Chasing Ocean ‘Snowflakes’

Below the ocean’s surface, sunlight quickly grows dim. But if you could shine a flashlight through the watery darkness, you might find yourself in an unexpected blizzard: a tempest of tiny underwater particles known as marine snow.

That “snow” isn’t snow at all. Each tiny “flake” is a clump of marine detritus: dead microscopic animals called plankton, fecal pellets (poop), bacteria, and other carbon-rich particles that together provide a rich source of food for deep-sea fish, jellies, and crustaceans.

“All those animals have to survive on carbon, just like we do,” said Ken Buesseler, a geochemist at Woods Hole Oceanographic Institution who studies marine snow. And beyond its role in the food web, he says, marine snow also plays a key part in regulating global climate.

Near the ocean surface, tiny plantlike organisms called phytoplankton make their own food using energy from the sun and pulling carbon dioxide out of the atmosphere, removing about a quarter of the carbon dioxide that we humans produce. Phytoplankton get eaten by zooplankton, whose carcasses and waste products also become part of the marine snowfall.

“As you go deeper there’s less and less marine snow,” Buesseler said, “because these particles are the food for organisms in the ocean twilight zone”-the area 650 to 3,280 feet (200 to 1,000 meters) below the surface, where little light penetrates.

About 90 percent of the carbon in marine snow stays in the twilight zone, cycling through the food web. But a small fraction sinks even deeper, remaining sequestered in the deep ocean-and out of the atmosphere-for potentially thousands of years.

“The big question we still need to answer is, how much carbon gets from the surface to the deep ocean-and how quickly,” Buesseler said.

Studies in the 1970s and 1980s to try to calculate the sinking rates of marine snow relied on scuba divers.

“You’d send a diver in with a stopwatch in one hand, and they would look to see visible particles,” Buesseler said. “They’d shoot a little spot of dye into the water and try and measure the travel distance of the particles between two calipers.”

Today, scientists can use satellite imagery to estimate how much phytoplankton is in the surface ocean, but what happens below the surface is much harder to measure. Scientists set traps in the ocean to collect falling particles, which can provide some idea of how much marine snow there is at a given location. But these traps are expensive-as much as $75,000 each. They are big and tricky to deploy, making it impractical to put more than a few in the ocean at a time. And because marine snowfall is highly variable at different times and places, it is difficult to generalize the information collected by one trap at one location to a larger region of the ocean.

What we need, Buesseler said, are more eyes in the ocean-small, inexpensive devices that can be deployed by the dozens at sites throughout the ocean twilight zone to take pictures of falling marine snow.

“That would tell us how many marine snow particles there are, what their sizes are, and how fast they’re moving,” Buesseler said.

University of Rhode Island physical oceanographer Melissa Omand was a postdoctoral student at WHOI when she first had the idea of putting a camera inside a trap to photograph particles as they were being collected.

“That gave you a time record of when the particles landed, and you could see them accumulating,” Omand said.

That idea led to another. Eventually, Omand, Buesseler, and a team from WHOI, the University of Rhode Island, the Monterey Bay Aquarium Research Institute, MIT, and NASA came up with the design for a new device that would take advantage of the low cost of miniature camera and sensor technology. They called it a MINION, short for MINiature IsOpycNal floats.

A MINION is a small, inexpensive, neutrally buoyant float. Each MINION can be carefully ballasted to sink to a specific depth in the ocean, where it will drift along with the currents, taking pictures of marine  particles as they fall past the MINION and through the twilight zone into the deep ocean.

“We’re thinking that it will be about the size of a 2-liter soda bottle, and maybe a little bit longer and skinnier,” Omand said.

Omand is planning to make the device out of thick, pressure-resistant glass. The top end will be flat, with a camera looking up through the glass to photograph marine snow as it settles on the imaging surface. A pair of stereo cameras will look out through one side, tracking particles’ sinking rates. Sensors on the MINION will measure seawater temperature and pressure and perhaps other properties.

Underwater, where GPS doesn’t work, the MINION will be tracked acoustically, using at least two drifting sound sources suspended from surface buoys. Far below the surface, the sound sources will emit signals at regular intervals. Hydrophones on the MINIONs will detect and precisely record the arrival times of those signals, allowing Omand to use speed of sound calculations to predict the underwater trajectory of each MINION.

After a few days to weeks, a small drop weight will be released from the MINION, allowing it to rise to the surface where it will transmit its position via GPS so that scientists can locate and retrieve it. Ultimately, the team plans to design an expendable version of the MINION that will transmit its data via satellite and remain at sea, allowing longer and more numerous deployments. Omand is designing the MINION with sustainability in mind-making its housing from glass is an environmentally friendly alternative to adding more plastic debris to the ocean. Omand, Buesseler, and their team expect to have their first prototype MINION in the water by 2019.

Buesseler thinks the MINION could be a game-changing technology. He envisions fleets of them tracking marine snow throughout the twilight zone within the next five years, providing critical measurements in the race to understand and mitigate our changing global climate.

Funding for the development of MINIONs came from the National Academies Keck Futures Initiative and the National Science Foundation-funded National Oceanographic Partnership Program.