Fertilizing ocean may slow climate change

A seasonal bloom of ocean plankton fertilised by the Amazon river pulls much more carbon dioxide out of the atmosphere than researchers had previously supposed.

The unexpected bloom may, in fact, be enough to turn the tropical Atlantic Ocean from a net source of atmospheric carbon into a net sink.

Ajit Subramaniam, an oceanographer at Lamont-Doherty Earth Observatory in Palisades, New York, and his colleagues sampled phytoplankton in the plume of water extending from the mouth of the Amazon into the Atlantic.

To their surprise, they found that for much of the year the offshore waters support a thriving plankton population that converts atmospheric nitrogen into biologically usable forms.

This nitrogen fixation allows the plankton to escape the restrictions of the nitrogen-poor Amazon and thrive on its other nutrients, especially phosphorus and silicon.

The result is a rapid uptake of carbon dioxide by the photosynthetic plankton.
'Major shift'

Moreover, much of this carbon ends up in long-term storage instead of being recycled quickly like most carbon in the ocean. That's because the main photosynthesisers are diatoms, single-celled algae that build a heavy silica shell around their bodies.

This glassy shell makes diatoms sink rapidly after they die, removing 20 million tonnes of carbon from the atmosphere each year, the researchers estimate.

That's not much compared to the amount emitted by human activity each year but it represents a major shift in our knowledge of the oceans' carbon balance, says Subramanian. And although the Amazon is the largest of the world's rivers, other major tropical rivers such as the Congo and the Orinoco may have similar effects, he says – a conjecture he is now aiming to test.

"I think the value of this work is not so much in figuring out how we can use it to humankind's advantage, but in figuring out that the major rivers of the world may be helping to balance the CO2 inventory of the planet in ways we haven't realised before," says David Karl, an oceanographer at the University of Hawaii.

Even so, the Amazon's major contribution suggests that proposals to artificially fertilise the ocean to enhance carbon uptake may pay off surprisingly well in tropical waters, says Subramaniam.

Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0710279105)