Giant floating islands of plastic pollution are a common topic in the news, but the ocean surface is not the only location that plastic is accumulating. Due to differences in densities between the different types of plastics, some plastic floats and some plastic sinks. This distribution of plastics at different depths means that wildlife swimming at different depths can be exposed to different plastics. It also means that piles of plastic are forming at the bottom of the ocean as well as the surface.
Plastics and benthos
An international team of researchers recently published a study that suggested these collections of plastic at the bottom of the ocean might become sources of enhanced biodiversity. The fact that various living organisms (or “biota”) will attach to ocean plastic is not a new concept, and it has actually caused some concern with regard to the transport of microscopic bacteria. There have also been photos and videos of larger ocean-floor organisms (“benthos”), like corals and sponges, inhabiting deep-sea plastic. However, there has not been much investigation into the presence of smaller macrobenthos on deep sea plastic. In this study, Xikun Song and collaborators collected and analyzed plastic debris collected in the Xisha Trough, a deep area of the South China Sea that has a high abundance of accumulated plastic, with a particular interest for these often-overlooked organisms.
The researchers obtained macro-plastic (big enough to see with the human eye) samples by dredging the sea floor and using a manned submersible to pick plastic samples at random from debris dumps. The plastic samples collected were a mix of different common varieties you might recognize from recycling labels: PET, PP, PE. Interestingly, most of the plastics collected had densities less than water (meaning they should float), but the sediments and biota sticking to them likely increased their effective densities and led to them sinking to the ocean floor. On the 33 plastic samples they examined, they found almost 1,200 individual organisms, ranging from 0.4 to 25 mm in size and representing 49 different species! A microscopic view of a plastic sample colonized by organisms is shown below.
The more you learn… the more there is to discover!
From studying each individual, the scientists were able to gain a better idea of what organisms inhabit underwater plastics and suggest many areas for future study. For example, they identified that the impacts of plastic on the development of gastropods would be a good topic for investigation, but the development process could take up to 2 years (outside the timeline of this work)!
In another example, the researchers noticed that brachiopods were only present up to two years old. Could it be that the plastic environment eventually starves the organisms due to lack of nutrients? Or, could it be that the plastic has only been available for the organisms to inhabit for ~2 years. It is hard to know at this stage, and it shows how science is an iterative process that advances significantly after several efforts. Later studies may look further into any of these vague findings and determine a clear answer, or they may not. This early research effort’s contribution is largely to illustrate potential areas for further research, and the authors indicate such.
A silver lining
Does the variety of organisms present on the plastic samples mean that these underwater trash heaps are good for the environment? A hard “no”. The scientists have not shown that the plastic provided any benefit to these species, or that the concentration of these species were higher on the plastics than on surrounding surfaces. However, the research team does point out that these underwater environments could provide information about how macrobenthic creatures respond to plastic. The plastic environment could even cause changes in species, increasing biodiversity! The relatively high concentration of plastic in these underwater dump areas makes it easier to collect samples for further study. So, while the presence of plastics in our oceans is not a good thing, Song and collaborators illustrated that the use of ocean plastics in further research might provide valuable information for solving the giant plastic pollution problem, and it could also expand biodiversity– a silver lining.
What can we do?
To do our part to reduce the amount of plastic entering these deep sea environments, we should limit our use of single use plastics, recycle what we can, and pick up plastic litter in our neighborhoods. Runoff can transport a lot of plastic waste into waterways that often empty into the ocean. So, that bottle cap in your street gutter might eventually become not-so-healthy food for sea turtles or benthic dwellers if you don’t intervene. You can read more about the work being done in the area of plastics pollution here.
Cover Image: Reprinted (adapted) with permission from Song et al. Copyright 2021 American Chemical Society.
Source Article: Large Plastic Debris Dumps: New Biodiversity Hot Spots Emerging on the Deep-Sea Floor. Xikun Song, Mingxin Lyu, Xiaodi Zhang, Bernhard Ruthensteiner, In-Young Ahn, Guido Pastorino, Yunan Wang, Yifan Gu, Kaiwen Ta, Jie Sun, Xi Liu, Jian Han, Caihuan Ke, and Xiaotong Peng. Environmental Science & Technology Letters 2021 8 (2), 148-154 DOI: 10.1021/acs.estlett.0c00967