Microplastics retention in different types of Mangrove forest in Xuan Thuy National Park, Vietnam

Mangrove forests provide important ecosystem services with regards to carbon storage and nutrient removal in coastal areas. They have also been found to retain emerging contaminants such as microplastics. However, in many parts of the world, mangrove forests are severely affected by deforestation and transformation into areas of intense agriculture/aquaculture. To restore their original ecosystem functions, mangrove forests are increasingly being targeted in small-scale conservation and restoration efforts, which often results in the coexistence of a variety of mangrove forest types with respect to tree age and vegetation density. This might have a severe impact on their retention capacity of fine particulate matter including microplastics (MP).

Here, we study Mangrove sediment samples from Xuan Thuy National Park, Vietnam located in the Red River Delta with respect to microplastics. Sediment samples were taken from four types of landcover, i.e., (i) completely deforested area, (ii) 5–7-year-old naturally regenerated forest, (iii) eight-year-old replanted forest, and (iv) 15 year-old naturally regenerated forest. Surface samples and 50 cm-long cores were collected using mini augers and dried at 50°C. Subsamples were used to extract microplastics by means of density separation with ZnCl₂ and digestion with Fenton reagent. Extracted MP were stained with Nile Red for florescence microscopy to determine MP concentration and shape. Additionally, microFTIR was used to identify the respective polymer type.

Microplastics concentrations (>64 µm) range from 64 to 2141 particles/kg dry weight (n=568) with the vast majority being fragments. Higher concentrations were found in the naturally regrown forests (ii and iv) than in deforested or reforested areas. Although no depth-dependent trends were visible, high concentration spikes in 30 cm depth at sites (ii) and (iii) have been identified. Future analyses will relate particle concentration to sediment grain size distribution and carbon/nutrient data from the same locations.