A Southern Ocean plastic puzzle – where is all the ocean plastic?

11 August, 2023 science

Where is all the ocean plastic? Eight million tonnes of plastic enter the ocean each year but only a small fraction of this (maybe even as low as 1%!) can be accounted for in surface waters. This poses an important question. Where is all the ‘missing’ plastic pollution in the ocean?

Here’s where we take on the role of ‘plastic detectives’ in Antarctica to solve a piece of the Antarctic plastic puzzle – what lies beneath…?!

The first piece

First, a bit of background about plastic pollution and how and why it exists in one of the most remote places on earth. We know that plastic pollution is in the Southern Ocean – from the ocean surface, all the way down to the sea floor. We also find it in lots of marine animals in Antarctica. In fact, we recently discovered that plastic is eaten by Antarctic krill – a zooplankton, about 6cm big, that is an important food source for many of Antarctica’s ocean animals. But where does the plastic come from?

Large plastics, such as bottles and fishing nets, break down over time to form microplastics (1–1000 micrometres). Plastics can also be released at the micro scale, for example, when we wash our clothes. Because all the world’s oceans are connected, plastics that are released thousands of miles away can travel to Antarctica through ocean currents. We think some microplastics are also from local sources such as ships and research stations.

Regardless of their origin, small pieces of plastic find their way into almost all parts of the marine ecosystem. But when it comes to working out where the microplastics are and where they’re going, we’ve only scratched the surface. What I mean by this is that most sampling for microplastics has been done by towing nets through the surface of the water, where large surface areas can be covered, and large volumes of water sampled in a relatively short time frame. But we know very little about plastic below the ocean’s surface….

There are various things that can happen to plastics below the ocean surface:

  • they can settle on the ocean floor,
  • get eaten by an animal and become part of the marine food web,
  • become trapped in the sea ice as it forms,
  • or be suspended in the water column.

It’s this fraction of plastic, not floating at the sea surface but not settled on the ocean floor, that we wanted to explore.

Setting the trap!

So, what lies beneath? To answer this question, in true detective fashion, we created a trap! Our floating sediment trap essentially contains a series of tubes which sit at different depths of the ocean, attached to a large float that sits on the ocean surface. These tubes collect particles moving through the water, including plastics. This newly designed kit allowed us to measure the vertical movement of microplastics over 24 hours (the vertical flux) between depths of 50 and 150 metres, within a natural harbour of the sub-Antarctic island of South Georgia. South Georgia is one of the most productive ecosystems in the Southern Ocean with large numbers of Antarctic krill. It is affected by fishing, tourism and research activity. Each of these activities can impact the amount of plastic in the water and the vertical plastic flux.

A person standing in front of a body of water
A floating sediment trap is deployed into the ocean.

And what did we find? Our study found microplastic at every depth. We found a relatively high number of microplastics at 50m (306 pieces/m2/day), which decreased with depth (94 pieces/m2/day at 150 m). Buoyant plastics which would typically float at the surface, were found at every depth indicating physical/biological drivers are promoting this flux. The most common microplastic type we found was fibres (commonly released from washing machines).

Why does it matter?

Small pieces of plastic can be mistaken for food by Antarctic krill (Euphausia superba) and a range of other zooplankton which play critical roles in Southern Ocean food web. Once eaten, they can negatively impact the animals and can build up through the food chain.

Plastic could also impact the carbon cycle. Once eaten by zooplankton, plastic may alter how quickly the animals’ poo and bodies sink to the ocean floor. Ultimately this could potentially reduce the ocean’s ability to regulate atmospheric CO2 emissions. This is being investigated through our CUPIDO project.

Our study also provides the first physical validation to models focused on predicting the fate of plastic in Southern Ocean, which are trying to understand where the ‘missing’ plastic is. Our findings are also helping us to protect and manage Southern Ocean ecosystems, as we learn more about how we need to regulate human activities in the region and ways of managing the plastic pollution.

There’s still more work to be done in terms of fully understanding exactly where the plastic is in the Southern Ocean, and this is just one piece of the puzzle. Nevertheless – its quite an important piece!