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Article

Study on the Mass Concentration Distributions of Marine Microplastics in Estuaries and Coastal Areas

1
State Key Laboratory of Hydraulic Engineering Simulation and Safety, Tianjin University, Tianjin 300072, China
2
Key Laboratory of Earthquake Engineering Simulation and Seismic Resilience of China Earthquake Administration, Tianjin University, Tianjin 300350, China
3
Power China Huadong Engineering Corporation Limited, Hangzhou 311122, China
4
State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China
*
Author to whom correspondence should be addressed.
Water 2025, 17(8), 1136; https://doi.org/10.3390/w17081136
Submission received: 15 February 2025 / Revised: 27 March 2025 / Accepted: 7 April 2025 / Published: 10 April 2025
(This article belongs to the Section Oceans and Coastal Zones)

Abstract

:
Marine microplastics are a global environmental issue, and understanding their distributions in estuaries and coastal areas is a critical prerequisite for the effective and sustainable management of microplastic pollution. Owing to the lack of methods that exist for quantifying microplastic content, characterizing the distribution of microplastics is difficult. The mass concentration of microplastics is an objective indicator that reflects their distribution. Therefore, a formula for calculating the microplastic mass concentration is proposed through the parameters of the number concentration, microplastic size, and mesh size, in addition to the proportions of particles with different sizes and shapes. On the basis of the large amount of existing measured data, the mass concentrations of microplastics in estuaries and coastal areas are calculated. It can be concluded that polypropylene (PP; 72%) and polyethylene (PE; 67%) are the most abundant microplastics in the ocean. Additionally, marine microplastics are more common in estuaries (102–103 mg/L) than in the open sea (0–10 mg/L). The maximum concentration of microplastics in surface water is approximately 8.0 g/L. Moreover, the concentration of microplastic pollution has significantly increased in areas surrounding sewage treatment plants and aquaculture farms.

1. Introduction

An estimated 4.8–12.7 million tons of plastic enter the ocean each year, and this amount is expected to increase by one order of magnitude by 2025 [1]; moreover, plastic pollution will negatively influence the marine environment. Under the influences of solar radiation, waves, and physical–chemical–biological factors, plastics break down into very small fragments called microplastics, which are smaller than 5 mm in size (Table 1). Since microplastics are small, have a large specific surface area, and exhibit excellent hydrophobicity, they are easily eaten by fish and benthic organisms, resulting in damage to the digestive system and negative effects on nutrient intake and healthy growth [2,3,4]. In addition, the surfaces of microplastics easily attach to bacteria, algae, barnacles, and other organisms, forming biofilms that can enhance predation effects and increase the risk of biological ingestion. As a result, microplastics can potentially harm human health through the food chain. Furthermore, microplastics easily enrich heavy metals and other persistent pollutants, forming complex pollutants [5,6] and endangering the marine environment, which has drawn extensive attention from countries worldwide (Figure 1).
To further understand the distribution characteristics of microplastics and effectively manage their treatment, researchers examined microplastic debris in estuaries, coastal areas, and oceans, revealing that microplastics are widespread throughout the marine environment [8]. Some scholars collected microplastic samples from estuaries of the Yangtze River, Minjiang River, Yellow River, Pearl River, and Haihe River in China through sieve meshing; the microplastic concentrations were 4137.3 ± 2464.5 n/m3, 1245.8 ± 531.5 n/m3, 654,000 n/m3, 8902 n/m3, and 1485.7 ± 819.9 n/m3, respectively [9,10,11,12,13]. Some researchers studied the microplastic concentrations and characteristics of the Bega Estuary, Clyde Estuary, and Hunter Estuary in Australia via sieve meshing and reported that the mass concentrations of microplastics were 80–906 n/m3, 23–198 n/m3, and 431–1892 n/m3, respectively [14]. Some researchers tested microplastics in the estuaries of the Baram River and Terengganu River in Malaysia through screening and reported that the microplastic concentrations were 9300–18,000 n/m3 and 1687 n/m3, respectively [15,16]. Some researchers studied the distribution characteristics of microplastics in the coastal areas of Tianjin, Xiamen, and Guangdong in China and reported that the microplastic concentrations were 210–1170 n/m3, 103–2071 n/m3, and 3000–19,000 n/m3, respectively [17,18,19]. The abundance levels of microplastics with different sizes along the southern coast of Korea were measured via hand nets. The concentration of microplastics decreased as the microplastic size range increased [20]. Rodrigues et al. (2020) used screens to measure the concentration of microplastics on the northwest coast of Portugal and reported that the microplastic concentration was 651 ± 1660 n/m3 [21]. Some scholars conducted fieldwork on the concentrations of microplastics in the Eastern Indian Ocean, Equatorial Atlantic Ocean, Antarctica Ocean, and Western Pacific Ocean by trawling and reported that the concentrations of microplastics were 0.34 ± 0.80 n/m3, 0.01 n/m3, 0.031 n/m3, and 0.06 ± 0.03 n/m3, respectively [22,23,24,25]. There was significant spatial variability in the concentration of microplastics. In addition to differences in the actual quantity of microplastics, the sieve mesh size and the statistical microplastic size range could greatly influence the amount of microplastics collected.
Currently, the primary parameter used to characterize microplastic concentration is the number concentration (or abundance). The abundance is the actual number of microplastics, and it is obtained through manual counting. This value is then converted into the concentration within the environmental sample, which is typically expressed in units of n/m3 (items/m3). However, standardized methods for collecting microplastics have yet to be established, and different sampling techniques significantly affect both the quantities and types of microplastics collected.
For example, researchers reported that the average abundance of microplastics was 2960.0 ± 825.3 n/m3 after direct filtration with a 0.45 μm membrane, 27.0 ± 11.5 n/m3 after rapid filtration with a 20 μm screen, and 0.18 ± 0.05 n/m3 after application of a trawl tool with a 150 μm mesh [26]. Therefore, determining the distribution characteristics of microplastics on the basis of their abundance alone is difficult. The mass of the same type of microplastic with a size of 0–300 μm is significantly lower than that of microplastics with sizes exceeding 1000 μm, and the mass difference of a single particle can reach degrees of 102–105. Therefore, the impact of the difference in trawl mesh size can be reduced by the mass concentration to assess the degree of marine pollution, and the comparability of the data can be improved. Thus, the mass concentration (mg/L) is a more realistic and objective indicator for reflecting the distribution of microplastics. However, because microplastics are extremely small, they are difficult to separate and weigh, and the presence of nonmicroplastic particles can significantly affect the measurement process.
In recent years, some scholars have determined formulas to convert the number concentration of microplastics to the mass concentration. For example, Cozar et al. (2014) proposed a rectangular prism model with a square base and a height that was each 0.1 times the length of the square base [27]. Simon et al. (2018) introduced a spheroid model with a height that was 0.67 times the width [28]. Isobe et al. (2019) developed a cylinder model with a height that was 0.4 times the diameter of the circular base [29]. There is a significant deviation when calculating the mass concentration of differently shaped microplastics based on the same model. Leusch and Ziajahromi (2021) assumed irregular granular and fibrous plastics as spheres and cylinders, respectively [30]. Chen et al. (2024) proposed ellipsoid and cylinder models to calculate the mass of pellets/fibers and fragments, respectively [31]. Microplastics are classified by shape (granules, fibers, films, and fragments) in this paper, and distinct conversion models are established for each shape. The overall size range is divided into subintervals, and the mass contributions from each interval are computed. The mass concentration of microplastics in the sea area is calculated from the accumulation of the proportions of shape and size.
The mass of microplastics of different shapes is calculated on the basis of factors such as abundance, microplastic size, and mesh size, in addition to the proportions of microplastics with different sizes and shapes. On the basis of the above conversion method, the distribution characteristics of microplastics in estuaries and seas are explored. In this study, Section 2 is focused on the collection of microplastic abundance data from estuaries and coastal areas, and the calculation method for the mass concentration of microplastics is described. Section 3 presents the distributions of microplastic mass concentrations in estuaries and coasts. Finally, the conclusions are presented in Section 4.

2. Data Collection and Calculation Methods

The quantities of microplastics in estuarine and coastal areas are determined. The scientific databases used include the Web of Science, Elsevier ScienceDirect, Springer, Google Scholar, and the China National Knowledge Infrastructure. The keywords and phrases “Microplastic” AND “Abundance/Distribution/Occurrence” AND “Marine water/Elucidous/Ocean/Coastal water/Bay/Sea” were searched.
The abundances of marine microplastics, including the survey area, sampling location (surface water refers to 0–5 m depth), mesh type, size range, material composition, and number concentrations for the various areas, are shown in Table 2. This table shows that various types of microplastics are present in estuaries and coastal areas, including common microplastic polymer components in water, such as polyethylene (PE), polypropylene (PP), polystyrene (PS; which are found with manta on the sea surface and are present as EPS with very low density), polyethylene terephthalate (PET), polyvinyl chloride (PVC), and polyamide (PA). In addition, the material and mesh sizes of the nets used for sampling on site differ.
Table 2 shows that the main shapes of microplastics in seawater are fibers (unidirectional extensibility, good softness and toughness, and difficult deformability), pellets (smooth spheres or irregular spherical microplastics), fragments (irregularly shaped with a certain thickness and hardness, extending in two or three directions), and films (thin microplastics that can be easily extended in two or three directions and squeezed by tweezers). Fibrous microplastics account for the greatest proportion, followed by pellets and then fragments [99]. Owing to the immaturity of microplastic size measurement technology and the absence of standardized definitions for what is considered an irregular microplastic size, accurately measuring the volume of complex microplastics is challenging. To address these issues, several reasonable assumptions are introduced in this work, and an effective method for estimating the volume of microplastics is proposed. By considering the distinctive appearances of microplastics with different shapes, specific calculation methods for their parameters are outlined, as shown in Table 3, and Table 4 presents the parameters for calculating the mass concentration of microplastics.
On the basis of Table 3 and Table 4, for fibers, the mass concentration can be expressed as follows:
C FB = i = 1 n ρ π d 2 2 × L i × p i × p FB × A = π ρ A p FB i = 1 n 1 4 d 2 p i L i
For fragments, the mass concentration is as follows:
C FR = i = 1 n ρ π L i 2 2 × 1 10 × L i × p i × p FR × A = π ρ A p FR i = 1 n 1 40 p i L i 3
For films, the mass concentration is as follows:
C FI = i = 1 n ρ π L i 2 2 × h × p i × p FI × A = π ρ A p FI i = 1 n 1 4 h p i L i 2
For pellets, the mass concentration is as follows:
C PT = i = 1 n ρ π × 4 3 × L i 2 3 × p i × p PT × A = π ρ A p PT i = 1 n 1 6 p i L i 3
Therefore, the mass concentration of microplastics is as follows:
C = C FB + C FR + C FI + C PT   = 1000 π ρ A p F B i = 1 n 1 4 d 2 p i L i + p FR i = 1 n 1 40 p i L i 3 + p FI i = 1 n 1 4 h p i L i 2 + p PT i = 1 n 1 6 p i L i 3
where C is the mass concentration of microplastic, mg/L; A is the number concentration of microplastic, n/m3; ρ is the microplastic density, kg/m3; d is the mesh size (the straight-line distance between the center point of two adjacent knots when the mesh remains naturally extended, m); h is the thickness of the film, m; Li is the ceiling or floor in each particle size interval, m; pi is the proportion of each microplastic size interval with the same shape; and PFB, PFR, PFI, and PFT are the proportions of fibers, fragments, films, and pellets, respectively.

3. Mass Concentration Distributions of Marine Microplastics

By using the microplastic mass concentration calculation formula, the mass concentrations of microplastics in 30 water bodies, including the Changjiang River Estuary, Bohai Bay, and Queen Charlotte Sound, are estimated (Table 5).
The density of microplastics in marine surface water is generally lower than that in aquatic environments. However, many samples analyzed in this study are collected from estuaries, where hydrodynamic forces are significant. Under these conditions, microplastics lead to irregular movement within the water column that may be resuspended from riverbeds. Microplastics with densities greater than 1000 kg/m3 are present in surface waters in a large amount [15,43,69]; therefore, the density of microplastics in the formula is determined as the average density of microplastics in aquatic environments (1040 kg/m3 [7]), disregarding the influence of water depth.

3.1. Shapes of Marine Microplastics

The shape characteristics of microplastics vary significantly across marine areas due to differences in pollution sources, hydrodynamic conditions, and suspended particulate matter contents. For example, researchers investigating the shapes of microplastics in the surface waters of Bohai Bay and Charleston Harbor reported that fibrous microplastics account for more than 50% of the total microplastics, followed by fragments. The total content of film and pellet microplastics is less than 5% [32,61]. Some scholars have collected samples from the eastern coast of Guangdong and the Baram River and reported that fragmented microplastics account for more than 50% of the total microplastics, followed by fibers. Films and pellets are less prevalent, constituting less than 15% of the total microplastics [15,19]. However, researchers have analyzed the shapes of microplastics in the surface waters of the Pearl River Estuary in China and reported that films and pellets comprise nearly 50% of the total microplastics, indicating that they are present in relatively high concentrations [12].
The data in Table 5 are summarized and organized, with each region being treated as an independent sample. The average proportions of microplastics of various shapes across the total sample range are calculated. These average proportions are then consolidated to determine the relative proportions, as shown in Figure 2. Fiber microplastics constitute the largest proportion of the surface water, followed by fragments and pellets, which is consistent with the findings of Luo [99].

3.2. Material Compositions of Marine Microplastics

The primary components of microplastic materials vary across different seas owing to differences in human activities and pollutants. Researchers have shown that the main components of microplastics along the coast of Xiamen and the South China Sea are polypropylene (PP) and polyethylene (PE) [18,22]. An analysis of microplastic samples from the Pearl River Estuary reveals that the dominant component is polyamide (PA) [12]. Research on microplastic pollution in Calvi Bay reveals that polystyrene (PS) is the main material [50].
The proportions of microplastic materials in estuaries and coastal areas are recalculated on the basis of the mass concentration distributions of microplastics with different materials (Figure 2). On the basis of the above data, the potential pollution risk of microplastics composed of specific materials is assessed. PP and PE are the primary materials in water pollutants, with probabilities of 72% and 67% being the main components of pollutants in a given area, respectively. This value is approximately three to six times greater than the probabilities for the PA, PVC, and PET pollutants. This phenomenon occurs because estuaries and coastal areas tend to accumulate waste, such as fishing nets, ropes, and plastic bottles, which are major sources of PP and PE. As a result, the potential risk posed by these pollutants has increased.

3.3. Mass Concentrations of Marine Microplastics

As shown in Table 2 and Table 5, studies on microplastic samples from the Bohai Sea and Haihe Estuary in China [13,32] indicate that the number concentration in the Bohai Sea (2000–4200 n/m3) is higher than that in the Haihe Estuary (1485.7 ± 819.9 n/m3). However, the mass concentration in the Bohai Sea (6.6–59.40 mg/L) is lower than that in the Haihe Estuary (62.11–154.82 mg/L). Some researchers have studied microplastic pollution in Queen Charlotte Sound and Charleston Harbor [54,61]. The researchers reported that the concentration in Queen Charlotte Sound (7630 ± 1410 n/m3) is greater than that in Charleston Harbor (6600 ± 1300 n/m3). However, the mass concentration in Queen Charlotte Sound (1.73–112.08 mg/L) is lower than that in Charleston Harbor (0.81–555.68 mg/L). Since the difference of statistical microplastic size range in both areas is reflected in <50 µm microplastics, which have more quantity but less quality, compared to abundance, microplastics (0–50 µm) have less effect on mass concentration. The difference in concentration is attributed primarily to the shape characteristics of the microplastics. Microplastic fibers have smaller masses than the other microplastic shapes, and their proportions in the Bohai Sea and Queen Charlotte Sound (both approximately 75%) are greater than that in Charleston Harbor (approximately 55%). Evaluating microplastic pollution by mass concentration is relatively accurate, as it reduces the influences of factors such as shape and microplastic size.
We observe that the mass concentrations of microplastics in estuaries are often higher than those in the open sea (Figure 3 and Figure 4), making estuaries popular locations for microplastic pollution due to the influences of various social and natural factors, such as geographical location, human activities, tides, waves, and wind, which make estuaries more prone to accumulating large quantities of microplastics. Additionally, estuaries are convergence points where inland rivers flow into the sea, and their areas are much smaller than those of the open sea. As a result, the mass concentrations of microplastics decrease significantly as they are transported from estuaries to the open sea.
For example, the mass concentrations of microplastics in the Yellow River Estuary and Sanggou Bay are relatively high. In the case of the Yellow River, this is primarily because the sampling site is located within a river channel, with samples collected 57 km upstream of the estuary, an area impacted by domestic wastewater and sewage from treatment plants in the middle and upper reaches [32]. These impacts result in a high concentration of microplastics, reaching 65,400 n/m3 (23.682–8252.771 mg/L).
The samples are collected from Sanggou Bay Marine Ranch in Weihai, which is the largest aquaculture base in northern China [33]. Plastic waste, such as that from fishing nets and lines used in aquaculture, is a significant source of microplastics in seawater. Consequently, the degree of microplastic pollution in the area is relatively high, with the microplastic concentration reaching 20,060 ± 4730 n/m3 (59.165–514.501 mg/L).
In contrast, the mass concentrations of microplastics in Bandon Bay and the Rio de la Plata Estuary are relatively low. This phenomenon can be attributed to the low concentrations of microplastics in these two bodies of water and the high proportions of fibrous microplastics (100% and 55.05%, respectively). In addition to those in these areas, the mass concentrations of microplastics in other estuarine and coastal regions generally fall within the range of 0–10 mg/L. On this basis, the mass concentrations of microplastics for experimental purposes can be set to values of approximately 5 mg/L.

4. Discussion

Zhu et al.(2024) calculated the mass concentration of microplastics in Beibu Bay as 0.04 × 10−3 (mg/L) through taking 0.5 mm as the average microplastics size and 0.992 g/cm3 as the average density [101]. Compared with it, the mass concentration of microplastics in Beibu Bay was calculated as 6.5–16.8 × 10−3 (mg/L) in this paper, the main reason for the difference is that the proportion of different particle size intervals is considered in this paper, where the mass of one 5000 µm microplastic is 103 times that of 500 µm.
Although we proposed a method for calculating the mass concentrations of microplastics on the basis of parameters such as microplastic abundance, microplastic shape, and microplastic density, the irregular shapes of microplastics were only approximated. The density of microplastics in the formula was determined from the average value of microplastics in aquatic environments, disregarding the influence of water depth. In the future, high-precision microplastic characterization techniques (first directly weighing microplastic samples from estuaries and marine areas and then inferring the mass concentration of microplastics in a given marine area) and advanced numerical simulation methods (automated extraction of the shape factor to calculate the volumes of microplastics with different shapes) should be combined to further optimize the calculation method for microplastic mass concentration.
Owing to the obvious differences in the pollution sources and migration modes of microplastics, effectively treating microplastic pollutants using a single microplastic treatment method (physical method, chemical method, biological method, magnetic separation technology method, and collaborative method) is difficult. First, the types, shapes, sizes, and contents of microplastics in different sea areas should be clarified, and targeted and efficient treatment methods combining various technologies should be developed [102].

5. Conclusions

Data on the survey area, mesh size, microplastic size range, main composition, and concentration of microplastics in estuaries and coastal areas were collected from approximately 100 samples. The shape and material characteristics of microplastics in the surface waters of estuaries and coasts were analyzed. Fibrous microplastics comprised the greatest proportion, followed by fragments and pellets. PP and PE were the dominant materials in water pollutants, with probabilities of 72% and 67% of being the main components in a given area, respectively; these values are approximately three to six times greater than those of pollutants such as PA, PVC, and PET.
A suitable conversion formula for mass concentration was proposed. By using this formula, the concentration of microplastics in coastal estuaries was converted to a mass concentration. The results revealed that the microplastic content was greater in estuaries than in the open sea and that the degree of microplastic pollution increased significantly in areas near sewage treatment plants and aquaculture farms.

Author Contributions

Conceptualization, J.Z., G.L. and Q.Z.; methodology, J.L.; formal analysis, Z.L. and J.L.; data curation, G.L., Z.L. and J.L.; writing—original draft preparation, Z.L. and J L.; writing—review and editing, G.L., J.Z., Y.L. and Q.Z.; supervision, G.L.; funding acquisition, J.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the National Natural Science Foundation of China (Grant No. 52271289).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

Author Jiaxiong Liang was employed by the company Power China Huadong Engineering Corporation Limited. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Formation and migration mechanisms of microplastics. (Plastic primarily originates from urban areas, industrial facilities, and maritime activities. Under prolonged exposure to sunlight and microbial degradation, these plastics can break down into microplastics. Transport behaviors such as floating, sinking, and resuspension occurred, leading to widespread marine pollution).
Figure 1. Formation and migration mechanisms of microplastics. (Plastic primarily originates from urban areas, industrial facilities, and maritime activities. Under prolonged exposure to sunlight and microbial degradation, these plastics can break down into microplastics. Transport behaviors such as floating, sinking, and resuspension occurred, leading to widespread marine pollution).
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Figure 2. Probabilities of the shapes and material compositions of microplastics.
Figure 2. Probabilities of the shapes and material compositions of microplastics.
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Figure 3. Mass concentration distributions of marine microplastics (the color represents the average mass concentration of microplastics (mg/L)).
Figure 3. Mass concentration distributions of marine microplastics (the color represents the average mass concentration of microplastics (mg/L)).
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Figure 4. Mass concentrations of microplastics in different estuaries and coastal areas.
Figure 4. Mass concentrations of microplastics in different estuaries and coastal areas.
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Table 1. Origin of microplastics [7].
Table 1. Origin of microplastics [7].
Plastics ClassProducts and Typical Origin
PPRope, bottle caps, and netting
PSPlastic utensils and food containers
PANetting and traps
PETPlastic beverage bottles
PVCPlastic film, bottles, and cups
Table 2. Abundance of marine microplastics.
Table 2. Abundance of marine microplastics.
Survey AreaSampling LocationMesh TypeSize RangeMaterial CompositionNumber ConcentrationReference
Changjiang estuarySurface32 µm steel0.5–5 mm/4137.3 ± 2461.5 n/m3[9]
Donghai coastSurface333 µm manta0.5–5 mm/0.167 ± 0.138 n/m3[9]
Jiaojiang estuarySubsurface333 µm manta0.333–5 mmPP, PE, PVC955.6 ± 848.7 n/m3[10]
Oujiang estuarySubsurface333 µm manta0.333–5 mmPP, PE, PVC680.0 ± 284.6 n/m3[10]
Minjiang estuarySubsurface333 µm manta0.333–5 mmPP, PE, PVC1245.8 ± 531.5 n/m3[10]
Bohai SeaSurface20 µm nylon0.02–5 mmPP, PE, PET2000–4200 n/m3[32]
Yellow River estuary
(wet season)
Surface50 µm steel50–200 µmPE, PP, PS654,000 n/m3[11]
Yellow River estuary
(dry season)
Surface50 µm steel50–200 µmPET, PP930,200 n/m3[11]
Pearl River estuarySurface50 µm steel50–500 µmPA8902 n/m3[12]
Haihe estuarySurface48 µm steel48–5000 µmPP, PE, PS, PVC1485.7 ± 819.9 n/m3[13]
Yonding River estuarySurface48 µm steel48–5000 µmPP, PE, PS, PVC788.0 ± 464.2 n/m3[13]
Tianjin coastal areasSurface125 µm steel125–5000 µmPP, PE, PET210–1170 n/m3[17]
Sanggou BaySurface30 µm steel30–5000 µmPE, PS, PP, CE20,060 ± 4730 n/m3[33]
South China seaSurface20 µm nylon20–300 µmAK, PCL, PEA2569 ± 1770 n/m3[34]
Xiamen coastal areasSurface330 µm manta330–5000 µmPE, PP103–2017 n/m3[18]
Zhangjiang estuarySurface300 µm manta330–5000 µmPP, PE, PS50–725 n/m3[35]
East of Guangdong coastSurface50 µm steel//3000–19,000 n/m3[19]
Zhuhai coastal areasSurface2.7 µm fiber1–5000 µmPE, PP, PS16,610 ± 4280 n/m3[36]
Donghai coastal areasSurface20 µm nylon20–5000 µmPP, PET, PES900 n/m3[37]
Eastern Indian OceanSurface330 µm manta0–5000 µmPP, PE0.34 ± 0.80 n/m3[38]
South China SeaSubsurface330 µm Plankton0–5810 µmPET, PP, PE0.52 ± 0.42 n/m3[39]
Eastern Indian OceanSubsurface330 µm Plankton0–5660 µmPET, PP, PE0.27 ± 0.19 n/m3[39]
South China SeaSurface330 µm manta140–5080 µmPP, PE0.61 ± 0.87 n/m3[22]
South China SeaColumn60 µm Filter70–4720 µmPET, PP, PE, AR50 m1.00 ± 0.43 n/m3[22]
200 m0.63 ± 0.25 n/m3
500 m0.30 n/m3
800 m0.30 n/m3
1000 m0.20 n/m3
Hong KongSurface153 µm Plankton30–4960 µmPP, HDPE, LDPE0.51–279.09 n/m3[40]
Zhubi reefSurface50 µm steel0–5000 µmPP, PA, PS, PVC4933 ± 1369 n/m3[41]
Haikou BaySurface333 µm Neuston0–4900 µmPE, Rayon0.44 ± 0.21 n/m3[42]
Nanxun reefSurface48 µm steel0–5000 µmPVC, PA, PE1733 n/m3[43]
Paracel IslandsSurface20 µm nylon20–5000 µmPET, PP54.7 n/m3[44]
Nansha Island
remote coral reefs
Surface333 µm neuston209–4917 µmPP, PE, PPC0.0556 ± 0.0355 n/m3[45]
Yellow SeaSurface500 µm bongo nets350–44,990 µmPE, PP0.13 ± 0.20 n/m3[46]
DanubeSurface500 µm mesh<20 mm/0.32 ± 4.67 n/m3[47]
Tamar estuarySurface300 µm mantaIncluding more than 5 mmPE, PS, PP0.03 n/m3[48]
Goiana estuarySurface/bottom300 µm plankton<5 mm/0.26 n/m3[49]
Calvi BaySurface200 µm mesh0.2–10 mmPS0.06 n/m3[50]
Israeli Mediterranean coastSurface300 µm manta0.3–5 mm/7.68 ± 2.38 n/m3[51]
Northwestern
Mediterranean Sea
Surface333 µm manta0.3–5 mmPS0.12 n/m2[52]
Jade system
(Southern North Sea)
Surface40 µm steel<5 mm/64,000 ± 194,000 n/m3[53]
Queen Charlotte SoundSubsurface62.5 µm copper398 ± 376 µm/7630 ± 1410 n/m3[54]
Strait of GeorgiaSubsurface62.5 µm copper513 ± 494 µm/3210 ± 628 n/m3[54]
West coast of
Vancouver Island
Subsurface62.5 µm copper558 ± 521 µm/1710 ± 1110 n/m3[54]
Western English ChannelSurface200 µm plankton<5 mmPA, PP0.27 n/m3[55]
Western Sardinian coastSurface500 µm manta0.5–5 mmPS0.15 n/m3[56]
Ligurian Sea and
Sardinian Sea
Surface200 µm mesh<5 mm/0–10 n/m3[57]
Portuguese coastal waters/180 µm mesh<5 mmPE, PP0.002–0.036 n/m3[58]
Brest BaySurface335 µm manta<5 mmPE, PP, PS0.24 ± 0.35 n/m3[59]
South coast of KoreaSurface50 µm mesh<50 µmPS, PP, PE103,000 ± 59,000 n/m3[20]
50–100 µm69,000 ± 41,000 n/m3
100–200 µm23,000 ± 20,000 n/m3
200–500 µm11,000 ± 11,000 n/m3
500–1000 µm2900 ± 3400 n/m3
>1000 µm2100 ± 2900 n/m3
West coast of
the North Atlantic
/333 µm manta0.333–5 mmPS0.01–14.1 n/m3[60]
Northwest coast of PortugalSubsurface30 µm mesh30–5000 µmPE, PP651 ± 1660 n/m3[21]
Charleston harborSurface63 µm mesh63–5000 µmPS, PA, PE6600 ± 1300 n/m3[61]
Wanyah BaySurface63 µm mesh63–5000 µmPS, PA, PE30,800 ± 12,100 n/m3[61]
Bega estuarySurface45 µm mesh//80–906 n/m3[14]
Clyde estuarySurface45 µm mesh//23–198 n/m3[14]
Hunter estuary0–4 m37 µm mesh//431–1892 n/m3[14]
Blanca estuarySurface//PA, CE6500 ± 4010 n/m3[62]
Chao Phraya River estuarySurface300 µm manta/PP, PE48 ± 8 n/m3[63]
Baram RiverSurface300 µm steel300–5000 µmPE, PET, PS9300–18,000 n/m3[15]
North coast of
Indonesia Surabaya
Surface200 µm filter200–5000 µmPS, PE, PP380–610 n/m3[64]
Tampa BaySurface1.2 µm fiber//250–7000 n/m3[65]
Bandon BaySurface5 µm nylon/RN, PP, PE630 ± 130 n/m3[66]
Southeastern coastline
of South Africa
Surface65 µm mesh65–5000 µmPS257.9–1215 n/m3[67]
Rio De La PlataSubsurface36 µm plankton36–5000 µmPE, PP5–110 n/m3[68]
Chukchi SeaSurface330 µm manta>333 µmPET, PP, PE0.23 ± 0.07 n/m3[69]
Northwest PacificSurface330 µm manta>333 µmPET, PP, PA0.030 ± 0.017 n/m3[69]
Bering SeaSurface330 µm manta>333 µmPET, PP, PA0.091 ± 0.094 n/m3[69]
MaldivesSurface200 µm neuston/PE, PP0.26 n/m3[70]
Equatorial AtlanticSurface300 µm plankton0–5000 µm/0.01 n/m3[23]
Terengganu estuarySurface200 µm mesh/PA, PE, PP1687 n/m3[16]
Offshore waters of MalaysiaSurface200 µm mesh/PA, PE, PP1900 n/m3[16]
Western tropical
Atlantic Ocean
Surface300 µm manta0–5000 µm/0.03 n/m3[71]
Southern OceanSurface350 µm neuston0–5500 µmPE, PP, PS0.031 n/m3[24]
Coast of CochinSurface250 µm steel250–5000 µmCE, PS, PE751.7 ± 452.21 n/m3[72]
Western PacificSurface330 µm manta/ER, PP, PESPMA0.06 ± 0.03 n/m3[25]
Chabahar BaySurface0.45 µm fiber/PE, PET, Nylon86,000–362,000 n/m3[73]
Sri LankaSurface380 µm neuston0–5000 µmHDPEPP, PS17.45 ± 3.35 n/m3[74]
Wanquan River estuarySurface335 µm steel335–5000 µmAPPs6573 ± 2569 n/m3[75]
Kattegat/SkagerrakSurface10 µm–300 µm UFO10–300 µmPLY, PP39 ± 23 n/m3[76]
Niantic Bay-5 µm silver filter>5 µmPLY/PET180 ± 100 n/m3[77]
Indian OceanSurface-0–5000 µmPE0.01–372,000 n/m3[78]
Azhikkal estuarySurfaceFilter paper0–1000 µmPS12,000–640,000 n/m3[79]
Da Nang, VietnamSurface0.45 µm filter paper0.45–50 µmNylon, PTFE111,000–304,000 n/m3[80]
Ennore to KovalamSurface150 µm plankton net150–2000 µmPE, PP6–30 n/Location[81]
Beibu GulfSurface300 µm plankton pump330–5000 µmCE, PP, PE0.25 ± 0.05 n/m3[82]
Moheshkhali channelSurface180 µm manta250–5000 µmPP, PE0–0.1 n/m3[83]
Ganges River basin to
the Meghna estuary
Surface0.45 µm filter paper0–5000 µmLDPE, PS, PA55,260 ± 25,400 n/m3[84]
KarnatakaSurface300 µm steel300–5000 µmLDPE, PES114 ± 68.93 n/m3[85]
Al Hoceima BaySurface335 µm manta150–5000 µmPE, PP4.70 ± 4.50 n/m3[86]
Canary archipelagoSurface200 µm manta200–5000 µmPE10 ± 31 n/m3[87]
Delaware Bay estuarySurface80/153/20 µm plankton500–5000 µmPE, PP0.34 ± 0.80 n/m3[88]
Zandvlei estuary
(and its catchment)
Surface250 µm sieve<5 mmPE, PP2620 ± 410 n/m3[89]
Plymouth coastSurface53 µm plankton>53 µmPP, Rayon0.26–0.68 n/m3[90]
Meghna estuarySurface300 µm plankton0–5000 µmPE, PS128.89 ± 67.94 n/m3[91]
Brazilian baySurface53 µm sieve50–5000 µm-5180 n/m3[92]
Northeast Atlantic OceanSurface10/300 µm filters20–500 µmPET500 ± 700 n/m3[93]
St. Lawrence estuarySurface63/150/250/500/1000 µm sieves50–3200 µmPET, regenerated cellulose, PE,120 ± 42 n/m3[94]
Potter CoveSurface45 µm microfilter110–3630 µmCM, PET390 ± 440 n/m3[95]
GalapagosSurface200 µm plankton>50 µmPE, PP2.56 ± 0.78 n/m3[96]
NorwegianSurface15 µm filter<300 µmCPE, PP, PEST189 n/m3[97]
Southern Weddell Sea
off Antarctica
Surface10 µm filter11–500 µmPP, PA43.5 ± 83.8 n/m3[98]
Table 3. Assumptions of microplastic parameters.
Table 3. Assumptions of microplastic parameters.
VolumeOriginal shapeAssumed shape
FiberCylinder
FragmentCylinder
FilmCylinder
PelletSphere
ThicknessFramentOne-tenth of the statistical microplastic size
Film5–100 µm [100],
average thickness of 50 µm
Density850–1410 kg/m3 [99], average density of 1040 kg/m3 [7]
Table 4. Parameters for calculating the mass concentration of microplastics.
Table 4. Parameters for calculating the mass concentration of microplastics.
Amount concentrationA
Film thicknessH
Ceiling and floor of microplastic sizeLi
Mesh sizeD
Proportion of microplastics with different shapesP*
Proportion of each microplastic size interval with the same shapepi
Table 5. Mass concentrations of microplastics in estuaries, coastal areas, and oceans.
Table 5. Mass concentrations of microplastics in estuaries, coastal areas, and oceans.
Survey AreaLocationMesh TapeSize Range (µm)Proportion of Each Shape (%)Mass Concentration (mg/L)Reference
FiberFilmPelletFragment0–5 mm
Changjiang estuarySurface32 µm steel500–500079.109.1011.800.0026.808–287.886[9]
Bohai SeaSurface20 µm nylon20–500075.000.000.4524.556.579–59.398[32]
Bohai Sea5–30 m20 µm nylon20–500093.200.180.276.355.475–17.972[32]
Yellow River estuarySurface50 µm steel50–500093.120.004.742.1423.682–8252.771[11]
Pearl River estuarySurface50 µm steel50–40009.0043.0048.000.00109.288–349.84[12]
Haihe estuarySurface48 µm steel48–500054.466.7710.0928.6762.11–154.818[13]
Tianjin coastal areasSurface125 µm steel125–500052.102.600.0046.800.833–74.568[17]
Sanggou BaySurface30 µm steel30–500029.5621.8448.600.0059.165–514.501[33]
Zhangjiang estuarySurface300 µm manta330–500032.290.0017.6242.909.713–81.41[35]
East of Guangdong coastSurface50 µm steel50–200024.005.001.0070.004.842–44.111[19]
Zhuhai coastal areasSurface2.7 µm fiber1–500053.013.735.9437.3227.089–3075.263[36]
Donghai coastal areasSurface20 µm nylon20–500037.004.002.0057.004.318–67.4[37]
Eastern Indian OceanSurface330 µm manta0–500024.340.556.7568.360.007–0.094[22]
South China SeaSurface330 µm manta140–50809.810.78085.940.033–0.162[22]
Zhubi ReefSurface50 µm steel0–500044248614.894–1669.800[41]
Haikou BaySurface333 µm nylon0–490088.222.1603.830.005–0.009[42]
Paracel IslandsSurface20 µm nylon20–5000991000.004–0.012[44]
Survey AreaLocationMesh TapeSize Range (µm)Proportion in Each Shape (%)Mass Concentration (mg/L)Reference
FiberFilmPelletFragment<5 mm
Nansha Island
remote coral reefs
Surface333 µm neuston209–491732.78.9058.40.004–0.010[45]
Queen Charlotte SoundSubsurface62.5 µm copper62–500075.000.000.0025.001.725–112.082[54]
South coast of KoreaSurface50 µm mesh1–50002.100.002.9095.0023.254–2497.663[20]
Northwest coast of
Portugal
Subsurface30 µm mesh30–500039.0039.000.0022.008.985–55.573[21]
Charleston harborSurface63 µm mesh63–500056.000.002.5026.400.813–555.675[61]
Baram RiverSurface300 µm steel300–500021.5111.380.7166.41748.298–1852.417[15]
North coast of
Indonesia Surabaya
Coastal200 µm filter200–50003.320.003.7334.510.336–18.748[64]
Bandon BaySurface5 µm nylon5–50001000.000.000.000.001–0.004[66]
Rio De La Plata estuarySubsurface36 µm plankton36–500055.0521.601.3921.950.717–1.742[68]
Equatorial AtlanticSurface300 µm plankton0–500022.540070.420.00004–0.003[23]
Coast of CochinSurface250 µm steel250–500053.7730.530.008.791.518–25.101[72]
Sri LankaSurface380 µm neuston0–500099.50.2500.250.149–0.349[74]
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Zhang, J.; Li, Z.; Liang, J.; Liu, G.; Luo, Y.; Zhang, Q. Study on the Mass Concentration Distributions of Marine Microplastics in Estuaries and Coastal Areas. Water 2025, 17, 1136. https://doi.org/10.3390/w17081136

AMA Style

Zhang J, Li Z, Liang J, Liu G, Luo Y, Zhang Q. Study on the Mass Concentration Distributions of Marine Microplastics in Estuaries and Coastal Areas. Water. 2025; 17(8):1136. https://doi.org/10.3390/w17081136

Chicago/Turabian Style

Zhang, Jinfeng, Zhengqi Li, Jiaxiong Liang, Guangwei Liu, Yongming Luo, and Qinghe Zhang. 2025. "Study on the Mass Concentration Distributions of Marine Microplastics in Estuaries and Coastal Areas" Water 17, no. 8: 1136. https://doi.org/10.3390/w17081136

APA Style

Zhang, J., Li, Z., Liang, J., Liu, G., Luo, Y., & Zhang, Q. (2025). Study on the Mass Concentration Distributions of Marine Microplastics in Estuaries and Coastal Areas. Water, 17(8), 1136. https://doi.org/10.3390/w17081136

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