How mangrove plants affect microplastic distribution in sediments of coastal wetlands: Case study in Shenzhen Bay, South China.

Microplastic pollution is common in marine and coastal ecosystems, especially in mangrove wetlands. However, factors affecting the distribution of microplastics, such as plants, have not been sufficiently studied. We investigated the effect of different plant species on the distribution of sediment microplastics in two Nature Reserves in South China, viz. Futian Mangrove and Mai Po Mangrove. In Futian Mangrove, the abundance of total microplastics among three monospecific mangrove stands dominated by Sonneratia caseolaris, Kandelia obovata, and Sonneratia apetala was similar. The abundance of microplastics in the mudflat was similar to that in the forest interior, except for the fact that more fiber was found in the mudflat than in the interior of Sonneratia apetala. This suggested that the dense pneumatophores at the fringe prevented fibers from entering the mangrove forest. The significant positive dependence (p < 0.05) between the density of Sonneratia pneumatophores and the abundance of fibers highlighted the importance of pneumatophores. The abundance of total microplastics, predominantly in the form of fibers, in sediments at the forest fringe (2835 ± 713 items/kg d.w. and 2070 ± 224 items/kg d.w. in Futian and Mai Po, respectively) was higher than that in the forest interior and mudflat. There was no difference between the two latter locations in both mangroves, which demonstrated the significance of the fringe effect. This paper reports for the first time that the spatial distribution of microplastics in mangrove sediments was affected by plant species, which provides useful information for environmental processes of microplastics in coastal wetlands.

Development of a digestion method for determining microplastic pollution in vegetal-rich clayey mangrove sediments.

Microplastics are ubiquitous pollutants found in environments. Mangrove sediments containing vegetal litter are different from other environmental matrices such as river and marine sediments. The presence of vegetal litter leads to an under-estimation of microplastic pollution, particularly classical digestion methods are not suitable for removing this type of organic matter. The present study aims to develop a digestion method to remove vegetal litter and improve the determination of microplastic pollution in mangrove sediments. Results showed that our three-stage method with repeatedly addition of hydrogen peroxide had the highest efficiency in removing mangrove vegetal litter when compared with the three classical digestion methods. The high match scores of Fourier Transform Infrared Spectroscopy proved that the developed method had little impacts on the integrity of five polymer types of microplastics. The developed method also achieved high efficiency in extracting microplastics from mangrove sediments containing different content of vegetal litter. CAPSULE: A digestion method was developed for extracting microplastics in clayey mangrove sediments rich in vegetal litter.