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Small-scale distribution patterns of seabirds in the Antarctic Polar Front (APF) were investigated in relation to other biological, physical, and chemical features during the ANT-XIII/2 research cruise of R.V. Polarstern from December 1995 to January 1996. The APF is characterized by steep gradients in sea-surface temperature and salinity. Within the APF, gradient zones were closely associated with elevated levels of primary production, chlorophyll-a (chl-a) concentrations, and zooplankton densities. Even broad-billed prions ('Pachyptila vittata-group'), which dominated the seabird community by 83% in carbon requirements, showed small-scale distributional patterns that were positively related to primary production, chl-a, and total zooplankton densities. The findings demonstrate a close, direct link between fine-scale physical processes in the APF and biological activity through several food web levels up to that of zooplankton-eating seabirds. Broad-billed prions appeared to forage on very small copepods (Oithona spp.) in close association with the front. Fish- and squid-eating predators showed poor correlations with small-scale spatial structures of the APF. However, in a wider band around the APF, most top predators did occur in elevated densities, showing gradual spatio-temporal diffusion of the impact of the APF on higher trophic levels. (C) 2002 Elsevier Science Ltd. All rights reserved.

The impact of food regimes on the feeding dynamics, bioelemental composition and community structure of the two Southern Ocean salps Salpa thompsoni and Ihlea racovitzai are studied. During a mesoscale iron fertilisation experiment (EIFEX) in the Southern Ocean Polar Frontal zone, S. thompsoni showed an enhanced ingestion inside the fast developing bloom but a low degradation efficiency of the diatom rich food. Seasonal feeding dynamics of S. thompsoni and I. racovitzai were investigated during three surveys in the Lazarev Sea in summer, fall, and winter (LAKRIS and IPY 2007/2008). Pigment concentrations in the salp guts of both species showed significant positive correlation to ambient surface chlorophyll concentrations with high feeding activity of Ihlea racovitzai in winter. Ecological impacts of the salps grazing pressure and contribution to the vertical flux by the production of fecal pellets are discussed for both studies. Population structure and demographic development strongly suggest that blooms of both salp species are pulsed events of ephemeral duration in Antarctic waters.

Chaetognaths contribute significantly to the zooplankton abundance and biomass in the world ocean. Stratified sampling between 500 and 2000 m depth with small mesh sizes in the Atlantic Southern Ocean has provided a unique opportunity for a detailed investigation of meso- and bathypelagic chaetognaths. The chaetognath community consisted of ten species with Eukrohnia hamata and Sagitta marri dominating. E. bathypelagica and E. bathyantarctica were characteristic deep-living species. Water depth and season were the determinants of chaetognath abundance and species composition patterns, indicating vertical seasonal migration and vertical segregation of species. Reproduction may be continuous in E. bathypelagica and E. bathyantarctica. These two deep-living species do not store high amounts of lipids and therefore rely on food supply year-round. However, their diet seems to be different because of their varying fatty acid compositions. The impact of meso- and bathypelagic chaetognaths on the copepod community in the Southern Ocean is probably significant and they may hold a central position channelling energy from lower to higher trophic levels.