Horizontal and vertical distribution of microplastics in Gehu Lake, China Open Access

Plastic is a polymer that is widely used in various fields, and global plastic production reached 3.60 billion tons in 2018 (Li et al. 2022a, 2022b). Plastic particles with a diameter of under 5 mm produced by biochemistry in the environment and biological action are called MPs. MPs mainly come from beads of personal care products, decomposition waste, etc. Access to the water environment includes rainwater runoff (such as garbage), wastewater and sludge from sewage treatment plants, and atmospheric deposition (Wang et al. 2021). Because of their similar size to sediment and plankton, MPs are easily ingested by marine organisms and mammals. Bioaccumulation also occurs (Li et al. 2022a, 2022b). MPs can have major health effects on aquatic mammals because of their longevity and weight, which can drastically limit population size and ultimately result in extinction, especially among vulnerable populations. For example, Chinese mitten crabs are impacted by MPs. On the one hand, Chinese mitten crabs exposed to low levels of microplastics will experience oxidative stress quickly, which will affect the function of the antioxidant system in the hepatopancreas. On the other hand, exposure to high levels of microplastics will significantly inhibit the activity of antioxidant enzymes in the hepatopancreas, which will harm the oxidative system in young crabs (Kan et al. 2022). At the same time, MPs also have an impact on human health. Through cytotoxicity, acute reactions, and unfavorable immunological responses, MPs seriously endanger both marine life and humans. But the facts about MPs on population levels remain unclear (Lenaker et al. 2019). Scientists first discovered MPs in the marine environment, and a series of reviews describe the growing threat of plastic pollution to the marine ecosystem; for example, the impact of MPs on marine organisms. However, the inland freshwater environment more easily comes into contact with humans and is one of the sources of human life. Therefore, it is necessary to study the MPs of freshwater systems. In recent years, more and more attention has been paid to the problem of MPs in fresh water, and there are more and more reports on freshwater MPs.

Most of the existing research focuses on the existence and distribution of MPs in seawater, such as MPs in samples obtained from Chiloé Inland Sea, Patagonia, Chile (Jorquera et al. 2022), evaluation of MPs’ diversity and traits in the southern Black Sea (Terzi et al. 2022), and MPs in Indonesian waters: presence, spatial distribution, characteristics, and potential ecological impacts (Falahudin et al. 2020). Most studies on MPs in freshwater waters have poured attention into the water surface, while the mechanism of MPs' transfer from the surface layer to the bottom layer has not been fully studied (Li et al. 2020); for example, the distribution characteristics and environmental risk rates of MPs in the surface sediments of the southern East China Sea (Li et al. 2022a, 2022b), MPs in Tibet Plateau surface water and river sediment (Jiang et al. 2019), and MPs in the Han River's and Yangtze River's surface water (Wuhan section) (Wang et al. 2017). Microplastic pollution in the Yangtze River Basin has become a major concern in China. The Yangtze River Basin's 624 sample locations collected data on the detection methods and presence of MPs in river water, channel sediments, lake-reservoir water bodies, and lake-reservoir sediments (Zhang et al. 2021). Inland lakes and rivers are large reservoirs of MPs, and rivers are considered a significant source of marine MPs. MPs are found in the water and sediment in the Thames River, London, UK (Horton et al. 2017), the surface water of Weihe River (Ding et al. 2019), and the Manas River in Xinjiang Uygur Autonomous Region (Wang et al. 2020). The closed water environment is one of the reasons for the extreme deposition of MPs, so lake water sources have become the main accumulation points of MPs in freshwater ecosystems (Xu et al. 2021). For instance, MPs have been found in Panyang Lake, the biggest freshwater lake in China (Yuan et al. 2019). However, given the wide variety of MP abundance and distribution in different freshwater ecosystems, more study is required.

Gehu Lake (119°84′E, 31°35′N) is one of the five lakes in Jiangsu Province. It is situated northwest of Taihu Lake and in the north subtropical monsoon climate area. It crosses the Wujin District of Changzhou City and Yixing City of Wuxi City from north to south (Guan et al. 2020). In this study, the types, colors, sizes, and abundance of MPs were seen in the water body in the Gehu Lake region. The specific purpose is to understand the vertical and horizontal distribution of MPs in the Gehu area. This study's objective is to gather data that can fill in these knowledge gaps and give basic information for monitoring freshwater MP pollution.

West Taihu Lake is the most common name for Gehu Lake. With a size of about 250,000 acres (about 166.7 km²), it is the second-largest lake in southern Jiangsu after Taihu Lake. The surface of Gehu Lake is long and eggplant-shaped, and the shore of the lake is smooth and neat. The body of the lake is shallow and dish-shaped, and the bottom of the lake is flat. Along the lake, river, and harbors, the water nets are intertwined, the ponds are scattered, and the natural environment is beautiful. The time and origin of the formation of the lake have always been a mystery.

Surface water (0–20 cm), middle water (depth 90–110 cm), and bottom water (180–200 cm) were collected at these water intake points with clean submersible pumps (ZQB4.5X6-48, Zhejiang Green-Pump Co., Ltd). A 5.0 L lake-water sample was collected at each water intake point, which was repeated three times. The obtained water samples were passed through a stainless steel screen (500 mesh), and the filtered samples were poured into glass bottles and sealed with tin foil (to avoid contamination). Finally, the glass bottles were put in a ziplock bag and taken back to the laboratory.

The stainless steel filter screen required for sampling was cleaned by using ultrasonics and air guns. The lake water samples were sieved sequentially through 74 μm and 25 μm stainless steel meshes (this process requires repeated rinsing of the meshes and beakers with distilled water in case the microplastic particles are not completely transferred). Water was removed by filtration, and then HPLC-grade pure water was used to wash away the salts. The solid remaining after filtration was placed in a glass beaker, and the glass beaker was treated for 24 hours at 90 °C in a vacuum-drying oven. After weighing all of the organic materials in the beaker, they were removed using 20 mL of 0.05 M FeSO₄ (AR, Aladdin, China) and 30% H₂O₂ (AR, Aladdin, China) (wet hydrogen peroxide, WTO) (Yang et al. 2018). Each 20 mL of sample was fully mixed with 6 g NaCl, then the samples were covered with tin foil, and settled overnight. After that, a multipurpose vacuum pump with water circulation was used to filter the supernatant (SHB-III, Bangsi Instrument Technology Co., Ltd) and 0.22 μm nitrocellulose filter paper and the precipitated particles were discharged.

A stereo microscope (SGO-KK203, Shenzhen Shenshi Optics Valley Optical Instrument Co., Ltd) was used to visually inspect the particles on the filter paper, and pictures were taken with a Nikon stereo microscope at 10–80× magnification to record the shape, color, and particle size of each suspicious item of particulate matter. It should be highlighted that MPs can only be identified and eliminated when the MPs do not contain any biofilms or other organic or inorganic materials. The filter paper was separated into four equal pieces, each of which was counted to produce results that were as accurate as possible. In the initial stage of identification, suspicious microplastic particles were singled out and characterized based on their morphological features (Song et al. 2018). Pictures were taken of the separated MPs, which were classified according to their shape (spheres, fibers, fragments, films, and others), color (main surface colors were transparent, red, blue, etc.), and size (most microplastic particles are 100–500 μm). Attenuated total reflection–Fourier transform infrared spectroscopy was utilized to measure particle movements and identify the polymer. Spectra were recorded in the 4,000–5,000 cm⁻¹ region with a scan number of 32s and a resolution of 4.00 cm⁻¹. To get a better spectrum, the infrared spectrum was observed three times from various places on each particle and the resulting spectrum was matched with the spectral library on the instrument, and those with a matching index ≥ 0.7 were all MPs.

During sampling and experiments, to lessen contamination from the outside, various important steps were taken (Cowger et al. 2020). Use of plastic products was avoided. During this experiment, the experimenters wore nitrile gloves and cotton lab coats, kept the number of people in the testing room to a minimum, and wiped down the bench three times with distilled water and alcohol. Before the experiment, all of the test-worthy containers were thoroughly rinsed three times with deionized water, and when the containers were not in use, they were wrapped with tin foil to prevent the contamination of the test samples. To avoid cross-contamination, alcohol was also used to clean the equipment between samples. In a spotless fume hood, the pretreatment procedure was carried out. Filter sheets and Petri dishes were examined using light microscopy to separate fibers and particles from non-samples and to visualize them. The samples were then subjected to three replicate blank experiments using the same preprocessing procedure to remove external interferences, and no MPs were observed on the filter paper. This confirms that potential laboratory contamination was negligible.

All statistical comparisons are made using Excel (Office 2019). The microplastic abundance of lake water samples is the number of particles per litre. A map of the sampling points is drawn with ArcGIS10.7. Three parallel samples were set up at each sampling point. The abundance value of microplastics is expressed as an average value ± standard error. The numerical data diagram is drawn by Origin 2019.

The horizontal abundance of MPs in the water samples obtained at each sampling location ranged from 1.40 ± 0.86 to 5.53 ± 0.96 n/L (‘n’ means number) (as seen in Figure 2), with an average abundance of 3.13 ± 0.32 n/L. As shown in Figure 2, the abundance changes of MPs have a certain spatial specificity. It can be seen from the figure that the abundance of MPs in S₄ is the largest, and the abundance of MPs in S₇ is the smallest. The water intake point S₄ is located in the upper reaches of the lake, where the wide water flow speed of the lake is relatively low. The water intake point is densely populated and an amount of sewage wastewater is discharged. Because of its location in the lower reaches of the lake, the surface of the lake there has narrowed and the water flow speed has accelerated, which is not conducive to the settlement of MPs. The northernmost portion of the entire lake region has the highest MP content, which steadily decreases from north to south. Some studies have shown that the content of MPs is related to seasons (the highest in winter and the lowest in summer) (Wei et al. 2020), and some studies have shown that the content of MPs has little to do with seasons (Eo et al. 2019). In other studies, it has also been pointed out that the content of MPs is affected by human factors (Yin et al. 2020).

While collecting surface water at eight water collection points, water at different depths was also collected: surface water (0–20 cm), middle water (depth 90–110 cm), and bottom water (180–200 cm), and from the vertical distribution of MPs, it can be seen that MPs are present in water bodies at different depths. In comparison with surface water (1.03 ± 0.14 n/L), the average microplastic abundance in middle water (2.28 ± 0.24 n/L) and bottom water (1.03 ± 0.14 n/L) was noticeably lower (p < 0.05). Surface water held 49.9%, middle water held 34%, and bottom water had 16.1% of the MPs, respectively (excluding surface sediments). Additionally, there were much fewer MPs in the bottom water than in the middle. Across the eight sites, Figure 2 shows that the average MP abundance was marginally higher in the middle water body than in the bottom water body.

Figure 2 shows that, at almost all water intake locations, the abundance of MPs in the middle and bottom water bodies is lower than that of the surface water body. Using the same sample technique, MP abundance in the middle and bottom water bodies is comparable (Kruskal–Wallis test, p = 6.64 × 10⁻³). Due to their small size and light weight, MPs are thought to float more easily on the water's surface, which may account for the difference in microplastic abundance between surface and middle water. A limited number of MPs were also found in the bottom water, possibly as a result of some MPs being impacted by the wind and waves entering the bottom water. Some water intake places have identical MP abundances in the bottom and middle water bodies, however, in the water bodies obtained from these eight water intake points, the middle water body has a higher MP abundance than the bottom water body. In general, the abundance of MPs from surface water to bottom water shows a decreasing trend.

MPs were widespread throughout the waters with the aforementioned average abundances. Compared with the microplastic abundance of global freshwater ecosystems, this result is significantly lower than that of Lake Qius (234 n/L) (Fischer et al. 2016) and Weihe River (360–1,320 n/L) (Ding et al. 2019). The abundance of MPs is lower than that of Taihu Lake (11.0–234.6 n/L) (Su et al. 2016), but the difference is not particularly large. Possible causes of microplastic deposition include rate, temporal variation, and bioturbation of deposited microplastic emissions. Due to the combination of vertical movements brought on by biological activity (such as biological pollution, excretion, etc.), the majority of MPs are trapped in the water column in coastal regions (Song et al. 2018). As another example, because MPs can form biofilms underwater, the formed biofilms will affect the hydrophobicity of their surfaces, reduce the buoyancy of MPs, and increase their sinking rates. In addition, microplastic particles may agglomerate with each other due to the production of viscous microcoagulants by microorganisms. Conversely, it may increase the sinking rate. Because MPs are not only present in the surface layer, it is necessary to detect MPs in the entire water column. It should be noted that the use of field data may overestimate the total amount of MPs in the Gehu water. This is because, in this study, the number of MPs was estimated with a small number of samples (Matsuguma et al. 2017).

Most MPs are produced based on human activities. Located in east China, Changzhou is one of the 27 cities in the central area of the Yangtze River Delta and a famous national historical and cultural city. Therefore, its tourism industry has been developing rapidly. A large number of tourism activities will bring a certain flow of people and garbage, which will lead to an increase in the content of MPs in the water (Feng et al. 2020). Fishing nets and fishing lines used in fishing activities will lead to an increase in the content of MPs in the water (Alfonso et al. 2020). There is a certain relationship between urban soil cover and the abundance of freshwater MPs (Mai et al. 2021). Additionally, there is a connection between population density and the abundance of MPs in its adjacent waters (Bertoldi et al. 2021). The above views indicate that the abundance of S₄ MPs may be affected by human activities, and these factors will affect the content of MPs in the lake.

Fibers are been found to be the most abundant MPs (especially in biota) in studies of inland water systems (Zhang et al. 2018), and textiles were considered to be the major contributors to fibers. Because fibrous MPs are light in weight and low in their density, they often float on the water surface and do not easily sink into the water. As a result, surface water bodies have the highest concentration of fibrous MPs, while depth decreases the concentration. This outcome is in line with other earlier investigations of fiber-dominated waters in China's inland seas (Zhao et al. 2014; Su et al. 2016). Gehu Lake is surrounded by factories and farms and has more than 3,000 homes. A bustling fishing industry may create much-aged fishing gear, such as fishing nets and other items, which will increase the amount of microplastic fibers as agriculture and aquaculture flourish. The second content is microplastic fragments, which are mostly produced by the breakdown of packaging plastics. In the most recent detection, for the convenience of statistics, regular spheres (or spheres) and irregular particles are collectively referred to as particles. Granular MPs may be mainly derived from resin granules and plastic goods. By comparison, spheres and films accounted for only 12 percent. The content of film-like MPs is the least (mostly low-density polyethylene (PE) and polypropylene (PP)), which may be related to their shape (usually flakes and larger particle sizes). Plastic bags and packaging materials may be a major source of film-like MPs (Wan et al. 2022). Surface-water, middle-water, and bottom-water MPs all had comparable shapes and compositions, with fibers making up 79%, 81%, and 82% of each, respectively, and debris making up 21%, 18%, and 16%. When viewed collectively, our findings imply that human waste and fishery activities may be the primary causes of microplastic pollution in Gehu Lake.

The findings indicated that the four MPs with the highest content of MPs in the water samples were PE, PS, PES, and PP with matching rates of 96.74%, 88.23%, 83.11%, and 82.38%. The majority of these elements come from everyday items including clothing, textiles, industrial raw materials, and packaging. PP has good chemical stability, low water absorption and excellent electrical insulation. It is mainly used for film products (such as industrial packaging film, drug and food packaging film, etc.), followed by daily necessities, fishing nets, etc. PES has high strength, high modulus, good resilience, is second only to cotton in abrasion resistance, and poor absorption of water; it is mainly from commercial and domestic uses. PS is often used to make various disposable containers that need to withstand the temperature of boiling water, as well as disposable foam lunch boxes. PP is widely used in the production of clothing, blankets and other fiber products, medical devices, automobiles, bicycles, parts, transmission pipelines, chemical containers, etc., as well as food and drug packaging. Consequently, to prioritize actions on certain microplastic sources, integrated methods focus on microplastic sources. It is important to investigate areas like modeling and isotope-tracking further. In general, obtaining MPs measured in water samples is closely related to human activities.

The vertical and horizontal distribution of MPs in Gehu Lake's water is provided by this study. The horizontal distribution of MPs, as well as the shape, color, size, and chemical composition, all have unique spatial distribution characteristics. The microplastic abundance in Gehu Lake varies from 1.40 ± 0.86 to 5.53 ± 0.96 n/L, with an average abundance of 3.13 ± 0.32 n/L. In all collected water samples, there were significant differences in the abundance of MPs. From a chronological standpoint, the dry season had a substantially higher microplastic abundance than the rainy season. The vertical variation trends of MPs at the same sampling point were the same. From a spatial point of view, the surface water body has the highest concentration of MPs, and the middle water body has a slightly greater concentration of MPs than the bottom layer. The abundance of MPs is influenced by a variety of circumstances. Fibrous MPs make up 70% of all MPs, whereas granular and thin films make up the least amount of MPs. The particle size of most microplastic particles is between 100 and 500 μm, and this particle size accounts for 51.90% of the microplastic particles. Transparent color is the predominant color of microplastic particles, followed by blue and red. Among the MPs contained in water at different depths, the four most common polymers are PE, polystyrene, PES, and PP. Gehu Lake is not currently seriously polluted by MPs, but microplastic pollution may still be a problem in Gehu Lake. This study's experimental results provide a useful benchmark for the degree of microplastic pollution in Gehu Lake's water. The features of microplastics, the surrounding environment, and sampling techniques will all have an impact on the abundance and distribution of microplastics since the abundance and distribution patterns of microplastics in different freshwater settings are quite diverse. It provides the experimental basis and has a certain reference value for freshwater lakes with a similar water environment to Gehu Lake.

However, there is still a certain error in this experiment, because the wind was strong on the day of sampling, the fluidity of the water body was high, and we could not avoid microplastics in the atmosphere during the entire experiment. Moreover, different collection, separation, and identification methods may also lead to experimental errors. In future experiments, the methods of MPs' sampling, separation, and identification can be improved to minimize experimental errors. There may be a connection between the prevalence of microplastics in rivers and lakes and the closeness of particular industries, wastewater emissions, and land-use patterns within the watershed. The sources, sinks, and movements of these emerging contaminants at the watershed scale should be the subject of future investigation. Field research will offer light on the issue of preventing microplastic development in soil and water when paired with comprehensive watershed modeling and risk-mapping. Thus, new techniques and modeling tools for determining the presence, fate, and transportation of microplastics in various ecosystems should be developed in the course of future research. Microplastics’ immediate and long-term effects should both be evaluated.

This work was supported by grants from the National Natural Science Foundation of China (21607017). The authors would like to thank the laboratory facilities of Changzhou University. The authors would like to thank Xia Xu for valuable advice during the planning and realization of this study.

Ruoying Yang: Conceptualization, Data curation, Writing-original draft. Xia Xu: Conceptualization, Funding acquisition, Writing-review & editing. Yingang Xue: Project administration, Supervision. Ling Zhang: Data curation, Investigation. Jun Guo: Data curation, Investigation. Liping Wang: Conceptualization, Resources. Mingguo Peng: Conceptualization, Resources. Qiuya Zhang: Conceptualization, Validation. Qiuya Zhang: Conceptualization, Validation. Yun Zhu: Conceptualization, Validation.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.