PHYTOPLANKTON FINDER: A team of researchers deployed two iRobot Seaglider Unmanned Underwater Vehicles (UUVs) in a Ross Sea polynya near Antarctica to study why phytoplankton blooms there disappear abruptly each January. Image: COURTESY OF VERNON ASPER, UNIVERSITY OF SOUTHERN MISSISSIPPI
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After decades of riding icebreakers in Antarctica's icy waters hoping to better understand the fragile ecosystem on and around this frigid continent scientists have begun delegating data collection to satellite-guided robotic subs. The hope is that these sea gliders, which can dive hundreds of meters and stay in the water for months at a time, will help to unlock the secrets of phytoplankton blooms that nourish the organisms in Antarctica's Ross Sea for a few months each year before mysteriously disappearing.
There are neither green plants in Antarctica nor macro-algae in the surrounding waters, says Vernon Asper, a marine science professor at the University of Southern Mississippi's (U.S.M.) Department of Marine Science at the NASA Stennis Space Center. "Essentially, everything that eats, lives and breathes in Antarctica is fed from phytoplankton in the ocean."
These blooms, which turn the sea from deep blue to lush green, begin sometime in November in a large polynya (an opening in the ice) in the Ross Sea when the liquid water is exposed to sunlight. The heat stratifies the sea surface in this roughly 160- by 320-kilometer area. (Its expanse varies from year to year.) This allows microscopic phytoplankton to flourish.
"In January, generally, the bloom is going great, the algae is growing like crazy, and then all of a sudden it quits," Asper says. "By mid-January the algae falls out of the water column and it gets clear again. We'd like to see what is different between mid-November and mid-January." Researchers have determined that there is no appreciable difference in water nutrients, sunlight, water temperature or stratification from the time the blooms begin in November to the time they disappear in January.
Researchers have been traveling to this polynya for decades, studying the phytoplankton during ship expeditions or collecting data from moorings placed in the water. The information has been incomplete because ice flows prevent ships from traveling to the area prior to late December (the beginning of southern summer). (They have been able to determine when the blooms begin with the aid of satellite images.) "It's hard to study the onset of the bloom," says Asper, who has been making the trip regularly since the early 1990s. "If you want to study the phytoplankton from a ship, it's like missing the first acts of a play."
Interest in the phytoplankton blooms began when ships traveling to the U.S. Antarctic Program's McMurdo Station would report on how green the water was. "We're interested in understanding the ecosystem around the Southern Ocean because it's an extremely productive place," Asper says. "Given that the entire ecosystem depends on the phytoplankton, we really want to nail down what they depend on. You could stretch that and say we're interested in this for a global significance, because we want to get baseline data to study global climate change, and we want to be able to monitor how things change."
Late last year, Asper and a diverse team of colleagues from U.S.M., the University of Washington in Seattle, Old Dominion University in Norfolk, Va., the Virginia Institute of Marine Sciences, and the U.K.'s University of East Anglia turned to robot subs to help get more comprehensive readings of the phytoplankton and surrounding water. During a trip funded by the National Science Foundation, the researchers dropped two Seaglider unmanned underwater vehicles (UUVs) built by Bedford, Mass.–based iRobot Corp. into the Ross Sea polynya. Seaglider is essentially a pointy-nosed, 1.8-meter-long yellow torpedo with two rear fins having a one-meter wingspan. The tetherless Seaglider drives through the water at a speed of about 1.8 kilometers per hour, driven by changes in buoyancy rather than a propeller system.*
The GPS-guided Seagliders at times operated beneath 100 meters of ice, although the researchers generally tried to avoid directing the subs below ice where they cannot get a satellite signal for prolonged periods of time. When they are out of GPS range the gliders use deduced reckoning (aka "dead" reckoning) to calculate their position and navigate using their last known coordinates until they can resurface and reestablish a GPS link. The deepest dives were about 500 meters and lasted no more than three hours at a time.
The scientists are still parsing the data collected by the gliders' sensors. A conductivity, temperature and depth (CTD) sensor was used to measure water temperature and salinity, which will help researchers calculate the its density and potential to mix with water flowing in from other locations. Researchers are hoping that the Seagliders' three optical sensors shed light on the water's biological properties, including the amount of oxygen and the presence of chlorophyll.
Asper and his colleagues were not the only researchers in the area using a UUV to study the phytoplankton blooms. A team led by Rutgers University oceanography professor Josh Kohut deployed an autonomous glider of its own design called the RU26 to track very small changes in the polynya's temperature and salinity. The RU26 was placed in the water December 11 and traversed 1,180 kilometers through the Ross Sea before it had to be removed on February 5 due to a mechanical problem.
Ocean scientists got their first taste of how gliders could help with their research last summer, following BP's Deepwater Horizon oil spill. iRobot provided Seagliders in May to help detect the presence of oil by measuring temperature, salinity and other ocean properties at depths of up to 1,000 meters.
As data from the Ross Sea is analyzed, Asper and his colleagues plan to build computerized models that help them visualize the phytoplankton blooms below the surface. Asper expects gliders and other autonomous subs are the future of oceanographic data collection. "They can work in conditions where ships can't," he says. "They don't care about hurricanes, they don't get tired or sick—they just work."
View a slide show of the Seagliders' deployment.
*Correction (2/25/11): This sentence was edited after posting. It originally referred to the maximum speed as 1.8 kilometers per second.