Plastic waste is increasingly found in marine environments. Entire plastic items appear regularly on beaches, accumulate on the sea bottom in both shallow and deep waters, or float at or below the ocean surface (Barnes et al. 2009). Many larger items, together with discarded fishing gear, are ingested by marine mammals, seabirds, turtles and fish, and can also entangle or smother them. Just as large debris presents a direct threat to large species (Gregory 2009), microplastics may prove hazardous to smaller species that inadvertently ingest them, including small planktivorous fish and filter feeders (sponges, corals, clams, etc.). These plastic items and fragments appear to be ubiquitous in the worlds' oceans, although the quantities found vary markedly according to ocean circulation patterns, river inputs, population density and level of industrialisation (although even unindustrialized areas receive current-borne debris). Of course, plastic debris is just one of many threats facing marine ecosystems (Allsopp et al. 2009), but one that nonetheless demands far greater attention in both research and policy terms. Paradoxically, although almost everyone aware of this pollution finds it alarming and unacceptable, relatively few make the immediate connection to their own excessive use and disposal of plastic goods. The present paper will examine the source and extent of plastics pollution in the ocean and outline its many threats to global marine life.
Since the mid-twentieth century, the global plastics market has grown steadily from around 1.5 million tonnes per annum to 250 million tonnes, representing a raw material demand equivalent to approximately 8% of global oil consumption (Plastics Europe 2009).
The term "plastic" covers several hundred commercially available materials. Around 90% of the market demand is for seven polymers, of which half are PVC, Polypropylene and High and Low Density Polyethylene (HDPE/LDPE).
Per capita use ranges from a high of some 100 kg annually in Western Europe and North America to 20 kg in developing Asian countries, where the industry sees the greatest potential for growth. Although plastics find their way into many products, over a third of production is used for short-life packaging purposes: true recycling of plastic wastes remains limited (accounting, even in Europe, for less than a quarter of post-consumer plastics in 2008). Accordingly, much of the waste plastic generated (about 10% of all solid waste on average) enters landfill sites, is burned as a source of energy in various industrial and waste disposal operations, or is simply discarded (Plastics Europe 2009). Plastics are extremely long-lived, although how long is a matter of debate - estimates range from hundreds to thousands of years. It has been suggested that, leaving aside incineration and fuel use, most plastic remains in the environment in its original form or as plastic fragments and particles. Studies carried out worldwide show that plastic items make up the bulk of marine litter found on beaches (Derraik 2002), as well as on the seafloor of urbanized coastal environments.
Plastic items may travel long distances before sinking under the weight of biofouling (colonization by barnacles and diverse organisms) or becoming stranded on the shore. It is often difficult to ascertain where an item entered the sea: it may have been thrown overboard from a ship or fishing vessel; identifying labels may have fallen off, and/or prevailing currents may have carried the item for some time. Smaller particles resulting from mechanical or photochemical degradation, or from the spillage of plastic pellets and powders, can also be ingested by marine animals, causing a variety of ill effects. Plastic particles also release added or adsorbed toxic chemicals that fish and other organisms can bioaccumulate (Teuten et al. 2007, 2009), with the potential for impacts along the food chain. Plants and animals colonize the surfaces of floating plastics and are consequently transported long distances into new environments; while not native to these habitats, they may nonetheless thrive and become invasive (Gregory 2009).
Some parts of the sea bottom seem to act as "sinks:" of the areas sampled to date, the Mediterranean has the highest densities of sunken plastic debris, due to high human populations, high levels of shipping activity and relatively low tidal dispersion. Large-scale residual ocean circulation patterns also determine underwater accumulation sites, as is the case for areas of the North Sea and Atlantic Oceans (Barnes et al. 2009).
Just as sunken plastics tend to accumulate in specific places, so does floating plastic debris. The North Pacific Central Gyre (Moore et al. 2001) appears to be one such area (probably one of many): ocean and atmospheric circulation patterns bring together high densities of plastic debris. This gyre is not a permanent physical feature but rather a region of high atmospheric pressure in the North Pacific between California and Hawaii. It weakens and moves further south in winter, but nonetheless entrains and concentrates debris from a wider area of the Pacific Ocean. It has been estimated that the most intensively sampled core area of the gyre, approximately 1000 km in diametre, could contain as much as three million tonnes of floating plastic debris (Moore 2008) or approximately 5kg of plastics per square kilometre, with a high proportion of this total mass distributed as a widely dispersed suspension of fine "microplastic" fragments. The media calls these areas the "Eastern Garbage Patch" or "Pacific Trash Vortex." Already there is some evidence for accumulations of a similar nature in other parts of the Pacific, including the "Western Garbage Patch" within a smaller gyre south of the Kuroshio Current near Japan (NOAA 2010). Some recent observations also note an "Atlantic Garbage Patch" centred on the Sargasso Sea (a region already long known to be an accumulation zone for floating biogenic material): the vast majority of individual fragments are less than 10mm in size and weigh less than 20mg (one fiftieth of a gramme) (Morét-Ferguson et al. 2010), although the long-term fate of debris in this region remains poorly understood.
Law et al (2010) note that, despite increases in global production of plastics in recent decades, levels of plastic debris in the Western North Atlantic and Caribbean appear to have remained relatively constant since the mid 1980s, indicating the existence of significant but so far undocumented pathways of loss from the water column.
The term "Pacific trash vortex" suggests an entire region covered by a large, obvious and easily visible patch of floating litter, one that could be detected from satellites or through aerial photography - in extreme terms, a "literal blanket of trash" (NOAA 2010). In reality - despite numerous relatively large items of debris, visible to observers on vessels or even from low-flying aircraft - these conspicuous items only rarely form larger agglomerations in the open ocean. However, their presence indicates a less visible but far more abundant and pervasive phenomenon: high concentrations of small fragments of plastic, detectable only by towing nets some distance through surface waters. Therefore, while aerial surveillance identifies large pieces of debris in other areas, such as in the North Pacific Tropical Convergence Zone (Pichel et al. 2007), such studies document only one extreme of the wide range of floating plastic debris.
It remains to be seen just how well the sample averages cited above represent the distribution of so-called "microplastic" debris across the gyre as a whole, and how that region compares to other similar convergences. Debris distributions appear to be inherently heterogeneous, both spatially and temporally. Despite a growing recognition and research interest in the issue over the past decade, the number, geographical extent and comparability of quantitative debris surveys remain remarkably limited to date. Given this paucity of basic descriptive data, assessing biological consequences is a considerable challenge.
While the consequences of entanglement and ingestion involving larger items and species are relatively simple to observe and document, microplastics ingestion research remains in its infancy: its physiological impacts and wider ecological significance urgently require study. A growing body of research documents both plastic-item ingestion by albatrosses and other seabirds and entanglement of dolphins, whales and turtles; however, few systematic surveys of smaller plastics exist, beyond those conducted along beaches in accumulation zones (most notably Hawaii). Moreover, researchers lack techniques for estimating the effects of smaller plastic fragments.
How can we address the known and as yet unknown threats to marine life posed by plastics pollution? Costly schemes aimed at collecting and recovering floating debris can only be effective at the local level, in shallow and enclosed in-shore waters. Beach-cleaning programmes fight a losing battle by treating the symptoms rather than the cause. Strategies such as recycling, taxing plastic bags, and using bio-based and biodegradable plastics may all play larger roles in the future, but will only ever address a fraction of the problem. Such strategies will work only if we are also willing and able to rethink our attitude to plastic consumption, to see it for the high-value, long-lived and non-disposable material it is, and to use plastic products and packaging sparingly and wisely.
CURRENTS CREATE THE PACIFIC VORTEX
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