“The Litter is a Whole New Dimension”
AWI biologist Dr Lars Gutow has investigated the survival strategies and metabolism of marine organisms like isopods for many years. Recently, his focus has increasingly been on the question of how plastic litter impacts marine organisms and habitats. In the following interview, he shares insights into his expeditions to the North Sea and Sargasso Sea, where he investigated how marine litter is spreading.
Plastic litter in the ocean is everywhere, and many people have personally found litter on the beach. Yet the research community doesn’t seem to know that much about the problem. Is that true?
Yes, that’s true. Basically, we’re still just getting started. Although the number of scientific studies has grown rapidly over the past few years, most of them only offer snapshot data from specific oceanic regions. In many cases we have to deal with estimations – like with the question of how much litter is floating in our seas. Quantifying marine litter is a quite challenging process: aspects like water temperature and salinity can be measured automatically using special-purpose buoys, but to determine the litter density in the ocean, you have to sail out there and count it on site.
You’ve participated in expeditions to survey the litter. How does it work?
There are a variety of methods. For instance, we observe a ten-metre-wide strip of the water surface, parallel to the ship. As the ship moves forward, we count the pieces of litter that drift by in that strip. It takes a bit of practice; little bits of sea foam and plastic debris can be quite hard to tell apart. But after a while you learn what to look out for. We used this method to carry out the first systematic litter survey in the south-eastern North Sea. Of course, with the naked eye you can only see pieces of litter that are at least a few centimetres across; we can’t count microscopically small objects like microplastics. To do that, we need different solutions, including the complex analysis methods that my AWI colleague Gunnar Gerdts is currently focusing on.
You also travelled to the Sargasso Sea, east of Cuba in the Atlantic. In terms of litter, are there differences between the North Sea and Sargasso Sea?
Absolutely. The North Sea is subject to heavy traffic from merchant ships and the coast is densely populated by humans. As a result, you often find litter that is still clearly recognisable, like bottles and candy wrappers. In the North Sea, we found roughly 30 to 35 pieces of litter per square kilometre, though the results of individual counts varied considerably. In contrast, the Sargasso Sea is home to one of the largest garbage patches: areas characterised by rotating currents (gyres) that lie far out in the ocean, and in which litter gathers. It takes quite a bit of time before litter reaches the centre of the gyre, by which time it has substantially degraded: often, we can no longer tell what kind of object it originally was. For the most part, we found small plastic bits measuring only a few centimetres in length – and an average of 50 to 60 such pieces per square kilometre. Compared to the North Sea, here we can more clearly see how the litter gathers in the gyre. We can also see the long-term effects of weathering – in other words, how the litter breaks down into smaller and smaller pieces.
On the basis of your observations, can you estimate how the litter is affecting marine habitats?
It’s still difficult to say. A recent study shows that 90 percent of all seabirds swallow pieces of plastic. Different species are at different levels of risk, depending on their feeding habits. The situation is especially critical for northern fulmars, which spend their entire lives on the open sea and exclusively feed from the sea surface. Examinations of dead animals have revealed that in some cases, their stomachs are completely full of plastics. Now, even fulmars from the Arctic have plastics in their stomachs, which should come as no surprise, since my AWI colleague Melanie Bergmann has found out that litter is also floating on the surface in Arctic waters. And the density of litter is even worse in the deep sea.
Together with Chilean colleagues, I’ve analysed a number of scientific studies that examine the extent to which marine surface litter facilitates dispersal of marine animals. It certainly represents a whole new dimension. There has always been flotsam like algae, wood or pumice that marine animals have clung to, transporting them over great distances. But plastic litter is extremely long-lived, and can be found in comparatively large amounts practically everywhere. Our analysis of various studies revealed that to date 387 different types of organisms have been found on floating plastic litter around the globe – including bacteria and other single-celled organisms, algae, bryozoans, bivalves, snails, barnacles and amphipods.
How does marine plastic differ from other litter and debris?
First of all, there’s the sheer amount of plastic in the ocean. Second, except for foamed polystyrene, the surface of plastics is usually much smoother than that of natural flotsam, which makes it easier for organisms to settle on it. Further, unlike floating algae that some marine animals feed on, the floating plastic offers no food, which also explains why only certain types of organisms can settle on them.
The differences become especially apparent in comparison to the Sargassum seaweed. Sargassum seaweed (for which the Sargasso Seas was named) is a delicately branched type of algae that floats on the surface of the Sargasso Sea and other waters, forming large natural rafts. The wind often clumps the plants into long strips, or into large, floating mats that offer a home for various small crustaceans and other organisms like the Columbus crab. This species has specialised to live on flotsam to such an extent that it can only be found there, and not on the ocean floor. The rafting community on drifting Sargassum is unique, and quite different to what forms on floating plastics.
During our expedition to the Sargasso Sea we determined that, despite the huge amount, plastic litter makes up only a fraction of all flotsam. However, the litter seems to be fundamentally changing the local rafting community, as it offers a highly persistent habitat for completely different species – like certain polyps, tunicates and species of goose barnacle that don’t settle on the floating Sargassum algae.
For the past several years, marine biologists have been investigating the phenomenon of bioinvasion – the spread of species indigenous to a given region to another. Could plastic litter become a problem in this regard?
There have already been many reports of plastic litter washing up on the West Coast of the USA, covered with organism from Japanese waters. Many organisms don’t survive their journey across the Pacific. However, following the major tsunami off the coast of Japan in 2011, which washed tremendous amounts of material out to sea, a great deal of flotsam and jetsam with diverse and vital communities of flora and fauna on it was found along the US coastline. To come back to seaweed: we know that it rarely drifts as far as the Tropics, and is mainly found in mid and higher latitudes.
That’s a good example of how the distribution through natural flotsam is subject to limits. But plastic litter can be found everywhere, and seems capable of overcoming all known boundaries. We’re convinced that it will revolutionise the distribution of species, even if there hasn’t yet been a documented case of an invasive species being introduced by plastic litter.
Can you already estimate the risk of these species being introduced?
No, we can’t, because we basically still don’t know how anthropogenic litter behaves at sea. Foamed polystyrene floating on the water can roll or be blown by the wind, whereas plastic bags drifting in the water behave very differently. A great deal of evidence suggests that marine litter becomes concentrated at certain points because of wind, currents or fronts. We still need much more information on how long the litter remains in a given region of the ocean.
One of my goals is to use oceanographic computer models to precisely monitor, or even predict, the behaviour and distribution of litter over days, weeks and months. We already have the ability to do so for large-scale regions and currents, but in coastal waters currents are often very complex, making it extremely difficult to make accurate predictions. Thanks to experiments with dragnets we’ve known for some time now that large amounts of litter are collecting on the seafloor in an area roughly 200 nautical miles off the Danish coast, but we don’t know why. Though there have been attempts to explain the phenomenon, we often see that the litter behaves quite differently than we expected.
Studies conducted by my Chilean colleagues show that the shape of the coast and small-scale local currents greatly influence the distribution of marine litter. Yet we still know very little about how the combination of wind and waves impacts the behaviour and spread of different types of litter.
In summer 2015, wreckage from a Malaysia Air plane that had crashed off the coast of Australia in March 2014 was found on the island of Réunion. Oceanographers have attempted to use ocean-current simulations to determine the exact crash site. Could similar methods be used for marine litter?
Those simulations are widely used, for instance to simulate the spread or the origin of pollutants or oil, and can also be used for marine litter on the ocean surface. But they only work at a larger scale. In the variable coastal waters, which are characterised by complex currents, using them to identify where exactly litter objects come from proves to be highly problematic. When it comes to plastic litter, we don’t even know how long the individual pieces have been in the ocean.
Further, litter that washes up on the shore doesn’t always stay there; it could be washed away with the next tide, resuming its journey. This makes it nearly impossible to precisely reconstruct its point of origin. At best, material analysis methods could tell us when a given piece of plastic was manufactured, but not how long it’s been in the ocean. If we want to truly understand marine litter, we still have a great deal to learn.
Dr Lars Gutow