Abstract Concentrations of floating plastic were measured throughout the Mediterranean Sea to assess whether this basin can be regarded as a great accumulation region of plastic debris. We found that the average density of plastic (1 item per 4 m2), as well as its frequency of occurrence (100% of the sites sampled), are comparable to the accumulation zones described for the five subtropical ocean gyres. Plastic debris in the Mediterranean surface waters was dominated by millimeter-sized fragments, but showed a higher proportion of large plastic objects than that present in oceanic gyres, reflecting the closer connection with pollution sources. The accumulation of floating plastic in the Mediterranean Sea (between 1,000 and 3,000 tons) is likely related to the high human pressure together with the hydrodynamics of this semi-enclosed basin, with outflow mainly occurring through a deep water layer. Given the biological richness and concentration of economic activities in the Mediterranean Sea, the affects of plastic pollution on marine and human life are expected to be particularly frequent in this plastic accumulation region.

Citation: Cózar A, Sanz-Martín M, Martí E, González-Gordillo JI, Ubeda B, Gálvez JÁ, et al. (2015) Plastic Accumulation in the Mediterranean Sea. PLoS ONE 10(4): e0121762. https://doi.org/10.1371/journal.pone.0121762 Academic Editor: Erik V. Thuesen, The Evergreen State College, UNITED STATES Received: September 25, 2014; Accepted: January 25, 2015; Published: April 1, 2015 Copyright: © 2015 Cózar et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited Data Availability: Original data reported in this paper are freely available at http://doi.pangaea.de/10.1594/PANGAEA.842054. Funding: The present study was funded by the EU's Framework Programme 7 and the Spanish Ministry of Economy and Competitiveness through projects MedSeA (EU contract number FP7-2010-265103), ESTRESX (ref. CTM2012-32603) and MEGAN (ref. CTM2013-49048-C2-1-R). Andrés Cózar is grateful for the funding of a "Salvador de Madariaga" stay to continue the study of the plastic pollution in the Mediterranean Sea (Spanish Ministry of Education, Culture and Sport, ref. PRX14/00743). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.

Introduction Recent studies have demonstrated the existence of five large-scale accumulation regions of floating plastic debris in the oceans corresponding to each of the subtropical gyres located at either side of the Equator [1–6]. Ocean currents transport floating plastic released from terrestrial (e.g., coastal cities, rivers, and tourist beaches) and maritime (e.g., vessels, and at-sea platforms) sources to central convergence zones in the open ocean where these materials accumulate [7–9]. This process results in surface concentrations of buoyant plastic up to the order of kilograms (or millions of pieces) per km2 in the center of ocean gyres, while open-ocean concentrations outside the gyres only occasionally reach a few grams (or thousands of pieces) per km2 [1–5, 10, 11]. However, the assessment of marine plastic pollution is relatively recent, and extensive areas of the ocean remain yet unexplored, including regional semi-enclosed seas located in basins with intense use of plastic. This is the case of the Mediterranean Sea. Its shores house around 10% of the global coastal population (ca. 100 million people within the 10-km coastal strip [12]). The basin constitutes one of world´s busiest shipping routes [13], and receives waters from densely populated river catchments (e.g., Nile, Ebro, and Po). Furthermore, the Mediterranean Sea is only connected to the Atlantic Ocean by the Strait of Gibraltar and has a water residence time as long as a century [14]. Estimating both terrestrial and maritime inputs, Lebreton and coworkers modeled the transport and distribution of floating debris in the ocean [8]. The model simulations identified the Mediterranean Sea as a potentially important accumulation zone at the global scale. Recently, the calibration of this model using a global dataset was applied to estimate the surface plastic load in the Mediterranean Sea at 23,150 tons [6]. The abundance of plastic debris floating in Mediterranean waters was first reported by Morris in 1980 [15]. Using a quantitative visual survey, he reported around 1,300 plastic items per square kilometer in a central region of the basin. However, all other visual counts carried out in different regions of the Mediterranean since then have reported fewer than 200 items per square kilometer [16–19]. Surface pollution has also been quantified using surface net tows, allowing for the detection of smaller plastic sizes, in coastal areas of northwestern Italy, southern France [6, 20], and western Sardinia [21]. These studies reported concentrations ranging from tens of thousands to hundreds of thousands of items per square kilometer, suggesting an abundant presence of buoyant plastic debris in the basin. In the present work, we have carried out extensive sampling across the Mediterranean Sea basin in order to provide a first-order approximation of the magnitude of the plastic pollution in the surface waters of the Mediterranean. Plastic concentrations found in this Sea are compared with those reported for the five regions of plastic debris accumulation in the open ocean.

Material and Methods Floating plastic debris was sampled across the Mediterranean basin in May 2013 on board the Spanish R/V Angeles Alvariño. Geographical coordinates and dates of sampling are available at http://doi.pangaea.de/10.1594/PANGAEA.842054. Permission for navigation and research operations in exclusive economic zones of the Mediterranean Sea was granted from the Governments of Spain, Greece, France, Italy and Cyprus. Sampling did not involve endangered or protected species. Plastic debris in surface waters was collected with a neuston net (1.0 × 0.5 m mouth, 0.2 mm mesh) towed at 2–3 knots for periods of ~15 min. A total of 28 sites were sampled using 39 net tows. The material collected in each tow was resuspended in 20 μm-filtered seawater and floating plastic debris was carefully picked from the water surface with the aid of a dissecting stereomicroscope. This examination of the samples was repeated at least twice to ensure the detection of all of the smallest plastic particles. Plastics extracted from the seawater samples were washed with deionized water and dried at room temperature before being weighted. Plastic items were assigned to five product type categories: industrial pellets (the raw form of plastic) and granules (likely derived from cosmetic and cleansing products); thin films (bags, wrappings, or pieces of them); fishing threads (including fishing lines and plastic fibers released from fishing nets); foamed plastic (termed here “foam”); and rigid manufactured items or pieces of them (all termed as “fragments” as the vast majority correspond to pieces from broken objects). Any fibers suspected of being of a textile origin were excluded from the analysis because they could be airborne contamination from clothing during the sampling or processing [22]. Potential textile fibers were identified according to shape and rigidness. They typically ranged from hundreds of microns to centimeters in length and from one to few tens of microns in width, being easily folded; whilst pieces of fishing threads are wider and generally straight in shape (S1 Fig). Tar particles were particularly abundant in relation to open ocean samples collected with the same method in a former study [5], likely due to the high maritime traffic across the Mediterranean. Nevertheless, tar particles were also excluded from the analysis. The maximum linear length of the plastic items was measured under an optical microscope using the image processing NIS-Elements software, whereas large plastic objects were measured with a ruler. A total of 3,901 plastic items were measured and separated into 28 size classes to build a size distribution. Narrower bins were used to describe the size structure of the smaller plastics. Therefore, size limits of the bins were set following a 0.1-log series of linear length. The width of the uppermost bin extended from 10 cm to 100 cm (the width of the net mouth) due to the relatively low abundance of plastic items in this size interval. To render plastic counts per bin independent of the width of the bin, the abundance of plastic items for each bin was normalized by the bin width. These results were compared with the plastic size distribution found in our previous study of the open ocean [5]. Plastic concentrations per surface area were calculated by dividing the total number and dry weight of plastics collected in each tow by the area towed. This area was derived from the volume of filtered seawater during the tow, measured with a flowmeter at the net, and the submerged area of the net mouth (1.00 m x 0.25 m). Given that wind stress can extend the vertical distribution of buoyant plastic debris below the surface sampling layer (0.25 m deep), surface plastic concentrations derived from tows carried out with average friction velocity in water (u*) > 0.6 cm s-1 (54% of the tows) were adjusted following the model proposed by Kukulta et al. [23]. The model provides wind-adjusted numerical concentrations from u* and the numerical concentrations measured in the surface tows. Wind-adjusted abundances were converted to mass concentrations using an empirical correlation based on simultaneous measurements of total weight and abundance of plastic in 609 worldwide tows (S2 Fig). Adjusted and non-adjusted concentrations of plastic measured in the Mediterranean Sea are reported at http://doi.pangaea.de/10.1594/PANGAEA.842054. Finally, to analyze the plastic abundance in the Mediterranean basin, we combined our data with the regional surveys carried out by Collignon and coworkers in coastal areas of Ligurian Sea in July 2010 [20], and by de Lucia and coworkers in the western coast of Sardinia in July 2012 and July 2013 [21]. Samples collected by Collignon et al. [20] at high wind speeds (located on the northwestern Mediterranean coast) were discarded. These additional data (33 net tows) were spatially averaged over grid cells of 1° in both latitude and longitude (11 grid cells) to avoid giving excess weight to sites with higher sampling density. Overall, the data compilation of plastic concentrations included 72 net tows, resulting in a spatial grid composed of 39 grid cells across the Mediterranean.

Conclusions and Final Remarks In the present work, we identify the Mediterranean Sea as a great accumulation zone of plastic debris. From the averaged plastic concentration measured into the basin, the surface load of plastic in the Mediterranean is estimated to be around one thousand tons, increasing the estimated global load of surface plastic by 7% [5], in agreement with the relative loads predicted for this Sea by modeling at the global scale [8]. However the calibration of distribution models to upscale absolute large-scale loads of plastic must be treated with caution due to the low agreement found between measurements and model predictions within the basin scale (Fig. 2). The estimate of plastic load in the Mediterranean Sea derived from model calibration [6] was one order of magnitude higher than our estimate from a gridded approach, considering both total and microplastic (< 5 mm) loads. The development of more accurate estimates of the magnitude and sub-basin distribution of the plastic pollution in the Mediterranean Sea requires improved sampling resolution and coverage. Model simulations are shown here to be useful tools to guide field surveys aimed at assessing the magnitude of global marine plastic pollution. The model by Lebreton et al. [8] identified the Mediterranean Sea as a region of high load of plastic pollution, and this is confirmed by our estimates. Lebreton and coworkers also pointed out the Bay of Bengal and the South China Sea as relevant accumulation zones, while the model of van Sebille and coworkers [9] drew attention to the Barents Sea in the Arctic Ocean as an accumulation zone. Recent measurements verified the existence of high plastic abundance in areas of the Bay of Bengal [33], but current assessments of plastic concentrations in the South China Sea [47] suggest lower loads than predicted. Available data for floating debris in Arctic waters show relatively low plastic concentrations [5, 10], but few measurements are yet available and these assessments need be extended to higher latitudes. Interestingly, recent analyses of plastic pollution in ice cores show significant loads of microplastics in the Arctic ice sheet [48], which implies accumulation of plastic pollution in the Arctic Ocean. The discovery of large-scale accumulations of marine debris has attracted worldwide attention in the media, which often refer to these areas as “great garbage patches”. However, these marine plastic accumulations are inaccurately illustrated in some media reports. The present work converges with other studies [1, 4–9] to define these accumulation zones as very large spans of the ocean (millions of km2 in area), although their borders are diffuse and changing, and their interior shows high heterogeneity at multiples scales [8, 26]. These accumulation zones are dominated by tiny plastic pieces, mainly on the order of millimeters, not easily perceptible by an observer on a ship. When the sea is calm, plastic fragments are present in nearly 100% of net surface tows in these areas, each covering around 1000 m2, but the density of plastic pieces is not as high as the term “patch” may suggest. The typical mean spatial concentration measured with net tows is around 1 plastic item in 4 m2, reaching 1–10 items m-2 in the most polluted areas. Marine plastic pollution has spread to become a problem of planetary scale after only half a century of widespread use of plastic materials, calling for urgent management strategies to address this problem. Cleanup activities on the shoreline could be particularly effective in the Mediterranean Sea since shore deposition of floating debris must be common in this semi-closed sea [49]. However, as the production of plastic materials will likely continue to increase in the coming decades [50], management strategies should be addressed at the pollution sources in order to prevent the release of plastic discards to the environment.

Supporting Information S1 Fig. Photograph of typical textile fiber (red) and fishing thread (blue). https://doi.org/10.1371/journal.pone.0121762.s001 (TIF) S2 Fig. Relationship between mass (M) and numerical (N) concentrations of floating plastic debris. Red circles correspond to surface tows carried out in the Mediterranean Sea (n = 39), and blue circles to the data set compiled by Cózar et al. for the global ocean [5] (n = 571). Black line shows the log-log linear-square fitting on all data in plot (log M (g km-2) = 1.22 log N (items km-2)- 4.04; n = 609, r = 0.8571, p < 0.0001). https://doi.org/10.1371/journal.pone.0121762.s002 (TIF)

Acknowledgments We thank P. Ziveri, M. J. Acevedo, U. Tilves, J. García-Orellana, and the captain and crew of R/V Angeles Alvariño for help with sampling. This is Campus de Excelencia Internacional del Mar (CEIMAR) Publication 82.

Author Contributions Conceived and designed the experiments: AC JIGG CMD. Performed the experiments: MSM EM BU JIGG AC. Analyzed the data: AC EM MSM. Wrote the paper: CMD MSM JAG XI. Wrote the manuscript: AC.