Marine Litter Vital Graphics

GEOGRAPHICAL DISTRIBUTION

Thermohaline circulation Ther ohaline circulation

Deep water formation Deep water formation

Deep water formation Deep water formation

Paci c Ocean

Paci c Ocean

Surface current

Surface current

Deep current Deep current

Indian Ocean

Indian Ocean

Paci c Ocean

Paci c Ocean

Atlantic Ocean

Atlantic Ocean

Surface water Deep water Surface water Deep water

Deep water formation Deep water formation

Source : NASA

Adapted from a map by Laura Margueritte Adapted from a map by Laura Margueritte

Source : NASA

The water approaching the centre of the gyre eventually has to exit and it does so by flowing downward, sinking to depths of a few hundred metres. Plastic brought to the centre of the gyre by the constantly spiralling water does not travel downward with the escaping water because it is too buoyant. Instead, it stays behind, trapped in the converging current (van Sebille, 2015). Over time this gathering process has led to the formation of five great litter accumulation regions associated with each of the gyres (Law et al., 2010, 2014; Cózar et al., 2014; Eriksen et al., 2014; van Sebille et al., 2015). In these areas of converging surface circulation, plastic debris occurs in much higher concentration than in other areas of the ocean – up to 10 kg per square km (Cózar et al., 2015; van Sebille et al., 2015). Recently, marine litter has also been observed in the Arctic region (Bergmann et al., 2015) where an additional region with high plastic concentration could be under formation (van Sebille et al., 2012). The concentration of marine litter in the Arctic could increase if floating plastic is transported into the polar regions from the North Atlantic, facilitated by melting sea ice (Bergmann et al., 2015). The Southern Ocean, which is generally considered to be one of the most pristine regions on the planet, is also a site of marine litter. Beach surveys on Antarctic islands reveal that marine debris, mostly consisting of plastic, is accumulating at rates up to four times higher than previously estimated (Eriksson et al., 2013).

open ocean, can be zones of accumulation of plastic debris. Modelling efforts have identified the Mediterranean, South East Asian seas and Bay of Bengal as coastal zones with increased concentration of debris and microplastics (Cózar et al., 2015; UNEP, 2016a). Models examining the movement of plastic from land- based sources across different regions also point to connections between oceanic basins and gyres, with particles moving from one gyre to another and across oceanic basins in a matter of years. For example, particles released in West Africa could reach the western coast of South America and the Caribbean within one to three years and the North Atlantic Gyre in four to five years (UNEP, 2016a). Of course, the major patterns of global surface circulation are subject to high temporal and spatial variability and surface waters are eventually mixed due to wave and wind action. This leads to short- term changes in plastic concentrations across the horizontal and vertical dimensions of the ocean (Reisser et al., 2015). Deep transfer and accumulation Plastic debris does not remain on the surface forever. Eventually it starts to sink. Cold, dense water sinks in the North Atlantic and Southern Ocean, driving what is often referred to as the ocean conveyor belt or thermohaline circulation. This deep water circulation pattern couples with the subtropical gyres and redistributes cooled waters towards the deep ocean layers. The combination of these currents could provide a mechanism for

In addition, enclosed or coastal seas, with densely populated coastal zones and limited exchange with the

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Marine Litter Vital Graphics