This thesis provides a comprehensive assessment of the response and vertical distribution of major components of the pelagic ecosystem and highlights the species interactions shaping this open ocean environment and its associated biogeochemical cycles.During the iron fertilization experiment (EisenEx), carried out in the Polar Frontal Zone of the Southern Ocean in austral spring (November), the composition, succession and temporal development of the nano-, microprotozooplankton (protozoa between 2 and 20 µm and 20 and 200 µm respectively), mesozooplankton (protozoa > 200 µm) and small metazooplankton (copepod nauplii, copepodites and adults of small species) assemblage was successfully followed for three weeks and compared with non-fertilized, surrounding water. The grazing impact of microprotozooplankton on the natural phytoplankton assemblage was also studied under controlled conditions using the dark incubation method. These experimental results combined with data on microphyto- and microprotozooplankton composition and temporal development were used to estimate microprotozoan grazing impact on primary producers, in particular the diatoms, during the fertilization experiment. The temporal development of particles produced by proto and small metazooplankton (empty diatom frustules, fecal pellets, skeletons and empty loricae) and their contribution to carbon and silica vertical fluxes during EisenEx were also investigated.Key words: Polar Frontal Zone; EisenEx; protozooplankton; small metazooplankton; iron fertilization; temporal development; vertical fluxes; grazing impact; copepod nauplii; small copepods, microphytoplanktonDistinct differences in microprotozoan assemblage inside and outside in relation with the phytoplankton bloom development were not observed. The microprotozooplankton population was dominated by dinoflagellates followed by ciliates. Dinoflagellates showed a ~ three-fold increase in abundance and biomass in the first 10 d of the experiment inside the patch, but decreased thereafter to about two-fold higher values compared to pre-fertilization values. The decline after day 10 indicates increasing grazing pressure by copepods. Dinoflagellate population was dominated by small sized (20-40 µm) athecate dinoflagellates which comprised up to 58 % of population stocks suggesting that also food limitation could be a further explanation for the strong decrease since the phytoplankton bloom was dominated by long chain-forming diatoms in the second half of the experiment which are not available for these dinoflagellates as food source. Ciliate abundances increased only slightly due to heavy grazing pressure by copepods, but biomass doubled with the majority of the increase caused by a three-fold increase in aloricate choreotrichs.Grazing impact by microprotozoa estimated for EisenEx showed that microprotozoa consume only a rather small fraction of primary production, including the diatoms (6 to 18 % of primary production grazed d-1, corresponding to 3 to 9 % of diatom daily carbon stock). Grazing rate on other phytoplankton (without diatoms) was relatively higher accounting for 9 to 28 % of other phytoplankton carbon stock d-1. Thus, microprotozooplankton grazing strongly affected nanoplankton populations, but was low on the diatom assemblage.Tintinnids, thecate dinoflagellates, foraminifera, radiolarian and acantharia comprised the bulk of larger (> 50 µm) protozoan assemblage. Inside the patch, acantharian numbers increased three-fold, but only negligibly in surrounding waters. This is of major interest, since acantharia are suggested to be responsible for the formation of barite found in sediments and which is possibly a paleoindicator of high productivity regimes. Foraminiferans also increased significantly in abundance, however the marked increase of juvenile individuals after a full moon event suggests a lunar periodicity in the reproduction cycle of some foraminiferan species rather than a reproductive response to enhanced food availability. Larger thecate dinoflagellates almost doubled in numbers and biomass, but also showed an increase outside the patch. In contrast, adult radiolaria showed no clear trend during the experiment, but juveniles increasing three-fold indicating elevated reproduction. Tintinnids decreased two-fold, whereas their carbon stocks remained more or less constant caused by a change in the relative importance of species. Empty tintinnid loricae, however, increased by a factor of two indicating that grazing pressure on this group intensified during EisenEx. In mixed layer (0 to 80 m depth), overall vertical distribution followed mainly vertical distribution of potential prey particles. The stocks of larger protozoa in the surface layers was negligible when compared to the rest of the protozoa and zooplankton. Below 80 m, biomass was dominated by foraminifera indicating important influence on biological processes occurring below the mixed layer during EisenEx.Within the small metazoans, copepod nauplii numbers stayed more or less constant, whereas biomass doubled indicating individual growth during EisenEx. Numbers of small sized copepodites and adults of cyclopoids (predominantly Oithona similis) and small calanoid copepods (predominantly Ctenocalanus citer) increased ~ three-fold suggesting either a enhanced recruitment from naupliar and copepodite stages, respectively, and/or an accumulation of numbers due to migration of individuals from below the mixed layer into the upper water column caused by a diet shift.Particles produced by proto- and metazooplankton during this study showed an increase in the course of the experiment and were correlated to the vertical distribution of their producers. Protozoan fecal pellets were significantly correlated to athecate dinoflagellates indicating that they were mainly responsible for fecal pellet production. The dominance of olive green ellipsoidal to spherical pellets discriminated during EisenEx suggest that particularly larger protozoan were also actively involved in either re-ingestion and repackaging of larger metazoan pellets. Copepods were the main source of large recognisable metazoan fecal pellets as indicated by the dominance of compact cylindrical pellets characteristic for copepod feces. Both protozoan, metazoan fecal pellets, empty diatom frustules and empty tintinnid loricae showed very high abundances as compared to radiolarian skeletons. Comparison with metazoan fecal pellet abundance indicate that contribution of protozoan fecal pellets and empty diatom frustules to total vertical fluxes could be significant, despite low sinking rates.