Ocean "fertilisation" has ignited fierce scientific controversy, with supporters saying these schemes could stave off damaging climate change and critics warning that swathes of ocean may turn stagnant or acidic.

Fertilisation has now touched off a political storm as well. An experiment by scientists aboard a German research ship in waters off Antarctica has pitted Germany's environment ministry, which opposes the scheme, against the country's research ministry, which supports it.

The idea behind fertilisation, an example of so-called "geo-engineering" -- is to scatter iron powder in swathes of the ocean, providing nutrients for algae in the warm upper layers of the sea called phytoplankton.

These tiny marine plants suck in carbon dioxide (CO2) by photosynthesis.

When they die, some of them would sink to the depths and their carbon remains would be stored, or sequestered, there. In other words, greenhouse gas would be transferred from the atmosphere to the depths of the ocean, and so would not be around to trap solar radiation.

But new research, published in the British-based science journal Nature, casts doubt on some claims of the effectiveness of the process.

Researchers led by Raymond Pollard of the National Oceanography Centre in Southampton, southern England, looked at seas around the Crozet Islands, a small archipelago on the northern rim of the Southern Ocean.

The flow of ocean currents means that the seas just north of the islands are rich in natural iron -- carried off Crozets' volcanic rocks -- and causes blooms of plankton in the southern summer that last for months.

South of the islands, though, is nutrient-poor, and plankton blooms there are far smaller and short-lived.

Comparing the two zones, the so-called CROZEX mission found that iron-rich seas doubled or even tripled plankton growth and the absorption of CO2.

But the amount of CO2 that was actually stored was just five or six percent, explained senior researcher Richard Sanders.

"If we think of a hundred units of carbon being fixed by phytoplankton in the upper ocean, around 90 percent of that will be recycled in the upper ocean and around 10 percent will sink out of this sunlit upper layer," he told AFP.

"Of those 10 units sinking, one will get to the sediment at the bottom," where it will be effectively stored forever.

The rest, though, will be recycled in the midwater region or, in the lower depths, eventually get pushed to the surface by deep ocean currents, which would prompt them to surrender their carbon, he explained.

This overturning takes place on a time scale of decades to a couple of hundred years, meaning the carbon would be stored out of harm's way for a long while but not permanently.

The study said carbon sequestration around the Crozet islands fell massively short -- by 15 to 50 times -- of some geo-engineering estimates, although it was also 20 times more than that calculated for a fertilisation experiment called SERIES.

These findings have "significant implications" for claims for ocean fertilisation, it said.

Around a dozen experiments in ocean fertilisation have been carried out, but the data they have yielded is often sketchy and contradictory. The experiments are complex and difficult to carry out and often do not look at the long-term effectiveness.

Some oceanographers, looking at published data, contend the sequestration yield is so poor that iron would have to be dumped over vast areas to make any inroad into the greenhouse-gas problem.

This would make the technique commercially unviable in addition to posing unknown risks for marine ecology, they say.

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