Jusqu'alors les débris flottants étaient détruits par les microorganismes mais cela n'est plus le cas avec le plastique. En effet, les plastiques constituent 90 % des déchets flottant sur les océans. Greenpeace estime qu'à l'échelle de la Terre, environ 1 million d'oiseaux et 100 000 mammifères marins meurent chaque année de l'ingestion de plastiques. Le principe du vortex nord-pacifique et l'accumulation des déchets selon Greenpeace
Pour voir l'animation relative au voyage des déchets, cliquez sur l'onglet "the journey of trash" puis faites glisser la flèche blanche.
Le Programme des Nations Unies pour l'Environnement (PNUE)mentionnait en juin 2006 qu'on trouve en moyenne 46 000 morceaux de plastique par 2,5 km² d'océan sur une profondeur d'environ 30 mètres ! Pire, en certains endroits, la quantité de plastique est six fois supérieure à celle du plancton, maillon élémentaire de la vie dans les océans (Charles Moore, Algalita Foundation)
Selon Greenpeace, sur les 100 millions de tonnes de plastique produits chaque année, près de 10 % finissent dans les océans. Et 70 % des plastiques qui s'aventurent en mer coule et le reste flotte naviguant au grès des courants...
Un "continent" mortel
Ce qui pose problème c'est le temps nécessaire à la dégradation de ces plastiques (estimé entre 500 et 1000 ans) et la toxicité des éléments qui les composent. Petit à petit, ils se morcellent et peuvent être ingérés par les animaux marins, l'exemple le plus classique étant la tortue qui s'étouffe avec des sacs plastiques assimilés à des méduses. Avec de telles concentrations de plastique, toute la chaîne alimentaire est affectée puisque les plus petits morceaux sont ingérés par de petits poissons qui seront à leur tour manger par de plus gros... Greenpeace estime qu'à l'échelle de la Terre, environ 1 million d'oiseaux et 100 000 mammifères marins meurent chaque année de l'ingestion de plastiques.
De plus, les débris de plastique fixent les polluants organiques persistants (POP), connus pour leur nocivité et leur capacité à voyager autour du globe. Ainsi, DDT et PCB se retrouvent dans des morceaux de plastique à des concentrations jusqu'à 1 million de fois supérieures aux normales !
Ce "continent" attire malheureusement des animaux marins comme les pélicans et les tortues marines dont l'espérance de vie se trouve alors diminuée. Au total, plus de 267 espèces marines seraient affectées par cet amas colossal de déchets selon les observations de Greenpeace.
The trash vortex
A tiny fraction of the plastic waste in the oceans being collected from a beach. Plastic waste kills many marine animals when they mistake plastic for food.
Enlarge ImageThese larger items are the visible signs of a much larger problem. These big items do not degrade like natural materials. At sea and on shore under the influence of sunlight, wave action and mechanical abrasion they simply break down slowly into ever smaller particles.
A single one litre drinks bottle could break down into enough small fragments to put one on every mile of beach in the entire world. These smaller particles are joined by the small pellets of plastic which are the form in which many new plastics are marketed and which can be lost at sea by the drumload or even a whole container load. These modern day “marine tumbleweeds” have been thrown into sharp focus, not only by the huge quantities removed from beaches by dedicated volunteers, but by the fact that they have been found to accumulate in sea areas where winds and currents are weak.
The “Eastern Garbage Patch”
The North Pacific sub-tropical gyre covers a large area of the Pacific in which the water circulates clockwise in a slow spiral. Winds are light. The currents tend to force any floating material into the low energy central area of the gyre. There are few islands on which the floating material can beach. So it stays there in the gyre, in astounding quantities estimated at six kilos of plastic for every kilo of naturally occurring plankton. The equivalent of an area the size of Texas swirling slowly around like a clock. This gyre has also been dubbed “the Asian Trash Trail” the “Trash Vortex” or the “Eastern Garbage Patch”.
This perhaps wouldn’t be too much of a problem if the plastic had no ill effects. The larger items, however, are consumed by seabirds and other animals which mistake them for prey. Many seabirds and their chicks have been found dead, their stomachs filled with medium sized plastic items such as bottle tops, lighters and balloons. A turtle found dead in Hawaii had over a thousand pieces of plastic in its stomach and intestines. It has been estimated that over a million sea-birds and one hundred thousand marine mammals and sea turtles are killed each year by ingestion of plastics or entanglement.
Animals can become entangled in discarded netting and line. Even tiny jelly-fish like creatures become entangled in lengths of plastic filament, or eat the small plastic particles floating in the water.
Chemical sponge
There is a sinister twist to all this as well. The plastics can act as a sort of “chemical sponge”. They can concentrate many of the most damaging of the pollutants found in the worlds oceans: the persistent organic pollutants (POPs). So any animal eating these pieces of plastic debris will also be taking in highly toxic pollutants.
The North Pacific gyre is one of five major ocean gyres and it is possible that this Trash Vortex problem is one which is present in other oceans as well. The Sargasso Sea is a well known slow circulation area in the Atlantic, and research there has also demonstrated high concentrations of plastic particles present in the water.
Ocean hitchhikers
The floating plastics can also affect marine ecosystems in a surprising way, by providing a ready surface for organisms to live on. These plants and animals can then be transported on the plastic far outside their normal habitat. These ocean hitch-hikers can then invade new habitats to become possible nuisance species.
Of course, not all plastic floats. In fact around 70 percent of discarded plastic sinks to the bottom. In the North Sea, Dutch scientists have counted around 110 pieces of litter for every square kilometre of the seabed, a staggering 600,000 tonnes in the North Sea alone. These plastics can smother the sea bottom and kill the marine life which is found there.
The issue of plastic debris is one that needs to be urgently addressed. At the personal level we can all contribute by avoiding plastics in the things we buy and by disposing of our waste responsibly. Obviously though, there is a need to make ship owners and operators, offshore platforms and fishing boat operators more aware of the consequences of irresponsible disposal of plastic items.
