Changjun Li1
Lixin Zhu
Daoji Li
95% of researchers rate our articles as excellent or good
Learn more about the work of our research integrity team to safeguard the quality of each article we publish.
Find out more
- 1Ocean School, Yantai University, Yantai, China
- 2State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai, China
Microplastic pollution has emerged as an undeniable marine environmental issue. While a distribution map of microplastics in the upper ocean has been established, the patterns of microplastics within the water column remain unclear. In this study, a large-volume in situ filtration device with filtration efficiency of 30 m3/h was employed to investigate microplastics in the deep waters of the South China Sea. The abundance of microplastics ranged from 0.2 to 1.5 items per cubic meter (n/m3), with an average of 0.56 ± 0.40 n/m3. Microplastics are primarily fragments (72.58%) and fibers (20.97%), with the predominant polymer types being polypropylene (PP) and polyethylene terephthalate (PET). The average size of microplastics is 0.91 ± 0.97 mm, with no statistically significant differences observed across different water layers from 50 to 1000 meter (m). Non-metric Multidimensional Scaling (NMDS) analysis indicated that microplastics in the water column primarily originated from surface waters in the studied region. The occurrence of microplastics in the marine water column is a complex environmental process, influenced by a range of oceanographic mechanisms, including biological, chemical, and physical interactions. Our results provided reliable baseline data on microplastics in the water column of the South China Sea, contributing a better understanding to the vertical transport and fate of microplastics in this region.
1 Introduction
Plastics are one of the most significant synthetic materials of the Anthropocene, having markedly enhanced the quality of human life (Geyer et al., 2017). However, inadequate end-of-life management has led to a substantial volume of plastic products becoming solid waste, resulting in severe environmental pollution (Rajmohan et al., 2019). The physical, chemical, and biological factors in the environment continuously alter the morphological characteristics and quantity of plastics (Barnes et al., 2009; Lambert et al., 2014). Throughout these processes, the size of plastic debris diminishes while their abundance exponentially increases. Plastic debris with a maximum dimension of less than 5 millimeters (mm) are classified as microplastics (Andrady, 2011). Due to their small size, microplastics can easily translocate across various environmental compartments, leading to their widespread distribution in terrestrial, marine, and atmospheric environments (Auta et al., 2017; Evangeliou et al., 2020; Gasperi et al., 2018; Xu et al., 2020). Consequently, microplastic pollution has emerged as a global environmental concern.
Oceans, covering approximately 70% of the Earth’s surface, acts as a primary sink for both plastic waste and microplastics originating from land-based and ocean-based sources (Harris et al., 2021; Mai et al., 2020). Due to their lower density than seawater, the majority of microplastics remain suspended in the surface waters, where they are dispersed across global oceans through oceanic currents, including even the most remote open ocean and polar regions, while the rest will undergo vertical movement driven by various forces (Cózar et al., 2014; Lusher et al., 2015; van Sebille et al., 2015). These pollutants are particularly concentrated in hotspot areas such as river mouths, coastal waters, and gyre systems (Harris et al., 2021; Lebreton et al., 2018). However, due to technical and financial constraints, most studies have predominantly focused on the more accessible upper ocean layers, characterizing the horizontal distribution and transport of microplastics (Cózar et al., 2017; Li et al., 2021; Lusher et al., 2015).
Surface trawls, including manta, neuston and bongo trawls, are prevalently employed for collecting microplastics in the surface waters (0 to 0.5 m) (Mai et al., 2018). So far, microplastics surveys have been conducted across nearly all the world’s oceans, including the Indian Ocean, the Pacific Ocean, the Atlantic Ocean, the Southern Ocean, and the Arctic Ocean (Chen et al., 2021; Isobe et al., 2017; Kanhai et al., 2017; Li et al., 2021; Liu et al., 2021; Lusher et al., 2015; Zhang et al., 2020b). These studies generally use nets with mesh sizes ranging from 50 to 3000 μm, with 330 μm being the most common (Isobe et al., 2015; Li et al., 2021; Stock et al., 2019).
Microplastics in surface waters can descend into deeper layers of the water column under the influence of vertical forces, such as vertical currents, biofouling, and biological transport (Kooi et al., 2017; Long et al., 2015). As such, the water column is considered a key sink for microplastics that have been lost from surface waters. The ocean water column is typically divided into several zones: epipelagic (< 200 m), mesopelagic (200-1000 m), bathypelagic (1000-4000 m), abyssopelagic (4000-6000 m), and hadopelagic (> 6000 m), each characterized by distinct physical conditions (Webb et al., 2010). To study microplastics in the water column, researchers have employed a variety of sampling devices, including CTD (conductivity, temperature, depth) water sampling Rosette, Multi-nets, vertical trawl, modified submersible pump and ROV (remotely operated vehicle) (Bao et al., 2022; Choy et al., 2019; Courtene-Jones et al., 2017; Li et al., 2020). Among those, sampling methods that allow for large volumes are particularly useful for obtaining more accurate data on microplastic abundance and vertical transport (Li et al., 2020; Liu et al., 2019).
Microplastics have become an integral component of marine debris, entering the detrital food web and potentially posing ecological risks to deep-sea organisms (Courtene-Jones et al., 2017). For instance, microplastic fibers have been identified within deep-sea organisms, such as Crinoidea, Ophiuroidea, and Gammaridea, which inhabit depths exceeding 4000 m in the western Pacific Ocean (Zhang et al., 2020a). These microplastics can cause mechanical damage to digestive systems, induce stress and immune responses, and hinder their growth of marine organisms (Wright et al., 2013). Additionally, microplastics can also adsorb heavy metals, persistent organic pollutants, and pathogens, leading to complex pollution that poses an even greater threat to the fragile deep-sea ecosystems (Carbery et al., 2018; Guzzetti et al., 2018). Therefore, understanding the vertical distribution and transport pathways of microplastics in the water column is crucial for predicting future pollution trends and assessing the ecological risks to deep-sea ecosystems.
The South China Sea, the largest semi-enclosed marginal sea in the western Pacific, is an area of particular concern. Surrounded by continents, peninsulas, and islands, and receiving large rivers like the Pearl River and Mekong River, the South China Sea is a hotspot for microplastic contamination (Cai et al., 2018; Zhu et al., 2019b). These rivers were recognized as the leading contributors to the influx of riverine plastics and microplastics into the ocean, with the region also being a significant fishing ground and shipping corridor, where abandoned fishing gear and maritime waste further contribute to plastic pollution (Lebreton et al., 2017; Wang et al., 2019). Extensive studies have already been conducted on the distribution of microplastics in the surface waters of the South China Sea, including areas such as the Pearl River Estuary, Xisha Islands, Zhongsha Islands, and Nansha Islands (Cai et al., 2018). For example, Tan et al. (2020) reported an average microplastic abundance of 0.056 ± 0.034 n/m3 around remote coral reefs of Nansha Island, with polypropylene (PP) and polyethylene (PE) being the predominant types. Additionally, microplastic pollution has penetrated into the benthic sediment environment (Huang et al., 2023; Peng et al., 2019). Microplastics were also detected in zooplankton groups, seabirds and deep-sea fish, posing potential ecological risks to the biota (Sun et al., 2017; Zhu et al., 2019a, 2019b).
Vertical transport is an important mechanism for microplastics to travel from surface waters to the seabed. Cai et al. (2018) detected microplastics in the water column (0-200m) using a bongo trawl, with an average abundance of 0.045 ± 0.093 n/m3. Similarly, Wang et al. (2022) investigated the distribution of microplastic fibers in water column off southeast China. However, the information about microplastic pollution in deeper water columns of the South China Sea is still relatively limited, which impedes our understanding of their vertical transport. To address the above knowledge gaps, we conducted a sampling campaign to investigate microplastics in water column (50-1000 m) of the northwestern South China Sea using a large-volume in situ filtration device at specific layers for the first time. Our goals were to 1) provide baseline data on the abundance and characteristics of microplastics; 2) elucidate the potential relationship between microplastic distribution and environmental factors; and 3) evaluate and synthesize sampling methods and transport mechanisms of microplastics in the water column.
2 Materials and methods
2.1 Study area
A total of 11 samples of microplastics in water column were collected onboard the R.V. SHI YAN 3 at three stations in the northwestern South China Sea from July 25 to August 15, 2020. This sampling campaign was part of a scientific expedition supported by the National Natural Science Foundation of China (Figure 1). The sampling stations were positioned along the same longitude (115°E), with an average water depth of 1,515 m. Three layers of samples were collected at station W1 (50, 200, 500 m) and W2 (50, 200, 1000 m), while samples were taken from five depths at station W3 (50, 200, 500, 800 and 1000m). Stations W2 and W3 are located near the Zhongsha Atoll in the central South China Sea. Detailed information on the sampling stations was shown in Supplementary Table S1 of Supplementary Material.
Figure 1
Figure 1. Sampling stations for microplastics in water column of the northwestern South China Sea.
2.2 Sample collection
In this study, microplastics from specific water layers were collected using a large-volume in situ filtration device, namely plankton pump (KC Denmark, Figure 2). The device is powered by an external battery (24 V, 10 A/h), and offers a filtration capacity of 30 m3/h. To ensure the stability and proper orientation of the instrument within the water column, a counterweight flow guide is mounted opposite the battery. This guide plate is fitted with an annular zinc block to mitigate the corrosion of metal structure by seawater. Seawater was drawn into a nylon filtration mesh with size of 60 μm by the rotation of propeller, with the filtered water passing through the net and residual material collecting in a bottom tube. A 60 μm filter size has been widely used in similar studies, providing consistency with existing literatures (Li et al., 2020; Liu et al., 2019). Sampling delay time was adjusted based on deployment depth and speed to filter 10 m3 of water at each target layer. After filtration, the device was retrieved to the deck using an electric winch, where the filtration net and other parts were thoroughly rinsed with filtered water. The bottom collection bag was then removed, and the residual material was washed into sample bottles with Milli-Q water, followed by storage at low-temperature (4°C) until laboratory analysis. Additionally, environmental variables at each station were measured by a shipborne SeaBird Electronics SBE911 plus conductivity temperature depth (CTD), which were applied to analyze the potential relationship between microplastic distribution in water column and environmental factors.
Figure 2
Figure 2. Schematic diagram (a) and field operation (b) of the sampling device.
2.3 Laboratory analysis
The isolation and identification of microplastics were performed following the methodology outlined by Li et al. (2022). Organic matter in the samples was digested using a 30% H2O2 solution with the pH range of 4.5 to 5.5 to facilitate the separation of microplastics. Microplastics were subsequently transferred onto glass fiber filter membranes (GF/A Whatman, 1.6 μm pore size, 47 mm diameter) using a vacuum filtration system, followed by natural drying in sterile petri dishes. All filter membranes were meticulously examined under a stereomicroscope (Leica M165 FC, Germany), equipped with a high-resolution camera for capturing images of suspected microplastics. The polymer composition of microplastics were then confirmed using a Micro Fourier Transform Infrared Spectrometer (μ-FTIR) (Thermo Nicolet iN10, USA). In transmission mode, the detector spectral range was 675-4000 cm-1, with 32 scans co-added to target substances on the diamond crystals, achieving a resolution of 4 cm-1. The polymer type was confidently identified if the quality index of the spectrum, compared to common polymer reference libraries, exceeded 70%. To minimize external contamination, stringent preventative measures were implemented. For instance, the entire laboratory analysis was conducted in an enclosed ultra-clean laboratory. All instruments and reagents underwent rigorous decontamination procedures, such as high-temperature treatment or thorough rinsing, to eliminate any potential microplastic contamination. Additionally, laboratory personnel were strictly prohibited from wearing any clothing made of plastic materials.
2.4 Statistical analysis and data visualization
Statistical analysis was performed using the SPSS (version 19.0). Data are reported as mean ± standard deviation (SD). One-way ANOVA was used to evaluate significant differences in the size of microplastics among different water layers and sampling stations. NMDS analysis based on Bray-Curtis distance was employed to assess the similarity and homogeneity of microplastic compositions among different stations and water layers. To explore potential relationships between microplastic abundance and the physicochemical properties of seawater, a Mantel test was performed. Pearson correlation analysis was used to calculate the raw correlation between two distance matrices, with values ranging from -1 to 1. A value of -1 indicates a negative correlation, 0 represents no correlation, and 1 denotes a positive correlation. One of the matrices undergoes random permutation of its rows and columns to preserve symmetry, and the correlation coefficient between the permuted matrices was recalculated. This permutation procedure was repeated 9,999 times. The original observed correlation coefficient was then compared with the distribution of correlation coefficients generated by the permutations, with a p-value less than 0.05 indicating statistical significance. Before conducting one-way ANOVA and Pearson correlation analysis, the data were tested for normality. All the diagrams were produced by software ODV (version 5.1.7), Origin (version 2024), Rstudio (2021) and Edraw Max (version 13.0.1).
3 Results
3.1 Vertical distribution of microplastics in water column
In the northwestern South China Sea, the abundance of microplastics in the water column decreases with increasing depth, as shown in the Figure 3. Pearson correlation analysis indicated a significant negative correlation between microplastic abundance and water depth (r = -0.74, R² = 0.49, p < 0.01). Overall, the range of microplastics abundance was from 0.2 to 1.5 n/m3, with an average of 0.56 ± 0.40 n/m3. Specifically, the average microplastic abundance at the water layer of 50 m is 1.00 ± 0.43 n/m3, with the highest value (1.5 n/m3) occurred at station W1 (Figures 3a, d). The abundance of microplastics at the layer of 200 m ranged from 0.4 to 0.9 n/m3, with and average value was 0.63 ± 0.25 n/m3 (Figure 3c). The average abundance of microplastics in deep water of 500 and 1000 m was 0.3 and 0.2 n/m3, respectively. The abundance of microplastics showed a noticeable synchrony with water temperature, while exhibiting an opposite trend with salinity (Figure 3d). The halocline in the South China Sea was located at a depth of 200 m. The microplastic abundance, water temperature and salinity at each water layer of sampling stations was showed in Supplementary Table S1 of Supplementary Material.
Figure 3
Figure 3. Microplastic abundance in the depth profile at sampling stations (a: W1, b: W2, and c: W3), and the average microplastic abundance across all sampling stations (d). Temperature and salinity profiles are also indicated.
3.2 Characteristics of microplastic community in water column
A total of 62 microplastics were identified from water column in the South China Sea. The microplastics predominantly comprised fragments (72.58%), fibers (20.97%), lines (3.22%), and films (3.23%) (Figure 4). Microplastic fragments were present in all water layers, while films and lines were only observed at the water layer of 50 m. Microplastics exhibited seven colors: transparent, green, blue, black, red, gray, and white. Transparent and green microplastics were the most dominant, accounting for 43.55% and 22.58%, respectively. Most fragment-type microplastics were transparent in color, accounting for approximately 53.3%. The primary colors of fiber-type microplastics were black and blue, each accounting for 46.15%. There was the most diverse color of microplastics in water layer of 200 m, with six different colors observed. A total of eight polymer types were identified, including polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polycarbonate (PC), polytetrafluoroethylene (PTFE), phenoxy resin (PR), alkyd resin (AR), and polymethyl methacrylate (PMMA). PET (22.58%) and PP (22.58%) were the most prevalent, followed by PE and AR. Microplastics composed of PET polymers predominantly exhibited the forms of fibers and fragments. Film-type microplastics were entirely transparent and composed of PE. PP, PE, AR, and PR were mainly found in water layers above 500 m, while PMMA was only present at 1000 m. The sole PMMA microplastic identified was a green fragment, standing out as the only specimen of this polymer in the sample. The detailed information about microplastic characteristics in each water layer of sampling stations was provide in Supplementary Table S2 of Supplementary Material.
Figure 4
Figure 4. Shape, color and polymer type composition of microplastics in different water layers of three stations in the northwestern South China Sea.
3.3 Size distribution of microplastics
The size of microplastics in water column of the South China Sea ranged from 0.07 to 4.72 mm, with an average size of 0.91 ± 0.97 mm (Figure 5b). The one-way ANOVA results indicated that microplastic sizes do not differ significantly across various water layers (F4, 57 = 0.193, p > 0.05; Figure 5a). Overall, the average size of microplastics exhibited a trend of initially increasing before 500 m and then decreasing. The largest average size of microplastics was observed at the depth of 500 m (1.14 ± 1.46 mm). The lowest average size of microplastics was 0.58 ± 0.57 mm, locating at the depth of 1000 m. The largest microplastic size was recorded at a depth of 200 m of station W3, while the smallest was observed at 50 m at the same station. There was no significant difference in microplastic sizes among the three stations (F2, 59 = 0.689, p > 0.05). The frequency distribution chart showed that microplastics smaller than 1 mm accounted for more than 70% of the total (Figure 5c). Microplastics sized between 3-4 mm were not detected in the seawater samples.
Figure 5
Figure 5. Size of microplastics among the different depths (a), stations (b) and frequency distribution (c) in water column of the northwestern South China Sea.
4 Discussion
4.1 Sources and transport mechanisms of microplastics
PET and PP were identified as the predominant polymers of microplastics in the water column of the South China Sea. PET, the primary constituent of polyester fibers, is widely used in textiles and clothing, entering into oceans through sewage and surface runoff (Cesa et al., 2017). The density of PET (1.35-1.40 g/cm3) is higher than that of seawater (1.025 g/cm3), which typically causes it to sink to the lower water column and accumulate in seabed sediments (Zhang et al., 2020a). Similarly, PTFE (2.13-2.33 g/cm3) and PMMA (1.18 g/cm3), which also have higher densities than seawater, were detected in this study, consistent with findings from the eastern Indian Ocean and western Pacific Ocean (Li et al., 2020). In contrast, PP and PE are predominantly used in packaging materials and lines, ropes and nets for fisheries (Wang et al., 2019). Due to their lower densities, PP and PE are widely dispersed in the surface waters of the ocean (Li et al., 2021; Zhang et al., 2020b).
NMDS analysis was used to assess the similarity in microplastic composition among the three sampling stations. The stress value of the NMDS plot was 0.0856, which is considered acceptable, as a stress value below 0.1 suggests that the two-dimensional projection accurately reflects the relationships among the samples (Figure 6a). There were significant differences in the microplastic community (abundance, shape, color and polymer) among the three stations. Microplastics collected from station W1 and W2 showed substantial variation, while station W3 exhibited an intermediate pattern, possibly influenced by different ecological environments. The relative clustering of samples from station W2 indicated a more uniform ecological environment within this station, whereas the dispersion of station W1 samples suggested greater diversity in its community structure. The microplastic community in the photic zone (< 200 m) was relatively stable, while the aphotic zone exhibited greater diversity (> 200 m) (Supplementary Figure S1 of Supplementary Material). The partial overlap between the two zones suggested that microplastics from the upper ocean layers could be a critical source of microplastics in the deeper water. Kane et al. (2020) used numerical simulations to integrate microplastic pollution data with oceanographic information, showing that the transport and accumulation of seabed plastics in the Tyrrhenian Sea were controlled by bottom-layer temperature and salinity circulation. Deep-sea gravity flows not only transport nutrients like dissolved oxygen and organic carbon but also play a crucial role in the vertical transport of microplastics, as well as their erosion and accumulation on the seabed. Submarine canyons, unique topographical features on the continental slope that connect shallow seas with deep-sea plains, serve as important conduits for turbidity currents, which possess substantial capacity for transporting and depositing dense water masses and sediments (Canals et al., 2006).
Figure 6
Figure 6. Non-metric multidimensional scaling analysis (NMDS) of microplastics among three stations in water column (a) and Mantel test for microplastic abundance and characteristics and environmental factors (b) in the South China Sea. Note: Each point on the plot corresponds to a specific sample, with labels such as W1-50 indicating the station (W1) and the sampling water depth (50 m). The points are colored based on their station of origin: blue for W1, red for W2, and green for W3. ***indicates a highly significant correlation (p < 0.001), ** denotes moderate significance (p < 0.01), and * signifies statistical significance (p < 0.05).
Understanding the mechanisms of vertical transport of microplastics in the water column is essential for gaining insights into the environmental fate of marine microplastics. The density difference between microplastics and seawater plays a primary role in determining their buoyancy in the ocean. Consequently, the vertical transport of microplastics is influenced by factors that alter either the density of the microplastics or the properties of the seawater. The results of Mantel test showed that the abundance of microplastics in water column of the South China Sea was significantly negatively correlated with depth, salinity, and density, and significantly positively correlated with conductivity and dissolved oxygen (Figure 6b). These physicochemical properties of seawater are important factors in marine primary productivity and oceanic dynamic processes. Temperature and salinity, for instance, influence the density of seawater and drive ocean circulation patterns across different depths, which is known as thermohaline circulation (Schmidt et al., 2004). Wind-driven turbulent mixing is a key force behind the vertical transport of microplastics in surface water and is also influenced by heat flux (Brunner et al., 2015). According to an idealized diurnal heating model, daytime increases in sea surface temperature can inhibit the sinking of microplastics, while nighttime cooling can enhance their sedimentation flux (Kukulka et al., 2016). This dynamic interplay of factors suggests that the behavior of microplastics in the ocean is intricately linked to the physicochemical characteristics of seawater and oceanic circulation patterns.
Moreover, there was no statistically significant correlation between microplastic characterization (i.e. color, shape and polymer type) and water depth, indicating that the intrinsic properties of microplastics along cannot determine their buoyancy or floating behavior in the ocean. This is further corroborated by the presence of PP and PE particles, which have lower densities than seawater, in deep water (1000 m). In fact, biological processes are believed to play a critical role in regulating the vertical transport of marine microplastics (Porter et al., 2018; Zhao et al., 2018). The formation of “Plastisphere” on the surface of microplastics, along with the attachment of inorganic materials, can significantly alter the density, weight and surface characterizations of these particles. These modifications can effectively change their buoyancy, allowing microplastics to either remain suspended in the water column or sink to deeper depths (Zettler et al., 2013; Oberbeckmann et al., 2015; Zhao et al., 2018).
The biological pump is a major component of the marine food web, serving as a key food supply chain for organisms at lower trophic levels (Wieczorek et al., 2019). Laboratory studies have shown that microplastics can be ingested by copepods and transferred up the food chain to higher trophic levels (Setälä et al., 2014). Microplastics can also be transported vertically through biological aggregation, particle flocculation, and marine snow (Katija et al., 2017; Porter et al., 2018). Zhao et al. (2018) found that over 70% of marine snow contained microplastics, confirming its role as a significant carrier of microplastics in vertical transport. Furthermore, marine organisms, such as plankton and fish, ingest microplastics present in the surface waters (Desforges et al., 2015; Zhang et al., 2019). Through their vertical migration and subsequent fecal deposition, these organisms facilitate the translocation of microplastics into deeper strata of the water column (Law, 2017). Hence, the vertical transport of microplastics in the marine water column is a complex environmental process, influenced by a combination of oceanographic mechanisms, including biological, chemical, and physical interactions. These multifaceted processes underscore the challenges in predicting the behavior and fate of microplastics in marine ecosystems, highlighting the importance of considering a range of factors in studying their transport and impact.
4.2 Comparison with other global regions
A ubiquitous presence of microplastics has been reported in the water column across global oceans (Table 1). Our results showed that microplastics were present throughout the water column (50-1000 m) in the northwestern South China Sea. The abundance of microplastics in this study was found to be lower than in the eastern Indian Ocean but higher than in Western Pacific Warm Pool (Li et al., 2020; Zong et al., 2024). The South China Sea is a relatively semi-enclosed marginal sea, characterized by limited water exchange rates (Shu et al., 2018). It is significantly influenced by substantial riverine inputs and numerous coastal anthropogenic discharges, making it particularly vulnerable to human activities (Chau et al., 2023; Chen et al., 2021).
Table 1
Table 1. Global comparisons of content and distribution of microplastics in water column.
Regrading microplastic characteristics, fragment- and fiber-shaped microplastics were found to be the most prevalent types, consistent with findings from other regions. The primary sources of fibrous microplastics in the ocean are the aging and washing of textiles, which enter the marine environment through wastewater systems and atmospheric transport (Chan et al., 2024; Wang et al., 2021). Previous have demonstrated that fibers are the dominant microplastic shape in the Pearl River and offshore water of the north South China Sea (Lin et al., 2018; Wang et al., 2022). The shape and surface structure of microplastics influence the formation of biofilms. A previous study has shown that fibrous microplastics are more prone to biofouling compared to spherical microplastics (Chubarenko et al., 2016). Interestingly, microplastic fibers were found in deep water (1000 m) in this study. Microplastic sizes decreased significantly as the sampling depth increased in the Baltic Sea, West Pacific and East Indian Ocean (Li et al., 2020; Zobkov et al., 2019). Smaller microplastics are more likely to overcome the barrier of the thermohaline layer and penetrate into deeper water layers.
In contrast to our study, Kanhai et al. (2018) did not observe a significant correlation between microplastic abundance and the physicochemical properties of water, including temperature, salinity, and density in the Arctic Central Basin. Vertical thermohaline circulation driven by temperature and salinity gradient might act as a sink buffer for microplastics in the Baltic Sea (Zobkov et al., 2019). Additionally, a positive correlation between the size of microplastics and particulate organic carbon were found in the HAUSGARTEN water column, associated with biological processes (Tekman et al., 2020). The inconsistent results suggest that the environmental factors influencing microplastic distribution may vary significantly across regions, such as ocean currents and sources. For example, near-bottom currents can resuspend microplastics from sediments into the water column, increasing turbidity. A positive correlation between microplastic abundance and turbidity has been observed in the near-bottom waters of the Baltic Sea (Zhou et al., 2021).
The inconsistency in sampling methodologies is a significant barrier for comparing microplastic data across studies. Variations in sampling tools, mesh sizes, and sampling volumes contribute to this challenge (Figure 7). Submersible pump filtration systems, for example, allow for precise and quantitative collection of microplastic samples from specific depths, such as those used by Cai et al. (2018) to collect small-sized microplastics (44-330 μm) in the South China Sea. Similarly, shipboard underway water systems provide an opportunistic method for continuous sampling at specific depths, a technique successfully employed in studies in the Atlantic, Pacific, Indian, and Arctic Oceans (Desforges et al., 2014; Enders et al., 2015; Kanhai et al., 2018; Li et al., 2022). Vertical plankton trawls have also been employed for microplastic sampling, with studies such as those by Cai et al. (2018) and Gorokhova (2015) focusing on the water column in the South China Sea and the Baltic Sea, respectively. However, vertical trawls are limited to collecting microplastics from a set depth to the surface due to the nature of vertical towing. Additionally, multi-net plankton trawls have enabled more targeted sampling of specific water layers (Kooi et al., 2016; Marcel et al., 2018). Reisser et al. (2015) utilized a novel 12-layer multi-net to study microplastics in the upper 5 m of the North Atlantic gyre, finding that microplastic abundance decreases exponentially with depth. This method provides data highly comparable to those obtained from surface trawls and is of significant importance for studying the vertical transport of microplastics in the upper ocean. Moreover, CTD water sampling systems, consisting primarily of a CTD monitoring system and rosette water samplers (e.g., Niskin bottles), allow for water sampling at any depth and time. Dai et al. (2018) used a CTD water sampler to study the vertical distribution of microplastics in the water column (0-30 m) of the Bohai Sea, finding an average abundance of 2.2 n/L. Peng et al. (2018) employed the same method to quantify microplastic pollution in the water column of the Mariana Trench (2500-11000 m), demonstrating that the hadal and ultra-abyssal zones might serve as significant reservoirs for microplastics. Additionally, sampling methods that combine in situ filtration systems with CTD water samplers or Remote Operated Vehicles (ROVs) have been applied in Monterey Bay and the Arctic (Choy et al., 2019; Tekman et al., 2020).
Figure 7
Figure 7. Summary chart of sampling methods for microplastic pollution in aquatic environment.
However, due to limitations in sampling time and cost, a single water layer typically yields only a few hundred liters of water during an oceanographic expedition. In most studies, smaller volumes of water samples are often directly filtered onto membranes, minimizing the loss of microplastic fibers and reducing external contamination, making it more effective for studying small-sized microplastics. However, the smaller sample volumes may introduce greater variability and potential biases in the results. In general, small-volume sampling methods, such as those using CTD, yield significantly higher microplastic abundance than large-volume sampling methods like plankton pumps and exhibit greater variability (Table 1). Besides, large-volume sampling methods capture a broader variety of microplastics, reduce the loss of low-abundance types, and provide more accurate quantification. Liu et al. (2019) conducted the first field test of large-volume in situ filtration technique in the East China Sea using a plankton pump, validating its impact on improving microplastic quantification accuracy, particularly in deep water. The large-volume in situ filtration technology has been used to collect microplastics from the 4000 m layer of the Western Pacific, showing that microplastic abundance was at least 1–2 orders of magnitude lower than those from small-volume samples (Li et al., 2020). Though large-volume sampling methods are recommended for accurate and stable data, they also come with certain limitations, such as requiring more ship time for deployment, retrieval, and sample collection (Liu et al., 2020).
The mesh size of the sampling tools plays a critical role in determining the quantification of microplastic abundance and the composition of microplastics in the samples (Bai et al., 2022). Dris et al. (2015) compared the effectiveness of surface trawls with 80 μm and 330 μm mesh sizes, finding that the 80 μm mesh collected microplastics at a rate approximately 30 times higher than the 330 μm mesh, with fibers being the predominant type. Selecting the appropriate mesh size requires balancing the risk of clogging with the desired microplastic size range. Smaller mesh sizes improve the capture of finer particles but increase the likelihood of clogging, which can impede the filtration. Therefore, optimizing mesh size is crucial for achieving both efficiency in sample processing and comprehensive representation of the microplastic population under study.
5 Conclusions
In this study, we investigated the abundance and distribution of microplastics in water column of the South China Sea utilizing a large-volume in situ filtration device, namely plankton pump. Microplastics were detected at various water layers, with their abundance exhibiting a significant negative correlation with water depth. Notably, Microplastics with a density lower than seawater, such as PP, have infiltrated the deep water (800 m), indicating their potential to travel long distances through the water column. The NMDS analysis revealed a high degree of similarity between microplastic communities in the photic zone and the aphotic zone, suggesting that microplastics in the water column are primarily derived from surface waters. The vertical transport of marine microplastics from the surface water to deep water is driven by various oceanographic and biogeochemical processes. The findings from this study underscore the importance and promising advantages of in situ large-volume sampling methods, such as the plankton pump, for investigating microplastics in the deep-sea water column. By providing foundational data on microplastics pollution in deep waters of the northwestern South China Sea, this study contributes to the knowledge necessary for guiding efforts to mitigate microplastic contamination in this critical marine environment. Looking ahead, future research should focus on establishing standardized methodologies for microplastics pollution in water column, including concerning the smaller microplastics (< 60 µm), which have not been widely addressed. Furthermore, interdisciplinary collaboration involving in situ field observations, laboratory simulations, physical modelling, and biogeochemical studies is strongly encouraged to better assess the vertical transport mechanisms, fate of microplastics in the water column, as well as their impacts on the marine ecosystem and the biogeochemical cycling.
Data availability statement
The original contributions presented in the study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.
Author contributions
CL: Data curation, Funding acquisition, Investigation, Writing – original draft. LZ: Conceptualization, Funding acquisition, Methodology, Writing – review & editing. XW: Visualization, Writing – review & editing. DL: Resources, Supervision, Writing – review & editing.
Funding
The author(s) declare that financial support was received for the research, authorship, and/or publication of this article. This research was support and funded by Hainan Key R&D Program (ZDYF2025SHFZ062), Yantai University Start-up Foundation (HX22B116), the Asia-Pacific Network Project (CRRP2024-07MY-Zhu), Shanghai Science and Technology Committee (22dz1202800), National Natural Science Foundation of China (42306175, 42206154, 42271085).
Acknowledgments
The authors thank the all crews of the R.V. SHI YAN 3 administered by the South China Sea Institute of Oceanology, the Chinese Academy of Sciences.
Conflict of interest
The 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.
Generative AI statement
The author(s) declare that no Generative AI was used in the creation of this manuscript.
Publisher’s note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
Supplementary material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmars.2025.1556592/full#supplementary-material
References
Andrady A. L. (2011). Microplastics in the marine environment. Mar. pollut. Bull. 62, 1596–1605. doi: 10.1016/j.marpolbul.2011.05.030
Auta H. S., Emenike C. U., Fauziah S. H. (2017). Distribution and importance of microplastics in the marine environment: A review of the sources, fate, effects, and potential solutions. Environ. Int. 102, 165–176. doi: 10.1016/j.envint.2017.02.013
Bai M., Lin Y., Hurley R., Zhu L. X., Li D. J. (2022). Controlling factors of microplastic riverine flux and implications for reliable monitoring strategy. Environ. Sci. Tech. 56, 48–61. doi: 10.1021/acs.est.1c04957
Bao M. R., Huang Q. H., Lu Z. B., Collard F., Cai M. G., Huang P., et al. (2022). Investigation of microplastic pollution in Arctic fjord water: a case study of Rijpfjorden, Northern Svalbard. Environ. Sci. pollut. R. 29, 56525–56534. doi: 10.1007/s11356-022-19826-3
Barnes D., Galgani F., Thompson R. C., Barlaz M. (2009). Accumulation and fragmentation of plastic debris in global environments. Phil. Trans. Biol. Sci. 364, 1985–1998. doi: 10.1098/rstb.2008.0205
Brunner K., Kukulka T., Proskurowski G., Law K. L. (2015). Passive buoyant tracers in the ocean surface boundary layer: 2. Observations and simulations of microplastic marine debris. J. Geophys. Res. Oceans. 120, 7559–7573. doi: 10.1002/2015JC010840
Cai M., He H., Liu M., Li S., Tang G., Wang W., et al. (2018). Lost but can’t be neglected: Huge quantities of small microplastics hide in the South China Sea. Sci. Total Environ. 633, 1206–1216. doi: 10.1016/j.scitotenv.2018.03.197
Canals M., Puig P., de Madron X. D., Heussner S., Palanques A., Fabres J. (2006). Flushing submarine canyons. Nature 444, 354–357. doi: 10.1038/nature05271
Carbery M., O’Connor W., Palanisami T. (2018). Trophic transfer of microplastics and mixed contaminants in the marine food web and implications for human health. Environ. Int. 115, 400–409. doi: 10.1016/j.envint.2018.03.007
Cesa F. S., Turra A., Baruque-Ramos J. (2017). Synthetic fibers as microplastics in the marine environment: a review from textile perspective with a focus on domestic washings. Sci. Total Environ. 598, 1116–1129. doi: 10.1016/j.scitotenv.2017.04.172
Chan C. K. M., Lo C. K. Y., Kan C. W. (2024). A systematic literature review for addressing microplastic fibre pollution: urgency and opportunities. Water 16, 1988. doi: 10.3390/w16141988
Chau H. S., Xu S. P., Ma Y., Wang Q., Cao Y. R., Huang G. L., et al. (2023). Microplastic occurrence and ecological risk assessment in the eight outlets of the Pearl River Estuary, a new insight into the riverine microplastic input to the northern South China Sea. Mar. pollut. Bull. 189, 114719. doi: 10.1016/j.marpolbul.2023.114719
Chen H., Wang S., Guo H., Huo Y., Lin H., Zhang Y. (2021). The abundance, characteristics and diversity of microplastics in the South China Sea: Observation around three remote islands. Front. Environ. Sci. Eng. 16, 9. doi: 10.1007/s11783-021-1443-1
Choy C. A., Robison B. H., Gagne T. O., Erwin B., Firl E., Halden R. U., et al. (2019). The vertical distribution and biological transport of marine microplastics across the epipelagic and mesopelagic water column. Sci. Rep. 9, 7843–7843. doi: 10.1038/s41598-019-44117-2
Chubarenko I., Bagaev A., Zobkov M., Esiukova E. (2016). On some physical and dynamical properties of microplastic particles in marine environment. Mar. pollut. Bull. 108, 105–112. doi: 10.1016/j.marpolbul.2016.04.048
Courtene-Jones W., Quinn B., Gary S. F., Mogg A. O. M., Narayanaswamy B. E. (2017). Microplastic pollution identified in deep-sea water and ingested by benthic invertebrates in the Rockall Trough, North Atlantic Ocean. Environ. pollut. 231, 271–280. doi: 10.1016/j.envpol.2017.08.026
Cózar A., Echevarría F., González-Gordillo J. I., Irigoien X., Úbeda B., Hernández-León S., et al. (2014). Plastic debris in the open ocean. P. Natl. Acad. Sci. U.S.A. 111, 10239–10244. doi: 10.1073/pnas.1314705111
Cózar A., Martí E., Duarte C. M., García-de-Lomas J., Van Sebille E., Ballatore T. J., et al. (2017). The Arctic Ocean as a dead end for floating plastics in the North Atlantic branch of the Thermohaline Circulation. Sci. Adv. 3, e1600582. doi: 10.1126/sciadv.1600582
Dai Z., Zhang H., Zhou Q., Tian Y., Chen T., Tu C., et al. (2018). Occurrence of microplastics in the water column and sediment in an inland sea affected by intensive anthropogenic activities. Environ. pollut. 242, 1557–1565. doi: 10.1016/j.envpol.2018.07.131
Desforges J., Galbraith M., Dangerfield N., Ross P. S. (2014). Widespread distribution of microplastics in subsurface seawater in the NE Pacific Ocean. Mar. pollut. Bull. 79, 94–99. doi: 10.1016/j.marpolbul.2013.12.035
Desforges J., Galbraith M., Ross P. S. (2015). Ingestion of microplastics by zooplankton in the Northeast Pacific Ocean. Arch. Environ. Contam. Toxicol. 69, 320–330. doi: 10.1007/s00244-015-0172-5
Ding J. F., Jiang F. H., Li J. X., Wang Z. X., Sun C. J., Wang Z. Y., et al. (2019). Microplastics in the coral reef systems from Xisha Islands of South China Sea. Environ. Sci. Technol. 53, 8036–8046. doi: 10.1021/acs.est.9b01452
Dris R., Gasperi J., Rocher V., Saad M., Renault N., Tassin B. (2015). Microplastic contamination in an urban area: a case study in Greater Paris. Environ. Chem. 12, 592–599. doi: 10.1071/EN14167
Enders K., Lenz R., Stedmon C. A., Nielsen T. G. (2015). Abundance, size and polymer composition of marine microplastics ≥10μm in the Atlantic Ocean and their modelled vertical distribution. Mar. pollut. Bull. 100, 70–81. doi: 10.1016/j.marpolbul.2015.09.027
Evangeliou N., Grythe H., Klimont Z., Heyes C., Eckhardt S., Lopez-Aparicio S., et al. (2020). Atmospheric transport is a major pathway of microplastics to remote regions. Nat. Commun. 11, 1–11. doi: 10.1038/s41467-020-17201-9
Gasperi J., Wright S. L., Dris R., Collard F. (2018). Microplastics in air: Are we breathing it in? Curr. Opin. J. Environ. Sci. Health 1, 1–5. doi: 10.1016/j.coesh.2017.10.002
Ge X. Y., Xu F., Li B., Liu L. L., Lu X., Wang L. J., et al. (2024). Unveiling microplastic distribution and interactions in the benthic layer of the Yangtze River Estuary and East China Sea. Environ. Sci. Ecotechnol. 20, 100340. doi: 10.1016/j.ese.2023.100340
Geyer R., Jambeck J. R., Law K. L. (2017). Production, use, and fate of all plastics ever made. Sci. Adv. 3, e1700782. doi: 10.1126/sciadv.1700782
Gorokhova E. (2015). Screening for microplastic particles in plankton samples: How to integrate marine litter assessment into existing monitoring programs? Mar. pollut. Bull. 99, 271–275. doi: 10.1016/j.marpolbul.2015.07.056
Guzzetti E., Sureda A., Tejada S., Faggio C. (2018). Microplastic in marine organism: environmental and toxicological effects. Environ. Toxicol. Phar. 64, 164–171. doi: 10.1016/j.etap.2018.10.009
Harris P. T., Westerveld L., Nyberg B., Maes T., Macmillan-Lawler M., Appelquist L. R. (2021). Exposure of coastal environments to river-sourced plastic pollution. Sci. Total Environ. 769, 145222. doi: 10.1016/j.scitotenv.2021.145222
Huang L., Li Q. P., Yuan X., Li H. (2023). Microplastic pollution and regulating factors in the surface sediment of the Xuande Atolls in the South China Sea. Mar. pollut. Bull. 196, 115562. doi: 10.1016/j.marpolbul.2023.115562
Isobe A., Uchida K., Tokai T., Iwasaki S. (2015). East Asian seas: A hot spot of pelagic microplastics. Mar. pollut. Bull. 101, 618–623. doi: 10.1016/j.marpolbul.2015.10.042
Isobe A., Uchiyama-Matsumoto K., Uchida K., Tokai T. (2017). Microplastics in the Southern Ocean. Mar. pollut. Bull. 114, 623–626. doi: 10.1016/j.marpolbul.2016.09.037
Kane I. A., Clare M. A., Miramontes E., Wogelius R., Rothwell J. J., Garreau P., et al. (2020). Seafloor microplastic hotspots controlle by deep-sea circulation. Science 368, 1140–1145. doi: 10.1126/science.aba5899
Kanhai L. D. K., Gårdfeldt K., Lyashevska O., Hassellöv M., Thompson R. C., O’Connor I. (2018). Microplastics in sub-surface waters of the Arctic Central Basin. Mar. pollut. Bull. 130, 8–18. doi: 10.1016/j.marpolbul.2018.03.011
Kanhai L. D. K., Officer R., Lyashevska O., Thompson R. C., O’Connor I. (2017). Microplastic abundance, distribution and composition along a latitudinal gradient in the Atlantic Ocean. Mar. pollut. Bull. 115, 307–314. doi: 10.1016/j.marpolbul.2016.12.025
Katija K., Choy C. A., Sherlock R. E., Sherman A. D., Robison B. H. (2017). From the surface to the seafloor: How giant larvaceans transport microplastics into the deep sea. Sci. Adv. 3, e1700715. doi: 10.1126/sciadv.1700715
Kooi M., Nes E., Scheffer M., Koelmans A. A. (2017). Ups and downs in the ocean: effects of biofouling on vertical transport of microplastics. Environ. Sci. Technol. 51, 7963–7971. doi: 10.1021/acs.est.6b04702
Kooi M., Reisser J., Slat B., Ferrari F., Schmid M., Cunsolo S., et al. (2016). The effect of particle properties on the depth profile of buoyant plastics in the ocean. Sci. Rep. 6, 33882. doi: 10.1038/srep33882
Kukulka T., Law K. L., Proskurowski G. (2016). Evidence for the influence of surface heat fluxes on turbulent mixing of microplastic marine debris. J. Phys. Oceanogr. 46, 809–815. doi: 10.1175/JPO-D-15-0242.1
Lambert S., Sinclair C., Boxall A. (2014). Occurrence, degradation, and effect of polymer-based materials in the environment. Environ. Contam. Toxicol. 227, 1–53. doi: 10.1007/978-3-319-01327-5_1
Law K. L. (2017). Plastics in the marine environment. Annu. Rev. Mar. Sci. 9, 205–229. doi: 10.1146/annurev-marine-010816-060409
Lebreton L., Slat B., Ferrari F., Sainte-Rose B., Aitken J., Marthouse R., et al. (2018). Evidence that the Great Pacific Garbage Patch is rapidly accumulating plastic. Sci. Rep. 8, 4666. doi: 10.1038/s41598-018-22939-w
Lebreton L. C., van der Zwet J., Damsteeg J.-W., Slat B., Andrady A., Reisser J. (2017). River plastic emissions to the world’s oceans. Nat. Commun. 8, 15611. doi: 10.1038/ncomms15611
Lefebvre C., Saraux C., Heitz O., Nowaczyk A., Bonnet D. (2019). Microplastics FTIR characterisation and distribution in the water column and digestive tracts of small pelagic fish in the Gulf of Lions. Mar. pollut. Bull. 142, 510–519. doi: 10.1016/j.marpolbul.2019.03.025
Li J. W., Liu Y., Chen Q. Q., Cai Y. X., Liao Y. X., Liu L. L., et al. (2025). Revealing Microplastic risks in stratified water columns of the East China Sea offshore. Water Res. 271, 122900. doi: 10.1016/j.watres.2024.122900
Li D. J., Liu K., Li C. J., Peng G. Y., Andrady A. L., Wu T. N., et al. (2020). Profiling the vertical transport of microplastics in the west Pacific Ocean and the East Indian Ocean with a novel in situ filtration technique. Environ. Sci. Technol. 54, 12979–12988. doi: 10.1021/acs.est.0c02374
Li C. J., Wang X. H., Liu K., Zhu L. X., Wei N., Zong C. X., et al. (2021). Pelagic microplastics in surface water of the Eastern Indian Ocean during monsoon transition period: Abundance, distribution, and characteristics. Sci. Total Environ. 755, 142629. doi: 10.1016/j.scitotenv.2020.142629
Li C., Zhu L., Wang X., Liu K., Li D. (2022). Cross-oceanic distribution and origin of microplastics in the subsurface water of the South China Sea and Eastern Indian Ocean. Sci. Total Environ. 805, 150243. doi: 10.1016/j.scitotenv.2021.150243
Lin L., Zuo L. Z., Peng J. P., Cai L. Q., Fok L., Yan Y., et al. (2018). Occurrence and distribution of microplastics in an urban river: A case study in the Pearl River along Guangzhou City, China. Sci. Total Environ. 644, 375–381. doi: 10.1016/j.scitotenv.2018.06.327
Liu K., Courtene-Jones W., Wang X., Song Z., Wei N., Li D. (2020). Elucidating the vertical transport of microplastics in the water column: A review of sampling methodologies and distributions. Water Res. 186, 116403. doi: 10.1016/j.watres.2020.116403
Liu M., Ding Y., Huang P., Zheng H., Wang W., Ke H., et al. (2021). Microplastics in the western Pacific and South China Sea: Spatial variations reveal the impact of Kuroshio intrusion. Environ. pollut. 288, 117745. doi: 10.1016/j.envpol.2021.117745
Liu K., Zhang F., Song Z., Zong C., Wei N., Li D. (2019). A novel method enabling the accurate quantification of microplastics in the water column of deep ocean. Mar. pollut. Bull. 146, 462–465. doi: 10.1016/j.marpolbul.2019.07.008
Long M., Moriceau B., Gallinari M., Lambert C., Huvet A., Raffray J., et al. (2015). Interactions between microplastics and phytoplankton aggregates: Impact on their respective fates. Mar. Chem. 175, 39–46. doi: 10.1016/j.marchem.2015.04.003
Lusher A. L., Tirelli V., O’Connor I., Officer R. (2015). Microplastics in Arctic polar waters: the first reported values of particles in surface and sub-surface samples. Sci. Rep. 5, 14947. doi: 10.1038/srep14947
Mai L., Bao L. J., Shi L., Wong C. S., Zeng E. Y. (2018). A review of methods for measuring microplastics in aquatic environments. Environ. Sci. pollut. R. 25, 11319–11332. doi: 10.1007/s11356-018-1692-0
Mai L., Sun X., Xia L. L., Bao L. J., Zeng E. Y. (2020). Global riverine plastic outflows. Environ. Sci. Technol. 54, 10049–10056. doi: 10.1021/acs.est.0c02273
Manullang C. Y., Patria M. P., Haryono A., Anuar S. T., Fadli M., Susanto R. D., et al. (2024). Vertical distribution of microplastic along the main gate of Indonesian Throughflow pathways. Mar. pollut. Bull. 199, 115954. doi: 10.1016/j.marpolbul.2023.115954
Marcel L., Philipp G., Sebastian P., Marlene H., Philipp H., Helmut H. (2018). A methodology for measuring microplastic transport in large or medium rivers. Water 10, 414. doi: 10.3390/w10040414
Oberbeckmann S., Loder M. G. J., Labrenz M. (2015). Marine microplastic-associated biofilms – a review. Environ. Chem. 12, 551–562. doi: 10.1071/EN15069
Öztekin A., Üstün F., Bat L., Tabak A. (2024). Microplastic contamination of the seawater in the Hamsilos Bay of the Southern Black Sea. Water Air Soil pollut. 235, 325. doi: 10.1007/s11270-024-07138-w
Peng X., Chen M., Chen S., Dasgupta S., Bai S. (2018). Microplastics contaminate the deepest part of the world’s ocean. Geochem. Perspect. Let. 9, 1–5. doi: 10.7185/geochemlet.1829
Peng X., Dasgupta S., Zhong G., Du M., Xu H., Chen M., et al. (2019). Large debris dumps in the northern South China Sea. Mar. pollut. Bull. 142, 164–168. doi: 10.1016/j.marpolbul.2019.03.041
Porter A., Lyons B. P., Galloway T. S., Lewis C. (2018). Role of marine s.nows in microplastic fate and bioavailability. Environ. Sci. Technol. 52, 7111–7119. doi: 10.1021/acs.est.8b01000
Rajmohan K. V. S., Ramya C., Viswanathan M. R., Varjani S. (2019). Plastic pollutants: effective waste management for pollution control and abatement. Curr. Opinion Environ. Sci. Health 12, 72–84. doi: 10.1016/j.coesh.2019.08.006
Reisser J., Slat B., Noble K., Du Plessis K., Epp M., Proietti M., et al. (2015). The vertical distribution of buoyant plastics at sea: An observational study in the North Atlantic Gyre. Biogeosciences 12, 1249–1256. doi: 10.5194/bg-12-1249-2015
Schmidt M. W., Spero H. J., Lea D. W. (2004). Links between salinity variation in the Caribbean and North Atlantic thermohaline circulation. Nature 428, 160–163. doi: 10.1038/nature02346
Setälä O., Fleming-Lehtinen V., Lehtiniemi M. (2014). Ingestion and transfer of microplastics in the planktonic food web. Environ. pollut. 185, 77–83. doi: 10.1016/j.envpol.2013.10.013
Shu Y. Q., Wang Q., Zu T. T. (2018). Progress on shelf and slope circulation in the northern South China Sea. Sci. China. Earth. Sci. 61, 560–571. doi: 10.1007/s11430-017-9152-y
Singh S., Gray A. B., Murphy-Hagan C., Hapich H., Cowger W., Perna J., et al. (2025). Microplastic pollution in the water column and benthic sediment of the San Pedro Bay, California, USA. Environ. Res. 269, 120866. doi: 10.1016/j.envres.2025.120866
Stock F., Kochleus C., Bansch-Baltruschat B., Brennholt N., Reifferscheid G. (2019). Sampling techniques and preparation methods for microplastic analyses in the aquatic environment - A review. Trac. Trend. Anal. Chem. 113, 84–92. doi: 10.1016/j.trac.2019.01.014
Sun X., Li Q., Zhu M., Liang J., Zheng S., Zhao Y. (2017). Ingestion of microplastics by natural zooplankton groups in the northern South China Sea. Mar. pollut. Bull. 115, 217–224. doi: 10.1016/j.marpolbul.2016.12.004
Tan F., Yang H., Xu X., Fang Z., Xu H., Shi Q., et al. (2020). Microplastic pollution around remote uninhabited coral reefs of Nansha Islands, South China Sea. Sci. Total Environ. 725, 138383. doi: 10.1016/j.scitotenv.2020.138383
Tekman M. B., Wekerle C., Lorenz C., Primpke S., Hasemann C., Gerdts G., et al. (2020). Tying up loose ends of microplastic pollution in the Arctic: Distribution from the sea surface through the water column to deep-sea sediments at the HAUSGARTEN Observatory. Environ. Sci. Technol. 54, 4079–4090. doi: 10.1021/acs.est.9b06981
van Sebille E., Wilcox C., Lebreton L., Maximenko N., Hardesty B. D., van Franeker J. A., et al. (2015). A global inventory of small floating plastic debris. Environ. Res. Lett. 10, 124006. doi: 10.1088/1748-9326/10/12/124006
Wang X., Liu K., Zhu L., Li C., Song Z., Li D. (2021). Efficient transport of atmospheric microplastics onto the continent via the East Asian summer monsoon. J. Hazard. Mater. 414, 125477. doi: 10.1016/j.jhazmat.2021.125477
Wang X., Zhu L., Liu K., Li D. (2022). Prevalence of microplastic fibers in the marginal sea water column off southeast China. Sci. Total Environ. 804, 150138. doi: 10.1016/j.scitotenv.2021.150138
Wang T., Zou X., Li B., Yao Y., Zang Z., Li Y., et al. (2019). Preliminary study of the source apportionment and diversity of microplastics: Taking floating microplastics in the South China Sea as an example. Environ. pollut. 245, 965–974. doi: 10.1016/j.envpol.2018.10.110
Webb T. J., Vanden Berghe E., O’Dor R. (2010). Biodiversity’s big wet secret: The global distribution of marine biological records reveals chronic under-exploration of the deep pelagic ocean. PloS One 5, e10223. doi: 10.1371/journal.pone.0010223
Wieczorek A. M., Croot P. L., Lombard F., Sheahan J. N., Doyle T. K. (2019). Microplastic Ingestion by gelatinous zooplankton may lower efficiency of the biological pump. Environ. Sci. Technol. 53, 5387–5395. doi: 10.1021/acs.est.8b07174
Wright S. L., Thompson R. C., Galloway T. S. (2013). The physical impacts of microplastics on marine organisms: A review. Environ. pollut. 178, 483–492. doi: 10.1016/j.envpol.2013.02.031
Xu C. Y., Zhang B. B., Gu C. J., Shen C. S., Yin S. S., Aamir M., et al. (2020). Are we underestimating the sources of microplastic pollution in terrestrial environment? J. Hazard. Mater. 400, 123228. doi: 10.1016/j.jhazmat.2020.123228
Zettler E. R., Mincer T. J., Amaral-Zettler L. A. (2013). Life in the “plastisphere”: microbial communities on plastic marine debris. Environ. Sci. Technol. 47, 7137–7146. doi: 10.1021/es401288x
Zhang D. D., Liu X. D., Huang W., Li J. J., Wang C. S., Zhang D. S., et al. (2020a). Microplastic pollution in deep-sea sediments and organisms of the Western Pacific Ocean. Environ. pollut. 259, 113948. doi: 10.1016/j.envpol.2020.113948
Zhang F., Wang X., Xu J., Zhu L., Peng G., Xu P., et al. (2019). Food-web transfer of microplastics between wild caught fish and crustaceans in East China Sea. Mar. pollut. Bull. 146, 173–182. doi: 10.1016/j.marpolbul.2019.05.061
Zhang W., Zhang S., Zhao Q., Qu L., Wang J. (2020b). Spatio-temporal distribution of plastic and microplastic debris in the surface water of the Bohai Sea, China. Mar. pollut. Bull. 158, 111343. doi: 10.1016/j.marpolbul.2020.111343
Zhao S., Ward J. E., Danley M., Mincer T. J. (2018). Field-based evidence for microplastic in marine aggregates and mussels: implications for trophic transfer. Environ. Sci. Technol. 52, 11038–11048. doi: 10.1021/acs.est.8b03467
Zhou Q., Tu C., Yang J., Fu C. C., Li Y., Waniek J. J. (2021). Trapping of microplastics in halocline and turbidity layers of the semi-enclosed baltic sea. Front. Mar. Sci. 8. doi: 10.3389/fmars.2021.761566
Zhu C., Li D., Sun Y., Zheng X. B., Mai B. (2019a). Plastic debris in marine birds from an island located in the South China Sea. Mar. pollut. Bull. 149, 110566. doi: 10.1016/j.marpolbul.2019.110566
Zhu L., Wang H., Chen B., Sun X., Qu K., Xia B. (2019b). Microplastic ingestion in deep-sea fish from the South China Sea. Sci. Total Environ. 677, 493–501. doi: 10.1016/j.scitotenv.2019.04.380
Zobkov M. B., Esiukova E. E., Zyubin A. Y., Samusev I. G. (2019). Microplastic content variation in water column: The observations employing a novel sampling tool in stratified Baltic Sea. Mar. pollut. Bull. 138, 193–205. doi: 10.1016/j.marpolbul.2018.11.047
Keywords: microplastics, South China Sea, source, distribution, vertical transport
Citation: Li C, Zhu L, Wang X and Li D (2025) Elucidating the distribution and characteristics of microplastics in water column of the northwestern South China Sea with a large-volume in situ filtration technology (plankton pump). Front. Mar. Sci. 12:1556592. doi: 10.3389/fmars.2025.1556592
Received: 07 January 2025; Accepted: 21 February 2025;
Published: 11 March 2025.
Edited by:
Rakesh Kumar, Auburn University, United StatesReviewed by:
Chiara Gambardella, National Research Council (CNR), ItalyBo-Shian Wang, National Academy of Marine Research (NAMR), Taiwan
Jingmin Zhu, Zhejiang Ocean University, China
Copyright © 2025 Li, Zhu, Wang and Li. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Lixin Zhu, bGl4aW56aHUwMzA1QGhvdG1haWwuY29t