Plans to curb climate change by using plankton to draw carbon dioxide into the world’s oceans have been boosted.

A spectacular natural algal bloom in the Southern Ocean helped to “lock” carbon away into deep sea sediments, according to a study in Nature journal.

But the amount of carbon stored was not nearly as high as some artificial “geo-engineering” schemes had predicted.

Plans to “seed” plankton blooms by adding iron to oceans are strongly opposed by many green groups.

The international research team behind the Crozex study say their findings have “significant implications” for plans to mitigate climate change.

They come as scientists resume a controversial ocean fertilisation experiment in the Scotia Sea, east of Argentina.

The Lohafex study had been suspended by the German government after environmental groups protested that it violates the terms of the UN Convention on Biological Diversity.

They fear that adding iron to oceans may damage ecosystems.

Ocean commotion

Using algae as a “biological carbon pump” has been touted as one of the more promising “geo-engineering” schemes for mitigating global warming.

Plankton act as a natural sponge for carbon dioxide - drawing the greenhouse gas down out of the atmosphere and into the sea.

When plankton die, they sink to the bottom of the ocean, locking away some of the carbon they have absorbed.

Experiments suggest that “seeding” oceans with iron can stimulate the growth of plankton - particularly waters which are rich in nutrients.

But exactly how much carbon sinks to the sea floor, and how long it remains locked away, is still unknown.

In the Crozex experiment, an international research team sailed to the Crozet Islands, in the Southern Ocean, about 2,200km (1,400 miles) southeast of South Africa.

These waters experience a spectacular annual plankton bloom the size of Ireland, 120,000 sq km (46,300 sq miles) fertilised by iron naturally supplied from the islands’ volcanic rocks.

The researchers used sediment traps to follow the passage of carbon from the sea surface to the ocean floor.

They found that seawater and sediment samples taken directly beneath the bloom were two-to-three times richer in carbon, compared to samples from a nearby ocean region which was rich in nutrients, but not in iron.

“Our results have significant implications for proposals to mitigate the effects of climate change through purposeful addition of iron to the ocean,” said lead author Professor levitra Raymond Pollard, of the National Oceanography Centre, Southampton.

“Our findings support the hypothesis that increased iron supply…may have directly enhanced carbon export to the deep ocean.

“[However] our estimate of carbon sequestration for a given iron supply still falls 15-50 times short of some geo-engineering estimates.”

Next steps

“This is a significant result,” said Professor Peter Burkill, director of the Sir Alister Hardy Foundation for Ocean Science, Plymouth (SAHFOS).

“It suggests that ocean iron fertilisation might work for reducing atmospheric CO2 through export of carbon into the ocean’s interior.

“But the next step from natural experiments to artificial ones is crucial.

“We now need to know what the ecological impacts of artificial fertilisation experiments are.”

The SeaSoar was used to measure ocean properties down to 300m

Many scientists doubt whether adding iron artificially will ever seed plankton blooms as successful

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