Iron is the scarcest vital nutrient in most areas of the oceans, say researchers with the USC Wrigley Institute for Environmental Studies. That makes it potentially the most interesting.

James Moffett, a professor of biological sciences in USC College, has been studying highly productive areas off California, Peru and in the Arabian Sea. At least in the Arabian Sea, he is finding that the zones with the most aquatic life are iron-limited. They are productive because other nutrients are plentiful.

Would they be even more productive if iron were added? Could less-productive areas also get a boost?

The question of “iron fertilization” continues to split the scientific community. Some scientists have called for iron to be used at sea as nitrogen was on land during the Green Revolution: as a stimulant to boost plant growth and, as a bonus, to remove additional carbon dioxide from the atmosphere.

Others warn that iron fertilization would have unforeseen consequences, including production of other greenhouse gases and wholesale changes in the food web.

Moffett and others caution that excessive stimulation of productivity can backfire, as rapid plant growth depletes the water of oxygen, causing massive fish kills.

Moffett is more interested in subtle changes in ocean chemistry from events on land. For example, drought caused by global warming, or agricultural mismanagement, can affect the amount of iron blown or carried into the sea.

“My work creates linkages between phenomena that were not suspected to be connected before,” such as the connection between drought and ocean productivity in the Arabian Sea, Moffett says. He plans to conduct a large cruise off iron-starved waters in Peru to confirm his findings.

Another USC Wrigley scientist, Douglas Capone, published research in 2008 in Proceedings of the National Academy of Sciences suggesting that iron fertilization in tropical waters could boost plant growth while avoiding some of the risks. Because the water in the deep tends not to mix with surface water at the tropics, those oceans should be able to keep captured carbon solids from returning to the surface in the short term, he says.

“The most appropriate places are probably not the high latitudes,” he adds, “but rather the low-latitude tropical areas.

"If we choose as a human society to fertilize areas of the oceans, these are the places that probably would get a lot more bang for the buck in terms of iron fertilization.”

And in a recently published study in Nature Geoscience, Katrina Edwards of USC Wrigley showed that iron fertilization may be occurring naturally, and from an unlikely source. Edwards and colleagues found that some iron emitted by hydrothermal vents at the bottom of the ocean can be captured by organic solids and carried away in seawater, potentially feeding life near the surface.

– C.M.