USC graduate student Raphael Kudela (left) and biological oceanographer William P. Cochlan, who indicates on a map the portion of the equatorial Pacific Ocean where the IronEx-II experiment was carried out.

Photo by Irene Fertik

"One could call it the Geritol effect," said William P. Cochlan, the biological oceanographer who conducted microbial ecology studies showing how increases in phytoplankton growth were tied to iron enrichment.

In the spring of 1995, scientists fertilized a 27.6-square mile patch of equatorial Pacific Ocean waters with a half-ton of iron - increasing the iron content of surface waters in the region by a mere 100 parts per trillion.

This minuscule increase almost immediately produced a dramatic response in the oceanic test site. Within days, concentrations of phytoplankton, the tiny marine plants that form the base of the ocean food chain, multiplied in lockstep with the increasing availability of iron. Then, as the iron dispersed, sank or was used up, phytoplankton concentrations fell back to prior levels.

The blooming phytoplankton attracted larger creatures. "Once the algae bloomed, sharks, turtles and squid flocked to it," Cochlan said.

Cochlan is one of 37 scientists, technicians and graduate students representing 13 institutions that participated in the "IronEx-II" research cruise, funded by the National Science Foundation and the Office of Naval Research. An overview article and several companion papers described the research in the Oct. 10 issue of the British journal Nature.

Measurements taken during the IronEx-II experiment indicated that the boost to biological productivity created by iron fertilization was so great that, if carried out on a large scale, it might even help to alleviate the greenhouse effect.

But Kenneth Coale, chief scientist of the Moss Landing Marine Laboratories in Moss Landing, Calif., emphasized that the experiment was neither designed nor conducted to provide a technological fix for the greenhouse effect.

Waters in the area fertilized with iron were high in most of the nutrients needed for marine life, but they contained relatively little phytoplankton. Such areas - called "high nutrient low chlorophyll" (HNLC) regions - comprise about 20 percent of the world' s ocean waters, mostly in the Pacific. For more than a century, scientists have wondered why so little phyoplankton grows there.

Liebig' s law (named for a 19th century German chemist) holds that any chemical reaction is limited by the quantity of the scarcest component of that reaction - a relation that' s familiar, in practical terms, to every backyard gardener who has ever tested soil to determine the right fertilizer to use.

Moss Landing Research Station scientist John Martin (recently deceased) was a longtime champion of the "iron hypothesis." He argued that the shortage of this particular micronutrient was the Liebig' s law factor limiting the phytoplankton productivity of HNLC regions.

The IronEx-II researchers believe the dramatic response observed in their mesoscale experiment - a 30-fold increase in plankton biomass - confirms the iron hypothesis. The climatic implications could be global, since phytoplankton growth is part of the process whereby greenhouse-effect carbon dioxide is stripped from the atmosphere.

"Using the energy of the sun," Cochlan said, "photosynthetic plants combine carbon dioxide and water to form organic material. As these phytoplankton grow and die, part of the biomass created sinks into deep waters, transporting fixed carbon dioxide down from the surface and locking it away in the depths of the ocean."

According to the data gathered by IronEx-II, the 450 kilograms of iron added to the ocean resulted in the removal of some 2,500 metric tons of carbon as carbon dioxide from the atmosphere - a ratio of more 5,000 to one.

The iron used was in the form of the common industrial chemical iron sulfate (also used in certain dietary supplements and patent medicine tonics). Even at extremely high concentrations, iron sulfate has low toxicity. At the 100-parts-per-trillion concentration employed in this experiment, it' s entirely non-toxic to the marine environment.

Theoretically, at the 5,000-to-1 iron/carbon ratio observed in the experiment, a 150,000-ton supertanker dispersing iron in HNLC areas might be able to compensate for 5,000 supertanker cargoes of oil delivered to burn as fuel.

The total cost of the acidic FeSO4 solution containing a half-ton of iron used by IronEx-II came to $6,000.

"I would absolutely not propose this as a method of geoengineering the global climate to ameliorate increases in atmospheric carbon dioxide," Cochlan said, echoing the cautions of Coale. "We have only these initial experimental results. An enormous amount of work remains to be done before, as an oceanographer, I would advocate doing such a thing."

"It' s safe to say," Coale added, "that no one connected with the project would advocate adding iron to the oceans for geoengineering purposes."

IronEx-II was, as the name indicates, the second effort to test the iron hypothesis in an open-ocean, mesoscale experiment. The first, three years ago, found a significant increase in biological activity, but much less than theory had predicted, and dramatically less than the increase observed in the second experiment.

For the second experiment, a slightly revised method for adding iron was devised by Coale and Keith Johnson of the Moss Landing Marine Laboratories. The two scientists created a technique to better mimic the way iron is naturally added to the oceans via dust particles - particularly those carried offshore in infrequent but intense dust storms. They added iron in three separate infusions over one week rather than adding it in a single large enrichment.

Cochlan' s research on the IronEx-II mission was aimed at establishing two key parameters of the experiment: first, to demonstrate that the increase in photosynthetic activity was tied to the iron increase, and second, to determine which micro-organisms were actually responding to the enrichment.

Using isotopically labeled nitrogen, which is absorbed and assimilated by growing plants, Cochlan and USC graduate student Raphael Kudela were able to establish differences in nitrogen uptake between areas inside the iron-fertilized patch and control areas with normal iron content. Various nitrogen compounds (ammonium urea and nitrate) are essential nutrients for phytoplankton growth. Measuring the absorption of those forms of nitrogen offers a sensitive and precise method for tracking and quantifying the growth of such organisms.

Cochlan found that nitrogen uptake dramatically increased after iron enrichment, then gradually decreased to pre-fertilization levels as the concentration declined - as the iron mixed in the water sank out, was diluted by the ocean, or was used by organisms.

The isotopically labeled nitrogen provided a way to distinguish how much of the inceased biological activity occurred in bacteria, and how much in phytoplankton. The distinction is important in marine ecology, because phytoplankton are widely used as a food source by larger organisms and sink out much more rapidly from surface waters. In theory, a mix heavier in phytoplankton than in photosynthetic bacteria means more fish up the food chain.

Cochlan and Kudela found that the increased biological activity stimulated by the iron was, overwhelmingly, increased phytoplankton growth. Essentially no change in bacterial abundance was observed.

Cochlan and Coale left in late September for the Southern Ocean to continue certain aspects of this work as part of the U.S. Joint Global Ocean Flux study.

"We now know," said Cochlan, "that major events occurring in the terrestrial world could dramatically influence marine systems thousands of kilometers from shore. This insight opens the door to a better understanding of what regulates marine productivity in vast regions of the ocean."

Besides USC and the MLML, other institutions with scientists involved include the Monterey Bay Aquarium Research Institute, Monterey; the University of Miami, Key Biscayne, Fla.; the University of East Anglia, Norwich, England; Plymouth Marine Laboratory, Plymouth, England; the University of Hawaii, Honolulu; CICESE Oceanographica Fisica, Ensenada, Mexico; the University of California, Santa Cruz; Brookhaven National Laboratory, Brookhaven, N.Y.; the Massachusetts Institute of Technology, Cambridge, Mass.; and Duke University, Durham, N.C.