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Microplastics in the Marine Environment: A Review of the Methods Used for Identification and Quantification

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  • Universidad Católica del Norte (Chile), Coquimbo, Chile

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This review of 68 studies compares the methodologies used for the identification and quantification of microplastics from the marine environment. Three main sampling strategies were identified: selective, volume-reduced, and bulk sampling. Most sediment samples came from sandy beaches at the high tide line, and most seawater samples were taken at the sea surface using neuston nets. Four steps were distinguished during sample processing: density separation, filtration, sieving, and visual sorting of microplastics. Visual sorting was one of the most commonly used methods for the identification of microplastics (using type, shape, degradation stage, and color as criteria). Chemical and physical characteristics (e.g., specific density) were also used. The most reliable method to identify the chemical composition of microplastics is by infrared spectroscopy. Most studies reported that plastic fragments were polyethylene and polypropylene polymers. Units commonly used for abundance estimates are "items per m(2)" for sediment and sea surface studies and "items per m(3)" for water column studies. Mesh size of sieves and filters used during sampling or sample processing influence abundance estimates. Most studies reported two main size ranges of microplastics: (i) 500 μm-5 mm, which are retained by a 500 μm sieve/net, and (ii) 1-500 μm, or fractions thereof that are retained on filters. We recommend that future programs of monitoring continue to distinguish these size fractions, but we suggest standardized sampling procedures which allow the spatiotemporal comparison of microplastic abundance across marine environments.
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Microplastics in the Marine Environment: A Review of the Methods
Used for Identification and Quantification
Valeria Hidalgo-Ruz,
,
Lars Gutow,
§
Richard C. Thompson,
and Martin Thiel*
,,
Facultad Ciencias del Mar, Universidad Católica del Norte, Larrondo 1281, Coquimbo, Chile
Facultad de Ciencias del Mar y Recursos Naturales, Universidad de Valparaíso, Av. Borgoño 16344, Viña del Mar, Chile
§
Alfred Wegener Institute for Polar and Marine Research, Box 12 01 61, 27515 Bremerhaven, Germany
School of Marine Science and Engineering, University of Plymouth, Drake Circus, Plymouth, Devon, PL4 8AA, United Kingdom
Centro de Estudios Avanzados en Zonas Áridas (CEAZA), Coquimbo, Chile
ABSTRACT: This review of 68 studies compares the method-
ologies used for the identification and quantification of micro-
plastics from the marine environment. Three main sampling
strategies were identified: selective, volume-reduced, and bulk
sampling. Most sediment samples came from sandy beaches at
the high tide line, and most seawater samples were taken at the
sea surface using neuston nets. Four steps were distinguished
during sample processing: density separation, filtration, sieving,
and visual sorting of microplastics. Visual sorting was one of the
most commonly used methods for the identification of micro-
plastics (using type, shape, degradation stage, and color as criteria).
Chemical and physical characteristics (e.g., specific density) were
also used. The most reliable method to identify the chemical composition of microplastics is by infrared spectroscopy. Most studies
reported that plastic fragments were polyethylene and polypropylene polymers. Units commonly used for abundance estimates are items
per m2for sediment and sea surface studies and items per m3for water column studies. Mesh size of sieves and filters used during
sampling or sample processing influence abundance estimates. Most studies reported two main size ranges of microplastics: (i) 500 μm5
mm, which are retained by a 500 μm sieve/net, and (ii) 1500 μm, or fractions thereof that are retained on filters. We recommend that
future programs of monitoring continue to distinguish these size fractions, but we suggest standardized sampling procedures which allow
the spatiotemporal comparison of microplastic abundance across marine environments.
INTRODUCTION
The worldwide production of plastics has increased considerably
since the development of synthetic polymers in the middle of the
20th century.
1,2
When discarded in the marine environment,
plastics can become an environmental hazard.
3,4
Plastic debris
enters the marine environment in a wide range of sizes, in the
micrometer to meter range.
5
Microplastic particles comprise either
manufactured plastics of microscopic size, such as scrubbers
6,7
and
industrial pellets that serve as precursors for manufactured plastic
products (primary sources), or fragments or fibers of plastics
derived from the breakdown of larger plastic products (secondary
sources).
4,8,9
Degradation processes of plastics are extremely
slow,
5,10
and thus microplastics potentially persist for very long
time periods in the marine environment.
11,12
The presence and accumulation of microplastics in the ocean
is of considerable concern for a variety of reasons, especially
because they are ingested by marine biota.
4,13
Microplastics can
absorb persistent bioaccumulative and toxic compounds (PBT)
from seawater,
14
which include persistent organic pollutants
(POPs)
1517
and metals.
18
Once ingested, the absorbed pollutants
may be transferred to the respective organisms.
19
However, while
microplastics have been reported in a wide variety of marine
organisms,
2024
the extent to which ingestion might present a
toxicological hazard is not well-known.
In order to gain a better understanding of the impacts of
microplastics, most studies have focused on quantifying their
abundance in the marine environment. One of the main prob-
lems of large-scale spatial and temporal comparisons is the fact
that a wide variety of approaches have been used to identify and
quantify microplastics. Furthermore, microplastics comprise a
very heterogeneous assemblage of pieces that vary in size,
shape, color, specific density, chemical composition, and other
characteristics. For meaningful comparisons and monitoring, it
is thus important to define specific methodological criteria to
estimate the abundances, distribution and composition of
microplastics.
25
Future monitoring programs will benefit from
standardized procedures for sampling and sorting of micro-
plastics such as those proposed by the Marine Strategy Frame-
work Directive of the EU.
26
Received: September 8, 2011
Revised: January 15, 2012
Accepted: February 9, 2012
Published: February 9, 2012
Critical Review
pubs.acs.org/est
© 2012 American Chemical Society 3060 dx.doi.org/10.1021/es2031505 |Environ. Sci. Technol. 2012, 46, 30603075
The main objective of the present review is to assess the
different methods that have been employed for the
identification and quantification of microplastics in marine
environments. Based on the results, we recommend basic
criteria and approaches to ensure that future quantitative
estimates are comparable, providing standardized data of
microplastics in the marine environment.
REVIEW OF METHODS
We conducted an extensive literature review using databases
such as ISI Web of Knowledge, Science Direct, Google Scholar,
and all published volumes of the journals Marine Pollution
Bulletin and Environmental Science & Technology, for studies
published up to December 2011. The term microplasticswas
first used in 2004 to describe very small fragments of plastic
(50 μm) in the water column and in sediments.
21
In 2009,
Arthur et al. proposed that microplastics should include all
fragments <5 mm.
9
However, at present there is no universally
adopted definition in terms of the size range for microplastics.
In this review we take a broad, methodologically focused
perspective dating back prior to 2004 and aim to include all
studies describing small fragments of plastics in the environ-
ment, irrespective of whether the term microplasticswas
specifically used. Keywords used in our searches were as
follows: microplastics, microdebris, microlitter, plastic frag-
ments, pellets, marine debris and plastics. We also examined the
reference lists from the selected literature and back-tracked to
other relevant papers until no new references came up.
A total of 68 research articles were found. From these we
extracted information on (i) study objectives, (ii) sampling
procedure, (iii) laboratory processing of samples, (iv)
identification of microplastics, and (v) microplastics abundance.
For the evaluation, we distinguished the main marine
environments that were studied: sediment, water column, and
sea surface.
OBJECTIVES OF STUDIES ON MARINE
MICROPLASTICS
Small particles of plastic were first reported in the marine
environment in the early 1970s.
20,27
The objectives of
subsequent studies were variable, with a majority of studies
that examined the spatial distribution (abundance, mass, type,
and/or size) of microplastics
2831
(Table 1). The objectives of
other studies were to confirm the presence of microplastics in
other locations,
32,33
compare different sampling and sorting
methods,
34,35
and estimate proportions of microplastics versus
macroplastics
36
or versus zooplankton.
37
A few studies also
examined temporal changes in microplastics abundance,
21,38,39
dispersal,
36
and degradation rates.
4042
Plastic pellets are also increasingly used as passive samplers
for POPs
12,4346
and metals.
18
Ingestion of microplastics by
marine organisms and their physiological and ecological
implications have also gained attention recently,
4749
but this
topic is beyond the scope of this review.
SAMPLING
General Sampling Methods. Sampling of microplastics in
the main marine environments (sea surface, water column,
sediment) requires different approaches: samples can be
selective, bulk, or volume-reduced. Selective sampling in the
field consists of direct extraction from the environment of items
that are recognizable by the naked eye, usually on the surface of
sediments. This method was applied in 24 of the 44 sediment
studies, 20 of which focused exclusively on plastic pellets.
Sampling for plastic pellets is often selective, because their size
range (16 mm diameter) makes them easily recognizable in
the flotsam deposits of sandy beaches. However, when
microplastics are mixed with other debris or have no
characteristic shapes (i.e., irregular, rough, angular) there is a
great risk of overlooking them,
50
and particular care needs to be
taken when selectively sampling them in the field.
Bulk samples refer to samples where the entire volume of the
sample is taken without reducing it during the sampling process
(18 sediment studies and one seawater study). Bulk samples are
most appropriate when microplastics cannot be easily identified
visually because (i) they are covered by sediment particles, (ii)
their abundance is small requiring sorting/filtering of large
volumes of sediment/water, or (iii) they are too small to be
identified with the naked eye.
Volume-reduced samples in both sediment and seawater
samples refer to samples where the volume of the bulk sample
is usually reduced during sampling, preserving only that portion
of the sample that is of interest for further processing. Three
sediment studies, 29 sea surface studies and six water column
studies applied this method. For sedimentary environments,
samples can be sieved directly on the beach or onboard the
vessel
45
while for seawater samples, volume-reduced samples
are usually obtained by filtering large volumes of water with
nets. Bulk and volume-reduced samples require further
processing in the laboratory.
Sediment Samples. Forty-four studies determined the
densities of microplastics in sedimentary environments, mostly
on sandy beaches. The number of beaches sampled in each
study ranged from one
33,42,43
to 300 beaches in a study from
New Zealand.
51
Most studies examined between 5 and 18
beaches (e.g., refs 40 and 5254).
The specific tidal zone sampled on a beach varied
considerably among studies; some covered the entire extent
of the beach, from the intertidal to the supralittoral zone (Table 2).
Some studies distinguished several littoral zones,
21
while others
pooled samples across different zones.
40,55
The majority of
studies, however, focused on the most recent flotsam deposited
at the high tide line. A considerable number of studies (eight of
44 studies) did not explicitly mention the tidal zone where
samples were taken.
This lack of uniformity across the reviewed studies shows the
need to examine systematically where microplastics tend to
accumulate across the beach zone. In a comparison of 10
different sampling methods, the highest amounts of macrolitter
were usually found in the upper beach zones.
56
Pellets also
Table 1. Principal Objectives of the Examined Studies on
Microplastics
a
objectives n=44
sediment n=30
sea surface n=7
water column n=68
total
methodology 2 2 - 3
presence/absence 4 2 1 5
spatial distribution 25 22 5 45
temporal variability 3 6 1 9
dispersal processes 1 - - 1
physical properties and
fragmentation processes 42 - 6
contaminants 12 3 1 13
a
Note that some studies have several objectives and thus the sum may
exceed the total number of studies in each column.
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accumulate near the high tide line, but large quantities are also
found in ditches and trenches on the back beach.
52
In general,
microplastics move differently than macroplastics in the sea:
the distribution of macroplastics can often be explained by the
prevailing currents and wind, while the mechanisms that drive
the distribution of microplastics are less well-known and are
possibly influenced by particle aggregation or animal activ-
ities.
36
Comparative studies should be conducted to determine
the accumulation dynamics of microplastics along gradients of
wave exposure and tidal height. Studies from the subtidal zone
revealed that microplastics were more abundant in subtidal
sediments than on sandy beaches and in estuarine hab-
itats.
8,21,57
Sampling tools were reported in 31 of the 44 reviewed
sediment studies. Selective sediment samples for plastic pellets
and fragments were taken with tweezers,
18,46
tablespoons,
42
or
picked up by hand.
15,52,55
Twenty-eight studies sampled at the
high tide line, using different approaches: (i) sampling a linear
extension along the strandline with a spoon and/or a trowel
(eight studies), (ii) sampling an areal extension using quadrats
(13 studies), and (iii) sampling different depth strata using
corers (two studies). Five studies did not report the exact
sampling procedure. The three studies from the sublittoral
zone
8,21,57
sampled with Ekman and van Veen grabs.
Sampling units were directly related to the sampling
instrument used. Studies that sampled a specific areal extension
(from 0.0079 to 5 m2) employed quadrats and corers. Other
sampling units were weight (from 0.15 to 10 kg) and volume of
sediment (from 0.1 to 8 L).
Samples were taken to variable depths below the sediment
surface. Twenty-seven studies did not mention the sampled
depth. Reported sampling depths ranged from 0 to 32 cm
(Table 3). Most studies sampled a single depth layer within the
top 5 cm of sediment.
42,58,59
Two studies sampled a second
layer at 10 cm depth, in addition to the sediment surface.
32,40
Two studies followed a stratified sampling scheme using a corer
down to a depth of 25 cm, separating the core into five layers
each of which had a thickness of 5 cm,
60
and to a depth of 28
and 32 cm, with four sediment layers of 7 and 8 cm,
respectively.
57
Given that beaches and subtidal coastal habitats are dynamic
systems with continuous and seasonal erosion of sediment
61,62
microplastics may become buried in sediment during periods of
accretion. Furthermore, beaches filter and retain particulate
organic matter (POM) over a range of depths, and sediments
between 0 and 5 cm depth are characterized by steep gradients
and strong seasonal variation of more fine-grained particles and
POM.
61
In permeable sands, microplastics might accumulate in
similar ways as sediment particles and POM, resulting in
microplastics being trapped in deeper sediment layers;
63,64
this
should be examined with stratified samples using cores.
59,60
Sea Surface and Water Column Samples. Thirty-three
studies investigated the abundance of pelagic microplastics (sea
surface and water column). Twenty-six of these studies sampled
exclusively at the sea surface. The depth of the surface layer
sampled was not specified in most cases, with the exception of
three studies that sampled neuston layers of 5060 μm (surface
microlayer), 15 cm, and 25 cm thickness.
32,65,66
Water column
samples were taken from 1 to 212 m depth.
32,65
Sea surface samples were mostly taken by neuston nets
(Table 4). The main advantage of using a net is that large
volumes of water can be sampled quickly, only retaining the
volume-reduced sample. There was only one study that
collected bulk samples of 10 L using a rotating drum sampler.
32
This sampler consists of a partly immersed rotating glass
cylinder with a clean hydrophilic surface, using capillary force to
sample water from the surface microlayer (1 to 1000 μm
thick).
6769
For water column samples, zooplankton nets were
used most commonly (Table 4). Other instruments, such as the
continuous plankton recorder (CPR)
21
and an epibenthic
sled,
70
were also used.
The most relevant characteristics of the sampling nets used
are the mesh size and the opening area of the net. Mesh sizes
ranged from 0.053 to 3 mm, with a majority of the studies (42%)
ranging from 0.30 to 0.39 mm (Table 5). The net aperture for
rectangular openings of neuston nets (sea surface) ranged from
0.03 to 2.0 m2.
31,37
For circular-bongo nets (water column) the
netaperturerangedfrom0.79to1.58m
2.
20,65
The length of the
net for sea surface samples was mentioned in 14 studies, varying
from 1.0 to 8.5 m, with most nets being 3.0 to 4.5 m long. The
length of the net for water column samples was only specified in
one study that used two different sampling devices: 3 m long
bongo nets and an epibenthic sled with a net that was 1 m long.
70
Table 2. Beach Zones Where Sediment Samples Were
Taken
a
beach zones no.
supralittoral 13
high tide line 28
high intertidal zone 14
mid intertidal zone 13
low intertidal zone 13
sublittoral 3
not specified 8
a
N= 44 studies; note that the sum of sampled zones exceeds the total
number of studies, because several studies sampled more than one
beach zone.
Table 3. Depth Strata Sampled for Microplastic Abundance
from Sediments
a
depths of samples no.
0516
510 4
1020 6
>20 4
not described 27
a
N= 44 studies; note that the sum of sampled depth layers exceeds
the total number of studies, because some studies sampled more than
one layer.
Table 4. Instruments Used for Sampling of Suspended
Microplastics
a
instrument sea surface water column
bongo/zooplankton net - 5
other plankton samplers - 3
neuston net 28 -
other neuston samplers 1 -
a
N= 33 studies; note that the sum of studies using each instrument
exceeds the total number of studies, because several studies took water
column and sea surface samples.
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SAMPLE PROCESSING
Laboratory processing and subsequent sorting of microplastics
is essential for bulk and volume-reduced samples. Four main
steps can be distinguished during laboratory processing of
samples: density separation, filtration, sieving, and visual sorting
(Figure 1).
Density Separation. The specific density of plastic
particles can vary considerably depending on the type of
polymer and the manufacturing process. Density values for
plastics range from 0.8 to 1.4 g cm3, specifically for
polypropylene from 0.85 to 0.94 g cm3, polyethylene from
0.92 to 0.97 g cm3, and for polystyrene from <0.05 to 1.00 g
cm3
71
(for specific densities of plastic polymers see Table 7).
These values refer to virgin resins, without taking into account
the effect on density of various additives that might be added
during product manufacturing. Typical densities for sand or
other sediments are 2.65 g cm3. This difference is exploited to
separate the lighter plastic particles from the heavier sediment
grains by mixing a sediment sample with a saturated solution
and shaking it for a certain amount of time. After mixing,
the sediment is expected to rapidly settle to the bottom, while
the low density particles remain in suspension or float to the
surface of the solution. Subsequently, the supernatant with the
plastic particles is extracted for further processing.
Ten of the 13 sediment studies that included density
separation (Figure 1a-c,e) applied a concentrated saline NaCl
solution (1.2 g cm3). Other solutions applied were a sodium
polytungstate solution with a density of 1.4 g cm3,
40
tapwater,
50
and seawater.
72
Plastics that float in fresh and
seawater are polystyrene in foamed form, high and low den-
sity polyethylene, and polypropylene. Polystyrene in solid form
also floats in a hypersaturated saline solution. Finally, the
plastics that float in sodium polytungstate solution include
flexible and rigid polyvinyl chloride (PVCs), polyethylene
terephthalate (PETs), and nylon.
1,50
A similar density
separation procedure was used in two seawater studies, placing
thesampleinfreshwaterandremovingfloatingparticlesfromthe
supernatant.
70,73
Shaking time varied widely between studies, according to the
size of the sediment sample: it ranged from 30 s
21
up to 2 h.
74
The time used for the suspended particles to settle down after
shaking ranged from 2 min
21,36
up to 6 h.
32
Recent studies
explicitly mentioned a repetition of density separation of the
sample remains.
8,59
Unpublished work had shown that additional
microplastic fragments can be recovered after an initial extraction
(R.C.Thompson,personalcommunication).Asaconsequence
three studies have since used several sequential extraction steps as
being an efficient approach to maximize recovery in relation to
sampling time.
8,57,59
Filtration. The plastic particles are separated from the
supernatant obtained from the density separation by passing
the solution that contains the plastic particles over a filter,
usually aided by a vacuum.
21,32
Filter papers had pore sizes of
1 to 1.6 μm (six studies) and 2 μm (one study).
66
In the study
Table 5. Different Mesh Sizes Used for Sampling of
Suspended Microplastics
a
mesh size (μm) sea surface water column
50290 2 2
300390 13 2
400505 5 1
6001000 6 -
15003000 3 -
not specified 6 2
a
N= 33 studies; note that the sum of cases exceeds the total number
of studies, because several studies took both water column and sea
surface samples.
Figure 1. Steps for sample processing. (a) Bulk sediment sample separated by density difference and filtering; (b) Bulk sediment sample separated
by density difference, where floating microplastics are picked up from the supernatant; (c) Bulk sediment sample separated by density difference and
sieving; (d) Bulk sediment sample that is reduced by sieving; (e) Volume-reduced sediment sample separated by density difference, where floating
microplastics are picked up from the supernatant; (f) Volume-reduced sediment samples where floating microplastics are picked up from the
supernatant; (g) Bulk seawater sample passed through filter for microplastics separation; (h) Volume-reduced seawater sample passed through the
filtration step; (i) Volume-reduced seawater sample passed through sieves; (j) Volume-reduced seawater sample passed straight through visual
sorting. All methodologies included a final step of visual separation of microplastics; in some cases the identity of the pieces extracted was then
confirmed by an additional step such as FT-IR spectroscopy (n= 28, see Figure 5B). N= 43 studies that employed the respective sequence of
processing steps.
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where density separation was done with freshwater, the
microplastics were picked up by tweezers from the surface of
the aqueous supernatant.
50
In order to sort out larger particles
before the filtration step, water samples can first be sieved over
a sieve with 500 μm mesh size.
1
None of the reviewed studies specified in detail how the
supernatant was extracted. Some fraction of the microplastics
might stick to the wall of the receptacle that contains the
solution, losing part of the sample during the procedure. Thus
washing of the container walls onto the filter is recommended.
Furthermore, samples can also become contaminated by
particles present in the air of the laboratory, on the clothes
of workers, in poorly cleaned instruments, by improperly sealed
samples, by plaques of color scratched off the ship wall by the
sampling gear, or by fibers broken off the plastic nets that are
used for seawater sampling. For accurate results it is necessary
to minimize sources of contamination, and avoid both loss of
parts from the sample or overestimation of microplastics due to
contamination. Control samples should always be used to
confirm that there is no procedural sample contamination.
66
Several contamination prevention strategies (e.g., sealing of
filters in Petri dishes during drying) have been specified.
21,32,74
Some laboratories use the procedures described above (R. C.
Thompson, personal communication), but it is recommended
to standardize those and also run cross-calibrations among
laboratories.
Sieving. Microplastics can be separated from samples using
sieves of variable mesh sizes. Materials retained in the sieve are
collected (and sorted), while those that pass through are usually
discarded. The use of sieves with different mesh sizes allows
distinguishing size categories of microplastics. Sieving was used
by six sediment (Figure 1c-d) and five sea surface studies
(Figure 1i). The sediment studies employed either one
sieve
57,72,75,76
or sieve cascades of two
33
and three sieves.
29
Sea surface studies used one,
77
five,
37,78
and six sieves.
70,79
The
sieves had mesh sizes ranging from 0.038 to 4.75 mm. All
studies included a sieve of 1 mm, except for the studies that
used only one sieve with mesh sizes ranging from 0.038 mm,
57
to 0.33 mm
72,77
and 2 mm,
53,76
respectively.
Visual Sorting and Separation. In all reviewed studies,
visual examination of the concentrated sample remains is an
obligatory step. Careful visual sorting of residues is necessary to
separate the plastics from other materials, such as organic debris
(shell fragments, animals parts, dried algae, or seagrasses, etc.)
and other items (metal paint coatings, tar, glass, etc.). This is
done by direct examination of the sample by the naked eye or
with the aid of a dissecting microscope.
12,65,7880
Most sea
surface studies separated microplastics by visual sorting of the
particles (Figure 1j) that were retained in the cod end of the
net.
28,30,81,82
The silk screen from the CPR is examined under
the dissecting microscope for plastic particles.
21
Previously isolated plastic fragments can also be washed for
removing other substances that adhere to their surface (like
sand and soil);
29
e.g. by ultrasonic cleaning in a liquid medium
or deionized water.
42
Samples can be preserved in their original
form without initial sorting, or they can be immediately sorted
to store only the plastics from the original sample. Plastics
separated from the sample should be dried and kept in a dark
and temperature-controlled environment (stable room temper-
ature) to reduce degradation during storage.
To avoid misidentification and underestimation of micro-
plastics it is necessary to standardize the plastic particle selec-
tion, following certain criteria to guarantee proper identifica-
tion. This is particularly important when it is not possible to use
more accurate methods, such as Fourier transform infrared
spectroscopy (FT-IR). Pieces of microplastics toward the larger
end of the size range (>1 mm) can to some extent be visually
distinguished according to the following criteria: no cellular or
organic structures are visible, fibers should be equally thick
throughout their entire length, particles must present clear and
homogeneous colors, and if they are transparent or white, they
must be examined under high magnification and a fluorescence
microscope.
66
New methods to separate microplastics from
bulk samples or from samples with large amounts of organic
debris need to be developed to improve the efficiency of
sampling programs. Molecular mapping made by focal plane
array (FPA)-based imaging has recently been examined to
detect microplastics by scanning the surface of filters obtained
from density separation and filtration of samples.
83
Enzymatic
digestion of organic debris and other approaches could also be
explored to facilitate the visual sorting of microplastics from
large sample volumes. Electrostatic separation of plastics from
sediments was extensively examined by one of us (R. C.
Thompson, unpublished data), and while this proved efficient
in separating known plastics spiked into specific sediments, it
was not advantageous in the separation of mixtures of polymers
from more diverse natural sediments and organic matter.
Table 7. Number of Studies That Identified Polymer Type
among the Sorted Microplastic Debris and Specific Densities
of Different Polymer Types
a
polymer type polymer density (g cm3) no. of studies
polyethylene 0.9170.965 33
polypropylene 0.90.91 27
polystyrene 1.041.1 17
polyamide (nylon) 1.021.05 7
polyester 1.242.3 4
acrylic 1.091.20 4
polyoximethylene 1.411.61 4
polyvinyl alcohol 1.191.31 3
polyvinylchloride 1.161.58 2
poly methylacrylate 1.171.20 2
polyethylene terephthalate 1.371.45 1
alkyd 1.242.10 1
polyurethane 1.2 1
a
Data from a total of N= 42 studies.
Table 6. Categories Used To Describe Microplastics
categories
sources consumer product fragments (e.g., fishing net) and raw industrial
pellets
type plastic fragments, pellets, filaments, plastic films, foamed plastic,
granules, and styrofoam
shape for pellets: cylindrical, disks, flat, ovoid, spheruloids
for fragments: rounded, subrounded, subangular, angular
general: irregular, elongated, degraded, rough, and broken edges
erosion fresh, unweathered, incipient alteration, and level of crazing
(conchoidal fractures), weathered, grooves, irregular surface,
jagged fragments, linear fractures, subparallel ridges, and very
degraded
color transparent, crystalline, white, clear-white-cream, red, orange, blue,
opaque, black, gray, brown, green, pink, tan, yellow, and
pigmentation
Environmental Science & Technology Critical Review
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CHARACTERIZATION OF MICROPLASTICS
Due to the diversity of sources, there exists a wide variety of
microplastics with multiple shapes, sizes, and origins (Table 6).
The characteristics of microplastics determine their distribution
and impact in the environment. For instance, dense plastic
particles spend more time in contact and collide more forcefully
with abrasive sediment particles than lighter microplastics do.
36
These differences are important because they can affect degra-
dation rates, surface characteristics, and shapes of microplastic
particles.
Size Fractions. The term microplasticswas first used in
the year 2004 and is associated with a classification based on
size.
21
There is no general consensus about a specific size
nomenclature, although it has been suggested that microplastics
should be defined as particles <5 mm.
9
The studies reviewed
here identified a wider range of sizes. Although not all studies
referred exclusively to microplastics, they nonetheless
classified micro- and small plastics, generally items <20 mm.
The minimum size of the collected microplastics directly
depends on the sampling and processing methods. For
sediment samples that were sieved, the minimum sizes of
collected microplastics ranged from 0.5 to 2 mm.
33,84
Similarly,
the minimum size of microplastics collected from seawater
samples is determined by the mesh size of the net. Mesh sizes
varied from 53 μm to 3 mm for seawater samples (Table 5).
During sample processing, the sizes of microplastics obtained
from bulk seawater and sediment samples are limited by the
pore size of the filters (1.6 to 2 μm).
21,66
The size ranges of the collected microplastics varied widely
among the reviewed studies. Forty-seven studies reported
values for minimum and maximum sizes of microplastics; 16 of
these studies were related to pellets. Plastic preproduction
pellets have a reference diameter of 1 to 5 mm
15
with a typical
diameter of 3.5 mm.
38
The values obtained by the reviewed
studies were close to these reference values, with a range of 1 to
6 mm. Fifteen sediment studies reported the size range for
plastic fragments, with size ranges of 1 μmto20mm(Figure2a).
The size ranges of microplastics in sea surface samples were
reported in 17 studies (Figure 2b). The widest size range from
sea surface studies was from 0.5 to 29 mm.
38
Eight studies were
consistent with upper size recommendations in Arthur et al.
(2009)
9
and classified the plastic particles with a maximum
Figure 2. Size ranges for microplastics from (a) sediment, (b) sea surface, and (c) water column studies. Only those studies that provided the lower
and upper size limits of microplastics are shown (excluding studies of plastic pellets because here size limits are dictated by pellet sizes).
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value of 5 mm and a minimum value according to the mesh size
of the nets (ranging from 0.335 to 5 mm).
79
Three of the seven water column studies presented size range
values. The maximum value reported was 10 mm
65
(Figure 2c).
The minimum value for microplastics in water samples (sea
surface and water column) was 1.6 μm
4
although this study did
not mention the maximum value of the collected plastic
fragments.
Some of the reviewed studies also distinguished different size
classes of microplastics (Figure 3). Size categories were given in
six sediment studies (Figure 3A), mostly in those that used
sieve cascades during sample processing. The largest propor-
tion of plastic fragments was obtained in the size classes of 1 to
5 mm.
29,33,75
One study that used an in situsieving step (1 mm)
and density separation (Figure 1e) also presented size
categories. Here, plastic pellets represented 58.3% of all
microplastics.
50
Seven sea surface studies reported size categories (Figure 3B).
Studies with a sieving step retained smaller particles than studies
that only used visual sorting. Most plastic items identified in these
studies were between 0.25 to 5 mm in diameter,
30,37,65,70,78
but
many were also >4.8 mm.
79
Only two water column studies
Figure 3. Distribution of plastic particles among size categories. (A) Sediment studies: (a) ref 60, (b) ref 59, (c) ref 33, (d) ref 29, (e) ref 75,
(f) ref 50. (B) Sea surface studies: (a) ref 78, (b) ref 70 (c), ref 37, (d) ref 79, (e) ref 80, (f) ref 65, (g) ref 30. (C) Water column studies: (a) ref 70,
(b) ref 65.
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distinguished size classes (Figure 3C), based on sieving and visual
sorting. Most microplastics were found in the size classes 0.5 to
1mm
65
and 2.8 to 4.75 mm.
70
No minimum size has been defined for microplastics. The
smallest reported size was 1 μm diameter and 20 μm length
in sediment samples.
45
Most studies presented values above
500 μm for sediment samples and 300 μm for seawater
samples. This differentiation depended directly on two main
factors: the tools used during sampling and the processing
steps. Particles >500 μm are retained in standard sieves and can
then be sorted using a dissecting microscope. Particles <500 μm
were usually only obtained by studies with density separation and
filtration, and particles <2 μm are unlikely to be sampled
representatively.
36
Microplastics occur in size ranges that are similar to many
organisms from benthos and plankton communities. The same
applies to geological sediment categories, which are mobilized
in similar ways as microplastics. Size ranges of microplastics can
be related to the geological categories of silt and sand,
85
micro-,
meio-, and macrofauna for benthos, and micro- and macro-
plankton (Figure 4). Due to these overlaps, microplastics are
frequently obtained in studies targeting specific benthos or
plankton size categories.
37,79
The strong overlap between
important size categories of benthic and planktonic organisms
also highlights the potential for microplastic ingestion by a wide
variety of organisms.
Based on the preceding comparison and on presently
employed methods to quantify microplastics in the environ-
ment, it may be useful methodologically to distinguish two
main size categories of microplastics: (i) < 500 μm (with the
lower size limit, which should be stated, being restricted by
technological constraints of identification equipment), and (ii)
500 μm to 5 mm (Figure 4a). In the future, samples could be
sieved over a 500 μm mesh, and the fraction passing through
the sieve should then be analyzed by density separation and
filtering. This would ensure that those studies that use the
density separation and filtration technique to quantify the smaller
size fractions of microplastics (<500 μm) can also be compared
with other studies that only employ sieves to quantify the large
fraction of microplastics (500 μmto5mm).Beyondthose
fractions it is very likely that there are even smaller fragments that
are in the range of nanoparticles. Identifying and quantifying
those nanoplastics will require new and innovative methods.
1
Morphology and Physical Characterization of Micro-
plastics. The number of categories used to classify micro-
plastics depends on the criteria of the respective authors, which
can vary widely. Fifty-four of the 68 reviewed studies offered
morphological descriptions of microplastics, referring to origin,
type, shape, color, and/or degradation stage of the particles
(Table 6). Eighteen of these studies corresponded exclusively
to studies on plastic pellets. The variation in sizes, shapes,
and colors of microplastics is of particular concern since they
could easily be mistaken for food by marine organisms and
seabirds.
21,22,86
Sources and Types of Microplastics. Sources of plastic
pellets were mainly associated to plastic-processing plants close
to study sites.
76,87
However, plastic pellets have also been found
on urban beaches distant from potential sources, implying
Figure 4. Categories of particle sizes. (a) Suggested methods-oriented categories of microplastics based on this review: 1 to <500 and 500 μmto5
mm, (b) size range of microplastics given by the reviewed studies (based on Figure 2), (c) litter,
120
(d) debris,
5
(e) plankton,
121,122
(f) benthos,
123
(g) geology.
85
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long-distance marine transport.
33
Some properties of plastic
pellets may change during residence at sea. For example, the
specific density of pellets decreased during prolonged exposure
to the marine environment, from 0.85 to 0.81 g cm3for high
density polyethylene (HDPE) and from 1.41 to 1.24 g cm3for
polystyrene (PS).
80
Buoyancy and density of plastics may
change during their residence at sea due to weathering and bio-
fouling.
80,88
This can be seen in the fact that the specific densities
of many pelagic microplastics do not coincide with that of primary
polymers (Figure 5). In the open ocean microplastics with high
specific density (negative buoyancy) will quickly sink and are thus
absent from neuston samples (Figure 5).
Fragments from plastic consumer products were of variable
types and diverse origins. These particles have been described
as embrittled and weathered, irregularly shaped and sized
degradational chunks of plastic,
51
with sharp, broken edges.
78
The origin of these fragments can be fishing nets, line fibers
(polypropylene strands), thin plastic films, industrial raw
material (e.g., from ship breaking industry), pellets or polymer
fragments of oxo-biodegradable plastic.
65,70,74,89
Other partic-
ular sources of microplastics are facial cleansers
7
and small
polyethylene microplastics or polyester fibers of low density
which escape from treatment screens at wastewater plants and
eventually arrive in the ocean.
8
Shape and Erosion of Microplastics. Microplastics vary in
shape from irregular to spherical and long-thin fibers. Plastic
pellets can have tablet-like, oblong, cylindrical, spherical, and
disk shapes, mostly spherical to ovoid with rounded ends.
52
Most fragments found in subtidal and estuarine sediments were
fibers.
8,21,36
The shape of plastic fragments depends on the frag-
mentation process as well as residence time in the environment.
Sharp edges might indicate either recent introduction into the
sea or the recent break-up of larger pieces, while smooth edges
are often associated with older fragments that have been con-
tinuously polished by other particles or sediment.
27,65
Circularity varied inversely with particle size. Larger particles
had more elongate shapes and/or irregular surfaces, while
progressively smaller particles were consistently more circular.
38
Likely, particles continue to fragment and degrade to ever
smaller particles over time.
1,65
Degradation and erosion of the particle surface are caused by
biological breakdown, photodegradation, chemical weathering,
or physical forces (wave action, wind, sand-blasting).
1
This can
cause visible cracks on the plastic surface, producing a wide
variety of different particle shapes.
78
Scanning electron
microscopy revealed that angular and subangular particles
featured conchoidal fractures, while rounded particles had
linear fractures and adhering particles.
40
Numerous surface
scratches on predominantly eroded angular plastic fragments
(<1 cm2) may be caused by continuous particleparticle
collision.
42
Pellets that presented a degree of weathering have
been termed eroded or weathered plastic pellets.
76,90,91
Many of
the plastic pellets found in a study on New Zealand beaches
were fresh but some showed degradation and embrittlement.
51
Surface abrasion is also caused by physical degradation and
oxidative aging of plastic particles in response to ultraviolet and
infrared components from solar radiation.
1,73
Plastic fragments
found in scats of fur seals Arctocephalus spp. also had clear
abrasion marks, presumably generated either in the digestive
tract of the seals or during physical breakdown on nearby
cobble beaches.
92
The surface texture of microplastics may affect the con-
centrations of sorbed chemicals. Pollutant sorption to plastic
pellets increases with the surface area as a result of weathering,
Figure 5. (A) Frequency of microplastics of different specific densities found (a) at the sea surface and (b) in beach sediments. Broken vertical line
indicates the specific density of seawater and bold horizontal lines show the specific densities of particular polymers (based on ref 80); PP:
Polypropylene, HDPE: High density polyethylene, LDPE: Low density polyethylene, PS: Polystyrene, PVC: Polyvinyl chloride, PET: Polyethylene
terephthalate. Reprinted from Moré
t-Ferguson et al. The size, mass, and composition of plastic debris in the western North Atlantic Ocean. Mar.
Pollut. Bull.2010,60, 18731878 (ref 80), with permission from Elsevier. (B) Fourier transform infrared spectroscopy (FT-IR) spectra of some
common plastic polymers. Spectra obtained from Bruker Optics ATR-Polymer Library, A Collection of Synthetic Fibres, Copyright 2004 Bruker
Optic GmbH.
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which enhances the sorbates effective diffusivity.
15
Although
plastics undergo various physical and chemical changes as they
age, no method exists at present that allows for determining
how long the particles have been in the marine environment
(unless they have characteristics that can be traced to a specific
point source such as a ship or container wreck). However, this
information would be invaluable to estimate drift trajectories of
floating microplastics and, thus, potential source regions.
Color of Microplastics. The colors of microplastics were
characterized in 24 of the 68 reviewed studies, revealing a
diverse range of colors (Table 6). The most common colors
found were white or related (e.g., discolored yellow, clear-
white-cream). Color can facilitate separation in situations where
microplastics are scattered among large quantities of other
debris. Particles with eye-catching colors have a high probability
of being isolated for subsequent identification as microplastics,
while those with dull colors are easily overlooked, thus potentially
introducing bias.
Colors have been used for a preliminary identification of the
chemical composition of the most common pellets.
52
Clear and
transparent plastic pellets have been ascribed to polypropylene
(PP), and white plastic pellets to polyethylene (PE),
58
but for
conclusive identification further analyses are required (see
below). Low density PE has opaque colors, while ethyl vinyl
acetate corresponds to clear and almost transparent pellets.
93
Color has also been used as an index of photodegradation and
residence time at the sea surface
17,94
and the degree of tarring
or weathering.
55
It has been suggested that discolored PE
pellets may contain higher amounts of PCBs than non-
discolored pellets, because the discoloration process (yellowing)
is indicative of longer exposure time to seawater, which enhances
the chances of the polymers becoming oxidized.
43
Black and aged
pellets, essentially those composed of polystyrene (PS) and PP,
presented the highest diversity of adsorbed pollutants for both
PAHs and PCBs.
45
Identifying the Chemical Composition of Micro-
plastics. Plastics are synthetic polymers made from a wide
range of chemical compounds with different characteristics
each. Forty-two of the reviewed studies described the chemical
composition of microplastics, although not all of them con-
ducted rigorous chemical analyses. The most common polymers
identified in the reviewed studies were PE, PP, and polystyrene
(Table 7), which is commonly used for packaging.
36
Several methods have been employed to identify microplastic
polymers. Identification based on infrared (IR) spectroscopy
was used in 28 studies. This method compares the IR spectrum
of an unknown plastic sample with spectra of known polymers
(Figure 5B). The different types of spectroscopy applied for
microplastics identification were infrared spectrophotometer,
51
Fourier transform infrared spectroscopy (FT-IR),
21,45,74
and
near-infrared spectrometer.
46
A range of common polymers like
PP, PE, and polyester can be identified by these techniques.
8
Another chemical analysis is the Raman spectroscopy
23
which
also gives information about the crystalline structure of the
polymer.
95
Also, a differential scanning calorimeter was used in
one study,
96
where temperature is applied simultaneously to an
unknown sample and a reference material.
Characteristic smoke during combustion and solvent assays
have also been used to determine the polymers that micro-
plastics are made of.
27,51,73,97
Synthetic polymers can also be
identified using the specific density of the particles (see also
Figure 5A) and to a lesser extent other characteristics such as
color. The density-based identification method has been
applied in two studies,
80,98
in which a sample is placed in
distilled water and ethanol or concentrated solutions of calcium
or strontium chloride are titrated until the plastic piece is neutrally
buoyant.
The use of certain characteristics, such as specific density and
color, seem useful methods for rapid and economic polymer
identification of plastic pellets, because those characteristics
have been described for virgin pellets.
80,93
Nonetheless, for
plastic fragments this method cannot be applied, because their
shape (an indicator for recent fragmentation or prolonged
persistence and erosion along the edges) and color are more
variable and are unlikely associated with a specific polymer
type. Microparticles of unknown origin might also be
erroneously characterized as microplastics, a problem that
increases considerably with decreasing particle size. For that
reason, the use of spectroscopy (FT-IR spectroscopy, near-
infrared spectroscopy, and Raman spectroscopy) is strongly re-
commended for small plastic fragments, because it can
determine the chemical composition of unknown plastic
fragments with high reliability. This step is critical since up to
70% of particles that visually resemble microplastics are not
confirmed as plastics by FT-IR spectroscopy (R. C. Thompson,
personal communication). Alternative spectroscopic techni-
ques, like attenuated total reflectance (ATR) FT-IR spectros-
copy, could also facilitate the identification of irregularly shaped
microplastics that cannot be identified by FT-IR spectrosco-
py,
83
but the main disadvantage is the high cost of this
instrument.
95
QUANTIFICATION OF MICROPLASTICS
Quantitative data were reported in 60 of the 68 reviewed
studies, which (i) examined the spatial and temporal dis-
tribution of microplastics, (ii) analyzed methodological aspects,
(iii) quantified organic pollutants and metals, and (iv)
determined rates of accumulation of microplastics on beaches.
Mass and abundance of microplastics was determined in 13 and
39 studies, respectively (Table 8). For sediment samples the
most commonly used units for mass were grams of micro-
plastics per m2and for abundance microplastic items per m2
(or items cm2). For sea surface samples, grams per m2and
items per m2were the most commonly used values for mass
and abundance, respectively, although a considerable number of
studies also reported items per volume(items m3). Only one
study for water column samples quantified mass values in
milligrams per m3, while abundance was mostly reported as
items per m3. The value of pieces per day, the plastic
Table 8. Various Quantitative Units Used in the Reviewed
Studies
a
results units N=22
sediment N= 21 sea
surface N= 5 water
column
Abundance (N= 39)
items per m213 13 -
items per m356 5
items per m strandline 3 - -
items per kg sediment 1 - -
Mass (N= 13)
grams per m226 -
grams per m3-2 1
grams per gram
sediment 3- -
a
N= 43 studies; note that the number of studies with the respective
units exceeds the total number of studies.
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replenishment rate on Hawaiian beaches, was estimated by one
study.
42
Values for abundances ranged from 0.21 to more than 77,000
items m2in sediment samples.
52,72
These values are
substantially higher than those from sea surface samples,
which ranged from 8 ×105to 5 items m2,
76,99
i.e. there were
several orders of magnitude difference between the two
environments (Figure 6a). Abundance values per volume
ranged from 0.022 to 8,654 items m3at the sea surface
38,66
and from 0.014 to 12.51 items m3in the water column.
65,81
Also in this case, sediment samples contained substantially higher
amounts of microplastics, ranging from 185 to 80,000 items
m3.
36,59
The three studies that obtained subtidal sediments
reported abundances of 5syntheticfibers50mL
1sediment,
21
0.8 synthetic fibers 50 mL1sediment,
8
and 97 particles kg1
dry sediment,
57
which scales up to 115,000, 18,000, and
125,000 items m3, respectively (Figure 6a).
The fact that seawater samples correspond to volume-
reduced samples taken with nets might explain the lower
abundances of microplastics in these environments, compared
to sediments (Figure 6a). Abundances of microplastics are
likely directly related to the mesh size of the net. A possible loss
of particles <1 mm passing through a mesh size of 0.95 mm has
been suggested to impede the comparison with other studies
that used a net with smaller mesh size.
87
The retention
efficiency differs substantially between an 80 μm mesh and a
Figure 6. (a) Comparison of microplastic abundance in sediment, sea surface, and water column environments. The units are expressed on a
logarithmic scale for items m2(left axis, diamonds) and items m3(right axis, squares and triangles). Values for subtidal sediments are highlighted
by gray triangles. Note that values were scaled up for sediment samples (areas or volumes of replicates generally comprise 10100 cm2or 10100
cm3, respectively), while values for seawater samples were scaled down (replicates generally sample areas/volumes 101000 m2or 10100 m3,
respectively). Overlapping dots were separated to show all data points. (b) Relationship between the mesh size of the nets and abundance
(logarithmic scale) of microplastics in sea surface samples as items per m2. (c) Relationship between the mesh size of the nets and abundance
(logarithmic scale) of microplastics in sea surface (gray diamonds) and water column (black squares) samples as items per m3. Line shows significant
relationship (p< 0.05) for sea surface samples. Each data point represents one study; if several values were provided for a particular study, we
calculated the mean value.
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450 μm mesh, with an up to 100,000 times higher concen-
tration of small plastic fibers in the former.
66
When comparing among the reviewed studies, there was
indeed a tendency of a negative exponential relationship between
mesh size and abundance for sea surface samples measured as
items per m3(p= 0.024) but not for sea surface measured as
items per m2(p= 0.54) or water column samples (p= 0.35)
measured as items per m3(Figure 6b,c). The lack of a correlation
for some of the examined relationships is probably due to
fundamental variations in microplastic abundances between the
locations where samples were taken, e.g. the open ocean in the
South Atlantic
98
or coastal waters near Cape Province, South
Africa.
28
Most likely, abundances of microplastics also depend on
distances from sources and human population centers, similar as
reported for macroplastics.
100,101
Furthermore, ocean currents can
play an important role in transporting and accumulating micro-
plastics on the shore or in oceanic gyres.
28,36,102
The variable units in which abundance and mass of
microplastics are reported become problematic when different
studies are compared, even though units can be transformed in
some cases. The choice of a specific unit to quantify micro-
plastics is a very relevant topic. We suggest the use of con-
vertible units whenever possible. For example, if sediment
samples are taken in a standardized manner in a specific surface
area and from a specified depth layer, per-area data (items m2)
can be easily converted to per-volume data (items m3) and
both should be reported.
Also, the classification of microplastics is important for
determining abundances and source pathways. Fragments from
plastic products were often numerically dominant, followed by
plastic pellets
28,36,37,65,78,80
and styrofoam as the second most
abundant material.
30
This suggests that at present most
microplastics originate from secondary rather than primary
sources.
38,78,80
While plastic pellets were very abundant
between the 1970s and 1990s,
51,73
their proportions seem to
have decreased in recent years,
25
possibly due to better practices
during pellet transport. Reference collections have proven to be of
extraordinary importance in order to analyze short- and long-term
temporal trends.
21,38,39
A solid analysis of long-term trends requires standardized
procedures in sampling and sample processing. This review on
methods applied in research on microplastics in the marine
environment indicates the importance of standardized
procedures that will maximize comparability of past and future
investigations in pelagic and sedimentary marine environments
(Figure 7). All of these procedures include sieving of bulk or
Figure 7. Suggested sampling schemes for different types of samples to achieve maximum comparability among results from past and future sampling
programs on marine microplastics. All samples should be sieved over a standard mesh, ideally of 500 μm, or alternatively 1000 μm. The material
retained in the sieve should then be identified by Fourier transform infrared spectroscopy (FT-IR). Sieved sediment samples should then be
processed by the density separation and followed by the filtration over a fine filter of about 1 μm. Bulk water samples can be filtered directly after
sieving. Samples taken with nets (sea surface or water column) should also be sieved over a 500 μm mesh; microplastics can then directly be sorted
from the sieve and separately from the sieved water.
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volume-reduced samples to cover the full size range of marine
microplastics adequately and an obligate visual sorting step.
Whenever possible, a stratified sampling strategy should be
applied for sediment samples (seafloor, beaches) to determine
the dynamics of microplastics within marine sediments.
OUTLOOK
The observed differences between types and amounts of
microplastics in the main marine habitat compartments (shore
sediments, sea surface, water column, and seafloor) indicate
that import, export, and residence times of microplastics may
vary depending on their characteristics, mostly size and the
specific density of the polymers.
Microplastics of low specific density are positively buoyant
and thus likely spend a long time at the sea surface (or in the
water column), where they can potentially be transported over
long distances. They can thus be found in remote places, e.g. on
sandy beaches, distant from their sources (Figure 8). However,
particles with low specific density have also been found in
subtidal sediments.
8,21,57
CHN elemental analysis revealed
relatively high contents of nitrogen (N) on microplastics, which
suggested abundant epibiont overgrowth, because N is not a
component of synthetic polymers.
80
Overgrowth by micro- and
macro-organisms causes an increase in specific density and thus
contributes to a loss in buoyancy and sinking of micro-
plastics
86,103
(Figure 8a). In contrast, erosion may lead to
decreases in specific density, thereby enhancing buoyancy.
80
Biofouling and erosion thus cause changes in specific density,
affecting exchange processes between different compartments.
Due to complex interactions between fouling, erosion, and
surface-volume ratios of particles, it can be hypothesized that
temporal changes in buoyancy of microplastics depend on
particle sizes. More research is needed to understand the
interaction between epibionts and microplastics and their
effects on particle buoyancy.
104
While positively buoyant microplastics are widely dispersed
across the worlds oceans, microplastics of high specific density
are negatively buoyant and thus sink more rapidly to the
seafloor; consequently they are expected to accumulate in subtidal
sediments near their sources (Figure 8b). Interestingly, micro-
plastics appear to be rare in deep sea sediment traps,
39
but more
deep sea studies are needed as macroplastic debris is now
relatively common in these habitats.
5
The transfer of microplastics between compartments (shore,
sea surface, water column, seafloor) is likely to vary, and there
are particular areas where these particles have been shown to
accumulate: on the shores,
75
on the seafloor,
105,106
and in the
oceanic gyres.
37,107,108
Given the lack of samples from subtidal
and deep sea sediments, future studies should examine these
marine environments. Also rocky shore samples are markedly
absent from studies on microplastics. This is surprising because
exposed rocky shores should enhance fragmentation of macro-
plastics that are battered against rocks and subsequently ground
down further by large moving boulders.
92,109
Due to the high
hydrodynamic energy, it is likely that rocky shores export
ground-up microplastics. In contrast, salt marshes with low hydro-
dynamic energy probably are retention systems for microplastics
(e.g., particle retention in salt marshes).
110
Quantitative sampling
on complex rocky shores and in salt marshes is challenging
but given the above considerations, it appears important to
estimate plastic fragmentation, transfer, and accumulation in these
habitats.
Smaller plastic pieces are likely to mix with food items
49
and
hence organisms may transport them into other (including
nonmarine) compartments.
92
Seabirds selectively feed on
plastics from the sea surface,
99,111
which can be deposited in
terrestrial habitats by regurgitation, defecation, or decom-
position after death.
112
The types of plastics most commonly
ingested by seabirds are plastic pellets and user plastics, such as
fragments and monofilament lines.
13,113116
For these reasons
seabirds have been used as indicators of changes in the amount
and composition of plastic debris in certain regions.
25,114,117
Stomachs of different seabird species contained plastic particles
mostly between 2 and 8 mm.
118,119
Thus, stomach contents can
only be used to monitor particles in this size range; other
organisms such as invertebrate suspension- and deposit-feeders
may provide opportunities to sample smaller pieces of debris.
Considering that microplastics cannot be effectively removed
from the ocean, future studies are necessary to understand how
biological agents (such as epibionts or seabirds) and abiotic
factors (UV radiation, wave action, currents) affect the transfer,
accumulation, and further breakdown of microplastics and to
describe the potential impacts of this debris. More work is also
needed to identify and reduce/eliminate the sources of
microplastics in the environment. Hopefully, this review will
contribute toward establishing standardized sampling programs
and hence to a more comprehensive understanding of the
sources, sinks, and fluxes of microplastics in the marine
environment.
AUTHOR INFORMATION
Corresponding Author
*Phone: + 56 51 209939. Fax: + 56 51 209812. E-mail: thiel@
ucn.cl.
Notes
The authors declare no competing financial interest.
Figure 8. Schematic figure indicating standing stocks in different
habitat compartments and flux pathways of (a) positively buoyant and
(b) negatively buoyant microplastics in the marine environment: sandy
beaches, subtidal sediments, sea surface, water column, and ocean
floor. Curved black arrows indicate the inputs of microplastics into the
environment, straight black arrows show the fluxes between different
habitat compartments, bold borders around larger boxes highlight the
most likely compartments for accumulation of microplastics (tiny
dots).
Environmental Science & Technology Critical Review
dx.doi.org/10.1021/es2031505 |Environ. Sci. Technol. 2012, 46, 306030753072
ACKNOWLEDGMENTS
We are very grateful to four anonymous reviewers who provided
many constructive comments that helped to improve this
manuscript. Lucas Eastman kindly read the final version of the
manuscript. R.C.T. was supported by DEFRA contract
ME5416, Leverhulme Trust grant F/00/568/C, and NERC
Grant NE/C000994/1.
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Environmental Science & Technology Critical Review
dx.doi.org/10.1021/es2031505 |Environ. Sci. Technol. 2012, 46, 306030753075
... According to different authors (Thompson et al., 2004;Arthur et al., 2009;Shim and Thompson, 2015;Alexander et al., 2016), plastic debris based on their dimensions encountered in macro-(>25 mm), meso-(5-25 mm) and microplastic (<5 mm) and particles range from 1 to 100 nm are considered as nano plastics (Ferreiraa et al., 2019). Microplastics comprise a very heterogeneous assemblage of pieces that vary in size, shape, color, specific density, chemical composition, and other characteristics (Hidalgo-Ruz et al., 2012) and may be pre-formed or derived from the degradation of larger macroplastic debris (Clunies-Ross et al., 2016). ...
... All particles exhibiting a cellular structure were excluded. Any particle visually identified as potentially plastic was further characterized and classified by shape, size, and color, photographed using a polarized light microscope (Leitz Laborlux 11 Pol S equipped with an Opto Capture 2.2 camera) according to the method described by other authors (Hidalgo-Ruz et al., 2012;Waters et al., 2016;Qiu et al., 2016;Worm et al., 2017;Zhoua et al., 2018). Samples observed under a polarized optical microscope (PLOM) (Leitz Laborlux 11 Pol S), in transmitted light, were not only evaluated for their morphology, but also for their refractive index, relief, optical activity, interference color of microplastics, which makes PLOM a semi-qualitative technique for determination and characterization of MPs. ...
... Several features like chemical composition, solid-state, shape, and structure, color, size, and origin were used as criteria for the separation and identification of MPs in different types or groups. Although microscopy is strongly recommended as a simple technique for the identification of microplastics (Hidalgo-Ruz et al., 2012;Dekiff et al., 2014;Lusher et al., 2017;Hartmann et al., 2019), there were some limitations and challenges like the number of measurements to determine particle size (Renner et al., 2018). To overcome these limitations, in this study we successfully used polarized optical microscopy (POM) as a qualitative and semiquantitative technique to better characterize microplastics in terms of abundance, dimensions, and morphology. ...
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... The concentrations of microplastics are significantly lower in the water columns below the sea surface layer compared to the water surface (Duis and Coors, 2016). Hidalgo-Ruz et al. (2012) showed that the concentrations of microplastics in water columns vary between 0.014 and 12.5 items per cubic meter. Desforges et al. (2014) reported that seawater microplastic concentration ranged from 8 to 9180 items per cubic meter at a depth of 4.5 m. ...
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Microdebris ingestion in fish is widespread and has adverse effects on marine life. This study assessed the occurrence and type of microdebris found in three commercially important fish species from different landing sites along Guyana's coast. Visual examination of fish gut content was initially carried out using the naked eye and a hand lens. Microscopic examinations were subsequently carried out to determine the number and type of debris present. Forty percent of the fishes examined had microdebris present in their bodies. A total of 112 microdebris particles were collected from 90 specimens of three species (Bagre bagre, Nebris microps, Macrodon ancyclodon). The microdebris particles observed included pellets, microbeads, fragments, fiber (wool), films, and foams. White-colored materials were the most frequently ingested. Most of the collected materials were large microdebris (>1 to 5 mm) that resembled pellets and microbeads. This study displayed the prevalence of microdebris ingestion by commercial fish in Guyana.
... Microplastics can be further divided into large MPs (1-5 mm) and small MPs (< 1 mm) according to the Guidance on Monitoring of Marine Litter in European Seas of the EU Marine Strategy Framework Directive (MSFD). From an analytical point of view, Hidalgo-Ruz et al. [99] suggested differentiating MPs of 500 μm-5 mm and < 500 μm since the first fraction is suitable for visual sorting and spectroscopic techniques are required to differentiate the second category. Numerous studies have demonstrated the ubiquitous presence of the two size fractions of microplastics in the environment, including water [100], soil [60] and air [101]. ...
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As an emerging pollutant in the life cycle of plastic products, micro/nanoplastics (M/NPs) are increasingly being released into the natural environment. Substantial concerns have been raised regarding the environmental and health impacts of M/NPs. Although diverse M/NPs have been detected in natural environment, most of them display two similar features, i.e.,high surface area and strong binding affinity, which enable extensive interactions between M/NPs and surrounding substances. This results in the formation of coronas, including eco-coronas and bio-coronas, on the plastic surface in different media. In real exposure scenarios, corona formation on M/NPs is inevitable and often displays variable and complex structures. The surface coronas have been found to impact the transportation, uptake, distribution, biotransformation and toxicity of particulates. Different from conventional toxins, packages on M/NPs rather than bare particles are more dangerous. We, therefore, recommend seriously consideration of the role of surface coronas in safety assessments. This review summarizes recent progress on the eco–coronas and bio-coronas of M/NPs, and further discusses the analytical methods to interpret corona structures, highlights the impacts of the corona on toxicity and provides future perspectives.
... All suspected microplastics were counted and categorized by shape (fragment, fiber, and microbead). The following details should be noted when using an optical microscope for microplastics identification: (1) clear and uniform color distribution, (2) the object has no metallic luster, (3) no visible tissue or natural organic structures attached to it, (4) same dimensions throughout the entire length and thickness of the synthetic fiber, (5) there are twists (convolutions), striations, zigzags, or jagged and irregular widths along the natural fibers, and (6) by adjusting the angle of incidence of light and the brightness, the edges of the transparent microplastics will be seen [18,27,28]. ...
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Book
Marine sediments provide the largest habitat on planet earth, yet knowledge of the structure and function of their flora and fauna continues to be poorly described in current textbooks. This concise, readable introduction to benthic ecology builds upon the strengths of the previous edition but has been thoroughly revised throughout to incorporate the new technologies and methods that have allowed a rapid and ongoing development of the field. It explores the relationship between community structure and function, and the selection of global examples ensures an international appeal and relevance. The economic value of marine sediments increases daily, reflected in the text with a new emphasis on the effects of pollution and fisheries and the management of marine sediments. This accessible textbook is suitable for both advanced undergraduate and graduate students who have had a general ecology course, but no further training in benthic ecology. It will also be of relevance and use to professional researchers and consultants in marine ecology and environmental science who seek a compact but comprehensive introduction to benthic ecology.
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Pellets, the form in which plastic is shipped as bulk cargo, are ubiquitous; they apparently enter the marine system at coastal manufacturing and shipping sites. On beaches, greatest amounts of plastics were found on Bermuda and the Bahamas. The occurrence of highest concentrations in the central sub-tropical gyre is attributed to the transport there by water circulation, and islands are considered to act as "sieves', "straining' plastic debris from the surface waters. Continuing input has led to a build-up of plastic in the North Atlantic evident in the last 15 years. The prospects of legislation to ban plastic disposal are considered. -J.Harvey
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To evaluate the incidence of ocean-borne plastic particle ingestion by western North Atlantic seabirds, we analyzed the gut contents of 1033 birds collected off the coast of North Carolina from 1975-1989. Twenty-one of 38 seabird species (55%) contained plastic particles. Procellariiform birds contained the most plastic and the presence of plastic was clearly correlated with feeding mode and diet. Plastic ingestion by procellariiforms increased over the 14 year study period, probably as a result of increasing plastic particle availability. Some seabirds showed a tendency to select specific plastic shapes and colors, indicating that they may be mistaking plastics for potential prey items. We found no evidence that seabird health was affected by the presence of plastic, even in species containing the largest quantities: Northern Fulmars (Fulmarus glacialis), Red Phalaropes (Phalaropus fulicaria) and Greater Shearwaters (Puffinus gravis).
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Resin pellets as a raw material of industrial plastic products are widespread in the coastal waters and beaches of the world. In this study, the distribution and abundance of the pellets were investigated on the coastal area (30 beaches) of Tokyo Bay and Sagami Bay. In most stations surveyed, the pellets were found (93% of total stations) and were particularly abundant at Kugenuma Beach, Nojima Seaside Park, Jonanjima Seaside Park and Kasai Seaside Park. The Highest density of the pellets on a beach exceeded 1,000/m2. From near infra-red spectrometry analyses, the pellets on most beaches were found to be comprised mostly of polyethylene (60%) and polypropylene (35%) which are very common constituents of plastic productions in Japan. Effluent from plastics manufacturers was suggested to be the major source of the pellets in the coastal areas of Tokyo Bay and Sagami Bay.