Stormwater runoff often gets blamed, at least in part, for many of the oceans’ problems: the gyres of plastics and other debris, the acidification, the dead zones. Now someone has discovered another one: It seems we’re messing up the Mariana trench.
At more than 36,000 feet below sea level, the Mariana trench is the deepest point in the ocean, far deeper than Mount Everest is high (only 29,000 feet). Despite the intense pressure at these depths, the Mariana and similar trenches have surprisingly diverse ecosystems, populated by various species of fish, octopuses, jellyfish, crustaceans, sea cucumbers, worms, and other critters. As this Economist article explains, such trenches are too deep for sunlight to reach, and the hydrothermal vents that provide nutrients to some other parts of the ocean don’t exist below about 5,000 meters—less than half the depth of the Mariana trench. So how do the deep trench dwellers survive? They “depend entirely on dead organic material raining down upon them from far above,” according to the article, and “these nutrients, having once flowed into a trench, never make their way out again.”
Researchers from Newcastle University in England decided to measure what else is down there. Using an unmanned lander, a team led by Alan Jamieson collected specimens from 10 locations in two trenches, the Mariana and the Kermadec trench near New Zealand. They captured amphipods, a type of crustacean, at depths ranging from 7,227 meters (23,710 feet) to 10,250 meters (33,628 feet). The team looked for several different types of pollutants in the animals’ tissue, such as polybrominated diphenyl ethers, once used in flame retardant materials, and they found some of what they were looking for in fairly moderate levels. However, the levels of polychlorinated biphenyls, or PCBs, were astonishing.
“In animals collected from clean coastal environments, polychlorinated-biphenyl levels do not normally exceed one nanogram (billionth of a gram) per gram of tissue. In grossly polluted areas, like the Liao river in China, that level may rise a bit to above 100 nanograms,” according to the article. “In the Mariana trench, Dr. Jamieson found, amphipods dwelling at 10,250 metres yielded 495 nanogams per gram of the pollutant. Those 8,942 metres down yielded 800 nanograms. And at 7,841 metres, he and his colleagues discovered the staggering level of 1,900 nanograms per gram of amphipod tissue analyzed.” Samples from the Kermadec trench ranged from 50 to 250 nanograms per gram.
The team theorized that levels of PCBs in the Mariana trench are so high because it lies near the North Pacific Subtropical Gyre, whose plastics might release PCBs as they degrade.