Microplastic Variations in Land-Based Sources of Coastal Water Affected by Tropical Typhoon Events in Zhanjiang Bay, China

Abstract

Increasingly serious microplastic pollution in coastal areas and the ecological threats associated with plastics have attracted global attention. The frequency and intensity of typhoons have increased owing to global warming, strongly influencing the distribution and composition of microplastics in coastal ecosystems. In this study, the abundance, composition, diversity, and flux of microplastics in three estuaries and one sewage outlet in Zhanjiang Bay (ZJB) were analyzed. The average abundance of microplastics from land-based sources increased 3.6-fold from 14.19 ± 3.60 items/L before Typhoon Kompasu to 51.19 ± 28.53 items/L after the typhoon (p < 0.05). In addition, the proportion of fiber and large microplastics increased after the typhoon. In all samples, microplastics 100–330 μm in size were predominant, and blue was the most abundant color. The diversity in the color and size of microplastics increased after Typhoon Kompasu. The total daily flux of microplastics at the four stations entering ZJB was 3.95 × 10¹¹ items before the typhoon and 9.93 × 10¹¹ items after the typhoon, showing a 2.5-fold increase. This study demonstrated the influence of Typhoon Kompasu on microplastics from land-based sources of ZJB coastal waters and provided vital data for further study on MP pollution in coastal water ecosystems and the impact of typhoons on microplastics.

1. Introduction

Microplastics (MPs) are receiving increasing attention worldwide. MPs are plastic particles less than 5.0 mm in diameter [1], and their lowest size is not defined. Through UV degradation, wind wave breakage, and biodegradation, plastics are gradually broken down into MPs [2,3,4], which are classified as secondary MPs [5]. Primary MPs are directly released into the environment as small plastic particles, which include the fiber fragments released during the domestic washing process, road markings, wear and tear of tires, marine coatings, and cosmetics [6,7,8]. Presently, MP pollution is recognized as an emerging environmental crisis [9]. According to the latest data from the International Union for Conservation of Nature, more than 12 million tons of plastic end up in the oceans each year [10]. Approximately 10% of plastics remain in aquatic systems as persistent pollutants [4]. When MPs enter marine systems, they can become a persistent environmental hazard. Studies have documented that at least 690 marine species ingest MPs, with at least 17% of affected species listed as threatened or near-threatened [11]. Because of their relatively stable chemical properties and non-degradability, MPs are continually detected in oceans, rivers, lakes, glaciers, polar regions, and organisms worldwide [12,13,14,15].

Land-based inputs are the main sources of MPs in marine environments [16]. Land-based sources are complex dynamic systems that can accumulate, store, and reactivate plastic particles on different temporal and spatial scales [17]. MP footprints originate in the environment and are largely influenced by human activities. Studies have found a positive correlation between urban population density, urban development, and MP levels [14,18]. Considering the high complexity of land-based source systems, the fate and transportation of MPs remain relatively unknown. The behavior of plastic waste in freshwater, especially in rivers, varies substantially from that in marine environments [19,20,21,22]. Research on freshwater is necessary to investigate the movement and accumulation of MPs in rivers [17]. The distribution of nearshore MPs is also influenced by numerous environmental factors, such as tides, waves, currents, wind, and rainfall [23,24,25,26]. Rainfall caused by typhoons increases runoff/river discharge and introduces considerable amounts of terrestrial and anthropogenic organic matter to coastal and land-based sources [27,28,29]. As an external force, prevailing weather (wind and rain) has been shown to play a key role in influencing the distribution and abundance of MPs in water bodies [30]. Projections for the 21st century suggest that typhoons will be forced to extend their reach northward although their number is expected to remain relatively stable. Tropical storm intensity is likely to increase, particularly during the most intense storms [31]. Extreme weather events, such as typhoons, are expected to increase in frequency, exerting a strong influence on the MP structure of coastal ecosystems [32]. Typhoons have a strong influence on MP abundance, diversity, and composition [32,33].

Zhanjiang Bay (ZJB) is located in the southernmost area of mainland China, in the Leizhou Peninsula, Guangdong Province [34], and is a typhoon-prone area owing to its geographical location. Typhoon Kompasu (TK), the strongest typhoon in South China in 2021, occurred in the waters east of the Philippines. According to data from the People’s Government of Guangdong Province, TK landed in the coastal area of Boao Town, Qionghai City, Hainan Province, at 15:40 on 13 October 2021. From the night of 13th October through 14th October, Zhanjiang City was affected by typhoon circulation and cold air. Heavy and torrential rain persisted. Heavy rain (100–220 mm) was reported in some areas of southeastern Guangdong [35]. Extreme wind speeds, storm surges, and rainfall caused by typhoons result in widespread damage to coastal areas. Since the 1980s, with the rapid socioeconomic development of Zhanjiang, the impact of human activities, such as coastal terrestrial pollutant discharge, marine aquaculture, harbors, and terminal shipping, has increased [36]. MPs have been found on Zhanjiang’s beaches and in the fish of mangrove wetlands [37,38]. There have been relevant studies on the effects of typhoons on chlorophyll-a and nutrients in the South China Sea [39,40], but the effects of typhoons on MP variations from land-based sources in coastal water remain poorly understood.

Therefore, to better understand the impact of typhoons on MPs from land-based sources, four typical stations, including three estuaries and one sewage outlet, were selected to explore the MP pollution from land-based sources and compare the changes in the MP abundance, composition, diversity, and flux before and after TK. Thus, the main research objectives were to analyze the variations in the (1) abundance, (2) composition, (3) diversity, and (4) flux of the MPs in land-based sources affected by typhoon events in ZJB. This study is the first to investigate the impact of typhoons on MP pollution in estuaries and sewage outlets in ZJB. The results identify the different land-based sources of MPs and suggest emission reduction strategies, providing a theoretical basis and scientific support for studying the typhoons of MPs transported from land to sea and monitoring MP pollution in ZJB.

2. Materials and Methods

2.1. Study Area

ZJB is located in southern China and is surrounded by the main urban area of Zhanjiang City (Figure 1). It is a typical semi-enclosed bay with a narrow outlet (<2 km) and is connected to the South China Sea [40]. The total length of ZJB is 54 km from south to north, with a width of 24 km, covering an area of 193 km² [36]. In the past three decades, human activities have interfered with the coastal environment, especially in the rapidly urbanized and industrialized areas of ZJB [34]. More than 10 small seasonal rivers and sewage outlets discharge into the bay with varying water and nutrient loads, including the Suixi, Nanliu, and Lvtang Rivers [34,41]. However, many rivers have become canals for industrial and domestic sewage discharge with the advancement of economic activity and an increase in the population of Zhanjiang City [36].

In this study, samples were collected from the survey stations of the estuaries and sewage outlets of land-based inputs in ZJB. These four stations are significant land-based sources for ZJB. To determine the characteristics of the samples, four land-based sources were sampled before and after TK, and four land-based source input stations, including three river estuaries that discharge into the bay (S2, S3, and S4) and inshore outflows of one sewage outlet (S1), were investigated. Furthermore, S1 includes aquaculture wastewater of Donghai Island [36], which is the center of aquaculture. The Nanliu River (S2) is located near industrial plants, such as fertilizer production plants, and transports a substantial amount of industrial wastewater to the coastal waters [41]. S3 is the Lvtang River, where urban sewage is discharged, and thus, it is seriously polluted. S4 is the Suixi River, a stream with an area of 1486 km², which carries the runoff from a key agricultural area, with the maximum freshwater flow into ZJB [41]. Moreover, the Suixi watershed is mainly agricultural land. In addition, the discharge at the four stations varied before and after TK landing (Table 1).

2.2. Sampling and Analytical Method

Water samples were collected from four land-based input stations in September and October 2021. All tools were cleaned with distilled water before sampling. A glass bucket was used to collect 5 L of surface water samples from the top 50 cm of the water column, which were quickly transported back to the laboratory and stored in sealed glass jars at 25 °C until extraction [42,43]. Plastic tools and containers were not used during sampling or laboratory work to avoid additional plastic contamination. Water samples was taken and then passed through a 45 µm stainless-steel sieve. The residue on the sieve was washed with pure water and then transferred to a 100 mL beaker. To dissolve the natural organic matter in the water sample, 10 mL of 30% H₂O₂ and 10 mL of 0.05 M Fe (II) solution were added to the sample and then heated to 75 °C on a hot plate for 1 h and cooled at room temperature for 24 h. The samples were then filtered through a 10 μm glass fiber filter membrane under a vacuum pump and air dried at 75 °C. Then, three parallel samples were made in the same way at each location.

MPs from 0.045 to 5 mm were systematically quantified using a stereomicroscope (Nikon SMZ1270, Japan) with a magnification up to 40 × 40 (NOAA, 2015). The MPs were visually identified according to the following criteria: (1) particles without visible edges or not sufficiently stiff were not counted; (2) particles with uniform color distribution were counted; and (3) particles without visible cellular or biological structures were counted [44,45]. This study used micro-Fourier transform infrared spectroscopy (Frontier, PerkinElmer, Waltham, MA, USA) to identify suspicious MPs. Suspicious MPs identified by visual inspection were randomly selected for verification, and the selected suspicious MPs were the most common types. The spectra were all compared with the spectrogram database on the instrument for validation. Matched spectra with a matching rate higher than 70% were accepted [46].

2.3. Diversity in the Total Amount of MPs Discharged into Coastal Water

The complexity of the MP types and sources in the ZJB was estimated by calculating the diversity index D′ (MPs) [47,48,49]. In total, three types of D′ (MPs), namely size D′ (MPs), color D′ (MPs), and shape D′ (MPs), were calculated based on their shape, color, and size characteristics, respectively.

(1)${\mathrm{D}}^{\prime }=1-{\displaystyle \sum }_{\mathrm{i}=1}^{\mathrm{S}}{\left(\frac{{\mathrm{N}}_{\mathrm{i}}}{\mathrm{N}}\right)}^2$

where S is the number of MP categories, N_i is the number of MPs categorized into the ith type, and N is the total number of MPs in the sample.

2.4. Estimation of the Total Amounts of MPs Discharged into Coastal Water

One indicator of MP pollution is the flux of MPs into coastal waters. The flux of MPs per unit time was calculated using Equation (2).

(2)$\mathrm{Flux}=C\times Discharge\times 24\times 3600$

where flux (items/day) is the annual MP flux into the coastal water, $C$ (items/m³) is the average abundance of the MPs from the land-based source, and discharge (m³/s) was taken as the daily averaged discharge into ZJB. To estimate the MP flux from the land-based sources, the abundance of MPs from the data was used for calculation.

2.5. Statistical Method

Microsoft Excel 2019 was used for the MP data analysis, and the Origin2021 software was used to create the graphs. One-way ANOVA was used to analyze the significance of differences in MP abundance and diversity before and after the typhoon as well as the significance of MP abundance between sites. Confidence intervals for all tests were set at 95%. In addition, Pearson correlation analysis was used to test the correlation between daily MP flux and river discharge. All correlation analyses were considered statistically significant at p < 0.05. The station map was created using ArcGIS10.2.

2.6. Quality Assurance and Quality Criteria

Samples were collected based on the latest quality assurance and quality control standards, and strict control measures were implemented [50,51]. Non-textile laboratory coveralls, masks, and nitrile gloves were worn throughout the sample collection, extraction, and identification processes to minimize MP contamination. All solutions, including ferrous sulfate (FeSO₄) and distilled water, were filtered through a 45 μm screen prior to use. All the laboratory supplies were rinsed thrice with filtered distilled water before use and left to dry on a clean test stand covered with aluminum foil. In all the experiments, each medium was used with three blank controls. Filtered distilled water was used as a blank in the laboratory and treated according to the same procedure used for the samples [52]. The abundance in the average blank MP samples was 4 items/L, and the final corresponding MP data were calibrated with the corresponding blank.

3. Results

3.1. Variation in MP Abundance in Land-Based Sources Affected by Typhoon Events in ZJB

The variations in the MP abundance from land-based sources affected by the typhoon events in ZJB are presented in Figure 2. MPs were widely detected in all the water samples collected from the four stations of ZJB (p > 0.05), with significant temporal variations in their distribution. Before TK, the average abundance of MPs from the land-based sources was 14.19 ± 3.60 items/L, while that after the typhoon was 51.19 ± 28.53 items/L. A significant increase of 3.6-fold was observed post-TK compared to pre-TK (p < 0.05). Before TK, the highest MP abundance was found in the Lvtang River (S3), reaching 17.00 items/L, whereas that at the Donghai Island aquaculture sewage outlet (S1) was the lowest. After TK, the maximum abundance of MPs appeared in the Nanliu River (S2) (94.67 items/L), while the minimum abundance appeared in the Suixi River (S4), reaching 17.22 items/L. The maximum abundance of MPs after TK was found at S2, where the maximum difference was discovered before and after the TK. The abundance of MPs at other stations also significantly increased after the typhoon.

3.2. Variation in MP Composition in Land-Based Sources Affected by Typhoon Events in ZJB

In this study, the sizes of the collected MP samples were grouped into six categories: 45–100 μm, 100–330 μm, 330–500 μm, 500–1000 μm, 1000–2000 μm, and 2000–5000 μm (Figure 3a). The results indicate that the total MPs < 500 μm at four stations accounted for 75.3% and 69.0% before and after TK, respectively. Additionally, a decline in the proportion of small MPs (<5000 μm) after a typhoon was observed in a previous study on Hong Kong beaches [32]. Furthermore, the proportion of MPs < 100 μm before TK was higher than other sizes and exhibited the greatest difference before and after TK, with values of 26.0% and 11.7%, respectively. The introduction of large MPs by typhoons from the open sea might result in a decline in the proportion of small MPs in the surface water [53].

Based on the investigation results, MPs were identified in a total of 12 colors (Figure 3b). Among all the MPs before TK, the predominant color was blue (35.2%), followed by transparent (20.3%) and black (11.0%). The other colors accounted for less than 10%. The least-frequent colors were brown and green, with only 1.0 item/L, accounting for 1.8% of the total MPs. After TK, blue (25.5%) was also the most abundant in the land-based source samples, with black and multicolored MPs accounting for 24.0% and 19.5%, respectively. The other colors accounted for less than 15%. Yellow MPs had the lowest proportions, at only 0.8%.

The relative abundances of MPs of different shapes were presented in Figure 3c. Three different shapes were identified in all the samples: fiber, fragment, and film. Most of the MPs identified were fibers, which represented 69.2% and 75.1% of all the samples before and after TK, respectively. Fragment was the second most common shape at all the stations before and after TK, accounting for 21.6% and 13.7% of the MPs, respectively.

Figure 4 showed the typical MP characteristics before and after TK and the composition of typical MPs under micro-FTIR. Four main polymer types were found in selected samples of multicolored fragments: (a), blue films (b), fading fiber from blue to transparent (c), and black fiber (d); the main types included polyethylene (a), polypropylene (b), cellulose (c), and polyethylene terephthalate (d).

3.3. Variations in MP Diversity in Land-Based Sources Affected by Typhoon Events in the ZJB

Typhoons increase the diversity of polymer types on shores [32]. The results of this study indicated that the diversity in the color, size, and shape of MPs changed after TK (Figure 5), mostly exhibiting an increasing trend. The diversity in the color, size, and shape of MPs before TK was 0.70 ± 0.04, 0.70 ± 0.13, and 0.45 ± 0.10, respectively. The color, size, and shape diversities of the MPs after TK were 0.72 ± 0.02, 0.79 ± 0.02, and 0.44 ± 0.18, respectively. In this study, the color diversity of MPs after TK was 1.13 times greater than that before TK. The diversity of MPs after TK increased by an average of 1.05 times compared to that before TK (p > 0.05).

3.4. Variations in MP Fluxes in Land-Based Source Affected by Typhoon Events in ZJB

Results of the four monitoring stations reveal the change in MP flux before and after the typhoon (Figure 6). The daily MP flux into the sea before and after TK was calculated using Equation (2). Before TK, the total flux of MPs from the four stations entering ZJB was 3.95 × 10¹¹ items, and that after the typhoon was 9.93 × 10¹¹ items, which was 2.5 times that before TK. The highest proportion of MPs in the Suixi River (S4) before and after TK was 3.65 × 10¹¹ items (92.5%) and 7.78 × 10¹¹ items (78.3%), respectively, and the flux after the typhoon was 2.1 times that before TK. The flux of MPs in the Lvtang River (S3) before and after TK was the lowest, both accounting for 0.4%, respectively.

4. Discussion

4.1. Comparison with River Estuaries Worldwide

The abundance and distribution of MPs in rivers in China and worldwide are listed in Table 2. However, to further clarify the level of MP pollution in ZJB, it is necessary to compare these results with those of other studies. The concentration of MPs before TK was higher than that previously reported in the Fenghua River, China [54]; Pearl River, China [55]; Qian Tang River, China [56]; and rivers in the remote Tibet Plateau, China [57]. However, the concentration of MPs before TK was lower than that of the Shenzhen coastal areas of China [53]; Yellow River, China [58]; Shanghai estuaries, China [59]; Winyah Bay, South Carolina [60]; and Baram River, Malaysia [61]. Owing to the influence of the typhoon, the MP concentration after the typhoon was higher than that before the typhoon, but the MP concentration after the typhoon was lower than that in the Yellow River, China [58]. The concentration of MPs in ZJB before the typhoon was lower than that in the Shenzhen coastal areas, whereas the opposite was observed after the typhoon. Nevertheless, the results indicated that the level of MP pollution in Zhanjiang land-based sources before the typhoon was relatively low compared with rivers worldwide, but the pollution levels after the typhoon were severe. However, as the data obtained in this study were instantaneous, not corresponding to an entire day, this limitation may have led to inaccurate values. A more temporally continuous study is required to capture the contamination dynamics of MPs during typhoons.

4.2. Tracing the Sources of MPs in River Estuary

The origins of environmental MPs are anthropogenic and are affected predominantly by human activities. The shape, color, size, and other characteristics of MPs are the key factors that determine the origin of MPs [7,62]. The high percentage of MPs smaller than 330 μm was not surprising and is consistent with previous reports. Similarly, the most common size of MPs observed in the Yellow and Bohai Seas was 50–500 μm [62]. In the observation of MPs in Taihu Lake, 100–1000 μm MPs were common [63]. In all the samples, before and after TK, fibers were much more abundant than other shapes. Similarly, fibers constituted the most common MPs observed in the surface waters of the Qiantang River [56] and the coastal areas of South China and Guangdong [64]. In this study, the survey data before the typhoon demonstrated that the Lvtang River (S3) contained the largest proportion of fibers, which are primarily used in textile and garment manufacturing. The Lvtang River is located near an urban residential area, and the untreated discharge of urban domestic sewage has caused severe pollution [41]. Therefore, MP pollution in the Lvtang River is closely related to residential activities. After the typhoon, the fiber content was the highest in the Nanliu River among the other rivers. The Nanliu River is located near industrial plants for fertilizer production that discharge a large amount of industrial wastewater into the coastal waters [41]. MPs of several colors were found in this study, with a high proportion of blue MPs that are widely used in synthetic clothing [65,66] and in plastic materials used in mussel farms worldwide [67].

4.3. Typhoon Events Alters the Water Discharge and MP Fluxes into Coastal Water

The highest MP flux before and after TK was found in the Suixi River (S4), at 3.65 × 10¹¹ and 7.78 × 10¹¹ items, respectively. The MP flux of the Lvtang River (S3) was the lowest, at 1.41 × 10⁹ before TK and 3.87 × 10⁹ after TK. In this study, discharge was found to be one of the factors affecting the flux. Geographical factors determined by river shape affect river discharge and may lead to the accumulation of MPs [68]. The linear regression relationship between the flux and discharge is presented in Figure 7, and the regression equation obtained is as follows: Y = 2.26 × 10¹⁰ + 1.41 × 10⁹ X (n = 8, R² = 0.98, p < 0.001). Therefore, the flux of MPs into coastal waters is affected not only by the degree of MP pollution at the research stations but also by the discharge. The discharge was determined by the water depth, river depth, and water flow velocity at the research stations. The Suixi River provides the maximum freshwater discharge into ZJB. In addition, the Suixi River basin is predominantly agricultural land, and the use of agricultural plastic products also increases MP pollution [41]. Because the Suixi River has the largest river width and the strongest flow among the four stations, its discharge was the largest; therefore, the MP flux of the Suixi River was the largest. The source of MP pollution in the Lvtang River is primarily domestic sewage, but its discharge is relatively weak, resulting in a low MP flux. The typhoon wind speed, storm surge, site direction, topography, and proximity to urban areas are the main determinants of plastic pollution caused by the excessive scouring of the environment by cyclones [32]. The flow velocity of the river after the typhoon was greater than that before the typhoon, affecting the water discharge and thus the flux of MPs into the coastal waters. Before TK, the discharge followed the order S4 > S2 > S1 > S3, and the flux of the Nanliu River (S2) was lower than that of the Suixi River (S4). Industrial factories are located near the Nanliu River, and MP pollution is predominantly caused by industrial wastewater discharge into the river. However, one site worth noting is the Nanliu River, whose discharge before the typhoon is greater than that after the typhoon. There are more sluices controlling water flow in the upper Nanliu River and more sluices closed for flood control before TK, which may lead to smaller discharge after the typhoon than before the typhoon. However, after TK, the discharge was in the order S4 > S1 > S2 > S3, resulting in the flux of the aquaculture sewage outlet on the east island (S1) being lower than that of the Suixi River (S4).

4.4. Effective Mitigation Strategies against MP Pollution Based on Land-Based Sources

Fibers are one of the predominant forms of MPs found in water bodies from the seabed to remote inland freshwater lakes [69,70]. Similarly, this study identified a high proportion of fibers at different sampling stations. In general, urban sewage, particularly household pollution, is considered a major contributor to microfiber abundance [71]. Experiments on wastewater samples from household washing machines demonstrated that a single garment can produce > 1900 fibers per wash [72]. According to the results, fibers were not only abundant in areas where humans live but were also found in agricultural, breeding, and industrial areas. Therefore, fibers play a critical role in river MP pollution. These results are crucial for studying estuaries for management purposes, as pollution-control strategies should be adjusted according to the source of MPs [73,74]. To reduce MP pollution in the environment, it is necessary to improve the textile production process and reduce the release of fibers [63]. For example, synthetic textiles can reduce fiber loss by 80% [75], and installing a filter on the drain of the washing machine can reduce the discharge of fibers [72]. In addition, the primary factors affecting the flux are the concentration of MPs and water discharge. These two aspects should be considered in the process of alleviating MP pollution.

In this study, the concentration of MPs in the Lvtang River (S2) before the typhoon was high. The Lvtang River is located in a residential area. The concentration of MPs in the Lvtang River was high because of the discharge of untreated human domestic sewage. After the typhoon, the Nanliu River (S3), located near industrial plants for chemical fertilizer production, had the highest concentration of MPs. By comparing the relationship between the discharge and flux, discharge was found to have a significant influence on the flux of MPs. In this study, the Suixi River (S4) contained substantial MP concentrations before and after the typhoon. The Suixi River watershed covers an area of 1486 km², providing the largest freshwater discharge in the ZJB area [41]. Therefore, to reduce the discharge of MPs into the coastal waters, the entire Suixi River Basin should be monitored and rectified.

A model of discharge and MP flux was established in this study. This model can estimate the flux of MP input into the estuary according to the discharge of land-based sources, to monitor the MP flux of ZJB and make relevant mitigation measures more convenient. A comprehensive comparison of ZJB with other regions of China and other countries reveals that the level of MP pollution in ZJB is higher than that of land-based sources. The results indicate that the risk of MP pollution in the coastal areas of Guangdong is high; hence, it is necessary to reduce the use and waste of plastic products at the source [64]. Efforts should be made to educate citizens, raise awareness about MP pollution, improve the classification of plastic waste, and promote the recycling of plastic products. The government should implement reasonable policies based on local conditions and valuable experience, thus reducing MP emissions.

5. Conclusions

This study reveals that MPs are transported from land-based sources to ZJB coastal water under the influence of tropical typhoons. TK was found to have a significant impact on the composition of MPs in coastal land-based sources in this study. MPs at the four stations showed significant temporal and spatial variations before and after TK. The abundance of MPs was 3.6-fold higher, and the flux of MPs was 2.5-fold higher after TK than before TK, and the diversity of MPs mostly increased after TK. ZJB was considered to have high levels of MP pollution after comprehensive comparisons with other large Chinese water bodies. This study provides valuable information for understanding coastal MP pollution caused by global climate change and human activities. Overall, these results highlight the MP pollution in ZJB before and after typhoons and provide vital data for further treatment of MP pollution and theoretical support for future studies on MP pollution in coastal water ecosystems and the impact of typhoons on MPs. With the increasing frequency and intensity of typhoons under global warming and human activities, further studies on the effects of typhoons on MP transport in marine ecosystems are needed in the future.

Author Contributions

Conceptualization, P.Z.; methodology, P.Z. and J.Z.; software, S.W. and Q.J.; validation, S.W. and Q.J.; formal analysis, S.W. and Q.J.; writing—original draft preparation, S.W. and Q.J.; writing—review and editing, P.Z., S.W. and Q.J.; visualization, D.L.; supervision, L.Z.; project management, P.Z.; funding acquisition, P.Z. and J.Z. All listed authors made substantial, direct, and intellectual contributions to the work and are approved for publication. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data sharing is not applicable to this article.

Acknowledgments

We are grateful for the financial support provided by the Research and Development Projects in Key Areas of Guangdong Province (2020B1111020004); Guangdong Basic and Applied Basic Research Foundation (2020A1515110483); Guangdong Ocean University Fund Project (R18021); Science and Technology Special Project of Zhanjiang City (2019B01081); First-class Special Fund (231419018); Innovation Strong School Project (230420021) of Guangdong Ocean University; and Guangdong Province College Student Innovation and Entrepreneurship Plan (S202110566063). Special thanks go to the reviewers for their careful review and constructive suggestions. We thank all members of the research team and others involved in this study.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. Geographic location and land-based input stations in ZJB.

Figure 2. Variation in MP abundance in land-based sources affected by the typhoon events in ZJB.

Figure 3. Variations in MP composition in land-based sources affected by the typhoon events in ZJB. (a) Normalized size, (b) color, and (c) shape.

Figure 4. Typical MP characteristics and composition before and after TK: (a) multicolored fragment, (b) blue film, (c) fading fiber from blue to transparent, and (d) black fiber.

Figure 5. Variation indices for size, color, and shape diversity of MPs in land-based sources affected by the typhoon events in ZJB.

Figure 6. Variations in MP fluxes in land-based sources affected by the typhoon events in ZJB.

Figure 7. The liner regression relationship between MP flux and discharge.

Table 1

Investigation of estuaries and sewage outlets.

Station	Estuaries and Sewage Outlets	Longitude/°E	Latitude/°N	Discharge (Pre-TK)m3/s	Discharge (Post-TK)m3/s
S1	Aquaculture sewage outlet	110.2144	21.0733	6.75	25.60
S2	Nanliu River	110.3839	21.1528	16.80	10.80
S3	Lvtang River	110.4131	21.2139	0.96	1.20
S4	Suixi River	110.3883	21.3928	268.50	523.05

Table 2

Comparison with river estuaries worldwide.

Location	Collection Method	Abundance(Items/L)	Reference
Shenzhen coastal areas, China	Sieve, 0.45 µm	Pre-typhoon: 35.6 ± 22.1Post-typhoon: 42.0 ± 13.3	[53]
Fenghua River, China	Sieve, 63 µm	1.62 ± 0.88	[54]
Pearl River, China	Sieve, 50 µm	8.90	[55]
Qian Tang River, China	Nylon filter, 20 µm	1.61	[56]
Rivers in remote Tibet Plateau, China	Sieve, 45 µm	0.48–0.97	[57]
Yellow River, China	Sieve, 50 µm	380–582 (wet)623–1392 (dry)	[58]
Shanghai estuaries, China	Nylon filter, 10 µm	27.84 ± 11.81	[59]
Winyah Bay, South Carolina	Sieve, 63 µm	30.80 ± 11.10	[60]
Baram River, Malaysia	Sieve, 30 µm	9.3 ± 1.2728 to 18 ±1.4142	[61]
Zhanjiang land-based sources, China	Sieve, 45µm	Pre-typhoon: 14.19 ± 3.60Post-typhoon: 51.19 ± 28.53	This study

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