Concluding remarks

The decision to carry out large-scale OIF should be based on a comprehensive assessment of the long- and short-term as well as local and global effects derived from experiments and models, hence taken by an international body, preferably an agency of the UN specifically established for the purpose. This science-oriented body would subsequently manage and monitor larger scale OIF whether carried out by itself (analogous to deployment of experts by the well-known International Atomic Energy Agency, IAEA, of the UN) or by private companies under contract. OIF can be opened to the carbon credit market only if such an independent body allots a specific carbon quota per tonne of iron added in a specific season and region. For reasons dealt with below, inherent uncertainties are likely to prevent the scientific community from ever being in a position to allot fixed quotas for the SO. Indeed, we feel it is highly unlikely that a free-for-all carbon market could ever take over the SO as feared by many scientists (Chisholm et al. 2001) owing to the verification problem. It is up to the scientific community to exercise tight control over any large-scale OIF operation by generating the knowledge required to assess its effects. Statements calling for more experiments have been made by several scientific and other international bodies (Buesseler et al. 2008).

We have outlined hypothetical differences between open ACC and land-influenced waters above; another region where a very different type of phyto-plankton assemblage is present is the silicon-limited band of the ACC north of the Polar Front, in particular the regions where Antarctic Intermediate Water is formed. The SOFEX north patch, carried out in low-silicon waters, induced a bloom of thin-shelled pennate diatoms followed by significant enhancement of vertical flux (Bishop et al. 2004; Coale et al. 2004). However, in addition to thin-shelled diatoms, haptophyte flagellates known to produce copious quantities of DMSP, in particular the colonial species Phaeocystis and the calcifying coccol-ithophorids, also dominate the phytoplankton of this region (Seeyave et al. 2007; Lampitt et al. 2008). If OIF stimulates growth of coccolithophorids then pCO2 decrease caused by the uptake and export of organic carbon will be offset by the increase resulting from CaCO3 formation that reduces the alkalinity as well as the pH of seawater.

Experiments in this area would shed light on the much debated factors selecting for diatom or haptophyte dominance of the SO phytoplankton blooms (Smetacek et al. 2004). Thus, Phaeocystis blooms are recurrent in the Ross Sea but not in the Atlantic Sector and it is possible that sustained fertilization may lead to sustained blooming of this species, which does not appear to either provide high-quality food to zooplankton or contribute as efficiently to vertical flux as do diatoms. The toxic algal blooms reported from coastal waters around the world are unlikely to occur in the SO, although it cannot be excluded that toxic species of diatoms (Pseudo-nitzschia spp.), not currently present, may appear in the future. These possibilities underline the need to first carry out experiments but also, if they prove successful, to monitor carefully the effects of any large-scale OIF continuously and stop fertilization if and when needed.

Prolonged OIF will certainly boost zooplankton stocks, in particular copepods, and possibly also their predator populations. Surface-living, copepod-feeding fish are absent in the HNLC ACC (Smetacek et al. 2004), so it is impossible to predict which other predator groups, from coelenterates (jelly-fish) and amphipods, to mesopelagic fish and squid and even the copepod-feeding, endangered southern right whales, might profit from copepod population build-up. It will also be necessary to follow the effects of sustained fertilization on the mesopelagic community of copepods and radiolarians because, if their population density increases over time, they will intercept an increasingly larger proportion of the deep sinking flux. In such a case, it would be advisable to interrupt OIF for appropriate periods. It would also be necessary to monitor closely oxygen concentrations in the water column and sediments underlying OIF regions and halt operations if and when large oxygen depletion begins to take place.

Even under the best possible conditions, OIF will have only a limited effect on the rate at which atmospheric CO2 is projected to rise, but the amount involved is too large to be discounted; in short, we cannot afford not to thoroughly investigate the potential of this technique. A further benefit accruing from large-scale experiments is the attention they draw from the media, eager to report new developments in the global struggle, now in its infancy, that will help meet the challenge of global warming. The case for and against OIF can be used as a platform to educate the public on the workings of the global carbon cycle and the anthropogenic impact on it by providing a perspective on the quantities involved. OIF experiments would also serve as an ideal training ground for the next generation of ocean scientists faced with the challenge of coping with ongoing climate change in a global context.


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