The Mediterranean Sea is just as polluted as some parts of the open ocean

When the tsunami struck the western coast of Japan at Fukushima in 2011, the heaving waters swept many tons of trash into the Pacific Ocean, and set afloat toward North America by ocean currents. Eventually, the Japanese trash joined up with another prominent patch of garbage floating just off the western coast of North America, held in a region some 5,000 sq. km across by a gentle but unceasing circle of winds called the North Pacific Gyre.

Like this, there are four other gyres around the world, all located near the equator, which have become lightning rods for waste – especially plastic waste – thrown into the oceans from coastal cities, industries and ships. This week, a survey by a group of Spanish researchers adds another patch of marine of trash to this list, this one with a dubious distinction.

It’s located in the Mediterranean Sea, and the distinction is this: if not for the Strait of Gibraltar, the Sea is virtually landlocked. As a result, the amount of plastic in it has accumulated faster than it has dissipated, today boasting of an abundance that could give its oceanic peers a run for their refuse.

In fact, the survey’s authors speculate that instead of draining out the Sea’s waters, the Strait of Gibraltar could be a gateway through which the Atlantic Ocean’s plastic trash is draining into the Sea. Overall, the average amount of plastic trash in the Mediterranean Sea – 423 grams per sq. km – appears comparable to that in the Indian Gyre, North Pacific Gyre or the South Pacific Gyre, and less than only in the twin Atlantic Gyres.

Ranges of surface plastic concentrations measured in the Mediterranean Sea, and reported for the open ocean. doi:10.1371/journal.pone.0121762.g003
Ranges of surface plastic concentrations measured in the Mediterranean Sea, and reported for the open ocean. doi:10.1371/journal.pone.0121762.g003

Between the ocean and a dirty place

Over 100 million people live along the Mediterranean Sea’s coastline, its waters are part of one of the busiest shipping routes on the planet and also receive waters from the densely populated catchments of the rivers Nile, Ebro and Po. Consequently, the total amount of plastic on the Sea’s surface is thought to be between 800 tons and a prodigious 3,000 tons.

Almost 83% of this is in the form of microplastics, pieces of plastic smaller than 5 mm in size. They are especially dangerous for the ecosystem because they could be swallowed by fish and other marine creatures. Similar concentrations of microplastics abound in the oceanic gyres as well.

Size distribution of the floating plastic debris collected in the Mediterranean Sea. doi:10.1371/journal.pone.0121762.g001
Size distribution of the floating plastic debris collected in the Mediterranean Sea. doi:10.1371/journal.pone.0121762.g001

However, the Mediterranean Sea has two unique features. It has more objects that’re bigger than 20 mm in size, and fewer objects that’re smaller than 2 mm in size, than are there in the oceanic gyres.`

Size distribution of the floating plastic debris collected in the Mediterranean Sea. doi:10.1371/journal.pone.0121762.g001
Size distribution of the floating plastic debris collected in the Mediterranean Sea. doi:10.1371/journal.pone.0121762.g001

Just last year, a widely reported study had found that contrary to popular belief almost 70% of the trash in the oceans didn’t stay on the surface but sank to the seafloor (sometimes going as far down as 15,000 feet). This is true of the Mediterranean Sea as well. As the Spanish team suggests, fragments of plastic bottles and bags could have sunk down and been colonized by organisms living on the seafloor.

This would explain the relative abundance of objects around 5 mm on the surface – but not where the objects smaller than 2 mm are disappearing to. As the authors write in their paper in the journal PLOS ONE,

Removal mechanisms of microplastics include ingestion by planktivorous animals and ballasting by biofouling, and these could be greater in the Mediterranean, where ecosystem production is higher than in the subtropical gyres. … However, estimates of ingestion rates of microplastic by marine life or microplastic abundance on the seafloor are still needed to test this hypothesis.

This movement of plastics within the Sea itself suggests perhaps the most frightening prospect emerging out of this study. If the surface alone hosts 800-3,000 tons of plastic objects, and that’s what’s floating after 70% has sunk to the bottom while the smallest fragments have been digested by marine animals, the Mediterranean Sea in all could be one of the most polluted water-bodies on Earth.