The SOLAS air–sea gas exchange experiment (SAGE) 2004

• Published in: Deep-sea research. Part II, Topical studies in oceanography Topical studies in oceanography, 2011-03, Vol.58 (6), p.753-763
• Main Authors: Harvey, Mike J., Law, Cliff S., Smith, Murray J., Hall, Julie A., Abraham, Edward R., Stevens, Craig L., Hadfield, Mark G., Ho, David T., Ward, Brian, Archer, Stephen D., Cainey, Jill M., Currie, Kim I., Devries, Dawn, Ellwood, Michael J., Hill, Peter, Jones, Graham B., Katz, Dave, Kuparinen, Jorma, Macaskill, Burns, Main, William, Marriner, Andrew, McGregor, John, McNeil, Craig, Minnett, Peter J., Nodder, Scott D., Peloquin, Jill, Pickmere, Stuart, Pinkerton, Matthew H., Safi, Karl A., Thompson, Rona, Walkington, Matthew, Wright, Simon W., Ziolkowski, Lori A.
• Format: Article
• Language: English
• Subjects: Air–sea gas exchange; Biological; DMSP satellites; Gas exchange; Grazing; Iron; Iron fertilisation; Marine; Mathematical models; Ocean biogeochemistry; Phytoplankton; SOLAS; Tracers
• ISSN: 0967-0645; 1879-0100
• DOI: 10.1016/j.dsr2.2010.10.015

The SOLAS air–sea gas exchange experiment (SAGE) was a multiple-objective study investigating gas-transfer processes and the influence of iron fertilisation on biologically driven gas exchange in high-nitrate low-silicic acid low-chlorophyll (HNLSiLC) Sub-Antarctic waters characteristic of the expansive subpolar zone of the southern oceans. This paper provides a general introduction and summary of the main experimental findings. The release site was selected from a pre-voyage desktop study of environmental parameters to be in the south-west Bounty Trough (46.5°S 172.5°E) to the south-east of New Zealand and the experiment was conducted between mid-March and mid-April 2004. In common with other mesoscale iron addition experiments (FeAX’s), SAGE was designed as a Lagrangian study, quantifying key biological and physical drivers influencing the air–sea gas exchange processes of CO 2, DMS and other biogenic gases associated with an iron-induced phytoplankton bloom. A dual tracer SF 6/ 3He release enabled quantification of both the lateral evolution of a labelled volume (patch) of ocean and the air–sea tracer exchange at tenths of kilometer scale, in conjunction with the iron fertilisation. Estimates from the dual-tracer experiment found a quadratic dependency of the gas exchange coefficient on windspeed that is widely applicable and describe air–sea gas exchange in strong wind regimes. Within the patch, local and micrometeorological gas exchange process studies (100 m scale) and physical variables such as near-surface turbulence, temperature microstructure at the interface, wave properties and windspeed were quantified to further assist the development of gas exchange models for high-wind environments. There was a significant increase in the photosynthetic competence ( F v/ F m) of resident phytoplankton within the first day following iron addition, but in contrast to other FeAX’s, rates of net primary production and column-integrated chlorophyll a concentrations had only doubled relative to the unfertilised surrounding waters by the end of the experiment. After 15 days and four iron additions totalling 1.1 ton Fe 2+, this was a very modest response compared to other mesoscale iron enrichment experiments. An investigation of the factors limiting bloom development considered co-limitation by light and other nutrients, the phytoplankton seed-stock and grazing regulation. Whilst incident light levels and the initial Si:N ratio were the lowest recorded in all FeAX