Biological response to iron fertilization in the Polar Frontal Zone of the Southern Ocean (EisenEx)Philipp Assmy1, Joachim Henjes1, Christine Klaas2 & Victor Smetacek11Alfred-Wegener-Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany2Max-Planck-Institute for Biogeochemistry, Carl-Zeiss-Promenade 10, 07745 Jena, GermanyDuring EisenEx - the second in situ iron fertilization experiment conducted in HNLC (High Nutrient Low Chlorophyll) waters of the Southern Ocean - an iron-enriched ocean eddy was followed over a period of three weeks in austral spring 2000. The aim of this experiment was to assess the impact of iron fertilization on the pelagic community and the processes within the food web driven by changes in plankton distribution and abundance. The diatom, large microzooplankton (in particular acantharians and tintinnids) and small mesozooplankton (copepodids and nauplii) community was quantified inside and outside the iron enriched patch.Inside the fertilized patch the diatom assemblage increased six-fold in abundance and four-fold in biomass. Outside the patch (control stations) diatom abundance remained constant whereas biomass doubled. Empty intact and broken diatom frustules were quantified as indicators of grazing pressure (i.e. mortality). Diatom debris increased three-fold inside and doubled outside the patch. Empty diatom frustules doubled inside and remained constant outside the patch. Acantharian abundance increased by a factor of 3 in the fertilized patch but only slightly outside the patch. This is of major interest, because acantharians are indirectly responsible for the formation of barite which is found in sediments and is suggested to be a paleo-indicator of high productivity regimes. The total abundance of the tintinnids ciliates (i.e. full and empty loricae) doubled inside the iron- enriched patch compared to no change outside the patch. Moreover the numbers of empty loricae, an indicator of grazing pressure on tintinnids, increased about 2.2-fold inside the fertilized patch with no change outside the patch. This suggests that particularly inside the iron enriched eddy, grazing pressure intensified highly on tintinnid ciliates and completely compensated tintinnid population growth in the course of the experiment. Copepodid numbers increased significantly (by a factor of ~ 3.6) inside the patch with a ~ 2.1-fold increase in the adjacent water mass. Nauplii numbers slightly increased in patch (by a factor of 1.7) in contrast to no changes out patch.Our results confirm the stimulation of diatom growth by iron addition in the Southern Ocean. Our results also indicate that a significant seasonal increase in diatom biomass occurs in the Polar Frontal Zone (PFZ) despite low iron conditions, consistent with satellite observations. Grazing pressure intensified more heavily inside the patch compared to out patch values due to greater food availability in the fertilized patch. Furthermore grazing pressure seems to be an important factor determining diatom productivity and assemblage composition in this area of the Southern Ocean.Key words: Southern Ocean, iron fertilization, diatoms, micro- and mesozooplankton, grazing