Abstract: Due to the unique geographical location and environmental characteristics, mangrove wetlands have become sinks of various pollutants. In this study, we investigated the distribution characteristics of microplastics in water and sediments of adult (>20 year-old) and juvenile (4 year-old) mangroves at different tidal zones in the west of Xincun lagoon in Lingshui County, Hainan Province. The results showed that the abundance of microplastics in surface seawater and sediments in adult mangrove (91.2±15.0 n·L^-1, 1972.9±805.2 n·kg^-1) was significantly higher than that of juvenile mangrove (49.1±7.3 n·L^-1, 686.2±64.3 n·kg^-1). The abundance of microplastics in different zones of adult mangrove was the highest on the landward (2822.7±309.6 n·kg^-1) and the lowest on the seaward (1221.3±292.9 n·kg^-1). The proportion of film microplastics in seawater was lower than that in sediments (6.5% vs 14.0%), while that of foam was higher in seawater than in sediments (23.8% vs 15.9%). There were no obvious patterns for the morphological distribution of microplastics in sediments in terms of stand age and tidal zone. The particle sizes of most microplastics (93.3%) ranged from 50 to 2500 μm, in which the proportion of 100-250 μm microplastics was higher on the seaward than that on the landward, both in adult and juvenile mangroves. The microplastics with white color had the highest proportion (39.2%), followed by blue (21.6%). The proportion of white microplastics in the sediments of juvenile mangrove was higher than that of adult mangroves, with the highest proportion on the landward, the lowest on the seaward, while higher proportion was found on the seaward than that on the landward in adult mangroves. The main materials of microplastics in mangroves were polyethylene (39.6%), polystyrene (28.3%), and polypropylene (20.8%), and were seriously eroded and weathered. Overall, mangrove ecosystems have the function of accumulating microplastic pollutants, and such ability is the strongest in adult mangroves on the landward.

Key words: Xincun lagoon, surface seawater, sediment, fiber, collecting function.