Marine Microplastic Pollution

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Abstract
Plastic pollution is accumulating rapidly in the world’s oceans. The potential effects of microplastics on the environment and human health are an area of active research. This POSTnote summarises their sources and spread, the evidence that they present a risk and possible strategies to reduce plastic pollution.
Number 528 June 2016
Marine Microplastic Pollution
Plastic pollution is accumulating rapidly in the
world’s oceans. The potential effects of
microplastics on the environment and human
health are an area of active research. This
POSTnote summarises their sources and
spread, the evidence that they present a risk and
possible strategies to reduce plastic pollution.
Overview
Microplastics, plastic pieces under 5 mm in
size, are a widespread ocean contaminant.
Sources of microplastic include fibres from
synthetic textiles, microbeads from
cosmetic and industrial applications and
large items of plastic debris that break
down into smaller pieces.
Studies have shown the presence of
microplastics in seafood. The potential risk
to human health is little studied and
remains uncertain.
Laboratory evidence suggests that
microplastics and their associated additives
can be harmful to wildlife. However, not all
species or life stages may be affected.
Background
Plastic is an extremely versatile resource whose production
levels have increased dramatically since the 1950s.
1
It can
be made into a wide range of products that are strong,
durable, inexpensive and lightweight. However, some of the
properties of plastic that make it such an attractive material
during use also make it problematic when it becomes waste.
The primary issue is that it is highly resistant to degradation.
Indiscriminate discarding and the accidental release of
plastic into the marine environment has resulted in the rapid
accumulation of persistent marine plastic debris in the
world’s oceans. By weight, most of this consists of large
pieces of debris such as fishing gear, bottles and plastic
bags; but by number, the dominant type of debris in the
world’s oceans are small pieces that are under 5 mm in size
these are known are microplastics.
2
It has been estimated
that there were between 15 to 51 trillion microplastic
particles floating on the surface of the world’s oceans in
2014, weighing between 93 and 236 metric tons.
3
Sources and Spread
Microplastics can either be manufactured (for example, as
microbeads for use in cosmetic scrubs, toothpastes, and
cleaning products), or can result from the fragmentation of
larger items of plastic debris. They are ubiquitous
throughout the marine environment and have been found in
estuaries, lakes, coasts, sediments, the open ocean, deep
seas, and arctic sea ice.
4
-
9
Sources
It is frequently possible to identify what type of plastic
polymer (Box 1) a particular piece of ocean debris is made
of, regardless of its size. However, when pieces become
small, fragmented and degraded they are almost impossible
to trace to their original source. As a result, the relative
importance of different microplastic sources is unknown.
The three largest sources are thought to be fibres from
textiles, microbeads and large pieces of plastic debris,
10
which will become microplastics as they fragment and
degrade. However, a 2014 report by the Norwegian
Environment Agency also highlighted the potential
importance of microplastic emissions from normal wear and
tear of plastic products such as tyres, fishing nets, rope and
carpets, as well as plastics in paints and varnishes.
11
Fibres
Small fibres from synthetic clothing, such as polyester and
nylon, are released into waste water through the process of
washing clothes.
12
Waste water treatment plants are not
designed to retain microplastics, and the resulting sewage
effluent can carry fibres out to rivers, lakes, estuaries and
the sea. Fibres are commonly the most abundant type of
microplastic found in marine wildlife and sediments.7,
13
-
16
Microbeads
Microbeads are small spheres or fragments of plastic that
are used in cosmetics, household cleaning products and
industrial blasting. They include beads used in exfoliants
POSTNOTE
The Parliamentary Office of Science and Technology, 7 Millbank, London SW1P 3JA T 020 7219 2840 E post@parliament.uk www.parliament.uk/post
POSTnote 528 June 2016 Marine Microplastic Pollution Page 2
and toothpastes, as well as beads used in ‘media blasting’,
where small plastics and other granules such as sand are
propelled onto the surfaces of buildings, machinery and
boats. In the UK, emissions of microplastics into the
environment have been estimated at between 16-86 tonnes
per annum from facial exfoliants alone.
19
There are no
academic estimates of the number of particles that are
entering the ocean as a result of media blasting.
Large Plastic Debris
Large pieces of plastic can enter the ocean as a result of
littering or accidental escape from waste management
systems. This can occur at sea, for example through the
accidental or deliberate disposal of fishing gear, but marine
litter can also originate from inland sources and enter the
marine environment after it has travelled down rivers, or be
carried in by the wind. Some definitions of large plastic
debris also include pre-production pellets that are known as
nurdles, but because they are usually around the 5 mm size
mark, several studies list them as a type of microplastic.
They are potentially a large source of plastic pollution,11
though there are no robust figures estimating the rate at
which pellets are lost at sea or at processing plants.
The importance of different sources of large plastic debris
depends on whether it is the number or weight of items that
is measured. For example, 891 visual surveys that counted
the number of large plastic items on the surface of the
world’s oceans found that 20% of items were fishing-related
debris, 58% were non-fishing related items and 22% were
classified as miscellaneous.2 However, by weight, fishing
gear was the dominant form of litter observed, accounting
for 70% of the total. Most of this (58%) was derelict fishing
buoys. Just over a quarter of non-fishing gear items were
pieces of foamed polystyrene, followed by bottles (18% of
all non-fishing items), and plastic bags/films (10%). Data on
the composition of plastic debris that has sunk to the bottom
of the ocean or is suspended in the water column is limited.
Filtering Microplastics from Waste Water
There is little information on how efficient waste water
treatment plants are at capturing microplastics before they
enter the environment. Studies across eight European
treatment plants have found that the percentage of
microplastic particles captured in sewage sludge ranges
from 24% to 100%, depending on the type of microplastic,
treatment process and methodology used in the study.
20
However, these studies are limited in scope, and in cases
where waste water bypasses treatment no microplastics
may be filtered out (for instance, if it is released directly into
the environment as a result of sewage overflow). Plastic
particles in sewage sludge that has been used as fertiliser
may additionally enter the sea at a later date as a result of
surface water run-off from agricultural land.
Areas of High Concentrations
Plastic pollution is moved around the world’s oceans by
currents and prevailing winds. Large differences in the
number of microplastic particles reported at different
locations suggests that their spread is uneven. Several
studies have modelled the spread of microplastics through
the marine environment, but predicting these movements is
complex and limited by uncertainties, including:
how the size, shape, density and fragmentation rate of
different types of plastic affect movement
how buoyancy is affected by the accumulation of algae
and bacteria on the plastic’s surface
the proportion of microplastics that are ingested by
wildlife or end up in sediments (including beaches).
Models and field studies agree that buoyant plastics are
likely to accumulate in areas of the ocean with circular
currents that are known as sub-tropical gyres.
21
-
24
These are
where the five oceanic ‘garbage patches’ occur. However,
concentrations can vary by a factor of ten across very small
distances,23 and also tend to be much higher close to
densely populated coastal areas.12
Monitoring Microplastics
Most estimates of microplastic abundance are based on
particles collected from plankton nets with a mesh size of
330 µm (1 µm = 1 thousandth of a mm), which means that
microplastics smaller than this threshold are less likely to be
collected and counted in samples. However, several studies
have shown that smaller microplastic particles do exist in
the ocean,
25
and have the potential to become nanoplastics
measuring less than 100 nm (1 nm = 1 millionth of mm).
26
As there is no routine sampling method for particles of this
size, it is likely that figures for the amount of marine
microplastics in the ocean are underestimates.
Current monitoring of microplastic pollution is not
standardised. Studies use a range of sampling techniques
and have different definitions of how small a fragment needs
to be in order to be classed as a microplastic. The Joint
Programming Initiative on Oceans an EU funded initiative
to pool research efforts includes funding for a project to
standardise methods for microplastic analysis.
Risks to Wildlife and Human Health
The small size of microplastics means that they can be
ingested by marine life. They have been found in a variety of
species including zooplankton, mussels, oysters, shrimp,
marine worms, fish, seals and whales.16,
27
-
37
Several of
these species are of commercial importance. For example,
a 2009 survey in the Clyde Sea found that 83% of
Norwegian lobster (the most valuable fishery in Scotland)
POSTnote 528 June 2016 Marine Microplastic Pollution Page 3
were contaminated with plastic, mainly in the form of
fibres.
38
Similarly, trawls in the English Channel found
microplastic contamination in 36.5% of fish caught,32 a
proportion similar to that found in fish from the North Pacific
Central Gyre, known as the ‘Great Pacific Garbage Patch’.
Toxicity to humans and wildlife could potentially be caused
by the plastic polymer itself, by the additives it contains (Box
2), or by other chemicals that are known to associate with
microplastics once they are in the ocean (Box 3). However,
there are a variety of polymers that behave differently
according to their size and shape
39
and thousands of
different additives used in products. This makes it difficult to
make general predictions about the effects of ingesting
them. Little is known about the rate at which plastic
additives leak into their surrounding environment (whether
this be the ocean or biological tissues), as well as the
potential levels of exposure for humans and wildlife.
Human Health
No studies have investigated whether microplastics that are
unintentionally ingested by humans can be subsequently
transported into tissues.
40
Several studies show that
microplastics are present in sea food sold for human
consumption,15,16,28,29,
41
including mussels in North Sea
mussel farms and oysters from the Atlantic. Although the gut
wall may be an important barrier,40 there is a possibility that
very small particles such as nanoplastics could penetrate
gut tissues. Experiments in rats have showed that
polystyrene microspheres of 50-100 nm can be absorbed
into the body through the gut and transported to the liver
and spleen.
42
The ability of different plastics to enter tissues
is likely to depend on their size and chemical properties.
Once inside, there are number of ways in which
nanoplastics could theoretically interact with biological
tissues in a way that could be toxic; but these have not been
tested, and the risk to human health remains unknown.40
Wildlife
Laboratory experiments have shown that plastic ingestion
can have detrimental effects in a range of species that have
key roles in marine ecology, though some of these
experiments expose animals to higher concentrations of
microplastics than those that have been reported in
sediments and the water column. The magnitude of effects
varies between species, and some animals appear to be
affected only at certain stages of their lifecycle (Box 4). Field
studies in this area face several difficulties. Wildlife in the
marine environment is exposed to a wide range of other
pressures, including rising temperatures, ocean acidification
and other types of pollutants such as heavy metals.
Disentangling the effects of microplastics from the effects of
these other factors will be challenging.
Environmental Effects
There are several concerns over the potential ecological
effects of microplastics that are not related to the ingestion
of these particles by animals or algae. Examples include:
Pieces of microplastic can provide a surface on which
marine insects can lay their eggs. The number of marine
pond skaters has been shown to increase with growing
amounts of microplastics in the North Pacific.
49
Such
proliferation of species that were previously limited by the
scarcity of places on which to lay their eggs has unknown
ecological consequences, but may allow several species
to become more abundant and expand their range.
The community of microbes associated with plastic
fragments is different to that normally found in seawater.
50
A study looking at the microbial communities on pieces of
polyethylene (the most commonly produced plastic
worldwide) and polypropylene (frequently used in
packaging) found that of a total of 3,484 species of
microbe, only 53 were shared by polypropylene,
polyethylene and seawater, whereas 799 were unique to
polypropylene and 413 were unique to polyethylene. The
ecological consequences of this are also unknown.
The presence of high concentrations of microplastics in
beach sediments can change their permeability and heat
absorbance,
51
raising concerns about the effects on
species where sex is determined by temperature (e.g.
sea turtles) and sediment-dwelling species that would be
at a higher risk of desiccation (including worms,
crustaceans, and molluscs).
Addressing the Risks of Microplastics
Three policies covering marine plastic litter are outlined in
Box 5. In addition, in January 2014 the European Parliament
passed a resolution on plastic waste in the environment
calling for single use plastics that cannot be recycled
(including microbeads) to be phased out.
Box 3. Plastics as Transport for Other Pollutants
The chemical and physical properties of microplastics enable them to
attract and accumulate a number of other chemicals in the oceans.
These include most persistent organic pollutants (POPs), and
persistent bioaccumulative and toxic substances.52-54 Chemicals on
microplastics ingested by an organism can dissociate from plastic
particles and enter body tissues. This has been demonstrated in
lugworms and seabirds.52 In the latter case, contaminants were
passed to the birds as a result of eating polyethylene resin pellets as
well as eating fish that were exposed to contaminants in the water,
suggesting that they have the potential to travel through the food
chain. However, natural sediments can also attract substances such
as POPs, and there is debate over the importance of microplastics as
a transmitter of POPs and other substances into tissues compared to
other subtrates. There is evidence that certain chemicals preferentially
attach to plastic,53,54 but there are only limited data on the extent to
which chemicals dissociate from plastic and migrate into tissues in
different environmental conditions and on how quickly they
accumulate in the food chain.
POSTnote 528 June 2016 Marine Microplastic Pollution Page 4
POST is an office of both Houses of Parliament, charged with providing independent and balanced analysis of policy issues that have a basis in science and technology.
POST is grateful to Ciara Stafford for researching this briefing, to NERC for funding her parliamentary fellowship, and to all contributors and reviewers. For further
information on this subject, please contact the co-author, Dr Jonathan Wentworth. Parliamentary Copyright 2016. Image copyright 5gyres.org
Preventing Microplastic Marine Pollution
There is widespread agreement that the most effective way
to reduce microplastic pollution is to prevent plastic from
entering the marine environment in the first place. This
applies to pieces that are already small enough to be
classed as microplastics, but also to large pieces of debris
that will eventually fragment into microplastics. Solutions
that aim to tackle these larger pieces will have the additional
benefit of reducing the negative social, economic and
ecological impacts associated with large plastic debris, such
as impacts on mental wellbeing, the entanglement of ship
propellers on discarded fishing gear, and the entanglement
of wildlife on items like rope and six-pack rings.
65
-
68
Approaches to preventing plastic pollution include changing
the design of plastic products and packaging, improving
plastic waste facilities and management, and changing
plastic use and littering behaviour through education and
public engagement.17 Preliminary data suggest public
awareness of microplastics is low,17,
69
which may be a
barrier to changing behaviours. However, there have been
no detailed studies investigating public understanding.
In 2014, the overall recovery rate for plastic waste in the UK
was 59%, of which 29% was recycled and 30% was
incinerated for energy recovery.1 The plastic bag charge led
to a 71% decrease in single-use plastic bag use in Wales
(2011-2014),
70
and 80% in Scotland (2014-2015).
71
A
Box 5. Existing Policies
EU Marine Strategy Framework Directive
Microplastic is listed as a type of marine litter under the EU Marine
Strategy Framework Directive. In order to achieve good environmental
status by 2020, member states need to ensure that the properties and
quantities of marine litter do not cause harm to the coastal and marine
environment. Determining these levels is difficult as the amount of
microplastic pollution required to do harm is likely to be variable
between species, and ‘harm’ itself has not been defined.
OSPAR
OSPAR (the Oslo and Paris Convention for the Protection of the
Marine Environment in the North East Atlantic) has a target (2010-
2020) to ‘substantially reduce marine litter in the OSPAR maritime
area to levels where properties and quantities do not cause harm to
the marine environment’. Its system of monitoring plastic debris is
based on three indicators: beach litter, seabed litter and plastic found
in the stomachs of fulmars (a seabird species). Further indicators,
including measuring plastic loads in a range of species, and an
indicator for microplastics, are under development.
MARPOL
MARPOL (International Convention for the Prevention of Pollution
from Ships) aims to reduce marine pollution from ships. Annex 5
specifically deals with marine litter and prohibits the disposal at sea of
all forms of plastic.
widening range of plastic packaging is also being picked up
at kerbside collections. A number of NGOs are additionally
calling for the implementation of a deposit return scheme
(DRS) in Scotland, with a feasibility study published in May
2015.
72
These involve consumers paying a small deposit on
drinks bottles that would be refunded when they are
returned to a recycling point. However, there has been no
systematic policy evaluation at scale,
73
and the available
evidence on whether they reduce littering behaviour cost-
effectively remains subject to debate.
74
The elimination of microbeads found in cosmetics has been
the central focus of a number of recent campaigns, including
the Marine Conservation Society’s Scrub it out! and the
North Sea Foundation and Plastic Soup Foundation’s Beat
the Microbead (see Briefing Paper 7510). Several countries
have issued or are in the process of issuing bans on
microbeads in cosmetic products, including Canada and the
US. Presently, 25 UK companies are or have stated an
intention to become microbead-free, such as the large
multinationals Unilever, Colgate-Palmolive and P&G.
Removing Microplastics from the Ocean
Even if the flow of plastic litter into the sea was halted
immediately, the amount of microplastic in the ocean would
likely continue to increase as larger items already in the
ocean fragment and degrade.17 Technology to remove
microplastics from the ocean does not currently exist.
75
However, the UK participates in a number of schemes that
are helping to remove existing large debris. An example is
Fishing for Litter, a voluntary scheme where participating
boats are given large bags to collect litter found in their nets
as part of their normal fishing activity. The bags are then
collected at participating harbours for disposal or recycling.
Over 360 fishing vessels and 26 harbours across Scotland
and South West England are participating.
76
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Fishing for Litter
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