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By Peter Rejcek
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.
The glider
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.
Glider in the Antarctic
t 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 sensors
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.
The mission
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.”
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