Déchets marins et microplastiques

5. What is the impact of micro plastics on the marine environment?

The source document for this Digest states:

3.5 Impact of micro-plastics on the marine environment- concepts of harm

Definitions of harm were explored by the workshop, as the EU MSFD had introduced this concept into EU legislation and it was felt that it might provide some insights that could be applied elsewhere. A typical dictionary definition of harm is as follows: physical injury, especially that which is deliberately inflicted, material damage and damage to health, actual or potential ill effects or danger, adverse effects. According to Galgani et al. (2010) “Harm” in the context of the marine litter problem can be divided into three general categories:

1. Ecological, e.g. mortality or sub-lethal effects on plants and animals through entanglements, captures and entanglement from ghost nets, physical damage and ingestion including uptake of micro-particles (mainly micro-plastics) and the release of associated chemicals, facilitating the invasion of alien species, altering benthic community structure.
2. Economic, e.g. cost to tourism, damage to vessels, fishing gear and facilities, losses to fishery operations, cleaning costs; and
3. Social, e.g. a reduction in aesthetic value and public safety;

Note the specific mention of micro-plastics in this context. The fact that the fulmar population in the North Sea contains high levels of ingested plastics could be considered as an undesirable exposure, regardless of its other implications. Recent findings of plastic ingestion by planktivorous fishes in the North Pacific Central Gyre (Boerger, et.al. 2010, in press) indicate an undesirable exposure within a food web.

The workshop considered three more concrete and science-based concepts

1. an undesirable exposure;
2. evidence of uptake and biological effects;
3. an extra cost on the energy budget of an organism.

For most of the chemicals involved, their hazard, or potential to cause (eco)toxicological harm is already well known. What remains unclear is their degree of bioavailability once adsorbed to plastics. The fact that such chemicals have been identified in plastics in the open ocean could on its own indicate that there is the potential for harm. This is in addition to potential detrimental health effects in marine organisms simply due to the presence of particles within the organism.

The workshop did not attempt to reach a definite conclusion, noting that at this juncture some of the potential risks which might make the problem more or less urgent were unclear. It was pointed out that not only plastics but also other forms of marine debris may adsorb contaminants and therefore all forms of marine debris should be considered. Ingestion of microplastics has been demonstrated in many invertebrate organisms, i.e. those lower down the food-chain which usually serving as prey for higher organisms. Thompson et al. (2004) showed in laboratory studies that amphipods (detritivores), barnacles (filter feeders), and lugworms (deposit feeders) ingest small PVC plastic fragments with a mean size of 230μm. Ward and Shumway (2004) in a review on particle selection in bivalve molluscs report several laboratory experiments which show that scallops and mussels can filter and take up polystyrene spherules. Browne et al (2007 and 2008) reported that the blue mussel Mytilus edulis ingests and accumulates polystyrene beads as small as 2 μm in their gut cavity. Mussels were exposed to treatments containing seawater and microplastic (3.0 or 9.6 μm). After transfer to clean conditions, the microplastics were tracked in the hemolymph. Particles were translocated from the gut to the circulatory system within 3 days and persisted - after a peak at 12 days - for over 48 days. Smaller particles were more abundant than larger particles. They reported that this short-term pulse exposure used did not result in significant biological effects.

Koehler et al. (2008) demonstrated the uptake of silicon dioxide particles (3-7μm) into the epithelial cells of the gills and the digestive gland tubules of the blue mussel Mytilus edulis with consequent effects on the stability of lysosomal membranes and the production of lipofuscin (an indicator of oxidative stress). The authors considered this to be a cause effect relationship. The workshop was informed of more recent work (Koehler & von Moos pers. com. Eds.), with the same species which demonstrates its ability to take up plastic particles in the size range 1-80μm into the vacuoles of the digestive gland, also with indications of granulocytoma formation (inflammation), increase in SB haemocytes after 48h and a significant decrease in lysosome stability after 48h.

Bowmer et al. (1991) discussed the histopathological condition of freshwater mussels in the River Maas and the Netherlands delta region in relation to pollution and other environmental factors, noting that responses such as granulocytomas and even degeneration of the digestive gland can be widespread in stressed populations.

3.6 Current state of knowledge

The workshop summarised the state of knowledge as follows:

1. The distribution of various sizes of plastic particles is inherently patchy;
2. Plastics do transport contaminants and a distinction can be made between sorbed pollutants and plastics additives, the latter of which might not otherwise reach the oceans.
3. The same theoretical rules of partitioning and behaviour should apply to additives as to the sorbed pollutants, however, knowledge of the whole transport process is generally lacking;
4. Plastics of various sizes are ingested by a range of organisms and where effects are concerned, all particle sizes are relevant.
5. A fraction of organic pollutants which is as yet difficult to quantify may desorb from plastics into organisms - there is evidence in seabirds for transfer of PCBs from plastics to the tissues;
6. Plastics of specific sizes have been reported by Browne et al, 2007 and Koehler et al., 2008) to pass through cell membranes – other particles also do this – the difference being that the plastics are solely anthropogenic in origin;
7. Once taken up, according to Browne (op. cit.) particles can be retained for long periods (weeks).
8. There is evidence of an inflammatory response in the blood compartment plus pathologies in other tissues following such accumulation of particles (Koehler, 2008).

3.7 Research priorities

3.7.1 Environmental effects

The workshop declared interest in a wide range of relevant indicator organisms from birds to invertebrates and the following selection criteria were suggested:

1. the impact of micro-plastics on different trophic levels needs further study, e.g. filter feeders, surface benthic feeders, deposit feeders, predators (including sea-birds).
2. the organisms likely to ingest plastics in their diet could be most useful - a focus on altered behaviour of organisms as opposed to passive encounters would be useful;
3. organisms with a greater fat content could be a better indicator of bioaccumulation of PBTs although it would be necessary to distinguish natural bioaccumulation with the added effect of plastics.
4. Human health impacts through the food-chain should also be considered as part of an attempt to assess the socio-economic consequences.

With regards to laboratory species/model organisms, these should be globally available, e.g. the blue mussel (Mytilus sp.) and marine worms but not restricted to those requiring running seawater. With laboratory studies and active bio-monitoring (placing clean animals in the field to assess contaminant uptake), the duration of the exposures should fit known ecotoxicological timescales, e.g. it may take several weeks for PCB’s to passively desorb (depending on fugacity capacity) from plastics. Residence times following ingestion and ingestion pressure as well as surface to volume ratios and nature of digestive fluid will determine the degree of leaching from PE, PP and PVC, as will ageing. The challenge is how to identify the added or reduced chemical impact of micro-plastics relative to the ‘natural’ bioaccumulation of PBTs from water and through the food-chain. This makes for a complex chain of circumstances that needs to be carefully considered in designing laboratory bioaccumulation experiments. One participant suggested that porosity might be a contributing factor in determining adsorption and desorption of PBTs; another recommended that the identification of additional chemical impact of micro-plastics relative to the ‘natural’ bioaccumulation of PBTs from water and through the food-chain might be deciphered using radio-labelled PBTs under experimental conditions.

3.7.2 Environmental fate

1. The identification of sources, sinks and hot-spots for plastics and micro-plastics would be beneficial rather than focussing on specific habitats.
2. Good reference sites need to be identified.
3. The availability of reference materials, e.g. pellets of various types and sizes of plastic was felt to be important to facilitating research and the industry representatives present offered their help in finding appropriate materials.
4. Desorption remains a key imponderable – testing under extreme conditions could be a way forward and there is a preference for a kinetic approach to provide a hypothesis against which to design experiments.

4. Socio-economic aspects (session G)

An invited expert on socio-economic analysis gave a presentation to the workshop on “Marine and coastal ecosystem services and coastal zone management”. This presentation looked primarily at how the ecosystem services and valuation concepts can be integrated into coastal zone management.

The Workshop considered the potential role of ‘ecosystem services valuation’ in developing solutions to reduce the marine debris problem. Valuing ecosystem goods and services might make tackling marine litter more attractive and encourage action, when compared to the potential costs associated with leaving it in situ. This incentive might lead to the provision of a range of abatement measures and regulatory controls which could be weighed up as part of a cost-benefit analysis used by policy makers. A key question is how to value the services, and on the other hand, how to value the loss of services through environmental damage. There is a general relationship to biodiversity, but is it efficient to conserve, regardless of who pays? Some participants felt that there was a danger of miss-applying cost-benefit analysis. You might get the desired benefit(s) but it remains very difficult to place monetary values on all the elements.

Although the focus of the workshop was on micro-plastics, it was recognised that solutions are related to how society deals with all marine debris and by extension solid waste, management. There is a need for scientists to express ‘damage’ in terms that can be easily understood by the general public. Where resources are limited, it will be important to focus on policies that deliver benefits to the largest proportion of the population on the most important sociological/health issues and micro-plastics might fare better in this regard when considered as a sub-set of the marine litter problem.

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. 3.5, 3.6, 3.7 and 4. Socioeconomic aspects (session G), p. 25-29