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.

Pinpointing plastic’s path to the deep sea

Until now, little has been known about how microplastics move in the ocean. A new paper by our colleagues at the Monterey Bay Aquarium Research Institute (MBARI), just published in the journal Science Advances, shows that filter-feeding animals called giant larvaceans collect and consume microplastic particles in the deep sea.

Larvaceans are transparent tunicates that live in the open sea and capture food in sticky mucus filters. Plastic particles accumulate in the cast-off mucus feeding filters and are passed into the animals’ fecal pellets, which sink rapidly through the water, potentially carrying microplastics to the deep seafloor.

Researchers at MBARI documented that tadpole-like giant larvaceans consume microplastic beaads. Photo courtesy MBARI.

The new findings contribute to an emerging picture about the ubiquitous nature of ocean plastic pollution. Over the last decade, scientists have discovered tiny pieces of plastic in all parts of the ocean—including deep-sea mud. One recent study documented microplastic fibers in deep-sea sediments at levels four times greater than an earlier study had found in surface waters. Plastic has also been discovered in the tissues of animals at the base of the ocean food web. Another just-published study found that fish confuse plastic particles with real food items because it smells just like organic matter in the ocean.

Despite their name, giant larvaceans are less than 10 millimeters (4 inches) long, and look somewhat like transparent tadpoles. Their mucus filters—called “houses” because the larvaceans live inside them—can be more than 1 meter (3 feet) across. These filters trap tiny particles of drifting debris, which the larvacean eats. When a larvacean’s house becomes clogged with debris, the animal abandons the structure and it sinks toward the seafloor.

Principal Engineer Kakani Katija studies giant larvaceans during field expeditions in Monterey Bay. Photo courtesy MBARI.

In early 2016, MBARI Principal Engineer Kakani Katija was planning an experiment using the DeepPIV system to figure out how quickly giant larvaceans could filter seawater, and what size particles they could capture in their filters. Other researchers have tried to answer these questions in the laboratory by placing tiny plastic beads into tanks with smaller larvaceans. Because giant larvacean houses are too big to study in the lab, Kakani decided to perform similar experiments in the open ocean, using MBARI’s remotely operated vehicles (ROVs).

When she discussed this experiment with Postdoctoral Fellow Anela Choy—who studies the movement of plastic through the ocean—they realized that in-situ feeding experiments using plastic beads could also shine light on the fate of microplastics in the deep sea.