Great blooms of oceanic algae, called phytoplankton, take carbon out of the atmosphere during photosynthesis, much of which is then taken deep into ocean with them when they die. Scientists have theorized that this mechanism helped cool the earth during historic ice ages by removing carbon dioxide from the atmosphere and storing it at the ocean floor, where it cannot be recycled back into the atmosphere. Inducing algal blooms on a large scale could do the same today, reducing the impact of carbon dioxide on the greenhouse effect and slowing the impact of global climate change.
Indeed, as reported today (July 18) in Nature, scientists have seeded the ocean with iron and watched as the resulting bloom flourished, then died, sinking down to the deep ocean with a significant amount of carbon in tow.
"[The authors] were focused and quite successful at seeing if you added iron, what would be the biological response of the phytoplankton? What was the fate of the carbon and nutrients they sucked up?" said marine geochemist Ken Buesseler of the Woods Hole Oceanographic Institution, who was not involved in the study. "This is probably the best example of a group that stayed out long enough and looked deep enough to see if there were any effects below the surface."
During glacial periods, more dust rich in iron, an essential nutrient for algae, reaches the oceans. Iron is particularly limiting in the Southern Ocean, and previous studies have shown that adding iron to the surface waters does induce large blooms of algae. In 2004, Victor Smetacek of the Alfred Wegener Institute for Polar and Marine Research, Germany, and his colleagues decided to repeat these experiments and closely monitor what happened. They seeded a portion of the Southern Ocean and followed the progress of the resultant algal bloom for 37 days. The team took regular profiles of the nutrients and biomass down through a column of water to the sea floor, several thousand meters deep. From their data, they were able to see good correlation between what the algae were using up at the surface, and what was showing up again in the deep, as the phytoplankton died and sank.
The team chose to spread the iron over an eddy, an isolated spinning column of water, so they could minimize the exchange of water and nutrients with the open ocean, effectively using the eddy as a natural beaker. They were able to estimate that about half of the bloom biomass sank to depths of 1,000 meters or more, well below the upper mixed layer (the first 100 meters or so in this region) that allows open exchange with the atmosphere. Carbon sunk in this way can stay stored for centuries, until deep, slow ocean currents eventually bring it back to the surface.
Smetacek says that while longer experiments are still needed, as are samples from the sea floor sediment itself, rather than just deep water, to confirm what settles into the mud, preliminary estimations suggest that as much as one gigaton of carbon could be removed from the atmosphere in this way each year—a quarter of the carbon that is currently accumulating in the atmosphere as a result of human activities on an annual basis. Thus, iron fertilization could serve as an effective geoengineering strategy to help slow the harmful effects, such as climate change, that come with rising atmospheric carbon. However, both Smetacek and Buesseler warn of the potential drawbacks of large-scale fertilization.
"If you do this for a longer time scale, you could change the structure of the ecosystem,” said Smetacek. By increasing the phytoplankton population, the number of microscopic animals that feed on it could also explode, rapidly using up all the nutrients at the surface and creating "dead zones" where nothing can thrive, he explained. In addition, some algal blooms release harmful gases and toxins which could damage surrounding marine ecosystems. No such consequences occurred in the present experiment, Smetacek said, but drawbacks of larger experiments "should be monitored by an independent group of scientists that are working in a non-profit manner."
The ocean's ability to sink carbon also depends on the many changes we are already making to the marine environment, including rising ocean temperatures and the decline of whale and krill populations in the Southern Ocean, which have previously played a large part in the cycling of nutrients harnessed by algal blooms. This experiment and its predecessors play important roles in understanding how the ocean, which naturally sinks about a third of our carbon emissions, plays in the global carbon cycle.
"The experiments themselves are not of huge environmental consequence, [but] we can learn a lot from them," said Buesseler. "This was a successful way to study the ocean and climate."
V. Semtacek et al., "Deep carbon export from a Southern Ocean iron-fertilized diatom bloom," Nature, 487: 313–319, 2012.