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Oceanography

Stirring times in the Southern Ocean

Nature volume 407, pages 685–686 (2000)Cite this article

Phytoplankton biomass in the global oceans can usually be correlated with the supply of nitrogen, phosphorus and silicon upwelling from the deep sea to the sunlit surface waters where photosynthesis takes place. In the equatorial Pacific and the Southern Ocean, however, these supplies far exceed demand, indicating that some other factor is limiting phytoplankton growth. This mystery plagued oceanographers for decades until the late John Martin showed that the limiting factor is iron, a trace element that reaches the oceans in atmospheric dust4. Analyses of ancient ice cores show that dust deposition varied significantly during glacial and interglacial periods, and is anticorrelated with concentrations of atmospheric CO2 over the past 400,000 years. So Martin reasoned that airborne iron supply could in part regulate climate by increasing the efficiency of the biological pump. He further suggested that very small amounts of iron distributed in today's Southern Ocean could draw a significant amount of CO2 from the atmosphere. This is partly because the Southern Ocean is huge and, apart from iron, holds vast quantities of unused nutrients, and partly because its surface waters tend to sink, delivering carbon to the deep ocean.

With this as a backdrop, Boyd et al.1 launched the SOIREE expedition (for Southern Ocean iron release experiment) in February 1999. They distributed 8,663 kg of an iron compound over a patch of ocean 8 km in diameter, some 2,000 km south-south-west of Hobart, Tasmania. The experiment was a dramatic success. Physiological indicators of iron stress decreased in the iron-enriched patch within two days. Primary productivity, phytoplankton carbon and chlorophyll levels increased slowly but steadily over the 13 days that the patch was monitored, drawing nutrients and CO2 from the surface waters. The result was a tripling of phytoplankton chlorophyll by the end of the experiment. These results are qualitatively much like those from a similar experiment5, conducted in the equatorial Pacific in 1995, but the development of the phytoplankton bloom was much slower and its magnitude smaller. The increase in phytoplankton biomass resulted in large part from a shift in the phytoplankton community from small- to large-celled species, primarily diatoms, which can outgrow their predators when suddenly released from nutrient limitation.

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Figure 1: The 'biological pump' is a collective property of a complex phytoplankton-based food web.

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  1. Department of Civil and Environmental Engineering, and the Department of Biology, Massachusetts Institute of Technology, Cambridge, 02139, Massachusetts, USA

    Sallie W. Chisholm

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Chisholm, S. Stirring times in the Southern Ocean. Nature 407, 685–686 (2000). https://doi.org/10.1038/35037696

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