Mapping the Plastic Tide

Alien landscapes have long been a source of intrigue and inspiration for humanity. It’s been more than 50 years since humans first set foot on the Moon, yet there is still an environment far closer to home that remains shrouded in mystery. The ocean deep is a place of chilling temperatures, crushing pressures and endless darkness – all characteristics which make it inhospitable to us land-dwellers. Perhaps that explains why more than 80% of the world’s oceans remain unexplored.

But the more we explore, the more interesting this aquatic environment becomes. For a place of such extremes, there is a surprising degree of diversity in the deep oceans. In fact, there is thought to be as much life in the deepest regions as there is in tropical shallow-water habitats.

With limited human interference, you might be mistaken for thinking this wealth of life is spared the polluting effects of modern society. Unfortunately, this is not the case.

A recent voyage to the South Atlantic waters of Tristan da Cunha – spearheaded by London’s Natural History Museum – discovered microplastics in the stomachs of deep-water fish. Species collected included viperfish, anglerfish and the common fangtooth, all found more than 2,000 metres beneath the surface. Microplastics were discovered in over two-thirds of the specimens collected, largely in the form of tiny plastic fibres. These results echo those of similar studies, meaning there is no longer any doubt – plastic pollution has now reached the most untouched environments on the planet.

How Do Microplastics Affect Deep Sea Life?

This stark milestone is highly concerning. Although research into the effects of microplastics on marine life is still in its infancy, there are worrying signs that the impacts can be dramatic. Plastics are produced with petroleum and natural gas, and the chemicals used in the manufacturing process have been found to leach into the environment. Not only that, but microplastics in the ocean accumulate pollutants such as polychlorinated biphenyls (BCPs) and fertilisers. These stick to plastic particles and become concentrated over time, slowly becoming more toxic.

Deep sea species such as anglerfish feed largely on plankton – tiny life forms which closely resemble small plastic particles. As a result, the majority of microplastics enter the fish when they’re mistaken for food. Once inside a fish’s body, the plastic particles leach toxins into the animal’s tissues. This can affect health, energy levels and reproductive success.

It may also influence fish behaviour. A recent study on hermit crabs, for example, found that plastic ingestion limits their ability to select new shells. Fish may suffer similar effects, with there already being evidence that large amounts of microplastic in a fish’s stomach can influence hunger. Fish with stomachs full of plastic eat less frequently due to a false belief that they are satiated, which can eventually lead to starvation.

Plastics can also facilitate the transport of bacteria and viruses between environments, spreading pathogens to new hosts. And while low microplastic levels might not impact fish substantially, there is certainly no indication that they have any positive impacts.

Toxins on the Menu

It can sometimes be hard to visualise how conditions in the deepest parts of the ocean can affect us here on the surface. But the mentality of ‘out of sight, out of mind’ is not one that can be applied to the problem of microplastics. The ocean ecosystems are all interconnected, meaning a problem impacting the deep seas will have knock-on effects elsewhere. As approximately 3 billion people rely on the oceans for their primary source of food, the possible health problems are severe.

Microplastics have been found to readily move up the marine food chain, being transferred from smaller species to larger ones during predation. Eventually, this means that plastic particles that start out in a tiny deep-sea fish can end up in larger fish such as tuna, which are widely consumed by humans. As unpleasant as this sounds, some of our seafood meals are tainted with microplastic toxins.

Currently, we don’t fully understand the impact this has on human health. It has been confirmed that humans are ingesting microplastics regularly, but it’s unknown what level of consumption is safe if any. What we do know is that almost 400 million tonnes of plastic continue to be produced every year. And as large amounts accumulate in the deepest regions of the ocean, it remains to be seen how marine ecosystems – and human health – will cope.