The Lagrangian Individual-Based model (IBM) used to describe the transport of micro- and macro-plastics (Tsiaras et al., 2021) is described in detail in the Supplementary Material. The model is coupled with a 3-D hydrodynamic model (POM), which provides ocean currents and diffusion coefficients that are used to describe the dispersion of plastic particles in the marine environment. Specific attributes of particles (size, density/buoyancy) were taken into account among different sizes and types of plastic categories. Six size classes (50, 200, 350, 500, 1,000, 2,000 μm) of microplastics particles and five sizes/types (5 mm–2 cm, 2–20 cm, > 20 cm bottles, > 20 cm bags, > 20 cm foam) of macroplastics were considered. For computational efficiency, the concept of Super-Individuals (SIs; Scheffer et al., 1995) was used, with each SI representing a group of particles with the same attributes (position, origin, type of plastic etc.). All sizes/types of plastics were affected by stokes drift (obtained from Copernicus marine service²), which was assumed to decrease exponentially with depth. Low-density macroplastics (i.e., bottles and foam; Tsiaras et al., 2021) were assumed to be partially above sea surface, being subject to wind drag (Yoon et al., 2010). Fields of wind were obtained from POSEIDON atmospheric forecast (Papadopoulos and Katsafados, 2009; Supplementary Figure 6).

Particles are removed from surface waters as they lose their buoyancy through interactions with the marine environment. The particles’ density is increasing (and thus buoyancy is decreasing) through biofouling (Kooi et al., 2017). In the model, biofouling of microplastics is explicitly described, as a function of bacterial abundance. Bacteria are usually found as early colonizers and most abundant organisms in marine biofilms (e.g., Oberbeckmann et al., 2014; Masó et al., 2016). Thus, they are considered as a proxy for the biofouling community which may include also other organisms, such as algae and their grazers (Oberbeckmann et al., 2014; Dang and Lovell, 2016; Rummel et al., 2017; Nava and Leoni, 2021). Bacterial abundance was obtained from a biochemical model based on European Regional Seas Ecosystem Model (ERSEM; Baretta et al., 1995; Kalaroni et al., 2020a,b). Based on the adopted formulation, smaller microplastics (50, 200, 350, 500 μm) are gradually sinking (time to sink with mean bacterial biomass: < 1 day 50 μm, 3.5 days 200 μm, 14 days 350 μm, 32.5 days 500 μm, see Supplementary Figure 1) due to the buoyancy loss resulting from the attachment of heavier biofilm (∼1,500 kg/m³, Bratback and Dundas, 1984). Larger particles (>1 mm) practically remain afloat and in most occasions, they get stranded on land before they sink (Tsiaras et al., 2021). Small macroplastics (5 mm–2 cm, 2–20 cm) and plastic bags were also assumed to gradually lose their buoyancy from the attachment of micro- and macro-fouling communities (Ye and Andrady, 1991; Fazey and Ryan, 2016a; Pauli et al., 2017). Based on the assumptions for biofilm thickness adopted by Tsiaras et al. (2021), small macroplastics with sizes 5 mm^–2 cm and 2–20 cm sink after 120 and 150 days, respectively, while bags sink after 90 days, which is comparable with estimates from observations (Holmström, 1975). Moreover, bottles were assumed to randomly lose their buoyancy daily (i.e., when filled with water), as indicated by observations, showing a relatively small contribution of bottles in open sea floating plastics (Zeri et al., 2018).

Particles that end-up on land were assumed to remain on the beach for a fixed retention time (Tsiaras et al., 2021), after which they return to the sea, in the direction of waves at the beach location. During the time they spent on the beach, the particles’ concentration decreases, assuming some loss rate due to burial. The latter is the main loss term in the model (along with sinking) and it has been tuned so that the mean basin scale concentration remains fairly stable throughout the interannual simulation, obtaining also a best fit of simulated micro- and macro-plastics concentrations with in situ data.

The model used to study the distribution of plastics is based on well-established methods, while the simulated results have been validated with available in situ data (Supplementary Figures 7, 8, and the references in Supplementary Material) presenting a reasonable skill (Tsiaras et al., 2021). Still, the available validation data cover partially the model domain, while the adopted sources of plastics are characterized by a large degree of uncertainty. Especially with regard to small microplastics (<300 μm) available data are not enough for model validation, and their background concentration is poorly known (Lindeque et al., 2020). Furthermore, in the absence of available data, the input of small microplastics (<300 μm) from rivers has not been taken into account. Thus, the total amount of microplastics input from rivers as estimated here with the model of Lebreton et al. (2017) should be considered as underestimated. On the other hand, Weiss et al. (2021) indicated that existing estimates for river inputs of microplastics (e.g., Lebreton et al., 2017; Schmidt et al., 2017) may be significantly overestimated, based on the assumed mean particle weight and sampling methodologies biases.

Inputs of macroplastics from river runoff are also characterized by a significant degree of uncertainty, both in their amount and distribution. These were based on a global empirical function (Lebreton et al., 2017), taking into account of only one Mediterranean river (River Po). This empirical function is strongly influenced by mismanaged plastic production distribution, which might result in an underestimation or overestimation in high-income/developed countries and low-income/developing countries, respectively (Mai et al., 2020). Furthermore, the recent study of González-Fernández et al. (2021) demonstrated that the model of Lebreton et al. (2017) underestimates the input of large macroplastics (>2.5 cm) of European rivers. However, such information is still missing for the non-European rivers. Given that this knowledge gap will be covered by future studies, the presented modeling framework should be updated by re-considering the input of large macroplastics (>2.5 cm). Still, with the adopted formulation, our estimates on the input of large macroplastics (>2 cm) considering only European rivers (∼980 tons/year) is very close to the estimate of González-Fernández et al. (2021) for rivers discharging in the Mediterranean Sea (660–1,800 tons/year; mean = 1,350 tons/year).

The atmospheric pathway is partially considered for macroplastics as an input from coastal cities (see section “Sources, c”). Yet, we do not consider the atmospheric deposition of microplastics. Airborne microplastics, depending on their size, can travel long distances, as smaller particles are dispersed more widely (Evangeliou et al., 2020), making the simulation of their deposition at sea rather challenging. This would require an atmospheric model for the dispersion of microplastic or even a particle tracking model (e.g., Zhang et al., 2020), such as the one used here for the marine microplastics. Although there is more and more scientific evidence about the importance of atmospheric deposition (Liu et al., 2019), the information on airborne microplastics distribution in the Mediterranean is insufficient.

The ocean-based sources, that are not taken into account here, are considered to cover about 20% of the total plastic pollution (Lebreton et al., 2012). This contribution might currently be even smaller, considering the official banning of litter discharge from vessels. In addition, Soto-Navarro et al. (2020) modeled the dispersal of plastic released from shipping lanes and found their contribution rather uniform throughout the basin. Moreover, the plastics load from activities in coastal areas, such as aquaculture and fisheries, is partly taken into account in the tuned input of macroplastics from cities. Yet, plastic debris accidentally released offshore might be endowed with peculiar dispersal patterns influenced by off-coast currents. Therefore, in a future study, ocean-based sources should be further quantified and included in the model. Conversely, this is not important for microplastics as land-based sources represent the overwhelming majority (98%) of inputs into the ocean, according to IUCN (Boucher and Friot, 2017). Other sources of plastic that are less known, and thus are not considered by the model, but could affect certain areas, include the deposition of plastic fragments from greenhouses (e.g., in the Almeria region along the Spanish Mediterranean Coast; Dahl et al., 2021) and the illegal dumping of industrial wastes into coastal waters (Sheavly and Register, 2007).

Besides the uncertainties related to the adopted sources, our study is limited by processes that are not considered by the model. For instance, fragmentation of plastics into smaller particles results from the degradation of plastics which is more efficient on a beach environment, under the effect of solar radiation and waves that is primarily activated by ultraviolet (UV) radiation of sunlight, being also regulated by oxygen availability, temperature and mechanical abrasion (Andrady, 2011, 2017; Song et al., 2017). Even though fragmentation of plastics acts on relatively long time scales (∼years, Song et al., 2017), it provides a link between plastics of different sizes and thus it should be considered in future modeling efforts. Other processes that may affect the fate of microplastics include aggregation with sinking particulate organic matter (Long et al., 2015) and fecal pellets (Cole et al., 2016), as well as ingestion from various marine organisms (Cozar et al., 2014; Andrady, 2017; Choy et al., 2019 and references therein). These processes have been documented in the field or lab, but are still poorly quantified (van Sebille et al., 2020) and thus, were not taken into account in the model. With regard to biofouling, given the absence of experimental data, particularly on the size dependence of biofilm growth on microplastics, the adopted parameterization could not be directly validated. Still, the description of biofouling in Tsiaras et al. (2021) is among the most explicit, compared to other plastic dispersion models for the Mediterranean Sea (e.g., Liubartseva et al., 2018; Kaandorp et al., 2020; Soto-Navarro et al., 2020; Guerrini et al., 2021).

Finally, input from cities (both micro- and macro-plastics) is assumed to be constant through time. This is a simplification considering the changes of population and human activity in the coastal zone especially during the touristic period. For instance, in the Mediterranean Sea, a 40% increment of plastics input has been observed during the summer months (Galgani et al., 2014). In our results, the seasonality of plastic distribution is the result of changes in circulation, wind/waves, and rivers runoff. Considering that the model does not provide sufficient details to investigate the inter-seasonal distribution of plastic, we decided to study the accumulation in the Mediterranean protected areas using annual mean concentrations.

The average near-surface (0–10 m) distribution of micro- and macro-plastics as resulting from the 2016–2018 model simulation is shown in Figure 2. The majority of microplastics in terms of particles km^–2 (basin average) were 200 μm (24.5%) and 350 μm (23.3%) followed by 500 μm (21%), 1,000 μm (15%) 50 μm (10.6%), and 2,000 μm (5.5%). Thus, in the resulting distribution maps, the fast-sinking microplastics (<1 mm) are dominant (79.5%) against afloat microplastics (20.5%). With regard to macroplastics, in terms of g km^–2, the majority were 0.5–2 cm (54%), followed by 2–20 cm (37%), while bottles, bags, and foam were only 2.7, 3.5, and 2.7%, respectively. Thus, only 6.2% of macroplastics was affected by the wind. In Figure 3, the median distance covered by Lagrangian particles per particle category during the 3-year simulation is shown. The size of particles has a direct impact on their ability to travel far from their sources, as larger particles can potentially travel longer distances.

The classification of the resulting distribution into 3 clusters, identified for both micro-and macro-plastics, is shown in Figure 4, and the corresponding statistics (median, 25, 75% percentiles etc.) of each cluster in Supplementary Figures 9, 10 and Supplementary Table 1. All clusters have scored high Jaccard indexes (>0.9), demonstrating that they are sufficiently stable and well-defined. The cluster analysis provides a more objective representation of the results, revealing the most contaminated areas (cluster#1, red).

The simulated distribution is significantly affected by the source inputs from municipal wastewater and river runoff, as well as near-surface circulation. Highly polluted areas were found in the coastal zone, and especially in the vicinity of large rivers (e.g., Po—Italy, Soumman—Algeria, Nile—Egypt, Seman—Albania) and Metropolitan cities (e.g., Athens, Rome, Beirut). Microplastics accumulation zones that extend to the open sea are found in areas with inputs from large rivers (e.g., Algerian and Albanian coasts) or cities where no wastewater treatment is applied (e.g., Beirut, Tripoli). Rivers and untreated water are sources of large microplastics (>300 μm) that sink slowly (see section “Lagrangian Particle Drift Model”), and thus can travel longer distances than small microplastics (<300 μm). Thus, high concentration of microplastics in the open sea may be seen in areas where energetic coastal currents result in the off-shore advection of large particles from their source regions, as in the Algerian basin (Algerian current), the North Eastern Levantine (Asia-Minor current) and the Ligurian Seas (Liguro-Provencal current). In contrast, in areas where untreated water sources are scarce and input from rivers is not important, like the Spanish coasts, only input of small microplastics (<300 μm) occur, which rapidly lose buoyancy and sink due to biofouling (see section “Lagrangian Particle Drift Model”), resulting in a strong decrease of their concentration in open sea areas. The macroplastics distribution is mostly affected by rivers input. Particularly large accumulation zones, extended in the open sea, were found in the Algerian basin (dispersed from the coast to the open sea by the Algerian current), the North-Western and South-Eastern Adriatic, the Thermaikos Gulf (North Aegean Sea), and the Levantine basin. The effect of near-surface circulation is also visible in anticyclonic circulations that act as convergence/accumulation zones for floating particles (e.g., Gulf of Sirte Gyre and Ierapetra Gyre in eastern Crete).

The average concentration in national MPAs, Natura 2000 sites, and CCHs, is shown in Figure 5, along with the averaged concentration of inshore (<5 km distance from the coast) and offshore (>5 km) waters. Compared to CCHs, national MPAs and Natura 2000 sites are clearly more impacted, as on most occasions these are located in the coastal zone, while CCHs are generally larger areas extending also to the open sea. The difference between inshore and offshore waters is smaller for macroplastics. Macroplastics in inshore waters are 3.4 times higher compared to offshore waters, while microplastics are found to be 5.3 times higher in inshore waters, illustrating that macroplastics can travel longer distances from their sources.

The characterization of the studied areas, as of relatively low/medium/high pollution based on the cluster analysis is shown in Figure 6 (national MPAs and Natura 2000 sites) and Figure 7 (CCHs). A total of 41 (38%) national MPAs, 107 (36%) Natura 2000 sites, and 4 (23%) CCHs were found in the high cluster of microplastics. The studied areas were found less impacted by macroplastics with a total of 19 national MPAs (18%), 74 Natura 2000 sites (25%), and 1 CCHs (6%) belonging to the high cluster. A great amount of the studied areas is located in the Western Italian and French coasts. Although these areas accumulate microplastics originated locally as they are close to highly populated coastal cities (see Supplementary Figure 3), they do not receive significant inputs from rivers. Thus, compared to other areas where both rivers and cities provide significant inputs, like the Algerian coasts and the Adriatic and Levantine Seas, they accumulate significantly less macroplastics. Moreover, macroplastics accumulation zones, in comparison to microplastics accumulation zones, coincide much less with MPAs. Predicted micro- and macro-plastics concentration in all studied areas, as well as the corresponding clusters, can be found in Supplementary Material.

From a country-specific point of view (Figure 8), a similar pattern is presented. Protected areas in EU countries like France, Italy and Greece are relatively more impacted by microplastics than macroplastics. The national MPAs of Albania, Turkey, Lebanon, and Algeria were found with relatively high concentrations of both micro- and macroplastics. In contrast, Natura 2000 sites and national MPAs in Croatia, Spain, Malta, and Cyprus maintain relatively low concentrations of both micro- and macro-plastics. Finally, the national MPAs in Tunisia, Libya, Israel and Egypt were found with relatively low concentrations in microplastics but high in macroplastics.

As regards to CCHs, the waters surrounding the Dodecanese Islands (Aegean Sea), the Tuscany Archipelago (Italy), the island of Ischia (Italy) and the Sazani Island (Albania) were found inside the high pollution cluster. With regard to macroplastics, only Sallum Gulf (Egypt) was found in the high cluster. Pelagos Sanctuary was found in the medium pollution cluster for both micro- and macro-plastics.

The average concentration of micro- and macro-plastics in Natura 2000 sites and national MPAs per ecoregion (ECR) is shown in Figure 9. National MPAs in ecoregions of the Eastern Mediterranean Sea—including the Ionian (ECR 6), Aegean (ECR 7), and Levantine (ECR 8) Seas presented particularly high microplastics concentrations. Natura 2000 sites in the Tyrrhenian Sea (ECR 3) presented the highest microplastics averaged concentration. With regard to macroplastics, the Natura 2000 sites of the Adriatic Sea (ECR 5), and national MPAs in the Levantine Basin (ECR 8) were the most impacted. Particularly low concentrations of both micro- and macro-plastics were found in the national MPAs and Natura 2000 sites of the Tunisian Plateau (ECR 4).

Protected areas that were predicted to have extreme concentrations of plastic (above the 95th percentile of surface concentration in the whole basin) are shown in Figure 10. The majority of the most polluted areas with regard to microplastics are located along the Italian (13 areas) and French coasts (6 areas). In contrast, concerning macroplastics, the most polluted areas are dispersed in several regions including the eastern Italian coast (8 areas) and the Messina Strait (2 areas), in Turkey (5 areas), Spain (3 areas), Lebanon (1 area), Greece (2 areas), and Egypt (1 area).

From these areas, we highlighted three cases to investigate the origin of their resulting plastic accumulation in detail: the Natura 2000 site of Costa Viola in Italy (Supplementary Figure 11), the national MPA of the Tyre Coast in Lebanon (Supplementary Figure 12), and the national MPA of the Axios Delta in Greece (Supplementary Figure 13). The Costa Viola Natura 2000 site and the Tyre Coast presented a plastic concentration of more complicated origin (especially regarding macroplastics) than the Axios Delta MPA, as numerous sources contributed to their predicted concentrations. The Axios Delta (Supplementary Figure 13) lies in a semi-enclosed gulf and receives great amounts of plastic litter from local sources: the Axios and Aliakmonas Rivers and the city of Thessaloniki (∼1 million inhabitants). These three sources explained 95% of the resulting concentration for both micro-and macro-plastics in the Axios Delta. In contrast, at Costa Viola (Supplementary Figure 11), 95% of the resulting microplastics concentration came from 13 sources, and the same percentage for macroplastics was originated from 158 sources. All 13 sources of microplastics were in Italy. Conversely, 16% of macroplastics came from Algerian coasts, 4% from Tunisia, 2.5% from France, and 1.3% from Spain. Transboundary pollution was even more significant in the Tyre Coast MPA in Lebanon, where 50% of macroplastics came from Egypt, 19% from Turkey,16% from Israel, and only 13% of predicted macroplastics concentration originated locally from Lebanon.

The transboundary pollution in national MPAs is shown in Figure 11. Transboundary pollution was found higher for macroplastics (= 57 ± 35%; MEAN ± STD) compared to microplastics (= 35 ± 34%; MEAN ± STD). All countries had at least one national MPA with over 55% of macroplastics originating from sources beyond their borders, excluding Algeria and Albania, which had a maximum fraction of 11 and 25% “foreign plastics” in their MPAs. The country receiving most transboundary pollution (as a percentage) was Malta, receiving 48% of the resulting concentration in macroplastics from Italy, 21% from Tunisia, 14% from Algeria and 7% from France (average percentages across all Maltese national MPAs). In contrast, in Greece, most national MPAs receive plastic load almost exclusively from sources within its borders, and thus the average fraction of transboundary pollution in Greek national MPAs was not found important for both micro- and macro-plastics (1 and 10%, respectively). The predicted fractions of transboundary pollution for both micro- and macro-plastics for all national MPAs can be found in Supplementary Material.

Throughout our analysis (area characterization considering cluster analysis, and average concentration in protected areas per country and per ecoregion), it is revealed that high microplastics concentrations are found in the Natura 2000 sites of the French and western Italian coasts, while national MPAs were found highly polluted in Albania, Greece, Turkey, Lebanon, Israel and Algeria (Figures 8, 9). The resulting concentration depends on the coastal cities’ population (Supplementary Figure 3) and wastewater treatment type (Supplementary Figure 4) adopted by the UNEP/MEDPOL (2011) report, as well as on the near-surface circulation (Supplementary Figure 5). For instance, Natura 2000 sites along the western Italian and French coasts are located close to highly populated coastal cities. In Italian coastal cities (22.5 million inhabitants), mainly primary wastewater treatment is applied (59% of coastal cities) while a significant amount (33%) of wastewater is disposed untreated (UNEP/MEDPOL, 2011). This has a direct impact on the resulting accumulation zones, with the Italian Natura 2000 sites presenting the highest concentrations among all EU counties (Figure 8). As a counterexample, in Spain, where the vast majority of coastal cities (7.8 million inhabitants; UNEP/MEDPOL, 2011) apply secondary treatment (87%), Natura 2000 sites were found much less impacted. The French coast (7.5 million inhabitants), where secondary treatment is also mainly applied (71%), receives also large (>300 μm) floating microplastics from northern Italian cities, due to the prevailing near-surface circulation. As a result, Natura 2000 sites in France were found with the second highest averaged microplastics concentration after Italian Natura 2000 sites (see Figure 8). Still, these results suffer from limitations with regard to the adopted wastewater treatment from the UNEP/MEDPOL (2011) report. Besides the need of an updated report, some information about untreated water might be missing. For instance, for the WWTPs of Italy, 33% of wastewater from cities with primary treatment is reported as untreated. Although wastewater might not be totally cleaned in other countries as well (e.g., in Spain or France), no relevant information is available (UNEP/MEDPOL, 2011). Such uncertainties in the untreated water discharge might explain the underestimation of the model results of large microplastics abundance (> 300 μm) on the French and Spanish coasts (Supplementary Figure 7; Tsiaras et al., 2021).

Macroplastics presented high concentrations in Natura 2000 sites of the Adriatic Sea (Eastern Italy; ECR 5), at the Strait of Sicily (Italy) and in national MPAs of the Levantine Basin (Turkey, Egypt, Lebanon; ECR 8). Soto-Navarro et al. (2021) reported the highest concentrations of eastern Mediterranean, in the Adriatic Sea, the strait of Sicily and on the slopes of the Levantine Basin from Egypt to Turkey. Generally, our resulting distribution agrees with the findings by Soto-Navarro et al. (2021) regarding the eastern Mediterranean Sea, but differ for the western part. For instance, we found the Iberian Peninsula as of moderate pollution, whereas according to Soto-Navarro et al. (2021) it is the most contaminated area of the western Mediterranean. Such differences are attributed to the adopted sources, both land- and sea-based, with the latter not being considered here. Furthermore, our model does not predict a macroplastics accumulation zone in the Gulf of Lions, as predicted by Soto-Navarro et al. (2021). This is partially due to the underestimation of large macroplastics (>2 cm) input from Rhone river since, our estimation based on Lebreton et al. (2017) is lower (∼0.08 tons year^–1) than the observed (∼0.7 tons year^–1; Castro-Jiménez et al., 2019). Additionally, as discussed in section “Potential Bias of the Results,” the strong influence of Lebreton et al. (2017) empirical function on mismanaged plastic production results in an underestimation of riverine plastic in high-income/developed countries (e.g., France) and to an overestimation in low-income/developing countries (Mai et al., 2020).

The national MPA in the Sallum Gulf (Egypt), which was found to belong in the high macroplastics pollution cluster, is of particular interest considering its high biodiversity (Farrag et al., 2019). Moreover, due to its importance for various cetacean species, ACCOBAMS considers this area as a CCH. Thus, high concentrations of macroplastics in the Sallum Gulf should raise the alarm, as it is known that cetacean species are particularly vulnerable to macroplastics, often with lethal consequences (Alexiadou et al., 2019). Notably, we found that the vast majority (99%) of this predicted concentration in Sallum Gulf, originates from sources beyond the Egyptian borders (Figure 11, see also Supplementary Material). Other CCHs, that did not belong to the high macroplastic cluster but were partially located within macroplastics accumulation zones include the Alboran Sea, the Saronikos Gulf, the waters surrounding Dodecanese islands (Aegean Sea), and the Pelagos Sanctuary for Marine Mammals. The Pelagos Sanctuary is also partially located within microplastic accumulation zones, with a median concentration corresponding to the medium pollution cluster for both micro- and macro-plastics. Our results regarding micro- (>0.3 mm) and macro-plastics abundance in the area have been validated with in situ data observations (Supplementary Figures 7, 8), with the prediction regarding microplastics being underestimated most likely due to underestimation of input from untreated wastewater from the French coasts (Tsiaras et al., 2021). Among the various marine mammals that inhabit the Pelagos Sanctuary, the area is also an important feeding ground for the fin whale Balaenoptera physalus, which is directly exposed to microplastics due to their filtering activity (Fossi et al., 2012). Our findings reinforce the evidence presented by Fossi et al. (2017) and Guerrini et al. (2019) demonstrating the risk of plastic ingestion by fin whales in the Pelagos Sanctuary.

The most effective way to reduce plastic pollution in protected areas is by reducing marine litter at the sources (Soto-Navarro et al., 2021). A management plan including litter reduction at its sources can occasionally be successfully implemented locally. For example, the National MPAs located in the semi-enclosed Thermaikos Gulf in the North Aegean Sea (Figure 11) receive large amounts of plastics from local sources: the Axios and Aliakmonas Rivers and the city of Thessaloniki (Supplementary Figure 13). Our results show that these sources explain 95% of the resulting concentrations for both micro- and macro-plastics. Thus, applying reduction technologies to these sources would significantly improve the levels of plastic pollution in the local protected areas. For instance, in Gkanasos et al. (2021) the implementation of two novel cleaning technologies is described, both outcomes of the recent CLAIM project (Cleaning Litter by developing and Applying Innovative Methods in European Seas). First, an effective floating barrier installation, to prevent macrolitter from entering the marine environment from rivers, and second a pre-filtering device (a combination of sand and mechanical filters of various mesh sizes placed in series) to more effectively clean wastewater from microplastics at treatment plants. Both technologies were tested in a semi-enclosed gulf (Saronikos Gulf, Greece; Gkanasos et al., 2021), with their estimated cleaning efficiencies looking promising, being 90% for the floating barrier (macroplastics) and 95% for the pre-filtering device (microplastics).

Yet, local management would only be efficient in areas with specific characteristics like the semi-enclosed Thermaikos and Saronikos Gulfs. Other protected areas, such as the Tyre Coast (Figure 11 and Supplementary Figure 12) and Costa Viola (Supplementary Figure 11) cannot be managed successfully in a local context, as they receive plastic from numerous and distant sources, especially with regard to macroplastics for which transboundary pollution was found significant. The consideration of connectivity in the marine environment has been stressed for the successful implementation of ecosystem-based management (Christensen et al., 2021), biodiversity conservation, and the sustainable use of marine resources. Here, we demonstrated the importance of connectivity in the distribution of microplastics and macroplastics across the Mediterranean Sea and, thus, that the management of plastic pollution requires coordinated efforts across countries even in national coastal MPAs. For example, we found that sources of macroplastic in an Italian MPA can be situated on the opposite side of the Mediterranean basin, in Algeria. In some cases, international collaboration for managing transboundary plastic pollution in MPAs at an ecoregion level could be effective, as would be the case for the Tyre Coast MPA. However, in other cases, e.g., the Costa Viola Natura 2000 site, international collaboration should be extended beyond ecoregions.

Previous studies have highlighted the importance of international collaboration for the implementation of transboundary networks of MPAs and the effective conservation of marine biodiversity in the Mediterranean and other European Seas (Katsanevakis et al., 2020). Herein, we demonstrated that the plastic pollution in a country’s MPA may originate from sources beyond its national jurisdiction, and thus, transboundary cooperation is also critical for the effective management of plastic pollution within Mediterranean MPAs. This collaboration could be more easily achieved among the EU Member States, which have committed to achieve a Good Environmental Status (GES) of the EU’s marine waters complying with the Marine Strategy Framework Directive (Directive 2008/56/EC). One of the descriptors of GES refers specifically to marine litter: Properties and quantities of marine litter do not cause harm to the coastal and marine environment. According to EU sources, this Directive has led to increased cooperation among littoral Member States and the knowledge gained from implementing it has been an important driver for the adoption of the Single Use Plastics Directive (Directive (EU), 2019/904, 2019). Nevertheless, as pointed out from our results, transboundary collaboration should extend beyond EU borders to address effectively the issue of plastic pollution within MPAs across the Mediterranean Sea. To this end, more cooperation and coordination of efforts across Mediterranean countries could be achieved under the auspices of UNEP/MAP (Mediterranean Action Plan of the United Nations Environment Programme). More specifically, the Barcelona Convention could provide a solid framework for the coordination of efforts to reduce or ultimately eliminate plastic pollution from Mediterranean MPAs, should the Mediterranean governments show increased political will to commit to ratified international environmental conventions.