Iron to blame

Huge algal blooms in the Indian Ocean dance to their own rhythm. In other places, such blooms occur every spring, when masses of tiny plants multiply in surface waters. But off the coast of Madagascar, the greenery erupts in late summer, and only every few years. Now, one researcher attributes this unusual pattern to typhoons that boost the amount of iron in the ocean.

Springtime blooms occur as surface-temperatures rise, warming nutrient-rich water that has risen from the cold ocean depths all winter. This creates the perfect environment for single-celled plants to thrive. But the erratic summer timing of the Indian Ocean bloom has puzzled scientists.

Sifting through meteorological data in search of a cause for these occasional blooms, which can span an area the size of Alaska, Baris Mete Uz of the University of Maryland at College Park noticed a pattern. Whenever a typhoon hit Madagascar, an algal bloom flared up shortly after, he reports in the Sept. 15 Journal of Geophysical Research.

Typhoons drench the land and wash soil into the ocean, Uz explains. While nitrogen and phosphorus in this runoff would cause immediate plant growth in waters near the shore, iron is slower to trigger growth, and thus could cause a slow, far-from-shore bloom like the Indian Ocean one.

“It’s surprising, however, because people don’t expect blooms triggered by iron to be so large,” says Uz.

Blooms caused by iron have important climate implications, he adds, because they soak up carbon dioxide from the atmosphere. Typical algal blooms instead thrive off nutrients that well up from the deep ocean.

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