Below the surface
Glider fleet will survey water around Antarctic Peninsula
Posted May 2, 2008
Oscar Schofield pauses the phone conversation for a moment, as he pulls up a Web page that tracks the location of two underwater gliders operated by Rutgers University’s Coastal Ocean Observation Lab.
He reports to his caller that one of the sleek robots has covered more than 2,000 kilometers since it left the New Jersey coast on March 7. It has made some 1,500 profiles of the water column, as it slowly bobs up and down in a sawtooth pattern below the surface, measuring physical ocean properties like salinity and temperature.
An associate professor at Rutgers whose research interests focus on phytoplankton, Schofield envisions a small fleet of these autonomous robots, known as Slocum gliders, swimming and prowling the cold waters off the Antarctic Peninsula to collect information about the rapidly changing marine ecosystem there.
“It’s just another tool, but it’s a real powerful tool, for filling in the picture” of climate change, said Schofield, now a principal investigator with the Palmer Long Term Ecological Research (PAL LTER) program.
The PAL LTER is a multi-disciplinary program to study the West Antarctic marine ecosystem over a long time scale, as typical National Science Foundation (NSF) grants last three to five years. The NSF established the PAL LTER, part of a larger network of LTER sites, mostly in and around the United States, in 1990.
In the last 18 years, the PAL LTER scientists have watched a warmer, moister climate migrate into the area around the U.S. Antarctic Program’s Palmer Station. This new subantarctic climate has driven the local Adélie penguin population to near extinction, and the researchers believe the upwelling of warmer ocean water plays a key role in accelerating melting along the icy edges of West Antarctica.
Much of the data collection over the last two decades has been from a science vessel during the month of January, the middle of the Antarctic summer. Now, with the Rutgers gliders, the researchers will cover more of the marine environment than ever before — and in new ways and at different times of the year.
The glider proved its worth during a test run in 2007, confirming what the PAL LTER researchers had suspected: that warm, deep ocean water was coming on to the continental shelf, bringing an enormous amount of heat to bear against the peninsula’s ice sheet and ice shelves.
“Just from throwing one glider out for three weeks, we got this tremendous new knowledge that there’s circulation all over the shelf of our region,” said Hugh Ducklow, the lead investigator for the PAL LTER.
Conceived by former Woods Hole Oceanographic Institution engineer Douglas Webb and named after the first man to sail solo around the world, the Slocum glider looks like a torpedo with wings.
It moves through the water by varying its buoyancy, sucking in about a coffee cup worth of water through its nose to dive and then expelling water to float back up, running horizontally in a sawtooth pattern. The relatively low energy requirements of the buoyancy pump allow most of the battery power to be devoted to scientific sensors.
Schofield said the latest robots, with extended bodies to carry additional batteries or instruments, can “fly” through the water for more than a month at a time, covering incredible distances, such as the glider winging its way toward Canada — 2,000 kilometers and counting.
“That’s enough distance that we can really start thinking about these things being networks,” Schofield said. He noted that the PAL LTER cruises have collected about 2,400 vertical profiles of the ocean over the last 18 years, one of the world’s most extensive regional marine records.
“That’s a huge amount of work,” he noted, but added, “The glider that flew last January [in 2007 on the LTER cruise] was a single system … it collected 1,200 profiles off of one battery pack.”
The glider remains submerged for several hours before surfacing to phone home through the iridium satellite system. It relays their data back to the Coastal Ocean Observation Lab, checks its e-mail for any new instructions, and then dives underwater again toward the next waypoint on its mission.
Each glider weighs about 50 kilograms, and can easily be launched and retrieved from a small boat like a Zodiac. Scientists at the Coastal Ocean Observation Lab can track its movements and send commands remotely.
The central compartment of each robot contains various sensors, which measure properties such as water temperature and salinity. However, thanks to a modular design that allows the scientists to swap out instruments, and advances in miniaturizing sensors, some of the gliders can do a lot more.
One robot next season already carries optical sensors that can “see” the color of the water, which scientists can use to determine the composition of phytoplankton encountered by the glider. Other sensors can measure fluorescence, which can tell the researchers something about the health and biomass of phytoplankton. Another set of instruments will record acoustics, such as the noises whales make underwater.
“We’re going to build up a fleet of these robots to carry sensors to do the physics, the phytoplankton abundance and health and type, and the acoustics for going after the krill, the whales and anything that makes noises,” Schofield said.
Phytoplankton refers to a conglomeration of free-floating, autotrophic organisms in the ocean that you can’t see individually with the naked eye but appear as a green coloration because of the presence of chlorophyll. (The color may vary depending on the composition of the phytoplankton bloom.)
They are important to local and global ecosystems for a number of reasons. Through photosynthesis, they cycle carbon dioxide out of the atmosphere and account for as much as 50 percent of the oxygen. They also form the basis of the ocean food web, serving as vittles for creatures both large and small — from whales to shrimp-like krill.
Phytoplankton is but one component of the PAL LTER. Other researchers are interested in ocean microbes, physical ocean properties such as circulation, ice dynamics and various seabirds and mammals. The researchers say sea ice is the linchpin in the system, affecting ocean productivity from the phytoplankton and on up.
For example, sea ice serves as a sort of krill grazing ground. In turn, Adélie penguins rely on krill as their main food staple. But warming temperatures in the region have caused the perennial sea ice to disappear and the annual sea to shrink in duration, causing a ripple through the small food web. (The recent disintegration of a chunk of the Wilkins Ice Shelf may also be due to the absence of sea ice, which buffers ice shelves from the motion of ocean waves.)
But the scientists don’t have all the answers yet. They need more information about the ocean, such as that belt of warm water flushing onto the continental shelf and causing the glaciers to melt more rapidly into the ocean. And what’s all that fresh water doing to ocean mixing and nutrient availability?
Some of the gaps in their knowledge may come from those little torpedo-shaped robots.
“Those gliders will increase by orders of magnitude our ability to cover the regions spatially and temporally and very intensively,” noted Bill Fraser, a PAL LTER research who studies the Adélies and other Antarctic seabirds.
For Schofield, the Slocum glider means that more information will be available to a wider audience, proving the cliché that two heads are better than one for solving a problem.
“Regardless if [scientists] can get ship time, we can get them robot time,” Schofield said. “Having a well-sampled ocean will allow us to answer our questions like we never could before.”
NSF-funded research in this story: Oscar Schofield, Coastal Ocean Observation Lab at Rutgers University; Hugh Ducklow, The Ecosystems Center at the Marine Biological Laboratory; and Bill Fraser, Polar Oceans Research Group.
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