Microplastic pollution at Qilianyu, the largest green sea turtle nesting grounds in the northern South China Sea

Study area

Qilianyu is a subgroup of islands is located in the northeastern Xisha Islands in the South China Sea (16°55′N–17°00′N, 112°12′E–112°21′E), approximately 330 km from Hainan Island. The eight small islands are connected by reefs, with a total area of approximately 1.32 km². With the exception of Zhaoshu Island and North Island, all other islands in the archipelago are uninhabited. The current permanent human population is approximately 200, primarily living on Zhaoshu Island. In this study, sampling points were set up on six islands, including North Island (NI), Middle Island (MI), South Island (SI), North Sand (NS), Middle Sand (MS), and South Sand (SS). These islands have good quality nesting sites and have recent records of C. mydas nesting (Zhang et al., 2020) (Fig. 1).

Sample collection and separation

The geographic coordinates of the sample points were recorded using a global positioning system. Sediment samples were collected within an area of 25 cm × 25 cm and a depth of 0–2 cm from both the strand line and the turtle nesting line (TNL) for six nesting grounds. This sampling process was repeated three times at each nesting ground. Additional samples were collected from the TNL using a custom-made cylindrical galvanised steel core with a diameter of 20 cm. Samples were collected at depths of 0–60 cm (0–2, 2–20, 21–40, and 41–60 cm) to explore the presence and extent of microplastic pollution at the sea turtle nesting depth of approximately 60 cm (Duncan et al., 2018).

The saturated sodium chloride density method described by Zhang et al. (2021) was used to separate microplastics from the sediment. For each sample, 250 cm³ of sand was placed in a beaker. A total of 500 mL of saturated sodium chloride solution (1.2 g·mL⁻¹) was added. The mixture was then stirred for 2 min, after which it was left to settle for 10 min. The supernatant was then passed through a 300-micron mesh sieve. The remaining solids in the beaker were added to a sodium iodide solution (1.8 g·mL⁻¹) and stirred for 2 min, after which the mixture was left to settle for 10 min. After density separation had taken place, the sample was transferred to a 100 mL beaker. A solution of 10% potassium hydroxide was added, and the mixture was left to digest for 2 days. Finally, the supernatant solution was decanted and filtered through a 0.45 μm glass fiber membrane (GF/F, 47 mm Ø; Whatman, Shanghai, China). This was done using a vacuum filtration device (GM-0.33A, Zhengzhou, China), while waiting for the one-step analysis (Thompson et al., 2004; Wang et al., 2018).

Observation and identification of microplastics

All samples on the filter membrane were observed under a stereo microscope (SMZ-168 SERIES; MOTIC, Xiamen, China), and images were obtained with a SONY DSC-RX10M2 digital camera. The microplastics were classified and counted according to their morphological characteristics, colour, and size (Zhang et al., 2021).

Samples suspected to be microplastics that were representative of each group were randomly selected, and their surface structures were tested for polymer types using a Fourier transform infrared spectrophotometer (IRTracer-100; SHIMADZU, Kyoto, Japan). The detector spectral range was 600–4,000 cm⁻¹, co-adding 16 scans at a resolution of 8 cm⁻¹ (Zhang et al., 2021). The resulting atlas was compared to the IR polymer spectral library (ATR-Polymer2, IRs Polymer2, T-Polymer2, and Shimadzu Standard Library), with only readings at a confidence level of 70% or higher being considered reliable and accepted.

Experiment quality control

All containers were rinsed at least three times with Milli-Q water and then dried before the start of the experiments. All plastic equipment was replaced with non-plastic ones if possible. If this was not possible, the equipment were rinsed three times with Milli-Q water and then inspected to ensure that no plastic fragments were generated during sample processing. In addition, all containers were always covered with aluminium foil to avoid contamination. Nitrile gloves and cotton lab coats were worn throughout the experiment, with laboratory windows remaining closed. Three procedural blanks were set to minimise contamination from the environment, and results showed that no microplastic particles were detected.

Statistical analysis

Statistical analysis was performed using Excel and SPSS 19.0 statistical software. All data were tested for normal distribution and variance homogeneity before the statistical analysis. One-way analysis of variance was used to analyse the difference in microplastic abundance between the six nesting grounds. The relevant data are shown as mean ± standard deviation. P < 0.05 was considered a significant difference, and P < 0.01 was considered a highly significant difference according to the two-tailed test.

Distribution and abundance of microplastics pollution at Qilianyu

The quantity of microplastics found in the nesting grounds at Qilianyu ranged as 92–782 thousand pieces·m⁻³ or 368–3,128 pieces·m⁻², with an average abundance of 338.44 ± 315.69 thousand pieces·m⁻³ or 1,353.78 ± 853.68 pieces·m⁻². The distribution of microplastics across the six islands showed a degree of spatial variation (Fig. 2). MS was the nesting ground that was most severely polluted with microplastics, followed by NS, SI, and NI, respectively (df = 17; F = 7.202; P = 0.002). In contrast, MI and SS were less polluted than the other sampling sites. The abundance of microplastics in the sediment samples exhibited a gradual increase from northwest to southeast, with the exception of MI and SS.

When comparing the abundance of beach microplastics with other areas (Table S1), the abundance of microplastics (0.05–5 mm in size) in nesting grounds of C. mydas at Qilianyu was lower than in Hainan Island, Hong Kong, and Guangdong Province, but similar to that in Ganquan and Quanfu Island in the Xisha Islands. Moreover, microplastics with a particle size range of 0.05–0.33 mm accounted for 27.79% of all particles in this study. The actual abundance of microplastics at Qilianyu is therefore considerably lower than that found in Guangdong (0.315–5 mm) and Hong Kong (0.315–5 mm).

Morphological characteristics of microplastics

When microplastics were separated, they were shown to have different morphological characteristics. Figure S1 shows the shape categories of the microplastics found in the samples. Among the microparticles observed, fragments formed the largest proportion at 60.85%, followed by foams at 30.98% and fibers at 7.35%. Meanwhile, pellets and films were relatively rare, accounting for only 0.41% of the total microplastic particles (Fig. 3).

The most common colour of the sampled microplastics was white (71.31%), which included both transparent and white particles (Fig. 4). Among this colour category, white foam was the most common type. The second most common colour was black (23.00%), Multicoloured microplastics such as yellow, green, grey, and blue were relatively rare. The average size of the microplastics at Qilianyu is indicated in Fig. 5. Small microplastic particles (<1 mm) comprised the majority of the microplastics (87.18%).

Polymer compositions of microplastics

The polymer compositions of the microplastics included polyethylene (PE), polypropylene (PP), and polystyrene (PS) (Fig. S2). The most common polymer compositions were PS (40.74%) and PE (40.74%). PS was found in foam, while PE found in fibers, pellets, and fragments, while PP was found in fragments (Fig. 6).

Changes in microplastic density with increasing sampling depth

As the sampling depth increased, the average density of the microplastics decreased (Fig. 7). However, there was no significant difference between microplastic densities at each depth (df = 66; F = 2.043; P = 0.117 > 0.05) (Table S2). This indicates that microplastic pollution was not limited to the beach surface, and that that microplastics can come into close contact with the turtle eggs, which usually lie at a depth of 60 cm.

Current status of microplastic pollution at Qilianyu

Beaches are gathering points for ocean microplastics and key areas of environmental pollution (Poeta, Battisti & Acosta, 2014; Nelms et al., 2016). Although Qilianyu is relatively remote from the mainland, the nesting grounds here for C. mydas are affected by microplastic pollution. Although many studies of beach microplastic pollution exist, the particle size survey range obtained was non-uniform, making it difficult to compare the variation in abundance of microplastics at the regional level. Therefore, only broad comparisons can be made for the same particle sizes across different studies (Fang et al., 2021). The overall abundance of microplastics (size range 0.05–5 mm) at Qilianyu (1,353.78 ± 853.68 pieces·m⁻²) was lower than that at other areas such as Hainan (5,595 pieces·m⁻²) and Guangdong (6,675 ± 7,021 pieces·m⁻²), China (Fok et al., 2017; Zhang et al., 2021). Microplastic pollution is closely related to regional population activities and economic development (Fang et al., 2021). It is likely that the lower abundance of microplastics at Qilianyu in the Xisha Islands is associated with increasing distance from the mainland and the small human population of the islands. However, microplastics are stable and can exist in the environment for a long time, and their abundance may increase with time. Therefore, measures must be taken to prevent the microplastic pollution increase at Qilianyu.

Sources of microplastic pollution for Qilianyu

The types of microplastics found at the nesting grounds at Qilianyu were primarily fragments and foams, with the main compositions being PS and PE. For comparison, Huang et al. (2020) showed that the types of microplastics found in the sea water of the Xisha Islands are primarily fibers and films, with the predominant composition being PET (56.2%) and PP (20.3%). The different types and compositions found indicate that microplastics in beach sediment at Qilianyu may not come directly from local sea water but primarily from the decomposition of beach litter.

A previous study on the general categorisation of beach litter (size > 1 cm) at Qilianyu found that the greatest proportion was plastic and foam (Zhang et al., 2020) (Fig. S3). Fragments and foam can easily breakdown in a beach environment, due to higher temperatures increasing weathering and degradation (Fok & Cheung, 2015; Fok et al., 2017). Owing to its tropical climate, Qilianyu experiences strong direct solar radiation, which accounts for 60–70% of the global solar radiation (Ye, 1996). The average annual temperature is approximately 27.4 °C, which is conducive to the breakdown of plastic (Xu et al., 2018). Furthermore, the highest percentage of the microplastics at Qilianyu were white, followed by black. In line with the results of 24 investigations analysed by Hidalgo-Ruz et al. (2012), this trend may be due to the weathering and fading of plastics in beach or ocean environments. Therefore, it is likely that most of the microplastics found in this study were from broken plastic litter on the beach. Items such as plastic bottle caps being broken into small plastic particles on the beach was commonly observed during field work (Fig. S4).

Potential threats of microplastic pollution to sea turtles at Qilianyu

Consistent with previous research results (Vianello et al., 2013; Mohamed Nor & Obbard, 2014; Peng et al., 2017), the microplastics found in the nesting grounds on Qilianyu were predominantly small particles (0.05–1 mm). However, the smaller the microplastic particle size, the larger their specific surface area. This implies that the microplastics can absorb more pollutants, which may later be released and cause greater harm to the hatching of green sea turtles (Duncan et al., 2018).

The presence of microplastics is extensive in the sea turtle nesting grounds at Qilianyu, with close contact between microplastic and eggs. Owing to the effects of climate change and the presence of microplastics, the beach temperature at Qilianyu has increased annually. Beach temperatures have increased by 1–2 °C in 2021 (the average temperature was 32.4 °C) as compared to data collected in 2018 (the average temperature was 30.2 °C) (T. Zhang & L. Lin, 2021, unpublished data). An increased incubation temperature may change the sex ratio of sea turtles hatched locally. In addition, the microplastic surfaces can accumulate and release heavy metals and organic pollutants (Bergeron, Crews & McLachlan, 1994; Yang et al., 2011). Jian et al. (2021) indicated that heavy metals can enter the embryo by penetrating the shell membrane. Therefore, we suggest that microplastics near the C. mydas nests may adversely affect the development of turtle embryos. However, the degree of harm to the hatching of C. mydas eggs due to microplastics at Qilianyu remains unclear. Therefore, research and field monitoring must be strengthened. Important areas for future research include determining the impact of microplastic enrichment on turtle hatching temperature, and the impact of attached microplastic surface pollutants on sea turtle hatching.

Management suggestions

Fok et al. (2017) suggsted that cleaning up plastic litter on beaches may reduce the generation of microplastics there. The current beach litter cleaning at Qilianyu primarily removes large plastic litter >10 cm. The overall amount of large litter has been reduced to low levels by regular cleaning efforts. However, a large amount of small litter (1–10 cm) still remains after cleaning has taken place. Thus, the removal proportion of smaller pieces of litter requires improvement (Zhang et al., 2020). In addition, during the peak period for sea turtle nesting, the accumulation rate of plastic litter on the C. mydas nesting ground beach at North Island of Qilianyu was 0.47 pieces·m⁻²·month⁻¹ or 9.2 g·m⁻²·month⁻¹ (D. Q. Li & L. Lin, 2021, unpublished data). This was higher than the average accumulation rate of 2019 Bulletin on the State of China’s Marine Ecological Environment (0.58 pieces·m⁻²·two month⁻¹ or 10.5 g·m⁻²·two month^-1) (Ministry of Ecology & Environment of the People’s Republic of China, 2019). This plastic litter should be cleaned up in a timely manner to prevent its breakdown into form microplastics. Therefore, it is suggested that the strength and frequency of beach litter cleaning should increase from once a week to once every 2–3 days, and focus on the removal of small plastic particles and foam. Considering the increasing number of foam plastics, we suggest that the foam used for local commercial activities such as aquaculture and seafood transportation should be recovered and replaced before aging and fragmenting into smaller plastic pieces.

As the geographic source of large beach litter at Qilianyu was primarily from Southeast Asian countries, such as Vietnam and Malaysia (Zhang et al., 2020). Regional cooperation between stakeholders in the South China Sea should be strengthened, with joint measures on the appropriate treatment of marine litter. This will help to reduce the generation of large plastic litter and prevent breakdown of large plastics to form microplastics, and benefit the conservation of C. mydas populations of the South China Sea.