The computer model showed that during El Nino periods, warm waters from the Western Pacific Ocean spread out over much of the ocean basin as upwelling weakens in the Eastern Pacific Ocean.

Upwelling brings cool, nutrient-rich water from the deep ocean up to the surface. When the upwelling is weakened, there are less phytoplankton, making food more scarce for zooplankton that eat the ocean plants.

During La Nina conditions as in 1998, the opposite effect occurs as the easterly trade winds pick up and upwelling intensifies bringing nutrients like iron to the surface waters, which increases phytoplankton growth.

Sometimes, the growth can take place quickly, developing into what scientists call phytoplankton "blooms."

In a study published in the January 2005 issue of Geophysical Research Letters, Wendy Wang and colleagues at the University of Maryland Earth System Science Interdisciplinary Center, College Park, Md., found that changes in phytoplankton amounts due to El Nino and La Nina not only affect the food chain, but also influence Earth's climate.

As phytoplankton flourish during La Nina years, a large amount of carbon is used to build their cells during photosynthesis. The plants get carbon from carbon dioxide in surface waters. In the atmosphere, carbon dioxide is an important greenhouse gas. When marine organisms die, they carry carbon in their cells to the deep ocean.

Surprisingly, this study found that this transfer of carbon to the deep ocean increased by a factor of eight due to the large phytoplankton blooms that can occur during a La Nina. At the same time, the effects of El Ninos can reduce phytoplankton numbers, and decrease the impacts of this "biological carbon pump."

Using a computer model and NASA's Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite, Wang examined marine biological changes associated with El Nino and La Nina, and found the mechanisms responsible for such phytoplankton blooms.

SeaWiFS measures the amount of light coming out of the ocean at different wavelengths on the spectrum, and can determine the strength of the greenness coming from the tiny plants' cells.

When the El Nino of 1997-1998 became a La Nina beginning in mid-1998, SeaWiFS imagery showed extremely dark greenness along the equator. "[At that time SeaWifs showed] chlorophyll concentrations increasing by more than 500 percent, a level not previously observed," said Wang.

The study found that because most microscopic animals called zooplankton died off during the El Nino there were less around to eat phytoplankton. That led to large phytoplankton blooms.

Besides influencing the marine food web, phytoplankton also help regulate the Earth's climate by accounting for about half of the carbon dioxide, a major greenhouse gas, absorbed annually from the atmosphere by plants.

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