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Impacts of diel vertical migration of the copepod Metridia pacifica on primary production and respiratory carbon flux inthe subarctic Pacific Ocean

Author(s)Yamaguchi, Atsushi

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北海道大学水産科学研究彙報, 71(1), 29-37

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2021-08-03

10.14943/bull.fish.71.1.29

http://hdl.handle.net/2115/82350

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bull.fish.71.1.29.pdf

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Hokkaido University Collection of Scholarly and Academic Papers : HUSCAPBull. Fish. Sci. Hokkaido Univ.
71(1), 29-37, 2021.
DOI 10.14943/bull.fish.71.1.29

Yamaguchi : Respiratory carbon flux by Metridia pacifica

Impacts of diel vertical migration of the copepod Metridia pacifica on primary
production and respiratory carbon flux in the subarctic Pacific Ocean
Atsushi Yamaguchi1,2)(Received 15 March 2021, Accepted 23 March 2021)
Abstract   Respiratory oxygen consumption rates (at the two temperatures of 0-50 m and 50-200 m depth strata) and day/nightbiomass in the top 50 m water column were determined on adult female Metridia pacifica at twelve stations in the western/
eastern subarctic Pacific and one station in the oceanic Bering Sea during summer. At each station, the respiration rates at 0-50
m depth temperatures were used to estimate ingestion rates during nighttime by assuming empirical carbon budget efficiencies,and rates at 50-200 m to estimate respiratory carbon flux during daytime. The abundance of the females in the upper 50 m atnight varied between 27 and 5,422 inds. m−2 and no specimen was collected from the same layer at daytime throughout thestations. The size of the females varied regionally from 25 to 77 µg C ind.−1. As a result, diel vertical migrant biomass of thefemales varied greatly from one station to the next (1 and 309 mg C m−2). Weight-specific respiration rates of the females were
2.2-6.3 µl O2 mg C−1 h−1, which was a function of experiment temperatures and body mass (C) of the females. Taking into
account of residence time at 0-50 m and 50-200 m in the day, daily population ingestion was estimated as 0.04-11.04 mg C m−2
day−1, which accounted for 0-2.4% of primary production at each station. Daily population respiration in the 50-200 m wascalculated as 0.02-9.39 mg C m−2 day−1, which corresponds to 0-10% of the POC flux down from the euphotic zone.
Key words : Respiratory flux, Active carbon flux, Copepoda, Metridia pacifica

Introduction
Diel vertical migration of zooplankton is a phenomenon
well documented since 200 years ago, but its ecologicalimplications are not fully understood as yet (cf. Hays,
2003). From a viewpoint of material transport, a possible
acceleration of vertical flux by the prey predator relationshipof vertically migrating zooplankton and micronekton has been
proposed as the “ladder of migration” hypothesis (Vinogradov,1968). Active flux (ingestion in the upper layer at night andmineralization in the lower layer during daytime) by diel vertical migration of zooplankton was estimated first by Longhurst and Harrison (1988). As active material flux mediatedby diel vertical migration behavior of zooplankton, gut flux
(Schnetzer and Steinberg, 2002), respiratory flux (Longhurst
and Harrison, 1988), and mortality flux (Zhang and Dam,
1997) are now considered. Among them, respiratory flux
has been reported as an important vertical flux process(Morales, 1999). From a methodological viewpoint, themost previous studies on flux are those based on indirect
methods, such as ETS assays (Hernández León et al., 2001 ;Yebra et al., 2005) or application of empirical respiration data
(Al Mutairi and Landry, 2001 ; Roman et al., 2002). The---

respiratory flux is known to be greater in the subtropical andtropical oceans (Longhurst et al., 1990 ; Dam et al., 1995),
but little has been studied in subarctic regions.
Metridia pacifica is a medium sized oceanic calanoid
copepods dominated (the second in abundance and the fifth inbiomass) in copepod communities in the subarctic Pacific(Yamaguchi et al., 2002). M. pacifica has been known toperform a marked diel vertical migration (Marlowe andMiller, 1975 ; Hirakawa et al., 1990 ; Bollens et al., 1993),
although the magnitude of vertical migration varies with location and season (Hattori, 1989 ; Hirakawa, 1991). M. pacifica feed mainly on phytoplankton (Dagg et al., 1989) and arepreyed upon micronektonic fishes (Moku et al., 2000).
Thus, M. pacifica is a vital link between primary producer
and animals at higher trophic levels. For Metridia spp. in
the Oyashio region, Takahashi et al. (2009) conducted theiractive carbon flux, and reported that the annual carbon transport by diel vertical migration as 3.0 g C m－2 year－1, corresponding to 15% of the annual total particulate organic carbon
flux at 150 m depths. While this information provide theimportance of Metridia spp. in carbon flux, their contributionsin the oceanic areas of the subarctic Pacific remains unknown.
In this study, we determined respiration rates and migratory

1)

Laboratory of Marine Biology, Faculty of Fisheries Sciences, Hokkaido University(北海道大学大学院水産科学院)
2)
Arctic Research Center, Hokkaido University(北海道大学北極センター)— 29 —-

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

abundance of Metridia pacifica adult females at stations in thewestern/eastern subarctic Pacific and the Bering Sea during
summer of 2004 as a basis to compute their feeding impact on
primary production while they reside in shallow layer, andtheir respiratory fluxes while they are in deep layer during
daytime in the broad subarctic Pacific. The present results
are compared with those from the subtropical and tropicaloceanic regions reported previously and discussed in the lightof pivotal roles of zooplankton in material cycles in the sea.

Respiration
At each nighttime sampling, an additional sampling from
200 m to the surface was made with an 80 cm ring net (0.33mm mesh) equipped with a large cod end. Live adultfemales of Metridia pacifica were sorted under a stereomicroscope and transferred into a beaker filled with chilled surface
seawater, which was collected prior to each experiment, filtered through GF/F filters and oxygenated by shaking. Thena batch of 5 10 specimens of M. pacifica were transferred
into each 75 ml oxygen bottles filled with the oxygen saturated filtered seawater. Three experiment bottles and twocontrol bottles (no copepod) were prepared concurrently andwere incubated in the dark and at two temperatures ; one wasthat integrated mean over the upper layer (0 50 m), and theother was that integrated mean over the lower layer (50 200m). Both temperatures were calculated based on the CTD
profile at each station. After the incubation for 24 h, dissolved oxygen concentration of seawater sampled from control and experimental bottles were measured by a Winkler
titration method. The difference in oxygen concentrations
between experimental and control bottles is assumed to be
due to respiration by the specimens (sealed chamber method,
cf. Omori and Ikeda, 1984). Respiration was converted tocarbon units assuming a respiratory quotient ([CO2]/[O2]) of
0.97 (protein metabolism, cf. Gnaiger, 1983). After theexperiments, the females were collected on a piece of nettings, rinsed briefly with distilled water, placed on preweighted aluminum pan and stored in a deep freezer (–80°C).
In the land laboratory, frozen samples were freeze dried andused for the analyses of carbon with an elemental analyzer(Yanaco CHN Corder).
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Materials and methods-

Day/night abundance
During the cruise of T/S Oshoro Maru, zooplankton samples were collected by towing a NORPAC net (flowmeter
mounted, 0.10 mm mesh) from 50 m depth to the sea
surface, both day and night, at four stations between 41°N and
47°N along 165°E (NW Pacific) during 30 June 4 July, and ateight stations between 41°N and 50°N along 165°W (NE
Pacific) during 13 20 July, and at one station in the Bering
Sea basin (53°30’N, 177°00’W) on 12 August 2004 (Fig. 1).
Sunrise and sunset was 4 : 06 4 : 45 and 19 : 22 20 : 05
(local time), respectively, across the stations (Table 1). Day
and night samplings of zooplankton were carried out avoiding
two hours just before/after sunrise and sunset (Table 1).
Zooplankton samples were preserved in 5% borax bufferedformalin immediately on board. In the land laboratory, adultfemales of Metridia pacifica occurred in these samples werecounted on 1/10 sub samples. From these data, nocturnal
migrant abundance (inds. m－2) of this copepod was calculated from their nighttime abundance subtracted the daytime
abundance. The prosome length (PL) of twenty adultfemale M. pacifica was measured to the nearest 0.05 mm
under a dissecting microscope using an eye piece micrometer.
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-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

Fig. 1.

Location of sampling stations. Four in the northwestern Pacific (41°-47°N along 165°E), eight in the northeastern Pacific(41-50°N along 165°W) and one in the oceanic Bering Sea during June-August 2004.
— 30 —Yamaguchi : Respiratory carbon flux by Metridia pacifica
Table 1. Location and position of sampling station, date (in 2004), sunrise and sunset in local time,
total length of night and day, and sampling (0-50 m vertical hauls of 0.1-mm mesh NORPAC net) conducted time during night and day. All of the sampling finished within fiveminutes.
Location
Position

Local time
Date

Bering Sea (177˚00’W)
53˚30’N
12 Aug.
NW Pacific (165˚00’E)
47˚00’N
4 July44˚00’N
2 July42˚30’N
1 July41˚00’N
30 JuneNE Pacific (165˚00’W)
50˚00’N
13 July
49˚00’N
14-15 July48˚00’N
47˚00’N
45˚30’N
44˚00’N
42˚30’N
41˚00’N

15 July
16 July
17 July
18 July
19 July
20 July

Total length

SunriseSunset

Night

Day

Night

Day

4 : 23

19 : 22

9 : 0114 : 59

22 : 20

16 : 50

4 : 104 : 21
4 : 26
4 : 3119 : 58
19 : 47
19 : 42
19 : 36

8 : 12
8 : 34
8 : 44
8 : 55

15 : 48
15 : 26
15 : 16
15 : 05

2 : 03
23 : 05
21 : 33
23 : 05

6 : 54
10 : 56
16 : 14
11 : 40

4 : 06

20 : 05

8 : 01

15 : 59

23 : 52

16 : 57

4 : 11
4 : 16
4 : 21
4 : 284 : 34
4 : 404 : 45

20 : 00
19 : 55
19 : 50
19 : 44
19 : 38
19 : 32
19 : 27

8 : 11
8 : 21
8 : 31
8 : 44
8 : 56
9 : 089 : 1815 : 49
15 : 39
15 : 29
15 : 16
15 : 04
14 : 52
14 : 42

0 : 51
23 : 33
23 : 50
21 : 35
21 : 53
23 : 22
23 : 35

16 : 37
11 : 16
11 : 45
9 : 18
12 : 58
12 : 35
15 : 05

Carbon budget
Carbon budgets of a live copepod is expressed as : Ingestion (I) = Growth (G) + Metabolism (M) + Egestion (E) +Excretion (U). Assuming assimilation efficiency ([G + M] /I) and gross growth efficiency (G / I) for general zooplankton
to be 70% and 30%, respectively (Ikeda and Motoda, 1978),ingestion rates (I : µg C individual－1 h－1) can be calculated
by knowing M as I = M / (0.7－0.3).
The distance of diel vertical migration of Metridia pacifica
in the subarctic Pacific is estimated as 125 m (Yamaguchi etal., 2004). M. pacifica has been reported to swim at thevelocity of 30 90 m h－1 (median : 60 m h－1) for more thanan hour (Enright, 1977). Then, the time needed for one way
vertical travel of M. pacifica is computed as 2.1 h (= 125/ 60).
Subtracting the up and down traveling time (2.1 h×2) fromone day (24 h), the residence time at above 50 m (upper
layer), and below 50 m (lower layer) everyday was calculated
taking into account the time of sunrise and sunset. The
ingestion of M. pacifica during residence in the upper layer,
and their respiration during residence in the lower layer were
calculated, and summed on daily bases (mg C m－2 day－1).
Based on the data on sea surface temperature, chlorophylla and light intensity (Taekeun Rho, unpublished data), primary production (mg C m－2 day－1) at each sampling station
was estimated using an algorithm (Kameda and Ishizaka,
2005). Assuming f ratio (new production/ primary production) as 0.21 (Varela and Harrison, 1999), particulate organiccarbon (POC) flux from the euphotic zone is calculated as
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Sampling time

primary production×0.21. Thus calculated POC flux fromthe euphotic zone was compared with the respiratory flux ofMetridia pacifica at each station, and the respiratory flux was
expressed as a fraction of POC flux (= 100×respiratory flux/
sediment POC flux, %).
Results
Migrant biomass
Adult females of Metridia pacifica occurred in all nightsamples but none in all daytime samples, indicating that theymigrated down below 50 m depths during daytime. Geographically, the nighttime abundance of the adult females
ranged between 27 and 5,422 inds. m－2, showing a pattern of
higher abundance with increasing latitudes of stations (Table
2). Since the daytime abundance was zero throughout thestations, the nocturnal migrant abundance equals to that of thenighttime abundance (Table 2). The prosome length (PL) of
the adult females increased from 1.68 to 2.27 mm graduallywith increasing latitudes of stations (Table 2). Because ofthis, carbon mass (CM) of the adult females increased from
24.7 to 75.6 µg C ind.－1 toward northernmost stations. The
relationship between CM (µg C ind.－1) and PL (mm) was
expressed as CM =－74.9 + 65.9×PL (r2= 0.68, p < 0.0001).
-

Respiration
Regional variations in the range of integrated mean temperatures in the 0 50 m and 50 200 m layer among stations
were 6.4 14.2°C and 2.8 9.5°C, respectively (Table 3).

— 31 —----

Bull. Fish. Sci. Hokkaido Univ. 71(1), 2021
Table 2. Abundance (inds. m−2 : 0-50 m at night), prosome length (PL) and carbon body massof Metridia pacifica adult females at each sampling station. Values are mean±1sd.
Location
Position
Bering Sea (177˚00’W)
53˚30’N
NW Pacific (165˚00’E)
47˚00’N
44˚00’N
42˚30’N
41˚00’N
NE Pacific (165˚00’W)
50˚00’N
49˚00’N
48˚00’N
47˚00’N
45˚30’N
44˚00’N
42˚30’N
41˚00’N

Abundance(inds. m−2)

PL
(mm)

Body mass(µg C ind.−1)

3,148.6

2.27±0.09

71.5±10.1

224.7
200.0
1,818.2384.6

2.06±0.08
1.94±0.02
2.06±0.09
1.67±0.08

75.6±5.762.6±9.759.5±4.1
43.3±2.3

1,656.0
5,421.7
200.0
294.1
460.5
985.9
87.0
26.9

2.15±0.20
2.11±0.06
2.24±0.10
2.15±0.07
2.10±0.05
1.89±0.07
1.87±0.04
1.68±0.07

75.1±4.1
56.9±4.775.1±4.0
63.8±9.349.2±5.952.1±3.2
41.7±7.5
24.7±1.6

Table 3. Integrated mean temperature at upper (0-50 m) and lower layer (50-200 m) at each sampling station, and oxygen consumption rate of Metridia pacifica adult females at temperatures of the upper and lower layers, respectively. Values are mean±1sd.
Temperature(˚C)

Location
Position

Upper L.

Lower L.

Upper L.

Lower L.

8.1

3.8

355±39247±16

6.4
7.3
7.4
14.2

2.8
5.0
4.9
9.1

314±19275±8271±7394±33202±21169±8223±10251±28

7.9
8.2
8.0
9.2
10.4
10.8
11.0
13.8

4.1
4.3
4.5
5.1
5.9
7.3
8.0
9.5

303±40250±22286±9244±36246±15259±12224±24155±14222±16236±7278±14218±42184±23
189±12
218±21122±10Bering Sea (177˚00’W)
53˚30’N
NW Pacific (165˚00’E)
47˚00’N
44˚00’N
42˚30’N
41˚00’N
NE Pacific (165˚00’W)
50˚00’N
49˚00’N
48˚00’N
47˚00’N
45˚30’N
44˚00’N
42˚30’N
41˚00’N

Respiration rates ranged from 155 to 355 nl O2 ind.－1 h－1 in
the 0 50 m layer, and from 122 to 278 nl O2 ind.－1 h－1 in the
50 200 m layer. Throughout the stations, respiration rates in
the 0 50 m layer was higher than that in the 50 200 m layer
by a factor of 1.3±0.2 (mean±1sd) (Table 3). All respiration data from the two layers were expressed in terms of thespecific rates and plotted against habitat temperatures and---Oxygen consumption rate
(nl O2 ind.−1 h−1)-

body mass of Metridia pacifica (Fig. 2), showing that specificrespiration rates are a function of temperature (r2= 0.72) orbody mass of individuals (r2= 0.51) (Fig. 2).
Carbon budget by diel migrant Metridia pacifica
Across the stations, the calculated daily ingestion from thedaily respiration in the 0 50 m layer ranged from 0.04 to— 32 —-Yamaguchi : Respiratory carbon flux by Metridia pacifica

Fig. 2.

Metridia pacifica adult females. Relationship between weight-specific oxygen consumption and experimental temperature(left), and weight-specific oxygen consumption and dry mass (right). Significant interactions were observed for both relationships. For comparison, regression lines from Ikeda (1985) and Ikeda et al. (2001) are also shown in the panels.

Fig. 3.

Ingestion in the upper layer and respiration in the lower layer by Metridia pacifica adult females in each sampling station.
For comparison, the amount of respiratory flux is expressed as percentage to sedimentation flux from euphotic zone (% POCflux), details of estimated methods see text.

11.04 mg C m－2 day－1 (mean : 2.83 mg C m－2 day－1), andthe observed daily respiration in the 50 200 m layer ranged
from 0.02 to 9.39 mg C m－2 day－1 (mean : 1.88 mg C m－2
day－1) (Fig. 3). Throughout the station, the daily respiration-in the 50 200 m layer were less than the daily ingestion in the
0 50 m layer (63.6±12.3% [mean±1sd] of the latter). Primary production and POC flux from the euphotic zone ateach station were estimated as 361 1,186 and 76 249 mg C

— 33 —----

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

m－2 day－1, respectively. Thus, respiratory flux by Metridia
pacifica adult females varied regionally, from 0% to 10% of
the POC flux from the euphotic zone (mean : 2%, Fig. 3).
Discussion
Abundance and biomass-

Within broad habitats (subarctic Pacific and its marginalseas), life cycles of M. pacifica are known to vary geographically ; generation numbers per year is one (northern Bering
Sea and Japan Sea, Heinrich, 1962 ; Hirakawa and Imamura,1993), two (Oyashio region, Padmavati et al., 2004), three(Gulf of Alaska, Batchelder, 1985) and four (western Bering
Sea, Heinrich, 1962). Developmental time from eggs toadults of M. pacifica under saturated food concentration werereported as 112 days at 8°C and 182 days at 3°C (Padmavati
and Ikeda, 2002).
Across the differences in latitude between 40°N and 55°N,
developmental timing of Neocalanus plumchrus was evaluated as 5 weeks faster in the southernmost latitudes than thosein the northernmost latitudes, and the temperature is probably
the most important factor governing the shorter development
duration of N. plumchrus in southern latitudes (Batten et al.,2003). These latitudinal changes in temperature were also
the case of this study (cf. Table 3). Combined effect of differences in life cycle and developmental rate induced by temperature, is a possible cause of geographical changes in M.
pacifica abundance. Throughout the station, M. pacifica
adult females absent during daytime and abundant at nightfrom upper 50 m indicate that they are not in resting phase in
the study period. Resting phase of M. pacifica is reported
from Japan Sea (Hirakawa, 1991 ; Hirakawa and Imamura,1993). Dormancy of M. pacifica due to the warming of
euphotic layer (>18°C) in the Japan Sea beyond the limit ofthermal tolerance for this species (Hirakawa, 1991). Since
the temperature at upper 50 m was lower than 15°C throughout the station (Table 3), warming of euphotic layer is notconsidered as a barrier of migration behavior of M. pacifica in
the present study.
Based on the large spatial and temporal coverage samples(Continuous Plankton Recorder survey) in the North Atlantic
and the North Sea, Hays (1995) revealed that the mean daily
length of time that Metridia spp. spent near the surface ineach month of the year varied seasonally with day length.
From this point of view, the surface residence time of M.
pacifica in the study period (summer) corresponds seems tobe minimum of the year. Nighttime length in each station
varied from 8 : 01 9 : 18 hours (mean±1 sd : 8 : 39±
0 : 23, Table 1). M. pacifica in Dabob Bay arrived to thesurface 0.5 h after sunset, fed at high rates throughout thenight until 1.0 h before sunrise, and then returned to depth(Dagg et al., 1989).
Body carbon mass of Metridia pacifica adult females var-

ied with station by a factor of 3.1 (= 75.6 / 24.7) (Table 2).
Prosome length is also varied with station by a factor of 1.4(= 2.27/1.67), and larger in the northern stations. Geletin(1976) noted that the two forms of M. pacifica, differing inbody length and structure of the cephalothorax, can be distinguished in the subarctic Pacific. According to Geletin(1976), the large form (2.70 3.65 mm in total length) is foundmainly in water at 2 9°C, and the small form (2.00 2.55 mm
in total length) in water at 9 15°C. Some of the copepods
ascend at night into warmer water : the large form to water at
15°C and the small form to water at 18°C. The large form is
associated with the boreal copepods (Calanus cristatus, C.
plumchrus (= Neocalanus cristatus, N. plumchrus) and
Eucalanus bungii), and the small form with species of intermediate zone of the Oyashio Kuroshio system (Calanus
pacificus and E. californicus). Such the geographical heterogeneous in genetic structure of M. pacifica are reported forthe populations at a coastal eddy and offshore groups in the
California current (Bucklin, 1991).
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Respiration and ingestion
Specific respiration rates of Metridia pacifica adult females
ranged between 1.07 and 3.54 µl O2 mg DM－1 h－1 (Fig. 2).
According to Ikeda (1974), the specific respiration rates at
4.5 10.5°C of this copepod are 1.69 3.94 µl O2 mg DM－1 h－1
which overlap partially the present results. The empirical
zooplankton respiration model (Ikeda, 1985) has been
employed by previous estimates of carbon flux by migrant
zooplankton (cf. Al Mutairi and Landry 2001, Roman et al.,2002). Later, the empirical respiration rates were updated
every nearly decades (Ikeda et al., 2001 ; Ikeda, 2014). The
comparison of the present respiration data and with thosederived from the empirical model showed that the former differed from the latter by a factor of only 0.8 1.5 (Fig. 2), suggesting a good agreement between the two.
Daily ingestion of Metridia pacifica adult females calculated from respiration data by using the carbon budget model(Ikeda and Motoda, 1978) in the present study (1.49 3.60 µg
C ind.－1 day－1) is consistent to the previous results determined experimentally (2.23 µg C ind.－1 day－1 by Tsuda andSugisaki, 1994 ; 3.62 µg C ind.－1 day－1 by Dagg, 1993).
In terms of daily ration, the present results (3.2 8.3%) fallwithin the extreme range of previous reports e.g. 24%(Ambler and Frost, 1974), 7.5% (Batchelder, 1986) or 2 22%(Mackas and Burns, 1986).
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Active flux
The abundance of Metridia pacifica adult females varied
greatly with location, and ranged from 27 to 5,422 inds m－2,
thus by a factor of 200 (= 5,422/ 27) (Table 2). Comparedwith this large location to location variations in the abundance, the respiration rate fell within a small range (122 394
nl O2 ind.－1 h－1, Table 3) or by a factor of 3.2 (= 394/122).

— 34 —---Yamaguchi : Respiratory carbon flux by Metridia pacifica
Table 4. Zooplankton active flux estimated by different authors in oceanic regions. Migrating biomass is the integrated night minus day
zooplankton biomass in the surface layer. Migratory flux is the amount of dissolved carbon respired or excreted by migrating zooplankton below the euphotic zone. % of mean POC flux = (migratory flux/ sediment trap flux at the depth of euphotic zone)x
100%. Mean and range (given in parentheses). Flux due to mortality at depth is not included here, although it is estimated insome studies.
Migrating biomass(mg C m−2)

Location
Sub- and tropical Atlantic (September)
Bermuda (March/April)-

191 (82-536)

Migratory flux
(mg C m−2 day−1)% of POC flux

References

5.5 (2.8-8.8)

6 (4-14)

Longhurst et al. (1990)

14.5 (6.2-40.6)
4.2
7.3
3.8
7.9

34 (18-70)
18
25
8
4

Dam et al. (1995)
Zhang and Dam (1997)
Zhang and Dam (1997)
Le Borgne and Rodier (1997)
Rodier and Le Borgne (1997)

Eq. Pacific (March/April)
Eq. Pacific (October)
Eq. Pacific oligotrophic
Eq. Pacific HNLC area (September)
North Atlantic (NABE)

96
155
47
53
(5-480)

Bermuda (year-round)
Hawaii (year-round)

50 (0-123)

2.0 (0-9.9)(19-40)8 (0-39)

Morales (1999)
Steinberg et al. (2000)

142
192 (156-300)

3.6 (1.0-9.2)

15 (6-25)

Al-Mutairi and Landry (2001)

2.6 (0.3 4.3)

25 (16 45)

Hernández-León et al. (2001)

Canary Islands (August)

-

-

-

Eq. North Pacific (winter)

297 (145-448)

13.2 (7.3-19.1)

24 (13-35)

Hidaka et al. (2001)

Eq. Pacific HNLC area (year-round)(2.8-3.4)(576-1284)
144(0.1-2.5)(1.9-8.3)
3.5

(2-4)
(12-53)
10

Roman et al. (2002)
Yebra et al. (2005)
Kobari et al. (2008)

5.7 (1.0-15.8)

15

Takahashi et al. (2009)

W. North Pacific (June)

558 (63-1676)(276-1096)(3.2-14.1)(2-11)

Hanamiya et al. (2020)

Subarctic Pacific (summer)

71 (1-309)

1.9 (0-9.4)

2 (0-10)

This study (note that Metridia pacifica
adult females only)

Canary Islands (July)
W. subarctic Pacific (Aug.)Oyashio region (year-round)In fact, daily ingestion in the upper layer and daily respirationin the lower layer showed a strong correlation with the abundance (r2= 0.93 0.99), while low correlation with respiration
rate (r2= 0.08 0.12) at each station, indicating the abundance
to be the most important factor affecting the active carbon
flux by M. pacifica in the subarctic Pacific.
Daily ingestion of Metridia pacifica adult females in theupper layer (0.04 11.04 mg C m－2 day－1, cf. Fig. 3) corresponds to 0.01 2.35% of the primary production. Daily respiration of M. pacifica adult females in the lower layer (0.02
9.39 mg C m－2 day－1) corresponds to 0 10% of the POC flux
down from the euphotic zone (Fig. 3). The present results of
respiratory flux relative POC flux (0 10% with a mean of2%) are less than those reported from other regions (mean
4 34%, cf. Table 4). Part of reasons may be that our estimation is on only M. pacifica adult females, as compared withentire zooplankton communities in other studies. The estimation of respiration in most previous studies are based on
ETS assay (cf. Hernández León et al., 2001 ; Yebra et al.,2005) or empirical community respiration data of zooplankton (Al Mutairi and Landry, 2001 ; Roman et al., 2002).
Compared with these indirect methods, ours are direct method
on M. pacifica collected at each study location. The present
estimate is limited for one species/stage therefore represents aminimum respiratory flux by migrant zooplankton. Other
than M. pacifica, euphausiids (such as Euphausia pacifica,
Bollens et al., 1992), amphipods (Themisto spp., Yamada et--al., 2004) and micronektonic fishes (Watanabe et al., 1999 ;Moku et al., 2000) are considered to be active components ofvertical carbon flux in the subarctic Pacific. To evaluate
accurate carbon flux schema in the subarctic Pacific, future
study needed to extend to these animals.
Acknowledgements

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I thank the captain, officers and crew of the vessel for theirgreat help during field sampling. I appreciate S. Tachibana
and N. Yamada for their help during sampling and sorting at
sea. Part of this study was supported by Grants in Aid for
Challenging Research (Pioneering) 20K20573 and Scientific
Research 20H03054 (B), 19H03037 (B), and 17H01483 (A),
16H02947 (B) and 15KK0268 (Joint International Research)from the Japan Society for the Promotion of Science (JSPS).
This work was partially supported by the Arctic Challenge for
Sustainability II (ArCS II), Program Grant Number
JPMXD1420318865.

-— 35 —--

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