Tracking the elusive seabed plastics

Since monitoring the plastic coming in Balnakeil Bay, we have removed several tons of pollution in the last 4 years. When we started getting informed on plastic pollution, the way things were done was supposedly quite straightforward, plastic comes from rivers, floats around and disperses into the ocean to arrive on our coasts. However, if you go on the beach, you will find a third of MOD flare parachutes from the manoeuvres and the rest will be mostly fishing gear and marine industry related pollution. Sometimes you will find the odd plastic bottle but that will be it. So Pollution doesn’t come from rivers here, it is generated at sea and lands in Balnakeil Bay. We have worked on this topic since a long time now and could offer a few scientific explanations.

We have since then focused a lot of our research and development efforts towards finding low-tech recycling solutions. So we started knowing pretty well which polymers we collect on the shore. One of the test to identify a polymer is a flotation test, it is smart and straight forward. Basically, we find mainly Polypropylene (ropes and nets), HDPE (fish farm pipes, drum barrels, oil containers), PET (fragments of drinking bottles), PVC (marker buoys) and nylon (gill nets). The three last polymers sink in water and we never really realised the consequences of this. Last year we conducted a number of survey and we realised that some weeks, the proportion of sinking litter was ranging between 30 and 70% of the total pollution collected.

Unlike the rest of the pollution found on the tide lines and the berm /backshore, dense plastic would preferentially accumulate in the lower part of the beach between sandbars. These dense plastic would get deposited during specific times in association with uprooted kelp.

Consequently, we have significant proportion of the marine pollution that is transported on the sea bed in a remote part of the world most likely from deeper grounds towards the surface. This is not an intuitive phenomenon. To try to better understand the movement of these plastics, we associated with Marine Scotland and Sea-Change Wester Ross to carry on a ROV survey into the bay and try to find out if there were plastics on the sea-bed there. We have explored five different sites thanks to Donald Morison and Klára Jančušková helping out with the boat and surveying.

We have observed some amazing beasts reaching depths of 35 m without encountering a single piece of plastic whereas we found some just putting the boat into the water. What we noticed is that there was a lot of current all over the bay and that the seabed was everywhere in motion and sandy. You can give it a go you can see the 5 videos we shot. Beware it might make you sea sick!

Although it was a lot of work in preparation and during the day, the simple reason why we didn’t find plastic could have been the small surface we surveyed, 287 sqm. Statistically we would have had to observe at least 3 times more seabed surface to be sure of seeing plastic by comparing with other polluted areas. The other parameter was that we only observed sandy mobile substrate and we have always observed the dense plastic associated with kelp. It is possible that the large accumulations of plastic on seabed are in a different setting on hard grounds where seaweed can grow.

We hope that we will find the finances and the right weather window this winter to carry on another survey and find out were dense plastic are hiding in our coastline. Understanding the motion of plastic in our environment is critical to better organise the remediation of the pollution and being able to predict and estimate where and how much plastic is out there.