12/31/2015
In Antarctica, melting ice drives unusual phytoplankton growth - GeoSpace - AGU Blogosphere
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Home ­ 2015 Fall Meeting ­ In Antarctica, melting ice drives unusual phytoplankton growth
29 DECEMBER 2015
In Antarctica, melting ice drives unusual
phytoplankton growth
Posted by llipuma
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by Alison F. Takemura
In most of the Southern Ocean, phytoplankton – the base
of the marine food web – grow poorly because they’re
starved for iron. But in the Amundsen Sea on the west
coast of Antarctica, phytoplankton abound in summer. A
new study now shows the reason behind the sea’s
startling productivity: meltwater from an abutting ice shelf
flows into the sea, buoys iron to the surface and
jumpstarts phytoplankton growth.
Previous research has shown iron must be getting into
the upper sunlit waters of the Amundsen Sea, but until
now, scientists have not been able to figure out where
the iron comes from, said Patricia L. Yager, an
Phytoplankton bloom in the Antarctic Amundsen Sea.
Credit: Patricia L. Yager.
oceanographer at the University of Georgia in Athens
and coauthor of the study. Yager and her colleagues
wondered if the iron might be trickling in from the Dotson Ice Shelf, a part of the West Antarctic Ice Sheet that
extends out over the Amundsen Sea.
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To test that hypothesis, Yager and her colleagues sampled water from the surface to about 1 kilometer (0.6
miles) deep along a 50­kilometer (30­mile) transect following the ice shelf’s edge, as well as about 300
kilometers (200 miles) from the shelf into the sea. In thousands of water samples taken during the southern
hemisphere’s summer — December 2010 to January 2011— Yager’s team measured iron and chemical markers
of phytoplankton activity: chlorophyll, oxygen, nitrate, and nitrite. With these values, the team built a snapshot of
iron abundance and phytoplankton growth in the sea. They also tracked the movement of the meltwater
underneath the ice shelf. Yager presented the results of the study at the 2015 American Geophysical Union Fall
Meeting in San Francisco.
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The team found that a “flow of seawater equal to the
Amazon River,” rich in both iron and glacial meltwater,
gushes from underneath the ice shelf into the sea, said
Robert Sherrell, a marine biogeochemist at Rutgers
University and coauthor of the study.
Meltwater from the Dotson Ice Shelf, shown here, carries
up iron­rich deep water in the Amundsen Sea, new
research shows.
Credit: Patricia L. Yager.
But the meltwater itself doesn’t bring iron along, Yager
said. As the fresh melt mixes with the denser salt water
below, it forms a buoyant plume that moves iron­rich
deep water to the surface. The push drives up the
concentration of dissolved iron in the upper 50 meters
(160 feet) — where phytoplankton can thrive — to about
0.4 nanomoles per kilogram of water. The melting ice
shelf provides a continuous supply of iron that fuels the
sea’s verdant productivity, she said.
“As soon as the iron gets to the surface, [the
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12/31/2015
In Antarctica, melting ice drives unusual phytoplankton growth - GeoSpace - AGU Blogosphere
phytoplankton] suck it up right away,” Yager said.
In Antarctica, melting ice drives
unusual phytoplankton growth
As phytoplankton grow, they draw carbon dioxide from the atmosphere and sequester it in the ocean. According
to Yager, the high phytoplankton activity in the Amundsen is akin to planting a forest: the Amundsen is only a
quarter of a percent the size of the total Southern Ocean, yet accounts for two to three percent of the carbon
dioxide it absorbs, she said.
Climate change could continue to increase meltwater and iron availability, encouraging phytoplankton growth in
the short term, Yager said. But if unchecked, she continued, climate change could bring it to a halt by getting rid
of an equally important factor: sea ice. The springtime melting of seasonal sea ice helps the phytoplankton get
enough light by making a stable layer of water near the sunlit surface. If the ice disappears, the delicate layers of
the sea will blend together, and the wind will mix phytoplankton down to where they can no longer bloom, she
said.
Seesawing sea surface height
corresponds with global
temperatures, study finds
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If that happens, “the extra iron won’t help,” she said.
Yager’s research provides a tangible answer to how ice sheet melting influences biological systems, said Shanlin
Wang, a climate modeler at Los Alamos National Laboratory in New Mexico who was not involved in the study.
Wang studies how the melting of sea and land ice affects nutrients available to marine microorganisms and
hopes to incorporate Yager’s data into her computational models.
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“We need this kind of work in modeling, to make sure we get the processes correct,” Wang said.
BLOGROLL
– Alison F. Takemura is a science communication graduate student at UC Santa Cruz. You can follow her
on twitter at @AlisonTakemura.
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