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Bloom! Satellite images show the sudden growth of plankton following the eruption of the Kasatochi volcano (triangle).

Hamme et al./Geophysical Research Letters, 37 (2010)

How Volcanoes Feed Plankton

Want to spur plankton growth in the ocean? Hire a volcano. In August 2008, scientists in the northeastern Pacific Ocean were shocked to witness a sudden, huge spike in the area's plankton population. Their investigation traced the bloom to an ash cloud from a volcano that had erupted in the Aleutian Islands only a few days before. The ash, it turned out, had fertilized the ocean with thousands of tons of iron, on which the plankton gorged.

A new study of the phenomenon, published this week in Geophysical Research Letters, shows for the first time outside of the laboratory how quickly plankton can bloom from an infusion of iron. It also reinforces the notion, however, that deliberately seeding the oceans with iron—which has been proposed as a way to draw carbon dioxide out of the atmosphere—might do almost nothing to change the CO2 content of the air.

Phytoplankton are microscopic plants that serve as the foundation for the entire marine food chain. But the plankton depend on more than sunlight for their sustenance. They also need mineral nutrients such as nitrate, phosphate, and iron. When areas of the ocean are low in iron, the plankton population usually remains sparse.

Until August 2008, that was the case in a region of the North Pacific bounded by Canada and the Aleutian Islands—nearly 2 million square kilometers. It's coincidentally one of the best-studied ocean regions in the world, with data on plankton going back over 50 years. But by midmonth, something surprising had happened. A major plankton bloom had somehow gotten started, and within just a few days the chlorophyll concentration in the water had increased by 150%.

A team of scientists happened to be in the area on a research cruise when they detected the beginning of the plankton bloom. "Once we knew that something extraordinary had happened, we wanted to know what had caused it," says chemical oceanographer and lead author Roberta Hamme of the University of Victoria in Canada.

Based on their analyses of air and water chemistry, they reported that dissolved iron must have dramatically increased, perhaps by five to 10 times the normal concentration. The source, they deduced, was the 7 to 8 August eruption of the Kasatochi volcano some 2000 kilometers away. A severe windstorm had spread the ash cloud across an unusually wide area over the ocean.

Ocean chemist Philip Boyd of the University of Otago in New Zealand says many other researchers have tried to link an infusion of iron from volcanic ash or even dust storms to plankton blooms, but this study is the first to "verify such a massive event."

Despite the size of the bloom, however, the plankton did not take in a record-breaking amount of carbon dioxide—only about 20% more carbon than that part of the ocean sequesters biologically each year. Hamme says the team's preliminary analysis suggests that trying to stimulate plankton growth by adding iron to the water would have a minuscule effect on marine CO2 absorption.

Ocean scientist Paul Harrison of the University of British Columbia, Vancouver, in Canada, who was not involved in the research, agrees. "Purposeful, large-scale additions of iron to produce an algal bloom and hence decrease the buildup of atmospheric CO2 may not be as effective as previously thought."