1.5–4.1% of the annual production can end up in the ocean (Jambeck et al. 2015), 7800 MMT of plastics hitherto produced should have resulted in at least 57–160 MMT of plastics in the ocean. However, inventories find less than 0.25 MMT of plastics in surface waters (Eriksen et al. 2014; Van Sebille et al. 2015) and their abundance does not correlate with production volumes over the recent years (Thompson 2004). This disparity has led to the notion of a “missing fraction” of MPs, especially the smaller fragments in the ocean (Kvale et al. 2020; Woodall et al. 2014). However, floating plastics essentially represent a “snapshot” view of the MPs in the short window of time they float before extensive fouling sink them into the water column. Also, the search for a missing fraction assumes that all floating MPs will be larger than the mesh size of collecting gear (around 330 μm) excluding most of the small MPs and NPs from the count. The missing fraction reflects limitations of the sampling gear and not taking into account the MPs below the surface water level, especially in the sediment (Kanhai et al. 2019; Nakki et al. 2019; Ramirez‐Llodra et al. 2011). A recent estimate (though based on only six sampling sites) estimates 14 MMTs of MPs that are >50 μm in the sediment (Barrett et al. 2020).
Beaches worldwide routinely accumulate plastic debris, including large amounts of microplastics (Kako et al. 2020; Koongolla et al. 2018; Tavares et al. 2020). Unlike larger plastic litter, the MPs are not removed during beach clean operations, or routinely collected on managed beaches. Worldwide beach clean exercises (Ocean Conservancy 2019) annually covering 36,000 miles of coastline yield a consistent list of highly abundant plastic litter items, with cigarette filters topping the list with 5.7 million collected. Plasticized cellulose acetate fibers used in cigarette filters are recalcitrant in the ocean environment, explaining their ubiquity in coastlines around the world. All of the top seven beach litter items reported are made of plastic; if MPs and NPs could also be somehow included in the count, the abundance of beach plastic debris would be higher by several orders of magnitude. Table 1.1 summaries the seven most abundant plastic debris items found in beach cleaning operations in different parts of the world by the Ocean Conservancy, Washington DC, in 2018. Their global beach cleaning operation is an annual event.
Table 1.1 Abundance of the top seven items found in global coastal cleanup by Ocean Conservancy (Washington DC) and types of plastics commonly used in them.
| Litter item | Count in millions | Plastics typically used |
|---|---|---|
| Candy wrappersCigarette filtersBeverage bottlesBottle capsStraws and stirrersCups and platesBags | 5.72 3.73 3.67 1.97 1.75 1.39 0.94 | PE, PP, PET Cellulose acetate PET PP PE PE, PS, EPS PE |
Data pertain to 36 000 miles of coastline in different global locations. Based on data by Ocean Conservancy on the 2018 Coastal Cleanup.
1.2.3 Chemicals in Plastic Debris
Plastics used in products typically include a suite of chemicals intimately mixed with the base resin. These include (i) intentionally added chemicals or additives to modify the properties of the base resin to suit the needs of the product (Groh et al. 2019; see Chapter 2); (ii) low levels of the relevant residual monomer trapped in the plastic (not an issue with PE or PP with gaseous monomers, but relevant with PS, PVC or polycarbonate [PC]); and (iii) unintended compounds sorbed from seawater and concentrated by partition (Hüffer and Hofmann, 2016; Pascall et al. 2005; Rochman et al. 2013; see Chapter 9). Some of these sorbed chemicals are persistent organic pollutants (POPs) that are toxic compounds as well, and remain in the environment for extended durations, allowing them to be widely distributed via water, soil, and air. These tend to accumulate in the fatty tissue of animals that ingest them and may bio‐magnify as they move to higher trophic levels. Any POPs in seafood are of special interest to human consumers. Despite their very low dissolved concentration in seawater, the equilibrium concentration of POPs in the MPs and NPs tends to be very high, reaching concentrations that are ~2 to 6 orders of magnitude higher than in sediment (Mato et al. 2001) or seawater (Wright et al. 2013). Sorption cleans the water of these pollutants but in the process, also generates MPs loaded with POPs, heavy metal compounds, and pharmaceuticals, ingestible by marine biota.
1.3 Ingestion of Microplastics Marine Organisms
That the marine environment contains numerous natural particles in the same size range as MPs/NPs that in any event constitute only a small fraction of all particles, is often pointed out. Marine organisms having evolved in this particle‐rich environment are reasonably expected to be not particularly affected by them. However, it is the high level of both sorbed or adsorbed chemicals in MPs that set them apart from the many inorganic “fines” abundantly found in seawater that may carry only surface‐adsorbed pollutants. The MPs and NPs with sorbed POPs are well known to be ingested by organisms ranging from zooplankton (Cole et al. 2013; Sun et al. 2017) to whales (Fossi et al. 2014; see Chapter 12), providing a pathway for these chemicals into biota (Hartmann et al. 2017). Particular attention has been paid to marine birds (Fry et al. 1987), where over 25% of the species (Pham et al. 2017), and sea turtles, where all species, are reported to ingest plastics (Kühn and van Franeker 2020). A particular concern is the ingestion of MPs and NPs by commercially important fish and seafood species (see Chapter 13). How the entanglement and ingestion risk of MPs relate to their particle size is illustrated in figure 1.7.)
Figure 1.7 Left: Categories of marine plastic debris developed by GESAMP (2015) indicating the size ranges for potential ingestion, and the techniques for their analysis. Right: A schematic showing the origin of plastic micro‐debris.
Source: Adapted from the same report. SEM ‐ Sacnning Electron Microscopy; TEM ‐ Transmission Electron Microscopy; AFM ‐ Atomic Force Microscopy; and AFM‐IR ‐ AFM with infra‐red Spectroscopy.
But, presently, there is no consensus on whether the POPs in ingested MPs do adversely affect the organism. Such effects would be compound‐ and species‐specific, but only a limited combination of POPs/species have been investigated as yet. Relevant data are therefore, quite variable even on species ingesting virgin plastics. For instance, adverse outcomes of ingesting virgin “clean” MPs have been reported for fish species (Jovanović et al. 2018) but were ruled out with sea urchins (Kaposi et al. 2014). An important focus of ingestion studies should also be to asses if the relevant POPs are bioavailable (Avio et al. 2015) to ingesting organisms at a high enough dosage to result in any physiological impacts. Bioavailability (Avio et al. 2015) of POPs is determined by (i) the residence time of MPs in the gut environment, (ii) hydrophobic gut contents that encourage release, and (iii) if POPs molecules can permeate the gut wall to enter systemic circulation (as opposed to being egested). However, data supporting the bioavailability of POPs are available only for several marine species (Bakir et al. 2016; Chua et al. 2014; Schrank et al. 2019), including mussels (Browne et al. 2008) and zebrafish embryos (Batel et al. 2018; Pitt et al. 2018). Bioaccumulation and biomagnification are introduced in Box 1.2.
Box 1.2 Bioaccumulation and
