Iron plays key role in ocean CO2 absorption

By Environmental News Network staff

Web Posted June 15, 1998
at11:31 AM ET

A lack of iron limits phytoplankton growth in waters just off the scenic cliffs of Big Sur in the Monterey Bay National Marine Sanctuary.

Iron gives a boost to microscopic ocean plants called phytoplankton which use the sun's energy to draw carbon dioxide from the air. The process allows the oceans to store up to 80 times more carbon dioxide than found in the atmosphere.

Without iron, the process no longer works, says David A. Hutchins, an assistant professor in the University of Delaware's College of Marine Studies and lead author of the Nature paper. This finding may cause researchers to change their global climate-change models.

For a decade, scientists have known that three remote, open-ocean areas -- the equatorial Pacific, the subarctic Pacific offshore from Alaska and the Southern Ocean around Antarctica -- don't contain enough iron to support phytoplankton growth. Inadequate iron in these areas stunts the marine food chain, from bacteria to whales, and so, these waters can't store their full share of atmospheric carbon dioxide.

Researchers have long believed that coastal waters contain abundant iron, provided by nearby continental dust and sediments. Consequently, researchers have assumed that coastal areas support healthy food chains and act as effective carbon "sponges."

But Hutchins and his coauthor, Kenneth W. Bruland of the University of California at Santa Cruz, discovered that a lack of iron limits phytoplankton growth in waters just off the scenic cliffs of Big Sur in the Monterey Bay National Marine Sanctuary. These central California waters are rich in plant "fertilizers" such as nitrate, silicate and phosphate, but they don't contain enough iron to help phytoplankton use nutrients through photosynthesis, Hutchins says.

"The role of the oceans in global climate change is still controversial and not yet fully understood, but biological uptake of carbon dioxide by the coastal ocean is one important piece of the fossil-fuel puzzle," Hutchins says. "Global climate-change models reflect a host of complex physical events -- including the amount of carbon dioxide stored by oceans. If certain near-shore waters aren't functioning as an effective carbon sponge due to a lack of iron, researchers may need to change existing carbon-cycling models."

But, Hutchins cautions, dumping extra iron into the ocean could make matters far worse by triggering unforeseen chemical and biological consequences. "We are nowhere near ready to start tinkering with the ocean's ecosystems on a global scale -- that would be a really bad idea," he said. "Most scientists agree that the best way to reduce atmospheric carbon dioxide is to burn fewer fossil fuels and forests."

"Oceans buffer our attempts to burn up all the carbon that nature has stored over millions of years as fossil fuels," Hutchins says. "But, a lack of iron in certain waters means that the sea may turn out to be a less effective carbon buffer than we had hoped."

In addition to carbon-cycle impacts, a lack of iron in coastal waters may impact the entire marine food chain. Phytoplankton are the "grass" of the sea, he notes, and their photosynthesis supports "almost all of the rest of the oceans' creatures, directly or indirectly." Fewer phytoplankton, resulting from a lack of iron, means that "less energy gets passed up to higher-level creatures," such as commercially important fish or marine mammals, he said.

"Top predators-like whales and people on commercial fishing boats -- are a common sight in the iron-rich waters along California, such as Monterey Bay," Hutchins says. "You see very few in the iron-poor waters near Big Sur, though. That's because iron-starved phytoplankton populations can't photosynthesize efficiently, and that puts the crunch on the whole food chain, which depends on them. There is simply less food and energy available to support larger predators, including humans."

Copyright 1998, Environmental News Network, All Rights Reserved