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September 6, 2000

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While evidence of the 1997-1998 El Niño was readily apparent on land—with storms and flooding that caused millions of dollars in damage—new studies have detailed El Niño’s extensive consequences in the ocean environment.

New evidence produced by researchers at Scripps Institution of Oceanography at the University of California, San Diego, shows that warm, nutrient-depleted waters ushered in during the El Niño resulted in a reduction in phytoplankton—the plants that are the base of the marine ecosystem.

Using high resolution, color-sensitive images from U.S. and Japanese satellites, Mati Kahru and Greg Mitchell report in the Sept. 15 issue of Geophysical Research Letters that the 1997-1998 event—one of the strongest El Niños on record—supplanted the normal upwelling of cold, nutrient-rich waters in the California Current System.

"When El Niño suppresses the availability of nutrients in the sunlit surface waters, the abundance of phytoplankton declines," said Greg Mitchell, research biologist in the Marine Research Division at Scripps. "Phytoplankton communities are the primary producers for the ocean, comparable to grasslands for terrestrial systems. Success of fish population recruitment, and therefore commercial fisheries, may in part depend on interannual cycles of nutrient and phytoplankton distributions associated with El Niño and La Niña."

The authors argue that one of El Niño’s effects on the California Current System is both a reduction and a more uniform distribution of phytoplankton, which results in a critical reduction in the high-concentration patches of phytoplankton that may be necessary for success in the planktonic stages of fish populations.

While Kahru and Mitchell documented reductions in satellite estimates of surface phytoplankton for water off central and southern California, they found a significant increase off Baja California.

"These moderate abundances of phytoplankton extended far off shore in warm waters, which had not been observed before," said Mitchell. "We believe this increase off Baja may be due to blooms of ‘nitrogen-fixing’ cyanobacteria. Some open ocean cyanobacteria are more abundant in nutrient-depleted, strongly stratified waters because they are capable of fixing nitrogen gas into organic matter, reducing their dependence on nutrient upwelling."

Kahru and Mitchell’s data showed the effects that made the 1997-1998 event one of the strongest on record. In a 15-year span, satellite sea surface temperatures for some regions were the highest in 1998 and lowest in 1999. The researchers observed a strong transition out of the El Niño in 1998 into the cold surface water La Niña event in 1999.

"The difference between ’98 and ’99 in satellite-derived temperature was the most dramatic that’s been observed, " said Kahru.

Kahru, Mitchell and their colleagues specialize in ocean observations combined from satellites and ships. They develop mathematical relationships to interpret satellite data, using imagery from the Ocean Color and Temperature Sensor (OCTS), the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), and the Coastal Zone Color Scanner (CZCS).

As part of the SeaWiFS science team, they have established a routine observing program with the California Cooperative Oceanic Fisheries Investigations (CalCOFI), a collaborative effort among scientists in the Marine Life Research Group at Scripps, the California Department of Fish & Game, and the National Marine Fisheries Service. The CalCOFI study of the California Current system provides one of the world’s most complete time series of data (50 years) from an oceanic ecosystem.

Funding for Kahru and Mitchell’s study was provided by NASA.