An iron budget during the natural iron fertilisation experiment KEOPS (Kerguelen Islands, Southern Ocean)

Abstract. Total dissolvable iron (TDFe) was measured in the water column above and in the surrounding of the Kerguelen Plateau (Indian sector of the Southern Ocean) during the KErguelen Ocean Plateau compared Study (KEOPS) cruise. TDFe concentrations ranged from 0.90 to 65.6 nmol L⁻¹ above the plateau and from 0.34 to 2.23 nmol L⁻¹ offshore of the plateau. Station C1 located south of the plateau, near Heard Island, exhibited very high values (329–770 nmol L⁻¹). Apparent particulate iron (Fe_app), calculated as the difference between the TDFe and the dissolved iron measured on board (DFe) represented 95±5% of the TDFe above the plateau, suggesting that particles and refractory colloids largely dominated the iron pool. This paper presents a budget of DFe and Fe_app above the plateau. Lateral advection of water that had been in contact with the continental shelf of Heard Island seems to be the predominant source of Fe_app and DFe above the plateau, with a supply of 9.7±3.6×10⁶ and 8.3±11.6×10³ mol d⁻¹, respectively. The residence times of 1.7 and 48 days estimated for Fe_app and DFe respectively, indicate a rapid turnover in the surface water. A comparison between Fe_app and total particulate iron (TPFe) suggests that the total dissolved fraction is mainly constituted of small refractory colloids. This fraction does not seem to be a potential source of iron to the phytoplankton in our study. Finally, when taking into account the lateral supply of dissolved iron, the seasonal carbon sequestration efficiency was estimated at 154 000 mol C (mol Fe)⁻¹, which is 4-fold lower than the previously estimated value in this area but still 18-fold higher than the one estimated during the other study of a natural iron fertilisation experiment, CROZEX.