If you pulled 1,000 liters (264 gallons) of water out of the ocean, how many small bits of plastic would you expect to find? Ten pieces? One hundred pieces? How about 8.3 million pieces of what researchers call “mini-microplastic.” Such is the finding of an alarming new study.
The amount of microplastic in our ocean—that is, pieces of plastic measuring smaller than 5 millimeters—is a million times greater than previously estimated, according to new research published in the science journal Limnology and Oceanography Letters.
This means the concentrations of micro-sized bits of plastic inundating our oceans isn’t two or three times more than scientists had previously estimated—but more like five to seven times greater, according to the authors of the paper, led by Jennifer Brandon from the Scripps Institution of Oceanography at the University of California, San Diego. The reason for the enormous discrepancy has to do with the type of microplastics involved and the way scientists have traditionally tried to measure the amount of microplastic in sea waters.
“For years we’ve been doing microplastics studies the same way (by) using a net to collect samples,” said Brandon in a press release. “But anything smaller than that net mesh has been escaping.”
Indeed, as independent research from 2015 pointed out, thousands of trawls done between 1971 and 2013—all with the same kind of net—were only able to capture plastics larger than 333 micrometers in size, or one-third of a millimeter. So while these nets were small enough to filter plankton, they were subsequently too big to capture the smallest plastic particles, known as mini-microplastics.
“I saw these published size ranges and thought, we are under-sampling this smaller range. There’s a big knowledge gap,” said Brandon.
With this deficiency in mind, Brandon and her colleagues developed a new technique to detect and measure the volume of mini-microplastics in seawater. Salps—tiny, gelatinous filter-feeding invertebrates— were key to the updated approach were. These barrel-shaped creatures swim at depths above 2,000 meters (6,500 feet) and they often link together to form long chains that, through their combined efforts, helps them to swim faster. To swim and filter-feed on plankton, salp pump water through their bodies with pulsed contractions.