, 2009). In this section, we look at several sources of plastic litter and discuss both direct and indirect routes by which plastic can enter the marine environment. Whilst the emphasis of this review is on microplastics, in this section we also consider the indiscriminate disposal of macroplastics, as, with time, they have the potential to degrade into secondary microplastics. Plastic litter with a terrestrial source contributes ∼80% of the plastics found in marine litter (Andrady, 2011). Such plastics include primary microplastics used in cosmetics and air-blasting,

improperly disposed “user” plastics and plastic leachates from refuse sites. With approximately Roxadustat order half the world’s population residing within fifty miles of the coast, these kinds of plastic have a high potential to enter the marine environment via rivers and wastewater-systems, or by being blown off-shore (Moore, 2008 and Thompson, 2006). Microplastics used both in cosmetics and as air-blasting media can enter waterways via domestic or industrial drainage systems (Derraik, 2002); whilst waste-water treatment plants will trap macroplastics and some small plastic debris within oxidation ponds or sewage sludge, a large proportion of microplastics will signaling pathway pass through such filtration systems

(Browne et al., 2007, Fendall and Sewell, 2009 and Gregory, 1996). Plastics that enter river systems – either directly or within waste-water effluent or in refuse site leachates – will then be transported out to sea. A number of studies have shown how the high unidirectional flow of freshwater systems drives the movement of plastic debris into the oceans (Browne et al., 2010 and Moore et al., 2002). Using water samples from two Los Angeles (California, USA) rivers collected in 2004–2005, Moore (2008) quantified

the amount of plastic fragments present that were <5 mm in diameter. Extrapolating the resultant data revealed that these two rivers alone oxyclozanide would release over 2 billion plastic particles into the marine environment over a 3-day period. Extreme weather, such as flash flooding or hurricanes, can exacerbate this transfer of terrestrial debris from land to sea (Barnes et al., 2009 and Thompson et al., 2005). Work conducted by Moore et al. (2002) showed neustonic litter (small, surface plastic debris) <4.75 mm in diameter in Californian waters near the mouth of a modified Los Angeles stormwater conveyance system increased from 10 plastic items/m3 to 60 plastic items/m3 following a storm. The work further showed how increased water volume in the river, due to the recent storm, resulted in litter being deposited at even greater distances from the river mouth. Similarly, in a study by Lattin et al. (2004), microplastic concentrations 0.8 km off the southern Californian coast jumped from an average <1 item/m3, to 18 items/m3 following a storm.