MOSS LANDING, California-The 1997-98 El Niño/La Niña 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.
According to new results published in the December 10 issue of the journal Science, El Niño 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, on 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 last year's El Niño/La Niña event.
Over the past decade scientists have been able to observe the development and progression of El Niño warmings-and consequent changes in upwelling of nutrient-rich ocean waters-thanks to data continuously collected in the Pacific by the buoys of the Tropical Atmosphere Ocean array, maintained by NOAA's Pacific Marine Environmental Laboratory.
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 gauge the fluctuating levels of 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.
"This is the first time we've ever had a continuous set of biological measurements from moored instruments and satellites during an intense El Niño, and we've never seen such low chlorophyll concentrations," said MBARI biological oceanographer Francisco Chavez, lead author of the new study.
It was the mooring measurements and SeaWiFS data that revealed surprisingly low and high levels of chlorophyll, coinciding with El Niño's strongest phase and the transition to La Niña cooling. When the warm-water layer produced by El Niño extended to its greatest depths and the upwelling of nutrients necessary for phytoplankton growth virtually ceased, chlorophyll values plummeted. Phytoplankton (microscopic algae) form the base of the oceanic food chain.
The researchers were again surprised in mid-1998 when chlorophyll levels skyrocketed, revealing the largest ever observed phytoplankton bloom, in area, for 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 Niña. Iron, an essential nutrient for phytoplankton growth, is thought to be the limiting nutrient for productivity in the equatorial Pacific under normal conditions.
El Niño can also be credited with drastically cutting the amount of carbon dioxide that this ocean region normally adds to the atmosphere. Unlike most parts of the world's oceans, the equatorial Pacific is a major contributor of atmospheric carbon dioxide because of the carbon-dioxide-rich deep ocean waters brought to the surface here and the relatively low levels of biological activity. The researchers calculate that 700 million metric tons of carbon normally released to the atmosphere as carbon dioxide were kept in the ocean during the year that El Niño conditions dominated the equatorial Pacific. This is equivalent to half of the United States' total annual carbon dioxide emissions from fossil fuel burning.
Authors and web sites for more information:
F.P. Chavez, P.G. Strutton, G.E. Friederich: http://www.mbari.org
R.A. Feely, M.J. McPhaden: http://www.pmel.noaa.gov/
G.C. Feldman: http://www.gsfc.nasa.gov/
D.G. Foley: http://www.nmfs.hawaii.edu/
Journal
Science