Distribution of dissolved iron during the natural iron-fertilization experiment KEOPS (Kerguelen Plateau, Southern Ocean)

Abstract

Abstract The Southern Ocean is the largest high-nutrient low-chlorophyll (HNLC) region of the world ocean. It is now well recognized that the low biological productivity in this region is mainly due to the limitation of phytoplankton growth by iron. However, in the core of the Southern Ocean, a massive bloom occurs annually above the Kerguelen Plateau. In the context of KEOPS (Kerguelen Plateau and Ocean compared Study) we investigated the three-dimensional distribution of dissolved iron (DFe) concentrations southeast of the island. We show that in surface waters, DFe was low and not significantly different above and outside the plateau (0.090±0.034 and 0.073±0.014 nM, respectively). By contrast, below 150 m major differences in vertical profiles of DFe were observed. The deep waters above the plateau were clearly enriched, with DFe concentrations up to 0.6 nM near the seafloor. The regeneration from sinking biogenic material and the input from the sediments are very likely the major processes delivering DFe to the deep water. The deep iron-rich reservoir existing above the plateau was responsible for the natural iron fertilization of the surface water by two different mechanisms: (i) winter mixing provided 5 times more DFe to the surface waters above the plateau than outside and (ii) enhanced vertical gradients of DFe and elevated vertical diffusivity resulted in an 8-fold higher diapycnal diffusive flux. The natural iron fertilization presented major differences from artificial iron-fertilization experiments carried out in the Southern Ocean. The amount of DFe required to stimulate the biological activity is much less and the mode of addition is clearly different. The quantification of the excess of DFe supplied in the naturally fertilized area leads to the conclusion that the natural iron fertilization is 10–150 times more efficient to export carbon below 200 m than in artificial fertilization experiments. The bloom above the Kerguelen Plateau was contingent to iron fertilization but it was also sustained by continuous input of major nutrients. Thus, the natural fertilization described here mimics quite well glacial period scenarios where iron fertilization of the Southern Ocean might have occurred from below.