A study led by New Zealand's University of Otago of Dunedin found that while a ton of iron would fertilise a phytoplankton bloom, it would also require at least 5,000 tons of silicate to sustain it.

"It's just not practical," Otago oceanographer Philip Boyd said in a paper published in the science journal Nature and issued Friday by the university.

Phytoplankton harvest sunlight to fix carbon that is then either re-mineralised to form carbon dioxide in the surface waters of the ocean and released back into the atmosphere, or "pumped" down to the deep ocean layer as the plankton sinks.

In 1999 Boyd and a team of other New Zealand and international scientists distributed around 8,000 kilograms (17,600 pounds) of an iron compound in solution over a patch of the Southern Ocean eight kilometres (five miles) in diameter.

The result was a five-fold increase in phytoplankton stocks during the developing bloom and it was believed that simply adding iron might be the answer to increasing the amount of atmospheric carbon dioxide locked up in the ocean.

"What we found, however, is that adding iron to the ocean produces a very different picture in the longer-term," Boyd said.

After 18 days of a similar experiment in the Gulf of Alaska, the iron-induced bloom declined and satellite pictures show merely a ghost of the plankton-rich patch that had blossomed initially.

"We think the decline was initiated by the drop-off in iron levels, but the secondary factor is the removal of all of the silicate by phytoplankton.

"Until now, we had not realised the importance of silicate in causing the bloom's decline," Boyd said.

"And while it might be feasible for us to add iron to the ocean to stimulate blooms, for every ton of it we throw overboard, we'd need to add at least 5,000 tons of silicate to enable the blooms to persist for long enough to impact on atmospheric carbon dioxide levels," he said.

SPACE.WIRE