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All issues Volume 29 / No 5 (October 2024) Wuhan Univ. J. Nat. Sci., 29 5 (2024) 471-483 Full HTML
Open Access
Issue
Wuhan Univ. J. Nat. Sci.
Volume 29, Number 5, October 2024
Page(s) 471 - 483
DOI https://doi.org/10.1051/wujns/2024295471
Published online 20 November 2024

© Wuhan University 2024

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

0 Introduction

Plastic products have been being widely produced and used because of their inexpensive and durable properties. The annual production of plastics globally is estimated to be 300 million tons. However, plastics misuse and improper waste management have resulted in a great deal of plastic waste entering aquatic environments, and then decomposing into tiny pieces through weathering and other processes. Plastic particles with a size smaller than 5 mm are defined as microplastics[1]. In recent years, microplastics have been frequently reported in aquatic environments, and they are now considered as a new type of contaminant[2].

It is well known that aquatic products play an important role in human life and social development and are closely related to human health. The aquaculture industry can provide a large number of products containing animal protein for human consumption. However, recent researches have reported that aquaculture environments are contaminated with microplastics, such as mariculture zones located at the Mediterranean coastline of Spain[3], the Bohai Sea and the Yellow Sea in China[4], and the mussel-farming in Brazil Jurujuba Cove[5]. In addition, microplastics were also found in freshwater aquaculture environments, such as the eel culture stations in Shanghai[6], scallop aquaculture areas in Shandong[7], and fishponds in Changzhou, China[8]. Moreover, fish feed was also found to contain large amounts of microplastics[9], which is a potential source of microplastics in the aquaculture environment. Recently, studies have shown that the Changjiang Estuary has a high abundance of microplastics[10], indicating a high probability that the aquaculture ponds along the Yangtze River Valley have been polluted by microplastics since they connect with the Yangtze River directly or indirectly. Accordingly, further studies on microplastics contamination in freshwater aquaculture environments were needed.

Microplastics in aquatic environments will cause harm to aquatic organisms. Once microplastics are ingested by organisms, they can cause physical damage to the digestive tracts or gills of animals due to their sharp edges[11,12], decreased feeding rate once blocking or accumulating in the gastrointestinal tracts (GITs)[13] , and also physical or chemical effects on fish health due to the adsorbates on microplastics[14]. Even though microplastics mainly accumulated in gills and GITs, which were usually eviscerated before consumption, there was still a potential to be transferred to edible tissue[15]. Furthermore, microplastics could be transferred from low trophic levels to high trophic levels along the aquatic food chain. Therefore, the cultured fish is an important source of human exposure to microplastics, which poses a potential threat to human health.

Freshwater pond aquaculture is the major culture method in China and its output accounts for 70% of the total freshwater aquaculture output[16]. There are many small and medium-sized fishponds covering hundreds to tens of thousands of square meters in the middle reaches of the Yangtze River, the freshwater aquaculture center of China[17]. Although studies have been focused on microplastics pollution in aquaculture, few studies have been conducted on freshwater aquaculture, especially in terms of microplastics contamination in water, sediment, and organisms from the individual freshwater aquaculture system. In this study, water, sediment, fish, and fish feed samples from several fishpond systems in Yidu and Zhijiang, main freshwater aquaculture bases from Hubei province in China, were collected and investigated to determine the abundance and distribution of microplastics. We studied the relationships between microplastic abundance in different matrixes and explored the potential sources of microplastics of aquaculture ponds. Based on the data obtained, risk assessments of water, sediment, and fish samples in this system were also performed.

1 Materials and Methods

1.1 Description of Sampling Sites

Freshwater aquaculture in Hubei is at the forefront of China in terms of aquaculture area and total aquaculture output. In 2022, the provincial freshwater aquaculture output is 498 020 tons, of which 356 440 tons are fish aquaculture, accounting for 71.57% of the total freshwater aquaculture output[18]. Zhijiang (ZJ: 30°25' N, 111°48' E)) and Yidu (YD: 30°27' N, 110°25' E) are located in the middle and lower reaches of the Yangtze River and have developed fisheries. They are green ecological demonstration zones and main freshwater aquaculture bases in Hubei province, with the characteristics of stillwater aquaculture. The ponds here are neither cement nor earthen bottom, but bottomed with PP or PE films to prevent the infiltration of pond water. The area of the surveyed ponds ranges from 1 000 to 3 000 m2. The water depth of these fishponds is 1-2 m. The sediment thickness of the ponds from YD station is 15-20 cm because the sediment is every 3-5 years, while that from ZJ is 3-5 cm since it is cleaned annually.

Samples were collected from 20 fishponds, 10 ponds of which were from each station, in December 2020. The fishponds at Zhijiang were labelled Z1-Z10, and Y1-Y10 for those at Yidu. The location of the two fishponds stations is shown in Fig. 1.

thumbnail Fig. 1 The location of two fishponds stations (ZJ and YD) in Yichang

1.2 Sample Collection

Floating microplastic samples in water were collected using a trawl net (0.5 m wide × 0.2 m vertical opening, 2 m long) equipped with a mesh size of 100 μm. After sampling, the net was thoroughly rinsed with double-distilled water to ensure that all contents were washed into a 2 L glass bottle. Surface sediment samples were collected from 5 to 15 cm of fishpond sediments using a stainless-steel shovel and stored in aluminum foil bags. Three replicates of water and sediment samples were taken from different sites of each fishpond. A total of 81 fish were collected from these ponds, belonging to 8 fish classes (species are detailed in Table 1). They were all adult fish readily for human consumption. The fish samples were brought back to the laboratory and stored at -20 ℃. Seven different commercial fish feeds were collected from fishpond owners in Yidu and Zhijiang. The compositions are presented in Table 2.

Table 1

Fish samples collected from ZJ and YD sampling stations

Table 2

Information regarding fish feed samples used in this study

1.3 Microplastic Extraction and Separation

1.3.1 Water

The extraction method for microplastics from water was described in Ref. [19]. Briefly, a water sample was filtered with a glass fiber membrane (pore size 0.45 μm), then the residues on the membranes were transferred to a beaker with 100 mL 30% H2O2 and incubated at 60 ℃ for 12 h to digest the natural organics, then cooled and filtered again. Finally, the filter membranes were placed in covered glass Petri dishes and dried at 50 ℃ for 2 h to obtain microplastics in the water for later analysis[20].

1.3.2 Sediment

Microplastics were extracted from sediment samples using the density separation procedure reported previously[1] with slight modifications. First, all sediment samples were dried at 60 ℃ for 72 h to constant weight, then crushed and thoroughly mixed. 50.0 g of this sample was added to 100 mL of saturated NaI solution (ρ = 1.84 g/mL) in a glass beaker. Allow the mixture to settle for 30 min after stirring for 10 min using a thermostatic oscillator. The supernatant was filtered through a piece of 0.45 μm glass fiber filter membrane by vacuum filtration. The materials retained on the membrane were then flushed to a clean glass beaker, and incubated at 60 ℃ for 12 h with the addition of a certain volume of 30% H2O2 to remove biogenic organic matters over the surfaces[21]. After being digested completely, samples were cooled and centrifuged at 300×g for 5 min. Microplastics in sediments were obtained by filtration of supernatant with glass fiber membrane (pore size 0.45 μm) for subsequent analysis.

1.3.3 Fish

Fish samples were subjected to deep anesthesia and frozen, followed by thawing and dissection at room temperature. The GITs and gills of each individual were weighed and placed in a clean glass beaker. Biological tissues were digested as previously reported[22]. Typically, 10% KOH was added at a volume of at least three times greater than that of the biological materials. The beakers were then placed at 60 ℃ for 48 h in a thermostatic water bath. When there was no noticeable organic residue observed, the digesting solution was filtered with a glass fiber membrane (pore size 0.45 μm) using a vacuum system. Finally, the membrane was placed in a clean Petri dish and dried at 50 ℃ for 2 h before testing.

1.3.4 Fish feed

Microplastics were extracted from fish feed based on previous reference[23] with some modifications. Briefly, 20.0 g of fish feed from each investigated brand and 60 mL 10% KOH were transferred into a 250 mL glass beaker, then the beaker was put in a thermostatic water bath at 40 ℃ for 72 h. When all the organic materials dissolved completely, the solution was filtered with a glass fiber membrane (pore size 0.45 μm). Then the filter membrane and 100 mL NaI solution were added to a clean beaker, stirred for 10 min using a thermostatic oscillator, and then allowed to settle for 20 min. The mixture was subsequently centrifuged at 300×g for 2 min and then the supernatant was filtered again. Finally, the membrane was placed in a glass Petri dishes and dried for further analysis. Each sample was treated and analyzed with three replicates.

1.4 Observation and Identification of Microplastics

Suspected plastic particles on the membrane were analyzed by micro-Fourier transformed infrared spectroscope (μ-FTIR, Thermo, USA) with a spectral range of 4 000-400 cm-1 and a spectral resolution of 4 cm-1. Particles > 0.1 mm were analyzed by Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR, PerkinElmer Frontier™, USA) with a spectral range of 4 000-400 cm-1 and a spectral resolution of 4 cm-1. The spectra obtained were then compared with standard spectra in the OMNIC software database. Only results having a similarity equal to or greater than 70% with the standard polymer spectrum were considered valid[24].

1.5 Ecological Risk Assessment

The potential ecological risks caused by microplastics were evaluated by using Håkanson's ecological risk index method, which has been wildly applied to assess the potential ecological risks of heavy metals in sediment[25], and has been improved recently to evaluate the microplastics risk[26]. Equations in the method were as follows:

C f i = C i / C r i # ( 1 )

T r i = P n × S i # ( 1 )

E r i = T r i × C f i # ( 1 )

R I = ∑ i = 1 n E r i # ( 1 )

where Cfi is the microplastic pollution index, Ci is the microplastic pollution concentration, and Cri is the the reference value. Generally, the safe concentration of microplastics (Cri) in the water estimated by Everaert using the mathematical model is 6.65 particles/L[27]. Tri is the ecotoxicity response factor, which is the sum of the the percentage of each polymer type Pn multiplied bythe hazard score on each of polymers (Table S1) as introduced by Lithner approach[28]; Eri is the potential ecological risk index factor, and n is the number of types of microplastic polymers contained in the sample. RI represents the potential ecological risk index, which is the sum of the potential ecological risk factor for each plastic polymer. The assessment of potential ecological pollution risk levels is shown in Table S2[29].

1.6 Quality Control of Experiments

During sampling collection procedure, the trawl and stainless-steel shovel were rinsed thoroughly with double distilled water before processing the next sample to reduce cross-contamination. The instruments and vessels were rinsed with double-distilled water and dried before the experiments to prevent contamination from the air. The lab windows remained closed during the experiment to reduce the air flow. In addition, three procedural blank experiments were performed for background correction, and no microplastics were observed in any blank samples.

1.7 Statistical Analysis

An average value was obtained from three repetitions for each sample and transformed to give the abundance of items/m3 in water, items/kg in sediment, items/g in fish feed and items/individual in fish. All values were reported as the mean±standard deviation. SPSS 26.0 and Origin 9.0 were used to conduct the statistical analyses. Before analysis, Shapiro-Wilk's test and Levene's test were used to check the homogeneity and normality of variances, respectively. One-way analysis of variance (ANOVA) was applied to compare the abundance of microplastics in fish concerning the different feeding habits. Differences in microplastic abundances within organs were evaluated by the Mann Whitney U test. The Kruskal-Wallis H method was employed to test the differences of microplastic abundance in fish from different living habitats. In the case of variance heterogeneity, the Mann Whitney U test was used to compare the abundance of microplastics in water samples and sediments from the two sampling stations. The difference in microplastics abundance among different commercial fish feed was examined by the Kruskal-Wallis H method. Spearman's correlation was used to evaluate the relationship of microplastic abundance between water, sediment, and fish. Linear relationships between microplastics abundance in fish feed and the content of crude protein were analyzed with Pearson's test.

2 Results and Discussion

2.1 Abundance and Distribution of Microplastics

2.1.1 In water

The abundance and distribution of microplastics in water are shown in Fig.2. The average abundance at the YD station was 5.2±1.9 items/m3, which was significantly higher than that at ZJ station (3.9±1.1 items/m3) (Mann Whitney U test, p < 0.05). A much higher microplastic abundance was observed at the Y7 (7.6±0.1 items/m3), while Y3 showed the lowest abundance of microplastics (1.8±0.3 items/m3). Microplastics in size of 0.1-1.0 mm predominated at both sample stations, accounting for 75.0% and 70.1% at ZJ and YD, respectively. Fiber was the dominant shape, accounting for 61.2% of the total at ZJ and 70.6% at YD, followed by fragment shape. Black and blue, which were the most common colors, accounted for 76.3% and 75.9% of the total (Fig. 2).

thumbnail Fig. 2 Abundance and distribution of microplastics in water of fishponds

(a) Abundance of microplastics in ZJ experimental station; (b) Abundance of microplastics in YD experimental station; (c) Size, shape, and color distribution as a percentage of total microplastics in water

In pond water, the abundance of microplastics is from1.8±0.3 to 7.6±0.1 items/m3. This is accordance with the result from Zigui sampling point along the Yangtze River which is located upstream of Three Gorges Dam and due to the barrier effect of dam being higher than that of other region reported by He's group by using a trawling method to collect sample[30]. If using filtering water method to collect sample, the abundance would be three orders of magnitude higher, up to 2 613.9±296.9 items/m3, which is only one tenth of those of ponds near the Yangtze Estuary and the Pearl River Estuary, China (10.3-87.5 items/L)[31,32]. This may be resulted from the difference of sampling method, geographical locations, and the size distribution of the microplastics. The morphology and color distribution characteristics both showed a similar pattern with those in other aquacultural fish ponds[31,32] while different from that of the Yangtze River mainstream[30]. The different characteristics of microplastics between pond water and mainstream water indicated much severe influence is from farming activities and human activities, in which fishing net, artificial feeds, pipelines of aeration equipment, water pump and surface runoff from the neighboring community are all sources of microplastics.

2.1.2 In sediment

The abundance and distribution of microplastics in sediment are shown in Fig.3.The average abundance of microplastics was 1 008.7±460.2 items/kg at the ZJ station and 1 360.0±639.3 items/kg at the YD station, respectively. The microplastic abundance of sediment samples from the two stations differed (Mann Whitney U test, p < 0.05). The highest abundance of microplastics was found at Y6 (2 560.0 ± 80.0 items/kg), while the lowest abundance was observed at Z4 (500.0 ± 40.0 items/kg). The most frequently observed microplastic size was in the range of 0.1-1.0 mm, which accounted for 75.8% and 71.5% of the total at ZJ and YD. Fiber was the most common type accounting for 57.1% and 48.6% of the total. Among the six kinds of color microplastics extracted from 20 sampling sites, green microplastics were the least, and black microplastics were slightly more than other colors.

thumbnail Fig. 3 Abundance and distribution of microplastics in sediment of fishponds

(a) Abundance of microplastics in ZJ experimental station; (b) Abundance of microplastics in YD experimental station; (c) Size, shape, and color distribution as a percentage of total microplastics in sediment

In this study, pond sediment all came from the sedimentation in the pond water resulting the similarity of microplastics distribution characteristics between pond water and sediment no matter in ZJ or YD. The only difference is that there are more films found in sediment than in water. The microplastics level was almost 30 times that of the riverbank sediments collected from Yichang[33], 15 times the detected highest level of the sediment from mainstream of Yangtze River[34], and also 5 times higher than that of the pond sediment near the Yangtze Estuary[31]. Considering that water in this type of pond is static, the sediment was never completely removed from these ponds during pond cleaning, so the accumulation may occur[35]. Before cleaning, the fishpond water was relatively stagnant most of the time, and larger-sized plastic waste at the bottom was decomposed into more microplastics. Additionally, lower-density microplastics tend to float on the surface water of the fishpond, while high-density particles sink to the bottom[36]. Biofouling, aggregation, and egestion can cause vertical movement of microplastics[37]. For example, biofilms formed by microorganisms on the surface of microplastics can affect the surface hydrophobicity and increase the density of the polymer, thereby increasing the sinking rate[38-40]. And that higher microplastics level, larger size (1-5 mm) and fragment type of microplastics in YD were found may be the consequence of long-term accumulation since farmers clean the sediment rarely, while in ZJ farmers clean annually.

2.1.3 In fish

In this study, a variety of fish with different feeding habits and habitats were collected from 20 fishponds. All eight fish species investigated were found to ingest microplastics and the microplastics were retained and accumulated in different organs. The microplastics abundance and distribution in eight species of fish is shown in Table 3. The average abundance of microplastics in fish samples from the ZJ station was 15.8 ± 4.2 items/individual, while that at YD station was 16.3 ± 4.1 items/individual. This indicated a higher level of microplastics in fish comparing with that in fish from seawater like from the Haizhou Bay mariculture farm in China[41], Zhoushan fishing ground, off the East Sea[42] and from the East Sea[43]. While similar results were reported for common freshwater fish in semi-arid regions of South America by Silva-Cavalcanti et al[44], who found that fish living in freshwater environments near urbanized areas had a higher risk of exposure to and ingestion of microplastics.

For fish samples in ZJ fishponds, the mean microplastic abundance in GITs was 8.9 ± 3.6 items/individual and in gills 6.9 ± 3.0 items/individual. For fish from YD fishponds, the abundance of microplastic in fish GITs and gills was 9.1 ± 3.6 items/individual and 7.0 ± 2.9 items/individual, demonstrating significant differences between them (Mann Whitney U test, p < 0.05). One-way ANOVA indicated that microplastics abundance in fish with different feeding habits differed significantly (p < 0.05). Furthermore, the abundance of microplastics was higher in the GITs than that in the gills at both stations, especially for Parabramis pekinensis and Carassius auratus. Fishes use their gills to filter to breath and/or feed, during this process smaller microplastics would be transfered to the esophagus, and larger ones may go back to the water again or stuck in gill. The distribution characteristics of total microplastics in fish GITs and gills were similar with those in water and sediment and details were shown in Fig. 4.

thumbnail Fig. 4 Microplastic distribution characteristics in the fish GITs and gills from (a) YD experimental station and (b) ZJ experimental station

As regards to feeding habits, two kinds of omnivorous fish (Pelteobagrus fulvidraco and Carassius auratus) were found to contain the highest microplastics abundance among all species, followed by filter-feeding and herbivorous fish at two stations. The wide range of food sources for omnivores may increase the likelihood of ingesting microplastic[45], and filter-feeders tend to accidentally prey on microplastics that were similar in color, size and shape to small plankton[46]. Conversely, herbivores and carnivores were unlikely to actively ingest microplastics because they are selectively, unless they ingested aquatic plants or animals adhering to or containing microplastics[47].

Therefore, the activities of fish, such as feeding and excretion, are one of the links of the microplastic cycle in pond water, sediment and fish, and the activities indirectly increased the concentration of water and sediment microplastics[48].

Table 3

The abundance of microplastics in different fish samples items/individual

2.1.4 In fish feed

Microplastics were observed in all evaluated seven different commercial fish feeds (Fig. 5(a)) with significant differences (Kruskal-Wallis H test, p < 0.05) in abundance. Specifically, Y-F1 fish feed was the highest (2.3 ± 0.2 items/g), while Y-F3 was the lowest. Furthermore, the abundance of microplastics in fish feed was significantly correlated with the content of crude protein (rPearson = 0.947, p < 0.01). Figure 5(b) shows the size, shape and color distributions of microplastics in fish feed. In terms of size, the proportion of microplastics in the range of 0.1-1.0 mm was the highest (65.3%). Through characterization analysis, fibrous microplastics are dominent, about 71.2%, and color was mostly black (47.9%), followed by blue (33.3%). Apparently, six colors of microplastics were found in fish feed, sediment and fish, but only five in pond water; three shapes were found in all type of samples with film the most abundent in sediment.

thumbnail Fig. 5 The abundance (a) and distribution characteristics (b) of microplastics in fish feed

2.2 Identification and Potential Sources of Microplastics

Through characterization analysis, it was determined that the main components of microplastics in this study were polyamide (PA), polypropylene (PP), polyethylene (PE) and polyethylene terephthalate (PET). The FTIR spectra of the test results are shown in Fig. 6(a). Depending on the abundance of subsamples taken from the suspected microplastics, a total of 339 (88.8% of the suspected microplastic particles; Fig. S1) were identified as PA (16.7%), PE (44.4%) and PP (38.9%) in the mixed water sample using μ-FTIR; a total of 293 (86% of the suspected microplastic particles; Fig. S2) were validated in the sediment as PA (11.5%), PE (42.3%), PP (34.6%) and PET (11.5%); a total of 306 (78.5% of the suspected microplastic particles; Fig. S3) were microplastics particles, where PA, PE and PP accounted for 13.3%, 46.7% and 40%, respectively; and a total of 66 (82% of the suspected microplastic particles; Fig. S4) in fish feed sample were identified to be PE, PP and PET accounting for 50%, 33.3% and 16.7%, respectively.

thumbnail Fig. 6 ATR-FTIR spectra and match degrees of polymers identified (a) and percentages of these polymers in different matrix(b)

As shown in Fig.6(b), the two most common types of microplastics are PE and PP, and PE is the main microplastic category in all kinds of samples. PP and PE are the two most consumed plastic materials today and are commonly used as packaging materials, plastic rope, fishing line and other fishing-related materials[49]. Fishing activity tools are immersed in fishpond for a long time, which allows more microplastics to accumulate in static closed fishponds. These are also two most detected polymers in previous studies from the Three Gorges Reservoir[50] and in Yangtze River from upstream to estuary[30], and also in aquaculture ponds near the Yangtze Estuary[31] and in the Pearl River Estuary[32]. It is worth note that PET was more likely to be found in aquaculture pond[31,32] not in natural water[30,50]. In this study PET was only found in fish feed and the sediment. PET in the sediment is presumed to be introduced from fish feed, which is difficult to be ingested by fish and suspend in water due to its high density, so it settled into the sediment. Siddique et al[51] and Hanachi et al[9] extracted and identified microplastics from a variety of fish meal, and PET was detected in all of the samples[52]. These results suggest that PET and other microplastics in fish feed are from its components like fish oil and fish meal and other vital ingredients like minerals, vitamins, cereal grains, and vegetable proteins, which in turn form granules or pellets. In addition, there are high probability to introduce microplastics during production, processing, transportation, packaging, and storage[47,49].

PA observed in this study in pond water is about 18%, not a dominant component, as well as the findings in Yangtze River[30] and Han River[53]. PA is a thermoplastic material widely used in the automotive trade, textiles, and clothing. Clothes laundering was found to be a very important source of microplastics[10,53]. The washing wastewater generated and discharged by the residents around the pond may result in microplastic fibers convergence in the pond. Fishing net usually made of Nylon as another important source cannot be neglected.

2.3 Relationship of Microplastics in Fish, Water and Sediment

Using data listed above for two sampling points (Table 4), Spearman correlation analysis demonstrated that there was a remarkable relationship between the abundance of microplastics in water and sediment (p < 0.01), and a considerable relationship between that in fish and in water and sediment (p < 0.05).

In this area, the fish ponds were bottomed by PP or PE film, in which the water was from the nearby water source. So it is reasonable to believe that microplastics in the sediment came from what in the water totally. The remarkable relationship between the abundance of microplastics in water and sediment can also confirm this. The distribution characteristics in size, shape and color are similar in water and in sediment. It is also apparent as regard to the difference between them. In sediment there are six colors found, while only five in water. And more film was observed in sediment than in water. PET polymers were detected only in sediment. These may come directly from other sources but water.

As shown in Table 4, the microplastics in fish were affected by water and sediment. In this study we studied omnivorous, carnivorous, herbivorous and filter-feeding fish. Omnivorous and herbivorous fishes swallow, filter-feeding fish filter water, carnivorous fish swallows and filters. No matter what feeding habit they have, gill is used to filter water to breath. So microplastics in water affect microplastics observed in fish gill. Functioning aerator pumps and the activities of benthic fish may stir microplastics on the surface of the sediment, and benthic fish would also directly contact and swallow microplastics in shallow sediment to present the affection of microplastics in fish from sediment. But PET detected in sediment was not found in fish. We can only assume that the physical properties of PET particulates in fish feed did not have much opportunity to suspend in water, that is why PET type of microplastics were not found in water.

Table 4

Correlation analysis results for YD and ZJ stations

2.4 Risk Assessment of Microplastic Pollution

2.4.1 Potential ecological risks of microplastics in fishponds

In this study, Cri represented the lowest abundance in all sampling points, which was 1.8 items/m3, 500.0 items/kg, and 9.0 items/individual for water, sediment, and fish samples, respectively. According to Section 1.5, the potential ecological risk factor of a single plastic polymer was calculated and the pollution risk degree at each sampling sites was evaluated as shown in Table 5, indicating that all samples were at moderate pollution risk.

Table 5

The potential ecological risk factors, risk index and potential ecological pollution risk level of microplastics in different matrix from YD and ZJ

2.4.2 Risk assessment of microplastic pollution

The overall RI values for water, sediment, and fish samples are under the moderate-risk category for both YD and ZJ study area. Polymeric potential risk assessment revealed the diversity of hazardous polymers in the fishpond ecosystem. It can be seen from Table 5 that PA has the highest ecological risk factor in all media among polymers identified. In total, the risk level was classified to be Ⅱ, which is medium risk. In order to obtain the required properties of plastic products (such as flexibility, UV protection, flame retardation, etc.), additives added in the production phase will gradually release in the outdoor environment, which may cause toxic effects. For example, leachate obtained from raw polyvinyl chloride, polyethylene, and polycarbonate sheets (0.1 and 0.5 m2/L, solid:liquid ratio) significantly increased the mortality of nauplii from barnacles after exposure of plastic to filtered seawater at 28 ℃ for only 24 h compared to controls[54].

In addition, Khosrovyan and Kahru found that PA-MP particles at concentration 1 000 mg/kg did not exert adverse effects on Chironomus riparius throughout the life cycle because PA itself is not toxic[55]. However, UV-weathered PA-MP particles significantly reduced the emergence of C. riparus larvae after exposure to the particles for 28 days, showing their adverse effects. Therefore, it is necessary to develop management systems and regulations to help reduce microplastics in freshwater aquaculture fishponds.

3 Conclusion

In this study, the microplastic contamination present in samples of water, sediment, fish collected from Zhijiang and Yidu in the middle reaches of the Yangtze River were analyzed. Microplastics were detected in all samples. The main components of microplastics were PP, PE, PA and PET. Among them, PET was found only in fish feed, PP and PE were the most common types in all samples. Small-sized (< 1.0 mm), black and fibrous were the main characteristics of microplastics in this pond system. Base on the type of microplastics found, activities in fish culturing progress, fish feed, clothes laundering, and other human activities may be the main microplastic sources. Through the evaluation of the pollution risk degree of each sampling point combined with the assessment of ecological risk factors, the pollution risk of all sampling points in this research is at a medium level. Thus, microplastic pollution of freshwater fishponds represents a way of human exposure, and further research should pay more attention to the pollution level of microplastics in aquaculture and scientific management system should be developed to make sustainable aquaculture. The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors have no conflicts of interest to declare.

Supplemental Information

Supplementary file provided by the authors. Access here

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All Tables

Table 1

Fish samples collected from ZJ and YD sampling stations

In the text
Table 2

Information regarding fish feed samples used in this study

In the text
Table 3

The abundance of microplastics in different fish samples items/individual

In the text
Table 4

Correlation analysis results for YD and ZJ stations

In the text
Table 5

The potential ecological risk factors, risk index and potential ecological pollution risk level of microplastics in different matrix from YD and ZJ

In the text

All Figures

thumbnail Fig. 1 The location of two fishponds stations (ZJ and YD) in Yichang
In the text
thumbnail Fig. 2 Abundance and distribution of microplastics in water of fishponds

(a) Abundance of microplastics in ZJ experimental station; (b) Abundance of microplastics in YD experimental station; (c) Size, shape, and color distribution as a percentage of total microplastics in water
In the text
thumbnail Fig. 3 Abundance and distribution of microplastics in sediment of fishponds

(a) Abundance of microplastics in ZJ experimental station; (b) Abundance of microplastics in YD experimental station; (c) Size, shape, and color distribution as a percentage of total microplastics in sediment
In the text
thumbnail Fig. 4 Microplastic distribution characteristics in the fish GITs and gills from (a) YD experimental station and (b) ZJ experimental station
In the text
thumbnail Fig. 5 The abundance (a) and distribution characteristics (b) of microplastics in fish feed
In the text
thumbnail Fig. 6 ATR-FTIR spectra and match degrees of polymers identified (a) and percentages of these polymers in different matrix(b)
In the text

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