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Buoyant Plastics at Sea:
Concentrations and Impacts
Júlia Wiener Reisser
B.Sc. Oceanography (Hons)
M.Sc. Biological Oceanography
This thesis is presented for the degree of Doctor of Philosophy of
The University of Western Australia
November 2015
School of Civil, Environmental, and Mining Engineering
The UWA Oceans Institute
The University of Western Australia
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Thesis Declaration
This thesis is presented as a series of three scientific papers, which is in agreement with
the Postgraduate and Research Scholarship Regulation 1.3.1.33 of the University of
Western Australia.
Chapter 2: Reisser J, Shaw J, Wilcox C, Hardesty BD, Proietti M, Thums M,
Pattiaratchi C (2013) Marine Plastic Pollution in Waters around Australia:
Characteristics, Concentrations, and Pathways. PLOS ONE 8(11):e80466
Chapter 3: Reisser J, Shaw J, Hallegraeff G, Proietti M, Barnes D, Thums M, Wilcox
C, Hardesty B, Pattiaratchi C (2014) Millimeter-sized Marine Plastics: A New Pelagic
Habitat for Microorganisms and Invertebrates. PLOS ONE 9(6): e100289
Chapter 4: Reisser J, Slat B, Noble K, du Plessis K, Epp M, Proietti M, de Sonneville
J, Becker T, Pattiaratchi C (2015) The Vertical Distribution of Buoyant Plastics at Sea:
an Observational Study in the North Atlantic Gyre. Biogeosciences 12: 1249-1256
Some parts of the other chapters (chapters 1 and 5) have also been previously published
in journals, magazines, and reports produced by J. Reisser during her PhD candidature.
Main supervisor C. Pattiaratchi provided guidance for the entire thesis and scientific
publications. The contributions of other collaborators to PhD chapters are mostly
associated with research directions, assistance with data processing, and editorial input
in manuscript drafts. The exceptions are the substantial contributions of G. Hallegraeff
towards the identifications of the ‘epiplastic’ organisms described in chapter 3, and B.
Slat towards designing the innovative sampling protocol presented in chapter 4. Besides
the three scientific papers that compose the data chapters of this thesis, J. Reisser co-
produced eleven additional scientific publications and five media articles during her
PhD candidature. The full references to these manuscripts are provided in ‘Appendix 1
Outputs produced during this candidature’.
Julia Reisser
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Abstract
Millimetre-sized plastics are a predominant type of marine debris floating at sea. These
small macroscopic particles are numerically abundant in some marine environments, but
little is known about their spatial distribution and environmental impacts. The goals of
this thesis were to investigate how buoyant plastics are distributed in sea surface waters
(both horizontally and vertically), and characterise organisms and textures on the
surface of millimetre-sized marine plastics. This work is the first to (1) quantify plastic
contamination levels in Australian waters, (2) characterize the biodiversity of organisms
living on millimetre-sized plastics from waters around Australia, and (3) obtain high-
resolution depth profiles (0 – 5 m) of plastic pollution in an oceanic accumulation zone.
I collected 839 pieces of plastic in 171 surface net tows from surface waters around
Australia, and 12,751 pieces of plastic in 12 multi-level tows from an oceanic
accumulation zone in the North Atlantic. Plastics were mostly fragments resulting from
the breakdown of larger objects (e.g. packaging and fishing gear) made of polyethylene
and polypropylene polymers. Contamination levels in waters around Australia were
similar to those in other marine regions (e.g. Caribbean Sea and Gulf of Maine), but
considerably lower than those found in plastic pollution hotspots within subtropical
gyres and Mediterranean Sea. There was a wide range of microbes and a few
invertebrates on the surface of floating plastics from Australia-wide sample collections.
Diatoms were particularly diverse, represented by 14 genera, 11 of which are new
records of ‘epiplastic’ organisms. Plastic pollution levels in the North Atlantic
accumulation zone decreased exponentially with water depth, with decay rates
decreasing as wind strength increased. Plastic mass per cubic metre of water decreased
more rapidly with depth than the number of plastic pieces per cubic metre, as the
smaller plastic pieces were associated with lower rising velocities and were more
susceptible to vertical mixing. This thesis contributed towards the global efforts of
quantifying plastic contamination levels and impacts in surface waters. It highlights the
widespread distribution of anthropogenic polymers, which has created a new pelagic
habitat for microorganisms and invertebrates. Plastic inhabitants seem to be invading
non-native marine regions by plastic transport, and playing an important role on ocean
plastic degradation.
Keywords: microplastics, marine debris, plastic pollution, Australia, garbage patches
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Acknowledgements
Thank you to my main supervisor Chari Pattiaratchi for the opportunity of working
under his guidance over the last years, and to the Australian government, UWA, and
CSIRO for awarding me scholarships that covered my living, study, and research costs.
I also acknowledge my co-supervisors Chris Wilcox and Michele Thums, and
collaborators/mentors Denise Hardesty, Jeremy Shaw, Gustaaf Hallegraeff, David
Barnes, Boyan Slat, Kim Noble, Kate du Plessis, Meredith Epp, Jan de Sonneville, and
Thomas Becker. A special thank you goes to Maíra Proietti, who has been my friend &
mentor since 2002. I also acknowledge my family and friends, specially my dad Carlos
Reisser, mum Helena Wiener, and siblings Tunico and Peteca for all the love, support,
and patience. Finally, but not last, I thank my husband Christopher Phillips for his love
and in-kind support towards my PhD work and failures.
Thanks to the Marine National Facility, Australian Institute of Marine Science, Austral
Fisheries, The Ocean Cleanup, and Pangaea Explorations for providing me sea time.
Thank you staff and crew of RV Southern Surveyor, RV Solander, Comac Enterprise,
and SV Sea Dragon for logistic support during the research voyages. I also
acknowledge the UWA School of Chemistry and Biochemistry, as well as the Centre
for Microscopy, Characterisation and Analysis for their facilities, and technical
assistance. Thank you to Ruth Gongora-Mesas, Don McKenzie, Mark Lewis, Oscar Del
Borrello, Qamar Schuyler, Kathy Townsend, Stephen McCullum, Lisa Woodward,
Alastair Graham, Sara Schofield, Tyrone Ridgway, Kim Brooks, Craig Steinberg, Gary
Brinkman, Martin Exel, Andy Prendergast, Sebastian Holmes, Luana Lins, Dagmar
Kubistin, Murphy Birnberg, Cyprien Bosserelle, Steve Rogers, Susana Agusti, Luana
Lins, Piotr Kuklinski, Paco Cardenas, Pat Hutchings, Anja Schulze, Christopher Boyko,
George Wilson, Marilyn Schotte, John Hooper, Christine Schoenberg, Jean Vacelet,
Andrzej Pisera, Alexander Muir, John Taylor, Martin Thiel, Anthony Richardson,
Carlos Duarte, Deepak Kumaresan, Andy Whiteley, Richard Allcock, Moritz Wandres,
Sarath Wijeratne, Asha de Vos, Shari Gallop, Eric Loss, Shanley McEntee, Winston
Ricardo, Bart Sturm, Beatrice Clyde-Smith, Kasey Erin, Mario Merkus, Max Muller,
Jennifer Gelin, Ruth Thompson, and all the others I forgot to mention for their help and
support towards this work.
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Contents
Thesis Declaration ..................................................................................................... iii
Abstract ....................................................................................................................... iv
Acknowledgements ...................................................................................................... v
Contents ...................................................................................................................... vi
Chapter 1 General Introduction ............................................................. 9
1.1 Plastic: definition and major types ................................................................... 9
1.2 The Plastic Age: plastic production and waste .............................................. 11
1.3 Oceans: the ultimate sink for plastic pollution .............................................. 14
1.4 Marine plastic pollution impacts .................................................................... 20
1.5 Monitoring plastic pollution ............................................................................ 22
1.5.1 Visual surveys ............................................................................................. 23
1.5.2 Surface net tows .......................................................................................... 26
1.5.3 Subsurface net tows .................................................................................... 28
1.6 Goals and aims ................................................................................................. 31
1.7 Structure of the thesis ...................................................................................... 31
Chapter 2 Marine plastic pollution in waters around Australia:
characteristics, concentrations, and pathways ....................................... 33
2.1 Summary ........................................................................................................... 33
2.2 Introduction ...................................................................................................... 34
2.3 Materials and Methods .................................................................................... 36
2.4 Results ............................................................................................................... 40
2.5 Discussion .......................................................................................................... 49
2.5.1 Characteristics of marine plastics ............................................................... 49
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2.5.2 Concentrations and sources ......................................................................... 52
2.5.3 Potential pathways ...................................................................................... 54
2.5.4 Final remarks ............................................................................................... 55
Chapter 3 Millimetre-sized Marine Plastics: A New Pelagic Habitat
for Microorganisms and Invertebrates ................................................... 57
3.1 Summary ........................................................................................................... 57
3.2 Introduction ...................................................................................................... 58
3.3 Material and Methods ...................................................................................... 60
3.4 Results ............................................................................................................... 62
3.5 Discussion .......................................................................................................... 74
Chapter 4 The vertical distribution of buoyant plastics at sea: an
observational study in the North Atlantic Gyre ..................................... 80
4.1 Summary ........................................................................................................... 80
4.2 Introduction ...................................................................................................... 81
4.3 Materials and Methods .................................................................................... 82
4.3.1 At-sea sampling ........................................................................................... 82
4.3.2 Estimating depth profiles of plastic contamination ..................................... 84
4.3.3 Characterising plastic length, type, resin, and rise velocity ........................ 85
4.4 Results ............................................................................................................... 86
4.4.1 Profiles of mass and numerical concentrations ........................................... 86
4.4.2 Lengths, types, resins and rise velocities of plastics ................................... 89
4.5 Discussion .......................................................................................................... 93
Chapter 5 General Discussion ............................................................... 97
5.1 Horizontal distribution of buoyant plastics at sea ........................................ 97
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5.2 Vertical distribution of buoyant plastics at sea . Error! Bookmark not defined.
5.3 Organisms on the surface of millimetre-sized ocean plastics ..................... 101
5.3.1 Ingestion of plastics at sea: does debris size really matter? ...................... 103
5.4 Overall Conclusions ....................................................................................... 108
References ................................................................................................ 111
Appendix 1 Outputs produced during this candidature ..................... 125
Appendix 2 Chapter 2 supplementary material ................................... 128
Appendix 3 Chapter 3 supplementary material ................................... 140
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Chapter 1 General Introduction
1.1 Plastic: definition and major types
Plastics are a diverse group of synthetic materials predominantly derived from
petrochemicals, such as petroleum and natural gas. They possess a peculiar molecular
architecture consisting of long chainlike macromolecules known as polymers, which are
a sequence of repeating units, called monomers. Plastics can be divided into two major
categories: thermoplastics and thermosets.
Thermosets are used in a few high-volume applications, such as automobile tires. They
can be considered a large molecule that is destroyed with heating, meaning recycling
options are mostly limited to energy recovery and physical grinding into powder
(Pickering, 2006). Environmental impacts of thermosets to marine ecosystems are
outside the scope of this thesis, mainly because nearly all thermosets are heavier than
water. Therefore, these materials do not predominate in the top layer of the world’s
oceans, which is the marine region examined in this thesis.
Thermoplastics, which will be referred here as ‘plastics’, are a group of materials made
of large polymeric molecules held together by relatively weak intermolecular forces.
They soften upon heating and return to their original condition when cooled. This
property makes them suitable for moulding and extrusion into films, fibres and
packaging. It also allows recycling into new products by re-melting and processing into
new shapes. Major thermoplastic types include polyethylene (PE), polypropylene (PP),
polyvinyl chloride (PVC), and polystyrene (PS). These are produced in large volumes
(Figure 1.1) and are therefore of particular environmental significance.
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Figure 1.1 Major plastic types and their share of the global plastic production.
Source: (Taylor, 2015)
The density of a certain plastic determines a key behaviour in the marine environment:
whether the material will sink or float in seawater. All major polymers containing
elements other than hydrogen and carbon are heavier than water due to their strong
intermolecular forces. As a consequence, the only polymer group containing materials
lighter than water are pure hydrocarbons.
A major group of pure hydrocarbons is polyolefins. They have a density range of
approximately 900 – 960 kg m-3, thus floating in both water (1000 kg m-3) and seawater
(1025 – 1045 kg m-3). Polyolefins are the most common type of synthetic polymer, with
a share of approximately 40% of the global plastic production (Taylor, 2015). This
group includes polypropylene (PP), low-density polyethylene (LDPE), linear low-
density polyethylene (LLDPE), and high-density polyethylene (HDPE) materials.
Polystyrene is also a pure hydrocarbon, but due to the benzene rings in its
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macromolecule, it commonly sinks in water. The exception is expanded polystyrene
(i.e. Styrofoam), which is highly buoyant due to its air bubbles.
Polyethylenes, an important member of the family of polyolefin resins, are the most
widely used type of plastics in the world (37% of global production). They are made
from the polymerization of ethylene monomers (CH2=CH2) and are used in products
ranging from clear food wrap and shopping bags, to detergent bottles and fish crates.
Polyethylenes’ durability, light-weight characteristics, and high use in single-use
packaging, makes them a major type of floating pollutant that persist for long periods of
time in both freshwater and marine environments, including the sea surface (Hidalgo-
Ruz et al., 2012).
1.2 The Plastic Age: plastic production and waste
Due to the ideal properties of plastic for many applications (e.g. inexpensive,
lightweight, flexible, durable, water resistant), it is displacing materials like paper,
glass, and metal from traditional usages, and is leading to the creation of new products
with high demands. Consequently, the global production of plastics has been growing
exponentially since the 1950-60s, when most basic polymer groups were already
available for use in a diverse range of applications (Thompson et al., 2009) (Figure 1.2).
For instance, in 2012 alone, 288 million tons of plastic were produced (PlasticsEurope,
2013), which is approximately the same weight of the entire human biomass (Walpole
et al., 2012). Approximately 8% of today’s annual fossil fuel production goes towards
plastic production, with 4% of this converted directly into plastic materials and a similar
quantity used as energy for its manufacture (Hopewell et al., 2009).
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Figure 1.2 Plastic production (1950-2012), in millions of tonnes.
Source: PlasticsEurope (PEMRG)/Consultic
One of the key drivers of this plastic production growth is the packaging market.
Approximately 37 - 44% of all plastic produced each year is used in packaging
(Industry Canada, 2011, Plastic Waste Management Institute, 2013, PACIA, 2011,
PlasticsEurope, 2013). This includes the manufacture of products entirely made of
plastic, such as bottles, cups, bags, containers and trays, as well as multilayer structures
containing plastic liners, including aluminium/tin cans and milk/juice cartons. This
packaging, together with agricultural films and other disposable consumer items,
represent half of the plastics produced nowadays, which are predominantly used once
and disposed of in less than one year (Selke, 2003). Packaging also represents the main
source of plastic waste, with 45.8% and 58% of the plastic waste produced respectively
in the UK and Japan coming from packaging (Hopewell et al., 2009, Plastic Waste
Management Institute, 2013).
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Unfortunately, the vast majority of global plastic winds up in landfills (Hopewell et al.,
2009, Hoornweg and Bhada-Tata, 2012), with only around 3.5 – 15% being recycled. In
the US, 32,000,000 tonnes of plastic waste were generated in 2012, of which 9% was
recycled (EPA, 2013). In Australia, 1,476,690 tonnes of plastic were used in 2011-2012,
of which 20.4% was recycled (PACIA, 2011). Japan, which is a major leader in plastic
waste recovery, recycled 26% of the 95,200,000 tonnes of plastic waste produced in
2011 (Plastic Waste Management Institute, 2013). Among the difficulties to increase the
rates of plastic recycling are the relative high costs of processing waste into recycled
materials, and the challenges to sort domestic waste into single polymer types
(Hopewell et al., 2009). Apart from the small quantities of waste diverted back into the
manufacture system through recycling, disposed plastic is either incinerated (with or
without energy recovery) or disposed in landfills and dumps. Due to low rates of
recycling and high durability, plastics are accumulating in many types of habitats
worldwide, particularly in aquatic environments (Figure 1.3).
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Figure 1.3 Nine-year-old boy collecting aluminium cans for recycling in a Brazilian river full of
plastic.
Source: Diego Nigro/JC Imagem
Particularly concerning issues associated with this sharp rise in plastic production and
waste are (1) the toxicity of certain plastics to human health, leading to adverse effects
such as increased risk for cancer and neurological problems (Breast Cancer Fundation,
2013), and (2) the impacts on marine life arising from the widespread occurrence of
discarded plastics in the oceans (United Nations Environment Programme, 2014). The
latter is the topic of this thesis.
1.3 Oceans: the ultimate sink for plastic pollution
Our massive plastic production and waste, the obstacles to recycle and properly dispose
of plastic products, and the sharp rise in the number of ships and coastal developments,
are all leading to an increase in the amount of plastic items accumulating in the oceans.
Plastics can be transported from populated areas to the marine environment by rivers,
wind, tides, rainwater, storm, drains, sewage disposal, and flooding; or can directly
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reach the sea from vessels (e.g. fishing gear) and offshore installations (e.g. oil rigs and
aquaculture farms) (Ryan et al., 2009). The relative importance of these different
pathways to the total load of plastics at sea has not yet been quantified. However, some
major sources are evident: (1) rivers, which can connect even inland populated areas to
oceans, (2) vessels, particularly those engaged in fishing activities using plastic gear
(e.g. long and heavy nets), and (3) tides, which takes litter left at beaches by users. It
was recently estimated that around 4.8 to 12.7 million tonnes of plastic waste enters the
oceans from land-based sources (Jambeck et al., 2015).
Once plastics reach the oceans, their fate will depend on their characteristics (e.g.
density, shape and size) and the environmental conditions they are exposed to. Plastics
made of resins denser than seawater (e.g. polyethylene terephthalate water bottles) will
sink to the seafloor, while less dense plastics (e.g. polypropylene water bottle caps) will
float at the sea surface layer for a variable period of time. Floating period will depend
on processes such as hydrodynamics, debris characteristics, and ecology of the
surrounding environment (Cózar et al., 2014, Eriksen et al., 2014). For instance,
buoyant plastics from coastal sources may encounter strong inshore winds and currents
and strand (either permanently or temporarily) in coastal environments such as sandy
beaches, rocky shores, and mangroves (Carson et al., 2011). Furthermore, biofouling,
which is the colonization of the debris surface by microorganisms and invertebrates, can
increase the density of buoyant debris to a point where they sink to the seafloor. This
process is particularly quick (days to months) for plastic items with high surface area to
volume ratios (Ryan, 2015), such as plastic bags and wraps. These biofouled items may
rest on the seafloor permanently and/or return to the water column, depending on the
biofouling dynamics and local sediment rates (Ye and Andrady, 1991).
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Throughout their marine journey, plastics slowly degrade and become brittle, then break
down into progressively smaller pieces (Andrady, 2011). There are four mechanisms by
which plastics degrade in the environment: photo-oxidative degradation, thermal
oxidation, hydrolysis, and biodegradation (Gregory and Andrady, 2003). Common
plastics encountered in marine environments (e.g. polyolefins), however, break down
primarily through photo-thermal oxidation processes (i.e. mostly due to the effect of
sunlight and heat). Ultraviolet light from the sun gives the energy to begin the
incorporation of oxygen atoms into the polymer (Andrady, 2011), which then causes the
plastic to become brittle and break into progressively smaller pieces. When the polymer
chains reach sufficiently low molecular weight, microorganisms may then convert the
polymer carbon into carbon dioxide or incorporate it into biomolecules (Zheng et al.,
2005). The process explained above can take 50 or more years to be completed (Webb
et al., 2012).
At sea, plastic degradation is particularly slow mostly due to the low temperatures and
low oxygen availability. Furthermore, in the ocean the rate of hydrolysis is insignificant
for most polymers. In the case of negatively buoyant plastics, degradation is likely to be
even slower as ultraviolet wavelengths from the sun are readily absorbed by water,
making the degradation process limited to thermal oxidation. Furthermore, biofouling
on the surface of floating plastics may protect the material from exposure to sunlight,
yielding a slower degradation relative to exposure on land (Gregory and Andrady,
2003).
The fragments resulting from the degradation of plastic objects are known as secondary
microplastics when smaller than 5 mm. In addition to these fragments, plastics can also
be directly manufactured in small sizes (< 5 mm). These are known as primary
microplastics and include: pellets, the raw material used to produce plastic items (Mato
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et al., 2001); synthetic fibers, including those used in clothing (Browne et al., 2011);
and microbeads, which are plastic spheres found in cosmetics such as toothpaste and
facial scrubs (Fendall and Sewell, 2009).
Buoyant microplastics are widespread across oceans, with well-known hotspots
occurring at surface waters of the Mediterranean Sea and at large oceanic accumulation
zones formed within subtropical gyres (Cózar et al., 2014, Eriksen et al., 2014). It is at
the sea surface that some major and highly visible environmental impacts occur,
including plastic entanglement and ingestion by pelagic animals and wide transport of
fouling organisms across oceans (Barnes, 2002, Derraik, 2002, Mato et al., 2001,
Wilcox et al., 2013).
The ocean’s subtropical gyres are five large continuous loops of flowing water (Talley
et al., 2011b) that occupy around 40% of the Earth’s surface. Their horizontal extension
is from about 10° north and south of the equator to about 45° in each hemisphere; the
water circulating in these massive regions reaches nearly 2 km beneath the sea surface
(Pedlosky, 1990). Those in the North Hemisphere rotate clockwise (North Pacific and
North Atlantic Gyres), while those in the South Hemisphere spin counter clockwise
(South Pacific, South Atlantic, and Indian Gyres). They are shaped by a strong and
narrow “western boundary current” and a weak and broad “eastern boundary current”.
The sea surface interiors of these gyres have low concentrations of nutrients and
biomass throughout the year, but their immense size makes their total biological
productivity significant to the world’s ocean ecosystem (McClain et al., 2004).
Accumulation zones of buoyant plastics within subtropical gyres can exceed 100,000
pieces km-2 and their horizontal extensions have been inferred by numerical (Lebreton
et al., 2012) and statistical modelling using satellite-tracked drifting buoys (Maximenko
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et al., 2012, van Sebille et al., 2012). Buoyant plastics are transported and accumulated
in these oceanic areas due to a combination of geostrophic current forcing, controlled by
pressure gradients (i.e. by the level “tilt”), and the effect of local wind through (1) direct
wind force applied to the surface of the floating debris - the so-called ‘windage’, (2)
Stokes drift created by local waves, and (3) Ekman currents (Maximenko et al., 2012).
Mean streamlines resulting from this forcing form a large-scale pattern with five well-
defined convergent zones known as accumulation zones or ‘garbage patches’
(Maximenko et al., 2012) (Figure 1.4).
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Figure 1.4 Top map: Mean streamlines as a combination of the mean geostrophic and Ekman
velocities (Maximenko et al., 2009). Bottom map: drifter model solution after 10 years of
integration from an initially homogeneous state. See details in (Maximenko et al., 2012).
Colours in the top map indicate the magnitudes of mean velocities used to compute the streamlines, and colours in the bottom map indicate the relative concentration of drifters/debris. Map sources: Maximenko et al. 2009 and Maximenko et al. 2012.
The main advantage of the statistical modelling approach developed by Maximenko et
al. 2012 is its ability to describe motions of floating objects, even without a full
understanding of the tremendously complex dynamics of the upper ocean. Their model
indicates that tropical and subpolar regions are cleared from buoyant debris within three
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years, with most of this pollution being transported into the five subtropical gyres. In
ten years time, it predicts that floating debris are redistributed within and between the
subtropical gyres to form more compact accumulation zones, centred at around 30
degrees latitude. The North and South Pacific accumulation zones are located in eastern
parts of the corresponding subtropical gyres, while the North and South Atlantic as well
as the South Indian accumulation zones are elongated zonally across their entire ocean
basin (see Figure 1.4).
1.4 Marine plastic pollution impacts
Animals as small as microscopic zooplankton (Wright et al., 2013) and as large as
whales (Fossi et al., 2012) ingest plastic debris. Plastics can also enter animals’ bodies
through respiration, as has been reported in crabs (Watts et al., 2014). Ingestion of
plastic items by marine animals can lead to gastrointestinal perforation and blockage,
reduction of food intake, reproductive disorders, and death (Derraik, 2002). At least 170
marine species are affected by plastic ingestion, including threatened species of sea
birds, turtles and mammals (Vegter et al., 2014).
Chemical impacts of plastics on organisms, food webs, and ecosystems have also
become a focus of concern over the last decade, e.g. (United Nations Environment
Programme, 2014). One of the main reasons for such concern is that over half of our
plastic objects contain at least one ingredient classified as hazardous (Rochman et al.,
2013a). Many plastic products contain non-polymeric components (e.g. residual
monomers, oligomers, low molecular weight fragments, catalyst remnants,
polymerisation solvents and additives) that can be carcinogenic, mutagenic, and/or toxic
for organisms, with potential long-term effects (Lithner, 2011). Since these substances
are usually of low molecular weight and are weakly/not bound to the polymeric
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macromolecules, they and/or their degradation products can be released from the plastic
into air, water or other contact media, such as food (Lithner, 2011).
Furthermore, plastics that enter aquatic environments can become increasingly
hazardous by adsorbing persistent organic pollutants and metals on their surface (Rios
et al., 2007, Holmes et al., 2012). Due to the hydrophobic nature of plastics,
hydrophobic pollutants such as polychlorinated biphenyls - PCBs (Mato et al., 2001)
and polycyclic aromatic hydrocarbons - PHAs (Rios et al., 2007) accumulate on their
surfaces. Adsorption of trace metals on plastic debris can also occur through exposure
to environmental conditions that lead to the development of viable surface sites by
photo-oxidation, biofouling and deposition of sediment particles (Holmes et al., 2012).
When plastic enters the body via ingestion or other means, these concentrated pollutants
can be transferred to predators and also up their food chains. Such a bio-magnification
process is more likely to occur when plastics are small enough to be ingested by low
trophic fauna, such as small fish and zooplankton (Browne et al., 2013, Rochman et al.,
2013c).
Entanglement in plastic items, especially discarded fishing gear (e.g. ropes, straps, lines,
pots, traps, nets), are also a serious threat to some species of marine vertebrates
(Derraik, 2002). It can lead to physical injuries, drowning, increased drag, impairment
of abilities to forage and avoid predators, and ultimately death. At least 135 marine
species have been recorded entangled in marine debris (Vegter et al., 2014). Animals
that often occur at the sea surface, such as air-breathing vertebrates (e.g. sea turtles,
birds, and mammals), are particularly prone to this type of adverse interaction. Young
fur seals, which are both curious and playful, are often entangled in nets and packing
bands (Derraik, 2002). Entanglement of juvenile northern sea lions (Eumetopias
jubatus), Hawaiian monk seals (Monachus schauinslandi), and northern fur seals
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(Callorhinus ursinus) has been listed as one of the factors contributing to the decline of
their populations (Derraik, 2002). Benthic organisms, especially those with branching
morphologies (e.g. gorgonians, sponges, corals), are also affected by tissue abrasion and
mortality caused by entanglement in lost fishing gear (Chiappone et al., 2005).
On the other hand, many marine species can also benefit from the occurrence of plastics
at sea, which is a new long-lasting type of floating habitat. These include fish species
and fouling organisms that can invade non-native waters through plastic drifting
(Barnes, 2002) and ‘epiplastic’ pathogens, which may infect animals that ingest plastics
(Pham et al., 2012). We still know very little about the dwellers of the widely dispersed
and abundant microplastics. The environmental implications of the occurrence of
organisms on the surface of millimetre-sized plastics are discussed in Chapter 3, where
the inhabitants of millimetre-sized plastics from waters around Australia are described.
Another environmental impact of plastic contamination is the alteration of physical
properties of marine environments. It changes light, oxygen and refuge availability, as
well as heat transfer and water movement throughout sediments (Goldberg, 1997,
Carson et al., 2011). Such effects could potentially have consequences to benthic
communities and animals that have offspring sex determined by sand temperature, such
as sea turtles (Goldberg, 1997, Carson et al., 2011). Plastic-induced alterations to
natural environments also lead to social and economic impacts (Vegter et al., 2014),
such as decreased tourism in beaches and diving destinations heavily polluted by marine
litter.
1.5 Monitoring plastic pollution
To gain a better understanding of the environmental hazards associated with marine
plastic pollution, several studies have attempted to quantify marine plastic debris,
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ranging from several meters to 0.001 millimetres in size (Hidalgo-Ruz et al., 2012,
Pichel et al., 2012). Such investigations have sampled plastics from the shoreline,
seafloor, water column, and sea surface (Thompson et al., 2004, Law et al., 2010,
Browne et al., 2011, Kukulka et al., 2012, Schlining et al., 2013, Van Cauwenberghe et
al., 2013b).
Floating marine plastics have been quantified mostly through visual counts conducted
from vessels and airplanes (Pichel et al., 2007, Hinojosa et al., 2011, Pichel et al.,
2012), and by sampling devices that collect plastics from the oceans. Such instruments
include Continuous Plankton Recorders (Thompson et al., 2004), Niskin bottles
(Gordon, 2000), rotating drum samplers (Ng and Obbard, 2006), and zooplankton nets
(Carpenter and Smith, 1972).
1.5.1 Visual surveys
The most common method of estimating amounts of large plastics in surface waters is
by counting floating objects while aboard vessels. Generally, an observer stands on the
flying bridge looking for floating debris as the ship moves through the area. Binoculars
are sometimes used to confirm the characteristics of the sighted objects (e.g. material,
size, colour). The area that is visually scanned for floating debris varies between
studies. Surveys following the Strip Transect approach stipulate a maximum distance
from the vessel in which the observer should scan for debris. It is a very simple method
that assumes all objects within the scanned area are counted (100% probability
detection). Surveys following the Line Transect approach (Buckland et al., 2005), have
the observers focusing their search effort in the heading line of the vessel, and use the
perpendicular distances between the sighted objects and the vessel’s heading line to
estimate detection probability functions, e.g. (Titmus and Hyrenbach, 2011). To obtain
24
an estimate of abundance, the number of objects observed during a certain time is
divided by the sampled area, which is equal to the distance travelled by the vessel
(transect length) multiplied by the distance from the boat where plastics were counted
(transect width). These plastic concentrations are commonly reported in number of
items per area, but grams per area are also estimated in a few studies that used weights
of beached plastic items to infer the mass of sighted objects and transform numerical
densities into mass densities (Eriksen et al., 2014).
Plastic concentrations in pieces km-2 reported in 18 studies that conducted visual
surveys are plotted in the top panel of Figure 1.5 (Venrick et al., 1973, Morris, 1980a,
Dixon and Dixon, 1983, Dahlberg and Day, 1985, Day and Shaw, 1987, Ryan, 1990,
Dufault and Whitehead, 1994, Aliani et al., 2003, Thiel et al., 2003, Barnes and Milner,
2005, Shiomoto and Kameda, 2005, Hinojosa and Thiel, 2009, Titmus and Hyrenbach,
2011, Zhou et al., 2011, Williams et al., 2011, Ryan, 2013b, Ryan, 2013a, Thiel et al.,
2013). There are many variables that influence these reported estimates, including sea
state, distance from which objects were observed, minimum plastic size counted, etc. As
such, comparisons between studies should be done with caution.
25
Figure 1.5 Mean buoyant plastic concentrations (pieces km-2) in different parts of the ocean.
The currents that form subtropical gyres are displayed as black arrows in the top map, together with mean concentration of meter-sized debris, as estimated by visual surveys. The bottom map shows mean concentrations of millimetre-sized plastics, as measured by net sampling. Circle positions show approximate location of each measurement and letters next to circles indicate the study that reported each mean value. Top map: aVenrick et al. 1973, bMorris 1980a, cDixon and Dixon 1983, dDahlberg and Day 1985, eDay and Shaw 1987, fRyan 1990, gDufault and Whitehead 1994, hAlani et al. 2003, iShiomoto and Kameda 2005, jBarnes and Milner 2005, kThiel et al. 2003, lHinojosa and Thiel 2009, mThiel et al. 2013, nWilliams et al. 2011, oTitmus and Hyrenbach 2011, pZhou et al. 2011, rRyan 2013b, sRyan 2013. Bottom map: aCarpenter and Smith 1972, bShaw 1977, cRyan 1988, dMorris 1980b, eWilber 1987, fGregory 1990, gDay et al. 1990, hMoore et al. 2001, iLaw et al. 2010, jYamashita and Tanimura 2007, kCollignon et al. 2012, lVan Cauwenberghe et al. 2013a, mEriksen et al. 2013, pZhou et al. 2011, qDufault and Whitehead 1994
Generally, centimetre-sized fragments resulting from the disintegration of larger plastic
objects were the most common type of debris observed during these visual surveys,
particularly in offshore regions (Venrick et al., 1973, Dahlberg and Day, 1985, Titmus
and Hyrenbach, 2011). Entire plastic items, such as bags, Styrofoam blocks, bottles,
26
packaging, and fishing gear, were also commonly sighted, especially in coastal waters
(Thiel et al., 2003, Williams et al., 2011).
Sighted plastics were widespread in the sampled marine regions and mean
concentrations higher than 10 pieces km-2 occurred close to coastal populated areas (i.e.
Indonesian, Chilean, Canadian, and South African waters) as well as in oceanic
accumulation zones within subtropical waters of the North Pacific and in the
Mediterranean Sea.
There are still vast areas of the oceans to be sampled, and more data is required to
adequately document the location of large-scale concentrations of plastic objects.
During my PhD, I did a few visual surveys in waters around Australia, but the results
are not presented in this thesis. They were used in a recent global meta-analysis
estimating the distribution and load of plastics at the world’s sea surface (Eriksen et al.,
2014). It was published while this thesis was under review.
1.5.2 Surface net tows
Zooplankton nets, such as Neuston and Manta nets (Brown and Cheng, 1981) are by far
the most common devices used to sample small pelagic plastics (Hidalgo-Ruz et al.,
2012). They are towed from vessels to systematically sample buoyant plastics at the air-
seawater interface, where floating material tend to accumulate (Kukulka et al., 2012). In
comparison to the Neuston net, the Manta net requires more people and logistics to be
deployed. On the other hand, it has two important advantages in relation to the Neuston
net: (1) its paravanes steer the net at an angle to the ship’s path, thus avoiding the
vessel’s wake influence in the sampling, and (2) the top edge of the net always rides on
the surface, ensuring a constant sampling of the sea surface and an easy calculation of
the area and volume sampled by each net tow (Brown and Cheng, 1981).
27
The main advantage of these surface nets is their capacity to concentrate buoyant
material from a relatively large volume of water (Hidalgo-Ruz et al., 2012). After each
net tow, the content captured by the net is carefully examined to separate plastics from
biological material. Detected plastics are then counted and/or weighed and usually
reported in pieces per area (Hidalgo-Ruz et al., 2012), although pieces per volume, mass
per area, and mass per volume are also used.
The findings of 15 studies that conducted surface net tows are shown in the lower panel
of Figure 1.5 (Carpenter and Smith, 1972, Shaw, 1977, Morris, 1980b, Wilber, 1987,
Ryan, 1988, Gregory, 1990, Day et al., 1990, Dufault and Whitehead, 1994, Moore et
al., 2001, Yamashita and Tanimura, 2007, Law et al., 2010, Zhou et al., 2011, Collignon
et al., 2012, Eriksen et al., 2013, Van Cauwenberghe et al., 2013a). These studies report
plastic pollution levels in pieces km-2. There are many variables that influence these
reported estimates, including sampling design, net mesh size, and processing technique
used, as well as efforts towards finding and identifying plastic particles smaller than 1
mm. As such, comparisons between studies should be done with caution.
Fragmented pieces of larger plastic objects were by far the most common plastic type
described in the “net tow” reports considered here. Plastic pellets had a high relative
abundance in reports from the 1970-80s, but decreased thereafter (Law et al., 2010).
This is probably due to both an increase in amounts of secondary microplastics, and a
decrease in pellets being lost during transportation.
Small marine plastics, mostly less than 5 mm across (microplastics), were widespread in
the sampled marine regions, and mean concentrations higher than 10,000 pieces km-2
were present in the Mediterranean Sea and in oceanic subtropical areas of the North
Pacific, South Pacific, and North Atlantic. By considering the estimates from these
28
plastic pollution observations, some recent global studies (Cózar et al., 2014, Eriksen et
al., 2014), and the outputs of global models of plastic dispersal (Lebreton et al., 2012,
Maximenko et al., 2012, van Sebille et al., 2012), it is possible to confidently conclude
that small plastics are concentrated within all large subtropical areas of the oceans
(“oceanic gyres”), as well as in the Mediterranean Sea. There are still several gaps in the
global datasets, particularly in the southern hemisphere and high latitudes. As such,
other large-scale accumulation zones may exist. For instance, the model developed by
Van Sebille et al. (2012) predicted an extra accumulation zone in the Barents Sea, while
the Lebreton et al. (2012) model identified many coastal accumulation zones, most of
which are still unsampled.
1.5.3 Subsurface net tows and depth profile modelling
Most of what is known about at-sea buoyant plastic characteristics and concentrations
comes from surface net sampling. However, buoyant plastics can be transported to
deeper waters due to vertical water movements created by turbulence and other
circulation patterns, such as Langmuir circulation (Kukulka et al., 2012).
Lattin et al. (2004) performed paired manta (surface) and bongo (5 m) net tows in Santa
Monica Bay before and shortly after a storm event, suggesting that high wind conditions
and urban runoff enhance vertical mixing of plastic debris, both at the sea surface and
ocean floor. Doyle et al. (2011) collected surface and subsurface (15 m from the
bottom) samples during four cruises off the US west coast. They found higher quantities
of plastic at the sea surface, with the occurrence of subsurface plastics only during a
winter cruise. They also attributed the presence of plastics in the water column to the
mixing of particles from surface and sediments.
29
Kukulka et al. (2012) conducted the first comprehensive multi-level survey of buoyant
plastics. These authors used a Neuston net and a multiple-net Tucker Trawl (Hopkins et
al., 1973) to sample plastics at the surface, 5 m, 10 m, and 20 m deep. Their
observations were used to validate a model capable of predicting depth-integrated
plastic numerical concentrations (pieces km-2) using surface values and wind conditions.
Their model assumes that buoyant plastics are vertically distributed due to wind-driven
mixing and that the depth-integrated plastic concentrations (Ci, pieces km-2) can be
inferred by applying the following one-dimensional column model:
Where: Cs = surface plastic concentration (pieces km-2); d = immersion depth of
the surface-towed net; wb = buoyant rise velocity of marine plastics; Ao = near-surface
turbulent (eddy) exchange coefficient, which was estimated by the following formula:
Where: k = von Karman constant (equal to 0.4); Hs = significant wave height
(m); u*w = frictional velocity of water (m s-1).
According to this model, one of the main drivers of the vertical distribution of plastic
pollution is the rising velocity of plastic particles (Figure 1.6). Preliminary experiments
conducted in Kukulka et al. 2012 indicated that this velocity ranges from 0.007 – 0.014
m s-1. However, details of this experimental work were not provided in the publication.
Furthermore, their comparison between observed depth profiles and model estimates
was incomplete because their model predicted an exponential depth decay on plastic
Ci =Cs
1− e−dwbAO−1
AO =1.5u*wkHs
30
pollution levels, with the largest decrease in the upper two metres, where no subsurface
measurements were taken (Kukulka et al., 2012) (see Figure 1.6).
Figure 1.6 Observed (dots) and modelled (lines) depth profiles of normalized plastic numerical
concentration for different values of frictional velocity of water (u*w) and debris rising speed (wb).
Note the strong dependence between wb and the depth profile of plastic concentration. Source: (Kukulka et al., 2012)
A better understanding of the vertical transport of buoyant plastics is fundamental for
improving estimates of ocean plastic load, size distribution, and dispersal (Kukulka et
al., 2012, Law et al., 2014, Isobe et al., 2014). In this context, Chapter 4 of this thesis
investigates the wind-driven turbulent transport of buoyant plastics by collecting high-
resolution observations of plastic concentrations and characteristics (e.g. rising speeds)
in the ocean top layer (0 – 5 m).
31
1.6 Goals and aims
Despite growing public awareness of ocean plastic pollution, the abundance, spatial
distribution, and ecological implications of marine plastic debris are still poorly
evaluated. It is clear that plastic pollution is a serious environmental issue occurring in
all oceans, and deserves further research.
The goals of this thesis were to investigate how buoyant plastics are distributed in sea
surface waters (both horizontally and vertically), and characterise organisms on the
surface of millimetre-sized marine plastics.
I collected plastic samples from surface waters around Australia (N = 171 15-minute
surface net tows, 839 pieces), and from an oceanic plastic pollution hotspot (N = 12 1-
hour net tows, 12,751 pieces) to achieve the following aims:
Aim 1: Quantify plastic contamination levels in waters around Australia.
Aim 2: Describe textures and organisms on the surface of millimetre-sized plastics from
Australian waters.
Aim 3: Investigate the vertical profile of plastic pollution in the top layer (0 – 5 m) of
the oceans.
1.7 Structure of the thesis
This thesis was prepared as a ‘series of papers’, following the guidelines of the
University of Western Australia. It has five chapters: this general introduction chapter,
three data chapters, and a general discussion chapter.
32
This chapter covered marine plastic pollution background in order to justify the overall
goals of the thesis. Additional literature reviews, focused on the aims and specific
objectives, are presented in the introduction sections of each data chapter.
The bodies of work related to aims 1, 2 and 3 are described in chapters 2, 3, and 4,
respectively. These chapters were written in standard scientific publication format, so
they can be read individually or as a part of the whole thesis. These three manuscripts
(two published in PLOS ONE and one in Biogeosciences) are generally reproduced
verbatim, except for:
- Acknowledgements, which have been consolidated into one general thesis
acknowledgement section;
- References, which have been consolidated into a general thesis references
section, following the Harvard bibliography style;
- Table and figure numbers, which are now following the thesis structure;
- Citations to the papers arising from this thesis, which have been changed to
references to the corresponding thesis chapters;
- Language, which has been changed from American to British English.
Finally, chapter 5 brings together the main findings of this thesis, highlighting their
scientific significance and limitations, and suggests directions for future research related
to marine plastic pollution.
33
Chapter 2 Marine plastic pollution in waters around Australia:
characteristics, concentrations, and pathways
2.1 Summary
Plastics represent the vast majority of human-made debris present in the oceans.
However, their characteristics, accumulation zones, and transport pathways remain
poorly assessed. We characterised and estimated the concentration of marine plastics in
waters around Australia using surface net tows, and inferred their potential pathways
using particle-tracking models and real drifter trajectories. The 839 marine plastics
recorded were predominantly small fragments (“microplastics”, median length = 2.8
mm, mean length = 4.9 mm) resulting from the breakdown of larger objects made of
polyethylene and polypropylene (e.g. packaging and fishing items). Mean sea surface
plastic concentration was 4256.4 pieces km-2, and after incorporating the effect of
vertical wind mixing, this value increased to 8966.3 pieces km-2. These plastics appear
to be associated with a wide range of ocean currents that connect the sampled sites to
their international and domestic sources, including populated areas of Australia’s east
coast. This study shows that plastic contamination levels in surface waters of Australia
are similar to those in the Caribbean Sea and Gulf of Maine, but considerably lower
than those found in the subtropical gyres and Mediterranean Sea. Microplastics such as
the ones described here have the potential to affect organisms ranging from megafauna
to small fish and zooplankton.
34
2.2 Introduction
Plastics are a diverse group of materials derived from petrochemicals (Thompson et al.,
2009). Their global production has grown exponentially from 1,700,000 tonnes in 1950
to 280,000,000 tonnes in 2011 (PlasticsEurope, 2012). The disposability of plastics,
together with their low recycling rates, has contributed to a significant rise in the
amount of waste produced globally (Hoornweg and Bhada-Tata, 2012). For instance, in
Australia, 1,433,046 tonnes of plastics were used in 2010-2011, of which only 20% was
recycled. Moreover, around 37% of this plastic was for the manufacturing of single-use
disposable packaging (PACIA, 2011). Plastics are transported from populated areas to
the marine environment by rivers, wind, tides, rainwater, storm drains, sewage disposal,
and even flood events. It can also reach the sea from vessels (e.g. fishing gear) and
offshore installations (Ryan et al., 2009). Once in the oceans, they will either float at the
ocean surface, or sink to the seafloor if made from polymers denser than seawater
(Andrady, 2011). Buoyant plastics may be cast ashore by inshore currents or winds
(Thiel et al., 2013), or may enter the open ocean, where they tend to accumulate in
convergence zones such as the ones formed by the five large-scale gyres: South and
North Pacific, South and North Atlantic, and Indian (Moore et al., 2001, Law et al.,
2010, Eriksen et al., 2013).
Marine plastics are known to undergo fragmentation into increasingly smaller pieces by
photochemical, mechanical and biological processes (Andrady, 2011, Davidson, 2012).
Plastics are also directly manufactured in small sizes (< 5 mm), which may find their
way into the oceans. These include virgin plastic pellets (pelletwatch.org) (Mato et al.,
2001), synthetic fibres from clothes (Browne et al., 2011), microbeads from cosmetics
(Fendall and Sewell, 2009), and synthetic ‘sandblasting’ media (Andrady, 2011). There
is increasing awareness that these small plastic particles (often called microplastics
35
when smaller than 5 mm) (Andrady, 2011) represent a significant proportion of the
human-made debris present in the oceans. However, their at-sea spatial and temporal
dynamics remain poorly assessed, mostly due to a lack of data on their characteristics
and at-sea occurrence (Kukulka et al., 2012, Lebreton et al., 2012). In Australia, the
only published information on microplastics comes from a global study that recorded
their occurrence in the sediments of Busselton beach (Western Australia) and Port
Douglas (Queensland) (Browne et al., 2011). Apart from this, our current knowledge on
plastic contamination in the Australian marine environment is restricted to (1) beach
litter clean-ups that record mainly the occurrence of relatively large objects, e.g. (Jones,
1995, Frost and Cullen, 1997, Edyvane et al., 2004); (2) land-based surveys of marine
megafauna impacted by marine debris, e.g. (Jones, 1995, Carey, 2011, Schuyler et al.,
2012, Verlis et al., 2013); and (3) inferences based on plastic pollution reports from
New Zealand, e.g. (Gregory, 2009).
The impacts of plastics on marine vertebrates, such as turtles, mammals and birds, have
been well recognized since the 80s (Carr, 1987, de Stephanis et al., 2013). However,
only recently has concern about the effects of small plastic particles on food webs and
marine ecosystems been raised. More than half of modern plastics contain at least one
hazardous ingredient (Rochman et al., 2013a) and those that end up in aquatic systems
can become increasingly toxic by adsorbing persistent organic pollutants on their
surface (Rochman et al., 2013b). These concentrated toxins might then be delivered to
animals via plastic ingestion and/or endocytosis (Teuten et al., 2009, von Moos et al.,
2012) and transferred up their food webs (Basheer et al., 2004, Choy and Drazen, 2013,
Gassel et al., 2013). This bio-magnification process is more likely to happen when
plastics are small enough to be ingested by organisms that are close to the bottom of the
ocean food web, such as planktivorous fish (Boerger et al., 2010) and zooplankton
36
(Cole et al., 2013). For instance, it was inferred that small plastic particles found in the
stomach contents of Southern Bluefin tuna captured close to Tasmania (Young et al.,
1997) were coming from the guts of their prey: myctophid fish (Eriksson and Burton,
2003). In this scenario, plastic contaminants can be transferred to the affected organism
and then biomagnified up the food chain. If this process is taking place, plastics can
affect the health of food webs, which include humans as an apex predator.
Australia’s acknowledgement of plastic threats to marine ecosystems is mostly limited
to impacts from relatively large debris (e.g. abandoned fishing nets, plastic bags) on
marine megafauna (e.g. turtles, mammals, birds) (Commonwealth of Australia, 2009).
A first step towards a better understanding of the extent of marine plastic hazards to
Australian organisms and environments is a better assessment of the occurrence and
characteristics of plastic debris at-sea. To this end, we characterized (size, type, color,
polymer) and estimated concentration (pieces km-2) of plastics in waters around
Australia using surface net tows. Additionally, potential pathways taken by the collected
plastics were inferred using outputs of a dispersal model and trajectories of satellite-
tracked drifting buoys.
2.3 Materials and Methods
Ethics Statement: Permits to conduct this field research were obtained from the Great
Barrier Reef Marine Park Authority (GBRMPA: permit G11/34378.1). No other special
permitting was required because sampling was limited to the collection of marine
debris.
During seven transit voyages aboard Australian vessels (Figure 2.1), we undertook three
consecutive 15-minute net tows (mean ± standard deviation tow length = 1.3 ± 0.50 km)
at 57 locations (hereafter called “net stations”), while the ship was travelling at a speed
37
of 2 – 4 knots. These net tows sampled the air-sea interface, using a Neuston net (1.2 ×
0.6 m mouth, 335 μm mesh) or a Manta net (1 × 0.17 m mouth, 333 μm mesh). After
each net tow, the collected material was transferred to a container filled with seawater
and examined for floating plastic pieces for at least an hour by a trained observer (J.R.).
Each plastic piece was picked up with forceps and placed in a graduated dish to be
counted, measured (length), photographed and classified into type (hard, soft, line,
expanded polystyrene, pellet), and colour. A random sample of 200 plastic pieces was
selected for polymer composition analysis by Fourier transform infrared spectrometry
(FT-IR; range = 500 - 4000 cm-1). Polymer type was determined by comparing sample
FT-IR spectra against known spectra from a database (Perkin-Elmer ATR of Polymers
Library).
Figure 2.1 Location of the 57 net stations sampled during this study.
Dot colours indicate the voyage when the net station was sampled and numbers follow the chronological order of sampling. Pictures of the two types of net used are shown in the right panel.
To estimate sea surface plastic concentrations (Cs, pieces km-2), we first divided the
number of plastic pieces found in the cod-end of each net tow by its towed area, which
38
was estimated by multiplying net mouth width by tow length (determined from GPS
position data). Mean Cs was then estimated for each of the 57 net stations by averaging
the Cs of its three net tows. To our knowledge, this is the first study to take net tow
replicates for marine plastic sampling. Apart from providing us measurements of Cs
variability, our approach (i.e. execution of 3 short net tows instead of 1 long trawl) also
avoided net clogging by gelatinous zooplankton.
Since buoyant plastics are vertically distributed due to wind-driven mixing, we also
estimated depth-integrated plastic concentrations (Ci, pieces km-2) by applying a one-
dimensional column model (Kukulka et al., 2012):
Where:
d = immersion depth of the surface-towed net; equal to 0.17 m for the Manta net
tows (full immersion of the net frame) and 0.3 m for the Neuston net tows (half
of the frame immersed).
wb = buoyant rise velocity of marine plastics; equal to 0.02 m s-1. Preliminary
experiments indicate that it ranges from 0.005 – 0.035 m s-1 (Kukulka et al.,
2012).
Ao = near-surface turbulent (eddy) exchange coefficient, which was estimated
by:
Where:
Ci =Cs
1− e−dwbAO−1
AO =1.5u*wkHs
39
k = von Karman constant; equal to 0.4.
Hs = significant wave height (m).
u*w = frictional velocity of water (m s-1).
Both Hs and u*w were taken from the ERA-Interim model (Dee et al., 2011). There was
a considerable similarity between wind fields of the ERA-Interim forecast model (U10)
and the wind speed measured by an anemometer (w) on five of our seven voyages (U10
= 0.85 + 1.04w, r2= 0.79, N = 39 net stations), indicating that the use of the model
outputs is adequate.
To infer potential pathways taken by the collected plastics, we used two approaches: (1)
application of the Australian Connectivity Interface Connie2 (csiro.au/connie2), and (2)
trajectories of satellite-tracked buoys from the Global Drifter Program
(aoml.noaa.gov/phod/dac). In our first approach, an area of 0.1° latitude by 0.1°
longitude was created around each net station and particle-tracking models were run
backwards in time. Particles were released within these areas over a 30-day period (25
particles per day), and subsequently tracked for a dispersal time equal to 45 days. These
models were forced by averaged ocean current fields (2002 - 2006) of the month when
the net station was sampled. Details of the particle tracking model, and the eddy-
resolving/data-assimilating ocean general circulation model can be found in (Condie et
al., 2005) and (Schiller et al., 2008), respectively. In our second approach, an area of 4°
latitude by 4° longitude was centred on each net station and drifters (drogued and un-
drogued) that reached these regions were selected. The tracks starting from the drifter
release point until they entered one of the net station areas were then plotted onto maps.
40
2.4 Results
We recorded 839 pieces of plastic, ranging in length from 0.4 to 82.6 mm (median = 2.8
mm, mean ± standard error = 4.9 ± 0.27 mm, Figure 2.2). The majority of these plastic
pieces had low circularity in their shape when compared to manufactured plastic
particles (e.g. pellets and microbeads from cosmetics), suggesting they mostly resulted
from the breakdown of larger items. The main plastic type was hard plastic (N = 633,
median length = 2.4 mm, range = 0.7 - 57.0 mm) followed by soft plastic (N = 142,
median length = 5.0 mm, range = 0.5 - 73.0 mm), plastic line (N = 54, median length =
10.3 mm, range = 2.0 - 82.6 mm), expanded polystyrene (N = 8, median length = 2.9
mm, range = 1.3 - 24.3 mm), and pellet (N = 2, both 4 mm). Most plastics were
white/transparent (84.7 %), but blue (8.3 %) and other colours (7 %) were also present.
Of the 200 pieces subjected to FT-IR, 67.5 % were made of polyethylene, 31 % of
polypropylene, 1 % of expanded polystyrene, and 0.5 % of ethylene vinyl acetate
(Figure 2.3).
41
Figure 2.2 Size and types of marine plastics collected around Australia.
Bars indicate the number of plastic pieces within each size category (< 2.5, 2.5 - 4.9, 5 – 10, > 10 mm) and colours show the amount of each plastic type within size categories. Examples of the types of plastic we collected are shown in the photos, including our biggest fragment of hard plastic (length = 57 mm, net station 32), soft plastic (length = 73 mm, net station 57, note the Indonesian words), and expanded polystyrene (Styrofoam cup fragment, length = 24.3 mm, net station 28).
42
Figure 2.3 Mean infrared spectra of the plastic pieces within each polymer type.
Approximately 80 % of our net tows (136 out of 171), and 93 % of our net stations (53
out of 57), had at least one piece of plastic (range: 0 – 68, median = 2, mean ± standard
error = 4.9 ± 0.63 pieces per net tow). Estimated sea surface plastic concentrations (Cs)
for each net tow ranged from 0 to 48895.6 pieces km-2 (median = 1932.1 pieces km-2,
mean ± standard error = 4256.4 ± 757.79 pieces km-2) and the mean Cs of net stations
varied between 0 and 23610.7 pieces km-2 (Figure 2.4, Appendix 2).
43
Figure 2.4 Mean sea surface plastic concentration (Cs) at the 57 net stations.
White crosses indicate location of major Australian cities (population > 1 million). From west to east: Perth, Adelaide, Melbourne, Sydney, and Brisbane.
Relatively high mean Cs (> 15500 pieces km-2) were estimated only at low wind speeds
(< 7 m s-1, Figure 2.5a). There was an inverse relationship between Cs and wind forcing
(b = -0.77 in Cs = a(u*w)b), which was relatively consistent with the biophysical model
applied here (Figure 2.5b). When taking into account the effect of wind-mixing, net tow
plastic concentrations increased by a mean factor of 2.8 (range: 1.04 – 10.0, median =
1.9). Hence, the amount of plastics collected by our net tows (Cs) represents anywhere
between 10.0 % and 96.1 % (median = 52.7 %, mean ± standard deviation = 50.0 ±
24.47 %) of the estimated total amount of plastic present in the water column (Ci,
Figure 2.6).
44
Figure 2.5 Sea surface plastic concentration (Cs) versus a) wind speed (U10) and b) water friction
velocity (u*w).
In (b) we also show the linear fit (Cs = a (u*w)b) and theoretical model estimates for Cs, when depth-integrated plastic concentration (Ci) is equal to 8966 (mean Ci of the 171 net tows) and significant wave height (Hs) is equal to the mean (1.85 m), maximum (4.78 m) and minimum (0.47 m) values estimated for the 57 net stations.
45
Figure 2.6 Mean and standard error of sea surface (Cs) and depth-integrated (Ci) plastic
concentrations.
Blue represents mean and standard error of Cs and red represents mean and standard error of Ci.
Depth-integrated plastic concentration estimates (Ci) for each net tow ranged from 0 to
105438.6 pieces km-2 (median = 4363.7 pieces km-2, mean ± standard error = 8966.3 ±
1330.75 pieces km-2) and the mean Ci of net stations ranged from 0 to 43194.5 pieces
km-2 (Figure 2.7). In this scenario, plastic concentrations higher than 15500 pieces km-2
(red dots) were quite common, and those higher than 31500 pieces km-2 (dark red dots)
were found close to populated areas (Brisbane and Fiji) as well as in some remote
coastal regions (southwest Tasmania) and oceanic areas (Figure 2.7).
46
Figure 2.7 Mean depth-integrated plastic concentration (Ci) at the 57 net stations.
White crosses indicate location of major Australian cities (population > 1 million). From west to east: Perth, Adelaide, Melbourne, Sydney, and Brisbane.
A wide range of pathways was taken by the virtual particles arriving at the net stations
(Figure 2.8, Appendix 2). The routes taken by real drifters, from their release points to
the net stations, showed similar patterns but covered larger areas due to their longer
drifting time and wider range of release date (Figure 2.9, Appendix 2).
47
Figure 2.8 Cumulative probability distribution of virtual particles arriving at the 57 net stations.
The month when the virtual particles (25 per day) were released is indicated in each panel. Backtracking dispersal time was equal to 45 days and arriving destinations (net stations) are marked with purple dots. See also Appendix 2.
48
Figure 2.9 Real drifter pathways arriving at the 57 net stations.
Purple dots indicate net station locations and asterisks indicate drifter release areas. See also Appendix 2.
49
2.5 Discussion
We found that the surface waters around Australia are contaminated with small plastics
that are mostly a by-product of the degradation of larger objects made of polyethylene
and polypropylene. The high prevalence of plastic fragments smaller than 5 mm in
Australian waters is consistent with other regions of the world’s oceans, where
microplastics were found to be the most abundant type of debris in all types of marine
environment (Moore et al., 2001, Thompson et al., 2004, Browne et al., 2010, Law et
al., 2010, Browne et al., 2011, Eriksen et al., 2013). Plastic pollution levels were
moderate when compared to concentrations in other marine areas (Moore et al., 2001,
Yamashita and Tanimura, 2007, Law et al., 2010, Collignon et al., 2012, Eriksen et al.,
2013). Higher amounts of plastic were found close to cities on Australia’s east coast, as
well as in remote locations (west Tasmania and North West Shelf). Recent studies
reported toxicological effects of these small and contaminated plastics on a host of
organisms, including large marine vertebrates (Fossi et al., 2012) and fish (Basheer et
al., 2004, Choy and Drazen, 2013, Gassel et al., 2013, Wright et al., 2013). As such,
small plastics are a type of harmful marine debris, implying that plastic hazards to
Australian species and ecological communities are likely to be broader than those
officially recognized.
2.5.1 Characteristics of marine plastics
Captured plastic particles ranged in size from 0.4 - 82.6 mm. The frequency distribution
of different sized plastics, which was skewed towards smaller particles, provides
evidence for the existence of smaller plastics. Current methods for assessing plastic
pollution at the ocean surface rely on the use of nets, which omits plastic particles
50
outside the collectible range of their mesh (Hidalgo-Ruz et al., 2012). It will be critical
for future investigations to develop efficient and reproducible techniques capable of
detecting smaller buoyant plastic particles (micro and nanoparticles). In addition, post
processing techniques for sorting particles are also likely to miss small fragments
(Hidalgo-Ruz et al., 2012). An example of a new method with the potential to eliminate
this limitation is the application of molecular mapping by reflectance micro-FT-IR
spectroscopy, which does not rely on visual selection of plastic particles for
characterization (Harrison et al., 2012).
Hard plastics were by far the most common plastic type found (75.4%), but soft plastics
(e.g. fragments of plastic wrappers) and lines (mostly fishing lines) were also relatively
common (16.5% and 6.4%, respectively). It is interesting to note that soft plastics were
more abundant in the larger size class (> 2.4 mm). Our findings are consistent with
recent studies documenting plastic pollution at the ocean surface, although explanations
for variations in hard/soft plastic trends are not given (Moore et al., 2001, Eriksen et al.,
2013, Morét-Ferguson et al., 2010). Plastics gradually lose buoyancy in seawater as a
result of biofilm formation (Ye and Andrady, 1991). We suggest that negative buoyancy
due to biofouling occurs more quickly in soft/thin than in hard/thicker plastic fragments,
resulting in a decline in the occurrence of soft plastics at the ocean surface, as they
become smaller/older and begin to sink. Indirect evidence for this is that the proportion
of soft plastics found in our coastal net stations was higher than that reported in open
ocean settings further away from potential sources (Morét-Ferguson et al., 2010). While
a small number of experimental studies have confirmed that biofilms decrease the
buoyancy of plastic items (Ye and Andrady, 1991, Lobelle and Cunliffe, 2011), none of
them report the magnitude or speed of this process across different types of small
fragments.
51
The plastics reported here were mostly white/transparent (84.7 %) or blue (8.3 %),
which is consistent with reports from other investigations on buoyant marine plastics
(Carpenter and Smith, 1972, Morét-Ferguson et al., 2010). Depending on the feeding
ecology of the affected animal, ingested plastic colour proportions can differ from what
is available in the environment (Schuyler et al., 2012). For instance, ingested plastic
colour proportions in Australian shearwaters (Ardenna pacifica and Ardenna
tenuirostris) are different from those reported by this study (Carey, 2011, Verlis et al.,
2013). As these birds are known to use colour vision to select their food (Bowmaker,
1980, Verlis et al., 2013), colour can play a role in the ingestion risk associated with a
certain plastic item. In contrast, the colour proportion of plastics found in scats of fur
seals (Arctocephalus spp.) at Macquarie Island (Australia) reflected what was available
as flotsam in this environment (Eriksson and Burton, 2003). These plastics are likely to
be coming from the stomach contents of their main prey: the myctophid Electrona
subaspera, which are pelagic small fish known to feed at night, selecting their food
based on size rather than colour (Eriksson and Burton, 2003).
The vast majority (98.5 %) of the plastics detected were made of polyolefins
(polyethylene and polypropylene), which is in agreement with what has been found for
this size range of plastics in other marine regions around the world (Morét-Ferguson et
al., 2010, Hidalgo-Ruz et al., 2012). Polyethylene and polypropylene account for most
of our global plastic production (38 % and 24 %, respectively) (Andrady, 2011) and
they are typically applied in the manufacturing of single-use disposable packaging. In
addition to packaging, which reaches the oceans primarily from coastal areas, fishing
equipment made of these polyolefins, e.g. fish crates, nets, ropes, fishing lines (Jones,
1995) are also likely sources of the plastic particles registered here. Other types of
polymers found in this study include two pieces of expanded polystyrene (Styrofoam), a
52
type of plastic also used in packaging and fishing gear, and one fragment of ethylene
vinyl acetate, which has several applications such as the making of shoe soles and foam
mats.
2.5.2 Concentrations and sources
Our overall mean sea surface plastic concentration (Cs) was 4256.4 pieces km-2, which
is similar to mean values reported for other regions outside subtropical gyres, such the
Caribbean Sea (mean Cs = 1414 pieces km-2) and Gulf of Maine (mean Cs = 1534
pieces km-2) (Law et al., 2010). Within subtropical gyres, Cs values tend to be higher
but within the same order of magnitude: 20328 pieces km-2 in the North Atlantic Gyre
(Law et al., 2010), and 26898 pieces km-2 in the South Pacific Gyre (Eriksen et al.,
2013). The exception seems to be the subtropical waters of the North Pacific and
Mediterranean, which present mean Cs values that are an order of magnitude higher
than those reported here: 116000 pieces km-2 in the Mediterranean (Collignon et al.,
2012), 174000 pieces km-2 in Northwest Pacific (Yamashita and Tanimura, 2007), and
334271 pieces km-2 in Northeast Pacific (Moore et al., 2001). The latter is also known
as the “Great Pacific Garbage Patch” (Moore et al., 2001), which is the largest
aggregator of floating marine particles (van Sebille et al., 2012).
Our findings show that the distribution of marine plastics is quite widespread (93 % of
our net stations had at least one plastic piece), patchy (i.e. high variability within and
between net stations’ Cs) and dynamic (Cs ranged from 10% to 91% of Ci). Therefore,
better spatio-temporal data coverage is required in order to identify plastic pollution
hotspots within Australian waters. However, our data already indicate some spatial
patterns: we observed high plastic concentrations close to Sydney and Brisbane cities.
This suggests that plastics along Australia’s east coast are mostly associated with
53
domestic inputs. Since high quantities of plastic were also found close to Viti Levu
(Fiji), we hypothesize that part of the plastics coming from coastal areas remain in the
vicinity of their sources for a long time, while fragmenting into smaller pieces. This
suggestion of local retention of plastic debris is in agreement with findings of recent
studies, e.g.(Thiel et al., 2013), and could be tested by developing high-resolution
models able to simulate plastic transport in coastal environments.
While the relatively high concentrations of plastic found close to the East Australian
coast (net stations 18-20, 22, 37) seem to originate from local sources of plastics, those
found in southwest Tasmania/eastern South Australia (net stations 5, 6, 8), and the
North West Shelf (net station 54) could be associated with international sources and/or
maritime operations. The presence of internationally-based plastics is suggested by (1) a
fragment with Indonesian words that was collected in North West Shelf (see Figure 2.2)
and (2) beach surveys, which registered in South Australia plastic debris from South
Africa and South America (Edyvane et al., 2004). High plastic concentrations in the
southern tip of Tasmania (net station 5) might be caused by convergence effects of the
encounter of the East Australian and Zeehan coastal currents (Edyvane et al., 2004),
whereas those found off the east coast (e.g. net station 37) could be associated with
meso-scale eddies of the East Australian current (Ridgway and Dunn, 2003).
Aside from this study and the one that developed the biophysical model we applied here
(Kukulka et al., 2012), we are not aware of any investigation that quantitatively
considers the effect of vertical mixing processes on plastic concentrations. This effect
needs to be taken into account in future studies assessing at-sea plastic pollution to
allow better comparisons between data collected under different sea states. An
important step towards improved simulations of plastic distribution along the water
column is to better quantify the buoyant rise velocity (wb) of plastic particles from
54
different oceanic and coastal surface waters. This variable has a considerable impact on
the output of the model applied here. Furthermore, other environmental variables that
were not taken into account in our one-dimensional column model (e.g. Langmuir
circulation, breaking waves, mixed layer depth) could be incorporated in this type of
modeling.
2.5.3 Potential pathways
The model outputs and routes taken by real drifters showed that plastics we found could
have moved via a wide range of routes. This is because our net stations are within
regions that experience different hydrodynamics, e.g. North West Shelf, Great
Australian Bright, Coral Sea, and Tasman Sea (Schiller et al., 2008). Plastics have the
potential to reach the sampled sites by travelling with a range of currents, including: (1)
Antarctic Circumpolar current (Talley et al., 2011a), which can carry plastics from a
wide area to several of our net stations, particularly those along the coast of Tasmania,
south coast of Australia, and Tasman Sea (net stations 1-15, 38 and 39); (2) South
Equatorial current in the Pacific Ocean (Webb, 2000, Talley et al., 2011a), which can
bring international plastics to the east coast of Australia (net stations 16-24, 40-45, 36,
37) and areas close to Fiji and New Caledonia (net stations 25-35); (3) East Australian
current (Ridgway and Dunn, 2003, Talley et al., 2011a), which can carry plastics from
domestic highly populated regions (e.g. Brisbane and Sydney) to the net stations along
the coast of Tasmania (net station 5, 15, 38, 39), east coast of Australia (net stations 16-
24, 36, 37) and the Tasman Sea (net stations 1-4); (4) Holloway, Leeuwin, South
Australian, and Zeehan coastal current systems (Ridgway and Condie, 2004, Sandery
and Kämpf, 2007, Condie and Andrewartha, 2008, Pattiaratchi and Woo, 2009), which
can bring plastics from international areas connected to the Indonesian Throughflow
and Indian Gyre, e.g. Southeast Asia/Indonesia (Lebreton et al., 2012), as well as from
55
domestic populated areas, to the net stations of the North West Shelf (net stations 48-
57), off Perth (net station 14), and along the south coast of Australia, Bass Strait,
Tasman Sea, and coast of Tasmania (net stations 1-13, 15-17, 37-39); and (5) West
Australia current (Pattiaratchi and Woo, 2009), which could transport international
marine plastics that accumulated in the Indian Gyre to the net stations in the North West
Shelf (net stations 48-57) and off Perth (net station 14).
It is important to note that running models backwards and using drifter trajectories
arriving at sampled locations can only provide an indication of the directions that the
collected plastics could have taken. To precisely estimate plastic pathways is quite
challenging, mostly because plastic source locations and quantities are still largely
unknown. Moreover, there are still no methods to estimate the “age” (drifting time) of a
certain plastic particle. For instance, only plastics with long drifting times (years) could
have matched the long tracks of drifters. Another limitation of the real drifter approach
is that the resulting pathway formed by all drifter tracks arriving at a certain region is
not only dependent on the ocean current systems, but also on the locations where most
of the drifters were released. For instance, sampled sites in the North West Shelf (net
stations 48-57) had only a few drifters arriving at them. This is mostly due to the non-
existence of drifters in the shallow waters of the Indonesian archipelago. The creation of
a shallow-water drifter - e.g. (Ohlmann et al., 2005) - release program in this area could
bring crucial information to help inform marine plastic pathways and sources.
2.5.4 Final remarks
This investigation shows that the abundant and widespread small marine plastics around
Australia are likely coming from a variety of domestic and international, land- and
ocean-based sources. Even though marine plastic pollution is a global environmental
56
issue, mostly caused by our massive production of plastic single-use disposable items,
there are still no attempts to regulate plastic disposal on land at an international level
(Rochman et al., 2013a). Additionally, dumping of plastics into the oceans remains a
significant issue owing to difficulties with regulation and enforcement (Jones, 1995,
Rakestraw, 2012). We suggest further at-sea studies on the characterization, spatial
distribution, and pathways of marine plastics in coastal and oceanic regions around
Australia, as well as on marine plastic toxin loads and interactions between small plastic
particles and organisms at all trophic levels of the food web. This would improve our
current knowledge on the effects of plastic on the marine ecosystem as a whole.
57
Chapter 3 Millimetre-sized Marine Plastics: A New Pelagic Habitat
for Microorganisms and Invertebrates
3.1 Summary
Millimetre-sized plastics are abundant in most marine surface waters, and known to
carry fouling organisms that potentially play key roles in the fate and ecological impacts
of plastic pollution. In this study we used scanning electron microscopy to characterize
biodiversity of organisms on the surface of 68 small floating plastics (length range = 1.7
- 24.3 mm, median = 3.2 mm) from Australia-wide coastal and oceanic, tropical to
temperate sample collections. Diatoms were the most diverse group of plastic
colonizers, represented by 14 genera. We also recorded ‘epiplastic’ coccolithophores (7
genera), bryozoans, barnacles (Lepas spp.), a dinoflagellate (Ceratium), an isopod
(Asellota), a marine worm, marine insect eggs (Halobates sp.), as well as rounded,
elongated, and spiral cells putatively identified as bacteria, cyanobacteria, and fungi.
Furthermore, we observed a variety of plastic surface microtextures, including pits and
grooves conforming to the shape of microorganisms, suggesting that biota may play an
important role in plastic degradation. This study highlights how anthropogenic
millimetre-sized polymers have created a new pelagic habitat for microorganisms and
invertebrates. The ecological ramifications of this phenomenon for marine organism
dispersal, ocean productivity, and biotransfer of plastic-associated pollutants, remain to
be elucidated.
58
3.2 Introduction
Millimetre-sized plastics resulting from the disintegration of synthetic products (known
as ‘microplastics’ if smaller than 5 mm) are abundant and widespread at the sea surface
(Moore et al., 2001, Law et al., 2010, Morét-Ferguson et al., 2010, Hidalgo-Ruz et al.,
2012, Eriksen et al., 2013, Law et al., 2014). These small marine plastics are a toxic
hazard to food webs since they can contain harmful compounds from the manufacturing
process (e.g. Bisphenol A), as well as contaminants adsorbed from the surrounding
water (e.g. polychlorinated biphenyls) (Mato et al., 2001, Rios et al., 2007, Teuten et al.,
2009, Rochman et al., 2013a). These substances can be carried across marine regions
and transferred from plastics to a wide range of organisms, from zooplankton and small
fish to whales (Basheer et al., 2004, Teuten et al., 2009, Boerger et al., 2010, Fossi et
al., 2012, von Moos et al., 2012, Gassel et al., 2013, Choy and Drazen, 2013, Cole et al.,
2013, Rochman et al., 2013c). Furthermore, they can physically damage suspension-
and deposit-feeding fauna (e.g. internal abrasions and blockages after ingestion)
(Wright et al., 2013), and alter pelagic and sediment-dwelling biota by modifying
physical properties of their habitats (Carson et al., 2011). Finally, these small marine
plastics can transport rafting species (Carpenter and Smith, 1972, Carpenter et al., 1972,
Gregory, 1978, Gregory, 1983, Carson et al., 2013, Zettler et al., 2013), potentially
changing their natural ranges to become non-native species and even invasive pests.
Apart from providing long-lasting buoyant substrata that allow many organisms to
widely disperse (Winston, 1982, Jokiel, 1990, Barnes, 2002, Masó et al., 2003, Barnes
and Fraser, 2003, Aliani and Molcard, 2003, Barnes, 2004, Thiel and Gutow, 2005,
Gregory, 2009, Fortuño et al., 2010, Goldstein et al., 2014), marine plastics may also
supply energy for microbiota capable of biodegrading polymers and/or associated
compounds (Sudhakar et al., 2007, Artham and Doble, 2009, Balasubramanian et al.,
59
2010, Harrison et al., 2011, Zettler et al., 2013, Harshvardhan and Jha, 2013), and
perhaps for invertebrates capable of grazing upon plastic inhabitants. The hydrophobic
nature of plastic surfaces stimulates rapid formation of biofilm, which drives succession
of other micro- and macro-organisms. This ‘epiplastic’ community appears to influence
the fate of marine plastic pollution by affecting the degradation rate (Andrady, 2011,
Zettler et al., 2013), buoyancy (Ye and Andrady, 1991, Moore et al., 2001, Lobelle and
Cunliffe, 2011), and toxicity level (Harrison et al., 2011) of plastics. Moreover,
epiplastic microbiota could have impacts on the microflora of its consumers, and
infectious organisms may reach their hosts through plastic ingestion (Harrison et al.,
2011, Pham et al., 2012, Zettler et al., 2013).
Although epiplastic organisms may play an important role in determining the fate and
ecological impacts of plastic pollution, little research has been directed to such study,
particularly on the inhabitants of the widely dispersed and abundant millimetre-sized
marine plastics (Harrison et al., 2011). In 1972, two papers first reported the occurrence
of organisms (diatoms, hydroids, and bacteria) on small plastics (0.1 – 5 mm long)
collected by plankton nets (Carpenter and Smith, 1972, Carpenter et al., 1972). Further
at-sea studies focusing on microplastic fouling biota only emerged in the 2000s (Carson
et al., 2011, Goldstein et al., 2012, Zettler et al., 2013). Zettler et al. (2013) conducted
the first comprehensive characterisation of epiplastic microbial communities, which
they coined the “Plastisphere”. These authors used scanning electron microscopy (SEM)
and next-generation sequencing to analyse three polyethylene and three polypropylene
plastic pieces (approx. 2 – 20 mm long) from offshore waters of the North Atlantic.
This pioneer study revealed a unique, diverse, and complex microbial community that
included diatoms, ciliates, and bacteria.
60
Here, we used SEM to examine types of organisms inhabiting the surface of 68 small
marine plastics (length range = 1.7 - 24.3 mm, median = 3.2 mm) from inshore and
offshore waters from around the Australian continent (Figure 3.1). We contributed
many new records of taxa associated with millimetre-sized marine plastics and imaged a
variety of marine plastic shapes and surface textures resulting from the interaction of
polymers with environments and organisms.
Figure 3.1 Sampling locations of the 68 plastics analysed in this study.
Black lines delimit marine regions of Australia (environment.gov.au/topics/marine/marine-bioregional-plans); dots indicate areas where the analysed plastics were collected; numbers represent how many plastics were taken for scanning electron microscopy analyses at these locations. Samples collected were fragments of hard plastic (N = 65), except at locations marked with an asterisk: one piece of Styrofoam cup in Fijian waters, one pellet in South Australia, and one piece of soft plastic in the Australia’s North-west marine region.
3.3 Material and Methods
Ethics Statement: Permits to conduct field research within the Great Barrier Reef area
were obtained from the Great Barrier Reef Marine Park Authority (GBRMPA: permit
61
G11/34378.1). No other special permit was required since sampling was limited to
marine debris.
Buoyant plastics were collected using surface net tows in waters around Australia (see
details in Chapter 2) and preserved in 2.5% glutaraldehyde buffered in filtered seawater.
Prior to analysis with a scanning electron microscope, plastics were dehydrated through
a series of increasing ethanol concentrations (up to 100%), critical-point dried using
CO2, mounted on aluminium stubs with carbon tape, and sputter coated with a 20 - 30
nm layer of gold. We used a Zeiss 1555 VP-FESEM scanning electron microscope
operated at 10 - 20 kV, 11 - 39 mm working distance, and 10 - 30 μm aperture. We
randomly selected 65 hard plastics among those small enough to fit onto SEM stubs (<
10 mm) and large enough to be easily handled (> 1 mm). For comparison, a piece of (1)
soft plastic, (2) industrial plastic pellet, and (3) expanded polystyrene (Styrofoam) were
also examined, totalling 68 plastic pieces examined with SEM. These plastics were
collected from offshore waters of the South-west Pacific (N = 19) and from different
Australian marine regions (environment.gov.au/topics/marine/marine-bioregional-
plans): North-west (N = 13), South-west (N = 3), South-east (N = 13), Temperate East
(N = 16), and Coral Sea (N = 4; Figure 3.1).
The different types of organisms detected on each plastic piece were imaged, measured
using ImageJ (length and width, http://rsb.info.nih.gov/ij/), classified into
taxonomic/morphological groups, and the frequency of occurrence (FO) for each type
was calculated. We used online resources, e.g. marinespecies.org,
westerndiatoms.colorado.edu, primary taxonomic literature, e.g (Cheng, 1976, Weise
and Rheinheimer, 1977, Kensley, 1994, Hallegraeff et al., 2010, Magalhães and Bailey-
Brock, 2012), and expert consultation to identify the organisms at the lowest possible
62
taxonomic level. Long filaments were very common but were excluded from the
analysis due to difficulty in determining if they were organisms or mucilage.
For each plastic piece observed, an image of the entire piece was taken at 50x
magnification. These images were uploaded to ImageJ to measure plastic particles’ size
parameters - length, area, perimeter, aspect ratio, and shape parameters - circularity and
solidity indexes (Paulrud et al., 2002, Ferreira and Rasband, 2012). Surface fractures,
pits and grooves (Corcoran et al., 2009, Cooper and Corcoran, 2010) were also
observed, recorded, and imaged while examining the entire surface of the plastics at
magnifications of 100 – 500x. Other peculiar microtextures observed at higher
magnifications, such as those suggesting interactions with biota, were also recorded and
imaged. After SEM analyses, plastics were washed with distilled water and submitted to
Fourier Transform Infrared spectrometry (FT-IR) for polymer identification. Two
plastic pieces were destroyed while being cleaned for FT-IR; as such, we identified the
polymer of 66 out of the 68 plastics examined using SEM.
3.4 Results
We examined 65 hard plastic fragments with lengths ranging from 1.7 to 8.9 mm
(median = 3.2 mm), one 4 mm-wide plastic pellet, one 8.7 mm portion of a 15 mm long
soft plastic fragment, and one 7 mm piece of a 24.3 mm Styrofoam cup fragment. Apart
from the Styrofoam cup fragment (expanded polystyrene), plastics were made of
polyethylene (N = 54) and polypropylene (N = 11). Hard plastics had a diverse range of
shapes (solidity index = 0.87 – 0.98, circularity index = 0.28 – 0.83; Figure 3.2) and
types of surface microtextures, including linear fractures, pits, and scraping marks
(Appendix 3). Diatoms and bacteria (rounded, and elongated cells) were by far the most
frequently observed organisms, being detected in all sampled marine regions (Figure
63
3.3). Plastics’ FT-IR spectra, 1143 SEM micrographs, and a matrix containing
information from collection sites, plastics characteristics, and organism/microtexture
presence-absence data are available in (Reisser et al., 2014).
64
Figure 3.2 Overall appearance of marine plastics, as shown by scanning electron micrographs.
Dot colour indicates the marine region where the piece was sampled (see legend and Figure 3.1). Pieces are hard plastic fragments, with the exception of the soft plastic fragment (red dot), pellet (yellow dot), and Styrofoam fragment (green dot) shown at the bottom of the diagram and marked with a white asterisk. All images are at the same magnification (see scale bar at lower right).
65
Figure 3.3 Types of epiplastic organisms detected at each of the marine regions sampled in this
study (see Figure 3.1).
Lines connect types of organisms (squares) to the marine regions (circles) where they were observed. Line colour indicates type of organism, with black lines representing invertebrates. Line thickness is proportional to the organism’s frequency of occurrence (FO = <25%, 25-50%, 50-75%, >75%).
Diatoms were the most abundant, widespread, and diverse group of plastic colonizers
(Figure 3.3 and Figure 3.4). These organisms were frequently observed (FO = 78%, N =
68 plastics) and included symmetrical biraphids/naviculoids (Navicula subgroup
66
lineatae, Mastogloia sp., Haslea sp.; Figure 3.4a-c), Nitzschioids (Nitzschia spp.,
Nitzschia longissima; Figure 3.4d-f), monoraphids (Cocconeis spp., Achnanthes sp.;
Figure 3.4g-i), centrics (Minidiscus trioculatus, Thalassiosira sp.; Figure 3.4j), araphids
(Thalassionema nitzschioides var. parva, Microtabella spp., Licmophora spp.,
Grammatophora sp.; Figure 3.4k,l,o), and asymmetrical biraphids (Amphora spp.,
Cymbella sp.; Figure 3.4m,n). Most diatoms were growing flat on the surface (adnate
and motile diatoms), but some were erect, attached to plastics by mucous pads or
stalks/peduncles. The genus Nitzschia was the most frequent diatom (FO = 42.6%),
followed by Amphora (13.2%), Licmophora (11.8%), Navicula (8.8%), Microtabella
(5.9%), Cocconeis (4.4%), Thalassionema (2.9%), and Minidiscus (2.9%). The other six
genera were only detected on a single plastic piece (FO = 1.5%). These frequencies of
occurrence are likely to be underestimated, as many diatoms could not be identified
from girdle-view positions (FO unidentified diatoms = 45.6%).
67
Figure 3.4 Examples of epiplastic diatoms.
a: Navicula sp.; b: Mastogloia sp.; c: small naviculoids; d: Nitzschia sp.; e: Nitzschia sp.; f: Nitzschia longissima; g,h: Cocconeis spp.; i: Achnanthes sp.; j: Thalassiosira sp.; k: Thalassionema nitzschioides; l: Microtabella sp.; m,n: Amphora spp.; o: Licmophora sp.
Calcareous coccolithophores were observed only on plastics from southern Australia
(South-east and South-west marine regions; FO = 37.5%, N = 16 plastics; Figure 3.3,
68
Figure 3.5a-h). The species identified included Calcidiscus leptoporus (Figure 3.5a),
Emiliania huxleyi (Figure 3.5b,c), Gephyrocapsa oceanica (Figure 3.5d),
Umbellosphaera tenuis (Figure 3.5e), Umbilicosphaera hulburtiana (Figure 3.5f),
Coccolithus pelagicus (Figure 3.5g), and Calciosolenia sp. (Figure 3.5h). Many of these
observations related to detached coccolith scales held in place by mucilage and chitin
filaments resembling those produced by diatoms (e.g. Thalassiosira; Figure 3.5b,f).
However, intact coccospheres were also present (Figure 3.5c,d,f). Additionally, one
specimen of the dinoflagellate Ceratium cf. macroceros was present on a 8.2mm plastic
from South-west Australia (Figure 3.3, Figure 3.5i).
69
Figure 3.5 Examples of epiplastic coccoliths (a-h) and dinoflagellate (i).
a: Calcidiscus leptoporus; b, c: Emiliania huxleyi; d: Gephyrocapsa oceanica; e: Umbellosphaera tenuis; f: Umbilicosphaera hulburtiana; g: Coccolithus pelagicus; h: Calciosolenia sp.; i: Ceratium cf. macroceros.
70
We found several unidentified organisms of various morphotypes and sizes, mostly
resembling bacterial, cyanobacterial, and fungal cells (Figure 3.6). After diatoms,
rounded/oval cells (length-width ratio < 1.5; Figure 3.6a-c,i-m) were the most
frequently observed morphotype (FO = 72%, N = 68 plastics; Figure 3.3).
Rounded/oval cells with widths < 1 μm and ≥ 1 μm had an overall FO of 38.2% and
54.4%, respectively.
Elongated cells (length-width ratio ≥ 1.5; Figure 3.6e-h) were also frequently observed,
being detected on 59% of the plastics examined (Figure 3.3). Those with widths < 1 μm
and ≥ 1 μm had an overall FO of 51.5% and 11.7%, respectively. Spiral cells (Figure
3.6d) had similar appearances (resembling spirochaete bacteria) and sizes (0.2 - 0.3 μm
width), and were only observed in the South-west Pacific region (FO = 31.6%, N = 19;
Figure 3.3). Several plastic pits and grooves contained bacteria-like cells closely
resembling their shape (Figure 3.6i-m). They were particularly common on plastics
covered by large rounded cells (Figure 3.6k).
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Figure 3.6 Examples of epiplastic rounded, elongated and spiral cells.
a, b, c: rounded cells; d: spiral “spirochaete” cell; e, f, g, h: elongated cells.; i, j, k, l, m: pits and grooves on plastics with rounded cells.
A few invertebrates were observed on the millimetre-sized plastics (FO = 16.2%, N =
68 plastics; Figure 3.3 and Figure 3.7). Colonies of encrusting bryozoans were the most
common epiplastic animal (FO = 8.8%; Figure 3.7a-d). They occurred on two fragments
from the Temperate East marine region and on four fragments from oceanic waters of
the South-west Pacific (plastic length = 3.2 - 5.4 mm). Four of these bryozoan colonies
were hosting abundant diatom assemblages dominated by Licmophora sp., Nitzschia
longissima (Figure 3.7a), Amphora sp. (Figure 3.7c), and Nitzschia sp. (Figure 3.7d).
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Additionally, lepadomorph barnacles (Lepas spp.; Figure 3.7e,f) were attached to the
24.3 mm Styrofoam cup fragment and to a 8.2 mm-long hard plastic; an Asellote isopod
(Figure 3.7g) was found on the Styrofoam cup fragment; eggs of the marine insect
Halobates sp. (Figure 3.7h) were observed on two plastics (4.6 and 5.5 mm long); and a
unidentified marine worm (Figure 3.7i,j) was found on a 6 mm hard plastic fragment.
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Figure 3.7 Examples of epiplastic invertebrates.
a: Bryozoan colony harboring an abundant assemblage of Nitzschia longissima (zoomed image shows part of this assemblage, scale bar = 20 μm); b: bryozoan colony relatively free of fouling; c: bryozoan-plastic interface displaying an abundant epizoic assemblage of Amphora sp.; d: bryozoan-plastic interface displaying an abundant epizoic assemblage of Nitzschia sp.; e, f: barnacles (Lepas spp.); g: Asellota isopod; h: egg of the marine insect Halobates sp.; i: marine worm; j: zoom on the surface of the unidentified marine worm shown in ‘i’.
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3.5 Discussion
There now exists a large body of evidence that millimetre-sized plastics are abundant
and widespread in marine environments (Carpenter et al., 1972, Carpenter and Smith,
1972, Moore et al., 2001, Law et al., 2010, Morét-Ferguson et al., 2010, Hidalgo-Ruz et
al., 2012, Eriksen et al., 2013) and our study significantly adds to this by conclusively
demonstrating that they are colonized by a wide range of biota, particularly diatom and
bacteria species - Table 1 (Carpenter and Smith, 1972, Gregory, 1978, Gregory, 1983,
Moore et al., 2001, Carson et al., 2013, Zettler et al., 2013). We more than doubled the
number of known diatom genera inhabiting millimetre-sized marine plastics and
provide the first identifications of coccolithophore genera attached to these floating
plastic particles. We also recorded a few invertebrate species living on these small
plastics. As such, our findings provide further evidence that not only large debris
(Winston, 1982, Jokiel, 1990, Aliani and Molcard, 2003, Barnes, 2002, Barnes and
Fraser, 2003, Masó et al., 2003, Barnes, 2004, Thiel and Gutow, 2005, Gregory, 2009,
Fortuño et al., 2010, Goldstein et al., 2014) serve as vehicles for organism dispersal.
Abundant ‘microplastics’ are equally providing a new pelagic habitat to many
microorganism and a few invertebrate taxa.
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Table 1 List of known genera occurring on millimetre-sized pelagic plastics.
Organism groups (first column), their abundance and/or frequency of occurrence (when available; second column), and genera (third column). References are indicated by superscript letters and given at the bottom of the table, along with approximate length range of plastics examined. Genera in bold indicate those first detected in this study.
Group Abundance/FO
Genera
Bacteria a,b,c,d,j d1833 per mm2
Acinetobacterb, Albidovulumb, Alteromonasb, Amoebophilusb, Bacteriovoraxb, Bdellovibriob, Blastopirellulab, Devosiab, Erythrobacterb, Filomicrobiumb, Fulvivirgab, Haliscomenobacterb, Helleab, Henriciellab, Hyphomonasb, Idiomarinab, Labrenziab, Lewinellab, Marinoscillumb, Microscillab, Muricaudab, Nitrotireductorb, Oceaniserpentillab, Parvularculab, Pelagibacterb, Phycisphaerab, Phormidiumb, Pleurocapsab, Prochlorococcusb, Pseudoalteromonasb, Pseudomonasb, Psychrobacterb, Rhodovulumb, Rivulariab, Roseovariusb, Rubrimonasb, Sediminibacteriumb, Synechococcusb, Thalassobiusb, Thiobiosb, Tenacibaculumb, Thalassobiusb, Vibriob
Diatoms a,b,c,d,f a 77.9% d 1188 per mm2
Amphoraa, Achananthesa, Chaetocerosb, Cocconeisa, Cyclotellac , Cymbellaa, Grammatophoraa, Hasleaa, Licmophoraa, Mastogloiaa,c, Microtabellaa, Minidiscusa, Naviculab, Nitzschiaa,b, Pleurosigmac, Sellaphorab, Stauroneisb, Thalassionemaa, Thalassiosiraa
Coccoliths a,d,b a 8.8% Calcidiscusa, Emilianiaa, Gephyrocapsaa, Umbellosphaeraa, Umbilicosphaeraa, Coccolithusa, Calciosoleniaa
Bryozoa a,e,f a 8.8% Membraniporaf, Jellyellae, Bowerbankiae, Filicrisiae Hydroids c,e - Clytiac, Gonothyraeac, Obeliae Polychaete g - Spirorbisg, Hydroidesg Dinoflagellates a,b,d a 1.5% Alexandriumb, Ceratiuma Insect eggs a,h,i a 2.9% Halobatesa,h,i Barnacles a a 2.9% Lepasa Rhodophyta b,g - Fosliellag Foraminifera g - Discorbisg Radiolaria b,d - Circorrhegmad Ciliate b - Ephelotab
aThis study (1.7 – 24.3 mm), bZettler et al. 2013 (2 – 20 mm), cCarpenter and Smith 1972 (2.5 – 5 mm), dCarson et al. 2013 (1 – 10 mm), eGoldstein et al. 2014 (4 – 5 mm), fGregory 1978 (2 – 5 mm), gGregory 1983 (1 – 5 mm), hMajer et al. 2012 (2 – 5 mm), iGoldstein et al. 2012 (1.2 – 6.5 mm), jCarpenter et al. 1972 (0.1 – 2 mm)
We observed fouling diatoms to be diverse and widespread on marine plastics. These
diatoms seemed to firmly attach to the plastic, resisting water turbulence and wave
action. All the identified diatom genera are well known to form biofilms on estuarine
and marine sediments and rocks (epilithic), vegetation (epiphytic), and animals
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(epizoic) (Carpenter, 1970, Totti et al., 2009, Reisser et al., 2010, Congestri and
Albertano, 2011, Tiffany, 2011, Romagnoli et al., 2014); marine plastics thus create a
novel, long-lasting and abundant floating habitat for ‘benthic’ diatoms, in a light and
nutrient-filled environment that is stable and beneficial to these organisms. Future
epiplastic diatom research should focus on the quantitative contribution of these
organisms to enhancing primary and secondary productivity of different marine regions,
such as within subtropical gyres where productivity tends to be low but plastic pollution
level high (Moore et al., 2001, Polovina et al., 2008, Law et al., 2010, Eriksen et al.,
2013). Because of their rapid growth and production of extracellular substances
(Kawamura et al., 1995), epiplastic diatoms may provide an important food source for
invertebrate grazers. As plastic debris can contain harmful substances (Mato et al.,
2001, Rios et al., 2007, Teuten et al., 2009, Gassel et al., 2013, Rochman et al., 2013c),
it remains unclear if such grazer-plastic relationships would have a positive or negative
impact on the populations involved in this new type of food web.
A significant number of coccolithophore species were present on millimetre-sized
marine plastics. These planktonic organisms are not commonly recognized as fouling or
rafting organisms (Thiel and Gutow, 2005), although their occasional occurrence on
marine plastics was briefly mentioned in recent studies (Carson et al., 2013, Zettler et
al., 2013). Some of our observations were of clusters of mixed coccolith species,
resembling zooplankton fecal pellets, and of solitary coccoliths, likely detached from
living coccospheres and stuck to clingy parts of the plastic biofilm. However, entire
coccolithophores were also seen attached to plastics, suggesting that these organisms
could be using ocean plastics as ‘floating devices’. We only observed coccoliths on
plastics from southern Australia; as such, additional studies in these temperate waters
may help better understand this potential coccolith-plastic relationship. Another atypical
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organism detected was the planktonic dinoflagellate Ceratium cf. macroceros. Recent
studies have found plastics heavily fouled by dinoflagellates, including individuals and
cysts of the potentially harmful species Ostreopsis sp., Coolia sp., and Alexandrium
spp. (Masó et al., 2003, Zettler et al., 2013), but here we only detected a single
specimen of this group.
Several unidentified organisms (rounded, oval, elongated, and spiral) resembling
bacterial cells were flourishing on millimetre-sized marine plastics. This supports
previous studies that describe well established bacterial populations growing on plastic
fragments (Carson et al., 2013, Zettler et al., 2013). Many of these unidentified cells
were apparently interacting with the plastic surface by forming pits and grooves. Within
this group of “pit-formers”, colonies of rounded cells (around 5 micron in diameter)
covered large areas of the plastic surface. They were similar to some previously
unidentified epiplastic organisms from the North Atlantic (Zettler et al., 2013). These
SEM observations, along with detections of putative hydrocarbon-degrading bacteria on
marine plastics (Zettler et al., 2013) and experiments demonstrating that marine bacteria
can biodegrade polymers (Sudhakar et al., 2007, Artham and Doble, 2009,
Balasubramanian et al., 2010, Harrison et al., 2011, Zettler et al., 2013, Harshvardhan
and Jha, 2013), strongly suggest that plastic biodegradation is occurring at the sea
surface. Such process could partially explain why quantities of millimetre-sized marine
plastics are not increasing as much as expected (Law et al., 2010, Law et al., 2014).
Studies of the “Plastisphere” from different marine regions worldwide will prove
invaluable for extending our knowledge on epiplastic marine microbial communities,
and may support the development of biotechnological solutions for better plastic waste
disposal practices (Sivan, 2011, Sangale et al., 2012, Webb et al., 2012).
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A number of invertebrates inhabited the small plastics examined here: bryozoans,
barnacles Lepas spp., an Asellota isopod, a marine worm, and eggs of the marine insect
Halobates sp. Even though microplastic-associated animals are rare and less diverse
when compared to those associated with macroplastics (Winston, 1982, Jokiel, 1990,
Barnes, 2002, Aliani and Molcard, 2003, Barnes and Fraser, 2003, Masó et al., 2003,
Barnes, 2004, Thiel and Gutow, 2005, Gregory, 2009, Fortuño et al., 2010, Goldstein et
al., 2014), ecological implications of this phenomenon may be significant, e.g.
(Goldstein et al., 2012), given the large quantities and wide distribution ranges of
millimetre-sized plastics in the marine environment (Carpenter et al., 1972, Carpenter
and Smith, 1972, Moore et al., 2001, Law et al., 2010, Morét-Ferguson et al., 2010,
Hidalgo-Ruz et al., 2012, Eriksen et al., 2013). Among the effects plastic associates may
have is to shape ‘epiplastic’ microbiota by hosting unique epizoic assemblages on their
bodies. For instance, the bryozoan colonies examined here covered a large proportion of
their plastic-host, with some of them harboring unique diatom-dominated assemblages.
Previous studies have shown that bryozoans do not represent neutral surfaces for
microbial colonizers (Scholz and Hillmer, 1995, Kittelmann and Harder, 2005), with
some species offering a favourable habitat for diatoms when compared to the
surrounding substratum, e.g. by protecting against predators and supplying nutrients
through flow generated by polypids (Wuchter et al., 2003). Further studies focusing on
both epiplastic microorganisms and invertebrates have the potential to further elucidate
symbiotic and/or competitive relationships between inhabitants of this new type of
pelagic habitat.
In summary, this study showed that millimetre-sized marine plastics are providing a
new niche for several types of microorganisms and some invertebrates. This
phenomenon has considerable ecological ramifications and deserves further research.
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As discussed here, additional observational and experimental studies on the inhabitants
of these small plastic fragments may better elucidate several key plastic pollution
processes that remain poorly assessed, such as at-sea polymer degradation and
mineralisation, impacts of epiplastic communities on their consumers, and changes in
the distributional range of species by plastic rafting.
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Chapter 4 The vertical distribution of buoyant plastics at sea: an
observational study in the North Atlantic Gyre
4.1 Summary
Millimeter-sized plastics are numerically abundant and widespread across the world’s
ocean surface. These buoyant macroscopic particles can be mixed within the upper
water column due to turbulent transport. Models indicate that the largest decrease in
their concentration occurs within the first few meters of water, where in situ
observations are very scarce. In order to investigate the depth profile and physical
properties of buoyant plastic debris, we used a new type of multi-level trawl at 12 sites
within the North Atlantic subtropical gyre to sample from the air-seawater interface to a
depth of 5 m, at 0.5 m intervals. Our results show that plastic concentrations drop
exponentially with water depth, and decay rates decrease with increasing Beaufort scale.
Furthermore, smaller pieces presented lower rise velocities and were more susceptible
to vertical transport. This resulted in higher depth decays of plastic mass concentration
(milligrams m-3) than numerical concentration (pieces m-3). Further multi-level
sampling of plastics will improve our ability to predict at-sea plastic load, size
distribution, drifting pattern, and impact on marine species and habitats.
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4.2 Introduction
Plastics pose physical and chemical threats to the oceans’ ecosystem. Their widespread
occurrence at the sea surface may be shifting the distribution and abundance of marine
populations due to (1) enhanced ocean drift opportunities and (2) damaging effects on
biota and habitats. Plastics harbour organisms - such as fouling microorganisms,
invertebrates, and fish - that can widely disperse via this new type of habitat, potentially
entering non-native waters (chapter 3) (Winston et al., 1997, Barnes, 2002, Thiel and
Gutow, 2005, Zettler et al., 2013). Plastic objects can also entangle or be
ingested/inhaled by marine animals, leading to impacts such as starvation, death, and
hepatic stress (Derraik, 2002, Browne et al., 2008, Gregory, 2009, Rochman et al.,
2013c, Watts et al., 2014).
Most of what is known about at-sea plastic characteristics and concentrations comes
from surface net sampling, where the top few centimetres of the water column is filtered
to collect plastics larger than 0.2–0.4 mm (Hidalgo-Ruz et al., 2012). These sea surface
samples have shown that the world’s sea surface contains many millimetre-sized plastic
pieces known as ‘microplastics’ when smaller than 5 mm in length (Arthur et al., 2009,
Hidalgo-Ruz et al., 2012). This type of plastic pollution is widespread across oceans,
with higher contamination levels at convergence zones such as those within subtropical
gyres (Carpenter and Smith, 1972, Maximenko et al., 2012, Lebreton et al., 2012, van
Sebille et al., 2012, Cózar et al., 2014, Eriksen et al., 2014). Plastic debris collected by
surface nets are mostly fragments of packaging and fishing gear made of polyethylene
and polypropylene (chapter 2) (Barnes et al., 2009, Morét-Ferguson et al., 2010,
Hidalgo-Ruz et al., 2012). These two resins are less dense than seawater and account for
approximately 62% of the plastic volume produced each year (Andrady, 2011).
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Turbulence in the upper-ocean layer can vertically mix buoyant plastic particles. A
model developed by Kukulka et al. 2012 predicted that the largest decrease in plastic
concentration occurs over the first meters of the water column, where only a few low-
resolution measurements exist (Lattin et al., 2004, Doyle et al., 2011, Kukulka et al.,
2012, Isobe et al., 2014). As studying ocean turbulent transport is heavily dependent on
observations (Ballent et al., 2012, D'Asaro, 2014), high-resolution multi-level plastic
sampling is needed to test this prediction. A better understanding of the vertical
transport of buoyant plastics is fundamental for improving estimates of concentration,
size distribution, and dispersal of plastics in the world’s ocean (chapter 2) (Kukulka et
al., 2012, Law et al., 2014, Isobe et al., 2014).
In this context, the present study aimed at obtaining depth profiles of plastic pollution in
the top layer of the oceans (0-5 m). We performed multi-level sampling with a new type
of equipment to (1) quantify the exponential decay rates of plastic mass and numerical
concentration with depth, and (2) demonstrate how these vary with sea state. We also
provide the first experimental measurements of the rise velocity of plastic pieces,
evaluating its relation to the type and size of pieces.
4.3 Materials and Methods
4.3.1 At-sea sampling
We conducted 12 multi-level net tows that sampled the upper 5 meters of the North
Atlantic accumulation zone (Law et al., 2010, Maximenko et al., 2012, Lebreton et al.,
2012) during day hours, from 19 to 22 May 2014, aboard the sailing vessel Sea Dragon
(Figure 1). We used a new collection device capable of sampling surface waters from
the air-seawater interface to a depth of 5 m, at 0.5 m intervals. This equipment is
composed of eleven frames with 0.5 m height x 0.3 m width fitted with 2.1 m-long 150
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µm mesh polyester nets. These nets were stacked vertically and secured within an
external frame that was dragged in the water from eight towing points, ensuring its
stability and perpendicular position in relation to the sea surface, with the top net
completely above mean water line (see Figure 1). Tow durations ranged from 55 to 60
minutes and were all undertaken while the vessel was travelling at a speed of 1–1.9
knots. The captain, who has 20 years sailing experience, estimated wind speeds and sea
state of each sampling period: Beaufort scale 1 (N = 3 net tows), 3 (N = 4 net tows), and
4 (N = 5 net tows) (Reisser et al., 2015). After each tow, we transferred the collected
contents to a 150 µm sieve and stored them in aluminium bags that were kept frozen
during transportation.
Figure 4.1 North Atlantic map indicating locations sampled during this study (orange dots) using
the multi-level net displayed in the right panel.
The map also shows the expedition departure and arrival location (Bermuda), plastic accumulation zones as predicted by ocean modelling (Lebreton et al., 2012, Maximenko et al., 2012), and a surface net tow dataset (grey dots) (Law et al., 2010).
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4.3.2 Estimating depth profiles of plastic contamination
We calculated plastic numerical and mass concentrations by dividing the number of
plastic pieces and total plastic mass by the volume of filtered seawater of each net
sample (pieces m-3 and milligrams m-3). Filtered volume was estimated using frame
dimensions and readings from a mechanical flowmeter (32 cm per rotation).
Samples were washed into a clear plastic container filled with filtered seawater, and
floating macroscopic plastics were organised into gridded petri dishes for counting and
characterisation. The searches for plastic pieces were of at least one hour per sample,
with the aid of thumb forceps, dissecting needles, magnifying glasses, and LED torches.
The latter was particularly important for detecting thin transparent plastic fragments,
which had low detection probability when not reflecting light. Two thin filaments
resembling textile fibres were discarded due to potential air contamination as noted in
(Foekema et al., 2013). Once all plastics were counted and characterised, they were
washed with deionised water, transferred to aluminium dishes, dried at 60° C, and
weighed.
To quantify the variation of plastic concentration with depth and assess the effect of
changing sea state on these vertical profiles, we first divided plastic concentration of
samples by their corresponding surface concentration value. We then took the average
of these normalised concentrations between adjacent nets to estimate normalised plastic
concentration values at depths of: 0 m (top 2 nets), 1 m (3rd and 4th nets), 2 m (5th and
6th nets), 3 m (7th and 8th nets), 4 m (9th and 10th), and 4.75 m (11th net). Finally,
numerical and mass concentration values from tows collected under the same Beaufort
scale were grouped and fitted to exponential decay models of the form N = e−λz , where
N = normalised plastic concentration, z = depth, and λ = decay rate.
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We also predicted normalised plastic concentration depth profiles using the model
described in Kukulka et al. (2012): N = ezwbA0−1
, where z = depth, wb = plastic rise
velocity, and A0 =1.5u*wkHs with u*w = frictional velocity of water, k = 0.4 (von
Karman constant), and Hs = significant wave height. We considered wb = 0.0053 m s-1
(plastics’ median rise velocity, as estimated in this study), Hs = 0.1 m, 0.6 m, or 1m
(typical wave heights experienced at Beaufort scales 1, 3, and 4, respectively), and used
the wind ranges of Beaufort 1, 3, and 4 (1-3 knots, 7-10 knots, and 11-16 knots,
respectively) to estimate their respective u*w values through the approximation proposed
by (Pugh, 1987): u*w = 0.00012W10, where W10 = ten-metre wind speed in m/s. Thus, the
considered numerical ranges of frictional velocity of water (u*w) were: 0.0006-0.0019 m
s-1 for Beaufort scale 1, 0.0043 – 0.0062 m s-1 for Beaufort scale 3, and 0.0068-0.0099
m s-1 for Beaufort scale 4.
4.3.3 Characterising plastic length, type, resin, and rise velocity
We measured the length of all plastic pieces using a transparent ruler (0.5 mm
resolution), and classified them into the following types: hard plastic - fragments of
rigid plastic; sheet - fragments of thin plastic, with some degree of flexibility; line -
fragments of fishing lines or nets; foam - expanded polystyrene fragments; and pellet -
raw material used to produce plastic items (Fotopoulou and Karapanagioti, 2012). We
also identified the resin composition of 60 pieces using Raman spectroscopy (WITec
alpha 300RA+), and measured the rise velocity of 0-3 plastics from each sample
collected.
Our method of rise velocity measurement is an adaptation of an experiment to examine
the fall velocity of various types of sediment particles in different fluids (Allen, 1985).
Firstly, we made two marks 12.5 cm from the ends of a 1 m long clear plastic tube
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(diameter = 40 mm). Secondly, we filled the tube with filtered seawater, capped both its
ends with rubber stops, and locked it in place with a clamp. One of the tube ends was
then opened, a plastic piece placed inside, the tube closed again (with no trapped air),
and quickly turned upside down and locked in place with a clamp, using a spirit level to
adjust its vertical position. Finally, we recorded the time taken for the plastic piece to
rise from one mark to the other (distance = 75 cm) using a stopwatch. This was
measured 4 times per plastic piece, and the average was used as the estimation of its rise
velocity (wb). Rise velocities of different plastic types were separately plotted against
plastic lengths (l), and linear regressions of the formwb = al + b were applied to assess
the effect of plastics’ characteristics on its rise velocity. We also plotted the rise
velocities of plastic pieces collected at different depths to visualise depth patterns.
Finally, we calculated the fractions of plastics of different size classes (0.5-1 mm, 1.5-2
mm, 2.5-3 mm, 3.5-4 mm, 4.5-5 mm, > 5.5 mm) that were located at the sea surface
(depth < 0.5 m) and in deeper layers (depth > 0.5 m) during sampling at Beaufort scales
1, 3, and 4. We calculated these fractions using all plastics collected, as well as
separated by plastic type.
4.4 Results
4.4.1 Profiles of mass and numerical concentrations
Plastic numerical and mass concentrations both decreased abruptly from their peak
values at the sea surface, where median values were equal to 1.69 pieces m-3 and 1.60
mg m-3 (Figure 4.2). Concentration differences between surface and deeper layers were
higher in terms of mass than number of particles. For instance, median mass and
numerical concentrations at 0.5-1 m were respectively 13.3 and 6.5 times lower than
their median plastic peaks at 0-0.5 m.
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Figure 4.2 Boxplots of plastic numerical (left) and mass (right) concentrations at different depth
intervals (N = 12 multi-level net tows).
The central line is the median value, edges of the box are the 25th and 75th percentiles, whiskers extend to extreme data points not considered outliers, and outliers are plotted individually as crosses.
Exponential models fitted well with both numerical and mass concentrations (R2 =
0.99–0.84), with depth decay rates (λ) consistently higher for mass than numerical
concentration. Furthermore, both numerical and mass concentration decay rates were
inversely proportional to Beaufort state (Figure 4.3). Depth decay rate of numerical
concentration went from 3.0 at Beaufort 1 (95% confidence interval - 95%CI = 2.56-
3.45), to 1.7 at Beaufort 3 (95%CI = 1.51-1.88), and 0.8 at Beaufort 4 (95%CI = 0.62-
0.98). Decay rate of mass concentration went from 3.8 at Beaufort 1 (95%CI = 3.23-
4.33), to 2.4 at Beaufort 3 (95%CI = 1.63-3.14), and 1.7 at Beaufort 4 (95%CI = 1.50-
1.94).
These exponential fits had relatively similar depth decay rates to those predicted by
Kukulka’s model for Beaufort 3 (λ = 2.36–3.37) and 4 (λ = 0.88–1.28). However, for
Beaufort 1 the statistical fit showed much smaller λ (2.56-4.33) than those predicted by
Kukulka’s model (λ = 141.73–47.2492).
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Figure 4.3 Depth profiles of plastic mass and numerical concentration under different Beaufort
scales: 1 (N = 3 net tows), 3 (N = 4 net tows), and 4 (N = 5 net tows).
Black lines show model predictions (Kukulka et al., 2012) using median plastic rise velocity (0.0053 m/s), and the typical range of frictional velocity of water (u*w) at each of the sea states sampled.
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4.4.2 Lengths, types, resins and rise velocities of plastics
We counted and classified 12,751 macroscopic plastic pieces with lengths varying from
0.5 to 207 mm (median = 1.5 mm; Figure 4.4). They were mostly fragments of
polyethylene (84.7%), followed by polypropylene (15.3%) items. Hard plastics (46.6%)
and sheets (45.4%) were predominant, with lower presence of plastic lines (7.9%),
pellets (0.05%) and foams (0.008%).
Figure 4.4 Glass jars with filtered water and plastic samples collected under wind speeds of 1 knot
(top image) and 15 knots (bottom image).
From left to right: 0 – 0.5 m, 0.5 – 1 m, 1 – 1.5 m, 1.5 – 2 m, 2 – 2.5 m, 2.5 – 3 m, 3 – 3.5 m, 3.5 – 4 m, 4 – 4.5 m, and 4.5 – 5 m deep.
Plastic rise velocity ranged from 0.001 to 0.0438 m/s (Figure 4.5a). It was directly
proportional to plastic length, with the slope of this linear relationship differing among
types of plastic (Figure 4.5b). While both hard plastics and sheets had a slope equal to
0.002 (95% CI = 0.0017-0.0026 and 0.0012-0.0023, respectively), plastic lines had a
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flatter slope of 0.00007 (95% CI = 0.00002-0.00013), since their rise velocity increased
only slightly towards longer pieces. Rise velocities differed among sampled depths,
with particles at the surface (0-0.5 m) having a wider range of values and a higher
median value than pieces at greater depths (Figure 4.5c).
Figure 4.5 Histogram of rise velocity of plastics (A), plots of plastic sizes x rise velocities of different
types of plastic (B), and boxplot of rise velocity at different depth intervals (C).
In C, the central dot is the median value, edges of the box are the 25th and 75th percentiles, whiskers extend to extreme data points not considered outliers, and outliers are plotted individually as crosses.
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The vertical mixing process was size-selective, and affected the size distribution of
plastics located at the sea surface (Figure 4.6), with the proportion of plastics at depths
over 0.5 m generally increasing towards smaller plastic lengths (Figure 4.7). For hard
plastics and sheets, this trend was observed at all Beaufort scales sampled. Plastic lines
however, only displayed this trend at Beaufort 1, with different size classes showing
similar and relatively high underwater proportions at Beaufort 3 and 4.
Datasets produced and analysed in this study are available at Figshare (Reisser et al.,
2015).
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Figure 4.6 Size histograms of plastics collected at depths 0-0.5 m and 0.5-5 m during Beaufort scale
1 (top panel), 3 (middle panel), and 4 (bottom panel).
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Figure 4.7 Percentage of plastic pieces of different size classes located at depths higher than 0.5 m
during sampling at Beaufort scale 1, 3, and 4.
Datasets produced and analysed in this study are available at Figshare (Reisser et al.,
2015).
4.5 Discussion
This study describes high-resolution depth profiles of plastic concentrations, which
were shown to decrease exponentially with depth, with decay rates decreasing towards
stronger winds. It also provides the first measurements of the rise velocity of ocean
plastics, which varies with particle size and type. Furthermore, it shows that depth
profiles of plastic mass are associated with higher decay rates than depth profiles of
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plastic numbers. This can be explained by our observation of smaller/lighter plastic
pieces generally associated with lower rising velocities, being therefore more
susceptible to vertical transport.
Predictions of plastic vertical mixing are commonly used to correct numerical
concentrations obtained by surface net sampling (chapter 2) (Kukulka et al., 2012,
Cózar et al., 2014, Law et al., 2014). As determined in our study, the model described in
Kukulka et al. (2012) performed relatively well in estimating the total number of plastic
pieces at the wind-mixed surface layer. The major difference between this model and
our observations occurred at the calmest sea state condition (Beaufort scale 1): while the
model predicted that all plastics would be at the surface, we still observed some
particles submerged at depths greater than 0.5 m below the water surface. This could
have been a consequence of the presence of other types of vertical flow at our sampled
sites (e.g. downwelling) or the occurrence of plastics rising from deeper waters due to
previous wind-driven mixing events.
Our results indicate that plastic numerical concentration decays at a lower rate than
plastic mass concentration, as smaller plastics are more susceptible to vertical transport.
The uncertainties related to how plastic numerical concentration translates into plastic
mass concentration have already led to differences between plastic load estimates
arising from different studies. For instance, Cózar et al. (2014) used a correlation based
on simultaneous surface tow measurements of total mass and abundance of plastic to
convert depth-integrated numerical concentrations into mass concentrations. These
authors estimated that the total plastic load in the world’s sea surface layer is between
7,000 and 35,000 tons. On the other hand, Law et al. (2014) multiplied depth-integrated
numerical concentrations by the average plastic particle mass (1.36 x 10-5 kg), and
estimated that the microplastic load at the North Pacific accumulation zone alone is of
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at least 21,290 tons. Such differences evidence the importance of better predicting the
vertical transport of ocean plastics to develop standard plastic load estimation methods.
More sampling is required to better quantify both profiles of plastic mass and numerical
concentration over a broader range of sea states, and translate these observations into
prediction models. Such models may need to be three-dimensional, and account not
only for wind mixing effects, but also ocean plastic properties (e.g. particle size) and
other types of vertical transport processes (e.g. Langmuir circulation).
As shown here, and in two modelling studies (Ballent et al., 2012, Isobe et al., 2014),
vertical mixing affects the size distribution of plastics floating at the surface. We
observed that the proportion of plastics mixed into deeper waters increases towards
smaller sizes even under low wind speed (1 knot) conditions (see Figure 7). This
observation has implications for studies assessing size distribution of plastics using
surface sampling devices. Cózar et al. (2014) and Eriksen et al. (2014) quantified the
size distribution of ocean plastics from worldwide sampling locations and concluded
that there are major losses of small plastics from the sea surface. Here we show that at
least a fraction of this ‘missing’ plastic could be just under the sampled surface layer (0-
0.5 m). For instance, 20% of 0.5-1 mm, 13% of 1.5-2 mm, and 8% of 2.5-3 mm long
plastics were between 0.5 and 5 m deep during our Beaufort scale 1 net tows. More at-
sea and experimental work is required to further quantify this effect and estimate depth-
integrated size distribution of buoyant plastics drifting at sea.
Predicting the vertical mixing of buoyant plastics is also important as it affects the
horizontal drifting patterns and ecological impacts of plastic pollution. For instance,
larger pieces of plastic coming from land-based sources may stay trapped near the shore
until further fragmentation, due to a combination of their high buoyancy and the effect
of Stokes drift produced by waves parallel to coastlines (Isobe et al., 2014).
96
Furthermore, the vertical distribution of plastics will influence the likelihood of animals
inhabiting different depths to encounter, and potentially interact, with plastic. For
instance, sea birds, turtles, and mammals, which breathe air and use the sea surface for
daily activities, present high rates of plastic ingestion and entanglement (Derraik, 2002,
Tourinho et al., 2010). These high interaction rates could be partly explained by the
relatively high concentrations of plastic debris at the sea surface, as shown in this study.
Our findings show that vertical mixing affects the number, mass, and size distribution
of buoyant plastics captured by surface nets, a standard equipment for at-sea plastic
pollution sampling (Hidalgo-Ruz et al., 2012). Subsurface samples are still scarce and
the processes influencing distribution of plastics throughout the ocean’s water column
are poorly understood. Further multi-level sampling across a broader range of sea states
is necessary for better quantifying the vertical mixing of buoyant plastics. This will
improve predictions of ocean plastic concentration levels (Kukulka et al., 2012), size
distributions (Cózar et al., 2014, Eriksen et al., 2014), drifting patterns (Isobe et al.,
2014), and interactions with neustonic and pelagic species of the world’s oceans.
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Chapter 5 General Discussion
Chapters 2, 3, and 4 contain detailed discussions of my PhD results so here I will limit
discussion to the key findings related to the overall goals of this thesis, which were to
investigate how buoyant plastics are distributed in sea surface waters (both horizontally
and vertically), and characterise organisms on the surface of millimetre-sized marine
plastics. Furthermore, I will discuss a few limitations of this thesis and suggest some
future research directions, which complement those mentioned in the discussion
sections of chapters 2, 3, and 4. Finally, the overall conclusions of this thesis are
presented.
5.1 Horizontal distribution of buoyant plastics at sea
In chapter 2, I showed that each square kilometre of Australian surface waters is
contaminated by thousands of small plastic fragments, mostly smaller than 5 mm across
– the so-called “microplastics”. This is the first study to sample plastic debris in waters
around Australia and the data collected here comprise most of the very scarce
measurements of plastic contamination at surface waters of the Indian Ocean and
western South Pacific. This dataset has been made open access in Figshare (Reisser et
al., 2013) and is already being used by a few groups of researchers attempting to
quantify plastic pollution levels and distribution at the world’s ocean, e.g. (Cózar et al.,
2014, Eriksen et al., 2014) (Figure 5.1).
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Figure 5.1 Measurements of plastic numerical concentrations used in Cózar et al. (2014).
Gray areas show accumulation zones as predicted by Maximenko et al. (2012). Dots close to the Australian and Fijian continent are data from this thesis. Source: National Geographic
99
I found that the small plastics in surface waters around Australia are mostly a by-
product of the degradation of larger objects made of polyethylene and polypropylene.
The high prevalence of small plastic fragments in Australian waters is consistent with
other regions of the world’s oceans, where microplastics were found to be the most
numerically abundant type of debris in all types of marine environment (Moore et al.,
2001, Thompson et al., 2004, Browne et al., 2010, Law et al., 2010, Browne et al.,
2011, Eriksen et al., 2013). Plastic pollution levels were moderate when compared to
concentrations in other marine areas (Moore et al., 2001, Yamashita and Tanimura,
2007, Law et al., 2010, Collignon et al., 2012, Eriksen et al., 2013), but higher amounts
were found close to cities on Australia’s east coast, as well as in remote locations (west
Tasmania and North West Shelf). Recent studies reported toxicological effects of these
small and contaminated plastics on a host of organisms, from zooplankton, small fish
and turtle hatchlings, to large mammals (Basheer et al., 2004, Choy and Drazen, 2013,
Cole et al., 2013, Fossi et al., 2012, Gassel et al., 2013, Rochman et al., 2013c, Wright
et al., 2013). As such, small plastics are a type of harmful marine debris, implying that
plastic hazards to Australian species and ecological communities are likely to be
broader than those officially recognized, which only includes physical impacts of large
plastic objects on marine vertebrates through entanglement and ingestion
(Commonwealth of Australia, 2009).
Additionally, I found that the abundant and widespread small marine plastics around
Australia are likely coming from a variety of domestic and international, land- and
ocean-based sources. Although marine plastic pollution is a global environmental issue,
mostly caused by our massive production of plastic single-use disposable items, there
are still no attempts to regulate plastic disposal on land and directly at sea at an
international level (Rochman et al., 2013a). Further at-sea studies on the
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characterization, spatial distribution, and pathways of marine plastics in coastal and
oceanic regions around Australia, as well as on marine plastic toxin loads and
interactions between small plastic particles and organisms at all trophic levels of the
food web, are necessary.
Even though the pioneering study described in chapter 2 advanced our knowledge about
plastic pollution in Australian waters, it is important to recognise the limitations of this
opportunistic research aboard Australian vessels. Among them are (1) the relatively
small number of surface trawls conducted, resulting in a very descriptive study; (2) the
use of two types of surface samplers (i.e. Manta and Neuston nets), which may have
brought some sampling bias to the plastic concentration values obtained; (3) the
estimation of plastic numerical concentrations only, as weighing the samples would
have damaged the plastic particles, impeding the conduction of the SEM and FT-IR
analyses described in chapter 3; (4) the use of a mean rising speed different from the
one I obtained two years later, using plastics from the North Atlantic accumulation zone
(chapter 4); (5) the lack of wind measurements in two out of the seven voyages
conducted, which did not allow the use of in-situ wind measurements to calculate the
depth-integrated plastic concentrations; and (6) the quantification of amounts of debris
within a limited size range (i.e. zooplankton size range only), as Manta trawl surveys
are not ideal for the quantification of microscopic plastic and mega debris.
I suggest that researchers continue monitoring plastic pollution levels in Australian
waters and beyond, by taking advantage of transit voyages aboard research vessels (e.g.
RV Investigator, Solander, Falkor). “Underway” data related to plastic debris could
include surface net sampling, as well as visual transects for counting larger items of
marine plastic debris (Eriksen et al., 2014). Furthermore, simple methods using
continuous seawater intake of vessels (Lusher et al., 2014) could be applied during a
101
wider range of maritime operations, thus maximising sea time and sampling effort.
Given the limitations of opportunistic sampling, I also suggest the planning and
execution of dedicated voyages to study plastic pollution. These voyages would provide
the plastic pollution research community with the opportunity to conduct robust surveys
with well-planned sampling designs. Plastic pollution research voyages with extensive
sea time, fund, and human resources, could use a range of aerial, sea surface and
underwater surveys to quantify concentrations of plastic debris within a broad range of
debris sizes, from nanometre-sized microplastics to meter-sized ghost nets.
5.2 Organisms on the surface of millimetre-sized ocean plastics
Chapter 3 showed that millimetre-sized plastics contaminating Australian waters are
home of a host of marine life (Figure 5.2), which is being transported by ‘plastic
drifting’, potentially affecting the fate and environmental impacts of plastic pollution. It
contributed 18 new records of taxa living on microplastics, and provided 1,143 open
access high-resolution images of plastic debris shapes, surface textures, and ‘epiplastic’
communities (Reisser et al., 2014).
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Figure 5.2 False-coloured electron micrographs of surface textures and organisms on buoyant
plastic debris. Original images (i.e. black and white) are available in Reisser et al. (2014a)
Top left: diatoms (green) and rounded cells (purple). Top right: Coccoliths (red, blue, yellow) and diatoms (green, turquoise). Bottom left: unidentified marine worm (red). Bottom right: isopod (red). Source: The Conversation
This study evidenced that microscopic plastic-dwellers are everywhere in our oceans.
Organisms ranging from single-celled microbes to invertebrates are all taking advantage
of this new human-made and durable type of floating habitat. Even though the SEM
work described here advanced our knowledge on the types of microbes inhabiting
plastic debris, this visual technique has many limitations, such as the inability to
identify the vast majority of microbes composing the ‘Plastisphere’ (e.g. bacteria and
fungi species). A more detailed study of epiplastic organisms could be achieved by
analysing ocean plastic biofouling with other modern techniques, such as metagenomics
(Thomas et al., 2012) and fluorescence in situ hybridization (Moter and Göbel, 2000).
103
Interesting observations of the SEM work described in this thesis included the many
flourishing microbes that appeared to interact with the plastic surfaces. These
observations, together with findings from previous studies (Harrison et al., 2011, Zettler
et al., 2013), suggest that microbes are helping break down plastics at sea. These
putative hydrocarbon-degrading microbes might support biotechnological solutions for
better plastic waste disposal practices on land (Sivan, 2011). For instance, such marine
microbes or their genes might enable the development of industrial ‘composts’ able to
break down plastic waste on land, thus reducing the risk of plastic findings its way to
the sea.
A shipboard three-dimensional survey to collect ocean plastics for functional genome
studies could significantly enhance our understanding of the plastic biofilm in a
relatively quick manner. Microplastics collected at the sea surface, water column, and
seafloor could have their biofilms screened for hydrocarbon-degrading genes, and
results compared with those obtained from the biofilm of natural substrates, such as
sediments, pumice, drifting wood, and floating seaweed. The detection of relatively
high amounts of hydrocarbon-degrading genes in the ‘Plastisphere’ may be the first step
towards the development of a bioremediation solution to some of our plastic waste
management issues.
5.2.1 Ingestion of plastics at sea: does debris size really matter?
Marine microplastics (< 5 mm in length) can contain high loads of additives and
adsorbed pollutants, and may be a threat to marine food webs due to their ingestion by
organisms at the base of the food chain (http://www.unep.org/yearbook/). Most of our
knowledge of plastic ingestion by zooplankton has been obtained through experiments
assuming that plastic particles have to be smaller than the organism’s feeding apparatus
104
for this type of interaction to occur (Cole et al., 2013). However, I propose that this is
not a rule.
By examining the surface of millimetre-sized marine plastics using a scanning electron
microscope, I observed a diverse range of fouling organisms, and a variety of intriguing
pits and scraping marks of unknown origin - see details in chapter 3 and SEM images at
(Reisser et al., 2014). I suggest that some of these plastic surface textures are feeding
marks produced by invertebrates grazing upon the plastic biofilm.
I observed sub-parallel linear scrapes with spacing of 5-14 μm (Figure 5.3a,c), which is
similar to typical distances between teeth of the mandibular gnathobases of copepods
(Michels et al., 2012). The thinner and shallower marks around the linear scrapes could
have been formed by filamentous microstructures present on their gnathobases.
Copepods are an abundant planktivorous group and possess strong feeding apparatuses
to feed upon organisms such as diatoms with silica cell walls (Michels et al., 2012).
Some pelagic species have flexible feeding habits, and can feed on sea-ice algae
(Brierley and Thomas, 2002), faecal pellets (Gonzalez and Smetacek, 1994, Noji et al.,
1991), and marine snow particles (Turner, 2002). I suggest that these copepods could
also feed upon biofilm of plastic debris, which is often rich in ‘epiplastic’ diatoms; see
chapter 3 and (Carson et al., 2013).
105
Figure 5.3 Scrapes putatively identified as feeding marks.
a: Linear scrape marks on a 2.3 mm long plastic debris with a high load of diatoms. b: Rounded scrape marks on a 6 mm long plastic with a unidentified marine worm. Arrow indicates unknown structure partially covering the worm. c: zoom on scraping displayed in ‘a’. d: zoom on scraping shown in ‘b’. Scale bars = 10 μm (a, c), 100 μm (b), 20 μm (d)
I also observed peculiar, rounded marks close to an unidentified marine worm (Figure
5.3b,d), which was partially covered by an unknown structure (indicated by the arrow)
possibly secreted by the animal. These unique scraping marks were also noted on two
other plastic pieces that did not have any visible animals, but possessed structures
similar to the one covering the worm in Figure 5.3b. These results suggest that feeding
on plastic biofilm is not restricted to zooplankton, and possibly occurs with rafting
organisms such as amphipods, gastropods, and chitons, which are known to associate
with floating debris such as plastics (Winston et al., 1997).
Small portions of the plastic particles were apparently removed, and perhaps ingested,
during these putative grazing activities (Figure 5.3). Thus, I contend that (1) plastic
106
biofouling induces plastic ingestion, and (2) plastic pieces must not necessarily be
smaller than the organism for a feeding interaction to occur. The latter hypothesis has
already been suggested for large items, as 15.8% of drifting plastic objects in Hawaii
displayed a variety of vertebrate bite marks (Carson, 2013).
Experiments exposing zooplanktonic organisms to millimetre-sized plastics with
biofilm may document whether they are capable of handling these particles, creating
such feeding marks. By exposing neustonic zooplankton to fresh pieces of brittle plastic
debris, researchers could possibly document this new type of feeding behaviour (e.g. by
filming) and detect plastic bits co-ingested with biofilm grazing (e.g. by examining
faecal pellets).
Due to their rapid growth and nutritional value, biofilms on plastic debris may be a
significant new food source for invertebrates, particularly in the oligotrophic waters
within subtropical gyres, where plastic contamination levels are particularly high. The
impacts related to this new type of feeding interaction remain unclear, but are likely
negative since plastics pose chemical and physical threats to their ‘predators/grazers’
(Wright et al., 2013). These impacts could include effects on food webs, since plastic-
associated pollutants and additives could be transferred to the biofilm and moved up the
food chain of plastic ‘predators/grazers’. The implications of plastic biofilm ingestion,
particularly in terms of pollutant transfer and health effects, should also be investigated.
5.3 Vertical distribution of buoyant plastics at sea
Chapter 4 is the first study to acquire high-resolution depth profiles of buoyant plastic
concentrations and characteristics across a range of sea states. This study described
high-resolution depth profiles of plastic concentrations, which decreased exponentially
with depth, and provided the first measurements of the rise velocity of ocean plastics,
107
which varied with particle size and type. Furthermore, it showed that depth profiles of
plastic mass are associated with higher decay rates than depth profiles of plastic
numbers. This can be explained by the observation that smaller/lighter plastic pieces
were generally associated with lower rising velocities, being therefore more susceptible
to vertical transport.
My findings show that vertical mixing affects the number, mass, and size distribution of
buoyant plastics captured by surface nets, a standard equipment for at-sea plastic
pollution sampling (Hidalgo-Ruz et al., 2012). Subsurface samples are still scarce and
the processes influencing distribution of plastics throughout the ocean’s water column
are poorly understood. Further multi-level sampling across a broader range of sea states
is necessary for better quantifying the vertical mixing of buoyant plastics. This will
improve predictions of ocean plastic concentration levels (Kukulka et al., 2012), size
distributions (Cózar et al., 2014, Eriksen et al., 2014), drifting patterns (Isobe et al.,
2014), and interactions with neustonic and pelagic species of the world’s oceans.
Models capable of using sea surface measurements of plastic sizes and concentrations to
predict depth-integrated values are of extreme importance to those attempting to
quantify and characterise marine plastic pollution. Kukulka et al. (2012) developed a
simple model to predict depth-integrated numerical concentration (pieces km-2) of
plastics. However, predictive models of depth-integrated mass concentrations and size
distribution still need to be developed. These depth-integrated values would then be
comparable without underestimating plastic loads and amounts of small microplastics
within the mixed layer. More multi-level observations such as the ones described in this
thesis, both in oceanic and coastal waters, will facilitate the creation of such models.
There is an urgent need to develop three-dimensional surveys capable of detecting
108
plastic debris thought the water column. Perhaps the unique optical properties of
plastics in the infrared band (e.g. see Figure 2.3) could be used to develop a sensor
capable of detecting ocean plastics, thus making plastic contamination quantification
more efficient and independent of human observers. Since hydrocarbon-bearing
substances such as plastics have typical absorption maxima around 1730nm and 2310
nm (Hörig et al., 2001, Tsuchida et al., 2009), short-wave infrared and Raman
spectroscopy hold promise for such application.
A sampling equipment that may improve the way we sample microplastics at-sea is the
Video Plankton Recorder (Davis et al., 1992), which is an underwater microscope
capable of capturing images of microorganisms and particles while being towed by a
vessel. This equipment could provide valuable information on the horizontal and
vertical distribution of small plastic debris.
5.4 Overall Conclusions
The ocean surface is heavily contaminated by plastic pollution, mostly in the form of
fragments coming from the degradation of plastic objects. The most common types of
items lost or discarded at sea are packaging and fishing gear made of polyethylene and
polypropylene. These are two very common types of plastic resin that are lighter than
seawater and quite resistant to environmental degradation. Such characteristics make
them particularly prone to long-distance dispersion in the ocean surface. Buoyant plastic
represent a major hazard to organisms that live in the top layer of the oceans (e.g. sea
turtles, birds, neuston communities); they are also a new type of vehicle for the
dispersal of pollutants and organisms that live associated with these synthetic floating
habitats.
109
This study set out to explore the spatial distribution and impacts of ocean plastic
pollution. By performing net tows for sampling surface plastics, I was able to
characterize the plastic debris in surface waters around Australia, describe textures and
organisms on the surface of millimetre-sized ocean plastic, and quantify the way
buoyant plastic is distributed in the top layer of the oceans.
The thesis showed that surface waters around Australia have moderate contamination
levels of microplastics, which were mostly fragments of larger objects made of
polyethylene and polypropylene. These millimetre-sized particles harbour a large
variety of organisms, such as bacteria, diatoms, and invertebrates, and are concentrated
at the sea surface, although they can reach deeper depths depending on their buoyancy
and sea state. The information presented in this thesis greatly contributed towards a
better understanding of the distribution and potential impacts of plastic pollution in the
world’s oceans.
It is important that continued research be conducted to further improve this baseline
knowledge for defining regulations and mitigation strategies for this concerning issue.
The findings of this thesis highlight the need for more plastic pollution research,
monitoring, and mitigation efforts within Australian waters, as well as a better
quantification of the depth profile of buoyant plastic debris. This would greatly improve
our capacity to quantify, characterize and search for solutions related to this new type of
marine pollution.
To generate achievable policy strategies and develop targets with regards to plastic
pollution reduction, there is need for more research and monitoring projects at the local,
national, and international scales aiming at assessing the environmental, economical and
human health impacts of plastics. Furthermore, collaborative research and development
110
efforts towards better environmental awareness, waste management, packaging designs,
bioplastics, cleanup efforts, and regulation enforcements will help mitigate and solve
this growing environmental issue.
111
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Appendix 1 Outputs produced during this candidature
Peer-reviewed publications published, in press or in prep
1. Thums M, Whiting SD, Reisser J, Pendoley K, Pattiaratchi C, Proietti M, Meekan M
(in prep) Artificial light on water attracts turtle hatchlings during their nearshore transit.
2. Proietti M, Reisser J, Cliff J, Soares L, Monteiro D, Pattiaratchi C, Secchi E (in
prep) Life history in a tortoiseshell: ontogenetic ecological changes of hawksbill turtles
detected through Secondary Ion Mass Spectrometry.
3. Reisser J, Slat B, Noble K, du Plessis K, Epp M, Proietti M, de Sonneville J, Becker
T, Pattiaratchi C (2015) The vertical distribution of buoyant plastics at sea: an
observational study in the North Atlantic Gyre. Biogeosciences 12: 1249-1256
4. Reisser J, Proietti M, Shaw J, Pattiaratchi C (2014) Ingestion of plastics at sea: does
debris size really matter? Frontiers in Marine Science 1: 70
5. Eriksen M, Lebreton LM, Carson HS, Thiel M, Moore C, Borerro JC, Galgani F,
Ryan PG, Reisser J (2014) Plastic Pollution in the World's Oceans: more than 5 trillion
plastic pieces weighing over 250,000 tons afloat at sea. PLOS ONE 9(12): e111913
6. Letessier TB, Bouchet PJ, Reisser J, Meeuwig JJ (2014) Baited videography reveals
remote foraging and migration behavior of sea turtles. Marine Biodiversity 1:2
7. Reisser J, Shaw J, Hallegraeff G, Proietti M, Barnes D, Thums M, Wilcox C,
Hardesty B, Pattiaratchi C (2014) Millimeter-sized Marine Plastics: A New Pelagic
Habitat for Microorganisms and Invertebrates. PLOS ONE 9(6): e100289
126
8. Proietti M, Reisser J, Marins L, Rodriguez-Zarate C, Marcovaldi M, Monteiro D,
Pattiaratchi C, Secchi E (2014) Genetic Structure and Natal Origins of Immature
Hawksbill Turtles (Eretmochelys imbricata) in Brazilian Waters. PLOS ONE
9(2):e88746
9. Proietti M, Reisser J, Marins L, Marcovaldi M, Soares L, Monteiro D, Wijeratne S,
Pattiaratchi C, Secchi E (2014) Hawksbill x loggerhead sea turtle hybrids at Bahia,
Brazil: where do their offspring go? PeerJ 2:e255
10. Reisser J, Shaw J, Wilcox C, Hardesty BD, Proietti M, Thums M, Pattiaratchi C
(2013) Marine Plastic Pollution in Waters around Australia: Characteristics,
Concentrations, and Pathways. PLOS ONE 8(11):e80466
11. Reisser J, Proietti M, Sazima I, Kinas P, Horta P, Secchi E (2013) Feeding ecology
of the green turtle (Chelonia mydas) at rocky reefs in the western South Atlantic.
Marine Biology 160(12): 3169-3179
12. Thums M, Whiting S, Reisser J, Pendoley K, Pattiaratchi C, Harcourt R, McMahon
C, Meekan M (2013) Tracking sea turtle hatchlings - a pilot study using acoustic
telemetry. Journal of Experimental Marine Biology and Ecology 440:156-163
13. Proietti M, Reisser J, Secchi ER (2012) Immature Hawksbill Turtles Feeding in
Brazilian Islands. Marine Turtle Newletter 135:4-6
14. Proietti M, Reisser J, Kinas P, Kerr R, Monteiro D, Marins L, Secchi E (2012)
Green turtle (Chelonia mydas) mixed stocks in the western South Atlantic, as revealed
by mtDNA haplotypes and drifter trajectories. Marine Ecology Progress Series
447:195-209
127
Media articles
1. Reisser J, Pattiaratchi C, Proietti M (2015) ‘Missing plastic’ in the oceans can be
found below the surface. The Conversation Link: https://theconversation.com/missing-
plastic-in-the-oceans-can-be-found-below-the-surface-37999
2. Reisser J, Pattiaratchi C, Shaw J (2014) Creatures living on tiny ocean plastic may
be cleaning our seas. The Conversation Link: https://theconversation.com/creatures-
living-on-tiny-ocean-plastic-may-be-cleaning-our-seas-27876
3. Pattiaratchi C, Reisser J (2013) The difficulty of searching for MH370 in a giant
rubbish patch. The Conversation Link: https://theconversation.com/the-difficulty-of-
searching-for-mh370-in-a-giant-rubbish-patch-25083
4. Reisser J, Pattiaratchi C (2013) Australian waters polluted by harmful tiny plastics.
The Conversation link: https://theconversation.com/australian-waters-polluted-by-
harmful-tiny-plastics-20790
5. Duarte C, Reisser J, Thums M, Duarte G (2013) Take a stand on Oceans Day and de-
plastify your life. The Conversation link: https://theconversation.com/take-a-stand-on-
oceans-day-and-de-plastify-your-life-15055
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Appendix 2 Chapter 2 supplementary material
Net tow data (N = 171) Columns indicate net station number, sampling date (day.month.year), location (degrees minutes), and mean sea surface plastic concentration (Cs; pieces per km-2).
Net Station Date (UTC) Latitude (S) Longitude (E) Sea Surface Concentration (Cs) 1 10.06.11 39 18.663 163 35.799 653.51 1 10.06.11 39 18.663 163 35.799 691.96 1 10.06.11 39 18.663 163 35.799 566.79 2 11.06.11 39 57.089 160 25.080 950.45 2 11.06.11 39 57.089 160 25.080 2198.90 2 11.06.11 39 57.089 160 25.080 1217.12 3 12.06.11 40 38.087 155 53.425 1424.92 3 12.06.11 40 38.087 155 53.425 3254.64 3 12.06.11 40 38.087 155 53.425 0.00 4 13.06.11 41 08.068 152 17.007 785.90 4 13.06.11 41 08.068 152 17.007 0.00 4 13.06.11 41 08.068 152 17.007 1810.17 5 12.08.11 43 34.637 145 47.977 32167.51 5 12.08.11 43 34.637 145 47.977 29582.80 5 12.08.11 43 34.637 145 47.977 9081.73 6 13.08.11 41 55.318 144 32.515 24299.60 6 13.08.11 41 55.318 144 32.515 6077.41 6 13.08.11 41 55.318 144 32.515 0.00 7 14.08.11 39 02.484 142 25.125 3554.15 7 14.08.11 39 02.484 142 25.125 3211.54 7 14.08.11 39 02.484 142 25.125 2477.57 8 15.08.11 37 53.346 139 49.553 931.74 8 15.08.11 37 53.346 139 49.553 2513.76 8 15.08.11 37 53.346 139 49.553 13625.39 9 16.08.11 36 03.358 135 41.202 4177.68 9 16.08.11 36 03.358 135 41.202 2015.15 9 16.08.11 36 03.358 135 41.202 0.00
10 17.08.11 35 28.841 132 10.928 2332.49 10 17.08.11 35 28.841 132 10.928 0.00 10 17.08.11 35 28.841 132 10.928 1381.05 11 18.08.11 35 04.329 127 15.958 0.00 11 18.08.11 35 04.329 127 15.958 0.00 11 18.08.11 35 04.329 127 15.958 0.00 12 19.08.11 34 36.224 121 49.046 0.00 12 19.08.11 34 36.224 121 49.046 1603.96 12 19.08.11 34 36.224 121 49.046 1099.32 13 20.08.11 35 18.893 118 37.370 1540.86 13 20.08.11 35 18.893 118 37.370 0.00 13 20.08.11 35 18.893 118 37.370 2852.58 14 22.08.11 33 06.825 114 29.446 889.63 14 22.08.11 33 06.825 114 29.446 0.00 14 22.08.11 33 06.825 114 29.446 458.30 15 11.04.12 41 53.867 148 26.891 708.09 15 11.04.12 41 53.867 148 26.891 2732.41 15 11.04.12 41 53.867 148 26.891 2355.35 16 12.04.12 39 39.389 148 53.558 0.00 16 12.04.12 39 39.389 148 53.558 0.00 16 12.04.12 39 39.389 148 53.558 0.00 17 13.04.12 37 38.671 150 23.827 3823.59 17 13.04.12 37 38.671 150 23.827 3313.01 17 13.04.12 37 38.671 150 23.827 2840.54
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18 13.04.12 35 24.230 150 50.390 8361.99 18 13.04.12 35 24.230 150 50.390 18324.00 18 13.04.12 35 24.230 150 50.390 6313.02 19 14.04.12 33 52.897 152 0.189 2903.60 19 14.04.12 33 52.897 152 0.189 7374.21 19 14.04.12 33 52.897 152 0.189 3858.96 20 15.04.12 33 10.310 152 41.536 5966.14 20 15.04.12 33 10.310 152 41.536 19119.87 20 15.04.12 33 10.310 152 41.536 33412.23 21 15.04.12 31 01.893 153 22.585 1954.41 21 15.04.12 31 01.893 153 22.585 3818.55 21 15.04.12 31 01.893 153 22.585 4404.43 22 16.04.12 28 20.522 153 55.265 9943.10 22 16.04.12 28 20.522 153 55.265 21021.85 22 16.04.12 28 20.522 153 55.265 32595.07 23 17.04.12 26 43.789 153 22.675 7064.40 23 17.04.12 26 43.789 153 22.675 4327.06 23 17.04.12 26 43.789 153 22.675 6168.01 24 04.05.12 24 55.084 155 12.525 743.14 24 04.05.12 24 55.084 155 12.525 0.00 24 04.05.12 24 55.084 155 12.525 0.00 25 06.05.12 23 52.232 162 20.311 5835.56 25 06.05.12 23 52.232 162 20.311 7695.14 25 06.05.12 23 52.232 162 20.311 10077.88 26 07.05.12 23 12.812 166 37.146 616.07 26 07.05.12 23 12.812 166 37.146 0.00 26 07.05.12 23 12.812 166 37.146 0.00 27 08.05.12 21 46.153 170 37.369 1248.23 27 08.05.12 21 46.153 170 37.369 8821.42 27 08.05.12 21 46.153 170 37.369 6114.52 28 07.06.12 18 33.897 176 30.406 48895.58 28 07.06.12 18 33.897 176 30.406 7857.04 28 07.06.12 18 33.897 176 30.406 3339.76 29 07.06.12 19 58.653 174 59.443 9647.43 29 07.06.12 19 58.653 174 59.443 7519.52 29 07.06.12 19 58.653 174 59.443 3789.07 30 08.06.12 21 16.990 173 34.862 946.73 30 08.06.12 21 16.990 173 34.862 801.95 30 08.06.12 21 16.990 173 34.862 1311.36 31 08.06.12 22 35.700 172 06.689 2575.43 31 08.06.12 22 35.700 172 06.689 2918.15 31 08.06.12 22 35.700 172 06.689 3507.91 32 09.06.12 23 44.992 170 32.263 0.00 32 09.06.12 23 44.992 170 32.263 0.00 32 09.06.12 23 44.992 170 32.263 0.00 33 09.06.12 25 04.307 168 42.713 2119.38 33 09.06.12 25 04.307 168 42.713 0.00 33 09.06.12 25 04.307 168 42.713 0.00 34 10.06.12 26 17.133 167 00.868 1423.41 34 10.06.12 26 17.133 167 00.868 764.70 34 10.06.12 26 17.133 167 00.868 729.39 35 10.06.12 27 31.614 165 11.757 711.58 35 10.06.12 27 31.614 165 11.757 2112.46 35 10.06.12 27 31.614 165 11.757 697.31 36 15.06.12 35 38.607 155 04.472 652.35 36 15.06.12 35 38.607 155 04.472 0.00 36 15.06.12 35 38.607 155 04.472 0.00 37 15.06.12 37 22.370 153 32.457 3594.90 37 15.06.12 37 22.370 153 32.457 9639.55
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37 15.06.12 37 22.370 153 32.457 8349.88 38 16.06.12 42 16.869 148 58.254 5509.05 38 16.06.12 42 16.869 148 58.254 1845.78 38 16.06.12 42 16.869 148 58.254 0.00 39 17.06.12 42 49.848 148 26.515 1651.09 39 17.06.12 42 49.848 148 26.515 3050.52 39 17.06.12 42 49.848 148 26.515 2289.58 40 26.07.12 16 34.596 145 44.605 1960.78 40 26.07.12 16 34.596 145 44.605 5392.16 40 26.07.12 16 34.596 145 44.605 3921.57 41 27.07.12 15 01.649 145 23.440 2304.10 41 27.07.12 15 01.649 145 23.440 5172.33 41 27.07.12 15 01.649 145 23.440 13518.32 42 27.07.12 13 39.306 144 05.270 2398.91 42 27.07.12 13 39.306 144 05.270 956.41 42 27.07.12 13 39.306 144 05.270 1832.08 43 28.07.12 12 01.760 143 16.230 8954.09 43 28.07.12 12 01.760 143 16.230 3588.54 43 28.07.12 12 01.760 143 16.230 6582.30 44 28.07.12 10 44.893 142 20.410 3937.23 44 28.07.12 10 44.893 142 20.410 446.59 44 28.07.12 10 44.893 142 20.410 1382.16 45 29.07.12 12 15.928 141 37.748 2796.53 45 29.07.12 12 15.928 141 37.748 1559.97 45 29.07.12 12 15.928 141 37.748 450.97 46 30.07.12 15 29.180 141 02.689 0.00 46 30.07.12 15 29.180 141 02.689 596.58 46 30.07.12 15 29.180 141 02.689 0.00 47 30.07.12 17 02.883 140 46.650 0.00 47 30.07.12 17 02.883 140 46.650 0.00 47 30.07.12 17 02.883 140 46.650 0.00 48 17.08.12 18 39.076 119 29.745 2747.58 48 17.08.12 18 39.076 119 29.745 1114.00 48 17.08.12 18 39.076 119 29.745 0.00 49 17.08.12 18 51.743 118 40.746 2488.88 49 17.08.12 18 51.743 118 40.746 1110.76 49 17.08.12 18 51.743 118 40.746 534.60 50 17.08.12 18 58.975 118 12.687 4394.68 50 17.08.12 18 58.975 118 12.687 1703.95 50 17.08.12 18 58.975 118 12.687 1143.61 51 17.08.12 19 06.486 117 43.548 0.00 51 17.08.12 19 06.486 117 43.548 612.37 51 17.08.12 19 06.486 117 43.548 1854.75 52 17.08.12 19 25.826 115 55.023 3401.75 52 17.08.12 19 25.826 115 55.023 599.94 52 17.08.12 19 25.826 115 55.023 4049.19 53 19.08.12 20 23.058 116 23.058 7748.76 53 19.08.12 20 23.058 116 23.058 7266.80 53 19.08.12 20 23.058 116 23.058 39225.93 54 19.08.12 20 18.934 115 40.132 3249.67 54 19.08.12 20 18.934 115 40.132 1245.08 54 19.08.12 20 18.934 115 40.132 654.36 55 22.08.12 19 47.716 115 58.478 1241.92 55 22.08.12 19 47.716 115 58.478 2129.73 55 22.08.12 19 47.716 115 58.478 496.75 56 22.08.12 20 22.838 115 08.123 0.00 56 22.08.12 20 22.838 115 08.123 0.00 56 22.08.12 20 22.838 115 08.123 1294.44 57 25.08.12 21 50.655 113 54.765 12846.12
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57 25.08.12 21 50.655 113 54.765 1932.06 57 25.08.12 21 50.655 113 54.765 3462.42
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Probability distribution of virtual particles arriving at the 57 net stations of this study (dispersal time = 45 days). Red dots indicate position of net stations. June 2011
August 2011
April 2012
May 2012
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June 2012
July 2012
134
August 2012
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Real drifter pathways arriving at the 57 net stations. Purple dots indicate net station locations and asterisks indicate drifter release areas.
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Appendix 3 Chapter 3 supplementary material
Examples of marine plastic surface textures. a, d: polypropylene plastics with linear fractures and pits; b, c: higher magnification of the plastic surface shown in ‘a’ (note very similar pits – one empty and one with a cell conforming its shape); e: higher magnification of the plastic surface shown in ‘d’ (note three equally spaced deep pits); f: polyethylene soft plastic with linear fractures, producing squared microplastics; g: higher magnification of the plastic surface shown in ‘f’ (note shallow pits likely formed by Cocconeis sp.); h: rounded scrape mark similar to the ones found close to the worm-like animal (see Figure 3.7i); i,k: sub-parallel scrape marks; j: large plastic pit likely formed by an egg of Halobates sp.