Progress and prospects of marine microplastic research in China

Research on microplastics in China is progressing rapidly. Within recent years, more than 30 research institutes have conducted research on marine microplastic in estuaries, coasts, open sea, and Polar regions. Microplastics have been detected in freshwater systems (lakes, rivers, and wastewater treatment plants) and coastal and marine environments. This paper reviews the research progress of microplastics in China, providing information on topics including the methodology, quantification of microplastics in various habitats, eco-toxicological effect, biodegradation, management, and control of plastic waste and microplastics. This paper discusses the sampling and analysis of microplastic in different media, followed by spatial and temporal distribution of microplastics in marginal seas and coastal and freshwater systems. After summarizing the recent advances on toxicology research and risk assessment of microplastics, this paper provides suggestions for future study to provide baseline information for better risk assessment and improved understanding of the lifecycle of microplastics in the environment.


Introduction
Over the past decade, marine debris and microplastics have attracted a tremendous amount of attention and discussion from governments, enterprises, international organisations, and the public. The problem of plastic debris can be dated back to the 1970s, when plastics were first discovered in the oceans and caused entanglement of and ingestion by marine biota. The term "microplastics" made its debut in academia after it was defined by Thompson (2004), and has attracted a great amount of scholarly interest around the world.
Marine microplastic is a global problem. In China, recent public awareness of the issue has led to academic and social achievements. In response to the United Nations call, China has been actively engaged in the research of marine microplastics. Up to now, more than 30 institutes have conducted marine microplastic research that ranges from land, estuaries, coasts, and open seas to Polar regions covering the distribution, methodology, eco-toxicological effect, biodegradation, and the management of microplastics. More than 20 national projects were supported by official government funds. The National Key Research and Development Program entitled "Monitoring and Ecological Risk Assessment of Microplastic Marine Debris" was the first officially sanctioned project on marine microplastics in China. With the implementation of the project, remarkable results have been achieved. Supported by the Ministry of Science and Technology of China, Marine Microplastics Monitoring Technical Standards were drafted, which established technical standards and revealed the distribution of microplastics in coastal China. Scientists and researchers in China pioneered the study of the composition, concentration, and distribution of microplastics in the Yangtze Estuary, East China Sea, and South China Sea as well as inland rivers (Zhao et al. 2014(Zhao et al. , 2015a(Zhao et al. , 2015bK. Zhang et al. 2015;Qiu et al. 2015;Su et al. 2016;Zhang et al. 2016;Peng et al. 2017Peng et al. , 2018Xu et al. 2018), to establish a predictive model of the amount of plastic waste entering the ocean from China and conduct investigations on the production of plastic products and the management of plastic waste pollution in China. The first research centre to focus on marine plastic debris in China, the Plastic Marine Debris Research Centre attached to East China Normal University, developed extensive cooperation with domestic and foreign universities and international organizations. Through multidisciplinary studies and citizen science, new materials and products were developed and legislation enacted.
China has been actively engaged in various international scientific organizations including United Nations Environment Programme (UNEP), Northwest Pacific Action Plan (NOWPAP), Unites Nations Educational, Scientific and Cultural Organization (UNESCO), Intergovernmental Oceanographic Commission (IOC) Sub-Commission for the Western Pacific (WESTPAC), and The Group of Experts on Scientific Aspects of Marine Environmental Protection (GESAMP), as well as multilateral and bilateral collaborations with Japan, Korea, Thailand, Norway, the United Kingdom, and the United States. Moreover, marine scientists in China are leading the UNESCO IOC/WESTPAC Asia-Pacific Regional project, titled "Distribution, Source, Fate and Influence of Marine Microplastics", which conducts pilot microplastic research on 50 beaches across 10 countries of the Asia-Pacific. This project is the first attempt around the world to apply standard methods for marine microplastics and promote global cooperation between regions for generating comprehensive international influence.

Analytical methods
Numerous reviews on analytical methods for sampling, extraction, quantification, and identification of microplastics from aquatic environments have been published. With many researchers attempting to improve the analysis process in both the field and the laboratory, the efforts to harmonize standard operating procedure have led to a series of articles and guidelines (Masura et al. 2015). Some of these attempts have been utilized in local or regional monitoring efforts. Mai et al. (2018) summarized the most common methodologies for sampling and quantifying microplastic. Another review by S.  summarizes and compares the analytical approaches for quantifying microplastics in marine environments used by the scientific community. Based on the common procedures of microplastic quantification in different marine matrices, S.  structured the review in four sections: collection, separation, analysis, and quality assurance or quality control. Here we present a flow chart for analysing microplastic samples from water, sediment, and biota ( Fig. 1; S. , and give recommendations for sampling, quantification, and identification of microplastics, including the following: • The selection of sampling sites should be representative enough for specific sites; • Manta trawls are suitable for large-scale collection (e.g., in lakes and seas), while bulk water sampling can facilitate sampling of smaller microplastics; • Biota sampling should take food webs into consideration when choosing species to study; • Density separation should always be applied when extracting microplastics from sediments, such as NaI, NaCl, and ZnCl 2 ; • H 2 O 2 should be used to remove organic matter to purify samples as a common practice; • Identification using various instruments, such as FTIR and Raman spectroscopy is necessary, although particles smaller than 1 μm are hard to detect; • Quantification of microplastics by manual counting is indeed time-consuming, so automated technologies are needed to be developed.
Other than the abovementioned environment matrices, Zhao et al. (2017) developed a method for extracting microplastics from marine snow that incorporates dual density separation with sodium iodide extraction and methanol precipitation, and was able to reach more than 90% recovery rate of microspheres of various sizes. For digestion chemicals, Li et al. (2018) tested the impact on fluorescent intensity of six digestion methods. Only KOH digestion had no impact on fluorescent polystyrene (PS) used in toxicological experiments, and was found to be an ideal protocol for extracting PS spheres in biota samples. NaOH, H 2 O 2 , HNO 3 , HNO 3 :HCl, and HNO 3 :HClO 4 all decreased fluorescence intensity and changed the morphology or composition of PS spheres.

Surface water in marginal seas
Microplastics have been found to be widely distributed in China's coastal waters, beaches, and marine organisms. The State Oceanic Administration began monitoring marine debris along China's coasts in 2007, and started microplastic monitoring in 2016. The highest concentration was found near the Changjiang Estuary, which is in correspondence with the reported microplastic abundance data of 4137 items/m 3 (Zhao et al. 2014), compared with 0.167 items/m 3 in the East China Sea. This big discrepancy could result from the inconsistency in the sampling method. But to some extent, the higher concentration in the freshwater end also indicates that rivers are the main source of microplastic entering the marine environment. The abundance of microplastics in sediments in the Changjiang Estuary was 121 items/kg dry weight . For the North Yellow Sea, microplastic abundance was 545 items/m 3 in surface water and 37 items/kg dry weight in sediments . Microplastic abundance in surface waters of the Bohai Sea was 0.33 n/m 3 (W. ). J.  reported that there are 171, 123, and 72 items/kg dry weight of sediments of Bohai Sea, northern Yellow Sea, and southern Yellow Sea, respectively. Polyethylene (PE) and polypropylene (PP) are the dominant types of microplastics in estuaries and coastal waters (Zhao et al. 2015a(Zhao et al. , 2015bW. Zhang et al. 2017;Zhu et al. 2018). In sediments, however, fibrous rayon, polyethylene terephthalate (PET) are the most common types J. Zhao et al. 2018;. Microplastics abundance in the South China Sea is also influenced by terrestrial sources, such as the Pearl River . Cheung et al. (2018) also identified the Pearl River estuary as a main contributor to microplastics in Hong Kong waters. The selected transect across the continental shelf found that the continental slopes tend to accumulate large quantities of microplastics. Based on in situ investigations, microplastic abundance in China's coastal area is comparable to that in the Mediterranean Sea (0.15 items/m 3 ) (de Lucia et al. 2014) and Arctic waters (2.68 items/m 3 ) (Lusher et al. 2015), and lower than that of the east Asian seas (3.74 items/m 3 ) (Isobe et al. 2015).

Beaches and coastal areas
Many efforts have been made to characterize microplastics on the beaches in China. A pioneering baseline investigation revealed the occurrence of small plastic debris in southern beaches in China (Zhao et al. 2015a(Zhao et al. , 2015b. Extensive study on beach sediment in Hong Kong revealed that mean microplastic abundance reached 5595 items/m 2 , making Hong Kong a hotspot for microplastic pollution (Fok and Cheung 2015). Zhou et al. (2018) reported microplastic distribution around Shandong Province and revealed the temporal distribution in beach sediments on a regional scale, indicating that the abundance varied among different sites. On a local scale, an investigation on beaches in Xiamen City in southeast China found that on average 76-333 items/kg of sediment , and noted that the compositions of microplastic polymers are different in beach sediments and surface water. A study on microplastics in wind-farm areas in Rudong discovered higher abundance of microplastics compared with those on other beaches and offshore surface waters , and fibre was the majority type.

Freshwater linked with the oceans
Many studies in China focus on freshwater systems. In general, urban rivers suffer more microplastic pollution compared with rural freshwater systems. A review by Zhang et al. (2018) summarized findings and research methods in China's inland water systems, and confirmed that inland waters in big cities have high concentrations of microplastics, and microplastics originate mainly from secondary sources.
For microplastics in lakes, Su et al. (2016) reported that 0.01-6.8 million items/km 2 (with a plankton net) or 0.003-0.026 items/m 3 (with pumping) in surface water, and 11-234 items/kg dry weight of sediments. W.  investigated 20 lakes in a mega city in Wuhan, and found the microplastic abundance to be 1666-8925 items/m 3 . In surface water, it was discovered that the abundance of microplastics is negatively proportional to the distance between the sampling site and urban areas. Even in lakes in Tibet, maximum microplastic abundance reached 563 items/m 2 ).
In rivers, microplastics have also been found. Peng et al. (2017) analyzed microplastics in subaqueous sediments in the Changjiang Estuary, and found 20-340 items/kg of sediment dry weight. For surface water in the Changjiang Estuary, Zhao et al. (2014) found much higher abundance than that of the East China Sea. In Jiaojiang and Oujiang estuaries, the abundances were found to be 955 and 680 items/m 3 , respectively (Zhao et al. 2015a(Zhao et al. , 2015b. In Pearl River, where it runs through Guangzhou City, the abundance in surface water was 379-7924 items/m 3 and 80-9598 items/kg dry weight sediments (Lin et al. 2018). Still, PE and PP are the most commonly found types of microplastics. J.  studied microplastics in sediments from Beijiang River, and reported an abundance of 178-544 items/kg dry weight. Peng et al. (2018) studied river sediments in Shanghai, and found that microplastic abundance was 802 items/kg dry weight, higher than that in the tidal flat in Shanghai. W.  reported that microplastics in the Wuhan section of the Changjiang River have a concentration of 2516 items/m 3 .
Wastewater treatment plants are also important sources of microplastic entering the sea. By analyzing microplastics in a secondary sewage treatment plant in Shanghai, microplastics concentration in effluent was found to be 52 000 items/m 3 , and in sludge was 180 items/50 g (wet weight). Fiber is the most common shape type. Rayon, synthetic leather, and polyester were the most common types in influent, while synthetic leather accounted for the largest percentage in effluent, followed by rayon, polyester, and PE (Bai et al. 2018b). In sewage sludge collected from 28 wastewater treatment plants throughout 11 Chinese provinces, microplastic concentration reached 1600-56 400 items/kg dry sludge . Polyolefin, acrylic fibers, PE, and polyamide were the most common microplastic polymer types. In the Three Gorges Reservoir, the largest reservoir in China, microplastic abundance ranged from 1597 to 12 611 items/m 3 in surface water and 25 to 300 items/kg wet weight in sediments, and PS, PP, and PE were the dominant polymer types (Di and Wang 2018). Another study in the Changjiang River in the Three Gorges Dam found the abundance of floating microplastics was more than 3 407 000-11 889 700 items/km 2 , with PE, PP, and PS being the major polymer types (K. . K.  studied microplastic abundance in the tributary of the Three Gorges Dam, the Xiangxi River, and found the concentration ranged from 55 000 to 34 200 000 items/km 2 in surface water and 80 to 860 items/m 2 in sediments. Similarly, the dominant composition of polymers was PE, PP, PS, and polyethylene terephthalate (PET).

Toxicology and risk assessment
Bioaccumulation is considered to be a common pathway to transfer microplastics from environment to organism, which may cause potential harm to individuals, food webs, and even ecosystems. Various shapes of microplastics, including fibers, pellets, and fragments, are widely found in organisms (Li et al. 2015). It has been reported that microplastic particles are more likely to be mistakenly ingested because of their smaller size (Li et al. 2015). C.  studied photosynthesis of microalgae under the influence of microplastics through algal growth inhibition tests and non-contact shading tests. The results indicated that microplastic with a diameter of 1 μm had negative effects on photosynthesis of microalgae with a concentration of 5-50 mg/L. Sun et al. (2017) investigated natural zooplankton groups (fish larvae, jellyfish, chaetognaths, shrimps, and copepods) using plankton nets (net I, 505 μm; net II, 160 μm) in the South China Sea, and found fibre to be the dominant type of microplastic. The study also discovered a positive relationship between the trophic level of zooplankton and the possibility for it to encounter microplastics. Polyester constituted the majority of microplastics identified, and the abundance ingested by zooplankton was 4.1 items/m 3 for net I and 131.5 items/m 3 for net II.
Using commercial species bought from fishery markets, Li et al. (2015) revealed the microplastic abundance in nine bivalve species to be 4.3-57.2 items/individual, with fibres being the most prevalent shape. In a following study, Li et al. (2016) collected mussels along China's coastlines at 22 sites, and found the average microplastic concentration in mussels to be 1.5-7.6 items/individual, among which wild groups had higher concentration than farmed groups. Qu et al. (2018) found a positive linear relationship between microplastic concentration in water and mussels collected from the field. Kolandhasamy et al. (2018) observed that microfibers adhered to foot and mantle of mussels, another way in which microplastics can contact tissues of the bivalves. Su et al. (2018) suggested the use of Asian clam as an indicator of microplastics because the composition of microplastics resembles that in freshwater environments. Microplastic concentration in wild oysters Saccostrea cucullata in the Pearl River estuary was investigated by Li et al. (2018). They found that the abundance of microplastics in oysters ranged from 1.4 to 7.0 items/individual with fibres being the dominant type ingested. Abundance in the oysters was positively correlated with concentration in the surrounding waters.
Microplastics may enter the digestive tracts of fish once ingested. However, compared with zooplankton and bivalves, fewer microplastics have been found in the digestive tracts of fish. Jabeen et al. (2017) found an average abundance of microplastics of 1.1-7.2 items/individual in the stomach and intestine of 21 sea species and six freshwater species. For amphibians, Hu et al. (2016) determined that tadpoles can ingest PS microspheres, but egest them quickly when exposed to microplastics after 3 days.
Apart from fish, another vertebrate that can be an indicator is birds. Zhao et al. (2016) studied gut contents of 17 terrestrial birds in Shanghai, and extracted 364 plastic items in total using 10% KOH digestion, among which 90% were microplastics. This result reflected the overwhelming distribution of microplastics in terrestrial ecosystems.
Microorganisms can also attach to microplastics and be distributed widely. Jiang et al. (2018) reported microbial communities on the plastic fragments in the Changjiang Estuary. Keystone species include Alphaproteobacteria, Gammaproteobacteria, Flavobacteriia, Acidobacteria, and Cyanobacteria. These hitchhikers may drift with microplastic particles in the ocean, creating a "plastisphere" (Zettler et al. 2013).
Some pollutants are found to be concentrated on microplastics. W.  collected plastic resin pellets from two beaches, and found a relatively high concentration of PCBs, PAHs, and DDTs on the collected pellets. Ma et al. (2016) investigated the toxic effects of nanoplastics and microplastics, and environmental fate of phenanthrene using Daphnia magna. The study confirmed that 50 nm nanoplastics posed significant toxicity to Daphnia with a concentration of around 10 mg/L, and the toxicity of 50 nm nanoparticles and phenanthrene resulted in additional damage to Daphnia.
Despite the potential threat of microplastics to the environment. The existing frameworks for evaluating environmental risks of pollutants, which are used for guiding principles worldwide, have not yet been applied to microplastic pollution. Currently, the risk assessment of microplastics is being actively studied in China. Peng et al. (2018) investigated the in situ concentration of microplastics in freshwater river sediments from seven sites in Shanghai, a megacity. With these data, the chemical toxicity of microplastics and risk can be calculated, and the environmental risk of each site could be compared. Starting from a regional scale, the framework can be applied to a larger scale in future studies (see Fig. 2).
In addition, great progress has been made on the methodology of microplastics risk assessment. Xu et al. (2018) investigated microplastics in surface water in the Changjiang Estuary and adjacent waters, and noted that PVC increased the risk that aquaculture farms were hotspots for microplastic pollution after a risk assessment. More information is required for present and future risk assessments of microplastics in China, and related assessment methodology should soon be developed. In prospective research, an integrated risk assessment of microplastic pollution worldwide based on data available in the literature should be conducted.

Conclusion
Increasing numbers of scientists are dedicated to microplastics research in China, covering various disciplines and areas of research. In the future, extensive international cooperation on a global scale and a unified research method of marine microplastics are needed. Researchers should focus on the flux, life cycle, and management of plastic waste, as well as understanding the distribution of microplastics in global rivers, estuaries, continental shelves, deep sea, and Polar regions. Furthermore, it should be emphasized that toxicological research on marine microplastics needs to be conducted at environmental concentration and risk of marine microplastics on marine ecosystems and human health should be better assessed. With the substantial steps taken by the Government of China, such as better classification of domestic waste; a ban on importing plastic waste that will reduce the amount of plastic waste imported; improving domestic waste collection, classification, and treatment; and improving the overall environmental quality, the amount of plastic debris entering the ocean from China could be expected to decrease to great extent. According to Bai et al. (2018a), these efforts will greatly reduce the quantity of mismanaged plastic waste entering the ocean from China and make an important contribution to the global environmental governance in the future.  . Reprinted from Environmental Pollution, Vol. 234, Guyu Peng, Pei Xu, Bangshang Zhu, Mengyu Bai, Daoji Li, Microplastics in freshwater river sediments in Shanghai, China: A case study of risk assessment in mega-cities, pp. 448-456, Copyright (2018), with permission from Elsevier, https://www.journals.elsevier.com/environmentalpollution.