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Microplastics are considered specifically in descriptor 10 of the MSFD [10.1.3 “Trends in the amount, distribution, and where possible, composition of micro-particles (in particular micro- plastics)”], and not directly but implicitly in the indicator related with impacts of litter on marine life. The descriptor will establish baseline quantities, properties, and potential impacts of MPs. It must be noted however that the decision was reviewed recently for changes in order to make it simpler and clearer, to introduce minimum standards and to be coherent with other EU legislation.

Within the process, the TSG-ML suggested that micro-litter be considered as a size fraction integrating micro-litter along with other litter fractions in the matrix related indicators. Not all of the experts support this view, arguing that micro litter is different from other litter types (meso/macro) and that micro-litter may have considerably different effects to those caused by larger items of litter. The idea of merging indicators 10.1.2 (litter at sea, floating and on the sea floor) with indicator 10.1.3 (microplastics) aimed to avoid treating microparticles as a separate issue while measures to combat marine litter need to be formulated covering all size classes.

Finally, the revised decision (article 9/3 and 11/4) kept (the review has been done but not published yet) criteria separated for macro litter (10DC1) and microplastics (D10C3), now defined as “The composition, amount, and spatial distribution of micro-litter in the surface layer of the water column, in sea-floor sediment, and possibly on coastlines, is at a level that does not cause harm to the coastal and marine environment.”

MPs should be categorized according to their physical characteristics including size, shape, and color (see Table 1). It is also important to obtain information on polymer type.

The size definition of MPs according to the TSG-ML (Galgani et al., 2013b) is in line with the NOAA definition. We strongly suggest using this size (<5 mm) as an international standard. One aspect that should be refined is the definition of the lower size boundary for MPs in the MSFD. The lower size has not been defined strictly and nanoparticles have not been considered as a category despite their potential relevance (Galgani et al., 2013a).

Sampling of MPs in the different marine compartments (sea water, sediment, and biota) requires different approaches: samples can be selective, bulk, or volume-reduced (see e.g., Hidalgo-Ruz et al., 2012). Selective sampling in the field involves visual identification and manual sorting of fragments from different matrices and is not very effective for MPs due to difficulties in handling small size items. The subsequent identification of plastic particles in the matrix follows similar procedures (section Quantification and nature of MPs).

Bulk samples refer to samples where the entire volume of the sample is taken without reducing it during the sampling process. Bulk samples are most appropriate when MPs cannot be easily identified visually because in the field because (i) they are covered by sediment particles, (ii) their abundance is small requiring sorting/filtering of large volumes of sediment/water, or (iii) they are too small to be identified with the naked eye (Hidalgo-Ruz et al., 2012).

Volume-reduced samples, in seawater, refers to sampling where the bulk volume of the sample is reduced during sampling, preserving only that portion of the sample that is of interest for further processing. While on board a vessel seawater samples can be volume-reduced by filtering water through nets or screens.

In the last years studies determining the global quantity of plastic particles in the ocean have been published (Eriksen et al., 2014; Cózar et al., 2014, 2015). In order to ensure inter-comparability between these studies to evaluate when (seasonality) and where (space) contamination is taking place, harmonization is urgently needed.

Seawater samples for MPs are mostly taken by nets. The main advantage of using a net is that large volumes of water can be sampled quickly, only retaining the volume-reduced sample. Most studies have been from surface water using neuston nets (Hidalgo-Ruz et al., 2012); manta and bongo nets have also been used at the sea surface. Since most plastics are buoyant they are likely to accumulate at the sea surface. Another instrument, that is widely deployed on a global scale and that has also been used for MPs sampling is the Continuous Plankton Recorder (CPR) (Thompson et al., 2004). Some instruments, including bongo and the CPR, are used sub surface making direct comparison rather difficult (Hidalgo-Ruz et al., 2012; Frias et al., 2014).

The most relevant characteristics of the sampling nets used are the mesh size and the net opening. Mesh sizes used for microparticle sampling range from 0.053 to 3 mm, with a majority of the studies (rather than individuals samples collected) ranging from 0.30 to 0.39 mm (Hidalgo-Ruz et al., 2012). The net aperture for rectangular openings of neuston nets (sea surface) ranged from 0.03 to 2.0 m².

Techniques using apparatus to collect surface seawater and pass it through a filter on-board ship are being developed for example by CEFAS, UK (T. Maes; personal communication). They use the ships water inlet, collecting seawater from the side at specified depths, mostly ranging between 4 and 1 m depth. The seawater is being passed along a set of sieves or nets after which the sieves or nets can be removed and analyzed for MPs in the laboratory (Pitois et al., 2016).

The advantage of such systems is that it can collect marine litter samples from the water column while steaming and thus long transects over several kilometers can be collected autonomous in connection with in-line analytical systems for other parameters like nutrients or oxygen. The development of filtration systems for the quantification of MPs appears promising (Lusher et al., 2014).

The recommendation from the TSG-ML is to obtain samples from sea water wherever possible, and to ensure the following details are recorded to accompany each sample: type of net (preferably Manta net), aperture (usually 60 cm), and mesh size (preferably 333 μm). It is also important to record the following parameters: depth (preferably either at the sea surface or within surface 10 m, for greatest inter-comparability among sampling programmes) distance towed, location of tow (in/out of water) and volume of water filtered (with a current meter).

Also prevailing weather conditions and sea state, together with any relevant information on the volume of plankton or other particulates sampled, for example if there is concern that the net may have become clogged due to high concentration of plankton, must be recorded. Samples should be stored in glass jars. MPs are determined as the total quantity of items per volume of seawater captured by the net during the period it is deployed.

Samples in seawater can be passed through a 500 μm sieve, and liquid passing through the sieve then filtered through a filter paper using a Buckner funnel. Filter papers can then be examined under a dissecting microscope to quantify microplastics below 5 mm. Sample on CPR silk filter screens can be examined directly under the microscope.

At present and from the experience in the implementation of the MSFD discussed in the TSG-ML, it is not appropriate to recommend one approach over all others. As an example, in Table 2 are shown MPs values available for the Mediterranean with sampling details (mesh size, net). Each approach has advantages and disadvantages and may be preferable according to local availability/sampling opportunities, the characteristics of the area to be sampled and other factors. The mesh size and water volume are important if one wants to compare different surveys and thus harmonization between these parameters is recommended.

The identification of MPs polymers is achieved by comparing the spectra from the unknown sample against that of a known standard polymer in a database. We encourage consulting Hummel (2002) for more details on this methodology. It should be noted that this method is only definitive where a good match is obtained and this is not always possible. Due to biofouling and degradation processes of microplastics in the environment, their spectra are not totally similar to spectra from the virgin material in the library.

If formal identification of particles using Fourier Transformed- Infra Red (FT-IR) or Raman Spectroscopy is applied then polymer type should also be recorded. Spectroscopy is not critical for routine monitoring of larger fragments > 500 μm. However, it should be considered essential for fragments > 50 μm and a proportion (5–10%) of all samples should be routinely checked to confirm the relative accuracy of any visual examination.

A suitable approach proposed by the TSG-ML would be to automatically accept any match >70% similarity (Frias et al., 2016), to individually examine matches between 60 to 70% similarity rejecting any samples which do not show clear evidence of peaks corresponding to known synthetic materials and to routinely reject (as synthetic) any samples which produce spectra with a match <60%).

It is advocated that when analyzing particles in the range 1–100 μm to subject them to further spectroscopic analysis to confirm polymer identity (e.g., using FT-IR). For particles in the size range 101 μm–4.99 mm we recommend that a proportion (10% of the material in each size class, up to a maximum of 50 items per year or sampling occasion whichever is the least frequent) of the items considered to be MPs is subjected to further spectroscopic analysis to confirm identity (e.g., using FT-IR). This step is important in order to; (1) ensure quality control of visual identification and (2) gain information on the relative abundance of different polymer types which can inform on sources.

One important issue is to mitigate contamination of samples, as plastics are present in our daily lives (in clothes, scrubbers) and in labs (labware). People undertaking the sampling and working in the lab should minimize any synthetic clothing. As procedural controls to check ambient cleanliness, place unused clean filter papers in Petri dishes. Remove the lid and leave the Petri-dish open for a fixed time period relevant to the time period for which samples might be exposed to the air during examination. Procedural contamination should be <10% of the average values determined form the samples themselves.

For MPs in seawater items/m³ seawater, average size of particles, relative abundance of main colors and shape are suggested as units. Relating quantities of MPs to volume is relevant when considering the sampling of water column through filtration. Expressing quantities by volume also allow to link field studies directly with exposure experiments in the laboratory. The estimation of volumes is however impossible when using neuston/manta nets as the trawl frames are permanently moving vertically at the surface of the sea, complicating correct calculation of the sampled water height covered during tows. For this reason, the sampling of the surface density most often rely on items/m², a more relevant estimation of the sample covered. It should be stressed that when possible, more info should be recorded to facilitate reporting in several units in order to ensure comparison with other studies. If FT-IR or Raman is used then polymer type should also be recorded together with shape and color.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.