Now scientists have evaluated the global-scale impacts of the most recent cycle on the growth of land and ocean plants. The researchers found that over a three-year period that started during El Niño conditions and ended in La Niña, global ocean productivity increased by nearly 10%.

In contrast, plant growth on land did not change consistently during this period. The investigators, including Christopher Field of the Carnegie Institution of Washington's Department of Plant Biology at Stanford, and lead author Michael Behrenfeld at NASA Goddard, report their findings in today's issue of Science.

According to Field, this study makes it clear that the ecology of the global oceans "reverberates with the beat of El Niño."

Plants provide the food for nearly all life on Earth. Whether they are giant trees of tropical forests or tiny phytoplankton of the open oceans, all plants harvest energy from the Sun and convert it into growth through photosynthesis.

The amount of growth is known as the net primary production (NPP). The more NPP there is, the more food is available for consumption by animals, including humans.

The 1997 launch of the satellite SeaWiFS allowed scientists for the first time to simultaneously measure the absorption of sunlight by land and ocean plants, a key factor that determines NPP.

The data were collected over the entire globe between September 1997 and August 2000. This period includes the last part of the strong El Niño of 1997-1998, as well as the onset of La Niña that continues to the present.

From their measurements, the investigators estimated the total NPP using an approach that integrates two production models: the Carnegie-Ames-Stanford model for land, and the Vertically Generalized Production Model for the oceans.

Both models calculate NPP based on a combination of light availability, the fraction of sunlight that is harvested, and the efficiency of photosynthesis and growth. This efficiency is sensitive to a number of environmental stresses.

The results showed large seasonal and regional variations. On land, increased NPP in some places more-or-less cancelled decreased NPP in others.

In the oceans, however, NPP was substantially lower during the El Niño than the La Niña conditions. The difference is about 10% of ocean NPP or about 5 billion tons of carbon per year.

The increased productivity during La Niña appears to reflect increased availability of nutrients in several parts of the ocean, including the equatorial Pacific, the South Atlantic and the Indian Ocean.

La Niña changes ocean circulation in these places in a way that brings more nutrients to the surface, where they can be used by phytoplankton.

The SeaWiFS data also revealed a striking difference between NPP in the northern and southern oceans. In the northern oceans, a strong summer bloom leads to very high productivity from 50-70 degrees North, or about the latitude range from the southern tip of England to the northern end of Finland. A comparable peak is absent from the southern oceans, probably because productivity is limited by nutrients, especially iron.

The El Niño/La Niña cycle has massive implications for agriculture and natural disasters in many parts of the world. Until the data from SeaWiFS became available, it was impossible to accurately assess the impacts for ocean production.

Does the cycle make a difference?

"You bet it does," says Field. "Ocean processes ranging from fish production to carbon storage are potentially affected by NPP. Now that we see the global patterns, we can probe more effectively for underlying mechanisms."

"This is an exciting period of exploration in the field of earth systems science," says Michael Behrenfeld. "The SeaWiFS record marks the beginning of what we hope will be a long-term monitoring program of changes in the photosynthetic biosphere. It represents a milestone against which future measurements can be compared. Even in its first three years of operation, SeaWiFS has provided a remarkable data set that has yielded our first truly global view of seasonal variability of ocean NPP. Together with its follow-on missions, the SeaWiFS record should eventually allow detection of longer term trends in biospheric NPP from normal seasonal and interannual oscillations."

SeaWiFS is one of a number of recently-launched satellite instruments that open new doors in global ecology. With the quality and coverage of the SeaWiFS data, scientists really can study the whole planet as if it were in the backyard.

According to Field, "We were blown away at the magnitude of the NPP effect in the ocean. But I have a feeling that is not the last surprise we will see from the latest earth-observation satellites."

[Contact: Chris Field, Michael Behrenfeld, Tina McDowell]

30-Mar-2001