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The first in situ iron-fertilisation in polar Southern Ocean waters, SOIREE (Southern Ocean Iron RElease Experiment) was a mesoscale, 13-day experiment that resulted in a phytoplankton bloom (chlorophyll-a increased six-fold; algal carbon, three-fold). Just prior to the iron enrichment, community chlorophyll-a at the study site was dominated by picoplankton, with an ‘innoculum’ of large diatoms. Iron enrichment resulted in a floristic shift, with increased diatom abundances, and to a lesser extent elevated nanoplankton stocks. Picoplankton stocks initially increased in response to iron supply, but were then grazed down to initial levels by microzooplankton. The chain-forming pennate diatom Fragilariopsis kerguelensis was the dominant bloom species, although <20μm haptophytes also increased in abundance. Other algal responses to iron enrichment included elevated nanoplankton cell size, increased chain length …