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. Continue reading Pinpointing plastic’s path to the deep sea