Iron budgets for three distinct biogeochemical sites around the Kerguelen Archipelago (Southern Ocean) during the natural fertilisation study, KEOPS-2

• Published in: Biogeosciences
• Main Authors: Bowie, AR, van der Merwe, P, Queroue, F, Trull, T, Fourquez, M, Planchon, F, Sarthou, G, Chever, F, Townsend, AT, Obernosterer, I, Sallee, J-B, Blain, S
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus GmbH 2015
• Subjects: Earth Sciences; Oceanography; Chemical Oceanography; Kerguelen; Southern Ocean
• Online Access: https://doi.org/10.5194/bg-12-4421-2015; http://ecite.utas.edu.au/103859

Iron availability in the Southern Ocean controls phytoplankton growth,community composition and the uptake of atmospheric CO 2 by thebiological pump. The KEOPS-2 (KErguelen Ocean and Plateau compared Study 2)"process study", took place around the Kerguelen Plateau in the Indiansector of the Southern Ocean. This is a region naturally fertilised with ironon the scale of hundreds to thousands of square kilometres, producing amosaic of spring blooms which show distinct biological and biogeochemicalresponses to fertilisation. This paper presents biogeochemical iron budgets(incorporating vertical and lateral supply, internal cycling, and sinks) forthree contrasting sites: an upstream high-nutrient low-chlorophyll reference,over the plateau and in the offshore plume east of the Kerguelen Islands.These budgets show that distinct regional environments driven by complexcirculation and transport pathways are responsible for differences in themode and strength of iron supply, with vertical supply dominant on theplateau and lateral supply dominant in the plume. Iron supply from "new"sources (diffusion, upwelling, entrainment, lateral advection, atmosphericdust) to the surface waters of the plume was double that above the plateauand 20 times greater than at the reference site, whilst iron demand (measuredby cellular uptake) in the plume was similar to that above the plateau but 40times greater than at the reference site. "Recycled" iron supply bybacterial regeneration and zooplankton grazing was a relatively minorcomponent at all sites (< 8 % of new supply), in contrast to earlierfindings from other biogeochemical iron budgets in the Southern Ocean. Overthe plateau, a particulate iron dissolution term of 2.5 % was invoked tobalance the budget; this approximately doubled the standing stock ofdissolved iron in the mixed layer. The exchange of iron between dissolved,biogenic particulate and lithogenic particulate pools was highly dynamic intime and space, resulting in a decoupling of the iron supply and carbonexport and, importantly, controlling the efficiency of fertilisation.