The atmosphere is a rich source of carbon dioxide, as millions of tons of this gas settle into the ocean every year. However, phytoplankton still require other nutrients, such as iron, to survive. When surface waters are cold, deeper depths are allowed to upwell, bringing these essential nutrients toward the surface where the phytoplankton may use them. However, when surface waters are warm (as during an El Nino), they do not allow the colder, deeper currents to upwell and effectively block the flow of life-sustaining nutrients. As phytoplankton starve, so too do the fish and mammals that depend upon them for food. Even in ideal conditions an individual phytoplankton only lives for about a day or two. When it dies, it sinks to the bottom. Consequently, over geological time, the ocean has become the primary storage sink for atmospheric carbon dioxide. About 90 percent of the world's total carbon content has settled to the bottom of the ocean, primarily in the form of dead biomass.

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The Iron Hypothesis

As early as the 1930's, the potential role of iron as a limiting factor in phytoplankton productivity was appreciated (Gran, 1931; Hart, 1934; Harvey, 1938). These early workers recognized the extreme insolubility of iron in today's oxygenated oceans, and deduced that it must be in very short supply in areas where it was not readily replenished from the land. Although they also recognized the analytical difficulties in actually measuring iron concentrations in the sea, they would be shocked to learn that their estimates of offshore iron concentrations were high by two orders of magnitude (Martin, 1992).

Because of the difficulties of working with trace levels of iron, direct evidence of its role in phytoplankton ecology has always been difficult to obtain. Its potential as a limiting factor becomes clear, however, when one recognizes that the early evolution of the biochemical pathways that drive phytoplankton physiology occurred when the ocean and atmosphere were anoxic. Because of its high solubility under these reduced (oxygen free) conditions, iron was extremely abundant relative to other trace nutrients (Table 1) and biochemical pathways were not under evolutionary pressure to be efficient with respect to iron. When oxygenic photosynthesis evolved around 3 billion years ago, iron was rapidly oxidized, becoming significantly less available to the biota. Since evolution cannot rewrite its story when the environment changes, many of the biochemical pathways that sustain life today carry designs inspired by ancient environments. This appears to be the case for iron (Table 1).

In the late 1980's John Martin began to weave together several independent threads of evidence, revitalizing the early idea that iron plays a major regulatory role in phytoplankton productivity (Martin, 1992). First, he recognized that the primary source of iron to the surface waters of the oceans is from the land, either via atmospheric dust deposition in offshore areas, or direct depositions from land masses. Second, his lab was able to demonstrate, for the first time, that dissolved iron concentrations in offshore areas are indeed extremely low (Martin and Gordon, 1988). In the Drake Passage of the Antarctic, for example, the concentration in the surface waters is 1.6 x 10 mole Fe kg (or 3.2 mg m). Using ``Redfieldian'' reasoning, he calculated that this amount of iron was one-tenth that required to allow the phytoplankton to assimilate the ambient nitrate in the surface waters. Third, he observed that atmospheric dust deposition in the two major HNLC areas---the Antarctic and equatorial Pacific oceans---are the lowest in the world (Prospero, 1981; Uematsu, 1987). Conversely, in the equatorial N. Atlantic, which receives large amounts of dust from the Saharan desert (Prospero, 1981), iron concentrations are sufficient for the complete assimilation of available nitrates and phosphates.

Martin and his team buttressed their argument with experiments. They filled bottles with surface waters from the HNLC regions and incubated them at ``simulated in situ'' light and temperature for roughly a week. To half of the bottles they added iron, and the other half were left alone as control bottles. They monitored phytoplankton abundance in the bottles by various means, and also nutrient (nitrates and phosphates) concentrations, to see if the addition of iron allowed the phytoplankton to assimilate additional nutrients. The general results were always the same: the total chlorophyll (a measure of phytoplankton biomass) in the iron-enriched bottles was always higher than in the control bottles at the end of the experiment, and nitrates were always more depleted in the iron-enriched bottles relative to the control bottles (Fig. 3). Martin also found that iron appeared to stimulate the growth of one particular group of species in the bottles---the pennate diatoms---and concluded, as others had predicted (Hudson and Morel, 1990), that not all phytoplankton species are equally iron limited.

Although to Martin and his supporters these experiments provided unequivocal proof that iron limits phytoplankton in HNLC regions, others were reluctant to accept the results so easily (see Chisholm and Morel 1991). Why? Because when you fill the experimental bottles with surface water, you are not just capturing the phytoplankton, you are capturing all the members of the planktonic foodweb that will be collected in the 10 to 30 liter sampling bottles typically used. This will include representatives of all bacterial species, most phytoplankton species, and most of the microzooplankton (which eat the small phytoplankton), but will exclude the large zooplankton (see Fig. 1). The latter, which eat the larger phytoplankton cells and also the microzooplankton, are simply not numerically abundant enough to be captured by the water samplers. The foodweb is a highly non-linear, interconnected system, and eliminating one factor has a cascading effect on all the linkages, profoundly changing the character of the system (Fig. 1).

A second problem with these types of bottle experiments is that no matter how carefully one sets up the experiment, one inevitably introduces contaminating iron into the bottles. Ships are very ``dirty'' platforms with respect to trace metals, and setting up these experiments requires a major investment in clean rooms, hoods, Teflon-coated sampling gear, ultra-clean glassware, and more. It also requires a firm conviction that one can actually be clean enough to get meaningful results. Unless you actually measure the iron concentration in the bottles after the experiment is set up, you can never be completely sure that the bottles to which you have intentionally added iron actually have more iron than the control bottles. Indeed, in many of Martin's experiments the phytoplankton in the control bottles grew almost as much as those in the bottles with added iron, and iron analyses confirmed that the control bottles often had elevated iron. Thus, only those who have the analytical capability to measure extremely low concentrations of iron in seawater can be assured unequivocal results, and be convincing in this debate.

Indeed, even though Martin's bottle incubations repeatedly showed that the phytoplankton in the iron-enriched bottles were stimulated relative to the controls (a result that is difficult to explain with anything other than the iron-limitation hypothesis) he still had his skeptics (see Chisholm and Morel, 1991). With the dogged determination that was his trademark, he decided to put an end to the debate once and for all and enrich a patch of the ocean with iron to see how the phytoplankton would respond when the food web was intact. Martin calculated that one could easily add more than enough iron to 100 km of ocean to stimulate the phytoplankton, provided there was enough N and P around. As we have already discussed, in the equatorial Pacific and the Southern Ocean, N and P are abundant. Working in the Southern Ocean is a logistical nightmare for oceanographers, and the availability of sunlight is unpredictable, thus Martin decided on the equatorial Pacific (near the Galapagos Islands) as the site for the experiment.

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U.S. National Report to IUGG, 1991-1994
Rev. Geophys. Vol. 33 Suppl., c 1995 American Geophysical Union