Littering Arctic seafloor
Biologists record increasing amounts of plastic litter in the Arctic deep sea: studies confirm that twice as much marine debris is lying on the seabed today compared to ten years ago
The seabed in the Arctic deep sea is increasingly strewn with litter and plastic waste. As reported in the advance online publication of the scientific journal Marine Pollution Bulletin by Melanie Bergmann, a biologist and deep-sea expert at the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association. The quantities of waste observed at the AWI deep-sea observatory HAUSGARTEN were even higher than those found, say, in a deep-sea canyon near the Portuguese capital Lisbon.
For this study Bergmann examined some 2100 seafloor photographs taken near HAUSGARTEN, the deep-sea observatory of the Alfred Wegener Institute in the eastern Fram Strait (the sea passage between Greenland and the Norwegian island of Spitsbergen). "The study was prompted by a gut feeling," Bergmann explains. "When looking through our images I got the impression that plastic bags and other litter on the seafloor were seen more frequently in photos from 2011 than in those dating back to earlier years. For this reason I decided to go systematically through all photos from 2002, 2004, 2007, 2008 and 2011."
The deep-sea scientists from the HGF-MPG Group for Deep-Sea Ecology and Technology of the Alfred Wegener Institute regularly deploy their towed camera system OFOS (Ocean Floor Observation System) during Polarstern expeditions to the HAUSGARTEN. At the central HAUSGARTEN station it is towed at a water depth of 2500 meters, 1.5 meters above the seabed, and takes a photograph every 30 seconds. Deep-sea biologists principally use these photographs to document changes in biodiversity with respect to larger inhabitants such as sea cucumbers, sea lilies, sponges, fish and shrimp. However, for Bergmann they also provided evidence of increasing deep-sea pollution: "Waste can be seen in around one percent of the images from 2002, primarily plastic. In the images from 2011 we made the same discovery on around two percent of the footage. The quantity of waste on the seabed has therefore doubled", she says. If we consider the time span between 2007 and 2011 the amount has even risen by an order of magnitude.
At first sight, the "two percent" result may not cause much concern. However a comparison demonstrates the true extent of the pollution in the Arctic deep sea: "The Arctic Ocean and especially its deep-sea areas have long been considered to be the most remote and secluded regions of our planet. Unfortunately, our results refute this notion at least for our observatory. The quantities observed were higher than those recorded from a deep-sea canyon not far from the industrialized Portuguese capital Lisbon," Bergmann explains.
"The Arctic sea ice cover normally acts as a natural barrier," she says, "preventing wind blowing waste from land out onto the sea, and blocking the path of most ships. Ship traffic has increased enormously since the ice cover has been continuously shrinking and getting thinner. We are now seeing three times the number of private yachts and up to 36 times more fishing vessels in the waters surrounding Spitsbergen compared to pre-2007 times." Furthermore, litter counts made during annual clean-ups of the beaches of Spitsbergen have shown that the litter washed up there originates primarily from fisheries.
The main victims of the increasing contamination of the seafloor are the deep-sea inhabitants. "Almost 70 percent of the plastic litter that we recorded had come into some kind of contact with deep-sea organisms," she says. "For example we found plastic bags entangled in sponges, sea anemones settling on pieces of plastic or rope, cardboard and a beer bottle colonized by sea lilies."
When sponges or other suspension feeders come into contact with plastic, they can be injured. The consequence: the inhabitants of the seabed are able to absorb fewer food particles, grow more slowly, and so, probably reproduce less often. Breathing could also be impaired. Furthermore, plastic always contains chemical additives, which have various toxic effects. "Other studies have also revealed that plastic bags that sink to the seafloor can alter the gas exchange processes in this area. The sediment below then becomes a low oxygen zone, in which only few organisms survive," Bergmann says. On the other hand, other animals use the waste as hard substratum to settle on. "This allows colonization by species that previously would have found hardly any suitable substratum. This means that the waste could change the deep-sea composition of species and therefore biodiversity in the long-term," she adds.
In view of the far-reaching climate changes in the Arctic, Bergmann and her colleagues want to expand their research projects on "litter in the sea": "Until now our results from the Fram Strait merely constitute a snapshot, reflecting the observations that we were able to make with the naked eye," she explains. For example, the focus is currently moving to the question of deep-sea pollution resulting from so-called microplastic particles. "We took samples for the first time during the last expedition with our research ice breaker POLARSTERN to the HAUSGARTEN observatory. Our AWI colleagues from Helgoland will analyse them for microplastics," says Bergmann. Microplastics can be ingested by marine animals including commercially harvested prawns and fish and enter the human food chain.
During this expedition Belgian mammal and bird observers also counted 32 pieces of litter floating at the water surface. The probability of researchers finding more litter on the deep ocean floor is therefore great. Bergmann: "Pieces of plastic on the deep seafloor are unlikely to degrade into microplastics as quickly as is the case on the North Sea coast, for example. This is due to the lack both of sunlight at a depth below 200 meters and of strong water movement. Instead it is dark and cold there. Under these conditions plastic waste can probably persist for centuries."