Temporal development and vertical distribution of major components of the plankton assemblage during an iron fertilization experiment in the Antarctic Polar Frontal Zone

During the iron fertilization experiment EisenEx conducted in the Antarctic Polar Frontal Zone the response of the plankton community to iron addition was studied in detail. Within the diatom assemblage four major response types could be distinguished inside the fertilized patch. Fast growing and weakly silicified diatoms like Pseudo-nitzschia lineola and Chaetoceros curvisetus exhibited exponential growth rates throughout the experiment. The heavily silicified species Fragilariopsis kerguelensis and Thalassionema nitzschioides showed an initial phase with negligible growth during the first week followed by a linear increase in abundance thereafter. Two large solitary and weakly silicified diatoms, Haslea sp. and Corethron pennatum, exhibited a linear growth with no initial lag phase. The small diatom species Nitzschia sp. and Cylindrotheca closterium were characterised by an initial linear increase and a decline during the second half of the experiment. The response of major components of the non-diatom phytoplankton assemblage, including Phaeocystis antarctica, phototrophic dinoflagellates, coccolithophores and Dictyocha speculum, to iron addition accounted for only a minor iron-induced biomass increase. In addition to life diatom cells intact empty and broken diatom frustules were also accounted for in this study as indicators of diatom mortality. The increase of both broken and intact empty frustules inside the patch indicates an increased grazing pressure by proto- and metazoen grazers. The vertical distribution of non-motile particles and planktonic organisms comprises another important aspect of this study. Whereas motile planktonic organisms are able to regulate their position in the water column by active swimming non-motile particles will eventually sink out of the surface layer. Various processes affect the composition and magnitude of non-motile particles of biological origin of which grazing seemed to have played the major role during this study.