Study shows ocean dynamics mixing microplastics deep into the water column
There’s no question that plastics pollution in the world’s oceans is a serious and growing problem. One recent study estimated that somewhere between 5 million and 13 million metric tons of plastic waste were dumped into the ocean in 2010 alone, for the sake of comparison, one metric ton is 2,200 pounds, about the weight of a small car.
Other studies focusing on the impacts of all the debris show that plastic poses a risk to sea turtles, crabs and seabirds, while research voyages have shown that the tiny microparticles are to be found nearly everywhere, including the Arctic.
And new research released in April suggests that most estimates of plastics pollution in the ocean may be far too low, because most attention has been focused on measuring the debris floating near the surface. But ocean turbulence may be mixing the plastic much deeper, beyond the reach of the surface sampling, according to University of Delaware physical oceanographer Tobias Kukulka.
“My research has shown that ocean turbulence actually mixes plastics and other pollutants down into the water column despite their buoyancy,” Kukulka said. “This means that surface measurements could be wildly off and the concentration of plastic in the marine environment may be significantly higher than we thought.”
“You have stuff that’s potentially poisonous in the ocean and there is some indication that it’s harmful to the environment, but scientists don’t really understand the scope of this problem yet,” said Kukulka, an expert on ocean waves and currents.
Up to now, sampling efforts have been based on dragging tow nets over the surface for a few miles and counting the plastic fragments. That number is a basis for calculating concentrations in a particular area.
But Kukulka isn’t so sure this method provides an accurate picture of what’s happening, based on the influence of ocean turbulence. A press release on the study uses the example of coffee to explain:
A good way to understand ocean turbulence is to think about adding cream to your coffee. If you pour the cream gently, you need a spoon to generate turbulence and mix the two liquids together. If you pour the cream quickly, however, as the liquid descends into the coffee it naturally generates turbulence and mixes the liquids. In the ocean, wind and waves act like a spoon, generating turbulence and mixing this surface layer of the water.
Together with scientists at Woods Hole Oceanographic Institute and the University of Washington, studied the effects of the waves with computer models, and how heating and cooling of the ocean surface affects mixing. The study concluded that both are significant factors in determining if plastics stay on the surface or are mixed down deeper into the water.
Here’s how it works. In the summer, the sun heats the top layer of water, making it buoyant. When the surface cools, the water sinks, taking the tiny bits of plastic with it.
“If we really want to go after this problem and quantify the amount of plastics in the ocean and think about distribution and impact, then we need to keep in mind that turbulence is influenced by heating and, therefore, the distribution of plastics is too,” Kukulka said.
The research helps inform efforts to get a handle on the plastics issue, but could also be applied to oil and other pollutants, as well as to the distribution of nutrients and phytoplankton in the water.
“Broadly, these plastics pieces can be used as a physical tracer to help answer bigger questions about ocean processes and their implications for other ocean pollutants,” he said.
While some scientists have suggested dragging nets through ocean’s surface waters to remove the plastic, Kukulka cautions that in areas with strong turbulence scientists “may want to consider spending our energy and efforts elsewhere.”
“Even though the plastic pieces are buoyant, cleanup might not be as simple as skimming the surface,” he said.