Deep–Sea Research I 185 (2022) 103771

Contents lists available at ScienceDirect

Deep-Sea Research Part I
journal homepage: www.elsevier.com/locate/dsri

Vertical distribution, standing stocks, and taxonomic accounts of the entire
plankton community, and the estimation of vertical material flux via faecal
pellets in the southern Okhotsk Sea
Daiki Kojima a, *, Yusuke Hamao a, Kanako Amei a, 1, Yutaka Fukai a, 2, Kohei Matsuno a, b,
Yoko Mitani c, 3, Atsushi Yamaguchi a, b
aGraduate School of Fisheries Sciences, Hokkaido University, 3-1-1 Minato-cho, Hakodate, Hokkaido, 041-8611, Japan
Arctic Research Center, Hokkaido University, Kita-21 Nishi-11 Kita-ku, Sapporo, Hokkaido, 001-0021, Japanc
Field Science Center for Northern Biosphere, Hokkaido University, 20-5 Benten-cho, Hakodate, Hokkaido, 040-0051, Japanb

A R T I C L E I N F O

A B S T R A C T

Keywords:
Carbon flux
Copepoda
Coprophagy
Faecal pellet
Okhotsk Sea
ZooScan

The faecal pellets egested by zooplankton are essential as they influence the vertical material flux in oceans. As
mesozooplankton are dominant within the plankton community in the southern Okhotsk Sea during earlysummer, the vertical material flux via mesozooplankton is expected to be substantial. However, quantitative
information on their faecal pellets is currently lacking. In this study, we evaluated the taxonomic accounts of theentire plankton community, including microplankton, mesozooplankton, and macrozooplankton, in the 0–1000
m water column. The ingestion and egestion rates of the zooplankton were also estimated. We used a fine-mesh(63 μm) plankton net along with an imaging technique (with ZooScan) to quantify the amount of in-situ faecal
pellets. Furthermore, on-board experiments were conducted to estimate the faecal pellet egestion by thedominant zooplankton species. Cosmopolitan diatom species were found to dominate the microplankton
biomass, whereas the large-sized calanoid copepod Metridia okhotensis, which performs nocturnal ascent dielvertical migration, dominated the mesozooplankton biomass. Two euphausiid species with different body sizes,
namely the small-sized Thysanoessa inermis and the large-sized Euphausia pacifica, were found to be dominantamong the macrozooplankton. The highest density and mass of faecal pellets (1888 pellets m− 3, 2.96 mg C m− 3)
was observed in the 0–100 m layer during the daytime. Throughout the layer, the volumes of the faecal pellets
peaked at 0.010–0.015 mm3, which corresponded with the size of the pellets egested by M. okhotensis in the onboard laboratory experiments. The large-sized faecal pellets (>0.2 mm3), which were inferred to be egested byeuphausiids, were only observed during the night-time. Based on the on-board experiments, the faecal pellets
egested by all the meso- and macrozooplankton species contained phytoplankton cells that possessed a fluo­
rescent ability. Furthermore, cyanobacteria were the most common taxa (26–65% in number) observed in the
faecal pellets. Coprophagy (feeding on faecal pellets) was observed for the calanoid copepod Gaetanus variabilis,
which was collected from depths of 500–1000 m. The estimated daily ingestion rates of meso- and macro­
zooplankton corresponded with 21% of the standing stock of microplankton. In contrast, their daily egestion
rates accounted for 40–112% of the in-situ faecal pellet masses. Faecal pellet collection by the gentle verticaltowing of a fine-mesh plankton net and quantification through imaging analysis can provide new insights for theevaluation of the number and mass of faecal pellets in the field. The findings of the present study suggest thatthese techniques have the potential to successfully quantify faecal pellets in the water column.

Abbreviations: DST, Drifting sediment trap; VMPS, Vertical Multiple Plankton Sampler; MOHT, Matsuda-Oozeki-Hu Trawl; TL, Total length; DVM, Diel verticalmigration.
* Corresponding author.
E-mail address: tksk-sijukara-kd3541@eis.hokudai.ac.jp (D. Kojima).
1
Present address: Graduate School of Agricultural and Life Sciences, The University of Tokyo, Kashiwa, Chiba, 277-8564, Japan.
2
Present address: Kushiro Fisheries Research Institute, Hokkaido Research Organization, 2-6 Hamacho, Kushiro, Hokkaido, 085-0024, Japan.
3
Present address: Wildlife Research Center, Kyoto University, 2-24 Tanaka-Sekiden-cho, Sakyo, Kyoto, 606-8203, Japan.
https://doi.org/10.1016/j.dsr.2022.103771
Received 5 October 2021; Received in revised form 29 March 2022; Accepted 31 March 2022
Available online 4 April 2022
0967-0637/© 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

D. Kojima et al.

Deep-Sea Research Part I 185 (2022) 103771(Wilson et al., 2008; Takahashi et al., 2009). However, few attempts
have been made to estimate zooplankton ingestion and egestion by
applying habitat depth through new physiological methods (Ikeda,2014).
Several projects focused on biological pump estimation, such as the
VERTIGO, A-line, and K2S1, have been undertaken in the western sub­arctic Pacific (Wilson et al., 2008; Takahashi et al., 2009; Kobari et al.,2016). In contrast, few such attempts have been made in the Okhotsk
Sea, which is a marginal sea adjacent to the western subarctic Pacific.
The Okhotsk Sea is located at the southern end of the seasonallyice-covered ocean in the Northern Hemisphere, and it is reported to havehigh biological productivity (Pinchuk and Paul, 2000). Since the depthof the southern Okhotsk Sea is > 3000 m, the function of the biologicalpump is expected to be large. However, information on the biologicalpump in this sea is lacking. Taxonomic accounts of the microplankton(Shimizu, 2009; Kasai et al., 2010; Shiomoto, 2011) and meso- andmacrozooplankton (Gorbatenko et al., 2014; Yamaguchi, 2015; Arimaet al., 2016; Mizukami et al., 2019) are available for this region. How­ever, such taxonomical studies have focused on targeted taxa, and no
study has been conducted to obtain a taxonomic account of the wholeplankton community (e.g. microplankton, mesozooplankton, and mac­rozooplankton) on one occasion.
In the present study, we conducted a species-level taxonomic surveyof the entire plankton community, including microplankton, meso­
zooplankton, and macrozooplankton, for a 0–1000 m water column inthe southern Okhotsk Sea during early summer. Vertical stratified
samples for meszooplankton were collected during the daytime andnight-time. To quantitatively estimate the ingestion and egestion of
meso- and macrozooplankton, we used physiological methods that uti­
lised respiration rates. In-situ faecal pellets were collected by the gentle
towing of a fine-mesh-equipped vertical stratification net (63 μm) with alarge filtration volume (19–108 m3). Next, faecal pellets were identified
and their size and volume were measured using imaging analysis(ZooScan). In addition, on-board experiments on the egestion of themeso- and macrozooplankton were conducted by rearing live specimens,
and the collected faecal pellets were examined for their ingredientcontents. There were two main aims of this study: (i) to explore a new
approach for estimating the standing stocks of faecal pellets, and (ii) tocharacterise the impacts of predominant metazoans on the biogeo­
chemical cycles of the Okhotsk Sea.

1. Introduction
The vertical material transport in oceans theoretically occurs via tworoutes: fluxes due to physical mixing, and fluxes mediated by biologicalactivities, the latter of which is also known as the “biological pump”(Longhurst and Harrison, 1989). The biological pump has been reported
to be the most important driving force behind vertical material flux inoceans (Ducklow et al., 2001). It can be further divided into active andpassive transport (Steinberg et al., 2008; Kobari et al., 2016). The main
mechanisms for active transport include respiration, egestion, andmortality of zooplankton at different depths owing to their verticalmigration (Ducklow et al., 2001; Takahashi et al., 2009). Passive
transport, which is the larger type of flux, includes the sinking of faecal
pellets that are egested by zooplankton rather than the direct sinking ofphytoplankton (Wilson et al., 2008; Turner, 2015). This is partly due totheir size differences—faecal pellets are much larger than phyto­plankton cells.
Drifting sediment traps (DSTs) are commonly used in the field for thequantitative collection of faecal pellets (Trull et al., 2008; Wilson et al.,2008; Laurenceau-Cornec et al., 2015). A DST is shaped as a cylinder
with a small diameter; therefore, only a small number of faecal pellets
are collected in DSTs, and long-drifting mooring (two to three days) is
required. This can mask the differences in the faecal pellet abundance
between day and night in the field (Emerson and Roff, 1987). As most
zooplankton have a diel feeding pattern (Mauchline, 1998), it is ex­pected that diel changes may influence the number of faecal pellets inthe field. However, due to the methodological limitations (i.e. mooringfor several days) of DSTs, field information on the diel changes in faecal
pellet abundance and on the passive flux of materials by faecal pellets iscurrently lacking.
The faecal pellet volume has a positive relationship withzooplankton body size, which can be expressed using log-to-log equa­tions (Uye and Kaname, 1994; Turner, 2002, 2015). Similarly, thesinking rates of faecal pellets can be expressed using log-to-log equations
that consider faecal pellet volume (Komar et al., 1981; Turner, 2002,2015). The sinking rates of faecal pellets are related to differences in thepellet constituents and density, and they may vary among species(Fowler and Small, 1972; Fender et al., 2019). Furthermore, coprophagy
and coprorhexy by other zooplankton are also important factors that
affect the sinking rate (Noji et al., 1991; Suzuki et al., 2003; Iversen andPoulsen, 2007). In addition, the faecal pellets that sink from the over­lying water layers are also considered to be an important food source forthe zooplankton living in the food-limited deep sea (Harding, 1974;
Yamaguchi et al., 2007; Abe et al., 2012). To evaluate the nutritional
importance and sinking rates of faecal pellets, it is essential to obtain
information on their constituents. For example, observations using
epifluorescence microscopes have revealed that the digestive tracts ofdeep-sea zooplankton in the subarctic and subtropical North Pacificcontain a high density of pico-sized cells such as cyanobacteria (Wilson
and Steinberg, 2010). Although information on the contents ofzooplankton faecal pellets is important for understanding the verticalmaterial flux in oceans, little information is currently available.
To quantitatively evaluate passive material flux via the zooplankton
community, it is necessary to obtain estimates of the ingestion andegestion rates of faecal pellets, both of which are commonly expressed inunits of mg C m− 2 day− 1. These rates are widely calculated according toa physiological method that utilises empirical equations to estimate
growth or respiration rates by applying independent variables (bodyweight and temperature) and combining them under assumed assimi­
lation and net growth efficiencies (Steinberg et al., 2008; Kobari et al.,2016). The respiration rate, which is estimated from body weight, watertemperature, and habitat depth, has been reported for zooplankton
worldwide (13 taxa including copepods) using empirical equations(Ikeda, 2014). In the western subarctic Pacific, several studies on
zooplankton ingestion and egestion estimation have conducted sensi­
tivity tests by manipulating the assimilation and net growth efficiencies

2. Material and methods
2.1. Field sampling
Field observations were made at two stations: St. D2 (44◦ 16′ N,
145◦ 34′ E; water depth: 2264 m) and St. E1 (44◦ 12′ N, 145◦ 27′ E; water
depth: 1859 m) (Fig. 1) in the southern Okhotsk Sea on 29 and 30 June
2019. To sample the microplankton, 1-L water samples were collected at
St. D2 using a bucket from the sea surface and a conductivitytemperature-depth profiler with rosette multi-bottle samplers (CTDRMS) from the maximum fluorescence layer (at a depth of 10 m). The
samples were preserved by adding glutaraldehyde at 1% in final con­centration, and they were stored under dark and cold conditions until
further analysis. Additionally, 250-mL water samples were collected,
filtered through a GF/F grade glass microfibre filter, and extracted in N,
N-dimethylformamide. Then, chlorophyll a (Chl. a) was measured witha fluorometer (10-AU; Turner Designs, San Jose, CA, USA). Meso­
zooplankton samples were collected by vertical stratified sampling fromthree layers (0–100, 100–500, and 500–1000 m) using a vertical mul­
tiple plankton sampler (VMPS; Tsurumi Seiki Co. Ltd., Kanagawa,Japan) (Terazaki and Tomatsu, 1997) with a mouth-opening of 0.25 m2.
The VMPS was equipped with 63-μm mesh and non-filtered cod ends atnight (St. D2) and during the day (St. E1). The filtered volumes were
measured using a flowmeter for each sample, and the values ranged
between 19.3 and 108.2 m3 (Table 1). After recovering the VMPS, each2

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Deep-Sea Research Part I 185 (2022) 103771

zooplankton sample was immediately split in half using a Motoda
splitter (Motoda, 1959). Then, one aliquot containing live zooplankton
individuals was used for the on-board rearing of zooplankton for faecal
pellet egestion experiments. The remaining samples were preserved byadding 5% (v/v) borax-buffered formalin immediately after splitting.
In the on-board rearing for the faecal pellet egestion experiments,
four copepod species (Metridia okhotensis C6F, Neocalanus cristatus C5,
Neocalanus plumchrus C5, Gaetanus variabilis C5M, C6F) and one
euphausiid species (Thysanoessa inermis), which were dominant in thecollected samples, were sorted into 50 mL bottles that were then filledwith chilled filtered seawater and equipped with a 300-μm mesh on thebottom to prevent coprophagy or coprorhexy. The specimens werereared for one day under dark conditions in an incubator set at 3 ◦ C. The
egested faecal pellets were preserved by adding glutaraldehyde at 1% in
the final concentration and stored under cool and dark conditions until
further analysis.
Macrozooplankton samples were collected by horizontal towing at aship speed of 2–3 knots. Midwater trawling was conducted using the
Matsuda-Oozeki-Hu trawl (MOHT) with a mouth area of 5 m2 (Oozeki
et al., 2004) at depths of 53–69 m (monitored using acoustics) at St. D2during the night (Table 1). The MOHT was equipped with 1.4 mm meshand a flowmeter. The collected samples were immediately preserved on
board by adding 10% (v/v) borax-buffered formalin. Additionally, thewater temperature, salinity, and fluorescence were measured at both
stations at depths between 0 and 1000 m using a CTD profiler (SBE–911
Plus, Sea-Bird Electronics, Bellevue, WA, USA) and a fluorometer (Sea­point Chlorophyll Fluorometer, Seapoint Sensors, Exeter, NH, USA).
Based on the Chl. a measurement data obtained throughout the cruise,
we obtained the conversion factor between Chl. a and fluorescence (r2 =0.96, p < 0.0001, n = 84) and expressed Chl. a in mg m− 3 in 1-m
intervals.

2.2. Microscopic observations
In the land laboratory, the 1-L water samples were allowed to sit
undisturbed on a stone table for more than one day to allow the
microprotist cells to settle at the bottoms of the bottles. Then, thesamples were concentrated to 20 mL using a siphon over 48 h under cooldark conditions. Subsamples (0.1–0.5 mL) were mounted on a glass slideand examined with an inverted microscope under 200–400 × magnifi­
cation. Microplankton (diatoms, dinoflagellates, and ciliates [Oligo­
trichia and Tintinnida]) were identified to the species level and counted.
For each taxon, the lengths along the major and minor axes were
measured with 0.01 μm precision using Nikon NIS Elements, and the cellvolumes were calculated using geometric models that varied for
different genera (Sun and Liu, 2003). Then, the volume data were con­verted to carbon units (mg C m− 3) using volume-carbon conversion
factors, which varied depending on the taxa (Strathmann, 1967; Men­
den-Deuer and Lessard, 2000).
For mesozooplankton samples, 1/4–1/136 (examined fraction/
whole sample in volume) subsamples (varied with the sample amount),which were created using a wide-bore pipette, were examined under astereomicroscope. The number of specimens were counted, and thespecies and the developmental stages were identified for calanoid co­pepods, which were the dominant taxa in the collected samples. The
total length (TL) of the adults of each copepod species was previously
reported by Brodskii (1967). To obtain the TL values for the juvenilestages, we used the moult increment data at each stage by Mauchline(1998) to compute the developmental stage factors. The values of eachstage were calculated by multiplying the adult TL with the following
factors: 0.318 (copepodite stage 1: C1), 0.411 (C2), 0.513 (C3), 0.648(C4), and 0.813 (C5). For cyclopoid, poecilostomatoid, and harpacticoid
copepods, species identification was not conducted, and the TL values
were measured using an eyepiece micrometre with a precision of 0.01

Fig. 1. Location of the sampling stations (St. D2 and E1) in the southern Okhotsk Sea during 29–30 June 2019. Depth contours are superimposed. For details of thesampling items at each station, see Table 1.
3

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Deep-Sea Research Part I 185 (2022) 103771

mm for 30 individuals. The TL data were converted to the carbon unit
(mg C ind.− 1) by applying the carbon-TL regressions for calanoid,
cyclopoid, and poecilostomatoid copepods that were previously re­ported by Nakamura et al. (2017), and that for harpacticoid copepods asreported by Uye et al. (2002). The carbon biomass (mg C m− 3) was
quantified by multiplying the carbon mass of an individual with theabundance of each species/stage.
The macrozooplankton samples were divided into 1/32 (examinedfraction/whole sample in volume) subsamples using a Motoda splitter,and the euphausiid species, the dominant taxa in the collected samples,
were identified and counted under a stereomicroscope. The TL of all
euphausiid individuals was measured under a stereomicroscope using ascale with a precision of 0.5 mm. The TL data for the euphausiids wereconverted to carbon units (mg C ind.− 1) by applying the carbon-TL
relationship reported by Ross (1982). The data were then expressed ascarbon biomass (mg C m− 3), which allowed the data to be comparable tothat of the other taxa.
In the land laboratory, faecal pellets egested by the zooplankton
during the on-board rearing experiments were examined under aninverted fluorescence microscope. The constituents of the faecal pellets
were identified to the lowest taxonomic levels whenever possible. The
counting procedure used was that of Wilson and Steinberg (2010). Cells
containing red fluorescence (chlorophyll) and yellow fluorescence(phycoerythrin and phycocyanin of the cyanobacteria) were identified
and counted. For flagellates and dinoflagellates, cells with red auto­
fluorescence were treated as autotrophs and green cells were treated asheterotrophs. The hard parts of the cells were used to identify them as
diatoms or dinoflagellates, and tintinnid loricae were also identified andquantified. The long and short axes of the faecal pellets were measured
with a precision of 1 μm, and then the faecal pellet volume (mm3) was
calculated using the volumetric formula for ellipsoids. The cell number
contained in each faecal pellet was expressed as the number of cells perunit volume of the faecal pellet (cells mm− 3). The faecal pellet sedi­mentation rate (SR; m day− 1) was also estimated from the volume of the
faecal pellet (V; mm3) using the formula given by Small et al. (1979):

background measurements were made using deionised water. The
sample images were digitised at a 2400-dpi resolution (one pixel cor­
responded to 10.58 μm). All obtained images were then uploaded to the
EcoTaxa website (http://ecotaxa.obs-vlfr.fr/prj/) and the faecal pellets
were identified (Fig. 2b–e).
The faecal pellet sizes were measured using the major axis length(Lmajor: mm) and minor axis length (Lminor: mm). As most faecal pellets
were elliptical, their volume (V: mm3) was calculated using thefollowing equation:ln R = 23.079 + 0.813 × ln

CM1000
− 6.248 ×− 0.136 × ln D (copepods)
1000
K

log 10 SR = 0.513log 10 V − 1.214

ln R = 23.079 + 0.813 × ln

CM1000
− 6.248 ×− 0.136 × ln D
1000
K

4
LmajorLminor 2
V= π×()×()
3
2
2
The carbon-to-volume conversion factor of faecal pellets (0.08 mg C
mm− 3), which was reported by Wilson et al. (2008) for the western
subarctic Pacific during summer, was used to calculate the faecal pellet
carbon mass (mg C m− 3) at each sampling layer.
2.4. Ingestion and egestion rates
The ingestion and egestion rates of mesozooplankton (copepods) andmacrozooplankton (euphausiids) were estimated using physiologicalmethods (respiration rates) by applying three independent variables:
water temperature, individual carbon mass, and habitat depths (Ikeda,2014). Because the mass contributions for the other taxa were extremely
low (<5%), we only focused on the fluxes of these two taxa in eachplankton community. To estimate the ingestion and egestion rates, weused an assimilation efficiency (AE) and a net growth efficiency of 60%
and 50%, respectively (Steinberg et al., 2008; Giering et al., 2014).
Further, mean water temperature (T:◦ C) was calculated for a 1-m in­terval at each layer of VMPS and MOHT sampling.
The respiratory rates (R: μL O2 ind.− 1 h− 1) of copepods and eu­
phausiids were calculated using the following equations (Ikeda, 2014):+ 0.600 (euphausiids)

2.3. In-situ quantification of faecal pellets (ZooScan measurements)

where CM is the carbon mass per individual (μg C ind.− 1), K is the ab­solute temperature (K: T + 273.15) at each layer, and D is the inter­mediate depth of the sampling layer (m).
Assuming a respiratory quotient of 0.97 (Gnaiger, 1983), the inges­
tion (I: mg C m− 3 day− 1) and egestion (E: mg C m− 3 day− 1) rates at night
were calculated using the following equations:

Multiple faecal pellets were observed in the mesozooplankton sam­ples collected by VMPS (Fig. 2a). This may be attributed to the use of
VMPS with a fine mesh size (63 μm) as well as gentle vertical towing of
the non-filtered cod end. A wide-bore pipette was used to create 1/88–1/
162 (examined fraction/whole sample in volume; varied according to
the size of the samples) subsamples for the image scanning measure­ments. Images of the faecal pellet samples were scanned using ZooScan(ZooScan MIII, 127 Hydroptic Inc., France) to quantify the number andvolume of the faecal pellets under in-situ conditions, according to themethod reported by Gorsky et al. (2010). Before each measurement,

I=

R
12
1
× 0.97 ××× Abu × 8.52
1000
22.4 0.3

Table 1
Sampling data for each plankton sample collected in the southern Okhotsk Sea. Five types of data collections were made: microscopic quantification of microplankton(A) and calanoid copepods (B), faecal pellet quantification by ZooScan (C), microscopic observation of faecal pellets egested by each dominant zooplankton species
reared onboard (D), and microscopic quantification of macrozooplankton (E).
Data collection
Date

StationLocal time

Sampling gear

Depth strata (m)Water volume quantified (m3)A

29 June 2019
(Night)D2

E1

29 June 2019
(Night)D2

CTD-RMS
VMPS
VMPS
VMPS
VMPS
VMPS
VMPS
MOHT

0, 10
0–100
100–500
500–1000
0–100
100–500
500–1000
52.6–69.4(Horizontal haul)

0.001
19.29
94.83
98.97
19.98
81.05
108.2021032●

30 June 2019(Day)

20:20–22:17
22:47–22:48
22:41–22:47
22:32–22:41
18:02–18:03
17:55–18:02
17:47–17:55
23:41–00:11

4

B

C

D

●
●
●
●
●
●

●
●
●
●
●
●

●
●
●

E

●

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Deep-Sea Research Part I 185 (2022) 103771

Fig. 2. (a) Example photographs of zooplankton samples collected by a vertical multiple plankton sampler (VMPS) at depths of 0–100 m in the daytime. Freshsamples were placed under the stereomicroscope. Note that brown-coloured faecal pellets were abundant. (b)–(e) Example images of formalin-preserved faecal pellet
samples captured by ZooScan. Scale bars are 1 mm.

E=

For comparative reference, the carbon mass of microplankton was
calculated to evaluate the impacts of the feeding of these zooplankton(copepods and euphausiids) on in-situ microplankton. For this estima­tion, we applied the C:Chl. a ratio (30) that was previously used for theregion around the Oyashio current (Taguchi and Saino, 1998). The in­tegrated standing stocks (mg C m− 2) of microplankton were calculated
by multiplying the C:Chl. a ratio by the Chl. a data for 1-m intervals
through the 0–100 m depth layer (Fig. 3). The standing stocks of
microplankton, mesozooplankton (copepods), macrozooplankton (eu­phausiids), and faecal pellets were all expressed in carbon units per
square meter (mg C m− 2) for their quantitative comparison. The units offluxes (ingestion and egestion rates) by mesozooplankton (copepods)
and macrozooplankton (euphausiids) were standardised as mg C m− 2day− 1. For egestion rate determination, the daily impacts of meso- and

R
12
0.4
× 0.97 ××× Abu × 8.52
1000
22.4 0.3

where Abu is the abundance of each species (ind. m− 3), 12/22.4 is theweight of carbon (12 g) in 1 mol (22.4 L) carbon dioxide, 0.97 is the
respiratory quotient (CO2/O2, Gnaiger, 1983), 1/0.3 is the F/R, 0.4/0.3
is the E/R, 1/1000 is a unit conversion from μg to mg, and × 8.52 is thetime of night hours of the study region/period.
While some copepods in this region (Neocalanus spp. and Eucalanus
bungii) are known to have life cycles with diapause (Kobari et al., 2003),
they were active during the studied season (early summer), although
they had low abundance and biomass (see Section 3.2.). For these rea­sons, we included these copepod species in the ingestion and egestion
estimates.

Fig. 3. Vertical changes in temperature (a), salinity (b), and chlorophyll a (c) at two stations (D2 and E1) in the southern Okhotsk Sea during 29–30 June 2019.
Circled numbers in the right column indicate sampling layers of the Vertical Multiple Plankton Sampler (VMPS).
5

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Deep-Sea Research Part I 185 (2022) 103771

macrozooplankton feeding on microplankton stocks, as well as a quan­titative comparison with the faecal pellet stocks, were evaluated.

surface, but it decreased rapidly with increasing depth. The temperaturereached a minimum at depths of 180–200 m, where it had negativevalues at St. E1. The temperature at depths below this minima layer
increased gradually to 2 ◦ C at a 1000 m depth. The salinity was in therange of 33.0–34.6 and was low at the surface. It reached a maximum at
depths of 50–60 m and a minimum at 180–200 m, where the tempera­ture was also at a minimum. The maximum salinity was observed at a1000 m depth. The Chl. a was in the range of 0.02–1.70 mg m− 3 and the
maximum was located at a depth of 20 m of St. E1. The Chl. a was highonly at depths <100 m.

3. Results
3.1. Hydrography
The water temperature, salinity, and Chl. a showed similar vertical
distributions at both stations (Fig. 3). The water temperature at 0–1000
m depths ranged from − 0.5–11.7 ◦ C. The temperature was high at theFig. 4. Abundance (a) and biomass (b) of microplankton based on water sampling at the sea surface (0 m) and the depth of maximum chlorophyll a content (10 m) at
St. D2 in the southern Okhotsk Sea during the night of 29 June 2019.
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3.2. Plankton stock, taxonomic accounts, and flux

contents of the faecal pellets were cyanobacteria, eukaryotic phyto­plankton, diatoms, tintinnids, and faecal pellets from other zooplankton(repacked).
The mean volume and contents of faecal pellets collected during therearing experiments of four copepod species and one euphausiid species
collected from the three depth layers at night are shown in Fig. 10. The
mean faecal pellet volume was 0.006–0.015 mm3. The number of cells
such as cyanobacteria, eukaryotic phytoplankton, diatoms, and tintin­
nids per faecal pellet volume was in the range 3.38–16.0 × 103 cells
mm− 3. For all examined zooplankton species, the dominant taxa in the
faecal pellets, as cells per faecal pellet volume, were cyanobacteria(26.3–65.2%). Heterotrophic flagellates were the second-most abundanttaxa, with a composition ranging from 9.6 to 55.3%. The highest numberof cells per volume was found at 0–100 m depths for all species.
Repacking was found for the calanoid copepod G. variabilis C5M, C6F at
depths of 500–1000 m.

Since the hydrography was similar at the two sampling stations, theplankton results at the two stations were not distinguishable, and only
distinctions between day and night were noted. Microplankton abun­dance was 3.7 × 103 cells L− 1 at the sea surface and 59.3 × 103 cells L− 1
at a depth of 10 m (Fig. 4a). In relation to their cell counts, the various
taxa occurred evenly, but the diatoms Chaetoceros socialis and Lep­
tocylindrus danicus were dominant taxa at both 0 and 10 m. The micro­
plankton biomass was calculated as 3.2 mg C m− 3 at the surface and10.2 mg C m− 3 at a 10 m depth (Fig. 4b). At the surface, the dominanttaxa in the biomass were dinoflagellates and ciliates, whereas a highabundance of the diatom Chaetoceros affinis was observed at a 10 m
depth. This was also where the maximum fluorescence was observed.
For mesozooplankton, the copepods were the predominant taxa(>95% both in abundance and biomass). The copepod abundance
ranged from 205 to 7454 ind. m− 3 and was highest at depths of 0–100 m
during both day and night (Fig. 5a). The dominant taxa were small co­
pepods, specifically Oithona spp. (Cyclopoida) and Pseudocalanus spp.
(Calanoida). Microsetella spp. (Harpacticoida) were also abundant at
depths of 0–100 m during the daytime. The biomass of copepods ranged
from 8.9 to 66.5 mg C m− 3 and was highest at 100–500 m depths during
the daytime and 0–100 m depths at night (Fig. 5b). The large calanoid
copepod M. okhotensis had high biomass throughout most of the layer,
but the species did not occur at 0–100 m depths during the day. Instead,
Pseudocalanus spp. were abundant there. At night, copepod feeding ineach layer was in the range 0.13–2.7 mg C m− 3 day− 1, and it was highest
at depths of 0–100 m (Fig. 5c). The copepod egestion rate was in therange 0.05–1.1 mg C m− 3 day− 1 and was highest at 0–100 m depths(Fig. 5d).
For macrozooplankton, the euphausiids were the predominant taxa(>95% both in abundance and biomass). The total length of euphausiids
collected by MOHT ranged from 6.5 to 25 mm and had a bimodal dis­tribution consisting of different species (Fig. 6a). Smaller individuals(<13 mm) were dominated by Thysanoessa inermis, and larger in­dividuals (>14 mm) were mostly comprised of Euphausia pacifica. The
smaller euphausiid T. inermis was numerous in terms of abundance, but
the larger euphausiid E. pacifica dominated in terms of biomass (Fig. 6b).
The euphausiid ingestion rate was 0.27 mg C m− 3 day− 1 and was
dominated by the large euphausiid E. pacifica. The euphausiid egestion
rate was 0.11 mg C m− 3 day− 1 and was also dominated by E. pacifica(Fig. 6c).

3.5. Carbon mass stocks and fluxes within the plankton community
Table 2 shows the standing stocks (mg C m− 2) of microplankton,
copepods, euphausiids, and faecal pellets in the 0–100 m water columnof the southern Okhotsk Sea, as well as the fluxes (ingestion and egestion
rates in mg C m− 2 day− 1) by copepods and euphausiids. The standing
stock of microplankton was calculated to be 1412 mg C m− 2. The
ingestion rates of copepods and euphausiids were 266 mg C m− 2 day− 1
and 27.1 mg C m− 2 day− 1, respectively. The ingestion rates of euphau­siids were approximately 1/10th that of copepods. The daily ingestion
by these zooplankton taxa corresponded to 21% of the standing stock of
microplankton. The carbon stocks of faecal pellets at 0–100 m depths
ranged from 105 to 296 mg C m− 2. In contrast, the egestion rates bycopepods and euphausiids were 107 mg C m− 2 day− 1 and 10.9 mg C m− 2day− 1, respectively (totalling 118 mg C m− 2 day− 1). The calculations
based on the whole ranges of faecal pellets corresponded to 40–112% of
the observed faecal pellet stocks.
4. Discussion
4.1. Methodology
In this study, we quantified microplankton, the day and night ver­tical distribution of copepods at 0–1000 m depths, macrozooplankton(euphausiid) abundance and biomass, the number and mass of faecal
pellets, the ingredients in the faecal pellets that were obtained throughrearing experiments, and fluxes within the plankton community in thesouthern Okhotsk Sea during the summer. Though this study surveyed
most of the planktonic and abiotic (faecal pellet) particles in the watercolumn, several considerations regarding the methodology were madein the study. These considerations included the following: (i) the loca­
tion of the day and night samplings, (ii) the methods for estimating
zooplankton ingestion and egestion rates, and (iii) the quantification
methods of in-situ faecal pellets. These are discussed in detail in thefollowing paragraphs.
Between the day and night sampling stations, temperature, salinity,and fluorescence were similar (Fig. 3). Intrusions of the Soya Warm
Current, which is characterised by high salinity (>33.6), on the southern
Okhotsk Sea have been reported (Takizawa, 1982; Sakai et al., 2008;
Sato et al., 2008). However, such high salinity was not observed for thesurface layers in this study, and dichothermal waters (<0 ◦ C) wereobserved at a depth of 180–200 m at each station (Fig. 3). These ob­servations suggest that at both stations, the water structure consisted of
Okhotsk Sea surface water placed above dichothermal waters (Taki­zawa, 1982). This may be attributed to the seasonal occurrence of theSoya Warm Current in this region—the current begins in late August andrarely occurs in April–June (Yamaguchi, 1993). Thus, though day andnight samples were collected from different stations in this study, thismethodology is expected to have a minor effect on the results, and the3.3. Faecal pellet abundance, mass, and size
The abundance of faecal pellets ranged from 67.7 to 1888.2 faecal
pellets m− 3 and was the highest at 0–100 m depths during the daytime(Fig. 7a). The faecal pellet mass was in the range 0.13–2.96 mg C m− 3
and was also the highest at 0–100 m depths during the daytime (Fig. 7b).
The individual volume of faecal pellets was in the range 0.005–0.6
mm3 (Fig. 8). Differences between day and night and the sampling
depths showed that large faecal pellets (>0.2 mm3) were observed in all
sampling layers at night, whereas such faecal pellets were not observed
during the day. Small faecal pellets (0.005–0.03 mm3) were the mostabundant in all depth layers during both day and night. No significant
differences were detected for the faecal pellet volumes between layers or
between day and night (p > 0.05, one-way ANOVA). Common peaks infaecal pellet volume (0.010–0.015 mm3) were observed in all of thelayers during both day and night (Fig. 8). The smallest faecal pellet
width was 131 μm and the largest was 722 μm. The sinking rate esti­
mated from the faecal pellet volume was 189–1885 m day− 1.
3.4. Microscopic observations of egested faecal pellets
Faecal pellets egested during the rearing experiment were observed
and photographed using a fluorescence microscope (Fig. 9). The
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Fig. 5. Vertical changes in copepod abundance (a) and biomass (b) and their taxonomic contents in the southern Okhotsk Sea during the day and night of 29–30 June
2019. Ingestion (c) and egestion (d) were estimated for the night-time data by applying an empirical model for the respiration rate (Ikeda, 2014), with the habitat
temperature and body weight as independent variables and the assumption of assimilation and net growth efficiencies (Steinberg et al., 2008). For details, see
the text.

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Fig. 6. Abundance (a) and biomass (b) of euphau­siid species in relation to the total length of thehorizontal tow of the Matsuda-Oozeki-Hu trawl(MOHT) at 50–70 m depths at St. D2 in the southern
Okhotsk Sea during the night of 29 June 2019.
Ingestion and egestion (c) were estimated for thenight-time data by applying an empirical model for
respiration rate (Ikeda, 2014) with habitat temper­
ature and body weight as independent variables andthe assumption of assimilation and net growth ef­
ficiencies (Steinberg et al., 2008). For details, see
the text.

water samples can be assumed to have the same water mass conditions
and biological settings.
In the present study, zooplankton ingestion and egestion rates were
estimated using respiration rates (Ikeda, 2014). Some previous studies
have applied a similar respiration rate by Ikeda (1985), but this method
does not include depth as an independent variable (Steinberg et al.,2008). The differences in the outcomes of the two methods are at factorsof 1.36–1.60 fold for deep layers (Kojima, unpublished data). This may
be attributed to the fact that the independent variables in the respiration
estimates included habitat depth (Ikeda, 2014). To estimate the physi­
ological rates of deep-sea metazoans, selection of the applied empiricalmodel that includes depth as an independent variable may be better for
flux estimations targeting the deep layer, as shown in this study.
The first objective of this study was to quantify faecal pellets using
fine-mesh (63 μm) plankton net samples (Figs. 7 and 8). In general,
faecal pellets are collected by mooring of the DST (Trull et al., 2008;
Wilson et al., 2008; Laurenceau-Cornec et al., 2015). Since the shape of
the DST is cylindrical, the small-diameter collection mouth area is re­ported to be a problem (Emerson and Roff, 1987). Alternatively, themass filtration method can be used. In this method, a large volume ofseawater (200–1000 L) is collected at a given depth and then filteredthrough a 10–53 μm mesh filter for the quantitative collection of faecal
pellets (Lomas and Moran, 2011; Stukel et al., 2011). Alternatively, asmall amount of water (5–9.2 L) is also filtered through a 20–60 μm
mesh net (Pasternak et al., 2000; Dagg et al., 2014). In this study, a large
volume of seawater (19–108 m3; which is a 2–540 times larger volume
than that used in the mass filtration method) was filtered through finemesh (63 μm) without clogging (Table 1).
The highest abundance of faecal pellets was observed for 0–100 m
depths during the daytime. At this depth, the abundance of faecal pellets
was comparable to that of copepods (Figs. 5 and 7). The collection of
faecal pellets using a plankton net may result in the loss of pellets, as
they can slip through the mesh (Tranter and Heron, 1967; Tranter andSmith, 1968; Smith et al., 1968). Moreover, this method can cause thephysical destruction of faecal pellets (Omori and Ikeda, 1984). Despite
such shortcomings, sufficient faecal pellets were collected using net
meshes in the present study (Fig. 2a). The filtration efficiency of the
VMPS ranged from 77.2 to 94.8% (mean ± standard deviation: 83.1 ±9

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Fig. 7. Vertical changes in the faecal pellet number (a) and their carbon mass (b) during the day and night in the southern Okhotsk Sea during 29–30 June 2019.

6.6%), which was comparable to that of common quantitative collection
nets (Omori and Ikeda, 1984). Because a large number of faecal pellets
were collected, we were able to identify and conduct accurate size
measurements of the faecal pellets through imaging analysis with Zoo­
Scan (Fig. 2b–e). Hence, it was possible to calculate the day and nightvertical distribution and to semi-quantitatively evaluate the faecal pel­lets based on the plankton samples. The observed faecal pellet volumes
collected by fine-mesh nets in the field (peaked at 0.010–0.015 mm3,
Fig. 8) corresponded well with the pellets egested by various
zooplankton species during on-board rearing (peaked at 0.010–0.015
mm3, Fig. 10). The faecal pellet volumes quantified by the two methods
showed no significant differences (p > 0.05, Mann-Whitney U test). Thissuggests that the underestimation of fragmented pellets smaller than themesh openings might be minor. For quantitative evaluation or valida­tion of faecal pellet collection by a fine mesh net, a comparison study on
the collection of faecal pellets by two methods, namely DST andfine-mesh nets, is required in the future.
In conclusion, although some considerations are required, this study
provides information on in-situ and laboratory-reared egested faecal
pellets, the standing stocks of the whole plankton community (micro­
plankton, mesozooplankton, and macrozooplankton), and the fluxes
through meso- and macrozooplankton. The findings of this study are
valuable and can be used to estimate the carbon budget in the pelagiczone of the southern Okhotsk Sea during summer.
4.2. Plankton community
Information on the microplankton in the southern Okhotsk Sea islimited to chlorophyll data. It has been reported that the chlorophyll
density during the spring phytoplankton bloom (March–April) is above
2 mg Chl. a m− 3 (Mustapha and Saitoh, 2008; Shimizu, 2009; Kasai
et al., 2010). During June–August, the sub-surface maximum number of10

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Fig. 8. Histograms showing the faecal pellet volume at each depth layer in the southern Okhotsk Sea during 29–30 June 2019. Differences in the histogram patterns
represent pellet width (= minor axis of length). Based on faecal pellet volume, the sinking rate (m day− 1) was estimated using the empirical equation described by
Small et al. (1979).

phytoplankton can be observed at a depth of approximately 20 m (Kasai
et al., 2010), and the chlorophyll concentration is approximately 1 mg
Chl. a m− 3 (Mustapha and Saitoh, 2008; Shimizu, 2009). The surface
water of the Okhotsk Sea is reported to contain a higher nutrient content
and more Chl. a than that of the Soya Warm Current (Hamasaki et al.,1998). In the present study, the surface water of the Okhotsk Sea wasdominant in the water mass and had a maximum concentration (1.7 mg
m− 3) near the surface layer (20 m); these results corresponded well withthose of previous reports (Fig. 3). Further, small-sized phytoplankton(<2 μm) contributed the most to overall primary productivity owing to
the heavy grazing pressure of copepods on large-sized phytoplankton(Shiomoto, 2011). The three diatom species that were dominant at the
study site (C. socialis, C. affinis, and L. danicus) are all cosmopolitan, andtheir dominance is well documented in various regions of the Northern
Hemisphere (Marshall et al., 2005; Guo et al., 2014; Righetti et al.,2020). However, the dominance of dinoflagellates and ciliates, espe­
cially in biomass (Fig. 4b), has not been previously reported in the
Okhotsk Sea. This might be a new finding of the present study.
Regarding mesozooplankton, it is well known that the zooplankton
fauna during summer in the southern Okhotsk Sea are dominated bycopepods and that M. okhotensis dominates the biomass (Yamaguchi,
2015; Arima et al., 2016; Hiragi and Yamaguchi, 2019). The present
study reported that M. okhotensis dominated the copepod biomass dur­ing both the day and night (Fig. 5b), whereas cyclopoid copepod Oithonaspp. were dominant with respect to abundance (Fig. 5a). Oithona spp.
are omnivores that can consume a wide range of food items; they are

dominant in the oceans worldwide (Turner, 2004). In the Okhotsk Sea,
Oithona similis has four generations per year, with a peak spawningseason during the spring phytoplankton bloom (Shebanova et al., 2011).
The life cycle of O. similis may contribute to the numerical dominance ofOithona spp. during the post-phytoplankton bloom in June. However,the contribution of Oithona spp. to overall biomass was small due to its
small body size. The large-sized calanoid copepod M. okhotensis, which
is known to perform nocturnal ascent diel vertical migration (DVM)(Padmavati et al., 2004; Takahashi et al., 2009), dominated the biomass.
The DVM behaviour of M. okhotensis was clearly observed in the present
study (Fig. 5b). Small-sized calanoid copepod Pseudocalanus spp. are
known to dwell on the surface layer during both daytime and night-time
during early summer (Yamaguchi and Shiga, 1997); this habit allows
Pseudocalanus spp. to dominate the biomass at depths of 0–100 m during
the daytime. Furthermore, harpacticoid copepod Microsetella spp. were
found to be abundant in the 0–100 m depth layer during the daytime(Fig. 5a). Microsetella species are known to feed on detritus and marine
snow (Uye et al., 2002). Hence, the high density of Microsetella spp. in
the 0–100 m layer during the daytime may be attributed to the highdensity of faecal pellets in this layer (Fig. 7). Dominance of Microsetella
spp. has not previously been reported in the Okhotsk Sea, and it thus
may be a new finding of this study.
Among the macrozooplankton fauna of the southern Okhotsk Sea,
euphausiids, especially E. pacifica, are known to be abundant(Tomiyama et al., 2017). In south-eastern Hokkaido, the biomass of
E. pacifica reaches 381–833 mg C m− 2 (Taki, 2006; Kim et al., 2009). The
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Fig. 9. Zooplankton faecal pellets as viewed under an epifluorescence microscope. (a) Gaetanus variabilis at 500–1000 m, (b) Metridia okhotensis at 0–100 m, (c)Neocalanus plumchrus at 0–100 m, (d) Thysanoessa inermis at 100–500 m, (e) Neocalanus cristatus at 100–500 m. (fp) faecal pellet, (cy) cyanobacteria, (ep) eukaryotic
phytoplankton, (di) diatom, (ti) tintinnid lorica, (am) amorphous material. Scale bars are 100 μm.

standing stocks of euphausiids observed in the present study (726 mg Cm− 2) corresponded well with previously reported values (Table 2). The
small-sized euphausiid T. inermis (TL: 8–10 mm) and the large-sized
E. pacifica (18–22 mm TL) were observed at the study sites (Fig. 6).
The body sizes of E. pacifica corresponded with the maximum sizes ofthis species that have previously been reported in the adjacent Oyashio
region (Kim et al., 2009), indicating that E. pacifica may have been in the
reproduction phase during the study period. The daily growth rate of
T. inermis along the weight base (0.0030) was similar to that of thesympatric E. pacifica (0.0025) in the Gulf of Alaska (Pinchuk and Hop­croft, 2007). The substantially different body sizes of the two dominanteuphausiids (E. pacifica and T. inermis) in the southern Okhotsk Sea
suggest that the timing of reproduction and life cycles varies among
species in this region. As the δ15N of euphausiids is higher than that ofcopepods in the southern Okhotsk Sea, euphausiids are expected to be
more carnivorous than copepods (Gorbatenko et al., 2014). Further­more, the presence of DVM is also characteristic of the euphausiids inthis region (Mizukami et al., 2019).

during July–August at St. K2 in the adjacent western subarctic Pacificwere reported to be 0.01–0.75 μg C pellet− 1 (see Fig. 4 in Wilson et al.,2008). Wilson et al. (2008) applied the carbon-to-volume conversion
factor of 0.08 mg C mm− 3; using the same method, the above values are
expressed as 0.000125–0.009375 mm3 in volume. The equivalent
spherical diameters of the faecal pellets collected at 60–200 m depths
during July at St. K2 were reported to be 0.065–0.075 mm (see Fig. 5 inKobari et al., 2016), which can be expressed in volume as
0.000144–0.000221 mm3. The faecal pellets collected in the present
study were larger (peaks, 0.010–0.015 mm3) than the above-mentioned
values (Fig. 8). This suggests that more large faecal particles can becollected using net towing methods than using the DST mooring method.
As the two methods resulted in large differences in the faecal pellet
volume, we suspect that the cause was the differences in the filtering
volume of the two devices. Based on the mouth area of the DST cylinder(7.5–11 cm diameter; Valdes and Price, 2000) and the sinking velocity of
the faecal pellets (− 150 m day− 1; McDonnell and Buesseler, 2010), theobserved volume of the DST per day can be estimated as 0.66–1.43 m3.
These values are much lower than the filtering volumes of the nets usedin this study (19.3–108.2 m3, Table 1). In addition to these differences inquantitative volume, the filtering by plankton net mesh (63 μm) used inthis study may have caused inevitable fragmentation of small-sizedfaecal pellets. A combination of both effects (large quantitative vol­ume and filtering through mesh) is considered to be a cause of the larger
volume of the faecal pellets in this study than of those quantified by DST.
In the future, quantitative evaluation or validation of faecal pellets using
a combination of the two methods, DST and fine-mesh net collection, isneeded.
For the faecal pellet volumes, log-log regressions are present from thezooplankton length (prosome or total length of copepods) (Uye andKaname, 1994; Stamieszkin et al., 2015; Kobari et al., 2016). Using these

4.3. Faecal pellets in the field
During net collection, any particles that are smaller than the diagonal
distance of the mesh size are not collected (Tranter and Heron, 1967;
Tranter and Smith, 1968; Smith et al., 1968). The diagonal distance ofthe 63-μm mesh in the VMPS used in the present study was 88.2 μm, andthe smallest size of the collected faecal pellets was 131.2 μm (Fig. 8).
Hence, it was assumed that particles with size <130 μm may not havebeen collected due to physical destruction during collection or the loss of
small particles through the mesh. In contrast, faecal pellets with a width>130 μm were considered to be quantitatively collected.
The faecal pellet sizes collected using DST at 150–500 m depths
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Fig. 10. Faecal pellet volume (upper) and the number and taxonomic composition of cells per faecal pellet volume (lower) egested by various zooplankton species
based on on-board rearing experiments of specimens collected with a vertical multiple plankton sampler (VMPS) on the night of 29 June 2019. Bars in the upper
panel represent standard errors. Asterisks denote that coprophagy was observed for the species.

regressions, the prosome lengths of copepods with egested faecal pellets
with a volume of 0.010–0.015 mm3 are estimated to be 4.2–4.3 mm.
These values corresponded to those of M. okhotensis adult females, thepredominant species in the mesozooplankton biomass in this study(Fig. 5b). The observed faecal pellet volumes from the net samples(0.010–0.015 mm3) corresponded well with the faecal pellet volume
egested by this species during the on-board laboratory rearing experi­ments (Fig. 10). These findings suggest that the faecal pellets observed in
the present study net are not fragmented during the net collections.
The abundance of M. okhotensis in the 0–100 m layer during the night
was found to be 33 ind. m− 3 (Fig. 5b). For copepods, the daily egested
faecal pellet number is 100–150 pellets ind.− 1 day− 1 or 8–12 pellets
ind.− 1 h− 1 when feeding occurs only at night (Mauchline, 1998). As thenight-time length during the study period was 8.52 h, the number ofTable 2Comparison of the standing stock and fluxes (ingestion and egestion) in carbon
units for the 0–100 m water column of the southern Okhotsk Sea during 29–30
June 2019. For details of each taxon and subject, see Fig. 4 (microplankton), 5(copepods), 6 (euphausiids), and 7 (faecal pellets). Note that fluxes were esti­
mated based on metabolic rates (Ikeda, 2014). (D): day, (N): night.
Flux (mg C m-2 day-1)-2

Taxa/subject

Standing stock (mg C m )Ingestion

Egestion

Microplankton
Copepods
Euphausiids
Fecal pellet1412
885 (D) - 4697 (N)
726 (N)
105 (N) - 296 (D)

266
27.1

107
10.9

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faecal pellets egested by M. okhotensis at night was estimated to be
68–102 faecal pellets ind.− 1 (= [8 to 12] × 8.52). By multiplying thenumber of faecal pellets by the M. okhotensis abundance, the number of
faecal pellets egested by M. okhotensis at 0–100 m depths during thenight was found to be 2244–3366 faecal pellets m− 3 (= 33 × [68 to102]). The calculated values are 1.2–1.8 times higher than those for theobserved faecal pellet density (1888 faecal pellets m− 3) in the 0–100 m
layer during the daytime (Fig. 7). This discrepancy may be attributed to
the loss of faecal pellets during net towing (Tranter and Heron, 1967;
Tranter and Smith, 1968; Smith et al., 1968). Zooplankton feeding on
the faecal pellets and the destruction of faecal pellets into small frag­ments may have also decreased the number of faecal pellets in field (Nojiet al., 1991; Suzuki et al., 2003; Iversen and Poulsen, 2007). It should be
noted that the faecal pellet density in the 0–100 m layer during the night(486 faecal pellet m− 3) was substantially lower than the estimated faecal
pellet density during the daytime (Fig. 7).
The most prominent finding of this study was that the faecal pellet
density at the surface layer during the daytime was much higher (fourtimes) than that during the night (Fig. 7). In general, since DSTs aremoored for a certain period (3–4 days) (Buesseler et al., 2007; Wilson
et al., 2008; Kobari et al., 2016), it is difficult to evaluate the day/nightdifferences of faecal pellets in the field. Therefore, the differences in the
faecal pellet density during the day and night is a novel finding of thepresent study. Large-sized taxa with high swimming abilities, such as the
copepod M. okhotensis and two euphausiid species, were abundant atnight, whereas the small-sized copepods such as Oithona spp., Micro­
setella spp., and Pseudocalanus spp. were dominant during the daytime.
The genera Oithona and Microsetella have been reported to attach to
detritus and perform coprophagy or coprorhexy (Turner, 2002; Uyeet al., 2002; Suzuki et al., 2003; Iversen and Poulsen, 2007). However, as
they feed mainly on small-sized faecal pellets, large-sized faecal pellets(0.010–0.015 mm3) may not be consumed during the daytime (Fig. 2)(Turner, 2004; Reigstad et al., 2005).
The sinking velocity of faecal pellets is correlated with their size anddensity; it exceeds 1000 m day− 1 for large-sized faecal pellets (e.g. thoseof Salpida) (Paffenhöfer and Knowles, 1979; Small et al., 1979; Uye andKaname, 1994; Trull et al., 2008). In the present study, the sinking ve­
locities of the observed faecal pellets were estimated to be faster than
100 m day− 1 (Fig. 8). This suggests that large-sized faecal pellets egested
by M. okhotensis at 0–100 m depths rapidly sink to deep layers without
being affected by coprophagy or coprorhexy by small copepods.
Furthermore, bacterial decomposition can also result in the fragmenta­tion of faecal pellets, but it takes 25 days for pellets to decompose(Cherry et al., 1978; Emerson and Roff, 1987; Turner, 2015).
Coprorhexy of faecal pellets may have been substantial at the codend of the surface samples, where the large-sized zooplankton with highswimming ability were abundant at night. However, the effect of cop­rorhexy may have been minimal for the surface layer at daytime becauseof the lack of large-sized zooplankton taxa with high swimming ability inthe sample. This could have caused the high density of faecal pellets at
0–100 m depths during the daytime, but this does not seem to be the casefor the present study, as extremely large faecal pellets (>0.20 mm3)
were collected throughout the layer at night (Fig. 8). These large faecal
pellets were clearly larger than those of the copepod M. okhotensis andthe euphausiid T. inermis observed during the laboratory rearing ex­
periments. This result suggests that such large-sized faecal pellets are
egested by the large-sized euphausiid E. pacifica (Figs. 6 and 10). Suchlarge-sized faecal pellets are expected to be more vulnerable to cop­rorhexy by large-sized zooplankton with high swimming ability. The
presence of large-sized pellets at night suggests that coprorhexy bylarge-sized zooplankton with high swimming ability at the cod end may
be minor. However, if coprorhexy was present, the faecal pellets affectedby coprophagy and coprorhexy may have become fragmented because
the sizes of the faecal pellets (peaked at 0.010–0.015 mm3) were near
the end of the collection limits of the applied mesh size (63 μm), which
would have made it difficult to collect the faecal pellets using theplankton net method applied in this study. The sinking velocity of large
faecal pellets observed only at night was estimated to be > 1000 mday− 1 (Fig. 8). Hence, these large faecal pellets may not have beencollected during the daytime in the present study.
The presence of a nocturnal active diel feeding rhythm without DVM
has been reported for the dominant large-sized copepods Gaetanus spp.
and M. okhotensis in the subarctic Pacific (Takahashi et al., 2008; Abeet al., 2012). These diel zooplankton feeding rhythms suggest that thechange in faecal pellet egestion must also follow a diel pattern. In thepresent study, vertical stratified vertical tows of the fine-mesh plankton
nets were used to filter large volumes of water. Consequently, day/night
density and vertical distribution of faecal pellets were evaluated in thefield. Hence, this new method can be used in future studies to evaluate
the quantity of faecal pellets in the field.
4.4. Microscopic observation of faecal pellets
In this study, the constituents of faecal pellets that were egested byzooplankton during the on-board laboratory rearing experiments were
examined and expressed as the number of cells per faecal pellet volume.
In similar studies, items obtained from the hindgut digestive tracts ofcopepods were quantified (Arashkevich, 1969; Harding, 1974; Wilson
and Steinberg, 2010; Abe et al., 2012). In these studies, only organisms
with hard shells could be identified to the species level if formalin wasused for preservation. In contrast, if the collected specimens wereimmediately frozen in liquid nitrogen and then thawed under cool anddark conditions, observations under a fluorescence microscope showedthat the faecal pellets contained many pico-sized cyanobacteria cells
that could not have been directly consumed by copepods; this is
explained by the feeding of copepods on detritus aggregates (Wilson andSteinberg, 2010). In the present study, zooplankton individuals werereared under cold and dark conditions, and faecal pellets were preserved
using 1% glutaraldehyde stored dark and cool conditions until micro­
scopic observation. This allowed us to examine the faecal pellet samples
in good condition, meaning that the cell-containing pigments could also
be observed.
The presence of viable phytoplankton (diatoms and dinoflagellates)in the faecal pellets of copepods is well known (Fowler and Fisher, 1983;
Maneiro et al., 2002; Kuwata and Tsuda, 2005). In fact, the low assim­
ilation efficiency of copepods when diatoms are consumed (e.g., 45% in
Thalassiosira nordenskioeldii; Abe et al., 2013) indicates that phyto­plankton cells are not fully digested when passed through the digestive
tract. In the present study, cyanobacteria were the most common taxa
that contained the highest number of cells per faecal pellet volume
compared to all other zooplankton species (Fig. 10). This indicates thatthe sinking of zooplankton faecal pellets is an efficient pathway for thevertical transport of phytoplankton cells from the surface layer to thedeep sea, and they are also an important food source for deep-sea or­ganisms (Trull et al., 2008; Wilson et al., 2008; Wilson and Steinberg,2010).
According to Wilson and Steinberg (2010), the number of phyto­plankton cells contained in the copepod digestive tract was higher at thenear-surface layer at night at St. K2 in the western subarctic Pacific. This
was assumed to be a reflection of the high feeding activity that occurredat night and the high number of phytoplankton cells in the near-surface
layer. In the present study, the highest number of cells per faecal pellet
volume was also observed at the surface layer (0–100 m) for all species(Fig. 10). This suggests that the surface-dwelling specimens may have
fed on fresh phytoplankton cells. In addition to common cyanobacteria
and heterotrophic flagellates, microplankton such as diatoms and tin­tinnid loricae were found in the zooplankton faecal pellets. These resultscorresponded well with those of previous reports, and they may reflect
the feeding modes of different species (Arashkevich, 1969; Wilson andSteinberg, 2010; Abe et al., 2012). It was observed that G. variabilis fed
on faecal pellets (coprophagy) at 500–1000 m depths (Fig. 9a). The
family Aetideidae, which includes Gaetanus, is reported to be
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Deep-Sea Research Part I 185 (2022) 103771

mesopelagic and has been observed to feed on sinking particles from theoverlying layer (Yamaguchi et al., 2007; Abe et al., 2012). Concerning
coprophagy, most information is derived from indirect counting underlaboratory rearing. Therefore, direct evidence of coprophagy is lacking.
However, the findings of the present study provide direct evidence ofcoprophagy by G. variabilis.

accounts of the migrant biomass may determine the quantity of theingestion and egestion rates from the upper layer. In the future, it isimportant to estimate the active flux that occurs due to the DVM of
M. okhotensis in the southern Okhotsk Sea. Moreover, little information
is available on standing stocks and ingestion rates of macrozooplankton(euphausiids) in this region. Studies at St. K2 have not adequately
considered such macrozooplankton. Therefore, more research is neededon the standing stocks and fluxes associated with macrozooplankton;
midwater trawling (MOHT) combined with acoustic surveys can be used
for this purpose (Mizukami et al., 2019).
Faecal pellet fluxes in the western North Pacific (mainly from thesubarctic region) are 7–28.5 mg C m− 2 day− 1 at 50–150 m depths(Taguchi and Saino, 1998; Wilson et al., 2008; Takahashi et al., 2009;
Kobari et al., 2016). Although this study only considered the standing
stock of faecal pellets, the above-mentioned daily fluxes correspondedwith 2.4–27.1% of the carbon mass of faecal pellets (105–296 mg C m− 2)
at 0–100 m depths in this study (Table 2). This suggests that a great
portion of the faecal pellets were disintegrated (consumed, destroyed, or
decomposed) in the 0–100 m portion of the surface layer. In contrast, aprevious study has reported that most (82%) of the faecal pellets egested
by large copepods and euphausiids are disintegrated in the euphotic
layer (Ayukai and Hattori, 1992). Additionally, the faecal pellet density
can also decrease due to coprophagy during pellet sedimentation(Suzuki et al., 2003; Kobari et al., 2016). Thus, most of the faecal pellets
egested in the near-surface layer may be consumed by other zooplankton
and decomposed within the near-surface layer (Urban-Rich et al., 1999;
Maar et al., 2004). Furthermore, faecal pellets can be destroyed by theswimming behaviour of zooplankton, which can break the pellets into
small particles (coprorhexy) (Suzuki et al., 2003; Iversen and Poulsen,
2007). Through the destruction of faecal pellets into small particles,
their sinking velocity is reduced by more than 50%, and their decom­
position is enhanced (Noji et al., 1991; Poulsen and Kiørboe, 2005;
Iversen and Poulsen, 2007). The resulting small organic particles canthen be consumed by various zooplankton (Paffenhöfer and Knowles,
1979; Poulsen and Kiørboe, 2005). Thus, the intensity of coprophagy, as
observed for G. variabilis in this study, may be a key factor for deter­mining the vertical material flux via faecal pellets in the ocean.

4.5. Carbon mass fluxes within the planktonic community
Information on standing stocks, taxonomic accounts, and fluxes via
the planktonic community in the Okhotsk Sea (i.e. this study) and at St.
K2 in the subarctic Pacific (Kobari et al., 2008, 2016; Wilson et al., 2008)
are summarised in Table 3. The primary productivity in the western
subarctic Pacific is reported to be 315–592 mg C m− 2 day− 1 (Wilson
et al., 2008; Takahashi et al., 2009; Kobari et al., 2016). In the present
study, primary productivity was not measured, but the microplankton
biomass was quantified as 1412 mg C m− 2 (Table 2). Although thestanding stock varies with phytoplankton production, zooplankton
grazing, sinking, and phytoplankton death, the values calculated in thepresent study are consistent with values obtained in the adjacent west­
ern subarctic Pacific (Boyd et al., 2008).
The standing stocks and ingestion rates of copepods at St. K2 in thewestern subarctic Pacific during July–August are 1733–3370 mg C m− 2
and 192–367 mg C m− 2 day− 1, respectively (Kobari et al., 2008, 2016).
The results of the present study [standing stocks were 885 (day)–4697(night) mg C m− 2 and the ingestion rate was 266 mg C m− 2 day− 1]correspond well with previously reported values (Table 3). These find­ings suggest that the observed standing stocks and ingestion rates ofcopepods in the present study are typical for the southern Okhotsk Sea
and western subarctic Pacific during summer.
However, the taxonomic composition of the copepod community
varies greatly between these regions. Large copepod Neocalanus spp. aredominant at St. K2 in the western subarctic Pacific (Kobari et al., 2008),
and M. okhotensis dominates the southern Okhotsk Sea; these are special
characteristics of this region (Fig. 5b) (Yamaguchi, 2015; Arima et al.,2016; Hiragi and Yamaguchi, 2019). As M. okhotensis follows a clear
DVM pattern, the active flux due to DVM (respiration and death at deep
layers during the daytime) has been reported to be large (Takahashi
et al., 2009). The most prominent differences in the plankton stocks
between the Okhotsk Sea and St. K2 were regarding the diel migrantbiomass. That of the former (3812 mg C m− 2) was approximately threetimes larger than that of the latter (1100 mg C m− 2) (Table 3). While thisdiscrepancy is available for the migrant biomass, it is interesting that the
copepod ingestion (266 and 192–367 mg C m− 2 day− 1) and egestion(107 and 77–147 mg C m− 2 day− 1) at each region were at the similarvalue (Table 3). It may due to the regional differences in the body sizesof the dominant copepods at each region. Thus, the body size of
M. okhotensis dominated in the Okhostk Sea is much smaller than thoseof Neocalanus spp. dominated at K2 in the western subarctic Pacific.
Thus, while the migrant biomass was varied with region, the taxonomic

5. Conclusion
To evaluate differences in the stocks and vertical distribution of
faecal pellets between daytime and night-time, we conducted imaging
analysis using ZooScan on faecal pellets collected by a VMPS equippedwith fine mesh (63 μm) through three depth layers between 0 and 1000m in the southern Okhotsk Sea. Additionally, observations of the wholeplankton community, ranging from microplankton to meso- and mac­rozooplankton, as well as direct observations of the faecal pellets eges­
ted during on-board laboratory rearing were made. The faecal pellets
showed diel changes, and both their numbers and carbon mass werehigh in the daytime; in contrast, only large-sized faecal pellets (>0.20mm3) were observed at night. Based on empirical equations thatconsidered faecal pellet volumes and zooplankton body sizes, the mostcommon faecal pellets observed (approximately 0.010–0.015 mm3)
were considered to be egested by the dominant copepod M. okhotensis;
this was confirmed by independent laboratory rearing experiments. The
standing stocks of faecal pellets at the surface layer (0–100 m) wereclearly higher than the faecal pellet fluxes reported for the adjacentwestern subarctic Pacific. These facts suggest that most of the faecal
pellets disintegrated in the surface layer before sinking. Since large-sizedfaecal pellets were only observed at night, they may be transported to a
deeper layer (>1000 m) within a day, which would correspond with
their estimated sinking velocity. Through the laboratory rearing exper­
iment, direct evidence of coprophagy was observed for the mesopelagic
copepod G. variabilis. This suggests that sinking faecal pellets may be
important food items for deep-sea organisms. To collect faecal pellets,
two main quantitative methods are available: DST moorings and large-

Table 3
Regional comparison on standing stocks and fluxes of copepods in the 0–100 m
layer of the Okhotsk Sea (this study) and in the 0–150 m layer at St. K2 in thewestern subarctic Pacific. Data for St. K2 are from Kobari et al. (2008, 2016) andWilson et al. (2008).
Parameters

Okhotsk Sea

K2(0–100 m)(0–150 m)

885-4697
3812
Metridia okhotensis

1733–3370
1100
Neocalanus spp.

–
266
107

315–592
192–367
77–147-2Standing stock (mg C m )Copepods
Diel migrants
Dominant copepod species
Flux (mg C m-2 day-1)
Primary production
Copepod ingestion
Copepod egestion

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D. Kojima et al.

volume (200–1000 L) filtration with fine mesh (0–60 μm). The filtration
volume of VMPS is 2–540 times larger than that of the large-volume
filtration method, and the mesh size of the net is as fine as 63 μm.
Although losses through the mesh and destruction of the faecal pellets
during net sampling are inevitable, gently towing fine-mesh and nonfiltered cod ends may allow us to quasi-quantitatively collect faecal
pellets, as demonstrated in this study. Applying imaging analysis (Zoo­
Scan) allows us to easily identify faecal pellets and obtain accurate size
measurements. To evaluate the differences of faecal pellets between day
and night in the field, the combination of the vertical towing of finemesh equipped non-filtered cod ends and imaging analyses on thecollected samples may be a new method that can be utilised in the
future.

Fender, C.K., Kelly, T.B., Guidi, L., Ohman, M.D., Smith, M.C., Stukel, M.R., 2019.
Investigating particle size-flux relationships and the biological pump across a rangeof plankton ecosystem states from coastal to oligotrophic. Front. Mar. Sci. 6, 603.
Fowler, S.W., Fisher, N.S., 1983. Viability of marine phytoplankton in zooplankton fecalpellets. Deep-Sea Res. 30A, 963–969.
Fowler, S.W., Small, L.F., 1972. Sinking rates of Euphausiid fecal pellets. Limnol.
Oceanogr. 17, 293–296.
Giering, S.L.C., Sanders, R., Lampitt, R.S., Anderson, T.R., Tamburini, C., Boutrif, M.,
Zubkov, M.V., Marsay, C.M., Henson, S.A., Saw, K., Cook, K., Mayor, D.J., 2014.
Reconciliation of the carbon budget in the ocean’s twilight zone. Nature 507,
480–483.
Gnaiger, E., 1983. Calculation of energetic and biochemical equivalents of respiratory
oxygen consumption. In: Gnaiger, E., Forstner, H. (Eds.), Polarogranic Oxygen
Sensors. Springer, Berlin, pp. 337–345.
Gorbatenko, K.M., Lazhentsev, A.E., Kiyashko, S.I., 2014. Seasonal dynamics of thetrophic status of zooplankton in the Sea of Okhotsk (based on data from stablecarbon and nitrogen isotope analysis). Russ. J. Mar. Biol. 40, 519–531.
Gorsky, G., Ohman, M.D., Picheral, M., Gasparini, S., Stemmann, L., Romagnan, J.-B.,
Cawood, A., Pesant, S., García-Comas, C., Prejger, F., 2010. Digital zooplankton
image analysis using the ZooScan integrated system. J. Plankton Res. 32, 285–303.
Guo, P.Y., Shen, H.T., Wang, J.H., 2014. Species diversity, community structure anddistribution of phytoplankton in the Changjiang estuary during dry and flood
periods. J. Mar. Biol. Assoc. U. K. 94, 459–472.
Hamasaki, K., Ikeda, M., Ishikawa, M., Shirasawa, K., Taguchi, S., 1998. Seasonalvariability of size-fractionated chlorophyll in Monbetsu Harbor, Hokkaido, northern
Japan. Plankton Biol. Ecol. 45, 151–158.
Harding, G.C.H., 1974. The food of deep-sea copepods. J. Mar. Biol. Assoc. U. K. 54,
141–155.
Hiragi, M., Yamaguchi, A., 2019. Vertical distribution and population structure of large
dominant planktonic copepods down to greater depths in the Okhotsk Sea during
early summer. Bull. Fish. Sci. Hokkaido Univ. 69, 83–91.
Ikeda, T., 1985. Metabolic rates of epipelagic marine zooplankton as a function of bodymass and temperature. Mar. Biol. 85, 1–11.
Ikeda, T., 2014. Respiration and ammonia excretion by marine metazooplankton taxa:
synthesis toward a global-bathymetric model. Mar. Biol. 161, 2753–2766.
Iversen, M.H., Poulsen, L.K., 2007. Coprorhexy, coprophagy, and coprochaly in the
copepods Calanus helgolandicus, Pseudocalanus elongatus, and Oithona similis. Mar.
Ecol. Prog. Ser. 350, 79–89.
Kasai, H., Nakano, Y., Ono, T., Tsuda, A., 2010. Seasonal change of oceanographic
conditions and chlorophyll a vertical distribution in the southwestern Okhotsk Sea
during the non-iced season. J. Oceanogr. 66, 13–26.
Kim, H.S., Yamaguchi, A., Ikeda, T., 2009. Abundance, biomass and life cycle patterns ofeuphausiids (Euphausia pacifica, Thysanoessa inspinata and T. longipes) in the Oyashio
region, western subarctic pacific. Plankton Benthos Res 4, 43–52.
Kobari, T., Shinada, A., Tsuda, A., 2003. Functional roles of interzonal migrating
mesozooplankton in the western subarctic Pacific. Prog. Oceanogr. 57, 279–298.
Kobari, T., Steinberg, D.K., Ueda, A., Tsuda, A., Silver, M.W., Kitamura, M., 2008.
Impacts of ontogenetically migrating copepods on downward carbon flux in thewestern subarctic Pacific Ocean. Deep-Sea Res. II 55, 1648–1660.
Kobari, T., Nakamura, R., Unno, K., Kitamura, M., Tanabe, K., Nagafuku, H., Niibo, A.,
Kawakami, H., Matsumoto, K., Honda, M.C., 2016. Seasonal variability in carbon
demand and flux by mesozooplankton communities at subarctic and subtropical sites
in the western North Pacific Ocean. J. Oceanogr. 72, 403–418.
Komar, P.D., Morse, A.P., Small, L.F., Fowler, S.W., 1981. An analysis of sinking rates ofnatural copepod and euphausiid fecal pellets. Limnol. Oceanogr. 26, 172–180.
Kuwata, A., Tsuda, A., 2005. Selection and viability after ingestion of vegetative cells,
resting spores and resting cells of the marine diatom, Chaetoceros pseudocurvisetus, by
two copepods. J. Exp. Mar. Biol. Ecol. 322, 143–151.
Laurenceau-Cornec, E.C., Trull, T.W., Davies, D.M., Bray, S.G., Doran, J., Planchon, F.,
Carlotti, F., Jouandet, M.-P., Cavagna, A.-J., Waite, A.M., Blain, S., 2015. The
relative importance of phytoplankton aggregates and zooplankton fecal pellets tocarbon export: insights from free-drifting sediment trap deployments in naturally
iron-fertilised waters near the Kerguelen Plateau. Biogeosciences 12, 1007–1027.
Lomas, M.W., Moran, S.B.M., 2011. Evidence for aggregation and export ofcyanobacteria and nano-eukaryotes from the Sargasso Sea euphotic zone.
Biogeosciences 8, 203–216.
Longhurst, A.R., Harrison, W.G., 1989. The biological pump: profiles of plankton
production and consumption in the upper ocean. Prog. Oceanogr. 22, 47–123.
Maar, M., Nielsen, T.G., Gooding, S., Tönnesson, K., Tiselius, P., Zervoudaki, S.,
Christou, E., Sell, A., Richardson, K., 2004. Trophodynamic function of copepods,
appendicularians and protozooplankton in the late summer zooplankton community
in the Skagerrak. Mar. Biol. 144, 917–933.
Maneiro, I., Guisande, C., Frangopulos, M., Riveiro, I., 2002. Importance of copepod
faecal pellets to the fate of the DSP toxins produced by Dinophysis spp. Harmful Algae
1, 333–341.
Marshall, H.G., Burchardt, L., Lacouture, R., 2005. A review of phytoplankton
composition within Chesapeake Bay and its tidal estuaries. J. Plankton Res. 27,
1083.
Mauchline, J., 1998. The biology of calanoid copepods. Adv. Mar. Biol. 33, 1–710.
McDonnell, A.M.P., Buesseler, K.O., 2010. Variability in the average sinking velocity ofmarine particles. Limnol. Oceanogr. 55, 2085–2096.
Menden-Deuer, S., Lessard, E.J., 2000. Carbon to volume relationships for
dinoflagellates, diatoms, and other protist plankton. Limnol. Oceanogr. 45, 569–579.
Mizukami, T., Itaya, K., Mukai, T., Iida, K., 2019. Vertical distribution of walleye pollock
Gadus chalcogrammus and euphausiids on the continental shelf of southern Okhotsk

Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgments
We thank the crew members of the T/S Oshoro-Maru for assistance incollecting the samples and CTD data. We appreciate Dr Orio Yamamura
and Ms. Shiho Furumaki for providing MOHT samples and analysis on
the acoustic data. We also highly appreciate the kind and careful com­ments argued by the anomalous reviewer and associate editor. It wasvery helpful. This work was supported by the Arctic Challenge for Sus­
tainability II (ArCS II) Program [Grant Number JPMXD1420318865];
the Environment Research and Technology Development Fund[JPMEERF20214002] of the Environmental Restoration and Conserva­
tion Agency of Japan; and Grants-in-Aid for Challenging Research(Pioneering) [JP20K20573] and Scientific Research [JP22H00374 (A),JP20H03054 (B), JP19H03037 (B), JP21H02263 (B), and JP17H01483(A)] from the Japanese Society for the Promotion of Science (JSPS).
References
Abe, Y., Ishii, K., Yamaguchi, A., Imai, I., 2012. Short-term changes in population
structure and vertical distribution of mesopelagic copepods during the spring
phytoplankton bloom in the Oyashio region. Deep-Sea Res. I65, 100–112.
Abe, Y., Natsuike, M., Matsuno, K., Terui, T., Yamaguchi, A., Imai, I., 2013. Variation inassimilation efficiencies of dominant Neocalanus and Eucalanus copepods in thesubarctic Pacific: consequences for population structure models. J. Exp. Mar. Biol.
Ecol. 449, 321–329.
Arashkevich, Y.G., 1969. The food and feeding of copepods in the northwestern Pacific.
Oceanology 9, 695–709.
Arima, D., Yamaguchi, A., Nobetsu, T., Imai, I., 2016. Usefulness of deep-ocean water
pumping for the seasonal monitoring of mesozooplankton. Reg. Stud. Mar. Sci. 3,
18–24.
Ayukai, T., Hattori, H., 1992. Production and downward flux of zooplankton fecal pellets
in the anticyclonic gyre off Shikoku. Japan. Oceanol. Acta. 15, 163–172.
Boyd, P.W., Gall, M.P., Silver, M.W., Coale, S.L., Bidigare, R.R., Bishop, J.L.K.B., 2008.
Quantifying the surface-subsurface biogeochemical coupling during the VERTIGO
ALOHA and K2 studies. Deep-Sea Res. II. 55, 1578–1593.
Brodskii, K.A., 1967. Calanoida of the Far Eastern Seas and Polar Basin of the USSR (inRussian). Nauka, Moscow. Translated to English in 1967, Israel Program of Scientific
Translation, Jerusalem.
Buesseler, K.O., Lamborg, C.H., Boyd, P.W., Lam, P.J., Trull, T.W., Bidigare, R.R.,
Bishop, J.K.B., Casciotti, K.L., Dehairs, F., Elskens, M., Honda, M., Karl, D.M.,
Siegel, D.A., Silver, M.W., Steinberg, D.K., Valdes, J., Mooy, B.V., Wilson, S., 2007.
Revisiting carbon flux through the ocean’s twilight zone. Science 316, 567–570.
Cherry, R.D., Higgo, J.J.W., Fowler, S.W., 1978. Zooplankton fecal pellets and element
residence times in the ocean. Nature 274, 246–248.
Dagg, M.J., Jackson, G.A., Checkley Jr., D.M., 2014. The distribution and vertical flux offecal pellets from large zooplankton in Monterey Bay and coastal California. DeepSea Res. I 94, 72–86.
Ducklow, H.W., Steinberg, D.K., Busseler, K.O., 2001. Upper ocean carbon export and thebiological pump. Oceanography 14, 50–58.
Emerson, C.W., Roff, J.C., 1987. Implications of fecal pellet size and zooplankton
behaviour to estimates of pelagic-benthic carbon flux. Mar. Ecol. Prog. Ser. 35,
251–257.

16

D. Kojima et al.

Deep-Sea Research Part I 185 (2022) 103771
Sun, J., Liu, D., 2003. Geometric models for calculating cell biovolume and surface areafor phytoplankton. J. Plankton Res. 25, 1331–1346.
Suzuki, H., Sasaki, H., Fukuchi, M., 2003. Loss processes of sinking fecal pellets ofzooplankton in the mesopelagic layers of the Antarctic marginal ice zone.
J. Oceanogr. 59, 809–818.
Taguchi, S., Saino, T., 1998. Net zooplankton and the biological pump off Sanriku,
Japan. J. Oceanogr. 54, 573–582.
Takahashi, K., Kuwata, A., Saito, H., Ide, K., 2008. Grazing impact of the copepod
community in the Oyashio region of the western subarctic Pacific Ocean. Prog.
Oceanogr. 78, 222–240.
Takahashi, K., Kuwata, A., Sugisaki, H., Uchikawa, K., Saito, H., 2009. Downward carbon
transport by diel vertical migration of the copepods Metridia pacifica and Metridia
okhotensis in the Oyashio region of the western subarctic Pacific Ocean. Deep-Sea
Res. I 56, 1777–1791.
Taki, K., 2006. Studies on fisheries and life history of Euphausia pacifica Hansen off
northeastern Japan. Bull. Fish. Res. Agency 18, 41–165.
Takizawa, T., 1982. Characteristics of the Soya Warm current in the Okhotsk Sea.
J. Oceanogr. Soc. Jpn. 38, 281–292.
Terazaki, M., Tomatsu, C., 1997. A vertical multiple opening and closing plankton
sampler. J. Adv. Mar. Sci. Technol. Soc. 3, 127–132.
Tomiyama, K., Matsuno, K., Abe, Y., Shimada, H., Yamaguchi, A., 2017. Inter-oceanic
differences in macrozooplankton biomass and community structure in four regions
around Hokkaido Island, Japan: consequences for marine ecosystem structure. Bull.
Fish. Sci. Hokkaido Univ. 67, 25–34.
Tranter, D.J., Heron, A.C., 1967. Experiments on filtration in plankton nets. Aust. J. Mar.
Freshw. Res. 18, 89–111.
Tranter, D.J., Smith, P.E., 1968. Filtration performance. In: Tranter, D.J., Fraser, J.H.
(Eds.), Zooplankton Sampling, Monographs on Oceanographic Methodology 2.
Unesco, Paris, pp. 27–56.
Trull, T.W., Bray, S.G., Busseler, K.O., Lamborg, C.H., Manganini, S., Moy, C., Valdes, J.,
2008. In situ measurement of mesopelagic particle sinking rates and the control ofcarbon transfer to the ocean interior during the Vertical Flux in the Global Ocean(VERTIGO) voyages in the North Pacific. Deep-Sea Res. II 55, 1584–1595.
Turner, J.T., 2002. Zooplankton fecal pellets, marine snow and sinking phytoplankton
blooms. Aquat. Microb. Ecol. 27, 57–102.
Turner, J.T., 2004. The importance of small planktonic copepods and their roles inpelagic marine food webs. Zool. Stud. 43, 255–266.
Turner, J.T., 2015. Zooplankton fecal pellets, marine snow, phytodetritus and theocean’s biological pump. Prog. Oceanogr. 130, 205–248.
Urban-Rich, J., Nordby, E., Andreassen, I.J., Wassmann, P., 1999. Contribution bymesozooplankton focal pellets to the carbon flux on Nordvestkbanken, northNorwegian shelf in 1994. Sarsia 84, 253–264.
Uye, S., Kaname, K., 1994. Relations between fecal pellet volume and body size for major
zooplankters of the Inland Sea of Japan. J. Oceanogr. 50, 43–49.
Uye, S., Aoto, I., Onbe, T., 2002. Seasonal population dynamics and production of
Microsetella norvegica, a widely distributed but little-studied marine planktonic
harpacticoid copepod. J. Plankton Res. 24, 143–153.
Valdes, J.R., Price, J.F., 2000. A neutrally buoyant, upper ocean sediment trap. J. Atmos.
Ocean. Technol. 17, 62–68.
Wilson, S.E., Steinberg, D.K., 2010. Autotrophic picoplankton in mesozooplankton guts:
evidence of aggregate feeding in the mesopelagic zone and export of small
phytoplankton. Mar. Ecol. Prog. Ser. 412, 11–27.
Wilson, S.E., Steinberg, D.K., Busseler, K.O., 2008. Changes in fecal pellet characteristics
with depth as indicators of zooplankton repackaging of particles in the mesopelagiczone of the subtropical and subarctic North Pacific Ocean. Deep-Sea Res. II 55,
1636–1647.
Yamaguchi, A., 2015. Inter-oceanic comparison of planktonic copepod ecology (vertical
distribution, abundance, community structure, population structure and body size)
between the Okhotsk Sea and Oyashio region in autumn. J. Nat. Hist. 49,
2743–2757.
Yamaguchi, A., Shiga, N., 1997. Vertical distribution and life cycles of Pseudocalanus
minutus and P. newmani (copepoda; calanoida) off cape esan, southwestern
Hokkaido. Bull. Plankton Soc. Jpn. 44, 11–20.
Yamaguchi, A., Ohtsuka, S., Hirakawa, K., Ikeda, T., 2007. Vertical distribution andfeeding ecology of a copepod Gaetanus variabilis in the southern Japan Sea during
winter. La Mer 45, 47–58.
Yamaguchi, M., 1993. Decadal mean oceanographic conditions in the southwestern partof the Okhotsk Sea from 1983 to 1992. Sci. Rep. Hokkaido Fish. Exp. Stn. 40, 1–19.

Sea, off the coast of Hokkaido, by using a quantitative echosounder. Sci. Rep.
Hokkaido Fish. Res. Inst. 96, 41–52.
Motoda, S., 1959. Devices of simple plankton apparatus. Mem. Fac. Fish. Hokkaido Univ.
7, 73–94.
Mustapha, M.A., Saitoh, S.-I., 2008. Observations of sea ice interannual variations andspring bloom occurrences at the Japanese scallop farming area in the Okhotsk Sea
using satellite imageries. Estuar. Coast Shelf Sci. 77, 577–588.
Nakamura, A., Matsuno, K., Abe, Y., Shimada, H., Yamaguchi, A., 2017. Length–weight
relationships and chemical composition of the dominant mesozooplankton taxa/
species in the Subarctic Pacific, with special reference to the effect of lipid
accumulation in Copepoda. Zool. Stud. 56, 13.
Noji, T.T., Estep, K.W., Macintyre, F., Norrbin, F., 1991. Image analysis of faecal material
grazed upon by three species of copepods: evidence for coprorhexy, coprophagy andcoprochaly. J. Mar. Biol. Assoc. U. K. 71, 465–480.
Omori, M., Ikeda, T., 1984. Methods in Marine Zooplankton Ecology. A Wiley-Inter
Science Publication, New York.
Oozeki, Y., Hu, F., Kubota, H., Sugisaki, H., Kimura, R., 2004. Newly designed
quantitative frame trawl for sampling larval and juvenile pelagic fish. Fish. Sci. 70,
223–232.
Padmavati, G., Ikeda, T., Yamaguchi, A., 2004. Life cycle, population structure andvertical distribution of Metridia spp. (copepoda: calanoida) in the Oyashio region(NW pacific ocean). Mar. Ecol. Prog. Ser. 270, 181–198.
Paffenhöfer, G.-A., Knowles, S.C., 1979. Ecological implications of fecal pellet size,
production and consumption by copepods. J. Mar. Res. 37, 35–49.
Pasternak, A., Arashkevich, E., Riser, C.W., Ratkova, T., Wassmann, P., 2000. Seasonalvariation in zooplankton and suspended faecal pellets in the subarctic Norwegian
Balsfjorden, in 1996. Sarsia 85, 439–452.
Pinchuk, A.I., Hopcroft, R.R., 2007. Seasonal variations in the growth rates of
euphausiids (Thysanoessa inermis, T. spinifera, and Euphausia pacifica) from thenorthern Gulf of Alaska. Mar. Biol. 151, 257–269.
Pinchuk, A., Paul, A.J., 2000. Zooplankton of the Okhotsk Sea: A Review of RussianStudies. University of Alaska Sea Grant, Anchorage.
Poulsen, L.K., Kiørboe, T., 2005. Coprophagy and coprorhexy in the copepods Acartia
tonsa and Temora longicornis: clearance rates and feeding behavior. Mar. Ecol. Prog.
Ser. 299, 217–227.
Reigstad, M., Riser, C.W., Svensen, C., 2005. Fate of copepod faecal pellets and the role of
Oithona spp. Mar. Ecol. Prog. Ser. 304, 265–270.
Righetti, D., Vogt, M., Zimmermann, N.E., Guiry, M.D., Gruber, N., 2020. PhytoBase: a
global synthesis of open-ocean phytoplankton occurrences. Earth Syst. Sci. Data 12,
907–933.
Ross, R.M., 1982. Energetics of Euphausia pacifica. I. Effects of body carbon and nitrogenand temperature on measured and predicted production. Mar. Biol. 68, 1–13.
Sakai, R., Kono, T., Oguma, S., 2008. Water mass structure on the eastern Hokkaido coastin the Okhotsk Sea from october 2006 to july 2007. J. Mar. Meteorol. Soc. 84,
123–130.
Sato, M., Kono, T., Kawasaki, Y., 2008. Inflow of Soya Warm current induced by wind in
the northern nemuro strait in summer. J. Mar. Meteorol. Soc. 84, 101–108.
Shebanova, M.A., Chuchukalo, V.I., Dulepova, E.P., 2011. Some biological features andproductive characteristics of Oithona similis (copepoda) in the sea of Okhotsk andwestern bering sea. Russ. J. Mar. Biol. 37, 594–603.
Shimizu, I., 2009. The effect of sea ice on the oceanographic structure and phytoplankton
biomass in the Nemuro Strait, a nursery area of salmon. Bull. Fish. Res. Agency 27,
43–94.
Shiomoto, A., 2011. Phytoplankton biomass and production in the coastal area of theShiretoko Peninsula during late spring to early autumn: comparison between the
Okhotsk Sea and the Nemuro Straight. Bull. Coast. Oceanogr. 49, 37–48.
Small, L.F., Fowler, S.W., Ünlü, M.Y., 1979. Sinking rates of natural copepod fecalpellets. Mar. Biol. 51, 233–241.
Smith, P.E., Counts, R.C., Clutter, R.I., 1968. Changes in filtering efficiency of plankton
nets due to clogging under tow. J. Cons. Perm. Int. Explor. Mer. 32, 232–248.
Stamieszkin, K., Pershing, A.J., Record, N.R., Pilskaln, C.H., Dam, H.G., Feinberg, L.R.,
2015. Size as the master trait in modeled copepod fecal pellet carbon flux. Limnol.
Oceanogr. 60, 2090–2107.
Steinberg, D.K., Mooy, A.S.V., Busseler, K.O., Boyd, P.W., Kobari, T., Karl, D.M., 2008.
Bacterial vs. zooplankton control of sinking particle flux in the ocean’s twilight zone.
Limnol. Oceanogr. 53, 1327–1338.
Strathmann, R.R., 1967. Estimating the organic carbon content of phytoplankton fromcell volume or plasma volume. Limnol. Oceanogr. 12, 411–418.
Stukel, M.R., Landry, M.R., Benitez-Nelson, C.R., Goericke, R., 2011. Trophic cycling andcarbon export relationships in the California Current Ecosystem. Limnol. Oceanogr.
56, 1866–1878.

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