Ocean Fertilization Raises More Questions Than Answers

Earlier this month, a one of a kind study was published on fertilizing the ocean with iron. It kicked off a series of articles making claims about the impact this may have on climate change, some of which are unfounded or incomplete. To get some perspective on this, we need to back up and look at the role of iron in the ocean and the carbon cycle.

Much of the open oceans around the world are essentially deserts. Though not short on water like their terrestrial counterparts, these regions are limited in other elements vital for life, whether phosphorous, a key element in DNA, or iron, a vital component for correct function of many of the proteins in every living cell. When the idea of iron shortages limiting life was first proposed as the Iron Hypothesis in 1990, it was not immediately accepted. After all, iron is the fourth most abundant element on Earth. However, a series of large-scale experiments in different regions of the world’s oceans have repeatedly shown that many marine systems have all the nutrients they need for growth of phytoplankton, except iron. 

Back to the recent study on one of these so-called ocean iron fertilization (OIF) experiments. In such experiments scientists “fertilize” a large patch of the ocean with soluble iron from a research vessel, creating something similar to an oasis in the middle of a desert, then measure the resulting phytoplankton bloom. While many studies have done this, none have tracked what happens afterward, when the phytoplankton die. This current study did, which is part of why it has received so much attention.

Why would one want to see what happens after the bloom? To answer this requires that we talk about the biological pump. The carbon cycle all over the world, including the oceans, is predominantly composed of two parts: organisms like plants and phytoplankton, which convert CO2 to biomass and other carbon compounds, and organisms which respire that material back into CO2, like you and me. In the oceans, there is generally a balance between how much CO2 is produced by natural processes and how much is “fixed” by phytoplankton. However, a small amount of carbon does not get respired, and winds up sinking all the way to the ocean floor, where it can be sequestered, sometimes for thousands of years or more. This small difference in carbon fixed vs. carbon respired means that over large timescales, there is a net transfer, or pumping, of carbon as CO2 from the atmosphere to the seafloor, and because this process is driven by microorganisms and other living creatures, it is called the biological pump.

The connection between iron fertilization and the biological pump has been tenuous because studies have not shown what happens to the carbon, in the form of dead phytoplankton and other molecules, after the bloom. Tracking carbon is not easy; the ocean is a big, dynamic place. However, these scientists were able to find a large eddy in the ocean where the water did not mix much with surrounding water, like a giant tube with one end at the surface and one end at the ocean floor. About half the dead phytoplankton and other carbon from the bloom fell below 1000m on its way to the seafloor. The other half, presumably, was respired back to CO2. 

It was an elegant experiment, but it is also a double-edged sword. There are many people who, upon seeing the success of OIF experiments, have advocated for scaling them up to mitigate increasing CO2 levels in our atmosphere. Some would even sell OIF for carbon credits. Since this study has demonstrated carbon export to the deep ocean, it has renewed interest in such CO2 geoengineering (just Google “ocean fertilization”).

However enticing the idea of OIF for attempting to remedy our man-made CO2 increases, there are many reasons why it should not be attempted, and scientists themselves argue against it. For starters, we don’t know if the carbon that sank below 1000m will indeed be sequestered. It may be respired in a year or 1000 years. More detailed experiments are needed to show the fate of such material. Also, scaled-up fertilization efforts could lead to dangerous side effects, like those posed by fertilizer runoff. Too much nutrient input to the oceans can actually stimulate other microbial metabolisms and make things worse, like methanogens which produce methane, a compound with much stronger greenhouse effects than CO2. In the same stroke, nutrient stimulated microbial blooms can also remove oxygen from the water, making it unlivable for larger marine organisms. Thus, while OIF experiments help us understand the natural world and our impacts on it, this should not be viewed as a solution to our human-created climate problems. For now, our understanding of marine microbes can help prevent us from adding insult to injury.

 

Example of a phytoplankton bloom in an eddy-like current. NASA image courtesy MODIS Rapid Response Team, Goddard Space Flight Center.

References and further reading:

Smetacek V, Klaas C, Strass VH, Assmy P, Montresor M, et al. (2012) Deep carbon export from a Southern Ocean iron-fertilized diatom bloom. Nature 487: 313–319. doi:10.1038/nature11229.

Pichevin LE, Reynolds BC, Ganeshram RS, Cacho I, Pena L, et al. (2009) Enhanced carbon pump inferred from relaxation of nutrient limitation in the glacial ocean. Nature 459: 1114–1117. doi:10.1038/nature08101.

Pollard RT, Salter I, Sanders RJ, Lucas MI, Moore CM, et al. (2009) Southern Ocean deep-water carbon export enhanced by natural iron fertilization. Nature 457: 577–580. doi:10.1038/nature07716.

Boyd PW, Jickells T, Law CS, Blain S, Boyle EA, et al. (2007) Mesoscale iron enrichment experiments 1993-2005: synthesis and future directions. Science 315: 612–617. doi:10.1126/science.1131669.

Chisholm S, Falkowski P, Cullen J (2001) Oceans - Dis-crediting ocean fertilization. Science 294: 309–310.

Martin J, Coale K, Johnson K, Fitzwater S, Gordon R, et al. (1994) Testing the Iron Hypothesis in Ecosystems of the Equatorial Pacific-Ocean. Nature 371: 123–129.

Martin J (1990) Glacial-interglacial CO2 change: The iron hypothesis. Paleoceanography 5: 1–13.

Buesseler KO, Doney SC, Karl DM, Boyd PW, Caldeira K, et al. (2008) ENVIRONMENT: Ocean Iron Fertilization--Moving Forward in a Sea of Uncertainty. Science 319: 162–162. doi:10.1126/science.1154305.

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