Significance High concentrations of floating plastic debris have been reported in remote areas of the ocean, increasing concern about the accumulation of plastic litter on the ocean surface. Since the introduction of plastic materials in the 1950s, the global production of plastic has increased rapidly and will continue in the coming decades. However, the abundance and the distribution of plastic debris in the open ocean are still unknown, despite evidence of affects on organisms ranging from small invertebrates to whales. In this work, we synthetize data collected across the world to provide a global map and a first-order approximation of the magnitude of the plastic pollution in surface waters of the open ocean.

Abstract There is a rising concern regarding the accumulation of floating plastic debris in the open ocean. However, the magnitude and the fate of this pollution are still open questions. Using data from the Malaspina 2010 circumnavigation, regional surveys, and previously published reports, we show a worldwide distribution of plastic on the surface of the open ocean, mostly accumulating in the convergence zones of each of the five subtropical gyres with comparable density. However, the global load of plastic on the open ocean surface was estimated to be on the order of tens of thousands of tons, far less than expected. Our observations of the size distribution of floating plastic debris point at important size-selective sinks removing millimeter-sized fragments of floating plastic on a large scale. This sink may involve a combination of fast nano-fragmentation of the microplastic into particles of microns or smaller, their transference to the ocean interior by food webs and ballasting processes, and processes yet to be discovered. Resolving the fate of the missing plastic debris is of fundamental importance to determine the nature and significance of the impacts of plastic pollution in the ocean.

The current period of human history has been referred as the Plastic Age (1). The light weight and durability of plastic materials make them suitable for a very wide range of products. However, the intense consumption and rapid disposal of plastic products is leading to a visible accumulation of plastic debris (2). Plastic pollution reaches the most remote areas of the planet, including the surface waters of the open ocean. Indeed, high concentrations of floating plastic debris have been reported in central areas of the North Atlantic (3) and Pacific Oceans (4, 5), but oceanic circulation models suggest possible accumulation regions in all five subtropical ocean gyres (6, 7). The models predict that these large-scale vortices act as conveyor belts, collecting the floating plastic debris released from the continents and accumulating it into central convergence zones.

Plastic pollution found on the ocean surface is dominated by particles smaller than 1 cm in diameter (8), commonly referred to as microplastics. Exposure of plastic objects on the surface waters to solar radiation results in their photodegradation, embrittlement, and fragmentation by wave action (9). However, plastic fragments are considered to be quite stable and highly durable, potentially lasting hundreds to thousands of years (2). Persistent nano-scale particles may be generated during the weathering of plastic debris, although their abundance has not been quantified in ocean waters (9).

As the size of the plastic fragments declines, they can be ingested by a wider range of organisms. Plastic ingestion has been documented from small fish to large mammals (10⇓–12). The most evident effects of plastic ingestion are mechanical [e.g., gastrointestinal obstruction in seabirds (13)], but plastic fragments contain contaminants added during plastic manufacture or acquired from seawater through sorption processes [e.g., hydrophobic chemicals (14, 15)]. Recent studies provide evidence that these contaminants can accumulate in the receiving organisms during digestion (14).

Our awareness of the significance of plastic pollution in the ocean is relatively recent, and basic questions remain unresolved. Indeed, the quantity of plastic floating in the ocean and its final destination are still unknown (16). Historical time series of surface plastic concentration in fixed ocean regions show no significant increasing trend since the 1980s, despite an increase in production and disposal (3, 16, 17). These studies suggest that surface waters are not the final destination for buoyant plastic debris in the ocean. Nano-fragmentation, predation, biofouling, or shore deposition have been proposed as possible mechanisms of removal from the surface (3, 9, 16).

On the basis of samples collected on a circumnavigation cruise (Malaspina 2010 expedition), on five regional cruises, and available data from recent studies (3⇓–5, 17⇓–19), we aim to provide a first-order approximation of the load of plastic debris in surface waters of the open ocean. We also examine the size distribution of floating plastic debris collected along the circumnavigation to provide insight into the nature of possible losses of floating plastic from the open ocean surface.

Materials and Methods From December 2010 to July 2011 the Spanish circumnavigation expedition Malaspina 2010 sampled surface plastic pollution at 141 sites across the oceans. Floating plastic was collected with a neuston net (1.0- × 0.5-m mouth, 200-μm mesh) towed at 2–3 knots for periods 10–15 min (total tows 225). Tow areas were calculated from the readings of a flowmeter in the mouth of the net. Wind speed and water surface density were measured during each tow to estimate average friction velocity in water (u * ) (39). The material collected by the net was mixed with 0.2-mm-filtered seawater. Subsequently, floating plastic debris was carefully picked out from the water surface with the aid of a dissecting microscope. This examination was repeated at least twice to ensure the detection of all of the smallest plastic particles. To confirm the plastic nature of the material collected in the examinations, Raman spectroscopy was applied to a random subset of particles (n = 67). The analysis confirmed the identity of all plastic particles, and polyethylene was found to be the most common polymer type. The vast majority of the plastic items consisted of fragments of larger objects, and industrial resin pellets represented only a small fraction (<2%) of all encountered items. Textile fibers were found only occasionally and were excluded from the analysis because they could be airborne contamination from clothing during the sampling or processing (31). Plastics extracted from the seawater samples were washed with deionized water and dried at room temperature. The total dry weight of the plastics collected in each tow was recorded. The maximum linear length (l) of the plastic items was measured by high-resolution scanning (SI Appendix, Fig. S11) and the image processing Zooimage software (www.sciviews.org). Alternatively, excessively large plastic objects were measured with a ruler. Overall, 7,359 plastic items were measured and separated in 28 size classes to build a size distribution. Size limits of the bins followed a 0.1-log series of l. The width of the uppermost bin extended from 10 cm to the length of the net mouth (100 cm) to account for all sizes that could be collected by the net. The trapping efficiency of fine particles by the mesh was tested from the analysis of the size distribution of nonplastic particles in six tows evenly distributed along the circumnavigation (SI Appendix, Fig. S12). Once the plastic particles were picked out from the samples, the size distribution of nonplastic particles was measured by the same methods. Wind stress can extend the vertical distribution of floating plastic debris into the surface mixing layer, resulting in underestimation of the plastic concentrations measured by the surface tows (0.25 m deep). Thus, the integrated plastic abundance from the surface to the base of the wind-mixed layer (generally <25 m) was estimated with a model dependent on u * and the numerical concentrations measured in the surface tows (39). Wind-corrected abundances were converted to mass concentrations using a correlation based on simultaneous measurements of total mass and abundance of plastic in 570 worldwide tows (SI Appendix, Fig. S13). Size-Distribution Analysis. A theoretical size distribution of plastic derived from fragmentation was modeled by assuming steady state (large-objects input = small-fragments output, below 0.2 mm). Given that the plastic abundance in a given size class depends on the fragmentation of larger plastic objects already present, we selected a size class with relatively large plastic (reference bin) and projected the plastic amount measured in this bin toward smaller and larger size classes (onward and backward in time). Therefore, the normalized abundance (divided by the width of the size-class interval) of the size class i derived from steady fragmentation was modeled as A i f = A r e f ⋅ α ⋅ l r e f 3 α ⋅ l i 3 = A r e f ⋅ l r e f 3 l i 3 . We used a standard shape for the plastic fragments having the three principal axes proportional to l. Thus, α ⋅ l i 3 accounts for the mean volume of the fragments of i, with α being a shape factor and l i the nominal length for the class i, set at the bin midpoint. A r e f is the normalized abundance measured in the reference bin (i = ref). The 20- to 25-mm class was selected as reference, although similar results were obtained by selecting other large-size classes. The normalized volume in each size class derived from fragmentation was modeled as V i f = A i f ⋅ α ⋅ l i 3 = A r e f ⋅ α ⋅ l r e f 3 , being α = 0.1, a value corresponding to flat-shaped volume. Because the steady fragmentation of the large-plastic input results in an even volume–size distribution, deviations of the observed size distribution from a conservative distribution can be related to changes in the fragmentation dynamics, inputs of small plastics, or losses (SI Appendix, Fig. S9). Estimating volumes from observed abundances ( V i ∗ = A i ∗ ⋅ α ⋅ l i 3 ) , and after smoothing the resulting volume–size distribution to remove small irregularities, the deviations from a conservative distribution (∆ i , expressed as percentage of total) were calculated as Δ i = V i − 1 ∗ − V i ∗ ∑ i = 1 n | V i − 1 ∗ − V i ∗ | = ( A i − 1 ∗ ⋅ l i − 1 3 ) − ( A i ∗ ⋅ l i 3 ) ∑ i = 1 n | ( A i − 1 ⋅ l i − 1 3 ) − ( A i ⋅ l i 3 ) | , where i = 1, 2, …, n, with n being the lowest size class (0.2–0.25 mm). The denominator accounts for the total deviations accumulated across the entire size range studied. Negative values of ∆ i are related to net plastic losses and positive values to plastic accumulations. Note that ∆ i is independent of the standard plastic shape (α value) used in the computations. Possible variations of α with size were unable to induce changes in the volume–size distribution enough to explain the gap found in small sizes, owing to the extreme scarceness of plastic below 1 mm and the geometrical constrain for α, getting the maximum at 0.52 (spherical shape). Observed plastic abundance in the lowest part of the size spectrum was four orders of magnitude lower than expected from fragmentation (Fig. 3). The size-distribution analysis is a useful tool to constrain the possible dynamics of marine plastic pollution. Nevertheless, the mechanisms leading to the observed plastic size distributions still are not entirely understood and deserve further attention, resolving the size dependence of the sink/sources processes, as well as testing the framework proposed here (SI Appendix, Fig. S9) to identify additional processes. Spatial Analysis. To analyze the global distribution of floating plastic, data from the Malaspina circumnavigation were combined with additional regional surveys and recent (from 2006 to date) measurements reported by other researchers after data standardization (SI Appendix, Table S1). Concentrations of plastic per surface-water volume were converted to concentrations per surface area from the tow depth, determined according to net type and mouth dimensions (one-half mouth height for neuston nets, three-fourths mouth height for manta nets). Plastic concentrations measured with mesh sizes larger than 0.2 mm were multiplied by a correction factor derived from the plastic size distribution measured in the Malaspina circumnavigation. For 0.3-, 0.5-, and 1.0-mm mesh sizes, numerical underestimation was estimated at 0.4, 2.7, and 21.3%, and mass underestimation at 0.0, 0.4, and 5.0%, respectively. Data reported in numerical concentrations were converted to mass concentrations by using the global relationship found between total mass and abundance (SI Appendix, Fig. S13). For data reported without wind correction (3⇓–5, 18), we use satellite winds from the CCMP database (http://podaac.jpl.nasa.gov) to discard samples collected with winds speeds larger than 5 m⋅s−1 (u * ∼0.6 cm⋅s−1), the threshold above which the effects of wind stress can be significant (39). The range of the global plastic load in the surface ocean was estimated from the concentration ranges measured over 15 major zones in relation to the degree of surface convergence and by using two different sets of measurements, a wind-corrected dataset and a noncorrected dataset. Using a global circulation model (6), nonaccumulation, outer accumulation, and inner accumulation zones were delimited in each ocean basin to reduce the inaccuracies derived from an uneven distribution of measurements. In addition, plastic measurements were spatially averaged over grid cells of 2° in both latitude and longitude to avoid overweight of areas with high sampling frequency. Overall, 442 grid cells (1,127 net tows) were included in the wind-corrected dataset (Fig. 1 and SI Appendix, Table S1). Midrange regional concentrations were calculated from the averaging of the wind-corrected plastic concentrations within each major zone. High-range regional concentrations were calculated from the 90th percentile. We used a wide confidence interval for the plastic load estimate to address variability and possible inaccuracies in the spatial concentrations of plastic. Low-range concentrations were calculated from the averaging of the direct measurements of surface concentrations, without wind correction or discards by high wind mixing (noncorrected dataset: 851 grid cells, 3,070 net tows; SI Appendix, Figs. S2 and S3). Global plastic loads in the open-ocean surface were estimated from high, mid, and low regional concentrations and surface areas.

Acknowledgments We thank Pakea Bizkaia and the Chilean Navy, which contributed to the sample collection, and K. L. Law, M. C. Goldstein, M. J. Doyle, M. Eriksen, J. Reisser, and their collaborators for their available data. We also thank S. Loiselle and J. Ruiz for his useful suggestions in writing the paper. This research was funded by the Spanish Ministry of Economy and Competitiveness through the Malaspina 2010 expedition project (Consolider-Ingenio 2010, CSD2008-00077) and the Migrants and Active Flux in the Atlantic Ocean project (CTM2012-39587-C04-01). Original data reported in this paper are freely available at http://metamalaspina.imedea.uib-csic.es/geonetwork. This is Campus de Excelencia Internacional del Mar (CEIMAR) Publication 58.

Footnotes Author contributions: A.C., F.E., J.I.G.-G., X.I., and C.M.D. designed research; A.C., F.E., J.I.G.-G., X.I., B.U., S.H.-L., A.T.P., S.N., J.G.-d.-L., A.R., M.L.F.-d.-P., and C.M.D. performed research; A.C., X.I., B.U., S.N., J.G.-d.-L., and M.L.F.-d.-P. contributed new reagents/analytic tools; A.C., J.I.G.-G., B.U., A.T.P., S.N., and J.G.-d.-L. analyzed data; and A.C., F.E., X.I., and C.M.D. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1314705111/-/DCSupplemental.