The deep impact of microplastic

 

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There’s a vast ecosystem stretching far below the ocean’s surface — one where the light dims, the pressure mounts, and life takes on forms that can seem downright alien. But even there, a place that seems a world apart from human society, our plastic trash is building up.

Scientists from the Aquarium and MBARI sampled microplastic pollution in the deep waters of Monterey Bay using the ROV Ventana. Photo courtesy MBARI

In the deep sea, it’s a challenge to study where that plastic accumulates and how it affects animals. So scientists at the Monterey Bay Aquarium and our partners at the Monterey Bay Aquarium Research Institute (MBARI) launched an ambitious collaboration.

The resulting study, which examined microplastic in the waters of Monterey Bay, was published June 6 in the journal Scientific Reports.

“We designed this study to answer a fundamental gap in our knowledge of marine plastic once it reaches the ocean,” says lead author Anela Choy, a former MBARI researcher and now a professor at the Scripps Institution of Oceanography in San Diego.

MBARI researchers collected larvaceans and their mucus feeding filters using its remotely operated vehicles. Photo courtesy MBARI.

The research team gathered data by using remotely operated vehicles (ROVs), robotic submarines designed by MBARI engineers, to collect water samples at depths from 200 to 600 meters (about 650 to 2,000 feet).

They also searched for plastic in animals with important roles in the marine food web: pelagic red crabs; and tadpole-like creatures called giant larvaceans, which surround themselves with clouds of mucus that capture food — and, as the researchers discovered, plastic.

“Problems like this are extremely complicated. To try and figure out how to solve them, you need a lot of different tools,” says Aquarium Chief Scientist Kyle Van Houtan, who co-authored the paper with Anela and nine others, tapping fields from physical chemistry to marine ecology.

“This is the most difficult project I’ve ever worked on, bar none. It was the most technically challenging and involved several teams with unique skill sets,” Kyle says. “This is what we call ‘big science.’” (Kyle discusses the research in an archived interview for the KQED Forum radio program.)

Plastic, plastic everywhere

When it comes to microplastic in the ocean, most existing data are from the surface. The thousands of vertical feet extending down toward the seafloor are a vast scientific blind spot.

Researchers found the highest concentrations of microplastic far below the surface, at levels comparable to those in surface waters of the “Great Pacific Garbage Patch.”

“The deep ocean is the largest living space on the planet,” Kyle says. “Not knowing what’s going on there with respect to plastic is, in a sense, not really knowing what’s going on with plastic.”

But it wasn’t easy to devise a way to measure plastic concentrations thousands of feet down. The research team took pains to ensure that the minute particles they detected weren’t from their own gear.

“Plastic is such a fundamental part of humanity’s existence,” Anela says. “It’s in our clothing and our scientific instruments.”

Anela Choy and her team of scientists and engineers at MBARI had to devise new ways to sample deep ocean waters. Photo courtesy MBARI

To minimize contamination, Anela and the team developed a new way to collect samples. It involved positioning a ship in water it hasn’t already passed through, then lowering an ROV with a specially engineered sampler. They took measurements of microplastic concentrations at various depths — some half a mile below the surface — by steering the ROV away from the ship, then suctioning water across a filter.

The crew then repeated this process nearly 30 times. Then they faced a new challenge: identifying the specific plastic bits they found, all less than a quarter-inch long — putting them in a growing category of ocean pollution: microplastic.

Of lasers and larvaceans

The researchers verified microplastic particles in their water samples by using a materials science technique called laser Raman spectroscopy. Each type of plastic has a distinct chemical makeup that gives it a unique visual signature, or spectrum, that is revealed when a laser hits it.

Scientists at Arizona State University used a laser Raman spectrometer to identify the chemical signature of microplastic particles. Photo courtesy Arizona State University

This allowed the team to catalog various types of plastic found in Monterey Bay. It also inspired them to build an open-source library of different spectra to make similar work easier and more affordable for other researchers. (In time, Kyle hopes to not only discern the different types of microplastic particles, but also their ages, to better understand how long the ocean’s plastic problem will persist.)

The study used data from two particle-feeding animals that carry plastic into the food web.

Pelagic red crabs, found from the surface to the deep, are food for many ocean animals. Photo © Monterey Bay Aquarium/Patrick Webster

Pelagic red crabs swim in shallow water and use tiny hairs on their appendages to catch particles of a certain size for food. In turn, the crabs are eaten by tuna, squid, sharks, sea turtles, sea birds and whales. When conditions are right, they’re abundant in Monterey Bay; collecting them was as easy as getting a fishing permit.

Then came the giant larvaceans.

These tadpole-like swimmers surround themselves with strangely beautiful clouds, or houses, of gossamer mucus — some a meter across— that snag tiny bits of food. The larvaceans eventually abandon these mucus houses, and they spiral downward as “sinkers” that become food for deeper-dwelling animals.

A larvacean “house,” which consists of two mucus filters, concentrates plastic particles it filters out of the water. When it discards the house, it sinks to the deep sea, where it is consumed by other animals. Photo courtesy MBARI

When other animals eat these sinkers, they ingest the microplastic bits too, introducing them into the deep-sea food web — unless the ROVs can get to them first.

Everywhere they looked, from the surface to the depths 3,000 feet down, the research team found plastic. The highest concentrations were in the midwater, 200 to 500 meters (600 to 1,500 feet) below the surface.

“A little bit shocking”

Some plastic concentrations were higher than those recorded in what’s known as the “Great Pacific Garbage Patch,” a mid-Pacific gyre reputed to be the dirtiest place in the ocean.

“Monterey Bay is a pretty clean place, so that result was a little bit shocking,” Kyle says. “Monterey Bay at depth has a higher concentration of plastic than the surface of the garbage patch.”

Concentrations of microplastic in deep waters of the Monterey Bay were greater than at the surface of the “Great Pacific Garbage Patch.”

And, Anela adds, “There was also plastic within the animals — all of the animals at all of the depths we looked at below the surface.”

The study points to one promising solution in particular.

“Making and using less plastic in the first place is one of the most effective means of solving this issue,” Kyle says. “It may be virtually impossible to remove existing microplastic from the deep sea. But when we slow the flow of plastic from land, we can help prevent the increasing accumulation of plastic in our global ocean.”

Kyle and Anela agree that much work remains, and more surprises are still in store. After all, scientists are just beginning to scratch the surface of the ocean plastic problem.

— Daniel Potter

Learn more about the Aquarium’s work to address ocean plastic pollution.