Plastics and the Ocean. Группа авторов
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Waste estimates from Borelle et al. (2020).
In 1997, Moore et al. (2001) reported an unusually high incidence of plastic micro‐debris in the North Pacific Gyre, a swirling vortex of water in the ocean, a couple of hundred miles North of Hawaii. In this 1.6 million sq. km. area (approximately 135°W to 155°W and 35°N to 42°N), the abundance of floating plastic fragments (some too small to be visible) was statistically higher than elsewhere at sea. A 2018 study estimated this garbage patch to carry 80 TMT of plastic, including ~1.8 trillion pieces of MPs (Lebreton et al. 2018). Misleadingly called the “Pacific Garbage Patch” in the media, the area is not a visible “patch” with obvious plastic floating debris, nor is it a floating island of dense plastic litter. The swirling water collects the micro‐plastic fragments at a statistically higher abundance and its center is calm and nonturbulent. Oceanographic modeling of particles subject to water currents predicts the formation of five such gyres, of which the North Pacific Gyre would be the largest (Eriksen et al. 2014; Van Sebille et al. 2015). How much plastic has accumulated in the deep water or the sediment at the gyre location, is not known. But, the floating stock of plastic debris is known to be a minuscule fraction (Eriksen et al. 2014) of what is estimated to reach the ocean each year, and a majority of ocean plastics are not visible at the surface. What is especially worrisome is that no mechanism in nature is able to remove the plastics from the ocean at a significant rate. With little or no degradation in the low‐temperature, anoxic sediment where the plastic debris ends up (Andrady 2011; Hurley et al. 2018), it is safe to assume that nearly all the plastic that ever entered the ocean still persists there in the sediment.
Plastics are now known to be present in all ocean basins (Andrady 2011; Cole et al. 2011; Derraik 2002; Peng et al. 2020; Law and Thompson, 2014), shorelines the world over (Li et al. 2016), in Antarctica (Ivar do Sul and Costa 2014; Waller et al. 2017), in the frozen polar ice masses (Peeken et al. 2018) (with the possibility of global warming releasing them gradually into the ocean) (Obbard et al. 2014), in remote alpine lakes (Gateuille et al. 2020), and even karst groundwater. Figure 1.5 shows the trend in plastics debris in aquatic environments.
Plastic waste in the ocean poses a variety of well‐known environmental problems and most of these are discussed throughout this volume. The main concerns might be summarized under the following eight categories.
1 Aesthetic damage to shorelines by beach plastic litter. Entanglement (Ryan 2018; Reinert et al. 2017) of marine life in plastic netting, rope, six‐pack rings, containers, and “ghost fishing” by lost and abandoned fishing gear (Richardson et al. 2019).
2 Sorption and adsorption of chemical species in seawater, river water, and wastewater by plastic debris. Some hydrophobic chemicals in seawater may concentrate in the plastic fragments and be transported elsewhere (see Chapter 9).
3 Ingestion of plastics (Reynolds and Ryan 2018; Santos et al. 2015), especially microplastics by a wide range of marine animals. Any chemicals the plastic carries may be bioavailable and lead to toxicity (Avio et al. 2015; Guo and Wang 2019; Rochman 2013; see Chapter 12).
4 Accumulation of waste plastic debris in the bottom sediment affecting its ecology (Barett et al. 2020; see Chapter 6).
5 Introduction of alien species to new ecosystems by “rafters” or foulant species on the surface of floating plastic debris (Kiessling et al. 2015; Rech et al. 2018).
6 Possible development of antibiotic‐ and metal‐resistant microorganisms on foulant layers on plastics that have sorbed antibiotics through exposure to wastewater (Yang et al. 2019).
7 Interference with the operation of seagoing vessels by derelict fishing gear (Hong et al. 2017).
8 Potential contamination of fish and seafood leading to the increased human intake of MPs and NPs (Cox et al. 2019; see Chapter 13).
Of these problems, their ingestion by organisms, especially seafood species, is widely discussed because of the potential threat it poses to human health, in addition to the ocean ecosystem.
1.2.2 Microplastics in the Ocean
While the entanglement hazard of plastic macro‐debris such as derelict net fragments or six‐pack rings on large marine animals is easier to observe, it is the small‐sized debris not discernible visually that will have the more significant impact as they can interact with a much wider range of marine species. Particularly significant are fragments of plastics (<5 mm to 250 μm) generally referred to as “microplastics” (MPs) (Arthur et al. 2009). The term, however, is a misnomer as reports of “microplastics” in the ocean invariably include fragments with dimensions that are orders of magnitude larger than a micron. MPs constitute a very small weight fraction of the plastic debris in the ocean but are predominant in terms of their numerical abundance. It is their abundance rather than their mass per unit volume of water/sediment, that is directly related to potential threats to marine life.
There is obvious merit in subdividing the size range encompassed by microplastics into several narrow subranges, for instance, as in the classification proposed by the GESAMP (2016). Consistent with the nomenclature used by some researchers (Andrady 2011; Lusher 2015; Nelms et al. 2019), it is reasonable to specify the narrower size range of 1 mm to 1 μm as “true” microplastics (with those smaller, as nano plastics (NPs), and larger as mesoplastics). MPs also occur in different geometries, including microfibers, plastic fragments, films, pieces of foam (such as PS foam and polyurethane), and virgin resin pellets that are believed to “leak” into the environment from plastic processing operations as well as during the transportation of resin (Karlsson et al. 2018). Geometry‐specific physiological impacts are possible with the ingestion of MPs, especially the NPs (Danopoulos et al. 2021). Nanoplastics derived from MPs or larger debris are an especially important category because their size allows a very large range of marine animals to ingest these.
Secondary MPs derived from mesoplastics by fragmentation may degrade further yielding NPs (Andrady 2017; Chapter 8). Some data suggests the NPs that are extensively weathered contribute to the pool of dissolved organic carbon (DOC in seawater (Zhu et al. 2020; Romera‐Castillo et al. 2018).
Plastic micro‐debris is further categorized as primary and secondary, based on their origin.
1 Primary MPs are intentionally manufactured in that size scale for use in a specific application; and
2 Secondary MPs result from the fragmentation of plastic macro litter in the ocean environment, often assisted by their weathering degradation as discussed in Chapter 8 (Andrady 2017; Barnes et al. 2009). Though fragmentation may also occur with virgin material, plastics that have undergone extensive weathering under exposure to solar UVR, and therefore weakened or embrittled, tend to fragment far more easily in the environment (see Chapter 8.)
Examples of primary MPs include manufactured microbeads used as exfoliant additives in personal care products such as facial creams and toothpaste (Fendall and Sewell 2009). These microbeads typically have a size range of 164–327 μm (Napper et al. 2015) and are predominantly made of polyethylene (Gouin et al. 2015); therefore, at least initially, float in seawater (Van Cauwenberghe et al. 2015). They are accessible for ingestion by a range of smaller marine organisms that populate the photic upper layer in the ocean. However, the use of microbeads in rinse‐off type cosmetics has been phased out in 2019–2020 in both the US and the EU, preventing 1500 tons per year of these from entering the aquatic environment (Sun et al. 2020). But noncosmetic uses of microbeads that include plastic‐blasting media, textile printing, and biomedical applications (Leslie 2014) continue worldwide.