Probing natural iron fertilization near the Kerguelen (Southern Ocean) using natural phytoplankton assemblages and diatom cultures

Natural phytoplankton assemblages collected in surface waters above the Kerguelen Plateau or in the open-ocean and single-species cultures of Southern Ocean diatoms were used to address the existence and effects of natural iron fertilization near the Kerguelen Islands (Southern Ocean). The phytoplankton was transferred during so-called translocation experiments into water collected at the surface over the Plateau, open-ocean surface water or water collected close to the sediment of the Plateau. These watertypes differed in iron (iron-rich deep water and iron-poor surface water) and silicic acid concentration (silicic acid-rich Plateau deep and open-ocean surface water, silicic acid-poor Plateau surface water). As a general trend in the natural phytoplankton assemblages, cell numbers, chlorophyll autofluorescence, photosynthetic efficiency of photosystem II, chlorophyll a and phytoplankton carbon concentrations increased especially after translocation into Plateau deep water. This response was most pronounced in terms of increase in carbon assimilation in the larger-sized phytoplankton (>8 μm in cell diameter), mainly diatoms. Effects of translocation on bacteria and viruses followed those of the phytoplankton. Experiments with single-species cultures of large diatoms (Fragilariopsis kerguelensis, Thalassiosira sp., Chaetoceros dichaeta), which have high iron requirements, confirmed the observations made for the natural phytoplankton assemblages.Assuming a continuous flux of deep water to the surface over the Kerguelen Plateau, the translocation experiments provide evidence that this water contains the growth-stimulating factor, most likely iron, responsible for the formation of a phytoplankton bloom as is observed over the Kerguelen Plateau.