Dead algae were then expected to sink to the ocean bed, dragging carbon along with them. Instead, the bloom attracted a swarm of tiny crustaceans, the copepods, which feed on phytoplankton, and in this way prevent carbon from being stored in the ocean sink. The copepods were in turn eaten by larger crustaceans called amphipods, which serve as food for squid and whales, as explained by the researchers from the Alfred Wegener Institute for Polar and Marine Research.

"I think we are seeing the last gasps of ocean iron fertilisation as a carbon storage strategy," said Ken Caldeira of the Carnegie Institution at Stanford University, quoted by the New Scientist.

No silica – no diatoms – no success

Previous fertilisation experiments had only caused blooms of diatoms, a type of phytoplankton that is protected against grazers by a hard shell of silica. But this time a diatom bloom did not occur because there was little silicic acid available in the water from which diatoms could build their shells.

The Southern Ocean is thought to be the largest ocean carbon sink on the planet. But most of its northern half is low on silicic acid, and therefore is not an option for carbon fertilisation.

"It seems that if it is possible to fertilise enough ocean to make a difference to the climate, we would need to turn vast ocean ecosystems into giant plankton farms," says Caldeira.

Hazard vs. benefits

In contradiction to yet unproven value to the environment, such experiments raise a series of concerns, too. Among the primary ones is that encouraging the oceans to absorb carbon dioxide boosts their acidity.

Some companies already claim they could one day seek to sell carbon offset credits similar to those offered by tree planting schemes. However, the International Maritime Organization is in the process of drafting international regulation for ocean fertilisation activities through its London Convention. Early discussions suggest it is not in favour of commercial fertilisation schemes.