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 globally.

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 dropped.

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
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Contact: David E. Steitz
202-358-1730
NASA/Goddard Space Flight Center
8-Aug-2002

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