Where ocean currents converge the accumulation of plastic has lead to several marine scientists describing the area as ‘garbage patches’, due to the huge amounts of debris trapped on decadal to millenial timescales. Within the worlds oceans 6 major garbage patches can be found globally, one per subtropical basin and one in the Barents Sea. Surface currents converge and subduct in locations, loosing energy, hence the ability to transport debris is reduced and so it accumulates in oscillatory patterns. The Hjulstrom Diagram is useful in this sense in understanding the relation between velocity and accumulation. Debris is generally less dense than sea water and so accumulates on the surface in ‘garbage patches’ in areas of convergence.
The Northern Atlantic Subtropical gyre (NASG) is the most popularly investigated within academia, with an estimated 2500 particles per square kilometre (Law et al, 2010). In their 2010 report Law et al used a time series of various data sources between 1986- 2008 along with the primary collection of 6136 surface plankton net tows to collect buoyant plastic pieces (99% plastic samples collected were less dense than sea water). 62% of the tows collected plastic particles, with the highest concentration of samples in the immediate vicinity of the NASG (83% particles collected) and the largest sample containing 1069 plastic pieces. The NASG subtropical convergence lead to the highest concentration of plastics as velocities were measured at a very low 2 cms-1. Law concluded that between 1990 and 2000 that the average concentration of plastic pollution with the NASG showed statistically significant temporal increases.
One issue to note with Laws methodology is that it is estimated that only 50% plastic pollution produced is buoyant, therefore any results produced maybe inaccurate or underestimated by a factor of 2 (Cozar et al, 2013). Another is the fact that plastic pollution is measured via abundance rather than mass, as the size distribution of particles should be taken into account when investigating pollution levels.
When comparing the NASG to other gyre systems around the world the NASG is often discussed as being the most polluted. The following bar chart shows the concentrations of plastic pollution within oceanic gyres as compilated from a variety of data sources by Cozar et al (2013). The dataset is compiled from 3070 samples, which had a frequency occurrence of plastic debris in 88% of the samples. The chart confirms the accumulation zones of plastic pollution within each of the subtropical gyres.
The chart shows nonaccumulation zones as blue boxes, outer accumulation zones as green boxes and inner zones as red boxes (representing the inner of the gyres) whilst the black lines represent the mean.
One issue to note with Laws methodology is that it is estimated that only 50% plastic pollution produced is buoyant, therefore any results produced maybe inaccurate or underestimated by a factor of 2 (Cozar et al, 2013). Another is the fact that plastic pollution is measured via abundance rather than mass, as the size distribution of particles should be taken into account when investigating pollution levels.
When comparing the NASG to other gyre systems around the world the NASG is often discussed as being the most polluted. The following bar chart shows the concentrations of plastic pollution within oceanic gyres as compilated from a variety of data sources by Cozar et al (2013). The dataset is compiled from 3070 samples, which had a frequency occurrence of plastic debris in 88% of the samples. The chart confirms the accumulation zones of plastic pollution within each of the subtropical gyres.
The chart shows nonaccumulation zones as blue boxes, outer accumulation zones as green boxes and inner zones as red boxes (representing the inner of the gyres) whilst the black lines represent the mean.
Van Sebille et al in their paper describe fluctuations in ocean garbage patches on an interrannual basis. The seasonal fluctuations in plastic pollution levels are apparent and labelled as ‘leaky’ garbage patches, due to the migration of pollution within the gyres. These fluctuations could be due to a variety of causes, such as seasonal fluctuations in gyre positions, wind patterns or due to the removal of pollution by oceanic sinks such as marine organisms. The vast inaccuracies associated marine data collection could also be the cause of the variation.
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