Aspiring for knowledge
New project seeks to learn more about biologically active polynya in remote region
Posted December 3, 2010
In a region famously isolated from the rest of the planet, there are plenty of hard-to-reach places in the Antarctic — crevassed and thinning ice shelves or glaciers squeezing through mountain peaks.
And then there are the really remote places to visit — like the Amundsen Sea polynya.
A roughly 38,000-square-kilometer area of ocean initially blown free of sea ice by katabatic winds rushing down from the Antarctic continent, the Amundsen Sea polynya hugs the west coast near the Dotson Ice Shelf. It’s far from any research station or ship route for the few vessels that might venture to that part of the Southern Ocean.
“It’s big enough that when you’re in the middle of it you think you’re in the North Atlantic. It’s grey, overcast, windy, open water. You can’t see any of the ice; you can’t see the ice shelf,” said Patricia Yager, chief scientist for a ship-based expedition to the region in December called the Amundsen Sea Polynya International Research Expedition (ASPIRE).
But it’s not the remoteness or the size of the polynya, the fourth largest in the Antarctic, that’s drawing Yager and her colleagues to the Amundsen Sea. It turns out that this particular polynya is a biological oasis, boasting the highest primary productivity of any similar region.
A polynya is a seasonally recurring pool in the sea ice caused by either winds (latent heat polynya) or ocean heat from below (sensible heat polynya). The absence of ice fosters a strong exchange of heat, moisture, and gases between the ocean and air, which has implications for global ocean circulation and carbon cycling.
It’s also an area of intense phytoplankton blooms. Phytoplankton are mostly microscopic plant cells in the upper water column. Like land plants, phytoplankton have chlorophyll to capture sunlight, using photosynthesis to turn it into chemical energy. They consume carbon dioxide and release oxygen, accounting for about half of the O2 produced by plants in the world.
“[The Amundsen Sea polynya] is a very dynamic place that we don’t know much about,” said Yager, associate professor of marine sciences at the University of Georgia.
A few measurements were made in 2007, when the Swedish icebreaker Oden crossed the region en route to McMurdo Station on Ross Island to open a channel in the sea ice to the largest base in the U.S. Antarctic Program (USAP).
Now American and Swedish scientists will return to the polynya aboard the USAP research vessel Nathaniel B. Palmer, a vessel better suited to the sort of work the researchers want to conduct. The Oden will accompany the Palmer to help it cut through the sea ice that surrounds the polynya.
Yager said the research team hopes to spend about three weeks in the open waters of the Amundsen Sea polynya to collect data that will help them better understand what’s happening to this climate-sensitive ecosystem.
The first and foremost question the scientists want to answer: What is driving the high productivity, which is about double that of other polynyas?
Phytoplankton growth depends on the availability of carbon dioxide, sunlight and nutrients. They also require trace amounts of iron, which caps their growth in the iron-limited Southern Ocean. Yet the chlorophyll concentrations in the Amundsen Sea polynya are so high that the greenish coloration from a bloom in the water can be seen from satellites in space.
One factor in the high productivity, as well as the wild swings in interannual variability, could be the nearby Pine Island and Thwaites glaciers just to the east, according to Kevin Arrigo, a researcher from Stanford University and co-principal investigator (PI) on ASPIRE whose team will perform some of the phytoplankton studies.
Pine Island and Thwaites are among the fastest moving glaciers in Antarctica, feeding ice into the Amundsen Sea. During a 2009 science cruise to the nearby Pine Island polynya, Arrigo and colleagues made a handful of chance measurements that implied the rivers of ice were dumping iron into the polynya.
“We found that there was so much iron being released that it actually draws down all of the other nutrients to zero, which is something you almost never see in the Antarctic,” Arrigo said. “The reason it’s important is because if you can drawn these [other nutrients] down to zero, it becomes a very important sink for CO2.”
Is the meltwater releasing nutrients, including trace metals like iron, into the near-shore environment? Or is the iron coming from somewhere else? Is it even the iron that’s driving the increased productivity?
To help answer those questions, the USAP recently acquired a new state-of-the-art trace metal system with a National Science Foundation (NSF) grant to Rob Sherrell at Rutgers University. Sherrell is a co-PI on ASPIRE.
The system helps ensure any trace metal samples collected by a conductivity, temperature and depth (CTD) sensor and rosette — a circular frame that holds canisters that collect water — haven’t been contaminated by the instrument itself. A thick, Kevlar-coated cable lowers the CTD from a trace-metal clean winch.
“This system makes it possible for us, and for other trace metal investigators, to sample seawater at any depth down to about 3,000 [meters] without contamination of the samples by the equipment, as would happen for at least some important metals if we used the ship’s conventional CTD,” Sherrell said by e-mail as the system was being installed in Punta Arenas.
“We’re excited to see the system work correctly, both mechanically, and in terms of returning clean samples. Iron is one of the most contamination-prone metals,” he added.
Another key goal of the project is to figure out what occurs post-bloom.
Phytoplankton are important to the carbon cycle because as they die, they fall from the surface to the deeper ocean, keeping that carbon from returning to the atmosphere. Carbon dioxide in the atmosphere is the key greenhouse gas in global warming.
“When that [carbon] sinks, it’s gone for a while,” Yager said. The timeframe in this case is at least centuries.
Scientists believe the polynya’s intense productivity will be matched by high level of carbon export to ocean depth. In addition to measuring the drawdown of CO2 at the surface, the ship will deploy a sediment trap attached to a mooring to capture the organic carbon particles as they settle from the surface to the bottom of the ocean. The mooring will sit on the ocean floor at about a depth of a thousand meters for a year.
Many of the measurements and techniques the scientists use will mirror those of the Palmer Long Term Ecological Research (PAL LTER) project, a nearly 20-year-long program to study the marine environment of the western Antarctic Peninsula.
For example, they will release a glider — a winged, torpedo-shaped robot that moves autonomously through the water — to take physical oceanographic measurements like temperature and salinity in the polynya. Oscar Schofield at Rutgers University, a co-PI on ASPIRE, first brought the instrument to the PAL LTER program about three years ago.
The western Antarctic Peninsula is one of the fastest warming regions on the planet, with average temperatures increasing nearly 3 degrees Celsius since the 1950s — and even more in the winter.
Additionally, the winter sea ice duration in an area stretching from the west Antarctic Peninsula to the eastern Amundsen Sea is in decline. Data between 1979 and 2006 show sea ice advancing later and retreating earlier. In the greater PAL LTER study area, the life of winter sea ice has shrunk by almost three months.
Citing previous studies published in the journal Nature, Sharon Stammerjohn said by e-mail, “These large trends toward shorter sea ice seasons are offshore of the coastal regions showing the largest ice mass losses from the Antarctic continent.
“Both sea ice and land ice appear to be responding to changes in atmospheric and ocean circulation,” added Stammerjohn, an assistant professor in Ocean Sciences at the University of California, Santa Cruz, who is a co-PI on both ASPIRE and the PAL LTER.
Stammerjohn said the ASPIRE team hopes to better identify the physical and chemical environmental factors in the polynya, like surface stratification and the presence of trace metals, that boost the region’s productivity. Once the researchers understand what’s happening, they can predict how changes in the future might influence the Amundsen Sea marine ecosystem.
“This is an important region because it’s where the West Antarctic Ice Sheet meets and interacts with the ocean,” noted Hugh Ducklow, the lead principal investigator for the PAL LTER program and co-PI for ASPIRE.
“ASPIRE, through comparison and coordinated study with LTER, will gauge the extent of change in the south,” said Ducklow, director of The Ecosystems Center at the Marine Biological Laboratory in Woods Hole, Mass. “This will give us a much better understanding of the extent of climate change continent-wide, and how it will proceed in the future.”
One concern is that as sea ice disappears, so will the polynyas and their highly productive carbons sinks.
“It’s possible that carbon fixation and storage will decrease — leading to more CO2 in the atmosphere, more warming, and so on — another positive feedback [loop]. Polynyas are on the front lines of global change,” Ducklow said.
NSF-funded research in this story: Patricia Yager, University of Georgia, Award No. 0839069; Oscar Schofield, Rutgers University, Award No. 0838995; Hugh Ducklow, Marine Biological Laboratory, Award No. 0839012; Robert Sherrell, Rutgers University, Award No. 0838995; Sharon Stammerjohn, University of California Santa Cruz, Award No. 0838975; and Kevin Arrigo, Stanford University, Award No. 0944727.
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