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Invited Commentary
Impacts of Changing Ocean Circulation on the Distribution of
Marine Microplastic Litter
Natalie AC Welden*
y
and Amy L Lusher
z
y
Faculty of Science, Technology, Engineering and Maths, Open University, Milton Keynes, United Kingdom
z
Department of Animal Ecology I, University of Bayreuth, Bayreuth, Germany
EDITOR’S NOTE:
This is 1 of 15 invited commentaries in the series “Current Understanding of Risks Posed by Microplastics in the
Environment.” Each peer-reviewed commentary reflects the views and knowledge of international experts in this field and,
collectively, inform our current understanding of microplastics fate and effects in the aquatic environment.
ABSTRACT
Marine plastic pollution is currently a major scientific focus, with attention paid to its distribution and impacts within
ecosystems. With recent estimates indicating that the mass of plastic released to the marine environment may reach 250 million
metric tons by 2025, the effects of plastic on our oceans are set to increase. Distribution of microplastics, those plastics
measuring less than 5 mm, are of increasing concern because they represent an increasing proportion of marine litter and are
known to interact with species in a range of marine habitats. The local abundance of microplastic is dependent on a complex
interaction between the scale of local plastic sources and prevailing environmental conditions; as a result, microplastic
distribution is highly heterogeneous. Circulation models have been used to predict plastic distribution; however, current
models do not consider future variation in circulation patterns and weather systems caused by a changing climate. In this study,
we discuss the potential impacts of global climate change on the abundance and distribution of marine plastic pollution. Integr
Environ Assess Manag 2017;13:483–487.
C2017 SETAC
Keywords: Marine debris Microplastic Vertical distribution Sea-temperature rise Climate change
INTRODUCTION
The plastics circulating in the world’s oceans have rich and
diverse histories. In the wake of the Second World War,
disposable plastic items were developed and marketed as a
tool to kick-start global economies. The perpetuation of this
mind-set, in combination with the popularity of single-use
products, uncontrolled disposal, and poor waste manage-
ment and recycling practices, has resulted in the addition of
large volumes of plastic to the environment (Bergmann et al.
2015). Becauseof technological advances,population growth,
and economic expansion, plastic production has increased;
since its development as an industrial component in the 1950s,
the annual production of plastics hasrisen to 322 million metric
tons (PlasticsEurope 2016). With greater volumes of consumer
plastic in circulation, loss to the environment has also
increased. One estimate of future plastic input suggests that
input of plastic waste from coastal countries will increase from
its current level of approximately 12.7 million metric tons to as
high as 250 million metric tons by 2025 (Jambeck et al. 2015).
Plastic pollution has been divided into 2 functional
subcategories: macroplastic debris, measuring more than
5 mm, and microplastic debris, which fall below this
boundary. Microplastics are further divided into primary
microplastics, which are manufactured to measure less than
5 mm, and secondary microplastics, which achieve these
dimensions through the breakdown of macroplastic litter
once in the environment. The abundance of macroplastic
debris and the scale of weathering factors have a great
influence on the local level of secondary microplastics. The
pathways through which these plastics enter our oceans are
varied, including littering, landfill runoff, and loss at sea
(Browne 2015). Although the scale and number of local
sources of debris are of obvious importance, a number of
factors are responsible for the distribution of plastics in the
marine environment.
MOVEMENT OF MARINE PLASTIC DEBRIS
In addition to being highly resistant to degradation (known
as recalcitrant), the density of the polymer greatly influences
its distribution (e.g., Ivar do Sul et al. 2013). Polymers with a
density higher than that of the surrounding water will sink,
and those that are lower will float. These differences influence
* Address correspondence to natalie.welden@open.ac.uk
Published 25 April 2017 on wileyonlinelibrary.com/journal/ieam.
Integrated Environmental Assessment and Management — Volume 13, Number 3—pp. 483–487
Received: 15 November 2016
|
Returned for Revision: 24 January 2017
|
Accepted: 9 February 2017 483
Integr Environ Assess Manag 2017:483–487
C2017 SETACDOI: 10.1002/ieam.1911
whether plastics remain in surface waters, become beached
in coastal areas and estuaries, or sink to deep-sea sediments
(Galgani et al. 2015) (Figure 1). In areas with high levels of
macroplastic debris, weathering may result in elevated local
levels of microplastics. This breakdown is dependent on
local environmental conditions including latitude, UV, and
temperature (Andrady 2015).
Some areas accumulate both macroplastics and micro-
plastics from a vast catchment because of prevailing
conditions. Suspended microplastics may be transported
from their point of release to remote areas (Ivar do Sul et al.
2013) or accumulate in central ocean regions (or gyres) (C
ozar
et al. 2014) (Figure 1). Studies in the North Pacific Subtropical
Gyre and the South Pacific Gyre have shown densities of small
plastic particles of 334 271 and 26 898 km
2
, respectively (e.g.,
Moore et al. 2002; Eriksen et al. 2014), and circulation models
suggest that plastics can be present in all ocean gyres,
because the gyres act as a conveyer to collect and
accumulate plastic items (Lebreton et al. 2012). C
ozar et al.
(2014) suggested that accumulation as a result of gyre
circulation has led to the Pacific Ocean containing 33% to
35% of the global ocean plastic load. However, the vast
majority of the sea surface, outside of subtropical gyres, has
not been surveyed, and therefore introduces potential errors
to the global estimates.
Plastics and microplastics not entrained in gyre systems
may reach remote oceanic regions and shorelines as a result
of ocean transport, that is, surface currents and bottom-
water transport (Figure 1). Movement of water between
ocean basins is the product of a complex interaction of
forces, the key drivers of which are the temperature and
salinity of water, known as thermohaline circulation, the
frictional effects of air currents, and the Coriolis force
(Jonasson et al. 2007). Observations of cargo lost from
container ships have shown a strong relationship between
the patterns of global circulation and those of debris
distribution (Ebbesmeyer et al. 2007).
A more subtle influence on the distribution of floating
plastics is that of the wind. Plastic debris riding at the water’s
surface is subject to the frictional effect of air currents that
alter the path of the object, known as windage (Shaw and
Mapes 1979). The impact of windage on debris may result in
the course of an object differing from that of the current alone
(Figure 1). Comparisons of windward and leeward beaches
have shown a distinct disparity in plastic abundance, with
windward beaches experiencing greater plastic abundance
of up to 24.2% (Debrot et al. 1999). Similarly, in the Tamar
estuary, England, downwind sites had higher levels of
fragmented plastic debris (Browne et al. 2010).
MAPPING MICROPLASTICS
The combination of factors that influence plastic distribu-
tion have been used to develop models to provide an
overview of the expected plastic load at a given time (e.g.,
C
ozar et al. 2014; Eriksen et al. 2014). In this way, researchers
may predict areas of high plastic accumulation and potential
threats of plastic debris to ecosystems. A Lagrangian tracking
model, NEMO, was used to predict the movement of
particles in the Mediterranean. This model assumed a
homogeneous initial distribution of marine debris, with
repeated 1-year predictions separated by 24 hours. This
model has been used to predict both the accumulation of
floating debris and its beaching points (Mansui et al. 2015).
The model MEDSLIK-II was used to predict the movement of
plastic debris in the Adriatic Sea, using an estimate of 10 000
tons of litter released each year. This model predicted
increased volumes of plastic in the northwest boundary of the
Adriatic, but no defined aggregation points; as a result, the
Figure 1. Factors that influence the distribution of plastics and microplastics within and between the zones of the marine environment. Figure adapted from
Lusher (2015).
484 Integr Environ Assess Manag 13, 2017—NAC Welden and AL Lusher
Integr Environ Assess Manag 2017:483–487
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authors suggest that the main sinks are the seafloor and
shoreline stranding (Liubartseva et al. 2016).
Although many of these models have proven effective in
mapping current plastic distribution, they are subject to
sources of uncertainty, such as variation in wind direction and
force. Reconstructions of a decade of oceanic conditions in
the North Sea were used to simulate both pre-existing and
subsequent distribution of floating debris. A Lagrangian
particle tracking model, PELETS-2D, was used to predict the
90-day trajectories of debris from various locations. In that
study wind drift was seen to greatly affect the distribution of
particles away from the prevailing circulation patterns
(Neumann et al. 2014). In addition to the variability caused
by short-term variation in abiotic factors, our oceans are
currently undergoing a more marked period of uncertainty
brought about by global climate change. These changes may
not only affect the ability of models to predict the location of
plastic aggregations, but also alter the geographic areas and
habitats at risk for the negative effects of marine plastic
pollution.
Changing climate and ice melt
Since the mid-19th century, there has been a mean
increase in atmospheric temperature of 0.6 0.2 ˚C (Solomon
et al. 2007). This warming has subsequently been linked to a
range of effects on the marine environment, manifesting in
the increasing reports of coral bleaching (Hoegh-Guldberg
1999), ocean acidification (Doney et al. 2009), and reduced
sea ice (Comiso et al. 2008). The symptoms of climate change
are not solely the result of a warmer atmosphere, but relate to
an interaction between the thermal properties of the air, land,
and sea. This relationship adds to an already complex picture
of emissions and mitigation methods, which cause uncer-
tainty in both climate change projections and its impacts on
the environment (Intergovernmental Panel on Climate
Change 2013). One of the most widely discussed effects of
temperature on circulation, salinity, and sea temperatures is
the increasing rate of ice melt and reducing glacier extent.
Increased freshwater input from terrestrial glaciers and
the thermal expansion of seawater result in global rises in
sea level. Previous projections of sea level change by
2099 indicated increases of 18 to 38 cm under the B1 and
26 to 59 cm under the A1FI climate scenarios (Solomon et al.
2007). More recent estimates range from a conservative 28
to 56 cm to a potentially disastrous 57 to 131 cm (Mengel
et al. 2016).
Ice melt in polar regions is predicted to have a range of
effects on the distribution of marine plastics. First, seasonal
expansion and contraction of the ice sheets is believed to
contribute to the flux of microplastics, because particles are
trapped as water freezes and are released when it melts
(Lusher et al. 2015) (Figure 2). It has been suggested that
melting ice may result in the release of entrained plastic
(Obbard et al. 2014); however, the bulk of the large ice sheets
and glaciers formed before the proliferation of plastics and
plastic litter, and the scale of such releases are predicted to
be relatively minimal. Second, the density of many polymers
is equal to or lower than that of seawater, causing plastics to
float and be carried for long distances on ocean currents.
Reduction in the density of seawater at sites of freshwater
Figure 2. Schematic diagram of the predicted effects of climate change on the marine environment and the potential impacts of such variation on plastic input,
distribution, and accumulation.
Changing Ocean Circulation and Marine Microplastic Litter—Integr Environ Assess Manag 13, 2017 485
Integr Environ Assess Manag 2017:483–487
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input is expected to reduce the relative buoyancy of marine
debris, increasing the rate at which plastics sink (Figure 1).
Correspondingly, areas of high evaporation will experience
increased water densities, resulting in plastics persisting in
the water column and/or surface waters. When predicting the
movement of plastics at the water’s surface, researchers must
take into account the residence time of plastics in the
neustonic zone; in low-water-density areas, the transport
time of plastics will be reduced, and in high-density areas it
will be increased.
Climate and circulation
Freshwater inputs from melting glaciers are predicted to
have effects beyond increasing the volume of water in our
oceans. In addition to its role in the transport of heat from
low-latitude upwelling areas to high-latitude sites of over-
turning (Wunsch 2002), thermohaline circulation is directly
affected by climate. Meridional overturning circulation is
caused by the cooling of warm water, which increases its
density and causes it to sink. Freshwater inputs in polar
regions reduce the salinity, and therefore the density, of
surface waters. This lessens the rate at which these waters
sink, slowing formation of cold deep waters (Broecker 1987).
Further freshwater inputs result from changes in the
hydrological cycle, including increased rain events at higher
latitudes (Rahmstorf 1995). It is believed that a 4-fold increase
in CO
2
is required to cause a collapse of thermohaline
circulation (Manabe and Stouffer 1994); however, a reduction
in transport of up to 50% has been observed in a number of
models (Rahmstorf 1999), and Atlantic water overturning is
thought to be slower now than at any point in the previous
century (Rahmstorf et al. 2015). Reducing the speed of deep-
water formation also slows down the rate at which freshwater
is removed from these sinking regions, again reducing the
seawater density (Toggweiler and Key 2001).
In addition to the effect of salinity on deep-water
formation, overturning is also influenced by the circulation
of surface waters (Pasquero and Tziperman 2004). Surface
water movement is primarily wind driven, which is the result of
uneven heating of the earth’s surface. Climate change is
predicted to influence the pattern of global heating, one
result of which will be altered wind patterns. Melting of the
sea ice will reduce the albedo effect (the heat-reflective
capacity of the earth’s surface) (Ingram et al. 1989); these
areas will be more readily heated, resulting in the formation
of new low-pressure areas as the newly heated—less dense—
air rises upward. Because wind patterns are governed by the
movement of air from areas of high to low pressure, this
change will alter prevailing wind conditions. Variation in wind
patterns will affect the movement of surface waters, as well as
the position of eddies and convergences, changing the
distribution of floating macroplastics and microplastics. In
addition to altering the effect of windage on plastic at the
water’s surface, increased wind speeds will result in increased
vertical mixing, raising the abundance of plastics found at
depth (Kukulka et al. 2012; Reisser et al. 2014) (Figure 1).
Debris that has previously settled in the sediment may also be
resuspended by wind-driven mixing in nearshore waters
(Floderus and Pihl 1990), increasing their abundance in the
water column. These changes in coastal systems could also
facilitate the transport of plastics to offshore areas (Figure 2).
Climate and weather
Changes in sea surface temperature may also affect the
scale and patterns of precipitation, in particular, tropical
storms, cyclones, and tornadoes. Global warming intensifies
alongshore wind stress on the ocean surface, resulting in
accelerated coastal upwelling (Bakun 1990). It has been
suggested that, as temperatures increase, torrential rain,
flooding droughts, and storms will become more frequent
(Coumou and Rahmstorf 2012). Furthermore, ocean subsur-
face temperatures and thermohaline depths and thickness
are predicted to affect the activities of natural climatic
variations, an effect currently believed to be visible in the
increasing frequency of El Ni~
no events (Cai et al. 2014). Rising
sea surface temperatures also increase the frequency of
hurricanes, which form over warm tropical waters. Uncertainty
in wind conditions may best be addressed by increasing the
integration time of models, smoothing the effect of this short-
term variation (Liubartseva et al. 2016).
In addition to the increasing annual mass of plastic released
from land or lost at sea, the input, distribution, and
accumulation of marine plastic debris will also be affected
by changes to global circulation and the morphology of
coastal regions (Figure 2). Not only is increased wind strength
predicted to facilitate the transport of windblown plastics
from terrestrial environments into waterways, but more
frequent rain events will increase flooding and surface water
runoff, further increasing plastic input (Moore et al. 2002).
Movement of plastic from the terrestrial to the marine
environment is also enabled by coastal flooding, as sea levels
rise in response to melting ice and changes in water density.
Debris that is littering shorelines will also become available
for transport as these areas are inundated by rising seas.
CONCLUSIONS
Distinct relationships have previously been recorded
between microplastic aggregation and both marine circula-
tion and weather conditions. The impacts of a changing
climate on ocean salinity and volume, and on the movement
of air and water, suggest that there will be significant changes
to the current pattern of distribution. The predicted increase
in the mass of marine plastic debris indicates that the threat
posed by microplastics will only increase in the coming
decades. The ability to predict areas of plastic input and
deposition would enable the identification of at-risk species
and enable efforts to reduce and remove plastic debris to be
targeted to specific locations. The current uncertainty as to
the effects of global warming on our oceans is the greatest
challenge in predicting the future patterns of plastic
aggregation and accumulation in relation to global circula-
tion. With future models of climate–ocean feedback
expected to produce more accurate predictions of circulation
patterns, including the impacts of a variable climate is vital in
486 Integr Environ Assess Manag 13, 2017—NAC Welden and AL Lusher
Integr Environ Assess Manag 2017:483–487
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forecasting potential microplastic hot spots and “garbage
patches.”
Acknowledgment—We thank Rebekah Cioffi and Gema
Hernandez-Milian for their valued comments on an earlier
draft of the manuscript.
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