Satellite surveys have detected a sharp decline in plankton in several of the world's oceansa situation that could threaten the marine food chain and undercut one of the world's natural buffers to global warming.
The decline in free-floating, microscopic plants called phytoplankton varies from ocean to ocean, scientists reported. The greatest decline was in the Northern Pacific Ocean, where summer levels have dropped by more than 30 percent since the 1980s.
"It's difficult to say what the implications are, but they could be pretty significant," said Watson Gregg, of NASA's Goddard Space Flight Center. "The whole marine food chain depends on the health and productivity of phytoplankton."
Plankton are as important to the long-term health of the atmosphere as the world's forests. The photosynthesis of the ocean's tiny green plants account for about half of the carbon dioxide that plants remove from the atmosphere each year.
Increases in carbon dioxide and other gases produced by cars, factories, and agriculture have been blamed for a gradual increase in global temperatures.
Scientists disagree over how much human-made sources contribute to the trend, but a long-term decline in the natural recycling of carbon dioxide by plankton and other green plants could exacerbate the problem.
"The less phytoplankton you have, the less carbon is taken up by the oceans," said Margarita Conkright of the National Oceanic and Atmospheric Administration.
In a study reported in the August 8 issue of Geophysical Research Letters, the researchers compared sets of satellite data from early 1980 to the late 1990s. The data showed that the sharpest decreases in plankton were in the North Pacific and the North Atlantic, where their abundance decreased by 14 percent.
In equatorial regions plankton levels increased, but there was an overall global decline of more than 8 percent.
The researchers say they can't be sure whether the decline is part of a natural cycle in the oceans, a reflection of regional changes, or a result of a gradual warming of the globe that has been occurring since about 1980.
They did, however, find a close correlation between the decline in plankton and increasing ocean surface temperatures, which suggests that climate change could be a cause as well as an effect of plankton declines.