Part of the plastic we use ends up in the ocean. What happens to that plastic and where does it go? In time, through the effect of sunlight, plastic breaks down in ever smaller particles. Some sinks to the bottom, some remains suspended in the water below the surface and another part floats on the surface.
Plastic is found on all beaches around the world. Microplastics have been found at a depth of 5000 metres. Scientists have not yet fully mapped all the plastic pollution in the oceans.
Oceans are continually moving due to the rotation of the earth and the prevailing winds. There are five main rotating ocean currents, also called gyres. They act as a kind of giant whirlpool in which floating debris is slowly sucked to the centre. The five large gyres (North Pacific Gyre, South Pacific Gyre, Indian Ocean Gyre, North Atlantic Gyre en South Atlantic Gyre) are situated in the subtropical zones, just above and below the equator. In all five, a higher concentration of plastic waste is found than in other parts of the oceans. For example, in the North Atlantic Gyre 20.328 plastic particles have been found per square kilometre, versus 334.271 particles/km in the North Pacific Gyre. The bulk of this plastic waste is microplastics. These are smaller than 5mm.
In the centre of the gyres there is no wind. These areas have traditionally been avoided by shipping. But in 1997 captain Charles Moore sailed from Hawaii to Southern California through the North Pacific Gyre. There, in the middle of the ocean, he saw pieces of plastic floating by every day. Later he returned to the area to do closer research. There appeared to be a significantly higher concentration of plastic then elsewhere in the ocean. The plastic appeared to not only float, but also to hang suspended below the surface. Moore called the phenomena plastic soup, the term now used worldwide.
The gyres aren’t floating island or tapestries of plastic. It isn’t a case of a visible heaped up mountain of floating plastic waste. It is also a myth that the garbage patches would be visible from space. Such misconceptions remain persistent rumours. But it does concern a higher concentration of plastic that consists of very small particles. That concentration is measured in suspended plastic per cubic metre or floating plastic per square kilometre.
The size of the North Pacific Gyre is said to be 34 times that of the Netherlands, France and Spain together, or twice the size of the state of Texas. Source of this estimate is the book Flotsametrics and the Floating World (2009) by the American oceanographer Curtis Ebbesmeyer. The size of the plastic soup gyres depends on the measured number of plastic items in relation to the surrounding ocean. Plastic soup is everywhere, also in lakes and rivers.
Are the concentrations of plastic debris highest in the gyres? Not necessarily. There are other places where the concentration is as large or larger. These places are called hotspots. Plastic concentrates in these places, but not as a result of ocean currents.
The Mediterranean Sea is such a hotspot. On the one hand there is the constant flow in of plastic (from rivers that discharge into the Mediterranean Sea as well as from coastal towns), on the other hand the connection to the Atlantic Ocean (Gibraltar Strait) is so narrow that little plastic moves out to the Atlantic Ocean. In other words, plastic that ends up in the Mediterranean Sea stays there. An average of 116.000 plastic particles p/km2 has been reported for the Med.
Hotspots are places where for one reason or other concentrations of plastic occur. Aside from inland seas like the Mediterranean Sea (think also the Black Sea or the Baltic Sea) there are:
- Bays where there are large cities;;
- Bays where plastic drifts towards and remains;
- River mouths;
- Coastal industrial centres;
- Places or islands where ocean currents converge.