Crustaceans in deep water, in deep trouble

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The ultra-deepwater Hirondellea gigas from the deepest depths of the Mariana Trench in the Northwest Pacific Ocean. This species is known to inhabit depths of 6,000 to nearly 11,000 metres.
Credit: Dr Alan Jamieson, Newcastle University

“Extraordinary levels” of pollutants have accumulated in crustaceans living in the world’s deepest ocean trenches, according to new research.

A study, led by Alan Jamieson at the University of Aberdeen in Scotland and published in Nature Ecology and Evolution, found contaminant levels at depths of 10,000 metres in the trenches “were considerably higher than [those] documented for nearby regions of heavy industrialisation”.

Jamieson’s team used a deep-sea lander to set baited traps in both the Mariana Trench in the western Pacific and the Kermadec Trench near New Zealand. The traps were used to capture three endemic species of small crustacean that were then tested for 14 different chemical markers.

“The legacy and reach of anthropogenic influence is most clearly evidenced by its impact on the most remote and inaccessible habitats on Earth,” the researchers report.

“Of particular concern are the persistent organic pollutants that are highly detrimental to organismal health through their endocrine disrupting properties.”

Persistent organic pollutants comprise a broad range of manufactured carbon-based substances, all noted for their ability to remain viable within the environment for very long periods, and for their habit of accumulating in living organisms. The first persistent organic pollutant to become the focus of public and scientific concern was the insecticide DDT.

The persistent organic pollutants of central interest to Jamieson and colleague were polychlorinated biphenyls, widely used as dielectric fluid, and polybrominated diphenyl ethers, used as flame retardants.

The research found multiple varieties of both in all samples, across all species, at all depths in both trenches.

“In the Mariana, the highest levels of polychlorinated biphenyls were 50 times more contaminated than crabs from paddy fields fed by the Liaohe River, one of the most polluted rivers in China,” say the researchers.

“The only Northwest Pacific location with values comparable to the Mariana Trench is Suruga Bay (Japan), a highly industrialised area with historically heavy usage of organochlorine chemicals.”

The scientists offer a few possible explanations for these deep-sea pollutants, citing sinking plastic debris and water-surface carrion consumed by the deep-sea scavengers as potential sources. 

Whatever the exact mechanisms, they add, the findings infer that “these pollutants are pervasive across the world’s oceans and to full ocean depth”.

Deep-sea – or hadal – trenches are notoriously difficult to study due to hydrostatic pressure and extremely low temperatures. The Mariana Trench, for instance, plunges 1.5 kilometres further below sea level than Mount Everest stretches above it.

Katherine Dafforn, a biologist at the University of New South Wales in Australia, says the finding is startling.

“At more than 6,000 metres under the sea, hadal trenches are a remote wilderness, largely unexplored and widely considered safe from human disturbance,” writes Dafforn in Nature News and Views.

“This [finding] is significant since the hadal trenches are many miles away from any industrial source and suggests that the delivery of these pollutants occurs over long distances despite regulation since the 1970s.”

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