Superimposed microplastic pollution in a coastal metropolis

Highlights

• Microplastics were prone to sedimentation.

• Microplastics in water and sediment increased downstream.

• Microplastics level was higher near coastal cities than that in remote inland areas.

• Hydrological and anthropogenic influences contribute to microplastic hotspots.

• Superimposed microplastic pollution was confirmed in coastal metropolis, Australia.

Abstract

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.

Introduction

The research into microplastics has increased exponentially since that term was initially proposed, as it has had widespread scientific and public media salience for over a decade (Sutherland et al., 2019; Thompson et al., 2004). Although there is yet no internationally agreed-upon definition for the cut-off size for microplastics, it is generally accepted to be those <5 mm (Frias and Nash, 2019; Law, 2017). There are indications that microplastics pose risks to organisms across the full spectrum of biological organization, from cellular to population level (Wright et al., 2013). Microplastics can also contaminate a wide range of species that are consumed by humans (Seltenrich, 2015; Wright and Kelly, 2017). What is clear is that more scientific field-based evidence is needed to steer current debates over the real risks of microplastic pollution (Burton, 2017; Kramm et al., 2018; Sedlak, 2017). The rates at which plastics are accumulating in urban and peri-urban waste streams have sparked research efforts to better to understand the major sources of microplastics in the environment and help improve management of this pollution.

For over half a century, impacts of plastic pollution to marine environments have been of great public concern (Carpenter and Smith, 1972). Large-scale efforts have been made to map microplastic distributions in global oceans, shorelines and marine organisms (Browne et al., 2011; Eriksen et al., 2014; Law et al., 2010). Marine microplastic dynamics can shed light upon various sources which are predominantly from land-based inputs (Jambeck et al., 2015; Lebreton et al., 2017). A world-scale modeling approach predicted that the amount of plastic waste entering oceans is estimated to be close to three orders of magnitude greater than the current monitoring results of floating marine plastic debris would suggest (Jambeck et al., 2015). Many sources of microplastic pollution in land-based watersheds have been identified; such as discharges of treated wastewater, industrial and commercial activities, municipal solid waste collection and agricultural activities. (Andrady, 2017; He et al., 2019; Law, 2017; McCormick et al., 2014). However, the relative contribution of these land-use activities to microplastic pollution is unknown.

The spatial distribution of microplastics in water and sediment can help predict their movement and deposition within local aquatic habitats. For example, vertical profiles of microplastics in waterbodies are greatly influenced by water flow and sediment dynamics (Nizzetto et al., 2016). For sediments, some case studies found extremely high concentrations of microplastics (74,800 items/kg) in freshwater sediment, which acted as a temporary or permanent sink (Wang et al., 2018). Particle sizes and densities were suggested as key factors governing with microplastic deposition process (Wagner et al., 2019). Nevertheless, the detailed mechanisms involved in microplastic transportation between sediment and water are not fully understood.

Reportedly high loads of microplastics in freshwaters has sparked interest in better understanding their sources. The spatial distribution of microplastics at a catchment scale has been described of some rivers, lakes and estuaries (Kapp and Yeatman, 2018; Schmidt et al., 2017; Siegfried et al., 2017). Plot and field studies involving direct monitoring suggest that microplastic pollution is closely related to anthropogenic factors such as population density, land-use and point source pollution (Barrows et al., 2018; Hendrickson et al., 2018; Klein et al., 2015). River systems were suggested as major transport pathways for plastic debris, especially in urban catchments (Atwood et al., 2019; Mani et al., 2015, 2019). The dominant role of natural forces like weather and hydrological conditions in microplastic transport dynamics has also been modeled (Hurley et al., 2018; Siegfried et al., 2017). However, it is not clear to what extent and how those factors ultimately determine the fate of microplastic transport from land to the ocean.

Snapshot sampling for microplastic distribution is still an economic way to help gauge microplastic transport pathways from source to sink, especially for places where baseline data are relatively insufficient. Although microplastics have been found on all continents, limited reports on their distribution exist for Australia’s inland water bodies (Lebreton et al., 2017; Schmidt et al., 2017). Such a knowledge gap limits global estimations of microplastic emissions from land to ocean. A better understanding of microplastic sources is required to best address the elimination of the principal sources. Herein we selected waterbodies in the Port Phillip and Western Port catchments which includes the Greater Melbourne Area (GMA) and one of the most populated cities in Australia. We aim to determine: (i) the spatial distribution of microplastics from inland water bodies to estuaries at a catchment scale, (ii) the environmental factors affecting the variation of microplastic pollution, and (iii) the anthropogenic impacts on the distribution of microplastics in sediment and water.