Microplastics in marine sediments of the Mississippi River Delta (Gulf of Mexico)

The marine environment across the globe is highly polluted with plastics. Plastics have been found at the sea surface, the water column, on beaches and on the seabed. Plastics may persist in the marine environment for hundreds to thousands of years, thus negatively impacting marine organisms. The current discrepancy between the measured and the modeled size distribution of floating plastic debris reveals a gap in microplastics (<5mm) in the ocean surface waters. The fate of these ‘missing plastics’ has yet to be unraveled. The seabed has been suggested to be a sink for microplastics. However, data on microplastic accumulation on the seabed remain scarce. Evidence of this accumulation is most likely to be found in areas with large inputs of plastics, such as outflows or river which carry plastics collected from land. For this reason, the potential role of the seabed as a sink for microplastics in the Mississippi River Delta (MRD) was studied. Microplastics larger than 30 μm were extracted from surface sediments using a solution of ZnCl2 (1.5 g/cm3), treated with Fenton’s reagent and if necessary with 1M HCl, and analyzed by light microscopy and Raman spectroscopy. The results show that microplastics were present along the entire depth transect from the mouth of the MRD to the deep sea with concentrations varying between 303 ± 960 and 2705 ± 960 pieces/kg dw (1031 ± 5088 and 18046 ± 5088 pieces/m2). The highest concentration of microplastics was observed closest to shore. Fibers were the dominant type of microplastics found in all sediment samples, with transparent, yellow and black as the dominant colors. It should be mentioned that fibers had a large input from contamination as well. Hard microplastics ranged from 180 to 643 µm and were identified as high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), and polyvinylchloride (PVC). Microbeads were identified as PS and polyamide (PA) and their sizes ranged between 245 and 496 µm. The findings reported in this study indicate that the distribution of microplastics at the river delta is likely controlled by the combination of varying input sources, surface currents, primary productivity and the bathymetry. At the MRD, microplastic concentrations in the seabed are substantially higher than observed at surface waters, the water column and beach sediments. This stresses the importance of the seabed as a sink for microplastics at the MRD.