Microplastics as an Emerging Contaminant of Concern

Microplastics are recognized as an emerging contaminant of concern in the aquatic environment because they occupy the same size fraction as plankton and sediment, making them bioavailable to a wide range of organisms; they are pervasive in the environment; and they have the potential to adversely impact aquatic organisms and ecosystems. While the detrimental effects of large plastic debris through ingestion and entanglement are well documented, the environmental and human health risks associated with microplastics continue to be elucidated.

Microplastic debris in aquatic systems presents unique hazards to organisms relative to that of larger forms of plastic. As larger plastic materials degrade into microplastics, the total number of particles increases, which in turn, increases their occurrence in the environment and their bioavailability (encounter rate) to organisms. Causal relationships between increasing global production of plastics and increased occurrence of microplastics in aquatic environments have been inferred (Figure 2). Decreased particle size increases the number of species that can ingest and/or ventilate the particles. Many lower trophic invertebrates feed on particles in a limited size range, and they will indiscriminately capture and ingest anything within that range. In this manner, organisms may ingest microplastics during normal feeding behavior or may mistake these particles for natural prey.

Figure 2. The sum of the number of microplastic particles found in different sediment layers along a beach at Groenenkijk, Belgium (*denotes significance at p < 0.05 level). Each 8 cm sediment layer is represented by its corresponding time period. Approximate annual global production is overlain for comparison. Source: Reprinted from Marine Pollution Bulletin, 62 (10); Claessens, M., De Meester, S. Van Landuyt, L., De Clerck, K., and Janssen, C.R., Occurrence and distribution of microplastics in marine sediments along the Belgian coast., 2199-2204, 2011, with permission from Elsevier

Beyond size and abundance, the bioavailability of microplastic particles is influenced by other factors, including density, shape, and color. The density of plastic particles will determine their fate within aquatic systems, which in turn will determine whether pelagic or benthic organisms are likely to encounter the particles (Figure 3). Low-density polymers, such as polyethylene (PE), can be found near or at the sea surface, while higher density polymers, such as PVC, tend to sink to the bottom. Buoyancy of plastic particles within the water column is also influenced by the development of a biofilm, which can increase their density.

Figure 3. Possible pathways for the transport and fate of microplastic particles in aquatic systems. Source: Reprinted from Environmental Pollution, 178; Wright, S.L., Thompson, R.C., Galloway, T.S., The physical impacts of microplastics on marine organisms: a review, 483-492, 2013 with permission from Elsevier

These particles may undergo defouling, which represents a potential pathway by which they may return to the surface. Similarly, particle shape (e.g. film, spheres, fibers, and irregularly shaped fragments) could cause different dispersal patterns in aquatic systems due to interactions between surface area and the sea surface microlayer. For visual predators, the color of the microplastic could contribute to the likelihood of ingestion as a result of its resemblance to prey items.

Microplastics may also serve as vectors for the introduction of nonnative and invasive species. Owing to their abiiity to accumulate and persist in global surface waters for extended periods of time, microplastics can become colonized by microbes and phytoplankton. Physical oceanographers propose that the cycling of debris within the North Pacific Gyre may introduce organisms living in or on the plastics to new coastal regions (e.g. Asia to California) where they could then successfully invade the new area in the absence of natural predators. This concept has not been studied extensively but poses a substantial invasive species threat that could cause unprecedented ecological harm. Furthermore, microplastics as a vector is a significant concern given the increased range of species distributions in infectious microbes (e.g. Vibrios) and harmful algal blooms (HABs).

Further research is required to gain a complete understanding of the fate and potential effects of microplastics including the relative contribution of various sources and their toxic effects on both environmental and human health.