Jamieson, A. J.; Malkocs, T; Piertney, S. B.; Fujii, T. and Zhang, Z. “Bioaccumulation of persistent organic pollutants in the deepest ocean fauna.” Nature ecology & evolution 1 (2017): 0051. doi:10.1038/s41559-016-0051
Scientists from the UK have recently reported finding extraordinarily high levels of toxic man-made chemicals in tiny crustaceans from the deepest parts of the ocean. Deep-sea trenches range in depth from 7,000 to 11,000 meters below sea level, and are one of the least explored, most isolated parts of the world, which is why it’s so surprising that persistent organic pollutants (POPs) were found in living organisms from two different deep-sea trenches. Concentrations were measured at over 50 times greater than similar species from the most polluted surface waters in China. While we don’t yet know how the contaminants were transported to such depths, these findings suggest that there may be no ecosystem left unaffected by human pollution.
What are POPS?
Persistent organic pollutants (POPS) are a class of man-made chemicals that do not easily degrade and therefore accumulate to high concentrations in the environment. Many POPs can accumulate in lipids, which leads to increasing levels up a food chain through a process known as bioaccumulation. Two of the most studied pollutants are PCBs and PBDEs because of their widespread use and toxic effects. PCBs were widely used in electrical equipment prior to their ban in 1977, and the structurally similar PBDEs are still used as flame-retardants. Both compounds are endocrine disruptors, which interfere with normal hormonal functions, and have been associated with a myriad of adverse health outcomes including cancer. In this study, both PCBs and PBDEs were measured at elevated levels in amphipods, tiny crustaceans, living in two deep-sea trenches.
How deep is deep?
Deep ocean trenches are created when tectonic plates press against each other pushing one below the other in a process called subduction. The Mariana Trenches located in the West Pacific contain the deepest known point on earth, Challenger Deep (~10,994m below sea level). If you were to submerge Mt. Everest, the tallest point on earth, at Challenger Deep, there would still be over a mile of water above the summit. Because of their incredible depths, trenches have been proposed as nuclear waste dumping grounds (though an international treaty called the London Convention currently outlaws this). The Kermadec Trench in the Southern Pacific by New Zealand is equally as deep as the Mariana trenches.
In order to explore such depths, the researchers needed a vehicle especially made to withstand the immense pressures at the seafloor. Such a vehicle was deployed for 8-12 hours in both the Mariana and Kermadec trenches. The vehicle contained mackerel bait to lure tiny crustaceans into a trap. Precautions were taken to not allow the crustaceans to actually eat the bait or for any additional sources of contamination.
Very high levels of PCBs and PBDEs
The pollutants, PCBs and PBDEs, were detected in all captured species at all depths measured. The levels of PCBs found in the Mariana Trench were 50 times greater than in crabs from one of the most polluted rivers in China, the Liaohe River. Levels of PBDEs were not as high as PCBs, but their concentrations were still in line with average values found in surface waters.
While levels of PBDEs were comparable across trenches, PCBs were significantly elevated in the Mariana trench compared to the Kermadec Trench. One interesting potential explanation for this is that the Mariana Trench is located near the “Great Pacific Garbage Patch” that is made up of floating microplastics stuck in the North Pacific Subtropical Gyre, which could be acting as a local source of contamination.
How were pollutants transported to such depths?
While we don’t yet know how PCBs and PBDEs were transported to the deepest depths of the ocean, the most likely theory is the ‘biological pump’. These pollutants likely underwent long-range transport via oceanic and atmospheric circulation, and then attached themselves to particles and tiny organisms, such as phytoplankton. The particles and organisms, after dying, eventually fall to the ocean floor. This falling debris serves as a conduit between the surface and deep ocean. This debris would also serve as food for the tiny crustaceans from this study, providing them a contaminated dose of PCBs and PBDEs. Whether these contaminants will have a negative impact on this formerly-thought-to-be pristine ecosystem remains to be seen.