| Summary: | Annual phytoplankton blooms are observed around most sub-Antarctic islands during austral spring and summer, but are absent in the surrounding high-nutrient low-chlorophyll (HNLC) ocean. The CROZEX study (2004/2005) tested the hypothesis that annual blooms occurring immediately north of the Crozet islands in the Polar Frontal Zone (PFZ) develop because of natural iron fertilisation, while to their south in HNLC waters, there is insufficient iron (Fe) to promote blooms. Size-fractionated nitrogen uptake (?N) experiments using 15N-NO3?, NH4+ and urea addressed three major goals. Firstly, measurements of NO3? uptake (?NO3?) aimed to establish whether ?NO3? responds to natural Fe fertilisation. Secondly, we compared regional ?NO3? rates, hypothesising that in Fe-fertilised regions, ?NO3? should exceed that in HNLC regions. Thirdly, by using satellite imagery, we extrapolated ?NO3? measurements made during a declining bloom to reconstruct seasonal ?NO3? by the spring bloom. Finally, we estimated the ‘new’ Fe demand required to support ?NO3?, comparing this with estimated Fe fluxes. Diatoms and colonial Phaeocystis dominated phytoplankton communities north of the islands, while to the south, Phaeocystis was absent. Total ?N was elevated north of the islands (400 ?mol m?2 d?1) relative to south of the islands (250 ?mol m?2 d?1). Nitrate uptake showed a clear response to Fe fertilisation, exhibiting a strong north (198 ?mol m?2 d?1) to south (74 ?mol m?2 d?1) gradient, while neither ?NH4+ nor ?urea showed such significant latitudinal gradients. The N–S integrated f-ratio gradient was 0.47–0.28 while specific N uptake (VNO3 d?1) rates were significantly higher in the Fe-fertilised region relative to those in the southern HNLC region. The potential for NH4+ inhibition of ?NO3? did not appear to be significant. High PON:chl-a ratios combined with relatively low 14C:15N uptake ratios suggested that most phytoplankton were relatively chlorotic and carbon stressed, with the exception of those growing actively within a ...
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