Large Accumulation of Micro-sized Synthetic Polymer Particles in the

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Large Accumulation of Micro-sized Synthetic Polymer Particles in the Sea Surface Microlayer Young Kyoung Song,†,‡ Sang Hee Hong,†,‡ Mi Jang,†,‡ Jung-Hoon Kang,† Oh Youn Kwon,† Gi Myung Han,† and Won Joon Shim*,†,‡ †

Korea Institute of Ocean Science and Technology, Geoje-shi 656-834, South Korea University of Science and Technology, Daejeon 305-320, South Korea

‡

S Supporting Information *

ABSTRACT: Determining the exact abundance of microplastics on the sea surface can be susceptible to the sampling method used. The sea surface microlayer (SML) can accumulate light plastic particles, but this has not yet been sampled. The abundance of microplastics in the SML was evaluated off the southern coast of Korea. The SML sampling method was then compared to bulk surface water filtering, a hand net (50 μm mesh), and a Manta trawl net (330 μm mesh). The mean abundances were in the order of SML water > hand net > bulk water > Manta trawl net. Fourier transform infrared spectroscopy (FTIR) identified that alkyds and poly(acrylate/styrene) accounted for 81 and 11%, respectively, of the total polymer content of the SML samples. These polymers originated from paints and the fiberreinforced plastic (FRP) matrix used on ships. Synthetic polymers from ship coatings should be considered to be a source of microplastics. Selecting a suitable sampling method is crucial for evaluating microplastic pollution.

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INTRODUCTION Microplastics, which are generated from both the fragmentation of large plastic products and macroplastic debris, and the manufacture of small plastic particles are being introduced into the oceans at an unprecedented rate.1 Small plastics in the marine environment are ingested by invertebrates,2−4 turtles,5 fishes,6,7 birds,8,9 and marine mammals,5 which can then have adverse physical and chemical effects on these organisms. Microplastics have a large potential to concentrate contaminants from the surrounding water, and they typically contain various toxic additives.10,11 After ingestion, the microplastics can act as transfer vectors of these toxic materials to marine organisms.11−14 Because of the global environmental concern regarding the negative impacts of microplastics on marine environments, a need exists to determine their source and global distribution.15 Knowledge about the distribution of microplastics would enable a better understanding of their effects on the marine environment and its biota. If the interrelation between source and sink regions could be identified, it would help to identify accumulation “hot spots”, such as the subtropical gyres.16,17 Determining the exact abundance of microplastics on the sea surface can be susceptible to the sampling method used, but no methodological comparison of the different methods used for the quantification of floating microplastics had been undertaken. Floating microplastics have typically been sampled using a neuston net with varying mesh sizes (280−505 μm).2,18−25 Other methods of microplastic sampling have also been used © 2014 American Chemical Society

on the sea surface and in the water column [e.g., bongo/ zooplankton net, other plankton samplers, and the surface microlayer (SML) sampler].26 Considering the specific gravity and small size of the microplastics, they are expected to accumulate within the SML. A neuston net usually samples microplastics in the SML and the underlying surface waters together, and consequently, no systematic study on the accumulation of microplastics in the SML has been reported. The aim of this study was therefore to answer two questions. (1) Are microplastics concentrated in the SML comparable to that in the underlying water layers? (2) What is the difference in the abundance of microplastics when different sampling methods are used, such as SML, surface bulk water filtering, hand net (50 μm mesh), and Manta trawl net (330 μm mesh)? To answer these questions, the spatiotemporal distribution of microplastics was investigated in the SML in near- and offcoastal areas in the vicinity of the Nakdong River before and after the rainy season in southern Korea. The microplastic abundance and their composition were compared among the four different sampling methods. In addition, the different polymers in the SML were identified using a Fourier transform infrared spectroscopy (FTIR) microscope to infer the most likely sources of microplastics in the study area. Received: Revised: Accepted: Published: 9014

April 9, 2014 July 23, 2014 July 24, 2014 July 24, 2014 dx.doi.org/10.1021/es501757s | Environ. Sci. Technol. 2014, 48, 9014−9021

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MATERIALS AND METHODS Microplastic Sampling. Water samples were collected in the near- and offshore environment of Geoje Island, which receives riverine discharge (mean flow of 187.1 m3/s) from the Nakdong River (length of 521 km, with 13.2 million people in the river catchment; see Figure S1 of the Supporting Information). On the basis of a particle-tracking model, solid particles discharged from the Nakdong River extend throughout the study area within a week following a heavy storm.27 Figure S2 of the Supporting Information is a schematic diagram of the sampling procedures for the SML, bulk water, hand nets, and Manta trawl methods, and photographs of the actual sampling procedure are given in Figure S3 of the Supporting Information. SML water samples were collected at 10 stations in May (before the rainy season) and July (after the rainy season) 2012. The monthly precipitation was 36.2 and 287.4 mm during May and July 2012, respectively. Stations 1 and 2 were located in Jinhae Bay, and stations 3−5 were positioned inside bays along the east coast of Geoje Island. Stations 6 and 7 were located in front of the mouth of the Nakdong River, while stations 8−10 were ∼10−16 km off the coast of Geoje Island. A metal sieve was used to obtain SML water samples, which were typically collected from a depth of 150−400 μm.28 The microplastic particles and the SML water were trapped within the metal sieve mesh spaces by surface tension. A 2 mm mesh sieve of 20 cm diameter was allowed to gently touch the sea surface 100 times over a 3.14 m2 sampling area at each station. The water trapped within the mesh spaces was collected in a stainless-steel tray using a leaning sieve and transferred to polyethylene bottles. The final volume of SML water collected at each station was in the range of 2.2−2.8 L. Bulk Water, Hand Net, and Manta Trawl Samples. Additional surface water samples were collected at the same time as the SML water was sampled using three different methods. Approximately 100 L of the top 20 cm of surface water was manually collected at three stations (stations 1, 3, and 5) in May using a plastic bucket with a 20 cm diameter. The bulk water samples were collected and stored in five 20 L plastic containers. Approximately 100 L of the top 20 cm of surface water was manually collected as a bulk water sample, and the collected water was passed through a hand net (mesh size of 50 μm) aboard the vessel. A Manta trawl net with a mouth of 40 × 19.5 cm (mesh size of 330 μm) was towed for 10 min at ∼2 knots. The total netted volume of water ranged from 44.6 to 83.1 m3. Manta trawl and hand net sampling was conducted at all stations in May and July. Microplastic Particle Counting and Identification. The SML water samples and control samples in the bottle were filtered using a glass fiber filter (GF/F; 0.75 μm; 47 mm Ø; Whatman, Maidstone, Kent, U.K.). Filters were dried at 60 °C and kept in Petri dishes. Microplastic particles on the filter paper were simultaneously counted and identified using a FTIR microscope (Thermo Nicolet 6700 and Continuμm, Thermo Scientific, Waltham, MA). Microplastic particles were counted in five randomly selected squares (7.9 × 7 mm; total area of 2.76 cm2) on filter paper, which accounted for 25% of the total filtered area. Whole plastic-like particles in each square were selected and immediately identified using the FTIR microscope. Synthetic polymers were conventionally categorized as fibers, elastomers, plastics, coatings, and adhesives by polymer chemists and producers.29 Synthetic polymers were identified

and confirmed using FTIR together with a polymer spectrum library, and the number of particles in the five categories above was counted. The microplastics considered in this study were not only hard plastics and fibers but also other synthetic polymers, such as paint particles. The spectra were recorded as the average of 128 scans in the spectral range of 650−4000 cm−1 at a resolution of 8 cm−1 or the average of 32 scans in the spectral range of 600−4000 cm−1 at a resolution of 8 cm−1. The microplastics were categorized according to their maximum size (1000 μm) and by type of plastic [paint particles and non-paint plastics or fragment, fiber, spherule, sheet, and expanded polystyrene (EPS)]. The microplastic particles were not weighed because the particles were too small to be removed from the filter papers. Weighing the plastics with the filter paper was difficult because of the many other non-plastic particles on the paper. The 100 L bulk water sample was filtered through a largediameter glass fiber filter (Whatman GF/F; 1 and 0.75 μm; 15 cm Ø) using a high-volume filtration system. The filters were dried at 60 °C and kept in Petri dishes. Microplastic particles were counted in five randomly selected squares (1.446 × 1.446 mm; total area of 10.46 cm2) on the filter paper, which accounted for 25% of the total filtered area. Whole plastic-like particles in each square were selected and immediately identified using the FTIR microscope. Hand net and Manta trawl samples were sieved through a 2 mm (rather than 1 mm) metal mesh sieve to avoid clogging of the mesh with zooplankton. Plastic particles larger than 2 mm were sorted and counted by the naked eye. The water and particle samples that passed through a 2 mm sieve were digested with 34.5% H2O2 for 2 weeks to remove biogenic materials. The remaining samples were filtered through a glass fiber filter (Whatman GF/F; 0.75 μm; 47 mm Ø). The filters were dried at 60 °C and kept in Petri dishes. Plastic particles on the filter paper were counted using a dissecting microscope (stereomicroscope, AxioCam ICc3, Spectra Services, Ontario, NY). Some dominant plastic-like particles from the Manta trawl and hand net samples were confirmed using the FTIR microscope. A total of 2 L of distilled water (n = 3), which was stored in the same type of polyethylene sampling bottle as that used for the SML water, was filtered using a glass fiber filter as a blank sample. Particles on the dried filter paper were counted and identified using a FTIR microscope in the same way as the microplastic particles were counted. None of the non-paint plastics and paint particles was detected in the blank samples. Statistical Analysis. All statistical tests were performed using the software program SigmaPlot, version 11. A Mann− Whitney rank sum test was used to test for differences between the abundance of non-paint plastics and paint particles for the different sampling methods and in different sampling locations. A Wilcoxon signed-rank test was used to test for differences in the abundance of non-paint plastics and paint particles between the sampling seasons (see Table S1 of the Supporting Information).

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RESULTS Identification of Microplastics in the SML. Unexpectedly, alkyds predominated among the polymer types (81%), followed by poly(acrylate/styrene) (11%) in this study (Table 1). Other than alkyds and poly(acrylate/styrene), the microplastic samples consisted of well-known plastics, such as polypropylene (PP), polyethylene (PE), polyester, synthetic 9015

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The specific density of alkyd samples (n = 13) from the SML water and general ship paints (n = 8), FRP matrix (n = 7), and antifouling paints (n = 4) from the ships or shipyards were 1.44 ± 0.10, 1.41 ± 0.10, 1.27 ± 0.05, and >1.7 g/mL, respectively (see Table S2 of the Supporting Information). All of the highdensity antifouling paint (AFP) chips sank in saturated ZnCl2 solution (1.7 g/mL). Although the density of alkyd samples was substantially higher than seawater (1.02−1.03 g/mL), they were found floating in the SML. The surface tension of the SML may keep alkyd particles afloat. To confirm that alkyd paint particles floated on the sea surface, an additional experiment was conducted. When alkyd paint particles (n = 269) with a size range of 0.4−4.36 mm were floated in natural seawater in a beaker, ∼46% of the paint particles remained floating, while the remainder sank. Even after vigorous stirring of the water, 23% of the paint particles remained in the surface water. Abundance of Microplastics in the SML. The abundance of microplastics in the samples taken in this study is considered separately for paint particles [including both the alkyds and poly(acrylate/styrene)] and non-paint plastics (hereafter plastics) to allow for a comparison to other studies. The mean abundance of paint particles was 195 ± 114 particles/L (150 ± 90 particles/m2 on a sampling area basis), and the mean abundance of plastics was 16 ± 14 particles/L (13 ± 11 particles/m2; Table 2). The abundance of paint particles increased as the particle size decreased, whereas no relationship was observed between the abundance and size of plastics.

Table 1. Composition of Microplastics by Polymer Type in the Sea SML Identified by FTIR composition (%) polymer type alkyd poly(acrylate/styrene) polypropylene polyethylene polyester synthetic rubber others a

abundance (particles/L)

total

w/o paint particlesa

± ± ± ± ± ± ±

81 11 2.0 0.8 0.9 0.6 3.4

25 11 11 8.3 44

171 24 4.2 1.8 1.9 1.4 7.3

113 21 4.8 3.6 3.1 3.8 12

Percent composition except for alkyd and poly(acrylate/styrene).

rubber, and other polymers (e.g., phenoxy resin, polyurethane, acrylic, EPS, and various copolymers), which accounted for 25, 11, 11, 8.3 and 44%, respectively. An alkyd is a polyester produced by the addition of polyols and organic acids.30 Various kinds of alkyds are made using different combinations of raw materials and additives, but most of the alkyds detected in this study were polyester (a phthalic alkyd modified by soybean oil). This alkyd is a typical polymer binder in industrial paints, including marine applications.31 Poly(acrylate/styrene) is an unsaturated polyester that is also widely used in the paint industry to manufacture emulsion-based paints32 and in fiberreinforced plastics (FRP) for boats as a thermoset matrix.33 On the basis of the distinct dark-green or dark-blue colors of alkyds and the white sky blue of poly(acrylate/styrene) and their applications in paints used on ships and/or the FRP matrix, small fishing boats were suspected to be their source. Samples of various kinds of paints from ships and FRP fragments were collected from fishing boats in ports and shipyards around Geoje Island. The color and FTIR spectrum of alkyds was matched with paint chips from ships collected from the shipyards (see Figure S4a of the Supporting Information). The bands at 2924 and 2853 cm−1 are C−H stretching vibrations.34 The 1410−1510 cm−1 region is attributable to the generic stretching of C−O in calcium carbonate, which matches with the carbonate additive in alkyd resin.35 The ether C−O−C asymmetric stretching band is often most intense between 1300 and 1000 cm−1. The bands at 1068, 1118, and 1258 cm−1 confirm the presence of C−O−C of an ester linkage. These strong bands were clearly present in all samples. The inorganic carbonate groups appear from 880 to 860 cm−1 (C−O out-of-plane bend) and at ∼740 cm−1 (C−O in-plane bend). A strong absorption band at 1728 cm−1 can be attributed to saturated CO bending. The color and FTIR spectrum of poly(acrylate/styrene) was matched with the FRP matrix (see Figure S4b of the Supporting Information). Strong evidence indicated the existence of an aromatic ring of styrene. First, the weak bands above 3000 cm−1 were assigned to stretching vibrations of the CH groups in the aromatic ring. Second, the bands at 1451, 1493, 1579, and 1600 cm−1 were assigned to the stretching vibrations of the carbons in the aromatic ring.34,36 Finally, the out-of-plane C−H band at 742 cm−1 and the ring band at 700 cm−1 firmly indicated a monosubstituted benzene ring. They were strong and clearly present in all samples. A strong band at 1724 cm−1 indicated the saturated CO stretching vibration. However, antifouling paint, which generally has a red color, displayed a different FTIR spectrum (see Figure S4c of the Supporting Information).

Table 2. Mean Abundance of Paint Particles and Non-paint Plastics in the Sea SML on the Southern Coast of Korea by Size abundance (particles/L) size (μm) 1000 total

paint particles 95 67 22 10

± ± ± ±

57 40 20 11

0.5 ± 1.5 195 ± 114

plastics 8.0 1.2 1.2 1.5 2.7 1.6 16

± ± ± ± ± ± ±

12 3.3 2.69 2.9 3.5 2.6 14

Because of their irregular shape, all paint particles were classified as fragments. Total fragments accounted for 95% of the total number of particles in the study area (Table 3), followed by spherules (2.8%) and fibers (2.1%). The length of the fibers was in the range of 200−2000 μm, while the spherule size was hand net (1143 ± 3353 particles/m3) > bulk water (213 ± 141 particles/m3) > Manta trawl (47 ± 192 particles/m3). The mean abundance of microplastic particles was significantly (p < 0.05) different among the SML, bulk water, hand net, and Manta trawl methods (except for the hand net versus bulk 9016

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Table 3. Mean Abundance of Paint Particles and Non-paint Plastics in the Sea SML on the Southern Coast of Korea by Shape and Size abundance (particles/L) size (μm) 1000 total percent total a

fragmenta 97 69 23 9.8 1.1 0.5 200

± 58 ± 41 ± 20 ± 10 ± 2.6 ± 1.5 ± 116 95

fiber

spherule 6.0 ± 9.8

sheet

EPS

total

percent total

0.2 ± 0.9

103 ± 59 69 ± 41 23 ± 20 11 ± 11 2.9 ± 3.4 2.1 ± 2.9 211 ± 117 100

49 33 11 5 1 1 100

0.2 ± 0.9 1.3 1.6 1.6 4.5

± 2.5 ± 2.9 ± 2.6 ± 4.1 2.1

6.0 ± 9.8 2.8

0.4 ± 1.8 0.2

Including paint particles

Figure 1. Comparison of floating microplastic abundance in surface water. The superscript letters indicate net mesh size or sample type: a, 505 μm; b, 450 μm; c, 333 μm; d, 280 μm; e, 80 μm; f, SML; g, 330 μm; h, 50 μm; and i, bulk surface water.

Figure 2. Composition of (a) paint particles and non-paint plastics and (b) non-paint plastics only by shape in different sampling methods.

water; p = 0.075; see Table S1a of the Supporting Information). The mean abundance of paint particles was in the order of SML water (194 450 ± 114 445 particles/m3) > bulk water (733 ± 167 particles/m3) > hand net (196 ± 121 particles/m3) > Manta trawl (0.88 ± 0.81 particles/m3). The mean abundance

of paint particles was significantly (p < 0.05) different among the sampling methods used in this study (see Table S1a of the Supporting Information). In addition to the number distribution, the microplastic abundance in terms of particle shape was also influenced by the 9017

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Few studies have investigated the quantitative polymer composition of floating microplastics in surface water, although a few reports have described qualitative information for some major plastics, such as PE, PP, and polystyrene.20,23,25,38 Only two previous studies have reported alkyds in estuarine and subtidal sediments,2,39 but no information on their source was given. Alkyd-based coatings have been widely applied to protect metallic substrates against corrosion and have been used as a primer.35 Alkyd paints are applied for anticorrosive purposes on metal or wood surfaces on ships. In the SML water of this study area, ∼12 times more paint particles than plastic particles originated from fishing boats, which might be a consequence of the substantial shipping activity in the study area. In 2012, 2610 and 14 994 fishing boats were registered in Geoje city and South Gyeongsang province, respectively. Anticorrosive alkyd paints are generally applied to the deck of fishing boats. During fishing activity at sea, such as pulling fishing nets over the deck, many paint particles are produced by friction. In addition, paint particles could be produced and introduced to seawater during ship maintenance in the various shipyards and ports within the study area. Further study is required to evaluate the major input pathway of micro-sized paint particles in the study area. Large Abundance of Microplastics in the SML. The highest abundance of both micro-sized plastic and paint particles was reported in the SML in this study. Four possible explanations exist for this large abundance of microplastic particles in SML water. First, microplastics can accumulate in SML water because of the low density of some polymer types and the high surface tension of the SML itself. The sticky microgel produced by microbial activity in the SML can enhance the aggregation of live and dead plankton as well as mineral particles.28 Less dense and hydrophobic microplastics can probably be easily aggregated and accumulate in the SML. Because most of the particles found in the SML were alkyds with a very high specific density, surface tension and sticky microgels are likely important for keeping them (at least temporarily) in the SML. Second, because only the very thin SML in which microplastic accumulated was selectively sampled in this study, the void volume of the underlying water with its low microplastic abundance was not included in the calculation of abundance. Third, microplastics of 0.05; see Table S1c of the Supporting Information) was observed in the mean abundance of plastic particles between May and July, while the mean abundance of paint particles in July was significantly lower than in May (p < 0.05; see Table S1c of the Supporting Information).

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DISCUSSION Source of Floating Microplastics. A possibility exists that paint particles sampled here might have originated from the research boat during sampling. However, this is likely to be negligible for three reasons. First, paint particles in the SML samples displayed a clear size distribution (see Figure S5 of the Supporting Information), indicating that the paint particles had been resident on the sea surface for some time, allowing them to be gradually broken down into smaller pieces. If the research boat did contribute substantially to the samples, this size distribution would have been unlikely in the samples. Second, although the deck of the research boat was coated with similar alkyd-type paints, paint chips that had detached from the boat had a different appearance from the paint particles in the SML samples, and their FTIR spectrum also differed (see Figure S7 of the Supporting Information). This difference may have been due to their different weathering status. Third, to confirm the presence of paint particles on the sea surface, the SML samples were collected with no research boat in the vicinity of the aquaculture float (n = 1) and ports (n = 2) used in the study (see Table S3 of the Supporting Information). The sampling sites were accessed from land using floating wooden rafts with no paint coating. This suggests that the alkyd paint particles sampled were already present in the study area. Although the possibility of contamination by paint particles from a research vessel seems unlikely, every effort was taken to avoid or reduce the possibility of contamination from the research vessel during sampling. 9018

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can be reported on both a volume (particles/m3) and area (particles/m2) basis. However, the SML sampling technique may lose larger (millimeter-sized) plastics, such as EPS particles. Although EPS particles also prevailed in the study area, these large particles were not effectively sampled using the SML sampling method, which, when used, underestimated large microplastic particles (e.g., 1−5 mm in size). The sea surface area covered and water volume collected in SML samples are much smaller than those achieved by neuston net sampling, but this can be overcome by increasing the number of SML sampling sites or using a large-volume SML water sampler (e.g., “rotating drum sampler”). A hand net has some advantages over a neuston net, but it still loses information for microplastic particles smaller than its mesh size (50 or 80 μm). Bulk water samples provide information for almost the same size range as samples from the SML but are typically large volumes of water that require a time-consuming filtration process. A neuston net is a useful microplastic sampling tool that can provide reliable data for a large area allowing for intra- and interregional comparative studies. However, it cannot sample microplastic particles smaller than its mesh size, which accounts for the majority of the microplastic abundance. Therefore, when more realistic information of microplastic abundance is required, an alternative, such as SML samples, should be used accompanied by a Manta trawl survey. Ecotoxicological Aspects of Micro-sized Paint Particles in the SML. Alkyds and poly(acrylate/styrene) are categorized as “coatings” rather than “plastics” according to polymer chemistry classification. This is the first study to confirm that micro-sized synthetic polymer particles originating from ship coatings predominate in the SML. Although thermoset polymers with ester groups, such as alkyds and poly(acrylate/styrene), are more likely to be degraded than PE and PP,29 their dominance as the standing stock in the SML implies that inputs are large enough to maintain a high abundance and/or that their degradation rate is slow. Not only are they capable of absorbing toxic chemicals, 41 but anticorrosive polymer resin paints also contain various toxic chemicals, including heavy metals (e.g., zinc and lead), organometals (e.g., dibutyltins), and organic toxicants (e.g., aromatics). 42,43 A screening analysis of the elemental composition in the alkyd particles from the SML using energy-dispersive X-ray revealed high levels of iron (12.9% by weight), lead (7.3%), copper (2.9%), and zinc (1.7%). The water-soluble fraction of polymer paints resulted in the growth inhibition of algae and acute immobilization of water fleas.43 Alkyd particles may absorb and leach toxic chemicals from or to the surrounding SML water, respectively, but very limited information is available regarding the fate and effects of alkyd polymer particles in marine waters. Therefore, further research is needed to better understand the degradation rate, absorption, leaching potential, bioavailability, and biological effects of these toxicants.

sorting and determining the cutoff size. As shown in this study, the type of sample (SML versus bulk water) and net mesh size (330 μm versus 50 μm) significantly affected the total abundance of paint particles and plastics. The reported values were categorized according to sampling method and net mesh size. Whether the other studies in the literature may count paint particles (look like colored plastic fragments with a microscope) as microplastics is uncertain. The abundance of plastics and paint particles was separated in this study, and only the abundances of plastics were compared to the results reported worldwide. The abundance of microplastic particles in samples taken using zooplankton nets (280−505 μm) from the coastal and open ocean in the Atlantic and Pacific were in the range of 0− 32.8 particles/m3 (average ± SD; 1.9 ± 5.3 particles/m3; Figure 1),2,18−25 which was similar to the range of non-paint microplastic abundance measured using a Manta trawl net (0.4−54.5 particles/m3, with the exception of one high value) in this study. Measurements in coastal environments and harbors in Sweden using an 80 μm mesh sized hand net revealed a range of microplastic abundance from 167 to 102 500 particles/m3 (average ± SD; 8651 ± 28 206 particles/ m3),18 similar to the values recorded from hand net samples in this study. Because no other measurements of bulk water samples have been reported, the values recorded in this study cannot be compared to other values. The quantitative analysis of microplastics in the SML has only been undertaken in Korea (this study) and Singapore,38 but the sampling techniques used were different. The SML samples in Singapore were collected with a rotating drum sampler (upper limit of 50 μm). The abundance of microplastic particles in the SML water of Singapore was in the range of 0−2000 particles/m3, i.e., 33 times lower than values recorded in this study (0−66 666 particles/m3). Comparison of the Sampling Methods. Neuston nets (e.g., the Manta trawl net) with a mesh size range of 280−450 μm have been widely used for microplastic sampling in surface waters,26 because they have the advantage of collecting integrated and representative samples from a wide area of the sea surface, including larger plastic particles with sizes at the millimeter scale. In addition, a neuston net can simultaneously sample zooplankton with microplastics and allows for a comparison of their abundances. In this study, a comparison of the four different sample types, including water passing through a Manta trawl, in the same study area demonstrated that Manta trawl sampling can underestimate the abundance of microplastics in surface water of coastal areas. A study in Sweden using two different sampling nets (80 and 450 μm mesh size) produced similar results.18 As shown in Figure 1, the type of sample taken produces more variation in the abundance of microplastics in surface water than the different regions where sampling takes place. Selecting a suitable sampling method for the evaluation and comparison of microplastic pollution is crucial. SML sampling is a very simple method for collecting microplastics without the need for a specialist or large equipment at wind speeds below 6.6 m/s. In comparison to net samples, SML water samples contain very small amounts of biogenic materials, which interfere with plastic identification and usually must be removed by a tedious sorting of the samples. SML sampling can collect microplastics in the size range of 1−1000 μm, most likely without underestimating their abundance. Furthermore, microplastic abundance in the SML

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ASSOCIATED CONTENT

S Supporting Information *

Methodology for density measurement, sampling location (Figure S1), schematic diagrams and photographs of the four sampling methods (Figures S2 and S3), statistical analysis (Table S1), comparison of the FTIR spectra of samples from the SML and ship paints (Figure S4), density and abundance of major microplastic samples (Table S2 and Figure S5), 9019

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spatiotemporal distribution of microplastics (Figure S6), (7) pictures and FTIR spectra of SML samples and the vessel (Figure S7), and abundance of control samples (Table S3). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +82-55-639-8671. Fax: +82-639-8689. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Seung Won Jung at the Korea Institute of Ocean Science and Technology for his energy-dispersive Xray analysis. This study was supported by grants-in-aid from the Korea Institute of Ocean Science and Technology (PE 99192).

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NOTE ADDED AFTER ASAP PUBLICATION This article published August 4, 2014 with an error in an author name, and a mistake in the units given in the sixth paragraph of the Results section. The corrected version published August 6, 2014.

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