The Deepest Ocean Trenches Contain High Levels of Pollution

Toxic industrial chemicals found well below the surface challenge the belief that the deep sea is largely free of human contamination.

Deep ocean trenches - considered the most remote places in the world - have levels of toxic, industrial chemicals 50 times higher than a highly polluted river system in China, an analysis of tiny deep-sea animals has found.

The discovery, published in today's Nature Ecology and Evolution journal, highlights the pervasive nature of pollution and destroys the belief these deep-sea wildernesses are largely safe from human degradation.

It also shows a strong connection exists between surface and deep-sea waters and suggests a need for better management and monitoring of these unique environments.

The discovery was made by a team of researchers from Scotland who investigated two of the world's deepest marine trenches - the Mariana Trench in the west Pacific Ocean above Australia, and the Kermadec Trench near the north-eastern tip of New Zealand.

The trenches lie in what is known as the ocean's hadal zone, which extends from six to 11 kilometres below the surface.

To collect samples in each of these remote trenches, in 2014 the researchers used a deep-sea lander operated remotely from the surface and baited traps to collect tiny crustaceans.

Analysis of these marine animals found they were contaminated with mainly two forms of persistent organic pollutants (POPs) - polychlorinated biphenyls (PCBs) and polybrominated diphenyl (PBDEs).

These two POPs - mostly human-made chemicals used mainly in industry or as pesticides - were present in all samples, across all species, at all depths in both trenches.

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Lead author Dr Alan Jamieson said the highest levels of PCBs recorded in crustaceans in the Mariana Trench were 50 times greater than that found in crabs from the Liaohe River system, one of the most polluted rivers in China.

The only site in the Northwest Pacific with PCB levels comparable to the Mariana Trench was Japan's Suruga Bay, a heavily industrialized area with high usage of organochlorine chemicals.

The analysis also showed PCB concentrations were higher in the Mariana Trench compared with the Kermadec Trench, said Dr Jamieson, a deep-ocean researcher who was based at the University of Aberdeen during the project.

It was likely this was related to Mariana's proximity to the industrialized Northwest Pacific region and the North Pacific Subtropical Gyre, also known as the Great Pacific Garbage Patch.

As the plastic rubbish in the gyre degraded it absorbed POPs, transporting the chemicals to the ocean floor as the rubbish sank and fragmented.

Dr Jamieson said it was also possible POPs were transported by ocean currents and through contaminated birds falling into the water and sinking to the ocean floor, where they were consumed by marine life.

In an accompanying opinion piece in Nature, University of NSW researcher Dr Katherine Dafforn said the finding was "quite concerning because these trenches are so removed from any kinds of industrial activity".

"The main method of control has been to regulate and eliminate but we know there is still huge amounts in our landfill and it has the potential to make it into the natural environment via soils and the ocean."

Dr Dafforn said the study was also significant because the hadal trenches had been regarded as pristine and "safe from human disturbance".

"We still know more about the surface of the Moon than that of the ocean floor," she said.

The discovery of pollution in these trenches highlighted the need to better understand the source of the pollution and its consequences on the food chain and marine ecology.

While this study raised many questions it provided clear evidence that far from being remote, the "deep ocean ... is highly connected to surface waters and has been exposed to significant concentrations of human-made pollutants."

Top Photo: Pollution levels in amphipods in the Mariana Trench were higher than in estuaries of polluted rivers in China. Credit: Wikimedia Commons: Daiju Azuma Article first appeared on ABC Science.

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