Enrichment and ecological risks of microplastics in mangroves of southern Hainan Island, China

Mangroves have recently been identified as one of the most threatened ecosystems by microplastics (MPs) pollution from terrestrial and marine sources, while little is known regarding the MPs enrichment, influencing factors and associated ecological risks in mangroves. The present investigation aims to evaluate the accumulation, characteristics, and ecological risks of MPs in various environmental matrices from three mangroves of southern Hainan Island during dry and wet seasons. The results revealed the prevalence of MPs pollution in the surface seawater and sediment from all studied mangroves during two seasons, where the highest MPs abundance was observed at Sanyahe mangrove. The abundance of MPs varied considerably in surface seawater by seasons and were distinctly modulated by rhizosphere effect. The characteristics of MPs also exhibited some pronounced variations across mangroves, seasons and environmental compartments, but the detected MPs were dominated by fiber-shaped, transparent-colored, and smaller-sized (100-500 μm) MPs. The most prevalent polymer types were polypropylene, polyethylene terephthalate and polyethylene. Further analyses revealed positive correlations between MPs abundance and contents of nutrient salts in surface seawater but negative relationships between MPs abundance and water physicochemical properties, including temperature, salinity, pH and conductivity (p < 0.05). The joint use of three evaluation models indicated MPs posed varying degrees of ecological risks to all studied mangroves, whereas Sanyahe mangrove exhibited the highest ecological risk of MPs pollution. This study provided new insights into the spatial-seasonal variations, influencing factors and risk assessment of MPs in mangroves, which would be helpful for source tracing, pollution monitoring and policy formulation.