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Abstract

The Subtropical South Pacific Gyre (SPG) is the most oligotrophic part of the ocean and very remote due to its large size. With the SO245 Ultra Pac Cruise this region was crossed, starting in Antofagasta, Chile and ending in Wellington, New Zealand. The aim of the cruise was to identify the biological and biogeochemical characteristics of the least studied part of the ocean. Although the central gyre is ultra-oligotrophic, deep chlorophyll a maxima with up to 0.97mg m-3 Chl a and cell numbers of microplankton (20-200 μm) of 51509 cells m-3 were observed. The deepest chlorophyll a maximum occurred at the centre of the gyre at 225 m. Before entering the central gyre oligotrophic waters with a 50 m higher DCM could be observed. Despite changing cell concentrations and decreasing nutrient concentrations the integrated Chl a from Chile to the gyre was stable. The post-gyre waters were characterized by higher nutrients, higher integrated Chl a values and higher plankton abundances. This leads to the conclusion that the phytoplankton is influenced by nutrients. However, only weak correlations could be observed. This indicates that other factors like grazing and viral lyses might have an influence on the phytoplankton. In general, a high diversity of dinoflagellates in comparison to diatoms could be observed. Despite low abundances of diatoms, associations of Rhizosolenia spp. with heterocystous cyanobacteria, Richelia, could be observed. These mainly occurred in the centre of the gyre, where in 71% of the Rhizosolenia spp. population heterocystous cyanobacteria could be observed. Furthermore, heterotrophic dinoflagellates with associated cyanobacteria could be observed at the gyre station as well. It is likely that the occurrence of cyanobacteria associations is influenced by nutrients. They were occurring in waters of the SPG, which are favourable for N2 fixation. Although this is the first time they were observed in the SPG, N2 fixation was measured before. It still needs to be determined if and in which amount DDAs are contributing to the N2 fixation in the SPG. Although their abundance is very low (112 cells m-3) their biomass and previously determined N2 fixation rate are high. This might be, nevertheless, a small part of the overall N2 fixation, but due to the size of the SPG even small rates can be important for the N budget of the ecosystem. Further studies need to be done to improve the understanding of the organisms occurring in the rarely studied SPG and their importance in the N cycle.