Satellite imagery pinpoints El Nino’s disruption of marine ecosystem

Contact: Harvey Leifert

hleifert@agu.org

202-777-7507

American Geophysical Union

WASHINGTON — While evidence of the 1997-1998 El Ni&#241o was readily apparent on land–with storms and flooding that caused millions of dollars in damage–new studies have detailed El Ni&#241o’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 Ni&#241o 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 September 15 issue of Geophysical Research Letters that the 1997-1998 event–one of the strongest El Ni&#241os on record–supplanted the normal upwelling of cold, nutrient-rich waters in the California Current System.

“When El Ni&#241o 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 Ni&#241o and La Ni&#241a.”

The authors argue that one of El Ni&#241o’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 Ni&#241o in 1998 into the cold surface water La Ni&#241a 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).

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

Notes for journalists:

1. Journalists may obtain a copy of this paper (4 pages) on request to Harvey Leifert, email:hleifert@agu.org. Include your name, title, publication, address, phone, and fax number.

2. Images to accompany this release are available at:
http://www.sio.ucsd.edu/scripps_news/pressreleases/mitchell_9_6_2000.html
(Powerful 1997-1998 event led to phytoplankton decrease off California,
increase off Baja)

3. This paper will appear in Geophysical Research Letters, vol. 27, no. 18 (September 15, 2000).