Biological response to iron fertilization in the Polar Frontal Zone of the Southern Ocean (EisenEx): I. Changes in diatom community composition and biomassPhilipp 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, GermanyLarge diatoms contribute significantly to the biological carbon pump. Fertilization experiments now enable the study of phytoplankton species composition and its impact on the pump.During 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 diatom community was quantified inside and outside the fertilized patch in order to assess the impact of iron fertilization on this biogeochemically important phytoplankton group. 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. The increase in diatom abundances inside the fertilized patch was mainly due to small and medium sized diatoms (Thalassionema nitzschoides, Cylindrotheca closterium, various discoid diatoms <30 µm, Pseudonitzschia spp. and Fragilariopsis kerguelensis). Pseudonitzschia lineola was the dominant species and increased 71-fold in numbers and accounted numerically for 51% of the assemblage at the end of the experiment. The same taxa comprised 80% of total diatom abundance in the control stations. Medium sized and large diatoms accounted for the build up of diatom biomass both inside and outside the patch. Differences in the increase in carbon biomass vs. abundances between fertilized and control stations are due to the greater increase in large diatoms (mainly large cylindrical and pennate diatoms) in the control stations.The factors selecting dominance of one species over another are largely unknown and cannot be attributed to bottom-up factors alone. Therefore 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. These results indicate that grazing pressure intensified more heavily inside the patch compared to out patch values due to greater food availability in the fertilized 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. 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, biogeochemistry, diatoms, HNLC, grazing