Horizontal and vertical distribution of microplastics in Korean coastal

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Characterization of Natural and Affected Environments

Horizontal and vertical distribution of microplastics in Korean coastal waters Young Kyoung Song, Sang Hee HONG, Soeun Eo, Mi Jang, Gi Myung Han, Atsuhiko Isobe, and Won Joon Shim Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b04032 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018

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Environmental Science & Technology

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Horizontal and vertical distribution of microplastics in Korean coastal

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waters

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Young Kyoung Song1,2, Sang Hee Hong1,2, Soeun Eo1,2, Mi Jang1,2, Gi Myung Han1,

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Atsuhiko Isobe3, Won Joon Shim1,2*

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Korea Institute of Ocean Science and Technology, Geoje-shi 53201, South Korea

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2

Korea University of Science and Technology, Daejeon 34113, South Korea

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3

Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga-Koen, Kasuga

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816-8580, Japan

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*To whom correspondence should be addressed

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Manuscript for “Environmental Science and Technology”

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Mailing Address:

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Won Joon Shim, Ph. D

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Oil and POPs Research Group

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Korea Institute of Ocean Science and Technology

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Tel: +82-55-639-8671

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Fax: +82-639-8689

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E-mail: [email protected]

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ACS Paragon Plus Environment


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ABSTRACT:

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This is the first survey to investigate the vertical distribution and composition of

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microplastics > 20 µm at the surface (0–0.2 m; bulk sample) and in the water column (3–58

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m depth; pump) of six semi-enclosed bays and two nearshore areas of South Korea. The

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average microplastic abundance of 41 stations at all sampling depths was 871 particles/m3,

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and the microplastic abundance near urban areas (1051 particles/m3) was significantly higher

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than that near rural areas (560 particles/m3). Although the average microplastic abundances in

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the mid-column (423 particles/m3) and bottom water (394 particles/m3) were approximately 4

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times lower than that of surface water (1736 particles/m3), microplastics prevailed throughout

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the water column in concentrations of 10–2000 particles/m3. The average sizes of fragment

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and fiber type microplastics were 197 µm and 752 µm, respectively. Although the polymer

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composition differed by depth depending on the particle size and density, polypropylene and

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polyethylene predominated throughout the water column regardless of their low density and

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particle size. Finally, the middle and bottom water samples contained higher abundances of

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microplastics than predicted by a model based on physical mixing, indicating that biological

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interactions also influence the downward movement of low-density microplastics.

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Keywords: FTIR, microplastic, vertical distribution, water column

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■ INTRODUCTION

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Microplastics are widespread on beaches1,2 and in coastal waters,3-5 pristine areas,6,7 fresh

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water,8,9 and open ocean.10 The annual global plastic production has increased over the last

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six decades,11 and 4.8–12.7 million metric tons of the total 275 million metric tons of plastic

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waste are estimated to have entered the ocean.12 Once entering marine environments, plastics

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are weathered and fragmented via ultraviolet radiation and mechanical abrasion into micro-

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and nano-sized particles.13,14 Moreover, reports of adverse biological effects of plastic

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particles and fibers or additives in marine organisms are increasing.15-17

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The main plastics (49.1%; PlasticsEurope, 2017), polyethylene (PE; 0.88–0.96 g/cm3) and

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polypropylene (PP; 0.855–0.946 g/cm3), are less dense than seawater and many other

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polymers [e.g., polyvinyl chloride (PVC) and polyethylene terephthalate (PET)]; therefore,

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they can be transferred up or down the water column by vertical mixing.18-20 Vertical mixing

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has been predicted and measured in laboratory experiments according to particle density, size,

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and shape, as well as environmental factors.21,22 Floating plastics can also be vertically

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transported downward from surface water in fecal pellets from zooplankton egestion21 and

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after aggregation with organic or inorganic particles.23 Meanwhile, heavy polymers can float

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or rise via surface tension, turbulence, and eddies. However, in situ microplastic observation

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data (e.g., abundance, size, and polymer composition) are insufficient to understand the

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vertical distribution and fate of microplastics in the water column. Additionally, it is difficult

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and time-consuming to identify and confirm the polymer composition of individual particles

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using Fourier-transform infrared spectroscopy (FTIR) or Raman spectroscopy.24,25 Studies

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have reported an increasing number of polymers in microplastics, but data comparing the

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polymer composition of microplastics from the water surface and water column are

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insufficient.

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Traditional monitoring studies have focused only on surface water,26 but further studies

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have increasingly described the prevalence of microplastics at different depths.18-20 It is

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difficult to determine the microplastic abundance of seawater without knowing the abundance

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in the water column. For example, using only surface water abundance as an estimate of the

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total abundance or contamination of microplastics in the ocean could result in underestimates,

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while extrapolating the microplastic abundance of surface water to the whole water column

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could result in overestimates. Several trials have attempted to estimate the total microplastic

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abundance of seawater by integrating surface and water column abundance as predicted with 3



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physical mixing models based on surface water data.27 Several models evaluated with very

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limited data based on multi-level net tows have predicted that the average concentrations of

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buoyant microplastics are highest at the sea surface.18-20 However, such estimates have not

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been validated or limited with in situ observations. Considering that marine organisms are

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more abundant in the water column than in the upper 20 cm of surface water, their encounter

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rate with microplastics could be higher in the water column than at the surface. Additionally,

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data are insufficient for nearshore regions, which are host to high biological production that

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could result in frequent interactions between microplastics and biota.28 Therefore, it is crucial

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to determine the distribution and fate of microplastics in nearshore areas.

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Manta trawls and neuston nets are frequently used for surface water sampling, as they are

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suitable for use on vessels, have wide sampling coverage to better represent a given area, and

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are helpful for studies of zooplankton abundance.29 However, manta trawl nets with mesh

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sizes of 300–350 µm are not appropriate for smaller-sized particles (< 300 µm). Size is an

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important property of microplastic particles, and is related to their stock, weathering,

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movement, and bioavailability to marine organisms. For example, microplastic abundance

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tends to increase with decreasing size30,31 and over 90% of detected microplastics are < 300

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µm,30 which are easily ingested by marine invertebrates.32,33 Therefore, it is necessary to use

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smaller mesh sizes or other sampling methods to obtain more accurate data on microplastic

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contamination and obtain better estimates for risk assessments.

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This study focused on the vertical distribution and composition of microplastics > 20 µm

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in surface, middle, and bottom water from semi-enclosed bays and nearshore areas of urban

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and rural areas in South Korea. We tested the hypotheses that (1) microplastic abundance near

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urban areas is higher than near rural areas, (2) the abundance, polymer composition, and size

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distribution of microplastics differ according to sampling depth, and (3) the vertical

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distribution of microplastics cannot be completely predicted by physical mixing alone due to

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biological interactions.

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■ METHOD AND MATERIALS

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Microplastic sampling

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Water samples were collected in July and August of 2016 and 2017 off the coast of South

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Korea in three rural areas [Cheonsu (CS), Hampyeong (HP), and Deungnyang (DR) Bays]

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that are less urbanized or designated as Environmentally Preserved Areas, and five urban 4


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areas [Gwangyang (GY), Ulsan (US), and Yeongil (YI) Bays and Incheon (IC) and Busan

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(BS) coastal areas] that are urbanized, industrialized, or designated as Special Management

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Areas due to environmental pollution (Figure 1). Surface, mid-column (hereafter ‘middle’),

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and bottom water samples were collected at 41 sampling stations in six bays and two coastal

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areas. The average wind speed was 3.1 m/s (range: 1.5–7.2 m/s) during the sampling period.

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We collected 100 L of surface water from the top 20 cm, including the surface microlayer,30

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using a customized surface water sampler made from a stainless tray (Figure S1a). Before

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collecting samples, the surface water sampler was washed three times with in situ seawater.

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We also collected 100 L of middle and bottom water using a submersible water pump (PD-

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272, Wilo; flow rate: 140 L/min) after draining water for the first 10 s at each station (Figure

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S1b). The 100-L surface, middle, and bottom seawater samples were filtered through portable

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hand nets (20-µm mesh) on board the vessel (Figure S1c, d). Volume-reduced samples in cod-

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end buckets were transferred to 1-L amber glass bottles, and the top of the bottle was first

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covered with aluminum foil before capping to avoid contamination from the plastic caps.

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Before sampling, all glass bottles were washed and thermally treated in a furnace at 450°C

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for 8 h. The depth of the middle sampling point was determined as the intermediate point of

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the water depth at each station; however, in instances of thermoclines identified with a

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conductivity, temperature, and depth system, the middle point was set to the mid-point of the

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thermocline. The bottom water was collected approximately 1 m above the ocean floor. The

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sampling depths of the middle and bottom layers were 3–27 m (average: 9.2 ± 6.3 m, n = 41)

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and 5–58 m (average: 19 ± 13 m, n = 41), respectively (Table S1).

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Sample pretreatment and identification

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The 20-µm sieves were washed thoroughly before use via sonication and an air gun to

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avoid cross-contamination. Volume-reduced samples were filtered through 20-µm metal

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sieves to remove water and washed with high-performance liquid chromatography-grade pure

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water to remove salts. Any solids remaining in sieves were transferred to a pre-weighed glass

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beaker and dried for 24 h at 60°C in a drying oven. All organic matter in the beaker was

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weighed and removed via wet peroxide oxidation with 35% H2O2 and Fe(II) solution at 75°C

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(NOAA Technical Memorandum NOS-OR&R-48) (Figure S1e). After removing organic

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matter, 6 g of NaCl was added per 20 mL of sample and mixed. The samples were transferred

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to a glass funnel for density separation (Figure S1f). After 24 h, the settled particles were

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drained and the supernatant was filtered through 5-µm filter paper (polycarbonate membrane 5



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filter paper; 47 mm Ø; Millipore). Filters were dried at room temperature and stored in glass

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Petri dishes. Microplastic particles on the filter paper were simultaneously identified and

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counted using a µFTIR microscope (Thermo Nicolet Continµum 6700; Thermo Scientific,

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Waltham, MA, USA), which was used to determine the structures of molecules based on their

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characteristic infrared radiation absorption, with the microscope used to identify smaller

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particles. Microplastic particles were counted in 25–100% of the total filter area. The

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analytical results for µFTIR microscope, blank samples and statistical analyses are provided

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in the Supporting Information.

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Vertical profile of microplastics

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The vertical distribution of microplastics depends on oceanic turbulence induced by wind

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and waves.18 Therefore, we assumed that the vertical profile of the particle counts per unit of

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seawater volume (hereafter ‘abundance’) could be calculated based on the formula of

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Kukulka et al. (2012) 18:

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= , (1)

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where z is the depth measured downward from the sea surface, N denotes the microplastic

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abundance normalized to the abundance at the sea surface (i.e., N = 1.0 at z = 0), and w is the

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rise velocity of microplastics. Based on laboratory experiments, Reisser et al.19 described this

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as:

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= 0.002 , (2)

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where the size and rise velocity are in units of mm and m/s, respectively.

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Meanwhile, A0 in Eq. (1) is the vertical diffusivity, calculated as:

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= 1.5∗ , (3)

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where u* denotes the frictional velocity given as u* = 0.0012 × U10, and U10 is the wind

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speed at a height of 10 m above the sea surface (i.e., the wind speed measured at

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approximately 7 m on the research vessel was used instead of U10 under the assumption that

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the difference in wind speed from that at a 10-m altitude was negligible); k is the von Karman

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constant (= 0.4); and Hs is the significant wave height, which was not observed in the present

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survey. Therefore, we computed multiple vertical profiles using possible Hs values ranging

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from 0 to 3 m as observed in the field.

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■ RESULTS

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Regional microplastic distribution

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Microplastics were detected in all regions and stations (Table S1). The average 6


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microplastic abundances of the surface, middle, and bottom waters were representative of

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each bay or coastal area. The mean abundances in the rural areas were 448 ± 237 (CS), 644 ±

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82 (HP), and 588 ± 173 (DR) particles/m3, respectively, while those in the urban areas were

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2000 ± 385 (IC), 1089 ± 283 (GY), 554 ± 273 (BS), 764 ± 304 (US), and 948 ± 103 (YI)

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particles/m3. The mean abundance of total samples (n = 123) was 871 ± 979 particles/m3. The

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mean abundance of microplastics in urban areas (1051 ± 571 particles/m3) was significantly

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(p < 0.05) higher than that of rural areas (560 ± 184 particles/m3). Finally, there was a strong

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correlation between the population of the catchments of the sampled bays and coastal area

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(Table S2) and the average microplastic abundance at the surface (excluding middle and

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bottom), except for the BS coastal area (Spearman’s rank, r = 0.857 and 0.893, p < 0.05, n = 7)

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(Table S3).

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Microplastic distribution by depth

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The microplastic abundances in the middle (423 ± 342 particles/m3) and bottom (394 ±

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443 particles/m3) waters were significantly lower (p < 0.05) than that of the surface water

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(1736 ± 1179 particles/m3) in almost all study areas (Table S1 and Figure S2). As two

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exceptions, the microplastic abundances in the middle water of one station in GY Bay located

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in the inner bay near the harbor (980 particles/m3) and in the bottom water of one station in

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the BS coastal area about 7.6 km from the Nakdong River (1340 particles/m3) were higher

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than those at the surface (620 and 1120 particles/m3, respectively). The microplastic

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abundances of the middle and bottom waters were similar. The abundance was higher in the

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middle water than bottom water at 21 stations, and higher in the bottom water than middle

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water at 20 stations. Overall, microplastics were found throughout the water column within

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the range of 10–2000 particles/m3.

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The total number of microplastics retained in water at each study area was estimated from

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the total area (Atotal; km2) and average depth (Ds_aver) obtained within a lattice based on the

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sampling stations in the KIOST Underway Meteorological and Oceanographic System (Table

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S4). The total abundances of microplastics in each study area in the surface (Ms_total) and

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water column (middle and bottom; Mc_total) were calculated as:

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Ms_total = Atotal × 0.2 × Ms_aver × 106 (Eq. 4)

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Mc_total = Atotal × (Ds_aver − 0.2) × Mc_aver × 106 (Eq. 5)

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The total numbers of microplastic residues in the whole water column at each study area are given in Table S4.

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Microplastic size, shape, and composition

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The detected microplastics were separated by shape into non-fibers (i.e., fragments,

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spheres, and film) and fibers, and fractionated by size. The size distribution was determined

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as the summed abundance of all stations (Figure 2), where the average sizes of non-fibers and

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fibers were 197 ± 168 µm and 752 ± 711 µm, respectively. Non-fibers and fibers had

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different size distributions. For example, non-fiber microplastics < 300 µm accounted for

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86%, and the peak size distribution was observed in the 100–150-µm range. Among fibers,

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the < 300-µm fraction only accounted for 30% of particles, while the peak was observed in

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the 1000–2000-µm fraction, followed by the 200–250-µm fraction. This implies that, unlike

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non-fibers, fibers can readily be collected using neuston nets due to their larger size.

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Fragments were the dominant shape, accounting for an average of 81% of the particles at

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all stations, which was followed by fibers (average abundance: 18%). In contrast, spheres and

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films accounted for only 1%. The shape composition profiles were similar at the surface,

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middle, and bottom, where fragments accounted for 79%, 81%, and 82%, respectively,

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followed by fibers, which accounted for 21%, 18%, and 16%, respectively.

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In total, 22 synthetic polymers were detected in the non-fiber-type microplastics in

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seawater, including PP/poly(ethylene:propylene), PE, EVA, polybutyl methacrylate (PBMA),

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polymethyl methacrylate (PMMA), PET/polyester, poly(acrylate:styrene), acrylic polymer,

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PS, and others (e.g., polyurethane (PU), nylon, PVC, polyvinyl acetate/polyvinyl

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chloride:vinyl acetate, polybutadiene acrylonitrile, alkyd, cellulose acetate/nitrocellulose,

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acrylonitrile butadiene styrene, polyoxymethylene, polydimethylsiloxane, polycarbonate, and

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epoxy/phenoxy, poly(vinyl alcohol)). Among them, PP, PE, and EVA were categorized as

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low-density (LD) polymers, and the others were categorized as high-density (HD) polymers

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based on their theoretical densities compared with that of seawater. Notably, expanded

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polystyrene is a particularly LD polymer, distinguishing it from PS. It should also be noted

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that EVA has a broad density range that depends on its vinyl acetate content; in this study, it

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was defined as LD polymer, because five of six EVA samples were lighter than seawater

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based on the density measurements. LD polymers were the predominant polymer type in

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seawater, where PP accounted for an average of 41 ± 17% of samples, followed by PE (21 ±

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15%) and EVA (19 ± 20%) (Table S5). Moreover, LD polymers were found throughout the 8


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water column.

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In total, eight types of polymer were detected in fiber-type particles: PP, PE, polyester,

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EVA, poly(vinyl chloride:vinyl acetate), PU, PS, and nylon. Among them, PP was

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predominant in all regions and depths, accounting for an average of 92 ± 10% of particles,

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followed by polyester (4.7 ± 7.7%).

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Microplastic size and distribution by polymer type

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Figure 3 presents the PCA results for the size difference of non-fiber microplastics

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between the urban and rural areas. The normalized abundances of four size groups (20–100,

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100–300, 300–500 and > 500 µm) of the 123 samples were used as the PCA input data. The

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first two principal components (PC1 and PC2) accounted for 38.9% and 29.0% of the

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variance, respectively. Because the size distributions of non-fiber microplastics in this study

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were similar among all regions, the samples were not strongly separated from each other in

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the score plot (Figure 3). However, some of the urban samples were clustered more toward

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the left side, while the rural samples were clustered closer to the upper side of the score plot.

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The urban samples were spread within the 20–100-µm fraction of the loading plot, while the

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rural samples were skewed toward larger size fractions (300–500 and > 500 µm) of the

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loading plot. However, both the urban and rural samples were distributed well in the 100–

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300-µm fraction of the loading plot. This implied that 20–100-µm sized microplastics were

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more prevalent in urban areas than rural areas, although the 100–300-µm fraction

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predominated in all regions regardless of the degree of microplastic contamination.

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The normalized abundance of four polymer types (PE, PP, EVA, and HD polymers for

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non-fiber particles) of the 123 samples were used as PCA input data to evaluate the

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distribution of particles by size and depth (Figure 4). PC1 and PC2 accounted for 35.4% and

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32.6% of the variance, respectively. The distributions of the size fractions within the score

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plot could be explained by the distribution of polymers in the loading plot. The samples were

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separated by PC1 in the order (from left to right) 20–100, 100–300, and > 300 µm (Figure 4a).

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This size distribution was determined by polymer type, and suggested that particles < 100 µm

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tended to be composed of HD polymers in the water column, while those > 100 µm were

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predominantly composed of PE and PP from the loading plot. The samples were not clearly

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separated in the score plot by depth (Figure 4b); however, surface samples on the bottom of

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the score plot were separated from the middle and bottom water samples along the vertical

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line and upper side of the score plot. This implies that HD polymers tended to be distributed 9



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in the water column rather than in surface water, while PE and PP prevailed throughout the

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whole water column regardless of depth.

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Physical mixing model prediction

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It has been hypothesized that pelagic microplastic abundance increases in deeper layers

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with enhanced wind-induced turbulence, regardless of size (Figure S3). Hereafter, for ease of

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interpretation, all microplastics were categorized according to their size (δ): δ < 0.3 mm, 0.3
30 m) also depended on wind speed.

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The black and red lines in Figure S3 are superimposed over the vertical profiles calculated

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with Eq. (1) (dotted curves) in Figure 5. In the present analysis, we used 4.2 (2.2) m/s for

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U10, which was the average wind speed measured in each survey with wind speeds higher

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(lower) than 3 m/s. Multiple curves were created by substituting different wave heights

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(which were not observed in field surveys), which were unlikely to be higher than 3 m. The

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average abundance followed the dotted curves in the uppermost layers (shallower than 5 m).

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However, in all panels, the average abundance deviated from the dotted curves immediately

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below the uppermost layer.

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■ DISCUSSION Horizontal distribution of microplastics 10


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The microplastic abundance in urban areas was significantly (p < 0.05) higher and about

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twice that in rural areas. This supports the hypothesis that urban or industrialized areas

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discharge more microplastics than rural areas into coastal environments. A previous study

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found a significant correlation between surface plastic concentration and coastal human

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activity,3 supporting the results of this study. Coastal areas are adjacent not only to populated

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areas, but also rivers, which are considered to be one of the most important input pathways

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for plastics into the ocean.34 Additionally, nearshore coastal zones are rich in marine life,

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supporting frequent interactions between microplastics and biota, such as ingestion, trophic

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transfer, and sinking via biofouling and fecal pellets.28,35 However, there is a lack of data on

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coastal areas, and few studies3-5 have investigated the abundance, spatial distribution, and

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composition of microplastics. In contrast, many studies have sampled microplastics in

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accumulation zones (e.g., gyres) to monitor contamination by small microplastics, and such

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data have been used to model and predict their spatial distribution. Overall, coastal areas are

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important sites of microplastic behavior and sources; therefore, investigations of

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microplastics in coastal areas are necessary.

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As expected, the microplastic abundance in surface water was significantly (p < 0.05) and

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about 4.1 and 4.4 times higher than that in middle and bottom water, respectively. Regardless,

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microplastics were found throughout the water column, with an average abundance of 418 ±

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393 particles/m3 (n = 82). This was much higher than the abundances found in Mediterranean

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coastal water (mean: 1.00 ± 1.84 particles/m3, maximum: 11.3 particles/m3),4 Hong Kong

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(mean: 0.05–27.9 particles/m3),36 Qatar’s exclusive economic zone (mean: 0.71 particles/m3,

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range: 0–3 particles/m3),37 and Arctic polar sub-surface water (mean: 2.68 ± 2.95 particles/m3,

329

range: 0–11.5 particles/m3).31 Even though these reported levels included microplastics
5 m) might have been

349

larger than that above the uppermost layer. This suggests that pelagic microplastics below the

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uppermost layer are older than those in the uppermost layer and have undergone biofouling

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processes.

352

The total number of microplastics in the surface water and the water column were

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calculated using Eqs. (4) and (5). The results showed that there were 2–58 (20 ± 19) times

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more particles in the water column than in surface water. These results demonstrate that a

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large proportion of microplastics are sequestered in the water column of coastal zones, where

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biological activity is prevalent. This is caused by vertical movement due to biological

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interactions such as biofouling, aggregation, and egestion, as fecal pellets facilitate sinking

358

and suspension of light microplastics in the water column and bottom water. For example,

359

biofilm formation, colonization, and accumulation by microorganisms on submerged surfaces

360

affects surface hydrophobicity and decreases plastic buoyancy, increasing the sinking rate.41-

361

44

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with waste organic matter in fecal pellets.21 Additionally, homo- or hetero-aggregation can

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enhance the sinking rate due to the production of sticky microgels by microbes. Thus,

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microplastic particles likely aggregate with one another or with live/dead plankton and

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mineral particles to form marine snow (i.e., organic aggregates > 200 µm in size).45 This, in

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turn, could increase the sinking rate, leading to the detection of even LD particles (e.g., PE

367

and PP) in the water column and sediment.46,47 Because microplastics are not limited to the

368

surface layer, monitoring of microplastics throughout the water column is necessary to

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monitor microplastic contamination of sea areas and reduce over- or underestimations.

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Further laboratory and field studies are urgently required to evaluate the relative contributions

Moreover, microplastics consumed and ingested by microorganisms are later egested along

12


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of various biological activities and interactions with microplastics to their vertical movement

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and sequestration in the water column.

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The results of the in situ observations in this study showed that the middle and bottom

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water samples contained higher abundances of microplastics than those predicted by a

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physical mixing model. Based on model results, Eriksen et al.48 estimated that at least 5.25

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trillion plastic particles (0.33 to > 200 mm) were likely to be found in global oceans (North

377

Pacific, North Atlantic, South Pacific, South Atlantic, Indian Ocean, and Mediterranean Sea).

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However, from the in situ data, we identified 13 trillion plastic particles (0.02–5 mm) in the

379

coastal area of South Korea alone (2007 km2). An average of 1.63 trillion microplastics was

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detected from six bays and two coastal areas in this study. This difference between the model

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prediction and in situ observation data was caused, in part, by a lack of empirical data and

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understanding of biological processes of microplastic particles in the water column, impeding

383

the determination of microplastic abundances in the ocean. These results imply that

384

biological interactions may significantly contribute to the downward movement of

385

microplastics. This difference might also have been caused by the microplastic size range for

386

sampling and analysis. Considering only the overlapping size range of microplastics in both

387

studies of 0.33–4.75 mm, there are likely to be 4.85 trillion particles in the global ocean48 and

388

3.13 trillion particles in the Korean coastal area. The number of microplastic particles in

389

Korean coastal waters alone is comparable to that predicted for the global ocean from the

390

model. Finally, the total number of plastics in water using in situ data could be overestimated,

391

because the total microplastic number was estimated in this study using a small number of

392

samples without spatial variation in each study area. However, the model estimations were

393

calibrated against available data to help fill gaps and generate an accurate estimate.

394

Composition of microplastics

395

We identified 22 synthetic polymers in seawater by FTIR based on the measurement of

396

thousands of particles on filter papers (n = 4860), among which PP, PE, and EVA were the

397

dominant polymer types. In 2016, PP and PE were in the greatest demand and the most used

398

polymer types,11 and they have been the most commonly found polymers in seawater in many

399

other studies.5,7,39 EVA is a copolymer of ethylene and vinyl acetate used widely as a

400

thermoplastic in elastomeric materials; however, it has rarely been reported in marine

401

environments. It is used in melt adhesives and coatings,49 as the encapsulant (or pottant)

402

material in photovoltaic modules,50 in agricultural films,51 and in expanded or foam rubber 13



403

used as equipment for various sports. Thus, EVA in marine environments is likely derived

404

from land-based sources. The EVA particles detected in seawater were fragile to even weak

405

forces, and could be readily weathered during transport into the ocean, which could explain

406

the high abundance of EVA particles (19% of non-fiber particles) in this study.

407

Among fiber particles, PP was the predominant polymer type in all regions, accounting

408

for 92% of particles. PP found as irregular white fibers was likely derived from rope debris,

409

such as aquaculture buoys, fishing nets, and ship ropes, originating from ocean activity

410

around Korea.52

411

Importance of microplastic size

412

Sampling was conducted using 20-µm mesh nets, which is close to the theoretical size

413

detection limit of FTIR spectroscopy. In this study, particles < 300 µm accounted for 86% of

414

all non-fiber particles. Among fiber particles, 71% of particles > 300 µm could be collected

415

by neuston nets. The non-fiber/fiber particle ratio differed by sampling method. For example,

416

the non-fiber/fiber ratio of samples collected with a manta trawler with a 330-µm mesh net

417

(surface water samples at 41 stations; data not shown) was lower (2.3) than that of samples

418

collected with 20-µm mesh hand nets in this study (7.8) at the same sampling stations. This

419

implies that the abundance of fragmented microplastics could be underestimated due to the

420

collection of only 14% of the total abundance of particles in the seawater because of the

421

manta net mesh size. Most studies have emphasized the importance of using mesh nets < 300

422

µm or pumps for sampling to identify small microplastics.4,31,36-40 Overall, the size detection

423

limit during sampling and analysis plays an important role in the accurate evaluation of

424

abundance and contamination of microplastics.

425

Non-fiber particles 100–300 µm in size prevailed at all stations, although the smallest

426

fraction (20–100 µm) was more prevalent in urban than rural areas (Figure 3). Moreover, this

427

size fraction tended to comprise HD polymers and was found in the water column rather than

428

floating on the surface (Figure 4), suggesting that smaller particles or plastics that are already

429

weathered and fragmented enter the ocean via sewage and rivers from urban areas.

430

Additionally, fragile polymer types that are vulnerable to weathering and fragmentation (e.g.,

431

co-polymers) are used more frequently in urban areas than rural areas due to differences in

432

the sources of microplastics from urban and rural areas. For example, a greater variety of

433

polymer types and complex polymers may be used in urban and industrialized areas, while

434

simple polymers (e.g., PP and PE) are more likely to be used in rural areas. However, 14


Page 14 of 26

Page 15 of 26

435


additional data are required to verify the sources of microplastics by polymer type.

436

Additionally, the particle size and vertical distribution were determined by polymer type.

437

For example, the middle and bottom layers tended to contain HD polymers < 100 µm, while

438

PE and PP were the dominant polymer types for particles > 100 µm and prevailed throughout

439

the water column regardless of depth. The rise and sink velocities depend on polymer density,

440

size, and shape.40,53 For example, small microplastic particles have shorter residence times in

441

the surface layer and increased vertical distribution with decreasing size.40 We found similar

442

trends, in that small HD polymers were vertically distributed and detected throughout the

443

water column. Finally, PE and PP were the predominant polymer types throughout the water

444

column, despite their low density. This can be explained by the fact that PE and PP are the

445

most highly used polymer types, resulting in the constant supply of PE and PP microplastics

446

from land- or marine-based sources to the ocean and continuous vertical transport throughout

447

the water column due to sinking and rising.

448 449

■ ACKNOWLEDGMENTS

450

This study was supported by the Ministry of Oceans and Fisheries, Korea, under the research

451

project titled “Environmental Risk Assessment of Microplastics in the Marine Environment.”

452

AI was supported by the Environment Research and Technology Development Fund (SII-2)

453

of the Ministry of the Environment, Japan.

454 455

■ ASSOCIATED CONTENT

456

Supporting Information

457 458

The analytical results of the µFTIR microscope, blank sample analysis and statistical analyses are presented in the Supporting Information.

459

Tables describing the (1) abundance of microplastics and depth, (2) population and

460

abundance of microplastics, (3) correlation between population and microplastics, (4) coastal

461

area and average depth, and (5) composition of microplastics. Figures describing the (1) the

462

water sampler and sample treatment via images, (2) vertical distribution, and (3) vertical

463

distribution by size and wind speed.

464 465

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618 619 620 621

Figure 1. Map of the locations of the surface, middle, and bottom water microplastic sampling stations in three rural (dotted line; CS, HP, and DR) and five urban (solid line; IC, GY, BS, US, and YI) areas.

21



622 623

Figure 2. Size distribution of total (a) non-fiber and (b) fiber microplastics.

624

22


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625 626 627 628 629

Figure 3. PCA score and loading plots of the distribution of microplastics in urban and rural sites. The size of non-fiber particles was categorized into fractions of 20–100, 100–300, 300–500, and > 500 µm.

630

23



631 632 633 634

Figure 4. PCA score and loading plots of (a) microplastic size (20–100, 100–300, and > 300 µm) and (b) sampling depth (surface, middle, and bottom). Four polymer types were considered (PE, PP, EVA, and HD polymers).

635

24


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636 637 638 639 640 641 642 643 644 645

Figure. 5 Average microplastic abundance (particle count per unit seawater volume; see Fig. S2) normalized to the abundance at the surface layer superimposed over the exponential curves calculated with Eq. (1) (dotted curves). Cases with wind speeds higher (lower) than 3 m/s are shown in the left (right) panels. The upper, middle, and lower panels show microplastics with sizes of < 0.3 mm, 0.3~0.5 mm, and 0.5~1.0 mm, respectively. The dotted curves are shown for significant wave heights (Hs; see numerals in the upper left panel) of 0.5, 1.0, 1.5, 2.0, 2.5, and 3 m.

646 647

25




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Horizontal and vertical distribution of microplastics in Korean coastal

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