Robotic Floats Shed New Light On Iron Hypothesis

The iron hypothesis holds that by adding small amounts of iron, an essential micronutrient, to ocean waters rich in other nutrients, aquatic plants can be made to bloom vigorously, thus removing enough carbon dioxide from the atmosphere to offset the greenhouse effect.

"This was the first experiment to test the iron hypothesis by comparing observations of iron-amended waters with control observations--on the fast, hourly-to-daily time scales of marine biological processes," says oceanographer Jim Bishop of Berkeley Lab's Earth Sciences Division, who directed the Carbon Explorer studies, "and it was the first to follow such an experiment for months."

A pair of robotic Carbon Explorers recorded and regularly reported, via satellite, more than a four-fold growth of plankton in a fertilized patch of nitrate-rich but silicate-poor waters, measurements that contradicted the expectation that lack of dissolved silicates would limit plankton growth.

Programmed to descend to depths of up to one kilometer several times a day, the floats measured concentrations of particulate organic carbon and documented its export, within and outside the fertilized area, below 100 meters. They showed that for every atom of iron added to the water, the plankton carried between 10,000 and 100,000 atoms of fixed carbon below 100 meters upon sinking, well beneath the zone of light-stimulated plant growth.

"From previous experiments we knew that adding iron turns the water green," says Bishop -- in other words, makes phytoplankton bloom. "What we didn't know was what happened to the carbon." He notes three possible fates: that "the plants just dissolve in the surface waters and the carbon recirculates into the atmosphere. Or the phytoplankton could be eaten by small microbes and zooplankton living in the upper ocean, which would have no impact on atmospheric CO2 because of their respiration."

However, the Carbon Explorer results strongly support the third alternative: "The only way iron fertilization can have an impact on atmospheric carbon is if the plants fix the carbon, and a major fraction of that carbon settles out of the surface layer into the deep sea"--either as waste from grazing zooplankton or other aggregate particles, or as the plants themselves sink.

"The results of the SOFeX experiment demonstrate an important role for the SOLO-based Carbon Explorers, which were developed at Berkeley Lab with the support of DOE's Office of Science," said Dr. Raymond L. Orbach, Office of Science director. "The new, cost-effective Carbon Explorer technology makes an important contribution to understanding what happens to the carbon dioxide that enters the ocean from the atmosphere."

Carbon Explorers are based on SOLO floats, Sounding Oceanographic Lagrangian Observers, designed by Russ Davis of the Scripps Institution of Oceanography to measure temperature and salinity at various depths. ("Lagrangian" refers to the fact that the floats drift freely with the currents, which determine their speed, direction, and position.)

Bishop and his colleagues equipped SOLOs with particulate organic carbon detectors and global-positioning and rapid communications systems. The floats' stored data can be recovered and their diving profiles altered via satellite link from almost anywhere on the globe.

As part of SOFeX, a multiship project led by the Moss Landing Marine Laboratory and the Monterey Bay Aquarium Research Institute, Berkeley Lab researchers Todd Wood, Christopher Guay, and Phoebe Lam joined the research vessel Revelle in the fall of 2001. Among other projects, the researchers launched four Carbon Explorers. Bishop monitored and communicated with the Explorers over a computer link from Berkeley.

Two of the Explorers--one a control float outside the fertilized North Patch, the other inside the patch--were originally planned for placement in the silicate-poor waters of the "Furious 50s," where plankton was expected to bloom sluggishly even after iron fertilization. The other two were to be launched in the silicate-rich waters farther south in the 60s, where a more vigorous bloom was expected after fertilization.

A control float, number 1177, was launched 20 kilometers north of the planned North Patch on January 11, 2002. An intended "in-patch" float was launched just before the iron was added to the patch the next day, but the current sheared and the float was swept away to the northwest, along with some of the fertilized water.

In a change of plan, the third float, number 2104, was deployed a week later, January 19, in the middle of the remaining segment of the patch. Bishop calls this float "the hero of the experiment"--not least because it documented quite unexpected results.

Revelle then traveled south to fertilize the South Patch in the 60s, where the final float was deployed. The ship stayed in this area for several weeks. The fourth float, which would encounter communications difficulties due to lack of satellite coverage, was deployed "in the patch" at 66 degrees south just before Revelle returned northward.

Meanwhile Carbon Explorer 2104 faithfully tracked the fertilized North Patch for three weeks, paralleled most of the time by control float 1177 outside the patch. Because of bad weather only one satellite picture of the green fertilized patch of water could be taken, just a day before Revelle returned to the scene on February 7--finding its way to the patch because of "hero float" 2104's position reports.

"Everyone believed that the lack of silicates in these waters would limit phytoplankton growth and thus carbon export," says Bishop. Because of this expectation, Revelle and SOFeX's two other ships were scheduled to spend only a little time in the North Patch. "Meanwhile our Carbon Explorers were left as sentinels in the North Patch. As it happened, they turned the tables on the hypothesis that silicates would limit growth."

Plankton known as diatoms use silicates to build their delicate supporting skeletons. But work by Phoebe Lam during SOFeX suggests that the siliceous plankton of the South Patch were too buoyant to sink, while the denser calcium-carbonate "ballast" of the calcareous plankton farther north allowed them to carry the carbon they had fixed below 100 meters.

Thus, says Bishop, while "the SOFeX ships certainly did everything they were funded to do, a couple of 25-thousand-dollar floats enhanced the shipboard experiments considerably."

Bishop notes that the Carbon Explorer results have implications beyond their bearing on the iron hypothesis, offering a new way to explore the oceans. "They allowed a ship that had been gone three weeks to steam right back to the North Patch with no delay; given the paucity of satellite images, that might not have been so easy otherwise. And they filled in the time series when ships weren't in the area."

Indeed, the two North Patch floats operated in the world's stormiest waters for over 14 months, diving, measuring, recording, and reporting their data while following the circumpolar currents from near New Zealand almost to South America before finally falling silent.

In conclusion, says Bishop, "the SOFeX Carbon Explorers showed that we can use autonomous technology to understand many physical, chemical, and biological processes important to the way carbon is sequestered in the oceans. Their success has energized efforts, both at Berkeley Lab and across the country, to develop a whole new generation of sensors."

SOFeX Carbon Explorer studies were supported by the Department of Energy's Office of Science through the Office of Biological and Environmental Research. Carbon flux measurement instrumentation for the floats was supported by the National Oceanographic Partnership Program.

"Robotic observations of enhanced carbon biomass and export at 55S during SOFeX," by James K. B. Bishop, Todd J. Wood, Russ E. Davis, and Jeffrey T. Sherman, appears in the 16 April 2004 issue of Science.

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