Floating plastic is considered to be a major pollutant in the world’s
oceans. It enters the oceans in large quantities from shipping, coastal
communities, the watersheds of river systems and even wind distribution from
inland communities. Once it enters the sea plastic is both durable and highly
buoyant, enabling its distribution over long distances within the oceans.
Plastic eventually breaks down at sea, releasing synthetic polymers into the
water, and can also absorb, transport and then release other pollutants,
causing some scientists to believe it should be regarded as hazardous waste.
Plastic is readily consumed by organisms in the ocean both large and small,
with some organisms, such as Seabirds, known to suffer high levels of direct
mortality as a result of plastic ingestion. Many organisms also consume
plastics secondarily (by eating other organisms that have themselves been
feeding on plastic), causing a build-up of plastic derived chemicals in the
tissues of animals at the top of many marine food chains. In addition floating
plastic has been shown to be capable of transporting many organisms to parts of
the world where they do not usually occur, causing a profound reshaping of many
marine ecosystems.
This has led to scientists and environmentalists taking a strong
interest in plastics in the world’s oceans, though calculating the amount of
plastic in the oceans and mapping its distribution has proved to be extremely
difficult.
In a paper published in the journal PLoS One on 10 December 2014, a
team of scientists led by Marcus Eriksen of the Five Gyres Institute, publish a
new estimate of the floating plastic content of the world’s oceans, based upon
survey data of the five ocean gyres (ocean gyres occur between the ocean’s
tropical currents, which flow east-to-west, and temperate currents, which flow
west-to east, creating areas of circular currents in the North and South
Atlantic, North and South Pacific, and southern Indian Ocean), as well as
coastal regions and enclosed seas along the coasts of Australia, the Bay of
Bengal and Mediterranean Sea.
Eriksen et al. divided the
plastics into four size categories, small microplastics (less than 1.00 mm),
large microplastics (1.01-4.75 mm), mesoplastics (4.76-200 mm) and
macroplastics (over 200 mm). The smaller plastic categories were sampled by
slowly towing nets across the oceans, then sorting and weighing the samples
(the nets had a mesh size of 0.33 mm, so very small particles are likely to
have been missed). Macroplastics were observed by dedicated spotters from
survey ships, and their weight estimated by comparison with macroplastic
samples recovered from shorelines in northern-central Chile, South Africa, the
Atlantic coast of North America and the Hawaiian Archipelago; this method may
have missed macroplastics which were dark in colour or of marginal buoyancy (i.e.
floating low in the water).
Eriksenet al.estimate that
there are a minimum of 5.25 trillion plastic particles floating in the world’s
oceans, with a minimum weight of 268 490 tons, with the North Pacific having at
least 1.99 trillion particles weighing 96 400 tons, the Indian Ocean having at
least 1.30 trillion particles weighing 59 130 tons, the North Atlantic having
at least 0.93 trillion particles weighing 56 470 tons, the South Pacific having
at least 0.491 trillion particles weighing 21 020 tons, the South Atlantic
having at least 0.297 trillion particles weighing 12 780 tons and the
Mediterranean having at least 0.247 trillion particles weighing 23 150 tons.
Model results for global count density in four size
classes. Model prediction of global count density (pieces km-2; see
colour bar) for each of four size classes (0.33–1.00 mm, 1.01–4.75 mm, 4.76–200
mm, and >200 mm). Eriksen et al. (2014).
Microplastics were predicted to be the most abundant category, as
larger plastics break down over time into smaller particles (microplastics are
almost entirely fragments of larger objects), and the surveys found that 92.4%
of the total particle count comprised microplastic particles. However within
these categories the largemicroplastic particles were more abundant than the
smaller particles, with a roughly 60:40 split. This was contrary to
predictions, and suggests that smaller particles are being lost from the oceans
at a greater rate than predicted, either through settling out (sinking or being
washed ashore), consumption by animals or microbial breakdown (microbes attack
the surfaces of all plastic particles in the sea, smaller particles have higher
surface-area-to-mass rations, and are therefore broken down more rapidly).
The ratio of mesoplastic to macroplastic also exceeded expectations,
with a predicted ratio of 16:1 and a detected ratio of ~24:1. This may be due
to a greater than predicted rate of break-up of macroplastic particles or a
greater than predicted rate of mesoplastic particles entering the sea,
particularly plastic bottles and single-use packaging.
While the smaller plastic particles were far more numerous than the
larger ones, the majority of the overall plastic mass was held in larger
particles, with 75.4% of the mass inmacroplastic particles, 11.4% in
mesoplastic particles, 10.6% in large microplsastic particles and 2.6% in small
microplastic particles.
Model results for global weight density in four size
classes. Model prediction of global weight density (g km-2; see colour bar)
for each of foursize classes (0.33–1.00 mm, 1.01–4.75 mm, 4.76–200 mm, and >200
mm). The majority of global weight is from the largest size class. Eriksen et al. (2014).
While there was more plastic in the more densely populated Northern
Hemisphere, but the amount of plastic in the Southern Hemisphere was closer
than expected, suggesting that more plastic is crossing the equator than
previously realized, although it is possible that plastic is being lost from
the northern oceans at a higher rate due to beach stranding or loss of
buoyancy. Plastic was also found to be more abundant in the ocean gyres than in
coastal environments, conforming the predicted expectation that plastics tend
to accumulate in these environments, while plastics near the coast are more
transient.
Finally Eriksen et al. note
that according to the trade organization Plastics Europe, 288 million tons of
plastic were produced globally in 2012. This suggests that the plastic content
of the oceans is roughly the equivalent of 0.1% (one thousandth) of annual
production. While it is not expected that all the plastic produced would end up
in the oceans, this seems excessively low, as plastic is known to survive in
the oceans for many years. Eriksen et al.
note that their methodology only detected surface plastics, not plastics in the
water column, on the ocean floor or incorporated in sediments, and suggest that
future studies should attempt to incorporate these plastics.
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