* Corresponding author. E-mail address: j.hall@niwa.cri.nz (J.A. Hall). Deep-Sea Research II 48 (2001) 2591}2613 The impact of in situ Fe fertilisation on the microbial food web in the Southern Ocean Julie A. Hall*, Karl Sa" NIWA, PO Box 11 115, Hamilton, New Zealand Abstract During the Southern Ocean iron release experiment (SOIREE) in February 1999, the composition and dynamics of the microbial food web were studied. SOIREE was a mesoscale experiment with four infusions of Fe into the patch to elevate Fe concentrations inside the patch. During the 13 d experiment, samples were collected from the mixed layer inside and outside the patch for the enumeration of bacteria, picophytoplan- kton, phyto and heterotrophic nano#agellates, ciliates, and for estimation of bacterial production and microzooplankton grazing. Inside the patch, bacterial numbers remained constant throughout SOIREE although bacterial production increased three-fold. A strong relationship between the increase in bacterial and primary production suggested that dissolved organic carbon and nitrogen, rather than Fe, potentially limited bacterial growth. The picophytoplankton population, was composed solely of eukaryotic cells and increased three-fold over the "rst 7d of the experiment before decreasing to initial concentrations of approximately 4000cellsml. In contrast to the bacterial and picophytoplankton populations, the nanophyto#agellate population increased six-fold in numbers and 23-fold in biomass. This resulted in a three-fold increase in carbon #ow through the microbial food web inside the patch by the end of the experiment. The increased carbon #ow resulted in a small increase in total microzooplankton biomass. Ciliate abundances tripled and biomass, doubled; however, the ciliate population only contributed 3}10% of the microzooplankton biomass, which was dominated by the heterotrophic nano#agellate population. The heterotrophic nano#agellate numbers decreased three-fold by the end of the experiment; however, there was no signi"cant change in biomass throughout the experiment. The changes in the dynamics and structure of the microbial food web during the SOIREE experiment suggest that microzooplankton grazing controlled the bacterial and possibly the picophytoplankton populations. In contrast, the nanophyto#agellates were initially controlled by the Fe concentration, with microzooplankton having an impact on the population towards the end of the experiment. The addition of Fe to a small patch of the Southern Ocean had a considerable impact on the microbial components of the food web, even though the overall importance of the microbial pathways decreased as a result of Fe addition.  2001 Published by Elsevier Science Ltd. 0967-0645/01/$-see front matter  2001 Published by Elsevier Science Ltd. PII:S0967-0645(01)00010-8