NASA and university scientists have made a breakthrough
in using satellites to study the tiny, free-floating ocean
plants, called phytoplankton. The plants form the base of the
ocean food chain and produce half of the oxygen in the air we

The development opens the door to solving a problem that has
stymied ocean biologists for more than a century, and is
revolutionary to our understanding of how ocean biology and
ecosystems, as well as carbon cycling, respond to climate
variability and change.

Data about the growth rate of the ocean plants can be derived
from space and incorporated into global estimates of their
life processes. New, accurate information on phytoplankton
will greatly advance understanding of marine ecosystems and
how they function, including issues related to fisheries,
water quality, and harmful algal blooms.

This research contributes to improved computer models that
enable predictions of how climate change will alter ocean
ecosystems and the Earth system. Despite their minute size,
the growth and photosynthesis of phytoplankton collectively
accounts for half of the carbon dioxide, a major greenhouse
gas, absorbed annually from Earth’s atmosphere by plants.

“While the full potential of this discovery awaits further
work, what is really amazing is that a signal detectable from
space has been found that tracks changes in the activity, not
just abundance, of phytoplankton,” said Michael Behrenfeld, a
professor at Oregon State University, Corvallis, Ore., and a
researcher at NASA’s Goddard Space Flight Center, Greenbelt,

In order to determine ocean productivity, which is the rate
of photosynthesis, scientists must know plant growth rates
and their abundance. Satellites can detect variations in the
color of light within the ocean, and researchers use this
information to tell phytoplankton amounts. The new method for
recording growth rates by satellite involves advances in the
way these satellite ocean data are analyzed.

“Satellite ocean color images are kind of like your
television screen, where you have controls for the color
setting and controls for brightness,” said researcher Dr.
David Siegel. “What we’ve done here is use both the color and
brightness signals to determine plant greenness and the
number of individual phytoplankton cells.”

With this new information, researchers can calculate growth
rates from the greenness of the individual phytoplankton
cells. When cold water temperatures, bright light, or low
nutrients put stress on phytoplankton, they lose pigment and
appear less green. The reverse is also true, phytoplankton
become greener when conditions improve and growth rates

To demonstrate the new approach, the research team used ocean
color data from the Sea-viewing Wide Field-of-view Sensor
(SeaWiFS). The data showed growth rates changed over seasons
and across ocean basins in precisely the manner expected from
years of laboratory studies on phytoplankton. Encouraged by
these findings, researchers applied their new data to
recalculate ocean production. The result was a significantly
different view of ocean photosynthesis previously revealed by
older models using the same satellite data.

The study appeared in the January 2005 electronic issue of
the journal Global Biogeochemical Cycles. The research was an
Editor’s Choice in the Feb. 4 issue of Science Magazine.
Coauthors include Dr. Emmanuel Boss of the University of
Maine, Orono; Dr. David Siegel, University of California,
Santa Barbara; and Donald Shea from Goddard.

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