As A 'Carbon Sponge,' Iron-Poor Coastal Waters Can't Always Do The Job, NaturePaper Shows

June 10, 1998

Like a sponge, the Earth's oceans store the greenhouse gas, carbon dioxide--but certain coastal waters can't perform this trick because they lack iron, a University of Delaware researcher reports in the June 11 issue of the journal Nature.

Just as iron-rich foods help children grow stronger, iron in the ocean gives a boost to microscopic plants called phytoplankton. Without enough iron, phytoplankton can't use the sun's energy to draw carbon dioxide from the air, says David A. Hutchins, an assistant professor in UD's College of Marine Studies and lead author of the Nature paper.

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, however, 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.'

In fact, 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 along one of the nation's best-known shorelines: 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 says. "Most scientists agree that the best way to reduce atmospheric carbon dioxide is to burn fewer fossil fuels and forests."

The amount of iron in water is among several important factors that determine photosynthesis and carbon-storage rates. The oceans store 60 to 80 times more carbon dioxide than the amount found in the atmosphere, though, allowing phytoplankton to use this greenhouse gas in photosynthesis. And, when these marine organisms die, they either sink into deep water, thereby capturing carbon for thousands of years, or they become imprisoned in limestone and sediments, which can contain carbon for millions of years.

In this way, "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."

Food-Chain Impacts

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 says.

"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."

The research also shows that the amount of iron in ocean water determines the amounts and ratios of other nutrients used by growing phytoplankton. Specifically, Hutchins says, phytoplankton collected from several iron-poor sites along the California coast depleted virtually all of the silicate from the water, before using nitrate. Silicate is used by a dominant group of phytoplankton called diatoms to make a shell of silicon dioxide--the same compound found in window glass and the gemstone, opal. When diatoms can't find enough iron, they consume more silicate and form more opal: two to three times more, compared to diatoms given enough iron.

When diatoms die and sink, Hutchins explains, opal collects on the sea floor, where it has been used by scientists to understand ancient phytoplankton growth and climate changes. Researchers have believed, for instance, that phytoplankton get extra iron during ice ages. When ice caps cover large areas of the continents, deserts also expand, allowing strong winds blow iron-rich dust across the planet. Estimates of the ocean's productivity during ice ages and interglacial periods-such as the current era-may need to be reevaluated, he notes, because some models are based in part on samples of opal pulled from deep-sea sediments. "We need to be careful about using opal as an indicator of phytoplankton growth and climate changes," he says, "because iron clearly plays a major role in this complex process."

This research received support from the University of Delaware Research Foundation and the National Science Foundation.

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