To improve the understanding of population dynamics of organisms, it is necessary to examine organisms under natural conditions. Only at such conditions, knowledge about their in situ use of resources, predator-prey interactions and loss rates can be obtained. It is possible to examine and understand these processes with controlled ocean iron fertilization experiments, which stimulate the growth of unicellular algae. This thesis provides a comprehensive description of the reactions and vertical distribution of the main components of the pelagic ecosystem and highlights the food-web interactions of individual organisms, which characterize this habitat. The response of the plankton community to iron addition was successfully observed during the Indo-German iron fertilisation experiment LOHAFEX (LOHA is hindi and means iron, FEX stands for fertilisation experiment) carried out in the Southern Ocean from January to March in 2009 lasting for 38 days. The iron-induced bloom was achieved in the closed core of a mesoscale eddy. The aim of the experiment was to study the growth and demise of the phytoplankton bloom and to examine whether the biomass is retained in the surface layers through recycling processes or whether biomass sinks out to the deep ocean. The fertilized patch was characterized by low silicic acid concentrations, which is an essential nutrient for diatoms, hence a flagellate-bloom developed with cells < 20 µm in size. The bloom remained stable over the course of the experiment. This was verified by microscopic analysis and molecular methods. The main reason for the lack of large scale biomass increase, was the strong grazing pressure by the large copepod population, consisting primarily of Calanus simillimus and Oithona similis. Incubation experiments proved that copepods increased their grazing rates and fecal pellet production within the patch. Neutrally buoyant PELAGRA traps were deployed to quantify the export fluxes. These contained a few diatoms and fecal pellets and were dominated by unicellular plankton like dinoflagellates, flagellates and coccoid cells. Hence fertilizing this type of plankton community did not increase the vertical flux. This lead to the hypothesis that the system was influenced by recycling processes in surface layers. The protozoan community and its vertical distribution was an important part of this work and was studied by focussing on key heterotrophic organisms. These included thekate dinoflagellates, loricate and aloricate ciliates, foraminifera, radiolarians, and juvenile and adult copepods. Again, the high grazing pressure by the copepods controlled the development of the protozoan community, and the copepod fecal pellets contained large amounts of damaged and empty loricae, but also foraminifera, which are usually not a preferred food item for copepods. This indicated that the copepods were food limited and resorted to large, armoured protozoans which they might have otherwise avoided. Dinoflagellates were mainly abundant at depths below 100 m, possibly this distribution pattern was caused by an escape response in order to avoid predators and to feed on the flux of larger particles which generally peaks at those depths. In conclusion, the plankton community during LOHAFEX was top-down controlled whereby a highly efficient retention type system prevailed, which resulted in low particle export fluxe
