Marianne Camoying ^1* Florian Koch ¹ Jasmin Stimpfle ¹ Franziska Pausch ¹ Christel Hassler ^2,3 Scarlett Trimborn ¹

• ¹ EcoTrace, Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI), Bremerhaven, Germany
• ² Département F.-A. Forel Sciences de l'environnement et de l'eau, Faculté des sciences, Université de Genève, Geneva, Switzerland
• ³ Swiss Federal Institute of Technology Lausanne, Lausanne, Switzerland

Primary production in the Southern Ocean is strongly influenced by the availability of light and iron (Fe). To examine the response of two distinct natural Antarctic phytoplankton communities (diatom vs. flagellates) to increasing light and Fe availability, we conducted two shipboard incubation experiments during late summer and exposed each community to increasing light intensities (30, 80, 150 µmol photons m -2 s -1 ) in combination with or without Fe amendment. Our results show clearly that both communities were Fe-limited since Fe addition resulted in higher particulate organic carbon (POC) production rates. The magnitude of the Fe-dependent increase in POC production, however, varied between the two stations being higher in the diatom-dominated relative to the flagellate-dominated community. This differential response to increasing Fe supply could be attributed to the higher Fe requirement of the flagellate-relative to the diatom dominated assemblage. Irrespective of Fe availability, light also strongly stimulated the POC production of both communities between low versus medium light supply (30 versus 80 µmol photons m -2 s -1 ), indicating that both assemblages were light-limited in situ. However, since POC production of both communities did not increase further at the highest light intensity (150 µmol photons m -2 s -1 ) even under high Fe supply, this suggests that light supply was saturated or that other conditions must be fulfilled (e.g. availability of trace metals other than Fe) in order for the communities to benefit from the higher light and Fe conditions.