Ocean Plant Life Slows Down and Absorbs Less Carbon

David E. Steitz
Headquarters, Washington
(Phone: 202/358-1730)               September 16, 2003

Krishna Ramanujan       
Goddard Space Flight Center (GSFC), Greenbelt, Md
(Phone: 301/286-3026)

Kent LaBorde
NOAA, Washington
(Phone: 202/482-5757)

RELEASE: 03-285

OCEAN PLANT LIFE SLOWS DOWN AND ABSORBS LESS CARBON

     Plant life in the world's oceans has become less 
productive since the early 1980s, absorbing less carbon, 
which may in turn impact the Earth's carbon cycle, according 
to a study that combines NASA satellite data with NOAA 
surface observations of marine plants.

Microscopic ocean plants called phytoplankton account for 
about half the transfer of carbon dioxide (CO2) from the 
environment into plant cells by photosynthesis. Land plants 
pull in the other half. In the atmosphere, CO2 is a heat-
trapping greenhouse gas.

Watson Gregg, a NASA GSFC researcher and lead author of the 
study, finds that the oceans' net primary productivity (NPP) 
has declined more than 6 percent globally over the last two 
decades, possibly as a result of climatic changes. NPP is the 
rate at which plant cells take in CO2 during photosynthesis 
from sunlight, using the carbon for growth. The NASA funded 
study appears in a recent issue of Geophysical Research 
Letters.

"This research shows ocean primary productivity is declining, 
and it may be a result of climate changes such as increased 
temperatures and decreased iron deposition into parts of the 
oceans. This has major implications for the global carbon 
cycle," Gregg said. Iron from trans-continental dust clouds 
is an important nutrient for phytoplankton, and when lacking 
can keep populations from growing.

Gregg and colleagues used two datasets from NASA satellites: 
one from the Coastal Zone Color Scanner aboard NASA's Nimbus-
7 satellite (1979-1986); and another from Sea-viewing Wide 
Field-of-view Sensor data on the OrbView-2 satellite (1997-
2002).

The satellites monitor the green pigment in plants, or 
chlorophyll, which leads to estimates of phytoplankton 
amounts. The older data was reanalyzed to conform to modern 
standards, which helped make the two data records consistent 
with each other. The sets were blended with surface data from 
NOAA research vessels and buoys to reduce errors in the 
satellite records and to create an improved estimate of NPP.

The authors found nearly 70 percent of the NPP global decline 
per decade occurred in the high latitudes (above 30 degrees). 
In the North Pacific and North Atlantic basins, phytoplankton 
bloom rapidly in high concentrations in spring, leading to 
shorter, more intense lifecycles. In these areas, plankton 
quickly dies and can sink to the ocean floor, creating a 
potential pathway of carbon from the atmosphere into the deep 
ocean.

In the high latitudes, rates of plankton growth declined by 7 
percent in the North Atlantic basin, 9 percent in the North 
Pacific basin, and 10 percent in the Antarctic basin when 
comparing the 1980s dataset with the late 1990s observations.

The decline in global ocean NPP corresponds with an increase 
in global sea surface temperatures of 0.36 degrees Fahrenheit 
(F) (0.2 degrees Celsius (C)) over the last 20 years. Warmer 
water creates more distinct ocean layers and limits mixing of 
deeper nutrient-rich cooler water with warmer surface water. 
The lack of rising nutrients keeps phytoplankton growth in 
check at the surface. 

The North Atlantic and North Pacific experienced major 
increases in sea surface temperatures: 0.7 degrees C (1.26 F) 
and 0.4 degrees C (0.72 F) respectively. In the Antarctic, 
there was less warming, but lower NPP was associated with 
increased surface winds. These winds caused plankton to mix 
downward, cutting exposure to sunlight. 

Also, the amount of iron deposited from desert dust clouds 
into the global oceans decreased by 25 percent over two 
decades. These dust clouds blow across the oceans. Reductions 
in NPP in the South Pacific were associated with a 35 percent 
decline in atmospheric iron deposition. 

"These results illustrate the complexities of climate change, 
since there may be one or more processes, such as changes in 
temperature and the intensity of winds, influencing how much 
carbon dioxide is taken up by photosynthesis in the oceans," 
said co-author Margarita Conkright, a scientist at NOAA's 
National Oceanographic Data Center, Silver Spring, Md.

Other recent NASA findings have shown land cover on Earth has 
actually been greening. For information and images on the 
Internet, visit:

http://www.nasa.gov/home/hqnews/2003/jun/HQ_03182_green_garde
n.html
&
http://www.gsfc.nasa.gov/topstory/2003/0815oceancarbon.html

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