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.