"Previous experiments could not document the fate of bloom biomass," Victor Smetacek of the Alfred Wegener Institute for Polar and Marine Research, Germany, and National Institute of Oceanography, India, told environmentalresearchweb. "By carrying out the experiment in the closed core of an eddy we could follow and quantify the fate of the biomass. We could show that diatom blooms transfer the bulk of their biomass to the deep sea and, most likely, sediments."
In 2004 Smetacek and colleagues from Norway, Italy, Germany, Belgium, France, South Africa, the Netherlands, Spain, the US, Ireland, Austria, the UK, Australia, and Switzerland fertilized part of an eddy of the Antarctic Circumpolar Current with iron sulphate, creating iron concentrations roughly five times the background level. The team monitored the results for the next five weeks, as part of the European Iron Fertilization Experiment (EIFEX).
"We could demonstrate that blooms can develop even in deep mixed layers of 100 m if enough iron is available," said Smetacek. "This was an eye-opener for us as we used to think – together with our colleagues – that blooms only develop in shallow (<50 m) mixed layers."
Although the Southern Ocean contains adequate levels of the essential plant nutrients nitrogen and phosphorus, algal growth is limited by a lack of iron. Its addition at this location resulted in a bloom of diatoms, which have silica walls. The bloom peaked four weeks after the fertilization; dead diatoms then formed mucilaginous aggregates that sank to the seabed.
"Diatoms are considered the 'pastures of the sea' but our experiment, and previous ones, show that they are actually the thistles, i.e. they are sparingly eaten, enabling them to build up blooms of large biomass," said Smetacek. "We could also show that when diatom blooms disappear from the surface layer in satellite images, they have not been eaten in the surface layer but have sunk out of it and provide sustenance to deep-sea organisms. The implications are that diatom blooms export organic matter, hence carbon, to the deep ocean and sediments."
The field of iron fertilization has been controversial. There have been twelve experiments to date; the 2009 trial LOHAFEX, a successor to EIFEX, was briefly suspended by the German government before being reinstated. The LOHAFEX experiment, in a region of the Southern Ocean that is low in silicic acid, did not see blooms of diatoms or extensive carbon sequestration. The other phytoplankton species that bloomed, which lacked a protective silica shell, were heavily grazed by predators.
"Apart from the many insights on how planktonic ecosystems function, we could also show, in conjunction with a subsequent experiment LOHAFEX, that only diatom blooms export organic carbon to the deep sea," said Smetacek. "Diatoms have an obligate demand for silicon and can only grow in restricted regions of the ocean."
Smetacek, who had worked on the fate of diatom blooms since the early 1970s, retired last year but said he "continues to worry about what global climate change and sea-level rise has in stock for our planet".
Commenting on the research, Michael Steinke of the University of Essex, UK, said: "Will this open up the gates to large-scale geoengineering using ocean fertilization to mitigate climate change? Likely not, since the logistics of finding the right spot for such experiments are difficult and costly. Of the 12 fertilization experiments of this kind carried out since 1993, many showed the desired increase in CO2 drawdown from the atmosphere, but this group's experiment is the only example to date that demonstrates the all-important carbon burial in the deep-sea sediments, away from the atmosphere."
In an accompanying Nature News and Views article, Ken Buesseler of the Woods Hole Oceanographic Institution, US, said that questions remain about the possible unintended consequences of geoengineering, such as the production of greenhouse-gas nitrous oxide, oxygen depletion in mid-waters as algae decompose, or stimulation of a toxic algal bloom. "The ocean's capacity for carbon sequestration in low-iron regions is just a fraction of anthropogenic CO2 emissions, and such sequestration is not permanent – it lasts only for decades to centuries," he wrote.
The team reported the results in Nature.