Life in Plastic: 14 Million Tonnes of Microplastics Likely on Sea Floor

Citation: Barrett, J., Chase, Z., Zhang, J., Banaszak Holl, M.M. Willis, K., Williams, A, Hardesty, B. D., and Wilcox, C. Microplastic pollution in deep-sea sediments from the Great Australian Bight. Frontiers in Marine Science, (2020).

The weight of plastics

A single microplastic. (Image Source: Wikimedia Commons,

The amount of plastics has become an increasingly noticeable environmental threat. We have seen the photographs of turtles eating plastics accidentally, heard about birds feeding them to their chicks, and encountered the countless plastic bags, masks and straws littered on our local beaches. Even worse are the tiny plastic particles, known as microplastics, which aren’t always so visible.

Microplastics are plastic particles less than or equal to 5 mm and come from a variety of sources. They can be manufactured intentionally for certain beauty products, like microbeads – a tiny plastic bead exfoliant present in some cleansers or toothpaste. Microplastics are also derived from larger pieces of plastic that become broken down by wave action into tiny plastic pieces. Eventually, because of their size, microplastics can find their way into our marine and human ecosystems (see picture below). They are easily ingested by a wide variety of marine species and can cause negative effects on organisms.

Microplastics were recently found in devastating concentrations even in the most pristine of beaches in the Maldives. Microplastics can contaminate the food we procure from the ocean, and even be present in areas we least suspect. A recent study estimated that bottle-fed human babies may even be swallowing millions of microplastics a day, which are shed from polypropylene bottles often used for feeding.

A depiction of the interactions between marine microorganisms and microplastics. Plastics can be broken down into microplastics which end up ingested, floating in the water, or on the seafloor. (Image Source: Wikimedia Commons, Urbanek, Rymowicz & Mironczuk)

Researchers were interested to find how much microplastic pollution had made its way into the bottom of the world’s oceans. Since the deep-sea is so difficult to access, it hasn’t made for the easiest study site… until now.

The Great Australian Bight

Map showing the Great Australian Bight, the body of water in the study. (Image Source: Wikimedia Commons, NormanEinstein)

The researchers focused on the Great Australian Bight (GAB), which is a large ocean area to Australia’s south. From March to April in 2017, researchers collected deep-sea sediment samples from six locations in the Great Australian Bight. The locations were 288 to 356 km off the coast ranged in depth from 1,655 to 3,062 meters deep!

The researchers were able to collect the samples using a robotic submarine deployed from a vessel, which dug cores of sediment (up to 9 cm deep) from the seafloor. The remotely operated vehicle brought the 51 samples on board, which were then frozen and brought to the lab. The researchers defrosted the samples, dyed them “Nile Red”, agitated, and filtered them. The “Nile Red” dye absorbs onto plastic surfaces and makes microplastics easier to see under fluorescent light. The researchers were able to view the number of microplastics and count them using a microscope.

Microplastics in the deep-sea

Marine debris litters a beach on Laysan Island in the Hawaiian Islands National Wildlife Refuge. (Image Source: Wikimedia Commons, U.S. Fish and Wildlife Service Headquarters)

The scientists found that the average number of microplastics from the Great Australian Bight was 1.26 microplastics per gram of dry sediment. Using this number, the scientists scaled up to approximate the global amount in the top sediments of the world’s ocean. The number was shocking.

The scientists estimate that 14.4 million tonnes of microplastics are in the sediment in the world’s oceans. That’s comparable to the weight of 40 Empire State Buildings (weighing 365 000 tonnes each)!

Scientists were also able to estimate that the bottom of the ocean has between 34 and 57 times the amount of plastic present at the surface. Even the deep ocean is not immune to plastic pollution and human impacts! This study clearly shows that plastic produced by humans are breaking down and sinking to the bottom of the ocean. As plastic production continues to increase, what will happen to these ecosystems in the future? It is urgent that we create effective solutions for microplastics and plastics pollution.

As scientists explore more and find more devastating results, we should stop to think of the impact of our own habits. We can help in our own ways on an individual basis. This includes avoiding single-use plastics (straws, bags, packaging), recycling properly, disposing of waste appropriately, and supporting leaders with environmental policies. These actions are vital for the health of our oceans and the deep sea. The alternative is even more plastic in the ocean, and the continued degradation of our ocean ecosystems.

Single-use plastics abandoned on a beach. (Image Source: Wikimedia Commons, Twentyfirstidentity)

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