Déchets marins et microplastiques

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

The source document for this Digest states:

3. Micro-plastics in the marine environment (sessions C, D, H, I)

3.1 An introduction to micro-plastics research and current questions

The occurrence of small plastic particles on beaches and in coastal waters was first reported in the 1970s (Carpenter et al., 1972; Carpenter and Smith Jr, 1972; Gregory, 1977; Morris and Hamilton, 1974) although the term ‘micro-plastics’ was not used until relatively recently (Thompson et al., 2004). It has become evident that the distribution of particles is global, including isolated mid-ocean islands, the open ocean and at high latitude Barnes, et al. (2009). There has been a rapid increase in the number of recent publications in the scientific literature on the distribution of fragments. Some general trends are likely, 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. Particles will reduce in size as weathering and disintegration takes place, increasing the surface area and the possibility of chemical transport (absorption of chemicals into or leaching out of microparticles; e.g. Teuten et al. 2009) and increasing the potential for ingestion by a wider range of biota further down the food-chain. 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. Interactions of large plastic items with biota such as seabirds, marine mammals and turtles through entanglement or ingestion are relatively well known (see Moore, 2008 for a recent review), but the sub-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. Several recent studies have identified potential effects of plastic particles, including:

• desorption of persistent, bioaccumulating and toxic (PBT) substances from plastics,
• leaching of additives from the plastics
• physical harm

The key questions are: i) to what extent do micro-plastics have a significant direct physical impact and ii) to what extent do they provide an additional vector for chemical contaminants increasing or decreasing the exposure of sensitive organisms to PBTs. The potential impacts of micro-plastics may be quite subtle (for example, compared with the entanglement of a marine mammal) and it may be difficult to extrapolate experimental results to population and ecosystem scales.

GESAMP (2001) in the last global assessment of the state of the marine environment which was focused on land-based sources reported that “Solid waste, or litter, is concentrated near urban areas, on beaches near villages and in shipping lanes, but is found throughout the oceans. Plastics are the largest component, followed, in urban areas, by steel and aluminium cans. Litter causes mortality to marine organisms, notably sea turtles, marine mammals, and sea birds. The extent of this mortality is unknown, but there is no evidence that it has major effects at the population level. Litter also has negative aesthetic impacts, thereby affecting recreation and tourism, and can be a navigational hazard. Better solid waste management is the overarching solution to problems of marine litter.” Since this was written, cause for concern has increased as further evidence for effects emerges.

3.2 The origin of micro-plastic particles

The Workshop adopted the NOAA-recommended definition of a micro-particle as being 5mm in diameter or less (Arthur et al., 2009). Micro-plastic particles 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

3.3 Methods of sampling and analysing micro-plastics

3.3.1 Existing methods

Methodologies for the sampling of sediments and the water column are available (e.g. Thompson et al., 2004; Eleftheriou and McIntyre, 2005) but there is a need for improved techniques and for standardisation.

The smallest particle size to be detected needs to be determined and a standardised sampling regime should be developed on this basis. It was felt that NOAA’s efforts in standardization of quantitative methods provided a good starting point. It was considered that there are major problems in handling the volume of samples potentially needed globally. Often particles are recovered during biological sampling so the size range is limited by the purpose and collection efficiency of the sampling device in question (e.g. 330 μm mesh neuston net for sampling zooplankton; Continuous Plankton Recorder; see: www.sahfos.ac.uk ).

It was pointed out where sediment sampling and sorting is concerned that basic techniques had been developed many decades ago in benthic ecology for sorting organic material and organisms from sediments, and that cost-effective, low-technology techniques are available which might be usefully applied to separating and identifying micro-plastics, e.g. elutriation using fluidized sand beds created by water flowing through sintered disks allows larger samples to be accurately sorted (Southwood and Henderson, 2000; p226). This has the potential to replace high-density chemicals. One participant also demonstrated the usefulness of a polarizing microscope in quickly separating by eye and identifying plastics from other materials (see Section 3.5). Some issues to contend with are the reporting units (mass per mass or mass per volume), the vertical and horizontal variability in occurrence and the presence of organic matter. Sampling for marine debris using biota has included birds (e.g. Fulmars), fish stomachs and filter-feeding invertebrates (e.g. Mytilus sp. , Browne et al., 2008). The group also considered the potential for particles to act as a vector for the transport of biota, including microbial colonisation of micro-plastics and discussed ways of assessing this. There are two common methods used to chemically analyse the bulk composition of plastic particles: Fourier transform infrared spectroscopy (FT-IR); and, Raman-spectroscopy. Both are expensive but they can be used as diagnostic tools. Raman spectroscopy can also provide more information on the crystalline structure of the polymer and thus, its sorption behaviour for PBT.

3.3.2 Information and research requirements

More information was required about plastic and microplastic inputs, spatial and temporal distributions, including transport dynamics, interactions with biota (e.g. plankton) and potential accumulation areas. It was felt that some form of ‘taxonomy’ of plastic particles would be useful (size, shape, density, chemical composition and properties) as would a method to derive the ‘age’ of particles, linked to suitable standards. This could be incorporated into Environmental Quality Standards to inform policy makers (e.g. Good Environmental Status under the EU MSFD). It could also be incorporated in the development of guidelines for sampling and reporting.

In terms of capacity building and raising awareness, the workshop proposed the development of an abundance map (linked to a database via the internet using, for example, GoogleEarthTM) as well as encouraging the development of the International Pellet Watch and related initiatives. This might also tie in the GEF/UNEP/IOC Transboundary Waters Assessment programme (See Section 5.3.2). The workshop would like to see the incorporation of marine litter and if feasible, micro- plastics in existing and new monitoring programmes as appropriate, bearing in mind the often limited resources available in many countries for marine monitoring.

3.4 Transport, distribution and fate including deterioration and degradation routes

3.4.1Transport and distribution

Most common plastics have specific gravities (SG) from ca. 0.6 to 1.5 but some finished products containing fillers can reach as high as 3.0 (see http://www.plasticsusa.com/specgrav.html). PE, PP natural and synthetic rubbers all have SG ranges of less than 1.0 and float on water. Many other common plastic types have an SG of slightly more than 1.0, e.g. polystyrene but given the higher density of seawater as opposed to freshwater many still float in the marine environment. PVC and POM have much higher SGs at around 1.4 and tend to sink. Finally, some speciality polymers such as polytetrafluoroethylene (PTFE) may have an SG of up to 2.3. The behaviour of different types of plastics in the water column needs further study. The ocean are complex heterogeneous water bodies. On a smaller scale, ‘plugs’ or ‘slabs’ of water tend to remain intact for long periods of time, characterised principally by their temperature and salinity, while currents, eddies and gyres dominate at a larger, oceanic scale. As hydrographical and ‘accidental’ drifter studies have shown, floating debris may often move quite predictably along well travelled paths in the oceans, e.g. the Gulf steam which casts floating objects originating in the Caribbean onto Eastern North Atlantic shores (Ebbmayer & Scigliano, 2009 provide a useful introduction to drifter studies). The same authors note the Azores in the North Atlantic (ca.1800 km W of Spain) and the coastal barrier islands of the Western Gulf of Mexico as known litter hotspots. Mapping of such hotspots of macro-debris may help to some extent to decipher the distribution of microplastics.

Thus far there has been an ad hoc scientific approach to determining the presence of micro- plastics in the pelagic and sedimentary environment – our knowledge of distribution is therefore very patchy. There is a need to set a broad sampling programme with fixed transects in open water, to determine how ubiquitous micro-plastics have become in the environment and to gain an overall picture of distribution and in particular trends.

Relatively constant levels of plastic particles has been observed in the Western North Atlantic Ocean between 1991 and 2007 (Morét-Ferguson et al., 2010; Law et al 2010). Ribic et al., (2010) have also shown that there has been little or no increase in beached and oceanic litter in recent years; only one of three sectors of US coastline showed increases. This may be related to improvements in solid waste management practices along the relevant coastlines (See Section 5).

We still know relatively little of the fate of micro-plastics, e.g. whether particles are being deposited in deep-sea sediments, or whether they are more limited to the shelf and the coastline. The vertical movement of various types and sizes of particles is also an area which needs attention, e.g. plastics fragments with biofilms may sink, but once the biofilm has been removed, it may become buoyant again (Ye & Andrady 1991). The density of the plastic itself may also play a role. As noted above, the workshop reiterated that further information needed to be gathered on locations where macro plastic debris accumulates and also where microplastics are likely be deposited in sinks. The behaviour of different sized particles also needs consideration.

Source & ©: ,  Proceedings of the GESAMP International Workshop
on micro- plastic particles as a vector in transporting persistent, bio- accumulating and toxic substances in the oceans. 28-30th June 2010, UNESCO-IOC,
Paris. 3.Micro-plastics in the marine environment (sessions C, D, H, I), p. 18-20.