Iron 'Fertilization' Causes Plankton Bloom
Scientists link iron to climate change
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For farmers and gardeners it would be revolutionary: a novel fertilizer with a cheap and widely available ingredient, iron, that produces more than two thousand times its weight of plant growth--within a week. Iron has that effect on parts of the ocean.
Despite being the fourth most abundant element in soil and rock (and the most common on Earth as a whole), iron is in short supply in much of the sea.
Scientists have long suspected that iron deficiencies might affect some 20 percent of the global ocean, yet this idea has only recently been tested. New results by National Science Foundation (NSF)-funded researchers, published in this week's science journal Nature, confirm earlier experiments that indicated a strong biological response to added iron. But this time the effects lasted longer, and large changes were observed in the air-sea transfer of gases involved in climate processes.
On an oceanographic research cruise called "IronEx II," led by scientists from Moss Landing Marine Laboratories (MLML) in California, 37 scientists from 13 institutions in the U.S., England, and Mexico "fertilized" with iron a patch of ocean waters some 800 miles west of the Galapagos Islands. Nearly one-half of one ton of iron was added to the experimental patch, increasing surface water iron concentrations by 100 parts per trillion. The experiment was tracked for 18 days.
Iron-starved plant plankton, called phytoplankton, native to the region responded rapidly; the amount of plankton began to nearly double each day. Working around the clock, scientists performed continuous measurements and over-the-side sampling operations. "Within one week, about two million pounds of phytoplankton had grown, representing a thirty-fold increase," says scientist Kenneth Coale of MLML. "At the same time, the rapid growth of these plankton began to 'draw down' carbon dioxide in surface waters. After 10 days, the concentration of carbon dioxide had dropped 20 percent over the initial values."
The waters in which this experiment were conducted are representative of about 20 percent of the ocean's surface area. They are called High Nitrate, Low Chlorophyll (HNLC) waters. "The experiment strongly supports the hypothesis that these waters do not grow more plant plankton because they lack iron," says Don Rice, director of NSF's chemical oceanography program, which funded the research along with the Office of Naval Research. Tiny additions of this nutrient to HNLC waters have the potential to cause rapid plant growth and a "draw-down" in the concentration of atmospheric carbon dioxide. This experiment may have pulled more than 2,500 tons of carbon dioxide from area waters before the patch was broken up by ocean currents, according to Coale. "It demonstrates that changes in iron supply to HNLC ocean regions play an important role in regulating atmospheric carbon dioxide and climate."
Could we slow future climate change resulting from human activities by adding iron to under-productive oceans? Calculations for the equatorial Pacific, reported in the Nature papers, indicate that iron fertilization there would not significantly counteract the projected future increase of atmospheric carbon dioxide.
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