Tuesday 23 December 2014

Counting floating plastics in the world’s oceans.

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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