The researchers found a surge in growth between 1997 and 1999, when the ocean underwent a cooling trend related to a strong shift between El Niño and La Niña events.
El Niño is a periodic warming of surface waters in the equatorial Pacific Ocean that influences global weather patterns. La Niña is a cooling of the equatorial Pacific waters.
Since 2000 the ocean surface temperatures have gradually warmed, and growth rates of the plankton have declined almost in lockstep, Behrenfeld noted.
"Projection into the future would suggest that as temperatures continue to warm, the climate will continue to suppress biology on a global basis," he said.
Behrenfeld explained that the link between climate and biology results from the way climate affects the stratification, or layering, of ocean waters.
As the ocean surface warms, it becomes lighter than the cold, dense water below it. The cold water is full of nutrients that phytoplankton need for growth. The increased layering essentially cuts off the phytoplankton from their food supply.
In addition to playing a prominent role in the food chain, Behrenfeld said, the single-celled plants are responsible for about half the photosynthesis on Earth.
(Read related story: "Source of Half Earth's Oxygen Gets Little Credit" [June 7, 2004].)
Photosynthesis is the process plants use to convert sunlight, water, and carbon dioxide into carbohydrates and oxygen.
Photosynthesis by phytoplankton absorbs about 50 billion tons of carbon dioxide—a greenhouse gas tied to global warming—each year, according to Behrenfeld.
"That has a profound influence on the carbon cycle on Earth," he said, referring to the process by which carbon, a key building block of life, is recycled in the environment.
(See a National Geographic magazine feature on threats posed to Earth's carbon cycle.)
According to Sarmiento, however, most of the carbon dioxide taken up by phytoplankton is absorbed from the water, not the atmosphere.
As a result, he said, the changes in the plankton's growth due to changes in the climate may not have much of an effect on the amount of carbon dioxide in the atmosphere.
"[But] the fact [that] biology is impacted is quite remarkable in and of itself," he added.
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