A survey of plastic waste picked up in the North Pacific Subtropical Gyre—aka the Giant Pacific Garbage Patch—has revealed that the garbage is providing a home to species that would otherwise not be found in the deep ocean. Over two-thirds of the trash examined plays host to coastal marine species, many of which are clearly reproducing in what would otherwise be a foreign habitat.
The findings suggest that, as far as coastal species are concerned, there was nothing inhospitable about the open ocean other than the lack of something solid to latch on to.
To find out whether that was taking place, the researchers collected over 100 plastic debris items from the North Pacific Subtropical Gyre in late 2018/early 2019. While a handful of items could be assigned to either Asian or North American origins, most were pretty generic, such as rope and fishing netting. There was a wide variety of other items present, including bottles, crates, buckets, and household items. Some had clearly eroded significantly since their manufacture, suggesting they had been in the ocean for years.
Critically, nearly all of them had creatures living on them.
Far from home
Ninety-eight percent of the items found had some form of invertebrate living on them. In almost all cases, that included species found in the open ocean (just shy of 95 percent of the plastic). But a handful had nothing but coastal species present. And over two-thirds of the items had a mixed population of coastal and open-ocean species.
While the open-ocean species were found on more items, the researchers tended to find the same species repeatedly. That isn’t surprising, given that species adapted for a sedentary existence near the surface are infrequent in that environment. By contrast, there was far more species diversity among the coastal species that had hitched a ride out into the deeps. All told, coastal species accounted for 80 percent of the 46 taxonomic richness represented by the organisms identified.
On a per-item basis, species richness was low, with an average of only four species per item. This suggests that the primary barrier to a species colonizing an item is simply the low probability of finding it in the first place.
Significantly, the coastal species were breeding. In a number of cases, the researchers were able to identify females carrying eggs; in others, it was clear that the individuals present had a wide range of sizes, suggesting they were at different stages of maturity. Many of the species that were reproducing do so asexually, which simplifies the issue of finding a mate. Also common was a developmental pathway that skips larval stages. For many species, the larval stage is free-ranging, which would make them unlikely to re-colonize the same hunk of plastic.
The species that seemed to do best were often omnivores, or engaged in grazing or filter feeding, all options that are relatively easy to pursue without leaving the piece of plastic they called home.
A distinct ecology
One thing that struck the researchers was that the list of species present on the plastic of the North Pacific Subtropical Gyre was distinct from that found on tsunami debris. Part of that may be that some items swept across the ocean by the tsunami, like docks and boats, already had established coastal communities on them when they were lost to the sea.
With the possible exception of fishing gear and buoys, however, these plastic items likely picked up their inhabitants while passing through coastal ecosystems that were largely intact. So the colonization of these items likely represents a distinct—and ongoing—ecological process.
It also has the potential to have widespread effects on coastal ecology. While the currents that create the North Pacific Subtropical Gyre largely trap items within the Gyre, it is home to island habitats that could potentially be colonized. And it is possible that some items can cross oceans without being caught in a gyre, potentially making exchanges between coasts a relatively common occurrence in the age of plastics.
Finally, the researchers caution against a natural tendency to think of these plastic-borne coastal species as “misplaced species in an unsuitable habitat.” Instead, it appears that they are well suited to life in the open ocean as long as there’s something there that they can latch on to.
Nature Ecology & Evolution, 2023. DOI: 10.1038/s41559-023-01997-y (About DOIs).
Source: Pacific garbage patch providing a deep ocean home for coastal species | Ars Technica