Working aboard the German research vessel Polarstern, German and Indian scientists in recent weeks mixed ten tons of ferrous sulfate with seawater. The team then pumped the artificially enhanced water back into the Atlantic outside Argentina's coastal waters.
As expected, the experiment created a massive, CO2-eating algae bloom.
But it was the wrong algae.
The blooms were mostly tiny haptophytes, not the larger diatom algae the team had expected.
The smaller algae variety is typically found only in coastal waters, and it's a favorite food of tiny shrimplike crustaceans called copepods.
The copepods wolfed down the algae shortly after the new South Atlantic bloom appeared—and a potential weapon against global warming quickly disappeared.
"The fact that they are rapidly eaten by marine animals is not good for carbon sequestration," said Ulrich Bathmann, head of bioscience at the Alfred Wegener Polar and Oceanography Institute (AWI) in Bremerhaven, Germany, who was involved in the experiment.
Good News? Bad News?
Experts not part of the new experiment are divided on what the results mean.
"The new finding here is that the standard calculations of 'the number of tons of iron in equals the number of tons of carbon out' probably don't actually work," said Gabriel M. Filippelli, an earth sciences professor at Indiana University-Purdue University Indianapolis.
"This calls into question the efficacy of iron fertilization as a solution to global warming."
Iron-fertilization supporters, though, remain hopeful.
"These results neither argue for nor against iron fertilization as a carbon-sequestration strategy," said Kenneth Coale, director of California-based Moss Landing Marine Laboratories.
Moss Landing scientists created a similar, though smaller, algae bloom in Antarctic waters in 2002.
On the bright side, Coale said, the experiment adds to evidence that iron can stimulate large-scale algae growth. It's not clear that in every instance animals would gobble up the carbon-sucking plants, he says.
Other experiments have also had better success at sequestering carbon, Coale added.
And regardless of its carbon-sequestration success or failure, Coale said, at least the South Atlantic experiment did not damage the local ocean environment—which would have been a more serious black mark on iron fertilization.
The consensus, though, seems to fall somewhere on the fence, said environmental scientist Andrew Watson of the University of East Anglia, U.K.
The recent experiment, Watson said via email, "shows that we still haven't learned by any means all there is to know about the effects of iron on marine ecosystems and the carbon balance in the oceans."
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