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The Occurrence, Effects and Fate of Small Plastic Debris in the Oceans

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  • National Fish and Wildlife Foundation (NFWF)

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Purpose This white paper reviews the literature and synthesizes the various topics that will be discussed at the workshop to be held September 9-10, 2008. Topics include (1) the occurrence of microplastics in the environment; (2) the documented effects of microplastics on marine fauna; (3) the potential interactions between persistent organic pollutants (POPs) and microplastics; and (4) the global fate of persistent organic pollutants (POPs). The workshop is a joint venture between the University of Washington Tacoma and the National Oceanic and Atmospheric Administration (NOAA) Marine Debris Program. The NOAA Marine Debris Program (MDP) leads efforts within the United States to address marine debris, especially as it affects living marine resources. This document is the initial draft of the synthesis paper resulting from the workshop.
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The Occurrence, Effects and Fate of Small Plastic Debris in the Oceans
Courtney Arthur1, Holly Bamford1 and Joel Baker2
1National Oceanic and Atmospheric Administration, Marine Debris Program, Silver Spring, Maryland USA
2The Center for Urban Waters, University of Washington Tacoma
Purpose
This white paper reviews the literature and synthesizes the various topics that will be discussed at the
workshop to be held September 9-10, 2008. Topics include (1) the occurrence of microplastics in the
environment; (2) the documented effects of microplastics on marine fauna; (3) the potential interactions
between persistent organic pollutants (POPs) and microplastics; and (4) the global fate of persistent
organic pollutants (POPs). The workshop is a joint venture between the University of Washington Tacoma
and the National Oceanic and Atmospheric Administration (NOAA) Marine Debris Program. The NOAA
Marine Debris Program (MDP) leads efforts within the United States to address marine debris, especially
as it affects living marine resources. This document is the initial draft of the synthesis paper resulting from
the workshop.
Introduction
Definition: Marine Debris. Any persistent solid material that is manufactured or processed and
directly or indirectly, intentionally or unintentionally, disposed of or abandoned into the marine
environment or the Great Lakes.
Definition: Microplastic (or ‘microdebris’ or ‘small plastics’). Marine debris composed primarily of
plastic materials smaller than 10 cm in the longest dimension.
Carpenter et al. (1972) first described small bits of plastic floating in the surface waters of the Northwest
Atlantic more than 35 years ago. These microplastics are also routinely found in seabirds and, less
frequently, in marine turtles and mammals. Despite increasing interest in ‘macro’ marine debris,
especially plastics concentrated within convergence zones and gyres, there is no systematic monitoring
or evaluation of marine microplastics. In the past year, the increased attention to small plastic debris in
the marine environment has highlighted the limited information and research on the amount, location, and
environmental impacts of marine microplastics. The purpose of this review is to summarize what is
known about the occurrence and distribution of microplastics in the oceans, the effects of these plastics
on marine organisms, and the role these plastics may play in the global cycling and marine exposure of
persistent organic pollutants. Specific goals are:
1. To summarize our current understanding of the spatial and temporal distribution of
microplastics in the world’s oceans.
2. To review what is known about the effects of microplastics on marine organisms
3. To evaluate the predictive capabilities to model microplastic fate and transport in the
ocean
4. To explore linkages between marine microplastics and the cycling and exposure of
persistent organic pollutants.
5. To identify gaps in understanding and to describe potential research and monitoring
initiatives.
I. Occurrence of small plastic debris in the marine environment
Purpose
Small plastic debris has been found in surface seawater and along coastlines, with an increasing number
of reported observations in many of the world’s oceans. While there has been some speculation about
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potential impact of marine microplastics, the details of this issue have not been systematically studied and
are not well-documented in peer-reviewed literature. Small particles of plastic may be ingested by filter-
feeders while in the water column and by benthic organisms after settling. Microplastics also accumulate
in marine surface layers, where they may be ingested by birds and other consumers who rely on these
food-rich environments. This leads to a concern that small plastics, and their associated chemical
contaminants, may become incorporated into the marine food webs. The consequences of this are
largely unknown.
Sources
Sources of small plastic debris to the oceans are difficult to determine, but fall into three categories: (1)
larger pieces of plastic debris undergo a slow weathering process in the ocean, and may break apart into
increasingly smaller bits; (2) inadvertent or accidental release of small, unweathered industrial plastic bits
called nurdles during production, shipping, and storage; and (3) discharge of wastewater that contains
microplastics purposefully added to consumer products. Nurdles have been documented in the oceans
since the 1970s (Carpenter et al. 1972, Colton et al. 1974), and may be a source of leachable
components into seawater. Smaller polyethylene, polypropylene or polystyrene particles on the order of
0.5 mm are increasingly being used by the cosmetics industry as exfoliants in soaps and scrubs as well
as for microabrasion cleaning of mechanical parts (Gregory 1996). Microplastics in consumer products
are designed to be used once and then enter domestic wastewater, and it is unclear whether wastewater
treatment plants are able to collect these particles before they enter rivers and oceans (Gregory 1996).
The absolute and relative importance of these three sources (breakdown of larger plastic items, industrial
plastic bits, and plastic additives to cosmetics) of microplastics are unknown, but likely vary spatially
throughout the world’s oceans and temporally as manufacturing, marine transportation, and consumer
products change.
Occurrence
Small plastics have been found across the world in the open ocean, on beaches, and in sediments.
However, most surveys of marine debris in the open ocean focus on large plastics and on derelict fishing
gear. Recently there has been an increased interest in plastic bags as debris, and surveys of marine
debris are more and more often including plastic bags as a debris category. While plastic bags (and
pieces of plastic bags) are a type of marine debris, they are not considered microplastics in this paper as
dimensions of bags are generally not given.
The scarcity of microplastics data is due to the difficulty of quantifying microplastics in the world’s oceans
(Table 1). Large sampling efforts and tedious sorting and enumeration methods are required. Carpenter
et al. (1972) made perhaps the first such effort by conducting plankton tows in the waters of southern
New England. Clear and opaque spheres, measuring 0.5 mm in diameter, were observed with a
maximum concentration of 14 spheres per cubic meter and a mean concentration of one sphere per cubic
meter. Based on the description in Carpenter et al. (1972), these spheres were most likely industrial
plastic pellets (nurdles). Importantly, these spheres were found in white perch, winter flounder,
silversides, and one chaetognath sampled from those same waters.
Colton et al. (1974) performed a major sampling effort in the Northwestern Atlantic Ocean, from Cape
Cod to the Caribbean, during a summer 1972 MArine Resources Monitoring Assessment and Prediction
(MARMAP) cruise. During this cruise, they conducted plankton tows that retained opaque and clear
polystyrene spheres, opaque polyethylene cylinders, Styrofoam, sheets of flexible plastic, and fragments.
There were geographical differences in plastic abundance along transects, with fewer particles present
around the Caribbean than near Cape Cod, which the authors attributed to greater plastic use and
production in the United States.
Day and Shaw (1987) provided an in-depth survey of the North Pacific Ocean for small debris, and found
the highest density in the subtropical North Pacific (96,100 objects km-2) when compared with the
subarctic North Pacific (mean density 3370 objects km-2 ) and Bering Sea (mean density 80 objects km-2)
during towed neuston samples collected in 1985. More recently, Moore et al. (2001) sampled in the
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subtropical North Pacific during August 1999 and found a mean density of 334,271 plastic pieces km-2
with a mean mass of 5114 g plastic km-2. Moore et al. (2001) encountered thin plastic films,
polypropylene and monofilament line, and unidentified plastic fragments most often. This is an increase
of approximately 350% in plastic density for that area of the Pacific in a little over one decade (Day and
Shaw 1987, Moore et al. 2001).
Thompson et al. (2004) investigated historic continuous plankton recorder samples routinely collected
since the 1960s from Aberdeen, Scotland to the Shetlands, and from Skule Skerry, Scotland to Iceland.
Thompson et al. (2004) found a significant increase in small plastics along these transects from the 1960s
to 1990s (Figure 1).
Figure'1.''“Microscopic'plastic'in'CPR'samples'revealed'a'significant'increase'in'abundance'when'samples'from'the'1960s'and'
1970s'were'compared'to'those'from'the'1980s'and'1990s'(*,'F3,3'=14.42,'P<0.05).'Approximate'global'production'of'synthetic'
fibers'is'overlain'for'comparison.'Microplastics'were'also'less'abundant'along'oceanic'route'CPR'1'than'along'CPR'2'(F1,24'='5.18,'
P"<'0.05).”''Reproduced'from'Thompson'e t"al.'2004.
One of the first beach surveys that examined accumulation and distribution of industrial pellets was
published by Gregory (1978) for New Zealand beaches. Most pellets were virgin polyolefins, and were
clustered near cities but they were also found in remote areas (Gregory 1978). Beach surveys have
become more prevalent in the last forty years, with efforts to quantify beach debris spanning the globe.
However, most beach surveys are not structured to methodically sample sediments for microplastics,
instead focusing on larger debris items. Quantitative information on microplastics is generally not
collected or published. There are several possible explanations for this. Often the smaller debris particles
are not a priority for clean ups, the surveyors must be apprised of the potential for microplastics to occur
in sand and sediments, and collecting sediment samples to analyze for plastics can be labor- and time-
intensive.
Abu-Hilal and Al-Najjar (2004) found more than 50% of the litter they surveyed from 1994-1995 on the
shores of Jordan was small plastic pieces or bags, mostly from local sources. Recently, Ng and Obbard
(2006) detailed the presence and abundance of microplastics (defined as >1.6 µm) in Singapore. They
found microplastics in four of seven beaches surveyed in the top centimeter of sediment and in the sea
surface microlayer, likely due to poor waste disposal practices and ship discharges. A similar study of
microplastics in sediment from the Alang-Sosiya shipyard in India identified polyurethane, nylon,
polystyrene, polyester and glass wool fragments with Fourier transform infrared (FT-IR) spectroscopy
(Reddy et al. 2006). These polymers are all used in ship construction, and thus it is not surprising to find
these types of fragments in one of the largest ship-breaking zones in the world. In a field investigation to
complement data from historic open ocean samples, Thompson et al. (2004) showed significantly higher
amounts of synthetic polymers in subtidal sediment than on sandy beaches around Plymouth, UK (Figure
2).
'
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Figure'2.''“Microplastics'were'more'abundant'in'subtidal'habitats'than'on'sandy'beaches'(*,'F2,3'='13.26,'P"<'0.05),'but'abundance'
was'consistent'among'sites'within'habitat'types.”''Reproduced'from'Thompson'et"al.'2004.'
Monitoring of plastics in seabirds
Ingestion of plastic by seabirds has been monitored over the past forty years, providing evidence of not
only the occurrence of plastics over large foraging areas but also the ability of small plastics to effect the
biological environment (Kenyon and Kridler 1969, Baltz and Morejohn 1976, Fry et al. 1987, Pettit et al.
1981, Ryan 1988, Auman et al. 1997, van Franeker et al. 2004, 2005). Documentation of microplastics in
some of the most remote regions of the marine environment stem from surveys of seabird regurgitations
or stomach contents. Baltz and Morejohn (1976) confirm that six species of seabird ingest polyethylene
cylinders off central California: short-tailed shearwater (Puffinus tenuirostris), sooty shearwater (Puffinus
griseus), pink-footed shearwater (Puffinus creatopus), northern fulmar (Fulmarus glacialis), black-legged
kittiwake (Rissa tridactyla) and Heermann’s gull (Larus heermanni). Plankton tows in Monterey Bay did
not collect these pellets, which perhaps points to a land source of the polyethylene cylinders (Baltz and
Morejohn 1976).
Quite a bit of research has investigated ingestion of plastic debris by the Laysan albatross, Phoebastria
immutabilis. Kenyon and Kridler (1969) provided one of the very first published reports of plastic
ingestion in Laysan albatross. Seventy-four of one hundred dead fledglings had swallowed plastic,
mostly as caps and miscellaneous fragments, and it was assumed these were picked up from the local
beaches of the Hawaiian Islands National Wildlife Refuge. In 1981, Pettit et al. suggested adult albatross
may offload plastic via regurgitations to chicks; however, chicks are not able to regurgitate the plastic until
just prior to fledging. Reports agree that the most likely venue for plastic ingestion in adult albatross is
offshore feeding, namely inadvertent consumption of floating plastic while surface-skimming for fish egg
casings (Pettit et al. 1981, Fry et al. 1987, Kinan and Cousins 2000). Forty-five of fifty albatross chicks
sampled in the Hawaiian Islands during the mid-1980s had ingested plastic. Twelve of twenty wedge-
tailed shearwaters had ingested plastics, most of which were industrial pellets (Fry et al. 1987). Though
obstruction of the digestive tract was not common, Fry et al. (1987) believe chicks are at greater risk
because they can pick up plastics from parental regurgitations and from their own inexperienced foraging
trips.
This greater threat to chicks is augmented by an increasing trend for plastic ingestion in the Laysan
albatross, with only 6 of 251 albatross chicks sampled on Midway Atoll devoid of plastics in their gut
according to a survey from the mid-1990s (Auman et al. 1997). High incidence of plastic ingestion is also
documented by Kinan and Cousins (2000), who noted all of the 43 black-footed and Laysan albatross
examined on Kure Atoll had ingested plastics. Plastic pellets were observed in Laysan albatross
samples; coupled with the high incidence of cigarette lighters and glowsticks on this beach but lack
thereof in stomach content samples (Kinan and Cousins 2000), these data point to offshore sources of
plastics that albatross carry as a burden until regurgitation or death.
Auman et al. (2004) cataloged the first incidence of plastic ingestion by seabirds at sub-Antarctic Heard
Island. Two Antarctic prions, Pachyptila desolata, contained plastics in their digestive tracts. Plastic was
not thought to be the cause of mortality. Out of 396 sub-Antarctic skua (Catharacta antarctica)
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regurgitated pellets, 2 contained small plastics that were on the order of less than one centimeter.
However, this area is far from any anthropogenic inputs and the plastic debris must have been
transported over long distances.
Plastics were sampled in 15 of 24 species of seabird from 1988-1990 in the sub-Arctic North Pacific,
another location far from anthropogenic inputs of plastic debris (Robards et al. 1995). Surface feeders
and planktivorous divers ingested more plastic debris than birds that use other foraging methods, and
approximately 76% of ingested plastic was composed of industrial pellets (Robards et al. 1995). Data
were compared with a similar study in the same area conducted from 1969-1977, and the comparison
(increasing number of species that ingested plastics, increasing frequency of occurrence, and increasing
mean number of plastics per individual) demonstrates an increasing presence of plastics in seabirds from
the sub-Arctic North Pacific (Robards et al. 1995).
This historic use of seabirds to document occurrence of plastic debris in marine ecosystems has been
lately augmented with a new strategy of using seabirds as indicators of plastic pollution in the North Sea
and North Pacific Ocean (Nevins et al. 2005; van Franeker et al. 2004, 2005). Van Franeker et al. (2004,
2005) tracked plastic pollution in the North Sea via northern fulmar (Fulmarus glacialis) stomach contents
from 1982 to 2003 as part of a Save the North Sea initiative. This study has effectively shown a
decreasing occurrence of industrial plastics and increasing occurrence of user plastics in northern fulmars
across the North Sea during the twenty year study period; overall abundance of plastics has not changed.
Northern fulmars show a gradient of pollution, with the most plastics present in the southeast and fewest
in the northwest North Sea; this points to a smaller fulmar range than was expected, and also points to
the importance of local sources of debris in determining fulmar plastic levels (van Franeker et al. 2004,
2005).
Nevins et al. (2005) investigated the use of pelagic seabirds as indicators of plastic pollution in the North
Pacific Ocean, but warn that seabirds’ utility as indicators will depend on species ecology and life history
characteristics, such as foraging method, lifespan, habitat use, body size and ability to regurgitate. These
authors conclude that when used correctly, seabirds give valuable information about the extent of small
plastic debris distribution across the North Pacific because they routinely traverse large spatial scales
(Nevins et al. 2005).
Modeling the Sources, Transport and Spatial Distribution of Marine Microplastics
The above studies highlight both local and global sources of microplastic pollution. The findings
demonstrate spatial gradients, with generally higher microplastic concentrations closer to populated
areas. However, marine microplastic debris also occurs in extremely remote areas far away from
possible point-sources. These plastics likely travel long distances via surface ocean currents to arrive on
islands with no human settlement, implying that microplastics are quite persistent in marine surface
waters. Convey et al. (2002) noted that synthetic plastic and polystyrene debris accounted for greater
than 70% of the debris littering the shores of Scotia Arc, Antarctica from 1990 to 2002. This area
encompasses several beaches in the Southern Ocean, including South Georgia, the South Sandwich
archipelago and Adelaide Island, all of which are far from any anthropogenic inputs.
Deterministic models could predict the spatial and temporal distribution of microplastics in the oceans if
we adequately understood the (1) the magnitude and locations of their sources, (2) their persistence and
behavior in seawater, and (3) the general surface water circulation patterns in the world’s oceans. In fact,
observations of marine debris have been used to calibrate and verify ocean surface circulation models.
To date, there is no systematic inventory of microplastic releases into the marine environment, which
greatly limits the ability to model global distributions. Microplastic debris washes from the land to the sea
directly, and is released into the marine environment as ship-generated discharge (Pruter 1987). It is
nearly impossible to predict debris inputs to the oceans from ships. According to regulations in Annex V
of MARPOL 1973/78 from the International Maritime Organization, discharge of any plastics is strictly
prohibited under all circumstances. These regulations are difficult to enforce and it is certain that some
microplastics enter the marine environment in this manner. Thus sources of microplastics are difficult to
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ascertain, and the growing trend is to rely on models to predict where debris will accumulate in lieu of
surveying marine plastic debris.
Ocean current models are generally employed to explain distribution of debris to remote areas. For
example, Kubota (1994) studied a series of climatological data to evaluate the relative contribution of
north of Hawaii. In addition, they examined the movement of floating buoys in the North Pacific to test the
apparent convergence of debris in a zone near the Northwestern Hawaiian Islands. Another study using
debris to study ocean currents employed an Ocean Surface Current Simulator (OSCURS) model
(Ingraham and Ebbesmeyer 2001). The authors placed floaters in the North Pacific and tracked
movement from 1965 to 1977, and developed numerical models from these data. Models developed from
these data predict approximately 86% of debris released into the North Pacific Ocean would accumulate
in the mid-latitudes within six years of release (Ingraham and Ebbesmeyer 2001). Recently, a Debris
Estimated Likelihood Index (DELI) was developed using aerial flyover surveys in the North Pacific that
documented debris accumulations just north of the North Pacific Transition Zone Chlorophyll Front
(TZCF) (Pichel et al. 2007). DELI maps were created based on significant correlations among presence
of debris, sea surface temperature, chlorophyll a content, and chlorophyll a gradient in the assumption
that these maps will be useful for tracking conditions in which marine debris is likely to aggregate (Pichel
et al. 2007).
Weathering of plastic in seawater will affect its properties (density, size) and, therefore, its persistence
and transport distance. In general, marine plastic debris degrades very slowly (Pruter 1987). Weathering
of plastics creates smaller pieces of microplastic, but the essential polymer remains intact. Weathering
rates, including photodegradation, are lower in seawater than on land (Andrady et al. 1998). Degradation
rates depend on the amount of processing and the addition of pigments, extenders, photo-stabilizers and
thermal-stabilizers; the purity of the resin is directly related to its decomposition time (Andrady et al.
1998). The effects of partially-degraded plastic polymers on marine ecosystems are largely unknown. If
microplastic particles are more dense than seawater (or become incorporated in dense aggregates), they
will settle through the water column and be sequestered in marine sediments. Plastic incorporation into
sediments could be detrimental if these plastic particles enter the benthic food web.
II. Impact of small plastic debris on the natural environment
Purpose
Microplastics may impact the marine environment in several ways: (1) direct impacts on marine
organisms by ingestion of microplastic particles, such as irritation of the gastrointestional tract or blockage
of feeding structures (2) indirect impacts through disruption of feeding behavior and nutrition, and (3)
altered exposure to chemical contaminants associated with microplastics. Several studies have
examined the direct interactions between small plastics and marine organisms, typically species that are
threatened or endangered. Most of this research has focused on ingestion of small plastics by pelagic
seabirds and the effects of mistaking plastic for prey items. This section highlights pertinent studies,
mainly from the wealth of literature on seabird–small plastic interactions, which have investigated the
direct effects of small plastic ingestion on biota. Entanglement is mentioned here for completeness only,
as no research has yet focused on the potential for small plastics to entangle marine organisms.
Invertebrates
One paper that focuses on the interactions between microorganisms and microplastics compares plastic
and zooplankton in the Pacific Ocean, close to the California coast in an area of high productivity and
expansive human development (Moore et al. 2002). This study showed an increase in abundance of
plastic debris after a runoff event compared to plastic levels during a dry spell; plastics were collected
along with plankton samples. Moore et al. (2002) collected a higher plastic density but lower mass than
was found in a high pressure convergence zone in the North Pacific subtropical gyre, though many
factors could have affected this comparison (Moore et al. 2001).
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It is important to note the rarity of studies focusing on the impact of plastic debris on invertebrate
organisms. Two studies are available on this topic. One shows that microplastics in dosed aquaria
sediment were ingested by amphipods, lugworms and barnacles despite their differences in feeding
method (detritivore, deposit feeder and filter feeder, respectively; Thompson et al. 2004). The second
shows that microplastics were taken up by the mussel Mydilus edulis in aquaria dosed with microplastics;
these plastics persisted in the circulatory system for more than 48 d and greater numbers of smaller
plastics were found embedded in circulatory tissue (Browne et al. 2008). Thus interactions between
microplastics and small invertebrate sediment fauna have been demonstrated, and this type of interaction
is likely happening in sediments contaminated with microplastics. Thompson et al. (2005) suggest more
research focusing on toxicity of microplastic ingestion and bioaccumulation in food chains, in order to
better understand the effects of interactions on the level of the organism before extrapolating to
ecosystem-level effects of microplastics.
Vertebrates - Seabirds
A wealth of information on plastic ingestion in seabirds exists in scientific literature. Most is useful to
document occurrence of plastics that have been incorporated from the marine environment into marine
food webs (see “Occurrences” section above). Few studies have determined the effects of ingestion on
elements of seabird health, such as survival and reproductive potential.
Some studies have integrated temporal trends in seabird plastic ingestion as an assessment of continued
impact to populations. Ryan (1988) published results on intraspecific variation in plastic ingestion among
seabirds in the Southern Ocean, focusing on long-term variation, which showed an increase in plastic
ingestion over time; geographic variation, which presented more incidences of plastic ingestion in the
northern part of the Southern Ocean than in the southern; sex-related variation, which showed no
differences between male and female plastic ingestion; and age-related variation, which is likely due to
gradual accumulation in time for Procellarid birds that are unable to regurgitate indigestible items as a
bolus. Age-related variation in which chicks have larger plastic loads than parents is explained by the
hypothesis of an intergenerational transfer in species that can regurgitate to chicks. Ryan (1988)
concludes that plastic ingestion is a phenomenon that has the most dramatic effect on young of the
species.
Spear et al. (1995) completed a comprehensive review of plastic ingestion in 36 species of seabirds of
the tropical Pacific Ocean from 1984 to 1991. Two main findings emerged from this study: (1) birds that
weigh more (given life history characteristics) were more likely to have plastic in their gut; (2) plastic-
containing individuals had a negative correlation between number of plastic particles and body weight
(Spear et al. 1995).
At Midway Atoll, Auman et al. (1997) found an increasing number of Laysan albatross chicks were
ingesting plastic. This study compared anthropogenic mortality chicks (e.g., chicks that died as a result of
car-induced injuries) and natural mortality chicks (e.g., found dead without probable cause of death).
Natural mortality chicks had more plastic in their proventriculus and gizzard than did chicks killed by cars
and also had lower mass and fat indices, which serve as indicators of health (Auman et al. 1997).
Van Franeker et al. (2004, 2005) noted an increasing trend of plastic ingestion in the North Sea, using
northern fulmars (Fulmarus glacialis) as an ecological indicator of plastic pollution in the Netherlands.
One major difference between this study and many of the studies mentioned above is that the North Sea
is a heavily polluted area near dense European populations. Another difference is that this effort is
international in scope to better understand and integrate temporal and spatial trends in northern fulmar
debris ingestion (Van Franeker et al. 2005). Results from a study spanning 1982 to 2003 show a trend of
shifting ingested plastic particles in F. glacialis, with increasing consumer (also termed “user”) plastics
and a smaller contribution (but not number) of industrial pellets to debris composition in northern fulmar
digestive tracts (Van Franeker et al. 2005).
Another study that addresses the topic of temporal changes in ingestion and does not find an increase in
frequency of ingestion is Vlietstra and Parga (2002). There has been a change in the type of plastic
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particles found in short-tailed shearwaters (Puffinus tenuirostris) in the Southeast Bering Sea, but not in
the frequency of occurrence or amount per individual. Type of plastic has shifted from industrial plastic
pellets to consumer plastics when comparing birds from the 1970s to 2001.
It is interesting to note that Moser and Lee (1992) found industrial pellets in 92% of northern fulmars (F.
glacialis) and 91% of greater shearwaters (Puffinus gravis) located off the Atlantic coast of North
Carolina, USA, during a fourteen-year study spanning 1975 to 1989. Very recently, Mallory et al. (2006)
observed plastic debris in Northern fulmars from Davis Strait, Nunavut, Canada. Out of a total sample
size of 42 fulmars, no industrial pellets were observed; consumer plastics were found in 15 samples, most
of which were less than one centimeter along the longest dimension (Mallory et al. 2006). These
temporal trends in fewer industrial pellets suggest that management controls may have been put in place
to reduce the accidental loss of pellets to the environment. Although this is a promising sign, Pierce et al.
(2004) argue that plastic ingestion is very likely underestimated as a direct cause of mortality, as dead
seabirds sink or are scavenged. Even if a necropsy is performed, it can be difficult to prove the cause of
death (Pierce et al. 2004) or directly connect plastic ingestion with mortality.
Vertebrates – Sea turtles
Quite a few studies have documented plastic ingestion in sea turtles, though as with seabirds it is very
difficult to bridge the gap between presence of foreign debris and cause of mortality. Carr (1987) wrote
that juvenile sea turtles are often drawn to convergence zones, as these zones tend to collect prey
organisms. Unfortunately, convergence zones also collect anthropogenic debris. Small plastic pieces
could be confused with the tiny floats present on Sargassum, and Carr (1987) observes that early life
stages are at higher risk than adults. After surveying loggerhead sea turtles (Caretta caretta) off the east
coast of Florida, Carr (1987) observed “ubiquitous plastic beads that are delivered to the sea by the
millions in industrial waste water” that floated in the same habitat as juvenile loggerhead sea turtles.
One of the most comprehensive reviews of plastic ingestion in sea turtles is the paper George Balazs
(1985) presented at the Workshop on the Fate and Impact of Marine Debris, 27-29 November 1984 in
Honolulu, Hawaii. Balazs compiled 79 reports of debris ingestion and suggested that obstruction of the
digestive pathway and chemical release by plastics were the two major threats of marine debris to these
threatened and endangered reptiles. Plastic particles were seen in green (Chelonia mydas), loggerhead
(C. caretta) and hawksbill (Eretmochelys imbricata) turtles and not in leatherback (Dermochelys coriacea)
or olive ridley (Lepidochelys olivacea) sea turtles. Plastic and Styrofoam particles comprised 18.9% of
ingested debris from the compiled records of ingestion.
Balazs (1985) also infers some possible impacts of eating plastic, including blocked intestines, loss of
nutrition, reduced nutrient absorption in the gut, possible absorption of polychlorinated biphenyls (PCBs)
from plastics, engulfment of microscopic particles in the intestines, and effects on buoyancy if too much
low-density plastic is ingested. The following explanations for the palatability of plastics are explored:
small plastic debris could be encrusted with other wildlife; prey could have ingested the plastic, thus
causing bioaccumulation through the food chain; or possibly habitat could lack nutritious food sources
(Balazs 1985).
More recently, Tomas et al. (2002) documented plastic debris ingested by juvenile loggerhead sea turtles
illegally caught for consumption in the western Mediterranean. Plastics accounted for the highest
percentage of anthropogenic debris recovered from the digestive tracts of 41 of 54 turtles surveyed.
Mascarenhas et al. (2004) documented plastic ingestion in two sea turtles in Brazil, one female C. mydas
that defecated 10 small pieces of hard plastic and plastic bags, and one adult male L. olivacea with 9
small pieces of hard plastic. Tomas et al. (2002) are in agreement with Bjorndal et al. (1994) that sea
turtles are resistant to mortality from ingesting small foreign debris, though with the increasing number of
turtles containing plastics, small plastics can be a major concern if they occlude the digestive tract.
Barreiros and Barcelos (2001) observed several pieces of soft plastic and a hard plastic cap in one
leatherback sea turtle (D. coriacea) intestine. This particular turtle was by-caught in a long-line fishery
near the Azores; the plastic did not cause the turtle apparent harm. Bugoni et al. (2001) identified marine
debris and human impacts to green sea turtles in Brazil. Plastics were the most frequently encountered
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form of debris in the digestive tract, though hard plastics were present in only four turtles and plastic bags
and ropes were the most prevalent forms of plastic debris, at 50% and 39.5%, respectively (Bugoni et al.
2001). There are very few, if any, published records of small plastics as the direct cause of mortality in
sea turtles.
Vertebrates – Marine mammals
Marine mammals are also at risk for ingestion of small marine plastic debris due to possible occlusion of
the digestive tract and subsequent starvation. However, there are very few reports of small plastics in
marine mammals. This is likely because most marine debris research centering on marine mammals
focuses on the more visual problem of entanglement in floating and submerged debris.
However, two recent papers highlight the small plastics problem. Baird and Hooker (2000) published
what they state is the third record of plastic ingestion for harbor porpoises, Phocoena phocoena. One
juvenile harbor porpoise was found dead on a beach in Nova Scotia, with no net markings or obvious
cause of death. A small piece of black plastic, 5x7 cm2, occluded the esophageal connection to the
stomach in this individual; incidentally, unusual prey items were also found in the digestive tract, thus
perhaps the porpoise ingested plastic due to inexperienced foraging (Baird and Hooker 2000).
Eriksson and Burton (2003) examined fur seal (Arctocephalus spp.) scat on Macquarie Island. They
recovered 164 plastic pieces from 145 scats with plastic present. No industrial plastic pellets were found.
The authors assume transfer of plastics from fish prey to fur seals must have been the source of plastics
in fur seal diet (Eriksson and Burton 2003).
Discussion
In a seminal review article of the biological effects of plastic debris on marine ecosystems, Laist (1987)
describes the two main issues as mechanical, those being entanglement and ingestion of plastics. As
entanglement generally does not refer to microplastics, ingestion is the only effect that fits in this overview
paper. Hence it makes sense that none of the above studies have noted entanglement in small plastics;
as stated previously, this could be possible but on such small scales that it is not probable to encounter
microplastic entanglement in the open ocean. Laist (1987) cautions persistent plastic debris should be
considered a major form of ocean pollution, as it is long-lasting, buoyant and increasingly ubiquitous in
the marine environment. This sentiment was echoed many years later by Derraik (2002).
A breadth of information exists on microplastics in seabirds. While marine mammals and sea turtles may
be impacted by entanglement as well as ingestion, seabirds are most likely affected by plastic ingestion,
especially chicks fed via regurgitation (Laist 1987). Seabirds can also be more accessible for study than
sea turtles and marine mammals. Thus, serious research gaps exist on the effects of microplastic debris
on the majority of marine organisms. In some cases, as with Laysan albatross, it is easier to see the
cause-and-effect relationship of plastic ingestion, occlusion of digestive tract, and eventual death. With
most other marine organisms, such direct links may be difficult to make in enough cases to be
scientifically convincing. Finally, the subtle effects of microplastics, such as the possibility of unloading
bound toxic chemicals to the organism, have yet to be thoroughly investigated. The next section of this
paper elaborates on the chemistry of microplastics and the potential to adsorb and desorb contaminants,
namely persistent organic pollutants, to the marine environment.
III. Impacts of small plastic debris exposure to persistent organic pollutants
Purpose
This section highlights the current and expanding literature on plastic pollution in marine environments,
and the indirect effect this may be having on dispersal of lipophilic chemicals that are able to adsorb to
plastics. The chemistry of small plastic debris is an advancing field of research, as is evidenced by the
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growing number of peer-reviewed publications that investigate the very specific connection between
persistent organic pollutants and small plastic debris in the oceans.
The first report of chemicals sorbed to small plastic marine debris was Carpenter et al. (1972). Aroclor
1254 (a polychlorinated biphenyl (PCB) mixture) was extracted at 5 parts per million from plastics
spherules found floating in Niantic Bay, Long Island Sound. The authors infer that small plastics are able
to adsorb these particles from the marine environment because unlike some PCBs that served as plastics
additives, the mixture Aroclor 1254 was never added to virgin plastics and must have sorbed to spherules
in the environment (Carpenter et al. 1972). In the western Sargasso Sea, Carpenter and Smith (1972)
found many brittle industrial plastic pellets and suggest that the PCB-containing plasticizers could have
been lost to the marine environment, which would account for the loss of elasticity in the pellets.
Small ingested plastics and organic contaminants were examined in tandem for the first time in great
shearwaters, Puffinus gravis, from Gough Island (Ryan et al. 1988). Nineteen of twenty breeding females
had ingested plastics. Total organic contaminant load was highly correlated with amount of fatty tissue
present in females, and variation in PCB load was explained via a significant, positive correlation with
plastic load. Positive correlations with other organic contaminants were not significant; thus Ryan et al.
(1988) surmise that ingested plastics are the source of PCBs in P. gravis tissues. Thirty years later, this
study is still one of the only direct field comparisons of plastics and contaminants in marine organisms.
More recently, studies have focused on the uptake potential of organic contaminants from the marine
environment to plastic debris. Mato et al. (2001) present data that define industrial plastic pellets
(nurdles) as a transport medium for organic contaminants in seawater. Polychlorinated bi-phenyls, DDE,
and nonylphenols were all detected in polypropylene pellets collected from the Japanese coast. An
apparent adsorption coefficient of 105106 was determined for polypropylene pellets, based on a field
experiment with virgin pellets placed in seawater for six days (Mato et al. 2001). Adsorption during this
six-day experiment was two orders of magnitude lower than chemical concentrations found in nurdles
from the Japanese coast; thus equilibrium with oceanic waters was not reached after six days (Mato et al.
2001).
An in-depth analysis of PCBs in industrial plastic pellets shows high variability among regions sampled in
Japan, ranging from a concentration of <28 to 2300 ng/g (Endo et al. 2005; reproduced in Figure 3).
'
'''
Figure'3.''“(a)'RCBs'concentration'in'PE'resin'pellets'from'47'Japanese'beaches,'(b)'PCBs'concentrations'in'mussel'samples'from'
Japanese'coasts.''Location'numbers'on'X‐axis'correspond'to'Fig.'1,'Tables'1'and'2.''Solid'diamonds:'discolored'and'fouled'(“fy”)'
pellets;'open'triangles:'discolored'and'non‐fouled'(“y”)'pellets;'crosses:'non‐discolored'and'fouled'(“f”)'pellets.”''Reproduced'
from'Endo'et"al.'2005.''
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Concentrations in pellets were comparable to mussel tissue PCBs and Endo et al. (2005) propose pellets
as monitors of organic chemicals in the marine environment. High concentrations of PCBs were found in
remote areas with no localized source; thus, the authors suggest pellets could be a major route of PCB
exposure for organisms in remote areas.
A similar study collected small plastics from the North Pacific Subtropical Gyre, coastal waters of Hawaii
and southern California, and from Laysan albatross regurgitations on Guadalupe Island, Mexico (Rios et
al. 2007). Of the PCBs, polycyclic aromatic hydrocarbons (PAHs), DDTs and aliphatic hydrocarbons
detected on pellets, PCBs were the most frequently encountered organic contaminant, and total PCBs on
plastic pieces ranged from 27 to 980 ng/g (Rios et al. 2007). Concentrations were not mapped by
location or plastic type, and trends based on these two parameters were not assessed (Rios et al. 2007).
Teuten et al. (2007) reach the same conclusion as Mato et al. (2001) that plastics can transport organic
contaminants in the oceans. Phenanthrene, a polycyclic aromatic hydrocarbon (PAH), was noted to bind
to three types of plastic (polyethylene, polypropylene and polyvinyl chloride). Compared to uptake of
phenanthrene by natural sediments, uptake and binding by plastics was much higher. Teuten et al. (2007)
show that addition of virgin plastics decreases phenanthrene availability in sediments, with a dramatic
effect on sediments low in organic carbon. It was estimated that 1 ng/g contaminated polyethylene or 14
ng/g contaminated polypropylene added to sediments would cause an 80% increase in lugworm
(Arenicola marina) tissue phenanthrene concentrations (Teuten et al. 2007). These exposure estimates
were derived from partition coefficient calculations and not actual experiments. Data from a study in
Greece are in agreement that of the plastics tested, polyethylene has the highest apparent distribution
coefficient (a measure of how much contaminant binds to a surface) for phenanthrene (Karapanagioti and
Klontza 2008).
Discussion
Plastic debris can serve as a sorbent for contaminants in the natural environment with different effects
based on the type of plastic and properties of the contaminant. Plastics may also serve the reverse role
as a source of contaminants to organisms upon ingestion. Based on the potential for plastics to adsorb
contaminants, Takada (2006) put out a call for beached pellets from around the world to aid in mapping
global distributions of persistent organic pollutants. It remains to be seen whether enough useful
information can be gleaned from floating bits of plastic to be useful in determining global cycles of
persistent organic pollutants. But it is certain that plastics have the potential to both adsorb organic
contaminants from the marine environment and desorb these contaminants to biota that ingest plastics.
IV. Effect of small plastic debris on biogeochemical cycling of POPs
No research has yet taken plastic debris into account in determining global POP distribution and
contribution to global cycles. Due to the complex nature of biogeochemical research, integrating plastics
into the most up-to-date models will be a challenge. Yet it is crucial to determine if plastics can be
accurately described as sources or sinks of POPs to the marine environment. If plastics are playing a
source or sink role it is necessary to factor plastic debris into global models of persistent organic pollutant
cycling in the oceans.
The so-called “Dirty Dozen” or legacy persistent organic pollutants, named and controlled by the
Stockholm Convention in 1995, are as follows: aldrin, chlordane, DDT, dieldrin, endrin, heptachlor,
hexachlorobenzene, mirex, polychlorinated biphenyls, polychlorinated dibenzo-p-dioxins, polychlorinated
dibenzofurans, toxaphene. This list has been expanded to include the carcinogenic polycyclic aromatic
hydrocarbons, brominated flame retardants, and some organometallics. These pollutants are set as the
highest priority compounds for environmental removal and remediation efforts.
While persistent organic pollutants are composed of similar molecules (e.g., a carbon-hydrogen
backbone), specific structures and functional groups largely determine differences in POP movement and
behavior in the environment. Wania and Daly (2002) estimate global lifetimes of PCBs to be on the order
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of decades, but even within this class of compounds there is quite a bit of variability in degradation and
environmental transfer that depends on the specific compound, or PCB congener. Binding to organic
particles in the deep sea is the dominant loss process (or sink) of highly chlorinated PCB congeners
(Wania and Daly, 2002). Determining presence of POPs in remote locations such as the deep sea is a
major difficulty to overcome in developing a global mass balance model for persistent organic pollutants
(Lohmann et al. 2007). The manner in which persistent organic pollutants interact with plankton in the
water column and with the oceanic microbial loop is largely unknown, making it difficult to assess
persistent organic pollutants in the deep sea (Lohmann et al. 2007).
Modeling
Global biogeochemical POPs models include cycling of organic contaminants through every compartment
of the ecosystem, including both abiotic (e.g., air and water columns, sediments, and soils) and biotic
(e.g., the flora and fauna that comprise the ecosystem) compartments. Chemicals are “recycled” on a
global scale. However, environmental sinks have a tendency to efficiently bind and transport a chemical
or compound out of the system, so that it is removed from the recycling process. It is important to know
the specifics of environmental sinks to accurately model the movement of chemicals on a global scale.
Global biogeochemical models include bioaccumulation of contaminants in tissues of plants and animals,
and biomagnification of those contaminants in a food web structure. Thus, obtaining accurate
biogeochemical models requires knowledge of how chemicals move on a small scale between particles,
how chemicals are moved through the environment via geologic and meteorological forces, how
chemicals are incorporated into tissues of biota, and how chemicals and compounds are degraded over
time.
Links between POPS and microplastics.
Several studies have demonstrated that persistent organic pollutants, due to their hydrophobic nature,
have strong affinities for microplastic particles. This is not surprising, as the standard analytical method
to isolate POPs from natural waters employs passing the water through beds of polymeric resin, trapping
the POP analytes. What is less clear is (1) whether the quantity and composition of microplastics in the
oceans are sufficient to alter the global cycling of POPs, (2) whether leaching of chemicals from
weathering microplastics is an important source of POPs to the world’s oceans, and (3) whether
microplastics play a role of accumulating POPs to high concentrations in a form that is ingested by marine
organisms.
V. Conclusions
The main sources of microplastic marine debris likely include: (1) larger pieces of plastic debris breaking
into smaller bits during weathering; (2) inadvertent or accidental release of small, unweathered industrial
plastic bits (“nurdles”) during production, shipping, and storage; and (3) discharge of wastewater that
contains microplastics purposefully added to consumer products. Effects of these types of microplastic
debris on marine organisms are not easily determined, and have not been systematically investigated.
Microplastics have the potential to adsorb persistent organic pollutants, which may account in part for
observed POP concentrations in seawater and marine organisms that contain microplastics.
Microplastics also may serve as a transport mechanism for contaminants, not only as a vector for
contaminants into organisms, but also as a transport medium for contaminants to move around the globe.
Often POPs reach remote locations through atmospheric transport and adversely effect ecosystems that
are far from the source of the contaminant. The ubiquity of plastic coupled with its ability to adsorb
hydrophobic contaminants could be another major introductory route for contaminants into marine
ecosystems.
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Table 1. Occurrences of microplastic debris in the marine environment.
Year (s)
Mean Abundance
Mean
Abundance
Range
Citation
(pieces/km2)
(pieces/m3)
(pieces/km2)
1971
3,500
0.00035
50 - 12,000
Carpenter and Smith
1972
1972
10000000*
1.0
0 - 140,000,000*
Carpenter et al. 1972
1972
700000*
0.070
Carpenter et al. 1972
1972
300000*
0.030
Carpenter et al. 1972
1972
200000*
0.020
Carpenter et al. 1972
1972
61
0.0000061
0 - 166,991
Colton et al. 1974
1972
184
0.000018
Colton et al. 1974
1972
2,773
0.00028
Colton et al. 1974
1985
96,100
0.0096
Day and Shaw 1987
1985
3,370
0.00034
Day and Shaw 1987
1985
80
0.0000080
Day and Shaw 1987
1999
334,271
0.033
31,982 - 969,777
Moore et al. 2001
2000
3000000*
0.30
Moore et al. 2002
2004
0-2 particles L-1 seawater
Ng and Obbard 2006
1960-1980
100000*
0.010
Thompson et al. 2004
1980-2000
400000*
0.040
Thompson et al. 2004
1972-1976
10-100 m-1*
0 - 100,000 m-1*
Gregory 1978
1994
5 items m-2**
0.35-0.81 items m-2
Abu-Hilal and Al-Najjar
2004
1995
3 items m-2**
0.15-0.62 items m-2
Abu-Hilal and Al-Najjar
2004
1993-2002
0-0.3 items m-1**
Convey et al. 2002
~2000
.001 fibers cm-3*
Thompson et al. 2004
~2000
.05 fibers cm-3*
Thompson et al. 2004
~2000
0.11 fibers cm-3*
Thompson et al. 2004
2004
0-4 particles 250g-1
sediment
Ng and Obbard 2006
2004
81.43 +/- 4.03 mg kg- 1
Reddy et al. 2006
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... These fragments can undergo further degradation (e.g., biological) where carbon in the matrix is converted to carbon dioxide and incorporated into biomass (Andrady, 2011). However, UV degradation of plastic floating in water is impeded by lower temperatures and oxygen levels relative to on-land, making conversion of macroplastic to microplastic much more rapid on beaches than in the water (Arthur et al., 2008b;Andrady, 2011;Corcoran et al., 2009). As a result, microplastics are considered relatively recalcitrant in aquatic environments, and degradation of plastic and leaching of additives is more likely along the shoreline than in open waters (Andrady, 2011;Cole et al., 2011). ...
... Microplastics have been found in sediments, throughout the water column, and in digestive systems, respiratory structures, and tissues of marine organisms (Andrady, 2011;Depledge et al., 2013). Quantitative reporting of global abundances of microplastics has been limited by time-and labour-intensive sampling, remoteness of sites, and fine-scale analytical processes required (Arthur et al., 2008b;Ballent et al., 2013;Driedger et al., 2015). Microplastics will accumulate in coastal sediments, on the ocean floor, and at the sea surface. ...
... Particles can be ingested by filter feeders directly from the water column or by benthic organisms after the particles have settled to the sediment (Arthur et al., 2008b). Filter feeders might have greater exposure to microplastics than organisms employing other feeding strategies (Leslie et al., 2013). ...
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... This mechanical fragmentation is accelerated by prior material degradation. In water, plastics are effectively protected from solar UV radiation, winds, high temperatures and frost, etc., so material degradation becomes much slower (Gregory and Andrady, 2003;Barnes and Milner, 2005;Arthur et al., 2008;Corcoran et al., 2009;Ryan et al., 2009;Andrady, 2011;Anderson et al., 2016). Thus, understanding of mechanical fragmentation processes becomes important. ...
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