Déchets marins et microplastiques

3. What are micro-plastics and how do they enter the marine environment?

The occurrence of small plastic particles on beaches and in coastal waters was first reported in the 1970s although the term ‘micro-plastics’ was not used until relatively recently. It has become evident that the distribution of particles is global, including isolated mid-ocean islands, the open ocean and at high latitude.

Micro-plastic particles, defined here as particles of less than 5mm in size, can arise through four separate processes:

1. deterioration of larger plastic fragments, cordage and films over time, with or without assistance from UV radiation, mechanical forces in the seas (e.g. wave action, grinding on high energy shorelines), or through biological activity (e.g. boring, shredding and grinding by marine organisms);
2. direct release of micro particles (e.g. scrubs and abrasives in household and personal care products, shot-blasting ship hulls and industrial cleaning products respectively, grinding or milling waste) into waterways and via urban wastewater treatment;
3. accidental loss of industrial raw materials (e.g. prefabricated plastics in the form of pellets or powders used to make plastic articles), during transport or trans- shipment, at sea or into surface waterways;
4. discharge of macerated wastes, e.g. sewage sludge

It is likely that the amount of plastic waste in the ocean will continue to increase, driven primarily by the inexorable rise in plastics consumption (ca. 9% per annum), and the continued inadequacy of re-use, recycling and waste management practices in many parts of the world.

Interactions of large plastic items with animals such as seabirds, marine mammals and turtles through entanglement or ingestion are relatively well known, but the non-lethal impacts on individuals and populations are unclear. Even less is known about the potential impacts of micro-plastics on a wide range of smaller organisms, exposed to various particle sizes and chemical constituents.

Weathering and disintegration lead to a slow decrease in the size of the particles. This increases the likelihood that a wider range of animals, which are further down in the food chain, will ingest the particles. It also increases the surface area of the particles, which means that there is more opportunity for the particles to either absorb chemicals or to release them, through a process called chemical exchange. The limited studies of their occurrence in sediments suggests that, to the best of our current knowledge, distribution is patchy and cannot be related directly to sediment transport, and therefore it is not yet possible to predict sinks. Different plastics have different characteristics and among those differences is density. Basically some plastics float and some sink, and this has an impact on where the micro-plastic particles end up in the environment, although very little is known about their ultimate fate, for instance if they eventually sink to the seafloor.

The key questions are:

1. to what extent do micro-plastics have a significant direct physical impact and
2. to what extent do they provide an additional vector for chemical contaminants increasing or decreasing the exposure of sensitive organisms to PBTs.

In order to measure the amount of micro-plastics in the oceans there are a number of methods available, but common standards are only now being developed to ensure that everyone is measuring the same thing, making it difficult to build a coherent picture of the presence of micro-plastics. It would be valuable to add the measure of micro-plastics to the current marine monitoring programmes. There is a need to set a broad sampling programme with fixed transects in open water subject to regular sampling, to determine how ubiquitous micro-plastics have become in the environment and to gain an overall picture of distribution, type and in particular time trends. More...