Iron and Ocean Productivity

Scientists have discovered that large zones in the Pacific are deficient in iron. The lack of iron makes phytoplankton (photo) produce more pigments and a fake appearance of an increased ocean productivity.

Past interpretations of satellite chlorophyll could be false and the tropical Pacific Ocean may photosynthesize 1-2 billion tons less atmospheric carbon dioxide than was previously counted. Global ocean carbon uptake is estimated at 50 billion tons, so the reduction in the estimate of the uptake is about 2 to 4 percent. "When stressed by a lack of iron, phytoplankton create additional pigments that fluoresce, or light up, unlike normal pigments", said Michael J. Behrenfeld, a research scientist in Oregon State University's Department of Botany and Plant Pathology. Unfortunately, "satellite imagery could not readily distinguish that difference."

"It's really a fascinating process," Behrenfeld said. "When phytoplankton species make these extra pigments, they don't use them right away - they bank them. Then when they get an infusion of iron, they just take off. They don't have to wait to begin dividing and growing. But that green color wasn't an indication of health, it was an indication of stress from a lack of iron."

Behrenfeld and his team looked at 12 years of fluorescence data taken along 36,000 miles of ship tracks throughout the tropical Pacific. They made a "fluorescence fingerprint" with the Pacific areas suffering of iron lack or nitrogen lack (another very important element to productivity). "Nitrogen and phosphorus are nutrients that come up from the ocean bottom to feed the upper water column," Behrenfeld said.

"Iron, on the other hand, can come from the deep or from the air, but it also enters the ocean through dust deposited by the wind. Windstorms blowing sand and dust off large deserts are a major source of iron for the world's oceans.
"It's like dumping a load of Geritol or some other iron supplement into the water."

There are three large areas in Pacific which appear limited by a lack of iron: the southern ocean around Antarctica, the sub-arctic north Pacific below Alaska, and a huge area in the tropical Pacific centered on the equator.

"It turns out different places in the ocean are missing different nutrients," said Robert Sherrell, a scientist from Rutgers University. "The new fluorescence technology now allows us to tell which combination of nutrients is stressing the phytoplankton."

The oceanic lack of iron links phytoplankton productivity to the climate, because of its dependence on the iron brought by terrestrial-based dust deposited in the ocean. "The northern tropical Pacific is more nitrogen-stressed and doesn't have the "false greenness," said Behrenfeld.

A new model of carbon cycling is created, based on NASA satellite imagery which they have calibrated using their ship-based measurements of fluorescence.

The ocean is critical in global carbon cycling. The phytoplankton adsorbs carbon dioxide out of the atmosphere for its growing.

The carbon cycle has a key role in the global climate change. Iron fertilization of phytoplankton is a key to a healthy marine food chain.

Both Behrenfeld and Peter Strutton, an assistant professor of oceanography at OSU and a co-author on the Nature paper, experimented productivity boosts in oceans by iron fertilization.

Introducing iron into small portions of the Pacific triggered phytoplankton growth, but not as robust or as sustained as the models predicted. "It wasn't the silver bullet that scientists originally hypothesized," Strutton said. "The carbon export was slower than we thought.

It could be that the scale was too small, and it could be that the (biological) response was too slow and we didn't wait long enough."

Iron fertilization is complicated, because the iron minerals aren't water-soluble and require repeated infusions. "When you first do it, there's an explosion of growth and then it plateaus," he said.

"Then you add a bit more iron, and the phytoplankton respond a bit more. Then you add a third shot, and it triggers some more modest growth.

But at the same time you're promoting phytoplankton growth, the grazers that feed on them come to life because they suddenly have a more abundant food supply. So the plankton can disappear as fast as you've made them grow."