Iron makes the southern ocean bloom

In February 1999, New Zealand scientists led an international research expedition to the remote Southern Ocean,to artificially initiate an algal bloom by adding dissolved iron to the sea. The results of this expedition are published today in a series of papers in the prestigious scientific journal Nature.

The Southern Ocean makes up 15% of the world’s oceans and has a major influence on global climate. Although the waters of this inhospitable region are rich in nutrients, they contain extremely low numbers of the microscopic plants, known as phytoplankton or algae, which support the marine food-chain. Plants need iron in order to grow, and the remoteness of the Southern Ocean from land results in low amounts of iron in this region. It has long been thought that the addition of iron could boost the numbers of phytoplankton in the waters of the Southern Ocean. Like forests and grasslands on land, these plants are believed to be able to play a key role in reducing the build up of carbon dioxide in the atmosphere, which is causing global warming. Because of this, there have been controversial proposals to fertilise the Southern Ocean with iron in order to help slow the pace of climate change.

The NIWA-led expedition, known as the Southern Ocean Iron Release Experiment or SOIREE, showed conclusively that fertilising the Southern Ocean with iron causes the algae to bloom. The addition of relatively small amounts of dissolved iron over a 50 square km area raised iron levels to 10 times the concentration normally found in these waters. Within two weeks, phytoplankton abundance was 10 times greater than outside the fertilised patch. Remarkable images from a NASA satellite show that six weeks after the fertilisation the bloom had grown to cover an area of 1100 square km. The persistence of the bloom for such a long time was entirely unexpected, and is thought to be due to the ability of the phytoplankton to release special substances into the water that hold the iron in a form they can use. The spread of the initial patch into the large bloom seen in the satellite images vividly demonstrates the sensitivity of the Southern Ocean ecosystem to small increases in iron concentrations. The amount of iron which was added to the seawater was only 2 tonnes – a small truckload – yet this was enough to increase growth in a volume of water equivalent to 20 million Olympic swimming pools.

Although there was a spectacular burst of algal growth after the fertilisation, the scientists were unable to measure any appreciable removal of carbon from the surface to deeper waters. This would be required if iron fertilisation were to be effective at reducing the accumulation of carbon-dioxide in the atmosphere. It is likely that, once the bloom finished, some of the carbon contained within the algae was eventually released back into the atmosphere. While the results of the SOIREE expedition do not support the use of ocean fertilisation as a way of preventing climate change, they do show that iron fertilisation would produce blooms of an intensity never normally seen in the Southern Ocean. Large-scale fertilisation would be likely to cause substantial changes to the naturally occurring ecosystems of this pristine environment.

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