By Powell, Hugh
Oceanus , Vol. 46, No. 1
In this age of satellites, it's fairly easy to answer the basic question of whether adding iron to the ocean can stimulate a plankton bloom. When storms over land blow iron-rich dust into the sea, satellite images show marbled swaths of green phytoplankton spinning across waters previously blue and barren. Satellites also show plankton blooms near the Galapagos and other islands where iron-rich deep waters naturally well up to surface. Even blooms spurred by experimental additions of iron to the ocean can be detected by satellite, and shipboard scientists conducting the experiments reported an almost instantaneous change in the color and even the smell of the water.
Twelve experiments so far have not looked so closely at the trickier questions of how much carbon dioxide taken up by a bloom is drawn out of the air and transferred into the deep sea, and how long it remains sequestered there. As yet, scientists have turned up only partial answers.
Philip Boyd of the New Zealand National Institute for Water and Atmospheric Research summarized the 12 experiments at an ocean iron fertilization conference convened at Woods Hole Oceanographic Institution (WHOI) in September 2007 and in an article in Science magazine earlier last year. Four took place in the northwest Pacific, two were in the equatorial Pacific, and six were in the Southern Ocean. All 12 reported up to 15-fold increases in the chlorophyll content of surface waters. (Chlorophyll is the sunlight-capturing molecule in photosynthesis and is often measured in lieu of actual plankton counts.)
Only a tiny fraction of the carbon drawn down by blooms sinks from the surface into deeper waters, where it is sequestered from the atmosphere. Estimates of the tonnage of carbon sequestered (measured at 200 meters depth) per ton of iron added hover around 200 to 1, a far cry from early experiments in laboratory beakers that yielded estimates around 100,000 to 1, Boyd said.
But those may be underestimates. Although scientists have spent up to several weeks monitoring blooms after iron addition, ship schedules and budgets have usually prevented them from monitoring long enough, or deep enough, to obtain good measurements of "export efficiency"--the proportion of carbon that sinks from the surface into deeper waters.
The 2002 United States-funded SOFeX experiment did show that more carbon was exported into deeper waters below the fertilized ocean patch, WHOI marine biochemist Ken Buesseler and colleagues reported. And unpublished results from the 2004 European EIFeX experiment showed levels of carbon sequestration that were far higher and far deeper (all the way to the seafloor) than previously observed--but this occurred only in the final days of monitoring, Victor Smetacek of the Alfred Wegener Institute in Germany told participants at the WHOI conference.
The emerging picture is that iron fertilization does in principle work well enough to squirrel away carbon for at least a few decades--possibly useful in the world's efforts to solve its carbon emissions problem. Although present yields seem low, improved methods could boost that number in two ways: by refining logistics to make blooms larger, and by increasing "export efficiency," or the proportion of carbon that sinks from the surface into deeper waters, where it is less easily returned to the atmosphere.
Logistics and luck
Iron addition is simple in principle, but once a ship is loaded up and heading for open waters, even small experiments become a tangle of logistics. The SOFeX experiment employed three research ships, helicopter scouts, and 76 scientists to monitor the results of adding one to two tons of iron to the ocean.
The typical method involves drizzling acidified iron sulfate into the ocean as a thin slurry, to reduce the amount that immediately sinks out of the sunlit surface waters where photosynthesis happens. …