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During the last decades, dispersion of plastics in the oceans has become a global pollution problem that poses a great threat to the marine ecosystem. The extensive production of short use-cycle plastic products leads to considerable waste generation, and subsequent leakages to the environment. It is estimated that over 269,000 tons of plastic debris float on the surface of the oceans (Eriksen et al., 2014). Despite being durable and long-lasting materials, plastics deposited in the environment are subject to weathering processes, such as UV-photooxidation and hydrolysis, causing degradation, and eventually fragmentation to smaller particles, called microplastics (MPs) (Thompson et al., 2004; Ioakeimidis et al., 2016).

Microplastics are defined as plastic pieces smaller than 5 mm, classified as primary and secondary. Primary MPs are small plastic particles consisting of raw industrial plastic pellets and processed particles added intentionally in health-care and cleaning products (cosmetics, detergents, etc.). Secondary MPs are particles derived either from larger plastics’ fragmentation or from materials’ wearing off during use (GESAMP, 2016; Veiga et al., 2016). MPs formation takes place at beaches and land, and reach the sea through rivers, road run off, city storm water, and wastewater treatment plants (Moore et al., 2011; Cheung et al., 2016; Kalogerakis et al., 2017). MPs could be found both in populated and remote places, and they are ubiquitous in the marine environment, from the ocean surface to the deep sea sediments (Van Cauwenberghe et al., 2013; Bergmann et al., 2015; Lusher et al., 2015; Courtene-Jones et al., 2017; Morgana et al., 2018). At sea, MPs are redistributed under the influence of physical factors and despite the well-known accumulation in the oceanic gyres, concentrations of MPs along the surface ocean exhibit increased variation in space and time. In addition, environmental fate, vertical transport, and biological effects, may be affected by microbial colonization on MPs and biofilm formation (Kaiser et al., 2017; Kooi et al., 2017). Biofilm may lead to an increase in the density of MPs and a decrease in their buoyancy with a strong impact on both sedimentation potential and upward transport (Rummel et al., 2017; Nguyen et al., 2020). Furthermore, despite that the understanding of MPs trophic transfer in marine ecosystems is increasing, several aspects remain unknown, primarily including the role of the microbial biofilm living on MPs surface (the so-called “plastisphere;” Zettler et al., 2013) on trophic transfer and its effects on marine organisms’ health. MPs are known to absorb harmful contaminants (Karapanagioti et al., 2011; Koelmans et al., 2016; Torres et al., 2021) whilst records on the ingestion of MPs by marine biota are increasing exponentially (Galloway, 2015; Wright et al., 2017; Digka et al., 2018). More recently, the role of MPs has also been investigated in relation to the carbon cycle (Galgani et al., 2018, 2020; Romera-Castillo et al., 2018; Taipale et al., 2019). Our understanding of the factors affecting the MPs variation at sea and their harmful effects in organisms, is still limited. Consequently, there is insufficient capacity for designing and implementing successful mitigation and regulatory policies (Thompson, 2015; Galgani et al., 2021).

In the Mediterranean Sea, the estimated average plastic concentration from both model and field data is comparable to the oceanic gyres (Lebreton et al., 2012; Eriksen et al., 2014; Cózar et al., 2015). This observation potentially results from several factors including the high population concentration (∼150 million), the increased tourism activity in the Mediterranean area (1/3 of the world’s tourism), the expanded shipping activity (15% of the global shipping; UNEP/MAP, 2017), in combination with the enclosed character of the basin. The distribution of floating MPs in the Mediterranean Sea, has been mainly investigated in the northwestern and central part of the basin (Collignon et al., 2012, 2014; Fossi et al., 2012, 2016; De Lucia et al., 2014; Cózar et al., 2015; Pedrotti et al., 2016; Suaria et al., 2016; Zeri et al., 2018). Even though there are some studies in the open and coastal waters in the eastern Mediterranean Sea (Cózar et al., 2015; Gündoğdu and Çevik, 2017; Güven et al., 2017; van der Hal al., 2017), there is still lack of information for the floating MPs distribution. In the present work, we aim to fill in the gap on the magnitude of MP pollution in the Eastern Mediterranean Sea, by providing data on concentrations and properties of floating MPs at the sea surface of the Ionian, Aegean, and Levantine Seas in addition to two enclosed gulfs, Corfu and Saronikos. We further discuss the factors affecting the observed distribution patterns and properties considering the vicinity to MPs sources and oceanographic variables.

A total of 23,800 microparticles were counted. Mesoplastics (>5 mm) caught in the manta net corresponded to ∼5% of total particles and were not included in the present analysis. In the subset of 2,971 items, subjected to FTIR analysis 2.7% were identified as natural materials, and 0.7% were suspect to laboratory contamination (e.g., filter material). These percentages are considered low, and so all microparticles counted were considered as MPs.

The Mediterranean Sea is characterized as a hot spot area for plastic pollution and floating plastic concentrations are found comparable to those of the oceanic gyres (Eriksen et al., 2014; Cózar et al., 2015). Despite this statement, increased variability in MPs has been recorded with concentrations differing 2–3 orders of magnitude from place to place and at different time instances (Suaria et al., 2016; Fossi et al., 2017; van der Hal et al., 2017; Baini et al., 2018). This can be attributed to the permanent, quasi-permanent mesoscale circulation features, seasonal structures, and significant temporal changes in the surface currents that characterize this basin (Poulain et al., 2012). Concentrations of MPs reported in the present study (0.012–1.62 items m^–2) capture this variability and are in line with other case studies reported at the Mediterranean Sea (Table 3). Models on plastics distribution in the oceans relate inputs from large rivers, navigation routes, and coastal population centers to surface currents circulation (Lebreton et al., 2012; Eriksen et al., 2014; van Sebille et al., 2015). Relevant studies for the Mediterranean Sea provide justification for the increased spatiotemporal variability observed and define that elevated plastic concentrations are expected close to input sources (river mouths, large cities) and along a 50 km strip parallel to land (Zambianchi et al., 2017; Liubartseva et al., 2018; Mansui et al., 2020). Liubartseva et al. (2018), in particular consider the stokes drift, as well as the beaching and sedimentation effects, to conclude that any long-term accumulation of plastics at the sea surface would be unlikely in the Mediterranean, mainly due to the beaching effect on long and complex coastlines. This is particularly true for the Aegean Sea, which is characterized as a “least polluted” area, especially the central Aegean Sea where simulated plastic concentrations appear less than 2 g km^–2 (Liubartseva et al., 2018). Seasonal simulations show a patchy distribution during winter (December–April) with relatively elevated concentrations in the N. Aegean and S. Aegean Seas and a cleansing action during summer (Politikos et al., 2017; Mansui et al., 2020). For the Levantine Sea, models agree on the formation of a coastal accumulation strip along the southern coasts of Turkey throughout the year, coinciding with the Asian Minor Current (AMC) and least polluted waters south of Crete island (Liubartseva et al., 2018; Mansui et al., 2020). For the eastern Ionian Sea, model simulations of floating litter transport conclude that moderate amounts of litter are consistently found around Corfu island, transported by the northward coastal current of the eastern Ionian Sea (Politikos et al., 2020). In accordance to model simulations, our results show the lowest concentrations of MPs in the Aegean Sea (0.15 ± 0.17 items m^–2), particularly in the Central Aegean transect (Figure 2B), while higher concentrations were recorded (0.27 ± 0.08 items m^–2) in the Levantine waters especially at the edge of the AMC which transports MPs from the Cilician basin as described above (Figure 2B). Our data set presents the highest MPs concentration in the areas of Corfu Gulf (0.63 ± 0.60 items m^–2) and in the N. Ionian Sea (0.38 ± 0.40 items m^–2) while, contrary to what one would have expected not in the highly populated Saronikos Gulf. The elevated MPs concentrations observed in these two areas may be related to the presence of sea surface slicks during October 2014. In fact, van Sebille et al. (2020) in their extensive review on the physical processes controlling the transport of plastic particles in the ocean, explain how the appearance of slicks denotes the presence of sub-mesoscale (<10 km) convergence zones due to specific physical structures (i.e., sub-mesoscale eddies, Langmuir circulation). Along these convergence zones floating objects (flotsam) i.e., seaweed, wood, oil, and plastics, show strong concentration factors (D’Asaro et al., 2018) sometimes forming visible windrows. Apart from visible objects, these formations favor also the accumulation of MPs, organic matter, planktonic organisms, fish eggs, and small fish. Pictures of the samples collected inside the slicks provide evidence of the proliferation of larvae and eggs (Figures 3D,E). Gove et al. (2019) investigating MPs occurrence and ingestion by larval fish inside and outside surface slicks around Hawaii islands showed that MPs concentrations were 126-fold higher inside than outside the slicks, while ingestion by larval fish was found 2.3-fold higher inside the slicks. The authors found surface slicks to be a conducive nursery habitat for small fish. In our study, MPs concentrations were found to be significantly higher (1.08 ± 0.46 items m^–2) when crossing or inside the slicks than those in the waters outside the slicks (0.21 ± 0.14 items m^–2), this regardless the studied areas. Surprisingly, surface slicks concentrations resulted to be equal to those reported recently for the N. Pacific gyre (average 1.08 items m^–2 for size range 0.5–5 mm; Lebreton et al., 2018). The high accumulation of MPs in such sensitive habitats becomes a matter of ecological relevance due to the increased possibility for interactions (and ingestion) between organisms and MPs. It is important to specify that the sampling design of this study was not arranged according to the presence of slicks, i.e., concentrations may have been even higher if trawls were conducted 100% inside the slicks, suggesting that this finding is worthy for further investigation. The importance of windrows in understanding marine litter and MPs pollution and the need for targeted windrow research has been acknowledged very recently by Cózar et al. (2021), in their relative perspective article.

It is documented that MPs show different buoyancy characteristics depending on their density, but also on their shape type, size, and biofouling degree (Kooi et al., 2016, 2017; Kaiser et al., 2017). In fact, small sized and elongated particles tend to float deeper in the water column due to increased water friction forces (Reisser et al., 2015; Kooi et al., 2016). This pattern can be further enhanced under the action of convergence and downwelling cells such as those described previously. Relatively large particles tend to stay at the surface and accumulate, while very small and elongated ones are more effectively transported below the surface. In addition, the predominance of fragments on the sea surface compared to small sized and elongated particles may be explained by the interactions between MP and biofilm and its effect on their buoyancy. In fact, biofouling should remove small fragments faster than large ones as a result of their higher surface area to volume ratios (Cózar et al., 2014); because the buoyancy of an item is a function of its volume, while its susceptibility to fouling is dependent largely on its surface area (Ryan, 2015). Given that the manta net is a neustonic sampling device, the buoyancy patterns described above, may explain the high contribution of fragments (more than 50%) in all the collected samples, and in most other places worldwide. Moreover, the minimal contribution of filaments in the N. Ionian Sea and the Corfu Gulf could be related to the sorting action of the convergence and downwelling structures that occurred there which further enhanced the downwelling of less buoyant shapes such as filaments. MPs floating in the Ionian, Aegean and Levantine Seas occur mostly in sizes smaller than 2 mm peaking at the size range 0.8–1.4 mm, that is in accordance with other surveys using manta nets (330 μm; Cózar et al., 2014, 2015; Isobe et al., 2015; Maes et al., 2017). The similarities in the size distribution of MPs caught in the manta nets in various marine regions reflect that the MPs assortment has undergone substantial mixing during long residence time at sea.

Global Plastics production in 2019 reached 368 Mt, out of which 58 Mt correspond to European production.² About 40% of all polymers produced are used in packaging, with polyolefin polymers PE and PP having the highest demand, followed by PVC, PET, PS and EPS, and PA (European market demand: 9, 7, 5, and 2%, respectively). In the oceans’ surface waters, this may be modified according to the buoyancy characteristics of each of the polymers. In fact, the relative contributions of light polymers such as PE and PP is further enhanced due to their low densities (0.86–0.96 g cm^–3), lower than the surrounding seawater (1.28 g cm^–3), while denser materials tend to escape from the surface layer. In line with the above, the chemical composition of sea surface MPs in the Ionian, Aegean and Levantine Seas is dominated by PE and PP polymers by 80%. Other polymers present are PA-Nylon, PET and PVA. Polyamides such as Nylon are used in fiber manufacturing while PET is used for packaging and for textile manufacturing in the form of fibers (polyester type fibers). Although other studies have reported the presence of PVA in environmental samples (Ng and Obbard, 2006; Claessens et al., 2011; Chae et al., 2015; Suaria et al., 2016; Zeri et al., 2018), its considerable proportion is quite surprising as it is not one of the commonly used materials. PVA is a hydrolizable, glue-like adhesive polymer, used also as a coating for other materials such as cloth and paper and more recently for 3D printing. The cross-referencing of our spectra with a larger data base revealed that PVA is one of the most questionable materials, as 26 out of 28 spectra presented high similarities to various natural materials, glue like natural resins (wax, brood comb), chitin, algae, linen, wool (animal fur), rather than the polymer spectra. A recent study on fibers distribution in the oceans (Suaria et al., 2020) highlighted that natural fibers are dominant in surface oceanic waters (0–5 m) (cellulosic 79.5% and of animal origin 12.3%) in comparison with those of synthetic origin (8.6%). Here we should mention that at least one of the OpenSpecy libraries (Primpke et al., 2018) used for the cross-referencing of the spectra is common to that used by Suaria et al. (2020). The spectral similarity by more than 70% with either PVA or natural materials, depending on the library used, indicates the presence of common functional groups among all these materials. Our findings suggest that checking against a greater data base of spectra for environmental particles is considered useful in refining the results obtained only from polymers data bases (Cowger et al., 2021). At the same time, the question is raised whether the obtained spectra indicate deviations from the pure PVA spectrum, related to its fast degradation/hydrolysis in the environment (Min et al., 2020) or are indicative of natural materials with functional groups similar to PVA. Furthermore, the use of PVA in paper and textile coatings further complicates the issue. Experimental work with known materials and combination of libraries including degraded polymers could be useful in clarifying the questions raised above.

Many studies have shown a direct relationship between MPs elevated concentrations and input sources such as large urban centers or river mouths (Frias et al., 2014; Zhao et al., 2014, 2015; Gewert et al., 2017; Sun et al., 2019) and have highlighted the importance of flood events for the washout and transport of MPs in coastal areas (Moore et al., 2002; Veerasingam et al., 2016; Gündoğdu and Çevik, 2017). In our study, for the case of Corfu Gulf, we cannot ascertain if the elevated MPs concentrations recorded reflect direct inputs of MPs from the Kalamas river, or rather the subsequent concentration effect caused by the formation of sea surface slicks. On the other hand, although Saronikos Gulf has been identified by models as a hot spot area, due to the high plastic inputs from Athens metropolitan area (Liubartseva et al., 2018), our field data, do not capture constant elevated MPs concentrations there (0.16 ± 0.22 items m^–2), even in the inner part of the Gulf and after the heavy rain event (Table 1). It is possible, that local conditions may favor fast dispersion or beaching of plastic particles. While some authors have reported that large sized MPs, meso- and macro- plastics become less abundant close to the shores than in offshore waters (Pedrotti et al., 2016; Zeri et al., 2018), others have shown the opposite. In particular, close to river mouths and urban centers the relative abundance of large sized MPs has been linked to shorter residence time in the marine environment (Morét-Ferguson et al., 2010; Schmidt et al., 2017). Isobe et al. (2014) combined modeled and field data and demonstrated that meso-plastics are selectively drifted close to the shores, independently from the presence river mouths. Based on these outcomes, it is understood that the size distribution of plastic particles close to the coastline is controlled by both the vicinity to sources (short residence time) and the prevailing dispersion-concentration modes (Doyle et al., 2011; Frere et al., 2017). Recent data on MPs in Kifissos river water further upstream confirm the high level of large MPs and mesoplastics, mostly PE films (Zeri et al., 2021). In addition, the small beach by the river mouth has been characterized as a hot-spot area for beach litter (>10,000 items/100 m) with ∼50% of items corresponding to small sized plastic and polystyrene fragments (Greek Marine Strategy Framework Directive monitoring program, HCMR unpublished data). These local conditions may explain the observed MPs distribution and properties in the inner Saronikos Gulf. In fact, as already described, in this area, several known input sources of MPs coexist (Kifissos River mouth, WWTP, ports and marinas, anchorage points). Apart from the typical contribution of PE and PP, a considerable presence of foam particles in the inner Saronikos Gulf was confirmed by their chemical signature, PS/EPS being 18%. In Greece, the market demand for plastics in 2018–2019 was ∼1 Mt per year; while recycling and energy recovery rates in the country are of the lowest in Europe (70% of waste is landfilled) (see text footnote 2). The EU Directives on the banning plastic bags (EU/720/2015) and single use items (EU/904/2019) came into force only very recently (in 2019 and 2020 respectively). This situation has favored plastic leakage in the environment and is reflected in the MPs chemical composition in the inner Saronikos Gulf where a higher diversity of polymer types has been recorded. In fact, different types of polymers indicative of specific economic sectors such as industry, construction, maritime, were found in this area (EVA, PU, EPDM, and AF-paints). AF-paint particles are usually painted metal chips not expected to stay afloat for long periods, and their presence in the inner Saronikos samples is a strong indication of the direct inputs of MPs from the anchorage points, ports operations and increased marine traffic.³ Lastly, the presence of WWTP outflow in the analyzed samples were not evidenced, this is probably related to the fact that sewage enters the gulf via a diffusive pipe situated at 65 m depth. In addition, MPs mostly found in sewage correspond to small sized filaments (Talvitie et al., 2015), which are unlikely to be caught by surface manta nets.

The results presented in this work highlight that local oceanographic conditions, such as slicks, significantly affect MPs concentrations, posing risks in these sensitive habitats. Our study suggests that MPs sources from coastal population centers were not detectable based on MPs concentrations, but rather on properties such as size and polymer type. Elevated MPs concentrations were not found close to the major MPs sources at the Athens metropolitan area. The assortment of MPs there, consisted of large sized MPs and more variable polymer types, holding rather “local” features and differentiated from the well mixed MPs assortment found in open waters and less impacted areas. To our knowledge, this is the first study presenting field data of floating MPs concentrations in relation to the presence of sea surface slicks in the Mediterranean Sea, and provides evidence that slicks act as strong MPs concentration factors. It is anticipated that the outcomes of the present study will provide insights toward a better interpretation of floating MPs data from systematic monitoring activities.

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

AA processed the samples, analyzed the data, and wrote and reviewed the manuscript. CZ conceived the study, collected the samples, and wrote and reviewed the manuscript. FG processed the samples and reviewed the manuscript. CG collected and processed the samples and reviewed the manuscript. CI collected the samples and reviewed the manuscript. EP processed spectroscopic data and reviewed the manuscript. All authors have read and agreed to the published version of the manuscript.

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

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.