Composition and distribution of microplastics in the surface seawater of Xisha Islands

Marine microplastics are emerging pollutants in the ocean and have become a major global environmental issue in recent decades. However, microplastic pollution has not been frequently studied in the oligotrophic oceanic regions around the Xisha Islands, the largest marine coral reef area of China. In this study, microplastics in surface seawater were sampled from 30 stations in the region around the Xisha Islands. Marine microplastics were classified via a microscopic method using an Olympus stereo microscope. Microplastic photographs were taken by an inner imaging system, and the size of the microplastics was estimated using Image J software. Additionally, a portion of the microplastic samples was randomly selected for component analysis based on the shape, color, and size of the particles. Our results indicate that microplastics are widely distributed in the surface seawater of the Xisha region. The abundance of microplastics was in the range of 10–130 pieces/m³, with an average value of 54.7 pieces/m³. The size range of microplastics was 0.02−5 mm with the length of 0.02−1 mm accounting for 32.1% and the length of 1−3 mm accounting for 64.8% of the total abundance. The microplastics from the Xisha region can be classified into five color types, namely transparent, blue, black, green, and red, with the average percentages being 56.8%, 40.2%, 1.2%, 1.2%, and 0.6%, respectively. Moreover, the local microplastics can be divided into fiber and film according to their morphology with fiber accounting for more than 99% of the total abundance. Polyethylene terephthalate (PET) and polypropylene (PP) were the two major chemical components of the microplastics, accounting for 56.2% and 20.3% of the chemical components, respectively. The average concentration of microplastics in Xisha region is relatively lower than that in many other regions of the South China Sea, as well as that in other regions of the global oceans. Our results suggest that the spatial distribution of microplastics is influenced by water movement, anthropogenic activities, and topographic features in the Xisha region. Although there is a large spatial variation in the surface seawater in the Xisha region, the concentration of microplastics in the near-shore stations is typically high, which may indicate that the microplastics in these areas originate from terrestrial sources. On the other hand, a reduced wave energy due to the topographic structure of shallow coral reefs allows more microplastics to settle and accumulate in the nearshore waters around the islands. Meanwhile, there were nearshore stations (S12, S19, S22) with low microplastic abundance, particularly in areas with high internal tides caused by local topography, strong mixing or upwelling induced by internal tides, and waves. These phenomena can bring subsurface waters to the surface and thus lead to the dilution of surface seawater pollutants at the nearshore stations. Analysis of microplastic morphology revealed the presence of large-diameter microplastic fibers in the surface seawater of the Xisha region, which may indicate that these microplastics are young in age. Most likely, they originated from recent fishery activities in the region, as both PET and PP are common types of plastics used in local fishery operations. Furthermore, microplastics have already been found in local soil samples and in the bodies of seabirds living on the Xisha Islands, due to an increase in island development and construction. Therefore, plastic waste discharge caused by increasing human activities may be another important source of microplastics to the surface seawater of the Xisha region.