By Richard Black
Environment correspondent, BBC News website
The experiment may have been compromised by voracious amphipods
The biggest ever investigation into "ocean fertilisation" as a climate change fix has brought modest results.
The idea is that putting iron filings in the ocean will stimulate growth of algae, which will absorb CO2 from air.
But scientists on the Lohafex project, which put six tonnes of iron into the Southern Ocean, said little extra carbon dioxide was taken up.
Germany's environment ministry had tried to stop the project, which green campaign groups said was "dangerous".
Leaders of the German-Indian expedition said they had gained valuable scientific information, but that their results suggested iron fertilisation could not have a major impact, at least in that region of the oceans.
"There's been hope that one could remove some of the excess carbon dioxide - put it back where it came from, in a sense, because the petroleum we're burning was originally made by the algae," said Victor Smetacek from the Alfred Wegener Institute in Bremerhaven.
"But our results show this is going to be a small amount, almost negligible."
Previous experiments, which have been going on for at least a decade, had indicated that iron particles could stimulate the growth of phytoplankton - algae - and that when the phytoplankton died, they fell to the sea floor, meaning that carbon taken from the air was effectively locked away on the bottom of the ocean.
Following fertilisation of a 300 sq km patch of ocean, Lohafex, too, saw a burst of algal growth.
But within two weeks, the algae were being eaten by tiny creatures called copepods, which were then in turn eaten by amphipods, a larger type of crustacean.
The net result was that far less carbon dioxide was absorbed and sent to the sea floor than scientists had anticipated.
"What it means is the Southern Ocean cannot sequester the amount of carbon dioxide that one had hoped," concluded Professor Smetacek.
One key issue appears to be the type of algae that grows in response to the extra iron.
Earlier experiments had found diatoms blooming - organisms with a protective silica casing.
But in the Lohafex area, the diatom population could not increase because the waters were depleted of silicic acid, the substance that is later converted to silica.
Some scientists have long argued that the iron fertilisation vision was flawed because lack of iron was not always the factor limiting growth; and this result appears to provide some backing for that contention.
But Kenneth Coale, director of Moss Landing Marine Laboratories in California, who has led several iron fertilisation experiments, said the initial burst of phytoplankton growth was consistent with previous findings.
"To date we've conducted experiments in what amounts to 0.04% of the ocean's surface," he told BBC News.
"All have indicated that iron is the key factor controlling phytoplankton growth, and most have indicated that there is carbon flux (towards the sea floor) - this is one that didn't."
A key aim for the future, he said, was to understand better the various ecosystems contained in the ocean in order that fertilisation could be conducted in areas containing the "right" kinds of organism.
Satellites can spot phytoplankton blooms in the process of formation
The Lohafex expedition, which used the German Polarstern ship, was controversial from the outset, with Greenpeace leading demands that it be stopped.
The campaign group said tipping iron filings into the sea amounted to pollution, and was forbidden under international agreements including the UN Convention on Biological Diversity, which at its 2008 meeting had called for a de facto moratorium on such experiments except at small scale in coastal waters.
"There are two things that concern us," said Greenpeace scientist David Santillo.
"Firstly, there's the direct impacts from the experiments themselves, and as the scale of the experiments has gone up and up there's much greater potential for those direct results," he told BBC News.
"But a second and broader concern is that if we're going to be pursuing this as a climate mitigation strategy, then we're looking at a state of the world where we rely on manipulating the ocean on a truly huge scale and that would undoubtedly have large and possibly irreversible effects on ocean ecosystems."
The German government put the expedition on hold earlier this year because of these concerns, but subsequently allowed it to proceed.
A commercial company, Climos, is planning a much larger experiment that could cover up to 40,000 sq km of ocean.
It hopes eventually to receive funding through the global carbon market if it can demonstrate that the technique can sequester large quantities of the greenhouse gas.
Compared with most other geo-engineering "fixes" for climate change, an added benefit of iron fertilisation in principle is that it would also combat ocean acidification, which appears likely to threaten marine organisms such as coral and snails in decades to come.