Huge Man-Made Algae Swarm Devoured--Bad for Climate?Kelly Hearn
for National Geographic News
March 27, 2009
A giant experiment went awry at sea this month.
Shrimplike animals devoured 159 square miles (300 square kilometers) of artificially stimulated algae meant to fight global warming—casting serious doubt on ocean fertilization as a climate-control tool.
Can Iron-Enriched Oceans Thwart Global Warming?
For years, scientists have proposed supercharging algae growth by dumping tons of iron into the ocean.
Iron is a necessary element for algae photosynthesis—the process by which the plants convert sunlight into energy—but it is relatively rare in the ocean.
Algae suck carbon dioxide (CO2), a greenhouse gas that contributes to global warming, out of the atmosphere. The algae then generally fall to the seafloor—sequestering the CO2 indefinitely.
About a dozen such "iron fertilization" experiments have already been done—with mixed success.
But experts have warned of unintended consequences, such as unpredictable reactions in the ecosystem.
And that's just what happened during a recent, large-scale iron dump in the South Atlantic, the Alfred Wegener Institute in Germany announced this week.
With the greenish, crystalline look of a pulverized windshield, ferrous sulfate is commonly given to iron-deficient humans.
It's also the iron of choice for boosting algae growth.
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."
(Read about other global warming solutions.)
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."