Research highlight

Phytoplankton Blooms May Remove Atmospheric Carbon

Phytoplankton Blooms May Remove Atmospheric Carbon

Carbon can be transported to the bottom of the ocean, and stored there, by sinking microscopic phytoplankton following iron fertilization, according to a report co-authored by Professor Peter Croot, National University of Ireland (NUI) Galway in Nature this week. These findings are by no means a green light for using this approach to generate carbon offsets. The researchers note that further experiments are needed to evaluate the effects on ecology, climate and the processes that determine the composition of the environment. These results do however provide a valuable contribution to our understanding of the global carbon cycle.

“These new data clearly shows that at the end of this phytoplankton bloom, a significant amount of the carbon was transported to the deep ocean over a relatively short time, a phenomenon which had not been observed in any great detail previously anywhere in the ocean” explained Dr Croot.

Previous ocean iron fertilization experiments have failed to adequately demonstrate the fate of resulting phytoplankton population explosions and hence removal of carbon from the atmosphere to the deep ocean. Dr Peter Croot and colleagues from the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association, Germany, present multiple lines of evidence from the European Iron Fertilization Experiment in the Southern Ocean that suggest carbon is exported to the deep ocean as a result of iron fertilization. The ocean iron fertilization experiments induce phytoplankton blooms, and sinking particles are tracked from the surface to the ocean floor. Taken together, their data indicate that at least half of the bloom biomass sank to below 1,000 metres, where it could potentially be stored for centuries.

The international team on board the research vessel Polarstern fertilized a part of the closed core of a stable eddy of the Southern Ocean with dissolved iron which stimulated the growth of unicellular algae (phytoplankton). The team followed the development of the phytoplankton bloom for five weeks from its start to its decline phase. The maximum biomass attained by the bloom was higher than that of blooms stimulated by the previous 12 iron fertilization experiments. According to Professor Dr Victor Smetacek and Dr Christine Klaas from the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association, this was all the more remarkable because the EIFEX bloom developed in a 100 metre deep mixed layer which is much deeper than hitherto believed to be the lower limit for bloom development. 

The bloom was dominated by diatoms, a group of algae that require dissolved silicon to make their shells and are known to form large, slimy aggregates with high sinking rates at the end of their blooms. “We were able to prove that over 50 per cent of the plankton bloom sank below 1000 metre depth indicating that their carbon content can be stored in the deep ocean and in the underlying sediments for time scales of well over a century”, says Smetacek. 

“These new findings highlight how differences between the species of phytoplankton that make up the community that formed the bloom can impact the sinking flux and transport of carbon as the bloom decays” adds Dr Croot. “This has implications for the biogeochemical cycling of other important elements in the ocean such as nitrogen, phosphorus and iron which are also part of the sinking material. In the context of Ireland’s marine areas, this work suggests a new area of focus for research targeting the end of phytoplankton blooms, rather than the traditional emphasis on the start of the spring bloom, in order to improve our overall understanding of how this economically important ecosystem functions.”

Iron plays an important role in the climate system. It is involved in many biochemical processes such as photosynthesis and is hence an essential element for biological production in the oceans and, therefore, for CO2 absorption from the atmosphere. During past ice ages the air was cooler and drier than it is today and more iron-containing dust was transported from the continents to the ocean by the wind. The iron supply to marine phytoplankton was hence higher during the ice ages. This natural process is simulated in iron fertilisation experiments under controlled conditions.

The EIFeX (European Iron Fertilisation Experiment) was a collaborative effort that involved representatives of 14 institutes and 3 companies from 7 European countries and the Republic of South Africa.