Paper: Jamieson, A. J., Malkocs, T., Piertney, S. B., Fujii, T., & Zhang, Z. (2017). Bioaccumulation of persistent organic pollutants in the deepest ocean fauna, 1(February), 24–27. http://doi.org/10.1038/s41559-016-0051
The deep sea is such an extreme environment that early scientists believed it impossible for any animal to survive the alien-like environmental conditions. With little to no natural sunlight, near-freezing temperatures, and pressures that would crush a human, the waters of the deep sea seem about as far removed from our life at the surface as we can imagine. In fact, that has been the pervading view of the deep sea for a long time: too far away to be directly connected to mankind. Unfortunately, new research has shown that even animals living miles below the surface, in the deepest reaches of our oceans, have been heavily influenced by human life.
The deepest point in our oceans, Challenger Deep, is found in the Western Pacific Ocean within the Mariana Trench (Figure 1), around 7 miles (almost 11,000 meters) below the surface. To give you an idea of scale, if Mount Everest was nestled into the Mariana Trench, the summit would still be a mile below the surface of the ocean (Figure 2).
The extreme depth of the Mariana Trench has made it difficult to study, with only 2 human-manned vessels ever reaching the bottom of Challenger Deep. Recently, a group of scientists set out to discover more about the relatively unknown depths by sending remotely operated vehicles (lander vehicles) equipped with baited traps to the bottom of the Mariana Trench and another deep trench in the Western Pacific (near New Zealand), the Kermadec Trench (Figure 1).
When the baited traps came up to the surface, the scientists had collected a surprising number of amphipods (Figure 3) – a type of crustacean commonly found in the deep sea- which they analyzed for different types of chemicals. The results from the lab tests were quite shocking: these small crustaceans that had been collected from over 7,000 m below the surface had extremely high levels of persistent organic pollutants (POPs) in their tissue, particularly stored in their fat.
The scientists chose to look at two types of POPs in particular: polybrominated diphyenyl ethers (PBDEs), manmade chemicals used as flame retardants, and polychlorinated biphenyls (PCBs). PCBs are a group of manmade chemicals that were produced and used for many purposes (such as hydraulic fluid, plasticizers in paints, pigments and dyes) from 1929-1979 before their production was banned in the United States. Although no longer manufactured, PCBs are still found in products made prior to 1979. Neither PBDEs or PCBs readily break down once in the environment, meaning these manmade chemicals hang around, settling in the water or soil and potentially building up in organisms that come in contact with them.
Sadly, both of these types of POPs were found in every sample recovered, from every depth (Figure 4). Worse yet, when trying to place the levels of contamination found in amphipods from the Mariana Trench into a global context, the scientists found that the highest level of PCB contamination recorded was 50 times greater than those found in crabs living in areas fed by the Liaohe River(one of the most polluted rivers in China) and comparable only to oceanic regions near highly populated areas with heavy historical use of the POP’s (such as Suruga Bay, Japan).
Scientists found that the levels of contamination were much higher in the Mariana Trench than in the Kermadec Trench. This could be linked to the fact that the Mariana Trench sits so near to the North Pacific Subtropical Gyre (i.e “the Great Pacific Garbage Patch”), an area where trash and debris caught at sea tend to accumulate, potentially acting as a source of pollutants from nearby land masses.
Scientists speculate that the POPs could possibly tag onto microplastics that sink into the trenches, accumulating in the sediment at great depths. Another possibility is that animals living in the surface waters may accumulate the toxins closer to human populations. When animals die and sink to the deep sea, their carcasses still contain the POPs, feeding the deep sea scavengers high levels of these man-made contaminants.
Whatever the mechanism, it is still clear that human actions have managed to impact even the areas of our planet once thought to be out of reach. We can no longer afford to view the deep sea with an “out of sight, out of mind” mentality.