REVIEW Progress on microplastics research in the Yellow Sea, China 1

: Marine microplastics are a global problem and are causing considerable concern. As the largest marginal sea of the Western Pacific, the Yellow Sea is surrounded by China and the Korean Peninsula, and its coastal ecosystem is greatly affected by human activities. This article reviews the progress of microplastics research in the Yellow Sea in China, including studies on surface water, the seawater column, sediments, and marine organisms. The results indicate that plastic debris exists throughout the west Yellow Sea, with higher abundance of microplastics in water columns and sediments in the north part than those in the south part. Fibers < 1 mm and transparent-colored particles dominated the samples collected. Polyethylene (PE), polypropylene (PP), and cellophane (CP) were the dominant debris types. The wide distribution of microplastics in the environment also results in animal ingestion. Sea cucumbers, accordingly, ingest more microplastic debris than other biologic taxa (zooplankton, shellfish, and fish) that have a bearing on their surrounding environment. By providing basic environmental assessment data regarding the Yellow Sea, this paper demonstrates that actions should be taken to reduce the consumption and emission of plastics into the environment.


Introduction
Microplastics are plastic particles <5 mm in diameter, and the occurrence of microplastics in the ocean has become a growing global concern over the last few decades. The existence of microplastics in the ocean was first reported by Buchanan (1971). Carpenter and Smith (1972) found that plastics were widespread throughout the western Sargasso Sea and recorded an average of 3500 pieces and 290 g per square kilometer one year later. To date, microplastics have been studied in freshwater, seawater, inland and coastal areas, rivers, estuaries (K. , nearshore and open ocean regions, at the surface, and in sediment (Gago et al. 2018). The sources of microplastics in the ocean are extensive, and most microplastics originate from personal care products, such as facial cleansers and cosmetics (Zitko and Hanlon 1991), drug delivery products (Patel et al. 2009), virgin plastic

Concentration and characterization of microplastics in the seawater
According to the reviewed papers, the units used to describe the results of microplastics at the seawater surface are the number of microplastics per m 3 . The concentrations of microplastics in the seawater of the Yellow Sea varied in different districts. The mean concentration of microplastics collected by a net with a mesh size of 500 μm throughout the west Yellow Sea was 0.13 ± 0.20 pieces/m 3 , ranging from 0.00 to 0.81 pieces/m 3 (Sun et al. 2018a). The microplastics concentration at the sea surface in the north wing of the Yangtze River Delta, which were sampled by a net with 333 μm mesh size, was 0.330 ± 0.278 items/m 3 , and ranged from 0.117 to 0.506 items/m 3 (T. . This concentration is similar to that throughout the west Yellow Sea. The abundance of microplastics detected by a net with 333 μm mesh in the north Yellow Sea varied from 3 to 19 particles/100 m 3 or 0.012-0.231 mg/m 3 (Mai et al. 2018). Surprisingly, the average abundance of microplastics in the surface seawater of the north Yellow Sea was 545 ± 282 items/m 3 ); this value is approximately 4500 times higher than that reported by Mai et al. (2018). The cause of this phenomenon is that the samples were collected with a small-mesh size of Niskin water sampler with a 30 μm steel sieve.
The hydrological conditions and the developments of coastal cities, marine fisheries, and shipping were found to differ in four coastal areas. The abundance of microplastics was 0.33 ± 0.34 particles/m 3 (Zhang et al. 2017) or 7-162 particles per 100 m 3 (0.012-2.96 mg/m 3 ) (Mai et al. 2018) in the seawater of the Bohai Sea, 0.31 pieces/m 3  or 0.167 ± 0.138 n/m 3 (Zhao et al. 2014) in the East China Sea, and 0.045 ± 0.093 particles/m 3 (Cai et al. 2018) in the South China Sea. All the samples were collected by a net with a mesh size of 333 μm, except these collected by Liu et al. (2018), who trawled for samples with a 500 μm mesh net. Compared to the other three areas, the abundance of microplastics in the seawater of the Yellow Sea is moderate.
Most of these present studies showed that the sizes of microplastics ranged from 0.025 to 5 mm, and these studies usually divided these sizes into groups: <0.5, 0.5-1, 1-2, 2-3, 3-4, and 4-5 mm Zhu et al. 2018). In the seawater, fibers and films were the main shapes in the north wing of the Yangtze River Delta (T. ) and the northwest Yellow Sea , constituting 75.4% and 97.2%, respectively. However, fragments were the most popular shapes throughout the west Yellow Sea, accounting for 42% (Sun et al. 2018a). Transparent and colored microplastics accounted for a majority of the microplastics in the Yellow Sea, followed by black and white microplastics. In previous studies, marine debris was commonly examined by the naked eye (Morét-Ferguson et al. 2010) or by using a dissection microscope (Doyle et al. 2011), which led to potentially high error rates, especially for smaller particles. The most common compositions of plastic were PE and PP, with PET accounting for a small portion. We were pleasantly surprised that all the marine debris in the West Yellow Sea was analyzed by Fourier transform infrared (FTIR) spectroscopy, including ATR-FTIR, micro-FTIR, and μ-FTIR, all of which are reliable for microplastic analysis. FTIR microspectroscopy has been the most commonly used technology for microplastic testing in recent studies (Gago et al. 2018). In addition to FTIR spectroscopy, several techniques have been used to analyze the chemical compositions of microplastics, including carbon, hydrogen, and nitrogen (CHN) analysis (Morét-Ferguson et al. 2010), scanning electron microscopy (SEM) (Woodall et al. 2015), Raman spectroscopy (Lenz et al. 2015), and pyrolysis-gas chromatography (Fischer and Scholz-Böttcher 2017). In comparison, FTIR spectroscopy is the most commonly used method for microplastic analysis because the chemical composition of plastics can be determined, and the original source of the debris can be traced. One of the deficiencies in FTIR spectroscopy technique was that it is time consuming, as the microdebris samples are analyzed one at a time; the other deficiency is that only a small part of the surface morphology of plastic particle can be analyzed. Nevertheless, FTIR has been extensively used in recent studies.

Concentration and characterization of microplastics in the sediments
The units that have been used to express the abundance of microplastics in sediments are diverse. Microplastics in the sediments of the south Yellow Sea ranged from 2.0 to 7.0 items/50 g of dry weight sediment, and that of the northwest Yellow Sea varied from 4.0 to 14.0 items/50 g of dry weight sediment , which is twice the abundance of microplastics in the southwest Yellow Sea. The north Yellow Sea is surrounded by the Liaodong Peninsula, Shandong Peninsula, and Korean Peninsula. Thus, human activities have a great impact on microplastic pollution in this area. The Bohai Sea has a higher concentration of microplastics than the Yellow Sea, and due to the water exchange between the Bohai Sea and the north Yellow Sea, the concentration of microplastics in the north Yellow Sea is higher than that in the other areas of the Yellow Sea.
The results of research on microplastics by different teams in the same area are distinct. One team researched the microplastics in the sediments of the north Yellow Sea, and the results proved that the mean concentration of microplastics was 37.1 ± 42.7 items/kg of dry weight sediment , which is much less than that measured by Zhao et al. (2018). T.  verified that the concentration of microplastics in the north wing of the Yangtze River Delta was 2.58 ± 1.14 items/g of dry weight sediment, ranging from 1.190 to 4.920 items/g of dry weight sediment. The concentrations of microplastics in the north Yellow Sea were higher than those in the south part, and the coastline areas were found to possess a large number of microplastics. The abundance of microplastics in the sediments was 171.8 items/kg of dry weight sediment  in the Bohai Sea, and this value was more than 250 items/50 g sediment (Qiu et al. 2015) in the South China Sea, which was higher than that in the Yellow Sea.
The shapes of marine particles in beach sediments were distinct from those in the seabed sediments. Pellets and foams were the main shapes in the intertidal zone (H. Zhou et al. 2018), and fiber was the primary shape in the seabed sediments (T. Zhao et al. 2018;Zhu et al. 2018). The colors of the microplastics in the sediments were dominated by transparent and colored items, which was similar to the pattern observed in the surface seawater. PE and PP were the main chemical compositions of microplastics in the sediments, followed by PS, PET, and nylon.

Concentration and characterization of microplastics in marine biota
The Yellow Sea possesses many fishing grounds, such as the Yanwei, Shidao, and Lvsi fisheries, which are rich in resources. The Yellow Sea is also an important aquaculture region, marine products mainly comprise fishes, mussels, oysters, and sea cucumbers. The concentration of microplastics in marine biota has been depicted by the unit item per g, item per individual, or item per m 3 (combined with the zooplankton abundances). In wild Mytilus edulis, the mean abundance of microplastics was 2.7 items/g or 4.6 items/individual, and this value was 1.6 items/g or 3.3 items/individual in farmed mussels (Li et al. 2016). The abundance of microplastics was higher in mussels from wild areas than in those from farmed areas. The numbers of microplastics in Mytilus galloprovincialis varied from 1.9 to 9.6 items/individual and from 2.0 to 12.8 items/g (Ding et al. 2018). The average abundance was similar to the published data of 2.7 items/g or 4.6 items/individual. However, the result showed that farmed M. galloprovincialis experienced more plastic contamination than the wild specimens, completely contrary to the previous study, which were reported by Li et al. (2016). Qu et al. (2018) demonstrated a positive linear relationship between the microplastic level in wild mussels and that in the waters. Oysters are important commercial species, and as keystone organisms for environmental quality monitoring, they can provide valuable ecosystem feedback. Teng et al. (2019) investigated the oysters along China's coastline; they chose the oysters from Qingdao and Lianyungang as representative samples from the Yellow Sea. The abundance in the samples from Qingdao was 0.42 items/g or 1.96 items/ individual and that in the samples from Lianyungang was 0.25 items/g or 2.63 items/individual. The mean value of microplastics in oysters from the Yellow Sea was close to that in oysters from the Bohai Sea, and much lower than those in oysters from the East China Sea and the South China Sea. The average number of microplastics ingested by cultured sea cucumbers along the Shandong Province near the Yellow Sea was 0.39 pieces/g or 12.92 pieces/individual, which was higher than that in sea cucumbers in the Bohai Sea (Mohsen et al. 2019). Additionally, 19 species of fish were collected with a bottom trawl; 34% of fish contained plastics, with 0.41 pieces/fish on average. The highest percentage of microplastics/fish was detected in the yellow croakers (Larimichthys polyactis), and the fish with the lowest percentage of plastics were Pampus argenteus and Psenopsis anomala . The distribution of fish containing plastics in the south and north Yellow Sea was higher than that in the middle part. This study reveals that the abundance of microplastics in fish is impacted by regions and species.
As grazers, zooplankton provide an energy pathway from the primary trophic level to predators. Eleven zooplankton taxonomic groups, including Stomatopoda, fish larvae, Medusozoa, Siphonophorea, Chaetognatha, Luciferidae, Brachyura larvae, Copepoda, Euphausiacea, Amphipoda, and Thaliacea, were analyzed in the laboratory after transfer from the ocean. Zooplankton were sampled with Bongo nets (500 μm mesh size). The results indicated that microplastics were detected in each group of the 11 zooplankton communities. The number of microplastics varied in the different zooplankton groups; Thaliacea retained the fewest number of microplastics with 0.07 MP/zooplankton, and the number of microplastics in the larger stomatopods was 1.17 MP/zooplankton, and this group contained the most microplastics. The number of microplastics ranged from 0 to 151.34 pieces/m 3 with an average of 12.24 ± 25.70 pieces/m 3 after the bioaccumulated concentrations were combined with the zooplankton taxa abundances (Sun et al. 2018a). The abundance of microplastics ingested by the zooplankton (sampled by Bongo nets with a mesh size of 500 μm) in the East China Sea was 19.7 ± 22.4 pieces/m 3 (Sun et al. 2018b). In the South China Sea, the abundance of microplastics was 4.1 pieces/m 3 and 131.5 pieces/m 3 for different nets (500 and 160 μm in size, respectively) (Sun et al. 2017). The zooplankton sampled in the East China Sea contained more microplastics than those in the Yellow Sea and the South China Sea due to the large abundance of zooplankton taxa in the East China Sea.
However, in contrast to the microplastics in sediments, fibers were the most popular microplastic shape in the marine biota, including shellfish, sea cucumbers, 11 zooplankton communities, and 19 species of fish (Li et al. 2016;Ding et al. 2018;Sun et al. 2018aSun et al. , 2019Mohsen et al. 2019;Teng et al. 2019). The colors of the microplastics ingested by marine organisms were not reported except for those in the wild M. galloprovincialis, which were described to have a "variety of colors" (Ding et al. 2018;Mohsen et al. 2019). The microplastics ingested by the zooplankton in the west Yellow Sea were mainly composed of organic oxidation polymers, PP, and PE. The main composition of the marine debris in the mussels, M. edulis and M. galloprovincialis, was confirmed to be cellophane (CP). Among all the reviewed papers, only five papers classified CP as the main chemical composition of the collected particles (Li et al. 2016;Ding et al. 2018;Mohsen et al. 2019;Teng et al. 2019). A few articles on microplastics worldwide claimed that CP was the dominant component of microplastics (Castillo et al. 2016;Jabeen et al. 2017;Gago et al. 2018). There are many different types of plastics, and they can be grouped into two main polymer families: thermoplastics (acrylonitrile butadiene styrene (ABS), polycarbonate (PC), PE, PET, PVC, polymethyl methacrylate (PMMA), PP, PS, expanded polystyrene (EPS)) and thermosetting polymers (epoxide (EP), phenol-formaldehyde (PF), PUR, polytetrafluoroethylene (PTFE), unsaturated polyester resins (UP)) (PlasticsEurope n.d.). Apparently, CP is outside the range of plastic. It remains to be determined whether CP should be cataloged as plastic. Only one review article focused on persistent organic pollutants (POPs); this article focused on plastic resin pellets in sandy beach sediment (Zhang et al. 2015) and showed that industrial products, agricultural activity, and the use of fossil fuel around the area could impact the effect of microplastics on the absorption of POPs in the environment.
In summary, microplastics are everywhere in the Yellow Sea. Compared to the Bohai Sea, the East China Sea, and the South China Sea, plastic waste pollution in the Yellow Sea is at a moderate level. According to the surveys, there has been an upward trend in the abundance of plastic pollution in this region over time. Moreover, sediments in the Yellow Sea have the highest rate of contamination by microplastics in relation to the water and marine biota. Clearly, most microplastics settle to the bottom of the ocean. Sea cucumbers, accordingly, ingest more microplastic debris than other biologic taxa (zooplankton, shellfish, and fish) that have a bearing on their surrounding environment. The fact that the seafood contains microplastics reminds us that microplastics contamination poses a risk to food safety.

Prospect
In the past few years, comprehensive studies on microplastics, including studies on surface seawater, sediment, mussels, zooplankton taxa, and fish, have been performed in the Yellow Sea. However, some issues regarding the microplastics in the environment remain to be solved. The following issues should be considered for further research in the Yellow Sea.

1.
Methodology. The current microplastics collection methods differ, leading to a difficulty in comparing the spatiotemporal distribution of microplastics.

2.
Ecotoxicology. To the best of our knowledge, microplastics in the environment can absorb the POPs around them. Whether these carriers could release POPs to a new environment remains unclear. It has not yet been resolved if the microplastics ingested by marine biota, including mussels and zooplankton groups, are toxic or harmful to the organisms (e.g., growth, mortality, and fecundity).

3.
Marine food web transfer. Once ingested by marine biota, microplastics can enter the food chain (web). It should also be explored whether biotransfer and bioaccumulation occur. 4.
Risk assessment. It is important to conduct the evaluations of the impact of microplastics on different marine biota and human beings based on the characteristics of microplastics in the Yellow Sea.

5.
Prevention. Technically, artificially accelerating the process of microplastic degradation in the ocean is not likely possible. At present, the only way to accelerate this process is by relying on the self-purification capacity of the environment. Therefore, the most important way to solve this environmental problem is to control it at the source, such as by reducing the emissions of plastics. More research is necessary on how to reduce the emissions and the effects of reducing emissions is necessary.