Microplastics don’t travel up the local marine food chain like other contaminants, study shows

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Tiny bits of plastic are floating around in our oceans, largely unseen by the human eye.

These pieces of plastic can be as large as a grain of rice, or as small as a single cell in the human body—and they’re everywhere.

There is still a lot researchers don’t know about how these tiny plastics interact with the marine environment, and how damaging they might be to the marine animals that ingest them. But new research could be good news, at least in regards to how microplastics travel through a food web.

In a new paper published last month, a research team out of UVic found that larger sized microplastics are not biomagnifying in the local food chain like other contaminants do.

Biomagnification is when a toxin or contaminant within animals becomes more concentrated the further up in the food chain you look. Mercury, for example, is found in higher levels in predators at the top of food chains, like orcas, and causes increased harm to those animals.

Lead researcher Garth Covernton and his team spent time at three sites around southern Vancouver Island—Victoria Harbour, Coles Bay in the Saanich Inlet, and Elliot Beach in Ladysmith—scuba diving to collect rockfish and crabs, hand picking mussels and sea cucumbers, and deploying nets from the beach to collect shiner surfperch and a few different flatfish species.

The sites were chosen because of the likelihood that the animals found there would be a part of the same food web. The copper and black rock fish represented the top of this particular web.

Back in the lab, the researchers dissected the harvested organism to remove the digestive tracts, then dissolved away the animal tissues using potassium hydroxide. What was left was filtered and the particles were examined under a microscope to determine how much microplastic was present.

“It was interesting that there wasn’t a relationship between the number of microplastic particles and how high in a food web the animal was,” Covernton said. “It suggests that this biomagnification was not occurring.”

This study deals with particles bigger than 100 microns—particles invisible to the naked eye and measuring about 10 times the size of a human cell—because anything smaller is too difficult to spot with microscopes.

Photo: Submitted

In general, each animal only had a few microplastic particles in their digestive tract. However, the smaller fish studied, such as shiner surfperch, actually had a higher concentration of microplastics inside. Covernton says this is likely because microplastics are of a similar size to the tiny creatures like zooplankton that these fish are already feeding on.

A lot of the microfibres found in this area are textile fibers, and a lot of these are coming from sewage: when people wash their synthetic clothing, microfibre runoff can make its way into the ocean. To a lesser extent, microplastics come from disintegrating fishing and boating equipment, like ropes, floating in the water.

The water around Vancouver Island has, in general, lower levels of microplastics than other areas and bodies of water. Studies in Lake Ontario, for example, have shown up to hundreds of microplastic particles per fish.

“[This study is] good news for that system, but I wouldn’t say it necessarily allows us to not be worried,” Covernton cautions, “because there are other systems [with higher levels of microplastics] and also that microplastic pollution is projected to keep increasing around the world.”

Plastics appear to 'pass through'

There isn’t much known about whether or not microplastics are harmful to the animals that ingest them, including us. Humans likely ingest many more microplastics than marine animals because our environments are full of them.

“In a lot of cases, especially for lower concentrations, the particles do seem to just kind of pass through their digestive tracts and they’ll excrete them out,” Covernton said.

“But even when we get some good news like this, where it seems like they’re pretty low or not affecting this certain area, I still think it’s important to investigate them and to understand at what level they would have an effect. Are they having that effect in certain areas already? And then, how do we limit that both now and in the future?”

Rhiannon Moore, a microplastics researcher who now works for the City of Victoria in waste reduction, released a study three years ago on the presence of microplastics in beluga whales and the fish they eat in the arctic. Similarly, in her study, she found microplastics were present in every animal.

“We don’t see it behaving the way that other pollutants do in ecosystems,” Moore said. “The good news that [Covernton’s] article brings is that it clarifies the difference between microplastics and other pollutants in the way that we create solutions and assess risk.”

Since microplastics go through the digestive system of animals and are excreted out, they don’t leave the ocean. And they don’t break down in the same way that organic material does. Rather, the amount in the environment just keeps growing as more microplastics are created.

Moore says there still isn’t a good way to effectively remove microplastics from the water, but removing large plastic waste is a good place to start.

The Great Canadian Shoreline Cleanup, led by Ocean Wise, and other smaller community-driven initiatives bring people together across the country to clean up plastic debris and garbage along beaches. Seabins—basically small floating trash cans—have also been installed in various marinas around the South Island to collect any floating debris and large microplastics in the water nearby. And non-profit Ocean Cleanup last year arrived in Victoria with 8.2 tonnes of plastic it had collected from the Great Pacific Garbage Patch.

Moore says the amount of microplastics research has really exploded in the past 10 years. While there is limited knowledge now, she is hopeful more studies will lead to better solutions.