Response of small grazers to an iron-induced diatom bloom in the Polar Frontal Zone of the Southern Ocean

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Henjes, J. , Assmy, P. , Klaas, C. and Smetacek, V. (2003): Response of small grazers to an iron-induced diatom bloom in the Polar Frontal Zone of the Southern Ocean , Gordon Research Conference on Polar Marine Science, The changing polar oceans: impacts of a changing climate on physical, chemical, biological and coupled systems, 16-21 March, Ventura, USA. .
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During the EisenEx experiment, conducted in HNLC (High Nutrients Low Chlorophyll) waters of the Southern Ocean, the centre of an eddy was fertilized with iron sulphate and followed over a period of 3 weeks in November 2000. The aim of this experiment was to assess the impact on the pelagic community and the processes within the food web driven by changes in plankton distribution and abundance. The large microzooplankton (in particular acantharians and tintinnid) and small mesozooplankton (copepodids and nauplii) community was surveyed inside and outside the iron enriched patch. Species abundance and taxonomic composition were quantified by microscopic techniques in concentrated water samples taken from seven discrete depth between 10 and 150 mBy the end of the experiment phytoplankton biomass had increased fourfold and the microzooplankton groups showed different trends in abundance. The abundance of acantharians increased by a factor of ~ 2.7 in the fertilized patch, but only slightly outside the patch. This is of major interest, because acantharians are suggested to be indirectly responsible for the formation of barite found in sediments and a paleo-indicator of high productivity regimes.The total abundance of the tintinnid ciliates (i.e. full and empty loricae) doubled inside the iron enriched patch compared to no change outside the patch. Moreover the numbers of empty loricae, an indicator of grazing pressure on tintinnids, increased about 2.2-fold inside with no change outside. This indicates that tintinnids increased growth rates with higher food availability but grazing pressure on them also intensified so that no net growth was recorded in the course of the experiment. With regard to small mesozooplankton, copepodid numbers increased significantly (by a factor of ~ 3.6) inside the patch with a ~ 2.1-fold increase in the adjacent water mass. Nauplii numbers slightly increased in patch (by a factor of 1.7) in contrast to no changes out patch.These results indicate that at least some of the large micro- and small mesozooplankton responded to the rapidly increasing food supply due to iron addition in this Southern Ocean HNLC region. In unfertilized waters, the slight increase in numbers of small grazers matched the seasonal increase in diatom biomass.Key words: Southern Ocean, iron fertilisation, grazing, micro-, mesozooplankton, abundance

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