Background
Microplastics are plastic material ranging in size from 1 μm to 5 mm and are listed as an emerging marine pollutant (Hitchcock, 2020; Horton, Walton, et al., 2017). Marine microplastic pollution is ubiquitous, even recorded in deep oceans and polar seas that are inaccessible to humans (Alice et al., n.d.; Angiolillo et al., 2021; Mason et al., 2016; Van Cauwenberghe et al., 2013). Previously documented sources of marine microplastics include the degradation and fragmentation of larger plastic litter in the ocean (Andrady, 2011; Cole et al., 2011). However, the findings of several recent studies demonstrate that riverine transport of microplastics is also a noteworthy source of microplastic pollution in the marine environment (Bondelind et al., 2020; Mai et al., 2019; van Wijnen et al., 2019). It has also been suggested that current estimates of microplastic concentrations in the marine environment are underestimates because they do not fully account for the role of freshwater systems in transporting microplastics to the oceans (R. Hurley et al., 2018).
Microplastics are found across the terrestrial land surface, in the atmosphere, and in human food, beverages and even infant milk products (Ferraz et al., 2020). Evidence of microplastic pollution has been reported in river systems, globally, including the Thames River in the UK (Horton, Svendsen, et al., 2017; Rowley et al., 2020), the Great Lakes in the US (Baldwin et al., 2016; Mason et al., 2016), the Yangtze River in China (J. Fan et al., 2021; Li et al., 2019), the Sinos River in Brazil (Ferraz et al., 2020) and the Nakdong River in South Korea (Eo et al., 2019). These microplastics are distributed in various environmental matrices in freshwater systems, including surface water, water columns, sediments, and aquatic organisms.
The ingestion of microplastics by aquatic organisms has been widely reported and is believed to cause physical damage. Some microplastics have sharp edges that can injure the organs and tissues of organisms. Microplastics can also accumulate in organisms and clog their digestive systems (Carreras-Colom et al., 2018; R. R. Hurley et al., 2017; Maaghloud et al., 2021; Peters & Bratton, 2016). In addition to the physical damage, ingestion of microplastics may also pose toxicological risks, as plastic materials may contain or adsorb organic or inorganic chemical contaminants. Moreover, the monomers released from polymers also have toxicological effects (He et al., 2020). However, the exact hazards of microplastics to organisms remains contentious because many studies are performed in laboratory settings with concentrations of microplastics far in excess of those found in the environment (Boucher et al., 2016; C. Y. Chen et al., 2021; Rochman et al., 2013).
The term ‘microplastics’ was first defined in the marine environment in 2004 (Thompson, 2004), yet plastic materials have been widely used globally for over half a century. Hou et al. (2021) investigated historical samples of urban freshwater fish held in the Chicago Museum in the USA and found no microplastic particles in samples from before the 1950s. In contrast, there was a significant increasing trend in the number of microplastics in the fish samples from the middle of the last century to 2018. Similar historical data on microplastic pollution can be obtained by investigating microplastic contamination in soil or sediment cores with similar results (Fan et al., 2019; Matsuguma et al., 2017; Van Cauwenberghe et al., 2013). This suggests that human society is facing a serious challenge of a pollutant that has been spreading and accumulating in the environment for more than half a century.