South Pacific has a severe case of anemia.

A vast region of the South Pacific suffers from a natural iron deficiency--one that stunts the growth of marine plants supporting all higher ocean life, according to two oceanographers. The discovery holds implications for understanding past ice ages and perhaps for devising ways to avert global warming.

Researchers have previously diagnosed iron limitations in other ocean areas, but most are far smaller than the South Pacific gyre, a continent-size swath of water stretching from South America to New Zealand. The new finding, reported in the Feb. 5 SCIENCE, builds on a growing recognition of iron's importance in controlling ocean life.

"It's the Iron Age of oceanography right now," quips Kenneth H. Coale of Moss Landing (Calif.) Marine Laboratories.

The iron-limitation hypothesis challenges the traditional theory that the amount of biologically useful fixed nitrogen keeps ocean life in check in most regions. According to the older concept, tiny marine plants called phytoplankton grow until they use up all available fixed nitrogen.

Some parts of the ocean, however, have abundant nitrogen but remain watery deserts. In 1988, the late John H. Martin suggested that lack of iron could explain this phenomenon--an idea supported by experiments in 1993 and 1995.

Oceanographers have now discovered signs of marine anemia under quite different conditions--the nitrate-poor waters of the South Pacific gyre. Michael J. Behrenfeld and Zbigniew S. Kolber of Rutgers University in New Brunswick, N.J., measured the fluorescent light emanating from phytoplankton in ocean water continuously collected between the equator and Tahiti. By comparing the intensity of fluorescence at night and during the day, the researchers could gauge the relative amounts of various molecules important for photosynthesis.

From the pattern of fluorescence, the researchers concluded that phytoplankton in parts of the South Pacific gyre has low amounts of key iron-rich molecules. The iron shortage in the water, they say, hence hinders the phytoplankton's growth.

The findings could help oceanographers understand how small periodic changes in Earth's orbit have sent the planet into repeated ice ages. As the climate began to cool, the continents dried out and winds intensified, carrying iron-rich dust into the oceans. According to this theory, the iron stimulated ocean plant growth and thereby chilled the globe further by pulling carbon dioxide out of the atmosphere.

By greatly expanding the area of known iron-limited ocean, Behrenfeld and Kolber have strengthened the idea that wind-borne iron could have dramatic impacts on Earth's climate. "It makes the iron story much larger geographically," says Rutgers' Paul G. Falkowski.

Some scientists have suggested that fertilizing the ocean with iron could slow global warming by helping plants sop up carbon dioxide. Many oceanographers, however, reject the idea because the iron could cause harmful side effects to ocean life (SN: 9/30/95, p. 220).