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Ocean Gardening Using Iron Fertilizer
(Released August 2004)

  by Ben Fertig  


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As evidence of the potential effects of global warming mounts, many people are already brainstorming means to minimize the atmospheric greenhouse gases which cause it. One of the most worrisome greenhouse gases comes from quite natural sources. In fact, all of us breathe out about two pounds of it a day. This gas is, of course, carbon dioxide. A number of ingenious and colorful ideas have been proposed to reduce the amount of carbon dioxide in the atmosphere. Some of these plans include reducing the production of greenhouse gases from automobile and industrial emissions, and maximizing trees (including in urban areas) to act as a carbon sink. One of the more fantastic possibilities given scientific attention has been sequestering carbon dioxide in the oceans by fertilizing them with iron.

Iron? What does that have to do with carbon dioxide?

The basic idea underlying many of the solutions to global warming involves tinkering with the carbon budget. During the carbon cycle, carbon dioxide is naturally removed from the atmosphere, and over thousands of years, a fraction of it is sequestered in the oceans for prolonged periods of time. The natural process which drives carbon towards the bottom of the ocean is termed the biological pump. Dead phytoplankton and other marine organisms act as carbon dioxide vessels, driving this pump as they sink towards the bottom of the ocean.

The biological pump has commanded attention from policy makers, entrepreneurs, and scientists for years as a method of intentional ocean carbon sequestration.1 Policy makers may be interested in carbon sequestration as one of a suite of greenhouse gas management methods. It is possible that this management method, if employed, may be used in conjunction with proposed international carbon trading markets. If so, some entrepreneurs speculate that ocean carbon sequestration may become a lucrative business. All this is dependent upon verifying the assumption that ocean carbon sequestration is a scientifically and ecologically sound method of reducing atmospheric carbon dioxide.

How can scientists think that phytoplankton, which represents less than 1% of all photosynthetic biomass, could possibly affect large-scale phenomenon such as global warming? And where does the iron come in? Phytoplankton assimilates dissolved carbon dioxide in the surrounding water during photosynthesis. As dissolved carbon dioxide is taken up, less is exchanged with the atmosphere, and so atmospheric carbon dioxide diffuses back into the water.2 Once the surrounding essential nutrients and minerals, including forms of nitrogen, phosphorous, and iron are used up, algal blooms die and sink to the bottom, exporting the carbon they assimilated and sequestering it indefinitely. Is there any way we could magnify this process a bit and get carbon dioxide out of the air and into the water? Well..., perhaps, if we knew what triggered large phytoplankton blooms in the open ocean.3

To answer some of these questions, scientists have focused on ocean regions with abundant plant nutrients but lacking plankton. These areas are formally termed high-nitrate low-chlorophyll (HNLC) and have become an open laboratory for scientists over the last decade to explore their limiting nutrient for phytoplankton growth: iron. What happens if we add iron to these parts of the ocean fertilizing them, if you will? Theoretically, by enriching the oceans with iron on a large scale, enough plankton could grow to reduce atmospheric carbon dioxide and mitigate global warming.

As simple and attractive as this might look, ocean fertilization still poses many questions and uncertain effects, especially at a large scale. Oceans are uncontrollable, sequestration results are not easily verifiable, and unintended tangential consequences are likely. Further, governments and global institutions must hammer out legal and regulatory aspects of this approach. Moreover, we must satisfy moral demands as well, and answer questions weighing uncertainty about potential effects, negative and positive. On a more immediate and practical level, some of the biggest questions remaining regard ocean fertilizations ties to global warming: Will it really work?4

If so, at what cost?

Iron Hypothesis

John Martin, the late director of Moss Landing Marine Laboratory, suggested that in HNLC waters phytoplankton display iron deficiency, preventing plankton blooms despite abundant nutrients.5 Earlier research on trace metals showed that iron levels in these types of water were much lower than previously thought. Apparently, preceding measurements were skewed by iron in the hulls of the ships used to collect samples and by the measuring equipment itself. 6 Later research on trace metals in polar waters showed coincidence between areas with elevated iron concentrations and high productivity, large plankton biomass, and increased nutrient drawdown.7 This evidence supported Martins iron hypothesis.

Martin connected ocean iron limitations to climate via glacial cycles. He and others before him noted that iron found in oceans partly originates on land and is distributed by wind.8 During ice ages, much of the world's water is locked into glaciers and the world becomes a drier, dustier place. Martin suggested that during these periods, greater amounts of iron are swept into the oceans by wind, resulting in larger plankton blooms that assimilate large quantities of carbon dioxide, ultimately decreasing the atmosphere's ability to retain heat and prolonging ice ages. 9 Subsequent measurements of equatorial surface waters show that another major iron source is actually upwelled waters from the ocean floor, rather than solely continental sources.10 Martin noted that ice cores recording atmospheric carbon dioxide agree with this notion. During ice ages when dust was abundant, carbon dioxide levels were low, and vice versa.11 This supported the idea that more carbon was being sequestered in the oceans during these periods.

Martin extended this Iron Hypothesis, as it became known, to suggest that fertilizing the oceans with iron would increase carbon sequestration and could potentially be used to mitigate some of the effects of global warming, should the need arise. He half jested Give me half a tanker of iron, and I will give you the next ice age.12 His bold ideas and pronouncements earned him the nicknames Iron Man and Johnny Ironseed.

Fertilization Experiments

Though Martin died before his theories were tested in the open ocean, other scientists carry on the proverbial torch. At the time of this writing, nine iron-enrichment experiments have been carried out in HNLC regions across the world: ironEx I13, IronEx II14, SOIREE15, EisenEx16, SEED, SOFeX17, Planktos18, SERIES19, and EIFEX. The latest, EIFEX (European Iron Fertilization Experiment), was completed in March 2004, and results have not yet been published. Iron enrichment experiments are likely to continue as scientists delve deeper into the biogeochemistry of iron limited waters.

Nitrate Concentrations in Surface Waters
Nitrate concentrations in the world's oceans, and the locations of iron fertilization experiments

Each of the experiments has shown increases in phytoplankton biomass and production rates. Observations of phytoplankton blooms without the addition of iron supplements show underutilized resources of carbon dioxide and nitrates, supporting Martin's Iron Hypothesis.20 February 1999 began the in situ iron fertilization experiments in the Australian-Pacific sector of the Southern Ocean. Dubbed SOIREE (Southern Ocean Iron RElease Experiment); the expedition used iron sulfate and a hexafluoride tracer, and conducted two enrichments. Initial monitoring at the site was followed with extensive remote sensing using NASA's SeaWiFS Ocean Color Project.21 SOIREE showed changes in the pelagic ecosystem structure, the cycling of carbon, silica, and sulfur, and a 10% drawdown of carbon dioxide.22 This withdrawal of carbon dioxide from the atmosphere was mostly due to a proliferation of diatoms.23

As part of the success of the enrichment experiments, much was learned about the process requirements of iron fertilization as well. The iron used to enrich the HNLC patch needs to stay in solution, though naturally iron is not readily soluble in sea water. Enrichment procedures were modified between IronEx I and IronEx II after initial failures to show an ability to significantly affect greenhouse gases, despite increased phytoplankton abundance and production. IronEx II, conducted in similar equatorial Pacific waters, added the same amount of iron as IronEx I, but divided the iron into three separate fertilizations over a week so that more would remain in solution.24 This change in procedure still increased plankton and diatom abundance and productivity, but also decreased nitrate concentrations. The long-term effect of the shift in community composition is still unknown. These events parallel those observed in previous shipboard experiments in the equatorial Pacific.

Earlier experiments showed that communities and their species compositions shifted due to iron fertilization. In 1995, the IronEx II survey in equatorial Pacific showed that phytoplankton grew in size and abundance. This growth differed across types of plankton, and changes were observed in the plankton community structure. Groups of plankton that were dominant previous to iron enrichment grew slowly and sometimes even decreased in abundance, while diatoms, which were initially rare, increased 15-fold in abundance by the blooms peak.25 Iron enriched plankton, especially diatoms, show a quick population doubling time.26 The long-term effects on community composition, food web structure, and resultant biogeochemistry are uncertain, especially if fertilization is continued regularly or on larger scales.

SOIREE phytoplankton bloom
Satellite image of phytoplankton bloom occurring from the SOIREE expedition. The arced distribution is due to oceanic currents

The 2002 Southern Ocean Iron (Fe) Experiment (SOFeX) enrichment demonstrated how silicic acid, necessary for diatom growth, affects iron enriched plankton blooms.27 Unmanned floats stationed underwater measured over a fourfold increase of organic carbon biomass, and a two- to sixfold increased carbon export.28 These increases were much higher than expected. Satellite images show a 150 km long algal bloom near the SOFeX experimental site six weeks after the initial fertilization.29 Furthermore, very small increases in iron concentration substantially increased carbon export to deeper waters.30

Aspects of the physical oceanography in HNLC waters greatly affected phytoplankton growth after fertilization with iron. Water mixing, in addition to diffusion, was shown to be important to the distribution and growth of the algal bloom. Ocean currents and other water movements help shape plankton masses into tendrils and filaments, often stretching over many miles. In the iron enrichment experiments, stirring mixed plankton and iron out of the patch and carried silicate, another form of silicic acid, along with the current.31 This may help delay silicate limitation, allowing blooms to grow until iron runs out.32 Stirring in oceans is variable, so blooms that start out similarly may develop differently.

Overall, each of these fertilization experiments has successfully demonstrated that iron has indeed been limiting phytoplankton growth in HNLC waters. The dramatic increase in phytoplankton abundance has fascinated the scientific community, while demonstrating that the ocean can be used as a laboratory in these small-scale experiments. Despite these successes, shifts in plankton communities and lack of conclusive evidence relating to atmospheric gases leave concerns regarding other consequences, as well as long-term effectiveness at mitigating global warming.

Caution Regarding Global Warming

Before getting too enamored with the implications of successful iron fertilization experiments and their connection to global warming, one must face several caveats. Though Martin suggested that iron fertilization may be one method of preventing global warming as the need arises, scientific evidence is as yet incomplete, and suggests there may be unintended consequences, especially at the scale necessary for global change. While SOFeX showed increases in the flux of carbon, they were small compared to global carbon budgets.33 It remains unclear if fertilizing the oceans will affect atmospheric levels of carbon dioxide enough to change climate patterns.

Long Term Effects of Iron Fertilization
Long term effects of iron fertilization are still unknown
Source: http://cafethoriu

Furthermore, other factors play a part in global warming, and carbon dioxide is not the only greenhouse gas. Other gases affecting global warming and their atmospheric lifetimes need to be focused on as well. Models have predicted several possible unpleasant unintended results of iron fertilization. Long-term increased productivity resulting from iron enrichment could lead to deep ocean anoxia and marine life hypoxia.34 Similarly, anoxia often results from algal blooms in coastal waters after eutrophication from agrochemical runoff. Moreover, the increased detritus and marine snow affecting the biological pump could shift microorganism community structure, ultimately changing oxygen levels in the ocean. Furthermore, such microorganisms may produce other greenhouse and climate related gases such as nitrous oxide, methane, or dimethylsulfide, which may offset the intended results.35

Thus far, iron enrichment experiments have been small and mainly intended to determine the role of iron as a limiting nutrient for phytoplankton growth in HNLC water. Studies of carbon export have been limited, it is difficult to verify sequestration, and we do not yet fully understand the fate of biogenic carbon induced from iron enrichment.36 Experimental sediment traps deployed during SOFeX showed that after nutrient resources were depleted only a small fraction of iron-induced organic carbon reached the bottom. Other pathways may be available to the carbon, particularly remineralization via bacteria and mesozooplankton grazing. 37 furthermore, just as experiments with sea water in a small bottle may not accurately reflect the same processes that occur naturally in an ocean, small scale experiments in selected regions may not reflect large scale trends that play a role in phenomena such as global warming.

Iron enrichment of our oceans purposefully alters the ecology of an uncontrollable environment. Current methods of iron enrichment do not emulate natural iron deposition either chemically or in time-scale. Aeolian iron deposition is chemically different from iron in fertilization experiments. Experimentally, solubility in sea water is crucial to maintaining traceable patches of fertilized water within the euphotic zone. Upwelling, the other natural source of iron, turbulently provides many nutrients, while experiments provide only iron. Fertilization introduces iron at a far greater rate to HNLC waters than occurs naturally. Resulting blooms export carbon to the ocean bottom in a matter of weeks, while natural processes siphon carbon dioxide from the carbon cycle over thousands of years. Ultimately, however, iron fertilization may only delay the release of carbon dioxide to the atmosphere. Deep ocean carbon dioxide reservoirs are eventually exposed to the atmosphere by ocean circulation and currents.

Given the limitations of our verifiable knowledge and the stakes at risk, we should be extremely cautious before thinking of iron fertilization as a solution to global warming. It is not guaranteed to be a reliable or ecologically safe method. More research is necessary to fully understand the oceans biogeochemistry.


Iron fertilization appears to have the potential to significantly alter plankton blooms, marine ecosystems, and perhaps even the planet. Therefore, understanding the role which iron plays is essential to understanding the biogeochemistry of our oceans and world. Some hope that this has the potential to mitigate global warming while others point out that such large-scale geo-engineering would have many unknown repercussions. Sallie Chisholm, Professor of Environmental Studies at MIT and collaborator in several of the fertilization experiments, asks if unintentional ocean degradation through previous enterprises justifies purposeful large-scale alterations of ocean ecosystems.38 While the debate about global warming and what should be done to minimize its effects continues, we now realize that the oceans have a potentially significant part in that drama. Furthermore, iron plays a definitive role in unlocking some of the secrets the oceans once held.


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