ReviewThe physical impacts of microplastics on marine organisms: A review
Introduction
In contemporary society, plastic has achieved a pivotal status, with extensive commercial, industrial, medicinal and municipal applications. Demand is considerable; annual plastic production has increased dramatically from 1.5 million tonnes in the 1950s to approximately 280 million tonnes in 2011 (PlasticsEurope, 2012). Through accidental release and indiscriminate discards, plastic waste has accumulated in the environment at an uncontrollable rate, where it is subjected to wind and river-driven transport, ultimately reaching the coast. Due to its lightweight, durable nature, plastic has become a prevalent, widespread element of marine litter (Moore, 2008; Thompson et al., 2009); the most commonly produced and therefore encountered polymers being polypropylene (PP), polyethylene (PE) and polyvinylchloride (PVC) composing 24%, 21% and 19% of global plastic production in 2007, respectively (Andrady, 2011). Recently, inconspicuous microscopic plastic particles, referred to here as ‘microplastics’, have been identified as a ubiquitous component of marine debris. Defined as less than 5 mm in size by the National Oceanic and Atmospheric Administration (NOAA), microplastics can be of primary (purposefully manufactured to be of microscopic size) or secondary (derived from the fragmentation of macroplastic items) origin. They have been accumulating in oceans worldwide over the last four decades (Carpenter et al., 1972), from low background levels to localized ‘hotspots’ (see Table 1). Present on beaches, in surface waters, throughout the water column and within the benthos (Lattin et al., 2004; Moore et al., 2001; Thompson et al., 2004), microplastics have pervaded even the most remote marine environments (e.g. Ivar do Sul et al., 2009).
Gyres are particular hotspots for microplastic accumulation. Recently a maximum concentration and mass of 32.76 particles m3 and 250 mg m3 respectively have been recorded in the North Pacific Subtropical Gyre (Goldstein et al., 2012). Industrial coastal areas have also been identified as microplastic hotspots; concentrations of approximately 100 000 plastic particles m3 of seawater have been reported in a Swedish harbour area adjacent to a PE production plant (Noren and Naustvoll, 2010). Sediment from densely populated coastal areas can be heavily contaminated with microplastics. Browne et al. (2011) found microplastics on eighteen shores across six continents, with a tendency towards fibrous shapes. Maximum concentrations of 124 fibres l−1 were reported and a significant relationship between microplastic abundance and human population-density was found (Browne et al., 2011). Thus as the human population continues to increase, the prevalence of microplastics will also most probably increase. Previous studies have found a predominance of fibrous microplastics (see Claessens et al., 2011; Thompson et al., 2004). Despite a variety of forms from irregular fragments to spherules, it seems likely that fibrous microplastics are most abundant in the marine environment.
A temporal increase in the abundance of marine microplastics has been indicated. Recently, combined data from peer-reviewed literature, publicly available data and new data sets revealed changes in the abundance and mass of microplastics in the North Pacific Subtropical Gyre. Abundance and mass increased by two orders of magnitude from a median of 0–0.116 particles m3 and 0–0.086 mg m3, respectively from 1972–87 to 1999–2010. This is believed to have been driven by a localised increase in microplastic abundance (Goldstein et al., 2012). Additionally, North Atlantic and North Sea surface samples collected by a Continuous Plankton Recorder (CPR, Sir Alister Hardy Foundation for Ocean Science), coincided with a growth in global plastic production (Thompson et al., 2004). Archived plastic samples from the west North Atlantic Ocean over the past 24 years have revealed a decrease in mean particle size from 10.66 mm in the 1990s to 5.05 mm in the 2000s. Sixty nine per cent of fragments were 2–6 mm (Morét-Ferguson et al., 2010), highlighting a prevalence of small plastic particles. Given the continual fragmentation of plastic items, particle concentrations are likely to increase with decreasing size.
The entanglement in and ingestion of macroplastic items is widely recognised in vertebrates. Over 250 marine species are believed to be impacted by plastic ingestion (Laist, 1997). The demise of higher organisms, typically vertebrates, is highly emotive and ultimately more conspicuous to observers. As a result, such instances are often subject to extensive scientific research and media coverage. Information regarding the biological impacts of microplastics on marine organisms, however, has received less attention and is only just emerging. A technical report considering the impacts of marine debris on biodiversity revealed that over 80% of reported incidents between organisms and marine debris was associated with plastic whilst 11% of all reported encounters are with microplastics (GEF, 2012). Since microplastics occupy the same size fraction as sediments and some planktonic organisms, they are potentially bioavailable to a wide range of organisms. Microplastics can be ingested by low trophic suspension, filter and deposit feeders, detritivores and planktivores (Browne et al., 2008; Graham and Thompson, 2009; Murray and Cowie, 2011; Thompson et al., 2004). Therefore, they may accumulate within organisms, resulting in physical harm, such as by internal abrasions and blockages. In addition to the potential physical impacts of ingested microplastics, toxicity could also arise from leaching constituent contaminants such as monomers and plastic additives, capable of causing carcinogenesis and endocrine disruption (see Oehlmann et al., 2009; Talsness et al., 2009). Furthermore, microplastics are liable to concentrate hydrophobic persistent organic pollutants (POPs), which have a greater affinity for the hydrophobic surface of plastic compared to seawater. Due to their large surface area to volume ratio, microplastics can become heavily contaminated – up to six orders of magnitude greater than ambient seawater – with waterborne POPs (Hirai et al., 2011; Mato et al., 2001). This presents a possible route of exposure to marine organisms, whereby bioaccumulation and biomagnification could occur through the food chain. The transfer of POPs to marine organisms via microplastic vectors is not considered in detail in this review (for examples see Teuten et al., 2009); however the pathways and uptake of microplastic particles are clearly of relevance to chemical transfer, as well as physical harm.
Given the growing evidence outlined above, this review – focussing on marine invertebrates – aims to: (1) summarise the factors contributing to the bioavailability of microplastics; (2) outline the susceptibility of different feeding guilds to microplastic ingestion; (3) determine the factors likely to influence the physical impacts of microplastics; and (4) discuss microplastic transfer through the food chain.
Section snippets
Size
A key factor contributing to the bioavailability of microplastics is their small size, making them available to lower trophic organisms. Many of these organisms exert limited selectivity between particles and capture anything of appropriate size (Moore, 2008). Alternatively, higher trophic planktivores could passively ingest microplastics during normal feeding behaviour or mistake particles for natural prey. Work by Fossi et al. (2012) investigated the impacts of microplastics on the
Biological interactions
Microplastic bioavailability could be enhanced by biological factors. The ingestion of polystyrene (PS) beads (100 nm) by suspension-feeding bivalve molluscs significantly increased when they were incorporated into manually-generated aggregates, formed by rolling natural seawater in the laboratory. The seasonal flocculation of natural particulates into sinking aggregates is an important pathway for energy transfer between pelagic and benthic habitats (Ward and Kach, 2009). Consequently, the
Factors influencing the physical impacts of microplastics
There is a wealth of literature regarding macroplastic ingestion in vertebrates (e.g. Denuncio et al., 2011; Laist, 1997; Lazar and Gracan, 2011; van Franeker et al., 2011; Yamashita et al., 2011), reporting global impacts including: internal and/or external abrasions and ulcers; and blockages of the digestive tract, which can result in satiation, starvation and physical deterioration. In turn this can lead to reduced reproductive fitness, drowning, diminished predator avoidance, impairment of
Conclusions
Low density microplastic debris is accumulating in ocean gyres and pelagic invertebrates inhabiting these regions may be susceptible to microplastic ingestion. In addition, the benthos is likely to be a sink for high density microplastics. Some organisms may have the capacity to egest microplastics, possibly leading to their incorporation into marine aggregates. Benthic suspension- and deposit- feeders are therefore likely to ingest sinking and sedimentary microplastics. Fibres are the most
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