Abundance and distribution of microplastics
Totals of 51 and 105 microplastics were identified in surface water and sediments, with abundances range of 0.0–25.9 items per 100 L (0.0–258.9 m− 3) and 2–12 items per 200 g dry weight (d.w., 10–60 items kg− 1 d.w.), respectively (Additional file 1: Table S1). Sampling sites on Chongming Island can be divided into two groups: group YRS (Yangtze River shores) including six sites along the island shorelines (sites Y1–Y6), and group ILR (inland rivers) including nine sites from six main inland rivers on Chongming Island (sites M1–M3 and C1–C6), representing microplastic pollution from land-based sources. Average microplastic abundances (mean ± standard deviation) in surface water and sediments of groups YRS and ILR were 67.5 ± 94.4 items m− 3 and 28.3 ± 14.4 items kg− 1; and 9.8 ± 12.2 items m− 3 and 39.4 ± 16.1 items kg− 1, respectively. An overview of the distribution of microplastics on Chongming island and in the Yangtze River Estuary is given in Fig. 1.
Microplastics were found in the sediment at every sampling site, and there were no statistically significant differences in microplastic abundances between the two groups (Mann–Whitney U test, p = 0.224). The highest abundance of microplastics in sediments was found at site C5 (60.0 items kg− 1 d.w.). There is an aquaculture market near C5, and therefore the long-term use and unintentional disposal of plastic products may result in accumulation of microplastics within this area. Other higher abundances in group ILR (50–60 items kg− 1) were collected at site M1 and M3, which were characterized by increasing human activities.
In surface water, however, microplastics were more abundant in group YRS, with the difference being significant (Mann–Whitney U test, p = 0.005). The highest abundance of microplastics in surface water (258.9 items m− 3) was found on northeastern Chongming Island at site Y5, which may partly be explained by the special topography near the north branch and tide interact in the Yangtze Estuary [25]. The second highest abundance (43.4 items m− 3) was at site Y2, near the Chongxi sluice gate in the western Chongming Island. There the external and internal gates are linked to the Yangtze River and intake channel, respectively, with the potential to influence microplastics transportation in the surrounding waters. No microplastics were found in surface water at three sampling sites (C2, C3, and C4), possibly due to the low level of human activity in the middle part of Chongming Island. Correlations of microplastic abundances in both phases of all sites were not statistically significant (Pearson’s correlation, p > 0.05).
Shape, color, and size of microplastics
The morphological characteristics of the observed microplastics are summarized in Fig. 2. Fragments were the predominant shape in surface water with an average proportion of 39.2%, dropping to 23.8% in sediments. Fiber proportions were higher in sediments (average 66.7%) than those in surface water (33.3%). The proportion of films decreased from 19.6% in surface water to 9.5% in sediments. Granules were the least frequent shape, accounting for 7.8% and 0.0% in surface water and sediment, respectively (Fig. 2a). Typical shapes of microplastics from all samples are summarized in Fig. 3.
Eight colors (white, blue, transparent, yellow, black, red, brown, and green) were observed in microplastics in surface water and sediment (Fig. 2b), with white being the most common in surface water (64.7%; cf. 11.4% in sediment). Transparent particles were most common in sediments (16.2%; cf. 9.8% in water). A large number of transparent plastics are commonly used in fishing nets and lines for the frequent fishery activities in Shanghai [21], and transparent microplastics in sediment samples may be attributed to the bleaching caused by digests.
The lower limit of microplastics size was 300 μm in surface water and 75 μm in sediments, as dictated by different filters. Most microplastics in both phases were < 2000 μm in size, whereas the large microparticles (2000–5000 μm) were seldom observed (Fig. 2c). In surface water, microplastics possessed the wide size distribution from 300 μm to 4000 μm (Fig. 2c), meanwhile the size of most microplastics valued less than 1000 μm (Fig. 2d). A possible explanation is that large plastics are prone to forces of flow and wind leading to floating on water, while smaller plastics tend to migrate into sediments and deep water [26, 27]. Size distribution of group YRS and ILR in both phases is shown in Additional file 1: Table S3.
Identification of microplastics
Suspected microplastic particles were analyzed by μ-FTIR, which has been widely employed in identifying microplastic polymers due to its high reliability in determining chemical compositions of unknown plastic fragments [16, 28]. Composition–library comparisons were difficult to achieve with high similarities due to the weathering and contamination of plastics [29], with the lower limit being set relatively high here (> 75%) to provide more accurate results (compared with 60%–80% in previous studies). Eleven polymer types were identified: polyethylene (PE), polypropylene (PP), α-cellulose, polyethylene terephthalate (PET), cellulose acetate (CA), polyamide (PA), polybutene (PB), polymethyl methacrylate (PMMA), cellophane, polyurethane (PU), and ethylene/ethyl acrylate (EEA). FTIR spectra of the five most common types are shown in Fig. 4, and relative compositions are given in Fig. 5. The predominant polymer type in surface water was PE (37.3%), and that in sediment was PP (28.6%). PE and PP were the most abundant polymer types in both phases, and these are also the leading polymers in plastics production. In 2018, PP (19.3%), low-density PE (17.5%), and high-density PE (12.2%) were the predominant resin types used widely in packaging and construction [1]. Another common microplastic, α-cellulose, used mostly in clothing, had proportions of 27.5% in surface water and 24.8% in sediments, but such semi-synthetic fibers had lower search scores (most < 80%) and were difficult to accurately recognize. The characteristic spectral band at 1105 cm− 1 was used to distinguish semi-synthetic and natural fibers [30]. Color uniformity and average shape were also considered when it was otherwise difficult to identify the composition of fibers.
PCA results indicate that 11 variables (i.e., 11 polymer types) described the spatial distribution of sampling sites in three-dimensional coordinates. Figure 6a shows the PCA of the 7 variables in surface water, with PC1, PC2, and PC3 explaining > 92.66% of the total variance (Component matrix and component plot are shown in Additional file 1: Table S4 and Fig. S3). PC1, explaining 41.53% of total variance, had high loadings of PP, PE, PA, and PB; PC2 indicated strong contributions of PET and α-cellulose; and PC3 was dominated by cellophane. In terms of spatial distribution, some sampling sites overlapped others, with microplastic abundance there being lower and with single polymer types causing high similarities in the same component. Figure 6b shows the PCA of the 9 variables in sediment, with PC1, PC2, and PC3 explaining > 81.92% of the total variance (component matrix and component plot are shown in Additional file 1: Table S5 and Fig. S4). PC1, explaining 41.53% of total variance, displayed strong correlations between PE, PU, and PMMA, while PC2 was mainly contributed by PP, α-cellulose, and PET, and PC3 with PA, EEA, and CA. Sediment analyses indicate that the sampling sites were divided into two main groups: group 1 comprising sites Y1, Y2, Y3, Y5, C3, and C4; and group two comprising sites M1, M2, C1, C2, C6, and Y4, consistent with the assignment of groups ILR and YRS, reflecting compositional differences between the island and estuary.