The dirty depths: how connected are we to the deep ocean?

By: Dana Sackett, Ph.D.

The deep sea is the largest and one of the most unique habitats on the planet. Once thought to be pristine and out of reach to human impacts, studies on the deep ocean have recently proven the opposite. This remote yet surprisingly diverse ecosystem is turning out to be more interconnected to human activities at the surface of the earth than we initially suspected.

In the 1960’s Rachel Carson brought new awareness to the impacts of the widespread use of a chemical causing extensive environmental harm. Her book, Silent Spring, helped to spark a movement towards environmental protection. The chemical highlighted in her book, DDT, is now just one among many chemicals listed as persistent organic pollutants (often called POPs for short) that were banned or have had extremely limited use for decades (some examples we talk about here include PCBs or polychlorinated biphenyls, and PBDEs or polybrominated diphenyl ethers). Despite their lack of use in recent decades, the persistence and long-range transport of these chemicals is proving problematic for the deep sea.

The 1962 book that helped to spark a movement. Source

For example, researchers found PCBs and PCBEs in organisms collected from two of the deepest locations on the planet, the Kermadec and Mariana Trenches (ranging from 7,000m to 10,000m deep). Even more concerning, many of these deep-sea creatures held concentrations 50 times greater than what was measured in crabs from one of the most polluted river systems in China. While scientists have characterized the toxicity of PCBs and PCBEs for inland, coastal, and shallower marine ecosystems, it remains unclear how these unexpectedly high concentrations are affecting deep-sea ecosystems.

Top, locations and depths of the Mariana and Kermadec trenches. Bottom, relative depths of oceanic layers, including the hadal zone (named for Hades, the underworld in Greek mythology). Source

Another astonishing finding, plastics, which are known to adsorb organic pollutants such as PCBs and PBDEs (meaning these pollutants stick to plastic), were found in the stomachs of marine organisms collected from six of the deepest ocean trenches on the planet, some of which are over 9,000 miles away from each other. At the deepest locations on the earth in Mariana Trench, 100% of the organisms collected and examined by researchers had plastic fibers in their digestive tracts (described as being equivalent to a person shallowing a 6.5ft polypropylene rope).

Map showing ocean trenches up to 10 km deep where scientists found tiny shrimps which had ingested microplastics. Source

The assumption that the deep ocean is too far removed to be affected by people stems, in part, from the idea that these chemicals would take millennia to diffuse and sink the approximate 4,000m to 11,000m from the surface of the ocean to the seafloor alone. Sinking ocean particles known as marine snow (which can include phytoplankton, zooplankton, detritus, or feces) can accumulate these chemicals and descend through the water column a bit faster based on particle size and density. For instance, larger and heavier particles or groups of particles stuck together will sink much faster than smaller particles. However, these sinking speeds slow significantly as particles reach greater and greater depths because of increasing seawater density and pressure and because material is lost from the particle as it sinks.

Sinking oceanic particles encompass a wide range of shape, porosity, and other characteristics that can affect sinking flux (red line). A simplified model shows with spheres representing either solid particles or aggregates produced in the sunlit euphotic zone, sink at a rate predicted by Stokes law. They slow as they reach greater depths due to their shrinking volume and increasing water density until they disappear completely. Credit for images of particles: Dr. Colleen Durkin. Source: Carolin Giering et al. 2019

These facts have left scientists stumped on exactly how pollutants, that were created by people less than a century ago, have made it to the seafloor already. Researchers have speculated that harmful chemicals can stick to masses of debris and garbage that can sink from the surface to the deepest depths on the order of months, much more quickly than expected for the chemicals or particles alone. Pollutants may also make it to the seafloor more quickly by hitching a ride with marine organisms that vertically migrate through the water column. These marine animals could accumulate pollutants through breathing and feeding in shallower waters, and then release them in waste while occupying deeper depths, or from their carcass after it sinks to the seafloor. Larger sinking particles known as particulate organic matter (POM) have also been seen to sink relatively fast carrying mercury produced in surface waters to the deepest trenches in the ocean.

Plastic ice bag found by a NOAA expedition to Mariana Trench in 2016. This picture was taken at 36,000 ft deep, over a mile deeper than Mount Everest is tall. NOAA Office of Ocean Exploration and Research. Source

While much is still unknown about the processes that are causing pollutants to accumulate in the deep-sea, the studies discussed here demonstrate that our actions at the surface of the earth are intimately connected to even the deepest and most remote parts of the ocean. Recognizing this fact is essential to ensuring the health of this vast and vital ecosystem that is critical to maintaining the earth’s climate.

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