El Niño’s Effect on Algae

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"Observing [the algae] in the oceans is much like putting dye in a tank and stirring it up to see where things are moving," Murtugudde said. Algae (phytoplankton) are by far the most abundant form of plant life in the ocean. They are sensitive to light, temperature, currents and winds, and their green chlorophyll can be detected by satellite instruments. Because phytoplankton changes an ocean's color, they are ideal candidates for tracking currents, detecting pollution, and observing meteorological events.

For years scientists have known that El Niño and La Niña change the levels of phytoplankton across the entire Pacific basin. During a normal year, winds gust at a steady rate from east to west across the Pacific and slowly blow the warm surface waters towards Australia and the Indonesian Archipelago. Over a period of time, these winds build up a "warm pool" of water in the western Pacific and leave the eastern Pacific relatively cool. This layer of warm water smothers any upwelling currents, which bring cool, nutrient-rich waters up from the depths of the sea (Njoku et al. 1993). Since phytoplankton can only survive in these nutrient-filled waters, the plants do not usually do well in the western Pacific and thrive in the eastern and central Pacific (Murtugudde et al. 1999).


Micrograph of a asterionella japonica—one of thousands of species of phytoplankton. (Photo courtesy George Rowland and the SUNY Marine Sciences Research Center)

To learn more about phytoplankton, see What are Phytoplankton? in the Earth Observatory Library.

May 9, 1998
May 24, 1998

El Niño and La Niña alter the temperature of the surface waters across the Pacific. During an El Niño year, the trade winds in the Pacific die down or reverse direction. The upwelling currents in the east subside, and the pool of warm water in the western Pacific spreads out over the entire basin (Njoku et al. 1993). The phytoplankton in the central Pacific all but disappear, and the population in the eastern Pacific are lowered significantly. The opposite occurs during La Niña. The easterly trade winds pick up and blow even more hot water into the west. The upwelling increases in the central and eastern regions, causing the phytoplankton concentration to explode (Murtugudde et al. 1999).

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Sea-viewing Wide Field-of-view (SeaWiFS) images of the Galapagos islands and surrounding waters from May 9, 1998 (top) and May 24, 1998 (bottom). The equatorial current shut down by El Niño reappeared over a period of days—indicated by the high concentrations of phytoplankton chlorophyll streaming to the west in the later image. (Courtesy Gene Feldman, SeaWiFS Project)

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