Marcus Eriksen, Five Gyres Institute, Los Angeles, CA, USA, and colleagues have estimated the total number and weight of plastic particles of all sizes floating in the world’s oceans. They used data from 24 expeditions (2007–2013) across all five sub-tropical gyres, costal Australia, Bay of Bengal, and the Mediterranean Sea. The team found that a minimum of 5.25 trillion particles weighing nearly 270.000 t are swimming at the ocean’s surface. This is 0.1 % of the world annual production.
However, the vast amount of plastic pollution is not swimming at the ocean’s surface, but is spread among the whole ecosystem.

The Mediterranean Sea has the highest plastic pollution; up to 890,000 plastic particles were found per km².

The total amounts of plastics determined for the southern hemisphere oceans are within the same range as for the northern hemisphere oceans. This is surprising, as inputs are substantially higher in the northern than in the southern hemisphere. The researchers assume that this might mean that plastic pollution is moved more easily than previously assumed, there are so far undiscovered sources of plastic pollution in the southern hemisphere, such as currents from the Bay of Bengal that cross the equator south of Indonesia, and/or disproportionally higher amounts of plastics, and more than thought so far, might be lost from the sea surface in the northern hemisphere.

• Plastic Pollution in the World’s Oceans: More than 5 Trillion Plastic Pieces Weighing over 250,000 Tons Afloat at Sea,
  Marcus Eriksen, Laurent C. M. Lebreton, Henry S. Carson, Martin Thiel, Charles J. Moore, Jose C. Borerro, Francois Galgani, Peter G. Ryan, Julia Reisser,
  PlosOne 2014.
  DOI: 10.1371/journal.pone.0111913