Researchers at the Norwegian Institute of Marine Research have observed significant variations in plastic and other waste on the seabed.
‘We have never seen such large amounts of plastic over a large area as we observed in the Norwegian Trench, outside the Skagerrak,’ said marine researcher Pål Buhl-Mortensen, who leads the Institute of Marine Research’s work with the Mareano seabed mapping programme.
‘There is great variation in how much plastic we find on the seabed. This varies from piles of plastic in the depths of the Norwegian Trench to not a single piece of plastic in sight where there is shallower water outside the channel.
Seabed areas from the Skagerrak to the edge of the shelf north of Svalbard have been mapped, working with data from 3421 video transects. Debris was observed 18.9% of these – or almost every fifth video station.
‘We observed plastic at almost one in 10 stations,’ he said, commenting that the average density of plastic litter increases down to a depth of around 600 metres, particularly with foil plastic, plastic bags and other unidentified plastic accounting for this increase.
‘We see that, in general, there is most plastic from 400 to 1000 metres,’ he said. ‘The largest proportion comes from fisheries, in the form of synthetic rope and fishing nets. In addition, we have a collection category called Other Fishery-Related Plastics which also make up a large part of the findings.’
In this case, plastic waste is defined as visible plastic – and does not include microplastics – and there also tends to be larger accululations of plastic close to habitation.
‘Especially near towns along the coast, we have seen local accumulations, but these cover smaller areas,’ Pål Buhl-Mortensen said, explaining that seabed landscape is just as varied as on the surface, with extensive plains, deep valleys and high mountains. These help to create traps for rubbish that drifts in the water.
‘We find the highest densities of rubbish in marine valleys, marine gorges (canyons) and coastal landscapes,’ he said.
When the bottom of the Nowegian Trench was examined, researchers were able to quickly establish that it almost acted as a magnet for certain types of plastic waste.
‘Here there are large accumulations of plastic of the type that sinks slowly. The characteristic of such plastic is that it sinks slowly while being transported by ocean currents. In deeper water layers in central parts of the Norwegian Trench, there is a slow vortex where the plastic is concentrated before it finally reaches the bottom,’ he said.
The findings of the researchers are that the larger amounts of garbage are observed outside southern Norway, decreasing towards the north – with the exception of accumulations on some fishing grounds.
‘All the same, we see that the amount of observed litter at stations where it is observed is roughly the same in the north as outside central Norway,’ Pål Buhl-Mortensen said, adding that one of the categories the researchers recognise easily is plastic bags, which turn up in even unexpected locations.
‘The seabed is almost covered in plastic bags, and we have a good number of observations of such bags. The northernmost observation we have of plastic bags is from the bottom of the central Barents Sea, some distance south of Svalbard,’ he said.
