Sea Changes
by Paul Karr
All around the Earth's oceans, countless billions of tiny phytoplankton
are going about their business, gobbling up nitrogen.
Normally, this might not be considered such a big deal. But now the appetites
of these microscopic plants may yield an important clue in the case of global
warming, said Deborah Bronk, a UGA assistant professor of marine sciences.
Once marine scientists figure out how quickly these phytoplankton grow in different
parts of the ocean, it will be possible to estimate just how much carbon dioxide
it takes to decompose these organisms, she said. All that can add up to influence
models of global warming.
Bronk has devoted her career to puzzling over the nitrogen cycles of the world's
oceans. From Maryland to Antarctica and points in between, she studies the
tiny changes that add up to big ones.
Taking samples from the mouths of three Georgia rivers, for instance, Bronk
examines the little-studied fate of organic nitrogen once it hits the Atlantic.
Most of the nitrogen pouring into the state's swampy coast comes from natural
sources like decomposing marsh grass and marsh creatures. That's in sharp contrast
to more developed areas of the East, where coastal waters are inundated with
huge quantities of inorganic nitrogen, such as fertilizers, sewage and other
runoff.
The inorganic nitrogen from development, Bronk said, is easier for the ocean's
phytoplankton to feed on - too easy, in some cases. When plankton feed and
multiply much too quickly, they can trigger a chain of events: Huge quantities
of these ocean plants can die almost simultaneously and sink to the bottom
of the sea, where bacteria rapidly exhaust the waterÕs oxygen supply
while decomposing the phytoplankton.
But scientists have fewer research results from which they can determine the
importance of organic nitrogen, which also is produced naturally by plankton
and is still abundant along Georgia's coast.
"This is important to study because Georgia is one of the most pristine
areas of the Atlantic coast," said Bronk, whose research is funded by the
National Science Foundation, the Department of Energy and the National Oceanic
and Atmospheric Administration through the Georgia Sea Grant College Program.
To determine nitrogen's importance, she developed a technique using a special "tracer" form
of nitrogen called 15N. Her undergraduate students grow spartina saltgrass
with the 15N, then chop it into bits and mix it with samples of sea water.
Later, Bronk examines the water's microscopic phytoplankton using a mass spectrometer,
which reveals the chemical composition of a substance and can easily find the
special 15N "fingerprint."
In this research, Bronk is interested in how (and if) they use humic and fulvic
acids, the nitrogenous substances that color bog and marsh water the color
of tea.
"We already know that bacteria use nitrogen," Bronk said. "But
can phytoplankton use it?"
In Antarctica, her work as a principal investigator studying nitrogen cycling
is part of the Joint Global Ocean Flux Study, a comprehensive effort to study
four of the world's major oceans. It's her one small piece to contribute to
scientists' understanding of the atmospheric carbon cycle.
Bronk's research team will make a total of four long icebreaker cruises to
the distant Ross Sea, a body of water lying just off continental Antarctica.
Her visit last fall on one of the cruises - early spring in the Southern Hemisphere
- employed a computerized sampling device, called a rosette, to collect sea
water. Phytoplankton collected on those cruises are now in her Athens laboratory,
being tested for nitrogen uptake.
For more information, access http://alpha.marsci.uga.edu,
or e-mail Deborah Bronk at dbronk@uga.cc.uga.edu.
