Environmental Pollution 292 (2022) 118275 Contents lists available at ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol Are research methods shaping our understanding of microplastic pollution? A literature review on the seawater and sediment bodies of the Mediterranean Sea☆ Laura Simon-Sánchez a, *, Michaël Grelaud a, Marco Franci a, Patrizia Ziveri a, b a Institute of Environmental Science and Technology (ICTA-UAB), Universitat Autònoma de Barcelona, 08193, Cerdanyola del Vallès, Barcelona, Spain b Catalan Institution for Research and Advanced Studies (ICREA), Pg. Lluís Companys 23, Barcelona, 08010, Spain A R T I C L E I N F O A B S T R A C T Keywords: The lack of standardization on the definition and methods in microplastic (MP) research has limited the overall Microplastic interpretation and intercomparison of published data. This has presented different solutions to assess the pres­ Mediterranean sea ence of these pollutants in the natural environment, bringing the science forward. Microplastics have been re­ Methods ported worldwide across different biological levels and environmental compartments. In the Mediterranean Sea, Seawater Marine sediments numerous research efforts have been dedicated to defining the MP pollution levels. The reported MP concen­ Beach trations are comparable to those found in the convergence zone of ocean gyres, pointing to this basin as one of the world’s greatest plastic accumulation areas. However, to what extent are the data produced limited by the methods? Here, we present the results of a systematic review of MP research methods and occurrence targeting the seawater and sediment bodies of the Mediterranean Sea. Based on this dataset, we 1) assess the discrepancies and similarities in the methods, 2) analyze how these differences affect the reported concentrations, and 3) identify the limitations of the data produced for the Mediterranean Sea. Moreover, we reaffirm the pressing need of developing a common reporting terminology, and call for international collaboration between Mediterranean countries, especially with North African countries, to provide a complete picture of the MP pollution status in this basin. 1. Introduction the most valuable character of plastic materials: their diversity. Recent proposals to standardize the MP definition consider their physi­ The presence of small plastic particles, microplastics (MPs), in the ochemical properties, shape, size, and origin (Frias and Nash, 2018; marine environment has been documented since the 1970s (Buchanan, Hartmann et al., 2019), although authors even differ on the size criterion 1971; Carpenter and Smith, 1972; Morris, 1980; Ryan and Moloney, (1 to <5000 μm - Frias and Nash, 2018; 1 to <1000 μm - Hartmann et al., 1990; Shiber, 1979). However, scientific and public concern have 2019). Despite the lack of standardization, the MP research community increased over the last 20 years once the accumulation and environ­ is aware of the pressing need of working towards the harmonization of mental effects of plastics in the environment were evident (Barnes et al., the data, reporting results using a common terminology (Hartmann 2009; Derraik, 2002; Thompson et al., 2004). Since then, growing efforts et al., 2019; Provencher et al., 2020; Rochman et al., 2019) that would are being devoted to characterize the presence and risk that these pol­ allow the large-scale interpretation and comparison of current lutants might pose to the natural environment and human health knowledge. (Vethaak and Legler, 2021; SAPEA, 2019). Consequently, the MP In the Mediterranean Sea, numerous research efforts have been research field is constantly evolving and different definitions and dedicated to define the plastic pollution issue (Fig. S1). The reported MP methodologies are being applied. Microplastics are generally defined as concentration, modeled (Eriksen et al., 2014; Lebreton et al., 2012; Van plastic particles smaller than 5 mm (Hidalgo-Ruz et al., 2012; Moore, Sebille et al., 2015) and empirically gathered (Cózar et al., 2015; Suaria 2008). This definition, restricted to the size criterion, overlooks one of et al., 2016), are comparable to those found in the convergence zone of ☆ This paper has been recommended for acceptance by Dr. Sarah Harmon. * Corresponding author. E-mail address: Laura.Simon@uab.cat (L. Simon-Sánchez). https://doi.org/10.1016/j.envpol.2021.118275 Received 31 May 2021; Received in revised form 23 September 2021; Accepted 30 September 2021 Available online 6 October 2021 0269-7491/© 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). L. Simon-Sánchez et al. Environmental Pollution 292 (2022) 118275 the subtropical gyres, indicating that this basin is one of the world’s AND “Mediterranean”. The search was limited to English peer-reviewed greatest plastic accumulation areas (Cózar et al., 2015; Lebreton et al., articles published before January 2021. The first article selection was 2012). The high levels of MP pollution and its accumulation are likely performed after a screening of both title and abstract. Only the articles driven by a combination of high anthropogenic pressure and the hy­ investigating the concentration of MPs in sediment and seawater bodies drodynamic conditions of the Mediterranean Sea (Morris, 1980). The were considered for full-text review. This set of articles was then clas­ Mediterranean region is inhabited by 480 million people, where sified according to the environmental compartment they investigated: one-third is concentrated along the coast, and about one-half is living in sea surface water, seawater column, marine sediments, and/or beach. the surrounding hydrological basins (European Environment Agency, For the articles in which more than one category was investigated, each 2014). The basin supports intensive fishing, shipping, and industrial compartment was included in the dataset as an independent study. The activities, and it is one of the world’s top tourist destinations (UNWTO, meeting criteria for inclusion after the full-text revision was that the 2018). Furthermore, the anti-estuarine circulation of the Mediterranean study provided primary results on the concentration of MPs in at least Sea contributes to this phenomenon by its limited outflow (Rohling one of the four targeted compartments of the Mediterranean Sea (Fig. 1). et al., 2009). The Strait of Gibraltar is characterized by the inflow of From each included study, the following information was extracted: Atlantic surface waters of relatively lower salinity, while the outflow of the environmental compartment targeted, the description of the more saline and denser waters is restricted to deeper depth. Thus, methods for MP sampling [e.g., date, instrument and its characteristics, floating plastic debris entering into, or generated within the Mediter­ flowmeter, wind correction, number of samples, sample preservation], ranean Sea, will be trapped within the basin with few possibilities of extraction [e.g., sample’s volume, purification treatment, flotation escape (Aliani et al., 2003; Lebreton et al., 2012; Morris, 1980). treatment parameters-brine solution, mixing and settling time, repeti­ The high spatio-temporal variability of the Mediterranean Sea cir­ tion], identification [visual, spectroscopy methods, % of particles culation prevents permanent accumulation areas of floating MP debris analyzed, % of particles confirmed as MPs], quality control and (Mansui et al., 2015). Most of the plastics are assumed to float at sea contamination protocols, the registered MP occurrence per study [e.g., because of their positive buoyancy, as their average specific density (ρ = geographical area, mean abundance, if available, min. and max. abun­ 0.9–1.0 g cm− 3) is lower than seawater (ρ = 1.027 g cm− 3); except for dance] and per sampling station [coordinates and abundance]. When few denser polymers (i.e., PET, ρ = 1.38 g cm− 3; or PVC, ρ = 1.39 g data were missing in the publication, the Online Portal for Marine Litter: cm− 3) which are expected to sink once entering into the aquatic envi­ LITTERBASE (https://litterbase.awi.de/) was used to complete the ronment. Ocean turbulence induces the vertical transport of positively dataset to acquire the geographical coordinates and abundance per buoyant MPs along the water column (Kooi et al., 2016; Kukulka et al., station, or the corresponding authors were contacted for further 2012), yet despite this, the seafloor is considered the long-term sink of information. MP pollution in the marine environment (Woodall et al., 2014). The We used QGIS Desktop 3.12 ‘București’ (QGIS Development Team, export of MPs from the sea surface to the deep sea can be facilitated by 2020) to map the extracted information. Sampling stations were clas­ the formation of biofouling on the particle’s surface, the sequestration of sified as “coastal” if they were located within 12 nm from the coast, and these pollutants into organic and inorganic aggregates, or transportation classified as “open-sea” for those exceeding 12 nm from the coast through the vertical migration of numerous organisms (Van Sebille (UNCLOS and Nations, 1982). The nearest distance between sampling et al., 2020 and references therein). In a recent study, Kaandorp et al. stations and the coast was calculated using the NNJOIN plugin (Tveite, (2020) estimated the floating plastic budget of the Mediterranean Sea: 2019). For data visualization, figures were produced in R-3.5.3 (RStudio their results showed that 37–51% of the plastic input to the basin tend to Team, 2020), using ggplot (Wickham, 2016), ggalluvial (Brunson, 2020) settle on the seafloor and that 49–63% of these inputs end up beaching. packages, and post-edited in Adobe Illustrator CC 2021 (Adobe Inc, Hence, the role of beaches cannot be neglected, as these transitional 2021). environments between terrestrial and marine systems act simulta­ neously as pathways and dynamic storage of MP pollutants. 3. Results Previously, authors have provided a general overview of the abun­ dance of MPs in Mediterranean rivers (Guerranti et al., 2020), surface The present literature review draws data from 85 articles obtained waters (Cincinelli et al., 2019), sediments (Martellini et al., 2018), and from our initial search that identified a total of 641 articles. From this their interaction with biota (Llorca et al., 2020). However, none of the first set of articles, 545 were discarded because they were duplicates or studies have discussed in depth the discrepancies in the methodologies and how these might be defining the current understanding of MP pollution in the Mediterranean basin. In this context, our approach for this paper is to: 1) Review and compile the available literature of MP occurrence in the seawater and sediment bodies of the Mediterranean Sea and 2) Describe the methods used for sampling, extracting, and identifying MPs, including the measures to prevent airborne contami­ nation and quality control procedures. The main aims are to summarize how the differences and similarities in the methods influence the observed abundances of MP in the Mediterranean Sea, to outline where the research efforts are focused, and to identify the pressing research needs that will enhance our understanding of the fate of MP within this basin. 2. Methods A systematic literature review was conducted to integrate the MP pollution data and the associated methods in the seawater and the sediment compartments of the Mediterranean Sea. Two scientific data­ bases (Web of Science, www.webofknowledge.com; SCOPUS, www. scopus.com) were consulted, integrating logical operators, through the Fig. 1. Flow diagram illustrating the selection process and systematic review of specific string search: (“microplastics” OR “microplastic pollution”) the published literature. 2 L. Simon-Sánchez et al. Environmental Pollution 292 (2022) 118275 out of the scope of this review. After the full-text review, another 11 (6.0%). Most of the samples were collected using a net (92.8%) and were articles were discarded as they were methodological or review articles, volume reduced (i.e., concentrated in the cod-end of the net). Trawling or did not provide primary results specifically on MPs. Within the suit­ speed depended on the sampling devices, currents, and weather condi­ able articles, eight presented data from more than one compartment, tions, but generally, it was adjusted between 1 and 4 knots. Reported extending our final dataset to 93 independent studies (Table S1), from trawling times ranged from 15 min up to 1 h. Depending on the use of a which 36 investigated the occurrence in sea surface water, 6 within the flowmeter (55.4%) or not (44.6%), the absolute abundances were water column, 21 in marine sediments, and 30 in beaches. expressed respectively as MPs/unit of volume or MPs/unit of surface. In the latter, the trawling distance was considered, and multiplied by the 3.1. Microplastic data distribution in the Mediterranean Sea width opening of the net, under the assumption of constant seawater flux and ship speed. The authors generally reported concentration of Although MP observations are available for the entire basin (Fig. 2), MPs per unit of surface area (71.6%), rather than MPs per unit of volume there is a clear bias towards the Western Mediterranean Sea, with fewer (32.4%). Additionally, the mass of plastic debris was measured in 43.2% studies (25.8%) investigating MP occurrence in the Eastern Mediterra­ of the samples, and the concentrations are expressed as g MPs/unit of nean Sea. Altogether, and depending on the precision of the sampling volume or g MPs/unit of surface (Table S2). For water column sampling, description in the reviewed literature, there were 3077 samples in the vertical or oblique tows were performed to report integrated values of Mediterranean Sea used to assess the status of MP pollution in its abiotic MP abundance. Different mesh sizes were used to collect the seawater compartments (Fig. S2). Most of the sampling efforts focused on the samples: 52 μm (0.3%), 200 μm (27.4%), 300 μm (3.6%), ≈333 μm (335, presence of these pollutants on the sea surface. Thirty-six studies 333 and 330 μm; 62.8%), 500 μm (2.5%) and 780 μm (3.5%). Once the collected a total of 1200 samples (40.0%). While less attention was net is on-board, it is carefully rinsed from the outside to concentrate the devoted to defining the abundance of MPs within the water column, six sample into the cod-end. The cod-end content is emptied over a steel studies collected a total of 76 samples (2.5%) restricted to the upper 100 sieve with the same or smaller mesh size than the sampling net to avoid m of the epipelagic layer. A similar picture was observed in the sediment the loss of particles. The sample is then resuspended and transferred into compartment: sample scarcity is observed as the sampling sites deepen. the storage container. The authors reported different preservation Sediments from beaches and the neritic zone have been more exten­ methods. Frequently, samples were resuspended in pre-filtered seawater sively studied with 30 and 17 studies, collecting respectively 1302 or deionized water, and fixed with 4% formalin. (42.3%) and 452 (14.7%) samples. In contrast, only four studies re­ Once in the laboratory, the prevalent protocol for MP extraction ported MP concentration from 47 (1.5%) deep-sea sediment samples. consisted of manual separation of the putative MP particles under the stereomicroscope (67.0%). The identification of MPs primarily relied just on visual identification (49.3%). Further identification on the 3.2. Seawater polymer composition was conducted by different spectroscopy methods (Attenuated total reflectance (ATR) - Fourier transform infrared (FTIR; In the seawater compartment (surface water and water column: 47.3%), Raman (2.0%), and Near-infrared (NIR; 1.3%)), analyzing a Fig. 3), most of the studies collected coastal samples (77.4%), and higher subsample that represented 1.2–100% of the putative MP particles efforts have been devoted to reporting the MP abundance in sea surface selected under the stereomicroscope (Fig. 3). When mentioned, the water (94.0%) rather than their dispersion within the water column Fig. 2. General map of the Mediterranean Sea. The location of sampling stations is shown in black dots for sea surface water stations, grey dots for water column stations, thin white circles for marine sediment stations, and thick black and white circles for beach stations. The population density of coastal areas is represented using an orange gradient. The population density data were obtained from several sources: Eurostat and National Statistical offices using the database City population [http://www.citypopulation.de]. NUTS3 regions and Mediterranean rivers were reprinted from Natural Earth free vector and raster map data [www.naturalearthdat a.com]. The background bathymetric map was retrieved from the GEBCO 2019 grid [https://www.gebco.net/data_and_products/gridded_bathymetry_data]. Globe earth projection map showing the location of the Mediterranean Sea was created using the World Borders Dataset retrieved from thematic mapping [http://thema ticmapping.org]. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 3 L. Simon-Sánchez et al. Environmental Pollution 292 (2022) 118275 Fig. 3. General overview of the sampling areas and methods for sampling, extraction processing, and characterization of microplastics in the seawater compartment of the Mediterranean Sea. The figure draws data from the 42 studies included in this review. The abbreviation DS refers to density separation, the asterisk (*) refers to the filtration process, and in the identification column, percentages are indicating the number of particles analyzed within the studies depending on the method used. The different size of plastic debris investigated by the authors is indicated in light blue (<5 mm) and dark blue (<5 mm and >5 mm). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) value of putative MP particles confirmed as MP particles after spec­ dataset (Table S2), we estimated that the MPs median concentration in troscopy analyses ranged from 58.0% to 95.6%. the surface water of the Mediterranean Sea is 8.48⋅104 items km− 2 Beyond the methodological approaches, high variability was re­ (Q1-Q3, 2.89⋅104–2.68⋅105 items km− 2) and the median weight is ported for the MP pollution levels in the sea surface waters (Fig. 4), the 1.0⋅102 g km− 2 (Q1-Q3, 2.79⋅101–3.63⋅102 g km− 2). minimum concentration (200 μm mesh size, median 6.25⋅103 items km− 2; Q1-Q3, 0–1.94⋅104 items km− 2) was reported in the Bay of Calvi, Corsica, NW Mediterranean Sea (Collignon et al., 2014). In contrast, the 3.3. Marine sediments highest values were reported in the Levantine Basin, in the coastal wa­ ters of Lebanon (52 μm mesh size, median 2.24⋅106 items km− 2; Q1-Q3, - In the Mediterranean Sea, most of the MP observations were recor­ 1.85⋅106–3.20⋅106 items km− 2; Kazour et al., 2019), Israel (333 μm ded within the coastal areas (94.4%), with the remaining 5.6% in the mesh size, mean 1.52⋅106 items km− 2; van der Hal et al., 2017), and open sea. For shallow and coastal sediments (<30 m), scientific scuba Turkey (333 μm mesh size, median 1.15⋅106 items km− 2; Q1-Q3, - divers (26.7%) collected surface sediments manually (77.4%) or used 2.94⋅105–5.12⋅106 items km− 2; Gündoğdu et al., 2018). In the compiled hand cores (22.6%). In contrast, when the sampling was conducted from a research vessel (73.3%), Van Veen grab (76.5%), boxcorer (6.8%), Fig. 4. Overview of the concentration of plastic debris reported in the sea surface water studies (n = 36). (A) Studies reporting the concentration in items km− 2 and g km− 2.(B) Studies reporting concentration in items m− 3 ang g m− 3. The studies marked with an asterisk (*) were plotted for both panels. The different sizes of plastic debris investigated by the authors are indicated in light blue (<5 mm) and dark blue (<5 mm and >5 mm). Note that the y-axes are on a logarithmic scale. The squares represent the average concentration reported within the studies when data per station were not available. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 4 L. Simon-Sánchez et al. Environmental Pollution 292 (2022) 118275 multicorer (7.4%), or a mixed combination of these different devices (9.3%) were used. From these bulk sediment samples, a large volume of sediment is available, however, researchers collected a subsample of the first top centimeters (1–5 cm) and preserved it by freezing (− 20 ◦ C) or cooling (4 ◦ C). It is worth to note that authors generally do not report to homogenize the sample before to further proceed with the MP extraction. Microplastic extraction is characterized by multi-step procedures including sieving, purification, density separation, and filtration or concentration of the sample for further identification of MP particles (Hidalgo-Ruz et al., 2012). Some of these steps were applied in different combinations or individually by the authors (Fig. 5). Density separation was the prevalent procedure and used 77.4% of the samples. However, different settings for separation by density need to be considered: the type and volume of the brine solution, the mass of the processed sample, the mixing and settling times, and if the extraction is consecutively repeated to enhance the recovery rate. The most common brine solution was saturated sodium chloride (NaCl, ρ ≈ 1.16–1.20 g cm− 3; 88.6%), followed by distilled water (H2O, ρ ≈ 0.99 g cm− 3; 6.2%), zinc chloride (ZnCl2, ρ ≈ 1.6–1.9 g cm− 3; 4.7%) and sodium iodide (NaI, ρ ≈ 1.8 g Fig. 6. Concentration of plastic debris reported in the marine sediment studies cm− 3; 0.5%). The mass of the sample treated varied from 10 g up to (n = 21). On the left side of the figure, studies reporting the concentration in 1000 g for seabed sediment. The volume of the brine solution added to items kg− 1, and on the right side, studies reporting concentration in items m− 2. the sediment varies from 200 mL to 1000 mL. The mixing and settling The different sizes of plastic debris investigated by the authors are indicated in time widely varied among the studies, from 30 s to 3 h, and from 2 min light brown (S-MPP; < 1 mm), brown (<5 mm), and dark brown (<5 mm and to overnight periods, respectively. The extraction was repeated up to >5 mm). Note that the left y-axis is on a logarithmic scale. The squares three times in 43.8% of cases. In general, the supernatant solution represent the average concentration reported within the studies when data per containing the MPs was collected and vacuum-filtered onto a filter station were not available. (For interpretation of the references to color in this (93.8%). In the studies where the 1–5 mm fraction was targeted figure legend, the reader is referred to the Web version of this article.) (22.6%), MP particles were manually selected from the sediment. In 24.4% of the samples, identification solely relied on visual iden­ (Kazour et al., 2019). The highest concentrations were found in the tification; in 27.3% of the samples, a subsample was selected for spec­ studies where the occurrence of S-MPP was investigated (Lagoon of troscopic analyses, and in 43.3% of the samples, all the putative MP Venice, Italy, median 1.48⋅103 items kg− 1; Q1-Q3, 1.35⋅103–1.86⋅103 particles were chemically characterized. Only Vianello et al. (2013) items kg− 1 - Vianello et al., 2013; North Tyrrhenian Sea, median performed the identification of MPs via chemical mapping (5.0%) on a 1.04⋅103 items kg− 1; Q1-Q3, 6.90⋅102–1.54⋅103 items kg− 1 - Kane et al., subsample of the filter. 2020). The plastic debris concentration in the Mediterranean marine sedi­ ments presented high variability (Fig. 6), the median concentration re­ ported is 3⋅102 items kg− 1 (Q1-Q3, 1.49⋅102–7.70⋅102 items kg− 1). The 3.4. Beaches lowest MP concentration (mean ± SD; 1.66 ± 1.77 items kg− 1) was re­ ported in the Augusta Harbour, Central Mediterranean Sea (D’Alessan­ The relatively easy access required for beach sampling has allowed dro et al., 2018). The highest concentration (mean ± SD; 2.43⋅103 ± researchers to widely collect sediment samples in this compartment and 2⋅103 items kg− 1) was reported in the coastal sediments of Lebanon characterize the MP abundance along the Mediterranean coastline. However, there is considerable variability in the sampling strategies. Fig. 5. General overview of the sampling areas and methods for sampling, extraction processing, and characterization of microplastics in the marine sediments compartment of the Mediterranean Sea. The figure draws data from the 21 studies included in this review. From left to right, the sixth column represents the solution used for the flotation treatment, the asterisk (*) refers to NaI and DI water to de-ionized water, the seventh column represents the consecutive number of extractions performed during the density separation step, and in the tenth column, percentages are indicating the number of particles analyzed within the studies depending on the method used. The different size of plastic debris investigated by the authors is indicated in light brown (S-MPP; < 1 mm), brown (<5 mm), and dark brown (<5 mm and >5 mm). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 5 L. Simon-Sánchez et al. Environmental Pollution 292 (2022) 118275 The number of beaches sampled ranges from one to 23 in a single study that relied on citizen science (Lots et al., 2017). One study covered the entire extent of the beach, while others targeted accumulation zones or pooled samples from different tidal areas. However, most studies focus on the intertidal zone (37.0%) and the high tide line (29.6%). In the studies where the aim was to report the presence of plastic pellets (10.0%) or large microplastic particles (L-MPP; >1 mm, 6.7%), selective sampling (3.3%), and volume-reduced sampling (13.3%) were con­ ducted. The rest (83.3%) collected bulk sediment samples for further MP extraction in the laboratory. Samples were collected using a quadrat (56.6%), ranging from 0.04 m2 to 1 m2. From the surface down to a maximum of 15 cm in-depth, the top centimeters were scraped using a metal spoon, spatula, or shovel. In contrast, six studies out of the 30 collected the samples using a corer (16.6%) or boxcorer (3.3%). Sieving was used as a pre-treatment of the sample to reduce the total volume, eliminating the particles larger than the MP size fraction (>5 mm) in 15 studies. The majority of the studies (76.6%) used density separation to extract MPs from the sediments. The mass of sediment Fig. 7. Concentration of plastic debris reported in the beach studies (n = 29). processed ranged from 50 g to 1000 g. Again, researchers prioritized the On the left side of the figure, studies reporting the concentration in items kg− 1, use of NaCl (78.3%), followed by ZnCl2 (13.3%), while two studies and on the right side, studies reporting concentration in items m− 2. The (8.7%) combined the use of different brine solutions in the consecutive different sizes and types of plastic debris investigated by the authors are indi­ cated in purple for pellets, white (S-MPP; <1 mm), light brown (L-MPP; 1–5 extractions to assure the recovery of denser polymers (Misic et al., 2019; mm), orange (<5 mm), and dark brown (<5 mm and >5 mm). Note that the y- Piperagkas et al., 2019). Besides those two studies, consecutive extrac­ axes are on a logarithmic scale. The squares and hexagons represent, respec­ tions, from 2 up to 5, were performed to increase the recovery rate of tively the average concentration and the full range of value (min. and max.) particles (30.4%). The MPs particles were manually picked from the reported within the studies, when data per station were not available. (For supernatant in 5 studies (21.7%), whereas in 18 studies (78.3%), the interpretation of the references to color in this figure legend, the reader is supernatant was concentrated onto a filter for further identification referred to the Web version of this article.) under the stereomicroscope. Additionally, two out of the 30 studies included a digestion step in their protocol, applied at different stages of including the prioritization of using cotton clothes while sampling, the the MP extraction process. Misic et al. (2019) performed acidic diges­ use of plastic-free and pre-cleaned storage containers, and pre-filtered tion, adding 100 mL of HCl to the supernatant collected at the density solutions to resuspend the samples. The use of blanks and procedural separation step. Chouchene et al. (2019) applied alkaline digestion to blanks is still not standardized in MP studies. Generally, each study the sample concentrated onto the filter, which was immersed into 50 mL attempted to comply with at least some appropriate measures to mini­ of a KOH (20%) solution. When no density separation was performed mize contamination. For example, researchers wore cotton lab coats at (23.3%), the samples were sieved, and particles were visually selected the laboratory, used glass materials or stainless-steel materials, covered (20%), or in one study (3.3%), a solvent extraction was performed to the samples to avoid airborne contamination, and all tools and surfaces determine the content of polymeric and polymer-derived materials by were rinsed/cleaned (HCl (1 M), ethanol or MilliQ water) before use. spectroscopic techniques (Pyrolysis-gas chromatographic-mass spec­ For the studies on the seawater compartment, 38.1% reported the troscopy (Py-GC/MS) - Ceccarini et al., 2018). exclusion of fibers from their results due to the high risk of background The identification of MPs relied on visual characterization in 9 contamination with this type of particle. To minimize this issue, 23.8% studies (30.0%), differential scanning calorimetry (DSC) in one study of the studies conducted the analysis below a laminar flow, 26.2% run (3.3% - Shabaka et al., 2019), and spectroscopy techniques were applied blank controls (i.e., Petri dish or blank filters are exposed to laboratory in 19 studies (63.3%). In the latter, 18 studies (60%) used ATR-FTIR on a conditions), and 7.1% reported to run procedural blank (i.e., MilliQ subsample of particles; when mentioned, it ranges from a few particles aliquot is treated as an environmental sample) to assess the cross- to 100%, being confirmed as MPs between 11.3% and 100% of the contamination while processing the samples. particles. Lots et al. (2017) analyzed a subsample of particles (221 units) In the marine sediment studies, 15.8% of studies performed the using the Raman spectrometry technique. Here, only 4.5% of the par­ analysis inside of a specialized clean laboratory designed for micro­ ticles were confirmed as MPs, while the rest of the particles did not plastic analysis, which minimizes airborne contamination through pre- provide discernible peaks (42%), were categorized as dyes (18%), or did filtration of the air in the room. Blank controls were run in 42.9% of not provide a reliable match with the library used (36%). the studies, and 28.6% of the studies ran procedural blanks along with In the compiled dataset, the median concentration of MPs along the environmental samples, to assess the potential contamination from Mediterranean beaches was 58.6 items kg− 1 (Q1-Q3, 11.6–2.49⋅102 laboratory background. Additionally, despite the use of procedural items kg− 1) (Fig. 7), one order of magnitude lower than the median blanks, one study reported excluding fibers (<500 μm) in order to avoid concentration found in marine sediments (3⋅102 items kg− 1). The lowest potential overestimation. abundance (particle size: <5 mm; median 7.5⋅10− 1 items kg− 1; Q1-Q3, In beach studies, 36.7% did not mention following any specific 6.3⋅10− 1 – 8.8⋅10− 1 items kg− 1) was reported in Slovenian beaches measure to prevent contamination. These measures were not relevant in (Korez et al., 2019). In this study, the authors reported a low rate of 18.5% of studies due to the MP size investigated (L-MPP and pellets). identification success, only 11.6% of the putative MPs were confirmed as Blank controls and procedural blanks were run in 20.0% and 23.3% of MPs. The highest abundance (L-MPP; median 9.75⋅102 items kg− 1; the studies, respectively. Three studies reported running spiked samples Q1-Q3, 6.16⋅102–1.55⋅103 items kg− 1) was reported in the Datça (virgin polymers) to assess the recovery rate of their protocols. Peninsula, North of Turkey (Yabanlı et al., 2019). 4. Discussion 3.5. Quality control In the Mediterranean Sea, numerous research efforts are focused on To prevent airborne and cross-contamination of samples, especially reporting MP occurrence in different abiotic compartments. Synthesis if S-MPP and fibers are investigated, preventative measures are required, 6 L. Simon-Sánchez et al. Environmental Pollution 292 (2022) 118275 and integration of these data are limited by the lack of standardization of of particles accompanied by a decrease in size (Isobe et al., 2017; the methods and reporting units. Notwithstanding the researchers’ Pabortsava and Lampitt, 2020). Similarly and importantly, the potential different approaches, similarities are found in the sampling, extraction, environmental risk is negatively correlated with the size of the particles and identification protocols. Here, we discuss how these different ap­ (Covernton et al., 2019; Ma et al., 2019). proaches, rather than solely the prevention for intercomparison among Other sampling approaches are required to account for the smaller studies, shape our current understanding of the Mediterranean Sea’s MP fraction of floating MPs and potentially nanoplastics (NPs). Barrows pollution levels. After an extensive literature review, we have identified et al. (2017) suggested implementing combined sampling, which in­ pressing research needs to be addressed within this basin. volves collecting volume-reduced and bulk samples to ensure the char­ acterization of a wide size spectrum of MPs (1–5000 μm). The pattern observed is that greater MP concentrations were reported in bulk water 4.1. The effect of sampling methods samples (Barrows et al., 2017; Covernton et al., 2019; Green et al., 2018). Additionally, this approach allows for the assessment of fibers 4.1.1. Surface waters that generally are overlooked or discarded in the net sampling approach In the Mediterranean Sea, most of the sea surface samples were (Suaria et al., 2020). However, the volume and number of replicates volume-reduced using a net. This approach, preferably using a mesh size required to provide a relevant statistical evaluation remain unclear. of 333 μm, was recommended by the Marine Strategy Framework Ryan et al. (2020) reported that sampling a larger volume of water may Directive (MSFD) for the monitoring of MPs (Gago et al., 2016). The increase the reproducibility of the measurement, but at the cost of main advantage is that large volumes of water are sampled in a rela­ underestimating the environmental concentration. tively short time. However, the mesh size of the net determines the size In addition to sampling devices, other factors may influence the ac­ range and diversity of the particles. In a study of the Seine River (Paris, curacy of the measurements and possibly explain the differences be­ France) surface waters, Dris et al. (2015) found that MP concentrations tween studies, such as using a flowmeter or considering the sea state. measured with a mesh size of 80 μm were 30-fold greater than those During sampling, the use of a flowmeter is critical, as it measures the collected with a 330 μm manta trawl, as a larger mesh size allows small water effectively passing through the net. Suaria et al. (2016) reported size-class fibers (100–500 μm) to pass through it more easily. Kang et al. that the sampled area was on average two times higher when computed (2015) reported that floating MP (<2 mm) concentrations were two from GPS data compared to the same area when estimated using the orders of magnitude higher using a 50 μm hand net when compared to a flowmeter, making the GPS method a less reliable approach. During 330 μm mesh size net in the Southern Sea of Korea. In a comparison sampling, the weather and environmental conditions affect the mixing experiment on the efficiency of sampling devices along the North layer and, hence, MP’s vertical distribution along the first meters of the Atlantic coastal waters, Lindeque et al. (2020) demonstrated that MP water column (Collignon et al., 2012; Kooi et al., 2016; Kukulka et al., concentrations collected with a 100 μm mesh size net were up to 2.5 and 2012). Manta trawls and neuston nets cover, respectively, the first 10-fold greater than with a 333 μm and a 500 μm mesh size nets, 15–25 and 50 cm of the water column. Thus, accounting just for the respectively. Despite the large-scale-temporal variability of the surface tow concentration may underestimate MPs’ total load in the compiled Mediterranean dataset (715 datapoints), we observed that surface waters, especially during high wind speed conditions (Kukulka lower MP abundances in the surface water were reported for larger mesh et al., 2012). sizes (Fig. 8). The smallest mesh size (52 μm) was used by Kazour et al. (2019) to collect samples from the coast of Lebanon (n = 3). The pre­ 4.1.2. Water column dominance of waterborne MP samples collected with ≈333 μm (68.2%) Despite the numerous investigations on seawater, to date, subsurface and 200 μm (27.4%) mesh size nets indicate that smaller MPs (<333 μm data are still scarce. Depth integration models (Kooi et al., 2016; and <200 μm) have been systematically underestimated in the surface Kukulka et al., 2012) and multi-level trawls in the North Atlantic (150 waters of the Mediterranean basin. This is particularly relevant when μm mesh size, Reisser et al., 2015; and 330 μm mesh size, Kooi et al., considering that there is a negative relation between MP size and their 2016) indicated that MP concentration exponentially decreases within abundance in the natural environment, with an increase in the number the first meters of the water column, yet reporting smaller sizes in deeper waters (Kooi et al., 2016; Reisser et al., 2015). In the Mediterranean Sea, few studies (n = 6), always restricted to the upper 100 m of the photic zone, have investigated the MP occurrence within the water column. de Lucia et al. (2018) found higher MP concentration in coastal surface waters (0.32 ± 0.24 items m− 3) of minor Italian islands compared to the abundance within the first 20 m of the water column (0.18 ± 0.10 items m− 3), although no significant variability was found. Along the Tuscany coastline, Baini et al. (2018) reported similar average concentrations between the water column (down to a maximum of 100 m; 0.16 ± 0.47 items m− 3) and the floating MPs concentration at the surface waters (0.27 ± 0.33 items m− 3), with the predominance of particles <1 mm. In the Gulf of Lion, Lefebvre et al. (2019) performed vertical tows from the bottom (max. depth of 100 m) to the surface, and solely fibers were found with an average abundance of 0.23 ± 0.20 items m− 3. In the lagoon of Bizerte, Tunisia, seawater samples were collected using a submersible pump (300 μm mesh size; Wakkaf et al., 2020a, 2020b). The authors reported relevant high concentrations in the sea surface waters (453 ± 335 items m− 3) and benthic waters (400 ± 200 items m− 3). Outside the Mediterranean Sea, when similar sampling approaches were conducted (i.e., in-situ pumps) and deeper layers were investigated, the Fig. 8. Violin graph showing plastic particle concentration (items km− 2) re­ MP concentrations reported varied between 1–4 orders of magnitude ported for 715 Mediterranean seawater locations, logarithmically transformed higher (Choy et al., 2019; Pabortsava and Lampitt, 2020). At the Bay of and plotted in relation to the mesh size of the net used to collect the Monterrey (California, USA), Choy et al. (2019) filtered a large volume seawater sample. of seawater (1007–2378 m3) in depths ranging from 5 to 1000 m, and 7 L. Simon-Sánchez et al. Environmental Pollution 292 (2022) 118275 they found the highest MP (>100–5000 μm) concentration in the morphological characteristics of the beaches. Generally, beaches are mesopelagic zone (200–600 m; 15 items m− 3). In a latitudinal transect in dynamic systems with changing conditions due to environmental and the Atlantic Ocean, Pabortsava and Lampitt (2020) reported much anthropogenic causes. Among these factors, for the Mediterranean Sea, higher concentrations of MPs (PE and PS, 32–651 μm; 1114 ± 542 items studies should consider the low-tide amplitude, hydrodynamic forces, m− 3) in the mesopelagic zone. These studies point to the water column beach morphology, wind exposure, seasonal visitor pressure on the as a major reservoir of MPs and highlight the importance of under­ beach, proximity to MP sources, cleaning events, erosion after storm standing the ecological and physical processes responsible for the export events, and potential maintenance works (i.e., restocking of sand). of these pollutants from the surface to deep-sea sediments. Specifically, the presence of smaller particles in the mesopelagic layer (Pabortsava 4.2. The effect of extraction protocols and Lampitt, 2020) suggests the progressive degradation of MPs in the seawater environment, highlighting the existing incongruity of the In seawater samples taken from the Mediterranean Sea, the extrac­ oceanic plastic budget (Eriksen et al., 2014; Pabortsava and Lampitt, tion of MPs predominantly relies on physical separation processes – 2020; Thompson et al., 2004; Van Sebille et al., 2015). sieving, flotation, and manual selection under the stereomicroscope (Fig. 3). Although most of the samples were collected within the conti­ 4.1.3. Marine sediments nental margins, which are recognized as high productivity areas, pre­ The most common sampling approach was collecting bulk sediment treatment for organic matter (i.e., phyto- and zooplankton) oxidation samples, with a significant constraint related to the extraction and pu­ protocols were rarely reported in the reviewed studies. Purification rification protocols (See section 4.2). Thus, the use of different sampling processes (i.e., enzymatic, digestion, oxidation) may reduce the pro­ devices does not imply such substantial variability as it does for the cessing time, enhance MPs’ recovery, and simplify the subsequent step water compartment. The reviewed studies (n = 21) investigated the MPs of identification (Cole et al., 2011; Löder et al., 2017). However, the presence in surface sediments, except for the study from De Ruijter et al. protocol selection should be cautious, as temperature and different re­ (2019), which investigated the vertical distribution of MPs in intertidal agents may damage MPs (Enders et al., 2017; Munno et al., 2018). sediments cores. Despite the collection of sediment cores, Following the manual MPs selection approach, the extraction efficiency palaeo-oceanographic approaches investigating historical accumulation highly depends on the user’s experience, the particles’ size, micro­ patterns in sediment archives (Bancone et al., 2020) are absent from the scope’s magnification, and the sample’s complexity (e.g., rich Mediterranean literature. Future studies would need to address the role biota-samples; Löder and Gerdts, 2015). Manual selection of particles is of geophysical processes (i.e., sediment and organic accumulation rate) time-consuming and inevitably introduces a bias towards selecting and anthropogenic pressures in the area (i.e., bottom trawling) on the larger and color particles because these are easier to recognize and fate of MPs once they reach the seafloor. These aspects need to be taken isolate (Song et al., 2015). The predominance in the use of nets to sample into account to gain knowledge of MPs’ sequestration within marine the seawater compartment sets the lower size cut of the collected MPs, sediments, their dilution and accumulation patterns, and potential generally above 200–300 μm. The manual selection of the particles in­ resuspension. troduces a wide range of variables that undoubtedly affect the MPs detection limits and compromises the accuracy of the isolation. Such 4.1.4. Beaches variability should be addressed by systematically implementing quality The sampling strategy to assess the occurrence of MPs in the Medi­ assurance practices (i.e., spiked samples, interlaboratory comparison terranean beaches encompasses a wide range of methodologies/ap­ exercises between Mediterranean laboratories) to validate the extrac­ proaches, depending on the main goal of a given study. For example, tion’s protocol and define the extraction rate and detection limits. reduced volume samples were employed in studies where pellets or The MP extraction from marine sediments is challenging. For specifically L-MPP (>1–5 mm) were targeted (Atwood et al., 2019; example, the organic content of the sediments, although less than sur­ Grelaud and Ziveri, 2020; Maršić-Lučić et al., 2018; Merlino et al., 2020; face water samples, may retain the MPs, complicating their separation. Piehl et al., 2019; Turner and Holmes, 2011). However, the collection of To address that, samples are subjected to a purification process to bulk sediment samples was the most common (83.3%). Like in marine facilitate and enhance the recovery of MPs (Hurley et al., 2018; Löder sediments, the whole MP size spectrum can be potentially characterized et al., 2017; Masura et al., 2015). In the Mediterranean Sea, most of the in this sample type, but the limitations are defined by the laboratory samples were collected in the coastal margins. These areas represent procedures (See section 4.2). The main differences in sampling pro­ sites of significant importance for many biogeochemical processes, tocols’ relied on the number of replicates, volume of sand collected, including organic carbon burial and remineralization (Muller-Karger depth of sampling, beach area sampled, and the ratio of the sampled et al., 2005). Thus, higher organic content is expected in these types of surface area to the beach’s surface. Different protocols have been pub­ samples. In the reviewed studies, the predominant extraction protocol lished for providing guidance on the monitoring of microlitter, targeting consisted in sieving and separation by density (Fig. 5), with few studies MPs particularly. The MSFD technical report recommended monitoring (n = 2) performing oxidation treatment of the samples (De Ruijter et al., the presence of MPs above the strandline, collecting a minimum of five 2019; Krüger et al., 2019). samples targeting the top 5 cm in a stratified random manner to cover The wide variety of existing polymers differ on their specific density, the entire beach or a specific area (MSFD Technical Support group on which is generally used for the particles’ physical separation. Due to the marine litter, 2013). In an attempt to standardize the sampling protocol, higher density of sediments (quartz, ρ = 2.65 g cm− 3) than the plastic Besley et al. (2017) investigated the sample size, sampling depth, and materials, Thompson et al. (2004) proposed a flotation approach to sampling location required to achieve statistical representativeness. extract the MPs from the sample matrix, using NaCl as a brine solution. Their proposed protocol developed a formula to calculate the number of Researchers used NaCl predominantly for the density separation in the samples required per 100 m transect and recommended collecting at Mediterranean studies because it is the most common brine solution in least 100 g of sand targeting the top 5 cm. In contrast, the findings of MP studies (Hidalgo-Ruz et al., 2012), it is environmentally friendly, and Karkanorachaki et al. (2018) highlight the importance of collecting it is cost-effective. The limitation of using this brine solution is that subsurface samples (down to 10 cm) to truly characterize the concen­ plastics materials with a higher density (i.e., PET, ρ = 1.38 g cm− 3; or tration of MPs in beaches. Their results showed that the concentration of PVC, ρ = 1.39 g cm− 3) might be underestimated. Different brine solu­ MPs fragments and pellets was up to one order of magnitude higher in tions with a higher density, as NaI (ρ ≈ 1.8 g cm− 3), sodium poly­ subsurface samples (339.8 ± 104.4 items m− 2) than in surface samples tungstate (SPT, 3Na2WO4⋅9WO3⋅H2O, ρ ≈ 1.4 g cm− 3), zinc bromide (35.3 ± 11.5 items m− 2). Future studies monitoring MP pollution on (ZnBr2, ρ ≈ 1.71 g cm− 3), sodium bromide (NaBr, ρ ≈ 1.37–1.40 g beaches need to strategically design their sampling to consider the cm− 3), calcium chloride (CaCl2, ρ ≈ 1.3–1.5 g cm− 3) have been proposed 8 L. Simon-Sánchez et al. Environmental Pollution 292 (2022) 118275 as an alternative to enhance the recovery of denser polymers from correct the reported concentrations. Similarly, the rate of successful sediment samples. While there is no standard recommendation, the se­ identification was often not reported in the studies, and when this was lection of the appropriate brine solution should consider the polymers’ clearly stated, it ranged from 5% to 100%. The most common technique density targeted in the study, hazard, and economic variables (Frias within the Mediterranean studies was the characterization of particles et al., 2018). by ATR-FTIR. In this approach, the particle is pressed against a crystal Besides the brine solution, other setting parameters may affect the and subjected to the IR laser beam to record its IR spectrum. This rate of MP extraction from sediment samples. However, little attention approach requires hand-picking the particles and manually analyzing was devoted to investigate these parameters’ effect on the extraction them, which is time-consuming, and limits the minimum particle size to efficiency (i.e., the mass of the sample processed, the ratio of the sedi­ that can be manually isolated (≈300 μm; Primpke et al., 2020). As ment/brine solution, mixing and settling times, and the number of mentioned above, in the seawater compartment of the Mediterranean consecutive extractions needed). Besley et al. (2017) addressed this Sea, the knowledge is predominantly restricted to plastic particles question on beach sediment samples, suggesting that for 50 g dry weight >200–300 μm due to the sampling mesh size. In a few studies (n = 4), of sediment samples, 200 mL of brine solution should be added to the the Raman spectroscopy technique was used to characterize the MP sample, the mix stirred for 2 min and then left to settle for at least 6 h. particles. Lots et al. (2017) reported a low success for the identification This process needs to be repeated three times, reaching a recovery rate of particles (4.5%). The authors indicated that spectra had low quality of 83.0%. Similarly, Simon-Sánchez et al. (2019) reported a recovery due to fluorescence that may result from biological material on the rate of 79.4% after three out of five consecutive extractions from sandy surface of the particles. In the same study, the authors reported the samples using NaCl. interference of additives and dyes that masked the spectrum, preventing The lack of validation on the extraction protocols prevents a quan­ polymer recognition. titive assessment of the effects of these diverse approaches on the re­ Exclusively, Vianello et al. (2013) applied reflectance μFTIR based ported MP concentration. In the current status of the research field, it is on chemical imaging of the filters. Under this approach, as the analysis is difficult to recommend the correct extraction protocol, as this should be automatized, there is no need for a visual pre-sorting of the particles, chosen depending on the study’s objective, the environmental matrix minimizing the human bias. However, the analysis is time consuming, investigated, and the economic and time limitations. Under this context, and requires a large initial investment for the acquisition of the instru­ the inter-comparison of results between studies will remain jeopardized ment (Primpke et al., 2020). Additionally, the authors reported diffi­ by the researchers’ numerous research approaches to characterize MP culties in the analysis caused by the use of Glass Fiber filters to pollution. However, the implementation of good practices and different concentrate the sample. To minimize this issue, researchers reported quality assurance measures, which can be easily fulfilled, can help to concentrating the samples onto an IR transparent or reflectance surface provide more accurate comparisons (e.g., providing a clear and concise (i.e., suitable membrane; Löder et al., 2015) or window (Simon et al., description of the protocol, acknowledging the size limitation of the 2018) to record the spectrum. In the study of Vianello et al. (2013), the methods, analyzing spiked samples to characterize the recovery rate of predominant size was in the range of 30–500 μm, and the smallest the protocol, control and procedural blanks while sampling and particle size reported was 15 μm. analyzing the samples, presenting the results separately per type of MP Pyrolysis-Gas Chromatography-Mass Spectrometry (Py-GC-MS) was pollutant and size fraction). used to characterize MPs (<2 mm) in Marina di Vecchiano (Tuscany, Italy) (Ceccarini et al., 2018). To our knowledge, this is the only study in 4.3. Identification the Mediterranean Sea that applied Py-GC-MS. This technique is a destructive method, which provides accurate data on mass concentra­ In the Mediterranean Sea, the identification of the particles relied on tion. However, it does not determine the number of particles (units) or visual characterization in 49.3% of seawater samples, 27.3% of marine their morphological characteristics by itself, which makes the compar­ sediment samples, and 30.0% of beach studies. There are general rec­ ison between studies in the region difficult. ommendations to visually identify putative MP particles (Hidalgo-Ruz et al., 2012; MERI, 2012) or fibers (Stanton et al., 2019). This method is 4.4. Microplastic pollution in the Mediterranean Sea time-efficient but highly depends on the user and the microscope’s magnification (Löder and Gerdts, 2015). Visual characterization may While the hydrodynamic conditions and wind-driven processes in­ overlook smaller and transparent particles, significantly impacting the fluence the redistribution of MP debris (Collignon et al., 2012; de Haan underestimation of the MP concentration (Song et al., 2015). Regarding et al., 2019; Fossi et al., 2017; Suaria et al., 2016), even far from their size, the recommended limit for a correct identification is 1 mm for the sources (Ruiz-Orejón et al., 2018), higher MP presence is related to areas naked-eye, 500-100 μm using optical microscopy (highly dependent of under high anthropogenic pressure and proximity to (micro)plastics the magnification; Primpke et al., 2020 and references therein). land-based sources (Pedrotti et al., 2016). Coastline morphology may Infrared (IR) and Raman are the most common techniques for the also affect the dispersion of these pollutants as the rugosity of the coast chemical characterization of the particles. These analytical methods are may facilitate the MP retention in nearshore areas (Compa et al., 2020), recommended by the MSFD (MSFD Technical Support group on marine while smooth and large bays may induce the exports of MPs to off-shore litter, 2013) and the Joint Group of Experts on the Scientific Aspects of waters (Brennan et al., 2018). Most of the sea surface samples from the Marine Environmental Protection (GESAMP, 2019). These spectroscopic Mediterranean Sea were collected in coastal areas (78.2%). Few studies techniques are typically non-destructive, provide particle number specifically investigated the relationship between MP occurrence and (units) and morphological characteristics of particles larger than 10 μm the proximity to the coast. In the Balearic Islands of Mallorca (Spain, NW and 1 μm in size for FTIR and Raman spectroscopy, respectively (Löder Mediterranean), Compa et al. (2020) found high heterogeneity of the and Gerdts, 2015). The analysis is relatively simple, as once the spec­ MP levels (8.58 ± 4.08⋅106 items km− 2) within the first km of coast, trum is retrieved, the polymer type can be determined by comparing it although MP concentration significantly decreased with increasing dis­ against a reference library. However, the purification and extraction tance to the coast (<1 km). The compiled dataset in Pedrotti et al. (2016) methods are critical to isolate MPs from the environmental matrix found higher MP concentration (1.58⋅105 ± 1.57⋅105 items km− 2) in the (Löder et al., 2017). In the Mediterranean Sea, the general approach was first km adjacent to the coast, with a decrease in waters between 1 and that after visual sorting of the putative plastic particles, a subsample 10 km from the coast (8.0⋅104 ± 3.80⋅104, items km− 2), and again (1.2–100% of the particles) was selected for characterizing its chemical reaching high values in waters further away from the coast (>10 km; composition. In general, these subsamples were chosen randomly, with 1.76⋅105 ± 2.16⋅105 items km− 2). In the coastal waters of Tuscany, Baini scarce reference to the statistical weight of the subsample to validate or et al. (2018) found a correlation between MP abundance, which 9 L. Simon-Sánchez et al. Environmental Pollution 292 (2022) 118275 significantly increased with distance from land. In the dataset compiled coast (Kaandorp et al., 2020) in the Mediterranean Sea. Biogenic habi­ for this literature review, we observed that 61.5% of the sampling sta­ tats likely sequester those particles in coastal ecosystems (de Smit et al., tions were located within the first 10 km of the coast (Fig. 9). No relation 2021). For example, seagrass meadows (Posidonia oceanica) trap plastic was observed between plastic debris concentration (items km− 2) and and MP debris and contribute to their removal by aggregating these distance to the coast (km), and this observation is drawn using data from pollutants within vegetal fibers that are washed up back to the coast 636 Mediterranean locations (Fig. 9). These findings agree with previous (Sanchez-Vidal et al., 2021). modelling studies (Liubartseva et al., 2018; Mansui et al., 2015) on the In the Mediterranean Sea, submarine canyons play an essential role effective plastic dilution trend along the Mediterranean Sea’s surface in the exchanges between the continental shelf and the deep sea. These waters, facilitated by the basin’s high surface dynamics, that prevent the geomorphic features of the continental margins are subjected to formation of permanent accumulation areas (Mansui et al., 2015). ephemeral gravity currents, responsible for transporting terrestrial The geographical distribution of surface water stations (Fig. 2) in­ sediments, organic carbon, and recently also MPs to the deep-sea floor dicates a gap of knowledge on the MP pollution levels in the southern (Fernandez-Arcaya et al., 2017; Pohl et al., 2020). High loads of plastic coast of the basin and in off-shore waters, specifically in the Central litter were reported in the submarine canyons of the Mediterranean Sea, Mediterranean and the Levantine basin. Equally important as the spatial which are transported down-slope and expected to accumulate at depth distribution of the sampling efforts is the temporal trend. Few studies (Ramirez-Llodra et al., 2013; Tubau et al., 2015). However, once on the investigated the seasonal variability of floating plastic debris in the deep-sea floor, bottom currents lead to the erosion, transport, and Mediterranean Sea (van der Hal et al., 2017; Baini et al., 2018; Compa deposition of sediment (Stow et al., 2019). In the Tyrrhenian Sea, Kane et al., 2020). Seasonal patterns, at local and regional scales, on the et al. (2020) showed the strong influence of near-bed thermohaline temporary retention of floating plastic debris were predicted by Mansui currents on the fate of MPs. Therefore, focusing research efforts to et al. (2020), who described three larger retention areas located east of coastal areas may bias our understanding of MPs occurrence in the the Balearic Islands, in the central Tyrrhenian Sea, and along the Mediterranean sediments. Future studies need to understand the MP Tunisian and Lebanese coast during summer and autumn. Seasonal and presence within a bathymetric gradient, considering the Mediterranean interannual variability is critical to understand the physical transport physiographic settings, vertical settling fluxes, and deep-sea currents and accumulation patterns of floating MPs. The detection of main cir­ that may lead to the final accumulation of MPs in this environment. In culation features (i.e., permanent and transient cyclonic and anticy­ the Mediterranean Sea, the deep sea represents the 80% of the basin, MP clonic gyres) during sampling efforts may also contribute to our impacts to this environment may occur imperceptibly, as the remoteness understanding of the accumulation and dissipative trends of these pol­ of the deep sea still limits our knowledge on the biodiversity inhabiting lutants (Brach et al., 2018) in the surface waters of the Mediterranean it (Danovaro et al., 2010). Sea. We observed a decrease in the sampling efforts in the marine sedi­ 5. Perspectives ment compartment with depth. Few studies investigated MP occurrence in deep-sea sediment, while most of the efforts focused on the conti­ Detailed assessment of MP pollution is challenging, and while the nental margins. In the Mediterranean Sea, continental shelf areas play lack of standardized methods persists in the field, researchers should be an essential role, representing up to 20% of the basin’s surface (Pinardi responsible for providing standardized results, and to highlight the et al., 2004). These areas are characterized by highly dynamic biogeo­ limitations of their study (Gago et al., 2016; Hartmann et al., 2019; chemical processes, the presence of critical endemic ecosystems, and Provencher et al., 2020). Under this context, to advance the knowledge tremendous biodiversity (Coll et al., 2010; Muller-Karger et al., 2005). of MP pollution in the abiotic compartments of the Mediterranean Sea Thus, it can be expected that plastic pollutants in these areas may pose a and to understand the environmental risk that these pollutants pose to higher environmental risk. The nearness to populated areas represents a its biodiversity, major research questions/challenges need to be continuous input of plastic debris to the marine environment. From addressed. these inputs, high sinking plastic flux (1 kg km− 2 day) occurs close to the • Combining efforts. Here, we have discussed the limitations of char­ acterizing the whole spectrum of MPs while acknowledging the vast amount of work developed in the past years. Ideally, studies should target the whole MP spectrum, however temporal and economic constraints are evident. Thus, MP pollution research in the Medi­ terranean region will be highly benefited if the data produced – sampling parameters (i.e., coordinates, sea state, wind, depth, etc.), abundances, particle’s size, polymer, shape distribution – will be made available in open data repositories. • Quantifying MP pollution. The numerous research efforts conducted in the Mediterranean Sea provide an excellent opportunity for future monitoring of the temporal trend on the abundance of these pollut­ ants within the basin. Future studies need to consider within their objectives to apply similar approaches for the comparison of data already produced for the Mediterranean Sea. Notwithstanding, implementing good quality practices and quality control protocols will further validate the accuracy of the future knowledge on MP occurrence in the basin. • Spatial distribution on marine MP data. Future sampling efforts need Fig. 9. Plastic particle concentration (items⋅km− 2) reported for 636 Mediter­ to consider sea surface features, and seasonal and interannual vari­ ranean Sea surface water data points, logarithmically transformed (left Y-axis, blue dots) and plotted in relation to the distance (km) logarithmically trans­ ability within the Mediterranean general circulation. There is also an formed. The barplot indicates the number of sampling stations (#unit, right Y- urgent need to define the MP pollution levels in the North African axis) in relation to the distance to the coast. (For interpretation of the references coast and the Tyrrhenian, Aegean, and Levantine basins. to color in this figure legend, the reader is referred to the Web version of • Assessing MP sinking/export in the water column. Very little is still this article.) known about the dynamics of MPs from the sea surface to the 10 L. Simon-Sánchez et al. Environmental Pollution 292 (2022) 118275 seafloor. The role of the water column is indubitable. Holistic ap­ Catalunya). L.S.-S acknowledges financial support from the FI-2018 proaches should be considered to gain comprehensive knowledge on fellowships of the Generalitat de Catalunya (2018_FI_B 00918). M.G., the physical and biological processes distributing these pollutants P.Z. acknowledges the i-plastic project (JPI-Oceans—Grant PCI2020- across different environmental compartments. Understanding the 112059) for financial support. MP abundance in the epipelagic and mesopelagic layers provides baseline knowledge to the MP exposure that fish stocks inhabiting Appendix A. Supplementary data these layers of the ocean are facing. This is particularly relevant, as crucial commercial fishing species for human consumption thrive in Supplementary data to this article can be found online at https://doi. these layers. org/10.1016/j.envpol.2021.118275. • Assessing the accumulation of MPs. The deep-sea sediments are considered a major reservoir of MP pollution in the marine cycle. In References the absence of studies linking geophysical processes with MP occurrence in sediments, the data is restricted to presenting only a Adobe Inc. 2021. Adobe Illustrator, https://adobe.com/products/illustrator. Aliani, S., Griffa, A., Molcard, A., 2003. Floating debris in the ligurian sea, North-Western snapshot of the potential exposure. However, to further understand mediterranean. Mar. Pollut. Bull. 46, 1142–1149. https://doi.org/10.1016/S0025- the risk of these pollutants buried in the deep ocean, we need to gain 326X(03)00192-9. knowledge of their patterns (accumulation, dilution, resuspension) Atwood, E.C., Falcieri, F.M., Piehl, S., Bochow, M., Matthies, M., Franke, J., Carniel, S., Sclavo, M., Laforsch, C., Siegert, F., 2019. 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