Superimposed microplastic pollution in a coastal metropolis

The mitigation of microplastic pollution in the environment calls for a better understanding of the sources and transportation, especially from land sources to the open ocean. We conducted a large-scale investigation of microplastic pollution across the Greater Melbourne Area and the Western Port area, Australia, spanning gradients of land-use from un-developed catchments in conservation areas to more heavily-developed areas. Microplastics were detected in 94% of water samples and 96% of sediment samples, with abundances ranging from 0.06 to 2.5 items/L in water and 0.9 to 298.1 items/kg in sediment. The variation of microplastic abundance in sediments was closely related to that of the overlying waters. Fiber was the most abundant (89.1% and 68.6% of microplastics in water and sediment respectively), and polyester was the dominant polymer in water and sediment. The size of more than 40% of all total microplastics observed was less than 1 mm. Both light and dense polymers of different shapes were more abundant in sediments than those in water, indicating that there is microplastic accumulation in sediments. The abundance of microplastics was higher near coastal cities than at less densely-populated inland areas. A spatial analysis of the data suggests that the abundance of microplastics increases downstream in rivers and accumulates in estuaries and the lentic reaches of these rivers. Correlation and redundancy analysis were used to explore the associations between microplastic pollution and different land-use types. More microplastics and polymer types were found at areas with large amounts of commercial, industrial and transport activities. Microplastic abundances were also correlated with mean particle size. Microplastic hotspots within a coastal metropolis might be caused by a combination of natural accumulation via hydrological dynamics and contribution from increasing anthropogenic influences. Our results strongly suggest that coastal metropolis superimposed on increasing microplastic levels in waterbodies from inland areas to the estuaries and open oceans.