From Philip Boyd, New Zealand National Institute for Water and Atmospheric Research TESTING THE WATERS—Twelve small-scale experiments over the past decade in several ocean locations (red dots) consistently showed that intentional iron additions do result in phytoplankton blooms that help draw down carbon dioxide from the air. But the experiments have not determined how much carbon is transferred and sequestered in the deep sea, rather than quickly recycled back to the atmosphere. Will Ocean Iron Fertilization Work? Getting carbon into the ocean is one thing. Keeping it there is another. n this age of satellites, it’s fairly easy to a bloom is drawn out of the air and trans- Only a tiny fraction of the carbon drawn answer the basic question of whether ferred into the deep sea, and how long it re- down by blooms sinks from the surface into addingI iron to the ocean can stimulate a mains sequestered there. As yet, scientists deeper waters, where it is sequestered from plankton bloom. When storms over land have turned up only partial answers. the atmosphere. Estimates of the tonnage of blow iron-rich dust into the sea, satellite im- Philip Boyd of the New Zealand Na- carbon sequestered (measured at 200 meters ages show marbled swaths of green phyto- tional Institute for Water and Atmospheric depth) per ton of iron added hover around plankton spinning across waters previously Research summarized the 12 experiments at 200 to 1, a far cry from early experiments blue and barren. Satellites also show plank- an ocean iron fertilization conference con- in laboratory beakers that yielded estimates ton blooms near the Galápagos and other is- vened at Woods Hole Oceanographic In- around 100,000 to 1, Boyd said. lands where iron-rich deep waters naturally stitution (WHOI) in September 2007 and But those may be underestimates. Al- well up to surface. Even blooms spurred by in an article in Science magazine earlier last though scientists have spent up to several experimental additions of iron to the ocean year. Four took place in the northwest Pa- weeks monitoring blooms after iron addi- can be detected by satellite, and shipboard cific, two were in the equatorial Pacific, and tion, ship schedules and budgets have usu- scientists conducting the experiments re- six were in the Southern Ocean. All 12 re- ally prevented them from monitoring long ported an almost instantaneous change in ported up to 15-fold increases in the chloro- enough, or deep enough, to obtain good the color and even the smell of the water. phyll content of surface waters. (Chlorophyll measurements of “export efficiency”—the Twelve experiments so far have not is the sunlight-capturing molecule in pho- proportion of carbon that sinks from the looked so closely at the trickier questions tosynthesis and is often measured in lieu of surface into deeper waters. of how much carbon dioxide taken up by actual plankton counts.) The 2002 United States-funded SOFeX Jack Cook, WHOI 10 OCEANUS MAGAZINE Vol. 46, No. 1, 2008 www.whoi.edu/oceanus THE BOTTOM LINE—Only a small fraction of the carbon drawn into the ocean by plankton blooms makes it into the depths where it no longer can be exchanged with the atmosphere. 1 Air and sea exchange CO2. Plankton Key 2 Phytoplankton 3 Zooplankton eat Coccolithophorids take up CO2 to grow. phytoplankton and respire CO2. Diatoms 4 Fragments of decaying phyto- Other plankton and fecal pellets from Phytoplankton zooplankton both contain carbon. 0 to 100 meters Foraminifera Radiolaria 5 Separately or in aggregations Copepod (called “marine snow”), these carbon-containing particles sink. Euphausiid 9 CO2 from organic matter respiration 6 Only 5 to 50% of the total carbon from Salps recirculates back to blooms reaches 100 meters. About 2 to surface waters. 25% sinks between 100 and 500 meters. 10 Zooplankton 7 Microbes decompose particles migrate up at night to further. Zooplankton eat some of feed and back to the this material. depths during the day. 100 to 500 meters Particle Key Salp pellet Copepod pellets Microzooplankton mini-pellets Marine snow aggregates Euphausiid pellets Bits of plankton shells Remnant of mucousy webs Below 500 made by some meters plankton to feed 8 Perhaps only 1 to 15% of the original carbon Jack Cook, WHOI in surface waters sinks below 500 meters. WooDS hoLE OCEANOGRAphIC INSTITUTION 11 experiment did show that more carbon tons of carbon dioxide, estimated WHOI easiest to work in: It’s warm and sunny all was exported into deeper waters below oceanographer Jim Ledwell, but even that is year, the seas tend to be fairly calm, and the fertilized ocean patch, WHOI marine insignificant compared with the amount of the warm waters encourage rapid growth. biochemist Ken Buesseler and colleagues CO2 drawn down by a bloom.) But there is already enough plankton reported. And unpublished results from “It’s really our safety net,” Boyd said. growth in equatorial waters to eventually the 2004 European EIFeX experiment “We’ve learned a great deal about how up- use up their nutrient supply anyway; add- showed levels of carbon sequestration that per ocean physics can rapidly redistribute ing iron there just creates a faster, concen- were far higher and far deeper (all the way the added iron and SF6,” which would have trated bloom in a specific location, but the to the seafloor) than previously observed— been hard to detect without the tracer. net effect on atmospheric carbon dioxide but this occurred only in the final days of Depending on local currents, blooms can levels is arguably negligible. monitoring, Victor Smetacek of the Alfred wind up strewn across Other factors make Wegener Institute in Germany told partici- the ocean like a ball the Southern Ocean a pants at the WHOI conference. of string or confined “ ‘Location, location, better choice. It has a The emerging picture is that iron fertil- within swirling loops location’ applies equally much larger area with ization does in principle work well enough of water known as ed- much higher nutrient to squirrel away carbon for at least a few dies. The eventual size well to these iron addition levels, which should in- decades—possibly useful in the world’s ef- of blooms from small experiments. Put it in the crease the total size of forts to solve its carbon emissions problem. iron additions can span blooms that could be Although present yields seem low, improved 1,000 square kilome- wrong place, and you’ll be stimulated. It has such methods could boost that number in two ters or more, extend to chasing your tail across an abundance of nutri- ways: by refining logistics to make blooms depths of up to 100 me- ents that they actually larger, and by increasing “export efficiency,” ters, and drift hundreds the ocean.” sink before they can be or the proportion of carbon that sinks from of kilometers from their —Philip Boyd, New Zealand utilized—unless more the surface into deeper waters, where it is starting positions. Plain National Institute for Water and iron is supplied. less easily returned to the atmosphere. bad luck can get in the Atmospheric Research Other possible loca- way, too, Boyd said. tions are the low-nu- Logistics and luck The first experiment trient, low-chlorophyll Iron addition is simple in principle, but shut down after only five days when a mass (LNLC) waters at middle latitudes, where once a ship is loaded up and heading for of less dense water moved over the iron-fer- both iron and nitrate are missing. Re- open waters, even small experiments be- tilized water, pushing it far below the sunlit search is less far along in LNLC waters, come a tangle of logistics. The SOFeX ex- surface and ending the iron-induced bloom said Anthony Michaels of the University of periment employed three research ships, prematurely. Southern California, but one three-week helicopter scouts, and 76 scientists to moni- experiment in the North Atlantic showed tor the results of adding one to two tons of Location, location, location that adding iron and phosphorus can stimu- iron to the ocean. Boyd likened a successful bloom to the late the photosynthetic bacteria Trichodesmi- The typical method involves drizzling real estate market. “ ‘Location, location, lo- um. Once armed with iron, this species can acidified iron sulfate into the ocean as a cation’ applies equally well to these iron ad- convert dissolved nitrogen gas into a usable thin slurry, to reduce the amount that im- dition experiments,” he said. “Put it in the form and possibly set off blooms as large as mediately sinks out of the sunlit surface wrong place, and you’ll be chasing your tail the ones seen in HNLC waters, Michaels waters where photosynthesis happens. Add- across the ocean.” Blooms depend crucially said. The advantage is that such blooms add ing the iron requires a 12-hour zigzagging on location at two levels: the appropriate their own nutrients, rather than stripping cruise across a theoretical square of water ocean region as well as the particular patch them from surface waters—one of the key whose boundaries shift constantly in the of water in that region chosen to receive the criticisms of iron addition in HNLC re- ocean currents. In the weeks of monitor- iron slurry. gions. But the blooms could quickly die out ing that follow, a ship typically spends 12 So far, iron fertilization has been con- once another limiting nutrient, phosphate, hours out of every day just mapping out the templated mainly for ocean waters known as is exhausted. boundaries of the bloom. high-nutrient, low-chlorophyll (HNLC) re- Once an ocean region is chosen, it pays Blooms are hard to track because the gions.