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May
28, 2009: A unique signal detected by NASA's Aqua
satellite is helping researchers check the health and productivity
of ocean plants around the world.
Fluorescent
red light emitted by ocean phytoplankton and detected by Aqua
reveals how efficiently the microscopic plants are turning
sunlight and nutrients into food through photosynthesis.
"This
is the first direct measurement of the health of the phytoplankton
in the ocean," says Michael Behrenfeld, a biologist at
Oregon State University who specializes in marine plants.
"We now have an important new tool for observing changes
in phytoplankton all over the planet."
Above:
Phytoplankton -- such as this colony of chaetoceros socialis
-- naturally give off fluorescent light as they dissipate
excess solar energy that they cannot consume through photosynthesis.
Credit: Maria Vernet, Scripps Institution of Oceanography
The
findings were published this month in the journal Biogeosciences
and presented at a news briefing on May 28th.
Single-celled
phytoplankton fuel nearly all ocean ecosystems, serving as
the most basic food source for marine animals from zooplankton
to fish to shellfish. In fact, phytoplankton account for half
of all photosynthetic activity on Earth. The health of these
marine plants affects commercial fisheries, the amount of
carbon dioxide the ocean can absorb, and how the ocean responds
to climate change.
Over
the past two decades, scientists have employed various satellite
sensors to measure the amount and distribution of the green
pigment chlorophyll, an indicator of the amount of plant life
in the ocean. But with the Moderate Resolution Imaging Spectroradiometer
(MODIS) on NASA's Aqua satellite, scientists have now observed
"red-light fluorescence" over the open ocean.
"Chlorophyll
gives us a picture of how much phytoplankton is present,"
says Scott Doney, a marine chemist from the Woods Hole Oceanographic
Institution and a co-author of the paper. "Fluorescence
provides insight into how well they are functioning in the
ecosystem."
All
plants absorb energy from the sun, typically more than they
can consume through photosynthesis. The extra energy is mostly
released as heat, but a small fraction is re-emitted as fluorescent
light in red wavelengths. MODIS is the first instrument to
observe this signal on a global scale.
Above:
A global map of red fluorescent light emitted by phytoplankton.
Credit: Aqua/MODIS/Mike Behrenfeld, Oregon State University
[Larger
image]
Red-light
fluorescence reveals much about the physiology of marine plants
and the efficiency of photosynthesis, as different parts of
the plant's energy-harnessing machinery are activated based
on the amount of light and nutrients available.
For
example, the amount of fluorescence increases when phytoplankton
are under stress from a lack of iron, a critical nutrient
in seawater. The iron needed for plant growth reaches the
sea surface on winds blowing dust from deserts and other arid
areas, and from upwelling currents near river plumes and islands.
Fluorescence data from MODIS has allowed the research team
to study these dynamics.
The
Indian Ocean was a particular surprise, as large portions
of the ocean were seen to "light up" seasonally
with changes in monsoon winds. In the summer, fall, and winter
– particularly summer – significant southwesterly winds stir
up ocean currents and bring more nutrients up from the depths
for the phytoplankton. At the same time, the amount of iron-rich
dust delivered by winds is reduced.
Right:
A map of fluorescent light emitted by plankton in the Indian
Ocean, where seasonal monsoons can limit the amount of iron
nutrients in the water and stress the plankton to emit more
light. Credit: Aqua/MODIS/Mike Behrenfeld, Oregon State University
"On
time-scales of weeks to months, we can use these data to track
plankton responses to iron inputs from dust storms and the
transport of iron-rich water from islands and continents,"
says Doney. "Over years to decades, we can also detect
long-term trends in climate change and other human perturbations
to the ocean."
Climate
change could mean stronger winds pick up more dust and blow
it to sea, or less intense winds leaving waters dust-free.
Some regions will become drier and others wetter, changing
the regions where dusty soils accumulate and get swept up
into the air. Phytoplankton will reflect and react to these
global changes.
"NASA
satellites are powerful tools," says Behrenfeld. "Huge
portions of the ocean remain largely unsampled, so the satellite
view is critical to seeing the big picture."
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Editor: Dr.
Tony Phillips | Credit: Science@NASA
more
information |
Credits:
The
research was funded by NASA and involved collaborators
from the University of Maine, the University of California-Santa
Barbara, the University of Southern Mississippi, NASA’s
Goddard Space Flight Center, the Woods Hole Oceanographic
Institution, Cornell University, and the University
of California-Irvine.
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Future: US
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