A controversial ‘ocean fertilization’ experiment suggests seeding the seas with iron to boost carbon-absorbing phytoplantkon will not sequester much carbon dioxide from the atmosphere. Some — including researchers and private companies — had hoped iron fertilization might be an easy fix for climate change.

The Lohafex experiment, conducted by Indo-German team of scientists from the National Institute of Oceaonography and the Alfred Wegener Institute earlier this year, dumped 20 tons of iron sulphate into the Southern Ocean and measured the carbon uptake by plankton. Fertilization stimulated a short burst of phytoplankton growth which was negated by increased predation by crustaceans known as amphipods.

“As a result, only a modest amount of carbon sank out of the surface layer by the end of the experiment,” said the Alfred Wegener Institute in a statement.

The experiment found that blooms of diatoms, a silica-based algae important in earlier carbon sequestration experiments, were limited by natural blooms that had already extracted all the silicic acid needed for shell building.



Closeup of the Amphipod Themisto gaudichaudii, a predator of phytoplankton. Photo: Humberto Gonzalez, UACh-COPAS / Alfred Wegener Institute

A second fertilization experiment three weeks after the first had no further effect on plankton “indicating that the ecosystem was already saturated with iron.”

By the end of the experiment phytoplankton had stabilized at its original levels. The experiment found that “concentrations of gases other than CO2 produced by the plankton either did not change or increased negligibly in the bloom”.

“Some of these gases such as nitrous oxide and methane are potent greenhouse gases, others such as halogenated hydrocarbons contribute to stratospheric ozone depletion,” noted the Alfred Wegener Institute.

“The cooperative project Lohafex has yielded new insights on how ocean ecosystems function,” the statement continued. “But it has dampened hopes on the potential of the Southern Ocean to sequester significant amounts of carbon dioxide (CO2) and thus mitigate global warming.”

Other research has produced similar results, suggesting that iron fertilization of oceans will not be an effective climate change mitigation strategy.

“Ocean iron fertilization is simply no longer to be taken as a viable option for mitigation of the CO2 problem,” Hein de Baar, an oceanographer at the Royal Netherlands Institute for Sea Research in Texel, told Nature News in January.

Other studies have also looked at the introduction of calcium hydroxide (i.e. lime) as a way to reduce ocean acidification and its capacity to absorb CO2.

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