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

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).

Table 1. Origin of microplastics [7].

Plastics Class	Products and Typical Origin
PP	Rope, bottle caps, and netting
PS	Plastic utensils and food containers
PA	Netting and traps
PET	Plastic beverage bottles
PVC	Plastic film, bottles, and cups

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).

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. Abundance of marine microplastics.

Survey Area	Sampling Location	Mesh Type	Size Range	Material Composition	Number Concentration		Reference
Changjiang estuary	Surface	32 µm steel	0.5–5 mm	/	4137.3 ± 2461.5 n/m3		[9]
Donghai coast	Surface	333 µm manta	0.5–5 mm	/	0.167 ± 0.138 n/m3		[9]
Jiaojiang estuary	Subsurface	333 µm manta	0.333–5 mm	PP, PE, PVC	955.6 ± 848.7 n/m3		[10]
Oujiang estuary	Subsurface	333 µm manta	0.333–5 mm	PP, PE, PVC	680.0 ± 284.6 n/m3		[10]
Minjiang estuary	Subsurface	333 µm manta	0.333–5 mm	PP, PE, PVC	1245.8 ± 531.5 n/m3		[10]
Bohai Sea	Surface	20 µm nylon	0.02–5 mm	PP, PE, PET	2000–4200 n/m3		[32]
Yellow River estuary (wet season)	Surface	50 µm steel	50–200 µm	PE, PP, PS	654,000 n/m3		[11]
Yellow River estuary (dry season)	Surface	50 µm steel	50–200 µm	PET, PP	930,200 n/m3		[11]
Pearl River estuary	Surface	50 µm steel	50–500 µm	PA	8902 n/m3		[12]
Haihe estuary	Surface	48 µm steel	48–5000 µm	PP, PE, PS, PVC	1485.7 ± 819.9 n/m3		[13]
Yonding River estuary	Surface	48 µm steel	48–5000 µm	PP, PE, PS, PVC	788.0 ± 464.2 n/m3		[13]
Tianjin coastal areas	Surface	125 µm steel	125–5000 µm	PP, PE, PET	210–1170 n/m3		[17]
Sanggou Bay	Surface	30 µm steel	30–5000 µm	PE, PS, PP, CE	20,060 ± 4730 n/m3		[33]
South China sea	Surface	20 µm nylon	20–300 µm	AK, PCL, PEA	2569 ± 1770 n/m3		[34]
Xiamen coastal areas	Surface	330 µm manta	330–5000 µm	PE, PP	103–2017 n/m3		[18]
Zhangjiang estuary	Surface	300 µm manta	330–5000 µm	PP, PE, PS	50–725 n/m3		[35]
East of Guangdong coast	Surface	50 µm steel	/	/	3000–19,000 n/m3		[19]
Zhuhai coastal areas	Surface	2.7 µm fiber	1–5000 µm	PE, PP, PS	16,610 ± 4280 n/m3		[36]
Donghai coastal areas	Surface	20 µm nylon	20–5000 µm	PP, PET, PES	900 n/m3		[37]
Eastern Indian Ocean	Surface	330 µm manta	0–5000 µm	PP, PE	0.34 ± 0.80 n/m3		[38]
South China Sea	Subsurface	330 µm Plankton	0–5810 µm	PET, PP, PE	0.52 ± 0.42 n/m3		[39]
Eastern Indian Ocean	Subsurface	330 µm Plankton	0–5660 µm	PET, PP, PE	0.27 ± 0.19 n/m3		[39]
South China Sea	Surface	330 µm manta	140–5080 µm	PP, PE	0.61 ± 0.87 n/m3		[22]
South China Sea	Column	60 µm Filter	70–4720 µm	PET, PP, PE, AR	50 m	1.00 ± 0.43 n/m3	[22]
					200 m	0.63 ± 0.25 n/m3
					500 m	0.30 n/m3
					800 m	0.30 n/m3
					1000 m	0.20 n/m3
Hong Kong	Surface	153 µm Plankton	30–4960 µm	PP, HDPE, LDPE	0.51–279.09 n/m3		[40]
Zhubi reef	Surface	50 µm steel	0–5000 µm	PP, PA, PS, PVC	4933 ± 1369 n/m3		[41]
Haikou Bay	Surface	333 µm Neuston	0–4900 µm	PE, Rayon	0.44 ± 0.21 n/m3		[42]
Nanxun reef	Surface	48 µm steel	0–5000 µm	PVC, PA, PE	1733 n/m3		[43]
Paracel Islands	Surface	20 µm nylon	20–5000 µm	PET, PP	54.7 n/m3		[44]
Nansha Island remote coral reefs	Surface	333 µm neuston	209–4917 µm	PP, PE, PPC	0.0556 ± 0.0355 n/m3		[45]
Yellow Sea	Surface	500 µm bongo nets	350–44,990 µm	PE, PP	0.13 ± 0.20 n/m3		[46]
Danube	Surface	500 µm mesh	<20 mm	/	0.32 ± 4.67 n/m3		[47]
Tamar estuary	Surface	300 µm manta	Including more than 5 mm	PE, PS, PP	0.03 n/m3		[48]
Goiana estuary	Surface/bottom	300 µm plankton	<5 mm	/	0.26 n/m3		[49]
Calvi Bay	Surface	200 µm mesh	0.2–10 mm	PS	0.06 n/m3		[50]
Israeli Mediterranean coast	Surface	300 µm manta	0.3–5 mm	/	7.68 ± 2.38 n/m3		[51]
Northwestern Mediterranean Sea	Surface	333 µm manta	0.3–5 mm	PS	0.12 n/m2		[52]
Jade system (Southern North Sea)	Surface	40 µm steel	<5 mm	/	64,000 ± 194,000 n/m3		[53]
Queen Charlotte Sound	Subsurface	62.5 µm copper	398 ± 376 µm	/	7630 ± 1410 n/m3		[54]
Strait of Georgia	Subsurface	62.5 µm copper	513 ± 494 µm	/	3210 ± 628 n/m3		[54]
West coast of Vancouver Island	Subsurface	62.5 µm copper	558 ± 521 µm	/	1710 ± 1110 n/m3		[54]
Western English Channel	Surface	200 µm plankton	<5 mm	PA, PP	0.27 n/m3		[55]
Western Sardinian coast	Surface	500 µm manta	0.5–5 mm	PS	0.15 n/m3		[56]
Ligurian Sea and Sardinian Sea	Surface	200 µm mesh	<5 mm	/	0–10 n/m3		[57]
Portuguese coastal waters	/	180 µm mesh	<5 mm	PE, PP	0.002–0.036 n/m3		[58]
Brest Bay	Surface	335 µm manta	<5 mm	PE, PP, PS	0.24 ± 0.35 n/m3		[59]
South coast of Korea	Surface	50 µm mesh	<50 µm	PS, PP, PE	103,000 ± 59,000 n/m3		[20]
			50–100 µm		69,000 ± 41,000 n/m3
			100–200 µm		23,000 ± 20,000 n/m3
			200–500 µm		11,000 ± 11,000 n/m3
			500–1000 µm		2900 ± 3400 n/m3
			>1000 µm		2100 ± 2900 n/m3
West coast of the North Atlantic	/	333 µm manta	0.333–5 mm	PS	0.01–14.1 n/m3		[60]
Northwest coast of Portugal	Subsurface	30 µm mesh	30–5000 µm	PE, PP	651 ± 1660 n/m3		[21]
Charleston harbor	Surface	63 µm mesh	63–5000 µm	PS, PA, PE	6600 ± 1300 n/m3		[61]
Wanyah Bay	Surface	63 µm mesh	63–5000 µm	PS, PA, PE	30,800 ± 12,100 n/m3		[61]
Bega estuary	Surface	45 µm mesh	/	/	80–906 n/m3		[14]
Clyde estuary	Surface	45 µm mesh	/	/	23–198 n/m3		[14]
Hunter estuary	0–4 m	37 µm mesh	/	/	431–1892 n/m3		[14]
Blanca estuary	Surface	/	/	PA, CE	6500 ± 4010 n/m3		[62]
Chao Phraya River estuary	Surface	300 µm manta	/	PP, PE	48 ± 8 n/m3		[63]
Baram River	Surface	300 µm steel	300–5000 µm	PE, PET, PS	9300–18,000 n/m3		[15]
North coast of Indonesia Surabaya	Surface	200 µm filter	200–5000 µm	PS, PE, PP	380–610 n/m3		[64]
Tampa Bay	Surface	1.2 µm fiber	/	/	250–7000 n/m3		[65]
Bandon Bay	Surface	5 µm nylon	/	RN, PP, PE	630 ± 130 n/m3		[66]
Southeastern coastline of South Africa	Surface	65 µm mesh	65–5000 µm	PS	257.9–1215 n/m3		[67]
Rio De La Plata	Subsurface	36 µm plankton	36–5000 µm	PE, PP	5–110 n/m3		[68]
Chukchi Sea	Surface	330 µm manta	>333 µm	PET, PP, PE	0.23 ± 0.07 n/m3		[69]
Northwest Pacific	Surface	330 µm manta	>333 µm	PET, PP, PA	0.030 ± 0.017 n/m3		[69]
Bering Sea	Surface	330 µm manta	>333 µm	PET, PP, PA	0.091 ± 0.094 n/m3		[69]
Maldives	Surface	200 µm neuston	/	PE, PP	0.26 n/m3		[70]
Equatorial Atlantic	Surface	300 µm plankton	0–5000 µm	/	0.01 n/m3		[23]
Terengganu estuary	Surface	200 µm mesh	/	PA, PE, PP	1687 n/m3		[16]
Offshore waters of Malaysia	Surface	200 µm mesh	/	PA, PE, PP	1900 n/m3		[16]
Western tropical Atlantic Ocean	Surface	300 µm manta	0–5000 µm	/	0.03 n/m3		[71]
Southern Ocean	Surface	350 µm neuston	0–5500 µm	PE, PP, PS	0.031 n/m3		[24]
Coast of Cochin	Surface	250 µm steel	250–5000 µm	CE, PS, PE	751.7 ± 452.21 n/m3		[72]
Western Pacific	Surface	330 µm manta	/	ER, PP, PESPMA	0.06 ± 0.03 n/m3		[25]
Chabahar Bay	Surface	0.45 µm fiber	/	PE, PET, Nylon	86,000–362,000 n/m3		[73]
Sri Lanka	Surface	380 µm neuston	0–5000 µm	HDPEPP, PS	17.45 ± 3.35 n/m3		[74]
Wanquan River estuary	Surface	335 µm steel	335–5000 µm	APPs	6573 ± 2569 n/m3		[75]
Kattegat/Skagerrak	Surface	10 µm–300 µm UFO	10–300 µm	PLY, PP	39 ± 23 n/m3		[76]
Niantic Bay	-	5 µm silver filter	>5 µm	PLY/PET	180 ± 100 n/m3		[77]
Indian Ocean	Surface	-	0–5000 µm	PE	0.01–372,000 n/m3		[78]
Azhikkal estuary	Surface	Filter paper	0–1000 µm	PS	12,000–640,000 n/m3		[79]
Da Nang, Vietnam	Surface	0.45 µm filter paper	0.45–50 µm	Nylon, PTFE	111,000–304,000 n/m3		[80]
Ennore to Kovalam	Surface	150 µm plankton net	150–2000 µm	PE, PP	6–30 n/Location		[81]
Beibu Gulf	Surface	300 µm plankton pump	330–5000 µm	CE, PP, PE	0.25 ± 0.05 n/m3		[82]
Moheshkhali channel	Surface	180 µm manta	250–5000 µm	PP, PE	0–0.1 n/m3		[83]
Ganges River basin to the Meghna estuary	Surface	0.45 µm filter paper	0–5000 µm	LDPE, PS, PA	55,260 ± 25,400 n/m3		[84]
Karnataka	Surface	300 µm steel	300–5000 µm	LDPE, PES	114 ± 68.93 n/m3		[85]
Al Hoceima Bay	Surface	335 µm manta	150–5000 µm	PE, PP	4.70 ± 4.50 n/m3		[86]
Canary archipelago	Surface	200 µm manta	200–5000 µm	PE	10 ± 31 n/m3		[87]
Delaware Bay estuary	Surface	80/153/20 µm plankton	500–5000 µm	PE, PP	0.34 ± 0.80 n/m3		[88]
Zandvlei estuary (and its catchment)	Surface	250 µm sieve	<5 mm	PE, PP	2620 ± 410 n/m3		[89]
Plymouth coast	Surface	53 µm plankton	>53 µm	PP, Rayon	0.26–0.68 n/m3		[90]
Meghna estuary	Surface	300 µm plankton	0–5000 µm	PE, PS	128.89 ± 67.94 n/m3		[91]
Brazilian bay	Surface	53 µm sieve	50–5000 µm	-	5180 n/m3		[92]
Northeast Atlantic Ocean	Surface	10/300 µm filters	20–500 µm	PET	500 ± 700 n/m3		[93]
St. Lawrence estuary	Surface	63/150/250/500/1000 µm sieves	50–3200 µm	PET, regenerated cellulose, PE,	120 ± 42 n/m3		[94]
Potter Cove	Surface	45 µm microfilter	110–3630 µm	CM, PET	390 ± 440 n/m3		[95]
Galapagos	Surface	200 µm plankton	>50 µm	PE, PP	2.56 ± 0.78 n/m3		[96]
Norwegian	Surface	15 µm filter	<300 µm	CPE, PP, PEST	189 n/m3		[97]
Southern Weddell Sea off Antarctica	Surface	10 µm filter	11–500 µm	PP, PA	43.5 ± 83.8 n/m3		[98]

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.

Table 3. Assumptions of microplastic parameters.

Volume	Original shape	Assumed shape
	Fiber	Cylinder
	Fragment	Cylinder
	Film	Cylinder
	Pellet	Sphere
Thickness	Frament	One-tenth of the statistical microplastic size
	Film	5–100 µm [100], average thickness of 50 µm
Density	850–1410 kg/m3 [99], average density of 1040 kg/m3 [7]

Table 4. Parameters for calculating the mass concentration of microplastics.

Amount concentration	A
Film thickness	H
Ceiling and floor of microplastic size	Li
Mesh size	D
Proportion of microplastics with different shapes	P*
Proportion of each microplastic size interval with the same shape	pi

On the basis of Table 3 and Table 4, for fibers, the mass concentration can be expressed as follows:

$$C_{\mathrm{FB}}={{\displaystyle \sum _{i=1}^n\rho \pi \left(\frac{d}{2}\right)}}^2\times L_i\times p_i\times p_{\mathrm{FB}}\times A=\pi \rho A\cdot p_{\mathrm{FB}}{\displaystyle \sum _{i=1}^n\frac{1}{4}{d}^2{p}_i{L}_i}$$

(1)

For fragments, the mass concentration is as follows:

$$C_{\mathrm{FR}}={{\displaystyle \sum _{i=1}^n\rho \pi \left(\frac{L_i}{2}\right)}}^2\times {\displaystyle \frac{1}{10}}\times L_i\times p_i\times p_{\mathrm{FR}}\times A=\pi \rho A\cdot p_{\mathrm{FR}}{\displaystyle \sum _{i=1}^n\frac{1}{40}{p}_i{L}_i^3}$$

(2)

For films, the mass concentration is as follows:

$$C_{\mathrm{FI}}={{\displaystyle \sum _{i=1}^n\rho \pi \left(\frac{L_i}{2}\right)}}^2\times h\times p_i\times p_{\mathrm{FI}}\times A=\pi \rho A\cdot p_{\mathrm{FI}}{\displaystyle \sum _{i=1}^n\frac{1}{4}h{p}_i{L}_i^2}$$

(3)

For pellets, the mass concentration is as follows:

$$C_{\mathrm{PT}}={{\displaystyle \sum _{i=1}^n\rho \pi \times \frac{4}{3}\times \left(\frac{L_i}{2}\right)}}^3\times p_i\times p_{\mathrm{PT}}\times A=\pi \rho A\cdot p_{\mathrm{PT}}{\displaystyle \sum _{i=1}^n\frac{1}{6}{p}_i{L}_i^3}$$

(4)

Therefore, the mass concentration of microplastics is as follows:

$$\begin{array}{cc}\hfill C& =C_{\mathrm{FB}}+C_{\mathrm{FR}}+C_{\mathrm{FI}}+C_{\mathrm{PT}}\hfill \\ & =1000\pi \rho A\cdot \left[p_{FB}{\displaystyle \sum _{i=1}^n\frac{1}{4}{d}^2{p}_i{L}_i+p_{\mathrm{FR}}{\displaystyle \sum _{i=1}^n\frac{1}{40}{p}_i{L}_i^3}}+p_{\mathrm{FI}}{\displaystyle \sum _{i=1}^n\frac{1}{4}h{p}_i{L}_i^2}+p_{\mathrm{PT}}{\displaystyle \sum _{i=1}^n\frac{1}{6}{p}_i{L}_i^3}\right]\hfill \end{array}$$

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; L_i is the ceiling or floor in each particle size interval, m; p_i is the proportion of each microplastic size interval with the same shape; and P_FB, P_FR, P_FI, and P_FT 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).

Table 5. Mass concentrations of microplastics in estuaries, coastal areas, and oceans.

Survey Area	Location	Mesh Tape	Size Range (µm)	Proportion of Each Shape (%)				Mass Concentration (mg/L)	Reference
				Fiber	Film	Pellet	Fragment	0–5 mm
Changjiang estuary	Surface	32 µm steel	500–5000	79.10	9.10	11.80	0.00	26.808–287.886	[9]
Bohai Sea	Surface	20 µm nylon	20–5000	75.00	0.00	0.45	24.55	6.579–59.398	[32]
Bohai Sea	5–30 m	20 µm nylon	20–5000	93.20	0.18	0.27	6.35	5.475–17.972	[32]
Yellow River estuary	Surface	50 µm steel	50–5000	93.12	0.00	4.74	2.14	23.682–8252.771	[11]
Pearl River estuary	Surface	50 µm steel	50–4000	9.00	43.00	48.00	0.00	109.288–349.84	[12]
Haihe estuary	Surface	48 µm steel	48–5000	54.46	6.77	10.09	28.67	62.11–154.818	[13]
Tianjin coastal areas	Surface	125 µm steel	125–5000	52.10	2.60	0.00	46.80	0.833–74.568	[17]
Sanggou Bay	Surface	30 µm steel	30–5000	29.56	21.84	48.60	0.00	59.165–514.501	[33]
Zhangjiang estuary	Surface	300 µm manta	330–5000	32.29	0.00	17.62	42.90	9.713–81.41	[35]
East of Guangdong coast	Surface	50 µm steel	50–2000	24.00	5.00	1.00	70.00	4.842–44.111	[19]
Zhuhai coastal areas	Surface	2.7 µm fiber	1–5000	53.01	3.73	5.94	37.32	27.089–3075.263	[36]
Donghai coastal areas	Surface	20 µm nylon	20–5000	37.00	4.00	2.00	57.00	4.318–67.4	[37]
Eastern Indian Ocean	Surface	330 µm manta	0–5000	24.34	0.55	6.75	68.36	0.007–0.094	[22]
South China Sea	Surface	330 µm manta	140–5080	9.81	0.78	0	85.94	0.033–0.162	[22]
Zhubi Reef	Surface	50 µm steel	0–5000	44	2	48	6	14.894–1669.800	[41]
Haikou Bay	Surface	333 µm nylon	0–4900	88.22	2.16	0	3.83	0.005–0.009	[42]
Paracel Islands	Surface	20 µm nylon	20–5000	99	1	0	0	0.004–0.012	[44]
Survey Area	Location	Mesh Tape	Size Range (µm)	Proportion in Each Shape (%)				Mass Concentration (mg/L)	Reference
				Fiber	Film	Pellet	Fragment	<5 mm
Nansha Island remote coral reefs	Surface	333 µm neuston	209–4917	32.7	8.9	0	58.4	0.004–0.010	[45]
Queen Charlotte Sound	Subsurface	62.5 µm copper	62–5000	75.00	0.00	0.00	25.00	1.725–112.082	[54]
South coast of Korea	Surface	50 µm mesh	1–5000	2.10	0.00	2.90	95.00	23.254–2497.663	[20]
Northwest coast of Portugal	Subsurface	30 µm mesh	30–5000	39.00	39.00	0.00	22.00	8.985–55.573	[21]
Charleston harbor	Surface	63 µm mesh	63–5000	56.00	0.00	2.50	26.40	0.813–555.675	[61]
Baram River	Surface	300 µm steel	300–5000	21.51	11.38	0.71	66.41	748.298–1852.417	[15]
North coast of Indonesia Surabaya	Coastal	200 µm filter	200–5000	3.32	0.00	3.73	34.51	0.336–18.748	[64]
Bandon Bay	Surface	5 µm nylon	5–5000	100	0.00	0.00	0.00	0.001–0.004	[66]
Rio De La Plata estuary	Subsurface	36 µm plankton	36–5000	55.05	21.60	1.39	21.95	0.717–1.742	[68]
Equatorial Atlantic	Surface	300 µm plankton	0–5000	22.54	0	0	70.42	0.00004–0.003	[23]
Coast of Cochin	Surface	250 µm steel	250–5000	53.77	30.53	0.00	8.79	1.518–25.101	[72]
Sri Lanka	Surface	380 µm neuston	0–5000	99.5	0.25	0	0.25	0.149–0.349	[74]

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].

Figure 2. Probabilities of the shapes and material compositions of microplastics.

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.

Figure 3. Mass concentration distributions of marine microplastics (the color represents the average mass concentration of microplastics (mg/L)).

Figure 4. Mass concentrations of microplastics in different estuaries and coastal areas.

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.