Iron Fertilisation Experiments in the Southern Ocean

By: Prof V Smetacek and Dr S W A Naqvi
Of the various macro-engineering schemes proposed to mitigate global warming, ocean iron fertilisation is one that could be started at short notice on relevant scales. It is based on the reasoning that adding trace amounts of iron to iron-limited phytoplankton of the Southern Ocean will lead to blooms, mass sinking of organic matter and ultimately sequestration of significant amounts of atmospheric carbon dioxide in the deep sea and sediments.
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Oceans play a key role in shaping global climate by regulating atmospheric concentrations of CO2. However, fossil fuel burning has swamped the natural carbon cycle and it is a challenge for earth system scientists to investigate how ocean uptake can be enhanced by manipulating natural processes responsible for sequestering CO2 from the atmosphere. One such technique, ocean iron fertilisation (OIF), involves fertilising certain ocean regions with trace amounts of iron to stimulate the growth of microscopic plant-like organisms known as phytoplankton, which then die and sink. The ocean iron hypothesis, proposed by John Martin in 1990 (Martin 1990), has been tested by five mesoscale experiments that provided strong support for its first condition: stimulation of a diatom bloom accompanied by significant CO2 drawdown.

Case study

In early 2009 LOHAFEX, (LOHA Hindi for iron, FEX Fertilisation EXperiment) was conducted in the Southwest Atlantic Ocean designed to further the understanding of OIF process and its potential. The experiment was designed by Alfred Wegener Institute for Polar and Marine Research, Germany, and National Institute of Oceanography, India, together with scientists from nine other institutions in India, Europe and Chile. Its results differed significantly from those of previous OIF experiments. The six key findings of LOHAFEX are: 1. Diatoms were conspicuous by their absence due to low ambient silicate levels, and phytoplankton biomass was dominated by small (<10 µm) flagellates; 2. Phytoplankton biomass did not build up beyond 1.7 mg chlorophyll a m-3, presumably due to intense grazing by zooplankton; 3. Although primary productivity almost doubled in response to fertilisation, bacterial biomass and production remained low; 4. CO2 drawdown inside the patch was modest (<15 µatm) and organic carbon accumulated in the surface layer in particulate and dissolved forms; 5. There was little export of particulate organic matter to the deep sea; and 6. Iron fertilisation had little effect on the production of other climatically-important greenhouse gases, such as nitrous oxide and ozone-destroying halocarbons.

The LOHAFEX results have two important implications. One is that although phytoplankton production in the Southern Ocean is iron-limited, supplying iron in the absence of adequate dissolved silicon for diatoms does not support unlimited biomass build-up due to top down control by grazers. However, bottom up control due to limitation by other micronutrients, e.g. cobalt, could not be excluded. Cobalt is an essential element required for vitamin B12 and its concentrations reached limiting levels at the end of the experiment. Second, because silicon is at low concentrations over 65 per cent of the Southern Ocean, the potential of OIF as a means to sequester anthropogenic CO2 should be substantially smaller than believed so far (1 Gt carbon per year).

Endnote

A number of arguments pertaining to the fate of bloom biomass, the ratio of iron added to carbon sequestered and various side effects of fertilisation, continue to cast doubt on the efficacy of OIF. The idea is also unpopular with some environmental groups because it is perceived as meddling with nature. However, the opposition to OIF is premature because none of the published experiments were specifically designed to test its second condition pertaining to the fate of iron-induced organic carbon production. Furthermore, the arguments on side effects are based on worst-case scenarios. These doubts, formulated as hypotheses, need to be tested in the next generation of OIF experiments. We argue that such experiments, if carried out at appropriate scales and localities, will not only show whether the technique is feasible, but will also lead to a better understanding of the structure and functioning of pelagic ecosystems in general and the krill-based Southern Ocean ecosystem, in particular. The outcomes of current models on the efficacy and side effects of OIF differ widely, so additional data from properly designed experiments are urgently needed for realistic parameterisation. OIF is likely to boost zooplankton stocks, including krill, which could have a positive effect on recovery of the great whale populations. Negative effects of possible commercialisation of OIF can be controlled by the establishment of an international body headed by scientists to supervise and monitor its implementation.

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