Scientists have discovered the Mediterranean equivalent of the Great Pacific Garbage Patch. Ocean currents are creating these microplastic “hotspots” deep in the Mediterranean Sea, according to a new paper published last month in Science (1). The authors have now reported the highest level of microplastics ever recorded on the seafloor.
Around 10 million tonnes of plastic makes its way into the ocean each year (2). And by 2050, there will be more plastic in the ocean than fish, by weight. But most of this plastic is not floating on the ocean surface, which accounts for a mere one per cent. So, where does all this plastic go? Scientists think significant quantities may be accumulating on the ocean floor, as well as in the water column and on beaches.
To figure out exactly how much plastic pollution is making its way to the deep sea, the international team of researchers collected and analysed sediment samples from the seafloor of the Tyrrhenian Sea, which is part of the Mediterranean Sea. Then, they used models of deep ocean currents to show how these current control the distribution of microplastics on the seafloor.
Microplastic concentrations were much lower in samples taken from channels where ocean currents are stronger. However, in areas with slower-moving currents, plastic particles have more time to settle on the seafloor. The authors report 1.9 million microplastic particles per square meter of seafloor sediment only 5cm thick pulled from the bottom of the sea close to Italy.
Lead author Dr Ian Kane of the University of Manchester said: “Almost everybody has heard of the infamous ocean ‘garbage patches’ of floating plastic, but we were shocked at the high concentrations of microplastics we found in the deep seafloor.”
Kane further explained: “We discovered that microplastics are not uniformly distributed across the study area; instead they are distributed by powerful seafloor currents which concentrate them in certain areas”.
Most of the microplastics were fibres (between 70 to 100 per cent), which are likely to have come from clothing and synthetic textiles, and the rest were fragments produced by the break down of larger plastic debris.
Senior author Dr Florian Pohl of Durham University said: “It’s unfortunate, but plastic has become a new type of sediment particle, which is distributed across the seafloor together with sand, mud and nutrients”.
He added: “Thus, sediment-transport processes such as seafloor currents will concentrate plastic particles in certain locations on the seafloor, as demonstrated by our research”.
Unfortunately, the same slow-moving currents that distribute microplastics also supply oxygen and nutrients to deep-sea creatures, creating biodiversity hotspots, which means more marine creatures are likely to gobble up microplastics along with the toxins, viruses, and bacteria that they are known to harbour.
Microplastics are particularly problematic for small fish that mistake them for prey. And the particles can work their way up the food chain, eventually reaching humans.
On a brighter note, the more scientists about the fate of plastics, the more actions that can be taken to stop the spread of plastic pollution and protect the planet’s precious ecosystems, including the deep sea.
(1) Kane, I.A., et al. Seafloor microplastic hotspots controlled by deep-sea circulation. Science (2020). DOI: 10.1126/science.aba5899
(2) Jambeck, J.R. et al. Plastic waste inputs from land into the ocean. Science (2015). DOI: 10.1126/science.1260352