Iron fertilisation of the ocean raises the food supply of marine animals and transports carbon dioxide to the deep ocean

An international team of scientists that recently carried out an experiment in the South Atlantic on board RV „Polarstern“ of the Alfred Wegener Institute for Polar and Marine Research has found that an algal bloom induced by iron fertilisation transported carbon dioxide to the deep ocean. The bloom also stimulated growth of zooplankton grazers such as copepods and krill that form the basis of food chains leading to fish stocks of commercial importance such as sardine and herring but also whales.

The large-scale experiment EIFEX (European Iron Fertilisation Experiment) was conducted in the Antarctic Circumpolar Current to find out whether organic matter produced by algae is converted back into the greenhouse gas carbon dioxide in the surface layer or whether a substantial portion sinks out to the deep ocean which would mean long-term removal of carbon dioxide from the atmosphere. Iron is essential for all life but it is highly insoluble in sea water and has to be continuously resupplied by dust blown off the continents. Land masses are rich in iron so the coastal seas such as the North Sea are more productive and support much larger fish stocks than the open ocean.

Can the carbon dioxide released by fossil fuel burning be taken up by the Southern Ocean? „Theoretically, the maximum that can be taken up per year would amount to about 15% of annual human output, equivalent to one billion tonnes of carbon. It would be more sensible to reduce consumption than to attempt to accelerate one of nature´s regulatory mechanisms“ says Chief Scientist Victor Smetacek of the Alfred Wegener Institute and points to an aspect of iron fertilisation that has not received much attention yet. „In the more productive oceans of the ice ages there must have been much larger stocks of whales than before their decimation by whaling,“ explains Smetacek. „Could local fertilisation of the breeding grounds of krill, the major whale food, stimulate their growth and help counteract the ongoing apparent decline of krill? This boost in their food supply would help endangered whales to recuperate their populations,“ conjectures Smetacek and adds that this aspect needs to be discussed on an international level.

The nine-week experiment was organised and led by the AWI. On board were 53 scientists from 14 institutions and three companies from seven European countries and South Africa. The fertilisation was carried out 2200 km southwest of Cape Town. The team released a solution of iron sulphate in the propeller wash while Polarstern drew ever wider circles around a reference buoy. Seven tonnes of iron sulphate were necessary to fertilise an area of 150 km2 with a diameter of 14 kms. The resulting chemical and biological processes were measured round the clock by the team of experts.

The improved food supply in the fertilised patch attracted zooplankton from the surrounding unfertilised water. They grazed down the fast-growing, „juicier“ algae, so larger, slower-growing but spiny algae dominated the bloom. In the fourth week of the experiment mass death of the large, spiny algae resulted in their almost total disappearance from the surface layer. Their spines entangled with one another and formed large flocks that sank so rapidly that some were found close to the bottom at 3,800 m depth within a week. This example shows how animal and plant interactions that evolved over millions of years can influence global cycles of elements particularly that of carbon.

Although unicellular algae are responsible for a third of global photosynthesis, little is known about the ecology of the large variety of species of very different shapes and sizes and why they appear and disappear in the plankton over periods of weeks. Systematic research on the function of the sometimes bizarre shapes has just begun. What percentage of them are eaten in the suface layer and are so channelled up the food chain leading to fish, cephalopods and whales and what percentage of the material produced by them sinks out in the dark abyss are still central questions in oceanography.

Bremerhaven, 5 April 2004