One could call it the Geritol effect, says biological oceanographer William P. Cochlan, one of 37 scientists, technicians and graduate students representing 13 institutions who participated in IronEx-II, a research project funded by the National Science Foundation and the Office of Naval Research.
In the spring of 1995, the scientists fertilized a 27.6-square-mile patch of equatorial Pacific Ocean waters with half a 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. 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 says.
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 worlds ocean waters, mostly in the Pacific. For more than a century, scientists have wondered why so little phytoplankton grows there.
Liebigs 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 thats familiar, in practical terms, to every backyard gardener who has ever tested soil to determine the right fertilizer to use.
The late John Martin, a scientist at Californias Moss Landing Research Station, was a longtime champion of the iron hypothesis, arguing that the shortage of this particular micronutrient was the Liebigs law factor limiting the phytoplankton productivity of HNLC regions.
The IronEx-II researchers believe the dramatic response observed in their 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 says, 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 than 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, its 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 just $6,000.
However, Cochlan says, I would absolutely not propose this as a method of geoengineering the global climate to ameliorate increases in atmospheric carbon dioxide, echoing the cautions of Kenneth Coale, chief scientist of Moss Landing Marine Laboratories, who has made it clear that the experiment was neither designed nor conducted to provide a technological fix for the greenhouse effect.
We have only these initial experimental results. An enormous amount of work remains to be done before, as an oceano-grapher, I would advocate doing such a thing.
Cochlan and Coale spent the fall in the Southern Ocean, continuing their work as part of the U.S. Joint Global Ocean Flux study.
We now know, says 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.
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WILLAM CHOCHLAN'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 p
hytoplankton growth. Measuring the absorption of those forms of nitrogen offers a sensitive and precise method for tracking and quantifying the growth of such organisms.
William Cochlan (right) with graduate student Raphael Kudela. Cochlan is indicating the portion of the equitorial Pacific Ocean where the IronEx-II experiment was carried out.
Cochlan found that nitrogen uptake dramatically in-creased 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 increased 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.