Since the early 1980s, ocean phytoplankton
concentrations that drive the marine food chain have declined
substantially in many areas of open water in Northern oceans,
according to a comparison of two datasets taken from
satellites. At the same time, phytoplankton levels in open
water areas near the equator have increased significantly.
Since phytoplankton are especially concentrated in the North,
the study found an overall annual decrease in phytoplankton

The authors of the study, Watson Gregg, of NASA’s Goddard
Space Flight Center, Greenbelt, Md., and Margarita Conkright,
a scientist at the National Oceanic and Atmospheric
Administration’s (NOAA) National Oceanographic Data Center,
Silver Spring, Md., also discovered what appears to be an
association between more recent regional climate changes,
such as higher sea surface temperatures and reductions in
surface winds, and areas where phytoplankton levels have

Phytoplankton consist of many diverse species of microscopic
free-floating marine plants that serve as food to other
ocean-living forms of life. “The whole marine food chain
depends on the health and productivity of the phytoplankton,”
Gregg said.

The researchers compared two sets of satellite data — one
from 1979 to 1986 and the other from 1997 to 2000 — that
measured global ocean chlorophyll, the green pigment in
plants that absorbs the Sun’s rays for energy during
photosynthesis. The earlier dataset came from the Coastal
Zone Color Scanner (CZCS) aboard NASA’s Nimbus-7 satellite,
while the latter dataset was from the Sea-Viewing Wide Field
of View Sensor (SeaWiFS) on the OrbView-2 satellite.

The researchers re-analyzed the CZCS data with the same
processing methods used for the SeaWiFS data, and then
blended both satellite measurements with surface observations
of chlorophyll from ocean buoys and research vessels over
corresponding time periods. By doing so, the researchers
reduced errors and made the two records compatible.

Results indicated that phytoplankton in the North Pacific
Ocean dropped by over 30 percent during summer from the mid-
80s to the present. Phytoplankton fell by 14 percent in the
North Atlantic Ocean over the same time period.

Also, summer plankton concentrations rose by over 50 percent
in both the Northern Indian and the Equatorial Atlantic
Oceans since the mid-80s. Large areas of the Indian Ocean
showed substantial increases during all four seasons.

“This is the first time that we are really talking about the
ocean chlorophyll and showing that the ocean’s biology is
changing, possibly as a result of climate change,” said
Conkright. The researchers add that it remains unclear
whether the changes are due to a longer-term climate change
or a shorter-term ocean cycle.

Phytoplankton thrive when sunlight is optimal and nutrients
from lower layers of the ocean get mixed up to the surface.
Higher sea surface temperatures can reduce the availability
of nutrients by creating a warmer surface layer of water. A
warmer ocean surface layer reduces mixing with cooler, deeper
nutrient-rich waters. Throughout the year, winds can stir up
surface waters, and create upwelling of nutrients from below,
which also add to blooms. A reduction in winds can also limit
the availability of nutrients.

For example, in the North Pacific, summer sea surface
temperatures were .4 degrees Celsius (.7 Fahrenheit) warmer
from the early 1980s to 2000, and average spring wind
stresses on the ocean decreased by about 8 percent, which may
have caused the declines in summer plankton levels in that

Phytoplankton currently account for half the transfer of
carbon dioxide from the atmosphere back into the biosphere by
photosynthesis, a process in which plants absorb carbon
dioxide (CO2) from the air for growth. Since carbon dioxide
acts as a heat-trapping gas in the atmosphere, the role
phytoplankton play in removing carbon dioxide from the
atmosphere helps reduce the rate at which CO2 accumulates in
the atmosphere, and may help mitigate global warming.

The paper appears in the current issue of Geophysical
Research Letters.

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