Heavy Metal

Scripps scientists offer new insight into natural iron fertilization occurring in the world's oceans

Drake Passage, a stretch of water between the southern tip of Chile and the northern tip of the Antarctic Peninsula, is well known as one of the roughest stretches of water in the world. Strong winds blow constantly and waves over 10 meters (30 feet) high are commonplace.

Despite its unwelcoming nature, the passage is a frequent destination for scientists at Scripps Institution of Oceanography, UC San Diego who believe it is a prime location to study natural iron fertilization in the world's oceans. As scientists and policymakers consider artificial "iron seeding" of the oceans as a means of combating global warming, an interdisciplinary team at Scripps has described in detail how nature itself distributes nutrients in this key region.

The team, which includes Scripps researchers Kathy Barbeau, Greg Mitchell, and graduate student Brian Hopkinson, published research findings in the November issue of the journal Limnology and Oceanography. The paper is the first study to experimentally document the natural fertilization process in the peninsula region of the Antarctic Circumpolar Current (ACC) and provides insight into the response by phytoplankton to the addition of iron in the ocean, team members said.

"This study clearly demonstrates that the addition of iron is being supplied from the Antarctic shelf," said Hopkinson, a recent Ph.D. recipient in the Scripps Geosciences Research Division and lead author on the paper. The shelf offers a natural supply of iron-rich water, which contributes to the unique blooms of algae observed in the region, explained Hopkinson.

Artificial iron fertilization has gained popularity recently as a potential remedy to mitigate global warming. Seeding ocean waters with iron could induce blooms of phytoplankton to remove through photosynthesis significant amounts of the greenhouse gas carbon dioxide from the atmosphere.

Such large-scale studies of natural iron fertilization like this one will help scientists to better understand the role of iron in the global carbon cycle and the ecological costs of seeding the ocean with additional iron.

"The emerging market in carbon credits has created a resurgence of interest in the use of iron fertilization of the oceans as a means to offset carbon emissions," said Barbeau. "Continuing studies of such areas can provide us with valuable information about the efficiency of oceanic iron fertilization as a means to sequester carbon and the effects of sustained iron fertilization on the oceanic ecosystem."

Conditions in the Drake Passage offer scientists a natural laboratory to study the distribution of phytoplankton and these tiny free-floating organisms' response to iron enrichment.

"It's a unique environment," said Barbeau, assistant professor of marine chemistry at Scripps. "We cannot point to another area of the Southern Ocean where iron fertilization-induced chlorophyll gradients are so dramatic and accessible for this type of research."

To collect the data, research vessel Laurence M. Gould, an icebreaker operated by the National Science Foundation, headed to the frigid waters of the ACC just west of the Antarctic Peninsula shelf. There the research team collected water samples from the bloom areas and shelf regions.

To most effectively mimic natural conditions, the researchers worked quickly to transfer samples to a shipboard laboratory for incubation. The lab, a retrofitted shipping container outfitted with banks of blue-fluorescent lights, simulates natural conditions in the open ocean through controlled light levels and temperature.

Mitchell, a research biologist, used bio-optical techniques to study the interactions between light, iron, and phytoplankton growth rates in the incubation studies. He has also used satellite data to study the transition between low- and high-chlorophyll waters that occurs in the southern Drake Passage in response to natural iron addition.

Methods used by the Scripps team can be applied to different regions and can provide a baseline for additional studies on the effects of natural and artificial iron fertilization on other organisms, especially those higher in the food chain.

Scripps scientists Rick Reynolds, Haili Wang, Christopher Hewes, and Osmund Holm-Hansen; and University of Hawaii researchers Karen Selph and Christopher Measures also contributed to the research paper.

Annie Reisewitz