At the bottom of the ocean, marine organisms have found a new home: our discarded plastics. Plastic items collected from trenches as deep as 3,200 m in the South China Sea host a diverse group of animals including corals, mollusks, and worms (Environ. Sci. Technol. Lett. 2021, DOI: 10.1021/acs.estlett.0c00967). Although previous studies have found animals colonizing objects in shallow waters or near coasts, this is the first time such a large number of species have been discovered on a deep-sea plastic dump, raising questions on how such waste heaps affect the deep-sea ecosystem in their vicinity.
Plastic litter is ubiquitous on most beaches and can often be seen riding the waves. But plastic bottles, bags, and other trash can also be weighed down by accumulated sand and other sediments, which makes them sink to the seafloor.
During expeditions in 2018, the submersible Shenhaiyongshi found large litter dumps on the bottom of the Xisha Trough in the northern South China Sea (Mar. Pollut. Bull. 2019, DOI: 10.1016/j.marpolbul.2019.03.041). The dumps contained up to 52,000 items/km2, mainly fishing gear and plastic bags, at depths of 1,700–3,200 m. Xikun Song of Xiamen University, Xiaotong Peng of the Chinese Academy of Sciences, and colleagues studied 33 plastic items collected during the dives and from dredging of the Xisha Trough over the past 3 years. They used scanning electron microscopy, DNA sequencing, and 3-D microcomputed tomography to identify the organisms on the objects. The researchers found nearly 1,200 individual organisms belonging to 49 different species.
They included organisms that stay fixed in one place, such as corals, barnacles, and fungi, as well as animals that move around, such as mollusks and bristle worms. The researchers also found flatworm and gastropod eggs on the objects, which suggests that plastic litter is providing surfaces for egg-laying that may not otherwise be available in this deep-sea environment.
Although these plastic objects provide extra habitats for deep-sea dwellers, “previous natural habitats might have been destroyed because of plastic littering, and species could have been supplanted due to plastic litter,” says Florian Pohl of Durham University, who studies microplastics on the seafloor and was not associated with the study. “We simply do not know which and how many species lived on the seafloor at these locations before it was littered with plastics.”
In addition, the effects of such changes to the deep-sea ecosystems are completely unknown, Song says, and will require continued monitoring.