Algal blooms re-evaluated for carbon capture

Northern hemisphere researchers who put tonnes of iron into the Southern Ocean say it not only triggered a massive algal bloom, but that dying phytoplankton carried carbon from the atmosphere into the deep ocean.

 Credit: Marina Montresor, SZN / Alfred Wegener Institute
Blooming good: A diatom species in the 2004 bloom - Corethron pennatum

The experiment, published in the journal Nature, counters previous research findings which raised questions about the efficiency of  fertilising algal blooms in iron-deficient oceans  as geoengineering approach to “sink” atmospheric carbon. It also  it provides new insight into natural processes of ocean-atmosphere interaction.

New Zealand researchers had previously found that carbon capture from iron fertilised algal blooms was unpredictable at best, casting doubt on the usefulness of the method as a solution to rising atmospheric carbon dioxide. They won the 2011 Prime Ministers Science Prize ($500,000) for their experiments in the Southern Ocean and Gulf of Alaska.

However, in the new study researchers from the  European Iron Fertilization Experiment in the Southern Ocean  say the phytoplankton bloom they triggered resulted in  at least half of the biomass which made up that specific bloom sinking to deeper than 1000 metres.

Potentially, the carbon in the microscopic phytoplankton which die and sink could  be stored in the ocean depths for centuries – but, the researchers say,  it is too soon to start geoengineering to combat global warming.

Further experiments would be needed to get a handle on the knock-on effects on ecology, and climate and how the processes that determine the composition of the environment would be affected.

The Science Media Centre contacted experts for further comment:

Dr Cliff Law, principal scientist at National Institute of Water and Atmospheric Research (NIWA), was a recipient of the 2011 Prime Minister’s Science Prize for his work on iron fertilisation. He comments:

“The paper reports on an experiment that extends the observations and understanding developed from the previous iron experiments

“As a result of longer occupation on site, and a measurement framework more focussed upon particle export, than previous experiments the authors have identified that a significant proportion of the “extra” carbon fixed by the iron-fuelled phytoplankton growth is transferred into the deep ocean.

“As this carbon will be isolated from the atmosphere for decades to centuries the results have implications for understanding of both  past and present climate change.

“That this experiment recorded a stronger phytoplankton bloom than any previous iron experiment highlights the importance of timing and location

“The 12 previous experiments to date have taken place in different regions and shown a broad range of responses, with the lowest response recorded on the NZ SAGE experiment at the same time of year and same latitude (in New Zealand waters) as the experiment reported in Nature

“Indeed a subsequent larger iron addition experiment carried out by Smetacek et al in the same region of the South Atlantic at the same time of year in 2009 saw only a modest increase in carbon export.

“So location and timing of an iron experiment is critical.

“The paper may re-open the debate around geoengineering, although the authors do not link their results to this.

” The strength of their experimental design conversely highlights the problems and limitations of larger scale iron releases (for science or geo-engineering) in that the response can only be observed, and more importantly verified to be have arisen from the iron addition, when carried out in optimal conditions such as the enclosed eddy described.

“Even under these conditions the authors mention potential interference of natural blooms, and also note significant uncertainties.

“Tracking the response to iron addition is required, not only in terms of fate of the extra carbon, but also the knock-on effects such as oxygen depletion, nitrous oxide (a greenhouse gas) production, and loss of nutrients in waters that support production in other regions.

” Observation and verification of these impacts represent huge technological challenges which will require significant financial investment to support major operations. Until this is achieved and the knock-on effects proven to be minor compared to the benefits of iron-induced carbon sequestration, then iron fertilisation cannot be considered as a viable geoengineering approach.”

Professor  Philip Boyd at Otago University’s NIWA Centre for Chemical and Physical Oceanography (also a PM’s Science Price Recipient), comments:

“It’s ‘location, location, location’.

“Not all blooms are made equal…This bloom is an end-member where conditions permitted a very efficient export of material to the deep ocean.

“Other blooms export much less material – such as our SERIES  bloom that CLiff Law and I led in the NE Pacific in early 2000’s.

“I don’t think this changes anything in this geoengineering debate.

“What we need to better understand is what controls the export efficiency of different blooms”.

Prof Tom Trull, Professor of Marine Biogeochemistry at CSIRO Marine and Atmospheric Research and the University of Tasmania, and leader of the Carbon Program at Australia’s Antarctic Climate and Ecosystems Cooperative Research Centre, comments:

“Carbon sequestration can be achieved by enhanced C export or its penetration further into the deep sea, or both.  EIFEX did not demonstrate enhanced surface carbon exort beyond what would occur naturally anyway, because the Fe fertilisation stimulated diatoms which require silicon, and that element is naturally depleted anyway as upwelled Southern Ocean surface water spread northward.  But EIFEX did observe surprisingly deep C export.  It remains to be seen whether this was an outcome particular to the EIFEX location and timing. Larger polar experiments as proposed by the authors will be important to examine this, as well as attempt to push beyond the Si limitation boundary and stimulate export from communities which do not require silicon.”

“The current IMO and UNCBD moratoriums on iron fertilisations for purposes other than research are motivated by concerns about environmental impacts. EIFEX did not seek to test these concerns and thus cannot be cited as a reason to re-evaluate them. This is a difficult issue in that no iron fertilisations have produced deleterious environmental effects, but proof that risks are minimal is far from being demonstrated. Continued experimentation with greater inclusion of impact assessment remains the wisest course.”

Our colleagues at the UK Science Media Centre also collected the following commentary:

Prof Andy Ridgwell, Professor in Earth System Modelling at the University of Bristol:

“The authors have obtained direct evidence for efficient and rapid carbon export to the deep ocean when stimulating phytoplankton growth with the nutrient iron.

“This is an extremely interesting result and one that is sure not only to renew interest in the potential for deliberate (iron) fertilisation for sequestering fossil fuel CO2 in the future – a geoengineering proposal not only currently banned under international regulations and previously assumed to be an inefficient way of sequestering carbon – but also in the understanding of past climates.

“Exactly why atmospheric CO2 concentrations during the last glacial were approximately one third lower than the current interglacial average is still hotly debated.

“The results of this study hint at a greater importance for higher glacial dust (and iron) fluxes to the surface ocean in strengthening the ocean carbon pump and hence in lowering atmospheric CO2.”

Dr Michael Steinke, Lecturer in Marine Sciences at the University of Essex, said:

“This article …  provides the very first evidence of a man-made conduit between the increasingly CO2-burdened atmosphere and the deep sea.  This transfer of CO2 assists with cooling our climate and keeping temperature at a level to facilitate life on our planet.

“The group … managed to add 7000kg of iron fertilizer to just the right part of the Antarctic Ocean to stimulate phytoplankton growth, increase the uptake of CO2 from the atmosphere and, crucially, demonstrate that at least half of this material disappeared into the deep sea.

“Several other scientific experiments tried this before but the oceanographic conditions were such that the CO2 from the decay of phytoplankton was transferred straight back into the atmosphere so that no additional cooling was produced from fertilising the sea.

“Will this open up the gates to large-scale geoengineering using ocean fertilisation to mitigate climate change?  Likely not, since the logistics of finding the right spot for such experiments are difficult and costly!  Of the twelve fertilisation 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.”

Dr Dave Reay, senior lecturer in carbon management at the University of Edinburgh:

“If the 50% figure for algal bloom biomass sinking to the deep ocean is correct then this represents a whole new ball game in terms of iron fertilisation as a geoengineering technique.

“Maybe such deliberate enhancement of carbon storage in the oceans has more legs than we thought but, as the authors acknowledge, it’s still far too early to run with it.”