The 1997-98 El Nino/La Nina had an unprecedented roller-coaster effect on the oceanic food chain across a vast swath of the Pacific, plunging chlorophyll levels to the lowest ever recorded in December 1997 and spawning the largest bloom of microscopic algae ever seen in the region the following summer.


El Niño El Niño

According to new results published in the Dec. 10 issue of the journal Science, El Nino also dramatically reduced the amount of carbon dioxide normally released into the atmosphere by the equatorial Pacific Ocean.

Data from an array of instruments on buoys, ships and in space, including NASA's Sea-viewing Wide Field-of-View Sensor (SeaWiFS), gave researchers an unprecedented view into the extreme biological effects of this El Nino/La Nina event.

"With SeaWiFS in orbit, we were able to see for the first time not only the vast size and intensity of the ocean's biological rebound from El Nino, but also the unbelievable speed of that recovery," said Goddard Space Flight Center (Greenbelt, Md.) oceanographer Dr. Gene Feldman, a co-author of the study. SeaWiFS provides daily views of the world's oceans and land masses.

Over the past decade scientists have been able to observe the development and progression of El Nino warmings, and consequent changes in upwelling of nutrient-rich ocean waters, thanks to data continuously collected in the Pacific by the buoys of the National Oceanic and Atmospheric Administration's Tropical Atmosphere Ocean array.

In 1996 new biological and chemical sensors were added to some of these buoys by the Monterey Bay Aquarium Research Institute (MBARI), allowing researchers for the first time to directly and continuously monitor biological productivity and the concentration of carbon dioxide in the region. The launch of SeaWiFS in 1997 added yet another ocean-monitoring tool capable of detecting subtle changes in ocean color that are directly related to the concentration of chlorophyll, a prime indicator of biological activity in ocean waters. The largest reservoir of chlorophyll in the ocean is in the phytoplankton (a microscopic form of algae), which forms the base of the oceanic food chain.

"This is the first time we've ever had a set of biological measurements from moored instruments and satellites during an intense El Nino, and we've never seen such low chlorophyll concentrations," said MBARI biological oceanographer Francisco Chavez, lead author of the study.

It was the buoy measurements and SeaWiFS data that revealed surprisingly low and then high levels of chlorophyll coinciding with El Nino's strongest phase and the recovery period and transition to La Nina cooling. When the warm-water layer produced by El Nino extended to its greatest depths and the upwelling of nutrients necessary for phytoplankton growth virtually ceased, chlorophyll values plummeted.

The researchers were again surprised in mid-1998 when chlorophyll levels skyrocketed, revealing the largest phytoplankton bloom, in area, ever observed in the equatorial Pacific. In their published results, the researchers suggest that elevated iron concentrations stimulated this intense bloom, a result of the increased upwelling associated with La Nina.

El Nino also drastically reduced the amount of carbon dioxide this ocean region adds to the atmosphere. Unlike most parts of the world's oceans, the equatorial Pacific is normally a major contributor to atmospheric carbon dioxide due to the carbon-dioxide-rich deep ocean waters brought to the surface here and the relatively low levels of biological activity.

The researchers calculate that the amount of carbon dioxide released to the atmosphere by the equatorial Pacific during the year of El Nino conditions was 700 million metric tons of carbon less than the previous year. This is equivalent to half of the United States' total annual carbon dioxide emissions from fossil fuel burning.

Credit line for all images:
NASA/Goddard Space Flight Center
The SeaWiFS Project and ORBIMAGE
Scientific Visualization Studio

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