|Other Abstract||The microplastic pollution of marine environment has become quite severe and microplastics are readily transported long distance from source areas by wind and water, and distributed across coastal zone, surface water and sediments in the marine environment. The studies of microplastic distribution always paid attention to surface seawater, there were few studies reported of the microplastic in sea water column in China. The studies of distribution of microplastic in the seawater column were also rare internationally. Therefore, this study mainly focused on the distribution and influencing factors of microplastics in the marine environment. First, the distribution of microplastics in different seawater layer and surface sediments at the Bohai Sea was investigated. Second, different types of microplastics were selected to conduct field experiments in the seawater, density and surface topography changes were analyzed at different times. Thrid, The laboratory experiments under UV light, biology and sea water were also conducted. The effects of human activities and natural conditions on the horizontal and vertical migration of microplastics and their influencing factors and changes were discussed.
According to the investigation of microplastics in the seawater and surface sediments in the Bohai Sea, it was found that the microplastic concentrations ranged from 0.2-23 N/L in the Bohai seawater, with an average of 4.4 (±5.0) N/L. In the surface sediments, the average microplastic concentration was 102.0 (±73.4) pieces/kg dry sediments, in the range of 31.1-256.3 N/kg. The microplastics contamination of the surface waters in the Bohai Sea was mainly affected by the discharging of boats in the sea and the anthropogenic activities of the coastal residents. In half of the sampling stations，microplastics accumulated in the 5-10 m because the discrepancy of flow velocity direction in different water layer currents. The distribution of microplastics in other sites in the ocean was related to their vertical migration velocity and the discharge velocity into the sea. There was a corresponding relationship between the contents of turbidity, chlorophyll a in the upper water and the concentration in the sediments at the same site. Higher turbidity and higher chlorophyll a contents were favorable for the sedimentation of microplastics, which reflected the contribution of particulates and biomass in the water to the vertical migration of microplastics.
The microplastics in the Bohai Sea were dominated by fibers. The maximum density fibers microplastics found in surface seawater, water column and seabed sediments were all polyesters, while the maximum density microplastics were in the order of polystyrene, polyethylene terephthalate, and polyvinyl chloride, the maximum density increased with depth；the vertical distribution of microplastic fragments in different water layers was affected by the densities of microplastics. The particle size of microplastics also showed a regular distribution as the depth increases. In the seawater, the proportion of microplastics less than 300 μm and fibers was increasing with depth, which indicated that smaller microplastics and fibers were more likely to migrate to deeper water bodies. At the same time, this also indicated that the sampling of the 330 μm plankton net used in marine microplastics surveys may underestimate the abundance of microplastics in the middle and bottom layers of water.
In the laboratory experiments, the surface of microplastics under UV light was rough, cracks appeared, the densities of polyethylene microplastics increased slightly, while the densities of nylon and polystyrene remained unchanged. Bacteria were observed on the microplastic surface in sea water. The characteristic peaks of the extracellular polymers were detected on the surfaces of the diatom co-cultured microplastics by FTIR, as a result of forming small pieces blocks showed by the SEM. The changes in the densities of fibers were significantly higher than that of granular microplastics, which was consistent with the increasing proportion of fiber as the depth increaseed in the surveys.
By analyzing the changes of microplastics placed in seawater, it was found that the changes in the density of the microplastics were greater than that in the laboratory experiments. The densities of microplastics were increased. The type of microplastics and the state of the marine environment were the main factors affecting the density changes of microplastics. The changes in densities of microplastics varied significantly between the depths. Because the surface and middle seawater were mainly affected by currents, the density changes were similar, while the bottom water body was also not only affected by the currents, but also the resuspension of the bottom sediments, The density changes of microplastic put in bottom water were different with the surface and middle seawater. It was conducive to form a solid aggregate structure between microplastics and environmental substances such as suspended particles and algae when the sea was stable, which changed the densities of microplastics. The SEM and FTIR showed that substances adhered to the surfaces of microplastics include algae residues and suspended particles, which was accompanied with the algae residues and suspended particles found in the microplastics sampled in the Bohai sea water and surface sediments, and the corresponding relationship between the contents of turbidity, chlorophyll a in the upper water and the concentration in the sediments, reflected the contribution of particulates and biomass in water to the vertical migration of microplastics altogether.|