Plastic washed up on shore. Image : NOAA

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.

The Scripps researchers figured the stomachs of salps might be a place where mini-microplastics accumulate. To that end, samples of both surface seawater and salp specimens were pulled from the California Current, the North Pacific subtropical gyre (also known as the Great Pacific Garbage Patch), and an in-between ocean zone.


Back at the lab, the scientists used a special fluorescent microscope to illuminate—both literally and figuratively—microplastic particles found in the samples. As the Scripps press release points out, “plastic self-illuminates when exposed to multiple wavelengths of light.” This method allowed them to document pieces as small as 10 micrometers, which is thinner than the width of human hair. The researchers also analyzed seawater collected from 2009 to 2017 for their analysis.

Disturbingly, every salp studied had mini-microplastics in their stomach, a finding that even surprised the researchers. Given the quick turnaround time of the salps’ digestive system—between 2 to 7 hours—the scientists expected their stomachs to be relatively clean. Such was not the case.


“The thing that truly surprised me the most was that every salp, regardless of year collected, species, life stage, or part of the ocean collected, had plastic in its stomach,” Brandon wrote in an email to Earther. “A species having 100 percent ingestion rates is quite extraordinary, and devastating for the foodweb that eats salps.”

Translating these findings into an estimate, the researchers concluded that, on average, 8.3 million pieces of mini-microplastics can be found in a typical cubic meter (35 cubic feet or the aforementioned 1,000 liters ) of ocean water. That runs in stark contrast to the previous estimate of 10 fragments per cubic meter.




“This study may be one of the first to estimate the abundance of the smallest mini‐microplastics in surface seawater, which are consistently under‐sampled,” wrote the authors in the study. The findings show plastic concentrations were up to seven orders of magnitude higher than earlier studies, highlighting the “previously unquantified significance of mini‐microplastics in marine debris counts.”

Of course, quantity is different than total volume.

“The quantity is one million times more numerically, but when you multiply the quantity times volume, the volume of the larger pieces is still much higher,” said Brandon. This distinction matters, she said, depending on what kind of animal you are. Smaller creatures like salps and small plankton will tend to eat more of the tiny bits of plastic while larger plankton and small fish are more likely to be impacted by bigger pieces of plastic.


That salps are accumulating so much plastic is a serious concern. The tiny creatures are likely providing a transportation mechanism for microplastics to reach the bottom of the deep ocean through their digestive processes and sinking feces, and by virtue of this, into the food chain down below. And because salps are regularly consumed by marine animals such as sea turtles, rockfish, and king crab—of which the latter two are regularly consumed by humans—these mini-microplastics might eventually find their way to our dinner plates, and ultimately our bodies.

Microplastics are harmful to marine organisms and ecosystems, but their effect on human health remains unclear. That said, the U.S. National Institutes of Health (NIH) says we should probably be worried, though:

“There is scientific uncertainty about the hazards of microplastic issues. There is concern that microplastics could have adverse health effects on humans as they move through the marine food web. Microplastics both absorb and give off chemicals and harmful pollutants. Plastic’s ingredients or toxic chemicals absorbed by plastics may build up over time and stay in the environment. It is not known if you can be exposed to these pollutants by eating contaminated seafood.”


Most of the microplastics observed in the new study were collected from regions close to shore, which suggests the source is runoff pollution from land. Microplastic waste comes from a variety of sources, including synthetic microfibers found in clothing and tiny spherules in toothpaste and skincare products. Over time, much of this plastic, whether big or small, breaks down into smaller and smaller pieces, but they linger in the environment for extended periods of time. They cannot be removed by wastewater treatment, and much of this waste ends up in our oceans.

“The results were very surprising in terms of how high our numbers were, but also not that surprising, when you think about how plastic breaks down,” said Brandon. “Every large piece breaks down into thousands, maybe millions of tiny pieces, so there should be millions more [of the] tiniest pieces. It actually makes sense compared to some modeled degradation simulations.”


In terms of how scientists could further validate these results, Brandon offered a few suggestions. She told Earther researchers could “sample more samples of water in other ocean basins and more salps and similar filter-feeding plankton,” and adopt her team’s fluorescence microscopy method and start hunting for small microplastics. In addition, scientists could “fine-tune this microscopy method with more specialized fluorescence filters for specific plastics so we know abundances of plastic types as well.”

Governments and other top officials need to enact legislation to limit the use of products that contribute to microplastic pollution, but there are things you can do as well, such as not using products that contain microplastics, avoiding single-use plastics, using paper bags, recycling, and, of course, not throwing plastic waste into any body of water. After all, it may just end up back in your body anyways.