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November 16, 2009

Undersea Gliders May Help Oceanographers Understand "Dead Zones"

Versatile undersea gliders provide critical ocean data

The mystery began in 2002.

That's when west coast fishermen contacted Oregon State University (OSU), reporting that nearly 100 percent of the crabs that they were pulling up in their pots were dead. "So, clearly the crabs had gotten in. But then something had happened to kill them," says OSU oceanographer Jack Barth.

After ruling out any kind of toxin in the water, tests showed that oxygen levels were too low for the crabs to survive. Areas with low oxygen, or "dead zones," are well known among scientists. There are about 400 marine dead zones around the world, occurring when oxygen levels in seawater drop so low that most fish, crabs, and invertebrates cannot survive.

Most hypoxic or dead zones are caused by human activity, due to river runoff and an overabundance of nutrients from land that flow into oceans, gulfs or estuaries. The nutrients fuel excessive growth of phytoplankton. When these tiny plants die, their decay uses up the water's oxygen supply.

Not your typical dead zone

But the marine dead zones in the Pacific Northwest are different. While seasonal low-oxygen areas are a normal occurrence in deep, offshore waters, it is not typical to find them close to shore. And that's what the oceanographers observed in 2002, and have every year since.

"It's something new, and it's something unusual," says Barth. With National Science Foundation (NSF) funding, Barth is studying the dead zones that appear each summer off the Oregon and Washington coasts. "So in 2006, we actually went to zero oxygen," explains Barth. "And it extended many miles across the sea floor. That's when we partnered with the Oregon Department of Fish and Wildlife to send a camera down on a remotely operated vehicle. And we got these images of dead crabs everywhere."

For the past four years, Barth and his colleagues have used impressive new tools to monitor those unusual ocean conditions. They are robotic undersea gliders, also known as autonomous underwater vehicles (AUV). These gliders have no propellers or tethers. Simple changes in buoyancy make them move up and down, and simple wings allow them to go forward. They are deployed almost continuously from April to October.

The gliders are about seven feet long, weigh a little over 100 pounds, and are equipped with two computers, a GPS, and satellite phone. With all that technology, the gliders' power systems are pretty low tech. "There are 250 'C' cells inside the vehicle. Just your little 'C' cell battery, like from a flashlight. And it will run for about three weeks," says Barth.

Sensors on the glider measure water temperature, chlorophyll, salinity, and the dissolved oxygen of the water. "That's what tells us how that low oxygen zone, or hypoxic zone, looks," notes Barth.

He says that the near-shore dead zones of the Pacific Northwest may be impacted by climate change.

Climate change implicated?

"There are two ways in which climate change could be affecting the dead zones," explains Barth. "First, out in the open ocean, there are measurements that show that oxygen levels in deep waters are going down ever so slightly with time. This likely results because the surface layers of the ocean are warming and becoming less effective at mixing oxygen from the atmosphere down into the deeper water. So, when the deeper water finally arrives to shore, it's already depleted of oxygen."

The other possible global warming complication is wind.

"The winds off our coast are usually very different each day. Today, it may blow from the south, another day from the north, and that change actually flushes out low oxygen or any other kind of toxin. So if those winds are steadier in one direction, it won't provide that flushing," continues Barth.

He thinks the information gained from these gliders could help with other ocean research as well.

Getting to the bottom of things

Recently, Barth received additional NSF funding through the American Recovery and Reinvestment Act (ARRA) of 2009. "Recovery Act funds invested in long-term ocean observatories off the Pacific Northwest will allow us to keep our fingers on the pulse of changing marine ecosystems, especially oxygen levels in and around 'dead zones.'"

"One of the things we are trying to do with these robots is to map out what's on the sea floor, get beneath that very surface. You've got to know what the bottom looks like; and you'd better know something about what is a good fishery and what is not, and what is a hypoxic zone and what is not," Barth explains. "So it's kind of helping with that whole question of sustainable use of the ocean."

Besides sending back data 24/7 while they are deployed, the gliders are also "smart" enough to self-diagnose potential problems.

"I've been really impressed with the fail-safes, the sequence of steps the glider goes through to either try to recover from some kind of mishap, or basically just telling you about its health. It is able to diagnose its health and then tell us by sending back data through the cell phone," says Barth.

The gliders usually send a text message to onshore pilots every six hours. But the robots also expect to hear from their human colleagues. If there's no communication for 14 hours, the glider will head for the surface, transmit its GPS coordinates, and say, "I'm here, come get me!"

The research in this episode was funded by NSF through the American Recovery and Reinvestment Act of 2009.

Miles O'Brien, Science Nation Correspondent
Marsha Walton, Science Nation Producer

Any opinions, findings, conclusions or recommendations presented in this material are only those of the presenter grantee/researcher, author, or agency employee; and do not necessarily reflect the views of the National Science Foundation.