Microplastics in the Southern Ocean

Abstract

A field survey to collect microplastics with sizes < 5 mm was conducted in the Southern Ocean in 2016. We performed five net-tows and collected 44 pieces of plastic. Total particle counts of the entire water column, which is free of vertical mixing, were computed using the surface concentration (particle count per unit seawater volume) of microplastics, wind speed, and significant wave height during the observation period. Total particle counts at two stations near Antarctica were estimated to be in the order of 100,000 pieces km− 2.

Full text

Baseline
Microplastics in the Southern Ocean
Atsuhiko Isobe
a,
⁎, Kaori Uchiyama-Matsumoto
b
, Keiichi Uchida
c
, Tadashi Tokai
c
a
Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga-Koen, Kasuga 816-8580, Japan
b
Observation and Research Center for Ocean Systems, Tokyo University of Marine Science and Technology,4-5-7 Konan, Minato-ku, Tokyo 108-8477, Japan
c
Tokyo University of Marine Science and Technology, 4-5-7 Konan, Minato-ku, Tokyo 108-8477, Japan
abstractarticle info
Article history:
Received 29 May 2016
Received in revised form 15 September 2016
Accepted 19 September 2016
Available online 26 September 2016
Afield survey to collect microplastics with sizes b5 mm was conducted in the Southern Ocean in 2016. We per-
formed five net-tows and collected 44 pieces of plastic. Total particle counts of the entire water column, which is
free of vertical mixing,were computed using the surfaceconcentration (particle count per unit seawater volume)
of microplastics, wind speed, andsignificant wave height during the observation period. Total particle counts at
two stations near Antarctica wereestimated to be in the order of 100,000 pieces km
−2
.
© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
Keywords:
Microplastics
Mesoplastics
Concentration
Total particle count
Mismanaged plastic waste can escape into the natural environment,
particularly in regions with high population density (Jambeck et al.,
2015). Numerous plastic fragments are typically found in the oceans
of the Northern Hemisphere in areas where plastic debris has degraded
on beaches (Andrady, 2011). A number of recent studies have reported
the collection of tiny plastic fragments with diameters of b5 mm (re-
ferred to as microplastics) in open oceans, including the Arctic polar wa-
ters (Thompson et al., 2004; Goldstein et al., 2012; Cózar et al., 2014;
Eriksen et al., 2014; Reisser et al., 2015; Lusher et al., 2015), marginal
seas (de Lucia et al., 2014; Isobe et al., 2015), and coastal waters
(Isobe et al., 2014). Table 1 summarizes the concentrations (particle
count per unit seawater volume) of pelagic microplastics reported in
the Northern Hemisphere. Importantly, microplastics can act as a trans-
port vector of chemical pollutants into the marine ecosystem, owing to
the absorption of pollutants onto their surfaces (Mato et al., 2001;
Teuten et al., 2009), and their subsequent ingestion by organisms as
small as zooplankton (Desforges et al., 2015). If the discharge of pelagic
microplastics into the oceans continues, such pollution will be unavoid-
able in the future.
Based on the synthesis of the results of several Southern Hemisphere
surveys, it has been suggested that pelagic microplastics are less wide-
spread in the Southern oceanic regions, compared with the oceans of
the Northern Hemisphere (see Fig. S5 in Cózar et al. (2014)). This may
indicate that microplastics have not yet spread across the oceans of
the Southern Hemisphere. However, there have been few comprehen-
sive surveys of the distribution of microplastics in the Southern Hemi-
sphere (Fig. S1 in Cózar et al. (2014)), and previous findings are
inconclusive. In particular, it is currently unclear whether pelagic
microplastics can be detected in the Southern Ocean (also known as
the Antarctic Ocean); a marine area with the lowest population in the
world, where minimal mismanagement of plastic is likely to occur. Ob-
servations of a significant concentrationof microplastics inthe Southern
Ocean would suggest that pelagic microplastics have already spread
across the world's oceans. Global plastic production has increased by
N500 times over the last 60 years (Thompson et al., 2009). However,
microplastic surveys in the Southern Ocean have not been reported in
peer-reviewed publications, except for a small number of surveys con-
ducted in the Drake Passage close to South America (unpublished, but
data were used in Cózar et al. (2014) and Eriksen et al. (2014)).
In the present study, we conducted microplastic surveys in the
Southern Ocean from January 30 to February 4, 2016, at five stations
along a route from Fremantle to Hobart, Australia, using a T/V
Umitaka-maru belonging to Tokyo University of Marine Science and
Technology (Fig. 1). Wind speed and significant wave height were mea-
sured on thevessel, and hourly averaged data were recorded during the
surveys (Fig. 2). These data were used to deduce the vertical distribution
of microplastics for comparison with data collected in other oceans
under different wind and wave conditions. A Neuston net (5552; RIGO
Co., Ltd., Tokyo, Japan) was used for sampling the small plastic frag-
ments. The mouth, length, and mesh size of the net were 75 × 75 cm,
3 m, and 0.35 mm, respectively. The T/V Umitaka-maru towed the Neus-
ton net around each station continuously for 20–40 min at a constant
speed of 2–3 knots. To avoid collecting plastic fragments originating
from the ship, the net was positioned at a distance of approximately
Marine Pollution Bulletin 114 (2017) 623–626
⁎Corresponding author at: Research Institute for Applied Mechanics, Kyushu
University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan.
E-mail addresses: aisobe@riam.kyushu-u.ac.jp (A. Isobe), uchiko.kaori@gmail.com
(K. Uchiyama-Matsumoto), kuchida@kaiyodai.ac.jp (K. Uchida), tokai@kaiyodai.ac.jp
(T. Tokai).
http://dx.doi.org/10.1016/j.marpolbul.2016.09.037
0025-326X/© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Contents lists available at ScienceDirect
Marine Pollution Bulletin
journal homepage: www.elsevier.com/locate/marpolbul

2 m during the towing, and thereafter was rinsed on the deck by
pouring seawater from the outside of the net. A flow meter (5571A;
RIGO Co., Ltd.) was installed at the net mouth. Once the surveys were
completed, the flow-meter readings and net mouth dimensions (75 ×
75 cm) were used to estimate the volume of water filtered during
each tow.
The seawater samples, including the suspended matter, were sent to
Kyushu University for the extraction of plastic fragments. The small
plastic fragments were first observed using a monitor display via a
USB camera (HDCE-20C; AS ONE Corporation, Osaka, Japan) attached
to a stereoscopic microscope (SZX7; Olympus Corporation, Tokyo,
Japan) and identified visually by their colour and shape. Polymer types
were identified using a Fourier transform infrared spectrophotometer
(FT-IR alpha; Bruker Optics K.K., Tokyo, Japan) when the fragments
were too small for visual differentiation between microplastics and bio-
logical matter. Expanded-polystyrene particles (three particles were de-
tected), fibers (a single piece), and biological elements were all
removed before any further analyses. Primary microplastics suchas pel-
lets were not detected in the present surveys.
The numbers of plastic fragments in each size range were counted
with an increment of 0.1 mm for microplastics b5 mm and 1 mm for
mesoplastics N5 mm. The sizes were defined by the longest length of
each irregularly shaped fragment visible on the monitor display, mea-
sured using image-processing software (ImageJ; downloaded from
http://imagej.nih.gov).The numbers within each size range were there-
after divided by the water volumes measured by the flow meter at each
sampling station to convert them to a measure of concentration in units
of the number of pieces m
−3
.
Direct comparison of microplastic concentrations observed in differ-
ent ocean areas under different wave and wind conditions can be diffi-
cult because light-weight microplastics are vulnerable to vertical
mixing caused by oceanic turbulence, and because the vertical distribu-
tion (and thesurface concentration observed usinga Neuston net) is af-
fected by these oceanic conditions (Kukulka et al., 2012; Reisser et al.,
2015; Isobe et al., 2015). The surface concentrations of microplastics ob-
tained in different oceans can be converted to the total particle count
(particle count per unit area) by vertically integrating the concentration
at depths using the wind speed and significant wave heights measured
during each microplastic survey (hereinafter, wind/wave correction).
The total particle count, which is regarded as the quantity of pelagic
microplastics in the entire water column, is independent of vertical
mixing. Thus, it is a useful measure for the comparison of microplastic
quantities in different oceans.
Let us consider microplastics with size δ. Assuming an equilibrium
state between plastic rise (terminal) velocity (w) and vertical diffusion
with the diffusivity (A
0
), we anticipate that their concentration (N
δ
) will
decrease exponentially into deeper layers (Kukulka et al., 2012; Reisser
et al., 2015) as follows:
Nδ¼Nδ0ew
A0z;ð1Þ
where N
δ0
denotes the concentration of microplastics collected using a
Neuston net, wis set to 0.0053 mm s
−1
obtained experimentally
(Reisser et al., 2015), and zis the vertical axis in the upward direction
from the sea surface. Parameter A
0
, with respect to the oceanic turbu-
lence in the upper layer is computed as:
A0¼1:5ukHS;ð2Þ
where u
⁎
represents the frictional velocity of water (=0.0012 U), kis the
vonKarmancoefficient (0.4), Hs is the significant wave height, and Uis
the wind speed. The applicability of the above formulation was exam-
ined by Reisser et al. (2015) in the North Atlantic gyre using multi-
level net towing. In the present study, wind speed and wave height
were averaged over 24 h before each survey. Vertically integrating Eq.
(1) from the sea surface (z=0)totheinfinitely deep layer (z→−∞)
yields the total particle count of microplastics per unit area, M
δ
(pieces
Table 1
Observed surface microplastic concentration reported by previous studies. Apart from
Goldstein et al. (2012), who synthesized the previous surveys, the studiesbelow comput-
ed the concentrations using particle counts and seawater volume measured by a flow
meter.
Oceans Concentration (pieces m
−3
)
East Asian seas (Isobe et al., 2015) 3.70
N. Atlantic (accumulation area) (Reisser et al., 2015) 1.70
Seto Inland Sea (Isobe et al., 2014) 0.39
Arctic polar waters (Lusher et al., 2015) 0.34
Mediterranean Sea (de Lucia et al., 2014) 0.15
N. Pacific(Goldstein et al., 2012) 0.12
100°E 110°E 120°E 130°E 140°E 150°E
70°S
65°S
60°S
55°S
50°S
45°S
40°S
35°S
30°S
25°S
1(20) 2(18)
3(2)
4(2)
5(2)
125°E 135°E 145°E
35°N
45°N
Fig. 1. Survey stations. The digits in parentheses denote the particle count (number of
pieces) of microplastics collected at each station. The survey stations around Japan are
shown in the inset map.
1
2
3
4
5
6
7
0
5
0
10
15
20
29 30 31 1 2 3 4 5 6
Jan. Feb. 2016
mm/s
12 3 4 5
Fig. 2. Temporal variation of significant wave heightand wind speed measuredduring the
surveyperiod. The black (red)curve indicatesthe wave height (wind speed)for which the
ordinateis shown on the left (right) side of the figure. The graybars show the observation
periods at the five stations in Fig. 1.
624 A. Isobe et al. / Marine Pollution Bulletin 114 (2017) 623–626

per unit area), as follows:
Mδ¼Nδ0A0=w:ð3Þ
Integrating Eq. (3) over microplastic sizes smaller than 5 mm gives the
total particle count of microplastics.
Overall, 44 pieces of microplastic, excluding fibers and expanded
polystyrene, were collected over the course of the surveys (see photos
[a–c] in Fig. 3 as examples). Of the 44 fragments, one fragment (photo
[c] in Fig. 3) was 5.5 mm in length, slightly larger than the b5 mm def-
inition of microplastics (Andrady, 2011;Cole et al., 2011). However, this
fragment was considered as a microplastic fragment for convenience in
the presentstudy. In this short report, we focus specifically on the con-
centrations of pelagic microplastics in the Southern Ocean. The analysis
of other types of data from these 44 samples, including biofouling
(Morét-Ferguson et al., 2010) and carbonyl index (Andrady et al.,
1993; Satoto et al., 1997) will be examined in the next phase of our
research.
The microplastics were predominantly found at Stas. 1 and 2, south
of 60°S (nearest Antarctica),while only two pieces of microplastic were
detected at each of Stas. 3, 4, and 5 (Fig. 1). The results indicated that the
abundance of plastic fragments was negligibly small in the latter three
stations, because the fragment count was so low that may have been
due to contamination by theship, in spite of the effort to avoidcollecting
ship-derived plastics. Of the 44 fragments, 29 were made of polyethyl-
ene, polypropylene, and polyethylene combined with unidentified
polymers. These microplastics can be carried long distances because
they are less dense than seawater (~1025 kg m
−3
). However, 14 of
the remaining 15 microplastics were made of polystyrene, and 1 was
made of polyvinyl chloride, which are all denser than seawater. These
dense microplastics are unlikely to have drifted independently in the
upper ocean. We speculate that they were likely to have been detected
in the surface water because they were entangled with drifting pelagic
objects such as zooplankton and krill, which were collected concurrent-
ly with the microplastics.
As with microplastics collected in mid-latitudes (Cózar et al., 2014;
Isobe et al., 2014; Isobe et al., 2015), high concentrations of
microplastics were found only in the smaller size range (Fig. 3). This re-
sult is likely to have been due to single pieces of plastic debris gradually
degrading into multiple tiny pieces as they move within the environ-
ment. Nevertheless, it is of particular interest that mesoplastics (diame-
ter: N5 mm) were rarely observed in the present surveys. The absence
of relatively “fresh”(i.e., less degraded) mesoplastics implies that the
areas around the Southern Ocean are unlikely to be significant sources
of pelagic plastic debris, and that any tiny fragments must have been
transported considerable distances.
Integrating the concentration data shown in Fig. 3 with sizes data
from fragments b5 mm yields the averaged concentration of
microplastics at each station (Table 2). Except for Sta. 1, the concentra-
tions of pelagic microplastics in the Southern Ocean were found to be 1–
2 orders of magnitudesmaller than those reported in the North Pacific,
Arctic polar waters, Mediterranean Sea, and the Seto Inland Sea of Japan
(Table 1). Furthermore, the concentrations we observed were 2–3 or-
ders of magnitude smaller than areas of other oceans with highconcen-
trations of pelagic microplastics, including the East Asian seas and the
accumulation (frontal) area of the North Atlantic (Table 1). However,
the microplastic concentration observed at Sta. 1 was comparable
with those observed in the oceans of the Northern Hemisphere.
As mentioned above, the total particle count, as a measure of the
quantity of pelagic microplastics in the entire water column, is useful
for the comparison of microplastic abundance in different oceans. The
current microplastic surveys in the Southern Ocean were conducted
under stormy conditions with wind speeds and significant wave heights
of 10 m s
−1
and 3.3 m, respectively, averaged over the survey period
(Fig. 2). However, in previous studies in both the East Asian seas
(Isobe et al., 2015) and the Seto Inland Sea (Isobe et al., 2014), the
microplastics were collected under relatively calm conditions, with
wind speeds of b5ms
−1
and significant wave heights of b1m.Thesur-
face concentrations in these areas may have been lower if the surveys
had been conducted under the same severe conditions as the present
surveys. Thus, there is clearly room to conclude that the abundance of
pelagic microplastics in the Southern Ocean is lower than in many
other areas. We computed the total particle counts at Stas. 1 and 2,
where relatively large amounts of microplastic were collected (Table
Table 3
Comparison of the total particle counts estimated in different oceans. The total particle
count in each oceanwas computed in Isobe et al. (2015) by averaging the total particle
counts for data from various oceans reported by Eriksen et al. (2014).
Oceans Total particle count (pieces km
−2
)
East Asian seas
a
1,720,000
N. Pacific
b
105,100
World's oceans
b
63,320
Seto Inland Sea
c
76,000
Sta. 1 286,000
Sta. 2 136,000
a
Isobe et al. (2015).
b
Eriksen et al. (2014).
c
Isobe et al. (2014).
Table 2
Results of microplastic surveys at each station.
Sta. Date Particle count
(pieces)
Seawater volume
(m
3
)
Concentration
(pieces m
−3
)
1 Jan 30, 2016 20 202 9.9 × 10
−2
2 Jan 31, 2016 18 392 4.6 × 10
−2
3 Feb 1, 2016 2 566 3.5 × 10
−3
4 Feb. 2, 2016 2 502 4.0 × 10
−3
5 Feb. 4, 2016 2 417 4.8 × 10
−3
Average 3.1 × 10
−2
510 20 30<
4321 mm
0
0
1
2
1
2
3
4
5
610-3 pieces/m3
×10-1 pieces/m3
×
(a)
(a)
(b)
(b)
(c)
(c)
00
1
2
3
10-2
×pieces/m3
Fig. 3. Size distribution of plastic fragments in the Southern Ocean (black bars; present
study), Seto Inland Sea (red; Isobe et al., 2014),and East Asian seasaround Japan (gray;
Isobe et al., 2015). The location of the Seto Inland Sea and stations around Japan are
shown in Fig. 1. The bar height at each size range indicates the concentration averaged
over all survey stations. Note that the intervals of size ranges are 0.1 mm for
microplastics b5 mm, 1 mm for mesoplastics b10 mm, and 10 mm for mesoplastics N
10 mm. The left ordinate is used for microplastics in the Southern Ocean, while the
black (red) ordinate on the right side is used for those in the East Asian seas (Seto
Inland Sea). The photographs (a), (b), and (c) are of microplastics collected in the
Southern Ocean in the size rangescorrespondingto the columns with the same letter at
the top. The photographs include a 5-mm grid, with grid lines of 0.3 mm.
625A. Isobe et al. / Marine Pollution Bulletin 114 (2017) 623–626

2). The approximate estimation of the total particle count around the
East Asianseas is still oneorder of magnitude greater than atthese sta-
tions in the Southern Ocean (Table 3). However, it should be noted that
the total particle counts at Stas. 1 and 2 were comparable with the aver-
age counts observed in the other oceans.
Importantly, the microplastics concentrations observed close to Ant-
arctica (Stas. 1 and 2) were more abundant than those observed at the
offshore stations (Stas. 3, 4, and 5), although they were likely to have
originated from northern inhabited areas, as suggested by the size dis-
tribution (Fig. 3). Stas. 1 and 2 were located south of the oceanic fronts
(convergence zone) around Antarctica (see Fig. 4 for schematic view),
and were located around the southern boundary of the Antarctic Cir-
cumpolar Current (ACC; see Fig. 7 in Orsi et al., 1995). Unlike seawater
subducted into the abyssal ocean by deep convection around Antarctica,
buoyant microplastics have typically been found to remain in the upper
ocean; see Fig. 2 of Reisser et al. (2015) for the dense concentration of
microplastics in the uppermost layer. This implies that the microplastics
are likely to be trapped around Antarcticaonce they are transported be-
yond the ACC and oceanic fronts. However, more comprehensive
microplastic surveys in the Southern Ocean are required to confirm
this speculation.
Significant concentration of microplastics in the Southern Ocean
would suggest that marine plastic pollution has spread across the
world's oceans. Thecurrent surveys revealed a relatively dense concen-
tration of microplastics in the Southern Ocean, comparable with con-
centrations observed in the Northern Hemisphere oceans (two of five
stations in the present case). The present findings raise concern about
the widespread nature of marine plastic pollution, indicating that plas-
tic-free ocean environments are increasing rare.
Acknowledgments
The authors sincerely thank the Captain, officers, and crews of the
Umitaka-maru for their assistance during the field surveys. We thank
the journal reviewer for valuable suggestions to improve the manu-
script. This research was supported by the Environmental Research
and Technology Development Fund (4-1502) of the Ministry of the En-
vironment, Government of Japan, and by MEXT for physical and chem-
ical oceanographic observations under the Japanese Antarctic Research
Expedition.
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100°E 110°E 120°E 130°E 140°E 150°E
70°S
65°S
60°S
55°S
50°S
45°S
40°S
35°S
30°S
25°S ×105 pieces/km2
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Fig. 4. Observed total particle counts and schematic view of oceanic conditions in the
Southern Ocean. The bars at Stas. 1 and 2 indicate the total particle counts listed in Table
3; see the reference scale shown in the upper left corner. The particle counts at Stas. 3,
4, and 5 were not shown because the numbers of collectedmicroplastics were negligibly
small. We referred to Fig. 6.7 in Tomczac and Godfrey (1994) for the positions of three
oceanic fronts in the schematic view. The arrows represent the Antarctic Circumpolar
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626 A. Isobe et al. / Marine Pollution Bulletin 114 (2017) 623–626