
1943), which varied by species. Thus, the DVM distance(m) was calculated as : daytime distribution depth (400 m)
D50%. For the swimming speed, we applied eight times BL
per second, which was reported for euphausiids (Kils, 1979).
Since we could not find comparable information on amphipods, their swimming speed was calculated by the same criteria of euphausiids (eight times of BL per second). Thisswimming speed of amphipods corresponded well with thereported swimming speed (0.172 m s−1) of Phronima spp.
(Davenport, 1994), which was abundant in this region.
The individual respiration rates of amphipods and euphausiids were calculated by the following equations (Ikeda,2014) :--

Rday = R × 12 × 0.97 × 1 × hday
22.4
1000

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(6)

where hday is the residence time at the deep layer during thedaytime (h [15 hours time required for DVM]). Multiplying the individual respiration rate (Rday) by the standing stock,which performs DVM (Nnight NDay, ind. m−2), and the daily
respiratory flux (mg C m−2 d−1) was calculated.
---

Results
Hydrography
The water temperature between 0 500 m of each station
was 5.3 16.7°C for 39N, 4.3 10.4°C for 41N and 4.0 10.5°C
for 44N, and thermocline development was observed at
approximately 30 m of 41N (Fig. 2a). The salinity at sea
surface ranged between 33.3 and 34.4, and increased withincreasing depth at 41N and 44N, and decreased with increasing depth at 39N (Fig. 2b). Throughout the station, thesalinity settled approximately 34.0 at 500 m depths. A T S
diagram showed that the water mass at 39N was a subtropicaldomain, while those at 41N and 44N were transitional
domains (Anma et al., 1990) (Fig. 2c).
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Amphipods— 79 —The standing stock of amphipods in the 0 250 m water col-

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

Fig. 2.

Vertical profiles of temperature (a) and salinity (b) and T-S diagram (c) at each station in the western North Pacific during
16-25 June 2003. Numbers in the panel (c) indicate water densities (sigma-T). Boundaries of water masses (Anma et al.,1990) are shown in the panel (c).

umn ranged between 60 ind. m−2 (39N) and 574 ind. m−2(44N) for daytime, and 35 ind. m−2 (39N) and 5,228 ind. m−2(41N) for night time (Table 1). Throughout the stations, 25
species belonging to 17 genera of amphipods occurred. The
number of species was high (21 species) at the southern station (39N) and decreased with the higher latitude station (11species at 44N) (Table 1). At 39N, Phronima spp. wasabundant and followed T. pacifica. The number of species
was high at night time. At 41N and 44N, T. pacifica was the
predominant species, both day and night, and C. challengeri
and V. gibbosa showed high abundance, especially at night.
Vertical distribution of the biomass of six dominant amphipod
species (C. challengeri, E. minuta, Phronima spp., P. abyssalis, T. pacifica and V. gibbosa) and their species compositions
are shown in Fig. 3. Biomass through the 0 250 m watercolumn ranged between 26 mg DW m−2 (39N) and 110 mg
DW m−2 (44N) at daytime, and 18 mg DW m−2 (39N) and
1,178 mg DW m−2 (41N).
At 39N, the amphipod biomass was high at 80 200 m
depths, during both the day and night (Fig. 3a). Phronima
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spp. dominated 47 100% of the amphipod biomass during thedaytime, and their concentration was high, especially below
85 m. V. gibbosa occurred at the deepest depth during thedaytime and constituted 48 76% of the biomass at 0 60 m
during the night. E. minuta occurred only at 39N and constituted 5 26% of the biomass at 0 80 m during the daytime
and 7 23% at 20 100 m during the night, thus showing no
DVM.
At 41N, the highest biomass (0.74 0.91 mg DW m−3) was
seen at 20 60 m during the daytime, and a prominent increase(21.39 mg DM m−3) was seen at 150 200 m at night (Fig.
3b). During daytime, T. pacifica predominated at 20 100 m,
and Phronima spp. and P. abyssalis dominated below 100 m.
At night, V. gibbosa dominated at 0 100 m and constituted
47% of the high biomass in the 150 200 m layer. As daynight differences, the species concentrations of C. challengeri
and V. gibbosa in the biomass increased at night throughout
the layers.
At 44N, the highest biomass was seen at 100 150 m, during both the day and night (1.26 mg DW m−3 at daytime and— 80 —

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HANAMIYA et al. : Vertical distribution of amphipods and euphausiids
Table 1. List of amphipod species and their abundance (ind. m−2) at 0-250 m water column of three stations along 158˚E in the western
North Pacific during 16-25 June 2003.
41N

39N

SpeciesDay

Cranocephalus scleroticus
Cyphocaris challengeri
Eupronoe minuta
Hyperioides longipes
Hyperia medusarum
Hyperoche medusarum
Mimonectes loveni
Oxycephalus clausi
Paraphronima crassipes
Paraphronima gracilis
Phronima atlantica
Phronima colletti
Phronima sedentaria
Phronima spp.
Phronima stebbingi
Phrosina semilunata
Platyscelus serratulus
Primno abyssalis
Scina crassicornis
Streetsia steenstrupi
Themisto pacifica
Vibilia australis
Vibilia caeca
Vibilia gibbosa
Vibilia pyripes
Vibilia viatrix

Night
0.08
0.69
3.31
0.07

4.53

44N

Day

Night

Day

0.11

98.44

28.01

Night

0.72
0.18
0.08
0.07
0.07
0.28
5.45
0.26
6.54
7.80

0.34
8.11
6.79
31.97
0.62
0.11

1.26

2.79
2.79
23.29

0.26
1.42

1.40

4.39

1.62

0.37
0.94
2.36
0.50
3.05

6.70

13.81
2.79

1.67

1.22

194.584,738.60

568.93315.850.42

5.57
323.83

0.08
0.48

0.18
95.02
0.57

574.47

448.1

0.23
0.12
0.35

2.27
0.34
4.70

0.21
6.31
0.08
2.29
0.85

11.42

Total amphipods

59.8

35.1

204.5

2.97 mg DW m−3 at night, Fig. 3c). During the daytime, T.
pacifica predominated the amphipod biomass (constituting
95 100% of the biomass), especially at 0 200 m depths. At
night, the concentration of T. pacifica decreased ; on the other
hand, the concentrations of C. challengeri and V. gibbosa
increased. Within the biomass, V. gibbosa dominated at
20 150 m depths and constituted 67 89% of the amphipod
biomass at night. Thus, while the amphipod biomass peak
was seen same depths of 100 150 m during both the day andnight, the biomass composition varied greatly ; T. pacifica
dominated during the daytime, while V. gibbosa dominated atnight.
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5,228.6

The number of euphausiid species was the highest at the subtropical station (39N). At that station, the number of species
increased at night. At 39N, Euphausia spp., especially E.
pacifica, was the most numerous species. At 41N and 44N,
the most abundant species was Thysanoessa spp., and E. pacifica increased in abundance at night. The vertical distribution of the biomasses of five dominant euphausiid species(Euphausia spp., Nematoscelis spp., Stylocheiron spp., Thysanoessa spp. and Tessarabrachion oculatum) and their species
concentrations are shown in Fig. 4. At the subtropical station, a prominent day night contrast (extremely high concentrations at night) was observed in the case of the euphausiid
biomass in the 0 250 m water column.
At 39N, the euphausiid biomass was high (2.51 mg DWm−3) at 100 150 m depth during the daytime, was concentrated at ≤100 m and was especially high (8.6 10.9 mg DWm−3) at 0 40 m at night (Fig. 4a). These high densities weremainly composed of E. pacifica, which accounted for81 100% of the euphausiid biomass at 100 200 m during the--

Euphausiids
The standing stock of euphausiids in the 0 250 m watercolumn ranged from 80 ind. m−2 (41N) to 382 ind. m−2 (44N)
during the daytime and from 286 ind. m−2 (39N) to 2,156
ind. m−2 (41N) at night (Table 2). Throughout the region, 19
species belonging 7 genera euphausiid species occurred.
-— 81 —

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

Fig. 3.

Vertical distribution in the biomass (upper panels) and species composition (lower panels) of dominant amphipods in termsof biomass at 39°N (a), 41°N (b), 44°N (c) along 158°E in the western North Pacific during 16-25 June 2003. The integrated mean biomass (0-250 m, mg DM m－2) is shown in the parentheses in the upper panels.

Table 2. List of euphausiid species and their abundance (ind. m−2) at 0-250 m water column of three stations along 158˚E in the western
North Pacific during 16-25 June 2003.
SpeciesEuphausia brevis
Euphausia diomedeae
Euphausia hemigibba
Euphausia mutica
Euphausia pacifica
Euphausia pseudogibba
Euphausia recurva
Euphausia similis
Euphausia spp.
Nematobrachion flexipes
Nematoscelis difficilis
Nematoscelis spp.
Stylocheiron abbreviatum
Stylocheiron carinatum
Stylocheiron elongatum
Stylocheiron longicorne
Stylocheiron maximum
Stylocheiron spp.
Tessarabrachion oculatum
Thysanoessa inspinata
Thysanoessa longipes
Thysanoessa spp.
Thysanopoda aequalis
Unidentified euphausiids
Total euphausiids

39N

41N

Day

Night

0.26
0.87
0.81
0.78
68.18
2.08
4.43
1.17
97.80
1.05
0.13
0.59

0.28
0.16
1.29
166.09
22.80
6.32
9.19
16.14
1.83
11.81
1.92

44N

Day

Night

Day

Night

2.62

203.77

5.58

41.39

0.42

45.25

5.07

0.12
0.19

0.11

0.23

0.36

0.19
0.13
3.17
7.60
23.60
0.10
611.12

5.28

0.07
9.74

4.17
4.59
0.90

1.85
3.58
0.87

2.17
4.16
1.97

3.18
12.22
16.49

3.54

12.49

1,705.57

135.68

13.85
0.16
18.05

7.15
4.08
23.52
1.14
192.7656.01

169.11147.06128.50

332.3

286.0

79.8

2,156.0

381.8

821.0

— 82 —HANAMIYA et al. : Vertical distribution of amphipods and euphausiids

Fig. 4.

Vertical distribution in biomass (upper panels) and species composition (lower panels) of dominant euphausiids in terms ofbiomass at 39°N (a), 41°N (b), 44°N (c) along 158°E in the western North Pacific during 16-25 June 2003. The integrated
mean biomass (0-250 m, mg DM m－2) are shown in the parentheses in the upper panels.

daytime and at 0 100 m during the night time. Within thedominant species, Nematoscelis spp. occurred only at 39N.
Nematoscelis spp., constituted 33% of the euphausiid biomass
at 40 60 m during the daytime, formed a substantial concentration at 40 250 m and constituted 52% of the biomass at
200 250 m at night.
At 41N, the euphausiid biomass was low (0.005 0.24 mg
DW m−3) during the daytime, while having a high density(maximum reached 39.43 mg DW m−3 at 0 25 m) for 0 100
m depths at night (Fig. 4b). During the daytime, Thysanoessa spp. dominated at 20 150 m, Stylocheiron spp. dominated at 150 200 m, and Euphausia spp. predominated at
200 250 m. At night time, Thysanoessa spp. showed a highconcentration (39 100% of the biomass) throughout the layer,
and it should be noted that Euphausia spp. dominated (4060%) at 0 40 m and 150 200 m depths, where the euphausiid
biomasses were high.
At 44N, the euphausiid biomass ranged from 0.003 1.28
mg DW m −3 during the daytime and from 0.06 8.05
mg DW m−3 at night (Fig. 4c). In the daytime, the euphausiid biomass was high at 20 60 m depths. At night, theeuphausiid biomass increased for the whole layers and wasconcentrated at 0 60 m. For the biomass composition, Thysanoessa spp. dominated at 40 150 m of daytime and at alllayers at night. Euphausia spp. occurred only at 0 60 m
depths during both day and night and constituted 4 99% and
18 50% of the euphausiid biomass during day and night,
respectively. Stylocheiron spp. occurred below 80 m andconstituted 28 100% of the biomass at 80 100 m and 150
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200 m during the daytime and 8 44% of the biomass below
80 m during the night time.
--

Active flux
For active flux, the feeding impact near the surface layer
during night and respiration at the deep layer during daytime
were computed (Table 3). The feeding impact of amphipods
was 0.19 11.76 mg C m−2 day−1, and that of euphausiids was
5.12 16.42 mg C m−2 day−1. The respiration rate of amphipods was 0.08 6.38 mg C m−2 day−1, and that of euphausiids
was 2.33 7.70 mg C m−2 day−1. For both the feeding impact
and the respiration rate, values at 41N were the largest, which
was due to the largest biomass at that station being observed
for both amphipods and euphausiids (1,178 or 1,562 mg DWm−2). Common for the whole station, the ingestion and respiration rates of euphausiids were greater than those of amphipods. For the total of active flux by macrozooplankton, theingestion rate was 8.34 28.18 mg C m−2 day−1 and the respiration rate was 3.15 14.08 mg C m−2 day−1.
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Discussion

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Amphipods
The abundance of amphipods was high at high latitude anddecreased with low latitude (Table 1). Within the dominant
species, T. pacifica is known to be a boreal subarctic species(Nagata, 1997) that inhabits a wide geographical range
including the Bering Sea, Okhotsk Sea and northern North
Pacific (Bowman, 1960 ; Nagata, 1997). The abundance of— 83 —

Bull. Fish. Sci. Hokkaido Univ. 70(1), 2020
Table 3. Biomass at night and active carbon transport by dielvertical migration of amphipods and euphausiids ;nighttime ingestion and daytime respiration were calculated.
Taxon/station

Biomass at night
Ingestion
Respiratory flux
(mg DW m−2) (mg C m−2 day−1) (mg C m−2 day−1)

Amphipods
39N
41N
44N

18
1,178
329

0.19
11.76
3.34

0.08
6.38
1.68

ern North Pacific (Nagata, 1997). E. minuta occurred onlyat the southern 39N (Fig. 3a). E. minuta distributed in theupper 100 m depths both day and night, which correspondedwith the previously reported vertical distribution of this species (Cornet and Gili, 1993).
For P. abyssalis, the presence of DVM is reported(Yamada, 2002). In this study, P. abyssalis occurred for daytime at 39N and 41N, and no evidence was detected for the
DVM. Since the abundance of P. abyssalis peaks at approximately 400 m depths both day and night (Yamada, 2002), theshallower sampling depths of this study (0 250 m) made it
impossible to collect their main population or to detect their
DVM.
T. pacifica is an epipelagic amphipod, and their distribution
in the western subarctic Pacific was concentrated at shallower
depths than 200 m in the daytime and shallower than 50 m atnight (Yamada, 2002). At 39N of this study, the biomass of
T. pacifica shifted to <100 m depths at night (Fig. 3a), whichcorresponded with this DVM pattern. At 41N and 44N, T.
pacifica composed the main species of the amphipod biomass<100 m depths during daytime, while their composition
decreased at night (Fig. 3b, c), which may be partly due to thebody size of T. pacifica being smaller than that of the otherspecies ; thus, because of the dominance of the large bodysize of other species, the contribution of T. pacifica in theamphipod biomass decreased at night.
Phronima spp. (P. atlantica and P. sedentaria) is reported
to distribute at 0 1,000 m depths in the Canary Islands, and P.
atlantica has abundance peaks <200 m and 600 1,000 m,
while P. sedentaria concentrates at <400 m, and both species
perform small DVM (Thurston, 1976b). In this study,
Phronima spp. had a high composition in the amphipod biomass, especially for the high biomass layer at 39N (Fig. 3a),and the vertical distribution and DVM of amphipods at thatstation may be governed by Phronima spp. Phronima spp.
is known to feed on small salps and enter barrels, which is theremains of gelatinous zooplankton living inside of barrels(Harbison et al., 1977). Since the distributions of Phronima
spp. and gelatinous zooplankton, which is a source of the barrel, are well corresponded (Gasca and Haddock, 2004), thedistribution of Phronima spp. May be governed and determined by the distribution of gelatinous zooplankton.
Few V. gibbosa occurred at each station during the daytime,
while being observed for whole layers throughout the station
at night (Fig. 3). Throughout the station, V. gibbosa had ahigh composition at <50 m depths and was the dominant species at biomass peak depths of 41N and 44N (Fig. 3b, c).
There is little information available on the DVM of Vibilia
spp. In the Canary Islands, vertical distribution of Vibilia
spp. extends down to 600 m and reaches 800 m, and the
DVM is reported for V. armata, which is the dominant species
there (Thurston, 1976b). These facts suggest that V. gibbosa
in the western North Pacific may be distributed below 250 m-

Euphausiids
39N
41N
44N

668
1,562
437

8.14
16.42
5.12

3.07
7.70
2.33

Total
39N
41N
44N

686
2,740
766

8.34
28.18
8.46

3.15
14.08
4.00

T. pacifica was low at the low latitude station (39N) and washigh at 41N and 44N, with T. pacifica being the dominant
species at those stations. These facts suggest that latitudinalchanges in the abundance may have primarily govern the latitudinal distribution of T. pacifica. Common for T. pacifica,
C. challengeri and V. gibbosa, their abundances showed clear
increases at night (Table 1). These three species are known
to perform DVM, which is characterized by nocturnal ascent(Thurston, 1976a, 1976b ; Yamada, 2002). Thus, the drastic increase in abundance of these species may be interpreted
as the result of their nocturnal ascent DVM. For the biomass of amphipods, their night time peak was seen for
75 150 m, which is comparable to that of surface swarming
euphausiids (Figs. 3 and 4). Next, we discuss the six dominant species in the amphipod biomass.
C. challengeri were extremely few in number in the daytime, while occurring throughout the 0 250 m layer of 41N
and 44N at night (Fig. 3b, c). C. challengeri is known to
distribute throughout the oceans worldwide (Hughes and
Lowry, 2015). In low latitude areas, their vertical distribution is reported to become deep (Thurston, 1976a). The
thermal and saline range of the C. challengeri distribution isreported to be <10°C and <34.5, respectively. In the Oyashio region, C. challengeri distributes at 200 400 m depth during daytime, then migrates upward and distributes
approximately 0 150 m at night (Yamada, 2002). This daynight distribution of C. challengeri corresponds well withthose at 41N and 44N for this study (Fig. 3b, c). Since thetemperature of 0 250 m depths at 39N was 9.4 16.7°C (Fig.
2a), to avoid the high thermal condition, the abundance of C.
challengeri was extremely low at that station.
E. minuta is classified as a warm water species in the west-

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— 84 —--HANAMIYA et al. : Vertical distribution of amphipods and euphausiids

during the daytime, perform DVM and occur at <250 m
depths in this study (Fig. 3). V. gibbosa is also reported to
distribute in the body of salps, and they feed on particle matter, which is collected by salps (Madin and Harbison,1977). These facts suggest that the distribution of V. gibbosa
may be related with gelatinous zooplankton, especially salps,,
while ecological information on V. gibbosa is rare in the western North Pacific, considering their high composition ; thus,
more study is required in the future.
Euphausiids
For euphausiids, 19 species belonging to 7 genera wereidentified in this study (Table 2). Since euphausiids in thisstudy contain many unidentified specimens (Table 2), there
are some possibilities that the species list of this study may
increase. The abundance of euphausiids increased at night,
especially at 41N and 44N (Table 2). The night timeeuphausiid abundance was dominated by Thysanoessa spp.,
followed by Euphausia spp., especially E. pacifica. At all
stations, these two genera (Thysanoessa spp. and Euphausiaspp.) dominated in abundance (Table 2). These facts indicate that the euphausiid abundance in this region is shaped
mainly by these two genera. For the euphausiid biomass,
few peaks were detected during the daytime, while forming
clear peaks at 0 40 m depths (Fig. 4). These facts suggestthat a clear DVM was the case for three stations in this study.
Next, we discuss the DVM of the biomass dominated 5 genera species.
Euphausia spp. is reported to have a clear DVM in various
locations (Mauchline, 1980 ; Taki, 2008). During the night,
Euphausia spp. distributes at the surface and feeds on phytoplankton, microzooplankton and detritus (Mauchline, 1980).
The night time distribution depth of Euphausia spp. is known
to vary with the amount of primary production (Hirota, 1987).
Dominance of Euphausia spp. at the shallowest depths (0 20m) at night throughout the station corresponds to these findings (Fig. 4). Throughout the station, vertical changes in theeuphausiid biomass were well synchronized with the dominance of Euphausia spp. These facts suggest that theeuphausiid biomass in this region is mainly governed by
Euphausia spp.
Nematoscelis spp. is reported to be distributed mainly inthe subtropical region, while the distribution depths varied by
species and with the performance of DVM (Mauchline,
1980 ; Sawamoto, 1997). Also found in this study was that
Nematoscelis spp. occurred only at the subtropical station(39N) (Fig. 4a). Vertical distribution of N. difficilis, which isthe abundant species in this study, is reported to be 50 400 m,
both day and night (Sawamoto, 1997). At 39N, the nighttime increase of Nematoscelis spp. in composition with theeuphausiid biomass may reflect upward migration frombelow 250 m at night.
Stylocheiron spp. is distributed below 50 m depths and
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occurred deeper than the other genera (Fig. 4). Within Stylocheiron spp., S. longicorne was the most abundant species(Table 2). S. longicorne is known to have no DVM and ischaracterized by mesopelagic carnivores (Mauchline, 1980),
while feeding on faecal pellets (Nemoto, 1967 ; Hirota andNemoto, 1989). Because of these feeding modes (feed on
faecal pellets and zooplankton), the vertical distribution
depths of Stylocheiron spp. may be deeper than that of theother euphausiid species.
T. oculatum showed deeper distribution depths at night
than during the daytime (reverse DVM) for all stations (Fig.
4). These reverse DVM patterns of T. oculatum have beenreported by Mauchline (1980). Since the main distribution
depth of T. oculatum is reported to be a much deeper layer(below 200 m depth during the daytime and below 400 m
depth at night) (Mauchline, 1980 ; Sawamoto, 1997), thesampling design of this study (0 250 m depths) may havefailed to accurately detect the distribution depths of this species.
Thysanoessa spp. composed a small percentage of theeuphausiid biomass at 39N, while having high percentages at
41N and 44N. For high biomass layers at 41N and 44N, thecomposition of Thysanoessa spp. in the euphausiid biomass
was followed by Euphausia spp. For Thysanoessa spp.,
species identification was difficult for most of the specimens
because of the incomplete species identification of body parts.
Since both the abundance and biomass of Thysanoessa spp.
increased clearly at night (Table 2, Fig. 4), DVM characterized by nocturnal ascent from below 250 m depth may thecase for this genus.
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Active flux
The estimated macrozooplankton ingestion at night andrespiration during the daytime were the highest at 41N, which
may have been caused by the highest biomass occurring atthat station (Table 3). For the second largest biomass station(44N), the daytime respiration was higher than that in 39N,
while night time ingestion was similar for that in 39N (Table3), which may be due to differences in the temperature
between the two stations (Fig. 2).
As information on primary production in this region, thevalue at St. KNOT (44°N, 155°E) during June August : 227
mg C m−2 day−1 has been reported (Imai et al., 2002). Estimated ingestion by macrozooplankton in this study (8.3 28.2
mg C m−2 day−1, Table 3) corresponded with 3.7 12.4% ofprimary production. If we assume the f ratio to be 0.58 (cf.
Fujii et al., 2007), passive POC flux from the euphotic layer isestimated to be 132 mg C m−2 day−1. Estimated respiration
by macrozooplankton in this study (3.2 14.1 mg C m−2 day−1,
Table 3) accounted for 2.4 10.7% of passive POC flux.
Active carbon flux by respiration of zooplankton DVM isestimated to be 1.5 20 mg C m−2 day−1 for oceans worldwide(Table 4). An estimation by this study contains only four

— 85 —

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Bull. Fish. Sci. Hokkaido Univ. 70(1), 2020
Table 4. Comparison of active carbon flux (respiratory only, not included egestion or motality at depth) by diel migrant zooplankton. All
migratory flux are means and are compared with mean sedimentary passive C flux at each region. Numbers in parentheses are values of amphipods and euphausiids, respectively.
Location

Migrant Biomass(mg C m )Migratory flux
(mg C m−2 day−1)

Component−2Compared with POC flux%

Depth (m)SourceAtlantic Ocean
BATS
BATS

191
49

Mesozooplankton
Mesozooplankton

15
1.5

34
6

150
150

Dam et al. (1995)
Steinberg et al. (2000)

96
155
126

Mesozooplankton
Mesozooplankton
Meso- and macrozooplankton

4.2
7.3

18
25

150
150

Zhang and Dam (1997)
Zhang and Dam (1997)

3

18

150

Steinberg et al. (2008)

K2

1,280

Meso- and macrozooplankton

K2
39N

116
274*

Metridia pacifica
Amphipods+euphausiids

20
3.0
3.2 (0.1+3.1)

72
10
2

150
150

Steinberg et al. (2008)
Kobari et al. (2008)

41N

1,096*

Amphipods+euphausiids

14.1 (6.4+7.7)

44N

306*

Amphipods+euphausiids

4.0 (1.7+2.3)

Pacific Ocean
Eastarn Equator
Eastarn Equator
ALOHA

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This study

11-This study

3-

This study*Calculated from Table 3, assuming C=0.4×DM (Peters and Downing, 1984).

amphipods and euphausiids, whose respiration flux (3.2 14.1
mg C m−2 day−1) is at the range of the reported value. The
respiration flux at 41N is very close to the value on zooplankton community at St. K2 (47°N, 161°E) in the western subarctic Pacific (Steinberg et al., 2008). As characteristics ofthe present estimation, active carbon flux was small relative tothe biomass. For instance, the active carbon flux in the central equatorial Pacific (Zhang and Dam, 1997) is same value
at 44N in this study (4.0 mg C m−2 day−1, Table 3). However, the zooplankton biomass was 96 mg C m−2 in the equatorial Pacific (Zhang and Dam, 1997), while it was 306
mg C m−2 at 44N (Table 4). These discrepancies may be
due to differences in the mass specific respiration, which isprimarily governed by habitat temperature (cf. Ikeda,
1985). For a calculation of the habitat temperature in theequatorial Pacific, Zhang and Dam (1997) used 13°C, while it
was 4.5°C at 44N (Fig. 2). Thus, the mass specific metabolism is high in the warm low latitude region, and the contribution of zooplankton respiration flux may be high in theseregions.
There are several shortcomings in the present estimation ofactive carbon flux. First, we assumed the vertical migration
distance to be : 400 m D50%. However, the magnitude of
DVM is known to vary by species, body size and developmental stages (Thurston, 1976b ; Mauchline, 1980 ;Yamada, 2002). To make a more accurate estimation, thevertical migration distance, which varied by body size, andinformation on the swimming velocity would be required.
Second, active carbon flux in this study only included respiration of zooplankton, and fluxes due to predation, mortality
and excretion were not considered. Among other types offlux, active flux via dissolved organic carbon (DOC) was---

reported to be important. According to Steinberg et al.
(2000), active flux via DOC accounts for 31% of respiration
flux. Zhang and Dam (1997) also noted that mortality flux
may account for one third of the zooplankton DVM flux.
Considering this information and since the present estimate is
only based on respiration flux, this estimate might be anunderestimation because it lacks consideration for the DOC
and mortality flux, but it may difficult to estimate those factors using the data set of this study.
With these shortcomings considered, the respiration flux inthis study is estimated to account 2.4 10.7% of the passive
sinking flux in this region. Based on the calculation of thisstudy, the importance of active carbon flux of macrozooplankton is evident. The biomass of Macrozooplankton, such asamphipods and euphausiids, is difficult to estimate accuratelyby ordinary zooplankton sampling. To collect macrozooplankton quantitatively, largemouth sampling gear, such as
MOCNESS, may be required. Because of the large bodysize, macrozooplankton perform large DVM and may havean important role in the vertical material flux of oceans.
---

Acknowledgements-

This study was carried out as a part of JARPNII (Japanese
Whale Research Program under Special Permit in the North
Pacific Phase II). We thank Fisheries Agency of Japan andJapan Fisheries Research and Education Agency who supported the survey by R/V Shunyo Maru. We thank the captain, officers and crew of the vessel for their great help during
field sampling. The data analysis and sample processing
were conducted by Dr. Hikaru Watanabe. We highly appreciate his great effort. Dr. Yuiciro Yamada provided guidance— 86 —-HANAMIYA et al. : Vertical distribution of amphipods and euphausiids

for species identification of amphipods. For euphausiids,
Dr. Hye Seon Kim provided valuable guidance. We express
our sincere thanks for their contributions. Part of this study
was supported through a Grant in Aid for Scientific Research
17H01483 (A), 18K14506 (Early Career Scientists),
19H03037 (B), 20K20573 (Pioneering) and 20H03054 (B)from the Japan Society for the Promotion of Science(JSPS). Part of this study was conducted by the Arctic Challenge for Sustainability (ArCS II) project.
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— 88 —HANAMIYA et al. : Vertical distribution of amphipods and euphausiids
Appendix 1. Relationships between body mass (Y : wet mass [WM], dry mass [DM] and ash-freedry mass [AFDM], all in mg) and body length (X, mm) of dominant amphipods in thewestern North Pacific Ocean. Note that the all regressions are highly significant (p<0.01).
Species

Body mass unitn

Cyphocaris challengeri

WM
DM
AFDM
WM
DM
AFDM
WM
DM
AFDM
WM
DM
AFDM
WM
DM
AFDM
WM
DM
AFDM
WM
DM
AFDM

18
18
18
11
11
11
21
21
21
15
15
15
19
19
19
19
19
19
19
19
19

Eupronoe minuta

Phronima atlantica

Phronima sedentaria

Primno abyssalis

Themisto pacifica

Vibilia gibbosa

Y= aX bab0.009690.000820.000780.045830.006280.00538
0.02986
0.00467
0.00506
0.010290.000830.00069
0.02598
0.00372
0.00258
0.02398
0.004400.00376
0.03805
0.000390.00019

3.12
3.30
3.26
2.67
2.58
2.56
2.47
2.06
1.84
3.05
2.98
2.91
2.84
2.93
3.03
2.98
2.82
2.65
2.72
3.90
4.06

r2
0.97
0.97
0.97
0.99
0.99
0.99
0.92
0.91
0.91
0.99
0.99
0.98
0.99
0.96
0.97
0.99
0.99
0.99
0.88
0.95
0.93

Appendix 2. Relationships between body mass (Y : wet mass [WM], dry mass [DM] and ash-freedry mass [AFDM], all in mg) and body length [X, mm] of dominant euphausiids in thewestern North Pacific Ocean. Note that the all regressions are highly significant(p<0.01).
SpeciesEuphausia pacifica

Nematoscelis difficilis

Stylocheiron longicorne

Tessarabrachion oculatum

Thysanoessa inspinata

Body mass unit

n

WM
DM

Y=aX b

r2

a

b

21
21

0.004200.00056

3.23
3.27

AFDM

21

0.00055

3.22

0.99

WM

21

0.01454

2.77

0.99

DM

21

0.00160

2.89

0.98

AFDM

21

0.00144

2.87

0.97

WM

21

0.01211

2.82

0.97

DM

21

0.00187

2.79

0.97

AFDM

21

0.00177

2.73

0.97

WM

20

0.00422

3.25

0.99

0.99
0.99

DM

20

0.00080

3.17

0.99

AFDM

20

0.00098

2.96

0.99

WM

21

0.01048

2.93

0.98

DM

21

0.00194

2.86

0.97

AFDM

21

0.00208

2.74

0.97

— 89 —