Abundance and Characteristics of Microplastics in the Gastrointestinal Tract of Fish from the Wanquan River on Hainan Island

Microplastics (MPs) are emerging global pollutants that have attracted considerable attention because of their widespread contamination and profound ecological impacts. Fish, as one of the most diverse groups of aquatic organisms, is a sensitive indicator of MP contamination in aquatic ecosystems. Hainan Island, which is located in the tropical and subtropical zone, is rich in freshwater fishery resources. However, freshwater fishes in Hainan Island are facing the threat of MP pollution caused by human activities. Wanquan River, the third largest river on Hainan Island, is facing eutrophication, heavy metal pollution, organic pollution, and MP pollution. However, the contamination status of MPs in the gastrointestinal tract (GI) of fish from the Wanquan River and the effects of feeding habits and spatial distribution on the abundance and characteristics of MPs in the GI tract of fish have not been reported. In this study, 11 representative sampling sites along the upstream, midstream, and downstream sections of the Wanquan River, covering urban residential areas and natural watersheds, were selected. Fish samples were collected using trawl nets from July to September 2021. All samples were immediately transported to the laboratory and stored in a freezer for subsequent analyses. In the laboratory, the surface of the fish was rinsed, and the GI tracts were carefully eviscerated and weighed. The GI tract samples were digested with potassium hydroxide, and the supernatant was filtered through 0.45 µm glass fiber filters. The insoluble materials such as sediment in the digestion solution were added with saturated zinc chloride solution for flotation, and the flotation supernatant was filtered through 0.45 µm glass fiber filters. Fourier transform infrared spectroscopy (FTIR) was used to identify the composition of the MPs. A total of 213 fishes spread over 34 species were collected, and MPs were detected in 85.0% of the samples. The average abundance of MPs was (3.15±1.70) ind./fish, which was consistent with the economic development level in Wanquan River basin. The MPs in the samples were mainly ＜1.0 mm (78.7%) transparent (46.0%) fibers (74.6%), which are widely present in the environment, easy to be ingested, and difficult to be excreted. FTIR results showed that most of the MPs were composed of polypropylene (PP, 35.8%) and polyethylene (PE, 25.4%). Given their good temperature resistance and corrosion resistance, PP and PE are the main components of carpets, industrial fabrics, fishing gear, shipping ropes, etc. Thus, PP and PE are widely detected in aquatic ecosystems because of their durability and widespread usage. Principal component analysis showed that transparent/blue fibers with 0.5–1.0 mm contributed the most to the abundance of MPs in the samples. The control of urban waste, fishing supplies, and agricultural waste must be strengthened to reduce the MP pollution in the Wanquan River. The abundance of MPs varied with feeding fish and followed the order molluscivorous (7.33 ind./fish) ＞ pelagic carnivorous (6.83 ind./fish) ＞ benthic carnivorous (4.33 ind./fish) ＞ detritivorous (3.36 ind./fish) ＞ crustaceavorous (3.15 ind./fish) ＞ insectivorous (2.40 ind./fish) ＞ epiphytivovrous (2.30 ind./fish) ＞ herbivorous (1.89 ind./fish). The abundance of MPs in the samples was affected by several factors, such as the status of MP pollution in foraging areas or habitats, the abundance of MPs in fish prey, frequency of fish predation, factors of the fish itself (e.g. body weight, body length, body shape, gut structure, etc.), retention time of MPs in the GI tract of fish, and excretion rate of MPs by the GI tract. The abundance of MPs in the samples gradually increased from upstream (2.67±0.43 ind./fish) to downstream (3.81±0.39 ind./fish). This result was related to the increasing urbanization and industrialization in the downstream areas and thus the increased input of MPs downstream. The proportion of red/yellow/blue fragments and thin film MPs with size ＞1.0 mm increased, whereas the proportion of black fiber MPs with ＜1.0 mm decreased from upstream to downstream. This result was related to the increase in shipyard, shipping, and tourism activities downstream. For individual fish species at the different sampling sites, the abundance of MPs in the samples did not show a continuous increasing trend. This result was because the MPs in the samples were mainly derived from passive ingestion, which is affected by many factors (e.g., feeding habits, intestinal structure, excretion ability, food intake, and the status of MP contamination in the environment). The results of this study provide data support for the risk assessment and control of MPs in fish in the Wanquan River and serve as a scientific basis for the prevention and control of MP pollution in freshwater rivers on Hainan Island.