From BBC website:
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.
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.